JP2009031213A - Device for diagnosing abnormality of oxygen sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a time needed for detecting abnormality of an oxygen sensor and accuracy in abnormality detection of an oxygen sensor. <P>SOLUTION: When the temperature of a detection element is greater than a reference temperature value (S40), the presence of abnormality is determined (S60). Frequency characteristics of an oxygen sensor shift toward a high-frequency side as the temperature thereof increases. Frequencies when the peak of a capacitance component in each region of the characteristic curve appears are relatively higher as the temperature increases. By determining the presence of abnormality from the characteristic when the temperature of the detection element is greater than the reference temperature value, the peak of the capacitance component can be detected by applying an alternating current signal with a frequency higher than that at a normal temperature. Thus, an alternating current signal with a predetermined cycle number for detecting the peak can be applied in a short time and the time needed for detecting abnormality can be reduced. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an oxygen sensor abnormality diagnosis device, and more particularly to an oxygen sensor abnormality diagnosis device that is provided in an exhaust passage of an internal combustion engine and generates an electromotive force according to the oxygen concentration of exhaust gas.

  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 feedback control to bring it closer 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 (air), and its outer surface is exposed to exhaust gas flowing through the sensor cover. The detection element is formed of a solid electrolyte having inner and outer surfaces covered with electrodes. 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.

  As shown in FIG. 12, the equivalent circuit of this oxygen sensor includes a resistance R0 such as a lead wire, an electrolyte bulk resistance Rb, a bulk capacitance Cb, an electrolyte grain boundary resistance Rg, a grain boundary capacitance Cg, and an electrode interface. It is considered to be composed of a resistor Re and an electrode interface capacitance Ce.

  FIG. 13 shows the impedance characteristics of the oxygen sensor. The horizontal axis represents the real part Z ′ of the impedance Z, and the vertical axis represents the imaginary part Z ″. The impedance Z of the oxygen sensor element is represented by Z = Z ′ + jZ ″. As shown in FIG. 13, when the frequency of the applied voltage (corresponding to the angular velocity ωn) is gradually increased, the imaginary part Z ″ of the electrode interface, that is, the electrostatic capacitance component, is considered to indicate the state of the electrode interface. In the electrode interface characteristic region A, a semicircle is drawn and converges to 0, and in the electrolyte part characteristic region B, which is considered to indicate the state of the electrolyte part, two semicircles of a grain boundary resistance part and a particle resistance part It is known that the region A in the characteristic curve converges to 0. When the detection element deteriorates, the portion of region A in this characteristic curve is Z′-Z, as shown by a one-dot chain line in FIG. "In order to shift to the upper right side (resistance component and capacitance component increase side) in the plane, the peak of the capacitance component in this region A is detected and compared with the normal value, thereby detecting the abnormality of the detection element. Can be detected

  By the way, in the devices disclosed in Patent Documents 1 and 2, the first frequency band in which the electrolyte impedance of the oxygen concentration detection element should be detected in order to detect the deterioration of the oxygen concentration detection element of the air-fuel ratio (A / F) sensor. , And an AC voltage in a second frequency band in which the electrode interface impedance is to be detected individually, and an AC impedance is detected for each of the first and second frequency bands, and from the difference between these impedance values, Detects the presence or absence of abnormalities due to deterioration.

  Further, in the apparatus disclosed in Patent Document 3, in order to detect the deterioration of the oxygen concentration detection element of the A / F sensor, when the air-fuel ratio is rich or lean, the element impedance is detected, and the current is detected from the corresponding element temperature. Is detected and compared with a reference current value to detect a sensor abnormality.

JP 2000-46780 A Japanese Patent No. 3570274 JP 2000-193636 A

  However, the devices of Patent Documents 1 to 3 all relate to the A / F sensor, and there is no conventional device that performs the same abnormality diagnosis for the oxygen sensor. In addition, there is still room for improvement in the time and accuracy required for detecting an abnormality. For example, the electrode interface characteristics at an element temperature of 550 ° C. appear in the region of 1 kHz to less than 0.01 Hz. To accurately measure impedance in this frequency region, it is necessary to apply a voltage with a frequency of about 0.1 Hz. In this case, since one cycle is as long as 10 seconds, it is required to shorten the time required for abnormality diagnosis. In addition, since the electrical characteristics of the electrode interface portion change depending on the oxygen concentration of the atmosphere and the electrode interface impedance becomes small in a rich atmosphere, it is desirable to suppress the influence on the abnormality diagnosis.

