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Interpretation of the anti-knock performance ofhydrocarbons in gasoline engines in terms of their combustion characteristics
interpretation o/ the Anli-knock Per/brmance of /lydrocarbons in Gasohne Engines in Terms of their Comhz+slioH C/.at acte rr.cs K. C. S a t o o I a '6hett,' l¢,.'.,c+zr,:h I+¢,1, I k,,rnt,,n
/¢+.'::tar¢ h (.'entP+. i'.¢). ]+'..,.'r *, C k , ' s t , r
IReceiv~d Mart:t~ lgJ'a) 7"/,t ;,+flal'.~aD'opl .Jtt the r,*lnb,+st;,.J, ch,tr*t+'t,"r/,eli,'x ~Jf h qde~'*e+lrt,or, .. I,ra,*,t'o~tM,i r,/J,~r(~',t, ~.l~t,[,+ tt~ 1~ h. tt,'r itJ.~+'!tht i+~to if*+' at~Ii,k~.s+'l." ].'r.forll++s/m,' ~{f h/ldr.e~+rhon.~ it~ f/tier+h.+' ,..9';n,.',+. 7%,' r,'.,'.D.,~ .~£o... ti,,tt the ,+r+'r,Ptl +'a~lemt of ,'..,wti,,,, tal¢it+fl ;Jltt'¢" tt,'ftlr,. Ol¢l/'¢iolt rc.t..t/+.,v +?,.rff ~qej.~;,l,iI t~'+the l¢,+o,';,:,t+'ttPite{~t p,'el>Jr.t,o+,w +*.fll~,," .f~eL 7'h,' t'lf+'et~ "el" th,' .t,ht/tir.+ o f t+'troethptl,h.,td q . d of t+~,t~[roearl~r**~. +'o~t+.t*,*t?C'i~t'~ ,vl~o.,~,' tv~l~O' it* t+'rttla qfl t~f~'tltltr+!l o,'flt*~e *t~lttltt.~r ia *+Or pre~f+',?t.t~ff.' frOrl/ the t't~irit!¢¢l~" ocfrtr+,! ntlttth,'r. ,',t** h,' r,~adil!t i.h,rpr,,#.~! .from tk.,'ir ,ff,'et.~ +m tk,,' ,'.cl.'.l qf prr.fl,z~*,r ~v.art/+'m.,+. F.rtll~:'rmor~. Ill,' [I+'PJ'OI'IItr'r.rri.~l let llt,¢ ¢~.t'ls'lft {{f +~ITt'J~,'tl?]+' rcetr't~o¢~.
,~ ,~l.:irm~r understanding ,>f the relationship :..,~tw,.~.n th,: anti-knock performance of huffs in -:L:.:,.t+rteeugimrs and their combustior, character>~k::., i,; of interest to both engine designers and tt>'l technologists. Numerous altempts have }J,un made to interpret the wide differences in 7h.. an'ti-knoek performance of differenl: hmls in I,rm~ of their combustion characteri,~ties. Titus(' a:u,mpts, however, have had only limited st=r~,.ss. Many investigators, who have smtght ¢!,: interpretation of engine [x.rforrnance in t,.rrras +,f basic combustion characteristics detereililt,d under simple laboratory eon
m:.dnty with arom,'ttic and some paraffin[¢ hydrocari)o)~s that there is ~ broad correlation t),,tween the engine perforrr);mce and fhe minimum ignition temperature, The combustion ¢haracterist{es o~ hydrocarbons, which have been investigated recently and rept,rted i,1 an earlier paper ~, do, however, correlate well with their anti-knock qualifies. These comlmstiou characteristics have been determined in terms of both chemical and phy'sical parameters and incltu.le: (i) the extent ~,f pre+tl.'une reaction, (i,) the minimmn ignition tt-nlperature for a g[ver~ lag, (ii,) the ign[fion h,m perature / tag relationship, and (iv) the effect of fuel/air ratio on (i), (ii} and (iii). Apart from correlating with anti-knock performance, these characteristics help to interpret successfully the inltuence on the anti-knock performs, nee of such factors as rite speed and operat.ia~g temperature of engines. Furthermore, the influence of antiknock agents and +.ff hydrocarbon components, whose value in te.rms of blending octane ntmlbcr [,,, not predictable from their intrinsic octane numbers, can be satisfactorily interpreted from their effects on combustion characteristics, The afore-mentioned combustion character. 193
194
~r~?
K . C, ~ l O O j a
istics, reported in the earlier paper, were measured for the following eight typical C,-C, hydrocarbons : n-heptane, methylcy~lohexane, isooctane, diisobutylene, benzene, toluene, m-xylene and ethylbenzene. In the present paper the relative performance of these hydrocarbons in gazolk~e engines* has been interpreted in terms of the combustion characteristics.
Cecrelation with Anti-knock Performance irt Engines The relative anti-k~ck performance of different fuels, under ~.ny given set of engine operating conditions, can be directly related to the extent of pre-flame reactions. Those hydrocarbons which undergo less reaction in the pre-flame stages perform better in the engine than those which react more extensively. This is so irrespective of file structural category to which the hydrocarbons belong, and is true almost without regard to the minimum ignition temperatures of the hydrocarbons so long as these are not to6 far apart. As an illustration, the ignition temperatures o.f diisobutylene and methylcydohexa,ae are in the same range. However, methylcyclohexane, which undergoes far more overall pro-flame reaction than diisobutylcne, has a much ~ower octane rating and critical compression ratio. isoOctane ignites at an appreciably higher temperature than diisobutylcne, yet dilso. butylene, whida suffers relatively less reaction in the pro-flame stages has a higher 'Research' octane rating. Ethylbenzene and isooctane both ignite in the same temperature range. Ethyihenzene, however, undergoes less proflame reaction than isooetane arid its 'Research' ,octane rating, too, is higher. Again, at the high end of the igaition temperature scale, benzene, toluene and m..xylcne ignite at comparable lcmpcn~tures, but benzene, which suffers the least extent of pre-flame reaction, performs best. The anti-knock quality of different hydrocarbons in terms of their octane numbers by both the 'Research' and 'Motor' methods and the .critical compression ratios is shown in Te,bles I and 2. "Tk~ Informalitln on the anti.knock ocrfr~rrna~e¢ of diffvrent hY~lrccat'bons In gasoline cngtlles tlas b~en taken frnm the ~ P ! Projet,'l ,t5 d~ll(~lt.
