US20060168943A1

US20060168943A1 - Method for operating an internal combustion engine and device for implementing the method

Info

Publication number: US20060168943A1
Application number: US11/334,970
Authority: US
Inventors: Eberhard Schnaibel; Jens Wagner; Kersten Wehmeier; Klaus Winkler; Richard Hotzel
Original assignee: Individual
Current assignee: Robert Bosch GmbH
Priority date: 2005-01-18
Filing date: 2006-01-18
Publication date: 2006-08-03
Also published as: DE102005002237A1; SE529485C2; FR2880921A1; SE0502840L

Abstract

A method for operating an internal combustion engine and a device for implementing the method may allow a catalytic converter diagnosis with a high degree of accuracy. At least one catalytic converter is positioned in the exhaust-gas region of the internal combustion engine, and a lambda sensor, which is configured as broadband lambda sensor, is positioned downstream from the catalytic converter or downstream from a section of the catalytic converter. The catalytic converter diagnosis is based on the evaluation of a measure for the oxygen storage capacity of the catalytic converter/within the catalytic converter. An at least approximately empty/filled oxygen reservoir of the catalytic converter is assumed. A change in the lambda setpoint value of the internal combustion engine to an excess air factor lambda that is greater than 1/less than 1 follows. Subsequently, a first change of the lambda signal is detected initially. Determined is a measure for the oxygen stored/discharged following the first change of the lambda signal. The determination of the stored/discharged oxygen is terminated if either a second change in the lambda signal is detected or if the measure of oxygen exceeds a threshold value.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Application No. 10 2005 002 237.5, filed in the Federal Republic of Germany on Jan. 18, 2005, which is expressly incorporated herein in its entirety by reference thereto.

FIELD OF THE INVENTION

The present invention relates to a method for operating an internal combustion engine and to a device for implementing the method.

BACKGROUND INFORMATION

One possibility for diagnosis of the catalytic converter is based on determining the oxygen storage capacity of the catalytic converter. A decrease in the oxygen storage capacity is considered a measure for the decline in the conversion capacity of the catalytic converter. The oxygen storage capacity of a catalytic converter can be determined by subjecting the excess-air factor lambda in the exhaust gas upstream from the catalytic converter to a cyclic change about the value 1.0 and comparing the lambda signal measured downstream from the catalytic converter to the lambda values predefined upstream. If the oxygen storage capacity is high, in predefined lambda values of less than 1 the oxygen stored in the catalytic converter is able to largely oxidize the oxidizable exhaust-gas components, and in the case of predefined lambda values that are greater than 1 it is able to largely store the excess oxygen. The measurable lambda fluctuations downstream from the catalytic converter are relatively low if the catalytic converter has a high oxygen storage capacity and thus is functioning properly.
A technical implementation of the described method is described in German Published Patent Application No. 41 12 478, for example, according to which a jump lambda sensor is positioned upstream from the catalytic converter and a jump lambda sensor is positioned downstream from the catalytic converter. First, it is determined whether, given a lambda control oscillation from rich to lean or vice versa upstream from the catalytic converter, the lambda value downstream from the catalytic converter shows a corresponding transition. If this is the case, it can be assumed that the oxygen reservoir of the catalytic converter is either completely full or empty, so that a defined initial state is present. The gas mass flow flowing through the catalytic converter is determined subsequently. The time integral of the product of gas mass flow and a term having the lambda value upstream from the catalytic converter, and the time integral of the product of gas mass flow and a term having the lambda value downstream from the catalytic converter are calculated. Used as a measure for the ageing condition of the catalytic converter is either the difference between the two integrals, or the quotient of the two integrals, or the quotient of the difference and one of the two integrals.
In German Published Patent Application No. 102 57 059, a method and a device are described, which provide a diagnosis of catalytic converters positioned in a plurality (number N) of exhaust tracts of an internal combustion engine. Situated downstream, following the merging of the n exhaust tracts, is a lambda sensor whose signal is compared to the individual lambda values that occur upstream from the catalytic converters. Here, too, the diagnosis is based on an evaluation of the oxygen storage capacity of the catalytic converters. Broadband lambda sensors, for example, are used as lambda sensors.

