DE102006055238A1 - Nitrogen oxide-storage catalyzer's nitrogen oxide-storage capability determining method for use in e.g. diesel engine, involves determining storage capability of nitrogen oxide storage catalyzers with larger loading of nitrogen oxide - Google Patents

Abstract

The method involves leading exhaust gas of groups (12, 18) of cylinders through respective group-individual nitrogen oxide (NOx) storage catalyzers (28, 30) in an internal combustion engine (10). A NOx sensor (52) is arranged in a section (32) of an exhaust gas system (27), which leads the gas. Different large loadings of the catalyzers are produced, the catalyzers are temporally and parallelly loaded with nitrogen oxides until a signal of the sensor signals an exhaustion of absorption of one of the catalyzers, and the storage capability of the catalyzers is determined with larger loading.

Description

State of the art

The The invention relates to both a method and a controller for determining NOx storage capability a NOx storage catalyst in the exhaust of an internal combustion engine having the features of the preambles of the independent claims.

Such a method and such a control device is in each case from DE 103 18 116 A1 known. NOx storage catalysts are used in motor vehicles whose internal combustion engines are designed to work as far as possible in a consumption-optimized lean operation. Examples of such internal combustion engines operate with direct injection of fuel into combustion chambers, which occurs shortly before combustion. This principle is used both in the gasoline engine and in the diesel engine. The internal combustion engines are largely de-throttled in lean operation operated with large excess air.

The in this mode comparatively high amount of the internal combustion engine emitted nitrogen oxides is stored in the NOx storage catalyst. To the storage capacity maintain lean operation is regularly interrupted, around the NOx storage catalytic converter to regenerate in a reducing exhaust gas atmosphere. It will the stored NOx is reduced and released as molecular nitrogen the storage catalyst discharged. The storage takes place for periods in the order of magnitude a minute while the regeneration takes place in periods of the order of one second.

Legislators in the US and Europe require on-board diagnostics for motor vehicles, including the ability of the NO _x storage catalytic converter to function. The NOx storage capacity of the NOx storage catalytic converter is determined for the assessment. If the determined storage capacity is too small, an error message is generated, stored and possibly displayed.

After DE 103 18 116 A1 In the control unit, a raw emission model is calculated, which simulates the NOx raw emissions of the internal combustion engine from variables measurable upstream of the catalytic converter. The integral of the difference between the raw emissions and the emissions measured downstream of the NOx storage catalyst indicates a calculated loading of the storage catalytic converter with NOx.

Parallel to the computational formation of the load with the integral of the raw emission model is the NOx concentration, as measured by the NOx sensor, simulated with a storage catalyst model. The value of the replicated NOx concentration depends thereby from the value of a NOx storage capability, which in the storage catalyst model is used. Alternatively or in addition, it will be behind the NOx-mass flow adjusting the storage catalytic converter with a storage catalytic converter model simulated.

As soon as is in the measured NOx concentration increases, is the simulated NOx concentration by changing the storage catalyst model used value of NOx storage capability with an adaptation value adjusted to the measured NOx concentration. The one by this Approximation specific value of the NOx storage capacity will depend on upstream of the storage catalyst measurable variables, namely the input variables of Raw emission model and the storage catalyst model and a Signal of the downstream of the storage catalytic converter arranged NOx sensor determines that an exhaustion the capacity of the Storage catalyst indicates. The known method is for exhaust systems presented with a strand.

at larger displacement Engines are transferred to to run the exhaust system as Y-system. It is under a Y system understood an exhaust system, the exhaust gases of two groups of cylinders of the internal combustion engine separated by each passes a group-specific NOx storage catalyst and merges the exhaust gases behind the two NOx storage catalytic converters. The extensive separation of the exhaust gas lines in such exhaust systems offer for reasons Gas Dynamics Benefits of Performance and Torque Combustion engine. The merging and mixing of the strands after the Both storage catalysts hardly affect this advantage.

at Such an exhaust system thus two NOx catalysts are present, that need to be regenerated and diagnosed. In a transmission of the known method would be to separate NOx sensors in the separate sections of the exhaust system required.

to However, minimizing the system cost, it is desirable to have such a system to realize with only one NOx sensor.

