Automotive Dictionary: Compression Ratio

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Automotive Dictionary: Compression Ratio


Throughout this site we use many technical terms, and given the breadth of readership our site enjoys, sometimes we are remiss and incorrectly assume everyone knows what we are referring to. For those that do not, here are some explanations of the technical terms use.

Compression Ratio


Compression Ratio
A calculated ratio that relates the uncompressed volume of the cylinder with the piston at bottom dead-centre to the compressed volume of the cylinder with the piston at top dead-centre. Hence a 9:1 compression ratio means that the air-fuel mixture is compressed to one-ninth of its original volume before being ignited. The compression ratio bears little relationship to the actual effective pressure created in the cylinder. Can also be described as the ratio between the volume of the cylinder above the piston when the piston is at BDC (including the volume of the combustion chamber) to the volume when it is at TDC (that is, the combustion chamber only). Expressed as a formula, compression ratio is: cv + sv / cv where CV equals clearance volume and SV equals swept volume.
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