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This engine, as simple as it is, has a rather effective automatic speed control, (I think) as I've noticed after several months of occasionally taking it down off the top of the refrigerator and letting it run on my morning cup of coffee.


https://youtu.be/lx1tet8aHJU

There was something about this engine that bothered and puzzled me that for a while I could not put my finger on, well, I think I finally figured it out.

It looks like the motion of the displacer is very erratic.

Sometimes it seems to get stuck, or not even move off the bottom at all, other times it appears to "jump" up to the top, and does this back and forth continually.

I figured that the displacer is loose inside the chamber and sometimes rubs on the side or turns sideways or something.

But I also noticed that when the water is super hot, like right out of the microwave and a little "superheated" the displacer seems to barely lift off the bottom at all, just barely lifting, or maybe sometimes not lifting at all.

In spite of all the seeming irregular behavior of the displacer, the engine tends to run at a surprisingly steady speed, regardless if the water is very hot or just warm.

For a while I was not able to reconcile the discrepancy: odd, irregular "jumpy" displacer movement, but very steady RPM, even as the water cools down, the engine continues to run at a steady speed, until it suddenly just stops altogether.

I've decided to put my revelation here, because what I've decided is that this engine is acting quite like a "hit-n-miss" engine.

Because the displacer is being lifted by a magnet being lowered down close to it, then "dropped" by the magnet being lifted away again, the distance the displacer is lifted is determined by the speed of the magnet.

So if the engine begins to slow down, the magnet moves more slowly, giving it more time to lift the displacer further up before dropping it down again, this allows more heat to transfer into the working fluid so the engine starts running a little faster, but as it starts to pick up speed the magnet has less time to lift the displacer, so the displacer sometimes barely moves off the bottom, which reduces the amount of heat introduced, so the engine slows down.

This change in speed is so slight and subtle the result is a very steady running engine.

This constant "fine tuning" of the displacer's degree of displacement or "lift" keeps the engine running quite steadily regardless of the gradual change in temperature, down to the very end, at which point, there is no longer enough energy to complete a full revolution and the engine abruptly stops.

What appeared to be erratic behavior was actually a very simple but elegant "hit-n-miss" type speed control mechanism.

Perhaps this could be more controllable by having some sort of adjustment to raise and lower the magnet. Or some adjustable stop could limit the displacer motion.

My idea, earlier in the thread, was to limit the displacer motion by spring tension, actual spring or "air spring" pressure, with a Ringbom type displacer.

These magnetic engines are sometimes, rightly or wrongly refered to as "Ringbom". I guess, because there is not actually any hard mechanical connection between the displacer and the crankshaft.

I sort of came to all these realizations just now watching this video:

https://youtu.be/EDFTdYRK3Uw?t=166

The Ringbom engine depicted has a speed control that apparently works by restricting the air flow to and from the displacer chamber, thereby limiting the displacement and subsequently the amount of heat input.

Page 54 of this NASA document:

https://ntrs.nasa.gov/citations/19920022001

Titled: "Free-Piston Stirling Engine Conceptual Design And Technologies For Space Power" makes an interesting point about gas springs.

I'm posting it here because my intended design for a "hit-n-miss" so-called, type heat engine would probably utilize a gas spring, the original intended purpose of which, in my mind, was to eliminate the transfer of the working fluid from the hot to the cold side. In theory, this would reduce heat loss.

NASA apparently takes this a step further by charging the gas spring with nitrogen. Nitrogen gas has a heat conductivity about 1/6 that of helium.

I'm not entirely sure how that compares with ordinary atmospheric air, but I know helium and hydrogen have much higher heat conductivity than air.


Though, I'm not actually sure how much if any difference it would make, as any heat transfered to the air spring would likely be due to secondary compression.

That is, the air spring is compressed and, as a whole, gets hot, temporarily, due to the compression. A moment later however it expands, cooling off again. So, possibly more would be gained, or conserved, by making sure that the air spring chamber walls are non-heat conducting, then the heat fluctuations of the air spring gas should, I think, be largely irrelevant.

Still, there is some likelihood that a non-heat-conductive air spring would have some advantage in preventing heat from conducting from the displacer chamber hot side to the air spring, raising the average temperature.

This whole concept though, (my design, not NASA's necessarily) seems to me, a radical departure from the idea that the working gas must be transfered to the sink to cool it down each cycle. The NASA engine still has a displacer shifting air from a hot input side to a cold "sink" side. My intention is to entirely prevent such heat transfer, as far as possible.

Using a nitrogen gas spring would provide an additional barrier to heat transfer through the engine.

What is wanted is not heat transfer, IMO, but heat into work conversion.