METHOD FOR DETERMINING A CORRECTED NITROGEN VALUE OF AN EXHAUST AIR SENSOR

Description

[0001] The present invention relates to a method for determining a corrected nitrogen oxide value of an exhaust gas sensor, for example a nitrogen oxide sensor.

[0002] Exhaust gas sensors, such as nitrogen oxide sensors, lambda sensors, and oxygen sensors, can be based on the amperometric principle, i.e., an electrochemical method for the quantitative determination of chemical substances. Specifically, an electric current is applied to an electrode of the exhaust gas sensor in such a way as to establish a constant electrochemical potential over time. For example, nitrogen oxide sensors allow the measurement of the nitrogen oxide concentration in the exhaust gas of internal combustion engines, such as gasoline or diesel engines. This enables, for example, optimal control and diagnostics of nitrogen oxide catalysts by the engine control unit.

[0003] These exhaust gas sensors have a main body made of a solid electrolyte, in which cavities with associated electrodes are provided. A heating device is also arranged in the main body, designed to heat the main body to and maintain a predetermined operating temperature, for example, approximately 850°C. Due to additional external heat input, such as from the exhaust gas flowing around the sensor, the electrodes can be warmer than in their initial state, which can lead to an increased signal from the exhaust gas sensor, for example, an excessively high nitrogen oxide value.

[0004] Other exhaust gas sensors are known from CN 102 798 654 B, US 2019 / 0078491 A1, US 7 462 266 B2, US 2005 / 0029250 A1 and US 6 939 037 B2. Furthermore, CN 105 892 529 B and CN 110 735 699 A represent prior art. The present invention is based on the objective of providing a method for determining a corrected nitrogen oxide value of an exhaust gas sensor, by means of which the accuracy of the exhaust gas sensor can be at least partially increased.

[0005] This problem is solved by a method according to independent claim 1. Advantageous embodiments are specified in the dependent claims.

[0006] The present invention is essentially based on the idea of ​​determining the voltage drop between an electrical supply line of a heating device of the exhaust gas sensor and a measuring line connected to this electrical supply line, and comparing this determined voltage drop with a predetermined reference voltage to determine a compensation voltage. The compensation voltage thus determined can then be used to determine a corrected nitrogen oxide value by assigning the determined compensation voltage to the corrected nitrogen oxide value.

[0007] A compensation nitrogen oxide value is determined, which is used to correct the nitrogen oxide value determined by the exhaust gas sensor.

[0008] In particular, the determined compensation voltage can be used to indicate the average temperature of the sensor element in the area of ​​the heating device's electrical leads, which in turn can be used to determine the housing temperature of the exhaust gas sensor. An increased average temperature of the sensor element in the area of ​​the heating device's electrical leads and / or an increased housing temperature can distort the measurement accuracy of the exhaust gas sensor. Specifically, the increased temperature in the lead area can disrupt heat dissipation from the tip of the sensor element, where the heating element's heating section is located. This can alter the temperature profile at the tip of the sensor element, which is very difficult to compensate for.However, the altered temperature profile can result in the pump electrode, which is located closer to the sensor element tip, being cooler than the measuring electrode. This can lead to the aforementioned measurement error in the detected nitrogen oxide concentration.

[0009] Consequently, the determined compensation voltage can be used to determine the average temperature of the sensor element in the area of ​​the electrical supply lines of the heating device, which in turn allows for compensation of the distorted measured values ​​of the exhaust gas sensor.

[0010] Consequently, according to a first aspect of the present invention, a method for determining a corrected nitrogen oxide value of an exhaust gas sensor is disclosed, the sensor element comprising a sensor element and a heating device arranged in the sensor element for heating the sensor element. The heating device comprises a heating section arranged in the sensor element, which is electrically connected via a first electrical line and a second electrical line to a control device for electrically controlling the heating section. The heating device further comprises a measuring line electrically connected between the control device and the first electrical line, by means of which the temperature-dependent resistance of the heating section can be controlled.The method according to the invention comprises determining a measuring voltage that drops between the first electrical conductor and the measuring conductor, determining a compensation voltage by calculating the difference between a predetermined reference voltage and the determined measuring voltage, determining a compensation nitrogen oxide value by assigning the determined compensation voltage to the compensation nitrogen oxide value, determining a nitrogen oxide value using the exhaust gas sensor, and determining the corrected nitrogen oxide value by subtracting the compensation nitrogen oxide value from the determined nitrogen oxide value. The predetermined reference voltage corresponds to a measuring voltage that drops between the first electrical conductor and the measuring conductor at a predetermined temperature of the sensor element.

[0011] It is preferred that the determined compensation voltage is assigned to the compensation nitrogen oxide value by means of an assignment table and / or an assignment rule, such as a mathematical mapping. It can be advantageous for the assignment table to contain previously determined empirical values ​​under predetermined conditions, which assign the corresponding state parameter to the determined compensation voltage.

