DE102015215501A1 - Determining the conversion efficiency of an exhaust gas catalytic converter - Google Patents

Abstract

In order to determine the conversion efficiency of at least one component of a catalyst box (6) comprising at least one SCR catalytic converter (7) and a nitrogen oxide storage catalytic converter (8) for after-treatment of the exhaust gases of an internal combustion engine (1), the catalyst box (6) at the downstream end Associated NOx sensor (9), it is proposed that at least for a limited period of time, the internal combustion engine (1) is operated such that a first of the two components does not affect the signal detected by the NOx sensor (9) and the conversion efficiency of the second component as a function of the uninfluenced by the first component signal of the NOx sensor (9) is determined.

Description

State of the art

The invention relates to a method for determining the conversion efficiency of at least one component of a catalyst box comprising at least one SCR catalyst and a nitrogen oxide storage catalyst for after-treatment of the exhaust gases of an internal combustion engine, wherein the catalyst box is assigned a NO _x sensor at the downstream end.

The invention further relates to a computer program that can be stored in a control unit for controlling and / or regulating an internal combustion engine. The invention also relates to a control device for controlling and / or regulating the operation of an internal combustion engine.

In order to reduce the NOx emissions _{x, x} inserted into harmless emissions in internal combustion engines and in particular in gasoline engines with lean operating modes and diesel engines exhaust aftertreatment components for conversion of NO. The components currently most commonly used are the nitrogen oxide storage catalyst (NO _x -Storage Catalyst NSC) and SCR (Selective Catalytic Reduction).

The nitrogen oxide storage catalyst stores the nitrogen oxides (NO _x ) present in the exhaust gas stream. If the storage catalytic converter is sufficiently filled, the stored nitrogen oxides are converted into harmless molecules in a special operating mode of the internal combustion engine, which is referred to as regeneration phase or as regeneration and in which the internal combustion engine is operated with a rich fuel mixture, and paged out of the storage catalytic converter. The NO _x storage of the storage catalyst depends on the current load, the temperature and other parameters, such as the so-called space velocity from. If the relative loading of the storage catalytic converter increases, then at some point the added NO _x mass can no longer be stored to a sufficient extent. The portion which can no longer be deposited then leaves the storage catalytic converter as so-called slip and is consequently emitted in the exhaust gas flow.

The conversion efficiency of the nitrogen oxide storage catalyst depends on the Einspeicherverhalten. This is usually first described by a model, which is then adjusted using measurements with a NO _x sensor for the respective storage catalyst, which is referred to as adaptation. In the adaptation, a quality factor is calculated, which describes the conversion efficiency of the storage catalytic converter. The quality factor can be used to calculate how much NO _x can be stored and when NO _x slip occurs. Thus, an exact adaptation between the model and the measured sensor value is important in order to be able to optimally use the storage catalytic converter and to keep the nitrogen oxide emissions low.

During operation of the internal combustion engine, the storage catalytic converter has to be diagnosed in order to detect damage, for example due to an excessively high temperature.

An SCR catalyst allows even in the presence of residual oxygen in the exhaust gas stream by catalytic action, for example by means of supplied ammonia, the NO _x present in the exhaust selectively to reduce, among other nitrogen (N ₂ ). The supply of ammonia can take place actively via the targeted metering of a urea water solution into the exhaust gas mass flow or passively via an SCR catalyst upstream oxidation catalyst, for example a three-way catalyst or a nitrogen oxide storage catalyst, during an operating phase in which the internal combustion engine with operated a rich fuel mixture, NH ₃ forms.

Moreover, the SCR catalyst can to a limited extent store excess NH ₃ in a zeolite. The capacity of the catalyst depends on the catalyst temperature. If the capacity is exceeded, the excess NH ₃ slip leaves as the catalyst. The NH ₃ stored in the SCR catalytic converter makes it possible to reduce the NO _x even at operating points in which no ammonia is supplied.

