JP2010203787A - System for diagnosing trouble of oxygen sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a system for diagnosing the trouble of an oxygen sensor for preventing misjudgment to enhance diagnostic precision. <P>SOLUTION: In the system for diagnosing the trouble of the oxygen sensor on the basis of the negative voltage output from the oxygen sensor arranged in the exhaust passage of an internal combustion engine, the presence of the poisoning of the exhaust pole in the detection element of the oxygen sensor is detected, so that the diagnosis of the trouble of the oxygen sensor is executed on condition that the poisoning of the exhaust pole is not detected. <P>COPYRIGHT: (C)2010,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.

  For air-fuel ratio feedback control and the like, an oxygen sensor that detects the air-fuel ratio based on the oxygen concentration of the exhaust gas is provided in the exhaust passage of the internal combustion engine. The oxygen sensor includes a cylindrical detection element disposed in the exhaust passage, and the detection element is formed of a solid electrolyte having electrodes formed on the inner and outer surfaces. The electrode on the outer surface side is the exhaust electrode facing the exhaust passage, and the electrode on the inner surface side is the atmosphere electrode facing the atmospheric chamber inside the detection element. The oxygen sensor generates an electromotive force according to the difference in oxygen partial pressure of the atmospheric gas of these electrodes. Specifically, the oxygen concentration of the exhaust gas decreases (that is, the air-fuel ratio of the exhaust gas becomes rich). Generates a large electromotive force.

  In this 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). ).

JP 2008-121463 A

  By the way, even if the oxygen sensor is not defective as described above and is normal, if the exhaust electrode is poisoned by poisonous substances in the exhaust gas, a negative output voltage may be generated from the oxygen sensor. is there. Therefore, even in this case, 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 one aspect of the invention,
In an apparatus for diagnosing a failure of the oxygen sensor based on a negative voltage output from an oxygen sensor disposed in an exhaust passage of an internal combustion engine,
There is provided poisoning detection means for detecting the presence or absence of poisoning of the exhaust electrode in the detection element of the oxygen sensor, and the oxygen detection is performed on the condition that the poisoning detection means detects that the exhaust electrode is not poisoned. An oxygen sensor failure diagnosis apparatus is provided, which performs a sensor failure diagnosis.

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

It is a figure which shows the exhaust-gas purification system of the internal combustion engine which concerns on this embodiment. It is a fragmentary sectional view which shows the attachment state of an oxygen sensor. It is an expanded sectional view of the detection element 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. It is a graph which shows the change of the output voltage in the warming-up process of a normal oxygen sensor. It is a graph which shows the relationship between element temperature and a negative voltage level. It is a flowchart of the main routine of failure diagnosis processing. It is a calculation map of the poisoning side fuel property coefficient. It is a calculation map of a desorption side fuel property coefficient. It is a flowchart of the subroutine for residual poisoning amount calculation. It is a calculation map of the poisoning side element temperature coefficient. It is a calculation map of a desorption side element temperature coefficient.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

  The configuration of an exhaust gas purification system for a vehicle internal combustion engine to which the present invention is applied will be described with reference to FIG. A spark ignition type internal combustion engine, specifically, an intake passage 11 of a gasoline engine 10 is provided with a throttle valve 15 (in this embodiment, electronically controlled) that adjusts the passage area, and through an air cleaner 14 by opening degree control. The amount of inhaled air is adjusted. 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 removes all major harmful components (HC, CO, NOx) in the exhaust gas only when the air-fuel ratio of the air-fuel mixture to be burned is within a narrow range (window) near the stoichiometric air-fuel ratio (stoichiometric). Purify. 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.

  Such air-fuel ratio control is performed by an electronic control unit (hereinafter referred to as “ECU”) 22. The ECU 22 includes the air flow meter 16, the oxygen sensors 19, 20 or the accelerator sensor 21 for detecting the depression amount of the accelerator pedal, the NE sensor 23 for detecting the engine speed, and the outside air temperature sensor 24 for detecting the outside air temperature. Detection signals from various sensors are input. The throttle valve 15 and the injector 17 are driven and controlled in accordance with the operating conditions of the internal combustion engine 10 and the vehicle ascertained from the detection signals of the sensors, thereby controlling the air-fuel ratio as described above.

