The knocking sound in engine is produced by which part?

[adil saleem]

hi thr.i hv a q wich m hvng trbl in finding ansr.i jus want info about the source or u can say the part may b that produces knocking.

[Mavrick14]

The typical "knocking" is caused by a rod bearing with excessive clearance. When the bearing is worn down, the connecting rod has freeplay around it's journal, and it knocks.

This could be cause by lack of oil, overheating, a spun bearing (which doesnt get oil), or extreme wear from abuse and lack of maintenance.

A worn or torn harmonic damper also makes a knocking sound as well as piston slap. Piston slap happens when the piston is undersized in it's bore, and it rocks back and forth. The piston skirts slap against the cylinder wall and create a slapping/knocking noise.

[jdmccright]

You may know this already, but the engine's power is caused by a series of controlled explosions inside the cylinders that happen at a precise time in relation to the position of the main shaft, the crankshaft. The car's computer, or ECU, senses and controls these explosions so that they provide, power, fuel economy, and emissions control.

The main problem that the computer has to deal with is that while the engine's RPMS can increase to make more explosions per minute, the rate of combustion of the air and fuel in each cylinder can't be made faster. While it may seem instantaneous, it actually burns at a fairly fixed rate. The ECU knows this limitation, so each explosion is ignited at a progammed point so that the maximum explosive force happens right when the crankshaft is in the best position. So, to compensate for this, the ECU causes the combustion to happen slightly sooner in the power cycle, known as Top Dead Center (TDC), by firing the spark plug just slightly before the crankshaft rotates that cylinder to that point. The explosion pushes down on the piston, which is connected to the crankshaft through a connecting rod.

"Knocking" in an engine is caused by combustion in a cylinder that happens out-of-sync with the programmed timing. Remember, all of the explosions are timed to make the crankshaft turn in one direction. When knocking occurs, some of those explosions happen too soon, so they want to turn the crankshaft in the wrong direction. It is this stress on the crankshaft and rods that can eventually wear out and damage the bearings and journals mentioned before.

Typical causes of knocking are fuel that burns too fast for the ECU to compensate for. This is why there are fuels of different octane ratings. The higher the octane, the more resistance that fuel will have to burn and will burn slower. The other causes are pre- and/or auto-ignition. Preignition is when the fuel/air mixture is ignited by something else in the cylinder, such as an excessively hot spark plug tip or by internal carbon buildup. Autoignition happens when the fuel/air mixture is compressed high enough that the mere act of compressing it heats it high enough to make it ignite. Again, fuels with higher octane ratings have higher resistance to autoignition.

Essentially, if combustion happens when the ECU doesn't expect it, knocking can happen. When knocking happens, engine damage can happen if left unchecked. I know this is a long-winded answer, but hopefully this gives you a more complete background on the subject...ad nauseum. Hope this helps!

[adil saleem]

thnx alot to both of of u maverick n jdmccright for detailed answer.got it.