Patent ID: 8074446

Claim:
An exhaust gas purification system for an internal combustion engine, comprising: a NOx storage reduction catalyst that is provided in an exhaust passage of said internal combustion engine; a reducing agent adding valve that is provided in said exhaust passage on an upstream side of said NOx storage reduction catalyst and that adds a reducing agent to exhaust gas; and a controller that is programmed to: when an oxide that is stored in said NOx storage reduction catalyst is reduced, perform a rich spike control that cyclically lowers an air-fuel ratio in the exhaust gas that flows into said NOx storage reduction catalyst to a target air-fuel ratio and that, when lowering the air-fuel ratio in the exhaust gas, perform addition of the reducing agent by said reducing agent adding valve that is performed when a crank angle of said internal combustion engine is in a specified range multiple times; determine whether or not a clogging of said reducing agent adding valve has occurred; and when the controller determines that the clogging of said reducing agent adding valve has occurred, lengthen the period during which the reducing agent is added by said reducing agent adding valve in each time when the air-fuel ratio in the exhaust gas is lowered while the rich spike control is being performed, wherein when the total amount of the reducing agent that is added by said reducing agent adding valve while the rich spike control is being performed does not reach the total amount of the reducing agent that is added when the rich spike control is performed in the state in which the clogging of said reducing agent adding valve has not occurred, the number of times the air-fuel ratio in the exhaust gas is lowered while the rich spike control is being performed is increased, even if the period during which the reducing agent is added by said reducing agent adding valve in each time when the air-fuel ratio in the exhaust gas is lowered while the rich spike control is being performed is lengthened to an upper limit value by said controller.