Method for Determining a Soot Load of a Particle Filter Provided with a Selective Catalytic Coating

BACKGROUND AND SUMMARY OF THE INVENTION

The invention concerns a method for determining a soot load of a particle filter with a selective catalytic coating.

Current and future emission guidelines stipulate clear limits on emissions of internal combustion engines, mainly with respect to hydrocarbon, carbon monoxide, nitric oxide and particle emissions. The increasing fuel consumption savings in the operation of internal combustion engines mean falling exhaust gas temperatures available for the catalytic exhaust aftertreatment. For this reason, close-coupled SCR (selective catalytic reduction) systems, having a close-coupled particle filter with integrated selective catalytic coating and SCR coating (SDPF), are playing a growing role in future exhaust aftertreatment designs in order to meet the increased requirements. For such an exhaust aftertreatment system, it is important to be able to determine the soot load of the particle filter as accurately as possible. Overly frequent triggering of a regeneration results in greater aging of the catalytic coating applied on the filter as well as higher carbon dioxide and pollutant emissions of the internal combustion engine. On the other hand, if the regeneration is initiated too late the result can be excessively high temperature gradients in the particle filter that can cause mechanical damage.

A method is known from European patent application EP 2 749 745 A1 for determining a patent application of a particle filter with a selective catalytic coating in which two different determination methods are combined with each other. For one, a soot raw emission model or load model is used that calculates a soot emission depending on various engine parameters and determines the quantity deposited in the particle filter and removed by the continuous soot combustion with nitrogen dioxide (NO2). Another model is used that calculates the soot load by measuring a differential pressure in the exhaust gas flow falling through the particle filter (differential pressure model).

A disadvantage of this is that for safety reasons—in particular component protection reasons—the regeneration of the particle filter is always triggered if one of the two models reaches the maximum allowable load limit stored in the internal combustion engine's control device. It is found that the determination of the soot load from the differential pressure is very imprecise, especially after an incomplete regeneration, also called partial regeneration. The soot raw emission model, for its part, has very sharp fluctuations in precision depending on the internal combustion engine's operating point, so there can be sharp fluctuations between the soot load values for the particle filters calculated by means of the two models. The overall result is a possible premature regeneration of the particle filter without it actually being necessary at the time indicated by one of the models.

The invention is based on the object of creating a method that does not have these disadvantages.

In particular, the object is solved by creating a method for determining a soot load of a particle filter having a selective catalytic coating, namely a particle filter with a coating having a selective catalytic effect for reduction of nitrogen oxides, with the following steps: a nitric oxide conversion—especially current—is determined on the particle filter and a soot load of the particle filter—especially current—is determined from the particular nitric oxide conversion. In particular, the soot load of the particle filter is determined depending on the particular nitric oxide conversion. The nitric oxide conversion can also be determined by averaging individual nitric oxide conversion values determined in a predefinable time interval ranging from a few seconds to a few minutes. It has been shown that with a particle filter having a selective catalytic coating, interactions are created between the filter's soot load and the nitric oxide conversion in the filter wall due to the spatial merging of the selective catalytic reduction function on the one hand and the particle filter function on the other. This dependence makes it possible to deduce the particle filter's soot load from the nitric oxide conversion. In at least certain operating ranges, this method has a very high accuracy and is therefore suitable for determining a highly precise soot load that can then be used as the basis for deciding on a regeneration measure without having to fear a premature regeneration. In particular, this can also be combined with at least one other determination of the soot load based on at least one other model, wherein the very precise method addressed here in particular can be used to correct a soot load determined based on the other model correct or adapt the other model itself.

The method is preferably performed in an exhaust tract of an internal combustion engine, wherein the exhaust tract preferably has an oxidizing catalytic converter, in particular a diesel oxidation catalyst (DOC), and/or a nitric oxide storage catalyst (NSC). In the flow direction of the exhaust gas downstream of the oxidizing catalytic converter and/or the diesel oxidation catalyst is a downstream system for selective catalytic reduction of nitric oxides, which has the particle filter that has the selective catalytic coating. With one embodiment of the method it is possible that this is done for an SCR system that, in addition to the particle filter provided with the selective catalytic coating, has a catalytic converter (SCR catalytic converter) without particle filter function designed for selective catalytic reduction. This is then preferably arranged downstream of the particle filter provided with the selective catalytic coating.

