Which term describes the ratio of the total cylinder volume above the piston at bottom dead center to the volume above the piston at top dead center?

• A. Stroke
• B. Clearance
• C. Compression ratio
• D. Swept volume

Answer: (C)

Compression ratio describes how much the cylinder volume changes as the piston moves from bottom dead center to top dead center. At bottom dead center, the space above the piston is the total cylinder volume, which equals the swept volume plus the clearance volume. At top dead center, the space above the piston is just the clearance volume. The ratio of these two volumes is (swept volume + clearance) divided by the clearance volume, i.e., the compression ratio. This is the handy way to express how much the air-fuel mixture is compressed. The other terms describe different things: stroke is how far the piston travels, clearance is the small volume at the top, and swept volume is the volume displaced by the piston during a stroke.