JP2017138215A - Sulfur oxide detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sulfur oxide detection device capable of improving a detection accuracy of a SOx content in an exhaust gas.SOLUTION: A sulfur oxide detection device 1 includes: an element part 10; a voltage application part 61 for applying an inter-electrode voltage to an electrochemical cell 51; a current detection part 62 for detecting an inter-electrode voltage; and a control part 80 for controlling the inter-electrode voltage and acquiring an inter-electrode current. The control part executes a temporary step-down control for reducing the inter-electrode voltage from a first voltage to a second voltage, and then to be raised to the first voltage again while a water concentration of gas to be measured is stabilized after applying a first voltage between the electrodes for more than a predetermined time. When a difference between the inter-electrode current at the start of a temporary step-down control and the inter-electrode current detected at the completion of the temporary step-down control is large, the sulfur oxide concentration of the gas to be measured is determined to be large as compared to the case where the difference is small. The first voltage is the voltage having the voltage equal to or higher than a decomposition start voltage of water and the sulfur oxide. The second voltage is the voltage detachably adsorbing the decomposition product of the sulfur oxide adsorbed to the first electrode.SELECTED DRAWING: Figure 6

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 SOx 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-described problems, in the first invention, a solid electrolyte layer having oxide ion conductivity and a first electrolyte disposed on one side surface of the solid electrolyte layer so as to be exposed to a gas to be measured. An element comprising an electrochemical cell having an electrode and a second electrode disposed on the other side surface of the solid electrolyte layer so as to be exposed to the atmosphere, and a diffusion-controlling layer for controlling the diffusion of the measured gas A voltage applying unit that applies an inter-electrode voltage between the first electrode and the second electrode so that a potential of the second electrode is higher than a potential of the first electrode, and the first electrode A current detection unit for detecting an interelectrode current flowing between the first electrode and the second electrode, and a control for controlling the interelectrode voltage applied from the voltage application unit and acquiring the interelectrode current from the current detection unit And the control unit includes the first electrode and the After applying the first voltage between the two electrodes for a predetermined time or more, while the water concentration in the measured gas is stable, the inter-electrode voltage is decreased from the first voltage to the second voltage. Temporary step-down control for raising the voltage to the first voltage again is performed, and the inter-electrode current detected by the current detection unit at the start of the temporary step-down control and the current detection unit detected at the completion of the temporary step-down control. When the difference between the currents between the electrodes is relatively large, it is determined that the sulfur oxide concentration in the measured gas is higher than when the difference is relatively small. There is provided a sulfur oxide detection device, which is a voltage equal to or higher than a decomposition start voltage of sulfur oxide, and wherein the second voltage is a voltage capable of desorbing a decomposition product of sulfur oxide adsorbed on the first electrode. The

  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 block diagram showing the configuration of the SOx detection device according to the first embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing the configuration of the element portion of the SOx detection device according to the first embodiment of the present invention. FIG. 3 is a diagram showing the relationship between the applied voltage between the electrodes of the first electrochemical cell and the interelectrode current flowing between the electrodes of the first electrochemical 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 schematic time chart of the engine speed and the like when detecting the SOx concentration in the measured gas. FIG. 6 is a flowchart showing a control routine of the SOx concentration detection process in the first embodiment of the present invention. FIG. 7 is a flowchart showing a control routine of water concentration stability determination processing in the first embodiment of the present invention. FIG. 8A is a schematic cross-sectional view showing the configuration of the element portion of the SOx detection device according to the second embodiment of the present invention, and FIG. 8B is a cross-sectional view along line A-- in FIG. 1 is a cross-sectional view along A. FIG. FIG. 9 is a schematic cross-sectional view showing the configuration of the element portion of the SOx detection device according to the third embodiment of the present invention. FIG. 10A is a schematic cross-sectional view showing the configuration of the element portion of the SOx detection device according to the fourth embodiment of the present invention, and FIG. FIG.

  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.

<First embodiment>
First, a first embodiment of the present invention will be described with reference to FIGS.

<Description of SOx detection device>
Hereinafter, with reference to FIG.1 and FIG.2, the SOx detection apparatus 1 which concerns on 1st embodiment of this invention is demonstrated. The SOx detection device 1 detects the concentration of sulfur oxide (SOx) in the exhaust gas flowing through the exhaust passage.

  FIG. 1 is a schematic block diagram showing the configuration of the SOx detection device 1 according to the first embodiment of the present invention. As shown in FIG. 1, the SOx detection device 1 includes an element unit 10, a heater control circuit 60 connected to the element unit 10, a voltage application circuit 61 and a current detection circuit 62, and an ECU 80. The SOx detection device 1 further includes a DA converter 63 connected to the ECU 80 and the voltage application circuit 61, and an AD converter 64 connected to the ECU 80 and the current detection circuit 62.

  The element unit 10 is disposed in an exhaust passage (not shown) of the internal combustion engine. FIG. 2 is a schematic cross-sectional view showing the configuration of the element unit 10 of the SOx detection device 1. As shown in FIG. 2, 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 diffusion rate limiting layer 16, a first impermeable layer 21, a second impermeable layer 22, a third impermeable layer 23, a fourth impermeable layer 24, and A fifth impermeable layer 25 is provided.

The first solid electrolyte layer 11 is a thin plate having oxide ion conductivity. The first solid electrolyte layer 11 is, for example, sintered by adding CaO, MgO, Y ₂ O ₃ , Yb ₂ O ₃ or the like as a stabilizer to ZrO ₂ (zirconia), HfO ₂ , ThO ₂ , Bi ₂ O ₃ or the like. It is formed by the body. 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 25 are gas impermeable thin plates, and are formed as layers containing alumina, for example.

  Each of the layers of the element unit 10 includes, from below in FIG. 2, a first impermeable layer 21, a second impermeable layer 22, a third impermeable layer 23, a first solid electrolyte layer 11, a diffusion rate limiting layer 16, and a fourth impermeable layer. The layer 24 and the fifth impermeable layer 25 are laminated in this order. A gas chamber 30 to be measured is defined by the first solid electrolyte layer 11, the diffusion-controlling layer 16, the fourth impermeable layer 24, and the fifth impermeable layer 25. 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 chamber 30 is configured to be adjacent to the first solid electrolyte layer 11 and to flow the measured gas.

  The first atmospheric chamber 31 is partitioned by the first solid electrolyte layer 11, the second impermeable layer 22, and the third impermeable layer 23. As can be seen from FIG. 2, 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.

  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 first electrochemical 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 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 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 of the element unit 10 is connected to the heater control circuit 60. The ECU 80 inputs a heater control signal to the heater control circuit 60 and controls the heater 55 via the heater control circuit 60. Thus, the heater 55 can heat the first electrochemical cell 51 to an activation temperature or higher.

  The first electrode 41 and the second electrode 42 are connected to the voltage application circuit 61. The voltage application circuit 61 applies an interelectrode voltage between the first electrode 41 and the second electrode 42 such that the potential of the second electrode 42 is higher than the potential of the first electrode 41. The ECU 80 inputs a voltage control signal to the voltage application circuit 61 via the DA converter 63 and controls the interelectrode voltage applied from the voltage application circuit 61.

  The current detection circuit 62 detects an interelectrode current flowing between the first electrode 41 and the second electrode 42 (that is, an interelectrode current flowing in the first solid electrolyte layer 11). The output of the current detection circuit 62 is input to the ECU 80 via the AD converter 64. Therefore, the ECU 80 can acquire the interelectrode current detected by the current detection circuit 62 from the current detection circuit 62.

  The voltage application circuit 61 functions as a voltage application unit that applies an interelectrode voltage between the first electrode 41 and the second electrode 42. The current detection circuit 62 functions as a current detection unit that detects an interelectrode current flowing between the first electrode 41 and the second electrode 42. The ECU 80 functions as a control unit that controls the interelectrode voltage applied from the voltage application circuit 61 and acquires the interelectrode current from the current detection circuit 62.

