Abstract: An apparatus for determining position of a free piston used in such as hydraulic compressor, which has no mechanical connection with crank shaft, and an apparatus for controlling reciprocating motion of the free piston are disclosed. A free piston position determining apparatus comprises a magnetic sensor disposed in a case containing a free piston, and a magnet disposed on the free piston. The magnet sensor generates pulse train when the magnet is positioned within a pre-determined distance form the magnet sensor. Stroke of the free piston is computed based on duration of the pulse, and offset of the average piston position from designed average position is computed based on interval of the pulse.

Abstract: A regenerator having a plurality of involute foils disposed in an annular gap between an inner cylindrical tube and an outer cylindrical tube. The involute shape of the foils provides uniform spacing throughout the entire regenerator and substantial surface area for fluid contact.

Abstract: In a regenerator 1, on the surface of a strip-shaped resin film 2, a resin layer 3 containing an ingredient having higher thermal conductivity than the resin film 2 is formed; or, over a predetermined width from an edge of the regenerator 1, a resin coating 4 is formed. Then, the resin film 2 is rolled into a cylindrical shape to produce the cylindrical regenerator 1. In a flow gas heat regeneration system having the regenerator 1 disposed in a doughnut-shaped space, when a hot working gas flows into the regenerator 1 through one end thereof, the heat of the working gas is stored in the resin film 2. Here, the resin layer 3 or resin coating 4 on the resin film 2 enhances heat conduction in the regenerator. Thus, more heat is stored in the resin film 2. When the cold working gas flows into the regenerator 1 through the other end thereof, the heat stored in the resin film 2 is rejected to the working gas.