Why Is There Lead In Petrol?

This answer should be read by all the automobile drivers. It is basic and important.
Petrol is a mixture of hydrocarbons. These are substances composed of hydrogen and carbon atoms. The petrol sold as motor fuel is a blend of several other different hydrocarbon liquids. Special substances called additives, are mixed with petrol to make it burn better.
At temperatures about 21 degree centigrade petrol turns from liquid into vapours. In an automobile engine, the gasoline is mixed with air and sprayed into the engine. The heat in the engine turns it into vapours. A spark plug then sets off a spark that burns the mixture.

Some times the petrol mixture may be ignited too soon. When this happens the engine make a sound, usually called a knock. There are two ways to reduce engine knock. One is to use a slow burning petrol. The other is to put a chemical into the fuel to slow down the burning. The best known chemical used for this is called 'tetra ethyl lead'. And this is the reason we find lead in petrol.

And it is SUPER BAD for the environment, too!