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equilibrium between the gaseous refrigerant and solution is obtained a t a pressure corresponding to the vapor pressure of the refrigerant a t 4.4’C. The solution is weighed, and the solubility is expressed as grams of solute per cubic centimeter of solvent. In seeking the best combination of refrigerant and solvent, much of the early work was done on methyl chloride; then dichlorornonofluoromethane was found to have distinctly better properties for the purpose a t hand, and it was extensively studied. Later the work was extended to obtain data on a few representative solvents with other compounds which have received attention as refrigerants. Many of the solvents reported in Tables I to IV are com-
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pounds which have not previously been described in the literature. The physical properties and analyses of the new compounds used in this work are collected in Table V. Work is in progress to establish a theory explaining the solubility characteristics of halogenated hydrocarbons in different types of solvents, and the results of this work will be submitted a t a later date. Articles covering the engineering phases of this development will appear in current issues of the air-conditioning journals, Heating, Piping, and Air Conditioning,and Refrigeration Engineering. RECEIVED December 19, 1936.
Hydrocarbon Reactions and Knock HE present paper contains an application of the authors’ theory of hydrocarbon combustion to problems of the internal combustion engine. It will be helpful to summarize the e s s e n t i a l features of the theory, but the reader is referred to the original paper for the supporting evidence and the reasoning (24).
T
Outline of Theory and Fundamental Facts The oxidation of hydrocarbons consists of two main chain reactions, one by which the hydrocarbon is transformed to aldehyde and another by which the aldehyde is oxidized to the ultimate productir carbon monoxide and water. Jn a h e a t e d vessel the chains are initiated a t the wall by the formation of monovalent radicals that carry on the chains, The primary reaction preceding the development of chains consists of the formation of minute amounts of aldehyde (the kind depending on the hydrocarbon) by a m e c h a n i s m that is imm a t e r i a l f o r the present purpose, but for which there is sorhe evidence that it is a s u r f a c e reaction involving the .intermediate formation of alkyl peroxide from the direct i n t e r a c t i o n of the hydrocarbon with o x y g e n.
IN THE INTERNAL COMBUSTION ENGINE GUENTHER VON ELBE Coal Research Laboratory, Carnegie Institute of Technology BERNARD LEWIS U. S. Bureau of Mines Experiment Station, Pittsburgh, Pa.
A brief summary is given of the chain mechanism of the oxidation of hydrocarbons. The combustion process in the Otto cycle engine is pictured to involve a race between combustion by a moving flame and the spontaneoug ignition of a part of the unburnt charge ahead of the flame. The latter occurs only after some time has elapsed following the establishment of ignition conditions in the unburnt charge. In normal combustion the flame travel is completed before the ignition lag is terminated. In knocking combustion the reverse is true. The various factors that determine the rate of flame travel and ignition lag are discussed. Numerous experimental facts, both chemical and physical, are interpreted. These include the action of antiknock agents.
Once aldehyde is formed, it may be oxidized a t the surface t o p e r a c i d that dissociates into the chain carrier radicals. The chain-initiating reaction may therefore be written :
+
RCHO 0 2 surface> RCO(O0H) surface > RCOO+OH (1) The monovalent radical, OH, reacts with the hydrocarbon, giving rise to a chain. In the first place an alkyl r a d i c a l , RCHZ, is formed; this oxidizes to an alkyl peroxide radical, RCH,OO.
