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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

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. Low pressure on a piston drive stem provides a force imbalance to move the piston towards the warm end.

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1. An expansion engine operating with a gas supplied from a compressor for producing refrigeration at cryogenic temperature, the expansion engine comprising: a piston and a cylinder, the cylinder comprising a cold end and a warm end, the warm end comprising a cylinder shoulder and a receiving portio

1. An expansion engine operating with a gas supplied from a compressor for producing refrigeration at cryogenic temperature, the expansion engine comprising: a piston and a cylinder, the cylinder comprising a cold end and a warm end, the warm end comprising a cylinder shoulder and a receiving portion,the piston comprising a drive stem and a piston shoulder, the piston reciprocally moving between the cold end and the warm end and altering respective sizes of a first inner space of the at the cold end, a second inner space of the cylinder between the cylinder shoulder and the piston shoulder, and a third inner space between the receiving portion and the drive stem;a first line connected to an inlet valve, the first line bringing high pressure gas from the compressor to the inlet valve, the inlet valve operatively connected to the first inner space for controlling admission of the high pressure gas into the first inner space when the piston is near the cold end and as the piston moves towards the warm end;a second line connected to an outlet valve, the second line taking low pressure gas from the outlet valve to the compressor, the outlet valve operatively connected to the first inner space or exhausting the low pressure gas from the first inner space when the piston is near the warm end and as the piston moves towards the cold end; anda third line connecting the third inner space and the second line, the third line providing gas to act on the drive stem;a pressure maintenance assembly for maintaining an operating pressure of the low pressure gas in the second inner space acting on the piston shoulder, the operating pressure being at about a same pressure as a pressure in the first inner space as the piston is moving;wherein the pressure maintenance assembly comprises: a first branch line connecting the first line with an inlet check valve, the inlet check valve operatively connected to the second inner space; a second branch line connecting an outlet check valve with the first line, the outlet valve operatively connected to the second inner space; and further comprising a third branch line connecting the first line with the second inner space; a third valve being disposed in the third branch line to control the flow of high pressure gas to increase a pressure in the second inner space while the piston is near the cold end. 2. The expansion engine of claim 1, further comprising a first throttle valve disposed in the first branch line and connected in series with the inlet check valve. 3. The expansion engine of claim 2, further comprising a second throttle valve disposed in the second branch line and connected in series with the outlet check valve. 4. The expansion engine of claim 3, wherein a speed at which the piston moves between the warm end and the cold end is changed by changing a setting of the throttle valves. 5. The expansion engine of claim 1, further comprising a second throttle valve disposed in the second branch line and connected in series with the outlet check valve. 6. The expansion engine of claim 1, wherein the third valve is an active valve or a passive valve. 7. The expansion engine of claim 1, wherein the third branch line comprising a port in the in the drive stem,the third valve is a passive valve. 8. The expansion engine of claim 1, further comprising a cooler in the second branch line to remove heat from the warm end. 9. An expansion engine operating with a gas supplied from a compressor for producing refrigeration at cryogenic temperatures, the gas supplied in a first line at a high pressure and returned in a second line at a low pressure, the expansion engine comprising: a cylinder having a cold end closed with a cap and a warm end closed with a cylinder head; a piston in the cylinder, the piston having a drive stem extending from the warm end into a sleeve in the cylinder head, the piston moving reciprocally between the cold end and the warm and altering respective sizes of a cold displaced volume, a warm displaced volume and a drive stem displaced volume, the drive stem displaced volume connected to a low pressure third line to the compressor, an inlet valve and an outlet valve connected to the cold displaced volume and alternating between being open and closed for admitting the high pressure gas from the compressor when the piston is near the cold end and as the piston moves towards the warm end, and exhausting it to a low pressure gas line to the compressor when the piston is near the warm end and as the piston moves towards the cold end; and a pressure maintenance assembly for maintaining an operating pressure in the warm displaced volume at a pressure sufficiently different from the pressure in the cold displaced volume to cause the piston to move, wherein the pressure maintenance assembly comprises a first branch line connecting the first line with an inlet check valve, the inlet check valve operatively connected to the warm displaced volume; a second branch line connecting an outlet check valve with the first line, the outlet valve operatively connected to the warm displaced volume; and further comprising a third branch line connecting the first line with the warm displaced volume; and one of an active and a passive a third valve disposed in the third branch line to control the flow of high pressure gas to increase a pressure in the warm displaced volume while the piston is near the cold end. 10. The expansion engine of claim 9, further comprising a first throttle valve disposed in the first branch line and connected in series with the inlet check valve,a second throttle valve disposed in the second branch line and connected in series with the outlet check valve. 11. The expansion engine of claim 9, wherein a passive third valve is contained in the drive stem. 12. The expansion engine of claim 9, wherein a speed at which the piston moves between the warm end and the cold end is changed by changing a setting of the throttle valves. 13. The expansion engine of claim 9, further comprising a cooler in the second branch line to remove heat from the warm end.