With a system comprising a combination of an SCR catalyst and a nitrogen oxide storage catalyst, higher total NO _x efficiencies can be achieved. A combination of these catalysts is referred to below as a catalyst box, wherein the two components are not necessarily arranged in a common housing.

For monitoring the NO _x conversion rate or the conversion efficiency, NO _x sensors are usually used behind each individual component.

Disclosure of the invention

The object of the invention is to determine the quality of the individual components of an overall system consisting of an SCR catalytic converter and a nitrogen oxide storage catalytic converter and, in particular, to determine the proportion of NO _x conversion of the SCR catalytic converter. Catalyst to determine its aging can be determined. It should also be shown a way to be able to determine the conversion efficiency sufficiently accurately even if only a single NO _x sensor is used.

The object is achieved by a method of the type mentioned in that at least for a limited period of time, the internal combustion engine is operated such that a first of the two components does not affect the signal detected by the NO _x sensor signal and the conversion efficiency of the second component in dependence is determined by the non-influenced by the first component signal of the NO _x sensor.

According to the invention, a state is thus created in which at least one of the components, for example the SCR catalytic converter, is put into a state in which it does not influence the signal detected by the NO _x sensor. It should be noted that depending on the current operating mode of the internal combustion engine by means of a NO _x sensor both NO _x (in a lean mode) and NH ₃ (in a rich mode) can be measured in the exhaust stream.

According to a first possible embodiment, a state is created in which the SCR catalyst does not affect the NO _x signal. This is achieved by causing a lowering of the NH ₃ stored in the SCR catalyst. For this purpose, according to a variant, the internal combustion engine is operated at least for a limited period of time such that a reduction of the NH ₃ stored in the SCR catalytic converter takes place. If no or almost no NH ₃ more stored in the SCR catalyst, the conversion efficiency of the NO _x storage catalytic converter in dependence on the signal of the _NOx sensor is determined. In particular, the internal combustion engine can be operated such that the SCR catalytic converter reaches a temperature at which the stored NH _{3 is} expelled.

According to another possible embodiment, the internal combustion engine is operated such that a supply of NH ₃ to the SCR catalyst is at least largely prevented and a reduction of NO _x using the stored NH ₃ takes place.

By means of the prescribed methods, it is achieved that no more NH ₃ is stored in the SCR catalytic converter, as a result of which the SCR no longer has any influence on the NO _x conversion of the entire system. Thus, the signal of the NO _x sensor can now be used to determine the conversion efficiency of the nitrogen oxide storage catalyst.

According to another aspect of the invention, the internal combustion engine for at least a limited period of time is operated in such a manner that a saturation of the NO _x storage catalytic converter with NO _x is carried out and when no or almost no NO can be stored _x more, the conversion efficiency of the SCR catalyst in Dependence on the signal of the NO _x sensor is determined. Here, consequently, a state is reached in which the nitrogen oxide storage catalyst is saturated and thus allows a mass transfer of NO _x almost completely unconverted. In particular, if the nitrogen oxide storage catalytic converter is installed upstream of the SCR catalytic converter, this state can also be achieved by selecting an engine operating mode in which NO _x crude emissions arise so that the catalytic converter is fully loaded. If the storage catalytic converter is installed in the exhaust gas flow downstream of the SCR catalytic converter, it can be filled by operating the internal combustion engine in an operating mode in which the NO _x concentration in the exhaust gas flow in front of the catalyst box is so high that even with an optimally operating resp a new SCR catalyst, the nO contained in the exhaust stream _x can not be completely converted.. The NO _x slip from the SCR catalyst is then stored in the nitrogen oxide storage catalyst until it is filled.

If the storage catalytic converter is fully loaded, an adaptation of the SCR catalytic converter can be carried out, since the SCR catalytic converter now influences the NO _x concentration in the exhaust gas flow at the NO _x sensor as sole component. Thus, an efficiency diagnosis of the SCR catalyst can now be performed, for example, by comparing the NO _x mass, which is retracted into the SCR, in relation to the NO _x mass at the position of the NO _x sensor.