Normally, both storage catalytic converters are either filled simultaneously (lean operation) or regenerated (rich operation). From a certain level, a storage catalytic converter begins to break through, that is, that measured by the NOx sensor NEN NOx emissions rise because in the storage catalyst inflowing NOx is not stored and therefore exits the storage catalytic converter again. However, with a single exhaust gas sensor located behind the merge, it is not readily possible to distinguish between both storage catalysts. In particular, it is not readily possible to distinguish whether the NOx storage capacity of both storage catalytic converters is simultaneously exhausted or whether this only applies to one of the two storage catalytic converters. In addition, a single storage catalyst with exhausted storage capability is not readily identifiable.

One different break-through behavior may e.g. through a different one Aging condition of the catalysts are caused. The state of aging The individual catalysts should be known to the engine control unit be to certain operating parameters in the control of the internal combustion engine to vote on it. It is therefore desirable the diagnosis / aging determination of the two storage catalytic converters for both storage catalytic converters perform separately.

In front In this background, the object of the invention in the specification a method and a control unit, each a separate Diagnosis of each of two NOx storage catalytic converters of a Y exhaust system allowed with only one NOx sensor located downstream of the Y-junction is.

Disclosure of the invention

These Task is in a method of the type mentioned by the characterizing features of claim 1 and in a control device of the beginning mentioned type solved by the characterizing features of the independent control device claim. These Features allow one separate diagnosis with only one NOx sensor, which is a significant cost savings allows. there becomes a sequential regeneration of the two storage catalysts carried out, causing both banks can be considered separately.

Further Advantages result from the dependent claims, the Description and attached Characters.

It it is understood that the above and the following yet to be explained features not only in the specified combination, but also in other combinations or alone, without to leave the scope of the present invention.

Brief description of the drawings

embodiments The invention are illustrated in the drawings and in the following description explained. In each case, in schematic form:

1 an internal combustion engine having a Y exhaust system and a controller; and

2 time profiles of signals from exhaust gas sensors, as they occur in an embodiment of a method according to the invention.

Embodiment (s) the invention

In detail, the shows 1 an internal combustion engine 10 with a first group 12 of cylinders 14 . 16 and a second group 18 of cylinders 20 . 22 , An intake system 24 serves to supply air to the internal combustion engine 10 , A fuel metering device 26 is used for the metering of fuel. The fuel metering device 26 has one or more injectors 26_14 . 26_16 . 26_20 . 26_22 put the fuel into the intake system 24 or directly into combustion chambers of the cylinders 14 . 16 . 20 . 22 dosing.

The internal combustion engine 10 also has a Y exhaust system 27 on, the exhaust gases of the first group 12 of cylinders 14 . 16 and the second group 18 of cylinders 20 . 22 separated by a respective NOx storage catalyst 28 or 30 conducts and separates the exhaust streams before one for both groups 12 . 18 common exhaust pipe section 32 united.

The internal combustion engine 10 is from a control unit 34 controlled by output of manipulated variables. In the context of the invention presented here, manipulated variables are essential, with which the air ratio lambda of combustion chamber fillings of the internal combustion engine 10 is set. The drive signals for the fuel metering device 26 belong to these manipulated variables as well as control signals for controlling the supply of fresh air to the internal combustion engine 10 , 1 shows in this context a throttle 36 , whose opening angle from the control unit 34 via a combined throttle actuator and angle sensor 38 is set. Further possibilities for influencing the supply of fresh air exist with a variable valve control with a throttling by an adjustment of the stroke of inlet valves and / or by influencing an internal exhaust gas recirculation of the internal combustion engine 10 by changing the valve overlap. Furthermore, the internal combustion engine 10 in the embodiment of 1 a first outer exhaust gas recirculation 40 with a first exhaust gas recirculation valve 42 and a second outer Ab gas recirculation 44 with a second exhaust gas recirculation valve 46 on. Both exhaust gas recirculation valves are also from the control unit 34 controlled and also provide actuators to influence the proportion of fresh air to combustion chamber fillings of the cylinder 14 . 16 . 20 . 22 represents.

The control unit processes the drive signals to form the control signals 34 usually signals a variety of sensors. Representing this variety are in the 1 next to the angle sensor 38 an air mass meter 48 , a speed sensor 50 and at least one NOx sensor 52 shown in the for both groups 12 . 18 common exhaust pipe section 32 is arranged.

The control unit 34 Moreover, it is set up, in particular programmed, to control the course of the method according to the invention and / or one of its embodiments.

