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.:In British English[[1]], the term steam engine may also refer to an entire 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.

Where usedEdit

Steam engines were used in pumps, locomotives, steam ships and steam tractors, and were essential to the Industrial Revolution[[2]]. They are still used for electrical power generation using steam turbine[[3]]s.

How it worksEdit

A steam engine needs a boiler to boil Water to produce steam under pressure. Any heat source can be used, but the most common is a fire fueled by wood, coal, or oil. (However, anything that can be burned can be used as fuel for the fire: paper, trash, used crankcase oil, ground-up corncobs, manure, natural gas, gasoline, high proof alcohol, dry grass, hay, dry weeds, etc). The steam expands and pushes against a piston or turbine, whose motion does the work of turning wheels or driving other machinery.

Types of steam engineEdit

Steam engines can be classified in two main ways:

InventionEdit

100px-Aeolipile illustration-StEngine1

Aeolipile

90px-Papinengine-StEngine2

The first piston steam engine, developed by Denis Papin in 1690.

The first steam device, the aeolipile Aeolipile, was invented by Hero of Alexandria [[4]], a Greek, in the 1st century AD, but used only as a toy. Incidentally, 700 years earlier in Corinth, Greece, rail tracks were invented; however the Greeks never thought of putting the two together.

In 1663, Edward Somerset, 2nd Marquess of Worcester [[5]] published designs for, and may have installed, a steam-powered engine for pumping water at Vauxhall House [[6]]. Denis Papin [[7]], a French physicist, built a working model of a steam engine in about 1687 --with the help of Leibniz [[8]], a paddle steam boat and is credited with a number of significant gadgets such as the safety valve. Sir Samuel Morland [[9]] also developed ideas for a steam engine during the same period, he built a number of steam-engine pumps for Louis XIV in the 1680s. Early industrial steam engines were designed by Thomas Savery (The "fire-engine", 1698) and Thomas Newcomen (1712). Humphrey Gainsborough [[10]] produced a model condensing steam engine in the 1760s, which he showed to James Watt[[11]]. In 1769 Watt patented improvements to the Newcomen engine that made it much more fuel efficient, which finally led to the general acceptance and use of steam power.

Use and developmentEdit

220px-Bennets aerosteam engine-StEng3

A diagram of Cameron's aero-steam engine, from an 1876 dictionary

The first industrial applications of the vacuum engines were in the pumping of water from deep mineshafts. The Newcomen steam engine [[12]] operated by admitting steam to the operating chamber, closing the valve, and then admitting a spray of cold water. The water vapor condenses to a much smaller volume of water, 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. In mineshaft pumps, the piston 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.

The first significant improvement, 60 years later, was creation of a separate condensing chamber with a valve between the operating chamber and the condensing chamber. This improvement was invented on Glasgow Green, Scotland by James Watt[[13]] and subsequently developed by him in Birmingham, England, to produce the Watt steam engine [[14]] with greatly increased efficiency. The next improvement was the replacement of manually operated valves with valves operated by the engine itself.

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.

Such early vacuum, or condensing, 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 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).

The next big improvement in efficiency came with Richard Trevithick's [[15]] use of pressurized steam, which used a far greater pressure, but more importantly (from a thermodynamic standpoint) operates at a higher temperature differential. But with this added pressure came much danger and many disasters due to exploding boilers and machinery. The most important refinement 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.

BoilersEdit

Boilers from Scientific American Supplement, Vol. XIX, No. 470, Jan. 3, are displayed in the National Museum of Science and Industry (The Science Museum), London. Boilers are of two main types:

  • Fire tube construction is typical of early maritime installations for boats and ships and the boilers of steam locomotives. In a fire tube boiler, the hot gases from the firebox (a combustion chamber) are passed through tubes connecting perforated end plates. The gases then enter a smokebox or smoke chest and pass on to a smokestack. The boiler may be vertical or horizontal. For an example of a vertical boiler of this type observe the boiler in the small riverboat used in the movie The African Queen. This type is also used in some boilers that provide steam for steam heating of a building and was also used in the steam shovel. Locomotives and early ships used a horizontal orientation and early ships would usually require a tall smokestack to provide draft, not having a fan to provide a forced draft. In a steam locomotive the draft is generally augmented at startup by directing the steam exhaust through the smokestack, which provides a partial vacuum.
  • In a Water-tube_boiler the water is heated in multiple tubes exposed to the hot gases. The tubes are joined to a steam collector chamber at the top. A significant advantage of this type is that there is less chance of catastrophic failure, as there is not a great amount of water in the boiler, nor are there large mechanical elements subject to failure. There may be additional tubes above the collector in the upper portion of the hot gas exhaust - this device, called a superheater, provides additional temperature (the pressure being unchanged) and increases the thermal efficiency of the entire mechanism. Superheaters were also used in some of the later versions of the steam locomotive.

There are also rarer variants, for example the drum boiler used in some steam cars.

There is also another division between boilers: natural aspiration, which is nearly all of them, and forced-draft, or "pressure-fired" boilers. This technology, equivalent to supercharging for an internal combustion engine, was developed by the Germans and acquired by the US Navy to be used in some frigates built after the Second World War. In it, a fan is used to increase the rate of burning; the boiler must be constructed to get that extra heat to the water. An engine using this kind of boiler has the greatest acceleration from a standing start of any marine powerplant.

EnginesEdit

High pressure steam engines are of various types but most are either reciprocating piston or turbine devices.

ReciprocatingEdit

Double-actingEdit

After the development of pressurized steam technology, the next major advance was the use of double-acting pistons, with pressurized steam admitted alternately to each side while the other side is exhausted to the atmosphere or to a condenser. Most reciprocating engines now use this technology. Power is removed by a sliding rod, sealed against the escape of steam. This rod in turn drives (via a sliding crosshead bearing) a connecting rod connected to a crank to convert the reciprocating motion to rotary motion. An additional crank or eccentric is used to drive the valve gear, usually through a reversing mechanism to allow reversal of the rotary motion.

