INDUSTRIAL AND ENGINEERING CHEMISTRY
1216
tures such a mixture is not combustible, except by accession of secondary air; it is not even explosive and could not be made to burn if it were in an inclosed space and subjected to an electric spark; hence, by maintaining such a ratio in the Bunsen burner we are free from any tendency on the part of the flame to “strike back.” The first notable departure from the conditions which are present in the Bunsen burner is found in the burners of the MBker type. Here under favorable conditions the primary air may be increased up to substantially the theoretical amount required for complete combustion of the gas passing through the burner tip. Increase i n Rate of Flow
But the high ratio of air thus secured produces an explosive mixture which requires special consideration in order to avoid the tendency to flash back and burn at the tip. The expedient employed as a remedy is the insertion in the gas flow of a grid having a total cross-sectional area for the open spaces sufficiently reduced so that the rate of travel forward of the gas mixture shall be greater than the rate of propagation of flame in the reverse direction. The speed of flame propagation in an explosive mixture has been made the subject of numerous investigations. I n the initial stage of flame propagation there is a period of uniform speed which precedes the explosive stage. This is designated in the literature as the period of “uniform movement.” I n studies by Wheeler,’ and later by Paymaq2 it is shown that the rate of flame propagation in the most highly explosive air-gas mixture is on the average about 1 meter per second. Comparing this value with the rate of flow of the gases in the larger part of a burner of the hlbker type, it is evident that conditions are present which would result in the striking back of the flame. For example, the speed of flow in meters per second may be represented by the formula:
Vol. 17, No. 12
extended application of the principle of full combustion by use of all air as primary air has not yet been attained. Increase in Gas Pressure
It will be a t once evident a t this point that the purpose of this paper must be merely to state the problem. By so doing, it sometimes happens that a service is rendered which results in at least aiding, if it does not directly lead to a solution of the matter. The chief factor involved is the variable and relatively low pressure of gas as supplied to city mains. The jet action or Venturi effect is of such a low order, and so easily obscured or obliterated, that it is exceedingly difficult to utilize in any broad and comprehensive manner. This difficulty is readily solved by industrial plants, where the volume of gas used warrants it, by inserting a booster in the system and bringing the gas to the burner a t a uniform pressure of several pounds per square inch. It is possible that some time, to facilitate a much higher distribution of gas through existing mains, city gas companies will want to carry pounds of pressure where they now carry inches. This method, using local pressure-control valves, would afford conditions where this particular problem could be readily solved. Any adequate appreciation of the fuel field now opening up to the gas manufacturers must lead directly, it would seem, t o a consideration of this matter of pressure as one that must be given a place from now on. Meanwhile, it may well be emphasized that, despite the handicaps, and even though the present number of burner designs is legion, it is an inviting field for the chemist and it may quite properly be said that the game is well worth the candle.
Chemical Equilibrium in Gases Exhausted b y Gasoline Engines
I,
R=
A X 3600 in which R = rate of flow in meters per second V = total volume in cubic meters of combined gases A = cross section of mixing chamber in square meters
With a mixing tube having a diameter of 30 mm. (approximately ll/s inch) the cross-sectional area A is 706.8 sq. mm. Assuming a burner tip delivery 0.3398 cubic meter (12 cubic feet) per hour and using an air-gas ratio of 4:1, the volume V mould be 5 X 0.3398 or 1.699 cubic meters (60 cubic feet). Substituting we have 1.699
R = 0.0007068 X 3600
By W. G. Love11 with T. A. Boyd GENERALMOTORS RESEARCH CORP., DAYTON, OHIO
Aside from the fact that it is complex, too little is known about the actual course of combustion in gasoline engines. The composition of the final combustion products, however, has been established by many investigators, and this information gives some insight into the character of the combustion reaction itself. With the purpose of getting further knowledge of the combustion reaction, which occurs so speedily in gasoline engines, the authors have closely examined a number of exhaust gas analyses, with some interesting results, as outlined i n this paper.