  Therefore, an object of the present invention is to shorten the time required for detecting an abnormality of the oxygen sensor and to improve the accuracy of detecting the abnormality of the oxygen sensor.

  According to a first aspect of the present invention, there is provided an abnormality diagnosis apparatus for an oxygen sensor, comprising: detection means for detecting an electrical characteristic by applying an AC voltage to a detection element of an oxygen sensor; An oxygen sensor abnormality diagnosis apparatus comprising: a determination unit that determines whether or not there is an abnormality, wherein the determination unit performs the determination when the temperature of the detection element is greater than a predetermined reference temperature value. It is characterized by.

  The frequency characteristic of the detection element of the oxygen sensor shifts to a higher frequency side as the temperature is higher, and the frequency at which the peak of the capacitance component in each region of the characteristic curve is relatively higher as the temperature is higher. By utilizing this property, in the present invention, when the temperature of the detection element is larger than a predetermined reference temperature value, the presence / absence of an abnormality is determined. Therefore, by applying an AC signal having a frequency higher than the normal temperature, The peak of the capacitance component can be detected. Therefore, in the first aspect of the present invention, it is possible to apply an alternating current signal having a predetermined number of cycles for detection in a short time, thereby shortening the time required for detecting an abnormality.

  According to a second aspect of the present invention, there is provided an oxygen sensor abnormality diagnosis device comprising: a detecting means for detecting an electrical characteristic by applying an AC voltage to a detecting element of an oxygen sensor; and a detecting element based on a detection value of the detecting means. An apparatus for diagnosing an abnormality of an oxygen sensor comprising a determination means for determining the presence or absence of an abnormality, wherein the influence of the output voltage of the detection element is determined based on the output voltage of the detection element when the AC voltage is not applied. And a correction unit that corrects the detection value of the electrical characteristic so as to suppress the determination, wherein the determination unit performs the determination based on the detection value of the electrical characteristic corrected by the correction unit. And

  In the second aspect of the present invention, the correcting means suppresses the influence of the output voltage of the detection element based on the output voltage of the detection element when no AC voltage is applied for detection of electrical characteristics. Since the detection value of the electrical characteristic is corrected, the accuracy of abnormality detection of the oxygen sensor can be improved.

  The configuration of a vehicle internal combustion engine to which the present invention is applied will be described with reference to FIG. 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 provided, and the amount of air taken in through the air cleaner 14 is adjusted by opening degree control. The The amount of air sucked here (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 the combustion chamber 12 where it is burned.

  On the other hand, the exhaust passage 13 through which exhaust gas generated by combustion in the combustion chamber 12 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. ing.

  The three-way catalyst 18 efficiently purifies all the main harmful components (HC, CO, NOx) in the exhaust gas only when the air-fuel ratio of the air-fuel mixture to be burned is in a narrow range (window) near the theoretical air-fuel ratio. . In order for such a three-way catalyst 18 to function effectively, it is necessary to strictly control the air-fuel ratio of the air-fuel mixture to match 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 is formed of a solid electrolyte in which electrodes 33A and 33B 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. 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.

  When a difference occurs in the oxygen partial pressure between the inner atmosphere separated from the detection element 31 and the outer exhaust gas, in order to reduce the difference in the partial pressure, the oxygen partial pressure side (usually the atmosphere side) is reduced. ) Is ionized, passes through the solid electrolyte, and moves to the side where oxygen partial pressure is low (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 31 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 surface side electrode 33A through the detection element 31 to the outer surface side electrode 33B, the direction of the current is reversed, and the inner surface is not connected to the external device connected to both electrodes. The side electrode 33A is a positive electrode, and the outer surface side electrode 33B is a negative 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. In many cases, a configuration for detecting the oxygen partial pressure of the exhaust gas based on the same detection principle as that of the above-described sensor, that is, a detection element arranged to isolate the reference gas (atmosphere) and the exhaust gas is a reference gas. The electromotive force is generated in accordance with the difference in oxygen partial pressure of the exhaust gas with respect to the exhaust gas.