SUMMARY

Example embodiments of the present invention may provide a method for operating an internal combustion engine and a device for implementing the method, which may allow a reliable diagnosis of the catalytic converter.
The method may assume that at least one catalytic converter is positioned in the exhaust-gas region of the internal combustion engine and a lambda sensor is positioned downstream from the catalytic converter or downstream from a section of the catalytic converter. A catalytic converter diagnosis is implemented, which is based on evaluating at least one measure for the oxygen storage capacity of the catalytic converter/inside the catalytic converter. An at least approximately empty/full oxygen reservoir of the catalytic converter is assumed. Subsequently, the lambda setpoint value of the internal combustion engine is changed to an excess-air factor lambda that is greater than 1/less than 1. A first change in a lambda signal is initially detected, which is provided by a lambda sensor positioned downstream from the catalytic converter or downstream from a section of the catalytic converter, the lambda sensor being configured as broadband lambda sensor. A measure is determined for the oxygen stored/discharged following the first change in the lambda signal. The determination of the stored/discharged oxygen is terminated if either a second change in the lambda signal is detected or if the oxygen exceeds a threshold value.
Due to the use of a broadband lambda sensor, the method may provide relatively high accuracy in the determination of at least one measure for the oxygen storage capacity of the catalytic converter, which may be utilized as a measure for the ageing condition of the catalytic converter. High oxygen storage capacity is a sign of a good catalytic converter.
The method may be based both on the oxygen stored in the catalytic converter and on the oxygen discharged from the catalytic converter. Comparable to the discharge of the oxygen is the storing of a reducing agent such as hydrogen and/or carbon monoxide, which is produced when the internal combustion engine is operated at an excess air factor lambda of less than 1. Since the storing/discharging of the oxygen into/out of the catalytic converter is an equilibrium reaction, not all supplied oxygen is stored/discharged in a transition region. Due to the kinetics of the storing/discharging and due to diffusion processes, the characteristic of the oxygen concentration downstream from the catalytic converter is flattened in terms of time. A transition region is produced. An excess of oxygen/lack of oxygen occurs downstream from the catalytic converter before the maximum oxygen storage capacity is exhausted/all the oxygen stored is reduced.
The determination of at least one measure for the stored/discharged oxygen, which begins following a detected first change in the lambda signal, is ended if either a second change in the lambda signal is detected or if the measure for stored/discharged oxygen has exceeded a threshold value. Accordingly, the diagnosis of the catalytic converter may be terminated prematurely, without waiting for the second change, if a sufficiently good oxygen storage capacity of the catalytic converter has already been established. If the second change in the lambda signal comes into effect as second criterion for terminating the determination of the measure for the stored/discharged oxygen, a relatively precise measure for the remaining oxygen storage capacity is able to be provided. By a comparison with a predefined storage capacity threshold value, it is then possible to decide whether the catalytic converter must be exchanged.
The method may be able to selectively implement the diagnosis of the catalytic converter by influencing the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine in a corresponding manner so as to predefine the starting condition and the subsequent change in the excess-air factor lambda.
However, the method may also be able to implement the catalytic converter diagnosis within the framework of normal operation of the internal combustion engine, provided both the starting condition, i.e., that a completely empty/full oxygen reservoir of the catalytic converter exists within the framework of normal operation is satisfied and, furthermore, a change in the excess-air factor lambda to a value greater than 1/smaller than 1 is implemented within the framework of normal operation.
