JP2015075082A - Air-fuel ratio sensor control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air-fuel ratio sensor control device capable of accurately diagnosing abnormality in the air-fuel ratio sensor.SOLUTION: A control device of air-fuel ratio sensors 40 and 41 comprises: the air-fuel ratio sensors arranged in an exhaust passage of an internal combustion engine; impedance detection means which detects impedance of elements constituting the air-fuel ratio sensors; a heating device 59 which heats the elements constituting the air-fuel ratio sensors; and heating amount control means which controls a heating amount of the heating device on the basis of the impedance detected by the impedance detection means. The control device also has abnormality diagnosing means which diagnoses abnormality in output of the air-fuel ratio sensors. When the abnormality diagnosing means diagnoses the abnormality in the output of the air-fuel ratio sensors, the heating amount control means controls the heating amount of the heating device regardless of the impedance detected by the impedance detection means.

Description

  The present invention relates to an air-fuel ratio sensor control apparatus.

  2. Description of the Related Art Conventionally, there is known an internal combustion engine that is provided with an air-fuel ratio sensor in an exhaust passage of the internal combustion engine and that controls the amount of fuel supplied to the internal combustion engine based on the output of the air-fuel ratio sensor. The air-fuel ratio sensor used in such an internal combustion engine gradually deteriorates with use. When the air-fuel ratio sensor is deteriorated, the output of the air-fuel ratio sensor does not become an appropriate value, and thus various controls executed by the control device for the internal combustion engine are hindered.

  Therefore, a diagnostic apparatus for diagnosing deterioration of the air-fuel ratio sensor has been proposed. As such a diagnostic apparatus, for example, the target air-fuel ratio is changed stepwise, and accordingly, the first response time until the output value of the air-fuel ratio sensor reaches the first predetermined value, and the first predetermined A second response time until reaching a second predetermined value greater than the value is determined, and deterioration of the air-fuel ratio sensor is determined based on two of the first response time and the second response time. (For example, Patent Document 1). Here, as a deterioration pattern of the air-fuel ratio sensor, there is gain deterioration in which the output itself increases or decreases in addition to response deterioration in which the response time is delayed. On the other hand, according to the diagnostic device described in Patent Document 1, by determining the deterioration of the air-fuel ratio sensor based on two of the first response time and the second response time, one of the two deterioration patterns can be determined. This makes it possible to accurately determine whether the air-fuel ratio sensor has deteriorated.

JP 2007-192093 A JP 2001-355505 A JP 2009-007991 A

  By the way, it is known that the abnormality of the output value of the air-fuel ratio sensor is larger or smaller than the value corresponding to the actual air-fuel ratio due to the deterioration of the air-fuel ratio sensor as described above. As a method for detecting such an abnormality of the air-fuel ratio sensor, the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is set to a constant air-fuel ratio, and the air-fuel ratio based on the output value of the air-fuel ratio sensor at that time It has been proposed to perform sensor abnormality diagnosis. In this case, when the difference between the output value of the air-fuel ratio sensor and the value corresponding to the actual air-fuel ratio is small, it is determined that no abnormality has occurred in the air-fuel ratio sensor. On the other hand, when the difference between the output value of the air-fuel ratio sensor and the value corresponding to the actual air-fuel ratio is large, it is determined that an abnormality has occurred in the air-fuel ratio sensor.

  On the other hand, in the air-fuel ratio sensor as described above, the output value changes according to the temperature of the elements constituting the air-fuel ratio sensor. For this reason, generally, the air-fuel ratio sensor is provided with a heater for heating the element, and the element is heated by this heater so that the temperature of the element becomes constant. In particular, since the air-fuel ratio sensor has a property that the impedance of the element changes according to the temperature of the element, the element is heated so that the impedance of the element becomes constant.

  However, according to the study by the present inventors, it has been found that if the temperature control is performed based on the impedance of the element of the air-fuel ratio sensor in this way, the accuracy in the abnormality diagnosis of the air-fuel ratio sensor is lowered. As described above, when the output value is deviated from the value corresponding to the actual air-fuel ratio due to the abnormality of the air-fuel ratio sensor, the deviation in the output value is reduced by performing the temperature control based on the impedance described above. Thus, the temperature of the air-fuel ratio sensor element is controlled.

  In view of the above problems, an object of the present invention is to accurately diagnose an abnormality even in an air-fuel ratio sensor that performs temperature control of an element based on the impedance of the element constituting the air-fuel ratio sensor. An object of the present invention is to provide a control device for an air-fuel ratio sensor.

  In order to solve the above problems, in the first invention, an air-fuel ratio sensor disposed in an exhaust passage of an internal combustion engine, an impedance detection means for detecting an impedance of an element constituting the air-fuel ratio sensor, and the air-fuel ratio sensor In the control device for an air-fuel ratio sensor, comprising: a heating device that heats an element that constitutes the heating element; and a heating amount control unit that controls a heating amount by the heating device based on the impedance detected by the impedance detection unit. An abnormality diagnosis means for diagnosing an output abnormality of the air-fuel ratio sensor is further provided, and the heating amount control means is configured to detect the impedance detected by the impedance detection means when the abnormality diagnosis means diagnoses an output abnormality of the air-fuel ratio sensor. There is provided an air-fuel ratio sensor control device that controls the heating amount by the heating device regardless of That.

  In a second invention, in the first invention, the heating amount control means controls the heating amount by the heating device to be constant when the abnormality diagnosis means diagnoses an output abnormality of the air-fuel ratio sensor.

  According to a third aspect, in the first aspect, the apparatus further comprises an atmospheric temperature detection device for detecting an atmospheric temperature, and the heating amount control means uses the abnormality diagnosis means to diagnose an output abnormality of the air-fuel ratio sensor. The heating amount by the heating device is controlled based on the atmospheric temperature detected by the atmospheric temperature detection device.

