Abstract: A reciprocating device which may be operated either as a compressor or an engine. Each cylinder has a reciprocating piston connected to a piston rod. Dual cylinder chambers are located in each cylinder on opposite sides of the piston. The pistons are connected to a scotch yoke which translates the reciprocating motion of the pistons to rotary motion at a shaft in the engine mode. In the compressor mode, the shaft is connected to a power source. The engine components such as the pistons, rods, bushings and cylinder lines may be high quality steel or a ceramic.