Method for exhaust gas aftertreatment of an internal combustion engine with at least one SCR catalyst

Abstract

Method for exhaust gas aftertreatment of an internal combustion engine (10) having at least one SCR catalyst (22), wherein the at least one SCR catalyst (22) is divided into at least two virtual bricks, wherein a control of the at least one SCR catalyst (22) is carried out by means of a first control according to a total NH3 target fill level (NH3total), wherein a switchover from the first control to a second control is carried out for the at least one SCR catalyst (22), wherein the second control of an NH3 fill level control is carried out based on a target NH3 fill level (mNH3CatSoll,SCR1B) of the first virtual brick of the at least one SCR catalyst (22).

Description

Prior ArtSelective catalytic reduction (SCR) by means of ammonia (NH3) or ammonia-releasing reagents constitutes a promising method for the reduction of nitrogen oxides in oxygen-rich exhaust gases. The operating window of an SCR catalytic converter or its efficiency is defined by the physical variables of the temperature and the space velocity. The degree of coverage of the catalyst with adsorbed NH3is decisive for the efficiency. As the temperature increases, the ability of the SCR catalyst to store ammonia decreases. In order to maintain high efficiencies, the ammonia quantity to be converted must be dosed instantaneously to the measured nitrogen oxide quantity upstream of the SCR catalyst. Ammonia which does not react with NOx, desorbs from the catalyst as ammonia slip or oxidizes it due to high temperature on the catalyst must likewise be added in a postdosed fashion. In order to achieve the highest possible nitrogen oxide conversions, it is essential to operate the SCR system at a high ammonia fill level. If the temperature of the filled SCR catalytic converter rises as a result of a load jump of the internal combustion engine, its ammonia storage capacity of the SCR catalytic converter falls, which can lead to ammonia slippage. SCR catalysts which are installed close to the engine in order to convert nitrogen oxides early after engine start are subject in particular to dynamic temperature gradients. Depending on the NH3 fill level loading or the gradients thereof, this fact can lead to increased NH3 desorption. A second SCR catalyst downstream of the first SCR catalyst may therefore be provided in the exhaust line in order to adsorb ammonia from ammonia slip of the first catalyst and subsequently react it. The guidelines for onboard diagnostics (OBD) require that both SCR catalysts must be monitored.Disclosure of the InventionIn a first aspect, the invention relates to a method for exhaust gas aftertreatment of an internal combustion engine having at least one SCR catalyst, wherein the at least one SCR catalyst is divided into at least two virtual brick, wherein a first controller is used to perform a control of the at least one SCR catalyst according to a total NH3 soll level, characterized in that a switch from the first controller to a second controller is performed for the at least one SCR catalyst, wherein the second controller is used to perform an NH3 füllstands level control on the basis of a setpoint NH3 füllstand level of the first virtual brick of the at least one SCR catalyst.The method has the particular advantage that improved NOx emissions can be obtained by means of the control based on the setpoint NH3 füllstand level of the first virtual brick of the SCR catalytic converter.By controlling to the first virtual brick of the SCR catalyst, the front region is selectively wetted with ammonia, and a significantly improved NOx conversion is obtained.In particular at low temperatures and rapid load changes of the internal combustion engine, a high degree of efficiency can thus be ensured and improved emission cleaning can thus be carried out.In a special embodiment, the first control system switches over to the second control system when a transient operating state for the internal combustion engine is detected and a variable threshold value determined is undershot.Particularly in transient operating states, the control mode on the first virtual brick is particularly efficient, since an improved utilization of the freely accessible surface of the SCR catalytic converter in contact with the gas phase is obtained. Diffusive transport into deeper layers of the catalytic material (washcoat) is critically dependent on the concentration gradient. This means that the velocity of the gas exchange decreases in the radial direction.In a further embodiment, a transient operating state is present when a transition from a steady-state or quasi-steady-state operating state to a non-steady-state operating state is detected for the internal combustion engine.In a particular configuration, the transient operating state is determined as a function of a change in an enthalpy in the exhaust tract, in particular from a temperature difference between an SCR catalyst temperature and a temperature upstream of the at least one SCR catalyst. By means of the monitored change in enthalpy in the exhaust system, strong load point changes can be detected