What condition can cause detonation due to excessive heat?

Detonation in an engine, particularly in reciprocating engines, occurs when the air-fuel mixture ignites prematurely due to high temperatures and pressures. The presence of low-grade fuel is critical in this context because low-octane fuel may not resist knocking or detonation as well as higher-octane alternatives. Low-grade fuel can ignite at lower pressures and temperatures, especially under conditions of high engine load or when the engine is operating at higher temperatures, such as during climbs or when the ambient temperatures are elevated.

Using higher-octane fuel typically helps to prevent detonation because it has a higher resistance to premature ignition. In contrast, factors like high humidity and increased altitude can influence engine performance but do not directly cause detonation related to excessive heat. High humidity can lead to engine performance issues, while increased altitude generally results in lower air density which can affect engine cooling. Thus, low-grade fuel is the prime contributor to detonation by facilitating ignition under conditions that promote excessive heat.