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made directly; for from the curves information may be obtained, (1) as to what percentage of the oxygen has been consumed a t any given time, and (2) as to how much of the products of combustion have been formed at the same time. It is possible, therefore, to plot a curve showing the portion of the fuel which has been burned a t various times during the combustion stroke. This is especially valuable when dealing with gaseous fuels, which appear directly in the analysis. On this basis, the curves shown in Figure 4 have been drawn. In these the percentage of the fuel charge that has burned up to a given time is plotted against the time after ignition. It should be pointed out, with respect to these curves, that since there was an excess of fuel present, in all of these experiments, 100 per cent burned means 100 per cent reacted, or burned so far as the amount of air permitted. Measurements of the rates of burning, such as these, have an important application to the study of knock in engines and of the action of antiknock compounds, such as tetraethyl lead. Thus, in Figure 4 the normal rate of burning of gasoline is compared with that of a gasoline-kerosene mixture, with that of gasoline in the presence of a knock inducer (which in this case was a small amount of isopropyl nitrite), and with that of gasoline run under the knocking conditions
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just described, but with the presence of enough tetraethyl lead to eliminate detonation. The data shown represent averages of different determinations. The relative rates under these various conditions are apparent from the chart. It is seen that when the engine knocks, whether the detonation is .caused by the presence of kerosene in the gasoline or by a chemical knock inducer, the gasoline burns at a rate more rapid than normal. It may be seen, also, that the presence of tetraethyl lead in a knocking combustion brings the rate of burning back to normal, within the probable limit of error. Inasmuch as it gives direct information upon the rate a t which fuels burn, it would seem that there is here available a powerful tool for investigating the course of combustion in the gasoline engine. The method gives, also, a measure of what the conditions are just before ignition, or the amount of the charge carried over from the previous cycle; and, in the case of gaseous fuels, of the composition of the new charge. Finally, the method gives information as to the nature of the chemical reactions occurring in the combustion chamber, for as will be seen from Part I1 of this series, it is possible to have two simple fuels in a n engine burning a t different rates.
Studies of Combustion in the Gasoline Engine’ 11-The Burning of Hydrogen and Carbon Monoxide By Wheeler G . Love11 and John D. Coleman, with T. A. Boyd RESEARCH LABORATORIES, GENERAL MOTORS CORP.,DETROIT, MICH.
N THE preceding paper Experimental Method The results of experiments described in this paper of this series2 there was indicate that the simultaneous burning of carbon The apparatus used was a described a method of monoxide and hydrogen in a gasoline engine probably single-cylinder, air-cooled enobtaining gas samples durtakes place according to the reactions: gine equipped with the speing the explosion period in a 2Hz 0 2 cial sampling valve previ2co o*gasoline engine, by means of ously described. Different where the ratio of the velocity constants which it is possible to find the compression ratios were emk ~ p : k c o= 2.3 rate a t which the fuel burns in ployed, but the engine speed Neither these reactions nor their relative rates are the cylinder. The results of was held constant at 1200 changed by a small increase in the compression ratio, experiments upon the comr. p..m. The fuel consisted of or by.the presence of a knock inducer or a knock bustion of hydrogen and cara mixture of carbon monoxide suppressor, as long as detonation does not occur. The bon monoxide in a gasoline and hydrogen. The hydrogen simultaneous combustion of two individual fuels in an engine, presented herewith, was the commercial prodengine at rates widely different for each does not appear were obtained by the method, uct prepared by electrolyto be in agreement with the concept of a narrow zone sis and the carbon monoxthe investigation having been of flame advancing across the cylinder, within which ide, compressed into cylinconducted in order to obtain combustion is completed. ders, was obtained through information upon the physithe courtesv of F. C. Zeisbera cal nature and mechanism of E. I. du Pont de Semours and Company: of combustion of these simpler gases. The fuel gases were metered to the engine in substantially I n an engine fueled with gasoline, hydrogen and carbon monoxide are products of combustion for mixtures in which equivalent proportions by means of calibrated flow meters. the fuel is in excess. On this account, an actual engine Uniform admixture with air upon admission to the cylinder study of the burning of these two simple gases themselves is was assured by a gas-mixing valve of considerable capacity of considerable practical importance, for information as to located between the throttle and the intake valve. The ratio how these fuels burn should be useful in helping to solve the of fuel to air was substantially that of a perfect mixture, problem of the mechanism of the combustion of gasoline. with neither fuel nor air in excess. This mixture ratio was Also, data upon the behavior of these gases in the presence of used as a matter of simplification, since there was then no tetraethyl lead may have application to the general problem complication from incomplete combustion, and the fuel of the action of antiknock compounds. gases were burned completely to water and carbon dioxide. 1 Presented before t h e Division of Petroleum Chemistry a t the T2nd Samples of the gases from the engine cylinder were collected Meeting of the American Chemical Society, Philadelphia, Pa., September in the manner described in Part I, and were analyzed in 5 t o 11, 1926. a Burrell precision gas analysis apparatus. 2 Page 373, this issue.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
