LTD magnetic vs gamma

[Canardian]

Has anyone done some comparisons between a LTD magnetic engine vs a LTD gamma engine, side by side under the same conditions?
I wonder which one works with smaller temperature difference.

[Bumpkin]

I had to search it. If what I saw is the same as what you're talking about, it looks very similar in principle to a Ringbom type engine. Advantages: They don't need a separate crank linkage to the displacer, they have a period of dwell at the heating/cooling ends, they're reversible, and sometimes they can even self-start. The downsides are a bit more (or sometimes a lot more,) noise from the displacer's sudden stop, the displacer moves faster so there's more air friction and less regeneration time, and the energy used to move the displacer is not returned when stopping the displacer, as it would be if linked to a crankshaft.

This is the first I've seen of this type displacer drive, I guess I would tentatively still call it a Ringbom. Since they take a bit of energy to initially "pop" the displacer over I would think the more standard LTD could run at a lower temp difference, but that's just a guess. Thanks for putting me on to what is for me at least a new system. You never know where the next inspiration will come from.

Bumpkin

[Canardian]

I found two interesting videos about the magnetic engine.

One which shows how to build one:
https://www.youtube.com/watch?v=jKhKs2LeY58

And another with seems to have regenerators built into the displacer:
https://www.youtube.com/watch?v=LcI9QH4nLF4

As you mention that the displacer moves too fast, perhaps the movement speed could be optimized with a less powerful magnet.
But you are right, the missing crank down pull motion of the displacer would still be missing.

[Tom Booth]

This micro-ltd is of the magnetic type.

It always impressed me that such a small engine could run so well. The way it is stacked on a coin? And then a washer, I thought it probably doesn't need much heat. It apparently also runs on the heat of a hand, (or finger?)

It also has regenerators built into the displacer.

https://youtu.be/czVQVCIi7CU

I think the fast suden movement is a benefit. Emphasis on think. I haven't run any tests.

I think the weight of the displacer being lifted by the magnet may also contribute in that the magnet is on the way down. Also the crank does not carry the weight when the displacer is dropped. Unfortunately this engine is no longer being produced.

A comparison with a "free displacer" might also be interesting.

http://www.bekkoame.ne.jp/~khirata/english/fpse.htm

[Bumpkin]

I didn't mean to say that the rapid displacer movement is bad, but simply that there are trade-offs. I should have been more clear. It seems likely that dwell time is more important than any loss to aero or regeneration loss.
I was thinking after my previous post about the return of energy to the crank towards the end of the downstroke as the magnet helps pull the piston down, but I'm not sure how that would balance against the energy expended breaking away on the upstroke. I think there would be a net loss since it would have to pull away from the magnet farther than it would be pulled to it, by the length of the displacer stroke. And I don't see any recovery of the energy used to raise the weight of the displacer to the top when it drops and simply hits bottom. These are just observations of what are likely compromises in the interest of simplicity. God save us from theoreticians waiting for perfection. (I fall too much in that camp myself.)

Bumpkin

[Tom Booth]

.... God save us from theoreticians waiting for perfection. (I fall too much in that camp myself.)

Bumpkin

LOL.

I'm currently reading a book I just sent for: "Stirling and Vuilleumier Heat Pumps, Design and Application".

Many of the theoretical Stirling and Vuilleumier Heat driven heat pumps covered in the book are mostly just that.; theoretical designs, without even a prototype.

For the life of me, I cannot understand how anyone ever expected some of these contraptions to work, even in theory. Some of the descriptions of how they are SUPPOSED to work, make no sense to me, logically.

I find myself having to force myself to keep reading. The explanations and illustrations in the book don't even seem to correspond with those in the original Vuilleumier patents.

I'm not sure if the fault is with me or the author's at this point.

It is worse than being just the work of theoreticians, it is like explanations, or descriptions of theories from academics who don't understand, or have much of any real interest in the original ideas, other than hoping that maybe some reader someday might come along and make some sense out of it all and come up with a design that actually works in practice.

Likely, I'll read and re-read the book several times, I'm only about 1/4 of the way through at this point, so it is really too early to say what insights might be gleaned from it.

Nothing described in the book so far, though, it seems, has ever been built, even in prototype, no less, as a viable commercial offering.

Not to say that the technology has not found some (secret and/or proprietary) practical applications. Mostly in the refinery industries and the military, which seems to be another reason the book can only go so far. "data are not available... because this research was confidential". (pg.43)

[omblauman]

What's the meaning of the original question? an LTD engine is normally a gamma engine, magnetic or not. A magnetic one can be more efficient, hence run on a smaller delta T or the opposite, depending on how good a job has been done on the losses. I am surprised not to see comments about magnetic engines concerning the phasing of the two pistons, an important issue in all respects. It would be nice to see a model where the phasing is changeable, with some kind of fine regulation, not a difficult task, to show how critical it is.

[Tom Booth]

What's the meaning of the original question? an LTD engine is normally a gamma engine, magnetic or not. A magnetic one can be more efficient, hence run on a smaller delta T or the opposite, depending on how good a job has been done on the losses. I am surprised not to see comments about magnetic engines concerning the phasing of the two pistons, an important issue in all respects. It would be nice to see a model where the phasing is changeable, with some kind of fine regulation, not a difficult task, to show how critical it is.

