JP2018003777A - Control device of internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress degradation of exhaust emission.SOLUTION: A control device 200 of an internal combustion engine 100 includes a target air-fuel ratio setting portion including a first setting control portion executing normal control to alternately switch a target air-fuel ratio to a prescribed first lean air-fuel ratio and a prescribed first rich air-fuel ratio, and a second setting control portion executing storage amount reduction control to reduce an oxygen storage amount of a second catalyst 35 while stopping the normal control, when an output air- fuel ratio of a third air-fuel ratio sensor becomes a prescribed lean determination air-fuel ratio or more. The second setting control portion is constituted to set the target air-fuel ratio to a prescribed second rich air-fuel ratio smaller than a first rich air-fuel ratio in starting the storage amount reduction control, and to set the target air-fuel ratio to a prescribed third rich air-fuel ratio larger than the first rich air-fuel ratio, after an exhaust gas having an air-fuel ratio larger than a theoretical air-fuel ratio flows out from a first catalyst 34 in a period when the target air-fuel ratio is set to the second rich air-fuel ratio.SELECTED DRAWING: Figure 1

Description

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

  Patent Document 1 discloses a first catalyst having an oxygen storage capability disposed in an exhaust passage of an engine body, a second catalyst having an oxygen storage capability disposed in an exhaust passage downstream of the first catalyst, A first exhaust sensor (air-fuel ratio sensor) disposed upstream of one catalyst, a second exhaust sensor (oxygen sensor) disposed between the first catalyst and the second catalyst, and a downstream side of the second catalyst An internal combustion engine provided with a third exhaust sensor (oxygen sensor) arranged in the above is disclosed.

  Further, in Patent Document 1, the control device for an internal combustion engine controls the engine body so that the air-fuel ratio of the exhaust detected by the first exhaust sensor becomes the target air-fuel ratio, and the detected value of the second exhaust sensor is used. Based on this, when it is determined that the lean air-fuel ratio exhaust gas is flowing out from the first catalyst, the target air-fuel ratio is switched to the rich air-fuel ratio, and conversely, when it is determined that the rich air-fuel ratio exhaust gas is flowing out, It is disclosed to perform control to switch the fuel ratio to a lean air-fuel ratio. Then, when it is determined that the lean air-fuel ratio exhaust gas is flowing out from the second catalyst based on the detection value of the third exhaust sensor, it is determined that the oxygen storage amount of the second catalyst is near the maximum storage amount, In order to reduce the oxygen storage amount of the second catalyst, it is disclosed that the target air-fuel ratio is corrected so that the target air-fuel ratio becomes smaller than normal.

JP 2005-299430 A

  As described above, the control device for the internal combustion engine described in Patent Document 1 switches the target air-fuel ratio to the lean air-fuel ratio or the rich air-fuel ratio based on the detection value of the second exhaust sensor. Therefore, even if the target air-fuel ratio is corrected based on the detection value of the third exhaust sensor to be smaller than normal, the rich air-fuel ratio exhaust gas flows out from the first catalyst based on the detection value of the second exhaust sensor. If it is determined that the target air-fuel ratio has been established, the target air-fuel ratio is switched to the lean air-fuel ratio. As a result, the target air-fuel ratio is switched to the lean air-fuel ratio before the oxygen storage amount of the second catalyst is sufficiently reduced. Therefore, when the lean air-fuel ratio exhaust gas flows out from the first catalyst, NOx contained in can not be reduced and purified by the second catalyst, and exhaust emission may be deteriorated.

  The present invention has been made paying attention to such problems, and an object of the present invention is to suppress the deterioration of exhaust emission that occurs when the oxygen storage amount of the second catalyst increases to the maximum storage amount.

