888
INDUSTRIAL A N D ENGINEERING CHEMISTRY
is highest; it is less variable in its action over the whole range of hydrocarbon types encountered in gasolines; its lower volatility, though less desirable from the standpoint of distribution, is better from the standpoint of safety in handling: and it is cheaper to manufacture. Tetraethyllead is always used together with a n artfully blended mixture of chlorine and bromine compounds which may be thought of as combining with the lead oxide formed in the combustion of tetraethyllead t o form lead halides which are sufficiently volatile so that substantially all of the lead passes out of the engine. The selection of the best amount and type of halide for optimum results depends on a large number of conflicting factors related to various types of fuels and engines. T h e other metallic antiknock compound which has been used commercially, but only for short periods of time and especially in Europe, is iron carbonyl. The principal disadvantage t o its use is that the combustion products are iron oxides which wear out engines a t a n extremely rapid rate; extensive research has failed to overcome this obstacle. Another class of antiknock compounds is the aromatic amines of which aniline and xylidine are well known examples, A great deal of time has been spent in investigations of the amines, especially during the war. They seem t o have a different method of action than do the metallic compounds, and generally speaking, are much less effective. Tlicir use has not been extensive. The problem of how much antiknock material to use is also an important one. If engine performance is considered rather than octane numbers, in general, equal proportional successive additions of tetraethyllead, in contrast to some other antiknock materials, produce equal proportional successive improvements in the utility of the fuel, and the constant of proportionality depends primarily on the fuel. For most paraffinic hydrocarbon fuels, the proportionality constant is substantially the same, so t h a t the better the fuel, the greater the gain when a fixed amount of lead is added. With other classes of fuels such as olefins and aromatics, the proportionality constant is profoundly affected by the specific hydiocarbon compounds so t h a t even a t this date there is only a useful geneial pictuie of the type of phenomenon t o be considered.
Vol. 41, No. 5
There is a tendency t o believe that the question of how much tetraethyllead to use aIso should be weighed from the standpoint of deposit accumulation in the combustion chamber and its relation t o increase in octane requirement of the engine with use. Extensive t'ests in passenger cars over the past 2 years offer convincing evidence t h a t the amount of tetraethyllead used in the fuel has no significant effect on the octane requirement increase due to its use. The better the scavenging of the lead accomplished by the halides, the better the valve and spark plug life, provided there is no accompanying increase in corrosion rate. Although better scavenging does reduce the t,otal weight of engine deposits due t o lead, this factor appears to bear little relation to increase in the octane requirement of the engine with use. Certain classes of unstable sulfur compounds, such as may occur in some gasolines, have a bad effect on the usefulness of lead ( I ) , Consequently, this is another problem that must be considered in the application of antiknock compounds. Finally, the usefulness of lead is influenced by the engine and engine condit'ions under which i t is used. Many unleaded fuels appear relatively superior to others when they are used in engines, or under conditions where the temperatures of t,he charge are relat,ively low. I n such fuels, especially, the effectiveness of lead is much better under those mild conditions. These, then, are some of t,he major problems of the use of antiknock compounds. It is apparent t h a t the best solution t o these problems is t h a t which furnishes the best product for the customer who drives an automobile or uses a n engine, and the best product for him is the maximum possible pleasant miles per dollar. Neither t,he fuel, nor the antiknock compound, nor the engine is any good by itself in producing transportation or power. It takes the patient cooperation of refiner, antiknock producer, and engine manufacturer to find the best solution tG their mutual problems, so t h a t fuels may be used t o the best possible advantage.
