Method for operating a reducing agent dosing system for an SCR catalyst

Abstract

Method for operating a reducing agent dosing 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 metering control is determined on the basis of a mass flow balance and that a diagnosis of the reducing agent dosing system is carried out by evaluating the pump current profile, wherein no pressure sensor is provided in the reducing agent dosing system, wherein the system pressure is inferred from the pump current profile, wherein the system pressure is inferred by measuring a time between an energization start (30) and an upper armature stop (31) of a lifting magnet of the feed pump (16).

Description

[0001] The present invention relates to a method for operating a reducing agent metering system for an SCR catalyst comprising a reducing agent tank, a feed pump, a pressure line and at least one electrically controllable metering valve. State of the art

[0002] Methods and devices for operating an internal combustion engine, particularly in motor vehicles, are known in which an SCR (Selective Catalytic Reduction) catalyst is arranged in the exhaust system. This catalyst reduces the nitrogen oxides (NOx) contained in the exhaust gas of the internal combustion engine to nitrogen in the presence of a reducing agent. This significantly reduces the proportion of nitrogen oxides in the exhaust gas. Ammonia (NH3) is required for the reaction and is added to the exhaust gas. Therefore, NH3 or NH3-releasing reagents are used as the reactant or reducing agent. Typically, an aqueous urea solution is used for this purpose, which is injected into the exhaust stream upstream of the SCR catalyst via a metering valve. NH3 is formed from this solution and acts as the reducing agent. A reducing agent tank is required to store the urea solution.

[0003] The reducing agent tank is typically equipped with a suction line to draw the urea solution from the tank. A pump is used to deliver the urea solution through a piping system to the metering valve, where it can be injected under pressure into the exhaust system via the metering valve, for example, an electromagnetic injection valve.

[0004] The injection of the urea solution is demand-based, depending on the nitrogen oxides produced during combustion. The required quantity is determined, for example, by a control unit based on engine speed or other operating parameters. The metering valve is typically controlled accordingly. The reducing agent metering system is a hydraulic system, with the reducing agent pressure being the determining factor for metering. A pressure sensor is usually used to measure the system pressure, and a preset system pressure is regulated by the feed pump. Based on this regulated system pressure, the metering valve is controlled to inject the required volume into the exhaust system. The volume tolerance of the injected volume is primarily determined by the metering valve's measurement error and the pressure sensor's measurement error or tolerance.To verify the functionality of the dosing system, and in particular the dosing valves, a system diagnostic test is performed using the pressure sensor. This test checks whether the system pressure is plausible. As soon as the pressure sensor detects deviations of a predefined magnitude from the system pressure, it is concluded that a fault has occurred.

[0005] In response to the general need for cost reduction, it is desirable to eliminate the pressure sensor in the reducing agent dosing system, including the heating element required for winter operation and the housing for the pressure sensor. The invention therefore aims to provide a cost-effective reducing agent dosing system for an SCR catalyst and a method for operating it, in which the pressure sensor can be omitted.

[0006] German patent application DE 10 2007 028 487 A1 discloses a method in which the reagent pressure is calculated based on a pressure model derived from a balance calculation. To validate this model, it is proposed to record a pump motor load current as a single value and derive a pressure value from it using a characteristic curve. The actual dosing, however, remains pressure-based.

[0007] The publication DE 10 2008 001 184 A1 concerns a method for verifying the plausibility of a common-rail fuel system. It teaches how to evaluate the rise time of the rail pressure in order to calibrate a control system. An analysis of the pump flow curve in an SCR system for direct pressure determination is not described here.

[0008] German patent application DE 101 37 871 C1 describes a method for calibrating an existing pressure sensor in an injection system. This method aims to improve the accuracy of an existing sensor, rather than eliminating its presence.