quickly and in a robust manner.For example, the change in enthalpy can be carried out as a function of a temperature difference between the temperature of the SCR catalytic converter and a temperature upstream of the SCR catalytic converter. By monitoring the change in enthalpy, it is possible to detect whether a critical NH3degradation state is present for the SCR catalyst.It is thus possible to assess whether a sufficient fill level quantity is kept ready for the fill level distribution of the SCR catalytic converter, in particular for the first virtual brick, in order to be able to ensure a sufficiently high NOx conversion.In a particular configuration, a diesel oxidation catalyst is arranged upstream of the at least one SCR catalyst. The method has the particular advantage that the diesel oxidation catalyst produces a damping of the temperature in the exhaust system.In an advantageous embodiment, the change in enthalpy is determined as a function of an SCR catalyst temperature of the at least one SCR catalyst and a first temperature of the diesel oxidation catalyst.The method has the particular advantage that the first temperature is attenuated by the preceding diesel oxidation catalyst, in particular this is similar to PT1 filtering and therefore no sudden temperature changes are shown in the temperature signal of the SCR catalyst.Method according to one of the preceding claims, characterized in that a difference between a target NH3 fill level of the first brick and an actual NH3 fill level for the first brick is determined.The method has the particular advantage that an increase in the conversion rate can be obtained and thus more effective utilization of the catalyst area and thus of the SCR catalyst is obtained.In an alternative embodiment, a variable threshold value is determined as a function of the enthalpy determined and / or a current load of the internal combustion engine and / or a rotational speed and / or a risk of ammonia slip.The method has the particular advantage, since a reduction in the risk of a high NH3 schlupf or a minimization of the NH3 schlupf can thus be obtained.In a particular embodiment, if the difference falls below the variable threshold value, in particular for a predeterminable time, the regulation of the SCR system is carried out on the basis of the second regulation.In an advantageous embodiment, if the difference exceeds the variable threshold value, in particular for a predeterminable time, the regulation of the SCR system is carried out on the basis of the first regulation.In a particular configuration, the risk of ammonia slip is determined as a function of the setpoint NH3target fill level and the actual NH3 füllstands level of the nth virtual brick of the at least one SCR catalytic converter.The method has the particular advantage, since a reduction in the risk of a high NH3 schlupf or a minimization of the NH3 schlupf can thus be obtained.In further aspects, the invention relates to a device, in particular a control unit and a computer program, which are set up, in particular programmed, for executing one of the methods. In yet another aspect, the invention relates to a machine-readable storage medium on which the computer program is stored.Brief Description of the DrawingsThe invention is explained in more detail below with reference to an exemplary embodiment shown in the figures. The following are shown: FIG. 1 shows a schematic illustration of an internal combustion engine with an exhaust gas aftertreatment system according to the invention, and FIG. 2 shows a flow chart for graphically illustrating the sequence of a first exemplary embodiment of the method.DETAILED DESCRIPTION OF THE DRAWINGSAn internal combustion engine 10 has in its exhaust line 11 an SCR exhaust gas aftertreatment system 25 with at least one SCR catalytic converter 22, which is illustrated in FIG. 1. This has a reducing agent metering unit 21, with which a urea water solution (Ad Blue) can be injected into the exhaust system 11. Ammonia is liberated therefrom at the high temperatures of the exhaust gas. Downstream of the internal combustion engine 10 there are a first temperature sensor 9 and a first NOx sensor 31. Further SCR catalysts may be disposed downstream of the first SCR catalyst 22. The first NOx sensor 31 measures a first NOx concentration sensor value NOx 1, preferably as a NOx concentration or as a NOx mass flow. A second NOx sensor 32 is arranged downstream of the first SCR catalytic converter 22 and measures a second NOx concentration sensor value NOx 2, preferably as a NOx concentration or as a NOx mass flow. This also applies in particular to the NH3 concentration or an NH3 mass flow.In an optional embodiment, an NH3sensor 33 can furthermore be installed downstream of the SCR catalytic converter 22. The NH3sensor can determine an NH3mass flow.All NOx sensors 31, 32 transmit their signals to an electronic control unit 100. Since the NOx sensors 31, 32 react to ammonia in a cross-sensitive manner in addition to nitrogen oxides, their signals are sum signals of nitrogen oxides and ammonia. The first NOx sensor 31 is arranged upstream of the reducing agent dosing unit 21 in such