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Thanks.

I think the diaphragm can be attached just about anywhere on the pressure vessel.

matador wrote:My displacer is made out of the bottoms of two soda cans. After the JB weld set up I tested for leaks by dipping it into my hot tub.

Why does the displacer need to be air tight? Steel wool isn't.

The sealed displacer and the steel wool displacer are each a different appraoch. The sealed displacer functions as a solid mass. It causes the air in side the engine to move from one end of the engine to the other. It is a loose fitting piston. It moves the air by "displacing" it.

The steel wool does not offer much for displacement, as it has less cubic displacement. It functions as a "regenerator". Heat is absporbed as the hot air passes through the steel wool. That helps the air cool a bit before it reaches the cool end of the motor. That heat then transfers back to the cool air as it returns to the hot end of the motor. This pre-heats the air on its way to the warm end of the cycle.

So which one, will perform better, everything else being equal? My guess is the one with steel wool would run slower. I can't imagine steel wool lasting very long at higher revs, it would shake itself apart. Will floral foam work better than steel wool? I've read about it but haven't seen comparisons.

I have not seen a test that compares them. Pop Can Engines are difficult to build precisely enough for real scientific testing. But if YouTube is any indication, they both work about the same.