  According to a fourth aspect, in the third aspect, the apparatus further comprises a catalyst temperature detecting device for detecting the temperature of the exhaust purification catalyst provided in the exhaust passage of the internal combustion engine, wherein the air-fuel ratio sensor is an exhaust of the exhaust purification catalyst. The heating amount control unit is provided downstream in the flow direction, and when the abnormality diagnosis unit diagnoses an output abnormality of the air-fuel ratio sensor, the atmospheric temperature detected by the atmospheric temperature detection device and the catalyst temperature detection device The amount of heating by the heating device is controlled based on the catalyst temperature detected by.

  According to a fifth aspect of the invention, in any one of the first to fourth aspects, the abnormality diagnosis unit is configured to maintain the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor at a predetermined air-fuel ratio. The output abnormality of the air-fuel ratio sensor is diagnosed based on the output of the air-fuel ratio sensor.

  In a sixth aspect based on the fifth aspect, the abnormality diagnosing means is configured such that the air-fuel ratio sensor is based on an output of the air-fuel ratio sensor when the exhaust gas flowing around the air-fuel ratio sensor is maintained as atmospheric gas. Diagnose the abnormal output.

  In a seventh aspect based on the sixth aspect, the internal combustion engine can execute fuel cut control for stopping fuel supply to the combustion chamber during operation of the internal combustion engine, and the abnormality diagnosis means An output abnormality of the air-fuel ratio sensor is diagnosed during cut control.

  In an eighth invention according to any one of the first to seventh inventions, when the heating amount control means is not diagnosing an output abnormality of the air-fuel ratio sensor by the abnormality diagnosis means, the impedance detection means The heating amount by the heating device is controlled so that the detected impedance becomes a predetermined target impedance.

  According to the present invention, there is provided an air-fuel ratio sensor control apparatus capable of accurately diagnosing an abnormality even in an air-fuel ratio sensor that performs temperature control of the element based on the impedance of the element constituting the air-fuel ratio sensor. Is done.

FIG. 1 is a diagram schematically showing an internal combustion engine in which a control device for an air-fuel ratio sensor of the present invention is used. FIG. 2 is a schematic cross-sectional view of the air-fuel ratio sensor. FIG. 3 is a diagram showing the relationship between the sensor applied voltage and the output current at each exhaust air-fuel ratio. FIG. 4 is a diagram showing the relationship between the exhaust air-fuel ratio and the output current when the sensor applied voltage is made constant. FIG. 5 is a diagram showing the relationship between the sensor applied voltage and the output current when the exhaust gas flowing is atmospheric gas. FIG. 6 is a diagram showing the relationship between the temperature and impedance of the solid electrolyte layer. FIG. 7 is a time chart of sensor applied voltage and output current. FIG. 8 is a view similar to FIG. 3 showing the relationship between the sensor applied voltage and the output current at each exhaust air-fuel ratio. FIG. 9 is a view similar to FIG. 5 showing the relationship between the sensor applied voltage and the output current when the flowing exhaust gas is atmospheric gas. FIG. 10 is a diagram showing the relationship between the sensor applied voltage and output current and the normality / abnormality of the air-fuel ratio sensor when the flowing exhaust gas is atmospheric gas. FIG. 11 is a view similar to FIG. 5 showing the relationship between the sensor applied voltage and the output current when the exhaust gas is atmospheric gas. FIG. 12 is a diagram showing the relationship between the atmospheric temperature and the heater heating amount. FIG. 13 is a diagram showing the relationship between the exhaust gas flow rate and the heater heating amount. FIG. 14 is a flowchart showing a control routine of the abnormality diagnosis control of this embodiment.

  Hereinafter, an internal combustion engine diagnosis apparatus of the present invention will be described in detail with reference to the drawings. In the following description, the same reference numerals are assigned to similar components. FIG. 1 is a diagram schematically showing an internal combustion engine in which a diagnostic device according to a first embodiment of the present invention is used.

<Description of the internal combustion engine as a whole>
Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is a piston 3 and a cylinder head 4. A combustion chamber formed therebetween, 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, and 9 is an exhaust port. The intake valve 6 opens and closes the intake port 7, and the exhaust valve 8 opens and closes the exhaust port 9.

  As shown in FIG. 1, a spark plug 10 is disposed at the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is disposed around the inner wall surface of the cylinder head 4. The spark plug 10 is configured to generate a spark in response to the ignition signal. The fuel injection valve 11 injects a predetermined amount of fuel into the combustion chamber 5 according to the injection signal. The fuel injection valve 11 may be arranged so as to inject fuel into the intake port 7. In this embodiment, gasoline having a theoretical air-fuel ratio of 14.6 is used as the fuel. However, other fuels may be used in the internal combustion engine in which the diagnostic device of the present invention is used.

  The intake port 7 of each cylinder is connected to a surge tank 14 via a corresponding intake branch pipe 13, and the surge tank 14 is connected to an air cleaner 16 via an intake pipe 15. The intake port 7, the intake branch pipe 13, the surge tank 14, and the intake pipe 15 form an intake passage. A throttle valve 18 driven by a throttle valve drive actuator 17 is disposed in the intake pipe 15. The throttle valve 18 is rotated by a throttle valve drive actuator 17 so that the opening area of the intake passage can be changed.

  On the other hand, the exhaust port 9 of each cylinder is connected to an exhaust manifold 19. The exhaust manifold 19 has a plurality of branches connected to the exhaust ports 9 and a collective part in which these branches are assembled. A collecting portion of the exhaust manifold 19 is connected to an upstream casing 21 containing an upstream exhaust purification catalyst 20. The upstream casing 21 is connected to a downstream casing 23 containing a downstream exhaust purification catalyst 24 via an exhaust pipe 22. The exhaust port 9, the exhaust manifold 19, the upstream casing 21, the exhaust pipe 22, and the downstream casing 23 form an exhaust passage.

