Mechanical Engineering - IC Engines and Nuclear Power Plants - Discussion

As piston compresses, before the spark is produced, the fuel ignites this is called pre-ignition.

After spark is produced ignition takes place in fuel at some other point as well, both the waves collide creating detonation/knocking.

Preignition occurs due to temp rise in the combustion chamber (due to increase in compression ratio) as spark get generated between two electrodes without supply of voltage from primary battery source. Detonation occurs due to autoignition of end charge in SI engine result in multiple flamefronts causes high velocity sound waves. They set the engine vibrating at the same velocity as the pressure equilisation not take place earlier.

When the unburnt fuel gets auto ignite due to the rise of temperature. It generates thudding sound known as knocking.

After the introduction of spark, Vibrations are generated called detonation.

Can any one please explain what is knocking?

And please explain the difference between knocking and per-ignition?

The detonation in the SI engine occurs due to un-burnt fuel which gets auto ignited due to the rise of temperature causing the detonation to take place where as Knocking at CI engine will occur at the beginning of combustion.

Because when the combustion is produced the piston moves from tdc to bdc in a same manner that process is known as knocking.