Stirling engine with near isothermal working spaces

A piston cylinder assembly comprising:

• (a) a cylinder comprising an internal bore extending along a longitudinal axis;
• (b) a head that caps a first end of the internal bore and that presents an internal surface facing the internal bore, the internal surface comprising:
• (i) a plurality of transverse valleys and transverse peaks that extend generally transverse to the longitudinal axis, adjacent transverse valleys being separated by a transverse peak;
• (ii) at least one central transverse valley;
• (iii) at least one lateral transverse valley that is laterally offset from the at least one central transverse valley;
• and
• (iv) a plurality of through holes extending through the head, each of the plurality of through holes opening at the internal surface within a corresponding one of the transverse valleys;
• (c) a piston received for longitudinal movement in the internal bore and comprising a piston crown disposed opposite the head, the piston crown presenting transverse peaks and transverse valleys that are complementary to, and longitudinally facing, the transverse valleys and transverse peaks of the head such that adjacent transverse peaks of the piston crown are separated by a respective transverse valley;
• and
• (d) a working fluid received in the internal bore between the piston crown and the head;
• wherein, on at least one lateral side of the head, the transverse valleys and transverse peaks follow a more tortuous path than the central transverse valley so as to increase a flow path length for the working fluid along the lateral side and to equal a flow path length of the central transverse valley, such that flow path lengths for the working fluid through the central and lateral transverse valleys are more nearly equal during a longitudinal stroke of the piston, thereby providing more nearly uniform residence times and heat transfer characteristics across different flow regions of the head.