---
title: "Knock and pre-ignition in SI engines"
subject: "Automotive Technology"
chapter: 3
key: "semester_5/automotive_technology:knock-and-pre-ignition-si-engine"
source: https://www.bctengineer.com/q/semester_5/automotive_technology/knock-and-pre-ignition-si-engine
site: BCT Engineering
---

# Knock and pre-ignition in SI engines

Automotive Technology · Chapter 3: Ignition and Combustion

- Asked in: year not given, 4+4 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_5/automotive_technology/knock-and-pre-ignition-si-engine
- Whole chapter: https://www.bctengineer.com/md/semester_5/automotive_technology/chapter-3

## Question

(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.

## 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.

## Answer

#### (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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is knocking in an SI engine?
   A. Ignition by a hot spot before the spark
   B. Auto-ignition of the unburnt end gas before the flame front reaches it
   C. Misfire due to a weak spark
   D. Late burning of fuel in exhaust
   Answer: B. End gas is heated and compressed by the burnt gas, auto-ignites, and creates pressure waves and a metallic ping. (Key point: Cause of SI knock)

2. How does pre-ignition differ from knocking?
   A. It occurs after the spark from end gas
   B. It occurs before the spark from a hot spot
   C. It is cured by higher octane
   D. It has a metallic ping
   Answer: B. Pre-ignition starts from a hot spot (plug, deposit, valve edge) before the spark; it is controlled by a colder plug and clean deposits. (Key point: Knock vs pre-ignition)

3. Which change reduces knock?
   A. Higher compression ratio
   B. Higher intake temperature
   C. Retarded ignition timing
   D. Spark plug at the edge of the chamber
   Answer: C. Retarding timing reduces end-gas temperature and pressure; the others increase knock tendency or flame travel. (Key point: Factors reducing knock)

4. How does normal SI combustion proceed?
   A. Whole charge auto-ignites together
   B. A flame front spreads progressively from the plug at 20 to 30 m/s
   C. Fuel burns as it is injected
   D. Burning starts at the cylinder walls
   Answer: B. A thin flame front spreads from the plug, burning the charge progressively, giving smooth pressure rise. (Key point: Normal combustion)

## Related in Automotive Technology

- [Phases of combustion in a CI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Acombustion-ci-engine-phases) — Automotive Technology, chapter 3
- [Requirements of SI and CI engine fuels](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Asi-engine-fuel-requirements) — Automotive Technology, chapter 2
- [Air flow and fuel flow of an SI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Afuel-consumption-air-flow-numerical) — Automotive Technology, chapter 2
- [Two-stroke versus four-stroke engines](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Atwo-stroke-vs-four-stroke) — Automotive Technology, chapter 1
- [Battery ignition system and spark timing calculation](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Abattery-ignition-spark-numerical) — Automotive Technology, chapter 3

## Related in other subjects

- [Knocking in SI engines and its control](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aknocking-in-si-engines-control) — Internal Combustion Engines, chapter 5
- [Differentiate SI and CI engines](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Asi-versus-ci-engine-differences) — Internal Combustion Engines, chapter 2
- [Stages of CI combustion and diesel knock](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Adiesel-combustion-stages-knock) — Internal Combustion Engines, chapter 5
- [Stages of SI combustion and flame propagation](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Astages-combustion-si-flame-propagation) — Internal Combustion Engines, chapter 5