in the manner described in conjunction with air inducted past inlet valve 19 greatly improves the overall engine volumetric efficiency it should also be noted that there are no external control valves used to sequentially separate the exhaust gas and air flows within their individual circuits in air receiver 27 turning now to fig2 and 3 which show a different method of staging the sequential flow of combustion gases and air through the upper and lower cylinder side ports 26 and 32 respectively by a modification of the piston crown looking first at fig2 piston 34 is slidably mounted in cylinder 4 in the same manner as shown in fig1 however piston crown 38 has a lip 39 which protrudes above its surface with opening 40 in said lip in some instances lip 39 may contain a plurality of openings such as opening 40 the swept volume 24 above piston 34 is brought into communication with the interior volume of air receiver 27 when opening 40 in lip 39 is aligned with the upper side port 26 therefore when piston 34 is in the position shown in fig2 at the end of the power stroke combustion gases flow through opening 40 into side port 26 and issue forth from jet 41 which is in this instance shaped as a convergent cone to show the versatility of the design the flow from jet 41 is directed toward diffuser 42 which for the purpose of illustration is also shaped as.