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Note |
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The animated illustration of a steam engine in this article is a simplified
schematic illustration rather than a true-to-life one. The emphasis is on the
basic principles of operation. In particular, the valve mechanism would be a lot
more complicated in a real steam locomotive. |
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Firebox
and Boiler |
 The boiler is actually a large water tank (see the picture on the
left) made of steel
containing a group of internal steel pipes called boiler tubes. Hot gasses from
the coal
fire in the firebox pass through the tubes heating the water in the boiler
which generates steam. Because the boiler is sealed the confined steam
attains a high pressure. The steam passes from the top of the boiler to the
cylinders through steam pipes - coloured red in the animated illustration.
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Valves and Valve Chamber |
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The valves, as shown in the animated illustration, are driven by movements of
the piston. They direct high pressure steam to the appropriate side of the
cylinder and permit exhaust (spent steam) to escape from the opposite side of
the cylinder. Note that the valves are left in one position while one side of
the cylinder fills and the other side empties. |
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The piston and cylinder
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The piston and cylinder are where thermal energy contained in the
high pressure steam is converted into the mechanical energy that drives the
locomotive forward causing it to pull the train of cars behind it. As the high
pressure steam drives the piston, the volume of the cylinder increases and the
pressure and temperature of the steam drops. However, even when the volume in
the cylinder has reached its maximum, there is still some residual pressure in
the cylinder - enough to cause a rush of exhaust steam out of the exhaust
pipe (shown in blue) when the exhaust valve is opened.
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Exhaust Vent and Chimney
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When smoke rises in a factory chimney it does so because it is hotter and
lighter than the surrounding air. This is so even though its constituents (hot air with added carbon
dioxide) would normally have a density (mass per unit volume) similar to that of
ordinary air. However, the smoke is a hot gas and as a gas is heated it expands
and its density decreases making it lighter. This is what causes the draft in a
factory furnace that draws in fresh air to the furnace to make its fuel burn
faster. |
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Now, on a locomotive, a tall chimney is not very practical! So an alternative
means of creating a draft had to be found. The solution was to use the rush of
exhaust steam from the cylinders to create a draft. How this is achieved is
shown in the animated illustration. The flow of exhaust steam through the
exhaust vent results in a partial vacuum in the chimney that draws air through
the firebox and boiler tubes. The bursts of exhaust steam from the
cylinders result in the familiar "chuff, chuff" of the steam
locomotive. |
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Other Points |
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Because the water is constantly lost as steam it must be replaced by water
pumped into the boiler from a supply tank in the tender car located behind the
locomotive. Coal is also carried in the tender car and has to be shovelled by
hand into the firebox. In addition to making stops at depots for more coal, the
water also has to be replenished. |
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On the opposite side of the locomotive is another power unit (cylinder and
piston, etc) just like the one in the illustration. But that unit is phased so
that its piston is at its extreme position when the piston on this side is at
its mid position. This results in a more even delivery of motive power. It also
means that the bursts of exhaust steam into the chimney (and the resulting
chuffs) will actually be twice as frequent as shown (and heard) in the animated
illustration. |
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