DE102010005428A1 - Method for operating exhaust gas cleaning equipment with selective catalytic reduction catalyzer for cleaning exhaust gas of motor vehicle internal combustion engine, involves dosing ammonia-containing reducing agent to exhaust gas - Google Patents

Abstract

The method involves dosing an ammonia-containing reducing agent to the exhaust gas with an adjustable dosing rate by a dosing unit (27). The dosing unit is controllable by a control unit. An exhaust gas is supplied to the selective catalytic reduction catalyzer. The exhaust gas is enriched with ammonia according to the metered addition. The metered addition of reducing agent is conducted depending on a determined actual efficiency factor and predetermined operating parameters of the exhaust gas cleaning equipment such that the actual efficiency factor achieves a reference efficiency factor.

Description

The invention relates to a method for operating an exhaust gas purification system with an SCR catalytic converter according to the preamble of claim 1.

For the catalytically assisted removal of nitrogen oxides (NOx) from engine exhaust gases, it is known to add aqueous urea solution to the exhaust gas of the internal combustion engine as a reducing agent containing ammonia (NH ₃ ). In the hot exhaust gas is released by thermolysis and / or hydrolysis NH ₃ as the actual selective reducing agent with respect to the NOx reduction of a so-called SCR catalyst. This results in the problem to determine a suitable for the desired cleaning effect rate of addition of the reducing agent, which also an overdose, which may have an undesirable slippage of NH ₃ result, should be avoided.

To solve these problems will be in the EP 1 348 477 A1 proposed to determine a consumption rate of NH ₃ stored in the catalyst and a NOx purification rate and to control the reduction agent addition in dependence on these quantities in connection with a level of NH ₃ stored in the catalyst.

From the publication EP 0 554 766 A1 A method is known for NOx reduction in exhaust gases by controlled superstoichiometric addition of NH ₃ , in which upstream of a catalyst NH ₃ is metered in until the amount of NH ₃ stored in the catalyst has reached an upper threshold value. The NH ₃ stored in the catalyst is consumed by reaction with the NOx emitted from the engine and supplied to the catalyst, thereby removing the pollutant NOx from the exhaust gas. The NH ₃ addition is resumed when the stored in the catalyst is NH ₃ amount has reached a lower threshold value.

In these and other known, similar methods, a model-based combination of measured values and stored characteristic values is usually carried out, for example for the SCR catalytic converter. In this case, an NH ₃ feed rate and a modeled consumption rate are balanced by a continuously performed integration and the reducing agent metering rate selected so that a desired NH ₃ level in the catalyst and thus a desired catalyst efficiency results. The indirect, arithmetic determination of the NH ₃ level, however, is not always possible with the desired accuracy. For example, errors caused by parameter changes may accumulate over time, so that the modeled NH ₃ content of the catalyst is progressively worse than the actual value and incorrect metering results in the form of under- or overdosing. The consequence of this is a reduced cleaning effect or an increased NH ₃ slip.

The object of the invention is to provide a method for operating an emission control system with an SCR catalyst, which allows a further improved nitrogen oxide conversion.

This object is achieved by a method having the features of claim 1.

In the method according to the invention, a setting of a metering rate for the ammonia-containing reducing agent, which is linked to an efficiency of the SCR catalyst with respect to a current turnover of nitrogen oxides with stored in the SCR catalyst and / or supplied to the SCR catalyst ammonia. As it could be confirmed, a dosing rate setting linked to the efficiency of the NOx conversion enables the highest NOx conversion values virtually without slippage. In this case, the invention provides that the adjustment of the metering rate of the corresponding metering unit for the reducing agent in a controlled metering operation with an efficiency-controlled adjustment of the metering or in a controlled metering operation with an efficiency-controlled adjustment of the metering rate is made. The choice between the efficiency-controlled or the efficiency-controlled metering rate adjustment takes place at least as a function of the determined actual efficiency and for specifiable values of predefinable operating parameters of the emission control system. In this case, an NOx conversion efficiency is understood as meaning a relative NOx conversion, which is the quotient of the amount of NOx converted into harmless constituents in a time interval and of the amount of NOx flowing into the SCR catalytic converter in the same time interval. With optimum operation of the SCR catalyst, NOx conversion values or efficiencies of approximately 100% can be achieved. The metering rate is to be understood as meaning a quantity of reducing agent preferably added to the same time interval. The metering of the reducing agent takes place preferably timed clocked, optionally with certain metering pauses. As has been found, it is possible by the procedure according to the invention to achieve predominantly efficiencies for the NOx conversion in a process-technically simple and, in particular, reliable manner, which are close to the maximum value that can be achieved under the respective operating conditions. In particular, it is possible that the actual efficiency at least approximately reaches predefinable values for a desired efficiency.

