Influence of Compression Ratio in Engine Efficiency

Poverty Bay Herald, Volume LXI, Issue 18313, 3 February 1934, Page 9

Influence of Compression Ratio in Engine Efficiency

JX ! ear specilicatious and articles iii the press reference is frequently made to higli-eilicicncy engines, to such an extent that the expression is in danger of becoming a mere catchphrase, whose true signiiicancc is overlooked, states the Sydney Morning Herald. "Efficiency ” in a petrol engine is merely a loose way of emphasising the extent to which inherent deficiencies have been overcome, and a closer approach made to that ideal and, at present, unattainable state in which the full power which might be generated by the explosion of a mixture of aii atid petrol is obtained. But ignoring many factors which are associated with engine efficiency, a useful and popular definition of the term is that it denotes the horsepower developed by the engine as compared with its cylinder capacity and rate of revolution. Revolutions per minute are signilicunt, for an internal combustion engine, within the limit of its effective peak speed, develops more power the faster it goes. For example, an engine of 2000c.c., which is designed to run at 4000 r.p.m. will usually develop much greater power than one which "peaks” at 3000 r.p.m.

Another far-reaching development has been the raising of compression ratios. The compression ratio of a cylinder is calculated by adding the total piston displacement of the cylinder to the volume or capacity of the combustion chamber, and then dividing their sum by the volume of the combustion chamber. For instance, the volume swept by the piston is 400 c.c., and between the crown of the piston when it reaches top dead centre and the top of the cylinder there is a combustion space of 100c.c., there is a total capacity of oOOe.c. Accordingly, since the volume of the combustion space is but one-fifth of the total the engine is said to have a compression ratio of sto 1. The higher the degree of compression of the mixture, the greater the energy unloosed when the charge is fired. The advantages of increasing compression ratio are thus expressed by a prominent expert: —"If the same amount of petrol-air mixture is taken into the cylinder and more of its stored energy is used on the piston by means of raising the compression, the lower the fuel consumption per horsepower will be. In other words, the total fuel consumption of an engine with a 0 to 1 compression ratio 'may remain the same as that of one having a ratio of 3 to 1. The reason for the exhaust and water tempera tare being lowered by an increase in compression ratio (all other things remaining the same) is obvious, for if more of the available heat units in the petrol-air charge are employed in useful work, then there will be less wasted by dissipation to the exhaust and cooling water, and the temperatures in regard to these will be lower. A higher compression ratio will give rise to greater temperatures and pressures when the charge is tired, but after combustion the gases will lie expanded to a greater degree (before the exhaust valve opens), in an engine having a high compression ratio than in one with a lower ratio. It is therefore this greater ratio of expansion, brought about bv. raising the compression pressure which contributes to the benefits secured.”

'There is, however, a limit to which compression pressure may be exploited, for when the pressure is too great the fuel may tend to pre-ignite, “pinking” be caused, and top gear flexibility affected. The modern tendency is to increase compression pressures (G to 1 is common now), and to depend on the anti-knock qualities of improved fuels.