Rotary displacer...

[brian hughes]

....has anyone tried it? I'm talking about a semicircular cross section displacer rotating in a cylinder with the hot source on one side and the cold sink on the other? It won't match anything with a regenerator for efficiency, but it will eliminate a lot of the reciprocating mass.

[airpower]

Like this, he has done lots of stirlings, not sure if he done a reverse alpha, anyway one can loose time fast watching his engines.
https://youtu.be/HLWh-6Lgoao