WO2017194183A1

WO2017194183A1 - Method and device for determining a nitrogen oxide storage capacity of a catalyst of a vehicle

Info

Publication number: WO2017194183A1
Application number: PCT/EP2017/000543
Authority: WO
Inventors: Thomas Beckmann
Original assignee: Daimler Ag
Priority date: 2016-05-13
Filing date: 2017-05-02
Publication date: 2017-11-16
Also published as: DE102016005968A1; CN109072748B; CN109072748A; US20190390583A1

Abstract

The invention relates to a method for determining a nitrogen oxide storage capacity of a catalyst (14) of a vehicle, in which a concentration of nitrogen oxides in the exhaust gas downstream of the catalyst (14) is measured, wherein, in at least a first step (30), a concentration of nitrogen oxides at which the catalyst (14) absorbs nitrogen oxides is set in the exhaust gas. In at least a second step (36), a concentration of nitrogen oxides at which a desorption of the nitrogen oxides from the catalyst (14) takes place is set in the exhaust gas. To determine the nitrogen oxide storage capacity of the catalyst (14), the behaviour of the catalyst (14) at least during the desorption of the nitrogen oxides is considered. The invention also relates to a device for determining a nitrogen oxide storage capacity of a catalyst (14) of a vehicle.

Description

Method and apparatus for determining a nitrogen oxide storage capability

Catalyst of a vehicle

The invention relates to a method and a device for determining a nitrogen oxide storage capability of a catalytic converter of a vehicle. In the method, a concentration of a nitrogen oxide in the exhaust gas downstream of the catalyst is measured.

From the prior art it is known the performance of a catalyst with nitrogen oxide storage capability with regard to the reduction of hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) in the exhaust gas of a

Determine internal combustion engine of a vehicle by means of specially introduced measures. These include, for example, an extra induced exotherm, ie a heating of the catalyst, or a fat jump, ie an operation of the

Internal combustion engine with a rich air-fuel mixture.

Furthermore, DE 198 52 240 A1 describes a method for monitoring a NOx storage catalytic converter, in which the NOx storage efficiency of the catalytic converter is determined from the NOx exhaust gas concentration before and after the NOx storage catalytic converter. The NOx exhaust gas concentrations before and after the NOx storage catalyst are converted into NOx mass flows, and from these values, the NOx storage efficiency is determined.

DE 103 18 116 B4 describes a method for operating a

Internal combustion engine, in which a storage catalytic converter is regenerated. Here, a storage capacity in the inflection point of a temporal signal curve of a NOx mass flow after the storage catalyst is specified.

DE 10 2006 055 238 A1 describes a method for determining the NOx storage capability of a NOx storage catalytic converter, in which the exhaust gas two parallel arranged NOx storage catalysts is supplied. There is generated a different size loading of both storage catalytic converters. After that both will

Storage catalysts in parallel with nitrogen oxides loaded until the signal of a NOx sensor signals a depletion of the absorption capacity of the storage catalytic converters.

The NOx trapping catalyst may be a NOx storage catalyst (NSK) or a diesel oxidation catalyst (DOC). Both the generation of an exotherm and the provision of a fat jump are associated with an additional fuel consumption. Therefore, such measures

advantageously only used if they are needed to reduce the pollutants in the exhaust gas. The frequency of the monitoring events, ie the respective investigations of the nitrogen oxide storage capacity of the catalyst, is thus limited. If the NOx storage capability of the catalytic converter is to be ascertained in the context of an on-board diagnosis (OBD), then this diagnosis is only possible at this interval or at the expense of additional fuel consumption if the monitoring events are additionally requested. For example, the diagnosis of a DOC using exothermicity is carried out as part of a particle filter regeneration. This is usually done in an interval between 500 kilometers and 1500 kilometers of driving distance traveled by the vehicle. In this form of monitoring, a

Information about the performance of the DOC with a view to reducing the

Hydrocarbons and carbon monoxide in the exhaust gas reached.

With a nitrogen oxide storage catalyst, the diagnosis is performed more frequently. Such a diagnosis is usually done with the help of a fat jump. Therefore, such an event usually occurs in an interval between 1 kilometer and 5 kilometers of a distance traveled by the vehicle.

This form of monitoring provides feedback on the nitrogen oxide storage capacity of the catalyst. From the nitrogen oxide storage capacity of the catalyst can in turn on the performance in terms of reducing the content of

Hydrocarbons or carbon monoxide are closed in the exhaust.

However, in particular with novel catalyst technologies such as, for example, a passive nitrogen oxide absorber (PNA), a DOC or a passively operated NSK, no or at least significantly less fat jumps can be provided. Accordingly, diagnostic events are only due to the exothermic nature of the regeneration of the Particulate filter possible or by extra requested diagnostics. However, these in turn result in increased fuel consumption.

