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UBC Theses and Dissertations

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 T h e s i s submitted  f o r t h e Degree o f  Master o f A p p l i e d  Science  i n t h e department  I . - of : Mechanical Engineering.  The U n i v e r s i t y o f B r i t i s h A p r i l 1955  Columbia  .'.Index  .  FUELS MB COMBUSTION KNOCK I H I . C. ENGINES• Chapter I  Page Introduction  II  (1)  General Introduction  1.  (2)  Combustion Knock d e f i n e d .  2.  (S)  R e l a t i o n o f k n o c k i n g to. e f f i c i e n c y , w e i g h t and c o s t s *  3.  Elementary F u e l Chemistry. (1)  (a) '(b) 1  Paraffin Unsaturated  ( c ) Napthene (d) (2)  III  6.  Hydrocarbon s e r i e s  .  Aromatic  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  (c)  Compounds c o n t a i n i n g S u l p h u r  Nitrogen.  P e t r o l e u m and L i q u i d F u e l s (1)  Sources o f l i q u i d f u e l s  (2)  Petroleum  (3)  Gasoline  (4)  (a)  Casinghead g a s o l i n e  (b)  Straight run gasoline  (c)  Cracked g a s o l i n e  Diesel Fuel  16. ,18.  Index  (continued) Page  Anti-Knock Ratings (l)  (S)  Octane S c a l e f o r G a s o l i n e s (a)  F u e l standards  (b)  Octane Number  (c)  Standard t e s t  28.  engines  Getene S c a l e f o r D i e s e l F u e l s  31.  (a)  F u e l standards.  (b)  Method o f d e t e r m i n i n g Cetene number.  (c)  Test engines.  Combustion and Combustion Knock. ,(l)  Combustion p r o c e s s 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 p r o c e s s i n compression I g n i t i o n , engine. . Combustion shock i n compression I g n i t i o n engine.  42.  (4)  47.  I n f l u e n c e o f 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 e n g i n e . (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 •(c) (2)  50.  Effect of other  curve.  properties  I n a compression i g n i t i o n engine. (a) E f f e c t o f a n a l y s i s , m o l e c u l a r 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 c u r v e . :  (c)  .Effect o f o t h e r p r o p e r t i e s .  .  53.  Index (continued.)  . • • Page Influence of Engine Factors on Combustion. (1)  I n a spark i g n i t i o n engine.  (2)  (a)  Compression r a t i o *  (b)  Form of the combustion chamber.  (c)  Intake and c y l i n d e r temperatures.  (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)  Carburation.  In a compression i g n i t i o n engine. (a)  Compression r a t i o .  (b)  Form o f t h e combustion chamber.  (c)  Intake and c y l i n d e r temperatures.  57  61.  !.(d) Supercharging and t h r o t t l i n g . (e)  Time o f 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 c o n t r o l of detonation by dopes.  70.  (5)  Dopes f o r suppressing combustion shock.  75.  (4)  Theory o f c o n t r o l 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  Introduction  I t i s t h e purpose o f t h i s t h e s i s t o c o n s i d e r t h e combustion p r o c e s s , combustion knock, and f a c t o r s i n f l u e n c i n g them, i n t h e i n t e r n a l combustion e n g i n e .  C o n s i d e r a b l e space i s devoted  t o t h e s u b j e c t o f l i q u i d f u e l s and f u e l c h e m i s t r y , i n o r d e r t o p r o v i d e a b a s i s f o r a complete u n d e r s t a n d i n g o f t h e r e l a t i o n between t h e f u e l c h a r a c t e r i s t i c s and t h e n a t u r e o f t h e combustion.  A'small  amount o f t h e t h e o r y i n c l u d e d i s o r i g i n a l , b u t h a s p r o b a b l y been p r e v i o u s l y enunciated by o t h e r s .  This thesis i s ,therefore,.a  summation o f p r e s e n t day knowledge r e l a t i n g t o t h e combustion p r o c e s s ; and a statement  o f t h e r e a s o n a b l e t h e o r i e s o f t h e mechanism o f t h a t  p r o c e s s , t h e knock, and knock s u p p r e s s i o n . The  s u b j e c t o f t h e i n t e r n a l combustion p r o c e s s i s a  v e r y c o n t r o v e r s i a l one t h a t i s b y no means s e t t l e d .  T h e r e f o r e , no  endeavour i s made t o prove a l i k e l y t h e o r y on any s u b j e c t b y d i s p r o v i n g the others.  A l s o , attempt  i s made t o a v o i d g i v i n g o p i n i o n s 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 h o l d s  (2)  the v i e w t h a t .  o t h e r w i s e t h i s t h e s i s would be e n d l e s s .  To put a l i m i t on t h e b r e a d t h o f t h e f i e l d , h i g h speed e n g i n e s a r e c o n s i d e r e d .  only  T h i s i s done because, o n l y i n t h e  h i g h e r speed t y p e s do t h e combustion d i f f i c u l t i e s become a c u t e . l a r g e l o w 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  The  insensitive  t o t h e n a t u r e o f t h e f u e l i t i s u s i n g , and r u n s s m o o t h l y on a wide range o f f u e l s w i t h o u t k n o c k i n g .  Combustion i n t h e h i g h speed,  s p a r k i g n i t i o n e n g i n e i s compared and c o n t r a s t e d .with t h a t i n t h e 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  e n g i n e s may be c o n s i d e r e d t o be o f t h e a u t o m o t i v e t y p e .  Combustion Knock D e f i n e d :  The knock i n t h e 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 b e i n g :  f u e l knock, compression knock, h e a t knock, carbon knock and e t c . I n r e a l i t y , t h e n a t u r e o f t h e f u e l , c o m p r e s s i o n , t e m p e r a t u r e and carbon d e p o s i t s a r e a l l c o n t r i b u t o r y t o t h e same k i n d o f k n o c k i n g , w h i c h s h o u l d be c a l l e d by t h e g e n e r a l name "combustion knock". I n t h e s p a r k i g n i t i o n e n g i n e , t h e knock t a k e s t h e form o f a c o n s i d e r a b l e amount o f f u e l vapotir i g n i t i n g a l m o s t s i m u l t a n e o u s l y giving a very rapid pressure r i s e .  F o r t h i s r e a s o n t h e name  '"detonation ' h a s been a p p l i e d t o t h e knock i n g a s o l i n e e n g i n e s . 1  I n t h e compression i g n i t i o n engine t h e knock i n no way resembles d e t o n a t i o n , b e i n g caused by a sudden change o f t h e 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 d e t o n a t i o n t h e name  "combustion shock" i s u s e d f o r t h e knock i n d i e s e l e n g i n e s . D e t o n a t i o n i n a s p a r k i g n i t i o n e n g i n e must be  disting-  u i s h e d from p r e - i g j o i i i o n , w h i c h may o c c u r 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 m i x t u r e o c c u r s s p o n t a n e o u s l y b e f o r e t h e s p a r k , and t h e r e f o r e c o n s i d e r a b l y b e f o r e t o p dead c e n t r e o f t h e cycle.  E f f i c i e n c y i s l o s t by n e g a t i v e work b e i n g done on t h e  p i s t o n d u r i n g t h e compression s t r o k e .  D e t o n a t i o n , on t h e o t h e r  hand, o c c u r s a f t e r t h e s p a r k and i n t h e f i n a l s t a g e s o f combustion. E f f i c i e n c y i s d e c r e a s e d by energy g o i n g 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 b e i n g l o s t i n h e a t t o t h e c o o l i n g water. are  P r e - i g n i t i o n and d e t o n a t i o n a r e n o t t h e same t h i n g , n o r  t h e y even c l o s e l y r e l a t e d .  F o r example, b o t h carbon  d i s u l p h i d e and a c e t y l e n e d e t o n a t e a t low c o m p r e s s i o n s , but w i l l n o t p r e - i g n i t e f benzene and c y c l o h e x a n e p r e - i g n i t e a t h i g h c o m p r e s s i o n s , but - w i l l n o t d e t o n a t e .  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 t h e s p a r k .  R e l a t i o n o f k n o c k i n g t o e f f i c i e n c y w e i g h t and c o s t s ;  Every f u e l  t h a t can be used i n a s p a r k i g n i t i o n i n t e r n a l .combustion engine has what i s known a s i t s H i g h e s t U s e f u l Compression R a t i o . of  The H.U.C.R.  a f u e l i s not t h e h i g h e s t compression a t which t h e f u e l may  be  u s e d , but i s t h e compression above which any i n c r e a s e i n compression does n o t 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  h i g h compression r a t i o , w h i c h r e q u i r e s a h i g h H. U. C. R. o f t h e f u e l , i s shown by t h e f o l l o w i n g r e l a t i o n between compression  ratio  and t h e maximum t h e r m a l e f f i c i e n c y o b t a i n a b l e :  Compression .Ratio Maximum Thermal E f f i c i e n c y 1 - &)-25  4  5  .295  .552  6  .361  7 .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 c o m p r e s s i o n r a t i o o f an engine can be r a i s e d from 4:1 t o 6:1 w i t h o u t i n c u r r i n g d e t o n a t i o n , t h e t h e r m a l e f f i c i e n c y and t h e 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 e n g i n e i s t h e h i g h e s t c o m p r e s s i o n t h a t can be used w i t h o u t causing detonation. I n t h e compression i g n i t i o n engine k n o c k i n g i s n o t t h 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 o f t h e engine.  As k n o c k i n g  h e r e I s reduced a s t h e compression i s i n c r e a s e d , t h e 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 t e m p e r a t u r e c o n s i d e r a t i o n s a n d t h e p r a c t i c a b l e w e i g h t o f t h e engine.. T h i s i s c a l l e d t h e , 1  '"Limit; o f U s e f u l . Compression." •"-  Compression  Ratio  Maximum T h e r m a l E f f i c i e n c y 1 - (i)'  10  .458  12 ,  1.464 -  14  16  20  .485  .500:  .527  2 5  Compression p r e s s u r e ... P i r -  566  i.,"  472  587  708  968  Approximate Maximum P r e s s u r e  700  ••-  800  900  1000  1250  1  4  T h i s t a b l e shows t h a t t o i n c r e a s e t h e compression r a t i o from 16:1 t o 20:1 o n l y g i v e s an i n c r e a s e o f 5% i n t h e t h e r m a l e f f i c i e n c y and involves very high cylinder pressures.  A l i m i t f o r maximum c y l i n d e r  p r e s s u r e s from both t e m p e r a t u r e and weight c o n s i d e r a t i o n s i s i n t h e neighbourhood o f 900 t o 1000 #/sq.in.  T h i s imposes a l i m i t o f  U s e f u l Compression a t about 15:1 o r 16:1 f o r d i e s e l engines a n d t h e s e  (5) r a t i o s o n l y i n t h e 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 k n o c k h a s a n i m p o r t a n t b e a r i n g on t h e engine w e i g h t p e r b r a k e horsepower o u t p u t .  I n an  e n g i n e t h a t runs smoothly and w i t h o u t k n o c k i n g , i t i s p o s s i b l e t o o b t a i n a h i g h e r mean e f f e c t i v e p r e s s u r e f o r t h e c y c l e w i t h a l o w e r maximum p r e s s u r e .  T h i s a l l o w s t h e u s e o f l i g h t e r p a r t s and  h i g h e r output r a t i n g s .  Weight r e d u c t i o n o f engines i s v e r y  important  i n t h e a u t o m o t i v e f i e l d , p a r t i c u l a r l y so i n t h e 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 t h e cost s t a n d p o i n t .  An e n g i n e t h a t does n o t knock  can be b u i l t 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 l o w e r upkeep 5  c o s t s , and w i l l have a l o n g e r u s e f u l l i f e . •  (6) C h a p t e r i±  Elementary Fuel Chemistry.  Hydrocarbon S e r i e s :  The most i m p o r t a n t compounds from t h e  s t a n d p o i n t o f f u e l s f o r I . C. e n g i n e s , a r e h y d r o c a r b o n s : i . e . o r g a n i c s u b s t a n c e s c o n s i s t i n g e x c l u s i v e l y o f carbon and hydrogen. i f i c a t i o n o f hydrocarbons  I  A class-  follows:  A l i p h a t i c Hydrocarbons  (Chain S t r u c t u r e s )  A.  Saturated  Paraffins  B«  Unsaturated  Olefins  C Ha._a  Di-olefins  ft  n  Tri-olefins Acetylenes II  C y c l i c Hydrocarbons A. N a p h t h e n i c  B. A r o m a t i c  (Ring S t r u c t u r e s ) Hydro-ben zane s  C„Ha„  Hydro-naphthalenes  C,fl /i-i  Hydro-anthracenes  CH  Benzenes>  G„H -<.  Naphthalenes  C K ~ ,z  Anthracenes  C„H n-/s  a  n  4h  .4  2n  n  zn  2  T h i s c l a s s i f i c a t i o n h a s two l a r g e groups o f hydrocarbons a c c o r d i n g t o s t r u c t u r e , c h a i n compounds and r i n g compounds, and each o f t h e s e groups i s d i v i d e d i n t o two l e s s e r groups a c c o r d i n g t o t h e r e l a t i v e number o f hydrogen t o carbon atoms i n t h e m o l e c u l e . The  (7) s t r u c t u r a l f o r m u l a o f a t y p i c a l member o f each group i s shown below?  S a t u r a t e d C h a i n Compound H  H  H  H  i  i  i  i  H  H  i ;i  H—C — C — C — C — C — C — H  -  H  I r  i  H  i  H  i  H  paraffin series n - hexane  i  H  fl  U n s a t u r a t e d C h a i n Compound H  H—  H  , I  i  I  fl  H  II  C— C —  v  \  H  H .H  C—C —  I  I  H  .-. H  I  |  I  C=C  olefin  series  n - hexene  I  H  H  N a p h t h e n i c R i n g Compound  H  H  \ / fl  C  X  H  C  cyclo-hexane  •H H  C  X  /\  H  A i o r a a t i c R i n g 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 a l w a y s p i c t u r e d i n s t r u c t u r a l f o r m u l a e a s connected t o o t h e r atoms  (8) by f o u r bonds.  The hydrogen atom i s mpno-valent.  I n c h a i n compounds  any number o f carbon atoms a r e connected t o g e t h e r and t h e b a l a n c e of t h e carbon, v a l e n c i e s a r e s a t i s f i e d by hydrogen atoms. The c h a i n o f carbon atoms may be s t r a i g h t o r branched: i f i t I s s t r a i g h t i t i s t h e normal form; i f branched, an i s o m e r i c form.  