An electromechanical energy converter for a gaseous or vaporous medium comprising a stator (1) with two pot-shaped stator members (1a, 1b), each open at one end face and adjoining each other at these end faces, the stator members surrounding a working space (12) and forming a common magnetically permeated air gap (7) in which a coreless armature coil (25) is displaceably arranged. Arranged in the working space (12) is a freely oscillating piston (14) linked to the armature coil (25). Furthermore, at least one source (21a, 21b) for the medium, two reaction chambers (66a, 66b) each assigned to a head (15a, 15b) of the piston (14) and control means (17a, 19a, 22a, 23a; 17b, 19b, 22b, 23b) assigned to the reaction chambers (66a, 66b) for self-control of the piston oscillations are provided, by means of which the reaction chambers (66a, 66b) may be opened to the source (21a, 21b) during the piston oscillations for accepting medium under pressure, and to the working chamber (12) for releasing expanded medium (FIG. 11).