JP2012068150A - Abnormality diagnostic device for oxygen sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an abnormality diagnostic device which is suitable for an oxygen sensor having a diffusion layer in an outer layer part of a detection element.SOLUTION: At least one of a plurality of positive direct currents obtained when a plurality of positive direct voltages different in magnitude are applied to the detection element; and a plurality of negative direct currents obtained when a plurality of negative direct voltages different in magnitude are applied to the detection element is detected. Based on a change rate of the detected direct currents that is the at least one of the plurality of positive direct currents and the plurality of negative direct currents, it is determined whether the oxygen sensor is normal or abnormal. Diagnostic errors can be reduced by reducing the influence of variations in current values caused by the variations or the like in the detection element itself.

Description

  The present invention relates to an oxygen sensor abnormality diagnosis device, and more particularly to an abnormality diagnosis device suitable for an oxygen sensor having a diffusion layer in an outer layer portion of a detection element.

  In an internal combustion engine equipped with an exhaust gas purification system using a catalyst, in order to effectively remove harmful components of exhaust gas by the catalyst, the mixture ratio of air and fuel in the air-fuel mixture burned in the internal combustion engine, that is, the air-fuel ratio is reduced. Control is essential. In order to perform such air-fuel ratio control, an oxygen sensor for detecting the oxygen concentration of the exhaust gas is provided in the exhaust passage of the internal combustion engine, the air-fuel ratio is obtained from the detection result, and the detected air-fuel ratio is set to a predetermined target air-fuel ratio. The air-fuel ratio feedback control is made so as to be close to.

  A typical oxygen sensor includes a cylindrical detection element disposed so as to protrude into the exhaust passage. The detection element has its inner surface exposed to the atmosphere and its outer surface exposed to exhaust gas. Moreover, the detection element has a solid electrolyte in which electrodes are coated on the inner and outer surfaces. The solid electrolyte is a solid substance that can move in the state in which oxygen is ionized. For example, zirconia is used as an oxygen sensor.

  If there is a difference in oxygen partial pressure between the atmosphere inside the sensing element and the exhaust gas outside, oxygen on the higher oxygen partial pressure (usually the atmosphere side) is ionized to reduce the difference in partial pressure. It moves through the solid electrolyte to the low oxygen partial pressure side (usually the exhaust gas side). Oxygen molecules receive tetravalent electrons in the process of ionization, and emit tetravalent electrons in the process of returning from the ionized state to the molecule. Therefore, movement of electrons occurs at the electrodes on the inner and outer surfaces of the detection element in accordance with the movement of oxygen, and as a result, an electromotive force is generated in the detection element.

  Thus, the oxygen sensor generates an electromotive force according to the difference in oxygen partial pressure between the atmosphere and the exhaust gas. More specifically, the oxygen concentration of the exhaust gas decreases (that is, the air-fuel ratio of the exhaust gas becomes richer). It generates a large electromotive force.

  On the other hand, in the field of automobiles, the laws and regulations of each country require that the abnormality of the catalyst and the sensor be diagnosed in an on-board state (on-board) in order to prevent the vehicle running with deteriorated exhaust emission. Accordingly, various diagnostic devices have been proposed for oxygen sensors.

  For example, in Patent Document 1, a resistance component and a capacitance component are calculated based on the AC impedance of the detection element, and the calculated resistance component and capacitance component are individually compared with a reference value. An apparatus for diagnosing abnormality of a detection element based on a combination of the above is disclosed.

JP 2009-25102 A

  Some oxygen sensors have a diffusion layer that controls the diffusion rate of gas in the outer layer portion of the detection element. As a result of intensive studies, the present inventors have found that an abnormality diagnosis of the oxygen sensor can be performed by utilizing such a unique phenomenon that occurs when the oxygen sensor malfunctions.

  Therefore, an object of the present invention is to provide an abnormality diagnosis device suitable for an oxygen sensor having a diffusion layer in the outer layer portion of a detection element.

According to one aspect of the invention,
An oxygen sensor abnormality diagnosis device having a diffusion layer that controls the diffusion rate of gas in the outer layer portion of the detection element,
Obtained when a plurality of positive DC voltages obtained by applying a plurality of positive DC voltages of different magnitudes to the detection element and a plurality of negative DC voltages of different magnitudes are applied to the detection elements. Detecting means for detecting at least one of a plurality of negative DC currents,
Determination means for determining whether the oxygen sensor is normal or abnormal based on a rate of change of at least one of a plurality of positive DC currents and a plurality of negative DC currents detected by the detection means;
An oxygen sensor abnormality diagnosis device is provided.

  Here, the rate of change of the plurality of positive DC currents refers to the rate or degree of change of the plurality of positive DC currents with respect to the change of the plurality of positive DC voltages. The change rate of the plurality of negative DC currents refers to the rate or degree of change of the plurality of negative DC currents with respect to the change of the plurality of negative DC voltages.

Preferably, the detection means detects at least the plurality of positive direct currents,
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of positive DC currents is smaller than a predetermined first threshold, and specifies clogging of the diffusion layer as the type of abnormality.

Preferably, the detection means detects at least the plurality of negative direct currents,
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of negative DC currents is greater than a predetermined second threshold, and specifies cracks in the detection element as the type of abnormality.

Preferably, the detection means detects both the plurality of positive DC currents and the plurality of negative DC currents,
The determination means is configured to change the oxygen when the rate of change of the plurality of positive DC currents is equal to or greater than a predetermined first threshold and the rate of change of the plurality of negative DC currents is equal to or less than a predetermined second threshold. The sensor is determined to be normal.

Preferably, the oxygen sensor is provided in an exhaust passage of the internal combustion engine,
The detection means is configured to stop the fuel supply to the combustion chamber of the internal combustion engine and the temperature of the detection element is higher than a predetermined reference temperature, or when the internal combustion engine is in an idle operation state and is empty. When the fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, at least one of the plurality of positive DC currents and the plurality of negative DC currents is detected.

According to another aspect of the invention,
An oxygen sensor abnormality diagnosis device having a diffusion layer that controls the diffusion rate of gas in the outer layer portion of the detection element,
Obtained when a plurality of positive DC resistances obtained when a plurality of positive DC voltages of different magnitudes are applied to the detection element and a plurality of negative DC voltages of different magnitudes are applied to the detection elements. Detecting means for detecting at least one of a plurality of negative DC resistances;
Determination means for determining whether the oxygen sensor is normal or abnormal based on a change rate of at least one of a plurality of positive DC resistances and a plurality of negative DC resistances detected by the detection means;
An oxygen sensor abnormality diagnosis device is provided.

  Here, the change rate of the plurality of positive DC resistances refers to the rate or degree of change of the plurality of positive DC resistances with respect to the change of the plurality of positive DC voltages. The change rate of the plurality of negative DC resistances refers to the rate or degree of change of the plurality of negative DC resistances with respect to the change of the plurality of negative DC voltages.

Preferably, the detection means detects at least the plurality of positive DC resistances,
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of positive DC resistances is greater than a predetermined third threshold, and specifies clogging of the diffusion layer as the type of abnormality.

Preferably, the detection means detects at least the plurality of negative DC resistances,
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of negative DC resistances is smaller than a predetermined fourth threshold, and identifies cracks in the detection element as the type of abnormality.

