The invention relates to an opposed piston engine comprising at least one cylinder, at least two pistons arranged to be reciprocated within the same cylinder in an opposed manner, at least one intake port through the cylinder wall, at least one exhaust port through the cylinder wall, at least one shaft arranged to be rotated by reciprocal motion of the opposed pistons, at least one reciprocatable sleeve valve within the cylinder for controlling porting of one or both of the at least one intake port and the at least one exhaust port, a sleeve valve driving mechanism for controlling reciprocal motion of the at least one sleeve valve, and a dwell mechanism. The dwell mechanism is configured to induce at least one period of dwell of the at least two pistons during their respective cycles of piston motion.