Petrol Octane Numbers and Star Ratings

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Petrol Octane Numbers and Star Ratings

Octane numbers and star ratings

It seems strange to think now with modern cars but up until the late 1980s (ish) each make and model of car would come with a stated minimum octance rating required by the engine of the car that the owner would not only need to be aware of but quite diligently observe when fililng up the petrol tank.

'Knocking' Resistance

A petrol's octane number is determined by matching the petrol against a mixture of two petroleum fluids in a laboratory test engine. One, iso-octane, has a high resist-ance to knocking. The other, normal-heptane, has a low resistance.

A petrol was said to have a 90 octane number if it has the same anti-knock ability in the laboratory test engine as 90 parts of octane mixed with 10 pa its of heptime. The compression ratio of the test engine is adjustable while running, so that a 'just knocking' condition could be produced in any petrol.


Two, Three & Four-Star Petrol Ratings

The octane number given to petrols sold at garages in Britain came under the old British Standard 'star rating' system. Two-star petrol has a minimum octane number of 90, three-star 94, four-star 97 and five-star petrol at least 100 octane. The British Standard also set limits to the volatility, lead content and other features which control deposit formation, odour and the storage life of petrol.

Petrol is a very complex mixture of hydrocarbons, and the octane number is only one of a number of characteristics that affect its behaviour in engines.

Long Term Implications

The octane requirement of an engine will also vary with time and mileage, due to the gradual build-up of carbon in the combustion chambers, and other factors, The best course is to follow the octane recommendation of the ear manufacturer, There is no advantage in using a petrol with a higher number than required, although there is no disadvantage, apart from the extra cost.

The need to understand the at least the basic complexities of the star-rating to ensure your put the right fuel in your car has been lost as modern car engines have a wider operating range and greater tolerances.

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