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Steam engine

2007 Schools Wikipedia Selection. Related subjects: Engineering

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

   A steam engine is an external combustion heat engine that makes use of
   the thermal energy that exists in steam, converting it to mechanical
   work.

   Steam engines were used as the prime mover in pumps, locomotives, steam
   ships, traction engines, steam lorries and other road vehicles, and
   were essential to the Industrial Revolution. Steam turbines,
   technically a type of steam engine, are still widely used for
   generating electricity, but older types have been almost entirely
   replaced by internal combustion engines and electric motors.

     Steam engine in action (animation)
     Enlarge
     Steam engine in action (animation)

   A steam engine requires a boiler to boil water to produce steam. The
   expansion—or contraction—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. One of the advantages of the steam engine is
   that any heat source can be used to raise steam in the boiler; but the
   most common is a fire fueled by wood, coal or oil or the utilisation of
   the heat energy generated in a nuclear reactor.

Invention and development

   Aeolipile
   Enlarge
   Aeolipile

   The first recorded steam device, the aeolipile, was invented by Hero of
   Alexandria, a Greek, in the 1st century AD, but used only as a toy. In
   1663, Edward Somerset, 2nd Marquess of Worcester published designs for,
   and may have installed, a steam-powered engine for pumping water at
   Vauxhall House.
   The first piston steam engine, developed by Denis Papin in 1690.
   Enlarge
   The first piston steam engine, developed by Denis Papin in 1690.

   In about 1680 the French physicist Denis Papin, with the help of
   Gottfried Leibniz, built a steam digester for softening bones, i.e. he
   invented the world's first-ever pressure cooker. Later designs
   implemented a steam-release valve to keep the device from exploding. By
   watching the valve rhythmically move up and down Papin conceived of the
   idea of a piston and cylinder engine. Papin wrote up the designs for
   such a device (as pictured adjacent), however he never built an actual
   steam engine. The English engineer Thomas Savery later used Papin's
   designs to build the world's first operational steam engine.

   Papin also designed a paddle boat and is also credited with a number of
   significant devices such as the safety valve. Sir Samuel Morland also
   developed ideas for a steam engine during the same period and built a
   number of steam-engine pumps for King Louis XIV of France in the 1680s.

   Early industrial steam engines were designed by Thomas Savery (the
   "fire-engine", 1698) but it was Thomas Newcomen and his
   "atmospheric-engine" of 1712 that demonstrated the first operational
   and practical industrial engine. Together, Newcomen and Savery
   developed a beam engine that worked on the atmospheric, or vacuum,
   principle. The first industrial applications of the vacuum engines were
   in the pumping of water from deep mineshafts. In mineshaft pumps the
   reciprocating beam was connected to an operating rod that descended the
   shaft to a pump chamber. The oscillations of the operating rod are
   transferred to a pump piston that moves the water, through check
   valves, to the top of the shaft. Early Newcomen engines operated so
   slowly that the valves were manually opened and closed by an attendant.
   An improvement was the replacement of manual operation of the valves
   with an operation derived from the motion of the engine itself, by
   lengths of rope known as potter cord (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.)

   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 1769 James Watt, another member of the Lunar Society,
   patented the first significant improvements to the Newcomen type vacuum
   engine that made 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. 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).
   Enlarge
   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 1802 William Symington built the
   "first 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

   Animated schematic illustrating the difference in operation between the
   vacuum and high pressure types of steam engine. High pressure steam is
   red, low pressure steam is yellow and condensate blue. The top of the
   cylinder in the vacuum cylinder must be open to allow atmospheric
   pressure to act on the piston. The vacuum piston is returned to the
   start position (top) by a counterweight and the high pressure piston
   (starting low) by the angular momentum of the flywheel.
   Animated schematic illustrating the difference in operation between the
   vacuum and high pressure types of steam engine. High pressure steam is
   red, low pressure steam is yellow and condensate blue. The top of the
   cylinder in the vacuum cylinder must be open to allow atmospheric
   pressure to act on the piston. The vacuum piston is returned to the
   start position (top) by a counterweight and the high pressure piston
   (starting low) by the angular momentum of the flywheel.

   Reciprocating engines use the action of steam to move a piston in a
   sealed chamber. The reciprocal action of the piston can be translated
   via a mechanical linkage into rotary work.