The efficiency-controlled adjustment of the reducing agent metering rate is preferably model-based. Values for the reducing agent metering rate, which can be expected to permit an efficiency of the SCR catalytic converter, which are based on current values of predefinable operating variables of the internal combustion engine and / or the exhaust system, are predefined by means of a computer model or by means of reproved characteristic curves and / or characteristic maps at least approximately achieves a predefinable desired efficiency. In the case of the metering rate setting in the controlled mode, a return of a determined current actual efficiency to the influencing of the metering rate setting based thereon is dispensed with. On the other hand, in the efficiency-controlled dosing operation, the determined current value for the actual efficiency is traced back to the input side of a control loop in a classical manner. A deviation from the desired efficiency as a reference variable is supplied as a control deviation to a controller which generates a manipulated variable for a controlled setting of the metering rate in a control algorithm.

It is envisaged that the actual efficiency for the current nitrogen oxide conversion of the SCR catalyst is also continuously determined or estimated in controlled operation. It is further provided in an embodiment of the invention that a transition from controlled dosing to regulated dosing takes place when the determined actual efficiency deviates from the target efficiency by more than a predetermined amount. The deviation which triggers a transition from the controlled dosing operation to the controlled dosing operation can be both positive and negative, whereby different threshold trigger thresholds can be provided for deviations in different directions. Often the controlled metering operation will provide a sufficiently good NOx performance result. However, unforeseen disturbances can occur, which can not be compensated in a controlled dosing operation.

As a result of the planned monitoring of the current catalyst efficiency, it is possible to detect and react to such disturbing influences. If faults in the controlled dosing operation lead to unexpected deviations between desired efficiency and actual efficiency, or if other faults occur, such as a failure or an implausible behavior of a sensor, then the controlled dosing operation is initiated, in which usually a compensation of the occurring disturbance variable is possible is. After successful correction of the disturbance variable, for example with sufficiently stable behavior of the dosing rate-determining manipulated variable as the output variable of the regulator or upon achievement of a sufficiently small control deviation, a switchover or downshifting into the controlled dosing operation can be provided.

According to the invention, it is provided in a further embodiment of the method, at least then to move from the controlled dosing to controlled dosing, if in the controlled dosing a time course with substantially increasing dosing rate is associated with a temporal course with substantially decreasing actual efficiency. As determined by the inventors, a decreasing actual efficiency with simultaneously increasing reducing agent metering rate corresponds to a behavior of the controlled system which is typical for an overdose with oxidation of the excess ammonia. As could be found, such unwanted interference behavior can often not or only be corrected unsatisfactory. As a result of the transition to a controlled dosing operation provided according to the invention, however, a dosing rate suitable for high NOx conversion values can again be set in the described case.

In a further embodiment of the invention, it is provided that the metering rate in the controlled metering operation and / or the desired efficiency as a function of the nitrogen oxide mass flow in the SCR catalyst and a temperature of the SCR catalyst are specified. In this case, the nitrogen oxide mass flow into the SCR catalytic converter can in turn be determined from characteristic diagrams for engine operating parameters such as load, rotational speed, degree of turbocharging, exhaust gas recirculation rate, etc. However, it is preferred if the nitrogen oxide mass flow into the SCR catalytic converter is determined from measured values of a nitrogen oxide sensor arranged upstream of the SCR catalytic converter, whereby preferably an exhaust gas mass flow determined from aspirated air mass flow and fuel consumption flow is taken into account. A consideration of an aging-related deterioration of the catalyst efficiency which possibly occurs during the operating time can likewise be provided when determining the metering rate and / or the desired efficiency.

