Abstract: A trochoid-type rotary piston gas expansion device including a stationary housing and endwalls defining an epitrochoidal cavity closing where the cavity is symmetrical about a first axis transverse to the end walls, a rotatable crank shaft extending through the cavity parallel to the first axis, and having an eccentric portion located on the shaft in the cavity, a rotary piston mounted on the eccentric portion of the shaft so that the axial centerline of the piston describes a circular path of selected diameter during rotation thereof where the rotary piston includes flank surfaces which intersect at apices to determine lines of sealing contact with the walls of the cavity.