DE10008563A1 - Nitrogen oxide storage catalyst diagnosis process, involving reporting value of characteristics of desorption peak as storage catalyst changes mode - Google Patents

Abstract

The diagnosis process involves determining the characteristic features of a nitrogen oxide desorption peak in the temporal process of the nitrogen oxide concentration as the nitrogen oxide storage catalyst (1) passes from the absorption mode into the regeneration mode for comparison with a predetermined test pattern, generating a comparison result and a catalyst state signal. Any error signal is stored or displayed.

Description

The invention relates to a method and a device for diagnosing a NO _x storage catalytic converter with the features mentioned in the preamble of claims 1 and 14, respectively.

The optimization of the air / fuel mixture is of central importance for the development of powerful and at the same time environmentally friendly internal combustion engines for motor vehicles. However, this problem cannot be solved with the 3-way catalytic converter that has been customary up to now, since this can only reduce the exhaust gases that occur during operation of the engine to harmless gas components when air and fuel are supplied to the engine in a precisely defined stoichiometric ratio. However, the stoichometric mode of operation with a so-called lambda value = 1 of the air / fuel mixture is not the most economical. A substantially lower fuel consumption can be achieved with a double air excess in the air / fuel mixture, ie with a lambda value <2. Since the conventional catalytic converters can no longer reduce the nitrogen oxides (NO _x ) that are formed when there is an excess of air, this operating mode cannot be justified without further measures from the point of view of environmental pollution.

A preferred solution to this problem without metering in a reducing agent is the use of NO _x storage catalysts in the exhaust system. A NO _x storage catalytic converter can store nitrogen oxides absorptively under certain boundary conditions with a lambda value <1 for a limited period of time and at a later point in time again with a lambda value <1 or = 1 and reduce it to harmless gases. NO _x storage catalytic converters are usually operated in a storage cycle which comprises at least one usually slow absorption mode and a faster regeneration mode.

NO _x storage catalytic converters are mainly used for lean-burn engines. In this type of engine, so-called lean operation with a lambda value <1 of the air / fuel mixture is preferred compared to stoichiometric or so-called rich operation with a lambda value <1. Stratified charge engines represent a special form of lean-burn engine. In stratified charge operation of a stratified charge engine, a lean air / fuel mixture is supplied to the engine and an ignitable rich air / fuel mixture is provided in the combustion chamber near the spark plug, while a lean mixture is present in the rest of the combustion chamber. First the rich air / fuel mixture is ignited with the spark plug and then the lean one. During stratified charge operation, the NO _x storage catalytic converter is operated in the absorption mode. In contrast, in homogeneous operation of the engine, a stoichiometric or rich air / fuel mixture is supplied and the NO _x storage catalytic converter is operated in regeneration mode.

Statutory regulations in an increasing number of industrialized countries stipulate that faults in exhaust-gas-influencing components of a motor vehicle, which could lead to emission limit values not being adhered to, have to be recognized and displayed as part of an on-board diagnosis during driving. The NO _x storage catalytic converter also belongs to the components which influence the exhaust gas. The function and efficiency of a NO _x storage catalytic converter depend on a large number of influencing factors and can in particular be subject to reversible and irreversible damage. Reversible damage can arise, for example, from sulfurization of the catalyst and in particular lead to a reduction in the NO _x storage capacity. Thermal damage, such as sintering of a catalyst component, segregation of catalyst and storage component or an increasingly inhomogeneous, near-surface NO _x loading, on the other hand, represent irreversible damage to the NO _x storage catalyst. In addition to a reduction in the NO _x storage capacity, thermal damage generally also results to a reduced oxygen storage capacity of the catalyst. In addition to operational damage, manufacturing-related variations in the properties of the catalyst can also affect efficiency and function.

From EP 0936349A2 a system for diagnosing damage to a NO _x catalyst is already known, which is connected to an internal combustion engine and in which the signals of a NO _x sensitive sensor arranged behind the catalyst are evaluated to assess the degree of damage. In this context, EP 0936349A2 discloses a reduction in the NO _x concentration after switching to a rich air / fuel mixture. The NO _x concentration reaches a minimum after a certain time in order to then rise again to higher values and finally to reach a value again as it existed before switching to a lack of oxygen. The state of the NO _x storage catalyst or its damage is determined in the known system from the rate of change of the NO _x concentration after the minimum has been reached. For this purpose, values of the NO _x concentration must be used within a relatively large time interval after switching to a lack of oxygen, which leads to a correspondingly long diagnosis duration. Another disadvantage is that the rate of change in the NO _x concentration in the time interval used is sensitive to the operating parameters of the engine and the exhaust system and therefore requires complex corrective measures.

