Hot Air Engines

| September/October 1992

Hot air engines

17333 34th St. Seattle, Washington 98188

On November 16, 1816, Robert Stirling was granted patent #4081 for an engine that ran on a column of air that was continuously displaced to hot and cold ends of a cylinder. From that time on, the hot air engine served as a stationary engine. It could not compete with the more powerful steam engine and soon became obsolete for industrial use. In more modern times, one of the more popular uses for small engines was to power home cooling fans in areas with no electricity.

Hot air engines are currently enjoying a renewed popularity, mainly as models. This is partially due to the many possible variations in construction. There are several commercially produced models on the market, either in kit form or completely assembled and ready to run. I have seen several models, well constructed in home workshops, that would rival many commercial models. Of these, there are many air cooled and water cooled variations. They can be heated by direct flame, flame injection, sunlight, and various other means.

Although my design does not equal many of the well-machined units in appearance, it does rival them in performance, material cost, and ease of construction. I chose this design because no special tools or materials are needed. The cylinder and piston are made from a shock absorber. The water jacket, heat jacket, displacer cylinder, and displacer piston are all tin cans. The water level gauge and the oiler are constructed of copper fittings and glass vials. Everything else is basically welding rod and nuts and bolts. The flywheel, supports, and frame can be of any convenient design and material. These engines run quietly at about 200 r.p.m. with a Sterno can or alcohol burner for heat.

I've constructed four of these models and I call them 'Rainy Day Numbers 1, 2, 3, and 4.' I chose that name because, with all of the materials prepared, this model is a satisfying rainy day project.