DE102007041897B4 - Method and device for operating an internal combustion engine - Google Patents

Abstract

Method for operating an internal combustion engine (14) with an SCR system comprising an SCR catalytic converter (23), in which
An ammonia loading level (NH3_LD) of the SCR catalytic converter (23) is determined or set,
Depending on at least one operating variable of the internal combustion engine (14), a current nitrogen oxide content (NOX_AV) of an exhaust gas of the internal combustion engine (14) is determined,
Depending on the determined current nitrogen oxide content (NOX_AV) of the exhaust gas, a base setpoint value (NH3_MASS_BAS) of an ammonia medium mass is determined, which is to be assigned to the exhaust gas tract for converting the nitrogen oxide of the exhaust gas,
- A nominal value (NH3_MASS_SP) of the ammonia medium mass is determined such that the setpoint value (NH3_MASS_SP) of the ammonia medium mass oscillates around the nominal base value (NH3_MASS_BAS) of the ammonia medium mass and the oscillating nominal value (NH3_MASS_SP) of the ammonia medium mass is at least equal to the determined base setpoint value (NH3_MASS_BAS) of the ammonia medium mass approximates,
- Depending on the determined setpoint value (NH3_MASS_SP) of the ammonia medium mass for converting the setpoint value (NH3_MASS_SP) of the ammonia medium mass, an actuating signal for an actuator is determined, the position of which is based on a ...

Description

The invention relates to a method and a device for operating an internal combustion engine. The internal combustion engine has an SCR system that includes an SCR catalyst. Depending on at least one operating variable of the internal combustion engine, a current nitrogen oxide content of an exhaust gas of the internal combustion engine is determined. Depending on the determined current nitrogen oxide content of the exhaust gas, a base target value of an ammonia medium mass is determined. The ammonia medium mass is to be metered to the exhaust tract for converting the nitrogen oxide of the exhaust gas.

To reduce a nitrogen oxide content in the exhaust gas of an internal combustion engine, an exhaust gas aftertreatment with aqueous urea solution and an SCR catalyst can be carried out. The aqueous urea solution may also be referred to as urea. For exhaust aftertreatment, the aqueous urea solution is pumped with a liquid pump to a urea injection valve which meters the urea solution upstream of the SCR catalyst into an exhaust gas stream in an exhaust tract of the internal combustion engine. The urea solution reacts in the hot exhaust stream to ammonia and carbon dioxide. In the SCR catalyst, the ammonia then reacts with the nitrogen oxide mixture of the exhaust gas to form nitrogen and water.

The US2005 / 0282285 A1 discloses a method for controlling NOx emissions and ammonia breakthrough in an SCR catalyst. To control the ammonia feed rate for an SCR catalyst, a NOx sensor is used that has sensitivity to ammonia. The sensor is located downstream of the SCR catalyst. The ammonia feed rate is varied preset. A positive response to the predetermined ammonia feed rate indicates ammonia breakthrough, a negative response to the predetermined ammonia feed rate indicates NOx breakthrough. Upon detection of ammonia breakthrough, a controller performs a ammonia breakthrough prevention mode in which the ammonia feed rate is reduced until a given ammonia intake capacity of the SCR catalyst is achieved. After performing this mode, a controller is activated.

The US 5,785,937 A discloses a method for the catalytic conversion of nitrogen oxides contained in the exhaust gas of an internal combustion engine. A reducing agent is introduced in the flow direction of the exhaust gas before a denitration catalyst in the exhaust gas. During the starting phase of the internal combustion engine and during operation with decreasing and optionally almost constant exhaust gas temperature, the reducing agent is metered in overstoichiometrically with respect to the nitrogen oxide concentration, taking into account the temperature-dependent storage capacity of the catalyst for the reducing agent, and otherwise metered in substoichiometrically. At a substantially constant catalyst temperature reducing agent is alternately metered in substoichiometric and superstoichiometric, wherein when a lower storage capacity threshold value of the catalyst, the substoichiometric addition is stopped and metered in excess stoichiometrically, and on reaching an upper storage capacity threshold of the catalyst, the superstoichiometric metering is stopped and metered in substoichiometric.

The object underlying the invention is to provide a method and an apparatus for operating an internal combustion engine, which allow a suitable loading of an SCR catalyst with ammonia for a particularly effective exhaust aftertreatment by means of an SCR system of the internal combustion engine.

