
A steam engine is a
heat engine
In thermodynamics and engineering, a heat engine is a system that converts heat to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower sta ...
that performs
mechanical work using
steam as its
working fluid
For fluid power, a working fluid is a gas or liquid that primarily transfers force, motion, or mechanical energy. In hydraulics, water or hydraulic fluid transfers force between hydraulic components such as hydraulic pumps, hydraulic cylinders, a ...
. The steam engine uses the force produced by steam pressure to push a
piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-t ...
back and forth inside a
cylinder. This pushing force can be transformed, by a
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...
and
crank, into
rotation
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
al force for
work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal ...
. The term "steam engine" is generally applied only to
reciprocating engines as just described, not to the
steam turbine
A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam tur ...
. Steam engines are
external combustion engines,
where the working fluid is separated from the combustion products. The ideal
thermodynamic
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws ...
cycle used to analyze this process is called the
Rankine cycle
The Rankine cycle is an idealized thermodynamic cycle describing the process by which certain heat engines, such as steam turbines or reciprocating steam engines, allow mechanical work to be extracted from a fluid as it moves between a heat sourc ...
. In general usage, the term ''steam engine'' can refer to either complete steam plants (including
boilers etc.), such as railway
steam locomotives and
portable engines, or may refer to the
piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-t ...
or turbine machinery alone, as in the
beam engine and
stationary steam engine
Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam c ...
.
Although steam-driven devices were known as early as the
aeolipile in the first century AD, with a few other uses recorded in the 16th century, in 1606
Jerónimo de Ayanz y Beaumont
Jerónimo de Ayanz y Beaumont (1553 – March 23, 1613 AD) was a Spanish soldier, painter, astronomer, musician and inventor.
He was born in Guendulain (Navarre).
He built an air-renovated diving suit that allowed a man to remain underwat ...
patented his invention of the first steam-powered water pump for draining mines.
Thomas Savery is considered the inventor of the first commercially used steam powered device, a steam pump that used steam pressure operating directly on the water. The first commercially successful engine that could transmit continuous power to a machine was developed in 1712 by
Thomas Newcomen.
James Watt
James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which wa ...
made a critical improvement in 1764, by removing spent steam to a separate vessel for condensation, greatly improving the amount of work obtained per unit of fuel consumed. By the 19th century, stationary steam engines powered the factories of the
Industrial Revolution
The Industrial Revolution was the transition to new manufacturing processes in Great Britain, continental Europe, and the United States, that occurred during the period from around 1760 to about 1820–1840. This transition included going f ...
. Steam engines replaced
sails for ships on
paddle steamer
A paddle steamer is a steamship or steamboat powered by a steam engine that drives paddle wheels to propel the craft through the water. In antiquity, paddle wheelers followed the development of poles, oars and sails, where the first uses we ...
s, and steam locomotives operated on the railways.
Reciprocating piston type steam engines were the dominant source of power until the early 20th century, when advances in the design of
electric motors and
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal com ...
s resulted in the gradual replacement of steam engines in commercial usage. Steam turbines replaced reciprocating engines in power generation, due to lower cost, higher operating speed, and higher efficiency.
History
Early experiments
One recorded rudimentary steam-powered "engine" was the
aeolipile described by
Hero of Alexandria
Hero of Alexandria (; grc-gre, Ἥρων ὁ Ἀλεξανδρεύς, ''Heron ho Alexandreus'', also known as Heron of Alexandria ; 60 AD) was a Greek mathematician and engineer who was active in his native city of Alexandria, Roman Egypt. He ...
, a Greek mathematician and engineer in
Roman Egypt in the first century AD. In the following centuries, the few steam-powered "engines" known were, like the aeolipile,
[''"De Architectura"'': Chapter VI (paragraph 2)]
from "Ten Books on Architecture" by Vitruvius
Vitruvius (; c. 80–70 BC – after c. 15 BC) was a Roman architect and engineer during the 1st century BC, known for his multi-volume work entitled ''De architectura''. He originated the idea that all buildings should have three attribute ...
(1st century BC), published 17, June, 0
accessed 2009-07-07 essentially experimental devices used by inventors to demonstrate the properties of steam.
A rudimentary
steam turbine
A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam tur ...
device was described by
Taqi al-Din Muhammad ibn Ma'ruf, Taqi al-Din[ Ahmad Y Hassan (1976). ''Taqi al-Din and Arabic Mechanical Engineering'', pp. 34–35. Institute for the History of Arabic Science, University of Aleppo.] in
Ottoman Egypt
The Eyalet of Egypt (, ) operated as an administrative division of the Ottoman Empire from 1517 to 1867. It originated as a result of the conquest of Mamluk Egypt by the Ottomans in 1517, following the Ottoman–Mamluk War (1516–17) and the ...
in 1551 and by
Giovanni Branca in Italy in 1629. The Spanish inventor
Jerónimo de Ayanz y Beaumont
Jerónimo de Ayanz y Beaumont (1553 – March 23, 1613 AD) was a Spanish soldier, painter, astronomer, musician and inventor.
