JP2017146199A - Sulfur oxide detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sulfur oxide detector with which it is possible to improve the accuracy of detecting an SOx concentration in an exhaust gas.SOLUTION: An SOx detection device 1 comprises: a sensor cell 51; a pump cell 52; a first voltage application circuit 61 for applying a voltage to the sensor cell; a second voltage application circuit 71 for applying a voltage to the pump cell; a first detector 62 for detecting a first current correlation parameter of the sensor cell; a second detector 72 for detecting a second current correlation parameter of the pump cell; and a control unit 80. The control unit executes first water concentration detection control to apply a voltage for not causing oxide ions to move from an anode to a cathode to the sensor cell and/or release the first voltage application circuit open and apply a voltage within the limiting current area of water to the pump cell, calculates a water concentration in a gas to be measured on the basis of the second current correlation parameter, or executes second water concentration detection control to apply a voltage within the limiting current area of water to the sensor cell and release the second voltage application circuit and calculates a water concentration in the gas to be measured on the basis of the first current correlation parameter.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a sulfur oxide detector (SOx detector) that detects the concentration of sulfur oxide (SOx) in exhaust gas.

  A trace amount of sulfur (S) component is contained in the fuel used for the internal combustion engine, particularly in the fossil fuel. Thus, the sulfur component contained in the fuel causes deterioration of components in the exhaust system of the internal combustion engine. Further, frequent control of suppressing deterioration of the component due to the sulfur component and control of regenerating the deteriorated component cause deterioration of fuel consumption. Accordingly, it is desired to detect the content of the sulfur component in the fuel in order to minimize deterioration of the component parts while minimizing deterioration of fuel consumption and the like.

  When the sulfur component is contained in the fuel used for the internal combustion engine, sulfur oxide (SOx) is contained in the exhaust gas discharged from the combustion chamber. Further, the higher the content ratio of the sulfur component in the fuel, the higher the SOx concentration in the exhaust gas. Therefore, if the SOx concentration in the exhaust gas can be detected, the content rate of the sulfur component in the fuel can be estimated.

  Thus, an exhaust gas sensor that detects the concentration of an oxygen-containing gas component such as SOx contained in the exhaust gas has been proposed (for example, Patent Document 1). This exhaust gas sensor has a measured gas chamber into which exhaust gas is introduced through a diffusion-controlled layer, a first electrochemical cell, and a second electrochemical cell. In the first electrochemical cell, a relatively low voltage is applied between the electrodes constituting the first electrochemical cell. As a result, the oxygen pumping action of the first electrochemical cell removes oxygen molecules in the measured gas chamber without decomposing SOx in the measured gas chamber. On the other hand, in the second electrochemical cell, a relatively high voltage is applied between the electrodes constituting the second electrochemical cell. As a result, SOx contained in the exhaust gas after oxygen molecules are removed by the first electrochemical cell is decomposed. In addition, the oxide ions generated by the decomposition of SOx are discharged from the measured gas chamber by the oxygen pumping action of the second electrochemical cell, and exhaust gas is detected by detecting the decomposition current flowing along with the discharge of the oxide ions. The SOx concentration in the gas is detected.

JP-A-11-190721 JP, 2015-017931, A

  However, the SOx concentration in the exhaust gas is considerably lower than the oxygen concentration and water concentration in the exhaust gas. For this reason, the decomposition current detected by the exhaust gas sensor as described above is also extremely small. Therefore, it is difficult to accurately detect the SOx decomposition current using the exhaust gas sensor as described above.

  Therefore, an object of the present invention is to provide a sulfur oxide detector that can improve the detection accuracy of the SOx concentration in the exhaust gas.

  In order to solve the above-mentioned problem, in the first invention, a sulfur oxide detector for detecting a sulfur oxide concentration in a gas to be measured, the first solid electrolyte layer having oxide ion conductivity, A first electrode disposed on one side surface of the first solid electrolyte layer so as to be exposed to the gas to be measured; and a first electrode disposed on the other side surface of the first solid electrolyte layer so as to be exposed to the atmosphere. A sensor cell having a second electrode, a second solid electrolyte layer having oxide ion conductivity, and a second solid electrolyte layer disposed on one side surface of the second solid electrolyte layer so as to be exposed to the gas to be measured. A pump cell having three electrodes, and a fourth electrode disposed on the other side surface of the second solid electrolyte layer so as to be exposed to the atmosphere, and a diffusion-controlling layer that controls the diffusion of the gas to be measured First voltage application for applying voltage to the element part and the sensor cell A second voltage application circuit for applying a voltage to the pump cell, a first detector for detecting a first current correlation parameter correlated with a current flowing through the sensor cell, and a second current correlated with a current flowing through the pump cell A second detector for detecting a correlation parameter; a voltage applied to the sensor cell by the first voltage application circuit; and a voltage applied to the pump cell by the second voltage application circuit; and from the first detector A control unit that acquires the first current correlation parameter and acquires the second current correlation parameter from the second detector, wherein the pump cell is disposed closer to the diffusion-controlled layer than the sensor cell, and the control The unit applies a voltage at which oxide ions do not move from the second electrode to the first electrode, or applies the first voltage to the sensor cell. A first water concentration detection control is performed to open the path and apply a voltage within the limit current region of water to the pump cell, and when the first water concentration detection control is executed, it is detected by the second detector. Calculating a water concentration in the measured gas based on the second current correlation parameter, or applying a voltage in a limit current region of water to the sensor cell and opening the second voltage application circuit; Water concentration detection control is executed, and the water concentration in the measured gas is calculated based on the first current correlation parameter detected by the first detector when the second water concentration detection control is executed. The control unit applies a voltage equal to or higher than the decomposition start voltage of water and sulfur oxide to the sensor cell, and generates a voltage equal to or higher than the lower limit voltage of the oxygen limit current region and lower than the decomposition start voltage of water and sulfur oxide. Mark on the pump cell The sulfur oxide concentration detection control to be applied is executed, and the first current correlation parameter detected by the first detector when the sulfur oxide concentration detection control is executed and the calculated water concentration There is provided a sulfur oxide detector that calculates a sulfur oxide concentration in the measured gas based on the above.

  ADVANTAGE OF THE INVENTION According to this invention, the sulfur oxide detection apparatus which can improve the detection precision of SOx density | concentration in exhaust gas is provided.