Preferably, an exhaust gas recirculation device is also used. Particularly preferred is a multipath exhaust gas recirculation device having a high-pressure exhaust gas recirculation line and a low-pressure exhaust gas recirculation line.

In a preferred embodiment of the method the soot load is determined from the predetermined nitric oxide conversion by the specific nitric oxide conversion being compared with a nitric oxide conversion—preferably stored in a characterization field—having the particle filter provided with the selective catalytic compound when it is not loaded with soot; for example, when new or after a complete regeneration. In particular, the current soot load can be determined very precisely by the comparison with this reference value of the particle filter without soot load.

An embodiment of the method is preferred that is characterized in that the soot load is determined with the procedure described here if an exhaust gas temperature—in particular current—that is preferably measured by a temperature sensor arranged in the exhaust tract upstream of the particle filter—is greater than or equal to a predetermined minimum temperature and less than or equal to a predetermined maximum temperature. It has been shown that there is a clear and, in particular, clearly evaluable connection between the nitric oxide conversion and the soot load of the particle filter, especially in a particular temperature range for the exhaust gas temperature upstream of the particle filter. Therefore, the method can be carried out with a particularly high accuracy in this temperature range. It is seen that the minimum temperature preferably is at least 175° C. to at most 210° C. Alternatively or additionally, the maximum temperature is preferably from at least 240° C. to at most 280° C. In a preferred embodiment of the method the soot load is preferably determined when the exhaust gas temperature upstream of the particle filter is from at least 150° C. to at most 300° C., preferably from at least 175° C. to at most 280° C., preferably from at least 175° C. to at most 240° C., preferably from at least 210° C. to at most 280° C., preferably from at least 210° C. to at most 240° C. The temperature ranges stated here are particularly appropriate areas for a highly accurate determination of the soot load based on the determined nitric oxide conversion. The minimum temperature and/or maximum temperature can preferably be variably set in a controller of an internal combustion engine.

An embodiment of the method is also preferred that is characterized in that for determining the soot load from the specific nitric oxide conversion a—particularly current—ratio of a nitrogen dioxide concentration to a total nitric oxide concentration in the exhaust gas is used upstream of the particle filter that is the sum of the nitrogen dioxide concentration and a nitrogen monoxide (NO) concentration. It has been shown that the dependence of the nitric oxide conversion on the soot load itself additionally depends on the ratio of the nitrogen dioxide concentration to the total nitrogen oxide concentration in the exhaust gas. In particular, the nitric oxide conversion rises strongly with an increasing ratio of nitrogen dioxide to total nitrogen oxide with rising soot load. A very precise evaluation of the soot load based on the nitric oxide conversion can therefore be performed if the ratio of nitrogen dioxide to total nitrogen oxide in the exhaust gas is considered.

An embodiment of the method is also preferred that is characterized in that the soot load is determined if the—in particular current—ratio of the nitrogen dioxide concentration to the total nitrogen oxide concentration in the exhaust gas upstream of the particle filter is greater than a predetermined minimum value. This can be defined such that at any rate a very accurate determination of the soot load is possible based on the nitric oxide conversion. Since the sensitivity of the method rises with the rising ratio of nitrogen dioxide to total nitrogen oxide, a very high accuracy can be assured through a suitable definition of the predetermined minimum value. An embodiment of the method is preferred in which the predetermined minimum value is from at least 10% to at most 50%, preferably from at least 20% to at most 40%, and particularly preferably from at least 30% to at most 50%. In these ranges, a highly accurate evaluation of the soot load depending on the predetermined nitrogen oxygen conversion is guaranteed.

An embodiment of the method is preferred that is characterized in that the nitrogen oxygen conversion at the particle filter is determined by means of nitric oxide sensors arranged upstream and downstream of the particle filter. Such sensors are preferably provided in the exhaust tract anyway, so that no additional, expensive elements are needed for performing the method. The method can therefore be performed very cost-effectively. At the same time, the nitric oxide conversion at the particle filter can be determined very accurately with the aid of sensors provided upstream and downstream of it. In this case, a differential measurement of the total nitrogen oxide concentration in the exhaust gas between the measuring point upstream and the measuring point downstream of the particle filter is preferably carried out. The nitric oxide conversion can thus be determined very simply and very precisely at the same time.

