Detonation
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Detonation is a process of supersonic combustion that involves a shock wave and a reaction zone behind it. The shock compresses the material thus increasing the temperature to the point of ignition. The ignited material burns behind the shock and releases energy that supports the shock propagation. This self-sustained detonation wave is different from a deflagration that propagates with a subsonic speed and without a shock. Detonations generate high pressures and are usually much more destructive than deflagrations.
Detonations can be produced by high explosives, reactive gaseous mixtures, certain dusts and aerosols. Thermonuclear detonations are believed to be involved in Type Ia supernova explosions.
Detonations are hard to control and are used primarily for demolition and in warfare. A great deal of research is conducted on achieving or preventing detonation in various materials to improve the performance of explosives and engines. An experimental form of jet propulsion, the pulse detonation engine, uses detonation to generate thrust.
Detonation in otto-cycle engines is caused usually by the pressure wave from ignition transforming into detonation of the unburnt portion of the fuel (knocking) due to its overly high sensitivity to heat and pressure. This force is extremely destructive to engines, and often results in holes blown through the top of pistons or engine blocks. This is different from knocking in diesel engines, which use heat and pressure as the primary source of ignition. In diesel engines, low sensitivity to these conditions results in ignition being delayed while a highly detonable mixture accumulates, causing detonation when ignition finally occurs. Diesel fuel has a cetane number much like gasoline has an octane rating, except that it represents increasing sensitivity while the octane rating represents decreasing sensitivity.
See also
- Detonation section of Explosive, Detonator.nl:Detonatie