EP1478834B1 - Method for adjusting a defined oxygen concentration by means of binary lambda regulation in order to diagnose an exhaust gas catalyst - Google Patents

Abstract

The invention relates to a method for diagnosing a regulated exhaust gas catalyst, according to which regulating the catalyst results in control cycles, catalyst diagnosis being performed at a predetermined oxygen concentration per control cycle. A fuel mixture can be adjusted fat or lean according to a specific lambda control factor. A fat or lean exhaust gas is detected, the lambda control factor being incrementally decreased when a lean exhaust gas is detected. The lambda control factor is modified by a P step following a detected change from a fat to a lean exhaust gas or from a lean to a fat exhaust gas, the lambda control factor being set to a minimum value during a first loading period following a detected change from a fat exhaust gas to a lean exhaust gas while being set to a maximum value during a second loading period following a detected change from a lean exhaust gas to a fat exhaust gas. The first and the second loading period are adjusted such that the oxygen concentration reaches the predetermined oxygen concentration in each control cycle.

Description

The invention relates to a method for setting a defined oxygen loading with binary lambda control for carrying out the exhaust gas catalyst diagnosis. The invention further relates to a control device that can be used to set a defined oxygen loading.

Exhaust catalysts for motor vehicles, hereinafter referred to simply as catalysts, are subject to aging phenomena. According to legislation, it is necessary to carry out a review of the function of catalytic converters in each driving cycle. The reliable function of catalysts is carried out by determining the oxygen storage capacity of the catalyst. The catalyst diagnosis runs over several lambda control periods, which coincide with catalyst diagnostic cycles. In order to have as few scatters as possible of individual diagnostic cycles, it is important to have a specific oxygen loading of the catalyst which can be repeated in each of the control cycles caused by the control.

In the case of a linear lambda control, this defined oxygen charge can be achieved with a defined forced excitation. In this case, cyclical deviations from the stoichiometric lambda desired value are set, wherein half periods alternate with lean and rich exhaust gas. In the lean exhaust half-cycle, the oxygen storage of the catalyst is charged by storing excess oxygen, while the rich exhaust gas half-period empties the oxygen storage of the catalyst by consuming oxygen to oxidize exhaust components. The instantaneous oxygen input is positive when excess oxygen is stored in the catalyst; he is negative, if the missing oxygen to oxidation reactions in the rich exhaust gas is removed from the catalyst (if it has been previously stored).

In the case of a binary lambda control, the control is based on feedback from the lambda probe that the exhaust gases correspond to a rich or lean mixture. In the case of a lambda probe signal which indicates a fuel mixture that is too rich, the fuel quantity is continuously emaciated, with the oxygen used for oxidation reactions being removed from the catalyst. The emptying takes place until the lambda probe signal jumps over and indicates too lean a fuel mixture, wherein the excess oxygen is stored in the catalyst. Then there is a short dwell time with which slight lambda shifts, i. different reaction times of the lambda probe, can be compensated. Subsequently, a so-called p-jump (proportional jump) of the lambda control factor takes place in the enrichment direction and the fuel mixture is then continuously enriched until the binary lambda probe indicates a too rich fuel mixture. This is followed by a corresponding dwell time and a p-jump of the lambda control factor in the leaning direction. This control cycle repeats itself.

The duration of the control cycle and the amplitude are essentially determined by the system transport delay and the reaction time of the lambda probe. The system delay is strongly dependent on the operating point of the engine. As a result, the oxygen loading of the catalyst is subject to changes, which makes it difficult to determine the catalyst efficiency. In addition, newer catalysts for meeting future emission limits (eg ULEV, LEV II) have a higher oxygen storage capacity, so that the catalyst efficiency diagnosis requires a higher oxygen loading than is self-adjusting in a control cycle.

So far, standard PI lambda controllers with extended residence times are known to achieve higher oxygen loading. The oxygen loading is subject to large variations from control cycle to control cycle and is significantly dependent on the operating point. As a result, the individual cycles of the catalyst efficiency diagnosis are also subject to large variations, so that a sufficient selectivity between differently aged catalysts does not exist. The patent US 5325664 discloses such a method.

