Stirlingengineforum.com

Just a couple more months of wood-cutting before I can spend some of the winter break fooling around with this again. Before snow covers everything up, I need to locate and stash whatever parts or materials I might need from my local “Hardware store.” (My backyard full of assorted obtainium.) So I need to get sorta serious about figuring out this Ringbom thing. I think I could get about 1 watt per RPM from this engine and I only need about 100 to be happy, but that’s still some pretty quick cycling for a 22” diameter displacer stroking about 6 inches. That 1 watt-per-RPM is figured from about 2 PSI swings; at which a 4 inch diameter Ringbom drive should give around 25 lbs thrust, at the expense of about 20% of the power pulse. So for the target 100 rpm, the displacer weight and flow would need to allow at least 200 strokes bottoming and pausing and topping and pausing per minute from only 25 lbs force. Also, I wonder for the “pause” part of the cycle, what keeps a balanced (or horizontal) Ringbom displacer from moving too soon? I’m wondering about using magnets or something at each end. If there are any Ringbom folks out there who could offer wisdom or derision in plain language, I’d appreciate it.

Bumpkin

I'm not expert but I'm not sure it's possible for a Ringbom displacer to move "too soon".

What I mean is, the displacer controls the action. When, and/or if it ever moves, it moves, then the piston follows.

Well, the displacer could start to move before the power piston is at optimal timing — but I agree; as soon as the DISPLACER starts to move, a portion of the air is heated or cooled which increases the pressure difference, which increases the movement, which increases the pressure difference… a chain reaction to the end of the stroke — which leaves a chamber full of either higher or lower pressure air for the positive or negative power stroke, assuming it had enough flywheel to get there. On the other hand, as soon as the displacer PISTON starts moving, it decreases the pressure difference that started it moving in the first place. We know Ringboms work though, so I guess I’m just wondering about the “sweet spot” ratio for displacer drive volume per engine volume per temperature difference per RPM per — well, there’s not much that doesn’t matter. Best solution might be just get drunked-up and “gitter done,” then ponder the results from the perspective of a pounding hangover. Ah science…

Which, here’s another one. I would think a vertical-stroking Ringbom would need or would at least benefit from a spring that balances gravity, so the upward stroke action could equal the downward. But diagrams I’ve seen show no such thing.

Bumpkin

Bumpkin wrote: ↑Sun Oct 01, 2023 12:41 am ...On the other hand, as soon as the displacer PISTON starts moving, it decreases the pressure difference that started it moving in the first place.
...

This is true in all Ringbom engines to some degree. Usually not enough to make a difference.

The big pressure drop that usually allows the displacer to fall back down comes when the power piston moves.

Your bellows innovation is a bit of a wildcard though, as it has some actual air volume.

Usually the weight of the displacer is what causes it to fall once the piston moves and the pressure drops, so taking the weight off with a spring might interfere with that.

If your displacer is going to be some sort of lightweight blanket, combined with the bellows, which I think would be very effective (no air can leak past as it can with a shaft) you may need to weigh the displacer down, or maybe put a bleeder valve in the bellows as a variable "leak".

Tom Booth wrote: ↑Sun Oct 01, 2023 4:29 am ...
Usually the weight of the displacer is what causes it to fall once the piston moves and the pressure drops, so taking the weight off with a spring might interfere with that.

...

Actually, I should let someone who knows what they are talking about answer.

Looking around on YouTube I can find a number of examples of Ringbom type engines that have the displacer basically suspended weightless in the cylinder by either springs or "virtual spring" magnets as you were considering.

Here are a couple examples:

https://youtu.be/Qdq7XAyhk7A?si=Gwnb-SWKqT7CyYsp

https://youtu.be/YrYvXpAlldw?si=3yffTCkzyZ81j0yw


I do find some that also have a pressure relief valve of one sort or another. Not as a "leak" exactly but a pressure equalization valve that can be open and closed, to prevent what in your engine might translate into over or under inflation of the bellows?

In this four cylinder engine he takes one displacer out of action by opening a valve, then starts it up again by closing the valve while the other three keep the engine running.

https://youtu.be/cnx_1qFA7OE?si=khcQhEksjYPB3UZW

In the horizontal-movement displacer Ringboms, it’s the pressure swings alone that move the displacer both ways. (I think that’s a good demonstration of Stirlings being one-stroke engines.) I was just thinking that for a vertical movement displacer, balancing the weight might have some minor affect, particularly in a low pressure difference engine. For instance, if I have a 3 1/2“ displacer drive and a 20 pound displacer — without some sort of balance spring it won’t move until the pressure difference is over 2 PSI. (That’s an admittedly extreme example.) I think the engine’s pressure swings should be about balanced too, positive to negative. That would happen naturally with the slight leak of regular pistons. So I agree a sealed engine might need a slight balancing “leak.” Also the pressure limitations of the bellows and rolling seal might necessitate a pressure relief.

