the engine comprises a base having a spool fastened thereon , the spool including a hub symmetrical about a first axis and having a pair of side plates normal to and having inside faces spaced apart along the first axis . there are plate ports in the inside faces . a cylinder rotor includes a second hub rotatable on the first hub about the first axis and includes a pair of side discs normal to the first axis and having outside faces spaced apart axially between said side plates to establish first and second axial clearances between the side plates and the adjacent side discs . there are cylinders mounted radially on the cylinder rotor with ducts extending from the cylinders to the disc ports in the outside faces radially opposite the plate ports . a driven shaft symmetrical about a second axis is eccentrically journalled in and extends through the hub . a rotor is fast on the driven shaft and carries pivot pins . there are pistons in the radial cylinders and connecting rods pivoted in the pistons and pivoted on the pivot pins . on the rotor are some spacing pins carrying eccentrics . eccentric strap bearings extend around the eccentrics and are secured to the spool . there are separate , constant volume oil pumps appropriately driven , each being connected to a respective one of the axial clearances in order to maintain the dimensions and shapes of the clearances such that gas leakage therethrough from the ports toward the atmosphere is maintained at a predetermined minimum value .