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INDUSTRIAL AND ENGINEERING CHEMISTRY
efficiency of tetraethyllead is lost at all tetraethyllead concentrations. (3) For base fuels with low sulfur concentrations, the loss in antiknock efficiency is approximately the same as it is in primary reference fuels. (4) For thiophenes, the ratio of inactive t o active lead is 7.4 X (weight % sulfur) ; for other compounds, the relation is more complex. (5) The effect of different sulfur compounds is additive, so that the loss in efficiency can be predicted if the amount of sulfur in each type of chemical combination (thiol, sulfide, etc.) is known. (6) Sulfur compounds have the same effect on tetraethyllead that has been compounded as Motor Mix as they do on uncompounded tetraethyllead. (7) The loss in tetraethyllead efficiency due to sulfur compounds is the same when judged by research octane numbers as when judged by motor octane numbers. These generalizations are not expected to be accurate for all fuels under all conditions. They are based on measurements made with fuels containing 0.1 to 6.0 ml. of tetraethyllead per
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gallon with 0.01 to 2% sulfur added as a thiol, sulfide, disulfide, or thiophene t o a fuel containing not more than 70% naphthenio or aromatic hydrocarbon and having a n octane number in the range from 40 to 95.
Literature Cited (1) Birch and Stansfield, IND. ENG.CHEM.,28, 668 (1936). (2) Brooks and Cleaton, S.A.E. Journal, 56, No. 6, 53 (1948). (3) Endo, J. Fuel Sac. Japan, 13,292 (1934). (4) Hanson and Coles, J . Inst. Petroleum, 33, 589 (1947). (5) Kobayashi and Kajimoto, J. SOC.Chem. Ind. Japan, 39, 354 (1936). ( 6 ) Livingston, Oil Gas J., 46, No. 45, 81 (1948). (7) Ibid., 47, No. 38, 67 (1949). (8) Ryan, IND. ENG.CHEM.,34, 824 (1942). (9) Schulze and Buell, Natl. Petroleum News, 27,No.41, 25 (1935). (10) Ibid., 29, No. 23, 54 (1937). (11) Widmaier, Jahrbuch deutschen Ltlftfahrtforschg., 1941,p. 441.
RECEIVED September 24, 1948.
Combustion of ydrocarbons Free Radical Chain Reactions Paul L. Cramer and John M. Campbell GENERAL MOTORS CORPORATION, DETROIT, MICH.
A
correlation of the knocking characteristics of liquid hydrocarbons in the Otto engine with previous published data on the initial reactions of ethyl radicals with representative paraffinic, olefinic, aromatic, and hydroaromatic hydrocarbons is presented and discussed from~the standpoint of a free radical mechanism, based on the relative amounts and reactivities of the hydrogen atoms contained in the hydrocarbon molecule. In general, the relative susceptibilities of paraffins to knock, based on molecular structure, are directly proportional to the number of secondary and tertiary hydrogen atoms in the
molecules. A similar relationship may be applied to olefins but because of the additional factors introduced by the double bond, the agreement is poorer than that obtained with the paraffins. The comparative high resistance of aromatic hydrocarbobs to oxidation and knocking combustion is in accord with the highly inert character of aromatic hydrogen to reactions with both methyl and ethyl radicals. Some new experimental engine data are presented on the comparative effects of tetraethyllead, iron carbonyl, and diethylmercury on the critical compression ratios of certain hydrocarbons.
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tion process. More recently Walsh (11) has discussed the possible point of attack in the hydrocarbon molecule during the oxidative chain process from the standpoint of the relative reactivities of the primary, secondary, and tertiary hydrogen a t o m contained in the hydrocarbon molecule. A correlation of the knocking characteristics with the molecular structure of pure hydrocarbons from the standpoint of the relative reactivities of primary, secondary, tertiary, and aromatic hydrogen, as activated by certain groups and bonds contained in the molecule, has been presented by Boord ( 2 ) .
