往复式发动机（1）具有环形并列的径向指向的活塞 - 缸单元（2,3）。 活塞（2）的向外行程由一个共同的偏心轴（12）的偏心轮（11）控制的，而它的向内冲程由控制环（16）（在一个侧控制和一割线，控制环的横截面形状的方向上延伸 每个活塞（2）的适配凹部（17）接合。 凹槽（17）由活塞（2）的底壁限制，控制环（16）通过内控制表面（20）与控制环滑动控制接触。 活塞（2）是中空的，并在其前侧带有隔膜阀的隔膜。 因此往复式发动机（1）具有特别紧凑的设计并且具有高度的弹性，使得其特别适合作为CO2车辆空调系统的压缩机。

The reciprocating piston engine (1) has annularly arranged side by side, radially directed piston-cylinder units (2,3). The outward stroke of the piston (2) is controlled by the eccentric (11) of a common eccentric shaft (12), while its inward stroke is controlled by a control ring (16) in a side extending in the direction of a secant, a cross sectional shape of the control ring ( 16) adapted recess (17) of each piston (2) engages. The recess (17) is delimited by a bottom wall of the piston (2), with the control ring (16) via an inner control surface (20) stands in controlling sliding contact. The piston (2) are hollow and carry on its face the diaphragm of a diaphragm valve. The reciprocating piston engine (1) thus has a particularly compact design and is highly resilient, so it is particularly suitable as a compressor of a CO ₂ -Fahrzeugklimaanlage.