METHOD, COMPUTING UNIT AND COMPUTER PROGRAM FOR OPERATING AN SCR CATALYTIC CONVERTER

Abstract

A method for operating an SCR catalytic converter in an exhaust gas system of an internal combustion engine with ammonia dosing upstream of the catalytic converter. The method includes: determining, on the basis of a catalytic converter model, the efficiency of nitrogen oxide conversion in the catalytic converter; determining an ammonia fill level in the catalytic converter; determining a nominal ammonia fill level in the catalytic converter, based on the determined efficiency and a pre-definable target nitrogen oxide conversion; and controlling the ammonia dosing depending on the nominal ammonia fill level and the ammonia fill level.

Claims

1. A method for operating an SCR catalytic converter in an exhaust gas system of an internal combustion engine with ammonia dosing upstream of the catalytic converter, the method comprising the following steps: determining, based on a catalytic converter model, an efficiency of nitrogen oxide conversion in the catalytic converter; determining an ammonia fill level in the catalytic converter; determining a nominal ammonia fill level in the catalytic converter, based on the determined efficiency and a pre-definable target nitrogen oxide conversion; and controlling the ammonia dosing depending on the nominal ammonia fill level and the ammonia fill level.

2. The method as recited in claim 1, wherein the catalytic converter model takes into account at least one temperature of the catalytic converter, an exhaust gas mass flow rate, a catalytic converter aging parameter, and an amount of ammonia added via the ammonia dosing.

3. The method as recited in claim 1, further comprising: determining the ammonia fill level and/or the nominal ammonia fill level for each of a plurality of zones of the catalytic converter.

4. A computing unit configured to operate an SCR catalytic converter in an exhaust gas system of an internal combustion engine with ammonia dosing upstream of the catalytic converter, the computing unit configured to: determine, based on a catalytic converter model, an efficiency of nitrogen oxide conversion in the catalytic converter; determine an ammonia fill level in the catalytic converter; determine a nominal ammonia fill level in the catalytic converter, based on the determined efficiency and a pre-definable target nitrogen oxide conversion; and control the ammonia dosing depending on the nominal ammonia fill level and the ammonia fill level.

5. A non-transitory machine-readable storage medium on which is stored a computer program for operating an SCR catalytic converter in an exhaust gas system of an internal combustion engine with ammonia dosing upstream of the catalytic converter, the computer program, when executed by a computer, causing the computer to perform the following steps: determining, based on a catalytic converter model, an efficiency of nitrogen oxide conversion in the catalytic converter; determining an ammonia fill level in the catalytic converter; determining a nominal ammonia fill level in the catalytic converter, based on the determined efficiency and a pre-definable target nitrogen oxide conversion; and controlling the ammonia dosing depending on the nominal ammonia fill level and the ammonia fill level.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG. 1 shows an example of a motor vehicle, which may be used within the context of the present invention, in a schematic illustration.

[0025] FIG. 2 schematically shows an advantageous configuration of a method according to the present invention in the form of a highly simplified flow chart.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0026] In FIG. 1, an example of a motor vehicle, as may be used within the context of the present invention, is schematically illustrated and is denoted as a whole by 100. The vehicle 100 comprises an internal combustion engine 110, for example with six cylinders shown here, an exhaust gas system 120, which has a plurality of purification components 122, 124, e.g. catalytic converters and/or particulate filters, wheels 140 driven by the internal combustion engine 110, and also a computing unit 130, which is designed to control the internal combustion engine 110 and exhaust gas system and is connected in a data-conducting manner thereto. Furthermore, the computing unit 130 in the illustrated example is connected in a data-conducting manner to sensors 112, 127, which record operating parameters of the internal combustion engine 110 and/or the exhaust gas system 120. It goes without saying that further sensors may be present, which are not illustrated.

[0027] Within the context of the further description, it is assumed that the purification components 122, 124 are a combined oxidation catalytic converter with particulate filter 122 and an SCR catalytic converter 124. An inlet of a secondary air system 121 may be provided upstream of the oxidation catalytic converter/particulate filter 122, it being possible to add air to the exhaust gas system 120 via the said inlet, for example for regeneration of the particulate filter 122. This inlet of the secondary air system may also be omitted, in particular in the case of lean engines, since, in such a case, sufficient oxygen for combustion of soot particles is generally contained in the exhaust gas generated by the internal combustion engine 110.

