Octane number measures whether a petrol is likely to cause knock in an engine. This premature ignition pushes against the crankshaft instead of with it, producing a knocking or pinging sound. Knocking causes the engine to overheat and lose power, and it can damage the engine in the long run. The way to avoid knocking is to use petrol with a sufficiently high octane number. The higher the octane number, the more resistant the petrol is to the self-ignition that causes knocking.
Vapour pressure is a measure of how easily a liquid vaporises, or changes into a gas.
Petrol is supplied in liquid form, but at least part of it must be vapour to ignite in an engine combustion chamber. This means that on a cold day in a cold engine, enough petrol must vaporise to enable ignition. On the other hand, on a hot day in a completely warmed-up engine, it is also important that the petrol does not vaporise and expand so much that it does not allow any air to be mixed with it in the cylinder. Oil companies balance these two extremes when blending petrol, taking into account the climate and the season where the petrol will be sold. Adding certain lead compounds to petrol was an effective and cost-efficient way to increase the petrol's octane number.