A nitric oxide sensor arranged downstream of the particle filter can also be arranged downstream of an additional SCR catalytic converter without particle filter function provided downstream of the particle filter.

An embodiment of the method is also preferred that is characterized in that the ratio of the nitrogen dioxide concentration to the total nitrogen oxide concentration in the exhaust gas is determined upstream of the particle filter based on the—in particular current—exhaust gas temperature upstream of the particle filter or upstream of a—in particular arranged upstream of the particle filter—oxidation catalyst, an exhaust gas mass flow over the oxidation catalyst, and/or an aging state of the oxidation catalyst. Especially preferably, the ratio of nitrogen dioxide to total nitrogen oxide is read depending on at least one of the named parameters from the characterization field. This is particularly preferably stored in a control device of the internal combustion engine that has the exhaust tract in which the method is performed. Each of the parameters named here by itself characterizes the method addressed here, where in particular the named parameters together enable a very precise determination of the ratio so that they are suited to spanning a characteristic field for the ratio.

An embodiment of the method is also preferred that is characterized in that the soot load is compared by comparison of the determined nitric oxide conversion with a predetermined nitric oxide conversion of the unloaded particle filter; i.e., without soot load. The method thus performed achieves a particularly high accuracy, as was already explained in greater detail above. The predetermined nitric oxide conversion of the unloaded particle filter is preferably determined depending on an exhaust gas mass flow over the particle filter, the exhaust gas temperature upstream of the particle filter, the ratio of nitrogen dioxide to total nitrogen oxide upstream of the particle filter, a reducing agent load, in particular an ammonium (NH3) load, an aging status of the particle filter and/or an operating point of an internal combustion engine, the exhaust tract of which has the particle filter. Particularly preferably, the predetermined nitric oxide conversion is read depending on at least one of the parameters here from a characterization field stored in particular in a control device for the internal combustion engine. Each of the parameters here by itself is already characteristic for the predetermined nitric oxide conversion, but in particular, the parameters named here in combination with each other are characteristic for the predetermined nitric oxide conversion, so that they are particularly suited to spanning a characterization field for the predetermined nitric oxide conversion. In particular, preferably the nitric oxide conversion of the particle filter not loaded with soot depending on the exhaust gas mass flow over the particle filter, the temperature upstream of the particle filter, the nitrogen dioxide to nitric oxide ratio before the particle filter, the reducing agent load of the particle filter, and the aging status of the particle filter are stored in the control device and read based on these factors for the current operating state of the internal combustion engine. The reducing agent load of the particle filter is preferably determined with the aid of a suitable model, in particular calculated.

At least one of the aging statuses of the oxidation catalyst and/or the particle filter is preferably determined as aging factor, and in particular dependent on the factors of temperature over time. In particular, a temperature-time integral can be formed for at least one of the named exhaust aftertreatment elements from which an aging status is then determined.

An embodiment of the method is also preferred that is characterized in that the soot load of the particle filter, in addition to the procedure previously described, is determined in the method by means of a load model and also additionally based on a differential pressure falling over the particle filter, in particular through a differential pressure model. There are then three options available for determining the soot load, which can be used alternatingly for correction of the determined soot load and thus in combination with each other and assure a particularly high accuracy of the determination of the soot load. A regeneration of the particle filter is preferably performed if one of the determination methods returns a soot load as the result that reaches or exceeds a predetermined maximum value for the soot load. This can guarantee that the particle filter is always regenerated if this is indicated.

An embodiment of the method is also preferred that is characterized in that the soot load is determined based on the nitric oxide conversion if a first load value for the soot load determined based on the load model and a second load value for the soot load determined based on the differential pressure have a difference that reaches or exceeds a predetermined difference threshold. The very precise method for determining the soot load through the determined nitric oxide conversion described here is therefore preferably used if the two other determination methods yield sharply differing results, where this is an indication that at least one of the determination methods currently yields no reliable load values. In a preferred embodiment of the method, the soot load determined based on the nitric oxide conversion is preferably used for correction of the first and/or the second load value and/or for correction of the determination methods on which these load values are based. This is based on the idea that the soot load determined using the method from the nitric oxide conversion—at least in particular operating ranges—is the most accurate possible soot load determinable. This can thus be used in a particularly favorable way for correction of the load values otherwise determined and/or the determination methods for determining these load values. It is possible that a regeneration of the particle filter is postponed even though this is indicated by one of the models, if the method according to which the soot load is determined based on the nitric oxide conversion determined indicates no regeneration. In this case, the determination method indicating a regeneration and/or the load value determined based on this determination method is instead corrected with the aid of the load value determined by the method described here.

