JP2009075012A - Deterioration suppressing device for air/fuel ratio sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide the deterioration suppressing device for air/fuel ratio sensor that suppresses deterioration of the detection element part and catalyst layer of the air/fuel ratio sensor that becomes high temperature due to emission. <P>SOLUTION: The deterioration suppressing device 100 of the air/fuel ratio sensor is equipped with a solid electrolyte layer 2 for moving oxygen ions between an atmospheric side and an exhaustion side; a pair of positive and negative electrodes 3, respectively provided on the atmospheric side and exhaust side of the solid electrolyte layer 2; the diffusion resistance layer 4, arranged around the electrodes provided on the exhaustion side of the electrodes 3; the catalyst layer 11, arranged on the exhaust inlet surface of the diffusion resistance layer 4; a power supply 12 for applying a voltage across the electrodes 3 and a control unit 13 for controlling the power supply 12. By applying a voltage V2 which is higher than the voltage V1 at the time of detection of an air/fuel ratio across the electrodes 3, when there is no request for air/fuel ratio feedback control, deterioration in the solid electrolyte layer 2 or the catalyst layer 11 can be suppressed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a deterioration suppressing device that suppresses deterioration of a detection element portion and a catalyst layer of an air-fuel ratio sensor.

  Conventionally, an engine includes an air-fuel ratio sensor that detects an air-fuel ratio of exhaust gas in an exhaust passage. Such an air-fuel ratio sensor detects the air-fuel ratio of the exhaust based on the oxygen concentration in the exhaust. However, when hydrogen is present in the exhaust gas, the oxygen concentration detected by the sensor detection unit due to the influence of hydrogen may be detected as a value different from the actual oxygen concentration in the exhaust gas. Hydrogen that causes such detection errors can be removed from the detected exhaust gas by the catalyst layer provided in the air-fuel ratio sensor. An air-fuel ratio sensor provided with such a catalyst layer is disclosed in Patent Document 1, for example. The air-fuel ratio control device disclosed in Patent Document 1 keeps the surface of the exhaust gas side electrode in the sensor at a predetermined temperature or higher by a heater, so that hydrogen is spontaneously combusted and removed, preventing the output deviation of the sensor. Air / fuel ratio control is performed.

Japanese Patent Laid-Open No. 11-30141

  In such an air-fuel ratio sensor, the detection element unit is activated by maintaining the detection element unit in an appropriate temperature range, and the detection output is stably obtained. In particular, since the air-fuel ratio sensor is required to be activated early in order to detect the exhaust exhausted when the engine is started, it is often disposed upstream of the exhaust pipe through which high-temperature exhaust flows. As a result, the high-temperature exhaust gas passes through the vicinity of the air-fuel ratio sensor, so that the detection element portion is activated early and the air-fuel ratio of the exhaust gas can be detected. However, since the air-fuel ratio sensor is disposed on the upstream side of the exhaust pipe, the air-fuel ratio sensor is exposed to high-temperature exhaust gas, so that the temperature in the detection element portion and the catalyst layer of the air-fuel ratio sensor exceeds an appropriate range. May rise. Further, since the detection element part and the catalyst layer are deteriorated in a high temperature state exceeding such an appropriate temperature range, the detection accuracy of the sensor is lowered.

  Accordingly, an object of the present invention is to provide an air-fuel ratio sensor deterioration suppressing device that suppresses deterioration of a detection element portion and a catalyst layer of an air-fuel ratio sensor that becomes high temperature due to exhaust.

  An air-fuel ratio sensor deterioration suppressing device of the present invention that solves such a problem is attached to each of a solid electrolyte that moves oxygen ions between the atmosphere side and the exhaust side, and the atmosphere side and the exhaust side of the solid electrolyte. A pair of positive and negative electrodes, a diffusion resistance layer disposed around the electrode attached to the exhaust side of the electrode, a catalyst layer disposed on a surface of the diffusion resistance layer where exhaust is introduced, and the electrode A voltage control means for applying a voltage between the electrodes, and when the air-fuel ratio is not detected, the voltage control means applies a voltage higher than the voltage at the time of air-fuel ratio detection to the electrodes. Thus, the deterioration of the solid electrolyte and / or the catalyst layer is suppressed. (Claim 1). With such a configuration, in the air-fuel ratio sensor, oxygen can be forcibly supplied from the atmosphere side to the exhaust side, the temperature rise of the solid electrolyte corresponding to the detection element can be suppressed, and deterioration can be prevented. . Further, the oxygen thus moved to the exhaust side burns excess hydrogen in the catalyst layer, recovers the ability of the catalyst, and can suppress deterioration of the catalyst layer.