Literature Cited (1) IND.ESG.CHEM.,41, 885 (1949). (2) Kettering, C. F.. S.A.E. Quart. T T U I L 1, S . ,659 (10171
teracti H. K. Livingston E. I. DL' PONT DE NEMOURS & COAMPANY, INC., WILMINGTON, DEL,
G
ASOLINES containing Pignificant amounts of sulfur give a smaller increase in octane number on the addition of tetraethyllead than sulfur-free or 101%sulfur garolines of similar hydrocarbon composition and base octane number. This antagonism of sulfur toward tetraethyllead was first reported by Rndo (S) in 1934. Much Jvork has been devoted since to the development of refining techniques for removing sulfur compounds, especially those that have the more deleterious effect on tetraethyllead, from gasoline. No complete explanation of the nature of the sulfurtetraethyllead interaction has ever been attempted, however. A thorough understanding of the sulfur-tetraethyllead interaction is of considerable importance in a t least two respects. Sulfur antagonism significantly limits the maximum beneficial results obtainable in the commercial utilization of tetraethyllead, and i t offers a n unusually attractive indirect method for investigating t h e mechanism of the antiknock action of tctraethyllead. A comprehensive investigation of the antagonifitic effect of sulfur compounds on the antiknock action of tetraethyllead is
now in progress ( 6 ) . The present paper will not be conccrned with the detailed rcsults of this investigation, except in so Jar as they relate t o the quantitative interpretation to be developed. All sulfur compounds t h a t have the same type of sulfur linkage have the same effect on tetraethyllead when compared a t the same sulfur concentration (6, 8). The data for compounds with aliphatic sulfide structure (6) are typical of the agreement observed. The other important classes of sulfur compounds arc thiols, disulfides, and thiophenes. The early work of Schulzc and Ihiell (9) established the order of antagonism as disulfiriP > sulfide > thiophene. Thiols are about equivalent to disulfides (8, 10) in their effect on tetraethyllead.
Effect of Tetraethyllead Concentration For any given sulfur compound and concentration, thc proportional lead response is the same for each increment of tetracthyl-
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
May 1949
Detailed investigations of hydrocarbon blends having octane numbers in the range from 40 to 95 containing 0.1 to 6.0 ml. of tetraethyllead per gallon a n d 0.01 to 2.00% sulfur have led to the following conclusions: (a) Sulfur compounds having the sulfur atom or atoms i n the same type of chemical combination have the same effect on tetraethyllead when fuels are compared at the same sulfur concentration. (b) T h e same fraction of the tetraethyllead antiknock efficiency is lost for a given amount of a sulfur compound, independent of the total concentration of tetraethyllead added. (c) T h e loss i n antiknock efficiency is substantially independent of the octane number or composition of the base fuel. (d) When mixtures of sulfur compounds are present, those containing sulfur in different types of chemical combination have an additive
lead added. This was recognized, though the condition was not described in these words, by Schulze and Buell (9) and Ryan (8). I n both these investigations i t was found t h a t the lead susceptibility decrease for a given fuel, sulfur compound, and sulfur concentration was substantially constant throughout the range of tetraethyllead concentrations investigated. The data of Birch and Stansfield (1) and Widmaier (11) also are consistent with this concept of constant lead susceptibility decrease. I n investigations limited t o a comparison of different sulfur compounds in the same fuel containing the same amount of tetraethyllead, the change in octane number offers a satisfactory index of sulfur antagonism. If comparisons are t o be made a t different lead concentrations, it is desirable t o employ the dey by Ryan (@-to crease in lead susceptibility-designated characterize sulfur antagonism. However, if results obtained with different fuels are t o be compared, it is necessary t o make the comparisons in terms of the antiknock efficiency of the tetraethyllead. This can be done best by using the loss in lead efficiency, defined (6) as:
889
effect on the efficiency of tetraethyllead. (e) T h e ratio of inactive to active tetraethyllead i n a fuel containing thiophenic sulfur is directly proportional to the sulfur concentration; for other types of sulfur compounds, the ratio is proportional to a power of the sulfur concentration. These facts are consistent with the hypothesis that during operation of automotive engines with fuels containing sulfur a n d tetraethyllead there is an interaction between these components that may be treated as a resultant of the chemical equilibrium
5'fTz-U S represents a sulfur compound or its reaction product, T rep-
resents an active form of tetraethyllead, a n d U represents a tetraethyllead reaction product devoid of antiknock activity.
