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Fuels and combustion knock in I.C. engines Givins, Henry Cecil 1933

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FUELS IMP COMBUSTIQM KNOCK IN I . G. ENGINES by Henry C e c i l G i v i n g . A Thesis submitted f o r the Degree of Master of Applied Science i n the department I . - of : Mechanical Engineering. The U n i v e r s i t y of B r i t i s h Columbia A p r i l 1955 .'.Index . FUELS MB COMBUSTION KNOCK IH I . C. ENGINES• Chapter Page I I n t r o d u c t i o n (1) General I n t r o d u c t i o n 1. (2) Combustion Knock d e f i n e d . 2. (S) R e l a t i o n of knocking to. e f f i c i e n c y , weight and c o s t s * 3. I I Elementary F u e l Chemistry. (1) Hydrocarbon s e r i e s 6. (a) P a r a f f i n '(b) Unsaturated 1 (c) Napthene (d) Aromatic (2) Other s e r i e s 15. (a) Compounds c o n t a i n i n g Oxygen (b) Compounds c o n t a i n i n g N i t r o g e n . (c) Compounds c o n t a i n i n g Sulphur I I I . Petroleum and L i q u i d Fuels (1) Sources of l i q u i d f u e l s (2) Petroleum (3) G a s o l i n e (a) Casinghead g a s o l i n e (b) S t r a i g h t run g a s o l i n e (c) Cracked g a s o l i n e (4) D i e s e l F u e l 16. ,18. Index (continued) Page Anti-Knock Ratings ( l ) Octane Scale f o r Gasolines 28. (a) F u e l standards (b) Octane Number (c) Standard t e s t engines (S) Getene Scale f o r D i e s e l Fuels 31. (a) F u e l standards. (b) Method of determining Cetene number. (c) Test engines. Combustion and Combustion Knock. , ( l ) Combustion process i n spark i g n i t i o n engine 55. (2) Detonation i n spark i g n i t i o n engine 39. (3) Combustion process i n compression I g n i t i o n , engine. . 42. (4) Combustion shock i n compression I g n i t i o n engine. 47. I n f l u e n c e of F u e l C h a r a c t e r i s t i c s on Combustion (1) I n a spark i g n i t i o n engine. 50. (a) E f f e c t of analysis> molecular : weight and s t r u c t u r e . (b) . E f f e c t o f d i s t i l l a t i o n curve. •(c) E f f e c t o f other p r o p e r t i e s (2) I n a compression i g n i t i o n engine. . 53. (a) E f f e c t of a n a l y s i s , molecular weight and s t r u c t u r e . '(b) :" E f f e c t of d i s t i l l a t i o n curve. (c) .Effect o f other p r o p e r t i e s . Index (continued.) . • • Page Influence of Engine Factors on Combustion. (1) In a spark i g n i t i o n engine. 57 (a) Compression r a t i o * (b) Form of the combustion chamber. (c) Intake and cylinder temperatures. (d) Supercharging and t h r o t t l i n g . (e) Time of i g n i t i o n . (f) Carburation. (2) In a compression i g n i t i o n engine. 6 1 . (a) Compression r a t i o . (b) Form of the combustion chamber. (c) Intake and cylinder temperatures. !.(d) Supercharging and t h r o t t l i n g . (e) Time of i n j e c t i o n . . ti • . . . . (f) Spray c h a r a c t e r i s t i c s . Anti-Knock Dopes. (l ) Dopes f o r suppressing detonation. 67. (g) Theories of control of detonation by dopes. 70. (5) Dopes f o r suppressing combustion shock. 75. (4) Theory of control of combustion shock by dopes. 76. Conclusion. (1) Summary 77 ( 2 ) Conclusions 80 (3) Bibliography 84 (1) FUELS AND COMBUSTION KNOCK IH I . C. ENGINES Chapter I I n t r o d u c t i o n I t i s the purpose of t h i s t h e s i s t o consider the combustion process, combustion knock, and f a c t o r s i n f l u e n c i n g them, i n the i n t e r n a l combustion engine. Considerable space i s devoted t o the subject o f l i q u i d f u e l s and f u e l chemistry, i n order t o provide a b a s i s f o r a complete understanding of the r e l a t i o n between the f u e l c h a r a c t e r i s t i c s and the nature of the combustion. A'small amount o f t h e theory i n c l u d e d i s o r i g i n a l , but has probably been p r e v i o u s l y enunciated by o t h e r s . T h i s t h e s i s i s , t h e r e f o r e , . a summation of present day knowledge r e l a t i n g t o the combustion process; and a statement of the reasonable t h e o r i e s o f the mechanism o f t h a t process, the knock, and knock suppression. The subject of the i n t e r n a l combustion process i s a very c o n t r o v e r s i a l one th a t i s by no means s e t t l e d . Therefore, no endeavour i s made t o prove a l i k e l y t h e o r y on any subject by d i s p r o v i n g t h e o t h e r s . A l s o , attempt i s made t o avoi d g i v i n g opinions on d i s -puted s u b j e c t s , t h a t i s , t o omit, "Mr. X says ........... ••.••while Dr. Y holds ( 2 ) the v i e w t h a t . otherwise t h i s t h e s i s would be endless. To put a l i m i t on the breadth of the f i e l d , only h i g h speed engines are considered. T h i s i s done because, only i n the h i g h e r speed types do the combustion d i f f i c u l t i e s become acute. The l a r g e low speed engine ( p a r t i c u l a r l y t h e d i e s e l ) i s q u i t e i n s e n s i t i v e t o the nature of t h e f u e l i t i s u s i n g , and runs smoothly on a wide range of f u e l s without knocking. Combustion i n t h e h i g h speed, spark i g n i t i o n engine i s compared and contrasted .with t h a t i n the h i g h speed, s o l i d i n j e c t i o n , compression i g n i t i o n engine. These engines may be considered t o be of the automotive t y p e . Combustion Knock Defined: The knock i n the c y l i n d e r o f an i n t e r n a l combustion goes under s e v e r a l a l i a s e s , some o f them being: f u e l knock, compression knock, heat knock, carbon knock and e t c . I n r e a l i t y , the nature o f the f u e l , compression, temperature and carbon deposits are a l l c o n t r i b u t o r y t o the same k i n d o f knocking, which should be c a l l e d by the general name "combustion knock". In t he spark i g n i t i o n engine, the knock takes the form of a considerable amount of f u e l vapotir i g n i t i n g almost simultaneously g i v i n g a v e r y r a p i d pressure r i s e . For t h i s reason the name '"detonation 1' has been a p p l i e d t o the knock i n g a s o l i n e engines. In the compression i g n i t i o n engine the knock i n no way resembles detonation, being caused by a sudden change of the r a t e of pressure r i s e . To d i f f e r e n t i a t e i t from detonation the name "combustion shock" i s used f o r the knock i n d i e s e l engines. Detonation i n a spark i g n i t i o n engine must be d i s t i n g -u i s h e d from p r e - i g j o i i i o n , which may occur from s i m i l a r ca.uses. P r e - i g n i t i o n of t h e f u e l mixture occurs spontaneously before the spark, and t h e r e f o r e considerably before t o p dead centre of the c y c l e . E f f i c i e n c y i s l o s t by negative work being done on the p i s t o n during the compression s t r o k e . Detonation, on the other hand, occurs a f t e r the spark and i n the f i n a l stages of combustion. E f f i c i e n c y i s decreased by energy going t o produce v i b r a t i o n and s t r e s s e s i n engine p a r t s , and being l o s t i n heat t o the c o o l i n g water. P r e - i g n i t i o n and detonation are not the same t h i n g , nor are they even c l o s e l y r e l a t e d . For example, both carbon d i s u l p h i d e and acetylene detonate at low compressions, but w i l l not p r e - i g n i t e f benzene and cyclohexane p r e - i g n i t e a t high compressions, but - w i l l not detonate. P r e - i g n i t i o n precedes the spark; detonation can o n l y f o l l o w the spark. R e l a t i o n o f knocking t o e f f i c i e n c y weight and c o s t s ; Every f u e l t h a t can be used i n a spark i g n i t i o n i n t e r n a l .combustion engine has what i s known as i t s Highest U s e f u l Compression R a t i o . The H.U.C.R. of a f u e l i s not the h i g h e s t compression a t which the f u e l may be used, but i s the compression above which any increase i n compression does not r e s u l t i n i n c r e a s e d e f f i c i e n c y . The d e s i r a b i l i t y o f a high compression r a t i o , which r e q u i r e s a h i g h H. U. C. R. of the f u e l , i s shown by the f o l l o w i n g r e l a t i o n between compression r a t i o and t h e maximum thermal e f f i c i e n c y o b t a i n a b l e : Compression .Ratio 4 5 6 7 Maximum Thermal E f f i c i e n c y 1 - &)-25 .295 .552 .361 .585 \ •••{4): • • From t h i s t a b l e i t can be seen t h a t , i f t h e compression r a t i o o f an engine can be r a i s e d from 4:1 t o 6:1 without i n c u r r i n g deton-a t i o n , the thermal e f f i c i e n c y and the f u e l economy w i l l be i n c r e a s e d 25$. Therefore, the l i m i t i n g f a c t o r for the e f f i c i e n c y o f a spark i g n i t i o n engine i s the highest compression t h a t can be used without causing d e t o n a t i o n . I n the compression i g n i t i o n engine knocking i s not th e f a c t o r l i m i t i n g t h e e f f i c i e n c y of the engine. As knocking here I s reduced as the compression i s i n c r e a s e d , the t h e o r e t i c a l e f f i c i e n c y i s o n l y l i m i t e d by c y l i n d e r temperature c o n s i d e r a t i o n s and 1the p r a c t i c a b l e weight o f the engine.. T h i s i s c a l l e d t h e , '"Limit; of Useful. Compression." •"-Compression Ratio 10 12 14 16 20 Maximum Thermal E f f i c i e n c y 1 - ( i ) ' 2 5 .458 , 1.464 - .485 .500: .527 Compression pressure ... P i r 1 - 4 566 i.," 472 587 708 968 Approximate Maximum Pressure 700 ••- 800 900 1000 1250 T h i s t a b l e shows t h a t t o i n c r e a s e the compression r a t i o from 16:1 t o 20:1 only g i v e s an in c r e a s e of 5% i n the thermal e f f i c i e n c y and i n v o l v e s v e r y high c y l i n d e r pressures. A l i m i t f o r maximum c y l i n d e r pressures from both temperature and weight cons i d e r a t i o n s i s i n the neighbourhood of 900 t o 1000 #/sq.in. T h i s imposes a l i m i t of U s e f u l Compression a t about 15:1 or 16:1 f o r d i e s e l engines and these (5) r a t i o s o n ly i n the v e r y h i g h speed t y p e s . E l i m i n a t i o n o f combustion knock has an important b e a r i n g on the engine weight per brake horsepower output. I n an engine t h a t runs smoothly and without knocking, i t i s p o s s i b l e t o ob t a i n a higher mean e f f e c t i v e pressure f o r t h e c y c l e w i t h a lower maximum pressure. T h i s a l l o w s the use o f l i g h t e r p a r t s and hig h e r output r a t i n g s . Weight r e d u c t i o n of engines i s very important i n the automotive f i e l d , p a r t i c u l a r l y so i n the case o f a i r c r a f t engines. E l i m i n a t i o n o f combustion knock i s a l s o v e r y important from the cost s t a n d p o i n t . An engine t h a t does not knock can be b u i l t 5 m o r e l i g h t l y and i n e x p e n s i v e l y , w i l l have lower upkeep c o s t s , and w i l l have a lo n g e r u s e f u l l i f e . • (6) Chapter i± Elementary Fuel Chemistry. Hydrocarbon S e r i e s : The most important compounds from the standpoint of f u e l s f o r I . C. engines, are hydrocarbons: i . e . organic substances c o n s i s t i n g e x c l u s i v e l y of carbon and hydrogen. A c l a s s -i f i c a t i o n o f hydrocarbons f o l l o w s : I A l i p h a t i c Hydrocarbons  A. Saturated B« Unsaturated (Chain S t r u c t u r e s ) P a r a f f i n s O l e f i n s D i - o l e f i n s CftHa.n_a T r i - o l e f i n s Acetylenes I I C y c l i c Hydrocarbons A. Naphthenic B. Aromatic (Ring S t r u c t u r e s ) C„Ha„ Hydro-ben zane s Hydro-naphthalenes C,fl a/i-i Hydro-anthracenes C nH 4h.4 Benzenes> G„H 2n-<. Naphthalenes C nK zn~ ,z Anthracenes C„H 2n-/s T h i s c l a s s i f i c a t i o n has two l a r g e groups of hydrocarbons according t o s t r u c t u r e , chain compounds and r i n g compounds, and each of these groups i s d i v i d e d i n t o two l e s s e r groups according t o the r e l a t i v e number of hydrogen t o carbon atoms i n the molecule. The (7) s t r u c t u r a l formula of a t y p i c a l member of each group i s shown below? Sa t u r a t e d Chain Compound H H H H H H i i i i i ; i H—C — C — C — C — C — C — H - I r i i i i H H H H H fl p a r a f f i n s e r i e s n - hexane Unsaturated Chain Compound H H H H H . H , I i I I | I H— C — C — C — C — C = C v I I I I I \ fl .-. H H H H o l e f i n s e r i e s n - hexene Naphthenic Ring Compound H H \ / fl C X C H H C X H / \ •H cyclo-hexane Aioraatic Ring Compound H C G benzene The carbon atom i s t e t r a - v a l e n t , t h e r e f o r e , i t i s always p i c t u r e d i n s t r u c t u r a l formulae as connected t o other atoms (8) by f o u r bonds. The hydrogen atom i s mpno-valent. In chain compounds any number of carbon atoms are connected together and the balance of the carbon, v a l e n c i e s are s a t i s f i e d by hydrogen atoms. The chain o f carbon atoms may be s t r a i g h t or branched: i f i t I s s t r a i g h t i t i s the normal form; i f branched, an isomeric form. For example, the formula f o r normal hexane given above may a l s o be w r i t t e n : CH 3-(Cfi a),-CH 5 An i s o m e r i c form of the same compound as c a l l e d iso-hexane and i s w r i t t e n CH 3- CH - CH - CHj ^ CHj. CH 4 , The p r o p e r t i e s o f isomers d i f f e r from those of the normal form; a s l i g h t d i f f e r e n c e i n b o i l i n g p o i n t and s p e c i f i c g r a v i t y and p o s s i b l y c o n s i d e r a b l e d i f f e r e n c e i n chemical s t a b i l i t y , i g n i t i o n temperature and normal r a t e of burning. The s i d e chain of a hydrocarbon c o n s i s t s of an a l k y l group j an a l k y l group having the general formula G„Eirifl » The l o n g e r s i d e chains (S carbon atoms and over) may themselves have s i d e chains . The number of p o s s i b l e : isomeric forms i n the higher members of the a l i p h a t i c s e r i e s i s tremendous. I n a p a r a f f i n molecule w i t h fourteen carbon atoms over one thousand isomers are p o s s i b l e . Research tends t o show t h a t a l l p o s s i b l e isomers e x i s t o r can be s y n t h e s i z e d . The unsaturated a l i p h a t i c hydrocarbons have fewer hydrogen atoms per molecule than the corresponding number of the (9) p a r a f f i n s e r i e s . The conception o f "unsaturation" has a r i s e n from the presence i n the molecule of l e s s hydrogen than i s r e q u i r e d t o s a t i s f y a l l t h e carbon v a l e n c i e s . The excess v a l e n c i e s are accounted f o r by the assumption o f one or more double or t r i p l e bonds between carbon atoms. I n c y c l i c compounds s e v e r a l carbon atoms ( u s u a l l y s i x ) are connected together i n a close d r i n g . Isomerism a l s o e x i s t s i n c y c l i c h y d rocarbons;in a d d i t i o n t o the isomers depending on the number and l e n g t h : o f the s i d e chains, t h e r e are the isomers caused by d i f f e r e n t p o s i t i o n s o f the s i d e c hains. T h i s p o i n t I s best i l l u s -t r a t e d by .thecase of o r t h o - , meta-, and para-xylene which are dimethyl ©gfi&eneS* o-xylene m~xylene p-xylene Numbers are also'used i n i n d i c a t i n g the p o s i t i o n of s i d e chainsj the hexagon being.numbered from 1 t o 6 i n a clockwise d i r e c t i o n beginning at the t o p . Unsaturated s e r i e s a l s o e x i s t i n c y c l i c compounds, and have one, two, or. t h r e e , double bonds i n a mono-cyclic s t r u c t u r e . , .'