According to an advantageous development of the method according to the invention, the NO _x storage catalyst is charged with NO _x at least until a NO _x slip is measured at the NO _x sensor. It is then chosen an operating mode in which a fully functioning, or at least sufficiently functional SCR catalyst, the total NO _x would convert concentration in the exhaust. Depending on the signal of the NO _x sensor, the conversion efficiency of the SCR catalyst is then determined.

Here, therefore, the storage catalyst is initially loaded so far that a slip occurs. This can be done as previously described. Usually, a regeneration of the storage catalytic converter will be activated in this loading state, which is now suppressed or prevented. Instead, a condition is created in the overall system in which a NO _x concentration in front of the catalyst box is maximally high is that with an optimally new SCR catalyst with enough NH ₃ stored, the NO _x would be completely converted. In an SCR catalyst that converts still enough good, now no NO _x slip of the _NOx sensor would be measured. However, if the SCR catalyst has aged and thus no longer has the property to convert the NO _x sufficiently well, NO _x slip is measured at the sensor behind the NSC. The relationship between the NO _x -Massenstrom upstream of the SCR catalyst and the measured by the NO _x sensor NO _x -Massenstrom by the SCR catalyst can now be used for the calculation of the quality factor and the conversion efficiency of the SCR catalyst.

According to a further embodiment of the method, the NH ₃ storage capability of the SCR catalytic converter is determined as a function of the signal of the NO _x sensor. This development uses the property of NO _x sensors to be able to measure NH ₃ instead of NO _x in the case of a rich fuel mixture. Furthermore, it can be determined with the NO _x sensors, whether currently a rich or lean fuel mixture is available. In order to conclude, in the event that no NOx sensor is present, of the NH ₃ storage capability of the SCR catalyst, the information about the NH ₃ in the exhaust gas flow upstream of the catalyst box could be calculated by means of suitable NO _x raw mass models and / or oxidation catalyst models , It should be noted here that when using an NSC as the NH ₃ source in front of the catalyst box, a high NH ₃ peak is formed by the depth memory of the NSC, which would possibly have to be taken into account in the model used.

Also an NH is preferably _{3 x} desorption arising due to the data stored in the nitrogen oxide storage catalyst NH ₃ NO ₃ considered storage capacity of the SCR catalyst in the determination of the NH. In particular, a short time after switching from a lean operation in a operation with a rich fuel mixture NH ₃ desorption is observed. This desorption can be determined, for example, by integrating the NH ₃ signal up to a defined time and adding it to the NH ₃ storage capacity or by attaching the curve of the NH ₃ signal in the region of the NH ₃ desorption to the subsequent course ( 'fit'). Thus, the NH ₃ storage capacity of the SCR catalyst can now be determined with sufficient accuracy from the difference between the inflowing NH ₃ into the SCR catalytic converter and the outflowing NH ₃ from the NO _x storage catalytic converter.

Alternatively or in addition, a NO _x desorption that the internal combustion engine is operated prior to detection of the signal of the _NOx sensor in such a manner may be provided, due to the data stored in the NO _x storage catalytic NO _x resulting NH ₃ to suppress the fact that the in the NO _x storage is stored _NOx is popped. This can be achieved for example by a temperature increase before the measurement.

According to a preferred embodiment, the NO _x storage capacity of the nitrogen oxide storage catalytic converter is determined with a fuel mixture that is only slightly rich, in particular a lambda value that is greater than 0.97, since the subsequent nitrogen oxide catalyst has the fat components via the oxygen storage capability that is present in the mixture (HC and CO) to harmless components (CO ₂ and H ₂ O) can implement and thus only low exhaust emissions. Furthermore, in this area of the mixture composition, the NH ₃ formation from the upstream oxidation catalyst is greatest.

