Michelle De Vries, of Ilam school, asks :-

What is the difference between super and unleaded petrol?

Jack Fergusson, a chemist at the University of Canterbury, responded.

In a car engine petrol vapour is compressed by the piston in the cylinder and then ignited by the spark plug. The heated gases expand rapidly and push the piston out, thus turning chemical energy into mechanical energy.

The efficiency will be reduced if a premature explosion occurs in the car cylinder just before maximum compression, a process called knocking. Knocking can also damage the car engine.

Petrol is made up of hydrocarbons which are molecules of carbon and hydrogen. These may be either long chain or branched. High octane petrol has a good proportion of branched hydrocarbons in it. These burn smoothly in the car's cylinder and dont cause knocking. This high grade petrol is called unleaded petrol.

Lower grade petrol (low octane rating) has a larger amount of long chain molecules and this grade does explode prematurely in the cylinder causing knocking. This problem can be prevented by adding a chemical, tetra-ethyl-lead, to the cheaper, low-grade petrol. This is the petrol the industry called Super.

Lead emitted from the car engines exhaust system spreads around our cities in dust. This is a potential danger, especially to children, because lead is a toxic element which affects the production of red blood cells and also the nervous system including the brain. The lead also prevents car emission control devices from working properly. For these reasons lead is no longer added to petrol.