Object of the present invention is therefore to provide an improved method and an improved device of the type mentioned.

This object is achieved by a method having the features of patent claim 1 and by a device having the features of patent claim 10. Advantageous embodiments with expedient developments of the invention are specified in the dependent claims.

In the method according to the invention, the nitrogen oxide storage capacity of a catalytic converter of a vehicle is determined. Here, a concentration of nitrogen oxides in the exhaust gas downstream of the catalyst is measured. In at least a first step, a concentration of nitrogen oxides in the exhaust gas is set at which the catalyst absorbs nitrogen oxides. In at least a second step, a concentration of nitrogen oxides in the exhaust gas is set at which desorption of the nitrogen oxides from the catalyst takes place. To determine the nitrogen oxide storage capacity of the catalyst, the behavior of the catalyst is taken into account, at least during the desorption of the nitrogen oxides.

In this way, the storage capacity of the catalyst can be determined qualitatively and indirectly also quantitatively without specially initiated measures which a

Fuel consumption result. In this case, different effects or mechanisms can be used. Nitrogen oxide saturation can be produced in the catalyst with nitrogen oxide storage capability. The maximum storage capacity or stored quantity is dependent on the current NOx partial pressure. By lowering the NOx partial pressure so also the maximum storage capacity or stored amount of nitrogen oxides decreases, and NOx is desorbed. Conversely, by raising the NOx partial pressure, the maximum storage capacity or stored amount of nitrogen oxides increases, and NOx is again absorbed by the catalyst. Thus, by observing the behavior of the catalyst at least during the deliberately induced desorption of the nitrogen oxides, the nitrogen oxide storage capacity of the catalyst can be deduced in an improved manner. The information regarding the nitrogen oxide storage capability of the catalyst enables a diagnosis of the catalyst with regard to the HC / CO / NOx performance. Furthermore, with the information regarding the nitrogen oxide storage capability of the catalytic converter, the operating strategy of the internal combustion engine or the engine of the vehicle can be optimized for the current state of the exhaust system. The vehicle may be in particular a motor vehicle or a

Trade commercial vehicle. Furthermore, an optimization of a strategy for

Desulfurization of the catalyst (DeSOx strategy) possible. The catalyst is considered to have a NOx storage capability that varies with the age of the exhaust system or with the age of the catalyst.

Preferably, in the at least one first step, a concentration of nitrogen oxides in the exhaust gas is set, at which the catalyst absorbs the nitrogen oxides up to a saturation of the catalyst with nitrogen oxides.

It has been shown to be advantageous if, after adjusting the saturation of the catalyst with nitrogen oxides, a coasting operation of the vehicle is carried out, which leads to the desorption of the nitrogen oxides from the catalyst. This is based on a time course of the desorption of nitrogen oxides on the nitrogen oxide storage capacity of the catalyst closed. For the determination of the NOx storage capacity can thus make the above-mentioned mechanism advantage that a NOx desorption in overrun operation of the vehicle or in operating conditions of the vehicle is present, in which the raw emission of nitrogen oxides of the internal combustion engine is preferably virtually zero.

According to a further advantageous embodiment, a concentration of nitrogen oxides in the exhaust gas is set in a plurality of first steps, in which the

Catalyst absorbs nitrogen oxides. In a plurality of second steps, a concentration of nitrogen oxides in the exhaust gas is set, at which the desorption of the nitrogen oxides takes place from the catalyst. Based on the quantities of the nitrogen oxides stored or discharged via the majority of the first steps and the second steps, the nitrogen oxide storage capacity of the catalytic converter is concluded here. In particular, the respectively absorbed amounts of nitrogen oxides and the respectively desorbed amounts of nitrogen oxides can be accumulated separately from one another. From the absorption amounts and the desorption can then be concluded that the storage capacity of the catalyst with respect to nitrogen oxides. According to a further advantageous embodiment, during the at least one first step, respective first gradients of a temporal course of the concentration of nitrogen oxides in the exhaust gas are determined upstream of the catalytic converter and downstream of the catalytic converter. During the at least one second step, respective second gradients of a time course of the concentration of nitrogen oxides in the exhaust gas are determined upstream of the catalytic converter and downstream of the catalytic converter. Based on the gradient is based on the nitrogen oxide storage capacity of the catalyst

closed. It is therefore in particular based on a nitrogen oxide gradient detection after the catalyst, in particular nitrogen oxide storage catalyst, closed on the nitrogen oxide storage capacity of the catalyst.