F o r example, t h e f o r m u l a f o r normal hexane g i v e n above may  a l s o be w r i t t e n : CH -(Cfi ),-CH 3  a  5  An i s o m e r i c form o f t h e same compound a s c a l l e d i s o - h e x a n e 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 i s o m e r s d i f f e r from t h o s e o f t h e 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 n o r m a l r a t e o f b u r n i n g . The s i d e c h a i n o f a h y d r o c a r b o n c o n s i s t s o f an a l k y l group j an a l k y l group h a v i n g t h e g e n e r a l f o r m u l a G„E  irifl  »  The  l o n g e r s i d e c h a i n s (S carbon atoms and o v e r ) may t h e m s e l v e s have side chains .  The number o f p o s s i b l e : i s o m e r i c forms i n t h e h i g h e r  members of t h e 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 f o u r t e e n carbon atoms o v e r one thousand i s o m e r s a r e p o s s i b l e . R e s e a r c h t e n d s t o show t h a t a l l p o s s i b l e i s o m e r s e x i s t o r can be synthesized.  The u n s a t u r a t e d a l i p h a t i c hydrocarbons have fewer  hydrogen atoms p e r m o l e c u l e t h a n t h e c o r r e s p o n d i n g number o f t h e  (9) paraffin series.  The  c o n c e p t i o n o f " u n s a t u r a t i o n " has a r i s e n from  t h e p r e s e n c e i n t h e m o l e c u l e o f l e s s hydrogen t h a n 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 a r e a c c o u n t e d  f o r by t h e assumption o f one o r more double o r 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 ) a r e connected t o g e t h e r i n a c l o s e 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 r o c a r b o n s ; i n a d d i t i o n t o t h e i s o m e r s depending on t h e number a n d l e n g t h : o f t h e s i d e c h a i n s , t h e r e a r e t h e i s o m e r s caused by d i f f e r e n t p o s i t i o n s o f the side chains.  T h i s p o i n t I s best i l l u s -  t r a t e d by . t h e c a s e o f o r t h o - , meta-, and p a r a - x y l e n e w h i c h a r e d i m e t h y l ©gfi&eneS*  o-xylene  m~xylene  p-xylene  Numbers a r e a l s o ' u s e d i n i n d i c a t i n g t h e p o s i t i o n of s i d e c h a i n s j t h e hexagon being.numbered from 1 t o 6 i n a c l o c k w i s e d i r e c t i o n beginning at the top.  Unsaturated s e r i e s also e x i s t i n c y c l i c  compounds, and have one, two, or. t h r e e , d o u b l e bonds i n a m o n o - c y c l i c structure.  , .'. The P a r a f f i n , s e r i e s i s t h e o n l y s a t u r a t e d s e r i e s o f  a l i - p h a t i c hydrocarbons,  The members o f t h e s e r i e s : h a v e a g e n e r a l  .  do) _. fo'rmTola\  G 'H^  Methane C H / i s  4  ;• "' '  the f i r s t - member o f the s e r i e s 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 p o i n t s of the normal p a r a f f i n s increase uniformly w i t h the number of carbon atoms i n the molecule.  At- '  normal temperatures, members of the s e r i e s from G, t o C* are gases, from G :tp-'G are l i q u i d s , and above C .'are waxy s o l i d s . f  M  Isomeric  /(  forms have a- lower b o i l i n g p o i n t and a higher melting point than the; corresponding normal form.  The p a r a f f i n 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 r a i s e s the i g n i t i o n temperature: considerably. * There are s e v e r a l unsaturated a l i p h a t i c s e r i e s varying i n t h e i r s t r u c t u r e and degree of unsaturation. are  t h e commonest of these compounds.  The o l e f i n s , C H 0  a n  The o l e f i n molecule contains  one double bond and ,is chemically more a c t i v e than the p a r a f f i n . They have a tendency to polymerize, i . e . ,  t o combine t o form l a r g e r  molecules* p a r t i c u l a r l y i n the s u n l i g h t .  The o l e f i n s have h i g h e r ,  i g n i t i o n temperatures and slower r e a c t i o n v e l o c i t i e s with' oxygen than have;the p a r a f f i n s .  The terminology of t h e ' o l e f i n s e r i e s  p a r a l l e l s that o f the p a r a f f i n s , but w i t h the s u f f i x - y l e n e or -ene. The f i r s t member of the s e r i e s isethylene or ethene R C:GH , , z  2  :  ' The d i o l e f i h s e r i e s G^H^.a. i s chemically more a c t i v e and, less"..'stable than the. o l e f i n s e r i e s .  The molecule I s 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 CJHJU,.^  but the molecule contains one t r i p l e bond.  combine very rapidly with oxygen.  The acetylenes  In terminology the higher members  of the series are regarded as substitution products and take the name of the a l k y l group present, e.g. ethyl acetylene, G H -C=CH a  r  Comparative formulae for the unsaturated series for C fl 4  x  are: HHHHHH H-C-G-G-C-C-b HHHHHH  <  Olefin Series hexene 0 H,, 4  H HHHH C;-C-C-C-C=C H,H H H  Diolefin Series Hexadiene  H  C B t  / 0  E , H HH C:C-C=C-C=C H.H H H  Trioiefin Series Hexatriene CH  H HH E H-C-C-6-C-CsC-H  Acetylene Series Butyl acetylene  H  4  H H H  ' ,  8  C H, T  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,H to cyclononane C E are known to t  ?  /t  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 i g n i t i o n 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 c r y s t a l l i z e  due to the large number of Isomers possible and probably present. ;  The  compounds of t h i s 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 cyclic Cvhydrocarbons, but are rarely regarded as such.  This i s because the  idea, of unsaturation Is usually associated vdth chemical, i n s t a b i l i t y , and the aromatics are very stable due to the exothermic reaction of their formation.  The boiling point of aromatic hydrocarbons  increases with the molecular weight, the number of side chains, and the nearness together of the side chains. of the  The ignition temperatures  members of the aromatic series also increase with their  molecular weight.  Their i g n i t i o n temperatures are higher and their  rates of burning slower than those of the corresponding naphthenes. The names of the monocyclic aromatics may be given i n 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 G R h  6"b C o  Mrn  ., three ringed - anthracenes  G H N  I H  .„  -• (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:  ' CnH^  ''Ether's:  C , H  Aldehydes:  W  - O-CJi^,  G,H„., - CKO  Ketones:  ;;;,CV.^' ;.;...  \  -OH  tl  G , H ^ , -GO-G^H_,,  •' Acids:- /  •  -V  -/ G- E^ H  - COOH .  Xi  ( A l l these. compounds have aromatic parallels with a phenyl group G/& in s  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':' Amines•,-,'•.  :  -(C,H ^ ^ - f f l ^ iB  (GA) -Nfl, .  ;(ahalines)-i V  X  s x  Nitro Gompounds: • • NitriteB-  G E ., \ - NQ^ H  xn4  Nitrate's:  GH  ^-NOV  Mercaptans  P „ H ^, - SE  n  C H S Compounds:  Thioethers  xn  ' . ' . . . . . C H . ^ , -SrCJi^„<.,  (These groups of compounds have also aromatic parallels with a phenyl group i n place of the a l k y l group)  (14)  Alcohols and ethers may be used directly or as blending agents for fuels. and t e r t i a r y .  There are three types of alcohols: primary, seconda  We are most familiar with the primary alcohols, of  which methyl and ethyl are the most common.  In structure they  consist of an a l k y l group, by which the alcohol i s named, and an hydroxyl group.  The alcohols have a high ignition temperature and  a slow rate of burning and therefore can be used at high compressions i n a spark i g n i t i o n 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 d i - e t h y l ether,  C^H/O-Cj.H^ .  I t has a low i g n i t i o n temperature and i s knock-inducing  i n a mixture with gasoline.  However, ether has anti-knock properties  when mixed with the fuel i n 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 I t s homologues are products of coal t a r d i s t i l l a t i o n and have t h e i r uses as anti-knock and gum-inhibiting agents i n gasoline. Nitrogen occurs i n petroleum probably i n the form of aliphatic amines, although t h i s i s questionable.  The aromatic  amines, called "analines , have" strong knock suppressing character11  i s t i c s i n a spark ignition 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 g a s o l i n e 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 i m p u r i t i e s i n crude petroleum.  These types of compounds are named,  mercaptans and t h i o e t h e r s , and they bear a remarkable s t r u c t u r a l • resemblance t o the primary a l c o h o l s and the e t h e r s . 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 i q u i d 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 o n of coal and l i g n i t e t a r s .  (4) D i s t i l l a t i o n 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 q u i d 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 f u l l y l a t e 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 t h i s continent i n a v i r t u a l l y untouched condition. These  (17) w i l l probably-come i n t o u s e 'should, t h e r e o c c u r any a p p r e c i a b l e f a l l i U g - o f f i n t h e supply o f petroleum.  There i s t h e o b j e c t i o n t o  s h a l e o i l on t h e grounds o f d i f i f i c u l t y I n h a n d l i n g due "to t h e h i g h • v o l a t i l i t y and l o w f l a s h p o i n t o f t h e 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 b l e n d i n g 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 o t h e r a r o m a t i c  compounds.  M e t h y l 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 a n d a wide range o f o i l s a r e t h e l i q u i d p r o d u c t s f r o m 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 o f wood and p e a t . v  These p r o d u c t s a r e n o t used t o any e x t e n t  as f u e l s f o r I . G. e n g i n e s . The  p r o d u c t i o n o f a n i m a l and v e g e t a b l e o i l s i s s m a l l  and t h e i r p r i c e s remain t o o h i g h f o r them t o e v e r be o f importance as motor f u e l s .  IShale o i l , peanut o i l , palm o i l and p r o b a b l y a g r e a t  many o t h e r s male e x c e l l e n t f u e l f o r d i e - s e l e n g i n e s a s t h e y have .low i g n i t i o n t e m p e r a t u r e s . value.  T h e s e o i l s h a v e , however, a l o w h e a t  Whale o i l h a s been v e r y s u c c e s s f u l l y u s e d i n t h e a r c t i c i n  s h o r t a g e o f cheaper d i e s e l f u e l . M e t h y l and e t h y l -alcohol-.are t h e c h i e f c h e m i c a l and s y n t h e t i c p r o d u c t s t h a t can be u s e d a s f u e l .  A l c o h o l can be  s y n t h e s i z e d f r o m non-condensable p e t r o l e u m gases o r can be made f r o m s u g a r s , s t a r c h e s , w o o d , f r u i t , v e g e t a b l e s , o r weeds.  L i t t l e use  has been made o f a l c o h o l a l o n e a s motor f u e l b u t . i t h a s been c o n s i d e r a b l y u s e d a s a b l e n d i n g 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. o f more t h a n 7.5: 1 w h i l e t h a t o f m e t h y l a l c o h o l i s  (18) a l m o s t as h i g h ; b o t h a r e c o n s i d e r a b l y h i g h e r t h a n t h a t o f benzene. Petroleum;  A rough, c l a s s i f i c a t i o n o f p e t r o l e u m s , w h i c h has but  l i t t l e b e a r i n g on c h e m i c a l c o m p o s i t i o n e x i s t s | crude o i l s a r e d i v i d e d i n t o t h r e e c l a s s e s j p a r a f f i n - b a s e , a s p h a l t - b a s e , and mixed-base.  They system c o n s i d e r s t h e n a t u r e o f t h e r e s i d u e from  n o 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 l e a v e s a waxy r e s i d u e , and an a s p h a l t - b a s e crude a t a r r y r e s i d u e .  The r e s i d u e  from a mixed -base crude c o n t a i n s b o t h 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 o f p e t r o l e u m s , based on t h e c h e m i c a l c o n s t i t u e n t s , t e l l s a g r e a t d e a l more o f t h e v a l u e o f t h e crude f o r i t s p r o d u c t s 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 a r o m a t i c h y d r o c a r b o n s e r i e s , and a s p h a l t i c material.  A t r i a n g u l a r c o m p o s i t i o n diagram i l l u s t r a t e s t h e  constituents 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 p e t r o l e u m c r u d e s , f r o m a l i q u i d f u e l s t a n d p o i n t 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 t h e 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 o f t h e crude o i l s o f t h e p r i n c i p a l  N o r t h American f i e l d s a r e : Pennsylvania:  h i g h p a r a f f i n c o n t e n t , 75$ a n d more, l i t t l e o r no a r o m a t i c s o r u n s a t u r a t e s .  Mid Continent: ;  V,,  L e s s p a r a f f i n s a n d more  naphthenes,  i n c r e a s e d a r o m a t i c content., c o n s i d e r a b l e asphaltic material.  G u l f Coast:  considerable naphthenic content - 3 0 $ . h i g h a r o m a t i c . c o n t e n t - 20^, much. . asphaltic. material.  California:  ,, ;  p r e d o m i n a t i n g i n naphthenes — 50%^ p a r a f f i n s i n l o w b o i l i n g range, aromatics , i n . h i g h b o i l i n g r a n g e , much a s p h a l t i c .material.  .  M e x i c a n and C e n t r a l American resemble C a l i f o r n i a n  crudes  chemically. The p a r a f f i n hydrocarbons o c c u r i n v a r y i n g amounts i n n e a r l y a l l p e t r o l e u m s , and members o f t h e s e r i e s a s h i g h a s C H . 4r  have been f o u n d .  7J  A l l normal forms seem t o e x i s t a l o n g 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 o f t h e p o s s i b l e i s o m e r i c forms.  (20) The e x i s t e n c e o f u n s a t u r a t e d h y d r o c a r b o n s i n p e t r o l e u m i s r e g a r d e d a s i m p o s s i b l e by many a u t h o r i t i e s .  Some e x p e r i m e n t e r s ,  however, r e p o r t t h e presence o f t h e l o w e r o l e f i n s i n t h e g a s from o i l wells.  