a way that it reliably measures only the amount of nitrogen oxide in the exhaust gas. The reducing agent metering unit 21 also forwards the ammonia quantity metered into the exhaust system 11 to the control unit 100.The above-mentioned temperature and NOx sensors are connected to a control device 100, and their signals are received and stored by the control device 100.In particular, a first temperature T Ist,Kat1, in particular for the diesel oxidation catalytic converter 3, and an SCR catalytic converter temperature T SCR of the SCR catalytic converter 22 can be determined by means of a temperature model stored on the control unit 100 as a function of the first temperature sensor.Furthermore, the system shown comprises a known SCR injection system, consisting of an SCR tank with a pump unit, which supplies urea liquid or reducing agent at a predeterminable pressure to the urea injection valve 21 in a known manner. The SCR injection system is controlled by means of a control strategy stored on the control unit 100. By means of the 21, an in particular urea-containing aftertreatment fluid is metered into the exhaust gas stream.Furthermore, measured variables such as the exhaust gas mass flow, ambient temperature T env and the rotational speed n eng of the internal combustion engine 10 can be called up in the control unit 100 in a known manner.To determine an NH3 füllstands level, an NH3 füllstands level model is stored in the control unit 100 for SCR catalytic converters 4.The modeled first NH3 füllstand level NH3 mod,Kat1 for the SCR catalytic converter 22 is determined by means of a first NH3 füllstands level model, in particular by means of a reaction kinetic model or an Arrhenius model, as a function of the amount of urea metered in by the first metering valve 21 and / or the NOxconcentration upstream of the first SCR catalytic converter 22, measured by means of the first NOxsensor and / or the SCR catalyst temperature T SCR of the first SCR catalytic converter 22 and / or the exhaust gas mass flow ≅ exh and / or the ambient temperature T env.A steady-state or quasi-steady-state condition exists, for example, when a speed change and / or an air mass flow change and / or an engine torque change and / or an accelerator pedal position change does not substantially change within a predefined time interval.A detection of the non-steady or dynamic operating state of the internal combustion engine is thereby preferably changed or greatly changed over a predefined time interval via a speed change and / or an air mass flow change and / or an engine torque change and / or an accelerator pedal position change. The detection is carried out by means of the control unit 100, which continuously receives a rotational speed n eng and / or an accelerator pedal position w pedal and / or an air mass flow ≅ air and / or a torque M and subsequently evaluates them.FIG. 2 shows, by way of example, the sequence of the method for an exhaust gas aftertreatment system having only one SCR catalytic converter 22, wherein the SCR catalytic converter 22 is virtually divided into n-brick.The method for an exhaust gas aftertreatment system having only one SCR catalyst 22 is described below, wherein SCR catalyst 22 is divided into at least two virtual brick.In a step 200, an exhaust gas mass flow dm EG, an exhaust gas pressure p, an SCR catalyst temperature T SCR, an oxygen concentration 0 2, a NOx concentration NOx Us downstream of the internal combustion engine 10 and upstream of the SCR catalyst 22, and a desired setpoint NOx conversion efficiency η NOx are ascertained continuously by the control unit 100.In particular, a brick temperature T SCR,i is determined for each brick of the SCR catalytic converter 22 as a function of the determined exhaust gas temperature T exh by means of a temperature model stored on the control unit 100.The desired setpoint NOxconversion efficiency η NOx can be determined or permanently predefined from a characteristic map as a function of the SCR catalyst temperature T SCR and / or the exhaust gas mass flow dm EG. The characteristic map is determined in an application phase and stored in the control unit 100.In an alternative embodiment, two SCR catalytic converters (different cannings) are positioned one behind the other. In this configuration, the desired target NOx conversion efficiency η NOx can also be determined depending on the state of the second catalyst. Here, the magnitudes of the actual NOxconversion efficiency η NOx,Ist and / or actual NH3 einzel levels m NH3,lst,i and / or actual total NOxdesired level m NOx,ges can be used.The method is then continued in a step 210.In a first step 210, a release condition for the method is checked. For this purpose, a dosing readiness for the SCR injection system 25 is determined by the control unit 100. This is done primarily by means of the feedback of the pressure p fed back by the pump unit. If the pressure p of the SCR injection system determined by the control unit 100 exceeds a predefinable pressure threshold, the SCR injection system is thus ready for metering. In addition, the SCR catalyst temperature T SCR of the SCR catalyst 22 can be determined by the