ERTAIN similarities between the thermal and oxidation characteristics of hydrocarbons, based on molecular structure, have been recognized for some time. There is now a large amount of experimental evidence favoring a free radical chain reaction mechanism for both the pyrolysis and oxidation of hydrocarbons. Published data on the combustion of hydrocarbons, obtained by experiments both inside and outside of the Otto cycle engine, have been compiled and critically discussed by Jost in a recent publication (4). Data obtained from experiments on the radical-induced decomposition of hydrocarbons have provided substantial evidence (9) in favor of the proposed initial reactions of free radicals with hydrocarbons, reactions 1 and 2.
RR-
+ R'H --+- RH + R'-
+ R'CH:CH,
--+- R'CHRCHz-
(1) (2)
It is, therefore, logical t o assume that similar initial reactions occur in the free radical chain mechanism proposed for the oxidation of hydrocarbons. If the above assumptions are correct, then reactions 1 and 2 should play a n important role in determining the relative knocking characteristics of hydrocarbons from the standpoint of molecular structure. To explain the high antiknock behavior of isoparaffins as compared with their normal isomers, Lewis and von Elbe (6) have suggested the assumption t h a t oxidation leads t o ketonic rather than aldehydic structures as intermediate products in the chain reac-
Combustion Characteristics of Hydrocarbons in the Otto Engine For the purpose of the present discussion, certain data on the combustion of hydrocarbons in the engine will be summarized. Knocking combustion has been shown to be associated with preflame reactions in the unburned charge ( 1 2 ) . The presence of cold flames has been successfully demonstrated in engine combustion (5). This may be of value in estimating t h e approximate temperature range-670' C. for benzene and 200' t o 260 C. for normal paraffins and naphthenes as determined outside the engine @)-for possible reactions in the unburned charge during knocking combustion, Knocking combustion may be induced by small amounts of additives, such as certain peroxides, organic nitrates, and nitrites, as well as suppressed by equally small amounts of antiknock compounds (4).
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duced reactions were obtained with all the paraffinic, olefinic, and hydroaromatic hvdrocarbons at reaction temperatures of 200 to 250 * C. As indiW 0 *OECAHYORONAPHTHALENE a cated by the similar ethane values for benzene, W a naphthalene, and 2,2,3,3-tetramethylbutane, it ia 70 - 0 2,3-DIMETHYLBUTENE-2 W reasonable to assume that the latter compound, a 2 4 I hydrocarbon containing only primary hydrogen, CYCLOHE XENE 0 60 is inert to ethyl radicals a t these temperatures. I0 A s shown in Table I, the extent of the reactions of -TETRAMETHYLBUTANE v) -I ethyl radicals with both paraffinic and hydroaro< 50 matic hydrocarbons is roughly proportional to the 4 percentage conversion of ethyl radicals to ethane, a -I reaction 1. There is a high percentage conversion C2 H,- + R C H C Ha* R F H - C M i 240 of all olefins t o higher boiling products. Both recan5 W Iactions 1 and 2 take place with olefins as indicated Y 0 by the relative amounts of ethyl radicals converted g30 to ethane. 0 PARAFFINS v) a Since reaction I involves only the hydrogen in W z > the hydrocarbon molecule, the relative amount? 20 and the reactivities of the primary, secondary, and 3 4 - DIMETHYLBUTENE-I I I I I I I I tertiary hydrogen atoms in the molecule would 0 2 4 6 8 IO 12 I4 16 be expected to determine largely the influence of NUMBER OF SECONDARY AN0 TERTIARY molecular structure on this reaction with the parafHYDROGEN ATOMS IN MOLECULE fins. Likenise, the relative amounts of reactions 1 Figure 1 and 2 with the olefins, as influenced b y the molecular structure,would involve the amount and kind of liyA large amount of experimental evidence, obtained both inside drogen atoms in the molecule, as well as the reactivity of the double bond. This relation is shown in Figure 1, in which the percentage and outside the engine, supports the assumption that the relative antiknock characteristics of hydrocarbons are associated with their relative resistance to preflame oxidation, and that this in turn is determined largely by differences in their molecular structures (4). In the case of the paraffins, either the branching or a decrease in the length of the normal chain increases resistance to knocking combustion. Liquid olefins are more resistant t o oxidation and knocking combustion than the corresponding paraffins. Aromatic hydrocarbons exhibit high resistance to both oxidation and knocking combustion. The above correlation, of course, holds only for well defined limits of engine operation.