Up t o 8$ u n s a t u r a t e d h y d r o c a r b o n s iiave been f o u n d i n  s t r a i g h t r u n g a s o l i n e s , but t h e s e may be s a f e l y c o n s i d e r e d a s h a v i n g t h e i r b i r t h i n t h e r e f i n e r y s t i l l and n o t under t h e ground.  It 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 o f o i l n e x t t o t h e h e a t e d m e t a l o f t h e s t i l l , a n d any u n s a t u r a t e s i n ^ s t r a i g h t r u n p r o d u c t s may be a c c o u n t e d f o r by t h i s m i l d c r a c k i n g . /.. • process. Haphthenes o c c u r i n a l l known p e t r o l e u m s .  Principally  t h e y a r e c y c l o - h e x a n e a n d i t s d e r i v a t i v e s i n m o n o c y c l i c and p o l y c y c l i c structures.  However, c y c l o p e n t a n e , c y c l o h e p t a n e and c y c l o - o c t a n e  have a l s o been f o u n d i n c e r t a i n p e t r o l e u m s , n o t a b l y R u s s i a n and Californian.  Baku crude o i l s c o n t a i n a r e m a r k a b l y h i g h p e r c e n t a g e  o f naphthenesj a s h i g h a s 80$ - 90$.  P e n n s y l v a n i a crudes c o n t a i n  very l i t t l e while other o i l s areintermediate. Most crude o i l s c o n t a i n o n l y a v e r y s m a l l p e r c e n t a g e ( i f any) o f a r o m a t i c h y d r o c a r b o n s .  However, C a l i f o r n i a and Borneo  c r u d e s c o n t a i n a v e r y c o n s i d e r a b l e amount.  The g r e a t e s t p a r t o f  t h e s e a r o m a t i c compounds a r e m o n o c y c l i c , i e . benzene and i t s d e r i v a t i v e s up t o C H I0  H  .  The remainder i s n a p h t h a l e n e (2 r i n g s ) and s e v e r a l  m e t h y l and d i - m e t h y l d e r i v a t i v e s . to  Borneo crudes c o n t a i n from 25$  40$ a r o m a t i c s o f which 6$ t o 7$ i s o f t h e n a p h t h a l e n e s e r i e s . I n t h e r e f i n e r y , t h e crude o i l i s " f r a c t i o n a t e d *  1  (21) into several different "cuts" i n s t i l l s and fractionating The s t r a i g h t r u n p r o d u c t s  towers.  ares  Gasoline. Gleaners'  naphtha.  Kerosene. Diesel Fuel. Fuel O i l . "Bunker G"Oil. Asphalt  Residue.  G a s o l i n e i s t h e most v a l u a b l e f r a c t i o n and t h e "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 o f t h i s f r a c t i o n . The r e s i d u e l e f t i n t h e s t i l l i s blown w i t h oxygen o r a i r t o f o r m asphalt o f t h e desired hardness.  A s p e c i a l "Qook" i s made o f a  d i f f e r e n t c h a r g i n g s t o c k t o produce l u b r i c a t i n g o i l . Gasoline:  Gasoline i s t h e f r a c t i o n o f petroleum  between t h e t e m p e r a t u r e s o f 30° and 220° G.  distilling  In theparaffin series  t h i s c o r r e s p o n d s t o t h e range o f compounds from C,- t o C« . G a s o l i n e s may be d i v i d e d i n t o t h r e e 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 g a s o l i n e . )  (2)  Straight run gasoline.  (5)  Cracked g a s o l i n e .  Gasinghead g a s o l i n e i s a v e r y v o l a t i l e r e c o v e r e d f r o m t h e n a t u r a l gas o f o i l w e l l s .  fraction  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 o c t a n e , which have been  (22) v a p o u r i z e d by t h e i r h i g h v a p o u r p r e s s u r e s when p r e s e n t i n s m a l l percentages i n t h e n a t u r a l gas.  V a r i o u s methods a r e u s e d 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 e x p a n s i o n  condensation.  (2) by d i s s o l v i n g i n l u b e 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 b s o r p t i o n I n c h a r c o a l t o w e r s . (4)  by c o n d e n s a t i o n by l i q u i d a i r . (when h e l i u m i s b e i n g  recovered.)  T h i s / t y p e o f g a s o l i n e h a s 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 v e r y 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 a s 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 w i n t e r grade gasolines. S t r a i g h t run gasoline i s t h e f i r s t "cut* taken i n f r a c t i o n a t i n g column o f t h e r e f i n e r y . about 60° t o 220° C.  I t h a s a b o i l i n g range o f  The p r i n c i p a l c o n s t i t u e n t s a r e p a r a f f i n  and naphthene h y d r o c a r b o n s ,  o n l y a s m a l l percentage o f aromatics  and o l e f i n s b e i n g p r e s e n t *  Approximate a v e r a g e a n a l y s e s f o r  s t r a i g h t r u n g a s o l i n e s f r o m d i f f e r e n t t y p i c a l crudes a r e g i v e n s Crude  Paraffins  Olefins  Naphthenes  Aromatics  20  3  Pennsylvania  75 7,  g  Oklahoma  71  3  25  3  Texas  68  5  25  4..  California  56  4  38  g  Smackover ( A r k . )  66  8  15  j±  (25) F o r many y e a r s s t r a i g h t r u n and casinghead were t h e o n l y f u e l s used i n spark i g n i t i o n e n g i n e s .  gasolines  When t h e demand  f o r g a s o l i n e exceeded t h e p r o d u c t i o n o f s t r a i g h t r u n and  casinghead  gasoline, t h e cracking process  The  came i n t o commercial u s e .  e l e m e n t a r y p r i n c i p l e s o f c r a c k i n g h y d r o c a r b o n s 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 , b u t t h e f i r s t r e s u l t s were p o o r , g i v i n g h i g h y i e l d s o f non-condensable gases a n d o f coke.  C r a c k i n g methods  t o d a y g i v e f r o m 50% t o 75% ( o f t h e crude) y i e l d i n t h e g a s o l i n e fractions.  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 t h e s t r a i g h t r u n g a s o l i n e f r o m t h e same crude.  T h i s i s due t o  t h e i n c r e a s e d a r o m a t i c and o l e f i n i c c o n t e n t and t h e d e c r e a s e d p a r a f f i n content.  Approximate a n a l y s e s o f c r a c k e d g a s o l i n e s from  t h e same c r u d e s a s t h e s t r a i g h t r u n g a s o l i n e a n a l y s e s p r e v i o u s l y given, arei Crude.  Paraffins  Olefins  Haphthenes  Aromatics  Pennsylvania  57  9  1%  21  Oklahoma  55  9  14  24  Teaas  27  20  27  26  California  42  20  17  21  45  14  14  29  Smackover  (Ark.)  These f i g u r e s do n o t r e p r e s e n t any p a r t i c u l a r j g a s o l i n e s , but a r e a v e r a g e s o f a l l t h e t y p i c a l c r a c k s d g a s o l i n e s from t h e f i e l d s represent.  they  By comparison w i t h t h e f i g u r e s g i v e n on s t r a i g h t r u n  g a s o l i n e s i t c a n be seen t h a t t h e average change i n t h e t y p e o f  (84) hydrocarbons i n a g a s o l i n e i s : Paraffins  -  d e c r e a s e d 20%  Olefins  -  i n c r e a s e d 10$  Naphthenes  -  d e c r e a s e d 10%  Aromatics  -  i n c r e a s e d 20%  2.0 7.  Paraffin  "Olefin  20%  Naphthene  Aromatic  The changes t a k i n g p l a c e i n a n o i l s u b j e c t e d t o p y r o l y s i s o r " c r a c k i n g " a r e c o n s i d e r e d t o be as f o l l o w s : (l)  P a r a f f i n s o f h i g h m o l e c u l a r weight a r e " c r a c k e d " , a t a t e m p e r a t u r e o f 400° t o 500° C, i n t o  paraffins  o f lovrer m o l e c u l a r weight and u n s a t u r a t e d a l i p h a t i c hydrocarbons ( c h i e f l y o l e f i n s ) .  paraffin  paraffin  +•  olefin  (m + m' = n) (g) U n s a t u r a t e d 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 w i t h a r e d u c t i o n i n m o l e c u l a r volume o f 16 units.  T h i s reaction i s maintained  unidirectional  by p r e s s u r e s o f 600 t o 800 #/sq. i n . i n t h e c r a c k i n g s t i l l . olefin  —  9—  naphthene  (25) (3)  Conversion of s a t u r a t e d c y c l i c hydrocarbons t o benzenoid hydrocarbons.  (The l i b e r a t e d h y d r o g e n  i s l a r g e l y a b s o r b e d by h i g h l y u n s a t u r a t e d 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  n o t r e v e r s e because o f t h e g r e a t e r s t a b i l i t y o f t h e benzenes. C H, + H t  2  c y c l o h e x a n e —>-cyclohexene  *-  (1R>*H,  ^C R>3H\ 4  c h y c l o h e x a d i e n e -?-benzene  A l l h i g h l y u n s a t u r a t e d compounds a r e removed from r e f i n e d g a s o l i n e a l o n g w i t h t h e s u l p h u r compounds.  The  olefins  may  be c o n s i d e r e d t o be t h e o n l y u n s a t u r a t e d h y d r o c a r b o n s r e m a i n i n g i n a good grade o f g a s o l i n e .  W h i l e n o t as c h e m i c a l l y a c t i v e as t h e  removed u n s a t u r a t e s , t h e o l e f i n s show a c o n s i d e r a b l e t e n d e n c y t o p o l y m e r i z e and f o r m gums, when t h e 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 the sunlight.  T h i s I s u n d e s i r a b l e and y e t t h e p r e s e n c e o f t h e  o l e f i n s i s desirable f o r t h e i r anti-knock q u a l i t y . g r e a t p r o g r e s s has been made i n t h e f i e l d o f gum gasoline.  Fortunately  inhibitors for  These enable t h e use o f a h i g h o l e f i n content t o g a i n a  h i g h a n t i - k n o c k r a t i n g f o r t h e g a s o l i n e , w i t h o u t i t f o r m i n g any gums. Most e f f e c t i v e among t h e compounds i n v e s t i g a t e d a r e : Thymol, P y r o g a l l o l , C a t e c h o l and B y d r o q u i n o n e . OH Thymol  OH  Pyrogallol  (26) OH  OH OH  Catechol  Hydroquinone  " OH' Today p r a c t i c a l l y a l l g a s o l i n e s on t h e market a r e blended gasolines. and  They c o n s i s t l a r g e l y o f s t r a i g h t r u n g a s o l i n e  c r a c k e d g a s o l i n e t o which a s m a l l f r a c t i o n o f c a s i n g h e a d g a s o l i n e  has been added.  The c r a c k e d g a s o l i n e r a i s e s t h e a n t i - k n o c k  qualities  o f t h e f u e l and t h e c a s i n g h e a d g a s o l i n e i n c r e a s e s t h e s t a r t i n g  '••V volatility.  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 o f  m e t a l l i c dope t o f u r t h e r r a i s e t h e a n t i - k n o c k  r a t i n g t o t h e current  standard. Diesel Fuel:  A wide range o f p e t r o l e u m f u e l s , from k e r o s e n e t o  l i q u i d a s p h a l t s may be u s e d i n d i e s e l e n g i n e s , b u t t h e t e r m " 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 b e s t 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 o f d i e s e l f u e l i s  •200° t o 390° G.  I n t h e p a r a f f i n s e r i e s t h i s corresponds t o t h e  compounds from C„  to G„ . 2  Ordinary  d i e s e l f u e l s have a range o f  18° t o 26° Baume g r a v i t y and a S a y b o l t v i s c o s i t y up t o a maximum o f 100  seconds a t 100° F. I n d i e s e l f u e l t h e r e a r e t h e same f o u r t y p e s o f  hydrocarbons t h a t occur i n g a s o l i n e .  Straight run d i e s e l f u e l  c o n t a i n s a h i g h p e r c e n t a g e o f p a r a f f i n s a n d naphthenes, and a s m a l l amount o f o l e f i n s and a r o m a t i c s , i n much t h e same p r o p o r t i o n s a s t h e s t r a i g h t r u n g a s o l i n e from t h e same c r u d e .  In cracking 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 t h e " d i e s e l f u e l "  (27) f r a c t i o n , c o n s e q u e n t l y t h e volume o f 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 h e f r a c t i o n a t i n g t o w e r o f t h e c r a c k i n g  still,  c o n t a i n s a g r e a t l y reduced percentage o f p a r a f f i n h y d r o c a r b o n s , as t h i s forms t h e s o u r c e o f c r a c k e d g a s o l i n e s .  The p e r c e n t a g e  volume o f t h e u n s a t u r a t e d and c y c l i c compounds a r e s u b s t a n t i a l l y increased.  (28)  Chapter IV  Anti-Knock Ratings.  Octane S c a l e f o r G a s o l i n e s ?  A l l g a s o l i n e s o f c o u r s e , have  n o t t h e same k n o c k i n g t e n d e n c y ; t h e r e f o r e i t became n e c e s s a r y f o r r e f i n e r s and m a r k e t e r s o f g a s o l i n e t o have some system o f r a t i n g the anti-knock q u a l i t y of t h e i r product.  Of t h e many systems  p r o p o s e d , t h e "Octane S c a l e " has p r o v e d t h e b e s t  ?  s t a n d a r d i z e d and p r a c t i c a l l y u n i v e r s a l l y , adopted.  and has been I n 1926 D r .  Graham Edgar s u g g e s t e d t h e u s e o f two pure h y d r o c a r b o n s a s f u e l standards f o r measuring t h e a n t i - k n o c k q u a l i t y o f g a s o l i n e s . standards  These  a r e i s o - o c t a n e , from w h i c h t h e system t a k e s i t s name,  and n o r m a l h e p t a n e . Iso-octane, CjH , i s a branched chain p a r a f f i n hydrol8  c a r b o n , 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 a n t i - k n o c k q u a l i t y i n a. s p a r k i g n i t i o n e n g i n e . t r i - m e t h y l i s o - b u t y l methane. : CH, .  I t i s a l s o sometimes c a l l e d  I t s s t r u c t u r a l formula i s :  •  t  CH - C - C H - CH - CH 3  Z  CH  CH.  3  Normal h e p t a n e , G 'A 7  3  i s a s t r a i g h t c h a i n p a r a f f i n which knocks v e r y  lh  l o u d l y when burned i n a s p a r k i g n i t i o n e n g i n e . CH  3  -(CH ) -CH x  r  3  I t s formula i s :  (29) I t i s i n t e r e s t i n g h e r e t o n o t e t h a t n o r m a l octane C^H^as a f u e l shows v i o l e n t k n o c k i n g  characteristics.  I n t h e r a t i n g o f g a s o l i n e , t h e sample b e i n g t e s t e d i s matched f o r k n o c k i n g t e n d e n c y , i n a s t a n d a r d t e s t e n g i n e , a m i x t u r e o f i s o - o c t a n e and n o r m a l h e p t a n e .  The  by  "Octane Number"  o f t h e g a s o l i n e i s t h e p e r c e n t a g e by volume o f i s o - o c t a n e i n t h e m i x t u r e o f i s o - o e t a n e 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 t h e h i g h c o s t o f t h e s e p u r e h y d r o c a r b o n s  (about $25 p e r g a l l o n ) secondary  standard f u e l s are used.  