JP6217739B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust emission control device for an internal combustion engine.

  2. Description of the Related Art There is known an exhaust emission control device in which an air-fuel ratio sensor is provided on the upstream side in the exhaust flow direction and the downstream side in the exhaust flow direction of an exhaust purification catalyst provided in an exhaust passage of an internal combustion engine. In such an internal combustion engine, feedback control is performed so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes the target air-fuel ratio based on the output of the upstream air-fuel ratio sensor. In addition, the target air-fuel ratio includes an air-fuel ratio richer than the stoichiometric air-fuel ratio (hereinafter simply referred to as “rich air-fuel ratio”) and an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter simply referred to as “lean air-fuel ratio”). (For example, Patent Document 1).

  In particular, in the internal combustion engine described in Patent Document 1, the air-fuel ratio corresponding to the output of the downstream air-fuel ratio sensor (hereinafter also referred to as “output air-fuel ratio”) is less than the rich determination air-fuel ratio that is richer than the theoretical air-fuel ratio. The target air-fuel ratio is switched to the lean air-fuel ratio, and the target air-fuel ratio is made rich when the oxygen storage amount of the exhaust purification catalyst exceeds a predetermined switching reference storage amount that is smaller than the maximum storable oxygen amount. I try to switch. According to Patent Document 1, this can suppress the outflow of NOx from the exhaust purification catalyst.

International Publication No. 2014/118892 JP 2006-343281 A

  If the air-fuel ratio sensor is cracked, and the air-fuel ratio of the exhaust gas around the air-fuel ratio sensor is almost the stoichiometric air-fuel ratio or lean air-fuel ratio, the output air-fuel ratio of the air-fuel ratio sensor is the actual exhaust gas ratio. It becomes almost equal to the air-fuel ratio of the gas. However, when the air-fuel ratio of the exhaust gas around the air-fuel ratio sensor is a rich air-fuel ratio, the output air-fuel ratio of the air-fuel ratio sensor may be an air-fuel ratio different from the actual air-fuel ratio of the exhaust gas, particularly a lean air-fuel ratio. .

  Thus, when an element crack occurs in the air-fuel ratio sensor, the air-fuel ratio of the exhaust gas around the air-fuel ratio sensor may not be detected accurately. When the target air-fuel ratio control as described above is performed using an air-fuel ratio sensor that generates an erroneous output due to element cracking in this way, it becomes impossible to suppress the outflow of NOx or the like from the exhaust purification catalyst There is.

  In view of the above problems, an object of the present invention is to provide an internal combustion engine exhaust gas purification apparatus capable of diagnosing an abnormality of an element crack in a downstream air-fuel ratio sensor.

  In order to solve the above problems, in the first invention, an exhaust purification catalyst provided in an exhaust passage of an internal combustion engine, and a downstream side air provided in the exhaust passage downstream of the exhaust purification catalyst in the exhaust flow direction. An abnormality diagnosis control for performing an abnormality diagnosis of the downstream air-fuel ratio sensor based on an air-fuel ratio sensor, an air-fuel ratio control for controlling an air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst, and an output air-fuel ratio of the downstream air-fuel ratio sensor The control device performs the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst in the air-fuel ratio control, the rich air-fuel ratio richer than the stoichiometric air-fuel ratio and the stoichiometric air-fuel ratio. The control device alternately and repeatedly switches to a lean lean air-fuel ratio, and in the abnormality diagnosis control, the control device is rich in the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst by the air-fuel ratio control. When the air-fuel ratio is set to an air-fuel ratio, the output air-fuel ratio of the downstream air-fuel ratio sensor is leaner than the stoichiometric air-fuel ratio. An exhaust purification device for an internal combustion engine is provided that determines that an abnormality has occurred in the downstream air-fuel ratio sensor when changed to.

  In a second invention, in the first invention, the control device is supplied to the combustion chamber of the internal combustion engine so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst in the air-fuel ratio control becomes a target air-fuel ratio. A feedback control of the fuel supply amount, and a learning control for correcting a parameter relating to the air-fuel ratio based on an output air-fuel ratio of the downstream air-fuel ratio sensor, wherein the control device performs the target air-fuel ratio in the air-fuel ratio control. Are alternately switched between a rich air-fuel ratio and a lean air-fuel ratio, and when the output air-fuel ratio of the downstream air-fuel ratio sensor becomes equal to or lower than a predetermined rich judgment air-fuel ratio richer than the theoretical air-fuel ratio, the target air-fuel ratio is reduced. The control device performs switching from the rich air-fuel ratio to the lean air-fuel ratio, and in the learning control, the control device switches the target air-fuel ratio to the lean air-fuel ratio and then again. An integrated oxygen excess amount that is an integrated value of the amount of oxygen that becomes excessive when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made to be the stoichiometric air-fuel ratio during the oxygen increase period until switching to the air-fuel ratio The oxygen deficiency when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made to be the stoichiometric air-fuel ratio in the oxygen reduction period from when the target air-fuel ratio is switched to the rich air-fuel ratio until it is switched again to the lean air-fuel ratio On the basis of the integrated oxygen deficiency that is the integrated value of the amount of the amount, the parameter relating to the air-fuel ratio is corrected so that the difference between the integrated oxygen excess and the accumulated oxygen deficiency is reduced, and the control device If it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor when the air-fuel ratio is set to a rich air-fuel ratio, the output air Even ratio is the target air-fuel ratio becomes less than the rich determining the air-fuel ratio is switched to the lean air-fuel ratio, it stops the correction of parameters related to the air-fuel ratio based on the integrated oxygen shortage at this time.

  In a third aspect based on the second aspect, the control device is configured such that when the target air-fuel ratio is set to a rich air-fuel ratio, the output air-fuel ratio of the downstream air-fuel ratio sensor is less than the lean determination air-fuel ratio. If it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor due to a change from a rich air-fuel ratio to an air-fuel ratio leaner than the lean determination air-fuel ratio, the target air-fuel ratio is set to the previous rich air-fuel ratio. The integrated oxygen shortage amount is calculated during a period from when the output air-fuel ratio of the downstream air-fuel ratio sensor is changed to a lean air-fuel ratio from a rich air-fuel ratio that is richer than the lean determination air-fuel ratio. The parameter relating to the air-fuel ratio is corrected so that the difference between the amount and the cumulative oxygen excess amount becomes small.

  According to a fourth invention, in the second or third invention, the control device, in the air-fuel ratio control, sets the target air-fuel ratio to a constant rich set air-fuel ratio and a stoichiometric air-fuel ratio that are richer than the stoichiometric air-fuel ratio. When the control device determines that an abnormality has occurred in the downstream air-fuel ratio sensor by the abnormality diagnosis control, the control device switches to a rich constant air-fuel ratio. Decrease the degree.

  According to a fifth aspect, in the fourth aspect, the control device sets the target air-fuel ratio to be set once for each of the rich air-fuel ratio and the lean air-fuel ratio in the air-fuel ratio control as one cycle. When the ratio of the number of times it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor to the number of times is equal to or greater than a predetermined ratio, the rich degree of the rich set air-fuel ratio is reduced.

  In a sixth invention, in any one of the first to fifth inventions, in the air-fuel ratio control, the control device is a predetermined air-fuel ratio output from the downstream air-fuel ratio sensor that is richer than the stoichiometric air-fuel ratio. The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst when it becomes less than the rich judgment air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio, and the oxygen storage amount of the exhaust purification catalyst is greater than the maximum storable oxygen amount The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is switched from the lean air-fuel ratio to the rich air-fuel ratio when the oxygen amount exceeds a small predetermined switching reference oxygen amount.

  According to the present invention, there is provided an exhaust emission control device for an internal combustion engine capable of diagnosing this abnormality when an element crack abnormality occurs in a downstream air-fuel ratio sensor.

FIG. 1 is a diagram schematically showing an internal combustion engine in which the abnormality diagnosis apparatus 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 applied voltage V and the output current I at each exhaust air-fuel ratio A / F. FIG. 4 is a diagram showing the relationship between the air-fuel ratio and the output current I when the applied voltage V is constant. FIG. 5 is a time chart showing changes in the oxygen storage amount and the like of the upstream side exhaust purification catalyst during normal operation of the internal combustion engine. FIG. 6 is a schematic cross-sectional view of an air-fuel ratio sensor in which element cracking occurs. FIG. 7 is a time chart of the air-fuel ratio correction amount and the like. FIG. 8 is a functional block diagram of the control device. FIG. 9 is a flowchart showing a control routine of air-fuel ratio correction amount setting control. FIG. 10 is a flowchart showing a control routine of abnormality diagnosis control for performing abnormality diagnosis of the downstream air-fuel ratio sensor. FIG. 11 is a flowchart showing a control routine in setting control of the rich set air-fuel ratio and the lean set air-fuel ratio. FIG. 12 is a time chart showing changes in the oxygen storage amount and the like of the upstream side exhaust purification catalyst, similar to FIG. FIG. 13 is a time chart of the control center air-fuel ratio and the like. FIG. 14 is a diagram showing the relationship between the flow rate of exhaust gas flowing around the downstream air-fuel ratio sensor and the output air-fuel ratio of the downstream air-fuel ratio sensor. FIG. 15 is a time chart showing changes in the air-fuel ratio correction amount and the like when element cracking occurs in the downstream air-fuel ratio sensor. FIG. 16 is a time chart similar to FIG. 15 showing changes in the air-fuel ratio correction amount and the like. FIG. 17 is a functional block diagram of the control device. FIG. 18 is a flowchart illustrating a control routine for learning control.

  Hereinafter, embodiments 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.

<Description of the internal combustion engine as a whole>
FIG. 1 is a diagram schematically showing an internal combustion engine in which an exhaust purification apparatus according to a first embodiment of the present invention is used. 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, in an internal combustion engine in which the exhaust emission control device of the present invention is used, a fuel other than gasoline or a mixed fuel with gasoline may be 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.

  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. The ECU 31 performs an abnormality diagnosis of the downstream air-fuel ratio sensor 41 based on the air-fuel ratio control for controlling the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 and the output air-fuel ratio of the downstream air-fuel ratio sensor 41. It functions as a control device that performs abnormality diagnosis control.

The upstream side exhaust purification catalyst 20 and the downstream side exhaust purification catalyst 24 are three-way catalysts having oxygen storage capacity. Specifically, the exhaust purification catalysts 20 and 24 support a noble metal having a catalytic action (for example, platinum (Pt)) and a substance having an oxygen storage capacity (for example, ceria (CeO ₂ )) on a ceramic support. Three-way catalyst. The three-way catalyst has a function of simultaneously purifying unburned HC, CO, and NOx when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is maintained at the stoichiometric air-fuel ratio. In addition, when a certain amount of oxygen is stored in the exhaust purification catalysts 20, 24, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 is richer or leaner than the stoichiometric air-fuel ratio. Even if there is a slight deviation, unburned HC, CO and NOx are simultaneously purified.

  That is, if the exhaust purification catalysts 20, 24 have oxygen storage capacity, that is, if the oxygen storage amount of the exhaust purification catalysts 20, 24 is less than the maximum storable oxygen amount, the exhaust purification catalysts 20, 24 flow into the exhaust purification catalysts 20, 24. When the air-fuel ratio of the exhaust gas becomes slightly leaner than the stoichiometric air-fuel ratio, excess oxygen contained in the exhaust gas is stored in the exhaust purification catalysts 20 and 24. For this reason, the surfaces of the exhaust purification catalysts 20, 24 are maintained at the stoichiometric air-fuel ratio. As a result, unburned HC, CO, and NOx are simultaneously purified on the surfaces of the exhaust purification catalysts 20, 24, and at this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

  On the other hand, if the exhaust purification catalysts 20, 24 are in a state capable of releasing oxygen, that is, if the oxygen storage amount of the exhaust purification catalysts 20, 24 is greater than 0, the exhaust gas flowing into the exhaust purification catalysts 20, 24 When the air-fuel ratio of the exhaust gas becomes slightly richer than the stoichiometric air-fuel ratio, oxygen that is insufficient to reduce unburned HC and CO contained in the exhaust gas is released from the exhaust purification catalysts 20 and 24. . Therefore, also in this case, the surfaces of the exhaust purification catalysts 20, 24 are maintained at the stoichiometric air-fuel ratio. As a result, unburned HC, CO, and NOx are simultaneously purified on the surfaces of the exhaust purification catalysts 20, 24, and at this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

  As described above, when a certain amount of oxygen is stored in the exhaust purification catalysts 20, 24, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20, 24 is richer or leaner than the stoichiometric air-fuel ratio. Even if it slightly deviates, the unburned HC, CO, and NOx are simultaneously purified, and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20, 24 becomes the stoichiometric air-fuel ratio.

