Compression Ratio

In an engine, the ratio of the cylinder volume at BDC (Bottom Dead Centre) to cylinder volume at TDC (Top Dead Centre).

Examples
Turbocharged petrol engine ~7:1
NA petrol engine ~11:1
Diesel engine ~16:1

Highest Useful Compression Ratio

The highest compression upto which no knocking occurs in a given engine. The tendency to knock increases with increase in compression ratio.

See also: Internal Combustion Engine.

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Subjects: Mechanical Engineering