. The P a r a f f i n , s e r i e s i s the only saturated s e r i e s of a l i - p h a t i c hydrocarbons, The members of the series:have a general . d o ) ._ fo'rmTola\ G4'H^ Methane CH/is ;• "' ' the f i r s t - member of the series and i s a gas l i q u i f y i n g at -160°. The b o i l i n g and melting points of the normal paraffins increase uniformly with the number of carbon atoms i n the molecule. At- ' normal temperatures, members of the series from G, to C* are gases, from Gf:tp-'GM are l i q u i d s , and above C/(.'are waxy s o l i d s . Isomeric forms have a- lower b o i l i n g point and a higher melting point than the; corresponding normal form. The paraffin hydrocarbons have; low , i g n i t i o n temperatures that decrease: as the molecular weight increases, ^pwever,-•••in isomeric p a r a f f i n s , the presence of numerous branched side chains raises the i g n i t i o n temperature: considerably. * There are several unsaturated a l i p h a t i c series varying i n t h e i r structure and degree of unsaturation. The o l e f i n s , C 0H a n are the commonest of these compounds. The o l e f i n molecule contains one double bond and ,is chemically more active than the p a r a f f i n . They have a tendency to polymerize, i . e . , to combine to form larger molecules* p a r t i c u l a r l y i n the sunlight. The olefins have higher, i g n i t i o n temperatures and slower reaction v e l o c i t i e s with' oxygen than have;the paraffins. The terminology of the'olefin series p a r a l l e l s that of the pa r a f f i n s , but with the s u f f i x - y l e n e or -ene. The f i r s t member of the series isethylene or ethene Rz C:GH2,:, ' The d i o l e f i h series G^H^.a. i s chemically more active and, less"..'stable than the. o l e f i n series. The molecule Is conceived to "contain two double bonds between carbon atpms. The names have the ending -diene: e.g. butadiene and pentadiene. (11) The acetylene series also has the general formula C J H J U , . ^ but the molecule contains one t r i p l e bond. The acetylenes combine very rapidly with oxygen. In terminology the higher members of the series are regarded as substitution products and take the name of the a lky l group present, e.g. ethyl acetylene, GaHr-C=CH Comparative formulae for the unsaturated series for C 4 fl x are: H H H H H H < Olefin Series H-C-G-G-C-C-b hexene H H H H H H 04H,, H H H H H Diolefin Series C;-C-C-C-C=C Hexadiene H,H H H H C tB / 0 E , H H H Trioief in Series C:C-C=C-C=C Hexatriene H.H H H C 4 H 8 H H H E Acetylene Series H-C-C-6-C-CsC-H Butyl acetylene H H H H ' , C T H , 0 The naphthene series are saturated cyclic hydrocarbons and are often regarded as completely hydrogenated aromatic compounds. Usually the naphthene molecule has a six-carbon ring and belongs to the group of cyclohexane'and Its derivatives. Cyclopentane and i t s derivatives are common and these have a five-carbon ring. Other structures from cyclopspane C,Ht to cyclononane C? E/t are known to e x i s t b u t , i n general, naphthenes are usually considered as eycio-hexane ( 1 2 ) and i t s derivatives. The naphthene series are.. chemically quite stable and have high igni t ion temperatures. Polycyclic naphthenes are stable, inert , hydrocarbons that are used to form a large part of lubricating o i l s . These compounds show no tendency to crystal l ize due to the large number of Isomers possible and probably present. ; The compounds of th is series are named from the nature and position of the side chains: e.g. ethyl cyclohexane, 1:1, dimethyl cyclohexane, 1-ethyl 4-iso-propyl cyclohexane. The Aromatic Hydrocarbons are really unsaturated cycl ic Cvhydrocarbons, but are rarely regarded as such. This i s because the idea, of unsaturation Is usually associated vdth chemical, ins tab i l i ty , and the aromatics are very stable due to the exothermic reaction of their formation. The boil ing point of aromatic hydrocarbons increases with the molecular weight, the number of side chains, and the nearness together of the side chains. The ignit ion temperatures of the members of the aromatic series also increase with their molecular weight. Their igni t ion temperatures are higher and their rates of burning slower than those of the corresponding naphthenes. The names of the monocyclic aromatics may be given in the same manner as the naphthen.es, but due- to their ccpmon occurrence, many of the simpler members of the series have individual names: benzene, toluene., •xylene, mesitylene, etc. In the polycyclic forms, two ringed aromatics are naphthalenes GhRMrn ., three ringed - anthracenes G N H I H . „ 6"b C o -• (15) Other Series: Organic chemical compounds other than hydrocarbons that concern the subject of.fuel chemistry directly or Indirectly, may contain i n addition to carbon and hydrogen^ oxygen, nitrogen, or sulphur. These substances are fuels, Impurities, and intermediate combustion products. G HO Compounds: Alcohols: ' C n H ^ t l -OH ''Ether's: C , H W - O - C J i ^ , Aldehydes: \ G,H„., - CKO ;;;,CV.^ ' Ketones: G , H ^ , -GO-G^H_,, ;.;... •' Acids:- / • -V -/ G-HE^Xi - COOH . ( A l l these. compounds have aromatic parallels with a phenyl group G/&sin place of the a l k y l group or groups) There are also compounds of the type of:\ •';•.••-'' P k e n p l - ' \ - • ' • ' • • • } • • Q'i&m G; H H - Compounds':' Nitro Gompounds: Amines•,-,'•. : - ( C , H i B ^ ^ - f f l ^ ;(ahalines)-i V (GA)X-Nfl,s.x • • Ni t r i teB- GHExn4., \ - NQ^ Nitrate's: GnH ^ - N O V C H S Compounds: Mercaptans P „ H x n ^, - SE Thioethers ' . ' . . . . . C H . ^ , -SrCJi^„<., (These groups of compounds have also aromatic parallels with a phenyl group i n place of the a lky l group) (14) Alcohols and ethers may be used directly or as blending agents for fuels. There are three types of alcohols: primary, seconda and te r t ia ry . We are most familiar with the primary alcohols, of which methyl and ethyl are the most common. In structure they consist of an a lky l group, by which the alcohol i s named, and an hydroxyl group. The alcohols have a high ignit ion temperature and a slow rate of burning and therefore can be used at high compressions i n a spark igni t ion engine. The ethers have a structure similar to the alcohols but exhibit very different properties as fuels i n I . C. engines. The most familiar member of the series' i s di-ethyl ether, C^H/O-Cj.H^ . I t has a low igni t ion temperature and i s knock-inducing i n a mixture with gasoline. However, ether has anti-knock properties when mixed with the fuel in a diesel engine. Aldehydes, Ketones, and Acids a l l occur as intermediate oxidation products i n combustion. Aldehydes occur as oxidation products of primary alcohols, ketones as oxidation products of secondary alcohols. Organic acids occur i n free and combined states i n animal and vegetable o i l s . Phenol and Its homologues are products of coal tar d i s t i l l a t i o n and have their uses as anti-knock and gum-inhibiting agents i n gasoline. Nitrogen occurs i n petroleum probably i n the form of al iphatic amines, although th i s i s questionable. The aromatic amines, called "analines1 1, have" strong knock suppressing character-i s t i c s i n a spark igni t ion engine. The nitrates and n i t r i t e s , on (15) the other hand, have a decided knock promoting tendency i n a gasoline engine and have been used experimentally f o r reducing the knocking i n d i e s e l engines. , The compounds containing sulphur occur as undesirable impurities i n crude petroleum. These types of compounds are named, mercaptans and thioethers, and they bear a remarkable structural • resemblance to the primary alcohols and the ethers. These sulphur compounds are characterized by extremely disagreeable odours, Yfhich are noticeable i n the v i c i n i t y of o i l r e f i n e r i e s . (16) Chapter i l l Petroleum and Liquid fluels Sources of Liquid Fuels: A vri.de range of fuels can be converted into mechanical power by the internal combustion engine. These fuels come from a variety of sources, the principal sources of l iqu id fuels being: (1) Petroleum. (2) D i s t i l l a t i o n of o i l shales. (.3) D i s t i l l a t i on of coal and l ign i te tars . (4) D i s t i l l a t i on of wood and peat. (5) Animal and vegetable o i l s . (6) Chemical and synthetic products. Petroleum i s the source of a very large percentage of a l l l i qu id fuels.used today and w i l l probably continue so for many years. The importance of petroleum may be judged from the fact that i t i s the largest chemical industry and the second largest industry of any kind i n America. Petroleum and fuels derived from petroleum w i l l be discussed fu l ly la te r . Shale o i l , which closely resembles petroleum, i s obtained from o i l shales, which have ¥dde occurrence i n every continent. Production of shale o i l i s not as simple as that of petroleum as the o i l shales must be mined and d i s t i l l e d . Large deposits of o i l shales exist on th i s continent i n a v i r tua l ly untouched condition. These (17) w i l l probably-come i n t o use 'should, t h e r e occur any appreciable f a l l i U g - o f f i n the supply o f petroleum. There i s the o b j e c t i o n t o shale o i l on the grounds o f d i f i f i c u l t y I n ha n d l i n g due "to the high • v o l a t i l i t y and low f l a s h p o i n t o f the l i g h t e r f r a c t i o n s . The d i s t i l l a t i o n o f c o a l and l i g n i t e t a r s y i e l d s : v e r y v a l u a b l e blending c o n s t i t u e n t s f o r g a s o l i n e j namely, benzene, t o l u e n e , and other aromatic compounds. Methyl a l c o h o l , t u r p e n t i n e , a c e t i c a c i d and a wide range o f o i l s are the l i q u i d products from t h e d e s t r u c t i v e d i s t i l -l a t i o n v o f wood and peat. These products are not used t o any extent as f u e l s f o r I . G. engines. The production of animal and vegetable o i l s i s small and t h e i r p r i c e s remain too high f o r them t o ever be of importance as motor f u e l s . IShale o i l , peanut o i l , palm o i l and probably a great many others male e x c e l l e n t f u e l f o r die-sel engines as they have .low i g n i t i o n temperatures. These o i l s have, however, a low heat v a l u e . Whale o i l has been v e r y s u c c e s s f u l l y used i n the a r c t i c i n shortage o f cheaper d i e s e l f u e l . Methyl and e t h y l -alcohol-.are the c h i e f chemical and s y n t h e t i c products t h a t can be used as f u e l . A l c o h o l can be synt h e s i z e d from non-condensable petroleum gases or can be made from sugars, s t a r c h e s , wood,fruit, vegetables, or weeds. L i t t l e use has been made of a l c o h o l alone as motor f u e l b u t . i t has been considerably used as a blending agent f o r . s p e c i a l g a s o l i n e s . E t h y l a l c o h o l has a H* U . C.S. of more than 7.5: 1 while t h a t o f methyl a l c o h o l i s (18) almost as h i g h ; both are considerably h i gher than t h a t o f benzene. Petroleum; A rough, c l a s s i f i c a t i o n of petroleums, which has but l i t t l e b e aring on chemical composition e x i s t s | crude o i l s are d i v i d e d i n t o three c l a s s e s j p a r a f f i n - b a s e , asphalt-base, and mixed-base. They system considers the nature o f the re s i d u e from no n - d e s t r u c t i v e d i s t i l l a t i o n ; a p a r a f f i n - b a s e crude leaves a waxy r e s i d u e , and an asphalt-base crude a t a r r y r e s i d u e . The residue from a mixed -base crude contains both waxy and t a r r y m a t e r i a l s . A c l a s s i f i c a t i o n of petroleums, based on the chemical c o n s t i t u e n t s , t e l l s a great d e a l more o f the valu e of the crude f o r i t s products f o r engine f u e l and l u b r i c a t i o n . Gruse has c l a s s i f i e d crude o i l s .according t o t h e i r c h i e f c o n s t i t u e n t s ; p a r a f f i n , naphthene, and aromatic hydrocarbon s e r i e s , and a s p h a l t i c m a t e r i a l . A t r i a n g u l a r composition diagram i l l u s t r a t e s the c o n s t i t u e n t s of t y p i c a l crudes. (19) The most u s e f u l c l a s s i f i c a t i o n o f petroleum crudes, from a l i q u i d f u e l standpoint i s one based on an a n a l y s i s i n t o h;y"drocarbon s e r i e s , o f the v a r i o u s f r a c t i o n s . d i s t i l l i n g below say 550° c. The c h a r a c t e r i s t i c s of the crude o i l s o f the p r i n c i p a l North American f i e l d s a r e : Pennsylvania: h i g h p a r a f f i n content, 75$ and more, l i t t l e o r no aromatics or unsaturates. Mid Continent: Less p a r a f f i n s and more naphthenes, ; V,, i n c r e a s e d aromatic content., considerable a s p h a l t i c m a t e r i a l . considerable naphthenic content - 3 0 $ . hi g h aromatic . content - 20^, much. . as p h a l t i c . m a t e r i a l . ,;, predominating i n naphthenes — 50%^ p a r a f f i n s i n low b o i l i n g range, aromatics ,in.high b o i l i n g range, much a s p h a l t i c . m a t e r i a l . . Mexican and C e n t r a l American resemble C a l i f o r n i a n crudes ch e m i c a l l y . The p a r a f f i n hydrocarbons occur i n v a r y i n g amounts i n n e a r l y a l l petroleums, and members o f the s e r i e s as h i g h as C 4 rH 7 J. have been found. A l l normal forms seem t o e x i s t along w i t h a con-s i d e r a b l e p r o p o r t i o n of the p o s s i b l e i s o m e r i c forms. G u l f Coast: C a l i f o r n i a : (20) The existence o f unsaturated hydrocarbons i n petroleum i s regarded as imp o s s i b l e by many a u t h o r i t i e s . Some experimenters, however, report t h e presence of the lower o l e f i n s i n the gas from o i l w e l l s . Up t o 8$ unsaturated hydrocarbons iiave been found i n s t r a i g h t run g a s o l i n e s , but these may be s a f e l y considered as having t h e i r b i r t h i n the r e f i n e r y s t i l l and not under the ground. I t i s e n t i r e l y l i k e l y t h a t thermal decomposition takes place i n the l a y e r of o i l next t o the heated metal of the s t i l l , and any unsaturates i n ^ s t r a i g h t run products may be accounted f o r by t h i s m i l d c r a c k i n g . /.. • proces s . Haphthenes occur i n a l l known petroleums. P