Method for quality detection of a reducing agent

Abstract

Method for quality detection of a reducing agent (2) in a reducing agent tank (1) of an SCR system for an internal combustion engine by means of a measuring system (3) comprising a sound source, a sound sensor and a stationary object (5) arranged at a defined distance to the sound source and the sound sensor, wherein, when the temperature (T) of the reducing agent (2) is above a freezing point of the reducing agent (2) at the target concentration, a sound wave (S) is emitted (12) from the sound source in the direction of the stationary object (5) and the stationary object (5) reflects the sound wave (S) in the direction of the sound sensor as a reflection wave (R), and wherein, if the reflection wave (R) is not received by the sound sensor, a fault is inferred (14), characterized in that, when a fault has been inferred (14) and when the reducing agent tank (1) is at least filled to the extent thatthat the measuring system (3) is covered and if the measuring system (3) is found to be error-free, it is concluded that the concentration of the reducing agent (2) deviates from the target concentration (19).

Description

[0001] The present invention relates to a method for quality detection of a reducing agent in a reducing agent tank of an SCR system. The invention further relates to a computer program that executes each step of the method when run on a computing device, and to a machine-readable storage medium that stores the computer program. Finally, the invention relates to an electronic control unit configured to execute the method according to the invention. State of the art

[0002] Nowadays, the SCR (Selective Catalytic Reduction) process is used in the aftertreatment of exhaust gases from combustion engines to reduce nitrogen oxides (NOx) in the exhaust gas. DE 103 46 220 A1 describes the basic principle. This involves the use of a 32.5% urea-water solution (HWL), also commercially known as AdBlue. ®The process is known to involve dosing the HWL (hydrocarbon liquid) into the exhaust gas. Typically, a dosing system with a dosing module is used to introduce the HWL into the exhaust stream upstream of an SCR catalyst. Ammonia is released from the HWL and subsequently bound to the reactive surface of the SCR catalyst. There, the ammonia combines with the nitrogen oxides, producing water and nitrogen. The HWL is conveyed from a reducing agent tank to the dosing module via a feed module and a feed pump through a pressure line.

[0003] The mass of the released ammonia depends directly on both the mass of the dosed urea-water solution and the concentration of urea in the urea-water solution - which is also referred to as the quality of the reducing agent or the urea-water solution.

[0004] Therefore, monitoring the quality of the urea solution is crucial for the proper functioning of the SCR system. If the urea concentration or the reducing agent quality is too low, less nitrogen oxide is converted, and the efficiency of the SCR catalyst decreases. If the urea concentration is too high or the reducing agent quality is too high, the converted ammonia does not react with the nitrogen oxides and passes through the SCR catalyst; ammonia slip occurs.

[0005] US Patent 9,038,442 B2 describes a system and a method for determining the quality and quantity of a reducing agent. For quality determination, the system includes a transducer and a stationary object mounted at a known distance from the transducer. The transducer is configured to emit a sound wave horizontally toward the stationary object and receive a reflected wave (an echo) from the stationary object. A temperature sensor is also provided to measure the temperature of the reducing agent. The quality is then determined by a control unit based on the sound wave, the reflected wave, and the temperature. A transducer is also called a transducer and, in specific cases, can be designed as an ultrasonic transducer or an ultrasonic probe. Accordingly, (measuring) transducers, (ultrasonic) transducers, and ultrasonic probes are each ultrasonic sources.

[0006] This method, however, requires the reducing agent to be in a liquid state. The freezing point of the aforementioned 32.5% urea-water solution is approximately -11°C, meaning that in practice, the reducing agent under investigation may be frozen. When frozen, the transmission for sound waves changes, preventing the reflection wave from reaching the transducer. Therefore, the quality cannot be determined using the method described above.

[0007] If the reflection wave does not reach the transducer, an error is triggered. However, the absence of a reflection wave at the transducer does not generally imply that the reducing agent is frozen; rather, other potential sources of error, particularly an empty reducing agent tank or a defective transducer, must be considered. Level sensors are known for determining the level of the reducing agent tank and thus also for detecting when the tank is empty. Methods for monitoring the transducer are also known.

[0008] DE 11 2014 001 297 T5 discloses a method for monitoring an exhaust aftertreatment system which uses a temperature sensor to detect a temperature during the phase transition and compares it with a target value in order to determine the sensor rationality or fluid quality.

[0009] DE 10 2015 224 358 A1 discloses a method for determining the state of matter which sends a signal via a sensor and uses the received signal quality to draw conclusions about the state of matter of the reducing agent, in particular in the case of changes in the acoustic signal between solid and liquid state.

