OCTANE WITHOUT LEAD

The vast majority of cars run on four-stroke petrol engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and petrol into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. The compression ratio of your engine is one of the most important factors determining what petrol octane rating it requires. One way to increase the horsepower of an engine is to increase its compression ratio. In general, the higher the compression ratio of an engine, the higher the octane it requires. So there is a trade-off between performance and octane requirements.

What is Octane?

The name "octane" is derived when crude oil is "cracked" in a refinery resulting in the creation of different length hydrocarbon chains. These different chain lengths can be separated from each other and blended to form different fuels such as methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four, Pentane five, hexane six, and heptane seven. Octane has eight carbons chained together. The most cost-effective way of increasing the octane of petrol is to add an organic lead compound called tetraethyl lead (TEL).

Clean fuels challenge all the refineries to raise the average octane of petrol production without using TEL. As indicated in the May newsletter, this will in essence be done by chemically re-arranging the molecular structure of hydrocarbons in the petrol pool in order to produce additional higher octane components. There is however a practical limit to how far octane can be raised by refinery processes.

For that reason, 97 fuel will no longer be available from January 2006. Users of 97 octane leaded petrol at the coast will have the option to use 95 octane lead replacement petrol or 95 octane unleaded petrol.

What petrol is compatible with my car?

From the beginning of 1996, all new cars sold in South Africa were compatible with 95 octane unleaded. Only a limited number of older vehicles require lead. Any vehicle presently operating on leaded petrol will be able to use lead replacement petrol, whether or not it requires AVSR protection.

The octane requirements of vehicles are also affected by altitude, due to changes in atmospheric pressure. The average octane requirement inland is about 2 numbers lower than at the coast. Most vehicles requiring 95 fuel at the coast will operate equally satisfactorily on 93 inland. While 95 octane petrol will be the predominant grade sold at the coast, 93 octane will remain the appropriate grade, as at present, for the majority of inland users.

Although 95 unleaded fuel will be widely available inland from next year, it is presently required only by a limited number of advanced-technology vehicles. For that reason government has decided to incorporate an additional levy (initially 10 cents per litre) in the price of 95 unleaded inland, to discourage unnecessary use of this grade.

It is important to understand that the octane requirement of a vehicle is not constant, but varies over the normal operating range and according to the way it is driven.

Octane requirement peaks under high load, wide-open-throttle conditions. Be aware, therefore, that the way you choose to drive can affect not only your fuel consumption but the grade of petrol you will find most suitable.