The change in the lambda signal may be determined from the gradient of the lambda signal. The gradient of the lambda signal, which may be approximated as difference quotient, may be determined continuously, at predefined time intervals, for example. The change may be detected on the basis of, for example, the gradient having to exceed a predefined gradient threshold value. In addition, a minimum time during which the gradient threshold value must have been exceeded may be provided. An additional or alternative detection of the change provides that the gradient must have a maximum. Furthermore, in addition or as an alternative, it may be provided that the change be considered detected when the gradient initially has a maximum and then falls below a gradient threshold value in the subsequent decrease.
Low-pass filtering of the lambda signal may be provided to block interference signals and rapid changes of the lambda signal due to dynamic processes during determination of the change.
Oxygen may be determined from an integral over time, which is a function of a combustion lambda and the air signal provided by an air detection that detects the combustion air supplied to the internal combustion engine. The stored/discharged oxygen mass is determined by taking the combustion air into account and by implementing the integration. The air signal and possibly the combustion lambda may be buffer-stored for a delay time, which at least approximately corresponds to the gas-propagation time in the catalytic converter before the broadband lambda sensor is reached. Due to this measure, an unsteady operating state of the internal combustion engine during the catalytic converter diagnosis may have an only negligible effect on the diagnosis result.
Conditioning of the catalytic converter prior to the catalytic converter diagnosis may be provided in that the combustion lambda predefined for the internal combustion engine is specified to a value greater than 1/less than 1, so that the oxygen reservoir of the catalytic converter is at least approximately full/empty. The stipulation of the combustion lambda to a value greater than 1 may be omitted if the catalytic converter diagnosis is performed after a deceleration fuel cutoff phase of the internal combustion engine that continues at least to the point where the oxygen reservoir is at least approximately filled.
The stored/discharged oxygen may be evaluated as a function of the catalytic converter temperature and/or the exhaust gas mass flow. With this measure, the threshold value for the comparison of the measure for the catalytic converter storage capacity may be defined as a function of operating conditions of the catalytic converter.
A device for implementing the method may provide a control device configured to implement the method.
The control device includes, for example: a diagnostic controller which changes the lambda setpoint value; a change detection to process the lambda signal provided by the broadband lambda sensor; and an integrator to determine a measure for the stored/discharged oxygen. The control unit may include at least one electrical memory in which the method steps are stored in the form of a computer program.
Additional aspects and features hereof are described in more detail below with reference to the appended Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a technical environment in which a method according to an example embodiment of the present invention may be run.
FIGS. 2 a to 2 e illustrate concentration characteristics as a function of location.
FIG. 3 illustrates a signal characteristic of a lambda signal occurring downstream from a catalytic converter as a function of time.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an internal combustion engine 10 in whose intake region 11 an air detection 12 is positioned, and in whose exhaust-gas region 13 an exhaust-gas temperature sensor 14, a catalytic converter 15 and, downstream from catalytic converter 15, a lambda sensor 16 are situated. Catalytic converter 15 has a catalytic converter input Kat_In and a catalytic converter output Kat_Out.
Air detection 12 outputs an air signal msL to a control unit 20. Internal combustion engine 10 emits a rotational speed n. Exhaust-gas temperature sensor 14 outputs an exhaust-gas temperature signal Tabg, and lambda sensor 16 outputs a lambda signal lam_nK. Control unit 20 emits a fuel signal mK to a fuel-metering device 21. An exhaust-gas mass flow msabg occurs in exhaust-gas region 13.
Air signal msL is also provided to a deceleration fuel-cutoff controller 30, a time delay 31 and a threshold-value definition 32. Deceleration fuel-cutoff controller 30 provides a deceleration fuel-cutoff signal 34 to diagnostic controller 33. Time delay 31 transmits a delayed air signal msL_d to an integrator 35. Threshold value definition 32 supplies a first and a second threshold value 37, 38 to a comparator 36. Rotational speed n is supplied to deceleration fuel-cutoff controller 30 and to threshold-value definition 32.