It is therefore an object of the present invention to enable a less sensitive to interference reproducible catalyst efficiency diagnosis.

This object is achieved by the method according to claim 1, and by the control device according to claim 4.

Further advantageous embodiments of the invention are specified in the dependent claims.

According to a first aspect of the present invention, there is provided a method of adjusting a defined oxygen loading to carry out the catalyst diagnosis. The control of the catalyst causes control cycles. The catalyst diagnosis is carried out at a predetermined oxygen loading per control cycle. A fuel mixture is fat or lean adjustable according to a lambda control factor. A rich or lean exhaust gas of the fuel mixture is detected, wherein upon detection of a lean exhaust gas of the fuel mixture, the lambda control factor is incrementally increased, and upon detection of a rich exhaust gas of the fuel mixture, the lambda control factor is incrementally reduced. After a detected change from a rich exhaust gas to a lean exhaust gas or from a lean exhaust gas to a rich exhaust gas of the fuel mixture, the lambda control factor is changed by a p-step value of the lambda control factor. Continue after a detected change from a rich exhaust gas to a lean exhaust gas of the fuel mixture, the lambda control factor during a first loading time to a minimum control factor value and after a detected change from a lean exhaust gas to a rich exhaust gas of the fuel mixture, the lambda control factor during a second loading time set a maximum controller factor value. The minimum control factor is determined by a local minimization of the controller factor value of the current control cycle, the maximum controller factor by a local maximum of the controller factor value of the current control cycle. The first and the second loading time are adjusted so that the oxygen loading in each control cycle reaches the specific oxygen loading, ie the predetermined oxygen input or oxygen discharge depending on the half-period of the control cycle.

With the lambda control factor you can set the mixture rich or lean. If a rich exhaust gas is detected with the lambda probe, the lambda control factor is continuously reduced and thus the mixture is emaciated until the lambda probe detects a lean exhaust gas. This is followed by a residence time during which the lambda control factor is stopped in order to compensate for the difference in the probe switching times, or to realize a slight mixture shift, as in a standard lambda controller. Thereafter, an additional P-jump .DELTA.P also takes place in the leaning direction of the lambda control factor to the minimum control factor value, which results from the maximum difference to the lambda control factor mean value, so that the value of the predetermined oxygen charge is reached more quickly. Thereafter, the P-jump is effected by the amount of incremental decreases and the additional P-jump ΔP in the direction of enrichment. Since a lean exhaust gas is detected at the lambda probe, the lambda control factor is now increased continuously and thus the fuel mixture is enriched until the lambda probe detects a rich exhaust gas. This is followed by a dwell time to compensate for the difference in the probe switching times, or mixture shift to realize. This is followed again by an additional P-jump in the direction of enrichment, which is limited by the maximum difference to the lambda factor mean, so that the oxygen discharge - corresponding to the oxygen input in the lean half period - realized faster. For the catalyst diagnosis, it is important to be able to set the amplitude of the lambda oscillation by the additional P-jump, or the limitation of the maximum amplitude as a function of the operating point, so that the oxygen storage properties in the catalyst can be taken into account in the catalyst diagnosis.

The process according to the invention results in that at one enrichment half period - oxygen discharge from the catalyst -, i. the mixture is enriched, or a half-leaning period - oxygen input in the catalyst, i. the fuel mixture is emaciated, the fuel mixture after detecting a change between rich and lean exhaust still changed by a ΔP jump, or is set to a maximum difference to the lambda control factor average to the previously not yet reached predetermined oxygen load as fast as possible to achieve with defined lambda amplitude. Adjusting the lambda controller factor to the maximum controller factor value that depends on the predetermined oxygen load causes the predetermined determined oxygen load to be reached quickly after a change between rich and lean exhaust gas has been detected.

