SO YOU WANT TO BUILD A HOT AIR ENGINE
By Brad E. Smith CORLISSBS@AOL.COM
Tips from a hot air engine builder)
The most common hot air pumping engine seen at engine shows is the Ericsson engine. It makes a very nice model because of its many moving parts and motion. Two very nice casting kits for this engine are produced by Meyers Model Engine Works, 15929 Five Point Road, Perrysburg, OH 43551. The larger of the two engines is a quarter scale model of an 8" engine. This is the size for most of us to build. A very nice 1/8 scale model is also offered, but because of it's small size, the parts become a little more difficult. The construction article for the larger engine was in LIVE STEAM in the '70s and reprinted in the book "Steam and Sterling". The engine is a very nice runner, if built correctly. It should run on very little heat. The following changes to the text in the book and article will greatly improve the engine's performance.
There are some secrets to the successful construction of a hot air engine. First there must be no binding. Everything must move freely. Second, the fit between the power piston and the power cylinder must be a very good fit. I recommend having this cylinder professionly honed to be within .0005" round and a taper within .0005". When you make the piston to fit the honed cylinder, turn it as close as you can and finish to diameter with emery cloth wrapped around a file. The fit has to be such that if you insert the piston in it's cylinder and seal the end of the cylinder with your hand, the piston will stay where it is.
Use "Oilite" bushings on all moving parts, both rotating bearings and sliding bearings. No matter what material the castings are made of, insert a replacable "Oilite" bushing. I have seen people build these engines without bushings and run them so much at engine shows that the holes become oblong. Now it is a difficult job to set up each part to bush it. It would be much simpler to simply press out the bushing and press a new one in. For sliding fits, such as the water pump and displacer rod, the Oilite bushing provides a seal, and constant lubrication.
The prints call for a 1/2" shaft and needle bearings. This requires making a hardened shaft. A better way is to use a shaft made out of 3/8" drill rod and use ball bearings. A 77R6 ball bearing fits fine. This is a shielded bearing. Wash out the grease that is inside the bearing in solvent and soak the bearing in light oil. Now the bearing will spin freely. Your local bearing dealer can cross reference the 77R6 bearing to the equal in whatever line he sells.
For best efficiency, make your displacer piston out of a pumice stone block called a "Sand-No-Mor" block, which is available at hardware stores and stores such as Handy Andy. It is a replacement for sand paper. Glue the block to the displacer rod with the hardening Permatex Form a Gasket. When the Permatex is hard, machine the block to size. It machines easy with HSS bits. Make your displacer piston 1/8" in diameter smaller than the I.D. of the displacer cylinder. The top and bottom clearence on the displacer cylinder at each end of the stroke should be 1/32". Straighten the displacer rod in the lathe before you assemble it in the engine. Get it to run true. If the pumice stone piston should rub a little on the inside of the cylinder, don't worry, it will rub itself away.
The teflon piston packing on the power piston has caused problems for several people. Instead use graphited string packing in the groove, soaked in Marvel Mystery Oil.
The displacer cylinder should be made of steel (NOT copper as stated on the plans) and turned to about .028" thick wall near the bottom. Use .032" thick copper to make the disc that will seal the end. Silver solder the disc to the end.
Lubricate the finished engine with either silicone lubricant or Marvel Mystery Oil. Silicone is best, but whatever it touches, paint will never stick to. Don't use regular oils as they carbon up on the bottom of the displacer cylinder and act as a heat insulator.