KR20190064468A - Method for correcting a modeled ammonia filling level - Google Patents

Abstract

The present invention relates to a method for correcting a modeled ammonia filling level of a downstream SCR catalytic converter (22) in an SCR catalytic converter system (20) having two SCR catalytic converters (21, 22) arranged in series in an exhaust gas line (11). An expected nitrogen oxide flow amount of the downstream SCR catalytic converter (22) is compared to a measured nitrogen oxide flow amount of the downstream SCR catalytic converter (22). Correction is performed in accordance with a comparison result.

Description

METHOD FOR CORRECTING A MODELED AMMONIA FILLING LEVEL [0002]

The present invention relates to a method for calibrating the modeled ammonia charge level of a downstream SCR catalytic converter in an SCR catalytic converter system having two SCR catalytic converters arranged in series in the exhaust line. The invention also relates to a computer program for executing each step of the method and to a machine-readable storage medium storing such a computer program. Finally, the present invention relates to an electronic control device configured to execute the method.

A promising way to reduce nitrogen oxides in oxygen-rich exhaust gases is Selective Catalytic Reduction (SCR) using ammonia or ammonia decomposition reactants. The efficiency of an SCR catalytic converter depends on its temperature, the space velocity of the exhaust gas, and very critically, the level of charge of ammonia absorbed on its surface. In addition to the ammonia being directly metered for the reduction of nitrogen oxides, absorbed ammonia is also provided, so that the efficiency of the SCR catalytic converter rises compared to that of an empty catalytic converter. This storage characteristic depends on the operating temperature of each of the catalytic converters. The lower the temperature, the larger the storage capacity.

When the SCR catalytic converter fully charges its reservoir, ammonia slip may occur even when the ammonia or ammonia decomposition reactant is no longer metered into the exhaust line at the time of the load on the internal combustion engine where the exhaust gas is reduced by the SCR catalytic converter have. If the highest possible nitrogen oxide conversion is to be achieved, it is inevitable to operate the SCR system at high ammonia charge levels. When the temperature of the fully charged SCR catalytic converter rises due to the load increase of the internal combustion engine, the ammonia storage capacity is lowered, which causes ammonia slip.

This effect is particularly pronounced when the SCR catalytic converters are mounted close to the internal combustion engine such that the SCR catalytic converter quickly reaches its operating temperature after the cold start of the internal combustion engine. Thus, a second SCR catalytic converter downstream of the first SCR catalytic converter can be provided in the exhaust gas line to absorb ammonia from the ammonia slip of the first catalytic converter and subsequently perform the conversion.

The guidelines for on-board diagnostics (OBD) require that two SCR catalytic converters be monitored. To this end, there is usually one nitrogen oxide sensor downstream of each of the two SCR catalytic converters. For cost reasons, only one metering valve is typically installed upstream of the first SCR catalytic converter for metering the ammonia decomposition reducing agent solution into the exhaust gas line. Thus, the ammonia charging of the second SCR catalytic converter is only carried out through the ammonia slip of the first SCR catalytic converter. However, additional metering valves may be arranged between the SCR catalytic converters. The data of the sensors can be used to model the charge level of the two SCR catalytic converters.

Nevertheless, the physical ammonia charge level in the second or downstream SCR catalytic converter may differ from the modeled ammonia charge level. If the physical ammonia charge level is too low, the nitrogen oxide conversion of the second SCR catalytic converter is reduced, which may lead to an exceeding of the limit value. Although the fast adaptation method can again correct the nitrogen oxide setpoint on the downstream side of the second SCR catalytic converter, it is difficult to eliminate the cause of the error because in this adaptation method the cause of the error can be found in the region of the first SCR catalytic converter Or whether it can be found in the region of the second SCR catalytic converter.

The method of the present invention is used to calibrate the modeled ammonia charge level of a downstream SCR catalytic converter in an SCR catalytic converter system with two SCR catalytic converters arranged in series in the exhaust line. In this case, the upstream SCR catalytic converter is directed to the internal combustion engine, and the downstream SCR catalytic converter is directed to the exhaust portion of the exhaust gas line. The expected amount of NO x flow downstream of the downstream SCR catalytic converter is compared to the amount of NO x flow measured downstream of the downstream SCR catalytic converter. The correction is performed by the result of the comparison.