Vacuum engines

   Early steam engines, such as Newcomen's "atmospheric" and Watt's
   "condensing" engines, worked on the vacuum principle and are thus known
   as vacuum engines. Such engines operate by admitting low pressure steam
   into an operating chamber and closing the inlet valve. The steam is
   then cooled, the resultant water vapor condensing to a smaller volume
   than the steam, creating a vacuum in the chamber. Atmospheric pressure,
   operating on the opposite side of a piston, pushes the piston to the
   bottom of the chamber. The piston is connected to a large beam and
   counterweight, the weight of which returns the piston to the top of the
   chamber; the low pressure steam is insufficient to move the piston
   upwards alone. The reciprocating action of the beam can be harnessed to
   do mechanical work. In the Newcomen engine cooling water is sprayed
   directly into the working chamber but in the Watt engine there is a
   separate condensing chamber, connected to the working chamber by a
   valve. The inefficiency of the Newcomen engine lay in the repeated and
   wasteful heating and cooling of the working chamber. By removing the
   condensing phase of the action to a separate chamber this was greatly
   reduced and the efficiency of the engine was greatly increased.

   Vacuum engines are severely limited in their efficiency but are
   relatively safe since the steam is at very low pressure and structural
   failure of the engine will be by inward collapse rather than an outward
   explosion. Their power is limited by the ambient air pressure, the
   displacement of the working chamber, the combustion and evaporation
   rates and—where present—the condenser capacity. The maximum theoretical
   efficiency is limited by the relatively low boiling point of water at
   near atmospheric pressure (100 °C, 212 °F).

High pressure engines

   A labeled schematic diagram of a typical single cylinder, simple
   expansion, double-acting high pressure 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.
   Enlarge
   A labeled schematic diagram of a typical single cylinder, simple
   expansion, double-acting high pressure 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.

   In a high pressure engine, steam is raised in a boiler to a high
   pressure and temperature. It is then admitted to a working chamber
   where it expands and acts upon a piston, although Trevithick's original
   " Cornish engines" used steam pressure alone to raise the cylinder. The
   piston consequently reciprocates, much like in the vacuum engine. 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 differential than is
   possible with a low pressure vacuum engine. After displacing the vacuum
   engine, the high pressure engine became the basis for further
   development of reciprocating steam technology. High pressure steam also
   has the advantage that engines can be much more compact. The
   significance is that engines could be developed that were small enough
   and powerful enough to propel themselves while doing useful work; steam
   power for transportation became a practicality.

Double-acting pistons

   The next major advance in high pressure steam engines was the use of
   double-acting pistons. In the simple-acting high pressure engine above,
   the cylinder is vertical and the piston returns to the start—or
   bottom—of the stroke by gravity. In the double-acting piston, 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 bearing). This
   converts the reciprocating motion to a rotary motion. The inlet and
   exhaust valves have a 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.

   Most reciprocating engines now use this technology, notable examples
   including steam locomotives and marine triple expansion 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 degrees. This is called quartering and ensures
   that the engine will always operate, no matter what position the crank
   is in. Some engines have used only a single double-acting piston,
   driving paddlewheels on each side by connection to an overhead rocker
   arm. 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 is not a second quartered piston to prevent
   this).

Compounding

   Image:Steam machine compound.png
   Schematic diagram of a cross compound steam engine.
   1 - High pressure cylinder
   2 - High pressure crank
   3 - Flywheel
   4 - Low pressure crank ("quartered")
   5 - Low pressure cylinder.
   Image:Steam machine tandem.png
   Schematic diagram of a tandem compound steam engine .
   1 - Low pressure cylinder
   2 - High pressure cylinder
   3 - Connecting rod
   4 - Driveshaft & flywheel.

   All of the high pressure engines mentioned above use simple
   expansion—the steam enters the cylinder, expands once and exhausts. As
   steam expands its temperature drops, this is known as adiabatic
   expansion. This 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 cylinder
   and then enters one or more subsequent lower pressure 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. Where space is at a premium, such as in a steam locomotive,
   two cylinders of a smaller volume are often substituted.

   The first compound engines had 2 cylinders, often called double
   compound, with later types of compound engines using triple and even
   quadruple expansion (see below). The difference between the inlet and
   exhaust temperature of the steam in each cylinder in a double compound
   is roughly half that in a simple expansion engine, with the pistons
   designed so that each produces half the work of the engine.

