Method for determining a quality of a reducing agent solution containing ammonia used to reduce nitrite oxide

Abstract

A determination of a quality of a reducing agent solution containing ammonia used to reduce nitrogen oxides involves actuating a metering unit that delivers metered quantities of reducing agent solution into an engine's discharged exhaust gas and determining an efficiency value that is at least correlated with the efficiency of the SCR catalytic converter and comparing the efficiency value to a predefinable limit value. If an efficiency value not corresponding to proper functioning of the SCR exhaust emission control system is identified by the comparison, a switch is made to a diagnostic mode in a second method step. If a predefinable deviation of the efficiency value from a limit value is identified after expiration of the second method step time period, a third method step is performed for conditioning the SCR catalytic converter. A fourth step involves an adaptation mode in which a deviation from the efficiency value is determined.

Claims

1. A method for determining a quality of a reducing agent solution containing ammonia used to reduce nitrogen oxides in an SCR exhaust emission control system of an internal combustion engine, the method comprising: a first method step that is a normal metering mode, which comprises actuating a metering unit to deliver a first target metering quantity of the reducing agent solution into exhaust gas of the internal combustion engine exhaust gas; determining, using a NOx sensor situated downstream from an SCR catalytic converter of the SCR exhaust emission control system and which has a cross-sensitivity to ammonia, a first efficiency value that is at least correlated with the efficiency of the SCR catalytic converter; comparing the determined efficiency value to a first limit value; wherein when the comparison indicates that the SCR exhaust emission control system is not functioning properly, the normal metering mode is terminated and a switch is made to a diagnostic mode in a second method step, wherein the second method step involves actuating the metering unit to deliver a second target metering quantity of reducing agent solution, which is greater than the first metering quantity, for a first time period, determining a second efficiency value, and comparing the determined second efficiency value with a second limit value; wherein when, after the first time period has elapsed, the comparison of the second efficiency value to the second limit value indicates a deviation of the second efficiency value and the second limit value, a third method step is performed in which the SCR catalytic converter is conditioned in such a way that a quantity of ammonia stored in the SCR catalytic converter falls below a predefined storage quantity limit value, and wherein an adaptation mode is switched to in a fourth method step, wherein the fourth method step involves actuating the metering unit to deliver a third target metering quantity of reducing agent solution for a second predefined period of time, determining a third efficiency value, and comparing the third efficiency value with a third limit value, wherein if the comparison of the third efficiency value and third limit value indicates a deviation of the third efficiency value and the third limit value, it is determined that there is a deficient quality reducing agent solution.

2. The method according to claim 1, wherein a target metering quantity is determined using a control unit having a computation model that at least partially describes the SCR exhaust emission control system, wherein the based on the determined target metering quantity predefined values or value ranges of a quantity of ammonia stored in the SCR catalytic converter, or an efficiency of a NOx conversion by the SCR catalytic converter, may be set in a regulated or controlled manner, and limit values for the efficiency value of a properly operating SCR exhaust emission control system may be established.

3. The method according to claim 1, in the diagnostic mode the second target metering quantity is greater than the first target metering quantity by a factor of 1.1 to 5.

4. The method according to claim 2, in the diagnostic mode the second target metering quantity is greater than the first target metering quantity by a factor of 1.1 to 5.

5. The method according to claim 1, wherein the first time period is selected as a function of the second target metering quantity in such a way that a quantity of ammonia metered with the reducing agent solution over the first time period corresponds to a quantity of ammonia that is storable in an SCR catalytic converter under the prevailing conditions with an ammonia storage capacity that is decreased to a tolerance limit.

6. The method according to claim 4, wherein the first time period is selected as a function of the second target metering quantity in such a way that a quantity of ammonia metered with the reducing agent solution over the first time period corresponds to a quantity of ammonia that is storable in an SCR catalytic converter under the prevailing conditions with an ammonia storage capacity that is decreased to a tolerance limit.

7. The method according to claim 1, wherein for the conditioning of the SCR catalytic converter in the third method step a target metering quantity is set that is less than the first target metering quantity or reduced to zero, or an uncontrolled NOx emission is set that is increased compared to normal internal combustion engine operation.

8. The method according to claim 2, wherein for the conditioning of the SCR catalytic converter in the third method step a target metering quantity is set that is less than the first target metering quantity or reduced to zero, or an uncontrolled NOx emission is set that is increased compared to normal internal combustion engine operation.

