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300px Richard Trevithick's No. 14 engine, built by Hazeldine and Co., Bridgnorth, about 1804, and illustrated after being rescued circa 1885; from Scientific American Supplement, Vol. XIX, No. 470, Jan. 3, 1885. Now on display in the National Museum of Science and Industry (The Science Museum), London. Boilers are of two types:
High pressure steam engines are of various types but most are either reciprocating piston or turbine devices.
Most reciprocating engines use double acting pistons, with pressurized steam admitted alternately to each side while the other side is exhausted to the atmosphere or to a condenser. 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 ensures that the engine will always operate, no mater 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 somewhere in its middle range of travel so that it could be readily restarted.
Steam turbines for high power applications will use a number of rotating disks containing propeller-like blades at their outer edge. These "rotor" disks alternate with "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, smoother operating, and require far less maintenance than reciprocating engines.
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.
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).
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.
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