T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
894
VOl. 14. No. 10
T h e Chemical Control of Gaseous Detonation with Particular Reference t o t h e Internal-Combustion Engine By Thomas Midgley, Jr., and T. A. Boyd’ GENERAL MOTORSRESFARCHCORPORATION, DAYTON, OHIO
N EXTEKSIVE study of gaseous detonation has been made by a number of investigators, but in spite of this fact the phenomenon is as yet not well understood. In presenting this paper, therefore, we will not attempt to explain a! the phenomena with which i t deals. The object of the paper is-to discuss the bearing of this factor on the operation of internalcombustion engines, and to describe the progress that has been made in controlling it by chemical means.
A
RESULTSOF
EARLY STUDY O F GASEOUS DETONATION
In1 1881 Bertholet2 and LeChatelier3 independently discovered that the propagation of flame through mixtures of some combustible gases with air, and through mixtures of practically all combustible gases with oxygen in proper proportions, results in setting up a detonation wave. These results were confirmed by €1. B. D i ~ o nwho , ~ has carried out quite an extensive investigation of this subject. Mallard and LeChatelier5 noted that the development of the detonation wave is not progressive, but always instantaneous. They noted further that the detonation wave is characterized, not only by its great velocity of movement, but also by its intense luminosity. Dixons was also impressed by the sharpness with which luminosity is set up when detonation occurs. Bertholet and Vieilk7 and also Dixon4 showed that the velocity of the detonation wave is constant, and Dixon advanced the theory that during detonation the flame travels a t the same speed as sound a t the temperature of the burning gases. Mallard and LeChatelier7 found that very large pressures are developed by detonation waves, but that such pressures exist only for a n exceedingly brief period. This finding was confirmed by D i ~ o n who , ~ worked on the principle that, if a pressure is produced in a glass container greater than the glass will withstand, the vessel will be broken, although the pressure may endure only for a very short interval of time. Dixon gave a range of from 25 to 78 atmospheres for the magnitude of these pressures for various gases. He showed further t h a t these instantaneous pressures are approximately four times the maximum “effective pressure” developed by the explosion. GASEOUS DETONAT~ON A N IMPORTANT FACTOR IN INTERNAL COXBUSTION Except for some comparatively recent work, practically all the study of gaseous detonation that has been made has been conducted in atmospheric tubes, or under other conditions that did not simulate those of internal combustion. From an economic standpoint, however, the detonation that occurs in internalcombustion engines is of great importance.s Nearly everyone 1 Chief and Assistant Chief of the Fuel Section, General Motors Research Corporation, Dayton, Ohio. 2 Compt. rend., 93 (1881),18. a Ibid., 93 (18811, 145. 4 Phal. T r a n s , 184 (18931, 97. 6 Ann. Mznes (8th series), 4 (1883), 274. e Phzl. Tran?, 200 (19021, 315. 7 Ann. chim. p h y s . , 28 (1883), 289 8 Some of the published reports of work done on detonation from the standpoint of the problem as it applies t o internal combustion may be found in the following references: Harold B. Dixon, J . SOG.Automotive Eng , 9 (1921), 237; Automotive Industries, February 3, 1921, p. 211. Woodbury, Lewis, and Canhy, J . SOC.Automotive Eng., 8 (1921), 209. H. R. Ricardo,
who has driven an automobile is familiar with the troublesome “pinking” in the engine which is known as a “knock.” The knock is a sharp ringing noise, suggestive of two metallic parts striking together, and is most pronounced when the engine is pulling a t slow speeds in high gear, as is the case on steep grades or when accelerating from slow speeds. This noise is not caused by the striking of metallic parts, but i t has been fairly well established that it results from a detonation of the highly compressed fuel mixture in the combustion chamber. For any given fuel the intensity of detonation varies with the compression of the engine-the higher its compression the more pronounced the detonation. It is herein that the seriousness of detonation in internal-combustion engines lies, for both the torque and the fuel economy of an engine are increased as its expansion ratio (called in automotive practice “compression ratio”) is raised. While the detonation that occurs in automobile engines of present compressions is so slight as to be merely an unpleasantness that does little or no harm, in engines of highly economical compressions its intensity becomes so great as to result not only in loss of power but also in actual damage to the engine. This phenomenon of detonation, then, stands as an effective barrier t o the obtaining of better fuel economies from internal-combustion engines, and because of this fact, i t has an important economic aspect.