In this case, an average value is preferably formed from a plurality of amounts of the first gradients and from a plurality of amounts of the second gradients. Based on the average can then be concluded that the nitrogen oxide storage capacity of the catalyst. With regard to the design of a corresponding control device or a control device of the vehicle for determining the nitrogen oxide storage capacity of the catalytic converter, such a method can be carried out in a particularly simple and low-effort manner.

Preferably, the nitrogen oxide storage capacity of a passive nitrogen oxide absorber of the vehicle and / or an oxidation catalytic converter of the vehicle and / or a passively operated nitrogen oxide storage catalytic converter of the vehicle is determined. In the case of a passive nitrogen oxide absorber (PNA), the storage or sorption of the nitrogen oxides takes place, for example, in the zeolite material of the passive nitrogen oxide absorber. However, the nitrogen oxides are not chemically bound. It is therefore not necessary to enrich the air-fuel mixture for reducing chemically bound nitrogen oxides in the course of a chemical reaction. Instead, the passive nitrogen oxide absorber undergoes thermal desorption of the nitrogen oxides.

Also, an oxidation catalyst, in particular diesel oxidation catalyst (DOC), has a sorption or absorption capacity for nitrogen oxides, for example, by the use of zeolites as support materials of the catalytically active

Substances of the oxidation catalyst. Again, therefore, a desorption of

Nitrogen oxides take place without the need for enrichment of the air-fuel mixture, ie the setting of an air ratio λ greater than 1, needs to be made. Furthermore, a nitrogen oxide storage catalyst can be operated passively, in which the nitrogen oxides are chemically bonded to a corresponding material of the nitrogen oxide storage catalyst. This chemically bound NOx can also be thermally desorbed, for example by raising the temperature of the passively operated nitrogen oxide storage catalytic converter.

In particular, in such catalysts, in which no fat jump in the air-fuel mixture is adjusted in order to achieve a reduction in the content of nitrogen oxides in the catalyst, therefore, the method described above is particularly advantageously used.

The device according to the invention for determining a nitrogen oxide storage capability of a catalytic converter of a vehicle comprises a sensor for measuring a concentration of nitrogen oxides in the exhaust gas downstream of the catalytic converter. The device further comprises a control device, which is designed to set in at least a first step, a concentration of nitrogen oxides in the exhaust gas, wherein the catalyst absorbs nitrogen oxides. The control device is further configured to adjust a concentration of nitrogen oxides in the exhaust gas in at least a second step, in which a desorption of the nitrogen oxides takes place from the catalyst. Furthermore, the control device is designed to take into account the behavior of the catalytic converter, at least during the desorption of the nitrogen oxides, in order to determine the nitrogen oxide storage capability of the catalytic converter.

The advantages and preferred embodiments described for the method according to the invention also apply to the device according to the invention and vice versa.

The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or shown alone in the figures are not only in the respectively indicated combination but also in other combinations or in

Used alone, without departing from the scope of the invention. Thus, embodiments are also included and disclosed by the invention, which are not explicitly shown or explained in the figures, but by separated

Feature combinations emerge from the described embodiments and can be generated. Thus, embodiments and combinations of features are also to be regarded as disclosed which do not have all the features of an originally formulated independent claim. Further advantages, features and details of the invention will become apparent from the

Claims, the following description of preferred embodiments and with reference to the drawings. Showing:

Fig. 1 shows the time course of the nitrogen oxide concentrations upstream of a

Oxidation catalyst and downstream of the oxidation catalyst during a coasting operation of a vehicle, which takes place following saturation of the oxidation catalyst with nitrogen oxides;

Fig. 2 shows the time course of the NOx mass flows upstream and

downstream of the oxidation catalyst and the respective sums of the absorbed or desorbed within the considered period amounts of nitrogen oxides; and

3 shows the slowed down step response of the nitrogen oxide concentration in the exhaust gas

downstream of the oxidation catalyst with changes in the

Untreated nitrogen oxide emissions.

From a vehicle such as a passenger car or a commercial vehicle, an exhaust system 10 is shown schematically and in part in Fig. 1. The exhaust system 10 includes an exhaust line 12, in which a catalyst 14 is arranged, which has a nitrogen oxide storage capability. The catalyst 14 may be

for example, to an oxidation catalyst, in particular a diesel oxidation catalyst (DOC) act. In the exhaust line 12, the catalytic converter 14 may be followed by a particle filter 16 and / or an SCR catalytic converter 18. The particulate filter 16 may further be designed in particular as an SCR-coated particulate filter 16. In order to reduce nitrogen oxides in a selective catalytic reduction reaction, in the SCR catalyst 18 (selective catalytic reduction (SCR) catalyst) nitrogen oxides from the exhaust gas may be mixed with ammonia to form water and

Nitrogen be implemented. For this purpose, by means of a metering device 20

For example, an aqueous urea solution are introduced into the exhaust gas line 12, wherein the ammonia is formed from the urea in the exhaust gas.