  In order to solve the above-described problem, according to an aspect of the present invention, an engine body, a first catalyst having an oxygen storage capacity disposed in an exhaust passage of the engine body, and an exhaust flow direction downstream side of the first catalyst A second catalyst having an oxygen storage capacity disposed in the exhaust passage, and an exhaust passage disposed upstream of the first catalyst in the exhaust flow direction and for detecting an air-fuel ratio of the exhaust flowing into the first catalyst From the first air-fuel ratio sensor, the second air-fuel ratio sensor that is disposed in the exhaust passage between the first catalyst and the second catalyst and detects the air-fuel ratio of the exhaust gas flowing out from the first catalyst, and the second catalyst And a third air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas flowing out from the second catalyst, the control device for the internal combustion engine for controlling the internal combustion engine is disposed in the exhaust passage downstream of the exhaust flow direction. The output air-fuel ratio of the first air-fuel ratio sensor is the target air-fuel ratio So as to includes a air-fuel ratio control unit for controlling the air-fuel ratio of the exhaust gas discharged from the engine body, and the target air-fuel ratio setting unit for setting a target air-fuel ratio, a. The target air-fuel ratio setting unit performs normal control for alternately switching the target air-fuel ratio between a predetermined first lean air-fuel ratio larger than the theoretical air-fuel ratio and a predetermined first rich air-fuel ratio smaller than the theoretical air-fuel ratio. The normal control is stopped when the output air-fuel ratio of the first setting control unit and the third air-fuel ratio sensor becomes equal to or greater than a predetermined lean determination air-fuel ratio that is larger than the theoretical air-fuel ratio and smaller than the first lean air-fuel ratio. And a second setting control unit that performs storage amount reduction control for reducing the oxygen storage amount of the second catalyst. The second setting control unit sets the target air-fuel ratio to a predetermined second rich air-fuel ratio smaller than the first rich air-fuel ratio at the start of the storage amount reduction control, and sets the target air-fuel ratio to the second rich air-fuel ratio. The target air-fuel ratio is set to a predetermined third rich air-fuel ratio that is larger than the first rich air-fuel ratio after the exhaust gas having an air-fuel ratio smaller than the stoichiometric air-fuel ratio flows out from the first catalyst during the period of time. The

  According to this aspect of the present invention, the oxygen storage amount of the second catalyst can be appropriately reduced, so that the deterioration of exhaust emission that occurs when the oxygen storage amount of the second catalyst increases to the maximum storage amount is suppressed. be able to.

FIG. 1 is a schematic configuration diagram of an internal combustion engine and an electronic control unit that controls the internal combustion engine according to the first embodiment of the present invention. FIG. 2 is a graph showing the relationship between the oxygen storage amount of the exhaust purification catalyst and the NOx concentration or HC, CO concentration in the exhaust gas flowing out from the exhaust purification catalyst. FIG. 3 is a schematic cross-sectional view of the sensor element portion of each air-fuel ratio sensor. FIG. 4 is a diagram showing the relationship between the sensor applied voltage and the output current at each exhaust air-fuel ratio. FIG. 5 is a diagram showing the relationship between the exhaust air-fuel ratio and the output current when the sensor applied voltage is made constant. FIG. 6 is a time chart for explaining the operation of lean failure control. FIG. 7 is a time chart for explaining the operation of the occlusion amount reduction control according to the first embodiment of the present invention. FIG. 8 is a flowchart illustrating target air-fuel ratio setting control according to the first embodiment of the present invention. FIG. 9 is a flowchart for explaining detailed processing contents of lean failure control as normal control. FIG. 10 is a flowchart for explaining detailed processing contents of the occlusion amount reduction control according to the first embodiment of the present invention. FIG. 11 is a time chart for explaining the operation of the occlusion amount reduction control according to the second embodiment of the present invention. FIG. 12 is a flowchart for explaining detailed processing contents of the occlusion amount reduction control according to the second embodiment of the present invention.

  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.

(First embodiment)
<Description of the internal combustion engine as a whole>
First, an internal combustion engine 100 and an electronic control unit 200 for controlling the internal combustion engine 100 according to a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a schematic configuration diagram of an internal combustion engine 100 and an electronic control unit 200 that controls the internal combustion engine 100 according to an embodiment of the present invention.

  As shown in FIG. 1, the internal combustion engine 100 includes an engine body 1, an intake device 20, and an exhaust device 30.

  The engine body 1 includes a cylinder block 2 and a cylinder head 3 fixed to the upper surface of the cylinder block 2.

  A plurality of cylinders 4 are formed in the cylinder block 2. A piston 5 that reciprocates inside the cylinder 4 in response to combustion pressure is housed inside the cylinder 4. The piston 5 is connected to a crankshaft via a connecting rod, and the reciprocating motion of the piston 5 is converted into rotational motion by the crankshaft. A space defined by the inner wall surface of the cylinder head 3, the inner wall surface of the cylinder 4, and the piston crown surface is a combustion chamber 6.