  Both the upstream side exhaust purification catalyst 20 and the downstream side exhaust purification catalyst 24 are three-way catalysts having oxygen storage capacity. When the exhaust purification catalysts 20 and 24 reach a predetermined activation temperature, the exhaust purification catalysts 20 and 24 exhibit an oxygen storage capability in addition to the catalytic action of simultaneously purifying unburned gas (HC, CO, etc.) and nitrogen oxides (NOx). According to the oxygen storage capacity of the exhaust purification catalysts 20, 24, the exhaust purification catalysts 20, 24 have an air / fuel ratio of exhaust gas flowing into the exhaust purification catalysts 20, 24 leaner than a stoichiometric air / fuel ratio (hereinafter referred to as “lean air / fuel ratio”). Is stored in the exhaust gas. On the other hand, when the air-fuel ratio of the inflowing exhaust gas is richer than the stoichiometric air-fuel ratio (hereinafter referred to as “rich air-fuel ratio”), the exhaust purification catalysts 20, 24 are oxygen stored in the exhaust purification catalysts 20, 24. Release. The exhaust purification catalysts 20 and 24 may be different from the three-way catalyst as long as they have a catalytic action and an oxygen storage capacity.

  An electronic control unit (ECU) 31 comprises a digital computer, and is connected to each other via a bidirectional bus 32, a RAM (Random Access Memory) 33, a ROM (Read Only Memory) 34, a CPU (Microprocessor) 35, an input A port 36 and an output port 37 are provided. An air flow meter 39 for detecting the flow rate of air flowing through the intake pipe 15 is disposed in the intake pipe 15, and the output of the air flow meter 39 is input to the input port 36 via the corresponding AD converter 38. Further, an upstream air-fuel ratio sensor 40 that detects the air-fuel ratio of the exhaust gas flowing through the exhaust manifold 19 (that is, the exhaust gas flowing into the upstream exhaust purification catalyst 20) is disposed at the collecting portion of the exhaust manifold 19. In addition, in the exhaust pipe 22, the downstream side that detects the air-fuel ratio of the exhaust gas that flows in the exhaust pipe 22 (that is, the exhaust gas that flows out of the upstream side exhaust purification catalyst 20 and flows into the downstream side exhaust purification catalyst 24). An air-fuel ratio sensor 41 is arranged. The outputs of these air-fuel ratio sensors 40 and 41 are also input to the input port 36 via the corresponding AD converter 38. The configuration of these air-fuel ratio sensors 40 and 41 will be described later.

  A load sensor 43 that generates an output voltage proportional to the amount of depression of the accelerator pedal 42 is connected to the accelerator pedal 42, and the output voltage of the load sensor 43 is input to the input port 36 via the corresponding AD converter 38. The For example, the crank angle sensor 44 generates an output pulse every time the crankshaft rotates 15 degrees, and this output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of the crank angle sensor 44. On the other hand, the output port 37 is connected to the spark plug 10, the fuel injection valve 11, and the throttle valve drive actuator 17 via the corresponding drive circuit 45.

<Description of air-fuel ratio sensor>
In this embodiment, as the air-fuel ratio sensors 40 and 41, limit current type air-fuel ratio sensors are used. The structure of the air-fuel ratio sensors 40 and 41 will be briefly described with reference to FIG. The air-fuel ratio sensors 40 and 41 include a solid electrolyte layer 51, an exhaust-side electrode 52 disposed on one side surface thereof, an atmosphere-side electrode 53 disposed on the other side surface, and diffusion of exhaust gas passing therethrough. A diffusion control layer 54 that controls the speed, a protective layer 55 that protects the diffusion control layer 54, and a heater unit 56 that heats the air-fuel ratio sensors 40 and 41 are provided.

  A diffusion rate controlling layer 54 is provided on one side surface of the solid electrolyte layer 51, and a protective layer 55 is provided on the side surface of the diffusion rate controlling layer 54 opposite to the side surface on the solid electrolyte layer 51 side. In the present embodiment, a measured gas chamber 57 is formed between the solid electrolyte layer 51 and the diffusion-controlling layer 54. A gas to be detected by the air-fuel ratio sensors 40, 41, that is, exhaust gas, is introduced into the measured gas chamber 57 through the diffusion rate controlling layer 54. Further, the exhaust side electrode 52 is disposed in the measured gas chamber 57, and therefore, the exhaust side electrode 52 is exposed to the exhaust gas through the diffusion rate controlling layer 54. The gas chamber 57 to be measured is not necessarily provided, and may be configured such that the diffusion-controlling layer 54 is in direct contact with the surface of the exhaust-side electrode 52.

  A heater portion 56 is provided on the other side surface of the solid electrolyte layer 51. A reference gas chamber 58 is formed between the solid electrolyte layer 51 and the heater portion 56, and the reference gas is introduced into the reference gas chamber 58. In the present embodiment, the reference gas chamber 58 is open to the atmosphere, and therefore the atmosphere is introduced into the reference gas chamber 58 as the reference gas. The atmosphere side electrode 53 is disposed in the reference gas chamber 58, and therefore the atmosphere side electrode 53 is exposed to the reference gas. In the present embodiment, since the atmosphere is used as the reference gas, the atmosphere side electrode 53 is exposed to the atmosphere.

  The heater unit 56 is provided with a plurality of heaters (heating devices) 59, and these heaters 59 can control the temperature of the air-fuel ratio sensors 40 and 41, particularly the temperature of the solid electrolyte layer 51. The heater unit 56 has a heat generation capacity sufficient to heat the solid electrolyte layer 51 until it is activated. The heater 59 is connected to the ECU 31, and the amount of heating by the heater 59 is controlled by the ECU 31. Therefore, the ECU 31 functions as a heating amount control unit that controls the heating amount by the heater 59.

The solid electrolyte layer 51 is an oxygen ion conductive oxide in which ZrO ₂ (zirconia), HfO ₂ , ThO ₂ , Bi ₂ O _3, etc. are distributed with CaO, MgO, Y ₂ O ₃ , Yb ₂ O _3, etc. as stabilizers. The sintered body is formed. The diffusion control layer 54 is formed of a porous sintered body of a heat-resistant inorganic substance such as alumina, magnesia, silica, spinel, mullite or the like. Furthermore, the exhaust-side electrode 52 and the atmosphere-side electrode 53 are formed of a noble metal having high catalytic activity such as platinum.

