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Narrow gauge, turquoise-colored steam engine with red cow catcher on the Tweetsie Railroad North Carolina by Carol M. Highsmith

Steam engines

Imagine living off nothing but coal and water and still having enough energy to run at over 100 mph! That's exactly what a steam locomotive can do. Although these giant mechanical dinosaurs are now extinct from most of the world's railroads, steam technology lives on in people's hearts and locomotives like this still run as tourist attractions on many heritage railways.

Steam locomotives were powered by steam engines, and deserve to be remembered because they swept the world through the Industrial Revolution of the 18th and 19th centuries. Steam engines rank with cars, airplanes, telephones, radio, and television among the greatest inventions of all time. They are marvels of machinery and excellent examples of engineering, but under all that smoke and steam, how exactly do they work?

Photo: A steam-powered railroad locomotive operating at Tweetsie Railroad in North Carolina. This is a narrow-gauge train, which means the track is not as wide as in a conventional railroad. Narrow tracks are often used in mountainous areas and in other difficult terrain, because they are generally cheaper to build. Credit: Photographs in Carol M. Highsmith's America Project in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.

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  1. What powers a steam engine?
  2. What is a steam engine?
  3. How a steam engine works
  4. Types of steam engine
  5. Did steam really die?
  6. Who invented the steam engine... and when?
  7. Find out more

What powers a steam engine?

It takes energy to do absolutely anything you can think of—to ride on a skateboard, to fly on an airplane, to walk to the shops, or to drive a car down the street. Most of the energy we use for transportation today comes from oil, but that wasn't always the case. Until the early 20th century, coal was the world's favorite fuel and it powered everything from trains and ships to the ill-fated steam planes invented by American scientist Samuel P. Langley, an early rival of the Wright brothers. What was so special about coal? There's lots of it inside Earth, so it was relatively inexpensive and widely available.

Coal is an organic chemical, which means it's based on the element carbon. Coal forms over millions of years when the remains of dead plants get buried under rocks, squeezed by pressure, and cooked by Earth's internal heat. That's why it's called a fossil fuel. Lumps of coal are really lumps of energy. The carbon inside them is locked to atoms of hydrogen and oxygen by joints called chemical bonds. When we burn coal on a fire, the bonds break apart and the energy is released in the form of heat.

Coal contains about half as much energy per kilogram as cleaner fossil fuels such as gasoline, diesel, and kerosene—and that's one reason why steam engines have to burn so much of it.

Photo showing the main component parts of a steam engine

Photo: The main parts of a steam locomotive. (For an alternative, side-view, look here.) This is ex-British Railways Standard 4MT tank locomotive number 80104 (built at Brighton in 1955) working on the Swanage Railway, England in August 2008. Read how it was restored from a rusting heap and returned to service by its owners, Southern Locomotives, in 80104 Restoration.

What is a steam engine?

A steam engine is a machine that burns coal to release the heat energy it contains—so it's an example of what we call a heat engine. It's a bit like a giant kettle sitting on top of a coal fire. The heat from the fire boils the water in the kettle and turns it into steam. But instead of blowing off uselessly into the air, like the steam from a kettle, the steam is captured and used to power a machine. Let's find out how!

How a steam engine works

Crudely speaking, there are four different parts in a steam engine:

  1. A fire where the coal burns.
  2. A boiler full of water that the fire heats up to make steam.
  3. A cylinder and piston, rather like a bicycle pump but much bigger. Steam from the boiler is piped into the cylinder, causing the piston to move first one way then the other. This in and out movement (which is also known as "reciprocating") is used to drive...
  4. A machine attached to the piston. That could be anything from a water pump to a factory machine... or even a giant steam locomotive running up and down a railroad.

That's a very simplified description, of course. In reality, there are hundreds or perhaps even thousands of parts in even the smallest locomotive.


It's easiest to see how everything works in our little animation of a steam locomotive, below. Inside the locomotive cab, you load coal into the firebox (1), which is quite literally a metal box containing a roaring coal fire. The fire heats up the boiler—the "giant kettle" inside the locomotive.

Animated cutaway showing the key parts of a steam engine and how they work

The boiler (2) in a steam locomotive doesn't look much like a kettle you'd use to make a cup of tea, but it works the same way, producing steam under high pressure. The boiler is a big tank of water with dozens of thin metal tubes running through it (for simplicity, we show only one here, colored orange). The tubes run from the firebox to the chimney, carrying the heat and the smoke of the fire with them (shown as white dots inside the tube). This arrangement of boiler tubes, as they are called, means the engine's fire can heat the water in the boiler tank much faster, so it produces steam more quickly and efficiently. The water that makes the steam either comes from tanks mounted on the side of the locomotive or from a separate wagon called a tender, pulled behind the locomotive. (The tender also carries the locomotive's supply of coal.) You can see a photo of a tender showing its water tank further down this page.

