On-board diagnostic procedure for a NOx sensor

Abstract

On-board diagnostic procedure (1) for a NOx sensor (2) coupled to an engine control unit (5) of an internal combustion engine (4) for measuring nitrogen oxides and ammonia in the exhaust gas of the internal combustion engine (4), in particular of a motor vehicle (3), - wherein the NOX sensor (2) generates an electrical binary signal which is monitored with respect to a predetermined threshold in order to decide whether the readings from the NOX sensor represent nitrogen oxides when the binary signal is below the predetermined threshold, or represent ammonia when the binary signal is above the predetermined threshold, - in which the NOX sensor (2) is decoupled from the engine control (5), - in which, after or with the decoupling of the NOX sensor (2) from the engine control (5), the temperature of a ceramic element (12) of the NOX sensor (2) is reduced to a predetermined diagnostic temperature, - in which the electrical binary signal of the NOX sensor (2) is monitored, - where proper functioning of the NOX sensor (2) is established when the binary signal rises to a predetermined minimum value within a predetermined time period that is greater than the predetermined threshold, the predetermined time period being measured from the time at which the diagnostic temperature has been reached, - where a malfunction of the NOX sensor (2) is detected if the binary signal does not rise to the minimum value within the predetermined time period.

Description

[0001] The present invention relates to an on-board diagnostic method for a NOx sensor integrated into the engine control unit of an internal combustion engine for measuring nitrogen oxides and ammonia in the exhaust gas of the internal combustion engine, in particular of a motor vehicle. The invention also relates to a diagnostic device for carrying out this on-board diagnostic method and to a motor vehicle equipped with such a diagnostic device.

[0002] In order to comply with increasingly stringent environmental protection regulations, it is necessary to reduce the proportion of nitrogen oxides, i.e. NOx or NO, in the exhaust gas emitted into the environment by combustion engines. X such as NO, NO2, N2O3 and N2O4, and ammonia (NH3). NOX sensors are used for this purpose, which, due to their cross-sensitivity, can measure both nitrogen oxides and ammonia.

[0003] To ensure the proper functioning of the NOx sensor can be verified during operation of the combustion engine or the vehicle equipped with it, on-board diagnostics of the NOx sensor are desirable. Such on-board diagnostics can be performed relatively easily, for example, by deliberately operating the combustion engine in a manner that increases the amount of nitrogen oxides in the exhaust gas, thus verifying the functionality of the NOx sensor with regard to nitrogen oxide measurement. Similarly, the combustion engine can be deliberately operated in a manner that increases the amount of ammonia in the exhaust gas, to verify the functionality of the NOx sensor with regard to ammonia measurement. However, such diagnostic procedures, in which the emissions to be avoided are deliberately generated, are undesirable if the pollutants are checked at the end of the exhaust system, as this would allow the pollutants to escape unhindered into the environment.

[0004] German patent DE 10 2011 077 246 B3 discloses a filtering method for a NOx sensor of an exhaust system with an SCR catalyst. In this method, NOx concentrations upstream and downstream of the SCR catalyst are determined, and based on these measurements, NOx conversion and NH3 slip behavior are modeled. By calculating errors in the model, it can be determined whether the currently measured data from the NOx sensor represent NH3 values ​​or NOx values.

[0005] German patent DE 10 2011 077 251 B3 discloses a diagnostic procedure for a filter of a NOx sensor in an exhaust system with an SCR catalyst. This diagnostic procedure includes operating the filter, calculating a NOx estimate downstream of the SCR catalyst, modeling a linear NOx model based on the NOx estimate, performing a comparison between the calculated NOx estimate and a modeled NOx estimate using an error calculation, and deactivating the filter if the resulting error is below a threshold value.

[0006] From US patent 2019 / 0136785A1, an on-board diagnostic procedure is known for a NOX sensor coupled to an engine control unit of an internal combustion engine for measuring nitrogen oxides and ammonia in the exhaust gas of the internal combustion engine, in which the temperature of a measuring cell is lowered and its voltage is observed for diagnostic purposes.

