After two successful runs with steam engines, I decided to try my hand at a stirling. I found a set of plans for sale on line here and bought a set. This is my variation on his plans. I opted for the much simpler to construct, but more difficult to align, single support. I moved from a 90 degree offset between the displacer and piston to a set of offset cranks. I also changed the way the lid went on the can so that it was much easier to fabricate. The displacer is made of balsa wood. Although no mention of a counter balance appeared in the plans I received, I found that the addition of a counter balance for the displacer aided the running of the engine. I used paperclips for the connecting rods on the piston and displacer. This affords a unique way to get the lengths just right. If the rod is too long, it can be bent into a zig-zag to reduce the effective length. Forming the ends of the connecting rods is easy. Select a drill bit of the same diameter as the rod you made the crank out of. Now just wrap the paperclip around it a few times. Trim off the end and you are done. You can use a second drill bit to twist the connecting rod to make sure the two "rod ends" are parallel.
I have run this engine off a variety of sources including the burner of my
stove set on low, a blowtorch, a heat gun, and two small candles. It is not
very efficient, or a strong runner, but it does run reliably and for several
minutes before it slows. The biggest problem with this engine is that is is
all metal. The metal can provides a thermal "short circuit" from the hot side
to the cold side. Not long after starting, both the hot and cold sides begin
to reach the same temperature and the engine stops. The supports, guides,
cylinders, and bearings are all soft soldered together and to the top of the
can. Getting the alignment right and soldering everything in place without
disturbing something else can be a bit of a trick, but it can be done. Since
the cylinder is out over the rippley part of the lid, I spent a little time
filing it to fit the contours of the can before soldering. The piston is a
section of 1/2 inch diameter aluminum rod that was faced, beveled, and bored
out to 3/8 inch. It was cross drilled for a 1/16 inch "wrist pin".
The displacer (of which I have no picture) is a hollow circular box made of
balsa wood. It is about 1/8 inch diameter smaller than the can I used for
the hot side of the engine. The vertical dimension of the displacer was
selected to fill the can less the displacer stroke. This assures that the
displacer travels all the way to the top and bottom of the can. If the
displacer does not utilize the entire space in the can, the engines
performance will suffer.
I now have two movies showing this engine running. They are both fairly large (1.2Mb and 4.5Mb) so please be patient.
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