Abstract: A cylinder head of an internal-combustion engine, operating with the four-strike Otto cycle, has a pair of diametrically opposite outlet ports and a pair of diametrically opposite inlet ports offset by 90.degree. from each other. Two coaxial annular shutters in the cylinder head, corotating with a speed ratio of 2:1, have each two mutually opposite solid quadrants and two intervening quadrants with valve apertures registering with the outlet ports and then with the inlet ports in two angular positions of the slower-rotating shutter spaced 45.degree. apart, both pairs of ports being blocked during the next quarter-turn of this shutter. A flow divider in the cylinder head directs richer components of an aspirated fuel/air mixture toward a spark plug while leaner components are diverted toward an associated piston head.