Abstract: An exhaust gas purification system for an internal combustion engine includes an engine management control system which controls engine air/fuel ratio alternatively at super-stoichiometric level and a sub-stoichiometric level in a three-way catalyst. The engine management system uses a timer and a memory to measure and store cumulative time values including first time over which the engine is operated at the super-stoichiometric level and a second time during which the engine is operated at a sub-stoichiometric level. The alternation between levels is such that the first cumulative time is greater than the second cumulative time with the second cumulative time being sufficient to regenerate at least partially the efficiency of the three-way catalyst activity.