[0012] Preferably, the predetermined temperature is a room temperature of approximately 20°C.

[0013] Further features and functions of the invention will become apparent to the person skilled in the art by carrying out the present teaching and examining the accompanying drawings, in which: Fig. 1 shows a schematic sectional view through an exhaust gas sensor for an internal combustion engine of a vehicle, exemplified by a nitrogen oxide sensor; Fig. 2 shows a schematic sectional view through the exhaust gas sensor of the Fig. 1 along line II - II, and Fig. 3 shows a flowchart of a method according to the invention for determining a corrected nitrogen oxide value.

[0014] Within the scope of the present disclosure, amperometrically operating sensors, such as nitrogen oxide sensors, lambda probes, and oxygen sensors, are characterized in that their measuring principle is based on amperometry, i.e., on an electrochemical method for the quantitative determination of chemical substances. In particular, an electric current is set at a working electrode such that a time-constant electrochemical potential is established.

[0015] Furthermore, within the scope of this disclosure, the term "control" encompasses the control engineering terms "controlling" and "regulating". A person skilled in the art will recognize when to apply control engineering control and when to apply control engineering regulation.

[0016] The Fig. 1 Figure 10 shows an exemplary nitrogen oxide sensor, which is representative of an exhaust gas sensor. Consequently, the present invention is also intended for use in all sensors for internal combustion engines in vehicles that have a heating device, such as lambda sensors and oxygen sensors. In particular, the present invention is applicable to exhaust gas sensors that have a ceramic base with an electrode pair attached to it.

[0017] Referring to the Fig. 1 Figure 10 is a schematic sectional view of the exemplary nitrogen oxide sensor 10, which is designed to be arranged in an exhaust tract of an internal combustion engine (not shown) and to quantitatively detect the nitrogen oxide content or the oxygen content in the exhaust gas of the internal combustion engine.

[0018] The nitrogen oxide sensor 10 has a main body 12 made of a solid electrolyte, preferably formed from a mixed crystal of zirconium oxide and yttrium oxide and / or from a mixed crystal of zirconium oxide and calcium oxide. Additionally, a mixed crystal of hafnium oxide, a mixed crystal of perovskite-based oxides, or a mixed crystal of trivalent metal oxide, such as aluminum oxide (Al₂O₃), can be used. The main body 12 forms a sensor element of the exhaust gas sensor 10. Hereinafter, the main body 12 will therefore also be referred to as the sensor element 12.

[0019] Within the main body 12 of the exemplary nitrogen oxide sensor 10, a first pumping cavity 20, a second pumping cavity 30, and a measuring cavity 40 are provided. The first pumping cavity 20 is connected to the exterior of the main body 12 via a connecting path 15. In particular, exhaust gas can flow into the first pumping cavity 20 through the connecting path 15. The second pumping cavity 30 is connected to the first pumping cavity 20 via a first diffusion path 25.

[0020] The measuring cavity 40 is connected to the second pumping cavity 30 via a second diffusion path 35.

[0021] The main body 12 also includes a reference cavity 50, which is directly connected to the exterior of the main body 12. A reference electrode 52 is arranged within the reference cavity 50. Specifically, the reference cavity 50 is connected to the ambient air, i.e., not to the exhaust gas, and is configured to provide an oxygen reference for the various electrodes arranged in the nitrogen oxide sensor 10.

[0022] An exhaust gas electrode 22 is arranged on an outer surface of the main body 12. In particular, during a measurement operation of the nitrogen oxide sensor 10, the oxygen contained in the exhaust gas can be ionized by applying a reference current to the exhaust gas electrode 22 and diffuse through the main body 12 as oxygen ions to the reference electrode 52, where it is converted back into oxygen molecules to form an oxygen reference.

[0023] A first pump electrode 24 is arranged within the first pump cavity 20. Specifically, during the measurement operation of the nitrogen oxide sensor 10, the oxygen present in the exhaust gas within the first pump cavity 20 can be ionized by applying a first pump current IP0 to the first pump electrode 24. This ionized oxygen then migrates through the main body 12 as oxygen ions. Due to the oxygen ions released from the first pump cavity 20, a first electrode voltage, or first Nernst voltage V0, indirectly forms between the first pump electrode 24 and the reference electrode 52. More precisely, the first electrode voltage, or first Nernst voltage V0, is generated directly from the residual oxygen still present in the first pump cavity 20.

