What is compression ratio of internal combustion engine?

Compression ratio is defined as the ratio of volume of cylinder when piston is at BDC (bottom dead center) i.e. position of crank when piston is at its lowest place to the volume of cylinder when piston is at TDC (top dead center) i.e. position of crank when piston at its top most place.

Usually gasoline engines have compression ratio of 7 – 9 while diesel engine have compression ratio of 15-20. To understand this figure let’s assume a engine having CR of 9, this means that cylinder having 9 volume area capacity will be compressed to 1 volume area.

Higher the compression ratio higher will be the heat generated inside cylinder.

Gasoline being volatile requires less compression ratio so has CR from 7 to 9 and Diesel engine being non volatile requires high compression ratio thus ranges from 15 to 20 CR.