Three stages of one idea
The steam engine did not appear in one step. A useful way to read its history is as three stages: the atmospheric engine that used steam only to create a vacuum, the low-pressure engine improved by James Watt, and the high-pressure engine that could be made small enough to move on wheels. A fourth stage, the steam turbine, arrived in the late 1800s and is still used in power stations.
Stage one: Newcomen
Thomas Newcomen’s engine, first put to work around 1712 at a Staffordshire coal mine, used a piston in a cylinder. Steam was admitted to fill the cylinder and then condensed by a spray of cold water, so the pressure of the atmosphere pushed the piston down. The engine was big and burned a great deal of coal, but it pumped water from mines that nothing else could drain. It earned its place by being practical rather than efficient.
Stage two: Watt
James Watt, a Scottish instrument maker working at the University of Glasgow, was asked to repair a model of a Newcomen engine in the early 1760s. He noticed how much steam was wasted heating the cylinder again after it had been cooled on each stroke. His solution, conceived in 1765 according to his own later account, was to condense the steam in a separate vessel, keeping the cylinder hot. This is the separate condenser, patented in 1769. In partnership with Matthew Boulton from 1775, in Birmingham, Watt developed engines that used much less coal than Newcomen engines for the same work.
Watt and Boulton went on to add other features: a closed cylinder top so that steam rather than air pushed the piston down, a double-acting design where steam acted on both sides of the piston, and mechanisms to convert the beam’s rocking into rotary motion. The sun-and-planet gear was devised as a way around a patent on the crank in the early 1780s. A centrifugal governor helped keep speed steady. The partnership charged customers royalties based on a share of the fuel saved compared with a Newcomen engine, an unusual business arrangement that was possible because the savings could be measured.
Watt’s patent was extended by Parliament to 1800, and many historians argue that it slowed some other designers, especially those working on higher-pressure engines. This is debated, and others point out that Watt’s firm also pushed engineering forward.
Stage three: high pressure
Watt was cautious about steam at high pressure because boilers and joints of the time could burst. When the patent protection ended, engineers moved ahead. Richard Trevithick in Cornwall and Oliver Evans in the United States built high-pressure engines around the turn of the 1800s. Using steam at several times atmospheric pressure allowed an engine of a given power to be much smaller and lighter. It could also exhaust directly to the air, without a condenser, which made it simpler. The tradeoff was a greater risk of boiler explosion until materials, safety valves and inspection improved.
High-pressure engines made locomotives and steamboats possible. Trevithick’s road and rail locomotives of the early 1800s were followed by the steam locomotives of the Stephensons and others from the 1820s onward. Cornish engineers also developed compound and high-efficiency pumping engines, which remained in use for mine drainage for many decades.
Refinements
Through the 1800s, engines grew more efficient. Compound engines expanded steam in two or more cylinders in turn, extracting more work from each charge. Better valve gears, such as those developed by Corliss in the United States, improved control of steam admission in stationary engines. Superheating, steam jackets and better boilers all raised efficiency further.
Steam engines in the form of reciprocating piston machines were then challenged by other prime movers. Charles Parsons patented the steam turbine in 1884. Turbines spin at high speed and are well suited to driving generators and ship propellers, and they were used in power stations through the twentieth century and in many nuclear and fossil fuel plants today. The internal combustion engine and electric motors took over many jobs that steam once did, particularly in smaller sizes.
A single line of development
Looking at the chain of events, three themes stand out. First, each improvement addressed a specific weakness of the last design: waste of heat, size, or speed. Second, the engine depended on contemporary metalworking, from cylinder boring to boiler plate. Third, there was a constant interplay between invention and demand, from mines to textile mills to railways. Naming one inventor oversimplifies a story where Savery, Newcomen, Watt, Trevithick and many others all contributed.