[0024] A second pump electrode 34 is arranged within the second pump cavity 30. During the measurement operation of the nitrogen oxide sensor 10, the oxygen present in the gas mixture within the second pump cavity 30 can be ionized by applying a second pump current IP1 to the second pump electrode 34. The oxygen then migrates through the main body 12 as oxygen ions. Due to the oxygen ions released from the second pump cavity 30, a second electrode voltage, or second Nernst voltage V1, is indirectly generated between the second pump electrode 34 and the reference electrode 52. More precisely, the second electrode voltage, or second Nernst voltage V1, is generated directly from the residual oxygen still present in the second pump cavity 30.

[0025] Within the measuring cavity 40, a measuring electrode 44 is arranged, which is designed to ionize the oxygen and / or nitrogen oxides present within the measuring cavity 40 when a measuring current IP2 is applied during the measurement operation of the nitrogen oxide sensor 10, so that the oxygen ions can migrate or pass through the main body 12. Due to the oxygen ions released or pumped out of the measuring cavity 40, a third electrode voltage or third Nernst voltage V2 is generated between the measuring electrode 44 and the reference electrode 52, which is kept at a constant value by applying the measuring current IP2 to the measuring electrode 44. More precisely, the third electrode voltage or third Nernst voltage V2 is generated directly from the residual oxygen still present in the measuring cavity 40. The applied measuring current IP2 then serves as an indicator of the nitrogen oxide content within the exhaust gas.

[0026] Thus, the one in the Fig. 1 The nitrogen oxide sensor 10 shown, which is an example of a sensor based on the amperometric measuring principle, has three relevant electrode pairs, namely a first electrode pair consisting of the first pump electrode 24 and the exhaust gas electrode 22, a second electrode pair consisting of the second pump electrode 34 and the exhaust gas electrode 22 and a third electrode pair consisting of the measuring electrode 44 and the exhaust gas electrode 22.

[0027] The pump currents IP0 and IP1 applied to the first and second pump electrodes 24, 34 are set such that preferably only the oxygen is ionized, but not the nitrogen oxides. In particular, the first pump electrode 24 is configured to pump almost all the oxygen out of the exhaust gas during normal operation of the nitrogen oxide sensor 10, or to allow a predetermined oxygen slip from the first pump cavity 20 into the second pump cavity 30. The second pump electrode 34 is configured to ionize and discharge the oxygen that has not yet been pumped out of the first pump cavity 20, so that almost only nitrogen oxides are present in the measuring cavity 40. The measuring electrode 44 is configured to ionize the nitrogen oxides, with the measuring current IP2 applied to the measuring electrode 44 being a measure of the nitrogen oxide content in the exhaust gas.

[0028] Furthermore, a heating device 60 is arranged within the main body 12, which is designed to heat the main body 12 to a predetermined operating temperature and to maintain it at this temperature, for example at approximately 850°C.

[0029] The operating method for determining the nitrogen oxide content in the exhaust gas of the internal combustion engine using the disclosed nitrogen oxide sensor 10 is already known from the prior art, to which reference is made here. The control principle for the nitrogen oxide sensor 10 of the Fig. 1 is characterized by the fact that the respective electrode voltages or Nernst voltages V0, V1, V2 are kept at a constant level by applying and adjusting the pump currents IP0, IP1 and the measuring current IP2.

[0030] The Fig. 2 shows a cross-sectional view through the exhaust gas sensor 10 of the Fig. 1 along line II-II. From the Fig. 2 It becomes apparent that the heating device 60 is completely embedded in and arranged within the sensor element 12 and has a heating section 62 designed for heating the sensor element 12, which is electrically connected via a first electrical line 64 and a second electrical line 66 to a control device (not explicitly shown) for electrically controlling the heating section 62. Fig. 2 The heating section 62 is shown separated from the first electrical line 64 and the second electrical line 66 by a dashed line. Specifically, the heating section 62 is a heating coil designed to heat the sensor element 12, particularly in the immediate vicinity of the first pump cavity 20, the second pump cavity 30, and the measuring cavity 40, to the operating temperature of approximately 850°C. Consequently, the electrodes 24, 34, 44 (see Figure 1) are also heated by this heating element. Fig. 1 ) heated accordingly.

[0031] Additionally, the heating device 60 has an electrical measuring line 68, which is electrically connected between the control device and the first electrical line 64. The measuring line 68 is, in particular, a high-resistance electrical line (approximately 6 ohms) relative to the first line (approximately 0.4 ohms [Ω]) and the second line (approximately 0.4 ohms [Ω]). It bridges the heating section 62 and is designed to control the resistance or the operating temperature of the sensor element 12 in the area of ​​the heating section 62. Specifically, the electrical resistance of the heating section 62 can be determined by measuring the resistance between the first electrical line 64 and the measuring line 68, or between the second electrical line 66 and the measuring line 68. This allows the heating energy supplied to the heating section 62 to heat the sensor element 12 to the predetermined setpoint temperature.The heating section 62 is arranged in particular in an end region 13 of the sensor element.

[0032] With further reference to the Fig. 3 An exemplary method according to the invention for determining a corrected nitrogen oxide value is explained below.

