Coolant penetrating cold-end pressure vessel

In a closed-cycle thermal engine, of the type contained within a pressure vessel and having a piston undergoing reciprocating linear motion within a cylinder and a working fluid heated by conduction through a heater head, the improvement comprising:

• a heat exchanger for cooling the working fluid, the heat exchanger comprising a first material in thermally conductive contact with the working fluid, and a second and distinct material in thermal conductive contact with a coolant fluid;
• and
• a continuous section of coolant tubing providing for circulation of the coolant fluid to outside the pressure vessel, wherein a section of coolant tubing is contained within a cooler for directing a flow of working gas across the coolant tubing.