4
That it is the extent of pre-flame teaetion rather than the minimmn ignition temperature that shoald provide the more exact correlation Tabl~ 1. CFR Octane Ratingz* of unleaded hydrocarbons r~ . Ilydr~carbon n-l/epta'n e Methylcyclolwxane, isoOclan,: Dii~obttt yh, m~ 13~,~n,Tcne Toluene m-Xytene Ethylbenzene
Rcseamh method 'F~'
Motor me~hod 'F,_)"
0
0 71'1 100,0 88.6
74.8 IU0.0
105.3 ~'~
--t
] 20" ! 117.8 107"4
[
:[
11)8'5
118,0
97"9
*Octane ratings above 100 a r e according in the A S T M (Wiese) ,',::ale, STh.¢ value h w b e g o n e i,~ t o o hhth to be toted sadsfactc3dht in
~he CFR engine.
with the engine performance is understandable since the ignition temperatures, in the manner in which they are usually determined in open vessels, cannot possibly reflect the tendencies of fuels to anto-lgnite in internal combustion engines where the fuel-air charge is subjet:ted to increasing pressure an<] temperature in the compression stroke before the passage of the spark and again under more severe conditlons in the 'end-gas region' ahead of the approaching flame front. A fuel cap,xlfle of undergoing greater pro-flame reaction woifld produce more "/ lddr "J, (,',~'¢tl( 'd (.'¢')lll/pl'¢'~,~if)~l ro/Id:,5 ell I(t,d, twdi,d h.Td)'ocarb.ns Zu GenerM Mot,)v.~ singb; cytind~.'r l..'Hl'l(ll~ft'-t:O?II/)¥~'SSlO,ig, t,'ll~'itt~5
lly,rlrcJc uric, ,n
6fro J'c?~ i' ~tltll
2 It00 v,e~J/,qlill
J,,,J,,.:.': :s:'l: .~.~":'F .~le'oFI s,~,°i ' Air : tllOr'F 150"F 10(f/; I 15W'[ nJlept.m~ ,lh"thylcyclo. Jt.e~a~t~ i.~uOC&irte
])iJsobu(ylt:m
:¢,i) 4,,55 7.3
I __1 -,tJ5
-.-
,'I,8
3,o
4"0 6'6
7'3~
5"9 ] 4'9 9.0 7-1 5,7
xt.:~s it,s
]3.s ] 9.0 IZ,:) I ~.s
8.2
0.3 I 6,o
~'S5_.
1~tl :~?,7,I"7L$"
"['~.dumte m-Xylene Ethylb,.,n~ene
Is
Is.`5 ]3,s
~ptem~r 1960
Anti-kngek performance of hydroear]~ons in gasoline engines
heat and pressure rise and would consequently carry the charge to the poi.at of ignition more readily than one ".;'l:ieh undergoes less reaction. Thus, irrespective of minor differences in ignition temperatures, fuels with widely different extents of pre-flame reactions would differ markedly in their tendencies to auto-ignite and knock in engines. It is not surprising therefore that, for all types of hydrocarbons, the extent of preflame reactions provides a satisfactory correIation with engine performance. ..%ch a correlation between engine performance and extent of pre-Ilame reaction has also been indicated in engine experiments by E. R. RZr,ULLlaO, H. A. Rmam~s, Jr and M. C. K. ,lesF.s'L These authors took the average engine tcmperatnre before ignition as the criterion of pn,dlalue reaction and showed that, under both firing and cycling conditions, isonctane under~:,.,,:,smort, reaction than benzene, anti a blend ~d ~,~,',.ctane containing 25 per cent n-heptane marc re;..:tlon than isooctane itself.
Correlation wkh Temperature Sensitivity of Fuels in Engines It is well known that an increase in the running Itmlpcratun, of an engine lowers its knocklinlhvd perforrnance (Table 2). The degree to whi,F.[i IlK. knock-limited compression ratio must be rcd.t:ed on increasing tl,e jacket and air tt~mpvr:,mre, however, varies with the nature of the ha,I, Comparhlg. fc,r {nstmlee, isooetane, cthylh,.,uzt,rxe and diisobutylene, the reduetkm el ¢:rhi,::l.I omlpressitm ratio is less with iso.etun. fi,;m with the otht, r two, hydroearb-ns .f.r , ;:i~,,n i,mrvase in ternperature. The studies of tll,.qr cc,i flms/ion char-qcteristies havt~ strongly indicated llt;,l the pre-flame reactions of. isotlcl.~4, art: I.'tr re.re resistant to self-ignition than %,',e of elhvlbenze:!c, or diisobutvlene. !'l'l,i. i< iu¢.lged fr¢~m the s]{q)e of lhe ig';tithm ~rJq,cI,;I.Illtr VCI'SIIS lag curves and from the efft,ct. ,d fuel eoncenlrat[on on ignition tempera. tim, ur~d,,r different isolag eomlithms.) For inst *l,:t:, for lhe same rise in temperature, the ign!li,m tags of ethylbenzene and dlisohatykme are sllortened m u c h more than with isooctane. In ,ther words, the same rise in temperature renders ethylbcnzene and diisobulylene much
more r~adity ignitable than isooctane. These observations are in line with the observed tet,~peratur¢ ~ensitivities of these hydrocarbons in the engine. Comparing toluene and m-xylene, the studies of combustion characteristics have shown that the pre-flame reactions of m-xylene are mort,' resistant to self-ignition than those of toluene and this again is in line with the temperature sensitivities iv. the engine of these two hydrocarbons. The same increase in temperature reduces the critical compression ratio o£ mxylene to a level which is still higher than that of t.luene. This lower temperature sensitivity of ,n-xylene is further retteeted by the relative differences in the 'Rc's~'arch' and 'Motor' octane ratings. The nct'me rating o~ m-xylene drops from 117..5 to only 118'0, whereas for toluene the drop is from 120"i tO I03'8.
