Patent ID: 8650859

Claim:
An exhaust gas purification system for an internal combustion engine comprising: an exhaust gas purification device that is arranged in an exhaust passage of the internal combustion engine and including a catalyst; a precatalyst that is arranged in the exhaust passage at a location upstream of said exhaust gas purification device and has an oxidation function; a reducing agent addition valve that is arranged in the exhaust passage at a location upstream of said precatalyst and adds a reducing agent to an exhaust gas when the reducing agent is supplied to said precatalyst and said exhaust gas purification device, wherein said reducing agent addition valve is disposed in a position in which at least part of the reducing agent added to the exhaust gas reaches said precatalyst in the state of liquid; and an electronic control unit including a routine, the routine including calculating an expansion rate of the reducing agent when the reducing agent having reached said precatalyst is vaporized to expand in said precatalyst, setting a target value of the flow rate of the exhaust gas flowing into said precatalyst to be less than the calculated expansion rate of the reducing agent, and when the addition of the reducing agent is performed by said reducing agent addition valve, decreasing the flow rate of the exhaust gas flowing into said precatalyst based on the target value of the flow rate of the exhaust gas flowing into said precatalyst so that at least part of the reducing agent, which has reached said precatalyst and has vaporized in said precatalyst, is caused to flow back.