Applied Science, Faculty of
Mechanical Engineering, Department of
DSpace
UBCV
Son, Donghyun (Jeff)
Master of Applied Science - MASc
University of British Columbia
Natural gas is an attractive alternative fuel with a lower price and carbon dioxide emission compared to liquid fossil fuels. Methane (CHâ) in natural gas, however, has a high global warming potential (GWP), thus the emission of unburnt CHâ can offset the cleaner combustion of natural gas and should be carefully monitored in dual-fuel natural gas/diesel engines. To develop cleaner engines, a high-speed methane sensor in the exhaust gas stream is required to capture emissions during real-world, transient engine operation. While CHâ measurements using Fourier-transform infrared (FTIR) spectroscopy in dual-fuel engines have been demonstrated, its speed is insufficient for cycle-resolved measurements. To this end, a fast, robust, and inexpensive CHâ sensor based on wavelength modulation spectroscopy (WMS) was developed using a relatively inexpensive IR diode laser with a centre wavelength of 1651 nm. The WMS sensor was assessed using gas standards and validated against a flame ionization detector (FID). The 508 mm heated gas cell reliably detects CHâ from 50 ppm to over 2% at a temporal resolution of over 200 Hz. The WMS sensor can resolve transient methane emission with a time delay and time constant of 0.55 s and 0.44 s respectively. When demonstrated on a heavy-duty dual-fuel research engine, the WMS sensor and FID agreed with a mean percentage difference of 4.2%. The field measurement capability of this portable sensor, which can continuously measure for seconds to hours, was demonstrated on dual-fuel marine engines of a coastal vessel. The GWP of dual-fuel engine exhaust heavily depends on operational profiles, and the fast WMS methane sensor will assist engineers and policy makers to characterize and reduce in-use CHâ emissions.
DEVELOPMENT OF A FAST METHANE SENSOR BASED ONWAVELENGTH MODULATION SPECTROSCOPY FOR EXHAUSTMETHANE EMISSION MEASUREMENTbyDonghyun (Jeff) SonBASc, The University of British Columbia, 2016A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF APPLIED SCIENCEinTHE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES(Mechanical Engineering)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)October 2019c© Donghyun (Jeff) Son, 2019The following individuals certify that they have read, and recommend to the Faculty ofGraduate and Postdoctoral Studies for acceptance, the thesis entitled:Development of a fast methane sensor based on wavelength modulation spectroscopyfor exhaust methane emission measurementsubmitted by Donghyun (Jeff) Son in partial fulfillment of the requirements forthe degree of Master of Applied Sciencein Mechanical EngineeringExamining Committee:Patrick Kirchen, Mechanical EngineeringSupervisorSteven Rogak, Mechanical EngineeringSupervisory Committee MemberNaomi Zimmerman, Mechanical EngineeringAdditional ExamineriiAbstractNatural gas is an attractive alternative fuel with a lower price and carbon dioxide emis-sion compared to liquid fossil fuels. Methane (CH4) in natural gas, however, has a highglobal warming potential (GWP), thus the emission of unburnt CH4 can offset the cleanercombustion of natural gas and should be carefully monitored in dual-fuel natural gas/dieselengines. To develop cleaner engines, a high-speed methane sensor in the exhaust gas streamis required to capture emissions during real-world, transient engine operation. While CH4measurements using Fourier-transform infrared (FTIR) spectroscopy in dual-fuel engineshave been demonstrated, its speed is insufficient for cycle-resolved measurements. To thisend, a fast, robust, and inexpensive CH4 sensor based on wavelength modulation spec-troscopy (WMS) was developed using a relatively inexpensive IR diode laser with a centrewavelength of 1651 nm.The WMS sensor was assessed using gas standards and validated against a flame ion-ization detector (FID). The 508 mm heated gas cell reliably detects CH4 from 50 ppm toover 2% at a temporal resolution of over 200 Hz. The WMS sensor can resolve transientmethane emission with a time delay and time constant of 0.55 s and 0.44 s respectively.When demonstrated on a heavy-duty dual-fuel research engine, the WMS sensor and FIDagreed with a mean percentage difference of 4.2%. The field measurement capability ofthis portable sensor, which can continuously measure for seconds to hours, was demon-strated on dual-fuel marine engines of a coastal vessel. The GWP of dual-fuel engineexhaust heavily depends on operational profiles, and the fast WMS methane sensor willassist engineers and policy makers to characterize and reduce in-use CH4 emissions.iiiLay SummaryWhile natural gas is an attractive alternative fuel with a lower price and carbon dioxideemission compared to liquid fossil fuels, methane in natural gas has a global warmingpotential, 28 times higher than that of carbon dioxide. To develop cleaner dual-fuel diesel/-natural gas engines with low methane emission, a high-speed methane sensor in the exhaustgas stream is required to capture emissions during real-world, transient engine operation.To this end, a fast, robust, and low-cost CH4 sensor based on wavelength modulation spec-troscopy (WMS) was developed using a relatively inexpensive laser. The methane sensorreliably detects CH4 from 50 ppm to over 2% at a temporal resolution of over 200 Hz. TheWMS methane sensor was tested against an existing slower methane sensor and demon-strated on heavy-duty dual-fuel engines both in a lab and on a coastal vessel.ivPrefaceThe initial development of the wavelength modulation spectroscopy methane sensor pre-sented in Chapter 3 was a collaborative effort between David Sommer, Miayan Yeremi,and myself. The first iteration of the mechanical system was designed and fabricated byDavid Sommer. Modulation signal optimization and digital lock-in amplifier software wereworks of Miayan Yeremi. I developed the control and data acquisition software. The opticalsystem was developed with equal contribution from all parties.All assessments of sensor performance in Chapter 4 with the exception of the field mea-surement was my original work. This involved development of a new mechanical systemwhich includes new heated cell design, piping, and instrumentation. Also, improving sen-sor (detection limit and speed) is my original work. The experimental design for sensorcharacterization was my work with guidance from Dr. Patrick Kirchen.The field measurement campaign presented in Chapter 4.5 was led by David Sommerwho designed the mechanical system. I was responsible for developing the control & dataacquisition software and post-processing workflow. I also took some measurements duringthe campaign. This work is presented in the following co-authored publications:â¢ D. Sommer, M. Yeremi, J. Son, J. Corbin, S. Gagne, P. Lobo, J. Miller, and P.Kirchen. Characterization and reduction of in-use CH4 emissions from a dual fuelmarine engine using wavelength modulation spectroscopy.Environmental ScienceTechnology, 53, 02 2019.doi:10.1021/acs.est.8b04244.â¢ D. Sommer, J. Son, P. Kirchen, and H. Penner. Characterization and reduction strate-gies for GHG emission from a LNG powered coastal vessel. CIMAC Congress 2019vIn these publications, David Sommer was the primary author in charge of WMS ex-periments and mechanical designs. I developed control and DAQ software, in additionto taking a part of the measurements. M. Yeremi developed post-processing software. P.Kirchen co-authored the publications and provided supervision and experiment supports. J.Corbin, S. Gagne, P. Lobo, and J. Miller were involved in the management, planning, andoperation of other instruments in the campaign. H. Penner coordinated access to the vesseland engine test points.viTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiLay Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiiAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xixDedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xx1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Greenhouse Gas Emissions in the Transportation Sector . . . . . . . . . . . 11.2 Natural Gas as a Fuel for Heavy-duty Engines . . . . . . . . . . . . . . . . 21.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 A Review of Methane Emissions and Measurement Techniques . . . . . . . . 62.1 Dual-fuel Natural Gas/Diesel Engine . . . . . . . . . . . . . . . . . . . . . 72.2 Comparison of Methane Measurement Methods . . . . . . . . . . . . . . . 82.2.1 Direct Laser Absorption Spectroscopy . . . . . . . . . . . . . . . . 82.2.2 Modulation Spectroscopy (WMS/FMS) . . . . . . . . . . . . . . . 9vii2.2.3 Photoacoustic Spectroscopy (PAS) . . . . . . . . . . . . . . . . . . 102.2.4 Fourier Transform Infrared Spectroscopy (FTIR) . . . . . . . . . . 102.2.5 Flame Ionization Detector (FID) . . . . . . . . . . . . . . . . . . . 112.2.6 Summary of CH4 Measurements Techniques . . . . . . . . . . . . 112.3 Absorption Spectroscopy Theory . . . . . . . . . . . . . . . . . . . . . . . 132.3.1 Absorption Coefficient and the Beer-Lambert Law . . . . . . . . . 132.3.2 Linestrength and Lineshape Function . . . . . . . . . . . . . . . . 152.3.3 Doppler Broadening . . . . . . . . . . . . . . . . . . . . . . . . . 162.3.4 Collisional Broadening . . . . . . . . . . . . . . . . . . . . . . . . 172.3.5 Voigt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.6 Absorbance Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . 192.4 Recent Development of WMS . . . . . . . . . . . . . . . . . . . . . . . . 193 Development of WMS Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.1 Wavelength Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.2 Hardware Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.2.1 Laser Components . . . . . . . . . . . . . . . . . . . . . . . . . . 263.2.2 Mechanical Components . . . . . . . . . . . . . . . . . . . . . . . 293.2.3 Gas Measurement Cell Design . . . . . . . . . . . . . . . . . . . . 303.3 Software Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333.3.1 LabVIEW Control Software . . . . . . . . . . . . . . . . . . . . . 343.3.2 Post-Processing Script . . . . . . . . . . . . . . . . . . . . . . . . 343.4 Signal Generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.4.1 Ramp Current Scan . . . . . . . . . . . . . . . . . . . . . . . . . . 363.4.2 Sine Current Modulation . . . . . . . . . . . . . . . . . . . . . . . 383.4.3 Sampling Frequency . . . . . . . . . . . . . . . . . . . . . . . . . 394 WMS Sensor Tests & Performance . . . . . . . . . . . . . . . . . . . . . . . . 424.1 Experimental System Setup . . . . . . . . . . . . . . . . . . . . . . . . . . 42viii4.1.1 SCRE Engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 434.1.2 Exhaust Sampling Points for WMS . . . . . . . . . . . . . . . . . . 474.1.3 WMS Configuration . . . . . . . . . . . . . . . . . . . . . . . . . 494.2 Steady State Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . 524.2.1 Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 534.2.2 Comparison With Flame Ionization Detector . . . . . . . . . . . . . 554.2.3 Limit of Detection . . . . . . . . . . . . . . . . . . . . . . . . . . 564.2.4 Signal Averaging . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3 Hot Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 594.3.1 Temperature Calibration . . . . . . . . . . . . . . . . . . . . . . . 614.3.2 Verification of Heated WMS System For Steady State Operation . . 624.3.3 Time Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . 634.3.4 Vibration Rejection . . . . . . . . . . . . . . . . . . . . . . . . . . 664.4 Dynamic Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674.4.1 First-order Correction . . . . . . . . . . . . . . . . . . . . . . . . . 714.5 Field Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 725 Conclusions & Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 765.1 Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84A List of Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91B Engine Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92C Gas Cell Target Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . 93D Coefficient of Variation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94E Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96ixE.1 Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96E.1.1 Laser Controllers . . . . . . . . . . . . . . . . . . . . . . . . . . . 97E.1.2 Laser Diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97E.2 Absorption and Photodetector Signal . . . . . . . . . . . . . . . . . . . . . 98xList of TablesTable 1.1 Euro VI and EPA emission standards for heavy-duty vehicles limitingcarbon monoxide (CO), non-methane hydrocarbon (NMHC), methane(CH4), nitrogen oxide (NOx), particulate matter (PM) and fuel sulphurcontent. [39, 54] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Table 2.1 Summary of CH4 measurement techniques . . . . . . . . . . . . . . . . 12Table 3.1 Major optical system components from Figure 3.4. A complete list ofcomponents can be found in Appendix A. . . . . . . . . . . . . . . . . . 27Table 3.2 Maximum achievable WMS temporal resolution per specification of eachmajor component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28Table 4.1 SCRE engine specification . . . . . . . . . . . . . . . . . . . . . . . . . 43Table 4.2 Comparison of exhaust temperature and pressure at the sample points . . 49Table 4.3 Combination of calibration gases used to calibrate WMS . . . . . . . . . 54Table 4.4 Considered operating conditions for the steady state methane sweep onSCRE. Detailed engine parameters can be found in Appendix B . . . . . 55Table 4.5 Step input response simulation parameters for WMS cell and exhaustsurge tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66Table A.1 Complete list of laser components . . . . . . . . . . . . . . . . . . . . . 91Table B.1 Engine parameter for steady-state SCRE tests . . . . . . . . . . . . . . . 92xiList of FiguresFigure 3.1 Methane linestrength at room temperature from 800 nm to 4500 nm . . 23Figure 3.2 Exhaust gas linestrength at room temperature from 1600 nm to 1700 nm 24Figure 3.3 Absorbance of the main absorbing exhaust gas species near 1650 nm atroom temperature and atmospheric pressure. . . . . . . . . . . . . . . 25Figure 3.4 Laser component setup: (1) current controller, (2) temperature con-troller, (3) TEC mount, (4) butterfly-mount DFB pigtailed diode laser,(5) collimating lens, (6) measurement cell, (7) photodetector, (8) fastmultifunction DAQ, (9) PC, (10) photodiode amplifier, (11) slow mul-tifunction DAQ, (12) thermocouples . . . . . . . . . . . . . . . . . . . 27Figure 3.5 Mechanical setup: (1) mounting plate or cage, (2) laser fibre-optic ca-ble, (3) 5-axis mount, (4) collimating lens, (5) Swagelok tee with awindow inside, (6) 508 mm measurement cell, (7) photodetector, (8)750mm optical rail. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Figure 3.6 Detection limit as a function of pathlength for a single-pass WMS forCH4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Figure 3.7 Final design of the sample gas chamber with the dimensions in mm . . 32Figure 3.8 An overview of the software workflow shown with sample signals atdifferent stages of the WMS system . . . . . . . . . . . . . . . . . . . 33xiiFigure 3.9 A simplified workflow of post-processing. (a) Raw photodiode signal.(b) FFT is performed on photodiode signal and a window function isapplied. (c) Perform iFFT to get the 2 f harmonic signal. . . . . . . . . 35Figure 3.10 Estimated modulation width displayed over CH4 absorbance at 298 Kand 1 atm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Figure 3.11 Maximum WMS signal locked-in at second and third harmonics de-pending on the sine modulation amplitude. Measurement is taken with3916 ppm [CH4] at 17â¦C . . . . . . . . . . . . . . . . . . . . . . . . . 39Figure 3.12 504 ppm CH4 balance N2 gas standard is measured at various DAQsampling frequencies to compare the effect of sampling frequency tothe 2 f signal shape and the noise floor. (T = 295 K, P = 1 atm) . . . . . 40Figure 4.1 SCRE engine facility highlighting the WMS sample ports . . . . . . . . 44Figure 4.2 P&ID of SCRE facility highlighting the intake and exhaust system andfuel systems along with the three WMS sample ports used throughoutthe investigation to sample the exhaust gas . . . . . . . . . . . . . . . . 46Figure 4.3 WMS sample points for SCRE exhaust . . . . . . . . . . . . . . . . . . 48Figure 4.4 WMS P&ID for sample gas at room temperature . . . . . . . . . . . . 50Figure 4.5 WMS P&ID for hot sample gas. Red line indicates a heated line . . . . 51Figure 4.6 WMS setup with a heated cell on a cart. (1) laser fibre optic cable andcollimating optics on a 5-axis mount, (2) measurement cell with heattape and insulation, (3) photodetector, (4) laser current and temperaturecontrollers and photoamplifier, (5) DAQs, (6) vacuum pump, (7) waterknockout and rotameter . . . . . . . . . . . . . . . . . . . . . . . . . 52xiiiFigure 4.7 A methane concentration calibration graph correlating the WMS 2 fsignal to the methane concentration at room temperature and pressurefor 508 mm pathlength cell . . . . . . . . . . . . . . . . . . . . . . . . 54Figure 4.8 Comparison of exhaust methane concentration readings from the WMSand the FID sensors for steady-state SCRE engine operating points.The WMS sampled the exhaust gas downstream of the back-pressurevalve near where FID pulled its sample. The sample exhaust gas wascooled and condensed before entering the WMS cell. . . . . . . . . . . 56Figure 4.9 2 f WMS signal for 504 ppm CH4 balance N2 standard gas after inverseFFT at room temperature and pressure. The 508 mm measurement cellwas used. The signal strength of peak 2 f and noise floor are highlightedwith the dashed lines. . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Figure 4.10 Inverse FFT 2 f signals are averaged over different numbers of rampperiods to show the effect of averaging on the noise floor. The signalis obtained by measuring SCRE engine exhaust containing about 100ppm methane with the WMS producing a 10 Hz ramp scan. (T = 352K, P â 1 atm) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Figure 4.11 Temperature calibration of WMS was performed by measuring 504ppm methane calibration gas from room temperature to 79â¦C . . . . . . 