a gaseous or liquid substance by means of which an energy conversion is accomplished to obtain mechanical work (in engines), cold (in refrigerating machines), or heat (in heat pumps). The most common working fluids are steam, used in steam turbines and steam engines; the combustion products of organic fuels, used in internal combustion engines and gas turbines; water and other liquids, used in hydraulic engines; air, used in pneumatic engines; and refrigerants, used in refrigerating machines. A rocket propellant is also called a working fluid.