Accordion style heat input plates

[Tom Booth]

I've been really eyeing the Sunpulse lately as the best basic layout for a large engine. It would lend itself well to Tom's idea of no cold side. I do like the spiral displacer layout but I see it as a materials problem again, trying to keep the top side of the displacer cool enough to not hinder the return stroke. Making it thicker by adding insulation on top seems appropriate.

Yes, I was thinking possibly a thin spring steel perhaps on the bottom for both additional hot surface area for heat exchange and "springiness" functionality, topped by a layer or coating of some insulating material, depending on the application. For low temperatures could be just the typical foam rubber used in most LTD's or for high temperatures, something with greater heat tolerance but somewhat flexible. The almost cloth-like compressed ceramic fiber board used in kilns seems potentially suitable.

It's a bit fragile but bonded to some spring steel with the small amount of flexing in such a long spiral should hold up.


https://youtu.be/GY4-pgSI2VA?si=XPlYUrslh1Eup0Gd


A thin coat of some high temp silicone over the ceramic fiber might help as the stuff can be a little dusty. Likewise, silica aerogel blanket is "dusty". I think it is basically ceramic fiber blanket impregnated with aerogel particles.

An intermediate temperature material might just be a thin sheet of wood. The right kind of wood could be both "springy" as well as insulating and low cost. I've been pleasantly surprised by reports of a balsa wood displacer withstanding extremely high temperatures even when the solder holding the displacer chamber together melted down.

I think perhaps, for wood to burn it needs oxygen and wouldn't get much of that in a sealed displacer chamber. Not for long anyway. Maybe thin plywood with a layer of metal foil on the bottom.

In my drawing I have the spiral tapered which should help the spiral arms lift clear of one another avoiding binding and friction but providing an air tight seal when down or closed. A potential side effect is that the angled gaps could help direct the air flow perpendicular to the power piston's domed surface for maximum impact. (Theoretically at least).

[Tom Booth]

To start with, making things a little easier, since my time lately is very limited, making some renovations on the house, and just looking for a quick and easy way to test an idea before spending a lot of time on a major build, I thought of a possible means of testing the spiral displacer concept with a relatively simple modification on one of my existing, more or less standard LTD engines:

Resize_20240212_054211_1283.jpg (125.69 KiB) Viewed 884 times

The spiral displacer should be easy, just make a slanted spiral cut in ordinary displacer material, foam board from the Dollar General or Walmart. I think it probably has about the right balance of rigidity and flexibility.

Then I just need to shape a piece of the blue insulation board I just picked up the other day.

Resize_20240212_055444_4668.jpg (137.61 KiB) Viewed 884 times

Shape as illustrated above.

I'm guessing that modification should eliminate about 2/3rds of the usual "dead air space" while the spiral displacer should significantly increase the heat input. Mostly because I'll also modify the connecting rod to add "dwell" and maybe glue some foil to the bottom of the displacer coil to take up some heat, possibly doubling the heat input heat exchange surface area.

The blue foam insert in the top of the displacer is just to take up dead space and also act as insulation to retain heat rather than trying to get rid of it

I may be able to also increase the throw of the power piston in order to increase the compression/expansion ratio. Possibly just reusing old parts from previous experimental engines already so modified.

[Tom Booth]

Making the spiral "heat valve"

Or planar spring displacer:


https://youtu.be/ySZqH-B_FKI?si=65tDp-8mA3y_DoDo

[Tom Booth]

The green insulation board together with the craft foam board coincidentally fit together perfectly in the LTD engines I've been experimenting with most often.


https://youtu.be/IedJtatFubg?si=c2_jjl9Gpc8JYJGG


I'll be making a passageway through the green foam board to the power cylinder and a small hole for the displacer rod. Put the engine together and see how (or if) this works.

Two goals, 1. To completely isolate the hot plate heat exchanger when not being used, or not required for heat input 2. Reduce "dead air space" and therefore increasing the effective compression/expansion ratio

[Tom Booth]

While cleaning up the work bench and engine parts of all the Styrofoam shavings, getting ready to put this engine together, I accidentally made a discovery.

The planar spring "HEAT VALVE" works perfectly as an ordinary low, or no-friction one-way check valve.


https://youtu.be/Oa-kWJYaChw?si=h1c5BvjBRNAawqjW

[Tom Booth]

Turns out, it does run:


https://youtu.be/Aci_RjTli_o?si=49pJ4bw5MNnvkKy1


So far, not any dramatic improvement over an ordinary displacer.

I'm just running it with normal sinusoidal displacer motion (no "dwell") to begin with.

It seemed necessary to advance the timing more than the ordinary 90°.

It was running great earlier on top of the wood stove briefly, but I was afraid of it melting or coming apart. The foam board displacer is just attached with hot glue, and except for the metal bottom plate, the rest of the engine body is mostly all acrylic. It seemed to like the high heat though.

It didn't seem to want to run as well on steam. When I disconnected the piston from the flywheel and turned it over to try and see better what was going on while running on steam, the piston seemed to be getting sucked in further each revolution, as if there was more cooling than heating, which seemed quite strange, considering the lack of heat dissipation to a cold side metal plate.