After the predetermined oxygen load has been reached, the lambda control factor is reset by the sum of the P-jumps (standard P-jump + ΔP-jump) carried out during the respective half-cycle. As before, the lambda control factor is now increased or decreased step by step, thus emacifying or enriching the fuel mixture.

It is preferably provided that the predetermined specific oxygen charge is determined by the maximum oxygen storage capacity of an aged catalyst. In this way, the catalyst efficiency diagnosis can be carried out even with an aged catalyst at a repeatable in each control cycle operating point-dependent oxygen loading of the catalyst.

The minimum or the maximum controller factor value is preferably determined by the difference between the lambda control factor and the lambda factor mean value and is predetermined by the oxygen storage rate of the catalytic converter. The oxygen storage rate of the catalyst depends on the flow rate of the exhaust gases through the catalyst and the catalyst temperature and essentially describes what maximum amount of oxygen per unit time can diffuse into the catalyst and be bound. The controller factor value is thus set to a minimum or maximum value at which the oxygen diffusion rate is not yet exceeded, and therefore measurable oxygen behind the catalyst, although the storage capacity has not been exceeded.

In accordance with another aspect of the present invention, a controller is provided for performing a controlled catalyst diagnostic. The controller adjusts a certain maximum oxygen load per control cycle to carry out a catalyst diagnosis. The control device regulates the composition of a fuel mixture, the regulation leading to control cycles. The control device can be connected to an injection system to set the fuel mixture rich or lean according to a lambda control factor. Using a sensor, lean or rich exhaust gas is detected. The controller incrementally increases the lambda control factor with lean exhaust gas and decreases the lambda control factor incrementally with rich exhaust. The control device sets the lambda control factor during a first loading time after a detected change from a rich exhaust gas to a lean exhaust gas of the fuel mixture to a minimum controller factor value, wherein after the first loading time, the controller factor value is set to an average value of the lambda controller factor. The controller further sets the lambda control factor to a maximum controller factor value during a second loading time after a change from a lean exhaust gas to a rich exhaust gas of the fuel mixture has been detected. After expiration of the second loading time, the lambda control factor is changed to an average value of the lambda control factor by the control device. The first and second loading times are set so that the oxygen loading, ie, the oxygen input or discharge in each control cycle, reaches the predetermined maximum positive or negative oxygen loading.

The control device according to the invention has the advantage that it controls the fuel mixture so that the oxygen loading is the same for each control cycle, so that a reproducible oxygen loading over several control cycles allows a störungsunempfindlichere and reproducible catalyst diagnosis.

The control device can preferably be operated in a diagnostic mode for performing the catalyst diagnosis and operated in a second operating mode, in which the control device regulates as previously known standard PI lambda controller. In this way, the catalyst diagnosis is merely an operating mode of an already provided control device, so that a change of the overall system with a control device, injection system, engine and catalyst essentially does not have to be changed constructively.

Figur 1

ein Motorsystem mit einer Regeleinrichtung gemäß einer bevorzugten Ausführungsform der Erfindung; und

Figur 2

den Verlauf des Lambda-Reglerfaktors über mehrere Regelzyklen.

A preferred embodiment of the invention will be explained in more detail below with reference to the accompanying drawings. Show it:

FIG. 1

an engine system with a control device according to a preferred embodiment of the invention; and

FIG. 2

the course of the lambda controller factor over several control cycles.

FIG. 1 shows a functional diagram of an engine system. The engine system has a Gemischbildner 1, which provides an internal combustion engine 2, a fuel mixture of air and fuel. The engine 2 burns the fuel mixture and releases exhaust gases supplied to a three-way catalyst 5. The exhaust gas emitted by the internal combustion engine 2 is conducted via a lambda probe 4, which determines from the exhaust gas composition whether the mixture is richer or leaner than the stoichiometric fuel mixture.

