Machine
for converting heat energy into mechanical energy using steam as a medium,
or working
fluid.
When water is converted into steam it expands, its volume increasing about
1,600 times.
The force
produced by the conversion is the basis of all steam engines. Steam engines
operate
by having
superheated steam force a piston to reciprocate, or move back and forth,
in a
cylinder.
The piston is attached by a connecting rod to a crankshaft that converts
the
back-and-forth
motion of the piston to rotary motion for driving machinery. A flywheel
attached
to the
crankshaft makes the rotary motion smooth and steady. The typical steam
engine has an
inlet
valve at each end of the cylinder. Steam is admitted through one inlet
valve, forcing the
piston
to move to the other end of the cylinder. This steam then exits through
an exhaust valve.
Steam
from the other inlet valve then pushes the piston back to its original
position, and the
cycle
starts again. In a single-cylinder steam engine the exhaust steam is usually
expelled directly
into the
atmosphere. A compounded steam engine has several cylinders, which the
steam
passes
through successively until, leaving the last cylinder, it is condensed
into water and
returned
to the boiler. From the Greek inventor Heron of Alexandria to the Englishmen
Thomas
Newcomen
and John Cawley, many persons contributed to the work of harnessing steam.
However,
James Watt’s steam engine, patented in 1769, provided the first
practical solution.
Earlier
engines depended on atmospheric pressure to push the piston into the cylinder,
where a
vacuum
was created by sudden cooling of its steam content. Watt’s use of a separate
condenser
resulted in a 75% saving in fuel. It also made possible the use of steam
pressure to
move the
piston in both directions. Watt’s continuing efforts produced a governor,
a mercury
steam
gauge, and a crank-flywheel mechanism, all of which prepared the steam
engine for a
major
role in the Industrial Revolution. Sailing vessels gave way to steamboats,
and
stagecoaches
yielded to railroad trains as the steam engine was perfected. Transmitted
by belts,
ropes,
shafts, pulleys, and gears, the energy from steam engines drove machines
in factories and
mills.
Now, however, steam engines have been replaced in most applications by
more
economical
and efficient devices, e.g., the steam turbine, the electric motor, and
the
internal-combustion
engine, including the diesel engine. They are still sufficiently economical
to
be used
in industries where steam is necessary for some purpose in addition to
that of driving an
engine.