- Practice · 4+4 marks
Knock and pre-ignition in SI engines
Short answer
Normal combustion: flame front spreads from spark plug. Knocking: unburnt end gas auto-ignites before flame arrival, causing pressure waves. Pre-ignition: ignition by hot spot before spark. Knock reduction: higher octane, lower compression, retarded timing, compact chamber, turbulence, avoiding mixture near richest knock-prone.
(a) Explain how normal combustion takes place in an SI engine and what is meant by knocking. (b) Differentiate between knocking and pre-ignition, and list the factors that reduce knock.
(a) Normal combustion and knock
In an SI engine, a spark ignites the compressed mixture. A thin flame front spreads outward from the plug at 20 to 30 m/s, burning the charge progressively. Pressure rises smoothly and peaks at about 10 to 15 degrees after TDC.
Knocking occurs when the unburnt end gas ahead of the flame front is compressed and heated by the burnt gas until it auto-ignites before the flame front reaches it. The end gas burns almost instantly, creating pressure waves that hit the cylinder walls with a sharp metallic "pinging" sound. Knock causes loss of power and overheating, and may damage pistons and bearings.
(b) Differences and remedies
| Point | Knocking | Pre-ignition |
|---|---|---|
| Cause | Auto-ignition of the end gas after the spark | Ignition by a hot spot before the spark |
| Timing | After spark | Before spark |
| Source | Fuel/charge condition | Hot spark plug, carbon deposits, hot valve edge |
| Sound | Metallic ping | Dull thud, or none |
| Effect | Pressure waves | More compression work, overheating |
| Control | Higher octane fuel | Colder plug, removal of deposits |
Factors reducing knock
- Use a higher octane fuel.
- Lower the compression ratio and the intake temperature.
- Retard the ignition timing.
- Higher turbulence and a compact combustion chamber with a central spark plug (short flame travel).
- Avoid the mixture most prone to knock (about 10 % richer than stoichiometric); much richer or leaner mixtures knock less.
- Better cooling and a clean combustion chamber.
✓ Answer checked by Claude Opus 5.5 (high effort)
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