Short notes on Knocking

A four stroke ,spark ignition engine

 Firstly, the fuel vapours with air in carburetter is drawn into the cylinder during the induction part and this step is known as suction stroke.
then, the fuel air mixture is compressed it is known as compression stroke
The mixture is then ignited by an electric spark during combustion the hot gases are produced and increase the pressure and push the piston down is known as power stroke.
In last stroke, the piston expels the exhaust gases from cylinder, it is known as exhaust stroke.

Compression Ratio

     Volume of the gas at the end of the   suction  stroke     (V1)
    Volume of the gas at the end of the compression stroke(V2)

Greater the compression ratio more is the efficiency.
The rate of Oxidation becomes so great that the last portion of the fuel-air mixture gets ignited instantaneously,producing an explosive violence, known as knocking.

Straight chain paraffin>branched chain paraffins>Olefins>cyclo paraffin>naphathalene>Aromatics.

Thus,Olefins of the same carbon-chain length possess better anti-knocking properties than the corresponding paraffins and so on








 Knocking is sharp metallic sound similar to rattling of a hammer produced in internal combustion engines due to uneven of unsteady or uncontrolled combustion of air gasoline mixture.