gas, fuel, gaseous substance that burns in air and releases enough heat to be useful as a fuel, while also remaining sufficiently stable at ordinary temperatures to permit long-term storage without deterioration or undue hazard. It is advantageous if a fuel gas is readily transportable through pipes and is easily liquefied. Practically all fuel gases meet the first condition, and some meet the second as well. Natural gas, which occurs alone and in conjunction with petroleum deposits, is an excellent fuel gas in wide commercial use. Liquefied petroleum gas is a manufactured mixture of flammable gases that is easily stored in its liquefied condition. Oil gas is a type of gas made by applying heat to various petroleum distillates. Its principal use is as a supplement to natural gas during periods of heavy demand. Coal gas may be any of a variety of gases produced by heating coal in the absence of air and driving off the volatile constituents. It is not as high in fuel value as other gases and often contains tars, light oils, ammonia, and hydrogen sulfide. Producer gas is made by forcing a mixture of air and steam through burning coal or coke. Water gas, or blue gas, which burns with a bright blue flame, is made by passing steam over glowing coke.

fuel injection, system in an internal-combustion engine that delivers fuel or a fuel-air mixture to the cylinders by means of pressure from a pump. It was originally used in diesel engines because of diesel fuel's greater viscosity and the need to overcome the high pressure of the compressed air in the cylinders. A diesel fuel injector sprays an intermittent, timed, metered quantity of fuel into a cylinder, distributing the fuel throughout the air within. Fuel injection is also now used in gasoline engines in place of a carburetor. In gasoline engines the fuel usually is injected into the intake manifold and mixed with air, and the resulting mixture is delivered to the cylinder. Modern fuel injection systems use computers to regulate the process. Fuel injection results in more efficient fuel combustion, improving fuel economy and engine performance and reducing polluting exhaust emissions.

fuel cell, electric cell in which the chemical energy from the oxidation of a gas fuel is converted directly to electrical energy in a continuous process (see oxidation and reduction). The efficiency of conversion from chemical to electrical energy in a fuel cell is between 65% and 80%, nearly twice that of the usual indirect method of conversion in which fuels are used to heat steam to turn a turbine connected to an electric generator. The earliest fuel cell, in which hydrogen and oxygen were combined to form water, was constructed in 1829 by the Englishman William Grove. In the hydrogen and oxygen fuel cell, hydrogen and oxygen gas are bubbled into separate compartments connected by a porous disk through which an electrolyte such as aqueous potassium hydroxide (KOH) can move. Inert graphite electrodes, mixed with a catalyst such as platinum, are dipped into each compartment. When the two electrodes are connected by a wire, the combination of electrodes, wire, and electrolyte form a complete circuit, and an oxidation-reduction reaction takes place in the cell: hydrogen gas is oxidized to form water at the anode, or hydrogen electrode; electrons are liberated in this process and flow through the wire to the cathode, or oxygen electrode; and at the cathode the electrons combine with the oxygen gas and reduce it. The modern hydrogen-oxygen cell, operating at about 250°C; and a pressure of 50 atmospheres, gives a maximum voltage of about 1 volt. Fuel cells have been used to generate electricity in space flights.

fuel, material that can be burned or otherwise consumed to produce heat. The common fuels used in industry, transportation, and the home are burned in air. The carbon and hydrogen in fuel rapidly combine with oxygen in the air in an exothermal reaction—one that liberates heat. Most of the fuels used by industrialized nations are in the form of incompletely oxidized and decayed animal and vegetable materials, or fossil fuels, specifically coal, peat, lignite, petroleum, and natural gas. From these natural fuels other artificial ones can be derived. Coal gas, coke, water gas, and producer gas can be made using coal as the principal ingredient. Gasoline, kerosene, and fuel oil are made from petroleum. For most transportation, fuel must be in a liquid form.

