Compound locomotive
Encyclopedia
A compound engine unit is a type of steam engine
where steam is expanded in two or more stages.
A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure (HP) cylinder
, then having given up heat and losing pressure, it exhausts directly into one or more larger volume low-pressure (LP) cylinders. Multiple-expansion engines employ additional cylinders, of progressively lower pressure, to extract further energy from the steam.
Invented in 1781, this technique was first employed on a Cornish beam engine
in 1804. Around 1850, compound engines were first introduced into Lancashire textile mills.
) or whether pressure fluctuation necessitates an intermediate "buffer" space in the form of a steam chest or pipe known as a receiver (receiver compounds).
In a single-expansion (or 'simple') steam engine, the high-pressure steam enters the cylinder through a cut-off
valve (archaically known as a regulator). The piston
moves down the cylinder, and when it is at about 25%–33% of its stroke, the cut-off valve shuts and the steam expands, pushing the piston to the end of its stroke, the exhaust valve opens and expels the depleted steam to the atmosphere, or to a condenser. As steam expands in a high-pressure engine, its temperature drops; because no heat is released from the system, this is known as adiabatic expansion
and results in steam entering the cylinder at high temperature and leaving at low temperature. This causes a cycle of heating and cooling of the cylinder with every stroke which is a source of inefficiency. The steam cut-off point when using a slide valve is less than 30% of the stroke. Early cut-off causes the turning moment
on the shaft to be more uneven, requiring a larger flywheel
to smooth this out.
, who patented his Woolf high pressure compound engine in 1805. In the compound engine, high-pressure steam from the boiler first expands in a high-pressure (HP) cylinder and then enters one or more subsequent lower pressure (LP) cylinders. The complete expansion of the steam occurs across multiple cylinders and, as there is less expansion in each cylinder, less heat is lost by the steam in each. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine.
There are other advantages: as the temperature range is smaller, cylinder condensation is reduced. Loss due to condensation is restricted to the LP cylinder. Pressure difference is less in each cylinder so there is less steam leakage at the piston and valves. The turning moment is more uniform, so balancing is easier and a smaller flywheel may be used. Only the smaller HP cylinder needs to be built to withstand the highest pressure, so reducing the overall weight. Similarly, components are subject to less strain so can be lighter. The reciprocating parts of the engine are lighter reducing the engine vibrations.
The compound could be started at any point in the cycle, and in the event of mechanical failure the compound could be reset to act as a simple, and thus keep running.
To derive equal work from lower pressure steam requires a larger cylinder volume as this steam occupies a greater volume. Therefore the bore, and often the stroke, are increased in low-pressure cylinders resulting in larger cylinders.
Double-expansion (usually just known as 'compound') engines expand the steam in two stages. The pairs may be duplicated or the work of the large LP cylinder can be split across two smaller cylinders, with one HP cylinder exhausting into either LP cylinder, giving a 3-cylinder layout where the cylinder and piston diameter of all three are about the same making the reciprocating masses easier to balance.
Two-cylinder compounds can be arranged as:
The adoption of compounding was widespread for stationary industrial units where the need was for increased power at decreasing cost, and almost universal for marine engines
after 1880. It was not widely used in railway locomotives where it was often perceived as complicated and unsuitable for the harsh railway operating environment and limited space afforded by the loading gauge
(particularly in Britain). Compounding was never common on British railways and not employed at all after 1930, but was used in a limited way in many other countries.
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 double-acting cylinders of progressively increasing diameter and/or stroke and hence volume. These cylinders are designed to divide the work into three or four equal portions, one for each expansion stage. The image to the right 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.
To generalise, between 1860 and 1926 all Lancashire mills were driven by compounds. The last compound built was by Buckley and Taylor
for Wye No.2 mill, Shaw. This engine was a cross-compound design to 2,500 ihp, driving a 24 ft, 90 ton flywheel, and operated until 1965.
