An engine/heat pump is shown. Most of its parts rotate around the same central axis. It comprises two doubly connected chambers. Blades in each chamber substantially rotate with the chamber and may be firmly attached to the walls of the chamber, thus forming a modified centrifugal pump with axial input and discharge. An expandable fluid is rotated outward by one of the pumps and then heat is added for an engine or removed for a heat pump as the fluid is being sent to the outer part of the second pump. The fluid travels toward the center of the second pump, thus impelling the pump in the rotation direction. Then heat is removed for an engine or added for a heat pump as the fluid leaves the second pump and travels back to the first pump near the center of rotation. Rotation energy of the fluid is typically much larger than the circulation energy. A modified centrifugal pump with axial discharge having a casing rotating with the blades is also claimed.