In a further embodiment of the invention, the actual efficiency is determined from signals of an upstream of the SCR catalyst and signals downstream of the SCR catalyst arranged nitrogen oxide sensor. From the difference between the values for the NOx concentration in the exhaust gas supplied by the upstream and the downstream nitrogen oxide sensor, the NOx conversion or the actual efficiency of the SCR catalytic converter can be reliably determined. As could be determined, with a metrological determination of the input side of the SCR catalyst existing NOx concentration in the exhaust gas a more reliable determination of efficiencies allows as a model-based determined NOx concentration. The metrological determination of the actual efficiency therefore allows increased accuracy of the metering rate adjustment and improved NOx conversion.

In a further embodiment of the invention, a controlled cold start metering operation is provided under predeterminable cold start conditions, in which a compared to the normal controlled metering operation increased metering rate is set. With this approach, the knowledge is taken into account that in a cold start or when compared to the normal operation cold SCR catalyst on the one hand its effectiveness is reduced for temperature reasons and on the other hand due to a usually small amount of stored ammonia. Due to the increased metering rate, an ammonia storage amount required for high NOx conversion can be rapidly established. In particular, in a cold start or warm-up, engine measures for rapid exhaust gas or catalyst heating are preferably taken at the same time. In this way, after a cold start in a short time an effective SCR catalyst available. As an increased metering rate, for example, temporarily the maximum possible metering rate or a predefinable portion of the maximum possible metering rate can be set. It is provided in a further embodiment of the invention that in an operating phase with uninterrupted operation of the motor vehicle internal combustion engine, the state of a controlled cold-start metering operation is set at most once. If the predefinable cold start conditions are no longer present, it is preferable to switch to the normal controlled dosing mode and to prohibit a new setting of the cold start dosing operation for the subsequent operating time. Only after switching off the engine will this be released again. This takes into account the fact that the typical conditions for a cold start or engine warm-up generally occur only once in an operating phase with uninterrupted engine operation.

Advantageous embodiments of the invention are illustrated in the drawings and described below. In this case, the features mentioned above and those yet to be explained below can be used not only in the respectively specified combination of features, but also in other combinations or alone, without departing from the scope of the present invention.

Showing:

1 a schematic block diagram of a motor vehicle internal combustion engine with an attached emission control system and

2 a block diagram to illustrate the inventively provided process management when operating the emission control system.

1 shows an example of a schematic block diagram of an internal combustion engine 1 a motor vehicle, not shown, with associated emission control system. The internal combustion engine 1 is preferably designed as an air-compressing internal combustion engine, hereinafter referred to simply as a diesel engine. That from the diesel engine 1 Exhaust gas emitted is from an exhaust pipe 2 taken and flows through an oxidation catalyst in succession 3 , a particle filter 4 and a NOx reduction catalyst 5 , oxidation catalyst 3 and particle filters 4 may also be closely adjacent in a common housing, optionally together with the NOx reduction catalyst 5 be arranged. The particle filter 4 is preferably provided with a Rußabbrand and / or a NO oxidation promoting catalytic coating. Downstream of the NOx reduction catalyst 5 may be another, not shown, oxidation catalyst for the oxidation of the NOx reduction catalyst 5 be provided slipping reducing agent.