  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, the sensor output of 0.45 V is used as a rich / lean determination threshold value to determine whether the detection result of the sensor 19 is richer or leaner than the stoichiometric air-fuel ratio. 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.

  The ECU 22 further includes a sensor control circuit for applying a voltage to the detection element 31 of the oxygen sensor 19 and a heater control circuit for controlling the power supply from the battery to the heater 36. The sensor control circuit receives an analog applied voltage from the CPU via a low-pass filter (LPF) formed of an RC voltage dividing circuit, and applies it to the detection element 31. The ECU 22 converts the digital data calculated according to the processing described later into a rectangular analog voltage having a pulse width of about 10 to 100 μsec by an internal D / A converter, and then outputs the analog data to the sensor control circuit via the LPF. To do. The LPF outputs a smoothed signal from which the high-frequency component of the rectangular analog voltage signal has been removed, and prevents detection errors in the output current of the detection element 31 due to high-frequency noise. With the application of the voltage of the annealing signal to the detection element 31, the ECU 22 detects the voltage generated by the detection element 31 that changes according to the oxygen concentration in the gas to be detected (that is, in the exhaust gas). The ECU 22 receives the analog voltage corresponding to the current flowing from the sensor control circuit to the detection element 31 and the applied voltage to the detection element 31 from each of the A / D converters provided inside to detect this voltage. These digital data are converted into data and used for processing to be described later. Note that other configurations such as a crystal resonator and an oscillation circuit may be employed to apply the AC voltage. The ECU 22 calculates the resistance value of the heater 36, supplies electric power to the heater 36 according to the operating state of the engine based on this resistance value, and prevents overheating (OT) of the heater 36 through the heater control circuit. The temperature of the heater 36 is controlled by PWM control.

  In order to measure the impedance of the detection element 31, an AC voltage is applied to the detection element 31 by executing a processing routine described later. On the other hand, the output current Im detected from the oxygen sensor 19 shows a value corresponding to the oxygen concentration of the gas to be measured at that time while no AC voltage is applied to the oxygen sensor 19. When a voltage is applied, it changes according to the element impedance. The ECU 22 detects the change in the input voltage Vm to the oxygen sensor 19 and the output current Im from the oxygen sensor 19 at this time, and calculates the impedance of the detection element 31.

  The abnormality diagnosis process performed in the first embodiment configured as described above will be described. In FIG. 5, the ECU 22 first determines whether the internal combustion engine 10 is in operation (S10). This determination is made based on whether an ignition switch (not shown) is turned on and whether there is an input from the rotation speed sensor 23. Next, the ECU 22 determines whether the above-described air-fuel ratio feedback control is being executed (S20). If the result is negative in either step S10 or S20, the process is returned.

  If the determination in step S20 is affirmative, the ECU 22 controls the heater 36 so that the element temperature becomes high (S30). Here, the duty ratio of the heater 36 is maximized. The energization control of the heater 36 is continuously performed until the element temperature becomes equal to or higher than a predetermined reference value x [° C.] (S40). The reference value x here is set to a value that is sufficiently higher than the activation temperature and that can shorten the time required to apply an AC signal having a predetermined number of cycles for detection (for example, a quarter cycle). The The element temperature may be estimated by a predetermined function based on the impedance (grain boundary impedance Rg) obtained when an AC voltage Vn having a frequency of about 100 kHz is applied to the oxygen sensor 19, for example, You may measure using an output.

  When the element temperature is equal to or higher than x [° C.] (in the configuration using the impedance for estimating the element temperature, the impedance is equal to or lower than a predetermined value), the ECU 22 performs impedance measurement (S50) and abnormality diagnosis. (S60) is performed. These processes are performed according to the subroutines of FIGS. 6 and 7, respectively.