The lambda probe 4 is connected to a control device 3, so that a measured value measured by the lambda probe 4 is available as an input variable for the control device. In the control device 3 is a binary controller, which only receives the information as input from the lambda probe, whether the exhaust gas corresponds to a too rich or too lean fuel mixture. The control device 3 generates a control value, which is transmitted to the mixture former 1. The manipulated variable is the lambda control factor, which indicates by what factor the basic fuel mixture ratio specified by an injection system (not shown) should be changed.

By checking the operability of the catalyst 5, a catalyst efficiency diagnosis can be performed. For such an efficiency diagnosis, it is important that the lowest possible spread between individual diagnostic cycles is available. This can be achieved by charging the catalyst with the same amount of oxygen in each control cycle. While one can achieve the same oxygen loading in the control cycles with linear lambda control with a defined forced excitation, this is not possible with a binary lambda control. A binary lambda control regulates the mixture composition via the lambda control factor based on a binary signal dependent on the lambda probe or the probe voltage U _λ , which indicates whether the fuel mixture is too rich or too lean, the control deviation being unknown.

Since the length of the control cycles is operating point-dependent, during normal operation there is no constant oxygen load over the control cycles. After activation of the catalytic converter efficiency diagnosis, however, switching is made to an oxygen charge-based lambda control. FIG. 2 shows the time profile of the lambda control factor over time.

In a first period T1, the controller 3 is in normal operation, i. the lambda control is achieved by cyclically oscillating the lambda control factor by an average of about a lambda value of 1, i. corresponds to a stoichiometric mean. The control cycles are referred to as a lean half-period when the lambda control factor is less than its average, and as the fifth-half period when the lambda control factor is greater than its average.

During the lean half-period, there is more oxygen in the fuel mixture than the stoichiometric means, ie, needed for optimal operation of the catalyst. This results in a positive oxygen loading during the lean half period. During the Fetthalbperiode is less oxygen in the fuel mixture than the stoichiometric means, that is less than necessary for optimal operation, so that oxygen is released from the catalyst for the oxidation reactions to the exhaust gas. This is called negative oxygen loading (oxygen discharge).

Lambda control is accomplished by incrementally increasing the lambda control factor in the phase in which the lambda probe reports lean exhaust gas, thereby increasingly enriching the fuel mixture, i. the fuel content in the fuel mixture is increasingly increased. This is represented by the stepwise increase of the lambda control factor over time in the first time period T1. Once it is detected by the lambda probe 4 that the fuel mixture is too rich, the stepwise increase of the lambda control factor is stopped.

Since the lambda probe 4 often has an asymmetric reaction time, i. With different reaction times a change from a lean to rich mixture, or detected from the rich to the lean mixture, a first residence time TDLY1 may be provided, while after detecting a change from the lean to the rich mixture and vice versa, the lambda control factor is maintained before being jumped back by a P jump. For the following Fetthalbperiode, i. after the P-jump of the lambda control factor, the lambda control factor becomes continuous, i. gradually reduced so that the fuel mixture is emaciated. If the lambda probe now indicates that the fuel mixture is too lean, the stepwise reduction of the lambda control factor is stopped and, after a second dwell time TDLY2, a P jump of the lambda control factor is made. The second residence time TDLY2 may be different from the residence time TDLY1.

A second time segment T2 now shows the profile of the lambda control factor in a diagnostic mode in which the Functionality of the catalyst should be checked. In order to be able to carry out the diagnosis of the functionality of the catalytic converter with the lowest possible spread between the diagnostic cycles, a constant oxygen charge is necessary for all control cycles. That is, the oxygen loading change should have substantially the same amount both in the lean half periods and in the fifth half periods. It does not matter if it is a positive or a negative oxygen change.

