Detonation may occur at high-power settings when

• A. The fuel mixture ignites instantaneously instead of burning progressively and evenly
• B. The engine runs too rich
• C. The engine is cold
• D. The spark plugs are fouled

Answer: (A)

Detonation happens when the end portion of the combustible mixture in the cylinder auto-ignites due to the high temperature and pressure reached at high power. In a normal burn, the spark starts a flame front and the mixture burns progressively across the chamber. With detonation, that remaining gas ignites almost instantly instead of being burned in a controlled, gradual manner, causing a sharp pressure spike or “knock.” This is more likely at high power settings because the pressures and temperatures are elevated, which lowers the resistance to spontaneous ignition of the end gas. That instantaneous ignition is the hallmark of detonation, and it can be damaging if it continues.

The other options describe conditions that lead to different issues (rich mixture causing rough running, cold engine reducing heat and detonation risk, fouled plugs causing misfiring), not the instantaneous end-gas ignition characteristic of detonation.