<SOx concentration detection principle>
Next, the principle of SOx detection by the SOx detection device 1 will be described. In describing the SOx detection principle, first, the limiting current characteristic of oxygen in the element unit 10 will be described. In the element unit 10, 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, it 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 first electrochemical 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 first electrochemical 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. Such oxygen limit current characteristics are such that the conduction velocity of oxide ions that can conduct in the solid electrolyte layer 11 with the application of voltage is oxygen introduced into the measured gas chamber 30 through the diffusion rate controlling layer 16. This is caused by exceeding the introduction speed of. That is, it occurs when the oxygen reductive decomposition reaction at the cathode is in a diffusion-controlled state.

  Accordingly, the interelectrode current (limit current) when a voltage in the oxygen limit current region is applied in the first electrochemical 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 decomposition start voltage of SOx and water between the first electrode 41 and the second electrode 42 of the first electrochemical cell 51, the water and SOx contained in the measured gas are reduced. Disassembled. 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 between the first electrode 41 and the second electrode 42 of the first electrochemical cell 51, the SOx contained in the measured gas is decomposed. The 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 between the first electrode 41 and the second electrode 42, 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 between the first electrode 41 and the second electrode 42, 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 this embodiment, in order to detect the SOx concentration in the gas to be measured by the first electrochemical cell 51 by the method described above, a voltage of 0.6 V or more is applied between the first electrode 41 and the second electrode 42. Is applied. 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, in order to detect the SOx concentration contained in the gas to be measured by the method described above, a voltage of 0.6 V or more and less than 2.0 V between the first electrode 41 and the second electrode 42. Is applied.

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 applied voltage and the interelectrode current when the applied voltage is gradually increased (step-up sweep) in the first electrochemical 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 first electrochemical cell 51 is maintained constant (approximately 0 ppm) in any measured gas.

A solid line L1 in FIG. 3 indicates the relationship between the applied voltage and the interelectrode current when the SO ₂ concentration in the measured gas is 0 ppm. In the example shown in FIG. 3, since the oxygen concentration in the measured gas is maintained at approximately 0 ppm, the interelectrode current is approximately 0 in the region where the applied voltage is less than about 0.6V. On the other hand, when the applied voltage is about 0.6 V or more, the interelectrode current starts to increase as the applied voltage increases. This increase in inter-electrode current results from the start of water decomposition at the first electrode 41.

The broken line L2 in FIG. 3 shows the relationship between the applied voltage and the interelectrode current when the SO ₂ concentration in the measured gas is 100 ppm. Even in this case, in the region where the applied voltage is less than about 0.6 V, the inter-electrode current is almost zero as in the case of the solid line L1. On the other hand, when the applied voltage is about 0.6 V or more, an interelectrode current flows due to water decomposition. However, the interelectrode current at this time (broken line L2) is smaller than that of the solid line L1, and the increase rate of the interelectrode current with respect to the applied voltage (slope of the broken line L2) is also smaller than that of the solid line L1.

A dashed-dotted line L3 in FIG. 3 shows the relationship between the applied voltage and the interelectrode current when the SO ₂ concentration in the measured gas is 300 ppm. A two-dot chain line L4 in FIG. 3 indicates the relationship between the applied voltage and the interelectrode current when the SO ₂ concentration in the measured gas is 500 ppm. Also in these cases, in the region where the applied voltage is less than about 0.6 V, the inter-electrode current is almost zero as in the case of the solid line L1 and the broken line L2. On the other hand, when the applied voltage is about 0.6 V or more, an interelectrode current flows due to water decomposition. However, the higher the SO ₂ concentration in the measured gas, the smaller the interelectrode current, and the smaller the rate of increase of the interelectrode current with respect to the applied 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 interelectrode current when the applied voltage is about 0.6 V or more, which is the SOx and water decomposition start voltage, is the SO _It can be seen that it varies depending on the concentration of ₂ (ie, SOx). For example, when the magnitude of the interelectrode current in the lines L1 to L4 when the applied voltage is 1.0 V in the graph shown in FIG. 3 is plotted against the SO ₂ concentration in the measured gas, it is shown in FIG. A graph is obtained.

As can be seen from FIG. 4, the magnitude of the interelectrode current when a predetermined voltage (1.0 V in the example shown in FIG. 4) is applied is SO ₂ (ie, SOx) contained in the measured gas. Varies depending on the 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.

<Sulfur poisoning of electrodes>
By the way, as described above, when a voltage equal to or higher than the decomposition start voltage of water and SOx is applied to the first electrochemical cell 51, the decomposition product of SOx (for example, on the first electrode 41 of the first electrochemical cell 51 (for example, , Sulfur and sulfur compounds) adsorb. As a result, the first electrode 41 is sulfur poisoned. For this reason, if the SOx concentration is to be detected again in a state where the SOx decomposition product is adsorbed on the first electrode 41, the detected interelectrode current is affected by the decomposition product adsorbed on the first electrode 41. . Specifically, the detected interelectrode current may be smaller than a value corresponding to the SOx concentration contained in the measured gas. For this reason, in order to accurately detect the SOx concentration in the measured gas, it is necessary to detect the SOx concentration in a state where the decomposition products of SOx adsorbed on the first electrode 41 are removed.

  According to the oxygen pumping action described above, it has been described that when a voltage is applied to both sides of the solid electrolyte layer of the electrochemical cell, 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.

  Here, when the potential difference between the electrodes arranged on both sides of the solid electrolyte layer is 0.45 V, the concentration difference corresponding to this potential difference is in equilibrium with the oxygen concentration contained in the atmosphere (that is, excessive It becomes equal to the concentration difference from the oxygen concentration in the state where there is no unburned component (HC, CO, etc.) and excessive oxygen, or the air-fuel ratio is the stoichiometric air-fuel ratio. Therefore, when the anode of the electrochemical cell is exposed to the atmosphere and the cathode is exposed to the gas to be measured, when a voltage of 0.45 V is applied between these electrodes, the area around the cathode exposed to the gas chamber to be measured is The oxide ions are moved so as to be in an equilibrium state.

  Therefore, when the oxygen concentration of the measured gas is higher than the oxygen concentration in the equilibrium state, that is, when the air-fuel ratio of the measured gas is leaner than the stoichiometric air-fuel ratio (in a state where oxygen is excessive with respect to unburned components) In some cases, oxide ions are transferred from the cathode to the anode in the measured gas chamber. As a result, the oxygen concentration of the measured gas approaches the oxygen concentration in the equilibrium state. On the other hand, when the oxygen concentration of the measured gas is lower than the oxygen concentration in the equilibrium state, that is, when the air-fuel ratio of the measured gas is richer than the stoichiometric air-fuel ratio (the unburned component is in excess with respect to oxygen). ), Oxide ions are transferred from the anode to the cathode in the measured gas chamber. As a result, the oxygen concentration of the measured gas approaches the oxygen concentration in the equilibrium state.

  On the other hand, when the potential difference between the electrodes arranged on both sides of the solid electrolyte layer is less than 0.45 V, the concentration difference corresponding to this potential difference is the concentration between the oxygen concentration in the atmosphere and the oxygen concentration in the equilibrium state. Smaller than the difference. Therefore, when the anode of the electrochemical cell is exposed to the atmosphere and the cathode is exposed to the gas to be measured, if a voltage of less than 0.45 V is applied between these electrodes, the cathode exposed to the gas to be measured is exposed. The oxide ions are moved so that there is a slight oxygen excess from the equilibrium state. As a result, the cathode of the electrochemical cell is maintained in a slightly oxygen excess state compared to the equilibrium state.

