finned riser pipe 27 the outlet of the aluminum intercooler is connected to a pipe fitting 30 which carries a safety valve 37 the safety valve 37 provides a means of warning personnel of excess pressure buildup the outlet of the pipe fitting 30 is connected to the inlet 31 of the high pressure cylinder 11 the outlet 32 of the high pressure 11 is connected by suitable fittings and conduits to form piping 39 which in turn is connected to an aluminum aftercooler not characterized returning now to fig2 it will be seen that the low and high pressure cylinders 10 11 each includes a piston 13 13 39 respectively it will be appreciated that each respective piston 13 13 39 is moved by a piston rod 14 14 39 which is connected thereto by a respective wrist pin 15 15 39 further it will be noted that each piston 13 13 39 is provided a plurality of conventional annular grooves for accommodating a set of upper compression rings 16 16 39 17 17 39 and a set of lower oil rings 18 18 39 19 19 39 the compression and oil rings are springy split metal rings for sealing the gap between the outer periphery of the piston and the inner cylinder wall in viewing fig2.