<Description of air-fuel ratio sensor>
In the present embodiment, a cup-type limit current type air-fuel ratio sensor is used as the air-fuel ratio sensors 40 and 41. 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 rate controlling layer 54 for controlling the rate, a reference gas chamber 55, and a heater unit 56 for heating the air-fuel ratio sensors 40 and 41, particularly for heating the solid electrolyte layer 51 are provided.

  In particular, in the cup-type air-fuel ratio sensors 40 and 41 of the present embodiment, the solid electrolyte layer 51 is formed in a cylindrical shape with one end closed. In the reference gas chamber 55 defined inside the solid electrolyte layer 51, atmospheric gas (air) is introduced and a heater unit 56 is disposed. An atmosphere side electrode 53 is disposed on the inner surface of the solid electrolyte layer 51, and an exhaust side electrode 52 is disposed on the outer surface of the solid electrolyte layer 51. On the outer surfaces of the solid electrolyte layer 51 and the exhaust-side electrode 52, a diffusion control layer 54 is disposed so as to cover them. A protective layer (not shown) for preventing liquid or the like from adhering to the surface of the diffusion limiting layer 54 may be provided outside the diffusion limiting layer 54.

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 applied voltage V is applied between the exhaust side electrode 52 and the atmosphere side electrode 53 by the applied voltage control device 60 mounted on the ECU 31. In addition, the ECU 31 is provided with a current detection unit 61 that detects a current I flowing between the electrodes 52 and 53 via the solid electrolyte layer 51 when the sensor application voltage V is applied. The current detected by the current detector 61 is the output current I 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 of the air-fuel ratio sensors 40 and 41 increases as the air-fuel ratio of the exhaust gas, that is, the exhaust air-fuel ratio A / F increases (lean). The V-I line at each exhaust air-fuel ratio A / F includes a region parallel to the sensor applied voltage V axis, that is, a region where the output current I hardly changes even when the sensor applied voltage V 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.

  FIG. 4 shows the relationship between the exhaust air-fuel ratio and the output current I when the applied voltage V is kept constant at about 0.45 V (FIG. 3). As can be seen from FIG. 4, in the air-fuel ratio sensors 40 and 41, the exhaust air-fuel ratio becomes higher with respect to the exhaust air-fuel ratio so that the output current I from the air-fuel ratio sensors 40 and 41 becomes larger as the exhaust air-fuel ratio becomes higher (that is, the leaner). The output current changes linearly (in proportion). 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.

  As the air-fuel ratio sensors 40 and 41, instead of the limit current type air-fuel ratio sensor having the structure shown in FIG. 2, for example, a limit current type air-fuel ratio having another structure such as a stacked type limit current type air-fuel ratio sensor is used. A sensor may be used.

<Basic control>
Next, an outline of basic air-fuel ratio control in the control apparatus for an internal combustion engine of the present embodiment will be described. In the air-fuel ratio control of the present embodiment, the fuel injection amount from the fuel injection valve 11 is set so that the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes the target air-fuel ratio based on the output air-fuel ratio of the upstream air-fuel ratio sensor 40. Feedback control is performed. That is, in the air-fuel ratio control of the present embodiment, feedback control is performed so that the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes the target air-fuel ratio based on the output air-fuel ratio of the upstream side air-fuel ratio sensor 40. Is called. “Output air-fuel ratio” means an air-fuel ratio corresponding to the output value of the air-fuel ratio sensor.

  Further, in the air-fuel ratio control of the present embodiment, the target air-fuel ratio is set based on the output air-fuel ratio of the downstream air-fuel ratio sensor 41 and the like. Specifically, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes a rich air-fuel ratio, the target air-fuel ratio is set to the lean set air-fuel ratio. As a result, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 also becomes the lean set air-fuel ratio. Here, the lean set air-fuel ratio is a predetermined constant value air-fuel ratio that is somewhat leaner than the stoichiometric air-fuel ratio (the air-fuel ratio that becomes the control center), for example, 14.65 to 20, preferably It is set to about 14.65 to 18, more preferably about 14.65 to 16. Further, the lean set air-fuel ratio can also be expressed as an air-fuel ratio obtained by adding a positive air-fuel ratio correction amount to the air-fuel ratio serving as the control center (the theoretical air-fuel ratio in the present embodiment). In addition, in the present embodiment, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes less than the rich determination air-fuel ratio (for example, 14.55) that is slightly richer than the theoretical air-fuel ratio, the downstream air-fuel ratio sensor 41 It is determined that the output air-fuel ratio of the fuel ratio sensor 41 has become a rich air-fuel ratio.

  When the target air-fuel ratio is changed to the lean set air-fuel ratio, the oxygen excess / deficiency of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is integrated. The oxygen excess / deficiency is the amount of oxygen that becomes excessive or the amount of oxygen that becomes excessive when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is made to be the stoichiometric air-fuel ratio (excess unburned HC, CO Etc. (hereinafter referred to as “unburned gas”). In particular, when the target air-fuel ratio is the lean set air-fuel ratio, oxygen in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 becomes excessive, and this excess oxygen is stored in the upstream side exhaust purification catalyst 20. Therefore, it can be said that the integrated value of oxygen excess / deficiency (hereinafter referred to as “accumulated oxygen excess / deficiency”) is an estimated value of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20.

Note that the oxygen excess / deficiency amount is calculated by estimating the intake air amount into the combustion chamber 5 calculated based on the output air-fuel ratio of the upstream air-fuel ratio sensor 40, the output of the air flow meter 39, or the like, or the fuel injection valve. 11 is performed based on the amount of fuel supplied from 11 or the like. Specifically, the oxygen excess / deficiency OED is calculated by, for example, the following formula (1).
OED = 0.23 × Qi × (AFup-AFR) (1)
Here, 0.23 is the oxygen concentration in the air, Qi is the fuel injection amount, AFup is the output air-fuel ratio of the upstream air-fuel ratio sensor 40, and AFR is the air-fuel ratio that is the control center (in this embodiment, basically (Theoretical air-fuel ratio).

  When the cumulative oxygen excess / deficiency obtained by integrating the oxygen excess / deficiency calculated in this way becomes equal to or greater than a predetermined switching reference value (corresponding to a predetermined switching reference storage amount Cref), the lean set empty is used until then. The target air-fuel ratio that was the fuel ratio is set to the rich set air-fuel ratio. The rich set air-fuel ratio is a predetermined air-fuel ratio that is somewhat richer than the stoichiometric air-fuel ratio (the air-fuel ratio that becomes the control center), for example, 12 to 14.58, preferably 13 to 14.57, More preferably, it is about 14 to 14.55. The rich set air-fuel ratio can also be expressed as an air-fuel ratio obtained by adding a negative air-fuel ratio correction amount to the air-fuel ratio that is the control center (the theoretical air-fuel ratio in the present embodiment). In the present embodiment, the difference (rich degree) of the rich set air-fuel ratio from the stoichiometric air-fuel ratio is set to be equal to or less than the difference (lean degree) of the lean set air-fuel ratio from the stoichiometric air-fuel ratio.

  Thereafter, when the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 again becomes equal to or less than the rich determination air-fuel ratio, the target air-fuel ratio is again set to the lean set air-fuel ratio, and thereafter the same operation is repeated. Thus, in the present embodiment, the target air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is repeatedly set alternately to the lean set air-fuel ratio and the rich set air-fuel ratio. In other words, in the present embodiment, it can be said that the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is switched alternately between the rich air-fuel ratio and the lean air-fuel ratio.

<Description of air-fuel ratio control using time chart>
With reference to FIG. 5, the operation as described above will be specifically described. FIG. 5 shows the air-fuel ratio correction amount, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20, the accumulated oxygen excess / deficiency when the air-fuel ratio control of this embodiment is performed. 6 is a time chart of the amount ΣOED, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41, and the NOx concentration in the exhaust gas flowing out from the upstream side exhaust purification catalyst 20.

  The air-fuel ratio correction amount AFC is a correction amount related to the target air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20. When the air-fuel ratio correction amount AFC is 0, the target air-fuel ratio is set to an air-fuel ratio (in this embodiment, the theoretical air-fuel ratio) equal to the air-fuel ratio serving as the control center (hereinafter referred to as “control center air-fuel ratio”). When the amount AFC is a positive value, the target air-fuel ratio is leaner than the control center air-fuel ratio (in this embodiment, the lean air-fuel ratio), and when the air-fuel ratio correction amount AFC is a negative value, the target air-fuel ratio is Is richer than the control center air-fuel ratio (in this embodiment, the rich air-fuel ratio). The “control center air-fuel ratio” is a reference when the target air-fuel ratio is changed according to the air-fuel ratio to which the air-fuel ratio correction amount AFC is added according to the engine operating state, that is, the air-fuel ratio correction amount AFC. It means air / fuel ratio.

In the illustrated example, the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich (corresponding to the rich set air-fuel ratio) before the time t ₁ . That is, the target air-fuel ratio is a rich air-fuel ratio, and accordingly, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes the rich air-fuel ratio. Unburned gas or the like contained in the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is purified by the upstream side exhaust purification catalyst 20, and along with this, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is increased. It gradually decreases. Since the exhaust gas flowing out of the upstream side exhaust purification catalyst 20 by purification in the upstream side exhaust purification catalyst 20 does not contain unburned gas or the like, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is substantially the stoichiometric air-fuel ratio. It becomes. Since the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio, the NOx emission amount from the upstream side exhaust purification catalyst 20 becomes substantially zero.

When the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, the oxygen storage amount OSA approaches zero, and along with this, a part of unburned gas etc. flowing into the upstream side exhaust purification catalyst 20 is upstream side. It begins to flow out without being purified by the exhaust purification catalyst 20. As a result, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 gradually decreases, and at time t ₁ , the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich.

  In the present embodiment, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio AFrich, the air-fuel ratio correction amount AFC is set to the lean set correction amount AFClean (lean set air amount) to increase the oxygen storage amount OSA. Equivalent to the fuel ratio). At this time, the cumulative oxygen excess / deficiency ΣOED is reset to zero.

  In the present embodiment, the air-fuel ratio correction amount AFC is switched after the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich. This is because even if the oxygen storage amount of the upstream side exhaust purification catalyst 20 is sufficient, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 may slightly deviate from the stoichiometric air-fuel ratio. is there. Conversely, the rich determination air-fuel ratio is such that the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 does not reach when the oxygen storage amount of the upstream side exhaust purification catalyst 20 is sufficient. It is said.

When the target air-fuel ratio is switched to the lean air-fuel ratio at time t ₁ , the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes from the rich air-fuel ratio to the lean air-fuel ratio. When the air-fuel ratio of the exhaust gas flowing into the upstream exhaust purification catalyst 20 is changed to the lean air-fuel ratio at time t _1, the oxygen storage amount OSA of the upstream exhaust purification catalyst 20 increases. Along with this, the cumulative oxygen excess / deficiency ΣOED also gradually increases.

  As a result, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 changes to the stoichiometric air-fuel ratio, and the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 also converges to the stoichiometric air-fuel ratio. At this time, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a lean air-fuel ratio. However, since the oxygen storage capacity of the upstream side exhaust purification catalyst 20 has a sufficient margin, the inflowing exhaust gas The oxygen therein is stored in the upstream side exhaust purification catalyst 20, and NOx is reduced and purified. For this reason, the NOx emission from the upstream side exhaust purification catalyst 20 becomes substantially zero.

