Steam engine

The term steam engine may also refer to an entire railroad steam locomotive.

The engineering definition of a steam engine is an external-combustion heat engine that converts heat energy in steam to mechanical work. The term 'steam engine' commonly refers to a reciprocating piston engine, whereas a steam turbine engine is typically called a 'steam turbine'. Technically both are heat-engines that use the Rankine cycle. About 86% of the man-made electric power in the world is generated by steam turbines, because they are easily scaled-up to large sizes, use little material, and have high thermal efficiency; however, they are more complex and difficult to design than piston steam engines.

In the 19th century, steam engines were used as the prime mover for transportation in locomotives, steam ships, traction engines, steam lorries and later in road vehicles such as the Stanley Steamer. Steam engines powered industrial utilities such as water pumping stations. Steam engines were the power source that made the Industrial Revolution possible, and saw widespread commercial use powering machinery in factories and mills.

In non-mobile applications, steam engines at the point-of-use have since been largely replaced by electric motors connected to the electric grid. However, the power plants that supply power to the electric grid, predominantly use steam turbines, so indirectly they are still steam powered. In mobile applications, nearly all small steam engines have since been superseded by internal combustion engines. Notable exceptions are the marine prime movers of some large ocean vessels.

A steam engine requires a boiler to heat water into steam. The expansion of steam exerts force upon a piston or turbine blade, whose motion can be harnessed for the work of turning wheels or driving other machinery. The expanded steam must be condensed and reused, or replaced by a source of cold water. This 'feedwater' is typically pumped up to a high boiler pressure. As in a pressure cooker, pressure raises the boiling point, and this is essential to run the turbine efficiently and effectively. One of the advantages of the steam engine is that any heat source can be used to raise steam in the boiler, the most common being the combustion of wood (biomass), coal or oil. In general, these fuels are more economical to use than the fuels that can be used in internal combustion engines.

Steam engine in action (animation). Note that movement of the connecting linkage from the centrifugal governor operating the steam throttle is shown for illustrative purpose only, in practice this link only operates when the engine speeds up or slows down.

Invention and development

Aeolipile

The first recorded steam-powered device, the aeolipile, was described by Hero of Alexandria (Heron) in 1st century Roman Egypt, in his manuscript Spiritalia seu Pneumatica. Steam ejected tangentially from nozzles caused a pivoted ball to rotate; this suggests that the conversion of steam pressure into mechanical movement was known in Roman Egypt in the 1st century, the device was used for some simple work, such as opening temple doors, but saw no other major uses.

A more practical steam turbine was invented much later by Taqi al-Din, an Arab philosopher, astronomer, and engineer in 16th century Ottoman Egypt, who exposed a method for rotating a spit by means of a jet of steam playing on rotary vanes around the periphery of a wheel. A similar machine is shown by Giovanni Branca, an Italian engineer, in 1629 for turning a cylindrical escapement device that alternately lifted and let fall a pair of pestles working in mortars. The steam flow of these early steam turbines, however, was not concentrated and much of its energy was dissipated in all directions. This would have led to a considerable waste of energy. They are usually called "mills".

Commercial development of the steam engine, however, required an economic climate in which the developers of engines could profit by their creations. Classical, and later Medieval and Renaissance civilisations provided no such climate. Even as late as the 17th century, steam engines were created as one-off curiosities.The first machine was created by the Spanish inventor Jerónimo Ayánz de Beaumont,a patent that influenced Savery. The difficulty in breaking out of this situation is evident judging by the difficulties encountered by the Marquis of Worcester and later by his widow in gaining financial investment into the practical application of his ideas for the exploitation of steam power. In 1663, he published designs for, and installed a steam-powered device for raising water on the wall of the Great Tower at Raglan Castle (the grooves in the wall where the engine was installed were still to be seen in the 19th century). However, no one was prepared to risk money in this revolutionary new concept, and without backers the machine remained undeveloped.

Denis Papin's design for a piston-and-cylinder engine, 1680.