  In an engine equipped with such an air-fuel ratio sensor, air-fuel ratio feedback (F / B) control may be performed. In the air-fuel ratio F / B control, the intake air amount and the fuel injection amount are determined based on the air-fuel ratio of the exhaust gas. For this reason, in order to perform air-fuel ratio F / B control, accurate information on the air-fuel ratio of the exhaust is required. On the other hand, when the air-fuel ratio F / B control is not performed, information on the air-fuel ratio of the exhaust is not necessary. That is, when the air-fuel ratio F / B control is not performed, the oxygen concentration on the exhaust side does not need to be an accurate concentration. For this reason, in the air-fuel ratio sensor deterioration suppressing device of the present invention, the voltage control means applies a voltage higher than the voltage at the time of air-fuel ratio detection to the electrodes when the air-fuel ratio feedback control is not performed. Thus, deterioration of the solid electrolyte and / or the catalyst layer can be suppressed (claim 2). With this configuration, the air-fuel ratio sensor deterioration suppressing device forcibly supplies oxygen from the atmosphere side to the exhaust side when the air-fuel ratio F / B control is not performed. Thereby, deterioration of a solid electrolyte or a catalyst layer can be suppressed.

  In addition, such an air-fuel ratio sensor deterioration suppressing device includes an element deterioration determining unit that determines deterioration of the solid electrolyte, and the voltage control unit is configured such that when the element deterioration determining unit determines deterioration of the solid electrolyte, A voltage higher than the voltage at the time of detecting the air-fuel ratio can be applied to the electrode. Further, the air-fuel ratio sensor deterioration suppressing device includes a catalyst layer capability determining unit that determines a decrease in the capability of the catalyst layer, and the voltage control unit is configured to determine whether the catalyst layer capability determining unit determines a decrease in the capability of the catalyst layer. A voltage higher than the voltage at the time of detecting the air-fuel ratio can be applied to the electrode. Thus, by supplying oxygen from the atmosphere side to the exhaust side according to the state of the solid electrolyte and the catalyst layer, it is possible to suppress the deterioration of the solid electrolyte and the catalyst layer and maintain the detection accuracy of the air-fuel ratio sensor. it can.

  The air-fuel ratio sensor deterioration suppression device of the present invention forcibly supplies oxygen from the atmosphere side to the exhaust side, thereby suppressing the high temperature of the solid electrolyte and the deterioration of the catalyst layer and maintaining the detection accuracy of the air-fuel ratio sensor. be able to.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.

  Embodiment 1 of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram showing a schematic configuration of an air-fuel ratio sensor deterioration suppressing device 100 according to the present embodiment, and showing the air-fuel ratio sensor 1 provided in the air-fuel ratio sensor deterioration suppressing device 100 in cross section. The air-fuel ratio sensor 1 is disposed in an exhaust passage (not shown) of the internal combustion engine and is used for detecting the air-fuel ratio of the exhaust gas. The air-fuel ratio sensor 1 includes a solid electrolyte layer 2, an electrode 3, and a diffusion resistance layer 4. The solid electrolyte layer 2 is formed from a zirconia sintered body having oxygen ion conductivity.

  The electrode 3 includes a pair of positive and negative positive electrodes 3a and 3b. The positive electrode 3a is attached to the air side surface 2a of the solid electrolyte layer 2, and the negative electrode 3b is opposed to the positive electrode 3a. It is attached to the exhaust side 2b. The positive electrode 3a and the negative electrode 3b are formed using platinum having high electronic conductivity, chemical stability, and catalytic activity.

  The diffusion resistance layer 4 is disposed around the negative electrode 3 b on the exhaust side surface 2 b of the solid electrolyte layer 2. The diffusion resistance layer 4 has a porous structure and reduces the component concentration in the exhaust gas that passes through the diffusion resistance layer 4 in accordance with the component concentration in the exhaust gas before inflow. That is, the higher the component concentration in the exhaust before being introduced into the diffusion resistance layer 4, the higher the component concentration in the exhaust that has passed through the diffusion resistance layer 4, and the component concentration in the exhaust before being introduced into the diffusion resistance layer 4 is higher. The lower the concentration, the lower the component concentration in the exhaust gas that has passed through the diffusion resistance layer 4.