@ Detonation as it occurs in internal combustion does not vary with the compression of the charge alone. It varies directly with the temperature, and is aggravated by advance of the spark timing and by carbon deposits in the engine. It is further influenced by certain elements of design, such as the shape of the combustion chamber and the location of the spark plug, but of great importance is the fact that it is a function of the chemical structure of the fuel. As a n illustration of this the case isomeric compounds ether and normal butyl alcohol may be cited. These compounds may have the same ultimate composition, C4H100,but thCy differ widely in molecular structure. Ether detonates when burned a t a very low compression; alcohol, when burned a t a compression and temperature which are relatively very high, is entirely free from detonation. Similarly, ethylenic hydrocarbons have a greater tendency to detonate than saturated cyclic hydrocarbons of the same ultimate composition. While these facts are interesting and important, the situation with respect to automotive fuels in the United States is that their composition is fixed within fairly narrow limits. From the standpoint of available supply, petroleum oils must be used as the principal source of motor fuels for many years to come. From a commercial standpoint, therefore, the problem is to find a means of controlling the detonation of paraffin hydrocarbons, a tendency which becomes more pronounced as their molecular size increases, or their volatility decreases. The Automobile Engineer (England), 11 (1921), 51, 92, 130, 169; J . SOG. Automotive Eng., 10 (1922), 305. Thomas Midgley, Jr , Trans. SOL.Automotive Eng., [ 2 ] 15 (1920), 659; J . Soc. Automotive Eng., 10 (1922), 357. T.Midgley, Jr., and T.A. Boyd, Ibzd., 10 (1922), 7, 451; THIS JOURNAL, 14 (1922), 589. 9 For a more complete discussion of this phase of the subject see “The Application of Chemistry to the Conservation of Motor Fuels,” Midgley and Boyd, THISJOURNAL, 14 (1922), 849.
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Oct., 1922
T H E JOURNAL OF IYDUXTRIAL A N D ENGI,VEERING CHEMISTRY
THEQRIES OF DETONATION IN INTERNAL COMBUSTION In spite of the fact t h a t gaseous detonation is not well understood, many theories have been advanced to explain the phenomena that occur in internal-combustion engines variously known as “detonation,” “knock,” and “pinking.” Aside from those which may be classified as mere speculations, three theories have been advanced in definite form.I0 The first of these, known as the mechanical knock theory, explains the sound as resulting from an actual impact between parts of the engine, such as between cylinder wall and piston or between shaft and bearings. This theory does not attempt to explain the cause of pressures which must exist in the engine cylinders t o produce these mechanical impacts. For this reason it may be passed over with this simple mention. A second theory is that which explains the sound and attendant phenomena of detonation as resulting from a very rapid increase of pressure in the engine cylinder.ll According to this theory, a portion of the charge is compressed to its auto-ignition point by the expansion of the portion first ignited, which compresses before i t the unburned part of the charge. When the rate of increase in temperature due to this compression exceeds by some margin the rate at which a sufficient amount of the heat can be dissipated t o the cylinder walls, the remaining portion ignites spontaneously and nearly simultaneously throughout. This theory apparently implies t h a t a fuel should have a tendency t o knock t h a t is proportional to its spontaneous-ignition temperature. The third theory explains the phenomena incident to the fuel knock as resulting from the impact of a high-velocity, highpressure wave against the cylinder walls and head. It has previously been demonstrated mathematically that, during the passage of a flame through a combustible mixture in a closed container, a higher pressure must exist immediately in front of the flame front than behind it, and t h a t this pressure difference may be represented by the following expression :I2
where P1 is the pressure in front of the flame PZ is the pressure to the rear of the flame W IS the reaction velocity expressed in pounds of mixture entering the flame front per second g is 32.2 VI IS the specific volume of the mixture in front of the flame Vz 1s cu. f t . per lb. of burned mixture t o the rear of the flame front Physical chemistry tells us t h a t W should equal C D‘&T”
(2)
where C is a constant D, density of thermixture T, absolute temperature n, exponent the numerical value of which depends upon the chemical equations of combustion nz, exponent the numerical value of which must be determined experimentally By combining Equations 1 and 2 and substituting thermodynamic equivalents, the following equation is obtained:
where A = 2JHL J = 778 H = effective heat value per pound of mixture
Dickinson, J . Soc. Aulomolise Eng., 8 (1921), 558. H. I