In the marine environment, the general requirement was for autonomy and increased operating range, as ships had to carry their coal supplies. The old salt-water boiler was thus no longer adequate and had to be replaced by a closed fresh-water circuit with condenser. The result from 1880 onwards was the multiple-expansion engine using three or four expansion stages (triple- and quadruple-expansion engines). These engines used a series of double-acting cylinders of progressively increasing diameter and/or stroke (and hence volume) designed to divide the work into three or four, as appropriate, equal portions for each expansion stage. Where space is at a premium, two smaller cylinders of a large sum volume might be used for the low-pressure stage. Multiple-expansion engines typically had the cylinders arranged in-line, 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 4-cylinder triple-expansion engine popular with large passenger liners (such as the Olympic class
), but was ultimately replaced by the virtually vibration-free steam turbine
.
The development of this type of engine was important for its use in steamships as by exhausting to a condenser the water could be reclaimed to feed the boiler, which was unable to use seawater
. Land-based steam engines could simply exhaust much of their steam, as feed water was usually readily available. Prior to and during World War II
, the expansion engine dominated marine applications where high vessel speed was not essential. It was however superseded by the steam turbine when speed was required, for instance in warships and ocean liner
s. HMS Dreadnought
of 1905 was the first major warship to replace the proven technology of the reciprocating engine with the then-novel steam turbine.
While designs for compound locomotives may date as far back as James Samuel
's 1856 patent for a "continuous expansion locomotive", the practical history of railway compounding begins with Anatole Mallet
's designs in the 1870s. Mallet locomotive
s were operated in the United States up to the end of mainline steam by the Norfolk and Western Railway
. The designs of Alfred George de Glehn in France also saw significant use, especially in the rebuilds of André Chapelon
. A wide variety of compound designs were tried around 1900, but most were short-lived in popularity, due to their complexity and maintenance liability. In the 20th century the superheater
was widely adopted, and the vast majority of steam locomotives were simple-expansion (with some compound locomotives converted to simple).
Steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
where steam is expanded in two or more stages.
A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure (HP) cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
, then having given up heat and losing pressure, it exhausts directly into one or more larger volume low-pressure (LP) cylinders. Multiple-expansion engines employ additional cylinders, of progressively lower pressure, to extract further energy from the steam.
Invented in 1781, this technique was first employed on a Cornish beam engine
Beam engine
A beam engine is a type of steam engine where a pivoted overhead beam is used to apply the force from a vertical piston to a vertical connecting rod. This configuration, with the engine directly driving a pump, was first used by Thomas Newcomen around 1705 to remove water from mines in Cornwall...
in 1804. Around 1850, compound engines were first introduced into Lancashire textile mills.
Compound systems
There are many compound systems and configurations, but there are two basic types, according to how HP and LP piston strokes are phased and hence whether the HP exhaust is able to pass directly from HP to LP (Woolf compoundsArthur Woolf
Arthur Woolf was a Cornish engineer, most famous for inventing a high-pressure compound steam engine. As such he made an outstanding contribution to the development and perfection of the Cornish engine.Woolf left Cornwall in 1785 to work for Joseph Bramah's engineering works in London...
) or whether pressure fluctuation necessitates an intermediate "buffer" space in the form of a steam chest or pipe known as a receiver (receiver compounds).
In a single-expansion (or 'simple') steam engine, the high-pressure steam enters the cylinder through a cut-off
Cutoff (steam engine)
In a steam engine, cutoff is the point in the piston stroke at which the inlet valve is closed. On a steam locomotive, the cutoff is controlled by the reverser....
valve (archaically known as a regulator). The piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
moves down the cylinder, and when it is at about 25%–33% of its stroke, the cut-off valve shuts and the steam expands, pushing the piston to the end of its stroke, the exhaust valve opens and expels the depleted steam to the atmosphere, or to a condenser. As steam expands in a high-pressure engine, its temperature drops; because no heat is released from the system, this is known as adiabatic expansion
Adiabatic process
In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...
