JP2008261757A - Device for diagnosing failure of oxygen sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent incorrect judgment and improve diagnostic accuracy. <P>SOLUTION: The device for diagnosing a failure of an oxygen sensor with a heater judges that an oxygen sensor is in a failure state when a negative voltage from the oxygen sensor is detected. In the warm-up process of the oxygen sensor after the start-up of an internal combustion engine, the heater is controlled so that the temperature Ts of a detection element is dropped at least once. By the drop of the element temperature, the outside air can be introduced in an atmosphere chamber by contracting the gas in the chamber. This promotes the exchange of moisture/steam in the atmospheric chamber and the atmosphere and prevents the occurrence of a negative voltage and the incorrect judgment. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an oxygen sensor failure diagnosis device, and more particularly to an oxygen sensor failure 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

  The 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, and zirconia, for example, 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.

  In such an oxygen sensor, when the sensing element is lost and the inside and outside of the sensing element communicate with each other, exhaust gas outside the sensing element enters the inside, and there is no difference in the oxygen partial pressure between the inside and outside, and the sensor generates an electromotive force. No longer. Further, if exhaust gas having a high oxygen concentration (air-fuel ratio lean) is present outside the detection element in a state where the exhaust gas has entered the detection element, an electromotive force in the reverse direction is generated in the oxygen sensor. Accordingly, by detecting the negative (minus) output voltage of the oxygen sensor corresponding to the back electromotive force, it is possible to detect a defect in the detection element of the oxygen sensor, that is, a failure of the oxygen sensor (see, for example, Patent Document 1). ).

  As another prior art, the heater of the oxygen sensor is energized on the condition that the integrated load amount of the internal combustion engine reaches a predetermined load amount corresponding to the moisture-free temperature of the exhaust pipe of the internal combustion engine with respect to the oxygen sensor with a heater. (See Patent Document 2).

JP 2003-14683 A JP-A-8-15213

  By the way, even if the oxygen sensor is not defective and is normal as described above, there is an event that a negative output voltage is generated from the oxygen sensor during warm-up after the internal combustion engine is started. confirmed. Accordingly, in this case as well, it is erroneously determined that the oxygen sensor has failed, resulting in a decrease in the accuracy of failure diagnosis.

  Therefore, the present invention has been made in view of such circumstances, and an object thereof is to provide an oxygen sensor failure diagnosis apparatus that can prevent erroneous determination and improve diagnosis accuracy.

According to the first aspect of the present invention,
In an oxygen sensor failure diagnosis apparatus having a detection element disposed in an exhaust passage of an internal combustion engine, and an air chamber is defined inside the detection element,
A heater provided in the oxygen sensor for heating the detection element;
Heater control means for controlling the heater;
Output voltage detection means for detecting the output voltage of the oxygen sensor;
A failure determination unit that determines that the oxygen sensor has failed when a negative output voltage is detected by the output voltage detection unit;
With
An oxygen sensor failure diagnosis apparatus is provided, wherein the heater control means controls the heater so that the temperature of the detection element is decreased at least once in the warm-up process of the oxygen sensor.

  As described above, even if the oxygen sensor is normal and not defective, a negative output voltage may be generated from the oxygen sensor during warm-up after the internal combustion engine is started. This is because condensed water or the like may be generated in the atmosphere chamber inside the detection element while the engine is stopped before the internal combustion engine is started, and moisture may exist in the atmosphere chamber. The moisture gradually evaporates after the internal combustion engine is started. At this time, due to the volume expansion due to the evaporation of the moisture, the atmosphere in the atmosphere chamber is expelled to the outside, that is, expelled. When this happens, the atmosphere chamber becomes a so-called oxygen deficient state, the oxygen partial pressure inside and outside the detection element is reversed, and a negative voltage is output from the oxygen sensor. However, after this, the water vapor is expelled from the atmosphere chamber, and instead the atmosphere returns to the atmosphere chamber again. When this happens, a difference in normal oxygen partial pressure occurs between the inside and outside of the detection element, and a positive voltage is output from the oxygen sensor.

  According to the first aspect, during the warm-up process of the oxygen sensor, the temperature of the detection element is decreased at least once. If it carries out like this, the gas in an atmospheric chamber can be shrunk and external air can be introduce | transduced into an atmospheric chamber. As a result, the exchange of moisture and water vapor in the atmosphere chamber with the atmosphere can be promoted, and the time during which the atmosphere chamber is deficient can be shortened. In addition, it is possible to prevent negative voltage generation during the sensor warm-up process, prevent erroneous determination, and improve diagnostic accuracy.