FIG. 1 is a schematic cross-sectional view showing the configuration of an SOx detection device according to the present invention. FIG. 2 is a schematic cross-sectional view showing another configuration of the SOx detection device according to the present invention. FIG. 3 is a diagram showing the relationship between the applied voltage of the sensor cell and the interelectrode current flowing in the sensor cell. FIG. 4 is a diagram showing the relationship between the magnitude of the interelectrode current when the applied voltage is 1.0 V and the sulfur dioxide concentration in the measured gas. FIG. 5 is a diagram showing the relationship between the applied voltage of the pump cell and the interelectrode current flowing in the pump cell. FIG. 6 is a flowchart showing a control routine of the SOx concentration calculation process in the present invention. FIG. 7 is a flowchart showing a control routine of detection condition determination processing in the present invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are assigned to similar components.

<Description of SOx detection device>
Hereinafter, a sulfur oxide detection device (hereinafter referred to as “SOx detection device”) according to the present invention will be described with reference to FIG. 1. The SOx detection device 1 detects the concentration of sulfur oxide (SOx) in the exhaust gas flowing through the exhaust passage of the internal combustion engine.

  FIG. 1 is a schematic cross-sectional view showing a configuration of an SOx detection device 1 according to the present invention. The SOx detection device 1 includes an element unit 10. The element unit 10 is disposed in an exhaust passage (not shown) of the internal combustion engine. As shown in FIG. 1, the element unit 10 is configured by laminating a plurality of layers. Specifically, the element unit 10 includes a first solid electrolyte layer 11, a second solid electrolyte layer 12, a diffusion rate controlling layer 16, a first impermeable layer 21, a second impermeable layer 22, a third impermeable layer 23, A fourth opaque layer 24, a fifth opaque layer 25, and a sixth opaque layer 26 are provided.

The first solid electrolyte layer 11 and the second solid electrolyte layer 12 are thin plate bodies having oxide ion conductivity. The first solid electrolyte layer 11 and the second solid electrolyte layer 12 are made of, for example, ZrO ₂ (zirconia), HfO ₂ , ThO ₂ , Bi ₂ O ₃ , CaO, MgO, Y ₂ O ₃ , Yb ₂ O _3, etc. It is formed of a sintered body added as a stabilizer. The diffusion-controlling layer 16 is a thin plate having gas permeability. The diffusion control layer 16 is formed of a porous sintered body of a heat resistant inorganic material such as alumina, magnesia, silica, spinel, mullite, and the like. The impermeable layers 21 to 26 are gas impermeable thin plates, and are formed as layers containing alumina, for example.

  Each layer of the element part 10 is, from the lower side of FIG. The layer 24, the second solid electrolyte layer 12, the fifth impermeable layer 25, and the sixth impermeable layer 26 are laminated in this order. A gas chamber 30 to be measured is defined by the first solid electrolyte layer 11, the second solid electrolyte layer 12, the diffusion rate limiting layer 16, and the fourth impermeable layer 24. The measured gas chamber 30 is configured such that the exhaust gas (measured gas) of the internal combustion engine flows into the measured gas chamber 30 via the diffusion-controlled layer 16 when the element unit 10 is disposed in the exhaust passage. Has been. That is, the element portion 10 is disposed in the exhaust passage so that the diffusion rate controlling layer 16 is exposed to the exhaust gas. Therefore, the measured gas chamber 30 communicates with the inside of the exhaust passage via the diffusion rate controlling layer 16. The measured gas chamber 30 may be configured in any manner as long as the measured gas is adjacent to the first solid electrolyte layer 11 and the second solid electrolyte layer 12 and configured to allow the measured gas to flow.

  A first atmospheric chamber 31 is defined by the first solid electrolyte layer 11, the second impermeable layer 22, and the third impermeable layer 23. As can be seen from FIG. 1, the first atmospheric chamber 31 is disposed on the opposite side of the measured gas chamber 30 with the first solid electrolyte layer 11 interposed therebetween. The first atmosphere chamber 31 is open to the atmosphere outside the exhaust passage. Accordingly, the atmosphere flows into the first atmosphere chamber 31. The first atmosphere chamber 31 may be configured in any manner as long as the first atmosphere chamber 31 is adjacent to the first solid electrolyte layer 11 and configured to allow the atmosphere to flow in.

  A second atmospheric chamber 32 is defined by the second solid electrolyte layer 12, the fifth impermeable layer 25, and the sixth impermeable layer 26. As can be seen from FIG. 1, the second atmospheric chamber 32 is disposed on the opposite side of the measured gas chamber 30 with the second solid electrolyte layer 12 interposed therebetween. The second atmosphere chamber 32 is open to the atmosphere outside the exhaust passage. Therefore, atmospheric gas flows into the second atmospheric chamber 32. The second atmosphere chamber 32 may be configured in any manner as long as it is configured to be adjacent to the second solid electrolyte layer 12 and to allow the atmosphere to flow in.

  The element unit 10 further includes a first electrode 41 and a second electrode 42. The first electrode 41 is disposed on the surface of the first solid electrolyte layer 11 on the measured gas chamber 30 side. Therefore, the first electrode 41 is exposed to the measured gas in the measured gas chamber 30. On the other hand, the second electrode 42 is disposed on the surface of the first solid electrolyte layer 11 on the first atmospheric chamber 31 side. Therefore, the second electrode 42 is exposed to the gas (atmosphere) in the first atmospheric chamber 31. The first electrode 41 and the second electrode 42 are disposed so as to face each other with the first solid electrolyte layer 11 interposed therebetween. The first electrode 41, the first solid electrolyte layer 11, and the second electrode 42 constitute a sensor cell 51.

  In the present embodiment, the material constituting the first electrode 41 includes a platinum group element such as platinum (Pt), rhodium (Rh) and palladium (Pd), or an alloy thereof as a main component. Preferably, the first electrode 41 is a porous cermet electrode containing at least one of platinum (Pt), rhodium (Rh) and palladium (Pd) as a main component. However, the material constituting the first electrode 41 is not necessarily limited to the above material, and when a predetermined voltage is applied between the first electrode 41 and the second electrode 42, the measured gas chamber 30. Any material may be used as long as oxygen, water, and SOx contained in the gas to be measured can be reduced and decomposed.

  In the present embodiment, the second electrode 42 is a porous cermet electrode containing platinum (Pt) as a main component. However, the material constituting the second electrode 42 is not necessarily limited to the above material, and when a predetermined voltage is applied between the first electrode 41 and the second electrode 42, Any material may be used as long as the oxide ions can be moved between the second electrode 42.