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Induced Decomposition of Liquid Hydrocarbons by Ethyl Radicals D a t a on the induced liquid-phase decomposition of certain liquid paraffinic, olefinic, aromatic, and hydroaromatic hydrocarbons by the thermal decomposition products of tetraethyllead provide relative information on both the type and extent of the reactions of ethyl radicals, reactions 1 and 2, with various classes of hydrocarbons (5). Also this is true for hydrocarbons in any particular class which have reasonably wide differences in molecular structure. Data on the percentage of the introduced ethyl radicals converted to ethane together with the corresponding percentage conversion of the hydrocarbons to higher boiling products are presented in Table I. No measurable amount of reaction of ethyl radicals occurs with either benzene or naphthalene a t temperatures below 300" C. As indicated by both the amounts of ethane and the quantities of high boiling liquid products formed in these reactions, the ethyl radical reactions in such inert hydrocarbons may best be explained by reactions 3,4,and 5 .
+ 4GH6+ CzH4 + CpHs
Pb(CZH6)P +P b
2C2H5-
C2H4 +liquid products
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Table I. Ethyl Radical Reactions with Hydrocarbons Hydrocarbon 2,2,3,3-Tetramethylbutane 2,2,3-Trimethylbutane 2,2,4-Trimethylpentane n-Heptane +Decane
Conversion of CzHs- t o CzHe, % 42 48
7c 2
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18 23
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35
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46
Benzene NaphtIislene Cyclohexane Cyolohexene Decahydronaphtlislene Tetrabydronapht halene
Conversion of Hydrocnt bans,
46
44 47
(4)
The ethylene formed in reaction 4 is converted to liquid products as initiated by ethyl radicals, reaction 2. With the possible exception of 2,2,3,3-tetramethylbutane,in-
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Figure 2
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(5)
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NUMBER O F SECONDARY AND TERTIARY HYDROGW ATOMS IN M O L E C U L E
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free radicals, reaction 1, is in general agreement with the relative thermal stabilities, as calculated from free energy data, of the isomeric butanes, hexanes, and heptanes (8). Negative results were obtained in attempts to determine the possible inducing effect of nascent metals and active carbQnyl by the decomposition of nickel and iron carbonyls in hydrocarbons under comparable conditions of temperature and pressure (8).
Correlation of Knocking Characteristics of Hydrocarbons with Relative Susceptibilities to Reactions with Free Radicals
If it is assumed t h a t the temperature range of the unburned charge during knocking combustion in the engine is Comparable or belon t h a t observed for cold flames outside the engine, then l l 16 a correlation of the relative knocking characteristics of hydrocarbons with their relative susceptibilities to induced reactions by free radicals appears t o be justified. The correlations of the knocking characteristics of paraffins and olefinsaniline equivalents (7)-with the number of secondary and tertiary hydrogen atoms in the molecule are shown in Figures 2 and 3.
conversion of ethyl radicals to ethane is plotted against the number of secondarv and tertiarv hvdroeen atoms in the molecule. The dotted line at 43.5% conversion of ethyl radicals to ethane represents the inert hydrocarbons benzene and naphthalene. Reaction 1 may be con15 sidered a s the predominating reaction with the hydrocarbons in the area above the dotted line, 14 IRON CARBONYL whereas reaction 2 is the predominating reaction / 01)) YOU) HBCTz0.03WOLS F E ( C O ) ( with the hydrocarbons in the lower area. 13 As indicated by the percentage conversion of ethyl radicals to ethane, Figure 1, the extent of 12 reaction 1 with the paraffins is proportional to the number of secondary and tertiary hydrogen atoms 4- 0.03 MOLS fE(CO), \ in the molecule. There appears t o be a definite II correlation of the relative amounts of reactions 1 and 2 with the number of secondary and tertiary E! I O ETHYL N I T R A T E 0.12 MOLS + 0 0 3 MOL? hydrogen atoms in the olefin molecule. Those % i +PnmE a 9 olefins containing a high percentage of primary & hydrogen, 2,3-dimethyl-2-butene being a n excepI tion, were highly resistant to reaction 1 but susceptible to reaction 2. Reaction 1 is the predomi4 nating reaction with the straight chain olefins, a 1-hexene and 1-heptene. As would be expected, all ,CYCLOH E X A N E of the hydroaromatic hydrocarbons are highly k a