Low temperature differential (LTD) alpha designs?

[cbstirling2]

Are there any low temperature differential Alpha designs? The ones I've read about have all been gamma designs having the displacer cylinder be short but rather wide.

If an alpha engine was designed for a low temperature difference, how would one do that? Have a large heat piston but a small cold piston?

[PetePeterson]

Changing the phase angle would change the compression ratio. 180 degrees would be 0 compression, closer to 0 degrees would raise compression. The temperature ratio follows the compression ratio.

I've never built an Alpha engine, but have read a lot about them. The first Stirling engine I built was an LTD Gamma from just a picture, no plans and it ran. When I ran the dimensions thru a LTD calculator it said it should not run. When I rebuilt it after it was dropped, I changed some dimensions and balanced the pistons and crank throws. The running temperature differential changed from over 7 degrees F to just under 3 degrees F. And it now matches what the LTD calculator says. Changing the phase angle changes the RPM and the running temperature.

So if you make an Alpha: balance it and get the friction down low, you might get the running temperature down low enough to call it an LTD
by playing with the phase angle.

---
Pete

[Bumpkin]

I'm not a fan of Alphas except for high temp versions, and even then they generally use heat sources that might be better and more efficiently spent elsewhere. But that said; you can lower the peak volume ratio to where it works with lower temps, with variable volume. The weight of a few inches of water in this novelty Alpha serves the purpose, but gravity limits speed. Springs in the rods or elsewhere might work for a more useful engine.

http://makezine.com/projects/two-can-stirling-engine/

Boiling water might not fit everyone's description of low temp, and yes the model is just a toy, but understanding might lead to better things.

Bumpkin

[cbstirling2]

Bumpkin, in regards to high temp engines, I find that my gamma/beta models perform poorly after running for sometime in one session due to the lack of thermal isolation in the displacer piston & cylinder between cold and the hot sides.

I presume a parallel twin cylinder Alpha engine would also suffer from the cold side getting too warm too quickly through thermal conduction. The wisdom of a V arrangement for an an alpha is very compelling. The hot and the cold Pistons are separated and the crankshaft is mechanically simple. The only drawback is that it takes up more space than a parallel Alpha engine. Also the passage between the two cylinders presumably takes more dead volume, however there's some research that suggests that there's little impact on performance from that.

[Ian S C]

Also in a V type ALPHA motor there is the advantage of a more simple crankshaft. The passage between the two cylinders can contain the regenerator.
With any of my motors ALPHA, BETA, or GAMMA, if they won't run all day without over heating the cold end, it's a failure, all have succeeded so far.
Ian S C

[Tom Booth]

As I understand it (or not) an LTD Stirling doesn't really have any "compression" or "compression ratio", in a conventional sense

The displacer shifts air from hot to cold, the pressure differential between internal and atmospheric increases and decreases and the piston moves in response to the pressure changes.

This is different from internal combustion "compression ratio" which involves an actual forced mechanical compression of the air/fuel mixture

I'm curious what you have in mind in terms of an alpha Stirling with "bellows" (instead of pistons ?).

The bellows would reduce friction considerably, which I think, could allow an alpha LTD to function, also the bellows-accordion type chambers could provide a large surface area for heat exchange.

The main concern I think, would be: what bellows material could be used that would be flexible, airtight, and yet allow for very rapid heat exchange.

Perhaps some sort of heat conducting polymer film.

https://www.sciencedaily.com/releases/2 ... 182815.htm

I can't locate any commercial source for this new material. Not to say there is none.

[Tom Booth]

...
With any of my motors ALPHA, BETA, or GAMMA, if they won't run all day without over heating the cold end, it's a failure, all have succeeded so far.
Ian S C

That is a very telling statement.

[Tom Booth]

This is an interesting engine. I believe it is a very old design. Many years ago I saw a picture and diagram of such an engine, I think, from some old magazine article or something. It was very large with a big heavy flywheel.

I'm not sure in what category this type of engine might be classified, but considering it runs on hot water, like the coffee cup type LTD, it might be categorized as "low temperature"

I was not able to locate the illustration for the old antique looking model, but this is a more modern "tin can" version.

https://makezine.com/2006/08/12/the-two ... ing-engin/

[omblauman]

alfa means high temp LTD low temp, an LtD alfa is a contradiction in terms

[Tom Booth]

alfa means high temp LTD low temp, an LtD alfa is a contradiction in terms

I think Alpha, Beta, Gamma, refer to the mechanical configuration of the engine. Though historically, alpha engines have been mostly high temp, I don't know of any rule or law or even convention that makes such a configuration necessarily high temperature.