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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 and deflagrations being the categories of explosions.
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 by the detonation of the unburnt portion of the fuel
(knocking), due to its overly high sensitivity to heat and pressure of the
particular fuel under certain conditions, resulting in a pressure wave.
This force is extremely
destructive to engines, and often results in holes blown through the top of
pistons or cracks in cylinder heads.
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. |