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 B u r n e d , a n d P e r c e n t a g e of Air P r e s e n t 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 i t proper t o 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 t o 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
(2) (3) (4) (5) (6)
Tube diameter Tube texture Tube composition Change in tube diameter Tube length (7) Chemical composition of fuel (8) Composition of gaseous mixture
(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.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1220
is characteristic of combustion. I n the second period the flame travels with a uniform velocity which is characteristic of a n explosion. I n the third period combustion is again produced by the formation of secondary gases, which burn with an accelerating velocity until explosion occurs, which is the fourth period, when the flame again travels with a uniform velocity, but at this time with such a rapid rate that the action is called detonation. (2) Tube Diameter2,9,30a,37b,c,40,46a,b The effect of tube diameter on combustion, as shown by photographic results obtained by Laffitte,305is that the length of combustion increases with the diameter of the tube. His results show that there is not much change in the lengths of the combustion waves until the diameter of the tube is greater than 25 111111. After this critical diameter has been reached the length of combustion increases very rapidly as the diameter of the tube increases. These experiments extended to tubes not over 56 mm. in diameter. I n the 25-mm. tube the combustion of a mixture of CSZ 302 passed into a n explosion after traveling only 58 cm., while in a 43-mm. diameter tube the combustion wave traveled 1.03 meters before explosion set in. The photographs made on films on a moving drum showed clearly that a definite line of demarcation exists between combustion and explosion, and that the speed of flame propagation is greatest in largest tubes. The portion representing combustion, represented by a curved line, showed that the flame accelerates with a regularly increasing velocity as i t travels along the tube but as soon as explosion sets in the wave is propagated a t a uniform velocity represented by a straight, sloping line. A variation of the chemical mixture from the theoretical of 1 per cent slows up establishment of the explosive wave. (3) Tube Textureso' The data in Table I were obtained during the course of the preceding experiments with sand sprinkled on the inside of a tube 34-mm. in diameter.
+
Table I-Effect
of Tube Texture Length of combustion before explosion wave begins Cm.
Diameter of tube Mm.
7
48 50 58 84 with sand 45 103 131
10
24 34 43 53
This shows that the smoothness of the walls of the containing vessel affects the rate of flame propagation of a combustible fuel and hastens the formation of the explosion wave. The development of turbulence depends also upon the smoothness of the walls, the turbulent motion of the gases being a valuable aid to the spreading of the ignition. (4) Tube Cornpositi~n~~'~.~~,~~ The effect of material on propagation of flame is shown by Table 11, which was taken from a paper by Parker and Rhead.43 Table 11-Effect of Material on Propaaation of Flame --IN~IAL VELOCITY,CM&EC.--
Methane in air mixture Per cent
Glass
2.65 cm.
Lead
2.64 cm.
Copper 2.3 cm.
Iron
2.72 cm.
The conclusion is drawn that the velocities of flame in tubes of the same diameter vary with the heat conductivity
Vol. 17, No. 12
of the material of the tube; the greater the conductivity for heat, the smaller the velocity of the flame. The velocity of the detonation wave in any given gaseous mixture is independent of the material composing the tube.'6 (5) Change in Tube Diarneter1~9~'6JOd Laffitte30dfound that the length of the period of combustion depends upon the mixture used or the chemical composition and the ratio of the diameters of the two tubes. His photographs show that the explosive wave set up in the first tube becomes again combustion upon entering the second larger tube. This combustion continues for a length depending upon the diameter and then for a second time resumes explosive wave properties. With tubes of variable diameters Table I11 shows, for a mixture of CSZ 302, that the length of combustion increases with the diameter of the second tube. The diameter of the first was the same in all the experiments-i. e., 7 mm.
+
Table 111-Variation of Combustion with Diameter of Second Tube Diameter of Length of combustion, second tube Mm.
second tube Cm. S 10
13 16 24 33 44
15 50 100
With a diameter of 24 mm. for the second tube, the period l/ZO2 as when of combustion is the same in a mixture of Hz the mixture is CSZ 302. If the diameter of the second tube is 34 mm., the length of the combustion is 62 cm. with a mixture of H2 ' / 2 0 2 and 50 cm. with a mixture of CSz 3oz,thus showing (1) the constitutional effect of chemical reactions, and (2) the critical size of tube below which period of combustion is independent of the oxidizing reaction. The following similar conclusions were drawn from experiments by Campbel1:s
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+
+
+
(a) An explosion flame is suddenly retarded at any point where a sudden expansion of hot gases may occur. In some gaseous mixtures, this flame moves then at a fairly constant velocity for some distance; in others, i t continues t o be retarded. Probably i t finally gains speed and reproduces the detonation wave. The retardation of a detonation wave as i t passes through a flexible joint between two rigid tubes as recorded by DixonlSb is probably due to a sudden expansion at the junction. Where the joint is smooth and rigid and the tubes of equal diameter, no expansion occurs, and therefore no retardation takes place. ( b ) The rates of the retarded flame and of the preceding pressure-wave are dependent on the gaseous mixture, the relative sizes of the different portions of the tube, and possibly the initial pressure. The greater the difference between the diameters of the two portions of the containing tube, the less the velocities of the retarded flame and of the pressure-wave in any one mixture. These velocities reach a lower limit in a very wide tube.