The steam generated in the boiler flows down into a cylinder (3) just ahead of the wheels, pushing a tight-fitting plunger, the piston (4), back and forth. A little mechanical gate in the cylinder, known as an inlet valve (shown in orange) lets the steam in. The piston is connected to one or more of the locomotive's wheels through a kind of arm-elbow-shoulder joint called a crank and connecting rod (5).

As the piston pushes, the crank and connecting rod turn the locomotive's wheels and power the train along (6). When the piston has reached the end of the cylinder, it can push no further. The train's momentum (tendency to keep moving) carries the crank onwards, pushing the piston back into the cylinder the way it came. The steam inlet valve closes. An outlet valve opens and the piston pushes the steam back through the cylinder and out up the locomotive's chimney (7). The intermittent chuff-chuff noise that a steam engine makes, and its intermittent puffs of smoke, happen when the piston moves back and forth in the cylinder.

There's a cylinder on each side of the locomotive and the two cylinders fire slightly out of step with one another to ensure there's always some power pushing the engine along.

Steam engine piston and cylinder

Photo: Close-up of the piston and cylinder in a steam engine. Photo from Carol M. Highsmith's America Project in the Carol M. Highsmith Archive courtesy of Library of Congress Prints and Photographs Division.

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Types of steam engine

Our diagram up above shows a very simple, one-cylinder steam engine powering a steam locomotive down a track. This is called a rotary steam engine, because the piston's job is to make a wheel rotate. The earliest steam engines worked in an entirely different way. Instead of turning a wheel, the piston pushed a beam up and down in a simple back-and-forth or reciprocating motion. Reciprocating steam engines were used to pump water out of flooded coal mines in the early 18th century.

Our diagram shows steam pushing the piston one way and the momentum of the locomotive driving it the other way. This is called a single-acting steam engine and it's quite an inefficient design because the piston is being powered only half the time. A much better (though slightly more complex) design uses extra steam pipes and valves to make steam push the piston first one way and then the other. This is called a double-acting (or counterflow) steam engine. It's more powerful because steam is driving the piston all the time.

This very simpified little animation shows you the basic concept:

Simplified animation of a double-acting counterflow steam engine cylinder.

Animation: In a double-acting cylinder, a valve (orange) moves back and forth allowing steam to enter (yellow) and exit (red) the cylinder from both directions, so providing power twice as much of the time. I've simplified the mechanism here so it's easy to understand. The valve actually slides from side to side rather than flipping over. The valve mechanism is contained in a smaller cylinder above the main piston cylinder.

Things are a bit more complicated than that in reality. In a typical steam engine, it works like this:

Detailed animation of a double-acting counterflow steam engine cylinder.

Animation: A more detailed look at how the cylinder valves work. In this animation, the cylinder and piston are shown in red (below) and the valves in blue (up above). Steam pushes the piston first from one direction, then from the other. The cylinder valves (blue) move back and forth allowing this to happen. After the steam enters the cylinder, the valves close the inlet and oulet ports so the steam can expand and push the piston, powering the train. Once the steam has fully expanded, it's pushed out through the chimney. Chuff chuff!

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If you look closely at the wheels of a typical steam engine, you'll see that everything is more complex than we've seen in the simple animation in the box up above: there's much more machinery than just a single crank and connecting rod. In fact, there's an intricate collection of shiny levers, sliding back and forth with meticulous precision. This is called the valve gear. Its job is to open and close the cylinder valves at just the right moments to let steam in from either end, both to make the engine work as efficiently and powerfully as possible and to allow it to drive in reverse. There are quite a few different types of valve gear; one of the most common designs is called the Walschaerts, named for its Belgian inventor Egide Walschaerts (1820–1901). The tank engine 80104 shown in the second photo on this page has a Walschaerts-type valve gear, and so does Eddystone, the locomotive pictured below.

Walschaerts valve gear on Rebuilt West Country Class 34028 Eddystone.

Photo: The Walschaerts valve gear on a typical large steam locomotive, 34028 Eddystone.

