Method for determining oxygen storage capacity

Abstract

The oxygen storage capacity of an oxygen store associated with a catalytic converter of a combustion engine is computed by forming an integral which begins at the time of a changeover in the exposure, e.g., from rich to lean, and ends when the output signal of a post-catalytic converter lambda probe is less than a threshold value. A correction is performed to take into a consideration a time offset in the signals of the post-catalytic converter lambda probe. In particular, the time offset is measured to determine a time at which the integration should have been terminated, wherein this time is inferred retroactively.

1 . A method for determining oxygen storage capacity of an oxygen store associated with a catalytic converter of an internal combustion engine, comprising the steps of:
 measuring, in an outflow direction of exhaust gas, an air-fuel ratio with a pre-catalytic converter lambda probe arranged upstream of the catalytic converter and with a post-catalytic converter lambda probe arranged downstream of a section of the catalytic converter,   initially exposing the oxygen store, under control of the pre-catalytic converter lambda probe, to rich exhaust gas so as to extract as much oxygen as possible from the oxygen store or to lean exhaust gas so as to fill the oxygen store with as much oxygen as possible,   thereafter changing over from the rich exhaust gas to lean exhaust gas so as to fill the oxygen store with oxygen or changing over from the lean exhaust gas to rich exhaust gas so as to remove oxygen from the oxygen store,   integrating the filled or removed quantity of oxygen over a time interval, starting at a first time of the changeover and ending at a second time when an output signal of the post-catalytic converter lambda probe is less than or greater than a threshold value, indicating a full state or an empty state of the oxygen store,   measuring a time offset between the first time and a third time when the mathematical sign of a slope of the output signal of the post-catalytic converter lambda probe changes, and   correcting the obtained integral with the time offset.   
     
     
         2 . The method of  claim 1 , further comprising the steps of storing intermediate values of the integral, subtracting the time offset from the second time for obtaining a final time, and using the intermediate value of the integral associated with the final time as a correction value for the obtained integral. 
     
     
         3 . The method of  claim 1 , further comprising the steps of computationally estimating a fraction of computed oxygen storage capacity contributing to the integral for the duration of the time offset before the second time, and subtracting this fraction from the obtained integral for obtaining a correction value for the obtained integral. 
     
     
         4 . The method of  claim 3 , wherein a change in the exhaust gas quantity occurring during the time interval is taken into consideration when estimating the fraction. 
     
     
         5 . The method of  claim 3 , wherein a change in the air-fuel ratio occurring during the time interval is taken into consideration when estimating the fraction. 
     
     
         6 . The method of  claim 4 , further comprising the steps of:
 deriving a filter constant for a digital low pass filter from the time offset,   filtering the oxygen uptake as a function of time with the low pass filter, and   calculating the correction value for the obtained integral based on a result of the filtering and the time offset.   
     
     
         7 . The method of  claim 6 , wherein the correction value for the obtained integral is calculated based on a filtering result at the end of the time interval and the time offset. 
     
     
         8 . The method of  claim 5 , further comprising the steps of:
 deriving a filter constant for a digital low pass filter from the time offset,   filtering the oxygen uptake as a function of time with the low pass filter, and   calculating the correction value for the obtained integral based on a result of the filtering and the time offset.   
     
     
         9 . The method of  claim 8 , wherein the correction value for the obtained integral is calculated based on a filtering result at the end of the time interval and the time offset.