( 6 ) Tube Length19 Figure 1 shows that the flame starts slowly from the spark with regularly increasing velocity. The velocity at any given point in its path varies with the length of the "arm" of the tube. (The propagation of the flame in various glass cylinders was photographed on a vertically moving film.) The distance-speed curves with the affixed numbers show the length in centimeters of the progressively shortened arm of the tube along which the flame was traveling from the spark. With central ignition the propagation is symmetrical, diverging flame fronts approaching opposite ends of the tube, each giving a photographic analysis which is the mirror image of that of the other. Asymmetry is obtained, however, even with central ignition, if the spark gap does not mark an exact balance of volume as well as length. If ignition is not central the flame front approaching the nearer end will be slower than the other, and this is true
December, 1929
COMBUSTIBLE Gas Glass DiDet .. co 15.75-68.40 Hz 9.46-64.52 CHI 6.12-13.60 CIHZ 2.68C*Hd 3.52C2H;OH 3 .Oi-12.10 CB 2.11-31.70 CHvCHO 3.59- 9 . 6 3 (CPH5)Zo 2.26- 6 . 9 0 CeHi 1.87- 5 . 9 5
INDUSTRIAL AND ENGINEERING CHEIMISTRY Glass globe 27 mm. 15.40-71.60 9.42-65.90 5.82-13.60 2.393.342.78-12.30 2.22-31.20 4.03- 8 . 5 3 2.38- 6 . 5 1 1.78- 6 . 2 0
Table IV--Explosive L i m i t s of C o m b u s t i b l e Gases (FIGURESIN PBR CENT) Glass globe 18 mm. Iron vessel 1000 c. 2000 c. 16.00-66.50 15.80-63.80 14.05-69.60 13,80-76.55 9.88-65.12 10.79-59.75 4.27-67.53 8.98-72.23 6.18-13.21 6.25-12.83 6.02-13.90 5.91-14.05 2.933.121.951.953 423.693.223.4& 2.91-11.60 2.78-12.10 2.68-12.60 2.47-12.60 2.69-30.20 3.38-29.20 1.35-33.10 Burns 3,65- 8 . 9 2 3.68- 8 . 6 3 3.30-10.13 2.98-10.18 2.44- 5.61 2.34- 6 . 1 5 1.971.631.97- 4 . 7 0 1.61- 6 . 2 0 1.57- 6 . 3 1 1.58- 6.36
1221
300’ C. 14.10-75.60 8.62-79.12 5.80-14.10 2.41-
a . Xa-
oxidation
.......
-- -9.65 Oxidation 1.71- 6 . 3 7
Strong ienition 15.80-84.90 8.50-81.50 5.35-14.68 2.242 8.3-
5.57-12.60 1.55-34.90
... ....
1.56- 7 . 4 9 1.32- 6 . 5 8
of every point in the flame path in reference to which the wave is developed sooner in the former. With the 33 per cent length of the arm of the tube is less than the minimum dis- mixture the uniform movement extended across the whole width of the photograph, showing that a rich mixture detance. more than a diluted mixture. With a mixture con(7) Chemical Composition of F ~ e / 2 , 3 , 1 7 , 3 2 a , 3 6 , 3 9 , 4 3 , 4 4 ~ ~ ~ , 4 ~ ~ , ~ tonates ~~,~74 taining 75.2 and 66.6 per cent of hydrogen, respectively, the Detonation may be controlled by retarding the rate of effects are similar in general character to those obtained with combustion of a fuel by adding chemicals to the mixture, the two corresponding mixtures of methane and oxygen. which serve to increase the specific heat and prevent excessive Table IV, from the results of Berl and Fischer,2 shows temperatures being produced. the lower and upper limit explosive mixtures of combustible Excessive temperatures cause thermal decomposition of gas and air as they depend upon various experimental condithe fuel, and a selective burning of the constituents develops tions of size, shape, and material of vessel, temperature, a high density in the gases ahead of the flame front and and ignition. prevents complete combustion of the fuel. This characterBerl and Fischer also found that intermediate products istic of the fuel depends on its formed during the explosion of rich mixtures were formaldestability or atomic arrangement. hyde, acetaldehyde, and acetic acid from ether-air mixtures, The paraffins, being the least acrolein from acetone and ethylene, and acetaldehyde and stable of our fuels, will detonate acetic acid from alcohol. with lower compression ratios. Table V, taken from results obtained by Wheeler,66f proAfter the paraffins in order of vides information on mixtures in which speed of uniform stability come the olefins, naph- movement of flame is most rapid, mixtures required for comthenes, aromatics, and alcohols, plete combustion, and mixtures most readily ignited by the then toluene, xylene, and ben- secondary discharge. zene. Wheeler66r showed that a Table V-Mixtures of t h e Paraffins w i t h Air Mixture in factor that might perhaps inwhich speed Mixture fluence the igniting power of a of uniform most readily movement Mixture ignited by LIMITS OB secondary discharge is the inINFLAMMABILITY of flame for complete secondary is fastest combustion discharge Upper sulating power of the medium COMBUSTIBLE Lower Per cent Per cent Per cent GAS Per cent Per cent through which it passes. A Methane 5.60 14.80 9.65 9.45 8.3 3.10 10.70 6.05 5.64 6.7 measure of the insulating power Ethane n-Propane 2.17 7.35 4.45 4.02 5.1 is given by the spark lengths ?+Butane 1.65 5.70 3.65 3.13 4.2 n-Pentane 1 . 3 5 4 . 5 0 2 . 9 0 2 . 5 6 4 .0 under standard conditions in %Hexane ... ... .. 2.15 3.2 the pure gases at 138” C. From these results it appears that inThe mixture with air in which the speed of uniform movement crease of molecular weight in the of flame is fastest contains an excess of combustible gas; the series of paraffins is attended by values for the mixtures most readily ignited by the secondary discharge are in general conformity with the suggestion that the increase in insulating power such initial speed of flame in a mixture, away from a point source as would cause the voltage of of heat of momentary duration, is the dominant factor in desecondary discharge obtained on termining what the intensity of such a source of heat must be breaking a given current in the to cause the general ignition of the mixture.5B/ primary circuit to attain higher I n this connection mention must be made of the applicapeak values a t a given spark gap as the series is ascended, tion of the beautiful fundamental researches on