Editing Steam engine (section)

== Engine configuration ==
{{Anchor|Engine cycles}}

=== Simple engine ===
In a simple engine, or "single expansion engine", the charge of steam passes through the entire expansion process in an individual cylinder. A simple engine may have one or more individual cylinders.<ref>Basic Mechanical Engineering by Mohan Sen p. 266</ref> It is then exhausted directly into the atmosphere or into a condenser. As steam expands in passing through a high-pressure engine, its temperature drops because no heat is being added to the system; this is known as [[adiabatic process|adiabatic expansion]] and results in steam entering the cylinder at high temperature and leaving at lower temperature. This causes a cycle of heating and cooling of the cylinder with every stroke, which is a source of inefficiency.{{sfn|Hunter|1985|p=445}}
{{anchor|High pressure cylinder}}
{{anchor|Low pressure cylinder}}
<!--''High pressure cylinder'' and ''Low pressure cylinder'' both redirect here, hence the bolding of these terms-->

The dominant efficiency loss in reciprocating steam engines is cylinder condensation and re-evaporation. The steam cylinder and adjacent metal parts/ports operate at a temperature about halfway between the steam admission saturation temperature and the saturation temperature corresponding to the exhaust pressure.  As high-pressure steam is admitted into the working cylinder, much of the high-temperature steam is condensed as water droplets onto the metal surfaces, significantly reducing the steam available for expansive work.  When the expanding steam reaches low pressure (especially during the exhaust stroke), the previously deposited water droplets that had just been formed within the cylinder/ports now boil away (re-evaporation) and this steam does no further work in the cylinder.{{Citation needed|date=February 2020}}

There are practical limits on the expansion ratio of a steam engine cylinder, as increasing cylinder surface area tends to exacerbate the cylinder condensation and re-evaporation issues.  This negates the theoretical advantages associated with a high ratio of expansion in an individual cylinder.<ref>{{Cite web|title=Stirling {{!}} Internal Combustion Engine {{!}} Cylinder (Engine) {{!}} Free 30-day Trial|url=https://www.scribd.com/document/36719088/Stirling|website=Scribd|language=en|access-date=2020-05-21}}</ref>

=== Compound engines ===
{{Main|Compound steam engine}}

A method to lessen the magnitude of energy loss to a very long cylinder was invented in 1804 by British engineer [[Arthur Woolf]], who patented his ''Woolf high-pressure '''compound engine''''' in 1805. In the compound engine, high-pressure steam from the boiler expands in a '''high-pressure (HP) cylinder''' and then enters one or more subsequent '''lower-pressure (LP) cylinders'''. The complete expansion of the steam now occurs across multiple cylinders, with the overall temperature drop within each cylinder reduced considerably.  By expanding the steam in steps with smaller temperature range (within each cylinder) the condensation and re-evaporation efficiency issue (described above) is reduced. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine. By staging the expansion in multiple cylinders, variations of torque can be reduced.{{sfn|Hunter|1985|p=}} To derive equal work from lower-pressure cylinder requires a larger cylinder volume as this steam occupies a greater volume. Therefore, the bore, and in rare cases the stroke, are increased in low-pressure cylinders, resulting in larger cylinders.{{sfn|Hunter|1985|p=}}

Double-expansion (usually known as '''compound''') engines expanded the steam in two stages. The pairs may be duplicated or the work of the large low-pressure cylinder can be split with one high-pressure cylinder exhausting into two low pressure cylinders, giving a three-cylinder layout where cylinder and piston diameter are about the same, making the reciprocating masses easier to balance.{{sfn|Hunter|1985|p=}}

Two-cylinder compounds can be arranged as:
* '''Cross compounds''': Also known as 'receiver' compounds. Cylinders out of phase (< 180°), requiring the use of an exhaust 'receiver', which is usually no more than the valve chest or casing itself.
* '''Woolf compounds''': The cylinders are in phase or 180° phase shifted.
* '''Tandem compounds''': A form of Woolf compound. The cylinders are end to end, driving a common [[connecting rod]]
* '''Angle compounds''': A form of Cross compound. The cylinders are arranged in a V (usually at a 90° angle) and drive a common crank.