  The cylinder head 3 includes an intake port 7 that opens to one side surface of the cylinder head 3 and opens to the combustion chamber 6, an exhaust port 8 that opens to the other side surface of the cylinder head 3 and opens to the combustion chamber 6, Is formed.

  The cylinder head 3 includes an intake valve 9 for opening and closing the opening between the combustion chamber 6 and the intake port 7, an exhaust valve 10 for opening and closing the opening between the combustion chamber 6 and the exhaust port 8, and an intake valve 9. An intake camshaft 11 that opens and closes and an exhaust camshaft 12 that opens and closes the exhaust valve 10 are attached.

  Further, the cylinder head 3 has a fuel injection valve 13 for injecting fuel into the combustion chamber 6, and an ignition for igniting the mixture of fuel and air injected from the fuel injection valve 13 in the combustion chamber 6. A plug 14 is attached. In this embodiment, gasoline having a theoretical air-fuel ratio of 14.6 is used as the fuel, but other fuels can also be used. The fuel injection valve 13 may be attached so as to inject fuel into the intake port 7.

  The intake device 20 is a device for introducing air into the cylinder 4 through the intake port 7, and includes an air cleaner 21, an intake pipe 22, an intake manifold 23, an electronically controlled throttle valve 24, and an air flow meter. 211.

  The air cleaner 21 removes foreign matters such as sand contained in the air.

  One end of the intake pipe 22 is connected to the air cleaner 21, and the other end is connected to a surge tank 23 a of the intake manifold 23. The intake pipe 22 guides air (intake air) flowing into the intake pipe 22 via the air cleaner 21 to the surge tank 23 a of the intake manifold 23.

  The intake manifold 23 includes a surge tank 23a and a plurality of intake branch pipes 23b branched from the surge tank 23a and connected to the openings of the intake ports 7 formed on the side surface of the cylinder head. The air guided to the surge tank 23a is evenly distributed in each cylinder 4 through the intake branch pipe 23b. Thus, the intake pipe 22, the intake manifold 23 and the intake port 7 form an intake passage for guiding air into each cylinder 4.

  The throttle valve 24 is provided in the intake pipe 22. The throttle valve 24 is driven by a throttle actuator 25 to change the passage cross-sectional area of the intake pipe 22 continuously or stepwise. By adjusting the opening of the throttle valve 24 (hereinafter referred to as “throttle opening”) by the throttle actuator 25, the amount of intake air taken into each cylinder 4 is adjusted. The throttle opening is detected by the throttle sensor 212.

  The air flow meter 211 is provided in the intake pipe 22 on the upstream side of the throttle valve 24. The air flow meter 211 detects the flow rate of air flowing through the intake pipe 22 (hereinafter referred to as “intake amount”).

  The exhaust device 30 is a device for purifying the combustion gas (exhaust gas) generated in the combustion chamber 6 and discharging it to the outside air. The exhaust device 30 includes an exhaust manifold 31, an exhaust pipe 32, an exhaust aftertreatment device 33, A first air-fuel ratio sensor 213, a second air-fuel ratio sensor 214, and a third air-fuel ratio sensor 215.

  The exhaust manifold 31 includes a plurality of exhaust branch pipes 31a connected to the openings of the exhaust ports 8 formed on the side surface of the cylinder head, and a collection pipe 31b that collects the exhaust branch pipes 31a into one. Prepare.

  The exhaust pipe 32 has one end connected to the collecting pipe 31b of the exhaust manifold 31 and the other end opened to the outside air. Exhaust gas discharged from each cylinder 4 through the exhaust port 8 to the exhaust manifold 31 flows through the exhaust pipe 32 and is discharged to the outside air.

  The exhaust aftertreatment device 33 includes a first catalytic converter 33a and a second catalytic converter 33b each incorporating an exhaust purification catalyst. Each catalytic converter is connected to the exhaust pipe 32 in the order of the first catalytic converter 33a and the second catalytic converter 33b from the upstream side in the exhaust flow direction. Thus, the exhaust port 8, the exhaust manifold 31, the exhaust pipe 32, the first catalytic converter 33a, and the second catalytic converter 33b form an exhaust passage through which the exhaust discharged from each cylinder 4 flows.

