A Stirling (Hot Air) Engine

Six Connecting Rods

M. Andrew Ross

Content Tools

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.