  In the process of FIG. 6, the AC voltage Vn (n = 1, 2,...) Is sequentially applied, and the impedance Z is calculated while gradually decreasing the voltage frequency of the AC voltage Vn, as shown in FIG. To create a complex impedance plot.

  In FIG. 6, first, the ECU 22 sets the initial value f0 to the frequency f of the AC voltage Vn (S110). Next, the ECU 22 applies the AC voltage Vn having the frequency f to the oxygen sensor 19 for a predetermined number of cycles, for example, a quarter period (S120). Next, the output current In of the oxygen sensor 19 is read after the delay time corresponding to the frequency f of the AC voltage Vn has elapsed since this application time (S130). Based on the AC voltage Vn and the output current In, the impedance Zn is calculated (S140), and this is stored and a complex impedance plot is created [S150].

  Next, the ECU 22 determines whether or not the current frequency f has fallen below a predetermined lower limit frequency fmin (S160). In the negative case, the frequency of the AC voltage Vn is decreased by a minute amount (S170), and the processes of steps S110 to S170 are repeatedly executed. As a result, a complex impedance plot as shown in FIG. 13 is created from the frequency f0 to fmin. When the current frequency f falls below a predetermined lower limit frequency fmin, this routine is exited. In this embodiment, since the frequency is gradually decreased from the high side to the low side, many measurements can be performed in a short time from the start of measurement, and only when the impedance plot creation is resumed after interruption, only for the unmeasured frequency. Although it is advantageous when the processing routine is configured so that impedance measurement is performed, the procedure for creating an impedance plot is arbitrary.

  In the subroutine of FIG. 7, an abnormality diagnosis process is performed. First, the ECU 22 extracts peak points P1, P2, and P3 based on the previously created complex impedance plot (S210). This extraction is performed, for example, by searching for a point where the electrostatic capacitance value shows a maximum value in each frequency region where it is empirically considered that each peak point exists. For example, as shown in FIG. 13, the peak point P1 is considered to exist in a frequency region lower than the frequency corresponding to the resistance value Rth1. Similarly, the peak point P2 is considered to exist in a frequency region corresponding to the resistance values Rth2 to Rth3, and the peak point P3 is considered to exist in a frequency region higher than the frequency corresponding to the resistance value Rth2.

  Next, the ECU 22 calculates resistances Rb, Rg, Re and capacitances Cb, Cg, Ce for the extracted peak points P1, P2, P3 (S220). The resistances Rb, Rg, and Re are calculated as the difference between the resistance values at two points that are equal to the diameter of the semicircle of each region in the complex impedance plot of FIG. The capacitances Cb, Cg, and Ce are calculated by the following formulas (1) to (3), respectively. Ωb, ωg, and ωe are angular velocities (ωn = 2πf, f is a frequency).

  Next, the ECU 22 determines whether the resistances Rb, Rg, Re and the capacitances Cb, Cg, Ce are all within a predetermined normal range (S230). If the determination is positive, the determination is normal (S240). In the case, abnormality determination (S250) is performed. These normal determination and abnormal determination are stored in a predetermined storage area in the nonvolatile memory of the ECU 22. These pieces of information are reflected in other controls at the time of traveling, and are read from the nonvolatile memory of the ECU 22 by the maintenance worker at the time of maintenance warehousing to determine the replacement timing of the oxygen sensor 19 and the oxygen sensor 19 or the detection element. Used for 31 improvements. In addition, you may display the determination result in step S240 or S250 as abnormality information on the display apparatus provided in the vehicle interior at the time of abnormality detection.

  When the resistance value Re of the electrode interface increases beyond the normal range and the capacitance value Ce decreases beyond the normal range, it can be determined that electrode aggregation has occurred. (In this case, the decrease in the capacitance Ce and the increase in the resistance Re are considered to be caused by a decrease in the contact area accompanying an increase in the particle diameter of the electrode surface due to electrode aggregation). Further, when the resistance value Re of the electrode interface increases beyond the normal range and the capacitance value Ce increases beyond the normal range, it can be determined that foreign matter is mixed. (In this case, the increase in the capacitance Ce and the resistance Re is considered to be caused by the fact that the capacitance of the foreign matter is larger than that of air in many cases.) In addition, when the resistance value Rg of the electrolyte portion increases beyond the normal range and the capacitance value Cg also increases beyond the normal range, the lattice defects of the electrolyte portion are reduced. It can be judged.