These  a r e c a r e f u l l y s t a n d a r d i z e d g a s o l i n e s o f octane numbers 50 and  78,  and a r e u s e d 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 t y p e s o f f u e l f o r spark i g n i t i o n engines  are:  Iso-octane Aviation  gasoline  .Premium g a s o l i n e  0. N.  100  "  80  t r  76  Standard gasoline  "  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 t y p e s o f apparatus have been used i n t h e p a s t few y e a r s f o r r a t i n g t h e a n t i - k n o c k q u a l i t y o f g a s o l i n e s . R e c e n t l y , however, t h e s e have narrowed down, on t h i s c o n t i n e n t a t 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 G a s o l i n e C o r p o r a t i o n  (so) have d e v e l o p e d and recommend f o r u s e t h e " 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 c o n j u n c t i o n  w i t h t h e D e t o n a t i o n Subcommittee  o f the Co-operative F u e l Research,  have d e v e l o p e d t h e "C. E . "R. T e s t E n g i n e . " The S e r i e s 30 E t h y l knock t e s t i n g a p p a r a t u s 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 o u t p u t I s absorbed by b e l t d r i v e n synchronous g e n e r a t o r mounted on t h e same base.  a The  s e t i s complete w i t h e l e c t r i c s w i t c h b o a r d , knock measuring a p p a r a t u s , and a l l a c c e s s o r i e s n e c e s s a r y f o r - t e s t i n g . x  The s i n g l e c y l i n d e r o f  t h e engine has overhead v a l v e s and a f i x e d compression r a t i o n o f 7.75:1.  The bore i s 2.5 i n c h e s , s t r o k e 4.625 i n c h e s , g i v i n g a d i s -  placement o f 22.71 c u b i c i n c h e s .  The speed o f t h e engine i s  mainta.ined c o n s t a n t a t 600 R.P.M. by t h e 220 v o l t , 3 phase, 60 c y c l e A. C. g e n e r a t o r which i s a l s o u s e d 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. w i n d i n g on t h e g e n e r a t o r s u p p l i e s d i r e c t c u r r e n t f o r t h e i g n i t i o n system and t h e knock m e a s u r i n g i n s t r u m e n t s . Knock measuring i s done by t h e b o u n c i n g p i n t y p e o f i n d i c a t o r w i t h e i t h e r a hydrogen o r e l e c t r i c a l  knock-meter.  I n t e s t i n g a f u e l , t h e compression p r e s s u r e , w h i c h 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 a d j u s t e d u n t i l an even (not v i o l e n t ) d e t o n a t i o n i s o b t a i n e d . The F u e l / a i r r a t i o and t h e a n g l e o f spark advance a r e t h e n a d j u s t e d t o g i v e maximum d e t o n a t i o n .  A mixture of standard reference f u e l s  i s t h e n f o u n d , by t r i a l and e r r o r , t o e x a c t l y match t h e c o n d i t i o n s  (31) p r o d u c e d by t h e t e s t  fuel.  The C. E. R. T e s t E n g i n e i s a s i n g l e c y l i n d e r e n g i n e o f 3.25 i n c h e s bore and 4.5 i n c h e s s t r o k e . 37.4 c u b i c i n c h e s .  I t s displacement i s  These c y l i n d e r dimensions have been chosen t o  c o r r e s p o n d c l o s e l y t o t h o s e o f a n average a u t o m o b i l e engine c y l i n d e r . The  c o m p r e s s i o n 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 p i e 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 a r e  o v e r h e a d and c e n t r a l l y p l a c e d w i t h t h e spark p l u g and t h e 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 t h e bouncing p i n t y p e  w h i c h h a s been f o u n d t o be t h e s i m p l e s t a n d most s a t i s f a c t o r y t y p e . The  speed o f t h e engine i s governed a t a c o n s t a n t 6G0 R.P.M. by an  induction  g e n e r a t o r w h i c h a b s o r b s t h e power o u t p u t .  Ignition i s  o p t i o n a l by c o i l o r magneto b u t c o i l i g n i t i o n i s f o u n d t o g i v e t h e 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 o r f i x e d  compression| t h e 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 S c a l e 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, h a s r e c e n t l y been d e v e l o p e d by G. D. B o e r l a g e and J . J". Broeze i n t h e r e s e a r c h l a b o r a t o r i e s Dutch S h e l l Company a t D e l f t , H o l l a n d . Quality"  o f the Royal  They measure t h e " I g n i t i o n  o f a f u e l i n a s c a l e o f "Cetene Numbers". The s t a n d a r d s f o r t h e s c a l e a r e cetene (Cetene Number 100)  and m e s i t y l e n e (C.N^O)  Cetene, C H /6  3i  , i s a straight chain o l e f i n that  i g n i t e s r e a d i l y w i t h .only a v e r y s h o r t i g n i t i o n l a g . I t s f o r m u l a  (52) i s written: CH - (CHj^-CH s CH a  a  The zero end of the scale, mesitylene, w i l l not ignite at a l l i n diesel engines of ordinary compressions. a symmetrical monocyclic aromatic.  Mesitylene,  !«, is  More specifically i t i s  1:5:5 tri-methyl benzene, and the structural formula i s : CH I  3  C I HG  C I C  N  3  /  1 H  S  GH  3  Here i t might be mentioned that an i s o - o l e f i n of the same composition as cetene, C H , has very poor i g n i t i o n quality. /6  M  This  i s hexa-methyl decene (also called tetra iso-butylene.) CH CH i i CH - C - CH - CH - ( C H j - C - CH - C S CH 3  3  3  t  I  Cfi j  2  '  I  CH j  x  GH j  a  I  GH ^  In determining the i g n i t i o n quality of a fuel, 90  out of phase indicator cards are taken for several different  compression pressures, both t h r o t t l i n g and supercharging with unchanged injection.  (55)  A l i n e i s drawn t h r o u g h t h e p o i n t s r e p r e s e n t i n g t h e end o f t h e d e l a y p e r i o d on each diagram; t h i s i s c a l l e d t h e " d e l a y  curve".  S i n c e t h e d e l a y curves a r e s i m i l a r a n d 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 t h e d e l a y s caused "by two p r e s s u r e s 2p and p was t h e same a s f i n d i n g t h e d e l a y caused by p. T h i s e l i m i n a t e s t h e n e c e s s i t y o f a v e r y a c c u r a t e and d i f f i c u l t determination o f t h e time o f t h e beginning o f i n j e c t i o n .  The,  p r a c t i c e i s t o d e t e r m i n e t h e d i f f e r e n c e i n d e l a y between 50 and 15 atmospheres.  The d e l a y r e a d i n g i s compared t o a curve o f d e l a y  readings f o r v a r i o u s cetene-mesitylene  mixtures.  The p e r c e n t a g e  by volume o f c e t e n e , i n a m i x t u r e o f cetene a n d m e s i t y l e n e t h a t w o u l d g i v e t h e same d e l a y p e r i o d a s t h e f u e l b e i n g t e s t e d , i s t h e c e t e n e number o f t h a t f u e l .  T h i s method does n o t 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 t h e q u a l i t y o f a f u e l t h a t i s burning w i t h u n s t a b l e smoothness, ( i . e . a n o e x t r e m e l y l o n g d e l a y p e r i o d and a s l o w p r e s s u r e 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  e n g i n e u s e d by Broeze and B o e r l a g e i n t h e i r  e x p e r i m e n t a l 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 e n g i n e b u i l t by Thomassen, De Steeg o f H o l l a n d .  I t i s a single  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 o f a p l u g i n t h e c e n t r e o f t h e head, g i v i n g a range o f compression p r e s s u r e s f r o m 375 t o 600 #/sq. i n . The power output i s a b s o r b e d by a h y d r a u l i c b r a k e and a  mechanical  (34) g o v e r n o r a l l o w s a range o f 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 o f phase i n d i c a t o r c a r d s , e i t h e r p h o t o g r a p h i c c a r d s from a Maihak o p t i c a l i n d i c a t o r o f 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 cards.  This l a t t e r i s possible  indicator  a s o n l y measurements o f t h e d e l a y  p e r i o d a r e made. As a l l t h e measurements a r e comparative and a r e matched w i t h c o m p a r a t i v e measurements r e f e r r i n g t o t h e s t a n d a r d f u e l s , t h e a c t u a l engine does n o t g r e a t l y a f f e c t t h e r e s u l t s .  The  c e t e n e r a t i n g o f any f u e l , o b t a i n e d on p r a c t i c a l l y any t y p e o f d i r e c t i n j e c t i o n e n g i n e , w i l l check w i t h i n a v e r y narroiv margin o f t h e r a t i n g of t h e same f u e l o b t a i n e d i n t h e Thomassen e n g i n e .  (55)  Chapter 7  Combustion and Combustion Knock  Combustion P r o c e s s i n a Spark I g n i t i o n Engines  In a gasoline  e n g i n e , an i n t i m a t e m i x t u r e o f a i r and g a s o l i n e vapour a r e drawn into the cylinder during the suction stroke.  On t h e  compression  s t r o k e t h e p r e s s u r e and t e m p e r a t u r e o f t h e f u e l charge I n t h e 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 p r e s s u r e s 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 a r e a p p r o x i m a t e l y ; Compression  Ratio  Compression P r e s s u r e p 1.4  4.0 120#  6.-5 . :.,i;s9#'-,  179#  ; goo#  r  The p e r i o d o f combustion b e g i n s w i t h t h e i g n i t i o n of t h e m i x t u r e by a s p a r k , which may  o c c u r any t i m e from 45° b e f o r e  T.D.C. up u n t i l T.P.G, depending on t h e speed o f t h e e n g i n e .  The  combustion p r o c e s s o c c u p i e s a r e m a r k a b l y s h o r t p e r i o d o f t i m e , r a n g i n g a p p r o x i m a t e l y from 5/1000 o f a second a t 1000 r.p.m., t o 2/1000 o f a second a t 4000 r.p.m.  The extreme s h o r t n e s s o f t h i s  t i m e makes t h e s t u d y and a n a l y s i s o f t h e combustion p r o c e s s v e r y difficult.  One o f t h e few methods p o s s i b l e i s t h e e x a m i n a t i o n o f  90° out o f phase i n d i c a t o r c a r d s o f t h e few t y p e s o f i n d i c a t o r s c a p a b l e o f t h e s e speeds.  An out o f phase i n d i c a t o r c a r d ( i d e a l )  j  (56) f o r a s p a r k i g n i t i o n engine i s shown below:  » 1  o  ,1 -  — v •  to  10  From e x a m i n a t i o n o f such c a r d s t h e combustion p r o c e s s i s g e n e r a l l y r e g a r d e d a s o c c u r r i n g i n two . Phase I s  phases.  The b u i l d i n g u p o f a s e l f p r o p a g a t i n g ; f l a m e n u c l e u s , i n c u r r i n g no a p p r e c i a b l e r i s e i n p r e s s u r e above t h e compression curve.  Phase I I : The s p r e a d o f t h e f l a m e t h r o u g h o u t t h e mixture. Immediately b e f o r e the b e g i n n i n g o f t h e combustion p r o c e s s , t h e c o n d i t i o n s e x i s t i n g i n t h e charge i n t h e c y l i n d e r a r e : - a h i g h p r e s s u r e (say 150 #/sq.in.) - a h i g h t e m p e r a t u r e a p p r o a c h i n g t h e i g n i t i o n temperature o f t h e m i x t u r e , produced by'Conduction o f h e a t from u n c o o l e d c y l i n d e r p a r t s , and by head o f a d i a b a t i c compression.  (37) -  a c o n s i d e r a b l e amount o f t u r b u l e n c e r e m a i n i n g from t h e v e l o c i t y w i t h w h i c h t h e m i x t u r e e n t e r e d the  cylinder. On t h e passage o f t h e s p a r k a c r o s s t h e gap o f t h e  spark p l u g , t h e f u e l vapour a t t h i s p o i n t receives t h e s l i g h t i t r e q u i r e s , and i g n i t e s .  kindling  I n f l a m m a t i o n spreads s l o w l y by c o n d u c t i o n  t o t h e f u e l vapour immediately adjacent t o t h i s s m a l l nucleus. T u r b u l e n c e o f t h e charge t e n d s t o p r e v e n t t h i s n u c l e u s from g r o w i n g , but i n s t e a d i t i s s p r e a d and moves t h r o u g h a c o n s i d e r a b l e p o r t i o n of t h e m i x t u r e , 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 t h e "Delay P e r i o d * as a p p r e c i a b l e amount o f combustion has y e t o c c u r r e d , o c c u p i e s a d e f i n i t e i n t e r v a l o f time (usually  something l e s s t h a n  2/1000 o f a second)  w h i c h i s more o r l e s s independent o f engine speed.  Consequently  Phase I i n terms o f c r a n k a n g l e v a r i e s d i r e c t l y a s t h e engine speed, The d u r a t i o n i n t i m e o f t h e d e l a y p e r i o d depends on: (l)  The c h e m i c a l n a t u r e o f t h e f u e l .  (?)  The f u e l / a i r r a t i o ( m i x t u r e s t r e n g t h )  (3)  The t e m p e r a t u r e o f t h e c o m b u s t i b l e a t t h e t i m e of i g n i t i o n .  (4)  The p r e s s u r e o f t h e c o m b u s t i b l e a t t h e t i m e Of  Ignition.  I n Phase I I , t h e t e m p e r a t u r e o f t h e m i x t u r e s have been raised  much h i g h e r t h a n a t t h e b e g i n n i n g o f Phase I , and t h e f l a m e  (58) n u c l e i spread r a p i d l y and merge.  The f l a m e f r o n t so formed sweeps  r a p i d l y t h r o u g h t h e r e m a i n i n g unburnt m i x t u r e s . The r a t e o f p r e s s u r e r i s e i n Phase I I a n d a l s o t h e maximum p r e s s u r e a t t a i n e d depend o n : (1)  The c h e m i c a l n a t u r e o f t h e f u e l .  (2)  The shape o f t h e combustion chamber.  (5)  The t u r b u l e n c e o f t h e m i x t u r e . As Phase I I depends on t h e t u r b u l e n c e i n t h e c y l i n d e r ,  w h i c h v a r i e s d i r e c t l y a s t h e engine speed, t h e t i m e o c c u p i e d by Phase I I w i l l d e c r e a s e a s t h e 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 c o n s t a n t i n terms o f crank a n g l e . The t a b l e below g i v e s some i d e a o f t h e t i m e s o c c u p i e d by t h e combustion p r o c e s s a t 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®  Speed  Time / Crank A n g l e  r.p«w<M  Sec.  500 1000 1500 2000  0  :  Phase I Crank Time Angle ;  deg.  000,355;; ;..000,167.  . Pha se I I Crank Time Angle sec.  sec.  5. \ 10  d e  sec.  Sv  I*  .00500  .00167  15  .00250. • 25. '  20  .00667  30  -00166  .00167 -.15,  I: - 0 0 0 ^ 0 8 5 , 3  .  : .00167 \:  u .00167:  ; .000^111 - \  V  Total Granlc Time Angle  .00417 :  .00533  .00292  '.iW.00125'/ :  15  ;ooioo  30  .00167 ^ 15  .00083  45  .00250  .000,047,6  35  .00167'  15  .00071  50  .00258  .000,041,7  40  .00167  15  .00062 ;  55  .00229  2500  .000,066,7 • 25  3000  .000,055,5  5500 4000  ;  .00167 \  :  40  :  .00267  (59) Detonation i n a  Spark I g n i t i o n E n g i n e :  T h e .smoothness -with  w h i c h an engine r u n s , depends on t h e manner i n w h i c h Phases I and I I merge, as w e l l as t h e r a t e o f p r e s s u r e r i s e p e r degree c r a n k a n g l e i n Phase I I .  The phenomenon o f d e t o n a t i o n , however, i s  only connected w i t h 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 t h e most p o p u l a r c o n c e p t i o n i s c o n s i d e r e d 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 t h r o u g h t h e 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 t h e c y l i n d e r  • w a l l a t h i g h v e l o c i t y c a u s i n g 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 t h e d e t o n a t i o n wave,  from e x p e r i m e n t s 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 t h e v i c i n i t y o f 50$  g r e a t e r t h a n t h e n o r m a l v e l o c i t y o f f l a m e t r a v e l i n t h e engine  cylinder. D u r i n g t h e p r o c e s s o f combustion t h e t h e r m a l energy i n the cylinder i s increasing.  The  oxidation reaction progresses  a t a c o n s t a n t l y i n c r e a s i n g t e m p e r a t u r e and t h e r e f o r e a t a c o n s t a n t l y accelerating rate.  D e t o n a t i o n o c c u r s when t h e v e l o c i t y o f r e a c t i o n  becomes so g r e a t t h a t t h e  f l a m e f r o n t compresses b e f o r e i t unburnt  g a s e s , h e a t i n g them f a s t e r (by a c e r t a i n margin) t h a n t h e y can d i s s i p a t e t h e heat by r a d i a t i o n , c o n d u c t i o n and c o n v e c t i o n .  The  unburnt gases so compressed and. h e a t e d i g n i t e almost s i m u l t a n e o u s l y , i n c r e a s i n g t h e v e l o c i t y o f t h e flame f r o n t t o an e x t r e m e l y h i g h value. The e x p e r i m a n t s o f "vVithrow, L o v e l l and Boyd have shown  (40) e v i d e n c e t h a t , i n d e t o n a t i o n , t h e i n c r e a s e d flame speed o c c u r s o n l y i n t h e l a s t q u a r t e r o f t h e maximum d i s t a n c e t o be t r a v e r s e d from t h e spark p l u g . The f o r e g o i n g e x p l a n a t i o n o f d e t o n a t i o n i s known as t h e " D e t o n a t i o n Wave T h e o r y " .  T h e r e a r e many o t h e r t h e o r i e s ,  c h e m i c a l a n d p h y s i c a l , o f t h e mechanism o f d e t o n a t i o n .  The c h e m i c a l  t h e o r i e s f o r t h e most p a r t r e f e r t o t h e d e t o n a t i o n o f p a r a f f i n h y d r o carbons.  Some o f t h e t h e o r i e s put? f o r w a r d \)  flydroxylation  are:  Theory  Peroxide Theory Theory o f -Preferential O x i d a t i o n 'Theory o f Thermal D e c o m p o s i t i o n R a d i a t i o n Theory E l e c t r o n Theory E r e e Hydrogen Theory' The H y d r o x y l a t i o n Theory o f t h e mechanism o f t h e o x i d a t i o n o f p a r a f f i n h y d r o c a r b o n s assumes a c h a i n o f r e a c t i o n s . The sequence i s one o f s u c c e s s i v e o x i d a t i o n s : g l y c o l ~* a l d e h y d e a c i d  o r decomposition  paraffin  alcohol  t o carbon monoxide and  hydrogen.  The r e a c t i o n s i n t h e case o f methane a r e i l l u s t r a t e d : ^Formic Acid Methane •-*> Methanol-=> Methene Glycol-#>Formaldehyde^ ~Carbon Monoxide * and Hydrogen  w  ; H..  H-G-H — H - C - O - H ;H  /  II  H H-G-H ^-H-G-0-H H . H.  ,0  H-C  H ^H^C-O-H  fl H  " " ^  .  'H  • 0 ^ " - ^ H-G" H  H"G :  — — C O  :  V  \p •  O-H  H,  (41)  The hydroxylation theory i s supported by evidence of the presence of most of the intermediate compounds i n the products of slow oxidation of paraffin hydrocarbons. rapidity of the  Knocking i s attributed to the  reactions.  The Peroxide Theory assumes the formation of unstable peroxides as one of the f i r s t products of oxidation.  The amount  of peroxides formed i s 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 i s illustrated by the structural equation below i n which R represents any alkyl group C^H^,., : V'tf. ^-  '  R-C-C-R  ,  1  • .':'fl " r  0 i  '•-lirAf  V  —^R-C-0-O-C-R  1  ' ..;';----.v :  H  :  • I  -.-^  •  f  - •• B  paraffin 4- oxygen -*-di-alkyl peroxide  H or  H  R-Q-C-R  6 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 i n  knocking tendencies of paraffin and olefin hydrocarbons.  This theory  i s 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 i s , i n 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 i n highly active state and combustion Is accelerated to detonating v e l o c 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 propagated 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 i n 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 i g h t 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 p r e s s u r e s i n f u l l  diesel  e n g i n e s a r e much h i g h e r t h a n i n g a s o l i n e . e n g i n e s :  Compression R a t i o  10  Compression P r e s s u r e P, r-  12  366#  :  .-',472#  14  16  ;'587#  708#  -. :  4  i s t h e crank h e a r s t o p dead c e n t r e 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 c y l i n d e r begins.  T h i s may be  e f f e c t e d b y e i t h e r o f two methods: -  by h y d r a u l i c p r e s s u r e * known a s " s o l i d * o r 1  "airless -  by a b l a s t , o f h i g h p r e s s u r e a i r , known a s "air  In  injection."  injection."  t h e h i g h speed engines,"'which we a r e c o n s i d e r i n g , s o l i d  i s u s u a l l y used.  injection  The i n j e c t i o n o f t h e f u e l i s not i n s t a n t a n e o u s !  if••'for n o - o t h e r t h a n m e c h a n i c a l c o n s i d e r a t i o n s , i t o c c u p i e s an appreciable period o f time. In  t h e f u l l d i e s e l engine, i g n i t i o n o f t h e f u e l i s  by t h e heat o f compression used.  a l o n e , no s p a r k o r o t h e r d e v i c e b e i n g  T h e p e r i o d o f combustion,  t h e r e f o r e , I s measured from t h e  b e g i n n i n g o f i n j e c t i o n , and from an examination o f 90° o u t o f phase I n d i c a t o r c a r d s , t h e combustion three  p r o c e s s may be d i v i d e d  phases: Phase I : .  Delay p e r i o d ; no a p p r e c i a b l e p r e s s u r e . r i s e above t h e compression  curve.  into  (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. w. x i  t  Q U  <  £00  \ \  \  \  \  s  >  70"  60°  3o°  TOC  •  3 0  s  v.  °  7°°  Phase I cf the diesel combustion process parallels Phase I i n the spark i g n i t i o n engine.  It i s a delay period i n  which either no i g n i t i o n takes place or Ignition i s confined to some very localized nucleus.  Fuel, usually unheated, i s injected  i n a. fine spray, into the a i r i n the combustion chamber.  This a i r  i s at a pressure i n 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 v e l o c i t y and meets the a i r which i s i n 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 i g n i t i o n temperature (  S.I.T.)  The "delay period" or  "ignition l a g therefore occupies a constant period of time, n  (45)  (usually between .001 and .002 seconds) that i s p r a c t i c a l l y independent of engine speed. (1)  The duration of Phase I in.time depends on:  the chemical nature of the fuel, particularly i t s S. I . T.  (2)  the temperature of the a i r i n 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 particles.  I t i s an unsettled question whether combustion takes place on the surface of the fuel droplets or i n a p a r t i a l vapourlzation near t h e i r 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 c u l t question to determine whether combustion occurs i n the l i q u i d phase, or i n the near-liquid vapour phase.  In either case,  i t does not greatly affect the larger part of the theory of combustion i n the  compression ignition 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 t o t a l fuel charge or even more.  Much of t h i s 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 s e , and the combustion accelerates u n t i l i t has caught up with the injection. time (the  By t h i s  end of Phase II) a very high temperature and high  pressure-have been attained. The maximum rate of pressure rise i n Phase I I depends on: (1)  the chemical nature of the f u e l .  (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 I I 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 swirl 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 i s e i s not as rapid as i n 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, s o l i d injection, compression ignition engine bears to the original constant pressure cycle proposed by Dr. Diesel.  The cycle of t h i s  (47)  engine has become a combined cycle" or even sometimes only a constant 11  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 I I and engine speed i s not as d i r e c t .  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 i n Phase I I . as the  But turbulence of the a i r w i l l increase directly '  engine speed giving a proportional increase i n the time rate  of pressure r i s e .  There w i l l also be an increase i n 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 t o t a 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. represented on the out of phase indicator card by: (l)  the angle with which the combustion l i n e leaves the compression l i n e , i . e. the  They are  Both. 6f these factors have been found to depend directly on the ignition l a g .  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 ignition l a g . Careful measurement of such cards for different fuels has revealed that a linear relation exists between ignition 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 t h i s i s largely dependent on cylinder size and  other factors. Combustion shock may be reduced i n 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 o f t h e F u e l C h a r a c t e r i s t i c s on Combustion.  . Of t h e s e v e r a l f a c t o r s t h a t determine t h e n a t u r e .of t h e combustion i n an e n g i n e , p r o b a b l y t h e most i m p o r t a n t i s t h e c h e m i c a l nature of the f u e l .  The t h e r m a l s t a b i l i t y o f t h e 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 t h e c h e m i c a l c h a r a c t e r i s t i c s which w h e t h e r t h e combustion  determines  o f t h e f u e l w i l l be smooth o r rough.  The  t h e r m a l s t a b i l i t y of a compound depends on t h e a n a l y s i s and t h e m o l e c u l a r weight, and Temperature and t h e  structure; i t i s exhibited i n the Self I g n i t i o n v e l o c i t y o f combustion o f t h e fuel.''  I n t h e s p a r k i g n i t i o n engine, i t was  seen t h a t a l o w  f l a m e 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 d e t o n a t i o n and a h i g h S. I . T. o f t h e f u e l . m i x t u r e t o p r e v e n t 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 s h o u l d have h i g h t h e r m a l 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 e n g i n e . The normal p a r a f f i n hydrocarbons w i t h t h e i r l o n g  c h a i n 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 u n d e s i r a b l e components o f 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 a r e compact have, on t h e o t h e r hand, h i g h i g n i t i o n t e m p e r a t u r e s and low r a t e s o f b u r n i n g .  The  ring  s t r u c t u r e s o f naphthene hydrocarbons  a r e a l s o compact and t h e s e  compounds a r e a l s o t h e r m a l l y s t a b l e .  S h o r t s i d e c h a i n s on t h e  (51) naphthene r i n g add t o t h e t h e r m a l s t a b i l i t y o f t h e molecule b u t a long side chain  does'not.  The p r e s e n c e o r p o s s i b l y t h e presence  o f double bonds between carbon atoms,  o f l e s s hydrogen i n t h e m o l e c u l e , seems  t o add g r e a t l y t o t h e t h e r m a l s t a b i l i t y  (but n o t t o t h e c h e m i c a l  • 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 g r e a t improvement o v e r t h e corresponding p a r a f f i n s , and t h e aromatics a r e b e t t e r than t h e c o r r e s p o n d i n g naphthenes.  T h e r e f o r e , from t h e s t a n d p o i n t o f  a n t i - k n o c k q u a l i t y , t h e o r d e r o f t h e d e s i r a b i l i t y o f hydrocarbons i n gasoline i s :  a r o m a t i c s , naphthenes, n o r m a l o l e f i n s and l a s t ,  normal p a r a f f i n s .  Some i s o m e r i c c h a i n compounds a r e , however,  comparable t o t h e a r o m a t i c s i n a n t i - k n o c k v a l u e . f o r a h i g h a n t i - k n o c k q u a l i t y o f a hydrocarbon i g n i t i o n e n g i n e , i s a compact m o l e c u l a r The  presence  The c r i t e r i o n  f u e l f o r a spark  structure.  o f a n atom o f oxygen i n t h e m i d d l e o f  a n a l i p h a t i c c h a i n g r e a t l y reduces t h e t h e r m a l s t a b i l i t y , a s i s shown b y t h e e t h e r s :  C„E  a n f l  - 0 - C IW m  +  , .  However, t h e 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„H ,,,.,-OH, but have e x c e l l e n t a n i l a  detonating q u a l i t i e s .  