has a sealing surface which can at least partially block the cylinder exhaust opening when flap 22 39 is in the down position the sealing surface is then very close to the chamber of cylinder 1 and piston 4 and piston rings 40 come practically into contact with this sealing surface when the interior surface of cylinder 1 is scavenged fig9 shows an embodiment wherein cylinder 1a has a pneumatic injector 15a supplied with fuel through a pipe 16a and with air or more generally with a pressurized gas through pipe 17a this pipe is supplied with pressurized gas from crankcase 3 of another cylinder this other cylinder is similar to that in fig3 the elements common to fig9 and fig3 have the same reference numerals in the two figures there is a non zero angular displacement between the operating cycles of cylinder 1a and cylinder 1 reference 8a designates the exhaust pipe of cylinder 1a this pipe 8a is connected to exhaust pipe 8 upstream of valve 22 when the latter is common to the two cylinders otherwise a throttle valve may be located in the vicinity of the exhaust port of each cylinder upstream of the connection between pipe 8a and pipe 8 the angular displacement between the cycles of the two cylinders 1 and 1a allows a satisfactory injection pressure to be available as described in european application 88 401569 4 filed on jun 22 1988 to which reference may be made according to the embodiment in fig9 pipe 17a has no check valve in the part which connects crankcase 3 to pneumatic injector 15a thus this pipe does not act as a pressurized gas storage volume and.