The Beam Engine

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 in dragging the engine above, or zooming with ⌘/Ctrl + scroll ). Let's start our journey through the engine with 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.

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

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.

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.

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 the atmosphere.

In the early 1700s, mines were getting deeper, and flooding was becoming a huge problem. Once a shaft reached below the water table, water seeped in continuously and had to be pumped out day and night. The pumps were driven by teams of horses walking in circles. As one team tired, another took over, but the deepest mines still flooded during wet weather and valuable coal had to be abandoned. A new solution was needed, and steam would provide the answer.

Thomas Newcomen supplied tools to the mines and knew that flooding was both a huge problem and an opportunity. He spent years turning the vacuum piston stroke into an engine that could run all day. He connected the piston to one end of a huge rocking beam and hung heavy pump rods from the other. The atmosphere drove the piston down and lifted the pump rods; their weight then pulled the piston back up while the cylinder filled with steam again.

Newcomen's first successful engine was installed at a coal mine near Dudley in 1712. It ran at about twelve strokes per minute, lifting roughly forty-five litres of water fifty metres on every stroke. Unlike the horses, it could continue around the clock without food or rest. Similar engines soon appeared in mines from Cornwall to Newcastle. 3

Newcomen's engine worked! But it used an extraordinary amount of coal. The cold water sprayed directly into the cylinder, chilling a huge mass of iron along with the steam. Roughly three quarters of the steam was wasted heating the cylinder back up on every stroke.

The mines were happy with this tradeoff because they burned slack , small pieces of coal that were considered waste. Anywhere else, the fuel cost was simply too much. This kept the steam engine stuck in coal mines for the next fifty years.

Why did Newcomen use the atmosphere to push the piston instead of the steam itself? His boiler was simply not strong enough. The haystack boiler produced only about a twentieth of an atmosphere above the surrounding air. It was built from thin copper or iron plates joined with rivets, and the wide walls and weak seams could not safely hold much pressure.

James Watt , who we will meet in the next section, used the waggon boiler shown below. Water sat in the broad chamber above the furnace, the hot gases passed underneath, and steam collected beneath the rounded roof.

The broad bottom was good at catching heat, but the waggon shape was terrible at holding pressure. Raise the steam pressure in the figure below and compare what happens to the rounded roof, the flat sides and the inward-curved bottom.

The figure also shows why later builders curved the whole boiler outward like the roof. They rolled iron plate into long cylinders, removing the flat sides and inward-curved bottom. They kept the boilers narrow because making a cylinder wider increases the force trying to split it open, even when the pressure stays the same. 4 Better iron and riveting then made much higher pressures possible, and around 1800 Richard Trevithick was running engines at several atmospheres.

Now Newcomen's use of a vacuum makes sense. His boiler could push with perhaps fifty grams per square centimetre above atmospheric pressure. By condensing the steam and letting the atmosphere push the piston instead, he got close to one kilogram per square centimetre, around twenty times as much force from the same boiler.

In 1765, Watt was repairing a model Newcomen engine at the University of Glasgow. He was amazed by how much steam it consumed and began trying to understand where it all went. He discussed the problem with his colleague Joseph Black , who was studying the heat absorbed while water boils. Black called it latent heat . For a kilogram of water, boiling it away takes more than five times as much energy as heating it from freezing to boiling.

With this knowledge, Watt calculated the exact amount of water needed to condense the volume of steam in the cylinder. He was surprised to find that this exact amount barely made a vacuum at all: the condensing steam dumped its latent heat into the spray, warming the water until it stopped condensing anything. Adding in more cold water just cooled the cylinder down more, wasting steam to heat the cylinder back up on the next stroke. Watt's brilliant insight was to add a second vessel that could stay cold while the cylinder stayed hot. 5

At the end of the stroke, a valve opened and the steam rushed into the cold vessel, called the condenser . As the steam turned back into water, the pressure fell in the condenser and, through the connecting pipe, in the cylinder as well. A small air pump driven by the engine drew out the condensed water, along with any air that had leaked in, on every stroke. Keeping the cylinder hot and the condenser cold cut coal consumption by about two thirds! Watt and his business partner Matthew Boulton turned the saving into a business model, charging customers one third of the money they saved on coal.

Better tools for making precise cylinders allowed Watt to make another important change: he closed the top of the cylinder and used steam on both sides of the piston. Steam pushed down while the condenser lowered the pressure below; on the return stroke, the same thing happened in the opposite direction. This was the double-acting engine. Below, we can compare it with the single-acting cylinder it replaced.

The same cylinder now produced power on both strokes, and the steady push-pull made the engine much better suited to driving machinery. But getting steam in and out of the cylinder was now more complicated. One end had to connect to the boiler while the other connected to the exhaust, and then the two connections had to switch before the piston returned.

Early steam engines used several separate valves and linkages to route the steam. Our engine does all of this with one slide valve . It moves only a few centimetres, connecting one end of the cylinder to fresh steam and the other to the exhaust. As the piston reaches the end of its stroke, the valve slides across and swaps the two connections.

The valve sits inside the steam chest , an iron box bolted to the side of the cylinder and kept full of fresh steam. Three ports open into the chest. The two outer ports connect to the ends of the cylinder, while the middle one carries away the exhaust. The valve is shaped like a wide, hollow D. One edge uncovers a cylinder port and lets fresh steam enter, while the hollow back joins the other cylinder port to the exhaust.

The valve needs to move in perfect synchronization with the piston, or the engine will not work. This motion comes from an eccentric on the engine's rotating shaft. The eccentric is a circular disc mounted slightly off-centre, so its centre travels in a small circle as the shaft turns. A strap around the disc follows this motion and drives the valve rod back and forth. Its position on the shaft is chosen so the next steam port begins opening before the piston reaches the end of its stroke.

Now, we can see how the piston, valve gear and eccentric work on our beam engine.

We can save a surprising amount of coal by closing the steam port before the piston reaches the end of its stroke. The trapped steam continues to expand and push the piston, although its pressure falls as the volume grows. Closing the valve at halfway, called cutoff , uses half as much steam while still producing about 85 percent of the ideal work. 6 Watt patented this idea in 1782. Later compound engines sent the exhaust from one cylinder into a larger cylinder, then sometimes into a third, extracting more work as the steam expanded.

Everything we have just discussed happens inside an opaque cylinder. In 1796, Watt's assistant John Southern built an instrument that let them see inside. A small spring-loaded piston moved a pencil up and down with the pressure, while a card moved sideways with the main piston. The resulting indicator diagram showed the pressure through the entire stroke