Abstract: The object of the invention is to provide an operating method of an internal combustion engine with critical water injection, in which the advantage of critical water is positively utilized to increase engine efficiency and output in addition to the reduction of the emission of nitrogen oxide and CO. It is characterized in that sub- or super-critical water of above 250° C. and above 10 MPa is injected during the compression or combustion stroke of the internal combustion engine by starting the injection before the fuel ignites in the compression stroke, particularly the injection is done so that said critical water interferes with the fuel spray injected into the combustion chamber, and the injection period is in the range of −90° C.˜+30° C. from the top dead center crank position.

Abstract: The present invention provide a feedback combined system of a reciprocating internal combustion engine, in which, in order to increase output and thermal efficiency by increasing fuel supply without increasing the maximum combustion temperature than the conventional level and concurrently reduce emissions such as NOx, a heat exchanger is provided downstream the exhaust turbine connected to the exhaust collector, sub-critical water of pressure of between 18 MPa and 22.1 MPa (critical pressure) and temperature of between 250° C. and 580° C. or super-critical water is generated by the heat exchange of water supplied to the heat exchanger with the exhaust gas, said sub- or super-critical water is injected into the cylinder in the range of 90° before the top dead center and 30° after the top dead center and fuel is injected so that the air excess ratio at the maximum output is 1˜2.5 and the maximum combustion temperature is 1600˜2000° C.