Downstream of the catalyst 14 and in the present case upstream of the metering device 20, a sensor 22 is arranged in the exhaust line 12, by means of which the Concentration of nitrogen oxides in the exhaust gas can be measured downstream of the catalyst 14. A corresponding curve 24 in FIG. 1 illustrates the time profile of the concentration of nitrogen oxides (NOx) in the exhaust gas downstream of the catalytic converter 14. A further curve 26 in FIG. 1 illustrates the raw nitrogen oxide emission, that is to say

Concentration of nitrogen oxides in the exhaust gas, as they are due to the operation of an internal combustion engine (not shown) of the vehicle upstream of the catalyst 14. The raw NOx emission upstream of the catalyst 14 may be detected by a sensor or determined based on a model. A

Control device approximately in the form of a control device 28 of the vehicle is used in the present case for determining the nitrogen oxide storage capacity of the catalytic converter 14.

In the method to be illustrated with reference to FIG. 1, desorption of the nitrogen oxides from the catalyst 14 is preferably considered in overrun of the vehicle in order to determine the NOx storage capacity of the catalytic converter 14. The NOx storage capability of the catalytic converter 14 changes with the aging of the exhaust system 10, wherein the nitrogen oxide storage capacity of the catalytic converter 14 usually decreases with increasing age of the exhaust system 10.

In the method illustrated with reference to FIG. 1, in a first step 30, saturation of the catalyst 14 with nitrogen oxides is produced. The fact that there is an at least substantial saturation of the storage or catalyst 14 can be recognized from the fact that in the time interval of step 30, in which the NOx saturation is established, the raw NOx emissions of the internal combustion engine (curve 26) the content of nitrogen oxides in the exhaust gas downstream of the catalyst 14 (curve 24) are the same.

In a time 34 plotted on a time axis 32 in FIG. 1, for example by means of the control device 28, an operation of the internal combustion engine or of the engine is set, in which the raw NOx emission is zero. This is advantageous because then the raw emission is known without measurement error. Such a condition occurs

for example, in overrun operation of the vehicle. The overrun operation can also be assisted by an electric motor of the vehicle, so that load requirements can be met even without an engine assist.

During a second step 36, which immediately follows the time 34, desorption of the nitrogen oxides from the catalyst 14 takes place.

Accordingly, the nitrogen oxide concentration in the exhaust gas downstream of the Catalyst 14 (curve 24) is not zero in the period following time 34, but the concentration decreases. The behavior of the catalyst 14 during this desorption of the nitrogen oxides is used to determine the nitrogen oxide storage capacity of the catalyst 14.

Preferably, the prerequisites to be met in this case are that the NOx storage, that is, the catalytic converter 14, is saturated and that the temperatures before the catalytic converter 14 and downstream of the catalytic converter 14 neither rise nor fall sharply. It is advantageous if the temperature gradient is not too large. This is due to the fact that the NOx storage capacity of the catalytic converter 14 is generally also dependent on the temperature. At at least substantially constant temperature, therefore, the desired effect of the desorption is not superimposed too strongly by the temperature effect.

If an exhaust gas recirculation, in particular high-pressure exhaust gas recirculation, is provided in the exhaust system 10, the exhaust gas recirculation should advantageously be dispensed with in overrun operation. Otherwise it will circulate

Nitrogen emissions of the internal combustion engine in a circle, and the raw emissions decrease accordingly slower. However, as soon as the raw NOx emission from the engine reaches the value zero (time 34), the exhaust gas recirculation can again be made, in particular the high-pressure exhaust gas recirculation rate can be increased.

In order to enhance the desorption, furthermore, the exhaust gas mass flow through the catalytic converter 14 can be changed by suitable engine measures. In particular, the exhaust gas mass flow can be reduced in order to increase the measurement accuracy when detecting the nitrogen oxide concentration by means of the sensor 22. The changing of the exhaust gas mass flow can be carried out, for example, by means of a corresponding setting of the engine speed and / or by actuating a throttle valve and / or by changing a high-pressure exhaust gas recirculation rate or low-pressure exhaust gas recirculation rate.

