IV.

Liquid and Gaseous Fuels

Common liquid fuels are fuel oils, gasoline, and naphthas derived from petroleum, and, to a lesser extent, coal tar, alcohol, and benzol obtained from coke manufacture. In stationary furnaces, less volatile fuel oils are sprayed through nozzles, with or without air or steam, into the combustion chamber. In an internal-combustion engine, volatile fuels such as gasoline or a gasoline and alcohol mixture (gasohol) are evaporated and the mixture admitted into the engine cylinder, where combustion is initiated by a spark. In these fuels, from 16 to 23 kg of air are required for complete combustion of 1 kg of fuel. In diesel engines the fuel is injected as an atomized spray into the combustion chamber, where the temperature rise associated with the high compression ratio of diesel engines is sufficient to cause ignition.

Gaseous fuels such as natural gas, refinery gas, and manufactured gases such as producer gas are usually mixed with air before combustion to supply a maximum amount of oxygen to the fuel. The fuel-air mixture then issues from the burner ports at a velocity greater than the velocity of flame propagation to prevent flame flashback into the burner, but not so great a velocity as to blow the flame off the burner. If not premixed with air, these fuels usually burn with smoky, relatively cool flames. Natural gas burned with air can produce flame temperatures in excess of 1930° C (3500° F).

Rockets for space exploration may use liquid fuels such as kerosene and hydrazine, and carry an oxidizer such as liquid oxygen, nitric acid, or hydrogen peroxide. Military rockets such as bazookas (see Bazooka) use solid fuels such as nitrocellulose and cordite, with oxygen incorporated into the fuel; these burn spontaneously on being heated by radiation from the products of combustion.

See also Guided Missiles.