This engine is operated by compressed gas and comprises two cylinders with pistons therein, on piston rods that are pivoted to cranks fixed to a control shaft. This shaft carries a gear meshing with direction reversible gearing. The shaft also carries cams for operating valves that connect opposite ends of the cylinders to compressed gas supply and to vent. Means is provided to block off the compressed gas supply and vent to both ends of each cylinder and to interconnect both ends of each cylinder, to permit one cylinder to be under power while the piston in the other cylinder idles and is dragged by the piston in the powered cylinder. Both cylinders are never devoid of power at the same time.