Here t h e oxygen atom belongs t o a h y d r o x y l  g r o u p , and t h e a d d i t i o n o f a s h o r t a l k y l (CH*,,,), o f a h y d r o x y l (Cfl) o r o f an animo group (NHi) a s s i d e c h a i n s , seems t o add t o t h e thermal s t a b i l i t y o f t h e 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 m o l e c u l a r weight o f a compound have no  g e n e r a l r e l a t i o n t o t h e a n t i - k n o c k p r o p e r t i e s o f t h e compound a s  (52) an engine f u e l .  However i n t h e case o f h y d r o c a r b o n s , a h i g h e r  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 i t most i m p o r t a n t  ;  d i s t i l l a t i o n curve o f a g a s o l i n e i s p r o b a b l y 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 t h e f u e l . boiling  A low i n i t i a l  p o i n t i s d e s i r a b l e t o g i v e s t a r t i n g v o l a t i l i t y and a l o w  f i n a l b o i l i n g p o i n t o r end p o i n t p r e v e n t s f u e l wastage by v a p o u r i z e d g a s o l i n e p a s s i n g u n b u r n t o u t t h e exhaust o r r u n n i n g down t h e c y l i n d e r w a l l s t o g i v e 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 l o w end p o i n t .  Examples o f d i s t i l l a t i o n  c u r v e s f o r motor and a v i a t i o n g a s o l i n e s a r e g i v e n below.  It will  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 l o w end p o i n t f o r aviation gasoline. Motor G a s o l i n e  • A v i a t i o n Gasoline.  I n i t i a l B o i l i n g Point  •; •• 55  y 10$/condensed  • SO  20$  >"\;7o;:;- ; ;  y'-50$": /,y ;  32° c "•  i  \y'':  :  yj&6y'--y'\  r  y  '•',.C/'  y-.End;point;'':'  "90:"""y-  " ;  '"""V""'VvT20'::'  :  210 ' y .  \ 1 5 0  Recovery a t l e a s t Other p r o p e r t i e s o f g a s o l i n e a r e m o s t l y covered by s p e c i f i c a t i o n s a n d have n o t much b e a r i n g on t h e a n t i - k n o c k  rating  (53) of  the  fuel.  These a r e :  Specific Gravity, Flash Point,  I o d i n e Number: A c i d , S u l p h u r , Water, and Ash C o n t e n t ; R e s i d u e and Gum,  C o l o r and Ddor.  r e l a t e d t o t h e C/H  The  s p e c i f i c g r a v i t y of a gasoline i s  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 t h e combustion i n t h o s e 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 a g a i n s t t h e 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 o f g a s o l i n e . about 150°  F.  unsaturated  The  I o d i n e Number i n d i c a t e s t h e amount o f  compounds p r e s e n t .  A good g a s o l i n e s h o u l d be f r e e  f r o m a c i d , w a t e r , ash and s u l p h u r . course, u n d e s i r a b l e .  I t i s u s u a l l y set at  An o b j e c t i o n a b l e odor i s , , o f  As f o r c o l o r , i t used t o be n e c e s s a r y t o  have, g a s o l i n e w a t e r - w h i t e i n o r d e r t o s e l l i t . . high anti-knock  Since gasolines  of  r a t i n g s have come i n t o v o g u e , t e t r a - e t h y l l e a d  i s u s e d i n n e a r l y a l l g a s o l i n e s on t h e 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 t h e r e f i n e r now  does n o t  have t h e remove t h e o r i g i n a l y e l l o w i s h c o l o u r o f t h e g a s o l i n e .  I n t h e Compression I g n i t i o n E n g i n e , i n o r d e r t o reduce k n o c k i n g changing t h e f u e l , one o f s h o r t i g n i t i o n l a g i s n e c e s s a r y . a f u e l o f l o w S. I.. T. and low t h e r m a l  by  Therefore,  stability i s desirable.  This  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.  I n d i e s e l f u e l , l o n g c h a i n p a r a f f i n s and o l e f i n s  d e s i r a b l e , w h i l e compounds o f compact m o l e c u l a r  forms a r e  T h i s i n c l u d e s v e r y branched c h a i n i s o - p a r a f f i n s and as w e l l as naphthenes a n d a r o m a t i c s .  The  are undesirable.  iso-olefins,  thermo-chemical s t a b i l i t y  (54) of  the  a r o m a t i c h y d r o c a r b o n s makes them v e r y •undesirable as  f u e l components.  diesel  The l o w t h e r m a l s t a b i l i t y o f t h e e t h e r s 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 r e a s o n t h e y a r e sometimes u s e d as b l e n d i n g agents t o reduce k n o c k i n g i n h i g h speed Quite opposite t o t h e i r e f f e c t  diesels.  i n spark i g n i t i o n engines, t h e s i d e  c h a i n s o f s m a l l a l k y l g r o u p s , o f h y d r o x y l groups (as i n a l c o h o l s ) and o f amino g r o u p s , h e r e i n c r e a s e k n o c k i n g .  On t h e o t h e r hand,  u n s t a b l e n i t r o and a l d e h y d e compounds g r a a t l y reduce k n o c k i n g i n the  diesel  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 o f a  d i e s e l f u e l and t h e n a t u r e o f t h e combustion i s not c l e a r l y  evident.  K e r o s e n e , f o r example, has a l o w e r 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 t h a n an average d i e s e l f u e l : but no may  be  conclusions  based on t h i s f a c t , as t h e t h e r m a l s t a b i l i t y o f t h e 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 c o n c l u s i o n s  theoretically  on t h e s u b j e c t , because i t i s unknovm whether combustion o c c u r s i n t h e v a p o u r phase o r t h e l i q u i d p h a s e , o r whether p a r t i a l v a p o u r i z a t i o n i s n e c e s s a r y b e f o r e i n i t i a l i g n i t i o n can t a k e p l a c e . O t h e r p r o p e r t i e s o f t h e f u e l may combustion.  The  o r may  not a f f e c t  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 o v e r a wide  r a n g e , but seem t o i n f l u e n c e t h e combustion o n l y inasmuch as a f f e c t t h e spray c h a r a c t e r i s t i c s :  they  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 o f a d i e s e l f u e l as o f a g a s o l i n e i s o f i m p o r t a n c e o n l y as an i n s u r a n c e r e g u l a t i o n . The  specified  f l a s h point i s u s u a l l y  (55 ) 150° F.  The  b u r n i n g 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 t h e r m a l s t a b i l i t y , b u t t h e c o n d i t i o n s i n a n open cup and i n an engine c y l i n d e r a t h i g h p r e s s u r e s a r e 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 b e a r s much r e l a t i o n t o t h e n a t u r e o f t h e combustion,, The  "ignitibility"  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. a n d S . A. E.  No  s t a n d a r d method h a s y e t been s p e c i f i e d f o r measuring t h e i g n i t i b i l i t y o f a f u e l b u t t h e method s h o u l d be such t h a t i t g i v e s r e s u l t s  bearing  some r e l a t i o n t o t h e " i g n i t i o n q u a l i t y " a s measured from t h e d e l a y p e r i o d i n an a c t u a l engine and r e c o r d e d i n C e t e n e Numbers".  The  a  maximum s u l p h u r , w a t e r , a s h and f r e e carbon i n d i e s e l f u e l a r e u s u a l l y specified.  S u l p h u r p r o b a b l y h a s no e f f e c t on i g n i t i o n and combustion  but landoubtedly h a s a c o r r o s i v e e f f e c t on t h e c y l i n d e r and v a l v e s , n o t so much w h i l e t h e engine i s r u n n i n g , but when i t i s f i r s t E n g i n e s have r u n s u c c e s s f u l l y  stopped.  on d i e s e l f u e l w i t h a s h i g h a s 3$fe5$  suphur w i t h o u t e x c e s s i v e c o r r o s i o n o f t h e exhaust v a l v e s , p r o v i d e d the, engine i s s t a r t e d and stopped on s u l p h u r f r e e o i l .  A s h , carbon  and r e s i d u e s p e c i f i c a t i o n s a r e i n c l u d e d t o p r e v e n t t h e p l u g g i n g o f f u e l l i n e s and i n j e c t o r s and t o m i n i m i z e t h e s c o r i n g o f t h e c y l i n d e r walls. The  grade of. f u e l I s o f t e n i n c l u d e d i n t h e s p e c i f i c a t i o n s .  A s t r a i g h t r u n f r a c t i o n i s much s u p e r i o r t o t h e d i e s e l f u e l t h e f r a c t i o n a t i n g tower o f a c r a c k i n g s t i l l .  cut"  fl  from  T h i s i s t o because t h e  compounds o f l e a s t t h e r m a l s t a b i l i t y which a r e most d e s i r a b l e i n a d i e s e l f u e l a r e t h e ones most e a s i l y t h e r m a l l y decomposed i n t h e  (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 t o 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 q u a l i t y and cannot be s a t i s f a c t o r i l y used i n a h i g h speed d i e s e l engine. Therefore, i t i s often s p e c i f i e d that the d i e s e l f u e l be a s t r a i g h t run f r a c t i o n , or a s t r a i g h t run topped crude, p r e f e r a b l y 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 diesel fuel.  (57)  Chapter V I I  I n f l u e n c e o f Engine F a c t o r s on Combustion  I n a spark. I g n i t i o n engineV t h e r e a r e s e v e r a l mecha 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 t h e combustion p r o c e s s i n a manner d e t e r m i n i n g whether combustion k n o c k i n g o c c u r s . .ones-are:  The  principal  , •  ,j  (a)  compression r a t i o .  (b)  f o r m o f combustion chamber.  (c)  i n t a k e and c y l i n d e r temperature.  (d)  s u p e r c h a r g i n g 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 t h e s e , t h e f a c t o r which most d e t e r m i n e s whether the  c o m b u s t i o n o f a c e r t a i n f u e l w i l l k n o c k , i s t h e compression  ratio.  As.shown b e f o r e , t h e t h e r m a l e f f i c i e n c y  o f an engine  improves w i t h an i n c r e a s e i n compression r a t i o u n t i l d e t o n a t i o n occurs.  T h e r e f o r e , f o r a g i v e n f u e l , a compression r a t i o s h o u l d  be u s e d t h a t keepsiy t h e V e l o c i t y o f t h e f l a m e f r o n t s l i g h t l y u n d e r d e t o n a t i n g speeds. higher than t h i s i s :  The i n f l u e n c e o f a compression •. :  - V " ^'''to'-'^dd'to'.the' temperattu^e o f : a c t t a b a t l e . :  compression, h a s t e n i n g the establishment /'"''•o.f'^the-flak'evn^cleuS'-Ih.'Piase'^I'.', ••'•'.  ratio  (58) -  t o reduce t h e S. I . T. of the p a r a f f i n hydrocarbons i n t h e 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-.\ f r o n t , causing t h e s p e e d o f t h e flame t o :  :  increase t o detonating v e l o c i t i e s . " :  , ; An increase-in^ pressure may also be regarded as •  b r i n g i n g the-unburnt  gases i n t o closer: contact; with the inflamed  p a r t i c l e s , and so i n c r e a s i n g 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 i n f l u e n c e on the nature of the combustion . Factors considered i n the design o f the c y l i n d e r head f o r considerations o f e f f i c i e n c y , maximum output and combustion knock, a r e : (1)  p r o v i s i o n f o r t h e f r e e 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) ; p o s i t i o n of the spark plug with s p e c i a l rggard t o the maximum distance o f flame travel.  ,  Of these t h e f a c t o r s d i r e c t l y i n f l u e n c i n g ; t h e tendency towards detonation are the turbulence and the p o s i t i o n of t h e spark plug.. Increased turbulence while i t increases the  (59)  rapidity of the combustion and the rate of pressure r i s e , 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, i n 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  i n 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 velocity, and detonating v e l o c i t i e s are only reached in the l a s t portion of the distance of maximum flame t r a v e l , 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 i n  the combustion chamber. There are several main types of combustion chambers used i n practice, as characterized by the position of their valves; the "L" ,"I","F ,and racing type heads. H  These types ( i l l u s t r a t e d  diagrammatically below) achieve the general aims i n design i n different degrees.  The racing type head most nearly approaches the  ideal combustion chamber but unfortunately requires the expensive mechanism of two overhead camshafts.  (60) The e f f e c t s o f h i g h e r i n t a k e and c y l i n d e r t e m p e r a t u r e s are  b o t h t o produce h i g h e r maximum t e p p e r a t u r e s (accompanied by  l o w e r e f f i c i e n c y and maximum o u t p u t ) . the  High temperatures a c c e l e r a t e  i g n i t i o n a n d combustion and s t i m u l a t e d e t o n a t i o n . S u p e r c h a r g i n g o f an a e r o - e n g i n e t o m a i n t a i n i t s normal  power output, a t h i g h a l t i t u d e s does n o t i n c r e a s e t h e k n o c k i n g , tendency.  I n f a c t t h e engine w i l l p r o b a b l y be r u n n i n g more  smoothly s u p e r c h a r g e d a t an a l t i t u d e t h a n unsupercharged a t ground l e v e l , 'because o f l o w e r a i r i n t a k e t e m p e r a t u r e s .  Supercharging t o  i n c r e a s e t h e power o u t p u t o f a g a s o l i n e engine i s accompanied by h i g h e r c y l i n d e r t e m p e r a t u r e s (and l o w e r t h e r m a l e f f i c i e n c y ) a n d therefore increases knocking.  S u p e r c h a r g i n g has t h e same e f f e c t s  on d e t o n a t i o n a s an i n c r e a s e d compression r a t i o w i t h t h e e x c e p t i o n of  a n i n c r e a s e d compression t e m p e r a t u r e which remains: e q u a l t o  T, r ^ ' . The e f f e c t o f i n c r e a s i n g t h e a n g l e o f s p a r k advance w i t h o u t i n c r e a s i n g t h e speed, i n a g a s o l i n e e n g i n e , i s t o i n c r e a s e the  knocking tendency.  