The upstream SCR catalytic converter has two set charge levels in a conventional operating strategy. The minimum set charge level causes low efficiency of the SCR reaction, but no ammonia slip. The maximum set charge level causes high nitrogen oxide conversion, with an acceptable ammonia slip not exceeding 200 ppm in particular. First, when the upstream SCR catalytic converter is operating at the maximum charge level, the SCR efficiency is very high and the resulting ammonia slip is received by the downstream SCR catalytic converter. If the nitrogen oxide slip from the upstream catalytic converter is low but the ammonia slip is high, the ammonia charge level in the downstream SCR catalytic converter rises rapidly beyond the minimum set charge level of the downstream SCR catalytic converter. The minimum set charge level of the downstream SCR catalytic converter already provides a high nitrogen oxide conversion but still has a charge level capacity for the ammonia slip from the upstream SCR catalytic converter. If the ammonia charge level in the downstream SCR catalytic converter exceeds the minimum set charge level and falls below the maximum set charge level, the ammonia set charge level in the upstream SCR catalytic converter decreases correspondingly to the interpolation coefficient. When the charge level in the second SCR catalytic converter rises to or above the maximum charge level, the ammonia set charge level in the upstream SCR catalytic converter decreases to the minimum charge level. In this case, the maximum set charge level in the downstream SCR catalytic converter is such that, during steady running operation of the vehicle with the exhaust gas line in which the SCR catalytic converter system is arranged, no ammonia slip occurs or less ammonia slip from the downstream SCR catalytic converter Only ammonia slip is specified to occur. If the SCR catalytic converter system includes an additional metering valve between the SCR catalytic converters, in addition to the metering valve upstream of the upstream SCR catalytic converter, the ammonia charge level in the downstream SCR catalytic converter is maintained at at least its minimum set charge level . At lower temperatures of the upstream SCR catalytic converter resulting in high ammonia storage capacity, this would not be possible without the second metering valve.

If the ammonia charge level model of the downstream SCR catalytic converter is different from the actual one, for example, the ammonia charge level of the downstream SCR catalytic converter may decrease during the acceleration phase of the vehicle. This can be prevented by correction. The expected amount of NO x flow downstream of the downstream SCR catalytic converter is preferably calculated from the amount of NO x flow between the two SCR catalytic converters and the efficiency of the downstream SCR catalytic converter. In this case, since the efficiency is obtained from the model of the downstream SCR catalytic converter, the model value with an error can be recognized. The amount of nitrogen oxide flow between the two SCR catalytic converters is preferably measured by a nitrogen oxide sensor. On the one hand, it is also possible to measure the ammonia inflow into the downstream SCR catalytic converter and to compensate for the ammonia cross sensitivity of the nitrogen oxide sensor, in order to feed this inflow into the catalytic converter model and, on the other hand, There is preferably an ammonia sensor between the SCR catalytic converters. However, basically, the nitrogen oxide flow amount and the ammonia flow amount between two SCR catalytic converters may be extracted from one model, respectively.

In one embodiment of the invention, the correction is carried out by varying the actual ammonia charge level of the downstream SCR catalytic converter in the joint control of the two SCR catalytic converters with a deviation between the anticipated nitrogen oxide flow rate and the measured nitrogen oxide flow rate do. This is especially true when the downstream SCR catalytic converter is only supplied for a considerably longer time by the ammonia slip from the upstream SCR catalytic converter when the catalytic converter temperature is high and the metering valve, which may be present between the two SCR catalytic converters, Is preferable.

In another embodiment of the invention, the correction is carried out by joint control of the two SCR catalytic converters by changing the value of the measured nitrogen oxide flow rate by a correction amount according to the deviation between the expected nitrogen oxide flow rate and the measured nitrogen oxide flow rate do.

In both embodiments of the method, the control is preferably carried out such that the ammonia charge level of the downstream SCR catalytic converter is at most different from the predetermined temperature-dependent ammonia charge level by a predetermined threshold value, The ammonia slip of the downstream side catalytic converter does not occur. This predetermined temperature-dependent ammonia charge level is the maximum set charge level of the downstream SCR catalytic converter in the conventional operating strategy of the SCR catalytic converter system.