  First, the ECU 22 obtains the required amount of intake air that is grasped according to the depression amount of the accelerator pedal and the detection result of the engine speed, and sets the opening of the throttle valve 15 so that the intake air amount corresponding to the required amount can be obtained. adjust. 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, the air-fuel ratio feedback control is performed by the two oxygen sensors 19 and 20 as described above, thereby further improving the accuracy of the air-fuel ratio feedback control.

  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 below as an example (hereinafter, the pre-catalyst oxygen sensor 19 is simply referred to as “oxygen sensor 19”), and the description of the post-catalyst oxygen sensor 20 will be omitted.

  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 defines an atmospheric chamber 34 inside thereof. The atmospheric chamber 34 communicates with the outside through an atmospheric passage (not shown) provided in the sensor and an atmospheric hole 35 formed in the sensor body, so that the atmosphere (air) can be led out. The outer surface portion of the detection element 31 is exposed in the exhaust passage 13 and exposed to exhaust gas flowing in through the sensor cover 32.

  As shown in FIG. 3, the detection element 31 includes a solid electrolyte 37, an atmospheric electrode 38 that is formed on the inner surface of the solid electrolyte 37 and faces the atmospheric chamber 34, and an outer surface of the solid electrolyte 37. And an exhaust electrode 39 as the other electrode facing the exhaust passage 13. The solid electrolyte 37 is a solid substance that can move inside the oxygen ionized state. For example, zirconia is used as an oxygen sensor. The atmospheric electrode 38 and the exhaust electrode 39 contain, for example, platinum Pt.

  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. The atmospheric electrode 38 and the exhaust electrode 39 are connected to the ECU 22, particularly a voltage detection circuit thereof, so that the electromotive force generated between the atmospheric electrode 38 and the exhaust electrode 39 can be detected by the ECU 22.

  When a difference in oxygen partial pressure occurs between the atmosphere gas (usually air) at the atmosphere electrode 38 and the atmosphere gas (usually exhaust gas) at the exhaust electrode 39, the oxygen partial pressure is reduced in order to reduce the difference in the partial pressure. The oxygen on the higher side (usually the atmosphere electrode 38 side) is ionized, passes through the solid electrolyte 37, and moves to the side having the lower oxygen partial pressure (usually the exhaust electrode 39 side). Oxygen molecules receive tetravalent electrons in the process of ionization, and emit tetravalent electrons in the process of returning from the ionized state to the molecule. Therefore, movement of electrons occurs between the electrodes 38 and 39 in accordance with the movement of oxygen, and as a result, an electromotive force is generated between the electrodes 38 and 39.

  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 atmospheric electrode 38 to the outer exhaust electrode 39, the direction of the current is reversed, and the inner atmospheric electrode 38 is the positive electrode and the outer exhaust gas for the ECU 22 connected to both electrodes. The pole 39 becomes the 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 with a theoretical air / fuel ratio A / Fs (for example, 14.6) as a boundary, and the air / fuel ratio A / F of the exhaust gas supplied to the oxygen sensor 19 is theoretically changed. A region leaner than the air-fuel ratio A / Fs (A / F> A / Fs, hereinafter also referred to as a lean air-fuel ratio) shows a voltage as small as about 0.1 V, and a region richer than the stoichiometric air-fuel ratio A / Fs (A / F <A / Fs (hereinafter also referred to as a rich air-fuel ratio) indicates a relatively high voltage of about 0.9V. 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 the present embodiment, in an internal combustion engine that performs air-fuel ratio control only for combustion at stoichiometric air-fuel ratio (stoichiometric combustion), oxygen having characteristics in which the output voltage changes transiently with the stoichiometric air-fuel ratio as a boundary. Sensors are often used. Although these sensors have a relatively low resolution, they 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 voltage varies linearly with the air-fuel ratio of exhaust gas, and the resolution is higher. 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 A 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 exhaust electrode 39 is higher than the potential of the atmospheric electrode 38, and a negative (minus) electromotive force or 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, it is possible to detect a deficiency failure of the oxygen sensor.

  However, as described above, even when the oxygen sensor 19 is not defective and is normal, the exhaust electrode 39 has a poisonous substance in the exhaust gas, specifically, a rich component (hydrocarbon HC or monoxide). When poisoned by carbon (CO) or the like (hereinafter referred to as rich poisoning), the oxygen sensor 19 may generate a negative output voltage. 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.

