CN109854347B

CN109854347B - Method for correcting modeled ammonia fill levels

Info

Publication number: CN109854347B
Application number: CN201811444942.3A
Authority: CN
Inventors: F.施魏策
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2017-11-30
Filing date: 2018-11-29
Publication date: 2022-04-15
Anticipated expiration: 2038-11-29
Also published as: CN109854347A; KR102660647B1; DE102017221573A1; KR20190064468A

Abstract

The invention relates to a method for correcting a modeled ammonia fill level of a downstream SCR catalyst (22) in an SCR catalytic system (20) having two SCR catalysts (21, 22) arranged one behind the other in an exhaust line (11). The expected mass flow of nitrogen oxides downstream of the downstream SCR catalyst (22) is compared with the mass flow of nitrogen oxides measured downstream of the downstream SCR catalyst (22). And performing correction based on the comparison result.

Description

Method for correcting modeled ammonia fill levels

Technical Field

The invention relates to a method for correcting a modeled ammonia fill level of a downstream SCR catalyst in an SCR catalytic system having two SCR catalysts arranged one behind the other in an exhaust line. Furthermore, the present invention relates to a computer program that executes each step of the method, and a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control device which is provided to carry out the method.

Background

Selective Catalytic Reduction (SCR) by means of ammonia or an aminolysis agent is a promising method for reducing nitrogen oxides in oxygen-rich exhaust gases. The efficiency of an SCR catalyst depends on its temperature, the space velocity of the exhaust gas and, critically, on the filling level of ammonia adsorbed on its surface. In addition to the directly dosed ammonia, the adsorbed ammonia can also be used for reducing nitrogen oxides, as a result of which the efficiency of the SCR catalyst is increased compared to an evacuated catalyst. The storage performance depends on the respective operating temperature of the catalyst. The lower the temperature, the greater the storage capacity.

If the SCR catalyst completely fills its reservoir, ammonia may escape even if no ammonia or aminolysis agent is dosed into the exhaust line in the event of a load transient of the internal combustion engine whose exhaust gas is reduced by means of the SCR catalyst. However, it is inevitably necessary to operate the SCR system at high ammonia fill levels if the highest possible nox conversion is to be achieved. If the temperature of a fully filled SCR catalyst increases due to a load transient of the internal combustion engine, the ammonia storage capacity of the SCR catalyst decreases, resulting in ammonia slip.

The above-mentioned effect is particularly pronounced when the SCR catalyst is installed in the vicinity of the internal combustion engine, so that it reaches its operating temperature quickly after a cold start of the internal combustion engine. A second SCR catalyst can therefore be arranged in the exhaust line downstream of the first SCR catalyst for adsorption and subsequent conversion of ammonia from the ammonia slip of the first catalyst.

The guidelines of on-board diagnostic systems (OBD) require that two SCR catalysts must be monitored. For this purpose, in each case one nitrogen oxide sensor is generally located downstream of the two SCR catalysts. For cost reasons, a dosing valve is usually installed only upstream of the first SCR catalyst to dose the aminolysis reductant solution into the exhaust line. Thus, the ammonia filling of the second SCR catalyst takes place exclusively by ammonia slip from the first SCR catalyst. However, it is also possible to arrange further metering valves between the SCR catalysts. The data of the sensor can be used to model the fill levels of the two SCR catalysts.

However, it may occur that the physical ammonia fill level in the second or downstream SCR catalyst deviates from the modeled ammonia fill level. When the physical ammonia fill level is too low, the nitrogen oxide conversion of the second SCR catalyst decreases and may cause the limit to be exceeded. Although the rapid adaptation method can recalibrate the nox setpoint downstream of the second SCR catalyst, it is still difficult to eliminate the cause of the error, since the adaptation method cannot determine whether the cause of the error should be found in the region of the first SCR catalyst or in the region of the second SCR catalyst.

Disclosure of Invention

The method is used for correcting a modeled ammonia fill level of a downstream SCR catalyst in an SCR catalytic system having two SCR catalysts arranged one after the other in an exhaust line. Here, the upstream SCR catalyst is close to the internal combustion engine and the downstream SCR catalyst is close to the exhaust outlet of the exhaust line. The expected nitrogen oxide mass flow downstream of the downstream SCR catalyst is compared to the nitrogen oxide mass flow measured downstream of the downstream SCR catalyst. Based on the comparison result, correction is performed.