  Therefore, when a voltage lower than 0.45 V is applied between the first electrode 41 and the second electrode 42 of the first electrochemical cell 51 with the first electrode 41 as a cathode and the second electrode as an anode, 41 is maintained in a slightly oxygen excess state. Even when SOx decomposition products are adsorbed on the first electrode 41, if the first electrode 41 is maintained in an oxygen-excess state, the adsorbed decomposition products react with oxygen, and SOx and And desorbed from the first electrode 41. Therefore, when SOx decomposition products are adsorbed on the first electrode 41 in accordance with the SOx detection process, the generated voltage is reduced by setting the voltage applied to the first electrochemical cell 51 to less than 0.45V. An object can be detached from the first electrode 41.

  If the voltage applied to the first electrochemical cell 51 is less than 0.45 V, the desorption of SOx decomposition products can be promoted. However, if the applied voltage is excessively reduced, blackening may occur in the first solid electrolyte layer 11. Here, blackening is a phenomenon in which metal oxide is reduced by reducing the metal oxide contained in the solid electrolyte layer in the solid electrolyte layer. When blackening occurs in the first solid electrolyte layer 11, the ionic conductivity of the first solid electrolyte layer 11 deteriorates, and it becomes impossible to appropriately detect the SOx concentration. For this reason, it is preferable that the applied voltage to the first electrochemical cell 51 is higher than the applied voltage (for example, −2.0 V) at which blackening occurs.

<Problems of SOx concentration detection>
Further, even when a voltage equal to or higher than the SOx and water decomposition start voltage is applied between the electrodes in order to detect the SOx concentration in the measured gas, the water concentration in the measured gas is not stable. The interelectrode current varies. Specifically, when the water concentration in the measured gas increases, the amount of water decomposed on the first electrode 41 increases, and thus the interelectrode current increases. On the other hand, when the water concentration in the gas to be measured becomes low, the amount of water decomposed on the electrode (cathode) decreases, so that the interelectrode current becomes small. Therefore, when the water concentration in the measured gas is not stable, the current between the electrodes fluctuates, and the detection accuracy of the SOx concentration in the measured gas decreases.

  Therefore, in order to ensure the detection accuracy of the SOx concentration, a voltage higher than the decomposition start voltage of water and SOx is applied between the electrodes while the water concentration in the measured gas is stable, and the interelectrode current is detected. It is possible. However, it takes a certain amount of time (for example, about 60 seconds) until the interelectrode current shows a value corresponding to the SOx concentration after applying a voltage higher than the decomposition start voltage of water and SOx between the electrodes. For this reason, the water concentration in the measured gas may fluctuate due to a change in the operating state of the internal combustion engine while the voltage is applied, and it is difficult to ensure the detection accuracy of the SOx concentration.

<Control for detecting SOx concentration>
Therefore, in the present embodiment, the SOx concentration in the gas to be measured is detected as follows. In the present embodiment, in order to detect the SOx concentration in the measured gas, the ECU 80 applies a first voltage between the first electrode 41 and the second electrode 42 for a predetermined time or longer, and then in the measured gas. While the water concentration is stable, temporary pressure reduction control is executed. The first voltage is a voltage equal to or higher than the decomposition start voltage of water and SOx, and is 1.1 V, for example. The predetermined time is a time required from when the first voltage is applied between the electrodes until the interelectrode current shows a value corresponding to the SOx concentration, and is, for example, 60 seconds. Further, in the temporary voltage step-down control, the ECU 80 reduces the interelectrode voltage of the first electrochemical cell 51 from the first voltage to the second voltage and increases it again to the first voltage. The second voltage is a voltage at which the decomposition product of SOx adsorbed on the first electrode 41 can be desorbed, and is, for example, 0.3V. Note that the first voltage does not necessarily have to be constant before the temporary step-down control.

  By performing the above-described temporary step-down control, at least a part of the decomposition product of SOx adsorbed on the first electrode 41 before the temporary step-down control is desorbed from the first electrode 41. Therefore, when the water concentration in the measured gas is stable, the interelectrode current detected by the current detection circuit 62 is increased by the temporary step-down control. The amount of change in the interelectrode current at this time increases as the amount of adsorption of the decomposition product before the temporary pressure reduction control increases. Therefore, the difference between the interelectrode current detected at the start of the temporary step-down control and the interelectrode current detected at the completion of the temporary step-down control increases as the SOx concentration in the measured gas increases. When the SOx concentration in the measured gas is very small, the difference between the currents between the electrodes is almost zero.

  For this reason, the ECU 80 measures based on the difference between the interelectrode current detected by the current detection circuit 62 at the start of the temporary step-down control and the interelectrode current detected by the current detection circuit 62 when the temporary step-down control is completed. The SOx concentration in the gas is detected. Specifically, the ECU 80 determines that the SOx concentration in the measured gas is higher when the difference between the currents between the electrodes is relatively larger than when the difference between the currents between the electrodes is relatively small. For example, the ECU 80 determines that the SOx concentration is greater than or equal to a predetermined value when the difference between the currents between the electrodes is equal to or greater than a predetermined value, and the SOx concentration is less than the predetermined value when the difference between the currents between the electrodes is less than the predetermined value. Judge that there is.

  In the present embodiment, the temporary pressure reduction control is executed while the water concentration in the measured gas is stable, and the applied voltage to the first electrochemical cell 51 is the same at the start and completion of the temporary pressure reduction control. . In this case, if the decomposition product of SOx is not adsorbed to the first electrode 41 before the temporary step-down control, the inter-electrode current accompanying the decomposition of water becomes substantially the same at the start and completion of the temporary step-down control. Therefore, by calculating the difference between the inter-electrode current at the start of the temporary step-down control and the inter-electrode current at the completion of the temporary step-down control, the water decomposition current that has been reduced by the decomposition of SOx before the temporary step-down control is calculated. The amount of change can be extracted, and the decrease in the detection accuracy of the SOx concentration due to the fluctuation of the water concentration in the measured gas can be suppressed. Therefore, according to the present embodiment, it is possible to improve the detection accuracy of the SOx concentration in the measured gas (exhaust gas).

  In the present embodiment, it is sufficient that the water concentration in the measured gas is stable at least during the temporary pressure reduction control. For this reason, before the temporary pressure reduction control, even if the operating state of the internal combustion engine changes and the water concentration in the measured gas is not stable, a voltage equal to or higher than the decomposition start voltage of water and SOx is first applied. It can be applied to the electrochemical cell 51 for a predetermined time (for example, about 60 seconds) or longer. Further, the time required for the temporary step-down control is, for example, several seconds, which is considerably shorter than the voltage application time before the temporary step-down control. For this reason, the timing for detecting the SOx concentration can be easily found even in a situation where the operating state of the internal combustion engine frequently changes. Therefore, according to the present embodiment, when there is a request for detecting the SOx concentration, the SOx concentration can be detected quickly while ensuring the detection accuracy of the SOx concentration.

<Description of control using time chart>
The control for detecting the SOx concentration will be specifically described below with reference to the time chart of FIG. FIG. 5 is a schematic time chart of the engine speed, the applied voltage to the first electrochemical cell 51, and the interelectrode current of the first electrochemical cell 51 when detecting the SOx concentration in the measured gas. is there.

  In the illustrated example, a voltage of 0.3 V is applied to the first electrochemical cell 51 before time t1 in order to suppress the SOx decomposition product from adsorbing to the first electrode 41. At time t1, the SOx concentration detection condition is satisfied, and the voltage applied to the first electrochemical cell 51 is increased from 0.3V to 1.1V. The SOx concentration detection condition is satisfied, for example, when the element unit 10 is in an active state and the first electrode 41 is not poisoned.