  For example, within a predetermined time after the engine is started, the fuel injection amount may be increased more than the theoretical air-fuel ratio and the air-fuel ratio may be controlled to the rich side in order to stabilize the rotation and promote warm-up.

  When such fuel increase control is performed when the oxygen sensor 19 (especially the exhaust electrode 39) is inactive, the exhaust electrode 39 is exposed to rich exhaust gas, and the exhaust electrode 39 is richly poisoned.

  Then, the oxygen ions that have moved from the atmospheric electrode 38 through the solid electrolyte 37 to the exhaust electrode 39 cannot be sufficiently molecularized (that is, electrons are released), and the oxygen ions concentrate on the exhaust electrode 39.

  Thereafter, when the oxygen sensor 19 is activated, the molecularization of oxygen ions concentrated on the exhaust electrode 39 proceeds rapidly, and lean gas based on fuel cut or the like may reach the exhaust electrode 39. A large amount of oxygen is present on the electrode 39, and as a result, the difference in oxygen concentration between the exhaust electrode 39 and the atmospheric electrode 38 is reversed, and a negative output voltage is generated.

  Thereafter, if the rich component is desorbed from the exhaust electrode 39 and the concentration of oxygen ions at the exhaust electrode 39 is eliminated, the generation of a negative voltage is eliminated.

  In FIG. 7, a change in the oxygen sensor output voltage when the fuel increase control is performed in the normal warm-up process of the oxygen sensor is shown by a solid line. In the figure, the change in the impedance of the detection element of the oxygen sensor (hereinafter also referred to as “element impedance”) is indicated by a broken line. The element impedance is a value that correlates with the temperature of the detection element (hereinafter, also referred to as “element temperature”). As can be seen from the figure, the element temperature gradually increases, and the oxygen sensor gradually changes from the inactive state to the active state.

  As shown in the broken-line circle in the figure, there is a time zone in which the oxygen sensor output voltage is negative. This is because the exhaust electrode 39 is richly poisoned before the sensor is activated, and the effect is after the sensor is activated. This is because it has appeared.

  FIG. 8 shows the relationship between the element temperature and the negative voltage level due to rich poisoning. The white triangle shows the data when the air-fuel ratio A / F is stoichiometric (= 14.6) and normal fuel is used, the black triangle shows the data when the air-fuel ratio is stoichiometric and heavy fuel is used, and the white circle shows the air-fuel ratio is stoichiometric More rich (= 13) and data when using normal fuel, and black circles are data when the air-fuel ratio is richer than stoichiometric and heavy fuel is used.

  As shown in the figure, at an element temperature of about 250 ° C., the richer the air-fuel ratio and the heavier the fuel, the greater the poisoning amount or degree of poisoning, and the negative voltage level output from the oxygen sensor 19. Tend to grow. The difference between these negative voltage levels decreases as the element temperature rises, and there is almost no difference at the element temperature of about 350 ° C. This is because rich components are desorbed from the exhaust electrode 39 as the element temperature rises, and poisoning is eliminated.

  Therefore, in this embodiment, in order to prevent erroneous determination based on the negative voltage due to such rich poisoning, poisoning detection means for detecting the presence or absence of poisoning of the exhaust electrode 39 is provided, and the exhaust electrode is detected by this poisoning detection means. The failure diagnosis of the oxygen sensor 19 is executed on the condition that 39 is not poisoned. This will be described below.

  In the present embodiment, the amount of poisoning substances accumulated on the exhaust electrode, that is, the amount of poisoning, and the amount of poisoning substances desorbed from the exhaust electrode, that is, the amount of desorption are estimated, and these estimated poisoning amount and estimated desorption are estimated. The presence or absence of poisoning of the exhaust electrode 39 is detected based on the amount. Specifically, the remaining poisoning amount is calculated by subtracting the estimated desorption amount from the estimated poisoning amount, and if this remaining poisoning amount is zero or more, there is poisoning, and if it is less than zero, there is no poisoning. Detected.

  FIG. 9 shows a flowchart of the main routine of the failure diagnosis process of the present embodiment. The illustrated routine is repeatedly executed by the ECU 22 every predetermined calculation cycle (for example, 16 msec).