control device 100. If the SCR catalyst temperature T SCR exceeds a predeterminable temperature T Kat,min, in particular 180° C., the operating temperature for the SCR catalyst 22 is thus reached and the release condition is issued.The method is then continued in a step 220.In a step 220, the current actual NH3 füllstands level distribution NH3is determined Ist for the SCR catalytic converter 22 on the basis of an SCR model F. The input variables for the SCR model F are ascertained continuously from step 200.In a particular embodiment, the desired NH3 fill level distribution and the desired total NH3 fill level can be calculated via an inverse model F -1. For a numerical solution of the inverse model, an estimated starting value is determined.In a first embodiment, the starting value can be determined on the basis of a solution of the inverse model F -1 at a previous time t-1, this approach being based on the fact that no abrupt changes in the NOxand NH3 konzentrationen occur within time intervals of less than 100 ms.In a second specific embodiment, the starting value is ascertained on the basis of an actual NH3 füllstands level distribution NH3 Ist that is current for SCR catalytic converter 22.In a third embodiment, the starting value can be determined from a calculation for an SCR catalytic converter 22 with only one brick. For this purpose, the inverse model F -1 is solved analytically for the one brick case and a total NH3 soll fill level NH3is obtained ges for the SCR catalyst 22.Thus, a starting value for the method can be determined and the method can be continued in a step 230.In a step 230, the NOx concentrations and the NH3 concentrations are iteratively solved according to the formulae (3) and (4): with the function for calculating NOx concentrations, r NH3 the function for calculating NH3 concentrations, m NH3,i ith NH3 mass, T SCR,i the temperature of the ith brick, the NH3 concentration of the previous brick, the NOx concentration of the previous brick, X O2 the oxygen concentration and p the exhaust gas pressure.The method is then continued in a step 240.In a step 240, a balancing of the NOxand NH3 konzentrationen upstream and upstream of each brick i is determined according to formula (7): with η Nox,i the NOxconversion efficiency of the ith brick, the NOxconcentration of the previous brick, the NH3concentration of the previous brick and ξ a stoichiometric factor corresponding to the NH3 äquivalenten from the NOxconversion and in particular in the range of [1:1.3̅].In a step 250, it is checked whether equation (11) is fulfilled for all brick i of the SCR catalytic converter 22: where ε tol is a suitable, predefinable tolerance threshold. This tolerance threshold can preferably be interpreted as a rate of change of the setpoint NH3individual fill levels NH3 Soll i.e. it indicates how strongly the fill levels are allowed to change in the defined stationary point. This can be determined in an application phase for the SCR catalytic converter 22.If the tolerance threshold is not fulfilled for all brick i, the method is continued again in step 230, wherein there an adaptation of the NH3 füll for the current estimated value (at the start of the method, it is the starting value). This can be effected, for example, by means of a Newton-based method. For this purpose, the derivation matrix (Jacobi matrix) of the function to be minimized for the formula Error! Reference source could not be found.The quotient of the function to be minimized and the Jacobi matrix determines the step size and direction of the adaptation of the current estimated value according to the following formula:In the event that the condition of formula (11) is fulfilled for all brick i, the method is continued in a step 260.In a step 260, the desired NH3 einzel levels are then combined to form a total NH3 soll level NH3 ges and the method can be continued in a step 270.In a step 270, a change in an enthalpy E in the exhaust system is determined.In a preferred embodiment, the change in enthalpy E is determined as a function of the SCR catalyst temperature T SCR of the first SCR catalyst 22 and a first temperature T Ist,Kat1 of the diesel oxidation catalyst 3.In an alternative configuration, an alternative catalyst instead of the diesel oxidation catalyst 3 can also be installed in the system and the temperature of the alternative catalyst can be used for determining the enthalpy E.In a preferred embodiment, an exponential smoothing is formed for the enthalpy E.In an alternative embodiment, the enthalpy E is determined as a function of a comparison between a temperature upstream of the diesel oxidation catalyst 3 and a temperature upstream of the SCR catalyst 22.The method is then continued in a step 280.In a step 280, a difference D between the target NH3 ziel fill level mNH3Cat Soll,SCR1B for the first brick and the actual NH3 füllstand fill level mNH3Cat Ist,SCR1B for the first brick is determined. The method is then continued in a step 290.In a step 290, a variable threshold value S Var is ascertained as a function of the ascertained enthalpy E, a current load of the internal combustion engine 10, the rotational speed n eng and a risk of