What is the compression ratio formula?

• A. (Swept volume) / (clearance volume).
• B. (Swept volume + clearance volume) / clearance volume.
• C. (Clearance volume) / (swept volume + clearance volume).
• D. (Clearance volume + residual volume) / swept volume.

Answer: (B)

Compression ratio tells you how much the cylinder volume is reduced when the piston moves from bottom dead center to top dead center. The swept volume is the volume the piston displaces during a stroke, and the clearance volume is the small space above the piston when it’s at top dead center. At bottom dead center the total gas volume is swept plus clearance, while at top dead center it’s only the clearance. So the compression ratio is the total volume divided by the clearance: (swept volume + clearance volume) / clearance volume.

This is the form that correctly captures how much the cylinder volume decreases, which is why it’s the right choice. The other options either omit the swept volume, invert the ratio, or mix in a volume (like residual volume) that isn’t used in calculating standard compression ratio.