  First, in step S101, it is determined whether or not the engine has been started. When the engine is not started, the routine is ended. When the engine is started, the process proceeds to step S102.

In step S102, the value of the remaining poisoning amount ΔSR _{0 in} the previous trip is acquired from the storage device (SRAM or the like) of the ECU 22, and the poisoning side fuel property coefficient Ks and the desorption side fuel property coefficient Kr are shown in FIG. Each is calculated from a predetermined map as shown in FIG.

The remaining poisoning amount is the amount of poisoning substance that is actually deposited or remaining on the exhaust electrode 39. The trip is a period from the start to the stop of the internal combustion engine, and the previous trip is a trip one time before the current trip. That is, the remaining poisoning amount ΔSR ₀ in the previous trip is the amount of poisoning substances deposited or remaining on the exhaust electrode 39 at the end of the previous trip, and the initial value of the remaining poisoning amount in the current trip. Means.

  The poisoning side fuel property coefficient Ks and the desorption side fuel property coefficient Kr are coefficients used for estimating the poisoning amount and the desorption amount in a subroutine to be described later, respectively, and are calculated based on parameters representing the fuel properties. . Here, the parameter representing the fuel property is, for example, engine rotational fluctuation within a predetermined time immediately after the engine is started. For example, the rotation fluctuation is small for normal fuel, and the rotation fluctuation is large for heavy fuel. Therefore, the rotation fluctuation can be used as a parameter representing fuel properties.

  As can be seen from FIG. 10, the poisoning side fuel property coefficient Ks increases as the rotational fluctuation increases. As a result, as will be understood later, a greater poisoning amount is calculated as the rotational fluctuation increases (for example, the heavier the fuel). Conversely, as can be seen from FIG. 11, the desorption side fuel property coefficient Kr decreases as the rotational fluctuation increases. Thereby, as will be understood later, the smaller the amount of desorption, the greater the rotational fluctuation (for example, the heavier the fuel).

  Next, in step S103, the current residual poisoning amount ΔSR is calculated. This calculation is performed by a subroutine shown in FIG. 12, and details thereof will be described later.

  Thereafter, in step S104, a desorption completion determination is made. That is, if the residual poisoning amount ΔSR calculated in step S103 is equal to or greater than zero, it is determined that the exhaust electrode 39 is poisoned and the desorption of the poisonous substance from the exhaust electrode 39 is not completed. Conversely, if the remaining poisoning amount ΔSR calculated in step S103 is less than zero, it is determined that the exhaust electrode 39 is not poisoned and the desorption of the poisoning substance from the exhaust electrode 39 is completed. Thus, ECU22 which performs this step S104 comprises poisoning detection means.

  If it is not determined in step S104 that desorption is complete, that is, if it is determined that the exhaust electrode 39 is poisoned, the process proceeds to step S108, and the current value of the remaining poisoning amount ΔSR is stored in the storage device. And the routine is terminated.

  On the other hand, if it is determined in step S104 that the desorption is completed, that is, if it is determined that the poisoning of the exhaust electrode 39 has been eliminated, the process proceeds to step S105 and thereafter, and a failure diagnosis based on the negative voltage is executed.

  First, in step S105, it is determined whether a precondition for executing failure diagnosis is satisfied. When this precondition is satisfied, for example, when engine warm-up has been completed to some extent, specifically, the engine coolant temperature detected by a water temperature sensor (not shown) is a predetermined temperature (for example, 40 ° C). However, this precondition can be set arbitrarily.

If the precondition is not satisfied, the process proceeds to step S108, where the current residual poisoning amount ΔSR is stored in the storage device, and the routine is terminated. On the other hand, when the precondition is satisfied, the process proceeds to step S106, and it is determined whether or not a negative output voltage from the oxygen sensor 19 is detected.
If a negative output voltage is detected, the process proceeds to step S107, and it is determined that the oxygen sensor 19 is faulty or abnormal. That is, it is determined that a defect such as a crack or a crack has occurred in the detection element 31 of the oxygen sensor 19. Thereafter, the process proceeds to step S108, where the current residual poisoning amount ΔSR is stored in the storage device, and the routine is terminated.