In conventional operating strategies, the upstream SCR catalyst has two nominal fill levels. The minimum nominal fill level results in inefficiency of the SCR reaction, but no ammonia slip occurs. The maximum nominal fill level leads to a high nitrogen oxide conversion under conditions which can still be considered as a positive ammonia slip (in particular not exceeding 200 ppm). Operating the upstream SCR catalyst first at the maximum fill level, the SCR efficiency is very high and the resulting ammonia slip is received by the downstream SCR catalyst. In the event of a low nitrogen oxide slip but a high ammonia slip from the upstream catalyst, the ammonia fill level in the downstream SCR catalyst rises rapidly and exceeds the minimum rated fill level of the downstream SCR catalyst. The minimum nominal fill level of the downstream SCR catalyst already provides high nox conversion, but still has the fill capacity for ammonia slip from the upstream SCR catalyst. If the ammonia fill level in the downstream SCR catalyst is above the minimum nominal fill level and below the maximum nominal fill level, the nominal fill level of ammonia in the upstream SCR catalyst is decreased corresponding to the interpolation factor. If the fill level in the second SCR catalyst rises to the maximum fill level or higher, the nominal fill level of ammonia in the upstream SCR catalyst decreases to the minimum fill level. The maximum setpoint filling level in the downstream SCR catalyst is defined here as the normal operation of the motor vehicle in which the SCR catalytic system is arranged in the exhaust line, in which no or only slight ammonia slip occurs from the downstream SCR catalyst. If the SCR catalyst system has, in addition to the dosing valve located upstream of the upstream SCR catalyst, a further dosing valve located between the SCR catalysts, the ammonia filling level in the downstream SCR catalyst is kept at least at its minimum nominal filling level. This is not possible without the second dosing valve under conditions where the upstream SCR catalyst is low (resulting in high ammonia storage capacity).

When the model of the ammonia fill level of the downstream SCR catalyst deviates from reality, it may result, for example, in the ammonia fill level of the downstream SCR catalyst decreasing during an acceleration phase of the motor vehicle. This can be prevented by correction. The expected mass flow of nitrogen oxides downstream of the downstream SCR catalyst is preferably determined by the mass flow of nitrogen oxides between the two SCR catalysts and the efficiency of the downstream SCR catalyst. In this case, the efficiency is derived from a model of the downstream SCR catalyst, so that incorrect model values can be identified. The nitrogen oxide mass flow between the two SCR catalysts is preferably measured by a nitrogen oxide sensor. Furthermore, an ammonia sensor is preferably present between the SCR catalysts in order to be able to measure the ammonia availability into the downstream SCR catalyst and to transfer it to the catalyst model on the one hand and to compensate for the ammonia cross-sensitivity (ammonium triquetrum) of the nitrogen oxide sensor on the other hand. In principle, however, the nitrogen oxide mass flow and the ammonia mass flow between the two SCR catalysts can also be derived from the respective model.

In one embodiment of the method, a correction is made in that, in the joint regulation of the two SCR catalysts, the current ammonia filling level of the downstream SCR catalyst is changed as a function of the difference between the expected nitrogen oxide mass flow and the measured nitrogen oxide mass flow. This is particularly preferred when the downstream SCR catalyst is supplied for a long time almost exclusively by ammonia slip from the upstream SCR catalyst at high catalyst temperatures and when a metering valve possibly present between the two SCR catalysts is hardly in use.

In a further embodiment of the method, a correction is carried out in that, in the joint regulation of the two SCR catalysts, the value of the measured nitrogen oxide mass flow is changed by a correction amount as a function of the difference between the expected nitrogen oxide mass flow and the measured nitrogen oxide mass flow.

In both embodiments of the method, the adjustment is preferably carried out such that the ammonia fill level of the downstream SCR catalyst deviates at most from a predeterminable threshold value with respect to a predetermined temperature-dependent ammonia fill level at which no ammonia escapes at the downstream catalyst. In a conventional operating strategy for an SCR catalytic system, the predetermined temperature-dependent ammonia fill level is the maximum rated fill level for the downstream SCR catalyst.