62Figure 4.12 Steady-state SCRE exhaust methane measurement comparing the WMSwith the heated cell against the FID. The WMS sampled exhaust di-rectly from the exhaust port, while FID sampled far downstream of thesurge tank and the back pressure valve. . . . . . . . . . . . . . . . . . . 63xivFigure 4.13 Time response to a step input at t = 0 with experiment and simulationof WMS concentration at sampling point upstream and downstream ofthe exhaust surge tank. The WMS concentration is given relative to thesettled final concentration. . . . . . . . . . . . . . . . . . . . . . . . . 67Figure 4.14 A comparison of photodetector signal and second harmonic signal withand without vibration noise. Measurement was taken with 500 ppmCH4 gas on the WMS mounted to the SCRE frame. . . . . . . . . . . . 68Figure 4.15 A dynamic measurement with CH4 measurements from WMS and FID.IMEP and maximum cylinder pressure are calculated from high-speedin-cylinder pressure transducer. Engine at a low load, high EGR (35%)operating point. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69Figure 4.16 Comparison of step response of WMS to FID to disabling the secondNG injection in SCRE. Corrected signals are shown with the time do-main shift by the amount of the gas transport time delay for each sys-tem. WMS was mounted at the exhaust port, while FID was samplingfrom far downstream of the surge tank. . . . . . . . . . . . . . . . . . . 70Figure 4.17 Uncorrected WMS signal compared with FID signal corrected for timedelay only and first-order response + time delay. . . . . . . . . . . . . . 72Figure 4.18 Methane concentration at various loads in the exhaust of a 4.5 MWdual-fuel marine engine with NG port-injection . . . . . . . . . . . . . 73Figure 4.19 Methane concentration measurement for a duration of one whole sail-ing that lasted three hours . . . . . . . . . . . . . . . . . . . . . . . . . 74Figure D.1 COV of maximum in-cylinder pressure and methane concentration . . . 95Figure D.2 COV of IMEP and methane concentration . . . . . . . . . . . . . . . . 95xvList of AbbreviationsC2H2 acetylene.C2H6 ethane.C3H8 propane.CH4 methane.CO2 carbon dioxide.CO carbon monoxide.H2O water.N2 nitrogen.NH3 ammonia.NOx nitrogen oxide.O2 oxygen.SOx sulphur oxide.ADC analog-to-digital.aTDC after top dead center.BP back pressure.DA direct absorption.DAQ data acquisition.DFB distributed feedback.xviDIDF diesel-ignited dual-fuel.EGR exhaust gas recirculation.FFT fast Fourier transform.FID flame ionization detector.FMS frequency modulation spectroscopy.FTIR Fourier-transform infrared.FWHM full width at half maximum.GHG greenhouse gases.GUI graphical user interface.GWP global warming potential.HITRAN high-resolution transmission molecular absorption database.HPDI High Pressure Direct Injection.ID internal diameter.iFFT inverse fast Fourier transform.IMEP indicated mean effective pressure.IR infrared.LAS laser absorption spectroscopy.LOD limit of detection.LPM litre per minute.NG natural gas.xviiOD outside diameter.PID piping and instrumentation diagram.PAS photoacoustic spectroscopy.PM particulate matter.SCRE single cylinder research engine.SNR signal-to-noise ratio.TDMS Technical Data Management Streaming.TEC thermo-electric cooler.THC total hydrocarbon.WMS wavelength modulation spectroscopy.xviiiAcknowledgmentsFirst and foremost, I would like to express my deepest gratitude to my supervisor, Dr.Patrick Kirchen. This project would have not been possible without you, and I am forevergrateful for your continued guidance and support since my junior year of undergrad atUBC. Your passion is truly inspiring and your positive attitude made my degree a wholelot more enjoyable.David Sommer and Miayan Yeremi, thank you for being great colleagues and men-tors. I enjoyed the journey of developing the WMS sensor together. I want to thank ReneZepeda and Michael Karpinski-Leydier for helping me with running the SCRE engine.Thank you Dr. Steve Rogak for helping us in the early stage of WMS development. Also,many thanks to Jeremy Rochussen, Mahdiar Khosravi, Pooyan Kheirkhah, Jeff Meiklejohn,Aditya Prakash-Singh and other colleagues at the Clean Energy Research Centre.I would like to acknowledge financial sponsorship from the Natural Sciences and En-gineering Research Council of Canada (NSERC) CREATE in Clean Combustion Enginesand Discovery Grant Program. The cooperation of the vessel operator is also appreciated.Finally, I could not have done this without love and support from my friends and family.Thank you for always believing in me and I love you all.xixTo my ever-loving parentsxxChapter 1IntroductionAmid accelerating climate instability around the world, a low carbon fuel is necessary toreduce the substantial carbon footprint produced by the transportation sector. Natural gasis a promising alternative fuel with lower carbon dioxide and nitric oxide emissions, butmethane in natural gas is a potent greenhouse gas. To develop cleaner engines, a fast,robust, and inexpensive methane sensor is needed to monitor methane emission in real-world operating conditions. This thesis demonstrates the development and application ofthe wavelength modulation spectroscopy (WMS) sensor for detecting methane in the ex-haust of natural gas/diesel dual-fuel engines. This chapter provides the motivation andbackground for the work discussed in this thesis by introducing the environmental impactof methane emission and the changing landscape of greenhouse gas emission regulations.Following the background information, the objective and the approach to the work are alsodiscussed.1.1 Greenhouse Gas Emissions in the TransportationSectorThe global average concentrations of the three dominant greenhouse gases (GHG) - car-bon dioxide, methane and nitrous oxides - have increased steadily over the past decades,resulting in a 43% increase in radiative forcing by GHGs since 1990. [2] The transporta-1tion sector is a major source of GHG emissions in Canada, accounting for 24% of nationalemissions in 2017. [11] GHG emissions from the transportation sector grew 43% between1990 and 2017, and the growth is attributed to an increase in the use of freight trucks, withtheir emissions tripling in the same period. [11] Fueled by the growing economy and in-creasing movements of goods, GHG emissions from the transportation sector are expectedto continuously grow internationally.With rising GHG emissions from freight transportation, engine developers are inves-tigating and adopting alternative fuels to reduce GHG emissions. The heavy-duty appli-cations, such as freight trucks, marine, and buses, are predominantly powered by dieselcompression-ignition engines due to high torque, fuel economy and reliability. However,natural gas (NG) is an attractive alternative fuel for internal combustion engines due toits lower specific CO2 and NOx emissions, higher knock resistance, more geographicallydistributed reserves, and generally lower cost compared to diesel. [7]1.2 Natural Gas as a Fuel for Heavy-duty EnginesNatural gas is composed predominantly of methane, making up more than 90% of naturalgas. [55] The rest of natural gas contains light hydrocarbons (ethane, propane, butanes,and pentanes), carbon dioxide, helium, hydrogen sulfide, and nitrogen. Natural gas (NG)has a low sulfur content, and methane has a high hydrogen-to-carbon ratio, resulting intypically lower particulate matter (PM), carbon dioxide (CO2), sulphur oxide (SOx), andnitrogen oxide (NOx) emissions associated with lean, premixed combustion strategies. [28]Marine vessel operators can benefit from these lower emission characteristics of naturalgas to comply with the stricter sulphur limit in marine fuels set by International MaritimeOrganization. In addition to potentially cleaner combustion, NG is generally lower in costrelative to other petroleum-based fuel on an energy basis. [53] These benefits make NG anattractive fuel for heavy-duty vehicle operators as it lowers operating costs and often meets2stricter emission standards.Methane, while providing clean combustion, is a strong greenhouse gas with globalwarming potential (GWP) 28 times that of carbon dioxide in 100 year horizon. [35] Over60% of current methane emission is attributed to anthropogenic sources such as energyproduction, transportation, and agriculture. [9] With methane responsible for about 25%of global radiative forcing, monitoring methane emission is critical in reducing the GHGemission. [35]Newer emission standards limit the amount of tailpipe methane emission from dual-fuel vehicles. The EURO VI emission standard for heavy-duty vehicles limits the methaneemission to 0.5 g/kWh in transient testing for natural-gas powered engines as shown inTable 1.1. In a comparison of emissions from diesel and gasoline engines under the stan-dard driving cycle tests in the lab and on the road, the investigation showed that manyvehicles exceeded their certified emissions limits when driven on roads. [40] In light ofdiscrepancy between in-lab and on-road emissions in modern vehicles, on-road emissiontesting with realistic operating conditions is crucial in evaluating more accurate GHG emis-sions.Engine crankcase ventilation and engine tailpipe are two major sources of methaneemission which account for 70% of pump-to-wheel methane emission from heavy-dutydual-fuel vehicles. [8] If a small amount of CH4 passes through an engine without com-busting and gets emitted into the atmosphere, the lower CO2 emission from NG can easilybe offset by CH4 in terms of total CO2 equivalent (CO2e) emission, which is defined as theamount of CO2 with an equivalent radiative forcing effect. In a study comparing diesel andafter-market dual-fuel heavy-duty engines, transient cycle testing showed that the dieselengines emitted undetectable level of methane while dual-fuel engines emitted 9-33 g/kmof CH4, which resulted in 50-127% higher CO2e emission in dual-fuel engines despite3Table 1.1: Euro VI and EPA emission standards for heavy-duty vehicles limiting car-bon monoxide (CO), non-methane hydrocarbon (NMHC), methane (CH4), nitro-gen oxide (NOx), particulate matter (PM) and fuel sulphur content. [39, 54]Species Euro VI T(g/kWh)Euro VI SS(g/kWh)EPA(g/bhp-hr)CO 4.0 1.5 15.5NMHC 0.16 0.13 0.14CH4 0.5NOx 0.46 0.4 0.2PM 0.01 0.01 0.01Fuel Sulphur (ppm) 10 10their 9% lower CO2 emissions. [50] This study highlights the importance of controlling theCH4 combustion and emissions in dual-fuel engines to lower vehicle GHG emissions, andthis necessitates a fast sensor that can detect methane in the exhaust under realistic in-useconditions.1.3 ObjectivesThe main objective of this project is to develop, characterize and demonstrate a high-speedmethane sensor for exhaust measurement of dual-fuel engines. More specifically, this thesisaims to:â¢ Define performance requirements for the methane sensor for engine exhaust-streammeasurementsâ¢ Develop a fast, low-cost, and robust methane sensorâ¢ Assess the performance, accuracy, and uncertainty of the developed sensorâ¢ Demonstrate the utility of the sensor in exhaust-stream engine measurements41.4 ApproachThe approach adopted to attain the specified objectives is detailed in the main body of thisthesis. A review of dual-fuel combustion, current CH4 measurement techniques, and ab-sorption spectroscopy theories was performed and presented in Chapter 2. A survey ofstate-of-the-art methane sensors and the relevant background theories of absorption spec-troscopy provides an insight on successful designs and technical challenges. Following thereview of literature, the development of a methane sensor based on wavelength modula-tion spectroscopy is discussed in Chapter 3, starting with the selection process for the lasercomponents and the mechanical design. A prototype WMS sensor with laser, optical, andmechanical systems was built, the operating parameters were optimized and the necessarysoftware for controls and analysis was developed. Following the development of a workingWMS sensor and tests with calibration gas standards, the methane sensor was installed on aresearch engine to test the sensor with realistic exhaust gas. The performance of the WMSmethane sensor was characterized and the measurement accuracy was validated against theexisting slower methane sensor. The experiments and performance characterization of theWMS sensor are detailed in Chapter 4. Following in-lab experiments, the utility of theWMS sensor was demonstrated in a field measurement to characterize the methane emis-sion from a coastal vessel and bring a contribution to reducing a real-world GHG emission.5Chapter 2A Review of Methane Emissions andMeasurement TechniquesIn light of increasingly strict emissions regulations with more scrutiny over other GHGin addition to CO2 as discussed in Section 1.2, natural gas is a promising alternative fuelwith generally lower PM, CO2, NOx, and SOx emissions from combustion. However, theuncombusted CH4 emission can offset the benefit of potentially cleaner combustion due tomethaneâs high GWP. This necessitates a fast sensor that can detect methane in the exhaustto monitor the emission during transient operations and reduce methane emission. Thepurpose of this chapter is to:1. Summarize the mechanism of CH4 emission in engines and provide the typical ex-haust gas conditions2. Review the fundamentals of different methane sensors and compare the measurementmethods for exhaust emission measurement3. Survey the current absorption spectroscopy sensors for detecting gas species andpresent the state of the art4. Provide background theories of absorption spectroscopy and introduce wavelengthmodulation spectroscopy62.1 Dual-fuel Natural Gas/Diesel EngineIn dual-fuel natural gas/diesel engines, much of the energy comes from the combustion ofnatural gas (NG) while pilot diesel fuel provides an ignition source. [21] Two types ofdual-fuel strategies were encountered in this work, and will be referred to as diesel-igniteddual-fuel (DIDF) and high pressure direct injection (HPDI). In DIDF engines, natural gasis injected in the intake manifold and diesel is directly injected in the cylinder by common-rail diesel injectors. In HPDI engines, both natural gas and diesel are directly injected intothe cylinder.Three main mechanisms of the incomplete combustion of CH4 resulting in tailpipemethane emissions in DIDF are: (1) intake and exhaust valve overlap causing some ofthe air fuel mixture to exhaust directly from the intake port to the exhaust port, (2) flamequenching at cylinder walls and crevices between the piston and cylinder wall preventingcomplete combustion, and (3) low combustion temperature and lean air-fuel mixture limit-ing flame propagation. [50] To identify the exact source of methane emission, the temporalresolution of methane sensors is important to be able to distinguish methane emission atdifferent stages of combustion.In a DIDF engine, substitution ratio and engine load are significant factors that deter-mine the amount of hydrocarbon (HC) emission. [10] Substitution ratio measures howmuch diesel fuel is replaced by natural gas to produce power in an engine. In a test com-paring diesel only operation and DIDF operation on the same engine, higher HC emissionswere observed at lower loads and higher substitution ratio while original diesel operationemitted â¼160 ppm. HC emission of over 20000 ppm was observed at a very low load(brake mean effective pressure = 1.2 bar) and a high substitution ratio (69%). [10] At thisoperation, the gas-air ratio approached the lower limit of flammability. This suggests thatthe upper limit of methane concentration that the WMS sensor will measure is in the range7of 2-3%.The exhaust of dual-fuel engines also contains other gases that might interfere with theCH4 detection. The cross-sensitivity between exhaust gas species need to be performedduring the development and verification of a methane sensor designed for exhaust mea-surements. As a reference, typical diesel exhaust contains about 73.8% N2, 9% O2, 9%H2O, 8% CO2, and less than 1% pollutants, including HC, CO, NOx, and SOx. [33] Thetemperature of the exhaust gas can range from 150-450â¦C for both diesel and dual-fuelengines. [6]2.2 Comparison of Methane Measurement MethodsCurrently, many different methane sensing methods are available, each with its own ad-vantages and disadvantages for exhaust-stream measurements. Brief explanations of eachtechnology will follow.2.2.1 Direct Laser Absorption SpectroscopyLaser absorption spectroscopy (LAS) uses lasers and detectors to measure the change inthe intensity of the laser passing through the absorbing gas species. [26] Direct absorption(DA) spectroscopy is a simple version of LAS in which the wavelength of the light sourcescans over an absorption feature of a gas species. Noda et al. used the DA techniqueto measure methane gas concentration with a laser diode at around 1300 nm. [36] Thismeasurement utilized a reference cell with a known concentration of methane, and thewavelength of the laser was scanned at 0.02 Hz. The measurements showed that methaneconcentration was proportional to absorbance within a methane concentration of 5-90 vol%.The advantages of DA include the high selectivity of the gas species, good sensitivity,and small system size. However, noise in the optical system can easily reduce the sensi-8tivity since DA relies on measuring small changes in laser power. DA suffers from variousnoise sources such as beam deviation, laser-power drift, and beam scattering, all of whichcan be avoided with a modulation technique. [56] Therefore, DA is usually limited to adetection of gas species for high absorbance applications. The detection limit of DA canbe improved by: (1) increasing path length to increase the absorbance, (2) selecting a moreprominent absorption feature at a different wavelength, and (3) utilizing a wavelength/fre-quency modulation technique to decrease noise.2.2.2 Modulation Spectroscopy (WMS/FMS)Modulation spectroscopy uses a faster modulation signal on top of the slower scanningwaves to enhance the sensitivity of DA. This technique allows for sensitive measurementsof gases such as CO, CO2, H2O, CH4, NH3, C2H2 and many more on the ppm scale.