Editing Steam engine (section)

=== 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>