The object is solved by the features of the independent claims. Advantageous embodiments of the invention are specified in the subclaims.

The invention is characterized by a method and a device for operating an internal combustion engine. The internal combustion engine has an SCR system that includes an SCR catalyst. An ammonia loading level of the SCR catalyst is determined or adjusted. Depending on at least one operating variable of the internal combustion engine, a current nitrogen oxide content of an exhaust gas of the internal combustion engine is determined. Depending on the determined current nitrogen oxide content of the exhaust gas, a base target value of an ammonia medium mass is determined. The ammonia medium mass is to be metered to the exhaust tract for converting the nitrogen oxide of the exhaust gas. A target value of the ammonia mass is determined such that the desired value of the ammonia medium mass oscillates around the basic target value of the ammonia medium mass and that the oscillating nominal value of the ammonia medium mass corresponds at least approximately to the determined base target value of the ammonia medium mass. Depending on the determined nominal value of the ammonia medium mass, an actuating signal for an actuator is determined for converting the nominal value of the ammonia medium mass, whose position has an effect on an actually metered ammonia medium mass. The basic setpoint of the ammonia medium mass is determined as a function of the determined or adjusted ammonia loading level.

The operating variable of the internal combustion engine includes, for example, a load, a rotational speed and / or a combustion temperature a combustion process of the internal combustion engine. The fact that the desired value of the ammonia medium mass oscillates around the basic setpoint value of the ammonia medium mass means in this context that a profile of the nominal value of the ammonia medium mass carries out a vibration about a progression of the basic nominal value of the ammonia medium mass. The fact that the oscillating desired value of the ammonia medium mass on average at least approximately corresponds to the determined base setpoint value of the ammonia medium mass means in this context that an averaging of the profile of the nominal value of the ammonia medium mass at least approximately corresponds to an averaging of the course of the basic nominal value of the ammonia medium mass. The term "or" is used in this patent application in its original meaning. Thus, "or" combines two elements of a multi-part statement, if these elements relate exclusively to different previously mentioned nouns.

The control of the actuator by means of the determined control signal causes the actual metered Ammoniamediummasse during a first period of time is greater than the Ammoniamediummasse that is needed to convert the nitrogen oxide in the exhaust gas, and that during a second period of time the actually metered Ammonia medium mass is smaller than the ammonia medium mass required for converting the nitrogen oxide. As a result, during the first period of time, the degree of loading of the SCR catalyst increases, and during the second period, the degree of loading of the SCR catalyst decreases. This can contribute to a particularly effective absorption of the ammonia by the SCR catalyst and to a particularly effective conversion of the nitrogen oxide of the exhaust gas by means of the ammonia. This contributes to a particularly effective exhaust aftertreatment of the exhaust gas of the internal combustion engine by means of the SCR system.

In an advantageous embodiment, the desired value of the ammonia medium mass is first determined so that the desired value of the ammonia medium mass is initially greater than the base setpoint of the ammonia medium mass. Subsequently, the desired value of the ammonia medium mass is determined so that the desired value of the ammonia medium mass oscillates around the basic target value of the ammonia medium mass. This contributes to the fact that initially the SCR catalyst is loaded with enough ammonia, so that in each case a sufficient conversion of the nitrogen oxides of the exhaust gas is possible. This contributes to the fact that the exhaust gas aftertreatment by means of the SCR system, for example after an engine start of the internal combustion engine, is particularly particularly particularly effective.

In a further advantageous embodiment, the nominal value of the ammonia medium mass is determined such that the desired value of the ammonia medium mass is greater than the base target value of the ammonia medium mass until the ammonia loading level of the SCR catalyst is greater than a predefined first threshold value or until the current nitrogen oxide content of the exhaust gas is less than is a predetermined second threshold. Furthermore, the nominal value of the ammonia medium mass is determined so that subsequently the desired value of the ammonia medium mass is smaller than the basic nominal value of the ammonia medium mass. This can contribute to the particularly effective loading of the SCR catalyst and / or to a particularly effective conversion of the nitrogen oxides of the exhaust gas.