  In addition, the air-fuel ratio sensor 1 includes supports 5 and 6. The support 5 is disposed so as to overlap the opposite side of the solid electrolyte layer 2 of the diffusion resistance layer 4. The support 6 is disposed on the atmosphere side surface 2a side of the solid electrolyte layer 2 so as to form a section 7 surrounding the positive electrode 3a. The support 6 is made of a material having good thermal conductivity, for example, alumina. Further, a heater 8 is provided in the support 6. The heater 8 is used to raise the temperature of the air-fuel ratio sensor 1 to a predetermined temperature at which air-fuel ratio detection is stably performed, for example, 600 ° C. or higher.

  The air-fuel ratio sensor 1 includes a protective layer 9 that covers the entire components stacked in this way. FIG. 2 is an explanatory view showing an enlarged cross section of the protective layer 9 in the vicinity of the diffusion resistance layer 4 indicated by an arrow 16 in FIG. The protective layer 9 has a structure in which an outer trap layer 10 and an inner catalyst layer 11 are laminated. In particular, the catalyst layer 11 is disposed on a surface of the diffusion resistance layer 4 where exhaust gas is introduced. The trap layer 10 is porous alumina, and the catalyst layer 11 is formed by supporting catalyst components such as platinum and rhodium on porous alumina as a catalyst carrier. Exhaust gas passes through the trap layer 10 and the catalyst layer 11 as shown by arrows 17 and 18 in FIG. At this time, the trap layer 10 adsorbs poisonous substances such as lead contained in the exhaust gas, and protects the solid electrolyte layer 2 and the electrode 3. Further, the catalyst 11 promotes combustion of hydrogen contained in the exhaust gas and removes hydrogen in the exhaust gas.

  As shown in FIG. 1, the air-fuel ratio sensor deterioration suppressing device 100 flows through the power source 12 that applies a voltage between the electrodes of the electrode 3, the voltmeter 14 that measures the voltage applied by the power source 12, and the electrode 3. And an ammeter 15 for measuring current. Further, the power source 12, the voltmeter 14 and the ammeter 15 are electrically connected to the control device 13. This control device 13 corresponds to the voltage control means of the present invention.

  Next, a method for detecting the oxygen concentration by the air-fuel ratio sensor 1 will be described. Exhaust gas flowing outside the air-fuel ratio sensor 1 passes through the trap layer 10, the catalyst layer 11, and the diffusion resistance layer 4 in order and reaches the negative electrode 3b. That is, the negative electrode 3b is configured to contact exhaust. In this way, the exhaust gas reaching the negative electrode 3 b is introduced into the diffusion resistance layer 4 after removing poisonous substances in the trap layer 10 and removing hydrogen in the catalyst layer 11. As a result, the influence of hydrogen in the exhaust gas to be detected can be suppressed, and the oxygen concentration can be detected more accurately. On the other hand, the section 7 formed by the support 6 and the solid electrolyte layer 2 is configured to communicate with the atmosphere. That is, the positive electrode 3a is configured to come into contact with the atmosphere.

  When a voltage is applied between the electrodes 3 by the power supply 12, oxygen molecules near the electrodes are ionized, and oxygen ions move in the solid electrolyte layer 2. By such movement of oxygen ions, electrons are carried and current flows. When the air-fuel ratio sensor 1 is warmed up by the heater 8 or is heated to a predetermined temperature or higher by exhaust, the sensor output is stabilized.

  The current flowing in the solid electrolyte layer 2 increases as the voltage applied between the electrodes increases, but the exhaust gas reaching the negative electrode 3b side is restricted from passing by the diffusion resistance layer 4, and the amount of oxygen to be ionized is saturated. . In such a state where the ionization of oxygen is saturated, even if the applied voltage is increased, the number of oxygen molecules to be ionized does not change, so the current value becomes constant. The current at this time is called a limit current, and the value of the limit current is determined by the oxygen concentration in the exhaust gas. The control device 13 applies a voltage V1 at which such a limit current is generated between the electrodes 3, and acquires the value of the limit current at this time by the ammeter 15. The control device 13 calculates the oxygen concentration in the exhaust gas by comparing the value of the limit current acquired in this way with the information of the oxygen concentration with respect to the value of the limit current acquired in advance, and sets the air-fuel ratio. To detect.

