Compression ratio

Compression ratio is an engineering term used to describe internal-combustion engines. It refers to the difference in pressure from maximum to minimum as the engine operates. For instance, in a piston engine it can be measured by examining the pressure of the charge inside the cylinder just after combustion, and then when the piston has moved fully down during its power stroke.

Compression ratio is a simple number that can be used to accurately predict the performance of any such engine. This is due to the nature of the gas laws; the relative performance of any engine is dependent on the amount of expansion the charge undergoes, and thus the compression ratio.