A hot air engine for converting heat into motion, comprising a wall member, comprising a cylindrical wall surrounding a central axis and an end wall extending through the central axis; a displacer member comprising a cylindrical displacer part co-axial with the cylindrical wall and a displacer end part extending through the central axis, the displacer member is movable, relative to the wall member, back and forth along the central axis to move a working fluid between a first chamber and a second chamber of the engine through the regenerator passage defined by the cylindrical wall member and the cylindrical displacer member; and a heating system provided with a heating zone separated from the first chamber by the end wall, wherein the cylindrical displacer part and the displacer end part together surround the cylindrical wall and the end wall, wherein the end wall defines the heating zone within the wall member.