Low temperature differential (LTD) alpha designs?

Discussion on Stirling or "hot air" engines (all types)
cbstirling2
Posts: 107
Joined: Sat Dec 17, 2016 9:35 pm

Low temperature differential (LTD) alpha designs?

Postby cbstirling2 » Sun Feb 26, 2017 9:39 pm

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?
CBStirling2

PetePeterson
Posts: 10
Joined: Wed Feb 08, 2017 12:33 pm

Re: Low temperature differential (LTD) alpha designs?

Postby PetePeterson » Mon Feb 27, 2017 8:28 am

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.

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Pete

Bumpkin
Posts: 121
Joined: Fri Mar 02, 2012 1:42 pm

Re: Low temperature differential (LTD) alpha designs?

Postby Bumpkin » Mon Feb 27, 2017 12:16 pm

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
Posts: 107
Joined: Sat Dec 17, 2016 9:35 pm

Re: Low temperature differential (LTD) alpha designs?

Postby cbstirling2 » Mon Feb 27, 2017 9:30 pm

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.
CBStirling2

Ian S C
Posts: 1954
Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Low temperature differential (LTD) alpha designs?

Postby Ian S C » Tue Feb 28, 2017 3:35 am

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


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