  Each of the first catalytic converter 33a and the second catalytic converter 33b incorporates a three-way catalyst having an oxygen storage capacity as an exhaust purification catalyst. In the following description, when it is necessary to particularly distinguish the three-way catalyst built in each of the first catalytic converter 33a and the second catalytic converter 33b, the three-way built in the first catalytic converter 33a. The catalyst is referred to as a “first three-way catalyst 34”, and the three-way catalyst built in the second catalytic converter 33b is referred to as a “second three-way catalyst 35”.

The three-way catalyst is obtained by supporting 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. When the three-way catalyst reaches a predetermined activation temperature, the three-way catalyst exhibits an oxygen storage capacity in addition to a catalytic action for simultaneously purifying unburned gas (HC, CO, etc.) and nitrogen oxide (NOx). In this embodiment, the term occlusion is used as a term including both absorption and adsorption.

  A three-way catalyst having oxygen storage capacity is a lean air-fuel ratio when the air-fuel ratio of exhaust flowing into the three-way catalyst is larger than the stoichiometric air-fuel ratio, that is, when the air-fuel ratio of exhaust flowing into the three-way catalyst is leaner than the stoichiometric air-fuel ratio. When it is at the fuel ratio, oxygen in the exhaust gas is occluded. On the other hand, when the air-fuel ratio of the exhaust flowing into the three-way catalyst is smaller than the stoichiometric air-fuel ratio, that is, when the air-fuel ratio of the exhaust flowing into the three-way catalyst is a rich air-fuel ratio richer than the stoichiometric air-fuel ratio, the three-way Releases oxygen stored in the catalyst. Further, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is a rich air-fuel ratio, ammonia is generated by the reaction of nitrogen and hydrogen or HC and NOx in the exhaust gas in the three-way catalyst.

  The three-way catalyst has a catalytic action and an oxygen storage capacity, and thus has a purification action of NOx and unburned gas according to the oxygen storage amount. That is, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is a lean air-fuel ratio, as shown in FIG. 2A, when the oxygen storage amount is small, oxygen in the exhaust gas is stored by the three-way catalyst. Along with this, NOx in the exhaust is reduced and purified. Further, when the oxygen storage amount increases, the concentration of oxygen and NOx in the exhaust gas flowing out from the three-way catalyst rapidly increases with a certain storage amount (Cuplim in the figure) in the vicinity of the maximum storage amount Cmax.

  On the other hand, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is a rich air-fuel ratio, as shown in FIG. 2B, when the oxygen storage amount is large, oxygen stored in the three-way catalyst is released, Unburned gas in the exhaust gas is oxidized and purified. Further, when the oxygen storage amount decreases, the concentration of unburned gas in the exhaust gas flowing out from the three-way catalyst suddenly increases at a certain storage amount near the zero (Crowlim in the figure).

  As described above, according to the three-way catalyst used in the present embodiment, the purification characteristics of NOx and unburned gas in the exhaust gas change according to the air-fuel ratio and oxygen storage amount of the exhaust gas flowing into the three-way catalyst. Note that the exhaust purification catalyst may be a catalyst different from the three-way catalyst as long as it has catalytic action and oxygen storage capacity.

  The first air-fuel ratio sensor 213 is provided in the collecting pipe 31b of the exhaust manifold 31 and detects the air-fuel ratio of the exhaust gas flowing into the first catalytic converter 33a.

  The second air-fuel ratio sensor 214 is provided in the exhaust pipe 32 between the first catalytic converter 33a and the second catalytic converter 33b, and the exhaust air flowing out from the first catalytic converter 33a and flowing into the second catalytic converter 33b. Detect the fuel ratio.

  The third air-fuel ratio sensor 215 is provided in the exhaust pipe 32 downstream of the second catalytic converter 33b in the exhaust flow direction, and detects the air-fuel ratio of the exhaust gas flowing out from the second catalytic converter 33b. In the present embodiment, air-fuel ratio sensors having the same configuration are used as the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215.

  FIG. 3 is a schematic cross-sectional view of the sensor element unit 50 of the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215.

  As shown in FIG. 3, the sensor element unit 50 is disposed on the solid electrolyte layer 51, the exhaust-side electrode 52 disposed on one side surface of the solid electrolyte layer 51, and the other side surface of the solid electrolyte layer 51. The air-side electrode 53, a diffusion-controlling layer 54 that controls the diffusion of exhaust gas that passes through, a protective layer 55 that protects the diffusion-controlling layer 54, and a heater unit 56 that heats the sensor element unit 50.

