Compression Ratio

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 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. 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.
The following formula is used to calculate the static compression ratio: `CR=(pi/4b^2s+V_c)/V_c`, where:

• `b` = cylinder bore
• `s` = piston stroke length
• `V_c` = clearance volume (volume of the combustion chamber (including head gasket). This is the minimum volume of the space at the end of the compression stroke, i.e. when the piston reaches top dead center (TDC). Because of the complex shape of this space, it is usually measured directly rather than calculated.)

References

Wikipedia (https://en.wikipedia.org/wiki/Compression_ratio)