  Further, a sensor application voltage V is applied between the exhaust side electrode 52 and the atmosphere side electrode 53 by a voltage application device 60 mounted on the ECU 31. In addition, the ECU 31 is provided with a current detection device 61 that detects a current I flowing between the electrodes 52 and 53 via the solid electrolyte layer when a sensor applied voltage is applied. The current detected by the current detector 61 is the output current of the air-fuel ratio sensors 40 and 41.

The thus configured air-fuel ratio sensors 40 and 41 have voltage-current (V-I) characteristics as shown in FIG. As can be seen from FIG. 3, the output current I increases as the exhaust air-fuel ratio increases (lean). The V-I line at each exhaust air-fuel ratio has a region parallel to the V axis, that is, a region where the output current hardly changes even when the sensor applied voltage changes. This voltage region is referred to as a limiting current region, and the current at this time is referred to as a limiting current. In FIG. 3, the limit current region and limit current when the exhaust air-fuel ratio is 18 are indicated by W ₁₈ and I ₁₈ , respectively.

  On the other hand, in a region where the sensor applied voltage is lower than the limit current region, the output current changes almost in proportion to the sensor applied voltage. Such a region is called a proportional region. The inclination at this time is determined by the DC element resistance of the solid electrolyte layer 51. Further, in a region where the sensor applied voltage is higher than the limit current region, the output current increases as the sensor applied voltage increases. In this region, the output voltage changes according to the change in the sensor applied voltage due to, for example, decomposition of moisture contained in the exhaust gas on the exhaust side electrode 52.

  FIG. 4 is a diagram showing the relationship between the exhaust air-fuel ratio and the output current I when the sensor applied voltage is kept constant at about 0.45V. As can be seen from FIG. 4, in the air-fuel ratio sensors 40 and 41, the output current I from the air-fuel ratio sensors 40 and 41 increases as the exhaust air-fuel ratio increases (that is, as the exhaust air-fuel ratio becomes leaner). In addition, the air-fuel ratio sensors 40 and 41 are configured such that the output current I becomes zero when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio. Further, when the exhaust air-fuel ratio becomes larger than a certain value (18 or more in the present embodiment) or becomes smaller than a certain value, the ratio of the change in the output current to the change in the exhaust air-fuel ratio becomes smaller.

  In the above example, the limit current type air-fuel ratio sensor having the structure shown in FIG. However, as long as the air-fuel ratio sensor has the characteristics described later, any air-fuel ratio sensor such as a limit current type air-fuel ratio sensor of another structure or an air-fuel ratio sensor not of the limit current type may be used.

In the following description, it is assumed that the air-fuel ratio sensors 40 and 41 basically maintain the sensor applied voltage at a constant value V ₁ during operation of the internal combustion engine. However, the sensor applied voltage of the air-fuel ratio sensors 40 and 41 does not need to be maintained at a constant value, and may be changed according to the output current, for example.

<Heater control of air-fuel ratio sensor>
By the way, in the air-fuel ratio sensors 40 and 41 configured as described above, the output value changes according to the temperature of the solid electrolyte layer 51 which is an element constituting the air-fuel ratio sensors 40 and 41. This is shown in FIG.

FIG. 5 shows the relationship between the sensor applied voltage V and the output current I to the air-fuel ratio sensors 40 and 41 when the exhaust gas flowing around the air-fuel ratio sensors 40 and 41 is atmospheric gas. When the solid line in the drawing element temperature of the air-fuel ratio sensor 40 and 41, in particular the temperature of the solid electrolyte layer 51 is T _1, the broken line when the temperature of the solid electrolyte layer 51 is T _2, the dashed line the solid electrolyte layer 51 The relationship at the temperature T ₃ is shown. Among the temperatures T _{1 to} T ₃ , T ₁ is the highest and decreases in the order of T ₂ and T ₃ (T ₁ > T ₂ > T ₃ ). As can be seen from FIG. 5, the output currents of the air-fuel ratio sensors 40 and 41 increase as the temperature increases when the sensor applied voltage is constant. For example, if a sensor applied voltage is V ₁ was, the output current when the temperature of the solid electrolyte layer 51 is T _1, T _2, T ₃ becomes _{I 1, I 2, I 3} , I 1 is Largest and smaller in the order of I ₂ and I ₃ .

  As described above, in the air-fuel ratio sensors 40 and 41, even if the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40 and 41 is constant, the output current changes according to the temperature. For this reason, in order to accurately detect the air-fuel ratio of the exhaust gas by the air-fuel ratio sensors 40, 41, it is necessary to keep the temperature of the solid electrolyte layer 51 constant.

  Incidentally, there is a close relationship between the temperature of the solid electrolyte layer 51 and the element resistance (impedance). FIG. 6 is a diagram showing the relationship between the temperature of the solid electrolyte layer 51 and the impedance. As shown in FIG. 6, the higher the temperature of the solid electrolyte layer 51, the lower the impedance of the solid electrolyte layer 51.

  Therefore, in the present embodiment, during normal operation of the internal combustion engine, the impedance of the solid electrolyte layer 51 is detected, and the heating amount of the solid electrolyte layer 51 by the heater 59 is determined based on the detected impedance of the solid electrolyte layer 51. I have control. In particular, in the present embodiment, the heating amount of the solid electrolyte layer 51 by the heater 59 is controlled so that the detected impedance of the solid electrolyte layer 51 becomes the target impedance.

  First, an impedance detection method will be described. In the present embodiment, the impedance of the solid electrolyte layer 51 is detected based on the current I flowing between the exhaust side electrode 52 and the atmosphere side electrode 53. In particular, in the present embodiment, detection is performed based on a change in the current I when the applied voltage applied between the exhaust side electrode 52 and the atmosphere side electrode 53 is slightly changed in a short time.