According to a second aspect of the present invention, in the first aspect,
Element temperature detection means for detecting the temperature of the detection element is provided;
When the temperature of the detection element decreases, the temperature of the detection element is decreased until the element temperature detected by the element temperature detection means reaches a predetermined first element temperature,
The first element temperature is set equal to a minimum temperature at which moisture existing in the atmospheric chamber can be evaporated.

  According to this, even when the element temperature is reduced to the maximum (that is, even at the lower end of the element temperature), a state in which moisture in the atmosphere chamber can be evaporated is maintained. Thus, the exchange of moisture and water vapor in the atmosphere chamber with the atmosphere can be promoted.

According to a third aspect of the present invention, in the first or second aspect,
Element temperature detection means for detecting the temperature of the detection element is provided;
In the warm-up process of the oxygen sensor, the detection element is heated until the element temperature detected by the element temperature detection means reaches a predetermined second element temperature,
The second element temperature is set higher than the activation start temperature of the oxygen sensor.

  According to this, the temperature when the element temperature is raised to the maximum (that is, the rising edge of the element temperature) can be made significantly higher than the lowest temperature at which moisture present in the atmospheric chamber can be evaporated. Therefore, when the element temperature is lowered from the second element temperature, the state in which the moisture in the atmosphere chamber can evaporate can be maintained for a relatively long time, thereby promoting the evaporation of the moisture in the atmosphere chamber, and the moisture and water vapor in the atmosphere chamber and the atmosphere. Exchange with can be promoted.

According to a fourth aspect of the present invention, in any one of the first to third aspects,
The heater control means controls the heater so as to lower the temperature of the detection element a plurality of times during the warm-up process of the oxygen sensor.

  According to this, it is possible to repeatedly execute the exhaust of the atmospheric chamber gas to the outside during the heating of the detection element and the introduction of the outside air into the atmospheric chamber when the temperature of the detection element is lowered. Can be promoted.

According to a fifth aspect of the present invention, in any one of the first to fourth aspects,
Measuring means for measuring the number of times the temperature of the detection element has been lowered is provided,
The heater control unit controls the heater so that the temperature of the detection element is maintained at an activation temperature after the number of times measured by the measurement unit reaches a predetermined value.

  Thereby, after exchanging the water | moisture content and water vapor | steam in air | atmosphere with air | atmosphere, it can transfer to normal heater control.

According to a sixth aspect of the present invention, in any one of the first to fifth aspects,
The heater control means controls the heater on and off.

  According to the present invention, an excellent effect of preventing erroneous determination and improving diagnosis accuracy is exhibited.

  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, a three-way catalyst 18 for purifying harmful components in the exhaust gas is provided in the exhaust passage 13 through which the exhaust gas generated by the combustion in the combustion chamber 12 is sent. A post-catalyst oxygen sensor 20 is provided on each downstream side.

  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 the air flow meter 16, oxygen sensors 19, 20, an accelerator opening sensor 21 that detects the amount of depression of the accelerator pedal (accelerator opening), and a rotational speed sensor 23 that detects the engine rotational speed. Sensor detection signals are input. The air-fuel ratio control is performed by drivingly controlling 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 these sensors. The outline of air-fuel ratio 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.

  Accordingly, the ECU 22 feedback corrects the fuel injection amount from the injector 17 based on the actual value of the air-fuel ratio grasped from the detection results of the oxygen sensors 19 and 20, and ensures the required accuracy of the air-fuel ratio control. ing.

  As described above, in this exhaust gas purification system, the so-called air-fuel ratio feedback control that performs feedback correction of the fuel injection amount according to the detection results of the oxygen sensors 19 and 20 is performed, so that the air-fuel ratio of the air-fuel mixture is reduced to the theoretical air-fuel ratio. A high exhaust gas purification rate is secured by maintaining the fuel ratio in the vicinity. In this exhaust gas purification system, as described above, the oxygen partial pressures of the exhaust gas upstream and downstream of the three-way catalyst 18 are detected by the two oxygen sensors 19 and 20, respectively. High accuracy is achieved.

  The two oxygen sensors 19 and 20 have the same configuration, and the failure diagnosis method is also the same. Therefore, the pre-catalyst oxygen sensor 19 will be described as an example, and the post-catalyst oxygen sensor 20 will not be described. 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 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, and zirconia, for example, 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 transiently at the theoretical air-fuel ratio A / Fs (for example, 14.6), and the air-fuel ratio A of the exhaust gas (atmosphere 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.

