METHOD OF PURIFYING EXHAUST GAS EXHAUSTED FROM AN INTERNAL COMBUSTION ENGINE

In an internal combustion engine, inside an engine exhaust passage, a hydrocarbon feed valve (15) and an exhaust purification catalyst (13) are arranged. A first hydrocarbon feed method which injections hydrocarbons from the hydrocarbon feed valve (15) by predetermined feed intervals (ΔT) so that the air-fuel ratio of the exhaust gas falls to the demanded minimum air-fuel ratio (X) to thereby remove the NOcontained in the exhaust gas and a second hydrocarbon feed method which lowers the feed amount of hydrocarbons from the hydrocarbon feed valve (15) and feeds fuel to the combustion chambers (2) during a second half of the expansion stroke or the exhaust stroke are selectively used.