In a further advantageous embodiment, the nominal value of the ammonia medium mass is determined so that the desired value of the ammonia medium mass is smaller than the base setpoint of the ammonia medium mass until the ammonia loading of the SCR catalyst is less than a predetermined third threshold or until the current nitrogen oxide content of the exhaust gas is greater is a predetermined fourth threshold. Furthermore, the nominal value of the ammonia medium mass is determined so that subsequently the desired value of the ammonia medium mass is greater than the basic nominal value of the ammonia medium mass. This can contribute to a particularly effective conversion of the nitrogen oxides of the exhaust gas and / or to a particularly effective charging of the SCR catalyst.

Embodiments of the invention are explained in more detail below with reference to schematic drawings.

Show it:

1 an internal combustion engine,

2 an SCR system of the internal combustion engine,

3 a flowchart of a program for operating the internal combustion engine.

Elements of the same construction or function are identified across the figures with the same reference numerals.

An internal combustion engine 14 ( 1 ) comprises an intake tract 1 , an engine block 2 , a cylinder head 3 and an exhaust tract 4 , The intake tract 1 preferably comprises a throttle valve 5 , a collector 6 and a suction tube 7 leading to a cylinder Z1-Z4 via an inlet channel into a combustion chamber 9 of the engine block 2 is guided. The combustion chamber 9 communicates depending on a switching position a gas inlet valve 12 or a gas outlet valve 13 with the intake tract 1 or with the exhaust gas tract 4 , The engine block 2 includes a crankshaft 8th , which has a connecting rod 10 with a piston 11 of the cylinder Z1-Z4 is coupled. The internal combustion engine 14 preferably comprises further cylinders Z1-Z4. The internal combustion engine 14 is preferably arranged in a motor vehicle.

In the cylinder head 3 is preferably a fuel injection valve 18 arranged. If the internal combustion engine 14 is not a diesel engine, so preferably each cylinder Z1-Z4 are each assigned a spark plug. Alternatively, the fuel injection valve 18 also in the intake manifold 7 be arranged.

Furthermore, the exhaust tract 4 assigned an SCR system (Selective Catalytic Reduction System) ( 2 ). The SCR system includes a urea tank 40 for taking up the urea, a urea injection valve 54 and preferably a pump 42 for supplying the urea injection valve 54 with urea from the urea tank 40 , The urea can be referred to in this context as an aqueous urea solution. The urea may be from the urea tank 40 via a urea line 41 by means of the pump 42 to the urea injection valve 54 be directed. The metering of urea into the exhaust tract 4 may additionally or alternatively to the urea injection valve 54 via a urea valve 52 at the urea line 41 to be controlled.

Upstream of the urea injection valve 54 is preferably a soot particle filter 21 arranged. Downstream of the urea injection valve 54 is preferably a mixing device 56 for mixing the metered urea with the exhaust gas. Further, downstream of the mixing device 56 an SCR catalyst 23 arranged. In addition to the SCR catalyst 23 may be upstream of the SCR catalyst 23 and downstream of the mixing device 56 a hydrolysis catalyst may be provided downstream of the SCR catalyst 23 An oxidation catalyst may be provided.

If the internal combustion engine 14 is operated with a lean mixture and thus is in lean operation is in a combustion process of the internal combustion engine 14 the combustion chamber 9 supplied less fuel than with the oxygen in the combustion chamber 9 can be burned. This forms over a stoichiometric operation of the internal combustion engine 14 Increases nitrogen oxides, which are then contained in the exhaust gas. In the presence of ammonia, the nitrogen oxides in the SCR catalyst can 23 react to elemental nitrogen and water. That's why the exhaust tract 4 with the urea injection valve 54 Preferably, the urea is metered from which emerges in a chemical reaction of ammonia. The urea, especially the ammonia, mix mainly in the mixing device 56 with the exhaust gas of the internal combustion engine 14 ,

A control device 25 is provided, the sensors are assigned, which detect different parameters and each determine the value of the measured variable. The control device 25 determined depending on at least one of the measured variables at least one manipulated variable, which are then converted into one or more control signals for controlling the actuators by means of corresponding actuators. The control device 25 can also be used as a device for operating the internal combustion engine 14 be designated.

