Fuel, the substance that reacts chemically with another substance to produce heat or that produces heat by nuclear processes. The term fuel is generally limited to those substances that burn readily in air or oxygen, emitting large quantities of heat. Fuels are used for heating, for the production of steam for heating and power purposes, for powering internal-combustion engines, and for a direct source of power in jet and rocket propulsion. In cases where a fuel must supply its own oxygen, as in many rockets and jet propulsion, an oxidizing agent such as hydrogen peroxide or nitric acid is added to the fuel mixture.
Chemical reactions in the combustion of all ordinary fuels involve the combination of oxygen with any carbon, hydrogen, or sulphur present in the fuels. The end products are carbon dioxide, water, and sulphur dioxide. Other substances present in fuels do not contribute to the combustion but either is driven off in the form of vapour or remain after combustion in the form of ash.
Fuel efficiency or heating value of a fuel is usually measured in terms of the thermal energy, or heat, evolved when a given amount of the fuel is burned under standard conditions. Heating values for solid and liquid fuels were traditionally stated in terms of Btu (British Thermal Units) per lb, and values for gases in Btu per cu ft. The standard unit is now joules per kilogram or cubic metre. A distinction is sometimes made between higher heating value, the entire heat evolved during combustion, and lower heating value, the net heat evolved, with allowance for the heat lost in the vaporization of the water produced by combustion. Approximate higher heating values of common fuels are Solid fuels (J per kg): coal 28 to 35 million; lignite 14 to 17 million; coke 29 million; dry wood 20 million. Liquid fuels: alcohol 25 million; fuel oil 44 million; petrol 48 million; paraffin 46 million. Gaseous fuels (J per kg): acetylene 55 million; blast-furnace gas 3.5 million; carbon monoxide 12 million; coke-oven gas or coal gas about 22 million; hydrogen 12 million; natural gas 39 to 82 million; oil gas 19 million; producer gas 5 million. See separate articles on most of these fuels.