Fuel
. Fuel, substance that reacts chemically with another 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 torpedoes, 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 sulfur present in the fuels. The end products are carbon dioxide, water, and sulfur dioxide. Other substances present in fuels do not contribute to the combustion but either are driven off in the form of vapor or remain after combustion in the form of ash. Fuel efficie