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Compression-ignition Engines
MO-DAY, March 1st, Mr. C. B. Dicksee, of the Associated .1 Equipment Co., Ltd., is to read a paper entitled "Some problems connected with high-speed compression-ignitionengine development"; the audience will consist of members of no fewer than 11 technical societies and institutions. The paper does not deal with any particular engine, but sets forth the various problems encountered in research work.
-The main problem, as we have constantly set forth, is the fullest possible utilization of the available oxygen. The author states that the oil engine must be truly comparable with the petrol engine, especially as regards its speed range. One of the points in connection with this is that an engine capable of high crankshaft speeds can be made much lighter than one of lower speed characteristics. One of the desirable conditions is an absolutely clear exhaust, and the noise must be reduced to a minimum. "Diesel knock" is detrimental to the engine, because it indicates excessive pressure rise; in the train of this follow undue bearing stresses which affect adversely the essential reliability. In connection with the better utilization of the air the problems demand the co-operation of the oil companies with the engineers. It has been found that oil-engine fuels require some 15.5 lb. of air per pound of liquid for complete combustion, and that the complete utilization of 1 cubic in. of air at normal temperature and pressure results in the production of 45 ft.-lb. of energy.
Two Classes of Engine.
In an oil, engine the rate of combustion tends to decrease towards the end of the phase, which is the opposite state of affairs to that obtaining in a petrol engine; in order to develop a high mean effective pressure it is, therefore, essential to maintain the rate of combustion and to utilize the•maximum amount of air. To this end it is important that the fuel and air be given a high velocity relative to each other. Engines may be divided into two categories ; there Are those in which motion is imparted to the air and those in which the fuel is the moving body. The advantages of the former type of engine are the possibility of providing for rapid and lasting air motion, the avoiding of multi-hole sprayers, and the need for onli moderate injection pressures. The air velocity, of course, increases directly as the piston speed increases. The good points of the system in which the fuel is the moving body are the ability to use a simple, open combustion chamber, whilst the advantageous surface-volume ratio facilitates starting from cold.
Air control can be arranged in three ways. Its direction can be regulated by the shape of the admission orifices ; a separate combustion chamber can be provided, allowing. rotational swirl, or thirdly, the swirl may be partly or wholly produced during combustion by the egress of the air from cells into the combustion chambers.
A vital point is that the fuel must be injected across the air stream and not with it ; this brings into contact fresh oxygen molecules and fuel particles. Providing the fuel and air are brought into contact at a rate which is proportional to the engine speed the combustion will keep pace with the speed of the engine.
The clearness or otherwise of the exhaust is a useful index as to the combustion efficiency. A pronounced early drop in m.e.p.. coupled with a clear exhaust—until much more air has been consumed—indicates that the air is not being used until late in the cycle, due to lack of turbulence, or too ' slow or too late injection. The presence of thick smoke when only a moderate percentage of air has been consumed shows that the method of searching out the oxygen is inadequate, or that fuel distribution is "patchy."
Not only is it important that the maximum amount of air be used, but it is vital to know exactly where, in the cycle, the air is being burned. One aid to good volumetric efficiency at high speed is the fact that the air-inlet manifold can be made of generous cross-section. It appears that about 85 per cent. of the available oxygen is the maximum that can be consumed.
With regard to compression ratios, present-day practice varies from about 13 to I to 16 to 1; one requirement is that the temperature generated should appreciably exceed 400 degrees C., which is the average figure for the selfignition temperature of oil fuels. An engine will run on a much lower ratio than is needed for starting; the use of heater -plugs helps to overcome starting troubles. For work in countries having locally produced fuel of suitable composition the compression ratio may need to be varied considerably if the optimum results are to be obtained.
High compression ratios tend to promote smooth running, in many cases, by reason of shortening of the delay period, but the effects of adopting higher ratios may be very complex.
The author advances an interesting theory of combustion which appears to contain sound facts; the fuel having been injected into the highly heated air, it starts to absorb heat and vapour begins to form. If the compression temperature be sufficiently above the spontaneous-ignition point of tbe particular, fuel, the vapour will ignite and orderly combustion will occur. If the temperature difference be Insufficient, vapour will collect and ultimately cause something resembling an explosion. In the latter case knocking and harshness are inevitable.
Advanced Injection and Knocking.
This state of affairs is responsible for knocking, as is an unduly long delay period. Advancing the injection moment, with many designs, adds to the knocking tendency because the fuel is delivered into a colder and less dense atmosphere than if it were accomplished at a point nearer the top dead centre. These facts do not apply with the same force in the case of an engine with an orderly air swirl. It has been found, however, that too high a rate of swirl can cause knocking; this is due to heat loss, amongst other things.
Other factors affecting combustion efficiency are injection pressures, which to some extent control globule size; a decrease in the size of globules decreases the delay period, but only up to a certain limit.
With regard to fuels the author has found that there are very marked variations in the behaviour of different liquids. Although there is room for improvement, fuels sold in Britain are fairly uniform ; those obtainable in the East are sometimes liable to give trouble.
If the theory be correct that vaporization causes knocking, then the properties of an oil which will have the greatest influence upon its suitability as a fuel are the distillation curve and the ignition point. Researches with different oils have confirmed the theory that the formation and subsequent explosion of vapour give .rise to knocking.