The Importance of End Cap Shaping

[Jerry]

Saw this on YouTube today. A very good explanation of the importance of shaping the hot ends of the displacer and it's cylinder. While this video is specifically about a TMG, it is basically a free piston beta stirling engine. Obviously it's most important for a free piston design, but I believe the deformation of these parts is important to take into consideration in any high temp design.
[youtube]http://www.youtube.com/watch?v=K9kDvs4FQ68[/youtube]
I can't tell you how many videos I've seen of various can models that make a wap wap wap noise due to one end or the other deforming.

[Ian S C]

My Ringbom (3) and free piston engines (1), have no special shaping of the ends of the dis placers, but special care is taken to prevent collision of the displacer and its cylinder. while all the Ringbom motors started their lives with either steel or stainless steel displacers, the free piston motor started with an aluminium displacer, then another, and another, then I gave up and went to stainless steel, it could then be run at full temperature, and after it rebuild should go even better. Inside at the cold end is a loosely placed O Ring, on the outside, on the displacer rod is a spring that prevents the displacer hitting the hot end, it also gives the displacer a bit of a kick, don't know if that helps much. When the motor is running correctly, it does'nt make too much noise, the main sound is the O ring hitting the cold end of the displacer and its cylinder. Ian S C

[Corey872]

Interesting video. I definitely see his point. With a perfectly flat cap, it may tend to bow in or out when the heat is on it. You might get lucky and the displacer and cylinder move the same way...or unlucky and they go opposite directions. With even a very slight crown, you assure the piece will move in one specific direction when the heat is applied and can assure both pieces move in the same way.