Diagnostic controller 33 supplies a first diagnosis control signal 39 to threshold-value definition 32, a second diagnosis control signal 41 to a change-determination 40, and a diagnosis lambda lam_D to a lambda setpoint selection 42. Diagnosis lambda lam_D is also supplied to time delay 31, which outputs a delayed lambda signal lam_d and provides it to integrator 35. Diagnostic controller 33 is supplied with a diagnosis stop signal 43 provided by comparator 36.
In addition to diagnosis lambda lam_D, lambda setpoint selection 42 is supplied with a nominal operating lambda lam_N. Lambda-setpoint selection 42 transmits a lambda setpoint value lam_S to a lambda controller 50, which provides fuel signal mK. Lambda signal lam_nk provided by lambda sensor 16 is not only supplied to lambda controller 50, but to a low-pass filter 51 as well. Lambda controller 50 is additionally provided with a lambda signal lam_vk, which reflects the excess air factor lambda in the exhaust gas upstream from catalytic converter 15.
Filtered lambda signal lam_nKF provided by low-pass filter 51 arrives at change determination 40. Integrator 35 provides integration result 52 to comparator 36, which outputs a fault signal F.
FIGS. 2 a to 2 e illustrate signal characteristics as a function of location x. Illustrated are oxygen concentration %02 as well as a reagent concentration % Rea. FIG. 2 a illustrates the situation at a first location x1 after a rich-lean jump has occurred after which a high oxygen concentration %02 and a low reagent concentration % Rea are present. FIG. 2 b illustrates the situation at a later point in time when the concentration changes are present at a second location x2. FIG. 2 c illustrates the situation where a change occurs in concentrations %02, % Rea downstream from catalytic converter input Kat_In. A first transition region x10 comes about in which oxygen concentration %02 changes only slightly. FIG. 2 d illustrates the situation at a later point in time when concentrations %02, % Rea change inside catalytic converter 15. A second transition region x20 occurs, which has a greater expansion than first transition region x10. FIG. 2 e illustrates the situation where concentrations %02, % Rea change largely downstream from catalytic converter output Kat_Out. A third transition region x30 comes about, which in turn has a greater expansion than second transition region x20.
FIG. 3 illustrates lambda signal lam_nK as a function of time t. Up to a first point in time t1, lambda is to be 0.97. A first change 60 of lambda signal lam_nK occurs between the first and a second point in time t2. At a third point in time t3, lambda signal lam_nK has at least approximately a plateau 61 at which lambda is at least approximately 1. At a fourth point in time t4, a second change 62 of lambda signal lam_nK sets in, which is concluded at a fifth point in time t5. After fifth point in time t5, lambda is to be at 1.03.
A method according to an example embodiment of the present invention operates as follows:
Lambda sensor 16 is arranged as broadband lambda sensor which is able to detect an excess air factor lambda lying in a wide range which, for example, is between 0.7 and 4.0. Lambda sensor 16 is located downstream from the at least one catalytic converter 15. In larger catalytic converters, lambda sensor 16 may also be arranged downstream from a partial volume of the catalytic converter. Lambda signal lam_nK supplied by lambda sensor 16 may be used not only for diagnosis, but for regulation of the combustion lambda as well. For this reason, lambda signal lam_nK is supplied both to low-pass filter 51 and to lambda controller 50, which is able to influence fuel signal mK as a function of lambda signal lam_nK. A lambda signal lam_vK may be taken into consideration, which is measured upstream from catalytic converter 15 by a lambda sensor and supplied to lambda controller 50.
Lambda controller 50 attempts to adjust a combustion lambda that corresponds to lambda setpoint value lam_S stipulated by lambda setpoint selection 42. During normal operation of internal combustion engine 10, lambda setpoint value lam_S corresponds to nominal-operation lambda lam_N. An open-loop control may be provided instead of lambda controller 50. Furthermore, lambda signal lam_vK, which reflects the excess air factor lambda in the exhaust gas upstream from catalytic converter 15, is not required. Lambda signal lam_nK detected downstream from catalytic converter 15 may be utilized instead.
To implement the catalytic converter diagnosis, diagnostic controller 33 may stipulate diagnosis lambda lam_D, which lambda setpoint selection 42 is to provide as lambda setpoint value lam_S. The diagnosis is prepared in that the oxygen reservoir of catalytic converter 15 is either approximately fully filled or emptied. Under the conditions illustrated in FIGS. 2 a to 2 e, a rich-lean jump at the beginning of the diagnosis is assumed in which oxygen concentration %02 is changed from a low to a high value. Due to the change in the lambda of the internal combustion engine from a rich to a lean value, a corresponding change from a high to a low value occurs in reagent concentration % Rea. To allow the diagnosis to be carried out, it may therefore be necessary that catalytic converter 15 be initially acted upon by a low oxygen concentration %02, until the point is reached where the oxygen reservoir is at least approximately empty.