2 1 illustrates an exemplary embodiment of a method according to the invention on the basis of courses of different operating parameters over time t, as they occur during the course of the method. 2a shows the loading of the two storage catalytic converters with nitrogen oxides. 2 B illustrates the change of storage phases and regeneration phases of the first storage catalyst 28 , 2c illustrates the change of storage phases and regeneration phases of the second storage catalyst 30 , A high signal level in the 2 B and 2c each indicates a regeneration. High levels in the 2d . 2e and 2f each indicate that a diagnostic result is determined. In the case of 2d the diagnostic result is the first storage catalytic converter 28 while in the case of the 2e the second storage catalyst 30 assigned. In the case of 2e a diagnostic result is determined, which is known to apply to the worse storage catalytic converter, without first knowing whether this is the first storage catalytic converter 28 or the second storage catalyst 30 is. 2g shows corresponding air data lambda in carrying out the method and 2h shows a time course of the signal of the NOx sensor 52 in carrying out the process.

In the 2a represents the dashed line 54 the loading of the first storage catalyst 28 with nitrogen oxides NOx while the solid line 56 the loading of the second storage catalyst 30 represented with nitrogen oxides. The values of the loads that result at any time t are in the control unit 34 calculated by a storage catalyst model and are therefore in the control unit 34 known. For an embodiment of such a storage catalyst model is at the above-mentioned DE 103 18 116 A1 directed. At time t = t0, both are storage catalysts 28 . 30 emptied, resulting from a previous regeneration of both storage catalysts 28 . 30 through an operation of both groups 12 . 18 of cylinders of the internal combustion engine 10 has been achieved with a rich mixture.

From time t0 both groups will be 12 . 18 of cylinders of the internal combustion engine 10 operated lean. As a result, both storage catalysts 28 . 30 initially filled simultaneously with NOx until a time t2. For the determination of the NOx storage capacities of the storage catalytic converters 28 . 30 Subsequently, a different sized loading of the two storage catalysts 28 . 30 generated with NOx. In the embodiment of 2 this is done by the second storage catalytic converter 30 is partially or completely regenerated between times t2 and t3. In the embodiment of 2 a complete regeneration takes place. The high level in the signal Denox Kat_30 the 2c indicates the regeneration.

The regeneration is triggered at an advantageous time t2. Such a time is present when in the control unit 34 continuously recalculated loading of the second storage catalyst 30 reaches a threshold S_30. For the first storage catalytic converter 28 has the controller 34 stored a threshold value S_28, which in the embodiment of the 2 is shown as greater than the threshold S_30 without loss of generality. Alternatively, the regeneration can be triggered by a first increase in the signal 58 of the NOx sensor 52 an exhaustion of the capacity of one of the storage catalysts 18 . 30 signaled. comparisons 2h , When the first increase of the signal 58 of the NOx sensor 52 At time t1 <t2, an imminent breakthrough of NOx is signaled, but it is not yet possible to decide with certainty which of the two storage catalytic converters 28 . 30 breaks through. In the control unit 34 This situation is solved by raising the signal 60 in the 2f represents. The high signal 60 of the 2f means that a diagnosis of a NOx storage catalyst follows, without already a secure assignment of the result to one of the two storage catalytic converters 28 . 30 can be done.

For the diagnosis then from the time t2 one of the two storage catalytic converters 28 or 30 regenerated. If it is known which of the two storage catalysts 28 . 30 has the smaller storage capacity, this is first regenerated. Which of two storage catalysts 28 . 30 that is, is the controller 34 usually known by earlier implementations of the method. As a rule, this storage catalyst also be the one that already passes NOx and thus the first rise of the signal 58 at the time t1 <t2 has caused. For the further explanations, it is assumed that this is the second storage catalyst 30 is.

For the regeneration is the second storage catalyst 30 assigned second cylinder group 18 of the internal combustion engine 10 operated between the times t2 and t3 so that it generates a reducing exhaust gas atmosphere, which flows through the second storage catalytic converter 30 Oxygen consumed, which is released from the nitrogen oxides stored there. The remaining after the release of oxygen from the nitrogen oxides nitrogen is released in a molecular, innocuous form.

The first group 12 of cylinders 14 . 16 is in contrast in the embodiment of 2 operated between the times t2 and t3 with stoichiometric fuel-air mixture. Usually, a 3-way catalyst volume is arranged in front of each storage catalytic converter, which converts the nitrogen oxide raw emissions occurring during stoichiometric operation into molecular nitrogen. However, the resulting exhaust gas does not yet have a reducing effect, so that the loading of the first storage catalytic converter remains at least approximately constant between the times t2 and t3.

