What does detonation refer to in an aircraft reciprocating engine?

Detonation in an aircraft reciprocating engine specifically refers to a type of combustion where the fuel-air mixture ignites in an uncontrolled manner, leading to a rapid increase in pressure. This phenomenon occurs after the piston has already reached top dead center (TDC) and is not in line with the intended timing of ignition. Normal combustion, which ideally occurs with a specific timing controlled by the spark plug, is a smooth process where the mixture ignites just before the piston reaches TDC, allowing for optimal performance and efficiency.

When detonation occurs, the combustion happens too late in the cycle, often as a result of high temperatures, high pressures, or the use of low-octane fuel, which can cause knocking and potentially severe engine damage. Thus, the correct answer highlights the importance of understanding combustion timing and its essential role in the health and efficiency of reciprocating engines.