  After time t1, a voltage of 1.1 V is applied to the first electrochemical cell 51 for a predetermined time or more. The predetermined time is 60 seconds, for example. Thereafter, in the illustrated example, it is determined that the water concentration in the measured gas is stable at time t2, and the temporary pressure-down control is executed from time t2 to time t3.

  In the temporary step-down control, the applied voltage to the first electrochemical cell 51 is decreased from 1.1 V to 0.3 V at time t2, and the applied voltage to the first electrochemical cell 51 is changed from 0.3 V at time t3. Raised to 1.1V. The time between time t2 and time t3 is, for example, several seconds. After time t3, at time t5, the applied voltage to the first electrochemical cell 51 is decreased from 1.1V to 0.3V in order to desorb the decomposition product of SOx adsorbed on the first electrode 41.

  When the difference between the interelectrode current detected by the current detection circuit 62 at the start of the temporary step-down control and the interelectrode current detected by the current detection circuit 62 at the completion of the temporary step-down control is relatively large, the ECU 80 It is determined that the SOx concentration in the gas to be measured is higher than when the difference between the currents between the electrodes is relatively small. In the example of FIG. 5, the interelectrode current detected immediately before time t2 or time t2 is the interelectrode current at the start of the temporary step-down control. On the other hand, the interelectrode current detected immediately after time t3 or time t3 is used as the interelectrode current when the temporary step-down control is completed. In order to reduce the influence of noise on the interelectrode current, even if the interelectrode current detected at time t4 after the lapse of the predetermined time after the end of the temporary stepdown control is set as the interelectrode current at the completion of the temporary stepdown control. Good.

<Control routine for SOx concentration detection 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 detection process in the first embodiment of the present invention. The illustrated control routine is repeatedly executed by the ECU 80 while 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 detected 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 detected once or a plurality of times.

  First, in step S101, it is determined whether or not the SOx concentration detection condition is satisfied. Specifically, for example, when the element unit 10 is in the active state and the poisoning flag is off, it is determined that the SOx concentration detection condition is satisfied. On the other hand, when the element unit 10 is in an inactive state or the poisoning flag is on, it is determined that the SOx concentration detection condition is not satisfied. The poisoning flag is turned on when the ignition key of the vehicle equipped with the internal combustion engine is turned off, and is turned off in step S111 described later. When the poisoning flag is off, it is estimated that SOx decomposition products are not adsorbed on the first electrode 41, and when the poisoning flag is on, SOx decomposition products are adsorbed on the first electrode 41. It is estimated to be.

  Further, when the temperature of the gas to be measured is high, the adsorption amount of the decomposition product of SOx on the first electrode 41 decreases. In this case, the amount of inter-electrode current that changes in accordance with the SOx concentration in the measured gas becomes small, and the SOx concentration detection accuracy may be reduced. For this reason, even when the temperature of the measured gas is equal to or higher than a predetermined temperature, it may be determined that the SOx concentration detection condition is not satisfied. The temperature of the measured gas is detected by, for example, an exhaust temperature sensor disposed in the exhaust passage of the internal combustion engine.

  If it is determined in step S101 that the SOx concentration detection condition is satisfied, the present control routine proceeds to step S102. In step S <b> 102, a first voltage higher than the decomposition start voltage of water and SOx is applied to the first electrochemical cell 51. The first voltage is 1.1 V, for example, and is applied stepwise. The interelectrode voltage may be gradually increased to the first voltage.

  Next, in step S103, it is determined whether or not an elapsed time since the first voltage is applied to the first electrochemical cell 51 is equal to or longer than a predetermined time. The predetermined time is a time required from when the first voltage is applied to the first electrochemical cell 51 until the interelectrode current shows a value corresponding to the SOx concentration, and is, for example, 60 seconds. If it is determined in step S103 that the elapsed time since the first voltage is applied to the first electrochemical cell 51 is less than the predetermined time, step S102 is repeatedly executed. On the other hand, when it is determined in step S103 that the elapsed time since the first voltage is applied to the first electrochemical cell 51 is equal to or longer than the predetermined time, the present control routine proceeds to step S104.

  Next, in step S104, it is determined whether or not the water concentration stability flag is on. The water concentration stability flag is a flag set by a water concentration stability determination process described later. The water concentration stability flag is set to ON when it is determined that the water concentration in the measured gas is stable, and is OFF when it is determined that the water concentration in the measured gas is not stable. Set to The water concentration stability flag is also set to off when the ignition key of a vehicle equipped with an internal combustion engine is turned off.

  If it is determined in step S104 that the water concentration stability flag is off, steps S102 and S103 are repeatedly executed. Therefore, the first voltage higher than the decomposition start voltage of water and SOx is continuously applied to the first electrochemical cell 51 until it is determined that the water concentration stability flag is on. On the other hand, if it is determined in step S104 that the water concentration stability flag is on, the present control routine proceeds to step S105.

  In step S105 to step S107, temporary step-down control is performed in which the interelectrode voltage of the first electrochemical cell 51 is decreased from the first voltage to the second voltage and then increased to the first voltage again. The second voltage is a voltage at which the decomposition product of SOx adsorbed on the first electrode 41 can be desorbed, and is, for example, 0.3V. The second voltage is lower than the first voltage.

  First, in step S105, the voltage between the electrodes of the first electrochemical cell 51 is lowered. Specifically, the interelectrode voltage is lowered from the first voltage to the second voltage. The interelectrode voltage may be decreased stepwise from the first voltage to the second voltage or may be gradually decreased.

  Next, in step S106, it is determined whether or not an elapsed time after the interelectrode voltage reaches the second voltage is a predetermined time or more. The predetermined time is a time required to desorb a predetermined amount of the decomposition product of SOx adsorbed on the first electrode 41 from the first electrode 41, for example, several seconds. If it is determined in step S106 that the elapsed time after the interelectrode voltage has reached the second voltage is less than the predetermined time, steps S104 and S105 are repeatedly executed. On the other hand, when it is determined in step S106 that the elapsed time after the interelectrode voltage has reached the second voltage is equal to or longer than the predetermined time, the present control routine proceeds to step S107.

  In step S107, the voltage between the electrodes of the first electrochemical cell 51 is increased. Specifically, the interelectrode voltage is increased from the second voltage to the first voltage. The voltage between the electrodes is increased stepwise from the second voltage to the first voltage.

  Next, in step S108, the interelectrode current detected by the current detection circuit 62 at the start of the temporary step-down control and the interelectrode current detected by the current detection circuit 62 when the temporary step-down control is completed are acquired. When the voltage between the electrodes is decreased from the first voltage to the second voltage, or the current between the electrodes detected by the current detection circuit 62 immediately before the voltage between the electrodes is decreased from the first voltage to the second voltage, the temporary step-down control is started. Current between the electrodes. On the other hand, when the interelectrode voltage is increased from the second voltage to the first voltage, or immediately after the interelectrode voltage is increased from the second voltage to the first voltage, the interelectrode current detected by the current detection circuit 62 is temporarily reduced. Is the current between the electrodes at the time of completion. In order to reduce the influence of the noise of the interelectrode current immediately after boosting, the interelectrode current detected by the current detection circuit 62 after a predetermined time after the interelectrode voltage reaches the first voltage is It may be an interelectrode current.

  Next, in step S109, the SOx concentration is detected based on the difference between the interelectrode current at the start of the temporary step-down control acquired in step S108 and the interelectrode current at the completion of the temporary step-down control. Specifically, when the difference between the currents between the electrodes is relatively large, it is determined that the SOx concentration is higher than when the difference between the currents between the electrodes is relatively small. For example, it is determined that the SOx concentration is greater than or equal to a predetermined value when the difference between the currents between the electrodes is greater than or equal to a predetermined value, and the SOx concentration is less than the predetermined value when the difference between the currents between the electrodes is less than the predetermined value. Is done.