and results in steam entering the cylinder at high temperature and leaving at low temperature. This causes a cycle of heating and cooling of the cylinder with every stroke which is a source of inefficiency. The steam cut-off point when using a slide valve is less than 30% of the stroke. Early cut-off causes the turning moment
Moment (physics)
In physics, the term moment can refer to many different concepts:*Moment of force is the tendency of a force to twist or rotate an object; see the article torque for details. This is an important, basic concept in engineering and physics. A moment is valued mathematically as the product of the...
on the shaft to be more uneven, requiring a larger flywheel
Flywheel
A flywheel is a rotating mechanical device that is used to store rotational energy. Flywheels have a significant moment of inertia, and thus resist changes in rotational speed. The amount of energy stored in a flywheel is proportional to the square of its rotational speed...
to smooth this out.
Compounding engines
A method to lessen the magnitude of this heating and cooling was invented in 1804 by British engineer Arthur WoolfArthur Woolf
Arthur Woolf was a Cornish engineer, most famous for inventing a high-pressure compound steam engine. As such he made an outstanding contribution to the development and perfection of the Cornish engine.Woolf left Cornwall in 1785 to work for Joseph Bramah's engineering works in London...
, who patented his Woolf high pressure compound engine in 1805. In the compound engine, high-pressure steam from the boiler first expands in a high-pressure (HP) cylinder and then enters one or more subsequent lower pressure (LP) cylinders. The complete expansion of the steam occurs across multiple cylinders and, as there is less expansion in each cylinder, less heat is lost by the steam in each. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine.
There are other advantages: as the temperature range is smaller, cylinder condensation is reduced. Loss due to condensation is restricted to the LP cylinder. Pressure difference is less in each cylinder so there is less steam leakage at the piston and valves. The turning moment is more uniform, so balancing is easier and a smaller flywheel may be used. Only the smaller HP cylinder needs to be built to withstand the highest pressure, so reducing the overall weight. Similarly, components are subject to less strain so can be lighter. The reciprocating parts of the engine are lighter reducing the engine vibrations.
The compound could be started at any point in the cycle, and in the event of mechanical failure the compound could be reset to act as a simple, and thus keep running.
To derive equal work from lower pressure steam requires a larger cylinder volume as this steam occupies a greater volume. Therefore the bore, and often the stroke, are increased in low-pressure cylinders resulting in larger cylinders.
Double-expansion (usually just known as 'compound') engines expand the steam in two stages. The pairs may be duplicated or the work of the large LP cylinder can be split across two smaller cylinders, with one HP cylinder exhausting into either LP cylinder, giving a 3-cylinder layout where the cylinder and piston diameter of all three are about the same making the reciprocating masses easier to balance.
Two-cylinder compounds can be arranged as:
- Cross-compound – the cylinders are side-by-side.
- Tandem compound – the cylinders are end-to-end, driving a common connecting rod
- Angle-compound – the cylinders are arranged in a vee (usually at a 90° angle) and drive a common crank.
The adoption of compounding was widespread for stationary industrial units where the need was for increased power at decreasing cost, and almost universal for marine engines
Marine steam engine
A marine steam engine is a reciprocating steam engine that is used to power a ship or boat. Steam turbines and diesel engines largely replaced reciprocating steam engines in marine applications during the 20th century, so this article describes the more common types of marine steam engine in use...
after 1880. It was not widely used in railway locomotives where it was often perceived as complicated and unsuitable for the harsh railway operating environment and limited space afforded by the loading gauge
Loading gauge
A loading gauge defines the maximum height and width for railway vehicles and their loads to ensure safe passage through bridges, tunnels and other structures...
(particularly in Britain). Compounding was never common on British railways and not employed at all after 1930, but was used in a limited way in many other countries.
Multiple expansion engines
Early work
- 1781 – Jonathan HornblowerJonathan HornblowerJonathan Hornblower was a British pioneer of steam power, the son of Jonathan Hornblower and brother of Jabez Carter Hornblower, two fellow pioneers....