[0007] German patent application DE 10 2019 208 254 A1 discloses a method for determining the nitrogen oxide and / or ammonia content in the exhaust gas of an internal combustion engine, in which an exhaust gas sensor generates a main signal indicating the nitrogen oxide and / or ammonia content and a binary or linear lambda signal. If the lambda signal is greater than a lambda threshold, the main signal indicates the nitrogen oxide content. If, on the other hand, the lambda signal is less than the lambda threshold, the main signal indicates the ammonia content.

[0008] Further methods and / or sensors are known from DE 198 52 244 C1, DE 10 2015 117 530 A1, DE 10 2016 110 167 A1 and DE 100 49 685 A1.

[0009] The present invention addresses the problem of providing an improved or at least a different embodiment for the diagnosis of a NOX sensor during the operation of the internal combustion engine, which is characterized by high environmental compatibility with a comparatively simple feasibility.

[0010] This problem is solved by the subject matter of the independent claims. Advantageous embodiments are the subject matter of the dependent claims.

[0011] The invention is based on the general concept of decoupling the NOx sensor from the engine control unit for on-board diagnostics and setting the temperature of a ceramic element of the NOx sensor to a predetermined diagnostic temperature, which is below the operating temperature of the NOx sensor. An electrical signal from the NOx sensor is monitored with respect to voltage and / or current. Proper function of the NOx sensor is then determined if the signal rises to a predetermined minimum value at the diagnostic temperature with respect to voltage and / or current. A malfunction of the NOx sensor is assumed if the signal does not rise at the diagnostic temperature or does not rise to the minimum value.The invention utilizes the fact that, in a properly functioning NOx sensor, the electrical signal correlates with the temperature of the ceramic element, such that the signal increases in voltage and / or current as the temperature of the ceramic element decreases. If the NOx sensor is damaged, however, this correlation is disrupted, so that the signal does not increase, or does not increase as much, as in a properly functioning NOx sensor. A particular advantage is that this on-board diagnostic method does not require any intervention in the engine control unit, for example, to deliberately generate more nitrogen oxides or ammonia in the exhaust gas. Accordingly, the on-board diagnostic method presented here is very environmentally friendly. Furthermore, this on-board diagnostic method can be implemented cost-effectively.The NOx sensor is decoupled from the engine control unit (ECU) if its output values ​​serve as a reference input for the ECU's control system. During the NOx sensor diagnostic procedure, the sensor is decoupled from this control system; specifically, the ECU's control system based on the NOx sensor's output values ​​can be temporarily deactivated for this purpose.

[0012] Additionally or alternatively, the NOX sensor is also decoupled from the engine control unit if the current values ​​for nitrogen oxides and ammonia are periodically and / or permanently logged, i.e., recorded and stored, in the engine control unit.

[0013] A binary signal and / or a linear signal from the NOx sensor can be used as an electrical signal that correlates with the temperature of the ceramic element and can be monitored with respect to electrical voltage and / or electrical current. Both the binary and linear signals correlate with the temperature of the ceramic element. The NOx sensor, which can be checked using the on-board diagnostic procedure presented here, is configured to provide at least a binary signal and optionally a linear signal during operation. The binary signal allows a determination of whether the NOx sensor readings represent nitrogen oxides or ammonia. The binary signal is typically a binary voltage. A low binary voltage, i.e., a binary voltage below a predetermined threshold, indicates the presence of nitrogen oxides, as this low binary voltage results from a superstoichiometric exhaust gas composition, i.e., lean exhaust gas with lambda > 1.In contrast, a high binary voltage, i.e., a binary voltage above the predetermined threshold, indicates the presence of ammonia, since the high binary voltage results from a substoichiometric exhaust gas composition, i.e., a rich exhaust gas with lambda < 1. The predetermined minimum value that the binary voltage should reach in the on-board diagnostic procedure is expediently higher than the aforementioned predetermined threshold value of the binary voltage. While the binary signal allows for a rough distinction between superstoichiometric and substoichiometric exhaust gas compositions, the linear signal serves to specify the stoichiometry of the exhaust gas composition more precisely, i.e., to provide a more accurate lambda value that can include decimal places.

[0014] According to the invention, proper functioning of the NOx sensor is detected when the binary voltage rises to a predetermined minimum value within a predetermined time period, and a malfunction of the NOx sensor is detected when the binary voltage does not rise to the minimum value within the predetermined time period. The predetermined time period is measured, in particular, from the point at which the diagnostic temperature has been reached.