  The element unit 10 further includes a third electrode 43 and a fourth electrode 44. The third electrode 43 is disposed on the surface of the second solid electrolyte layer 12 on the measured gas chamber 30 side. Therefore, the third electrode 43 is exposed to the measured gas in the measured gas chamber 30. Further, the third electrode 43 is disposed closer to the diffusion rate controlling layer 16 than the first electrode 41 in the measured gas chamber 30. Therefore, the measured gas that has flowed into the measured gas chamber 30 via the diffusion rate controlling layer 16 first flows around the third electrode 43 and then flows around the first electrode 41. On the other hand, the fourth electrode 44 is disposed on the surface of the second solid electrolyte layer 12 on the second atmospheric chamber 32 side. Therefore, the fourth electrode 44 is exposed to the gas (atmosphere) in the second atmospheric chamber 32. The third electrode 43 and the fourth electrode 44 are disposed so as to face each other with the second solid electrolyte layer 12 interposed therebetween. The third electrode 43, the second solid electrolyte layer 12, and the fourth electrode 44 constitute a pump cell 52. The pump cell 52 is disposed closer to the diffusion rate controlling layer 16 than the sensor cell 51.

  The third electrode 43 and the fourth electrode 44 are porous cermet electrodes containing platinum (Pt) as a main component. However, the material constituting the third electrode 43 is not necessarily limited to the above material, and when a predetermined voltage is applied between the third electrode 43 and the fourth electrode 44, the measured gas chamber 30. Any material may be used as long as it can reductively decompose oxygen and water contained in the gas to be measured. Further, the material constituting the fourth electrode 44 is not necessarily limited to the above material, and when a predetermined voltage is applied between the third electrode 43 and the fourth electrode 44, Any material may be used as long as oxide ions can be moved between the fourth electrode 44 and the fourth electrode 44.

  The element unit 10 further includes a heater (electric heater) 55. In the present embodiment, the heater 55 is disposed between the first impermeable layer 21 and the second impermeable layer 22 as shown in FIG. The heater 55 is a cermet thin plate including, for example, platinum (Pt) and ceramics (for example, alumina), and is a heating element that generates heat when energized. The heater 55 can heat the sensor cell 51 and the pump cell 52 to an activation temperature or higher. The SOx detection device 1 further includes an electronic control unit (ECU) 80. The ECU 80 is connected to the heater 55. Therefore, the ECU 80 can control the temperature of the element unit 10, particularly the sensor cell 51 and the pump cell 52 by controlling the heater 55.

  The SOx detection device 1 further includes a first voltage application circuit 61 that applies a voltage to the sensor cell 51 and a second voltage application circuit 71 that applies a voltage to the pump cell 52. The first voltage application circuit 61 applies a voltage between the first electrode 41 and the second electrode 42 of the sensor cell 51 so that the potential of the second electrode 42 is higher than the potential of the first electrode 41. The second voltage application circuit 71 applies a voltage between the third electrode 43 and the fourth electrode 44 of the pump cell 52 so that the potential of the fourth electrode 44 is higher than the potential of the third electrode 43. The ECU 80 is connected to the first voltage application circuit 61 and the second voltage application circuit 71. Therefore, the ECU 80 can control the voltage applied to the sensor cell 51 and the voltage applied to the pump cell 52 by controlling the first voltage application circuit 61 and the second voltage application circuit 71.

  The SOx detection device 1 includes a first current detector 62 that detects a current flowing in the sensor cell 51 (that is, a current flowing in the first solid electrolyte layer 11 between the first electrode 41 and the second electrode 42), and a pump cell. And a second current detector 72 that detects a current flowing through the second solid electrolyte layer 12 between the third electrode 43 and the fourth electrode 44. The ECU 80 is connected to the first current detector 62 and the second current detector 72. Therefore, the ECU 80 acquires the interelectrode current of the sensor cell 51 detected by the first current detector 62 from the first current detector 62, and the interelectrode current of the pump cell 52 detected by the second current detector 72 is the first. It can be obtained from the dual current detector 72.

  In the element unit 10 shown in FIG. 1, the second solid electrolyte layer 12 constituting the pump cell 52 is a solid electrolyte layer different from the first solid electrolyte layer 11 constituting the sensor cell 51. However, like the element part 10 'shown in FIG. 2, the first solid electrolyte layer 11 and the second solid electrolyte layer 12 may be a single solid electrolyte layer. In this case, the number of atmospheric chambers in the element unit is one. The first electrode 41 is disposed on the surface of the first solid electrolyte layer 11 on the measured gas chamber 30 side, and the second electrode 42 is disposed on the surface of the first solid electrolyte layer 11 on the atmosphere chamber 31 side. The third electrode 43 is disposed on the surface of the second solid electrolyte layer 12 on the measured gas chamber 30 side, and the fourth electrode 44 is disposed on the surface of the second solid electrolyte layer 12 on the atmosphere chamber 31 side. .

<SOx concentration detection principle>
Next, the principle of SOx detection by the SOx detection device 1 will be described. In explaining the detection principle of SOx, first, the limiting current characteristic of oxygen in the sensor cell 51 will be explained. In the sensor cell 51, when a voltage is applied between the first electrode 41 on the measured gas chamber 30 side as a cathode and the second electrode 42 on the first atmospheric chamber 31 side as an anode, the voltage is included in the measured gas. Oxygen is reduced and decomposed into oxide ions. The oxide ions are conducted from the cathode side to the anode side through the first solid electrolyte layer 11 of the sensor cell 51 to become oxygen, and are discharged into the atmosphere. In the present specification, such oxygen transfer from the cathode side to the anode side through the conduction of oxide ions through the solid electrolyte layer is referred to as “oxygen pumping action”.

  Due to the conduction of oxide ions accompanying such an oxygen pumping action, an interelectrode current flows between the first electrode 41 and the second electrode 42 constituting the sensor cell 51. The interelectrode current increases as the applied voltage applied between the first electrode 41 and the second electrode 42 increases. This is because the higher the applied voltage, the greater the amount of oxide ion conduction.

  However, when the applied voltage is gradually increased to a certain value or more, the interelectrode current is maintained at a certain value without increasing further. Such a characteristic is referred to as an oxygen limiting current characteristic, and a voltage region in which the oxygen limiting current characteristic occurs is referred to as an oxygen limiting current region. In such a limiting current characteristic of oxygen, the conduction velocity of oxide ions capable of conducting in the first solid electrolyte layer 11 is introduced into the measured gas chamber 30 through the diffusion-controlling layer 16 with the application of voltage. This is caused by exceeding the oxygen introduction rate. That is, it occurs when the oxygen reductive decomposition reaction at the cathode is in a diffusion-controlled state.