[0009] This problem is solved by a method for operating a reducing agent dosing system as described in claim 1. Preferred embodiments of this method are set forth in the dependent claims. Disclosure of the invention Advantages of the invention

[0010] The core of the invention is to implement a mass flow balance-based dosing control in a method for operating a reducing agent dosing system that dispenses with a pressure sensor, such that the delivery rate of the feed pump directly determines the quantity to be dosed. Simultaneously, a diagnosis of the reducing agent dosing system is performed by evaluating the pump flow curve, whereby the system pressure is inferred by measuring a time interval within the flow curve, in particular the time between the start of current application and the upper armature stop. Through the inventive implementation of a mass balance-controlled dosing system, the pressure sensor for representing the volume of urea solution to be dosed into the exhaust gas stream is no longer required, thus eliminating the need for a pressure sensor in the reducing agent dosing system altogether.Since the commonly used pressure sensor, including its housing and the heating element required for winter operation, incurs considerable costs, eliminating the pressure sensor can significantly reduce the cost of the reducing agent dosing system. Furthermore, the pressure sensor typically used in reducing agent dosing systems is a relatively failure-prone and sensitive component. By eliminating the pressure sensor in a properly operated reducing agent dosing system, a potential source of error is avoided, thereby improving the robustness of the reducing agent dosing system compared to conventional systems.

[0011] The method according to the invention starts with a volumetric feed pump, comparable to a reciprocating piston pump. This could, for example, be a solenoid-type diaphragm pump commonly used in reducing agent dosing systems. The feed pump is controlled to determine the volume it delivers. Thus, the volume introduced into the system is known. The mass flow balance consists of the feed pump's delivery rate and the metering quantity from the metering device. If necessary, the mass flow rate of any leakage must also be taken into account. The metering valve(s) are controlled in parallel so that the quantity delivered by the feed pump is metered into the exhaust gas stream. Based on the mass flow balance, or...Due to the conservation of mass, an equilibrium is established such that the same volume is metered into the exhaust system via the metering valve as is delivered by the feed pump. If the metering valve exhibits a deviation or tolerance from the target quantity, the system pressure will rise or fall accordingly until the delivered quantity equals the quantity metered into the exhaust system. Since the delivered quantity is known due to the volumetric properties of the feed pump, the quantity metered into the exhaust system can be precisely determined or controlled. The metered quantity is thus determined by the feed pump. The quantity tolerance is essentially dependent on the delivery tolerance of the feed pump. The metering takes place within a pressure range dependent on the delivery tolerance, within the range of the system pressure.

[0012] The dosing control based on the mass flow balance is accompanied by system monitoring through evaluation of the pump flow curve. The system pressure can be determined within certain tolerances based on the pump flow curve. This is sufficient to check for system faults, such as a blocked dosing valve. As soon as the pump flow curve exceeds or falls below predefined thresholds indicating that acceptable pressure values ​​have been exceeded or fallen below, a fault in the reducing agent dosing system is inferred. In this case, improper dosing is assumed. Therefore, a pressure sensor can be completely dispensed with, as the system pressure is no longer relevant for quantity representation. The system pressure is determined indirectly solely for system diagnostics by evaluating the pump flow curve and inferring the system pressure from it.The method according to the invention can be used to a particular advantage if the delivery rate of the feed pump is essentially exact and the system setup is without backflow and without significant leakage of the system.

[0013] For evaluating the pump current, the time interval between the start of the current application and the end stop of the solenoid can be analyzed, as this time interval depends on the pressure in the system. Other criteria are also possible.

[0014] The invention further comprises a computer program that executes all steps of the method according to the invention when run on a computer or a control unit. Finally, the invention includes a computer program product with program code stored on a machine-readable medium for carrying out the described method when the program is run on a computer or a control unit. With the aid of the computer program or the computer program product, the method according to the invention can be readily implemented in a reducing agent dosing system for an SCR catalyst, thereby eliminating the need for a pressure sensor in the reducing agent dosing system. This significantly reduces the cost of the reducing agent dosing system, as the installation of a pressure sensor, including its housing and the heating element required for its operation, is no longer necessary.

[0015] Further advantages and features of the invention will become apparent from the following description of exemplary embodiments in conjunction with the drawings. The individual features can be implemented individually or in combination with one another. Brief description of the drawings

[0016] The drawings show: Fig. 1 a schematic representation of the components of a reducing agent dosing system for an SCR catalyst; Fig. 2 the time course of the pump flow (A), the stroke of the feed pump (B) and the pressure (C) measurable via a pressure sensor in the system to illustrate the method according to the invention; Fig. 3 the time course of the pump current (I); Fig. 4 the temporal profile of the pump current together with the signal from a needle lift sensor and Fig. 5 the dependence of the time interval between the start of current application and the anchor stop on the system pressure. Description of exemplary implementations

