Editing Steam engine (section)

== Efficiency ==
{{Main|Thermal efficiency}}
{{See also|Engine efficiency#Steam engine}}

The efficiency of an engine cycle can be calculated by dividing the energy output of mechanical work that the engine produces by the energy put into the engine.

The historical measure of a steam engine's [[thermal efficiency|energy efficiency]] was its "duty". The concept of duty was first introduced by Watt in order to illustrate how much more efficient his engines were over the earlier [[Newcomen atmospheric engine|Newcomen designs]]. Duty is the number of [[foot-pounds]] of [[work (physics)|work]] delivered by burning one [[bushel]] (94 pounds) of coal. The best examples of Newcomen designs had a duty of about 7&nbsp;million, but most were closer to 5&nbsp;million. Watt's original low-pressure designs were able to deliver duty as high as 25&nbsp;million, but averaged about 17. This was a three-fold improvement over the average Newcomen design. Early Watt engines equipped with high-pressure steam improved this to 65&nbsp;million.<ref>John Enys, [https://books.google.com/books?id=blhqAAAAMAAJ&pg=PA457 "Remarks on the Duty of the Steam Engines employed in the Mines of Cornwall at different periods"], ''Transactions of the Institution of Civil Engineers'', Volume 3 (14 January 1840), p. 457</ref>

The [[Carnot cycle]], a theoretical ideal thermodynamic cycle, sets the limit of a steam engine's efficiency. Carnot cycle describes a system of two thermal reservoirs with heat transferred between them. In a steam engine part of the heat transferred is converted into mechanical work. The greater the difference in temperature between the two reservoirs, the more efficient the engine. One way of increasing the difference is by using [[superheater|superheated steam]].

The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure reaching [[Critical point (thermodynamics)|supercritical]] levels for the working fluid, the temperature range over which the cycle can operate is small; in steam turbines, turbine entry temperatures are typically 565&nbsp;°C (the [[Creep (deformation)|creep]] limit of stainless steel) and condenser temperatures are around 30&nbsp;°C. This gives a theoretical [[Carnot efficiency]] of about 64% compared with an actual efficiency of 42% for a modern [[coal-fired power station]]. This low turbine entry temperature (compared with a [[gas turbine]]) is why the Rankine cycle is often used as a bottoming cycle in [[Combined cycle|combined-cycle gas turbine]] power stations.{{citation needed|date=January 2013}}

One principal advantage the Rankine cycle holds over others is that during the compression stage relatively little work is required to drive the pump, the working fluid being in its liquid phase at this point. By condensing the fluid, the work required by the pump consumes only 1% to 3% of the turbine (or reciprocating engine) power and contributes to a much higher efficiency for a real cycle. The benefit of this is lost somewhat due to the lower heat addition temperature. [[Gas turbine]]s, for instance, have turbine entry temperatures approaching 1500&nbsp;°C. Nonetheless, the efficiencies of actual large steam cycles and large modern simple cycle gas turbines are fairly well matched.<ref>{{Cite journal |last1=Yin |first1=Feijia |last2=Rao |first2=Arvind Gangoli |date=2020-02-01 |title=A review of gas turbine engine with inter-stage turbine burner |journal=Progress in Aerospace Sciences |language=en |volume=121 |pages=100695 |doi=10.1016/j.paerosci.2020.100695 |bibcode=2020PrAeS.12100695Y |s2cid=226624605 |issn=0376-0421|doi-access=free }}</ref>

In practice, a reciprocating steam engine cycle exhausting the steam to atmosphere will typically have an efficiency (including the boiler) in the range of 1–10%. However, with the addition of a condenser, Corliss valves, multiple expansion, and high steam pressure/temperature, it may be greatly improved. Historically into the range of 10–20%, and very rarely slightly higher.{{Citation needed|date=February 2020}}

A modern, large electrical power station (producing several hundred megawatts of electrical output) with [[steam reheat]], [[economizer]] etc. will achieve efficiency in the mid 40% range, with the most efficient units approaching 50% thermal efficiency.{{Citation needed|date=February 2020}}

It is also possible to capture the waste heat using [[cogeneration]] in which the waste heat is used for heating a lower boiling point working fluid or as a heat source for district heating via saturated low-pressure steam.{{Citation needed|date=February 2020}}

<gallery class="center">
File:GNR N2 1744 at Weybourne - geograph.org.uk - 1479849.jpg|A steam locomotive – a [[GNR Class N2|GNR N2 Class]] No.1744 at Weybourne nr. [[Sheringham]], [[Norfolk]]
File:Dampf-Fahrrad 2.jpg|A [[steam power|steam-powered]] bicycle by John van de Riet, in [[Dortmund]]
File:Steam-powered fire engine.jpg|British horse-drawn [[fire engine]] with steam-powered water pump
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