[46] Common modulation techniques involve either modulation of the wavelength or thefrequency. Wavelength modulation spectroscopy (WMS) modulates the wavelength at afrequency usually in the range of 1-100 kHz by modulating the laser injection current,since changing the injection current of the laser diode varies the output wavelength. [46]The wavelength modulated at high speed interacts with the absorption feature at harmonicsof the modulation frequency. This increases the frequency at which the absorption signalis detected thus rejecting any low frequency noise and lowering the detection limit. [27]Frequency modulation spectroscopy (FMS) is similar to WMS in terms of the exper-imental setups employing similar superposed modulation signals. The only difference isthe speed at which the wavelength is modulated since FMS modulates the laser at a fre-quency greater than the width of the absorption lineshape. [4] The modulation frequencyin FMS generally increases to the MHz range or above, compared to the kHz range inWMS. [46] For example, Uehara and Tai demonstrated the FMS methane detection systemwith a modulation frequency of 5.35 MHz using a DFB laser with its centre wavelength9locked at 1.6659 µm. By averaging 2 f signal over 1.3 s, the system achieved the detec-tion limit of 0.3 ppm·m. [52] The high modulation speed of FMS requires more expensivehardware such as high bandwidth photo-detectors and electronics like a lock-in amplifierand DAQ.2.2.3 Photoacoustic Spectroscopy (PAS)Photoacoustic spectroscopy utilizes the photoacoustic effect to detect and measure gasesby exposing the sample gas to a pulsed IR light source which causes the gas molecules toexpand and contract at resonance to produce an acoustic signal. [22] The target gas speciesabsorbs the IR radiation of a specified wavelength, the temperature and pressure increasein the measurement chamber, and sensitive microphones can detect the audible pulses. Theamount of radiation absorbed is proportional to the number of the target gas moleculeswhich is proportional to the magnitude of the pressure pulses.PAS is capable of detecting very low concentrations of target gases due to its stabilityand low cross-sensitivity. [22] The sensitivity of PAS system is usually limited by themicrophone, and signal saturation can affect the signal amplitude reducing sensitivity. [22,29]2.2.4 Fourier Transform Infrared Spectroscopy (FTIR)Unlike DA which uses a monochromatic light source, Fourier transform infrared spec-troscopy (FTIR) uses a broadband light source to scan through a wider range of wave-lengths and measure absorbance. [34] A Michelson interferometer produces an interfero-gram of different wavelengths by moving a mirror and creating different interference pat-terns caused by the difference in the path lengths of two split beams. By performing aninverse Fourier transform, the interferogram is converted to a transmission spectrum whichcontains the absorption data.10The ability to analyze multiple compounds with low noise is an advantage of FTIR,but the library of spectra limits the compounds detectable by this technique. The temporalresolution is lower compared to WMS or FMS at under 100Hz, and the sensitivity is lowerthan PAS using a laser light source. [37] Also, the volume of the FTIR gas cell required formultiple passes of IR beam results in a gas residence time which is much greater than thetemporal resolution, which obscures the time-resolved data. [57]2.2.5 Flame Ionization Detector (FID)A flame ionization detector (FID) measures hydrocarbon concentrations by ionizing thesample gas with a hydrogen flame and measuring the resultant current. [14] As the sampleflows through the hydrogen flame, the carbon molecules are broken to positively chargedions. The generation of these ions is proportional to the amount of carbon atoms in thesample. The positive and negative electrodes generate an electric field and drive the ionsresulting in a measurable current.FID reacts mostly only to organic compounds, and it can measure hot and wet samplesso no preparation to the sample gas is required. However, as the sensor reports the totalhydrocarbon content in terms of an equivalent propane content, FID cannot distinguish thetypes of detected hydrocarbons. [57] Furthermore, FID has a response time in seconds andrequires a constant supply of hydrogen gas to sustain the flame, thus making it challengingto package the sensor into a small portable form factor. [14]2.2.6 Summary of CH4 Measurements TechniquesMany technologies provide the ability to monitor methane emission, each with advantagesand limitations. Characteristics of the different methods of measuring methane discussedabove are summarized in Table 2.1. All the techniques are suitable for exhaust measure-ment, but only WMS and PAS can achieve the high speed required for transient measure-11mentsTable 2.1: Summary of CH4 measurement techniquesSensor Type Advantage LimitationDirect Absorption(DA)Simple setup, lowcross-sensitivityLow sensitivity and speedWavelength orFrequency ModulationSpectroscopy(WMS/FMS)Simple setup, highsensitivity, high temporalresolution (>1kHz)Sensitivity depends on pathlengthFourier TransformInfrared Spectroscopy(FTIR)Multiple compoundsmeasured in one scanLimited speed (<20Hz), canonly measure species in thelibraryPhotoacousticSpectroscopy (PAS)Sensitive and selective Limited by microphoneperformance and laser powerFlame IonizationDetector (FID)Robust, no samplepreparation neededLimited response time,cannot distinguish differenthydrocarbons, hydrogen gasconsumptionElectrochemical Sensor Inexpensive and compact Long response time (inseconds), not specific tomethaneLAS is an inexpensive technique with high sensitivity, selectivity, resolution, speed,and portability allowing it to be employed directly in field. [19] More specifically, WMSoffers a good balance between performance and cost with its low limit of detection and anadequate temporal resolution for cycle-resolved methane measurements. For these reasons,WMS was selected for exhaust methane measurements and will be discussed in greaterdetail in the following sections.122.3 Absorption Spectroscopy TheoryThe purpose of this section is to provide readers with the background theory of absorptionspectroscopy necessary to understand subsequent discussions about developing a WMSmethane sensor. A detailed derivation and discussion of absorption theory can be found inHansonâs spectroscopy text [18].2.3.1 Absorption Coefficient and the Beer-Lambert LawWhen a collimated light enters a gas sample, the amount of absorption, reflection, scat-tering, and transmission sum to 100%. As the amount of reflection and scattering in thegas species can be assumed negligible in spectroscopy, a light through a gas sample can bedescribed asAν +Tν = 1 (2.1)where Aν and Tν are unitless variables, spectral absorptivity and transmissivity, respec-tively. If the absorptivity is expressed in terms of an absorption coefficient kν and theincident light spectral intensity Iν ,Aν = kνdx =âdIÎ½IÎ½(2.2)Spectral intensity denotes power per unit area at frequency ν . By rearranging, the spectralabsorption coefficient kν is defined as the absorption of spectral intensity per unit length,orkν =âdIÎ½/dxIÎ½ [cmâ1] (2.3)If a gas sample is assumed to be a black body, its absorptivity equals emissivity. Therefore,the energy balance equation for radiation across a slab of gas of width L can be expressed13as a difference between emission and absorption, ordIν = Aν(Ibbν â Iν) = kνdx(Ibbν â Iν) (2.4)where Ibbν is the black body spectral radiancy which is essentially intensity from an emis-sion. The integral form of Eq. 2.4 isIν(L) = I0Î½exp(âkÎ½L)+ Ibbν [1â exp(âkÎ½L)] (2.5)where I0ν is an incident intensity at x = 0 (or before the light enters a slab of absorptivegas). Since the black-body radiation is much smaller than the incident light in an absorptionspectroscopy, Eq. 2.5 simplifies toIν(L) = I0Î½exp(âkÎ½L) (2.6)This equation is the Beer-Lambert Law which is the fundamental relation in absorptionspectroscopy. Without further discussions about the Einstein Theory of Radiation, the Beer-Lambert law can also be expressed as(II0)ν= exp(âkÎ½L) (2.7)= exp(âSÏÎ½PiL) (2.8)where S is linestrength [cmâ2atmâ1], ÏÎ½ is frequency-dependent lineshape function [cm],Pi is partial pressure of species i [atm], and L is pathlength. Converting absorption intoabsorbance α is more convenient for spectroscopy because it is the exponential term in the14Beer-Lambert Law. Absorbance is defined asα(ν) = SÏÎ½PiL = SÏÎ½PXiL (2.9)where Xi = mole fraction of the absorbing species i.2.3.2 Linestrength and Lineshape FunctionLinestrength S describes the probability of an absorptive transition between two energystates at a given wavelength. Linestrength depends on the temperature, and the linestrengthSi(T ) for a transition i at some temperature T can be calculated asSi(T ) = Si(T0)Q(T0)Q(T )(T0T)exp[âhcEâ²â²ik(1Tâ 1T0)]Ã[1â exp(âhcÎ½0,ikT)][1â exp(âhcÎ½0,ikT0)]â1(2.10)where S is linestrength in [cmâ2atmâ1], Q(T ) is the absorbing moleculeâs partition func-tion, ν0,i is frequency of the transition, E â²â²i is the lower-state energy of the transition, h isPlanckâs constant, k is Boltzmann constant, and c is the speed of light in cm/s.It is useful to note that HITRAN (high-resolution transmission molecular absorptiondatabase) online provides a per-unit-number density version Sâ in [cmâ2atmâ1]. The de-tails and examples of Eq. 2.10 can also be found in HITRAN website. To convert betweenS and Sâ at T = 296K, the equation isS = Sâà (2.4797Ã1019)[cmâ2atmâ1] (2.11)For an ideal gas without any broadening effect, the lineshape would be infinitely narrow,described with a delta function. However, real transitions involve some broadening due to15the Uncertainty Principle and temperature and pressure effects. Lineshape function Ï is anormalized parameter describing the broadening of the absorption line and is defined asÏ =kÎ½â«line kνdv(2.12)such that â«lineÏdÎ½ = 1 (2.13)Broadening is characterized by the maximum height Ï(Î½0) at the center frequency ν0and the width at half the maximum value âÎ½ which is referred as full width at half maxi-mum FWHM. Doppler broadening and collisional broadening, two important mechanismsthat influence the lineshape in spectroscopy, will be discussed in the following sections.2.3.3 Doppler BroadeningThe gas molecules are always in motion and their velocity component in the direction ofthe collimated laser light will shift the frequency at which the light is absorbed. This distri-bution of velocities causes Doppler broadening which is affected by temperature. The fre-quency broadening due to Doppler effect, âÎ½D , is calculated with the line shapeâs halfwidth(FWHM) asâÎ½D = ν0(8kT ln2mc2)1/2(2.14)where T = temperature [K], k = Boltzmann constant, and c = speed of light in cm/s, ormore conveniently,âÎ½D = Î½0(7.1623Ã10â7)(TM)1/2(2.15)where M is the molecular weight in g/mole.162.3.4 Collisional BroadeningCollisional broadening (or pressure broadening) occurs due to molecules colliding witheach other and transferring energy that changes the state of each molecule. The frequencybroadening due to collisions, âÎ½C , is calculated asâÎ½C = PâAXA2Î³BâA (2.16)where P = total pressure, XA = mole fraction of species A, and Î³BâA = broadening coeffi-cient for B-A collision.B is the molecule whose lineshape is being studied and A is the molecule that speciesB interacts with. For example, if methane mixed with air is being studied, the equation forâÎ½C would look likeâÎ½C = P[XCH42Î³CH4âCH4 +Xair2Î³CH4âair] (2.17)The broadening coefficient γ is temperature dependent and can be modelled with an ap-proximation2γ(T ) = 2γ(T0)(T0T)N(2.18)where T0 = the reference temperature, and N = the temperature coefficient (usually < 1).2.3.5 VoigtThe lineshape due to only Doppler broadening or only collisional broadening can be ex-pressed as the Gaussian lineshape function or the Lorentzian lineshape function, respec-tively. More commonly and accurately, the Voigt profile, which is a convolution of the17Gaussian and Lorentzian functions, is used to describe the lineshape function when bothDoppler and collisional broadening are significant. If the two broadening effects are as-sumed to be independent, the lineshape function is a convolution of Doppler and collisionalbroadening as expressed by the Voigt lineshape function ÏV ,ÏV (ν) = ÏD(Î½0)apiâ« âââexp(ây2)dya2+(wâ y)2 = ÏD(Î½0)V (a,w) (2.19)where V (a,w) is a Voigt function. The parameters are defined as:Voigt âaâ parametera =âln2âÎ½CâÎ½D(2.20)which can be seen as the relative significance of Doppler and collisional broadening;nondimensional line position, ww =2âln2 (Î½âÎ½0)âÎ½D(2.21)which calculates the offset from the linecenter;linecenter magnitude, ÏD(Î½0)ÏD(Î½0) =2âÎ½Dâln2pi(2.22)which is the magnitude of the Doppler line shape at the linecenter;integral variable, yy =2uâln2âÎ½D(2.23)Several approximation techniques for the Voigt function were published. [3, 15, 16] Thenumerical approximation in the paper by Humlicek has been used for simulations in thisthesis due to its easily implementable algorithm. In extreme conditions where temperature18and pressure are far from the room condition, the assumption of independence between theDoppler and collisional broadening can produce a significant error which can be correctedby using a more suitable profile for that condition. The derivation of various equationsintroduced above and discussions about some other profiles are found in [18].2.3.6 Absorbance SpectrumThe absorbance equation in Eq. 2.9 is specific to individual transition linestrength. Sincethe absorbance of different transitions can overlap due to broadening, the absorbance at agiven wavelength is the sum of the absorbances of different transitions in the region. Forexample, the absorbance of methane at a given frequency isα(ν) =âjS j(T )Ï j(ν ,T,P,Ï)PÏCH4 L (2.24)where j indicates a given transition. This total absorbance should be used in the Beer-Lambert law, which simplifies toIν = I0ν exp(âÎ±Î½) (2.25)Equation 2.24 and 2.25 are the fundamental equations that describe the interaction be-tween the laser and the absorbing gas that can be detected by a photodetector. These equa-tions form the basis for WMS and the work discussed in the following chapters of thisthesis.2.4 Recent Development of WMSModulation spectroscopy with semiconductor lasers in near and mid-IR is a common tech-nique used to monitor trace gases. Modulation spectroscopy, specifically WMS, is selectedas the technique to monitor methane emissions from dual-fuel engines because of its tem-19poral resolution, selectivity, sensitivity and simple setup. Modulation spectroscopy retainsDAâs ability to measure methane with low cross-sensitivity, and improves upon it withthe noise-rejection characteristics and a higher sensitivity. In this section, a review of thecurrent state of WMS sensors and their applications will be discussed. An application ofWMS for exhaust-stream methane measurement for internal combustion engines has notbeen documented. Therefore, a few relevant works on absorption spectroscopy for engineapplications will be presented.The improvement in sensitivity of modulation spectroscopy over DA arises from thelaser wavelength modulation by applying a superimposed sine and ramp signal to the in-jection current of the laser diode. As the wavelength scans through the absorption line, thetarget gas partially absorbs the modulated laser beam and generates signals at harmonics ofthe applied modulation frequency . [46] Following the Beer-Lambert law, the harmonicssignal due to the light absorption in the laserâs path is proportional to the concentration ofthe target gas.The frequency of the laser for a WMS system with modulation frequency f can beexpressed asν(t) = Î½¯(t)+acos(Ït) (2.26)where ν is the laser frequency and Ï is the angular frequency (2pi f ). The laser intensitycan be written asI0(t) = I¯0[1+ i0cos(Ït+Ï1)+ i2cos(2Ït+Ï2)] (2.27)where I is the laser intensity, i0cos(Ït +Ï1) and i2cos(2Ït +Ï2) are linear and nonlin-ear components with i as an intensity modulation amplitude and Ï as phase shift betweenfrequency and intensity modulations. The process of using these equations to find a trans-20mission coefficient and harmonic signals is detailed in [41] along with simulations andexperiments.Rieker et al. demonstrated a WMS sensor mounted in a spark plug to measure temper-ature and water concentration in internal combustion engines. [42] The sensor uses 6 mmpathlength to measure water concentration and temperature inside the cylinder at a tem-poral resolution of 7.5 kHz. Gurneesh et al. demonstrated a high-speed direct absorptionmeasurement of water vapour at the intake manifold. [17] Yamakage et al. applied directabsorption spectroscopy for in situ measurement of exhaust gases throughout the exhaustpipe. [58] This direct absorption spectroscopy applications uses a unique sensor designto increase pathlength by using mirror to achieve a multi-pass laser beam path. As DAand WMS use similar optical setup, these applications are relevant in developing a WMSsensor. WMS is an appealing gas measurement technique which has not been applied to ex-haust methane characterization, and this work aims to take advantage of its high sensitivity,speed, signal-to-noise ratio, and robustness for dual-fuel engines diagnostics.21Chapter 3Development of WMS SensorExhaust-stream methane measurement for dual-fuel engines imposes specific requirementsfor the sensor such as a concentration range, temporal resolution, and absence of interfer-ence from other exhaust gases. The methane should resolve a methane concentration ofbelow 100 ppm for HPDI engines and over 3% for DIDF engines. A temporal resolutionof over 50 Hz is required for cycle-resolved measurements, and the measurements shouldbe insensitive to non-methane exhaust gas species. Developing a WMS methane sensorinvolves choosing a laser centre wavelength and designing