  Further, the control device 13 performs air-fuel ratio feedback (F / B) control based on the air-fuel ratio detected in this way. In the air-fuel ratio F / B control, the fuel injection amount by a fuel injection valve (not shown) or the throttle valve (not shown) is controlled so that the air-fuel ratio of the exhaust gas becomes the stoichiometric air-fuel ratio. This is to adjust the air intake amount. If the air-fuel ratio of the exhaust gas detected from the air-fuel ratio sensor 1 is not the stoichiometric air-fuel ratio, the control device 13 requests F / B control.

  Next, control performed by the control device 13 will be described. FIG. 3 shows a flow of control performed by the control device 13. In step S1, the control device 13 determines whether or not there is a request for F / B control. If the determination result in step S1 is YES, that is, if there is a request for F / B control, the control device 13 proceeds to step S2.

  In step S2, the control device 13 sets the voltage applied between the electrodes 3 to V1. The applied voltage V1 is a voltage at which a limit current is generated, and is about 0.4 (V), for example. By applying the voltage V1 between the electrodes 3, a limit current is generated, and the air-fuel ratio of the exhaust gas is detected from the value of the limit current at this time. Information on the air-fuel ratio detected in this way is used for F / B control. When the process in step S2 ends, the control device 13 returns.

  On the other hand, when the determination result of step S1 is NO, that is, when there is no request for F / B control, the control device 13 proceeds to step S3.

  In step S3, the control device 13 sets the voltage applied between the electrodes 3 to V2. The applied voltage V2 is higher than the voltage V1 at which the limit current is generated, and is, for example, 0.8 to 3 (V). By applying such a voltage V2, once ionization of saturated oxygen molecules is resumed, oxygen on the atmosphere side is forcibly supplied to the exhaust side. That is, oxygen near the positive electrode 3a moves to the negative electrode 3b side. At this time, the temperature of the solid electrolyte layer 2 decreases, and deterioration of the solid electrolyte layer 2 can be suppressed. Further, the oxygen thus moved to the negative electrode 3b side reaches the catalyst layer 11, contributes to hydrogen combustion in the catalyst layer 11, and can recover the equilibration ability of the catalyst which decreases with the lapse of the operation time. The deterioration of the catalyst layer can be suppressed.

  The control device 13 proceeds to step S2 after step S3. The control device 13 sets the voltage applied between the electrodes 3 by the power supply 12 to V1. As a result, the air-fuel ratio sensor 1 returns to the air-fuel ratio detection state, and F / B control is performed based on the air-fuel ratio detected here.

  Thus, when there is no request for F / B control, it is not necessary to detect the air-fuel ratio of the exhaust gas. Therefore, the oxygen concentration at the exhaust-side electrode, that is, the negative electrode 3b, matches the actual oxygen concentration in the exhaust gas. There is no need to do. For this reason, oxygen can be supplied from the positive electrode 3a on the air side to the negative electrode 3b side on the exhaust side when there is no request for F / B control. Thereby, while reducing the temperature of the solid electrolyte layer 2 and suppressing deterioration, the equilibration ability of the catalyst layer 11 can be recovered, and deterioration can be suppressed.

  Next, a second embodiment of the present invention will be described. The air-fuel ratio sensor deterioration suppressing device of the present embodiment has the same configuration as the air-fuel ratio sensor deterioration suppressing device 100 of the first embodiment. Therefore, the air-fuel ratio sensor of the present embodiment will be described using the same reference numerals as those of the air-fuel ratio sensor deterioration suppressing device 100 of the first embodiment. The present embodiment is different from the first embodiment in that the content of the control performed by the control device 13 is different from the first embodiment.

  Control performed by the control device 13 of the present embodiment will be described. FIG. 4 shows the flow of control performed by the control device 13 of this embodiment. In step S10, the control device 13 determines whether there is a voltage change request. Here, the voltage change request is a request to change the applied voltage between the electrodes 3 by the power supply 12, and this change request is determined based on the element deterioration determination result.

