Upstream of such a catalytic converter and, thus, upstream of the oxygen storage system (when viewed in the outflow direction of the exhaust gas) conventionally there is a first lambda probe—the so-called pre-catalytic converter probe. Downstream of the catalytic converter (or rather a region thereof) there is a second lambda probe—the post-catalytic converter probe. In order to determine the oxygen storage capacity, a switchover operation from lean to rich exhaust gas and/or vice versa is effected. In the course of determining the exhaust gas composition, the measurement results of the first lambda probe are used. In contrast, the measurement results of the second lambda probe show the buffering by means of the oxygen storage system. To date, the point in time of the switchover operation of the exhaust gas composition was found by means of the first lambda probe; and then a numerical integration over a time interval beginning with this point in time was calculated. The time interval terminates when the second lambda probe shows a breakthrough (by means of a certain voltage value of, for example, 0.45 V).
The formula that is used during the switchover operation from rich to lean exhaust gas for the oxygen storage capacity OSC in g reads:
                              O          ⁢                                          ⁢          S          ⁢                                          ⁢          C                =                  0          ⁢                      ,                    ⁢          23          ⁢                                    ∫                              t                1                                            t                2                                      ⁢                                          (                                  λ                  -                  1                                )                            ⁢                                                m                  .                                ⁡                                  (                  t                  )                                            ⁢                              ⅆ                t                                                                        (        1        )            
During the switchover operation from lean to rich, the following formula holds true for the oxygen release storage capacity:
      R    ⁢                  ⁢    S    ⁢                  ⁢    C    =      0    ⁢          ,        ⁢    23    ⁢                  ∫                  t          3                          t          4                    ⁢                        (                      1            -            λ                    )                ⁢                              m            .                    ⁡                      (            t            )                          ⁢                  ⅆ          t                    
In this respect {dot over (m)}(t) is the exhaust gas mass flow through the oxygen storage system in g/s. The variable λ is derived from the measurement signals of the first lambda probe, thus, λ(t).
In this context t1 or t3 is the point in time of the lambda switchover; t2 is the time, at which the value U(t) is equal to 0.45 V; at t4, U(t) is equal to 0.70 V.
The lambda probes, which are configured typically as Nernst sensors, can be subject to aging. Aging has the effect as if the signals of a correctly functioning lambda probe were filtered. The filtering can occur due to the adverse effect in the flow of a fluid in the lambda probe or in the electrical signal conditioning. In addition or as an alternative, there is a time shift.
The past methods for determining the oxygen storage capacity do not work reliably if an aged lambda probe is used. The aged lambda probe causes a larger oxygen storage capacity to be measured than is actually the case. As a result, it can happen that there is simultaneously an aged second lambda probe and an aged oxygen storage system/aged catalytic converter, but this situation cannot be detected. Then the result will be a high emission of toxic substances that is undesired.
The engineering object of the invention is to provide an improved method for determining the oxygen storage capacity for an oxygen storage system (or rather a region thereof).