One of Denis Papin’s centres of interest was in the creating of a vacuum in a closed cylinder and in Paris in the mid 1670s he collaborated with the Dutch physicist, Huygens’ working on an engine which drove out the air from a cylinder by exploding gunpowder inside it. Realising the incompleteness of the vacuum produced by this means and on moving to England in 1680, Papin devised a version of the same cylinder that obtained a more complete vacuum from boiling water and then allowing the steam to condense; in this way he was able to raise weights by attaching the end of the piston to a rope passing over a pulley. As a demonstration model the system worked, but in order to repeat the process the whole apparatus had to be dismantled and reassembled. Papin quickly saw that to make an automatic cycle the steam would have to be generated separately in a boiler; however as he did not take the project further all we can say is that he invented the reciprocating steam engine conceptually and thus paved the way to Newcomen’s engine. Papin also designed a paddle boat driven by a jet playing on a mill-wheel in a combination of Taqi al Din and Savery's conceptions and; he is also credited with a number of significant devices such as the safety valve.

Early industrial engines

None of the foregoing developments were applied practically as a means of undertaking any early useful task. Another early industrial steam engine was the "fire-engine", designed by Thomas Savery in 1698. This was a pistonless steam pump, and apparently not very efficient. It was thus Thomas Newcomen and his "atmospheric-engine" of 1712 that demonstrated the first practical industrial engine for which there could be a commercial demand. Newcomen appears to have based his development on Papin's description of early experiments at the Royal society to which he most likely had access through acquaintance with a society member, Robert Hooke who had himself worked with Papin.

The first industrial applications of the Newcomen engine were in the pumping of water from deep mineshafts. In mineshaft pumps a reciprocating beam was connected to an operating rod that descended the shaft to a pump chamber. The oscillations of the operating rod were transferred to a pump piston that moved the water to the top of the shaft. The valves of early Newcomen engines were manually opened and closed by an attendant. An early improvement was the automation of valves operation by deriving the motion from that of the engine itself (Legend has it that this was first done in 1713 by a boy, Humphrey Potter, charged with opening the valves; when he grew bored and wanted to play with the other children he set up ropes to automate the process.), however by 1715 this had already been achieved by an escapement system activated by a vertical plug beam suspended from the engine beam.

Humphrey Gainsborough produced a model condensing steam engine in the 1760s, which he showed to Richard Lovell Edgeworth, a member of the Lunar Society.

In 1761 Professor Joseph Black proposed his 'theory of latent heat' which laid the foundations for the development of steam engine technology. Black's friend James Watt applied Black's theory to the design of an improved steam engine. Watts' technology enabled the widespread commercial use of stationary steam engines. Later, in 1769 the same James Watt, a member of the Lunar Society, patented the first significant improvements to the Newcomen type vacuum engine making it much more fuel efficient. Watt's leap was to separate the condensing phase of the vacuum engine into a separate chamber, while keeping the piston and cylinder at the temperature of the steam. Gainsborough believed that Watt had used his ideas for the invention, but there is no proof of this.

Watt, together with his business partner Matthew Boulton, developed these patents into the Watt steam engine in Birmingham, England. The increased efficiency of the Watt engine finally led to the general acceptance and use of steam power in industry. Additionally, unlike the Newcomen engine, the Watt engine operated smoothly enough to be connected to a drive shaft—via sun and planet gears—to provide rotary power. This was in all essentials the engine that we know today. In early steam engines the piston is usually connected to a balanced beam, rather than directly to a connecting rod, and these engines are therefore known as beam engines.

Richard Trevithick's No. 14 Engine, built by Hazeldine and Co., Bridgnorth, about 1804. This was a single-acting, stationary high pressure engine that operated at a working pressure of 50 psi (350 kPa).

The next improvement in efficiency came with the American Oliver Evans and the Briton Richard Trevithick's use of high pressure steam. Trevithick built successful industrial high pressure single-acting engines known as Cornish engines. However, with increased pressure came much danger as engines and boilers were now likely to fail mechanically by a violent outwards explosion, and there were many early disasters. The most important refinement to the high pressure engine at this point was the safety valve, which releases excess pressure. Reliable and safe operation came only with a great deal of experience and codification of construction, operating, and maintenance procedures.