He was born in Guendulain (Navarre).
He built an air-renovated diving suit that allowed a man to remain underwat ...
received patents in 1606 for 50 steam-powered inventions, including a water pump for draining inundated mines.
Denis Papin, a
Huguenot
The Huguenots ( , also , ) were a religious group of French Protestants who held to the Reformed, or Calvinist, tradition of Protestantism. The term, which may be derived from the name of a Swiss political leader, the Genevan burgomaster Beza ...
, did some useful work on the
steam digester in 1679, and first used a piston to raise weights in 1690.
Pumping engines
The first commercial steam-powered device was a water pump, developed in 1698 by
Thomas Savery.
It used condensing steam to create a vacuum which raised water from below and then used steam pressure to raise it higher. Small engines were effective though larger models were problematic. They had a very limited lift height and were prone to
boiler explosion
A boiler explosion is a catastrophic failure of a boiler. There are two types of boiler explosions. One type is a failure of the pressure parts of the steam and water sides. There can be many different causes, such as failure of the safety val ...
s. Savery's engine was used in mines,
pumping station
Pumping stations, also called pumphouses in situations such as drilled wells and drinking water, are facilities containing pumps and equipment for pumping fluids from one place to another. They are used for a variety of infrastructure systems ...
s and supplying water to
water wheel
A water wheel is a machine for converting the energy of flowing or falling water into useful forms of power, often in a watermill. A water wheel consists of a wheel (usually constructed from wood or metal), with a number of blades or bucke ...
s powering textile machinery.
Savery's engine was of low cost.
Bento de Moura Portugal introduced an improvement of Savery's construction "to render it capable of working itself", as described by
John Smeaton
John Smeaton (8 June 1724 – 28 October 1792) was a British civil engineer responsible for the design of bridges, canals, harbours and lighthouses. He was also a capable mechanical engineer and an eminent physicist. Smeaton was the fir ...
in the Philosophical Transactions published in 1751. It continued to be manufactured until the late 18th century. At least one engine was still known to be operating in 1820.
Piston steam engines

The first commercially successful engine that could transmit continuous power to a machine was the
atmospheric engine, invented by
Thomas Newcomen around 1712. It improved on Savery's steam pump, using a piston as proposed by Papin. Newcomen's engine was relatively inefficient, and mostly used for pumping water. It worked by creating a partial vacuum by condensing steam under a piston within a cylinder. It was employed for draining mine workings at depths originally impractical using traditional means, and for providing reusable water for driving waterwheels at factories sited away from a suitable "head". Water that passed over the wheel was pumped up into a storage reservoir above the wheel.
In 1780 James Pickard patented the use of a flywheel and crankshaft to provide rotative motion from an improved Newcomen engine.
In 1720,
Jacob Leupold
Jacob Leupold (22 July 1674 – 12 January 1727) was a German physicist, mathematician, instrument maker, mining commissioner and engineer. He wrote the seminal book ''Theatrum Machinarum Generale'' ("The General Theory of Machines").
Early ...
described a two-cylinder high-pressure steam engine. The invention was published in his major work "Theatri Machinarum Hydraulicarum". The engine used two heavy pistons to provide motion to a water pump. Each piston was raised by the steam pressure and returned to its original position by gravity. The two pistons shared a common four-way
rotary valve
A rotary valve (also called rotary-motion valve) is a type of valve in which the rotation of a passage or passages in a transverse plug regulates the flow of liquid or gas through the attached pipes. The common stopcock is the simplest form of ro ...
connected directly to a steam boiler.

The next major step occurred when
James Watt
James Watt (; 30 January 1736 (19 January 1736 OS) – 25 August 1819) was a Scottish inventor, mechanical engineer, and chemist who improved on Thomas Newcomen's 1712 Newcomen steam engine with his Watt steam engine in 1776, which wa ...
developed (1763–1775)
an improved version of Newcomen's engine, with a
separate condenser.
Boulton and Watt's early engines used half as much coal as
John Smeaton
John Smeaton (8 June 1724 – 28 October 1792) was a British civil engineer responsible for the design of bridges, canals, harbours and lighthouses. He was also a capable mechanical engineer and an eminent physicist. Smeaton was the fir ...
's improved version of Newcomen's.
[ Duty comparison was based on a carefully conducted trial in 1778.] Newcomen's and Watt's early engines were "atmospheric". They were powered by air pressure pushing a piston into the partial
vacuum generated by
condensing steam, instead of the
pressure of expanding steam. The engine
cylinders
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an inf ...
had to be large because the only usable force acting on them was
atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
.
Watt developed his engine further, modifying it to provide a rotary motion suitable for driving machinery. This enabled factories to be sited away from rivers, and accelerated the pace of the Industrial Revolution.