[0028] A reduction-agent dosing device 123 is provided upstream of the SCR catalytic converter. This dosing device may be designed for example for urea-solution dosing, ammonia being produced from the urea solution at high temperatures. The dosing device 123 is therefore also referred to simply as ammonia dosing 123.

[0029] A second catalytic converter (not shown) may be provided downstream of the first catalytic converter 124 in the exhaust gas system 120 in order to adsorb and convert ammonia from the ammonia slip of the catalytic converter 124. For cost reasons, single ammonia dosing 123 may be provided upstream of the first SCR catalytic converter 124 in order to feed the urea solution into the exhaust gas system. In such a configuration, the second SCR catalytic converter is filled only with ammonia slip from the first SCR catalytic converter 124.

[0030] The guidelines for on-board diagnostics (OBD) stipulate that SCR catalytic converters which are present must be monitored. To this end, a nitrogen oxide sensor is generally present downstream of each SCR catalytic converter. The sensor data may be used to model the fill level of the SCR catalytic converters.

[0031] However, the physical fill levels may differ significantly from the modeled fill levels, e.g. in the event of deviations from the modeled aging of the SCR catalytic converters. This may lead to changes in the effectiveness of the nitrogen oxide reduction and therefore possibly to the emission limits being exceeded. This may be improved by an advantageous configuration of a method according to the present invention, as shown in FIG. 2.

[0032] In FIG. 2, an advantageous configuration of a method according to the present invention is schematically illustrated in the form of a highly simplified flow chart and is denoted as a whole by 200. The method serves for operating an SCR catalytic converter and is also described below with reference to FIG. 1.

[0033] In a first step 210 of the method 200, a current ammonia fill level of the SCR catalytic converter 124 is determined. To this end, for example, dosing amounts of the ammonia dosing 123 and/or data of one or more nitrogen oxide sensors 127 may be used upstream (not shown) and/or downstream of the catalytic converter 124 and, for example, offset against each other.

[0034] In a second step 220 of the method, which may also be carried out in parallel with the first step 210, the efficiency of the nitrogen oxide conversion in the catalytic converter 124 is determined. To this end, a physical model of the catalytic converter 124 is used, in which input variables, such as, for example, catalytic converter temperature, exhaust gas mass flow rate, exhaust gas composition (e.g., lambda value, NO.sub.x sensor signal, . . . ) and, if applicable, further parameters are included. The model may be provided, e.g., as a single-disk model (assuming a homogeneous mix in the catalytic converter 124) or as a multi-disk model (assuming a plurality of zones, each with different ammonia or nitrogen oxide fill levels and/or temperatures within the catalytic converter 124). Such a catalytic converter model may, if applicable, also be used for determining the ammonia fill level in step 210. Suitable catalytic converter models are described, for example, in the from the following articles:

[0035] Sjövall, H.; Blint, R. J.; Olsson, L.: Detailed kinetic modeling of NH3 SCR over Cu-ZSM-6. Applied Catalysis B: Environmental 92 (2009) p. 138-153

[0036] Tronconi, E.; Cavanna, A.; Forzatti, P.: Unsteady Analysis of NO Reduction over Selective Catalytic Reduction-De-NOx Monolith Catalysts. Industrial and engineering chemistry research; Vol 37 Nr. 6 (1998) p. 2341-2349

[0037] Olsson, L.; Sjövall, H.; Blint, R.: A kinetic model for ammonia selective catalytic reduction over Cu-ZSM-5. Applied Catalysis B: Environmental 81 (2008) p. 203-217.

[0038] On the basis of the determined conversion efficiency, a nominal ammonia fill level for the catalytic converter 124 is determined in a subsequent step 230. To this end, the determined efficiency may be compared to a target efficiency or a target conversion, which may be predetermined empirically and/or specified on the basis of legal requirements.

[0039] In a control step 240, depending on the determined nominal fill level and the determined fill level, the ammonia dosing 123 may then be controlled such that the nominal fill level is also actually established in the catalytic converter 124 or the actual fill level approaches the nominal fill level. If, for example, the nominal fill level is higher that the determined current fill level, in step 240, an amount of reducing agent, added to the exhaust gas system 123 via the ammonia dosing 123, may be increased, or vice versa, in step 240.

[0040] The method may return to steps 210 and 220 again after the control step 240.

[0041] It should be emphasized that the described step-by-step procedure serves merely for explanation and should not be construed as restrictive. In fact, the method 200 may also be carried out substantially continuously and some or all of the steps 210 to 240 may be carried out in parallel with each other so long as this is useful and/or possible.