Nicolas-Joseph Cugnot demonstrated the first functional self-propelled steam vehicle, his "fardier" (steam wagon), in 1769. Arguably, this was the first automobile. While not generally successful as a transportation device, the self-propelled steam tractor proved very useful as a self mobile power source to drive other farm machinery such as grain threshers or hay balers. In 1788, a steamboat built by John Fitch operated in regular commercial service along the Delaware river between Philadelphia PA and Burlington NJ, carrying as many as 30 passengers. This boat could typically make 7 to 8 miles per hour, and traveled more than 2,000 miles (3,200 km) during its short length of service. The Fitch steamboat was not a commercial success, as this travel route was adequately covered by relatively good wagon roads. In 1802 William Symington built a practical steamboat, and in 1807 Robert Fulton used the Watt steam engine to power the first commercially successful steamboat. On February 21, 1804 at the Penydarren ironworks at Merthyr Tydfil in South Wales, the first self-propelled railway steam engine or steam locomotive, built by Richard Trevithick, was demonstrated.

Reciprocating engines

Reciprocating engines use the action of steam to move a piston in a sealed chamber or cylinder. The reciprocating action of the piston can be translated via a mechanical linkage into either linear motion, usually for working water or air pumps, or else into rotary motion to drive the flywheel of a stationary engine, or else the wheel(s) of a vehicle.

Vacuum engines

Early steam engines, or "fire engines" as they were at first called such as "atmospheric" and Watt's "condensing" engines, worked on the vacuum principle and are thus known as vacuum engines. Although Savery's patent of 2 July 1698 claimed, in addition to "the raising of water", the ability to "occasion... motion to all sorts of mill-works" there is no evidence that they were used for any purpose other than pumping. Such engines operate by admitting low pressure steam into an operating chamber or cylinder. The inlet valve is then closed and the steam cooled, condensing it to a smaller volume and thus creating a vacuum in the cylinder The upper end of the cylinder being open to the atmospheric pressure operates on the opposite side of a piston, pushing the piston to the bottom of the cylinder.

Engraving of Newcomen engine. This appears to be copied from a drawing in Desaguliers' 1744 work: "A course of experimental philosophy", itself believed to have been a reversed copy of Henry Beighton's engraving dated 1717 representing what is probably the second Newcomen engine erected around 1714 at Griff colliery, Warwickshire. (See Hulse p.84)

The piston is connected by a chain to the end of a large beam pivoted near its middle. A weighted force pump is connected by a chain to the opposite end of the beam which gives the pumping stroke and returns the piston to the top of the cylinder by force of gravity, the low pressure steam being insufficient to move the piston upwards. In the Newcomen engine the cooling water is sprayed directly into the cylinder the still-warm condensate running off into a hot well. [1] Repeated and wasteful cooling and reheating of the working cylinder was a source of inefficiency, however these engines enabled the pumping of greater volumes of water and/or from greater depths than had been hitherto possible. Watt's version of this engine as developed and marketed from 1774 onwards in partnership with Matthew Boulton, was meant to improve efficiency through use of a separate condensing chamber immersed in a bath of cold water, connected to the working cylinder by a pipe and controlled by a valve. A small vacuum pump connected to the pump side of the beam drew off the warm condensate and delivered it to the hot well, at the same time helping to create the vacuum and draw the condensate out of the cylinder.

Early Watt pumping engine.

