Compression Ratio

Compression ratio compares the cylinder’s total volume when the piston is at the bottom of its stroke to the small residual volume left when it is at the top. The total bottom volume is the swept or displacement volume plus the clearance volume, so dividing by the clearance volume tells you how much the air-fuel charge is squeezed before ignition. A higher ratio generally means a more tightly compressed charge, which can improve efficiency and power but also raises the risk of knocking in spark-ignition engines.

Use this formula when analyzing or comparing engine designs, especially to estimate how aggressively an engine compresses its intake charge. It is also useful when checking whether changes to piston shape, head gasket thickness, or combustion chamber volume alter an engine’s compression ratio.

• Using only the displacement volume and forgetting to add the clearance volume in the numerator
• Mixing up V_d and V_c, which gives the wrong ratio
• Writing the answer as a plain number without expressing it as a compression ratio such as 9:1