Electronic control unit for controlling air fuel ratio to reduce NOx occluded in NOx catalyst

A method for controlling an air-fuel ratio of an engine having lean NOx catalyst in its exhaust system to reduce the lean NOx catalyst, comprising:

• estimating NOx amount occluded in the lean NOx catalyst;
• performing shift-to-rich process on a transition from lean-burn operation to stoichiometric operation;
• calculating an accumulated value of an exhaust flow amount;
• and
• completing said shift-to-rich process responsive to said accumulated value of the exhaust flow amount reaching a threshold value which is set based on the occluded NOx amount estimated by said estimating step.

An electronic control unit for controlling an air-fuel ratio of an engine having a three-way catalyst and a lean NOx catalyst provided in the engine exhaust system, and an air-fuel ratio sensor provided between said three-way catalyst and said lean NOx catalyst, the electronic control unit comprising:

• an estimator means for estimating NOx amount occluded in the lean NOx catalyst,
• means for performing shift-to-rich process responsive to the NOx amount estimated by said estimator exceeding a predetermined value;
• calculator means for calculating an accumulated value of an exhaust flow amount, said calculating means starting to accumulate the exhaust flow amount when said air-fuel ratio sensor indicates rich;
• and
• means for completing said shift-to-rich process responsive to said accumulated value of the exhaust flow amount exceeding a threshold value which is set based on the occluded NOx amount estimated by said estimator;
• wherein said electronic control unit controls the air-fuel ratio to reduce the lean NOx catalyst.

A method for controlling an air-fuel ratio of an engine having a three-way catalyst and a lean NOx catalyst provided in the engine exhaust system, and an air-fuel ratio sensor provided between said three-way catalyst and said lean NOx catalyst to reduce the lean NOx catalyst, the method comprising the steps of:

• estimating NOx amount occluded in the lean NOx catalyst;
• performing shift-to-rich process responsive to the NOx amount estimated by said estimating step exceeding a predetermined value;
• starting to calculate an accumulated value of an exhaust flow amount when said air-fuel ratio sensor indicates rich;
• and
• completing said shift-to-rich process responsive to said accumulated value of the exhaust flow amount exceeding a threshold value which is set based on the occluded NOx amount estimated by said estimator.