DE102010030860A1 - Method for operating reducing agent dosing system for selective catalytic reduction catalyst-converter in exhaust line of combustion engine of motor car, involves actuating metering valve for dosing certain dosage amount into exhaust line - Google Patents

Abstract

The method involves determining a dosing control based on a mass flow balance. A diagnostic of a reducing agent dosing system is carried out through an evaluation of current characteristic of a dosing pump (16). No pressure sensor is provided in the reducing agent dosing system. Required dosing amount is conveyed over a control unit of the pump into a pressure line (17). A metering valve (13) is parallely actuated in such a manner that the dosage amount is dosed into an exhaust line (10). The dosing pump and the metering valve are controlled by a control device (18). Independent claims are also included for the following: (1) a computer program comprising a set of instructions for executing steps involved in a method for operating a reducing agent dosing system selective catalytic reduction catalyst-converter (2) a computer program product with a program code for performing a method for operating a reducing agent dosing system selective catalytic reduction catalyst-converter.

Description

The present invention relates to a method for operating a Reduktionsmitteldosiersystems for an SCR catalyst with a reducing agent tank, a feed pump, a pressure line and at least one electrically controllable metering valve.

State of the art

Methods and apparatuses for operating an internal combustion engine, in particular in motor vehicles, are known, in the exhaust gas region of which an SCR catalytic converter (selective catalytic reduction) is arranged, which reduces the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine to nitrogen in the presence of a reducing agent. As a result, the proportion of nitrogen oxides in the exhaust gas can be significantly reduced. For the course of the reaction ammonia (NH ₃ ) is required, which is admixed to the exhaust gas. As reactants or reducing agents therefore NH ₃ or NH _3- splitting reagents are used. As a rule, an aqueous urea solution is used for this purpose, which is injected in the flow direction upstream of the SCR catalytic converter into the exhaust gas line via a metering valve. From this solution forms NH ₃ , which acts as a reducing agent. To store the urea solution, a reducing agent tank is required.

The reducing agent tank is usually equipped with a suction line to suck the urea solution from the tank can. To promote the urea solution, a delivery pump is provided which conveys the solution through a line system to the metering valve, so that the urea solution can be injected via the metering valve, for example an electromagnetic injection valve, under pressure into the exhaust gas line.

The injection of the urea solution takes place as needed as a function of the nitrogen oxides formed during combustion. The required amount is determined for example in a control unit in dependence on the engine speed or other operating variables. The metering valve is usually controlled accordingly. Here, the Reduktionsmitteldosiersystem forms a hydraulic system, wherein the reductant pressure is decisive for the dosage of the reducing agent. Usually, the system pressure is detected by means of a pressure sensor and adjusted by means of the feed pump a predeterminable system pressure. On the basis of the regulated system pressure, the metering valve is controlled to meter in the required volume into the exhaust gas line. The quantity tolerance of the metered volume is determined in this case essentially by the quantity error of the metering valve and by the measurement error or the measurement tolerance of the pressure sensor. For a check of the functionality of the dosing system, in particular for a check of the metering valves, a diagnosis of the system is performed by means of the pressure sensor. This checks whether the system pressure is plausible. As soon as the pressure sensor detects deviations of a certain amount from the system pressure, it is concluded that there is an error.

In the course of a general need to reduce costs, it is desirable to dispense with the pressure sensor in the reductant dosing system, including the winter-mode heater required for the pressure sensor, and including the housing for the pressure sensor. The invention is therefore based on the object of providing a cost-effective Reduktionsmitteldosiersystem for an SCR catalyst and a method for operating the same, in which can be dispensed with the pressure sensor.

This object is achieved by a method for operating a reducing agent metering system as claimed in claim 1. Preferred embodiments of this method emerge from the dependent claims.