  Next, in step S110, in order to desorb the SOx decomposition product adsorbed on the first electrode 41 from the first electrode 41, the voltage at which the SOx decomposition product adsorbed on the first electrode 41 can be desorbed is first. It is applied to the electrochemical cell 51 for a predetermined time. The predetermined time is a time sufficient to desorb the SOx decomposition product adsorbed on the first electrode 41 from the first electrode 41. Further, the applied interelectrode voltage is, for example, 0.3 V, and is applied stepwise. In step S110, the interelectrode voltage may be zero.

  Next, in step S111, the poisoning flag is set to OFF. After step S111, this control routine ends.

  If it is determined in step S101 that the SOx concentration detection condition is not satisfied, the control routine proceeds to step S110.

<Control routine for water concentration stability determination processing>
FIG. 7 is a flowchart showing a control routine of water concentration stability determination processing in the first embodiment of the present invention. The illustrated control routine is repeatedly executed by the ECU 80 at predetermined time intervals.

  First, in step S201, it is determined whether the water concentration in the measured gas is stable. This determination is performed by the following method, for example. When the exhaust air-fuel ratio of the exhaust gas discharged from the combustion chamber of the internal combustion engine changes, the water concentration in the exhaust gas changes. Therefore, in step S201, it may be determined whether or not the amount of change in the exhaust air / fuel ratio of the exhaust gas discharged from the combustion chamber of the internal combustion engine is less than a predetermined value. The predetermined value is determined in advance by experiment or calculation, and is a value corresponding to the lower limit value of the change amount of the water concentration where it is difficult to ensure the desired accuracy of detecting the SOx concentration. Further, the change amount of 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 in 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. When it is determined that the amount of change in the exhaust air / fuel ratio of the exhaust gas is less than the predetermined value, it is determined that the water concentration in the measured gas is stable. On the other hand, when it is determined that the amount of change in the exhaust air-fuel ratio of the exhaust gas is greater than or equal to a predetermined value, it is determined that the water concentration in the measured gas is not stable.

  If it is determined in step S201 that the water concentration in the measured gas is stable, the present control routine proceeds to step S202. In step S202, the water concentration stability flag is set to on. After step S202, this control routine ends. On the other hand, when it is determined in step S201 that the water concentration in the measured gas is not stable, the present control routine proceeds to step S203. In step S203, the water concentration stability flag is set to OFF. After step S203, this control routine ends.

<Second embodiment>
The configuration and control of the SOx detection device according to the second embodiment are basically the same as the configuration and control of the SOx detection device 1 according to the first embodiment except for the points described below. For this reason, the second embodiment of the present invention will be described below with a focus on differences from the first embodiment.

  FIG. 8A is a schematic cross-sectional view showing the configuration of the element portion 10a of the SOx detection device according to the second embodiment of the present invention. FIG. 8B is a cross-sectional view taken along line AA in FIG. The SOx detection device according to the second embodiment includes an element unit 10a. The element part 10 a has the same configuration as the element part 10 in the first embodiment except that the element part 10 a includes a second electrochemical cell 52 in addition to the first electrochemical cell 51. As can be seen from FIGS. 8A and 8B, the second electrochemical cell 52 has a first electrochemical cell so that the distance from the diffusion-controlling layer 16 is equal to the first electrochemical cell 51. 51 are arranged side by side.

  The element unit 10 a further includes a third electrode 43 and a fourth electrode 44. The third electrode 43 is disposed on the surface of the first solid electrolyte layer 11 on the measured gas chamber 30 side. Therefore, the third electrode 43 is exposed to the measured gas in the measured gas chamber 30. The third electrode 43 has the same surface area as the first electrode 41. The third electrode 43 is arranged side by side with the first electrode 41 so that the distance from the diffusion-controlling layer 16 is equal to the first electrode 41. On the other hand, the fourth electrode 44 is disposed on the surface of the first solid electrolyte layer 11 on the first atmospheric chamber 31 side. Therefore, the fourth electrode 44 is exposed to the gas (atmosphere) in the first atmospheric chamber 31. The fourth electrode 44 has the same surface area as the second electrode 42. The fourth electrode 44 is arranged side by side with the second electrode 42 so that the distance from the diffusion rate limiting layer 16 is equal to the second electrode 42. The third electrode 43 and the fourth electrode 44 are disposed so as to face each other with the first solid electrolyte layer 11 interposed therebetween. The third electrode 43, the first solid electrolyte layer 11, and the fourth electrode 44 constitute a second electrochemical cell 52.

  In the present embodiment, the material constituting the third electrode 43 includes a metal element such as platinum (Pt), gold (Au), lead (Pb), silver (Ag), or an alloy thereof as a main component. Preferably, the third electrode 43 is a porous cermet electrode containing at least one of platinum (Pt), gold (Au), lead (Pb), and silver (Ag) as a main component. The fourth electrode 44 is a porous cermet electrode containing platinum (Pt) as a main component. In addition, the material which comprises the 3rd electrode 43 is not necessarily limited to the said material, What kind of material will be sufficient if an electrode is comprised so that the decomposition | disassembly rate of SOx may become low compared with the 1st electrode 41? It may be. In particular, in the present embodiment, the material constituting the third electrode 43 is preferably a material that substantially eliminates the rate at which SOx is decomposed in the third electrode 43. 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.

  In the example shown in FIG. 8, the second electrochemical cell 52 shares the first solid electrolyte layer 11 with the first electrochemical cell 51. However, the second electrochemical cell 52 may include a solid electrolyte layer different from the first solid electrolyte layer 11 constituting the first electrochemical cell 51.

  The third electrode 43 and the fourth electrode 44 are connected to the voltage application circuit 61. The voltage application circuit 61 applies an interelectrode voltage between the third electrode 43 and the fourth electrode 44 so that the potential of the fourth electrode 44 is higher than the potential of the third electrode 43. The applied interelectrode voltage is controlled by the ECU 80.

  The current detection circuit 62 detects an interelectrode current flowing between the third electrode 43 and the fourth electrode 44 (that is, an interelectrode current flowing in the first solid electrolyte layer 11). The output of the current detection circuit 62 is input to the ECU 80 via the AD converter 64. Therefore, the ECU 80 can acquire the interelectrode current of the second electrochemical cell 52 from the current detection circuit 62.

<Function of the second electrochemical cell>
As described above, in the second electrochemical cell 52, even when the same applied voltage as that in the first electrochemical cell 51 is applied, the rate of decomposing SOx contained in the measured gas is higher than that in the first electrochemical cell 51. Is too slow. Specifically, the rate at which SOx is decomposed at the third electrode 43 is extremely slower than the rate at which SOx is decomposed at the first electrode 41, and is substantially zero. Therefore, the rate at which the decomposition product of SOx contained in the measured gas is adsorbed to the electrode is also lower in the third electrode 43 than in the first electrode 41. Specifically, the SOx decomposition product is not substantially adsorbed on the third electrode 43. Therefore, the rate of decrease in activity for water decomposition in the third electrode 43 is smaller than the rate of decrease in activity for water decomposition in the first electrode 41. Specifically, the activity for water decomposition in the third electrode 43 is not substantially reduced. As a result, the interelectrode current in the second electrochemical cell 52 is not affected by the decomposition of SOx.

  For this reason, the interelectrode current of the second electrochemical cell 52 hardly changes according to the SOx concentration in the gas to be measured. On the other hand, the interelectrode current of the second electrochemical cell 52 changes in the same manner as the first electrochemical cell 51 according to the water concentration in the gas to be measured. Therefore, the same voltage is applied to the first electrochemical cell 51 and the second electrochemical cell 52, and the difference between the interelectrode current of the first electrochemical cell 51 and the interelectrode current of the second electrochemical cell 52 is calculated. As a result, it is possible to extract the amount of change in the water decomposition current that decreases due to the decomposition of SOx. As a result, it is possible to further suppress a decrease in the detection accuracy of the SOx concentration due to fluctuations in the water concentration in the measured gas.

