It has long been known that the performance loss of a catalyzer can be evaluated with the signal of a lambda probe mounted rearward of the catalyzer. As long as the catalyzer has deteriorated only slightly, the catalyzer provides a good storage capacity for oxygen which enables the catalyzer to store oxygen in the lean phases of a lambda control to the value one and to release this oxygen in rich phases. In this way, the lambda value one is continuously measured at the output of the catalyzer. However, with increasing deterioration of the catalyzer, this storage capacity of the catalyzer is reduced so that the catalyzer can no longer store all the oxygen which is supplied in the above-mentioned lean phase. This leads to the condition that after some time in the lean phase, oxygen is present in the exhaust gas leaving the catalyzer which causes the lambda probe mounted rearward of the catalyzer to provide a lean signal. In the opposite case, there is not enough oxygen present in the rich phase in order to convert the total incoming quantity of toxic components to be oxidized. For this reason, after some time in the rich phase, the lambda probe rearward of the catalyzer measures components to be oxidized. Because of the control-conditioned signal trace of the lambda value forward of the catalyzer, the amplitude of the lambda value signal (as it is measured rearward of the catalyzer) is dependent upon the time point at which the oxygen store overflows or is entirely empty. The earlier the time point lies, the greater is the amplitude of the signal measured rearward of the catalyzer (assuming that the time point does not lie excessively early). The amplitude of the signal rearward of the catalyzer is then a measure for the storage capacity and therefore of the performance loss of the catalyzer.
The above-mentioned amplitude does not only depend on the storage capacity of the catalyzer, but also on the amplitude of the lambda value signal measured forward of the catalyzer. To compensate for this influence, it is known to form a relationship of the lambda values measured forward and rearward of the catalyzer. For this purpose, U.S. Pat. No. 3,962,866 teaches to form the difference between the two above-mentioned signals and to emit a warning signal when the difference drops below a threshold value. On the other hand, German published patent application 3,500,594 teaches to form the ratio of the above-mentioned signals and to use a mean value of this ratio in order to evaluate the state of deterioration of the catalyzer.
Notwithstanding the above-mentioned corrective measures, it has until now been possible only in especially selected operating conditions to determine the state of deterioration of the catalyzer with the above-mentioned methods. The reason for this can be derived from the above-mentioned storage capacity of the catalyzer. If, for example, there is no control precisely to the lambda value one in an operating condition but instead to a richer value which is often the case, the lean phases are shortened relative to the rich phases. Under certain circumstances, not as much oxygen can be stored in the lean phase as the catalyzer would actually still be able to store. In the rich phase, this leads to an especially large amplitude of the lambda value signal measured rearward of the catalyzer. Similar effects occur when the lean phase is shortened for another reason, for example, because a change of the controller frequency or of non-steady-state operation.