Exhaust purification system of internal combustion engine

An exhaust purification system of an internal combustion engine comprising:

• an exhaust purification catalyst arranged in an engine exhaust passage, exhaust gas flow surfaces of the exhaust purification catalyst carrying precious metal catalysts thereon, basic exhaust gas flow surface parts formed around the precious metal catalysts, the exhaust purification catalyst having a property of reducing NOx which is contained in an exhaust gas when a concentration of hydrocarbons which flow into the exhaust purification catalyst is made to vibrate within a predetermined range of amplitude and within a predetermined range of period, the exhaust purification catalyst having a property of increasing a storage amount of NOx contained in the exhaust gas when the vibration period of the hydrocarbon concentration is made longer than the predetermined range of period;
• a hydrocarbon feed valve arranged in the engine exhaust passage upstream of the exhaust purification catalyst, the hydrocarbon feed valve being oriented to inject the hydrocarbons in an upstream direction of the engine exhaust passage;
• and
• an electronic control unit coupled to the hydrocarbon feed valve and configured to control injection of the hydrocarbons by the hydrocarbon feed valve so that, at a time of engine operation, the hydrocarbons are injected from the hydrocarbon feed valve within the predetermined range of period to thereby remove the NOx which is contained in the exhaust gas while gradually reducing an injection pressure of the hydrocarbons from a start of the injection toward an end of the injection during each injection period, the injection pressure of the hydrocarbons at the start of the injection and the injection pressure of the hydrocarbons at the end of the injection being set so that the hydrocarbons which are injected later are superposed in the engine exhaust passage with the hydrocarbons that were previously injected.