  Therefore, the interelectrode current (limit current) when a voltage in the limit current region of oxygen is applied in the sensor cell 51 corresponds to the oxygen concentration in the measured gas. Thus, by utilizing the limiting current characteristic of oxygen, it is possible to detect the oxygen concentration in the gas to be measured and detect the air-fuel ratio of the exhaust gas based on the detected concentration.

By the way, the oxygen pumping action as described above is not an action that appears only in oxygen contained in the gas to be measured. Among gases containing oxygen atoms in the molecule, there are gases that can exhibit an oxygen pumping action. Such gases include SOx and water (H ₂ O). Therefore, by applying a voltage equal to or higher than the SOx and water decomposition start voltage to the sensor cell 51, the water and SOx contained in the measured gas are decomposed. Oxide ions generated by the decomposition of SOx and water are conducted from the first electrode 41 to the second electrode 42 by the oxygen pumping action. For this reason, an interelectrode current flows between the first electrode 41 and the second electrode 42.

  However, the SOx concentration in the exhaust gas is extremely low, and the interelectrode current generated due to the decomposition of SOx is also extremely small. In particular, the exhaust gas contains a large amount of water, and an interelectrode current is generated due to the decomposition of the water. For this reason, it is difficult to accurately detect and detect the interelectrode current caused by the decomposition of SOx.

  In contrast, the inventors of the present application have found that the decomposition current when water and SOx are decomposed in an electrochemical cell having an oxygen pumping action changes according to the SOx concentration in the exhaust gas. The specific principle that causes such a phenomenon is not necessarily clear, but is considered to be caused by the following mechanism.

  As described above, when a predetermined voltage equal to or higher than the SOx decomposition start voltage is applied to the sensor cell 51, the SOx contained in the measured gas is decomposed. As a result, SOx decomposition products (for example, sulfur and sulfur compounds) are adsorbed on the first electrode 41 as the cathode. As a result, the area of the first electrode 41 that can contribute to the decomposition of water is reduced. When the SOx concentration in the measured gas is high, the decomposition products adsorbed on the first electrode 41 increase. As a result, when a predetermined voltage equal to or higher than the water decomposition start voltage is applied to the sensor cell 51, the water decomposition current flowing between the electrodes becomes relatively small. On the other hand, if the SOx concentration in the measured gas is low, the decomposition products adsorbed on the first electrode 41 are reduced. As a result, when a predetermined voltage equal to or higher than the water decomposition start voltage is applied to the sensor cell 51, the water decomposition current flowing between the electrodes becomes relatively large. Therefore, the decomposition current of the water flowing between the electrodes changes according to the SOx concentration in the measured gas. Using this phenomenon, the SOx concentration in the measured gas can be detected.

  Here, the water decomposition starting voltage is about 0.6 V, although some fluctuations are observed depending on the measurement conditions and the like. The decomposition start voltage of SOx is approximately the same as or slightly lower than the decomposition start voltage of water. Therefore, in the present embodiment, a voltage of 0.6 V or more is applied to the sensor cell 51 in order to detect the SOx concentration in the measured gas by the sensor cell 51 by the method described above. Further, when the applied voltage is excessively high, the first solid electrolyte layer 11 may be decomposed. In this case, it becomes difficult to accurately detect the SOx concentration in the measured gas based on the interelectrode current. For this reason, in this embodiment, a voltage of 0.6 V or more and less than 2.0 V is applied to the sensor cell 51 in order to detect the SOx concentration contained in the gas to be measured by the method described above.

Hereinafter, the relationship between the applied voltage and the interelectrode current will be specifically described. FIG. 3 is a schematic graph showing the relationship between the sensor cell voltage and the sensor cell current when the applied voltage is gradually increased (stepped-up sweep) in the sensor cell 51. In the illustrated example, four types of measured gases (0 ppm, 100 ppm, 300 ppm, and 500 ppm) having different concentrations of SO ₂ (ie, SOx) contained in the measured gas were used. Note that the oxygen concentration in the measured gas reaching the first electrode (cathode) 41 of the sensor cell 51 is constant (almost 0 ppm) in any measured gas by the pump cell 52 arranged on the upstream side of the sensor cell 51. Maintained.

A solid line L1 in FIG. 3 indicates the relationship between the sensor cell voltage and the sensor cell current when the SO ₂ concentration in the measured gas is 0 ppm. In the example shown in FIG. 3, when the sensor cell voltage becomes about 0.6 V or more, the sensor cell current starts to increase as the applied voltage increases. This increase in sensor cell current is due to the start of water decomposition at the first electrode 41.

A broken line L2 in FIG. 3 indicates the relationship between the sensor cell voltage and the sensor cell current when the SO ₂ concentration in the measured gas is 100 ppm. Also in this case, when the sensor cell voltage is about 0.6 V or more, a sensor cell current flows due to decomposition of water. However, the sensor cell current at this time (broken line L2) is smaller than that of the solid line L1, and the rate of increase of the sensor cell current with respect to the sensor cell voltage (slope of the broken line L2) is also smaller than that of the solid line L1.

An alternate long and short dash line L3 in FIG. 3 indicates the relationship between the sensor cell voltage and the sensor cell current when the SO ₂ concentration in the measured gas is 300 ppm. Also, a two-dot chain line L4 in FIG. 3 shows the relationship between the sensor cell voltage and the sensor cell current when the SO ₂ concentration in the measured gas is 500 ppm. Even in these cases, when the sensor cell voltage is about 0.6 V or more, the sensor cell current flows due to the decomposition of water. However, the higher the SO ₂ concentration in the gas to be measured, the smaller the sensor cell current, and the smaller the increase rate of the sensor cell current with respect to the sensor cell voltage (the slopes of the one-dot chain line L3 and the two-dot chain line L4).

Thus, also from the example shown in FIG. 3, the magnitude of the sensor cell current when the sensor cell voltage is about 0.6 V or higher, which is the decomposition start voltage of SOx and water, is the SO ₂ contained in the measured gas. It turns out that it changes according to the density | concentration (namely, SOx). For example, when the magnitude of the sensor cell current in the lines L1 to L4 when the sensor cell voltage in the graph shown in FIG. 3 is 1.0 V is plotted against the SO ₂ concentration in the measured gas, the graph shown in FIG. Is obtained.

As can be seen from FIG. 4, the magnitude of the sensor cell current when a predetermined voltage (1.0 V in the example shown in FIG. 4) is applied is the SO ₂ (ie, SOx) contained in the measured gas. Varies with concentration. Therefore, as described above, the SOx concentration can be detected based on the interelectrode current when a predetermined voltage equal to or higher than the decomposition start voltage of water and SOx is applied.

