@prefix vivo: . @prefix edm: . @prefix ns0: . @prefix dcterms: . @prefix skos: . vivo:departmentOrSchool "Applied Science, Faculty of"@en, "Mechanical Engineering, Department of"@en ; edm:dataProvider "DSpace"@en ; ns0:degreeCampus "UBCV"@en ; dcterms:creator "Shield, Malcolm"@en ; dcterms:issued "2011-10-04T17:42:31Z"@en, "2011"@en ; vivo:relatedDegree "Doctor of Philosophy - PhD"@en ; ns0:degreeGrantor "University of British Columbia"@en ; dcterms:description """A rapid compression machine was redesigned to allow the use of acetone UV laser diagnostics to investigate the mixture distribution that results from the injection of a methane partially stratified charge (PSC) and direct injection (DI) jet. A central composite test matrix was used to investigate the effect of relative injection timing and bulk charge air-fuel ratio upon the mixture distribution. Comparison was made between the distribution that resulted from a capillary injected PSC charge and a bespoke ‘sparkplug insert’ injected charge. The capillary injected PSC jet was found to preserve a jet-like structure despite its interaction with the direct injection jet, while the effect of the DI jet upon the insert injected fuel was to encourage coalescence of the jets to form a largely homogeneous mixture at the point of injection and near the leading edge of the DI jet. The DI jet, with a weak bulk charge, served to reduce the fluctuations in relative air-fuel ratio compared to PSC injection into air; while the insert injected PSC charge exhibited increased fluctuation levels with advanced relative injection timing. The improved ignition of a partially stratified charge from the introduction of a weak bulk charge had been presumed to work through reductions in fuel concentration gradients, however the findings of the this work suggest that this works in unison with a decrease in fuel concentration fluctuations that increases ignition efficacy. The insert injected PSC fuel demonstrates scalar dissipation rates that are potentially too low to provide robust enough combustion for a viable partially stratified charge approach. The PSC insert engenders more mixing than a capillary injected PSC, but penetrates the DI jet less well. In all cases, and throughout the region of the interaction, there exists a finite probability of encountering pure fuel or the bulk fuel concentration that suggests mixing driven by engulfment rather than entrainment. The PSC ‘sparkplug’ insert offers better opportunity for mixing than the capillary injection and using a stochastic design approach should be pursued further to improve the performance of partially stratified charge combustion for natural gas engines."""@en ; edm:aggregatedCHO "https://circle.library.ubc.ca/rest/handle/2429/37778?expand=metadata"@en ; skos:note """ MIXTURE FORMATION IN A PARTIALLY STRATIFIED CHARGE DIRECTLY INJECTED NATURAL GAS ENGINE by MALCOLM SHIELD Master of Engineering (Hons.), Imperial College, University of London, 2003 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) October 2011 © Malcolm Shield, 2011 ii ABSTRACT A rapid compression machine was redesigned to allow the use of acetone UV laser diagnostics to investigate the mixture distribution that results from the injection of a methane partially stratified charge (PSC) and direct injection (DI) jet. A central composite test matrix was used to investigate the effect of relative injection timing and bulk charge air-fuel ratio upon the mixture distribution. Comparison was made between the distribution that resulted from a capillary injected PSC charge and a bespoke ‘sparkplug insert’ injected charge. The capillary injected PSC jet was found to preserve a jet-like structure despite its interaction with the direct injection jet, while the effect of the DI jet upon the insert injected fuel was to encourage coalescence of the jets to form a largely homogeneous mixture at the point of injection and near the leading edge of the DI jet. The DI jet, with a weak bulk charge, served to reduce the fluctuations in relative air-fuel ratio compared to PSC injection into air; while the insert injected PSC charge exhibited increased fluctuation levels with advanced relative injection timing. The improved ignition of a partially stratified charge from the introduction of a weak bulk charge had been presumed to work through reductions in fuel concentration gradients, however the findings of the this work suggest that this works in unison with a decrease in fuel concentration fluctuations that increases ignition efficacy. The insert injected PSC fuel demonstrates scalar dissipation rates that are potentially too low to provide robust enough combustion for a viable partially stratified charge approach. The PSC insert engenders more mixing than a capillary injected PSC, but penetrates the DI jet less well. In all cases, and throughout the region of the interaction, there exists a finite probability of encountering pure fuel or the bulk fuel concentration that suggests mixing driven by engulfment rather than entrainment. The PSC ‘sparkplug’ insert offers better opportunity for mixing than the capillary injection and using a stochastic design approach should be pursued further to improve the performance of partially stratified charge combustion for natural gas engines. iii TABLE OF CONTENTS ABSTRACT .................................................................................................................................................................. ii   TABLE OF CONTENTS................................................................................................................................................. iii   LIST OF TABLES......................................................................................................................................................... vi   LIST OF FIGURES....................................................................................................................................................... vii   LIST OF SYMBOLS & ABBREVAIATIONS.................................................................................................................. xiii   ACKNNOWLEDGEMENTS.......................................................................................................................................... xvi   DEDICATION .......................................................................................................................................................... xviii   CHAPTER 1   INTRODUCTION ............................................................................................................................ 1   1▪1   INTRODUCTION................................................................................................................................................. 1   1▪2   THE NEED ........................................................................................................................................................ 2   1▪3   THE DIRECTLY INJECTED PARTIALLY STRATIFIED CHARGE NATURAL GAS ENGINE ...................................... 4   1▪4   AIMS................................................................................................................................................................. 5   1▪5   OBJECTIVES...................................................................................................................................................... 5   1▪6   DISSERTATION STRUCTURE.............................................................................................................................. 6   CHAPTER 2   GASEOUS JETS, GAS ENGINES & POLLUTANT EMISSIONS ............................................ 7   2▪1   INTRODUCTION................................................................................................................................................. 7   2▪2   TURBULENT GASEOUS JETS ............................................................................................................................. 8   2▪3   NATURAL GAS ENGINES ................................................................................................................................ 15   2▪4   POLLUTANTS AND ENGINE EMISSIONS .......................................................................................................... 23   2▪5   OXIDES OF NITROGEN.................................................................................................................................... 24   2▪6   CARBON MONOXIDE ...................................................................................................................................... 26   2▪7   HYDROCARBON EMISSIONS............................................................................................................................ 27   2▪8   PARTICULATE MATTER .................................................................................................................................. 29   2▪9   HYDROCARBON COMBUSTION & CARBON DIOXIDE PRODUCTION ................................................................ 30   2▪10   PHOTOCHEMICAL SMOG & TROPOSPHERIC OZONE...................................................................................... 31   2▪11   HEALTH IMPLICATIONS OF ENGINE EMISSIONS ........................................................................................... 31   2▪12   CLIMATIC IMPLICATIONS OF ENGINE EMISSIONS ......................................................................................... 32   2▪13   CONCLUSIONS .............................................................................................................................................. 32   CHAPTER 3   LASER IMAGING TECHNIQUES & PLIF FUNDAMENTALS ............................................ 33   3▪1   INTRODUCTION............................................................................................................................................... 33   3▪2   BACKGROUND ................................................................................................................................................ 34   3▪3   COHERENT AND INCOHERENT PROCESSES..................................................................................................... 34   3▪4   VELOCITY TECHNIQUES ................................................................................................................................. 35   3▪5   RAYLEIGH AND MIE SCATTERING ................................................................................................................. 35   3▪6   RAMAN SCATTERING ..................................................................................................................................... 36   iv 3▪7   COHERENT ANTI-RAMAN SCATTERING.......................................................................................................... 38   3▪8   LASER INDUCED FLUORESCENCE ................................................................................................................... 39   3▪9   APPLICATIONS OF PLANAR LASER INDUCED FLUORESCENCE ....................................................................... 40   3▪10   TRACER CHOICES ......................................................................................................................................... 42   3▪11   ACETONE AS A FLUORESCENT MEDIUM....................................................................................................... 49   3▪12   CONCLUSIONS .............................................................................................................................................. 57   CHAPTER 4   EXPERIMENTAL SETUP & PROCEDURES........................................................................... 59   4▪1   INTRODUCTION............................................................................................................................................... 59   4▪2   THE RAPID COMPRESSION MACHINE ............................................................................................................. 60   4▪3   NEW RCM CYLINDER .................................................................................................................................... 61   4▪4   RCM FUELLING SYSTEM ............................................................................................................................... 61   4▪5   FLOW SEEDING............................................................................................................................................... 64   4▪6   LASER & OPTICAL SYSTEM............................................................................................................................ 65   4▪7   EVENT & TIMING CONTROL........................................................................................................................... 66   4▪8   TEST MATRIX................................................................................................................................................. 68   CHAPTER 5   REPEATABILITY & CALIBRATION TESTS.......................................................................... 79   5▪1   INTRODUCTION............................................................................................................................................... 79   5▪2   CAMERA TRIGGER REPEATABILITY ............................................................................................................... 80   5▪3   ICCD CALIBRATION....................................................................................................................................... 81   5▪4   FLUORESCENT SIGNAL CHECKS ..................................................................................................................... 84   5▪5   PRESSURE TRANSDUCER CALIBRATION ......................................................................................................... 87   5▪6   POLYTROPIC EXPONENT CALCULATION......................................................................................................... 88   5▪7   LASER SHEET PROFILE CORRECTION ............................................................................................................. 89   5▪8   OPTICAL DISTORTION CORRECTION............................................................................................................... 90   5▪9   PIXEL INTENSITY AIR-FUEL-RATIO CALIBRATION ........................................................................................ 91   5▪10   IMAGE BINNING ........................................................................................................................................... 92   5▪11   SEEDER CHARACTERISATION ....................................................................................................................... 94   5▪12   FLOW ENVIRONMENT IN RCM CYLINDER ................................................................................................... 95   5▪13   J43M INJECTOR CALIBRATION ..................................................................................................................... 95   5▪14   CONCLUSION ................................................................................................................................................ 96   CHAPTER 6   IMAGE CALIBRATION............................................................................................................... 97   6▪1   INTRODUCTION............................................................................................................................................... 97   6▪2   IMAGE PROCESSING PROCEDURES AND PRESENTATION................................................................................. 98   6▪3   CORRECTION PROCEDURES ............................................................................................................................ 99   6▪4   RESULTS....................................................................................................................................................... 100   6▪5   DISCUSSION.................................................................................................................................................. 100   6▪6   CONCLUSIONS .............................................................................................................................................. 111   v CHAPTER 7   PARTIALLY STRATIFIED CHARGE, DIRECT INJECTION & INSERT FUEL JETS.. 112   7▪1   INTRODUCTION............................................................................................................................................. 112   7▪2   DATA PRESENTATION................................................................................................................................... 113   7▪3   INEFFECTUAL BACKGROUND CORRECTION.................................................................................................. 113   7▪4   PARTIAL STRATIFICATION WITH DIRECT INJECTION- FULL BORE IMAGING................................................ 116   7▪5   PARTIAL STRATIFICATION WITH CAPILLARY TUBE INJECTION .................................................................... 120   7▪6   PARTIAL STRATIFICATION WITH SPARK PLUG INSERT INJECTION................................................................ 127   7▪7   CONCLUSION ................................................................................................................................................ 130   CHAPTER 8   THE INTERACTION OF PARTIALLY STRATIFIED & DIRECT INJECTION FUEL JETS………….. ........................................................................................................................................................ 131   8▪1   INTRODUCTION............................................................................................................................................. 131   8▪2   BACKGROUND .............................................................................................................................................. 132   8▪3   MIXTURE DISTRIBUTION .............................................................................................................................. 133   8▪4   CAPILLARY INJECTED PSC IGNITION & INFLAMMATION POTENTIAL .......................................................... 137   8▪5   PSC INSERT INJECTION IGNITION & INFLAMMATION POTENTIAL ................................................................ 151   8▪6   CONCLUSIONS .............................................................................................................................................. 167   CHAPTER 9   ERROR AND UNCERTAINTY ANALYSIS ............................................................................ 169   9▪1   INTRODUCTION............................................................................................................................................. 169   9▪2   DISCOUNTING BUOYANCY ........................................................................................................................... 170   9▪3   CYCLIC INVARIANCE.................................................................................................................................... 170   9▪4   EXPERIMENTAL CONVERGENCE................................................................................................................... 171   9▪5   UNCERTAINTY ANALYSIS............................................................................................................................. 176   9▪6   CONCLUSIONS .............................................................................................................................................. 179   CHAPTER 10   CONCLUSIONS & RECOMMENDATIONS......................................................................... 180   10▪1   INTRODUCTION........................................................................................................................................... 180   10▪2   CONCLUSIONS ............................................................................................................................................ 181   10▪3   RECOMMENDATIONS & FUTURE WORK ..................................................................................................... 185   REFERENCES……………………………………………………………………………………………………..187 APPENDIX A   RCM APPARATUS & RE-DESIGN ........................................................................................ 215   APPENDIX B   RCM TIMING CONTROL SYSTEM...................................................................................... 240   APPENDIX C   RELATIVE AIR-FUEL-RATIO PDFS.................................................................................... 261   APPENDIX D   STATISTICAL TREATMENTS .............................................................................................. 271   vi LIST OF TABLES TABLE 1-1 CORE CURRENT AND PROPOSED EMISSIONS STANDARDS FOR ‘LIGHT DUTY VEHICLE’ ROAD TRANSPORT ............................................................................................................................................................................. ..4 TABLE 2-1 CENTRE-LINE ROOT MEAN SQUARE TURBULENT FLUCTUATION VALUES FOR JET CONCENTRATION, AXIAL VELOCITY AND RADIAL VELOCITY...................................................................................................................... 13 TABLE 2-2 MAXIMUM ROOT MEAN SQUARE TURBULENT FLUCTUATION VALUES FOR CONCENTRATION .................. 14 TABLE 3-1 ADVANTAGES AND DISADVANTAGES TO COHERENT AND INCOHERENT LASER DIAGNOSTICS TECHNIQUES ............................................................................................................................................................................. 35 TABLE 3-2 PHYSICAL & THERMODYNAMIC PROPERTIES OF METHANE AND ACETONE GAS, ....................................... 50 TABLE 3-3 ACETONE EXCITATION SOURCES, ............................................................................................................... 50 TABLE 4-1 RCM TIMED EVENTS.................................................................................................................................... 66 TABLE 4-2 DATA POINT LOCATIONS IN DESIGN (X) AND EXPERIMENTAL SPACE (Ξ) ................................................... 70 TABLE 4-3 CONTROL BOX CAPILLARY INJECTED PSC EVENT TIMINGS....................................................................... 71 TABLE 4-4 CONTROL BOX INSERT INJECTED PSC EVENT TIMINGS.............................................................................. 71 TABLE 4-5 DATA POINT EXPERIMENTAL SETUP VALUES ............................................................................................. 71 TABLE 4-6 HOMOGENEOUS CHARGE PARTIAL PRESSURE (GAUGE) VALUES ASSUMING PRIOR AIR FLUSH. ............... 72 TABLE 4-7 MISCELLANEOUS EQUIPMENT SETTINGS .................................................................................................... 72 TABLE 4-8 CAPILLARY INJECTED PSC WITH DI EXPERIMENTAL TEST ORDER ............................................................ 75 TABLE 4-9 INSERT INJECTED PSC WITH DI EXPERIMENTAL TEST ORDER ................................................................... 78 TABLE 5-1 MICROGC NATURAL GAS COMPOSITION RESULTS .................................................................................... 85 TABLE 5-2 LEAST SQUARES FIT COEFFICIENTS FOR LASER SHEET PROFILE ................................................................ 90 TABLE 5-3 MIXTURE PROPERTIES BASED ON DATA IN TABLE 5-1 AND ORIGINAL GC DATA....................................... 92 TABLE 5-4 DICAM PRO ICCD NOISE PARAMETERS ..................................................................................................... 93 TABLE 6-1 ANTOINE EQUATION COEFFICIENTS FOR ACETONE, REPRODUCED FROM [383] ....................................... 104 TABLE 8-1 CAPILLARY INJECTED PSC SAMPLE STATION LOCATIONS........................................................................ 141 TABLE 8-2 PROBABILITY OF MIXTURE FALLING WITHIN FLAMMABILITY LIMITS FOR CAPILLARY INJECTED PSC JET WITH DI AT STATIONS ONE TO FOUR ................................................................................................................. 145 TABLE 8-3 INSERT INJECTED PSC SAMPLE STATION LOCATIONS .............................................................................. 154 TABLE 8-4 PROBABILITY OF MIXTURE FALLING WITHIN FLAMMABILITY LIMITS FOR INSERT INJECTED PSC JET WITH DI AT STATIONS ONE TO FIVE............................................................................................................................ 160 TABLE 8-5 INCREASE IN PROBABILITY OF MIXTURE FALLING WITHIN FLAMMABILITY LIMITS FOR INSERT INJECTED PSC JET OVER CAPILLARY INJECTED JET AT STATIONS ONE AND FIVE ............................................................ 161 TABLE 9-1 PEARSON PRODUCT-MOMENT FOR CAPILLARY INJECTED PSC DI IMAGES ............................................. 170 TABLE 9-2 MAXIMUM ENTROPY FOR NON-ZERO MEAN NORMAL DISTRIBUTION, AFTER [411] ................................ 175 TABLE A-1 HPFS 7890 (KRF GRADE) PROPERTIES ..................................................................................................... 218 TABLE A-2 RCM FUELLING OPTIONS ........................................................................................................................ 229 TABLE A-3 RCM FUELLING VALVE SETTINGS........................................................................................................... 230 TABLE A-4 COMPONENT LISTING FOR RCM SPARK CIRCUIT .................................................................................... 239 TABLE A-5 INSTRUMENT LIST FOR RCM ................................................................................................................... 239 vii LIST OF FIGURES FIGURE 1-1 WORLD ENERGY CONSUMPTION BY SOURCE, ............................................................................................. 2 FIGURE 1-2 ENERGY GROWTH DUE TO INCREASED TRANSPORT DEMAND BY REGION 2006-2030,............................... 2 FIGURE 1-3 PARTIALLY STRATIFIED CHARGE WITH DIRECT INJECTION. ........................................................................ 4 FIGURE 2-1 THE VORTEX BALL MODEL OF A TRANSIENT PLUME BY TURNER,............................................................ 10 FIGURE 2-2 NORMALISED RADIAL R.M.S. PROFILES OF AXIAL VELOCITY (W), RADIAL VELOCITY (U) AND CONCENTRATION (C),........................................................................................................................................... 12 FIGURE 2-3 POWER REDUCTION FROM INCREASED BURN DURATION AND NECESSITATED SPARK ADVANCE, ........... 16 FIGURE 2-4 GHG REDUCTION POTENTIAL OF CNG ENGINES AGAINST GASOLINE EQUIVALENTS, ............................. 17 FIGURE 2-5 OPERATING ENVELOPE FOR AN SI GAS ENGINE, ....................................................................................... 19 FIGURE 2-6 LEAN LIMIT EXTENSION & BSFC REDUCTION (LEFT) AND NOX REDUCTION POTENTIAL (RIGHT) OF PSC ............................................................................................................................................................................. 21 FIGURE 2-7 FUEL EFFICIENCY OF LOAD CONTROL STRATEGIES,.................................................................................. 22 FIGURE 2-8 ENGINE POLLUTANT CONCENTRATIONS ACROSS THE RANGE OF OPERATING FAR’S,.............................. 24 FIGURE 2-9 FUEL JET STRUCTURE IN A COMPRESSION IGNITION ENGINE, ................................................................... 28 FIGURE 2-10 THE EFFECT OF OVERFUELLING ON EXHAUST HC CONCENTRATIONS, ................................................... 28 FIGURE 2-11 HIERARCHICAL NATURE OF HC COMBUSTION, ........................................................................................ 30 FIGURE 3-1 RO-VIBRONIC STRUCTURE OF THE RAMAN SPECTRUM,............................................................................. 36 FIGURE 3-2 CARS ADAPTED FROM [146] ....................................................................................................................... 39 FIGURE 3-3 MOLECULAR ACETONE, (CH3)2CO............................................................................................................ 49 FIGURE 3-4 DEACTIVATION PATHWAYS FOR THE FIRST EXCITED SINGLET, S1, OF ACETONE, ..................................... 49 FIGURE 3-5 ABSORPTION SPECTRUM OF ACETONE AT ROOM TEMPERATURE AND 1 ATM, ........................................... 51 FIGURE 3-6 ACETONE DISSOCIATION THROUGH Α-CLEAVAGE,.................................................................................... 52 FIGURE 3-7 JABLONSKI DIAGRAM FOR A ELECTRONICALLY EXCITED ORGANIC MOLECULE,...................................... 54 FIGURE 3-8 FLUORESCENT (SOLID LINE) AND PHOSPHORESCENT (BROKEN LINE) EMISSION SPECTRA FOR PURE ACETONE, ............................................................................................................................................................ 55 FIGURE 3-9 FLUORESCENT LIFETIME VARIATION OF ACETONE WITH PRESSURE, ........................................................ 57 FIGURE 4-1 SCHEMATIC OF RAPID COMPRESSION MACHINE (RCM) ............................................................................. 60 FIGURE 4-2 EXPLODED VIEW OF NEW RCM CYLINDER................................................................................................. 61 FIGURE 4-3 SCHEMATIC OF RCM FUELLING SYSTEM .................................................................................................... 62 FIGURE 4-4 PSC INSERT ............................................................................................................................................... 64 FIGURE 4-5 SECTION VIEW OF RCM SEEDER ................................................................................................................. 65 FIGURE 4-6 LASER SHEET FORMATION......................................................................................................................... 66 FIGURE 4-7 RCM EVENT TIMING DIAGRAM................................................................................................................. 67 FIGURE 4-8 RCM INSTRUMENTATION SCHEMATIC ........................................................................................................ 68 FIGURE 4-9 PARTIALLY STRATIFIED CHARGE JET AND DIRECT INJECTION JET INTERACTION ..................................... 69 FIGURE 4-10 CENTRAL COMPOSITE DESIGN FOR TEST MATRIX................................................................................... 70 viii FIGURE 5-1 MEAN PISTON POSITION AND STANDARD DEVIATION OF POSITION FOR 4300, 4800 AND 5800 CAMERA TRIGGER COUNT TIMING ..................................................................................................................................... 80 FIGURE 5-2 ICCD FLAT FIELD IMAGE WITH MEAN INTENSITY AT 26 BITS, WITH (RIGHT) AND WITHOUT (LEFT) RANGE CROPPING............................................................................................................................................................. 81 FIGURE 5-3 MEAN AND STANDARD DEVIATION FOR THE ICCD DARK FIELD.............................................................. 83 FIGURE 5-4 CENTRAL ROW (LEFT) AND COLUMN (RIGHT) FOR ICCD FLAT FIELD SUBREGION .................................. 83 FIGURE 5-5 MEAN TEMPORAL-SPATIAL CORRELATION PLOTS FOR ICCD FLAT FIELD ................................................ 84 FIGURE 5-6 AIR NORMALISED IMAGES OF FLUORESCENT INTENSITY FROM NATURAL GAS (LEFT) AND NATURAL GAS WITH 1% ACETONE V/V (RIGHT). .......................................................................................................................... 86 FIGURE 5-7 UNSEEDED DIRECT INJECTION EVENT NORMALISED AGAINST QUIESCENT AIR CHARGE ........................ 87 FIGURE 5-8 POLYTROPIC EXPONENT PLOTS FOR AIR (LEFT) AND STOICHIOMETRIC CH4 MIXTURE (RIGHT)............... 88 FIGURE 5-9 LASER SHEET PROFILE; RECORDED (•) AND WITH A 4TH ORDER FIT (-) ..................................................... 89 FIGURE 5-10 CYLINDER OPTICAL DISTORTION............................................................................................................. 90 FIGURE 5-11 NORMALISED FUEL NUMBER COUNT VS SIGNAL INTENSITY................................................................... 92 FIGURE 5-12 SIGNAL-TO-NOISE RATIO FOR 2×2 AND 4×4 ON-CHIP BINNING AT DIFFERENT SIGNAL INTENSITIES & MAXIMUM SNR CURVE....................................................................................................................................... 93 FIGURE 5-13 ACETONE SEED CONCENTRATIONS......................................................................................................... 95 FIGURE 5-14 FLAME PROPAGATION FOR A STOICHIOMETRIC MIXTURE IN RCM CYLINDER (TIME LISTED AFTER SPARK) ................................................................................................................................................................. 95 FIGURE 5-15 J43M INJECTOR CHARACTERISATION DATA AND MODEL PREDICTION .................................................. 96 FIGURE 6-1 IMAGE POST-PROCESSING ROAD MAP....................................................................................................... 98 FIGURE 6-2 INTENSITY IMAGE FOR TEST MATRIX CENTRAL LOCATION .................................................................... 100 FIGURE 6-3 ACETONE FLUORESCENT YIELD VARIATION WITH TEMPERATURE AT 248NM EXCITATION, ................... 