If the nitrogen oxide storage catalytic converter is arranged in the exhaust gas flow in front of the SCR catalytic converter, the NH ₃ could pass from the desorption of the nitrogen oxide storage catalytic converter into the SCR catalytic converter and lead to a falsification of the results. This can be prevented that the NH ₃ storage of the SCR catalyst and the NO _x storage of the nitrogen oxide storage catalyst are emptied and then generated by operation of the internal combustion engine with a rich fuel mixture NH ₃ in the exhaust stream and the signal of NO _x Sensor is evaluated for the determination of the NH ₃ storage capacity of the SCR catalyst. If the unloading of the catalysts is achieved by means of a temperature increase, then the catalysts could be cooled with a homogeneous phase, for example with a lambda value = 1, without NH ₃ and NO _x emissions up to the desired temperature and through an operating phase with a rich fuel mixture NH ₃ be generated, which would not be distorted by the empty nitrogen oxide storage catalyst.

The object underlying the invention is also achieved by a computer program which is stored in a control device for controlling and / or regulating an internal combustion engine, characterized in that the inventive method is carried out when the computer program is executed on the control unit. The object is further achieved by a control unit for controlling and / or regulating the operation of an internal combustion engine in that in the control unit, a computer program is stored and the inventive method is performed when the computer program is executed on the control unit.

Other features, applications and advantages of the invention will become apparent from the The following description of exemplary embodiments, which are explained with reference to the drawings, wherein the features both in isolation and in different combinations for the invention may be important, without being explicitly referred to again. Show it:

1 a schematic representation of some existing in a motor vehicle components, which are adapted to carry out the method according to the invention;

2 some method steps of a possible embodiment of the method according to the invention in a flowchart;

3 a further flowchart with method steps of a possible embodiment of the invention;

4 some process steps in performing the diagnosis of the SCR catalyst; and

5 a flowchart with some steps that can be used to diagnose the SCR catalyst on NH ₃ information.

1 schematically shows an internal combustion engine 1 , which is an intake tract 2 , a fuel supply 3 and an exhaust tract 4 are arranged. In the exhaust tract 4 are an oxidation catalyst 5 (For example, a three-way catalyst) and a catalyst box 6 arranged. The catalyst box 6 includes an SCR catalyst 7 and a NO _x storage catalyst 8th , Behind the catalyst box 6 in the exhaust stream is a NO _x sensor 9 arranged over a data line 10 with a control unit 11 connected is. The internal combustion engine 1 is also via data lines or a bus system 12 with the control unit 11 connected to the operation of the internal combustion engine 1 controls or regulates.

In the control unit 11 is a storage area 13 trained in which a computer program 14 is stored, which is programmed to carry out the method according to the invention.

In addition to the in 1 shown arrangement further embodiments are possible according to the invention. For example, the arrangement of the SCR catalyst 7 and the nitrogen oxide catalyst 8th swapped or it is also an exhaust probe in front of the catalyst box 6 available.

In 2 some process steps are shown in the diagnosis of the nitrogen oxide storage catalyst 7 be executed according to a possible embodiment. The process begins with a step 20 , In one step 21 becomes the internal combustion engine 1 operated such that in the SCR catalyst 7 stored NH ₃ is stored out, which can be achieved by increasing the catalyst temperature. Alternatively, by a NO _x reduction with simultaneous suppression of NH ₃ supply passively, the NH ₃ level can be lowered.

In one step 22 is the withdrawal of NH ₃ ended. In one step 23 the internal combustion engine is operated with a lean fuel mixture. In one step 24 the NO _x slip is determined. For this purpose, the NO _x content in the exhaust gas stream in front of the catalyst box 6 determined and by means of the _NOx sensor 9 becomes the NO _x concentration after the catalyst box 6 measured. The difference of these values gives the NO _x slip.

In one step 25 Then, the above-described adaptation is performed by, for example, a quality factor or the conversion efficiency of the nitrogen oxide storage catalyst 8th is calculated. Alternatively, the conversion efficiency can be used to the nitrogen oxide storage catalyst 8th to diagnose.

