Engine is a machine that converts energy into mechanical work. An engine may get its energy from any of a number of sources, including fuels, steam, and air or water under pressure.
Reciprocating gasoline engines use the chemical energy of burning gasoline to push one or more pistons. The motion of the pistons can then be converted into rotary (circular) motion, which can be used to turn the wheels of an automobile or to do some other kind of work. Gasoline engines and steam engines are called heat engines because they convert heat energy into mechanical work. Reciprocating gasoline engines are called internal-combustion engines if the gases produced by the combustion (burning) of the fuel push directly against the pistons.
The steam engine is an external-combustion engine. External-combustion engines also get their energy from hot gases produced by burning. These gases transfer heat energy to another fluid. The energy in this fluid, in turn, is converted into mechanical work. In a piston steam engine, for example, heat of combustion is transferred to water inside a boiler. The heat converts the water to steam, which pushes the engine’s pistons.
In hydraulic engines, water pressure is used to produce mechanical work. The pressure may be created by a pump or by water flowing downward from a level above the engine.