r i n c i p a l l y t hey are cyclo-hexane and i t s d e r i v a t i v e s i n monocyclic and p o l y c y c l i c s t r u c t u r e s . However, cyclopentane, cycloheptane and cyclo-octane have a l s o been found i n c e r t a i n petroleums, notably Russian and C a l i f o r n i a n . Baku crude o i l s c o n t a i n a remarkably high percentage of naphthenesj as high as 80$ - 90$. Pennsylvania crudes c o n t a i n v e r y l i t t l e w h i l e other o i l s are i n t e r m e d i a t e . Most crude o i l s c ontain o n ly a very s m a l l percentage ( i f any) of aromatic hydrocarbons. However, C a l i f o r n i a and Borneo crudes contain a v e r y considerable amount. The great e s t part o f these aromatic compounds are mon o c y c l i c , i e . benzene and i t s d e r i v a t i v e s up t o C I 0H H . The remainder i s naphthalene (2 r i n g s ) and s e v e r a l methyl and di-methyl d e r i v a t i v e s . Borneo crudes cont a i n from 25$ t o 40$ aromatics of which 6$ t o 7$ i s o f the naphthalene s e r i e s . In t h e r e f i n e r y , the crude o i l i s " f r a c t i o n a t e d * 1 (21) i n t o s e v e r a l d i f f e r e n t " c u t s " i n s t i l l s and f r a c t i o n a t i n g towers. The s t r a i g h t run products ares G a s o l i n e . Gleaners' naphtha. Kerosene. D i e s e l F u e l . F u e l O i l . "Bunker G " O i l . Asphalt Residue. Gasoline i s the most v a l u a b l e f r a c t i o n and the "cook" i s made i n a manner t o o b t a i n t h e l a r g e s t y i e l d of t h i s f r a c t i o n . The residue l e f t i n the s t i l l i s blown with oxygen or a i r t o form asphalt of the d e s i r e d hardness. A s p e c i a l "Qook" i s made of a d i f f e r e n t charging stock t o produce l u b r i c a t i n g o i l . G a s o l i ne: Gasoline i s the f r a c t i o n o f petroleum d i s t i l l i n g between the temperatures o f 30° and 220° G. In the p a r a f f i n s e r i e s t h i s corresponds t o the range o f compounds from C,- t o C« . Gasolines may be d i v i d e d i n t o t hree c l a s s e s . (1) Gasinghead g a s o l i n e , ( n a t u r a l gas gasoline.) (2) S t r a i g h t run g a s o l i n e . (5) Cracked g a s o l i n e . Gasinghead g a s o l i n e i s a very v o l a t i l e f r a c t i o n recovered from t h e n a t u r a l gas of o i l w e l l s . I t s o n s i s t s o f pentane, hexane, heptane and p o s s i b l y a t r a c e o f octane, which have been (22) v a p o u r i z ed by t h e i r h i g h vapour pressures when present i n s m a l l percentages i n the n a t u r a l gas. Various methods are used f o r s e p a r a t i n g t h i s g a s o l i n e from t h e gas. (1) by expansion condensation. (2) by d i s s o l v i n g i n lube o i l I n c l o s e d c i r c u i t . (S) by s e l e c t i v e a bsorption I n charcoal towers. (4) by condensation by l i q u i d a i r . (when helium i s being recovered.) T h i s / t y p e o f ga s o l i n e has a H. U. C. ft. around 5?1 and i s a good f u e l f o r spark i g n i t i o n engines. Because o f i t s very v o l a t i l e n a t u r e , i t s p r i n c i p a l use i s as a blending agent t o i n c r e a s e t h e v o l a t i l i t y o f g a s o l i n e , p a r t i c u l a r l y a v i a t i o n and wi n t e r grade g a s o l i n e s . S t r a i g h t run ga s o l i n e i s the f i r s t " c u t * taken i n f r a c t i o n a t i n g column o f the r e f i n e r y . I t has a b o i l i n g range o f about 60° t o 220° C. The p r i n c i p a l c o n s t i t u e n t s are p a r a f f i n and naphthene hydrocarbons, only a small percentage o f aromatics and o l e f i n s being present* Approximate average analyses f o r s t r a i g h t run g a s o l i n e s from d i f f e r e n t t y p i c a l crudes are givens Crude P a r a f f i n s O l e f i n s Naphthenes Aromatics Pennsylvania 75 7, g 20 3 Oklahoma 71 3 25 3 4.. Texas 68 5 25 C a l i f o r n i a 56 4 38 g Smackover (Ark.) 66 8 15 j± (25) F o r many years s t r a i g h t run and casinghead g a s o l i n e s were the only f u e l s used i n spark i g n i t i o n engines. When the demand f o r g a s o l i n e exceeded the production o f s t r a i g h t run and casinghead g a s o l i n e , t h e c r a c k i n g process came i n t o commercial use. The elementary p r i n c i p l e s o f c r a c k i n g hydrocarbons had been known f o r s e v e r a l y e a r s p r e v i o u s l y , but t h e f i r s t r e s u l t s were poor, g i v i n g h i g h y i e l d s of non-condensable gases and of coke. Cracking methods today give from 50% t o 75% (of the crude) y i e l d i n the g a s o l i n e f r a c t i o n s . T h i s cracked g a s o l i n e i s s u p e r i o r i n anti-knock p r o p e r t i e s t o the s t r a i g h t run g a s o l i n e from the same crude. T h i s i s due t o the i n c r e a s e d aromatic and o l e f i n i c content and the decreased p a r a f f i n content. Approximate analyses of cracked g a s o l i n e s from the same crudes as the s t r a i g h t run g a s o l i n e analyses p r e v i o u s l y g i v e n , a r e i Crude. P a r a f f i n s O l e f i n s Haphthenes Aromatics Pennsylvania 57 9 1% 21 Oklahoma 55 9 14 24 Teaas 27 20 27 26 C a l i f o r n i a 42 20 17 21 Smackover (Ark.) 45 14 14 29 These f i g u r e s do not represent any p a r t i c u l a r j ga s o l i n e s , but are averages of a l l the t y p i c a l cracksd g a s o l i n e s from t he f i e l d s they r e p r e s e n t . By comparison with the f i g u r e s given on s t r a i g h t run g a s o l i n e s i t can be seen t h a t t h e average change i n the type o f (84) hydrocarbons i n a g a s o l i n e i s : P a r a f f i n s - decreased 20% O l e f i n s - in c r e a s e d 10$ Naphthenes - decreased 10% Aromatics - i n c r e a s e d 20% P a r a f f i n 2.0 7. "Olefin Naphthene 20% Aromatic The changes t a k i n g p l a c e i n an o i l subjected t o p y r o l y s i s or " c r a c k i n g " are considered t o be as f o l l o w s : ( l ) P a r a f f i n s o f high molecular weight are "cracked", a t a temperature o f 400° t o 500° C, i n t o p a r a f f i n s of lovrer molecular weight and unsaturated a l i p h a t i c hydrocarbons ( c h i e f l y o l e f i n s ) . p a r a f f i n p a r a f f i n +• o l e f i n (m + m' = n) (g) Unsaturated a l i p h a t i c s ( o l e f i n s ) condense t o r i n g compounds with a r e d u c t i o n i n molecular volume of 16 u n i t s . T h i s r e a c t i o n i s maintained u n i d i r e c t i o n a l by pressures of 600 t o 800 #/sq. i n . i n the cracking s t i l l . o l e f i n — 9— naphthene (25) (3) Conversion of saturated c y c l i c hydrocarbons t o benzenoid hydrocarbons. (The l i b e r a t e d hydrogen i s l a r g e l y absorbed by h i g h l y unsaturated compounds changing them t o o l e f i n s . ) T h i s r e a c t i o n does not reverse because of the g r e a t e r s t a b i l i t y of the benzenes. CtH, + H 2 *- (1R>*H, ^C 4R>3H\ cyclohexane —>-cyclohexene chyclohexadiene -?-benzene A l l h i g h l y unsaturated compounds are removed from r e f i n e d g a s o l i n e along w i t h t h e sulphur compounds. The o l e f i n s may be considered t o be the only unsaturated hydrocarbons remaining i n a good grade of g a s o l i n e . While not as chemically a c t i v e as the removed unsaturates, the o l e f i n s show a considerable tendency t o polymerize and form gums, when the g a s o l i n e i s s t o r e d , p a r t i c u l a r l y i n t h e s u n l i g h t . T h i s I s u n d e s i r a b l e and y e t the presence of the o l e f i n s i s d e s i r a b l e f o r t h e i r anti-knock q u a l i t y . F o r t u n a t e l y great progress has been made i n the f i e l d o f gum i n h i b i t o r s f o r g a s o l i n e . These enable the use o f a high o l e f i n content t o gain a h i g h anti-knock r a t i n g f o r t h e g a s o l i n e , without i t forming any gums. Most e f f e c t i v e among the compounds i n v e s t i g a t e d are: Thymol, P y r o g a l l o l , Catechol and Bydroquinone. OH Thymol OH P y r o g a l l o l OH (26) OH OH Catechol Hydroquinone " OH' Today p r a c t i c a l l y a l l gasolines on the market are blended g a s o l i n e s . They c o n s i s t l a r g e l y of s t r a i g h t run ga s o l i n e and cracked g a s o l i n e t o which a sm a l l f r a c t i o n of casinghead g a s o l i n e has been added. The cracked g a s o l i n e r a i s e s the anti-knock q u a l i t i e s o f t h e f u e l and the casinghead g a s o l i n e i n c r e a s e s the s t a r t i n g '•V v o l a t i l i t y . The g a s o l i n e may a l s o c o n t a i n a v e r y s m a l l amount of m e t a l l i c dope t o f u r t h e r r a i s e the anti-knock r a t i n g t o the current standard. D i e s e l F u e l : A wide range of petroleum f u e l s , from kerosene t o l i q u i d asphalts may be used i n d i e s e l engines, but the term " d i e s e l f u e l " i s a p p l i e d t o t h e f r a c t i o n best s u i t e d t o use i n t h e compression i g n i t i o n engine. The approximate b o i l i n g range of d i e s e l f u e l i s •200° t o 390° G. In the p a r a f f i n s e r i e s t h i s corresponds t o the compounds from C„ t o G2„ . Ordinary d i e s e l f u e l s have a range of 18° t o 26° Baume g r a v i t y and a Saybolt v i s c o s i t y up t o a maximum of 100 seconds at 100° F. In d i e s e l f u e l t h ere are the same f o u r types o f hydrocarbons t h a t occur i n g a s o l i n e . S t r a i g h t run d i e s e l f u e l contains a high percentage of p a r a f f i n s and naphthenes, and a s m a l l amount o f o l e f i n s and aromatics, i n much the same proporti o n s as the s t r a i g h t run g a s o l i n e from the same crude. In cra c k i n g f o r g a s o l i n e , t h e c r a c k i n g stock u s u a l l y i n c l u d e s the " d i e s e l f u e l " (27) f r a c t i o n , consequently the volume of t h i s f r a c t i o n i s much reduced. D i e s e l f u e l from t he f r a c t i o n a t i n g tower of the c r a c k i n g s t i l l , c o ntains a g r e a t l y reduced percentage of p a r a f f i n hydrocarbons, as t h i s forms the source of cracked g a s o l i n e s . The percentage volume of the unsaturated and c y c l i c compounds are s u b s t a n t i a l l y i n c r e a s e d . (28) Chapter IV Anti-Knock Ratings. Octane Scale f o r Gasolines? A l l gasolines of course, have not t h e same knocking tendency; t h e r e f o r e i t became necessary f o r r e f i n e r s and marketers of g a s o l i n e t o have some system of r a t i n g the anti-knock q u a l i t y of t h e i r product. Of the many systems proposed, the "Octane Scale" has proved the b e s t ? and has been standardized and p r a c t i c a l l y u n i v e r s a l l y , adopted. In 1926 Dr. Graham Edgar suggested t h e use of two pure hydrocarbons as f u e l standards f o r measuring t h e anti-knock q u a l i t y o f g a s o l i n e s . These standards are i s o - o c t a n e , from which t h e system takes i t s name, and normal heptane. Iso-octane, CjH l 8, i s a branched chain p a r a f f i n hydro-carbon, namely 2:2':4 t r i - m e t h y l pentane, o f e x c e l l e n t anti-knock q u a l i t y i n a. spark i g n i t i o n engine. I t i s a l s o sometimes c a l l e d t r i - m e t h y l i s o - b u t y l methane. I t s s t r u c t u r a l formula i s : : CH, . • t CH3 - C - CH Z- CH - CH3 CH3 CH. Normal heptane, G7'Alh i s a s t r a i g h t chain p a r a f f i n which knocks v e r y l o u d l y when burned i n a spark i g n i t i o n engine. I t s formula i s : CH 3 -(CH x) r-CH 3 (29) I t i s i n t e r e s t i n g here t o note t h a t normal octane C^H^as a f u e l shows v i o l e n t knocking c h a r a c t e r i s t i c s . I n the r a t i n g o f g a s o l i n e , the sample being t e s t e d i s matched f o r knocking tendency, i n a standard t e s t engine, by a mixture o f iso-octane and normal heptane. The "Octane Number" o f the g a s o l i n e i s t h e percentage by volume of iso-octane i n the mixture of iso-oetane and n-heptane t h a t e x a c t l y matches t h e knocking tendency o f the g a s o l i n e . ,"->\ Due t o the h i g h cost of these pure hydrocarbons (about $25 per g a l l o n ) secondary standard f u e l s are used. These are c a r e f u l l y standardized g a s o l i n e s of octane numbers 50 and 78, and are used f o r a l l r o u t i n e t e s t i n g o f f u e l s . Approximate Octane numbers f o r v a r i o u s types of f u e l f o r spark i g n i t i o n engines a r e : Iso-octane 0. N. 100 A v i a t i o n g a s o l i n e " 80 .Premium g a s o l i n e t r 76 Standard g a s o l i n e " 68 2nd grade g a s o l i n e " 50 n-heptane " 0 Many d i f f e r e n t types of apparatus have been used i n the past few years f o r r a t i n g the anti-knock q u a l i t y o f g a s o l i n e s . Recently, however, these have narrowed down, on t h i s continent at l e a s t , t o two p r i n c i p a l t y p e s . The E t h y l Gasoline Corporation (so) have developed and recommend f o r use the "S e r i e s 30" E t h y l knock t e s t i n g apparatus. The Wankesha Motor Company i n conjunction w i t h the Detonation Subcommittee of the Co-operative F u e l Research, have developed the "C. E . "R. Test Engine." The Series 30 E t h y l knock t e s t i n g apparatus c o n s i s t s o f a s i n g l e c y l i n d e r engine whose power output I s absorbed by a b e l t d r i v e n synchronous generator mounted on the same base. The set i s complete w i t h e l e c t r i c switchboard, knock measuring apparatus, and a l l x a c c e s s o r i e s necessary f o r - t e s t i n g . The s i n g l e c y l i n d e r o f the engine has overhead v a l v e s and a f i x e d compression r a t i o n of 7.75:1. The bore i s 2.5 i n c h e s , stroke 4.625 inc h e s , g i v i n g a d i s -placement of 22.71 cubic i n c h e s . The speed o f the engine i s mainta.ined constant at 600 R.P.M. by the 220 v o l t , 3 phase, 60 c y c l e A. C. generator which i s a l s o used f o r s t a r t i n g t h e s e t . A s p e c i a l 70 v o l t D. C. winding on the generator s u p p l i e s d i r e c t current f o r the i g n i t i o n system and t h e knock measuring instruments. Knock measuring i s done by the bouncing p i n type of i n d i c a t o r with e i t h e r a hydrogen or e l e c t r i c a l knock-meter. In t e s t i n g a f u e l , t h e compression pressure, which can be v a r i e d from about 200 t o 100 #/sq. i n . by t h r o t t l i n g , I s adjusted u n t i l an even (not v i o l e n t ) detonation i s obtained. The F u e l / a i r r a t i o and t h e angle o f spark advance are then adjusted t o g i v e maximum detonation. A mixture of standard reference f u e l s i s then found, by t r i a l and e r r o r , t o e x a c t l y match the c o n d i t i o n s (31) produced by the t e s t f u e l . The C. E. R. Test Engine i s a s i n g l e c y l i n d e r engine of 3.25 inches bore and 4.5 inches s t r o