What is a compression ratio?

Answer 1

Compression Ratio is the ratio between the original volume and the compressed volume

in combustion chamber.

The compression ratio is denoted as CR. It is the measure of how much air-fuel mixture is compressed during the compression stroke in the case of the gasoline engine (in the case of the diesel engine only air is compressed.

Compression ratio is found by dividing the volume of stroke of the cylinder (swept volume) and clearance volume when the piston is at BDC by the volume when the piston is at TDC (ie clearance volume).

Compression ratio= (Swept volume + Clearance volume) / Clearance volume.

For example engine has a Swept volume of 700 cc.

It has clearance volume 50 cc.

Compression ratio= (700 + 80) / 80 =9.75: 1

It means Air fuel mixture is compressed to 1/9.75 of its original volume during compression stroke.