A steam engine is a heat engine that does mechanical work using steam as its working fluid. In the classic form, steam pushes against a piston inside a cylinder, the piston moves back and forth, and that motion turns a shaft that delivers power for useful work. Because the fuel burns outside the engine, in a separate boiler, steam engines are called external combustion engines. That means the heat can come from many sources: wood, coal and other fossil fuels, or solar power, nuclear power or geothermal energy. Between the early 18th and early 20th centuries, steam engines pumped water out of mines, drove factory machinery, and powered railways and ships. This made them central to the Industrial Revolution. Their descendant, the steam turbine, still produces a large share of the world's electricity.
Principle of operation
A steam engine turns the thermal energy of heated water into motion. When water boils and expands into steam, it takes up roughly 1,700 times more volume than it did as a liquid. A simple engine has a boiler, a cylinder with a piston, valves that control when steam enters and leaves, and a linkage such as a beam, crank or flywheel that carries the piston's motion to a pump or machine. In condensing engines, the used steam is cooled back into water. This creates a partial vacuum that adds to the force on the piston. Non-condensing engines release the used steam into the air instead.
There are two broad types of engine. The earliest designs used low-pressure steam. In them, the main driving force came from atmospheric pressure pushing against a vacuum. Later designs used high-pressure steam that pushed the piston directly. Later refinements included double-acting cylinders, where steam pushes on both sides of the piston, and compound engines, where steam expands through several cylinders in turn. The thermodynamics of these machines is now described by the Rankine cycle.
Early history
Steam was known as a source of motion in antiquity. In the 1st century AD, Hero of Alexandria described the aeolipile, a ball spun by jets of steam. It produced little power, but it is the first known device moved by steam pressure. Hero worked in Roman Egypt, and he treated the device as a curiosity rather than a source of power.
Practical steam power began in the late 17th century, driven by the need to pump water out of flooded mines. The idea of using steam to push a piston in a cylinder seems to have started with Denis Papin, who built a model engine with a vertical cylinder and piston around 1690. Papin's model was not developed further. In 1698, Thomas Savery patented a machine that used steam pressure to draw water from flooded mines. Savery's "Miner's Friend" had no piston. Like any pump that relies on suction, it could lift water no more than 32 feet, and usually much less.
Newcomen and Watt
The first commercially successful piston engine was the atmospheric engine of Thomas Newcomen, introduced in 1712. It condensed steam inside the cylinder to create a partial vacuum, and atmospheric pressure then pushed the piston into the cylinder. The piston was linked by a rocking beam to a mine pump. By the mid-1700s, Newcomen engines were working at mines in France, Sweden, Russia, Britain and other countries. They were very wasteful, though. Because the cylinder was heated and cooled on every stroke, energy was lost reheating the metal, and the cylinder suffered large thermal stresses.
James Watt solved this problem. In 1763, while working as an instrument maker at the University of Glasgow, Watt was asked to repair a model Newcomen engine and saw how inefficient it was. In 1765, he came up with the idea of a separate condensation chamber, which he called a condenser. The condenser stayed cold and below atmospheric pressure at all times, while the cylinder stayed hot. Building the engine was slowed by difficulties in machining cylinders accurately enough. With help from Matthew Boulton, Watt perfected a practical version, and they first offered it for sale in 1775. In its fully developed form, the Watt engine used about 75% less fuel than a comparable Newcomen engine. In 1781, Watt added a sun-and-planet gear that converted the piston's back-and-forth stroke into continuous rotary motion. This let steam engines drive mills and factories that had previously depended on water power.
High pressure and transport
Watt's patent on the separate condenser ran from 1769 to 1800. Watt preferred low-pressure steam. Just after 1800, the Cornish inventor Richard Trevithick and the American millwright Oliver Evans both built high-pressure engines. These were smaller and lighter, which made it practical to mount an engine on a vehicle. Rather than condensing the used steam to create a vacuum, builders simply released it into the air.
Trevithick went on to pioneer the steam locomotive. The world's first railway journey pulled by a locomotive took place on 21 February 1804, when his unnamed locomotive hauled a train along the tramway of the Penydarren Ironworks in Merthyr Tydfil, Wales. Salamanca, built in 1812 by Matthew Murray for the Middleton Railway, was the first commercially successful steam locomotive. In the decades that followed, steam-powered railways and steamships transformed overland and ocean transport. They sharply reduced travel times and freight costs.
Science of heat engines
Steam engines also pushed forward the physics of heat. In 1824, Sadi Carnot developed the Carnot cycle, a basic model for all heat engines that gave the first ideas behind the second law of thermodynamics. Carnot's analysis showed that an engine's maximum efficiency depends on the temperature difference between its heat source and its heat sink. Later work by James Prescott Joule on the equivalence of heat and work, and the development of the concept of entropy, built classical thermodynamics into a full theory.
Steam turbines and decline of the piston engine
The steam turbine spins a bladed rotor directly with steam instead of moving a piston. Charles Parsons invented its modern form in 1884, and it revolutionized marine propulsion. The turbine has almost completely replaced the piston steam engine, mainly because it is more thermally efficient and delivers more power for its weight. Because it produces rotary motion directly, it is especially well suited to driving electrical generators. In the 20th century, the internal combustion engine and electric motors replaced steam in most vehicles. By the second half of the century, steam locomotives had largely disappeared from major rail networks.
Modern relevance
Steam remains central to how electricity is generated. Coal plants, nuclear reactors, natural gas facilities, geothermal stations, and even some solar installations all use steam turbines to turn heat into electricity. Estimates of steam's share of global generation vary from source to source, but most sources agree it is the majority. Efficiency has improved enormously. Modern supercritical coal plants reach thermal efficiencies of around 48%, compared with under 1% for early steam engines. Because most steam plants burn fossil fuels, they are closely linked to concerns about climate change. Many renewable energy sources such as wind and photovoltaic solar generate electricity without steam.
References
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