A LEGO Technic ``Steam'' Engine
This double-action compressed-air engine, which is built with
standard LEGO Technic components, illustrates some simple
principles of thermodynamics and mechanics. And it was a lot of fun to build!
Compressed air from the
blue tank goes via the the gray hose through the valve and one of the two
blue hoses to the cylinder. As the piston reaches the
end of its stroke, the vertical pendulum hits the valve and shifts the
high-pressure air to the other side of the piston, driving it in the opposite
direction while the side which was previously connected to the tank is
opened to the outside for exhaust. This ``double-action'' was first
introduced in steam engines around 1800. The pendulum-activated valve
performs the same function as the slider valve in a real steam engine.
The rotating thing which looks like a centrifugal governor is really
just a decoration, so that the engine has something to drive and I could
build a simple gearbox.
The back-and-forth (reciprocating) motion of the piston is converted to
rotation with the ``walking-beam,'' rod, and crank mechanism. The
angular momentum stored in the large flywheel helps the engine past its
``dead points.'' The whole machine looks much like an early 19th-century
steam engine.
A physicist will notice one important difference between this machine
and a real steam engine: this engine is not a heat engine.
Instead of working between a hot and a cold reservoir (the boiler and
the condenser), it extracts mechanical work
by moving air from a high-pressure resservoir, the tank
(which is far too small to be an ideal reservoir), to a
low-pressure reservoir, the atmosphere.
This is the only single-cylinder LEGO compressed-air engine I am aware
of. Several beautiful designs with two or more cylinders
can be found at these sites:
Dr. C. S. Soh:
http://web.singnet.com.sg/~cssoh
and
Joe Nagata:
http://member.nifty.ne.jp/mindstorms/
The multicylinder designs do away with the need for the large flywheel
and walking beam, so these designs can be quite compact, compared to this one.
My own recent experiments with two-cylinder ``steam cars'' can be found
here.
You can find a bibliography of links to other Lego pneumatics sites at
http://web.singnet.com.sg/~cssoh/links.htm
Dr. Rikvold's home page