[0010] DE 10 2009 027 435 A1 discloses a method for providing information about the state of matter or temperature of a reducing agent solution, which evaluates sloshing movements in the tank to determine the state of matter and indicate readiness for dosing.

[0011] DE 10 2012 215 635 A1 discloses a measuring element and method for liquid differentiation in which a phase transition is provoked and the resulting material properties are detected, for example by cooling to the freezing point or heating to the boiling point.

[0012] DE 10 2009 055 738 A1 discloses a method for determining the state of a reducing agent in a reducing agent tank, in which filling and withdrawal quantities, the temperature and the propagation speed of ultrasonic waves are recorded in order to determine the state of the reducing agent in the control unit.

[0013] US Patent 2011 / 0107812A1 discloses a rationality diagnostic procedure that monitors concentration, level, and temperature sensors in a tank by identifying discrepancies between their values ​​to check the plausibility of the sensor data.

[0014] US Patent 2013 / 0160433A1 discloses a device for measuring the fill level of a urea tank using ultrasound, in which a sensor is positioned in the sump area to enable accurate measurements even at low fill levels or in a frozen state.

[0015] EP 3 369 902 A1 discloses a method for determining the validity of a urea concentration, which compares the measured concentration with a validity interval in a phase diagram at a given temperature and physical phase.

[0016] DE 103 46 220 A1 discloses an exhaust gas aftertreatment system with a heated pressure accumulator for reducing agent, which prevents the urea-water solution from freezing and thus ensures the functionality of the system. Disclosure of the invention

[0017] A method for quality detection of a reducing agent in a reducing agent tank of an SCR system for an internal combustion engine is proposed. A measuring system is provided, comprising a sound source, a sound sensor, and a stationary object arranged at a defined distance from the sound source and the sound sensor. In a preferred embodiment, the sound source is an ultrasonic source. Preferably, the sound source and the sound sensor form a single unit and are configured, for example, as a transducer and, more specifically, as an ultrasonic transducer.

[0018] Traditionally, the concentration of the reducing agent is determined in its liquid state. A temperature threshold is defined, representing the freezing point of the reducing agent at the target concentration. The target concentration is the concentration at which the reducing agent is assumed to be present and which is used as a basis for subsequent operation, for example, in dosing strategies. The temperature threshold is therefore selected depending on the reducing agent used and can be found in a table or determined through preliminary tests. According to the definition of the temperature threshold, it can be assumed that at temperatures above this threshold, the reducing agent is in its liquid state at the target concentration.The preferred temperature threshold is the freezing point of a typically used urea-water solution, which is -11°C. The temperature is preferably measured by a temperature sensor on or in the reducing agent tank.

[0019] If the temperature is above the freezing point of the reducing agent at the target concentration, the diagnostic procedure is performed. A sound wave is emitted from the sound source towards the stationary object. The stationary object then reflects the sound wave back towards the sound sensor. The reflected sound wave is referred to as the "reflection wave." If the reflection wave is not received by the sound sensor, a fault is assumed. Common sources of error include a malfunction of the measuring system or an insufficient level of the reducing agent, which would prevent the sound sensor, the stationary object, and the intervening sound path (i.e., the measuring system) from being completely covered.

[0020] Once a fault has been identified, it is checked whether other potential sources of error can be ruled out. Devices and methods for detecting these potential sources of error are already known. Specifically, this means that the reducing agent tank is filled with the reducing agent solution at least to the point where the measuring system is covered, i.e., the sound source and the sound sensor – or the transducer – as well as the stationary object and the sound path between them are covered by the reducing agent. The fill level can preferably be monitored by a level sensor. Furthermore, the measuring system is identified as fault-free using a diagnostic procedure, i.e., the sound source and the sound sensor – or the transducer – are functional and correctly set and adjusted for use, and the stationary object is correctly aligned. Depending on the diagnosis, the measuring system can be checked during operation or during maintenance.If the temperature is measured by a temperature sensor, as described above, its functionality and calibration are also checked.

[0021] According to the procedure, in this case, where the temperature exceeds the temperature threshold and a fault has been inferred, but the aforementioned sources of error can be ruled out, it is concluded that the concentration of the reducing agent deviates from the target concentration. The reason for this is that, if all other sources of error can be excluded, only the transmission properties of the reducing agent can be responsible for the fault. These properties change particularly drastically when the reducing agent is frozen, so much so that the sound wave can no longer be reflected as intended, and consequently, no reflection wave can be detected by the sound sensor. The fault therefore originates from a frozen reducing agent.However, since the temperature is above the temperature threshold that represents the freezing point of the reducing agent at the target concentration, the reducing agent can only freeze if the concentration of the reducing agent deviates from the target concentration.

