COMBUSTION CHAMBER PRC
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3LEMS SOLVED AND UNSOLVED
,ure Rise and the Phenomenon of Detonation Are But rs Associated With the Complex Subject of Combustion. unmarizes the Relevant Knowledge in the Possession of vrkers and Refers to Questions Still Unanswered
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THE so-called explosion which occurs in the engine is a process about which many books have been written, and presents _problems of absorbing interest. It is a subject to which many able experimenters have given close attention, but such is its complexity that even the most eminent of these are not in total agreement about all phenomena of combustion,
.Research is not assisted by the high speeds and pressures attained by the modern engine, a. point that will be appreciated when it is realized that at 3,000 r.p.m. (quite a moderate figure these days) the combustion stroke lasts only 0.0J second, and that sin this short period all the phases of burning take place.
. It is upon the burning of .the petrol mixture that efficiency depends, and this is, in turn, decided by combustion-chamber design, the object of the designer being to employ the highest possible compression ratio compatible with engine smoothness and refinement. An increase in compression ratio from 4 to 1 to 6 to 1 represents a gain in thermal efficiency of 16 per cent., for example. Compression cannot be increased without limit, however, kecause above ratios of 5 or 6 to 1 the phenomenon of detonation„ or.krtocking, is liable to assert itself, and it may be said that this factor represents the criterion for maximum compression. It is, therefore, upon the elimination of detonation on which so much effort has been expended. Chemists have done much to improve matters, and it is likely, as has been said before in this paper, that alter the war 100-octane fuel will be available for commercial use.
ideal Combustion Conditions Unattainable in Practice Best possible conditions for combustion exist at constant volume, which means that burying takes place without change in volume. 'Unfortunately, however, the number of degrees of stroke over which burning ta.ltes place precludes this; although " desaxe," offset ,Insproves matters slightly.
'Combustign proceeds in the following stages: the electric spark ignites the inflammable particles about it, which pass on the heat to others until a nuclear ball ,of flame appears, comparatively slovsby, and then a rapid spread of the burning to other parts of the cylinder, the speed of this Process being determined `by pressure, temperature, mixture density, and turbulence.
This last agency would, at first, appear to be the 'tire ,for all ills in the search for complete combustion, but, actually, only careful experiment is the determining factor, and sometimes, indeed, a reduction of turbulence is necessary. One of its real values is that the rotating mass of mixture scours away the layer'which tends to cling to the coel walls and thus to cause incomplete combustion.
The most efficient type of cylinder head is generally accepted to be the hemispherical with overhead valves. This design is frequently considered to possess some
remarkable propertyof finding almost 'unlimited horse
power, but such view is, of course, quite a fallacy. For small engines running at commercial-vehicle crankshaft speeds the side-valve engine is as efficient as the overhead-valve unit, but at high speeds the rather more complex valve passages, restrict the path of the moving gases, thus giving a correspondingly rhos° rarefied. charge, or reduced volumetric efficiency as compared with the overhead-valve engine.
On large power units another factor requires con sideration. Speed of flame propagation does not increase with engine size, and, therefore, as the time for burning is limited,and time must be found for the flame to penetrate into the farthermost parts of the cylinder, there is a Etna to the distance between the sparking plug and the remotest corner. This is reduced to a minimum when the plug is in the centre of the _ cylinder. The exact location of the plug again a
point of great, technical importance, and is based onthe elimination of detonation.
This phenomenon" is believed to result 'from _a high prnssure rise, and a rapid movement of the flame front,
' which pushes a. "piston," as it were, of gas in front of it. The temperature of the still-unburnt gas is increased by 'radiation and the normal rise of pressure caused by the on-coming flame front, With the result
that it ignites spontaneouSly, bringing about a rapid
pressure rise and much increased flame velocity, usually in the order of 5,000 ft. per second. The consequent impingement on 'the surrounding walls causes a highfrequency vibration which is coincident with the period
of the metal and the familiar " knock " results.
A Detonation-promoting Design Now Obsolete
An example of inefficient combustion-chamber design, now obsolete, was the T-head type in which tie exhaust and inlet valves were nn opposite sides of -the cylinder, and were of large -dimensions, requiring a large degree of flame traveV The pockets over the valves caused
local areas of stagnant mixture, which gave an almost ideal detonating condition, -made doubly bad if .the spatking plug were situated over the inlet valve, as was
commonly the case. .