[0015] According to the invention, the on-board diagnostic procedure is carried out during operation of the internal combustion engine. The temperature of the ceramic element is lowered to the diagnostic temperature after the NOx sensor has been decoupled from the engine control unit.

[0016] According to an advantageous embodiment, if the proper functioning of the NOx sensor has been confirmed, the ceramic element can be heated to a predetermined operating temperature that is above the diagnostic temperature. For example, the operating temperature of the NOx sensor or the ceramic element is approximately 700 °C. The diagnostic temperature can, for example, be selected to be less than 500 °C, for instance, approximately 400 °C.

[0017] According to an advantageous embodiment, the on-board diagnostic procedure can additionally be configured to perform a diagnosis of the NOx sensor even during engine start-up. For this purpose, the system monitors whether the respective electrical signal is moving in the correct direction, preferably decreasing, as the ceramic element heats up. Since the ceramic element must be heated to a predetermined operating temperature anyway when the engine is started, this NOx sensor diagnosis can be integrated into the starting process with virtually no additional effort.

[0018] According to an advantageous embodiment, after the ceramic element has warmed to its operating temperature, the NOx sensor can be reconnected to the engine control unit. This allows the engine control unit to access the measured values ​​from the NOx sensor again and, in particular, to take them into account during control. Since a NOx sensor of this type is typically used at the end of an exhaust system, it reacts relatively slowly to changes in the operating behavior of the combustion engine. Such a remote NOx sensor is therefore unsuitable for rapid control or regulation of the combustion engine. Sensors located close to the engine, especially lambda sensors, are used for this purpose. The remote NOx sensor of interest here, located at the end of the exhaust system, can be used to optimize engine operation, but is not crucial for the general operation of the combustion engine.Therefore, the engine control unit can temporarily do without the measured values ​​from the NOX sensor, so that the temporary decoupling of the NOX sensor from the engine control unit is uncritical.

[0019] According to an advantageous embodiment, if a malfunction of the NOx sensor is detected, a corresponding error message can be transmitted to the engine control unit. The engine control unit can then permanently exclude the faulty NOx sensor from the control of the combustion engine. During a visit to the workshop, this error message can then be recognized and lead to a repair or replacement of the NOx sensor.

[0020] In another embodiment, the temperature of the ceramic element of the NOx sensor can be lowered passively, namely by switching off or reducing the heating element of the NOx sensor, which serves to heat the ceramic element to a predetermined operating temperature, or by adjusting or controlling it to set the diagnostic temperature. This makes lowering the temperature of the ceramic element particularly easy to implement.

[0021] For example, the ceramic element of the NOx sensor can be formed by a ceramic probe. When the NOx sensor is mounted on the exhaust system, the ceramic probe protrudes into the exhaust stream or is exposed to the exhaust gas, while other components of the NOx sensor, such as a housing containing electronics, are located on the outside of the exhaust system.

[0022] According to an advantageous embodiment, the NOx sensor can be positioned on the exhaust system of the combustion engine such that it measures nitrogen oxides and ammonia in the exhaust gas composition as it exits the exhaust system into the environment. This allows, for example, the effectiveness of the exhaust aftertreatment system's exhaust gas treatment components to be checked. By integrating the NOx sensor into the engine control unit, the nitrogen oxide and ammonia levels in the exhaust gas can be continuously monitored. Furthermore, this enables fine-tuning of the exhaust aftertreatment system's control and / or the combustion engine's control to continuously reduce or maintain these values ​​at low levels.

[0023] The combustion engine may expediently have an exhaust system with several exhaust aftertreatment devices through which the exhaust gas flows. For example, the exhaust system may include an oxidation catalyst and a particulate filter as exhaust aftertreatment devices. Other exhaust aftertreatment devices include, for example, an SCR catalyst and a three-way catalyst. In this context, exhaust aftertreatment devices refer to exhaust gas purification devices, i.e., devices for the mechanical and / or catalytic and / or chemical cleaning of exhaust gases. Accordingly, passive and / or active silencers are not considered exhaust aftertreatment devices in this context. In any case, the NOx sensor is expediently positioned on the exhaust system so that it measures nitrogen oxides and ammonia in the exhaust gas downstream of the last exhaust aftertreatment device through which the exhaust gas flows. Thus, the NOx sensor is located at the end of the exhaust system.at the outlet of the exhaust system and measures nitrogen oxides and ammonia in the exhaust gas in the exact composition with which the exhaust gas leaves the exhaust system and escapes into the environment.