I've often thought that some form of timing advance mechanism on a Stirling would be worthwhile. There isn't any "ignition delay" as in an internal combustion engine, but there is an intermittent exposure to heat and the heat takes time to transfer to and expand the gas. As the speed of the engine increases, it seems to me that some form of advance is needed.

There is no intake, so no vacuum advance, but some form of centrifugal advance might be possible.

[omblauman]

Tom I am talking about something much more obvious, while the phase relationship between piston and displacer in a conventional engine is fixed by design, in a magnetically controlled one is not and depends on the variable distance between the magnets, so it's most likely badly wrong, by design. It would be good to provide a magnetic stirling with a screw somewhere in the right place to change the timing between the pistons and while at it let us know how sensitive the engine is to this regulation, a fun generic information.

[Tom Booth]

Tom I am talking about something much more obvious, while the phase relationship between piston and displacer in a conventional engine is fixed by design, in a magnetically controlled one is not and depends on the variable distance between the magnets, so it's most likely badly wrong, by design. It would be good to provide a magnetic stirling with a screw somewhere in the right place to change the timing between the pistons and while at it let us know how sensitive the engine is to this regulation, a fun generic information.

Ok.

So the engine is running. Turn the screw. The engine runs better or worse. Find the optimal position for the magnets.

So then after such adjustment, the timing is practically fixed in place.

But, suppose now, more heat is added. Or a clutch is engaged and the load on the engine changes. Or disengaged. The engine starts pumping water, sawing lumber, accelerating etc. The screw, or whatever, needs constant adjustment as conditions change, if the application involves changing conditions, and most real world applications do.

Short of a computer control, a centrifugal control is a common method of making such constant "screw adjustments".

So, I think we are talking about the same thing.

Instead of controlling a valve, as in a steam engine, a centrifugal mechanism could control the positioning of the magnet.

I can't find any examples of a Stirling engine with any such timing control, so this video will have to do:

https://youtu.be/8T6ucXiQWy4

Before computer control, not going back too far, automotive distributors all had centrifugal timing control.

The movement of the displacer in a Stirling, magnetically controlled or otherwise, is equivalent to "ignition", the moment heat is introduced and expansion takes place.

I think the lack of any such timing adjustment is one of the reasons that the applications for Stirling engines has been limited.

Early Ford's had a manual lever controlled by the driver to adjust the timing.

https://youtu.be/oaCZ5lB9EJ4

Advances in IC engines over the years has a lot to do with improvements in timing control. From a basic manual lever or "screw" to centrifuge and/or vacuum control to computerized.

https://youtu.be/msHlcjrnjV0

So, while we are essentially talking about the same thing, my screw just has some weights and springs so it doesn't need to be adjusted manually.

Having, as you point out, the correct phase relationship between the piston and displacer is the difference between a barely functional vs a high performance engine.

[omblauman]

I think the lack of any such timing adjustment is one of the reasons that the applications for Stirling engines has been limited.

If the engine shows a very limited sensitivity to timing, then there is no need to think about it any more.
Which I intuitively think is the case, in comparison to a gas engine, I was curious.
thinking about it it would be really easy to give it a try with a variable current external air coil to offset the internal magnet field.
Even simpler would be to design an LTD with the displacer driven by an altogether external coaxial coil.
A well designed coil could also work as a generator.
Oh no, now I am starting to think about a novel free-piston LTD design and waste my afternoon on it, oscar.-

[Tom Booth]

A Stirling engine with the displacer attached to the crank would, or is, typically, not very sensitive to timing. About a 90 degree advance, or somewhere in the ballpark.

Some faster, more efficient heat exchanger might make a difference. But BECAUSE the displacer is attached to the crank, heat delivery is rather washed out.

The magnetic displacer moves faster and potentially, independently, so I think it could benefit from more precise, controlled timing of some sort.

Probably not a waste of time.

Years ago I designed a heat engine with adjustable timing, the displacer controlled by a solenoid.

Designed on paper that is. Still determined to build it some day soon when I find the time.

Could the solenoid double as a generator?

Lift displacer up electrically, drop back down by gravity and recover the current - maybe? Store the charge in a condenser?

[omblauman]

According to me to be an efficient generator the engine needs to be self oscillating free piston, very different from a conventional magnetic LTD with a work piston going up and down carrying a small magnet, the same for a record low temperature LTD, no piston, no flywheel, just a displacer slowly oscillating by itself. The electrical energy extraction is relatively simple: two well coupled coaxial windings.

[Tom Booth]

... no piston, no flywheel, just a displacer slowly oscillating by itself. ...

Is there such a thing?

I've seen what I guess I'd call a PISTON oscillating by itself, a typical "laminar flow" running without a flywheel, but just a displacer?

Are you talking about a "free piston" with linear generator?

[omblauman]

... no piston, no flywheel, just a displacer slowly oscillating by itself. ...

Is there such a thing?

I've seen what I guess I'd call a PISTON oscillating by itself, a typical "laminar flow" running without a flywheel, but just a displacer?

Are you talking about a "free piston" with linear generator?

I don't see obstacles in principle but I spent minutes thinking about it and I could be totally mistaken, I know nothing about stirling engines in practice.
The closed volume has to expand and contract to produce mechanical work so a "piston" has to be present but it could be reduced to an oscillating membrane/bellow connected to a coil which is part of an under-dumped electrical oscillator. A typical LTD displacer, coupled magnetically to the same oscillator, would be driven with the right phase to sustain the oscillations. Not obvious how the losses would be shared among aerodynamic, mechanical and electrical.