Disclosure of the invention

Advantages of the invention

The core of the invention is that in a method for operating a Reduktionsmitteldosiersystems Dosieransteuerung is based on a mass flow and that a diagnosis of the Reduktionsmitteldosiersystems, in particular a diagnosis of the metering valves, is carried out via an evaluation of the pump current profile. Due to the inventive implementation of a mass balance-controlled dosing control of the pressure sensor for quantity display of the volume of urea solution to be dosed into the exhaust line is no longer required, so that can be completely dispensed with by these measures to a pressure sensor in Reduktionsmitteldosiersystem. Since the commonly used pressure sensor, including the housing and the heating required for winter operation of the sensor causes considerable costs, can be saved by dispensing with the pressure sensor, a significant portion of the costs for the Reduktionsmitteldosiersystem. Furthermore, the pressure sensor commonly used in Reduktionsmitteldosiersystem is a relatively susceptible to interference and sensitive component. By dispensing with the pressure sensor with appropriately operated Reduktionsmitteldosiersystem thus a source of error in the system is avoided, whereby the robustness of the Reducing agent metering system over conventional systems is improved.

The inventive method is based on a volumetric feed pump, comparable to a reciprocating pump from. This may be, for example, a solenoid diaphragm pump commonly used in reducing agent metering systems. By controlling the feed pump is given, which volume is funded by the pump. So it's known what volume is put into the system. The mass flow balance is composed of the delivery rate of the feed pump and the metered amount of the metering device. If necessary, the mass flow of a leak must also be taken into account. The metering valve (s) are controlled in parallel so that the amount delivered via the delivery pump is metered into the exhaust gas line. Due to the mass flow balance or the mass conservation, a balance is established in such a way that the same volume is metered into the exhaust gas line via the metering valve, as is conveyed via the feed pump. If the metering valve shows a quantity deviation or a tolerance to the target amount, the system pressure will rise or fall accordingly to the point until the delivered amount corresponds to the metered into the exhaust line amount. Since the delivered quantity is known by the volumetric properties of the feed pump, the amount metered into the exhaust gas line must therefore be precisely determined or precisely controlled. The metered quantity is thus specified by the pump. The quantity tolerance depends in principle on the delivery tolerance of the pump. The dosage is carried out in a dependent of the delivery tolerance pressure range in the range of the system pressure.

The metering control based on the mass flow balance is accompanied by a monitoring of the system by evaluating the pump flow path. Based on the pump flow, the system pressure can be determined within certain tolerances. This is sufficient to check if there is a fault in the system, for example a clogged metering valve. As soon as the pump current profile exceeds or falls below predefinable threshold values which indicate an overshoot or undershoot of acceptable pressure values, an error in the reducing agent metering system is inferred. In this case, improper dosage is assumed. Overall, it is thus possible to completely dispense with a pressure sensor, since the system pressure is no longer relevant for the quantity display. The system pressure is determined indirectly only for the diagnosis of the system by evaluating the pump flow profile and from this it is closed to the system pressure. The method according to the invention can then be used with particular advantage if the delivery rate of the pump is substantially exact and the system structure takes place without return and without substantial leakage of the system.

For the evaluation of the pump current, for example, the time interval between the beginning of the energization and the stop of the solenoid can be evaluated, since this period is dependent on the pressure in the system. Other criteria are possible.

The invention further comprises a computer program that performs all the steps of the method according to the invention when it is executed on a computing device or a control device. Finally, the invention relates to a computer program product with program code, which is stored on a machine-readable carrier, for carrying out the method described, when the program is executed on a computing device or a control device. With the aid of the computer program or the computer program product, the method according to the invention can readily be used in a reducing agent metering system for an SCR catalyst, wherein a pressure sensor can be dispensed with in the reducing agent metering system. As a result, the costs for the Reduktionsmitteldosiersystems can be significantly reduced, since the installation of a pressure sensor including its housing and a required for the operation of the pressure sensor heater is no longer required.

Further advantages and features of the invention will become apparent from the following description of embodiments in conjunction with the drawings. In this case, the individual features can be implemented individually or in combination with each other.

Brief description of the drawings

In the drawings show:

1 a schematic representation of the components of a Reduktionsmitteldosiersystems for an SCR catalyst;

2 the time profile of the pump current (A), the stroke of the pump (B) and the pressure measurable via a pressure sensor in the system (C) to illustrate the method according to the invention;

3 the time course of the pump current (I);

4 the time course of the pump current together with the signal of a Nadelhubsensors and

5 the dependence of the period between the start of energization and the anchor stop against the system pressure.

