What is the Compression Ratio of an Engine?

[WapCar] For an internal combustion engine, the ratio between the volume of the combustion chamber when the piston is at the bottom of its stroke (the maximum volume of a cylinder) and the volume of the combustion chamber when the piston is at the top of its stroke (the minimum volume of a cylinder) is the engine's compression ratio. This ratio shows how compressed the air / fuel mixture is in the cylinder.

The compression ratio is a fundamental specification of efficiency for an engine. For a naturally aspirated engine, irrespective of other factors, the increase on compression ratio means the increase on the engine's performance and efficiency. But it cannot go too high, because an excessive compression ratio will cause knockings in gasoline engines, which severely reduces the working lifetime of the engine, hence fuels with high octane numbers are oftenly required to reduce the possibility of engine koncking.

The compression ratios for naturally aspirated engines nowadays are around 10.5 : 1. Some Mazda SkyActiv engines have compression ratios up to 14 : 1, but still work with fuels with ordinary octane rating (e.g. RON 95). Therefore engines with high compression ratios do not always require high octane-rated fuels, depending on special designs in some systems (such as exhaust) and specific adjustments.