susceptible to reaction 1. Cyclohexane, in which 0 6 all of the hydrogen atoms may be considered as secondary hydrogen, falls in line with the open CY CLO HE X E NE 5 MERCURY DIETHYL chain paraffins. +LAME The high resistance of both primary and aroEN€ 4 matic hydrogen to attack by free radicals is in general agreement with later work by Taylor and 3 Smith on the gaseous phase reaction of methyl radicals with certain hydrocarbons at somewhat 2 lower reaction temperatures (IO). As calculated from these data, the same authors obtained a heat I of activation for isobutane lower than t h a t for 0.01 0.03 0.05 0.10 0.20 n-butane. This result is not in agreement with GRAM MOLS/GAL. later and probably more reliable experimental L E A D PER GAL. aocc6.0ce L E A D ( 2 . 0 6 c c IRON) PER GAL. equilibria data on the isomerization of the butanes (8). These data show that at temperatures below Figure 4. Comparison of Tetraethyllead, Iron Carbonyl, and Diethyl300 e C.isobutane is more stable than n-butane. mercury T h e correlation of the molecular structure of General Motors single-cylinder variable compression engine, 600 r.p.m.; mixture ratio and paraffins with their susceptibility t o attack by epark timing for maximum power; jacket temperature, 2 1 2 O F. "
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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amounts of branching reaction resulting from these two reactions with olefins. The results obtained with 2,3dimethyl-2-butene are contrary t o this relation. A comparison of similar data for n-heptane and cyclohexane show that reaction 1 occurred with cyclohexane t o a greater extent than with n-heptane, but the amount of liquid reaction products for n-heptane was twice as large as that obtained with cyclohexane. These results support the assumption that the reaction chains are relatively short in cyclic paraffins.
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I n general the knocking characteristics of the paraffin hydrocarbons are directly proportional to the number of secondary and tertiary hydrogen atoms in the molecules. 3,3,4,4-Tetramethylhexane and neopentane show t'he greatest deviations from this relation. A similar relation may be applied to olefins, but because of the additional factors introduced by the double bond, the agreement is poorer than that obtained with the paraffins. This relation shows apparent but irregular differences in the knocking characteristics of alpha and beta olefins. The comparative high resistance of aromatic hydrocarbons to oxidation and knocking combustion is in accord with the highly inert character of aromatic hydrogen t o reactions with both methyl and ethyl radicals. Since both reactions 1 and 2 result in the formation of large? radicals, both reactions have been considered as the initial steps in the propagation of chains in the purely thermal reactions of olefins. If both reactions 1 and 2 occur in the oxidative chain process, then, as based on the data obtained on t,he free radical induced reactions of hydrocarbons, it appears necessary to assume that reaction 1 leads to a stronger chain branching process than reaction 2. T o explain the relative high resistance of liquid olefins to knocliing combustion as compared with the corresponding paraffins, it has been postulated that the oxidation of olefins results in many Ehort reaction chains, whereas the oxidation of paraffins gives fewer but long or highly branched rea,ction chains (4, 6 ) . The same assumption may be employed in explaining the fact that cyclic paraffins are more resistant to knocking combustion than acyclic paraffins having the same number of carbon atoms in the molecule. The extraction of hydrogen from the ring carbon structure, reaction 1, would be expected to result in short or unbranched reaction chains. It is difficult t o correlate the data from the free radical induced reactions with the knocking characteristics of hydrocarbons from the above viewpoint. The reaction chain lengths for all liquid phase reactions would be expected t,o be comparatively short. Also, since both reactions 1 and 2 occur extensively with olefins, there is no way of estimating the length of the reaction chains for comparison with those resulting from r e a h o n 1 with paraffins. A comparison of the relative amounts of reactions 1 and 2 with the amounts of liquid reaction products formed with the isomeric branched and straight chain olefins (Table I) seems to support the above assumption on t8herelative
