If the volume of a cylinder with a piston at the bottom center is 84 cubic inches and the piston displacement is 70 cubic inches, then the compression ratio is

• A. 5:1
• B. 7:1
• C. 6:1
• D. 8:1

Answer: (C)

The key idea is how compression ratio is defined: it compares the total cylinder volume at bottom dead center to the clearance (or head) volume remaining when the piston is at top dead center.

Here, the total volume at bottom dead center is 84 cubic inches. The piston displacement, which is the swept volume the piston moves through, is 70 cubic inches. The clearance volume is what’s left when the piston is at top dead center: 84 − 70 = 14 cubic inches.

Compression ratio = total volume / clearance volume = 84 / 14 = 6. So the ratio is 6:1. (Note: using the swept volume in the numerator would give 70/14 = 5:1, which isn’t the correct definition.)