In the diagnostic mode, the control is substantially the same as in the normal mode as described above. As soon as a change from a too rich to a lean fuel mixture has been detected during a lean half period, the lambda control factor is first kept constant after a dwell time TDLY and further emaciated by a ΔP jump after the dwell time. The duration for which the maximum value for the lambda control factor is to be maintained depends on the oxygen load achieved in the relevant half-period. That the maximum value of the lambda control factor is maintained until a defined oxygen load has been reached in this control cycle.

wobei m _{O 2} die Sauerstoffbeladung, t_M die Zeit der Halbperiode, λ der Lambda-Wert des Brennstoffgemischs, (λ = 1 bei stöchiometrischem Mittel) und ṁ_L den Luftmassenstrom darstellt. Da das λ von dem Lambda-Reglerfaktor abhängt, ergibt sich: $$m_{O_2}=23\%\cdot \underset{0}{\overset{t_M}{\int }}\left(1-\frac{1}{{\mathit{\lambda }}_{\mathit{soll}}+\mathrm{\Delta }{\mathit{\lambda }}_{\mathit{soll}}}\right)\cdot {\dot{m}}_L\mathit{dt}$$

wobei λ_soll der Mittelwert des λ-Reglers über eine Periode der λ-Reglerschwingung und Δλ _soll den Verlauf der Abmagerung darstellt. Der Faktor 23% ergibt sich aus dem Sauerstoffmassenanteil in der Luft.In order to determine the oxygen load of the control cycle, the time course of the oxygen input must be determined for each half-period. It applies $$m_{{\mathrm{<figure-callout\; id="2"\; label="O"\; filenames="imgf0001.png"\; state="\{\{state\}\}">O</figure-callout>}}_2}=23\%\cdot \underset{0}{\overset{t_M}{\int }}\left(1-\frac{1}{\mathit{\lambda }}\right)\cdot {\dot{m}}_L\mathit{dt}.$$

where m is _{O 2} the oxygen loading, t _{M is} the time of the half-period, λ is the lambda value of the fuel mixture, (λ = 1 at stoichiometric average) and ṁ _{L represents} the air mass flow. Since the λ depends on the lambda control factor, it follows: $$m_{{\mathrm{<figure-callout\; id="2"\; label="O"\; filenames="imgf0001.png"\; state="\{\{state\}\}">O</figure-callout>}}_2}=23\%\cdot \underset{0}{\overset{t_M}{\int }}\left(1-\frac{1}{{\mathit{\lambda }}_{\mathit{should}}+\mathrm{\Delta }{\mathit{\lambda }}_{\mathit{should}}}\right)\cdot {\dot{m}}_L\mathit{dt}$$

where λ _is the mean value of the λ controller over a period of the λ-regulator oscillation and Δ λ _{soll represents} the course of the leaning. The factor of 23% results from the oxygen mass fraction in the air.

Δ λ _{is to} be positive during the lean half-period and negative during the rich half period. For the oxygen emptying process during the Fetthalbperiode the formulas can be applied in the same way.

wobei FAC_LAM der momentane multiplikative Lambda-Reglerfaktor und FAC_LAM_MV sein Mittelwert über die gesamte Lambda-Reglerperiode ist. Durch diese Integration wird für jede Mager- und Fetthalbperiode der Lambda-Regelung die Sauerstoffbeladung ermittelt. Dadurch, dass der aktuelle Luftmassenstrom ṁ_L berücksichtigt wird, wird auch die Änderung des Betriebspunkts des Motors berücksichtigt.For a binary lambda control, the value of λ is not known directly. λ can be calculated by the lambda control factor, which represents a multiplicative factor of the basic injection quantity. The lambda control factor corresponds inversely proportional to the λ shift. The respective mean value is a mean control intervention over a control cycle and corresponds to λ _soll , and Δ λ _soll is the difference between the current value and the mean value of the lambda control factor. It follows: $$m_{{\mathrm{<figure-callout\; id="2"\; label="O"\; filenames="imgf0001.png"\; state="\{\{state\}\}">O</figure-callout>}}_2}=23\%\cdot \underset{0}{\overset{t_M}{\int }}\left(1-\frac{\mathit{FAC\_LAM}-\mathit{FAC\_LAM\_MW}}{\mathit{FAC\_LAM\_MV}}\right)\cdot {\dot{m}}_L\mathit{dt}.$$

where FAC_LAM is the instantaneous multiplicative lambda controller factor and FAC_LAM_MV is its average over the entire lambda controller period. Through this integration, the oxygen loading is determined for each lean and Fetthalbperiode the lambda control. The fact that the current air mass flow ṁ _{L is} taken into account, the change in the operating point of the engine is taken into account.

