The
compression ratio of an
internal-combustion engine or
external combustion engine is a value that represents the ratio of the volume of its
combustion chamber; from its largest capacity to its smallest capacity. It is a fundamental specification for many common combustion engines.
In a piston engine it is the
ratio between the volume of the
cylinder and combustion chamber when the
piston is at the bottom of its
stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.
[1]
Picture a cylinder and its combustion chamber with the piston at the bottom of its stroke containing 1000 cc of air (900 cc in the cylinder plus 100 cc in the combustion chamber). When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher
thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the adiabatic heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.