Hydrocarbon Fuel

All aviation fuel, whether avgas or turbine fuel, is a mixture of hydrocarbons: substances made of chains of carbon atoms, with hydrogen atoms attached. Different hydrocarbons have different freezing points, boiling points, densities and energy content. In general, the longer the chain, the higher the boiling point, but some chains are branched, and the three dimensional shape of the molecule affects its properties, too. Crude oil is a mixture of many different hydrocarbons, as well as compounds containing other atoms.

Crude oil is refined by distilling it in a column, and separating out fractions with different boiling points. Light hydrocarbons like butane and propane come out the top of the distilling column, with gasoline lower down, followed by kerosene and diesel. The products may be again refined by removing undesirable components such as sulphur compounds through further chemical reactions. The big molecules left over at the bottom of the column may be broken up to produce more gasoline, kerosene and diesel.

Avgas, for piston engines, is refined to increase its octane number, a comparison of the amount of a particular eight-carbon chain molecule versus n-heptane, a straight chain seven-carbon molecule. Additives such as tetraethyl lead can improve the smooth running qualities of the fuel in an engine and increase the octane above 100, where it is called a performance number. Avgas has a higher ratio of energy per unit weight than turbine fuel, but a lower ratio of energy per volume.

Turbine engines orginally used kerosene, not for its properties, but because turbine engines could burn anything, and development took place in wartime, when gasoline was in short supply. After the war, the US Air Force started using "wide cut" fuel, spanning the range from gasoline to kerosene. The logic was that you can make more of it from the same amount of crude oil. Unfortunately, it also evaporates at a lower temperature, leading to loss of fuel at high altitudes and increasing the risk of fire during ground handling or crashes.

Americans standardized on kerosene-type Jet A fuel, which freezes at -40 degrees, for domestic flights. Much of the rest of the world uses Jet A-1 (almost the same as US military JP-8 fuel), which freezes at -47 degrees Celsius.

Wide cut fuel, known as Jet-B, is still used in northern Canada, because it doesn't freeze until -51 degrees Celsius. The Russians use something similar, called TS-1. Because jet fuel is a mixture of so many different fractions, it remains pumpable for about ten degrees below the temperature where it starts to freeze. I wonder at what temperature human testicles freeze. I'm glad I haven't got any, in a country where the difference between -47 and -51 is significant enough to warrant a grade of fuel.