gaseous but though the increase is fairly reactions of Lind, utilizing alpha rays from Ra-C. Mason CM, FROM SPARK regular, no explanation is offered and Wheeler3’0 show that the rate of reaction between a Figure 1-Effect Lofi!Tube of the fact that most readily combustible gas and oxygen increases with the concentration L e n g t h on R a t e a t W h i c h F l a m e Travels ignited mixtures of any of the of whichever has the greater “stopping power” for radiant paraffins with air are those of energy such as will “activate” it, and that this stopping propane. Spark lengths in the paraffins are: methane 31, power depends on the density of the gas just as does the ethane 25, propane 20, n-butane 16, n-pentane 14, isopen- stopping power for alpha rays. With methane the lowest ignition temperatures are obtained when there is excess of tane 10. (8)Composition of Gaseous Mixture2.8,17,26,32b,3~.3~d,e,~,4a.44c, oxygen. With the other hydrocarbons there is a diminution in ignition temperatures as the proportion of combustible 455,63,561 gas is increased. This peculiar behavior of methane may From experiments by Parker and Rhead43 the effect of be compared with the stopping powers relative to air as the composition of gaseous mixtures with closed tubes on follows: oxygen 1.067, methane 0.860, ethane 1.519, pentane combustion and explosion showed that the uniform move- 3.544. ment of flame is slower in a mixture containing 40 per cent A very excellent study of the detonation limits of gaseous methane than in one containing 33 per cent. The detonation mixtures has recently been made by Wendlandt.53
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1222
(9, Composition of Secondary Gas The effect of the composition of the secondary gas in the propagation of the shock wave is characterized by experiments performed by Lc~ffitte.~O~ A film was placed around a glass tube by means of a photographic cylinder that rotated past a slit with a linear speed of displacement of 40 to 45 meters per second. A solid explosive was ignited a t the closed end of the tube and a piece of black paper was used to close up the tube a t the extremity of the dynamite. The results showed that the pressure developed in the space occupied by the explosive moved with a constant velocity, while the pressure in the first part of space which the flame entered after breaking the paper increased its acceleration and then decreased after entering the last part of the tube. The lengths through which the shock waves passed when propagated in air were found to be longer with tubes of larger diameter. Experimental data obtained with a 5-mm. diameter tube were as follows: Explosion Shock Shock
2750 m./sec. 3500 m./sec. for 10 cm. 2500 m./sec. for 15 cm.
I n hydrogen sulfide the shock existed only for a very minute time, as it was absorbed by the decomposition of hydrogen sulfide, whereas in a vacuum the shock existed for a still shorter time than with hydrogen sulfide, showing that the shock wave cannot propagate itself in a vacuum. (10)
Turb~~~e1e~23,30o,66a.b,c,69
It will be observed from Table I that, with the inside of the tube of 34 mm. diameter, sprinkled with sand, the length of combustion before explosion begins is 45 cm. as against 84 cm. for a smooth clean tube. Because of the effect of turbulence, the turbulent motion of the gases being a valuable aid to the rapid spreading of the ignition, the sooner those portions of the gas that are in motion become ignited the more rapidly will the flame spread through the entire charge. I n internal combustion motors a t high speeds the explosion intervals-the interval between the moment of spark occurring and the attainment of maximum pressure-is about 1/2m of a second, while the complete combustion period is about l / ~ m of a second. Here turbulence is important, for it greatly decreases the time for the attainment of maximum pressure by speeding up the process of combustion by the mechanical distribution of flame. The combustion should be completed in one-sixth of a revolution and before explosion sets in. If this can be accomplished, complete combustion of all the fuel will result-i. e., it will burn all the carbon as well as hydrogen if sufEicient oxygen is supplied. The influence of a turbulent charge in securing rapid ignition of the charge and more nearly complete combustion should be taken into account in the design of the cylinder head and inlet valve opening. Figure 2 shows the speeds of propagation of flame calculated from time intervals between ignition and the attainment of maximum pressure for each mixture plotted against percentages of ethane. The conclusion that a mixture in which normally the speed of flame is slow should be affected by turbulence to a greater extent than one in which normally the speed of flame is rapid is thus proved experimentally. Strong agitation of a mixture poor in combustible gas renders it difficult to ignite, or, to be precise, renders it difficult for the flame that no doubt occurs during the passage of the discharge to spread away therefrom and travel throughout the mixture. This difficulty increases as the degree of
Vol. 17, No. 12
agitation is increased and as the percentage of combustible gas is decreased. When, however, the flame in such an agitated mixture does manage to spread away from the source of ignition, it travels rapidly.5v (11 ) Gaseous Ionization and Excitation (Spectroscopy)
24126134,54166
Laffitte3W has studied the spectra of explosions, particularly for the mixture CS2 302. His results show that combustion, expIosion, and detonation all produce continuous spectra upon which sodium and calcium lines from the material of the walls are superimposed in the cases of combustion and shock, but are absent during explosion. This shows that there is some relation between combustion and shock that does not pertain to explosion.