Thereafter, when the oxygen storage amount OSA of the upstream exhaust purification catalyst 20 is increased, at time t _2, the oxygen storage amount OSA of the upstream exhaust purification catalyst 20 reaches the switching reference occlusion amount Cref. For this reason, the cumulative oxygen excess / deficiency ΣOED reaches the switching reference value OEDref corresponding to the switching reference storage amount Cref. In the present embodiment, when the cumulative oxygen excess / deficiency ΣOED becomes greater than or equal to the switching reference value OEDref, the air-fuel ratio correction amount AFC is switched to the rich set correction amount AFCrich so as to stop storing oxygen in the upstream side exhaust purification catalyst 20. . Therefore, the target air-fuel ratio is set to a rich air-fuel ratio. At this time, the cumulative oxygen excess / deficiency ΣOED is reset to zero.

  Note that the switching reference storage amount Cref is set to a sufficiently small amount so that the oxygen storage amount OSA does not reach the maximum storable oxygen amount Cmax even if an unintended air-fuel ratio shift due to sudden acceleration of the vehicle occurs. For example, the switching reference storage amount Cref is set to 3/4 or less, preferably 1/2 or less, more preferably 1/5 or less of the maximum storable oxygen amount Cmax when the upstream side exhaust purification catalyst 20 is unused. The As a result, the lean air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is slightly leaner than the stoichiometric air-fuel ratio (for example, 14.65. The deviation from the stoichiometric air-fuel ratio is a rich judgment air-fuel ratio and the stoichiometric air-fuel ratio. The air-fuel ratio correction amount AFC is switched to the rich set correction amount AFCrich before reaching the lean air-fuel ratio of the same level as the difference between the air-fuel ratio and the air-fuel ratio.

Switching the target air-fuel ratio to the rich air-fuel ratio at time t _2, the air-fuel ratio of the exhaust gas flowing into the upstream exhaust purification catalyst 20 changes from a lean air-fuel ratio to a rich air-fuel ratio. Since the exhaust gas flowing into the upstream side exhaust purification catalyst 20 contains unburned gas or the like, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases. At this time, the NOx emission from the upstream side exhaust purification catalyst 20 becomes substantially zero.

When the oxygen storage amount OSA of the upstream exhaust purification catalyst 20 gradually decreases at time t _3, as with time t _1, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is in the rich determination air AFrich To reach. As a result, the air-fuel ratio correction amount AFC is switched to the lean set correction amount AFClean. Thereafter, the cycle from the time t _{1 to} t ₃ described above is repeated.

  As can be seen from the above description, according to the present embodiment, the NOx emission amount from the upstream side exhaust purification catalyst 20 can always be suppressed. That is, as long as the above-described control is performed, basically, the NOx emission amount from the upstream side exhaust purification catalyst 20 can be made substantially zero. In addition, since the integration period when calculating the integrated oxygen excess / deficiency ΣOED is short, a calculation error is less likely to occur than when integrating over a long period of time. For this reason, NOx is prevented from being discharged due to a calculation error of the cumulative oxygen excess / deficiency ΣOED.

  In general, when the oxygen storage amount of the exhaust purification catalyst is kept constant, the oxygen storage capacity of the exhaust purification catalyst is lowered. That is, in order to keep the oxygen storage capacity of the exhaust purification catalyst high, it is necessary that the oxygen storage amount of the exhaust purification catalyst fluctuates. On the other hand, according to the present embodiment, as shown in FIG. 5, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 always fluctuates up and down, so that the oxygen storage capacity is prevented from being lowered. Is done.

In the above embodiment, the air-fuel ratio correction amount AFC is maintained at the lean set correction amount AFClean from time t _{1 to} t ₂ . However, in such a period, the air-fuel ratio correction amount AFC does not necessarily have to be kept constant, and may be set so as to fluctuate, for example, gradually decrease. Alternatively, the air-fuel ratio correction amount AFC may be temporarily set to a value smaller than 0 (for example, a rich setting correction amount) during the period of time t _{1 to} t ₂ .

Similarly, in the above embodiment, the air-fuel ratio correction amount AFC is maintained at the rich set correction amount AFCrich from time t _{2 to} t ₃ . However, in such a period, the air-fuel ratio correction amount AFC does not necessarily have to be kept constant, and may be set to vary, for example, gradually increase. Alternatively, during the period from time t _{2 to} time t ₃ , the air-fuel ratio correction amount AFC may be temporarily set to a value larger than 0 (for example, a lean set correction amount).

  The ECU 31 sets the air-fuel ratio correction amount AFC in this embodiment, that is, the target air-fuel ratio. Therefore, the ECU 31 determines that the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is equal to or higher than the switching reference storage amount Cref when the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio. Until the target air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is continuously or intermittently changed to the lean air-fuel ratio, and the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is switched. When it is estimated that the reference storage amount Cref is greater than or equal to the reference storage amount Cref, the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio sensor 41 without the oxygen storage amount OSA reaching the maximum storable oxygen amount Cmax is less than or equal to the rich determination air-fuel ratio. Until it becomes, it can be said that the target air-fuel ratio is continuously or intermittently made the rich air-fuel ratio.

  More simply, in the present embodiment, the ECU 31 detects the target air-fuel ratio (that is, the upstream side exhaust purification catalyst 20) when the air-fuel ratio detected by the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio. The air-fuel ratio of the inflowing exhaust gas is switched to the lean air-fuel ratio, and the target air-fuel ratio (that is, the upstream side exhaust purification catalyst) when the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 becomes equal to or higher than the switching reference storage amount Cref. It can be said that the air-fuel ratio of the exhaust gas flowing into the engine 20 is switched to the rich air-fuel ratio.

<Element cracking of air-fuel ratio sensor>
By the way, as an abnormality occurring in the air-fuel ratio sensors 40 and 41 as described above, there is a phenomenon of element cracking in which cracks occur in the elements constituting the air-fuel ratio sensors 40 and 41. Specifically, a crack that penetrates the solid electrolyte layer 51 and the diffusion-controlling layer 54 (C1 in FIG. 6), or a crack that penetrates both the electrodes 52 and 53 in addition to the solid electrolyte layer 51 and the diffusion-controlling layer 54 (see FIG. 6). C2) occurs. When such element cracking occurs, the exhaust gas enters the reference gas chamber 55 through the cracked portion as shown in FIG.

  As a result, when the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40 and 41 is a rich air-fuel ratio, the rich air-fuel ratio exhaust gas enters the reference gas chamber 55. As a result, the rich air-fuel ratio exhaust gas diffuses into the reference gas chamber 55, and the oxygen concentration around the atmosphere-side electrode 53 decreases. On the other hand, even in this case, the exhaust-side electrode 52 is exposed to the exhaust gas through the diffusion-controlling layer 54. For this reason, the difference in oxygen concentration between the atmosphere-side electrode 53 and the exhaust-side electrode 52 decreases, and as a result, the output air-fuel ratio of the air-fuel ratio sensors 40 and 41 becomes a lean air-fuel ratio. That is, when element cracking occurs in the air-fuel ratio sensors 40, 41, even if the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40, 41 is a rich air-fuel ratio, the output air-fuel ratio of the air-fuel ratio sensors 40, 41 is lean. It becomes the fuel ratio.

  On the other hand, when the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40 and 41 is a lean air-fuel ratio, such a reverse phenomenon of the output air-fuel ratio does not occur. This is because when the air-fuel ratio of the exhaust gas is a lean air-fuel ratio, the output current of the air-fuel ratio sensors 40, 41 is greater than the difference between the air-fuel ratios on both sides of the solid electrolyte layer 51 via the diffusion-controlling layer 54. This is because it depends on the amount of oxygen reaching the surface of the electrode 52.

  As described above, when an element crack occurs in the air-fuel ratio sensors 40, 41, an erroneous output is generated when the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40, 41 is a rich air-fuel ratio. For this reason, for example, if an element crack occurs in the downstream air-fuel ratio sensor 41, the air-fuel ratio control cannot be performed properly when the above-described air-fuel ratio control is performed. For this reason, it is necessary to quickly diagnose that an element crack has occurred in the downstream air-fuel ratio sensor 41.

<Abnormal diagnosis>
Therefore, in the present embodiment, the abnormality diagnosis based on the element crack of the downstream air-fuel ratio sensor 41 is performed using the property of the element crack abnormality of the downstream air-fuel ratio sensor 41 as described above. Specifically, in the abnormality diagnosis control, when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is set to the rich air-fuel ratio by the air-fuel ratio control described above, the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 is increased. When the air-fuel ratio changes from an air-fuel ratio richer than the lean determination air-fuel ratio to a lean air-fuel ratio, it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor 41.

  In addition, in this embodiment, when it is determined by the abnormality diagnosis control that an abnormality has occurred in the downstream air-fuel ratio sensor 41, the absolute value of the rich setting correction amount is reduced, that is, the rich setting air-fuel ratio. The rich degree of is made small. In addition, in this embodiment, when it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor 41, the absolute value of the lean set correction amount is decreased, that is, the lean degree of the lean set air-fuel ratio is decreased. I have to. This state will be described with reference to FIG.

  7 shows the air-fuel ratio correction amount AFC, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20, the cumulative oxygen excess / deficiency ΣOED, and the output air-fuel ratio of the downstream air-fuel ratio sensor 41. It is a time chart of fuel ratio AFdwn. The broken line in the figure represents a transition when no element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, and the solid line in the figure represents a transition when an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41. Respectively.

In the example shown in FIG. 7, before time t ₁ , the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich, and the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 is the rich air-fuel ratio. . In particular, the rich setting correction amount AFCrich at this time is the first rich setting correction amount AFCrich ₁ . Along with this, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases and approaches zero, whereby a part of unburned gas or the like flowing into the upstream side exhaust purification catalyst 20 is partly upstream. At 20 it begins to flow out without being purified.

When unburnt gas or the like from the upstream exhaust purification catalyst 20 starts to flow out, when the abnormality of the element crack on the downstream side air-fuel ratio sensor 41 does not occur, downstream at time t ₁ as indicated by a broken line in FIG. The output air-fuel ratio AFdwn of the air-fuel ratio sensor 41 becomes a rich air-fuel ratio. However, when an abnormality of the element crack on the downstream side air-fuel ratio sensor 41 has occurred, the substantially stoichiometric air-fuel ratio output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 at time t ₁ as indicated by the solid line in FIG. Change to lean air-fuel ratio.

That is, when an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, when the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich, that is, the air-fuel ratio correction amount AFC is a negative value. When the output air-fuel ratio sensor 41 is set to, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 changes from less than the lean determination air-fuel ratio AFlean to more than the lean determination air-fuel ratio AFlean. In the present embodiment, it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41 at time t ₁ when such a change in the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 has occurred. Thereby, in this embodiment, abnormality of the element crack of the downstream air-fuel ratio sensor 41 can be diagnosed correctly.

In the example shown in FIG. 7, at time t ₁ , the abnormality diagnosis flag that is turned on when an abnormality occurs in the downstream air-fuel ratio sensor 41 is turned on. In addition, in this embodiment, when it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, for example, a warning light of a vehicle equipped with an internal combustion engine is turned on.

When it is determined at time t ₁ that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, the absolute value of the rich setting correction amount AFCrich and the absolute value of the lean setting correction amount AFClean are decreased. . In the example shown in FIG. 7, the rich set correction amount AFCrich is changed from the first rich set correction amount AFCrich ₁ to the first rich set correction amount AFCrich absolute value than ₁ smaller second rich set correction amount AFCrich _{2 (| AFCrich 1 |> | AFCrich} 2 |). Further, the lean setting correction amount AFClean is changed from the first lean set correction amount AFClean ₁ to the first lean set correction amount AFClean ₁ absolute value than the smaller second lean set correction amount AFClean ₂ (| AFClean ₁ |> | AF Clean ₂ |). Therefore, the rich degree of the rich set air-fuel ratio and the lean degree of the lean set air-fuel ratio are reduced.

Thereby, after time t ₁ , the rich degree of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is reduced, and the rich degree of the output air-fuel ratio of the upstream side air-fuel ratio sensor 40 is reduced. For this reason, the concentration of unburned gas or the like in the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is lowered, and hence deterioration of exhaust emission can be suppressed. In the present embodiment, both the absolute value of the rich setting correction amount AFCrich and the absolute value of the lean setting correction amount AFClean are decreased. However, both may not necessarily be decreased, and only the absolute value of the rich setting correction amount AFCrich may be decreased.

