Fuel is a substance that, when combined with oxygen, will burn and produce heat. Fuels may be classified according to their physical state as solid, gaseous, or liquid.

Solid Fuels

Solid fuels are used extensively for external combustion engines, such as a steam engine, where the burning takes place under boilers or in furnaces. They include such fuels as wood and coal. Solid fuels are not used in reciprocating engines, where the burning takes place inside the cylinder, because of their slow rate of burning, low heat value, and numerous other disadvantages.

Gaseous Fuels

Gaseous fuels are used to some extent for internal combustion engines, where a large supply of combustible gas is readily available. Natural gas and liquefied petroleum gas are two of the more common types. Gaseous fuels can be disregarded for use in aircraft engines. The large space they occupy limits the supply of fuel that can be carried.

Liquid Fuels

Liquid fuels, in many respects, are the ideal fuel for use in internal combustion engines. Liquid fuels are classified as either nonvolatile or volatile. The nonvolatile fuels are the heavy oils used in diesel engines. The volatile class includes those fuels that are commonly used with a fuel metering device and are carried into the engine cylinder or combustion chamber in a vaporized or partially vaporized condition. Among these are alcohol, benzol, kerosene, and gasoline.

Aviation fuel is a liquid containing chemical energy that, through combustion, is released as heat energy and then converted to mechanical energy by the engine. This mechanical energy is used to produce thrust, which propels the aircraft. Gasoline and kerosene are the two most widely used aviation fuels.