55-gallon barrel Stirling

[Bumpkin]

55-gallon barrel Stirling.

Years after admitting defeat on a prior attempt at a practical engine, I’ve grown foolish enough to try again. Aside from some low-tech steampunk fun, “practical” would mean cogeneration to replace my wood-burning shop stove. It would need to use standard 16 inch wood without splitting it silly-small or fussy tending; and it would need to trickle-charge my PV system battery through dark winter spells. A hundred watts would be useful, and the battery and electronics are there in the shop anyway. So I partitioned a barrel as a burner and displacer chamber, using heavier steel formed into a 3 inch deep cone; pointed down so the fire flows around it to the chimney. The displacer chamber is 22 3/8 inches diameter and 10 inches deep.

I’m pondering a Beta, with the piston stroking about an inch on a rolling seal. The seal would use about the top 4 inches of the chamber to work in. I made a piston with a 1/2” gap and tried the seal with a bicycle inner-tube and it works easy and rocks enough that it won’t need a wrist-pin. (The displacer drive will have to accommodate that.) I don’t yet know what pressure it can handle — I’m an awful welder and I’m still tracking down leaks; but a bigger concern is how the seal will handle heat, being only 7 inches from the hot end. I could insulate or cool it, but I like things simple as possible. If it works out, I might need to make a stronger piston since the big bore would make 800 lbs thrust from just 2 psi swings. Anyway, I’ll pursue the Beta as far as I can, but I could end up going with a Gamma, and for easy servicing and tinkering, attach the cool-end to the lip at the top of the chamber with a ring-clamp. A Gamma could use the full 10 inch depth (minus the displacer) so it could feed a power piston with about an 8 inch bore and stroke. A 1/8 compression ratio would allow an easy 1/4 temperature ratio.

I believe radiant thermal transfer is under-used, and gains could come from radiating heat to and from a regenerative displacer. It’s already done in some engines, but seemingly by chance. I think the displacer should be a flow-through blanket; not “displacing” the air, but moving through it, alternately blanketing the hot and cold ends of a big-bore short-stroke chamber. One side would be a flow-through air heater, heated radiantly by the hot end of the chamber. The other side would be a flow-through air cooler, cooled by radiating heat to the cool end of the chamber. The middle depth should be a thermal break, and of course the entirety remains a regenerator. About an inch-thick steel wool and glass wool sandwich held in shape between light frames might work. I’d prefer all glass, but to get enough thermal mass it would likely need to be packed too tight to flow. Whatever the materials; for aero and reciprocating strength, it should be conical in a matching chamber. I think the inward-angled face of the hot-end cone might help radiate less heat out to the chamber wall. The internal wall should be white, the ends and displacer should be black, blah blah blah…

Anyway, nothing’s locked in except that I have a burner with an empty displacer chamber. I’d hoped to get further along before posting, but I have to get back to seasonal work. It might sit as-is until next winter, so in the meantime I invite suggestions, critiques, rude insults, etc...

Bumpkin

[Tom Booth]

Do you have, or could you take some photos and post them?

[Bumpkin]

Well shoot it looks like I need to shrink the pictures somehow. I have a couple here on my i-pad which I barely know how to use. I got it to replace my busted p.c. and my son was going to help me learn — just before he got married and moved out. I run chainsaws; he runs computers.

Bumpkin

[Tom Booth]

Well shoot it looks like I need to shrink the pictures somehow. I have a couple here on my i-pad which I barely know how to use. I got it to replace my busted p.c. and my son was going to help me learn — just before he got married and moved out. I run chainsaws; he runs computers.

Bumpkin

I know the problem. I take pictures with my phone all the time. About 5 megabytes each. Higher resolution than the dedicated digital camera I had to buy for school a few years ago.

If you don't want to download an app, there are several free drag and drop image resize websites that work remarkably fast.

I usually use "pixels" rather than "percent" to know exactly what the results will be and set the width to between about 400 to 600 pixels for this forum, depending on if the image has fine details that are important or not. (If anyone has problems viewing images I upload let me know please).

I usually change the quality to 100% though the default is usually OK but when reducing from 5MB down to maybe 300kb the results can be a little grainy and reducing the quality doesn't save that much bandwidth but could loose some detail.

Here's one:

https://www.reduceimages.com/

[VincentG]

Sounds like a great start. I'd like to see how the rolling seals are attached.

[Bumpkin]

Thanks Tom. I’ll get the pictures done soon. Vince, the inflatable rolling seal might have another name. A photo won’t help much for understanding because other than the valve-stem, it’s hidden. But when you see it drawn in cross-section it’s obvious. Here’s where I first saw it: https://patents.google.com/patent/US5414997A/en
Enlarge the first image.
Bumpkin

[VincentG]

Got it. Interested to see how that works. From riding motorcycles I have seen many differences in innertubes, with the quality Japanese or American made tubes have a much more supple feeling rubber than the cheaper or "heavy duty" ones. Doesn't matter in a tire, but the former would roll easier.

