Abstract: An orbital pump or the like comprises a housing having first and second end walls and intermediate, axially spaced first and second annular, inwardly projecting wall members. First and second axial drive shaft portions are installed through the housing first and second end walls respectively. A spool shaped engine rotor having radially outwardly projecting end flanges is installed in the housing with the first and second end flanges closely adjacent to axially outer surfaces of the first and second annular wall members. The rotor is eccentrically rotatably mounted to inner ends of the drive shaft portions. The two annular wall members and rotor ends divide the housing interior into an inlet plenum at the housing first end, a central chamber and an outlet plenum at the housing second end. A pair of intersecting rotor vanes, mounted diametrically through major portions of the rotor and in mutual orthogonal relationship, circumferentially divide the central chamber into four working chambers.