Last of all, an embodiment of the method is preferred that is characterized in that conditions are deliberately set by control of an exhaust gas recirculation device and/or by active heating of an oxidation catalyst under which the soot load of the particle filter can be determined from the determined nitric oxide conversion. An exhaust gas recirculation device is preferably used that has a high-pressure exhaust gas recirculation line and a low-pressure exhaust gas recirculation line. A strategy for controlling the exhaust gas recirculation device can then be adapted as needed such that the exhaust gas recirculation occurs completely through the high-pressure exhaust gas recirculation line, thus achieving the best possible nitrogen dioxide formation on the oxidation catalyst. Furthermore, in addition or alternatively to a change of the total exhaust gas recirculation rate and/or by active heating of the oxidation catalyst, in particular by means of an electric heating device, a desired temperature range for the exhaust gas temperature can be specifically set. Overall, it is thus possible to create conditions regarding the exhaust gas temperature or regarding the ratio of nitrogen dioxide to total nitrogen oxide in the exhaust gas in which a very accurate determination of the soot load is possible with the aid of the method described here.

The deliberate setting of such favorable conditions is preferably done in particular if the two other determination methods, i.e., in particular the load model and the differential pressure model, yield sharply different load values, where a difference between the load values preferably reaches or exceeds a predetermined difference threshold.

The invention also includes a control device for an internal combustion engine, which is designed for performing a preferred embodiment of the method. It is possible that the control device is the engine control unit (ECU) of the internal combustion engine. Alternatively, it is also possible that a separate control device is provided for performing the method.

It is possible that the method is implemented directly in an electronic structure, in particular a hardware, of the control device. Alternatively or additionally, it is possible that certain procedural steps, preferably the entire method, are present as a computer program product. In this respect, preferably a computer program product is loaded into the control device that has steps based on which an embodiment of the method is performed if the computer program product runs on the control device.

The invention also includes an internal combustion engine, adapted to perform an embodiment of the method and/or having a control device that is designed for performing the method. In particular, an exhaust tract with at least one of the exhaust aftertreatment elements described in the method is assigned to the internal combustion engine.

Finally, the invention includes also a motor vehicle having an internal combustion engine according to any one of the embodiments described above. The motor vehicle is preferably configured as a passenger car, truck or commercial vehicle.

The invention is described in more detail below with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an internal combustion engine with exhaust tract, for which an embodiment of the method for determining a soot load is feasible;

FIG. 2 is a schematic representation of the connection between the nitric oxide conversion in a particle filter provided with a selective catalytic coating and the soot load of same depending on the temperature;

FIG. 3 is a schematic representation of the connection between the nitric oxide conversion and the soot load depending on a ratio of a nitrogen dioxide concentration to a total nitrogen oxide concentration in the exhaust gas upstream of the particle filter, and

FIG. 4 is a schematic representation of an embodiment of the method.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of an embodiment of an internal combustion engine 1 with an exhaust tract 3, for which the method described here is preferably performed. The exhaust tract 3 comprises a particle filter 5 provided with a selective catalytic coating arranged to filter soot particles from the exhaust stream of the internal combustion engine 1 and selectively catalytically reduce nitric oxides present in the exhaust gas. Such a particle filter 5 is generally also called SDPF. Downstream of the particle filter 5 in the embodiment presented here is additionally arranged an SCR catalytic converter 7, which is likewise arranged for selective catalytic reduction of nitric oxides but has no particle filter function. Upstream of the particle filter 5 is arranged a dosing device 9 for a reducing agent, which can be dosed into the exhaust stream by means of the dosing device 9, where it is then converted with the nitric oxides on the selective catalytic coating of the particle filter 5 and the SCR catalytic converter 7, whereby the nitric oxides are reduced to nitrogen.