9. The method according to claim 4, wherein for the conditioning of the SCR catalytic converter in the third method step a target metering quantity is set that is less than the first target metering quantity or reduced to zero, or an uncontrolled NOx emission is set that is increased compared to normal internal combustion engine operation.

10. The method according to claim 6, wherein for the conditioning of the SCR catalytic converter in the third method step a target metering quantity is set that is less than the first target metering quantity or reduced to zero, or an uncontrolled NOx emission is set that is increased compared to normal internal combustion engine operation.

11. The method according to claim 1, wherein conditions for the conditioning of the SCR catalytic converter in the third method step are maintained until a quantity of ammonia stored in the SCR catalytic converter has fallen below a threshold value that is critical for ammonia slip due to a reaction of NOx contained in the SCR catalytic converter with internal combustion engine exhaust gas.

12. The method according to claim 2, wherein conditions for the conditioning of the SCR catalytic converter in the third method step are maintained until a quantity of ammonia stored in the SCR catalytic converter has fallen below a threshold value that is critical for ammonia slip due to a reaction of NOx contained in the SCR catalytic converter with internal combustion engine exhaust gas.

13. The method according to claim 4, wherein conditions for the conditioning of the SCR catalytic converter in the third method step are maintained until a quantity of ammonia stored in the SCR catalytic converter has fallen below a threshold value that is critical for ammonia slip due to a reaction of NOx contained in the SCR catalytic converter with internal combustion engine exhaust gas.

14. The method according to claim 6, wherein conditions for the conditioning of the SCR catalytic converter in the third method step are maintained until a quantity of ammonia stored in the SCR catalytic converter has fallen below a threshold value that is critical for ammonia slip due to a reaction of NOx contained in the SCR catalytic converter with internal combustion engine exhaust gas.

15. The method according to claim 2, wherein an adaptable correction value that corrects the target metering quantity is provided for the controlled or regulated setting of the quantity of ammonia stored in the SCR catalytic converter in the third method step, or the efficiency of the SCR catalytic converter as a result of the target metering quantity determined by the computation model.

16. The method according to claim 4, wherein an adaptable correction value that corrects the target metering quantity is provided for the controlled or regulated setting of the quantity of ammonia stored in the SCR catalytic converter in the third method step, or the efficiency of the SCR catalytic converter as a result of the target metering quantity determined by the computation model.

17. The method according to claim 6, wherein an adaptable correction value that corrects the target metering quantity is provided for the controlled or regulated setting of the quantity of ammonia stored in the SCR catalytic converter in the third method step, or the efficiency of the SCR catalytic converter as a result of the target metering quantity determined by the computation model.

18. The method according to claim 15, wherein in the adaptation mode the third target meter quantity is selected to achieve a predefined target efficiency of the SCR catalytic converter, a value of the correction value is determined which is or would be necessary for a predefined approximation of the efficiency to the target efficiency, and a deficient quality of the reducing agent solution is determined when the determined correction value is outside a predefined target range.

19. The method according to claim 16, wherein in the adaptation mode the third target meter quantity is selected to achieve a predefined target efficiency of the SCR catalytic converter, a value of the correction value is determined which is or would be necessary for a predefined approximation of the efficiency to the target efficiency, and a deficient quality of the reducing agent solution is determined when the determined correction value is outside a predefined target range.

20. The method according to claim 17, wherein in the adaptation mode the third target meter quantity is selected to achieve a predefined target efficiency of the SCR catalytic converter, a value of the correction value is determined which is or would be necessary for a predefined approximation of the efficiency to the target efficiency, and a deficient quality of the reducing agent solution is determined when the determined correction value is outside a predefined target range.

Description

BRIEF DESCRIPTION OF THE DRAWING FIGURES

(1) The figures show the following:

(2) FIG. 1 shows a schematic block diagram of a vehicle internal combustion engine having a connected exhaust emission control system; and

(3) FIGS. 2a and 2b show time diagrams for examples of curves of an efficiency value (FIG. 2a) and of a specified target metering quantity of reducing agent (FIG. 2b).

DETAILED DESCRIPTION

(4) FIG. 1 shows by way of example a schematic block diagram of an internal combustion engine 10 that is preferably designed as a diesel engine, having an associated SCR exhaust emission control system 21. The exhaust gas discharged from the internal combustion engine 10 is received by an exhaust gas line 11 and flows through an SCR catalytic converter 20. On the input side of the SCR catalytic converter 20, a temperature sensor 13 for measuring the exhaust gas temperature is situated in the exhaust gas line 11, and situated further upstream from the temperature sensor 13 is a metering unit 14 for delivering a reducing agent solution containing ammonia into the exhaust gas. An aqueous urea solution having a specified urea concentration of approximately 32 mass percent is preferably used.

