Free-piston engine having a fluid energy unit

Free-piston engine having a fluid energy unit including one or more pressure accumulators, the free-piston engine comprising a cylinder and a piston within the cylinder limiting one side of a combustion room and reciprocating in the cylinder between a first position in which the volume of the combustion room in the cylinder is at a maximum, and a second position in which the volume of the combustion room in the cylinder is at a minimum, an expansion stroke of the piston from the second to the first position taking place by expansion pressure within the combustion room and mechanical energy rendered to the piston by the expansion pressure being converted into hydraulic energy within the engine, the piston being equipped with a plunger-shaped extension comprising first, second and third axial faces, wherein at least the first and third axial faces limit displacement rooms the volume of which is reduced during the expansion stroke of the piston, said displacement rooms comprising supply and discharge means for supplying and discharging fluid to and from the respective displacement rooms from and to said at least one or more pressure accumulators, and at least the second axial face directed opposite to said third axial face and limiting a pressurized room in the first position of the piston in order to hold the piston in said first position prior to moving back to the second position, characterized in that of at least all displacement rooms of which the fluid contained therein is a liquid, the supply and discharge means connecting thereto comprise separate first and second channel means, and that the plunger-shaped extension drives displaceable closure means preventing discharge or supply, respectively, of liquid from or to, respectively, the displacement room via the first channel means unless during a first part of the expansion stroke and a second part of the compression stroke of the piston, respectively, and the first channel means having a low flow resistance, the second channel means comprising a quickly closing non-return valve and having a small waste volume of liquid at the end of the expansion stroke of the piston by arranging said quickly closing non-return valves near to the respective displacement rooms.