ICE-06 — Compression Ratio

ASSUMPTION — The ranges given here are typical orders of magnitude that vary by technology; they are not a specific engine’s exact values.

In this lesson you will learn:

• The definition and calculation
• The gasoline vs diesel difference
• The relationship to efficiency and knock

1. Definition

Compression ratio (CR) is the total cylinder volume at BDC divided by the remaining (clearance) volume at TDC.

FACT — CR = (Vd + Vc) / Vc, where Vd = swept volume and Vc = clearance volume (including the combustion chamber). Example: 450 cc swept + 50 cc clearance → CR = 500/50 = 10:1.

2. Gasoline vs Diesel

Engine	Typical CR	Limiting factor
Gasoline (Otto)	~9:1 – 13:1	Knock
Diesel	~14:1 – 23:1	Mechanical loads

FACT — In gasoline the compression ratio is limited by knock (the mixture can self-ignite during compression, ICE-27). Diesel compresses only air and injects fuel later, so it has no such limit and uses a higher CR.

3. Efficiency Relationship

FACT — In the ideal cycle, thermal efficiency rises with compression ratio. This is why diesels generally reach higher thermal efficiency — though friction and heat losses limit real gains.

4. Modern Approaches

INTERPRETATION — Higher CR is not always “better”: knock risk, mechanical loads and emissions are trade-offs. The goal is the optimum CR for the fuel and application.

5. Summary

• CR = (swept + clearance) / clearance.
• Gasoline uses lower CR (knock limit), diesel higher.
• Higher CR generally means higher efficiency but more load/knock risk.

6. Sources

• Heywood, Internal Combustion Engine Fundamentals
• Bosch Automotive Handbook

Next Lesson

• ICE-07 — Torque & Power: engine performance quantities.