A computer program, when executed in a computing device or an electronic control device in particular, is configured to execute each step of the method. To this end, the computer program is stored in a machine-readable storage medium.

The computer program is installed in an electronic control device to obtain an electronic control device configured to correct the modeled ammonia charge level of the downstream SCR catalytic converter by the method.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description.
1 is a schematic diagram of a prior art SCR catalytic converter system in which the modeled ammonia charge level of a downstream SCR catalytic converter can be corrected by a method according to an embodiment of the present invention.
2 is a graph showing a time-dependent curve of the amount of nitrogen oxide flow in the SCR catalytic converter system.
3 is a graph showing another curve of the amount of nitrogen oxide flow in the SCR catalytic converter system.
4 is a graph showing another curve of the amount of nitrogen oxide flow in the SCR catalytic converter system.

The internal combustion engine 10 includes, within its own exhaust line 11, the SCR catalytic converter system 20 shown in Fig. This system includes a first reducing agent metering unit 41 that allows urea aqueous solution to be injected into the exhaust gas line 11. [ From these units ammonia is generated at high exhaust gas temperatures. Downstream of the first reducing agent metering unit 41, a first or upstream SCR catalytic converter 21 and a second or downstream SCR catalytic converter 22 are arranged. The catalytic material of the first SCR catalytic converter is arranged on the particle filter (SCR on filter; SCRF). The first NOx sensor 31 is arranged upstream of the reducing agent metering unit 41 in the exhaust gas line 11. A second NOx sensor 32 is arranged between the two SCR catalytic converters 21,22. And the third NOx sensor is arranged downstream of the second SCR catalytic converter 22. [ A second reducing agent metering unit 42 is arranged between the second NO x sensor 32 and the second SCR catalytic converter 22. All of the NOx sensors 31, 32, and 33 transmit their signals to the electronic control unit 50. Since the NOx sensors 31, 32, and 33 respond to ammonia in a cross-sensitized manner, the signals of these sensors are a sum signal for nitrogen oxides and ammonia. However, since the first NOx sensor is arranged upstream of the reducing agent metering unit 41, such a sensor reliably measures the amount of nitrogen oxides in the exhaust gas. If the SCR catalytic converter system 20 is operated such that ammonia slip does not occur in the second SCR catalytic converter 22, it can be assumed that the signal of the third NOx sensor is based solely on nitrogen oxides. In addition to the second reducing agent metering unit 42, the ammonia slip may be provided in the first SCR catalytic converter 21 to provide ammonia to the second SCR catalytic converter 22, It also provides a sum signal of ammonia and nitrogen oxides. To obtain the nitrogen oxide flow rate (q32) in ppm between the two SCR catalytic converters (21, 22), the sum signal should be reduced by the amount of ammonia flow in ppm units. This can be detected by a model of the first SCR catalytic converter or by an ammonia sensor not shown between the two SCR catalytic converters. The reducing agent metering units 41 and 42 also report the amount of ammonia metered into the exhaust gas line 11 to the controller 50 as well.