k e . I t s displacement i s 37.4 cubic i n c h e s . These c y l i n d e r dimensions have been chosen t o correspond c l o s e l y t o those o f an average automobile engine c y l i n d e r . The compression r a t i o i s v a r i a b l e from 4:1 t o 8:1 by means o f a moveable one pie c e c y l i n d e r and c y l i n d e r head. The v a l v e s are overhead and c e n t r a l l y placed w i t h the spark plug and the i n d i c a t o r opening.on e i t h e r s i d e . The i n d i c a t o r i s the bouncing p i n type which has been found t o be the si m p l e s t and most s a t i s f a c t o r y t y p e . The speed of the engine i s governed a t a constant 6G0 R.P.M. by an i n d u c t i o n generator which absorbs the power output. I g n i t i o n i s o p t i o n a l by c o i l o r magneto but c o i l i g n i t i o n i s found t o give the most c o n s i s t e n t r e s u l t s . The t e s t may be made on v a r i a b l e or f i x e d compression| the r e s u l t s being p r a c t i c a l l y i d e n t i c a l . Cetene Scale f o r D i e s e l F u e l s i A method o f r a t i n g d i e s e l f u e l s f o r t h e i r tendency t o knock, has r e c e n t l y been developed by G. D. Boerlage and J . J". Broeze i n t h e research l a b o r a t o r i e s of the Royal Dutch S h e l l Company at D e l f t , Holland. They measure the " I g n i t i o n Q u a l i t y " of a f u e l i n a s c a l e of "Cetene Numbers". The standards f o r the s c a l e are cetene (Cetene Number 100) and mesitylene (C.N^O) Cetene, C / 6H 3 i , i s a s t r a i g h t chain o l e f i n t h a t i g n i t e s r e a d i l y with .only a very short i g n i t i o n l a g . I t s formula (52) i s written: CH a - (CHj^-CH s CHa The zero end of the scale, mesitylene, w i l l not ignite at a l l i n diesel engines of ordinary compressions. Mesitylene, ! « , i s a symmetrical monocyclic aromatic. More specifically i t i s 1:5:5 tri-methyl benzene, and the structural formula i s : CH3 I C C I I H3G N C / SGH3 1 H Here i t might be mentioned that an iso-olefin of the same composition as cetene, C / 6HM , has very poor igni t ion quality. This i s hexa-methyl decene (also called tetra iso-butylene.) CH3 CH3 i i CH 3 - C - CH t - CH - ( C H j 2 - C - CH x - C S CHa I ' I I Cfi j CH j GH j GH ^  In determining the igni t ion quality of a fuel, 90 out of phase indicator cards are taken for several different compression pressures, both thrott l ing and supercharging with unchanged injection. (55) A l i n e i s drawn through the p o i n t s r e p r e s e n t i n g t h e end of the delay p e r i o d on each diagram; t h i s i s c a l l e d the "delay curve". Since t h e d e l a y curves are s i m i l a r and t y p i c a l f o r each f u e l , i t was seen t h a t f i n d i n g t h e d i f f e r e n c e i n the delays caused "by two pressures 2p and p was the same as f i n d i n g the delay caused by p. T h i s e l i m i n a t e s the n e c e s s i t y of a very accurate and d i f f i c u l t d etermination o f the time o f the beginning of i n j e c t i o n . The, p r a c t i c e i s t o determine t he d i f f e r e n c e i n delay between 50 and 15 atmospheres. The delay reading i s compared t o a curve of delay readings f o r v a r i o u s cetene-mesitylene mixtures. The percentage by volume o f cetene, i n a mixture of cetene and mesitylene t h a t would give t h e same delay p e r i o d as the f u e l being t e s t e d , i s the cetene number of t h a t f u e l . T h i s method does not a l l o w any p o s s i b -i l i t y o f o v e r r a t i n g the q u a l i t y of a f u e l t h a t i s burning w i t h unstable smoothness, ( i . e . anoextremely long delay p e r i o d and a slow pressure r i s e as t h e p i s t o n i s w e l l s t a r t e d on i t s downward stroke.) The engine used by Broeze and Boerlage i n t h e i r experimental work on cetene r a t i n g s i s a s p e c i a l f u e l t e s t i n g engine b u i l t by Thomassen, De Steeg of Holl a n d . I t i s a s i n g l e c y l i n d e r , s o l i d i n j e c t i o n 20 h.p. engine o f h o r i z o n t a l t y p e . Compression i s v a r i a b l e by means of a pl u g i n t h e centre o f the head, g i v i n g a range o f compression pressures from 375 t o 600 #/sq. i n . The power output i s absorbed by a h y d r a u l i c brake and a mechanical (34) governor a l l o w s a range of speed v a r i a t i o n from 100 t o 350 R.P.M. Knock r a t i n g i s done e n t i r e l y from out of phase i n d i c a t o r c a rds, e i t h e r photographic cards from a Maihak o p t i c a l i n d i c a t o r of t h e diaphragm t y p e , o r even o r d i n a r y p e n c i l i n d i c a t o r c ards. T h i s l a t t e r i s p o s s i b l e as only measurements of t h e delay p e r i o d are made. As a l l the measurements ar e comparative and are matched w i t h comparative measurements r e f e r r i n g t o t h e standard f u e l s , the a c t u a l engine does not g r e a t l y a f f e c t the r e s u l t s . The cetene r a t i n g of any f u e l , obtained on p r a c t i c a l l y any type of d i r e c t i n j e c t i o n engine, w i l l check w i t h i n a v e ry narroiv margin of the r a t i n g of the same f u e l obtained i n the Thomassen engine. (55) Chapter 7 Combustion and Combustion Knock Combustion Process i n a Spark I g n i t i o n Engines I n a g a s o l i n e engine, an i n t i m a t e mixture of a i r and g a s o l i n e vapour are drawn i n t o t he c y l i n d e r during the s u c t i o n s t r o k e . On the compression strok e the pressure and temperature of the f u e l charge In the c y l i n d e r a r e r a i s e d u n t i l the mixture i s almost at i t s i g n i t i o n temperature* The compression pressures a t t a i n e d by d i f f e r e n t compression r a t i o s are approximately; Compression Ratio 4.0 6.-5 Compression Pressure p r1.4 120# . :.,i;s9#'-, 179# ; goo# The p e r i o d of combustion begins w i t h the i g n i t i o n of the mixture by a spark, which may occur any time from 45° before T.D.C. up u n t i l T.P.G, depending on the speed of the engine. The combustion process occupies a remarkably short p e r i o d of time, ranging approximately from 5/1000 of a second at 1000 r.p.m., t o 2/1000 of a second at 4000 r.p.m. The extreme shortness of t h i s time makes t h e study and a n a l y s i s of the combustion process very d i f f i c u l t . One of the few methods p o s s i b l e i s the examination of 90° out of phase i n d i c a t o r cards of the few types of i n d i c a t o r s capable of these speeds. An out of phase i n d i c a t o r card ( i d e a l ) j (56) f o r a spark i g n i t i o n engine i s shown below: » 1 o , 1 -— v • to 10 From examination o f such cards t h e combustion process i s g e n e r a l l y regarded as o c c u r r i n g i n two phases. . Phase Is The b u i l d i n g up of a s e l f propagating ; flame nucleus, i n c u r r i n g no appreciable r i s e i n pressure above the compression curve. Phase I I : The spread of the flame throughout the mixture. Immediately before the beginning o f the combustion process, the c o n d i t i o n s e x i s t i n g i n the charge i n the c y l i n d e r are: - a high pressure (say 150 #/sq.in.) - a high temperature approaching the i g n i t i o n temperature of the mixture, produced by'Conduction of heat from uncooled c y l i n d e r p a r t s , and by head of a d i a b a t i c compression. (37) - a considerable amount of turbulence remaining from the v e l o c i t y w i t h which t he mixture entered th e c y l i n d e r . On the passage o f the spark across t h e gap of the spark p l u g , the f u e l vapour at t h i s p o i n t r e c e i v e s the s l i g h t k i n d l i n g i t r e q u i r e s , and i g n i t e s . Inflammation spreads slowly by conduction t o the f u e l vapour immediately adjacent t o t h i s s m a l l nucleus. Turbulence o f the charge tends t o prevent t h i s nucleus from growing, but i n s t e a d i t i s spread and moves through a considerable p o r t i o n of t h e mixture, f u r t h e r h e a t i n g i t . Phase I> which i s c a l l e d the "Delay P e r i o d * as app r e c i a b l e amount of combustion has yet occurred, occupies a d e f i n i t e i n t e r v a l o f time ( u s u a l l y something l e s s than 2/1000 of a second) which i s more or l e s s independent of engine speed. Consequently Phase I i n terms of crank angle v a r i e s d i r e c t l y as the engine speed, The d u r a t i o n i n time o f t h e delay p e r i o d depends on: ( l ) The chemical nature of t h e f u e l . (?) The f u e l / a i r r a t i o (mixture strength) (3) The temperature o f t h e combustible a t t h e time of i g n i t i o n . (4) The pressure o f t h e combustible a t the time Of I g n i t i o n . I n Phase I I , t h e temperature o f the mixtures have been r a i s e d much h i g h e r than at t h e beginning of Phase I , and the flame (58) n u c l e i spread r a p i d l y and merge. The flame f r o n t so formed sweeps r a p i d l y through the remaining unburnt mixtures. The ra t e of pressure r i s e i n Phase I I and a l s o the maximum pressure a t t a i n e d depend on: (1) The chemical nature of the f u e l . (2) The shape o f the combustion chamber. (5) The turbulence of the mixture. As Phase I I depends on t h e turbulence i n the c y l i n d e r , which v a r i e s d i r e c t l y as the engine speed, the time occupied by Phase I I w i l l decrease as the engine speed i n c r e a s e s . T h i s means t h a t Phase I I w i l l be p r a c t i c a l l y constant i n terms o f crank angle. The t a b l e below gives some id e a of the times occupied by t h e combustion process at d i f f e r e n t engine speeds i n an average spark i g n i t i o n engine® . Phase I Pha se I I T o t a l Speed Time / 0 Crank Angle Crank ; Angle Time Crank Angle Time Granlc Angle Time r.p«w<M Sec. deg. sec. sec. . d e S v sec. 500 : 000,355;; 5 . : .00167 \ : I* .00500 20 .00667 1000 ;..000,167. \ 10 .00167 15 .00250. • 25. ' .00417 1500 V; .000^111 - \ u .00167: -00166 30 : . 0 0 5 3 3 2000 I: -000^085 ,3 .00167 -.15, '.iW.00125'/ : .00292 2500 .000,066,7 • 25 ; .00167 \ 15 ;ooioo : 40 : .00267 3000 .000,055,5 30 .00167 ^ 15 .00083 45 .00250 5500 .000,047,6 35 .00167' 15 .00071 50 .00258 4000 .000,041,7 40 .00167 15 .00062 ; 55 .00229 (59) Detonation i n a Spark I g n i t i o n Engine: The .smoothness -with which an engine runs, depends on the manner i n which Phases I and I I merge, as w e l l as t h e r a t e o f pressure r i s e per degree crank angle i n Phase I I . The phenomenon of detonation, however, i s only connected with t h e l a t t e r f a c t o r . Detonation occurs i n the l a s t p a r t o f Phase I I and by the most popular conception i s considered t o be an e x p l o s i o n wave t r a v e l l i n g a t h i g h v e l o c i t y through the combustible. T h i s wave o f h i g h d e n s i t y gases s t r i k e s the c y l i n d e r • w a l l at h i g h v e l o c i t y causing a m e t a l l i c r i n g i n g sound d e s c r i b e d p h o n e t i c a l l y as " p i n g i n g " . The v e l o c i t y o f the detonation wave, from experiments i n s t a t i c c y l i n d e r s * i s assumed t o be i n the v i c i n i t y o f 50$ g r e a t e r than the normal v e l o c i t y o f flame t r a v e l i n the engine c y l i n d e r . During t h e process o f combustion the thermal energy i n t h e c y l i n d e r i s i n c r e a s i n g . The o x i d a t i o n r e a c t i o n progresses a t a c o n s t a n t l y i n c r e a s i n g temperature and t h e r e f o r e at a c o n s t a n t l y a c c e l e r a t i n g r a t e . Detonation occurs when the v e l o c i t y o f r e a c t i o n becomes so great t h a t t he flame f r o n t compresses before i t unburnt gases, h e a t i n g them f a s t e r (by a c e r t a i n margin) than they can d i s s i p a t e the heat by r a d i a t i o n , conduction and convection. The unburnt gases so compressed and. heated i g n i t e almost simultaneously, i n c r e a s i n g the v e l o c i t y o f the flame f r o n t t o an extremely high v a l u e . The experimants of "vVithrow, L o v e l l and Boyd have shown (40) evidence t h a t , i n detonation, the in c r e a s e d flame speed occurs only i n the l a s t q uarter of the maximum dis t a n c e t o be t r a v e r s e d from the spark p l u g . The f o r e g o i n g explanation of detonation i s known as the "Detonation Wave Theory". There are many other t h e o r i e s , chemical and p h y s i c a l , o f the mechanism o f detonation. The chemical t h e o r i e s f o r the most pa r t r e f e r t o the detonation o f p a r a f f i n hydro-carbons. Some of the t h e o r i e s put? forward are: \) flydroxylation Theory Peroxide Theory Theory o f - P r e f e r e n t i a l O x idation 'Theory o f Thermal Decomposition R a d i a t i o n Theory E l e c t r o n Theory Eree Hydrogen Theory' The H y d r o x y l a t i o n Theory of the mechanism o f the o x i d a t i o n o f p a r a f f i n hydrocarbons assumes a chain of r e a c t i o n s . The sequence i s one of successive o x i d a t i o n s : p a r a f f i n a l c o h o l g l y c o l ~* a l d e h y d e a c i d o r decomposition t o carbon monoxide and hydrogen. The r e a c t i o n s i n the case of methane are i l l u s t r a t e d : ^ F o r m i c A c i d Methane •-*> Methanol-=> Methene Glycol-#>Formaldehyde^ ~Carbon Monoxide * and Hydrogen w ; H.. ,0 \p • H-G -H —H-C-O-H H H-C H"G ;H / II ^ H ^ C - O - H " " ^ . ' H : : V O - H H fl • 0 H-G-H ^-H-G-0-H H ^ " - ^ H-G" — — C O H , H . H. H (41) The hydroxylation theory is supported by evidence of the presence of most of the intermediate compounds in the products of slow oxidation of paraffin hydrocarbons. Knocking is attributed to the rapidity of the reactions. The Peroxide Theory assumes the formation of unstable peroxides as one of the first products of oxidation. The amount of peroxides formed is not sufficient to cause detonation, but the peroxide acts as a primer increasing the velocity of the reaction of the rest of the fuel. The primary reaction of the peroxide equation is illustrated by the structural equation below in which R represents any alkyl group C^H^,., H H : V'tf. -^ ' • . ' : ' f l r " H V R-C-C-R , 0 i — ^ R - C - 0 - O - C - R or R-Q-C-R 1 1 • I • f '•-lirAf ':..;';----.v: -.