Effects of Tetraethyllead, Iron Carbonyl, and Diethylmercury on Critical Compression Ratios of Hydrocarbons I t was thought that a comparison of the relative effects of tetraethyllead, iron carbonyl, and diethylmercury might possibly furnish some information on the reduced antiknock effectiveness of tetraet>hyllead in certain hydrocarbons as adversely effected by the knock-inducing action of ethyl radicals derived from the thermal decomposition of the tetraethyllead. Since mercury alone has been shown t o have a neutral effect' on engine combustion, the effect of ethyl radicals alone could be determined by thc use of diethylmercury. The antiknock effect of iron should be independent of any induced reactions, since the decomposition products of iron carbonyl have been shown t o have no induced pyrolytic effect in hydrocarbons. (Experimental evidence ( 1 ) has been presented recently to show that the oxidation of n-butane may be induced by the spontaneous ignition of relative high concentrations of nickel carbonyl. j The relat,ion between the antiknock effectiveness of iron curbony1 and tetraethyllead would then be expected to vary according to the susceptibility of the fuel to the knock-inducing act,ion of et,hyl radicals as determined with equivalent amounts of diethylmercury. The experimental data, as det'ermined by the critical compression ratio method, show considerable substantiation of the above concept. These data are presented graphically in Figures 4 and 5. I n n-heptane, which was relatively insensitive to ethyl radicals, tetraethyllead was twice as effective as iron carbonyl. I n diisobutylene, which was sensitive t o the knock-inducing effect of et,hyl radicals, tetraethpllead was scarcely more effective than iron carbonyl. I n iso-octane, which was relatively insensitive to ethyl radicals, tetraethyllead was more effective than iron carbonyl a t low concentrations in accord with the above concept,, although a t higher concent,rations, tetraethyllead became less effective than iron carbonyl by a n amount which it is difficult to account for on the basis of the quantit,y of ethyl radicals derived from t,he tetraethyllead. Similarly in both cyclohexane and cyclohexene, xThich were moderately sensitive to et'hyl radicals, tetraethyllead was less effective with respect t o iron carbonyl than in n-heptane, although the decrease in effectiveness of tetraethgllead in these two compounds was not directly proportional t o the sensitivity to dicthylmercury. Although both cyclohexane and cyclohexene had almost the same sensitivity to ethyl radicals, tetraethyllead was considerably less effective than iron carbonyl in cyclohexane and only slightly better than iron carbonyl in cyclohexene. I n cyclopentadiene, both tetraethyllead and iron carbonyl induced knock. Since this hydrocarbon was sensitive t o ethyl radicals, the fact that tetraethyllead was a more effective knock-inducer than iron carbonyl is in accord with the above concept. The above correlation on the relative effectiveness of tetraethyllead and iron carbonyl with the susceptibility to radicals is not as clear cut as the knock-inducing effect of radicals done. There is some evidence, however, that the antiknock effect of tetraethyllead may be impaired by the counter effect of the ethyl radicals associated with it.
M a y 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
The knock-inducing effect of ethyl radicals may be still further illustrated by the following simple experiment which incidentally correlates explosion ahead of the flame front, in other words, knock, with explosion of the entire charge induced by the introduction of a radical-producing substance. In this expe’riment a n engine was cranked at a compression ratio of 10 to 1 with cyclohexane as a fuel a t the normal air-fuel mixture ratio but without spark ignition. Cranking at this compression ratio, which is five compression ratios above t h a t for incipient knock for normal operation at 600 r.p.m., no autoignition takes place with cyclohexane alone, but if a drop of diethylmercury is introduced with the intake air a violent explosion follows for several cycles. Firing stops when the diethylmercury has been consumed. No explosion takes place when benzene is substituted for cyclohexane as might be expected from the relative inertness of benzene with respect to ethyl radicals. Ethyl nitrate and acetylene may be substituted for diethylmercury as a source of ignition. This experiment strikingly illustrates how a radical-producing substance, which is known to induce cracking at temperatures of the order of 50’ C. below t h a t for appreciable cracking normally, can induce reaction between certain hydrocarbons and oxygen resulting in explosion.
891
This suggests t h a t the reactions ahead of the flame front that result in knock involve a similar process.