a mechanical and electrical sys-tem around the tunable diode laser. The mechanical system holds the optical componentsand sensors in place, passes the sample gas through the measurement cell, and providesconvenient ways to control the gas flow. The electrical system modulates the light source,detects the transmitted light intensity, and records various signals. This chapter explains theprocess behind selecting a suitable laser wavelength for spectroscopy, specifying laser con-trol devices, and designing a gas measurement cell. A combination of methane absorbanceand laser pathlength determines the lower detection limit of the WMS system. A WMSsystem intended for detecting a very small concentration of methane needs to maximizethe pathlength. The temporal resolution and methane detection limit were key influencersin designing a fast and responsive WMS system.223.1 Wavelength SelectionThe methane linestrength and interference with other exhaust gases are the two most im-portant factors when selecting the wavelength of the laser. The methane linestrength de-termines the absorptive power at a specific wavelength and determines the lower limit ofdetection of the system for a given pathlength. The wavelength should be selected to maxi-mize the methane linestrength while minimizing linestrength for other exhaust gases. From500 nm to 5000 nm, three absorption wavelength regions (1650 nm, 2300 nm, and 3200nm - 3400 nm) have a high methane linestrength of over 10â21 cmâ1/(molec * cmâ2).Figure 3.1: Methane linestrength at room temperature from 800 nm to 4500 nmWMS applications using a wavelength of around 1650 nm are common for methanedetection. [23, 24, 43] Despite methane having stronger absorption peaks at higher wave-lengths such as 2300 nm and 3300 nm, a wavelength of under 2000 nm was chosen forthis application due to the availability and cost of lasers and the more linear power-to-wavelength characteristics of the laser at this wavelength. To put the price difference into23perspective, a 3270 nm laser costs almost three times more than a 1651 nm laser, whichis already one of the most expensive components in the system. Operating in the nearIR wavelength provides the added benefit of inexpensive optical components due to thecommon applications for telecommunication.The linestrength of other exhaust gases should be considered to ensure the laser isabsorbed mostly by methane gas molecules so that the CH4 detection is not affected by thepresence of other species. The linestrengths of exhaust gases like H2O, CO2, N2O, CO,O2, NO, SO2, NO, N2, and C2H6 are investigated in addition to the methane gas. Figure3.2 shows the linestrength of exhaust gas species from 1600 nm to 1700 nm. This figurereveals that a wavelength in between 1620 nm and 1670 nm could potentially provide astrong methane absorption peak with weak absorption from other exhaust gases.Figure 3.2: Exhaust gas linestrength at room temperature from 1600 nm to 1700 nmSince the actual absorbance is the sum of all linestrengths including temperature andpressure broadening effects, the absorbance of the exhaust gases is calculated in the region24of interest as shown in Figure 3.3. The absorbance was calculated using HITRAN data withgases at 300 K, 1 atm, and a pathlength of 50 cm. Although a higher concentration of H2Oand CO2 is typically present in exhaust gas (up to around 10% by volume), their absorptivepower is insignificant because the absorbance of CH4 is at least 1000 times stronger thanother relevant exhaust gases near 1650 nm. [6] In the operating condition of the WMSmethane sensor, other exhaust gases, including CO2 and H2O, do not interfere with themethane peaks due to the relative difference in absorbance and the sufficient gap betweentheir centre wavelengths.Figure 3.3: Absorbance of the main absorbing exhaust gas species near 1650 nm atroom temperature and atmospheric pressure.By adjusting the temperature and injection current of the laser diode, the center wave-length can be fine-tuned so that the range of wavelength modulation contains these strongabsorption peaks. While the centre wavelengths of 1648, 1651, and 1654 nm are suitablefor exhaust-stream methane measurement, the 1651 nm distributed feedback (DFB) diode25laser was chosen due to its availability.3.2 Hardware DesignThis section discusses the hardware for the WMS setup and the selection process behindit. The laser design, data acquisition, and mechanical systems are explained in detail. Acustom laser diode was used, and the remaining optical components were purchased offthe shelf to minimize the cost and time investment. Modularity and performance wereprioritized when designing the gas cell since the cell length can be optimized for differentapplications to balance between the detection limit and the response time.3.2.1 Laser ComponentsLaser components are the core of a WMS methane sensor, and they comprise of a lightsource, detector, and other complementary components to help control the laser and condi-tion the signal. A tunable diode laser with a centre wavelength of 1651 nm is not commer-cially available, so a custom 10 mW DFB diode laser (Norcada) was ordered. The 1651nm DFB laser comes in a 14-pin butterfly packaging with a single-mode fibre optics cou-pling. Within the safe operating conditions, the laser can be tuned to a wavelength between1648 nm - 1652 nm with a combination of different temperature and injection currents ofthe laser diode. The temperature needs to be held constant to achieve a high modulationfrequency for WMS, so the maximum practical scanning window using current modulationis ±0.5 nm which is sufficient to scan across the methane absorption peak near 1651 nm.To drive the laser, the diode temperature and injection current need to be adjusted tocontrol the wavelength and the power output. A Thorlabs LDC202C current controller wasspecified to meet the current demand of the laser diode and provide a modulation of the laserinjection current up to 250 kHz. The modulation frequency should be sufficiently higherthan the ramp speed, therefore the current controller is theoretically capable of achieving26Figure 3.4: Laser component setup: (1) current controller, (2) temperature controller,(3) TEC mount, (4) butterfly-mount DFB pigtailed diode laser, (5) collimatinglens, (6) measurement cell, (7) photodetector, (8) fast multifunction DAQ, (9)PC, (10) photodiode amplifier, (11) slow multifunction DAQ, (12) thermocou-plesTable 3.1: Major optical system components from Figure 3.4. A complete list of com-ponents can be found in Appendix A.Component ModelLight source Norcada DFB Laser NL16510-B(1651 nm, 10 mW)Temperature controller Thorlabs TED200CCurrent controller Thorlabs LDC202CPhotodetector Thorlabs DET10DPhotodiode amplifier Thorlabs PDA200CMultifunction DAQ (fast) National Instruments USB 6361Multifunction DAQ (slow) National Instruments USB 6210a WMS temporal resolution in the range of a few kHz. The laser temperature and currentcontrollers connect to the laser diode via a thermoelectric cooling (TEC) laser diode mount.The laser modulation signal is generated with a multifunction data acquisition card27(DAQ) and LabVIEW, which simplified the hardware setup by reducing the number ofcomponents compared to hardware-based signal generators. The multifunction DAQ andsoftware-based lock-in amplifier negated the need for hardware-based signal generators ora lock-in amplifier, thereby reducing the system cost. [59] A National Instrument USB-6361 DAQ provides analog outputs of up to 2MHz which provides sufficient resolutionto generate a smooth modulate signal for the laser. The maximum achievable temporalresolution of the WMS for each major component is summarized in Table 3.2 to identifythe bottleneck in the system.Table 3.2: Maximum achievable WMS temporal resolution per specification of eachmajor componentComponent Parameter Value Max. WMSLight source(Norcada NL16510-B)Modulationfrequency> 10 MHz > 10 kHzCurrent controller(Thorlabs LDC202C)Modulationfrequency250 kHz 2.5 kHzPhotodetector(Thorlabs DET10D)Rise time 25 ns > 1 MHzPhotodiode amplifier(Thorlabs PDA200C)Analog outputbandwidth500 kHz@ 10mA range> 5 kHzMultifunction DAQ(NI USB-6361)Analog inputsampling rate2 MHz 1 kHzThe light from the optical fibre is collimated with an adjustable aspheric collimatorlens (f = 2.0 mm) into a free beam to pass through the centre of the gas sample chamber.The photodetector on the other side of the sample chamber records the light intensity ofthe transmitted light and outputs a current proportional to the intensity. The photodiodeamplifier converts the output current to a voltage and amplifies the analog signal which is28recorded by the DAQ.3.2.2 Mechanical ComponentsThe mechanical system for the WMS sensor primarily focuses on the design of the gashandling system and the mounting of the laser and optical components. Rigidly mountingthe laser components is critical because the laser must pass through gas in a narrow cell;however, the ability to adjust the source and detector is required for alignment. To achievethis, the end of the fibre optic cable coupled to an adjustable collimating lens is mounted ona 5-axis stage, while the photodetector is solidly mounted to an optical post. This allowsfor enough freedom to align the collimated laser beam through the narrow measurementcell and onto the detector surface .Figure 3.5: Mechanical setup: (1) mounting plate or cage, (2) laser fibre-optic cable,(3) 5-axis mount, (4) collimating lens, (5) Swagelok tee with a window inside,(6) 508 mm measurement cell, (7) photodetector, (8) 750mm optical rail.A rigid system is important to keep components aligned even in vibrating environments.If the mounting plate can easily bend with an external force, the vibrations from the enginecan cause an oscillatory signal fluctuation that interferes with the WMS system. WMS hasan intrinsic ability to filter out minor lower-frequency vibration noises because modula-tion frequency is much higher in the kHz range away from the low-frequency noise. [20]There still exists a limit on how much optical misalignment it can handle since a completemisalignment would result in zero signal. As pathlength increases, vibration becomes a29bigger problem because the light source is farther away from the detector surface and aslight change in the beam direction will be exaggerated. A rigid optical rail can be uti-lized as shown in Figure 3.5 for applications where weight is not important. The rail alsoallows for an easy modification of the pathlength since the location of the components isadjustable depending on the length of the gas cell. If a lower weight is preferred for a fieldmeasurement, components can be solidly mounted to mounting plates. The combinationof a rigid mounting plate, dampening mounts, and a large beam diameter can protect theWMS system from vibration noise.A vacuum pump draws sample gas from the exhaust pipe through the gas cell. Using apump allows for adjustability in the flow rate and the cell pressure by varying the bypassratio and the downstream pressure of the sample chamber. The use of a vacuum pump topull the sample also adds flexibility to the mounting location of the WMS as it no longerrelies on exhaust exhaust pressure to drive the flow. The measurement cell is considered ingreater detail in the subsequent section. Various configurations of piping and instrumenta-tion were used depending on the application. Each experimental setup will be presented inChapter 4.3.2.3 Gas Measurement Cell DesignThe optical pathlength has a direct impact on the WMS signal strength, and the gas sam-ple chamber needs to be designed carefully to provide enough pathlength to ensure thedetection limit of below 100 ppm. However, the length of measurement cell increases thegas residence time, and causes the WMS system to react slower to dynamically varyingmethane concentrations. By calculating the absorbance at the detection limit at a givenpath length, wavelength, and pressure, the impact of the pathlength on the detection limitcan be elucidated.By rearranging the definition of absorbance, the mole fraction can be expressed as30α(ν) = SÏÎ½PXiL => Xi =Î±(Î½)SÏÎ½PL(3.1)where α(ν) = absorbance, S = linestrength, ÏÎ½ = frequency-dependent lineshape function,P = pressure, Xi = mole fraction of gas species i, L = pathlength.The absorbance at the detection limit was experimentally estimated to be 0.00013 usingEquation 3.2.3. A detection limit is defined as the absorbing gas concentration that resultsin a signal-to-noise ratio of 3, and it will be discussed in detail in Section 4.2.3. For a givensystem, a particular absorbance results in the same photodetector signal. Since absorbanceis a function of linestrength, lineshape, pressure, concentration, and pathlength, the CH4detection limit (concentration) for varying pathlengths for a particular absorbance can beplotted as shown in Figure 3.6.Figure 3.6: Detection limit as a function of pathlength for a single-pass WMS forCH4.A small inner diameter (ID) for gas chamber is desirable to reduce the chamber volumeand consequently the time constant; however, a small ID makes the laser beam alignment31more challenging since the beam may hit and reflect off the inner cell wall. Here, internaldiameters of 25.4 mm (1â), 15.9 mm (5/8â) and 9.5 mm (3/8â) were considered.Figure 3.7: Final design of the sample gas chamber with the dimensions in mmFigure 3.7 shows the final gas chamber design with a pathlength of 508 mm and ID of9.5 mm (3/8â). At these dimensions, the chamber volume equates to 36 mL. By using a12.7 mm (1/2â) outside diameter (OD) tube in between two Swagelok tees with windowsheld in by springs, the pathlength and the ID of the gas chamber can easily be changed byswapping out the center tube depending on the applications. This design utilizes off-the-shelf components to build the entire chamber and negates the need for custom fabrication,thus reducing lead time and making the design easily replicated or modified. The windowshave an anti-reflection coating to reduce reflection at around 1650 nm, and one surface iswedged at 0.5â¦ to prevent interference fringes and reflections back to the laser diode. Thewindows seal the measurement chamber with o-rings, and are held in by springs which aretightened by screwing in the outer Swagelok nuts.It should be noted that operating the WMS system in an optically thin region is prefer-able due to the linear relationship between harmonic signals and absorbing gas concentra-tion. The optical depth (absorbance) is the exponential term as shown in Eq. 2.7, and anoptically thin region is characterized by an optical depth kνL or α 1. A multipass cell,32such as a Herriott cell or a circular cell, provides a longer pathlength in a smaller size,but commercially available multipass cells are expensive. In-situ measurement providesthe best response time, but a simple, adjustable, and portable external measurement cellwas a more versatile option. An external cell also provides more controlled pressure ofthe absorbing gas, which could be set to near atmospheric pressure to eliminate pressurebroadening.3.3 Software DevelopmentSignal generation, data acquisition, and post-processing software was developed to controlthe laser, read signals and analyze to obtain the methane concentration. LabVIEW inter-face controls the multifunction DAQ which modulates the injection current and records thevoltage from the photoamplifier. The recorded signal is post processed to demodulate theraw photodiode signal and obtain harmonic signals using a software-based digital lock-inamplifier .The algorithms for this post-processing were developed using Python. Figure 3.8shows the workflow from the signal generation to post-processing.Figure 3.8: An overview of the software workflow shown with sample signals at dif-ferent stages of the WMS system333.3.1 LabVIEW Control SoftwareThe WMS signal generation and data acquisition is performed with NI LabVIEW on a PClaptop. A graphical user interface (GUI) was developed to control the WMS parameterssuch as modulation frequency and amplitude, and data acquisition settings such as samplingrates and channel configurations. The control software interfaces with the NI USB-6361multifunction DAQ to generate a modulation signal for the laser controller and read thephotodiode signal from the photodiode amplifier. The raw photodiode signal is recorded tothe PC to allow for flexibility in the post-processing algorithm. The NI USB-6210 DAQreads slower signals from other additional sensors such as the pressure transducer, thermo-couples, and if equipped, O2 and CO2 sensors. The slow signals are sampled at 10 Hz,which is equal to the ramp frequency of WMS. The LabVIEW GUI communicates withthe DAQs to display real time sensor signals and to simplify post-processing by outputtingthe data to a technical data management streaming (TDMS) file.The software also supports interval logging where a burst of measurements happensevery desired interval. This feature is used for longer measurements in which continuoushigh-speed recording is unnecessary and the long measurement duration would result in alarge output file. The interval logging feature is achieved by saving the burst measurementsas separate time-stamped files in a folder which can be post-processed to output time andsignal.3.3.2 Post-Processing ScriptA post-processing script was developed to read the raw photodiode signal from the TDMSfile and perform the software-based lock-in amplifier function to output the demodulatedWMS signals at different harmonics. By reading the photodiode signal, performing a fastFourier transform (FFT), extracting the harmonics signal and performing an inverse Fouriertransform, the script can perform the same functionality of a hard-ware based lock-in am-34plifier and output the height of the harmonics signal (2f or 3f) which is proportional to themethane concentration as shown in Figure 3.9.The post-processing algorithm is based on the digital detection of harmonics whichis discussed in detail in [32]. The process can be summarized as: (1) perform Fouriertransform on photodiode signal, (2) apply a window function to select a certain