[0024] A diagnostic device according to the invention for a motor vehicle is coupled in its assembled state to an engine control unit, which serves to control an internal combustion engine of the motor vehicle, and to a NOx sensor, which serves to measure nitrogen oxides and ammonia in the exhaust gas of the internal combustion engine. Furthermore, the diagnostic device is configured to perform the on-board diagnostic procedure of the type described above during operation of the internal combustion engine.

[0025] In the present context, a “configuration” is synonymous with a “design” and / or “programming”, so that the phrase “configured so that” is synonymous with the phrase “designed and / or programmed so that”.

[0026] A motor vehicle according to the invention, which may in particular be a passenger car, has an internal combustion engine, an engine control unit for controlling the internal combustion engine and a NOX sensor for measuring nitrogen oxides and ammonia in the exhaust gas of the internal combustion engine and is also equipped with a diagnostic device of the type described above.

[0027] Further important features and advantages of the invention will become apparent from the dependent claims, the drawings and the associated description of the figures based on the drawings.

[0028] It is understood that the features mentioned above and those to be explained below can be used not only in the combinations specified, but also in other combinations or individually, without departing from the scope of the invention as defined by the claims. Components of a higher-level unit, such as a device, apparatus, or arrangement, mentioned above and those to be mentioned below, which are designated separately, can form separate parts or components of this unit or be integral areas or sections of this unit, even if this is depicted differently in the drawings.

[0029] Preferred embodiments of the invention are shown in the drawings and are explained in more detail in the following description, wherein identical reference numerals refer to identical or similar or functionally identical components.

[0030] They show, schematically, Fig. 1 a block diagram of an on-board diagnostic procedure, Fig. 2. A schematic diagram-like representation of a vehicle.

[0031] Accordingly Fig. 1 includes an on-board diagnostic procedure 1 for a in Fig. 2 simplified representations of the NOX sensor 2 several steps, which are explained below using the following examples Fig. 1 will be explained in more detail. First, the following will be used as an example: Fig. 2. The general structure of a simplified representation of a motor vehicle 3 is explained in more detail. The motor vehicle 3, which may preferably be a passenger car, has an internal combustion engine 4, an engine control unit 5 for controlling the internal combustion engine 4, and an exhaust system 6 through which the exhaust gas of the internal combustion engine 4 flows before exiting into an environment 7 of the vehicle 3. The exhaust gas is in Fig. 2 is indicated by an arrow representing the exhaust emissions and the environment 7. The exhaust system 3 serves in the usual way for exhaust gas purification and also for noise reduction and may contain several silencers and exhaust aftertreatment devices 8 (not shown), each through which the exhaust gas flows. Without limiting generality, these could be, for example, an oxidation catalyst 9 and an SCR catalyst 10, where SCR stands for Selective Catalytic Reduction.

[0032] The NOx sensor 2 is positioned on the exhaust system 6 such that it measures nitrogen oxides and ammonia in the exhaust gas downstream of the last exhaust aftertreatment device 8 through which the exhaust gas flowed. By positioning the NOx sensor 2 on the exhaust system 6 downstream of all exhaust aftertreatment devices 8, the NOx sensor 2 measures nitrogen oxides and ammonia in the exhaust gas composition with which the exhaust gas ultimately exits the exhaust system 6 into the environment 7.

[0033] The NOx sensor 2 is suitably coupled to the engine control unit 5, so that it is integrated into the engine control unit 5 and the engine control unit 5 can continuously monitor and, in particular, record the measured values ​​for nitrogen oxides and ammonia in the exhaust gas. It is therefore possible for the engine control unit 5 to act on the operation of the combustion engine 4 and / or on the operation of at least one exhaust aftertreatment device 8, for example, on the dosing of urea solution upstream of the SCR catalyst 10, depending on the measured values ​​of the NOx sensor 2.

[0034] Vehicle 3 is also equipped with a diagnostic device 11, which is coupled to the NOx sensor 2 and the engine control unit 5. In the example of the Fig. 2. The diagnostic device 11 is partially or fully integrated into the engine control unit 5 in terms of hardware and / or implemented in the engine control unit 5 in terms of software. The diagnostic device 11 can now be configured to perform the aforementioned on-board diagnostic procedure 1 during the operation of the internal combustion engine 4.