ammonia slip.The risk of ammonia slip can be determined in this case as a function of the setpoint NH3target fill level mNH3Cat Soll,SCRnB and the actual NH3 füllstands level mNH3Cat Ist,SCRnB of the nth brick of the SCR catalytic converter 22. The method is then continued in a step 300.In a step 300, the difference D determined in step 280 is checked against the variable threshold value S var determined in step 290.If the difference D falls below the variable threshold value S var the control strategy for the SCR system 25 is carried out on the basis of the first virtual brick.In a preferred embodiment, a time can be specified which the difference D must exceed the variable threshold value S var until the control strategy for the SCR system 25 is carried out on the basis of the first virtual brick.If the difference D exceeds the variable threshold value S var, in particular for a predeterminable time, a control strategy for the at least one SCR catalytic converter 22 is carried out on the basis of the total NH3 soll fill level NH3 ges.The method is then continued in a step 310.In a step 310, the determined desired setpoint NH3 füllstand level is added to the dosing strategy and set between setpoint and actual values by means of a dosing strategy, preferably by means of a P-control. In particular, the determined metered quantity can be divided in particular into a defined time constant. The proportion of the P regulator is added to the pilot quantity. Subsequently, the method can be started or ended from the front in step 200.

Claims

Method for exhaust gas aftertreatment of an internal combustion engine (10) having at least one SCR catalyst (22), wherein the at least one SCR catalyst (22) is divided into at least two virtual brick, wherein a first controller is used to perform a control of the at least one SCR catalyst (22) in accordance with a total NH3 nominal fill level (NH3 ges) characterized in that a switch from the first controller to a second controller is performed for the at least one SCR catalyst (22), wherein the second controller is used to perform an NH3 fill level control on the basis of a nominal NH3 fill level (mNH3Cat Soll,SCR1B) of the first virtual brick of the at least one SCR catalyst (22).Method according to Claim 1, characterized in that a changeover is made from the first control to the second control if a transient operating state for the internal combustion engine (10) is detected and a variable threshold value (S Var) determined is undershot.Method according to Claim 2, characterized in that a transient operating state is present if a transition from a steady-state or quasi-steady-state operating state to a non-steady-state operating state is detected for the internal combustion engine (10).Method according to Claim 2, characterized in that the transient operating state is determined as a function of a change in an enthalpy (E) in the exhaust tract, in particular from a temperature difference between an SCR catalyst temperature (T SCR) and a temperature upstream of the at least one SCR catalyst (22).Method according to claim 1, characterised in that a diesel oxidation catalyst (3) is arranged upstream of the at least one SCR catalyst (22).Method according to Claim 4, characterized in that the change in enthalpy (E) is determined as a function of an SCR catalyst temperature (T SCR) of the at least one SCR catalyst (22) and a first temperature (T Ist,Kat1) of the diesel oxidation catalyst (3).Method according to one of the preceding claims, characterized in that a difference (D) between a target NH3 ziel fill level (mNH3Cat Soll,SCR1B) of the first brick and an actual NH3 füllstand level (mNH3Cat Ist,SCR1B) for the first brick is determined.Method according to one of the preceding claims, characterized in that a variable threshold value (S Var) is determined as a function of the enthalpy (E) determined and / or a current load of the internal combustion engine (10) and / or a rotational speed (n eng) and / or a risk of ammonia slip.Method according to one of the preceding claims, characterized in that, if the difference (D) falls below the variable threshold value (S Var) in particular for a predeterminable time, the regulation of the SCR system (25) is carried out on the basis of the second regulation.Method according to one of the preceding claims, characterized in that, if the difference (D) exceeds the variable threshold value (S Var) in particular for a predeterminable time, the regulation of the SCR system (25) is carried out on the basis of the first regulation.Method according to Claim 8, characterized in that the risk of ammonia slip is determined as a function of the setpoint NH3 target fill level (mNH3Cat Soll,scRnB) and the actual NH3 fill level (mNH3Cat Ist,SCRnB) of the n-th virtual brick of the at least one SCR catalytic converter (22).Computer program which is configured to carry out a method according to one of Claims 1 to 11.Electronic storage medium comprising a computer program according to claim 12.Device, in particular control device (100), which is configured to execute a method according to one of Claims 1 to 11.

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