Hot-gas reciprocating machine with self-centered free piston

In a hot-gas reciprocating machine, comprising at least one working space in which a working medium completes a thermodynamic cycle, the working space comprising a compression space and an expansion space of mutually different mean temperature during operation, said spaces being interconnected via heat exchangers, including a regenerator;

• and at least one free piston which is reciprocatable in a cylinder, one face of the piston varying the volume of the working space, its other face forming part of the boundary of a buffer space in which working medium is also present under a pressure which is at least substantially constant during operation and which corresponds to the mean working medium pressure in the working space, the improvement comprising a control mechanism responsive to deviation of the mean piston position from a desired nominal central position, for instantaneously bringing the working space in communication with the buffer space at instants corresponding to such an instantaneous pressure of the working medium participating in the cycle that the nominal central position is restored by supplying or extracting working medium to or from the working space as a result of the instantaneous pressure difference between the two spaces.