As an alternative, an at least approximately completely full oxygen reservoir of catalytic converter 15 may be assumed in which catalytic converter 15 is to be acted upon by a high oxygen concentration %02 in he beginning. This operating state is already present in a sufficiently long deceleration fuel-cutoff phase of internal combustion engine 10.
It may therefore be provided that deceleration fuel-cutoff controller 30, which determines the deceleration fuel-cutoff of internal combustion engine 10, from air signal msL and rotational speed n, for example, signals to diagnostic controller 30 via deceleration fuel-cutoff signal 34 that the diagnosis is able to be started. Deceleration fuel-cutoff signal 34 is supplied when the deceleration fuel-cutoff phase has been present for a sufficient length of time.
According to FIG. 2 a, the change in lambda setpoint value lam_S to diagnosis lambda lam_D occurs upstream from catalytic converter 15 in the form of a jump of oxygen concentration %02 and in the form of a jump of reagent concentration % Rea in a first location x1. At a later point in time, the concentration changes move to second location x2 where the concentration transitions still have relatively steep characteristics. The originally jump-like concentration changes are already slightly flattened by diffusion and turbulence in exhaust-gas region 13.
The lean exhaust gas having a high oxygen concentration %02 displaces the rich exhaust gas having a high reagent concentration % Rea, which has only a very low oxygen concentration %02. In the situation illustrated in FIG. 2 c, the concentration changes meanwhile have arrived at a location downstream from catalytic converter input Kat_In. Once the high oxygen concentration %02 has reached catalytic converter 15, the excess oxygen is stored in catalytic converter 15. First transition region x10 is created in which the oxygen concentration %02 exhibits first change 60. An at least short plateau 61 then occurs, which is left upon second change 62.
The reagent concentration % Rea drops to low values during first change 60. Since catalytic converter 15 is unable to store the entire available oxygen, oxygen concentration %02 that remains in first transition region x10 is higher than that in the rich exhaust gas.
FIG. 2 d illustrates the situation in the concentration changes at a later point in time. Second transition region x20 has become longer compared to first transition region x10.
FIG. 2 e illustrates the situation at an even later point in time when the concentration changes occur at least partially already downstream from catalytic converter output Kat_Out or downstream from a section of catalytic converter 15. It is only at this point in time that the concentration changes may be measured by lambda sensor 16.
In the beginning, lambda signal lam_nK of lambda sensor 16 will indicate a rich exhaust gas lambda, which is set to 0.97, for example. FIG. 3 illustrates the time characteristic in which the oxygen deficiency is to exist up to first point in time t1. The change in the oxygen concentration %02 is to begin at first point in time t1, with first change 60. The analysis of lambda signal lam_nK takes place in change determination 40 after diagnostic controller 33 has output second diagnosis control signal 41 to change determination 40.
In order to prevent faulty measurements, low-pass filter 51 is provided, which rids lambda signal lam_nK of high-frequency interference signals on the one hand and of rapid changes caused by dynamic processes that are unrelated to first change 60 on the other hand. Change determination 40 may then be provided with filtered lambda signal lam_nKF instead of lambda signal lam_nK.
To detect first change 60, change detection 40 may determine, for example, the gradient of lambda signal lam_nk or filtered lambda signal lam_nKF. The gradient may be ascertained continuously, in rapid time succession. It may be approximated by difference quotients, for example. For example, the gradient may be compared to a predefined gradient threshold value. When the gradient threshold value is exceeded, integrator enable signal 53 is supplied. It may be provided that the gradient must exceed the gradient threshold value for a predefined period of time before integrator enable signal 53 is made available. Additionally or alternatively, the presence of a point of inflection of lambda signal lam_nK or of filtered lambda signal lam_nKF may be determined and utilized to provide integrator enable signal 53. Furthermore, in addition or as an alternative, it may be provided that the maximum of the gradient is determined first and that it is then checked whether the gradient falls below a threshold value before integrator enable signal 53 is provided.
