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Octane rating for petrol

When it comes to octane rating for petrol, the actual number applied to the fuel is based on how it auto ignites under compression. The octane number is based on a sacle where isooctane is rated at 100, equating to the minimal ignition under pressure, while heptane equates to zero, meaning it ignites very easily.

Relating the octane number to petrol

In terms of applying this test, most vehicle drivers are interested in the octane rating for petrol. If the fuel in a petrol-driven car has an octane number of 92, then this will have the same knock as a mixture of 92% isooctane and 8% heptane.

How ratings change

  • The Octane number will increase with carbon chain branching.
  • The Octane number will decrease if there is an increase in the carbon chain length.
  • The Octane number will increase in aromatics with the same number of carbons.

Why the number is important

The reason it is so important to be aware of the octane number of the fuel in a vehicle is because the auto ignition of any fuel causes an effect in engines that is known as 'knocking'. This occurs where the fuel ignites doubly. The first time is due to high pressure, and the subsequent occasion when the spark from the plugs ignites the petrol. Knocking causes a car's engine to perform far less efficiently. It can also cause damage to the engine.

In the case of vehicles running on diesel, knocking is not an issue. Engines actually rely on the knocking effect because, unlike petrol engines, they don't have spark plus, instead relying on compression to cause the ignition between fuel and air.

Improving the octane number

The octane rating petrol of petrol can be improved in two main ways. Firstly, additives can be introduced into the fuel which are specifically designed to discourage auto ignition. Secondly, high octane fuels can be blended with petrol.

Since the 1920s, the anti-knocking additives available to drivers and mechanics have included small traces of lead compounds, proving to be an economical counter to the knocking effect.

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