heptane blend containing 0.1% sulfur as I-propanethiol are given in Figure 1, together with the lead blending curve for the sulfurfree blend. With 3 ml. of tetraethyllead per gallon, the octane number is 83 without sulfur and 72 with sulfur. An octane number of 72 corresponds t o 0.81 ml. of tetraethyllead per gallon on the blending curve, so the loss of tetraethyllead efficiency is given by
The loss in efficiency, like the decrease in lead susceptibility, is independent of the tetraethyllead concentration (Table 11). While these data were obtained with Motor Mix (a 1 t o 1t o 0.5 molar ratio of tetraethyllead, ethylene dichloride, and ethylene dibromide, plus small amounts of kerosene and dye), the results obtained with tetraethyllead alone-that is, not containing scavenging or other compounding agents-are identical within the limits of accuracy of the lead blending and octane rating methods.
Effect of Fuel Composition L = loss in lead efficiency; uo = actual tetraethyllead concentration; and a = tetraethyllead concentration in sulfur-free fuel t h a t corresponds to the observed octane number. (It is necessary that a and a0 be in the same units.) The following example illustrates the method of calculating L and correcting for the effect of the sulfur compound on the base octane number. The data obtained for a 60 iso-octane to 40 Table I. Octane Numbers of Leaded Reference Fuels Containing Aliphatic Sulfide Compounds at 0.1% Sulfur Concentration Motor Blend, 60 iso-octane:40 heptane 3 ml. tetraethyllead/gal. Same Leaded blend with 0.1% S as: Methyl sulfide Ethyl methyl sulfide Ethyl sulfide Butyl methyl sulfide Ethyl propyl sulfide Amyl methyl sulfide Butyl ethyl sulfide Propyl sulfide Isopropyl sulfide Butyl propyl sulfide Butyl sulfide Isobutyl sulfide sec-Butyl sulfide Heptyl sulfide Allyl sulfide Dodecyl methallyl sulfide Thietane Tetrahydrothiophene a Prior reference gave 74, but repeated tests have indicated correct value.
+
Octane No. 60
83
75 75 75a 75 75 75 75 75 75 75 75 75 75 74 74 75 76 75 that 75 is the
The method of calculating the loss in lead efficiency outlined above has been applied to the data published by various investigators. The results, which are summarized in Tables I11 and IV, indicate t h a t L has approximately the same value in all the different fuels used, I n those cases where the base fuel contained appreciable sulfur and no correction was made for this sulfur Concentration, the apparent lead loss is low. This results from the fact t h a t the loss in lead efficiency is a function of the sulfur concentration raised to a fractional power (6, 8 ) , so t h a t the first increment of sulfur is the most effective. I n particular, data from sources E and F (Table v) tend to be low for this reason.
Table 11. Effect of Tetraethyllead Concentration on Octane Number a n d Loss i n Lead Efficiency Fuel 60 iso-octane :40 he Lane blend Sulf& concentration, 0.1% as ethyl sulfide Tetraethyllead compounded as Motor iMix Octane rating method, A.S.T.M. Motor Method (D357-47) Decrease in Octane No. Observed Relative t o Loss in Lead Sulfur-Free Efficiency Tetraethyllead, Octane Ml./Gal. NO. Fuel (L)
Average
60
Vol. 41, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY Table 111. Comparison of Lead Loss Data for Various Fuels Containing 0.05% Added Sulfur Sulfur Compound Added Ethanethiol
Sourcea
Lb
$I
1-Butanothiol 2-Rlethyl-1-propanethiol
2-Methyl-2-propanethiol RIethyl sulfide
.I C A
Ethyl sulfide Propyl sulfide Methyl disulfide Ethyl disulfide
Propyl disulfide Butyl disulfide
63 56 63 56 63 63 62 63 56
D
47 40 35
A D
47 39 34
C
47 40
E
E A
A F A D E F A C A F
58 48 58 55 50 48 58 58
tert-Butyl disulfide
A C
63 66
Amyl disulfide
6
58 54
Isoamyl disulfide
A C A C G
58 58
Thiophene
I J
28 31
27 35 37
a Sources of d a t a are given in Table 1’. b I n Tables I11 a n d IV the L-values represent a weighted average of the results obtained at different tetraethyllead concentrations. T h e weight given in averaging was proportional t o the tetraethyllead concentration,
Table IV.