In order to avoid a shift in the lambda value, the dwell time and the range of the stepwise change of the lambda control factor are unchanged in the diagnostic mode maintained. However, in order to realize the desired predetermined oxygen loading as quickly as possible, after the residence time the lambda control factor in the lean half period may be increased by a ΔP jump or decreased by a ΔP jump during the fifth-half period in order to increase the oxygen loading - positive or negative - faster to achieve catalyst efficiency diagnostics.

The length of time during which the maximum or minimum value of the lambda control factor is output by the controller 3 depends on the desired oxygen loading, i. the lambda control factor remains applied until the desired oxygen charge according to the above formula is reached.

Upon reaching the desired oxygen load, the lambda control factor is reset by the sum of the lambda controller changes made during the incremental increases or decreases in the respective half-cycle and the additional P-jump ΔP. The sum results from the sum of all incremental increases or decreases of the lambda control factor, and the additional increase or decrease to the maximum difference or the minimum value of the lambda control factor over the entire lambda control cycle.

The maximum or the minimum value of the lambda control factor results from the maximum diffusion rate of the oxygen into the active layer or washcoat of the catalyst into or out. The maximum or the minimum value of the lambda control factor is thus determined by how quickly oxygen from the exhaust gas stream, which is passed through the catalyst, can be taken up or released into the active layer or washcoat. The maximum or minimum control factor value thus results from a predetermined oxygen loading value. If the lambda control factor is set greater than the maximum value or less than the minimum value, this does not mean that more oxygen is absorbed or delivered. As a result, the catalyst is no longer able to buffer the λ fluctuations caused by the control cycles relative to the output of the catalyst, so that no fluctuations can be detected there, although the oxygen storage capacity of the catalyst has not yet been exhausted.

The particular oxygen load used to perform the catalyst efficiency diagnostics corresponds to the oxygen storage capability of an aged catalyst that is just meeting efficiency requirements.

The efficiency diagnosis is carried out with the aid of a λ monitor probe (not shown), which is also a lambda probe, wherein the monitor probe is mounted in the exhaust gas flow downstream of the catalytic converter 5. The monitor probe then detects whether a constant lambda value is reached or whether the lambda value varies according to the control cycles. If the lambda value measured by the monitor probe varies, the catalyst under test does not have sufficient oxygen storage capacity and a defective or aged catalyst is detected.

The oxygen loading calculation and setpoint adjustment also take into account the aging of the lambda probe and the resulting detection delay of the exhaust gas change in rich ↔ lean. Prolongs the reaction time of the lambda probe by aging phenomena, the stepwise increase or decrease in the lambda control factor is carried out longer, so that even when detecting a change between a too rich and too lean a fuel mixture, a higher oxygen loading of the catalyst is achieved and a higher amplitude in the λ control factor and λ oscillation. Therefore, the amplitude of the lambda control factor becomes maximum difference to lambda control factor average limited, that is, the additional P-pitch ΔP is not fully realized.

The idea of the invention is to provide a method for an oxygen-loading-based, binary lambda control, wherein after the residence time a further jump of the lambda control factor value in the original direction is provided in order to achieve the increased oxygen loading more quickly. However, in order to prevent an excessive increase in the amplitude of the lambda controller factor and lambda oscillation by aging the lambda sensor and thus prolonging the reaction time of the probe, the additional P-jump is limited so that it does not reach the maximum in the sum of the I component integrated over half-period Difference to the mean value of the lambda control factor may not exceed. Thus, even with an aged binary lambda control probe with slower dynamics can be avoided that there is an increase in the lambda amplitude.