R = 0.667
That is to say, the rate of movement forward would be 0.667 meter (approximately 2.2 feet) per second. Hence the necessity of speeding up the rate of flow by some means in order to prevent the travel of the flame backward. These conditions, so simple to name, are not always easy to maintain. Any condition tending to impede the established rate of flow endangers the proper functioning of the flame. For example, a crucible or beaker placed too close to the area of discharge of the mixed gases may slow down the rate of flow t o such an extent that striking back occurs; or, the heating up of the grid through which the gases flow may act as a damper and thus slow down the rate of flow, while wide changes in gas pressure add other complications. I n fact, the conditions for successful operation are so limited that any J . Chem. SOC.(London), 106, Pt.11, 2606 (1914). s Ibid., 116, 1454 (1919). 1
ONSIDERABLE data on the composition of the gases
C
exhausted by automobile engines are available in the literature. Of these the most comprehensive and consistent analyses have been made by Fieldner, Straub, and Jones,l and by Fieldner and Jones.2 The results obtained by these investigators are especially valuable in that, besides being very complete, they cover a wide range of the usual variables, such as fuel-air ratio, speed, grade, engine design, and operating conditions. On account of these facts, the study reported in this paper is based entirely upon the results of Fieldner and his associates. From a consideration of these data the authors have found, first, that the components of the gas exhausted by gasoline engines are in such proportions that the calculated value of the equilibrium constant, K , of the water-gas reaction 1 8
THISJOURNAL, 13, 51 (1921). Ibid., 14, 594 (1922).
INDUSTRIAL AND ENGIhlEERING CHEMISTRY
December, 1925
+
COz H z e CO 4-Hz0 lies within narrow limits, and that these limits are substantially the same for all conditions of mixture ratio and other variables.
25
+--
temperatures above 1000" C. having been extrapolated from Eastman's data by the authors of this paper.4 From the upper curve a value of 3.6 for K in the water-gas equilibrium corresponds to a temperature of about 1500' C., and from the lower curve it represents a temperature of 1250" C. The range of values for K , 3.0 to 4.0, corresponds to temperatures of 1350" to 1550" C. on the upper curve, and of 1150" to 1350' C. on the lower curve. It is not probable that these temperatures represent the maxima reached in the combustion reaction in gasoline engines. Rather, it may be that they designate the positions to which the equilibria are displaced as the gas cools; or, to use Haber's term, the point a t which the equilibrium "freezes." According to Haber6 a water-gas equilibrium in a very hot flame is progressively displaced until the gas has cooled to about 1500", where the equilibrium "freezes." If such a thing as this does occur in the gasoline engine, it happens very quickly, for there the gases are cooled with great rapidity, the time for the adjustment of equilibrium being extremely small. The combustion chamber is inclosed in a highly conductive metal wall cooled by a surrounding jacket of water, which is maintained a t about 80" C. Besides, the burning gas is cooled almost instantaneously by expansion as it drives the receding piston down the cylinder. When the engine is running a t only 1000 r. p. m., for example, which corresponds to car speeds of slightly more than 20 miles per hour, the entire interval during which combustion and expansion, with its consequent cooling, occurs is one two-thousandth part of a minute, or 0.03 second. Although Haber states that by increasing the rapidity of cooling from high temperatures, say from 2000' C., the point a t which "freezing" occurs is changed, he gives no quantitative data on just what may be meant by rapid cooling.