D e t o n a t i o n may be reduced by r e t a r d i n g  the  spark (within l i m i t s ) > a l l other conditions remaining constant. ' C a r b u r a t i o n has not much b e a r i n g on d e t o n a t i o n . The  c a r b u r e t t o r does n o t v a p o u r i z e t h e f u e l , as i s g e n e r a l l y i m a g i n e d , but m e r e l y 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  t a k e s p l a c e i n t h e i n t a k e m a n i f o l d and t h e c y l i n d e r s and,depends on t h e d i s t i l l a t i o n  curve a n d s p e c i f i c heat o f t h e 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 s u b j e c t t h a t does have a b e a r i n g on combustion k n o c k i n g , i s t h e h u m i d i t y o f t h e a i r .  For a long  t i m e t h e i m p o r t a n c e o f t h i s f a c t o r was n o t r e c o g n i z e d u n t i l i t was found t o account f o r t h e 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 h e e f f e c t  o f i n c r e a s i n g t h e h u m i d i t y o f t h e a i r from 50% t o 85$ o c t a n e r a t i n f o f a l o w 0. h i g h 0. R.  N. g a s o l i n e from 45 t o 57,  g a s o l i n e from 85 t o 89.  raised the and o f a  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' h u m i d i t y and t h e n e c e s s i t y o f r a t i n g f u e l s a t some s t a n d a r d h u m i d i t y o r 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 of the a i r .  I n t h e compression i g n i t i o n e n g i n e , s i m i l a r m e c h a n i c a l f a c t o r s I n f l u e n c e t h e smoothness o f t h e combustion p r o c e s s . (a) c o m p r e s s i o n r a t i o . (b) f o r m o f t h e combustion chamber. (c) i n t a k e and c y l i n d e r t e m p e r a t u r e , (c) supercharging and t h r o t t l i n g . (e) t i m e o f 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 . R e d u c i n g t h e 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 reduction i n the extent of the delay p e r i o d .  This i s  p r o b a b l y most s i m p l y e f f e c t e d by i n c r e a s i n g t h e compression r a t i o . B u t as mentioned i n Chapter I , i t i s not advantageous t o c o n t i n u e i n c r e a s i n g t h e c o m p r e s s i o n r a t i o , f o r a t 15:1  C. R. t h e w e i g h t  (62) and cost o f the engine a r e increasing much f a s t e r 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 t o give s a t i s f a c t o r y combustion. A higher compression pressure reduces t h e i g n i t i o n l a g (and therefore the combustion shock) bys - i n c r e a s i n g t h e temperature difference between the  -.air -and the ••ignition temperature -.of the'-fuel*' (a) by i n c r e a s i n g t h e compression temperature.. ':•-.  (b) by decreasing the S. I . T. of the f u e l .  - bringing t h e oxygen molecules o f the a i r i n c l o s e r contact with t h e f u e l d r o p l e t s .  •  The.people supporting the theory that combustion occurs on t h e surface o f the f u e l drops, c i t e the f a c t that higher pressures increase t h e b o i l i n g temperatures of. the f u e l as w e l l 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 a g i s decreased and the combustion improved, they conclude t h a t the correct combustion occurs on the surface of t h e drops. The r e l a t i o n between combustion shock and t h e 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 o f a combustion chamber'of a high speed d i e s e l engine i s t o secure a h i g h r e l a t i v e v e l o c i t y o f the f u e l and the a i r , i n order that t h e combustion process may be completed i n the short time, allowed.  The c h i e f types i n use achieve r a p i d 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.  o  'I  I'  'I  I  1  (Z)  (1)  (3)  o  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 v e l o c i t y . The success of the system depends largely on the design and accuracy of the j e t s . Good efficiencies are usually abtained but not the smoothest combustion. The Ricardo head consists of a c y l i n d r i c a l 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.) sleeve v a l v e .  by t a n g e n t i a l p o r t s o u t s i d e t h e  T h i s does n o t mean t h a t t h e combustion s w i r l s  r o u n d and r o u n d ; i f t h e combustion p r o c e s s o c c u p i e s an average v a l u e o f about 56° o f c r a n k a n g l e , t h e a i r i n t h e head would, make one complete r o t a t i o n d u r i n g t h e t i m e o f 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 t h e o u t e r edge o f :  r:  t h e swirl,../is•,always met b y f r e s h a i r . ,  Very high e f f i c i e n c i e s  , a r e r d b t a i n e d i n t h i s t y p e o f engine and speeds up t o 2200 r.p.m.have been r e a c h e d . . I n t h e p r e - c o m b u s t i o n chamber t y p e s t h e 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 o f t h e c o m p r e s s e d :  a i r where Phases I and I I o f t h e combustion d e v e l o p .  The r a p i d  p r e s s u r e r i s e i n t h e p r e - c o m b u s t i o n chamber and t h e p i s t o n b e g i n n i n g i t s downward ...stroke cause .the p a r t i a l l y b u r n t gases and. t h e unburnt f u e l t o i s s u e through narrow o r i f i c e s a t h i g h v e l o c i t y i n t o t h e • c y l i n d e r where phase I I i s completed.  The combustion t a k e s p l a c e  .smoothly and, t h e system i s c a p a b l e o f - h i g h speeds ?and. o f h a n d l i n g f u e l of high thermal s t a b i l i t y .  However, t h e E f f i c i e n c y i s n o t  good and t h e power o u t p u t f o r a g i v e n s i z e o f c y l i n d e r i s v e r y limited. The Aero system u s e s 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 o r t h e 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 c o n t a i n s about, h a l f  (65)  of the compressed a i r and t h e i n j e c t i o n o f the f u e l i s d i r e c t e d e i t h e r a t o r across t h e mouth of i t .  The i g n i t i o n takes place  i n t h e neck and the combustion of Phase I I o s c i l l a t e s about t h a t point and i s f i n a l l y  drawn down i n t o t h e c y l i n d e r .  Phase I I I  takes p l a c e w i t h t h e a i r i s s u i n g from t h e a i r c e l l , d i r e c t l y at /the fuel jet.  The system I s very s e n s i t i v e and-requires p e r f e c t  balance of seventl f a c t o r s t o work s a t i s f a c t o r i l y .  I f much  f u e l penetrates t o the a i r - c e l l the r a t e o f pressure r i s e i s excessive and knocking occurs.  Properly designed and adjusted  an engine o f t h i s type runs smoothly and very high speeds are possible.  The A. E i Gi b u i l d a s i x cylinder, t r u c k engine under  Aero patents t h a t has a speed range from 300 t o 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 s p h e r i c a l chambers of approximately the same s i z e . ' The f u e l I s i n j e c t e d Into t h e f i r s t chamber where Phases I and I I :" develop.,  As the p i s t o n moves downward,, the p a r t l y burned products  pass through a narrow t h r o a t i n t o the c y l i n d e r with great turbulence\ The a i r from' the a u x i l i a r y chamber expands i n t o the pre-combustion chamber t o supply a i r a t t h e i n j e 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 I n c l i n e d t o be rough from the exeessive turbulence. Increase i n intake and c y l i n d e r temperatures improve the smoothness Of t h e combustion but decrease the maximum output and  (66) efficiency of the engine.  As they affect the masimum  temperature of the cycle a l i m i t i s imposed on t h e i r increase by lubrication factors. Supercharging to increase the power output and t h r o t t l i n g hare a marked effect on the combustion shock i n the diesel engine.  As the compression temperature remains the  same, t h i s must be wholly due to the change i n 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 i s e i n the f i r s t part of Phase I I , followed by a f a l l i n g off i n the rate of pressure r i s e , resulting i n 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 "dopes have been found. 35  These  substances may be classified 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 i g n i t i o n 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)  Nickel carbonyl  NI (00)  Di-ethyl t e l l u r i t e ;  (G^J.Te  5  f  Di-ethyl selenide Ethyl iodide Some of the  C^H^I  organic substances found to have  knock suppressing powers are:  (68) Jfylidine  C H -(CH )-NH,  Toluidine  G H^-GH -M  4  3  3  4  3  2  Analine  18. S 21.7  Methyl analine  G H -NH- CH  22.2  Ethyl analine  Ggflr-NH- G,E  10.4  Benzyl analine  C H -NH- GHj- CgH^  9.5  Cresol  G f i - C H OH  5.8  Phenol (Benzene  fc  y  y  S  fe  y  t  f  r  . 0<H OH r  O A -  4.4 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. It 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 stability.  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) i n 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 i s a colorless l i q u i d  decomposing at 250° C , Ethyl Fluid contains ethylene dibromide C H Br and a characteristic red dye. 1  /f  2  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 safetymeasure against i t s very poisonous properties.  Ethyl f l u i d 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 i m i t i s imposed because above t h i s 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 l u i d 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 i n h i b 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. into two classes:  This has caused some experimenters to divide dopes 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 c o l l o i d a l suspensions of certain metals i n gasoline have knock suppressing power comparable to that of the organic compound of the metal. Nickel or iron i n c o l l o i d a l suspension have anti-knock properties •slightly superior to iron or nickel carbonyls.  A colloidal  suspension of lead Is only s l i g h t l y 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 i n 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 H ) CI and ;i  r  J  as e f f e c t " ^  Pb ('CJis-^Cl*. are only f and J respectively as lead tetra-ethyl A  Pb . (CiHs-) i n suppressing detonation. 4  This would seem to indicate  that the a l k y l groups are of importance as well as the lead atoms i n 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, p a r t i c u l a r l y 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 ignition 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 i n 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 ignition  (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 (PbO ) oxidises any unstable a  intermediate product i n the chain that might give rise to accelerated burning, thereby being reduced to i t s lower form (PbO). It i s Immediately oxidized again to the higher form by the a i r .  Thus  the lead mo3.ecules are assumed to o s c i l l a t e very rapidly between the higher and lower oxide forms. A t h i r d 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. establishment of  The  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 p a r t i a l l y 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 c o l l o i d a l 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 organometallic dopes act c a t a l y t i c a l l y ( 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 ignition 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 p a r t i a l l y 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 t h i s means. The Radiation Theory explains the action of anti-knocks  (74) as one of absorption by the m e t a l l i c atoms of r a d i a t i o n s emitted from the flame f r o n t .  T h i s prev-ents t h e ; a c t i v a t i o n of the unburht  .mixture ahead of the flame f r o n t , and so insures against \ accelerated combustion. Other l e s s probable t h e o r i e s e x i s t , and of these, the theory of "poisoning" of the metal w a l l s of the c y l i n d e r might be mentioned.  I t i s based on- the chemical knowledge of the poisoning  •of c a t a l y s t s and the f a c t t h a t surface chemical r e a c t i o n s take place more R a p i d l y than homogeneous r e a c t i o n s .  Various elements occur-  r i n g i n anti-knock dopes are known t o "poison" c a t a l y s t s i n chemical processes: these include.:,  l e a d , selenium, t e l l u r i u m , i o d i n e , 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 a c t i o n by the w a l l s of the combustion chamber.  The r e a c t i o n , the theory  concludesj must be slower for,being; a purely homogeneous r e a c t i o n . ; , , A l l these and several more l e s s probable t h e o r i e s attempt to account f o r the knock-suppression p r o p e r t i e s of such small amounts of organo-metallic compounds. The a c t i o n of purely organic dopes i n decreasing detonation i s more e a s i l y understood, as appreciable amounts of these m a t e r i a l s are required t o suppress detonation.  These compounds arc aromatic amines and phenols and  are thermochemically, very stable with t h e i r benzene r i n g s t r u c t u r e s . T h e i r thermal s t a b i l i t y Is added t o by the presence of amino,  (75) hydroxy! and short'y-alkyi''':grpups,'-. and/:.seemS' t o r a i s e the thermal ;  s t a b i l i t y and anti-oxidation p r o p e r t i e s of the whole f u e l mixture. Used i n q u a n t i t i e s up t o 5% and 1 0 $ these compounds improve the anti-knock q u a l i t y o f g a s o l i n e very n o t i c e a b l y . Dopes f o r suppressing combustion shock;  I n t h e search f o r organic  dopes f o r reducing the combustion knock i n gasoline engines, many agents were found t h a t showed pro-detonating i n f l u e n c e .  