Preferably, the detection means detects both the plurality of positive DC resistances and the plurality of negative DC resistances,
The determination means includes the oxygen generator when the rate of change of the plurality of positive DC resistances is not more than a predetermined third threshold value and the rate of change of the plurality of negative DC resistances is not less than a predetermined fourth threshold value. The sensor is determined to be normal.

Preferably, the oxygen sensor is provided in an exhaust passage of the internal combustion engine,
The detection means is configured to stop the fuel supply to the combustion chamber of the internal combustion engine and the temperature of the detection element is higher than a predetermined reference temperature, or when the internal combustion engine is in an idle operation state and is empty. When the fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, at least one of the plurality of positive DC resistances and the plurality of negative DC resistances is detected.

  According to the present invention, an excellent effect that an abnormality diagnosis device suitable for an oxygen sensor having a diffusion layer in an outer layer portion of a detection element can be provided is exhibited.

1 is a schematic view showing an internal combustion engine according to an embodiment of the present invention. It is a fragmentary sectional view which shows the attachment state of an oxygen sensor. It is an expanded sectional view of the detection element of an oxygen sensor. It is a graph which shows the output characteristic of an oxygen sensor. FIG. 4 is a detailed view of a V portion in FIG. 3, schematically showing a configuration of a power supply circuit for a detection element. It is a graph which shows the relationship between the applied voltage at the time of positive DC voltage application, and an electric current. It is a graph which shows the relationship between the applied voltage at the time of negative DC voltage application, and an electric current. It is a flowchart regarding the abnormality diagnosis processing of the first embodiment. It is a graph which shows the relationship between the applied voltage at the time of positive DC voltage application, and resistance. It is a graph which shows the relationship between the applied voltage at the time of negative DC voltage application, and resistance. It is a graph which shows the relationship between an applied voltage and resistance change rate. It is a flowchart regarding the abnormality diagnosis processing of the second embodiment. It is an image figure which shows the relationship between the applied voltage at the time of positive DC voltage application, and an electric current.

  A configuration of an internal combustion engine according to an embodiment of the present invention will be described with reference to FIG. The internal combustion engine (engine) 10 of this embodiment is for automobiles, and is a spark ignition internal combustion engine, specifically a gasoline engine. The intake passage 11 of the internal combustion engine 10 is provided with a throttle valve 15 (in this embodiment, electronically controlled) whose variable passage area is variable, and the amount of air taken in through the air cleaner 14 is adjusted by opening degree control. Is done. The amount of inhaled air (intake air amount) is detected by the air flow meter 16. The air sucked into the intake passage 11 is mixed with fuel injected from an injector 17 provided downstream of the throttle valve 15 and then sent to a combustion chamber in the engine body 12 where it is burned.

  The exhaust passage 13 through which exhaust gas generated by combustion in the combustion chamber is sent is provided with a three-way catalyst 18 for purifying harmful components in the exhaust gas, and an oxygen sensor 19 is provided upstream thereof.

  The three-way catalyst 18 removes all the main harmful components (HC, CO, NOx) in the exhaust gas only when the air-fuel ratio of the exhaust gas supplied thereto is only in a narrow range (window) near the stoichiometric air-fuel ratio (stoichiometric). Purify efficiently. In order for such a three-way catalyst 18 to function effectively, strict control is required to adjust the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber to the center of the window.

  The air-fuel ratio is controlled by an electronic control unit (hereinafter referred to as “ECU”) 22. The ECU 22 includes, for example, a CPU, ROM, RAM, B (battery backup). A RAM, a nonvolatile memory, an input port, an output port, an A / D converter, and a D / A converter are provided. The ECU 22 includes various sensors including the air flow meter 16, the oxygen sensor 19, an accelerator opening sensor 21 that detects the amount of depression of the accelerator pedal (accelerator opening), and a rotation speed sensor 23 that detects the engine rotation speed. Detection signal is input. The ECU 22 controls the air-fuel ratio feedback control by driving the throttle valve 15 and the injector 17 in accordance with the operating conditions of the internal combustion engine 10 and the vehicle ascertained from the detection signals of the sensors. . The outline of the air-fuel ratio feedback control is as follows.

  First, the ECU 22 obtains a required amount of intake air amount grasped according to the detection result of the accelerator opening degree and the engine rotation speed, and adjusts the opening degree of the throttle valve 15 so as to obtain the intake air amount corresponding thereto. On the other hand, an amount of fuel sufficient to obtain the stoichiometric air-fuel ratio is obtained from the actually measured value of the intake air amount detected by the air flow meter 16, thereby adjusting the fuel injection amount from the injector 17. Thereby, the air-fuel ratio of the air-fuel mixture burned in the combustion chamber 12 can be brought close to the stoichiometric air-fuel ratio to some extent. However, that alone is not sufficient for the required highly accurate air-fuel ratio control.

  Therefore, the ECU 22 feedback corrects the fuel injection amount from the injector 17 based on the actual value of the air / fuel ratio obtained from the detection result of the oxygen sensor 19 to ensure the required accuracy of the air / fuel ratio control.

  As described above, in this exhaust gas purification system, the so-called air-fuel ratio feedback control that feedback-corrects the fuel injection amount according to the detection result of the oxygen sensor 19 is performed, so that the air-fuel ratio of the air-fuel mixture is made close to the theoretical air-fuel ratio. To maintain a high exhaust gas purification rate.

  As shown in FIGS. 2 and 3, the oxygen sensor 19 includes a cylindrical detection element 31 arranged so as to protrude into the exhaust passage 13. The detection element 31 has its inner surface exposed to the atmosphere (air), and its outer surface is exposed to exhaust gas flowing through the perforated sensor cover 32. The detection element 31 includes a solid electrolyte 37 having electrodes 33A and 33B coated on the inner and outer surfaces. That is, the detection element 31 includes a solid electrolyte 37 sandwiched between a pair of electrodes 33A and 33B. The solid electrolyte 37 is a solid substance that can move inside the oxygen ionized state. For example, zirconia is used as an oxygen sensor. The electrodes 33A and 33B are made of a porous material such as platinum.

  The atmosphere chamber 34 inside the detection element 31 communicates with the outside through an atmosphere passage (not shown) provided in the sensor and an atmosphere hole 35 formed in the sensor body, and the atmosphere can be led out and in. The atmospheric chamber 34 is provided with a heater 36 for heating the detection element 31 and activating it early, and the heater 36 is energized and controlled by the ECU 22.

  If there is a difference in oxygen partial pressure between the atmosphere inside the detection element 31 and the exhaust gas outside, oxygen on the higher oxygen partial pressure (usually the atmosphere side) is ionized to reduce the difference in partial pressure. Then, it passes through the solid electrolyte 37 and moves to the side having low oxygen partial pressure (usually the exhaust side). Oxygen molecules receive tetravalent electrons in the process of ionization, and emit tetravalent electrons in the process of returning from the ionized state to the molecule. Therefore, movement of electrons occurs between the inner and outer electrodes in accordance with the movement of oxygen, and as a result, an electromotive force is generated in the detection element 31.

  Thus, the oxygen sensor 19 generates an electromotive force according to the difference in oxygen partial pressure between the atmosphere and the exhaust gas. More specifically, as the oxygen concentration of the exhaust gas decreases (that is, the exhaust gas outside the detection element 31). Larger electromotive force is generated (the richer the air-fuel ratio). Here, since oxygen ions are directed from the inner electrode 33A through the detection element 31 to the outer electrode 33B, the direction of the current is reversed, and the inner electrode 33A is positive for the external device connected to both electrodes. The outer electrode 33B becomes a negative electrode. Hereinafter, the inner electrode 33A is also referred to as an atmospheric electrode, and the outer electrode 33B is also referred to as an exhaust electrode.

