Abstract: A rotary piston heat engine system is composed of two units. Each includes two pistons mounted for movement in opposite directions. Each piston is mounted for rotation in a cylinder. The longitudinal axes of the pistons and cylinder are collinear. The pistons are mounted for movement in opposite directions. Effective cylinder displacements are formed in each case between two radial boundary surfaces of the two respective pistons which execute an angular motion relative to each other when the engine is operating. At least one mechanism superimposes a circular motion on the angular motion of the two pistons, and each unit includes a shaft for driving a torque-producing device. The system also includes a heater, a heat storer and a cooler connected to a pipe system by which the inlet ports and outlet ports of the displacements of the cylinders of the units are connected to each other.