Steam engines were the first engine type to see widespread use. They were first invented by Thomas Newcomen in 1705. James Watt (whom we remember each time we talk about "60 watt light bulbs") made big improvements to steam engines in 1769. Steam engines powered all early locomotives, steam boats and factories, and therefore acted as the foundation of the industrial revolution. They work with some components not dissimilar to those in a two stroke or four stroke engine. The main difference is the power is not derived from combusting a mixture of fuel and air, but instead it comes from the burning of coal, or meths, or anything really, to boil water and produce steam. The water is boiled in a contained space, (the boiler), this containment means that you get very high pressure steam, and it is this steam that does the work in moving a piston up and down a cylinder. First we will discuss the piston and liner arrangement, and then the boilers.

Steam engines are in the class of external combustion engines, unlike two and four stroke engines which are internal combustion.

Piston and Cylinder.

The following diagram shows the major components of a piston steam engine.

The engine shown is a double-acting steam engine. So called because the valve allows high-pressure steam to act alternately on both faces of the piston. Single-action engines do also exist, the steam only moves the piston in one direction, they are more similar to two and four stroke engines in this respect. A weighted crankshaft must be used to return the piston to its starting position. Double action engines are as you may imagine much more powerful than single action ones. The following animation shows a double action engine in action. Once you understand this it is easy to imagine how a single action engine would work. Neither of the diagrams shows a crankshaft (that converts the linear motion into rotary motion), as we have already covered this in our explanation of four stroke engines.

You can see that the slide valve is in charge of letting the high-pressure steam into either side of the cylinder. The control rod for the valve is usually hooked into a linkage attached to the piston road, so that the motion of the piston rod slides the valve as well. This linkage also allows the user to put the engine into reverse.You can see in this diagram that the exhaust steam simply vents out into the air.

Boilers

The high-pressure steam for a steam engine comes from a boiler. The boiler's job is to apply heat to water to create steam. There are two approaches, called fire tube boilers and water tube boilers. A fire tube boiler was more common in the 1800s and is found in most locomotives. It consists of a tank of water perforated with pipes. The hot gases from a coal or wood fire run through the pipes to heat the water in the tank, as shown here:

In a fire tube boiler, the entire tank is under pressure, so if the tank bursts it creates a major explosion.

More common today are water tube boilers, in which water runs through a rack of tubes that are positioned in the hot gases from the fire. The following simplified diagram shows you a typical layout for a water tube boiler:

In a real boiler, things would be much more complicated because the goal of the boiler is to extract every possible bit of heat from the burning fuel to improve efficiency.

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