You are here: Home page > Engineering > Steam engines
Advertisement

Restored steam locomotive engine in Swanage

Steam engines

by Chris Woodford. Last updated: July 10, 2013.

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 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).

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. 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.

Photo showing the main component parts of a steam engine

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!

Photo: The main parts of a steam locomotive. Click the small photo to see a much bigger one. This is ex-British Railways Standard 4MT locomotive number 80104 (built at Brighton in 1955) working on the Swanage Railway, England in August 2008.

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.

Step-by-step

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.

The parts of a steam engine

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 red 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.

Artwork: How a steam engine works. We also have a slightly bigger version of this animation (it makes the page load very slowly if we include it here).

Types of steam engine

Photo: Close-up of the piston and cylinder in a steam engine.

Steam engine piston and cylinder

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

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

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 (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?

The Class 55 (Deltic) diesel locomotive Royal Scots Grey

Photo: Steam engines were gradually replaced by diesels. It's ironic that some of the older diesels are now being preserved on heritage lines and treated with almost as much reverence as steam engines. This one is a preserved British Rail Class 55 ("Deltic"), number 55022, called Royal Scots Grey dating from 1960. So it's actually older than some of the very last steam trains that were made!

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?

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). Read more about Manston's restoration and return to service.

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.

Find out more

On this website

On other websites

Videos

Books

How-it-works

History (for older readers)

History (for younger readers)

Sponsored links

Please do NOT copy our articles onto blogs and other websites

Text copyright © Chris Woodford 2007, 2011. All rights reserved. Full copyright notice and terms of use.

Follow us

Rate this page

Please rate or give feedback on this page and I will make a donation to WaterAid.

Share this page

Press CTRL + D to bookmark this page for later or tell your friends about it with:

Cite this page

Woodford, Chris. (2007) Steam Engines. Retrieved from http://www.explainthatstuff.com/steamengines.html. [Accessed (Insert date here)]

More to explore on our website...

Back to top