Abstract
The invention concerns a method for controlling an SCR catalytic converter. A first modelled level of ammonia (NH3_mod1) and a second modelled level of ammonia (NH3_mod2) of the SCR catalytic converter are determined from two different models. The second modelled level of ammonia (NH3_mod2) is assessed by comparing it with the first modelled level of ammonia (NH3_mod1).
Claims
1. A method for controlling an SCR catalytic converter (21), the method comprising: determining, via a computer, a first modelled level of ammonia (NH3_mod1) and a second modelled level of ammonia (NH3_mod2) of the SCR catalytic converter (21) from two different models, and assessing, via the computer, the second modelled level of ammonia (NH3_mod2) by comparing it with the first modelled level of ammonia (NH3_mod1).
2. The method according to claim 1, wherein the second modelled level of ammonia (NH3_mod2) is further compared during the assessment with a maximum level of ammonia (NH3_max) of the SCR catalytic converter (21), wherein at the maximum level of ammonia (NH3_max) a specifiable ammonia slip occurs at the SCR catalytic converter (21).
3. The method according to claim 1, wherein the first modelled level of ammonia (NH3_mod1) is adjusted to a setpoint value (NH3_min).
4. The method according to claim 1, wherein the first model comprises no controller components and the second model comprises controller components.
5. The method according to claim 4, wherein in the first model a dispensed ammonia amount (D) is reduced by a correction amount (K) and in the second model the dispensed ammonia amount (D) is multiplied by the adjustment factor (a).
6. The method according to claim 1, wherein an interpolation factor (i) of a setpoint efficiency value of the SCR catalytic converter is determined from the result of the assessment.
7. The method according to claim 1, wherein the SCR catalytic converter (21) is part of an SCR catalytic converter system (20) with a plurality of SCR catalytic converters (21, 22), wherein the SCR catalytic converter (21) is the furthest upstream SCR catalytic converter of the SCR catalytic converter system (20).
8. A non-transitory computer-readable medium including a computer program that when executed by a computer cause the computer to determine a first modelled level of ammonia (NH3_mod1) and a second modelled level of ammonia (NH3_mod2) of the SCR catalytic converter (21) from two different models, and assess the second modelled level of ammonia (NH3_mod2) by comparing it with the first modelled level of ammonia (NH3_mod1).
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] An exemplary embodiment of the invention is represented in the drawings and is described in detail in the following description.
[0018] FIG. 1 shows schematically an SCR catalytic converter system in which an SCR catalytic converter can be controlled by means of an exemplary embodiment of the method according to the invention.
[0019] FIG. 2 shows schematically a level of ammonia model without controller components in an exemplary embodiment of the method according to the invention.
[0020] FIG. 3 shows schematically a level of ammonia model with controller components in an exemplary embodiment of the method according to the invention.
[0021] FIG. 4 shows in a plurality of diagrams how a modelled nitrogen oxide signal and measured nitrogen oxide signals change in an exemplary embodiment of the method according to the invention in the event of a change of an SCR catalytic converter temperature and a correction amount against time.
[0022] FIG. 5 shows in a plurality of diagrams how a plurality of modelled levels of ammonia of an SCR catalytic converter, an efficiency factor of a catalytic converter model and an interpolation factor change in an exemplary embodiment of the method according to the invention in the event of the change of an SCR catalytic converter temperature and a correction amount against time.
DETAILED DESCRIPTION
[0023] A combustion engine 10 comprises in the exhaust system 11 thereof an SCR catalytic converter system 20 that is represented in FIG. 1. Said system comprises a reducing agent metering unit 40, with which a urea-water solution can be injected into the exhaust system 11. Ammonia is released from this at the high temperatures of the exhaust gas. A first SCR catalytic converter 21 and a second SCR catalytic converter 22 are disposed downstream of the reducing agent metering unit 40. The catalytic converter material of the first SCR catalytic converter is disposed on a particle filter (SCR on filter; SCRF). A first NOx sensor 31 is disposed in the exhaust system 11 upstream of the sensor reducing agent metering unit 40. A second NOx sensor 32 is disposed between the two SCR catalytic converters 21, 22. A third NOx sensor is disposed downstream of the second SCR catalytic converter 22. All the NOx-sensors 31, 32, 33 pass the signals thereof to an electronic control unit 50.
