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 worlds 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, aeroplanes, 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 small, newly rebuilt steam locomotive working on the Swanage Railway, England, in 2007. Great Western Railway 0-6-2 Tank 6695 was rescued from a scrapyard in 1979 and took 26 years to restore to full working order at a cost of £200,000 (approx US$400,000).

Last updated: January 4, 2008.

What is a steam engine?

It takes energy to do absolutely anything you can think of—to ride on a skateboard, to fly on an aeroplane, 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. A few decades ago, coal was the world's favourite 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. 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.

A steam engine is a machine that burns coal to release the heat energy it contains. 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.

It's easiest to see how all this works on our little side-on diagram 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.

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, which heats the water in the boiler tank as it passes through. This arrangement of boiler tubes, as they are called, means the engine's fire can heat the water more quickly, 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.)

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 a valve (shown in green) 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 crankshaft and connecting rod (5).

As the piston pushes, the crankshaft and connecting rod turn the locomotive's wheels (6) and power the train along. 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 crankshaft 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 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.

You can see the whole thing in action by watching this animated graphic of a steam engine.

Types of steam engine

Our diagram 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.

The 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 much more powerful because steam is driving the piston all the time.

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.

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 on the Swanage Railway.

Did steam really die?

"Food" for the journey. The coal supply in this compact steam engine sits in a hopper just behind the cab.

Coal was a cheap and abundant fuel during the early Industrial Revolution, but the invention of the petrol (gasoline) engine in the mid-19th century heralded a new era: during the 20th century, oil overtook coal as the world's favourite 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. Petrol/gasoline engines are a totally different design called an internal combustion engine. The petrol/gasoline is burned inside the cylinder, not separate from it, and this makes them considerably more efficient. 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 engines disappeared from powering locomotives. They 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?

But things are not quite what they seem. Steam and coal never did disappear—not exactly. Where does the electricity we use come from? Well, a great deal of it still comes from coal, burned in power plants miles away from our homes and factories. Inside a coal-fired power plant, giant, efficient, steam engines burn coal to make steam, which drives windmill-like devices called steam turbines. As they rotate, they turn electromagnetic generators and produce electricity. So, you see, although steam locomotives have vanished from our railways, steam engines are alive and well—and just as important as they ever were!

A brief history of steam engines

• 1st century CE: Hero of Alexandria demonstrates a steam-powered spinning sphere called an aeolipile.
• 16th century CE: Italian architect Giovanni Branca (1571-1640) uses a steam jet to rotate the blades of a small wheel, anticipating the steam turbine developed by Sir Charles Parsons in 1884.
• 1680: Dutch physicist Christiaan Huygens (1629-1693) makes the first piston engine using a simple cylinder and piston powered by exploding gunpowder. Huygens' assistant Denis Papin (1648-c.1712) realizes steam is a better way to drive a cylinder and piston.

• 1698: Thomas Savery (c.1650-1715) develops a steam-powered water pump called the Miner's Friend. It's a simple reciprocating steam engine (or beam engine) for pumping water from mines.
• 1712: Englishman Thomas Newcomen (1663-1729) develops a much better design of steam-powered, water-pumping engine than Savery and is usually credited with inventing the steam engine. A Scottish engineer named James Watt (1736-1819) figures out a much more efficient way of making power from steam after improving a model of the Newcomen engine. Watt's improvements of Newcomen's engine lead to the widespread adoption of steam.
• 1770: French army officer Nicolas-Joseph Cugnot (1725-1804) invents a steam-powered, three-wheeled tractor.
• 1797: English mining engineer Richard Trevithick (1771-1833) develops a high-pressure steam version of Watt's engine, paving the way for steam locomotives.
• 1803: English engineer Arthur Woolf (1776-1837) makes a steam engine with more than one cylinder.
• 1804: American industrialist Oliver Evans (1775-1819) invents a steam-powered passenger vehicle. Like Trevithick, he recognizes the importance of high-pressure steam and builds more than 50 steam-powered vehicles.
• 1807: American engineer Robert Fulton (1765-1815) runs the first steamboat service along the Hudson River.
• 1819: Steam-powered ocean ship Savannah crosses the Atlantic from New York to Liverpool in only 27 days.
• 1882: The prolific American inventor Thomas Edison (1847-1931) opens the world's first commercial power plant at Pearl Street, New York. It uses high-speed steam engines to power the electricity generators.
• 1884: English engineer Sir Charles Parsons (1854-1931) develops the steam turbine for his high-speed steam boat Turbinia.

Further Reading

Books you can read

• Collier, James Lincoln. Steam Engines: Great Inventions. New York: Marshall Cavendish/Benchmark Books 2005.
• Whiting, Jim. James Watt & The Steam Engine. Hockessin, DE: Mitchell Lane., 2006.
• Woodford, Chris. Energy and Electricity. New York: Facts on File, 2004.

Favourite websites

• Steam engine: A very good Wikipedia article with some excellent historical illustrations.
• How steam engines work: An alternative explanation from How Stuff Works website.
• Photo report from the footplate of locomotive 6695 at the Swanage Railway: What's it like to drive a steam engine? A behind-the-scenes look at the locomotive featured in our article, with dozens of great colour photos.
• Official handover of locomotive 6695 to the Swanage Railway on Sunday 5th March 2006: Read how the locomotive pictured in our photographs was rescued and returned to steam. Includes over 70 colour photos of the engine.