+ RCHs = RCHz + Hz0 (2) RCHz + = RCHZOO OH
0 2
(3)
The radical RCHzOO has a limited lifetime, after which it decomposes into aldehyde and methoxyl, CH,O; the latter is oxidized f u r t h e r with the regeneration of an OH radical which carries on the chain by reaction 2. RCHzOO = R’CHO CHI0 -!%CO HzO OH (4)
+
+
+
If, however, the peroxide radical, RCH,OO, meets an aldehyde molecule within its lifetime, it p o s s e s s e s the
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ability to react in a series of reactions involving condensation, cleavage of the peroxidic bond, and oxidation of the radicals so formed. In this way there may be as many as five OH radicals regenerated, and the original chain branches into several new ones. This reaction will be referred to as the “peroxide branching” reaction. It may be written: RCHzOO
+ RCHO + [RCH20-OC(OH)R]% R’CHO
+ 2CO + 60H
(5)
Since this reaction is favored by a long lifetime of the RCHzOO radical, it becomesmost pronounced toward low temperatures. It accounts for the pronounced maximum in the reactivity and explosivity shown by a number of hydrocarbons in the neighborhood of 350” C. (1, 11, 19, 2’0). The oxidation of aldehydes proceeds by the initial formation of an aldehyde radical, RCO, which by oxidation is degraded stepwise to lower aldehyde radicals. The last step, however, leads to formaldehyde which is formed with excess energy and is capable of emitting its characteristic luminescent radiation. This gives rise to the phenomenon of “cool flames.” The degradation reaction may be written: RCHO
+ OH = Ha0 + RCO A+ CO + Ha0 + R’CO 09,
. . .CO + H20 + CHsCO HCHO*
(6)
kz
+ CO + OH
The formaldehyde reacts with OH to form the radical HCO.
OH
+ HCHO
HCO
+ HnO
(7)
This radical may be oxidized, t o carbon monoxide and the radical HOz; the latter on reaction with formaldehyde regenerates an OH radical: HCO HOI
+ 02 = CO + HOz
(8) (9)
+ HCHO = CO + Hz0 + OH
The HOz radical may react with oxygen and an aldehyde molecule in a three-body collision to yield, altogether, three new radicals according to the following: HO,
+ RCHzCHO +
0 2
= CO
+ H20 + 20H
+ RCO
l+?
(IO)
oxidation and de radation OH to HCHO* C d (see reaction 6 )
+
Reaction 10 again leads to chain branching and will be referred to as the “peracid branching” reaction. Reaction chains are terminated by destruction of the chain carriers at the wall: OH
surface+
destruction
(11)
At high pressures it is also possible that two OH radicals combine in the gas phase in a triple collision. Although unimportant for the present purpose, the scheme may be completed by mention of a formaldehyde condensation reaction with the radical HCO : HCO
+ HCHO
---f
*+ +
[CH(OH)CHO] 2CO
Hz0
+ OH
(12)
The preceding mechanism successfully explains numerous experimental facts connected with the slow oxidation and
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explosion of hydrocarbons (24). For the present purpose only a few facts need be mentioned.
The oxidation of paraffins with side groups takes a somewhat different course, since the degradation, according to reaction 6, cannot proceed beyond the carbon atom to which the side groups are attached. The results of Pope, Dykstra, and Edgar (14)suggest that the oxidation of such isoparaffins consumes the longest alkyl group, leaving a ketone. A ketone, however, cannot be expected to substitute for aldehyde in the peroxide branching reaction, and it is not surprising to find that the low-temperature reactivity and explosivity are most pronounced in normal paraffins and decrease more and more for isoparaffins as the st,ructure of the molecule is more condensed (14, 2’2). If a mixture of hydrocarbon and oxygen or air is admitted to a heated vessel, the reaction is immeasurably slow a t first. During this slow primary reaction, traces of aldehyde are formed. Simultaneously, chain carriers are released from the wall by reaction 1. The subsequent chain mechanism produces more aldehyde. Although most of it is oxidized in the gas phase by reactions 6 to 9, some of it diffuses to the wall and reacts there to generate more radicals by reaction 1. Thus, the autoaccelerating nature of the reaction is evident. This autoacceleration is enhanced by the branching reactions. If branching of the chains is sufficiently rare, the process tends toward an equilibrium in which the rates of formation of OH radicals and aldehyde molecules equal their respective rates of destruction. The reaction then proceeds at a steady nonexplosive rate until consumption of the reactant8 decreases it. On the other hand, if branching becomes frequent, the rate of production of chain carriers may exceed their destruction. I n this case the steady state is not possible; the OH radical concentration iricreases continuously (until the very last stages, when the consumption of reactants exerts its influence) and the reaction rate reaches explosive proportions. Both branching reactions depend on the presence of aldehydes and thus the explosion condition that chain branching shall exceed chain breaking is fulfilled only after aldehyde has accumulated to a sufficient concentration. The process of building up the aldehyde concentration requires some time, which may be considered to comprise the larger part of the so-called induction period (ignition lag) preceding an explosion. I n the usual experiment of admitting a mixture to a heated