harmonicsignal, (3) perform inverse Fourier transform to circulate the Fourier spectrum, (4) calculatethe phase, (5) calculate the maximum absolute value of harmonics signal, and (6) retrievethe harmonics signal from the real part of the absolute value and the phase. Followingthis calculations, the maximum heights of the peaks are recorded so that the signal can beconverted to a methane concentration with the analysis script. The post-processing scriptalso converts the voltages from the complementary sensors to meaningful values with units.Figure 3.9: A simplified workflow of post-processing. (a) Raw photodiode signal. (b)FFT is performed on photodiode signal and a window function is applied. (c)Perform iFFT to get the 2 f harmonic signal.The WMS signals are converted to methane concentration in ppm using calibration,pressure, and temperature data. Concentration calibrations of WMS are performed at roomtemperature and atmospheric pressure with a range of calibration gases depending on theapplication. The signal analysis script creates a calibration curve with the second harmonicsintensity at known calibration concentrations. For measurements with the heated WMS35cell, a temperature correction is applied based on thermocouple measurements to accountfor the Doppler broadening of the methane absorption peak at an elevated temperature.3.4 Signal GenerationFor the WMS method, the laser wavelength must be scanned and modulated, typicallyby varying the injection current. The laser injection current is controlled with the su-perposed signal of a slower scanning ramp wave and a faster sine wave. Modulation oflaser wavelength provides the important noise rejection characteristics which amplifies thesignal-to-noise ratio (SNR) to allow WMS to detect low methane concentration in a noisyenvironment. As the laser wavelength is a function of temperature and current, WMS typ-ically uses current to modulate the wavelength at high speed due to challenges associatedwith fast temperature modulation. The speed and amplitude of the ramp current scan andsine modulation wave determines the overall WMS temporal resolution. This section dis-cusses how the ramp and modulation parameters were selected to optimize the system forincreasing the SNR.3.4.1 Ramp Current ScanThe ramp shape in the laser injection current provides the basis for the laser absorp-tion spectroscopy by scanning the laser wavelength across the absorption peak of the gasmolecules. The wavelength of the laser depends on the diode temperature and the injectioncurrent. The ramp amplitude should be sufficiently large so that the resulting wavelengthrange contains the full width of the absorption peak.The ramp frequency determines the overall temporal resolution of the WMS system.Since a single ramp period passes through the methane absorption peak twice, the methaneabsorption is measured twice per ramp period. Thus, the temporal resolution of the WMSsystem is twice the ramp frequency. A ramp frequency depends purely on the desired36temporal resolution. The maximum temporal resolution of the WMS sensor depends onthe modulation and data logging capacity of the laser electronic and DAQ system. A highramp frequency requires a high modulation frequency and higher sampling rate; therefore,the performance of the DAQ and the current controller determines the maximum rampfrequency. 1 kHz measurements correspond to a methane reading every 7.2 crank angle at1200 rpm.The current modulation is achieved via an external analog modulation signal to thecurrent controller. The analog modulation signal is generated with the DAQ at a rate of 2MHz. The current controller outputs the laser drive current with an external modulationwith the following equation:ILD = ILD SET + ILD MAX âUMOD/10 V (3.2)where ILD is injection current to the laser diode, ILD SET is nominal current value set withthe controller knob, ILD MAX is maximum injection current from controller (200 mA forLCD202C in the constant current mode), and UMOD is modulation signal voltage fromDAQThe ramp amplitude was set at 0.4 V which responds to drive current ramp amplitude of8 mA. With the laser drive current constant of 0.030 nm/mA, the ramp was scanning acrossthe absorption peak at +/- 0.24 nm from the nominal wavelength, which is set by both thecurrent controller and the diode temperature. Figure 3.10 shows the ramp scan amplitudewith respect to the absorbance peak shape.It should be noted that these numbers are based on the test report on the laserâs spec-ification sheet. The laser wavelength was not verified in this investigation, but a largewavelength sweep was performed to tune the laser center wavelength to below the largeabsorbance peak at 1648 nm by manually changing the diode temperature and injection37Figure 3.10: Estimated modulation width displayed over CH4 absorbance at 298 Kand 1 atmcurrent. If performed with methane in the measurement cell, this effectively mimics adirect absorption spectroscopy, and the laser behaviour can be roughly verified.3.4.2 Sine Current ModulationA higher frequency sinusoidal modulation superposed with a ramp wave to provide noiserejection is a key difference between WMS and direct absorption spectroscopy. This sinemodulation overlaid on top of a ramp allows signal processing to shift to a higher frequencydomain which provides rejection of low frequency noises. [47] Operating at a higher fre-quency also results in a greater SNR which improves the detection limit. The sine frequencywhich is 50 times the ramp frequency is sufficiently faster to be a scanned-WMS. [13]The modulation amplitude affects the WMS signal strength as shown experimentally inFigure 3.11. For the second harmonic signal, the WMS signal is maximized at a 0.12 Vmodulation amplitude. For the third harmonic, the WMS signal continues to increase with38increasing modulation amplitude. The shape of the second harmonic modulation sweepagrees with [44].Figure 3.11: Maximum WMS signal locked-in at second and third harmonics depend-ing on the sine modulation amplitude. Measurement is taken with 3916 ppm[CH4] at 17â¦C3.4.3 Sampling FrequencyThe frequency at which the photodiode signal is recorded must be sufficiently fast to resolvethe sine modulation wave. For the WMS application with the sine modulation of 20 kHz,the sampling rate of 400 kHz was chosen. This provides 20 samples per sine period andexceeds the Nyquist criteria.To identify the optimal sampling frequency, a sensitivity study was performed to evalu-ate the effect of sampling frequency on the signal quality and the noise floor. In Figure 3.12,the demodulated second harmonics signal is shown for varying sampling frequencies. Alower noise floor is observed at higher sampling frequencies, but the post-processing scriptwas still able to resolve the relatively low-resolution 200 kHz photodiode signal which is39Figure 3.12: 504 ppm CH4 balance N2 gas standard is measured at various DAQ sam-pling frequencies to compare the effect of sampling frequency to the 2 f signalshape and the noise floor. (T = 295 K, P = 1 atm)only 10 times faster than the modulation frequency. The benefit of higher sampling rateswas negligible beyond 20 times the modulation frequency. Higher sampling frequencyincreases data size since the raw photodiode signal is recorded from the DAQ. Data isrecorded in a double data type which takes up 8 bytes per sample. At a sampling frequencyof 400 kHz, data is written at 3.2 MB/s per channel which provides a good balance betweenperformance and data size. Following the design of mechanical system, optical system, andcontrol software with key parameters selected, the assessment of theWMS methane sensor40and improvements to its performance will be discussed in the next chapter.41Chapter 4WMS Sensor Tests & PerformanceThe sensor was characterized through comparison with existing verified methods. In thissection, the experimental setup is introduced, followed by calibration processes and defini-tion of performance metrics. Then, steady state and dynamic performances are comparedbetween the WMS and the FID methane sensors, while strategies to improve performanceare discussed. Tests were performed to:1. Obtain CH4 concentration measurements at different engine parameters2. Evaluate temporal response of the system at various sampling locations3. Determine the limit of detections of different configurationsSignal averaging and time response correction can improve the WMS sensor perfor-mance by lowering the noise floor and reducing the response delay. Furthermore, a 200 HzWMS measurement is demonstrated and a field measurement application is discussed.4.1 Experimental System SetupThe Single Cylinder Research Engine (SCRE) dual-fuel engine was used for all engine testsin the lab. To help readers understand the facility used to characterize the WMS sensor,the experiment facility is discussed with a piping and instrumentation diagram (P&ID) of42Table 4.1: SCRE engine specificationConfiguration 4-stroke, compression ignitionDisplacement 2.491 LCompression Ratio 17:1Stroke 169 mmBore 137 mmConnecting Rod Length 261 mmSpeed Limit 1800 rpmClearance Volume 0.155 LNumber of Valves 2 Intake, 2 ExhaustInjector 1st gen. HPDI (Westport Fuel Systems)the SCRE engine, air and exhaust system, fuel system, and other diagnostic tools. Thefuel system and control system for the SCRE are summarized here as they allowed themanipulation of exhaust CH4 concentration to assess the WMS sensor against the emissionbench for the SCRE. The gas handling system and the sampling locations of several WMSconfigurations are also discussed in greater details.4.1.1 SCRE EngineThe SCRE was originally developed by Cummins as an inline 6-cylinder, 4-stroke heavy-duty diesel engine (Cummins ISX 450 series). The engine was modified to be a dual-fuelengine with only one active cylinder. This single cylinder engine allows researchers totest various injectors and injection parameters with the sophisticated instrumentation. Thesingle cylinder engine removes cylinder-to-cylinder variability and lowers the operationalcosts. Performance and emissions of the SCRE engine are similar to that of an equiva-lent multi-cylinder stock engine. [31] The SCRE facility is shown in Figure 4.1 and keyspecifications of the SCRE engine are listed in Table 4.1.43Figure 4.1: SCRE engine facility highlighting the WMS sample portsThroughout this investigation, the SCRE used a High Pressure Direct Injection (HDPI)injector, from Westport Fuel Systems, which is mounted centrally in the cylinder head. Theinjector can adjust the injection duration and the injection angle of diesel and natural gasindependently and precisely down to a microsecond and 0.1â¦ respectively. Natural gas iscompressed by a 3-stage reciprocating compressor to 3600 psi, and diesel is pressurizedwith a hydraulic pump driven by an electric motor. A dome regulator regulates the dieselrail pressure to the injector and a needle valve controls the natural gas injection pressurewhich is set slightly below the diesel rail pressure to prevent backflow. The diesel railpressure was set to 20 MPa, and the natural gas pressure was regulated to about 1 MPalower than the diesel rail pressure.44Since the engine has five non-firing cylinders producing friction, an external 30 kWvector-controlled electric motor (Baldor) provides an additional constant torque to thecrankshaft to overcome the frictional losses. The torque output of the engine and the motoris measured by a water-cooled eddy-current dynamometer mounted in series to the motor.Controlled by a Digilog controller, the dynamometer maintains a constant engine speed byabsorbing the excess torque generated by the engine and the motor. A flywheel encoderand an optical encoder measure the engine speed, which is maintained within 1% of the setspeed by the control system.The combustion air is supplied from a rotary screw compressor (Ingersoll-Rand Arc-tic 1000) which provides controlled forced induction. Back pressure is regulated by aelectronically-controlled back-pressure butterfly valve and a wastegate valve. Intake andexhaust surge tanks damp the pressure variation upstream and downstream of the enginedue to the valve actuation. Exhaust gas recirculation (EGR) is controlled by the pressuredifferential between the intake and exhaust, and an electrically operated flow-control valvein the EGR loop. A flow diagram for intake and exhaust is shown in Figure 4.2.A custom LabVIEW interface was used on two PCs to control and monitor the engine,and log data from numerous sensors in the system. One PC controlled the engine param-eters, including intake and exhaust pressure, injection angle, injection duration, and EGRrate. The other PC provides data logging functionality and displays the real-time detailedsensor data, including combustion statistics, and emissions data from the AVL emissionsbench. National Instrument data acquisition (DAQ) modules interface the PCs with thevarious sensors on the SCRE. The DAQ and instrumentation for the SCRE are discussedmore in detail in [30].The SCRE engine features extensive diagnostic sensors, including in-cylinder pressuresensor, AVL emissions bench, and temperature and pressure sensor scattered throughout theintake and exhaust streams. These sensors allow for crank-angle (CA) resolved combustion45Figure 4.2: P&ID of SCRE facility highlighting the intake and exhaust system andfuel systems along with the three WMS sample ports used throughout the in-vestigation to sample the exhaust gasdiagnostics, making this an ideal engine on which to test WMS methane sensor. In-cylinderpressure is measured with a piezo-electric transducer (Kistler 6067C) every half degree CAwhile other âslowâ data is measured at 1 Hz. The sensor data can be logged as a TDMS fileon the DAQ control PC through the LabVIEW interface for a desired duration.An AVL CEB II emissions bench samples the exhaust gas far downstream of the exhaustport and measures the gaseous emissions of SCRE engine. The emissions bench measurestotal hydrocarbon (THC), CH4, CO2 (intake and exhaust), CO, O2, and NOx at a temporalresolution of 2 Hz. A sample is drawn in at 14 litre per minute (LPM) through a heated filterand a temperature-controlled sample line to the bench where the gas is distributed to indi-46vidual analyzers. FID (Pierburg FID 4000) measures the THC concentration in the exhaustgas. To measure only the methane concentration, the sample is passed through a thermo-chemical catalyst that oxidizes all hydrocarbons except methane, and the converted sampleis measured in the other FID. However, the catalyst is not 100% efficient and some non-methane hydrocarbons were observed in the methane sensor during the use. This error wascorrected by calculating the catalyst efficiency through a manual methane/ethane/propanecalibration. The CO and CO2 emissions are measured with non-dispersive IR absorptionanalyzers (ABB Uras 14 EGA) measure, while the NOx emissions are measured with achemiluminescence detector (Pierburg CLD 4000). The oxygen concentration is measuredwith a paramagnetic detector (Magnos 106 EGA). Before measurements began, the AVLemission bench was calibrated against gas standards each day to correct for sensor drift.4.1.2 Exhaust Sampling Points for WMSTo create a fast-responding system, the sample point for the WMS sensor needs to be asclose as possible to the exhaust valve. However, the temperature and pressure of the exhaustgas increase as a sample point moves closer to the exhaust port, creating a harsher conditionthat needs to be compensated by the sensor. Several sampling points for the WMS sensorare identified throughout the exhaust path of the SCRE engine as shown in Fig 4.3. Exhausttemperature can range from 200 to 500â¦C and pressure can range from 1 to 4 bar at theexhaust port. Three sample points were identified in the SCRE exhaust stream and used inthis investigation as they bring unique benefits and challenges. Table 4.2 summarizes thelocation of the ports and the exhaust gas conditions expected at those ports.The sample port (Sample Port 3) downstream of the back pressure valve provides asample point with the lowest temperature and pressure available because it is at slightlyabove the atmospheric pressure and gas has cooled down while flowing through the exhaustpipe. This sampling location is close to the sample point of the FID sensor, and it provides47Figure 4.3: WMS sample points for SCRE exhaustthe most conservative sampling point due to the distance from the exhaust port, resultingin a slow response. Regardless of the engine load and back pressure setting, the WMSsensor drawing sample from this port can expect a consistent and steady gas temperatureand pressure. However, later discussion will show that this sample port is more suitablefor monitoring steady state and slow processes since the mixing time of the surge tankdominates the sensor response of both FID and WMS sensors.The sample port (Sample Port 2) between the exhaust surge tank and the back pressure(BP) valve provides higher temperature and pressure compared to the port downstream ofthe back pressure valve. Due to the location upstream of the BP valve, the pressure atthis sample point can range from 1 to 3 bar depending on the back pressure setting for theSCRE. Temperature is also higher than Port 3 due to its proximity to the insulated high-temperature exhaust surge tank which can range between 150 - 350â¦C. Being downstreamof the exhaust surge tank, this sample port provides homogeneous exhaust gas withoutpulsations from the exhaust valve opening.To measure the transient and dynamic CH4 emission and to take advantage of the hightemporal resolution without the limitation of gas residence time in the surge tank, the WMS48Table 4.2: Comparison of exhaust temperature and pressure at the sample pointsPort Location Temperature Pressure1 Upstream of surge tank 200 - 500â¦C 1-4 bar2 Between BP valve and surge tank 150 - 400â¦C 1-3 bar3 Downstream of BP valve 100 - 200â¦C 1.1 barsensor needs to be mounted as close as possible to the exhaust port. The sample port (Sam-ple Port 1) upstream of the surge tank provides a sample point that is the closest to theexhaust port, thus reducing