8. Knocking And Octane Number Of Petrol 3952t

 Knocking 1) Knocking is a term related to the internal combustion engine working on petrol. 2) It an internal combustion engine (gasoline engine or spark ignition type engine ) a mixture of air and petrol vapor is compressed and ignited by an electric spark 3) An essential reaction i.e. oxidation of hydrocarbons take place. It is essential that combustion of fuel in cylinder of an ICR should proceed in a regular way. 4) A Flame should spread rapidly and smoothly through gas mixtures which produce the gases and expanding gases drives the piston. 5) In certain circumstances rate of oxidation is so great that mixture detonates producing rattling sound called knocking of engine r or engine knock 6) Thus knocking can be defined as “a sharp metallic sound similar to rattling of hammer, which is produced in the internal combustion engine due to immature ignition of the air- gasoline mixture”. 7) Rate of oxidation of hydrocarbon molecule depends on number of carbon atoms, structure of molecule and temperature. The temperature in turn depends upon compression ratio i.e. ratio of cylinder volume at end of suction stroke to that at the end of compression stroke of piston. 8) The tendency to knock depends not only on fuel and fuel air ratio but also on engine design, shape of piston head, location of spark plug and upon running conditions. 9) Knocking results in loss of efficiency of engine causing fuel wastage and increased wear and tear of engine. Octane Number of Petrol 1) It has been found that is o- octane produces a very little knock while on contrary nheptane produces high knocking in the in the engine 2) Hence for n-heptane octane number is taken as zero and for is o –octane it is 100 3) Thus octane number of a petrol sample it is defined as “the percentage of is ooctane in the mixture of is o-octane and n-heptane which has similar knocking to the petrol sample”. 4) The relation between octane number of a hydrocarbon and its structure can be given as follows. Aromatic > Cycloalkane > alkene > branched alkane > straight chain alkane 5) Octane number of petrol can be increased by addition of anti knocking agents, adjusting compression ratio, addition of benzene or toluene.