Method of operating an exhaust gas aftertreatment

Abstract

A method and a device for operating an exhaust gas aftertreatment, wherein a diesel particulate filter is regenerated during the operation, in particular passively regenerated, wherein a corrected differential pressure is calculated from a current differential pressure across the diesel particulate filter at a current exhaust gas volumetric flow rate and with a current correction factor. The current correction factor is determined by determining a lower differential pressure in a predetermined time interval at a defined exhaust gas volumetric flow rate, in particular in a specified exhaust gas volumetric flow rate interval around the defined exhaust gas volumetric flow rate, and comparing the lower differential pressure with a specified current reference value and, depending thereon, calculating a new correction factor or retaining the previous correction factor as the current correction factor.

Claims

1. A method for operating an exhaust gas aftertreatment in which a diesel particulate filter is regenerated during operation, comprising the steps of: measuring a current differential pressure across the diesel particulate filter at a current exhaust gas volumetric flow rate and determining a current correction factor for a differential pressure; and calculating a corrected differential pressure based on the current differential pressure and the current correction factor; and initiating a regeneration of the diesel particulate filter when the corrected differential pressure exceeds a predetermined threshold value by increasing an exhaust gas temperature; wherein the current correction factor is determined at least by the steps of: determining a lower differential pressure in a predetermined time interval at a defined exhaust gas volumetric flow rate; and comparing the lower differential pressure with a specified current reference value, and as a function of that comparison, calculating a new correction factor or retaining the previous correction factor as the current correction factor.

2. The method according to claim 1, wherein the time interval is a specified exhaust gas volumetric flow rate interval around the defined exhaust gas volumetric flow rate.

3. The method according to claim 1, wherein when the lower differential pressure is above the specified current reference value a new correction factor is calculated based on the previous correction factor, wherein the new correction factor is assigned to the current correction factor for correspondence; and when the lower differential pressure is below the specified current reference value the previous correction factor is retained, wherein the current correction factor corresponds to the previous correction factor.

4. The method according to claim 1, wherein the current differential pressure is filtered and/or subjected to a plausibility check and a filtered and/or plausibility-checked value of the current differential pressure is used to determine the correction factor.

5. The method according to claim 4, wherein the filtered and/or plausibility-checked valve of the current differential pressure is used to determine the corrected differential pressure.

6. The method according to claim 1, wherein the lower differential pressure is sent to a confidence check to produce a trustworthy value of the lower differential pressure that is used for the comparison.

7. The method according to claim 6, wherein the lower differential pressure has the trustworthy value if the lower differential pressure is acquired at values of a current exhaust gas volumetric flow rate which are constant as a function of time for a predetermined first confidence period, which values correspond to a value of the defined exhaust gas volumetric flow rate; and/or the lower differential pressure remains essentially unchanged as a function of time for a predetermined second confidence period.

8. The method according to claim 7, wherein the values correspond to values of the current exhaust gas volumetric flow rate which lie in a specified exhaust gas volumetric flow rate interval around the value of the defined exhaust gas volumetric flow rate.

9. The method according to claim 6, wherein the lower differential pressure is a time-wise local minimum and/or a time-wise absolute minimum differential pressure.

10. The method according to claim 1, wherein the specified current reference value is a reference differential pressure value of the diesel particulate filter in a new state or in a preferred load state of the diesel particulate filter.

11. The method according to claim 1, wherein the calculation of the corrected differential pressure based on the current differential pressure and the current correction factor comprises the steps of: calculating a current correction value as a product of the current correction factor times a current exhaust gas volumetric flow rate; and calculating the corrected differential pressure by subtracting the current correction value from the current differential pressure.

12. The method according to claim 1, wherein, if the lower differential pressure is above the specified current reference value, the new correction factor is calculated by adding a defined constant to the previous correction factor.

13. The method according to claim 1, wherein, if the lower differential pressure is above the specified current reference value, a new reference value is calculated that is obtained by adding the current reference value to a product of the current correction factor times the defined exhaust gas volumetric flow rate.

14. The method according to claim 1, wherein the current differential pressure is measured regularly; values of the current differential pressure for determining the lower differential pressure are plotted regularly and the lower differential pressure is determined based on plotted values of the current differential pressure in a predetermined time interval at a defined exhaust gas volumetric flow rate.

15. The method according to claim 14, wherein the current differential pressure is measured continuously and the values of the current differential are plotted continuously.

16. The method according to claim 1, wherein the regeneration includes a thermomanagement.

17. A device for controlling an exhaust gas aftertreatment system with a diesel particulate filter, wherein the device is configured to carry out a method according to claim 1.

