Alpha Type Stirling Engine Questions

[topspin247]

Hi everyone. I am currently trying to build an alpha type stirling engine, and I have been scourging the web for answers and tips on optimizing power.

Background Info
The cold piston is -60 Celsius, and the hot piston is 160 degrees Celsius. I am currently trying to use this temperature difference to generate 6 volts and a little over 1 amp (approximately 10 watts). I figured that a stirling engine would be an interesting project to build, and I have currently designed the engine from stuff I can find on McMaster. However, I am in need for advice!

Progress So Far
Please see CAD:


The inner diameter of each piston is 2.4 inches. The travel distance of each piston is 2.0 inches. They are identical, so that if the hot and cold sources are swapped, and the engine will run in reverse.

Questions
1. What should the diameter of each piston be?

2. What is the ideal volume change? Currently, each piston's travel distance is 2.0 inches. I should be minimizing dead volume, but should the pistons be as close to the exit hole as possible? What if the air cannot compress that much?

3. Should the hose be attached to the side of each piston and not at the top?

4. Do you have any recommendations for other areas of the engine design?

Some Previous Research

This link: http://www.solarheatengines.com/2012/02 ... g-engines/
explained how to size the pistons, but it was meant for the beta/gamma type stirling engines. The author, Doug Conner, explained that the volume ratio depends on the alpha engine's phase change. What does phase change mean?

I have also checked out Mr. Andy Ross's stirling engines on YouTube:https://www.youtube.com/watch?v=5pdqDQwehlk and it's incredible. I understand the general layout of the engine, but the specific dimensions are the most important design aspect missing from my CAD model.

Thank you very much in advance for helping out!

[Ian S C]

Topspin, first welcome, next how are you heating it, and cooling it? I can't believe that you are cooling it to -60*C, and if you are, how do you do it?
Your motor is what's known as an ALPHA type. The ratio between the hot and cold cylinders is 1:1.
There is a difference between the two cylinders, The hot piston should have a light weight stainless steel cap on it, 2" high would do it, this means that the cylinder will be higher too, take the duct to the cold cylinder from TDC of the solid part of the hot piston, so that one at the side, and the cold one at the top. The hot cap should be a thin stainless cup for best, and you can heat it with gas(not gasoline/petrol), or liquid fuel. You might use a metal tube between the cylinders. The hot cap, and the Heylandt Crown (the bit on top of the hot piston) are constructed in a similar fashion to the displacer of a BETA or GAMMA motor, the crown keeps the direct heat away from the piston.
I'm just going on what I know having built a Ross Yoke type of ALPHA motor. I would expect about 1 Watt per cc(or is it cu inch), my motor has a 35 mm bore, and 20 mm stroke, and about 5 Watt is a fairly normal out put. Ian S C Have a look in my gallery

[topspin247]

Hi Ian!

I am currently going to place dry ice around the cold cylinder to keep it at that temperature. I only need the engine to run for 5 minutes at a time, so the dry ice should last that long before sublimating.

Followup Questions:

1. What do you mean by the ratio between the hot and cold cylinders is 1:1? Are you talking about the diameters of the pistons?

2. Why does the hot piston need to have a stainless steel cap that is 2" high?

I plan to make my pistons out of PVC and PETG plastic to get minimize friction with the stainless steel cylinders.


3. "take the duct to the cold cylinder from TDC of the solid part of the hot piston, so that one at the side, and the cold one at the top."
Why take the duct from the Top Dead Center (TDC) of the hot piston, but go through the side of the cold cylinder?


4. "You might use a metal tube between the cylinders." Do you mean a metal tube for the duct?

[Ian S C]

Topspin, how are you heating the motor? Ian S C

[topspin247]

It is a chemical reaction between sulfuric acid and sugar. The reaction temperature reaches up to 160 degrees C at completion.

Physically, it looks like giant lumps of coal because the acid turns the sugar black.

https://www.youtube.com/watch?v=poDBrGIyTEk

[Ian S C]

Had a look at the video, but I can't see how it could be utilised as a heat source, and safty with the acid might be a problem. As far as cooling goes, I suppose a fire extinguisher could supply the cold, but part of the idea of running one of these motors is to do it as economicaly as possible. By using air/water cooling, and either gas or liquid fuel the motor can be run almost indefinitely. Ian S C

[topspin247]

Hi Ian,

Can you explain your previous post about the size of the hot and cold cylinders? Thanks.

[Ian S C]

Both cylinders have the same capacity, but because the hot piston has the little extension(the Heylandt Crown), the hot cap needs a like extension. there is a site that has a good animation of an ALPHA motor. Google Tips for building engines-Florida Association of Model Engineers. Then look for animated engines.
It would be worth while down loading(it's free), Andy Ross's book "Making Stirling Engines", It is 68 pages.
Ian S C

[raima55]

1 mm stainless is a bit thick, if you look up our engine '2-Cylinder Gamma Stirling - Project' somewhere down on the index page, our cylinders are .45 mm thick. However, you're going to use aluminium i think.

The pvc, what is the diameter of it, on the inside? I'm not so familiar with the imperial system. Is the diameter 6 and 1.5 inch?

a crankshaft made with loctite can be better. You can assemble it straight, it might be easier than bending, however, it wasn't strong enough for our engine. That had also to do with the gap between the parts, they must be very thight. In your case it's maybe better than bending, more accurate. Bending is also possible, but it's less precise, and maybe more flexy.

Do i understand right that you're going to build something similar as in the link from Tomostre? Keep us updated!

[Ian S C]

For the crankshaft, if you have a lathe, make the crankshaft from solid, you won't have any problem with joints coming loose, or the inaccuracy of a bent up crankshaft.
Make sure the hot cap is suitable for use with the heating system, it probably should be stainless steel.
From what I see there is quite a bit of danger in both the heating and cooling system of this motor, but you know what you are doing, just be careful.
Ian S C

[clark5901]

I would recommend that you down load Andy Ross's book (its free, 68 pages if you print it out), "Making Stirling Engines".