FIG. 7 is a time chart of the sensor applied voltage V applied between the exhaust side electrode 52 and the atmosphere side electrode 53 and the output current flowing between the electrodes 52 and 53. As shown in FIG. 7, a predetermined sensor applied voltage V ₁ is basically applied between the electrodes in order to detect the air-fuel ratio of the exhaust gas. The output current with respect to this becomes a constant value as long as the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensors 40, 41 does not change.

The output current I of the air-fuel ratio sensors 40 and 41 is used for feedback control of the fuel injection amount from the fuel injection valve 11 and the like. The calculation of the fuel injection amount in the feedback control is performed at regular time intervals (for example, every 128 ms). Along with this, the output currents of the air-fuel ratio sensors 40 and 41 are also referred to at regular time intervals for feedback control. In the example shown in FIG. 7, the output current is referred to at time t ₂ and time t ₄ .

In addition, in the example shown in FIG. 7, the applied current V is slightly vibrated up and down over a minute period during the output current reference timing (t ₂ , t ₄ ) (time t ₁ , t _3). ). In particular, in the illustrated example, the sensor applied voltage V is increased and decreased once each as the vibration of the sensor applied voltage V. Thus, when the sensor applied voltage V is oscillated, the output current I oscillates even if the air-fuel ratio of the surrounding exhaust gas does not change. Thus, when the sensor applied voltage V and the output current I fluctuate, the impedance R can be calculated by the following equation (1).
R = dV / dI (1)

  Thus, in the present embodiment, the sensor applied voltage V is vibrated slightly up and down over a minute period, and the impedance R of the solid electrolyte layer 51 is calculated based on the fluctuation of the output current I at that time. ing. Thereby, the impedance of the solid electrolyte layer 51 can be detected regardless of the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40 and 41.

  In addition, since detection of the impedance of the solid electrolyte layer 51 mentioned above is performed by ECU31, ECU31 functions as an impedance detection means. The method for detecting the impedance (resistance) of the solid electrolyte layer 51 is not necessarily limited to the above method. For example, a resistance (impedance) detection electrode and an electric circuit of the solid electrolyte layer 51 may be provided separately from the exhaust-side electrode 52 and the atmosphere-side electrode 53, and detection may be performed using these. Alternatively, the impedance may be detected by a method different from the above method using the exhaust side electrode 52 and the atmosphere side electrode 53.

  Next, a method for controlling the heating amount of the solid electrolyte layer 51 by the heater 59 based on the impedance detected as described above will be described. When the detected impedance is lower than a predetermined target impedance corresponding to a predetermined target temperature, the heating amount per unit time by the heater 59 is increased. In the present embodiment, the higher the heater applied voltage to the heater 59, the greater the amount of heating of the solid electrolyte layer 51. Therefore, when the detected impedance is lower than the target impedance, the heater applied voltage is increased. . On the other hand, when the detected impedance is higher than the target impedance, the heating amount per unit time by the heater 59 is reduced. Therefore, in this embodiment, in this case, the heater applied voltage is reduced.

  Therefore, in this embodiment, the heater applied voltage is feedback-controlled so that the detected impedance becomes the target impedance. As a specific control method at this time, various feedback controls such as PID control can be used. In addition, by performing feedback control of the heater applied voltage in this way, the temperature of the solid electrolyte layer 51 of the air-fuel ratio sensors 40 and 41 can be maintained constant.

<Abnormality diagnosis of air-fuel ratio sensor>
Incidentally, as described at the beginning, it is known that the output current of the air-fuel ratio sensors 40, 41 becomes larger or smaller than the value corresponding to the actual air-fuel ratio due to the deterioration of the air-fuel ratio sensors 40, 41. It has been. This is shown in FIG.

  FIG. 8 is a view similar to FIG. 3 showing the relationship between the sensor applied voltage V and the output current I at each exhaust air-fuel ratio. The solid line in the figure indicates the case where the abnormality in the air-fuel ratio sensors 40, 41 has deteriorated, and the broken line in the figure indicates the case in which no abnormality has occurred. In the example shown in FIG. 8, when the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor 40 is the same, when an abnormality occurs in the air-fuel ratio sensor 40, an output is output relative to when no abnormality occurs. The absolute value of the current I decreases.

  Thus, when an abnormality occurs in the air-fuel ratio sensors 40 and 41, the output current of the air-fuel ratio sensors 40 and 41 becomes different from the value corresponding to the actual air-fuel ratio of the exhaust gas. When such a deviation occurs in the output current of the air-fuel ratio sensors 40 and 41, various controls using the outputs of the air-fuel ratio sensors 40 and 41 are hindered. For this reason, it is necessary to diagnose abnormality of the air-fuel ratio sensors 40 and 41.

  FIG. 9 is a view similar to FIG. 5 showing the relationship between the sensor applied voltage V and the output current I to the air-fuel ratio sensors 40 and 41 when the exhaust gas flowing around the air-fuel ratio sensor 40 is atmospheric gas. is there. The broken line in the figure indicates that no deterioration abnormality has occurred in the air-fuel ratio sensors 40 and 41, the solid line in the figure indicates that the absolute value of the output current I has decreased due to deterioration, and the alternate long and short dash line in FIG. Each shows a case where the absolute value of the output current I increases.

Here, as can be seen from FIG. 9, when there is no abnormality in the air-fuel ratio sensors 40 and 41, the output current when V ₁ is applied as the sensor applied voltage is I ₄ . In contrast, when the air-fuel ratio sensors 40 and 41 are deteriorated and the absolute value of the output current I is decreasing, the output current when V ₁ is applied as the sensor applied voltage is smaller than I _4. I ₅ Further, when the air-fuel ratio sensors 40 and 41 are deteriorated and the absolute value of the output current I is increasing, the output current when V ₁ is applied as the sensor applied voltage is I ₆ larger than I _4. Become. Therefore, the presence or absence of abnormality in the air-fuel ratio sensors 40 and 41 is diagnosed based on the output current when a predetermined sensor applied voltage is applied if the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor 40 is known. be able to.