+
600
2 2
50C
4oc
Y
2 k5
300
sG
i
e 2oo
b
9
100
*1
0
In
‘4 ETHANE Figure 2-Effect of Turbulence on the Speed of Propagation of Flame
The value of this work is doubtful, since it presents an integrated effect over a considerable period. The authors of this review are now working on an experiment along these lines in an actual engine cylinder with synchronous shutters by means of which the state of excitation a t any definite stage may be ascertained. (See our forthcoming paper on ultra-violet spectra in the .Journal of the Society of Automotive Engineers, Vol. 17, 1925.) The ionization theory is considered in another paper in this symposium by Clark, Brugmann, and Thee. (19) Methods of Interesting studies of ignition by arcs, sparks, and by heated surfaces have been made by Wheeler56 and Mason and Wheeler.37 Methane stands apart in both cases inasmuch as the mixture with air that requires the lowest “igniting current,” hence ignition temperature, contains a n excess of oxygen. Distinction is made between ignitibility by a momentary source of heat which is related to the speed of propagation of flames, particularly the initial speed, the ignition temperatures, and the insulating power. Thus, n- and iso-pentanes have the same ignitibility but different insulating powers (spark lengths). It is clearly evident from these experiments that a certain minimum volume of gas must be burned before flame can be propagated. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
December, 1925
vast difference in the effects of initiating explosions between sparks and incandescent wires is shown in Table X. I n connection with the ignition of the charge, in the practical case of an engine, an important consideration is the time taken for the flame to spread throughout the entire charge after being ignited by the spark. The time taken for complete combustion depends upon the distance between the plug and the most remote portion of the charge. The whole charge should become ignited before the piston has traveled far on the firing stroke, and therefore the distance should not be great between the plug and the most distant portion of the charge. Ellis and Wheelerz2 have recently obtained experimental proof of the correctness of the assumptions regarding the manner in which flame spreads from the center of a closed spherical vessel, by means of apparatus developed by Ellis and Robinson.*O They secured results which showed that, if the ignition of the mixture is exactly a t the center of a sphere, the propagation of flame follows regular concentric spherical surfaces in such a manner that the boundary is reached simultaneously a t all points. They also showed experimentally that the speed a t which flame traveled from the center to the walls of the spheres was uniform, and that cooling by the walls of the vessel, and consequent retardation of the flame, can affect, not only the maximum pressure that a given mixture can produce when it is inflamed, but also the rate of development of pressure; so that when attempting to interpret the character of time-pressure curves obtained from gaseous explosions in closed vessels due consideration must be paid to the shape of the vessels and to the position of the point of ignition. (1s) Temperature of
Ignition2,6.811B,3a,37.41,60,66,67d,58,69,61
Mason and WheeleF performed experiments on the ignition of gases by exposure to heated surfaces of known temperature. They found that the ignition temperatures of mixtures depend on (1) the character of heated surface, and (2) the ratio of area of heated surface to velocity of mixture. The relative ignition temperatures of different mixtures of the paraffin series obtained by heated surfaces were interpolated from curves produced by Mason and Wheeler,37 where relative ignition temperatures were plotted against percentage of combustible gas. (Table VI) Table VI-Relative COMBUSTIBLE
GAS Methane Ethane Propane Butane Pentane
5 3. 692 590 565 545 537
Ignition Temperatures
3.
6 9
%
585 548 517 492 480
540 515 488 475
8.
680 560 535 512 500
590
It will be observed that the relative ignition temperature is less, with the same percentage of combustible gas, as the series is ascended. Another method of measuring relative ignitibilities is by measurement of the duration of the pre-flame period when the reaction vessel is maintained at a temperature higher than the ignition temperatures. The results of Taffanel and Le FlochS0show the lag of ignition in seconds for methane-air mixtures. (Table VII) Table VII-La$ Methane in air Per cent 10.0 12.0
700° C. Sec.
... ...
of Ignition for Methane-Air Mixtures TEMPERATURES 8000 c. Sec.
.
9000 c.
Sec.
10000 c. Sec.
1.12 1.;
This method has also been used by Mason and Wheeler3’ to compare different hydrocarbons.
1223
The influence of increased initial temperature is to increase the speed of propagation of the “uniform movement” of flame through a mixture.86 Woodbury, Lewis, and Canby, 69 firing acetyleneair mixtures, state that increased temperature produced little effect upon the rate of flame propagation. With this Brown, Leslie, and HunnG seem to agree. White67d has made a very thorough investigation of the effect of temperature of ignition, compared the results with thirty previous investigations, and found very little effect of the initial temperature. Berl and FischerZ show that a n increase in temperature reduces the lower explosion limits of gas-air mixtures and raises the upper limits. Thus, carbon disulfide has limits of 2.11 and 31.70 per cent at 20” and 1.35 and 33.10 per cent at 100’ C. Preignition occurs a t 200” C. (Table IV) (14) Compres~ion~~~~7
The detonation tendency of the fuel is the limiting factor in the efficiency with which it can be burned and the power developed-i. e., detonation limits the compression a t which an engine can be run. Therefore, to secure the benefits of greater expansion, the detonation that results when compression pressures are high must be eliminated. The manner in which the power developed varies with the compression ratio and the effect of compression on temperature of the cylinder barrel are shown by Table VIII. Table VIII-Effect Compression ratio 4.6 5.0 5.4 5.8 6.2 6.4
Brake M. E. P. 116.2 119.3 122.0 125.0 129.0 123.0
(16)
of Compression
Gasoline consumption per b. h. p./hour Pounds 0.530 0.507 0.490 0.475 0.480 0.520
MEANTEMPERATURE OF CYLINDER BARREL Bottom
a.,q 180 170 157 154 183 212
F.
