This project arose from my thoughts about heat, which is a form of energy and the most common waste
product in the world. A differential engine, commonly called a Stirling engine, after Robert
Stirling, who invented the first practical model, is an engine which runs on heat, or more
accurately, runs on a differential of heat.
I made my Stirling engine from plans I found on the internet, two types of glue, and three or four
tin cans.
The Stirling engine is so simple that I have often asked myself why they are not in more widespread
use commercially. It works on the principle that air expands when it is heated, and contracts when
it is cooled.
Here is an animation I made to try to understand the Stirling engine.
Parts of a Stirling engine
Parts of a Stirling engine include the displacement piston, a large piston inside the body of the
engine, and the power piston, a small piston or diaphragm. Both pistons are offset 90 degrees from
each other on the crankshaft, which is connected to a flywheel. The displacement piston must be a
few millimeters smaller in diameter than the cylinder in which it sits, to allow air to pass around
it.
How a Stirling engine works
With the displacement piston at the top of the engine, a candle heats the air inside the cylinder,
causing the air to expand, pushing on the power piston. This in turn moves the displacement piston
to the bottom of the cylinder, displacing the hot air, which moves to the top of the cylinder, and
is cooled by the ice. The air contracts, pulling the power piston, pulling the crankshaft, pulling
the displacement piston to the top of the cylinder, moving the cold air to the bottom. And then the
process starts again.
Here is a video of my engine running on a few tea candles.
high precision engines do not need to be cooled with ice. Keep in mind that they run off of a
difference in temperature; one can also run them off of cold water on a warm day. Some are so
sensitive they will run on the heat from your hand. However I am very satisfied with the results I
got, considering it cost me ten dollars' worth of glue and an afternoon looking through people's
recycling bins.
Here is a video of a higher quality commercial Stirling engine frome the Grand Illusions Youtube channel.