  Note that, as in this embodiment, in an internal combustion engine that performs air-fuel ratio control for the purpose of combustion only at the stoichiometric air-fuel ratio (stoichiometric combustion), an oxygen sensor having a characteristic that the output voltage changes greatly with the stoichiometric air-fuel ratio as a boundary. 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.

  By the way, due to aged deterioration due to long-term use or the like, the detection element 31 of the oxygen sensor 19 may be cracked or the detection element 31 may be broken, and the oxygen sensor 19 may break down. In the case of a sensor failure due to this defect, as shown in FIG. 5, the inside and outside of the detection element 31 communicate with each other through the defect part 37 of the detection element 31, and exhaust gas outside the detection element 31 enters the inside. If exhaust gas having a high oxygen concentration (air-fuel ratio lean) exists outside the detection element 31 with the exhaust gas entering the detection element 31, an electromotive force in the reverse direction is generated in the oxygen sensor 19. This may occur, for example, when the air-fuel ratio is switched from rich to lean in a sensor failure state, or when fuel cut is performed. In this case, the potential of the negative electrode 33B is higher than the potential of the positive electrode 33A, and a negative (minus) output voltage is generated.

  FIG. 6 shows an example of a change in the oxygen sensor output voltage at the time of such a failure. As shown in the circled area, the oxygen sensor 19 often outputs a negative voltage. Therefore, by detecting such a negative output voltage by the ECU 22, a failure of the oxygen sensor can be estimated for the time being.

  However, as described above, even if the oxygen sensor 19 is not defective and is normal, a negative output voltage may be generated from the oxygen sensor 19 during warm-up after the internal combustion engine is started. Therefore, in this case as well, it is erroneously determined that the oxygen sensor 19 has failed, resulting in a decrease in the accuracy of failure diagnosis.

  FIG. 7 shows the test results of examining changes in the oxygen sensor output voltage (solid line) during warm-up after the internal combustion engine is started when the oxygen sensor is normal. The change in impedance (hereinafter also referred to as “element impedance”) (one-dot chain line) of the detection element of the oxygen sensor is also shown. The element impedance is a value that correlates with the temperature of the detection element of the oxygen sensor (hereinafter also referred to as “element temperature”), and the relationship between the two is that the element impedance decreases as the element temperature increases. Therefore, it will be understood from the figure that the element temperature gradually increases.

  As shown in the figure, the oxygen sensor output voltage is 0 V for a certain period from the start of the internal combustion engine (t = 0). This is because the element temperature of the oxygen sensor has not yet reached the activation temperature and voltage cannot be output. Thereafter, when warming up of the oxygen sensor is completed and the element temperature reaches the activation temperature, a negative voltage may be output from the oxygen sensor as indicated by a broken-line circle. This is because condensed water or the like is generated in the atmospheric chamber inside the detection element while the engine is stopped before the internal combustion engine is started, and moisture exists in the atmospheric chamber. 2 and 3, when the internal combustion engine is stopped, outside air containing moisture enters the atmosphere chamber 34 through the atmosphere passage and the atmosphere hole 35, and the temperature of the atmosphere chamber 34 and the detection element 31 also decreases. Therefore, condensed water is generated in the atmospheric chamber 34. After the internal combustion engine is started, the condensed water evaporates as the temperature of the atmospheric chamber 34 and the detection element 31 rises along with the energization of the heater 36. At this time, the atmospheric chamber 34 is caused by volume expansion due to evaporation of moisture. The air inside is expelled to the outside, that is, expelled. When this happens, the atmosphere chamber 34 is in a so-called oxygen deficient state, the oxygen partial pressure inside and outside the detection element 31 is reversed, and a negative voltage is output from the oxygen sensor 19. After this, the water vapor is expelled from the atmospheric chamber 34 and instead the atmospheric air returns or is reintroduced into the atmospheric chamber 36. As a result, a difference in normal oxygen partial pressure occurs between the inside and outside of the detection element 31, and a positive voltage is output from the oxygen sensor 19.

  For this reason, a negative voltage may be output from a normal oxygen sensor during sensor warm-up after starting the internal combustion engine. Therefore, in this embodiment, in order to prevent erroneous determination as a failure of the oxygen sensor in this case, the heater 36 is controlled so as to decrease the temperature of the detection element 31 at least once during the warm-up process of the oxygen sensor 19. I am going to do that. This will be described below.

