Editing Steam engine (section)

== Types of motor units ==

=== Reciprocating piston ===
{{Main|Reciprocating engine}}
[[File:Steam engine in action.gif|thumb|right|upright=1.35|[[Double-acting cylinder|Double acting]] stationary engine. This was the common mill engine of the mid 19th century. Note the [[slide valve]] with concave, almost D-shaped, underside.]]
[[File:Indicator diagram steam admission.svg|thumb|upright=1.35|Schematic [[Indicator diagram]] showing the four events in a double piston stroke. See: Monitoring and control (above)]]

In most reciprocating piston engines, the steam reverses its direction of flow at each [[stroke (engines)|stroke]] (counterflow), entering and exhausting from the same end of the cylinder. The complete engine cycle occupies one rotation of the crank and two piston strokes; the cycle also comprises four ''events'' – admission, expansion, exhaust, compression. These events are controlled by valves often working inside a ''steam chest'' adjacent to the cylinder; the valves distribute the steam by opening and closing steam ''ports'' communicating with the cylinder end(s) and are driven by [[valve gear]], of which there are many types.<ref>{{Cite web |date=2017-06-03 |title=Valves and Steamchest - Advanced Steam Traction |url=https://advanced-steam.org/5at/5at-project/5at-features/valves-and-steamchest/ |access-date=2024-06-19 |language=en-GB}}</ref>

The simplest valve gears give events of fixed length during the engine cycle and often make the engine rotate in only one direction. Many however have a reversing [[Machine|mechanism]] which additionally can provide means for saving steam as speed and momentum are gained by gradually "shortening the [[cutoff (steam engine)|cutoff]]" or rather, shortening the admission event; this in turn proportionately lengthens the expansion period. However, as one and the same valve usually controls both steam flows, a short cutoff at admission adversely affects the exhaust and compression periods which should ideally always be kept fairly constant; if the exhaust event is too brief, the totality of the exhaust steam cannot evacuate the cylinder, choking it and giving excessive compression (''"kick back"'').<ref>{{cite book |chapter=Backfiring |title=The Tractor Field Book: With Power Farm Equipment Specifications |place=Chicago |publisher=Farm Implement News Company |year=1928 |pages=108–109 [ [https://books.google.com/books?id=pFEfAQAAMAAJ&pg=PA108 108] ]}}</ref>

In the 1840s and 1850s, there were attempts to overcome this problem by means of various patent valve gears with a separate, variable cutoff [[expansion valve (steam engine)|expansion valve]] riding on the back of the main slide valve; the latter usually had fixed or limited cutoff. The combined setup gave a fair approximation of the ideal events, at the expense of increased friction and wear, and the mechanism tended to be complicated. The usual compromise solution has been to provide ''lap'' by lengthening rubbing surfaces of the valve in such a way as to overlap the port on the admission side, with the effect that the exhaust side remains open for a longer period after cut-off on the admission side has occurred. This expedient has since been generally considered satisfactory for most purposes and makes possible the use of the simpler [[Stephenson valve gear|Stephenson]], [[Joy valve gear|Joy]], and [[Walschaerts valve gear|Walschaerts]] motions. [[Corliss steam engine|Corliss]], and later, [[poppet valve]] gears had separate admission and exhaust valves driven by [[trip valve|trip mechanisms]] or [[Cam (mechanism)|cam]]s profiled so as to give ideal events; most of these gears never succeeded outside of the stationary marketplace due to various other issues including leakage and more delicate mechanisms.<ref name="van Riemsdijk, Compound Locomotives">{{cite book|last=van Riemsdijk| first=John|year=1994|title=Compound Locomotives |location=Penrhyn, UK|publisher=Atlantic Transport Publishers|isbn=978-0-906899-61-8|pages=2–3}}</ref>{{sfn|Chapelon|2000|pp=56–72, 120-}}

==== Compression ====
Before the exhaust phase is quite complete, the exhaust side of the valve closes, shutting a portion of the exhaust steam inside the cylinder. This determines the compression phase where a cushion of steam is formed against which the piston does work whilst its velocity is rapidly decreasing; it moreover obviates the pressure and temperature shock, which would otherwise be caused by the sudden admission of the high-pressure steam at the beginning of the following cycle.{{citation needed|date=January 2013}}

==== Lead in the valve timing====
The above effects are further enhanced by providing ''lead'': as was later discovered with the [[internal combustion engine]], it has been found advantageous since the late 1830s to advance the admission phase, giving the valve ''lead'' so that admission occurs a little before the end of the exhaust stroke in order to fill the ''clearance volume'' comprising the ports and the cylinder ends (not part of the piston-swept volume) before the steam begins to exert effort on the piston.<ref name="Bel, Locomotives">{{cite book|last=Bell|first=A.M.|title=Locomotives|publisher=Virtue and Company|year=1950|location=London|pages=61–63}}</ref>

=== Uniflow (or unaflow) engine ===
{{Main|Uniflow steam engine}}
[[File:Uniflow steam engine.gif|thumb|upright=1.15|Animation of a [[uniflow steam engine]].<br />The [[poppet valves]] are controlled by the rotating [[camshaft]] at the top. High-pressure steam enters, red, and exhausts, yellow.]]