Catalyst oxygen balancing takes place exclusively via oxygen-loading integrals, which have to compensate each other in the rich and lean half-period. This leads to an increase in the accuracy of the oxygen loading adjustment, especially during transient or slight disturbances. Oxygen load-based lambda control adjusts the times during which the maximum or minimum lambda control factor is maintained, or the amplitude increases, adaptively to the maximum and minimum lambda control factor values, respectively.

Alternatively it can be provided that the lambda control factor is not set to a maximum or minimum value after detection of a change between a lean and rich fuel mixture, but that the lambda control factor is maintained until the predetermined oxygen charge is reached.

Claims (5)

1. Method for adjusting a defined oxygen concentration by means of binary lambda regulation in order to diagnose a catalyst (5) whereby regulation of the catalyst (5) results in control cycles, whereby

   - catalyst diagnosis is carried out at a predetermined defined oxygen concentration for each control cycle,

   - a fuel mixture can be adjusted to rich or lean according to a lambda control factor,

   - a rich or lean exhaust gas is detected,

   - in the case of a lean exhaust gas, the lambda control factor is increased incrementally, and

   - in the case of a rich exhaust gas the lambda control factor is decreased incrementally,

   - after a change has been detected from a rich exhaust gas to a lean exhaust gas or from a lean exhaust gas to a rich exhaust gas, the lambda control value is changed by a P step,

   characterised in that after a change has been detected from a rich exhaust gas to a lean exhaust gas the lambda control factor is set during a first loading time to a minimum control factor value, which represents a local minimum for the control factor value in the current control cycle, and after a change has been detected from a lean exhaust gas to a rich exhaust gas the lambda control factor is set during a second loading time to a maximum control factor value, which represents a local maximum for the control factor value in the current control cycle,
   whereby the first loading time is adjusted so that the oxygen concentration achieves an oxygen input defined by the predetermined oxygen concentration in each control cycle, and
   whereby the second loading time is adjusted so that the oxygen concentration achieves an oxygen output defined by the predetermined oxygen concentration in each control cycle.
2. Method according to claim 1,
   characterised in that the predetermined oxygen concentration is determined by the maximum oxygen storage capacity of an ageing catalyst.
3. Method according to claim 1 or 2,
   characterised in that the minimum and maximum control factor values are defined by the difference between the lambda control factor and a mean value of the lambda control factor for the current control cycle, whereby the difference is predetermined by the oxygen absorption capacity of the catalyst.
4. Regulator (3) for adjusting a defined oxygen concentration by means of binary lambda regulation in order to diagnose a catalyst, whereby the regulator carries out catalyst diagnosis at a predetermined defined oxygen concentration for each control cycle, whereby the regulator (3) regulates the composition of a fuel mixture with control cycles,
   whereby the regulator (3) can be connected to a mixer (1) to adjust the fuel mixture to rich or lean according to a lambda control factor,
   whereby a lean exhaust gas or rich exhaust gas can be detected using a sensor (4),
   whereby in the event of a lean exhaust gas for the fuel mixture, the regulator increases the lambda control factor incrementally and in the event of a rich exhaust gas for the fuel mixture, it decreases the lambda control factor incrementally,
   whereby the regulator (3) changes the lambda control factor by a P step, after a change has been detected from a rich exhaust gas to a lean exhaust gas or from a lean exhaust gas to a rich exhaust gas for the fuel mixture,
   characterised in that the regulator (3) sets the lambda control factor to a minimum control factor during a first loading time after a change has been detected from a rich exhaust gas to a lean exhaust gas for the fuel mixture and sets the lambda control factor to a maximum control factor during a second loading time after a change has been detected from a lean exhaust gas to a rich exhaust gas for the fuel mixture,
   whereby the first and second loading times are determined such that the oxygen concentration achieves the predetermined defined oxygen concentration in each control cycle.
5. Regulator (3) according to claim 4,
   characterised in that the regulator can be operated in a diagnosis mode to carry out diagnoses and in a second operating mode, in which the regulator (3) regulates the catalyst according to a normal operating mode.

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