When a pair of double acting pistons is used, their crank phasing is offset by 90 degrees of angle; this is called quartering. This ensures that the engine will always operate, no matter what position the crank is in.

Some ferryboats 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.

Multiple expansionEdit

Triple expansion engine animation-StEng4

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.

Another type uses multiple (typically three) single-acting cylinders of progressively increasing diameter and stroke (and hence volume).

High pressure steam from the boiler is used to drive the first and smallest diameter piston downward. On the upward stroke the partially expanded steam is driven into a second cylinder that is beginning its downward stroke. This accomplishes further expansion of the relatively high pressure exhaust from the first chamber. Similarly, the intermediate chamber exhausts to the final chamber, which in turn exhausts to a condenser.

The image at the right shows a model of such an engine. The steam travels through the engine from left to right. The valve chest for each of the first two cylinders is to the left of the corresponding cylinder while that of the third is to the right.

One modification of the triple-expansion engine is to use two smaller pistons that sum to the area of the third piston to replace it. This results in the more balanced unit of a total of four pistons arranged in a vee-configuration.

The development of this type of engine was important for its use in steamships, for the condenser would, by taking back a little of the power, turn the steam back to water for its reuse in the boiler. Land-based steam engines could exhaust much of their steam and be refilled from a fresh water tower, but at sea this was not possible. Prior to and during World War II[[16]] , the expansion engine dominated marine applications where high vessel speed was not essential. It was however superceded by the steam turbine where speed was required, for instance in warships and Ocean liners. HMS Dreadnought (1906) of 1905 was the first major warship to replace the proven technology of the reciprocating engine with the then novel steam turbine.

Multiple expansion can also result in greater efficiency, as the steam expends more of its energy driving pistons before leaving the engine. Some steam locomotives used double expansion. The most common arrangement was two sets of driving wheels. A set of high pressure cylinders drove one set and the low pressure cylinders drove the other set. A rarer arrangement was called the tandem compound, in which the high and low pressure cylinders were coaxial and had a common piston rod.

Other steam locomotives were simple, or single, expansion only. Most compound steam locomotives had a "simpling valve" which fed high pressure steam to all cylinders to help start a train.

UniflowEdit

Another type of reciprocating steam engine is the "uniflow'' type. In this, valves (which act similarly to those used in internal combustion engines) are operated by cams. The inlet valves open to admit steam when minimum expansion volume has been reached at the top of the stroke. For a period of the crank cycle steam is admitted and the poppet inlet are then closed, allowing continued expansion of the steam during the downstroke. Near the bottom of the stroke the piston will expose exhaust ports in the side of the cylindrical chamber. These ports are connected by a manifold and piping to the condenser, lowering the pressure in the chamber to below that of the atmosphere. Continued rotation of the crank moves the piston upward. Engines of this type always 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 concept is also employed in two stroke supercharged diesel engines used for marine, locomotive, and stationary applications. Such diesels do not need the economizer feature and use a simpler sliding camshaft for reversing.)

Turbine typeEdit

Steam turbines for high power applications use a number of rotating disks containing propeller-like blades at their outer edge. These moving "rotor" disks alternate with stationary "stator" blade rings affixed to the turbine case that serve to redirect the steam flow for the next stage. Owing to the high speed of operation such turbines are usually connected to a reduction gear to drive another mechanism such as a ship's propeller. Steam turbines are more durable, and require less maintenance than reciprocating engines. They also produce smoother rotational forces on their output shaft, which contributes to their lower maintenance requirements and lower wear on the machinery they power.

The main use for steam turbines is in electricity generation stations where their high speed of operation is an advantage and their relative bulk is not a disadvantage. They are also used in marine applications, powering large ships and submarines. Virtually all nuclear power plants generate electricity by heating water and powering steam turbines. A limited number of steam locomotives were manufactured that used turbine technology. While they met with some success for long haul freight operations in Sweden and elsewhere, steam turbine technology did not last long in the railway world and was rapidly replaced by diesel locomotives.

Rotary typeEdit

In theory, it might be 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. Lack of control of the cutoff is a major problem with such designs, and none has been demonstrated in practice.

Jet typeEdit

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 change the way the steam mixes with the water.

Unlike conventional steam engine, there is 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 typeEdit

The Aeolipile represents the use of steam by a rocket jet technique, 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 critical 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.[17]

Steam powered vehiclesEdit

Nicolas-Joseph Cugnot [[18]] 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.

Steam engine powered automobiles continued to compete with other motive systems into the early decades of the 20th century. However steam engines are less favored for automobiles, which are generally powered by internal combustion engines, because steam requires at least thirty seconds (in a flash boiler) or so to develop pressure.

On February 21, 1804 at the Pen-y-Darren ironworks in Wales, the first self-propelled railway steam engine or steam locomotive built by Richard Trevithick was demonstrated.

AdvantagesEdit

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 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) [[19]].

Similar advantages are found in a different type of external combustion engine, the Stirling engine, which offers efficient power in a compact engine, but which is difficult to operate over a wide range of operating conditions, difficulties which are readily addressed by the modern hybrid vehicle.

Steam locomotives are especially advantageous at high elevations as they are not especially 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.

EfficiencyEdit

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 5%, but with the addition of a condenser the efficiency is greatly improved to 25% or better. A power station with exhaust reheat, etc. will achieve 30% efficiency. Combined cycle in which the burning material is first used to drive a gas turbine can produce 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 museumsEdit

See alsoEdit

External linksEdit

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