one or more pistons 205 driven by a crankshaft to pressurize fluid the fluid displacement device 201 comprises an inlet 202 connected to a pumping chamber 208 in which a piston 205 is reciprocally movable the pumping chamber 208 also comprises a discharge unit 206 connected to an outlet 204 the discharge unit may be a spring loaded valve the spring 210 exerts a force in the upstream direction and ensures that the valve 211 is closed a force in the opposite direction i e the downstream direction may open the valve 211 such that the pumping chamber 208 is in open connection with the outlet 204 the inlet 202 and discharge unit 206 are positioned at opposite ends of the pumping chamber it is possible however to position the inlet and the discharge unit at one end of a cylindrical pumping chamber in fig2 a to 2 c different stages in a working cycle of the fluid displacement device are shown the piston 205 is movable between a first position in which the volume of the pumping chamber 208 is maximal towards a second position in which the volume of the pumping chamber 208 is minimal in fig2 a the piston is in or near the first position in fig2 c the piston is in or near the second position movement from the first position to the second position is called the compression stroke movement from the second position to the first position is called the expansion stroke as can be seen in fig2 a the inlet 202 to the pumping chamber 208 is open when the piston is in the first position fluid may enter the pumping chamber 208 through the inlet.