In the example shown by the solid line in FIG. 7, the air-fuel ratio control described above is continued as it is even after it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41. Therefore, as described above, the air-fuel ratio correction amount AFC is calculated from the rich set correction amount AFCrich to the lean set correction amount AFClean when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio AFrich. Can be switched to. For this reason, at time t ₁ , the air-fuel ratio correction amount AFC is not switched to the lean set correction amount AFClean and is maintained as the rich set correction amount AFCrich. For this reason, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is maintained substantially zero.

  In the above-described embodiment, when it is determined once that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, the warning light of the vehicle is turned on. However, in order to increase the determination accuracy, the warning light of the vehicle is turned on when the number of times it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41 within a predetermined time is greater than or equal to a predetermined number. It may be. Alternatively, if the period in which the air-fuel ratio correction amount AFC is set once for the rich setting correction amount AFCrich and the lean setting correction amount AFClean is one cycle, an element crack abnormality occurs in the downstream air-fuel ratio sensor 41 with respect to the number of cycles. The warning light of the vehicle may be turned on when the ratio of the number of times determined to be equal to or greater than a predetermined ratio Ra (0 <Ra <1).

  Similarly, in the above-described embodiment, when it is determined once that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, the absolute value of the rich setting correction amount AFCrich and the absolute value of the lean setting correction amount AFClean. Is reduced. However, in order to avoid an inadvertent change of the set correction amount, when the number of times that it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41 within a predetermined time is greater than or equal to a predetermined number of times. The absolute value of the set correction amount may be decreased. Alternatively, when the ratio of the number of times it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor with respect to the number of cycles is equal to or greater than a predetermined ratio Ra (0 <Ra <1), these set correction amounts The absolute value of may be decreased.

  In the above embodiment, in the air-fuel ratio control, the target air-fuel ratio is switched to the lean air-fuel ratio when the air-fuel ratio detected by the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio. Further, the target air-fuel ratio is switched to the rich air-fuel ratio when the cumulative oxygen excess / deficiency ΣOED becomes equal to or greater than a predetermined switching reference value OEDref. However, another control may be used as the air-fuel ratio control. As such another control, for example, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio, the target air-fuel ratio is switched to the rich air-fuel ratio, and the output air-fuel ratio of the downstream air-fuel ratio sensor 41 is changed. Control that switches the target air-fuel ratio to a lean air-fuel ratio when the fuel ratio becomes equal to or lower than the rich determination air-fuel ratio can be considered. Even when such control is performed, an abnormality in the element crack of the downstream air-fuel ratio sensor 41 can be similarly diagnosed.

<Description of specific control>
Next, the control device in the above embodiment will be described in detail with reference to FIGS. As shown in FIG. 8 which is a functional block diagram, the control device in the present embodiment is configured to include each functional block of A1 to A8. Hereinafter, each functional block will be described with reference to FIG. The operations in these functional blocks A1 to A8 are basically executed in the ECU 31.

<Calculation of fuel injection amount>
First, calculation of the fuel injection amount will be described. In calculating the fuel injection amount, in-cylinder intake air amount calculation means A1, basic fuel injection amount calculation means A2, and fuel injection amount calculation means A3 are used.

  The in-cylinder intake air amount calculation means A1 calculates the intake air amount Mc to each cylinder based on the intake air flow rate Ga, the engine speed NE, and a map or calculation formula stored in the ROM 34 of the ECU 31. The intake air flow rate Ga is measured by the air flow meter 39, and the engine speed NE is calculated based on the output of the crank angle sensor 44.

  The basic fuel injection amount calculation means A2 calculates the basic fuel injection amount Qbase by dividing the in-cylinder intake air amount Mc calculated by the in-cylinder intake air amount calculation means A1 by the target air-fuel ratio AFT (Qbase = Mc / AFT). The target air-fuel ratio AFT is calculated by target air-fuel ratio setting means A6 described later.

  The fuel injection amount calculation means A3 calculates the fuel injection amount Qi by adding an F / B correction amount DFi described later to the basic fuel injection amount Qbase calculated by the basic fuel injection amount calculation means A2 (Qi = Qbase + DFi). . An injection instruction is issued to the fuel injection valve 11 so that the fuel of the fuel injection amount Qi calculated in this way is injected from the fuel injection valve 11.

<Calculation of target air-fuel ratio>
Next, calculation of the target air-fuel ratio will be described. In calculating the target air-fuel ratio, oxygen excess / deficiency calculation means A4, air-fuel ratio correction amount calculation means A5, and target air-fuel ratio setting means A6 are used.

  The oxygen excess / deficiency calculation means A4 calculates the integrated oxygen excess / deficiency ΣOED based on the fuel injection quantity Qi calculated by the fuel injection quantity calculation means A3 and the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40. The oxygen excess / deficiency calculation means A4, for example, multiplies the difference between the output air-fuel ratio of the upstream air-fuel ratio sensor 40 and the control center air-fuel ratio by the fuel injection amount Qi and integrates the obtained value to integrate the excess oxygen excess. The deficiency ΣOED is calculated.

  In the air-fuel ratio correction amount calculation means A5, the air-fuel ratio of the target air-fuel ratio is calculated based on the integrated oxygen excess / deficiency amount ΣOED calculated by the oxygen excess / deficiency amount calculation means A4 and the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41. A correction amount AFC is calculated. Specifically, the air-fuel ratio correction amount AFC is calculated based on the flowchart shown in FIG.

  The target air-fuel ratio setting unit A6 adds the air-fuel ratio correction amount AFC calculated by the air-fuel ratio correction amount calculation unit A5 to the control center air-fuel ratio AFR (the theoretical air-fuel ratio in the present embodiment), thereby obtaining the target air-fuel ratio AFT. Is calculated. The target air-fuel ratio AFT calculated in this way is input to the basic fuel injection amount calculating means A2 and an air-fuel ratio deviation calculating means A7 described later.

<Calculation of F / B correction amount>
Next, calculation of the F / B correction amount based on the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 will be described. In calculating the F / B correction amount, air-fuel ratio deviation calculating means A7 and F / B correction amount calculating means A8 are used.

  The air-fuel ratio deviation calculating means A7 calculates the air-fuel ratio deviation DAF by subtracting the target air-fuel ratio AFT calculated by the target air-fuel ratio setting means A6 from the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 (DAF = AFup -AFT). This air-fuel ratio deviation DAF is a value that represents the excess or deficiency of the fuel supply amount with respect to the target air-fuel ratio AFT.

The F / B correction amount calculating means A8 supplies the fuel based on the following equation (2) by subjecting the air-fuel ratio deviation DAF calculated by the air-fuel ratio deviation calculating means A7 to proportional / integral / differential processing (PID processing). An F / B correction amount DFi for compensating for the excess or deficiency of the amount is calculated. The F / B correction amount DFi calculated in this way is input to the fuel injection amount calculation means A3.
DFi = Kp / DAF + Ki / SDAF + Kd / DDAF (2)

  In the above equation (2), Kp is a preset proportional gain (proportional constant), Ki is a preset integral gain (integral constant), and Kd is a preset differential gain (differential constant). DDAF is a time differential value of the air-fuel ratio deviation DAF, and is calculated by dividing the deviation between the air-fuel ratio deviation DAF updated this time and the air-fuel ratio deviation DAF updated last time by the time corresponding to the update interval. Is done. SDAF is a time integrated value of the air-fuel ratio deviation DAF, and this time integrated value SDAF is calculated by adding the currently updated air-fuel ratio deviation DAF to the previously updated time integrated value SDAF (SDAF = SDAF + DAF).

<Flowchart of air-fuel ratio correction amount setting control>
FIG. 9 is a flowchart showing a control routine of air-fuel ratio correction amount setting control. The illustrated control routine is performed by interruption at regular time intervals.
As shown in FIG. 9, first, in step S11, it is determined whether a calculation condition for the air-fuel ratio correction amount AFC is satisfied. The case where the calculation condition of the air-fuel ratio correction amount AFC is satisfied includes that the normal control in which feedback control is performed, for example, that the fuel cut control is not being performed, and the like. If it is determined in step S11 that the target air-fuel ratio calculation condition is satisfied, the process proceeds to step S12.

  In step S12, it is determined whether or not the lean setting flag Fl is set to OFF. The lean setting flag Fl is turned ON when the air-fuel ratio correction amount AFC is set to the lean setting correction amount AFClean, and is turned OFF otherwise. If the lean setting flag Fl is set to OFF in step S12, the process proceeds to step S13. In step S13, it is determined whether or not the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is equal to or less than the rich determination air-fuel ratio AFrich. If it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is greater than the rich determination air-fuel ratio AFrich, the process proceeds to step S14. In step S14, the air-fuel ratio correction amount AFC is maintained while being set to the rich set correction amount AFCrich, and the control routine is ended.

  On the other hand, when the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 decreases and the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 decreases, the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 in step S13. It is determined that AFdwn is equal to or less than the rich determination air-fuel ratio AFrich. In this case, the process proceeds to step S15, and the air-fuel ratio correction amount AFC is switched to the lean set correction amount AFClean. Next, at step S16, the lean setting flag Fl is set to ON, and the control routine is ended.

  When the lean setting flag Fl is set to ON, in the next control routine, it is determined in step S12 that the lean setting flag Fl is not set to OFF, and the process proceeds to step S17. In step S17, it is determined whether or not the cumulative oxygen excess / deficiency ΣOED after the air-fuel ratio correction amount AFC is switched to the lean set correction amount AFClean is less than the switching reference value OEDref. When it is determined that the cumulative oxygen excess / deficiency ΣOED is smaller than the switching reference value OEDref, the routine proceeds to step S18, where the air-fuel ratio correction amount AFC is continuously maintained while being set to the lean set correction amount AFClean. It will be terminated. On the other hand, when the oxygen storage amount of the upstream side exhaust purification catalyst 20 increases, it is determined in step S17 that the cumulative oxygen excess / deficiency ΣOED is equal to or greater than the switching reference value OEDref, and the process proceeds to step S19. In step S19, the air-fuel ratio correction amount AFC is switched to the rich set correction amount AFCrich. Next, in step S20, the lean setting flag Fl is reset to OFF, and the control routine is ended.

<Flow chart of abnormality diagnosis control>
FIG. 10 is a flowchart showing a control routine of abnormality diagnosis control for performing abnormality diagnosis of the downstream air-fuel ratio sensor 41. The illustrated control routine is performed by interruption at regular time intervals.

  As shown in FIG. 10, first, in step S31, it is determined whether or not an abnormality diagnosis execution condition is satisfied. The case where the abnormality diagnosis execution condition is satisfied includes, for example, that the above-described air-fuel ratio control is being executed. If it is determined in step S31 that the abnormality diagnosis execution condition is satisfied, the process proceeds to step S32.

  In step S32, it is determined in the air-fuel ratio correction amount setting control described above whether the air-fuel ratio correction amount AFC is set to less than 0, that is, whether the target air-fuel ratio is a rich air-fuel ratio. If it is determined in step S32 that the air-fuel ratio correction amount AFC is set to be less than 0, that is, if it is determined that the target air-fuel ratio is a rich air-fuel ratio, the process proceeds to step S33.

  In step S33, it is determined whether or not the stoichiometric flag Fs is set to OFF. The stoichiometric flag Fs indicates that when the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is smaller than the lean determination air-fuel ratio AFlean and is close to the theoretical air-fuel ratio. This is a flag that is turned on when the air-fuel ratio is reached, and turned off in other cases. If it is determined in step S33 that the stoichiometric flag Fs is set to OFF, the process proceeds to step S34.

  In step S34, it is determined whether or not the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is less than the lean determination air-fuel ratio AFlean. Immediately after the air-fuel ratio correction amount AFC is switched to the rich set correction amount AFCrich, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is substantially the stoichiometric air-fuel ratio. It is determined that the output air-fuel ratio AFdwn of 41 is less than the lean determination air-fuel ratio AFlean, and the process proceeds to step S35. In step S35, the stoichiometric flag Fs is turned ON, and the control routine is ended.