the time constant of the WMS system, but increasing the tem-perature and pressure. Without the surge tank upstream of the sample port, large fluctu-ations in the pressure, temperature, and methane concentration are observed. A potentialsampling bias exists at this port due to the inhomogeneous exhaust mixture and acousticpulsation between the engine and the exhaust surge tank. [31]4.1.3 WMS ConfigurationDifferent configurations of WMS systems were used in this work with various combina-tions of measurement cell size, sampling/modulation rates, and sample gas conditioning.A shorter 250 mm measurement cell was used for DIDF measurements while a 508 mmmeasurement cell was used for HPDI CH4 measurements to increase absorption sensitivity.When the sample gas was cooled to room temperature, the water vapour was cooled andcondensed upstream of the measurement cell to prevent condensation on the cell windows.This simplified WMS analysis because the sample gas at room temperature does not requireany correction for the Doppler broadening. However, this increased the gas residence timeof the system, resulting in a less responsive system. The P&ID for the cooled sample gasis shown in Figure 4.4 This configuration was used for measurements at Sample Port 3.To reduce the system time constant, the water knockout was moved downstream of the49Figure 4.4: WMS P&ID for sample gas at room temperaturemeasurement cell and the cell was heated to prevent condensation in the setup shown inFigure 4.5. The tubing between the exhaust pipe and the WMS measurement cell was alsoheated to prevent condensation and control the sample gas temperature flowing into thecell. This configuration was used for measurements at Sample Port 1 and 2 due to its fasterresponse time. At these sampling locations, the exhaust gas pressure can easily exceedatmospheric pressure. Therefore, a small amount of exhaust gas was flowing out throughthe sample port to the atmosphere, and the WMS sampled before the exhaust gas wasreleased to the atmosphere. The exhaust release from the exhaust pipe was controlled witha needle valve, and its flow rate was greater than the WMS flow rate (5 LPM) to preventambient air backflow into the cell. This maintained a steady flow through the WMS cellwhile keeping the cell pressure at around atmospheric pressure. It should be noted that allthe measurements with the heated cell are wet, meaning the water vapour from exhaust gas50is not removed. In the WMS setup with a water knockout, the measurements are dry. Theamount of water condensed can be calculated using the CO2 concentration of the exhaustgas since the combustion equation determines the ratio of CO2 and H2O in the exhaust.Figure 4.5: WMS P&ID for hot sample gas. Red line indicates a heated lineA photo of hot measurement system on a cart is shown in Figure 4.6. This movable cartallows convenient installation of WMS sensor at various locations throughout the SCREexhaust pipe. This setup provides an added benefit of extra vibration isolation and easieraccess to calibration gas bottles compared to directly mounting on the SCRE engine mountframe. Two thermocouples at the inlet and outlet of the cell and a pressure transducer(Validyne P55) at the outlet measured the temperature and pressure of the sample gas.51Figure 4.6: WMS setup with a heated cell on a cart. (1) laser fibre optic cable andcollimating optics on a 5-axis mount, (2) measurement cell with heat tape andinsulation, (3) photodetector, (4) laser current and temperature controllers andphotoamplifier, (5) DAQs, (6) vacuum pump, (7) water knockout and rotameter4.2 Steady State PerformanceAfter the bench-top tests of the WMS sensor with calibration gas, the WMS sensor wasinstalled in the exhaust of a natural-gas burning engine to compare the WMS sensor withan FID for real engine exhaust. The exhaust gas of a dual-fuel engine contains many dif-ferent gas constituents including nitrogen, oxygen, carbon dioxide, water, nitrogen oxide,methane, carbon monoxide, and other trace gases. These gas species and exhaust par-ticulate matter have a potential to interact with the laser and interfere with the selectiveabsorption by methane; therefore, the cross-sensitivity needs to be validated with the realexhaust gas. Furthermore, the temperature and the pressure of the exhaust gas at the inlet52of the WMS are higher compared to the calibration gas, and the cooling of the exhaust cancause condensation on glass windows. Testing the WMS with the actual engine exhaust isimportant in ensuring the setup can handle gases at harsher, realistic conditions.In a steady state engine test, stable operating points were held constant, and the methaneconcentration was measured using the WMS and FID for a specified duration, usually rang-ing from one to two minutes. By averaging the WMS signal, an average methane concentra-tion is calculated for a test point. Also, variables such as the time delay between samplingpoints and differences in the sensor time constants are minimized to compare the accuracyof measuring the methane concentration. Between test points, the engine and sensors wereallowed to settle to the new operating point by waiting for at least a minute before takingany measurements.4.2.1 CalibrationBefore taking any measurements, the WMS needed to be calibrated against working gasstandards. Many factors, such as alignment, diode temperature, and laser current, canaffect the WMS signal. The required calibration gas concentrations were selected based onthe expected CH4 concentration during measurements. At least two span gases are requiredto generate a linear fit between the signal and the concentration.The calibration gas for the FID was 500 ppm and 3900 ppm methane balance nitrogen,so these span gases were used for calibrating WMS before the engine measurement. 0 ppm(pure N2), 120 ppm, 500 ppm, 1000 ppm, 3900 ppm, 1 %, and 4% methane gas standards(balance N2) were available for calibrating the WMS sensor, and a different combinationof these gas standards were used depending on the expected methane concentration duringthe measurement as listed in Table 4.3.As shown in Figure 4.7, the calibration of different span gases at room temperatureresults in a linear relationship between the WMS signal and methane concentration in an53Table 4.3: Combination of calibration gases used to calibrate WMSEngine 0 ppm 120 500 1000 3900 10000 40000HPDI X X X X XDIDF X X X XFigure 4.7: A methane concentration calibration graph correlating the WMS 2 f sig-nal to the methane concentration at room temperature and pressure for 508 mmpathlength celloptically thin regime (kνL 1). The least-squares linear regression model shows a goodfit with R2 of 0.9998 and root mean square difference of 0.000128. The 95% intervalfor 2 f signal is below 1Ã 104 and unreadable from the plot. A methane concentrationof 3900 ppm in Figure 4.7 represents an optical depth of 0.088, therefore it is safe toassume the calibration region in the figure is optically thin and use a linear fit. The 3900ppm calibration point did not skew the linear fit coefficient as it changed the slope byonly 1%. This least-squares linear fit is used to convert a WMS signal back to a methane54Table 4.4: Considered operating conditions for the steady state methane sweep onSCRE. Detailed engine parameters can be found in Appendix BParameter ValueIMEP â¼ 5 barmean piston speed 6.8 m/sλ â¼ 3Ï main inj. duration 0.7 msÏ 2nd inj. duration 0.4 ... 0.68 msθ 2nd inj. angle 150â¦ aTDCconcentration. The optical alignment, laser current and laser temperature can affect the2 f WMS signal, thus pre-measurement calibration corrects for a potential sensor drift andensures the minimum variability between different measurements.4.2.2 Comparison With Flame Ionization DetectorThe performance of the WMS for steady state operation was assessed through simultaneousmeasurements with a flame ionization detector. By comparing the methane measurementsfrom the WMS to the FID on the AVL bench, the performance of the WMS can be vali-dated. With a second methane injection in the very late stage of the combustion, methanecan be easily added to the exhaust thereby varying the exhaust methane concentration. Thismethod of generating methane in the exhaust is convenient because the exhaust CH4 con-centration can be varied over a wide range while still maintaining stable and safe engineoperation.In this setup, the exhaust is cooled to room temperature and the condensed water vapouris extracted from the gas before the sample gas enters the WMS cell. Cooling down theexhaust gas allows the temperature broadening effect to be neglected, assuming the gastemperature is the same during measurement and calibration.55Figure 4.8: Comparison of exhaust methane concentration readings from the WMSand the FID sensors for steady-state SCRE engine operating points. The WMSsampled the exhaust gas downstream of the back-pressure valve near where FIDpulled its sample. The sample exhaust gas was cooled and condensed beforeentering the WMS cell.Figure 4.8 shows good agreement between the FID and WMS as the steady state datapoints lie close to the y=x line. The error bars here show the 95% confidence intervals.Some deviations can be observed at lower concentrations since those points approach theWMS limit of detection where the noise level skews the signal off the linear relationshipbetween the signal and the concentration. This was corrected using a separate second-orderfit near the limit of detection which improved the accuracy at lower methane concentrations.4.2.3 Limit of DetectionLow concentration measurement and thus measurement range of the WMS sensor is de-termined by a limit of detection (LOD), which is mostly affected by the line strength atthe selected wavelength, cell length, and noise. The LOD exists because of a noise floor56that interferes with the methane signal. Defining the LOD is important in characterizingthe WMS methane sensor and its steady-state performance because it defines its applica-bility to a given system. For an engine operating normally with HPDI, the exhaust streammethane concentration is considerably lower than port-injected dual-fuel engines therebyrequiring a lower limit of detection. For the considered SCRE engine, a limit of detectionbelow 100 ppm is desired to detect methane in the exhaust even at low-emission operatingconditions.Figure 4.9: 2 f WMS signal for 504 ppm CH4 balance N2 standard gas after inverseFFT at room temperature and pressure. The 508 mm measurement cell wasused. The signal strength of peak 2 f and noise floor are highlighted with thedashed lines.In a noise-free system, a sample gas without any methane would result in a zero 2 fsignal. However, due to the noise, the zero (pure nitrogen) gas results in a small WMS sig-nal. Noise in a WMS system can result from various sources such as quantization noise ofthe analog-to-digital (ADC) and digital-to-analog (DAC) conversion by DAQ, laser relative57intensity noise, and photodetector shot noise. [25] In the signal for the exhaust gas con-taining other gases (such as H2O and CO2), non-methane laser absorption can also resultin additional absorption features around the methane peak.In Figure 4.9, some noise can be observed outside the WMS peaks. If the signal gener-ated by the methane is lower than the noise level, accurate methane concentration is difficultto recover at that point. A common definition of LOD is the point at which a signal resultsin signal-to-noise ratio (SNR) of 3. [38, 45] Thus, the limit of detection can be defined as:LOD = 3Ã Snoise,RMSSsignal,peakà [CH4]cal (4.1)where Snoise is the RMS noise signal, Speak is the signal strength near the absorption peak,and [CH4]cal is the concentration of calibration gas.Thus, the LOD is defined as the limit of detection as a concentration at which the peakmethane signal can be detected with a SNR of 3. Using this definition, the limit of detectionof the WMS system in Figure 4.9 is calculated to be 62 ppm at a cell length of 508 mm.This definition is conservative since the 2 f signal decreases in a non-linear fashion whenapproaching the noise floor and is recoverable. Some additional methane concentrationinformation can be recovered in the low concentration region by averaging a harmonicsignal over several ramp periods and by using a second-order fit in the low PPM region ofthe concentration calibration.With the LOD calculated for one configuration of the WMS system, the LOD can beeasily modified for different applications by changing the cell length which changes the ab-sorbance and consequently the peak 2 f WMS signal. Since the noise signal was observedto be constant regardless of the pathlength, temperature, and pressure of the gas, a lowerLOD can be achieved by increasing the cell length. If the system is optically thin (i.e. opti-cal depth 1), the path length is directly related to the total energy absorbed by methane.58As discussed in Chapter 3, the rate of DAQ analog input sampling and DAQ output signalgeneration also impact noise floor which affects LOD.4.2.4 Signal AveragingInstead of focusing on increasing the pathlength to increase absorption, considering the av-erage of harmonic signals over numerous ramp periods can reduce the error associated withrandom noise and improve the signal shape. Signal averaging can reduce noise by reduc-ing the random signals outside the absorption peaks thereby increasing the SNR. When themethane concentration decreases, the noise interferes with the signal shape and the peakdetection, so signal averaging can lower the limit of detection.Figure 4.10 shows a comparison of the inverse FFT signals averaged over various rampperiods from exhaust with 100 ppm of methane. Compared to the signal without any aver-aging, the signal averaged over 3 periods has a 20% lower noise floor. For this WMS setup,the limit of detection decreased from around 45 ppm to 35 ppm based on equation 4.1. Theideal number of ramp periods to average was estimated to be between 3 and 5 as furthernoise reduction is not seen for more than 5 periods. Also, averaging results in a delayedresponse to a rapid change in signal.4.3 Hot MeasurementOne of the goals for the WMS sensor is a high temporal resolution. As the sampling rateincreases, the time constant of the WMS cell needs to be decreased to be able to resolve theadditional information about the concentration change. When the sample gas is cooled tothe room temperature and any condensed water is removed, the increased system volumesignificantly increases the WMS system time constant; however water removal is necessaryas condensation may otherwise occur in the measurement chamber. Condensation on thecell windows will result in erroneous measurements due to light attenuation and droplet59Figure 4.10: Inverse FFT 2 f signals are averaged over different numbers of ramp pe-riods to show the effect of averaging on the noise floor. The signal is obtainedby measuring SCRE engine exhaust containing about 100 ppm methane withthe WMS producing a 10 Hz ramp scan. (T = 352 K, P â 1 atm)scattering. To bypass the problem of slow response time due to cooling, the WMS cell canbe heated to a temperature above the dew point of the sample gas. In this case, exhaust canflow through the heated WMS cell without condensation and the tube from the samplingport to the cell can be very short. The P&ID diagram for measurements with hot samplegas is shown in Figure 4.5. The measurement cell was heated to 70â¦C which is a safetemperature that would prevent condensation of the exhaust gas even at a high load oper-ating condition. The calculation of the target gas temperature inside the cell is shown inAppendix C604.3.1 Temperature CalibrationAs the gas temperature increases in the WMS cell, the methane lineshape at 1651 nmbroadens due to the Doppler broadening, decreasing the WMS 2 f signal. Therefore, the gastemperature needs to be monitored and the signal needs to be corrected for the the Dopplerbroadening. This can be achieved in two methods: experimental temperature calibration orthe calculation of signal change due to absorbance broadening via simulations. For eachWMS cell configuration, a temperature calibration was performed to obtain the relationshipbetween the WMS 2 f signal and the measured temperature for a given CH4 concentration.This relationship should ideally be independent of engine parameters and WMS mechanicalsetup; however, accurately measuring the gas temperature inside the cell is challenging dueto factors like flow rate, sensor location, and cell geometry which can affect the measuredtemperature and the temperature homogeneity in the cell.Figure 4.11 shows an example of temperature calibration which considers a range ofgas temperature and measures the WMS 2 f signal for the same calibration gas. The tem-perature of methane gas is measured at both inlet and outlet of the gas cell, and the twotemperature values are averaged to estimate the temperature inside the cell. Its vertical axisshows the 2 f peak signal relative to the signal at room temperature of 21â¦C. At the targetcell temperature of 70 â¦C, the WMS 2 f peak drops approximately 23% from the roomtemperature value. With a relative change ofâ¼0.8 %/â¦C, the sensor reacts to small changesin the gas temperature. The flow rate should be kept constant throughout the calibrationand the measurement to minimize the temperature and pressure variation in the system.A WMS signal of sample gas at temperature above the room temperature can be cor-rected for the signal broadening by parametrizing the temperature and signal relationshipusing a second-order polynomial based on least-squares optimization, which allowed thecalibration curve to go through all the error bars. WMS signal was corrected for temper-61Figure 4.11: Temperature calibration of WMS was performed by measuring 504 ppmmethane calibration gas from room temperature to 79â¦C.ature, then the concentration calibration was used on the temperature corrected signal toobtain the CH4 concentration. This heuristic temperature correction provides some advan-tages over a model-based correction method. In addition to the ease of implementation,empirical correction compensates