Knocking Engines

What is meant by knocking? Why does it occurs?

Questions by s.murugan

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  • Aug 20th, 2010

'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.!


  • Sep 5th, 2010

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.

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himanshu shekhar

  • Nov 10th, 2013

knocking is natural phenomenon which occurs in ic engine. no scientist in the world is capable to clearly explain this phenomenon but the explain much part of them....

how it occurs... in petrol engine it occurs always after tdc some of the charge may not burn when this charge burn it collides with the burned charged which will produce pinging sound some where it is harmful but some where it is useful too...

in c.i engine it occurs before tdc but may occurs after tdc the initial drop of diesel which is injected by injector take some time to burn call time lag when this droplet burn it produce heat which further burn the charges the time lag between this burn is produce detonation or we call detonate burning of fuel produce pinging sound...

to remove this remides we should keep in mind of octane as well as cetane rating pre combustion chamber... the thing which decrease the knock tendency in petrol engine is increase the tendency of knock in diesel engine..... so it is typical to understand i had tried to focus some of them in nutshell....

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