Bumpkin

Hadn’t noticed you’d posted again before I answered your prior. Thanks for those examples.

Bumpki

I finally got back to this a bit. I got the fire-box door done. I think I’ll pick up a few extra lengths of stove-pipe next time I get to town so I can take it outside and burn off all the old paint and see how it draws. I’ve made a few wood-stoves and they’ve all drawn ok, but this one was made with different priorities.

For the Ringbom displacer drive, I had figured shock-absorber boots would be an off-the-shelf item I could get my hands on and think about, but no. So I ordered a “big” one, and was a bit disappointed. It easily strokes far enough and seems to be a good compromise between strength and flexibility, but it’s a smaller diameter than I had expected and will only give maybe 10 lbs thrust from 2 psi. I have doubts about that being enough power to cycle the displacer fast enough, but I finally decided to go ahead and try it since I’m flying blind anyway. I welded a 2-1/4” ring into the center of the Beta piston to clamp the big end of the boot to. For the displacer rod I used a 1-1/2” steel tube that’s a good fit in the small end of the boot. There’s a bit of taper in the bellows so there’s some wiggle-room at the bottom for the piston to rock on it’s seal, but the fit is good enough that with the boot collapsed there’s not much dead space. So it looks pretty much like the drawing on page one, except a bit smaller boot, and the rod just ends with a flange at the bottom to more easily change out displacer-blanket trials/errors.

For the crankshaft I’ve got a 1-1/4” threaded shaft and bearings salvaged from a water-wheel that rotted away. The bearings are in good shape but will need cleaned out and some lighter lube. The shaft is plenty long to pass through the barrel and cut to length. The stroke will only be about an inch and I think I can just weld bolts/studs to the ends of the shaft to fit whatever size rod bearings I come up with. I've got a few scrapped auto alternators I might re-purpose for that. If I do it right I should still be able to slip the main bearings and a flywheel past the journals. For the flywheel I've got a 13" pulley that has a sorta steam-punk look to it. It might be too light, but I'll leave room at the other end of the shaft to maybe add-on. Anyway, the project is still alive and I figured posting about it might guilt me into more progress.

Bumpkin

I've been side-tracked for a while, but a bit of a progress report. I took this outside to test the burner and it was a smoky sooty mess even with the driest wood. I'll try an open-top insert to match the door width and narrow the burn zone. Hopefully the cooler incoming air will stay at the bottom to flow more directly through the fire without the insert sides having to be too tall, so the heat can spread to the entire chamber hot end. The large surface area for the size of the fire should give higher efficiency, but the cooler exhaust will have less draft, so like everything else it's all a compromise.

I got the crank and rods done so I can get a feel for some of the movement. The rolling-seal handles the piston rocking and still has a strong self-centering affect. I ended up with a 1 inch stroke in a 6 to 7 inch chamber depth, so about 2 psi and the seal handles that for now. It's difficult to turn over at that pressure and I had to reinforce the badly flexing piston. It almost certainly will need more than the 13 inch iron flywheel. The Ringbom bellows seems pretty energetic and may be fast enough if the displacer blanket is light and breathable enough.

I've been through a few blanket attempts. My latest idea is aluminum sheet in a cone to match the chamber ends, with a zillion little holes drilled through to let it breathe, and carbon-felt welding blanket glued to each side. I'll drill it after it's glued, so the holes will line up through both layers of blanket, RTV glue, and the aluminum. The blanket breathes as-is, but seems too restrictive, so I tested drilling it with a small bit, and it opens up breathing and mostly leaves the felt in place. I've no idea how efficient the convective heater=regenerator-cooler exchange will be though. (I'm sure there are better ways.) The blanket seems friction-smooth and the idea is for it to lap over the slightly undersized aluminum cone to lightly bear and self-center on the chamber wall. Its softness should help cushion Ringbom stops, so it can rest firmly on the hot and cool ends for a bit of conductive boost to the intended radiant exchange. I'll report back if the burner mod and blanket show any promise, but at my work rate that may be a while.

If this ever runs it might be worth doing over because there's already a thousand things I'd do different, but you don't know something until you do something. Pictures when I can; it pisses me off to no end that I can't just take a low resolution picture without having to mess with it.

Bumpkin

If you take a picture on your phone or pad, open it in your gallery and take a screenshot of it. Voila, resized.

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