Correlation with Effects of Engine Speed Variation on Perfgrmance ~inc'c cragirws ha.vc to operate over a wide range of speed, much attention h~,s been given to the relative sensitivities of different fuels to changes i~: engine speed. As a result considerable data have been accumulated which show very rnarkcd differences between different fuels in this respect. As the engine speed increases, ~l-p',~ral~-ins,most of the cyclopara~ns and some isop:traffins and oMins improve in anti-knock lwrf(¢mam'e, whereas aromatics and n,,st ¢,~ the hi
,,rwv-; between in,"ividuat hydrocarhons. .'\mrmgst tlw h}dro,~-arbons under disens:ailm, :,s shown iu "l'ah/c 2. n-hephme and methy[c.v¢lo]lexane appreci;tte i*l {.heir perforrnance whorl the engine speed is increased from 600 to 2 t'R}I') r o v / n l h l , f,';o-()etal~e a]sc~ improves irl perforln;tnce at the higher ~peet:t, whereas diisobltl .,.lt,ne altd ethyllae/lzel-le depreciate markedly. Widt both toluene am:l m-xy[ene, the engine perfiwnmnce lalls c,aa:;d,.rably at the higher speed. To understand the reasons for the widely
196
K . C . Salooja
different effects tLat an increase in engine speed produces on the performance of different fuels, one must follow the manner in which the increase in engine speed a~feets the combustion processes o:~ an individual hydrocarbon. An increase in engine speed, when all other operating conditions are kept the same, could be considered to affect the combustion processes in the engine in the following ways: (I) The temperatur~t~s at which combustion takes place in the engine tend to be higher at faster speeds even when the jacket temperature and the other operating conditions are kept the same. This is due to the greater number of heat cycles per unit time which result in a higher combustion chamber surface temperature. The increase in the running temperature of the engine would tend to lower the performance of various fuels in accordance with their temperaturn sensitivies, (2) At higher speeds the duration of comOustion cycIes woutd decrease. This would have the following effects: (~1 The extent of pie-flame reactions would decrease owing to shorter times being available for these reactions to take place. The reduction in the extent of pre-flame reactions q iould lead to an improvement in the engine performance. fit) The stability characteristics of the pro-flame reactions with respect to self-ignition would be affected: those hydrocarbons which have a relatively stable pre-flame reaction stage and offer greater resistance to ignition at shorter reaction times would tend to improve in their engine perfnrm:,n,:~ at higher speeds. Any improvement on this account, as also on account of (i) above, wau~d I)e offset somewhat bv the increase in the runni,lv, tern'. perature of the engine, With n-heptane and methylcyclohexane, which undergo extensive pre-flame reactions, the effect of a large increase in engine speed would
Vol. 4
primarily be that of a marked reduction in the extent of the pre-flame reactions. This, by itself, should lead to a corresponding improvement in the engine performance, There would, however, be some depreciation in the perform. ante due to an increase in the effective engine temperature at the higher speed, The net effect could be that of an improvement in the per. formance of the fuel, such as is observed, In comparison with n-heptane and methylcyclohexane, isooctane, diisobutylene and ethyl. benzene undergo little pre-flame reaction and ~t~ a result have much higher critical compression ratios. In accordance wish the relative extents of pre-flame reaction, at an engine speed of 600 rev/min and jacket and air temperatures of 212 ° and 100"F, ethyibenzene has the highest critical compression ratio value (13'5), followed by diisobutylene (11.95) and isooctane (7'3). With little pre-flame reaction taking place in these hydrocarbons, the effect of increasing engine speed on the extent of pre-flame reactions would be much less pronounced than that with n-heptane or methylcyclohexane. However, what little improvement ,could occur on this account at higher speeds would be greatest with isooctane which undergoes more pre-flame reaction than either diisobutylcne or ethyl. benzene. At higher engine speeds, the depreciating influence due to a rise in the effective engine temperature would be mot,. marked with ethylbenzene and diisobutylene than with isooctane. The studies on the combustion characteristics of these hydrocarbons, as mentioned in the previous section, have dearly shown that the inherent temperature sensitivity of iso~mtane i~ very rnuch less than that ~,f either ethylbcnzenc or diisolmtylcne. Consideration of the results on the stability ch~.raeleristics c~f the pre-flame reactions towards self-ignition of these hydrocarbons particulart.~ with respect to reaction time, which is reduced by aa increase [rl engine speed, indicates thai at slmrter residence times isooctane ha~s markedly greater stability towards ignition tha~ has tither diisobutylene or ethylbenzene, For instance, as shown in the previous paper', for the reduction of the ignition lag even trom four TM
September 1980
Aztti-kngek performance,o~ hyd~oegrbons in gasollr~e engines
to two seconds, isooctane has to be raised in temperatme by about 28* at ;~bout 520°C, whereas, for a similar reduction in ignition lag ef ethylbenzene, the temperature needs to be raised by only 13 ° at 525"C and with diisobutylene by only 11 ° and that moreover at a much lower temperature level, about 490°C. Furthermore, the previous studies have shown that whereas the ignition temperature of isooctane is raised when the fuel concentration is increased at shorter isolags, the ignition temperatures of diisobutylene and ethytbenzene are 10wined under identical experimental conditions. Thus, in view of the greater resistance to ignition of tsvoctane its performance at higher engine ~pecds would be expected to be less severely ~lffccted than that of either diisobutylene or ethytbcnzene. Yet another factor to be borne in mind, while cmnpariag hydrocarbons with widely varying critica~ compression ratk~ such as isooctane, diisotmtylene and ethylbenzcne, is that at higher compression ratio levels the rate of reaction of the pre-llarne processes would be higher. Con.~eque~tly, the depreciating influence on the basis ~)f critical compression ratios alone would be in Ihe order: ethylbenzene>diisobutylene >z.~'0octarte. In the light of all the foregoing considerations, the iwrformance of isooctane with increasirtg Sl~'ed would be extx,cted to improve more, or be less adversely affected, than that of either dii.~obutylene or ethylbenzene. ThL~ is in agreemeet with the observed engine performance of these hydrocarbons, The criticai compression r,~tk, for isooctane is irlcreased appreciably as ti~t. :,rlgine speed is raised from 600 to 2 000 rtrv/ruin, whereas, with diisobutylene and ethyibenzent:, it actually falls under simiIar T,qut,nc arid m-xylene, which tmderglo little pre.llnme re-'tctior~and that to~ at a much higher tempt.future level than that in the case of dii.~v~hutylene, isooctar~c or ethylb~.nzcne, indeed haw, higher critical cornprcsskm r a t i ~ than those for the other hydrocarbons. Tlte t:omlillslion characteristics "of t~)ltR,rle and n,-xylenc arc '~¢~rnewhatsimilar to each -ther and so also i.~ the effect of increasing eilf~ine speed, whictl
107
causes a depreciation in the engine performance in both cases. This effect of increasing engine speed on the performance of toluene and m-xylene is more llke that observed with ethyibenzene and diisobutylene than that of isooetane which appreciates in performance at higher speeds. The combustion characteristics of toluene and raxylene, particularly with regard to the .4.abillty of the pre-flame reactions towards self.ignition, however, have more similarities with those of isooctane than with those of di~sobutylcne and ethylbenzene. The depreciation with increasing speed for toluene and m-xylene as against the appreciation observed with isooctane, m(ght well be due to the rather large depreciating factor inw>tved in the former case on aeeovnt of higher rate of reaetiov of pre-flame processes at higher compression ratios. [The critical compression ratios are very much higher for toluene and m-xylene (15'0 and 15-5 respectively) than for ixooctane (7.3).] Correlation with Blending Properties of Fuels The improvement that takes place in engirm performance on adding a high qtlality comr~onent to one of pooL' quality is often quite different from that expected from the known characteristics of the individual components. For example, benzene by itself has a much higher anti-knock quality than dllsobutylene, ~x~t the addition of dlisobutylene to a fuel causes much greater improvemem in its performance than does the addition of the same amount of benzene. The differences in the blending eharacteristk:s of various hydrocarbons which are indicated by their 'blending octane numbers' are tabulated i'l~ Table 3. in ¢~rder t(, detvrmiru' whether the blending o(;I.all(: Ill.lriltlers i;[ different hydn~carbons in thc tql~irl(t are rvth'ctcd in tt~e corrll;)tlSl[t;ll pl',,pertk'.s t.tn(lf'r , , x . , n i . a t l o n . , st't'it'.gof cxp0rim~,nts waq c:,rried ~,lt ~m blends of n-heptane with 2(1 per cvnt tsot,c|arle, benzene arid diisobutyh.ne. For ti~,t'se exper~tr~et;ts the s:.l.I~lt' method and technique were ttsod :is )~ad been ad,~pled f~:~r stndJes on pare hydrocarbons dt';:cribed h'k th~ pr~'vious paper ~. The re,~tflts
Vol. 4
198
are shown in Figure 1, The extent of reaction at different temperatures has been plotted in terms of the concentration of carbon monoxide in the gaseous products. The concentration of carbon dioxide was also measured Moog with that of carbon monoxide but since carbon dioxide was fomled in much smaller m o u n t s Fable 3.