For heating the oxidation catalyst 3 or the exhaust gas can, as shown, a heater 26 on the input side of the oxidation catalyst 3 in the exhaust pipe 2 be arranged. The heater 26 can be designed as an electric heating element or as a heat-supplying reformer or Vorverbrenner. A heating of the exhaust gas comes in particular in a regeneration of the particulate filter 4 by Rußabbrand into consideration. To determine the need for such regeneration is the particulate filter 4 a differential pressure sensor 22 assigned, which provides a signal relative to a particle loadable signal. Further, an exhaust gas turbocharger 6 provided, the turbine is driven by the exhaust stream and the compressor via an intake air line 7 sucked air through an air supply line 11 the diesel engine 1 supplies. The turbocharger 6 is preferably designed as a so-called VTG loader with a variable setting of its turbine blades or as a wastegate loader. To adjust the diesel engine 1 amount of air supplied is in the air supply line 11 a throttle 12 arranged. For cleaning the intake air or for measuring the intake air amount are an air filter 8th or an air mass meter 9 in the intake air line 7 arranged. An intercooler arranged in the air supply line 10 serves to cool the compressed intake air. Furthermore, an exhaust gas recirculation, not shown, via an exhaust gas recirculation valve is provided, with which a predetermined amount of recirculated exhaust gas of the intake air can be added. Returned exhaust gas may be via an upstream of the turbine of the turbine Abgastruboladers 6 branching high pressure EGR path and / or downstream of the particulate filter 4 branching low pressure EGR path to the inlet side of the diesel engine 1 be guided. In this case, advantageously for adjusting the NOx raw emissions of the diesel engine 1 or the NOx reduction catalyst 5 the amount of exhaust gas branched off via the high-pressure EGR path and the low-pressure EGR path can be set in a predefinable ratio.

Upstream of the NOx reduction catalyst 5 is an addition point with a dosing unit 27 arranged for adding a reducing agent in the exhaust gas. The supply of the dosing unit 27 with the reducing agent takes place from a container, not shown here. In the following, it is assumed that the reducing agent is aqueous urea solution, which is the exhaust gas via the metering unit 27 metered added. In the hot exhaust gas is released by thermolysis and / or hydrolysis NH ₃ , which selectively acts with respect to the reduction of NOx contained in the exhaust gas. Accordingly, the present invention is the NOx reduction catalyst 5 as a classic SCR catalyst based on V ₂ O ₅ / WO ₃ or formed as a zeolite-coated, supported SCR catalyst. However, the method according to the invention is also advantageously applicable to the dosing of other reducing agents which preferably contain ammonia in free or bound form. In particular, when using aqueous urea solution as a reducing agent can between the metering unit 27 and the NOx reduction catalyst 5 a mixing unit, not shown in the exhaust pipe 2 be arranged, with which through the metering unit 27 already preferred dispersed finely sprayed into the exhaust gas reducing agent is mixed with exhaust gas to a homogeneous mixture as possible.

This can promote droplet evaporation and the release of NH ₃ .

To control the operation of the emission control system and the diesel engine 1 is an in 1 not shown control unit provided, the information about operating variables of the diesel engine 1 and the emission control system receives. Information about engine operating variables can, for. B. concern a delivered torque or speed. The control unit preferably comprises a computing unit and a memory unit as well as an input-output unit. As a result, the control unit is able to perform complex signal processing operations and the operation of the diesel engine 1 and the emission control system to capture and control or regulate. Necessary maps for this purpose are preferably stored in the memory device, wherein an adaptive adaptation of the maps can be provided. The maps relate mainly to the relevant operating variables of the exhaust gas, such as mass flow, raw emissions, temperature depending on the operating variables of the diesel engine 1 such as load, speed, air ratio, etc. Furthermore, maps for the relevant operating variables of the oxidation catalyst 3 , the particulate filter 4 and the SCR catalyst 5 intended. With regard to the SCR catalyst 5 These maps relate in particular to the NOx conversion or the efficiency with respect to the NOx conversion and the NH ₃ storage capacity as a function of the variables influencing this, such as catalyst temperature, exhaust gas mass flow or NOx mass flow and NH ₃ or urea dosing rate ,

