Quote:
Originally Posted by iorhld
at low rpm when the throttle is partially open,the ignition timing is retarded.the flame front from the spark plug travels slowly.as a result,there are two individual seperate combustion inside the chamber,one near the spark plug and other at the very extreme end of the cylinder.as a result of two seperate combustions,we hear two distinct sounds in a single cycle of piston operation.thus the "knocking" term is used to describe the effect.
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Actually, that's not true. With retarded ignition timing the fuel air mixture still burns at the same speed as it would otherwise.
Knocking in an engine is a phenomenon that is caused by premature ignition or detonation.
Premature ignition can happen in an engine with too much of carbon deposits or because of a dirty spark plug, it's generally anything that gets too hot in the combustion chamber and ignites the fuel. Because the fuel burns while the piston is still in the compression stroke it will hammer the piston down into the connecting rod and crank. This causes the piston to rattle and you can hear it.
Detonation can happen when the fuel is of inferior quality or in high powered engines with high compression due to too low octane rating of the fuel. It happens when the fuel air mixture ignites by itself because of exceeding temperature and pressure at which the fuel will not ignite automatically. When this happens the fuel air mixture tends to burn very rapidly in what is called detonation, which flame speeds much higher than you would get with a normal combustion ignited by the spark plug. This causes a very high pressure spike in the combustion chamber. Again this can be heard.
Retarding ignition timing is done to prevent pressure from building up too fast, so the piston can reach TDC before the pressure really starts to push it back down. Advancing ignition timing is done at high RPM to make sure the fuel burns on time to give the best push down on the piston during the work stroke.
I could be off by a few bits here and there, but as far as I know this should be mostly correct.
Also see:
http://en.wikipedia.org/wiki/Engine_knocking