Compression ratio

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 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.

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The ratio is calculated by the following formula:

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:mbox{CR} = rac { ( pi b^2 s) / 4 + V_c } {V_c} , where

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:b = cylinder bore (diameter)

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:s = piston stroke length

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:V_c = 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.

Related Topics:
Combustion chamber - Ignition

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Due to pinging (detonation), the CR in a gasoline/petrol or LPG or CNG-powered engine will usually not be much higher than 10:1.

Related Topics:
Pinging - Gasoline - Petrol - LPG - CNG

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In engines with a ping sensor and an electronic control unit, the CR can be as high as 12.5:1 (2005 Audi A6 3.2)

Related Topics:
Sensor - Electronic control unit - 2005 - Audi A6

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In a turbo charged or super charged engine the CR will be around 8.5:1

Related Topics:
Turbo charged - Super charged

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In a diesel engine the CR will be 20:1 and higher.

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