METHOD OF OPERATING AN EXHAUST PURIFICATION SYSTEM FOR AN INTERNAL COMBUSTION ENGINE

Inside an engine exhaust passage, in order from the upstream side, a hydrocarbon feed valve (16), oxidation catalyst (13), and exhaust purification catalyst (14) are arranged. A first NO X purification method which lowers the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (14) by a predetermined period while maintaining it lean so as to remove NO X without causing NO X to be stored in the form of nitrates and a second NO X purification method which switches the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst (14) from lean to rich by a period longer than the above period so as to remove NO X are selectively used in accordance with an operating state of the engine.