  Incidentally, there are various types of oxygen sensors such as those using a plate-shaped detection element and those using a material other than zirconia for the detection element. Many of them are configured to detect the oxygen partial pressure of the exhaust gas based on the same detection principle as that of the sensor exemplified above. That is, the detection element arranged to isolate the reference gas (atmosphere) and the exhaust gas generates an electromotive force according to the difference in oxygen partial pressure of the exhaust gas with respect to the reference gas. The present invention is applicable to such various types of oxygen sensors.

  The output characteristics of the oxygen sensor 19 are illustrated in FIG. As shown, the output voltage of the oxygen sensor 19 changes in a switching manner with a theoretical air-fuel ratio A / Fs (for example, 14.6) as a boundary, and the air-fuel ratio A of the exhaust gas (atmospheric gas) supplied to the oxygen sensor 19 In a region where / F is leaner than the stoichiometric air-fuel ratio A / Fs (A / F> A / Fs, hereinafter also referred to as lean air-fuel ratio), it shows a small voltage of about 0.1 V, richer than the stoichiometric air-fuel ratio A / Fs In such a region (A / F <A / Fs, hereinafter also referred to as a rich air-fuel ratio), a relatively high voltage of about 0.9 V is shown. Here, a sensor output of 0.45 V is preset as a rich / lean determination threshold value, and an actual sensor output voltage is compared with the threshold value to determine whether the air-fuel ratio is rich or lean. Note that the magnitude of the sensor output voltage in each region of the oxygen sensor 19 may vary depending on the temperature state of the detection element 31.

  In an internal combustion engine that performs air-fuel ratio feedback control for the purpose of combustion only at the stoichiometric air-fuel ratio (stoichiometric combustion), the output voltage changes in a switching manner with respect to the stoichiometric air-fuel ratio as in this embodiment. Oxygen sensors are often used. Such sensors have a lower resolution, either richer than the stoichiometric air-fuel ratio and leaner than the stoichiometric air-fuel ratio, but are often sufficient to perform only the stoichiometric combustion. On the other hand, in an internal combustion engine that performs combustion at a wider range of air-fuel ratio, such as combustion at a lean air-fuel ratio, the output value linearly changes according to the air-fuel ratio of the exhaust gas, and has higher resolution. An oxygen sensor may be used. The present invention is also applicable to such an oxygen sensor or an A / F (air / fuel ratio) sensor.

  3 and 5, a diffusion layer 38, a catalyst layer 39, and a trap layer 40 are sequentially stacked on the outside of the exhaust electrode 33B of the detection element 31 from the inside to the outside. Yes. The diffusion layer 38, the catalyst layer 39, and the trap layer 40 form an outer layer portion 41 formed outside the exhaust electrode 33B or interposed between the exhaust electrode 33B and the gas outside the exhaust electrode 33B.

The diffusion layer 38 controls the gas diffusion rate, and is formed of a porous material such as alumina (Al ₂ O ₃ ) or magnesium oxide MgO. When the gas passes through the diffusion layer 38 toward the exhaust electrode 33B, resistance is given to the gas and the gas is rate-controlled.

The catalyst layer 39 is for oxidizing and removing unburned components (especially H ₂ ) in the gas and suppressing the output deviation of the sensor. The catalyst layer 39 is configured by supporting catalyst noble metal particles such as Pt, Pd, and Rh on a porous material such as alumina.

  The trap layer 40 is for protecting each layer inside thereof and for preventing substances other than gas from reaching the exhaust electrode 33B. The trap layer 40 is formed of a porous material such as alumina.

  Although the catalyst layer 39 is separately provided in addition to the diffusion layer 38 here, the catalyst layer 39 may be omitted. In this case, catalyst noble metal particles may be supported on the diffusion layer 38, and the function of the catalyst layer 39 may be provided on the diffusion layer 38. The trap layer 40 may be omitted.

  Next, abnormality diagnosis of the oxygen sensor 19 will be described.

[First embodiment]
In the first embodiment described below, the ECU 22 applies a plurality of positive DC voltages having different magnitudes to the detection element 31 to detect a plurality of positive DC currents corresponding to each of the positive DC voltages. A plurality of negative direct current voltages having different magnitudes are applied to a plurality of negative direct currents corresponding to the respective negative direct current voltages.

  This detection method is illustrated and described. FIG. 5 schematically shows a configuration of a power supply circuit that can apply a positive DC voltage and a negative DC voltage to the detection element 31. A power supply to which a positive DC voltage is applied is referred to as a positive DC power supply and is represented by B +. Similarly, a power supply to which a negative DC voltage is applied is referred to as a negative DC power supply and is represented by B−. These power sources can be switched and selected by switching circuits S1 and S2 built in the ECU 22. Note that the positive DC power supply B + and the negative DC power supply B− may be shared, and both may be switched only by switching.

In the illustrated example, only the positive DC power supply B + is selected, the positive (+) pole of the positive DC power supply B + is connected to the atmospheric electrode 33A, and the negative (−) pole of the positive DC power supply B + is connected to the exhaust electrode 33B. . This state is defined as “a state in which a positive DC voltage is applied to the detection element 31”. In a state where a positive DC voltage is applied, oxygen O ₂ is pumped from the exhaust electrode 33B toward the atmospheric electrode 33A as shown in the figure.

On the other hand, when only the negative DC power supply B- is selected, the negative (-) pole of the negative DC power supply B- is connected to the atmospheric electrode 33A, and the positive (+) pole of the negative DC power supply B- is connected to the exhaust electrode 33B. Is done. This state is defined as “a state in which a negative DC voltage is applied to the detection element 31”. When a negative DC voltage is applied, the direction of oxygen O ₂ pumping is reversed.

  When a positive DC voltage is applied to the detection element 31 (also referred to as “+ DC voltage application”), the diffusion layer 38 limits the amount of oxygen reaching the exhaust electrode 33B to some extent. On the other hand, when a negative DC voltage is applied to the detection element 31 (also referred to as “-DC voltage application”), the amount of oxygen is not limited because there is no diffusion layer 38 on the atmospheric electrode 33A side. .

  When a + DC voltage is applied, the amount of oxygen reaching the exhaust electrode 33B changes according to the state of the diffusion layer 38. That is, if the diffusion layer 38 is normal, a relatively large amount of oxygen can reach the exhaust electrode 33B, but if the diffusion layer 38 is clogged, a relatively small amount of oxygen can reach the exhaust electrode 33B. That is, as the degree of clogging of the diffusion layer 38 increases, the amount of oxygen per unit time reaching the exhaust electrode 33B decreases, and the current value obtained when + DC voltage is applied (this is referred to as “positive DC current”). Get smaller. The cause of the clogging of the diffusion layer 38 is mainly soot, lead, manganese, sulfur, etc. contained in the exhaust gas.

  FIG. 6 is a graph showing the relationship between applied voltage and current when + DC voltage is applied. In the figure, “normal” means a normal state in which the diffusion layer 38 is not clogged, and “large clogging”, “during clogging”, and “small clogging” respectively indicate the clogging in the diffusion layer 38. When the level of clogging is large, medium or small. As shown in the figure, the larger the degree of clogging in the diffusion layer 38, the smaller the current value for a certain applied voltage.