Uniflow engines attempt to remedy the difficulties arising from the usual counterflow cycle where, during each stroke, the port and the cylinder walls will be cooled by the passing exhaust steam, whilst the hotter incoming admission steam will waste some of its energy in restoring the working temperature. The aim of the uniflow is to remedy this defect and improve efficiency by providing an additional port uncovered by the piston at the end of each stroke making the steam flow only in one direction. By this means, the simple-expansion uniflow engine gives efficiency equivalent to that of classic compound systems with the added advantage of superior part-load performance, and comparable efficiency to turbines for smaller engines below one thousand horsepower. However, the thermal expansion gradient uniflow engines produce along the cylinder wall gives practical difficulties.{{citation needed|date=January 2013}}.

=== Turbine engines ===
{{Main|Steam turbine}}
[[File:Dampfturbine Laeufer01.jpg|thumb|right|A rotor of a modern [[steam turbine]], used in a [[power plant]]]]

A steam turbine consists of one or more ''[[Steam turbine#Reaction turbines|rotors]]'' (rotating discs) mounted on a drive shaft, alternating with a series of ''[[Steam turbine#Reaction turbines|stators]]'' (static discs) fixed to the turbine casing. The rotors have a propeller-like arrangement of blades at the outer edge. Steam acts upon these blades, producing rotary motion. The stator consists of a similar, but fixed, series of blades that serve to redirect the steam flow onto the next rotor stage. A steam turbine often exhausts into a [[surface condenser]] that provides a vacuum. The stages of a steam turbine are typically arranged to extract the maximum potential work from a specific velocity and pressure of steam, giving rise to a series of variably sized high- and low-pressure stages. Turbines are only efficient if they rotate at relatively high speed, therefore they are usually connected to reduction gearing to drive lower speed applications, such as a ship's propeller. In the vast majority of large electric generating stations, turbines are directly connected to generators with no reduction gearing. Typical speeds are 3600 revolutions per minute (RPM) in the United States with 60 Hertz power, and 3000 RPM in Europe and other countries with 50 Hertz electric power systems. In nuclear power applications, due to enormous size, the turbines typically run at half these speeds, 1800 RPM and 1500 RPM. A turbine rotor is also only capable of providing power when rotating in one direction. Therefore, a reversing stage or gearbox is usually required where power is required in the opposite direction.{{Citation needed|date=February 2020}}

Steam turbines provide direct rotational force and therefore do not require a linkage mechanism to convert reciprocating to rotary motion. Thus, they produce smoother rotational forces on the output shaft. This contributes to a lower maintenance requirement and less wear on the machinery they power than a comparable reciprocating engine.{{citation needed|date=January 2013}}

[[File:Turbinia At Speed.jpg|thumb|right|''[[Turbinia]]'' – the first [[steam turbine]]-powered ship]]

The main use for steam turbines is in [[electricity generation]] (in the 1990s about 90% of the world's electric production was by use of steam turbines)<ref Name="Wiser" /> however the recent widespread application of large gas turbine units and typical combined cycle power plants has resulted in reduction of this percentage to the 80% regime for steam turbines. In electricity production, the high speed of turbine rotation matches well with the speed of modern electric generators, which are typically direct connected to their driving turbines. In marine service, (pioneered on the ''[[Turbinia]]''), steam turbines with reduction gearing (although the Turbinia has direct turbines to propellers with no reduction gearbox) dominated large ship propulsion throughout the late 20th century, being more efficient (and requiring far less maintenance) than reciprocating steam engines. In recent decades, reciprocating Diesel engines, and gas turbines, have almost entirely supplanted steam propulsion for marine applications.{{Citation needed|date=February 2020}}