[0022] This method makes it possible to determine the quality of the reducing agent even when frozen.

[0023] Typically, a urea-water solution is used as the reducing agent. The freezing point for a 32.5% urea-water solution at the target concentration is -11°C. Advantageously, the temperature of the urea-water solution at the target concentration is above this freezing point. This allows for the common occurrence of the reducing agent tank being filled with water instead of the urea-water solution. The added water reduces the concentration of the urea-water solution, and as the concentration decreases, the freezing point shifts towards 0°C.

[0024] Therefore, it is advantageous that the process is only carried out when the temperature of water-based reducing agents is below 0°C, since a freezing point above this temperature is not reached.

[0025] Preferably, the process is carried out immediately after the combustion engine is started. At this point, additional heating devices used to defrost the reducing agent are not yet fully operational.

[0026] A previously mentioned level gauge is typically unable to measure the level of the reducing agent when frozen. Preferably, a level last stored as valid, previously determined by the level sensor, is used to assess whether the measuring system is covered.

[0027] The computer program is configured to execute each step of the procedure, particularly when performed on a computer or control unit. It enables the implementation of the procedure in a conventional electronic control unit without requiring any structural modifications. For this purpose, it is stored on a machine-readable storage medium.

[0028] By uploading the computer program to a conventional electronic control unit, the electronic control unit is obtained which is set up to perform the quality detection of the reducing agent. Brief description of the drawings

[0029] Exemplary embodiments of the invention are shown in the drawings and explained in more detail in the following description. Fig. Figure 1 shows a schematic representation of a reducing agent tank with a measuring system in which the method according to the invention can be carried out. Fig. Figure 2 shows a flowchart of an embodiment of the method according to the invention. Exemplary embodiment of the invention

[0030] Fig. Figure 1 shows a schematic representation of a reducing agent tank 1 for an SCR system (not shown) of an internal combustion engine, which is filled with a reducing agent 2. The preferably used reducing agent 2 for the SCR is a 32.5% urea-water solution. A measuring system 3 for quality detection of the reducing agent 2 is arranged at the bottom of the reducing agent tank 1. The measuring system 3 comprises a transducer 4 and a stationary object 5, which in this embodiment are formed as a single unit. In this embodiment, the stationary object 5 is arranged horizontally to the transducer 4 and at a predetermined distance from it. The transducer 4 serves as a sound source for a sound wave S, which it emits in the direction of the stationary object 5. The stationary object 5 is designed and oriented such that the sound wave S is reflected and returns to the transducer 4 as a reflection wave R.Simultaneously, the transducer 4 is configured to receive the reflection wave R and thus serves as a sound sensor. The transducer 4 is mounted on a circuit board 6 within the measuring system 3, which serves as an interface to an electronic control unit 7. The electronic control unit 7 controls the transducer 4 and receives and processes the received signals. A temperature sensor 8 is also provided on the circuit board 6. This sensor measures the temperature T of the reducing agent 2 and transmits the measured value to the electronic control unit 7. Furthermore, a level sensor 9 is arranged within the reducing agent tank 1. This sensor detects the fill level of the reducing agent tank 1, i.e., the height to which the reducing agent 2 is present in the tank. The fill level is transmitted to the electronic control unit 7, validated, and stored there.

[0031] Fig. Figure 2 shows a flowchart of an embodiment of the method according to the invention. The method is executed directly after the combustion engine is started and begins when, in a query 10, the currently measured temperature T of the reducing agent is above a temperature threshold Ts, which represents the freezing point of the reducing agent 2 at the target concentration. In this embodiment, the target concentration is defined as the concentration assumed for the typically used 32.5% urea-water solution and which is used as the basis for the dosing strategy for the SCR. In this embodiment, the temperature threshold Ts is -11°C, since this is the freezing point of the 32.5% urea-water solution.If the temperature T is below or equal to the temperature threshold Ts, the process is terminated 11 and the quality of the reducing agent 2 can be determined by other, already known methods, which are not described further here. If, however, the temperature T is above the temperature threshold Ts, S It can be assumed that the 32.5% urea-water solution, i.e., the reducing agent 2, is not frozen but is in a liquid state. Furthermore, the procedure is not carried out for temperatures T above 0°C, as the urea-water solution freezes at any concentration, even with pure water, in this case. The process is carried out as described in Fig. Figure 1 shows the emission 12 of the sound wave S by the measuring transducer 4, the reflection of the sound wave S at the stationary object 5, and the return of the reflection wave R to the measuring transducer 4. When the reflection wave R reaches the measuring transducer 4, it is detected by it 13. If no error is detected during the detection 13 of the reflection wave R in a query 14, the procedure is terminated 11 and the quality of the reducing agent 2 can be determined by other, already known methods.