CFR Blending Oct=ne Ratings o.f unleaded hydrocarbons*~
,Hydrocarbon
t{esea~tch method
lllotar method
O 104 tOO 1~8 S9 ~24
0 84 100 151 91 112 124 107
rt.Hept¢,ne MathyIcyclohexane isoOctane Dilsobutytene Bt:rlgCPI~
m-.¥ylenc Ethylben:ene
148 124
• l-lydrtxw:bon 2~ Our cent in a 6LU4r, t.~onctan.'.'/tvhcpmn¢ mixture.
than the latter and furthermore its manner of formation at all the pro-flame stages followed the same pattern as that for the latter, the results are presented only in terms of carbon monoxide produced. For a rough comparison with the results on blends of n-heptane with isooctane, benzene and diisobntylene, the results obtained ~6
~
j
~
,
,
,
=
'l"',
'l
n-HE,pl~l~e{S0'/'.) E]enzen¢.{~0°/=) |
"~
=
|1~
]
n .Heptar~eCr40"/,)+isoOctanel2e'/.)~,~--, L'--- ~ ]
~ , - - - n -NeptanelS0"/o),.Diisobutylene (20%t
c. . . . . . . . . . .
I
L_ __A....... !
' .... . . . . . . . . . . . . . . [.. . . . "--v-~--1; ! ~ ~_~'(7 .......
"~g F
I o 2.LZ
]-!-l-[-l
2an 280
320
360
I
__
..... I.........
-[ !-/r-f
Z00" a~0
Temperature
q
r -I '~0
520 =C
l:igu;',, t. ['~n.fi~t~w r,'r~ ~ions as inl~ue~tced by blemtin~: rliffer~nt hydvocarbotts u:ilh n-h,,ptane. K[fe~,t o] te;wpt!t~utuv~ on carbon ~tonoxide production up to ignitfo, tin;el
with n-heptane alone, using the same amount of n-heptane as present in the mixtures, axe shown by the broken line in Figure I. The results clearly show that the presence of 20 per cent benzene has hardly any effect on the extent of pro-flame reactions or on the ignition temperature of n-heptane. The presence of isooetane tends to cause some inhibition in the pre-flame reaction and a slight lowering in ignition temperature. Di:'sobutylene, • however, markedly reduces the extent 9f overall pro-flame reaction and also lowers the ignition temperature. However, it has already been shown that, unless the differences in ignition temperatures are large, it is the extent of pre-tlame reactions that primarily provides the correlation with the on#no performance of fuels. The results obtained in the present experiments on the extent of pro-flame reaction using mixtures of n-heptane with benzene, isooctane and diisobutylene thus fall in broad agreement with the observed blending octane mnnbers. C. WALCUTr and E. B. RIFKI~N r, who carried out experiments ~n motored engines, also arrived at similar conclusions with regard to the superior blending octane rating of dilsobutylene. They estimated the total heat released prior to autoignition and regarded it as the criterion of the extent of pro-flame reactions. On comparing various blends of n-heptane and diisobutylene with the corresponding blends containing n-heptane and isooctarw, they showed that there was much less heat released in the n-heptane/ diisobutylene bkmds, Correlation with. Influenace of Tetraethyl-lead Addition of rulatlvely small amounts of tetraethyl-lead to poor quality fuel is well known to bring about a phe,~ornenal irnprovemem in its engine performance. In an attempt t,.) investigate whether such in[tuenee of tetraethyllead on engine performance could be interpreted ira terms of any effects on the combustitm characteristics, expc'r[ments were carried out using n-heptane as the h,el. The extent of proflame reactions and the variation of ignition lag with temperature were first determined in a clean reaction chamber in the nmnner described
A~ti-l-'txoek p e r f o r m a n c e of hydroe,'xrbons in gasoline e n g i n e s
~ p t e m b e r 1960
previously=. A smalt amount (about 1 g) of lead oxide was then dropped into the chamber and the experiment repeated. (It has already been shown by G. H. N. Cr~AMnr-RLA~ and A. D. WALSns that the effect produced by the
presence of lead oxide strongly inhibited the pre-flame reactions and it also raised the ignition temperature by over IOOOC. This effect thus corresponds to the observed improvement ia engine performance.
I
c
I f,
o
O~
4
I j1 i~
o
fi oi-i
240
280
• ~
I i
360
320
+l L
........... / T+-L t=i + -,-~--~.,_,..-,-"
o ¢.
'~
199
|
I
:
......
400 4&O Temperat ure
480
520
550
600 °C
Figure 2. Influence o] lead oxide ~m the c,¢u~thu.~li,:m charar'h, ri~ti+:s +.,[ a.heptl:~e, l'[[ect ,'.~f t,'ml,eratur,' +,n ¢',H,,.'n tm'Jnrs.~'idc prt~Jl
addithm <# tetraethyl+lead can be reproduced by carrying out the combusEon exix:riments in &e presence of lead oxide.) The results obtained ia the abs0nee of h,ad oxide and again i[~ its preserit:tr are shown [~1 Figures 2 and 3. The
Furtl:'ter work on the combustion characteristie~ of different fuels, both ha the absence &nd presence of various blending agents and antikm~ck compounds, is in hand.
The at|'thor wishes to Htank the Directors o[ '~ghell" Research, Lld [or permission to publisk this work,
References i ,"~,'~i'rrl.|'t,
+
K.
(',
('OI~t~Iff:.N~H?*Z
,!'~ ]",/e4J)l, ~, i,~'l'~l.I.
=~, 1 1 7
p ",q7~ c)xfimI I.It~ivc'r.~itv Prt,,~.~'. L~n
t*+O FJ,,,~,,
~
500 560 580 600 Ignition temperature l+~tlu+'nt'r'
o/
Ir'+~tl +~'zdc . ~
~"JHP'+t!''~'IPt.'S ¢'J[ fl'heIpl~ltltt.