2g shows corresponding air ratios lambda in the implementation of the method. The dashed line represents 62 the air ratio lambda before the first storage catalytic converter 28 and the solid line 64 represents the air ratio lambda before the second storage catalytic converter 30 ,

As a result, both storage catalytic converters 28 . 30 Therefore, at time t3, a different loading, wherein the loading of the first storage catalyst 28 greater than the loading of the second storage catalyst 30 , Subsequently, that is from the time t3, both storage catalytic converters 28 . 30 again in parallel loaded with nitrogen oxides by both groups of cylinders are operated with excess air. The loading therefore takes place starting from an empty second storage catalytic converter 30 while the first storage catalyst 28 is further filled starting from the previously reached level. Under normal circumstances, therefore, the first storage catalyst 28 then be rather full than the second storage catalyst 30 , Therefore, in the control unit 34 a signal 66 in the 2d high, that is a diagnosis of the first storage catalytic converter 28 displays. The storage capacity determined in the following will therefore be the first storage catalyst 28 assigned.

If then the signal of the NOx sensor 52 at the time t4 exhaustion of the capacity of one of the storage catalysts 28 . 30 signaled, this breakthrough of nitrogen oxides by the storage catalyst assembly can clearly the first storage catalyst 28 be assigned, because this has the larger load at time t4 due to the history. The uniqueness arises at least when the loading of the second storage catalytic converter 30 between times t3 and t4 has not again exceeded the associated threshold S_30. If S_30 is exceeded, the second storage catalyst would become 30 but regenerated again. After such a single or multiple repetition of the regeneration of the second storage catalyst 30 However, it will definitely happen that the NOx sensor 52 again signaled a breakthrough before the loading of the second storage catalytic converter 30 again reaches the threshold value S_30. Such a breakthrough becomes clearly the first storage catalyst 28 assigned.

Because the second storage catalytic converter 30 has been regenerated at least once in the meantime, the first storage catalytic converter has 28 At the time t4 of the last breakthrough in any case, the relatively larger load. The value of its load, in the control unit 34 is updated continuously corresponds, at time t4 this breakthrough of the sought NOx storage capacity of the first storage catalytic converter 28 ,

The clear assignment of the breakdown signal from the time 14 to the first storage catalyst 28 enables the separate diagnosis for the first storage catalytic converter 28 ,

A preferred embodiment of the invention provides that at the beginning of the process, ie at the time 10 , For determining the NOx storage capabilities initially an equal loading of both storage catalysts 28 . 30 is produced. As a result, the amount of time between times t0 and t4 in the first storage catalyst corresponds 28 introduced nitrogen oxides of the loading of the first storage catalyst 28 at time t4. This creates defined starting conditions for carrying out the method.

In the embodiment of 2 are both storage catalysts 28 . 30 Completely discharged from nitrogen oxides at time t0. Between t0 and t1, they are initially charged in parallel with nitrogen oxides in time, before a different charge is generated between times t2 and t3.

A further preferred embodiment provides that the generation of a different Be Charge through a partial or complete regeneration of the worse storage catalytic converter then takes place when the signal of the NOx sensor 52 the exhaustion of the absorption capacity of one of the storage catalysts 28 . 30 signaled.

In the 2 this is the case at time t1, at which the signal of the NOx sensor 52 or an NOx concentration derived therefrom in the control unit exceeds a threshold value.

For the diagnosis of the second storage catalytic converter 30 the process is then repeated, of course, only the first storage catalyst 28 is regenerated. For a quick diagnosis result and the lowest possible vehicle emissions, it is therefore optimal to first regenerate the worse catalyst in the system after the first breakthrough, which occurs at time t1. At the first diagnostic attempt, there is a 50% probability that the worst storage catalytic converter will actually be regenerated. If, however, then the worst storage catalytic converter in the system is known and this in the control unit 34 remains stored, the optimal order can be kept for all further diagnoses.

In the repetition of the process with reversed groups of cylinders and storage catalytic converters used to determine the NOx storage capacity of the second storage catalytic converter 30 However, it is not necessary between the evaluation of the two NOx storage catalytic converters 28 . 30 to generate equal loads as initial values at the beginning of a performance of the method. However, it is essential again that during the process different loadings of both NOx storage catalysts 28 . 30 be generated.