(5) Depending on the operating conditions and the design of the SCR catalytic converter 20, the ammonia contained in the reducing agent is stored to a greater or lesser extent in the SCR catalytic converter 20, and reacts there in a selective catalyst reaction with NOx contained in the exhaust gas to form nontoxic products. The reducing agent solution is supplied to the metering unit 14 via a line 22 from a container, not illustrated. NOx sensors 16 and 17 are situated in the exhaust gas line 11, upstream from the metering unit 14 and on the output side of the SCR catalytic converter 20. In particular, the NOx sensor 17 is designed in such a way that it may emit a signal correlated with the NOx concentration and also with the NH.sub.3 concentration of the exhaust gas. The NOx sensors 16, 17 as well as the temperature sensor 13 are connected to a control unit 12 via signal lines 18. The control unit 12 is also connected to the metering unit 14 via a control line 15, and is connected to the internal combustion engine 10 via a bidirectional signal line 19. The metering unit 14 for delivering a metering quantity of the reducing agent solution specified by the control unit 12 is actuated via the control line 15.

(6) The control unit 12 communicates with the internal combustion engine 10 via the signal line 19, and on the one hand may receive information concerning the operating variables of the internal combustion engine, such as a delivered torque or a rotational speed, and on the other hand may set operating variables of the internal combustion engine 10 as needed. The control unit 12 has a computing and memory unit, not illustrated, and may perform complex control and regulation tasks for operating the internal combustion engine 10 and the SCR exhaust emission control system 21 by processing the received signals and accessing stored characteristic maps. In particular, a computation model that at least partially describes the SCR exhaust emission control system 21 is provided, by means of which operating variables of the SCR exhaust emission control system 21 may be detected, processed, and set in a regulated or controlled manner.

(7) It is understood that further components, not illustrated, may be situated in the exhaust gas line 11, such as an additional oxidation catalytic converter or a particle filter, which may be installed in the exhaust gas line 11 downstream or upstream from the SCR catalytic converter 20. In addition, further sensors for exhaust gas components or operating variables may be situated in the exhaust gas line 11 and connected to the control unit 12 for improving the control and regulation behavior.

(8) Preferred procedures for carrying out the method according to the invention are explained in greater detail below with reference to the time diagrams illustrated in FIGS. 2a and 2b. Examples of time curves of an efficiency value of the SCR exhaust emission control system 21 are in FIG. 2a, and a parallel curve of a target metering quantity D, which has been determined by a computation model program running in the control unit 12 and for delivery of which the metering unit 14 has been actuated, is in FIG. 2.

(9) A normal metering mode proceeds in a time range t.sub.0<t<t.sub.1. Model-based NH.sub.3 filling level regulation or model-based efficiency control is preferably carried out in this normal metering mode. In the model-based NH.sub.3 filling level regulation, a model-based value of the NH.sub.3 filling level of the SCR catalytic converter 20 is adjusted to a predefinable target value by feedback control, and a target metering quantity necessary for this purpose is determined, for delivery of which the metering unit 14 is actuated. In the model-based efficiency control, feed-forward control is carried out in which the metering unit 14 is actuated for delivering a target metering quantity D required according to the computation model in order to achieve a predefinable target efficiency of the SCR catalytic converter 20 with regard to its NOx conversion. In both cases, an efficiency ε of the SCR catalytic converter 20 results for the monitored SCR exhaust emission control system 21 and whose course is depicted by way of example in FIG. 2a by a line denoted by reference numeral 30. In the present case, the efficiency ε of the SCR catalytic converter 20 is computed based on NOx and NH.sub.3 concentration values K1 and K2, respectively, determined from signals of the NOx sensors 16 and 17, according to the formula
ε=1−K2/K1  (1),

(10) and is considered to be crucial for the functioning of the SCR exhaust emission control system 21. Instead of the NOx concentration obtained by measurement using the NOx sensor 16 situated upstream from the SCR catalytic converter 20, a NOx concentration that is read out from a characteristic map corresponding to the prevailing operating conditions of the internal combustion engines may also be used.