If these requirements are met, the nitrogen oxide concentration behind the catalytic converter 14 is preferably measured with the help of the nitrogen oxide sensor 22. If the catalytic converter 14 still has a nitrogen oxide storage capacity, then the nitrogen oxide concentration downstream of the catalytic converter 14, ie downstream of the catalytic converter 14, decreases more slowly than in front of the catalytic converter 14, that is to say upstream of the catalytic converter 14. Thus, it is desorbed during overrun operation of nitrogen oxides from the catalyst 14 instead. Preferably, the desorbed amount of nitrogen oxides with a modeled, so expected nitrogen oxide amount is adjusted. Such a model can thus indicate how the nitrogen oxide concentration in the exhaust gas downstream of the catalytic converter 1 should decrease from zero in the presence of a raw NOx emission. The model can thus take into account, for example, the decay of the measurements of the sensor 22 or the presence of regions of the exhaust system 10 which are less well flowed through by exhaust gas, which accordingly slows down the decrease in the exhaust gas

Nitrogen concentration downstream of the catalyst 14 can lead. Furthermore, the aging of the exhaust system 10, in particular of the model, is preferred in the model

Catalyst 14, taken into account.

If the actually measured course of the decrease in the nitrogen oxide concentration downstream of the catalytic converter 14 deviates from the expected NOx decay curve, then a corresponding change, in particular reduction, of the nitrogen oxide storage capacity of the catalytic converter 14 can be deduced.

In particular, from the desorbed amount of nitrogen oxide in the overrun mode and the gradient of the nitrogen oxide decay curve in the overrun mode on the basis of a model on the absolute nitrogen oxide storage capacity of the catalyst 14 can be concluded. The nitrogen oxide storage capability of the catalyst 14 used in a currently used model is then corrected upwards or downwards if the measured

Nitrogen decay curve deviates from the target NOx decay curve.

Instead of considering the NOx saturation and the NOx desorption in the overrun operation, the amounts accumulated in a plurality of absorption processes or desorption processes can also be considered. This will be illustrated with reference to FIG.

This variant is based on the finding that by lowering and raising the NOx partial pressure in the region of the catalyst 14 continuously small

Absorption processes and desorption occur. Such variations in the NOx partial pressure occur, for example, in dynamic driving situations, that is to say when the raw NOx emissions of the internal combustion engine vary due to different load requirements on the internal combustion engine. Preferably, the respective amounts of NOx are accumulated separately. The total amount of nitrogen oxides absorbed and the total amount of nitrogen oxides desorbed are then compared to expected levels according to a model. FIG. 2 plots, on a first ordinate 38, the NOx mass flow upstream or downstream of the catalyst 14 as a function of time t, which is indicated on the time axis 32. A first curve 40 accordingly illustrates the NOx mass flow upstream of the catalytic converter 14 and a second curve 42 the NOx mass flow downstream of the catalytic converter 14. In a plurality of first steps 44, the emissions upstream of the catalytic converter 14 are higher than the emissions downstream of the catalytic converter 14 Accordingly, it comes here to an absorption of nitrogen oxides. In an analogous manner, a desorption of the nitrogen oxides from the catalyst 14 takes place in a plurality of second steps 46. This is the case when the emissions downstream of the catalyst 14 are higher than the emissions

upstream of the catalyst 14.

In a further graph in FIG. 2, the respective sums of the quantities of nitrogen oxides accumulated over time t are plotted on a further ordinate 48. A curve 50 illustrates the accumulation of the absorption and a curve 52 the

Accumulation of desorption. The time t is in turn plotted on the time axis 32 in the second graph in FIG. Within the period of time considered, there is a difference 54 between the accumulated absorption amounts and the accumulated desorption quantities. This difference 54 is compared with a target value, and based on the comparison, the nitrogen oxide storage capability of the catalyst 14 is concluded.

In order to determine the amounts of nitrogen oxide as correctly as possible, it is important that the NOx concentrations are known before the catalyst 14 and after the catalyst 14 with low fault tolerances. If there are measurement errors, these errors should tend in the same direction before the catalyst 14 and after the catalyst 14. The presence of errors can be detected, for example, by observing conditions in which no storage of nitrogen oxides in the catalyst 14 occurs. Then, the measured values upstream of the catalytic converter 14 and downstream of the catalytic converter 14 should correspond to one another or the modeled value upstream of the catalytic converter 14 should correspond to the value measured downstream of the catalytic converter 14 with the sensor 22. Furthermore, the nitrogen oxide concentrations should be present at the right time. For this the values of the

Sensors before the catalyst 14 and after the catalyst 14th

(or the values provided by the raw emission model and those of the Sensor 22 values supplied to the catalyst 14) are timed to each other. Corresponding methods are known to the person skilled in the art.

In order to determine the absorption processes and desorption processes, the nitrogen oxide absorption and the nitrogen oxide desorption of the catalyst 14 are continuously determined. This is done by subtracting the nitrogen oxide mass flows before the catalyst 14 and the nitrogen oxide mass flows after the catalyst 14 from each other. For determining the NOx mass flow downstream of the catalyst 14, the NOx sensor 22 is provided. For the determination of the NOx mass flow upstream of the catalyst 14 may also be

Emission model can be used.

