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Knock knock!
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Mon, 23 May 2005 08:37:51 Share Post by Email
In Reply to: What is Pre-Ignition, Buz, Mon, 23 May 2005 08:10:15
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Pre-ignition is also known as knock, ping, and pre-detonation. It occurs when the air/gas mixture in the cylinder ignites before it is supposed to in the compression cycle. Usually, it's early, while the piston is still coming up.

Pre-ignition/knock can be caused by a few things. In cars where the timing is adjustable, setting the timing too soon - the spark plug fires too soon for the piston/valve orientation.
The common cause is too high a compression for the fuel octane. Fuel octane is a measure of resistance to knock/predetonation. When the piston comes up, it squeezes the mixture. This compresses and heats it. Squeeze it enough and it'll blow up by itself. That's how a diesel engine works. The higher the octane rating of the fuel, the more resistant it is to blowing up when squeezed. What can cause too high a compression? If there is carbon buildup on the piston and head, it makes the cylinder smaller, so it will compress more. Turbo engines blow in pressurized air, which makes the effective compression ratio higher - so there is a tendancy to knock at high boost.
You can also knock if there is a hot spot inside the cylinder, that touches off the ignition early. That could be due to carbon buildup, or a spark plug that's too hot.


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