Knocking Engines

'Knocking' is the sound produced in the engine due to incomplete/untimely combustion of fuel. Knocking tendency of engine can be reduced by increasing the Octane/Cetane number of fuel being used in engine. The fuel companies add some additives in the fuel to increase the octane[for petrol]/cetane[for diesel] no. to reduce the Knocking tendency of the engine. Over-heating may also sometimes lead to knocking.

Near the end of compression stroke of the piston, the spark is introduced to
the combustion chamber, after which the combustion starts, which further leads
to expansion stroke! but sometimes, due to high pressure and temperature, the
air fuel mixture self ignites and starts burning.  This happens because the
air fuel mixture has attained the self ignition temperature, before the spark
has been introduced.

This pre-combustion leads to incomplete compression stroke and expansion
stroke begins before the completion of compression stroke and thus producing a
hitting sound in the engine.

This is called knocking.!

In case of petrol engine: flame-front travels from spark plug. If proper
mixing of petrol with air doesn't take place charge at the end gets accumulated
when flame-front reaches to that that fuel burns faster than other which makes
sound known as knocking.  Hence knocking in petrol engine takes place at
the end of compression stroke.

"knocking" or "pinking" is the phenomenon in which pressure wave of high altitude generated strikes wall of combustion chamber.pre. waves are generated because fuel attain self ignition temp. before flame front created by spark plug reaches to it.
THE MAIN FACTOR RESPONSIBLE IS PRE-IGNITION.