  When the stoichiometric flag Fs is turned on, the next control routine proceeds from step S33 to step S36. In step S36, it is determined whether or not the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 has become equal to or greater than the lean determination air-fuel ratio AFlean, that is, from the air-fuel ratio in which the output air-fuel ratio AFdwn is lower than the lean determination air-fuel ratio AFlean. It is determined whether or not the above has changed. When it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is not equal to or greater than the lean determination air-fuel ratio AFlean, the control routine is ended. On the other hand, if it is determined in step S36 that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is equal to or greater than the lean determination air-fuel ratio AFlean, the process proceeds to step S37. In step S37, it is determined that there is an abnormality in the downstream air-fuel ratio sensor 41, and the process proceeds to step S38. In step S38, the stoichiometric flag Fs is turned off, and the control routine is ended.

  On the other hand, if it is determined in step S32 that the air-fuel ratio correction amount AFC is set to 0 or more, that is, if it is determined that the target air-fuel ratio is a lean air-fuel ratio, the process proceeds to step S39. In step S39, it is determined whether or not the stoichiometric flag Fs is ON. When the target air-fuel ratio is set to the rich air-fuel ratio before being set to the lean air-fuel ratio, if the stoichiometric flag Fs is turned on in step S35, the process proceeds to step S40. In this case, when the target air-fuel ratio was previously set to the rich air-fuel ratio, it means that the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 did not become equal to or greater than the lean determination air-fuel ratio AFlean. For this reason, it is determined in step S40 that the downstream air-fuel ratio sensor 41 is normal. Next, in step S41, the stoichiometric flag Fs is reset to OFF, and the control routine is ended. When the stoichiometric flag Fs is reset to OFF, in the next control routine, it is determined in step S39 that the stoichiometric flag Fs is not ON, and the control routine is ended.

<Flow chart of set air-fuel ratio change control>
FIG. 11 is a flowchart showing a control routine in setting control of the rich set air-fuel ratio and the lean set air-fuel ratio. The illustrated control routine is performed by interruption at regular time intervals.

First, in step S51, it is determined whether or not it is determined that there is an abnormality in the downstream air-fuel ratio sensor 41 in step S37 of FIG. If it is determined in step S37 that the downstream air-fuel ratio sensor 41 is not abnormally determined, the process proceeds to step S52. In step S52, the rich setting correction amount AFCrich is set to the first rich setting correction amount AFCrich ₁ . Therefore, in steps S14 and S19 of the flowchart shown in FIG. 9, the air-fuel ratio correction amount AFC is set to the first rich set correction amount AFCrich ₁ .

Next, in step S53, the lean set correction amount AFClean is set to the first lean set correction amount AFClean ₁ . Therefore, in steps S15 and S18 of the flowchart shown in FIG. 9, the air-fuel ratio correction amount AFC is set to the first lean set correction amount AFClean ₁ .

On the other hand, when it is determined in step S51 that the abnormality determination of the downstream air-fuel ratio sensor 41 has been made, the process proceeds to step S54. In step S54, the rich set correction amount AFCrich second rich set correction amount AFCrich ₂ is set to _{(| AFCrich 2 | <| |} AFCrich 1). Therefore, in step S14, S19 in the flowchart shown in FIG. 9, the air-fuel ratio correction amount AFC is set to the second rich set correction amount AFCrich _2.

Next, in step S55, the lean set correction amount AFClean is set to the second lean set correction amount AFClean ₂ (| AFClean ₂ | <| AF Clean ₁ |). Therefore, in steps S15 and S18 of the flowchart shown in FIG. 9, the air-fuel ratio correction amount AFC is set to the second lean set correction amount AFClean ₂ .

<Second embodiment>
Next, with reference to FIGS. 12-18, the exhaust gas purification apparatus which concerns on 2nd embodiment of this invention is demonstrated. The configuration and control of the exhaust purification device according to the second embodiment are basically the same as the configuration and control of the exhaust purification device according to the first embodiment except for the points described below.

<Difference in upstream air-fuel ratio sensor>
By the way, when the engine body 1 has a plurality of cylinders, the air-fuel ratio of the exhaust gas discharged from each cylinder may vary between the cylinders. On the other hand, the upstream side air-fuel ratio sensor 40 is disposed at the collection portion of the exhaust manifold 19, but the extent to which the exhaust gas discharged from each cylinder is exposed to the upstream side air-fuel ratio sensor 40 according to the position of the upstream manifold 19 is determined. It is different. As a result, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is strongly influenced by the air-fuel ratio of the exhaust gas discharged from a specific cylinder. For this reason, when the air-fuel ratio of the exhaust gas discharged from a certain cylinder is different from the average air-fuel ratio of the exhaust gas discharged from all cylinders, the average air-fuel ratio and the upstream air-fuel ratio There is a deviation from the output air-fuel ratio of the sensor 40. That is, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is shifted to the rich side or the lean side from the actual average air-fuel ratio of the exhaust gas.

  Further, hydrogen in the unburned gas or the like has a high passing speed through the diffusion-controlled layer of the air-fuel ratio sensor. For this reason, when the hydrogen concentration in the exhaust gas is high, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 shifts to a side lower than the actual air-fuel ratio of the exhaust gas (that is, the rich side).

  As described above, if the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is deviated, NOx and oxygen flow out from the upstream exhaust purification catalyst 20 or unburned gas even if the above-described control is performed. There is a high possibility that the frequency of such outflow will increase. Hereinafter, such a phenomenon will be described with reference to FIG.

  FIG. 12 is a time chart of the oxygen storage amount OSA and the like of the upstream side exhaust purification catalyst 20, similar to FIG. FIG. 12 shows a case where the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is shifted to the rich side. In the figure, the solid line in the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 indicates the output air-fuel ratio of the upstream air-fuel ratio sensor 40. On the other hand, the broken line indicates the actual air-fuel ratio of the exhaust gas flowing around the upstream air-fuel ratio sensor 40.

Also in the example shown in FIG. 12, before the time t ₁ , the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich, and thus the target air-fuel ratio is set to the rich set air-fuel ratio. Accordingly, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 becomes an air-fuel ratio equal to the rich set air-fuel ratio. However, as described above, since the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is shifted to the rich side, the actual air-fuel ratio of the exhaust gas is on the lean side of the rich set air-fuel ratio. That is, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 is lower (rich side) than the actual air-fuel ratio (broken line in the figure). For this reason, the decrease rate of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is slow.

In the example shown in FIG. 12, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich at time t ₁ . Therefore, as described above, at time t _1, the air-fuel ratio correction amount AFC is switched to the lean set correction amount AFClean. That is, the target air-fuel ratio is switched to the lean set air-fuel ratio.

  Along with this, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 becomes an air-fuel ratio equal to the lean set air-fuel ratio. However, as described above, since the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is shifted to the rich side, the actual air-fuel ratio of the exhaust gas is leaner than the lean set air-fuel ratio. That is, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 is lower (rich side) than the actual air-fuel ratio (broken line in the figure). For this reason, the increase rate of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 is increased, and the actual oxygen amount supplied to the upstream side exhaust purification catalyst 20 is switched while the target air-fuel ratio is set to the lean set air-fuel ratio. It becomes larger than the reference storage amount Cref.

Thus, if there is a deviation in the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40, it flows into the upstream side exhaust purification catalyst 20 when the air-fuel ratio correction amount AFC is set to the lean set correction amount AFClean. The lean degree of the air-fuel ratio of the exhaust gas increases. For this reason, even if the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 does not reach the maximum storable oxygen amount Cmax, it is not possible to store all NOx and oxygen flowing into the upstream side exhaust purification catalyst 20, There is a case where NOx and oxygen flow out from the upstream side exhaust purification catalyst 20. At time t _2, the oxygen storage amount of the upstream exhaust purification catalyst 20 OSA is the have become more switching reference occlusion amount Cref, deviation, etc. of unintended air as described above at time t ₂ vicinity occurs upstream There is a possibility that NOx and oxygen flow out from the side exhaust purification catalyst 20.

  As described above, it is necessary to detect a deviation in the output air-fuel ratio of the upstream air-fuel ratio sensor 40, and it is necessary to correct the output air-fuel ratio based on the detected deviation.

<Learning control>
Therefore, in the embodiment of the present invention, during normal operation (that is, when feedback control is performed based on the target air-fuel ratio as described above) in order to compensate for the deviation in the output air-fuel ratio of the upstream air-fuel ratio sensor 40. Learning control is performed. Hereinafter, this learning control will be described.

  Here, the period from when the target air-fuel ratio is switched to the lean air-fuel ratio until the cumulative oxygen excess / deficiency ΣOED becomes equal to or greater than the switching reference value OEDref, that is, the period from when the target air-fuel ratio is switched to the rich air-fuel ratio again is referred to as the oxygen increase period. To do. Similarly, a period from when the target air-fuel ratio is switched to the rich air-fuel ratio until the output air-fuel ratio of the downstream side air-fuel ratio sensor 41 becomes equal to or lower than the rich determination air-fuel ratio, that is, until the target air-fuel ratio is switched again to the lean air-fuel ratio. The oxygen reduction period. In the learning control of this embodiment, the cumulative oxygen excess amount is calculated as the absolute value of the cumulative oxygen excess / deficiency ΣODE during the oxygen increase period. The cumulative oxygen excess represents the cumulative value of the amount of oxygen that becomes excessive when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is set to the stoichiometric air-fuel ratio during the oxygen increase period. In addition, the cumulative oxygen deficiency is calculated as the absolute value of the cumulative oxygen excess / deficiency ΣOED during the oxygen reduction period. The accumulated oxygen deficiency represents an accumulated value of the amount of oxygen deficient when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is set to the stoichiometric air-fuel ratio during the oxygen reduction period. Then, the control center air-fuel ratio AFR is corrected so that the difference between the accumulated oxygen excess amount and the accumulated oxygen deficiency amount becomes small. FIG. 13 shows this state.

  FIG. 13 shows the control center air-fuel ratio AFR, the air-fuel ratio correction amount AFC, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20, the cumulative oxygen excess / deficiency ΣOED, 4 is a time chart of an output air-fuel ratio AFdwn of a fuel ratio sensor 41 and a learned value sfbg. FIG. 13 shows a case where the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 is shifted to the low side (rich side) as in FIG. Note that the learned value sfbg is a value that changes in accordance with the deviation of the output air-fuel ratio (output current) of the upstream air-fuel ratio sensor 40, and is used to correct the control center air-fuel ratio AFR in this embodiment. In the figure, the solid line at the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 indicates the air-fuel ratio corresponding to the output detected by the upstream air-fuel ratio sensor 40, and the broken line circulates around the upstream air-fuel ratio sensor 40. The actual air-fuel ratio of exhaust gas is shown. In addition, the alternate long and short dash line indicates the target air-fuel ratio, that is, the air-fuel ratio obtained by adding the air-fuel ratio correction amount AFC and the learned value sfbg to the theoretical air-fuel ratio.

In the example shown in FIG. 13, as in FIGS. 5 and 12, in the state before time t ₁ , the control center air-fuel ratio is set to the stoichiometric air-fuel ratio, and the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich. Yes. At this time, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 becomes an air-fuel ratio corresponding to the rich set air-fuel ratio as shown by the solid line. However, since the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 is deviated, the actual air-fuel ratio of the exhaust gas is leaner than the rich set air-fuel ratio (broken line in FIG. 13). However, in the example shown in FIG. 13, as can be seen from the broken line in FIG. 13, the actual air-fuel ratio of the exhaust gas before time t ₁ is richer than the rich set air-fuel ratio, but is rich. Therefore, the oxygen storage amount of the upstream side exhaust purification catalyst 20 gradually decreases.

At time t _1, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich. As a result, as described above, the air-fuel ratio correction amount AFC is switched to the lean set correction amount AFClean. After time t _1, the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes an air-fuel ratio corresponding to the lean set air-fuel ratio. However, due to the deviation in the output air-fuel ratio of the upstream air-fuel ratio sensor 40, the actual air-fuel ratio of the exhaust gas becomes an air-fuel ratio that is leaner than the lean set air-fuel ratio, that is, an air-fuel ratio with a large lean degree (the broken line in FIG. See). For this reason, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 increases rapidly.

  On the other hand, the oxygen excess / deficiency amount OED is calculated based on the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40. However, as described above, there is a deviation in the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40. Accordingly, the calculated oxygen excess / deficiency OED is a value smaller than the actual oxygen excess / deficiency OED (that is, the amount of oxygen is small). As a result, the calculated cumulative oxygen excess / deficiency ΣOED is smaller than the actual value.