There is a growing concern about the environmental contamination caused by the burning of great amounts of fossil fuels and about the increasing expense of finding them and processing them into easily usable forms (see energy, sources of). During the last 100 years the amount of carbon dioxide in the atmosphere has increased, and there is evidence that this phenomenon may be due to the burning of fossil fuel. Use of biomass, which consists of plants or plant waste, would not produce excess carbon dioxide because the plants absorb the gas for their growth. Wood is not as concentrated a form of energy as fossil fuels, but it can be converted into a more energy-rich fuel called charcoal. Burning fossil fuel also releases acidic oxides of sulfur and nitrogen, which are deposited on the earth in rainwater (see acid rain). The clearing of forests, particularly in the tropical regions, also threatens to increase the amount of carbon dioxide in the atmosphere because the forests utilize carbon dioxide for growth.

The amount of fossil fuel available is limited and new methods of recovery are being developed. One proposed alternative fuel is hydrogen, which is now employed as a fuel only for a few special purposes because of its high cost. Hydrogen can be produced by electrolysis of water for which nonfossil fuels would supply the energy. Solar energy could be utilized either by direct conversion to electricity using photovoltaic cells or by trapping solar heat. Fuels are rated according to the amount of heat (in calories or Btu) they can produce. Nuclear fuels are also possible substitutes for fossil fuels. Nuclear fuels are not burned; they undergo reactions in which the nuclei of their atoms either split apart, i.e., undergo fission, or combine with other nuclei, i.e., undergo fusion. In either case, a small part of the nuclear mass is converted to heat energy. All nuclear fuels currently employed in practical, nonweapons applications react by fission.

High-energy fuels for jet engines and rockets are rated by their specific impulse in thrust per pound of propellant per second. Hydrogen, which is the lightest element, is usually used in the form of compounds, because the density of liquid hydrogen is low and therefore a large volume is required. Addition of aluminum powder or lithium increases the efficiency. Rockets usually have a self-contained supply of oxygen or some other oxidizer, such as ammonium, lithium, or potassium perchlorate. Fuels such as turpentine, alcohol, aniline, and ammonia use nitric acid, hydrogen peroxide, and liquid oxygen as oxidizers. More power can be obtained by oxidizing hydrazine, diborane, or hydrogen with oxygen, ozone, or fluorine.

See oil gas; liquefied petroleum gas; gas, fuel; nuclear energy.

fossil fuel: see energy, sources of; fuel.

Any of a class of materials of biologic origin occurring within the Earth's crust that can be used as a source of energy. Fossil fuels include coal, petroleum, and natural gas. They all contain carbon and were formed as a result of geologic processes acting on the remains of (mostly) plants and animals that lived and died hundreds of millions of years ago. All fossil fuels can be burned to provide heat, which may be used directly, as in home heating, or to produce steam to drive a generator for the production of electricity. Fossil fuels supply nearly 90percnt of all the energy used by industrially developed nations.

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In an internal-combustion engine, introduction of fuel into the cylinders by a pump rather than by the suction created by the movement of the pistons (see piston and cylinder). On diesel engines, which lack spark plugs, the heat created by compressing air in the cylinders ignites the fuel, which has been pumped in as a spray. In engines with spark ignition, fuel-injection pumps are often used instead of conventional carburetors. Fuel injection distributes the fuel more evenly to the cylinders than does a carburetor; more power can be developed and undesirable emissions are reduced. In engines with continuous combustion, such as gas turbines and liquid-fueled rockets, which have no pistons to create suction, fuel-injection systems are necessary.

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Device that converts chemical energy of a fuel directly into electricity (see electrochemistry). Fuel cells are intrinsically more efficient than most other energy-conversion devices. Electrolytic chemical reactions cause electrons to be released on one electrode and flow through an external circuit to a second electrode. Whereas in batteries the electrodes are the source of the active ingredients, which are altered and depleted during the reaction, in fuel cells the gas or liquid fuel (often hydrogen, methyl alcohol, hydrazine, or a simple hydrocarbon) is supplied continuously to one electrode and oxygen or air to the other from an external source. So, as long as fuel and oxidant are supplied, the fuel cell will not run down or require recharging. Fuel cells can be used in place of virtually any other source of electricity. They are especially being developed for use in electric automobiles, in the hope of achieving enormous reductions in pollution.

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