(11) For evaluating the exhaust emission control system 21 with regard to its proper functioning, the determined efficiency ε is compared to a target efficiency ε.sub.target corresponding to the NH.sub.3 filling level that is adjusted during model-based NH.sub.3 filling level regulation or corresponding to the set target metering quantity D that is controlled during the efficiency control. An example of the course of the target efficiency ε.sub.target is illustrated by the dashed line 33 in FIG. 2a. Improper functioning is considered to be present, for example, when the determined efficiency ε deviates from the target efficiency ε.sub.target by more than a predefinable extent over the entirety or the major portion of the interval length t.sub.0<t<t.sub.1. A preferred evaluation criterion is defined according to the formula
|∫(ε.sub.target−ε)*dt|<δ  (2),

(12) in which deviations between the target efficiency ε.sub.target and the efficiency ε are integrated over the interval length t.sub.0 to t.sub.1, and the absolute value is compared to a predefinable limit value δ. The interval length is preferably established based on a predefinable cumulative value of an uncontrolled NOx emission from the internal combustion engine 10 or a cumulative value of a NOx concentration supplied by the NOx sensor 16, or is defined by a fixed time period. It is preferably provided to continuously line up corresponding integration intervals one after the other and thus continuously monitor the SCR exhaust emission control system 21. If the limit value δ is exceeded and therefore the inequality (2) is not satisfied, improper functioning is diagnosed. It is understood that other suitable efficiency values may also be evaluated, and/or conditions for assessing proper functioning of the exhaust emission control system 12, such as for further improved statistical validation, may be additionally provided.

(13) If improper functioning of the SCR exhaust emission control system 21 is identified, the normal metering mode is terminated and a switch is made to a diagnostic mode in which the metering unit 14 is actuated for delivering a target metering quantity D that is increased compared to the normal metering mode. This is illustrated by the abrupt rise in the target metering quantity D at point in time t.sub.1 in the diagram in FIG. 2b. At the same time as the increase in the target metering quantity D, a new, second value of the target efficiency ε.sub.target is established. This value is illustrated in the diagram in FIG. 2a by the curve branch denoted by reference numeral 34, and as a function of the metering quantity D is selected to be much smaller in the diagnostic mode than in the normal metering mode. For overmetering, which in the diagnostic mode is typically set to be 2 to 5 times higher, the limit value for the efficiency value, which in the present case is represented by the target efficiency ε.sub.target, is negative according to formula (1). For the conditions of the diagnostic mode, a time interval t.sub.1<t<t.sub.2 having a predefinable maximum duration is provided. This maximum duration is preferably selected in such a way that that the metered NH.sub.3 quantity at least corresponds to a quantity of NH.sub.3 that is storable in an SCR catalytic converter 20 under the prevailing conditions with an NH.sub.3 storage capacity that is decreased to a tolerance limit.

(14) With regard to the second limit value of the efficiency value that is established in the diagnostic mode as a response to the overmetering, two different and mutually exclusive results are possible. In the first case, the determined efficiency value falls below the second limit value within the time interval t.sub.1<t<t.sub.2, and in the second case this result does not occur; i.e., the determined efficiency ε is above the second limit value, even after the time interval has elapsed. For the further procedure, initially this second case is considered below.

(15) When, as assumed in the present case, the efficiency ε determined in the diagnostic mode does not fall below the second limit value within the time interval t.sub.1<t<t.sub.2, it is assumed that actuating the metering unit 14 for delivering the increased target metering quantity D has not led to the expected result of a correspondingly increased concentration of NH.sub.3 which is measurable downstream from the SCR catalytic converter 20. Consequently, there is already an increased probability that an NH.sub.3 quantity corresponding to the predefined target metering quantity D of the reducing agent solution has not been supplied to the SCR catalytic converter 20, since the reducing agent solution has an impermissibly decreased content of NH.sub.3, and therefore impermissibly decreased quality. In order to verify this situation at this point in time when the circumstances are still subject to uncertainty, it is preferably provided to optionally carry out conditioning of the SCR catalytic converter 20 after the time interval t.sub.1<t<t.sub.2 has elapsed. At least the failure of the efficiency ε, determined according to formula (2), to drop below the second limit value predefined in the diagnostic phase is provided as a necessary trigger criterion for the transition into the conditioning phase. In addition, one or more additional conditions may be provided that likewise must be met for a transition into the conditioning phase. These conditions may include, for example, the presence of a predefinable target temperature range for the SCR catalytic converter 20.