, the grandson of one of NewcomenThomas NewcomenThomas Newcomen was an ironmonger by trade and a Baptist lay preacher by calling. He was born in Dartmouth, Devon, England, near a part of the country noted for its tin mines. Flooding was a major problem, limiting the depth at which the mineral could be mined...
's engine erectors in Cornwall, patented a double-cylinder compound reciprocating beam engineBeam engineA beam engine is a type of steam engine where a pivoted overhead beam is used to apply the force from a vertical piston to a vertical connecting rod. This configuration, with the engine directly driving a pump, was first used by Thomas Newcomen around 1705 to remove water from mines in Cornwall...
in 1781. He was prevented from developing it further by James WattJames WattJames Watt, FRS, FRSE was a Scottish inventor and mechanical engineer whose improvements to the Newcomen steam engine were fundamental to the changes brought by the Industrial Revolution in both his native Great Britain and the rest of the world.While working as an instrument maker at the...
, who claimed his own patents were infringed.
- 1804 – A method to lessen the magnitude of the continual heating and cooling of a single-expansion steam engine that leads to inefficiency was invented by British engineer Arthur WoolfArthur WoolfArthur Woolf was a Cornish engineer, most famous for inventing a high-pressure compound steam engine. As such he made an outstanding contribution to the development and perfection of the Cornish engine.Woolf left Cornwall in 1785 to work for Joseph Bramah's engineering works in London...
. Woolf patented his stationary Woolf high-pressure compound engine in 1805.
Double-expansion
- 1845 – William McNaught (Glasgow)William McNaught (Glasgow)William McNaught was a Scottish engineer, from Glasgow, who patented a compound steam engine in 1845. This was a technique of improving the efficiency of a standard simple Boulton & Watt beam engine. The engine was compounded by adding a high-pressure cylinder between the support column and the...
devised a method of fixing an additional high-pressure cylinder within an existing beam engine. To do so involved using a long pipe to connect the cylinders, and an extra set of valves to balance them. In effect this acted as a receiving chest, and a new type of compound had been invented. This system allowed greater control of the steam intake and cut-offs. An engine could be slowed by either a throttle which reduced the pressure of the steam, or by adjusting the cut-off on either cylinder. The latter was more efficient as no power was lost. The cycle was smoother as the two cylinders were not in phase.
- Understanding thermodynamicsThermodynamicsThermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
rather than believing in caloricsCaloric theoryThe caloric theory is an obsolete scientific theory that heat consists of a self-repellent fluid called caloric that flows from hotter bodies to colder bodies. Caloric was also thought of as a weightless gas that could pass in and out of pores in solids and liquids...
- Corliss valvesCorliss Steam EngineA Corliss steam 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 in Providence, Rhode Island....
Multiple-expansion
- 1861 – Daniel AdamsonDaniel AdamsonDaniel Adamson was a notable English engineer who became a successful manufacturer of boilers and was the driving force behind the inception of the Manchester Ship Canal project during the 1880s.-Early life:...
took out a patent for a multiple-expansion engine, with three or more cylinders connected to one beam or crankshaft. He built a triple-expansion engine for Victoria Mills, Dukinfield which opened in 1867.
- 1871 – Charles Normand, of Le Havre fitted a triple-expansion engine to a Seine river boat in 1871.
- 1872 – Sir F J Bramwell reported that compound marine engines, operating at 45psi to 60psi, consumed 2 lbs to 2.5 lbs per indicated horsepower.
Mill engines
Though the first mills were driven by water power, once steam engines were adopted the manufacturer no longer needed to site the mills by running water. Cotton spinning required ever larger mills to fulfil the demand, and this drove the owners to demand increasingly powerful engines. When boiler pressure had exceeded 60psi, compound engines achieved a thermo-dynamic advantage, but it was the mechanical advantages of the smoother stroke that was the deciding factor in the adoption of compounds. In 1859, there was 75,886 ihp (indicated horsepower) of engines in mills in the Manchester area, of which 32,282 ihp was provided by compounds though only 41,189 ihp was generated from boilers operated at over 60psi.To generalise, between 1860 and 1926 all Lancashire mills were driven by compounds. The last compound built was by Buckley and Taylor
Buckley & Taylor
Buckley & Taylor was a company that manufactured stationary steam engines. It was the largest firm of engine makers in Oldham, Greater Manchester in England. The company produced large steam-driven engines for textile mills in Oldham and exported to India, Holland and Brazil.-History:Buckley &...
for Wye No.2 mill, Shaw. This engine was a cross-compound design to 2,500 ihp, driving a 24 ft, 90 ton flywheel, and operated until 1965.