The hot well was also a source of pre-heated water for the boiler. Another radical change was to close off the top of the cylinder and introduce low pressure steam above the piston and inside steam jackets that maintained cylinder temperature constant. On the upward return stroke, the steam on top was transferred through a pipe to the underside of the piston to be condensed for the downward stroke. Thus the engine was thus no longer "atmospheric", the power stroke depending on the differential between the low-pressure steam and the partial vacuum. Sealing of the piston on a Newcomen engine was achieved by maintaining a small quantity of water on its upper side. This was no longer possible in Watt's engine due to the presence of the steam; so sealing of the piston and its lubrication was obtained by using a mixture of tallow and oil. The piston rod also passed through a gland on the top cylinder cover sealed in a similar way.[2]

Vacuum engines, although in general limited in their efficiency, were at least relatively safe, use of very low pressure steam being preferable due to the primitive state of 18th century boiler technology. Power was limited by the low pressure, the displacement of the cylinder, combustion and evaporation rates and—where present— condenser capacity. Maximum theoretical efficiency was limited by the relatively low temperature differential on either side of the piston; this meant that for a vacuum engine to provide a usable amount of power, the first industrial production engines had to be very large, and were thus expensive to build and install.

Trevithick pumping engine (Cornish system).

Around 1811 Richard Trevithick was required to update a Watt pumping engine in order to adapt it to one of his new Cornish boilers. Steam pressure above the piston was increased eventually reaching 40 psi (2.8 bars) and now provided much of the power for the downward stroke; at the same time condensing was improved. This considerably raised efficiency and further pumping engines on the Cornish system (often known as Cornish engines) were built new throughout the 19th century, older Watt engines being updated to conform. Many of these engines were supplied worldwide and gave reliable and efficient service over a great many years with greatly reduced coal consumption. Some of them were very large and the type continued to be built right down to the 1890’s.

High pressure engines

Around the year 1800, "high pressure" amounted to what today would be considered very low pressure, i.e. 40-50 psi (276 - 345 kPa), the point being that the high pressure engine was driven solely by the pressure of the steam and once that steam had performed work, it was usually exhausted at higher-than-atmospheric pressure. The blast of the exhausting steam into the chimney could be exploited to create induced draught through the fire grate and thus increase the rate of burning, hence creating more heat in a smaller furnace.

The outcome was that engines could be made much smaller than previously for a given power output. There was thus the potential for steam engines to be developed that were small and powerful enough to propel themselves and other objects. As a result, steam power for transportation now became a practicality in the form of ships and land vehicles, which revolutionised cargo businesses, travel, military strategy, and essentially every aspect of society.

The importance of raising steam under pressure (from a thermodynamic standpoint) is that it attains a higher temperature. Thus, any engine using such steam operates at a higher temperature and pressure differential than is possible with a low pressure vacuum engine. The high pressure engine thus became the basis for most further development of reciprocating steam technology.

A labeled schematic diagram of a typical single cylinder, simple expansion, double-acting high pressure horizontal steam engine. Power takeoff from the engine is by way of a belt.
1 - Piston
2 - Piston rod
3 - Crosshead bearing
4 - Connecting rod
5 - Crank
6 - Eccentric valve motion
7 - Flywheel
8 - Sliding valve
9 - Centrifugal governor.

1906 horse-drawn steam fire engine in England. The water is pumped onto the fire by a double-acting onboard steam engine.

Double-acting engine

The next major advance in high pressure steam engines was to make them 'double-acting'. In the single-acting high pressure engine above, the cylinder is vertical and the piston returns to the start—or bottom—of the stroke by the momentum of the flywheel (not shown).

In the double-acting engine, steam is admitted alternately to each side of the piston while the other is exhausting. This requires inlet and exhaust ports at either end of the cylinder (see the animated expansion engine below) with steam flow being controlled by valves. This system increases the speed and smoothness of the reciprocation and allows the cylinder to be mounted horizontally or at an angle.

Power is transmitted from the piston by a sliding rod—sealed to the cylinder to prevent the escape of steam— which in turn drives a connecting rod via a sliding crosshead). This in combination with the connecting rod converts the reciprocating motion to rotary motion. The inlet and exhaust valves have their reciprocating motion derived from the rotary motion by way of an additional crank mounted eccentrically (i.e. off centre) from the drive shaft. The valve gear may include a reversing mechanism to allow reversal of the rotary motion.

A double-acting piston engine provides as much power as a more expensive 2-piston single-acting engine, and also allows the use of a much smaller flywheel than what would be required by a one-piston single-acting engine. Both of these considerations made the double-acting piston engine smaller and less expensive for a given power range.