103 FIGURE 6-4 FLUORESCENT SIGNAL RATIO TEMPERATURE VARIATION FOR 308/248NM AND 308/266NM PAIRS. REPRODUCED FROM [258].................................................................................................................................. 103 FIGURE 6-5 HIGH MAGNIFICATION SHADOWGRAPH IMAGES OF JET: (A) NATURAL-GAS JET, FAR-FIELD (B) ACETONE SEEDED NG JET, FAR-FIELD; (C) NG JET, NEAR-FIELD; (D) ACETONE SEEDED NG JET, NEAR-FIELD .................. 105 FIGURE 6-6 CFD GAS INJECTION PREDICTIONS OF (A) TEMPERATURE, (B) DENSITY AND (C) VELOCITY .................. 107 FIGURE 6-7 THE TURBULENT ENERGY CASCADE ....................................................................................................... 109 FIGURE 7-1 DATA VISUALISATION SCHEMATIC.......................................................................................................... 113 FIGURE 7-2 BACKGROUND CORRECTION IMAGE ........................................................................................................ 114 FIGURE 7-3 RAW LIF IMAGE ...................................................................................................................................... 114 FIGURE 7-4 INEFFECTUAL BACKGROUND CORRECTED IMAGE ................................................................................... 115 FIGURE 7-5 ROLLING BALL EROSION, ........................................................................................................................ 115 FIGURE 7-6 4×4 AND 2×2 ON-CHIP BINNING CALIBRATION LINES, INSET DETAIL FOR LOW SIGNAL INTENSITY ...... 117 FIGURE 7-7 NORMALIZED R.M.S FLUCTUATIONS IN RAFR FOR DIRECT INJECTION WITH PSC AT RELATIVE TIME INDICATED ......................................................................................................................................................... 118 FIGURE 7-8 RAFR FOR PSC JET FROM CAPILLARY TUBE AT TIMES INDICATED ........................................................... 121 ix FIGURE 7-9 PSC CAPILLARY INJECTED JET CENTRELINE COUNT LEVELS (BROKEN LINES INDICATE 95% CONFIDENCE)..................................................................................................................................................... 122 FIGURE 7-10 CAPILLARY INJECTED PSC JET PENETRATION; (•) PSC DATA (-) LEAST SQUARES FIT ........................ 123 FIGURE 7-11 NORMALIZED R.M.S FLUCTUATIONS IN RAFR FOR CAPILLARY INJECTED PSC AT TIMES INDICATED ..... 125 FIGURE 7-12 SPATIAL FUEL GRADIENTS FOR CAPILLARY INJECTED PSC AT TIMES INDICATED ................................. 126 FIGURE 7-13 JET DEVELOPMENT FOR INSERT INJECTED PSC AT TIMES INDICATED .................................................... 127 FIGURE 7-14 FUEL CONCENTRATION GRADIENTS FOR INSERT INJECTED PSC WITH BULK CHARGE RAFRS INDICATED ........................................................................................................................................................................... 129 FIGURE 7-15 BACKGROUND CHARGE EFFECT UPON INSERT JET MIXTURE DISTRIBUTION ........................................ 129 FIGURE 8-1 CENTRAL COMPOSITE TEST MATRIX ....................................................................................................... 132 FIGURE 8-2 RAFR DISTRIBUTION WITH CAPILLARY INJECTED PSC WITH DIRECTION INJECTION ............................. 134 FIGURE 8-3 CAPILLARY INJECTED PSC JET CENTRELINE IMAGE INTENSITY.............................................................. 135 FIGURE 8-4 RAFR DISTRIBUTION WITH INSERT INJECTED PSC WITH DIRECTION INJECTION .................................... 136 FIGURE 8-5 DEFLECTION OF CAPILLARY INJECT PSC JET .......................................................................................... 137 FIGURE 8-6 CAPILLARY INJECT PSC & DI JET RAFR GRADIENTS............................................................................. 139 FIGURE 8-7 CAPILLARY INJECTED PSC AND DI JET NORMALISED RAFR FLUCTUATIONS ........................................ 140 FIGURE 8-8 SAMPLE STATION LOCATIONS FOR CAPILLARY INJECTED PSC & DI JETS .............................................. 141 FIGURE 8-9 PROBABILITY DENSITY FUNCTIONS FOR CENTRAL CONDITION OF THE TEST MATRIX AT (A) STATION ONE, (B) STATION TWO, (C) STATION THREE AND (D) STATION FOUR. ...................................................................... 142 FIGURE 8-10 PROBABILITY DENSITY FUNCTIONS FOR CAPILLARY INJECTED PSC JET WITH DI AT STATION ONE FOR (A) RAFR=1.573 T=-3.53CAD AND (B) RAFR=1.5 AND T=0CAD....................................................................... 143 FIGURE 8-11 MAXIMA, MINIMA AND MEAN RAFR FOR ALL CAPILLARY INJECTED PSC TEST CONDITIONS AT (A) STATION ONE, (B) STATION TWO, (C) STATION THREE AND (D) STATION FOUR. .............................................. 144 FIGURE 8-12 CAPILLARY INJECTED PSC WITH DI JET FUEL CONCENTRATIONS BY VOLUME. ................................... 146 FIGURE 8-13 PROBABILITY OF ENCOUNTERING A MIXTURE WITHIN THE FLAMMABILITY LIMITS OF METHANE FOR INSERT INJECTED PSC........................................................................................................................................ 147 FIGURE 8-14 SCALAR DISSIPATION RATE FOR CAPILLARY INJECTED PSC WITH DI JET ............................................ 150 FIGURE 8-15 INSERT INJECT PSC & DI JET RAFR GRADIENTS.................................................................................. 151 FIGURE 8-16 INSERT INJECTED PSC AND DI JET NORMALISED RAFR FLUCTUATIONS ............................................. 152 FIGURE 8-17 SAMPLE STATION LOCATIONS FOR INSERT INJECTED PSC DI EVENTS.................................................. 154 FIGURE 8-18 RAFR PROBABILITY DENSITY FUNCTIONS FOR T=0CAD AT λBULK=1.5 (LEFT), λBULK=1.75 (CENTRE) AND λBULK=2 (RIGHT) FOR (A) STATION ONE, (B) STATION TWO AND (C) STATION THREE. ....................................... 156 FIGURE 8-19 RAFR PROBABILITY DENSITY FUNCTIONS FOR T=0CAD AT λBULK=1.5 (LEFT), λBULK=1.75 (CENTRE) AND λBULK=2 (RIGHT) FOR (A) STATION FOUR AND (B) STATION FIVE. ....................................................................... 157 FIGURE 8-20 RAFR PROBABILITY DENSITY FUNCTIONS FOR T=-5CAD (CENTRE) AND T=-3.53CAD (LEFT & RIGHT) AT λBULK=1.57 (LEFT), λBULK=1.75 (CENTRE) AND λBULK=1.93 (RIGHT) FOR STATION FIVE. ....................................... 157 FIGURE 8-21 MAXIMA, MINIMA AND MEAN RAFR FOR ALL INSERT INJECTED PSC TEST CONDITIONS AT (A) STATION ONE, (B) STATION TWO AND (C) STATION THREE.............................................................................................. 158 x FIGURE 8-22 MAXIMA, MINIMA AND MEAN RAFR FOR ALL INSERT INJECTED PSC TEST CONDITIONS AT (A) STATION FOUR AND (B) STATION FIVE ............................................................................................................................. 159 FIGURE 8-23 EFFECT OF RELATIVE INJECTION TIMING ON MEAN RAFR FOR STATION ONE (LEFT) AND STATION FIVE (RIGHT) .............................................................................................................................................................. 160 FIGURE 8-24 INSERT INJECTED PSC WITH DI JET FUEL CONCENTRATIONS BY VOLUME. .......................................... 163 FIGURE 8-25 PROBABILITY OF ENCOUNTERING A MIXTURE WITHIN THE FLAMMABILITY LIMITS OF METHANE FOR INSERT INJECTED PSC........................................................................................................................................ 164 FIGURE 8-26 SCALAR DISSIPATION RATE FOR CAPILLARY INJECTED PSC WITH DI JET ............................................ 166 FIGURE 9-1 QUINTILE-QUINTILE PLOTS FOR MEAN PIXEL INTENSITY AT STATIONS 1 (LEFT) & 3 (RIGHT) FOR CAPILLARY INJECTED PSC-DI EVENT WITH BULK CHARGE RAFR OF 1.5 (A), 1.75 (B) AND 2.0 (C) ............... 172 FIGURE 9-2 QUINTILE-QUINTILE PLOTS FOR MEAN PIXEL INTENSITY AT STATIONS 1 (LEFT) & 3 (RIGHT) FOR INSERT INJECTED PSC-DI EVENT WITH BULK CHARGE RAFR OF 1.5 (A), 1.75 (B) AND 2.0 (C).................................... 173 FIGURE 9-3 NORMALISED ENTROPY IMAGES OF LIF INTENSITY FOR CAPILLARY INJECTED PSC WITH DI AT T=0CAD AND (A) RAFR = 1.5 AND 51 REPEATS, AND (B) RAFR = 1.75 AND 153 REPEATS BASED ON A NORMAL DISTRIBUTION. ................................................................................................................................................... 176 FIGURE 9-4 NORMALISED ENTROPY IMAGES OF LIF INTENSITY FOR CAPILLARY INJECTED PSC WITH DI AT T=0CAD AND (A) RAFR = 1.5 AND 51 REPEATS, AND (B) RAFR = 1.75 AND 153 REPEATS BOTH BASED ON POISSON STATISTICS......................................................................................................................................................... 176 FIGURE 9-5 UNCERTAINTY IN RELATIVE AIR-FUEL RATIO FOR INSERT INJECTED PSC WITH DI AT T=0CAD AND (A) λBULK=1.5 (B) λBULK=1.75 AND (C) λBULK=2.0......................................................................................................... 178 FIGURE A-1 RCM SCHEMATIC ................................................................................................................................... 216 FIGURE A-2 OLD ACRYLIC RCM CYLINDER .............................................................................................................. 217 FIGURE A-3 RCM QUARTZ CYLINDER ....................................................................................................................... 218 FIGURE A-4 RCM QUARTZ CYLINDER HEAD (WINDOW)........................................................................................... 219 FIGURE A-5 PROBLEMATIC ELEMENTS IN QUARTZ CYLINDER FEA .......................................................................... 220 FIGURE A-6 ELEMENT ASPECT RATIOS IN AREA OF CONCERN FOR QUARTZ CYLINDER FEA ................................... 220 FIGURE A-7 ELEMENTAL JACOBIAN VALUES FOR QUARTZ CYLINDER FEA .............................................................. 221 FIGURE A-8 RCM QUARTZ CYLINDER FOS VALUES BASED ON A MOHR-COULOMB YIELD CRITERIA FOR COMPRESSION RATIO 5.33(A), 5.19(B), 8.18(C) & 13.78(D) .............................................................................. 223 FIGURE A-9 RCM PORT PLATE .................................................................................................................................. 224 FIGURE A-10 RCM ELECTRODE MOUNT.................................................................................................................... 225 FIGURE A-11 RCM PISTON......................................................................................................................................... 226 FIGURE A-12 RCM CYLINDER EXPLODED VIEW........................................................................................................ 227 FIGURE A-13 RCM COMPRESSION PLATE DRAWING ................................................................................................. 227 FIGURE A-14 RCM FUELLING SYSTEM ...................................................................................................................... 228 FIGURE A-15 RCM FUELLING PANEL VALVING, PIPING & VALVE NUMBERING ....................................................... 229 FIGURE A-16 RCM DI TIP DRAWING ......................................................................................................................... 231 FIGURE A-17 RCM PSC INJECTOR MOUNT................................................................................................................ 232 FIGURE A-18 ACETONE SEEDER SECTION VIEW......................................................................................................... 233 xi FIGURE A-19 ACETONE SEEDER: BODY...................................................................................................................... 234 FIGURE A-20 ACETONE SEEDER: BOTTOM FLANGE ................................................................................................... 234 FIGURE A-21 ACETONE SEEDER: TOP FLANGE........................................................................................................... 235 FIGURE A-22 ACETONE SEEDER: CUP ........................................................................................................................ 235 FIGURE A-23 ACETONE SEEDER: GUIDE..................................................................................................................... 236 FIGURE A-24 ACETONE SEEDER: PIPE ASSEMBLIES ................................................................................................... 236 FIGURE A-25 ACETONE SEEDER: TOP WELD-UP........................................................................................................ 237 FIGURE A-26 ACETONE SEEDER: ASSEMBLY DRAWING............................................................................................. 237 FIGURE A-27 RCM SPARK CIRCUIT, CIRCUIT DIAGRAM ........................................................................................... 239 FIGURE B-1 INFORMATION FLOW IN RCM CONTROL................................................................................................. 241 FIGURE B-2 ENCODER SIGNAL GATE, CIRCUIT DIAGRAM.......................................................................................... 243 FIGURE B-3 PULSE TRAIN DISTRIBUTION, CIRCUIT DIAGRAM ................................................................................... 244 FIGURE B-4 RESET SIGNAL DISTRIBUTION, CIRCUIT DIAGRAM ................................................................................. 245 FIGURE B-5 ENCODER PULSE TRAIN RECEIVING, CIRCUIT DIAGRAM ........................................................................ 245 FIGURE B-6 PULSE COUNTER AND COMPARATOR, CIRCUIT DIAGRAM ...................................................................... 247 FIGURE B-7 LOGIC PULSE GENERATION, CIRCUIT DIAGRAM ..................................................................................... 247 FIGURE B-8 LOGIC SIGNAL GENERATION, CIRCUIT DIAGRAM ................................................................................... 248 FIGURE B-9 START COUNTER-GATE A, PULSE CIRCUIT............................................................................................. 249 FIGURE B-10 START COUNTER-GATE B, PULSE CIRCUIT ........................................................................................... 250 FIGURE B-11 LATCH & RESET CIRCUITS, PULSE CIRCUIT.......................................................................................... 250 FIGURE B-12 SMOOTH CAPACITOR CIRCUITS, PULSE CIRCUIT................................................................................... 251 FIGURE B-13 START COUNTER-GATE A, LOGIC CIRCUIT........................................................................................... 252 FIGURE B-14 START COUNTER-GATE B, LOGIC CIRCUIT ........................................................................................... 252 FIGURE B-15 LATCH CIRCUIT..................................................................................................................................... 253 FIGURE B-16 SIGNAL RECEIVING, LOGIC CIRCUIT ..................................................................................................... 253 FIGURE B-17 SMOOTH CAPACITOR CIRCUITS, LOGIC CIRCUIT................................................................................... 254 FIGURE B-18 COUNTER CIRCUIT, START, INJECTOR CIRCUIT..................................................................................... 255 FIGURE B-19 COUNTER CIRCUIT, DURATION, INJECTOR CIRCUIT .............................................................................. 255 FIGURE B-20 J-K FLIP FLOP INJECTOR CIRCUIT ......................................................................................................... 256 FIGURE B-21 RESET CIRCUIT, INJECTOR .................................................................................................................... 256 FIGURE B-22 SIGNAL RECEIVING, INJECTOR CIRCUIT ................................................................................................ 257 FIGURE B-23 SMOOTHING CAPACITORS, INJECTOR CIRCUIT ...................................................................................... 257 FIGURE B-24 SIGNAL DISTRIBUTION CIRCUITS .......................................................................................................... 260 FIGURE B-25 RCM CONTROL CIRCUIT ENCLOSURE .................................................................................................... 260 FIGURE C-1 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR CAPILLARY INJECTED PSC EVENT WITH DI AT STATION ONE (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-1) ..................................................................................................... 262 xii FIGURE C-2 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR CAPILLARY INJECTED PSC EVENT WITH DI AT STATION TWO (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-1) ..................................................................................................... 263 FIGURE C-3 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR CAPILLARY INJECTED PSC EVENT WITH DI AT STATION THREE (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-1) ..................................................................................................... 264 FIGURE C-4 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR CAPILLARY INJECTED PSC EVENT WITH DI AT STATION FOUR (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-1) ..................................................................................................... 265 FIGURE C-5 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR INSERT INJECTED PSC EVENT WITH DI AT STATION ONE (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-3) ............................................................................................................. 266 FIGURE C-6 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR INSERT INJECTED PSC EVENT WITH DI AT STATION TWO (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-3) ............................................................................................................. 267 FIGURE C-7 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR INSERT INJECTED PSC EVENT WITH DI AT STATION THREE (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-3) ............................................................................................................. 268 FIGURE C-8 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR INSERT INJECTED PSC EVENT WITH DI AT STATION FOUR (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-3) ............................................................................................................. 269 FIGURE C-9 RELATIVE AIR FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR INSERT INJECTED PSC EVENT WITH DI AT STATION FIVE (FOLLOWING STANDARD IMAGE PRESENTATION PROTOCOL USED IN MAIN TEXT, AND STATION LOCATION AS PER TABLE 8-3) ............................................................................................................. 270 FIGURE D-1 (A) CIRCUMSCRIBED CENTRAL COMPOSITE DESIGN (B) INSCRIBED CENTRAL COMPOSITE DESIGN, TEST DOMAIN SHOWN IN GREY, DATA POINTS AS BLACK DOTS ............................................................................... 273 xiii LIST OF SYMBOLS & ABBREVAIATIONS Nomenclature: Roman B Bayesian entropy P data 95% confidence interval B0 exit buoyancy Pa pa ambient pressure C concentration Pr pressure ratio c concentration volume flux mean concentration Q normalising co-ordinate exit concentration rc volume (compression) ratio r.m.s. concentration R gas constant Pearson product moment centre-line r.m.s. concentration r radius d diameter r0 exit radius D binary diffusion coefficient s solid angle d diameter S0 singlet ground state de equivalent diameter Sf fluorescent intensity dps pseudo-diameter S1 first excited singlet state f mixture fraction sr steridian fc centre-line mixture fraction T1 first excited triplet state g gravity u (radial) velocity g′ reduced gravity u′ radial velocity fluctuation Plank’s constant ut turbulent velocity H entropy mean radial velocity k kinetic energy centre-line radial r.m.s. velocity kjc jet constant for concentration r.m.s. radial velocity fluctuation kjw jet constant for velocity w axial velocity Ks mass flow rate constant w′ axial velocity fluctuation mean kinetic energy squared mean exit velocity L largest eddy lengthscale mean centre-line velocity lI integral length-scale r.m.s. axial velocity fluctuation lm momentum buoyancy ratio length scale x design space variable M0 exit momentum Y mass fraction mass flow rate z axial co-ordinate exit mass flow rate z* dimensionless axial co-ordinate Ni number of incident photons Zt jet penetration xiv Nomenclature: Greek α diffusivity µ dynamic viscosity αe entrainment velocity µ population mean αT turbulent diffusivity µeff effective viscosity Γ jet penetration constant ν kinematic viscosity Δθc combustion duration ξ experimental variable Δν frequency shift (in wavenumber) ρ0 exit density ε turbulent kinetic energy dissipation rate ρa ambient density η dimensionless radial co-ordinate, σ population standard deviation η Kolomogorov lengthscale τ timescale κ Kleinstein decay constant τI integral time-scale λ relative air fuel ratio τk Kolomogorov time-scale λ profile width ratio Φ equivalence ratio λ wavelength χ scalar dissipation rate λ relative fuel-air-ratio xv Abbreviations AC alternating current nmHC non-methyl hydrocarbon ATDC after top dead centre NG natural gas BMEP break mean effective pressure NOx oxides of nitrogen bsfc break specific fuel consumption OHD-RIKES optically heterodyned Raman induced Kerr effect spectroscopy BTDC before top dead centre PAH polyaromatic hydrocarbon CAD crank angle degrees PAN peroxyacetyl nitrate CARB California Air Resources Board PARS photoacoustic Raman spectroscopy CARS coherent anti-Raman scattering PFC perflurocarbon CCD charge coupled device PIV particle image velocimetry CI compression ignition PM particulate matter CI95% 95% confidence interval PMMA polymethylmethacrylate (acrylic) CNG compressed natural gas ppm parts per million CO carbon monoxide Pr Prandtl number CoV coefficient of variation PSC partially stratified charge CSRS Stokes-shifted Raman scattering PWM pulse width modulation DC direct current R&D research & development DI direct injection RC resistor-capacitor DISC direct injection stratified charge RCARS rotational-CARS EGR exhaust gas recirculation RCM rapid compression machine FAR fuel-air-ratio Re Reynolds number FWHM full width half maximum RELIEF Raman excitation plus laser induced electronic fluorescence GDI gasoline direct injection RFAR relative fuel air ratio HC hydrocarbon Ri Richardson number HCCI homogeneous charge compression ignition RIKES Raman-induced Kerr effect spectroscopy HFC hydrofluorcarbon RON research octane number HHV higher heating value RPV reaction progress variable HRR heat release rate Sc Schmidt number IMEP indicated mean effective pressure SI spark ignition IPCC Intergovernmental Panel on Climate Change SNR signal-to-noise ratio KrF krypton-fluorine (laser) SRGS stimulated Raman gain spectroscopy LDA laser Doppler anemometry SRLS stimulated Raman loss spectroscopy Le Lewis number SRS spontaneous Raman spectroscopy LIF laser induced fluorescence TTL transistor transistor logic M third body (chemical reactions) UHC unburnt hydrocarbon MBT mean timing for best torque UV ultra violet MW molecular weight VOC volatile organic compound Nd:YAG neodymium-doped yttrium aluminium garnet VVT variable valve timing w.r.t. with respect to xvi ACKNNOWLEDGEMENTS It is rare that a journey as long as PhD follows the course originally set out or delivers the expected destination. My years at UBC are no exception. It is a misnomer to presume that Doctoral studies in engineering are the endeavour of an individual, in a single discipline, in a single technicality. My studies have taken me from mechanical design, to electronics, quantum physics and combustion. In doing so I have spoken to, met, worked with and relied on many people. Space does not allow me to thank them all, nor to be earnest enough about those whom I can mention. My initial thanks must go to Dr Robert Evans for accepting me into his research group and in doing so allowing me to become part of UBC, an institution that introduced me to Vancouver- a city I now call home. I would like to thank Dr Martin Davy for having the vision to bring laser diagnostics to UBC, a field with huge scope for the individual researcher and for the wider research community. My gratitude also goes to Drs. Steven Rogak and Kendal Bushe for their acumen when called upon. I am also very much indebted to Glenn Golly for his patience in teaching a mechanical engineer about electronics; Sean Buxton for helping bring those teachings to life; Roland Genschorek for making sense of my engineering drawings; and Eric Wilson and Markus Fengler for their mechanical insight. My thanks must also go to those at Imperial College London who were responsible for laying the foundations of my engineering faculty. I would also like to offer my sincere thanks to Brian Beck for allowing me to pursue and complete my studies while continuing my professional development; and to Dr. Meryn Bowen for being, and continuing to be, a stalwart friend who is always happy to lend an ear. No matter how dark the hour I have consistently been granted a little light from some choice fellow Graduate Students. Over the years each of Ed Chan, Dave Gorby, Dave Williams, James Saunders, Andrew Mezo, Jean Logan, and Erik Kastanis have provided humour and levity that will forever punctuate my time at UBC. On occasion they have even been known to provide some of astute insights (and sometimes less so). At every turn and in every stage of my life I have been graced with the unqualified and unwavering support of my family. They have provided me with every opportunity and all the support I needed to succeed- a luxury to which I am not blind. My mother has, from the outset, instilled in me a set of principles which I drawn on daily and for which I am a better person; she has listen to the minutiae of my work with patience and understanding, despite not understanding what exactly I do. My step-mother has taught me to always be philosophical and look for happiness; and despite only a limited amount of time with my Father I learned from him the importance of application and all that it can bear. The work presented herein is all my own with the exception of the CFD computations and results presented in Chapter 6, for which I offer Mr Ehsan Faghani my sincere thanks. The insight they provided helped further my understanding of gaseous injections. xvii DEDICATION It is a mistake to suppose that men succeed through success; they much more often succeed through failures. Precept, study, advice and example could never have taught them so well as failure has done. Samuel Smiles, 1859 1 Chapter 1 INTRODUCTION 1▪1 INTRODUCTION This opening chapter serves only to put the rest of the manuscript in context. The greater need for research into improved energy efficiency is presented, with an emphasis on the need for more efficient and cleaner road transport. A technology to facilitate exactly these is outlined- that of partially stratified charge combustion. An enhanced understanding of this technology and its potential integration into a direct injection engine are sought through the current laser diagnostics study, the aims of which are made explicit with a clear and measureable set of objectives. The chapter concludes with an outline of the dissertation structure. ▪ Chapter 1 ▪ Introduction ▪ 2 1▪2 THE NEED Energy is at the very core of society, particularly those societies which form the countries of the OECD1. The primary sources of energy open to us, as a race, are few: solar, nuclear and fossil derived; any other ‘sources’, including all ‘renewables’, can be tied back to these primary sources. Currently fossil fuels account for 80% of world energy demand (which totals 130-140GWh annually), cf. Figure 1-1, resulting in anthropogenic CO2 emissions of 29 Gt/yr [1]. Figure 1-1 World Energy Consumption by Source, after [2]. Transport accounts for 23% of these emissions [1] and uses 61% of all produced oil, where gasoline avgas and diesel (including fuel bunker) account for 26%, 6% and 29% respectively [3]. Transport has been wedded to oil since the demise of coal; however, climate change and the scourge of urban pollution are forcing changes upon road transport. Rail, marine and air transport, although expected to improve, are not under the same pressures. World transport demand is set to grow by 45% by 2030 [2] and will account for three-quarters of total oil consumption increase [2]. Only a small fraction of this growth is expected to come from the OECD, cf. Figure 1-2. Figure 1-2 Energy Growth Due to Increased Transport Demand by Region 2006-2030, after [2]. 1 Organisation for Economic Co-operation and Development Total 767Mto e ▪ Chapter 1 ▪ Introduction ▪ 3 Economic growth and improvement of the living standards are inexorably linked to increased energy demand, which until recently has been made with scant regard for the future, let alone regard for a sustainable future. Car consumers are their own fiduciaries and have forced the auto-industry to levels of efficiency and end-product performance which are the envy of most other manufacturers. Customer expectation will not support a move away from these standards in the name of pollution reduction, climate mitigation or sustainability. The International Energy Agency proposes in their BLUE Map scenario a cut in emissions of 50% (based on 2005 values) by 2050 to stabilise atmospheric CO2 at 450- 520ppm. These gains are to come from a range of the approaches, though it is the Second Generation transport solutions which must satisfy the most demanding consumers while addressing the emissions needs of years to come. The technology to achieve these reductions is under development, some of which are nearing commercialization. Hybrid-electric cars are now widely available; and there is renewed vigour from Ford (Focus RV), Nissan (Leaf), Mitsubishi (MiEV), Subaru (G4e), Volvo (C30 electric) and Tesla (Roadster) in fully electric vehicles which Deutsche-Bank believe [3] to be the ‘game changing technology’ that will spell the end for oil and gas. Biofuels offer promise, although their exact benefit strongly depends on their nature: sugar-cane and corn-derived ethanol are not the same entity with respect to their carbon footprint. The touted hydrogen economy, the elixir of the sustainably growing economy, is still decades away from being realised and suffers a significant problem in that hydrogen is not an energy source but an energy carrier and arguably, some claim, an inefficient one at that. Accepting this, hydrogen still offers the potential to meet the toughest targets of BLUE Map. In moving towards these targets, natural gas (NG) offers significant potential. Oil has a current reserve to production ratio of around 40 years, while NG offers 60 [4] reaching 130 years [2] if flaring and venting are curtailed. Further, increased production capacity is likely to swell the capacity margin to about 27% (compared to ~12% presently) [2]. The larger technical and commercial problems of distribution and onboard storage need still to be overcome if the the potential benefits of a move to NG-powered transport ar to be realised. Natural gas technology currently offers CO2 reductions of ~20% compared to gasoline and provides a technological stepping stone towards hydrogen combustion. Setting aside the climatic benefits, natural gas offers, in the short term, the most significant route to consistently meeting emissions regulations (which as of 2009 also regulate CO2 on a manufacturer fleet-wide basis in the USA). The targets to be met by the auto- manufacturers are outlined in Table 1-1, which summarizes an expansive and complex set of regulation for light duty vehicles (those up to about 3500 kg). Optimized natural gas engines can exploit the inherent cleanliness of gas to meet these standards with an ease arguably not available from the incremental steps being made on conventionally fuelled spark ignition (SI) and compression ignition (CI) engines. ▪ Chapter 1 ▪ Introduction ▪ 4 NOx (mg/km) CO (mg/km) PM (mg/km) nmHC (mg/km) CH2O (g/km) CO2 (g/km) Regulatory Standard Pertaining to Model Year Vehicle Type SI CI SI CI SI CI SI CI SI CI SI CI CARB2 Low Emission Vehicle II [5] 2009 onwards <3864kg 31 --- 2100 --- 6.25 3 --- 47 --- 9.4 --- 2024 --- Euro 5 [6] 2009-2014 <3500kg 82 280 2270 740 5 5 1085 --- --- --- --- --- Euro 6 [6] 2014 onwards <1760kg 82 125 2270 740 5 5 1085 --- --- --- --- --- Table 1-1 Core Current and Proposed Emissions Standards for ‘Light Duty Vehicle’ Road Transport Engine development trends are trying to bring together, into a common platform, the benefits of SI engines (low NOx and PM) and those of SI engines (low HC and CO). To meet emissions demands across the full operating envelope, future engine architecture is likely to be a combination of enhanced SI and CI technology, with further additions from technology like HCCI and partial stratification. 1▪3 THE DIRECTLY INJECTED PARTIALLY STRATIFIED CHARGE NATURAL GAS ENGINE Striving for emissions reductions and efficiency improvements, the research group of Prof. R L Evans has studied and developed a partially stratified charge, PSC, combustion system for natural gas SI-ICE’s [7]. The approach uses a modified spark plug to inject a small amount of fuel (about 5% w/w of the total charge) adjacent to the plug electrodes, so generating a mixture that is locally rich. The bulk cylinder charge then comprises an ultra-lean mixture that would not ignite if an unmodified sparkplug were used. Figure 1-3 Partially Stratified Charge with Direct Injection. 