  Therefore, in the present embodiment, the air-fuel ratio sensors 40 and 41 are based on the sensor applied voltage V and the output current I during the execution of the fuel cut control for stopping or reducing the fuel supply to the combustion chamber during the operation of the internal combustion engine. We are going to perform an abnormal diagnosis. During the fuel cut control, the atmospheric gas flowing into the combustion chamber 5 flows out to the exhaust passage as it is, so that the exhaust gas circulating around the air-fuel ratio sensors 40 and 41 is also atmospheric gas.

  FIG. 10 shows the relationship between the sensor applied voltage V and the output current I and the normality / abnormality of the air-fuel ratio sensors 40, 41 when the exhaust gas flowing around the air-fuel ratio sensors 40, 41 is atmospheric gas. . In the present embodiment, when the output current I detected during the fuel cut control is between the upper limit curve Xmax and the lower limit curve Xmin, it is determined that the air-fuel ratio sensors 40 and 41 are normal. On the other hand, when the output current I detected during the fuel cut control is not less than the upper limit curve Xmax or not more than the lower limit curve Xmin, it is determined that an abnormality has occurred in the air-fuel ratio sensors 40 and 41.

  As shown in FIG. 10, the output current at which the air-fuel ratio sensors 40 and 41 are determined to be normal increases as the sensor applied voltage increases until a certain applied voltage is reached. And when it becomes more than a certain fixed voltage applied to the sensor, it becomes almost constant. Further, the width of the output current determined that the air-fuel ratio sensors 40 and 41 are normal increases as the applied voltage increases until a certain sensor applied voltage is reached. And when it becomes more than a certain fixed voltage applied to the sensor, it becomes almost constant.

Specifically, when the sensor applied voltage is V ₁ during the fuel cut control and the output current is a value between Imax and Imin, an abnormality occurs in the air-fuel ratio sensors 40 and 41. Judge that it is not. On the other hand, in this case, when the output current is equal to or greater than Imax or equal to or less than Imin, it is determined that an abnormality has occurred in the air-fuel ratio sensors 40 and 41. Thereby, the abnormality of the air-fuel ratio sensors 40 and 41 can be diagnosed.

  In the above embodiment, the abnormality diagnosis of the air-fuel ratio sensors 40 and 41 is performed based on the output current during the fuel cut control. However, the abnormality diagnosis is not necessarily performed during the fuel cut control. If the air-fuel ratio of the exhaust gas flowing around each air-fuel ratio sensor 40, 41 can be grasped regardless of the output current of the air-fuel ratio sensor 40, 41 to be diagnosed, it may be performed in any case. For example, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is feedback controlled based on the output current of the upstream air-fuel ratio sensor 40, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst can be controlled. The active air-fuel ratio is controlled so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is kept constant at a predetermined air-fuel ratio, whereby the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is reduced. This predetermined air-fuel ratio can be controlled. In such a case, abnormality diagnosis of the downstream air-fuel ratio sensor 41 can be performed during active air-fuel ratio control. However, in this case, the relationship between the sensor applied voltage V and the output current I and the normality / abnormality of the air-fuel ratio sensors 40, 41 when the exhaust gas flowing around the air-fuel ratio sensors 40, 41 has a predetermined air-fuel ratio. A map similar to that shown in FIG. 10 is required. In any case, the abnormality diagnosis of the air-fuel ratio sensors 40, 41 can be performed in any state as long as the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensors 40, 41 can be kept constant at a predetermined air-fuel ratio. It may be done.

  Further, the ECU 31 diagnoses the output abnormality in the air-fuel ratio sensors 40 and 41 described above. Therefore, the ECU 31 functions as an abnormality diagnosis means for diagnosing output abnormality in the air-fuel ratio sensors 40 and 41.

<Heater control during abnormality diagnosis control>
Incidentally, as described above, in the air-fuel ratio sensors 40 and 41, the heating amount by the heater 59 is controlled based on the impedance. However, it has been found that if the heating amount is controlled during the above-described abnormality diagnosis of the air-fuel ratio sensors 40 and 41, it is impossible to properly diagnose the abnormality of the air-fuel ratio sensors 40 and 41. This will be described below with reference to FIG.

11 shows the relationship between the sensor applied voltage V and the output current I to the air-fuel ratio sensors 40, 41 when the exhaust gas flowing around the air-fuel ratio sensors 40, 41 is atmospheric gas, and is similar to FIG. FIG. The broken line in the figure indicates when no abnormality has occurred in the air-fuel ratio sensors 40 and 41, and the alternate long and short dash line in the figure indicates when an abnormality has occurred. As shown in FIG. 11, when there is no abnormality in the air-fuel ratio sensors 40 and 41, the output current when V ₁ is applied as the sensor applied voltage is I ₄ . Similarly, when an abnormality occurs in the air-fuel ratio sensors 40 and 41, the output current when V ₁ is applied as the sensor applied voltage should be I ₅ lower than Imin.

  Here, as described above, in the air-fuel ratio sensors 40 and 41, the heating amount by the heater 59 is controlled based on the impedance. The impedances of the air-fuel ratio sensors 40 and 41 are inversely proportional to the slope of the proportional region in the VI characteristic. Therefore, as the impedance of the air-fuel ratio sensors 40 and 41 increases, the slope of the proportional region in the VI characteristic decreases. Conversely, when the impedance of the air-fuel ratio sensors 40 and 41 is lowered, the slope of the proportional region in the VI characteristic is increased.

  In the example shown in FIG. 11, when the abnormality occurs in the air-fuel ratio sensors 40 and 41, the slope of the proportional region in the VI characteristic is smaller than when no abnormality occurs. For this reason, even if the temperature of the solid electrolyte layer 51 is an appropriate temperature, the impedance of the solid electrolyte layer 51 is detected as higher than the target impedance. For this reason, the amount of heating by the heater 59 is reduced to lower the impedance. As a result, the temperature of the solid electrolyte layer 51 becomes lower than an appropriate temperature, and the slope of the proportional region in the VI characteristics of the air-fuel ratio sensors 40 and 41 increases as shown by the solid line in FIG. .

