Engine knock is an abnormal combustion event in a gasoline engine that can sound like pinging, rattling, or tapping under load. In normal operation, the spark plug ignites the air fuel mixture and a smooth flame front spreads across the combustion chamber. Knock happens when part of the mixture burns too early or too suddenly, creating sharp pressure waves.
Understanding knock matters because repeated knocking can damage pistons, valves, spark plugs, and cylinder heads.
Key Facts
- Normal spark ignition: spark plug fires before top dead center so peak pressure occurs just after top dead center.
- Engine knock occurs when the end gas autoignites and creates fast pressure waves in the cylinder.
- Pre-ignition happens when the mixture ignites before the spark, often from a hot spot such as a glowing deposit or overheated spark plug.
- Compression ratio = cylinder volume at bottom dead center / cylinder volume at top dead center.
- Higher temperature, higher pressure, too much spark advance, and low octane fuel increase knock risk.
- Octane rating measures a fuel's resistance to autoignition, not how much energy the fuel contains.
Vocabulary
- Engine knock
- Engine knock is abnormal combustion that produces sharp pressure waves and a pinging or rattling sound in the cylinder.
- Pre-ignition
- Pre-ignition is combustion that starts before the spark plug fires because a hot surface ignites the air fuel mixture.
- Autoignition
- Autoignition is when a fuel mixture ignites by heat and pressure without being lit by a spark.
- Octane rating
- Octane rating is a measure of how well gasoline resists autoignition under compression.
- Top dead center
- Top dead center is the piston position where it reaches the highest point in the cylinder.
Common Mistakes to Avoid
- Confusing knock with normal engine noise: knock is caused by abnormal combustion pressure waves, not just moving metal parts.
- Thinking higher octane fuel always gives more power: octane mainly prevents autoignition, and it only improves performance if the engine can use it to avoid knock or run more aggressive timing.
- Assuming pre-ignition and knock are the same event: pre-ignition starts before the spark, while knock usually happens after the spark when unburned end gas autoignites.
- Advancing ignition timing too far: firing the spark too early can raise pressure before the piston is ready, increasing knock risk and reducing reliability.
Practice Questions
- 1 A cylinder has a volume of 520 cm3 at bottom dead center and 65 cm3 at top dead center. Calculate the compression ratio.
- 2 An engine normally sparks 12 degrees before top dead center, but the timing is advanced by 6 degrees. What is the new spark timing before top dead center, and why might this increase knock risk?
- 3 A driver hears pinging while accelerating uphill on a hot day using low octane fuel. Explain which conditions in the cylinder make knock more likely and name two changes that could reduce the problem.