  Therefore, it is conceivable to apply a positive DC voltage of a predetermined magnitude to the detection element 31 and detect an abnormality of the oxygen sensor 19, particularly an abnormal clogging of the diffusion layer 38, based on the current value obtained at this time. .

  However, the current value with respect to the applied voltage may vary due to variations in the detection element 31 itself and the control element temperature. Therefore, if only a current value corresponding to a single applied voltage is used as a basis for diagnosis, there is a possibility that a diagnosis error becomes large and it is difficult to distinguish between normal and abnormal.

  On the other hand, according to the relationship of FIG. 6, the greater the degree of clogging in the diffusion layer 38, the smaller the increase rate (gradient) of the current value with respect to the increase in applied voltage.

  Therefore, paying attention to this tendency, in the first embodiment, as shown in FIG. 6, first, a plurality of positive DC voltages having different sizes are applied to the detection element 31, and a plurality of positive DC currents corresponding to each of them are applied. Is detected. Based on the detected change rates of the plurality of positive DC currents, it is determined whether the oxygen sensor 19 is normal or abnormal.

  Here, the rate of change of the plurality of positive DC currents refers to the rate or degree of change of the plurality of positive DC currents with respect to the change of the plurality of positive DC voltages. In the example shown in FIG. 6, eight values of positive DC current corresponding to eight values of positive DC voltage are detected. That is, eight points of data are acquired. In particular, in this example, since the positive DC current tends to increase as the positive DC voltage increases, the rate of change can be rephrased as an increase rate.

  The rate of change can be arbitrarily defined. For example, an average gradient can be calculated by applying the least square method or the like to all the eight data points, and this gradient can be defined as the rate of change. Or, more simply, a gradient between two predetermined data points out of eight points can be calculated, and this gradient can be defined as a rate of change. In this case, only two points of data may be acquired from the beginning. In any case, in order to obtain the rate of change, it is necessary to acquire at least two points of data.

  By comparing this rate of change with a predetermined first threshold value, it is possible to determine whether there is a clogging abnormality. That is, when the rate of change is smaller than the first threshold, there is a clogging abnormality, and when the rate of change is greater than or equal to the first threshold, there is no clogging abnormality.

  In this way, since the rate of change of positive DC current is used as the basis of diagnosis, the influence of current value variations due to variations in the detection element 31 itself and the control element temperature is reduced, diagnostic errors are reduced, and diagnostic accuracy is improved. Improvement is possible.

  On the other hand, when -DC voltage is applied, the relationship between the applied voltage and the current does not substantially change according to the state of the diffusion layer 38, but the relationship between both changes according to the state of the detection element 31. It has been found.

  FIG. 7 is a graph showing the relationship between applied voltage and current when a -DC voltage is applied. The values of applied voltage and current are expressed as absolute values. In the figure, “normal” refers to a normal case in which the detection element 31 is not cracked (referred to as “element crack”), and “small crack” and “large crack” respectively indicate cracks in the detection element 31. When the degree is small and large. In the test, a hole is intentionally made in the detection element 31 to reproduce a crack in the detection element 31, and the size of the hole is changed to change the degree of the crack. Here, the case where a hole having a diameter of φ0.3 mm is formed is “small crack”, and the case where a hole having a diameter of 2.0 mm is formed is “large crack”.

  As shown in the figure, the current value for a constant applied voltage tends to increase as the degree of element cracking increases. Therefore, it is conceivable to apply a negative DC voltage of a predetermined magnitude to the detection element 31 and detect an abnormality of the oxygen sensor 19, particularly an element cracking abnormality, based on the current value obtained at this time.

  However, as described above, in the method using only the current value corresponding to a single applied voltage as the basis of diagnosis, the diagnosis error becomes large and it may be difficult to distinguish between normal and abnormal. On the other hand, according to the relationship of FIG. 7, the greater the degree of element cracking, the greater the increase rate of the current value (the absolute value of the negative DC current) with respect to the increase in the applied voltage (the absolute value of the negative DC voltage). It is in.

  Therefore, paying attention to this tendency, in the first embodiment, as shown in FIG. 7, a plurality of negative DC voltages of different sizes are applied to the detection element 31 to detect a plurality of negative DC currents corresponding to each of them. To do. Then, based on the detected change rates of the plurality of negative DC currents, it is determined whether the oxygen sensor 19 is normal or abnormal.

  As described above, the rate of change of the plurality of negative DC currents refers to the rate or degree of change of the plurality of negative DC currents with respect to the change of the plurality of negative DC voltages. Also in the example shown in FIG. 7, eight values of negative DC current corresponding to eight values of negative DC voltage are detected, and eight points of data are acquired. In particular, in this example, since the negative DC current tends to increase as the negative DC voltage increases, the rate of change can be rephrased as an increase rate. The rate of change can be arbitrarily defined as described above.

  By comparing this rate of change with a predetermined second threshold, it is possible to determine whether or not there is an element crack abnormality. That is, when the rate of change is greater than the second threshold, there is an element crack abnormality, and when the rate of change is less than the second threshold, there is no element crack abnormality.

  As described above, since the rate of change of the negative DC current is used as the basis of diagnosis, the influence of the current value variation due to variations in the detection element 31 itself, the control element temperature, etc. can be reduced, the diagnosis error can be reduced, and the diagnosis accuracy can be reduced. Improvement is possible.

  The clogging abnormality diagnosis based on the positive DC current change rate and the element crack abnormality diagnosis based on the negative DC current change rate described here can be performed independently. For example, regarding the former, when the positive DC current change rate is smaller than the first threshold value, it is determined that the oxygen sensor 19 is abnormal, the clogging of the diffusion layer 38 is specified as the type of abnormality, and the positive DC current change rate is determined. For example, the oxygen sensor 19 is determined to be normal when is equal to or greater than the first threshold. However, it is naturally preferable to perform both because the accuracy and reliability of diagnosis are increased. Further, it is preferable to perform both because not only the normality / abnormality of the oxygen sensor 19 can be determined, but also the type or location of the abnormality can be specified in the case of an abnormality. In this way, it is possible to provide an abnormality diagnosis device suitable for the oxygen sensor 19 having the diffusion layer 38.

  Next, the abnormality diagnosis process according to the first embodiment executed by the ECU 22 will be described with reference to FIG.

  In step S101, it is determined whether a predetermined precondition necessary for performing the diagnosis is satisfied. In order to satisfy the precondition, for example, (1) the basic condition that the detection element 31 of the oxygen sensor 19 is sufficiently high in temperature and the oxygen sensor 19 is activated needs to be satisfied. . For example, the ECU 22 determines that the oxygen sensor 19 has been activated when it detects that the output voltage of the oxygen sensor 19 has been inverted a predetermined number of times between rich and lean.

In addition, in order to satisfy the precondition, it is necessary that the following specific condition (2) or (3) is also satisfied.
(2) The fuel supply to the combustion chamber of the engine is stopped. In this fuel supply stop state, a fuel cut (deceleration fuel cut) state in which fuel injection from the injector 17 is stopped during deceleration, and a fuel injection from the injector 17 is stopped in order to stop the engine intermittently in a hybrid vehicle or the like. A fuel cut state is included.
(3) The engine is in an idle operation state.

  As will be described in detail later, in the abnormality diagnosis process, in order to improve detection accuracy and diagnosis accuracy, diagnosis is executed only when the following control change is performed under the specific conditions.