In the embodiment of 2 this is done by the first storage catalyst 28 is completely discharged by a regeneration between the times t4 and t5, while the loading of the second storage catalytic converter 30 held constant during this period. At time t5, the second storage catalytic converter 30 therefore the larger load on. The next breakthrough of NOx by the storage catalytic converter assembly, reflected in the signal from the NOx sensor 52 at the time t6 can therefore clearly the second storage catalytic converter 30 be assigned. The value of the control unit 34 continuously updated and reached at time t7> t6 loading of the second storage catalytic converter 30 is considered storage capacity of the second storage catalyst 30 saved.

As the described embodiment shows, it is between the evaluation of the two NOx storage catalytic converters 28 . 30 not required, the same level in both storage catalytic converters 28 . 30 manufacture.

In addition, the following optional / supplementary strategy is particularly advantageous: An adaptation factor is already determined at the first breakthrough of NOx at the time. The determination preferably takes place as has been explained above in connection with the prior art. Without loss of generality, the breakthrough in the signal will be described below 58 considered as such a first breakthrough at time t6.

The adaptation value determined from time t6, which is used to match the modeled NOx concentration to the measured NOx concentration, then initially becomes generally the value for the worst-case catalytic converter of the exhaust system 27 saved. If after regeneration of the first storage catalyst 28 , which takes place between the times t7 and t8, no immediate further breakthrough is measured, this means that the second storage catalytic converter 30 in further lean operation for times t> t7 can continue NOx storage.

On the other hand, as it happens in the 2 is shown, at time t9 briefly after the regeneration of the first storage catalytic converter 28 another breakthrough, this can clearly the second storage catalyst 30 be assigned. The adaptation factor determined in this case can be clearly assigned to the second storage catalytic converter 30 be assigned. The previously generally for the worst storage catalytic converter of the exhaust system 27 determined adaptation factor can then directly to the first storage catalyst 28 be assigned. The diagnosis is then completed.

Conversely, if after a regeneration of the second storage catalytic converter 30 a new immediate breakthrough occurs, this clearly the first storage catalyst 28 be assigned and the previously determined value for the worst storage catalyst is the second storage catalyst 30 assigned.

The designs were overlooking an exhaust system 27 with a NOx sensor 52 explained. However, it is understood that in addition one or more lambda probes in the exhaust system 27 can be used. In addition, the invention can be used both in gasoline engines and in diesel engines and in combination with particulate filters and with and without further catalysts before and / or behind the storage catalysts 28 . 30 be used.

The presented method involves that in a Y exhaust system at times only one Speicherka regenerated. In engine controls where the charge control is via just one throttle 36 together for both groups 12 . 18 Cylinders, it may be problematic to operate a group in lambda >> 1 shift operation and the other group for homoge- neous regeneration with lambda <1. The regeneration of only one group can be realized most easily under these conditions by operating the other group in parallel with Lambda = 1. Since this may lead to exhaust-desorbing NO ₂ -Desorptionseffekten, provides a further preferred embodiment, the cylinder group of non-regenerating storage catalyst continue to operate lean, while the regenerating group is operated fat with late firing angles. In a diesel engine, the regeneration can be done by means of post-injection. Thus, with the same charge and the same ignition torque on both banks opposite air ratios Lambda can be realized. Since these phases are required only rarely and very briefly, the fuel consumption and the thermal load is considered to be justifiable.

A another alternative solution The diagnostic problem is in a separate, bank-wise system AGR feed (Exhaust gas recirculation) possible. By shut down the AGR can the NOx raw emissions greatly increased become. Consequently, by closing the EGR on a bench the loading of the NOx storage catalyst compared to the other bank clearly elevated become. The then measured NOx breakthrough (diagnosis) can then the stronger be assigned loaded catalyst.

The Diagnosis can be through a mass regenerant mass balance added become. The storage catalyst is in this case with a defined mass NOx (raw emission model) loaded. Then a regeneration is going through Grease gas performed. Over a Balancing of the absorbed regenerant (transition lean -> fat) and the absorbed oxygen mass (transition fat -> lean) is the determination the actual stored NOx mass possible. The difference can be used to determine the aging.

