A Stirling (Hot Air) Engine


| September/October 1974



Six Connecting Rods

M. Andrew Ross

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.