[0035] Accordingly Fig.In the first step of the on-board diagnostic procedure 1, S1, the NOx sensor 2 is decoupled from the engine control unit 5. This first step S1 is primarily required when the on-board diagnostic procedure 1 is performed while the combustion engine 4 is running. If, however, the on-board diagnostic procedure 1 is performed when the combustion engine 4 is started, the NOx sensor 2 may, depending on the starting procedure, only be coupled to the engine control unit 5 once its predetermined operating temperature has been reached. In this case, the first step S1 checks whether the NOx sensor 2 is coupled to the engine control unit 5. If the NOx sensor 2 is coupled to the engine control unit 5, it is decoupled from it in the first step S1 so that the on-board diagnostic procedure 1 can then continue. If, on the other hand, the NOx sensor 2 is not coupled to the engine control unit 5, for example, because it is a faulty sensor, the first step S1 checks whether the NOx sensor 2 is coupled to the engine control unit 5.Since this is a start-up process, the first step S1 only determines that the on-board diagnostic procedure 1 can be continued.

[0036] In a second step S2 following the first step S1, the temperature of a ceramic element 12 of the NOx sensor 2 is set to a predetermined diagnostic temperature. This diagnostic temperature can be, for example, around 400 °C and is in any case below the operating temperature of the NOx sensor 2, which can be, for example, around 700 °C. If the on-board diagnostic procedure 1 is performed while the combustion engine 4 is running, the temperature of the ceramic element 12 is lowered from the operating temperature to the diagnostic temperature. If, on the other hand, the on-board diagnostic procedure 1 is performed when the combustion engine 3 is started, the temperature of the ceramic element 12 is raised from the ambient temperature to a specific temperature, in particular the diagnostic temperature. For this purpose, a heater 13, with which the NOx sensor 2 is equipped, can be used to heat the ceramic element 12 to the operating temperature.

[0037] In a third step S3, which is performed simultaneously with or after the second step S2, an electrical signal from the NOx sensor 2 is monitored. This signal correlates with the temperature of the ceramic element 12. This electrical signal is expediently a binary signal or binary voltage from the NOx sensor 2 and / or a linear signal from the NOx sensor 2. During proper operation of the NOx sensor 2, the binary signal or binary voltage is used to determine whether the measured values ​​currently indicate a nitrogen oxide (NOx) content or an ammonia content in the exhaust gas. During proper operation of the NOx sensor 2, the linear signal provides a relatively accurate lambda value for the current exhaust gas composition. In a fourth step S4, it is checked whether the respective signal rises to a predetermined minimum value.If the signal rises to the predetermined minimum value, a fifth step (S5) determines that the NOx sensor 2 is functioning correctly. However, if the check in the fourth step (S4) shows that the signal does not rise to the minimum value, a sixth step (S6) determines that the NOx sensor 2 is malfunctioning.

[0038] In the fourth step S4, a predetermined time period can be taken into account within which the respective signal must rise to the predetermined minimum value upon reaching the diagnostic temperature in order to confirm proper functioning of the NOx sensor 2. This allows the duration of the on-board diagnostic procedure 1 to be limited to a timeframe that is not critical for the engine control unit 5.

[0039] In other words, in the fourth step S4, proper functioning of the NOX sensor 2 is determined if the respective signal rises to the predetermined minimum value within the predetermined time period after reaching the diagnostic temperature. Conversely, a malfunction of the NOX sensor 2 is detected if the signal does not rise to the minimum value within the predetermined time period after reaching the diagnostic temperature.

[0040] If the proper functioning of the NOx sensor 2 has been established, the ceramic element 12 can be heated to a predetermined temperature in a seventh step S7, preferably to the previously mentioned operating temperature. After the ceramic element 12 has been heated to the operating temperature, the NOx sensor 2 can be reconnected to the engine control unit 5 in an eighth step S8.

[0041] If, instead, a malfunction of the NOX sensor 2 is detected, a corresponding error message is transmitted to the engine control unit 5 in a ninth step S9.