The detection of operating conditions or operating variables of the diesel engine 1 as well as the emission control system and the associated units is preferably carried out at least partially by means of suitable sensors. Exemplary are in 1 pressure sensors 13 and 15 for a pressure in front of the compressor and a pressure in front of the turbine of the turbocharger 6 as well as temperature sensors 14 . 16 . 18 . 19 . 21 . 23 and 24 for each one temperature after the intercooler 10 , in front of the turbine, before the oxidation catalyst 3 , before and after the particle filter 4 as well as before and after the SCR catalyst 5 shown. Other sensors, in particular for the detection of exhaust gas components may also be provided. For example, a lambda sensor 17 as well as sensors 20 and 25 provided for the nitrogen oxide and / or NH ₃ content in the exhaust gas. The signals of the sensors are processed by the control unit, so that the essential operating variables are present value-wise at all times and the operating point of the diesel engine 1 if necessary, can be changed so that an optimal or desired operation of the emission control system is possible.

The core of the invention is a definition of a metering rate for the urea solution such that the highest possible, but at least a requirement of the corresponding NOx conversion through the SCR catalyst 5 is possible. It is envisaged that an actual efficiency of the SCR catalyst 5 for a current NOx conversion with in the SCR catalyst 5 stored and / or the SCR catalyst 5 supplied NH _{3 is} determined. For this purpose, the signals of the nitrogen oxide sensors are present 20 and 25 upstream and downstream of the SCR catalyst 5 evaluated. According to the invention, the reductant is metered in at least as a function of the determined actual efficiency and for specifiable values of predefinable operating parameters of the emission control system in a controlled dosing operation with an efficiency-controlled dosing rate adjustment or in a regulated dosing operation with an efficiency-adjusted dosing rate adjustment such that the actual Efficiency one predetermined target efficiency at least approximately achieved. Specified values for the target efficiency with respect to the NOx conversion of the SCR catalytic converter 5 are preferred in advance empirically depending on the main operating variables such as exhaust gas mass flow, NOx mass flow, temperature of the SCR catalyst 5 and optionally determined further sizes and maintained in the form of a map. In order to realize the efficiency-controlled or efficiency-controlled metering rate setting, use is made of a control or regulating algorithm which is preferably stored as programs in the control unit.

The following is a preferred procedure using an in 2 illustrated block diagram explained in more detail. After a start of the diesel engine 1 After determining the presence of predetermined metered release conditions, an efficiency controlled metering operation is initiated 30 set. For a dosing release, for example, a proper circulation of the urea solution, a compliance with a predefinable dosing pressure and a readiness for operation of the dosing system and the exhaust gas sensors can be checked.

In the efficiency-controlled metering mode 30 is then first checked whether predefinable cold start conditions are present and, if so, a change of state 33 made and in a cold start metering operation 32 changed. These decisive conditions for a change of state 33 For example, fall below specified minimum temperatures in the exhaust system or coolant and / or lubricating oil supply system of the diesel engine 1 , In the present case, the cold start conditions are considered to be satisfied at least when the temperature of the SCR catalyst 5 Below a predetermined lower limit temperature of about 200 ° C and at the same time the determined actual efficiency of the SCR catalyst 5 the target efficiency for the present operating state of the emission control system by more than a predetermined amount. To assess the latter criterion, a difference or ratio of actual efficiency and target efficiency can be evaluated. If the specified cold start conditions are considered to be fulfilled, the controlled cold start metering operation 32 set. It is provided to set the metering rate to a predetermined, comparatively high Kaltstartdosierratenwert. The cold start metering rate is higher than the metering rate in the normal efficiency-controlled metering operation by a predeterminable amount 30 set. Preferably, the Kaltstartdosierratenwert by a separate map as a function of the in the SCR catalyst 5 inflowing NOx mass flow and the temperature of the SCR catalyst 5 specified. Preferably, the cold start metering rate value corresponds to a higher than stoichiometric addition of urea with respect to the NOx mass flow. In this way, it is possible to quickly a sufficient amount of ammonia in the SCR catalyst 5 einzuspeichern. After reaching its light-off temperature, therefore, a maximum NOx conversion corresponding to the conditions is at least approximately possible. It may be provided by motorized measures, such as bypassing the intercooler 10 and / or an EGR cooler, actuation of the heater 26 Performing a post fuel injection, throttling the intake air amount, a rapid heating of the SCR catalyst 5 to support.

