A Stirling (Hot Air) Engine

By Staff
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Courtesy of M. Andrew Ross, 37 W. Broad St. #630, Columbus, Ohio 43215
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Courtesy of M. Andrew Ross, 37 W. Broad St. #630, Columbus, Ohio 43215
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Courtesy of M. Andrew Ross, 37 W. Broad St. #630, Columbus, Ohio 43215

37 W. Broad St. #630 Columbus, Ohio 43215

I first became interested in Stirling engines in 1956, when I
was in high school in Baxley, Ohio. I saw an article in Popular
Science describing General Motor’s Stirling engine with the
novel rhombic drive.

My interest revived several years after graduating from law
school, when I bought (after a great deal of indecision) a 10′
South Bend lathe. At last I was able to build engines of my own
design.

In a local library I found several excellent articles on
Stirling engine design in Philips Technical Review (Vol. 9 pp
97-104 pp 125-134, Vo. 20 pp 245-262). Philips, the Dutch
electrical company, had invented the rhombic drive, and had built
the first modern hot air engines. Their articles gave me the
incentive and background to build my own hot air engine.

As most readers know, a simple hot air engine generally has two
pistons. One is the displacer piston, which serves only to move the
working air inside the engine from the hot space to the cold space,
and vice versa. The other is the power piston, which is acted upon
by the pressure differences caused in turn by working air being
heated and cooled. These two pistons move with a phase angle
difference of about 90 deg.

Top left view shows the crankshafts and the six connecting rods.
The crankshafts are steel and the con rods are 2024 aluminum alloy.
Top right shows the displacer cylinder, water-jacketed, with cooler
holes drilled around it. Bottom left is the displacer piston,
disassembled. I have since learned that epoxy could be used to join
these parts, and thus make for a much simpler design. Bottom right
shows the piston and its rings.

There are a number of ways to drive both pistons from a
crankshaft, but I chose the Philips rhombic drive because it allows
an engine to be in perfect mechanical balance. On the other hand,
it also requires two crankshafts, geared together, and four, or
preferably six, connecting rods!

After deciding on the proper crankshaft geometry, I hastily drew
up plans, bought the necessary bar stock and tubing, and began to
machine my engine. What with various design changes along the way
(I usually think of the easiest way to machine a part after I have
halfway finished making it the hard way!), it took about one year
to complete the engine. Unfortunately, it didn’t run. You can
imagine my utter disappointment when after spending hundreds of
hours making a ‘Modern’ hot air engine, I discovered it
wouldn’t run. In my basement I had six antique engines that
were inefficient and in some ways crude, but they all ran!

Oh well! I completely redesigned my engine, and after another
year it was once again complete. This time I was downright scared
when I first lit my propane torch to heat the shiny new hot cap. I
almost didn’t want to look, as I reached over to turn the
flywheel. But I did. And to my surprise the engine immediately
began to turn under its own power. It gathered more and more speed,
and I was as happy as I’ve ever been in my life. Of course,
I’d obtained some good advice, not only from the Philips
articles, but also from a world famous Stirling cycle engineer; but
nevertheless, the engine was almost entirely my own work. And it
ran! To sit and watch this engine quietly convert the open flames
of the burner into mechanical power, with almost no vibration, is
one of the most delightful things I can imagine!

At left the photo clearly shows the displacer piston [top] and
the power piston [beneath it]. At right is the view of the engine,
minus only the burner and back plate, shows the rhombic drive with
the balance weights in place. The engine runs with almost no
vibration.

View of the finished engine running on its test stand. [And
I’ll bet Andy had a satisfied smile on his face – Anna
Mae].

Subsequently, I built a small prony brake to test the output. At
first it only produced 1.5 watts of power, but after various minor
changes it now produces 7 watts at 1100 rpm at atmospheric
pressure, and 9 watts at 720 rpm when pressurized at 15 psi. The
engine will run at 1800 rpm unloaded at 15 psi, and 1500 rpm
unloaded at atmospheric pressure.

Some details of the engine are as follows: Bore is 2.125′,
stroke is 1.125′. I made all the parts except rings, bearings,
gears and the hot cylinder and displacer caps. The rings are from
McCulloch go kart engines (they have extremely low friction, which
is essential in hot air engines). The caps were pressed from 321
stainless steel by a firm in New Jersey. I have extras, if any
potential hot air engine builders are interested.

The engine presently burns propane, but could as well use any
heat source that is hot enough. The only noise it makes while
running is a slight gear noise, and a better quality gear (or nylon
gears); would cut that down considerably. I hope to display this
engine in at least one steam show this year, probably the one at
London, Ohio.

Much testing remains to be done. Eventually I will try much
higher internal pressures. I hope to get 30 to 50 watts of power
out of this design sooner or later. In the meantime?. . .1 am
already designing a hot air engine of 200 watts output (quarter
horsepower)!

I hope other readers will get inspired to build hot air engines
of their own designs. Nothing could be more enjoyable.

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