High-pressure engines
The meaning of high pressure, together with an actual value above ambient, depends on the era in which the term was used. For early use of the term Van Reimsdijk refers to steam being at a sufficiently high pressure that it could be exhausted to atmosphere without reliance on a vacuum to enable it to perform useful work. states that Watt's condensing engines were known, at the time, as low pressure compared to high pressure, non-condensing engines of the same period.
Watt's patent prevented others from making high pressure and compound engines. Shortly after Watt's patent expired in 1800,
Richard Trevithick
Richard Trevithick (13 April 1771 – 22 April 1833) was a British inventor and mining engineer. The son of a mining captain, and born in the mining heartland of Cornwall, Trevithick was immersed in mining and engineering from an early age. He ...
and, separately,
Oliver Evans in 1801
introduced engines using high-pressure steam; Trevithick obtained his high-pressure engine patent in 1802, and Evans had made several working models before then. These were much more powerful for a given cylinder size than previous engines and could be made small enough for transport applications. Thereafter, technological developments and improvements in manufacturing techniques (partly brought about by the adoption of the steam engine as a power source) resulted in the design of more efficient engines that could be smaller, faster, or more powerful, depending on the intended application.
The
Cornish engine was developed by Trevithick and others in the 1810s. It was a compound cycle engine that used high-pressure steam expansively, then condensed the low-pressure steam, making it relatively efficient. The Cornish engine had irregular motion and torque through the cycle, limiting it mainly to pumping. Cornish engines were used in mines and for water supply until the late 19th century.
Horizontal stationary engine
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.
The acme of the horizontal engine was the
Corliss steam engine
A Corliss steam engine (or Corliss engine) is a steam engine, fitted with rotary valves and with variable valve timing patented in 1849, invented by and named after the American engineer George Henry Corliss of Providence, Rhode Island.
Engines ...
, patented in 1849, which was a four-valve counter flow engine with separate steam admission and exhaust valves and automatic variable steam cutoff. When Corliss was given the
Rumford Medal
The Rumford Medal is an award bestowed by Britain's Royal Society every alternating year for "an outstandingly important recent discovery in the field of thermal or optical properties of matter made by a scientist working in Europe".
First awa ...
, the committee said that "no one invention since Watt's time has so enhanced the efficiency of the steam engine".
In addition to using 30% less steam, it provided more uniform speed due to variable steam cut off, making it well suited to manufacturing, especially cotton spinning.
Road vehicles

The first experimental road-going steam-powered vehicles were built in the late 18th century, but it was not until after
Richard Trevithick
Richard Trevithick (13 April 1771 – 22 April 1833) was a British inventor and mining engineer. The son of a mining captain, and born in the mining heartland of Cornwall, Trevithick was immersed in mining and engineering from an early age. He ...
had developed the use of high-pressure steam, around 1800, that mobile steam engines became a practical proposition. The first half of the 19th century saw great progress in steam vehicle design, and by the 1850s it was becoming viable to produce them on a commercial basis. This progress was dampened by legislation which limited or prohibited the use of steam-powered vehicles on roads. Improvements in vehicle technology continued from the 1860s to the 1920s. Steam road vehicles were used for many applications. In the 20th century, the rapid development of
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal com ...
technology led to the demise of the steam engine as a source of propulsion of vehicles on a commercial basis, with relatively few remaining in use beyond the
Second World War
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposi ...
. Many of these vehicles were acquired by enthusiasts for preservation, and numerous examples are still in existence. In the 1960s, the air pollution problems in California gave rise to a brief period of interest in developing and studying steam-powered vehicles as a possible means of reducing the pollution. Apart from interest by steam enthusiasts, the occasional replica vehicle, and experimental technology, no steam vehicles are in production at present.
Marine engines

Near the end of the 19th century, compound engines came into widespread use.
Compound engines exhausted steam into successively larger cylinders to accommodate the higher volumes at reduced pressures, giving improved efficiency. These stages were called expansions, with double- and triple-expansion engines being common, especially in shipping where efficiency was important to reduce the weight of coal carried. Steam engines remained the dominant source of power until the early 20th century, when advances in the design of the
steam turbine
A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam tur ...
,
electric motors and
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal com ...
s gradually resulted in the replacement of reciprocating (piston) steam engines, with merchant shipping relying increasingly upon
diesel engines, and warships on the steam turbine.
Steam locomotives
As the development of steam engines progressed through the 18th century, various attempts were made to apply them to road and railway use. In 1784,
William Murdoch, a
Scottish inventor, built a model steam road locomotive. An early working model of a steam rail locomotive was designed and constructed by steamboat pioneer
John Fitch in the United States probably during the 1780s or 1790s.
His steam locomotive used interior bladed wheels guided by rails or tracks.