The first steam engines were very large and inefficient, which means it took huge amounts of coal to get them to do anything. Later engines produced steam at much higher pressure: the steam was produced in a smaller, much stronger boiler so it squeezed out with more force and blew the piston harder. The extra force of high-pressure steam engines allowed engineers to make them lighter and more compact, and it was this that paved the way for steam locomotives, steam ships, and steam cars.

Two views of a steam engine tender seen (above) from the side and (below) looking down from behind

Photo: Steam engines could not carry all the water they needed for a long journey. Periodically, they would have to stop to refill at track-side water tanks like this one (above) on the Swanage Railway. Larger engines had tenders: trucks they hauled behind that held supplies of coal (in front of the red line we've drawn) and water (behind the red line). The coal rests on an angled plate inside the tender that makes it tip naturally toward an opening at the front where the fireman can easily shovel it into the firebox. Below: You can see what the tender's like inside on this unusual photo of an empty tender, photographed from slightly above and behind, taken at Think Tank, the museum of science in Birmingham, England. This tender holds about 18000 liters (4000 UK gallons) of water and belongs to the museum's City of Birmingham locomotive.

Did steam really die?

Coal was a cheap and abundant fuel during the early Industrial Revolution, but the invention of the gasoline engine (petrol engine) in the mid-19th century heralded a new era: during the 20th century, oil overtook coal as the world's favorite fuel. Steam engines are extremely inefficient, wasting around 80–90 percent of all the energy they produce from coal. That means they have to burn enormous amounts of coal to produce useful amounts of power.

A steam engine is so inefficient because the fire that burns the coal is totally separate (and often some distance from) the cylinder that turns the heat energy in the steam into mechanical energy that powers the machine. This design is called an external combustion engine because the fire and boiler are outside the cylinder. It's inefficient because energy is wasted as the heat and steam travel from the fire, via the boiler, to the cylinder. Gasoline- and diesel-powered engines are based on a totally different design called an internal combustion engine. The gasoline or diesel fuel is burned inside the cylinder, not outside it, and this makes internal combustion engines considerably more efficient. (You can read more about internal and external combustion in our overview of engines.) Oil has many other advantages too: it's cleaner than coal, makes less air pollution, and is much easier to transport in pipes.

That's largely why steam locomotives disappeared from our railroads—diesel locomotives were altogether more convenient. It takes hours to fire up a steam engine before you can use it; you can get a diesel engine running in less than a minute. Steam engines disappeared from factories when electricity became a more convenient way of powering buildings. Who wants to load coal into a factory every day when they can just flick on switches to make things work?

Chart showing how UK rail freight became more efficient in the 1960s, with just as much freight carried by less than half as many locomotives.

Artwork: Less is more: The UK switched from steam engines to diesel and electric during the 1960s. The last engines were built there in 1956 and the very last steam train ran in August 1968. By 1968, there were only about a third as many locomotives in service as there had been in 1962, but just as much freight was being hauled: the diesel-electric rail system was apparently much more efficient. Source: Drawn using data from "The Performance of British Railways 1962–1968" by C.D.Jones, Journal of Transport Economics and Policy, Vol. 4, No. 2 (May, 1970), pp. 162–170.

But things are not quite what they seem. Steam and coal never did disappear—not exactly. Where does the electricity we use come from? It would be great if it all came from renewable energy (wind turbines, solar panels, and so on), but much of it still comes from coal, burned in power plants miles away from our homes and factories. Inside a coal-fired power plant, coal is still burned to make steam, driving windmill-like devices called steam turbines, which are much more efficient than steam engines. As they rotate, they turn electromagnetic generators and produce electricity. So, you see, although steam locomotives have vanished from our railways, steam power is alive and well—and just as important as it ever was!

The steam engine Manston with tender

Photo: Some of the steam engines that run on heritage lines were still relatively new when they were withdrawn from service. This one, Bulleid Pacific No. 34070 "Manston," was built in 1947 and withdrawn less than 20 years later (in 1964). After a long restoration by Southern Locomotives, it returned to service on the Swanage Railway in September 2008. A wonderfully impressive sight, it weighs 128 tons and can reach speeds of over 160km/h (100mph).

Who invented the steam engine... and when?

Here's a brief history of steam power:

PS Waverley steam ship pulling into Swanage Pier, September 2009

Photo: Think of steam engines and you probably think of steam locomotives, but ships were steam powered too before diesel engines came along. This one is the beautifully restored PS Waverley, the last ocean-going paddle steamer in the world, dating from 1947 and steaming into Swanage Pier in September 2009.

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