The object of the invention is to provide a method and a device for diagnosing a NO _x storage catalytic converter, which is essentially based on the evaluation of values of the NO _x concentration within a relatively short time interval and enables a relatively quick and uncomplicated diagnosis.

This object is achieved with the features of the independent claims.

The invention is based on the knowledge that when the NO _x storage catalytic converter changes from an absorption mode to a regeneration mode, only a part of the stored NO _{x is converted} catalytically within a short time interval. The unreacted part of the NO _x leads to a brief increase in the NO _x concentration in the exhaust gas, the so-called NO _x desorption peak. Characteristic properties of this peak, such as time duration, height or the like, are related to the function or to any damage to the NO _x catalyst which may be present. According to the invention, the NO _x concentration in the exhaust gas downstream of the NO _x storage catalytic converter is measured, the values of characteristic features of a NO _x desorption peak in the NO _x concentration are determined and compared with predetermined test patterns and a comparison result is formed. A catalyst status signal, in particular an error signal, which is formed as a function of the comparison result, is stored and / or displayed. Since the NO _x desorption peak occurs within a relatively short time interval, for example after the transition from a lean to a rich or stoichiometric air / fuel mixture, a relatively short diagnostic period can be achieved. In the best case, the time duration of a single NO _X desorption peak is sufficient for the diagnosis according to the invention. The determination according to the invention of values of characteristic features of the NO _x desorption peak allows a particularly simple evaluation of the time profile of the NO _x concentration in the time interval of interest, so that a sufficiently precise diagnosis can be achieved with little detection effort. Since the NO _X desorption peak is largely independent of the NO _X loading of the NO _X storage catalytic converter, there is a further advantage that the diagnostic results are only slightly sensitive to variations in the NO _X loading of the NO _X storage catalytic converter and thus the operating parameters of Internal combustion engine and exhaust system.

Further features and advantages of the present invention result from the dependent claims and regardless of their summary in the Claims from the following description of preferred inventive Exemplary embodiments in conjunction with the associated drawings.

The drawings show in a schematic representation:

FIG. 1 shows an internal combustion engine with an NO _x storage catalyst;

FIG. 2 shows a diagram of time profiles of various signals during a regeneration process of a NO _x storage catalytic converter;

Fig. 3 is a flowchart of the diagnosis of a NO _x storage catalyst.

Fig. 1 shows a leanly operable internal combustion engine 3 of a motor vehicle, shown only schematically, such as a stratified charge engine with a downstream exhaust system 2 and with an engine control 13 and a NO _X control device 13 a for the on-board diagnosis of a NO _X storage catalytic converter 1 for storing and converting nitrogen oxides. The NO _x storage catalytic converter 1 can be operated in a storage cycle with an absorption and a regeneration mode.

In addition to the NO _x storage catalytic converter 1 , the exhaust gas system 2 is assigned a pre-catalytic converter 16 , a temperature sensor 12 and lambda sensors 10 and 15 for detecting the lambda value of the exhaust gas in the area of the pre-catalytic converter 16 or downstream of the NO _x storage catalytic converter 1 . A known NO _x sensor 4 arranged downstream of the NO _x storage catalytic converter 1 supplies a NO _x signal that selectively represents the NO _x concentration in the exhaust gas and, if appropriate, a corresponding signal for the oxygen concentration.

The engine control unit 13 detects operating parameters of the internal combustion engine 3, such as exhaust gas temperature, load, speed, raw emissions profile or the like, in a known manner via the temperature sensor 12 and further sensors (not shown) and can do this via steep links (not shown), such as a throttle valve in the air supply of the internal combustion engine 3, if necessary. Communication between the engine control unit 13 and the internal combustion engine 3 or the actuators takes place via a cable system 14 . The engine control unit 13 comprises in particular a lambda control 11 , which is connected to the lambda probe 10 . Furthermore, the engine control unit 13 contains the NO _x control device 13 a, to which the signal of the NO _x probe 4 is supplied.

