Where does detonation collide?

In automotive engineering, “detonation” is the long-standing name for knock in a spark-ignition engine: part of the unburned end-gas ahead of the advancing flame heats and compresses until it autoignites, producing a sharp pressure spike and the characteristic metallic knocking sound. Miller's 1947 high-speed-photography study Roles of detonation waves and autoignition in spark-ignition engine knock showed that this event is usually autoignition rather than a true supersonic wave, and König and Sheppard's 1990 work tied knock onset to end-gas chemistry. The referent is a damaging combustion anomaly limited by fuel octane rating, and despite the name it is a different construct from the shock-driven detonation of the explosives physicist.[1][2]

Unit

The intensity and timing of the end-gas knock pressure spike

Recovery means

Not applicable

Measured by

Detected by in-cylinder pressure oscillations and knock sensors, and bounded by the fuel's octane rating

In combustion and explosives physics, a detonation is a genuine supersonic combustion wave in which a leading shock front compresses and heats the reactants so that the chemical reaction occurs right behind the shock and sustains it, travelling at thousands of metres per second, fundamentally different from a subsonic deflagration or ordinary flame. Fickett and Davis's Detonation: Theory and Experiment develops the Chapman–Jouguet and Zel'dovich–von Neumann–Döring theory that fixes the wave's speed and structure. The referent is a shock-coupled reaction front in an explosive or reactive gas, defined by its supersonic self-sustaining speed, and it is an entirely different phenomenon from the end-gas autoignition that engineers loosely call detonation.[3]

Unit

The steady supersonic velocity and structure of the reaction wave

Recovery means

Not applicable

Measured by

Characterised by the detonation (Chapman–Jouguet) velocity, the von Neumann pressure spike, and the reaction-zone length