2 shows a curve of the nitrogen oxide flow rate q with respect to time t in one operation strategy of the SCR catalytic converter system 20. In Fig. The amount of nitrogen oxide flow (q32) between the two SCR catalytic converters 21, 22 is detected from the signal of the second NOx sensor 32. The downstream NOx flow amount q33 of the second SCR catalytic converter 22 is detected by the third NOx sensor 33. [ quot; qmax "represents an expected amount of nitrogen oxide flow on the downstream side of the second SCR catalytic converter 22 when the second SCR catalytic converter 22 is charged to its maximum set ammonia charge level. "qmin100" represents the expected amount of nitrogen oxide flow in the third NOx sensor 33 when the second SCR catalytic converter 22 is charged to 100% of its minimum set charge level. "qmin50" represents the expected amount of nitrogen oxide flow downstream of the second SCR catalytic converter 22 when the ammonia charge level of the second SCR catalytic converter 22 corresponds to only 50% of its minimum set charge level . Expected nitrogen oxide flow quantities (qmax, qmin100, qmin50) are calculated from the efficiency of the second SCR catalytic converter 22 and the nitrogen oxide flow rate 32. [ This efficiency depends on the activation energy of the SCR reaction; The temperature of the second SCR catalytic converter 22; The normalized area coefficient of the second SCR catalytic converter 22; Frequency coefficient of SCR response; The minimum ammonia set charge level and maximum ammonia storage capacity of the second SCR catalytic converter 22; And the residence time of the SCR catalytic converter 22. It can be seen that the measured nitrogen oxide flow rate (q33) substantially corresponds to the expected nitrogen oxide flow rate (qminlOO) for the minimum set fill level of the downstream SCR catalytic converter (22). This curve of the measured nitrogen oxide flow rate (q33) indicates that in the operating state of the SCR catalytic converter system in which the ammonia charge level of the downstream SCR catalytic converter 22 has to be adjusted to its minimum set charge level, the SCR catalytic converter system Fully adjusted so that the ammonia charge level of the second SCR catalytic converter 22 corresponds to its modeled value. In this case, correction is unnecessary.

FIG. 3 shows how the ammonia charge level of the second SCR catalytic converter gradually escapes when the load of the internal combustion engine 10 is high. Starting from a value corresponding to the expected nitrogen oxide flow rate (qmin100) for 100% of the minimum set fill level of the second SCR catalytic converter 22, the measured nitrogen oxide flow rate (q33) gradually increases to 50 The value is decreased to the value (qmin50) for%. In one embodiment of the method according to the invention, this tendency is recognized from the deviation between the measured nitrogen oxide flow rate (q33) and the expected nitrogen oxide flow rate (qmin100), the need to calibrate the modeled ammonia charge level; The ammonia charge level in the second SCR catalytic converter 22 is maintained through the amount of correction; Can be stopped.

4 shows that when the ammonia charge level drops from the maximum set charge level to the minimum set charge level through control intervention in the second SCR catalytic converter 22, the measured nitrogen oxide flow rate < RTI ID = 0.0 > (q33) changes. In this case, there is a control intervention in which an amount of negative correction can be provided in an embodiment of the method according to the present invention.

Claims (8)

A method for calibrating the modeled ammonia charge level of the downstream SCR catalytic converter 22 in an SCR catalytic converter system 20 with two SCR catalytic converters 21 and 22 arranged in series in the exhaust line 11 As a result,
The expected nitrogen oxide flow amount qmin100 downstream of the downstream SCR catalytic converter 22 is compared with the measured nitrogen oxide flow amount q33 downstream of the downstream SCR catalytic converter 22, ≪ / RTI > wherein the modeled ammonia charge level is calibrated. The system according to claim 1, wherein the expected nitrogen oxide flow rate (qminlOO) downstream of the downstream SCR catalytic converter (22) is determined by the amount of nitrogen oxide flow (q32) between the two SCR catalytic converters ≪ / RTI > 22). ≪ Desc / Clms Page number 20 > 3. The method according to claim 1 or 2, wherein the correction is such that the actual ammonia charge level of the downstream SCR catalytic converter (22) in the joint control of the two SCR catalytic converters (21, 22) ) And the measured nitrogen oxide flow rate (q33). ≪ Desc / Clms Page number 13 > 3. The method according to claim 1 or 2, wherein the correction is such that, in the joint control of the two SCR catalytic converters (21, 22), the value of the measured nitrogen oxide flow is greater than the expected nitrogen oxide flow < Is changed by a correction amount in accordance with a deviation between the oxide flow amount (q33) and the oxide flow amount (q33). 5. The method of claim 3 or 4, wherein the control is performed such that the ammonia charge level of the downstream SCR catalytic converter (22) is at most different from the predetermined temperature dependent ammonia charge level by a predetermined threshold value, Wherein the ammonia slip of the downstream catalytic converter does not occur. A computer program configured to execute each step of the method according to any one of claims 1 to 5. A computer readable storage medium having stored thereon a computer program according to claim 6. An electronic control device (50) configured to correct a modeled ammonia charge level of a downstream SCR catalytic converter (22) by a method according to any one of claims 1 to 5.

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