Marine applications
Olympic class ocean liner
The Olympic-class ocean liners were a trio of ocean liners built by the Harland & Wolff shipyard for the White Star Line in the early 20th century...
), but was ultimately replaced by the virtually vibration-free steam turbine
Steam turbine
A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884....
.
The development of this type of engine was important for its use in steamships as by exhausting to a condenser the water could be reclaimed to feed the boiler, which was unable to use seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
. Land-based steam engines could simply exhaust much of their steam, as feed water was usually readily available. Prior to and during World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
, the expansion engine dominated marine applications where high vessel speed was not essential. It was however superseded by the steam turbine when speed was required, for instance in warships and ocean liner
Ocean liner
An ocean liner is a ship designed to transport people from one seaport to another along regular long-distance maritime routes according to a schedule. Liners may also carry cargo or mail, and may sometimes be used for other purposes .Cargo vessels running to a schedule are sometimes referred to as...
s. HMS Dreadnought
HMS Dreadnought (1906)
HMS Dreadnought was a battleship of the British Royal Navy that revolutionised naval power. Her entry into service in 1906 represented such a marked advance in naval technology that her name came to be associated with an entire generation of battleships, the "dreadnoughts", as well as the class of...
of 1905 was the first major warship to replace the proven technology of the reciprocating engine with the then-novel steam turbine.
Application to railway locomotives
For railway locomotive applications the main benefit sought from compounding was economy in fuel and water consumption plus high power/weight ratio due to temperature and pressure drop taking place over a longer cycle, this resulting in increased efficiency; additional perceived advantages included more even torque.While designs for compound locomotives may date as far back as James Samuel
James Samuel
James Samuel was a railway engineer who was appointed engineer to the Eastern Counties Railway in 1846. He held two important patents: the first mechanism for the compounding of steam locomotives, granted in 1850, and the railway fishplate of 1844. In both cases, however, the invention was the...
's 1856 patent for a "continuous expansion locomotive", the practical history of railway compounding begins with Anatole Mallet
Anatole Mallet
Jules T. Anatole Mallet was a Swiss mechanical engineer, who was the inventor of the first successful compound system for a railway steam locomotive, patented in 1874....
's designs in the 1870s. Mallet locomotive
Mallet locomotive
The Mallet Locomotive is a type of articulated locomotive, invented by a Swiss engineer named Anatole Mallet ....
s were operated in the United States up to the end of mainline steam by the Norfolk and Western Railway
Norfolk and Western Railway
The Norfolk and Western Railway , a US class I railroad, was formed by more than 200 railroad mergers between 1838 and 1982. It had headquarters in Roanoke, Virginia for most of its 150 year existence....
. The designs of Alfred George de Glehn in France also saw significant use, especially in the rebuilds of André Chapelon
André Chapelon
André Chapelon was a noted French mechanical engineer and designer of advanced steam locomotives. Engineer of Ecole Centrale Paris, he was one of very few locomotive designers who brought a rigorous scientific method to their design, and he sought to apply up-to-date knowledge and theories in...
. A wide variety of compound designs were tried around 1900, but most were short-lived in popularity, due to their complexity and maintenance liability. In the 20th century the superheater
Superheater
A superheater is a device used to convert saturated steam or wet steam into dry steam used for power generation or processes. There are three types of superheaters namely: radiant, convection, and separately fired...
was widely adopted, and the vast majority of steam locomotives were simple-expansion (with some compound locomotives converted to simple).