for temperature measurement error and temperature in-homogeneity in the sample gas, thereby providing simple yet accurate temperature correc-tion.4.3.2 Verification of Heated WMS System For Steady StateOperationThe heated measurement cell was verified with FID in a steady state operation of the SCREto ensure the heating of the cell did not affect the sensor performance. With the heated cell,the gas inside the cell can fluctuate in temperature which can affect the accuracy of the62sensor. The fluctuation can arise from the manual setting of the heat tape which is fixed at aconstant power while different operating points change the exhaust temperature. Therefore,a system is needed to maintain a constant gas temperature despite the varying temperatureof the exhaust gas at the inlet. Figure 4.12 compares the CH4 concentration measured usingthe heated WMS cell against that of the FID sensor in a steady state condition. The resultdemonstrates that compensating the Doppler broadening with a temperature calibration canyield an accurate WMS sensor.Figure 4.12: Steady-state SCRE exhaust methane measurement comparing the WMSwith the heated cell against the FID. The WMS sampled exhaust directly fromthe exhaust port, while FID sampled far downstream of the surge tank and theback pressure valve.4.3.3 Time ResponseThe goal of the WMS methane sensor was to be able to monitor the transient behaviour ofthe SCRE engine, which meant the time constant Ï of the system needs to be minimized63to exploit the fast optical and electrical system. A target time constant was <1 secondgiven the minimum volume of the measurement cell. To evaluate the time constant of theWMS system at different sampling points, the time response to a step input is simulatedand measured with the SCRE engine.Moving the WMS system upstream and testing it requires a modification of the gashandling system to cope with harsher gas conditions and perform additional SCRE enginetests. Therefore, a simulation of gas residence time in the surge tank and the WMS gas cellprovides an easier way to determine if the new sample port will bring an adequate reductionin the time constant. To model the system, the surge tank and the WMS cell are assumed tobe continuous stirred-tanks, in which flow through the inlet spontaneously mixes with thetank. The partial volume of a gas species in a well-mixed tank with a constant flow at itsinlet and outlet can be modeled as a first-order system:dVCH4dt= VËCH4,inâVËCH4,out= ÏCH4,inÃQinâÏCH4,tankÃQout= QÃ (ÏCH4,inâÏCH4,tank)= QÃ(ÏCH4,inâVCH4Vtank)(4.2)where Ï is volume fraction, Q is volumetric flow rate, VCH4(t) is partial volume of CH4 inthe tank at time t, and Vtank is volume of the tank (WMS cell or surge tank)Ajtay and Weilenmann showed that the exhaust gas transport can be modelled to correctfor the time delay and mixing by assuming the mixing as a first order system. [1] Similarly,for the WMS cell mounted downstream of the surge tank (Port 2 from Fig. 4.2), the systemcan be modelled as two first-order systems in a series where the WMS cell and the surgetank represent individual first-order systems. The two systems are connected such that the64methane volume fraction at the outlet of the surge tank is equal to the inlet volume fractionof the WMS cell. Using this model, the system can be simulated to investigate the timeconstants of individual first-order systems and the overall time response of two first-ordersystems in series.Simulations were performed to estimate the time constant of the system at various sam-pling locations. The simulations numerically solved the Vtank(t) in Equation 4.2 with theknown information about the system such as volumes of the surge tank and WMS cell, andtheir flow rates as provided in Table 4.5. The extra volumes of pipes and tubes betweenthe engine, the surge tank, and the WMS cell were estimated with their dimensions andincluded in the simulation as well. There is about 5 L of volume between the engine andthe surge tank, 0.7 L between the surge tank and WMS sampling port, and 30 mL betweenthe sampling port and the WMS cell. By imposing the initial conditions ofV (0) = 0 and ÏCH4,in(t) ={0, if t < 0.1, otherwise.the step input of a sudden change in methane emission was simulated and solved. Tovalidate the time response simulation and compare the performance between WMS andFID, a step change in the exhaust stream methane concentration is generated by turning onor off the late second NG injection from the HDPI injector. The methane concentrationsmeasured by the WMS sensor in response to a step input from enabling the second NGinjection are shown in Figure 4.13.Since the volume of surge tank relative the flow rate is greater than that of the WMScell, the gas residence time in the surge tank was expected to be dominant compared to theWMS cell for determining the time constant of the combined system. A time constant ofa first-order system is defined as the time between 0 to 63.2% of the final settled value inresponse to a step input. At the engine speed of 1200 rpm and WMS cell flow rate of 565Table 4.5: Step input response simulation parameters for WMS cell and exhaust surgetankLocation Volume Flow rate Ïsim ÏexpExhaust surge tank 133 L 3500 lpm 2.57 sWMS gas cell 36 mL 5 lpm 0.42 s 0.43 sSurge tank & WMS cell 133 L & 36 mL 3500 & 5 lpm 3.03 s 3.17 sLPM, the individual time constants of the surge tank and the WMS cell were simulated tobe 2.57 s and 0.42 s respectively by considering each of them as a single first-order system.For the WMS sensor located downstream of the surge tank at Sample Port 2, the total timeconstant was simulated to be 3.03 s. Experimentally, the combined time constant was 3.17 swhile the WMS cell time constant was 0.43 s. This result shows that the surge tank whosetime constant is 6 times that of the WMS cell is slowing down the response time of thesystem more dominantly. To increase the temporal resolution of the WMS sensor withoutbeing limited by the surge tank response time, the WMS was installed upstream of thesurge tank to better detect the changing CH4 concentration in transient engine operations.By finding the time constant of the WMS cell and the surge tank, the delayed response tothe change in the methane emission can be corrected, which is discussed in Section 4.4.1.4.3.4 Vibration RejectionWMS filters out low-frequency vibration noise because modulation frequency is magni-tudes higher than physical vibration frequencies. Figure 4.14 shows the measurementsof 500 ppm CH4 gas with and without vibrational noise. The WMS is mounted to theSCRE mounting frame whose large mass dampens the SCRE engine vibration. The low-frequency vibration is evident in the photodetector signal when the engine is operating, butthe 2 f signal is unaffected from the vibration noise due to the modulation frequency of 2066Figure 4.13: Time response to a step input at t = 0 with experiment and simulationof WMS concentration at sampling point upstream and downstream of theexhaust surge tank. The WMS concentration is given relative to the settledfinal concentration.kHz.4.4 Dynamic MeasurementBenefits of the WMS sensor include its flexibility of mounting location, small measurementvolume, and consequently the fast response time at sampling locations farther upstreamcompared to the FID. When installed at the exhaust port with the heated cell, the rampfrequency of the WMS system was increased to 200 Hz to take advantage of its lower gasresidence time. To test the transient measurement capability of the WMS sensor, SCREengine was operated at less stable operating condition with low load and high EGR of 35%(detailed parameters found in Appendix B). Figure 4.15 provides a comparison of the re-67Figure 4.14: A comparison of photodetector signal and second harmonic signal withand without vibration noise. Measurement was taken with 500 ppm CH4 gason the WMS mounted to the SCRE frame.sponse to a step input for the both WMS and FID methane measurement systems. The FIDsampled at 2 Hz from Sample Port 3 far downstream of the surge tank while the WMS sam-pled at 200 Hz from Sample Port 1 at the exhaust port. Alongside the CH4 measurements,the figure also shows cycle-resolved indicated mean effective pressure (IMEP) and maxi-mum in-cylinder pressure gathered from the high-speed in-cylinder pressure transducer todemonstrate the utility of the WMS sensor in the SCRE instrumentation setup. Ideally,a correlation between the in-cylinder and exhaust stream processes should exist. An at-68tempt to show this correlation with SCRE experiments is shown in Appendix ??, but nomeaningful relationship was revealed.Figure 4.15: A dynamic measurement with CH4 measurements from WMS and FID.IMEP and maximum cylinder pressure are calculated from high-speed in-cylinder pressure transducer. Engine at a low load, high EGR (35%) operatingpoint.The FID sensor on the AVL bench has a time delay of more than 11 s and a timeconstant of 10 s, while the WMS mounted near the exhaust port has a much shorter timedelay of 0.55 s and a time constant of 0.43 s. Combined with the responsive gas cell andimproved temporal resolution, the WMS sensor mounted at the exhaust port is capableof detecting transient methane emissions. The >200 Hz temporal resolution equates to amethane reading every 18 degree crank angle (CA) at the engine speed of 1200 rpm.The time delay between the actual methane change and the beginning of the sensorâs69response can be corrected by shifting in the temporal domain, but the first order responseis harder to correct for and significant transient data is lost in a system with a large timeconstant. Figure 4.16 compares the step input response of the WMS at the exhaust port andthe FID when the second injection of SCRE engine was disabled. The corrected signals areshifted in the time domain by the time delay between disabling the second injection andbeginning of the sensor response.Figure 4.16: Comparison of step response of WMS to FID to disabling the secondNG injection in SCRE. Corrected signals are shown with the time domainshift by the amount of the gas transport time delay for each system. WMS wasmounted at the exhaust port, while FID was sampling from far downstream ofthe surge tank.In Figure 4.15 and 4.16, the WMS signal shows significant fluctuations even if themethane concentration appears very constant on the FID. The time-averaged methane con-centration of the WMS sensor in stable engine operating points agree with the FID mea-surements. Also, the time scale of methane concentration fluctuations suggests that those70fluctuations are real instability in methane emission. The 400 Hz temporal resolution is rel-atively high for the system with a time constant of 0.43 s, and the system is limited by thegas cell. The FID mounted far downstream of the back pressure valve is unable to resolvethe instability of methane emission with its sampling speed of only 2 Hz. The followingsection discusses an attempt to correct the slow response of the FID.4.4.1 First-order CorrectionIf the sensorâs response to the change in CH4 concentration is assumed to be a first order, theresponse can be corrected to derive the actual methane concentration. Consider a genericfirst order system:F(t)a0= Ïdxdt+ x (4.3)where F(t) is actual CH4 emission, x is CH4 concentration measured from the sensor, andÏ is time constant.Using this equation, the actual concentration can be calculated by correcting the mea-sured signal with the rate of change and the time constants found in Section 4.3.3. Amethane concentration measurement from the FID with the first-order correction applied isshown in Figure 4.17. This figure compares corrected FID signal (one with time delay cor-rected and other with both time delay and first-order response correct) against uncorrectedWMS signal.The FID signal with both time delay and first-order behaviour corrected displays aquicker response to a step change compared to the uncorrected FID signal. However, whenthere is a sudden change in CH4 concentration, the signal displays a large error leading toa negative concentration due to its sensitivity to noise and slopes in the signal and errorsin the time constant estimate. Therefore, the correction may be useful in approximatingthe original signal, but it cannot substitute a faster sensor for high-speed accurate measure-71Figure 4.17: Uncorrected WMS signal compared with FID signal corrected for timedelay only and first-order response + time delay.ments. Attempt were made to correct the WMS signal to further improve the response;however, the dynamic nature of the WMS signal meant the first-order correction resultedin an excessively noisy signal even with very aggressive filtering.4.5 Field MeasurementOne benefit of the WMS methane sensor is its portability and the flexibility of its plumbing.By designing the mechanical and electrical system of the WMS sensor to be more compact,the sensor can be deployed for field measurements to gather fast methane emission dataoutside the lab setting in real world engines under realistic operating conditions.The WMS sensor was installed at the exhaust pipe in a cargo ferry with two (4.5 MW)dual-fuel engines to measure methane emissions. [48] As the expected methane concentra-72tion was higher than the SCRE engine due to the port-injected natural gas, a 36 cm longmeasurement cell was used for this measurement campaign. The WMS system sampleddownstream of the engineâs turbocharger, and the WMS gas handling system included agas cooler and a condenser upstream of the WMS cell. This setup is justified since thecampaign focused on steady-state operation points due to the slow behaviour of the largemarine engines (9 cylinder, 36 L/cyl) and the typical operation at a constant load. Themechanical setup is very similar to the SCRE engine setup for the room temperature mea-surement cell except oxygen and carbon dioxide sensors were added upstream of WMS formarine engines. The exhaust methane concentration at different load conditions is shownin Figure 4.18. The measurement revealed high methane slip at low load due to incompletecombustion due to the lean operation.Figure 4.18: Methane concentration at various loads in the exhaust of a 4.5 MW dual-fuel marine engine with NG port-injectionThe methane emission data suggested that to decrease methane slip, one engine run73at higher load instead of distributing loads to two engines. The cylinder deactivation dis-tributes the same power output across fewer cylinders, meaning per-cylinder load increasesand methane emission decreases. To monitor the methane emission over the whole sailingwhich typically lasts around 3 hours for this vessel, a WMS control and data acquisitionsystem for long term logging was developed. By periodically recording a period of data, theWMS sensor could measure the methane emission for longer duration without continuoussupervision and at reasonable data sizes. The long-term logging feature is demonstrated inFigure 4.19 in which the methane emission was monitored at a 30 second interval for theentire duration of a sailing.Figure 4.19: Methane concentration measurement for a duration of one whole sailingthat lasted three hoursThrough the modification of engine software (cylinder deactivation and closed-loopcontrol) and vessel operation strategy (single high-load engine and shore power to reduceidling), an estimated 56 - 60% reduction in CH4 emission and 42% reduction in total GWP74was achieved.75Chapter 5Conclusions & OutlookThe overall objective of the presented work was to develop a fast, inexpensive, and ac-curate methane sensor based on WMS using a near-IR laser for measuring methane con-centration in the exhaust-stream of engines. The investigation of the mechanism behindthe methane emission in a transient operation of diesel-ignited dual-fuel engines requires ahigh-speed (> 100 Hz) exhaust-stream methane sensor to perform cycle-resolved or crank-angle-resolved emission diagnostics. Compared to other conventional emission diagnosticsmethods for detecting CH4 such as FTIR and FID, WMS offers highly selective, fast, andsensitive methane measurements with a simple, robust, and inexpensive setup. The WMSmethane sensor, developed using a diode laser with a centre wavelength of 1651 nm, wasdemonstrated and characterized with calibration gas standards, the single-cylinder researchengine, and in-use dual-fuel marine engines. The hardware for WMS cost around $10,000to implement in this investigation. Commercially available FTIR sensors can cost from$50,000 to $100,000 with a lower temporal resolution. The sensor was shown to reliablymeasure CH4 concentration from 50 ppm to 4% with two different path lengths at a tem-poral resolution of over 200 Hz.During the development of the WMS methane sensor, the requirements for the temporalresolution and detection limit of the sensor were evaluated to determine the specificationrequirements for laser and optical components. Considering the engine speed and typical76methane emissions observed with HPDI engines, the minimum performance target for theWMS was set to a temporal resolution of over 100 Hz and a detection limit below 100 ppm.For DIDF engines, the sensor should be able to resolve a methane concentration of up to4%. While various configurations of the measurement cell were considered to reduce costand physical size, an external measurement cell design made of common tubing compo-nents was selected for its simplicity, low cost, and adjustability. The external measurementcell also allowed a constant cell pressure near atmospheric pressure, thereby eliminatingpressure broadening. By using a software-based lock-in amplifier and a multifunction DAQgenerating the laser modulation signals in place of a physical lock-in amplifier and functiongenerators, the cost and the size of the system were further reduced.Experimentally it was determined that a DAQ sampling at 20 times the speed of sinemodulation yielded WMS harmonic signals with acceptable level of noise. The increasingsignal-to-noise ratio was important in improving LOD for a given pathlength. SNR couldbe improved by increasing the signal generation sampling rate for modulation signal. 