  However, the inter-electrode current detected when a voltage equal to or higher than the decomposition start voltage of water and SOx is applied to the sensor cell 51 in accordance with not only the SOx concentration in the measured gas but also the water concentration in the measured gas. To do. Specifically, when the water concentration in the gas to be measured increases, the amount of water decomposed on the first electrode (cathode) 41 increases, so that the interelectrode current increases. On the other hand, when the water concentration in the gas to be measured is reduced, the amount of water decomposed on the first electrode 41 is reduced, so that the interelectrode current is reduced. Therefore, in the present embodiment, in order to detect the SOx concentration in the measured gas with high accuracy, the water concentration in the measured gas is calculated, and the calculated water concentration and the sensor cell 51 start to decompose water and SOx. The SOx concentration in the measured gas is calculated based on the interelectrode current detected when a voltage equal to or higher than the voltage is applied.

<Problem of water concentration detection>
The water concentration in the measured gas can be calculated, for example, based on the interelectrode current detected when a voltage in the limit current region of water is applied to the sensor cell 51 or the pump cell 52. The limit current region of water is a region of the lower limit voltage (for example, 1.0 V) or more at which the water decomposition current hardly changes even when the applied voltage is further increased. However, when calculating the water concentration in the gas to be measured using the sensor cell 51 or the pump cell 52 in the SOx detection device 1 including the sensor cell 51 and the pump cell 52, the following problems occur.

  As described above, according to the oxygen pumping action, when a voltage is applied to both sides of the solid electrolyte layer of an electrochemical cell such as the sensor cell 51 and the pump cell 52, oxide ions are conducted from the cathode side to the anode side. However, if a voltage is applied to both sides of the solid electrolyte layer of the electrochemical cell, the measured gas chamber on the cathode side has a rich air-fuel ratio and the concentration of oxygen contained in the measured gas is extremely low. Rather, oxide ions move from the anode side to the cathode side.

  The reason why such oxide ion migration occurs will be briefly described. When a difference in oxygen concentration occurs between both side surfaces of the solid electrolyte layer, an electromotive force is generated to move the oxide ions from the high concentration side surface to the low concentration side surface (oxygen battery action). On the other hand, when a voltage is applied between the electrodes on both sides of the solid electrolyte layer as described above, the oxide ion is set so that the potential difference between these electrodes becomes equal to the applied voltage even if the above-described electromotive force is generated. Movement occurs.

  As a result, when a voltage is applied between the electrodes arranged on both sides of the solid electrolyte layer of the electrochemical cell, the migration of oxide ions so that the oxygen concentration difference on both electrodes becomes a concentration difference corresponding to the potential difference between both electrodes. Is done. Specifically, the oxide ions are moved so that the oxygen concentration on the anode is higher than the oxygen concentration on the cathode by a concentration difference corresponding to the potential difference.

  Therefore, when a certain voltage is applied between both electrodes of the electrochemical cell, the oxygen concentration difference is different when the oxygen concentration on the anode is not as high as the concentration difference corresponding to this certain voltage compared to the oxygen concentration on the cathode. The oxide ions are moved from the cathode to the anode so as to increase. As a result, the oxygen concentration on the cathode decreases, and the oxygen concentration difference on both electrodes approaches the concentration difference corresponding to the certain voltage. Conversely, when the oxygen concentration on the anode is higher than the concentration difference corresponding to this certain voltage compared to the oxygen concentration on the cathode, the movement of oxide ions from the anode toward the cathode is reduced so that the oxygen concentration difference becomes smaller. Is done. As a result, the oxygen concentration on the cathode increases, and the oxygen concentration difference on both electrodes approaches the concentration difference corresponding to the certain voltage.

  Therefore, when a low voltage is applied to the electrochemical cell, oxide ions may move from the anode toward the cathode. When a voltage within the limit current region of water is applied to the pump cell 52, hydrogen is generated at the third electrode 43 by the decomposition of water. At this time, if oxide ions move from the second electrode (anode) 42 of the sensor cell 51 toward the first electrode (cathode) 41, the third electrode 43 to the first electrode 41 in the measured gas chamber 30. In some cases, hydrogen that has moved to oxidizes by oxide ions to become water again. In this case, the water concentration in the exhaust gas reaching the third electrode 43 of the pump cell 52 is higher than the actual, and the water concentration in the measured gas calculated based on the interelectrode current flowing in the pump cell 52 is higher than the actual. There is a risk of high calculation.

  Similarly, when a voltage within the limit current region of water is applied to the sensor cell 51, hydrogen is generated at the first electrode 41 due to the decomposition of water. At this time, if oxide ions move from the fourth electrode (anode) 44 of the pump cell 52 toward the third electrode (cathode) 43, the first electrode 41 to the third electrode 43 in the measured gas chamber 30. In some cases, hydrogen that has moved to oxidizes by oxide ions to become water again. In this case, the water concentration in the exhaust gas reaching the first electrode 41 of the sensor cell 51 becomes higher than the actual, and the water concentration in the measured gas calculated based on the interelectrode current flowing in the sensor cell 51 is higher than the actual. There is a risk of high calculation.

  In the present embodiment, as described above, the water concentration in the measured gas is calculated, and the electrode detected when the calculated water concentration and a voltage equal to or higher than the decomposition start voltage of water and SOx are applied to the sensor cell 51. The SOx concentration in the measured gas is calculated based on the current between the two. For this reason, if the detection accuracy of the water concentration in the measured gas decreases, the detection accuracy of the SOx concentration in the measured gas also decreases.

<Control for detecting water concentration>
Therefore, in the present embodiment, the water concentration in the measured gas is detected as follows. The water concentration in the measured gas is calculated using the pump cell 52 or the sensor cell 51. First, a method for detecting the water concentration in the gas to be measured using the pump cell 52 will be described. The ECU 80 performs first water concentration detection control that opens the first voltage application circuit 61 and applies a voltage within the limit current region of water to the pump cell 52. The ECU 80 calculates the water concentration in the measured gas based on the current detected by the second current detector 72 when the first water concentration detection control is executed. The ECU 80 calculates the water concentration higher when the detected current is relatively large than when the detected current is relatively small.