-^ - B •• 6 paraffin 4- oxygen -*-di-alkyl peroxide or alkyl hydrogen peroxide The Theory of Preferential Oxidation assumes a direct reaction of the hydrogen of the hydrocarbon with oxygen to form water and an unsaturated hydrocarbon. This explains the difference in knocking tendencies of paraffin and olefin hydrocarbons. This theory is also supported by evidence of an increase in the number of molecules at the moment of reaction, which i s contradictory to the peroxide theory. .The Theory of Thermal Decomposition is, in a way, a modification of the Detonation Wave Theory. It supposes that, due to the excessive temperatures accompanying detonation, thermal ( 4 2 ) decomposition of the paraffin hydrocarbons takes place i n the high pressure area immediately ahead of the flame front. The freshly cracked products are in highly active state and combustion Is accelerated to detonating ve loc i t i e s . The Radiation Theory i s based on the assumption that the flame front gives off radiations activating the unburnt mixture ahead of the flame. The nature of the combustion of paraffins i s supposed to give more radiations than other hydrocarbons, activatin the unburnt mixture and resulting i n accelerated .combustion. The Electron Theory assumes that the flame i s prop-agated by a wave of electrons and that the combustion of paraffin hydrocarbons liberates more electrons than other hydrocarbons. The excessive number of electrons cause an increase rate of burning resulting in detonation. The Free Hydrogen Theory i s an old one based on the explosive combination of hydrogen and oxygen. It suggests that hydrog carbon and l ight hydrocarbons are formed soon after the gasoline vapour enters the cylinder. This last mentioned theory has found l i t t l e confirming evidence i n research, and may be disregarded along with numerous other theories of detonation, too fanciful to mention. Combustion Process i n a Compression Ignition Engine; In a diesel engine, a charge of a i r i s drawn into the cylinder during the suction stroke and i s raised to a high pressure and temperature on the (45) compression stroke. The compression pressures i n f u l l d i e s e l engines are much higher than i n gasoline.engines: Compression Ratio 10 12 14 16 -:. Compression Pressure P, r-4 366# : .-',472# ;'587# 708# i s the crank hears top dead centre on compression, i n t r o d u c t i o n of the f u e l i n t o the cyl i n d e r begins. This may be e f f e c t e d by e i t h e r of two methods: - by hydraulic pressure* known as "solid* 1 or " a i r l e s s i n j e c t i o n . " - by a blast, of high pressure a i r , known as " a i r i n j e c t i o n . " In the high speed engines,"'which we are considering, s o l i d i n j e c t i o n i s u s u a l l y used. The i n j e c t i o n of the f u e l i s not instantaneous! if••'for no-other than mechanical considerations, i t occupies an appreciable period of time. In the f u l l d i e s e l engine, i g n i t i o n of the f u e l i s by the heat of compression alone, no spark or other device being used. The period of combustion, therefore, Is measured from the beginning of i n j e c t i o n , and from an examination of 90° out of phase Indicator cards, the combustion process may be divided i n t o three phases: Phase I : Delay period; no appreciable pressure . . r i s e above the compression curve. (44) Phase I I : Spread of the flamej a l l the fuel already ignited burns. Phase I I I : Controlled combustion; fuel burns as injected, at t i p of nozzle. £00 w i . x t Q U < \ \ \ \ \ s > s v. 70" 60° 3o° TOC • 3 0 ° 7°° Phase I cf the diesel combustion process parallels Phase I in the spark igni t ion engine. I t i s a delay period in which either no igni t ion takes place or Ignition i s confined to some very localized nucleus. Fuel, usually unheated, i s injected in a. fine spray, into the a i r i n the combustion chamber. This a i r i s at a pressure in the v i c i n i t y of 500 #/sq. i n . and a corresponding high temperature. The fuel enters the combustion chamber with a high velocity and meets the a i r which i s in a state of great turbulence. The fuel drops are rapidly raised i n temperature. The time occupied by Phase I Is the time required for the f i r s t of the fuel injected, to heat up from i t s injection temperature to i t s self ignit ion temperature ( S . I . T . ) The "delay period" or "ignit ion l ag n therefore occupies a constant period of time, ( 4 5 ) (usually between .001 and .002 seconds) that i s pract ical ly independent of engine speed. The duration of Phase I in.time depends on: (1) the chemical nature of the fuel, particularly i t s S. I . T. (2) the temperature of the a i r in the cylinder at the time of injection. (§) the pressure of the a i r i n the cylinder at the time of injection. - ' ( 4 ) . the spray characteristics, particularly the fineness of the finest part icles. I t i s an unsettled question whether combustion takes place on the surface of the fuel droplets or i n a par t ia l vapourlzation near their surfaces. Calculations have been made, to show that, the time of the delay period i s too short for the droplets to be heated throughout. This may prove that a l l the fuel i s not vapourized, but i t does not prove that the surface of the droplet i s not vapour!zed, before combustion takes place. Therefore, i t Is a d i f f i cu l t question to determine whether combustion occurs in the l i q u i d phase, or in the near-liquid vapour phase. In either case, i t does not greatly affect the larger part of the theory of combustion in the compression igni t ion engine. When Phase II begins, a considerable quantity of fuel has already been injected into the cylinder; this may amount to about half of the to t a l fuel charge or even more. Much of this i s already heated nearly to i t s Ignition temperature, and the flame spreads rapidly through i t aided by the turbulence of the a i r . This (46) causes a rapid pressure and temperature r i se , and the combustion accelerates u n t i l i t has caught up with the injection. By this time (the end of Phase II) a very high temperature and high pressure-have been attained. The maximum rate of pressure rise in Phase II depends on: (1) the chemical nature of the fuel . (2) the spray characteristics. (3) the turbulence o f the a i r In the cylinder. (4) the maximum pressure and temperature due to compression. The pressure attained at the completion of Phase II depends on, i n addition to the above four items: (5) , the duration of the delay period. (6) the rate of fuel Injection during Phases I and I I . In Phase I I I the rate of combustion follows the rate of Injection. The fuel burns almost immediately after leaving the nozzle t i p , i n the swir l of very high temperature a i r and burnt gases, that s t i l l prevails i n the cylinder. The rate of pressure r ise i s not as rapid as in Fnase I I and sometimes may only be a maintenance of the pressure attained at the end of Phase I I . This i s the only resemblance that the cycle of the modern high speed, sol id injection, compression ignit ion engine bears to the original constant pressure cycle proposed by Dr. Diesel. The cycle of th i s (47) engine has become a11 combined cycle" or even sometimes only a constant volume or Otto cycle. As i n the spark Ignition engine. Phase I i s constant i n time and varies i n terms of crank angle, directly as the engine speed. The relation between the extent of phase II and engine speed i s not as direct . An increase i n speed means that more fuel w i l l be injected during Phase I , giving a greater quantity of fuel to be burnt in Phase I I . But turbulence of the a i r w i l l increase directly ' as the engine speed giving a proportional increase i n the time rate of pressure r i se . There w i l l also be an increase in the rate of pressure rise per degree crank angle, caused by. the increased cylinder temperatures accompanying the higher maximum pressures at higher speeds, therefore Phase I I w i l l occupy an approximately constant crank angle at different engine speeds. The extent of Phase I I I i s mechanically governed and depends on the load on the engine. However, for a given amount of fuel injected, (or a constant mean effective pressure) Phase I occupies a greater proportion of the tota l period of combustion at higher speeds and the extent of Phase I I I i s consequently reduced. Combustion Shock i n a Compression Ignition Engine? Knocking caused by the combustion i n a diesel engine depends on two factors. They are represented on the out of phase indicator card by: (l) the angle with which the combustion l ine leaves the compression l ine , i . e. the Both. 6f these factors have been found to depend directly on the ignit ion lag . The accompanying figure representing out of phase indicator card shows the effect of increased delay period with fixed injection setting. It can readily be seen how the angle of "breakaway" varies with the ignit ion lag. Careful measurement of such cards for different fuels has revealed that a l inear relation exists between ignit ion lag and the maximum rate of pressure r i s e . The rate of pressure rise associated with audible knocking i s something of the order of 50 and over #/sq.in. /o 1 crank angle, bat this i s largely dependent on cylinder size and other factors. Combustion shock may be reduced in two ways: ( l) by decreasing the ignition lag or delay period. (49) (g) by injecting less fuel during the delay period. This second method requires a variable rate of injection which involves mechanical complications of the injection system, which are undesirable for thfe high speed engine. Methods of decreasing the time of the delay period will be discussed in the following chapters. (50) Chapter V I I n f l u e n c e of the F u e l C h a r a c t e r i s t i c s on Combustion. . Of the s e v e r a l f a c t o r s t h a t determine the nature .of the combustion i n an engine, probably the most important i s the chemical nature of t h e f u e l . The thermal s t a b i l i t y of the c o n s t i t u e n t s i s t h e p a r t i c u l a r f a c t o r o f the chemical c h a r a c t e r i s t i c s which determines whether the combustion of the f u e l w i l l be smooth or rough. The thermal s t a b i l i t y of a compound depends on the a n a l y s i s and the molecular weight, and s t r u c t u r e ; i t i s e x h i b i t e d i n the S e l f I g n i t i o n Temperature and the v e l o c i t y of combustion o f t h e fuel.'' I n t h e spark i g n i t i o n engine, i t was seen t h a t a low flame v e l o c i t y i s d e s i r a b l e t o a v o i d detonation and a high S. I . T. of t h e f u e l . m i x t u r e t o prevent any p o s s i b i l i t y o f p r e - i g n i t i o n . T h i s means t h a t a f u e l should have high thermal s t a b i l i t y f o r use i n the h i g h e f f i c i e n c y , spark i g n i t i o n engine. The normal p a r a f f i n hydrocarbons w i t h t h e i r long chain s t r u c t u r e s , have l o w - i g n i t i o n temperatures and.high r a t e s o f b u r n i n g , making them undesirable components of g a s o l i n e . Isomeric p a r a f f i n s whose s t r u c t u r e are compact have, on the other hand, h i g h i g n i t i o n temperatures and low r a t e s of burning. The r i n g s t r u c t u r e s of naphthene hydrocarbons are a l s o compact and these compounds are a l s o t h e r m a l l y s t a b l e . Short s i d e chains on the (51) naphthene r i n g add t o the thermal s t a b i l i t y o f the molecule but a long side chain does'not. The presence o f double bonds between carbon atoms, or p o s s i b l y t h e presence of l e s s hydrogen i n the molecule, seems t o add g r e a t l y t o the thermal s t a b i l i t y (but not t o the chemical • s t a b i l i t y ) o f t h e f u e l , f o r o l e f i n s show great improvement over the corresponding p a r a f f i n s , and the aromatics are b e t t e r than t h e corresponding naphthenes. Therefore, from the standpoint of anti-knock q u a l i t y , the order of t h e d e s i r a b i l i t y of hydrocarbons i n g a s o l i n e i s : aromatics, naphthenes, normal o l e f i n s and l a s t , normal p a r a f f i n s . Some is o m e r i c chain compounds are, however, comparable t o the aromatics i n anti-knock v a l u e . The c r i t e r i o n f o r a h i g h anti-knock q u a l i t y of a hydrocarbon f u e l f o r a spark i g n i t i o n engine, i s a compact molecular s t r u c t u r e . The presence of an atom of oxygen i n the middle o f an a l i p h a t i c chain g r e a t l y reduces the thermal s t a b i l i t y , as i s shown b y the ethers: C„E a n f l - 0 - C m I W + , . However, t he a l c o h o l s have a v e r y s i m i l a r s t r u c t u r e C„Ha,,,.,-OH, but have e x c e l l e n t anil-d e tonating q u a l i t i e s . Here the oxygen atom belongs t o a hyd r o x y l group, and the a d d i t i o n of a short a l k y l (CH*,,,), of a hydroxyl (Cfl) o r of an animo group (NHi) as si d e chains, seems t o add t o the thermal s t a b i l i t y o f the molecule, i n c r e a s i n g i n e f f e c t i n the order named. The a n a l y s i s and molecular weight o f a compound have no gener a l r e l a t i o n t o the anti-knock p r o p e r t i e s of the compound as (52) an engine f u e l . However i n the case of hydrocarbons, a higher C/H r a t i o ( i n d i c a t i n g t h e a n a l y s i s ) g e n e r a l l y i n d i c a t e s a h i g h e r ;ahfeknock r a t i n g i n . g a s o l i n e s . ; The d i s t i l l a t i o n curve of a gas o l i n e i s probably i t most important p h y s i c a l p r o p e r t y , and i t i s c l o s e l y watched by t h e r e f i n e r y t o give proper balance t o the f u e l . A low i n i t i a l b o i l i n g p o i n t i s d e s i r a b l e t o give s t a r t i n g v o l a t i l i t y and a low f i n a l b o i l i n g p o i n t o r end poi n t prevents f u e l wastage by vapourized g a s o l i n e passing unburnt out t h e exhaust o r running down the c y l i n d e r w a l l s t o give crank case d i l u t i o n . Road t e s t s have shown much b e t t e r f u e l economy on g a s o l i n e o f low end p o i n t . Examples of d i s t i l l a t i o n curves f o r motor and a v i a t i o n g a s o l i n e s are given below. I t w i l l be n o t i c e d t h e c h i e f d i f f e r e n c e i s i n a v e r y low end point f o r a v i a t i o n g a s o l i n e . Motor Gasoline • A v i a t i o n G a s o l i n e . I n i t i a l B o i l i n g Point i 32° c "• y 1 0 $ / c o n d e n s e d •; •• 55 • SO 20$ >"\;7o;:;-;; y;'-50$":: /,y \ y ' ' : " 9 0 : " " " y -y j & 6 y ' - - y ' \ r y '•',.C/' " ; :'"""V""'VvT20'::' y-.End;point;'':' 210 ' y . \ 1 5 0 Recovery at l e a s t Other p r o p e r t i e s of ga s o l i n e are mostly covered by s p e c i f i c a t i o n s and have not much bearing on the anti-knock r a t i n g (53) of the f u e l . These are: S p e c i f i c G r a v i t y , F l a s h P o i n t , Iodine Number: A c i d , Sulphur, Water, and Ash Content; Residue and Gum, C o l o r and Ddor. The s p e c i f i c g r a v i t y of a g a s o l i n e i s r e l a t e d t o the C/H r a t i o and t h e d i s t i l l a t i o n range and bears a r e l a t i o n t o the combustion i n those ways. The f l a s h p o i n t spec-i f i c a t i o h s a r e l a r g e l y a s a f e t y measure against the danger o f f i r e i n t r a n s p o r t i n g and s t o r i n g of g a s o l i n e . I t i s u s u a l l y set at about 150° F. The Iodine Number i n