  As a result of the above processing, in this embodiment, when the temperature of the detection element 31 is higher than a predetermined reference temperature value, the presence / absence of abnormality is determined (S40 to S60), and the resistance component and the electrostatic in the impedance of the detection element 31 are determined. The cause of the abnormality of the detection element 31 is specified according to each behavior with the capacitive component.

  As described above, in this embodiment, the presence / absence of an abnormality is determined when the temperature of the detection element 31 is higher than a predetermined reference temperature value (S40 to S60). The frequency characteristic of the detection element of the oxygen sensor shifts to a higher frequency side as the temperature is higher, and the frequency at which the peak of the capacitance component in each region of the characteristic curve is relatively higher as the temperature is higher. Then, by determining whether or not there is an abnormality when the temperature of the detection element 31 is higher than a predetermined reference temperature value, the peak of the capacitance component is detected by applying an AC signal having a frequency higher than the normal temperature. be able to. Therefore, in the present embodiment, application of an alternating current signal having a predetermined number of cycles for detection can be performed in a short time, thereby shortening the time required for detecting an abnormality.

  Next, a second embodiment of the present invention will be described. The abnormality diagnosis device according to the second embodiment performs abnormality diagnosis only during fuel cut control or during fuel increase. Since the mechanical configuration of the second embodiment is the same as that of the first embodiment, detailed description thereof will be omitted using the same reference numerals.

  The control in 2nd Embodiment is demonstrated. In FIG. 8, the ECU 22 first determines whether the internal combustion engine is in operation (S310). This process is the same as step S10 in the first embodiment. If no in step S310, the process is returned.

If the determination in step S310 is affirmative, the ECU 22 controls the heater 36 so that the element temperature becomes high (S320). Here, the duty ratio of the heater 36 is maximized. The energization control of the heater 36 is continuously performed until the element temperature becomes equal to or higher than a predetermined reference value x [° C.] (S330). The reference value x here is set to a value that is sufficiently higher than the activation temperature and that can shorten the time required to apply an AC signal having a predetermined number of cycles for detection (for example, a quarter cycle). The The element temperature may be estimated by a predetermined function based on the impedance (grain boundary impedance Rg) obtained when an AC voltage Vn having a frequency of about 100 kHz is applied to the oxygen sensor 19, for example, You may measure using an output.
Next, the ECU 22 determines whether the internal combustion engine is under fuel cut control or fuel increase (S340).

  The fuel cut control is a control for individually cutting the fuel supply to each cylinder. Specifically, for example, the fuel cut control is determined in advance based on, for example, throttle opening, internal combustion engine speed, internal combustion engine water temperature, and vehicle speed input from various sensors. The fuel cut region map is referred to, and when the running state is in the fuel cut region (for example, at the time of deceleration or high rotation), it is determined that the execution condition of the fuel cut control is satisfied, and the fuel to the predetermined cylinder is determined. Cut the supply. By the fuel cut control, the fuel supply is cut for all cylinders or a part of the cylinders. During the fuel cut control, the actual air-fuel ratio of the exhaust becomes lean.

  The fuel increase is a control for increasing the fuel injection amount so that the air-fuel ratio becomes rich under a predetermined operating condition such as at high load. During the fuel increase, the actual air-fuel ratio of the exhaust becomes rich.

  When the internal combustion engine is under fuel cut control or fuel increase, the ECU 22 performs impedance measurement (S350) and abnormality diagnosis (S360). These processes are performed according to subroutines similar to those in FIGS. 6 and 7, respectively.

