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

Abstract

A method for determining a soot load on a particle filter provided with a selective catalytic coating is disclosed. The method includes determining a nitric oxide conversion on the particle filter and determining a soot load on the particle filter from the determined nitric oxide conversion.

Claims

1.-10. (canceled)

11. A method for determining a soot load of a particle filter provided with a selective catalytic coating, comprising the steps of: determining a nitric oxide conversion on the particle filter; and determining a soot load of the particle filter from the determined nitric oxide conversion.

12. The method according to claim 11, wherein the soot load is determined if an exhaust gas temperature in an 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 and wherein the predetermined minimum temperature is from at least 175° C. to at most 210° C. and/or the predetermined maximum temperature is from at least 240° C. to at most 280° C.

13. The method according to claim 11, wherein the soot load is determined if an exhaust gas temperature in an exhaust tract upstream of the particle filter is from at least 150° C. to at most 300° C.

14. The method according to claim 11, wherein for the determining the soot load from the determined nitric oxide conversion, a ratio of a nitrogen dioxide concentration to a total nitrogen oxide concentration in exhaust gas upstream of the particle filter is used.

15. The method according to claim 14, wherein the soot load is determined when the ratio of the nitrogen dioxide concentration to the total nitrogen oxide concentration is greater than a predetermined minimum value and wherein the predetermined minimum value is at least 10% to at most 50%.

16. The method according to claim 11, wherein the nitric oxide conversion on the particle filter is determined by respective nitric oxide sensors disposed upstream and downstream of the particle filter.

17. The method according to claim 14, wherein the ratio is determined based on a temperature of the exhaust gas upstream of the particle filter or a temperature of the exhaust gas upstream of an oxidation catalyst, a mass flow of the exhaust gas over the oxidation catalyst, and/or an aging status of the oxidation catalyst.

18. The method according to claim 11, further comprising the step of comparing the determined nitric oxide conversion with a predetermined nitric oxide conversion of an unloaded particle filter, wherein the predetermined nitric oxide conversion is determined depending on an exhaust gas mass flow over the particle filter, an exhaust gas temperature upstream of the particle filter, a ratio of a nitrogen dioxide concentration to a total nitrogen oxide concentration upstream of the particle filter, a reducing agent load of the particle filter, an aging status of the particle filter, and/or on an operating point of an internal combustion engine that has the particle filter.

19. The method according to claim 11, wherein the soot load of the particle filter is additionally determined by a load model and a differential pressure model, wherein a regeneration is performed if one of the soot load determinations returns a soot load that reaches or exceeds a predetermined maximum value.

20. The method according to claim 19, wherein the soot load is determined from the determined nitric oxide conversion if a determined first load value based on the load model and a determined second load value based on the differential pressure model have a difference that reaches or exceeds a predetermined difference threshold.

21. The method according to claim 20, wherein the soot load determined from the determined nitric oxide conversion is used for correction of the first and/or the second load values and/or for correction of at least one of the load model and the differential pressure model.

22. The method according to claim 11, wherein specific conditions are set by controlling an exhaust gas recirculation device and/or by active heating of an oxidation catalyst under which the soot load of the particle filter is determined from the determined nitric oxide conversion.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] 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;

[0028] 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;

[0029] 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

[0030] FIG. 4 is a schematic representation of an embodiment of the method.

DETAILED DESCRIPTION OF THE DRAWINGS

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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).

[0035] The oxidation catalyst 15 has in this case a heating device 17 which is preferably designed as an electrical heating device.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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%.

[0041] 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.

[0042] 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.

[0043] The aging factor 51 is preferably determined depending on the factors temperature and time, in particular as a temperature-time integral.

[0044] 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.

[0045] 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.

[0046] 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.