To provide refrigeration below 200 K, a Brayton cycle engine contains a light reciprocating piston. The refrigerator includes a compressor, a gas-balanced reciprocating engine having a cold rotary valve, a counterflow heat exchanger, a gas storage volume with valves that can adjust system pressures, a variable speed engine and a control system that controls gas pressure, engine speed, and the speed of the piston. The engine is connected to a load such as a cryopanel, for pumping water vapor, through insulated transfer lines.