March, 1927 Data
A graph giring the composition of the fixed gases a t various times during the burning period is shown in Figure 1. The percentage of the different components is given upon a wet basis, the amount of water having been obtained by computation. These curves are quite similar in general form to those obtained with gasoline as a fue1,2 in that they exhibit no abrupt change in composition with time, but, on the contrary, are smooth. This is true in spite of the fact that, as compared with gasoline, the burning here takes place in a relatively short time. Of special significance is the fact that t h e h y d r o g e n and carbon monoxide do not burn a t the same r a t e , but hydrogen burns the faster of the two. Thus, it plz m a y be s e e n from
I:
1: 1'
time 55 per cent of the carbon monoxide has disappeared, about 75 per cent of the hydrogen has burned. Consequently, it appears that, although the hydrogen and the carbon monoxide were intimately mixed, they burned more or less independently. These data are supplemented by those given in Figures 2, 3, and 4. Figure 2 relates to the burning of mixtures of hydrogen and carbon monoxide at a compression ratio of 4.5 : 1 in the presence of ethyl nitrate. The data of Figure 3 were obtained a t a similar compression, but with the presence of tetraethyl lead in the fuel. Figure 4 relates, again, to the combustion of the gases alone, but a t a compression ratio of 6.0 : 1. Under all these conditions, the curves show the same general characteristics, although the total time of burning is not identical in all cases. This latter point is without special significance here, however, because the engine was not equally hot during all of the runs. Figure 1-Combustion of Hydrogen and Carbon Monoxide At compression ratio 4.5: 1
Figure >Combustion of Hydrogen a n d Carbon Monoxide At compression ratio 4.5:1 in presence of ethyl nitrate
377
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0 2 = 2Hz0 2H2 0 2 = 2c02 2co There is considerable experimental evidence from direct meas,~ the urements to justify the assumption. H a ~ l a m considering simultaneous combustion of hydrogen and carbon monoxide burning in flues, concluded that between 900" and 1500" C. 0 2 = 2C02 and the reactions are trimolecular (2CO 2H2 0 2 = 2H20), and that the ratio of the velocity constants for the two reactions is ka2:hco = 2.86. The results of indirect methods of various kinds for determining the order of these reactions are not in entire agreement. Falk4 investigated the ignition temperatures of hydrogen and carbon monoxide in oxygen and concluded that the hydrogen combustion 0 2 = HzOz), but that the carbon reaction is bimolecular (Hz 0 2 = 2C02). Bodenmonoxide reaction is trimolecular (2CO stein,6 measuring the reaction velocity directly, concluded t h a t a t 600" C. the combustion of hydrogen is trimolecular (2Hs 0 2 = 2H20). Rideal,b investigating the simultaneous catalytic combustion of hydrogen and carbon monoxide a t low tempera0 2 = H,Oz = tures, concluded t h a t the reactions are HZ HzO 0 and CO l / 2 0 z = COZ. Bone and his collaborators9 concluded from explosion experiments, t h a t the "mass influence" of hydrogen is proportional to the square of its concentration, and that of carbon monoxide is proportional to the first power.
+
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Assuming, tentatively however, the validity of the relations given first above, the rate of disappearance of hydrogen upon that basis becomes: d(H?) dt
= Ki(H2)' (O?)
Likewise, for carbon monoxide:
do = k2(C0)* dt
(02)
It is possible to combine these two equations if it is assumed that the temperature coefficient of reaction velocity for hydrogen is the same as for carbon monoxide. That this is true, a t least, appears from the data of Falk,4which is as follows: Temperature coefficient of reaction velocity per 10' C: 800°C. 900°C. ,1000O c. H2 1.31 1.13
co
1.24
1.14
Furthermore, if the combined equation is to be valid for all
Figure 3-Combustion of Hydrogen and Carbon Monoxide At compression ratio 4.5:1 in presence of
Figure 4-Combustion of Hydrogen and Carbon Monoxide At compression ratio 6.0: 1
tetraethyl lead
As was mentioned above, i t appears from these data that the hydrogen and carbon monoxide in the engine burn somewhat independently. The general situation seems to be that of two gases both reacting individually with the oxygen, and a t different rates. Consequently, i t would be especially valuable if it were possible to obtain quantitative information as to what the relative rates of the two combustion reactions are.
conditions of pressure, it is necessary that the increase of pressure during the explosion shall have the same effect upon the combustion of both gases.
Relative Rates of Combustion
THISJOURNAL, 15, 679 (1923). J . A m . Chem. Soc., 29, 1536 (1907). 6 2. fihysik. Chcm., 29, 665 (1899); Bodenstein and Ohlmer, I b i d . , 53, 166 (1905). 0 J . Chem. Soc. (London), 115, 993 (1919). 7 Phil. Trans., 215A,275 (1915).
As a means of analyzing the data obtained, let it be assumed that hydrogen and carbon monoxide burn according to the reactions:
Combining the equations,
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1-For these conditions the mechanism of the combustion is the same as has already been considered. 2-The relative rates for these conditions are identical.
