Compression Ratio

Press, Volume CI, Issue 29897, 10 August 1962, Page 9

Compression Ratio

'THE compression ratio of A an engine is the volume in the engine cylinders with the piston at the bottom of its stroke divided by the volume with the piston at tshe top of its stroke. It is a measurement of how much the mixture in a cylinder is compressed by the pdston. For example, a compression ratio of 7:1 means seven volumes of gas mixture is compressed to one Volume by the piston. In the last few years improvements in motor fuels have permitted the compression ratios of engines to be increased from figures as low as 5:1 to nearly twice this figure. Family cars with a compression ratio of 8:1 are common today. Increasing the compression ratio of an engine increases the power and economy without also increasing engine size and weight (within certain limits, of course). If the ratio is increased too far, the engine will lose flexibility (torque, or pulling power) and the fuel will not ignite properly. An engine with a high compression ratio always needs higher grade fuel than an engine with a low compression ratio, but it will give more power for the amount of fuel used. The higher ratio squeezes the air-fuel mixture more, and causes higher pressures, and hence more power, to be developed on the downwards stroke of the piston. There are several methods of increasing the compression ratio of a standard engine: the head can be planed or “shaved,” the length of the crankshaft throws can be increased, or the piston can be lengthened above the gudgeon, or wrist pin.

A production engine, however, is usually designed for the compression ratio with which it is originally made, and to alter it too drastically will put undue strain on bearings, crankshaft and gudgeon pins. It can result in an engine “blowing up.” Many experts say it is safer to obtain extra power by fitting a supercharger (cost: £7O to £100) than by raising compression ratio. Altering the compression ratio puts more than 10 times greater strain on the engine, they say. If a low grade fuel must be used, the compression of an engine can be lowered by using two head gaskets, or even by fitting a special cylinder head. Although the engine will then run on cheaper fuel, there will be a considerable falling off in performance. As the compression ratio goes up the problem of knock, detonation, or pinking becomes more acute. The anti-pinkin>g value of a petrol is known as the octane rating.

As the octane rating of a petrol goes up its resistance to knock or pink also increases. Higher compression engines can be used, and more power and economy can be obtained from each gallon of petrol. Diesel engines run at compression ratios as high as 18:1. These high pressures are accompanied by great heat, and this heat ignites the oilair mixture without the assistance of a spark plug.