Explain knocking & role of ant knocking agents in gasoline.

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written 6.9 years ago by

vibhavarikulkarni • 510

Knocking: What is knocking Knocking is the term related to internal combustion engine working on petrol In internal combustion engine a mixture of gasoline vapours and air is used as a fuel. The combustion or burning of fuel is initiated by a spark in cylinder. Due to combustion gases are formed which move piston down the cylinder. The rate of combustion and movement of piston depends upon the combustion of fuel, temperature and design of engine. The movement of piston must be uniform without vibration. But sometimes rate of combustion become so great that the fuel on ignitation, instantaneously produces sudden increase in gaseous volume which causes uneven movement of piston with rattling noise in engine. It is called as knocking of the engine The knocking results in the loss of efficiency of I.C. engine. Thus Knocking is 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. Knocking causes loss of energy & damage to piston & cylinder. The knocking tendency of fuel increases with the composition. It is observed that The straight chain saturated hydrocarbons have more knocking tendency than unsaturated hydrocarbons. The cyclic compounds have less tendency to knock than the straight chain compounds. Aromatic hydrocarbon burn uniformly If double bond is near the centre of chain have less knocking property Ant knocking Agent: The high compression ratio used in internal combustion engines, demands petrol of high quality with least knocking tendency .The octane rating of many fuel can be increased by adding certain antiknock agents to petrol. These compound which are added to vehicular or aviation petrol to improve their knocking property are called antiknock agents. The commonly used antiknock agents are
Tetraethyl lead
Tetra methyl lead
Mixed methylene lead Working of TEL Among the antiknock agents, TEL is more commonly used, since it is cheap and more effective in increasing octane number of fuel. Generally it is used with ethylene dibromide or ethylene dichloride. During combustion TEL forms PbO. These species acts as free radical chain inhibitors and thus prevent propagation of explosive chain and thereby minimizing knocking. PbO formed may get deposited on engine parts and cause mechanical damage. In order to minimize damage caused to the engine parts, TEL is always used with ethylene bromide. The function of these halogen compounds is to convert the less volatile Pb and PbO into more volatile PbBr2, and PbCl2 which escape into air along with exhaust gas

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