EXHAUST PURIFICATION SYSTEM OF INTERNAL COMBUSTION ENGINE

摘要

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.