2 California Air Resources Board 3 Beyond 80,000 km durability. 4 Fleet-wide value, dropping to 133 mg/km by 2015. 5 Total allowable tHC’s 160 mg/km. ▪ Chapter 1 ▪ Introduction ▪ 5 The modified sparkplug ignites the rich pilot charge which burns vigorously enough to inflame the remaining bulk charge. Load control may be achieved through varying the bulk charge air-to-fuel ratio, although the current work is concerned with an approach capable of providing still better control- direct injection. Direct injection, of any fuel, will provide volumetric efficiency gains and offers the potential of throttless operation and the elimination of pumping losses. The work herein is presented as a study of the fundamental mixture distribution that results from combining partial stratification with direct injection. It is intended as a step towards explaining the phenomenological effects reported by D. Gorby [8] in his study of DI-PSC in a single-cylinder research engine. 1▪4 AIMS To Establish The Spatial Fuel Distribution Within The Cylinder of a Rapid Compression Machine Which Results From Both The Direct Injection & Partial Stratification of Natural Gas The current study aims to understand the interaction of the partially stratified fuel jet with the jet generated by the direct injection of natural gas in a rapid compression machine. The study will achieve this through a detailed identification of the in-cylinder fuel distribution that results from the combined partial and direct-injection of fuel. The findings will be used to expand upon, and draw together, earlier work [8] on the utility of combined PSC-DI natural gas fuelling. 1▪5 OBJECTIVES The objectives to support the stated aim of this study are:  To quantitatively map the fuel air distribution which results from the partial- and direct-injection of natural gas into the cylinder of a rapid compression machine (RCM).  To identify the level and extent of the fluctuations in fuel concentration at the location of the spark electrodes in the RCM.  To understand the extent and role played by concentration gradients in the formation and ignition of the PSC fuel plume.  To understand the differences in the fuel distribution generated by a bespoke partially stratified charge (PSC) spark-plug ‘insert’ and a capillary tube injected PSC jet.  To understand the extent and implications of the inherent uncertainties associated with laser induced fluorescence imaging of acetone-doped natural gas jets in engine research. ▪ Chapter 1 ▪ Introduction ▪ 6 1▪6 DISSERTATION STRUCTURE Chapter 2 offers the reader background information on laminar and turbulent jets, pollutant formation in engines (with a discussion not limited to natural gas engines) and a basic understanding of natural gas engines. Laser diagnostics provide an expansive set of experimental techniques, each suited to providing slightly different information. Chapter 3 provides an overview of these techniques and their application, while a more substantial justification and explanation of laser induced fluorescence is made in the second half. Chapter 4 offers the reader the most salient details of the experimental apparatus used in the current study and outlines the experimental procedures used to obtain the results of Chapters 6-8. Chapter 5 outlines the wide range of calibration experiments undertaken to ensure that the results of the LIF study are of the highest quality possible of the experimental apparatus. Chapter 6 goes on to highlight some of the difficulties of establishing a quantitative LIF study for a highly underexpanded jet. Chapter 7 details the findings from a central composite test matrix that investigates the mixture distribution within the RCM cylinder for a range of relative direct injection injection timings and bulk charge air fuel ratios. The Chapter also establishes some basic behaviour of PSC injections made from a capillary tube and bespoke PSC ‘insert’. Chapter 8 repeats the same central composite test matrix to consider for a smaller region and in more detail the area where there is the interaction of the direct injection jet with that of the partially stratified charge produced from a capillary and contrasts it to that produced from the ‘insert’. Chapters 9 & 10 offer an assessment of the uncertainty in the experimental findings and some concluding remarks on the most important findings and recommended future work. Appendices A-D present, respectively, the detailed redesign and design work undertaken to allow the RCM to be used for the current study, the timing and control hardware designed and implemented for the study, the relative air-fuel ratio probability density functions that resulted at a number of locations within the cylinder for the nine conditions of the test matrix and some fundamental information pertaining to the statistics used to assess the experimental results. 7 Chapter 2 GASEOUS JETS, GAS ENGINES & POLLUTANT EMISSIONS 2▪1 INTRODUCTION Chapter 2 aims to bring the reader through what, at first sight, may appear to be three disparate themes, but which are in fact closely related. The first portion of the chapter establishes the fundamental principles of gaseous jets and interaction with their surroundings. It is this interaction and mixing which forms the ignitable mixture within a natural gas engine, and it is the various features and designs of natural gas engines which are considered in the central portion of this chapter. The chapter concludes with a discussion of the major pollutants to be expected from natural gas engines and the importance with respect to human health and climate change. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 8 2▪2 TURBULENT GASEOUS JETS The use of direct injection necessitates a fundamental understanding of turbulent free jets. The current discussion is limited to gaseous and submerged liquid jets. In general the flow from the nozzle of the injector used in the current work is fully turbulent (Re ~ 5×105), choked and under-expanded [9]. 2▪2▪1 INCOMPRESSIBLE JET STRUCTURE & DEVELOPMENT Mass entrainment around the periphery of turbulent jets causes them to grow at, beyond the development region, a set half-angle. Early experimental work on this growth [10-13] confirmed the self-similarity first proposed for turbulent flows by Zel’Dovich [14], though there were still inconsistencies in the data. These centred on the failure of the turbulence intensities to attain self-similarity, and motivated the work of Wygnanski and Fiedler [15], whose hot-wire measurements provided the reference profiles for mean velocity, velocity fluctuation and turbulent stress for many years. Wygnanski & Fiedler’s work, as in previous research, made use of a virtual origin (at a distance up- or downstream of the nozzle exit) to circumvent the problems of specifying an initial condition, and allowed the simple hyperbolic form for the axial velocity of Eq. 2.1 Eq. 2.1 where de is the equivalent diameter first introduced by Thring & Newby [16] as d(ρ0/ρa)½ accounts for density differences between the jet and its surroundings. However, concerns about such treatment of the source were first raised by Baker [17] when he showed Wygnanski & Fiedler’s formulation failed to preserve axial momentum. Using laser Doppler anemometry (LDA), Capp et al. [18] identified the source of the error as facility related, which motivated Schneider [19] to re-examine the theory of jets and show that the integrated momentum at any axial station is constant and equal to the rate at which it was added to the source. This extended understanding has led George [20] to revise the original implicit assumption of self-similarity, and assert that “the self-preserving state obtained [is] in fact … uniquely determined by the initial conditions.”. That is, jets cannot ‘forget’ their origins and will always hold a unique, though small, dependence upon their initial (source) conditions, primarily Reynolds number and velocity profile. An extensive re-examination of the experimental literature by Carazzo et al. [21] supports George’s work on local self-preservation, though it suggests that at large enough distances (where measurements have yet to be made) jets and plumes do indeed forget their initial condition and become fully self-similar with a non-Gaussian profile. Mi et al. [22] propose that before this state is achieved the source of local self- preservation is in the intermittent large-scale coherent structures of the flow. Close to the source such coherent structures have not formed, while at intermediate distances, large scale structures appear, but do ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 9 so only intermittently [23] and are still aware of their origin. Far enough away from the source these intermittent structures become permanent and self-similarity is obtained. The evolution of velocity was shown by Wygnanski and Fiedler [15] to progress towards self-similarity in a stepwise fashion “since the energy is transferred from the mean motion directly to w' [axial] fluctuations, and only pressure-velocity-gradient correlations transfer the energy further to other components of the turbulent motion…”, i.e. self-similarity can be obtained only after a balance is achieved in the mean flow velocity and its fluctuation. The turbulence of the jet is heavily anisotropic as far as 100 diameters downstream, and this longitudinal preference skews the energy towards the lower end of the turbulent energy spectrum and highlights the preferential transfer of energy from the mean motion to (mean axial velocity fluctuation) before (mean radial velocity fluctuation). The nature of the anisotropy may reasonably be expected to depend upon the initial conditions and so ultimately affect the form of the energy redistribution. This is contradicted somewhat by Papanicalaou & List [24] who find the mean and fluctuating velocities to converge to self-similarity at the same point about 50 diameters downstream. Consideration of the evolution of the Reynolds stresses, reduced gravity, and kinetic energy by George [20] support the assertion of a stepped, and unequal, progress amongst the flow variables. The nature of the energy transfer affects the rate at which self-similarity is asymptotically approached, with the nature of the energy transfer equations shown in [20] to be a function of the initial conditions, i.e. source Reynolds number. The evolution of the flow may vary with changes in the profile shape and/or with the relative width of the buoyancy and velocity profiles, i.e. the local turbulent Prandtl number (the ratio of viscous diffusion to thermal diffusion). The evolution is, in general, seen to progress from a situation which is momentum governed (Prt~1) near the source to one in which vorticity transport dominates (Prt~0.5) [21]. Across the jet, however, the energy and Reynolds stress balances are affected differently. Near the core, balances are convection driven, while towards the jet boundary the balance is primarily the result of the production, dissipation and pressure-strain rate interactions in the Navier- Stokes equations [25]. The entrainment rate of a jet, which is in essence the result of all the interactions discussed above, is the primary process governing the mixing of fuel and oxidizer in a directly injected engine cylinder. Considering the macroscopic details of a jet and taking a priori that the appropriate scaling is based on nozzle exit momentum [19, 26] allows an analysis of the problem that includes density gradients, compressibility and under-expansion [9]. Indeed, this a priori assumption is shown in [9] to be fully adequate for fuel jets, given that the opening transient is short compared to the duration of injection. The starting nature of a plume is most often modelled using the approach of Turner [27] where a quasi-steady- state jet is headed by a ‘vortex-ring’, cf. Figure 2-1. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 10 Figure 2-1 The Vortex Ball Model of a Transient Plume by Turner, after [27]. Turner’s model is based upon buoyant atmospheric plumes, where an initial buoyant thermal rises and is subsequently ‘fed’ by the trailing quasi-steady-state plume that supplies about half of the mixed fluid in the ‘cap’ (the rest being entrained over the front of the cap). Assuming a spherical vortex and Gaussian velocity profile in the jet it can be shown that [27] the speed of advance for the cap is approximately 0.6 times that of the maximum velocity of the jet behind it, with a hyperbolic decrease in velocity with downstream location. It would be reasonable to assume that the dominant role of buoyancy in Turner’s model limits its applicability to vertical plumes. However, as Turner notes [27] “since the cap merges gradually with the plume behind it, the exact form [of the velocity profile] to be taken for the cap is somewhat arbitrary”, which also implies the role to be played by buoyancy in the formulation is somewhat arbitrary. This has led Abramovich & Solan [28] to successfully apply Turner’s theory to non- buoyant transient laminar jets where the cap is assumed to gain mass solely through addition from the jet, though this may be questioned in that, Hill & Ouellette say of Rizk’s photographs [29] “the head vortex…appears to have suffered little entrainment, and appears almost on the verge of pinching itself off from the preceding flow region.”. Nonetheless, Witze [30] and Rubas et al. [31] have demonstrated the applicability of Turner’s model to impulsively started incompressible turbulent jets. These works support the validity of the vortex-ball model far beyond the case presented originally and hence its suitability for engine relevant injections. Using Turner’s model (with an entrainment constant specified by Ricou & ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 11 Spalding [32] as where KS=0.32 and the ratio d/Zt is 0.25±0.05 from [29]) to support their scaling arguments, Hill & Ouellette [9] are able to show that jet penetration is given by Eq. 2.2 Eq. 2.2 where Γ is a pure constant- the jet penetration constant- with a value of 3.0±0.1. It is of note that Eq. 2.2 is not directly dependent upon injection pressure. Using this expression with the experimental velocity data of Witze [30] to evaluate the arrival of the jet at a set location, Hill & Ouellette confirm the usefulness of Turner’s simple model even though the Reynolds numbers for the experimental data used are three to seven times lower than the suggested lower limit [32] of 30,000 for a turbulent jet. Using flow visualisation and laser Doppler velocimetry (LDV) Cossali et al. [33] have investigated the early stages of injection prior to the formation of self-similarity. If the injection duration is short enough, or in the initial stages of a longer injection before self-similarity is obtained, the velocity field around the vortex cap is markedly different to that of the quasi-steady-state discussed previously. The jet head does indeed entrain air upon ejection from the nozzle, though it is posited that this is due to wrinkling of the jet boundary and not velocity differences. It is this which causes the initial jet volume to be larger than the final quasi- steady jet. This ‘enhanced mixing’ in the early stages curtails the applicability of the widespread assumption used in applying Turner’s model, viz. that all the vortex-cap entrainment is from the jet. Cossali et al. do not, however, provide an alternative explanation. Of significance is that steady-state conditions are reached on the jet centre-line about twice as fast as for the edge “where the mixing mechanism requires longer times to reach equilibrium.” [33]. This is attributed by the authors to the dominance of inertial effects on the jet axis in the near field; whereas the boundary is primarily influenced by turbulent diffusion of mass and momentum, with correspondingly longer characteristic times, and where the fluctuations may be of the order of the jet half-width [34]. The importance of r.m.s. fluctuations cannot be overlooked, with [35] showing that turbulent mass transport accounts for ~8% in jets. The hyperbolic decay of mean (or centreline) velocity could reasonably be expected to apply to the scalar field of a jet, and early work by Hinze [11] and Field [36] confirmed this basic premise, with concentration decay from the source given by Eq. 2.3: Eq. 2.3 where the co-ordinate z is measured from the virtual origin, and the term in parentheses is termed z*. The data of Birch et al. [37] suggest kjc=4.0 with the virtual origin 5.8d upstream of the nozzle exit. Birch et al. report kjc to be in the range four to six, with more recent studies settling on values close to five, though without conclusive agreement (e.g. 5.4 [38], 5.37 [24] & 4.96 [39]). The location of the virtual origin suffers a wide range of reported values when based on velocity since the location at which self- ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 12 similarity is attained varies widely in the literature (e.g. z* ~ 10 [40], 10-15 [29], 25 [37, 41], 30 [42], 50 [24, 43] and 70 [15, 44]). Birch et al. note that, as with velocity, there is an increase in the rate of spread for concentration in the near field before it settles to its (steady) far-field value at z*~10-30. The difference in the velocity and scalar fields is attributable to the fact that turbulence and buoyancy affect velocity and mixing very differently. Using Raman spectroscopy of a methane jet in air, Birch et al. [37] report that the unmixedness tends to 28.5% in the far field (z*>70). The initial development of unmixedness is rapid within the first 10de and is attributed to the r.m.s concentration reaching similarity before the mean concentration. The ratio of the concentration (or buoyancy) to velocity spread rates, λ (= Pr-½ [21]) is important to the analysis of the jet, as is the Richardson number where lm is the characteristic lengthscale of buoyant jets (or plumes), lm=M0¾/B0½ [39]. It has been found consistently that the radial concentration profile is wider than the velocity profile at the same location with λ~1.19 [24]. As noted by Carazzo et al. [21] “the route to self similarity is different for jets and plumes…[where] self- similarity occurs earlier in pure plumes than in jets [and] the effective turbulent Prandtl number is smaller in jets than plumes.” Fundamental work by Wang & Law [41] using particle image velocimetry and laser induced fluorescence (LIF) suggest that a jet exists for z/lm<0.6 and a plume for z/lm>6 with a smooth transition between the two regimes ([24] find these values to be 1 and 5 respectively). Papanicolaou & List [24] suggest, that a lack of care in assessing the regime of the flow by various experimenters can explain the discrepancy in the constants reported in the literature. Figure 2-2 Normalised Radial r.m.s. Profiles of Axial Velocity (w), Radial Velocity (u) and Concentration (c), after curve fit coefficients presented in [41]. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 13 Typical r.m.s. velocity and concentration profiles are shown in Figure 2-2, though the extent of the intensities reported by different researchers are inconsistent (cf. Table 2-1). Common to almost all is that, in jets, the concentration fluctuations are smaller than the velocity fluctuations. Wang & Law [41] 0.224 0.27 0.19 Papanicolaou & List [24] 0.22 0.25 0.17 Birch et al. [37] 0.285 0.27 0.32 Table 2-1 Centre-line Root Mean Square Turbulent Fluctuation Values for Jet Concentration, Axial Velocity and Radial Velocity. Despite their proximity, Papanicolaou & List and Wang & Law draw markedly different conclusions from their findings. Papanicolaou & List [24] assert from their LDA and LIF measurements that buoyancy produced turbulence accounted for twice the transport of turbulence than jet driven turbulence, with an increase in normalised momentum of ~12% compared to the initial specific momentum flux (though the exact increase is related to the axial motion of the externally entrained fluid). Wang & Law, on the other hand, point to the fact that the normal and shear turbulent intensities in their jet and plume results are so similar that buoyancy has no effect on the turbulent velocity fluctuations, and acts mainly through the mean velocity. Shabbir and George’s work [40] on the vertical plume suggests that the mean energy advection is primarily balanced by the radial turbulent transport, while the radial turbulent transport is responsible for balancing the mean advected momentum and buoyancy forces. This supports Wang & Law in that although the direct effect of buoyancy upon turbulence is substantial it is still small compared to the effect of shear. Buoyancy is the main driving force in the mean velocity field, which in turn produces the shear to enhance the turbulent flow properties [40]. Regardless, there is a high level of self-similarity in the mean and turbulent velocity fields. The same cannot be said of the respective concentration fields however. The peak r.m.s. concentrations in a jet may be 1.65 times that of the centreline mean. The importance of these fluctuations is further built on when concentration minima are considered [24]. Jets may see r.m.s. concentrations as low as 20% of centreline means, with this value dropping to zero beyond η~0.1 ( ). Root mean square concentration (cf. Table 2-2) and velocity (cf. Table 2-1) values tend to decay like their mean counterparts [24] such that for a jet and , while the contributions to scalar transport from the turbulent fluctuations are often overlooked, Wang & Law [41] have shown that they account for ~7-12%. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 14 at η Wang & Law [41] 0.21 0.07 Papanicolaou & List [24] 0.25 0.1 Birch et al. [37] 0.22 0.07 Table 2-2 Maximum Root Mean Square Turbulent Fluctuation Values for Concentration (with Radial Position) 2▪2▪2 UNDER-EXPANDED COMPRESSIBLE JETS The discussions presented above pertain to incompressible flows, yet at the pressures used in direct injection systems the resultant jet cannot be described as so. Early work by Kleinstein [45] showed that analytical solutions to the compressible laminar jet were tractable, and that the turbulent case could also be tackled [46]. Assuming: negligible axial pressure gradient, Pr~1, Sc~1, Le~1, and using adaptations of Prandtl’s momentum transfer theory and Taylor’s vorticity transport theory (both with an enhanced eddy viscosity that takes into account the thermodynamics of the problem), Kleinstein [46] showed that the axial decay of velocity and concentration (and entropy) follow the form of Eq. 2.4 Eq. 2.4 where κ is the decay constant (0.074 for velocity and 0.104 for mass fraction [46]). Birch et al. [42] note of Kleinstein’s solution that the agreement in the near field is only marginally better than the hyperbolic formulation (Eqs.2.1& 2.3) and at large distances may be closely approximated by a hyperbola. However, Kleinstein’s formulation is important in that it is shown to agree well with similar incompressible work by Schlichting [47] up to Ma~2.5, which may explain its persistent wide use [31]. The suggested similarity would imply that despite the well-known structural differences between compressible jets (as detailed by Shapiro [48]) and their incompressible counterparts, their inherent behaviour is the same. This is indeed shown to be the case, with the details of the early development region (where any shocks form) and the potential core ostensibly being ‘forgotten’ downstream. It would appear that the work of George [20] may bring this in to some doubt, but for engine related jets the more complex formulation of George provides no significant benefit. An under-expanded jet undergoes a rapid expansion upon leaving the nozzle to equilibrate in pressure with its surroundings. This expansion causes the subsequent jet to act as though its source were much larger than in reality. This discrepancy is easily overcome by specifying a pseudo-diameter- the diameter that would flow the same amount of mass were it subsonic, and is shown by Birch et al. [49] to be given by Eq. 2.5 Eq. 2.5 In later work [42] the pressure ratio was shown not only to affect the pseudo-diameter, but also the location of the jet’s virtual origin. With this in mind Birch et al. [42] proposed the use of a pseudo-source ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 15 to fully describe the jet behaviour. This source would have a diameter given by Eq. 2.5 with a (subsonic) pseudo-velocity at the source to maintain the mass flow rate for this larger diameter. Hill & Ouellette [9] are able to show that the work of Birch et al. is well defined, but that the more simple scaling of Thring & Newby [16] provides results which match equally as well. This has the implication that Eq. 2.2 for jet penetration applies equally well across the sub-sonic and highly underexpanded jet regimes; with the preservation of the jet penetration constant at 3.0±0.1, and noting that although stagnation pressure losses are significant, the ¼ power dependence of penetration on momentum flux minimises the influence of the loss. Gaseous jets have relatively simple analytical solutions if steady and laminar, and even a transition to turbulent gets allows a certain amount of analytical tractability. However, when considering jets in more detail and their transients it is apparent that entrainment is the mechanism driving their development and entrainment itself is non-trivial to predict. 2▪3 NATURAL GAS ENGINES 2▪3▪1 NATURAL GAS COMBUSTION ENGINES McTaggart-Cowen et al. [50] offer an excellent review of on-road natural gas engines in terms of their market position and some of the larger technical and infrastructure concerns, yet here our attention is turned to the macroscopic combustion behaviour of natural gas engines. Natural gas engines come in SI, CI and HCCI variants, where the gas may comprise the major or minor stake in the fuel mixture. Historic development has followed the order given, for ease of implementation, with the latest engines offering exceptionally low emissions, high thermal efficiencies and high power. In directly injected engines the fuel may be burnt as a premixed or non-premixed mixture, depending upon the time and duration of the injection event. Early injection affords the time for enhanced mixing, both diffusive and turbulent, to yield increased homogeneity. Late injection, with shorter times to ignition will yield a more poorly mixed, or stratified, charge which may auto-ignite or necessitate an ignition source. With the partially stratified charge approach used in the current system, combustion may be either pre- mixed or non-premixed depending upon the timing of the fuelling events. Non-premixed combustion is responsible for the burn in the partially stratified charge and direct injection plumes; while if there is a homogeneous bulk charge present, ignition by the PSC results in a premixed flame. The slow flame speed of premixed natural gas combustion results in slow heat release rates, HRR (that are proportional to propagation velocity), which in turn cause combustion instability and low combustion quality. The result is misfire and high UHC emissions, which are even more pronounced (as are cyclic variations), when leaning the mixture to reduce NOx output. Leaning the charge mixture (for any fuel) yields lower NOx ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 16 emissions through the provision of a heat sink from the excess air, which reduces combustion temperatures and hence thermal NOx. Gupta et al. [51] report a low NOx plateau of 1.12g/kWh (~220ppm) for Φ<0.7. Compared to gasoline, natural gas reduces the reactive hydrocarbons emitted when burnt close to stoichiometric, though, with natural gas, total HC emissions do tend to be higher when measured against the same baseline [52] since methane is considered a relatively unreactive compound, though has a significant global warming potential. Higher charge pressures retard flame propagation and lower the auto-ignition temperature, which although better for HCCI designs increases the propensity for knocking in SI engines. Retarded timing tends to raise the mixture temperature in the end-zone above the knock temperature limit (which is independent of fuel composition [53]). Further, a drop in combustion duration, Δθc in Figure 2-3, reduces the thermal efficiency and, hence, power, since there is less heat release near TDC. Figure 2-3 Power Reduction From Increased Burn Duration and Necessitated Spark Advance, reproduced from [54]. To circumvent the slow heat release rate, fuel additives are often used to increase the flame propagation velocity and get it close to the gas maximum of ~0.2m/s (for lean mixtures), which is independent of fuel composition (Payman in [53]). Diesel pilot ignition is often used to increase the strength of the initial combustion, which is dependent upon mixture temperature, jet mixing and jet penetration. This strong ignition encourages a faster second stage burn in the bulk charge with an overall HRR closely related to the percentage of the total charge which was pilot injected. Pre-chambers offer similar benefits, though their design is more complex with a wider range of variables affecting the second stage combustion of the bulk charge, particularly for multi-stage chamber designs [55]. Spark ignition of lean gas mixtures is inhibited by the high spark energies required, though early gas engine designs used a pre-chamber to ignite a rich mixture allowing lean spark-ignited gas engines to be some of the earliest used. Meyers [56] shows that pre-chamber designs with a volume of 2-3% of the cylinder can offer low emissions with high efficiency. However, Kubesh [57] notes that large pre-chambers (~20%) are required to offer full ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 17 throttless operation, though using a more traditional pre-chamber Goto et al. [58] achieve excess air ratios exceeding two. More contemporary designs use the diesel pilot to the same effect, though the pilot is the source of the majority of the overall emitted NOx, where NOx emissions are proportional to the percent pilot injection. Regardless, optimal gas engine performance (in terms of efficiency versus emissions) can only be achieved when the gas fuel has at least 80% methane; less than this and the overall fuel octane number is insufficiently low and can cause knocking. The increased reaction rates required for ignition can also be achieved through spark plug shrouding, whereby the electrodes are covered with a shroud, the enclosed volume of which is filled on the compression stroke. The enclosed, more quiescent mixture, is ignited producing strong flame jets, rich in radicals, which then ignite the bulk charge [59]. The enhancement of the radical pool through mini-chambers on the cylinder head has also been numerically investigated as a controlled source of auto-ignition, though is still awaiting experimental trial [60]. The high knock resistance of methane/natural gas (RON ~125-135) allows for the use of higher compression ratios than gasoline SI engines and allows an efficiency increase of up to 6% as a direct result [61]. Figure 2-8 shows schematically how CNG engines can achieve a 25% reduction in CO2 emissions when compared to a gasoline baseline, and an overall emissions reduction of ~20% on a CO2- equivalent basis. Figure 2-4 GHG Reduction Potential of CNG Engines Against Gasoline Equivalents, reproduced from [61]. Further, the high knock tolerance of NG yields a fuel that is well suited to turbocharging, which in itself lends the ability to downsize and meet the strictest emissions standards [62]. As a fuel, natural gas also offers a better ability to homogenate in the cylinder (if required) and an inherent reduction in nmHC’s. Adaptation of existing engine configurations does not fully access all these thermodynamic advantages however, and there exists the need for bespoke NG engine designs. Bespoke engines could offer cylinder conditions aimed at optimising the burning velocity, not mixing, and hence reduce the required turbulence levels, minimizing wall heat loss. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 18 Gaseous natural gas displaces about 10% of the cylinder air, reducing the volumetric efficiency that, when coupled with the stoichiometry of gas combustion, reduces the quantity of fuel available to produce power by about 15% and hence power by 10-15% [51]; while maximum torque levels are also seen to drop by ~10%. Volumetric efficiency is further reduced due to a lack of charge cooling with gaseous fuels, though this does mean that on cold start no enrichment is required and CO emissions are correspondingly reduced. It is further noted, that on a well-to-wheel basis, CNG shows the ability to offer 15% CO2 savings over gasoline, beneficial but lower than the 25% when measured on a tank-to-wheel basis [63]. 