At time t _2, the accumulated oxygen deficiency amount ΣOED reaches the switching reference value OEDref. For this reason, the air-fuel ratio correction amount AFC is switched to the rich set correction amount AFCrich. Therefore, the target air-fuel ratio is set to a rich air-fuel ratio. At this time, the actual oxygen storage amount OSA is larger than the switching reference storage amount Cref as shown in FIG.

After time t ₂ , as in the state before time t ₁ , the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich, and thus the target air-fuel ratio is set to the rich air-fuel ratio. Also at this time, the actual air-fuel ratio of the exhaust gas is leaner than the rich set air-fuel ratio. As a result, the decrease rate of the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 becomes slow. In addition, as described above, at time t _2, the actual oxygen storage amount of the upstream exhaust purification catalyst 20 is made larger than the switching reference occlusion amount Cref. For this reason, it takes time until the actual oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 reaches zero.

At time t ₃ , the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich. As a result, as described above, the air-fuel ratio correction amount AFC is switched to the lean set correction amount AFClean. Therefore, the target air-fuel ratio is switched from the rich set air-fuel ratio to the lean set air-fuel ratio.

In the present embodiment, as described above, the cumulative oxygen excess / deficiency ΣOED is calculated from time t ₁ to time t ₂ . Here, when the estimated value of the oxygen storage amount OSA of the upstream exhaust purification catalyst 20 from the time of switching the target air-fuel ratio to the lean air-fuel ratio (time t ₁₎ is equal to or higher than the switching reference occlusion amount Cref (time t ₂₎ When the period up to this time is referred to as the oxygen increase period Tinc, in this embodiment, the cumulative oxygen excess / deficiency ΣOED is calculated in the oxygen increase period Tinc. In FIG. 13, the absolute value (cumulative oxygen excess amount) of the cumulative oxygen excess / deficiency ΣOED in the oxygen increase period Tinc from time t ₁ to time t ₂ is indicated by R ₁ .

This accumulated oxygen excess amount R ₁ corresponds to the oxygen storage amount OSA at time t ₂ . However, as described above, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 is used for estimating the oxygen excess / deficiency amount OED, and there is a deviation in the output air-fuel ratio AFup. For this reason, in the example shown in FIG. 13, the cumulative oxygen excess R _{1 from} time t ₁ to time t ₂ is smaller than the value corresponding to the actual oxygen storage amount OSA at time t ₂ .

In the present embodiment, the cumulative oxygen excess / deficiency ΣOED is also calculated from time t ₂ to time t ₃ . Here, a period from when the target air-fuel ratio is switched to the rich air-fuel ratio (time t ₂ ) to when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich (time t ₃ ) is set. In the present embodiment, the cumulative oxygen excess / deficiency ΣOED is calculated during the oxygen reduction period Tdec when referred to as the oxygen reduction period Tdec. In FIG. 13, the absolute value (cumulative oxygen deficiency) of the cumulative oxygen excess / deficiency ΣOED in the oxygen reduction period Tdec from time t ₂ to time t ₃ is indicated by F ₁ .

This accumulated oxygen deficiency F ₁ corresponds to the total amount of oxygen released from the upstream side exhaust purification catalyst 20 from time t ₂ to time t ₃ . However, as described above, there is a deviation in the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40. Therefore, in the example shown in FIG. 13, the integrated oxygen shortage F ₁ at time t ₂ ~ time t _3, the total oxygen that is actually released from the upstream exhaust purification catalyst 20 from time t ₂ to time t ₃ More than the value corresponding to the quantity.

Here, oxygen is stored in the upstream side exhaust purification catalyst 20 during the oxygen increase period Tinc, and all of the stored oxygen is released during the oxygen decrease period Tdec. Therefore, it is ideal that the cumulative oxygen excess amount R ₁ and the cumulative oxygen shortage amount F ₁ basically have the same value. However, as described above, when there is a deviation in the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40, the integrated value also changes in accordance with this deviation. As described above, when the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is shifted to the low side (rich side), the cumulative oxygen deficiency F ₁ is larger than the cumulative oxygen excess R ₁ . Conversely, when the output air-fuel ratio of the upstream air-fuel ratio sensor 40 is shifted to the higher side (lean side), the cumulative oxygen deficiency F ₁ becomes smaller than the cumulative oxygen excess R ₁ . In addition, the difference ΔΣOED (= R ₁ −F _1, hereinafter referred to as “excess / deficiency error”) between the accumulated oxygen excess amount R ₁ and the accumulated oxygen deficiency amount F ₁ is equal to the output air-fuel ratio of the upstream side air-fuel ratio sensor 40. It represents the degree of deviation. It can be said that the deviation in the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes larger as the excess / deficiency error ΔΣOED increases.

Therefore, in the present embodiment, the control center air-fuel ratio AFR is corrected based on the excess / deficiency error ΔΣOED. In particular, in the present embodiment, the control center air-fuel ratio AFR is corrected so that the difference ΔΣOED between the cumulative oxygen excess amount R ₁ and the cumulative oxygen shortage amount F ₁ becomes small.

Specifically, in the present embodiment, the learning value sfbg is calculated by the following equation (3), and the control center air-fuel ratio AFR is corrected by the following equation (4).
sfbg (n) = sfbg (n−1) + k ₁ · ΔΣOED (3)
AFR = AFRbase + sfbg (n) (4)
In the above formula (3), n represents the number of calculations or time. Therefore, sfbg (n) is the current calculation or the current learning value. In addition, k ₁ in the above equation (3) is a gain representing the degree to which the excess / deficiency error ΔΣOED is reflected in the control center air-fuel ratio AFR. The correction amount of the control center air-fuel ratio AFR increases as the value of the gain k ₁ increases. Further, in the above equation (4), the basic control center air-fuel ratio AFRbase is the basic control center air-fuel ratio, and in this embodiment, is the theoretical air-fuel ratio.

At time t ₃ in FIG. 13, as described above, the learning value sfbg is calculated based on the cumulative oxygen excess amount R ₁ and the cumulative oxygen shortage amount F ₁ . In particular, in the example shown in FIG. 13, since the cumulative oxygen deficiency F ₁ is larger than the cumulative oxygen excess R ₁ , the learning value sfbg is decreased at time t ₃ .

  Here, the control center air-fuel ratio AFR is corrected based on the learning value sfbg using the above equation (4). In the example shown in FIG. 13, since the learning value sfbg is a negative value, the control center air-fuel ratio AFR is smaller than the basic control center air-fuel ratio AFRbase, that is, a rich value. As a result, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is corrected to the rich side.

As a result, after time t ₃ , the deviation of the actual air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 from the target air-fuel ratio becomes smaller than before time t ₃ . Therefore, after time t ₃ , the difference between the broken line representing the actual air-fuel ratio and the one-dot chain line representing the target air-fuel ratio is smaller than the difference before time t ₃ .

Further, after time t ₃ , the same operation as the operation from time t ₁ to time t ₂ is performed. Therefore, when the cumulative oxygen excess / deficiency ΣOED reaches the switching reference value OEDref at time t ₄ , the target air-fuel ratio is switched from the lean set air-fuel ratio to the rich set air-fuel ratio. Then, at time t _5, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches a rich determination air AFrich, again, the target air-fuel ratio is switched to a lean set air-fuel ratio.

Time t ₃ to time t ₄ correspond to the oxygen increase period Tinc as described above, and therefore the absolute value of the cumulative oxygen excess / deficiency ΣOED during this time can be expressed by the cumulative oxygen excess R ₂ in FIG. Further, the time t ₄ to the time t ₅ correspond to the oxygen decrease period Tdec as described above, and therefore the absolute value of the cumulative oxygen excess / deficiency ΣOED during this time can be expressed by the cumulative oxygen deficiency F ₂ in FIG. Based on the difference ΔΣOED (= R ₂ −F ₂ ) between the cumulative oxygen excess amount R ₂ and the cumulative oxygen shortage amount F ₂ , the learning value sfbg is updated using the above equation (3). In the present embodiment, similar control is repeated after time t ₅ , whereby the learning value sfbg is repeatedly updated.

  By updating the learning value sfbg in this way through the learning control, the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40 gradually departs from the target air-fuel ratio, but the exhaust gas flowing into the upstream side exhaust purification catalyst 20 The actual air fuel ratio gradually approaches the target air fuel ratio. Thereby, the deviation in the output air-fuel ratio of the upstream air-fuel ratio sensor 40 can be compensated.

  As described above, the update of the learned value sfbg is based on the cumulative oxygen excess / deficiency ΣOED in the oxygen increase period Tinc and the cumulative oxygen excess / deficiency ΣOED in the oxygen decrease period Tdec immediately after the oxygen increase period Tinc. Is preferably performed. As described above, this is because the total amount of oxygen stored in the upstream side exhaust purification catalyst 20 during the oxygen increase period Tinc and the total amount of oxygen released from the upstream side exhaust purification catalyst 20 during the oxygen decrease period Tdec that follows immediately after this increase. This is because they are equal.

  In addition, in the above embodiment, the learning value sfbg is updated based on the cumulative oxygen excess / deficiency ΣOED in one oxygen increase period Tinc and the cumulative oxygen excess / deficiency ΣOED in one oxygen decrease period Tdec. ing. However, the learning value is based on the total value or average value of the cumulative oxygen excess / deficiency ΣOED in the plurality of oxygen increase periods Tinc and the total value or average value of the cumulative oxygen excess / deficiency ΣOED in the plurality of oxygen decrease periods Tdec. You may update sfbg.

  In the above embodiment, the control center air-fuel ratio is corrected based on the learned value sfbg. However, other parameters relating to the air-fuel ratio may be corrected based on the learned value sfbg. Examples of other parameters include the amount of fuel supplied into the combustion chamber 5, the output air-fuel ratio of the upstream air-fuel ratio sensor 40, the air-fuel ratio correction amount, and the like.

  In the present embodiment, the above-described another control can be performed as the air-fuel ratio control. Specifically, as another control, for example, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the lean determination air-fuel ratio, the target air-fuel ratio is switched to the rich air-fuel ratio, and the downstream air-fuel ratio sensor 41 is switched. It is conceivable to control the target air-fuel ratio to the lean air-fuel ratio when the output air-fuel ratio becomes equal to or less than the rich determination air-fuel ratio.

  In this case, the cumulative oxygen shortage is an absolute value of the cumulative oxygen excess / deficiency in the oxygen reduction period from when the target air-fuel ratio is switched to the rich air-fuel ratio until the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio. A quantity is calculated. In addition, the cumulative oxygen excess / absolute amount is obtained as an absolute value of the cumulative oxygen excess / deficiency during the oxygen increase period from when the target air / fuel ratio is switched to the lean air / fuel ratio until the output air / fuel ratio of the downstream air / fuel ratio sensor 41 becomes equal to or higher than the lean determination air / fuel ratio. A quantity is calculated. Then, the control center air-fuel ratio and the like are corrected so that the difference between the accumulated oxygen excess amount and the accumulated oxygen deficiency amount becomes small.

  Therefore, to summarize the above, in the present embodiment, in the learning control, the cumulative oxygen excess amount during the oxygen increase period from when the target air-fuel ratio is switched to the lean air-fuel ratio and again to the rich air-fuel ratio, and the target air-fuel ratio are rich. Based on the cumulative oxygen deficiency during the oxygen reduction period from when the air-fuel ratio is switched to when the air-fuel ratio is switched back to the lean air-fuel ratio, the air-fuel ratio parameter is set so that the difference between the cumulative oxygen excess and the cumulative oxygen shortage is reduced. It can be said that it is corrected.

<Behavior when element cracks occur>
As described above, when an element crack occurs in the downstream air-fuel ratio sensor 41, basically, when the air-fuel ratio of the exhaust gas around the downstream air-fuel ratio sensor 41 is a rich air-fuel ratio, the downstream air-fuel ratio sensor 41. The output air-fuel ratio AFdwn becomes the lean air-fuel ratio. However, such a phenomenon may not occur depending on the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41.