d i c a t e s the amount of unsaturated compounds present. A good g a s o l i n e should be f r e e from a c i d , water, ash and s u l p h u r . An o b j e c t i o n a b l e odor i s , , o f course, u n d e s i r a b l e . As f o r c o l o r , i t used t o be necessary t o have, g a s o l i n e water-white i n order t o s e l l i t . . Since g a s o l i n e s of h i g h anti-knock r a t i n g s have come i n t o vogue, t e t r a - e t h y l l e a d i s used i n n e a r l y a l l g a s o l i n e s on the market. It£z presence i s i n d i c a t e d by a dye, w i t h t h e r e s u l t t h a t the r e f i n e r now does not have t h e remove the o r i g i n a l y e l l o w i s h colour of the g a s o l i n e . I n the Compression I g n i t i o n Engine, i n order t o reduce knocking by changing the f u e l , one of short i g n i t i o n l a g i s necessary. Therefore, a f u e l of low S. I.. T. and low thermal s t a b i l i t y i s d e s i r a b l e . T h i s i s p r e c i s e l y the opposite of the c o n d i t i o n f o r a spark i g n i t i o n engine. In d i e s e l f u e l , l o n g chain p a r a f f i n s and o l e f i n s are d e s i r a b l e , w h i l e compounds of compact molecular forms are u n d e s i r a b l e . T h i s i n c l u d e s v e r y branched chain i s o - p a r a f f i n s and i s o - o l e f i n s , as w e l l as naphthenes and aromatics. The thermo-chemical s t a b i l i t y (54) of t h e aromatic hydrocarbons makes them v e r y •undesirable as d i e s e l f u e l components. The low thermal s t a b i l i t y o f the ethers g i v e s them e x c e l l e n t i g n i t i o n q u a l i t i e s f o r which reason they are sometimes used as blending agents to reduce knocking i n h i g h speed d i e s e l s . Quite opposite t o t h e i r e f f e c t i n spark i g n i t i o n engines, the s i d e chains of s m a l l a l k y l groups, of hydroxyl groups (as i n a l c o h o l s ) and of amino groups, here i n c r e a s e knocking. On the other hand, uns t a b l e n i t r o and aldehyde compounds g r a a t l y reduce knocking i n the d i e s e l engine. The r e l a t i o n between the d i s t i l l a t i o n range of a d i e s e l f u e l and t h e nature o f t h e combustion i s not c l e a r l y evident. Kerosene, f o r example, has a lower d i s t i l l a t i o n range and b e t t e r i g n i t i o n q u a l i t i e s than an average d i e s e l f u e l : but no conclusions may be based on t h i s f a c t , as the thermal s t a b i l i t y o f the kerosene f r a c t i o n i s lower. I t i s i m p o s s i b l e t o draw conclusions t h e o r e t i c a l l y on the s u b j e c t , because i t i s unknovm whether combustion occurs i n the vapour phase o r the l i q u i d phase, or whether p a r t i a l v a p o u r i z a t i o n i s necessary before i n i t i a l i g n i t i o n can take p l a c e . Other p r o p e r t i e s of t h e f u e l may o r may not a f f e c t the combustion. The s p e c i f i c g r a v i t y and v i s c o s i t y may v a r y over a wide range, but seem t o i n f l u e n c e t h e combustion only inasmuch as they a f f e c t the spray c h a r a c t e r i s t i c s : a t o m i z a t i o n and p e n e t r a t i o n . The f l a s h p o i n t of a d i e s e l f u e l as of a g a s o l i n e i s o f importance only as an insurance r e g u l a t i o n . The s p e c i f i e d f l a s h point i s u s u a l l y (55 ) 150° F. The burning p o i n t o f a f u e l may g i v e some i n d i c a t i o n o f i t s thermal s t a b i l i t y , but the c o n d i t i o n s i n an open cup and i n an engine c y l i n d e r a t h i g h pressures are so d i f f e r e n t , t h a t i t I s d o u b t f u l i f t h i s s p e c i f i c a t i o n bears much r e l a t i o n t o the nature o f the combustion,, The " i g n i t i b i l i t y " o f a d i e s e l f u e l i s a new s p e c i f i c a t i o n (1952) recommended by t h e A. S . T. I . , i , S , M, E. and S . A. E. No standard method has y e t been s p e c i f i e d f o r measuring the i g n i t i b i l i t y of a f u e l but the method should be such t h a t i t gives r e s u l t s bearing some r e l a t i o n t o the " i g n i t i o n q u a l i t y " as measured from the delay p e r i o d i n an a c t u a l engine and recorded i n aCetene Numbers". The maximum sulphur, water, ash and f r e e carbon i n d i e s e l f u e l are u s u a l l y s p e c i f i e d . Sulphur probably has no e f f e c t on i g n i t i o n and combustion but landoubtedly has a c o r r o s i v e e f f e c t on the c y l i n d e r and v a l v e s , not so much w h i l e t h e engine i s running, but when i t i s f i r s t stopped. Engines have run s u c c e s s f u l l y on d i e s e l f u e l w i t h as h i g h as 3$fe5$ suphur without excessive c o r r o s i o n of the exhaust v a l v e s , provided the, engine i s s t a r t e d and stopped on sulphur f r e e o i l . Ash, carbon and r e s i d u e s p e c i f i c a t i o n s are i n c l u d e d t o prevent the plugging o f f u e l l i n e s and i n j e c t o r s and t o minimize the s c o r i n g o f the c y l i n d e r w a l l s . The grade of. f u e l I s often i n c l u d e d i n the s p e c i f i c a t i o n s . A s t r a i g h t run f r a c t i o n i s much s u p e r i o r t o the d i e s e l f u e l flcut" from the f r a c t i o n a t i n g tower of a cra c k i n g s t i l l . T h i s i s t o because the compounds of l e a s t thermal s t a b i l i t y which are most d e s i r a b l e i n a d i e s e l f u e l are the ones most e a s i l y t h e r m a l l y decomposed i n the (56) cracking s t i l l . The straight chain compounds i n the d i e s e l f u e l range are reduced to a very small f r a c t i o n . Consequently a d i e s e l f u e l from the cracking s t i l l has very low i g n i t i o n quality and cannot be s a t i s f a c t o r i l y used i n a high speed d i e s e l engine. Therefore, i t i s often specified that the d i e s e l f u e l be a straight run f r a c t i o n , or a straight run topped crude, preferably from a p a r a f f i n base petroleum. In t h i s connection the Royal Dutch S f i e l l Company has estimated, a f t e r an extensive survey, that only 15% of the world's l i q u i d f u e l supply i s adapted f o r a good high speed d i e s e l f u e l . (57) Chapter V I I Influence of Engine F a c t o r s on Combustion In a spark. I g n i t i o n engineV there are s e v e r a l mech-a n i c a l f a c t o r s t h a t i n f l u e n c e the combustion process i n a manner determining whether combustion knocking occurs. The p r i n c i p a l .ones-are: , • (a) compression r a t i o . ,j (b) form of combustion chamber. (c) i n t a k e and c y l i n d e r temperature. (d) supercharging and t h r o t t l i n g . (e) time o f i g n i t i o n . ( f ) c a r b u r a t i o n (and a i r h u m i d i t y ) . Of these, t he f a c t o r which most determines whether the combustion of a c e r t a i n f u e l w i l l knock, i s the compression r a t i o . As.shown before, the thermal e f f i c i e n c y o f an engine improves w i t h an increase i n compression r a t i o u n t i l detonation occurs. Therefore, f o r a given f u e l , a compression r a t i o should be used t h a t keeps iy t h e V e l o c i t y of t h e flame f r o n t s l i g h t l y under detonating speeds. The i n f l u e n c e of a compression r a t i o h i g h e r than t h i s i s : •. : - V " ^'''to'-'^dd'to'.the' temperattu^e of: acttabatle . : compression, hastening the establishment /'"''•o.f'^the-flak'evn^cleuS'-Ih.'Piase'^I'.', ••'•'. (58) - to reduce the S. I. T. of the paraffin hydrocarbons i n the f u e l . - t o give a s l i g h t increment i n pressure t o .'x--,the high pressure zone ahead of,the flame /y,y-y:...\ , y-.\ front, : causing the speedof the flame t o increase to detonating v e l o c i t i e s . " : , ; An increase-in^ pressure may also be regarded as • bringing the-unburnt gases i n t o closer: contact; with the inflamed p a r t i c l e s , and so increasing the v e l o c i t y of the flame by increased-conduction of: head ahead of the flame. The form of the combustion chamber has a great influence on the nature of the combustion . Factors considered i n the design of the cylinder head for considerations of ef f i c i e n c y , maximum output and combustion knock, are: (1) provision f o r the free unobstructed entry of the f u e l / a i r mixture. (2) maintenance of turbulence. (5) surface / volume r a t i o . (4) no stagnant pockets. (5) ; position of the spark plug with special rggard to the maximum distance of flame t r a v e l . , Of these the factors d i r e c t l y influencing;the tendency towards detonation are the turbulence and the position of the spark plug.. Increased turbulence while i t increases the ( 5 9 ) rapidity of the combustion and the rate of pressure r ise , does not increase detonation. This i s so because the effect of the turbulence i s to spread the flame and to break up the flame front. Therefore, in the design of the cylinder head i t i s desirable to provide means of maintaining or producing maximum turbulence. The position of the spark plug i s important because in a given size of cylinder, i t determines the maximum distance of flame travel* As the oxidation reaction i s one of increasing energy and veloci ty, and detonating velocit ies are only reached in the last portion of the distance of maximum flame t ravel , i t follows that the tendency to detonate w i l l be less i f that distance i s reduced. Henqjg the spark plug i s placed almost centrally in the combustion chamber. used in practice, as characterized by the position of their valves; the "L" ,"I","FH,and racing type heads. These types ( i l lustrated diagrammatically below) achieve the general aims in design in different degrees. The racing type head most nearly approaches the ideal combustion chamber but unfortunately requires the expensive mechanism of two overhead camshafts. There are several main types of combustion chambers (60) The e f f e c t s of h i g h e r i n t a k e and c y l i n d e r temperatures are both t o produce higher maximum tepperatures (accompanied by lower e f f i c i e n c y and maximum o u t p u t ) . High temperatures a c c e l e r a t e the i g n i t i o n and combustion and st i m u l a t e detonation. Supercharging o f an aero-engine t o maintain i t s normal power output, a t h i g h a l t i t u d e s does not i n c r e a s e the knocking , tendency. I n f a c t the engine w i l l probably be running more smoothly supercharged a t an a l t i t u d e than unsupercharged at ground l e v e l , 'because of lower a i r i n t a k e temperatures. Supercharging t o in c r e a s e the power output of a g a s o l i n e engine i s accompanied by hi g h e r c y l i n d e r temperatures (and lower thermal e f f i c i e n c y ) and t h e r e f o r e i n c r e a s e s knocking. Supercharging has the same e f f e c t s on detonation as an increased compression r a t i o w i t h t h e exception of an in c r e a s e d compression temperature which remains: equal t o T, r ^ ' . The e f f e c t of i n c r e a s i n g the angle of spark advance without i n c r e a s i n g the speed, i n a ga s o l i n e engine, i s t o in c r e a s e the knocking tendency. Detonation may be reduced by r e t a r d i n g the spark ( w i t h i n l i m i t s ) > a l l o ther c o n d i t i o n s remaining constant. 'Carburation has not much bearing on detonation. The c a r b u r e t t o r does not vapourize the f u e l , as i s g e n e r a l l y imagined, but merely forms a f i n e l y d i v i d e d spKay. The v a p o u r i z a t i o n a l l takes place i n the i n t a k e m a n i f o l d and the c y l i n d e r s and,depends on the d i s t i l l a t i o n curve and s p e c i f i c heat o f the g a s o l i n e . (61) However, a c l o s e l y r e l a t e d subject t h a t does have a bearing on combustion knocking, i s t h e humidity of the a i r . For a l o n g time the importance o f t h i s f a c t o r was not recognized u n t i l i t was found t o account f o r the d i s c r e p a n c i e s i n knock r a t i n g s o f gasolines.. I n a German'research i t was found t h a t t he e f f e c t o f i n c r e a s i n g the humidity of the a i r from 50% to 85$ r a i s e d the octane r a t i n f o f a low 0. N. g a s o l i n e from 45 t o 57, and of a h i g h 0. R. g a s o l i n e from 85 t o 89. T h i s shows v e r y c l e a r l y t h e d e s i r a b l e e f f e c t o f high' humidity and the n e c e s s i t y of r a t i n g f u e l s at some standard humidity or a p p l y i n g a c o r r e c t i o n t o t h e r a t i n g f o r the humidity o f the a i r . I n t h e compression i g n i t i o n engine, s i m i l a r mechanical f a c t o r s I n f l u e n c e the smoothness of the combustion process. (a) compression r a t i o . (b) form of the combustion chamber. (c) i n t a k e and c y l i n d e r temperature, (c) supercharging and t h r o t t l i n g . (e) time of i n j e c t i o n . ( f ) spray c h a r a c t e r i s t i c s . Reducing the knocking, i n a d i e s e l engine u s u a l l y r e q u i r e s a r e d u c t i o n i n the extent of the delay p e r i o d . This i s probably most simply e f f e c t e d by i n c r e a s i n g the compression r a t i o . But as mentioned i n Chapter I , i t i s not advantageous t o continue i n c r e a s i n g the compression r a t i o , f o r at 15:1 C. R. the weight (62) and cost of the engine are increasing much faster than the thermal e f f i c i e n c y . Therefore i t i s desirable only t o increase the compression r a t i o enough to give satisfactory combustion. A higher compression pressure reduces the i g n i t i o n lag (and therefore the combustion shock) bys - increasing the temperature difference between the -.air -and the ••ignition temperature -.of the'-fuel*' (a) by increasing the compression temperature.. ':•-. (b) by decreasing the S. I. T. of the f u e l . - bringing the oxygen molecules of the a i r i n closer contact with the f u e l droplets. • The.people supporting the theory that combustion occurs on the surface of the f u e l drops, c i t e the fact that higher pressures increase the b o i l i n g temperatures of. the f u e l as well as lowering i t s S. I. T * Hence more f u e l w i l l remain unvapourized i n the drops and as the i g n i t i o n l ag i s decreased and the combustion improved, they conclude that the correct combustion occurs on the surface of the drops. The r e l a t i o n between combustion shock and the form of combustion chamber i s not c l e a r l y defined.; The p r i n c i p a l aim i n the. design of a combustion chamber'of a high speed d i e s e l engine i s to secure a high r e l a t i v e v e l o c i t y of the f u e l and the a i r , i n order that the combustion process may be completed i n the short time, allowed. The chief types i n use achieve rapid combustion but with varying e f f i c i e n c i e s and knocking tendencies. The p r i n c i p a l (63) types of high speed diesel heads are: (1) the direct injection type. (2) the fiicardo head. (5) the pre-eombustion chamber type. (4) the Aero system. (5) the auxiliary a i r chamber type. I' 'I (1) (Z) o o I1 'I (3) In the direct injection system the fuel i s injected i n a l l directions into an open combustion chamber i n which i n -discriminate turbulence exists . Very high injection pressures are used to give the fuel high i n i t i a l veloci ty . The success of the system depends largely on the design and accuracy of the jets . Good efficiencies are usually abtained but not the smoothest combustion. The Ricardo head consists of a cyl indr ical space above the piston about half the diameter of the cylinder. The (64) a i r i s made t o r o t a t e i n t h i s space a t a h i g h r a t e o f speed ( a i r r.p.m.