vessel, the ignition lag may be a matter of seconds. It can be reduced to a time smaller than can be observed in the usual experimental arrangement by previously adding aldehyde to the mixture. While in such experiments it is justified to speak of the elimination of the ignition lag by addition of aldehyde, this no longer applies to engine experiments where one is concerned with time intervals of the order of a thousandth of a second. In other words, even though aldehyde is present in the original mixture, some time elapses before the OH radical concentration is built up to proportions required for explosive reaction. By varying the temperature and pressure of a mixture, a boundary may be found which separates regions of explosive and nonexplosive reaction. For any given temperature the ignition lag i s longest a t the boundary itself. At a constant temperature the deeper one progresses within the explosive region-that is, by increasing the pressure, the shorter the lag becomes (92). This is understandable because the rates of the branching reactions increase with increasing concentration of the reactants-i. e., pressures-and the rate of the breaking reaction decreases. Since, as indicated above, the peroxide branching reaction is pronounced a t about 350 C. but decreases as the temperature increases, there is no corresponding regular change of the ignition lag with increasing temperature and constant pressure (2’2).
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Otto Cycle Engine The type of combustion desired in an Otto cycle engine is that produced by the passage of a moving flame through the combustible mixture in the cylinder head a t a rate that ensures a smooth rise in pressure devoid of vibrations. The subject of gas vibrations is discussed in references 8, 9, and 18. During the compression stroke the charge is heated adiabatically; after the passage of the spark the expanding flame gases compress still further the unburned charge ahead of the flame. The temperatures and pressures attained by the unburned charge are well within the explosive region of the mixture. For example, the unburned charge reaches a temperature of 350" C. a t a pressure of about 14 atmospheres, whereas the ignition pressure of a gasoline-air mixture for this temperature is of the order of 1 atmosphere (21). The ignition lags are, therefore, rather short. Although a t the explosion limit itself they are of the order of 1second a t 350 O C. (11),a t pressures occurring in the engine they may easily reach values comparable with engine periods. Only the length of the ignition lag determines whether the moving fiame arrives a t the end of the combustion chamber prior to spontaneous ignition of the last part of the charge {66,2.3). If the ignition lag is sufficiently short or the flame sufficiently delayed, part of the charge will ignite spontaneously and give rise to the knock. Direct evidence for this is to be found in the high-speed motion pictures of engine flames (17). The severity of the knock depends on the amount of charge that has thus ignited. This at once explains why the tendency to knock can be reduced by any change in conditions that decreases the normal burning time of the whole charge or increases the ignition lag. The former can be accornplished by constructing the cylinder head in special ways to allow rapid expansion of the flame area (conical shape with spark plug a t the apex; dual ignition) or to produce high turbulence, which is enhanced by increased piston velocities-that is, higher engine speeds. (For the relation between engine speed and flame velocity, compare reference IO.) Increase of the ignition lag may be accomplished in several ways : The unburned charge may be forced into a narrow space (for example, V-shaped), thereby decreasing the reaction rate by increasing the rate of chain breaking, the simultaneous cooling effect being advantageous only if the charge is cooled considerably below 350" C.; the compression ratio may be decreased or the spark retarded, both resulting in less compression of the unburned charge. Likewise the unburned charge undergoes less compression for mixture ratios richer or leaner than some optimum ratio corresponding to the maximum flame temperature (slightly on the rich side of the theoretical). It is probably for this reason that the greatest tendency to knock is found for a mixture ratio slightly on the rich side (19). Considering the decrease in the rate of peroxide branching with increasing temperature, it is not inconceivable that with increasing intake temperature the knock should, under certain conditions, be eliminated. This has been found experimentally (5). Formaldehyde is always detected in the noninflamed charge that is about to knock (7, 23, 15). It is also detected frequently under nonknocking conditions (66). That formaldehyde should be present is accounted for by the theory outlined. It has already been pointed out, however, that the appearance of formaldehyde in detectable amounts does not necessarily coincide with the end of the ignition lag, and it is understandable, therefore, that its appearance does not mean that the mixture will knock. Therefore it is clear that the addition of aldehyde to the charge should not necessarily induce knock (63). It may be said, however, that if formaldehyde is not detectable knocking will not occur. Experiments have shown this (25).