(16) In the conditioning phase, conditions are set in such a way that a quantity of NH.sub.3 supplied by computation in the diagnostic mode to the SCR catalytic converter 20 and stored therein is decreased by reaction with NOx contained in the supplied exhaust gas to the extent that it falls below a predefinable NH.sub.3 storage quantity limit value which in particular is crucial for NH.sub.3 slip. For this purpose, a target metering quantity D that is less than the first target metering quantity D or reduced to zero and/or an uncontrolled NOx emission that is increased compared to normal internal combustion engine operation is/are preferably set. It is particularly preferred to carry out model-based, efficiency-controlled metering of the reducing agent solution, in which a target metering quantity D determined by the computation model is specified in such a way that a predefinable target efficiency ε.sub.target of the SCR catalytic converter 20 may be expected. At the same time, the internal combustion engine operation is preferably adapted in such a way that an increase in uncontrolled NOx emissions results, which may be achieved, for example, by changing an exhaust gas recirculation rate. It is preferred to set an increase in the uncontrolled NOx emissions by a factor of 1.1 to 5 compared to uncontrolled NOx emissions resulting from normal internal combustion engine operation. The duration of the conditioning phase is preferably established by the computation model as a function of the uncontrolled NOx emissions and the set target metering quantity D, so that the NH.sub.3 filling level falls below a threshold value for the NH.sub.3 filling level of the SCR catalytic converter 20 which is crucial for NH.sub.3 slip due to a reaction of stored NH.sub.3 with supplied NOx.

(17) After conditioning of the SCR catalytic converter 20 has concluded, a switch is made into an adaptation mode for a predefinable second time period. Upon entry into the adaptation mode, the emissions of the increased NOx quantity are terminated by a return to normal internal combustion engine operation, and efficiency-controlled metering of the reducing agent solution is preferably carried out. The computation model specifies a target efficiency ε.sub.target for a functional SCR catalytic converter 20, and the metering unit 14 is actuated for delivering the target metering quantity D determined by the computation model as necessary for this target efficiency ε.sub.target. At the same time, the efficiency ε of the SCR catalytic converter 20 is determined, preferably based on an evaluation of formula (1), and is compared to the target efficiency ε.sub.target. If the determined efficiency ε deviates from the target efficiency ε.sub.target by more than a predefinable extent, a deficient, in particular decreased, quality of the reducing agent solution is diagnosed and an appropriate message is output.

(18) In one particularly preferred method variant, an adaptable correction value F is provided for the computation model for the controlled or regulated setting of the target metering quantity D, by means of which the target metering quantity D determined by the computation model is multiplicatively or additively corrected. This correction value is preferably determined in the adaptation mode according to the formula
F=ε.sub.target/ε  (3)
or the formula
ε=ε.sub.target+/−F  (4).

(19) This allows adaptation of the computation model to compensate for drift effects and inaccuracies that are not otherwise captured. If it is determined that the correction value F is outside predefinable limits, a deficient quality of the reducing agent solution is diagnosed.

(20) In addition to determining a deficient quality of the reducing agent solution, further diagnostic results may advantageously be determined using the method according to the invention. For further explanation, reference is made once again to FIG. 2a. Namely, if it is determined in the diagnostic mode that the efficiency ε falls below the second limit value for the target efficiency ε.sub.target within the time interval t.sub.0<t<t.sub.1, corresponding to the curve branch 32 illustrated by a dotted line, it may be assumed that overmetering has already occurred in the previous normal metering mode; i.e., the SCR catalytic converter 20 has not been able to process the metered NH.sub.3 quantity. This is to be interpreted as increased NH.sub.3 slip that triggers the error, and which further intensifies for the increased target metering quantity D which is set in the diagnostic mode. Possible reasons are an actual impermissibly increased metering quantity in the normal metering mode, or an impermissibly aged SCR catalytic converter 20 having correspondingly decreased NH.sub.3 storage capacity.

(21) For further verification, if the value falls below the predefinable second limit value it is preferably provided to immediately terminate the diagnostic mode and carry out conditioning of the SCR catalytic converter 20 analogously to the above-described procedure. If it is determined in a subsequent filling level-regulated or efficiency-controlled metering mode that the efficiency value does not meet predefinable conditions corresponding to a properly operating SCR exhaust emission control system 21, in particular that the catalytic converter efficiency ε has fallen below a corresponding predefined limit, an impermissibly aged SCR catalytic converter 20 is deduced and an appropriate warning message is output. On the other hand, if the efficiency limit has been reached or exceeded, the SCR catalytic converter 20 is considered to be operating properly and is switched into an adaptation mode analogously to the above-described procedure. A determined correction value F for the target metering quantity D is preferably taken over by the computation model, and the further metering is carried out using the updated correction value F.

(22) The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.