=10 x crank r.p.m.) by t a n g e n t i a l p o rts outside the sleeve v a l v e . This does not mean that the combustion s w i r l s round and round; i f the combustion process occupies an average v a l u e o f about 56° of crank angle, the a i r i n the head would, make one complete r o t a t i o n during the time of combustion. T h i s means •., that. t h e . f u e l , .which i s : i n j e c t e d downwards a t the outer edge o f r :the swirl,../is•,always met by f r e s h a i r . , Very h i g h e f f i c i e n c i e s , are rdbtained i n t h i s type of engine and speeds up t o 2200 r.p.m.-have been reached. . In the pre-combustion chamber types the f u e l i s i n j e c t e d Into; a. chamber c o n t a i n i n g about h a l f of the :compressed a i r where Phases I and I I o f the combustion develop. The r a p i d pressure r i s e i n t h e pre-combustion chamber and the p i s t o n beginning i t s downward ...stroke cause .the p a r t i a l l y burnt gases and. the unburnt f u e l t o i s s u e through narrow o r i f i c e s at high v e l o c i t y i n t o the • c y l i n d e r where phase I I i s completed. The combustion takes place .smoothly and, the system i s capable o f - h i g h speeds ?and. o f handling f u e l of hig h thermal s t a b i l i t y . However, the E f f i c i e n c y i s not good and the power output f o r a given s i z e o f c y l i n d e r i s v ery l i m i t e d . The Aero system uses an a i r chamber (which may be, l o c a t e d e i t h e r i n t h e c y l i n d e r head or the p i s t o n ) connected t o the, b l i n d e r - b y a .narrow" neck.: The a i r - c e l l contains about, h a l f (65) of the compressed a i r and the i n j e c t i o n of the f u e l i s directed either at or across the mouth of i t . The i g n i t i o n takes place i n the neck and the combustion of Phase I I o s c i l l a t e s about that point and i s f i n a l l y drawn down into the cylinder. Phase I I I takes place with the a i r issuing from the a i r c e l l , d i r e c t l y at /the f u e l j e t . The system Is very sensitive and-requires perfect balance of seventl factors to work s a t i s f a c t o r i l y . I f much f u e l penetrates to the a i r - c e l l the rate of pressure r i s e i s excessive and knocking occurs. Properly designed and adjusted an engine of t h i s type runs smoothly and very high speeds are possible. The A. Ei Gi b u i l d a s i x cylinder, truck engine under Aero patents that has a speed range from 300 to 3000 r.p.m. . The- a u x i l i a r y a i r chamber type i s r e a l l y a combination of the pre-combustion and a i r chamber types. The a i r i s compressed IntoVtwo spherical chambers of approximately the same s i z e . ' The f u e l Is injected Into the f i r s t chamber where Phases I and I I :" develop., As the piston moves downward,, the partly burned products pass through a narrow throat into the cylinder with great turbulence\ The a i r from' the a u x i l i a r y chamber expands into the pre-combustion chamber to supply a i r at the inje c t o r f o r phase I I I . The system has- ~a;6Vantages\over '-t;he.-,.pre^ combustl&n-'chsdBber -type -fcptithe.' combustion i s Inclined to be rough from the exeessive turbulence. Increase i n intake and cylinder temperatures improve the smoothness Of the combustion but decrease the maximum output and (66) efficiency of the engine. As they affect the masimum temperature of the cycle a l imi t i s imposed on their increase by lubrication factors. Supercharging to increase the power output and throt t l ing hare a marked effect on the combustion shock in the diesel engine. As the compression temperature remains the same, this must be wholly due to the change in the cylinder temperatures, the change i n the S. I.. T. of the fuel, and a change i n the distance between a i r and fuel molecules. An increase i n the static angle of injection advance, with e l l other conditions remaining unchanged leads to higher maximum pressures and rougher running. The spray characteristics might be thought to have great influence on the combustion knock. I t has been found that varying the size of the fuel drops (from .005"to .0005") has very l i t t l e effect on the delay period. However, finer atomization did give a more rapid pressure r ise in the f i r s t part of Phase I I , followed by a fa l l ing off in the rate of pressure r i se , resulting in a smoky exhaust. Finer atomization of the fuel does make the combustion shock definitely worse. The spray penetration required depends entirely on the types and size of the combustion chamber. (67) Chapter VIII Anti-Knock Dopes. Dopes for suppressing detonation. In 1922 Midgely and Boyd announced the discovery of the effectiveness of certain substances i n suppressing detonation i n gaseous explosions. Since that time many more knock suppressing "dopes35 have been found. These substances may be classif ied chemically into two groups; (1) Organo-metaliic compounds. (2) Organic compounds. The dopes that are most effective v olum et r i cally i n suppressing detonation i n spark igni t ion engines belong to the f i r s t group, which includes: Lead 'tetra-ethyl Pb (C^H,), Tin tetra-ethyl Iron carbonyl Fe (C0)5 Nickel carbonyl NI (00)f Di-ethyl t e l l u r i t e ; ( G ^ J . T e Di-ethyl selenide Ethyl iodide C^H^I Some of the organic substances found to have knock suppressing powers are: (68) Jfylidine C4H3-(CH3)-NH, Toluidine G 4 H ^ - G H 3 - M 2 18. S Analine 21.7 Methyl analine GfcHy -NH- CHy 22.2 Ethyl analine Ggflr-NH- G,ES 10.4 Benzyl analine C f eH y -NH- GHj- CgH^ 9.5 Cresol G t f i f - C H r OH 5.8 Phenol . 0<Hr OH 4.4 (Benzene O A - 1.0) The figures i n the right hand column Indicate the percent increase i n H. U. C. R. made possible by the addition of 5% by volume of the organic compound to a good average gasoline. I t w i l l be noticed that the common property of a l l these substances i s a benzenoid ring molecular structure and therefore high thermal s t ab i l i t y . The compounds are a l l aromatic amines and phenols. By far the most effective anti-detonating agent known i s tetra-ethyi lead; .04$ by volume having a knock suppressing power equivalent to 25$ by volume of benzene. The use of tetra-ethyl lead (also callad lead ethide) in gasoline, Is controlled by the Ethyl Gasoline Corporation who manufacture tetra-ethyl lead end supply "Ethyl Fluid" to manufacturers of gasoline a l l over the continent. In addition to lead ethide, which is a colorless l iqu id decomposing at 250° C , Ethyl Fluid contains ethylene dibromide C1H/f Br 2 and a characteristic red dye. The ethylene dibromide i s (69) i s added to prevent the deposition of metallic lead In the cylinder or on the exhaust valve and valve seat; lead bromide vapour passes out with the exhaust gases. The red dye i s required by law and i s added to indicate the presence of tetra-ethyl lead as a safety-measure against i t s very poisonous properties. Ethyl f lu id i s used i n quantities up to 5 c c . per gallon of gasoline ( i . e . one part •  i n 900), as much being added as I s necessary to bring the Octane Number up to standard. This l imi t i s imposed because above this amount i t i s impossible to prevent lead from being deposited, resulting i n fouled plugs and sticking valves. It has also been noticed that excessive amounts of ethyl f lu id seem to accelerate combustion. - Otherfacts that may have considerable bearing on the mechanism of knock'suppression, have been discovered i n static and engine cylinders. Tetra?ethyl lead decreases the flame velocity i n both high and low pressure oxidation reactions; i n low pressure combustion i t has been found to inhib i t as complete oxidation as would otherwise take place. Tetra-ethyl lead Is not as effective in low temperature slow oxidation reactions but the aromatic amines and phenols are. This has caused some experimenters to divide dopes into two classes: oxidation inhibitors (organic substances preventing low temperature oxidation) and anti-knock reagents (organo-metallic compounds preventing detonation.) The distinction i s probably more one of the temperature at which the compounds are most effective. I t i s usually concluded that tetra-ethyl lead must (70) be decomposed to be effective. Research has also revealed the fact that col loidal suspensions of certain metals in gasoline have knock suppressing power comparable to that of the organic compound of the metal. Nickel or iron in col loidal suspension have anti-knock properties •slightly superior to iron or nickel carbonyls. A colloidal suspension of lead Is only s l ight ly less effective than lead tetra-ethyl. "^V The effect of lubricating o i l on the action of metallic dopes i s interesting. An addition of 1% vegetable o i l to a fuel destroys the action of iron and nickel dopes, but actually Improves the anti-detonating action of lead dope. On the other hand, mineral o i l i n the fuel reduces the anti-knock properties of lead ethide, but not to the same degree as i t affects other organo-metallic dopes. However, o i l in the fuel does not lower the H. U. G. R.. of an undoped fuel , and does not lower the H. 0. C. R. of a doped fuel below i t s undoped value. Another interesting fact i s that Pb (C ; iH r) J CI and as e f f e c t " ^ Pb ('CJis-^Cl*. are only f and J respectivelyAas lead tetra-ethyl Pb . (CiHs-)4 in suppressing detonation. This would seem to indicate that the a lky l groups are of importance as well as the lead atoms in the mechanism of knock suppression. Theories of control of detonation by dopes: I f there are many theories of the mechanism of detonation, there are a great many (71) theories of the action of knock suppressors, for there i s one for each theory of detonation and many more to spare. Prominent among the well reasoned theories which attempt to account for the effectiveness of so small a quantity of dope (one.•molecule of lead ethide effective i n suppression detonation i n 200,000 molecules of mixture) are: The Nuclear Drop Theory. The Metallic Oxide Theory. : 'y The Theory of Metallic Atom Flame Nuclei. And, other theories less conclusively developed are: The Negative Catalyst Theory. The Positive Catalyst Theory. The Electron Theory. The Radiation Theory, and others. The Nuclear ©rop Theory suggests that the vapourized fuel , part icularly the fractions of higher molecular weight, w i l l tend to condense on high compression to form minute nuclear drops. The paraffin hydro-carbons of high molecular weight i n the drops have -feery low ignit ion temperatures and being freshly condensed are i n a very active state. The tendency of these paraffins, as mentioned i n the Peroxide theory'of detonation, i s to form unstable peroxides as the f i r s t products of oxidation. The accumulation of peroxides in the nuclear drops i s not i t s e l f enough to cause detonation, but i t acts as a primer producing simultaneous igni t ion (72) of the drops and thereby a very high velocity of combustion. The organo-metallic dope has decomposed shortly after entering the cylinder (250°C.) and the free metal reacts with the peroxides i n the-nuclear drops, reducing them as fast as they are formed. This prevents the combustion from being accelerated by the unstable peroxides. Another theory i s based on the observed property of the metals, effective i n reducing detonation, of being capable of forming two oxides, within the range of cylinder temperatures. The theory assumes interference by the metal In the chain of reactions of the Hydroxylation theory. The lead atom i s probably immediately oxidized after the decomposition of the lead ethide on entering the cylinder. The lead oxide molecule (PbOa) oxidises any unstable intermediate product in the chain that might give rise to accelerated burning, thereby being reduced to i t s lower form (PbO). I t i s Immediately oxidized again to the higher form by the a i r . Thus the lead mo3.ecules are assumed to osci l la te very rapidly between the higher and lower oxide forms. A th i rd very reasonable theory assumes that numerous incandescent nuclei of combustion are formed by the lead particles^ When the decomposition of the lead ethide takes place. The establishment of these nuclei i s aided by the presence at the point of. unstable free ethyl groups - C^H^. The result i s that the flame (75) front moves through par t i a l ly burned gases and i s , i n that manner, retarded. This theory would explain the difference i n effectiveness of lead ethide and lead i n col lo idal suspension, and also the difference between the action of the tetra-ethyl, the di-ethyl , and the carbonyl dopes. The Negative Catalyst Theory!assumes that the organo-metallic dopes act cata lyt ical ly ( i . e . without entering into the reaction) i n reducing the rate of combustion of the fuel and the primary-products of oxidation. The Positive, Catalyst Theory proposes quite the opposite, effect of the catalyst. It assumes that the igni t ion temperature of the fuel i s lowered to a point at which the heat of. adiabatic compression could cause appreciable, but relatively slow, chemical reaction to occur ahead of the flame front. The flame therefore advances through a par t ia l ly burned mixture at a lower velocity than i t would otherwise have. The Electron Theory of detonation has i t s own explanation for the action of anti-knock dopes. The lead atoms are assumed to absorb electrons i n the wave front. A deficiency of electrons retards the propagation of the wave front by this means. The Radiation Theory explains the action of anti-knocks (74) as one of absorption by the metallic atoms of radiations emitted from the flame fro n t . This prev-ents the;activation of the unburht .mixture ahead of the flame front, and so insures against \ accelerated combustion. Other l e s s probable theories ex i s t , and of these, the theory of "poisoning" of the metal walls of the cylinder might be mentioned. I t i s based on- the chemical knowledge of the poisoning •of catalysts and the fact that surface chemical