Exhaust emission control device for engine



【課題】 演算負荷を抑えた比較的に簡単な構成により、空燃比がリッチであるときの排気の浄化率を確保する。 【解決手段】 三元触媒3の排気上流に二次空気を導入する導入路4を開閉する開閉弁5をコントロールユニット6により開閉制御して、導入する二次空気の流量を周期的に増減させ、三元触媒3に流入する排気の空燃比を理論値よりも高いリーンな空燃比と、この理論値よりも低いリッチな空燃比との間で交互に反転させるとともに、エンジン1の回転速度に基づいて二次空気の流量の増減周期を変更して、排気の空燃比のリーン及びリッチの間の反転周期を変化させる。 【選択図】 図1
<P>PROBLEM TO BE SOLVED: To secure the conversion rate of exhaust gas when the air fuel ratio is rich with a relatively simple structure limiting operation loads. <P>SOLUTION: An open close valve 5 opening and closing an introduction path 4 introducing secondary air to an exhaust upstream of a three way catalyst is controlled by a control unit 6 to periodically increase and decrease flow rate of the introduced secondary air. Consequently, air fuel ratio of exhaust gas flowing in the three way catalyst 3 is alternated between lean air fuel ratio higher than a theoretical value and rich air fuel ratio lower than the theoretical value, and alternation cycle of air fuel ratio between lean and rich is changed by changing increase and decrease cycle of flow rate of secondary air based on a rotation speed of the engine 1. <P>COPYRIGHT: (C)2007,JPO&INPIT




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