As can be seen from FIG. 11, when the temperature control of the solid electrolyte layer 51 as described above is performed when the air-fuel ratio sensors 40 and 41 are abnormal, the VI characteristic is changed to the normal VI characteristic. I'm getting closer. In the example shown in FIG. 11, when the sensor applied voltage is V ₁ and the temperature control of the solid electrolyte layer 51 is not performed, the output current is I ₅ , which is lower than the determination lower limit value Imin. Become. However, in this case, when the temperature control of the solid electrolyte layer 51 described above is performed, the output current becomes I ₄ , which is higher than the determination lower limit value Imin. As a result, in spite of the fact that the air-fuel ratio sensors 40 and 41 are actually abnormal, the abnormality diagnosis of the air-fuel ratio sensors 40 and 41 determines that no abnormality has occurred.

  Therefore, in the control device of the present embodiment, when performing abnormality diagnosis of the air-fuel ratio sensors 40 and 41, the heating amount by the heater 59 is open-controlled without performing the feedback control of the heating amount by the heater 59 based on the impedance described above. I am going to do that.

  Here, during the fuel cut control, the atmospheric gas supplied into the combustion chamber 5 flows as it is around the air-fuel ratio sensors 40 and 41. Therefore, the temperature of the solid electrolyte layer 51 of the air-fuel ratio sensors 40 and 41 depends on the temperature of the atmospheric gas. Therefore, in the present embodiment, as shown in FIG. 12, the heating amount by the heater 59, that is, the heater applied voltage is controlled based on the temperature of the atmosphere (outside air) detected by the atmospheric temperature sensor (not shown). It is said. In particular, as shown in FIG. 12, in this embodiment, the higher the outside air temperature, the smaller the amount of heating by the heater 59, that is, the smaller the heater applied voltage.

  As for the downstream air-fuel ratio sensor 41, the exhaust gas flowing around it passes through the upstream side exhaust purification catalyst. For this reason, the temperature of the exhaust gas flowing around the downstream side air-fuel ratio sensor 41 becomes higher as the temperature of the upstream side exhaust purification catalyst 20 becomes higher. Therefore, in the present embodiment, the downstream air-fuel ratio sensor 41 is heated by the heater 59 based on the temperature of the upstream side exhaust purification catalyst 20 detected by the catalyst temperature sensor that detects the temperature of the upstream side exhaust purification catalyst 20. May be controlled. Specifically, as in FIG. 12, as the temperature of the upstream side exhaust purification catalyst 20 becomes higher, the amount of heating by the heater 59 becomes smaller, that is, the heater applied voltage becomes smaller.

  Further, in the air-fuel ratio sensors 40 and 41, convective heat transfer is performed with the exhaust gas flowing therearound. For this reason, as the flow rate of the exhaust gas flowing around the air-fuel ratio sensors 40 and 41 increases, heat is more easily taken from the air-fuel ratio sensors 40 and 41 to the exhaust gas. Therefore, in the present embodiment, as shown in FIG. 13, the flow rate of the exhaust gas flowing around the air-fuel ratio sensors 40 and 41 is calculated based on the output of the air flow meter 39 and the calculated flow rate of the exhaust gas. Based on the above, the amount of heating by the heater 59, that is, the heater applied voltage is controlled. In particular, as shown in FIG. 13, in the present embodiment, as the flow rate of the exhaust gas increases, the amount of heating by the heater 59 increases, that is, the heater applied voltage increases.

  The temperature of the atmospheric gas, the temperature of the upstream side exhaust purification catalyst 20 and the flow rate of the exhaust gas described above are examples of parameters for controlling the heating amount by the heater 59. Therefore, when performing an abnormality diagnosis of the air-fuel ratio sensors 40 and 41, the heating amount by the heater 59 may be controlled based on other parameters in addition to these parameters or not based on these parameters. When performing abnormality diagnosis of the air-fuel ratio sensors 40 and 41, the heating amount by the heater 59 may be maintained at a predetermined constant amount, that is, the heater applied voltage may be maintained at a constant value.

  Thus, in the present embodiment, when performing abnormality diagnosis of the air-fuel ratio sensors 40, 41, the temperature control of the air-fuel ratio sensors 40, 41 is open controlled without being based on the impedance. For this reason, the output current of the abnormal air-fuel ratio sensors 40, 41 is prevented from approaching the normal value, and as a result, the abnormality diagnosis of the air-fuel ratio sensors 40, 41 can be performed appropriately. Become.

<Flowchart>
FIG. 14 is a flowchart showing a control routine of the abnormality diagnosis control of this embodiment. The illustrated control routine is performed by interruption at regular time intervals.

  First, in step S11, it is determined whether an execution condition for abnormality diagnosis control is satisfied. The condition for executing the abnormality diagnosis control is satisfied, for example, when the temperature of the air-fuel ratio sensors 40 and 41 is equal to or higher than the activation temperature. If it is determined in step S11 that the execution condition is not satisfied, the process proceeds to step S22. In step S22, normal control of the heater 59, that is, feedback control is performed so that the detected impedance becomes the target impedance, and the control routine is terminated.

  On the other hand, if it is determined in step S11 that the execution condition is satisfied, the process proceeds to step S12. In step S12, it is determined whether an abnormality diagnosis of the air-fuel ratio sensors 40, 41 has not been performed after the internal combustion engine is started (or after the ignition key of the vehicle equipped with the internal combustion engine is turned on). If it is determined in step S12 that abnormality diagnosis has already been performed, it is not necessary to perform further abnormality diagnosis, and thus the process proceeds to step S22, and normal control of the heater 59 is performed. On the other hand, if it is determined in step S12 that an abnormality diagnosis has not yet been performed, the process proceeds to step S13.