The nitrogen oxide absorption mass flows and the nitrogen oxide desorption mass flows are accumulated or added up separately. Furthermore, the raw nitrogen oxide emission of the internal combustion engine is accumulated. In addition, a characteristic value is preferably determined which is representative of the averaged nitrogen oxide gradient upstream of the catalytic converter 14, which therefore indicates the average gradient of a curve representing the raw nitrogen oxide emissions of the internal combustion engine. Such a characteristic value can be calculated or specified in particular in ppm _N o _x per second. A high characteristic value is accordingly strong

Changes to the raw emissions before. In contrast, a lower characteristic value indicates less pronounced fluctuations in the raw emissions of nitrogen oxides.

Furthermore, the difference between the total amount absorbed

Nitrogen oxides and the total desorbed amount of nitrogen oxides depending on

Temperature history and of the raw emission during the evaluation period.

For example, the nitrogen oxide storage capacity of the catalyst 14 typically increases as the temperature of the catalyst 14 drops. Accordingly, the accumulated amount of absorbed nitrogen oxide is larger than the accumulated amount of desorbed nitrogen oxide. Conversely, an increasing temperature of the catalyst 14 causes the accumulated desorption to be generally greater than the accumulated absorption.

The difference 54 present at the end of the considered evaluation period is compared with an expected, modeled difference. If the difference 54 is greater or less than the expected difference, this may indicate a drift of the emission model or the sensor 22. This can be compensated by a corresponding new calibration of the emission model or the sensor 22. If none such drift is present, the NOx storage capability of the catalyst 14 can be deduced based on the difference 54.

The diagnosis is preferably carried out considering a past time interval. This is because it can be ensured that predetermined boundary conditions have been present within the time interval. As one of these boundary conditions can be provided that the NOx storage, ie the catalyst 14, was sufficiently saturated with nitrogen oxides in the considered, past time interval. For example, a saturation of at least 80 percent may be provided. Furthermore, it is preferable to consider a time interval in which the temperature was sufficiently stable and within an allowable range. For this purpose, it can be considered, for example, whether, with a temperature change of the exhaust gas upstream of the catalytic converter 14, no or at most a slight temperature change has occurred downstream of the catalytic converter 14. The specification of this boundary condition is in turn based on the temperature dependence of the desorption.

As a further boundary condition it can be provided that the characteristic value for the nitrogen oxide gradient upstream of the catalytic converter 14 was within a predetermined range. This area should not be too small. Otherwise, namely the

Absorption effects and the desorption effects are very low, so that they can not be detected well by measurement. On the other hand, if the range is too large, the tolerances of the emission model and the tolerances of the sensor 22 may become too large. Furthermore, it can be checked as a boundary condition whether the difference between the absorption and the desorption in the past time interval is plausible. This can be checked in particular by using a model.

For the evaluation of the diagnosis the height of the accumulated absorption and the amount of accumulated desorption are compared with the modeled absorption and the modeled desorption. If the difference between the accumulated absorption amount and the accumulated desorption amount is less than the modeled difference or the modeled quantity, then this is an indication of a reduced nitrogen oxide storage capacity of the catalyst 14. From the absorption amounts and the

Desorptionsmengen can therefore be closed by a model on the absolute nitrogen oxide storage capacity of the catalyst 14. In the given period, the model preferably includes a setpoint value for the absorption amount and a setpoint value for the desorption amount for the given boundary conditions. These Setpoint values are a function of the temperature, the characteristic value of the NOx gradient, the exhaust gas mass flow and the sum of the raw emissions of nitrogen oxide. The currently modeled nitrogen oxide storage capacity of the catalyst 14 may be corrected upwards or downwards if the measured absorption amount and the measured desorption amount deviate from the target absorption amount and the target desorption amount.

Another possibility, the desorption of nitrogen oxides from the catalyst 14 in

Considering the dependence of the concentration of nitrogen oxides in the exhaust gas for determining the nitrogen oxide storage capacity of the catalytic converter 14 will be illustrated with reference to FIG. 3. Here is in a graph on an ordinate 56 the

Plotted nitrogen oxide concentration and on the time axis 32 again the time t. A first curve 58 represents the course of the raw emission as a function of time, and a second curve 60 shows the concentration of the nitrogen oxides in the exhaust gas downstream of the

Catalyst 14, which is detected by the sensor 22. Again, one makes use of the effect that by lowering and lifting or by

Variations of the nitrogen oxide partial pressure, as are common in dynamic driving situations, resulting in continuous small absorption processes and

Desorption occurs. Due to these absorption processes and

Desorptionsvorgänge, however, changes the nitric oxide concentration after the

Catalyst 14 slower than the nitrogen oxide concentration before the catalyst 14. The corresponding step response, ie the change in nitrogen oxide concentration

downstream of the catalyst 14 resulting from a corresponding change in nitrogen oxide concentration upstream of the catalyst 14 is compared to a step response expected by a model. In particular via a statistical evaluation of a plurality of such step responses, the absolute nitrogen oxide storage capacity of the catalytic converter 14 can be concluded by means of a model.

