| May/June 1975

Box 246, Elfers, Florida 33531

The sketch of this tiny vacuum engine may be of interest to some of your readers. It is perhaps the simplest engine to build, having only one cylinder and one slide valve in contrast to a hot air engine with its displacer cylinder, etc. I have a hot air engine of 1 inch bore, the vacuum engine is also 1 inch-one would expect the hot air engine to have the most power since there is a push from the expansion of the hot air as well as the vacuum, but strange to say the vacuum engine has just as much 'power' although operating on vacuum only. It is fascinating to watch as it pops at every revolution, sounding like a little gas engine.

It is truly a mystery engine although the principle is that the hot gases are admitted to the cylinder through an open port on the outward stroke which is closed by the slide valve at about 95 degrees when the hot gases are supposed to cool, and create a partial vacuum pulling the piston back on the return of 'power' stroke. The mystery is that the hotter the cylinder, the faster the engine runs! In fact, if one wants to get a quick start, the cylinder has to be heated. It is also strange that the transition from hot to cold (in a hot cylinder) can be rapid enough to cause the engine to run at a high rate of speed, but it does!