Most reciprocating steam engines now use this technology, notable examples including steam locomotives and marine engines. When a pair (or more) of double acting cylinders, for instance in a steam locomotive, are connected to a common driveshaft their crank phasing is offset by an angle of 90°. This is called quartering and ensures that the engine will always start, no matter what position the crank is in.

Some marine engines have used only a single double-acting cylinder, driving paddlewheels on each side. When shutting down such an engine it was important that the piston be away from either extreme range of its travel so that it could be readily restarted (as there was not a second quartered piston to facilitate this).

Steam turbine engine

The very first steam turbine engine was invented an developed in 1999 by Alex Rogers.

Steam distribution

Schematic Indicator diagram showing the four events in a double piston stroke

In most reciprocating piston engines the steam reverses its direction of flow at each stroke (counterflow), entering and exhausting from the cylinder by the same port. The complete engine cycle occupies one rotation of the crank and two piston strokes; the cycle also comprises four events — admission, expansion, exhaust, compression. These events are controlled by valves often working inside a steam chest adjacent to the cylinder; the valves distribute the steam by opening and closing steam ports communicating with the cylinder end(s) and are driven by valve gear, of which there are many types. The simplest valve gears give events of fixed length during the engine cycle and often make the engine rotate in only one direction. Most 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 50s there were attempts to overcome this problem by means of various patent valve gears with separate variable cutoff valves 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 ever since 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. Later, poppet valve gears had separate admission and exhaust valves driven by cams profiled so as to give ideal events; nevertheless most of these gears never succeeded in ousting conventional gears 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

The above effects are further enhanced by providing lead: as was later discovered with the internal combustion engine, it has been found advantageous since the late 1830s to advance the admission phase, giving the valve lead so that admission occurs a little before the end of the exhaust stroke in order to fill the clearance volume comprising the ports and the cylinder ends (not part of the piston-swept volume) before the steam begins to exert effort on the piston.

Simple expansion

This means that a charge of steam works only once in the cylinder. It is then exhausted directly into the atmosphere or into a condenser, but remaining heat can be recuperated if needed to heat a living space, or to provide warm feedwater for the boiler.

Compounding

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.

A method to lessen the magnitude of this heating and cooling 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 and as less expansion now occurs in each cylinder so less heat is lost by the steam in each. This reduces the magnitude of cylinder heating and cooling, increasing the efficiency of the engine. 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 known as compound) engines expanded the steam in two stages. The pairs may be duplicated or the work of the large LP cylinder can be split with one HP cylinder exhausting into one or the other, giving a 3-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
• Angle compounds - The cylinders are arranged in a vee (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 4-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 3-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 (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

An animation of a simplified triple-expansion engine.
High-pressure steam (red) enters from the boiler and passes through the engine, exhausting as low-pressure steam (blue) to the condenser.

1890s vintage triple-expansion (three cylinders of 26, 42 and 70 inch diameters in a common frame with a 42 inch stroke) marine engine that powered the SS Christopher Columbus

It is a logical extension of the compound engine 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, as appropriate, equal portions for each expansion stage. As with the double expansion engine, where space is at a premium, two smaller cylinders of a large sum volume may be used for the low pressure stage. Multiple expansion engines typically had the cylinders arranged inline, but various other formations were used.

The images to the right show a model and 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.

The development of this type of engine was important for its use in steamships as by exhausting to a condenser the water can be reclaimed to feed the boiler, which is unable to use seawater. Land-based steam engines could 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 British invention steam turbine where speed was required, for instance in warships and ocean liners. HMS Dreadnought of 1905 was the first major warship to replace the proven technology of the reciprocating engine with the then-novel steam turbine.

