05-21-2006, 07:38 AM #1
Definition of Pre - Detonation (ABNORMAL COMBUSTION)
Pre-Detonation / Pre-Ignition; is basically Abnormal Combustion.
This is basically an uncontrolled explosion in the combustion chamber caused by any one or combination of more than one of the items listed above as restrictors of ignition timing.
In normal combustion the spark ignites the compressed fuel/air mixture and a smooth burn travels through the combustion chamber and building combustion chamber pressure as it goes. This flame travels through the chamber by the time the crankshaft has moved about 15 to 30 degrees after top dead center (ATDC).
In Abnormal combustion the air, something in the combustion chamber or even the whole chamber is too hot for the fuel or it is compressed too high which itself causes the temp to raise too high, causes the fuel to explode.
This explosion produces extremely high temperature in a localized spot and causes a pressure spike so high and quick that it is like a hammer strike.
This extremely high temp/pressure spike can cause extreme damage too. The weakest point in a chamber gives way first. This hammer like strike hits the top of the piston and attempts to go down between the piston and cylinder wall and hits the top ring. which slams down on the ring land below it, braking it down against the second ring. At the same time, the extreme localized heat starts to melt the aluminum piston.
On true hemispherical engines (spark plug in the center of the chamber) The extreme heat can first weaken the piston closer to the center and the pressure spike can knock a hole in the center of the piston.
In a more wedge shaped chamber (plug on the side of the chamber) the damage usually in done opposite the plug.
Abnormal combustion damage is easy to ID. The underside of the piston crown is blackened, the ring land just below the top ring on the opposite side from the plug is broken in a downward pattern and the land above the top ring will be melted and burned away opposite the plug. There is also damage to the top rod bearing as the hammer effect usually beats out the rod bearing.
Here is a good link to read also
Here are some good detonation pics
More Detonation info
05-21-2006, 04:16 PM #2
Piston Detonation Pictures
some pics of piston detonation
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