Comparison of Lead Loss Data for Various Fuels Containing O.iO% Added Sulfur Sulfur Compound Added Source“ L Ethanethiol
Source A
Identification of Sources Used in Compiling Tables I11 and IV
Literature Cited
B
J 1-Propanethiol
Table V.
-4
B ne1
(6)
60 iso-octane : 4 0 heptane (0.002%
(1)
q)
E
(9)
F G H I
(10)
(Ipj’
65;$o-octane :35 heptane Rodessa reformed gasoline (contained 0.0033% S , b u t lead susceptibility data were corrected for this) 50 Octane gasoline (contained 0.00370 S, but lead susceptibility data were corrected for this) 50 Octane gasoline (contained 0.005% 9; no correction made) 50 Octane gasoline 98 Octane aviation gasoline Paraffinic gasoline Paraffinic gasoline
J
( 11 )
Leuna hydrogenation gasoline
C
(8)
D
(9)
(4)
Octane Ratink Conditions M o t o r and researrb Motor Motor
Motor
Motor Motor Oxygen boost CFR Motor, as modified in Germany Motor, a s modified in Germany
The agreement is particularly good for the disulfides. In almost every case, the greatest lead loss was observed with the fuel containing the least sulfur (source A). It seems likely that the base lead efficiency would be greater and the loss in efficiency due to sulfur compounds would be even larger if a base fuel with lower sulfur content were available. The results given in Tables 111and IV suggest that the loss in lead efficiency is to a considerable extent independent of the nature of the fuel containing the tetraethyllead and sulfur. Thie has been found ( 7 ) to be the case for fuels varying in base octane number from 40 t o 80 and containing as much as 60 to 70% naphthenic or aromatic hydrocarbons. Figures 2 and 3, taken from data in ( 7 ) ,demonstrate this point. The data are plotted as bars, in which the length of the bar represents the uncertainty in L resulting from the fact that the octane number is known with a n accuracy of only *0.5 unit. Brooks and Cleaton (2) have shown that octane numbers based on duplicate determinations, a5 these were, have a 90% probability of being within 0.5 unit of the true value.
Comparison of Mixtures of Sulfur Compounds It can be reasoned that if the various types of sulfur compound5 affect tetraethyllead by the same mechanism, the antagonism should be additive when two different compounds are added to fuel. That is, an amount of sulfide giving a 50% decrease in
B
H
Methyl sulfide
I J A D
E
Ethyl sulfide
Propyl sulfide Tetrahydrothiophene Ethyl disulfide
Propyl disulfide Isoamyl disulfide Thiophene
Sources of d a t a are given in Table 1’.
4 B D E H A C A B A B D E A C A B A B C D E I J
63 48 43 63 56 48 43 60
80
76
3 C.C./GAL.
t
/
63 45 63 56 70 68 66 63 67 70 67 66 44 39 34 46 39 46 48
0
0.81 CDIQAL.
“v
60 ---B
L
0
,
I
I e Tetraethyllead, ec./sal.
3
Figure 1. Type of Lead Blending Curve Used in Calculating Loss in Tetraethyllead Efficiency A = octane No. of base fuel with 3 ml. of tetraethyllead p e r gallon; B = octane No. of base fuel, unloaded; C = octane No. of base fuel with 3 ml. of tetraethyllead p e r gallon a n d O.l%o sulfur a d d e d as 1-propanethiol
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
May 1949
891
--
0.1
-
-
IN 80 ISOOCTANE: 20 HEPTANE IN 60 ISOOCTANE : 40 HEPTANE rmp~p~~ IN 40 ISOOCTANE: 60 HEPTANE
IN 60 TOLUENE : 40 HEPTANE IN 60 ISOOCTANE: 40 HEPTANE IN 70 C Y C L M X p N E : W HEPTANE
5
I
40
I
I
so
80
70
20
I
4
40
60
80
L
L
Figure 2. Decrease in Antiknock Efficiency Due to Ethyl Disulfide
Figure 3. Decrease in Antiknock Efficiency Due to Ethyl Disulfide
efficiency should have the same effect, in mixtures with a disulfide, as a n amount of disulfide giving a 50% decrease in efficiency. As shown in Table VI, L-values calculated according to this principle are in good agreement with those obtained experimentally.
