--- title: "Knocking in SI engines and its control" subject: "Internal Combustion Engines" chapter: 5 key: "semester_6/internal_combustion_engines:knocking-in-si-engines-control" source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/knocking-in-si-engines-control site: BCT Engineering --- # Knocking in SI engines and its control Internal Combustion Engines · Chapter 5: Combustion in SI and CI Engines - Asked in: year not given, 8 marks - Video: [Detonation Engine || Detonation in SI engine || Detonation and Knocking || Gear institute](https://www.youtube.com/watch?v=aep822ScjxY) — Gear Institute Mechanical Engineering Videos - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/knocking-in-si-engines-control - Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-5 ## Question 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. ## 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. ## Answer #### 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 | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What causes knock in an SI engine? A. Ignition of the charge before the spark by a hot spot B. Auto-ignition of the end gas before the flame front reaches it C. Late injection of fuel D. Too long an ignition lag in the spark plug Answer: B. Knock is auto-ignition of the unburnt end gas, which releases energy almost instantly and sends a pressure wave to the walls. (Key point: Cause of knock) 2. How does spark timing affect knock? A. Advancing the spark reduces knock B. Retarding the spark reduces knock C. Spark timing has no effect D. Only a very early spark stops knock Answer: B. Retarding the spark lowers peak pressure and temperature of the end gas, so knock is reduced; knock sensors do this automatically. (Key point: Effect of spark timing) 3. Which statement correctly differentiates pre-ignition from knocking? A. Pre-ignition is auto-ignition of the end gas after the spark B. Pre-ignition is ignition by a hot spot before the spark, early in compression C. Pre-ignition is cured by raising the octane number D. Pre-ignition occurs late in combustion Answer: B. Pre-ignition is ignition of the charge before the spark by a hot spot such as a deposit or plug tip; the remedy is removing hot spots or a cooler plug. (Key point: Knock vs pre-ignition) 4. Which change helps to control knock? A. Raise the compression ratio B. Raise the inlet temperature C. Use a compact chamber with high turbulence and short flame travel D. Use a fuel of lower octane number Answer: C. Compact chambers with turbulence and short flame travel burn the end gas before it can auto-ignite; the other changes increase knock. 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