Detonation is the sudden release of heat energy from the fuel-air mixture caused by reaching its critical pressure and temperature.

• A. Detonation
• B. Pre-ignition
• C. Knock
• D. Misfire

Answer: (A)

Detonation is the rapid, explosive combustion of the remaining, unburned fuel–air mixture in the cylinder once the mixture reaches a critical pressure and temperature, causing auto-ignition and a shock-like pressure rise. This unplanned, abrupt energy release can produce a harsh knock and have the potential to damage the engine.

Pre-ignition, by contrast, is ignition of the mixture before the spark due to hot spots in the cylinder, not tied to the flame front timing after the spark. Misfire means a cylinder fails to ignite at all. Knock is the audible result often associated with detonation, but detonation refers to the explosive energy release itself.