18. An exhaust gas aftertreatment system comprising: a diesel particulate filter; and a device according to claim 17.

19. An internal combustion engine comprising: an engine; and an exhaust gas aftertreatment system according to claim 18.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows a schematic diagram of a preferred embodiment of an internal combustion engine with an engine, a charger, and a system for exhaust gas aftertreatment with a diesel particulate filter and a device for passive regeneration of the diesel particulate filter;

(2) FIG. 2 shows a graph of curves of the differential pressure across a diesel particulate filter during an exhaust gas aftertreatment, as shown for an example of the internal combustion engine of FIG. 1, as a function of the service life of the diesel particulate filter;

(3) FIG. 3 shows a graph of differential pressure curves measured during an exhaust gas aftertreatment, shown for an example of the internal combustion engine of FIG. 1 as a function of an exhaust gas volume, wherein the differential pressure curves are measured at various times during the service life of a diesel particulate filter;

(4) FIG. 4 shows a flow chart of an embodiment of a method for operating an exhaust gas aftertreatment system with a basic structure; and

(5) FIG. 5 shows a flow chart of another embodiment of a method for operating an exhaust gas aftertreatment system with an elaborated structure.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIG. 1 shows an internal combustion engine 1000 with an engine 100, a charger 200, and a symbolically indicated exhaust gas aftertreatment system 300 comprising a diesel particulate filter DPF, which can be subjected to thermomanagement measures by means of a control unit GCU for the passive regeneration of the diesel particulate filter DPF. In the present case, the control unit GCU of the exhaust gas aftertreatment is accommodated as a module in a system comprising the exhaust gas aftertreatment system, the diesel particulate filter, and the control unit GCU. In the present case, the control unit for controlling the passive regeneration of the diesel particulate filtersymbolized by the arrow 301is functionally connected to a central control unit ECU of the internal combustion engine 1000 by a data and control bus CAN. The central control unit ECU, furthermore, as symbolically indicated by the arrows 301, 302 is configured to control the engine 100 and the charger. In the present case, the engine 100 is in the form of a diesel engine, the cylinders Z in the engine block being illustrated symbolically only by way of example; the cylinders are supplied with fuel by a common rail system with appropriate injection (not shown).

(7) The charger 200 is connected to the engine block to supply charging air LL and to carry away exhaust gas AG by way of appropriate intake and exhaust manifolds, i.e., manifold 101L in the charging air line and manifold 101A in the exhaust gas line. The charger 200 is formed in the present case by a first charging stage 200I and a second charging stage 200II, providing an appropriate arrangement of turbochargers, comprising compressors 201.1, 202.1 in the charging air LL line and turbines 201.2, 202.2 in the exhaust gas AG line. Downstream from each of the compressors 201.1, 202.1 is a charging air cooler 201.3, 202.3. The various charging stages, compressors, turbines, and coolers can also be described as low-pressure or high-pressure compressors, turbines, and coolers. The internal combustion engine 1000 and the charging system 200 shown here are described only as one example of an internal combustion engine with an exhaust gas aftertreatment system 300 and are provided only to help explain that system.

(8) The concept of the invention also comprises exhaust gas aftertreatment systems for engines 100 without charging or only with a single-stage charger. In the present case, the charger is in fact set up as a two-stage charger for a large diesel engine; the high-pressure stage (second charging stage 200II) can be shut off by means of a waste gate 202.4 in an exhaust gas bypass line 101B. To control the charging, a throttle valve 202.5 is arranged in the charging air line 101L of the internal combustion engine 1000; this valve can be actuated in cooperation with the waste gate 202.4 to control the charging stages 200II, 200I as needed, depending the load state of the engine 100.

(9) In the present case, the internal combustion engine 1000 is also provided with an exhaust gas return system 400, wherein, in the exhaust gas return line 101R, an exhaust gas return valve 401 and an exhaust gas cooler 402 are arranged to treat the returned exhaust gas AG. The charger 200 and the exhaust gas return system 400 are operated as needed by actuation of the exhaust gas return valve 401 and the waste gate 202.4, as symbolized by the arrows 302.

(10) In the following, the behavior of a differential pressure P at the diesel particulate filter DPF as a function of its soot and ash load is described over the course of its service life T_L and as a function of an exhaust gas volumetric flow rate V_AG. It can be seen that the knowledge of these values of a differential pressure P, as realized by the concept, can be used advantageously to provide a superior method and device for controlling the exhaust gas aftertreatment system 300. For the details, reference is made to the description of FIG. 2, FIG. 3 and FIG. 4, as well as to the description of FIGS. 4 and 5.

