Monday, Nov 12, 2018 at 00:28
The definition of fuel combustion in an internal combustion engine is a controlled, but very rapid burn.
A normal fuel burn can be defined as an explosion - but a better definition of an explosion is one where the speed of the reaction is virtually instantaneous, resulting in serious damage or great destruction.
Detonating cord explodes at a speed not less than 6400 M/sec. Black powder can either burn very rapidly or explode - but when it explodes, it explodes at a speed much less than the explosive in detcord.
Deflagration is the rate at which a flame front moves through a solid mass of propellant.
Deflagration is literally a burning due to a transfer of heat in the material, which separates it from detonation, which is a chemical decomposition caused by the passage of a pressure wave through an explosive.
In an internal combustion engine, the flame front is ignited by the spark plug long before the piston reaches TDC (between 10 and 40 degrees on average).
The flame front then proceeds rapidly through the fuel-air mixture, at an even and controlled burn rate.
However, if the conditions are right, the fuel-air mixture can detonate - effectively, a very rapid and destructive explosion.
Detonation in an engines combustion chamber occurs beyond, and outside the flame front generated by the spark plug.
Detonation in a combustion chamber is initiated by a combination of heat and pressure on the fuel-air mixture, beyond the smoothly-burning flame front, and is characterised by an extremely rapid explosion of the fuel-air mixture, resulting in a major shockwave, generating pressures
well beyond the normal but rapid pressure rise, that is caused by the fuel-air mixture burning at the flame front.
The resultant extremely high pressures caused by detonation, is what causes engine knocking, and this very often results in engine damage.
Cheers, Ron.
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