[0033] The procedure of Fig. 3 The process starts at step 100 and then proceeds to step 110, where a measured voltage is determined that drops between the first electrical line 64 and the measuring line 68. In a subsequent step 120, a compensation voltage is determined based on a predetermined reference voltage and the determined measured voltage. The predetermined reference voltage is a measured voltage drop between the first electrical line 64 and the measuring line 68, occurring between these two lines 64 and 68 at a predetermined (known) temperature of the sensor element 12 (such as room temperature of approximately 20°C). In step 120, the compensation voltage is determined by calculating the difference between the predetermined reference voltage and the determined measured voltage.

[0034] In a further step, a compensation nitrogen oxide value is determined by assigning the calculated compensation voltage to the compensation nitrogen oxide value. This can be done, for example, using a mapping table that assigns the calculated compensation voltage to a corresponding compensation nitrogen oxide value. Alternatively, the assignment can be done using a mathematical mapping.

[0035] In a further step 140, the nitrogen oxide value recorded by the exhaust gas sensor 10 as measuring current IP2 is then corrected to a corrected nitrogen oxide value, taking into account the determined compensation nitrogen oxide value.

[0036] The determined compensation nitrogen oxide value is subtracted from the nitrogen oxide value recorded by exhaust gas sensor 10, which is displayed by measuring current IP2.

[0037] According to a further method of the invention, instead of or in addition to step 130 the Fig. 3A process step is carried out in which a corresponding sensor element temperature value is assigned to the determined compensation voltage for that area of ​​sensor element 12 surrounding the electrical leads 64, 66, 68. From the sensor element temperature value thus determined, the temperature of a housing (not explicitly shown) of the exhaust gas sensor 10 can be estimated in a further step. The compensation of the determined nitrogen oxide values ​​of the exhaust gas sensor 10 can then be carried out based on the determined average temperature of that area of ​​sensor element 12 surrounding the electrical leads 64, 66, 68, and / or on the determined housing temperature.

[0038] The inventive method thus enables at least partial compensation of the nitrogen oxide offset that can result from an increased temperature of the sensor element 12 in the area of ​​the electrical leads 64, 66, 68. In particular, the exhaust gas flowing around the exhaust gas sensor 10 can lead to the temperature increase of the aforementioned area of ​​the sensor element 12. The hot exhaust gas can increase the temperature of the housing of the exhaust gas sensor 10, which in turn can lead to heating of the area of ​​the sensor element 12 surrounding the electrical leads 64, 66, 68 via convection and conduction. Furthermore, the heated exhaust pipe and other external heat sources, such as turbochargers and exhaust manifolds, can lead to a temperature increase of the housing of the exhaust gas sensor 10, which in turn can impair the measurement accuracy of the exhaust gas sensor (see above).Due to the temperature increase of the sensor element 12 of the exhaust gas sensor 10, less heat can be dissipated from the front measuring area of ​​the sensor element 12, which can lead to a shift in the temperature profile of the electrodes of the exhaust gas sensor 10 compared to a normal operating state or to the new state of the exhaust gas sensor 10. Due to the increase in the temperature of the measuring electrode 44, the measuring current IP2 can become larger and thus the NOx signal can be larger than it actually is.

Claims

1. Method for determining a corrected nitrogen oxide value of an exhaust gas sensor (10) comprising a sensor element (12) and a heating device (60) arranged in the sensor element (12) and intended for heating the sensor element (12), wherein the heating device (60) comprises a heating section (62) arranged in the sensor element (12), said heating section being electrically connected, via a first electrical line (64) and a second electrical line (66), to a control device for electrically controlling the heating section (62), wherein the heating device (60) furthermore comprises a measuring line (68), which is electrically connected between the control device and the first electrical line (64) and by means of which the temperature-dependent resistance of the heating section (62) is controllable, wherein the method comprises: - determining a measurement voltage dropped between the first electrical line (64) and the measuring line (68), - determining a compensation voltage by determining a difference between a predetermined reference voltage and the determined measurement voltage, wherein the predetermined reference voltage corresponds to a measurement voltage dropped between the first electrical line (64) and the measuring line (68) at a predetermined temperature of the sensor element (12), - determining a compensation nitrogen oxide value by means of assigning the determined compensation voltage to the compensation nitrogen oxide value, - determining a nitrogen oxide value by means of the exhaust gas sensor (10), and - determining the corrected nitrogen oxide value by means of subtracting the compensation nitrogen oxide value from the determined nitrogen oxide value.

2. Method according to Claim 1, wherein assigning the determined compensation voltage to the compensation nitrogen oxide value is effected by means of an assignment table and / or an assignment specification.

3. Method according to either of the preceding claims, wherein the predetermined temperature corresponds to a room temperature of approximately 20°C.