Detonation in a reciprocating aircraft engine occurs when what condition is met?

Detonation in a reciprocating aircraft engine specifically occurs when the unburned fuel/air mixture combusts instantaneously. This rapid combustion is a critical factor because it leads to a sudden spike in pressure and temperature within the combustion chamber, which can cause significant mechanical stress and damage to engine components.

In normal combustion, the fuel and air mixture ignites in a controlled manner, allowing for a smooth increase in pressure and power production. However, when detonation occurs, the mixture doesn't burn progressively as intended but instead results in a shock wave that disrupts the engine's normal operation.

While conditions such as overly compressed fuel/air charges can contribute to the risk of detonation, it is the instantaneous combustion of the unburned mixture that defines the event of detonation itself. Low engine oil temperatures might affect engine performance but are not directly responsible for causing detonation.