Possibly this may have been due to an air leak somewhere?

I'll be tinkering around for a while.

It is at least a "proof of concept" as far as the planar spring displacer. Maybe not too bad for a first rough prototype.

[Tom Booth]

Some bad news, which I think may actually be good news


Before running the engine on steam to make the previous video, I noticed the "valve" did not appear to be closing completely. This can be seen in the video.

I tried to "fix" this by adjusting the connecting rod length, which just involves loosening a small set screw. This helped, but involved the engine having to force the spring down. I tried to find a compromise position where the engine would at least run, but I was puzzled why this was necessary as previously the "spring" returned to a closed position on its own very well.

The displacer, in the video was also hard to even see, as if it had been compressed.

Anyway I took the engine apart and found that the displacer "spring" had become permanently deformed and compressed due to the heat also the green foam board had melted around the displacer rod.

I'm nearly 100% certain all the damage and deformation was due to the high heat while running on the wood stove. Just as soon as I transfered the engine to the steam generator, that I had been waiting to warm up, the performance degraded.

I thought this was due to the engine just running much better on a higher temperature heat source, but I think, really, the heat from the wood stove was too much for just a little too long and by the time I switched to the steam, the damage was already done. Somewhat predictably.

I thought I might have gotten away with running it on the wood stove briefly, but apparently not.

The good news is; the engine was running really GREAT on the wood stove, for about 10 or 15 seconds. I was very pleased with the initial performance, so I was disappointed at how it only ran so-so on steam.

So, I THINK the reason for the poor performance on steam was mostly due to having become pretty badly damaged by the brief run on a very hot wood stove. I should have known better.



https://youtu.be/JPq-NSgYgFE?si=Dk__G9IGSLl_djtj


Anyway, I think I will make a new displacer and try again, this time using only hot water.

Seeing how badly deformed the displacer had become, to the point where it needs to be forcibly closed, indicates to me that the way the engine ran on the wood stove is more indicative of the performance of the "heat valve" than what is seen in the video, which really wasn't all that bad, but certainly no better than a regular displacer.

The spiral displacer may perform really really well if that is the case.

Now I'm wondering what might be used to make one that can withstand higher heat.

[Tom Booth]

Actually the entire plastic ring that makes up the engine body had softened and become deformed

Resize_20240225_205432_2686.jpg (93.4 KiB) Viewed 822 times

Possibly this was no longer making a tight seal and lowering compression.

I'm also right now testing the foam board over the steamer to make sure that alone was not too much heat for the displacer.

Resize_20240225_205433_3449.jpg (91.48 KiB) Viewed 822 times

It's been over the boiling water for a good five minutes already and has not softened up at all.

I have to conclude that all the damage took place while the engine was on the wood stove before I even started making a video.

I've already started .making the replacement displacer and also have extra replacement parts for the engine.

[Tom Booth]

A little something extra this time:


https://youtu.be/WDJyB6TKk7k?si=FE7cQrob6_WQ2ubf


Now that I know the spiral displacer at least works, and probably works really great, I'm taking a little time to do something I intended to do earlier. Line the bottom of the displacer with foil.

The "glue" I used to hold it on is also mixed with hollow glass microspheres.

It's pretty much ready to put back together and run but I'm going to leave it overnight to be sure the "glue" is good and fully cured.

[Tom Booth]

After all this, I'm not getting any pressure.

https://youtu.be/r7f91ZmnOdg?si=Jdc1TlsTXMbnHKc3


And, no, not because I forgot to install the O-ring.

Sitting over the steam generator and turning it over by hand, and disconnecting the piston from the crankshaft I'm not getting any pressure variation at all

I even took out the piston and covered the power cylinder with plastic wrap and nothing.

I tightened and retightened all the bolts, even honed the power cylinder, as the piston seemed to bind slightly when the engine heated up.

Maybe too much heat retention doesn't allow any temperature difference to form? None?

That seems impossible.

I kept the displacer down as the engine warmed up over the steam, then lifted the displacer once the bottom plate heated up, but still nothing. No pressure whatsoever.

Maybe I'll have to take the upper insulation out, but I can't believe there is no pressure at all, but I can't find any leaks at all anywhere.

[Tom Booth]

Mystery solved, (I think?)

Apparently hot glue does not adhere to borosilicate glass, at all

I had replaced the aluminum power cylinder with a new glass cylinder.

Normally I probably would have used epoxy, which I've done before without any problems.

This time I just gobbed some hot glue on the base of the cylinder and mashed it down into place .

By all appearances it was completely sealed but basically the cylinder was just cradled by the hot glue, which prevented it from falling over but really did not seal very well, and it only got worse over time.

While checking things over again, I noticed the cylinder seemed to be loose and it pulled right out. When I put it back and pressed it down tight, suddenly I had compression!

This time I just used a bit of Gorilla glue in the gap between the glass cylinder and the hot glue, which seemed to have adhered to the acrylic OK.