The first full-scale working railway steam locomotive was built by
Richard Trevithick
Richard Trevithick (13 April 1771 – 22 April 1833) was a British inventor and mining engineer. The son of a mining captain, and born in the mining heartland of Cornwall, Trevithick was immersed in mining and engineering from an early age. He ...
in the
United Kingdom
The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and No ...
and, on 21 February 1804, the world's first railway journey took place as Trevithick's unnamed steam locomotive hauled a train along the
tramway from the
Pen-y-darren
: ''For Trevithick's Pen-y-darren locomotive, see Richard Trevithick.''
Penydarren is a community and electoral ward in Merthyr Tydfil County Borough in Wales.
Description
The area is most notable for being the site of a 1st-century Roman fort, ...
ironworks, near
Merthyr Tydfil to
Abercynon in south
Wales
Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...
. The design incorporated a number of important innovations that included using high-pressure steam which reduced the weight of the engine and increased its efficiency. Trevithick visited the Newcastle area later in 1804 and the
colliery railways in north-east England became the leading centre for experimentation and development of steam locomotives.
Trevithick continued his own experiments using a trio of locomotives, concluding with the
Catch Me Who Can in 1808. Only four years later, the successful twin-cylinder locomotive ''
Salamanca'' by
Matthew Murray was used by the
edge railed rack and pinion Middleton Railway
The Middleton Railway is the world's oldest continuously working railway, situated in the English city of Leeds. It was founded in 1758 and is now a heritage railway, run by volunteers from The Middleton Railway Trust Ltd. since 1960.
The ra ...
.
In 1825
George Stephenson
George Stephenson (9 June 1781 – 12 August 1848) was a British civil engineer and mechanical engineer. Renowned as the "Father of Railways", Stephenson was considered by the Victorians a great example of diligent application and thirst fo ...
built the ''
Locomotion'' for the
Stockton and Darlington Railway. This was the first public steam railway in the world and then in 1829, he built ''
The Rocket'' which was entered in and won the
Rainhill Trials.
The
Liverpool and Manchester Railway opened in 1830 making exclusive use of steam power for both passenger and freight trains.
Steam locomotives continued to be manufactured until the late twentieth century in places such as
China
China, officially the People's Republic of China (PRC), is a country in East Asia. It is the world's List of countries and dependencies by population, most populous country, with a Population of China, population exceeding 1.4 billion, slig ...
and the former
East Germany
East Germany, officially the German Democratic Republic (GDR; german: Deutsche Demokratische Republik, , DDR, ), was a country that existed from its creation on 7 October 1949 until German reunification, its dissolution on 3 October 1990. In t ...
(where the
DR Class 52.80 was produced).
Steam turbines
The final major evolution of the steam engine design was the use of steam
turbine
A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generati ...
s starting in the late part of the 19th century. Steam turbines are generally more efficient than reciprocating piston type steam engines (for outputs above several hundred horsepower), have fewer moving parts, and provide rotary power directly instead of through a
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...
system or similar means.
Steam turbines virtually replaced reciprocating engines in electricity generating stations early in the 20th century, where their efficiency, higher speed appropriate to generator service, and smooth rotation were advantages. Today most
electric power is provided by steam turbines. In the United States, 90% of the electric power is produced in this way using a variety of heat sources.
Steam turbines were extensively applied for propulsion of large ships throughout most of the 20th century.
Present development
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.
Components and accessories of steam engines
There are two fundamental components of a steam plant: the
boiler
A boiler is a closed vessel in which fluid (generally water) is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central ...
or
steam generator, and the "motor unit", referred to itself as a "steam engine".
Stationary steam engine
Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam c ...
s in fixed buildings may have the boiler and engine in separate buildings some distance apart. For portable or mobile use, such as
steam locomotives, the two are mounted together.
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.
Other components are often present; pumps (such as an
injector) to supply water to the boiler during operation, condensers to recirculate the water and recover the
latent heat of vaporisation, and
superheaters to raise the temperature of the steam above its saturated vapour point, and various mechanisms to increase the draft for fireboxes. When coal is used, a chain or screw stoking mechanism and its drive engine or motor may be included to move the fuel from a supply bin (bunker) to the firebox.
Heat source
The heat required for boiling the water and raising the temperature of the steam can be derived from various sources, most commonly from burning combustible materials with an appropriate supply of air in a closed space (e.g.,
combustion chamber,
firebox, furnace). In the case of
model or toy steam engines and a few full scale cases, the heat source can be an
electric heating element.
Boilers

Boilers are
pressure vessel
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
Construction methods and materials may be chosen to suit the pressure application, and will depend on the size ...
s that contain water to be boiled, and features that
transfer the heat to the water as effectively as possible.
The two most common types are:
;
Water-tube boiler
A high pressure watertube boiler (also spelled water-tube and water tube) is a type of boiler in which water circulates in tubes heated externally by the fire. Fuel is burned inside the furnace, creating hot gas which boils water in the steam-gen ...