As the reducing agent, a urea-water solution is preferably dosed through the dosing device 9 into the exhaust stream, wherein the urea reacts with the hot exhaust gas and decomposes with formation of ammonium, which then acts as the actual reducing agent in the particle filter 5 and preferably in the SCR catalytic converter 7.

To be able to determine the nitric oxide conversion in the particle filter 5, in the embodiment presented here upstream of the dosing device 9 is arranged a first nitric oxide sensor 11, where directly downstream of the particle filter 5—here in particular between the particle filter 5 and the SCR catalytic converter 7—is arranged a second nitric oxide sensor 13. The second nitric oxide sensor 13 can also be arranged downstream of the SCR catalytic converter 7. A current absolute or relative nitric oxide conversion in the particle filter 5 can be determined through a differential measurement of the signals of the two nitric oxide sensors 11, 13, in particular taking into consideration the signal of the first nitric oxide sensor 11.

In the embodiment shown here, an oxidation catalyst 15 is arranged upstream of the particle filter 5, and in particular also upstream of the dosing device 9. Instead of the oxidation catalyst 15, or in addition to the oxidation catalyst 15, it is possible that upstream of the particle filter 5, and in particular upstream of the dosing device 9, is arranged a nitric oxide storage catalyst (NSC).

The oxidation catalyst 15 has in this case a heating device 17 which is preferably designed as an electrical heating device.

The internal combustion engine 1 also has a charge air line 19 through which charge air can be fed to it. In an inherently known manner, a compressor 21 is arranged in the charge air line that can be driven by a turbine 23 arranged in the exhaust tract 3. To this extent, a turbocharger device is realized with the embodiment shown here.

The embodiment shown also has an exhaust gas recirculation device 25, which here has a high-pressure exhaust gas recirculation line 27 and a low-pressure exhaust gas recirculation line 29. The high-pressure exhaust gas recirculation line 27 upstream of the turbine 23 branches off from the high-pressure section of the exhaust tract 3, entering the high-pressure section of the charge air line 19 downstream of the compressor 21. The low-pressure exhaust gas recirculation line 19 branches downstream of the SCR catalytic converter 7 from the low-pressure section of the exhaust tract 3 and enters the low-pressure section of the charge air line 19 upstream of the compressor 21.

In the method, the heating device 17 is preferably deliberately controlled to bring the exhaust gas temperature into a range in which the method can be performed with high accuracy. Alternatively or additionally, the exhaust gas recirculation device 25 is also controlled in a manner creating conditions in which the method can be performed with high accuracy. It is provided in particular that the exhaust gas recirculation overall is shifted to the high-pressure exhaust gas recirculation line 27 to achieve the highest possible nitrogen dioxide formation at the oxidation catalyst 15. Additionally or alternatively, it is possible to specifically influence an overall exhaust gas recirculation rate to bring the exhaust gas temperature—additionally or alternatively to control of the heating device 17—into a range in which the method can be performed especially efficiently and with high accuracy. For determination of the exhaust gas temperature, a temperature sensor not shown in FIG. 1 is also preferably provided with which the exhaust gas temperature can be detected upstream of the particle filter 5.

FIG. 2 shows a schematic, diagrammatic illustration of a dependence of the nitric oxide conversion (NOx)—plotted against the exhaust gas temperature T upstream of particle filter 5—on the soot load of particle filter 5. With a solid curve 31 is shown the nitric oxide conversion at particle filter 5 with a first, higher soot load of particle filter 5, wherein with a second, dashed curve 35 the nitric oxide conversion on particle filter 5 is plotted with a second, lower soot load against the temperature of the exhaust gas before particle filter 5. It is also suggested in FIG. 2 that there is a temperature range between a minimum temperature Tmin and a maximum temperature Tmax in which the method can be applied with particularly high sensitivity, because curves that describe the nitric oxide conversion U(NOx) dependent on the temperature T for differing soot loads of particle filter 5 have comparatively large distances from each other. In particular, there are also no curve intersections in this temperature range so that a clear assignment of the nitric oxide conversion U(NOx) to the soot load is possible. This is especially true when—as is preferably carried out—the current nitric oxide conversion is compared with the reference value for the unloaded particle filter 5 and to this extent with a baseline or base curve.

