Piston heads- simple easy designs/ light materials?

I'm looking for an apropiate design soloution for making my own stirling engine piston heads. I plan on using O'rings for the seal and joined them to piston arms which in turn are connected to a crankshaft...please advice

Once I'v built I'll upload pictures/tutorial (over the next few month)

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Unless the engine is quite large, O rings will add way too much friction to the system for it to work. An accurately machined aluminium piston will be better, or graphite if you can find some.

As for design, keep the contact surfaces smooth, but apart from that, the design is not too important providing the mass is balanced on the crank.

If you can't get a good seal with a conventional piston, there are designs using a large flat piston mounted on a rubber membrane, which also allows you to pressurise the engine for more power.
dubpunkdub (author)  The Skinnerz5 years ago
A lot of useful information to take on board from quite a few people here.

My main concern is balancing the piston/crank to ensure the moment from the crank arm doesn't dis-balance the piston head so much it becomes a problem...would slight elongation on the piston head itself help guidance along with using a bearing attached to the crank arm and the piston head?
You will never get the crank completely balanced, and as the engine is very light it is not that important to get it fully balanced. Providing the crank stays fairly still around half way through the stroke against the friction of the bearings, even a small flywheel will eliminate the rest of the imbalance.

As for a long piston, it will help the seal, but at a point it will be counteracted by the increasing friction. A piston about as long as it is wide is probably best, as long as it is sufficiently lightened. (If you are machining a metal or plastic piston, leave turning the outside diameter until last to avoid warping the piston.)

Also, making the length of the connecting rods several times the stroke of the crank will minimise the friction against the piston and the sides of the bore.

Out of interest, what sort of size engine are you building?
dubpunkdub (author)  The Skinnerz5 years ago
140-160 mm per cylinder, its an unusual design so hopefully it'll all work out.
Any advice, guidance will be taken on board.
Much apreciated
I am also building my own Stirling engine and I have made my power piston out of steel that tightly fits into a brass cylinder no O-rings.
Kiteman5 years ago
I've seen pistons made of plugs of lightly-scrunched aluminium foil.
Should that be Tightly scrunched??
I meant "lightly".

The piston needs to displace air, but needs to have as little mass as possible.

Depends if he means the displacer piston or the power piston. I guess the power piston because the displacer isn't a tight fit in the cylinder.

You right for the displacer but I don't think it would do for the power piston.

Tightly packed foil is a little like foamed aluminium and could be used (perhaps) BUT it would be the devil to get a good smooth fit even if you packed it into the cylinder your going to use - I have tried that.
Ah, I was assuming he meant the displacer - simple Stirling engine don't have a power piston, they have a diaphragm (= bit of a balloon).

(The foil was shaped by pressing it gently into a can the right size.)
rickharris5 years ago
Many Models use graphite as a piston in an aluminium or glass cylinder.

Brass in steel is also a traditional and successful approach if you get the bore parallel and the piston a good fit. My brass piston will not fall into the bore under gravity as air pressure holds it back but left for a couple of days it will slowly decent - I spent a long time lapping the cylinder and piston to get them smooth and parallel.

Steel in Aluminium will work if well oiled.

If your hell bent on O rings then a lubricated glass cylinder - traditionally a test tube would be your best bet - Lubricate with mineral oil or the O ring will expand.

O rings are really too heavy for a Stirling engine. Most engines slide freely, but are honed in the bores to get that lovely free action.