Types of steam engineThe steam engine is an external-combustion engine, the steam being raised in a boiler heated by a furnace; it is also a reciprocating engine. There are two main types: condensing, in which the pressure drop is caused by cooling the steam and so condensing it back to water; and non-condensing, in which the steam is exhausted to the atmosphere.
Historical developmentThe first major precursor of the steam engine was Thomas Savery's steam pump (1698), which worked by the partial pressure vacuum created by condensing steam in closed chambers. It had no moving parts, however, and the first working reciprocating engine was that of Thomas Newcomen (1712): steam was admitted to the cylinder as the piston moved up, and was condensed by a water spray inside the cylinder, whereupon the air pressure outside forced the piston down again.
James Watt radically improved Newcomen's engine (1769) by condensing the steam outside the cylinder (thus no longer having to reheat the cylinder at each stroke) and by using the steam pressure to force the piston up. He later found that, if steam were admitted for only part of the stroke, its expansion would do a great deal of extra work. (The principles involved were later studied by Carnot and became the basis of thermodynamics.) Watt also invented the double-action principle – both strokes being powered, by applying the steam alternately to each end of the piston – the flyball governor, and the crank and "sun-and-planet" devices for converting the piston's linear motion into rotary motion.
The compound engine (1781) makes more efficient use of the steam by using the exhaust steam from one cylinder to drive the piston of a second cylinder. Later developments included the use of high-pressure steam by Richard Trevithick and Oliver Evans.
Operation of a steam engine
Related categories• HEAT AND THERMODYNAMICS
• HISTORY OF SCIENCE
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