3 shows some process steps in performing the diagnosis of an SCR catalyst. The procedure begins in one step 30 , In one step 31 becomes the nitrogen oxide storage catalyst 8th loaded with NO _x . For this purpose, a corresponding operating mode of the internal combustion engine 1 chosen, in particular, the internal combustion engine 1 operated with a lean fuel mixture. In one step 32 it is checked whether the storage catalyst 8th is fully loaded. This is done by means of the NO _x probe 9 measured whether NO _x -in the exhaust stream after the catalyst box 6 is included. If this is the case, it will be in one step 33 the NO _x mass in the exhaust stream upstream of the catalyst box 6 certainly. This can be done by means of a suitable model. Should be in front of the catalyst box 6 If there is another NO _x sensor, this value can be determined particularly accurately by means of this sensor.

In one step 34 is the difference of the NO _x mass in front of the catalyst box 6 in the exhaust stream and after the catalyst box 6 certainly. In one step 35 This difference will affect the conversion efficiency of the SCR catalyst 7 closed.

4 shows method steps of a further embodiment, in the diagnosis of the SCR catalyst 7 be performed. The procedure begins in one step 40 , In one step 41 becomes the nitrogen oxide storage catalyst 8th loaded with NO _x . In one step 42 is measured whether a NO _x slip occurs, the storage catalyst 8th So loaded. This is done by an evaluation of the signal of the NO _x sensor 9 reached.

In one step 43 is the regeneration of the nitrogen oxide storage catalyst 8th which would usually now use, since the nitrogen oxide storage catalyst 8th fully loaded, suppressed.

In one step 44 a condition is created in which a NO _x concentration in front of the catalyst box 6 is maximum so high that would be _x fully converted at an optimally working or a new SCR catalyst with enough much eingespeichertem NH _3, NO. For a passive SCR catalyst 7 Who converted yet sufficiently well, is now no NO _x slip of the _NOx sensor in the step 45 be measured.

In one step 46 now the quality is determined. Is the SCR catalyst 7 aged and thus no longer has the property to convert the NO _x sufficiently well in the step 45 measured slip in the step 46 evaluated accordingly and the conversion efficiency determined.

5 shows some process steps in the diagnosis of the SCR catalyst 7 , wherein NH ₃ values are used, by means of the NO _x sensor 9 be recorded. The procedure begins in one step 50 , In one step 51 is the amount of NO _x in the exhaust stream upstream of the catalyst box 6 certainly. This can either be calculated by means of another _NOx sensor measured, or any suitable NO _x and oxidation catalyst -Rohmassenmodelle models. In one step 52 is by means of the NO _x sensor 9 the NH ₃ mass in the exhaust gas stream after the catalyst box 6 determined.

In one step 53 an NH ₃ desorption is considered as necessary. This results from the fact that from the in the nitrogen oxide storage catalyst 8th stored NO _{x can} also arise NH ₃ . The desorption can be determined by the NH ₃ signal after the NO _x storage catalyst 8th integrated up to a defined time and the NH ₃ storage capacity is added. Alternatively, a curve of the NH ₃ signal can be calculated and fitted to the later course in the region of NH ₃ desorption. But it can also be provided in the step 53 the internal combustion engine 1 to operate such that the NH ₃ desorption is suppressed. This is achieved by the fact that the NO _x storage catalytic converter 8th before the measurement of the NH ₃ signal in the step 52 is emptied or the stored therein NO _x is stored, including, for example, a temperature increase before the measurement is sought.

In one step 54 the NH ₃ slip is evaluated and depending on the result, the conversion efficiency of the SCR catalyst is determined.