   The arrangement of cylinders in double compound engines are used as a
   basis for classification:
     * Cross compound - The cylinders are side by side and drive the same
       crank.
     * Tandem compound - The cylinders are end to end, driving a common
       connecting rod
     * Steeple engine - A vertical tandem compound engine.
     * Angle compound - The cylinders are arranged in a vee and drive a
       common crank.

   In cross and angle compounds, the pistons are connected to the crank
   90° out of phase with each other (quartered) to derive a smooth motion
   that will not lock up, with a new power stroke every quarter turn.

   The compound engine increases the efficiency of steam engines but adds
   a great deal of complexity to the system. Its adoption was almost
   universal in industrial and marine engines, but was not so marked in
   railway locomotives. This is partly due to the harsh railway operating
   environment and limited space afforded by the loading gauge
   (particularly in Britain, where compounding was not common). Locomotive
   compounding most commonly drove two different sets of driving wheels,
   to better distribute the power of the engine and lessen the effects of
   hammer blow peculiar to steam locomotives. Most compound steam
   locomotives had a simpling valve that fed high pressure steam to all
   cylinders to help start a heavy train.

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.
   Enlarge
   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.

   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 compound 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.
   Model of a triple expansion engine
   Enlarge
   Model of a triple expansion engine

   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 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.

Uniflow Engines

   Schematic animation of a uniflow steam engine. The poppet valves are
   controlled by the rotating camshaft at the top. High pressure steam
   enters, red, and exhausts, yellow.
   Enlarge
   Schematic animation of a uniflow steam engine. The poppet valves are
   controlled by the rotating camshaft at the top. High pressure steam
   enters, red, and exhausts, yellow.

   Another type of steam engine is the uniflow type, the name deriving
   from the fact that steam flowed in one direction only in each half of
   the cylinder. Thermal efficiency was increased in the compound and
   multiple expansion types by separating expansion into steps in separate
   cylinders. In the uniflow design, thermal efficiency is achieved by
   having a temperature gradient along the cylinder. Steam always enters
   at the hot ends of the cylinder and exhausts through ports at the
   cooler centre. By this means the relative heating and cooling of the
   cylinder walls is reduced.

   Steam entry is usually controlled by poppet valves (which act similarly
   to those used in internal combustion engines) that are operated by a
   camshaft. The inlet valves open to admit steam when minimum expansion
   volume has been reached at the start of the stroke. For a period of the
   crank cycle steam is admitted and the poppet inlet is then closed,
   allowing continued expansion of the steam during the stroke, driving
   the piston. Near the end of the stroke the piston will expose a ring of
   exhaust ports mounted radially around the centre of the cylinder. These
   ports are connected by a manifold and piping to the condenser, lowering
   the pressure in the chamber to below that of the atmosphere causing
   rapid exhausting. Continued rotation of the crank moves the piston.
   From the animation the features of a uniflow engine can be seen, with a
   large piston almost half the length of the cylinder, poppet inlet
   valves at either end, a camshaft (whose motion is derived from that of
   the driveshaft) and a central ring of exhaust ports.

   The beauty of the uniflow engine was that it potentially allowed great
   expansion in a single cylinder without the relatively cool exhaust
   steam flowing across the hot end of the working cylinder and steam
   ports of a conventional "counterflow" steam engine during the exhaust
   stroke. This condition allows high thermal efficiency. The exhaust
   ports were only open for a short period of the stroke, therefore not
   all expanded steam was able to exhaust. This remaining steam was
   compressed by the returning piston and was thermodynamically desirable
   as it preheated the hot end of the cylinder before the admission of
   steam. However, the risk of excessive compression often resulted in
   small auxiliary exhaust ports being included at the cylinder heads.
   Such a design may be called a semi-uniflow engine.

   In practice the uniflow engine has a number of operational
   shortcomings. The large expansion ratio requires a large cylinder
   volume. To gain the maximum potential work from this a high
   reciprocation rate was required, typically 80% faster than a
   double-acting engine. This caused the opening times of the inlet valves
   to be very short, putting great strain on a delicate mechanical part.
   In order to withstand the huge mechanical forces encountered, engines
   had to be heavily built and a large flywheel was required to smooth out
   the variations in torque as the steam pressure rapidly rose and fell in
   the cylinder. Additionally, as there was a thermal gradient across the
   cylinder, the metal of the wall expanded to different extents. This
   required precise boring of the cylinder barrel to be wider in the cool
   centre than at the hot ends. If the cylinder was not heated correctly,
   or if water entered, the delicate balance could be upset causing
   seizure mid-stroke or, potentially, destruction.

