Preparations for Building a Coke Can Stirling Engine
"You have to think like a watchmaker," says Jim Larsen, a long-time Stirling engine builder, author and educator. "You have to pay attention to the details. If you pay attention to the details, you have a better chance of success."
The major components of a Stirling engine are relatively simple and straightforward. While we're focusing on a soda can engine, engines have been built using materials ranging from paint cans to oil drums. Larsen said during a Thanksgiving challenge while visiting in-laws, he built a Stirling engine from assorted hardware store materials, including pots and pans.
Aluminum soda cans offer ready-made, preformed shapes perfect for the engines. They're also easy to work with and, of course, very cheap. And while not robust enough for serious usage, they'll withstand the micro-horsepower produced by most engine plans.
The pressure chamber is a vessel that holds the captive air, or working fluid, within the closed system. It is here the air is heated and cooled during the thermodynamic cycle. While air and pressure leaks can be the bane of many engines the pressure chamber actually needs a small controlled leak. Without this leak, the chamber would simply become a barometer and only react to changes in the barometric pressure of the air around it.
Larsen said many Stirling builders opt to change the working fluid in the pressure chamber from air to helium, which reacts better during the thermodynamic cycle.
The drive mechanism uses the expansion and contraction of the air inside the pressure chamber to drive a crankshaft. The drive mechanism can either be attached to the side of the engine, or be integrated into the structure of the engine.
For Larsen, the crankshaft is the most critical part of the engine, and influences every part of the whole, from timing, to displacer travel, to the speed of the flywheel, and the balance of the whole. "This is a part you want to spend time on to get it right," said Larsen.