File:TripleExpansionMed.jpg Model of a triple expansion engine

S/S Ukkopekka Triple expansion steam engine

Uniflow (or unaflow) engine

Main article: Uniflow steam engine

This is intended to remedy the difficulties arising from the usual counterflow cycle mentioned above which means that at 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 working temperature. The aim of the uniflow is to remedy this defect by providing an additional port uncovered by the piston at the end of its half-stroke making the steam flow only in one direction. By this means, thermal efficiency is improved by having a steady temperature gradient along the cylinder bore. The simple-expansion uniflow engine is reported to give efficiency equivalent to that of classic compound systems with the added advantage of superior part-load performance. It is also readily adaptable to high-speed uses and was a common way to drive electricity generators towards the end of the 19th century before the coming of the steam turbine.

Uniflow engines have been produced in single-acting, double-acting, simple, and compound versions. Skinner 4-crank 8-cylinder single-acting tandem compound [3] engines power two Great Lakes ships still trading today (2007). These are the Saint Marys Challenger,[4] that in 2005 completed 100 years of continuous operation as a powered carrier (the Skinner engine was fitted in 1950) and the car ferry, Badger.[5]

In the early 1950s the Ultimax engine, a 2-crank 4-cylinder arrangement similar to Skinner’s, was developed by Abner Doble for the Paxton car project with tandem opposed single-acting cylinders giving effective double-action. [6]

Turbine engines

Main article: Steam turbine

A steam turbine consists of an alternating series of rotating discs mounted on a drive shaft, rotors, and static discs fixed to the turbine casing, stators. 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 exhausts into a 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 rotate at very high speed, therefore are usually connected to reduction gearing to drive another mechanism, such as a ship's propeller, at a lower speed. A turbine rotor is also capable of providing power when rotating in one direction only. 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 (about 86% of the world's electric production is by use of steam turbines) and to a lesser extent as marine prime movers. In the former, the high speed of rotation is an advantage, and in both cases the relative bulk is not a disadvantage. Virtually all nuclear power plants and some nuclear submarines, generate electricity by heating water to provide steam that drives a turbine connected to an electrical generator for main propulsion. 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, but were not repeated. Elsewhere, notably in the U.S.A., 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.

Other engines

Other types of steam engine have been produced and proposed, but have not been nearly so widely adopted as reciprocating or turbine engines.

Rotary steam engines

It is possible to use a mechanism based on a pistonless rotary engine such as the Wankel engine in place of the cylinders and valve gear of a conventional reciprocating steam engine. Many such engines have been designed, from the time of James Watt to the present day, but relatively few were actually built and even fewer went into quantity production; see link at bottom of article for more details. The major problem is the difficulty of sealing the rotors to make them steam-tight in the face of wear and thermal expansion; the resulting leakage made them very inefficient. Lack of expansive working, or any means of control of the cutoff 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. They were eventually replaced in these niche applications by steam turbines.

Jet type

Invented by Australian engineer Alan Burns and developed in Britain by engineers at Pursuit Dynamics, this underwater jet engine uses high pressure steam to draw in water through an intake at the front and expel it at high speed through the rear. When steam condenses in water, a shock wave is created and is focused by the chamber to blast water out of the back. To improve the engine's efficiency, the engine draws in air through a vent ahead of the steam jet, which creates air bubbles and changes the way the steam mixes with the water.

Unlike in conventional steam engines, there are no moving parts to wear out, and the exhaust water is only several degrees warmer in tests. The engine can also serve as pump and mixer. This type of system is referred to as 'PDX Technology' by Pursuit Dynamics.

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. The technique is simple in concept, simply fill a pressure vessel with hot water at high pressure, and open a valve leading to a suitable nozzle. The drop in pressure immediately boils some of the water and the steam leaves through a nozzle, giving a significant propulsive force.

It might be expected that water in the pressure vessel should be at high pressure; but in practice the pressure vessel has considerable mass, which reduces the acceleration of the vehicle. Therefore a much lower pressure is used, which permits a lighter pressure vessel, which in turn gives the highest final speed.

There are even speculative plans for interplanetary use. Although steam rockets are relatively inefficient in their use of propellant, this very well may not matter as the solar system is believed to have extremely large stores of water ice which can be used as propellant. Extracting this water and using it in interplanetary rockets requires several orders of magnitude less equipment than breaking it down to hydrogen and oxygen for conventional rocketry.

