Method for adjusting the oxygen concentration in a three-way catalytic converter system

A method-for adjusting an oxygen concentration in a catalytic converter system with lambda control for optimizing three-way conversion properties of the catalytic converter system after the transition from lean-burn operation with a lean fuel/air mix to stoichiometric operation or during permanent stoichiometric operation, the method which comprises:

• providing a catalytic converter system in an exhaust train of an internal combustion engine, the catalytic converter system having:
• a preliminary three-way catalytic converter;
• a main three-way catalytic converter disposed downstream of the preliminary catalytic converter with respect to an exhaust flow direction;
• a pre-cat oxygen sensor disposed upstream of the preliminary catalytic converter with respect to an exhaust flow direction and producing a measurement signal;
• an inter-cat oxygen sensor having a constant characteristic curve, the inter-cat oxygen sensor disposed between the preliminary catalytic converter and the main catalytic converter and producing a measurement signal;
• an air mass flow meter disposed in an intake section of the internal combustion engine and producing a measurement signal;
• and
• an operation control unit receiving measurement signals from the air mass flow meter, the pre-cat oxygen sensor, and the inter-cat oxygen sensor;
• switching operation to a predetermined rich fuel/air mix following one of a lean-burn operation and a permanent stoichiometric operation;
• determining a time at which the preliminary catalytic converter has been fully discharged through release of its stored oxygen to the exhaust gas utilizing the measurement signal from the inter-cat oxygen sensor;
• calculating on an ongoing basis a quantity of oxygen released to the exhaust gas from the main catalytic converter from the determined time onward utilizing the measurement signals from the inter-cat oxygen sensor and the air mass flow meter, and comparing the oxygen quantity with a desired predetermined value corresponding to a desired oxygen concentration in the main catalytic converter;
• ending operation with the rich air/fuel mix as soon as the quantity of oxygen released to the exhaust gas by the main catalytic converter has reached the desired predetermined value;
• and
• switching operation to stoichiometric mode.