The sensors are, for example, a pedal position transmitter 26 , the accelerator pedal position of an accelerator pedal 27 detected, an air mass sensor 28 , the air mass flow upstream of the throttle 5 detected, a temperature sensor 32 sensing an intake air temperature, an intake manifold pressure sensor 34 that produces a manifold pressure in the collector 6 detected, a crankshaft angle sensor 36 , which detects a crankshaft angle, then a speed of the internal combustion engine 14 can be assigned, a urea temperature sensor 43 for detecting a urea temperature of the urea in the urea tank 40 , Furthermore, an exhaust gas probe 38 provided, for example, downstream of the SCR catalyst 23 is arranged and detects, for example, the nitrogen oxide content and / or a urea content of the exhaust gas.

Depending on the embodiment of the invention, any subset of said sensors may be present, or additional sensors may be present.

The actuators are, for example, the throttle 5 , the gas inlet and outlet valves 12 . 13 , the fuel injection valve 18 , the urea injector 54 , the urea valve 52 , the pump 42 and / or optionally the spark plug.

On a storage medium of the control device 25 is preferably a program for operating the internal combustion engine 14 saved ( 3 ). The program serves to determine a nominal value NH3_MASS_SP of the ammonia medium mass so that the nominal value NH3_MASS_SP of the ammonia medium mass oscillates around a nominal base value NH3_MASS_BAS of the ammonia medium mass and at least approximately corresponds to it on average, and thus exhaust gas aftertreatment by means of the SCR system is particularly effective. That the setpoint NH3_MASS_SP the ammonia medium mass on average at least approximately corresponds to the base setpoint means in this context that a difference between a time average of the setpoint NH3_MASS_SP the ammonia medium mass and a time average of the base setpoint NH3_MASS_BAS the ammonia medium mass is less than a predetermined setpoint threshold.

The program is preferably timely an engine start of the internal combustion engine 14 started in a step S1, in which variables are initialized if necessary.

In a step S2, a loading level NH3_LD of the SCR catalyst 23 determined. The degree of loading NH3_LD is preferably determined by means of a catalyst loading model. In this case, an ammonia mass is preferably determined at least as a function of one of the measured variables, with which the SCR catalyst 23 was charged and it is determined an ammonia mass, which is responsible for the loss of nitrogen oxides from the SCR catalyst 23 was delivered. The difference between the two variables is representative of the degree of loading NH3_LD of the SCR catalyst 23 , Alternatively, a loading degree sensor or a loading degree sensor arrangement may be provided.

Alternatively, in step S2, the degree of loading NH3_LD of the SCR catalyst 23 set and thus specified.

In a step S3, a current nitrogen oxide content NOX_AV of the exhaust gas is determined. The current nitrogen oxide content NOX_AV, for example, by means of a nitrogen oxide sensor downstream of the gas outlet valve 13 and upstream of the urea injection valve 54 be recorded. Preferably, however, the current nitrogen oxide content NOX_AV of the exhaust gas is determined by means of a model calculation and / or a performance map. The model calculation, the map and possibly other model calculations or maps are preferably determined and recorded on an engine test bench. As input variables for determining the current nitrogen oxide content NOX_AV, for example, a load of the internal combustion engine 14 , the speed of the internal combustion engine 14 and / or a combustion temperature of a combustion process in the combustion chamber 9 be used. The load of the internal combustion engine 14 is represented by one of the operating variables, in particular a load variable. The load size includes, for example, the air mass flow and / or the intake manifold pressure.

In a step S4, a base target value NH3_MASS_BAS of the ammonia medium mass becomes dependent on the degree of loading NH3_LD of the SCR catalytic converter 23 and determined depending on the current nitrogen oxide content NOX_AV of the exhaust gas. The basic setpoint NH3_MASS_BAS of the ammonia medium mass can be determined, for example, by means of a further model calculation or by means of a further characteristic diagram. The base setpoint NH3_MASS_BAS of the ammonia medium mass is preferably determined so that at constant nitrogen oxide content of the exhaust gas and at constant base setpoint NH3_MASS_SP the ammonia medium mass is mixed with the exhaust gas just as much ammonia medium mass that the nitrogen oxides of the exhaust gas are preferably completely converted.

