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A steam engine is a heat engine that makes use of the
potential energy that exists as pressure in steam, converting it to mechanical work. Steam
engines were used in pumps, locomotive
trains and steam ships, and were essential to the Industrial Revolution. They are still used for electrical power
generation.
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 wood or coal fire. The steam is allowed to
expand by pushing against a piston or turbine, whose motion is used to do work.
The first steam device, the aeolipile, was invented by Heron of Alexandria, a Greek, in the 1st century AD, but used only as a toy. Denis Papin, a French physicist, built a working model of a steam engine after observing steam escaping from
his pressure cooker in about 1679. Early industrial steam engines were designed by Thomas
Savery (1698), Thomas
Newcomen (1712), and James Watt
(1769), each of whom added new refinements.
Early engines worked by the vacuum of condensing steam, whereas later types (such as
steam locomotives) used the power of expanding steam.
Steam engines are of various types but most are reciprocal
piston or turbine devices.
Nicolas-Joseph Cugnot demonstrated the first
functional self-propelled steam vehicle, his "steam wagon", in 1769. Arguably, this was the
first automobile. 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-propelling railway steam engine or steam locomotive built by Richard Trevithick was first demonstrated.
The strength of the steam engine for modern purposes is in its ability to convert raw heat 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.
A steam engine exhausting 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.
One source of inefficiency is that the condenser causes losses by being somewhat hotter than the outside world. Thus any
closed-cycle engine will always be somewhat less efficient than any open-cycle engine, because of condenser losses.
See also
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