What is pre-ignition and how does it differ from detonation?

• A. Pre-ignition is ignition of the fuel-air mixture from a hot spot before spark; detonation is a secondary flame front after spark.
• B. Pre-ignition is the normal ignition; detonation is delayed ignition.
• C. Pre-ignition is the fuel-air mixture exploding after compression; detonation is pre-ignition from high heat.
• D. Pre-ignition is ignition by the spark plug; detonation is ignition by hot spot.

Answer: (A)

Pre-ignition happens when the fuel-air mixture in the cylinder ignites before the spark plug fires, usually from a hot spot in the chamber such as a hot electrode, carbon buildup, or an overheated component. Detonation, on the other hand, is the end gas auto-igniting after the spark has already occurred, creating a second flame front (and often a shock wave) that can cause a violent pressure rise. The key difference is timing relative to the spark: pre-ignition is ignition before the spark; detonation is ignition after the spark.