Page 32 - Curriculum Visions Dynamic Book

Lead in fuels
We use petrol in most of the world’s engines. Petrol is made from refined crude oil, which is
a compound of carbon. The fuel is “atomised” and sprayed into the engine cylinders along with air. The piston then rises in the cylinder and compresses the mixture, causing it to get hot.
As the air is being compressed, and before a spark is generated by the spark plug, the mixture may explode. This is called pre-ignition, and it
is noticed as a knocking sound inside the engine.
The numbers used to describe the quality
of the fuel are octane numbers. Commonly
the octane number varies between 79 and 91. This number describes the way the fuel can
resist “knocking”.
When engines knock, it means that the mixture has exploded while the piston is still travelling up the cylinder, not when it has just passed the top of its stroke, as is meant to happen. By exploding at the wrong time,
the engine become both noisy and less efficient, and so consumes more fuel.
Compounds of lead (tetraethyl lead, Pb(C2H5)4) were traditionally added to the fuel to stop the mixture from exploding prematurely. This meant that engines ran more efficiently. This was a good example of how a simple chemical treatment provided a cheap solution
to a problem. However, because it has now been realised that lead can be toxic in the environment, this use of lead is now being phased out. Instead, the engines have had to
be redesigned so that they do not knock even when lead additives are not present.
32