Exhaust gas purification system for an internal combustion engine

An exhaust gas purification system for an internal combustion engine comprising:

• an internal combustion engine capable of fuel combustion at lean air-fuel ratios and having an exhaust manifold and an exhaust conduit connected to the exhaust manifold;
• a dual passage portion provided in the exhaust conduit and located downstream of and close to the exhaust manifold of the engine, the dual passage portion including a first passage and a second passage which are connected in parallel to each other;
• a first catalyst installed in the first passage and constructed of zeolite carrying at least one metal selected from the group consisting of transition metals and noble metals to reduce nitrogen oxides included in exhaust gas from the engine under oxidizing conditions and in the presence of hydrocarbons;
• a second catalyst installed in the first passage downstream of the first catalyst and constructed of any one of a three-way catalyst and an oxidizing catalyst;
• a third catalyst installed in the second passage and constructed of any one of a three-way catalyst and an oxidizing catalyst, wherein said third catalyst is located within the second passage such that the temperature of the exhaust gas passing through the second passage is not affected by prior catalytic contact;
• a flow switching valve for switching flow of exhaust gas from the engine between the first passage and the second passage;
• valve control means for switching the flow switching valve so as to cause exhaust gas to flow through the second passage while the engine is being warmed-up and to cause exhaust gas to flow through the first passage after the engine has been warmed-up
• a secondary air introduction port for introducing secondary air to a portion of the first passage upstream of the first catalyst;
• and
• a secondary air introduction control routine for controlling secondary air introduction so that secondary air supply is executed for a predetermined period of time after the engine has stopped running.