Understanding Fuels

In petrol ("gasoline") engines an electrical spark ignites the compressed fuel/air mixture. It is possible for the fuel to ignite simply as the result of compression, and when this happens a "knocking" sound is produced and the engine cylinder may be damaged.

Photo: Engine piston

The octane rating of petrol is a measure of its tendency to pre-ignite. Click here to find out more about octane ratings.

Controlling Octane Numbers

The octane rating of a fuel can be increased with relatively small quantities of additives. This is cheaper than refining the whole fuel. Additives in past/current use include:

Petrol additives to increase octane rating
Tetraethyl lead ("leaded" petrol) Diagram: tetraethyl lead molecule  Very effective, but now banned in many countries for health reasons and because the lead "poisons" catalytic converters
Benzene Diagram: benzene molecule Effective, but benzene is a carcinogen. Still some use, but at greatly reduced levels
MTBE (Methyl Tertiary Butyl Ether) Diagram: MTBE molecule Increases octane rating and oxygenates fuel (more complete combustion). In US has been found in drinking water, and so is banned in some US states. In use in UK
Ethanol Diagram: ethanol molecule Increases octane and also oxygenates fuel. In common use in US where it is not made from oil
Diesel Fuel

In 1900 at the World Exhibition (Paris) Rudolph Diesel demonstrated an internal combustion engine that ran on peanut oil. His intention was to design an engine that would run on whatever fuel was locally available. When he died in 1913 his engine continued to be developed, but until recently "diesel" fuel has frequently been petrochemical in origin.

Diesel engines require no spark. The fuel is injected into the cylinder and it ignites when compressed. There is therefore no octane rating, but the ease with which the fuel ignites under compression can be measured, and this is called the cetane rating. A higher cetane number means the fuel ignites easier, and this is preferable.

Ethanol and E85

In the UK most industrial ethanol is made by the petrochemical industry, so there is little advantage in using it as a fuel. In the US, however, fermentation from corn is the main method of production (see the ethanol story for more detail). There is then an environmental advantage in using ethanol as a fuel, as well as a fuel additive. Some vehicles in the US are designed to run on a blend of 85% ethanol and 15% gasoline (petrol), known as E85. Click here to find out more

Photo: E85 fuel pump

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