I I
-_I
Figure 1-Graphical
1217
Arrangement of t h e Values of K for Different Tests
The ranges of the principal factors which varied during Fieldner's investigations are given in Table I. The carburetors, for example, were adjusted throughout, the entire range a t which the engines would operate with any degree of satisfaction, so that the amounts of the different products of combustion in the exhaust gas varied from twofold to over fourfold. The other variables were also of considerable magnitude. Table I-Range of Variables Covered Speed: Engine idling, racing, and pulling the car at 5, 6, 10, and 15 miles per hour Grade: Up 3 per cent, down 3 per cent, and level Carburetor adjustment: Maximum range possible Cars used: Two &ton trucks, eleven 7-passenger automobiles, seven 5passenger automobiles, and one roadster -EXHAUST GAS ANALYSESMaximum Minimum Ranee Component Per cent Per cent Per Cent 11.9 6.2 100 to 192 coz 100 to 2.10 co 3.7 7.8 1.5 100 to 450 HI 6.8 15.6 100 to 205 Ha0 7.6 0.2 100 to 2100 0: 4.1
I
, 65t-
I n Table I1 the analyses of the exhaust gases are given, together with the calculated values of the corresponding equilibrium constant - e
For the most part the values for K obtained in this way lie .within a narrow range, from 3.0 to 4.0, with an average value of about 3.6. The variations in the value of K are shown graphically in Figure 1, where the constants have been plotted consecutively. Although from the figure the fluctuations in the magnitude of K appear t o be considerable, amounting to a deviation of about 15 per cent plus or minus from the mean value, yet, as may be seen from Figure 2, in which the values are plotted against the computed percentage completeness of combustion, there is no consistency about the variations. Furthermore, the magnitude of K is not affected uniformly by the speed or the design of the engine, by the fuel-air ratio, or by the grade of the road on which the cars were run. From these data it seems reasonable to COBclude that in the ordinary automotive engine there is a substantial equilibrium of the reaction COz -I- H z e C O
+ HzO
A graph of temperature against K is given in Figure 3. The upper curve is plotted with values determined by Hahn and by Haber and R i ~ h a r d tand , ~ the lower one is based on the recent calculations of Eastman and Evans, values for * Data from Lewis and Randall, "Thermodynamics and the Free Energy of Chemical Substances," 1 9 2 s : Hahn, 2. phrsik. Chcm., 44, 513 (1903); 48, 735 (1904); and Habcr and Richardt. Z. onorg. Chcm.. 38, 5 (1904).
t
e
I
to.
i-
I
I
.
1
I
I
I
I
05
00 50
60
70
90
Figure 2-Graphical Arrangement of K for Various Degree. of Completeness of C o m b u s t i o n
As may be seen from Table 11, the values obtained for K a t all the different engine speeds investigated by Fieldner were similar. The magnitude of K is therefore not affected uniformly by the rate of flow of the scavenged gases in the exhaust pipe. Hence, the equilibrium, of which K is the constant, must be struck at some point in the rapidly changing combustion cycle. 4
J. Am. Chcm. Soc., 46, 900 (1924).
"Thermodynamics of Technical Gas Reactions," English translation by Lamb, 1908, p. 309. 5
INDUSTRIAL AND ENGINEERING CHEMISTRY
1218
The maximum temperatures that occur in the cylinders of gasoline engines are not easily determined, and have never been measured satisfactorily. Not only are the temperatures there very high, but also the time interval during which any given condition persists is only an extremely small fraction of a second. Nevertheless, some endeavors have been made to determine the magnitude of this factor, various methods having been employed by different investigators.8 It is interesting to note that, notwithstanding the uncertainty as to the reliability of the results obtained in such work, the values given by the various investigators, 1500" to 1800" C., are not greatly different from those corresponding to the values of K computed from the composition of exhaust gases. On the basis of these facts it may possibly be concluded that under the usual conditions of operation, at least, a definite gaseous equilibrium is set up in the gasoline engine. Since the heat of reaction of the water-gas reaction is very small-about -10 kg. calories per gram mol., as compared with 96 kg. calories for C 0 2 = COZ-it could not be of much advantage to drive the equilibrium to one side or the other as a means of increasing the total heat available for doing work. Thus, if water or steam were admitted to the engine, there would be a tendency to drive the reaction to the left, provided the water itself were not decomposed, with the consequent production of carbon dioxide and hydrogen. This, in itself, would have no great effect on the completeness of combustion. Some of the water might be decomposed according to the reaction HrO = '/rOz f HI But the use of water to provide oxygen is not an advantage, as the heat of decomposition of the water is much larger than the heat of combustion of carbon monoxide. That is to say, the net result would be a loss of heat, so that the only probable gain would be a possible lessening of the carbon deposit and the slight suppression of the knock that water gives.
