| 摘要 |
<p>In order to improve an exhaust gas purification performance immediately after a cold engine is started, firstly for HC purification, an air-fuel ratio is controlled to be in a lean range immediately after the engine start.</p><p>Next for NOx purification, the air-fuel ratio is switched to be from lean to rich based upon a parameter (engine load, namely, fuel injection amount) related to NOx density at an engine exit or a parameter (catalyst temperature or water temperature) related to the catalyst temperature.</p><p>The air-fuel ratio to be rich is set as a plurality of values based upon the parameter and as a richer range as the parameter shows high density or high temperature.
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