Applications

Steam engines can be classified by their application:

Stationary engines

Stationary steam engines can be classified into two main types:

• Winding engines, rolling mill engines, steam donkeys, (marine engines) and similar applications which need to frequently stop and reverse.
• Engines providing power, which stop rarely and do not need to reverse. These include engines used in thermal power stations and those that were used in mills, factories and to power cable railways and cable tramways before the widespread use of electric power. Very low power engines are used to power model ships and speciality applications such as the steam clock.

The steam donkey is technically a stationary engine but is mounted on skids to be semi-portable. It is designed for logging use and can drag itself to a new location. Having secured the winch cable to a sturdy tree at the desired destination, the machine will move towards the anchor point as the cable is winched in.

Vehicle engines

Steam engines have been used to power a wide array of types of vehicle:

• Steamboat and steamship
• Land vehicles:
  • Steam locomotive
  • Steam car
  • Steam roller
  • Steam shovel
  • Traction engine
  • Steam aircraft

Advantages

The strength of the steam engine for modern purposes is in its ability to convert heat from almost any source into mechanical work. Unlike the internal combustion engine, the steam engine is not particular about the source of heat. Most notably, without the use of a steam engine it would be more difficult to harness nuclear energy for useful work, as a nuclear reactor does not directly generate either mechanical work or electrical energy—the reactor itself simply heats or boils water. It is the steam engine which converts the heat energy into useful work. Steam may also be produced without combustion of fuel, through solar concentrators. A demonstration power plant has been built using a central heat collecting tower and a large number of solar tracking mirrors, (called heliostats). (see Whitecliffs Project[7])

Similar advantages are found in a different type of external combustion engine, the Stirling engine, which can offer efficient power (with advanced regenerators and large radiators) at the cost of a much lower power-to-size/weight ratio than even modern steam engines with compact boilers.

Steam locomotives are especially advantageous at high elevations as they are not adversely affected by the lower atmospheric pressure. This was inadvertently discovered when steam locomotives operated at high altitudes in the mountains of South America were replaced by diesel-electric units of equivalent sea level power. These were quickly replaced by much more powerful locomotives capable of producing sufficient power at high altitude.

In Switzerland (Brienz Rothhorn) and Austria (Schafberg Bahn) new rack steam locomotives have proved very successful. They were designed based on a 1930s design of Swiss Locomotive and Machine Works (SLM) but with all of today's possible improvements like roller bearings, heat insulation, light-oil firing, improved inner streamlining, one-man-driving and so on. These resulted in 60 percent lower fuel consumption per passenger and massively reduced costs for maintenance and handling. Economics now are similar or better than with most advanced diesel or electric systems. Also a steam train with similar speed and capacity is 50 percent lighter than an electric or diesel train, thus, especially on rack railways, significantly reducing wear and tear on the track. Also, a new steam engine for a paddle steam ship on Lake Geneva, the Montreux, was designed and built, being the world's first full-size ship steam engine with an electronic remote control[8]. The steam group of SLM in 2000 created a wholly-owned company called DLM to design modern steam engines and steam locomotives.

Efficiency

Main article: thermodynamic efficiency

The efficiency of an engine can be calculated by dividing the number of joules of mechanical work that the engine produces by the number of joules of energy input to the engine by the burning fuel. The rest of the energy is dumped into the environment as heat.

No pure heat engine can be more efficient than the Carnot cycle, in which heat is moved from a high temperature reservoir to one at a low temperature, and the efficiency depends on the temperature difference. Hence, steam engines should ideally be operated at the highest steam temperature possible (superheated steam), and release the waste heat at the lowest temperature possible.

In practice, a steam engine exhausting the steam to atmosphere will have an efficiency (including the boiler) of 1% to 8%, but with the addition of a condenser and multiple expansion engines the efficiency may be greatly improved to 25% or better. A power station with steam reheat, etc. will achieve 30% to 42% efficiency. Combined cycle in which the burning material is first used to drive a gas turbine can produce 50% to 60% efficiency. It is also possible to capture the waste heat using cogeneration in which the residual steam is used for heating. It is therefore possible to use about 90% of the energy produced by burning fuel—only 10% of the energy produced by the combustion of the fuel goes wasted into the atmosphere.