  As described above, in the second embodiment, abnormality diagnosis is executed only during fuel cut control or during fuel increase (S340). Like the oxygen sensor 19 used in the present embodiment, an oxygen sensor whose output voltage changes in a switching manner with respect to the stoichiometric air-fuel ratio is outside the predetermined air-fuel ratio region including the stoichiometric air-fuel ratio (that is, the lean side). (Or on the rich side), the output voltage is stable compared to the inside of the region. Therefore, the abnormality diagnosis is executed only during the fuel cut control or during the fuel increase (S340), thereby suppressing the influence of the output of the oxygen sensor 19 according to the air-fuel ratio, and detecting the abnormality of the oxygen sensor. Can be improved.

  Next, a third embodiment of the present invention will be described. The abnormality diagnosis apparatus according to the third embodiment is electrically controlled so as to suppress the influence of the output voltage of the detection element 31 based on the output voltage of the detection element 31 when no AC voltage for abnormality diagnosis is applied. A correction means for correcting the detection value of the characteristic is further provided, and abnormality diagnosis is executed based on the detection value of the electrical characteristic corrected by the correction means. Since the mechanical configuration of the third embodiment is the same as that of the first embodiment, detailed description thereof is omitted using the same reference numerals.

  A sensor output-correction coefficient map as shown in FIG. 9 is stored in advance in the ROM of the ECU 22 in the third embodiment. The map is a data file in the form of a table in which the output value of the oxygen sensor 19 and the correction coefficient α are stored in association with each other, and the influence of the output voltage of the detection element 31 itself when detecting the AC impedance of the detection element 31. The correction coefficient α is set so as to cancel out. In the example of FIG. 9, the correction coefficient α is set to gradually increase as the output value of the oxygen sensor 19 increases, but the tendency of the correction coefficient α is arbitrarily set according to the output characteristics of the oxygen sensor 19. be able to.

  An abnormality diagnosis process performed in the third embodiment will be described. In FIG. 10, the ECU 22 first determines whether the internal combustion engine is in operation (S410). This determination is made based on whether an ignition switch (not shown) is turned on and whether there is an input from a crank angle sensor (not shown). Next, the ECU 22 determines whether the above-described air-fuel ratio feedback control is being executed (S420). If the result is negative in any of steps S410 and S420, the process is returned.

  If the determination in step S420 is affirmative, the ECU 22 determines whether or not the element temperature is equal to or higher than a predetermined reference value y [° C] (S430). This reference value y is set at or slightly above the activation temperature of the detection element 31. The element temperature may be estimated by a predetermined map based on the impedance (grain boundary impedance Rg) obtained when an AC voltage Vn having a frequency of about 100 kHz is applied to the oxygen sensor 19, for example, You may measure using an output.

  When the element temperature is equal to or higher than y [° C.] (in the configuration using the impedance for estimating the element temperature, when the impedance is equal to or lower than a predetermined value), the ECU 22 acquires the pre-measurement sensor output (S440). . That is, the ECU 22 stores the current voltage value of the oxygen sensor 19 in a predetermined storage area. The current voltage value varies depending on the current exhaust air-fuel ratio.

  Next, the ECU 22 performs impedance measurement (S450). This process is performed according to the subroutine of FIG.

  In the process of FIG. 6, the AC voltage Vn (n = 1, 2,...) Is applied in order, and the impedance Z is calculated while gradually increasing the voltage frequency of the AC voltage Vn to create a complex impedance plot. Is.

  In FIG. 6, first, the ECU 22 sets the initial value f0 to the frequency f of the AC voltage Vn (S110). Next, the ECU 22 applies the AC voltage Vn having the frequency f to the oxygen sensor 19 for a predetermined number of cycles, for example, a quarter period (S120). Next, the output current In of the oxygen sensor 19 is read after the delay time corresponding to the frequency f of the AC voltage Vn has elapsed since this application time (S130). Based on the AC voltage Vn and the output current In, the impedance Zn is calculated (S140), and this is stored and a complex impedance plot is created (S150).