<Control for detecting SOx concentration>
In the second embodiment, the ECU 80 applies the same voltage to the second electrochemical cell 52 as the first electrochemical cell 51 when detecting the SOx concentration in the gas to be measured. Specifically, the ECU 80 applies a first voltage to the first electrochemical cell 51 and the second electrochemical cell 52 for a predetermined time or more and then temporarily decreases the water concentration in the measured gas. Execute control. The first voltage is a voltage equal to or higher than the decomposition start voltage of water and SOx, and is 1.1 V, for example. The predetermined time is a time required from when the first voltage is applied between the electrodes of the first electrochemical cell 51 until the interelectrode current of the first electrochemical cell 51 shows a value corresponding to the SOx concentration. Seconds. Further, in the temporary voltage step-down control, the ECU 80 decreases the interelectrode voltage of the first electrochemical cell 51 and the second electrochemical cell 52 from the first voltage to the second voltage and increases it again to the first voltage. The second voltage is a voltage at which the decomposition product of SOx adsorbed on the first electrode 41 can be desorbed, and is, for example, 0.3V.

  The ECU 80 detects the current between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 detected by the current detection circuit 62 at the start of the temporary step-down control and the current detection circuit 62 when the temporary step-down control is completed. Based on the difference between the interelectrode currents of the first electrochemical cell 51 and the second electrochemical cell 52, the SOx concentration in the measured gas is detected. In the second embodiment, the ECU 80 changes the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control to the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control. Correct based on. Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control from the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control is The corrected interelectrode current of the first electrochemical cell 51 is used. Further, the ECU 80 corrects the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed based on the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed. . Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed from the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed is The corrected interelectrode current of the first electrochemical cell 51 is used.

  The ECU 80 corrects the inter-electrode current after correction of the first electrochemical cell 51 detected at the start of the temporary step-down control, and the corrected inter-electrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed. When the difference between the two is relatively large, it is determined that the SOx concentration in the measured gas is higher than when the difference between the currents between the electrodes is relatively small. For example, the ECU 80 determines that the SOx concentration is greater than or equal to a predetermined value when the difference between the currents between the electrodes is equal to or greater than a predetermined value, and the SOx concentration is less than the predetermined value when the difference between the currents between the electrodes is less than the predetermined value. Judge that there is.

  In the second embodiment, the control routine of the SOx concentration detection process shown in FIG. 6 is changed as follows. In the second embodiment, the same voltage as that of the first electrochemical cell 51 is applied to the second electrochemical cell 52 in addition to the first electrochemical cell 51 in step S102, step S105, step S107, and step S110. The

  In step S108, the current between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 detected by the current detection circuit 62 at the start of the temporary step-down control and the current detection circuit 62 detect when the temporary step-down control is completed. The interelectrode currents of the first electrochemical cell 51 and the second electrochemical cell 52 are acquired. When the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is decreased from the first voltage to the second voltage, or the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is changed to the first voltage. The interelectrode current of the first electrochemical cell 51 and the second electrochemical cell 52 detected immediately before the voltage is lowered from the voltage to the second voltage is set as the interelectrode current at the start of the temporary step-down control. On the other hand, when the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is increased from the second voltage to the first voltage, or the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is increased. The interelectrode current of the first electrochemical cell 51 and the second electrochemical cell 52 detected immediately after the second voltage is raised to the first voltage is the interelectrode current when the temporary step-down control is completed. In addition, in order to reduce the influence of the noise of the interelectrode current immediately after boosting, the first electrode detected a predetermined time after the interelectrode voltage of the first electrochemical cell 51 and the second electrochemical cell 52 reaches the first voltage. The interelectrode current of the electrochemical cell 51 and the second electrochemical cell 52 may be the interelectrode current when the temporary step-down control is completed.

  Next, in step S109, the interelectrode currents of the first electrochemical cell 51 and the second electrochemical cell 52 at the start of the temporary step-down control acquired in step S108 and the first electrochemical cell 51 at the time of completion of the temporary step-down control are obtained. The SOx concentration is detected based on the difference between the current between the electrodes of the second electrochemical cell 52. In the present embodiment, the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control is corrected based on the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control. The Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control from the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control is The corrected interelectrode current is used. Further, the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed is corrected based on the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed. Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed from the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed is The corrected interelectrode current is used.

  As a result, the corrected interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control, and the corrected interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed When the difference between the two is relatively large, it is determined that the SOx concentration in the measured gas is higher than when the difference between the electrodes is relatively small. For example, it is determined that the SOx concentration is greater than or equal to a predetermined value when the difference between the currents between the electrodes is greater than or equal to a predetermined value, and the SOx concentration is less than the predetermined value when the difference between the currents between the electrodes is less than the predetermined value. Is done.

<Third embodiment>
The configuration and control of the SOx detection device according to the third embodiment are basically the same as the configuration and control of the SOx detection device 1 according to the first embodiment except for the points described below. For this reason, the third embodiment of the present invention will be described below with a focus on differences from the first embodiment.

  FIG. 9 is a schematic cross-sectional view showing the configuration of the element portion 10b of the SOx detection device according to the third embodiment of the present invention. The SOx detection device according to the third embodiment includes an element unit 10b. The element part 10b has the same configuration as that of the element part 10 in the first embodiment except that a third electrochemical cell 53 is provided in addition to the first electrochemical cell 51.

  As shown in FIG. 9, the element portion 10 b is configured by laminating a plurality of layers. Specifically, the element portion 10b 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 second solid electrolyte layer 12 has the same configuration as the first solid electrolyte layer 11. The sixth impermeable layer 26 has the same configuration as the first impermeable layer 21 to the fifth impermeable layer 25. Each layer of the element portion 10b is, from the lower side of FIG. 9, from the first impermeable layer 21, the second impermeable layer 22, the third impermeable layer 23, the first solid electrolyte layer 11, the diffusion rate controlling layer 16, and the fourth impermeable layer. 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 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 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. 9, 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, the atmospheric gas also 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 b further includes a fifth electrode 45 and a sixth electrode 46. The fifth electrode 45 is disposed on the surface of the second solid electrolyte layer 12 on the measured gas chamber 30 side. Therefore, the fifth electrode 45 is exposed to the measured gas in the measured gas chamber 30. In addition, the fifth electrode 45 is disposed closer to the diffusion-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-controlled layer 16 first flows around the fifth electrode 45 and then flows around the first electrode 41. On the other hand, the sixth electrode 46 is disposed on the surface of the second solid electrolyte layer 12 on the second atmospheric chamber 32 side. Therefore, the sixth electrode 46 is exposed to the gas (atmosphere) in the second atmospheric chamber 32. The fifth electrode 45 and the sixth electrode 46 are arranged to face each other with the second solid electrolyte layer 12 interposed therebetween. The fifth electrode 45, the second solid electrolyte layer 12, and the sixth electrode 46 constitute a third electrochemical cell 53.

  The fifth electrode 45 and the sixth electrode 46 are porous cermet electrodes containing platinum (Pt) as a main component. However, the material constituting the fifth electrode 45 is not necessarily limited to the above material, and when a predetermined voltage is applied between the fifth electrode 45 and the sixth electrode 46, the measured gas chamber 30. Any material may be used as long as oxygen contained in the gas to be measured can be reduced and decomposed. Further, the material constituting the sixth electrode 46 is not necessarily limited to the above material, and when a predetermined voltage is applied between the fifth electrode 45 and the sixth electrode 46, Any material may be used as long as the oxide ions can move between the sixth electrode 46.

