DE102018203394A1 - A method of operating a sensor to detect at least a portion of a sample gas component having bound oxygen in a sample gas - Google Patents

Abstract

A method is proposed for operating a sensor (100) for detecting at least a portion of a measurement gas component with bound oxygen in a measurement gas, in particular in an exhaust gas of an internal combustion engine, wherein the sensor (100) comprises a sensor element (110). The sensor element (110) has a first pumping cell (112) which has an outer pumping electrode (114) and an inner pumping electrode (116) and which bears against a first cavity (126) which is in communication with the measuring gas, a Nernst cell ( 130) having a Nernst electrode (132) and a reference electrode (134) and abutting a reference gas space (138), and a second pumping cell (140) having a pumping electrode (142) and a counter electrode (144) and which rests against a second cavity (145). A Nernst voltage of the Nernst cell (130) is changed from a first setpoint value to a second setpoint value, wherein the second setpoint value is greater than the first setpoint value. Oxygen is removed from the first cavity (126) by means of the outer pumping electrode (114) and the inner pumping electrode (116).

Description

State of the art

From the prior art, a variety of methods and sensors for detecting at least a proportion of the measured gas component with bound oxygen in a gas mixture, in particular in an exhaust gas of an internal combustion engine, by detecting a proportion of oxygen, by a reduction of the measured gas component with the bound oxygen is generated, known.

Sensors for detecting at least a portion of the measured gas component with bound oxygen in a gas mixture, which are also abbreviated or simply referred to as NO _x sensors or nitrogen oxide sensors, are, for example, in Reif, K., Deitsche, KH. et al., Kraftfahrisches Taschenbuch, Springer Vieweg, Wiesbaden, 2014, pages 1338-1347.

Nitrogen oxide sensors (= NO _x sensors), which are used today in the automotive industry, work according to the limiting current principle, analogous to oxygen sensors, such as lambda sensors. Such a nitrogen oxide sensor comprises a Nernst concentration cell, which is also called a reference cell, a modified oxygen pump cell and a further modified oxygen pump cell, the so-called NO _x cell. An outer pumping electrode exposed to the exhaust gas and an inner pumping electrode in a first cavity, which is separated from the exhaust gas by a diffusion barrier, form the oxygen pumping cell. The first cavity also contains the Nernst electrode and, in a reference gas space, the reference electrode, which together form the Nernst cell. The NO _x cell includes a NO _x pumping electrode and a counter electrode. The NO _x pump electrode is located in a second cavity, which is connected to the first inner cavity and separated from it by a diffusion barrier. The counter electrode is located in the reference gas space. All electrodes in the first and second cavities have a common return conductor.

During operation of the nitrogen oxide sensor, the oxygen from the first cavity, which is connected to the exhaust gas via a diffusion barrier, is removed from the so-called O2 cell. The resulting pumping current is then proportional to the oxygen content of the ambient air in the sample gas or exhaust gas flow. The nitrogen oxides are pumped out in the NO _x cell. The nitrogen oxide NO _x in the atmosphere in the second cavity is reduced or reduced by applying a constant pumping voltage. The oxygen produced by reduction or degradation of the measurement gas component in the second cavity, which preferably originates from the reduction of the nitrogen oxide NO _x , is pumped off into a reference gas space. Thus, the applied pumping voltage to the resistance of the NO _x cell and the concentration of nitrogen oxide NO _x or oxygen, a pumping current result, which is proportional to the content of nitrogen oxide NO _x or oxygen and represents the NO _x measurement signal.

The resulting pumping current I _P2 is thus a measure of the NO _x concentration of the ambient air in the sample gas or exhaust gas stream. A gold doping of the electrode of the O2 cell together with a limited pumping voltage prevents the decomposition of the NOx, which would then be noticeable as an additional O2 pumping current and would falsify the actual NOx measurement of the second cell.

Despite the advantages of the sensors known from the prior art and methods for operating the same, they still have room for improvement. To establish readiness for operation of the nitric oxide sensor, existing oxygen must be removed from the NOx measuring cell. This has hitherto been done by pumping off the oxygen present via the NOx electrode. The surface of the NOx electrode is relatively small. Excessive current density damages the electrode. The current available for pumping is therefore relatively low, so that a relatively long time of about 20 s is required for pumping out the oxygen present.

Disclosure of the invention

Therefore, a method is proposed for operating a sensor for detecting at least a portion of a sample gas component with bound oxygen in a measurement gas, which avoids the disadvantages of known methods for operating these sensors at least largely and in which the time to reach the operational readiness at the start of the sensor is significantly reduced.