  In the method described above, the first voltage application circuit 61 for applying a voltage to the sensor cell 51 is opened while the voltage in the limit current region of water is applied to the pump cell 52. In this case, oxide ions do not move between the first electrode 41 and the second electrode 42 of the sensor cell 51. For this reason, even if hydrogen generated on the third electrode 43 of the pump cell 52 reaches the first electrode 41 of the sensor cell 51, water is not generated because hydrogen is not oxidized. Therefore, the water concentration in the gas to be measured can be accurately calculated based on the interelectrode current flowing in the pump cell 52. As a result, the detection accuracy of the SOx concentration in the measured gas (exhaust gas) can be improved.

  In the first water concentration detection control, a voltage at which oxide ions do not move from the second electrode 42 to the first electrode 41 is applied to the sensor cell 51 and a voltage in the limit current region of water is applied to the pump cell 52. Good. In this case, oxide ions do not move from the second electrode 42 of the sensor cell 51 to the first electrode 41. For this reason, even if hydrogen generated on the third electrode 43 of the pump cell 52 reaches the first electrode 41 of the sensor cell 51, water is not generated because hydrogen is not oxidized. Therefore, the water concentration in the gas to be measured can be accurately calculated based on the interelectrode current flowing in the pump cell 52. The voltage at which oxide ions do not move from the second electrode 42 to the first electrode 41 is, for example, a voltage of 1.0 V or more.

  Next, a method for detecting the water concentration in the measured gas using the sensor cell 51 will be described. The ECU 80 executes second water concentration detection control that applies a voltage within the limit current region of water to the sensor cell 51 and opens the second voltage application circuit 71. The limit current region of water is a region of the lower limit voltage (for example, 1.0 V) or more at which the interelectrode current hardly changes even when the applied voltage is further increased. The ECU 80 calculates the water concentration in the measured gas based on the current detected by the first current detector 62 when the second water concentration detection control is executed. The ECU 80 calculates the water concentration higher when the detected current is relatively large than when the detected current is relatively small.

  In the method described above, the second voltage application circuit 71 for applying a voltage to the pump cell 52 is opened while the voltage in the limit current region of water is being applied to the sensor cell 51. In this case, oxide ions do not move between the third electrode 43 and the fourth electrode 44 of the pump cell 52. For this reason, even if hydrogen generated on the first electrode 41 of the sensor cell 51 reaches the third electrode 43 of the pump cell 52, water is not generated because hydrogen is not oxidized. Therefore, the water concentration in the measured gas can be accurately calculated based on the interelectrode current flowing in the sensor cell 51. As a result, the detection accuracy of the SOx concentration in the measured gas (exhaust gas) can be improved.

  In addition, when a voltage of 1.0 V or more is applied to the pump cell 52 so that oxide ions do not move from the fourth electrode 44 to the third electrode 43, a voltage within the limit current region of water is applied to the pump cell 52. become. For this reason, almost all the water in the measured gas is decomposed in the pump cell 52. As a result, since water is not decomposed in the sensor cell 51 located on the downstream side of the pump cell 52, the water concentration in the gas to be measured cannot be detected using the sensor cell 51. Therefore, when the sensor cell 51 is used to detect the water concentration in the gas to be measured, it is necessary to open the second voltage application circuit 71 while the voltage in the limit current region of water is being applied to the sensor cell 51.

  FIG. 5 is a diagram showing the relationship between the applied voltage of the pump cell 52 and the interelectrode current flowing in the pump cell 52. In the example of FIG. 5, the oxygen concentration and the water concentration in the measured gas are each 10%. As can be seen from FIG. 5, the hydrogen decomposition start voltage is higher than the oxygen decomposition start voltage. For this reason, the interelectrode current detected when the voltage in the limiting current region of water is applied to the pump cell 52 changes not only according to the water concentration in the measured gas but also according to the oxygen concentration in the measured gas. . Specifically, when the oxygen concentration in the gas to be measured increases, the amount of oxygen decomposed on the third electrode (cathode) 43 increases, so that the interelectrode current increases. On the other hand, when the oxygen concentration in the measured gas decreases, the amount of oxygen decomposed on the third electrode 43 decreases, so that the interelectrode current decreases. Therefore, in this embodiment, in order to accurately detect the water concentration in the measured gas, the oxygen concentration in the measured gas is calculated, and the calculated oxygen concentration and the voltage in the limit current region of the water are pump cells. Based on the interelectrode current detected when applied to 52, the water concentration in the measured gas is calculated.

  The oxygen concentration in the measured gas can be detected using, for example, the pump cell 52. In this case, the ECU 80 applies a voltage lower than the decomposition start voltage of water to the sensor cell 51 and applies a voltage equal to or higher than the lower limit voltage in the oxygen limit current region and lower than the decomposition start voltage of water and SOx to the pump cell 52. One oxygen concentration detection control is executed. The lower limit voltage in the limiting current region of oxygen is about 0.1V, and the decomposition start voltage of water and SOx is about 0.6V. Therefore, the ECU 80 applies, for example, a voltage of 0.4 V to the sensor cell 51 and the pump cell 52 in the first oxygen concentration detection control. The ECU 80 calculates the oxygen concentration in the measured gas based on the current detected by the second current detector 72 when the first oxygen concentration detection control is executed. The ECU 80 calculates the oxygen concentration higher when the detected current is relatively large than when the detected current is relatively small.

  As described above, when hydrogen is generated by the decomposition of water at the first electrode 41 of the sensor cell 51, the generated hydrogen may be oxidized by oxide ions on the third electrode 43 of the pump cell 52 to become water again. In this case, the current between the electrodes of the pump cell 52 detected by the second current detector 72 may fluctuate, and the detection accuracy of the oxygen concentration in the measured gas may be deteriorated. On the other hand, in the first oxygen concentration detection control described above, a voltage lower than the water decomposition start voltage is applied to the sensor cell 51. In this case, since water is not decomposed in the sensor cell 51, hydrogen is not generated in the first electrode 41 of the sensor cell 51. Therefore, the oxygen concentration in the gas to be measured can be accurately calculated based on the interelectrode current flowing in the pump cell 52. In the first oxygen concentration detection control, the first voltage application circuit 61 that applies a voltage to the sensor cell 51 may be opened.

  Further, the oxygen concentration in the measured gas may be detected using the sensor cell 51. In this case, the ECU 80 applies a voltage that is equal to or higher than the lower limit voltage of the oxygen limit current region and lower than the decomposition start voltage of water and SOx to the sensor cell 51 and opens the second voltage application circuit 71. Execute. The ECU 80 applies, for example, a voltage of 0.4 V to the sensor cell 51 in the second oxygen concentration detection control. The ECU 80 calculates the oxygen concentration in the measured gas based on the current detected by the first current detector 62 when the second oxygen concentration detection control is executed. The ECU 80 calculates the oxygen concentration higher when the detected current is relatively large than when the detected current is relatively small. Note that the oxygen concentration in the measured gas may be detected by an air-fuel ratio sensor disposed in the exhaust passage of the internal combustion engine without using the sensor cell 51 and the pump cell 52.

