The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine. It is a ratio between the volume of a combustion chamber and cylinder, when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture. Higher compression ratios, however, also make detonation more likely.
The ratio is calculated by the following formula:
- , where
- b = cylinder bore (diameter)
- s = piston stroke length
- Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated.
- Due to pinging (detonation), the CR in a gasoline/petrol powered engine will usually not be much higher than 10:1.
- In engines running exclusively on LPG or CNG, the CR may be higher, due to the higher octane rating of these fuels.
- In engines with a ‘ping’ or ‘knock’ sensor and an electronic control unit, the CR can be as high as 13:1 (2005 BMW K1200S)
- In a turbo charged or super charged engine, the CR will be around 8.5:1
- In a diesel engine, the CR will be 14:1 and higher.