The compression ratio of an engine refers to the relationship
between the maximum and minimum volume within an engine's
cylinder / combustion chamber. That is the ratio of volumes
between when the piston is at Bottom Dead Centre ( BDC )
and at Top Dead Centre ( TDC ) position.
High Compression in Diesel Engines
This ratio specifies the extent to which gases are compressed
before they are ignited. An engine with a higher compression
ratio can extract more energy and can have higher fuel economy.
Petrol engines have a compression ratio in the region of
10:1, a diesel engines compression ratio is typically 20:1
or higher. A diesel engine is engineered to withstand these
higher compression ratios using heavier stronger components.
This is why diesel cars are heavier and also why diesel
engine reliability is much improved over petrol engines.