+
Vol. 17, No. 12
Table 11-Composition
of E x h a u s t Gases
Completeness of combustiond Ke Percent
01" CO" CH4" Hz" Nn" HiOb Loaded 5-ton truck 10 m. p. h. u p 3 per cent grade Carburetor (Stromberg) adjustment range maximum* 11.6 0.6 3.5 0.5 1.1 82.7 15.4 4.2 82.4 0.4 1.5 16.0 3.4 11.4 3.7 0.6 5.2 1.0 10.4 0.3 1.8 81.3 14.6 4.0 0.7 3.2 79.3 6.6 14.8 3.1 9.9 0.3 4.1 77.6 14.4 3.7 8.4 0.5 8.8 0.6 7.8 9.5 0.8 5.1 76.3 14.2 3.4 0.5 6.7 1.0 6.8 73.1 12.1 12.4 3.3 0.3 Loaded 5-ton truck 10 m. p. h. up 3 per cent grade Carburetor (Zenith) adjustment range maximum* 12.4 1.1 2.0 0.7 0.5 83.3 14.8 4.7 11.9 0.2 4.1 0.7 1.4 81.7 14.8 3.7 9.3 0.6 6.9 0.9 2.9 79.4 15.2 3.9 8.8 0.4 8.4 0.7 3.5 78.2 14.6 3.9 Loaded 5-ton truck 6 m. p. h. up 3 per cent grade Carburetor (Stromberg) adjustment range maximum* 8.8 8.1 0.7 4.6 76.9 13.2 2.7 0.9 76.6 17.6 1.0 5.0 3.0 0.9 8.0 9.5 76.4 14.2 2.9 0.9 8.4 8.1 1.1 5.1 74.9 13.4 3.1 9.4 1.9 5.3 0.7 7.8 74.3 13.0 3.1 1.5 7.1 10.0 1.1 6.0 11.8 4.0 11.1 72.7 2.8 5.8 2.0 5.6 Loaded 54on truck 6 m. p. h. up 3 per cent grade Carburetor (Zenith) adjustment range maximum* 0.3 6.7 0.8 3.0 79.5 15.2 3.5 9.7 14.1 3.5 4.7 76.8 8.1 0.4 9.4 0.6 13.6 3.8 11.1 0.7 5.6 75.1 7.1 0.4 72.4 12.0 3.5 12.8 1 . 0 7 . 2 6.2 0.4 Average results from eleven 5-passenger cars Owners' carburetor adjustment Up 3 per cent grade, down 3 per cent grade, on level Engine idling, racing, and running a t 5, 10, and 15 m. P. h. 3 . 0 - 78.8 13.8 315 -6.9 -0.8 9.1 -1.5 12.8 2.9 3.7 77.8 0.6 7.6 8.9 1.4 79.8 14.0 3.0 0.6 2.6 5.7 10.2 1.1 13.8 3.1 2.6 79.8 9.9 1.5 5.7 0.5 3.1 79.2 14.0 6.5 0.6 3.0 9.8 0.9 3.1 78.8 13.2 6.5 0.9 2.9 9.5 1.4 1 4 . 8 3.8 3.1 79.2 8.6 1.4 7.0 0.7 79.6 13.8 3.0 0.7 2.7 9.5 1.5 6.0 2.9 79.3 13.2 5.6 0 . 8 2 . 8 9.3 2.2 2.8 78.8 12.8 6.3 0.6 3.1 9.3 1.9 3.4 79.0 1 3 . 8 6.7 0 . 6 3 . 0 9.1 1.6 7-passenger cars above** 77.1 11.2 4.1 7.3 78.2 10.6 4.2 8.0 77.9 13.2 3.5 8.5 77.8 13.2 3.3 8.2 75.0 8.8 2.9 7.6 76.8 9.0 3.9 6.4 78.5 11.4 4.7 6.9 78.1 11.4 4.3 6.9 78.8 13.4 3.7 8.2 78.6 13.2 3.7 8.0 77.9 12.0 3.5 8.0 Average results from five light trucks Conditions varied as above** 2.9 76.8 12.4 7.7 1.2 4.0 8.3 2.0 3.4 76.3 11.6 2.1 3.7 6.6 4.2 7.1 2.9 79.1 13.4 9.6 6.2 0.6 3.0 1.6 2.5 77.3 13.4 1.3 4.1 9.0 1.3 7.0 3.0 76.2 12.4 1 . 2 4 . 4 8.5 8.1 1.6 3.4 77.4 12.8 7.1 1.4 3.5 7.5 3.1 3.7 76.1 11.4 2.2 3 . 6 6.5 4.1 7.7 4.4 76.8 13.0 2.2 3.4 6.5 3.6 7.5 3.0 78.0 13.0 7 . 0 1.1 3 . 4 9.0 1.5 3.6 77.1 13.0 1.3 3.8 7.7 2.1 8.0 3.2 76.6 12.4 1.3 4.1 7.4 2.9 7.7 Four-c ylinder roadster 15 m. p. h . up 3 per cent grade Carburetor adjustment range maximum** 83.5 12.6 0.2 0.0 1.7 1.2 13.4 83.5 15.2 1.4 2.0 1.1 0.0 12.0 79.9 14.8 3.9 0 . 8 2.4 0.3 6.4 10.2 73.3 12.0 3.3 1.0 6.4 1.2 11.6 6.5