Preferably, the following procedure is used to determine the characteristic value for the NOx gradient. First, a signal noise of the nitrogen oxide sensor 22 after the catalyst 14 and the (optional) nitrogen oxide sensor upstream of the catalyst 14 is averaged out. The gradient or slope of the

Nitrogen oxide concentration upstream of the catalyst 14 is determined, for example, in pprri NOx per second. The basis for this is a (not shown here)

Nitrogen sensor upstream of the catalyst 14 and a nitrogen oxide Rohemissionsmodell. Furthermore, the gradient or the slope of the nitrogen oxide concentration downstream of the catalyst 14 is preferably determined in ppm _N ox per second. The basis for this is the nitrogen oxide sensor 22. Subsequently, for a defined period of, for example, at least 100 seconds, the mean value is formed from the amounts of the nitrogen oxide gradients upstream of the catalytic converter 14 and downstream of the catalytic converter 14. This mean value is a characteristic which is representative of the nitrogen oxide gradient.

According to FIG. 3, for example, during a first step 62, absorption of nitrogen oxides in the catalyst 14 takes place. This can be seen from the fact that a markedly slower increase in the nitrogen oxide concentration downstream of the catalytic converter 14 (curve 60) occurs due to a sharp rise in the raw emission (curve 58). Conversely, for example, in a second step 64, nitrogen oxide desorption from the catalyst 14 occurs. Here, the raw nitrogen oxide emission (curve 58) drops rapidly, while the nitrogen oxide concentration downstream of the catalytic converter 14 decreases more slowly. Due to these slower jump responses of the nitrogen oxide concentration downstream of the catalytic converter 14, which is detected by means of the sensor 22, the presence of absorption during a plurality of first steps 62 or desorbing during a plurality of second steps 64 can be concluded.

Also in the detection of the nitrogen oxide gradient downstream of the catalyst 14, which has the nitrogen oxide storage capability, the diagnosis is preferably carried out on a past time interval. As a boundary condition, it may also be provided here that the NOx storage or the catalytic converter 14 was sufficiently saturated with nitrogen oxides in the past time interval. Furthermore, it is preferably detected that the exhaust gas temperature in the past time interval was sufficiently stable and in the permitted range. Furthermore, the characteristic value for the nitrogen oxide gradient upstream of the catalytic converter 14 was preferably within a predetermined range. If this range is too small, the tolerances and noise of the sensors are too dominant. If the characteristic value is too large, however, the tolerances of the emission model and the tolerances of the sensors become too large.

From the parameter, which can be determined from the mean values of the amounts of the NOx gradients upstream of the catalytic converter 14 and downstream of the catalytic converter 14, the absolute nitrogen oxide storage capacity of the catalytic converter 14 can be deduced from a model. For the given boundary conditions, the model preferably has a desired value for the characteristic value of the nitrogen oxide gradient downstream of the catalyst 14 in the given period. However, this is a function of the temperature,

Exhaust gas mass flow, the sum of the nitrogen oxide raw emission and the nitrogen oxide Gradients before the catalyst 14. The currently modeled nitrogen oxide storage capacity of the catalyst 14 is corrected upward or downward, when the measured characteristic of the nitrogen oxide gradient deviates from the desired value.

The knowledge about the nitrogen oxide storage capacity of the catalytic converter 14 can be used for the operation of the exhaust system 10. For example, can be adjusted via the engine control, the raw nitrogen oxide emissions of the engine. Furthermore, a fat jump dosing strategy and / or a urea dosing strategy can be adapted, or heating measures of the exhaust gas can be made.

From the storage capacity of the catalyst 14 may further on the current performance of the catalyst 14 in terms of reducing the content of

Hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) are closed in the exhaust. Thus, a diagnosis of the catalyst 14 according to OBD is possible. For the diagnosis, an error can be reported when falling below a critical nitrogen oxide storage capacity or amount of storage. This can result in that, for example, an engine control lamp is activated. Additionally or alternatively, further diagnostic measures may be initiated, such as

To validate the result. Such validation measures may be made according to the state of the art.

