Power Piston & Cylinder Materials

[jstluise]

Hello everyone. As an engineering student, I have the opportunity
to create a displacement style Stirling engine with my peers in a
manufacturing class. The engine is similar to the one in this link:

http://www.cse.iitk.ac.in/~amit/courses ... main2.html

The crankshaft will be turning a fan blade, and a stand will be
created for the unit. There are other groups making fans so there
will be a competition for highest RPM...so performance and tight
tolerancing is key!

ANYWAYS, to my question. The parts have been divided and I am in
charge of machining the power piston cylinder and power piston.
Currently, the power piston cylinder will be machined from brass and
the piston from steel. This is what the plans call for and what
previous classes have used.

Of course, since I am responsible for a vital part, I want to make
improvements. After doing some research, it seems as though
graphite or PTFE are desirable materials to use because of their low
friction factors.

So, I'm looking for some input on this. What would you recommend?
Should both parts be made from the same materials (both PTFE or both
graphite)? What materials would be best for maximum sealing (ie max
compression)?

I was thinking of maybe making the brass cylinder with a sleeve made
of one of those materials, with a piston made from one of those
materials. Eh? That would for sure reduce the friction...as long
as maintaining a seal won't be an issue.

And if I get my hands on some PTFE, I can give some to my other
group members for some upgrades...such as the bushings in the power
and displacer connecting rod (which now calls for split brass
bearings).

Thanks for any assistance! I'm really excited for this project

[stan.hornbaker]

Your link took me to a page about the Myranda or Moriya SE fan. These kits are sold by "A Stirling Mecca" They have a steel piston in a brass cylinder. Most Stirling engines require a close fitting piston to prevent loss of gas pressure but also be nearly friction free.
Smaller LTD (Low Temperature Difference) engine such as the Lufkin shown at the Mecca site use a pyrex cylinder and a graphite piston.

A good test for the piston-cylinder fit is to insert the piston in the cylinder and let it free fall through the bore, then close the bottom end air tight and again let if fall. This time it should take 30 seconds to a minute. HINT:check out all links on the web site.

[jstluise]

Thank you for your response. And thank you for the info on the engine. We are building a machine based off of that design, but with minor modifications to some of the parts...that don't really affect the way the motor runs.

I understand that the piston must be tight fitting with a minimal amount of friction, and that is what I want to improve over the brass/steel combo. I guess my question should be: Is having a very precise brass cylinder matched with a precise fitting piston give me the best performance? Or is there room for improvement?

The first thing I thought of for improvements was the materials (which is why I brought up the teflon and graphite). If the performance can be improved, I'd like to know the proven material and combination for the cylinder and piston.

Would using teflon or graphite only be benificial to a smaller motor where weight is a concern?

Thanks!

[stan.hornbaker]

Friction at all rolling or sliding surfaces must be kept to a minimum. Weight of the moving components need to be light weight to minimize intertia forces when in motion. Graphite would be ideal for a piston but it is a little expensive. Look for in via Google. Select a fine grained product. Otherwise almost any combination of dissimilar materials can be used if they can withstand the heat.
Plastic is difficult to machine to precision dimensions.
Polished steel shafts running in polished brass bushing (Hobby Shop Tubing) will work well. Run dry or lubricate sparingly with Wynn's Marvel Mystery Oil thinned to 1:10 or 1:5 kerosene or equivalent.

All too many assignments consist of a frustrating call for the design of a Stirling engine with no clue as to specifications etc. leaving the student(s) in a quandry. Your instructor is to be commended for using a practical construction project which will teach more that the other kind.

[jstluise]

Thanks again for the help.

I did a little searching around for some graphite and came across one site with reasonable prices for a fine grained rods:

http://www.beckergraphite.com/stock.html

The JC4 grade: "Strong fine grained. Machinable. Good density and strength. Medium temp. Mechanical apps."

Not too bad, since I'll only be needing a 12"L x .75-1" OD stock ($4.60) for the piston...anything larger and the price goes up.

I think if I could get my hands on that, I could make a graphite piston and then stick with a polished brass cylinder. I think that would work well. Then I could probably get some smaller pieces for graphite bushings if my other group members are intersted. Never have machined graphite so I don't know how smaller parts will work.

It would be really sweet to make the cold end out of graphite since the thermal conductivity of graphite is much higher than aluminum. But, it would be spendy and there may be structural problems.

Thanks again!