  If the precondition is not satisfied, the process enters a standby state. If the precondition is satisfied, the process proceeds to step S102. In step S102, the following control change is performed.

  First, when the specific condition (2) is satisfied, the temperature of the detection element 31 is raised from a predetermined reference temperature. That is, the ECU 22 normally feedback-controls the heater 36 of the detection element 31 so that the temperature of the detection element 31 becomes a predetermined reference temperature (for example, 550 ° C.). However, at the time of diagnosis and when the specific condition (2) is satisfied, the heater 36 of the detection element 31 is feedback-controlled so that the temperature of the detection element 31 becomes a predetermined temperature (for example, 700 ° C.) higher than the reference temperature.

  Further, when the specific condition (3) is satisfied, the air-fuel ratio is controlled to a predetermined value (for example, a value in the range of 14.8 to 15.0) leaner than the theoretical air-fuel ratio (for example, 14.6). That is, the ECU 22 normally feedback-controls the fuel injection amount so that the air-fuel ratio detected by the oxygen sensor 19 becomes the stoichiometric air-fuel ratio. However, at the time of diagnosis and when the specific condition (3) is satisfied, the fuel injection amount is controlled so that the air-fuel ratio becomes a predetermined value that is leaner (particularly weakly lean) than the stoichiometric air-fuel ratio. In this forced lean control, the fuel injection amount is feedforward controlled based on the intake air amount detected by the air flow meter 16. The reason for this is that in the case of the oxygen sensor 19 having the Z characteristic (FIG. 4) as in the present embodiment, it is difficult to perform feedback control to an air-fuel ratio other than the stoichiometric air-fuel ratio, and the diagnosis of the oxygen sensor 19 is originally performed. This is because sometimes it is not preferable to perform air-fuel ratio control using the output value.

  The present invention can also be applied to the oxygen sensor 19 provided on the downstream side of the catalyst 18. An oxygen sensor (such as a linear air-fuel ratio sensor) is additionally provided on the upstream side of the catalyst 18, and the air-fuel ratio can be feedback controlled based on the output of the upstream oxygen sensor. In this case, at the time of forced lean control, it is possible to feedback control the air-fuel ratio (specifically, the fuel injection amount) based on the output of the upstream oxygen sensor.

  Next, in step S103, a prohibition flag for prohibiting use of the output voltage of the oxygen sensor 19 within a predetermined time is set. The reason for this prohibition is that when a positive and negative DC voltage is applied to the detection element 31 of the oxygen sensor 19 as described later, the sensor output changes to rich or lean regardless of the oxygen concentration of the exhaust gas.

  Specifically, the use of the output voltage of the oxygen sensor 19 for purposes other than the main diagnosis is prohibited during the application of positive and negative DC voltages, which will be described later, and until a predetermined time has elapsed from the end of the application.

  As a result, when the specific condition (2) is satisfied, another diagnosis (for example, the responsiveness diagnosis of the oxygen sensor 19) that is performed using the output voltage of the oxygen sensor 19 while the fuel supply is stopped is prohibited. Further, when the specific condition (3) is satisfied, other control and processing (for example, air-fuel ratio feedback control and correction value learning processing) performed using the output voltage of the oxygen sensor 19 during the idling operation are prohibited.

  Thus, adverse effects on other controls due to the use of the output voltage of the oxygen sensor 19 (for example, misjudgment in diagnosis using the sensor output, deviation of the control value and the learning value, and accompanying emission deterioration) can be prevented. Can do.

Next, in step S104, a predetermined positive DC voltage to the detecting element 31 is applied, this time, the current flowing through the sensing element 31 to or positive direct current (+ direct current) Ip _n is detected and stored. In this embodiment, the operation of applying a positive DC voltage and detecting the positive DC current Ip is performed A times while changing the voltage value. A is an integer of 2 or more, and is set to 8, for example, in this embodiment. Although the method of changing the voltage value is arbitrary, in this embodiment, the voltage value is gradually increased. At the first detection (n = 1), a predetermined minimum positive DC voltage is applied.

Next, in step S105, a predetermined negative DC voltage is applied to the detection element 31, and a current flowing through the detection element 31, that is, a negative DC current (−DC current) Im _n (however, an absolute value) is detected. Remembered. As described above, in this embodiment, the operation of applying a negative DC voltage and detecting the negative DC current Im is performed A times while changing the voltage value. A is an integer of 2 or more, and is set to 8, for example, in this embodiment. Although the method of changing the voltage value is arbitrary, in this embodiment, the absolute value of the negative DC voltage is gradually increased. At the first detection (n = 1), a negative DC voltage having a predetermined minimum absolute value is applied.

In step S106, it is determined whether A detection has been completed, that is, whether n ≧ A is satisfied.
If not, the process proceeds to step S107, the value of n is increased by 1, and the voltage value of the positive / negative DC voltage is changed to the next predetermined value, and the process returns to step S101. Thus a positive DC current Ip _n and negative direct current Im _n are one after another, will be detected until n = 8.

  On the other hand, if it is established, the process proceeds to step S108, and a positive DC current change rate (+ DC current change rate) LIp is calculated based on the detected data of A positive DC currents Ip. In this embodiment, this calculation is performed as follows. First, from A data, a current value Ip1 corresponding to a predetermined intermediate voltage value (for example, 300 mV) and a current value Ip2 corresponding to the maximum voltage value (for example, 500 mV) are extracted. Then, the ratio Ip2 / Ip1 between the former and latter current values is calculated as the positive DC current change rate LIp.

  In this way, two positive DC currents corresponding to two predetermined positive DC voltages are detected, and the positive DC current corresponding to the larger positive DC voltage is converted into the smaller positive DC voltage. Dividing by the corresponding positive DC current, the positive DC current change rate is calculated.

  However, as described above, the calculation method of the positive DC current change rate LIp is arbitrary, and the positive DC current change rate LIp is calculated using more data or all of the A data. May be.

  Next, the process proceeds to step S109, and a negative DC current change rate (-DC current change rate) LIm is calculated based on the detected data of A negative DC currents Im. In this case as well, the current value Im1 corresponding to a predetermined intermediate voltage value (absolute value, for example, 300 mV) from the A data and the maximum voltage value (absolute value, for example, 500 mV) are similarly supported. The current value Im2 to be extracted is extracted. Then, the ratio Im2 / Im1 between the former and the latter current values is calculated as the negative DC current change rate LIm.

  In this way, two negative DC currents corresponding to two predetermined negative DC voltages are detected, of which the negative DC current corresponding to the negative DC voltage having the larger absolute value is converted to the one having the smaller absolute value. The negative DC current change rate is calculated by dividing by the negative DC current corresponding to the negative DC voltage.

  Next, in step S110, the positive DC current change rate LIp is compared with a predetermined first threshold value α1. When LIp <α1, the process proceeds to step S111, where the oxygen sensor 19 is determined to be abnormal, and clogging of the diffusion layer 38 is specified as the type of abnormality.

  On the other hand, when LIp ≧ α1, the process proceeds to step S112, and the negative DC current change rate LIm is compared with a predetermined second threshold value α2. When LIm> α2, the process proceeds to step S113, where the oxygen sensor 19 is determined to be abnormal, and the crack of the detection element 31 is specified as the type of abnormality.

  On the other hand, if LIm ≦ α2, the process proceeds to step S114, and the oxygen sensor 19 is determined to be normal.

