Method and system for reducing vehicle emissions using a sensor downstream of an emission control device

A method of controlling an engine that operates at a plurality of engine operating conditions characterized by combustion of air-fuel mixtures having different air-fuel ratios to generate engine exhaust gas, wherein the exhaust gas is directed through an exhaust treatment system including an emission control device that stores a selected exhaust gas constituent when the exhaust gas is lean and releases the stored selected exhaust gas constituent when the exhaust gas is rich, and a sensor operative to generate an output signal representative of a concentration of the selected constituent in the exhaust gas exiting the device, the method comprising:

• determining a first value representative of an instantaneous concentration of the selected constituent in the engine exhaust gas when operating in the lean operating condition;
• determining a second value representative of the instantaneous concentration of the selected constituent exiting the device based on the output signal generated by the sensor;
• and
• selecting an engine operating condition as a function of the first and second values, wherein selecting includes:
• calculating a differential value based on the first and second values;
• accumulating the differential value over time to obtain a first accumulated measure representative of an amount of the selected constituent stored in the device;
• calculating a total excess fuel value representative of an amount of fuel in excess of a stoichiometric amount of fuel that is required to release stored selected constituent and stored oxygen from the device as a function of the first accumulated measure and a previously stored oxygen-only excess fuel value representative of an amount of excess fuel required to release only stored oxygen from the device;
• and
• supplying an amount of fuel to the engine in excess of the stoichiometric amount based on the excess fuel value.

A system for controlling an internal combustion engine that operates at a plurality of engine operating conditions characterized by combustion of air-fuel mixtures having different air-fuel ratios, wherein exhaust gas generated by such combustion is directed through an exhaust treatment system including an emission control device that stores a selected exhaust gas constituent when the exhaust gas is lean and releases the stored selected constituent when the exhaust gas is rich, and a sensor operative to generate an output signal representative of a concentration of a selected constituent of the exhaust gas exiting the device, the system comprising:

• a controller including a microprocessor arranged to determine a first value representative of an instantaneous concentration of the selected constituent in the engine exhaust gas when operating in a lean operating condition, and to determine a second value representative of the instantaneous concentration of the selected constituent exiting the device based on the output signal generated by the sensor, and wherein the controller is further arranged to select an engine operating condition as a function of the first and second values, wherein the controller is further arranged to calculate a differential value based on the first and second values, to accumulate the differential value over time to obtain a first accumulated measure representative of an amount of the selected constituent stored in the device, to calculate a total excess fuel value representative of an amount of fuel in excess of a stoichiometric amount of fuel that is required to release stored selected constituent and stored oxygen from the device as a function of the first accumulated measure and a previously stored oxygen-only excess fuel value representative of an amount of excess fuel required to release only stored oxygen from the device, and to supply an amount of fuel to the engine in excess of the stoichiometric amount based on the excess fuel value.