Compression Ratio - EngineKnowHow

The compression ratio is the ratio between the maximum cylinder volume to the minimum cylinder volume.

The geometric compression ratio is:

 

 

 

The effective compression ratio is determined by the maximum cylinder volume once the cylinder is sealed, and is therefore dependent upon the timing of the intake valve closing.

Spark ignition engines typically have compression ratios between 7-14 and are limited by engine knock, pre-ignition and high octane fuel availability.  Testing has indicated that maximum efficiency for spark ignition engines is at approximately 17.  Above this compression ratio efficiency begins to drop due to cylinder pressures forcing fuel into crevices in the cylinder, eg. between the piston and the cylinders walls prior to combustion and higher heat losses due to the increasing combustion chamber surface to volume ratio.

Compression ignition engines typically have compression ratios between 12-24.

The compression volume can also be calculated from the compression ratio via: