Richard Melson

July 2006

Steam Engines

The engine shown is a double-acting steam engine because the valve allows high-pressure steam to act alternately on both faces of the piston.

The following animation shows the engine in action:

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 cross-head, so that the motion of the cross-head slides the valve as well.

(On a steam locomotive, this linkage also allows the engineer to put the train into reverse.)

You can see in this diagram that the exhaust steam simply vents out into the air.

This fact explains two things about steam locomotives:


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: fire tube and water tube.

A fire-tube boiler was more common in the 1800s. 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.

Steam Engines

July 25, 2006