  In the above embodiment, the third electrochemical cell 53 includes the second solid electrolyte layer 12 that is different from the first solid electrolyte layer 11 that constitutes the first electrochemical cell 51. However, the first solid electrolyte layer and the second solid electrolyte layer may be the same solid electrolyte layer. That is, the first solid electrolyte layer 11 of FIG. 9 also functions as the second solid electrolyte layer, and thus the third electrochemical cell 53 may be configured to include the first solid electrolyte layer 11. In this case, the fifth electrode 45 is arranged on the surface of the first solid electrolyte layer 11 on the measured gas chamber 30 side, and the sixth electrode 46 is on the surface of the first solid electrolyte layer 11 on the first atmosphere chamber 31 side. Be placed.

  The fifth electrode 45 and the sixth electrode 46 are connected to the voltage application circuit 61. The voltage application circuit 61 applies an interelectrode voltage between the fifth electrode 45 and the sixth electrode 46 so that the potential of the sixth electrode 46 is higher than the potential of the fifth electrode 45. The applied interelectrode voltage is controlled by the ECU 80.

  The current detection circuit 62 detects an interelectrode current flowing between the fifth electrode 45 and the sixth electrode 46 (that is, an interelectrode current flowing in the second solid electrolyte layer 12). The output of the current detection circuit 62 is input to the ECU 80 via the AD converter 64. Therefore, the ECU 80 can acquire the interelectrode current of the third electrochemical cell 53 from the current detection circuit 62.

<Function of the third electrochemical cell>
In the first electrochemical cell 51, as described above, a predetermined voltage equal to or higher than the SOx and water decomposition start voltage is applied between the first electrode 41 and the second electrode 42, so that And the current between electrodes accompanying the decomposition of water is detected while SOx is decomposed. However, when oxygen is contained in the gas to be measured that reaches the first electrochemical cell 51, oxygen is decomposed (ionized) on the first electrode 41, and oxide ions generated thereby are generated. It flows from the first electrode 41 to the second electrode 42. As described above, if a decomposition current flows between the first electrode 41 and the second electrode 42 due to the decomposition of oxygen, it becomes difficult to accurately detect the SOx concentration based on the interelectrode current. .

  Here, when a voltage in the limiting current region of oxygen is applied to the third electrochemical cell 53, the conduction velocity of the oxide ions that can be conducted by the third electrochemical cell 53 is measured via the diffusion rate controlling layer 16. The rate of introduction of oxygen introduced into the chamber 30 is faster. Therefore, when a voltage in the limiting current region of oxygen is applied to the third electrochemical cell 53, most of the oxygen contained in the measured gas that has flowed into the measured gas chamber 30 through the diffusion rate controlling layer 16. Can be removed.

  Therefore, in the third embodiment, when a voltage equal to or higher than the decomposition start voltage of water and SOx is applied to the first electrochemical cell 51, the third electrochemical cell 51 is disposed closer to the diffusion rate controlling layer 16 than the first electrochemical cell 51. A voltage within the limiting current region of oxygen is applied to the electrochemical cell 53. The limiting current region of oxygen is a region of a lower limit voltage (for example, 0.1 V) or more at which the interelectrode current hardly changes even when the applied voltage is further increased. The voltage applied to the third electrochemical cell 53 is set to a voltage lower than the SOx and water decomposition starting voltage (about 0.6 V). Thereby, in the third electrochemical cell 53, oxygen can be decomposed and removed without decomposing water and SOx. Therefore, the third electrochemical cell 53 functions as a pump cell that discharges oxygen from the measured gas chamber 30 without discharging water and SOx. As a result, oxygen in the measured gas is removed in the third electrochemical cell 53 before the measured gas reaches the first electrochemical cell 51, so that the SOx concentration detection accuracy in the first electrochemical cell 51 is detected. Can be further improved.

<Control for detecting SOx concentration>
In the third embodiment, the ECU 80 applies a voltage to the third electrochemical cell 53 in addition to the first electrochemical cell 51 when detecting the SOx concentration in the gas to be measured. Specifically, when applying the first voltage to the first electrochemical cell 51, the ECU 80 applies a voltage of 0.1 V or more and less than 0.6 V, for example, a voltage of 0.4 V to the third electrochemical cell 53. . On the other hand, when a voltage capable of desorbing the decomposition product of SOx adsorbed on the first electrode 41 is applied to the first electrochemical cell 51, the first electrode 41 is promoted in order to promote the desorption of the decomposition product due to reoxidation. It is preferable to supply oxygen. For this reason, the ECU 80 applies a voltage lower than 0.1 V, for example, a voltage of 0.05 V, when applying to the first electrochemical cell 51 a voltage capable of desorbing the decomposition product of SOx adsorbed on the first electrode 41. Application to the third electrochemical cell 53.

  In the third embodiment, the control routine of the SOx concentration detection process shown in FIG. 6 is changed as follows. In step S <b> 102, a voltage is applied to the third electrochemical cell 53 in addition to the first electrochemical cell 51. The applied voltage to the third electrochemical cell 53 is a voltage of 0.1 V or more and less than 0.6 V, for example, 0.4 V. In step S105, in addition to the interelectrode voltage of the first electrochemical cell 51, the interelectrode voltage of the third electrochemical cell 53 is reduced. The voltage between the electrodes of the third electrochemical cell 53 is decreased from 0.4V to 0.05V, for example. In step S107, the interelectrode voltage of the third electrochemical cell 53 is boosted in addition to the interelectrode voltage of the first electrochemical cell 51. The interelectrode voltage of the third electrochemical cell 53 is increased from 0.05 V to 0.4 V, for example. In step S110, the applied voltage to the first electrochemical cell 51 is set to 0.3V, and the applied voltage to the third electrochemical cell 53 is set to 0.05V.

<Fourth embodiment>
The configuration and control of the SOx detection device according to the fourth embodiment are basically the same as the configuration and control of the SOx detection device 1 according to the first embodiment except for the points described below. For this reason, the fourth embodiment of the present invention will be described below with a focus on differences from the first embodiment.

  FIG. 10A is a schematic cross-sectional view showing the configuration of the element portion 10c of the SOx detection device according to the fourth embodiment of the present invention. FIG. 10B is a cross-sectional view taken along line BB in FIG. The SOx detection device according to the fourth embodiment includes an element unit 10c. The element part 10 c has the same configuration as the element part 10 b in the third embodiment except that the element part 10 c includes a second electrochemical cell 52 in addition to the first electrochemical cell 51 and the third electrochemical cell 53. Further, the second electrochemical cell 52 of the element unit 10c has the same configuration as the second electrochemical cell 52 of the element unit 10a shown in FIG. For this reason, the detailed description about a SOx detection apparatus is abbreviate | omitted.

<Control for detecting SOx concentration>
In the fourth embodiment, the ECU 80 applies the same voltage to the second electrochemical cell 52 as the first electrochemical cell 51 when detecting the SOx concentration in the gas to be measured. Specifically, the ECU 80 applies a first voltage to the first electrochemical cell 51 and the second electrochemical cell 52 for a predetermined time or more and then temporarily decreases the water concentration in the measured gas. Execute control. The first voltage is a voltage equal to or higher than the decomposition start voltage of water and SOx, and is 1.1 V, for example. The predetermined time is from when a voltage higher than the decomposition start voltage of water and SOx is applied between the electrodes of the first electrochemical cell 51 until the interelectrode current of the first electrochemical cell 51 shows a value corresponding to the SOx concentration. The time required, for example, 60 seconds. Further, in the temporary voltage step-down control, the ECU 80 decreases the interelectrode voltage of the first electrochemical cell 51 and the second electrochemical cell 52 from the first voltage to the second voltage and increases it again to the first voltage. The second voltage is a voltage at which the decomposition product of SOx adsorbed on the first electrode 41 can be desorbed, and is, for example, 0.3V.