  Next, the ECU 22 determines whether or not the current frequency f has fallen below a predetermined lower limit frequency fmin (S160). In the negative case, the frequency of the AC voltage Vn is decreased by a minute amount (S170), and the processes of steps S110 to S170 are repeatedly executed. As a result, a complex impedance plot as shown in FIG. 13 is created from the frequency f0 to fmin. When the current frequency f falls below a predetermined lower limit frequency fmin, this routine is exited. In this embodiment, since the frequency is gradually decreased from the high side to the low side, many measurements can be performed in a short time from the start of measurement, and only when the impedance plot creation is resumed after interruption, only for the unmeasured frequency. Although it is advantageous when the processing routine is configured so that impedance measurement is performed, the procedure for creating an impedance plot is arbitrary.

  Next, the ECU 22 performs abnormality diagnosis (S460). This process is performed according to the subroutine of FIG.

  In the subroutine of FIG. 11, abnormality diagnosis processing is performed. First, the ECU 22 extracts peak points P1, P2, and P3 based on the previously created complex impedance plot (S510). This extraction is performed, for example, by searching for a point where the electrostatic capacitance value shows a maximum value in each frequency region that is empirically considered that each peak point exists. For example, as shown in FIG. 13, the peak point P1 is considered to exist in a frequency region lower than the frequency corresponding to the resistance value Rth1. Similarly, the peak point P2 is considered to exist in a frequency region corresponding to the resistance values Rth2 to Rth3, and the peak point P3 is considered to exist in a frequency region higher than the frequency corresponding to the resistance value Rth2.

  Next, the ECU 22 calculates resistances Rb, Rg, Re and capacitances Cb, Cg, Ce for the extracted peak points P1, P2, P3 (S520). The resistances Rb, Rg, and Re are calculated as the difference between the resistance values at two points that are equal to the diameter of the semicircle of each region in the complex impedance plot of FIG. The electrostatic capacitances Cb, Cg, and Ce are calculated by the following formulas (4) to (6), respectively. Ωb, ωg, and ωe are angular velocities (ωn = 2πf, f is a frequency).

  Next, the ECU 22 calculates the correction coefficient α from the map (S530). That is, the ECU 22 calculates the correction coefficient α by searching the sensor output-correction coefficient map of FIG. 9 based on the pre-measurement sensor output (voltage value) acquired in step S440.

  Next, the ECU corrects and updates the resistance value by multiplying the resistances Rb, Rg, and Re by the correction coefficient α (S540). This correction cancels the influence of the output voltage of the detection element 31 on the resistors Rb, Rg, and Re.

  Next, the ECU 22 determines whether the corrected resistances Rb, Rg, Re and the capacitances Cb, Cg, Ce are all within a predetermined normal range (S550). If the determination is affirmative, the ECU 22 determines normality (S560). If not, an abnormality determination (S570) is performed. These normal determination and abnormal determination are stored in a predetermined storage area in the nonvolatile memory of the ECU 22. These pieces of information are reflected in other controls at the time of traveling, and are read from the nonvolatile memory of the ECU 22 by the maintenance worker at the time of maintenance warehousing to determine the replacement timing of the oxygen sensor 19 and the oxygen sensor 19 or the detection element. Used for 31 improvements. Note that the determination result in step S560 or S570 may be displayed as abnormality information on a display device provided in the passenger compartment at the time of abnormality detection.

  When the resistance value Re of the electrode interface increases beyond the normal range and the capacitance value Ce decreases beyond the normal range, it can be determined that electrode aggregation has occurred. (In this case, the decrease in the capacitance Ce and the increase in the resistance Re are considered to be caused by a decrease in the contact area accompanying an increase in the particle diameter of the electrode surface due to electrode aggregation). Further, when the resistance value Re of the electrode interface increases beyond the normal range and the capacitance value Ce increases beyond the normal range, it can be determined that foreign matter is mixed. (In this case, the increase in the capacitance Ce and the resistance Re is considered to be caused by the fact that the capacitance of the foreign matter is larger than that of air in many cases.) In addition, when the resistance value Rg of the electrolyte portion increases beyond the normal range and the capacitance value Cg also increases beyond the normal range, the lattice defects of the electrolyte portion are reduced. It can be judged.

  As a result of the above processing, in the present embodiment, the presence or absence of abnormality is determined based on the values of the resistors Rb, Rg, Re corrected according to the output voltage of the detection element 31, and the resistance component in the impedance of the detection element 31 is determined. The cause of the abnormality of the detection element 31 is specified in accordance with the behavior of each of the capacitance component and the capacitance component.

