Steam engines were the first engine type to see widespread
use. They were first invented by Thomas Newcomen in 1705, and
James Watt (who we remember each time we talk about "60-watt
bulbs" and the such) 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. In this edition of HowStuffWorks,
we will see exactly how steam engines work!
Steam Engine Operation The following diagram
shows the major components of a piston steam engine.
This sort of engine would be typical in a steam locomotive.
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:
It explains why they have to take on water at the
station -- the water is constantly being lost through the
It explains where the "choo-choo" sound comes from. When
the valve opens the cylinder to release its steam exhaust,
the steam escapes under a great deal of pressure and makes a
"choo!" sound as it exits. When the train is first starting,
the piston is moving very slowly, but then as the train
starts rolling the piston gains speed. The effect of this is
the "Choo..... choo.... choo... choo choo-choo-choo" that we
hear when it starts moving.
On a steam locomotive, the cross-head normally links to a
drive rod, and from there to coupling rods that
drive the locomotive's wheels. The arrangement often looks
something like this:
In this diagram, the cross-head is connected to a drive rod
that connects to one of three drive wheels for the train. The
three wheels are connected via coupling rods so they turn in
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:
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.
For more information on steam engines and all sorts of
other engines, check out the links on the next page.