[0032] If, however, an error is detected in query 14 because no reflection wave R was detected by the measuring transducer 4 13, the type of error is first determined. For this purpose, the various sources of error are checked using diagnostic procedures that are known per se, which are described in Fig.2 are represented as different queries 15, 16, 17. The queries 15, 16, 17, or the diagnostic procedures, can be executed simultaneously or sequentially. It is also possible that diagnostic procedures have already been carried out beforehand, provided the robustness of the respective diagnostic procedure allows it and the result is still valid during the procedure according to the invention. Firstly, the fill level of the reducing agent 2 in the reducing agent tank 1 is checked 15. Since it cannot be assumed according to the invention that the reducing agent 2 is not frozen, the current fill level, which cannot be determined in the frozen state anyway with the conventionally used level gauges 9, is not used, but rather the fill level F last stored as valid. lgsused. If the measuring system 3, more precisely the transducer 4, the stationary object 5, and the sound path traversed by the surge waves S and the reflection waves R, are covered by the reducing agent 2, then this source of error can be excluded. Secondly, the measuring system 3 is checked for functionality 16. More precisely, it is checked whether the transducer 4 is capable of emitting and receiving sound waves. Thirdly, the functionality and calibration of the temperature sensor 8 are checked 17. If both are the case, the measuring system itself can be excluded as a source of error.

[0033] If the source of the error is identified by one of these queries 15, 16, 17—in other words, if one of the diagnostic procedures represented by these queries 15, 16, 17 indicates the error—the procedure is terminated 11. No statements can then be made about the quality of the reducing agent 2, since the cause of the error lies in the measurement itself. However, if the source of the error is not identified by any of the aforementioned queries 15, 16, 17—in other words, if the error is not found by any of the aforementioned diagnostic procedures—then, according to the invention, it is determined 18 that the error can only be caused by the reducing agent 2 being frozen.Taking into account the premise established at the beginning by query 10, that the temperature T of the reducing agent is above the temperature threshold Ts, which represents the freezing point for the reducing agent at the target concentration, the invention concludes from the finding that the reducing agent 2 is nevertheless frozen that the concentration of the reducing agent 2 deviates from the target concentration 19. In the case that the reducing agent is the aforementioned urea-water solution, the higher temperature T at which the urea-water solution is frozen indicates a lower concentration relative to the target concentration and thus an increased water content.

Claims

[1] Method for quality detection of a reducing agent (2) in a reducing agent tank (1) of an SCR system for an internal combustion engine by means of a measuring system (3) comprising a sound source, a sound sensor and a stationary object (5) arranged at a defined distance to the sound source and the sound sensor, wherein, when the temperature (T) of the reducing agent (2) is above a freezing point of the reducing agent (2) at the target concentration, a sound wave (S) is emitted (12) from the sound source in the direction of the stationary object (5) and the stationary object (5) reflects the sound wave (S) in the direction of the sound sensor as a reflection wave (R), and wherein, if the reflection wave (R) is not received by the sound sensor, a fault is inferred (14), characterized by, that if a fault has been inferred (14) and if the reducing agent tank (1) is filled at least to the extent that the measuring system (3) is covered and if the measuring system (3) is recognized as fault-free, it is inferred that the concentration of the reducing agent (2) deviates from the target concentration (19). [2] Method according to claim 1, characterized by , that the temperature (T) is above the freezing point for a urea-water solution at target concentration. [3] Method according to claim 1 or 2, characterized by that the temperature (T) is below 0°C. [4] Method according to any one of the preceding claims, characterized by that it is executed immediately after the combustion engine is started. [5] Method according to any one of the preceding claims, characterized by , that the temperature (T) is determined by a temperature sensor (8). [6] Method according to any one of the preceding claims, characterized by, that to assess (15) whether the measuring system (3) is covered by the reducing agent solution (2), a fill level last stored as valid (F lgs ), which was previously determined by a level sensor (9). [7] Computer program which is configured to perform each step of the method according to any one of claims 1 to 6. [8] Machine-readable storage medium on which a computer program according to claim 7 is stored. [9] Electronic control unit (7) which is configured to perform quality detection of the reducing agent (2) by means of a method according to any one of claims 1 to 6.

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