: Water is passed through tubes surrounded by hot gas.
;
Fire-tube boiler
A fire-tube boiler is a type of boiler in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating ...
: 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.
Many boilers raise the temperature of the steam after it has left that part of the boiler where it is in contact with the water. Known as
superheating
In thermodynamics, superheating (sometimes referred to as boiling retardation, or boiling delay) is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. This is a so-called ''metastable st ...
it turns '
wet steam' into '
superheated steam'. It avoids the steam condensing in the engine cylinders, and gives a significantly higher
efficiency
Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
.
Motor units
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.
These "motor units" are often called 'steam engines' in their own right. Engines using compressed air or other gases differ from steam engines only in details that depend on the nature of the gas although
compressed air has been used in steam engines without change.
Cold sink
As with all heat engines, the majority of
primary energy
Primary energy (PE) is an energy form found in nature that has not been subjected to any human engineered conversion process. It is energy contained in raw fuels, and other forms of energy, including waste, received as input to a system. P ...
must be emitted as
waste heat
Waste heat is heat that is produced by a machine, or other process that uses energy, as a byproduct of doing work. All such processes give off some waste heat as a fundamental result of the laws of thermodynamics. Waste heat has lower utili ...
at relatively low temperature.
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.
Steam engines in stationary power plants use
surface condensers as a cold sink. The condensers are cooled by water flow from oceans, rivers, lakes, and often by
cooling towers which evaporate water to provide cooling energy removal. The resulting condensed hot water (''condensate''), is then pumped back up to pressure and sent back to the boiler. A dry-type cooling tower is similar to an automobile radiator and is used in locations where water is costly. Waste heat can also be ejected by evaporative (wet) cooling towers, which use a secondary external water circuit that evaporates some of flow to the air.
River boats initially used a
jet condenser in which cold water from the river is injected into the exhaust steam from the engine. Cooling water and condensate mix. While this was also applied for sea-going vessels, generally after only a few days of operation the boiler would become coated with deposited salt, reducing performance and increasing the risk of a boiler explosion. Starting about 1834, the use of surface condensers on ships eliminated fouling of the boilers, and improved engine efficiency.
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.
Water pump
Most steam boilers have a means to supply water whilst at pressure, so that they may be run continuously. Utility and industrial boilers commonly use multi-stage
centrifugal pump
Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor. They are a sub-class of dynamic ...
s; however, other types are used. Another means of supplying lower-pressure boiler feed water is an
injector, which uses a steam jet usually supplied from the boiler. Injectors became popular in the 1850s but are no longer widely used, except in applications such as steam locomotives. It is the pressurization of the water that circulates through the steam boiler that allows the water to be raised to temperatures well above boiling point of water at one atmospheric pressure, and by that means to increase the efficiency of the steam cycle.
Monitoring and control

For safety reasons, nearly all steam engines are equipped with mechanisms to monitor the boiler, such as a
pressure gauge and a
sight glass to monitor the water level.
Many engines, stationary and mobile, are also fitted with a
governor to regulate the speed of the engine without the need for human interference.
The most useful instrument for analyzing the performance of steam engines is the steam engine indicator. Early versions were in use by 1851, but the most successful indicator was developed for the high speed engine inventor and manufacturer Charles Porter by Charles Richard and exhibited at London Exhibition in 1862.
The steam engine indicator traces on paper the pressure in the cylinder throughout the cycle, which can be used to spot various problems and calculate developed horsepower. It was routinely used by engineers, mechanics and insurance inspectors. The engine indicator can also be used on internal combustion engines. See image of indicator diagram below (in ''Types of motor units'' section).
Governor

The
centrifugal governor was adopted by James Watt for use on a steam engine in 1788 after Watt's partner Boulton saw one on the equipment of a flour mill
Boulton & Watt were building. The governor could not actually hold a set speed, because it would assume a new constant speed in response to load changes. The governor was able to handle smaller variations such as those caused by fluctuating heat load to the boiler. Also, there was a tendency for oscillation whenever there was a speed change. As a consequence, engines equipped only with this governor were not suitable for operations requiring constant speed, such as cotton spinning. The governor was improved over time and coupled with variable steam cut off, good speed control in response to changes in load was attainable near the end of the 19th century.
Engine configuration
Simple engine
In a simple engine, or "single expansion engine" the charge of steam passes through the entire expansion process in an individual cylinder, although a simple engine may have one or more individual cylinders. 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 expansion
In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal proces ...
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.
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.
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.
Compound engines
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. 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.
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 one or the other, giving a three-cylinder layout where cylinder and piston diameter are about the same, making the reciprocating masses easier to balance.
Two-cylinder compounds can be arranged as:
* Cross compounds: The cylinders are side by side.
* Tandem compounds: The cylinders are end to end, driving a common
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...
