A steam engine is a heat engine that uses steam power to produce mechanical work. The steam engine works by pushing a piston back and forth inside a cylinder using the force created by steam pressure. A rod and crank can convert this pushing effort into rotary motion for work. The name “steam engine” is usually reserved for reciprocating engines such as those detailed above, rather than the steam turbine. External combustion engines, or steam engines, are those in which the liquid is isolated from the combustion products.
The Rankine cycle is the appropriate thermodynamic cycle for analyzing this process. The phrase “steam engine” can apply to either whole steam plants such as railway steam engines and transportable engines, or just the cylinder or compressor mechanism, as in the beam engine and static steam engine.
Although steam-powered machines were known as soon as the aeolipile in the first century AD, with a few further usages reported in the 16th and 17th centuries, Thomas Savery is credited with inventing the first practically usable steam-powered machine, a heat engine that operated immediately on the water.
Thomas Newcomen created the first financially successful engine that could transfer constant power to a device in 1712. In 1764, James Watt made a significant advance by diverting wasted steam to a separate tank for condense, resulting in a significant increase in the amount of work produced per unit of fuel.
Until the early twentieth century, when innovations in the design of electric motors and internal combustion gradually replaced steam engines in commercial use, reciprocating piston-type steam engines were the primary source of power. Because of their reduced cost, faster-operating speed, and higher efficiency, steam turbines have largely supplanted piston engines in power generation.