- Practice · 8 marks
Knocking in SI engines and its control
Short answer
Knocking is auto-ignition of the end gas after the spark, causing pressure waves and damage. It is influenced by compression ratio, fuel octane, spark timing, and temperature; control methods include using high-octane fuel, retarding spark, and compact chamber design. Knocking occurs after normal ignition, while pre-ignition is ignition before the spark from hot spots.
What is knocking in an SI engine? Explain the process, its effects, the factors affecting it and the methods of controlling it. Differentiate between knocking and pre-ignition.
Knocking
Knock is the metallic pinging sound caused by the auto-ignition of the end gas (the last part of the unburnt charge) before the normal flame front reaches it.
Process
The advancing flame compresses the unburnt end gas and heats it by compression and by radiation from the flame. If the temperature and pressure are high enough for long enough, the end gas auto-ignites in several places and releases its energy almost instantly. This produces a high-frequency pressure wave (shock) that strikes the cylinder walls and causes the pressure-trace oscillations and noise.
Effects
- Loss of power and efficiency, rough running and noise.
- High local heat transfer, which can burn the piston crown and cause piston, ring or head damage.
- Overheating and increased fuel consumption.
Factors affecting knock
| Factor | Change to reduce knock |
|---|---|
| Compression ratio | Lower |
| Inlet temperature and pressure | Lower |
| Spark advance | Retard |
| Mixture strength | Use slightly rich (or very lean) |
| Fuel octane number | Higher |
| Combustion chamber | Compact, spark plug central, high turbulence, cool exhaust valve |
| Engine speed | Higher speed means more turbulence and less time for auto-ignition |
| Cylinder temperature | Lower (good cooling) |
Control of knock
- Use a high-octane fuel or additives.
- Reduce the compression ratio, charge temperature and pressure; use intercooling in turbocharged engines.
- Retard the spark (knock sensors do this automatically).
- Design compact chambers with turbulence, short flame travel and the plug near the hot exhaust valve.
- Increase engine speed, use more cooling for the end gas, and enrich the mixture.
Knocking versus pre-ignition
| Knocking | Pre-ignition |
|---|---|
| Auto-ignition of end gas after the spark | Ignition of the charge before the spark, by a hot spot (deposit, plug tip) |
| Occurs late in combustion | Occurs early, in compression |
| Remedy: raise octane, retard spark | Remedy: remove hot spots, use cooler plug |
| It can lead to pre-ignition | It can start knock; causes serious power loss |
✓ Answer checked by Claude Opus 5.5 (high effort)
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