The computer program is provided to carry out each step of the method, in particular when the computer program runs on a computing device or an electronic control device. To this end, the computer program is stored on a machine-readable storage medium.

By running the computer program on an electronic control unit, an electronic control unit is obtained which is provided for correcting the modeled ammonia fill level of the downstream SCR catalyst by means of the method.

Drawings

Embodiments of the invention are illustrated in the drawings and are further described in the following description.

Fig. 1 schematically shows an SCR catalytic system according to the prior art, in which a modeled ammonia fill level of a downstream SCR catalyst can be corrected by means of a method according to an embodiment of the invention.

Fig. 2 shows a time diagram of the nitrogen oxide mass flow in an SCR catalytic system.

Fig. 3 shows another diagram of the nitrogen oxide mass flow in an SCR catalytic system.

Fig. 4 shows another diagram of the nitrogen oxide mass flow in an SCR catalytic system.

Detailed Description

The internal combustion engine 10 has an SCR catalytic system 20 in its exhaust line 11, as shown in fig. 1. The SCR catalytic system has a first reducing agent dosing unit 41, with which an aqueous urea solution can be injected into the exhaust gas tract 11. At the high temperature of the exhaust gases, ammonia is released from the aqueous urea solution. Downstream of the first reducing agent dosing unit 41, a first or upstream SCR catalyst 21 and a second or downstream SCR catalyst 22 are provided. The catalyst material of the first SCR catalyst is arranged on a particle filter (SCR on filter; SCRF). A first NOx sensor 31 is arranged in the exhaust line 11 upstream of the reducing agent dosing unit 41. The second NOx sensor 32 is arranged between the two SCR catalysts 21, 22. The third NOx sensor is arranged downstream of the second SCR catalyst 22. A second reducing agent dosing unit 42 is arranged between the second NOx sensor 32 and the second SCR catalyst 22. All the NOx sensors 31, 32, 33 transmit their signals to the electronic control device 50. Since the NOx sensors 31, 32, 33 react cross-sensitively to ammonia, their signals are the sum signal of nitrogen oxides and ammonia. A first NOx sensor is arranged upstream of the reducing agent dosing unit 21 in order to reliably measure the amount of nitrogen oxides in the exhaust gas. If the SCR catalytic system 20 is operated such that no ammonia slip occurs at the second SCR catalyst 22, it can be assumed that the signal of the third NOx sensor is based solely on nitrogen oxides. However, since ammonia slip may be provided at the first SCR catalyst 21, supplementing the second SCR catalyst 22 with ammonia as a second reductant dosing unit 42, it is possible for the second NOx sensor to provide a sum signal from ammonia and nitrogen oxides. In order to obtain a nitrogen oxide mass flow q32 in ppm between the two SCR catalysts 21, 22, the ammonia mass flow in ppm in the sum signal must therefore be reduced. This may be determined by a model of the first SCR catalyst or by an ammonia sensor (not shown) between the two SCR catalysts. The reducing agent dosing units 41, 42 likewise forward the amount of ammonia dosed into the exhaust line 11 to the control device 50.

Fig. 2 shows a graph of the nitrogen oxide mass flow q over time t in an operating strategy of the SCR catalytic system 20. The nitrogen oxide mass flow q32 between the two SCR catalysts 21, 22 is determined from the signal of the second NOx sensor 32. The nitrogen oxide mass flow q33 downstream of the second SCR catalyst 22 is determined by a third NOx sensor 33. qmax represents the expected mass flow of nitrogen oxides downstream of the second SCR catalyst 22 when the second SCR catalyst is filled with ammonia at its maximum rated fill level. qmin100 represents the expected nitrogen oxide mass flow at the third NOx sensor 33 when the second SCR catalyst 22 is filled to 100% of its minimum nominal fill level. qmin50 represents the expected mass flow of nitrogen oxides downstream of the second SCR catalyst 22 when its ammonia fill level corresponds to only 50% of its minimum nominal fill level. The expected nitrogen oxide mass flows qmax, qmin100, qmin50 are determined by the nitrogen oxide mass flow 32 and the efficiency of the second SCR catalyst 22. The efficiency is derived from the activation energy of the SCR reaction, the temperature of second SCR catalyst 22, the normalized area coefficient of second SCR catalyst 22, the frequency coefficient of the SCR reaction, the minimum ammonia nominal fill level of second SCR catalyst 22, its maximum ammonia storage capacity, and the residence time of SCR catalyst 22. It can be seen that the measured nitrogen oxide mass flow q33 substantially corresponds to the expected nitrogen oxide mass flow qmin100 for the minimum nominal filling level of the downstream SCR catalyst 22. In the operating state of the SCR catalytic system, in which the ammonia filling level of the downstream SCR catalyst 22 is to be set to its minimum nominal filling level, this curve of the measured nitrogen oxide mass flow q33 shows that the SCR catalytic system is perfectly regulated and the ammonia filling level of the second SCR catalyst 22 corresponds to its modeled value. No correction is required here.