[0017] Fig. Figure 1 schematically shows the components of a reducing agent dosing system for an SCR catalyst. An SCR catalyst 12 is arranged in the exhaust stream 10 of an internal combustion engine 11. This catalyst selectively reduces the nitrogen oxides in the exhaust gas by means of selective catalytic reduction (SCR). Ammonia (NH3) is required as a reducing agent for this reaction. Since ammonia is a toxic substance, it is obtained from the non-toxic carrier substance urea. Urea is injected as a liquid urea-water solution into the exhaust stream 10 upstream of the SCR catalyst 12 via a metering valve 13. The urea-water solution is stored in a reducing agent tank 14. A suction line 15 is provided in the reducing agent tank 14 for extracting the urea-water solution. The solution is pumped from the reducing agent tank 14 via a pump 16 and conveyed through the pressure line 17 to the metering valve 13.The control of the feed pump 16 and the metering valve 13 is carried out via a control unit 18.

[0018] In this reducing agent dosing system for the SCR catalyst, the invention eliminates the need for a pressure sensor. This is achieved through a method for operating the reducing agent dosing system in which the dosing control is based on a mass balance and system diagnostics are performed by evaluating the pump flow curve. These measures eliminate the need for a pressure sensor for demand-based dosing of the reducing agent solution. A prerequisite for carrying out the method is the use of a volumetric feed pump 16, for example, a solenoid diaphragm pump. The method according to the invention is based on the fact that the mass flow balance of the pressure side of the dosing system consists of the delivery rate of the feed pump 16, the metering quantity of the metering valve, and, if applicable, any leakage.The pressure change depends on the different mass flows according to the following formula: m˙Pump−m˙DV−m˙Leakage=m˙Pressure change, where ṁ denotes the mass flow rates of the feed pump 16, the metering valve (DV), and any potential leakage. If the sum of the mass flow rates is not zero, the system pressure slowly builds up or falls until the mass flow balance is zero. This means that in a system with negligible leakage and appropriate control of the metering valve, the system pressure changes until the quantity metered into the exhaust stream 10 corresponds to the quantity delivered by the feed pump 16. The metered quantity is determined by the feed pump 16, and the quantity tolerance is essentially determined by the delivery tolerance of the feed pump 16. Therefore, system pressure regulation and the associated pressure measurement are unnecessary. Metering occurs even without system pressure regulation within a pressure range around the desired system pressure, dependent on the delivery tolerance.For example, a target system pressure of 6 bar can be specified. The resulting equilibrium pressure will then be within this range as the metering pressure.

[0019] The dosing control is implemented in such a way that the desired dosing quantity (target quantity), which depends on the nitrogen oxide content in the exhaust gas and is determined primarily based on the operating parameters of the internal combustion engine, is delivered to the pressure line 17 by appropriately controlling the feed pump 16. The metering valve 13 is controlled in parallel so that this quantity is metered into the exhaust gas. This assumes a typical system pressure, which prevails in a fault-free system. If the feed pump 16 delivers too much or too little reducing agent, the pressure increases or decreases until the actual dosing quantity corresponds to the quantity delivered by the feed pump 16.

[0020] To detect a potential fault in the system, such as a blocked metering valve 13, the pump flow required to operate the feed pump 16 is recorded and evaluated. The system pressure can be inferred from the pump flow curve within certain tolerances, thus enabling system diagnosis without the need for a pressure sensor.

[0021] To illustrate the relationships between the pump flow curve, the resulting stroke of the feed pump 16 and the pressure measurable in the system, the time profiles of these measured and operating parameters are shown in the Fig. 2 shown. The flow pattern of a volumetric pump 16 ( Fig. 2A), for example a feed pump 16 with solenoid drive, changes depending on the system pressure ( Fig. 2C), against which the pump 16 must pump. Fig. Figure 2B shows the signal from a needle lift sensor to visualize the movement of the armature of the feed pump 16. When recorded and evaluated in a control unit, the current waveform can be used to deduce the system pressure with sufficient accuracy for monitoring purposes. For example, the duration of "energization of the solenoid until the armature reaches its end stop" or the so-called BIP (begin of injection period) can be evaluated. These parameters are influenced by the pressure on the discharge side of the feed pump 16. Significant deviations of the prevailing pressure from the usual system pressure can indicate various faults or defects, such as a jammed feed pump 16, a defective pump diaphragm, a leak in the discharge line 17, a blocked metering valve 13, or problems in the suction section of the feed pump 16.Overall, the method according to the invention allows for precise and demand-based dosing of the reducing agent by appropriately controlling the feed pump 16, with the metering valve 13 being controlled in parallel. The demand-based dosing made possible solely by this method is based on the fact that, assuming negligible leakage, the system pressure changes with differing mass flow rates of the feed pump 16 and metering valve 13 until the mass flow balance equals zero. Upon reaching this point, the metering quantity is precisely determined by controlling the feed pump 16. The simultaneously required diagnosis and plausibility check of the metering system is performed by evaluating the pump flow curve, which allows conclusions to be drawn about the system pressure.If the pump flow rate and / or the system pressure indirectly determined from it deviates from predefined values ​​representing a properly functioning system, a fault in the system is assumed. Therefore, a pressure sensor can be omitted for the operation of the reducing agent dosing system, thus significantly reducing system costs.