The NO _X control device 13 a, which may also be designed as a separate component, has means 5 for determining the values of characteristic features of a NO _X desorption peak, means 6 for comparing the determined values with predetermined test patterns and for forming a comparison result corresponding to the deviation between the determined values and the test samples, as well as evaluation means 7 and storage means 8 . The NO _X control device 13 a can be implemented, for example, by a microcontroller with a CPU, a program memory, a data memory and input and output interfaces. A catalytic converter status signal, in particular an error signal, is, as will be described in more detail below, formed by the evaluation means 7 as a function of the comparison result which is provided by the means 6 . To display the catalytic converter status signal, the NO _x control device 13 a is connected via an interface (not shown) to display means 9 , for example a light indicator. The test sample, which may be for example stored in a ROM that represent desired values of the characteristic features of the NO _x desorption peak in the exhaust gas downstream of the NO _X storage catalytic converter 1 in a transition from absorption to regeneration mode of the NO _X storage catalytic converter 1, whereupon subsequently is discussed in more detail.

To illustrate the method according to the invention, FIG. 2 shows the basic temporal course of signals for the regeneration process of a NO _x storage catalytic converter 1 during the transition from lean to rich operation in a stratified charge engine. The NO _x storage catalytic converter 1 is in the up to time t ₁ absorption mode. At this time, engine control unit 13 recognizes that regeneration of NO _x storage catalytic converter 1 is required. This can happen, for example, when the engine control unit 13 determines that the NO _x concentration in the exhaust gas has reached a threshold value NO _x -S because the NO _x loading capacity of the NO _x storage catalytic converter 1 has been exhausted and therefore no or only a small amount Amount of NO _X can be stored further. At time t ₁ , engine control 13 therefore requests a NO _x reduction and the value of control signal S _M is set to 1. The lambda value L of the air / fuel mixture is accordingly reduced from a value <2 to a value of approximately 0.9, which corresponds to a transition from an excess of oxygen to a lack of oxygen.

The internal combustion engine 3 is switched from shift to homogeneous operation starting at time t ₁ , since a rich air / fuel mixture is now made available. The control signal S _B is set from 1 to 0. The actual regeneration mode of the NO _x storage catalytic converter 1 begins at this time. Under these conditions, initially not all the NOx is catalytically converted in the exhaust gas to the NO _X storage catalytic converter 1 for a short time. There is a brief increase in the NO _x concentration beyond the threshold NO _x -S, which can be recognized as a NO _x desorption peak in the NO _x signal.

In Fig. 2 D is shown for the time course of NO _X signals _Xn NO and NO _Xa with a new and an aged NO _x storage catalyst 1 of the respective, substantially triangular in this case, NO _X in the range -Desorptionspeak. The characteristic features of the respective NO _X desorption peaks are the maximum value H _n , the area A _n and the duration D _n for a new, or H _a , A _a and D _a for an aged NO _X storage catalytic converter in the drawing. The values of these characteristics are each based on a reference NO _x concentration. In the exemplary embodiment, the value of the measured NO _x concentration at time t _{2 is} used as the reference NO _x concentration. However, other reference values can also be used according to the invention, in particular the value of the NO _x concentration at time t ₁ at which the engine control unit 13 requests a NO _x reduction. The reference of the values of the characteristic features to a reference value allows only values relative to this reference value to be used instead of absolute values of the NO _x concentration and thus possible offset errors of the NO _x probe 4 to be compensated in a simple manner.

Instead of or in addition to the mentioned features of a NO _x desorption peak, other features, in particular the rising edge, the falling edge or the half-value width, can also be selected according to the invention. In particular, triangular-shaped NO _x desorption peaks, possibly also with more than one maximum, could also be taken into account.

Sorting algorithms known per se, for example from pattern recognition, are used by the NO _x control device 13 a to determine the values of the characteristic features from the time profile of the NO _x signal.

To further carry out the method according to the invention, the determined values of the characteristic features of the NO _x desorption peak are compared with the corresponding test samples. Since the test patterns represent target values, in particular error threshold values of the respective characteristic features, they are preferably determined from a model for the NO _x storage catalytic converter 1 and measured or calculated operating parameters of the internal combustion engine 3 and the exhaust system 2 . Load, speed, raw emission curve, exhaust gas temperature, function of a pre-catalytic converter 16 or the like are particularly suitable as operating parameters here. Alternatively, the test patterns can also be obtained in a learning phase of the engine control unit 13 or the NO _x control device 13 a from the measured values of a new NO _x storage catalytic converter 1 .

In the simplest case, a test pattern only consists of the target value of a single feature, for example the maximum value of the NO _x desorption peak.

For a differentiated diagnosis, the values of a number of two or more characteristic features are compared with corresponding test samples. The comparison result formed in accordance with the deviation between the characteristic features and the test samples then reflects the type and degree of damage. Here, the knowledge is used that different damages to the NO _x storage catalytic converter 1 influence the value of the characteristic features of the NO _x desorption peak differently. For example, thermal damage to a certain type of NO _x storage catalytic converter results in a reduced maximum value of the NO _x desorption peak, but does not influence its duration, while sulfur poisoning only leads to a reduced duration. In other types of NO _x storage catalytic converters, however, other changes in the NO _x desorption peak can occur due to different damage mechanisms.

