My Stainless Steel Hot Air Engines


     I started collecting antique engines about 1984, after I attended a "Pioneer Day Celebration" at Barberville, Fl. Those old engines fascinated me, and I decided that this would be an interesting hobby. Eventually I bought, sold or traded some 25 engines. Since I didn't have much space in my garage it was necessary to trade or sell some to get more interesting ones. It was soon evident that all these heavy "toys" were getting too difficult to handle. About this time a friend of mine, Alan Phillips gave me some reprint catalogs of early Stirling Cycle engines. After reading this literature I decided that I must have a hot air engine. Unfortunately, none could be found locally. The only course of action left was to construct my own. The only information available was several cut-away views in the reprinted catalogs, so this is where I started. Fortunately, I had previously taken a very comprehensive welding course at the local community college. For some reason or another my fascination with working with stainless steel led me to try to make a model of stainless steel.

     My first attempt (Photo #1) took approximately two hundred hours. Of course, since it was a trial and error operation, it goes without saying that I ended up making several parts more than once! The first power piston was not properly sized in relation to the displacer. The little engine tried to run but just couldn't get going. Dilemma, make it bigger or smaller? On a hunch, I made it smaller and the engine ran. Success at last, what a rush! I polished all the parts and took the engine to an engine show, where it was a hit. Lots of offers to buy, but when you put so much effort into something you don't want to part with it for any amount of money. 

     After this initial success it seemed like a good idea to make another engine, bigger and better. So, back to the shop and engine number two began to take shape, (Photo #2). This one is also stainless steel and has precision ball bearings in the rod ends as well as the main shaft. The first engine had needle bearings for the main shaft and pins for all the rod connections. Since I was now an "experienced" hot air engine builder, this one only took about half the time to complete. I finished this one by bead blasting to get a nice satin finish.

      I was now on a roll and decided to make an even bigger and better one. It took a while to gather all the materials, especially the large diameter pipe for the flywheel rims. I ultimately had to splice and reform smaller pipe to get the size I wanted. This engine is a real "high tech antique". The main bearings are double row self-aligning ball bearings. I calculated the output at approximately 1/8 HP at 300 RPM. As always, it is all stainless steel! Photo #3).

     After a few months "off" I needed another project but not another Stirling engine, so the old thinking machine started to work and I remembered a miniature vacuum engine I had seen five or six years ago. I just happened to have an article describing a "Flame Licker" engine. Since I don't like to copy directly, I changed the design details quite a bit. Again this engine would be made of stainless steel. It was a decision that I would come to regret. Stainless steel is difficult enough to work, but in miniature i.e., very small holes to tap, it becomes almost impossible! However, it was well worth the effort to see the little engine run. It is a good runner and makes a neat sound. (Photo #4).
         
      Somewhere along the way I acquired a book about "Free Piston Sterling Engines". I had been toying with this concept for quite a while and finally decided to try to make one. My first attempt was less than successful, (Translation, it would not run). More time was spent trying to make it run than it took to build. Once again, my friend Alan Phillips provided the clue. He said, "The power piston must be much heavier than the displacer". I made a new piston and it finally ran. This was a crude prototype, so I didn't finish it. Who wants an engine with a BRASS piston? That's right, the newest creation would have to be all stainless steel! This engine ran successfully the very first time heat was applied. It is about 15 1/2 inches high, and is very, very quiet running, (Photo #5). As with all my engines, propane is the fuel of choice, because it is clean and convenient.

     The urge hit again and in the spring of 1995 I decided to make another engine. This one was my most complex to date. It was to be a "V" configuration. Little did I realize the problems that would confront me, because I planned to use a crankshaft with only one throw. On the surface this looked like a simple straightforward approach. However, it soon became apparent this was not as easy as one would expect. I started with the most difficult part (I thought) first, the crankshaft. It is not an easy task to build up a crankshaft by welding the parts together, but, the job was eventually completed and after much bending and fiddling a straight and true running piece emerged. The frame and bearing support was next. This "baby" has Timken tapered roller bearings! Following this the power cylinder and displacer cylinder was fabricated and mounted. So far no problems. One neat item I used to help keep the displacer piston rod aligned, was a "Linear ball bushing." These clever devices look like an elongated ball bearing housing, with the actual balls arranged in a "race track" pattern that is oriented longitudinally. The shaft cannot be rotated but slides back-and-forth very easily. After these parts were finished, work started on the displacer piston and the power piston. These were both straightforward and presented no great problems. The power piston on this engine does have a Teflon compression ring unlike my other engines, which use a series of oil grooves, to maintain the seal. Now it was time to assemble the pistons and check everything for free running. Then the fun began!!! The crankshaft would not rotate a full 360 degrees. Instead it locked up and refused to spin. I tried another scheme to smooth things up by substituting an eccentric with ball bearings for the rod end cap that connected to the two rods. This did not work one bit better. As Thomas Edison said "I know a thousand things that won't work." Well now I knew what he meant. I didn't try a thousand things but after many experiments I concluded that it would be necessary to add a toggle link on one of the connecting rods. After much trial-and-error, (the engineers call this "empirical" engineering), I determined which rod would get the toggle and how long it would be. All that remained was to weld the heads on the two cylinders and mount the plumbing to connect the two cylinders. I made a trial run using a propane torch and it ran....barely. After lots of tweaking and adjustments it finally ran as I had hoped it would. The only thing left to do was to make a burner. Normally this is easily done if you can get replacement orifices of the kind used in propane torches. However, it seems that manufacturers no longer use replaceable ones. Now they are just drilled in the nozzle. So, I was faced with the prospect of making my own orifices. If you ever tried drilling a very, very small hole in stainless steel, you can appreciate the difficulty of the task. The hole is .0135" diameter, not much bigger than coarse hair. In the process several (5) drills fell victim. But, the burner worked beautifully (Photo # 6). The whole assembly, engine, burner and valve were mounted on a 20"x9"x2" piece of maple that was stained and varnished. The more the little engine runs the faster it goes because the compression ring is seating.