* Angle compounds: 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°.
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
A loading gauge is a diagram or physical structure that defines the maximum height and width dimensions in railway vehicles and their loads. Their purpose is to ensure that rail vehicles can pass safely through tunnels and under bridges, and k ...
(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.
Multiple-expansion engines

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 Yarrow-Schlick-Tweedy balancing "system" was used on some
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), but this was ultimately replaced by the virtually vibration-free
turbine engine. It is noted, however, that triple-expansion reciprocating steam engines were used to drive the World War II
Liberty ship
Liberty ships were a class of cargo ship built in the United States during World War II under the Emergency Shipbuilding Program. Though British in concept, the design was adopted by the United States for its simple, low-cost construction. M ...
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.
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.
Land-based steam engines could exhaust their steam to atmosphere, as feed water was usually readily available. Prior to and during
World War I
World War I (28 July 1914 11 November 1918), often abbreviated as WWI, was one of the deadliest global conflicts in history. Belligerents included much of Europe, the Russian Empire, the United States, and the Ottoman Empire, with fight ...
, the expansion engine dominated marine applications, where high vessel speed was not essential. It was, however, superseded by the British invention
steam turbine
A steam turbine is a machine that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Charles Parsons in 1884. Fabrication of a modern steam tur ...
where speed was required, for instance in warships, such as the
dreadnought battleships, and
ocean liners. of 1905 was the first major warship to replace the proven technology of the reciprocating engine with the then-novel steam turbine.
Types of motor units
Reciprocating piston

In most reciprocating piston engines, the steam reverses its direction of flow at each
stroke
A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop funct ...
(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
The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing ...
, of which there are many types.
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
mechanism which additionally can provide means for saving steam as speed and momentum are gained by gradually "shortening the
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"'').
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 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,
Joy and
Walschaerts motions.
Corliss, and later,
poppet valve
A poppet valve (also called mushroom valve) is a valve typically used to control the timing and quantity of gas or vapor flow into an engine.
It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usual ...
gears had separate admission and exhaust valves driven by
trip mechanisms or
cams 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.
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.
Lead in the valve timing
The above effects are further enhanced by providing ''lead'': as was later discovered with the
internal combustion engine
An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal com ...
, 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.
Uniflow (or unaflow) engine

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..
Turbine engines

A steam turbine consists of one or more ''
rotors'' (rotating discs) mounted on a drive shaft, alternating with a series of ''
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, 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.
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.

The main use for steam turbines is in
electricity generation
Electricity generation is the process of generating electric power from sources of primary energy. For utilities in the electric power industry, it is the stage prior to its delivery ( transmission, distribution, etc.) to end users or its s ...
(in the 1990s about 90% of the world's electric production was by use of steam turbines)
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.
Virtually all
nuclear power
Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced ...
plants generate electricity by heating water to provide steam that drives a turbine connected to an
electrical generator
In electricity generation, a generator is a device that converts motive power (mechanical energy) or fuel-based power (chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, ...
.
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 railroad locomotives were manufactured. Some non-condensing direct-drive locomotives did meet with some success for long haul freight operations in
Sweden
Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic countries, Nordic c ...
and for
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.
Oscillating cylinder steam engines
An oscillating cylinder steam engine is a variant of the simple expansion steam engine which does not require
valves
A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...
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
A trunnion (from Old French "''trognon''", trunk) is a cylindrical protrusion used as a mounting or pivoting point. First associated with cannons, they are an important military development.
Alternatively, a trunnion is a shaft that positions a ...
). 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
ships where their compactness is valued.
Rotary steam engines
It is possible to use a mechanism based on a
pistonless rotary engine such as the
Wankel engine
The Wankel engine (, ) is a type of internal combustion engine using an eccentric rotary design to convert pressure into rotating motion. It was invented by German engineer Felix Wankel, and designed by German engineer Hanns-Dieter Paschke ...
in place of the cylinders and
valve gear
The valve gear of a steam engine is the mechanism that operates the inlet and exhaust valves to admit steam into the cylinder and allow exhaust steam to escape, respectively, at the correct points in the cycle. It can also serve as a reversing ...
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, is also a serious problem with many such designs.
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..
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
Admiralty for driving dynamos on board the ships of the
Royal Navy
The Royal Navy (RN) is the United Kingdom's naval warfare force. Although warships were used by English and Scottish kings from the early medieval period, the first major maritime engagements were fought in the Hundred Years' War against ...
. They were eventually replaced in these niche applications by steam turbines.
Rocket type
The
aeolipile represents the use of steam by the
rocket-reaction principle, although not for direct propulsion.
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.
Ferdinand Verbiest
Father Ferdinand Verbiest (9 October 1623 – 28 January 1688) was a Flemish Jesuit missionary in China during the Qing dynasty. He was born in Pittem near Tielt in the County of Flanders (now part of Belgium). He is known as Nan Huairen () in ...