In a step S5, a nominal value NH3_MASS_SP of the ammonia medium mass is determined as a function of the determined base setpoint NH3_MASS_BAS of the ammonia medium mass. Preferably, the nominal value NH3_MASS_SP of the ammonia medium mass is first determined so that the nominal value NH3_MASS_SP of the ammonia medium mass is greater than the nominal base value NH3_MASS_BAS of the ammonia medium mass. This contributes to the fact that at the beginning of the exhaust aftertreatment the SCR catalyst 23 is loaded with enough ammonia to convert the nitrogen oxides of the exhaust gas. Subsequently, the nominal value NH3_MASS_SP of the ammonia medium mass is varied such that the nominal value NH3_MASS_SP of the ammonia medium mass oscillates around the determined base nominal value NH3_MASS_BAS of the ammonia medium mass, in such a way that a profile of the nominal value NH3_MASS_SP of the ammonia medium mass at least approximately corresponds to a profile of the base target value NH3_MASS_BAS over time. This ensures that sufficient ammonia is supplied to the exhaust tract to enable effective exhaust aftertreatment by means of the SCR system.

Furthermore, this takes into account the fact that during a first period of time during which the ammonia on the SCR catalyst 23 is attached, the ammonia can not react with the nitrogen oxides of the exhaust gas and that during a second period of time, during which the ammonia, on the SCR catalyst 23 is associated with the nitrogen oxides of the exhaust gas, no ammonia in the corresponding area of the SCR catalyst 23 on the SCR catalyst 23 can be stored.

In particular, steps S6 to S11 can be executed to selectively vary the nominal value NH3_MASS_SP of the ammonia medium mass.

In step S6, it is checked whether the degree of loading NH3_LD of the SCR catalyst 23 is greater than a predetermined first threshold THD_1. If the condition of step S6 is satisfied, the processing is continued in step S8. If the condition of step S6 is not met, then step S6 is executed again or a step S7 is executed.

In step S7, which is executed in addition to or as an alternative to step S6, it is checked whether the current nitrogen oxide content NOX_AV is less than a predefined second threshold value THD_2. If the condition of step S7 is satisfied, the processing is continued in step S8. If the condition of the step S7 is not satisfied, the processing is continued again in the step S7 or in the step S6.

In a step S8, the nominal value NH3_MASS_SP of the ammonia medium mass is preferably determined as a function of the base nominal value NH3_MASS_BAS such that the nominal value NH3_MASS_SP of the ammonia medium mass is smaller than the nominal base value NH3_MASS_BAS of the ammonia medium mass. Thus, in step S8, the second period of time is initiated while substantially the reaction of the in the SCR catalyst 23 accumulated ammonia with the nitrogen oxides of the exhaust gas takes place.

In a step S9, it is checked whether the ammonia loading degree NH3_LD is smaller than a predetermined third threshold value THD_3. If the condition of step S9 is satisfied, the processing is continued in step S11. If the condition of step S9 is not satisfied, step S9 is executed again or a step S10 is executed.

In step S10, it is checked whether the current nitrogen oxide content NOX_AV is greater than a predetermined fourth threshold value THD_4. If the condition of step S10 is satisfied, the processing is continued in step S11. If the condition of step S10 is not met, step S10 is executed again or step S9 is executed. The step S10 may be executed additionally or alternatively to the step S9.

In a step S11, the program for operating the internal combustion engine 14 to be ended. Preferably, the program for operating the internal combustion engine 14 but regularly during operation of the internal combustion engine 14 processed. Furthermore, with essentially constant basic nominal value NH3_MASS_BAS of the ammonia medium mass, only steps S5 to S10 can be executed.

The threshold values may be constant or depending on at least one of the operating variables of the internal combustion engine 14 be specified.

Claims (5)

