The ratio of the volume of gas at the end of the suction stroke to the volume of gas at the end of the compression stroke is known as the compression ratio.
The power output and engine efficiency depend on this ratio.
The compression ratio of seven or eight is required for obtaining maximum efficiency of engine the compression ratio corresponding to the maximum power output of the engine is known as highest useful compression ratio or critical compression ratio. At critical ratio slight knock or rattling can be heard.
It is the ratio is the volume above the piston when it is at the bottom-most position (BDC) to the volume above the piston when it is at the top-most position (TDC).
The compression ratio indicates the extent to which the air-fuel mixture is compressed in the engine.