[0042] Lowering the temperature of the ceramic element 12 can be done passively. For this purpose, the heater 13 of the NOX sensor 2, which serves to heat the ceramic element 12 to the operating temperature, can be switched off or reduced, or controlled to set the diagnostic temperature. The ceramic element 12 can be a ceramic probe of the NOX sensor 2, which, when the NOX sensor 2 is mounted on the exhaust system 6, is exposed to the exhaust gas and can therefore be subjected to or surrounded by the exhaust gas flow.

Claims

[1] On-board diagnostic method (1) for a NOx sensor (2) coupled to an engine control unit (5) of an internal combustion engine (4) for measuring nitrogen oxides and ammonia in the exhaust gas of the internal combustion engine (4), in particular of a motor vehicle (3), - wherein the NOX sensor (2) generates an electrical binary signal which is monitored with respect to a predetermined threshold in order to decide whether the readings from the NOX sensor represent nitrogen oxides when the binary signal is below the predetermined threshold, or represent ammonia when the binary signal is above the predetermined threshold, - in which the NOX sensor (2) is decoupled from the engine control (5), - in which, after or with the decoupling of the NOX sensor (2) from the engine control (5), the temperature of a ceramic element (12) of the NOX sensor (2) is reduced to a predetermined diagnostic temperature, - in which the electrical binary signal of the NOX sensor (2) is monitored, - where proper functioning of the NOX sensor (2) is established when the binary signal rises to a predetermined minimum value within a predetermined time period that is greater than the predetermined threshold, the predetermined time period being measured from the time at which the diagnostic temperature has been reached, - where a malfunction of the NOX sensor (2) is detected if the binary signal does not rise to the minimum value within the predetermined time period. [2] On-board diagnostic method (1) according to claim 1, characterized by , - that in the event that the proper functioning of the NOX sensor (2) has been established, the ceramic element (12) is heated to a predetermined operating temperature. [3] On-board diagnostic method (1) according to claim 2, characterized by , - that the NOX sensor (2) is coupled to the motor control (5) with or after the ceramic element (12) is heated to the operating temperature. [4] On-board diagnostic method (1) according to any one of claims 1 to 3, characterized by , - that in the event that the malfunction of the NOX sensor (2) has been detected, a corresponding error message is transmitted to the engine control unit (5). [5] On-board diagnostic method (1) according to any one of claims 1 to 4, characterized by , - that the temperature is lowered passively by switching off or reducing a heater (13) of the NOX sensor (2), which serves to heat the ceramic element (12) to a predetermined operating temperature, or by controlling it to set the diagnostic temperature, and / or - that the temperature is actively increased by controlling a heater (13) of the NOX sensor (2), which serves to heat the ceramic element (12) to a predetermined operating temperature, to set the operating temperature. [6] On-board diagnostic method (1) according to any one of the preceding claims, characterized by , - that the NOX sensor (2) on an exhaust system (6) of the internal combustion engine (4) is positioned such that the NOX sensor (2) measures nitrogen oxides and ammonia in a composition of the exhaust gas with which the exhaust gas exits the exhaust system (6) into the environment (7). [7] On-board diagnostic method (1) according to any one of the preceding claims, characterized by , - that the internal combustion engine (4) has an exhaust system (6) with several exhaust aftertreatment devices (8) through which the exhaust gas flows, - that the NOX sensor (2) is arranged on the exhaust system (6) in such a way that it measures nitrogen oxides and ammonia in the exhaust gas downstream of the exhaust aftertreatment device (8) through which the exhaust gas last flows. [8] Diagnostic device (11) for a motor vehicle (3), - wherein the diagnostic device (11) can be coupled or coupled to an engine control unit (5) for controlling an internal combustion engine (4) of the motor vehicle (3) and to a NOX sensor (2) for measuring nitrogen oxides and ammonia in the exhaust gas of the internal combustion engine (4), - wherein the diagnostic device (11) is configured to perform the on-board diagnostic procedure (1) according to any one of claims 1 to 7 during the operation of the internal combustion engine (4). [9] Motor vehicle (3), in particular passenger cars, - with an internal combustion engine (4), - with an engine control unit (5) for controlling the internal combustion engine (4), - with a NOx sensor (2) for measuring nitrogen oxides and ammonia in the exhaust gas of the combustion engine (4), - with a diagnostic device (11) according to claim 8.

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