105 95 89 85 110 135
D~~ut~on7,10,11,1?,13,15,18,?1,28,29,44d,46~
The effect of adding either oxygen or nitrogen to electrolytic gas, 2H2 0 2 , was shown by P a ~ m a to n ~reduce ~ ~ the rate of detonation in the resulting mixture. The explosion wave differs from the uniform m o v e m e n t 4 e., “the combustion period”-in that with the former the addition of 0 2 , and of methane to the hydrogen to the mixture 2H2 mixture CH4 202, increases the rate of detonation, whereas the speed of the uniform movement is decreased in both mixtures.’& Other experiments on addition of other gases are discussed in the preceding section on ignition temperatures. Ellis and Stubbsz1 studied the effect of nitrogen dilution in inflammation from a closed end tube. The speeds are directly proportional to the thermal conductivities of the mixtures. (16) p~ess~re2,6.13,14,19,37.42,46,46~,49.~7,~9~~0
+
+
+
Here must be distinguished the actual pressure of the gas which is ignited, and the pressure developed upon the production of a flame. Mason and Wheeler3’ have shown in experiments on the lag in ignition that pressure is developed only when flame is actually produced and propagated. Woodbury, Lewis, and Canbyb9 state that with acetylene and air mixtures under pressures of from 1 to 4 atmospheres the rate of propagation increases with an increase in the initial pressure up to a critical density, and that with any further increase in the pressure above that critical value no increase in the rate of propagation is obtained. When a mixture is fired under great initial pressure a mass of gas through which the flame has already passed becomes reignited, to a small extent, by a subsequent spark.’@ Ellislg states that the maximum speed pressure should be one atmosphere.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1224
The data in Table IX from experiments performed by Dixon” on the velocity of the explosive wave show that it is propagated a t a slightly increased rate with increased initial pressure, but a maximum rate seems to be reached soon. of Propagation of Explosion Wave at Different Pressures PRESSURS 2Hz 0 2 CzH4 -i-202 500 mm. (meters/second) 2775 2280 19.685 inches (miles/second) 1.724 1.417 760 mm. (meters/second) 2821 2322 29.921 inches (miles/second) 1.758 1.443 1000 mm. (meters/second) 2850 2319 39.370 inches (miles/second) 1.771 1.497 1500 mm. (meters/second) 2872 59.055 inches (miles/second) 1.786
T a b l e IX-Rate
+
.... .,..
These data show that if the fuel undergoes thermal decomposition during the explosion period, and the hydrogen unites with the oxygen independently of the carbon, the rate of flame propagation of the hydrogen-oxygen mixtures keeps accelerating until the velocity characteristic of the detonation wave is reached. Fuels which are unstable and break down easily at high temperature will therefore detonate easily. The great disadvantage of this kind of fuel is that it leaves too much carbon behind which does not have time to unite with the oxygen. Stevens, 19 in experiments with a constant pressure bomb on kinetics of explosive gaseous reactions, shows that the increase of pressure and density ahead of the flame is negligible until the velocity of the flame approaches that of sound, and that the more complete description of equilibrium conditions of explosive reactions, as indicated by the mass law, has found wide application. He states that the velocity of reaction is gravely affected by pressure. The velocity varies as the 02 reaction. square of the pressure for 2CO If the pressure at the region of reaction is not one, but p atmosphere, then the rate of the explosion reaction
+
v = K(IA]nl[B]nzlC]ns. . . ) f i n where n is the molecular number of the equation, and A , B, and C are the products of reaction. If the pressure, p , is more than one atmosphere, n must correspond to this change in pressure. Table X, from the work of deHemptinne,14 shows for the explosion of gases with oxygen (1) the minimum pressure necessary for explosion initiated by electric sparks; (2) by incandescent wires; (3) heat of formation; (4) heat of combustion; (5) temperature of explosion O C.; (6) speed of explosive wave, meters per second; (7) speed of combustion, centimeters per second. Table COMBUSTIBLE GAS (1) 35 Hydrogen 58 Carbon monoxide 145 Methanol 40 Ethyl alcohol 35 Ether Benzene 25 Acetylene 15 Carbon disulfide 12
X-Explosion Characteristics (2) (3) (4) (5) 58,000 530-606 192 68,000 650-730 145 26’,300 ... 145 48,800 182,200 125 53,000 340,500 125 105 45 14
56,500
- 17,100 - 53,200 - 28,700
659,600 210-240
77,600
310,100 265,100
isi-is5
190-210
(7) 5780
(6) 2831
1089 2831
2773
2840 2622 2482 1953
6700 8480 8305 8942 9700 10,070 8200
General Interpretation of Experimental Factors
The fact that there are sixteen or more variable factors which determine and influence det,onation explains why a clear and fundamental explanation of the phenomenon has not been made. The term is widely used in the literature interchangeably with explosion, flame propagation, preignition, etc. It is obvious that the control of detonation, which is objectionable in the operation of internal combustion engines, is dependent upon an understanding of detonation itself. Detonation may be defined as the last phase of the existence of a flame, or of the reaction between certain combustible
Vol. 17, No. 12
gases and oxygen which is so rapid as to become luminous. The characteristics of a detonation wave are: (a) Great velocity of movement, equal to that of sound. Lupuss3 points out the misuse of the term “velocity of detonation.” This does not refer to the rate of propagation but to the time elapsing between the initiation of the explosion and the attainment of the maximum gas volume or pressure. ( b ) Intense luminosity (white in contrast with blue color of perfect combustion) (c) Sharp beginning of luminosity. ( d ) Constant velocity of movement. (e) Production of “knock” in internal combustion engines by metallic impact, by spontaneous ignition, or by high pressure differences as detonation wave forms and passes. (f) Accompanied by another wave sent back in oppositq‘direction through the partly burned gas, called by Dixon retonation wave.” If started near the end of a closed tube, the detonation curve may be reflected and add itself to the “retonation wave.” (g) Pressure in wave front may be of magnitude of 1000 pounds per square inch.
