Steam engine

Heat engine that performs mechanical work using steam as its working fluid

Early builders of stationary steam engines considered that horizontal cylinders would be subject to excessive wear. Their engines were therefore arranged with the piston axis in vertical position. In time the horizontal arrangement became more popular, allowing compact, but powerful engines to be fitted in smaller spaces.

Although the reciprocating steam engine is no longer in widespread commercial use, various companies are exploring or exploiting the potential of the engine as an alternative to internal combustion engines.

The widely used reciprocating engine typically consisted of a cast-iron cylinder, piston, connecting rod and beam or a crank and flywheel, and miscellaneous linkages. Steam was alternately supplied and exhausted by one or more valves. Speed control was either automatic, using a governor, or by a manual valve. The cylinder casting contained steam supply and exhaust ports.

Engines equipped with a condenser are a separate type than those that exhaust to the atmosphere.

Hot gas is passed through tubes immersed in water, the same water also circulates in a water jacket surrounding the firebox and, in high-output locomotive boilers, also passes through tubes in the firebox itself (thermic syphons and security circulators).

Fire-tube boilers were the main type used for early high-pressure steam (typical steam locomotive practice), but they were to a large extent displaced by more economical water tube boilers in the late 19th century for marine propulsion and large stationary applications.

In a steam engine, a piston or steam turbine or any other similar device for doing mechanical work takes a supply of steam at high pressure and temperature and gives out a supply of steam at lower pressure and temperature, using as much of the difference in steam energy as possible to do mechanical work.

The simplest cold sink is to vent the steam to the environment. This is often used on steam locomotives to avoid the weight and bulk of condensers. Some of the released steam is vented up the chimney so as to increase the draw on the fire, which greatly increases engine power, but reduces efficiency.

Sometimes the waste heat from the engine is useful itself, and in those cases, very high overall efficiency can be obtained.

Evaporated water cannot be used for subsequent purposes (other than rain somewhere), whereas river water can be re-used. In all cases, the steam plant boiler feed water, which must be kept pure, is kept separate from the cooling water or air.

Many engines, stationary and mobile, are also fitted with a governor to regulate the speed of the engine without the need for human interference.

An animation of a simplified triple-expansion engine. High-pressure steam (red) enters from the boiler and passes through the engine, exhausting as low-pressure steam (blue), usually to a condenser.