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I am designing a Gamma Stirling engine for an engineering project for school. Our goal is 50-100 Watts of power from a non-pressurized appropriate technology design. I'm curious about design trade-offs between the power piston seal and the friction in the engine. In other words, does anyone have any tips on how to achieve a sealed power piston-cylinder that will not have too much friction for an atmospheric-pressure engine? As appropriate technology, I'm restricted to low-cost solutions.

Graphite is the material of choice for pistons in modeling Stirling. Not so much a question of design but of proper material use here. Unknown what size your cylinder is going to be. A glass, copper or steel cylinder with aluminum, Nylon or brass piston can work but may require a lubricant which causes friction here. Graphite cost more but if you can't capitalize your project properly you will end up with a 50Watt door stop. Finding the sweet spot between cyl. bore and piston diameter is a trial and error fitting and may not be as critical in a larger bore engine. Once you go undersize on the piston its all over, you lost the seal and oversize you found friction. You can buy a graphite rod 3" X 20"L or a 2" x 48"L for $75 shipped (USA). That will give you several completed pistons. You will know your cost when you settle your cyl. size. Smaller sizes for less. Pool your funds with other students and buy a stick!

Have you seen a Ferrofluid Piston? It makes a low friction seal, but it may not work in a large motor. Check this out:

http://www.youtube.com/watch?v=21WzdjqAG0s