2▪3▪2 LEAN BURNING NATURAL GAS ENGINES The stoichiometry of a reaction describes the extent to which there are enough reactants to complete the reaction, which in combustion are fuel and oxidizer. When there is enough oxidizer (or fuel) to complete the reaction with no excess or shortfall, the ratio of fuel to oxidizer, i.e. the fuel air mixture, is said be stoichiometric. Any excess fuel and the mixture is rich, any shortfall and the mixture is lean. The ratio of air to fuel (by mass), AFR, is used to quantify the stoichiometry of the mixture. Mixture composition is often expressed in terms of the relative air fuel ratio (RAFR), λ, which is the ratio of the AFR to that of the stoichiometric condition, the inverse of which is known as equivalence ratio, Φ. Spark ignited engines must operate at an RAFR of around one, and so load control is obtained by throttling. There are significant pumping losses associated with this which are further exacerbated with natural gas. They range from 10% at full-load to over 25% at part-load, when compared to a diesel engine- a natural gas engine’s obvious competitor [57]. Using a fuel that allows a move away from stoichiometric could potentially reduce, or eliminate, the need for throttling. Natural gas is well suited to such lean-burn applications because of its wide flammability limits and propensity to homogenate (even upon cold start). Modelling of a medium duty throttless naturally aspirated natural gas engine shows that at low load an unthrottled engine requires equivalence ratios as low as ~0.2 [64]. Late intake valve closure can achieve 15% thermal efficiency gains at up to 15% load, with throttling used at part- and full-load [65]. Engine load range can also be controlled by internal trapping and exhaust gas recirculation with supplemental charge heating [66]. It does however, result in higher NOx at stoichiometric conditions compared to gasoline, since a lower residual gas fraction is employed because of significant differences in the required valve overlap. Honda appears to have overcome these problems with an electronically controlled VVT CNG engine with a variable length air intake [67]. Power is reported to increase by 15% over the non-VVT case, with a 5% reduction in fuel consumption. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 19 Although methane has a similar ratio of specific heats to gasoline, advantage is to be had in thermal efficiency since as the charge is leaned the mixture ratio of specific heats increases monotonically to that of air. Further, volumetric efficiency gains can be realised over turbocharged and normally aspirated SI engines in that the need for more charge air necessitates a wider throttle with lower pumping losses. Karim et al. [68] and Klimstra & Jones [69] both report natural gas’ ability to operate over a wide range of equivalence ratios with Gupta [51] reporting the same 10% COVIMEP6 at λ=1.6 compared to 1.4 for gasoline. Yet there are problems with gaseous fuelling and lean operation: high ignition energies and slow combustion durations. The first of these problems may be overcome with high energy ignition sources such as laser ignition [70, 71], pilot charges or pre-chambers, where laser ignition is also shown to have emissions benefits over traditional spark ignition. The flame speed of a hydrocarbon mixture is strongly related to both the equivalence ratio and pressure of the charge, dropping rapidly with pressure and RAFR’s above one. This is a serious problem for high pressure (large compression ratio) lean burn engines where extinction and misfire are to be avoided. Hassaneen et al. [72] report the initial flame kernel growth to be retarded by up to 60% at MBT, and the rapid burn duration by up to 33% when a gas mixture is leaned to Φ=0.6 against a stoichiometric baseline. However, the COVIMEP was typically less than 5%, showing good combustion stability. Increased levels of charge motion would elevate burn rates and permit the use of higher compression ratios or boosting, while avoiding knock through extreme leaning. Figure 2-5 Operating Envelope for an SI Gas Engine, reproduced from [61]. Figure 2-5 shows, schematically, that with an increase in turbulence, and hence burn rate, the engine operating envelope extends such that higher compression ratios and leaner mixtures can be supported. 6 Coefficient of variation, CoV=σ/µ. r c λ ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 20 Using this approach Pischinger et al. [61] show the best full load fuel consumption compared to similarly specified SI and boosted CI engines and with 25% less CO2. Combustion chamber and piston design are key to enhanced combustion through turbulence generation. The bowl-in-piston approach is most often used to generate squish and tumble [73-75]. A novel approach outlined by Evans et al. [76-79] uses a fence around the bowl, with gates designed to guide and funnel the squish motion into highly turbulent jets that converge at the ignition location. Meyers & Kubesh [80] propose a markedly different approach to the use of lean burning engines. Their concept fuels a portion of a multi-cylinder engine with a very rich gas mixture, while the remaining cylinders are fuelled with a lean mixture of natural gas, air and the rich cylinder exhaust. Catalytic treatment of the high HC and CO concentrations in the rich cylinder exhaust is used to produce hydrogen that is then employed to enhance the combustion of the lean cylinders. This approach can offer NOx as low as 8ppm, good combustion quality, combustion stability and thermal efficiencies from 24-28%. Homogeneous charge compression ignition of lean gas mixtures is reported as pre-mixed charge compression ignition by Kawasaki et al. [81]. It shows that maximum specific power may be increased with engine speeds up to 2400rpm while maintaining an indicated thermal efficiency of 32% and NOx emissions below 100ppm. However, “an increase in the engine speed extends the combustion duration, especially under a lean condition, and so the oxidation reaction is frozen at the last stage of combustion, and the indicated thermal efficiency decreases compared to the low speed condition.” 2▪3▪3 STRATIFICATION AND THE PARTIALLY STRATIFIED CHARGE The aim of stratification is to combine the benefits of a spark ignited engine and a compression ignition engine in a single platform. The inducted charge is globally beyond the lean limit, but through judicious charge motion the portion of the charge which is ignitable is brought to the sparkplug for ignition. Stratified charge engines can operate over a wide range of loads through controlled mixture strength, are knock resistant, and can accommodate a wide range of fuel compositions [82]. Abata [83] provides a detailed discussion of traditional stratified charge approaches and their application, with Toyota [84], Honda [85] and Texaco [86] all releasing commercial stratified charge concepts. Zhao et al. [87] offer discussions on the more contemporary approach of direct injection stratified charge (DISC) based on the now widespread practice of gasoline direct injection (GDI). These approaches use a direct injection fuel jet that is wall or piston guided to the plug, during which time entrainment provides the mixing required to form an ignitable mixture. The complexities of the system (fluid-fluid interactions, fluid-surface interactions, injection pressures/timing, piston motion etc.) still remain problematic in that poor fuel utilisation results in high UHC emissions, especially at part load. Further, overmixing and poor delivery to the sparkplug result in significant misfire (with its own emissions penalty) [57, 88, 89], though DISC ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 21 shows the potential for significant efficiency improvements [90-92]. The ignition problems associated with stratified charge may be overcome with local charge enhancement or partial stratification; whereby a small portion (typically <5% w/w) of the fuel is concentrated in the region of the ignition source. This easily ignitable source then provides the energy required to ignite the remaining fuel. This is particularly important in the ignition and stability of ultra-lean mixtures which require high ignition energies [93, 94]. Partial stratification is known to enhance the combustibility of a mixture through increased turbulence generation; Acroumanis et al. [95] report an increase in peak cylinder pressure of 55% and similar flame speeds at equivalence ratios 0.2 lower. At the University of British Columbia a partially stratified charge (PSC) approach has been developed by Evans [7] whereby the pilot charge is injected next to the spark electrodes using a modified spark plug [8, 96]. The plug is then able to ignite the fuel rich pilot which can inflame the rest of the homogeneous charge. Extensive testing has been carried out on a fully instrumented Ricardo Hydra single cylinder engine, the details of which are described elsewhere [8, 96, 97]. No benefit in emissions or engine performance is found for λ<1.4, although when λ>1.6 at full load, there is a significant extension of the lean limit with an increase of 7% in BSFC. Of more significance are the part load improvements. As is clear from Figure 2-6 there are gains to be made in BSFC and NOx emissions, which are coupled with (not shown) marked combustion stability improvements. Figure 2-6 Lean Limit Extension & BSFC Reduction (left) and NOx Reduction Potential (right) of PSC reproduced from [98]. The work of Reynolds [96] and Brown [97] (who completed his studies on natural gas PSC with port injected gasoline) was built upon by Gorby whose “goal was [to] determine if mixture enrichment local to the spark plug would aid the combustion of a stratified DI fuel charge” [8]. The thinking behind the work was to improve fuel usage through a larger ignition zone for the DI plume, and to provide an ignition which was stronger and which could then progress in areas which would otherwise be overly lean. These benefits should engender a reduction in UHC’s and “increase efficiency from improved combustion and reduced misfire.”. The potential gains are shown schematically in Figure 2-7 as given by Gorby [8]. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 22 Figure 2-7 Fuel Efficiency of Load Control Strategies, reproduced from [8]. These hypothesised gains were not observed, however. The optimal BSFC was found at early injection timings, the combustion performance degrading rapidly past a minimum threshold value. Further, the DI jet was most effective when directly aligned with the PSC injection; showing that only small gains were to be made from the increased volume of the ignition source. Finally, and most significantly, it was found that the PSC charge was not being reliably ignited and required a certain level of background fuelling to ignite, and a pure air charge impeded the ignition to the extent that it would fail to inflame the PSC injection. However, when the PSC charge was ignited reliably (COVIMEP<5%) the late DI also ignited reliably and provided stable operation of the engine. It is imperative that a strong combustion be caused through vigourous charge motion and charge ignition if natural gas is to be used as an alternative fuel for ICEs. 2▪3▪4 GAS COMPOSITION Gas engines are known to have a sensitivity to gas composition that acts primarily through the observed ignition delay and the fuel/dilutant kinetics [99]. Ethane (the second most prevalent component in natural gas) decreases ignition delay and combustion duration [99, 100], while the CO2 in EGR gas has the reverse effect [99]. However, the type of ignition source has the dominant effect on ignition/composition interaction. In an unassisted (auto) ignition, non-reacting species such as nitrogen, are known to stabilise the ignition, while no benefit is observed when an ignition source such as a pilot charge is used [101]; highlighting the benefits of enhanced ignition sources to account for fuel composition variability. Further, Schiffgens et al. [102] and Pischinger et al. [61] both suggest that no matter, knock-controlled and lambda-controlled operation is required to account for the range of methane numbers (a measure of the composition of natural gas) encountered when fuelling gas engines. With engine combustion affected, delivered power is also affected. Kim et al. [103] suggest that such a clear relation exists between gas ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 23 composition and engine power that a simple relation based on the Wobbe Index7 may be used as a practical method for power estimation based on fuel composition, although composition is not an integral part of the current. 2▪4 POLLUTANTS AND ENGINE EMISSIONS Pollutants include, but are not limited to: oxides of nitrogen (NOx), carbon monoxide (CO), hydrocarbons (HC’s) and particulate matter (PM). Fuel bound sulphur is of concern since it will form SO2 during combustion, which can then oxidize to SO3, the source of acid rain. It is of note that the US EPA has recently recognised carbon dioxide as a pollutant (along with, methane (CH4), nitrous oxide (NO), hydrofluorocarbons (HFC’s), perfluorocarbons (PFC’s) and sulphur hexafluoride (SF6) ) [104]. The term NOx covers both nitric oxide (NO) and nitrogen dioxide (NO2), which in engine emissions total concentrations of 500-1000ppm [82]. Hydrocarbons from unburnt or partially oxidized fuel, blow-by, fuel evaporation or (in older engines) carburettor release may reach 3000ppm. Intrinsic to these releases there may also be HC’s from oil films, quench products, crevice volumes and oil layer scavenging. Carbon monoxide resulting from ICE’s can reach concentrations as high as 1-2%. NOx levels are similar for both SI and CI engines, while HC’s may be up to five times higher for SI engines as a result of the exhaust process. There is little PM found in SI exhaust, though it may range from 0.2- 0.5% (at 0.1µm diameter) of fuel mass for CI engines. The high pressures and ample quantities of oxygen found in the combustion process of non-premixed DI engines means that there is little CO present compared to that produced by an SI engine as the thermodynamics is driven towards complete combustion and the production of C02. Detailed chemistry is needed to link pollutant formation due to combustion chemistry to that of the post combustion processes. This need arises since cylinder concentrations can differ significantly from equilibrium values in the exhaust stream. Fuel chemistry tends to drive CO, PM and organic pollutant formation, while post-combustion processes govern NOx and SOx emissions. High combustion temperatures will form NO that is frozen during the expansion stroke. Rich combustion yields higher CO concentrations since there is not enough oxygen present to complete the fuel oxidation 7 The Wobbe Index, IW, is define as and allows the direct comparison of different fuels on an energy basis. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 24 and the CO is quenched on expansion. Lean combustion will have similar results though these are attributable to dissociation rather than partial oxidation. Figure 2-8 Engine Pollutant Concentrations Across the Range of Operating FAR’s, reproduced from [82]. Emissions for SI engines are almost solely governed by equivalence ratio. With the exception of NOx, pollutant concentrations rise with increased equivalence ratio as combustion quality starts to deteriorate, cf. Figure 2-8. On warm-up, HC and CO emissions increase because of the rich combustion, while under normal loads, exhaust gas recirculation (EGR) drops NOx by lowering the combustion temperature, though at the expense of combustion quality. Emissions for CI engines are almost solely governed by fuel distribution. 2▪5 OXIDES OF NITROGEN Oxides of nitrogen are integral to the formation of tropospheric (low level) ozone, as first noted by Haagen-Smit [105] and discussed extensively since (often with geographically specific chemistry, see for example [106-112]8). NOx is also the cause of photochemical smog, which is discussed more extensively in §2▪10. The nitrogen required to form NOx can be supplied from the air charge or may be fuel bound. Nitric oxide predominates in NOx and is formed through several mechanisms [113]. 8 The first seven hits for most recently published material on the NOx-ozone relationship at the time of writing (July 2009). ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 25 2▪5▪1 NO FORMATION MECHANISMS 2▪5▪1▪1 ZEL’DOVICH (THERMAL) MECHANISM First postulated in 1946 by Zel’Dovich and proven by Baulch et al. in 1994 [114] (with Lavoie et al. [115] establishing the significance of step (c) in Eq. 2.6) the thermal mechanism is the most commonly cited route to NO formation, viz.: (a) (b) (c) Eq. 2.6 Kuo [116] also suggests that the mechanism include the primary step: Eq. 2.7 Steps (b) and (c) (of mechanism Eq. 2.6) are strongly temperature dependent and necessitate temperatures in excess of ~1800K to exceed the high activation energy of nitrogen’s triple bond. Reducing reactant concentrations or lowering the temperature will ultimately reduce final NO concentrations. The intrinsic timescales of the formation mean that the process is kinetically controlled and does not result in the equilibrium condition for the exhaust gas state [117]. 2▪5▪1▪2 FENIMORE (PROMPT) MECHANISM The prompt mechanism first proposed by Fenimore [118] has the CH radical as a prerequisite, which limits its applicability to the flame front (cf. Fig 17-4 of [119] for the complex oxidation mechanism of C1 and C2 hydrocarbons to produce the required CH radical). C2H2 is a precursor to CH and is primarily formed from CH3 recombination in rich flames [119], though the mechanism can be significant at temperatures as low as ~1000K. 2▪5▪1▪3 N2O MECHANISM A nitrous oxide route to the formation of NO was suggested by Wolfrum as an alternative to the two major mechanisms discussed above, and is more prevalent in situations where the relative air/fuel ratio is above 1.6 [120]. The mechanism is normally overlooked, but under lean conditions CH concentrations are suppressed (negating the prompt mechanism) and temperatures are low (eliminating the Zel’Dovich route). The nitrous oxide route is prompt at high pressure (as with all third body reactions). 2▪5▪2 NO FORMATION IN SI ENGINES Early combustion contributes significantly to NO which is then frozen on expansion and may contribute many orders-of-magnitude more to the overall NO concentration [82] than late formation NO. The lack of significant bulk motion near the sparkplug also increases NO concentrations in this region. Given that the ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 26 thermal route to NO production predominates in SI engines, it would be expected, and has been shown [121-123], that NO concentrations mirror the temperature gradients across the cylinder. Spark ignition engines achieve maximum combustion temperatures at an equivalence ratio of ~1.1, but the low oxygen concentration at this equivalence limits NO formation. As the charge is ‘leaned out’ the extra oxygen compensates for the drop in temperature such that NO concentrations peak at about Φ=0.9. Under lean conditions NO freezes early so there is little decomposition compared to rich mixtures, meaning that the gas composition at peak pressure is important. 2▪5▪3 NO FORMATION IN CI ENGINES The kinetics of NO formation in CI engines are also primarily driven by the Zel’Dovich mechanism, though in a markedly heterogeneous fuel and temperature field. Some fuel is pre-mixed and burnt near a stoichiometric composition, while the remainder is non-premixed and burnt at stoichiometric. The critical stage for NO formation is, as with SI engines, at peak cylinder temperature, which is between the start of combustion and peak pressure. Early burning gives high NO concentrations that are frozen by subsequent expansion and the mixing of late burn (cooler) gases or air. It is this fact that limits NO decomposition in CI engines, with almost all NO formation taking place within 20CAD of start of combustion [82]. Increases in overall fuel/air ratio increase both NO and NO2 since it generates higher peak pressures and temperatures. Moving towards a rich mixture reduces NO, though the heterogeneity of the fuel distribution means the reduction is less marked than in SI engines. Most fuel is still burnt close to stoichiometric, with the result that NO formation is almost proportional to total fuel mass injected. 2▪6 CARBON MONOXIDE Carbon monoxide (CO) is a known poison and is intrinsic to photochemical smog formation. CO production is primarily controlled by fuel-air-ratio. The higher the FAR for a fuel rich mixture the more CO is produced; while for lean mixtures CO concentration is independent of FAR, with mole fractions of ~10-3. Since SI engines run at stoichiometric under part load and rich at full load CO emissions are significant. CO levels are observed to be lower than their maximum possible concentrations, but above equilibrium levels, which suggests the CO mechanism is kinetically controlled. Lean conditions promote CO concentrations well below those suggested by a kinetically controlled model, implying the partial oxidation of oil and crevice HC’s during expansion and exhausting. Reductions in CO levels have, to date, been achieved through more uniform mixture generation, charge leaning, reduced cylinder-to-cylinder variation, after treatment and better fuel metering during transients [82]. Mixture non-uniformity and transients are still the main sources of CO however [124]. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 27 2▪7 HYDROCARBON EMISSIONS Hydrocarbons (HC’s), sometimes also referred to as tHC’s (total hydrocarbons) or UHC’s (unburnt hydrocarbons) are more strictly organic emissions. They result primarily from incomplete combustion. Total HC emissions are a good proxy for combustion efficiency, though not total pollution emissions [82]. Fuel composition is important to HC emissions in that rich combustion is known to increase aromatic and alkene concentrations, both of which are highly reactive. The combustion process, through pyrolysis and synthesis, changes the hydrocarbon structure significantly from those bound in the fuel. Oxygenates in the exhaust such as carbonyls and phenols (which are increased by the addition of alcohols to fuel) are important to the formation of photochemical smog (cf. §2▪10) 2▪7▪1 SPARK IGNITION DERIVED HYDROCARBONS Typical levels range from 1000-3000 ppm C1 for SI engine emissions. Hydrocarbon emissions rise significantly on the rich side of stoichiometric (cf. Figure 2-8). The four processes primarily at work are identified by Heywood [82] as: flame quenching, crevice volumes storage, absorption/desorption into/out of the oil layer and incomplete combustion (from high EGR, poor spark timing or mixture leaning). Maximum HC generation occurs on blowdown and at the end of the exhaust stroke with a 50/50 split on a mass basis (though the blowndown HC’s tend to be significantly heavier) [82]. 2▪7▪1▪1 OVERLEANING & OVERMIXING Upon injection a fuel spray develops a mixture distribution similar to that shown schematically in Figure 2-9. The amount of fuel mixed beyond the lean limit (Φ=0.3) rapidly increases with time [125]. The auto- ignition location is normally on the lean side of the stoichiometric contour, downstream of the leading edge of the jet. At this location there should be fuel which has spent the longest time in the combustible limit. The overmixed fuel will not support auto-ignition or a fast reaction, and is only capable of sustaining thermal oxidation, which is most likely to be incomplete. The results are unburnt fuel, decomposed or partial burnt products, most of which will be exhausted from the cylinder. The quantity exhausted will depend upon the amount of fuel injected during the ignition delay [126] and the cylinder conditions at ignition. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 28 Figure 2-9 Fuel Jet Structure in a Compression Ignition Engine, reproduced from [82]. 2▪7▪1▪2 UNDERMIXING Undermixing is attributable to: low injection velocities, i.e. fuel injected late in the cycle, and that which escapes the sac volume; as well as excess fuel from overfuelling, i.e. poor load matching and metering. Fuel retained in the sac volume is vapourised as the cylinder temperature increases and enters the cylinder slowly, and may miss the primary combustion altogether. Heavier fuel fractions may stay in the sac and later be evacuated as UHC’s or undergo post-combustion oxidation in the elevated temperature prior to blowdown. It has been shown [127] that the fuel retained, not in the sac, but in the holes of the injector, matter to overall HC emissions for CI engines. Direct injection compression ignition engines are limited to Φ~0.7 at full load to avoid smoking. They are lean overall, but locally rich during transients due to overfuelling in the power stroke. Hydrocarbon pollutant levels are ostensibly constant for increasing equivalence ratio (at constant speed and minimum ignition delay) until Φ=0.9, when there is a sharp jump in emissions, cf. Figure 2-10. This is critical to emissions under acceleration, although levels are still lower than those observed from overleaning [125]. Figure 2-10 The Effect of Overfuelling on Exhaust HC Concentrations, reproduced from [82]. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 29 2▪8 PARTICULATE MATTER Particulate matter (PM) is now considered the most important pollutant with respect to human health. The side effects of ingesting and breathing PM can be profound and long-lasting. In SI engines particulate matter can form from lead, sulphates and organic matter in the fuel, though the phasing out of leaded fuel has all but eliminated PM from lead compounds. Organic PM is mostly soot, while sulphates in unleaded fuel (at levels of 150-600ppm) are oxidized to SO2 and then SO3, which reacts with atmospheric water to produce sulphuric acid aerosols. Unleaded fuel generates PM emissions at ~20 mg/km comprised of mostly soluble and condensed organic matter. These emissions result from poorly adjusted engines and, under rich running, may contain significant amounts of black carbon (soot). Compression ignition engines emit mostly carbonaceous material with some adsorbed organic compounds, both of which originate mainly from incomplete combustion of fuel hydrocarbons. Heywood [82] provides and excellent summary on the formation and nature of PM, where he points out that the composition of CI engine emitted PM is strongly affected not by the fuel make-up but by the exhaust system, and somewhat paradoxically by the analysis collection system. At temperatures above ~5000C individual spherules with diameters of 15-30nm are observed whilst below this temperature the spherules are generally coated, despite their high porosity, in condensed material. The condensates may be any one or a combination of UHC’s, oxygenated HC’s (primarily ketones, esters and ethenes), PAH’s or some inorganic species such as SO2 or NO2. Fuel oil has been shown to contribute between two and 25% of exhaust particulate matter and as much as 80% of the organic material. Traces of zinc, sulphur, calcium, iron, chromium and potassium in PM have also been found with the zinc, calcium and chromium linked to engine lubricating oil. 2▪8▪1 FORMATION PROCESSES Soot originates primarily from fuel-bound carbon, that is to say chains of about 12-22 carbon atoms at a hydrogen to carbon ratio of about 2:1. This yields about 105 carbon atoms with an H:C ratio closer to 0.1:1 [82]. The processes at work to make this transformation are poorly and incompletely understood. The soot formation process, which has characteristic timescales in the order of milliseconds, is known to require high temperatures (1000-2500K) with pressures from 50-100atm and overall enough air to completely oxidize the charge. Particulate formation requires the production of a condensed phase, most often from fuel species via oxidation or pyrolysis. Generally this phase consists of unsaturated hydrocarbons and PAH’s, which undergo condensation reactions to form the soot nuclei (diameter less than ~2nm). Large numbers of these particles exist, though they form a negligible amount of the total sooting because of their small size. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 30 Particle growth occurs according to: formation, surface growth, coagulation and agglomeration. Surface growth is primarily solid phase through gas-phase deposition, condensation and solidification, and is associated with dehydrogenation [82]. Oxidation may occur at any of the growth stages to yield CO or CO2, and soot formation is an inherent balance between formation and burn-out. 2▪8▪2 FORMATION LOCATION In-cylinder particle distributions are almost solely related to fuel distribution and local heat release. Direct injection results in peak PM concentrations (mostly soot and hydrocarbons) along the jet centre-line. This locally rich core ‘converts’ up to 50% of the local fuel carbon to PM, with fuel pyrolysis in this core contributing the majority of the cylinder-formed soot. The concentration of particulate matter drops away from the jet centre-line, though near the outer edge of the jet, or in the vicinity of the piston bowl, PM concentrations again rise, albeit to levels an order-of-magnitude lower than the core. 90% of all soot is formed in the cylinder prior to exhaust [82]. Particulate formation increases significantly at about four or five degrees ATDC with peak number density occurring at ~200 ATDC. There is then a rapid drop because of coagulation and (possible) oxidation. The volume fraction of PM is seen to rise smoothly much like the number density; however it peaks earlier at ~180 ATDC and drops steadily until oxidation stops at ~400 ATDC, when the PM concentration remains constant [82]. 2▪9 HYDROCARBON COMBUSTION & CARBON DIOXIDE PRODUCTION Water and carbon dioxide are the lowest energy states of the hydrogen-oxygen and carbon-oxygen molecular pairs, and as a result it is inevitable that these will be the most prevalent products of hydrocarbon combustion. Aliphatic fuel combustion is essentially a series of fragmentations in which the chain is broken down into subsequently smaller and smaller intermediate species, each of which is oxidized according to the strict hierarchy of Figure 2-11. Figure 2-11 Hierarchical Nature of HC Combustion, Reproduced from Westbrook [128]. ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 31 The intermediate species CO and H2 are common to all parent fuels as are the radicals H, O, OH, HO2 and HCO [116]. The CO is slowly oxidized to CO2 while the other species are key to the oxidation of hydrogen. The detailed kinetics of HC combustion are elucidated most comprehensively by Westbrook and Dryer [128, 129]. 2▪10 PHOTOCHEMICAL SMOG & TROPOSPHERIC OZONE As has been alluded to, engine emissions, primarily NOx and UHC’s, but also CO, provide the precursors to photochemical smog. Photochemical smog has been shown to consist of generic hydrocarbons, peroxyacetyl nitrate (PAN, C2H3NO5), NOx, ozone and nitric acid (NHO3) [130]. Tropospheric ozone (with desirable levels below 0.1ppm [131]) is highly stable and its measurement thus acts as a proxy for general air quality. ‘Difficulty regulating O3 occurs because in regions of high NOx (primarily urban centres and power plant plumes), O3 formation is limited by the availability of hydrocarbons.’[132], while in rural regions it is the availability of NOx which is the limiting factor in O3 production [133]. The addition of VOC’s (most commonly biogenic isoprene [132] ) and CO to the atmospheric chemistry alters the reaction mechanism significantly, since the VOC’s initiate different precursor reactions [134]. The complex interactions of the O3-NOx-VOC system ensure that smog chemistry is strongly non-linear and the concerned reader is referred to Sillman’s seminal paper The Relation Between Ozone, NOx, and Hydrocarbons in Urban and Polluted Rural Environments [134]. 2▪11 HEALTH IMPLICATIONS OF ENGINE EMISSIONS The pollutants discussed previously are significant in that their role on atmospheric chemistry and conditions may be contended, but the negative effects of engine emissions on respiratory health are beyond doubt. Hydrocarbon volatiles and aldehydes irritate the eyes and respiratory tract, while phenols have a similar effect and are an odorant. Hydrocarbons, as has been noted, are critical in photochemical smog formation, which is itself an eye and respiratory tract irritant. Aromatic compounds resealed from CI engines are known carcinogens, while contemporary thinking puts PM at the fore of health concerns related to air quality. Tropospheric ozone is know to cause significant respiratory morbidity and has been related to mortality [135, 136]. The problem of urban air quality management is difficult in that the chemistry is non-linear and temperature dependent [137]. The PM problem is complex. There is ‘emerging evidence of PM-related cardiovascular health effects and growing knowledge regarding interconnected general pathological pathways that link PM exposure with cardiopulmonary morbidity and mortality’ [138]. Despite toxicological studies of the effects of PM upon ▪ Chapter 2 ▪ Gaseous Jets, Gas Engines & Pollutant Emissions ▪ 32 respiratory health [139-143] establishing a direct casuality between PM and mortality/ill-health is not without significant problems, ‘however, recent research has increased confidence that the PM- cardiopulmonary health effects observed in the epidemiology are “biologically plausible”.’ [138]. The concentration-response function for PM is almost conclusively linear and as such ‘further improvements in air quality are likely to result in corresponding improvements in public health’ [138]. There is still a significant lack of knowledge, however, in our understanding of which pollutants, or combinations thereof, cause which symptoms and the role of coarse and ultra-fine PM in the pathological systems. 