(11) FIG. 2 shows a graph of differential pressure curves in a diesel particulate filter over the course of the service life T_L of the diesel particulate filter. The differential pressure P is plotted on the vertical axis (ordinate), the service life T_L on the horizontal axis (abscissa). Without any load of soot or ash, the differential pressure P in diesel particulate filters would remain, in theory, constant over the entire service life T_L. This is represented by the curve 110. Under real operating conditions, however, the diesel particulate filter will become loaded with both soot and ash.

(12) In an actively regenerated system, soot is burned off at predetermined intervals, usually with the help of an additional burner or by the post-injections of fuel. The resulting increase in the exhaust gas temperature causes the diesel soot present in the diesel particulate filter to become oxidized by the excess oxygen present in the exhaust gas. This soot burnoff is usually complete. Expressed differently, the data available on the processes which occur in systems with active regeneration show that there is a point in time after the active regeneration at which there is no longer any soot in the diesel particulate filter, as a result of which the effect of the ash is easier to measure. Curve 120 shows the course of the differential pressure in an actively regenerated system of this type. The differential pressure curve in the actively regenerated system shows various minima 121, 122, which symbolize points after a complete soot burnoff. At these points, the diesel particulate filter is free of soot and is loaded only with ash. During the operation of the engine, there are therefore always states (after an active regeneration) for which it is known that there is no longer any soot in the diesel particulate filter. At these times, it is possible to determine the extent to which the presence of ash affects the differential pressure. Accordingly, all these minima lie on the hypothetical curve of the differential pressure over time in the diesel particulate filter for the case of a load consisting purely of ash, without soot, as shown by curve 130. The course of the differential pressure in the diesel particulate filter loaded purely with ash can be determined in the active system on the basis of the differential pressure minima occurring after complete soot burnoff. On the basis of the differential pressure values after complete soot burnoff, it is also possible to obtain the correction value for the differential pressure P based on the ash load.

(13) For a passively regenerating system, there are no predetermined times at which complete soot burnoff takes place. There are only states characterized by both soot and ash. There is therefore no time at which only the influence of ash on the differential pressure P can be measured. In the case of passively regenerating systems, furthermore, it is not known in particular where in the diesel particulate filter the ash has been deposited. Accordingly, the influence of ash on the differential pressure P can be different in each system. The course of the differential pressure in a diesel particulate filter with passive regeneration is shown by curve 140. Because the soot burnoff in the passively regenerating system proceeds continuously rather than cyclically, there are no states in the passively regenerating diesel particulate filter in which it would be possible to measure reliably the differential pressure P attributable purely to ash. Thus there is no simple way to correct the measured differential pressure, as can be done in the active system on the basis of the differential pressure after complete soot burnoff. The threshold value 150 for the differential pressure P indicates the value of the differential pressure P at which an another regeneration of the diesel particulate filter must be started in order to burn off the accumulated soot. Thermomanagement is activated whenever the differential pressure limit 150 is exceeded. Unless the differential pressure is corrected somehow, regeneration will be initiated too often (or continuously). As can be seen from the graph, the ash load causes this threshold value to be reached more quickly than would be case if the filter were loaded only with soot. In the absence of a correction factor ash P_K for the differential pressure P, the additional regeneration step would thus be started too early and unnecessarily. This leads to unnecessary fuel consumption and to an unnecessary load on the diesel particulate filter. According to the present concept, therefore, the measured differential pressure is shifted downward by the ash P correction, that is, by the current correction factor. As a result, only the effect attributable to soot is taken into account. The differential pressure P in the diagram shown is plotted at a defined exhaust gas flow rate.

(14) According to the concept of the invention described here, a correction value for the differential pressure P attributable to the ash load can be determinedin particular for a passively regenerating systemwithout the necessity for periods during the course of operation in which the differential pressure P is influenced only by the ash load, i.e., without any contribution from the soot load.

(15) According to the concept of the invention, this type of correction factor is obtained by determining the lower differential pressure PMIN in a predetermined time interval at a defined exhaust gas volumetric flow rate V_AG, and by comparing this lower differential pressure PMIN with a specified current reference value. If the lower differential pressure PMIN obtained with a previous correction factor exceeds the specified current reference value, a new correction factor is calculated, and the current correction factor will then correspond to this new correction factor; and if the lower differential pressure PMIN falls below the current reference value, the previous correction factor is retained, wherein the current correction factor will then correspond to the previous correction factor. It has been found that a lower differential pressure PMIN can be determined reliably at, for example, the time-wise local minimum 141 and the time-wise local saddle point 142.