  As described above, in the present embodiment, the output voltage of the detection element 31 is acquired immediately before detection of the electrical characteristics for abnormality diagnosis (S440), and is corrected (S540) according to the output voltage of the detection element 31. Based on the values of the resistances Rb, Rg, and Re, the presence or absence of abnormality is determined. Since the output of the detection element 31 affects the detected value of the AC impedance, the accuracy of detecting the abnormality of the oxygen sensor can be improved in this embodiment by canceling or suppressing this influence by correction.

  In the present embodiment, the detection value of the electrical characteristic is corrected based on the output voltage of the detection element 31 acquired immediately before the application of the AC voltage for abnormality diagnosis. However, the AC voltage is applied. If the output voltage is not before the abnormality diagnosis, the output voltage may be after the application of the AC voltage and the detection of the electrical characteristics.

  In each of the above embodiments, the present invention has been described with a certain degree of concreteness, but various modifications and changes can be made to the present invention without departing from the spirit and scope of the invention described in the claims. It must be understood that there is.

For example, in each of the above embodiments, the point where the electrostatic capacitance component shows the maximum value is specified as the peak points P1, P2, and P3. Instead of such a configuration, the impedance Zn is converted into a polar coordinate by a predetermined function. The phase angle αn (see FIG. 13) may be calculated by conversion, and the point where the calculated phase angle αn turns from increasing to decreasing may be specified as the peak point.
In each of the above embodiments, the capacitance and the resistance are individually used as the electrical characteristics. However, the electrical characteristics in the present invention may be impedance, and such a configuration also belongs to the category of the present invention. is there.

1 is a schematic view showing an internal combustion engine according to an embodiment of the present invention. It is sectional drawing which shows the attachment state of an oxygen sensor. It is an expanded sectional view of the detection element periphery of an oxygen sensor. It is a graph which shows the output characteristic of an oxygen sensor. It is a flowchart which shows the abnormality diagnosis control process in 1st Embodiment. It is a flowchart which shows the impedance measurement process in 1st Embodiment. It is a flowchart which shows the abnormality diagnosis process in 1st Embodiment. It is a flowchart which shows the abnormality diagnosis control process in 2nd Embodiment. It is a graph which shows the example of a setting of the sensor output-correction coefficient map in 3rd Embodiment. It is a flowchart which shows the abnormality diagnosis control process in 3rd Embodiment. It is a flowchart which shows the abnormality diagnosis process in 3rd Embodiment. It is a figure which shows the equivalent circuit of the oxygen sensor element in embodiment. It is a graph which shows the complex impedance characteristic of the oxygen sensor in an embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 12 Combustion chamber 17 Injector 18 Three-way catalyst 19 Oxygen sensor 31 Detection element 36 Heater

Claims (3)

An oxygen sensor comprising: a detection unit that detects an electrical characteristic by applying an AC voltage to a detection element of the oxygen sensor; and a determination unit that determines whether the detection element is abnormal based on a detection value of the detection unit. An abnormality diagnosis device of
The oxygen sensor abnormality diagnosis apparatus, wherein the determination unit performs the determination when the temperature of the detection element is higher than a predetermined reference temperature value. An oxygen sensor comprising: a detection unit that detects an electrical characteristic by applying an AC voltage to a detection element of the oxygen sensor; and a determination unit that determines whether the detection element is abnormal based on a detection value of the detection unit. An abnormality diagnosis device of
Based on the output voltage of the detection element when the AC voltage is not applied, further comprises correction means for correcting the detection value of the electrical characteristic so as to suppress the influence of the output voltage of the detection element,
The oxygen sensor abnormality diagnosis device, wherein the determination unit performs the determination based on the detected value of the electrical characteristic corrected by the correction unit. The oxygen sensor abnormality diagnosis device according to claim 1 or 2,
The oxygen sensor abnormality diagnosis device, wherein the electrical characteristic is impedance, capacitance, or resistance.

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