Stirlingengineforum.com

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I made a new engine body from a section cut out of a Coleman lantern globe, which happens to be the same diameter as the original acrylic engine body. The only advantage of glass over acrylic is it can take more heat.

I've also made another spiral displacer from a sheet of the microwave wave guide cover material (mica flakes with a silicone binder)

Unfortunately, after trying to attach the displacer rod several times it seems ordinary high temperature epoxy or stove or muffler cement and such do not adhere to the wave guide material, likely due to the silicone component.

Possibly some RTV silicone will work, since silicone is about the only thing that sticks to silicone.

For a displacer, the mica sheet is very thin. Not sure how, or if that will work. If not I may try backing it up with some ceramic fiber board or ceramic fiber "paper".

I may leave out, or use less, or make a "window" in the top inside insulation so as to be able to see what's going on inside; if the displacer is actually opening and closing properly.

Still can't post images, though, embedded video links still work I think.

After repeated attempts using all kinds of adhesives and cements, including RTV silicone, nothing seems to stick well enough or be strong enough to hold the mica displacer to the connecting rod.

I finally fell back on using the plastic rivet-like mechanical compression fitting that came with the engine to hold the original foam board displacer. which may be, and hopefully is something somewhat heat tolerant, like Teflon.

This should be OK for testing at least.

Perhaps something similar, machines from brass might be a better, more permanent solution.


https://youtu.be/rk05dTmA3eg?si=c_bLwD1vuMjEwCea

Looking forward to the results. Cutting those spirals looks tedious but they came out great.

VincentG wrote: ↑Fri Mar 08, 2024 3:12 pm Looking forward to the results. Cutting those spirals looks tedious but they came out great.

Not bad actually.

The compressed mica sheets cut rather easily with a razor blade. Maybe too easily. It is also rather thin and fragile.

A little thicker would probably be better, but I think it will do for experimenting. Not sure how long it will hold up.

Nice to see the forum is back to normal. Yay!

I'm wondering if leaving an air gap under the displacer would improve performance. Based on the idea that some volume of preheated air would mix with and heat up the remaining volume more rapidly.

Especially given the way the heated air under the displacer (spiral "heat valve" thing) tends to remain isolated.

Lifting the displacer would force air down to mix with the preheated air. If the heating is mostly radiant, probably wouldn't make a difference, but then again, hard to say.

Blade Attila made the suggestion in the YouTube comments that it would improve performance if the spiral was free to move up AND DOWN from a central position.

I assumed it would be better to drive all the air away from the hot plate, as far as possible, with zero clearance to help avoid mixing of hot and cold air, but if the heating is mostly radiant, mixing would likely be minimal anyway.

There could also be some advantage of the spring being free to oscillate.

Anyway, going with my first instincts for now, I'm going to try fashioning a kind of reverse "snail cam" that can "drop" the displacer a little sooner.

Something like this:


https://youtu.be/EzD0HsdR4O4?feature=shared


The timing in the last trial has the displacer start to rise at TDC (power piston in down position) and "close" completely near BDC.

No advance of any kind at either end.

In theory, I would think some advance on the heat input would be an advantage but I think more advance on the "valve" closure would be more of an advantage in terms of producing some additional adiabatic cooling.

My aim is not so much to maximize power production but to achieve some degree of "self cooling", or zero "waste heat" transfer.

Well I got the high temperature Spiral displacer in the engine, all reassembled.


https://youtu.be/o4_SYIfyftE?si=eiLpsJ_G61rRHwlZ


I have a few preliminary observations. What I mean by "preliminary" is I don't have it exactly running yet, but I spent most of the night trying to get it working and finally did, sort of.

By disconnecting the displacer from the crankshaft, but leaving the power piston and crankshaft attached normally, I could work the displacer up and down manually and get the engine to run, and apparently keep it running indefinitely, but the timing necessary to do this is quite different from the standard sinusoidal motion with a 90° advance.

Actually the engine seemed to "run" best if I lifted the displacer after TDC and then held it down again before BDC.

That is, the "heat valve" was lifted and kept "open" for about maybe 120° between TDC and BDC. But the displacer had to be held down firmly at all other times, especially between BDC and TDC (after BDC and before TDC)

It didn't seem to make much difference if the displacer was lifted a little early, slightly in advance of TDC, but it absolutely would not run if the spiral was open even a little bit after BDC, infact, the flywheel would reverse itself if I was a little slow in closing the "heat valve".

The mica sheet is somewhat transparent so may not be blocking all radiant heat.

The spiral displacer continues to be very difficult to keep centered, swells and twists and binds when it gets hot and does not fully close without actually being held down

I continue to see the "weird" behavior that seems to indicate that the air below the displacer is radiantly heated rather than heated by convection. The piston motion will exactly parallel the displacer motion even if the displacer is moved very very slowly in either direction, so slow that it seems unlikely the air is being "stirred up" so as to come into contact with the hot plate. A tiny motion of the displacer up or down is almost exactly mirrored by the power piston, regardless if the displacer is nearly all the way up or nearly all the way down.

As the engine runs and speeds up this behavior changes somewhat I think, or maybe due to the momentum of the flywheel and the speed and limitations on the speed which the displacer can be moved, it appears different

Anyway, without some actual "snail cam" type mechanism to "drop" the displacer quickly before BDC I don't think I'll be able to get this engine to actually run completely on its own power.

Also the spiral displacer, once it gets hot, no longer wants to work. Basically the spiral "spring" expands and the spirals no longer nest together properly, so the "heat valve" gets stuck open, along with the additional problems of misalignment and binding.

The thinness of the mica displacer does not seem to be a major problem. It still seems to block the heat nearly as well as the foil lined foam board.

All in all though, it has been educational as far as emphasizing the possibility that radiant heat is much more important than convective heat, there seems to be no advantage to using a spiral displacer. It seems like it could work if some material that holds it's shape when heated could be found. The only advantage it might have is a reduction in resistance to air flow. Just "working" though, is not actually "better". So far I don't see any advantage, but I have encountered a lot of difficulties.

I keep thinking, why can't we make a displacer out of ceramic tile? Heavy yes, but if counter balanced by a second one? Also cut a spiral in one to make your spring displacer.

https://m.youtube.com/watch?v=4h3r4BUFES0

Attachment could be as simple as a rod washer and nut? Could stack several to get greater thickness, strength, and insulation. Could drill holes in each layer and sandwich stainless steel pot scrubber in between the layers. Off setting the holes would lengthen the regeneration path? Just a thought.

Amazing video!

I never would have imagined such rigid materials, when spiral cut, could become so flexible. That certainly expands the possibilities for material selection.

His YouTube has several other interesting experiments. He tried to make a saw blade out of window glass, cut out using a water jet cutter. Fun!

I was thinking of using a rod saw for tile and hand sawing out a spiral on a twelve or eighteen inch ceramic tile? Might take a very long time. Maybe a roto tool, like a Dremel, and a carbide roto-zip bit?

His process of cutting glass seems to leave micro cracks at the edges. I was wondering if a spiral could be poured directly into a mold and cooled slowly, and annealed? Could leave a much more durable product?