has an outer air or membrane or solid piston (resonator).
When we increase the pressure of gas the frequency will be higher and gas will be much harder in the engine . At this time we must use more weight for outer piston (or to use longer tube) and adjust its frequency also.
This is not easy in the closed system and under pressure.
Loop engine has not any outer piston, so we need not adjust it! When we change the pressure, the frequency will varies but the engine will work.
The wave can go through the inner membrane because connects only a small point to rod then tosses back it.
There is a need for further investigation.
It is not yet clear this engine is only worked by the pressure of the gas or mass of the rod. This engine is not easy to build, so I do not recommend it, only to those who like to play, and will not be bad mood.
It is important, regenerator (cool side) must be close to the inside membrane!