Compression ratio is the ratio of which quantities?

• A. Ratio of intake to exhaust pressure
• B. Ratio of bore to stroke
• C. Ratio of total cylinder volume above the piston at BDC to volume above the piston at TDC
• D. Ratio of fuel to air

Answer: (C)

Compression ratio measures how much the air–fuel charge is compressed inside the cylinder. It is defined as the ratio of the cylinder volume when the piston is at bottom dead center (the largest volume) to the volume when the piston is at top dead center (the smallest volume). In practical terms, that means the ratio of (swept volume plus the clearance volume) to the clearance volume. This is exactly the description of comparing the total cylinder volume above the piston at BDC to the volume above the piston at TDC. Higher compression ratios usually improve thermal efficiency up to practical limits, but too high a ratio can cause knocking or pre-ignition. The other ideas describe pressures, engine geometry, or fuel–air mixture—none of which define compression ratio.