Method for operating an internal combustion engine ( 14 ) with an SCR system containing an SCR catalyst ( 23 ), in which - an ammonia loading level (NH3_LD) of the SCR catalyst ( 23 ) is determined or set, - depending on at least one operating variable of the internal combustion engine ( 14 ) a current nitrogen oxide content (NOX_AV) of an exhaust gas of the internal combustion engine ( 14 ) is determined, depending on the determined current nitrogen oxide content (NOX_AV) of the exhaust gas, a base setpoint (NH3_MASS_BAS) of an ammonia medium mass is determined, which is to be metered to the exhaust tract for converting the nitrogen oxide of the exhaust gas, - a target value (NH3_MASS_SP) of the ammonia medium mass is determined the nominal value (NH3_MASS_SP) of the ammonia medium mass oscillates around the nominal base value (NH3_MASS_BAS) of the ammonia medium mass and the oscillating nominal value (NH3_MASS_SP) of the ammonia medium mass corresponds at least approximately to the determined base nominal value (NH3_MASS_BAS) of the ammonia medium mass, - depending on the determined nominal value (NH3_MASS_SP) the ammonia medium mass for converting the nominal value (NH3_MASS_SP) of the ammonia medium mass is determined as an actuating signal for an actuator whose position has an effect on an actually metered ammonia medium mass, characterized in that the basic nominal value (NH3_MASS_BAS) of the ammonia medium mass is determined as a function of the determined or set ammonia loading level (NH3_LD). The method of claim 1, wherein the setpoint value (NH3_MASS_SP) of the ammonia medium mass is first determined so that the setpoint value (NH3_MASS_SP) of the ammonia medium mass is initially greater than the base setpoint value (NH3_MASS_BAS) of the ammonia medium mass, and then the setpoint value (NH3_MASS_SP) of the ammonia medium mass is determined so that the setpoint (NH3_MASS_SP) of the ammonia medium mass oscillates around the base setpoint (NH3_MASS_BAS) of the ammonia medium mass. Method according to one of the preceding claims, wherein the nominal value (NH3_MASS_SP) of the ammonia medium mass is determined such that the nominal value (NH3_MASS_SP) of the ammonia medium mass is greater than the nominal base value (NH3_MASS_BAS) of the ammonia medium mass until the ammonia loading level (NH3_LD) of the ammonia medium mass SCR catalyst ( 23 ) is greater than a predetermined first threshold (THD_1) or until the current nitrogen oxide content (NOX_AV) of the exhaust gas is less than a predetermined second threshold value (THD_2), and then that the setpoint value (NH3_ MASS_SP) of the ammonia medium mass is smaller than the basic setpoint value (NH3_MASS_BAS) of the ammonia medium mass. Method according to one of the preceding claims, wherein the setpoint value (NH3_MASS_SP) of the ammonia medium mass is determined so that the setpoint value (NH3_MASS_SP) of the ammonia medium mass is smaller than the base setpoint value (NH3_MASS_BAS) of the ammonia medium mass until the ammonia loading level (NH3_LD) of the ammonia medium mass decreases SCR catalyst ( 23 ) is less than a predetermined third threshold (THD_3) or until the current nitrogen oxide content (NOX_AV) of the exhaust gas is greater than a predetermined fourth threshold (THD_4), and then the setpoint (NH3_MASS_SP) of the ammonia medium mass is greater than the base setpoint (NH3_MASS_BAS) of the Ammonia medium mass is. Device for operating an internal combustion engine ( 14 ) with an SCR system containing an SCR catalyst ( 23 ), wherein the device is designed to - an ammonia loading level (NH3_LD) of the SCR catalyst ( 23 ) to determine or adjust, - depending on at least one operating variable of the internal combustion engine ( 14 ) a current nitrogen oxide content (NOX_AV) of an exhaust gas of the internal combustion engine ( 14 ), - to determine, based on the determined actual nitrogen oxide content (NOX_AV) of the exhaust gas, a basic target value (NH3_MASS_BAS) of an ammonia medium mass, which is to be assigned to the exhaust gas tract for converting the nitrogen oxide of the exhaust gas, - to determine a target value (NH3_MASS_SP) of the ammonia medium mass the nominal value (NH3_MASS_SP) of the ammonia medium mass oscillates around the nominal base value (NH3_MASS_BAS) of the ammonia medium mass and the oscillating nominal value (NH3_MASS_SP) of the ammonia medium mass corresponds at least approximately to the determined base nominal value (NH3_MASS_BAS) of the ammonia medium mass, - depending on the determined nominal value (NH3_MASS_SP) the ammonia medium mass for converting the nominal value (NH3_MASS_SP) of the ammonia medium mass to determine an actuating signal for an actuator whose position has an effect on an actually metered ammonia medium mass, characterized in that the device is designed to be the basic target t (NH3_MASS_BAS) of the ammonia medium mass depending on the determined or set ammonia loading level (NH3_LD) to determine.

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