  Further, the ECU 80 applies a voltage to the third electrochemical cell 53 in addition to the first electrochemical cell 51 and the second electrochemical cell 52 when detecting the SOx concentration in the measured gas. Specifically, when the first voltage is applied to the first electrochemical cell 51 and the second electrochemical cell 52, the ECU 80 applies a voltage of 0.1 V or more and less than 0.6 V, for example, a voltage of 0.4 V to the third voltage. Applied to the electrochemical cell 53. In addition, when the ECU 80 applies a voltage capable of desorbing the decomposition product of SOx adsorbed to the first electrode 41 to the first electrochemical cell 51 and the second electrochemical cell 52, a voltage of less than 0.1 V, for example, A voltage of 0.05 V is applied to the third electrochemical cell 53.

  The ECU 80 detects the current between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 detected by the current detection circuit 62 at the start of the temporary step-down control, and the current detection circuit 62 after the completion of the temporary step-down control. Based on the difference between the interelectrode currents of the first electrochemical cell 51 and the second electrochemical cell 52, the SOx concentration in the measured gas is detected. In the fourth embodiment, the ECU 80 converts the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control into the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control. Correct based on. Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control from the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control is The corrected interelectrode current of the first electrochemical cell 51 is used. Further, the ECU 80 corrects the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed based on the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed. . Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed from the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed is The corrected interelectrode current of the first electrochemical cell 51 is used.

  The ECU 80 corrects the inter-electrode current after correction of the first electrochemical cell 51 detected at the start of the temporary step-down control, and the corrected inter-electrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed. When the difference between the two is relatively large, it is determined that the SOx concentration in the measured gas is higher than when the difference between the currents between the electrodes is relatively small. For example, the ECU 80 determines that the SOx concentration is greater than or equal to a predetermined value when the difference between the currents between the electrodes is equal to or greater than a predetermined value, and the SOx concentration is less than the predetermined value when the difference between the currents between the electrodes is less than the predetermined value. Judge that there is.

  In the fourth embodiment, the control routine of the SOx concentration detection process shown in FIG. 6 is changed as follows. In the fourth embodiment, in step S102, step S105, step S107, and step S110, the same voltage as that of the first electrochemical cell 51 is applied to the second electrochemical cell 52 in addition to the first electrochemical cell 51. The

  In step S <b> 102, a voltage is applied to the third electrochemical cell 53 in addition to the first electrochemical cell 51 and the second electrochemical cell 52. The applied voltage to the third electrochemical cell 53 is a voltage of 0.1 V or more and less than 0.6 V, for example, 0.4 V. In step S105, the interelectrode voltage of the third electrochemical cell 53 is lowered in addition to the interelectrode voltage of the first electrochemical cell 51 and the second electrochemical cell 52. The voltage between the electrodes of the third electrochemical cell 53 is decreased from 0.4V to 0.05V, for example. In step S107, the interelectrode voltage of the third electrochemical cell 53 is boosted in addition to the interelectrode voltage of the first electrochemical cell 51 and the second electrochemical cell 52. The interelectrode voltage of the third electrochemical cell 53 is increased from 0.05 V to 0.4 V, for example. In step S110, the applied voltage to the first electrochemical cell 51 and the second electrochemical cell 52 is set to 0.3V, and the applied voltage to the third electrochemical cell 53 is set to 0.05V.

  In step S108, the current between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 detected by the current detection circuit 62 at the start of the temporary step-down control and the current detection circuit 62 detect when the temporary step-down control is completed. The interelectrode currents of the first electrochemical cell 51 and the second electrochemical cell 52 are acquired. When the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is decreased from the first voltage to the second voltage, or the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is changed to the first voltage. The interelectrode current of the first electrochemical cell 51 and the second electrochemical cell 52 detected immediately before the voltage is lowered from the voltage to the second voltage is set as the interelectrode current at the start of the temporary step-down control. On the other hand, when the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is increased from the second voltage to the first voltage, or the voltage between the electrodes of the first electrochemical cell 51 and the second electrochemical cell 52 is increased. The interelectrode current of the first electrochemical cell 51 and the second electrochemical cell 52 detected immediately after the second voltage is raised to the first voltage is the interelectrode current when the temporary step-down control is completed. In addition, in order to reduce the influence of the noise of the interelectrode current immediately after boosting, the first electrode detected a predetermined time after the interelectrode voltage of the first electrochemical cell 51 and the second electrochemical cell 52 reaches the first voltage. The interelectrode current of the electrochemical cell 51 and the second electrochemical cell 52 may be the interelectrode current when the temporary step-down control is completed.

  Next, in step S109, the interelectrode currents of the first electrochemical cell 51 and the second electrochemical cell 52 at the start of the temporary step-down control acquired in step S108 and the first electrochemical cell 51 at the time of completion of the temporary step-down control are obtained. The SOx concentration is detected based on the difference between the current between the electrodes of the second electrochemical cell 52. In the present embodiment, the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control is corrected based on the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control. The Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control from the interelectrode current of the second electrochemical cell 52 detected at the start of the temporary step-down control is The corrected interelectrode current is used. Further, the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed is corrected based on the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed. Specifically, the value obtained by subtracting the interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed from the interelectrode current of the second electrochemical cell 52 detected when the temporary step-down control is completed is The corrected interelectrode current is used.

  As a result, the corrected interelectrode current of the first electrochemical cell 51 detected at the start of the temporary step-down control, and the corrected interelectrode current of the first electrochemical cell 51 detected when the temporary step-down control is completed When the difference between the two is relatively large, it is determined that the SOx concentration in the measured gas is higher than when the difference between the electrodes is relatively small. For example, it is determined that the SOx concentration is greater than or equal to a predetermined value when the difference between the currents between the electrodes is greater than or equal to a predetermined value, and the SOx concentration is less than the predetermined value when the difference between the currents between the electrodes is less than the predetermined value. Is done.

  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, 10a, 10b, 10c Element part 11 1st solid electrolyte layer 16 Diffusion-controlled layer 41 1st electrode 42 2nd electrode 51 1st electrochemical cell 61 Voltage application circuit 62 Current detection circuit 80 Electronic control unit ( ECU)

Claims (1)

A solid electrolyte layer having oxide ion conductivity; a first electrode disposed on one side surface of the solid electrolyte layer so as to be exposed to a gas to be measured; and the solid electrolyte layer so as to be exposed to the atmosphere An electrochemical cell having a second electrode disposed on the other side surface of the element, and an element portion comprising a diffusion rate-limiting layer that performs diffusion rate-limiting of the measured gas,
A voltage application unit that applies an inter-electrode voltage between the first electrode and the second electrode so that the potential of the second electrode is higher than the potential of the first electrode;
A current detector for detecting an interelectrode current flowing between the first electrode and the second electrode;
A controller that controls the interelectrode voltage applied from the voltage application unit and acquires the interelectrode current from the current detection unit, and
The control unit applies the first voltage between the first electrode and the second electrode for a predetermined time or more, and then sets the inter-electrode voltage while the water concentration in the measured gas is stable. Temporary step-down control for reducing the first voltage to the second voltage and increasing the first voltage again is performed, the inter-electrode current detected by the current detection unit at the start of the temporary step-down control, and the temporary When the difference between the interelectrode current detected by the current detection unit at the time of completion of the step-down control is relatively large, the sulfur oxide concentration in the measured gas is larger than when the difference is relatively small. The first voltage is higher than the decomposition start voltage of water and sulfur oxide, and the second voltage can desorb the decomposition product of sulfur oxide adsorbed on the first electrode. A sulfur oxide detector that is a voltage.

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