From the experiments on the factors concerned in detonation, it appears that the control of detonation hinges upon the rate a t which the fuel burns or the control of the initial period-i. e., combustion. Combustion is the initial stage of flame propagation while the fuel burns with an increasing or accelerating rate. This is followed by the second period or explosion, which propagates the flame with a constant velocity and a t a faster rate. As long as combustion exists essentially all of the fuel will be burned and no carbon deposits will remain in the containing vessel. I n order to secure this ideal operation, all the factors listed in the preceding part of this paper must be considered. I n general, desirable points of design and operation of internal combustion engines, as shown from the standpoint of prevention of detonation by the experimental study of factors, may be briefly enumerated as follows: (1)High compression aids in rapid combustion. Experimental results show that the practical limiting value seems to be about 6 for maximum power, minimum gasoline consumption, and minimum wall temperature. (2) Smooth combustion chambers will not disturb the turbulent effect of mixture. (3) Compact combustion chambers, nearly spherical, will enable combustion of the maximum proportion of the fuel supplied, and not permit the flame to travel far. Hence tubes and parallel surfaces should be avoided. The critical size and shapes are determined by so many factors that present data are insufficient to specify them. (4)A superior orifice coefficiency of the valves determines the maximum turbulent effect ( 5 ) The proper location of the spark plug near the center of the gaseous mixture so as to limit the flame travel is a very important consideration. (6) Temperature of the cooling water and of gas and air mixture before entering the combustion chamber must be kept reasonably low. (7) Conductivity of the metal of the combustion chamber and cvlinder walls should be highest attainable by the metals which- i t is permissible to use. (8) The proper mixture for surest control of complete combustion and thorough vaporization of the fuel are essential, and may be quantitatively ascertained. (9) The stability and ignition temperatures of the fuel must be taken into consideration, the remaining unburnt fuel of an ignited charge should not crack or decompose and cause selective burning of the constituents, and the ignition temperature should be higher than the existing temperature of the containing vessel so that the charge will be ignited by the proper means and a t the proper time. Theories of Detonation
DISSOCIATION THEoRY-one of the first theories proposed was that of dissociation of the products of combustion. The amount of dissociation of carbon dioxide and of water at the temperature and pressure reached in the ordinary internal combustion engine is small, probably not more than 5 per cent.
December, 1925
IND UXTRIAL AND ENGINEERING CHEMISTRY
Furthermore, while this theory might account for some of the phenomena that take place, it cannot very well account for most of the knock in the cylinder, nor the vibration in the cylinder head. The dissociation of these compounds, while producing a slight increase in volume of the gases, does not proceed with any great violence. As the temperature and pressure are lowered by the piston moving on the expansion stroke, these dissociated elements reunite, but scarcely with the violence often attributed to them. THEORY PROPOSED BY RICARDO-A theory which satisfies most all the conditions affecting detonation was proposed by Ricardo. According to this theory when combustion first takes place it produces a rise in pressure and temperature in the cylinder. This rise takes place before all of the fuel is consumed. Just as soon as this temperature gets above the flash point of the fuel, the whole remaining mass burns almost instantaneously. This produces a very rapid increase in pressure which may be sufficient to produce the characteristic knock. However, this theory assumes that the fuel has an inclination to pre-ignite if the .proper conditions are present. Some fuels which detonate very badly do not easily pre-ignite, while some that pre-ignite very easily show no tendency to detonate. Carbon disulfide will pre-ignite very easily but will not detonate. Kerosene detonates very badly but shows no great tendency to pre-ignite. DETONATION WAVE TREoRY-This theory explains the detonation knock by assuming that when ignition takes place the burning spreads out in all directions from the ignition point. While the travel is comparatively slow at first, it increases in velocity, because of rise in temperature during initial reaction, and an acceleration begins which continues until a so-called detonation velocity is reached, accompanied by increase in pressure when expansion is prevented to the point of a sudden compression wave. This very high velocity, combined with the mass of the gas in motion, produces the impact on the cylinder walls necessary to give the characteristic knock. This last theory accounts for the fact that T-head or L-head cylinders are more liable to produce detonation than an Ihead type. I n the latter the distance through which the combustion proceeds is shorter than in the former types, so that there is not such an opportunity for a very high velocity to be attained. ELECTRON THEORY-This theory assumed that the flame is propagated by an electron front. According to this theory, tetraethyl lead, a knock suppressor, was believed to absorb the electrons and thereby reduce the rate of flame propagation. Recent experiments by the writers, however, presented in another paper in this symposium, have shown that this theory cannot satisfactorily account for the facts. FREEHYDROGEN Tmom-There is the old theory that the knock is produced by the excessively rapid combination of free hydrogen. Gasoline, of course, contains no free hydrogen as i t enters the carburetor. However, certain constituents of gasoline, when heated in the presence of air, decompose and form different hydrocarbon compounds, as well as liberate free hydrogen and carbon (octanes and nonanes are especially apt to do this). One of the new compounds formed may be acetylene. If acetylene is actually produced it will account for the knock. It has been demonstrated that allowing a small amount of acetylene to enter the cylinder along with the gasoline will produce a detonation knock. The principal reason for believing that free hydrogen is the basis of the detonating difficulties is that addition of any substance that will combine readily with hydrogen and at the same rate vaporize under the same conditions as will gasoline will eliminate the knock. Iodine will meet all these requirements and actually does stop detonation when about
1225
2 per cent is added to the gasoline. The explanation of the production of a knock by either hydrogen or acetylene lies in the very rapid rate of combustion of these two as compared to gasoline-air mixtures. (The rate of flame propagation for hydrogen is about ten times as rapid as that for gasolineair mixtures, while that for acetylene is about six times as rapid as for gasoline-air mixtures.) Not only does the small amount of hydrogen or acetylene burn with this high velocity, but it accelerates the rate for all other gases present. I n fact, all the gases present burn a t a rate approaching closely that of the one having the highest rate. RADIATIONTHEORY-Some writers maintain that detonation results when a portion of the mass of mixture is activated by radiations sent ahead by a flame. It will be recalled that radiant heat may pass directly through a body without heating it. A wave of radiant heat may pass through part of the mixture without raising its temperature. The denser the mixture vapors the less easily the radiant heat will pass through, and the more of it will be absorbed by them. This absorption of the radiations tends to decompose the hydrocarbons composing the vapor. This decomposition produces carbon, lighter hydrocarbons, and possibly hydrogen. The last two immediately ignite and burn with great rapidity. The knock may be produced by a pressure wave resulting from this sudden combustion. ABSOLUTE DENSITYT H E O R Y - L ~states ~ ~ ~that ~ absolute density may be the controlling factor for the cause or elimination of detonation. I n seeking to explain this, he points out the similarity of the case to that of the velocity of interaction of hydrogen and oxygen under the influence of alpha particles, whose departure from the equivalent mixture in the direction of excess hydrogen (lower density or “stopping power” for radiation) lowers the velocity, while excess of oxygen (higher density or “stopping power”) increases the velocity. MOLECULAR COLLISION TmoRY-According to this theory undecomposed hydrocarbon fuel molecules immediately in the front of an explosion wave will be bombarded by swiftly moving molecules of carbon, hydrogen, oxygen, carbon monoxide, etc., from the explosion wave itself. The controlling reaction will therefore depend upon the relative numbers of these bombarding molecules. This theory has had a remarkable test in the experiments of Garner and Saundersz4 on the spectra of acetylene explosions (really detonations). They explain the appearance and disappearance of the Swan and cyanogen bands as due first to the formation of carbon from collision of the molecules with C Z H ~ to form 2C and Ha,then a preponderance of the collision reaction C Z H ~ 0 2 2CO Hz.
