A thermal cycle engine having a heat exchanger for transferring thermal energy across the heater head from a heated external fluid to the working fluid. The heat exchanger has a set of heat transfer pins each having an axis directed away from the cylindrical wall of the expansion cylinder. The height and density of the heat transfer pins may vary with distance in the direction of the flow path, and the pin structure may be fabricated by stacking perforated rings in contact with a heater head. Ribs are provided interior to the heater head to enhance hoop strength and thermal transfer.
