US Patent 7111606 Rotary positive displacement device

A device to convert energy having exterior and interior rotors where the number of legs (Λ) of an interior rotor divided by the number of chambers (X) defined by the fins of the outer rotor is equal to the effective radius of the inner reference circle ri divided by the effective radius of the outer reference circle ro (i.e. Λ/X=ri/ro). Where the surface of the fins of the outer rotor and the toe and heel portion of the interior rotor allow for a sealed chamber for a finite amount of rotation of the inner and outer rotors.