  The element deterioration determination is for determining whether or not the solid electrolyte layer 2 is deteriorated. This element deterioration determination is performed by the control device 13. When the deterioration of the solid electrolyte layer 2 proceeds, the impedance at the same temperature increases as compared with that before the deterioration. Based on such properties, the control device 13 determines the deterioration of the solid electrolyte layer 2 from the change in impedance. When determining that the solid electrolyte layer 2 is deteriorated in such element deterioration determination, the control device 13 makes a voltage change request. If the determination result in step S10 is YES, that is, if there is a voltage change request, the control device 13 proceeds to step S11. On the other hand, if the determination result in step S10 is NO, that is, if there is no voltage change request, the control device 13 proceeds to step S12.

  Step S11 is the same as step S1 of the first embodiment shown in FIG. Step S12 is the same as step S2, and step S13 is the same as step S3. For this reason, description of step S11, step S12, and step S13 is abbreviate | omitted.

  Moreover, the voltage change request | requirement in step S10 can also perform a voltage change request | requirement based on a catalyst layer capability determination result. The catalyst layer capability determination is to determine whether or not the catalyst in the catalyst layer 11 has a predetermined hydrogen removal capability. This catalyst layer capacity determination is performed by the control device 13. When it is determined that the hydrogen removal capability of the catalyst layer 11 is reduced, the control device 13 makes a voltage change request.

  By applying the applied voltage V2 according to such a voltage change request, the deterioration of the solid electrolyte layer 2 and the deterioration of the catalyst layer 11 are suppressed, so that the detection accuracy of the air-fuel ratio sensor can be maintained.

  The above-described embodiments are merely examples for carrying out the present invention, and the present invention is not limited thereto. Various modifications of these embodiments are within the scope of the present invention. It is apparent from the above description that various other embodiments are possible within the scope.

It is explanatory drawing which showed schematic structure of the air-fuel ratio sensor deterioration suppression apparatus by making an air-fuel ratio sensor into a cross section. It is explanatory drawing which expanded and showed the cross section of the protective layer of the diffused resistance layer vicinity. 3 is a control flow performed by the control device in the first embodiment. 6 is a control flow performed by the control device in the second embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Air fuel ratio sensor 2 Solid electrolyte layer 3 Electrode 4 Diffusion resistance layer 9 Protective layer 10 Trap layer 11 Catalyst layer 12 Power supply 13 Control apparatus 100 Air fuel ratio sensor deterioration suppression apparatus

Claims (4)

A solid electrolyte that moves oxygen ions between the atmosphere side and the exhaust side;
A pair of positive and negative electrodes attached to the air side and the exhaust side of the solid electrolyte, and
A diffusion resistance layer disposed around the electrode attached to the exhaust side of the electrode;
A catalyst layer disposed on a surface where the exhaust gas in the diffusion resistance layer is introduced;
Voltage control means for applying a voltage between the electrodes,
The voltage control means suppresses deterioration of the solid electrolyte and / or the catalyst layer by applying a voltage higher than the voltage at the time of air-fuel ratio detection to the electrode when the air-fuel ratio is not detected. An air-fuel ratio sensor deterioration suppressing device. The air-fuel ratio sensor deterioration suppressing device according to claim 1,
The voltage control means suppresses deterioration of the solid electrolyte and / or the catalyst layer by applying a higher voltage to the electrode than the voltage at the time of detecting the air-fuel ratio when the air-fuel ratio feedback control is not performed. An air-fuel ratio sensor deterioration suppressing device. The air-fuel ratio sensor deterioration suppressing device according to claim 1,
Comprising an element deterioration determining means for determining deterioration of the solid electrolyte;
The air-fuel ratio sensor deterioration suppressing device, wherein the voltage control means applies a voltage higher than the voltage at the time of detecting the air-fuel ratio when the element deterioration determining means determines the deterioration of the solid electrolyte. . The air-fuel ratio sensor deterioration suppressing device according to claim 1,
Comprising a catalyst layer capacity determining means for determining a decrease in capacity of the catalyst layer;
The voltage control means applies a higher voltage to the electrode than the voltage at the time of detecting the air-fuel ratio when the catalyst layer capacity determination means determines that the capacity of the catalyst layer is lowered. Suppression device.

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