  In step S13, it is determined whether or not the internal combustion engine is under fuel cut control. Note that when performing an abnormality diagnosis of the air-fuel ratio sensors 40 and 41, when performing active control so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is kept constant at a predetermined air-fuel ratio, this timing is used. Thus, the active air-fuel ratio control is executed. In step S13, it is determined whether or not the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensors 40 and 41 is maintained constant by active air-fuel ratio control. If it is determined in step S13 that the internal combustion engine is not under fuel cut control, the process proceeds to step S22, and normal control of the heater 59 is performed. On the other hand, if it is determined in step S13 that the internal combustion engine is under fuel cut control, the process proceeds to step S14.

  In step S14, the normal control of the heater 59 is stopped, and in step S15, the atmospheric temperature is detected by an atmospheric temperature sensor (not shown). Next, in step S16, the flow rate of the exhaust gas flowing around the air-fuel ratio sensors 40 and 41 is detected based on the output of the air flow meter 39 and the like. Next, in step S17, based on the atmospheric temperature detected in step S15 and the exhaust gas flow rate detected in step S16, the heater applied voltage to the heater 59 using the maps shown in FIGS. Is set.

  Next, in step S18, it is determined whether or not a predetermined time has elapsed since the fuel cut control was started. Immediately after the start of the fuel cut control, the output currents of the air-fuel ratio sensors 40 and 41 change greatly and are not stable. Therefore, the abnormality diagnosis of the air-fuel ratio sensors 40 and 41 is delayed until the output current is stabilized. If it is determined in step S18 that the predetermined time has not elapsed since the fuel cut control was started, the control routine is ended. Thereafter, in the control routine after a certain amount of time has elapsed, it is determined in step S18 that the predetermined time has elapsed, and the process proceeds to step S19.

  In step S19, it is determined whether or not the output current I of the air-fuel ratio sensors 40 and 41 is higher than the determination lower limit value Imin based on the current sensor applied voltage and lower than the determination upper limit value Imax. If it is determined in step S19 that the output current I is higher than the determination lower limit value Imin and lower than the determination upper limit value Imax, the process proceeds to step S20. In step S20, it is determined that the air-fuel ratio sensors 40 and 41 are normal, and the control routine is terminated. On the other hand, if it is determined in step S19 that the output current is not more than the determination lower limit value Imin or not less than the determination upper limit value Imax, the process proceeds to step S21. In step S21, it is determined that an abnormality has occurred in the air-fuel ratio sensors 40, 41, and the control routine is terminated.

DESCRIPTION OF SYMBOLS 1 Engine body 5 Combustion chamber 7 Intake port 9 Exhaust port 19 Exhaust manifold 20 Upstream exhaust purification catalyst 24 Downstream exhaust purification catalyst 31 ECU
40 upstream air-fuel ratio sensor 41 downstream air-fuel ratio sensor

Claims (8)

An air-fuel ratio sensor disposed in an exhaust passage of the internal combustion engine; impedance detection means for detecting an impedance of an element constituting the air-fuel ratio sensor; a heating device for heating the element constituting the air-fuel ratio sensor; and the impedance detection A control device for an air-fuel ratio sensor, comprising: a heating amount control means for controlling a heating amount by the heating device based on an impedance detected by the means;
An abnormality diagnosis means for diagnosing an output abnormality of the air-fuel ratio sensor is further provided, and when the heating amount control means diagnoses an abnormality in the output of the air-fuel ratio sensor by the abnormality diagnosis means, the impedance detected by the impedance detection means A control device for an air-fuel ratio sensor that controls the amount of heating by the heating device regardless of the temperature.   2. The air / fuel ratio sensor control device according to claim 1, wherein the heating amount control unit controls the heating amount by the heating device to be constant when the abnormality diagnosis unit diagnoses an output abnormality of the air / fuel ratio sensor. 3. An air temperature detecting device for detecting the air temperature;
The heating amount control means controls the heating amount by the heating device based on the atmospheric temperature detected by the atmospheric temperature detection device when the abnormality diagnosis means diagnoses an output abnormality of the air-fuel ratio sensor. The control device for an air-fuel ratio sensor according to claim 1. A catalyst temperature detecting device for detecting the temperature of the exhaust purification catalyst provided in the exhaust passage of the internal combustion engine, and the air-fuel ratio sensor is provided downstream of the exhaust purification catalyst in the exhaust flow direction;
When the abnormality diagnosis means diagnoses the output abnormality of the air-fuel ratio sensor, the heating amount control means is based on the atmospheric temperature detected by the atmospheric temperature detection device and the catalyst temperature detected by the catalyst temperature detection device. The control device for an air-fuel ratio sensor according to claim 3, wherein the amount of heating by the heating device is controlled.   The abnormality diagnosing means diagnoses an output abnormality of the air-fuel ratio sensor based on an output of the air-fuel ratio sensor when the air-fuel ratio of the exhaust gas flowing around the air-fuel ratio sensor is kept constant at a predetermined air-fuel ratio. The control device for an air-fuel ratio sensor according to any one of claims 1 to 4.   6. The abnormality diagnosis unit according to claim 5, wherein the abnormality diagnosis means diagnoses an output abnormality of the air-fuel ratio sensor based on an output of the air-fuel ratio sensor when the exhaust gas flowing around the air-fuel ratio sensor is maintained as atmospheric gas. Control device for air-fuel ratio sensor. The internal combustion engine is capable of executing fuel cut control for stopping fuel supply to the combustion chamber during operation of the internal combustion engine,
The control apparatus for an air-fuel ratio sensor according to claim 6, wherein the abnormality diagnosis means diagnoses an output abnormality of the air-fuel ratio sensor during the fuel cut control.   When the heating amount control means has not diagnosed the output abnormality of the air-fuel ratio sensor by the abnormality diagnosis means, the heating amount by the heating device is set so that the impedance detected by the impedance detection means becomes a predetermined target impedance. The control device for an air-fuel ratio sensor according to any one of claims 1 to 7, wherein the control is performed.

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