  This technique for preventing erroneous determination generally relates to improvement in energization control of the heater 36 of the oxygen sensor 19. That is, in general, in the process of warming up the oxygen sensor after the engine is started, the heater is continuously heated until the element temperature reaches the activation temperature. On the other hand, in the present embodiment, the heater 36 is controlled so as to decrease the temperature of the detection element 31 at least once. During element heating, the gas (including water vapor) in the atmospheric chamber 34 expands and is expelled to the outside. On the other hand, the gas in the atmospheric chamber 34 contracts while the element temperature decreases, Only outside air can be introduced into the atmospheric chamber 34. In other words, the atmospheric chamber 34 can breathe to the outside by increasing or decreasing the element temperature, and the exchange between the gas in the atmospheric chamber 34 and the outside air can be promoted. In this way, the oxygen deficiency state of the atmospheric chamber 34 can be eliminated as quickly as possible, and negative voltage generation during the sensor warm-up process can be prevented, erroneous determination can be prevented, and diagnostic accuracy can be improved.

  In the present embodiment, the ECU 22 detects the impedance of the detection element 31. As described above, since the element impedance is a value correlated with the element temperature, detecting the element impedance is equivalent to detecting the element temperature. Hereinafter, specific heater control contents in the present embodiment will be described.

  FIG. 8 schematically shows how the heater is controlled after the internal combustion engine is started. When the internal combustion engine is started (t = 0), the heater 36 is turned on, whereby the temperature of the heater 36 and the detection element 31 rises. The element temperature Ts exceeds a predetermined first element temperature Ts1, and eventually reaches a predetermined second element temperature Ts2 that is higher than the first element temperature Ts1. In the present embodiment, the first element temperature Ts1 is the lowest temperature at which moisture in the atmosphere chamber 34 can evaporate, in other words, the evaporation start temperature at which moisture in the atmosphere chamber 34 starts to evaporate during the sensor warm-up process (in this embodiment, 100 ° C.). The second element temperature Ts2 is set to a value (500 ° C. in this embodiment) slightly higher than the activation start temperature Tsa (400 ° C. in this embodiment) of the oxygen sensor 19. The first element impedance Rs1 and the second element impedance Rs2 corresponding to the first element temperature Ts1 and the second element temperature Ts2 are 50000Ω and 1000Ω, respectively.

  At the same time as the element impedance Rs detected by the ECU 22 reaches the second element impedance Rs2, the heater 36 is turned off, and thereafter the temperatures of the heater 36 and the detection element 31 are lowered. Then, at the same time when the detected element impedance Rs reaches the first element impedance Rs1 again, the heater 36 is turned on, whereby the temperature of the heater 36 and the detection element 31 rises. In this way, the heater temperature and thus the element temperature Ts are repeatedly increased or decreased between the first element temperature Ts1 and the second element temperature Ts2. Thereby, exhaust of the atmospheric chamber gas to the outside and introduction of the outside air into the atmospheric chamber can be repeatedly performed, and exchange of moisture / water vapor in the atmospheric chamber with the outside air can be promoted. When the number N of times that the temperature of the detection element 31 has decreased reaches a predetermined value Ns (3 in the present embodiment), it is considered that the exchange of moisture / water vapor in the air chamber and the outside air has been completed, and normal heater control is performed. Transition. In the present embodiment, the number N is increased by 1 each time the element impedance Rs reaches the first element impedance Rs1 while the heater is turned off. The number N is measured by a counter provided in the ECU 22. In normal heater control, the heater is controlled so that the element temperature Ts is maintained at the activation temperature. Specifically, the heater is controlled so that the element temperature Ts is maintained at a predetermined target temperature that is equal to or higher than the second element temperature Ts2 (in this embodiment, about 500 to 550 ° C.).

  FIG. 9 shows a flowchart of the above-described heater control executed by the ECU 22. First, in step S101, it is determined whether a precondition for starting heater control is satisfied. The case where this precondition is satisfied is, for example, the case where all of 1) after engine start and 2) the engine water temperature exceeds a predetermined temperature (for example, 0 ° C.) are satisfied. is there. The engine water temperature is detected by a water temperature sensor (not shown).