[0022] Fig. Figure 3 illustrates the time course of the current I in the solenoid pump and shows the end stop of the solenoid. The current to the solenoid pump begins at the time indicated by reference numeral 30. The upper end stop of the solenoid is reached at the time indicated by reference numeral 31. The period between the start of current 30 and the upper end stop 31 is marked with a double arrow. To clarify the armature movement, in Fig. Figure 4, in addition to the current waveform of the solenoid pump, shows the signal from a needle lift sensor (reference 40), which illustrates the movement of the solenoid, i.e., the armature movement. The armature movement, here designated by reference 41, begins shortly after the start of the current energization 30. The armature movement can be identified by the decreasing signal from the needle lift sensor. The end of the armature movement is marked by the subsequently constant signal from the needle lift sensor, here designated by reference 42. The end of the armature movement 42 indicates the upper limit of the solenoid. The time from the start of the current energization 30 until the upper limit of the solenoid 31 is reached, or until the end of the armature movement 42, proves to be a useful criterion for inferring the pressure in the system. Fig.Figure 5 shows the relationship between the length of this time interval in [ms] and the prevailing pressure. As the pressure increases, the time interval between the start of current application and the upper stop of the solenoid increases. Therefore, by measuring the time between the start of current application and the upper stop of the solenoid or the armature stop, the system pressure can be deduced. According to the invention, this relationship is used to eliminate the need for a pressure sensor.

[0023] Various parameters can be used within the scope of the invention that influence the period between the start of current application and the armature stop, for example, the battery voltage or the coil temperature. These "interference parameters" can be corrected by appropriate correction functions. For example, the battery voltage is known in the control unit, so that it can be corrected accordingly. Similarly, the coil temperature can be calculated via current feedback, so that this parameter can be taken into account.

[0024] The evaluation of the time from the start of current application until the armature reaches its end stop represents an example of a pressure-dependent parameter of the solenoid pump, which is used according to the invention. Other pressure-dependent criteria are also possible for solenoid pumps or other pump types, in order to be used according to the invention to infer the pressure in the system.

Claims

[1] Method for operating a reducing agent metering system for an SCR catalyst (12) comprising a reducing agent tank (14), a feed pump (16), a pressure line (17) and at least one electrically controlled metering valve (13), characterized by , that the dosing control is determined on the basis of a mass flow balance and that a diagnosis of the reducing agent dosing system is carried out by evaluating the pump current profile, wherein no pressure sensor is provided in the reducing agent dosing system, wherein the system pressure is inferred from the pump current profile, wherein the system pressure is inferred by measuring a time between a current start (30) and an upper armature stop (31) of a lifting magnet of the feed pump (16). [2] Method according to claim 1, characterized by, that the mass flow balance is composed of the delivery rate of the feed pump (16), the metering quantity of the metering valve (13) and, if applicable, the mass flow of a leak, wherein the required metering quantity is delivered into the pressure line (17) via a control of the feed pump (16) and the metering valve (13) is controlled in parallel so that this metering quantity is metered into an exhaust stream (10). [3] Method according to claim 1 or claim 2, characterized by , that if the pump flow rate deviates from predefined values, a fault in the reducing agent dosing system is inferred. [4] Computer program that performs all steps of a method according to any one of claims 1 to 3 when executed on a computing device or a control device (18). [5] Computer program product comprising program code stored on a machine-readable medium for carrying out a method according to any one of claims 1 to 3 when the program is executed on a computing device or a control device (18).

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