  Here, regarding steps S101 and S102, the reason why the diagnosis is performed only when the specific condition (2) or (3) is satisfied and the reason why the control change corresponding to each of the specific conditions (2) and (3) is performed will be described. To do.

  FIG. 13 is an image diagram showing a relationship between applied voltage and current when a positive DC voltage is applied. A line (characteristic line) a indicates a case where the oxygen sensor is normal, the detection element temperature is the reference temperature, and the atmosphere gas of the oxygen sensor is the atmosphere (corresponding to a fuel supply stop state). In this case, the current has a simple proportional relationship with the applied voltage, and the current increases proportionally as the applied voltage increases.

  On the other hand, line b shows a case where the oxygen sensor is clogged but the detection element temperature is the reference temperature and the atmospheric gas of the oxygen sensor is the atmosphere. In this case, so-called limit current characteristics appear, and as the applied voltage increases, the current increases proportionally at first. However, the increasing tendency decreases from a certain applied voltage value V1, and finally tends to be substantially constant. The reason why such limit current characteristics appear is that the diffusion rate of the gas reaches the upper limit due to clogging of the diffusion layer, and current limitation occurs.

  By the way, the applied voltage value V1 in which the limit current characteristic appears (that is, the proportional increase in the current value stops) is relatively high and is significantly higher than the applied voltage level Vs employed in the present embodiment. Therefore, even if the applied voltage is changed within the applied voltage level Vs in the present embodiment, there is no difference in the current change rate between the normal time and the clogged time.

  However, when the temperature of the detection element is raised from the reference temperature, the line a changes to the line c and the line b changes to the line d. Then, the applied voltage value at which the limit current characteristic appears changes from a value V1 higher than the applied voltage level Vs to a value V2 within the applied voltage level Vs. Therefore, by changing the applied voltage within the applied voltage level Vs adopted in the present embodiment, a clear difference occurs in the current change rate between the normal time and the clogged time, and the detection accuracy and diagnostic accuracy are improved. Will be able to.

  This can be said similarly even when the applied voltage level Vs is expanded to a higher voltage side than the illustrated level or to a lower voltage side. That is, regardless of the width and magnitude of the applied voltage level Vs, it is possible to improve detection accuracy and diagnostic accuracy by raising the detection element temperature.

  In consideration of this point, when calculating the positive direct current change rate LIp, current value data corresponding to at least an applied voltage value that exhibits a limit current characteristic and an applied voltage that is smaller than that is used. Is preferred. By doing so, it becomes possible to make a clear difference in the positive DC current change rate LIp between normal and clogged.

  When calculating the rate of change of positive DC current based on two positive DC currents corresponding to two predetermined positive DC voltages, the larger positive DC voltage is a value larger than the applied voltage that exhibits the limiting current characteristics. The smaller positive DC voltage is preferably set to a value smaller than the applied voltage at which the limit current characteristic appears.

  Although the above explanation was related to clogging detection when a positive DC voltage is applied, the detection accuracy and diagnostic accuracy of the element crack detection when a negative DC voltage is applied are also detected for the same reason as described above. Improvement is possible. That is, even when a negative DC voltage is applied, it is difficult to generate the limit current characteristic when the detection element temperature is the reference temperature, but the limit current characteristic is likely to be generated when the detection element temperature is raised from the reference temperature. Accuracy and diagnostic accuracy can be improved.

  When calculating the negative DC current change rate based on two negative DC currents corresponding to two predetermined negative DC voltages, the negative DC voltage with the larger absolute value is calculated from the applied voltage that shows the limit current characteristics. Also, it is preferable that the absolute value is a large value, and the negative DC voltage having the smaller absolute value is a value having a smaller absolute value than the applied voltage at which the limit current characteristic appears.

  On the other hand, the advantage of performing detection only when the specific condition (2) is satisfied and raising the temperature of the detection element when the specific condition (2) is satisfied is also due to the following reason.

  When a positive or negative DC voltage is forcibly applied to the oxygen sensor as described above, an output irrelevant to the oxygen concentration and air-fuel ratio of the atmospheric gas is output from the oxygen sensor. It is preferable to make a diagnosis when stopping. However, when the fuel supply is stopped, since a large amount of oxygen is present in the atmospheric gas, current limitation due to the diffusion layer is difficult to occur, and it is difficult to determine a change in electrical characteristics due to clogging or element cracking. However, when the temperature of the detection element is raised, the electric resistance of the oxygen sensor decreases, and more current flows when a positive or negative DC voltage is applied. Therefore, current limitation due to the diffusion layer as described above is likely to occur. Therefore, it becomes easy to detect a change in electrical characteristics due to clogging or element cracking, and this can improve detection accuracy and diagnostic accuracy.

  Further, when the temperature of the detection element is raised, current limitation occurs with a smaller applied voltage. In the method of increasing the applied voltage stepwise and detecting the current each time, a difference in current value between the abnormal sensor and the normal sensor appears from a smaller applied voltage. If the applied voltage is increased to an applied voltage at which this difference appears, the number of detected data is smaller when the temperature of the detecting element is increased than when the temperature is not increased. Therefore, the detection time can be shortened.

  Next, in FIG. 13, a line e shows a case where the oxygen sensor is clogged abnormally, the detection element temperature is the reference temperature, and the atmosphere gas of the oxygen sensor is air-fuel ratio 15.0 (ie, weak lean). . On the other hand, unlike the above, line a indicates a case where the oxygen sensor is normal, the detection element temperature is the reference temperature, and the atmospheric gas of the oxygen sensor is air-fuel ratio 15.0.

  In the case of a normal oxygen sensor, even if the atmospheric gas is weakly lean, no limit current is generated, so that the characteristics as shown by line a are obtained. On the other hand, in the case of the clogging abnormality sensor indicated by the line e, even if the applied voltage is increased, the current does not increase from a certain applied voltage value V3. That is, the limit current characteristic appears.

  When it is weakly lean, the applied voltage value at which the limit current characteristic appears becomes a value V3 near the applied voltage level Vs. Therefore, even if the detection element temperature is not raised, the difference in current change rate between the normal sensor and the clogging sensor can be clearly obtained in the vicinity of the applied voltage level Vs by making it weakly lean. And diagnostic accuracy can be improved.

  On the other hand, the advantage of performing detection only when the specific condition (3) is satisfied and making the air-fuel ratio lean when the specific condition (3) is satisfied is also due to the following reason.

  When a positive or negative DC voltage is applied to the oxygen sensor as described above, an output irrelevant to the oxygen concentration and air-fuel ratio of the atmospheric gas is output from the oxygen sensor. Is preferably performed. However, when the fuel supply is stopped, since a large amount of oxygen is present in the atmospheric gas, current limitation due to the diffusion layer is difficult to occur, and it is difficult to determine a change in electrical characteristics due to clogging or element cracking. For this reason, it is preferable to perform the detection when the engine is in an idling state where the influence on the engine control is relatively small. In addition, by forcibly reducing the air-fuel ratio, it is possible to create a situation where current limitation is likely to occur without increasing the temperature of the detection element. Therefore, it becomes easy to detect a change in electrical characteristics due to clogging or element cracking, and this can improve detection accuracy and diagnostic accuracy.

[Second Embodiment]
Next, a second embodiment will be described. The second embodiment is different from the first embodiment in that a resistance value is used instead of a current value. That is, the ECU 22 applies a plurality of positive DC voltages having different sizes to the detection element 31 to detect a plurality of positive DC resistances corresponding to each of the positive DC voltages, and also detects a plurality of negative DC voltages having different sizes. A plurality of negative DC resistances corresponding to the respective DC voltages are detected.

