Building a small Stirling Engine

[Michaelpier]

Hello.
Iam trying to build a small engine and i need help on size details.
i found these two pipes and i want to use them.
The cylinder is 28cm(11inch)height and 5.4cm (2.1inch)diameter
The displacer is 19.50cm(7.7inch)height and 5.2cm (2.0 inch)diameter.
My question is these mesurments ok or i need to cut the height of the pipes?
Is there are any formula to caculate the right dimensions of both?

Thanks

[jimlarsen]

There are many variables to consider in answering your question. The amount of air that is displaced in the pressure chamber will have an impact on the displacement needed for your power cylinder. And this figure will vary depending on the temperature differential of your engine. The higher the differential, the greater the displacement of the power cylinder. Based on your pictures I think it is worth a try as it is.

The easy way to calculate the displacement of the power piston is to build the pressure chamber, piston, and cylinder, and then test it by heating the pressure chamber to the operating temperature you will be running it at. Cycle the displacer and measure the movement of your drive piston. Use that measurement to determine the stroke length of your drive piston. That way you know that your stroke/displacement will match the available energy being produced by the engine.

Jim Larsen
http://StirlingBuilder.com

[speedless]

Hi
Are you building a walking beam stirling?

With the heights of these cans you would need a stroke at 85mm.!
Mayby this will be usefull.
Some (roughly calculated) data for The Walking beam:
cyl./displacer ratio 100/33 % ( VOLUME/DISPLACEMENT)
heigth of displacer 2/3 of height of cyl.
2-3mm clearance between cyl./displacer.
Displacer and power-piston stroke:50-60mm.
The data should give some information on the relatives.I hope?
These criterias is hard to fullfill with the options you have in the shops, If you see a person shopping,armed with a ruler and desperatly measuring cans.Youll know whats up.
I can see the benefits from your cyl.choise;screw-on cap,alu...very nice.
I would really like a dismantable(?) cyl.
But I would find another can for the displacer and then cut the cyl. to fit accordingly.
More clearance and less stroke!
Try to find a diplacer-can with diameter 45-50mm,hight 60-70mm and then adjust the height of the cyl.
Or costum make one from alu- steelwool:See "Junkies" tread.
Good luck,hope to see some pics."eventually"
Jan
Sorry for my french

[Michaelpier]

Finnaly after your thoughts i decide to cut the cylinder pipe at 17cm, found new displacer 5cm diameter and 11.5cm height.
The stroke would be 5.5cm.

I didnt decide yet what kind of Stirling engine i would build
Was thinking/dreaming if is possible a free piston engine with two cylinders like the picture attached ?

Thanks,
Michael

[Michaelpier]

Thanks Ian.

I completed the first part and now need to go for the next, the air piston. :(

Whats your thoughts on size/volume? I guess must be the same as the chamber/stroke in cylinder.That is 5.5 the stroke by 5.4 the diameter of cylinder?
Sorry for the newbie questions
Michael

[jimlarsen]

I once had a displacer burst as a result of heat. The solution for me is a vent hole in the displacer. It does not need to be a sealed unit. I would think that aluminum would work better than stainless because it is lighter and to conducts heat faster. An aluminum displacer will act as a Stirling Regenerator as it cycles through the heat field.

[Junkie]

Hey it's looking good! Have you considered a balloon diaphragm ? Nice and simple, easy to build.

[jimlarsen]

Ian,

There seem to be several issues here. When you talk about keeping the hot side hot and the cold side cool, I assume you are talking about the pressure chamber, not the displacer piston. Yes, the pressure chamber needs to be able to maintain a temperature differential. And the conductivity of the pressure chamber material can be a deterrent to maintaining that temperature differential. So for that application I would agree that stainless is slightly better than aluminum. However the ideal would be to have a thermal insulator of some type between the two ends to interrupt the thermal short caused by any metal.

When it comes to the displacer, I think aluminum is a better choice because of its ability to heat and cool quickly. Rev. Stirling didn’t get famous for inventing the engine, his claim to fame is the regenerator, or what he called the “economizer”. An aluminum displacer will act much like a regenerator. As hot air rushes past the displacer to the cool side of the engine some of that heat is absorbed and stored in the aluminum displacer piston. This pre-treats the air and stores some of the energy as heat. When the cycle reverses and the cool air flows back over the displacer piston that heat is returned to the moving air. This process salvages waste heat from the working fluid and puts it back to work. This process of regeneration is what makes the Stirling engine more efficient than a “hot air engine” that has no regenerator. That is why metals with faster rates of conductivity are better in this application. We want them to heat and cool quickly.

The venting that I was referring to has to do with making a hollow displacer piston. If that hollow piston is a sealed unit, and it gets exposed to high temperatures, it is prone to become pressurized itself and burst. The solution is to make sure there is at least a pinhole vent so that the hollow displacer piston shares the same pressure as the pressure chamber of the motor. You mentioned a hollow displacer rod that acts as a vent to the outside atmosphere. The pressure inside the pressure chamber is rising above the pressure of the atmosphere of the room. If you have a hollow displacer piston that is vented to the outside, it will have a lower internal pressure than the pressure chamber of the motor. If it is not sturdy, it could theoretically be crushed. It is like putting a straw in a balloon and putting the balloon under water. The pressure of the balloon’s environment is greater than the pressure of the atmosphere and the balloon collapses.

Junkie has solved this displacer problem in a unique way. Instead of a hollow displacer piston he makes his displacers from rolled steel wool. That eliminates the pressure problem altogether and makes for a great regenerator. They are probably also light in weight and easy to make.

Jim Larsen
http://StirlingBuilder.com

[jimlarsen]

Ian,

I agree with you. Under extreme conditions you can't use something like aluminum. I have encountered this before. I was speaking under the assumption that we were talking about a can engine, not anything like the engines you describe. When the temperatures get very high, all materials must adapt. My experience is with the lower temperatures and the hand build engines.

I agree that dead space is a problem. But in my experience it is less of a problem than an exploding displacer piston. So yes, if you wanted to maximize efficiency you would eliminate all dead space.

However, I still like the idea (with can engines, at least) of a displacer that will also function as a regenerator. Stainless does not dissipate heat as well as aluminum or copper. So once it gets hot is it likely to stay hot. Stainless is more likely to contribute to overheating because of this. But if you look at the numbers, the difference may not be enough to have a noticeable impact on our little can engines. With can engines the biggest thermal short is the pressure chamber.

I hope to experiment in a future design with a displacer that is just a thin disk rather than a long cylinder. With some modification to the engine design I think it could work well.

All of my design work at present is focussed on LTD engines that use foam board as a displacer. I am not going to solve the energy crisis. But I have a lot of fun getting them to run with solar and hand heat.

Jim Larsen
http://StirlingBuilder.com

[Michaelpier]

Finally the previous engine fail after many attempts.So i let that project.

But i found a broken one on ebay and iam trying to rebuild it.
The cylinder was missing so i made a new one.(aluminium)
Now i am on the cleaning process.
What i was wondering and wanted to ask: is the usual displacer piston preferred instead of the aluminium rings with the small holes?

[Michaelpier]

Thanks Ian,
Yes i was thinking to install some kind of cooling.But first i will test it with out them to see if there is a change to the output.