Claims (10)

Method for determining the NOx storage capacity of a NOx storage catalytic converter ( 28 . 30 ) in the exhaust gas of an internal combustion engine ( 10 ) as a function of upstream of the storage catalyst ( 28 . 30 ) measurable quantities and a signal downstream of the storage catalyst ( 28 . 30 ) arranged NOx sensor ( 52 ), which is a depletion of the absorption capacity of the storage catalyst ( 28 . 30 ), characterized in that in an internal combustion engine ( 10 ), two groups ( 12 . 18 ) of cylinders and a Y exhaust system ( 27 ), the exhaust gases of the two groups ( 12 . 18 ) separated by a respective group NOx storage catalyst ( 28 . 30 ) and where the NOx sensor ( 52 ) in an exhaust gas of both groups ( 12 . 18 ) conductive section ( 32 ) of the exhaust system ( 27 ), a different loading of both storage catalysts ( 28 . 30 ) and then both storage catalysts ( 28 . 30 ) are loaded with nitrogen oxides in parallel until the signal from the NOx sensor ( 52 ) exhaustion of the absorption capacity of one of the storage catalytic converters ( 28 . 30 ) and that the storage capacity for the storage catalytic converter ( 28 . 30 ) is determined with the larger load. A method according to claim 1, characterized in that initially an equal loading of both storage catalysts ( 28 . 30 ), both storage catalysts ( 28 . 30 ) are initially loaded in parallel with nitrogen oxides, one of the two storage catalysts ( 28 . 30 ) is completely or partially regenerated, so that the other of the two storage catalysts has a greater load, and then both storage catalysts ( 28 . 30 ) are loaded with nitrogen oxides in parallel until the signal from the NOx sensor ( 52 ) the exhaustion of the absorption capacity of one of the storage catalytic converters ( 28 . 30 ) signals. A method according to claim 2, characterized in that the one of the two storage catalysts ( 28 . 30 ) is completely or partially regenerated when the signal of the NOx sensor ( 52 ) the exhaustion of the absorption capacity of one of the storage catalytic converters ( 28 . 30 ) signals. A method according to claim 2, characterized in that in dependence on upstream of the storage catalyst ( 28 . 30 ) measurable quantities a loading of one or both storage catalysts ( 28 . 30 ) is determined with nitrogen oxides and that the one of the two storage catalysts ( 28 . 30 ) is completely or partially regenerated when its load exceeds a predetermined threshold (S_28). A method according to claim 1 or 2, characterized in that it is carried out repeatedly, wherein the one storage catalytic converter ( 28 . 30 ), which in a first implementation, a larger load than the other storage catalyst ( 30 . 28 ) learns in a further implementation, a smaller load than the other storage catalyst ( 30 . 28 ) learns. Method according to one of the preceding claims, characterized in that the Be the NOx storage capacity as a function of the upstream of the storage catalytic converter ( 28 . 30 ) measurable variables and the exhaustion of the absorption capacity of the storage catalyst ( 28 . 30 ) signaling signal of the NOx sensor ( 52 ) by forming an adaptation value that matches a NOx exhaust gas parameter determined based on the upstream measured quantities to an exhaust gas parameter determined based on the signal of the NOx sensor. Method according to Claim 7, characterized in that an adaptation value is determined already at the first breakthrough and is determined as the value for the worst of the two storage catalytic converters ( 28 . 30 ) is stored. A method according to claim 8, characterized in that if, in a further breakthrough, an adaptation value of one of the two storage catalysts ( 28 . 30 ) assigned to the value according to claim 7 the other of the two storage catalysts ( 30 . 28 ). Control unit ( 34 ) for determining the NOx storage capacity of a NOx storage catalytic converter ( 28 . 30 ) in the exhaust gas of an internal combustion engine ( 10 ) as a function of upstream of the storage catalyst ( 28 . 30 ) measurable quantities and a signal downstream of the storage catalyst ( 28 . 30 ) arranged NOx sensor ( 52 ), which is a depletion of the absorption capacity of the storage catalyst ( 28 . 30 ), characterized in that the control unit ( 34 ) is adapted to be used in an internal combustion engine ( 10 ), two groups ( 12 . 18 ) of cylinders and a Y exhaust system ( 27 ), the exhaust gases of the two groups ( 12 . 18 ) separated by a respective group NOx storage catalyst ( 28 . 30 ) and where the NOx sensor ( 52 ) in an exhaust gas of both groups ( 12 . 18 ) conductive section ( 32 ), the internal combustion engine ( 10 ) so that a different loading of both storage catalysts ( 28 . 30 ) and then both storage catalysts ( 28 . 30 ) are loaded with nitrogen oxides in parallel until the signal from the NOx sensor ( 52 ) exhaustion of the absorption capacity of one of the storage catalytic converters ( 28 . 30 ) and the storage capacity for the storage catalytic converter ( 28 . 30 ) with the larger load. Control unit ( 34 ) according to claim 9, characterized in that it is adapted to control a sequence of a method according to one of claims 2 to 8.