What is the definition of the compression ratio in an engine?

The compression ratio in an engine is defined as the ratio of the volume of the cylinder with the piston at Bottom Dead Center (BDC) compared to the volume of the cylinder with the piston at Top Dead Center (TDC). This measurement indicates how much the air-fuel mixture is compressed within the cylinder as the piston moves from BDC to TDC.

When the piston is at BDC, the cylinder contains the maximum volume, which includes both the combustion chamber and the space above the piston. Conversely, when the piston is at TDC, the volume is at its minimum, mainly consisting of the combustion chamber. The compression ratio illustrates the efficiency and potential power output of the engine; a higher compression ratio generally leads to improved thermal efficiency and power, assuming proper fuel is being used.

In contrast, the other options do not accurately capture the essence of the compression ratio. The definition related to total volume of the fuel-air mixture addresses the content within the cylinder but lacks the critical comparative aspect of TDC and BDC. The option regarding maximum pressure achieved during combustion focuses on performance outcomes rather than the specific physical dimensions that create the compression ratio, which is primarily concerned with volume changes rather than pressures. Lastly, the first option inaccurately describes the compression ratio