Resize_20240226_194731_1848.jpg (103.98 KiB) Viewed 792 times

Between the wrapped malfunctioning displacer "heat valve", the melted and deformed acrylic ring and now loose air-leaking power cylinder, amazing this thing could still run.

[Tom Booth]

As it turns out, the hot glue didn't really stick to the acrylic either, so now I've taken the engine apart and put it back together with some 2 part epoxy I had on hand, but it is slow curing.

All the potential or probable leaks however, didn't really seem severe enough to cause a total loss of pressure differential. I suspect something else is going on also.

Even theoretically, (my own theory that is), for a Stirling engine to act as a kind of heat pump and maintain its own temperature differential it needs to actually be running.

So if the heat retention is nearly perfect, setting the engine on a heat source for any length of time would cause it to become heat saturated fairly rapidly.

To avoid such a consequence, it may be necessary to spin-up, or continue to manually turn the engine as it sits on the heat source before warming up fully to "operating temperature".

I think I've seen some evidence of this in starting or attempting to start this engine. Sometimes it would run great right away and other times not at all, but for no obvious reason.

But I think I have noticed that it seems to do better if I try to start it early and keep spinning it manually rather than just leaving it idle to heat up.

But these are all mostly just guesses at this point. I need to at least eliminate the obvious potential problems or causes.

[VincentG]

Looks great Tom but I think the foil lined upper insulation is a step backwards from just insulation alone. There's probably more thermal mass in the upper foil than the gas, so its likely influencing gas temp alot when not wanted, imo. Unless I'm misunderstanding its purpose.

[Tom Booth]

Looks great Tom but I think the foil lined upper insulation is a step backwards from just insulation alone. There's probably more thermal mass in the upper foil than the gas, so its likely influencing gas temp alot when not wanted, imo. Unless I'm misunderstanding its purpose.

Well, the idea of the foil is like an aluminized "space blanket" or old style aluminum on glass lined thermos bottle or Dewar; to reflect radiant heat. But you may be right. Looking it up, aluminum apparently has a higher heat capacity than any other metal, so how much heat is being absorbed vs. reflected?

Anyway, the engine did infact run nicely on the wood stove before melting down and the aluminum was added after that.

However, the epoxy has dried, and I was about to strip off the aluminum lining, IF there was still no compression, but, there seems to be quite a good pressure at this point.

In the process of setting up a video.

Other problems now though.

I over honed the cylinder which is still binding at times, but also leaky.

The displacer, because it is not free floating but fixed in place has to be ECACTLY centered or will bind. It also has to stay centered as it lifts and turns slightly. Since it is rather crudely made by hand, recentering after disassembly and reassembly is a real challenge.

The foam board also does expand when heated so the spirals start to get jammed and will not close, almost as soon as the engine gets warm enough to start.

To isolate the various problems, as soon as possible I'll be removing the piston and connecting rods and working the displacer manually with a balloon over the power cylinder.

The expansion contraction seems erratic at this point, but there are so many purely mechanical issues it's impossible to say what the cause may be.

[Tom Booth]

One thing that seems worth mentioning though, while fooling around, testing compression with the cranks disconnected and working the displacer by hand, I observed the same behavior you posted a long time ago.

You wrote:

I've demonstrated, at least to myself in this next video, that there is a constant flow of heat energy between the hot and cold sinks based on displacer position that we use to manipulate air pressure. https://photos.app.goo.gl/Xa9e8qaf5dHpkrb89

viewtopic.php?f=1&t=5539

In both instances this seems rather odd to me.

In your experiment it is at least explainable with the metal cold plate and ice. But why there should be such a proportional or parallel motion in this engine with a spiral displacer and rather thoroughly insulated upper interior is rather mysterious.

Unfortunately I did not capture that on video. I'll try to when I get some time.

I thought it was a little puzzling when you demonstrated it. The apparent exact proportionality between the displacer position and the power piston position. But with no cold plate and no ice? It seems unlikely the aluminum foil backed by hollow microspheres and styrofoam could be absorbing that much heat, repeatedly. The behaviour did not seem to diminish with repetition.

Also, there is almost free passage of air through the spiral displacer when in any position other than all the way down.

I tend to imagine that the heating and expansion of the air below the displacer on the hot side is almost entirely due to continuous radiant heat being emitted into and reflected around in this lower space. Increasing or decreasing the volume of air within this "isolated" space controls the degree or percentage of expansion and contraction.

Above the displacer, on the cold side is "shaded" from the infrared radiation which keeps that lower volume of air expanded.

That the same effect can be produced regardless of their being a metal plate with ice or a virtual Dewar insulation chamber seems rather remarkable.

Bumpkin said in that thread:

I know I’ve harped on it for years now, but I think we should give radiant exchange more consideration. Solar heat doesn’t get here by conduction or convection and it has quite an affect

I don't see how a division between the hot and cold side of a spiral displacer could be maintained except due to radiant heat. Or how the "shaded" side could be proportionately cool other than by blocking radiant heat, as a spiral spring type displacer is hardly a barrier to convection.