  FIG. 14 is a graph showing the relationship between the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 and the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41. FIG. 14 shows a case where the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is a rich air-fuel ratio that is slightly richer than the stoichiometric air-fuel ratio. In addition, the circles in the figure indicate the case where a normal sensor with no element cracking is used, and the triangles in the figure indicate the case where a sensor with an element cracking is used.

  As can be seen from FIG. 14, when the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is large, the output air-fuel ratio AFdwn is the actual exhaust gas if no element cracking occurs in the downstream air-fuel ratio sensor 41. It corresponds to the air-fuel ratio. Accordingly, in the example shown in FIG. 14, the output air-fuel ratio AFdwn is an air-fuel ratio slightly richer than the stoichiometric air-fuel ratio. On the other hand, if an element crack occurs in the downstream air-fuel ratio sensor 41 at this time, the output air-fuel ratio AFdwn becomes a lean air-fuel ratio as described above. On the other hand, when the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is small, the output air-fuel ratio AFdwn is equal to the actual air-fuel ratio of the exhaust gas regardless of whether the downstream air-fuel ratio sensor 41 is cracked or not. The air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio.

  Thus, even when the above-described element crack occurs in the downstream air-fuel ratio sensor 41, the above-described phenomenon does not occur when the flow rate of the exhaust gas is small. It is done. That is, if the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is small, the flow rate of the exhaust gas entering the reference gas chamber 55 through the cracked portion is reduced. For this reason, even if the exhaust gas enters the reference gas chamber 55 through the crack, the oxygen concentration around the atmosphere-side electrode 53 hardly changes. As a result, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 does not change, and the correct air-fuel ratio is output.

<Incorrect learning due to element cracking>
When an element crack occurs in the downstream side air-fuel ratio sensor 41, the output air-fuel ratio exhibits the behavior as described above. Therefore, when the learning control as described above is performed, the learning value is erroneously updated. There is. Such an erroneous learning value update will be described with reference to FIG.

  FIG. 15 shows the intake air amount Mc, the control center air-fuel ratio AFR, the air-fuel ratio correction amount AFC, and the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 when element cracking occurs in the downstream air-fuel ratio sensor 41. 4 is a time chart of an integrated oxygen excess / deficiency ΣOED, an output air-fuel ratio AFdwn of a downstream air-fuel ratio sensor 41, and a learned value sfbg. In the example shown in FIG. 15, no error has occurred in the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40.

In the example shown in FIG. 15, the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich before the time t ₁ . For this reason, the oxygen storage amount OSA of the upstream side exhaust purification catalyst 20 gradually decreases, and unburned gas or the like starts to flow out of the upstream side exhaust purification catalyst 20 in the vicinity of time t ₁ . At this time, the amount of intake air Mc into the combustion chamber 5 of the internal combustion engine is relatively small, and therefore the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is also relatively small. For this reason, when the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 becomes a rich air-fuel ratio, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 also becomes a rich air-fuel ratio. As a result, in the illustrated example, at time t _1, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich set air-fuel ratio AFrich. For this reason, the air-fuel ratio correction amount AFC is switched from the rich setting correction amount AFCrich to the lean setting correction amount AFClean.

Air-fuel ratio correction quantity AFC is the switched to the lean set correction amount AFClean, the oxygen storage amount OSA of the upstream exhaust purification catalyst 20 gradually increases at time t _1. Further, this integrated oxygen deficiency amount ΣOED gradually increased with, arrives at the switching reference value OEDref at time t _2. Therefore, air-fuel ratio correction quantity AFC at time t ₂ is switched from the lean setting the correction amount AFClean rich set correction amount AFCrich.

When the air-fuel ratio correction amount AFC is switched to the rich set correction amount AFCrich, reduced oxygen storage amount OSA of the upstream exhaust purification catalyst 20 gradually, at time t ₃ near the oxygen storage amount OSA of the upstream exhaust purification catalyst 20 It reaches almost zero. For this reason, unburned gas or the like starts to flow out from the upstream side exhaust purification catalyst 20 in the vicinity of time t ₃ .

At this time, in the example shown in FIG. 15, the intake air amount Mc into the combustion chamber 5 of the internal combustion engine is relatively large, and therefore the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is also relatively large. . For this reason, when the air-fuel ratio of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 becomes a rich air-fuel ratio, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes a lean air-fuel ratio. Therefore, at time t ₃ , the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or greater than the lean determination air-fuel ratio AFlean, and it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41.

  However, since the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is not less than or equal to the rich determination air-fuel ratio AFrich, the air-fuel ratio correction amount AFC is maintained at the rich set correction amount. For this reason, unburned gas or the like flows out from the upstream side exhaust purification catalyst 20, and the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is maintained at a larger air-fuel ratio than the lean determination air-fuel ratio AFlean.

Thereafter, in the example shown in FIG. 15, the intake air amount Mc decreases at time t ₄ , and the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 becomes relatively small. For this reason, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 changes toward the correct air-fuel ratio corresponding to the actual air-fuel ratio of the exhaust gas. As a result, at time t ₅ , the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio AFrich. For this reason, the air-fuel ratio correction amount AFC is switched from the rich set air-fuel ratio AFCrich to the lean set air-fuel ratio AFClean.

Here, as described above, during the oxygen reduction period used when calculating the learning value sfbg, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 is rich-determined after the target air-fuel ratio is switched to the rich air-fuel ratio. A period until the fuel ratio AFrich is reached (or when the target air-fuel ratio is switched to the lean air-fuel ratio) is set. For this reason, when the oxygen reduction period is calculated in this way, the oxygen reduction period becomes extremely long. As a result, the cumulative oxygen deficiency F ₁ from time t ₂ to time t ₅ becomes larger than the cumulative oxygen excess R ₁ from time t ₁ to time t ₂ , and the learned value sfbg is large as shown in FIG. It will be reduced. As a result, the learning value sfbg is changed and an erroneous learning value is updated even though there is no error in the output air-fuel ratio of the upstream air-fuel ratio sensor 40. As a result, as shown in FIG. 15, the control center air-fuel ratio AFR is erroneously changed.

<Learning value update when element crack is abnormal>
Therefore, in this embodiment, when it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor 41 when the target air-fuel ratio is set to a rich air-fuel ratio, the downstream air-fuel ratio sensor 41 thereafter Even when the output air-fuel ratio AFdwn becomes equal to or less than the rich determination air-fuel ratio AFrich and the target air-fuel ratio is switched to the lean air-fuel ratio, the correction of the learned value sfbg based on the accumulated oxygen shortage at this time is stopped.

  In addition, in the present embodiment, when the target air-fuel ratio is set to the rich air-fuel ratio, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is from the air-fuel ratio richer than the lean-determined air-fuel ratio AFlean to the lean-determined air-fuel ratio. If it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor 41 due to the air-fuel ratio being leaner than AFlean, the downstream air-fuel ratio sensor 41 has been switched to the rich air-fuel ratio from the previous time after switching the target air-fuel ratio. The absolute value of the cumulative oxygen excess / deficiency ΣOED during the period until the output air-fuel ratio AFdwn of the engine changes from the rich air-fuel ratio to the lean air-fuel ratio is calculated as the cumulative oxygen shortage F. The learned value sfbg is corrected so that the difference between the calculated cumulative oxygen deficiency F and the cumulative oxygen excess R is reduced.Hereinafter, with reference to FIG. 16, a learning value update method according to the present embodiment will be described.

FIG. 16 is a time chart similar to FIG. 15 showing the air-fuel ratio correction amount AFC and the like. FIG. 16 shows a case where a slight error has occurred in the output air-fuel ratio AFup of the upstream side air-fuel ratio sensor 40. Also in the example shown in FIG. 16, each parameter changes until time t ₃ as in the example shown in FIG.

Also in the example shown in FIG. 16, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the lean determination air-fuel ratio AFlean at time t ₃ , and an element crack abnormality occurs in the downstream air-fuel ratio sensor 41. It is determined that At this time, as can be seen from FIG. 16, the oxygen storage amount OSA is substantially zero, so that in reality, exhaust gas having a rich air-fuel ratio flows out from the upstream side exhaust purification catalyst 20. Thus, the integrated oxygen deficiency amount ΣOED at time t ₃ from time t ₂ represents the amount of oxygen released from the upstream exhaust purification catalyst 20 between times t ₃ from time t _2, the upstream at time t ₂ This corresponds to the oxygen storage amount of the side exhaust purification catalyst 20. In other words, time t ₃ from the time t _2, together equivalent to the oxygen reduction period Tdec in Fig. 13, the integrated oxygen deficiency amount ΣOED from time t ₂ to time t ₃ is the cumulative oxygen shortage F shown in FIG. 13 Equivalent to.

Therefore, unless the deviation in the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 is occurring, the integrated in the oxygen increases the period Tinc from time t ₁ to time t ₂ oxygen excess R ₁ and time t ₂ time t ₃ from The accumulated oxygen deficiency F ₁ in the oxygen decrease period Tdec until is the same value. However, if the shift to the upstream side air-fuel ratio sensor 40 has occurred, the these integrated excess oxygen and R ₁ and cumulative oxygen shortage F ₁ and different values.

Therefore, in the present embodiment, the cumulative oxygen excess amount R ₁ in the oxygen increase period Tinc from time t ₁ to time t ₂ and the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 from time t ₂ are the lean determination air-fuel ratio AFlean. Based on the excess / deficiency error ΔΣOED (= R ₁ −F ₁ ), which is the difference from the accumulated oxygen deficiency F ₁ in the oxygen decrease period Tdec until time t ₃ , the learning value is obtained by the above equation (3). sfbg is calculated. Further, the control center air-fuel ratio AFR is corrected by the above equation (4) based on the calculated learned value sfbg.

As a result, in the example shown in FIG. 16, since the accumulated oxygen deficiency F ₁ is larger than the accumulated oxygen excess R ₁ , the learning value sfbg is decreased at time t ₃ . As a result, the control center air-fuel ratio AFR is reduced. As a result, according to the present embodiment, the learning value sfbg can be appropriately updated even when an element cracking abnormality has occurred in the downstream air-fuel ratio sensor 41, and thus the upstream air-fuel ratio can be updated. A deviation in the output air-fuel ratio of the sensor 40 can be appropriately compensated.

In the example shown in FIG. 16, as in the example shown in FIG. 15, the intake air amount Mc decreases at time t _{4 and} the flow rate of the exhaust gas flowing around the downstream air-fuel ratio sensor 41 is relatively small. Become. For this reason, the output air-fuel ratio AFdwn of the downstream side air-fuel ratio sensor 41 changes toward the rich air-fuel ratio after time t ₄ and becomes equal to or less than the rich determination air-fuel ratio AFrich at time t ₅ . For this reason, the air-fuel ratio correction amount AFC is switched from the rich set air-fuel ratio AFCrich to the lean set air-fuel ratio AFClean.

In this case, in the present embodiment, at time t _5, it is made to stop the updating of the learning value SFBG, thus correcting the control center air-fuel ratio AFR is made to stop. As described with reference to FIG. 15, when the learning value sfbg is updated based on the accumulated oxygen deficiency from time t ₂ to time t ₅ , the learning value sfbg is erroneously updated. In the present embodiment, at time t ₅ , the learning value sfbg is not updated based on the accumulated oxygen deficiency from time t ₂ to time t _5, so that the learning value sfbg can be prevented from being erroneously updated.

In the above embodiment, the learning value sfbg is updated at the time t ₃ , but the learning value sfbg at this time may not necessarily be updated. That is, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 when the air-fuel ratio correction amount AFC is set to the rich set correction amount AFCrich for reasons other than the occurrence of element cracks in the downstream air-fuel ratio sensor 41. May be greater than or equal to the lean determination air-fuel ratio AFlean. As such a case, for example, a case where the engine load suddenly changes and the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 suddenly changes can be considered. By not updating the learning value sfbg at time t ₃ , it is possible to suppress the learning value from being updated erroneously in such a case.

<Description of Control in Second Embodiment>
Next, with reference to FIGS. 17 and 18, the control device in the present embodiment will be described in detail. As shown in FIG. 17, the control device in the present embodiment is configured to include functional blocks A1 to A10. Among these, the functional blocks A1 to A8 are the same as the functional blocks A1 to A8 in the first embodiment, and thus description thereof is basically omitted.