 	The year 1997 was a banner year as I felt the urge to make another engine. I started on engine #7 (Photo # 7), which I planned as a very low temperature version. I decided that I would make this one of aluminum instead of stainless steel, because the aluminum has  better heat transfer than stainless. The most difficult component was the Plexiglas cylinder for the displacer. I had to heat and bend the 1/2" plastic around a form. Two pieces were made and then each cut to half a cylinder and cemented together to make a full cylinder. It was chucked in the lathe and the inside was trued, and grooves were cut top and bottom to accommodate an "O" ring gasket. After the inside was trued and polished it was time to start on the other engine parts. No great difficulties were encountered, but it must be pointed out the heat required to run was more than I expected. I plan to do more work on the piston and displacer rod packing and hopefully it will come up to expectations.

      During the time I was working on the aluminum engine I started another small engine. This one was made of stainless steel, as were most all of my other engines. I also planned to have this one run on solar power as the primary fuel, with propane as a backup. This engine would be no. 8 of the series.
		
     The only problem encountered was the bushing and seal on the displacer rod. It turns out that two separate Teflon bushings would be more of a problem than I thought. The Teflon swells up a bit when it warms up and this in turn binds the rod. As I discovered long ago, a small leak is less damaging than even a little bit of friction. Also, it is difficult to keep the two bushings aligned.  The first solar powered runs, in summer 1997 were less than spectacular. The Fresnel lens couldn't produce enough heat to run the engine as fast as I would have liked. (Photos # 8a & 8b) show the engine in both configurations. 

     When I want to run with solar power, the engine is mounted on an equatorial mount that I constructed for that purpose. It is also made of stainless steel. The tripod (not shown) is made of schedule 40 pipe painted black.  This engine was modified in March 1998. I finally solved the problem by making the bushing of brass. Instead of two bushings there is only one long one.  I would have liked to use bronze but didn't have any handy. Before this modification the engine  would  run, but not very fast. However, with the new bushing it runs very fast, my guess is about 600 to  800 R.P.M. and should be a good runner under solar power. One of these days I must get a tachometer so I can measure the speed. 
 
      In the spring of 1998 I was casting about for a project to occupy my time. I wanted to make another engine so the planning, (in my head) began. Even though I swore I would never make a small engine because of the problems involved, I decided to see how small an engine I could make. Again stainless steel was my choice of materials but this one is not entirely made of stainless. The bushing for the displacer rod is brass! I have given up on Teflon for this application. The coefficient of friction is less for the Teflon, but when it gets warm it swells and binds the displacer rod causing the engine to stop. The brass is much more dimensionally stable for this application. Another departure was in the choice of material for the power piston. Most of the other engines used a stainless steel power piston that required lubrication. As stated before, the use of Teflon was also ruled out. I decided to make the piston of graphite. This by the way is rather pricey, i.e. $17.00 for a 1/2" x 6" rod. The graphite is very easy to work and requires no lubrication. No special problems were encountered making this engine and it is a fantastic runner.
      

       I needed a source of heat but decided against propane. I felt alcohol would produce enough heat, so I constructed an alcohol lamp, also of stainless steel.  Photo # 9 shows the completed rig. As I said it is a very fast engine. Just to satisfy my curiosity as to the top speed, I cut a small square out of a piece of magnetic tape and stuck it to the flywheel with crazy glue, (remember, stainless steel is generally non-magnetic). I had a small coil that was probably a telephone pickup coil that I bought at the local surplus store. By placing the coil adjacent to the spinning flywheel I could read the pulses on my oscilloscope. The frequency of the pulses is easily converted to RPM. Imagine my surprise when I determined that the speed was 1875 RPM. This turns out to be the fastest engine of my entire collection. 
						
       I am already starting to think about the next project! If anyone wishes to correspond with me on the subject of Stirling Engines, I can be reached at the following address. 


William C. Kehm
46 Volusia Dr.
DeBary, Fl. 32713

wkehm@bellsouth.net