It can be seen from FIG. 2 that some time after requesting NO _x regeneration at time t _1, the lambda value L _n measured downstream of the NO _x storage catalytic converter 1, for example by the lambda sensor 15, drops from a value <2 to a value close to 1 and assumes a value <1 at a later point in time after the end of the NO _x desorption peak, before it increases again after the end of the regeneration mode. The drop in the lambda value L _n to a value <1 takes place as shown in the diagram in FIG. 2 for a new NO _x storage catalytic converter 1 at a later point in time than the corresponding drop in the lambda value L _a for an aged catalytic converter. These differences in the time profile of the lambda values L _n and L _a can be used as additional information for evaluating the NO _x desorption peak, as can a peak in the oxygen concentration which may occur before the NO _x desorption peak.

Depending on the comparison result, the evaluation means 7 form the catalytic converter status signal which, depending on the comparison result, can be stored by the storage means 8 and / or displayed by the display means 9 . For example, if the comparison result exceeds a predetermined error threshold value, an error signal is generated. In the case of a more differentiated diagnosis, different catalyst status signals are also used depending on the type and degree of damage. If a malfunction of the NO _x storage catalytic converter 1 is detected, the driver of a motor vehicle can be warned immediately using the display means 9 . On the other hand, the stored information can also be forwarded to a workshop diagnostic system when the motor vehicle is in the workshop.

The values of the characteristic features of one or more NO _x desorption peaks can also be stored for later evaluation. As an alternative or in addition to this, the course over time of the values of the NO _x concentration can be stored at least in one or more time windows assigned to the NO _x desorption peaks in order to have more complete information available

Since the measured values of the NO _x concentration can be subject to fluctuations, a further embodiment of the invention provides for averaging to compensate for these fluctuations. For this purpose, the values of the characteristic features are determined over a number of storage cycles of the NO _x storage catalytic converter and a corresponding mean value, for example an arithmetic mean value, is formed. This measure can generally be provided or can be provided depending on the determined values of the characteristic features, in particular on the value of the fluctuation range.

In a preferred embodiment of the invention, a catalyst status index K is determined from the determined values of the characteristic features of the NO _x desorption peak by evaluating the values of the characteristic features and combining them algebraically. The equation shows one way of doing this:

K = H _k .c ₁ + D _k .c ₂ + A _k .c ₃ .

Here H _k denotes the maximum value, D _k the time duration and A _k the area of the respective NO _X desorption peak. The evaluation factors c ₁ to c ₃ allow adaptation to the specific properties of a specific NO _x storage catalytic converter 1 . Likewise, the evaluation factors allow an adaptation to properties of the internal combustion engine 3 and the exhaust system 2 . In this embodiment of the invention, the catalytic converter status signal is formed as a function of the value of the catalytic converter status index K and an error threshold value.

The flowchart in FIG. 3 shows a typical sequence in the determination and evaluation of the features of a NO _x desorption peak. In this embodiment of the invention, one or more catalytic converter status indicators K are calculated. After the start of the diagnosis in step S1, the system waits until a signal indicates the start of the NO _X regeneration mode at time t ₂ because the NO _X signal has reached the threshold value NO _X -S. As soon as a positive decision is made at branch point S2, the value of the NO _x signal at time t _{2 is} stored in step S3. The temporal profile of the NO _x signal is then stored in step S4. In this case, it is not necessary in every case to store the entire time profile of the NO _x signal, since, depending on the selected characteristic features of the NO _x desorption peak, a subrange of the time profile of the NO _x signal may also be sufficient. As soon as the NO _x signal falls below the stored value of the NO _x signal at time t ₂ , a decision is made at branch point S5 to determine the NO _x desorption peak characteristics in step S6, since the NO _x desorption peak is considered to have ended at this time , The determined values are provided with evaluation factors in step S7 and the associated catalytic converter status index K is then calculated and stored. In step S8, a query is made as to whether the number of catalytic converter status indicators K exceeds a given threshold value. If this is not the case, a jump back to branch point S2 takes place. If the number of determined catalyst status indicators K is greater than a predetermined threshold value, the catalyst status indicator K is averaged in step S9 and then a query is made in branching point S9 as to whether the average value of the catalyst status indicator K is greater than a predetermined threshold value.