  When the fuel supplied to the combustion chamber of the internal combustion engine is feedback controlled so that the exhaust air / fuel ratio becomes the target air / fuel ratio using an air / fuel ratio sensor disposed in the exhaust passage of the internal combustion engine, The oxygen concentration in the gas becomes a substantially constant value. In this case, the water decomposition current can be extracted by subtracting a current value corresponding to a predetermined oxygen concentration from the interelectrode current detected when a voltage in the limit current region of water is applied to the pump cell 52. it can. For this reason, it is not always necessary to calculate the oxygen concentration in the measured gas in order to calculate the water concentration in the measured gas.

<SOx concentration calculation processing>
Hereinafter, the control for detecting the SOx concentration will be described in detail with reference to the flowchart of FIG. FIG. 6 is a flowchart showing a control routine of the SOx concentration calculation process in the present invention. This control routine is repeatedly executed by the ECU 80. In addition, this control routine is forcibly terminated when it is determined by the detection condition determination process described later that the SOx concentration detection condition is not satisfied.

  First, in step S101, it is determined whether there is a request for detecting the SOx concentration in the gas to be measured. The SOx concentration detection request is generated, for example, when fuel is supplied to the fuel tank of the internal combustion engine, and disappears when the SOx concentration is calculated once or a plurality of times. The SOx concentration detection request may be generated when the ignition key is turned on, and may be extinguished when the SOx concentration is calculated once or a plurality of times. If it is determined in step S101 that there is no request for detecting the SOx concentration in the gas to be measured, this control routine ends. On the other hand, if it is determined in step S101 that there is a request for detecting the SOx concentration in the measured gas, the present control routine proceeds to step S102.

  In step S102, oxygen concentration detection control is executed. Specifically, a voltage lower than the decomposition start voltage of water is applied to the sensor cell 51, and a voltage that is equal to or higher than the lower limit voltage of the oxygen limit current region and lower than the decomposition start voltage of water and SOx is applied to the pump cell 52. Oxygen concentration detection control is executed. In the first oxygen concentration detection control, for example, a voltage of 0.4 V is applied to the sensor cell 51 and the pump cell 52.

  Next, in step S103, the oxygen concentration in the measured gas is calculated based on the interelectrode current of the pump cell 52 detected by the second current detector 72 when the first oxygen concentration detection control is executed. For example, the oxygen concentration is calculated using a map showing the relationship between the detected current and the oxygen concentration. In this map, the oxygen concentration is shown as increasing as the current increases.

  Next, in step S104, water concentration detection control is executed. Specifically, first water concentration detection control is performed in which the first voltage application circuit 61 is opened and a voltage in the limit current region of water is applied to the pump cell 52. In the first water concentration detection control, for example, a voltage of 1.0 V or more is applied to the pump cell 52. In the first water concentration detection control, a voltage at which oxide ions do not move from the second electrode 42 to the first electrode 41 is applied to the sensor cell 51 and a voltage in the limit current region of water is applied to the pump cell 52. Good. In this case, the applied voltage to the sensor cell 51 is, for example, 1.0 V or more.

  Next, in step S105, the interelectrode current of the pump cell 52 detected by the second current detector 72 when the first water concentration detection control is executed, and the oxygen concentration in the measured gas calculated in step S103. Based on this, the water concentration in the measured gas is calculated. Here, first, the detected current is corrected based on the oxygen concentration. Specifically, the value obtained by subtracting the current corresponding to the oxygen concentration in the measured gas from the detected current is the corrected current value. The corrected current value corresponds to the water decomposition current. Next, the water concentration in the measured gas is calculated based on the corrected current value. For example, the water concentration is calculated using a map showing the relationship between the corrected current value and the water concentration. In this map, the water concentration is shown to increase as the corrected current value increases.

  Next, in step S106, a voltage equal to or higher than the decomposition start voltage of water and SOx is applied to the sensor cell 51, and a voltage equal to or higher than the lower limit voltage in the oxygen limit current region and lower than the decomposition start voltage of water and SOx is applied to the pump cell 52. The SOx concentration detection control is executed. In the SOx concentration detection control, for example, a voltage of 1.1 V is applied to the sensor cell 51 and a voltage of 0.4 V is applied to the pump cell 52.

  Next, in step S107, based on the interelectrode current of the sensor cell 51 detected by the first current detector 62 when the SOx concentration detection control is executed, and the water concentration in the measured gas calculated in step S105. Thus, the SOx concentration in the measured gas is calculated. For example, the SOx concentration is calculated using a map showing the relationship between the detected interelectrode current, water concentration, and SOx concentration. After step S107, this control routine ends.

  In the oxygen concentration detection control in step S102, a voltage that is equal to or higher than the lower limit voltage of the oxygen limit current region and lower than the decomposition start voltage of water and SOx is applied to the sensor cell 51 and the second voltage application circuit 71 is opened. Dioxygen concentration detection control may be executed. In this case, in step S103, the oxygen concentration in the measured gas is calculated based on the interelectrode current of the sensor cell 51 detected by the first current detector 62 when the second oxygen concentration detection control is executed.

  Further, the oxygen concentration in the measured gas may be detected by an air-fuel ratio sensor arranged in the exhaust passage of the internal combustion engine. In this case, step S102 is omitted, and the oxygen concentration in the measured gas is calculated based on the output of the air-fuel ratio sensor in step S103. If the fuel supplied to the combustion chamber of the internal combustion engine is feedback controlled so that the exhaust air / fuel ratio becomes the target air / fuel ratio using the air / fuel ratio sensor disposed in the exhaust passage of the internal combustion engine, S102 and step S103 may be omitted.

  Further, in the water concentration detection control in step S104, second water concentration detection control may be executed in which a voltage within the limit current region of water is applied to the sensor cell 51 and the second voltage application circuit 71 is opened. In this case, in step S105, the water concentration in the measured gas is calculated based on the interelectrode current of the sensor cell 51 detected by the first current detector 62 when the second water concentration detection control is executed.

  In the present embodiment, the interelectrode current is used as a current correlation parameter that correlates with the interelectrode current in order to calculate the oxygen concentration, water concentration, or SOx concentration in the measured gas. However, parameters other than the interelectrode current may be used as the current correlation parameter. The current correlation parameter is, for example, the resistance value of the sensor cell 51 or the pump cell 52. The current correlation parameter is detected by an arbitrary detector provided in the SOx detection device 1 and acquired by the ECU 80.

