Abstract: A piston engine comprising a piston movable in a recriprocating manner in a cylinder, while during operation a wedge-shaped gap is present between piston and cylinder. The piston divides the cylinder into a first chamber and a second chamber, which communicate with each other through the gap. A stable central position of the piston is obtained by means of the wedge-shaped gap, which mainly determines a pressure difference between the first chamber and the second chamber. A particular application of the piston engine is a cryo-cooler, by means of which, for example, processors of computers are cooled.