In a method according to the invention for operating a sensor for detecting at least a portion of a measurement gas component with bound oxygen in a measurement gas, in particular in an exhaust gas of an internal combustion engine, wherein the sensor comprises a sensor element, wherein the sensor element, a first pump cell, an outer pumping electrode and an inner pump Having a pumping electrode and which abuts a first cavity, which is in communication with the measurement gas, a Nernst cell, which has a Nernst electrode and a reference electrode and which abuts a reference gas space, and a second pump cell, the pump electrode and has a counter electrode and which abuts a second cavity, a Nernstspannung the Nernst cell is changed from a first target value to a second target value. The second setpoint value is greater than the first setpoint value. Oxygen is removed from the first cavity by means of the first pumping cell or the outer pumping electrode and the inner pumping electrode.

In the first pumping cell, after a short time of, for example, less than 2 s, the desired operating point with an O 2 concentration of about 10 ^-6 mbar = 1 ppm is reached. If the Nernst potential of the Nernst cell is changed by setting a new setpoint for the Nernst voltage so that the regulator is simulated a gas with higher oxygen partial pressure, this increases the pumping voltage at the first pump cell, whereby an increased pumping current flows. Thus, the first pump cell tries even more to pump off oxygen, although the cell is already emptied of oxygen. Due to the oxygen concentration gradient between the first pumping cell (about 1 ppm) and the second pumping cell (>> 1 ppm), oxygen now diffuses out of the second pumping cell or the second cavity into the first pumping cell or into the first cavity and is pumped out there. This shortens the time until the second pump cell is ready for operation.

In a further development, the second desired value has an electrical voltage in a range from 425 mV to 800 mV and preferably 430 mV to 650 mV. This voltage is sufficient for a quick removal of oxygen from the second pumping cell.

In one development, the electronic control unit is connected to the sensor element via at least four electrical connections, wherein the electronic control unit has at least a first separate connection for the first pump cell, a second separate connection for the second pump cell, a connection for the Nernst voltage and a common connection features. This allows different voltages, currents and potentials on the sensor element to be measured, which serve to describe qualitative and quantitative properties of the measurement gas.

In a further development, the Nernst voltage of the Nernst cell is regulated by means of the control unit. The controller thus has a controller which may be integrated in this. This makes it possible to set the Nernst voltage very precisely.

In a further development, the Nernst electrode is connected to the common connection and the reference electrode is connected to the connection for a Nernst voltage. This allows the Nernst cell to be precisely regulated.

In a further development, the pump electrode is connected to the common terminal, wherein the counter electrode is connected to the second separate terminal, wherein an electric pump voltage is applied to the pump electrode and the counter electrode. In this case, a pumping current between the pumping electrode and the counter electrode is established. This ensures a suitable pumping operation of the second pumping cell.

In a further development, an oxygen partial pressure difference between the first cavity and the second cavity is increased by means of the second setpoint value of the Nernst voltage of the Nernst cell compared to an oxygen partial pressure difference between the first cavity and the second cavity at the first nominal value of the Nernst voltage. As a result, oxygen can pass from the second cavity into the first cavity through the diffusion barrier particularly well or quickly.

In a development, the sensor element is heated at least during the removal of the oxygen from the first cavity. This additionally accelerates the removal of oxygen, since it ensures that the solid electrolyte is sufficiently conductive for oxygen ions.

In addition, a computer program is proposed which is set up to carry out each step of the method according to the invention.

Furthermore, an electronic storage medium is proposed, on which a computer program for carrying out the method according to the invention is stored.

The invention furthermore encompasses an electronic control unit which contains the electronic storage medium according to the invention with the said computer program for carrying out the method according to the invention.

Finally, the invention also relates to a sensor for detecting at least a portion of a measuring gas component with bound oxygen in a measuring gas, in particular in an exhaust gas of an internal combustion engine, comprising a sensor element, wherein the sensor element comprises a first pumping cell having an outer pumping electrode and an inner pumping electrode and the abuts a first cavity, which is in communication with the measurement gas, a Nernst cell having a Nernst electrode and a reference electrode and at a reference gas space is applied, and a second pumping cell having a pumping electrode and a counter electrode and which abuts a second cavity, wherein the sensor further comprises an inventive electronic control unit.

