An expansion engine operating on a Brayton cycle which is part of a system for producing refrigeration at cryogenic temperatures that includes a compressor, a counter-flow heat exchanger, and a load that may be remote, which is cooled by gas circulating from the engine. The engine has a piston in a cylinder which has nearly the same pressure above and below the piston while it is moving. The piston and valves can be either mechanically or pneumatically actuated and the pressures above and below the piston can be nearly equal by virtue of a regenerator that connects the two spaces or by valves.