From the change in the nitrogen oxide storage capacity of the catalyst 14 within a period or interval in which a desulfurization of the catalyst 14 takes place, it is also possible to deduce the current sulfur concentration in the fuel and the sulfur loading of the catalytic converter 14. It is therefore also possible to optimize a DeSOx strategy, ie a strategy of desulfurization of the catalyst 14. In particular, the interval or the period between two DeSOx measures and the DeSOx intensity can be optimized. The DeSOx intensity manifests itself in particular in a depth of the fat jump, that is to say in the extent or intensity of the enrichment of the air-fuel mixture, in the duration of the fat jump and in the number of fat leaps. The desulfurization of the catalyst 14 can be accomplished by appropriate leaps in particular at elevated temperature, for example in the context of a regeneration of the particulate filter 16. LIST OF REFERENCE NUMBERS

10 exhaust system

12 exhaust system

14 catalyst

16 particle filters

18 SCR catalyst

20 metering device

22 sensor

24 curve

26 curve

28 control unit

30 step

32 timeline

34 time

36 step

38 ordinates

40 curve

42 curve

44 step

46 step

48 ordinates

50 curve

52 curve

54 difference

56 ordinates

58 curve

60 curve

62 step

64 step

Claims

claims

Method for determining a nitrogen oxide storage capacity of a catalytic converter (14) of a vehicle, in which a concentration of nitrogen oxides in the exhaust gas downstream of the catalytic converter (14) is measured,

characterized in that

in at least a first step (30, 44, 62) a concentration of nitrogen oxides in the exhaust gas is adjusted, at which the catalyst (14) absorbs nitrogen oxides, wherein in at least a second step (36, 46, 64) a concentration of nitrogen oxides in the exhaust gas is set at which a desorption of the nitrogen oxides from the catalyst (14) takes place, wherein for determining the nitrogen oxide storage capacity of the catalyst (14), the behavior of the catalyst (14) is taken into account at least in the desorption of nitrogen oxides.

Method according to claim 1,

characterized in that

in which at least one first step (30) is set a concentration of nitrogen oxides in the exhaust gas, wherein the catalyst (14) absorbs the nitrogen oxides to a saturation of the catalyst (14) with nitrogen oxides.

3. The method according to claim 2,

characterized in that

after adjusting the saturation of the catalyst (14) with nitrogen oxides, a coasting operation of the vehicle is performed, which leads to the desorption of nitrogen oxides from the catalyst (14), based on a temporal Course of the desorption of nitrogen oxides on the nitrogen oxide storage capacity of the catalyst (14) is closed.

4. The method according to any one of claims 1 to 3,

characterized in that

in a plurality of first steps (44), a concentration of nitrogen oxides in the exhaust gas is adjusted, at which the catalyst (14) absorbs nitrogen oxides, and in a plurality of second steps (46) a concentration of nitrogen oxides in the exhaust gas is adjusted, wherein the Desorption of the nitrogen oxides of the

Catalyst takes place, based on the stored over the majority of the first steps (44) and second steps (46) and discharged amounts of nitrogen oxides on the nitrogen oxide storage capacity of the catalyst (14)

is closed.

5. The method according to any one of claims 1 to 4,

characterized in that

determining, during the at least one first step (62), respective first gradients of a time course of the concentration of nitrogen oxides in the exhaust gas upstream of the catalyst (14) and downstream of the catalyst (14), and during the at least one second step (64) respective second gradients a time course of the concentration of nitrogen oxides in the exhaust gas upstream of the catalyst (14) and downstream of the catalyst (14) are determined, based on the gradient on the nitrogen oxide storage capacity of the catalyst (14) is concluded.

6. The method according to claim 5,

characterized in that

an average is formed from a plurality of amounts of the first gradients and a plurality of amounts of the second gradients, based on the mean value of the nitrogen oxide storage capacity of the catalyst (14) is concluded.

7. The method according to any one of claims 1 to 6,

characterized in that the nitrogen oxide storage capacity of a passive nitrogen oxide absorber of the vehicle is determined.

8. The method according to any one of claims 1 to 7,

characterized in that

the nitrogen oxide storage capacity of an oxidation catalytic converter of the vehicle is determined.

9. The method according to any one of claims 1 to 8,

characterized in that

the nitrogen oxide storage capacity of a passively operated nitrogen oxide storage catalytic converter of the vehicle is determined.

10. An apparatus for determining a nitrogen oxide storage capacity of a catalytic converter (14) of a vehicle, comprising a sensor for measuring a concentration

Nitrogen oxides in the exhaust gas downstream of the catalyst (14),

marked by

a control device (28) which is designed to set a concentration of nitrogen oxides in the exhaust gas in at least one first step (30, 44, 62) at which the catalyst (14) absorbs nitrogen oxides, and in at least a second step ( 36, 46, 64) to adjust a concentration of nitrogen oxides in the exhaust gas at which a desorption of the nitrogen oxides from the catalyst (14) takes place, and to determine the nitrogen oxide storage capacity of the

Catalyst (14) to take into account the behavior of the catalyst (14) at least during the desorption of nitrogen oxides.