  When the + DC voltage is applied, the greater the degree of clogging of the diffusion layer 38, the smaller the amount of oxygen per unit time reaching the exhaust electrode 33B, the smaller the positive DC current value, and the obtained resistance (which is positive The value of DC resistance is increased.

  FIG. 9 is a graph showing the relationship between applied voltage and resistance when + DC voltage is applied. “Normal”, “Clogged”, “Clogged” and “Clogged” are the same as those in FIG. As shown in the figure, the resistance value with respect to a certain applied voltage tends to increase as the degree of clogging in the diffusion layer 38 increases.

  Therefore, it is possible to make a diagnosis based only on the resistance value corresponding to a single applied voltage. However, with this method, a diagnostic error due to variations in the detection element 31 itself, the control element temperature, and the like increases, and it may be difficult to distinguish between normal and abnormal.

  On the other hand, according to the relationship of FIG. 9, the greater the degree of clogging in the diffusion layer 38, the greater the increase rate (gradient) of the resistance value with respect to the increase in applied voltage.

  Therefore, paying attention to this tendency, in the second embodiment, first, as shown in FIG. 9, a plurality of positive DC voltages of different sizes are applied to the detection element 31, and a plurality of positive DC resistances corresponding to each of them are applied. Is detected. Based on the detected change rates of the plurality of positive DC resistances, it is determined whether the oxygen sensor 19 is normal or abnormal.

  The rate of change of the plurality of positive DC resistances refers to the rate or degree of change of the plurality of positive DC resistances with respect to the change of the plurality of positive DC voltages. In the example shown in FIG. 9, eight points of data are acquired. In particular, in this example, since the positive DC resistance tends to increase as the positive DC voltage increases, the rate of change can be rephrased as an increase rate. The point that the rate of change can be arbitrarily defined is the same as in the first embodiment.

  By comparing this rate of change with a predetermined third threshold value, it is possible to determine whether there is a clogging abnormality. When the rate of change is greater than the third threshold, there is a clogging abnormality, and when the rate of change is less than the third threshold, there is no clogging abnormality.

  As described above, since the rate of change of the positive DC resistance is used as the basis of diagnosis, the influence of the resistance value variation due to variations in the detection element 31 itself and the control element temperature is reduced, thereby reducing the diagnostic error and improving the diagnostic accuracy. Improvement is possible.

  On the other hand, when -DC voltage is applied, the relationship between the applied voltage and the resistance does not substantially change according to the state of the diffusion layer 38, but the relationship between both changes according to the state of the detection element 31. It has been found.

  FIG. 10 is a graph showing the relationship between applied voltage and resistance when a -DC voltage is applied. The applied voltage is expressed as an absolute value. “Normal”, “small crack”, and “large crack” in the figure are the same as those in FIG. As shown in the figure, the greater the degree of element cracking, the smaller the resistance value for a certain applied voltage.

  Therefore, it is possible to make a diagnosis based only on the resistance value corresponding to a single applied voltage. However, with this method, a diagnostic error due to variations in the detection element 31 itself, the control element temperature, and the like increases, and it may be difficult to distinguish between normal and abnormal.

  On the other hand, according to the relationship of FIG. 10, the greater the degree of device cracking, the smaller the increase rate (gradient) of the resistance value with respect to the increase in applied voltage (increase in the absolute value of the negative DC voltage).

  Therefore, paying attention to this tendency, in the second embodiment, as shown in FIG. 10, a plurality of negative DC voltages having different sizes are applied to the detection element 31 and a plurality of resistors corresponding to the negative resistances (which are negatively connected) are applied. DC resistance). Then, based on the detected change rates of the plurality of negative DC resistances, it is determined whether the oxygen sensor 19 is normal or abnormal.

  The change rate of the plurality of negative DC resistances herein also refers to the rate or degree of change of the plurality of negative DC resistances with respect to the change of the plurality of negative DC voltages. In the example shown in FIG. 10, eight points of data are acquired. In particular, in this example, since the negative DC resistance tends to increase as the negative DC voltage increases, the rate of change can be rephrased as an increase rate. The point that the rate of change can be arbitrarily defined is the same as in the first embodiment.

  By comparing this rate of change with a predetermined fourth threshold, it is possible to determine whether or not there is an element crack abnormality. That is, there is an element crack abnormality when the change rate is smaller than the fourth threshold value, and there is no element crack abnormality when the change rate is equal to or greater than the fourth threshold value.

  FIG. 11 shows the relationship between the applied voltage and the resistance change rate when applying a -DC voltage based on the data of FIG. Here, the resistance change rate is based on the resistance value when the applied voltage is 300 mV (or the vicinity thereof) as a reference value, and the vertical axis indicates how much each resistance value when the applied voltage is greater than 300 mV increases relative to the reference value. It is represented by That is, assuming that the reference value is R0 and each resistance value when the applied voltage is greater than 300 mV is Rx, the value on the vertical axis is represented by (Rx / R0) × 100-100 (%). It can be seen from the figure that the rate of change in resistance decreases as the degree of element cracking increases.

  Thus, since the rate of change of the negative DC resistance is used as the basis of diagnosis, the influence of current value variations due to variations in the detection element 31 itself and the control element temperature, etc. is reduced, and diagnostic errors are reduced and diagnostic accuracy is improved. Improvement is possible.

  The clogging abnormality diagnosis based on the positive DC resistance change rate and the element crack abnormality diagnosis based on the negative DC resistance change rate described here can be performed independently. However, it is naturally preferable to perform both because the accuracy and reliability of diagnosis are increased. Further, it is preferable to perform both because not only the normality / abnormality of the oxygen sensor 19 can be determined, but also the type or location of the abnormality can be specified in the case of an abnormality. In this way, it is possible to provide an abnormality diagnosis device suitable for the oxygen sensor 19 having the diffusion layer 38.

  Next, the abnormality diagnosis process according to the second embodiment executed by the ECU 22 will be described with reference to FIG.

Steps S201 to S203 are the same as steps S101 to S103. In step S204, a predetermined positive DC voltage is applied to the detecting element 31, the resistance or positive DC resistance (+ DC resistance) Rp _n of the detecting element 31 at this time is detected and stored. In this embodiment, as in the first embodiment, the operation of applying the positive DC voltage and detecting the positive DC resistance Rp is performed A times while changing the voltage value.

Next, in step S205, a predetermined negative DC voltage is applied to the detection element 31, the resistance or negative DC resistance of the detection element 31 at this time (- DC resistance) Rm _n is detected and stored. Similarly to the above, the operation of applying the negative DC voltage and detecting the negative DC resistance Rm is performed A times while changing the voltage value.

  In step S206, it is determined whether or not A detections have been completed, that is, whether or not n ≧ A is satisfied. If not, the process proceeds to step S207, where the value of n is increased by 1, and the voltage value of the positive / negative DC voltage is changed to the next predetermined value, and the process returns to step S201.

  On the other hand, if it is established, the process proceeds to step S208, and a positive DC resistance change rate (+ DC resistance change rate) LRp is calculated based on the detected data of the A positive DC resistances Rp. A resistance value Rp1 corresponding to a predetermined intermediate voltage value (for example, 300 mV) and a resistance value Rp2 corresponding to the maximum voltage value (for example, 500 mV) are extracted from the A pieces of data. The ratio Rp2 / Rp1 between the former and latter resistance values is calculated as a positive DC resistance change rate LRp. However, other calculation methods are possible.