  A diffusion rate controlling layer 54 is provided on one side surface of the solid electrolyte layer 51, and a protective layer 55 is provided on the side surface of the diffusion rate controlling layer 54 opposite to the side surface on the solid electrolyte layer 51 side. In the present embodiment, a measured gas chamber 57 is formed between the solid electrolyte layer 51 and the diffusion-controlling layer 54. Gas to be measured by the air-fuel ratio sensors 213, 214, and 215, that is, exhaust gas is introduced into the measured gas chamber 57 through the diffusion rate controlling layer 54. Further, the exhaust side electrode 52 is disposed in the measured gas chamber 57, and therefore, the exhaust side electrode 52 is exposed to the exhaust through the diffusion rate controlling layer 54. The gas chamber 57 to be measured is not necessarily provided, and may be configured such that the diffusion-controlling layer 54 is in direct contact with the surface of the exhaust-side electrode 52.

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

  A plurality of heaters 59 are provided in the heater unit 56, and the temperatures of the air-fuel ratio sensors 213, 214, and 215, particularly the temperature of the solid electrolyte layer 51 can be controlled by these heaters 59. The heater unit 56 has a heat generation capacity sufficient to heat the solid electrolyte layer 51 until it is activated.

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

  Further, a sensor application voltage Vr is applied between the exhaust side electrode 52 and the atmosphere side electrode 53 by the voltage application device 60 mounted on the electronic control unit 200. In addition, the electronic control unit 200 is provided with a current detection device 61 that detects a current flowing between the electrodes 52 and 53 via the solid electrolyte layer 51 when the sensor application voltage Vr is applied by the voltage application device 60. . The current detected by the current detector 61 is the output current of each air-fuel ratio sensor 213, 214, 215.

  Next, output characteristics of the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 in the present embodiment will be described with reference to FIGS. FIG. 4 is a diagram showing voltage-current (V-I) characteristics of the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 in the present embodiment, and FIG. FIG. 6 is a diagram showing the relationship between the output current I and the air-fuel ratio of exhaust flowing around the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 when the voltage is kept constant. is there.

As can be seen from FIG. 4, in the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 of this embodiment, the output current I increases as the air-fuel ratio of the exhaust gas becomes higher (lean as it becomes leaner). ),growing. The V-I line at each exhaust air-fuel ratio has a region substantially parallel to the V axis, that is, a region where the output current hardly changes even when the sensor applied voltage changes. This voltage region is referred to as a limiting current region, and the current at this time is referred to as a limiting current. In FIG. 4, the limit current region and the limit current when the air-fuel ratio of the exhaust gas is 18 are indicated by W ₁₈ and I ₁₈ , respectively. Accordingly, it can be said that the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 are limit current type air-fuel ratio sensors.

  FIG. 5 is a diagram showing the relationship between the air-fuel ratio of the exhaust and the output current I when the applied voltage is kept constant at about 0.45V. As can be seen from FIG. 5, in the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215, the first air-fuel ratio sensor becomes higher as the air-fuel ratio of the exhaust gas becomes higher (that is, the leaner it becomes). The output current changes linearly (proportional to) the exhaust air / fuel ratio so that the output current I from the second air / fuel ratio sensor 214 and the third air / fuel ratio sensor 214 increases. In addition, the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 are configured so that the output current I becomes zero when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. . Further, when the air-fuel ratio of the exhaust gas becomes larger than a certain value or when it becomes smaller than a certain value, the ratio of the change in the output current to the change in the air-fuel ratio of the exhaust gas becomes smaller.

  In the above example, limit current type air-fuel ratio sensors are used as the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215. However, if the output current changes linearly with respect to the air-fuel ratio of the exhaust, the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 are, for example, cup-type limit currents. Any air-fuel ratio sensor such as a limit current type air-fuel ratio sensor of another structure such as an air-fuel ratio sensor or an air-fuel ratio sensor that is not a limit current type may be used. The first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, and the third air-fuel ratio sensor 215 may be air-fuel ratio sensors having different structures.

  Returning to FIG. 1, the electronic control unit 200 is composed of a digital computer, and is connected to each other by a bi-directional bus 201. A port 205 and an output port 206 are provided.