*~'~[f+'gl iiJ¢
big/temperalua,+.t Pt.'gOltiOi'l'L~,hl/,
the
1320 ~C z::ndmn
¢ h , I -'+:.,Ir~++l.~/
'+ A++l,.r+c:II+ [><:trc+[t'llI'~1 In,~L+eub +, ~¢P.~eri:tI'Cl"~ ~['['('~II:PL;t •I:'~, ['ii'-',hte'n'r+r]t .\t]mt+d I~'+'I~,+I+ ?,IP jar.., ]+PSi.+ e~ E.;~'t ,~,rt Lt.,,,~r, F,. l{., [{I¢'AP.~I:,.,-,, II, A., .rr, :rod Jn,',.t:s, M, {', K, S,AE. ,T r.',',~.'.,' T,r¢';ns. 3950, 4. 4:+~ ; VI,';,,I.CrT'rT, (.'.. ;trill I~tl,'l¢lN, ['~. P,../,*z,dt~,'~lr../","~'~',I~n~ ;t,'.+. (IP:dustr.}, 1951, 43, %844 (.*i|ASItI|..RI.AIN.
(].
}[,
N,
;tIl(1 "~V,%E.NTI. A
N o v 5,~,, ..l. 1~.~'2. 215, 17~
D.
P r , k',
/¢+.'::tar¢ h (.'entP+. i'.¢). ]+'..,.'r *, C k , ' s t , r
IReceiv~d Mart:t~ lgJ'a) 7"/,t ;,+flal'.~aD'opl .Jtt the r,*lnb,+st;,.J, ch,tr*t+'t,"r/,eli,'x ~Jf h qde~'*e+lrt,or, .. I,ra,*,t'o~tM,i r,/J,~r(~',t, ~.l~t,[,+ tt~ 1~ h. tt,'r itJ.~+'!tht i+~to if*+' at~Ii,k~.s+'l." ].'r.forll++s/m,' ~{f h/ldr.e~+rhon.~ it~ f/tier+h.+' ,..9';n,.',+. 7%,' r,'.,'.D.,~ .~£o... ti,,tt the ,+r+'r,Ptl +'a~lemt of ,'..,wti,,,, tal¢it+fl ;Jltt'¢" tt,'ftlr,. Ol¢l/'¢iolt rc.t..t/+.,v +?,.rff ~qej.~;,l,iI t~'+the l¢,+o,';,:,t+'ttPite{~t p,'el>Jr.t,o+,w +*.fll~,," .f~eL 7'h,' t'lf+'et~ "el" th,' .t,ht/tir.+ o f t+'troethptl,h.,td q . d of t+~,t~[roearl~r**~. +'o~t+.t*,*t?C'i~t'~ ,vl~o.,~,' tv~l~O' it* t+'rttla qfl t~f~'tltltr+!l o,'flt*~e *t~lttltt.~r ia *+Or pre~f+',?t.t~ff.' frOrl/ the t't~irit!¢¢l~" ocfrtr+,! ntlttth,'r. ,',t** h,' r,~adil!t i.h,rpr,,#.~! .from tk.,'ir ,ff,'et.~ +m tk,,' ,'.cl.'.l qf prr.fl,z~*,r ~v.art/+'m.,+. F.rtll~:'rmor~. Ill,' [I+'PJ'OI'IIt
,~ ,~l.:irm~r understanding ,>f the relationship :..,~tw,.~.n th,: anti-knock performance of huffs in -:L:.:,.t+rteeugimrs and their combustior, character>~k::., i,; of interest to both engine designers and tt>'l technologists. Numerous altempts have }J,un made to interpret the wide differences in 7h.. an'ti-knoek performance of differenl: hmls in I,rm~ of their combustion characteri,~ties. Titus(' a:u,mpts, however, have had only limited st=r~,.ss. Many investigators, who have smtght ¢!,: interpretation of engine [x.rforrnance in t,.rrras +,f basic combustion characteristics detereililt,d under simple laboratory eon
m:.dnty with arom,'ttic and some paraffin[¢ hydrocari)o)~s that there is ~ broad correlation t),,tween the engine perforrr);mce and fhe minimum ignition temperature, The combustion ¢haracterist{es o~ hydrocarbons, which have been investigated recently and rept,rted i,1 an earlier paper ~, do, however, correlate well with their anti-knock qualifies. These comlmstiou characteristics have been determined in terms of both chemical and phy'sical parameters and incltu.le: (i) the extent ~,f pre+tl.'une reaction, (i,) the minimmn ignition tt-nlperature for a g[ver~ lag, (ii,) the ign[fion h,m perature / tag relationship, and (iv) the effect of fuel/air ratio on (i), (ii} and (iii). Apart from correlating with anti-knock performance, these characteristics help to interpret successfully the inltuence on the anti-knock performs, nee of such factors as rite speed and operat.ia~g temperature of engines. Furthermore, the influence of antiknock agents and +.ff hydrocarbon components, whose value in te.rms of blending octane ntmlbcr [,,, not predictable from their intrinsic octane numbers, can be satisfactorily interpreted from their effects on combustion characteristics, The afore-mentioned combustion character. 193
194
~r~?
K . C, ~ l O O j a
istics, reported in the earlier paper, were measured for the following eight typical C,-C, hydrocarbons : n-heptane, methylcy~lohexane, isooctane, diisobutylene, benzene, toluene, m-xylene and ethylbenzene. In the present paper the relative performance of these hydrocarbons in gazolk~e engines* has been interpreted in terms of the combustion characteristics.
Cecrelation with Anti-knock Performance irt Engines The relative anti-k~ck performance of different fuels, under ~.ny given set of engine operating conditions, can be directly related to the extent of pre-flame reactions. Those hydrocarbons which undergo less reaction in the pre-flame stages perform better in the engine than those which react more extensively. This is so irrespective of file structural category to which the hydrocarbons belong, and is true almost without regard to the minimum ignition temperatures of the hydrocarbons so long as these are not to6 far apart. As an illustration, the ignition temperatures o.f diisobutylene and methylcydohexa,ae are in the same range. However, methylcyclohexane, which undergoes far more overall pro-flame reaction than diisobutylcne, has a much ~ower octane rating and critical compression ratio. isoOctane ignites at an appreciably higher temperature than diisobutylcne, yet dilso. butylene, whida suffers relatively less reaction in the pro-flame stages has a higher 'Research' octane rating. Ethylbenzene and isooctane both ignite in the same temperature range. Ethyihenzene, however, undergoes less proflame reaction than isooetane arid its 'Research' ,octane rating, too, is higher. Again, at the high end of the igaition temperature scale, benzene, toluene and m..xylcne ignite at comparable lcmpcn~tures, but benzene, which suffers the least extent of pre-flame reaction, performs best. The anti-knock quality of different hydrocarbons in terms of their octane numbers by both the 'Research' and 'Motor' methods and the .critical compression ratios is shown in Te,bles I and 2. "Tk~ Informalitln on the anti.knock ocrfr~rrna~e¢ of diffvrent hY~lrccat'bons In gasoline cngtlles tlas b~en taken frnm the ~ P ! Projet,'l ,t5 d~ll(~lt.
