by piston 154 during expansion at the crank angle shown in fig3 only primary intake ports 158 are uncovered while secondary intake ports 160 remain occluded by intake piston 154 although primary intake ports 158 are uncovered flow through intake ports 158 is prevented by reed valves provided in the intake duct petals 174 of the reed valve press against frame 178 sealing off flow between an intake plenum 164 and cylinder 162 in the situation illustrated in fig3 the pressure in cylinder 162 exceeds that in intake plenum 164 when pistons 152 and 154 move away from each other further and the pressure in cylinder 162 reduces petals 174 lift from frames 178 thereby allowing intake flow into cylinder 162 in the embodiment shown in fig3 and 4 reed valves are also provided in the secondary intake ports 160 with petals 176 sealing against stops in frames 180 in an alternative embodiment reed valves are provided only in primary intake ports 158 and not in secondary intake ports 160 in fig5 engine 150 is shown at a crank angle at which pistons 152 and 154 are further apart from each other than in fig3 exhaust ports 156 are completely uncovered the pressure in cylinder.