teflon as a piston.

[minh-thanh]

Hi all !
I have a question :
Has anyone used teflon as a piston for stirling engines or flame eater? And if not use it, why?
Thank you !

 

[neil_1821]

I did try Teflon in my Stirling engine but I found that it caused too much drag. I also tried a material called peak but again I found that it caused too much friction. The best result I had was using a graphite piston in a glass cylinder although I’m sure others may comment as I never made one with a metal cylinder.

 

[minh-thanh]

neil_1821 !
Thanks for the reply!
I am intending to make piston by teflon with glass cylinder for stirling engine , I hope to have more opinions before I decide to do, or not

 

[doc1955]

Trouble with teflon and other plastics is that they absorb moister and will swell causing friction.

 

[minh-thanh]

Thanks doc1955 !

 

[neil_1821]

The only other bit I would add is that depending on what glass you use for the cylinder they are not always uniform in size so it’s possible to have loose spots as well as tight spots. The good thing with graphite is that it’s a natural lubricant. Oiling anything on a Stirling is generally a big no as it can cause more friction.

 

[Orrin]

My experience with plastics--Teflon, for instance--is their dimensional changes when the temperature goes up or down. IOW, they have a large coefficient of thermal expansion.

 

[fcheslop]

I have tried using ptfe with limited success
The piston was of the cup style very much like a bicycle pump and the seals formed in a die using thin sheet. The engine ran better with a normal piston

Iv attached a spec sheet and a link on how to make the piston cups
http://www.vapeuretmodelesavapeur.com/piston/index.html

 

[minh-thanh]

Thanks for all reply !
fcheslop
thanks for link . There are many interesting to learn

 

[lennardhme]

I concur with the above. Stirling engines develop that little power, they need every help they can get.
Good luck.

 

[minh-thanh]

Thanks lennardhme !
Another question :
With a glass cylinder, material can make a piston - except graphite ?


x

 

[Mechanicboy]

I used thin oiled leather as sealant + steel disc and screw on top of the piston in Stirling engine, it works well due it keep oil in leather and keep tight against leakage. Friction not big problem with leather. Stirling engine from 1995. In the flame eater can be a problem with hot cylinder, but take a try first and see what happend. The leather must be reversed due the flame eater is running with vacuum instead pressure in the cylinder.

 

[Murph]

The best cylinders are made from matched graphite pistons and pyrex tubing, purchased as dashpot cylinders.

Easiest thing to use is a pyrex syringe, cut to be used as a matched piston and cylinder - heat has no effect on it, and needs NO lubrication, as well.

Murph

 

[fcheslop]

Iv used brass and bronze in glass cylinders without problems or maybe I was just lucky
Glass syringes make good cylinders as the bore is more consistent unlike test tubes although I used one for Jans Thermo pulse engine I had to sort through them and if sorry when I broke it another piston was needed as every tube had a different bore.Pyrex seem to be that bit better but not by much.
Good luck

 

[lennardhme]

A polished glass piston works almost as well as graphite. Difficult to find with a perfect fit, but you may try a chemist [drug store to you I think] supply co. They usually have a great supply of such things, at least in Oz.
Glass syringes used for specific purposes use glass pistons which are a good fit.
Good luck.
I'd still go with the graphite though.
Lennard.

 

[Cogsy]

I bought some very cheap Chinese glass syringes off eBay and they are surprisingly well made with excellent sealing between the glass plunger and tube. Paid around $7 for 5 including shipping.

 

[minh-thanh]

Thanks for all opinions !
I like to make glass cylinder, it's easy to see how it works (stirling enigine, flame eater, and intrernal combustion engine) but it looks like a challenge , i will try to do it.