Description of exemplary embodiments

1 schematically shows the components of a Reduktionsmitteldosiersystems for an SCR catalyst. In the exhaust system 10 an internal combustion engine 11 is an SCR catalyst 12 arranged, which selectively reduces the nitrogen oxides in the exhaust gas by a selective catalytic reduction (SCR). For this reaction, ammonia (NH ₃ ) is needed as a reducing agent. Since ammonia is a toxic substance, this substance is derived from the non-toxic carrier substance urea. Urea is used as a liquid urea water solution via a metering valve 13 in the exhaust system 10 upstream of the SCR catalyst 12 injected. The urea water solution is placed in a reducing agent tank 14 stored. To remove the urea water solution is a suction line 15 in the reducing agent tank 14 intended. The solution is via a feed pump 16 from the reducing agent tank 14 promoted and through the pressure line 17 to the metering valve 13 directed. The activation of the feed pump 16 and the metering valve 13 via a control unit 18 ,

In this Reduktionsmitteldosiersystem for the SCR catalyst is omitted according to the invention to a pressure sensor. This is made possible by a method for operating the reducing agent metering system, in which the metering control takes place on the basis of a mass balance and a diagnosis of the system is carried out on the basis of an evaluation of the pump flow profile. By these measures, a pressure sensor for a needs-based metering of the reducing agent solution is no longer necessary, so that can be dispensed with. Prerequisite for the implementation of the method is the use of a volumetric feed pump 16 , For example, a solenoid diaphragm pump. The inventive method is based on the fact that the mass flow balance of the pressure side of the metering system from the flow rate of the pump, the metered amount of the metering valve and optionally a leakage composed. Here, the pressure change depends on the different mass flows according to the following formula: ṁ _pump - ṁ _DV - ṁ _leakage = ṁ _{pressure change} , where ṁ denotes the mass flows of the pump, the metering valve (DV) and possible leakage. If the sum of the mass flows is not equal to 0, the system pressure slowly builds up or down until the mass flow balance equals zero. In other words, in a system with negligible leakage and with a corresponding control of the metering valve, the system pressure changes until it reaches the exhaust gas line 10 metered amount of through the pump 16 subsidized amount corresponds. The metered amount is specified by the pump and the quantity tolerance is determined in principle by the delivery tolerance of the pump. This can be dispensed with an adjustment of the system pressure and a required measurement of the pressure. Metering also takes place without regulation of the system pressure in a pressure range dependent on the delivery tolerance around the desired system pressure. For example, 6 bar can be specified as the target system pressure. The resulting equilibrium pressure is then in this range as dosing.

The control of the dosage is in particular implemented so that the Dosierwunschmenge (target amount), which is dependent on the nitrogen oxide content in the exhaust gas and is determined in particular on the basis of operating variables of the internal combustion engine, by appropriate control of the feed pump 16 in the pressure line 17 is encouraged. The metering valve 13 is controlled in parallel so that this quantity is metered into the exhaust gas. This is based on the usual system pressure, which prevails in a faultless system. Promotes the pump 16 Too much or too little reducing agent, the pressure builds up or down until the actual metered amount corresponds to the amount pumped by the feed pump.

To detect a possible fault in the system, such as a clogged metering valve 13 , the course of the pump current, the operation of the pump 16 is required, recorded and evaluated. From the pump flow history can be closed to the system pressure within certain tolerances, whereby a diagnosis of the system is possible without a pressure sensor would be required.

To clarify the relationship between the pump current profile, the resulting stroke of the pump and the measurable pressure in the system, the time courses of these measurement and operating variables in the 2 shown. The current flow of a volumetric pump ( 2A ), for example, a pump with solenoid drive, varies depending on the system pressure ( 2C ) against which the pump must be pumped. 2 B shows the signal from a needle lift sensor to visualize the movement of the pump's armature. The current profile can be used in detection and evaluation in a control unit to infer the system pressure with sufficient for monitoring accuracy. For example, the period of time "current supply to the end stop of the armature" or the so-called GDP (begin of injection period) can be evaluated. These variables are influenced by the pressure on the pressure side of the pump. For larger deviations of the prevailing pressure from the usual system pressure can be concluded that there are various errors or defects, such as a jamming of the pump 16 , a defective pump diaphragm, a leakage in the pressure line 17 , a clogged metering valve 13 or problems in the suction area of the pump 16 , Overall, the inventive method allows a precise and needs-based metering of the reducing agent by appropriate control of the feed pump 16 , wherein the metering valve 13 is controlled in parallel. The only possible due to this dosage is based on the fact that under the condition of a negligible leakage at different mass flows of pump 16 and metering valve 13 the system pressure changes until the mass flow balance equals 0. When this point is reached, the metered quantity becomes exactly by controlling the delivery pump 16 certainly. The simultaneously required diagnosis or plausibility of the dosing system is based on an evaluation of the pump flow path, which allows a conclusion on the system pressure. If the pump flow and / or the system pressure, which can be determined indirectly from these, deviate from predefinable values which represent a system which functions properly, an error in the system is deduced. Thus, a pressure sensor can altogether be dispensed with for the operation of the reducing agent metering system, so that the costs for the system can be considerably reduced as a result.

