An aggregate, in particular a hybrid engine, electrical power generator or compressor, comprises a housing in which are arranged a piston motor part and an electric motor part, wherein the piston motor part has a first piston with a first end face and at least one second piston with a second end face, wherein the first piston and the at least one second piston perform reciprocating motions, wherein between the first end face and the second end face is provided a working chamber for a working medium, which chamber is periodically reduced and enlarged because of the reciprocating motions of the pistons, and wherein the electric motor part has an annular rotor which can rotate in the housing about a rotation axis that is fixed relative to the housing. The rotor surrounds the piston motor part.