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If the engine were run sufficiently slowly to reduce the speed of flame travel and therefore allow more time for the development of the OH radical concentration due to the branching reactions, one would expect the appearance of formaldehyde always to result in knock. Experiments have shown this also (25). As previously pointed out, the high knock resistance of hydrocarbons with side groups (high octane number) iP probably due to their property of forming ketones rather than aldehydes. A characteristic difference exists in the emission spectra from the normal moving flame and the flame of the knocking charge (16). I n the former the bands of C-C and C-H radicals are intense, whereas in the latter they are very weak. Such radicals undoubtedly arise from the thermal dissociation (cracking) of hydrocarbons. It is proposed that, on account of the high concentration of chain carriers in the knocking part of the charge, the hydrocarbon is consumed essentially by the chain mechanism given above, so that cracking reactions are suppressed. I n the normal moving flame, with its adjacent layers of gas a t high and low temperatures, the temperature in the unburned layer rises very rapidly, owing in part to direct heat transfer and to chemical reaction induced by the diffusion of radicals from the burned layer, Under these conditions the branching reactions cannot be important since there has been too little time to build up a sufficient concentration of aldehyde. Thus, OH is no longer the predominant radical, and the reaction may be carried forward largely by radicals arising from cracking. The action of antiknock agents has been studied most extensively with tetraethyllead. This compound has been found to inhibit the chain reaction by which hydrocarbons are oxidized (15). It was proposed early that antiknock agents destroy peroxidic substances (for example, see reference 4). This hypothesis is plausible chemically. It is a fruitful suggestion, since the antiknock effect is readily explained by the retardation of both branching reactions which involve peroxidic substances. Therefore the ignition lag is increased. It is unnecessary to assume that the OH radicals themselves are destroyed. As for the actual reactions that tetraethyllead undergoes, the compound as such is rather ineffective. It does not inhibit the photochemical oxidation of acetaldehyde a t room temperature (3) which is carried on by peracid radicals (24),or the oxidation of pentane a t 265" C. (13) where the reaction, according to the theory, is probably governed principally by peroxide branching. Finely dispersed lead or other metals such as potassium and iron are efficacious (2, 6). Furthermore, atomic lead has been found spectroscopicalIy in the engine after the addition of tetraethyllead to the charge (16). Suggestions might be advanced concerning the destruction of peroxidic substances in a chain reaction involving metals or their oxides, but these should be deferred until more chemical facts are available.
Diesel Engine I n a Diesel engine it is desirable that the fuel spray ignite immediately; that is, the ignition lag must be as short as possible. Therefore, knocking fuels (high cetane number) are preferred, and the requirements are thus quite the opposite from those of the Otto cycle engine. As long as one was content with slow-running Diesel engines, the fuel problem was not dBcult, but with the tendency to use high-speed Diesel engines the problem of igniting the fuel rapidly enough becomes serious. In this connection much fruitful fundamental work on thermal ignition of fuels and flame propagation could be done.