In the context of the present invention, a solid electrolyte is to be understood as meaning a body or article having electrolytic properties, that is to say having ion-conducting properties. In particular, it may be a ceramic solid electrolyte. This also includes the raw material of a solid electrolyte and therefore the formation as a so-called green or brownling, which only becomes a solid electrolyte after sintering. In particular, the solid electrolyte may be formed as a solid electrolyte layer or from a plurality of solid electrolyte layers. In the context of the present invention, a layer is to be understood as a uniform mass in the areal extent of a certain height which lies above, below or between other elements.

An electrode in the context of the present invention is generally understood to mean an element which is capable of contacting the solid electrolyte in such a way that a current can be maintained by the solid electrolyte and the electrode. Accordingly, the electrode may comprise an element to which the ions can be incorporated in the solid electrolyte and / or removed from the solid electrolyte. Typically, the electrodes comprise a noble metal electrode which may, for example, be deposited on the solid electrolyte as a metal-ceramic electrode or otherwise be in communication with the solid electrolyte. Typical electrode materials are platinum cermet electrodes. However, other precious metals, such as gold or palladium, are in principle applicable.

In the context of the present invention, a heating element is to be understood as meaning an element which serves for heating the solid electrolyte and the electrodes to at least their functional temperature and preferably to their operating temperature. The functional temperature is the temperature at which the solid electrolyte becomes conductive to ions and which is approximately 350 ° C. Of this, the operating temperature is to be distinguished, which is the temperature at which the sensor element is usually operated and which is higher than the operating temperature. The operating temperature may be, for example, from 700 ° C to 950 ° C. The heating element may comprise a heating area and at least two feed tracks. The supply tracks are also referred to below as a supply line.

In the context of the present invention, a heating region is to be understood as the region of the heating element which overlaps in the layer structure along an axis perpendicular to the surface of the sensor element with an electrode. Usually, the heating area heats up during operation more than the supply line or supply line, so that they are distinguishable. The different heating can for example be realized in that the heating area has a higher electrical resistance than the supply track. The heating area and / or the supply line are designed, for example, as an electrical resistance path and heat up when an electrical voltage is applied, since an electrical current flows through them. The heating element may for example be made of a platinum cermet. The invention is directly detectable by a shortened waiting time until the operational readiness is reached after starting the sensor. The respective potentials can be measured at the supply lines.

list of figures

Further optional details and features of the invention will become apparent from the following description of preferred embodiments, which are shown schematically in the figures.

• 1 einen prinzipiellen Aufbau eines erfindungsgemäßen Sensors und
• 2 Flussdiagramm eines erfindungsgemäßen Verfahrens.

Show it:

• 1 a basic structure of a sensor according to the invention and
• 2 Flowchart of a method according to the invention.

Embodiments of the invention

1 shows a basic structure of a sensor according to the invention 100 , which is particularly suitable for carrying out the method according to the invention.

The sensor 100 , which for detecting at least a portion of a measured gas component with bound oxygen, hereinafter referred to as nitrogen oxide NOx, in a gas mixture, for example, an exhaust gas of an internal combustion engine, is arranged, for this purpose comprises a sensor element 110 a first pumping cell 112 , which between an outer pumping electrode 114 and an inner pumping electrode 116 is trained. The outer pump electrode 114 , which by means of a porous aluminum oxide layer 118 from the environment of the sensor 100 is separated, in this case has a first electrically conductive connection 120 over which a first pumping current I _P1 in the first pump cell 112 can generate. The first electrically conductive connection 120 is this with a connection P1 an external electronic control unit 122 connected. To a complete Circuit has the inner pumping electrode 116 also via a second electrically conductive connection 124 leading to a common connection COM of the external electronic control unit 122 leads. The first pump cell 112 lies on a first cavity 126 at the inside of the sensor element 110 is located and in communication with the sample gas. By generating the first pumping current I _P1 in the first pump cell 112 a first portion of oxygen ions, which are formed from molecular oxygen from the gas mixture, between the first cavity 126 and the environment of the sensor 100 transport. In the entryway from the environment to the first cavity 126 is a diffusion barrier 128 available.

The sensor element 110 also has an electrical Nernst cell 130 on which a Nernst electrode 132 and a reference electrode 134 having. While the Nernst electrode 132 over the second electrically conductive connection 124 together with the inner pumping electrode 116 to the common port COM has, points the reference electrode 134 a separate electrically conductive connection 136 to a connection vs the external electronic control unit 122 for the Nernst tension vs on. The Nernst cell 130 is located on a reference gas space 138 on. A second portion of the oxygen ions from the sample gas space 126 and / or from the environment of the sensor 100 gets into the reference gas space 138 transported by applying a reference pumping current between the terminal Vs and the common terminal COM. In this case, the value for the reference pumping current is adjusted such that a defined proportion of the oxygen ions in the reference gas space 138 formed. Preferably, in this context, the value for the first pumping current I _P1 adjusted such that there is a fixed ratio between the first portion of the oxygen ions in the sample gas space 126 and the second portion of the oxygen ions in the reference gas space 138 results.

