Abstract: A thermal engine with a power source located either externally or internally to its cylinders, such as a compressor or Stirling cycle engine. The engine includes a plurality of chambers in each which a double-effect piston is mounted. Each piston and chamber together form an expansion phase side and a compression phase side. Each expansion side of one piston and chamber assembly communicates via a fluid transfer circuit with a compression side formed by another piston and chamber assembly and directly opposite the expansion side. Each fluid transfer circuit is traversed by a gas conveyed from one chamber to the other by movement of the pistons. Each piston is equipped with an arm cooperating with means for transforming reciprocal circular motion of the pistons into circular motion of an output shaft. The chambers are distributed around the transforming means and the output shaft. Each chamber, piston and each expansion and compression side have a toroidal shape.