   Engines of this type usually have multiple cylinders in an inline
   arrangement and may be single or double acting. A particular advantage
   of this type is that the valves may be operated by the effect of
   multiple camshafts, and by changing the relative phase of these
   camshafts, the amount of steam admitted may be increased for high
   torque at low speed and may be decreased at cruising speed for economy
   of operation, and by changing the absolute phase the engine's direction
   of rotation may be changed. The uniflow design also maintains a
   constant temperature gradient through the cylinder, avoiding passing
   hot and cold steam through the same end of the cylinder.

   The uniflow engine was first used in Britain in 1827 by Jacob Perkins
   and was patented in 1885 by Leaonard Jennett Todd. It was popularised
   by German engineer Johann Stumpf in 1909, with the first commercial
   stationary engine produced a year previously in 1908.

   The uniflow principle was mainly used for in industrial power
   generation, but was also tried in a few railway locomotives in England,
   such as The NER Uniflow Locomotive No 825 of 1913, The NER Uniflow
   Locomotive No 2212 of 1919, and The Midland Railway Paget locomotive.
   Experiments were also made in the USA and Russia. In no case were the
   results encouraging enough for further development to be undertaken.

   The final commercial evolution of the Uniflow engine occurred in the
   USA during the late 1930s and 1940s by the Skinner Engine Company with
   the development of the Compound Unaflow Marine Steam Engine. This
   engine operated in a steeple compound configuration and provided
   efficiencies approaching contemporary diesels. Many bulk carriers and
   ferries on the Great Lakes were so equipped, several of which are still
   operating.

   In small sizes (less than about 1000 horsepower ), reciprocating steam
   engines are much more efficient than steam turbines. The Whitecliffs
   solar steam power plant uses a three cylinder uniflow engine to
   generate about 25 kW electric output.

Turbine engines

   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 propellor-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.

   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 . While they met with some success for long haul freight
   operations in Sweden and elsewhere, steam turbines were not ideally
   suited to the railroad environment. Turbine locomotives did not persist
   in the railway world and were replaced by diesel locomotives.

   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.

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 1840's 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 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, (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.

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 lorry
          + Steam roller
          + Steam shovel
          + Traction engine

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
   nuclear energy could not be harnessed 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)

   Similar advantages are found in a different type of external combustion
   engine, the Stirling engine, which offers efficient power in a compact
   engine.

   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 engines operated at high
   altitudes in the mountains of South America were replaced by
   diesel-electric engines of equivalent sea level power. They 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 ship steam engine with an electronic remote control. The steam
   group of SLM in 2000 created a wholly-owned company called DLM to
   design modern steam engines and steam locomotives.

Efficiency

   To get the efficiency of an engine, divide 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. In general, 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,
   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 the efficiency may be greatly improved. 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.

   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 pressurised gas may be used. Compressed air is sometimes used to
   test or demonstrate small model "steam" engines.

Festivals and museums

     * Antique Gas & Steam Engine Museum - Bi-Annual show in Vista, CA,
       Specializing in farm equipment, engines, and machinery from
       1850-1950
     * Great Dorset Steam Fair - annual show in England - specialises in
       showing engines being used in their original context: heavy
       haulage, threshing, sawing, road making, etc
     * Annual Steam Show in America North American Model Engineering
       Society (NAMES)
     * Annual Steam-Up in America New England Wireless and Steam Museum
     * The Newcomen Engine House, Dartmouth, Devon, England, UK
     * Steam Era in Milton, Ontario
     * Ontario Agricultural Museum in Milton, Ontario
     * Missouri River Valley Steam Engine Association Back to the Farm
       Reunion in central Missouri, USA. This is not a steam-only
       festival, but it has always had a good showing of running steam
       engines.
     * 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.
     * Kempton Park Steam Engines
     * Kew Bridge Steam Museum
     * Crofton Beam Engines
     * Bancroft Mill Engine , Barnoldswick. Movie of engine operating here
     * Buckley Old Engine Show Northwest Michigan Engine & Thresher Club.
       Annual show (39 years) showing steam engines and equipment, antique
       gas and oil engines, antique agricultural equipment, mills,
       blacksmithing, and foundries. Show includes steam building
       seminars.
     * Hollycombe Steam Fair

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