With two-cylinder compounds used in railway work, the pistons are connected to the cranks as with a two-cylinder simple at 90° out of phase with each other (''quartered''). When the double-expansion group is duplicated, producing a four-cylinder compound, the individual pistons within the group are usually balanced at 180°, the groups being set at 90° to each other. In one case (the first type of [[Vauclain compound]]), the pistons worked in the same phase driving a common crosshead and crank, again set at 90° as for a two-cylinder engine. With the three-cylinder compound arrangement, the LP cranks were either set at 90° with the HP one at 135° to the other two, or in some cases, all three cranks were set at 120°.<ref>{{Cite journal |date=1889-05-11 |title=Triple Expansion Engine |url=https://doi.org/10.1038/scientificamerican05111889-294 |journal=Scientific American |volume=60 |issue=19 |pages=294–295 |doi=10.1038/scientificamerican05111889-294 |issn=0036-8733|url-access=subscription }}</ref>

The adoption of compounding was common for industrial units, for road engines and almost universal for marine engines after 1880; it was not universally popular in railway locomotives where it was often perceived as complicated. This is partly due to the harsh railway operating environment and limited space afforded by the [[loading gauge]] (particularly in Britain, where compounding was never common and not employed after 1930). However, although never in the majority, it was popular in many other countries.<ref name="van Riemsdijk, Compound Locomotives" />

=== Multiple-expansion engines ===
<!-- Triple expansion steam engine redirects here -->
{{Anchor|Multiple expansion engines|Triple-expansion steam engine}}<!-- This keeps from having to update redirects to this section should its title change (again). -->
{{Main|Compound steam engine}}
[[File:Triple expansion engine animation.gif|thumb|upright=1.35|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.]]

It is a logical extension of the compound engine (described above) to split the expansion into yet more stages to increase efficiency. The result is the '''multiple-expansion engine'''. Such engines use either three or four expansion stages and are known as ''triple-'' and ''quadruple-expansion engines'' respectively. These engines use a series of cylinders of progressively increasing diameter. These cylinders are designed to divide the work into equal shares for each expansion stage. As with the double-expansion engine, if space is at a premium, then two smaller cylinders may be used for the low-pressure stage. Multiple-expansion engines typically had the cylinders arranged inline, but various other formations were used. In the late 19th century, the [[Compound steam engine#The Yarrow-Schlick-Tweedy system|Yarrow-Schlick-Tweedy balancing "system"]] was used on some [[Marine steam engine#Triple or multiple expansion|marine triple-expansion engines]]. Y-S-T engines divided the low-pressure expansion stages between two cylinders, one at each end of the engine. This allowed the crankshaft to be better balanced, resulting in a smoother, faster-responding engine which ran with less vibration. This made the four-cylinder triple-expansion engine popular with large passenger liners (such as the [[Olympic class ocean liner|''Olympic'' class]]), but this was ultimately replaced by the virtually vibration-free [[#Turbine engines|turbine engine]].{{citation needed|date=January 2013}} It is noted, however, that triple-expansion reciprocating steam engines were used to drive the World War II [[Liberty ship]]s, by far the largest number of identical ships ever built. Over 2700 ships were built, in the United States, from a British original design. {{Citation needed|date=February 2020}}

The image in this section shows an animation of a triple-expansion engine. The steam travels through the engine from left to right. The valve chest for each of the cylinders is to the left of the corresponding cylinder.{{Citation needed|date=February 2020}}

Land-based steam engines could exhaust their steam to atmosphere, as feed water was usually readily available. Prior to and during [[World War I]], the expansion engine dominated marine applications, where high vessel speed was not essential. It was, however, superseded by the British invention [[steam turbine]] where speed was required, for instance in warships, such as the [[dreadnought battleship]]s, and [[ocean liner]]s. {{HMS|Dreadnought|1906|6}} of 1905 was the first major warship to replace the proven technology of the reciprocating engine with the then-novel steam turbine.<ref>Brooks, John. ''Dreadnought Gunnery at the Battle of Jutland''. p. 14.</ref>