Fig. 3 shows how the ammonia filling level of the second SCR catalyst gradually leaks away (leerlaufen) at high load of the internal combustion engine 10. Starting from a value corresponding to the expected nitrogen oxide mass flow qmin100 (100% of the minimum nominal filling level for the second SCR catalyst 22), the measured nitrogen oxide mass flow q33 gradually drops to a value qmin50 for 50% of the minimum nominal filling level. In an embodiment of the method according to the invention, this tendency can be stopped by identifying the necessity for correcting the modeled ammonia filling level from the difference between the measured nitrogen oxide mass flow q33 and the expected nitrogen oxide mass flow qmin100 and maintaining the ammonia filling level in the second SCR catalyst 22 by the correction amount.

Fig. 4 shows how the nitrogen oxide mass flow q33 measured downstream of the second SCR catalyst 22 changes when the ammonia filling level is reduced from the maximum nominal filling level to the minimum nominal filling level by a regulating intervention in the second SCR catalyst 22. There is an adjustment intervention, for which a negative correction value can be set in an embodiment of the method according to the invention.

Claims

1. Method for correcting a modeled ammonia filling level of a downstream SCR catalyst (22) in an SCR catalytic system (20) having two SCR catalysts (21, 22) arranged one after the other in an exhaust line (11), characterized in that an expected nitrogen oxide mass flow (qmin 100) downstream of the downstream SCR catalyst (22) is compared with a nitrogen oxide mass flow (q 33) measured downstream of the downstream SCR catalyst (22) and a correction is made on the basis of the comparison result, wherein the expected nitrogen oxide mass flow (qmin 100) represents an expected nitrogen oxide mass flow downstream of the downstream SCR catalyst (22) when the downstream SCR catalyst (22) is filled with 100% of its minimum nominal filling level, at which no ammonia slip occurs.

2. Method according to claim 1, characterized in that the expected mass flow (qmin 100) of nitrogen oxides downstream of the downstream SCR catalyst (22) is determined from the mass flow (32) of nitrogen oxides between the two SCR catalysts (21, 22) and the efficiency of the downstream SCR catalyst (22).

3. Method according to claim 1 or 2, characterized in that the correction is made in that in the common regulation of the two SCR catalysts (21, 22) the current ammonia filling level of the downstream SCR catalyst (22) is changed as a function of the difference between the expected nitrogen oxide mass flow (qmin 100) and the measured nitrogen oxide mass flow (q 33).

4. Method according to claim 3, characterized in that the adjustment is made such that the ammonia filling level of the downstream SCR catalyst (22) deviates at most from a predeterminable threshold value with respect to a predetermined temperature-dependent ammonia filling level at which no ammonia slip occurs at the downstream catalyst.

5. Method according to claim 1 or 2, characterized in that the correction is carried out in that, in the common regulation of the two SCR catalysts (21, 22), the value of the measured nitrogen oxide mass flow is changed by a correction amount as a function of the difference between the expected nitrogen oxide mass flow (qmin 100) and the measured nitrogen oxide mass flow (q 33).

6. Method according to claim 5, characterized in that the adjustment is made such that the ammonia filling level of the downstream SCR catalyst (22) deviates at most from a predeterminable threshold value with respect to a predetermined temperature-dependent ammonia filling level at which no ammonia slip occurs at the downstream catalyst.

7. A machine-readable storage medium on which is stored a computer program arranged to perform each step of the method according to any one of claims 1 to 6.

8. An electronic control device (50) which is provided for correcting a modeled ammonia fill level of a downstream SCR catalyst (23) by means of a method according to one of claims 1 to 6.