(16) FIG. 3 shows a graph of the dependence of the measured differential pressure on the exhaust gas volume at various times during the service life of a diesel particulate filter. The increase in the differential pressure can be illustrated on the basis of the symbolized time axis 240, which, in the present case, extends over the course of a year. At time A on the time axis 240, shown on curve 210, the diesel particulate filter is still almost completely free of ash. In the example shown, the differential pressure P across the diesel particulate filter at time B, symbolized by the curve 220, eight months after time A, has become considerably higher than that at time A. All of the differential pressures shown in this graph were measured in the soot-free state of the diesel particulate filter. Four months later, at time C, shown on curve 230, a further increase in the differential pressure as a result of the additional ash load can be seen. As can be derived from the graph, the increase in the differential pressure caused by ash is proportional to the exhaust gas volume. A regeneration interval, e.g., for a soft or a hard thermomanagement measure, depends on the differential pressure P across the diesel particulate filter. This differential pressure P increases over the life of the diesel particulate filter as a result of ash, as illustrated in FIG. 3. Thus, because the differential pressure P reaches its limit at progressively earlier times, the regeneration interval will become shorter.

(17) If, however, the influence of ash can be determined and/or calculatedas can be done in an especially advantageous manner according to the concept explained herethe regeneration interval can be optimized, e.g., kept constant. This has the result of preventing the unnecessary extra fuel consumption associated with overly frequent regeneration.

(18) FIG. 4 shows a flow chart of a first embodiment of the method according to the invention. It proceeds on the basis of the following ideas: the differential pressure is evaluated over time as a function of the volumetric flow rate; the change in the differential pressure over time is a measure of the ash content; and it should not matter how much ash is deposited or where or what oil is being used.

(19) Accordingly, in step 310, the current differential pressure P is measured as a function of the exhaust gas volumetric flow rate V-AG. The measured values can be previously filtered and possibly limited and subjected to a plausibility check in step 311 to avoid a situation in which the following steps are based on measurement outliers.

(20) On the basis of the measured values, the lower differential pressure, in particular a MIN value, is determined in step 320 for a defined exhaust gas volumetric flow rate in a predetermined time interval. This lower differential pressure P is compared in step 330 with a specified current reference value and, in step 340, a current correction factor is . . . as a function of the result of the comparison between the lower differential pressure P and the reference. Expressed differently, a comparison with the reference (difference) is carried out in step 330 first, and then, in step 340, the slope, i.e., the correction factor, is calculated. The current correction factor is determined in step 340 as follows:

(21) If the lower differential pressure PMIN is greater than the specified current reference value, then, on the basis of the previous correction factor, a new correction factor is calculated, and this new correction factor is used as the current correction factor. If the lower differential pressure P is lower than the current reference value Ref_Value, then the previous correction factor is retained; that is, the current correction factor corresponds to the previous correction factor. One possible way of determining the current correction factor when the lower differential pressure P exceeds the reference value is to add a predetermined constant to the previous correction factor. Another way of calculating the current correction factor comprises the steps: recording the lower differential pressures in a predetermined time interval for various exhaust gas volumetric flow rates; plotting the lower differential pressures as a function of the exhaust gas volumetric flow rates; forming a straight line from the determined points and determining its slope; and finally subtracting the slope of a reference line from this slope.
In an advantageous embodiment of the method according to the invention, in step 350, on the basis of the previously determined current correction factor and the current exhaust gas volumetric flow rate V_AG in question, a correction value for the differential pressure P is determined at the current exhaust gas volumetric flow rate V_AG. This correction value is subtracted in step 360 from the current differential pressure P, and the result of this subtraction is given out as the corrected differential pressure P. In a following step not shown here, this corrected differential pressure P can be compared with a predetermined threshold value, and, if this threshold value is exceeded, an additional regeneration step of the diesel particulate filter is initiated.

(22) FIG. 5 shows a flow chart of another embodiment of the method according to the invention. In addition to the method steps shown in FIG. 4, step 480 is carried out prior to the execution of steps 330-360. In this step, a certain operating time of the diesel particulate filter and a certain number of measurement values must be reached first on the basis of steps 310 and 320, before, in step 481, the lower differential pressure P for a defined exhaust gas volumetric flow rate V_AG in a predetermined time interval is released as trustworthy; simultaneously, in step 482, the lower differential pressure is sent onward for comparison with a current reference value in step 330. Step 480 serves, first, to verify the trustworthiness of the lower differential pressure acquired during the predetermined time interval and, second, as a way of avoiding continuous system corrections, it ensures, through the requirement of a defined operating time, that the adjustment of the correction factor is not carried out too frequently. Step 480 thus preferably comprises a learning completed acknowledgement step (remaining below a certain learning threshold for x hours, plus a certain number of values learned).