Patent ID: 8776534

Claim:
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; and a 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.