2▪12 CLIMATIC IMPLICATIONS OF ENGINE EMISSIONS The realm of climate change is vast and nebulous. The discussion about it in the scientific and popular communities has been vitriolic and heated. The Earth’s atmosphere contains a large number of chemical species, some natural, and many anthropogenic. The role of these chemicals is wide and varied, but even small concentrations play a significant part in regulating tellurian climate. The complexity of the climate system makes it almost impossible to model. Progress has been steady as atmospheric models have started to include more detailed chemistry and the role of water vapour, though the role of the complete hydrological and carbon cycles are not fully understood and are not at the stage of being accurately modelled. Many of the climate sceptics’ main arguments for non-anthropogenic forcing (solar activity, poor data and temperature lagging CO2 increases) have been recently debunked. As the UN IPCC makes clear in its latest report [144] much of the data pertaining to climate change is incomplete but the science is still sound and must act as a foundation for future action. Current atmospheric CO2 levels are around 380ppm, and the IPCC reckons that stabilising these at ~540ppm is economically viable and ‘safe’. Any increase beyond this will significantly endanger human activity on the planet. The IPCC report [144] predicts temperature rises anywhere from 1.1-6.40C in the next 100 years. A clear ‘bottom line’ has been drawn under anthropogenic radiative forcing. Regardless of the benefits to human health, reductions in the level of pollutants emitted from ICE’s is imperative if the climate change problem is to be mitigated. 2▪13 CONCLUSIONS Arguments have been presented that describe the behaviour of incompressible and compressible gaseous jets. The suitability of natural gas fuelling for transport engines has been argued and a description of the main technological approaches used was made, with particular attention paid to partial stratification- the technology at the core of the current work. The nature of the pollutant formation in engines has been discussed. The general nature of hydrocarbon combustion has been offered before considerations were made to pollutant effects upon human health and the climatic environment. 33 Chapter 3 LASER IMAGING TECHNIQUES & PLIF FUNDAMENTALS 3▪1 INTRODUCTION Chapter 3 provides an overview of several techniques used in laser-based species concentration measurement. The Chapter provides the most relevant details of each approach, and briefly outlines their suitability for combustion diagnostics. The reader is taken through the approaches offered by: Rayleigh and Mie scattering; Raman spectroscopy; advanced Raman techniques; and laser induced fluorescence. The majority of the Chapter is concerned with the basic principles of planar laser induced fluorescence (PLIF), its background and applicability to experimental flows. Building upon this, a qualitative explanation is made with regard to how PLIF can be used in elucidating mixing processes and, in particular, mixture formation in internal combustion engines. A case is made for the use of acetone over the many other possible options as a gaseous fuel tracer, and a detailed explanation of the photophysical process behind acetone LIF provided. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 34 3▪2 BACKGROUND Laser diagnosis provides the ability to investigate harsh environments, such as those in the combustion chamber of internal combustion engines, whilst being non-intrusive and can yield high spatial and temporal resolution. Fluid and chemical parameters can be simultaneously or individually resolved at any point or across a plane, with contemporary techniques providing full three dimensional information. State specific, species specific and non-equilibrium chemical compositions may all be scrutinized and, if desired, related to the temperature field. Modern lasers have advanced the field of combustion diagnostics through their high powers, coherent light production, spectral purity, short pulse durations (down to ~10ns) and small probe volumes [145]. High laser powers allow the exploration of weak events, including some that were previously unattainable with arc and flash lamps. Further, the coherence of laser light opens up the opportunity to use spectroscopic techniques only available with coherent stimulation. Species and state resolution have been enhanced with the improved spectral purity of lasers and allow specific electronic states to be targeted. The ability to sweep a range of spectra or multiplex multiple light sources presents the opportunity to simultaneously investigate a range of species. The latter point is complimented by the ability to focus laser light sources into probe volumes as small as 50µm2, which, coupled with short pulse durations, allows both chemistry and flow to be frozen on the smallest of scales. Laser diagnostics can be split into four main areas: velocity determination (both point and plane); the use of Rayleigh scattering for density determination; Raman techniques for major species (~1% concentration [146]); and laser induced fluorescence for minor species (in the order of ppm). 3▪3 COHERENT AND INCOHERENT PROCESSES An incoherent process will result in the emission of light which is non-directional, that is to say it will be emitted over the full 4πrs steridian solid angle of the molecule/particle. A scattered signal will result from each point along the path of the laser beam and as such allows for easy extension of these techniques to 2D measurement. Coherent phenomena emit light in a specific direction much like a laser beam. The probe volume is now governed by the intersection volume of the incident laser beams, where all the light emitted from the molecule is collected by the collection optics. This gives a stronger signal than incoherent approaches, but makes the experimental set-up more complex since the exact direction of emission must be established a priori. Eckbreth [145] lists the advantages and disadvantages of each as: ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 35 Incoherent Coherent Advantages Linear Intensity independent Single ported Spectral simplicity Simple calibration Strong signals Laser-like signal beams Interference tolerant Disadvantages Large solid angles Prone to interferences Non-linear Intensity independent Multi-ported Complicated Spectra Difficult to normalize Refraction sensitive Table 3-1 Advantages and Disadvantages to Coherent and Incoherent Laser Diagnostics Techniques, after [145] 3▪4 VELOCITY TECHNIQUES The importance of the flowfield within a combusting environment cannot be understated and as a result a brief outline of laser based and optical velocity tracking techniques is presented herein. For a full discussion the reader is referred to the excellent books of Durst et al. [147] and Taylor [148] and the review of Adrian [149]. Non-intrusive velocity measurement is now possible, with high resolution, at a single point or across a plane. Particle image velocimetry, PIV, uses a CCD camera to cross-correlate, across a number of sub- regions, laser light which has been elastically scattered by particles seeded into the flow. High laser powers provide a strong signal that can also be enhanced with high seed densities (which are tolerable since particles are not tracked individually but throughout the sub-region of the image). Laser Doppler velocimitry, LDV, uses the Doppler shift in the light elastically scattered from seed particles to ascertain the flow velocity at a point. 3▪5 RAYLEIGH AND MIE SCATTERING Both Rayleigh and Mie scattering involve the elastic scattering of light incident upon a particle or molecule. There is no energy transfer between the incident photons and the target molecule, which means that the scattered light is not frequency shifted. The process is incoherent and for d/λ<<1 is termed Rayleigh scattering while for πd/λ≥1 it is Mie scattering. The lack of photonic interactions means that both processes are species independent and as a result can only provide generic density information and only under some circumstances species concentration [145]. The processes do provide high signal intensity, yet may be prone to spurious light interference. Combination with LDV provides a powerful diagnostic tool [150]. A comprehensive summary of Rayleigh scattering’s use in combustion diagnostics has been presented by Zhao and Hiroyasu [151]. The Mie scattering effect generates a weak signal in ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 36 unseeded flows and is most commonly used in naturally seeded flows, i.e. soot studies where the technique is now in competition with laser induced incandescence (LII). 3▪6 RAMAN SCATTERING Raman processes are characterised by inelastic scattering, and may be based on changes in one, two or all of the rotational, vibrational or electronic states of the molecule. The process is almost instantaneous (~10-12s), hence linear Raman spectroscopy is referred to as spontaneous Raman spectroscopy, SRS. The photons generated from Raman scattering are shifted from the incident frequency by a quantity Δυ, where υ is wavenumber. For Δυ<0 the spectral lines produced are termed Stokes lines while for Δυ>0 they are anti-Stokes (cf. Figure 3-1). The latter requires energy addition from the target molecule and as such the molecule must first be in an excited state or at elevated temperature. Given the wide range of transitions that can take place to produce a Raman shift the hyperfine structure is grouped according to changes in rotational quantum number, J. Q-branch transitions are those for ΔJ=0 and Δv=±1 (i.e. the principall quantum number changes by one). When ΔJ=±1 the branches are termed P and R-branches respectively, while for ΔJ=±2 the O and S-branches are obtained. Figure 3-1 Ro-vibronic Structure of the Raman Spectrum, adapted from [152]. Scattering intensity scales with the fourth power of frequency [145], which means that UV diagnostics are preferable. Further, as a result of energy quantization the Raman shift observed is unique to each chemical υS υR υQ υP υO υO υP υQ υR υS Stokes Lines Rayleigh Line Anti-Stokes υ0 ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 37 species and proportional to molecular number density. This all means that Raman is well suited to species identification and concentration measurement; however with a scattering cross section of ~10-31cm/sr signals are weak. Near-resonant Raman uses laser excitation to excite target molecules near an electronic resonance, thereby increasing the scattering cross section by as much as six orders-of-magnitude [145], similar to those of laser induced fluorescence (cf. §3▪8). The difference is that the incident photon is not absorbed by the target molecule; with the result that the scattering is much shorter in duration, and not subject to quenching effects. However, for combustion diagnostics there are few species which can be excited at near resonant frequencies. 3▪6▪1 HYPER-RAMAN TECHNIQUES Hyper-Raman is the term given to any generic Raman emission that results from multi-photon excitation. The resultant emission is coherent though the excitation is weak, leading to a taxing experimental setup. This is often prohibitive, though coherent anti-Raman scattering (CARS, cf. §3▪7) provides the exception to the rule. Stimulated Raman gain spectroscopy (SRGS) and stimulated Raman loss spectroscopy (SRLS) are induced emissions techniques at the Stokes and anti-Stokes frequencies respectively. The target molecules are excited with both a pump and probe laser. SRGS and SRLS have the advantage that experimentally they are very similar to spontaneous Raman scattering as they are based on a resonant phenomenon to produce the emission and do so without the lineshape complications of other hyper-Raman techniques, while still providing a signal proportional to number density [145]. Nonetheless, beam multiplexing is required to reduce the interrogation time whilst still ensuring a strong signal. This is difficult to achieve and coherent anti-Raman scattering (cf. §3▪7) is favourable under these conditions. Beam steering that results from turbulent fluid motion impedes diagnostic significance and cannot be corrected for. This, and the experimental complexity, mean that SRGS and SRLS are of primary use in the fundamental research of molecular photophysics. Raman induced Kerr-effect spectroscopy, RIKES [153] is a Raman based effect whereby the target molecule is pumped (by a two colour phase matched system) such that the molecule takes on a polarized rotation at the Raman frequency. This induces an emission at the corresponding Raman shifted frequency and with the corresponding polarization. A polarizer is used to block the pump light, while the resultant emission still has components from the resonant and non-resonant parts of the excitation, the latter of which requires filtering. The difficulties of RIKES arises from poor signal strength due to extensive filtering and polarization of signal beams both of which contribute to a poor signal-to-noise ratio. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 38 Turbulence in the region of interest also introduces significant anisotropy, for which the induced polarization is likely to be weak. This practically limits the technique to high density media, i.e. liquids, and curtails its diagnostic utility [145]. The problems of poor susceptibility (in essence, the ability to excite a molecule) and incomplete probe laser rejection in RIKES are circumvented with optically heterdyned-RIKES (OHD-RIKES). The details of this technique are somewhat nuanced and not of current concern, though the interested reader is referred to works of Eckbreth [145] and Eesley [154]. Photoacoustic Raman spectroscopy, or PARS, is very much like SGRS except that the collection ‘optics’ are now acoustically based and collect the pressure wave that results from the photomolecular de- excitation. This means that the signal collection is free of all electromagnetic contamination. It is possible to detect signals down to 1ppm, though the noise requirements for this, or anything useful, are prohibitive for diagnostic application. 3▪7 COHERENT ANTI-RAMAN SCATTERING Coherent anti-Raman scattering, CARS, is a three or four colour non-linear technique whereby the pump laser, ω1, and the probe laser, ω2 (which is Stokes shifted from the pump), combine to stimulate the emission of a wave at ω3=2ω1 - ω2 (cf. Figure 3-2). Based upon this approach Eckbreth [145] also reports the technique of coherent Stokes-shifted Raman scattering (CSRS) whereby ω2 is now shifted to the anti- Stokes lines instead of the Stokes lines, though this application is much less widely used and is, as yet, unreported for diagnostic purposes. CARS can exhibit significant constructive and destructive interference because of the non-linear ro- vibronic interactions of the target molecule, though their interpretation is still analytically tractable [145]. The non-resonant part of the Raman signal can be removed through simple polarization techniques, though this significantly affects the quality of the signal such that the experimental set-up quickly becomes shot-noise-limited. Phase matching is not guaranteed automatically, though since gasses (which are the main target for CARS) are dispersionless, ensures that simple overlapping of the pump and probe beams generate phase matching. This does in turn limit the spatial resolution of the apparatus in collinear systems. Higher resolution necessitates the use of crossed-beam approaches and their inherent phase matching complexities. CARS is most commonly employed to excite the vibrational branches of the target molecule (particularly the Q-branch). However, similar techniques to those outlined here can be used to hit the rotational branches of the Raman spectrum [155]. Rotational CARS (RCARS) allows for simultaneous investigation of multiple species [146] and is less prone to carbon poisoning. Further ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 39 advances in CARS ease of use and applicability have been made by Eckbreth et al. [156] and Alden et al. [155]. Figure 3-2 CARS adapted from [146] The advantages of CARS are that it provides a signal many orders-of-magnitude larger than SRS; its coherent signal allows for complete collection; there is no fluorescent inference since emission is on the anti-Stokes side of the Rayleigh line and is more robust than SRS in harsh environments [145]. CARS is impeded by its limited sensitivity to species concentration and the requirement for concentrations greater than ~0.1%, though this is similar to that required by Raman, and much higher than that tolerable in many LIF techniques. LIF’s inability to cope with high concentrations (due to saturation) means that LIF and CARS may be seen as complimentary, as has been demonstrated by Mokhov & Levinsky [157]. Miles et al. [158] have also extended the idea to their RELIEF (Raman excitation plus laser induced electronic fluorescence) technique which can provide velocity information. Rotational CARS has also been shown to provide significant thermometric data [159]. 3▪8 LASER INDUCED FLUORESCENCE Laser induced fluorescence uses the energy bound in the incident beam to promote the target molecule to a higher energy state. This ‘optical absorption’ is followed by an almost instantaneous relaxation which may, or may not, be radiative. The entire process has a timescale of 10-10-10-4s [145]. Fluorescence is the radiative emission that maintains the spin multiplicity of the ground state, while phosphorescence is that which results from a state of different spin multiplicity. Phosphorescence is characterised by much longer timescales, typically 10-4-10-2s. The radiative emission may result from a number of states whether excited, predissociated or photoionized; while the yield may be linear or saturated depending upon the exact treatment of the subject molecule. Fluorescent emission can be at the incident wavelength (resonance fluorescence) or, more commonly, hypsochromically shifted from the incident frequency. Fluorescence gives signal strengths many orders of magnitude higher than Raman based techniques and as a result can be used to image very low concentrations (down to trace or ppm) and is readily expandable to planar analyses. Of most concern to the experimentalist is the strong tendency for excited molecules to be quenched. Quenching refers to any process which deactivates a molecule without directly leading to radiative emission. ω3 ω1 ω2 ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 40 3▪9 APPLICATIONS OF PLANAR LASER INDUCED FLUORESCENCE Laser diagnostics have a wide range of uses applicable across a range of transient [160] and steady flows which may be sub-, or even, super-sonic [161-165] (although the latter has its limitations [161, 166]). Flow properties such as: velocity (using metal atom tracing [167], iodine [168-170] and nitric oxide [171] fluorescence), periodic instability [172]; or the extent of entrainment [173] are obtained easily. With added experimental and computational complexity more fundamental flow properties may be obtained. These include: levels of vorticity and strain [174]; the dissipation length scales of energy and mixture fraction [175]; small scale turbulent structure identification [176, 177]; conserved scalar behaviour [178]; and heat transfer characteristics [179, 180]. LIF is primarily used to study mixing [181, 182] and the resulting concentration field [160, 183-194], with the works of [173, 180, 193, 195-199] on concentration fields in jets of particular note. The technique of LIF may even be extended to visualise concentrations in three dimensions [199]. Advances in molecular photophysics and the role of temperature within it (cf. §3▪11▪6) have extended LIF application to the temperature imaging for, amongst other things, gaseous flows [169, 184, 186, 200-205] and liquid droplets [206]. Laser induced fluorescence has provided much experimental data on combustion and flames. The fuels studied range from coal [207] and acetylene [208, 209] to the more pertinent: LPG (liquid petroleum gas) [210]; hydrogen [160, 184, 211-213]; natural gas [214] and methane [187, 188, 194, 215-226]. Using LIF, it is now possible to gain insight into sooting [227-231], flame surfaces structure [232] and blowout [233]. Existing engine related laser diagnostics cover: flame kernel growth [234, 235]; swirl [236]; heat release rates [237, 238], pyrolysis [239]; flame position [240] and mixture formation (cf. §3▪9▪1); and are applicable to homogeneous (including homogeneous charge compression ignition [241-243]) and stratified mixtures [244-246] alike. 3▪9▪1 LIF FOR MIXTURE FORMATION AND DISTRIBUTION Early work centred on mixture distribution imaging from Rayleigh scattering [247] or with fibre optic imaging of CH and C2 [248]. The C2 signal provided flame position while the ratio of the CH to C2 signals provided equivalence ratio. Established laser techniques such as Raman scattering have been used to investigate major flame species (CH4, O2, N2, H2O and CO2) as well as minor components (H2) [215], the concentrations of which, when combined with Rayleigh scattered light, can provide temperature information [249]. However, for most minor species (NO, CO and OH) LIF is the experimenter's only recourse. Frank et al. [215] use three Nd:YAG lasers to probe methane/air flames with equivalence ratios of 0.6, 0.7 and 0.8. The species concentration results compare favourably with laminar flame calculations. Acetone emission displays only a small temperature dependence at 266nm excitation. However, Fujikawa et al. [250] in their study of gasoline direct injection stratified mixture formation perform a number of cell ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 41 experiments to identify the variation in LIF behaviour at different temperatures and pressures. An analytical approach is used to calculate the DI jet temperature due to evaporative charge cooling and fuel superheating, with the effects upon the LIF signal then taken into account to produce an accurate temperature corrected fuel-air-ratio (FAR) field. Fujikawa et al. use acetone as the fuel dopate, which is substantially different in nature to the iso-octane fuel, wiyj the inaccuracies are compounded by low laser energies (60mJ/pulse) necessitating high seed concentrations (10% w/w). The concerns of differential evaporation are cursorily addressed by noting the highly elevated cylinder temperatures, though the consequences of differential diffusion, fractional distillation, and the effects of the high seed concentration upon combustion are not discussed. It is subtleties like these that must be addressed since, as is noted by Kraemer et al. [251], two seemingly identical initial mixtures may yield significantly different combustion characteristics. Kraemer et al. use a benzene/TEA9 trace for iso-octane mixture formation in a DI SI ICE. Since the tracer is consumed during the combustion event, flame tracking is also performed. Mixture formation and flame front propagation are further studied by Wolff [252] who also investigates relative fuel density and residual gas distribution using an acetone doped (non-specified) fuel for an in-line four cylinder SI engine with equivalence ratios from 0.3-1. Of more use to gasoline, iso- octane and two phase fuel systems is the work of Ipp et al. [253] who use laser induced exciplex fluorescence of benzene/TEA to track both fuel phases. Their technique (which is validated in the liquid phase by Mie scattering and Raman scattering in the gas phase) allows the specification of probability density functions for the FAR at a series of locations. The same tracer combination was used by the same authors [254] to investigate (with Raman validation) mixture formation in lean (Φ = 0.8) GDI engines where substantial fuel concentration gradients are found. The Raman investigation yielded sharper gradients due to its smaller probe volume, while the utility of completing Beer-Lambert attenuation calculations for incident laser intensity was brought into doubt. TEA also has application to gaseous fuel tracking [255] where it yields qualitative injection development information and can be temperature corrected to provide quantitative FAR values that correspond well to simultaneous Raman. TEA was also shown to track iso-octane and helium well (which was used as a safer proxy for hydrogen in motored studies), with only a ±3% error reported at a relative air fuel ratio of three. However, problems with TEA pyrolysis and retained gas fraction were noted to skew the LIF results under motored conditions. No such problems were encountered by Medaerts et al. [214] when they used toluene to track natural gas and gasoline. Using excitation at 248nm, mixture heterogeneity (for Φ=0.9-1.0) was elucidated. Pre- combustion diagnostics of lean hydrogen mixtures (Φ=0.55) is achieved with an acetone trace in the work of White [256], who simultaneously uses OH* chemiluminescence to investigate the flame structure. A lack of detailed knowledge about the quenching behaviour and kinetics of OH* limits the investigation to the qualitative, which is further complicated by line of sight signal integration and the symmetry of the 9 triethanolamine ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 42 OH* intensity about Φ = 1. Similar imitations are placed upon the LIF signal due to the low seed densities which result from the limited vapour pressure at the high injection pressures. Despite these shortcomings the importance of injector geometry, injection timing and injection pressure are established for gaseous fuelling. Oh & Bae [257] have similar problems with a weak signal in their LPG LIF study, which uses acetone in a lean stratified mixture, though they do obtain meaningful mixture distributions in the richer zones and good flame front tracking. The work of Kaminski et al. [221] compares well with DNS predictions. The LIF signal, which is calibrated against 1D laminar flame calculations, provides good mixture fraction values (±30%) which are ostensibly temperature independent at the excitation wavelength of 282nm. The effect of highly controlled turbulence (the extent of which is verified by laser Doppler anemometry) is also evident, as are the quenching and surface reaction effects of the electrodes upon ignition. By applying a 2µs delay to one of the CCD flames and capturing OH* chemiluminescence, flame kernel structure was also investigated, while the calculation of the Reynolds and Karlowitz numbers allowed the establishment of flame rate and surface to volume ratio. A judicious use of tracer and excitation sources allowed a temperature map to be established simultaneously with a mixture distribution map. The pioneer of this technique, Marc Thurber, details this approach in [258] whereby he (and his colleagues) employs a method that uses a simple acetone dopate (at either 3% or 9%v/v for a co- flowing jet) excited at 248 and 308nm concurrently. This yields both the temperature and mole fraction fields. The short (30ns) lifetime of the fluorescence freezes the flow while careful concentration adjustment and laser sheet alignment ensure good agreement with the existing literature. A similar excitation scheme was used by Einecke et al. [186] to excite acetone’s linear cousin 3-pentenone and make use of its 10nm hypsochromic shift per 100K increase in temperature (for any change in absorption behaviour is reflected in the emitted light intensity). However application was limited to small loads in a two-stroke engine since lean conditions (Φ=0.62) were required to avoid excessive self quenching. 3▪10 TRACER CHOICES The current work is solely related to stratified natural gas combustion, and as such the discussion herein limits the innumerable set of tracer and fluorescing species to those suitable for gas phase diagnostics. Methane, the major component of natural gas, does not fluoresce under the influence of any known radiation sources. The consequence is that it must be doped with a fluorescent compound which acts as a proxy for the methane. The most important parameters for a tracer are those listed below (which have been paraphrased and adapted from [259] & [260]  Possess a high fluorescent yield at easily available high-power laser wavelengths. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 43  Absorption and excitation of the tracer should still leave the fuel/tracer/air mixture ‘optically thin’10.  A good spectral separation between the absorption and emission wavelengths is required so as to ease the implementation of the excitation and collection optics, but more importantly to avoid fluorescent trapping whereby the emitted light may be absorbed before reaching the detection source.  It is desirable to have the fluorescent intensity independent of bath composition, temperature and pressure, yet strong enough to allow 2D visualisation. The lifetime must also be short enough to freeze the flow.  The tracer must be structurally stable at the temperatures and pressures found in ICE’s (for the duration of the cycle at least).  The tracer should be consumed during the combustion process so as to avoid tracer build up.  It should be inert with respect to the combustion process while tracking the fuel well.  The dopate should have a high vapour pressure to facilitate low seed concentrations (which will also help the preceding point).  Finally, the tracer must be non-toxic. 3▪10▪1 METAL ATOMS Metal atoms are known to have large absorption cross sections with strong emission spectra, yet their production generally requires high temperatures [261]. Seeding levels may be kept low because of their strong emission, hence metal atoms allow the use of low power lasers. Consequently, saturation of the signal is easily achieved, which results in low signal strengths, despite the positive spectral properties. 3▪10▪2 INORGANIC MOLECULES Many non-organic compounds are known to fluoresce when excited by a wide range of photon energies. Some, most notably water [262] and oxygen, display a weak signal that requires low pressures (almost vacuum) or high temperatures to fluoresce. Oxygen, even at moderate pressures (~ 10bar) is rapidly collisionally quenched [263]. Despite this, the work of Lee and Hanson [264] paved the way for excitation in the deep UV (193nm), which provided meaningful O2 concentration and temperature results ([265, 266] respectively). Nitric oxide (NO) has been used extensively to study mixing, diesel sprays [267] and pollutant formation in flames [268] and combustors [269]. The spectral properties of nitric oxide have been exhaustively 10 The incident radiation should exhibit no appreciable change in properties after passing through the medium/object. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 44 reported in two seminal papers by Bessler et al. [270, 271], and build upon the earlier understanding provided by Di Rosa et al. [272]. With an absorption peak at 225.83nm [272] nitric oxide is easily excited with: the fourth harmonic of an Nd:YAG laser (214.34nm) [273]; a tuneable UV laser [271]; or a pumped dye laser [274, 275]. The collection requirements for the broadband UV emission are not excessively onerous. For accurate results the NO LIF signal must be extensively corrected for: temperature dependencies; differential diffusion; self quenching; the quenching effects of other bath gases; and signal interference from O2 fluorescence [276]. The problems of seeding a fuel with NO can be circumvented by mixing the nitric oxide into the oxidiser stream, potentially allowing the mixture formation to be imaged ‘inversely’ [261], though this technique, to the author’s knowledge, has not been applied quantitatively as yet. Further, substantial problems are also encountered due to the markedly different evaporative and diffusive characteristics of NO when compared to most fuels. This is most easily overcome by causing photodissociation of seeded NO2 [171, 176] or the O2 content of air [277], which provide the precursors to NO. This approach does not, however, allow for substantive 2D mapping and is limited by its short lifetime [261]. All these factors mean that NO LIF is suited to combustion events, rather than the mixing processes involved in engine mixture preparation. Rather than photodissociate NO2, the compound itself may be used directly as a fluorescent tracer. Nitrogen dioxide has many rovibronic excitation levels, and as such has correspondingly wide absorption and emission spectra. Typically, a frequency doubled Nd:YAG (532nm) laser is used for the excitation [273, 278, 279] although the spectra is broadband from ~250-666nm with a peak at ~357nm [280]. The fluorescent emission is seen to fluctuate significantly between 450 and 760nm, albeit with an increase in intensity towards longer wavelengths. Despite the insight into cyclic variations from Zhao et al. [279] on the applicability of NO2 LIF to engine diagnostics, nitrogen dioxide is of more use in examining reaction kinetics; as has been demonstrated by Cattolica et al. [273]. Yet, it must be noted that, in general, NO2 suffers the same problems as NO for tracking mixture formation in ICEs, and is less spectrally favourable for the same applications. Sulphur is exclusively utilised for LIF studies as sulphur dioxide (SO2). SO2 displays a clear rovibronic structure with absorption throughout the UV, which strengthens at shorter wavelengths (Greenbough et al. [281] use a mercury lamp to excite over the range 180-390nm). A clear peak is observed in the absorption spectra at ~200nm, which produces “a broad structureless emission with a maximum at around 360nm” [282], though Greenbough et al. later pin-point the maximum to 374nm [281]. With a bathochromic shift in excitation Strickler & Howell note a marked increase in the phosphorescent to fluorescent emission ratio (×4, in the range 289-313nm) [282]. This observation is further corroborated by Mettee [283] who studied fluorescent lifetime from excitation at several wavelengths (313, 302, 296, 285 & 265nm), and attributed the shift to vibrational relaxation having a more marked effect than quenching, which would in ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 45 turn, explain the reduced fluorescent emission at longer wavelengths. Sulphur dioxide emission is heavily self quenched, the quenching being some twenty times stronger than that for CO2 [281], while it is also noted that oxygen only quenches the singlet emission [281, 282]. Nitrogen is also shown by Rao [284, 285] and Mettee [283] to heavily quench the emission, which correspondingly limits the applicability of SO2 to combustion environments. More pertinently, the general observation that there are significant numbers of quench partners means that emission intensity is severely limited by increases in pressure. Through the use of sulphur doped fuels SO2 LIF is deemed to be of more use in studying exhaust gases [260] or residual gas fraction and its influence on mixture preparation than the mixing process itself. The body of work to support the OH radical as a fluorescent tracer is vast, though its application to the current study is limited. Generation and subsequent seeding of OH into a flow is impractical due to the short lifetime of the OH radical. Photodissociative production of OH from vibrationally hot water has been reported by Pitz et al. [286] to circumvent the seeding problem, though the approach is spectrally limited. The interested reader is referred to [287] for the details of OH LIF (and CH & C2, which are similar in nature). Iodine is highly corrosive and toxic, but has nevertheless been used as a fluorescent medium because of its favourable spectral properties. Hiller [288], and references therein, review extensively the use of iodine to track pressure, temperature, density and velocity. Of particular note in the application of these approaches are Kido et al. [173] who use iodine to study gas entrainment in intermittent low speed jets; Lefebvre et al. [169] whose experimental apparatus provides simultaneous pressure, velocity and temperature fields and Lemoine et al. [170, 289] who use iodine LIF in compressible flows. Absorption is broadband in the green (usually with a 514.5nm argon ion laser excitation), though has a weak transition probability due to its spin forbidden nature. The forbidden transition generates a long emission lifetime (0.3-7µs, [290]) which allows for velocity specification. Excitation in the green provides a highly resolved ro-vibronic structure that results from nuclear-molecular spin interactions and yields some 45000 lines between 500 and 650nm [288]. Below 499nm the incident energy is enough to predissociate the iodine [288]. Molecular nitrogen and oxygen are known to be good quench partners though good fluorescent efficiency offsets this somewhat. This efficiency is due, in part, to the large absorption cross-section of iodine, yet this ‘advantage’ may limit the suitability of the gas since the fluorescent signal is easily saturated. The low diffusivity (0.07cm/s) of gaseous iodine explains its utility for flow tracking, but the difficulties of seeding at a constant rate (with levels typically at ~400ppm [288]) can hinder its experimental applicability [261]. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 46 3▪10▪3 ORGANIC MOLECULES The field of organic molecular photophysics is itself almost boundless. This is reflected in the vast body of work presented by Birks in his two volume Organic Molecular Photophysics [291, 292], and his reliance on the contributions of two dozen contributing authors and innumerable cited works. Polyatomic molecules exhibit wide broadband absorption since the large number of quantum interactions permits many ‘allowed’ states and transitions. The large number of excited states provides a correspondingly wide emission spectra. Matching the tracer to the fuel, primarily in terms of boiling point and mass diffusivity, should ensure (as much as is possible) that the mixing processes are similar in the two substances. 3▪10▪3▪1 AROMATIC COMPOUNDS Aromatic compounds are far less widely used than aliphatic ones (cf. §3▪10▪3▪2). The most widely used aromatic compound is toluene (C6H5CH3). Toluene absorbs broadband UV from ~220-280nm, peaking at 262nm; with emission over the range 265-350nm and a maximum intensity at 284nm [293]. Initial work by Reboux et al. [294] at 248nm excitation found a linear dependence of fluorescent intensity upon incident laser energy (up to 5mJ/mm2). The same study also found 5%v/v toluene in iso-octane to be optically thin, and not self-quenching. The effects of nitrogen, water vapour and CO2 upon fluorescent intensity were found by the same authors to be negligible. Oxygen was hypothesised to be toluene’s main quench partner (which was later studied in detail by Koban et al. [295]) and the direct relationship observed between intensity and oxygen concentration suggested a linear FAR-LIF relationship. However, Reboux et al. noted a temperature dependence of ~20%/1000C which was later used by the same researchers [294] to investigate charge inhomogeneity in SI-ICE’s. The purported results are quoted as being accurate to within 2%, yet the work of Koban et al. [296, 297], in the author’s opinion, casts doubt upon this assertion, in that the toluene signal does not scale linearly with oxygen partial pressure. Koban et al. [295] propose a detailed two step fluorescent model which accurately predicts the behaviour of toluene emission in the presence of oxygen at bath compositions relevant to ICE’s. Indeed, in 2002 Frieden et al. [298] used toluene’s oxygen dependent behaviour to predict oxygen concentrations in a directly injected spark ignited engine. For the current study however, toluene is of limited use since its physical properties lend it to tracking iso-octane (rather than methane), though the work of Fujikawa et al. [250] compares the use of toluene to acetone at 1bar and 200C and notes that at 248nm toluene is 90 times more fluorescent than acetone. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 47 Polycyclic aromatic hydrocarbons (PAHs) may be seeded into a flow or, as is more often the case, be part of the (commercial) fuel11 to be studied. Their absorption in the blue-UV range is complex, though well documented and noted to shift hypsochromically with increased pressure [292], Ch.3. Similarly, the emission, which occurs over a similar wavelength to the absorption (the details of which are heavily dependent upon the exact structure of the compound), is broadband and highly structured [292], Ch2. The large absorption cross section of PAHs and their high quantum yields (Φf = 0.17 – 0.82 [261]) makes them ideal tracers from a spectroscopic point of view ([299], [292] Ch. 2-3). The fluorescent lifetime of the emission may even be used to map the composition of the PAH ensemble, given that each is unique [300] and may be coupled with detailed knowledge of the hypsochromic shifts observed for increased compound size [261]. The large number of PAHs available allows tailoring of tracers for a bespoke fit to complex fuels; though ensuring a matched boiling point does not ensure fuel following behaviour [301, 302]. Further, any PAHs larger than toluene or benzene need to be seeded at above room temperature because of their low vapour pressure [261]. Thijissen et al. [192] have conducted in situ LIF PAH concentration measurements in an industrial scale fuel rich natural gas flame. Excitation using an argon ion laser at 488nm established a correlation between the LIF signal intensity and the PAH concentration. It was found that LIF emission is strongest from high molecular weight PAH’s (e.g. coronene), and that their fluorescence emission spectra are similar to those from flames. The difficulty of seeding PAHs directly and matching their mass diffusion properties to gaseous fuels has not been overcome, while the health concerns of these compounds also often precludes their use. 3▪10▪3▪2 ALIPHATIC COMPOUNDS Aliphatic compounds are easily excited in the UV though tend to photo-dissociate. Longer unsaturated or conjugated systems are generally unstable and are prone to polymerization [261]. The need for an easily accessible chromophore leads to structures like ketones (R2CO), aldehydes (R-CHO) or amines (R3N)12, while conjugation is also know to produce marked hypsochromic shifts in the fluorescence spectra [261]. Ketones have been studied extensively for their spectral and photochemical properties. Their pressure and temperature dependencies are relatively easily modelled ([303, 304] and [305] respectively), and the fluorescent lifetime of ketones is well established [306] for a range of temperatures and pressures [307]. Knowledge of the role of oxygen in ketonic systems is also well established [308]. The combined effect of this extended understanding has allowed several authors to present detailed ketone photophysics [303, 309]. The range of accessible vapour pressures allows ketones to track gas phase fuels well (e.g. the use of acetone to track natural gas: [310, 311]); as well as liquid phase fuels where 3-pentenone (or a mixture 11 Gasoline contains high concentrations of single ring aromatics (toluene and xylene) while diesel has plenty of two-ring compounds (naphthalene and its derivatives) 12 Where R is a saturated hydrocarbon ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 48 of 3-pentanone and 3-hexanone [312]) has been used widely to track gasoline [307, 313, 314], or more commonly iso-octane. Biacetyl (CH3(CO)2CH3) is widely used in fluorescent and phosphorescent studies since its boiling point of 880C matches well that of liquid fuels and in some cases can be deemed suitable for gaseous flows and it exhibits a high phosphorescent yield (~15% [261]) in oxygen-free flows. However, biacetyl has a low vapour pressure, which makes it difficult to seed. Formaldehyde (CH2O) is difficult to dope since it polymerizes easily [261], though its temperature dependent fluorescent behaviour also allows the identification of temperature variations in the immediate pre-combustion field [190, 315-317]. Formaldehyde LIF is often used simultaneously with CH LIF for flame front imaging [318, 319]. Further, coupling of the CH2O and OH LIF signals has been shown by Paul et al. [237] to correlate well with the local heat release rate. The applications of CH2O LIF are widespread and accurate [190, 200, 243, 316, 317, 320, 321], but are tied to the combustion process as the method of CH2O production, and as such leave CH2O of no use in mixture formation diagnostics. The next homologous molecule to formaldehyde is acetaldehyde (CH3CHO), which has been successfully used as a tracer by Arnold et al. [322]; however, acetone is commonly used in preference since it is less harmful. Aldehydes exhibit small quantum yields compared to aromatic compounds in the absence of oxygen, yet are comparable in the presence of air. Hexafluoracetone is suggested as an alternative to acetone since its quantum yield is an order-of-magnitude greater [323], though there are problems with its toxicity. Hansen and Lee [324] have investigated the radiative and nonradiative lifetimes of linear aldehydes. Radiative emissions share much the same lifetime as large aldehydes and follow well the Stickler-Berg equation, expect for acetlylealdehyde which has a much longer radiative lifetime than expected. CH LIF provides similar information and is applied in much the same was as CH2O LIF. It is particularly useful in methane and natural gas combustion studies. It is, however, applicable across a wide range of hydrocarbon fuels due to its prevalence in hydrocarbon combustion pathways. Yet, without a flame the practicalities of generating and seeding CH prohibit its use in mixture studies when easier and more effective tracers are available. Carbon dioxide LIF can be spectrally resolved in the 215-255nm range using an Nd:YAG pumped dye laser and has been successfully used by Lee et al. [325] to study CH4/O2/Ar and CH4/air flat-flames at fuel/air ratios from 0.8-1.9 in the pressure range 5-40bar. Emission is broadband in the range 200-450nm and has a weak structure. Further, the signal was found to scale linearly with laser fluence and pressure (within the range stated). However, in general, the signal from CO2 is weak and strongly temperature dependent, precluding its use as a direct measure of concentration [261]. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 49 Amines, as is common to most aliphatic compounds, fluoresce under UV excitation. They are however, effectively quenched by oxygen and often deemed prohibitively toxic for widespread use. Amines used in their own right include ethylamine [326] and N,N-dimethylaniline (DMA) [327], though they are more commonly used in exciplex systems particularly as aniline groups (i.e. NH2 on a benzene ring) [261]. 3▪11 ACETONE AS A FLUORESCENT MEDIUM Acetone (also called propanone, dimethyl ketone, 2-propanone, propan-2-one and β-ketopropane) is an aliphatic ketone with formula (CH3)2CO, cf. Figure 3-3, and molecular weight 58.08. Its high vapour pressure (24 kPa at 200C) facilitates high seeding densities, while its vapour pressure behaviour fits the Antonie equation well [196]. The photochemical properties of acetone have been studied extensively, starting with the work of Heicklen [328, 329] and Groh [330]. Typical of ketones, acetone exhibits strong absorption from 225-320nm, with a peak at 277nm [331], though the absorption spectrum is ostensibly flat between 270-280nm [196]. The emission spectrum of acetone is broadband blue between 350-550nm with peaks at 445 and 480nm [196]; the spectra is, however, affected markedly by vibrational relaxation prior to emission (cf. §3.11.4). Acetone has flammability limits which are easily accommodated and it is not carcinogenic or excessively toxic, though prolonged exposure is advised against. Figure 3-3 Molecular Acetone, (CH3)2CO Upon excitation, various process are competing to deactivate acetone, cf. Figure 3-4. From the first excited singlet state, S1, there may be: fluorescence, the spontaneous emission of radiation resulting from a transition which maintains spin multiplicity, S0←S1(n,π*); intersystem crossing, the conversion from one state to another of different multiplicity T1(n,π*)←S1(n,π*) [309]; internal conversion, the non- radiative conversion from one state to another of equal spin, S0←S1(n,π*); or photodissociation of the acetone to several possible photo-products. Figure 3-4 Deactivation Pathways for the First Excited Singlet, S1, of Acetone, adapted from [332] O C H3C CH3 ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 50 Acetone’s status as the de facto tracer for methane comes from its similarity to methane in the most salient of physical properties, cf. Table 3-2. Methane Acetone Molecular Weight (g/mol) 16.043 58.080 Density @ 250C (g/cm3) -- 0.786 Boiling Pt. (0C) -161.48 56.1 Lower Heating Value (MJ/kg) 50 28.6 Heat of Vapourisation @ 250C (kJ/mol) -- 30.99 Heat of Combustion (MJ/mol) 0.8908 1.8207 Max. Burning Velocity @ 250C with Φ Specified (cms-1) 44.8 @ 1.08 44.4 @ 0.93 Flash point (0C) -188 -18 Autoignition Temp. in Air (0C) 537-632 465-727 Flammability Limits in 1 bar Air (% vol) 5.0-15 2.6-13 Gas Phase Viscosity at 1000C (µPa s) 13.4 9.5 Gas-Phase Diffusion Coefficient in 1 bar air @ 1000C (cm2s-1) 0.344 0.166 Gas-Phase Diffusion Coefficient in 8 bar air @ 1300C (cm2s-1) 0.0493 0.0239 Table 3-2 Physical & Thermodynamic Properties of Methane and Acetone Gas, excerpts from [259, 261] and references therein. 3▪11▪1 EXCITATION SOURCE CHOICE The broad absorption spectrum of acetone facilitates the use of many excitation sources. It is important to note that as the absorption cross-section increases for shorter wavelengths, the fluorescent efficiency decreases slightly due to higher dissociation (cf. §3▪11▪3) The ‘figure of merit’ (sic, [196]), Eσ, in the final column of Table 3-3 is the product of pulse energy and absorption cross-section at the laser wavelength, and provides a metric against which to measure the efficiency of each excitation source. With the exception of the difficult to use flashlamp pumped rhodamine 590 dye laser the krypton fluorine (KrF) laser shows the best applicability, and as such is used in the current work. Laser λ (nm) Energy/pulse (mJ) Eσ (x10-20 Jcm2) XeCl excimer 308 300 0.48 KrF excimer 248 300 0.7 Quadrupled Nd:YAG 266 120 0.52 Flashlamp Pumped Rhodamine 590 280 400 1.9 Raman Shifted ArF Excimer 254 2 0.006 284 1 0.0045 Table 3-3 Acetone Excitation Sources, after [196] ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 51 3▪11▪2 ABSORPTION Upon excitation the singlet ground state of acetone is excited to the first electronic state of similar spin multiplicity [333], which is 30435 cm-1 above the ground state [334] (although Thurber et al. [305] use 30440cm-1 in their modelling work). The chromophoric C=O bond undergoes a transition to elevate an electron from its non-bonding, n, oxygen orbital to that of an anti-bonding π* orbital, leaving the electron unpaired and the oxygen with a partially filled p-type orbital (cf. Figure 3-6). The n→π* transition is orbital-forbidden though spin-allowed [307], and is thus weak. In its excited state the weakened C=O bond leads to carboxyl out-of-plane wagging and stretch [333, 335], while also causing molecular lengthening and pyramindization at the carbonyl carbon atom through increased C-C torsion [309, 333, 335]. The low energy of all these behaviours means that their frequencies are much lower than that of the molecular vibration, obscuring any rotational structure in the absorption band for acetone [336, 337], cf. Figure 3-5. A hypsochromic shift, of about 2nm/100K, in the absorption spectra for 3-pentanone (which, as (C2H5)2CO is a corollary of acetone) is noted by Grossman et al. [307]. These findings were later extended and confirmed to apply to acetone by Thurber et al. [305, 306] and Ossler & Aldén [306], the former also noting that the absorption cross-section increase significantly with temperature. The increased absorption cross-section roughly compensates for the loss in cross-section caused by the hypsochromic shift for excitation wavelengths shorter than the peak at 277nm (at ambient temperatures). Figure 3-5 Absorption Spectrum of Acetone at Room Temperature and 1 atm, reproduced from [196] 3▪11▪3 PHOTODISSOCIATION OF MOLECULAR ACETONE The cleaving of the α-CC bond occurs since the bond overlaps the vacant non-bonding oxygen orbital in the excited π* state (Figure 3-6) [338]. The latest ab initio studies by Liu et al. [339] suggest that the dissociation barrier for the lower energy methyl acetyl split, Fig.3-6a, lies at 556 kJ/mol above the ground sate. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 52 Figure 3-6 Acetone Dissociation through α-Cleavage, adapted from [338] This cleavage reaction allows for multiple products, though the methyl acetyl split is more thermodynamically preferable [338], and explains the predominance of Eq.3.1 in the photodissociation of acetone [340, 341]. Eq. 3.1 For excitation at less than 312nm [196], and particularly under 300nm [342], the alternative Eq. 3.2 is more likely. Eq. 3.2 It is noted by Liu et al. [339] that the energies required to form the products of Eq.3.2 are unattainable by direct 248nm excitation, and a step wise decomposition of the (CH3)2CO molecule is more likely [343]. Consequently, at wavelengths below 250nm the reaction outlined by Calvert & Pitts [344] and generalised by Gilbert [338] may also occur (Figure 3-6(b)), viz.: Eq. 3.3 Haas [309] proposes a detailed 11 step mechanism for the photodissociation of acetone, which outlines exact product formations, including that of biacetyl (cf. §3▪11▪3▪1). Observed reductions in phosphorescent intensity with decreased wavelength indicate that photodissociation increases with excitation energy [345]. Excitation of acetone at 248nm through the global step of Eq. 3.1 leaves many (~30%) [343] of the photofragments with enough excess energy that the acetyl radical may spontaneously decompose to form, with the original methyl radical, the products of Eq.3.2. The linear Stern-Volmer plot obtained from the data of Blitz et al. [342] for λ<300nm suggests deactivation from only one excited state- the singlet state S1- according to the scheme (that may include any third body, M): Eq. 3.4 CH3 H3C C O • • CH3 H3C C O • • H3C C O • • + CH3 hv (n,π*) CH2 H3C C O • + • H CH3 H3C C O • • CH3 H3C C O • • hv (n,π*) (a) (b) ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 53 3▪11▪3▪1 BIACETYL POISONING The main products of acetone photochemical pyrolysis are: ethane, methane and carbon monoxide [309]; however, the acetyl radical resulting from Eq. 3.1 is important since it may recombine to form biacetyl, (CH3CO)2, which produces a strong phosphorescent signal [346]. This competes with the fluorescence of the acetone S1 state, and may complicate the collection of a high fidelity fluorescence signal. Biacetyl formation from triplet acetone is only of concern for excitation above 300nm [342] or at temperatures below 1000C [347]. However, when present biacetyl effectively quenches the T1 state of acetone, reducing the tendency for acetone to phosphoresce, but in the process exciting itself to phosphoresce (at wavelengths similar to acetone) [328]. Although biacetyl triplets are known to phosphoresce with high efficiency (~15%) they are also effectively quenched by oxygen; while biacetyl fluorescence has a very low efficiency below 320nm [196]. For excitation at 248nm it is observed that there is a move away from Eq.3.1 towards that of Eqs. 3.2 & 3.3 reducing the biacetyl concentration to a level which is not of concern [309]. While, under thermal conditions the low concentration of acetyl radicals negates the need to worry about biacetyl formation [309]. All of these features mean that for the oxygenated environment of the RCM, biacetyl poisoning may be disregarded. 3▪11▪3▪2 THERMAL DISSOCIATION Thermal pyrolysis of acetone, through the Rice-Herzfield mechanism [348], primarily yields carbon monoxide, methane and ketene ([349] in [309]), with small amounts of butanone, ethane and 2,5- hexanedione [309]. However, the process is not significant for temperatures below ~800K, which the compression stroke of the RCM fails to generate. 3▪11▪4 NON-RADIATIVE DEACTIVATION Throughout the following discussions reference to Figure 3-7 may be of help to the reader. Hansen and Lee [332] assert that “…the non-radiative processes predominate over the radiative processes at least by two orders-of-magnitude in simple ketones.”, the low zero-pressure fluorescence quantum yield of ~0.002 reported by Heicklen [328] and Shortridge et al. [350] supporting such a view. Inter-system crossing from the singlet state is known to peak at intermediate energies [345], which suggests competing pathways for the deactivation of the S1 state based upon its vibrational state [303]. Rapid inter-system crossing (ISC) (kISC between 4 x 108 s-1 [350] and 3 x 108 s-1 [303], and [345]), depopulates the first excited singlet state to that of the first triplet manifold at 28000cm-1 above the ground state [334]. Although the process is spin-forbidden the first singlet and triplet states of acetone are closely coupled such that ISC efficiencies approach 100% [351-353]. The small energy difference between S1 and T1 means that collisions also effectively populate T1 from S1 [354]. The resultant mix of excited acetone in its singlet and triplet states has a predominant triplet character because of the efficient ISC [354]. The triplet ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 54 component may come from increased excess energy above the S1 state, which increases molecular vibration and rotation to promote ISC to the T1 state, or by direct optical preparation [354]. The n→π* transition is known to produce a movement of electron density away from the oxygen molecule, with the methyl moieties able to inductively donate electron desnity, stabilising the excited state [338]. This allows ISC to take place (potentially following vibrational relaxation, VR, in the excited singlet state) before rapid, fluorescent deactivation that is independent of the bath phase [303]. The vibrationally hot triplet state, T1**(which predominates in the high pressure and liquid phases of acetone [354]) may phosphoresce (τT1** ~ 30µs [345]), dissociate or relax to a thermally equilibrated state, T1 [355], where it cannot be fully quenched due to its high reactivity [309], though may still dissociate through the Rice-Hertzfeld mechanism [348] (cf. §3▪11▪3▪2), or phosphoresce. Bitto [356] asserts that it is the vibrational coupling of the torsional modes in the symmetrically favourable triplet manifold which increases the lifetime of the triplet state. This thermal triplet, T1, does not however correlate directly with the triplet ground state [309] and so no fluorescence is observed from a possible T0←T1 transition. Figure 3-7 Jablonski Diagram for a Electronically Excited Organic Molecule, reproduced from [261]. 3▪11▪5 RADIATIVE DECAY The mixed singlet-triplet state leads to a quasi-biexponential decay of the excited state [337] whereby the nature of the emission is governed by the rovibronic interactions of the molecule [354]. When excited to near the S1 origin, decay to the S0 ground state is most likely through fluorescent emission [354]. Yet for stronger stimulation the small number of molecules left in the S1 excited state after ISC may relax to a vibrationally excited ground state, S0*, through internal conversion (IC). Those molecules left in the metastable T1 state (populated directly from ISC or thermally relaxed T1** molecules) have a lifetime which is purported by Bitto [356] to be extended due to symmetrically favoured vibrational coupling. This accounts for the long lifetime observed for the phosphorescent relaxation to the ground state S0 (τT1 ~ ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 55 200µs [345] & 196). The emissions spectra of acetone are shown in Figure 3-8 where the lower energy of the triplet emission shows a clear hypsochromic shift. Figure 3-8 Fluorescent (solid line) and Phosphorescent (broken line) Emission Spectra for Pure Acetone, reproduced from [331] 3▪11▪6 FLUORESCENT SIGNAL LINEARITY, QUENCHING & THERMODYNAMIC STATE EFFECTS For weak excitation of acetone, the fluorescent intensity is known to be proportional to incident laser intensity (up to ~1 J/cm2) [196, 357]. To remain linear however Lozano et al. [196] assert that the inequality Eq.3.5 must hold true. Eq. 3.5 For 248nm excitation the absorption cross-section and lifetime yield Ni to be ~ 4x1028 photons/cm2s. For a 20ns pulse at this wavelength, incident laser intensities must reach ~630 J/cm2 to saturate the signal, yet the KrF excimer used in the current work can only supply a maximum of 166 J/cm2, implying that all fluorescent intensity values may be deemed proportional to laser beam energy. Copeland and Crosley [336] assert that the lowest photodissociation barrier for acetone is at 360kJ/mol above the ground state. For a constant 1mm thick laser sheet in the RCM cylinder, with an acetone concentration of 1%, the maximum laser power (of 400mJ) would excite the acetone to ~42kJ/mol above its ground state. This suggests that 248nm excitation avoids the need to consider dissociative effects- an assertion supported by the work of Liu et al. [339]. Finally, the fluorescent intensity observed is also known to be proportional to the partial pressure of acetone vapour [196, 357]. The failure to generate a Boltzman population of the probe volume in the S1 state upon excitation (because of the fast ISC from the singlet to triplet states) yields the prompt fluorescent emission observed, which in turn limits the susceptibility of singlet acetone to quenching [258, 358]. Oxygen, which has a triplet ground state, is the notable exception to this. The small but distinct quenching effects of oxygen upon acetone fluorescence are reported by several authors [304-307], and are well understood for excited molecules in general [330, 359, 360]. The triplet state of oxygen is a more effective quencher than its singlet equivalent [308], which may result from its energetic similarities to the 1(n,π*) state of acetone. ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 56 The consequence is that oxygen quench rates are close to, or exceed, the gas kinetic rate constant for acetone [354]. For ketone oxygen interactions Grossman et al. suggest the mechanism of oxygen quenching to be based upon “…the formation of a charge transfer complex with the ketone, involving the electronegative oxygen atom of the excited carbonyl bond and the electrons of the [oxygen] double bond.” [307]. The work of Nau et al. in the liquid phase suggests a similar mechanism to that of Grossmann et al. with the formation, upon ketone-oxygen collision, of an encounter complex which if stable will give an overall quench rate proportional to collision frequency ([361] in [357]). Breuer et al. [303] suggest the findings of Nau et al. to be due to oxygen enhanced ISC that serves to depopulate the S1 state. They correspondingly adjust the work of Thurber et al. [305] to correct for fluorescence yield at shorter (248nm) wavelengths, though no significant new trends were observed. Excitation of acetone at shorter wavelengths is noted to increase the likelihood of quenching since further excitation above the S1 origin allows for more vibrational relaxation before ISC or radiative deactivation [307]. The vibrational level of the excited state is also known to have a small effect on the encounter probability of oxygen assisted ISC [357]. In the gas or vapour phase the effects of quenching manifest themselves through differences in fluorescent intensity at different temperatures and pressures. The work of Thurber & Hanson [357] indicates that in a nitrogen bath gas fluorescent intensity increases asymptotically towards a high pressure limit (at ~16.5atm for 248nm excitation)- attributable to full vibrational relaxation of the S1 state prior to ISC. This limit will be reached sooner at longer excitation wavelengths since there is less vibrational relaxation to take place before radiative decay. Similar trends are observed in a pure oxygen bath gas, with the high pressure limit being reached sooner (at ~4atm) because of more effective oxygen quenching (though the fluorescent intensity is also observed to decrease for increases in pressure past the upper limit). The data of Grossmann et al. [307] which pertains to a synthetic air bath gas, unsurprisingly, shows a tendency to reach a high pressure limit (at ~8-10 bar) which sits between the two cases of Thurber & Hanson. The behaviour is not paralleled exactly however. For 248nm excitation an increase in fluorescent intensity is observed up to eight bar, where the high pressure limit is apparently reached, yet past ~25 bar a steady drop of ~1% per bar is then observed without explanation (up to an experimental maximum of 50 bar). These findings are at odds with those of Yuen et al. [304] who, for their air bath studies, report the drop in fluorescence to start at ~5 bar and continue until the experimental maximum of 8 bar. Temperature affects emission by varying the product of absorption cross-section and fluorescent yield for a given molecule. Koch and Hanson report no significant change in intensity for 3-pentanone fluorescence in air over a range of bath temperatures [358]. This is consistent with the observation, in another study by Koch & Hanson [362], that acetone has an almost constant absorption cross-section at ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 57 248nm excitation (the absorption cross section was observed to rise by only ~5% over a 600K temperature increase). However, other significant works [305, 307, 363, 364] show small though clear dependencies of acetone and 3-pentanone emission on temperature. Grossman et al. [307] report (for 248nm excitation in synthetic air) a steady drop in fluorescent intensity of ~35% per 100K while Ghandi & Felton [363] report similar, though slightly less dramatic, findings for acetone in a heated jet and a motored engine (at 266nm). All the studies are able to discount thermal dissociation for the decrease. The findings of Thurber et al. [305] are in agreement with those of Tait & Greenhalgh [364] (who use 308nm excitation of acetyhldehyde) and Grossman et al. [307], and extend the results from a 600K experimental maximum to 975K. Beyond ~800K the drop in fluorescence is seen to reduce by ~8% per 100K, with a smooth transition between the two regimes. Although there is a significant increase in absorption cross-section with temperature it does not correct for the decrease in fluorescence and so a marked drop in fluorescent yield is observed for increased temperatures [305], and as such is a useful temperature diagnostic. According to the model of Thurber & Hanson [357], when the above tendencies for pressure and temperature are combined for acetone, at 248nm excitation, a rise in fluorescent yield is still observed. Indeed, it is noted that pressure effects are stronger for the shorter wavelengths and the reduction in fluorescent yield due to temperature rise mitigate only slightly the rise due to pressure. For increased pressure and temperature Ossler & Aldén [306] report similar effects on fluorescence decay time to those of fluorescence intensity. From a ‘zero-pressure’ minimum up to ~10bar the fluorescent lifetime increases linearly from ~0.9ns (with a slope of ~0.15ns/bar at 323K decreasing to ~0.02ns/bar at 723K) as in Figure 3-9. Figure 3-9 Fluorescent Lifetime Variation of Acetone with Pressure, reporoduced from [306] 3▪12 CONCLUSIONS The most salient details of the main techniques that could be used to study species concentration have been outlined. The suitability of each to combustion diagnostics has been discussed, and from this the utility of laser induced fluorescence made patent. The versatility and diversity of LIF applications has ▪ Chapter 3 ▪ Laser Imaging Techniques & PLIF Fundamentals ▪ 58 been outlined and a comprehensive discussion of possible gaseous tracer compounds made. A strong case for the use of acetone as a proxy for methane/natural gas has been made, primarily on the basis of their well matched physical properties. Further discussion has been offered as to the utility of acetone, its spectral properties, and how these properties vary with engine relevant conditions. Acetone LIF has been studied extensively and its behaviour is well understood, so allowing the contemporary experimentalist to draw meaningful and accurate inferences from its use. 59 Chapter 4 EXPERIMENTAL SETUP & PROCEDURES 4▪1 INTRODUCTION Chapter 4 offers the most important details of the rapid compression machine used in the current study. Important changes are highlighted, primarily with respect to the improvements that allow combustion studies to be supported and the control systems required to operate the fuelling and laser diagnostics systems. The experimental test matrix is outlined, with all pertinent parameters and operational details provided to the reader, along with the rational behind the choices made. ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 60 4▪2 THE RAPID COMPRESSION MACHINE The rapid compression machine, RCM, was designed and built by Döhring [365] and can simulate any two consecutive strokes of an internal combustion engine. The RCM is shown schematically in Figure 4-1. The most important features are considered in detail below. Figure 4-1 Schematic of Rapid Compression Machine (RCM) The principle of operation is as follows. The RCM piston and con-rod are driven by a vertical crank shaft, which is itself driven by a rack, where the rack is actuated by a pneumatic cylinder. The braking force is provided by a hydraulic piston on the opposite side of the rack. The pneumatic cylinder is supplied by an 80gal accumulator, which is charged from the regulated laboratory compressed air supply. The braking side is a closed loop system filled with 20W30 oil. When air pressure is supplied to the driver cylinder, the rack is prevented from moving by the solenoid valve on the braking side. This valve prevents flow in the oil loop. To actuate the RCM the solenoid valve is triggered so that the oil in the braking cylinder is allowed to flow up into the reservoir. When the rack is returned to the start position, a small secondary loop is opened on the braking side such that the braking piston is allowed to fill from the reservoir. The details of RCM operation have been covered previously [365, 366], and only those operational changes pertinent to this study shall be reported herein. The single shot nature of the RCM, its lack of intake and exhaust manifolds, and the omission of the valve train mean that cyclic variability is almost eradicated, whilst also ensuring strict control of the in-cylinder Braking Section Cylinder Driver Section ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 61 charge motion. Any motion may be fully eliminated to give a quiescent starting state from which all subsequent fluid movement is imparted by the piston. 4▪3 NEW RCM CYLINDER As part of the current work the rapid compression machine was altered significantly from its previous incarnations. Prior studies using the RCM [365, 366] have been based on cold flow. The PMMA cylinder arrangement of the RCM was redesigned and retrofitted with a UV grade HPFS (high purity fused silica) cylinder and fire deck, and a new 316 stainless steel cylinder and cylinder head installed to allow combustion (cf. Figure 4-2). Figure 4-2 Exploded View of New RCM Cylinder 4▪4 RCM FUELLING SYSTEM The RCM is set up to have its mixture prepared from three different sources. A premixer is used to prepare homogenous NG/air mixtures based upon partial pressures. The premixer is instrumented for NG line pressure, total pressure and mixture temperature. This premixed charge can be supplied directly to the RCM cylinder for homogenous fuelling, or may also be supplied to the PSC capillary for premixed PSC ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 62 studies. The PSC capillary tube, which has its injection controlled by an Omega SV-121 high-pressure solenoid valve, can also inject pure methane. The final source of fuel for the RCM cylinder is the J43M direct injector, which can only inject methane into the RCM cylinder. The laser induced fluorescence technique used in this study requires that acetone be mixed with NG fuel. The high-pressure seeder used to achieve this is covered in §4▪5 The seeded NG may be sent to the PSC the direct injector or to the premixer. This allows LIF studies to be carried out on any combination, of the fuel sources. The piping diagram to achieve the abilities outlined above is shown in Figure 4-3, while its details are presented in Appendix A. Figure 4-3 Schematic of RCM Fuelling System The mixture within the RCM cylinder can be ignited with a spark between tungsten electrodes. The electrodes have a 40 angle w.r.t. the fire deck to clear the PSC capillary tube. The electrode position also has a small amount of adjustability so that the location of the spark within the PSC plume or homogenous charge can be adjusted. N 2 C H 4 A IR VENT T T S E E D E R P R E M IX E R T Thermocouple Pressure Gauge Check Valve Valve Regulator Relief Valve 2W- 2 Way 3W- 3 Way 5W- 5 Way B- Ball N- Needle 3W-B N N N VENT SAMPLE J43 VENT VENT SAMPLE 3W-B 3W-B 5W-B 5W-B 5W-B 3W-B 3W-B PSC 2W-B 2W-B 2W-B RCM 2W-B N N N 2W-B 2W-B VENT ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 63 4▪4▪1 THE WESTPORT® J43M INJECTOR Much of the information pertaining to the J43M natural gas injector is proprietary and confidential. The injector was developed by Westport Innovations Inc. as part of its own R&D program and was not a commercial product. The actuation of the injector needle is achieved through the energizing of a magnetorestricitive material. Upon imposition of a magnetic field to the Terfenol-D™, the magnetic regions within this ferrous material realign which causes the volumetric expansion which actuates the needle. The movement of the needle allows the gas supply (at up to 3600psi) to flow through the injector. The injector is PWM controlled, however the injector requires a 5V logic signal for the entire injection duration. Further, rate shaping is possible by varying the logic level of the needle lift signal. In the current work no rate shaping was required and maximum needle lift was always specified. Two J43M injectors were originally specified for the current study: one to supply the direct injection fuel plume and one the partially-stratified-charge injection. The characterisation information for both injectors is listed in Chapter 5. However, persistent technical problems with the PSC injector necessitated its replacement with a high-pressure (1000psi) solenoid valve (Omega SV-121). The solenoid was connected directly to the capillary tube supplying the RCM cylinder. The implications of this change for the current study are minimal in that it is only the initial stages of the injection that are relevant to PSC DI interaction. However, the overall reliability and repeatability of the response of this model of solenoid have posed significant problems to other researchers [7, 367]. Provision of the PSC charge can also be made through the use of an ‘insert’ that replaces the PSC capillary tube. The RCM cylinder quartz window in the cylinder head may be replaced with an aluminium equivalent which is threaded with a ¾” parallel thread. This thread accepts the insert, which hold a 14mm sparkplug. The insert, pictured in Figure 4-4, has natural gas supplied by the same solenoid as for the capillary tube; however, the insert now provides a flow path to direct the fuel in front of the plug electrodes. ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 64 Figure 4-4 PSC Insert 4▪5 FLOW SEEDING As was alluded to previously, the LIF studies of the current work require the fuel to be doped since natural gas does not naturally fluoresce. This is achieved with the high-pressure bubbler shown in Figure 4-5, which is based on the design presented by Neij [259], and adapted for higher pressures. The detailed design information for the seeder is presented in Appendix A, though the basic principles of operation are outlined here. The vessel is a thick-walled 316 stainless steel chamber with flanged ends. There are a number of tapings to the vessel: two gas inlets, one gas outlet, a refill inlet, a pressure relief valve, pressure gauge and a thermocouple tapping (shielded type-K). The last two provide state information for the seeder, while the former allows methane to be fed into the seeder in batches (based on supply pressure) or continuously. The two inlets vary the flow path. One inlet bubbles gas through the liquid acetone, while the second inlet (not shown in Figure 4-5) bypasses the liquid acetone. The two flow paths then mix as they pass through a bed of glass beads before effluxing. The flowpath of the feed gas is shown in blue in Figure 4-5. This arrangement allows the acetone concentration in the effluent to be varied according to the fraction of the flow which was bubbled through the acetone. ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 65 Figure 4-5 Section View of RCM Seeder 4▪6 LASER & OPTICAL SYSTEM A UV laser system is used to excite the acetone doped gas. 248nm UV light, with a spectral width of 3nm at full-width half maximum is supplied by a CompexPro 102 krypton-fluorine (KrF) excimer laser manufactured by Lambda Physik (now Coherent Inc.). The beam dimensions are 12mm x 24mm at the exit aperture, with a divergence half-angel of 3mrad. The beam path to the RCM and its formation into a light sheet are shown in Figure 4-6. The beam is immediately sampled with a Melles-Griot 248nm beam sampler. About 16% of the beam energy is directed to the power meter (cf. §4▪7▪1, below), the remainder going to a broadband metallic mirror in a kinematic gimbal that turns the beam though 900 horizontally to a beam steerer. The beam steerer steps the beam up by approximately 300mm, with the beam exiting in line with the RCM cylinder axis. Two 50.8mm x 50.8mm UV grade cylindrical lenses (fl1=200mm & fl2=300mm) are then used to form a Galilean telescope. The light from the telescope is directed onto a high-power dielectric mirror (that is both wavelength and direction specific in its reflectivity) mounted on a kinematic gimbal. This mirror turns the light through ~98.50 so that the beam enters the cylinder at an angel of ~8.50. This angle allows the majority of the optical section of the cylinder to be illuminated, while also minimizing the area obscured by the beam impinging upon the PSC capillary. Adjustment of the final cylindrical lens and the high-power mirror allow the beam to set-up so that it is on the cylinder axis and vertical. By changing the length of the telescope it is possible to have the beam waist (the optically invariant minimum thickness) of ~0.8mm on the central axis of the cylinder. However, a lack of Pressure Relief Thermocouple Glass Beads Acetone ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 66 collimation after the 300mm lens means the beam is ~1.45mm thick at the cylinder bore. The beam is finally collected, to prevent the scattering of potentially dangerous UV light, by a beam dump under the RCM cylinder (not shown). LIF luminosity is collected with a Cooke Corporation DiCam Pro intensified CCD camera, fitted with a Nikkor AF Micro105mm, 1:2.8D, lens and B+W F-Pro UV filter. Figure 4-6 Laser Sheet Formation 4▪7 EVENT & TIMING CONTROL The details of the RCM event and timing control system are offered in Appendix B. Here a brief overview is presented to provide some context for subsequent discussions. Figure 4-8 shows the timing diagram for the pertinent RCM events. Those events are: Event Pulse or Step PSC injection Step Direct injection Step Camera trigger Pulse Laser trigger Pulse Table 4-1 RCM Timed Events All the events listed in Table 4-1 are timed from the crank angel encoder, which gives a single pulse every 0.036CAD. The RCM is triggered by actuating a solenoid valve on the braking side that allows the rack to move and which causes the crank angle encoder to rotate. The 5V TTL pulse train from the encoder is gated so that no pulses pass the gate until an index pulse is reached. For rotation after this index, pulses are allowed to propagate. This system is used to circumvent the variability in the rack start position. The pulse train is supplied to the RCM control box which counts the number of pulses in the train and compares it against the set start value for each event. When the value is reached, a positive TTL trigger pulse is ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 67 released by the control box for that event. This signal is a pulse, the duration of which is set internally by an RC constant (at ~10ns), or a top-hat with a set duration (in encoder counts). Figure 4-7 RCM Event Timing Diagram 4▪7▪1 INSTRUMENTATION Further to the control system, the RCM is instrumented with a high dynamic response piezoelectric pressure gauge (PCB Electronics 0112A), which is connected to a charge amplifier (Kistler 5010) that has its output voltage displayed on an oscilloscope (Tektronix 2014B). The oscilloscope is connected to a PC with OpenChoice Desktop installed that allows the trace to be electronically recorded. The PC is also equipped with the camera control software (CamWare and CineControl). In the case of the LIF studies an iCCD (Cooke Corporation DiCam Pro) was used, while for the flame propagation investigation a high frame-rate CCD was used (Phantom). The final use for the PC in the RCM control system is to record and display the reading from the laser power meter (Coherent FieldMax Pro II) using proprietary FieldMax II PC software. ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 68 Figure 4-8 RCM Instrumentation Schematic 4▪8 TEST MATRIX 4▪8▪1 FUELLING RATIONAL The work of Gorby [7] on the use of PSC with late DI was marked by difficulty in getting the PSC plume to ignite reliably. The problem of poor inflammation was resolved if a weak background mixture was induced through a traditional port-fuelled approach. The minimum RAFR found to produce reliable PSC ignition was 1.79, thus the value forms the basis for the current study. Centering the central-composite designed text matrix (cf. §4▪8▪3) on this value, or close to it, allowed the investigation of the effect of the bulk charge fuel concentration. The current work used bulk RAFR’s close to the threshold value reported in [7], and to the rich and lean sides of it. The fuelling levels for each of the three fuel sources in the RCM cylinder (i.e. the background bulk charge, the PSC injection and the direct injection) were chosen to provide an overall RAFR of one. The PSC injection was held constant at 3%w/w of the total charge. The DI injection duration was altered to maintain the λ=1 condition after accounting for the fuel in bulk charge. 4▪8▪2 TIMING RATIONAL The test matrix was designed to meet the objectives listed Chapter 1. The overarching aim of the study was to understand the interaction of the PSC fuel plume with that of DI fuel jet. As discussed in Chapter 2, the structure of gaseous jets is well-established and Turner’s vortex ball model predicts two distinct regions to any given starting jet. The head of the jet is comprised of a ball, or vortex, which grows in size through leading-edge entrainment and mass supply from the trailing steady-state jet. It is thus apparent Injector Driver Timing Control Trigger Gate PSC DI CCD CA Encoder Laser Pressure Trace Ignition Circuit ▪ Chapter 4 ▪ Experimental Setup & Procedures ▪ 69 that the PSC fuel jet (which itself must have the same structure, although on a smaller scale) can interact with the DI jet in three distinctly different regions. The PSC jet may intersect the DI fuel jet: i) along its leading edge, ii) at the centre of the vortex-ball iii) following the vortex-ball in the steady-state jet. Case ii is shown schematically in Figure 4-9 with the expected rich regions shown in red, the stoichiometric ocntour in yellow and the lean areas in blue. Figure 4-9 Partially Stratified Charge Jet and Direct Injection Jet Interaction To achieve this the PSC injection timing was held constant at 120CAD BTDC (duration 6CAD) so that the nature of the PSC plume would remain constant regardless of its subsequent interaction with the DI jet. Further, all images were taken at the same time of 90CAD BTDC. This ensured the cylinder conditions were, within experimental error, the same for each image case. It must be made clear that these timings were not optimised for potential combustion quality, but were made on the grounds of obtaining the most information to meet the objectives of the study. The image timing of 90CAD BTDC is advanced when compared to the late injection regime of a typical DISI engine. An understanding was sought for the full extent of the DI and PSC plumes, and use of a timing later than 90CAD BTDC would have had the piston impinge upon the image region. 4▪8▪3 CENTRAL COMPOSITE DESIGN Appendix E contains the mathematical background to the use of central composite designs and how they offer a statistically rigorous way of avoiding a full factorial test matrix. The two experimental variables (background charge AFR and DI injection timing), are mapped to the design variables x1 and x2, where - 199.8 %/cm Table A-1 HPFS 7890 (KrF Grade) Properties Figure A-3 RCM Quartz Cylinder ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 219 The quart cylinder head is shown in Figure A-4. The window is designed to offer full optical access across the entire bore, except that one edge must be cut by a chord to provide some metallic material to house the direct injector for the fuel system (cf. §A▪3▪3 for port plate design), hence the final D-shape of the window. The draft provides the seat for the window in the port plate. All quartz components have their edges chamfered to the largest degree possible to minimize the chance of stress concentrations, cracking and scalloping. Figure A-4 RCM Quartz Cylinder Head (Window) A▪3▪2▪1 FINITE ELEMENT ANALYSIS A finite element analysis (FEA) was conducted on the quartz cylinder to establish the factor of safety for the design. The pressure loading on the internal surface was determined through minimization of the Gibbs energy for a stoichiometric constant volume combustion at TDC, where the state of the mixture prior to combustion was determined assuming an isentropic compression of the mixture from ambient conditions at BDC. The Gibbs free energy minimization was achieved using the well established freeware GasEq (www.gaseq.co.uk). Flash thermal loadings were disregarded in this study. The quartz for the cylinder had its properties amended to reflect those of Table A-1. Only one quarter of the cylinder was ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 220 modelled (reflecting the symmetry of the problem). It was axially constrained on all axial surfaces while no other constraints were imposed. This accurately reflects the physical boundary conditions of the problem. CosmosWORKS® was used to complete the analysis. CosmosWORKS® is a basic FEA software package, the results of which should be used for guidance only. An inability to control meshing and element shape function is severely limiting. The tetrahedral mesh produced had problems in the areas highlighted in Figure A-5. Figure A-5 Problematic Elements in Quartz Cylinder FEA Of most concern are the elements around the inner recess because of their high aspect ratio. However, inspection of Figure A-5 shows them all to have a ratio under 10, the critical cut-off for aspect ratio integrity. Further, CosmosWORKS® uses adaptive shape functions to minimize the effect of degenerate elements. Figure A-6 Element Aspect Ratios in Area of Concern for Quartz Cylinder FEA ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 221 Of more concern however is the elemental Jacobian, effectively a measure of the elemental deformation against its parent shape. A high Jacobian suggests excessive deformation for the solution to hold true. Two elements in the model are cause for some concern. They are shown in Figure A-7. Figure A-7 Elemental Jacobian Values for Quartz Cylinder FEA Despite these concerns there was still deemed to be merit in continuing with the study. Given the brittle nature of fused silica, a Mohr-Coulomb yield surface is used to asses the failure of the quartz cylinder. The yield criterion for the Mohr-Coloumb surface is defined by Eqs. B:1 & B:2. 0 where Eq. A2. Eq. A2 while Rc is the compressive yield strength and Rt the tensile yield of the material. For this analysis, yield is assumed to occur at rupture. As a matter of note, for the case K=0 the Von Misses yield surface is retrieved. For the four compression ratios used in the RCM (5.33, 5.99, 8.18 and 13.78) Figure A-8 presents the results of the FEA safety analysis. Highlighted in Figure A-8(a) is the excessive deformation at the element interface where the elemental Jacobian was too high. This distortion persists in all the analyses. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 222 rc Minimum FOS Image (a) 5.33 2.1 (b) 5.99 1.8 (c) 8.18 1.3 ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 223 rc Minimum FOS Image (d) 13.78 0.82 Figure A-8 RCM Quartz Cylinder FOS Values based on a Mohr-Coulomb Yield Criteria for Compression Ratio 5.33(a), 5.19(b), 8.18(c) & 13.78(d) Given that this analysis is inherently conservative (by virtue of the fact that the combustion is assumed to be at constant volume, while in the RCM the combustion is in an expanding volume) the quartz cylinder is deemed to be amply safe. A▪3▪3 PORT PLATE The quartz window must be held securely to form the gas tight seal with the cylinder head. Further, the cylinder head is required to house all the pieces normally found in an ICE head (except the valve system). The component detailed in Figure A-9 was designed to this. The key features are as follows (going through the views of Figure A-9 in an anti-clockwise direction from the top right). The large aperture in the centre of the plate is the mount for the quartz window. The 170 draft corresponds to the draft on the window, however, the diameter is over-sized by 1mm. The drafted surface then has in it two 1/16” o-ring grooves which provide the mounting surface for the quartz when the Buna-N rings are in place. The o-rings serve to cushion the quartz against metal contact and provide a gas-tight seal against combustion pressure. The hole toward the bottom of the plate is designed to mount the Westport® J43M injector at an angle of 320, which keeps the injector out of the image area. The hole in the injector nozzle is spark eroded at an angle which ensures the top edge of the injected fuel plume travels parallel to the fire deck (at a clearance of ~4mm). The injector is held in place, against combustion, pressure by four M4 nickel coated mild steel threaded rods which anchor to a plate that braces against the top mounting surface of the injector. Next to the hole for the injector is the tapping for the dynamic pressure transducer (PCB Electronics, 0112A). The set of dowel holes on this face allow the repeatable location of the compression plate which holds the entire cylinder assembly in compression. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 224 Figure A-9 RCM Port Plate The series of eight M3 hole pairs take a set of small tangs which allow the quartz cylinder to be held and positioned radially in a repeatable fashion. A▪3▪3▪1 ELECTRODE & PSC MOUNT There are three 6-32 UNC holes which extend radially from the circular portion of the central aperture. The hole extending to the top surface of the port plate takes the PSC mount. The other two are for the electrode mounts and have an angle of 40 w.r.t. the fire deck so that the electrodes intersect the PSC fuel plume. Both mounts have a thread to allow the radial position of the PSC tube/electrode to be altered. The mounts for the PSC capillary and the electrodes are the same (except for their internal bore) and are made of PTFE. The electrode mount is shown in Figure A-10. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 225 Figure A-10 RCM Electrode Mount The PSC capillary (OD=1/16”) or tungsten electrode (OD=0.030”) have an interference fit (of 0.002”) on the internal bore of their respective mounts. The mounts locate in the port plate to position the capillary and electrodes accordingly. Assuming a built in cantilever for the capillary and the electrodes their natural frequencies are 5.3Hz and 12.7Hz respectively. These are both well away form anything that can be expected to be found on the RCM (as are the higher order vibrations) and hence excessive vibration of the tube and electrodes was not deemed to be a concern. A▪3▪4 CYLINDER The quartz cylinder does not cover the full stroke of the piston. The majority of the RCM cylinder is made of 316 stainless steel and is responsible for locating the quartz components and cylinder head. The cylinder was machined from a solid billet, with tight tolerances on the internal bore surface finish (1.6 µm), circularity (0.1mm) and parallelism (0.1mm). The running section of the cylinder accounts for the bottom 50% of the stroke, and has the mixture inlet and outlet ports integral to it. The large flange is the sealing surface (sealed with a silicone gasket) for the quartz portion of the cylinder, cf. Figure A-12. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 226 A▪3▪5 PISTON The piston used in this study has a flat top-land geometry and modest skirt. The PEEK seals provide the tightest gas-seal possible though are far form gas tight. The final groove on the skirt takes a PEEK runner that is bonded in place and ensures the true running of the piston in the cylinder. The o-ring nearest the top-land is designed to be 3mm from the interface between the quartz and the steel at TDC, which avoids it running over the interface and the associated wear and debris concerns. The top-land of the piston is made from polycarbonate and is threaded to allow the fitting of aluminium spacers between it and the piston body. The resultant change in piston length changes the compression ratio of the RCM (given its fixed stroke of 104mm). With no spacer and the current 1/8”, 3/8” and 5/8” spacers the compression ratios are 5.33, 5.99, 8.18 and 13.78 respectively. Figure A-11 RCM Piston A▪3▪6 RCM CYLINDER ASSEMBLY The components of the preceding sections are held in place by a compression plate (cf. Figure A-12) which is secured by a series of tie-rods (not shown) to the crank case of the RCM (as in Figure A-1). This ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 227 plate ensures that the load-path passes through the components that form the RCM cylinder and in doing so generates a gas-tight seal. Figure A-12 RCM Cylinder Exploded View Figure A-13 RCM Compression Plate Drawing ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 228 A▪4 RCM FUELLING SYSTEM Along with the substantial alterations made to the RCM cylinder there was a need to fuel the RCM with natural gas or methane. The fuelling system was required to deliver fuel as an homogenous premixture; a pure or premixed PSC charge; and pure a DI charge. The final requirement was that any one or all of the fuel sources had the ability to be passed through the acetone seeder so that the fuel could be imaged using LIF. A schematic for the system designed to do this is given in Figure A-15, where the valve numbers pertain the settings table given as Table A-3. Figure A-14 RCM Fuelling System All valves are hand operated and mounted on a front facia with the corresponding piping diagram shown in Figure A-14 (the hard-line pipes are shown as lightweight equivalents). Using partial pressures the operator can prepare a mixture in the premixer. Line pressure for the methane is shown, while the total pressure in the premixer is also listed (which allows the amount of air required for a given composition to be determined). N 2 C H 4 A IR VENT T T S E E D E R P R E M IX E R T Thermocouple Pressure Gauge Check Valve Valve Regulator Relief Valve 2W- 2 Way 3W- 3 Way 5W- 5 Way B- Ball N- Needle 3W-B N N N VENT SAMPLE J43 VENT VENT SAMPLE 3W-B 3W-B 5W-B 5W-B 5W-B 3W-B 3W-B PSC 2W-B 2W-B 2W-B RCM 2W-B N N N 2W-B 2W-B 2 1 4 6 7 13 9 11 10 12 5 8 3 ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 229 Figure A-15 RCM Fuelling Panel Valving, Piping & Valve Numbering The fuelling options available are listed in Table A-2. Bulk Charge PSC DI Air Seeded CH4 Seeded CH4 Air Seeded CH4 CH4 Air CH4 CH4 Air CH4 Seeded CH4 Air Seeded CH4 Seeded Premix Air Seeded CH4 Seeded Premix Air CH4 Seeded Premix Air CH4 Seeded Premix Premix None None Table A-2 RCM Fuelling Options The required valve settings to achieve these states are given in Table A-3. Every fuel source has a high pressure two way valve (one outlet to the cylinder and one to vent) as its final stage to allow the fuel system to be fully flushed with nitrogen. Direct venting of the premixer is allowed though the charge may be diluted with air to take it beyond its flammability limit. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 230 Table A-3 RCM Fuelling Valve Settings ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 231 A▪4▪1 FUEL INJECTORS The natural gas fuel injectors used on the RCM are discussed in detail in the body of this work. The DI injector hole was sized at 0.75mm diameter using bespoke MatLAB® code and electrostatic discharge machined (EDM’d) 400 off the injector centreline. This provides a DI fuel plume with a top edge parallel to the RCM fire-deck. All plume geometry and development calculations were based on the work of Hill et al. [8]. The tip drawing is shown in Figure A-16. Figure A-16 RCM DI Tip Drawing Due to poor solenoid response in previous [7, 367, 415, 416] and parallel studies the PSC solenoid was replaced with a J43M injector with a 0.26mm diameter orifice. This injects at much higher pressure than the old fast acting solenoid, though it does provide much better pulse control. The addition of a significant length of capillary tube to the end of the injector does mitigate these advantages significantly however All injectors are supplied by high pressure flexible thermoplastic hose so as not to excessively load the injectors or their mounts. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 232 A▪4▪2 PSC INJECTOR MOUNT The J43M used for PSC fuel control was retrofitted to the RCM using the mount shown in Figure A-17. The need for the injector to support combustion pressures and temperatures is bypassed in this application, so that the detail of the combustion seal can be omitted. The 0.26mm diameter hole in the injector tip was machined (by EDM) to be on the injector centreline, and is the smallest hole that can be produced by EDM. The small hole requirement is a result of needing only a small mass of fuel to form the PSC plume. A brace similar to that used on the DI injector was used to hold the injector in place, with two OEM ‘quick release’ skewers providing the seating force. These skewers allow easy adjustment of the seating force against the o-ring on the injector tip. It is this o-ring that provides the gas-tight seal. Figure A-17 RCM PSC Injector Mount A▪5 SEEDER The acetone seeder is a high pressure bubbler which allows methane/natural gas to be passed through liquid acetone or by-pass it before exiting. The flow path for the methane may be varied between the two routes so that the concentration of acetone in the exit stream may be varied. The seeder is based upon the design presented by Neij [259] in his thesis, though the current design was altered to operate at higher pressures. Since the DI injector requires supply pressures in excess of 1800psi (up to a maximum of 3600psi) the seeder was designed to operate at up to 2800psi. The pressure vessel was designed to the 1995 American Society of Mechanical Engineers Boiler & Pressure Vessel Code, Section 8: Rules for Construction of Pressure Vessels, Division 1, Part UG. The flanges are held in place by eight M12 12.9 bolts. The bottom flange is sealed with an ASME 342 ethelyleme-propylene (EPDM) ring for its high resistance to acetone attack, while the top flange is sealed with a similarly specified nitrile (Buna-N) o-ring since nitirle rubbers have a better resistance to methane ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 233 corrosion. The o-rings and their recesses on the seeder flanges were specified according to SAE codes ARP 1231(Rev. A) and 1234A. Figure A-18 Acetone Seeder Section View Tappings are also provided for: emergency pressure relief (set at 4000psi); a thermocouple (type-K); and pressure gauge. Glass beads placed in the exit flow path after the liquid acetone ensure the gas from the two steams is fully mixed. A complete set of parts, process and assembly drawings for the seeder are presented on the subsequent pages. