Leaded Petrol (Anti-Knock Agent)

LEADED PETROL (ANTI-KNOCK AGENT)

The anti-knock properties of a gasoline can be improved by the addition of suitable additives. Tetra Ethyl Lead (TEL) (C₂H₅)₄ Pb is an important additive added to petrol. Thus the petrol containing tetra ethyl lead is called leaded petrol.

Mechanism of knocking

TEL reduces the knocking tendency of hydrocarbon. Knocking follows a free radical mechanism, leading to a chain growth which results in an explosion. If the chains are terminated before their growth, knocking will cease. TEL decomposes thermally to form ethyl free radicals which combine with the growing free radicals of knocking process and thus the chain growth is stopped.

Disadvantages of using TEL

When the leaded petrol is used as a fuel, the TEL is converted to lead oxide and metallic lead. This lead deposits on the spark plug and on cylinder walls which is harmful to engine life. To avoid this, small amount of ethylene dibromide is added along with TEL. This ethylene dibromide reacts with Pb and PbO to give volatile lead bromide, which goes out along with exhaust gases.

But this creates atmospheric pollution. So now a days aromatic phosphates are used instead of TEL.