ICE-27 — Knock

ASSUMPTION — This lesson covers knock conceptually; numeric octane values are not specific fuel claims.

In this lesson you will learn:

• The knock mechanism
• Its effects
• Control methods

1. What Is Knock?

Knock is the spontaneous, uncontrolled ignition of the end-gas — the mixture the flame front has not yet reached — in a gasoline engine.

FACT — In normal combustion a flame front spreads from the spark. If the end-gas self-ignites under pressure/temperature before the flame arrives, a sharp, irregular pressure wave results — the “knock” sound and mechanical stress.

2. Effects

FACT — Severe knock can damage piston and combustion-chamber surfaces, cause overheating and power loss. Engines are therefore operated to avoid it.

3. Control

FACT — A knock sensor (piezoelectric) detects knock’s characteristic vibration; the ECU retards spark advance temporarily. Once conditions improve, advance is restored.

4. Octane Relationship

INTERPRETATION — Higher-octane fuel resists auto-ignition (knock), allowing more aggressive advance and/or higher compression ratio. Octane number (RON/MON) measures that resistance.

5. Summary

• Knock is end-gas auto-ignition.
• It can damage the engine.
• Controlled by knock sensor + spark retard.
• Octane determines knock resistance.

6. Sources

• Heywood, Internal Combustion Engine Fundamentals
• Bosch, Gasoline-Engine Management

Next Lesson

• ICE-28 — Pre-ignition.