discerned separately the exhaust gas bore 4 ahead of the plane of the drawing and thus of the common cross sectional plane leads to a cylinder in front relative to the drawing while the exhaust gas bore 5 behind leads to a cylinder behind the drawing plane correspondingly the cylindrical rotary valve 1 also contains two exhaust gas ducts 6 and 7 of which duct 6 ahead of the drawing plane communicates precisely with the exhaust gas bore 4 whereas the exhaust gas duct 7 communicates with the exhaust gas bore 5 associated with it only following clockwise rotation of the rotary valve 1 by 90 u00b0 the two individual exhaust gas ducts 6 and 7 converge in the region of the axis of rotation 8 of the cylindrical rotary valve 1 so that both contain the common conduit 9 which in case of a four cylinder four stroke engine forms an angle of 90 u00b0 whose peak is placed on the axis 8 the common conduit 9 has two mouth points 10 and 11 on the periphery of the rotary valve 1 of which the mouth point 11 is placed directly below the exhaust gas opening 3 in the housing 2 so that the conduit arrangement produces a connection of the flow between the cylinder which is the front cylinder in the figure via the exhaust gas opening 4 the exhaust gas duct 6 the common conduit 9 and the mouth point 11 whereas following swivelling of the rotary valve by 90 u00b0 the.