What is detonation in an aircraft reciprocating engine?

• A. Detonation is an uncontrolled burning of fuel in the cylinder of an engine.
• B. Detonation is a progressive pre-ignition event caused by hot spots.
• C. Detonation is normal combustion.
• D. Detonation is fuel vaporization in the intake system.

Answer: (A)

Detonation is an abnormal, explosive burning of the fuel–air mixture in the cylinder after the spark has ignited the mixture. Instead of a smooth, outward-moving flame front, the unburned portion auto‑ignites or is driven by a shock wave, causing a sudden, sharp rise in cylinder pressure. This rapid pressure spike produces a knocking sound and can damage pistons, rods, and other components if it continues. This is different from normal combustion, where the flame front advances in a controlled way from the spark plug, and from pre-ignition, which is ignition of the mixture before the spark due to hot spots. Detonation isn’t fuel vaporization in the intake. It’s typically promoted by high temperatures, high compression, and/or low-octane fuel, and it’s countered by retarding timing, using higher octane fuel, and keeping the engine cool.