Petrol needs to be blended to have the right properties in terms of its volatility and to reduce its propensity to auto-ignite, a problem known as ‘knocking’.
Volatility
A mixture of air and vaporised petrol is injected into the cylinders in a car engine, and ignited. In winter the petrol blend needs to be richer in small, volatile hydrocarbons with a lower boiling point such as butane and pentane, otherwise the car will be difficult to start in cold weather. In summer, the petrol blend needs less volatile molecules to prevent it from vaporising in the petrol tank (polluting and costly) or in the fuel supply system.
Octane number
The octane number of a petrol blend is a measure of how much compression that blend can endure before it auto-ignites. In a car engine, the vaporised fuel-air mixture in the cylinders is compressed and then ignited with a spark. The explosion pushes down a piston, turning a fly-wheel.
Fuel-air mixtures heat up when compressed which may cause them to auto-ignite. When this happens 2 explosions occur – because of compression and when sparked. The thrust from the expanding gases in the cylinder no longer occurs at the correct time, the performance is lowered and the cylinder may be damaged. This also known as knocking.
EricTheCarGuy explains this brilliantly!
We can increase the octane number of a fuel (so it will withstand more compression and not auto-ignite) by adding shorter alkanes (but watch the volatility), branched alkanes, cycloalkanes and aromatic hydrocarbons to the blend.
You can find out how these molecules are produced by cracking, isomerisation and reforming here.