Ricardo on Oil Engines
Page 53
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f\N November 23rd, Mr. H. R.
F.R.S., gave the first part of his lecture on oil engines, to the members of the Royal Society of Arts. He outlined in brief the history of the evolution of the engine and showed that it was the outcome of a considerable surplus of petroleum products, which, until the compression-ignition engine was available, were actually waste materials.
In the first case, Dr. Diesel designed the engine to operate on coal dust, but it was rapidly developed to use the liquid fuel. The lecturer stated that vaporization of Diesel oils is not a genuine possibility for any highefficiency engine.
Interesting points in connection with combustion are that a certain amount of the smoke in the exhaust may be due to cracking of the fuel and that turbulence is more vital than in the case of the petrol engine. Temperatures play an important part, as every fuel has its critical temperature of selfignition. ;When this is reached, there is a marked hestitancy in combustion, but as the heat is raised the delay is minimized. Turbulence, due to its cooling effect, calls for higher temperatures in order to offset the delay. It is important to reduce the delay action, but much can be done to regulate this by control of temperature. The greater the excess oNthe air temperature above the self-ignition temperature, the shorter the hesitation at the commencement of burning.
Mr. Ricardo again confirmed his view that the best form of turbulence is that in which a continuous air flow, is arranged at right angles to the path of the injected fuel. There are three stages in combustion: (1) a delay period, (2) a rapid pressure rise, (3) the period during which injection continues, but burning can be controlled by the rate of injection and its period.
' Mr. Ricardo pointed out that, by regulation of the third stage, engines could be made sweeter running; of course, a good deal of 'clearness of the exhaust depends upon the control of this stage of the expansion. The rate of burning is the same at all speeds, because if we double the speed we double the turbulence.
Hitherto most authorities have held that the oil engine is definitely limited in its speed. Mr. Ricardo expressed the opposite view, viz., that oil engines will probably run at higher speeds than those of petrol engines, because stages one and two in the combustion occupy less time than the corresnonding ones of a petrol engine—the delay period is only half that occurring in the petrol engine. The process of combustionl occupies but one-five-hundredth of a second or less--approximately only two-thirds of the period required for combustion in a petrol engine. At the present time it is solely mechanical limitations which hamper the development of higher speeds for oil engines, i.e., weights of rotating and reciprocating parts, etc.
As regards the increase of maximum pressure, the lecturer pointed out that successive 100-lb.-per-sq.-in. rises from 650 lb. per sq. in, gave corresponding increases of efficiency of 4 per cent., 3 per cent. and 2 per cent.
An interesting comparison of fuel consumptions concerns the most efficient instances with which Mr. Ricardo is familiar. In the case of a low-speed power unit giving 1,000 b.h.-p. per cylinder, the return was .35 lb. per b.h.p.hour, whilst in the case of a bigh-speed type of 5i ins, bore, and running at 1,500 Limn., the return was .27 lb, per b.h.p.
The second part of the lecture was given last night, November 30th, and the third and final portion will be given at 8 p.m. on Monday, December 7th, at the Royal Society of Arts.