Virtually all [[nuclear power]] plants generate electricity by heating water to provide steam that drives a turbine connected to an [[electrical generator]]. [[Nuclear marine propulsion|Nuclear-powered ships and submarines]] either use a steam turbine directly for main propulsion, with generators providing auxiliary power, or else employ [[turbo-electric transmission]], where the steam drives a [[turbo generator]] set with propulsion provided by electric motors. A limited number of [[steam turbine locomotive|steam turbine railroad locomotives]] were manufactured. Some non-condensing direct-drive locomotives did meet with some success for long haul freight operations in [[Sweden]] and for [[LMS Turbomotive|express passenger work in Britain]], but were not repeated. Elsewhere, notably in the United States, more advanced designs with electric transmission were built experimentally, but not reproduced. It was found that steam turbines were not ideally suited to the railroad environment and these locomotives failed to oust the classic reciprocating steam unit in the way that modern diesel and electric traction has done.{{citation needed|date=January 2013}}

[[File:Oscillating cylinder.svg|thumb|right|Operation of a simple [[oscillating cylinder steam engine]]]]

=== Oscillating cylinder steam engines ===
{{Main|Oscillating cylinder steam engine}}

An oscillating cylinder steam engine is a variant of the simple expansion steam engine which does not require [[valve gear|valves]] to direct steam into and out of the cylinder. Instead of valves, the entire cylinder rocks, or oscillates, such that one or more holes in the cylinder line up with holes in a fixed port face or in the pivot mounting ([[trunnion]]). These engines are mainly used in toys and models because of their simplicity, but have also been used in full-size working engines, mainly on [[Marine steam engine#Oscillating|ships]] where their compactness is valued.<ref>{{cite book |last1=Seaton |first1=A E |title=Manual of Marine Engineering |date=1918 |publisher=Charles Griffin |location=London |pages=56–108}}</ref>

=== Rotary steam engines ===
It is possible to use a mechanism based on a [[pistonless rotary engine]] such as the [[Wankel engine]] in place of the cylinders and [[valve gear]] of a conventional reciprocating steam engine. Many such engines have been designed, from the time of James Watt to the present day, but relatively few were actually built and even fewer went into quantity production; see link at bottom of article for more details. The major problem is the difficulty of sealing the rotors to make them steam-tight in the face of wear and [[thermal expansion]]; the resulting leakage made them very inefficient. Lack of expansive working, or any means of control of the [[cutoff (steam engine)|cutoff]], is also a serious problem with many such designs.{{citation needed|date=January 2013}}

By the 1840s, it was clear that the concept had inherent problems and rotary engines were treated with some derision in the technical press. However, the arrival of electricity on the scene, and the obvious advantages of driving a dynamo directly from a high-speed engine, led to something of a revival in interest in the 1880s and 1890s, and a few designs had some limited success.{{citation needed|date=January 2013}}.

Of the few designs that were manufactured in quantity, those of the Hult Brothers Rotary Steam Engine Company of Stockholm, Sweden, and the spherical engine of [[Beauchamp Tower]] are notable. Tower's engines were used by the [[Great Eastern Railway]] to drive lighting dynamos on their locomotives, and by the [[British Admiralty|Admiralty]] for driving dynamos on board the ships of the [[Royal Navy]]. They were eventually replaced in these niche applications by steam turbines.{{citation needed|date=January 2013}}

[[File:Aeolipile illustration.png|thumb|upright|alt=Line drawing of a sphere suspended between two uprights forming a horizontal axis. Two right-angle jet arms at the circumference expel steam that has been produced by boiling water in a closed vessel under the two uprights, which are hollow and let steam flow into the interior of the sphere.|An [[aeolipile]] rotates due to the steam escaping from the arms. No practical use was made of this effect.{{Citation needed|date=July 2020}}]]

=== Rocket type ===
{{Main|Steam rocket}}

The [[aeolipile]] represents the use of steam by the [[reaction engine|rocket-reaction principle]], although not for direct propulsion.{{Citation needed|date=February 2020}}

In more modern times there has been limited use of steam for rocketry – particularly for rocket cars. Steam rocketry works by filling a pressure vessel with hot water at high pressure and opening a valve leading to a suitable nozzle. The drop in pressure immediately boils some of the water and the steam leaves through a nozzle, creating a propulsive force.<ref>[http://www.tecaeromex.com/ingles/vapori.html Steam Rockets] {{Webarchive|url=https://web.archive.org/web/20191124015726/http://www.tecaeromex.com/ingles/vapori.html |date=24 November 2019 }} Tecaeromax</ref>

[[Ferdinand Verbiest]]'s carriage was powered by an aeolipile in 1679.{{citation needed|reason=Verbiest's car appears to use a turbine, not an aeolipile.  Motion is derived from the jet impacting upon a wheen, not from the reaction to the jet.|date=July 2020}}