FIG. 3 shows schematically and diagrammatically the dependency of the nitric oxide conversion U(NOx)—plotted against the temperature T of the exhaust gas upstream of particle filter 5—on a ratio of a nitrogen dioxide concentration to a total nitric oxide concentration in the exhaust gas. In the diagram of FIG. 5 a first pair of curves 35 is drawn showing nitric oxide conversions with a first, higher ratio of nitrogen dioxide to total nitric oxide. A first curve 37, presented solid, shows the temperature dependency of the nitric oxide conversion for a first, higher soot load of particle filter 5, and a second, dot-dashed curve 39 shows this progression for a second, lower soot load of particle filter 5. A second pair of curves 39 is shown according to a second, lower ratio of nitrogen dioxide to total nitric oxide in the exhaust gas. This second pair of curves 39 has a third, dashed curve 41 showing the nitric oxide conversion depending on the temperature for the first, larger soot load of particle filter 5, and the second pair of curves 39 has a fourth, dotted curve 43 showing the nitric oxide conversion depending on the temperature for the second, lower soot load of particle filter 5. The pairs of curves 35, 39 are based on identical first and second soot loads. It is immediately clear from FIG. 3 that there is a clearer separation of the nitric oxide conversion depending on the soot load of particle filter 5 if the ratio of nitrogen dioxide to total nitric oxide is higher. The method can be performed with particularly high accuracy given a comparatively high ratio of nitrogen oxide to total nitric oxide, especially if a predetermined minimum value for the ratio is exceeded. This is between at least 10% to at most 50%, preferably between at least 20% to at most 40%, and especially preferably between at least 30% to at most 50%.

FIG. 4 shows a schematic representation of an embodiment of the method as a flowchart. In the method, the current nitric oxide conversion 45 at the particle filter 5 is preferably determined with the aid of the nitric oxide sensors 11, 13. From the characteristic field 47 is used as reference value the load-free nitric oxide conversion 49 that the particle filter 5 has if it is not loaded with soot, i.e., either new or completely regenerated. The characteristic field 47 is spanned over an aging factor 51 for particle filter 5, the ratio 53 of the nitrogen dioxide concentration to the total nitric oxide concentration upstream of particle filter 5 in the exhaust gas, the temperature 55 upstream of particle filter 5 in the exhaust gas, an exhaust gas mass flow 57 over particle filter 5, and a reducing agent load 59, in particular an ammonia load of particle filter 5.

The load-free nitric oxide conversion 49 is accordingly read depending on the parameters shown on the left of characteristic field 47. Preferably, the aging factor 51 and the ratio 53 of nitrogen dioxide concentration to total nitric acid concentration is dimensionless. The temperature 55 is preferably given in ° C., the exhaust gas mass flow 57 preferably in kg/h, and the reducing agent loading 59 preferably in g.

The aging factor 51 is preferably determined depending on the factors temperature and time, in particular as a temperature-time integral.

The ratio of nitrogen dioxide to total nitric oxide in the exhaust gas is preferably determined based on the temperature of the exhaust gas upstream of the oxidation catalyst 15 or the temperature upstream of particle filter 5, the exhaust gas mass flow over the oxidation catalyst, and the aging status of the oxidation catalyst 15, likewise from a characterization field not shown here. The reducing agent load 59 preferably results from a model calculation.

In a differential element 61, a difference 62 is now preferably formed between the current nitric oxide conversion 45 and the load-free nitric oxide conversion 49. This difference 62 goes into a detection member 63, in which the ratio 53 of the nitrogen dioxide concentration also preferably enters into the total nitric oxide concentration. From this ratio 53 and the difference 62 determined in the difference member 61, the detection member 63 now calculates the current soot load 65 of particle filter 5. This represents a very accurate value for the soot load of particle filter 5 in particular in the temperature range optimal for the method, and with a ratio 53 exceeding the predetermined minimum value. In particular, it is possible to use this value for correction of other determination methods, in particular a soot load model and/or a differential pressure model.

Overall, it is shown that a premature regeneration of particle filter 5 in particular can be avoided with the aid of the method. This extends the regeneration interval of particle filter 5, resulting in a savings of fuel and a lower thermal load on the catalytic coating of the exhaust aftertreatment system.

Method for Determining a Soot Load of a Particle Filter Provided with a Selective Catalytic Coating

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