Claims (13)

Method for determining the conversion efficiency of at least one component of at least one SCR catalyst ( 7 ) and a nitrogen oxide storage catalyst ( 8th ) catalyst box ( 6 ) for the aftertreatment of the exhaust gases of an internal combustion engine ( 1 ), wherein the catalyst box ( 6 ) at the downstream end of a NOx sensor ( 9 ), characterized in that for at least a limited period of time the internal combustion engine ( 1 ) is operated such that a first of the two components by means of the NOx sensor ( 9 ) and the conversion efficiency of the second component as a function of the signal of the NOx sensor which is uninfluenced by the first component ( 9 ) is determined. Method according to Claim 1, characterized in that the internal combustion engine ( 1 ) is operated for a limited period of time such that a reduction in the SCR catalyst ( 7 ) stored NH ₃ and if no or almost no more NH ₃ is stored, the conversion efficiency of the nitrogen oxide storage catalyst ( 8th ) is determined depending on the signal of the NOx sensor. Method according to Claim 2, characterized in that the internal combustion engine ( 1 ) is operated for the limited period of time such that the SCR catalyst ( 7 ) reaches a temperature at which the stored NH ₃ is stored out. Method according to Claim 2, characterized in that the internal combustion engine ( 1 ) is operated such that a supply of NH ₃ to the SCR catalyst ( 7 ) is at least largely prevented and a reduction of NOx using the stored NH ₃ takes place. Method according to one of the preceding claims, characterized in that the internal combustion engine ( 1 ) is operated at least for a limited period of time such that a saturation of the nitrogen oxide storage catalyst ( 8th ) with NOx and if no or virtually no more NOx can be stored, the conversion efficiency of the SCR catalyst ( 7 ) in dependence on the signal of the NOx sensor ( 9 ) is determined. Method according to one of the preceding claims, characterized in that the nitrogen oxide storage catalyst ( 8th ) is loaded with NOx at least until the NOx sensor reaches a NOx Slip is selected, a mode in which a fully functional or at least sufficiently functional SCR catalyst ( 7 ) would convert the total NOx concentration in the exhaust gas and in dependence on the signal of the NOx sensor ( 9 ) the conversion efficiency of the SCR catalyst ( 7 ) is determined. Method according to one of the preceding claims, characterized in that in dependence on the signal of the NOx sensor ( 9 ) on the NH ₃ storage capacity of the SCR catalyst ( 7 ) is closed. A method according to claim 7, characterized in that a NH ₃ desorption due to the in the nitrogen oxide storage catalyst ( 8th ) stored NH ₃ in the determination of the NH ₃ storage capacity of the SCR catalyst ( 7 ) is taken into account. A method according to claim 7, characterized in that a NH ₃ desorption due to the in the nitrogen oxide storage catalyst ( 8th ) stored NOx NH ₃ is suppressed by the fact that the internal combustion engine ( 1 ) before detecting the signal of the NOx sensor ( 9 ) is operated such that in the nitrogen oxide storage catalyst ( 8th stored NOx is stored. Method according to one of claims 7 to 9, characterized in that the internal combustion engine ( 1 ) is operated with a fuel mixture which is as slight as possible, preferably with a desired lambda value greater than 0.97, while the signal of the NOx sensor ( 9 ) for the determination of the NH ₃ storage capacity of the SCR catalyst ( 7 ) is evaluated. Method according to one of claims 7 to 10, wherein the nitrogen oxide storage catalyst ( 8th ) in the exhaust stream upstream of the SCR catalyst ( 7 ), characterized in that the NH ₃ storage of the SCR catalyst ( 7 ) and the NOx storage of the nitrogen oxide storage catalyst ( 8th ) are emptied and then by an operation of the internal combustion engine ( 1 ) produced with a rich fuel mixture NH ₃ in the exhaust stream and the signal of the NOx sensor ( 9 ) for the determination of the NH ₃ storage capacity of the SCR catalyst ( 7 ) is evaluated. Computer program ( 14 ) in a control unit ( 11 ) for controlling and / or regulating an internal combustion engine ( 1 ) is stored, characterized in that a method according to one of claims 1 to 11 is performed when the computer program ( 14 ) on the control unit ( 11 ) is performed. Control unit ( 11 ) for controlling and / or regulating the operation of an internal combustion engine ( 1 ), characterized in that in the control unit ( 11 ) a computer program ( 14 ) and a method according to one of claims 1 to 11 is carried out when the computer program ( 14 ) on the control unit ( 11 ) is performed.

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