THE SUNDAY TIMES (UK): The Knowledge: Octane: All you wanted to know about cars but were afraid to ask: “Petrol is classified by its research octane number (RON). In Britain standard unleaded petrol is 95RON. However, some fuels, such as Shell Optimax which is 98RON, have a higher rating and boast increased performance for the car.” (ShellNews.net) 12 Dec 04

Petrol is classified by its research octane number (RON). In Britain standard unleaded petrol is 95RON. However, some fuels, such as Shell Optimax which is 98RON, have a higher rating and boast increased performance for the car. But how does the octane achieve this? All cars have a pre-set “compression ratio”. This is the point at which the compressed fuel and air mix in the cylinder is ignited by the spark plug. The more the mix can be compressed (ie, the higher the compression ratio) the more powerful the explosion that drives the piston and therefore the car.

However, all fuels have a point at which they will self-combust under pressure. If the pressure in the cylinder exceeds this point then the fuel will combust before the spark plug fires, thus throwing the engine’s rhythm out of time. This is known as pinking, and can be identified by a knocking sound coming from the engine.

The higher the RON the more pressure the fuel can be put under before it self-ignites. This is because the RON measures how much octane is present (a 95RON fuel means it is 95% octane and 5% other hydrocarbons). The atomic structure of octane (with eight carbon atoms in a chain) makes it better able to take pressure than other hydrocarbons refined from oil.

A 98RON fuel should in theory be able to withstand more pressure and produce more power than a 95RON fuel. However, not all engines have the same compression ratio. An engine with a ratio set for a 95RON fuel will usually gain no benefit from using 98RON fuel since the pistons will not compress the fuel to its maximum. Some cars’ engine management computers may be able to adjust the compression ratio, but this is not the norm.

Adjusting the other way (where a 95RON fuel is put into a car set to use 98RON) is easier. Many cars can detect the resultant pinking and adjust the timing accordingly to cope.