The reason for varying efficiencies is because of the thermodynamic rule of the Carnot Cycle. The efficiency is the absolute temperature of the cold reservoir over the absolute temperature of the steam, subtracted from one. As the temperature changes in seasons, the efficiency changes with it, unless the cold reservoir is kept in an isothermal state. It should be noted that the Carnot Cycle calculations require absolute temperatures.

One source of inefficiency is that the condenser causes losses by being somewhat hotter than the outside world, although this can be mitigated by condensing the steam in a heat exchanger and using the recovered heat, for example to pre-heat the air being used in the burner of an external combustion engine.

The operation of the engine portion alone is not dependent upon steam; any pressurized gas may be used. Compressed air is sometimes used to test or demonstrate small model "steam" engines.

Unconventional engines

In the 2007 international scrapheap challenge, which pitted British and American engineers against each other to construct paddle-boats in the spirit of Isambard Kingdom Brunel, the British team chose to power their paddles via use of a scratch built steam engine. This engine, rather than using a mechanical linkage to control the steam feed to the pistons, utilised scavenged microswitches and solenoid valves in an electrical system believed by the scrapheap presenters to be the first such functional system ever attempted. The paddle-steamer - thus constructed - was racing against the American diesel version. Both were making similar headway until a catastrophic loss of pressure halted their attempt.

See also

• Timeline of steam power
• Steam power during the Industrial Revolution
• Boiler
• Valve gear
• Compound locomotive
• Live steam
• Steam clock
• Steam car
• Steamboat
• Reciprocating engine
• Heat Regenerative Cyclone Engine

Steam Fairs

UK

• Carter's Steam Fair - touring vintage fairground, including several rides powered by steam engines
• Great Dorset Steam Fair - 5-day annual show in England - specialises in showing engines being used in their original context: heavy haulage, threshing, ploughing, sawing, road making, etc

USA

• Antique Gas & Steam Engine Museum - Bi-Annual show in Vista, CA, Specializing in farm equipment, engines, and machinery from 1850-1950

Steam museums

See also: List of pumping stations, many of which are, or were, steam-powered.

UK

• Bolton Steam Museum
• Crofton Beam Engines (Movie of Crofton engines operating)
• Hollycombe Steam Collection
• Kempton Park Steam Engines
• Kew Bridge Steam Museum

Canada

• Ontario Agricultural Museum in Milton, Ontario
• Steam Era in Milton, Ontario

External links

A steam-powered bicycle.

• Interactive Animation – (in German)
• Titanic's Triple Expansion Engines on Titanic-Titanic.com
• Howstuffworks - "How Steam Engines Work"
• Animated engines - Illustrates a variety of steam engines
• A history of the growth of the steam-engine
• Uniflow engines
• Steam powered lawn mower
• Rotary steam engines
• Beauchamp Tower's spherical steam engine
• Miniature live steam site
• Steamboat revival on Lake Geneva
• New Scientist jet steam engine article
• Dual pistons with linear generator as microCHP
• An art project underway that aims to build a steam powered Victorian house on tractor treads
• A short movie of a mill engine under steam at Barnoldswick, Lancashire, England
• Steam Engineering Tutorials
• A history of the growth of the steam engine Cornell University Library Historical Monographs Collection. {Reprinted by} Cornell University Library Digital Collections

Steam museums

• Bancroft Mill Engine[9], Barnoldswick. Movie of engine operating here [10]
• Country Living Museum in Dudley, Staffs UK: full-size working replica of the first Newcomen atmospheric engine of 1712.
• Newcomen Engine House, Dartmouth, Devon, England, UK
• Hamilton Museum of Steam and Technology in Hamilton, Ontario. An old municipal pumphouse dating to 1860 with its original two Woolf Compound Rotative Beam Engines, one of which still operates.