PURPOSE: To raise the overall efficiency of an internal-combustion engine, by connecting an oxygen-enriched air supply system to the intake system of the engine, and combining an exhaust-gas economizer of a system for recovering heat energy possessed by the exhaust gas with the exhaust system of the engine.

CONSTITUTION: An oxygen-enriched air supply system 4 is connected to the intake system of an internal-combustion engine, and an exhaust gas economizer 10 of a system 5 for recovering heat energy possessed by the exhaust gas is combined with the exhaust system of the engine. A compressor 7 driven by a motor 8 id disposed on the inlet side of an oxygen enriching means 6 and it is connected to a turbine 9. Further, a feed water preheating means 11 is disposed on the outlet side of the compressor 7. With such an arrangement, it is enabled to make an efficient use of energy and to raise the overall efficiency of the engine.