Suppression of detonation is partially. accomplished in several-ways, the major one being .theslocation of the sparking plug _near the exhaust valve. When this is not done the unburnt gases receive heat froth the hot Valve, and thiil increaSe in temperature is quite sufficient to instigate knocking. Careful attention must be iiven
to any new design to obtain best results.
, In side-valve engines the combustion chamber is
almost universally' offsetto encompass the valves, advantage being taken of this fact to give a high degree of turbulence; The sparking plug is situated at practically the centre of the chamber, with a slight bias
towards the exhaust-valve side, to eliminate the possibility of detonation caused by a residual mixture heated by this valve. Accordingly, the distance between the sparking plug and the farthest part of the main body of the mixture is such that the conditions obtaining in the combustion chamber are similar to those of the overhead-valve engine.
There is, of course, a small quantity of mixture still trapped between piston and cylinder head, but the large surface area, in comparison with thevolume, suffices to reduce its temperature sufficiently to prevent spontaneous ignition and thus detonation. Experiment has shown that variation of charge transfer area has a direct result on flame spread, reduction of area giving an acceleration of pressure increase.
Faster Burning Leads to Smooth-running Problems
• As compression affects the speed of burning, it will be obvious that the load imposed on the piston will be more violent and, unless careful study is given, there will be roughness. Ricardo points out that a reasonable maximum pressure rise would be SO lb. increase for each degree of crankshaft movement. To obtain this, the combustion chamber would be formed so that the amount of mixture in concentric rings about the sparking plug when ignited would approximately afford this figure.
Due allowance will be made by the designer for increase in temperature and pressure. A good method of examining this condition that is in common use is to cast an impression in the combustion chamber and turn concentric rings away, the turnings being weighed.
Reduction of charge temperature is of great value in prevention of detonation, because the final temperature is a multiple of the initial temperature, and the heat applied to the residual mixture must, therefore, also be dependent on the initial temperature. Although it would ,appear to be, as yet, ofonly academic use, it is interesting to note that one experimenter claims that power may be increased, and greater economy may result, by the introduction into the cylinder of inert gas, such as the exhaust iiroducts. This has the effect of slowing down the speed of flame propagation, and thus giving a smoother engine. Unfortunately, if the exhaust gases be of greater temperature than the surrotrnding air, the improvement would be offset by a loss in efficiency, and it will be obvious that the necessary storage and cooling systems would require more complication than would .be warranted by any slight improvement.
One of the accompanying sketches shows the wellknown Bedford cylinder head, which is representative of modern thought, being an Outcome of careful experiment and applied theory. Initial combustion takes place in the large hot area under the exhaust valve, and the area to which the flame spreads from here to give the high pressure increase is termed the " shock" _zone. It is found that this reduction in area gives a corresponding decrease in acceleration. The third area is as previously, described, a narrow, cool space to prevent detonation.
It will be noticed that the conditionsgoverning acceleration in the shock zone are completely at variance with those of the side-valve .head—a contrast that -appears to be difficult to reconcile:
Flame-front Phenomenon of Fundamental Importance
Another strange anomaly common to all types of engine is the agency synchronizing flame speed with engine revolutions. Sir Dugald Clerk discovered, by propagating. flames in glass cylinders, that when air was projected on to a flame if receded from the advancing air stream, but if a combustible mixture were projected into the flame it advanced at a rate more or less dtival to thatof the oncoming mixture.? This condition is simulated in the engine, where mixture is pushed into that burning in the combustion chamber by the piston during the last few degrees of the compression stroke.
Photographs obtained by high-speed apparatus have confirmed this; and pictures in a series taken over small intervals of 'crank movement are almost identical at different speeds.
Having settled the best combustion chamber, there still remain many other methods of increasing compression ratio without detonation. Careful attention to the hot point of the cylinder head is imperative in the design stage, and all bosses for sparking plugs, valve; and cylinder head studs, should be of minimum size.
A sound scheme, that would probably be universa were it not for the war, is the jet cooling of-certain parts This is accomplished by means of a tube taking wale; direct from the pump, having apertures suitably directec towards the hot bosses, and thereby ensuring the mos even cooling possible. • Sodium-cooled exhaust valves, as used in aeroplane engine practice, will also limit the temperature of th. gases around the valve, with beneficialresults.
Much experimental work has been done already b: eniinent research workers, but there still remains more • and many problems are still unsolved. However; w may be sure that, on the cf6se of hostilities, work wi: once more go forward on the development of bette commercial-motor engines for future production.
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