's carriage was powered by an aeolipile in 1679.
Safety
Steam engines possess boilers and other components that are
pressure vessel
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
Construction methods and materials may be chosen to suit the pressure application, and will depend on the size ...
s that contain a great deal of potential energy. Steam escapes and
boiler explosion
A boiler explosion is a catastrophic failure of a boiler. There are two types of boiler explosions. One type is a failure of the pressure parts of the steam and water sides. There can be many different causes, such as failure of the safety val ...
s (typically
BLEVEs) can and have in the past caused great loss of life. While variations in standards may exist in different countries, stringent legal, testing, training, care with manufacture, operation and certification is applied to ensure safety.
Failure modes may include:
* over-pressurisation of the boiler
* insufficient water in the boiler causing overheating and vessel failure
* buildup of sediment and scale which cause local hot spots, especially in riverboats using dirty feed water
* pressure vessel failure of the boiler due to inadequate construction or maintenance.
* escape of steam from pipework/boiler causing scalding
Steam engines frequently possess two independent mechanisms for ensuring that the pressure in the boiler does not go too high; one may be adjusted by the user, the second is typically designed as an ultimate fail-safe. Such
safety valves traditionally used a simple lever to restrain a plug valve in the top of a boiler. One end of the lever carried a weight or spring that restrained the valve against steam pressure. Early valves could be adjusted by engine drivers, leading to many accidents when a driver fastened the valve down to allow greater steam pressure and more power from the engine. The more recent type of safety valve uses an adjustable spring-loaded valve, which is locked such that operators may not tamper with its adjustment unless a seal is illegally broken. This arrangement is considerably safer.
Lead
fusible plugs may be present in the crown of the boiler's firebox. If the water level drops, such that the temperature of the firebox crown increases significantly, the
lead melts and the steam escapes, warning the operators, who may then manually suppress the fire. Except in the smallest of boilers the steam escape has little effect on dampening the fire. The plugs are also too small in area to lower steam pressure significantly, depressurizing the boiler. If they were any larger, the volume of escaping steam would itself endanger the crew.
Steam cycle

The Rankine cycle is the fundamental thermodynamic underpinning of the steam engine. The cycle is an arrangement of components as is typically used for simple power production, and uses the phase change of water (boiling water producing steam, condensing exhaust steam, producing liquid water)) to provide a practical heat/power conversion system. The heat is supplied externally to a closed loop with some of the heat added being converted to work and the waste heat being removed in a condenser. The Rankine cycle is used in virtually all steam power production applications. In the 1990s, Rankine steam cycles generated about 90% of all electric power used throughout the world, including virtually all
solar,
biomass
Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms bi ...
,
coal
Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other elements, chiefly hydrogen, sulfur, oxygen, and nitrogen.
Coal is formed wh ...
and
nuclear
Nuclear may refer to:
Physics
Relating to the Atomic nucleus, nucleus of the atom:
*Nuclear engineering
*Nuclear physics
*Nuclear power
*Nuclear reactor
*Nuclear weapon
*Nuclear medicine
*Radiation therapy
*Nuclear warfare
Mathematics
*Nuclear ...
power plants. It is named after
William John Macquorn Rankine, a Scottish
polymath
A polymath ( el, πολυμαθής, , "having learned much"; la, homo universalis, "universal human") is an individual whose knowledge spans a substantial number of subjects, known to draw on complex bodies of knowledge to solve specific pro ...
.
The Rankine cycle is sometimes referred to as a practical
Carnot cycle
A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...
because, when an efficient turbine is used, the
TS diagram begins to resemble the Carnot cycle. The main difference is that heat addition (in the boiler) and rejection (in the condenser) are
isobaric (constant pressure) processes in the Rankine cycle and
isothermal (constant
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied o ...
) processes in the theoretical Carnot cycle. In this cycle, a pump is used to pressurize the working fluid which is received from the condenser as a liquid not as a gas. Pumping the working fluid in liquid form during the cycle requires a small fraction of the energy to transport it compared to the energy needed to compress the working fluid in gaseous form in a compressor (as in the
Carnot cycle
A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...
). The cycle of a reciprocating steam engine differs from that of turbines because of condensation and re-evaporation occurring in the cylinder or in the steam inlet passages.
The working fluid in a Rankine cycle can operate as a closed loop system, where the working fluid is recycled continuously, or may be an "open loop" system, where the exhaust steam is directly released to the atmosphere, and a separate source of water feeding the boiler is supplied. Normally water is the fluid of choice due to its favourable properties, such as non-toxic and unreactive chemistry, abundance, low cost, and its
thermodynamic properties.
Mercury is the working fluid in the
mercury vapor turbine. Low boiling hydrocarbons can be used in a
binary cycle.