Loss in Lead Efficiency with Mixtures of Two Different Sulfur Compounds
Table VI.
L-Values yo 9 Constituent % S Calou- Experias A B as B lated mentalQ Constituent A 57 63 I-Propanethiol 0.02 0.03 Ethyl sulfide 1-Propanethiol 63 66 0.05 Ethyl sulfide 0.06 65 69 2-Methyl-1-propanethiol 0.04 1-Pentanethiol 0.04 63 64 2-Methyl-1-propanethiol 0.04 Ethvl sulfide 0.04 81 77 2-Methyl-1-propanethiol 0 . 1 0 Eth3l sulfide 0.16 66 63 2-Methyl-1-propanethiol 0.04 Ethyl disulfide 0.04 86 90 2-Methyl-1-propanethiol 0 . 1 0 Ethyl disulfide 0 . 1 5 58 58 Ethyl sulfide 0.04 Butyl sulfide 0.04 62 58 Ethyl sulfide 0.04 Ethyl disulfide 0 . 0 4 65 68 Ethyl sulfide 0 . 0 5 Ethyl disulfide 0 . 0 5 81 80 Ethyl sulfide 0 . 1 0 Ethyl disulfide 0 . 1 5 Based on a single determination: as a result the octane number is accurate t o only A 1 unit and L is accurate to t 5 units a t L 60 or A 3 units a t L = $0.
*Ot IO
e
+
1
I
.03 M
I
I
I
I
.06 .W
I
I
.lo
20
Sullur, 9%
Figure 4. Method of Calculating L for Mixed Sulfur Compounds
-
The method of calculating L is as follows: Example. The fuel contains 0.04% 2-methyl-1-proanethiol 0.04% ethyl sulfide. As shown in Figure 4 = 42 for 0.04% sulfide sulfur, which corresponds to' 0.019% thiol sulfur, since L = 42 for 0.019% thiol sulfur. I n terms of thiol sulfur, the mixture contains 0.019 4- 0.040 = 0.059% sulfur, for which the thiol value is L = 64. I n terms of sulfide sulfur, the mixture contains 0.084 to 0.040 = 0.12470 sulfur, for which the sulfide value is L = 65. These values check very well with each other and with the experimental value (Table VI).
I
I
D2
0.04%
IO 8
-
sulfide sulfur = 0.019% thiol sulfur; 0.04% thiol sulfur 0.084% sulfide sulfur
I
I
I
I
I
I
t
I
I
I
I
I
, I ,
-0
L
6-
54-
3-
e-
Effect of Sulfur Concentration Ryan (8) found t h a t a log-log plot of the decrease in lead susceptibility (designated as y) against sulfur concentration (designated as c) gave straight lines over a considerable concentration range. The results could then be expressed by the equation y = klck3. This result has been extended (6) to show t h a t
L = ks&
0
a
DISULFIDES SULFIOES THlOPliENES DEVIATIONS DUE TO FAJLURE TO CORRECT
-
(2)
However, this relation is not valid for L values higher than 60 to 70. The data are more accurately expressed by the equation
(3)
0.I
I
I
I
I
I I I I I
I
I
I
1
1
1
1
1
Sulfur, %
Figure 5.