  If the precondition is not satisfied, step S101 is executed until the precondition is satisfied, and a standby state is entered. On the other hand, when the precondition is satisfied, the process proceeds to step S102, and the first heater control is executed. In the first heater control, the heater 36 of the oxygen sensor 19 is turned on, and the maximum power that can be supplied is supplied to the heater 36. As a result, the heater 36 is quickly heated, and the element temperature is quickly raised. If the element temperature Ts exceeds the first element temperature Ts1, the moisture in the atmosphere chamber 34 is evaporated and the gas containing water vapor in the atmosphere chamber 34 is exhausted to the outside. The heater 36 can also be duty-controlled by the ECU 22. In this case, an energization pulse having an ON duty ratio of preferably 100% or slightly less is supplied to the heater 36.

  Next, in step S103, it is determined whether or not the element temperature Ts has exceeded the second element temperature Ts2. More specifically, it is determined whether or not the element impedance Rs detected by the ECU 22 is lower than the second element impedance Rs2.

  When it is determined that the element temperature Ts does not exceed the second element temperature Ts2, step S102 is repeatedly executed, and the heater heating is continued.

  On the other hand, when it is determined that the element temperature Ts exceeds the second element temperature Ts2, the second heater control is executed in step S104. In the second heater control, the heater 36 of the oxygen sensor 19 is turned off. Thereby, the heating of the heater 36 is stopped, the heater 36 and the detection element 31 are in a natural heat radiation state, and the heater temperature and the element temperature are lowered. At this time, the temperature in the atmospheric chamber 34 also decreases, the gas in the atmospheric chamber 34 contracts, and outside air is taken into the atmospheric chamber 34. When the heater 36 is duty-controlled, in the second heater control, an energization pulse having an ON duty ratio smaller than that in the first heater control is supplied to the heater 36. The ON duty ratio at this time is preferably 0% (that is, no energization), but may be a slightly larger value (for example, about 20%).

  Next, in step S105, it is determined whether or not the element temperature Ts is lower than the first element temperature Ts1. More specifically, it is determined whether the element impedance Rs detected by the ECU 22 exceeds the first element impedance Rs1 described above.

  When it is determined that the element temperature Ts is not lower than the first element temperature Ts1, step S104 is repeatedly executed, and the heater stop is continued.

  On the other hand, when it is determined that the element temperature Ts is lower than the first element temperature Ts1, the count value N of the number counter is incremented by 1 in step S106. Then, in the next step S107, it is determined whether or not the count value N has reached the predetermined value Ns or more.

  When it is determined that the count value N has not reached the predetermined value Ns or more, the process returns to step S101, and steps S101 to S106 are repeatedly executed. Thereby, heating and stopping of the heater and counting up of the number counter are executed again.

  On the other hand, when it is determined that the count value N has reached the predetermined value Ns or more, the process proceeds to step S108, and the third heater control is executed. The third heater control is normal heater control, and more specifically, the detected element impedance Rs is equivalent to the target temperature so that the element temperature Ts is maintained at the target temperature. The heater is controlled so as to be maintained. As a result, the heater is first heated, and after the element temperature Ts reaches the target temperature, the heater is turned on / off so that the element temperature Ts is close to the target temperature. The warm-up process of the oxygen sensor 19 is completed when the element temperature Ts first reaches the target temperature by the third heater control.

  In the above heater control, it is preferable to exchange the moisture and water vapor in the atmosphere chamber 34 with the atmosphere as soon as possible so that the oxygen sensor 19 can be used as usual as soon as possible. From such a viewpoint, the first and second element temperatures Ts1 and Ts2 that define the rising end, the falling end, and the increase / decrease range of the heater temperature and the element temperature, and the predetermined value Ns that defines the number of element temperature drops are optimally set. Is desirable.

  In the present embodiment, the first element temperature Ts1 is set equal to the lowest temperature at which the moisture in the atmospheric chamber 34 can be evaporated. Therefore, even when the element temperature is lowered to the maximum, the state in which the moisture in the atmospheric chamber 34 can be evaporated is maintained. As a result, the moisture in the atmospheric chamber 34 can always be evaporated while the element temperature is increased or decreased. Exchange of moisture / water vapor in the air chamber with the outside air can be promoted. Further, since the second element temperature Ts2 is set to be higher than the activation start temperature Tsa of the oxygen sensor 19, the rising end temperature at the time of heating the detection element can be significantly higher than the lowest temperature at which moisture can be evaporated. Therefore, when the element temperature is lowered from the second element temperature Ts2, the state in which the moisture in the atmospheric chamber 34 can evaporate can be maintained for a relatively long time. Therefore, evaporation of moisture in the atmosphere chamber 34 can be promoted, and exchange of moisture / water vapor in the atmosphere chamber 34 with outside air can be promoted. Furthermore, since the second element temperature Ts2 is only slightly higher than the activation start temperature Tsa of the oxygen sensor 19, the sensor is often not activated during the increase or decrease in the element temperature. Therefore, the negative voltage is hardly detected during the increase / decrease of the element temperature, and this can prevent erroneous determination. Since the increase / decrease width Ts1 to Ts2 of the element temperature is relatively large, the moisture / water vapor can be discharged and the outside air can be introduced with high efficiency (that is, a state similar to deep breathing). Exchange with the outside air can be promoted.

