Exhaust gas purification apparatus and method for an internal combustion engine

摘要

In order to suppress the quantity of the nitrogen oxide to be released to the atmosphere when the operation state is shifted from the engine operation with lean air fuel ratio to that with stoichiometric air fuel ratio, the exhaust gas purification apparatus is arranged such that by forced rich operation, when the operation state is shifted from the engine operation with lean air fuel ratio to that with stoichiometric air fuel ratio, the operation state is shifted to the engine operation with stoichiometric air fuel ratio after performing that with rich air fuel ratio.