2 fharmonic signal offered good signal strength with low noise, and it does not have an y-axisoffset to be subtracted unlike 1 f signal. Signal averaging of several ramp periods helpedreduce random noise in the signal to improve SNR, and averaging 3-5 ramp periods offereda good balance between SNR improvement and temporal resolution.Increased temporal resolution of the system required a measurement cell with low gasresidence time to detect rapid changes in methane concentration. To lower the time constantof the system, the cell was heated to over 70â¦C to prevent condensation which negated theneed for an upstream water knockout upstream. However, the increased gas temperaturebroadened the methane absorption lineshape and lowered the signal amplitude; therefore,the gas temperature was measured upstream and downstream of the measurement cell anda temperature calibration was performed to correct for the signal reduction. This empiricaltemperature calibration in which a gas standard is measured through a range of temper-77atures provided a quick yet accurate method of improving accuracy even at elevated gastemperature inside the heated measurement cell.The WMS sensor was demonstrated on a research engine to measure the methane con-centration under real-world exhaust conditions. The WMS sensor was calibrated againstgas standards, and its methane measurements on the SCRE were validated against the FIDsensor which was calibrated independently using separate gas standards. The steady-statemethane measurements on SCRE showed good agreements with a mean absolute differ-ence of 4.2% between the WMS with the heated cell mounted at the exhaust port and theFID mounted far downstream of the exhaust surge tank. : The biggest source of WMSerror comes from uncertainty in measuring the temperature and pressure of the sample gasin the cell due to the high sensitivity of absorbance to the gas conditions. The system triesto minimize these errors by conditioning the sample gas before entering cell. However, asmall pressure fluctuation exists in the measurement cell due to the pressure pulses fromthe engine and the vacuum pump.The utility of the WMS sensor was demonstrated in the field measurement to character-ize the CH4 emission from a coastal vessel. The sensor was portable enough to be broughtto the engine room and installed in the exhaust pipe to measure steady-state CH4 emis-sion at different engine loads. After the engine control software update and operationalchanges driven by the measurement data, the continuous CH4 monitoring during the threehour sailing suggests an estimated 56-60% reduction in methane emission. This field mea-surement highlights the importance of testing the CH4 emissions in real-world operationsand therefore the need for a fast, robust and inexpensive methane sensor that can facilitateand promote CH4 measurements in realistic conditions.Compared to other methane detection methods or the current state-of-the-art WMS sen-sors, some limitations exist for the WMS sensor developed in this work. While the highselectivity of the WMS sensor allows a methane specific measurement without interfer-78ence from other exhaust gases, the sensor is limited to only one species unlike FTIR thatcan measure multiple species in one scan. The external measurement cell has slower re-sponse time compared to in-situ measurements with laser beam path directly through theexhaust pipe, as demonstrated in [58]. Although the empirical temperature calibration issimple and reliable, the process is time-consuming and limited in the temperature range.To address these limitations and improve sensor performance, suggestions for the futurework are discussed in the following section.While the global mean temperature and sea level continues to rise, Canada is expectedto miss the 2020 Copenhagen emissions target by 20% and additional measures are neededto meet the 2030 Paris target. [5, 12] Adoption of cleaner fuel like natural gas while min-imizing the methane emission is one way to reduce GHG emission in the transportationsector, the second largest GHG source. A fast and inexpensive methane sensor such asone developed in this work will allow engine developers and policy makers to characterizeand regulate tailpipe CH4 emission in transient operations. This investigation demonstratesan implementation of an exhaust-stream methane sensor which provides higher temporalresolution compared to the conventional sensors such as FTIR and FID.5.1 Future WorkThe objectives of this thesis were accomplished by developing a fast WMS methane sen-sor, characterizing its performance, and showing its applications on heavy-duty dual-fuelengines such as the SCRE and marine engines. However, there remain some improvementsthat can further enhance the performance, usability, and versatility of the sensor developedin this work. Recommendations for future work are as follows :1. Improve temperature control of the sample gas2. Increase temporal resolution793. Modify the cell design and/or laser beam path to improve detection limit and gasresidence time4. Integrate lasers with different wavelengths to increase laser sensitivity or detect othergas species or properties5. Develop an accurate physics modelCurrently temperature control is performed manually with two heat tapes with separateconstant-power controllers. One heat tape heats the tube between the inlet of the WMS celland the sampling point, while the other heats the WMS cell and the tube from the WMSoutlet to the downstream thermocouple. Although this method provides a quick and simpleway to increase the temperature of the measurement cell and the connecting tubes abovethe dew point to prevent condensation, it lacks the consistency of a well-tuned temperaturecontroller. The user had set the target temperature by rotating the control knob on thetwo heat tape power controller during this investigation. This manual control relied on theprevious experiments to know which power setting roughly corresponded to the appropriatetarget cell temperature. The gas temperatures at the inlet and outlet of the WMS cell werestable through the SCRE engine measurements. However, a temperature controller wouldallow a more accurate regulation of the cell and sample gas temperature without a need formanual adjustment to compensate for changing environment such as exhaust temperature,sample flow rate, and ambient temperature.Electronic and laser devices specified during the sensor development are theoreticallycapable of achieving a temporal resolution of over 500 Hz. As discussed in Chapter 3,literature review suggests a minimum sine modulation frequency that is 50 times higherthan a ramp frequency. Assuming modulation frequency is 100 times higher than the rampfrequency and DAQ sampling frequency is 20 times higher than modulation frequency,80the theoretical upper limit of system temporal resolution can be calculated per devices asshown in Table 3.2.The current WMS setup is theoretically capable of measuring CH4 at 1 kHz. Addi-tional tests are required to ensure increased modulation speed does not affect noise leveland the sensor performance. Further parameter optimization can be performed to identifythe maximum temporal resolution of the current system beyond the conservatively esti-mated maximum. The temporal resolution of the WMS sensor is limited by the maximumsampling frequency of the DAQ, therefore a faster DAQ could be necessary for a temporalresolution capable of crank-angle resolved measurements.To fully benefit from the increased temporal resolution, the system requires a comple-mentary cell design with low gas residence time. The current closed-cell single pass designis simple and inexpensive, but the time constant needs improvement for faster methanemeasurement. The time constant of the cell can be lowered by increasing the flow rateby decreasing the pressure inside the measurement cell, which would negatively affect thedetection limit. Also, increasing the cell length beyond the current length for a lower de-tection limit is impractical due to the physical size, difficulty aligning the optics, and theincreased cell volume. The 9.5 mm ID of the current cell is a good balance between theease of alignment and cell volume. A reduction to 6.4 mm can be considered, and anydiameter under 6.4 mm would be impractical with the laser beam diameter of 3-4 mmTherefore, a new beam path design such as a multi-pass cell or even an integrated beampath within the exhaust port or the cylinder inside the engine could be explored. There arecommercially available multi-pass cells for absorption spectroscopy such as a Herriott cellor a circular multi-pass cell which provide a pathlength that is a magnitude higher than asingle-pass cell. These cells however pose other challenges associated with keeping thecell volume low and preventing condensation. Beam path within the exhaust port or thecylinder provides the best response time which would depend only on the engine geometry.81This integrated design comes with other challenges like extreme temperature and pressurevariability, and mounting of laser diode and detector in the limited space. An in-depthanalysis is needed to drastically change and improve the cell design.Alternative methane absorption wavelengths can be explored to increase the WMS sen-sitivity. As shown in Figure 3.1, the methane linestrengths at 3200-3300 nm are about 100times stronger than 1600-1700 nm linestrengths. Using a laser at the longer, more absorp-tive wavelengths is one way to increase the sensitivity, which can translate to an improveddetection limit and/or a reduced pathlength. Additional lasers with different wavelengthscan also be integrated in the system for detecting other gas species in the exhaust or mea-suring the gas temperature or pressure. For example, using a multiplexed system with fourdifferent lasers, the concentrations of CO, CO2, CH4, and H2O were measured in a syngasplant. [51]Currently, the WMS post-processing script only considers the peak height to calcu-late the methane concentration. The lineshape, however, contains much more informationabout the sample gas. The width and secondary peak heights of the lineshape and its har-monics can be used for calculating temperature and pressure without separate temperatureand pressure sensor. Developing a physics model that predicts the lineshapes at differenttemperatures that match the experimental results can also negate the need of a tempera-ture calibration if the lineshape can be fitted and compared with the theoretical model tocalculate the gas temperature. An attempt at this was briefly made during this work, buta model developed using HITRAN resulted in a significant error and requires further in-vestigation. Throughout this investigation, the peak of the 2 f was used to calculate theCH4 concentration. Normalizing 2 f signal with 1 f (2 f /1 f ) reduces errors from scatter-ing, window fouling, and beam steering. [49] These baseline uncertainties were mitigatedthrough daily calibration during this investigation, but 2 f /1 f is recommended for the futureresearch since it was shown to improve calibration repeatability. Absorption spectroscopy82is a very broad and complex subject with many potential applications in engine diagnostics.83Bibliography[1] D. Ajtay and M. Weilenmann. Compensation of the exhaust gas transport dynamicsfor accurate instantaneous emission measurements. Environmental sciencetechnology, 38(19):5141â5148, 2004. â page 64[2] American Meteorological Society. State of the climate in 2018. SEP 2018. â page 1[3] B. Armstrong. Spectrum line profiles: The voigt function. Journal of QuantitativeSpectroscopy and Radiative Transfer, 7(1):61 â 88, 1967. 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Infrared Physics and Technology, 61:306â312, 11 2013. doi:10.1016/j.infrared.2013.08.006. â page 2890Appendix AList of ComponentsTable A.1: Complete list of laser componentsComponent ModelLight Source Norcada DFB Laser NL16510-B(1651 nm, 10 mW)Temperature Controller Thorlabs TED200CCurrent Controller Thorlabs LDC202CPhotodetector Thorlabs DET10DPhotodiode Amplifier Thorlabs PDA200CMultifunction DAQ National Instruments USB 6361TEC LM14S2Collimating Lens CFC-2X-CPressure Transducer Validyne P5591Appendix BEngine ParametersTable B.1: Engine parameter for steady-state SCRE testsParameter ValueTorque knob 3715RPM 1200Intake pressure 120 kPaExhaust back pressure 10 kPaPSOI -19PPW 0.8GSOI -4.5GOW 0.932PSOI 02PPW 02GSOI 1502GPW 0.4-0.792Appendix CGas Cell Target TemperatureTo determine the target temperature of the cell, a calculation revealed that 70â¦C is a safetarget cell temperature that would prevent condensation of the exhaust gas even at a highload operating condition. The dew point of the exhaust gas is 62â¦C if the equivalence ratioΦ of 1 is assumed with only methane as the fuel; in other words, all oxygen in the 2.5 Lof air (21% O2 in dry air) in the cylinder at room temperature is converted for completecombustion with methane.With the methane combustion reaction:CH4+2 O2 = CO2+2 H2O (C.1)and the diesel combustion reaction:C13H28+20 O2 = 13 CO2+14 H2O (C.2)methane produces the highest water vapour content of all hydrocarbons from combustiondue to its high hydrogen to carbon ratio. Since dual-fuel engines use diesel combustionas a pilot, the assumption is conservative as some of the air is used for diesel combustion.Also, due to the lean burning nature of the engine, 70â¦C is a safe temperature even when aforced induction is used.93Appendix DCoefficient of VariationThe coefficient of variation (COV) was investigated between exhaust methane concentra-tion and indicated mean effective pressure and maximum pressure to find a correlationbetween methane emission and engine stability. COV is defined as the ratio of the standarddeviation to the mean and is a metric for variability in data. It was hypothesized that anincrease in variability of IMEP, which signals unstable combustion, would correlate withthe variability in the exhaust methane concentration. The following figures do not suggestany conclusive correlations between the parameters.94Figure D.1: COV of maximum in-cylinder pressure and methane concentrationFigure D.2: COV of IMEP and methane concentration95Appendix ESimulationA simulation was designed to model the expected signals at various stages of the WMSfrom the modulation and laser controllers to laser absorption and photodetection. The endgoal of the simulation was to take in operating parameters such as modulation frequency,gas temperature and pressure, path length, and concentration to predict the photodiodesignal. The motivation for the simulation was to verify the post-processing script, but anideal simulation can theoretically provide temperature correction without the need of anexperimental temperature calibration.E.1 LaserIn the physical WMS system, the modulation signal for the laser is generated by NI DAQvia LabVIEW software. A 20 kHz sine wave is generated with an amplitude of 0.12 V anda 10 Hz ramp signal is generated with an amplitude of 0.4 V. Both signals are generated ata 2 MHz sampling frequency. The sum of the sine and ramp signals can be easily simulatedin MATLAB similar to the actual signal generated by LabVIEW and hardware.The current controller reads the modulation signal from DAQ output and sends themodulated current to the laser diode. The temperature controller is set to a constant tem-perature. The intensity of the laser detected by the photodetector is a function of current,while the laser wavelength is a function of current and temperature.96E.1.1 Laser ControllersThe current controller and the temperature controller regulate two important operating pa-rameters of the laser. The temperature controller keeps the laser diode at a constant temper-ature because modulating temperature at the high speed required for WMS is difficult dueto thermal inertia. In a quick experiment, it was concluded that modulating temperature ata speed above 1 Hz is unrealistic due to thermal delay. Instead, with a laser temperaturetuning coefficient of 0.1 nm/â¦C, temperature allows a broad yet slow adjustment of the cen-tre wavelength. Once the current is set to the desired value, the temperature is used to setthe nominal centre wavelength of the laser within the range of the ramp signal. Therefore,the laser diode is set to a constant temperature in this simulation with the temperature valuepredetermined from the laser specification sheet and experiments.E.1.2 Laser DiodeCombining the effect of temperature and current of the laser, the output power and wave-length of the laser can be calculated in the following equations.Plaser = 8.7mW â (100mAâ Ilaser)â0.088mW/mA; (E.1)λlaser = 1651.13nmâ (100mAâ Ilaser)â0.026nm/mA+(305.65KâTlaser)â0.1nm/K;(E.2)These equations are true for laser in a steady state. At a high modulation speed, thelaser response might differ slightly from the steady-state behaviour due to the phase ofwavelength modulation. Resolving wavelength and power of laser at high speed requiresexpensive sensors and could be an improvement to be made in the future.97E.2 Absorption and Photodetector SignalThe Voigt profile with data from HITRAN and the Beer-Lambert law is used to simulatethe absorption of laser beam by methane as discussed in Chapter 2.The photodetector converts incident light intensity to a current. Responsivity of thephotodetector is 0.8083 A/W near 1651nm. Responsivity is assumed to be constant withinthe range of wavelength modulation. Responsivity is actually a function of wavelength;however, this assumption results in an error of less than < 1% since the range of wavelengthof modulation is very narrow compared to the detection range of the photodetector.The current output from the photodetector is sent to the photoamplifier which convertsthe current to a voltage which can be measured by the DAQ. At this stage, the simulatedWMS data can be post-processed by the same algorithm that processes the experimentalWMS data.98
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Development of a fast methane sensor based on wavelength modulation spectroscopy for exhaust methane emission measurement
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