22 through elongated opening 33 thus as outer pistons 22 each of which have a similar structure reciprocate in the opening defined at central housing 12 valve porting communicating with the varying volumes accordingly defined are provided similarly elongated opening 36 defined as illustrated in right outer piston 22 provides an exhaust function which communicates in turn with exhaust port 35 connected to exhaust opening similar to that of intake opening 30 but positioned at the bottom of inner piston 26 rather than the top to provide appropriate timing similarly inner piston 26 is provided for example with inlet opening 40 communicating with inlet port 41 inlet port 41 merely opens at inner piston 26 since there is a static relationship while elongated openings 33 and 36 comprising inlet and exhaust openings respectively of right outer piston 22 are elongated since outer pistons 22 move relative to for instance inlet port 32 and exhaust port 35 each of the two outer pistons 22 and inner piston 26 have an inlet and outlet opening and porting relationship for instance inlet opening 46 communicates with left outer piston 22 in a manner identical to that described with reference to inlet opening 30 while inlet opening 47 communicates with bottom inner piston 26 as described above with reference to inlet opening 40 relative to upper inner piston 26 piston assembly 20 includes an opening 50.