Patent ID: 8297040

Claim:
A method of operating an internal combustion engine comprising a catalytic converter, a first exhaust gas sensor that is disposed in such a way that it may be used in a lambda closed-loop control operation, and a second exhaust gas sensor that is disposed in such a way that it may be used in a trim closed-loop control operation, the method comprising the steps of: modulating a forced excitation upon an air-fuel ratio setpoint value in order to adjust a lambda variation for optimizing an efficiency of the catalytic converter, using a second exhaust gas sensor to generate a measuring signal responsive to a controlled air-fuel ratio of the internal combustion engine, a controller determining an HC quality value that is representative of an oxygen storage capacity of the catalytic converter by: increasing the forced excitation amplitude applied to the air-fuel ratio setpoint value, comparing an oscillation of the measuring signal generated by the second exhaust gas sensor with respect to a predefined reference value for the measuring signal, and determining the HC quality value based on the results of the comparison, the controller determining a NOx correction value by: comparing the measuring signal generated by the second exhaust gas sensor to one or more threshold values, each having a predetermined factor assigned thereto, determining durations during which the measuring signal crosses each of the one or more threshold values, for each determined duration, calculating a product of a determined air mass flow for the internal combustion engine and the factor assigned to the threshold value crossed during that duration, and summing the calculated products for the durations, the controller multiplying or adding the HC quality value and the NOx correction value to determine an NOx quality value representing a level of current NOx emissions from the engine.