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Literature Cited (1) Beatty, H. A., and Edgar, G., J . Am. Chem. SOC.,56, 102-6 (1934). (2) Berl, E., Heise, K., and Winnacker, K., Z.physik. Chem., 139, 453-81 (1928). (3) Bowen, E. J., and Tietz, E. L., J. Chem. SOC.,1930, 234-48. (4) Callendar, H. L.,and co-workers, Aeronaut. Research Corn. (London), Repts. and Mem. 1162 (1927). (5) Dumanois, P., Ann. combustibles liquides, 1934,No. 1, 143-4. (6) Egerton A. C., and Gates, S. F., Aeronaut. Research Com. (London), Repts. and Mem. 1079 (1926). (7) Egerton, A., Smith, L., and Ubbelohde, A. R., Trans. Roy. SOC. (London), 234A, 433-521 (1935). (8) Lewis, B., 8. A . E . Journal, 36, 133-5T (1936). (9) Lewis, B., &nd yon Elbe, G., J . Chem. Phys., 3, 63-71 (1935). (10) Marvin, C. F., and Best, R. D., Natl. Advisory Com. Aeronaut., Rept. 399 (1931). (11) Neumann, M., and Aivazov, P., Nature, 135, 655-6 (1935). (12) Pease, R.N.,J. Am. Chem. SOC.,51, 1839-56 (1929). and Egerton, A. C., J. Chem. SOC.,1932,676-86. (13) Pidgeon, L.M., (14) Pope, J. C., Dykstra, F. J., and Edgar, G . , J . Am. Chem. SOC., 51, 2203-13 (1929). (15) Ibid., 51, 2213-20 (1929). (16) Rassweiler, G. M., and Withrow, L., IND.ENQ. CHEM.,24, 528-38 (1932).
(17) Ibid., 28, 672-7 (1936). (18) Rassweiler, G . M., and Withrow, L., S. A . E . Journal, 36, 125-33T (1935). (19) Ricardo, H.,“Internal Combustion Engine,” Vol. 2, London, Blackie and Sons, Ltd., 1913. (20) Townend, D. T. A,, and Chamberlain, E. A. C., Proc. Roy. SOC. (London), 154A, 95-112 (1936). (This paper includes earlier references.) (21) Townend, D. T. A., and Cohen, L. L., J . Soc. Chem. Ind., 53, 267-8 (1934). (22) Townend, D. T. A,, Cohen, L. L., and Mandlekar, M. R., Proc. Roy. SOC.(London), 146A, 113-29 (1934). (23) Ubbelohde, A. R., Drinkwater, J. W., and Egerton, A,, Ibid., 153A, 103-15 (1935). (24) Von Elbe, G., and Lewis, B., “Combustion of Hydrocarbons,” to be uublished. (25) Withrow, L., and Rassweiler, G . M., IND.ENQ. CHEM.,26, 1256-62 (1934). (26) Ibid., 27, 872-9 (1935). R E C E I V ~January D 20, 1937. Presented before the Division of Industrial and Engineering Chemistry at the 92nd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936. Published by permiaaion of the Director of the U. 9. Bureau of Mines 8nd the Director of the Coal Researoh Laboratory, Carnegie Institute of Teohnology. (Not subject to copyright.)
T H E ALCHEMIST
Bv EGgene Isabey
(1803-1886)
For years it has been known that Eugene Isabey, the painter of the original re roduced as No. 20 in the series of Alchemical and hstorical Reproductions (June, 1932, page 645) had executed a second painting entitled L’Alchimiste. After much wearisome searching Mr. Berolzheimer has succeeded in obtaining a copy of the original water-color painting made by Isabey in 1874.
A comparison of our present reproduction with the previous one will show the great similarity of the composition, the laboratory equi ment illustrated, and the general arrangement. The aychemist himself, however, appears less ascetic and obviously, loath to leave his work, slept in the laboratory, as is evidenced by the fine poster-bed at the right. This is No. 77 in the series.
A detailed list of Reproductions Nos. 1 to 60 appeared in our issue of January 1936 page 129 and the list of Nos. 61 to 72 appeared in January 1937, page 74 whire als; will be fdund Reproduction No. 73. Reproduction No. 74 ippears on pa& 166, February issue, No.75 on page 345, March issue, and No. 76 on page 459, April issue.