3 illustrates the time course of the current I of Hubmagnetpumpe and shows the stop of the solenoid. To the with the reference numeral 30 designated time begins to energize the solenoid drive. To the with the reference numeral 31 designated time, the upper stop of the solenoid is reached. The period between the start of lighting 30 and the top stop 31 is marked with a double arrow. To clarify the anchor movement is in 4 in addition to the current profile of the Hubmagnetpumpe the signal of a Nadelhubsensors located (reference numeral 40 ), which illustrates the movement of the lifting magnet, so the armature movement. Time after the start of the energization 30 starts the armature movement, here with the reference number 41 designated. The armature movement can be recognized by the falling signal of the needle lift sensor. The end of the armature movement is marked by the signal of the needle lift sensor which is the same below in the following, here by the reference numeral 42 features. The end of the anchor movement 42 refers to the top stop of the solenoid. The time from the beginning of the energization 30 until reaching the upper stop of the solenoid 31 or until the end of the anchor movement 42 proves to be a sensible criterion to be able to infer the pressure in the system. 5 shows the relationship of the length of this period in [ms] with the prevailing pressure. With increasing pressure, the time between energization start and upper stop of the solenoid lengthens. By measuring the time between the start of energization and the upper stop of the solenoid or the armature stop can therefore be deduced the system pressure. This relationship is used according to the invention to be able to dispense with a pressure sensor.

It can be used in the context of the invention, various parameters that affect this period between the start of energization and anchor stop, for example, the battery voltage or the coil temperature. Corresponding correction functions can be used to correct these "disturbance parameters". For example, the battery voltage in the control unit is known, so that the battery voltage can be corrected accordingly. Also, for example, the coil temperature can be calculated via a current feedback, so that this parameter can be taken into account.

The evaluation of the time from the start of energization to the armature stop represents an example of a pressure-dependent parameter of the solenoid drive, which is used according to the invention. Other pressure-dependent criteria are also possible with solenoid-operated pumps or other types of pumps in order to be able to be used according to the invention to draw conclusions about the pressure in the system.

Claims (7)

Method for operating a reducing agent metering system for an SCR catalyst ( 12 ) with a reducing agent tank ( 14 ), a feed pump ( 16 ), a pressure line ( 17 ) and at least one electrically controllable metering valve ( 13 ), characterized in that the dosing control is determined on the basis of a mass flow balance and that a diagnosis of the Reduktionsmitteldosiersystems is made via an evaluation of the pump current waveform. A method according to claim 1, characterized in that in the Reduktionsmitteldosiersystem no pressure sensor is provided. A method according to claim 1 or claim 2, characterized in that the mass flow balance from the delivery rate of the feed pump ( 16 ), the metering amount of the metering valve ( 13 ) and, where appropriate, the mass flow of a leakage, wherein the required metered amount via a control of the pump ( 16 ) into the pressure line ( 17 ) and the metering valve ( 13 ) is controlled in parallel so that this metered amount in the exhaust line ( 10 ) is metered. Method according to one of the preceding claims, characterized in that is closed from the pump flow path to the system pressure. A method according to claim 4, characterized in that is closed at a deviation of the pump current profile of predetermined values to a fault in the Reduktionsmitteldosiersystem. A computer program executing all the steps of a method according to any one of claims 1 to 5 when stored on a computing device or controller ( 18 ) is performed. Computer program product with program code, which is stored on a machine-readable carrier, for carrying out a method according to one of claims 1 to 5, when the program is stored on a computing device or a control device ( 18 ) is performed.

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