reactions take place more Rapidly than homogeneous reactions. Various elements occur-rin g i n anti-knock dopes are known to "poison" catalysts i n chemical processes: these include.:, lead, selenium, tellurium, iodine, and the organic amino compounds. By t h i s theory, these elements i n the dopes are supposed to prevent surface c a i a l y t i c action by the walls of the combustion chamber. The reaction, the theory concludesj must be slower for,being; a purely homogeneous reaction. ; , , A l l these and several more less probable theories attempt to account for the knock-suppression properties of such small amounts of organo-metallic compounds. The action of purely organic dopes i n decreasing detonation i s more easily understood, as appreciable amounts of these materials are required to suppress detonation. These compounds arc aromatic amines and phenols and are thermochemically, very stable with t h e i r benzene rin g structures. Their thermal s t a b i l i t y Is added to by the presence of amino, (75) hydroxy! and short'y-alkyi''':grpups,'-. and/:.seem;S' to raise the thermal s t a b i l i t y and anti-oxidation properties of the whole f u e l mixture. Used i n quantities up to 5% and 10$ these compounds improve the anti-knock quality of gasoline very noticeably. Dopes f o r suppressing combustion shock; In the search f o r organic dopes for reducing the combustion knock i n gasoline engines, many agents were found that showed pro-detonating influence. These were.organic peroxides, nitrogen peroxide, ozone, the aldehydes and alkyl n i t r a t e s and n i t r i t e s , A l l of these compounds are unstable and some of them explosivej t h e i r action was found to be to hasten i g n i t i o n and accelerate combustion. In a compression i g n i t i o n engine, the purpose of an anti-knock dope i s to decrease the i g n i t i o n lag of the f u e l . Many prb-detbnating dopes of the types mentioned above have been used experimentally i n d i e s e l engines and found to be successful In suppressing the combustion shock. Some of these are; Acetyl Peroxide Benzoyl Peroxide ;EthyJ: fitrate : - . "GaEg-m5'\ -• Amyl ffitrate Cg-H^-NO, ^^•^v;^>y^ ; ^ 1 N i t r i t e ' • • C < r f l Y / - M 0 i Aoetaldehyde 0H5-GHO Benzaldehyde C 4H^CH0 (76) The doping of d i e s e l f u e l s on a commercial sc a l e has not developed t o any great extent as y e t . The DuPont Company • w i l l dope d i e s e l f u e l t o improve the i g n i t i o n q u a l i t y u s i n g amyl n i t r a t e . T h i s substance however, i s h i g h l y e x p l o s i v e and cannot be shipped. Although the q u a n t i t i e s used are small (Z% of amyl n i t r a t e r a i s i n g t h e cetene number of an average d i e s e l f u e l from 50 t o 70) the organic n i t r a t e s and n i t r i t e s are too expensive f o r improving a f u e l whose cost must remain low f o r the d i e s e l engine t o r e t a i n i t s p r i n c i p a l advantage. The peroxides and benzaldehyde decompose r a p i d l y and l o s e t h e i r e f f e c t i v e n e s s . The o b j e c t i o n t o aoetaldehyde i s i t s low b o i l i n g temperature. Ho inexpensive and e n t i r e l y s a t i s f a c t o r y dope f o r e l i m i n a t i n g combustion shock from d i e s e l engines has yet been found. Theory of c o n t r o l of combustion shock by dopes: The a c t i o n of or g a n i c dopes i n decreasing knocking i n the d i e s e l engine, i s simply one of supplying low I g n i t i o n temperature n u c l e i t o i n s t i t u t e t h e combustion. The i g n i t i o n l a g becomes th a t of the dope i n s t e a d of t h a t o f the f u e l component of lowest 3. I . T. The decreased delay p e r i o d gives a smoother angle of•"breakaway", a lower maximum r a t e of pressure r i s e per degree crank angle, and a longer p e r i o d of c o n t r o l l e d combustion. The a c t i o n o f l e a d teb?a-ethyl and other a n t i - d e t o n a t i n g dopes on the combustion i n a compression i g n i t i o n engine not o n l y decreases t h e v e l o c i t y o f combustion but a l s o i n c r e a s e s the I g n i t i o n l a g of the f u e l . ( 7 7 ) Chapter l X : --v- • - ." . sr~"^ ' —is. umclusxon Summary of factors affecting combustion, knock; I t has been seen that a compact molecular structure and therefore a'high thermal s t a b i l i t y are desirable properties f o r a, f u e l f o r a spark '•••ignition engine from an anti-knock standpoint. Cracking of crude o i l t o Increase the gasoline y i e l d also increases the anti-knock r a t i n g < o f the gasoline f r a c t i o n , f o r the high pressure cracking process, causes the formation of more compact molecules. and leaves , only the compounds of high thermal s t a b i l i t y uncraeked. Aromatic and naphthene hydrocarbons and also alcohols are desirable components of f u e l f o r spark i g n i t i o n engines j ethers,, normal paraffins and o l e f i n s are not. eSPor anti-knock q u a l i t y i n a'compression i g n i t i o n engine, a l l these factors are reversed. Ether, normal paraffins,, and o l e f i n s bum smoothly i n a d i e s e l engine while alcohols, naphthenes and aromatics do not. ' Here high thermal s t a b i l i t y and a molecular structure that i s compact, are desirable. Cracking of crude o i l greatly lowers the i g n i t i o n quality of the resul t i n g d i e s e l f u e l f r a c t i o n . , Anti-knock dopes f o r gasolines, are substances which reduce the rate of combustion;-organo-metallic, aromatic, amines, (78) and phenols. These substances added t o d i e s e l f u e l increase the combustion shock. The compounds used f o r reducing combustion shock i n d i e s e l engines are unstable peroxides, aldehydes and n i t r a t e s , which when added t o g a s o l i n e i n c r e a s e t h e detonation. jfl.1 f a c t o r s g i v i n g lower temperatures and pressures i n t h e c y l i n d e r o f a gas o l i n e engine, decrease the detonating tendency; but i n a d i e s e l engine higher temperatures and pressures are necessary t o decrease combustion shock. In general, almost a l l f a c t o r s t h a t t e n d t o decrease detonation i n a spark i g n i t i o n engine tend to i n c r e a s e combustion shock i n a compression i g n i t i o n engine, and v i c e v e r s a . One of the few exceptions i s the time of the beginning o f t h e combustion process; under co n d i t i o n s o f knocking, a retarded spark decreases detonation i n a g a s o l i n e engine,- and retarded i n j e c t i o n decreases combustion shock i n a d i e s e l engine. The v a r i o u s f a c t o r s decreasing knocking i n both types of engines are t a b u l a t e d on the next page. (79) Anti-Knock Factors Spark I g n i t i o n .jsngine Compression I g n i t i o n Engine. F u e l s : F u e l s : Compact molecular forms. Aromatics. Naphthenesi • Branched c h a i n hydrocarbons.. / A l c o h o l s . ; : S t r a i g h t chain forms. ; . , .:: Normal pa r a f f i n s . : ; l o r m a l o l e f i n s . / E t h e r s . Dopes-: : • : Dopes: Compounds t o reduce speed of combustion. O x i d i z i n g agents t o reduce Organo-metallies. Aromatic nmines. Phenols. i g n i t i o n l a g . A l k y l n i t r a t e s . Aldehydes. Organic peroxides. Engine Factors: Engine F a c t o r s : Lower pre s s u r e s : Low compression r a t i o . T h r o t t l i n g . Lower temperatures. Increased c o o l i n g . 7LIght l o a d . ' — R e t a r d e d s p a r k s Higher pressures. /High compression r a t i o , / Supercharging, '.7./ Higher -temperatures. Decreased c o o l i n g . F u l l l o a d . Retarded i n j e c t i o n . Higher speeds. Increased a i r humidity. Lower speeds. Coarse a i o m i e a t i o n . ( 8 0 ) Conclusions: Very l i t t l e can be learned of the actual mechanism of the combustion process and combustion knock from observing them i n an engine. Therefore, the whole subjeet of the combustion i n the cylinder of an engine i s a highly t h e o r e t i c a l one. The 'very fact that so many theories have been advanced to account f o r •combustion knocking and i t s suppression by dopes, Is an ind i c a t i o n ,;.ftf:.the,.Ia%<^l.pf 'definite, toowledge- .of-these, phenomena-. There are objections or contradictory evidence to p r a c t i c a l l y every theory so f a r proposed on the subject of detonation and i t s control, but that i s probably because i t i s easier to give destructive than constructive c r i t i c i s m . I t I s possible that none of the theories,, so f a r advanced are correct, but i t Is more probable that one, or a combination of more than one of the theories represents the mechanism of the reaction taking place i n the combustion •••chamber.1.. An a n a l y t i c a l method for pre-determining the a n t i -knock rating of a gasoline has been proposed. An analysis of the of the f u e l i s made into the four types of hydrocarbons and the anti-knock value i s calculated emperically from t h i s analysis. However, due to the widely varying properties of the possible isomers present, i t has been concluded that I t Is quite impossible to give.an accurate estimate of the'knocking tendency of a gasoline i n a spark i g n i t i o n engine from a chemical examination. (81) I n a d i e s e l f u e l due t o the l a r g e r molecules, there are much great e r p o s s i b i l i t i e s of isomerism. The h i g h l y complicated nature of t h e f u e l structure,:combined w i t h a greater number of engine v a r i a b l e s make t h e p o s s i b i l i t y of r a t i n g d i e s e l f u e l s from a chemical examination, even, l e s s . _ The r a t i n g o f both g a s o l i n e and d i e s e l f u e l s must i n c l u d e engine t e s t s as w e l l a s chemical and p h y s i c a l p r o p e r t i e s . The octane number of a g a s o l i n e f o r a given engine :,has a d e f i n i t e maximum advantageous l i m i t . . I t i s at a point where the r a t e of combustion i s slowed down enough t o a v o i d the sudden increase i n pressure t h a t leads t o detonation. R a i s i n g t h e octane number o f the g a s o l i n e above t h i s p o i n t f u r t h e r slows combustion a l l o w i n g l e s s time f o r expansion, and g i v i n g l e s s complete burning. I n c r e a s i n g the octarenumber i n c r e a s e s the power output and e f f i c i e n c y only i f t h e r e i s detonation. The use o f a gaso l i n e of high octane r a t i n g i n an engine of low compression i s a c t u a l l y u n d e s i r a b l e , f o r not only I s the e f f i c i e n c y and power decreased, but the combustion may be slowed t o a point wheire the gases are s t i l l burning as they pass out around the exhaust v a l v e ; owners then blame t e t r a - e t h y l l e a d f o r burning out t h e exhaust v a l v e s . Therefore i t must be concluded t h a t the octane number o f the g a s o l i n e should be s u i t e d -to the compression of t h e engine i n which i t I s used. A g a s o l i n e (82) of 76 0. N. i s suitable f o r use i n an engine with, a G. R. i n the v i c i n i t y of 5.5:1. The low speed or i n j e c t i o n diesel engine w i l l burn almost anything i n the way of di e s e l f u e l s , but the high speed types require a high grade refined d i e s e l f u e l . A good grade d i e s e l f u e l suitable f o r high speed engines should be a straight run f r a c t i o n from a pa r a f f i n base crude; approximately 20° - 26° Baume gravity and free from.all impurities, sediment, etc. Cracked fractions do not make satisfactory f u e l f o r high speed d i e s e l engine. In a given engine the combustion becomes rougher as the speed increases. Therefore, the higher the speed range of a d i e s e l engine, the better the grade of f u e l I t w i l l require, otherwise higher compression w i l l be necessary. The high speed d i e s e l engine undoubtedly has a large place i n the future of transportation, but i t i s questionable whether t h i s w i l l ever extend to the passenger automobile f i e l d . Aside from the few technical d i f f i c u l t i e s that s t i l l remain, the item of f u e l price would l i m i t t h e i r use. As a high grade refined f u e l ; i s necessary; and the supply of such i s l i m i t e d and cannot be increased by cracking processes, the price would r i s e i f a large; demand were created f o r di e s e l f u e l : by passenger cars. The d i s t r i b u t i o n costs and taxes would become comparable to those on gasoline, and the di e s e l engine (83) would lose i t s most important advantage; the low fuel cost. For this reason the future of the high speed diesel i n transportation i s probably limited to bus, truck, marine and engines. The quantity of fuel used by buses and trucks, particularly on long runs, makes.the diesel engine desirable for i t s lower fuel costs and consumption,, In power boats and aeroplanes the lower fuel consumption, the l ighter weight of the fuel , and the elimination of radio interference and f i re hazard;make the diesel engine in f in i t e ly more advantageous. (84) B i b l i o g r a p h y . Books: B i r d , A. L. ' Bone and Townend Brooks, B. T.» Gruse, W. A. Hollewan, i . F, L e s l i e , E. H. •Moore, H. Rioardo, H. R« Pye, D. R. T i n k l e r and Challenger The I . C. Engine. Flame and Combustion i n Gases. Hon-Benzenoid Hydrocarbons. Petroleum and i t s Products. Textbook of Inorganic Chemistry. Motor F u e l s . Fuels f o r I . G. Engines. The High Speed I . C. Engine. The I n t e r n a l Combustion Engine. Chemistry of Petroleum. T e c h n i c a l Papers2 Beroeze and Boerlage Gallendar, H. L« Clark and Thee C l a r k , Brugman & Thee Dicksee, G. B. I g n i t i o n Q u a l i t y of D i e s e l F u e s l , S. A. E. J o u r n a l J u l y , 1952. Dopes and Detonation Engineering Feb. 1927 Theories of Detonation Ind. & Eng. Chemistry Dec. 1925 Theories of A c t i o n of Knock-Suppressors. Ind. & Eng. Chemistry Dec. 1925 Problems i n High Speed D i e s e l Engines Automobile Engineer Aug. 1932 (85) Edgar, G, E g l o f f P G< Bibliography (continued.) Octanes Ind. & Eng. Chemistry Anti-knock Properties of Cracked Gasoline. Egloff, Schaad and Lowry Horning, H. L. Hubner and Murphy Le Mesurier & Stansfield Midgely and Boydc Schweitzer, P. H, Whatmough, W.Av Y.lthrow, L o v e l l and Boyd 1927 Universal O i l Products B u l l e t i n . Oxidation Mechanisms of Paraffin Hydrocarbons. Universal O i l Products B u l l e t i n . C. F. R„ Test Engine. S. A. E. Journal June 1931 A Standard Knock-testing Apparatus. Universal O i l Products B u l l e t i n . Fuel Testing i n Engines. Journal Of Inst, of Pet. Tech. Jul.1951. Anti-detonating dopes Ind. & Eng. Chemistry 1922 Combustion Knock Diesel Power Aug. 1952 Pet r o l Characteristics Automobile Engineering Jul.1951 - Apr.1932« Experiments on Flame V e l o c i t i e s . Ind. & Eng. Chemistry. Sept. 1950 

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