A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder.
This pushing force can be transformed, by a connecting rod and flywheel, into rotational force for work. The term “steam engine” is generally applied only to reciprocating engines as just described, not to the steam turbine.
Steam engines are external combustion engines, where the working fluid is separated from the combustion products. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle.
In general usage, the term steam engine can refer to either complete steam plants (including boilers, etc.), such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine.
Although steam-driven devices were known as early as the aeolipile in the first century AD, with a few other uses recorded in the 16th and 17th centuries, Thomas Savery is considered the inventor of the first commercially used steam-powered device, a steam pump that used steam pressure operating directly on the water.
The first commercially successful engine that could transmit continuous power to a machine was developed in 1712 by Thomas Newcomen. James Watt made a critical improvement by removing spent steam to a separate vessel for condensation, greatly improving the amount of work obtained per unit of fuel consumed.
By the 19th century, stationary steam engines powered the factories of the Industrial Revolution. Steam engines replaced sails for ships on paddle steamers, and steam locomotives operated on the railways.
Reciprocating piston-type steam engines were the dominant source of power until the early 20th century when advances in the design of electric motors and internal combustion engines resulted in the gradual replacement of steam engines in commercial usage. Steam turbines replaced reciprocating engines in power generation, due to lower cost, higher operating speed, and higher efficiency.
In a steam engine, hot steam, usually supplied by a boiler, expands under pressure, and part of the heat energy is converted into work. The remainder of the heat may be allowed to escape, or, for maximum engine efficiency, the steam may be condensed in a separate apparatus, a condenser, at comparatively low temperature and pressure.
For high efficiency, the steam must fall through a wide temperature range as a consequence of its expansion within the engine. The most efficient performance that is, the greatest output of work in relation to the heat supplied is secured by using a low condenser temperature and a high boiler pressure.
The steam may be further heated by passing it through a superheater on its way from the boiler to the engine. A common superheater is a group of parallel pipes with their surfaces exposed to the hot gases in the boiler furnace.
By means of superheaters, the steam may be heated beyond the temperature at which it is produced by boiling water.
In a reciprocating engine, the piston and cylinder type of steam engine, steam under pressure is admitted into the cylinder by a valve mechanism. As the steam expands, it pushes the piston, which is usually connected to a crank on a flywheel to produce rotary motion. In the double-acting engine, steam from the boiler is admitted alternately to each side of the piston.
In a simple steam engine, expansion of the steam takes place in only one cylinder, whereas in the compound engine there are two or more cylinders of increasing size for greater expansion of the steam and higher efficiency; the first and smallest piston is operated by the initial high-pressure steam and the second by the lower-pressure steam exhausted from the first.
In the steam turbine, steam is discharged at high velocity through nozzles and then flows through a series of stationary and moving blades, causing a rotor to move at high speeds. Steam turbines are more compact and usually permit higher temperatures and greater expansion ratios than reciprocating steam engines. The turbine is the universal means used to generate large quantities of electric power with steam.
Steam engines were used in all sorts of applications including