[0024] In the electronic control unit 50, two different models of the ammonia level of the first SCR catalytic converter 21 are created. The first model, which has no controller components, is schematically represented in FIG. 2. A dispensed amount D is determined from a dispensing demand from the electronic control unit 50 to the dosing valve 40. An amount of ammonia V consumed by the SCR reaction, an ammonia slip S, a correction amount K from a controller and the oxidation O of ammonia in the SCR catalytic converter 21 are subtracted from said dispensed amount of ammonia. This results in a mass balance flow U1 of ammonia through the first SCR catalytic converter 21. The amount of ammonia consumed V corresponds in each case to the nitrogen oxide mass flow multiplied by a stoichiometry factor for the mass difference between nitrogen oxide and ammonia. In this case, the molecular weight of NO.sub.2 is taken as basis for the nitrogen oxides. If there is no ammonia slip at the first SCR catalytic converter 21, then the converted nitrogen oxide mass flow in the first SCR catalytic converter corresponds to the difference of the signals of the first nitrogen oxide sensor 31 and the second nitrogen oxide sensor 32. On the other hand, if there is ammonia slip, then the fact that the second nitrogen oxide sensor 32 reacts to ammonia cross-sensitively is to be taken into account. The respective current level of ammonia in the first SCR catalytic converter 21 is calculated in said model by changing a previous level of ammonia by the mass balance flow U1.
[0025] FIG. 3 shows schematically a second model of the ammonia level in the first SCR catalytic converter 21, which comprises controller components. While controller components are subtracted in the first model as a correction amount, in the second model they are brought into balance by multiplying the dispensed amount D by an adjustment factor a. The amount of ammonia V consumed, the ammonia slip S and the oxidation O of ammonia are subtracted from said product as in the first model. In this way, in turn a mass balance flow U2 is obtained by which a level of ammonia of the first SCR catalytic converter 21 that was previously stored in the second model is changed.
[0026] FIG. 4 shows how in a conventional operation of the first SCR catalytic converter 21 a positive or negative correction amount K is produced by a controller in the event of a change of the temperature T of the first SCR catalytic converter 21 with time t. A nitrogen oxide signal NOx_31 of the first nitrogen oxide sensor 31 indicates the nitrogen oxide emissions of the combustion engine 10. A nitrogen oxide signal NOx_32 of the second nitrogen oxide sensor 32 indicates the amount of nitrogen oxide based on the first SCR catalytic converter 21. A model value NOx_mod of the nitrogen oxide downstream of the first SCR catalytic converter 21 differs from the signal NOx_32 of the second nitrogen oxide sensor 32. The controller seeks to compensate said deviations with the correction amount K. Said control is referred to as a level watcher. If the second nitrogen oxide sensor 32 indicates higher values than the nitrogen oxide model, then the level watcher outputs a positive correction amount in order to fill the level of ammonia in the first SCR catalytic converter 21 and thereby to reset the setpoint emissions of the model.
[0027] FIG. 5 shows how the modelled levels of ammonia of the two models according to FIGS. 2 and 3 are taken into account in an exemplary embodiment of the method according to the invention. The first modelled level of ammonia NH3_mod1 is controlled to the minimum level of ammonia NH3_min of the first SCR catalytic converter 21 as a setpoint value. It contains no controller component of the level watcher in order to be able to compensate the corresponding missing amount in the first SCR catalytic converter 21. On the other hand, the level of ammonia controller component is taken into account in the second modelled level of ammonia NH3_mod2, which results from the second model. A difference between the two modelled levels of ammonia NH3_mod1 and NH3_mod2 results from this. By considering the difference between the second modelled level of ammonia NH3_mod2 and the first modelled level of ammonia NH3_mod1 on the one hand, and on the other hand the difference between the second modelled ammonia level NH3_mod2 and the maximum physical level of ammonia NH3_max, an assessment of the second modelled level of ammonia NH3_mod2 is possible. This is carried out in the present exemplary embodiment by interpolation. This enables the calculation of an efficiency factor w of the catalytic converter model on the one hand, and therefrom the calculation of an interpolation factor i. This can then influence the setpoint efficiency value of the level of ammonia model of the first SCR catalytic converter 21. If in the present exemplary embodiment, chemical reaction kinetics based on the Arrhenius equation are used in the currently usual manner for the SCR reaction, then the interpolation factor directly changes the efficiency the SCR-model there by means of a change of the frequency factor of the nitrogen oxide conversion reaction. In the case of a rising value of the second modelled level of ammonia NH3_mod2, the efficiency requirement is slightly reduced, in the case of a falling value it is accordingly increased. The first SCR catalytic converter 21 is thus always operated at the maximum efficiency of the SCR reaction.
