A LEGO Technic ``Steam'' Engine

Quick-Time movie of the engine at work

Complete building instructions by Dr. C. S. Soh

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 in the late 18th century. 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.

The three figures of historic steam engines and engine parts which illustrate this page are from D.H. Thurston, A History of the Growth of the Steam-Engine (Appleton, New York, 1878). They show Boulton and Watt's double-action steam engine from 1784 (left), Stephenson's slider-valve mechanism from 1833 (top right), and Watt's centrifugal governor (bottom right). Thurston's book is made available on the Web by the Steam Engine Library project of the University of Rochester Department of History.

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://www.fifth-r.com/cssoh1/contents.htm
Joe Nagata: http://homepage3.nifty.com/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.

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