The measured gas component nitrogen oxide NO _x with the bound oxygen, which is furthermore contained in the gas mixture, reaches, in particular by diffusion, largely uninfluenced into a second pumping cell 140 of the sensor element 110 , which may also be referred to as a "NO _x pump cell". The second pump cell 140 has a NO _x pump electrode 142 and a NO _x counter electrode 144 on and lies on a second cavity 145 inside the sensor element 110 on. At least one of the two electrodes NO _x pumping electrode 142 and / or NO _x counter electrode 144 are configured such that upon application of a voltage by means of catalysis from the sample gas component NO _x further molecular oxygen can be generated, which in the second pumping cell 140 is formed.

While the NO _x pumping electrode 142 an electrically conductive connection 146 which leads to the common terminal COM has the NO _x counter electrode 144 an electrically conductive connection 146 on which a second pumping current I _P2 to the second pumping cell 140 can be created. The electrically conductive connection 146 is this with a connection P2 the external electronic control unit 122 connected. When applying a second pumping current I _P2 to the second pumping cell 140 a proportion of further oxygen ions, which were formed from the further molecular oxygen, in the reference gas space 138 transported. The second cavity 145 is from the first cavity 126 through a diffusion barrier 147 separated.

The sensor element 110 also has a heating element 148 , which by means of two supply lines 150 with connections HTR + and HTR- of the control unit 122 is connected, via which a heating current in the heating element 148 can be introduced, which by means of generating a heating power, the sensor element 110 can bring to the desired temperature.

2 shows a flowchart of a method according to the invention for operating the sensor 100 , First, in step S10 the electronic control unit 122 via the electrical connections with the sensor element described above 110 connected. This can be done, for example, at the factory.

In the present embodiment, the electronic control unit 122 via at least four electrical connections to the sensor element 110 connected. This is how the electronic control unit works 122 at least over the first separate connection P1 for the first pump cell 112 , the second separate connection P2 for the second pumping cell 140 , about the connection for a Nernst voltage vs and connected via the common port COM. The Nernst electrode 132 is connected to the common terminal COM and the reference electrode 134 comes with the connection for a Nernst voltage vs connected. In addition, the pumping electrode 142 connected to the common terminal COM and the counter electrode 144 comes with the second separate port P2 connected.

In step S12 becomes the sensor 100 switched on, which can be done for example at the same time as starting the internal combustion engine. At this time is located in the second pumping cell 140 and more specifically in the second cavity 145 Oxygen, for example, from a previous operating cycle or production-related. For this reason, the pump electrode 142 still occupied with oxygen. The control unit 122 is suitable for a nominal value for the Nernst voltage of the Nernst cell 130 to regulate. For this purpose, the controller 122 a controller not shown. The control unit 122 gives in step for normal operation, ie after making ready for operation S14 a first nominal value for the Nernst voltage of the Nernst cell 130 in front. The first setpoint value is, for example, an electrical voltage of 420 mV and corresponds to a lambda = 1 of the Nernst cell 130 ,

Then in step S16 in the control unit 122 the controller a modified setpoint value for a Nernst voltage of the Nernst cell 130 specified. Thus, the first setpoint value is changed to a second setpoint value, wherein the second setpoint value is greater than the first setpoint value. The second desired value is an electrical voltage in a range of 425 mV to 800 mV and preferably 430 mV to 650 mV, for example 500 mV. This puts the controller of the controller 122 a higher pumping voltage at the first pumping cell 112 on.