+
-.
+
Further Researches Suggested and under Way
I n the course of this survey new experiments have been suggested which might prove helpful in a clearer understanding of this phenomenon and of its prevention. Some of these are as follows: (1) The effect of tube diameter on the period of combustion, explosion, and shock in tubes from 56 to 100 mm. in diameter (range of engine cylinders). (2) Photographic experiments adapting the classical methods of Bertholet, Dixon. and others to an actual internal combustion engine, which will give the relation of combustion, explosion, and shock while using fuels with different chemical composition and under different conditions, such as pressure, temperature, etc. (3) Comprehensive studies of the fundamental causes and effects of turbulence. (4) Mechanical control of detonation by use of a governor operated by the instantaneous pressures developed by the detonation waves, which are approximately four times the maximum “effective pressure” developed by the combustion and explosion periods. ( 5 ) .Accurate spectrometric studies of combustion and detonation in actual engine of various fuels and in the presence of
1226
INDUSTRIAL A N D ENGINEERING CHEMISTRY
various knock inducers and suppressors a t particular stages in the cycle. This is now being done in this laboratory, utilizing an engine with quartz window and synchronous shutter. (0) Laboratory experiments upon the activation and decomposition of hydrocarbon mixtures by radiation in the spectrum from X-rays to radiant heat, utilizing also radiation from the flame itself, as analyzed spectrometrically and spectrobolometrically, reflected by highly polished walls. (7) Adsorption and catalysis studies of fuel mixtures on metal surfaces of various textures and possible catalytic poisoning by knock suppressors. Bibliography
54-Wendt and Grimm, Ind. Eng. Chem., 16, 890 (1924). 55-Weston, Proc. ROY. SOC.(London), 109A, 176 (1925). 56-Wheeler, J . Chem. SOL. (London), ( a ) 106, 81 (1914); (b) lS, 152 (1918); (c) 116,90 (1919); ( d ) 117,903 (1920); (e) 126, 1858, 1869 (1924); tg) 141, 14 (1925). 57-White, Ibid , ( a ) 121, 2561 (1922); (b) 126, 2387 (1924); (c) 117, 48, ( d J 672 (1925). 58-White and Price, Zbid., (a) 116, 1448, ( b ) 1462 (1919). 59-Woodbury, Lewis, and Canby. J. SOC.Automofioe Eng., 11, 209 (1922). 6O--Yamaga, J Faculty Eng. Tokyo I m p . Uniw., 16, 20 (1924). 61-Young and Holloway, J. SOL.Aufomotiwe Eng., 16, 315 (1924). ~~
The following references, almost entirely to the most recent work, together with ninety others listed in a paper by Berl and Fixher,*cover practically completely the more important literature up to July, 1925, upon the experimental side of the phenomena of explosion and detonation: 1-Audibert, Comfit. rend., 178, 1275 (1924). d% B eand Fischer, 2. Elektrochem., 30, 29 (1924). 3-Bone and Wheeler, J. Chem. Soc. (London), (oJ 81, 536 (1902): ( b ) 88, 1074 (1903); (c) Bone and Haward, Proc. Roy. SOL.(London), 101A, 67 (1921). 4-Boyd, Ind. Eng. Chem., 16, 893 (1924). &Brooks, Zbid., 17, 752 (1925). &Brown, Leslie, and Hunn, Zbid., 17, 397 (1925). 7-Bunset1, Ann. Physik., 131, 164 (1867). &Burgess and Wheeler, J. Chem. SOL.(London), ( a ) 99, 2013 (1911); (b) 105, 2596 (1914). 9-Campbell, Zbid., 121, 2454 (1922). IC--Coward and Brinsley, Ibid.. 106. 1859 (1914). 11-Coward, Carpenter, and Payman, Ibid., 106, 27 11914). 12-C.rouch and Carver, Znd. Eng. Chem., 17, 641 (1925). 13-Crowe and Newey, Phil. Mag., 49, 1112 (1925). 14--DeHemptinne, Bull. sci. acad. Toy. Belg., 11, 761 (1902). I b D i x o n , Phil. Trans., ( a ) 184, 97 (1893); ( b ) 200, 326 (1903). 16-Diron and Crofts, J . Chem. .SOL. (London), 106, 2036 (1914). 17-Dixon and Walls; Ibid., 128, 1026 (1923). l&Dixon and Wheeler, Zbid., 111, 1048 (1917). 19-Ellis, Ibid , 123, 1435 (1923). ZO-Ellis and Robinson, Ibid., 147, 760 (1925). 21-Ellis and Stubbs, Ibid., 126, 1957 (1924). 22-Ellis and Wheeler, Zbid., 127, 764 (1925). 23-French, J. SOL.Automolive Eng., 11, 182 (1922). 24-Garner and Saunders, J . Chem. SOC. (London), 147, 77 (1925). 2 6 H a b e r and Hodsman, 2. physik. Chem., 67, 343 (1889). ZB-Hemsalach and Watteville, Compl. r e n d , 146, 748 (1908). 27--Horning, J. SOC.Automotive Eng., 14, 144 (1924). Z&Jorissen and Meuwissen, R C C .trau. chim., (a) 43, 591 (1924); ( b ) 44, 132 (1925). 