If the mean value of the catalytic converter status index K is greater than the threshold value, an error is determined in step S10 and, if appropriate, an error signal is generated. If the mean value of the catalytic converter status index K is smaller than the threshold value, it is determined in step S11 that there is no malfunction of the NO _x storage catalytic converter 1 .

Overall, the inventive determination and subsequent evaluation of values of characteristic features of the NO _x desorption peaks occurring when switching from an absorption to a regeneration mode enables fast and uncomplicated on-board diagnosis of the NO _x storage catalytic converter of a motor vehicle.

Claims (15)

1. A method for diagnosing a NO _x storage catalytic converter which is arranged in the exhaust system of an internal combustion engine and can be operated in an absorption and a regeneration mode depending on the lambda value, the NO _x concentration in the exhaust gas being measured downstream of the NO _x storage catalytic converter, characterized in that during a transition of the NO _x storage catalytic converter from the absorption mode to the regeneration mode, the values of characteristic features of a NO _x desorption peak are determined in the course of the NO _x concentration over time, compared with predetermined test patterns, a comparison result is formed and a function of that Comparison result formed catalyst status signal, in particular an error signal is stored and / or displayed. 2. The method according to claim 1, characterized in that as the characteristic features of the NO _X desorption peak, the features shape, number of maxima, height of the maxima, duration, area, half-width, rising and / or falling edge each based on a predetermined reference -NO _X concentration can be selected. 3. The method according to claim 2, characterized in that for a NO _x desorption peak from the height of the maximum, the time duration and the area of the NO _x desorption peak, a catalyst status indicator is formed and the catalyst status signal as a function of the catalyst. State key figure is formed. 4. The method according to claim 2 or 3, characterized in that a NO _x threshold value is selected as the reference NO _x concentration, in which the regeneration mode of the NO _x storage catalytic converter is initiated. 5. The method of claim 2 or 3, characterized in that in an operable in stratified charge mode internal combustion engine as a reference NO _X - concentration is chosen, the value of the _NOx concentration at the time of switching the engine on homogeneous operation. 6. The method according to at least one of claims 1 to 5, characterized in that only relative changes in the NO _x concentration are taken into account based on a predetermined reference value. 7. The method according to at least one of claims 1 to 6, characterized in that different types of damage to the NO _x catalyst different catalyst state signals are assigned. 8. The method according to at least one of claims 1 to 7, characterized in that, depending on the comparison result, the temporal course of the values of the NO _x concentration is stored in one or more time windows. 9. The method according to at least one of claims 1 to 8, characterized in that the values of the characteristic features of the NO _x desorption peak are stored as a function of the comparison result. 10. The method according to at least one of claims 1 to 9, characterized in that that the test samples are dependent on the operating parameters of the Internal combustion engine, such as load, speed, raw emissions or the like to get voted. 11. The method according to at least one of claims 1 to 10, characterized characterized in that the test samples depending on the operating parameters of the Exhaust system, such as exhaust gas temperature, function of a pre-catalyst or the like can be chosen. 12. The method according to at least one of claims 1 to 11, characterized in that, depending on the values of the characteristic features, in particular on their fluctuation range for a number of storage cycles of the NO _x storage catalytic converter, the values of the characteristic features are stored, their mean value is determined and the catalyst status signal is formed as a function of the mean value. 13. The method according to at least one of claims 3 to 11, characterized in that, depending on the values of the characteristic features, in particular on their fluctuation range for a number of storage cycles of the NO _x storage catalytic converter, the values of the catalytic converter status index are stored and their mean value is determined and the catalytic converter status signal is formed as a function of the mean value. 14. Device for diagnosing a NO _x storage catalytic converter, which is arranged in the exhaust system of an internal combustion engine and can be operated in an absorption and a regeneration mode depending on the lambda value, with a lambda control having a lambda probe for measuring and changing the lambda value and a NO _x sensor arranged downstream of the NO _x storage catalytic converter for measuring the NO _x concentration in the exhaust gas, characterized in that a NO _x control device is provided, to which the measured values of the NO _x sensor can be fed and
Means for determining the values of characteristic features of a NO _x desorption peak in the course of time of the NO _x concentration during a transition of the NO _x storage catalytic converter from the absorption mode to the regeneration mode,
Means for comparing the values of the characteristic features with specified test samples and for forming a comparison result
Evaluation means for forming a catalyst status signal, in particular an error signal depending on the comparison result. 15. The apparatus according to claim 14, characterized in that means for Change of the test pattern depending on the operating parameters of the Internal combustion engine and / or the exhaust system are provided.