4
That it is the extent of pre-flame teaetion rather than the minimmn ignition temperature that shoald provide the more exact correlation Tabl~ 1. CFR Octane Ratingz* of unleaded hydrocarbons r~ . Ilydr~carbon n-l/epta'n e Methylcyclolwxane, isoOclan,: Dii~obttt yh, m~ 13~,~n,Tcne Toluene m-Xytene Ethylbenzene
Rcseamh method 'F~'
Motor me~hod 'F,_)"
0
0 71'1 100,0 88.6
74.8 IU0.0
105.3 ~'~
--t
] 20" ! 117.8 107"4
[
:[
11)8'5
118,0
97"9
*Octane ratings above 100 a r e according in the A S T M (Wiese) ,',::ale, STh.¢ value h w b e g o n e i,~ t o o hhth to be toted sadsfactc3dht in
~he CFR engine.
with the engine performance is understandable since the ignition temperatures, in the manner in which they are usually determined in open vessels, cannot possibly reflect the tendencies of fuels to anto-lgnite in internal combustion engines where the fuel-air charge is subjet:ted to increasing pressure an<] temperature in the compression stroke before the passage of the spark and again under more severe conditlons in the 'end-gas region' ahead of the approaching flame front. A fuel cap,xlfle of undergoing greater pro-flame reaction woifld produce more "/ lddr "J, (,',~'¢tl( 'd (.'¢')lll/pl'¢'~,~if)~l ro/Id:,5 ell I(t,d, twdi,d h.Td)'ocarb.ns Zu GenerM Mot,)v.~ singb; cytind~.'r l..'Hl'l(ll~ft'-t:O?II/)¥~'SSlO,ig, t,'ll~'itt~5
lly,rlrcJc uric, ,n
6fro J'c?~ i' ~tltll
2 It00 v,e~J/,qlill
J,,,J,,.:.': :s:'l: .~.~":'F .~le'oFI s,~,°i ' Air : tllOr'F 150"F 10(f/; I 15W'[ nJlept.m~ ,lh"thylcyclo. Jt.e~a~t~ i.~uOC&irte
])iJsobu(ylt:m
:¢,i) 4,,55 7.3
I __1 -,tJ5
-.-
,'I,8
3,o
4"0 6'6
7'3~
5"9 ] 4'9 9.0 7-1 5,7
xt.:~s it,s
]3.s ] 9.0 IZ,:) I ~.s
8.2
0.3 I 6,o
~'S5_.
1~tl :~?,7,I"7L$"
"['~.dumte m-Xylene Ethylb,.,n~ene
Is
Is.`5 ]3,s
~ptem~r 1960
Anti-kngek performance of hydroear]~ons in gasoline engines
heat and pressure rise and would consequently carry the charge to the poi.at of ignition more readily than one ".;'l:ieh undergoes less reaction. Thus, irrespective of minor differences in ignition temperatures, fuels with widely different extents of pre-flame reactions would differ markedly in their tendencies to auto-ignite and knock in engines. It is not surprising therefore that, for all types of hydrocarbons, the extent of preflame reactions provides a satisfactory correIation with engine performance. ..%ch a correlation between engine performance and extent of pre-Ilame reaction has also been indicated in engine experiments by E. R. RZr,ULLlaO, H. A. Rmam~s, Jr and M. C. K. ,lesF.s'L These authors took the average engine tcmperatnre before ignition as the criterion of pn,dlalue reaction and showed that, under both firing and cycling conditions, isonctane under~:,.,,:,smort, reaction than benzene, anti a blend ~d ~,~,',.ctane containing 25 per cent n-heptane marc re;..:tlon than isooctane itself.
Correlation wkh Temperature Sensitivity of Fuels in Engines It is well known that an increase in the running Itmlpcratun, of an engine lowers its knocklinlhvd perforrnance (Table 2). The degree to whi,F.[i IlK. knock-limited compression ratio must be rcd.t:ed on increasing tl,e jacket and air tt~mpvr:,mre, however, varies with the nature of the ha,I, Comparhlg. fc,r {nstmlee, isooetane, cthylh,.,uzt,rxe and diisobutylene, the reduetkm el ¢:rhi,::l.I omlpressitm ratio is less with iso.etun. fi,;m with the otht, r two, hydroearb-ns .f.r , ;:i~,,n i,mrvase in ternperature. The studies of tll,.qr cc,i flms/ion char-qcteristies havt~ strongly indicated llt;,l the pre-flame reactions of. isotlcl.~4, art: I.'tr re.re resistant to self-ignition than %,',e of elhvlbenze:!c, or diisobutvlene. !'l'l,i. i< iu¢.lged fr¢~m the s]{q)e of lhe ig';tithm ~rJq,cI,;I.Illtr VCI'SIIS lag curves and from the efft,ct. ,d fuel eoncenlrat[on on ignition tempera. tim, ur~d,,r different isolag eomlithms.) For inst *l,:t:, for lhe same rise in temperature, the ign!li,m tags of ethylbenzene and dlisohatykme are sllortened m u c h more than with isooctane. In ,ther words, the same rise in temperature renders ethylbcnzene and diisobulylene much
more r~adity ignitable than isooctane. These observations are in line with the observed tet,~peratur¢ ~ensitivities of these hydrocarbons in the engine. Comparing toluene and m-xylene, the studies of combustion characteristics have shown that the pre-flame reactions of m-xylene are mort,' resistant to self-ignition than those of toluene and this again is in line with the temperature sensitivities iv. the engine of these two hydrocarbons. The same increase in temperature reduces the critical compression ratio o£ mxylene to a level which is still higher than that of t.luene. This lower temperature sensitivity of ,n-xylene is further retteeted by the relative differences in the 'Rc's~'arch' and 'Motor' octane ratings. The nct'me rating o~ m-xylene drops from 117..5 to only 118'0, whereas for toluene the drop is from 120"i tO I03'8.