If predefinable jump conditions are detected, then a state change takes place 34 in the normal efficiency-controlled metering operation 30 changed. In particular, it is for a change 34 in the normal efficiency-controlled metering operation 30 at least required that the actual efficiency of the SCR catalyst 5 differs from the target efficiency by less than a specifiable low level. It is intended that after leaving a cold start dosing operation that has been set once 32 a renewed activation of this mode is blocked during further engine operation.

In the efficiency-controlled metering mode 30 is based on a map, which is a dosing rate in response to essential operating parameters of the diesel engine 1 and the exhaust system, the dosing rate is set. It is envisaged that these operating parameters at least those in the SCR catalyst 5 inflowing NOx mass flow and the temperature of the SCR catalyst 5 include. The corresponding characteristic diagram for the dosing rate in the efficiency-controlled mode 30 is preferably determined empirically in advance. The metering rate stored in the map is determined in such a way that, under the relevant conditions, a predefinable target efficiency of the SCR catalytic converter 5 is reached. Preference is given to a metering rate which corresponds to an at most stoichiometric metering of urea in such a way that the input side of the SCR catalyst 5 the same values for the NOx concentration and the NH ₃ concentration. An adaptive adaptation or correction of the characteristic map, for example due to an aging-related degradation of the SCR catalytic converter occurring during the operating time 5 can be provided. This can cause an aging-related loss of activity of the SCR catalyst 5 be taken into account during Dosierratenermittlung, which overdoses are avoided due to the loss of activity. Depending on the prevailing operating conditions, the assigned metering rate for the urea solution is read from the map and set. It is envisaged that the actual efficiency of the SCR catalyst 5 with regard to NOx conversion, and with the target Efficiency is compared. The actual efficiency is based on the upstream and downstream of the SCR catalyst 5 present NOx concentrations or mass flows determined. It can be in the SCR catalyst 5 inflowing NOx mass flow can be read from a motor operating map or sensory from the signal of the NOx sensor 20 be determined in conjunction with the exhaust gas mass flow. The NOx concentration and the NOx mass flow downstream of the SCR catalyst 5 is preferred from the signals of the downstream NOx sensor 25 determined.

Used with efficiency-controlled metering operation 30 determined that predefinable jump conditions are present, it is in a state change 35 in the efficiency-controlled metering operation 31 changed. The jump conditions for the state change 35 In this case, at least one operating readiness of the controller or an availability of operating variables or sensor signals required for controller operation comprise. Further, in the efficiency-controlled metering operation 31 in the present case only changed when the determined actual efficiency deviates from the target efficiency by more than a predefinable measure. In the efficiency-controlled metering operation 31 there is a regulated setting of the urea dosing rate, preferably in accordance with a control algorithm stored in the control unit. The preferably implemented by software control loop is used as a reference variable analogous to the efficiency-controlled dosing 30 determined target efficiency for the NOx conversion at the SCR catalytic converter 5 fed. In addition, it is analogous to the efficiency-controlled dosing 30 a continuous determination of the actual efficiency of the SCR catalyst 5 , A difference determined from the setpoint efficiency and the actual efficiency is supplied to the control algorithm, which preferably operates as a PID controller, as a control deviation, and a control variable for the dosing rate to be set is output and the dosing unit 27 fed. The control algorithm can also provide for the setting of a pre-control value for the metering rate, whereby vibrations of the manipulated variable are reduced and the setting speed is increased.