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 234 Figure A-19 Acetone Seeder: Body Figure A-20 Acetone Seeder: Bottom Flange ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 235 Figure A-21 Acetone Seeder: Top Flange Figure A-22 Acetone Seeder: Cup ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 236 Figure A-23 Acetone Seeder: Guide Figure A-24 Acetone Seeder: Pipe Assemblies ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 237 Figure A-25 Acetone Seeder: Top Weld-Up Figure A-26 Acetone Seeder: Assembly Drawing ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 238 A▪5▪1 PROOF TESTING The maximum operating pressure for the seeder is 2800psi. To ensure the safe operation of the seeder it was sent for hydrostatic pressure testing at Enermax Fabricators Inc., North Vancouver, Canada. Under contract number 11553 the seeder was hydrostatically proof tested with oil to a pressure of 4300psi. No failure or leaking was observed. A▪6 TIMING & CONTROL The timing and control system for the RCM is detailed extensively in Appendix B. A▪7 SPARK CIRCUIT An MSD Blaster 2 (turns ratio 100:1) coil is used to generate the high voltage required for the spark. The primary coil +ve is connected to +12V through an 0.8Ω ballast resistor; the primary –ve to the drain of an NPN MOSFET (rated to 200V, 15A). The MOSFET source is grounded. The MOSFET switches based upon a TTL signal from the BNC connector. The TTL signal, which is received from the RCM triggering circuits (cf. Appendix B), is inverted by the 74LS04 and increased to +10V using the LTC1693 line driver. The 1kΩ resistor protects the MOSFET gate. The signal stability after the LTC1693 is protected from supply voltage fluctuations by the 0.1µF ceramic capacitor, for high frequency fluctuations, and against low frequency fluctuations by the 4.7µF tantalum capacitor. In its normal state the circuit flows about 6A through the MOSFET (with VGS ~10V). Upon triggering the MOSFET gate voltage drops to ~0V switching the transistor off. The current is now forced into the RC circuit formed by the 100kΩ power resistor and the 0.5µF high voltage capacitor. The current through the coil primary decays exponentially as the capacitor charges. As the field lines are cut the voltage produced in the primary is governed by , reaching a value of ~250V. The coil secondary now has an induced voltage of about 25kV, which generates the spark at the HV terminal. The purpose of the RC circuit is to protect the MOSFET against the high voltages which would otherwise be produced on switching (since di/dt would be large). The fast response diode pair prevents the capacitor discharging to the primary coil as the voltage drops on the primary -ve. The high voltage capacitors now discharge through the 100kΩ resistor. ▪ Appendix A ▪ RCM Apparatus & Re-Design ▪ 239 Figure A-27 RCM Spark Circuit, Circuit Diagram Component Part # Value Inverter 74LS04 --- Line Driver LTC1693 --- Capacitor (Ceramic) --- 0.1µF Capacitor (Tantalum) --- 4.7µF Resistor --- 1kΩ MOSFET (NPN) IRFP450X --- Power Resistor --- 100 kΩ Capacitor (High Voltage) --- 0.5 µF, 1.5kV Fast Response Diode 10ETF12 --- MSD Coil Blaster 2 100:1 Resistor (Ballast) MSD Ballast 0.8kΩ Table A-4 Component Listing for RCM Spark Circuit A▪8 INSTRUMENTATION & EQUIPMENT LIST Equipment Model Manufacturer Supporting Software Laser CompexPro 102 Lambda Physik -- Intensified CCD Camera DiCam Pro Cooke Corp. CamWare v2.12 High Speed Camera Vision Research Phantom v7.1 Cine Control v605.2 Pressure Transducer (high dynamic response) 0112A PCB Electronics -- Charge Amplifier 5010 Kistler Instrument Corp. -- Oscilloscope 2014B Tektronix Inc. OpenChoice Desktop v1.4 Laser Power Meter FieldMax II Coherent Inc. FieldMax II PC v1.2 Thermocouple Reader HH12A Omega Inc. -- Table A-5 Instrument List for RCM Not listed are the various linear and variable power supplies, the proprietary Westport® injector driver hardware, and the in-house hardware prepared by the author. 240 Appendix B RCM TIMING CONTROL SYSTEM B▪1 INTRODUCTION The design and operation of the rapid compression machine control system is outlined in this appendix. Detailed discussions are presented regarding the design and operation of the electrical circuits and the mechanical details of the chassis on which the circuits are mounted. ▪ Appendix B ▪ RCM Timing Control System ▪ 241 B▪2 GENERAL PRINCIPLES OF RCM EVENT CONTROL The RCM is controlled by a series of TTL circuits which, typically, use LS series DIP chips. All logic is, unless otherwise stated, +5V high with 0V low. A rotary encoder provides positional data which is used as the time base for all events. The piston TDC is the reference datum, with all events timed against this. The encoder provides 10000 counts per revolution. The event timings are specified, as appropriate with, a start count and a duration count. The count values, as set on the thumbwheel switches on the front panel of the control box, are set according to the required number counts after the gate (cf. §B▪3). Timing w.r.t. TDC is made by knowing that TDC is 5200 counts after the gate. Figure B-1 Information Flow in RCM Control The RCM control hardware is capable of providing timing for the events listed in Table B-1. Event Required Input 16 (logic pulse or signal) Spark Signal PSC Signal Direct Injection 1 Pulse & Signal Direct Injection 2 Pulse & Signal Laser 1 Pulse Laser 2 Pulse Camera 1 Pulse Camera 2 Pulse Table B-1 RCM Timed Events Power is stepped down from mains 120V A.C. to 12V D.C. for distribution to all boards. Each board supply is smoothed with a 25V aluminium electrolytic capacitor, and is then stepped down to +5V with a smoothed LM7805 voltage regulator. B▪3 ENCODER & ENCODER GATE The rapid compression machine (RCM) has all its timed events synchronised against a common time base- that of the piston crank angle. The RCM crank has a US Digital H6 incremental rotary encoder attached to 16 A logic pulse is simply a +5V spike, while a logic pulse is a +5V high state for a specified number of encoder counts ▪ Appendix B ▪ RCM Timing Control System ▪ 242 it via a 1¼” flexible coupling. The 2500 count encoder uses ×4 quadrature to provide 10000 counts per revolution (i.e. 0.036 0/count). The encoder also provides an index pulse once per revolution. The operation of the RCM does not guarantee that the driving rack will return to the same point after ever run. To circumvent this, the index from the encoder is used to start the RCM event timings. The encoder is positioned such that the index pulse is received 5200 (~1870) counts BTDC. The circuit outlined below is designed to gate the encoder pulses so that no pulses are passed to the timing circuits of §B▪2 prior to the output of encoder index pulse. The LS7184 decoder takes the pulses of the rotary encoder and provides an output clock pulse train. The floating state on pin 5 provides the ×4 quadrature, while the up or down count (i.e. rotary direction indicator) on pin 7 is ascertained from the phase of the pulses on Channel A with respect to Channel B. The 74HC109 J-K flip flop provides the gating. Pulling pin 1 low through application of the push button switch, J2 (coloured red on the gate box front panel), makes the Q output go low. The flip-flop will stay in this state until J (the encoder index pulse) is asserted. The flip-flop checks the state of its input pins J and K at 10MHz, as governed by the crystal oscillator on pin 4 (CP). Gate A of the 5400 series quad-NAND chip allows the encoder pulse chain to pass only when the Q output of the J-K is asserted. When Q is asserted the LED connected to J3 is no longer forward biased so will not illuminate once the index has been received by the J-K and starts to pass the pulse train. The pulse train is now passed through gate D of the 5400 series quad-NAND chip (since pin 12 is asserted and the, inverted, train arrives on pin 13) and inverted by gate D of the 7400 series hex-inverter. The train is now high (+5V) with pulses going low (0V). The pulse train is then fed to a line driver, LTC1693, so that the gate can provide enough current and voltage to drive the timing. The output of the LTC1693 provides a high quality square wave independent of supply fluctuation (because of the capacitors on the chip supply pin 8). The 0.1µF capacitor smoothes high frequency fluctuations while the 4.7µF tantalum capacitor smoothes the lower frequencies. The output pulse train is supplied to a BNC jack, J4. ▪ Appendix B ▪ RCM Timing Control System ▪ 243 Figure B-2 Encoder Signal Gate, Circuit Diagram The rotary encoder has its output split so that its signals are also provided to and ED3 digital display (not shown in Figure B-2), which provides a digital display of the encoder’s rotary position. This display resets each time it is powered up, so must be used with caution if trying to find the position of the RCM piston. Component Part # Value Resistors (unless stated below) --- 4.7kΩ Resistor R4 --- 1kΩ Resistor R6 --- 160Ω Capacitor (Ceramic) --- 0.1µF Capacitor (Tantalum) --- 4.7µF Quad-NAND, 14 pin DIP 74HC54 --- Hex-inverter, 14 pin DIP 74HC04 --- Oscillator ACH-10.000MHZ- EK 10MHz J-K flip-flop 74HC109 --- Rotary decoder LS7184 --- Table B-2 Rotary Encoder Gate, Circuit Components & Values B▪4 SIGNAL DISTRIBUTION The signal received from the encoder gate must be distributed to all the timing circuits outlined in Table B-1. The pulse train from the encoder gate is received on the BNC jack J1, which has a 50Ω terminating resistor. The signal is inverted by gates A and B of the 74LS04 hex-inverter, before going to the differential line drivers U33 and U35. These AM26LS31 differential drivers split the signal (pins 1, 7, 9 & 15) into two (pins 2, 6, 10 & 14 being +ve and pins 3, 5, 11 & 13 being –ve) passing half down each line. This allows better high frequency transmission to each of the individual timing circuits. The distribution circuit shown Figure B:3 in also distributes a 10MHz clock signal to the four timing circuits which requires a J-K flip-flop (cf. §B▪8). ▪ Appendix B ▪ RCM Timing Control System ▪ 244 Figure B-3 Pulse Train Distribution, Circuit Diagram The distribution board also contains circuitry to supply a rest signal to all timing boards and the chips which require it, Figure B:3. A SPST pushbutton switch is attached through jumper J3, which upon depression makes pin 1 on the Schmitt-triggered inverters (74LS14) of U23 & 24 go low, while the LED on the other 2 pins of J3 is forward biased and illuminates to show that the rest signal was sent. Switch bounce is minimized through using a 74LS279 R-S latch. ▪ Appendix B ▪ RCM Timing Control System ▪ 245 Figure B-4 Reset Signal Distribution, Circuit Diagram The reset pulse is stretched by the RC time constant of R11 and C23 while the Schmitt-triggered inverters then provide the hard switch required by the reset gates. B▪5 SIGNAL RECEIVING The encoder signal from the distribution circuit of §B▪4, is received on an AM26LS32 differential line receiver. The 100Ω terminating resistor ensures the difference between the two receiving lines (pins 1 & 2) is maintained, so producing a clear waveform on the receiver output (pin 3). The switch of J57 must also be closed to allow gate A of the AND chip to pass the encoder signal, which is output on pin 3 of the AND chip. When the circuit is active (i.e. powered) the LED of J58 is forward biased and illuminated. The 220Ω resistor protects the LED while the 1kΩ pulls down the input on the AND gate. Figure B-5 Encoder Pulse Train Receiving, Circuit Diagram ▪ Appendix B ▪ RCM Timing Control System ▪ 246 B▪6 PULSE COUNTER AND COMPARISON CIRCUIT Integral to the event timing is the ability to count and compare the number of pulses received against a set value. The circuit of performs this task. The gated pulse train is received on ENC_PULSES_1 from the signal receiving circuit of. All counter chips have the present value pins (pins 3-6) held low, and the load (LD) held high so that counting will always start at zero. The first pulse is used to trigger the R-S latch, which then asserts the parallel enable (ENP) on all the 74LS160 synchronous decade counters. All subsequent pulses on the pulse train are then passed to the 74LS160 clock pins (CLK). The unit stage of the counter array (U54, in Figure B-6) has its trickle enable asserted at the same time as the ENP. With both enables asserted the counter counts the rising edge of all the pulses received on the CLK, with the output being given on Q0-Q3 (pins 11-14). On count 9 the ripple carry out is asserted so that the next pulse will be counted by the 10’s stage (U53), while the unit stage rolls back to zero. U53 thus counts every tenth pulse, on the 99th pulse the RCO of U53 goes high, asserting ENT on U52 allowing U52 to count the 100th pulse. The 100’s stage works as the units and 10’s stages do. Finally, the 1000’s stage (U51) has to have RCO asserted on all previous stages to assert its ENT, which allows the count to take place. When required the chip resets for the 74LS160 and 74LS279 are received from the distribution circuit (§B▪4) on START_RST_0_1. Both the 74LS160 and 74LS85 have decoupling capacitors on their power pins (not shown) to provide the power required on the output pins when the states change rapidly upon switching. The capacitor values of 0.1µF and 0.01µF are alternated to avoid a resonant response if they are required to discharge simultaneously. The output of each counter stage is sent to the 74LS85 four bit magnitude comparators on inputs B0-B3. The set values on the comparators (A0-A3) are defined by the user through use of the thumbwheels switches attached to J49-J52. The switches use complimented BCD to set the state, with pull up resistors on each line limiting the low state current draw. The input states AB are not used so pins 2 and 4 are tied low, while the required input state A=B necessitates that pin 3 be held high. When this case is satisfied the QA=B pin (6) is asserted. It follows logically that when the count on each stage matches the count specified by the switches for each stage, the A=B output of each comparator will go high, and only in this case will the output of the quad-input AND gate (74LS21) be asserted. ▪ Appendix B ▪ RCM Timing Control System ▪ 247 Figure B-6 Pulse Counter and Comparator, Circuit Diagram B▪7 LOGIC PULSE GENERATION The output of the counter/comparator circuit is the sole input required to trigger a +5V logic pulse from the circuit of Figure B-7. A pulse is required to trigger the lasers, the iCCD cameras and injector lift. The assertion of A1 on the 74LS221 monostable multivibrator leads to a logic pulse (pulled up to +5V by the 4.7kΩ resistor) on the output Q. The length of the pulse is governed by the RC time constant of capacitor C47 (0.1µF, pin 14) and the 10kΩ trim pot on RC (pin 15). The output is made through BNC J53. Figure B-7 Logic Pulse Generation, Circuit Diagram ▪ Appendix B ▪ RCM Timing Control System ▪ 248 B▪8 LOGIC SIGNAL GENERATION For the RCM events which require a +ve logic signal for a set duration of counts the circuit of Figure B-7 is used. This circuit is used to provide logic for injector hold signals, spark and PSC solenoid lift. The 74LS109 J-K flip-flop provides switching based upon inputs from two counting circuits. The Q output of the flip-flop is held low until J is asserted (START_EQ). The high signal on J is sent from the counter circuit of §B▪6 when the specified start count is reached. Q is then active making the BNC connector of J15 also high. Gate A of the 74LS08 AND chip now starts to pass the encoder signal (output on pin 3, WIDTH_COUNT) to a duplicate of the counter circuit of §B▪6. The second counter/comparator circuit asserts after the specified number of counts such that K is then asserted (WIDTH_EQ, which is inverted by U11), sending Q low and switching the BNC jack. The flip-flop receives its clock pulse (CP) from the oscillator distribution circuit of Figure B-4 via an AM26LS32 differential line receiver, U28 (for details about the operation of the line receiver cf. §B▪4). The flip-flop is reset by asserting R with a low state, as received from the reset distribution circuit of Figure B-3. Figure B-8 Logic Signal Generation, Circuit Diagram B▪9 APPENDIX 1: DESIGN, ELECTRICAL DRAWING SETS, ARTWORK AND COMPONENT LISTS The following sections discuss how the circuits were designed and implemented. B▪9▪1 CIRCUIT DESIGN AND TESTING The RCM control hardware was developed using the MentorGraphics design suite PADS2005. PADS Logic was used to develop the logic and operating principles outlined in §B▪4-§B▪8. This logic was transferred on to a board PCB layout with the use of PADS Layout, where all component specifications ▪ Appendix B ▪ RCM Timing Control System ▪ 249 where defined and finalised. Finally, PADS Router was used to specify the traces on the PCB’s according to the logic put together in PADS Logic. PADS Router was also used to generate the Gerber plots required (drill aperture list, drill positions, silk screens, solder masks, artwork and board extents) for PCB production. Upon receipt of the boards, they were populated and tested (an Agilant digital oscilloscope was used to test circuit logic). Manufacturing drawing sets are presented in the following sections. Test results for each board are presented in Tables B-4, B-5, B-7, B-8, B-10 & B-11. A full component list for each circuit board is also given (Tables B-6, B-9 & B-12). B▪9▪2 LOGIC PULSE CIRCUITS This circuit board contains two logic pulse circuits, and is intended to fire one laser and camera pair. Figure B-9 Start Counter-Gate A, Pulse Circuit ▪ Appendix B ▪ RCM Timing Control System ▪ 250 Figure B-10 Start Counter-Gate B, Pulse Circuit Figure B-11 Latch & Reset Circuits, Pulse Circuit ▪ Appendix B ▪ RCM Timing Control System ▪ 251 Figure B-12 Smooth Capacitor Circuits, Pulse Circuit Component Component # Part # Value Quad-AND, 14 pin DIP U61 74LS08 --- Dual R-S latch, 14 pin DIP U1 74LS279 --- Counter, 16 pin DIP U41-U44, U51-U54 74LS85 --- Comparator, 16 pin DIP U37-U40, U47-U50 74LS160 --- Resistors (¼W) All R, unless otherwise listed --- 4.7kΩ Dual-4 in AND, 16 pin DIP U46 74LS21 --- Dual multivibrator, 16 pin DIP U57 74LS221 --- Hex-inverter, 14 pin DIP U63 74LS04 --- Liner Receiver, 14 pin DIP U62 AM26LS32 --- Resistors (¼W) R109, R112 --- 220Ω Resistors (¼W) R108, R111 --- 1kΩ Resistors (¼W) R110, R113 --- 100Ω Trim Pot R103, R107 --- 10kΩ BNC jack J53, J55 --- --- 5 pin sip J36, J36, J38, J48, J49, J50, J51, J52 --- --- 2 pin sip J57, J58, J62, J61 --- --- 4 pin power jack J65 --- --- Capacitors (Ceramic) C47, C59 --- 10pF Capacitors (Ceramic) C48-C55 --- 0.01µF Capacitors (Ceramic) C60-C67 --- 0.1µF Capacitor (Tantalum) C1, C2 --- 0.1µF Capacitor (Aluminium) C58 --- 0.1µF 5V voltage regulator U58 LM7805 --- Heat Sink U2 --- --- Table B-3 RCM Logic Pulse Board, Component List ▪ Appendix B ▪ RCM Timing Control System ▪ 252 10▪3▪3 LOGIC SIGNAL CIRCUITS This circuit board contains one logic signal circuit and is intended for use with the PSC solenoid and spark circuit. Figure B-13 Start Counter-Gate A, Logic Circuit Figure B-14 Start Counter-Gate B, Logic Circuit ▪ Appendix B ▪ RCM Timing Control System ▪ 253 Figure B-15 Latch Circuit Figure B-16 Signal Receiving, Logic Circuit ▪ Appendix B ▪ RCM Timing Control System ▪ 254 Figure B-17 Smooth Capacitor Circuits, Logic Circuit Component Component # Part # Value Quad-AND, 14 pin DIP U12, U14 74LS08 --- Dual R-S latch, 14 pin DIP U10 74LS279 --- Counter, 16 pin DIP U5-U8, U19- U22 74LS85 --- Comparator, 16 pin DIP U1-U4, U15- U18 74LS160 --- Resistors (¼W) All R, unless otherwise listed --- 4.7kΩ Dual-4 in AND, 16 pin DIP U26 74LS21 --- Dual J-K flip flop, 16 pin DIP U13 74LS109 --- Hex-inverter, 14 pin DIP U11 74LS04 --- Liner Receiver, 14 pin DIP U28 AM26LS32 --- Resistors (¼W) R52 --- 220Ω Resistors (¼W) R51 --- 1kΩ Resistors (¼W) R56, R18 --- 100Ω BNC jack J15 --- --- 5 pin SIP J4-J11 --- --- 2 pin SIP J13, J17, J18 --- --- 4 pin power jack J65 --- --- Capacitors (Ceramic) C48-C55 --- 0.01µF Capacitors (Ceramic) C60-C67 --- 0.1µF Capacitor (Tantalum) C1, C2 --- 0.1µF Capacitor (Aluminium) C58 --- 0.1µF 5V voltage regulator U58 LM7805 --- Heat Sink U2 --- --- Table B-4 RCM Logic Signal Board, Component List ▪ Appendix B ▪ RCM Timing Control System ▪ 255 10▪3▪4 INJECTOR SIGNAL This circuit board contains one pulse circuit to specify the injector lift and a width circuit to specify the injector hold duration. Figure B-18 Counter Circuit, Start, Injector Circuit Figure B-19 Counter Circuit, Duration, Injector Circuit ▪ Appendix B ▪ RCM Timing Control System ▪ 256 Figure B-20 J-K Flip Flop injector Circuit Figure B-21 Reset Circuit, Injector ▪ Appendix B ▪ RCM Timing Control System ▪ 257 Figure B-22 Signal Receiving, Injector Circuit Figure B-23 Smoothing Capacitors, Injector Circuit ▪ Appendix B ▪ RCM Timing Control System ▪ 258 Component Component # Part # Value Quad-AND, 14 pin DIP U14 74LS08 --- Dual R-S latch, 14 pin DIP U10 74LS279 --- Counter, 16 pin DIP U19-U22, U5-U8 74LS85 --- Comparator, 16 pin DIP U1-U4, U15-U18 74LS160 --- Resistors (¼W) All R, unless otherwise listed --- 4.7kΩ Dual-4 in AND, 16 pin DIP U26 74LS21 --- Dual multivibrator, 16 pin DIP U27 74LS221 --- Dual J-K flip-flop, 16 pin DIP U13 74LS109 --- Hex-inverter, 14 pin DIP U11 74LS04 --- Liner Receiver, 14 pin DIP U28 AM26LS32 --- Oscillator U31 ACH-10.000MHZ- EK 10MHz Resistors (¼W) R52 --- 220Ω Resistors (¼W) R51 --- 1kΩ Resistors (¼W) R18 --- 100Ω Trim Pot R55 --- 10kΩ BNC jack J15, J16 --- --- 5 pin SIP J5-J8, J19-J22 --- --- 2 pin SIP J17, J18, J27 --- --- 3 pin SIP J14, J19-J21, J28- J35 --- --- 4 pin power jack J65 --- --- Capacitors (Ceramic) C47, C59 --- 10pF Capacitors (Ceramic) C48-C55 --- 0.01µF Capacitors (Ceramic) C60-C67 --- 0.1µF Capacitor (Tantalum) C1, C2 --- 0.1µF Capacitor (Aluminium) C58 --- 0.1µF 5V voltage regulator U58 LM7805 --- Heat Sink U2 --- --- Table B-5 RCM Injector Board, Component List ▪ Appendix B ▪ RCM Timing Control System ▪ 259 B▪9▪3 DISTRIBUTION CIRCUIT This board receives the signal from the encoder gate and then distributes it. Component Component # Part # Value Dual R-S latch, 14 pin DIP U10 74LS279 --- Resistors (¼W) All R, unless otherwise listed --- 4.7kΩ Hex-inverter, 14 pin DIP U32, U33 74LS04 --- Liner Receiver, 14 pin DIP U28 AM26LS32 --- Liner Driver, 14 pin DIP U9 AM26LS31 --- Resistors (¼W) R15 --- 50Ω Trim Pot R55 --- 10kΩ BNC jack J1 --- --- 5 pin SIP J5-J8, J19- J22 --- --- 2 pin SIP J17, J18, J27 --- --- 4 pin power jack J65 --- --- Capacitors (Ceramic) C23, C48- C55 --- 0.01µF Capacitors (Ceramic) C60-C67 --- 0.1µF Capacitor (Tantalum) C1, C2 --- 0.1µF Capacitor (Aluminium) C58 --- 0.1µF 5V voltage regulator U58 LM7805 --- Heat Sink U1 --- --- Table B-6 RCM Signal Distribution Board, Component List ▪ Appendix B ▪ RCM Timing Control System ▪ 260 Figure B-24 Signal Distribution Circuits B▪10 APPENDIX 2: MECHANICAL HOUSING Each circuit board has an aluminium front panel which mounts all LED’s and switches. Card guides allow the easy removal and replacement of the circuit-boards, which are held in place with a #8 thumbscrew on the front panel and a locking nut on the BNC jacks at the rear. The control box reset and power supply are mounted on the right of the box with the mains power input and fuse located on the rear of the box. The top cover gives access to all signal and power distribution cables. The mechanical enclosure for the PCB’s of the preceding circuits is presented in Figure B-25. Figure B-25 RCM Control Circuit Enclosure 261 Appendix C RELATIVE AIR-FUEL- RATIO PDFS C▪1 INTRODUCTION Appendix C presents the relative air-fuel ratio probability density functions for all points in the central composite test matrices at the four stations considered in the capillary injected PSC study and the five stations considered in the insert injected PSC study. ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 262 C▪2 RELATIVE AIR-FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR CAPILLARY INJECTED PSC-DI INTERACTION Figure C-1 Relative Air Fuel Ratio Probability Density Functions for Capillary Injected PSC Event with DI at Station One (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-1) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 263 Figure C-2 Relative Air Fuel Ratio Probability Density Functions for Capillary Injected PSC Event with DI at Station Two (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-1) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 264 Figure C-3 Relative Air Fuel Ratio Probability Density Functions for Capillary Injected PSC Event with DI at Station Three (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-1) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 265 Figure C-4 Relative Air Fuel Ratio Probability Density Functions for Capillary Injected PSC Event with DI at Station Four (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-1) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 266 C▪3 RELATIVE AIR-FUEL RATIO PROBABILITY DENSITY FUNCTIONS FOR INSERT INJECTED PSC-DI INTERACTION Figure C-5 Relative Air Fuel Ratio Probability Density Functions for Insert Injected PSC Event with DI at Station One (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-3) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 267 Figure C-6 Relative Air Fuel Ratio Probability Density Functions for Insert Injected PSC Event with DI at Station Two (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-3) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 268 Figure C-7 Relative Air Fuel Ratio Probability Density Functions for Insert Injected PSC Event with DI at Station Three (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-3) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 269 Figure C-8 Relative Air Fuel Ratio Probability Density Functions for Insert Injected PSC Event with DI at Station Four (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-3) ▪ Appendix C ▪ Relative Air-Fuel-Ratio PDFs ▪ 270 Figure C-9 Relative Air Fuel Ratio Probability Density Functions for Insert Injected PSC Event with DI at Station Five (Following Standard Image Presentation Protocol Used in Main Text, and Station Location as Per Table 8-3) 271 Appendix D STATISTICAL TREATMENTS D▪1 INTRODUCTION Appendix D presents a basic summary of the for four of the most important statistical techniques used in this study, those of central composite test matrix design, the normal and Poisson distributions and the specification of the 95% confidence interval for a data sample. ▪ Appendix D ▪ Statistical Treatments ▪ 272 D▪2 DESIGN OF EXPERIMENTS The aim of any experimental design is to collect data as parsimoniously as possible, whilst still maintaining the accuracy of the information collected. In general the experimenter is required to fit the experimental response, y, to the variables x1, x2, x3…xk. For a second order response in 2D space the model is fit according to Eq.D.1 Eq. D.1 where k is the number of experimental factors (or variables). Eq.D1 exhibits, in order, main (or linear) effects, quadratic effects, interaction effects and an associated error (of which more later). In designing an experiment to provide enough data to accurately evaluate Eq.D1 it is common practice to use a central composite design (CCD) for responses which may have moments up to, and including, the second order. The quadratic response, which will posses only a single maximum or minimum, can be found from the set of points defined by the CCD without extra assumptions. To apply the CCD the experimental variables ξ1, ξ2, ξ3... ξk are transformed into the variables x1,x2,x3…xk of Eq.D.1. The coded variables are generally transformed to fit the interval (where k is the number of factors). In general the experimental design will have F factorial points, 2k axial points and nc centre-runs (cf. Figure xx). The F points provide the linear response and the two-factor interactions, the axial points estimate the pure quadratic terms and the centre-runs estimate the internal pure-error and help with the quadratic term estimation. It can be shown that [417] the centre-runs reduce the prediction variance at the centre of the design space, weighting the variance towards the extrema of the space. In general three to five centre-runs avoid severe imbalance and level the predication variance across the response hypersurface. The response y is bounded in the design space by the interval , however the designer is still faced with three options as to the location of the design points. The options are shown in Figure D:1 for a two-dimensional design space (k=2). The circumscribed design fits data well across the design space, while the inscribed design provides bias to a central subset of the experimental range (at the expense of the extrema). Finally, the face-centred option, is fair across the entire space, but with poor accuracy for the response quadratic terms. Which design is chosen is a matter for careful consideration by the designer, and must reflect his/her ability to judge the importance of extrema in the experimental range and the experimental accessibility of these points. ▪ Appendix D ▪ Statistical Treatments ▪ 273 Figure D-1 (a) Circumscribed Central Composite Design (b) Inscribed Central Composite Design, Test Domain Shown in Grey, Data Points as Black Dots D▪3 RANDOM VARIABLES & THEIR DISTRIBUTIONS As is standard for most engineering and scientific applications the random variable of concern (which will be addressed in more detail shortly) is considered to be discreet since it is most likely to have a countable or a countably infinite set of values. The population of concern may be sampled (the purpose of any experiment) to produce the random variable X. X may be comprised of a single event (X = x1), a series of individual events (X= x1, x2, x3…xn) or may be a the results of an arbitrary function applied to individual events (X = X1, X2, X3…Xn). The probability of an event taking place is described by the probability density, or simply the density, function f(z). The first moment of the density distribution (which represents to true population) is the distribution mean, µ, which is the expected value of random variable X and discrete from the sample (arithmetic) mean , viz.: Eq. D.2 If each realisation of Xi is also normally distributed it can be shown that the expected value (i.e. the mean) of is equal to the common mean, µ, of the Xith distribution. Similarly, if the distribution realisations share a common variance, σ2, the variance of is simply σ2/n for n realisations. The second moment about the mean for any density function is the variance, σ2, which corresponds to the expected value of (X-µ)2, with the corresponding value for a population sample denoted by S2. Eq. D.3 Variance is often used interchangeably with standard deviation which is, arithmetically, the square root of the variance. ▪ Appendix D ▪ Statistical Treatments ▪ 274 D▪3▪1 THE NORMAL DISTRIBUTION As with most random variables that are evaluated to high degree of accuracy the transformation of discreet values to continuous ones is made to allow the application of continuous density functions, such as the normal density distribution Eq.D.4 Eq. D.4 The non-trivial integration of Eq.D4 to yield event probabilities is circumvented through the use of normal integral tables. This, and other applications, requires the transformation of the recorded distribution into one which has zero mean and unity variance. This is done through use of the expressions in Eq.D.5 Eq. D.5 such that Eq.D4 is now simplified to the normalized distribution of Eq.D.6. Eq. D.6 Integration of the normalized density function from a specified upper percentage point, Kα (defined as (b- µ)/σ, where b is the upper percentage point in the original distribution), to infinity evaluates the probability P{X>b}. Values of this integral for many Kα limits are widely presented. The symmetry of the normal distribution is used to evaluate negative values of Kα through noting that P{N<-Kα} = P{N>Kα}, and similarly P{N ≥ -Kα} = 1-P{N>Kα}. D▪3▪2 THE T DISTRIBUTION The random variable, t, for the distribution is defined by the quotient of Eq.D.7 which involves the two independent random variables of the normal, X (with zero mean and unity variance), and chi-squared, χ2, distributions discussed previously. Eq. D.7 The resulting density expression is non-trivial in its derivation and inconsequential for the current work. As with the normal and chi-squared distributions, tabulated values of P{t≥tα;v}are widely available, and applicable as described in §D▪3▪1. D▪3▪3 THE POISSON DISTRIBUTION One of the most simple, yet useful, distributions is that of the Poisson distribution, which has a density function given by Eq.D.8 Eq. D.8 ▪ Appendix D ▪ Statistical Treatments ▪ 275 This discreet distribution describes well particulates, electron interaction, radioactive particle behaviour or any event which has a large possible number of outcomes each with a low probability. The application of this distribution is simplified by virtue of one interesting property: the mean and variance are the same and equal to the distribution parameter λ. D▪4 CONFIDENCE INTERVALS When a sample mean is calculated, it is important to specify the limits over which the mean is valid. This is most commonly expressed through the used of confidence intervals. In almost all cases the mean and standard deviation of the normally distributed random variable are not know. Under these circumstances the sample standard deviation, S, is taken as a proxy for the population standard deviation, with a correction factor included. This correction factor, it can be shown [368], is related to the t-distribution such that the confidence interval, about the (known) sample mean is: Eq. D.9 where tα/2;n-1 is the tabulated 100α/2 percentage point of the t-distribution with n-1 degrees of freedom (i.e. 2.5% point for a 95% confidence interval). If the population mean and standard deviation are both unknown Eq.D.10 must be revised such that the (non-uniform) confidence interval is given by: Eq. D.10 """@en ; edm:hasType "Thesis/Dissertation"@en ; vivo:dateIssued "2011-11"@en ; edm:isShownAt "10.14288/1.0072272"@en ; dcterms:language "eng"@en ; ns0:degreeDiscipline "Mechanical Engineering"@en ; edm:provider "Vancouver : University of British Columbia Library"@en ; dcterms:publisher "University of British Columbia"@en ; dcterms:rights "Attribution-NonCommercial-NoDerivatives 4.0 International"@en ; ns0:rightsURI "http://creativecommons.org/licenses/by-nc-nd/4.0/"@en ; ns0:scholarLevel "Graduate"@en ; dcterms:title "Mixture formation in a partially stratified directly injected natural gas engine"@en ; dcterms:type "Text"@en ; ns0:identifierURI "http://hdl.handle.net/2429/37778"@en .