Correlation with Effects of Engine Speed Variation on Perfgrmance ~inc'c cragirws ha.vc to operate over a wide range of speed, much attention h~,s been given to the relative sensitivities of different fuels to changes i~: engine speed. As a result considerable data have been accumulated which show very rnarkcd differences between different fuels in this respect. As the engine speed increases, ~l-p',~ral~-ins,most of the cyclopara~ns and some isop:traffins and oMins improve in anti-knock lwrf(¢mam'e, whereas aromatics and n,,st ¢,~ the hi
,,rwv-; between in,"ividuat hydrocarhons. .'\mrmgst tlw h}dro,~-arbons under disens:ailm, :,s shown iu "l'ah/c 2. n-hephme and methy[c.v¢lo]lexane appreci;tte i*l {.heir perforrnance whorl the engine speed is increased from 600 to 2 t'R}I') r o v / n l h l , f,';o-()etal~e a]sc~ improves irl perforln;tnce at the higher ~peet:t, whereas diisobltl .,.lt,ne altd ethyllae/lzel-le depreciate markedly. Widt both toluene am:l m-xy[ene, the engine perfiwnmnce lalls c,aa:;d,.rably at the higher speed. To understand the reasons for the widely
196
K . C . Salooja
different effects tLat an increase in engine speed produces on the performance of different fuels, one must follow the manner in which the increase in engine speed a~feets the combustion processes o:~ an individual hydrocarbon. An increase in engine speed, when all other operating conditions are kept the same, could be considered to affect the combustion processes in the engine in the following ways: (I) The temperatur~t~s at which combustion takes place in the engine tend to be higher at faster speeds even when the jacket temperature and the other operating conditions are kept the same. This is due to the greater number of heat cycles per unit time which result in a higher combustion chamber surface temperature. The increase in the running temperature of the engine would tend to lower the performance of various fuels in accordance with their temperaturn sensitivies, (2) At higher speeds the duration of comOustion cycIes woutd decrease. This would have the following effects: (~1 The extent of pie-flame reactions would decrease owing to shorter times being available for these reactions to take place. The reduction in the extent of pre-flame reactions q iould lead to an improvement in the engine performance. fit) The stability characteristics of the pro-flame reactions with respect to self-ignition would be affected: those hydrocarbons which have a relatively stable pre-flame reaction stage and offer greater resistance to ignition at shorter reaction times would tend to improve in their engine perfnrm:,n,:~ at higher speeds. Any improvement on this account, as also on account of (i) above, wau~d I)e offset somewhat bv the increase in the runni,lv, tern'. perature of the engine, With n-heptane and methylcyclohexane, which undergo extensive pre-flame reactions, the effect of a large increase in engine speed would
Vol. 4
primarily be that of a marked reduction in the extent of the pre-flame reactions. This, by itself, should lead to a corresponding improvement in the engine performance, There would, however, be some depreciation in the perform. ante due to an increase in the effective engine temperature at the higher speed, The net effect could be that of an improvement in the per. formance of the fuel, such as is observed, In comparison with n-heptane and methylcyclohexane, isooctane, diisobutylene and ethyl. benzene undergo little pre-flame reaction and ~t~ a result have much higher critical compression ratios. In accordance wish the relative extents of pre-flame reaction, at an engine speed of 600 rev/min and jacket and air temperatures of 212 ° and 100"F, ethyibenzene has the highest critical compression ratio value (13'5), followed by diisobutylene (11.95) and isooctane (7'3). With little pre-flame reaction taking place in these hydrocarbons, the effect of increasing engine speed on the extent of pre-flame reactions would be much less pronounced than that with n-heptane or methylcyclohexane. However, what little improvement ,could occur on this account at higher speeds would be greatest with isooctane which undergoes more pre-flame reaction than either diisobutylcne or ethyl. benzene. At higher engine speeds, the depreciating influence due to a rise in the effective engine temperature would be mot,. marked with ethylbenzene and diisobutylene than with isooctane. The studies on the combustion characteristics of these hydrocarbons, as mentioned in the previous section, have dearly shown that the inherent temperature sensitivity of iso~mtane i~ very rnuch less than that ~,f either ethylbcnzenc or diisolmtylcne. Consideration of the results on the stability ch~.raeleristics c~f the pre-flame reactions towards self-ignition of these hydrocarbons particulart.~ with respect to reaction time, which is reduced by aa increase [rl engine speed, indicates thai at slmrter residence times isooctane ha~s markedly greater stability towards ignition tha~ has tither diisobutylene or ethylbenzene, For instance, as shown in the previous paper', for the reduction of the ignition lag even trom four TM
September 1980
Aztti-kngek performance,o~ hyd~oegrbons in gasollr~e engines
to two seconds, isooctane has to be raised in temperatme by about 28* at ;~bout 520°C, whereas, for a similar reduction in ignition lag ef ethylbenzene, the temperature needs to be raised by only 13 ° at 525"C and with diisobutylene by only 11 ° and that moreover at a much lower temperature level, about 490°C. Furthermore, the previous studies have shown that whereas the ignition temperature of isooctane is raised when the fuel concentration is increased at shorter isolags, the ignition temperatures of diisobutylene and ethytbenzene are 10wined under identical experimental conditions. Thus, in view of the greater resistance to ignition of tsvoctane its performance at higher engine ~pecds would be expected to be less severely ~lffccted than that of either diisobutylene or ethytbcnzene. Yet another factor to be borne in mind, while cmnpariag hydrocarbons with widely varying critica~ compression ratk~ such as isooctane, diisotmtylene and ethylbenzcne, is that at higher compression ratio levels the rate of reaction of the pre-llarne processes would be higher. Con.~eque~tly, the depreciating influence on the basis ~)f critical compression ratios alone would be in Ihe order: ethylbenzene>diisobutylene >z.~'0octarte. In the light of all the foregoing considerations, the iwrformance of isooctane with increasirtg Sl~'ed would be extx,cted to improve more, or be less adversely affected, than that of either dii.~obutylene or ethylbenzene. ThL~ is in agreemeet with the observed engine performance of these hydrocarbons, The criticai compression r,~tk, for isooctane is irlcreased appreciably as ti~t. :,rlgine speed is raised from 600 to 2 000 rtrv/ruin, whereas, with diisobutylene and ethyibenzent:, it actually falls under simiIar T,qut,nc arid m-xylene, which tmderglo little pre.llnme re-'tctior~and that to~ at a much higher tempt.future level than that in the case of dii.~v~hutylene, isooctar~c or ethylb~.nzcne, indeed haw, higher critical cornprcsskm r a t i ~ than those for the other hydrocarbons. Tlte t:omlillslion characteristics "of t~)ltR,rle and n,-xylenc arc '~¢~rnewhatsimilar to each -ther and so also i.~ the effect of increasing eilf~ine speed, whictl
107
causes a depreciation in the engine performance in both cases. This effect of increasing engine speed on the performance of toluene and m-xylene is more llke that observed with ethyibenzene and diisobutylene than that of isooetane which appreciates in performance at higher speeds. The combustion characteristics of toluene and raxylene, particularly with regard to the .4.abillty of the pre-flame reactions towards self.ignition, however, have more similarities with those of isooctane than with those of di~sobutylcne and ethylbenzene. The depreciation with increasing speed for toluene and m-xylene as against the appreciation observed with isooctane, m(ght well be due to the rather large depreciating factor inw>tved in the former case on aeeovnt of higher rate of reaetiov of pre-flame processes at higher compression ratios. [The critical compression ratios are very much higher for toluene and m-xylene (15'0 and 15-5 respectively) than for ixooctane (7.3).] Correlation with Blending Properties of Fuels The improvement that takes place in engirm performance on adding a high qtlality comr~onent to one of pooL' quality is often quite different from that expected from the known characteristics of the individual components. For example, benzene by itself has a much higher anti-knock quality than dllsobutylene, ~x~t the addition of dlisobutylene to a fuel causes much greater improvemem in its performance than does the addition of the same amount of benzene. The differences in the blending eharacteristk:s of various hydrocarbons which are indicated by their 'blending octane numbers' are tabulated i'l~ Table 3. in ¢~rder t(, detvrmiru' whether the blending o(;I.all(: Ill.lriltlers i;[ different hydn~carbons in thc tql~irl(t are rvth'ctcd in tt~e corrll;)tlSl[t;ll pl',,pertk'.s t.tn(lf'r , , x . , n i . a t l o n . , st't'it'.gof cxp0rim~,nts waq c:,rried ~,lt ~m blends of n-heptane with 2(1 per cvnt tsot,c|arle, benzene arid diisobutyh.ne. For ti~,t'se exper~tr~et;ts the s:.l.I~lt' method and technique were ttsod :is )~ad been ad,~pled f~:~r stndJes on pare hydrocarbons dt';:cribed h'k th~ pr~'vious paper ~. The re,~tflts
Vol. 4
198
are shown in Figure 1, The extent of reaction at different temperatures has been plotted in terms of the concentration of carbon monoxide in the gaseous products. The concentration of carbon dioxide was also measured Moog with that of carbon monoxide but since carbon dioxide was fomled in much smaller m o u n t s Fable 3.