The steam engine contributed much to the development of thermodynamic theory; however, the only applications of scientific theory that influenced the steam engine were the original concepts of harnessing the power of steam and atmospheric pressure and knowledge of properties of heat and steam. The experimental measurements made by Watt on a model steam engine led to the development of the separate condenser. Watt independently discovered
latent heat, which was confirmed by the original discoverer
Joseph Black
Joseph Black (16 April 1728 – 6 December 1799) was a Scottish physicist and chemist, known for his discoveries of magnesium, latent heat, specific heat, and carbon dioxide. He was Professor of Anatomy and Chemistry at the University of G ...
, who also advised Watt on experimental procedures. Watt was also aware of the change in the boiling point of water with pressure. Otherwise, the improvements to the engine itself were more mechanical in nature. The thermodynamic concepts of the Rankine cycle did give engineers the understanding needed to calculate efficiency which aided the development of modern high-pressure and -temperature boilers and the steam turbine.
Efficiency
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 by the burning fuel.
The historical measure of a steam engine's 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 designs. Duty is the number of
foot-pounds
The foot-pound force (symbol: ft⋅lbf, ft⋅lbf, or ft⋅lb ) is a unit of work or energy in the engineering and gravitational systems in United States customary and imperial units of measure. It is the energy transferred upon applying ...
of
work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal ...
delivered by burning one
bushel (94 pounds) of coal. The best examples of Newcomen designs had a duty of about 7 million, but most were closer to 5 million. Watt's original low-pressure designs were able to deliver duty as high as 25 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 million.
No heat engine can be more efficient than the
Carnot cycle
A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodynam ...
, in which heat is moved from a high-temperature reservoir to one at a low temperature, and the efficiency depends on the temperature difference. For the greatest efficiency, steam engines should be operated at the highest steam temperature possible (
superheated steam), and release the waste heat at the lowest temperature possible.
The efficiency of a Rankine cycle is usually limited by the working fluid. Without the pressure reaching
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 °C (the
creep limit of stainless steel) and condenser temperatures are around 30 °C. This gives a theoretical
Carnot efficiency
A Carnot cycle is an ideal thermodynamic cycle proposed by French physicist Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. By Carnot's theorem, it provides an upper limit on the efficiency of any classical thermodyna ...
of about 63% compared with an actual efficiency of 42% for a modern coal-fired power station. This low turbine entry temperature (compared with a
gas turbine
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the direct ...
) is why the Rankine cycle is often used as a bottoming cycle in
combined-cycle gas turbine power stations.
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
A gas turbine, also called a combustion turbine, is a type of continuous flow internal combustion engine. The main parts common to all gas turbine engines form the power-producing part (known as the gas generator or core) and are, in the direct ...
s, for instance, have turbine entry temperatures approaching 1500 °C. Nonetheless, the efficiencies of actual large steam cycles and large modern simple cycle gas turbines are fairly well matched.
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.
A modern, large electrical power station (producing several hundred megawatts of electrical output) with
steam reheat,
economizer
Economizers (US and Oxford spelling), or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, ...
etc. will achieve efficiency in the mid 40% range, with the most efficient units approaching 50% thermal efficiency.
It is also possible to capture the waste heat using
cogeneration
Cogeneration or combined heat and power (CHP) is the use of a heat engine or power station to generate electricity and useful heat at the same time.
Cogeneration is a more efficient use of fuel or heat, because otherwise- wasted heat from elect ...
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.
File:GNR N2 1744 at Weybourne - geograph.org.uk - 1479849.jpg, A steam locomotive – a GNR N2 Class No.1744 at Weybourne nr. Sheringham, Norfolk
Norfolk () is a ceremonial and non-metropolitan county in East Anglia in England. It borders Lincolnshire to the north-west, Cambridgeshire to the west and south-west, and Suffolk to the south. Its northern and eastern boundaries are the N ...
File:Dampf-Fahrrad 2.jpg, A steam-powered bicycle by John van de Riet, in Dortmund
Dortmund (; Westphalian nds, Düörpm ; la, Tremonia) is the third-largest city in North Rhine-Westphalia after Cologne and Düsseldorf, and the eighth-largest city of Germany, with a population of 588,250 inhabitants as of 2021. It is the ...
File:Steam-powered fire engine.jpg, British horse-drawn fire engine
A fire engine (also known in some places as a fire truck or fire lorry) is a road vehicle (usually a truck) that functions as a firefighting apparatus. The primary purposes of a fire engine include transporting firefighters and water to an ...
with steam-powered water pump
See also
Notes
References
References
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Further reading
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* Rose, Joshua. (1887, reprint 2003) ''Modern Steam Engines''
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* Van Riemsdijk, J.T. (1980) ''Pictorial History of Steam Power''.
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External links
Animated engines – Illustrates a variety of enginesVideo of the 1900 steam engine aboard paddle steamer Unterwalden
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18th-century inventions
Energy conversion
English inventions
Gas technologies
Piston engines