How a Beam Engine Works — An Interactive Guide

This is a beam engine. It produced about fifteen horsepower continuously, roughly as much power as 150 people. Engines like this turned steam into the power that drove the Industrial Revolution. This article builds the engine up from first principles, using interactive figures to explore each idea (try rotating the engine above with two fingers, or pinching to zoom indragging the engine above, or zooming with ⌘/Ctrl + scroll). Let's start our journey through the engine with steam.

Steam

Below, we have a pot filled with water and a fire underneath. As the fire heats the water, some of it begins to boil and turns into steam.

Steam undergoes an amazing transformation: it expands to 1,700 times the volume of the original water. One cup of water becomes roughly 400 litres of steam, enough to fill two bathtubs. If the steam doesn't have enough room to expand it will push on all the walls of the container. This push on every wall is pressure, and we will measure it in atmospheres, multiples of the ordinary pressure of the air around us. The steam also presses on the surface of the water, which transmits the pressure evenly to everywhere the water touches.

In 1679, Denis Papin demonstrated a device he called a digester to the Royal Society. By trapping steam, it raised the boiling point high enough to cook beef bones soft. The early digesters had an unfortunate tendency to burst, so Papin fitted a weighted lever over a vent. When the pressure became too high, the steam lifted the weight and escaped, giving us the first steam safety valve.

Now we need a way to harness the properties of steam.

Pistons and cylinders

A piston is a round disc that fits snugly inside a cylinder. Steam pushes on one face of the piston and a rod transmits the force elsewhere. The force depends on two things: the pressure of the steam and the area of the piston. At a pressure difference of one atmosphere, each square centimetre of piston provides about one kilogram of force.

Early boiler builders didn't know how to safely harness high-pressure steam.1 Instead, to get more force they made the piston wider. Because area grows with the square of the diameter, doubling the width of a piston gives it four times the area and four times the force at the same pressure. This is why early steam engines had enormous cylinders, sometimes wide enough for a person to stand inside. In the figure below, the boiler pressure never changes; try increasing only the bore until the piston can lift the car.

With steam pushing on our piston, we can do real work. But low-pressure steam is not very strong. To move heavy machinery, engineers turned to a surprising source: the atmosphere.

The weight of air

Air feels weightless, but only because we are surrounded by it. Imagine a column of air one centimetre square, extending from your hand all the way to the top of the atmosphere. That column weighs about one kilogram, so the atmosphere presses on every square centimetre with roughly one kilogram of force.

We do not feel this enormous pressure because the air and fluid inside us push back at the same pressure. But if the pressure falls on one side of a surface, the pressure on the other side remains. This is what happens when you drink through a straw. Your mouth lowers the pressure inside the straw, and the atmosphere pushing on the drink in the cup forces it upward.

Italian well-diggers knew that a suction pump could not lift water more than about ten metres, no matter how hard they worked the handle. In 1643, Evangelista Torricelli realized that the pump was not pulling the water upward. The atmosphere was pushing it, and ten metres was simply the tallest column of water it could support. He repeated the experiment with mercury, which is fourteen times denser, and the column fell to 76 centimetres. This became the first barometer, with a permanent vacuum above the mercury.

Otto von Guericke gave a spectacular demonstration of this effect in 1654. He joined two copper hemispheres into a sphere about half a metre across and pumped out the air. To the amazement of the observers, teams of horses could not pull the halves apart. The atmosphere was clamping them together with about two tonnes of force! As soon as he opened a valve and let the air back in, they came apart by hand.

Creating a vacuum was extremely difficult at first. Guericke had to laboriously pump the air out of his sphere, but steam gives us a much faster way to make one. If we fill a vessel with steam and then cool it with a spray of water, the steam condenses back into roughly 1/1,700 of its volume.

Fill a cylinder with steam, condense it underneath a piston, and the atmosphere will drive the piston down into the vacuum. A near-perfect vacuum gives us the same pressure difference we used earlier: about one kilogram of force for every square centimetre of piston. A piston half a metre across could collect almost two tonnes of force from...