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What is Knocking?


"A sharp metallic sound produced in the internal combustion of engine is known as knocking."It results in a huge loss of energy.

Gasoline or petrol is used as a fuel in internal combustion of engine. The vapours of petrol mixed with air are compressed by piston and allowed to burn in the cylinder of engine after sparking. This produces large expansion resulting in the movement of piston and consequently the automobile.

But if the petrol is of such quality that its vapours mixed with air can ignite prior to high compression on passing of firing spark, the energy produced by burning of fuel is not utilized in moving the piston, rather. is lost in creating a metallic sound, called knocking. 

"It is infact maximum in petrol containing straight chain paraffins but is not produced in branched chain compounds."

The tendency to knock decreases in the order is straight chain alkanes > branched chain alkanes > olefins > cycloalkanes  aromatic hydrocarbons.

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