These  were.organic peroxides, n i t r o g e n peroxide, ozone, the aldehydes and a l k y l n i t r a t e s and n i t r i t e s , unstable and some  A l l o f these compounds are  of them e x p l o s i v e j t h e i r a c t i o n was found  t o be t o hasten i g n i t i o n and accelerate combustion. I n a compression i g n i t i o n engine, t h e purpose of an anti-knock dope i s t o decrease the i g n i t i o n l a g 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 t o be successful In suppressing t h e combustion shock.  Some o f these a r e ;  A c e t y l Peroxide Benzoyl Peroxide ;  EthyJ: Amyl  ^^•^v;^>y^ ; ^ 1  fitrate  :  "G Eg-m '\  - .  a  ffitrate Nitrite'••C  Cg-H^-NO, < r  fl  -M0i  Y /  Aoetaldehyde  0H -GHO  Benzaldehyde  C H^CH0  5  4  5  -•  (76) The d o p i n g o f d i e s e l f u e l s on a commercial s c a l e has n o t d e v e l o p e d t o any g r e a t e x t e n t 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 nitrate.  T h i s s u b s t a n c e however, i s h i g h l y e x p l o s i v e and cannot A l t h o u g h t h e q u a n t i t i e s u s e d a r e s m a l l (Z% of  be s h i p p e d .  amyl  n i t r a t e r a i s i n g t h e c e t e n e number o f an average d i e s e l f u e l from 50 t o 70) t h e  o r g a n i c n i t r a t e s and n i t r i t e s a r e t o o e x p e n s i v e f o r  i m p r o v i n g a f u e l whose c o s t must remain l o w f o r t h e d i e s e l e n g i n e t o r e t a i n i t s p r i n c i p a l advantage.  The p e r o x i d e s and  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 . aoetaldehyde i s i t s low b o i l i n g temperature.  benzaldehyde  The o b j e c t i o n t o  Ho i n e x p e n s i v e 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 e n g i n e s has y e t been f o u n d .  T h e o r y o f c o n t r o l o f combustion shock by dopes:  The a c t i o n o f  o r g a n i c dopes i n d e c r e a s i n g k n o c k i n g i n t h e d i e s e l e n g i n e , i s s i m p l y one o f s u p p l y i n g l o w I g n i t i o n t e m p e r a t u r e n u c l e i t o i n s t i t u t e t h e combustion. of t h a t  The i g n i t i o n l a g becomes t h a t o f t h e dope i n s t e a d  o f t h e f u e l component o f l o w e s t 3 . I . T.  The d e c r e a s e d  d e l a y p e r i o d g i v e s a smoother a n g l e of•"breakaway", a l o w e r maximum r a t e o f p r e s s u r e r i s e p e r degree crank a n g l e , and a l o n g e r p e r i o d o f 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 o t h e r a n t i - d e t o n a t i n g dopes on t h e combustion i n a compression i g n i t i o n engine n o t o n l y d e c r e a s e s 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 t h e I g n i t i o n l a g of t h e f u e l .  ( 7 7 )  Chapter l X --v:  ~"^'  •  —is.  sr  - .".  umclusxon  Summary of f a c t o r s a f f e c t i n g combustion, knock;  I t has been  seen t h a t a compact molecular s t r u c t u r e and therefore a'high thermal s t a b i l i t y are d e s i r a b l e p r o p e r t i e s f o r a, f u e l f o r a spark '•••ignition engine from an anti-knock standpoint.  Cracking o f crude  o i l t o Increase t h e gasoline y i e l d a l s o increases the anti-knock r a t i n g o f t h e gasoline f r a c t i o n , f o r the high pressure cracking <  process, causes t h e formation of more compact molecules. and leaves , only the compounds o f high thermal s t a b i l i t y uncraeked.  Aromatic  and naphthene hydrocarbons and a l s o a l c o h o l s are d e s i r a b l e components of f u e l f o r spark i g n i t i o n engines j ethers,, normal p a r a f f i n s 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 f a c t o r s a r e 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 a l c o h o l s , naphthenes and aromatics do not. ' Here high thermal s t a b i l i t y and a molecular s t r u c t u r e t h a t i s compact, are d e s i r a b l e . Cracking o f crude o i l g r e a t l y lowers the i g n i t i o n q u a l i t y o f t h e resulting diesel fuel fraction. ,  Anti-knock dopes f o r gasolines, are substances which  reduce t h e r a t e o f combustion;-organo-metallic, aromatic, amines,  (78) and p h e n o l s .  T h e s e substances added t o d i e s e l f u e l i n c r e a s e t h e  combustion shock.  The compounds used f o r r e d u c i n g combustion  shock i n d i e s e l engines a r e u n s t a b l e p e r o x i d e s , aldehydes and n i t r a t e s , w h i c h when added t o g a s o l i n e i n c r e a s e t h e d e t o n a t i o n . jfl.1 f a c t o r s g i v i n g l o w e r t e m p e r a t u r e s and p r e s s u r e s i n t h e c y l i n d e r o f a g a s o l i n e e n g i n e , d e c r e a s e the d e t o n a t i n g t e n d e n c y ; but i n a d i e s e l engine h i g h e r t e m p e r a t u r e s and p r e s s u r e s a r e n e c e s s a r y t o d e c r e a s e combustion shock.  I n g e n e r a l , 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 e n g i n e t e n d t o i n c r e a s e combustion shock i n a compression i g n i t i o n e n g i n e , and v i c e v e r s a .  One o f t h e few e x c e p t i o n s  i s t h e t i m e o f t h e b e g i n n i n g o f t h e combustion p r o c e s s ;  under  c o n d i t i o n s o f k n o c k i n g , a r e t a r d e d spark d e c r e a s e s d e t o n a t i o n i n a g a s o l i n e engine,- and r e t a r d e d i n j e c t i o n d e c r e a s e s shock i n a d i e s e l e n g i n e .  combustion  The v a r i o u s f a c t o r s d e c r e a s i n g  k n o c k i n g i n b o t h t y p e s o f engines a r e t a b u l a t e d on t h e next page.  (79) Anti-Knock Factors Spark I g n i t i o n .jsngine  Compression  Fuels:  Fuels:  Compact m o l e c u l a r f o r m s .  S t r a i g h t chain forms.  Aromatics. Naphthenesi  ;. •  I g n i t i o n Engine.  , .:: Normal p a r a f f i n s . : ; lormal olefins. /  Branched c h a i n hydrocarbons.. /Alcohols. : ;  Ethers.  Dopes-: :  • : Dopes:  Compounds t o reduce speed o f Organo-metallies.  combustion.  O x i d i z i n g agents t o reduce ignition lag. Alkyl nitrates.  A r o m a t i c nmines.  Aldehydes.  Phenols.  Organic peroxides.  Engine F a c t o r s :  Engine F a c t o r s :  Lower p r e s s u r e s :  Higher pressures.  Low compression  ratio.  Throttling. Lower t e m p e r a t u r e s . Increased c o o l i n g . 7LIght l o a d . ' —Retarded sparks  / H i g h compression  ratio,  / Supercharging, H i g h e r -temperatures. Decreased  cooling.  F u l l load. Retarded i n j e c t i o n .  H i g h e r speeds.  Lower speeds.  Increased a i r humidity.  Coarse a i o m i e a t i o n .  '.7./  (80)  Conclusions:  Very l i t t l e can be learned o f the a c t u a l 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 c y l i n d e r o f an engine i s a h i g h l y t h e o r e t i c a l one.  The  'very f a c t t h a t so many t h e o r i e s have been advanced t o account f o r •combustion knocking and i t s suppression by dopes, I s an i n d i c a t i o n ,;.ftf:.the,.Ia <^l.pf 'definite, toowledge- .of-these, phenomena-. There are %  objections or contradictory evidence t o p r a c t i c a l l y every theory so f a r proposed on the subject o f detonation and i t s c o n t r o l , but that i s probably because i t i s e a s i e r t o give d e s t r u c t i v e than constructive c r i t i c i s m .  I t I s p o s s i b l e that none o f the  theories,, so f a r advanced are c o r r e c t , but i t I s more probable that one, or a combination o f more than one of the t h e o r i e s  represents  the mechanism o f the r e a c t i o n t a k i n g place i n t h e combustion •••chamber... 1  An a n a l y t i c a l method f o r pre-determining knock r a t i n g o f a gasoline has been proposed.  the a n t i -  An a n a l y s i s of t h e  of the f u e l i s made i n t o the four types of hydrocarbons and t h e anti-knock value i s c a l c u l a t e d emperically from t h i s a n a l y s i s . However, due t o the widely v a r y i n g properties of the p o s s i b l e isomers present, i t has been concluded t h a t I t I s quite impossible t o give.an accurate estimate o f 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 t h e l a r g e r m o l e c u l e s , t h e r e a r e much greater p o s s i b i l i t i e s of isomerism.  The h i g h l y c o m p l i c a t e d  n a t u r e o f t h e f u e l s t r u c t u r e , : c o m b i n e d w i t h a g r e a t e r number o f 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 o f r a t i n g d i e s e l f u e l s from a c h e m i c a l e x a m i n a t i o n , even, l e s s . _  The r a t i n g o f  b o t h 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 c h e m i c a l and p h y s i c a l p r o p e r t i e s . The o c t a n e number o f a g a s o l i n e f o r a g i v e n engine ,has a d e f i n i t e maximum advantageous l i m i t . . :  I t i s at a  p o i n t where t h e r a t e o f combustion i s slowed down enough t o a v o i d t h e sudden i n c r e a s e i n p r e s s u r e t h a t l e a d s t o d e t o n a t i o n . R a i s i n g t h e octane number o f t h e 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 t i m e f o r e x p a n s i o n , and g i v i n g l e s s complete b u r n i n g .  I n c r e a s i n g t h e octarenumber i n c r e a s e s  t h e power o u t p u t and e f f i c i e n c y o n l y i f t h e r e i s d e t o n a t i o n . The use o f a g a s o l i n e o f h i g h octane r a t i n g i n an engine o f l o w 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 n o t o n l y I s t h e e f f i c i e n c y and power d e c r e a s e d , but t h e combustion may be slowed t o a p o i n t wheire t h e gases a r e s t i l l b u r n i n g a s t h e y pass out around t h e exhaust v a l v e ; owners t h e n blame t e t r a - e t h y l l e a d f o r b u r n i n g o u t t h e exhaust v a l v e s .  T h e r e f o r e i t must be  c o n c l u d e d t h a t t h e octane number o f t h e g a s o l i n e s h o u l d be s u i t e d -to t h e compression o f t h e e n g i n e i n w h i c h i t I s u s e d .  A gasoline  (82) of 76 0. N. i s s u i t a b l e f o r use i n an engine with, a G. R. i n the v i c i n i t y o f 5.5:1. The low speed or i n j e c t i o n d i e s e l engine w i l l burn almost anything i n the way o f d i e s e l f u e l s , but the high speed types require a high grade r e f i n e d d i e s e l f u e l .  A good  grade d i e s e l f u e l s u i t a b l e f o r high speed engines should be a s t r a i g h t run f r a c t i o n from a p a r a f f i n base crude; approximately 20° - 26° Baume g r a v i t y and f r e e f r o m . a l l i m p u r i t i e s , sediment, etc.  Cracked f r a c t i o n s do not make s a t i s f a c t o r y f u e l f o r high  speed d i e s e l engine.  In a given engine t h e combustion becomes  rougher as the speed increases.  Therefore, the higher t h e  speed range of a d i e s e l engine, the better t h e grade o f 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 t h e future of t r a n s p o r t a t i o n ,  but i t i s  questionable whether t h i s w i l l ever extend t o the passenger automobile f i e l d .  Aside from the few t e c h n i c a l d i f f i c u l t i e s that  s t i l l remain, the item o f f u e l p r i c e would l i m i t t h e i r use.  As  a h i g h grade r e f i n e d 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 p r i c e would r i s e i f a large; demand were created f o r d i 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 t o those on g a s o l i n e , and t h e d i 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 i g h t e r weight of the fuel, and the elimination of radio interference and f i r e hazard;make the diesel engine i n f i n i t e l y more advantageous.  (84) Bibliography . Books:  B i r d , A. L . '  The I . C.  Bone and Townend  Flame and Combustion  B r o o k s , B. T.»  Hon-Benzenoid  G r u s e , W. A.  P e t r o l e u m and i t s P r o d u c t s .  Hollewan, i . Leslie,  F,  E. H.  Textbook Motor  Engine. i n Gases.  Hydrocarbons.  o f I n o r g a n i c Chemistry.  Fuels.  •Moore, H.  F u e l s f o r I . G. E n g i n e s .  R i o a r d o , H. R«  The H i g h Speed I . C. E n g i n e .  Pye, D. R.  The I n t e r n a l Combustion  T i n k l e r and C h a l l e n g e r  Chemistry o f P e t r o l e u m .  Technical  Engine.  Papers2  Beroeze and B o e r l a g e  I g n i t i o n Quality of Diesel Fuesl, S . A. E. J o u r n a l  G a l l e n d a r , H. L«  Dopes and D e t o n a t i o n Engineering  C l a r k and Thee  1927  Dec.  1925  T h e o r i e s o f A c t i o n o f KnockSuppressors. I n d . & Eng. Chemistry  D i c k s e e , G. B.  Feb.  Theories of Detonation I n d . & Eng. C h e m i s t r y  C l a r k , Brugman & Thee  J u l y , 1952.  Dec.  1925  Aug.  1932  Problems i n H i g h Speed D i e s e l Engines Automobile E n g i n e e r  (85) Bibliography (continued.) Edgar, G,  Octanes Ind. & Eng. Chemistry  Egloff  P  G<  1927  Anti-knock P r o p e r t i e s of Cracked Gasoline. U n i v e r s a l O i l Products B u l l e t i n .  E g l o f f , Schaad and Lowry  Oxidation Mechanisms o f P a r a f f i n Hydrocarbons. U n i v e r s a l O i l Products B u l l e t i n .  Horning, H. L.  C. F. R„ Test Engine. S. A. E. J o u r n a l  Hubner and Murphy  June 1931  A Standard Knock-testing Apparatus. U n i v e r s a l O i l Products B u l l e t i n .  Le Mesurier & S t a n s f i e l d  Fuel T e s t i n g i n Engines. Journal Of I n s t , of Pet. Tech. Jul.1951.  M i d g e l y and Boydc  Anti-detonating dopes Ind. & Eng. Chemistry  Schweitzer, P. H,  1922  Combustion Knock D i e s e l Power  Whatmough, W.Av  Petrol  Characteristics  Automobile Engineering Y.lthrow, L o v e l l and Boyd  Aug. 1952  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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