Abstract: A piston mechanism with a flow passage through the piston includes a cylinder which has a closable cylinder passage at one extremity thereof; a piston, axially movable in the cylinder, which has a closable piston flow passage and has in its outer circumference a groove in which a piston ring is situated for sealing between the piston and the cylinder; and a movement mechanism to axially move the piston in the cylinder. The movement mechanism includes a crank shaft which is mounted in a crank chamber and a piston rod which, on the one hand, is hingedly connected to the piston by a first bearing and, on the other hand, is hingedly connected to the crank shaft by a second bearing. The piston ring is mounted with a certain axial clearance in the groove. The piston flow passage extends over the groove and the piston ring forms a valve for closing and opening the piston passage.