<Detection condition determination processing>
FIG. 7 is a flowchart showing a control routine of detection condition determination processing in the present invention. This control routine is repeatedly executed by the ECU 80 at predetermined time intervals. In this control routine, it is determined whether or not the SOx concentration detection condition is satisfied. If it is determined that the SOx concentration detection condition is not satisfied, the control routine of the SOx concentration calculation process described above is forcibly executed. To finish.

  First, in step S201, it is determined whether or not the sensor cell 51 and the pump cell 52 are in an active state. For example, when the temperature of the sensor cell 51 and the pump cell 52 is equal to or higher than the activation temperature, it is determined that the sensor cell 51 and the pump cell 52 are in the active state. On the other hand, when the temperature of the sensor cell 51 or the pump cell 52 is lower than the activation temperature, it is determined that the sensor cell 51 or the pump cell 52 is not in the active state. The temperature of the sensor cell 51 and the pump cell 52 can be calculated based on the impedance of the sensor cell 51 and the pump cell 52.

  When it is determined in step S201 that the sensor cell 51 or the pump cell 52 is not in the active state, the present control routine proceeds to step S203. In this case, the oxygen concentration, water concentration, and SOx concentration in the gas to be measured cannot be accurately calculated using the sensor cell 51 and the pump cell 52. For this reason, in step S203, the control routine of the SOx concentration calculation process is forcibly terminated. After step S203, this control routine ends. On the other hand, when it is determined in step S201 that the sensor cell 51 and the pump cell 52 are in the active state, the present control routine proceeds to step S202.

  In step S202, it is determined whether or not the air-fuel ratio (exhaust air-fuel ratio) of the exhaust gas discharged from the combustion chamber of the internal combustion engine is stable. The “air-fuel ratio of exhaust gas (exhaust air-fuel ratio)” means the ratio of the mass of fuel to the mass of air supplied until the exhaust gas is generated.

  The determination in step S202 is performed, for example, by determining whether or not the amount of change in the exhaust air / fuel ratio is equal to or less than a threshold value. When the change amount of the exhaust air-fuel ratio is equal to or less than the threshold value, it is determined that the exhaust air-fuel ratio is stable. On the other hand, when the amount of change in the exhaust air / fuel ratio is larger than the threshold value, it is determined that the exhaust air / fuel ratio is not stable. The amount of change in the exhaust air / fuel ratio is calculated, for example, by the difference between the maximum value and the minimum value of the exhaust air / fuel ratio at a predetermined time. The exhaust air / fuel ratio is detected by, for example, an air / fuel ratio sensor disposed in the exhaust passage of the internal combustion engine. The threshold value is determined in advance by experiment or calculation, and is a value corresponding to the upper limit value of the change amount of the exhaust air / fuel ratio that can obtain the desired calculation accuracy of the oxygen concentration, water concentration, and SOx concentration in the measured gas. It is said.

  If it is determined in step S202 that the exhaust air-fuel ratio is not stable, the present control routine proceeds to step S203. In this case, since the oxygen concentration, water concentration, and SOx concentration in the measured gas are fluctuating, the oxygen concentration, water concentration, and SOx concentration in the measured gas cannot be accurately calculated. For this reason, in step S203, the control routine of the SOx concentration calculation process is forcibly terminated. After step S203, this control routine ends. On the other hand, when it is determined in step S202 that the exhaust air-fuel ratio is stable, this control routine ends.

  The preferred embodiments according to the present invention have been described above, but the present invention is not limited to these embodiments, and various modifications and changes can be made within the scope of the claims.

DESCRIPTION OF SYMBOLS 1 SOx detection apparatus 10, 10 'Element part 11 1st solid electrolyte layer 12 2nd solid electrolyte layer 16 Diffusion-controlled layer 41 1st electrode 42 2nd electrode 43 3rd electrode 44 4th electrode 51 Sensor cell 52 Pump cell 61 1st voltage Application circuit 62 First current detector 71 Second voltage application circuit 72 Second current detector 80 Electronic control unit (ECU)

Claims (1)

A sulfur oxide detector for detecting a sulfur oxide concentration in a measured gas,
A first solid electrolyte layer having oxide ion conductivity; a first electrode disposed on one side surface of the first solid electrolyte layer so as to be exposed to the gas to be measured; and being exposed to the atmosphere. A sensor cell having a second electrode disposed on the other side surface of the first solid electrolyte layer, a second solid electrolyte layer having oxide ion conductivity, and the gas to be exposed to the measured gas. A pump cell having a third electrode disposed on one side of the second solid electrolyte layer and a fourth electrode disposed on the other side of the second solid electrolyte layer to be exposed to the atmosphere; An element unit comprising a diffusion rate-determining layer that performs diffusion rate-determination of the measured gas;
A first voltage application circuit for applying a voltage to the sensor cell;
A second voltage application circuit for applying a voltage to the pump cell;
A first detector for detecting a first current correlation parameter correlated with a current flowing through the sensor cell;
A second detector for detecting a second current correlation parameter correlated to the current flowing through the pump cell;
Controlling the voltage applied to the sensor cell by the first voltage application circuit and the voltage applied to the pump cell by the second voltage application circuit, obtaining the first current correlation parameter from the first detector, and A controller for obtaining the second current correlation parameter from a second detector,
The pump cell is disposed closer to the diffusion-controlling layer than the sensor cell,
The control unit applies a voltage at which oxide ions do not move from the second electrode to the first electrode to the sensor cell or opens the first voltage application circuit and applies a voltage in a limit current region of water to the pump cell. First water concentration detection control to be applied to the gas, and when the first water concentration detection control is executed, based on the second current correlation parameter detected by the second detector, The second water concentration detection control is performed by calculating a water concentration or by applying a voltage within a limit current region of water to the sensor cell and opening the second voltage application circuit. Calculating the water concentration in the measured gas based on the first current correlation parameter detected by the first detector when executed,
The control unit applies a voltage equal to or higher than the decomposition start voltage of water and sulfur oxide to the sensor cell, and sets a voltage equal to or higher than the lower limit voltage of the limiting current region of oxygen and lower than the decomposition start voltage of water and sulfur oxide. The sulfur oxide concentration detection control applied to the pump cell is executed, the first current correlation parameter detected by the first detector when the sulfur oxide concentration detection control is executed, and the calculated water concentration The sulfur oxide detector which calculates the sulfur oxide density | concentration in the said to-be-measured gas based on these.

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