PURPOSE: To reduce specific fuel consumption largely by adopting a uniflow scavenging system in an upper combustion chamber and adopting a loop scavenging system in a lower combustion chamber in the double-acting type internal-combustion engine.

CONSTITUTION: The upper section and lower section of a piston 2 vertically reciprocating in a cylinder 1 are provided with combustion chambers 4, 3. When the piston 2 falls, scavenge air flows into the upper combustion chamber 4 from a scavenging port 6, and a combustion gas is scavenged into an exhaust manifold 12 at the same time as the opening of an exhaust valve 8 through a uniflow system from the upper end of the cylinder 1. A scavenging system in the lower combustion chamber 3 is adopted loop scavenging system, and a combustion exhaust gas is inverted in the lower combustion chamber 3 and exhausted from an exhaust port 7 while being replaced with the scavenge air from a scavenging port 5 and forwarded into an exhaust manifold 11. Accordingly, specific fuel consumption can be reduced largely, and the cost of the engine can be decreased.