COP
.... ..
2
fig--
Figure 3-Relation
s
6
between T e m p e r a t u r e and Value of K
Again, in Figure 4, the percentages of carbon burned to carbon monoxide and carbon dioxide, of carbon burned to carbon monoxide alone, and of hydrogen burned to water are plotted against the air present in per cent of that required theoretically for complete combustion of the fuel. These data were taken from runs made on two trucks and a passenger car at different carburetor adjustments and 1 Garner, J. Intl. Petroleum Tech., 7 (1921); Petersen, Z. Ver. dcuf. Ing., 69, 603 (1914); "Gaseous Explosions," Seventh Committee Report, Engincning,.98, 299 (1914).
(a) Values given as per cent composition on dry basis. (a) Values given as per cent composition on dry basis. according t o the formula: Ha0 = 2[- 20.9(Nz) 79.1 (co, o*
-
+
84 83 75 72 66 62 55 87 81 70 67 66 62 60 56 55 47 71 65 60 53
** 70 69 75 75 72 70 70 72 76 72 72 63 70 68 67 65 61 67 67 69 69 68 64 57 73 64 63 63 66 56 67 63 62 95 85 74 56
Calculated
+?)I
Values of K calculated directly from the analyses, since K is a pure number and the units cancel: (Co) ( H O ) (Cod (Hz) (d) Values for per cent comp!eteness of combustion calculated according to the formula a s given by Fieldner, Straub, and Jones: (COz 0.3 CO) (14,540 C) % Complete combustion = (14,540 C 62,000 H) (COa CO CHd + (62,000 H) (14,540 C 62,000 H) (Hn f CHI) (e)
+
(Con
+
+
+ +
+ 2CO + CH4)
where C and H represent per cent carbon and hydrogen, respectively, in the gasoline. footnote 2. *** See See footnote 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1925
speeds. The curve a t the top shows that throughout the range investigated, which covered the extremes a t which the engines would run satisfactorily, over 95 per cent, or practically all, of the carbon burned to carbon monoxide and carbon dioxide. The figure also shows that within the range of operation the curves of carbon monoxide and water have substantially the same slope. This agrees with the modern hydroxylation theory of combustion of hydrocarbons, and further disproves the early belief that the reason for the power curves of gasoline engines reaching their maxima a t about 85 per cent theoretical air was that the hydrogen, with a high heat value, burned in preference to the carbon, with a lower value.
I\
4-
I
B
+ I
/ I
c toto
1219
Present Status of the Facts and Theories of Detonation By G. L. Clark and W. C. Thee L ~ A S S A C H U S E T T SINSTITUTE O F
TECHNOLOGY, CAMBRIDGE, MASS.
