compressieratios

Compressieratios, or compression ratios, describe how much a gas is compressed in a closed volume. In thermodynamics and engineering, the term is most often encountered in internal combustion engines and in gas or air compressors. A compression ratio is typically defined as the ratio of the volume when the piston is at its lowest point (bottom dead center) to the volume when the piston is at its highest point (top dead center). In engines, V1 is the total cylinder volume at BDC, including the clearance volume, and V2 is the clearance volume at TDC; thus CR = V1/V2.

In spark-ignition engines, higher compression ratios generally improve thermal efficiency by allowing greater expansion of the combustion gases, but they also increase the likelihood of pre-ignition or knocking. This limits practical ratios for gasoline engines, which commonly range from about 8:1 to 12:1. Diesel engines, designed to rely on compression ignition, frequently employ higher ratios, from roughly 14:1 up to 25:1 or more, contributing to their efficiency. Some modern engines use variable compression ratio (VCR) systems to balance efficiency and performance across operating conditions.

In other contexts, compression ratio can refer to pressure ratios in compressors and turbomachinery, where it is the ratio of discharge (or outlet) pressure to suction (or inlet) pressure. For ideal gases undergoing isentropic compression, the pressure ratio is related to the volume ratio by P2/P1 = (V1/V2)^gamma, where gamma is the gas’s heat capacity ratio.

Typical considerations when selecting a compression ratio include fuel octane, desired efficiency, emissions targets, and risk of knock or pre-ignition.