A critical review of literature bearing on the autoignition and detonation-wave theories of spark-ignition engine knock and on the nature of gas vibrations associated with combustion and knock results in the conclusion that neither the autoignition theory nor the detonation-wave theory is an adequate explanation of spark-ignition engine knock. A knock theory is proposed, combining the autoignition and detonation-wave theories, introducing the idea that the detonation wave develops in autoignited or afterburning gases, and ascribing comparatively low-pitched heavy knocks to autoignition but high-pitched pinging knocks to detonation waves with the possibility of combinations of the two types of knock.

Marcel Eberbach provides insight into the investigations of the knocking behavior of methane-based fuels and compares them with the knocking behavior of very high knock resistant liquid fuels during engine combustion. With pressure-based knock detection algorithms and thermodynamic evaluation, the atypical knocking combustion phenomena are evaluated with respect to the abnormalities on the heat release curve. Based on the investigated fuels an engine specific relation between the fuel index numbers (RON and MN) and the actual knock resistance of the fuels by means of the motor methane number was established and applied to the investigated gaseous and liquid fuels during knocking combustion.