Literature Cited (1) Badin, Hunter, and Pease, J . Am. Chem. Soc., 70, 2055 (1948). ( 2 ) Boord, presented before Division of Pet1 oleum Chemistry, at
112th Meeting of AMERICAN CHEMICAL SOCIETY, New York, N. Y . (3) Cramer, J. Am. Chenz. SOC.,60, 1406 (1938). (4) Jost and Croft, “Explosion and Combustion Proresses in Gases,” Chap. 11and 12, New York, McGraw-Hill Book Co., 1946. ( 5 ) Ibid., p. 437. (6) Lewis and von Elbe, “Combustion, Flames, and Explosions of Gases,” Chap. 4, London, Cambridge University Press, 1938. (7) Lovell, Campbell, and Boyd, IND.ENG. CHEM.,23, 26, 555 (1931).
(8) Rossini. Prosen, and Pitaer, J.Research Nutl. Bur. Standards, 27, 529 (1941) (9) Steacie, “Atomic and Free Radical Reactions,” Chap. 6, New York, Reinhold Publishing Corp , 1946. (10) Taylor and Smith, J . Chem. Phye., 7 , 390 (1939): 8, 543 (1940). (11) Walsh, Trans. Faraday Soc., 42, 269 (1946). I (12) Withrow and Rassweiler, S.A.E. Journal, 39, 297 (1936) ; IKD. ENG.CHEM.,25, 1359 (1933). RECEIVED September 27, 1948.
Effect of Additives on Gasoline Engine Deposits R. E. Albright, F. L. Nelson, and L.Raymond SOCONY-VACUUM LABORATORIES, PAULSBORO, N. J.
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Gasoline additives such as antioxidants and tetraethyllead have been evaluated for their effect on engine varnish and sludge deposits by means of both laboratory and field tests. Passenger car field tests have shown small and relatively insignificant differences in the deposit forming tendencies of several gasoline antioxidants; the differences in engine design factors being of far greater importance than fuel composition variables. Laboratory tests were made also, using the low temperature CRC FL-2 procedure which has been shown in some instances to correlate with commercial vehicle operations. By this means, it has been shown that some gasoline antioxidants tend to increase engine varnish and sludge deposits; the extent of deposit formation is dependent on the hydrocarbon composition of the base gasoline. Tetraethyllead was found to have no significant effect on varnish and sludge deposits in laboratory engine tests. Experimental fuel additives, which will provide large reductions in engine varnish and sludge deposits, have been developed also. These materials, however, are not yet suitable for commercial use.
T
HE formation of deposits in internal combustion engines, until fairly recent years, has been blamed generally on the Iubricating oil used in the engine. However, the development of detergent and heavy-duty type oils in the mid-thirties and their widespread use during the war years focused attention on other factors which may at times be of such importance as t o nullify in large part the effediveness of the newer lubricants. Recently published papers (1, 9, 4-6, 8) have indicated that, while heavyduty oils are generally effective when used under the conditions for which they were designed, their performance is interrelated with other parameters. It is now recognized that there are four major factors affecting the formation of engine deposits, namely:
engine design, operating conditions, fuel, and lubricant. A fifth factor, mechanical condition of the engine, may be added t o this list. Since 1945, the Socony-Vacuum Laboratories have been conducting independent but coordinated investigations t o determine the area of influence and the magnitude of importance of each o f these five factors. While it is impossible t o separate completely the contribution of any single factor from the others, this paper will deal with investigations of deposits in gasoline engines in which the fuel effect was of primary interest.
Deposit Types Gasoline engine deposits, due t o the fuel, fall into two main categories: induction system deposits and power section deposits. The former, for t h e most part, are caused by preformed gum and, occasionally, fuel additives. While induction system deposits can have important performance consequences, the present discussion is limited to power section deposits as influenced by additives and associated fuel characteristics. The relation of lubricating oils will not be discussed although there is substantial evidence t h a t the lubricant can cause controlling differences even under conditions accentuating fuel deposition tendencies.
Test Procedures Most of the laboratory engine evaluations described hereiirl have been made by the CRC FL-2 Procedure (a) in which a Chevrolet engine is operated at fairly high load and speed but at low oil and water temperatures. These low temperatures minimize deposits resulting from oil deterioration and emphasize the fuel effect. Key operating conditions for this procedure are given in Table I. Over-all performance in this test is based on a rating system in which varnish and sludge deposits on the following engine parts,
