Piston rings, O-rings or what?

Discussion on Stirling or "hot air" engines (all types)
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steedspeed
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Joined: Sat Jan 20, 2018 1:45 pm

Piston rings, O-rings or what?

Post by steedspeed » Sun Jan 21, 2018 8:12 pm

So! Best seal for a stirling engine power piston?

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

Re: Piston rings, O-rings or what?

Post by Ian S C » Mon Jan 22, 2018 3:37 am

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
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Re: Piston rings, O-rings or what?

Post by Alfista » Mon Jan 22, 2018 9:07 am


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
Posts: 14
Joined: Sat Jan 20, 2018 1:45 pm

Re: Piston rings, O-rings or what?

Post by steedspeed » Mon Jan 22, 2018 12:59 pm

I'm going with two 4 inch diameter pistons.

Alfista
Posts: 91
Joined: Thu Feb 18, 2016 9:14 pm

rings for a 4" piston

Post by Alfista » Mon Jan 22, 2018 2:25 pm

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
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Joined: Sat Jan 20, 2018 1:45 pm

Re: Piston rings, O-rings or what?

Post by steedspeed » Mon Jan 22, 2018 2:35 pm

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

Ian S C
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Location: New Zealand

Re: Piston rings, O-rings or what?

Post by Ian S C » Tue Jan 23, 2018 3:45 am

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
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Re: Piston rings, O-rings or what?

Post by steedspeed » Tue Jan 23, 2018 9:29 pm

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

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

Re: Piston rings, O-rings or what?

Post by Ian S C » Wed Jan 24, 2018 4:53 am

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
Posts: 91
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Great Photos

Post by Alfista » Wed Jan 24, 2018 8:00 am

Great photos Ian ! Thank you for posting those !

Wayne

steedspeed
Posts: 14
Joined: Sat Jan 20, 2018 1:45 pm

Re: Piston rings, O-rings or what?

Post by steedspeed » Wed Jan 24, 2018 10:32 am

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
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Joined: Thu Dec 02, 2010 5:15 am
Location: New Zealand

Re: Piston rings, O-rings or what?

Post by Ian S C » Thu Jan 25, 2018 4:55 am

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

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