LTD Stirling Engine

I am noodling with a Low Temperature Differential (LTD) Stirling engine. These are Stirling engines that can run on a small temperature differential, like that between a cup of boiling water and room temperature. This particular design should not use a flame as the heat source because of the low-temperature materials used. I have wanted to build one for a while, and it may work as a project for K-12 students. I am creating this page to post some notes as I go through.

These are just field notes, not a set of instructions. For good step-by-step instructions, check out Scrap To Power's build log.

First Parts Picture

File:00-parts.jpg

LTD Stirling Engine Parts

So far I have made the power piston, cut down two large tuna cans and a standard 2 liter, and made a displacer piston.

The power piston (little orange thing on the lower right) is a diaphragm type using a gatorade bottle lid, 1/4-20 x 1" machine screw and nut, two 1 1/4" dia fender washers, a punching balloon, and two rubber bands just the right size to go around once. The gap between the fender washers and the wall looks good: Wide enough to give a decent stroke without stretching at the extents or wrinkling in mid stroke, but small enough that it should keep power loss low. There's a piece of steel wire wound around the screw in the picture -- I was testing the form-a-thread effect of winding the wire around the screw's threads, and it worked very nicely. You can screw the coupling rod on just like a nut.

Large tuna cans and a standard 2 liter are nicely sized for each other; you get about 3/4" overlap when you wrap the polyethylene from the 2 liter around the tuna can. I used a side-cutting can opener -- the weird ones that don't leave a sharp edge -- and I think I'm going to be able to get both cylinder heads out of a single can.

I used the dumb-guy method of cutting the displacer out of a leftover piece of polystyrene packing material; cut a smooth edge on one of the tuna can walls and press it through cold. I think heating the can with a torch to hot-cut the foam would be worth the effort, but the cold method worked well enough for the first draft. To get the billet out of the tin can, I just lightly heated the sides of the can to shrink the polystyrene a bit. Once the puck was free of the cutter, I ran a low flame around the circumference to shrink and harden the edges a bit more, testing the fit in the tuna can as I went (it should fall under its own weight when you drop it in). After you run the flame around the circumference, it should harden enough that you can sand it to clean up the shaggies.

Finished

File:Ltd-stirling-finished.jpg

LTD Stirling Engine Parts

Random Thoughts

Cut Sheet Steel Is Sharp

Cut sheet steel is sharp. Little slivers of cut sheet steel are sharp and nearly invisible.

When cutting tuna cans with tin snips, if you back the snips out of the cut by even a little bit, or don't keep the blade intersection tight against the end of the cut when opening the jaws for the next snip, it is really easy to catch a hair of metal on one side of the existing cut -- which will create one of those sharp, nearly invisible slivers.

Therefore, if you maintain constant gentle pressure holding the blade intersection against the end of the cut -- sliding the snips forward as you open the blades for the next snip -- you reduce your chance of getting an annoying metal sliver in your hand.

Gloves probably would have helped too. :)