Oil Engines: A Valuable Lecture O N page 593 of our
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issue dated December let we gave a summary of the first lecture by Mr. H. R.
• Ricardo, F.R.S., on oil engines, as delivered to the Royal Society of Arts. The second portion was delivered on Monday, November 30th, and the third • portion last night (December 7th).
The lecturer pointed out that undue' praise has been given to Continental engineers for the evolution of the highspeed oil engine, but inadequate credit has been given to the staff of the Royal Aircraft Establishment, at Farnborough. which, 10 years ago, showed that a high-speed experimental oil engine was capable of beating some of the results of the petrol engine, even at that date.
As was pointed out in the first part of the lecture, higher speeds are to be anticipated. For this purpese the delay period must be shortened. To do this, the compression ratio can be raised, but the hearing stresses serve as the limiting factor, whilst the higher compression means denser air and greater difficulty in distributing the fuel. Again, the turbulence can be increased, hut heat • losses to the walls must be kept down. He stressed the importance of the reduction of the delay period, because of its effect upon "Diesel knock" and sweetness of running.
As regards the second phase of combustion—rapid pressure rise—this is largely controllable by design, etc., whilst in the third stage, which is that during which burning occurs directly the fuel enters the combustion chamber, mechanical control is possible and this stage should be prolonged at the expense of the first two phases.
Mr. Ricardo next dealt with the three types of engine : (1) That in which the fuel finds the air, i.e., direct injection ; (2) The pre-combustion-type engine; (3) The design which embodies an orderly air swirl and the air has to find the fuel.
In the case of engines of the first class, the advantages are good starting, fuel economy, smooth running and simplicity. The disadvantages include the speed limitation by fuel travel, the need for proper control of injection timing owing to inadequate turbulence, the need for high fuel pressures with small jet orifices and accurate control of direction.
Turning to the pre-combustionchamber type, he stated that stages 1 and 2 of combustion occur in the ante chamber, but stage 3 occurs in the main chamber. The advantages of the type are the possibility for the employment of large fuel orifices, whilst direction of injection is not important. Low fuel pressures can be utilized, so that the work of the pump is facilitated.
With regard to the disadvantages, he pointed out that the air and fuel have to pass through apertures, hence there are considerable heat losses and the system demands high compression and heavy parts. The power and efficiency are between 15 per cent. and 20 per cent, lower than need be the case, whilst starting from cold has to be facilitated by heating devices.
As regards the type of engine which has a rotational swirl, Mr. Ricardo claimed that it has the advantages of classes 1 and 2 plus good combustion, with low fuel pressures, whilst the velocity of injection and the direction thereof are not important, and fixed injection timing can be used throughout the range. On the other hand, the engine having rotational air swirl can be used to its best advantage only in twostroke-cycle or sleeve-valve engines.
The highest efficiencies so far obtained in either high-speed or lowspeed oil engines have, Mr. Ricardo stated, been achieved with engines in this category. The best results are obtained when the air swirl rotates 10 times for one turn of the crankshaft. This principle, with a separate combustion chamber, is used in the new A.E.C. engine and a somewhat similar principle in the Lister.
With regard to supercharging, Mr. Ricardo outlined the • position and showed that, whilst the gain in power can be in the neighbourhood of 100 per cent. in the case of the petrol engine, the oil engine gains only to the extent of 75 per cent., but the running is sweeter and smoother.