If predefinable return conditions exist, then a change of state occurs 36 from the efficiency-controlled metering operation 31 back into the efficiency-controlled metering operation 30 jumps back. The state change 36 is preferably performed when sensor signal values of one or more of the sensors 14 . 16 . 17 . 18 . 19 . 20 . 21 . 23 . 24 and 25 falls below or exceed predetermined limits or a failure of one or more of said sensors is detected. In case of a recognized serious error, such as the failure of a component of the dosing system can also be jumped into an error handling routine or in an emergency. In the present case is a change of state 36 from the efficiency-controlled metering operation 31 in the efficiency-controlled metering operation 30 provided at least when in the regulated metering operation 31 a temporal course with substantially increasing metering rate is associated with a time course with substantially decreasing actual efficiency. Such behavior may be, for example, in the event of a failure of one of the NOx sensors 20 . 25 occur. In the case of an ammonia-sensitive NOx sensor 25 the said return condition may also be obscured by overdosing in which, as the dosing rate increases, an increasing ammonia slip occurs, which is erroneously interpreted as an increasing nitrogen oxide concentration value and thus as a decreasing efficiency. By the state change 36 Thus, any possible "runaway" of the control is prevented and it can be set in the re-assumed efficiency-controlled operation again defined conditions for high NOx conversion values.

Overall, in particular by the present invention under specifiable conditions change of efficiency-controlled metering operation 30 and efficiency-controlled metering operation 31 Achieving high, predominantly optimal nitrogen oxide conversions on the SCR catalyst 5 allows. It is understood that for a state change 33 . 34 . 35 . 36 preferably a temporal Entprellung is provided. This can for example be designed such that a state change 33 . 34 . 35 . 36 is only made if the conditions prevailing for a state change conditions for a predeterminable period.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• EP 1348477 A1 [0003]
• EP 0554766 A1 [0004]

Claims (7)

Method for operating an emission control system with an SCR catalytic converter ( 5 ) for cleaning an exhaust gas of a motor vehicle internal combustion engine ( 1 ), in which by a controllable by a control unit dosage unit ( 27 ) an ammonia-containing reducing agent is added to the exhaust gas at an adjustable metering rate and the SCR catalyst ( 5 ) an exhaust gas enriched in accordance with the metered addition with ammonia, wherein an actual efficiency of the SCR catalyst ( 5 ) for a current conversion of nitrogen oxide with in the SCR catalyst ( 5 ) and / or the SCR catalyst ( 5 ) supplied ammonia, characterized in that the metered addition of the reducing agent at least in dependence on the determined actual efficiency and for specifiable values of specifiable operating parameters of the emission control system in a controlled dosing ( 30 ) with an efficiency-controlled adjustment of the metering rate or in a controlled metering operation ( 31 ) is performed with an efficiency-controlled adjustment of the metering rate such that the actual efficiency reaches a target efficiency at least approximately. Method according to claim 1, characterized in that a transition from the controlled metering operation ( 30 ) to the regulated dosing operation ( 31 ) takes place when the determined actual efficiency in the controlled metering operation ( 30 ) differs from the target efficiency by more than a predetermined amount. A method according to claim 1 or 2, characterized in that a transition from the regulated dosing ( 31 ) to the controlled dosing operation ( 30 ) takes place when in the controlled metering operation ( 31 ) is associated with a temporal course with a substantially increasing metering rate a time course with substantially decreasing actual efficiency. Method according to one of claims 1 to 3, characterized in that the metering rate in the controlled metering operation ( 30 ) and / or the desired efficiency as a function of the nitrogen oxide mass flow into the SCR catalyst ( 5 ) and a temperature of the SCR catalyst ( 5 ). Method according to one of claims 1 to 4, characterized in that the actual efficiency of signals from an upstream of the SCR catalyst ( 5 ) and one downstream of the SCR catalyst ( 5 ) arranged nitrogen oxide sensor ( 20 . 25 ) is determined. Method according to one of claims 1 to 5, characterized in that under predefinable cold start conditions, a controlled cold start metering ( 32 ), in which a compared to the normal controlled dosing ( 30 ) Increased dosing rate is set. A method according to claim 6, characterized in that in an operating phase with uninterrupted operation of the motor vehicle internal combustion engine ( 1 ) the state of a controlled cold start metering operation ( 32 ) is set at most once.

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