CFR Blending Oct=ne Ratings o.f unleaded hydrocarbons*~
,Hydrocarbon
t{esea~tch method
lllotar method
O 104 tOO 1~8 S9 ~24
0 84 100 151 91 112 124 107
rt.Hept¢,ne MathyIcyclohexane isoOctane Dilsobutytene Bt:rlgCPI~
m-.¥ylenc Ethylben:ene
148 124
• l-lydrtxw:bon 2~ Our cent in a 6LU4r, t.~onctan.'.'/tvhcpmn¢ mixture.
than the latter and furthermore its manner of formation at all the pro-flame stages followed the same pattern as that for the latter, the results are presented only in terms of carbon monoxide produced. For a rough comparison with the results on blends of n-heptane with isooctane, benzene and diisobntylene, the results obtained ~6
~
j
~
,
,
,
=
'l"',
'l
n-HE,pl~l~e{S0'/'.) E]enzen¢.{~0°/=) |
"~
=
|1~
]
n .Heptar~eCr40"/,)+isoOctanel2e'/.)~,~--, L'--- ~ ]
~ , - - - n -NeptanelS0"/o),.Diisobutylene (20%t
c. . . . . . . . . . .
I
L_ __A....... !
' .... . . . . . . . . . . . . . . [.. . . . "--v-~--1; ! ~ ~_~'(7 .......
"~g F
I o 2.LZ
]-!-l-[-l
2an 280
320
360
I
__
..... I.........
-[ !-/r-f
Z00" a~0
Temperature
q
r -I '~0
520 =C
l:igu;',, t. ['~n.fi~t~w r,'r~ ~ions as inl~ue~tced by blemtin~: rliffer~nt hydvocarbotts u:ilh n-h,,ptane. K[fe~,t o] te;wpt!t~utuv~ on carbon ~tonoxide production up to ignitfo, tin;el
with n-heptane alone, using the same amount of n-heptane as present in the mixtures, axe shown by the broken line in Figure I. The results clearly show that the presence of 20 per cent benzene has hardly any effect on the extent of pro-flame reactions or on the ignition temperature of n-heptane. The presence of isooetane tends to cause some inhibition in the pre-flame reaction and a slight lowering in ignition temperature. Di:'sobutylene, • however, markedly reduces the extent 9f overall pro-flame reaction and also lowers the ignition temperature. However, it has already been shown that, unless the differences in ignition temperatures are large, it is the extent of pre-tlame reactions that primarily provides the correlation with the on#no performance of fuels. The results obtained in the present experiments on the extent of pro-flame reaction using mixtures of n-heptane with benzene, isooctane and diisobutylene thus fall in broad agreement with the observed blending octane mnnbers. C. WALCUTr and E. B. RIFKI~N r, who carried out experiments ~n motored engines, also arrived at similar conclusions with regard to the superior blending octane rating of dilsobutylene. They estimated the total heat released prior to autoignition and regarded it as the criterion of the extent of pro-flame reactions. On comparing various blends of n-heptane and diisobutylene with the corresponding blends containing n-heptane and isooctarw, they showed that there was much less heat released in the n-heptane/ diisobutylene bkmds, Correlation with. Influenace of Tetraethyl-lead Addition of rulatlvely small amounts of tetraethyl-lead to poor quality fuel is well known to bring about a phe,~ornenal irnprovemem in its engine performance. In an attempt t,.) investigate whether such in[tuenee of tetraethyllead on engine performance could be interpreted ira terms of any effects on the combustitm characteristics, expc'r[ments were carried out using n-heptane as the h,el. The extent of proflame reactions and the variation of ignition lag with temperature were first determined in a clean reaction chamber in the nmnner described
A~ti-l-'txoek p e r f o r m a n c e of hydroe,'xrbons in gasoline e n g i n e s
~ p t e m b e r 1960
previously=. A smalt amount (about 1 g) of lead oxide was then dropped into the chamber and the experiment repeated. (It has already been shown by G. H. N. Cr~AMnr-RLA~ and A. D. WALSns that the effect produced by the
presence of lead oxide strongly inhibited the pre-flame reactions and it also raised the ignition temperature by over IOOOC. This effect thus corresponds to the observed improvement ia engine performance.
I
c
I f,
o
O~
4
I j1 i~
o
fi oi-i
240
280
• ~
I i
360
320
+l L
........... / T+-L t=i + -,-~--~.,_,..-,-"
o ¢.
'~
199
|
I
:
......
400 4&O Temperat ure
480
520
550
600 °C
Figure 2. Influence o] lead oxide ~m the c,¢u~thu.~li,:m charar'h, ri~ti+:s +.,[ a.heptl:~e, l'[[ect ,'.~f t,'ml,eratur,' +,n ¢',H,,.'n tm'Jnrs.~'idc prt~Jl
addithm <# tetraethyl+lead can be reproduced by carrying out the combusEon exix:riments in &e presence of lead oxide.) The results obtained ia the abs0nee of h,ad oxide and again i[~ its preserit:tr are shown [~1 Figures 2 and 3. The
Furtl:'ter work on the combustion characteristie~ of different fuels, both ha the absence &nd presence of various blending agents and antikm~ck compounds, is in hand.
The at|'thor wishes to Htank the Directors o[ '~ghell" Research, Lld [or permission to publisk this work,
References i ,"~,'~i'rrl.|'t,
+
K.
(',
('OI~t~Iff:.N~H?*Z
,!'~ ]",/e4J)l, ~, i,~'l'~l.I.
=~, 1 1 7
p ",q7~ c)xfimI I.It~ivc'r.~itv Prt,,~.~'. L~n
t*+O FJ,,,~,,
~
500 560 580 600 Ignition temperature l+~tlu+'nt'r'
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