The diagnosis of the catalytic converter may begin as soon as first change 60 is detected. Ascertained is the oxygen being stored in catalytic converter 15. This may be the oxygen mass or the oxygen quantity. The determination may be implemented, for example, in that integrator 35 multiplies the value (1−1/lambda) by air signal msL and a constant that corresponds to the percentage oxygen content of the air and integrates it as a function of time. For a relative valuation, the constant may be set to equal 1. To take the gas propagation time through catalytic converter 15 into account, air signal msL provided by air detection 12 may be delayed in time delay 31 and made available to integrator 35 as delayed air signal msL_d. In addition, the lambda on which the integration is based may be delayed in time delay 31. The combustion lambda during the diagnosis corresponds to the predefined diagnosis lambda lam_D, which is forwarded to integrator 35 as delayed lambda signal lam_d. The delay of lambda may be omitted since diagnosis lambda lam_D is generally kept constant during the diagnosis.
The delay time to be input may depend on air signal msL. Furthermore, the delay time may be a function of the load of internal combustion engine 10. For example, the load may be indicated by fuel signal mK, possibly in conjunction with rotational speed n, or by a torque of the combustion engine known to control unit 20.
As illustrated in FIG. 3, the determination of the oxygen takes place in the region of plateau 61 of lambda signal lam_nK or filtered lambda signal lam_nKF. Plateau 61 is more or less pronounced. During plateau 61, lambda may change from a value of just below 1 to a value of just above 1. The lambda values may change between 0.99 and 1.01 in a rich-lean jump and between 0.998 and 1.002 in a lean-rich jump.
According to an example embodiment of the present invention, the integration is ended when second change 62 occurring at fourth point in time t4 is detected. The determination of second change 62 may be implemented analogously to the already described determination of first change 60. With the occurrence of second change 62, integrator enable signal 53 is reversed and the integration concluded. Integration result 52, which reflects a measure of the oxygen storage/oxygen discharge or reagent storage, is compared to first threshold value 37 provided by threshold-value definition 32. If a threshold has been exceeded, which signals a poor catalytic converter, comparator 36 provides fault signal F which is stored in a fault memory, for example, or is able to be displayed.
According to an example embodiment of the present invention, the integration may be ended even before second change 62 is reached. In this situation, integration result 52 is compared to second threshold value 38, which is provided by threshold-value definition 32 and defined to a value that corresponds to a good or properly functioning catalytic converter 15. If integration result 52 already corresponds to a properly functioning catalytic converter 15, the catalytic converter diagnosis may be terminated even before second change 62 is able to be detected.
If first or second threshold value 37, 38, respectively, have been reached or exceeded, comparator 36 provides diagnosis stop signal 43, which induces diagnostic controller 33 to terminate the diagnosis. First and second diagnosis control signals 39, 41 are canceled for this purpose. Change determination 40 cancels integrator enable signal 53, thereby resetting integrator 35 to an initial state, which is subsequently available for a new determination of the oxygen.
Threshold-value definition 32 may stipulate first and/or second threshold value 37, 38 as a function of air signal msL, rotational speed n and/or the temperature of the catalytic converter. Exhaust-gas temperature Tabg occurring upstream from catalytic converter 15 may be utilized as measure for the temperature of the catalytic converter. Air signal msL is a measure for exhaust gas mass flow msabg, which, just as the catalytic converter temperature/exhaust gas temperature Tabg, has an influence on the oxygen storage capacity of catalytic converter.
Exhaust gas temperature Tabg may be measured by temperature sensor 14. Exhaust gas temperature Tabg may be able to be measured at least approximately on the basis of air signal msL and, for example, fuel signal mK as a measure for the load of internal combustion engine 10.

