Abstract: It comprises a first cylinder (102), a displacement piston (104) sliding in the first cylinder (102), a conduit in which a thermal regenerator is included, a second cylinder (202), a second displacement piston (204) sliding in the second cylinder (202), means for displacing the first cylinder and the second cylinder in phase relation. A gas bearing (104a) is provided at the hot end of the first piston (104) and a gas bearing (104b) is provided at the intermediate temperature end of the first piston. A gas bearing (204b) is also provided at the intermediate temperature end of the second piston (204) and a gas bearing (204a) is provided at the cold end of the second piston (204).