PURPOSE: To enable it to control the timing of ignition even in case of such fuel as being low in a self-ignition temperature by premixing fuel in intake air uncontained oxygen at an inlet stroke, having it inhaled in a combustion chamber, and spraying this oxygen into the combustion chamber at the optional timing of a compression stroke before fuel combustion.

CONSTITUTION: When intake air is inhaled in a combustion chamber 3, atmospheric air is inhaled through an air cleaner 8 whenever a piston 2 goes down. At this time, this air is subjected to a removal of oxygen O2 from the atmosphere for separation by an oxygen separator 17, making it into such a gas as containing no oxygen and, after premixing this intake air with fuel, it is inhaled into the combustion chamber 3 from an inlet port 5. Then this intake air is compressed in time of an ascent of the piston 2 after an inlet valve is closed, and at a point of optional time in the compression stroke, gas inclusive of oxygen regulated with content and temperature is pressurized by an injection pump 21 and sprayed out of an injection nozzle 16, whereby it is mixed with the fuel 10 being compressed and raised in temperature up to more than a self-ignition temperature into reaction for combustion.