29-Jorissen and Valisek, Ibid., 48, 80 (1924). 30-La5tte, Compt. rend, ( a ) 176, 1392 (1923); ( b ) 178, 1277, (c) 2176 (1924); ( d ) 179, 1396 (1924). 31-LeChatelier, Ibid., 179, 971 (1924). 32-Lind, (0) Trans. A m . Electrochem. Soc., 44, 63 (1923); ( b ) J . A m . Chem. Soc., 41, 531 (1919). 33-I,upus, 2. ges. Schiess-Sgrengsloffw., 19, 155 (1924) 34--Malinovskii, J . chim. ghys., 21, 468 (1924). 36-Mallard, A n n . mines, 7, 355 (1875). 3&Mason, J. Chem. SOC.(London),123, 210 (1923). 37-Mason and Wheeler, Ibid., ( a ) 111, 1048 (1917); ( b ) 116, 578, (c) 2606 (1919); ( d ) 117, 1227 (1920); (e) 118,1920 (1920); V, 121, 2079 (1922); (g) 126, 1873 UQW. 3&Midgley, J. SOC.Automotioc Eng., 10, 218 (1922). 39-Midgley and Boyd, J . Ind. Eng. Chem., ( a ) 14, 849, ( b ) 894 (1922). 40-Midgley and Janeway, J . SOC.Automotioc Eng., 16, 458 (1923). 41--Morgan, Phil. Mag., 46, 968 (1923). 4 S M c K e n z i e and Honamun, J. SOL.Automotive Eng., (a) 11, 119, (a) 338 (1922). 43-Parker and Rhead, J . Chem. SOC.(London),106,3150 (1914). 4&Payman, Ibid., ( a ) 116, 1446, (b) 1454 (1910); (c) 117, 47 (1920); (a) 128, 415 (1923). d&Payman and Walls, Ibid., ( a ) 123, 421, (b)'434 (1923). 4&Payman and Wheeler, Ibid., (a) 106,36 (1914); ( b ) 113, 656 (1918); (6) 123, 426, (a) 1251 (1923). 4 7 4 h l e s i n g e r . J. SOC.Automolive Eng., 16, 433 (1925). 4 8 S p a r r o w s . Ibid.. 11, 129 (1922). 49-Stevens, R e p f . 176 Natl. Advisory Comm. Acronaufics, 1924. 50-Taffanel and Le Floch, Compf. rand., 166, 1544 (1913). 51-Thornton, Proc. Roy. Soc. (London), 101A, 272 (1924). 5 S V a u t i e r . Compf. rend., 119, 256 (1924). 53-Wendlandt, 2. 9hysik. Chcm.. (e) 110, 637 (1924); ( b ) 118, 277 (1925).
Vol. 17, No. 12
Effects of Knock Inducers and Suppressors upon Gaseous Ionization By G. L. Clark, E. W. B r u g m a n n , a n d W. C. T h e e MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
T h e experiments of Wendt a n d G r i m m a r e repeated u n der carefully controlled conditions, using, however, a s an ionizing source, m o n o c h r o m a t i c molybdenum Kct X-rays of constant intensity a n d a very sensitive q u a d r a n t electrometer a n d a gold-leaf electroscope for m e a s u r e m e n t of ionization currents. T h e ionizations of air, air plus benzene, a n d air plus benzene plus various knock suppressors a n d inducers a r e determined, a n d f r o m t h e f u n d a m e n t a l information concerning gaseous ionization by X-rays, t h e relative ionizations of various vapors, gaseous catalysis, t h e effect of traces of tetraethyl lead, etc., i n absorbing electrons, a n d t h e r a t e of recombination of ions a r e obtained. T h e experiments indicate t h a t t h e theory of electron wave fronts i n explosions a n d t h e absorption of electrons by knock suppressors is n o t sufficient t o explain t h e practical operation of s u c h chemical substances in t h e control of detonation, in agreement with t h e negative results o n t h e in0uence of electric 5elds o n t h e propagation of explosive waves, a n d o n t h e effect of knock inducers i n increasing ionization. Eight theories of t h e action of these compounds a r e critically considered a n d new experiments suggested by this work a r e outlined.
I
T HAS been pointed out that the presence of as little as one molecule of tetraethyl lead in 200,000 molecules of combustible mixture of kerosene and air exerts a marked effect in the suppression of detonation. A mechanism to explain this remarkable effect based upon a theory of many years' standing and experimental evidence in its support have been given by Wendt and Grimm.' Preliminary experiments in this laboratory with a modified form of their apparatus produce results that are not in accord with the suggested mechanism. Theoretical
It is a well-established fact that burning gases are good carriers of the electric current, and, therefore, that they are ionized. Part of the energy from the reaction of the molecules in the flame is believed to be used in the liberation of electrons, which precede the flame front and ionize the combustible mixture immediately ahead. This ionization activates the combustible mixture so that flame propagation is accelerated until detonation occurs. Increased temperature and pressure of the reacting gases would be accom"l'EnS]OURNAL,
16, 890 (1924).