Claims

1. A method for operating an internal combustion engine, at least one catalytic converter arranged in an exhaust-gas region of the internal combustion engine, a lambda sensor arranged as a broadband lambda sensor and arranged downstream from one of (a) the catalytic converter and (b) a section of the catalytic converter, comprising:

performing a catalytic converter diagnosis based on a measure for an oxygen storage capacity at least one of (a) of and (b) within the catalytic converter based on an oxygen reservoir of the catalytic converter that is at least approximately one of (a) empty and (b) full, and in which a change in a lambda setpoint value of the internal combustion engine to an excess air lambda that is one of (a) greater than 1 and (b) less than 1 is implemented;

detecting a first change in a lambda signal provided by the broadband lambda sensor;

ascertaining an oxygen stored/discharged after the first change; and

terminating ascertaining of the stored/discharged oxygen when either: (a) a second change of the lambda signal is determined; or (b) a predefined measure of oxygen was stored/discharged.

2. The method according to claim 1, wherein the change of the lambda signal is determined from a gradient of the lambda signal.

3. The method according to claim 2, wherein the change is detected when the gradient exceeds a predefined gradient threshold value for a predefined time period.

4. The method according to claim 2, wherein the change is detected when the gradient has a maximum.

5. The method according to claim 2, wherein the change is detected when the gradient has exceeded a maximum and subsequently falls below a gradient threshold value.

6. The method according to claim 1, further comprising low-pass filtering of the lambda signal provided by the lambda sensor.

7. The method according to claim 1, wherein the oxygen is determined from an integral over time, which is a function of a diagnosis lambda predefined during the diagnosis, and of an air signal, which is provided by an air detection, which detects combustion air provided to the internal combustion engine.

8. The method according to claim 7, wherein the air signal is buffer-stored for a delay time that corresponds to a gas propagation time before the broadband lambda sensor is reached.

9. The method according to claim 8, wherein the delay time is a function of at least one of (a) the air signal and (b) a load of the internal combustion engine.

10. The method according to claim 1, wherein one of (a) the oxygen reservoir of the catalytic converter and (b) a section of the catalytic converter is at least approximately completely one of (a) filled and (b) emptied prior to the catalytic converter diagnosis by specifying a combustion lambda that is one of (a) greater than 1 and (b) less than 1.

11. The method according to claim 10, wherein the combustion lambda that is one of (a) greater than 1 and (b) less than 1 is specified within the framework of normal operation of the internal combustion engine.

12. The method according to claim 1, wherein the catalytic converter diagnosis is implemented following a deceleration fuel-cutoff phase of the internal combustion engine by which the oxygen reservoir of the catalytic converter is at least approximately filled.

13. The method according to claim 1, wherein the evaluation of the stored/discharged oxygen is implemented as a function of at least one of (a) a catalytic converter temperature, (b) an exhaust gas temperature and (b) an exhaust gas mass flow.

14. A device for operating an internal combustion engine, comprising:

a control device configured to perform a method, at least one catalytic converter arranged in an exhaust-gas region of the internal combustion engine, a lambda sensor arranged as a broadband lambda sensor and arranged downstream from one of (a) the catalytic converter and (b) a section of the catalytic converter, the method including:

ascertaining an oxygen stored/discharged after the first change; and

15. The device according to claim 14, wherein the control unit includes:

a diagnostic controller adapted to change the lambda setpoint value;

a change determination adapted to process the lambda signal; and

an integrator adapted to determine a measure for the stored/discharged oxygen.