  On the other hand, if a negative output voltage is not detected, the sensor is not immediately normal, and in order to improve accuracy, in step S109, it is determined whether or not a condition that allows normal determination (normal determination condition) is satisfied. The The normal determination condition is a condition in which a negative voltage is always generated when the oxygen sensor 19 is defective, and is, for example, a fuel cut immediately after an operation with a large intake air amount Ga. When the intake air amount Ga is small, the exhaust gas flow rate is also small, and there is a possibility that the exhaust gas does not sufficiently flow into the atmospheric chamber 34 from the defect portion A of the detection element 31. In addition, the exhaust gas sufficiently flows into the atmospheric chamber 34 This is because when the fuel is cut, the outer side of the detection element 31 has a higher oxygen partial pressure than the inner side and a negative voltage is generated.

  If it is determined that the normal determination condition is not satisfied, the process proceeds to step S108 without being determined normal, and the remaining poisoning amount ΔSR is saved, and the routine is terminated. On the other hand, if it is determined that the normal determination condition is satisfied, it is determined in step S110 that the oxygen sensor 19 is normal, and then the remaining poisoning amount ΔSR is stored in step S108, and the routine is terminated.

  Next, a subroutine for calculating the remaining poisoning amount ΔSR will be described with reference to FIG. The illustrated routine is also repeatedly executed by the ECU 22 every predetermined calculation cycle (for example, 16 msec).

  First, in step S201, the element temperature of the oxygen sensor 19 is detected, and this element temperature is compared with a first predetermined value X.

  That is, the ECU 22 constantly detects the element impedance R of the oxygen sensor 19 and converts this element impedance R into element temperature. The first predetermined value X is set to the maximum temperature at which the exhaust electrode 39 can be poisoned. In the present embodiment, the first predetermined value X is set to 350 ° C. in consideration of the result of FIG. The first predetermined value X is usually a temperature higher than the minimum activation temperature (for example, 300 ° C.) of the oxygen sensor 19.

  When the element temperature is lower than the first predetermined value X, the exhaust electrode 39 may be poisoned, so the poisoning amount is calculated in steps S202 to S204.

  First, in step S202, the poisoning side element temperature coefficient Ts is calculated from a predetermined map or the like as shown in FIG. The poisoning side element temperature coefficient Ts is a coefficient used for calculating the poisoning amount, and is calculated based on the element temperature. As can be seen from FIG. 13, the poisoning side element temperature coefficient Ts increases as the element temperature decreases. Thereby, as will be understood later, a greater poisoning amount is calculated as the element temperature decreases.

  Next, in step S203, the values of the current air-fuel ratio AF and the fuel injection amount TAU are acquired. The air-fuel ratio AF is obtained by dividing the intake air amount Ga detected by the air flow meter 16 by the fuel injection amount TAU. Further, since the ECU 22 determines the fuel injection amount TAU, the fuel injection amount TAU itself is an internal value in the ECU 22.

Next, in step S204, the current value S and the integrated value TS of the poisoning amount are calculated. The current value S is the amount of poisonous substances estimated to have accumulated on the exhaust electrode between the previous calculation time and the current calculation time, and the integrated value TS is the integrated start time (specifically, The sum of the residual poisoning amount ΔSR _0, that is, the initial value and the current value ΣS sequentially accumulated from the accumulation start time to the present time (when the step S103 in FIG. 9 is first executed after the engine start) Value. That is, the integrated value TS represents the estimated value (estimated poisoning amount) of the poisoning amount at the present time. The current value S and the integrated value TS are calculated by the following equations, respectively.

  As can be seen from these equations, the estimated value of the poisoning amount is calculated based on the parameter indicating the fuel property, the element temperature, the air-fuel ratio AF, and the fuel injection amount TAU. As the air-fuel ratio AF is richer than the theoretical air-fuel ratio (= 14.6), a larger current value S is obtained. When the air-fuel ratio AF is leaner than the stoichiometric air-fuel ratio, or when the fuel injection amount TAU becomes zero due to fuel cut, a current value S that is less than or equal to zero is obtained in the calculation. The current value S is treated as zero.

  After the current value S and the integrated value TS of the poisoning amount are calculated in this way, the process proceeds to step S205, where the current remaining poisoning amount ΔSR is calculated, and the routine ends. The remaining poisoning amount ΔSR is calculated by the following equation.