Logarithmic Plot of Sulfur Antagonism Data for Primary Reference Fuels
This equation appears to be valid up to sulfur concentrations a t which the effect of the sulfur compound on the base octane number becomes significant (Figure 5 ) . If c is expressed in x-eight % sulfur, the constants of this equation for the various types of sulfur compounds are: Type of Compound
12
n
Thiol Disulfide Sulfide Thiophene
11.7 10.5 7.3 7.4
0.68
.68 .68
1 .oo
+
Chemical Interpretation of the Interaction The various chemical facts regarding the sulfur-tetraethyllead interaction may be summarized as follows:
1. The interaction proceeds to the same extent for all compounds that have the sulfur atom or atoms in the same type of chemical combination. 2. The interaction leads to the destruction of the antiknock activity of the same fractional portion of the tetraethyllead, regardless of the total concentration of tetraethyllead added. 3. The interaction generally is unaffected by changes in composition of the hydrocarbon portion of the fuel. 4. Sulfur compounds containing aulfur in different types of chemical combination act in a n additive manner. 5 . The ratio of inactive to active tetraethyllead is a function of a power of the sulfur concentration. From these facts, the following equations may be deduced:
a- - n
=
a0
L
x
100 =
7"loss in lead efficiency;
8
These relations suggest a condition in which thcre are threp molecular species 8, T , and U entering into a reaction, S T% U . To describe the conditions of the sulfur-tetraethyllead interaction, it is only necessary to set up the following equations.
+
scn
(7)
[TI = la
(8)
-
[ U ] = u (a" a)
(9)
where s, t , and u are proportionality constants. Then the mass-law expression for a chemical equilibrium,
reduces to
ao--a = Kts Cna u Combining the constants so that
L 100
-L
Effect of Engine Conditions KO mention has been made heretofore of the effect of engine conditions on the sulfur-tetraethyllead interaction. A detailed investigation of the effect of engine variables is in progress. It has been established that the octane ratings of primary reference fuels containing sulfur K 90 W and tetraethyla lead are identiz cal, w h e t h e r W z measured by the 80 motor method 0 I (A. 9. T. 11 D 357-47) or reK o W 4 search method 270 a (A. S. T. M. D 908-47T). The data, which appear in Figure eo 7, have a coeffieo 70 80 MOTOR OCTANE NUMBER cient of regresFigure 7. Relation between Motor and sion of research Research Octane Numbers for 60 Isooctane number octane to 40 Heptane Blends Containing on motoroctane Varying Amounts of Sulfur and Tetran u m b e r of ethyllead 0,9998, which indicates an almost perfect correlation between the t\+o sets of values. It has been determined also that road octane numbers, determined by the Uniontown method, for a series of primary reference fuels containing 0.1% sulfur and varying amounts of tetraethyllead, agree within the limits of experimental accuracy a i t h the results obtained by the motor and research methods, for most sulfur compounds.
P
aa = concentra-
tion of tetraethyllead added; a = concentration of tetraethyllead correspondin to the observed octane number; c = sulfur concentration; = concentration of any given hydrocarbon; and k and n are constants.
[SI =
which is the equation that was derived from the experimental data. Therefore, it iz I2t possible to consider IC1 the sulfur-tetraethyllead interaction as the resultant of the equilibrium, S T -ts U,where S represents a sulfur compound or its reaction product; I' 0 0 I 2 reoresents tetraethyllead or an antiSulfur, % knock derived from it, and U represents Figure 6 . Sulfur Antagonism Data for Thiophenes in Primary Reference Fuels a tetraethyllead reaction product devoid of antiknock activity. The equilibriuni also may involve other moleculea, such as carbon monoxide or hydrocarbon, that are present in the fuelair mixture in such large amounts thal the slight concentration changes due to the above equilibrium would not affect the masslam expression. An analysis of combustion gases is being carried out t o idcnlifg the molecules S,T,and U , if possible.
I
Inasmuch as the exponent ( n )for thiophenes is unity, the data for thiophenes are linear when plotted on ordinary, nonlogarithmic coordinates. The plot of Ll(100 - 1,) = 7.4 c i s given in Figure 6.
L
Vol. 41, No. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
892
5
Practical Implications this gives
The results of the present investigation show that: (1) The relative order of antagonism of the different sulfur compounds is the same for all fuels. (2) For any given fuel, the same fraction of the antiknock
May 1949
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
893
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.
C
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).