Or, in other words, if the assumptions are correct, the reciprocal of the hydrogen concentration during the combustion plotted against the reciprocal of the carbon monoxide should give a straight line. The slope of the line, also, should represent kl/kz or a ratio of the velocity constants of the two reactions. The plotting of the experimental data in accordance with the equation developed above should demonstrate the validity of the various assumptions in regard to the mechanism by which the mixture of carbon monoxide and hydrogen burns. Such a plot is given in Figure 5 . It has been computed from the experimental data shown in Figure 1, and represents the combustion of hydrogen and carbon monoxide a t the relatively low compression ratio of 4.5: 1. It is seen that the points do fall almost exactly upon a straight line, indicating thereby that the assumed reactions co Figure 5-Reciprocal Relationship are probably correct. Hydrogen and carbon monoxide burning at If the assumptions were compression ratio-4.5: 1 not correct, the points would trace a curve, the form of which would depend-upon whether the reaction were bimolecular or of a higher order. It appears, consequently, that hydrogen and carbon monoxide, burning simultaneously under engine conditions, do follow the reactions 2Hz 2co
++ 02-2H20 02---32c02
The slope of the common line is 2.2, which is in agreement, within the experimental error, with that of the similar curve for the combustion of the gases alone a t a lower compression. It appears, then, that these changes in the conditions of burning do not affect either the mechanism or the relative rates of these reactions. It should be noted that in these tests the presence of the knock inducer, ethyl nitrate, and of the antiknock compound, tetraethyl lead, does not relate to their actual functioning as such, but only to their presence, since in none of these runs did any detonation occur. Physical Nature of the B u r n i n g
From the data presented above, i t appears that when hydrogen and carbon monoxide burn simultaneously in an internal combustion engine under the conditions of these experiments they follow definite chemical reactions, the velocity constant of one being twice that of the other. From the point of view of the engine, it is difficult to reconcile these facts with the conception of flame in gaseous combustion, in which it is considered as a boundary surface between completely burned and entirely unburned gases that advances across the chamber from one side to another.* Here are two gases, mixtures of which appear to burn a t widely different rates under the conditions prevailing in the gasoline engine. I n Part I of this series it was pointed out that from the data of this investigation the gases in the engine cylinder seem to be substantially homogeneous during the c o m b u s t i o n period. Upon this basis it appears, therefore, that the usual conception of flame as a narrow zone w h i c h moves I t h r o u g h the charge and w i t h i n w h i c h combustion completes itself is not a valid ‘ one, at least so far as combustion in the gaso et II ad os a6 i oline engine is conEO Figure 6-Reciprocal Relationship. cerned. Hydrogen and Carbon Monoxide Burning under Various Conditions It is much simder. 0-in presence of ethyl nitrate (compresin the Of the en- sion ratio 4.5: 1). A-in presence of lead gine, to consider the tetraethyl (compression ratio 4.5: 1). Xat compression ratio 6.0: I burning gases in the combu&n chamber as practically homogeneous, and such flame as may exist to be broken up into a multitude of parts by the high degree of turbulence set up in the gases as they are first drawn in through the inlet valve, and then compressed by the rapidly ascending piston. At any rate, it appears that the ordinary physical chemical laws relating to gaseous reactions are applicable to conditions that prevail in the cylinders of the automobile engine. This method of attack on the problem of what goes on during combustion in the gasoline engine seems especially valuable because it makes it possible to utilize an actual engine for determining what chemical reactions occur there, how fast they proceed, and how they are influenced by various catalysts. It has a direct practical application, therefore, to the study of the mechanism by which hydrocarbon fuels burn. Results of the experiments upon this phase of combustion in the gasoline engine will be considered in a subsequent paper of this series. ~
~
Hence it seems that nothing unusual happens when these reactions are confined to the cylinder of an internal combustion engine. The slope of the line in Figure 5 is 2.35. Since this is also the value of kl/lcz, it means that, under the engine conditions, the reaction velocity constant for hydrogen is 2.35 times that for carbon monoxide. This value is in good agreement with that of 2.8 obtained by Haslam3 for simultaneous combustion of hydrogen and carbon monoxide in flues. Effect of Conditions of B u r n i n g
The effect of increase in compression ratio, and of the presence of antiknock and knock-reducing compounds, upon these reactions when they occur in a gasoline engine may now be considered. I n Figure 6 are shown data, similar to these of Figure 5, where the reciprocal of the hydrogen concentration is plotted against the reciprocal of the carbon monoxide concentration for various times during the burning. The three sets of data are for the combustion of: 1-Hydrogen
and carbon monoxide a t a compression ratio of
4 5 : 1 and in the presence of ethyl nitrate.
2-The gases under the same conditions except for the substitution of tetraethyl lead for ethyl nitrate. 3-The two gases alone at a compression ratio of 6 0 : 1 with the substitution of ethyl nitrate for tetraethyl lead
The points are based upon the data shown graphically in Figures 2, 3, and 4, respectively. It is seen that for all of these conditions the points lie approximately upon the same straight line. Since the slope of this function is practically identical for all three sets of points, it follows that:
4,
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8 Stevens, J . Am. Ckem. Soc., 48, 1896 (1926); Midgley and Janeway. J . SOC.Automotive Eng.. la, 367 (1923).