Here, TR is an integrated value of the desorption amount calculated in step S209 described later.
On the other hand, when the detection element temperature is equal to or higher than the first predetermined value X in step S201, the process proceeds to step S206, and the detection element temperature is compared with the second predetermined value Y. The second predetermined value Y is set to a minimum temperature at which desorption of poisonous substances from the exhaust electrode 39 can occur, and is a value equal to or greater than the first predetermined value X. In the present embodiment, the temperature is set to 360 ° C. in consideration of the result of FIG. In the present embodiment, the second predetermined value Y is set higher than the first predetermined value X on the assumption that neither poisoning nor desorption occurs between the first predetermined value Y and the second predetermined value Y. However, they may be set to equal values.

  If the detection element temperature is lower than the second predetermined value Y in step S206, the process proceeds to step S205, where the current residual poisoning amount ΔSR is calculated, and the routine is terminated.

  On the other hand, if the detection element temperature is equal to or higher than the second predetermined value Y in step S206, detoxication of the poisoning substance from the exhaust electrode 39 may occur, and thus the desorption amount is calculated in steps S207 to S209. .

  First, in step S207, the detachment side element temperature coefficient Tr is calculated from a predetermined map as shown in FIG. The desorption-side element temperature coefficient Tr is a coefficient used for calculating the desorption amount, and is calculated based on the element temperature. As can be seen from FIG. 14, the detachment side element temperature coefficient Tr increases as the element temperature increases. Thereby, as will be understood later, a larger amount of desorption is calculated as the element temperature increases.

  Next, in step S208, the values of the current air-fuel ratio AF and the fuel injection amount TAU are acquired. The air-fuel ratio AF and the fuel injection amount TAU are the same as described in step S203.

  Next, in step S209, the current value R and the integrated value TR of the desorption amount are calculated. The current value R is the amount of poisonous substances estimated to have desorbed from the exhaust electrode between the previous calculation time and the current calculation time, and the integrated value TR is the integrated start time (specifically, Is the current value ΣR accumulated sequentially from the time when this step S209 is first executed) to the present time. That is, the integrated value TR represents the estimated value (estimated desorption amount) of the current desorption amount. The current value R and the integrated value TR are calculated by the following equations, respectively.

  As can be seen from these equations, the estimated value of the desorption amount is calculated based on the parameter indicating the fuel property, the element temperature, the air-fuel ratio AF, and the fuel injection amount TAU, similarly to the estimated value of the poisoning amount. It will be. As the air-fuel ratio AF is leaner than the stoichiometric air-fuel ratio (= 14.6), a larger current value R is obtained. As described above, when the air-fuel ratio AF is richer than the stoichiometric air-fuel ratio, or when the fuel injection amount TAU becomes zero due to fuel cut, a current value R of zero or less is obtained in calculation. Handles the current value R as zero for processing.

  After the current value R and the integrated value TR of the desorption amount are calculated in this way, the process proceeds to step S205, where the current remaining poisoning amount ΔSR is calculated, and the routine ends.

  As mentioned above, although embodiment of this invention was described in detail, this invention can also take other embodiment. For example, the use, type, type, and the like of the internal combustion engine are not limited, and the internal combustion engine may be other than for a vehicle, a diesel engine, or the like.

  The estimated poisoning amount and the estimated desorption amount may be calculated based on parameters other than those described above, for example, the amount of exhaust gas, the exhaust gas temperature, the time when the detection element is exposed to the exhaust gas, and the like. Further, element impedance, heater input power, exhaust gas temperature, or the like may be used as a substitute value for element temperature.

  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.

DESCRIPTION OF SYMBOLS 10 Internal combustion engine 13 Exhaust passage 19, 20 Oxygen sensor 22 Electronic control unit (ECU)
31 detection element 34 atmosphere chamber 38 atmosphere electrode 39 exhaust electrode

Claims (1)

In an apparatus for diagnosing a failure of the oxygen sensor based on a negative voltage output from an oxygen sensor disposed in an exhaust passage of an internal combustion engine,
There is provided poisoning detection means for detecting the presence or absence of poisoning of the exhaust electrode in the detection element of the oxygen sensor, and the oxygen detection is performed on the condition that the poisoning detection means detects that the exhaust electrode is not poisoned. A fault diagnosis apparatus for an oxygen sensor that performs fault diagnosis of the sensor.

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