How Do Steam Engines Work?

Imagine a tin can with its top cut off. Inside the can, near the top, is a snug-fitting wooden disc. The tin is full of steam - there is no air in it, just steam. What would happen if the tin was cooled and the steam turned to water?

The wooden disc would be pulled down to the bottom of the tin. Why? Apart from a small drop of water, the tin now has nothing in it. The vacuum pulls the disc down. Or more accurately, the air pressure on top of the disc pushes the disc down to the bottom of the tin.

And that's it, that's a steam engine.

In 1712 Thomas Newcomen built just such an engine. His wooden disc was attached to the bottom end of a rod. The top end of the rod was attached to the end of a hinged beam. When the disc was pulled down into the cylinder it pulled the end of the hinged beam down, making the other end of the beam rise. And to that end of the beam was attached a very long rod which went all the way down a mine: when that rod moved up it operated a pump which pumped water out of the mine.

But of course the cycle must be repeated over and over again. Once the water in the cylinder - our tin - has condensed, the cylinder must be refilled with steam, allowing the disc to rise, pulled up by the weight of the rod that hangs down the mine. Then the cylinder is cooled again, the steam condenses and off we go pumping another slug of water out of the mine.

This type of steam engine persisted for many years until a certain James Watt came along. He realised that cooling the cylinder to condense the steam meant the cylinder then had to be heated up again when fresh steam was let in. He devised a system that meant the cylinder did not have to be cooled down to condense the steam. Instead a separate, always-cold, chamber was added. To condense the steam in the hot cylinder, a valve was opened connecting the hot cylinder to the cold condensing chamber. On reaching the condenser the steam turned to water, quickly drawing all the steam out of the hot cylinder. Watt's engines were far more efficient - they used far less coal - and they quickly began to dominate the world of the steam engine.

But Newcomen and Watt's engines were the same in one respect. They both worked by atmospheric pressure pushing the disc - the piston - down into the evacuated cylinder.

People did think there was an alternative: how about steam under pressure being forced in when the piston was at the bottom of the cylinder so that the pressure of the steam pushed the piston up to the top of the cylinder? This was a great idea except for one thing: the materials of the mid 18th century were not terribly strong, and things tended to burst when filled with high pressure steam. Watt wouldn't touch it, saying it was far too dangerous. However, he owned patents that made atmospheric-pressure steam engines work efficiently, such as his patent on the separate condenser. High pressure steam engines don't need a Watt condenser...

But along came a chap called Richard Trevithick. Around 1800 he developed a working, high-pressure steam engine, building on the earlier work of William Murdoch and others. The great advantage of his engine was its size: it was far smaller, power-for-power, than a corresponding "atmospheric" steam engine. Small enough, in fact, to mount on a vehicle.

With improvements in materials, high pressure steam began to supersede older engines. Indeed, it was only twenty years or so after Trevithick's pioneering engine that Stevenson built Locomotion, the first engine to run on a passenger line. Though in 1828 Locomotion's boiler exploded, killing the driver. There was still a long way to go to perfect high pressure technology. That development continued throughout most of the 19th century.

High pressure, piston-in-cylinder steam technology reigned supreme until 1884. In that year Charles Parsons devised a very clever steam turbine, ushering in the modern era of steam. Think of a fan in a tube: force steam down the tube and the fan will spin. Parsons' turbine had many fans all attached to a central shaft to extract the maximum amount of energy from the steam. Between each of the fans were static blades attached to the inside of the tube rather than to the rotating shaft: these kept the steam moving straight along the tube, preventing the steam from starting to spin in tune with the spinning fans.

Think of a jet engine, the sort we have all seen on an aeroplane: imagine putting a huge air-blower in front of it and blowing air into the jet engine. The jet engine would start to turn: it's now acting as an air-driven turbine.

However, forcing steam down a tube full of fans puts pressure on the fans which the bearings have to absorb, and the bearings wear. Parsons' idea was to inject steam into the middle of the cylinder so that it flowed towards both ends through two sets of fans. That way the load is taken by the shaft on which the fans are mounted, stretching it as it were: there's no pressure on the bearings. Genius.

Parsons turbines power the world today. Gas, coal and nuclear power all do the same thing: they boil water and turn it into steam. The steam passes through a turbine, spinning it at great speed. The turbine is connected to an electricity generator. That electricity powers the modern world.

The steam age is still very much with us.