Unloaded Petrol

Gasoline mixtures that are rich in straight-chain alkanes such as heptane ignite very readily and explode rapidly. This causes 'knocking' and inefficient combustion. Combustion of branch-chain alkanes like 2,2,4-trimethylpentane (iso-octane) is much smoother and more controlled. Hence, gasoline mixtures rich in branch-chain alkanes are more efficient in burning and less likely to cause knocking.

The octane rating (number) of 2,2,4-trimethylpentane is set at 100 and that of heptane is set to 0. Fuels with high octane numbers burn smoothly and efficiently in high performance engines.

There are two ways to meet the demand of modern high-performance engines for fuels of high octane numbers. The first one is to produce artificial petrol that are rich in branched-chain alkanes. Another method is to add an anti-knock agent to petrol. The most commonly added compund is tetraethyl lead [(CH₃CH₂)₄Pb]. When burned, this lead compound produces small particles of lead oxide which tend to combine with the free radicals produced in the chain reaction of combustion. This slows down the reacrion and makes it smoother.

To prevent lead from accumulating in the engine, 1,2-dibromoethane (CH₂BrCH₂Br) is added to petrol. The formation of lead bromide is volatile and is swept away in the car exhaust. The exposure of lead in air leads to serious effect. Lead is a neurotoxin, or nerve poison. It can cause damage in the central nervous system (brain and nervous system), especially in young children.

Many countries are now phasing out the use of leaded petrol. The production of unleaded petrol involves complicated process (cracking, isomerisation and reforming reactions) to generate high-octane petrol. Unleaded petrol also contains an expansive additive methyl tertiary butyl ether (MTBE, octane rating 118). Unleaded petrol is generally more expansive than the leaded one. Some countries (like UK) has taxed lower to encourage people to use it.