  The preferred embodiment of the present invention has been described in detail above, but various other embodiments of the present invention are conceivable. For example, the internal combustion engine is not limited to being mounted on a vehicle, and the arrangement method and installation position of the oxygen sensor can be arbitrarily changed. The numerical values used in the above embodiment can be arbitrarily changed. For example, the first element temperature Ts1 may be set to a temperature higher or lower than the lowest temperature at which moisture in the atmospheric chamber 34 can be evaporated. If the temperature is set to a high temperature, the state capable of evaporating moisture can be maintained as in the present embodiment. If the temperature is set to a low temperature, moisture evaporation cannot be performed when the temperature falls below the minimum temperature, but it is advantageous for gas contraction in the atmosphere chamber and introduction of outside air. It is. Moreover, you may set 2nd element temperature Ts2 to the temperature below the activation start temperature of an oxygen sensor. In this way, the oxygen sensor can be kept in an inactive state whenever the element temperature is increased or decreased, and negative voltage detection can be reliably prevented.

  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.

It is a figure which shows the internal combustion engine which concerns on this embodiment. 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 an expanded sectional view when a defective part arises in a detection element of an oxygen sensor. It is a graph which shows the change of the output voltage at the time of failure of an oxygen sensor. The test result which investigated the change of the oxygen sensor output voltage in the warming-up after an internal combustion engine start in case an oxygen sensor is normal is shown. It is a time chart which shows the mode of heater control of this embodiment roughly. It is a flowchart of heater control of this embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 13 Exhaust passage 19, 20 Oxygen sensor 22 Electronic control unit (ECU)
31 Detection element 34 Atmospheric chamber 36 Heater Ts Element temperature Ts1 First element temperature Ts2 Second element temperature N Count value Ns Predetermined value

Claims (6)

In an oxygen sensor failure diagnosis apparatus having a detection element disposed in an exhaust passage of an internal combustion engine, and an air chamber is defined inside the detection element,
A heater provided in the oxygen sensor for heating the detection element;
Heater control means for controlling the heater;
Output voltage detection means for detecting the output voltage of the oxygen sensor;
A failure determination unit that determines that the oxygen sensor has failed when a negative output voltage is detected by the output voltage detection unit;
With
The oxygen sensor failure diagnosis apparatus, wherein the heater control means controls the heater so that the temperature of the detection element is decreased at least once in a warm-up process of the oxygen sensor. Element temperature detection means for detecting the temperature of the detection element is provided;
When the temperature of the detection element decreases, the temperature of the detection element is decreased until the element temperature detected by the element temperature detection means reaches a predetermined first element temperature,
2. The oxygen sensor failure diagnosis device according to claim 1, wherein the first element temperature is set equal to a minimum temperature at which moisture existing in the atmosphere chamber can evaporate. Element temperature detection means for detecting the temperature of the detection element is provided;
In the warming-up process of the oxygen sensor, the detection element is heated until the element temperature detected by the element temperature detection means reaches a predetermined second element temperature,
The oxygen sensor failure diagnosis apparatus according to claim 1 or 2, wherein the second element temperature is set to be higher than an activation start temperature of the oxygen sensor. The oxygen sensor according to any one of claims 1 to 3, wherein the heater control unit controls the heater so as to lower the temperature of the detection element a plurality of times during a warm-up process of the oxygen sensor. Fault diagnosis device. Measuring means for measuring the number of times the temperature of the detection element has been lowered is provided,
The heater control means controls the heater so that the temperature of the detection element is maintained at an activation temperature after the number of times measured by the measurement means reaches a predetermined value. The oxygen sensor failure diagnosis device according to any one of claims 1 to 4. The oxygen sensor failure diagnosis apparatus according to any one of claims 1 to 5, wherein the heater control means performs on / off control of the heater.

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