The End of an Unsuccessful Project                                                

I cannot find any air leaks now and yet the engine is not self sustaining. The force delivered by the piston seems to be too small to turn the flywheel. I think I have come as far as I can with this design and it’s time to call the Terrapin project dead.

This is video of me hand-cranking the engine.

Parts of this project worked quite well:

1. A cheap and airtight piston can be made with a bike inner-tube and a bucket.
2. The phase angle between the power piston and regenerator can be easily adjusted on the fly by using two concentric axles pinned together by a pointer on the flywheel face.
3. A very good regenerator can be made by alternating aluminum screens with nylon netting. The temperature of the lower cold chamber did not appreciably increase over the nominal 22C even thought the top chamber was over 100C, while the engine was being cranked.
4. An adjustable and calibrated electrical heat source can be made from a thrift store toaster and a wall dimmer.
5. The power piston stroke length was adjustable on the fly by moving a control rod.

Parts of the project I had trouble with:

1. The strength and insulation properties of wood are superb but it is very difficult to make an airtight wooden box.
2. The regenerator rod O-ring seals, that I made, needed to be adjusted very tightly to become airtight. This caused friction during the regenerator rod travel.
3. The engine produced very little power with the 80C temperature gradient. The size of the regenerator vessel must need to be very large to produce usable power at such temperatures.
4. The project was over-budget. The parts that make up the engine cost about $175.

Closing Remarks

Stirling Engines still intrigue me and I may very well attempt to build another. Here, I learned of several new techniques that I can apply to future projects. For low temperature-gradient engines, the displacement vessel must be quite large to develop appreciable power. Building large airtight insulated vessels that can withstand alternating positive and negative  pressures is challenging.

Disgruntled                                                                                   

Well, I got so disgruntled with the engine that I worked on other projects for a while.

No mater how many coats of epoxy resin that I applied to the outside of the box, there were always air leaks (detected by soapy water) bubbling from multiple odd places on the sides and lid of the box. The wood is very porous to air and it lifts and splits the brittle epoxy. I have used resin reinforced by fiberglass cloth before and it is rather tedious to work with and it also leaks air if every single hole in the cloth is not completely saturated with resin. I looked around for an alternate sealing material and settled on Kevlar reinforced bed liner.

The stuff is not cheap; a quart cost me $36 !  I applied four coats of the black stuff to the sides, bottom and top lips of the box. I coated the inside of the lid as well. I now use aquarium glue and two inch screws to secure the lid.

I reworked the difficult-to-adjust regenerator rod seals to a better design using modified lamp parts. I glue one inch long, 3/8” diameter, threaded lamp tubes into the lid where the regenerator rods pass though. Then I screw on 3/8” brass lamp caps that has a hole drilled in it for the rod to pass through. An O-ring fits nicely under the cap and provides an easily adjustable air-tight seal. Am I air tight? Time to test the engine again.