Can't believe it's been almost 3 months since an update.
I've finished the chamber enough for some preliminary testing. The results, though on paper don't look great, are actually excellent. The first thing I did was let hot water(150F) from the kitchen sink run through the engine for about 20 minutes with the displacer elevated. This was to test how much heat is conducting through the body or the gas, and going on to raise the temperature of the chamber. The answer is essentially none. The chamber was only a few degrees higher than ambient surfaces in my sunny kitchen.
- 60l first test hot water after 30 minutes.jpg (276.18 KiB) Viewed 3844 times
Then I tested to see how hot the surface of the heat exchanger was getting. 100 degrees F, but that can't be, so I painted the stainless steel surface with black primer and saw 135F. These thermal scanners are tricky and really can't be trusted for precise work at all, but good enough for now.
- 60l first test hot plate temp.jpg (225 KiB) Viewed 3844 times
Then I re-assembled and put ice on the top plate. There is so little thermal bridging that the ice froze to the top plate for some time. The surface of the top plate(tested both top and bottom) was about 30F. The walls of the chamber remained at about 80F.
- 60l first test temperature gradient.jpg (236.06 KiB) Viewed 3844 times
The displacer travel was about 1.5" for this test, which is about 3200cc of swept volume. The actual volume is about 500cc more than that, until I put insulation on the chamber walls and fit the displacer to it.
In reality the area of the chamber walls is greater than the hot and cold plates, so they are acting to reduce the Tmax and Tmin by a large percentage. Figuring that the average Tmax is more like 105F, and the Tmin 60F, the expected pressure gain is just over 1psi. The uninsulated walls are likely bringing those temps even closer than that. This is evident by the fact the chamber does not go beyond a few degrees over ambient with the displacer elevated for extended periods. The thin steel walls of the chamber are enough to easily dissipate the heat moving through the gas.
I saw just over .25psi in testing. With well insulated chamber walls and a snug fitting displacer, I think this will rise significantly. Recall that an MEP of 2psi is my target for one horsepower at 600RPM. Refer to post 1 and 2 in the link to see how little pressure gain I believe is needed for decent power when the power piston is near or bigger than the displacer chamber.
viewtopic.php?t=5638
I'm still looking for the right insulation for the inside of the chamber walls. It must be closed cell, smooth, and easy to work with. A roll of disposable coffee cup styrofoam is what I need. More updates to come.