In the first pump cell 112 is reached after a short time, for example, less than 2s the desired operating point with an O2 concentration of about 10 ^-6 mbar = 1 ppm. Is the controller of the controller 11 another set value for the Nernst voltage is given to the control unit 122 a simulated gas with a higher oxygen partial pressure and the first pump cell 112 tries to pump off even more oxygen, although the first pump cell 112 already emptied of oxygen. By means of the second nominal value of the Nernst voltage of the Nernst cell 130 becomes an oxygen partial pressure difference between the first cavity 126 and the second cavity 145 compared to an oxygen partial pressure difference between the first cavity 126 and the second cavity 145 increased at the first nominal value of the Nernst voltage. Due to the concentration gradient between the first pumping cell 112 of about 1ppm and the second pump cell 140 significantly more than 1ppm now diffuses oxygen from the second pumping cell 140 or the second cavity 145 through the diffusion barrier 147 between the first cavity 126 and the second cavity 145 in the first pump cell 112 or the first cavity 126 and is pumped out there. The pumping or removal of the oxygen from the first cavity 126 happens through a first pumping current I _P1 which is about the first electrically conductive connection 120 and the connection P1 an external electronic control unit 122 when presetting a pump voltage through the controller of the controller 122 established. To get a complete circuit, the inner pumping electrode has 116 over the electrically conductive connection 124 leading to the common port COM of the external electronic control unit 122 leads. At the same time oxygen from the second cavity in the reference gas space 138 by means of the pumping electrode 142 and the counter electrode 144 the second pumping cell 140 pumped. This shortens the time until the second pump cell is ready for operation 140 , A further acceleration to establish the operational readiness of the second pumping cell 140 can be achieved when the sensor element 110 by means of the heating element 148 at least during the removal of the oxygen from the first pumping cell 112 or the first cavity 126 is heated.

Claims (12)

Method for operating a sensor (100) for detecting at least a portion of a measurement gas component with bound oxygen in a measurement gas, in particular in an exhaust gas of an internal combustion engine, comprising a sensor element (110), wherein the sensor element (110) comprises a first pump cell (112) an outer pumping electrode (114) and an inner pumping electrode (116) and abutting a first cavity (126) in communication with the measuring gas, a Nernst cell (130) comprising a Nernst electrode (132) and a reference electrode (134) and which abuts a reference gas space (138), and a second pumping cell (140) having a pumping electrode (142) and a counterelectrode (144) and abutting a second cavity (145), wherein a Nernstspannung the Nernst cell (130) is changed from a first setpoint value to a second setpoint value, wherein the second setpoint value is greater than the first setpoint value, wherein by means of the outer pumping Keles electrode (114) and the inner pumping electrode (116) oxygen is removed from the first cavity (126). Method according to Claim 1 wherein the second setpoint value is an electrical voltage in a range of 425 mV to 800 mV, and preferably 430 mV to 650 mV. Method according to Claim 1 or 2 wherein the electronic control unit (122) is connected to the sensor element (110) via at least four electrical connections, the electronic control unit (122) having at least a first separate connection (P1) for the first pumping cell (112), a second separate connection (P2) for the second pumping cell (140), via a connection for a Nernst voltage (Vs) and via a common connection (COM). Method according to Claim 3 , wherein the Nernst voltage of the Nernst cell (130) is regulated by means of the control device (122). Method according to Claim 3 or 4 wherein the Nernst electrode (132) is connected to the common terminal (COM) and the reference electrode (134) is connected to the Nernst voltage terminal (Vs). Method according to one of Claims 3 to 5 wherein the pumping electrode (142) is connected to the common terminal (COM), the counterelectrode (144) being connected to the second separate terminal (P2), an electrical pumping voltage being applied to the pumping electrode (142) and the counterelectrode (144) is created. Method according to one of Claims 1 to 6 wherein an oxygen partial pressure differential between the first cavity (126) and the second cavity (145) is determined by the second nominal value of the Nernst voltage of the Nernst cell (130) as compared to an oxygen partial pressure differential between the first cavity (126) and the second cavity (145). is increased at the first target value of the Nernst voltage. Method according to one of Claims 1 to 7 wherein the sensor element (110) is heated at least during the removal of the oxygen from the first cavity (126). Computer program adapted to perform each step of the method according to one of the preceding claims. An electronic storage medium on which a computer program according to the preceding claim is stored. An electronic control device comprising an electronic storage medium according to the preceding claim. Sensor (100) for detecting at least a portion of a measurement gas component with bound oxygen in a measurement gas, in particular in an exhaust gas of an internal combustion engine, comprising a sensor element (110), wherein the sensor element (110) has a first pumping cell (112) which forms an outer pumping electrode (110). 114) and an inner pumping electrode (116) and abutting against a first cavity (126) in communication with the sample gas, a Nernst cell (130) having a Nernst electrode (132) and a reference electrode (134) and which abuts a reference gas space (138), and a second pumping cell (140) having a pumping electrode (142) and a counterelectrode (144) and abutting a second cavity (145), the sensor (100) further having electronic control device according to the preceding claim.

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