Compression ratio calculation — Using swept and clearance volumes An engine has a clearance volume of 100 cm³ and a swept volume of 800 cm³. What is its compression ratio?

Introduction / Context:
The compression ratio is a key geometric parameter affecting thermal efficiency, knock tendency, and combustion speed. It is computed directly from the cylinder's swept and clearance volumes.

Given Data / Assumptions:

• Swept volume Vs = 800 cm³.
• Clearance volume Vc = 100 cm³.
• Single cylinder equivalent; ratio is independent of cylinder count.

Concept / Approach:
Compression ratio CR is defined as total volume at bottom dead centre divided by clearance volume at top dead centre:
CR = (Vs + Vc) / Vc
This uses plain arithmetic with volumes in consistent units.

Step-by-Step Solution:
Compute total volume at BDC: Vs + Vc = 800 + 100 = 900 cm³.Divide by clearance volume: 900 / 100 = 9.Express as a ratio: 9:1.

Verification / Alternative check:
Reverse check: Vc = (Vs + Vc) / CR → 900 / 9 = 100 cm³, matching the given.

Why Other Options Are Wrong:
7:1 or 8:1: would imply smaller Vs or larger Vc than provided.10:1 or 11:1: would require either larger Vs or smaller Vc.

Common Pitfalls:
Forgetting to add swept and clearance volumes before dividing.Mixing units (e.g., cm³ and mm³); here units are consistent.

Final Answer:
9:1