Piston rings, O-rings or what?

[steedspeed]

So! Best seal for a stirling engine power piston?

[Ian S C]

Best seal is a well fitting piston, no rings, a labyrinth seal on the piston skirt. Another type of seal can be used, that is in the form of a cup seal, either leather or plastic.
This is the piston in my stove top fan, as set up here it has a leather cup seal, an labyrinth seal grooves in the piston skirt. As it turned out the cup seal was not required.
Ian S C
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[Alfista]

Rizzo, among others, writes about using o-rings. Cast iron rings can be an excellent choice in larger engines. Pfte is an excellent choice for making rings but may wear more quickly than some other materials. Rings are probably not desirable in smaller sizes.

[steedspeed]

I'm going with two 4 inch diameter pistons.

[Alfista]

I would suggest a lightweight, aluminum piston with 2 rings per piston, each ring .188" to .25" in width. That way, you can also reduce the diameter of the piston and avoid some friction along the length of the piston. If your cylinder is cast iron, use cast iron rings.

[steedspeed]

Thanks! BTW, has anyone tried using a rolling diaphragm seal?

[Ian S C]

Have not tried the rollsock, but that way gets rid of the friction of the piston, as this is made a non contact unit. Here is the piston for a 4" Heinrici motor, the top end of the displacer is at the bottom of the photo.
Ian S C


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[steedspeed]

I thought about the diaphragm but I want the cold cylinder walls to contact the air as it reaches BDC

[Ian S C]

The cooling system on a diaphragm type engine would be in the area of the diaphragm, about the same area as a piston in a cylinder. I think I'll stick to cast iron pistons in either steel or cast iron cylinders.
Here's the displacer for the little Heinrici motor.
Ian S C

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[Alfista]

Great photos Ian ! Thank you for posting those !

Wayne

[steedspeed]

I love those old engines. This will be my first hot air engine. Before that I built a "Masie" live steam engine years ago.

[Ian S C]

I'd love to get one of those of my own, that one belongs to a friend who lives a bout a hundred metres down the street.
Just looking at the displacer, its about 3.5 x its diameter long, the usual quote on length is 3 : 1, but a little longer is no problem, I think some of mine have gone to 4 : 1.
Ian S C