Aluminium Cylinder, Best piston material?

[mattedwards90]

Hi there,

As part of my university degree course we are making a rhombic drive beta Stirling engine, which is powered by a solar collector. Due to machining capabilities and heat transfer the cylinder material was chosen (and ordered) to be aluminium.

I have been trawling around the internet trying to find out which material we should use for the piston. The piston has to be sealed as well (obviously!!) so we were thinking graphite, or bronze(in the form of an oilite bearing bush).

Has anyone got any better ideas or experience with using aluminium?

Thanks
Matt

[Ian S C]

Look up a list of coefficients of friction, there is a list on this forum, you'll have to go back 4 or 5 pages. I would not recomend aluminium for any sliding surfaces, particulry on a stirling engine unless it is hard anodised, or as some 2 stroke motors are, hard chromed. For a piston, don't use oilite bush, this is porus, and of course contains oil, you don't want oil.Graphite is good, and you can get brass tube with a bore smooth enough to use. Glass is another option. For efficiency, you'v got to do research into materials, ie., friction, heat conductance, rate of corrosion with heat.
If you are making gears for a Rhombic drive, you must have a milling machine, and a lathe, with that gear you could turn up a cylinder in cast iron, and make the piston of the same material, this would give the best in bearing surfaces/for cost, second best would be a steel cylinder, with a cast iron piston. And if your tutor told you your orrigional idea, tell him/her you know better!! Ian S C

[Ian S C]

Sorry

[mattedwards90]

Ok, so we have to make the cylinder out of aluminium (6082-T6 grade), as the design has fins cut into the aluminium "tube" to act as the cooler for the engine. So we can't use aluminium on the cylinder/piston interface due to friction, is that right?

I'm guessing some sort of cylinder liner could be used? but this would effect the thermal conductivity of the cylinder, thus reducing the thermodynamic performance.

As for cast iron, we have never used it, and I'm not sure how to work it.

Thanks
Matt

[Ian S C]

Matt, you have it, a liner would be ideal, If it were posibile, thin walled seamless stainless steel wouyld be a good liner, some keep away from stainless because of its heat resistance, but that is negligible over the short distance of say .030".
What degree are you doing? Cast Iron, bought as cast bar is no more dificult to machine than steel, and its easy to machine fins in that, ideal for making flame eaters that get too hot for aluminium.