  In the learning value calculation means A9, the learning value sfbg is calculated based on the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41, the integrated oxygen excess / deficiency amount ΣOED calculated by the oxygen excess / deficiency calculation means A4, and the like. Specifically, the learning value sfbg is calculated based on the learning control flowchart shown in FIG. The learning value sfbg calculated in this manner is stored in a storage medium in the RAM 33 of the ECU 31 that is not erased even if the ignition key of the vehicle equipped with the internal combustion engine is turned off.

  In the control center air-fuel ratio calculating means A10, the control center air-fuel ratio AFR is calculated based on the basic control center air-fuel ratio AFRbase (for example, the theoretical air-fuel ratio) and the learned value sfbg calculated by the learned value calculating means A9. Specifically, the control center air-fuel ratio AFR is calculated by adding the learning value sfbg to the basic control center air-fuel ratio AFRbase, as shown in the above-described equation (4).

  The target air-fuel ratio setting means A6 adds the air-fuel ratio correction amount AFC calculated by the air-fuel ratio correction amount calculation means A5 to the control center air-fuel ratio AFR calculated by the control center air-fuel ratio calculation means A10, so that the target air-fuel ratio setting means A6 is added. The fuel ratio AFT is calculated.

<Learning control flowchart>
FIG. 18 is a flowchart illustrating a control routine for learning control. The illustrated control routine is performed by interruption at regular time intervals.

  As shown in FIG. 18, first, in step S61, it is determined whether or not an update condition for the learning value sfbg is satisfied. The case where the update condition is satisfied includes, for example, that normal control is being performed. If it is determined in step S61 that the update condition for the learning value sfbg is satisfied, the process proceeds to step S62. In step S62, it is determined whether the lean setting flag Fl is set to ON. The lean setting flag Fl is a flag set in the control routine of the air-fuel ratio correction amount setting control shown in FIG. If it is determined in step S62 that the lean setting flag Fl is set to ON, that is, if the air-fuel ratio correction amount AFC is set to the lean setting correction amount AFClean, the process proceeds to step S63.

  In step S63, it is determined whether it is the switching timing of the air-fuel ratio correction amount AFC. Specifically, it is determined whether or not the air-fuel ratio correction amount AFC has been switched from the rich set correction amount AFCrich to the lean set correction amount AFClean between the end of the present control routine and the start of this control routine. If it is determined in step S63 that it is not the timing for switching the air-fuel ratio correction amount AFC, the process proceeds to step S64. In step S64, the current oxygen excess / deficiency amount OED is added to the integrated oxygen excess / deficiency amount ΣOED.

  Thereafter, when the target air-fuel ratio is switched to the rich air-fuel ratio, in the next control routine, it is determined in step S62 that the lean setting flag Fl is set to OFF, and the process proceeds to step S65. In step S65, it is determined whether it is the switching timing of the air-fuel ratio correction amount AFC. Specifically, it is determined whether or not the air-fuel ratio correction amount AFC has been switched from the lean set correction amount AFClean to the rich set correction amount AFCrich between the end of the present control routine and the start of this control routine. If it is determined in step S65 that it is the switching timing of the air-fuel ratio correction amount AFC, the process proceeds to step S66. In step S66, the cumulative oxygen excess amount Rn is made the absolute value of the current cumulative oxygen excess / deficiency ΣOED. Next, in step S67, the cumulative oxygen excess / deficiency ΣOED is reset to 0, and the control routine is ended.

  In the next control routine, it is usually determined that the lean setting flag Fl is set to OFF in step S62, and it is determined in step S65 that it is not the switching timing of the air-fuel ratio correction amount AFC. In this case, the present control routine proceeds to step S68. In step S68, the current oxygen excess / deficiency amount OED is added to the integrated oxygen excess / deficiency amount ΣOED.

  Thereafter, when the target air-fuel ratio is switched to the lean air-fuel ratio, in the next control routine, it is determined in step S62 that the lean setting flag Fl is set to ON, and in step S63, at the switching timing of the air-fuel ratio correction amount AFC. It is determined that there was. In this case, the present control routine proceeds to step S69. In step S69, it is determined whether an abnormality determination has been made on the downstream air-fuel ratio sensor 41 while the target air-fuel ratio has been set to the rich air-fuel ratio. If it is determined in step S69 that the downstream air-fuel ratio sensor 41 has not been determined to be abnormal, the process proceeds to step S70.

  In step S70, the cumulative oxygen deficiency Fn is set to the absolute value of the current cumulative oxygen excess / deficiency ΣOED. Next, in step S71, the cumulative oxygen excess / deficiency ΣOED is reset to zero. Next, at step S72, the learning value sfbg is updated based on the cumulative oxygen excess amount Rn calculated at step S66 and the cumulative oxygen shortage amount Fn calculated at step S70, and the control routine is terminated. The learning value sfbg updated in this way is used to correct the control center air-fuel ratio AFR by the above equation (4).

  On the other hand, if it is determined in step S69 that the downstream air-fuel ratio sensor 41 has been determined to be abnormal, the process proceeds to step S73. In step S73, the integrated oxygen excess accumulated in the period from when the air-fuel ratio correction amount AFC is switched to the rich set correction amount AFCrich until the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes equal to or greater than the lean determination air-fuel ratio AFlean. The shortage amount ΣOED is calculated as the cumulative oxygen shortage amount Fn. Next, in step S71, the cumulative oxygen excess / deficiency ΣOED is reset to zero. Next, at step S72, the learning value sfbg is updated based on the cumulative oxygen excess amount Rn calculated at step S66 and the cumulative oxygen shortage amount Fn calculated at step S73, and the control routine is terminated. The learning value sfbg updated in this way is used to correct the control center air-fuel ratio AFR by the above equation (4). In order to prevent the learning value from being updated erroneously, if it is determined in step S69 that the downstream air-fuel ratio sensor 41 has been determined to be abnormal, the learning value sfbg must not be updated in step S72. Also good.

  In the first embodiment and the second embodiment, when it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, the absolute value of the rich setting correction amount AFCrich and the lean setting correction amount AFClean. The absolute value of is reduced. As a result, the rich degree of the rich set air-fuel ratio obtained by adding the rich set correction amount AFCrich to the control center air-fuel ratio is lowered. In addition, the lean degree of the lean set air-fuel ratio obtained by adding the lean set correction amount AFClean to the control center air-fuel ratio is reduced.

  However, when it is determined that an element crack abnormality has occurred in the downstream air-fuel ratio sensor 41, the absolute value of the rich setting correction amount AFCrich and the absolute value of the lean setting correction amount AFClean are not the rich setting air-fuel ratio and lean. The set air-fuel ratio may be corrected directly. In this case, when it is determined that an element crack abnormality has occurred, the rich degree of the rich set air-fuel ratio and the lean degree of the lean set air-fuel ratio are lowered.

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 (6)

An exhaust purification catalyst provided in the exhaust passage of the internal combustion engine, a downstream air-fuel ratio sensor provided in the exhaust passage downstream of the exhaust purification catalyst in the exhaust flow direction, and an exhaust gas flowing into the exhaust purification catalyst An air-fuel ratio control that controls the air-fuel ratio, and a control device that performs abnormality diagnosis control that performs abnormality diagnosis of the downstream air-fuel ratio sensor based on the output air-fuel ratio of the downstream air-fuel ratio sensor,
In the air-fuel ratio control, the control device alternately changes the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst into a rich air-fuel ratio richer than the stoichiometric air-fuel ratio and a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio. Switching repeatedly,
In the abnormality diagnosis control, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made rich by the air-fuel ratio control in the abnormality diagnosis control, the control device determines the output air-fuel ratio of the downstream air-fuel ratio sensor. When the air-fuel ratio richer than the predetermined lean determination air-fuel ratio leaner than the stoichiometric air-fuel ratio changes to an air-fuel ratio leaner than the lean determination air-fuel ratio, it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor An exhaust purification device for an internal combustion engine. The control device performs feedback control of the fuel supply amount supplied to the combustion chamber of the internal combustion engine so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst in the air-fuel ratio control becomes a target air-fuel ratio, and the downstream side Further performing learning control for correcting a parameter relating to the air-fuel ratio based on the output air-fuel ratio of the side air-fuel ratio sensor,
In the air-fuel ratio control, the control device alternately switches the target air-fuel ratio between a rich air-fuel ratio and a lean air-fuel ratio, and the output air-fuel ratio of the downstream air-fuel ratio sensor is a predetermined rich air richer than the stoichiometric air-fuel ratio. When the target air-fuel ratio becomes lower than the determination air-fuel ratio, the target air-fuel ratio is switched from the rich air-fuel ratio to the lean air-fuel ratio,
In the learning control, the control device determines the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst during the oxygen increase period from when the target air-fuel ratio is switched to the lean air-fuel ratio to when it is switched to the rich air-fuel ratio again. The exhaust oxygen purification in an integrated oxygen excess amount that is an integrated value of the amount of oxygen that becomes excessive when trying to achieve, and an oxygen reduction period from when the target air-fuel ratio is switched to a rich air-fuel ratio until it is switched to a lean air-fuel ratio again Based on the integrated oxygen deficiency, which is the integrated value of the amount of oxygen that is deficient when trying to make the air-fuel ratio of the exhaust gas flowing into the catalyst the stoichiometric air-fuel ratio, Correct the air-fuel ratio parameter so that the difference is small,
When it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor when the target air-fuel ratio is set to a rich air-fuel ratio, the control device thereafter outputs an output air from the downstream air-fuel ratio sensor. 2. The correction of the parameter relating to the air-fuel ratio based on the accumulated oxygen deficiency at this time is stopped even when the target air-fuel ratio is switched to the lean air-fuel ratio because the fuel ratio is equal to or lower than the rich determination air-fuel ratio. An exhaust purification device for an internal combustion engine.   When the target air-fuel ratio is set to a rich air-fuel ratio, the control device is configured such that the output air-fuel ratio of the downstream air-fuel ratio sensor is richer than the lean determination air-fuel ratio to the lean determination air-fuel ratio. If it is determined that an abnormality has occurred in the downstream air-fuel ratio sensor due to a change to a lean air-fuel ratio, the target air-fuel ratio is switched to the rich air-fuel ratio last time, and the output air-fuel ratio sensor output An integrated oxygen deficiency amount is calculated during a period from when the fuel ratio changes from an air fuel ratio richer than the lean determination air fuel ratio to a lean air fuel ratio, and the difference between the integrated oxygen deficiency amount and the integrated oxygen excess amount is small. The exhaust emission control device for an internal combustion engine according to claim 2, wherein the parameter relating to the air-fuel ratio is corrected so as to be. In the air-fuel ratio control, the control device alternately switches the target air-fuel ratio between a constant rich set air-fuel ratio richer than the stoichiometric air-fuel ratio and a constant lean set air-fuel ratio leaner than the stoichiometric air-fuel ratio,
4. The internal combustion engine according to claim 2, wherein the control device reduces the rich degree of the rich set air-fuel ratio when it is determined by the abnormality diagnosis control that an abnormality has occurred in the downstream air-fuel ratio sensor. 5. Engine exhaust purification system.   In the air-fuel ratio control, the control device sets the target air-fuel ratio as one cycle for each of the rich air-fuel ratio and the lean air-fuel ratio as one cycle, and the downstream air-fuel ratio sensor has an abnormality with respect to the number of cycles. The exhaust emission control device for an internal combustion engine according to claim 4, wherein the rich degree of the rich set air-fuel ratio is reduced when the ratio of the number of times determined to occur is equal to or greater than a predetermined ratio.   In the air-fuel ratio control, the control device detects the exhaust gas flowing into the exhaust purification catalyst when the output air-fuel ratio of the downstream air-fuel ratio sensor becomes equal to or lower than a predetermined rich determination air-fuel ratio richer than the stoichiometric air-fuel ratio. The air-fuel ratio of the exhaust gas is switched from the rich air-fuel ratio to the lean air-fuel ratio, and flows into the exhaust purification catalyst when the oxygen storage amount of the exhaust purification catalyst exceeds a predetermined switching reference oxygen amount that is smaller than the maximum storable oxygen amount. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 5, wherein the air-fuel ratio of exhaust gas to be switched is switched from a lean air-fuel ratio to a rich air-fuel ratio.

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