Which of the following statements about detonation and high engine temperatures is true?

• A. Detonation occurs only at low engine temperatures and under light load.
• B. Detonation can be avoided by using higher compression ratio.
• C. Detonation is caused by explosive, unplanned combustion of the fuel-air mixture and can be avoided by proper fuel grade, mixture, and avoiding high density altitude or high engine temperatures.
• D. Pre-ignition equals detonation.

Answer: (C)

Detonation is unplanned, explosive combustion of the fuel-air mixture in the end gas behind the flame front, causing a shock wave that can ping the engine. It tends to occur when the end gas is compressed to high temperatures and pressures, which is aggravated by hot engine temperatures, high power settings, and conditions like high density altitude. The way to prevent it is to ensure the fuel can resist auto-ignition (use the proper octane fuel), use the correct fuel-air mixture to avoid overly hot spots, and avoid operating at conditions that push the engine temperature or density altitude high. That’s why this statement is true: it correctly describes what detonation is and how it can be avoided. By contrast, using a higher compression ratio would raise detonation risk, not prevent it, and detonation is not the same as pre-ignition.