[Bumpkin]

VincentG, yes the tube I bought seems somewhat more plasticy than rubbery and there well might be better tubes available. I have no idea yet how it will handle various tortures, and am very dubious about the durability. That said; the 20 inch by 1.75-2.25 tube stretched tight around the 21 1/8 inch piston; when inflated to a moderate pressure, (sorry I don’t have a gauge that reads that low) inside a 22 3/8 inch bore, moved so smooth and easy it almost made this old man want to giggle. There is about a 1 1/2 inch stroke available before it rolls to tug on the valve-stem, but I don’t need nearly that much. Obviously if this was the answer for everyone, everyone would be doing it; I just want to see if it might work for my limited application.

Bumpkin

[Bumpkin]

Not the best picture, but It’ll have to do for now. I mighta finally got pictures figured out to maybe do better later. You can sorta see the door across from the chimney where a standard 4” pipe slips over. (That matches my existing shop stove.) The piston sitting on the chamber fits inside with about a half-inch clearance around. You can see the 20 inch bicycle tube stretched tight around the piston. The valve-stem points into the piston. The piston is taller than it needs to be, but I can cut it down to whatever when/if I figure out the drive. The cone of the piston crown matches the cone of the burn chamber top/displacer chamber bottom, but of course there will be a matching cone-shaped displacer between.

81334C4E-FD7D-4482-B548-A3BF4CE64A80.jpg (17.07 KiB) Viewed 17247 times

Bumpkin

[Tom Booth]

Can you possibly include a sketch illustrating the arrangement?

There is a burn chamber, like a wood stove on the bottom? Displacer in the middle and power piston on top?

I've always thought some kind of Stirling made from a steel drum would put out some power, but never saw one actually run.

I was thinking just an oversized "tin can" engine using really big "tin cans" but if that is the power piston, your design is different from what i imagined, like a small piston on the side maybe.

This looks more like the big LTD Tamera engine?

[Bumpkin]

Sorry for the late reply, computer troubles. Anyway, I’ll get back to this next winter, but for now the displacer and its drive and the crankshaft placement are imagined in. It’s drawn to scale on the graph paper with one square to an inch. I didn’t draw the long tandem connecting rods that’ll go up each side of the barrel to fasten solid to each side of the piston. The long rods and short stroke will minimize the piston rocking and side thrust on the rolling seal. Running the rods to the bottom will eliminate any structural needs of the chamber, leaving it possible to replace the upper chamber with something less conductive. A linked displacer drive would be difficult, but I want to try a Ringbom anyway. My first thought for the displacer blanket was carbon-felt air-filter material. The felt is already the preferable black, and its softness might quiet and smooth abrupt Ringbom landings. It looks like about 1/8 inch carbon felt would have enough thermal mass to exchange with 6 inches of air. Whatever I use, it should lightly rub the edge to force the flow through and be self centering, though it will still probably need a guide at the top to keep it straight, and a spring of some sort to balance the weight. Possibly a long spring or snubber suspended from straight above could do both. I’m skeptical that a standard shock absorber boot will hold up for the Ringbom drive, but they stroke far enough and have about the right diameter, so I think it’s worth trying. A bellows is preferable since it could deal with the slight rocking and side-motion of the piston, and an air-tight Ringbom would be self-starting.

FullSizeRender-compressed.jpeg (59.59 KiB) Viewed 14011 times

After all of that, I see it would be better and simpler if the piston-skirt and rolling seal were outside the chamber instead of inside. It should be feasible to stretch the tube around the outside of the chamber with the valve-stem facing out, and rock a short-skirted piston over the stem and into place before inflating the tube. Then the tube would be protected from stuff falling into it and the piston skirt would more directly connect to the rods. Ah well; I may or may not change that, but I think more will be learned from finishing than from continually starting over. Then if it works at all, it would be worth making multiple changes in a later version.

Bumpkin

[Tom Booth]

I don't think I've ever seen a Beta-Ringbom.

It certainly makes for a rather elegant design.

[Tom Booth]

Out of curiosity, I took a look through Senft's "Miniature Ringbom Engines" as he, I had thought, covers about every Ringbom arrangement conceivable.

There is actually an entire chapter (Chapter 4) devoted to "The Beta Ringbom".

He describes this (Beta type Ringbom) as "a better configuration as far as thermodynamics is concerned.... This makes the most effective use of the hot and cold spaces of the engine, giving the beta engine better thermal efficiency and higher specific power..."

There is a "short" video of an LTD type Beta Ringbom here:


https://youtube.com/shorts/-QqD61x08Bg? ... T2ItXHrYtd

[VincentG]

Looking forward to build updates.

[Bumpkin]

The Ringbom displacer stroke length finds its own way, solving math problems for lazy folks like me. Anyway, it fills in the Beta strokes from each end, and allows experiments with different displacer thicknesses. I think a kinematic linked displacer COULD be more efficient, but the timing exactitude might take more brains than I’ve suffered lately.

Bumpkin