Piston rings

[Gordon WParticipant @gordonw]

I've finally got most of the materials collected to make a model hot-air engine. The cylinder is brass 2 1/2" bore, piston is to be aluminium. I know this is not the best combination. I am hoping to fit PTFE rings, maybe 2 ,one at each end of the 2" long piston. Question is – can I use the rings as bearing surfaces ? The PTFE sheet is about 2mm thick, any thought on how much clearance to give the piston. I'm thinking on making the rings about 10mm wide. Any ideas on cutting the strips ?

[Gordon WParticipant @gordonw]

[Bob RodgersonParticipant @bobrodgerson97362]

Why not heat the cylinder in a domestic oven to about 200 degrees Celsius and measure it hot. Do the same with the piston and work out hot clearances from that.

[HopperParticipant @hopper]

Are you building this engine to a proven design? The reason I ask is it's unusual for a Stirling engine to have piston rings of any type. More common to make the piston a very neat sliding fit in the cylinder and run no rings, so as to minimize friction. As the power output of most Stirlings is very low, eliminating friction is paramount.

You may run into friction problems with an ally piston in brass cylinder as the ally will expand more than the brass as it heats up and thus tighten up. A steel piston might work better.

[Gordon WParticipant @gordonw]

Thanks for replies. It is a model of a Robinson engine, 1/3 scale, so in that respect it is a proven design, I have scaled it from various drawings and photo's. I can calculate the different expansion rates and agree it is not ideal, but I have these materials. I'm hoping that the rings will act as a sealing ring and allow a larger clearance than normal but work as a bearing surface. For interest the cylinder is a hand pump for beer, piston ally an offcut from somewhere, rest is bits and pieces ,scrap and etc. Not bought anything and hope I won't have to.

[Ian S CParticipant @iansc]

One of my Stirling Engines has a brass cylinder with an aluminium piston that has a carbon impregnated Teflon sleeve over it. Normally I use steel, or cast iron for cylinders, and cast iron for pistons.

The motor has a 5/8" bore, the cylinder is a bit of brass tube, no special finishing was done to the bore.

Ian S C

[Gordon WParticipant @gordonw]

Hello Ian, I seem to remember you doing something similar in an earlier post but can't find it now. I am thinking of using two broad rings, with scarfed ends. Hoping these will act as bushes and sealing rings. Have measured my bit of ally for the piston and can get two pistons out of it, so will try the rings first. Will have lots of swarf.

[Ian S CParticipant @iansc]

I have not got a photo of one, but car shock absorbers (mostly) have a plastic ring on the piston about 1/4" / 6 mm wide by about 1 mm thick, these rings have an inter locking step at the joint. A sheet of 1 mm Teflon and a sharp knife is all you need, with these wide rings I think the stepped design might be better than the scarfed/angle cut.

The above motor has just enough power to drive the little generator(motor ex casete tape machine), and power the radio in the foreground.

Ian S C

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