An attempt is made to collect recent widely scattered and heretofore unrelated experimental, chemical, physical, and engineering facts bearing upon the general phenomenon of detonation and the factors affecting it, in order to subject theories to critical examination, to suggest further experiments of fundamental nature, and to make practical applications particularly to the design and operation of internal combustion engines. The present unsatisfactory status is ascribed to inexactness in use of terms, to failure to make use of experiments of purely scientific nature, to many diametrically opposite results by different investigators, to the need of more strictly quantitative data, and to the inherent great complexity of the subject, involving, as it does, such factors as oxidation, combustion, flame propagation, esplosions, shock, gaseous ionization, gaseous catalysis,environmental variables of size, shape, composition, texture, temperature, methods of ignition, etc.
NOWLEDGE now available does not permit an exact scientific definition of detonation or of the various terms used to explain the phenomenon known as detonation. To gain a clear understanding of the control of detonation we must know where combustion ceases and explosion begins, for the control of detonation really depends fundamentally on these two factors. The prevention of detonation in practice belongs to the problem of burning all the constituents of a fuel before the secondary gases can be formed, hence to the fundamental problems dealing with combustion and explosion. Many important laboratory experiments have not heretofore been recognized or applied to practical engineering purposes. Different investigators at different times and with various t,ypes of apparatus have noted that each of the following factors are involved in combustion, explosion, and shock:
=lm0 K
0
d& P e m & k d B o ~ ~
Figure:4--Relation b e t w e e n Carbon a n d Hydrogen Burned, a n d Percentage of Air Present i n t h e Mixture
Summary
A correlation of the results of fifty-eight analyses of the gases exhausted by gasoline engines (using the data of Fieldner and his associates) shows that: 1-Four of the components are in such proportions that the calculated value of the equilibrium constant K of the watergas reaction COz f H z e CO Hz0 lies for the most part within narrow limits-namely, 3.0 to 4.0-and that these limits are substantially the same for all conditions of mixture ratio and other variables. 2-The magnitude of K obtained in this way corresponds to equilibrium conditions that prevail at temperatures of 1350" to 1550' C. Investigators who have attempted the actual measurement of the maximum temperatures reached in the combustion reaction in gasoline engines have obtained values of the same order-namely, 1500' to 1800' C. 3-The composition of the exhaust from gasoline engines at different fuel-air ratios confirms the hydroxylation theory of combustion of hydrocarbons and further disproves the old belief that the reason for the maxima of the power curves of gasoline engines occurring at about 85 per cent theoretical airlay in a preferential burning of hydrogen.
+
Fix Duty on Ferrocyanides-In connection with the recent decision of the Treasury Department on the classification of calcium ferrocyanide, Assistant Secretary Andrews announced : The Department deems it proper to state that only the compounds enumerated in T. D. 41,120, which have been previously admitted free of duty under paragraph 1565 of the tariff act, were intended to be embraced in the decision, and therefore, potassium ferrocyanide and sodium ferrocyanide are properly dutiable under paragraphs 80 and 83.
(1) Closed and open end tubes
Tube diameter Tube texture Tube composition Change in tube diameter Tube length (7) Chemical composition of fuel (8) Composition of gaseous mixture
(2) (3) (4) (5) (6)
(9) Composition of secondary gas (10) Turbulence (11) Gaseous ionization and excitation (spectroscopy) (12) Method of ignition (13) Temperature of ignition (14) Compression (15) Dilution (16) Pressure
Experimental Evidence
(1) Open and Closed End
Tubes'6,'9,3~"~d,43,4~~,47.49.~Z,~(c,4,~9,*
The terms used when speaking of closed tubes are often confused with terms used when speaking of open end tubes. I n closed tubes the propagation of flame occurs in three distinct periods when fired from an open end to a closed end of a tube. I n the first period the flame travels with a constant and uniform velocity. The second period consists of a vibratory movement and a sudden increase in velocity. I n the third period the rate again falls to a uniform value as the flame approaches the closed end of the tube. I n open end tubes the propagation of flame occurs in four distinct periods when fired from the closed end to the open end tube. From the experiments performed by L ~ i f f i t t e , ~ ~ which will be considered later, it is evident that in the first period the flame travels with a n accelerating velocity which
* Numbers refer to bibliography at end of article.
