James Watt's industrial steam engine, 1788.
Diagram of a steam engine.
The steam engine was the first important heat engine, supplying the power that made the Industrial Revolution possible, and the principal power source for industry and transport (notably railroad locomotives and steamships) until largely superceded in the twentieth century by steam turbines and the various internal-combustion engines.
Types of steam engine
The 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.
The 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
Steam engines, such as the ones formerly used to drive locomotives, operate through the production and expansion of steam as water is heated. A coal- or wood-fired furnace (1) heats a water-filled boiler (2), producing steam. The steam rises and is forced, via a steam dome (3), through a series of tubes to the cylinder (4), where it drives the piston (5) in a reciprocal motion. Linked to the piston's connecting rod (6) is a slide valve (7) that at one stage allows steam to enter the cylinder (as shown), blocking off the exhaust port (8). This creates pressure that forces the piston forward, at which point the slide valve is in a position where the exhaust port is open and the steam escapes. The motion of the wheel forces the piston back and the cycle begins again.