  Again, two positive DC resistances corresponding to two predetermined positive DC voltages are detected, of which the positive DC resistance corresponding to the larger positive DC voltage corresponds to the smaller positive DC voltage. The positive DC resistance change rate is calculated by dividing by the positive DC resistance. As described above, the larger positive DC voltage is preferably set to a value larger than the applied voltage at which the limit current characteristic appears, and the smaller positive DC voltage is preferably set to a value smaller than the applied voltage at which the limit current characteristic appears. .

  Next, proceeding to step S209, a negative DC resistance change rate (−DC resistance change rate) LRm is calculated based on the detected data of the A negative DC resistances Rm. Similarly, in this case, the resistance value Rm1 corresponding to a predetermined intermediate voltage value (absolute value, for example, 300 mV) from the A data and the maximum voltage value (absolute value, for example, 500 mV) are supported. The resistance value Rm2 to be extracted is extracted. The ratio Rm2 / Rm1 of the former and the latter resistance value is calculated as a negative DC resistance change rate LRm. However, other calculation methods are possible.

  Here again, two negative DC resistances corresponding to two predetermined negative DC voltages are detected, and the negative DC resistance corresponding to the negative DC voltage having the larger absolute value is changed to the one having the smaller absolute value. The negative DC resistance change rate is calculated by dividing by the negative DC resistance corresponding to the negative DC voltage. As described above, the negative DC voltage having the larger absolute value is set to a value having a larger absolute value than the applied voltage that exhibits the limit current characteristic, and the negative DC voltage having the smaller absolute value is applied voltage that exhibits the limit current characteristic. It is preferable that the absolute value is smaller.

  Next, in step S210, the positive DC resistance change rate LRp is compared with a predetermined third threshold value α3 (for example, 1.5). When LRp> α3, the routine proceeds to step S211, where it is determined that the oxygen sensor 19 is abnormal, and clogging of the diffusion layer 38 is specified as the type of abnormality.

  On the other hand, when LRp ≦ α3, the process proceeds to step S212, and the negative DC resistance change rate LRm is compared with a predetermined fourth threshold value α4 (for example, 1.5). When LRm <α4, the process proceeds to step S213, where the oxygen sensor 19 is determined to be abnormal, and the crack of the detection element 31 is specified as the type of abnormality.

  On the other hand, if LRm ≧ α4, the routine proceeds to step S214, where it is determined that the oxygen sensor 19 is normal.

  Although the embodiment of the present invention has been described in detail above, various other embodiments of the present invention are conceivable. For example, the internal combustion engine may be other than for automobiles, and there are no particular restrictions on its use and type. The oxygen sensor can also be applied to other than the internal combustion engine. Each numerical value in the embodiment can be arbitrarily changed. The oxygen sensor is not limited to the cup type or the cylinder type as described above, and may be, for example, a plate type.

  The embodiment of the present invention is not limited to the above-described embodiment, and includes all modifications, applications, and equivalents included in the concept of the present invention defined by the claims. Therefore, the present invention should not be construed as being limited, and can be applied to any other technique belonging to the scope of the idea of the present invention.

19 Oxygen sensor 22 Electronic control unit (ECU)
31 detection element 33A, 33B electrode 37 solid electrolyte 38 diffusion layer 41 outer layer part

Claims (10)

An oxygen sensor abnormality diagnosis device having a diffusion layer that controls the diffusion rate of gas in the outer layer portion of the detection element,
Obtained when a plurality of positive DC voltages obtained by applying a plurality of positive DC voltages of different magnitudes to the detection element and a plurality of negative DC voltages of different magnitudes are applied to the detection elements. Detecting means for detecting at least one of a plurality of negative DC currents,
Determination means for determining whether the oxygen sensor is normal or abnormal based on a rate of change of at least one of a plurality of positive DC currents and a plurality of negative DC currents detected by the detection means;
An oxygen sensor abnormality diagnosis device comprising: The detecting means detects at least the plurality of positive direct currents;
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of positive DC currents is smaller than a predetermined first threshold, and specifies clogging of the diffusion layer as the type of abnormality. The oxygen sensor abnormality diagnosis device according to claim 1. The detecting means detects at least the plurality of negative direct currents;
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of negative DC currents is greater than a predetermined second threshold, and specifies cracks in the detection element as the type of abnormality. The abnormality diagnosis apparatus for an oxygen sensor according to claim 1 or 2, The detecting means detects both the plurality of positive DC currents and the plurality of negative DC currents;
The determination means is configured to change the oxygen when the rate of change of the plurality of positive DC currents is equal to or greater than a predetermined first threshold and the rate of change of the plurality of negative DC currents is equal to or less than a predetermined second threshold. The oxygen sensor abnormality diagnosis device according to any one of claims 1 to 3, wherein the sensor is determined to be normal. The oxygen sensor is provided in an exhaust passage of an internal combustion engine;
The detection means is configured to stop the fuel supply to the combustion chamber of the internal combustion engine and the temperature of the detection element is higher than a predetermined reference temperature, or when the internal combustion engine is in an idle operation state and is empty. 5. When the fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio, at least one of the plurality of positive DC currents and the plurality of negative DC currents is detected. The oxygen sensor abnormality diagnosis device according to Item. An oxygen sensor abnormality diagnosis device having a diffusion layer that controls the diffusion rate of gas in the outer layer portion of the detection element,
Obtained when a plurality of positive DC resistances obtained when a plurality of positive DC voltages of different magnitudes are applied to the detection element and a plurality of negative DC voltages of different magnitudes are applied to the detection elements. Detecting means for detecting at least one of a plurality of negative DC resistances;
Determination means for determining whether the oxygen sensor is normal or abnormal based on a change rate of at least one of a plurality of positive DC resistances and a plurality of negative DC resistances detected by the detection means;
An oxygen sensor abnormality diagnosis device comprising: The detecting means detects at least the plurality of positive DC resistances;
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of positive DC resistances is greater than a predetermined third threshold, and identifies clogging of the diffusion layer as the type of abnormality. The oxygen sensor abnormality diagnosis device according to claim 6. The detection means detects at least the plurality of negative DC resistances,
The determination means determines that the oxygen sensor is abnormal when the rate of change of the plurality of negative DC resistances is smaller than a predetermined fourth threshold, and specifies cracks in the detection element as the type of abnormality. The oxygen sensor abnormality diagnosis device according to claim 6 or 7, characterized in that: The detecting means detects both the plurality of positive DC resistances and the plurality of negative DC resistances;
The determination means includes the oxygen generator when the rate of change of the plurality of positive DC resistances is not more than a predetermined third threshold value and the rate of change of the plurality of negative DC resistances is not less than a predetermined fourth threshold value. The oxygen sensor abnormality diagnosis apparatus according to any one of claims 6 to 8, wherein the sensor is determined to be normal. The oxygen sensor is provided in an exhaust passage of an internal combustion engine;
The detection means is configured to stop the fuel supply to the combustion chamber of the internal combustion engine and the temperature of the detection element is higher than a predetermined reference temperature, or when the internal combustion engine is in an idle operation state and is empty. The at least one of the plurality of positive DC resistances and the plurality of negative DC resistances is detected when the fuel ratio is controlled to be leaner than the stoichiometric air-fuel ratio. The oxygen sensor abnormality diagnosis device according to Item.