A power unit with a diesel free piston device which is fed with air from a compressor device (96), and whose exhaust gas runs a turbine (150). The device has a cylinder (12) with two pistons (24, 26) which are movable in anti-phase and which between them define a combustion chamber (54). The cylinder end sections (36, 38) comprise buffer end chambers (56, 58). An electronic device (160) controls the pressure of the air in the end chambers (56, 58, 256, 258). A central piston section (48, 50) of each piston together with a related cylinder section (40, 42) forms a piston pump chamber (68, 70) which supplies the device's inlet manifold (80) and a mixing chamber (148) with compressed air, exhaust gases and the compressed air being mixed in the mixing chamber before being passed to the turbine (150). The electronic device (160) also controls the pressure of the air in the compression chambers (68, 70) by means of the compressor device, valves, pressure sensors, etc.