  Output signals from the air flow meter 211, the throttle sensor 212, the first air-fuel ratio sensor 213, the second air-fuel ratio sensor 214, the third air-fuel ratio sensor 215, and the like are input to the corresponding AD converters 207 at the input port 205. Is input via. Further, the output voltage of the load sensor 217 that generates an output voltage proportional to the amount of depression of the accelerator pedal 220 (hereinafter referred to as “accelerator depression amount”) is input to the input port 205 via the corresponding AD converter 207. The In addition, an output signal of a crank angle sensor 218 that generates an output pulse every time the crankshaft of the engine body 1 rotates, for example, 15 ° is input to the input port 205 as a signal for calculating the engine rotation speed and the like. As described above, output signals of various sensors necessary for controlling the internal combustion engine 100 are input to the input port 205.

  Control components such as the fuel injection valve 13, the spark plug 14, and the throttle actuator 25 are electrically connected to the output port 206 via a corresponding drive circuit 208.

The electronic control unit 200 controls the internal combustion engine 100 by outputting a control signal for controlling each control component from the output port 206 based on the output signals of various sensors input to the input port 205. Hereinafter, the air-fuel ratio control of the internal combustion engine 100 performed by the electronic control unit 200 will be described.
<Outline of air-fuel ratio control>
The electronic control unit 200 controls the air-fuel ratio of the exhaust discharged from the combustion chamber 6 of the engine body 1 so that the output air-fuel ratio of the first air-fuel ratio sensor 213 becomes the target air-fuel ratio. Specifically, the electronic control unit 200 injects fuel from the fuel injection valve 13 based on the output air-fuel ratio of the first air-fuel ratio sensor 213 so that the output air-fuel ratio of the first air-fuel ratio sensor 213 becomes the target air-fuel ratio. Feedback control the amount. The “output air-fuel ratio” means an air-fuel ratio corresponding to the output value of each air-fuel ratio sensor 213, 214, 215. The electronic control unit 200 performs a predetermined first lean air-fuel ratio AFL1 in which the target air-fuel ratio is larger than the theoretical air-fuel ratio and a predetermined smaller than the theoretical air-fuel ratio as target air-fuel ratio setting control for setting the target air-fuel ratio. Normal control is performed to switch alternately to the first rich air-fuel ratio AFR1.

<Lean failure control as normal control>
In the present embodiment, as normal control, the oxygen storage amount (hereinafter referred to as “first oxygen storage amount”) OSAsc of the first three-way catalyst 34 is periodically increased to the maximum storage amount Cmaxup of the first three-way catalyst 34. Then, lean failure control is performed to cause the first three-way catalyst 34 to lean.

  The electronic control unit 200 switches the target air-fuel ratio to the predetermined first rich air-fuel ratio AFR1 when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes the lean air-fuel ratio during the execution of the lean failure control, and then The first rich air-fuel ratio AFR1 is maintained. In the present embodiment, when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or higher than the lean determination air-fuel ratio AFref (for example, 14.65) that is slightly larger than the theoretical air-fuel ratio, the electronic control unit 200 It is determined that the output air-fuel ratio AFmid of the two air-fuel ratio sensor 214 has become a lean air-fuel ratio, and the target air-fuel ratio is switched to the first rich air-fuel ratio AFR1.

  The first rich air-fuel ratio AFR1 is a predetermined air-fuel ratio that is somewhat smaller than the theoretical air-fuel ratio, and is, for example, 12 to 14.58, preferably 13 to 14.57, more preferably 14 to 14.55. It is said to be about. The first rich air-fuel ratio AFR1 is expressed as an air-fuel ratio obtained by adding a rich correction amount (negative value) from the air-fuel ratio (hereinafter referred to as “control center air-fuel ratio”. In this embodiment, the theoretical air-fuel ratio) AFcen to be the control center. You can also

The electronic control unit 200 performs first oxygen storage amount estimation control for estimating the first oxygen storage amount OSAsc in parallel with the target air-fuel ratio setting control. Specifically, the electronic control unit 200 calculates the oxygen excess / deficiency OEDsc of the exhaust gas flowing into the first three-way catalyst 34 based on the output air-fuel ratio AFup of the first air-fuel ratio sensor 213 as needed. The first oxygen storage amount OSAsc is estimated by integrating the excess / deficiency amount OEDsc. The oxygen excess / deficiency amount OEDsc is the amount of excess oxygen or the amount of insufficient oxygen (excessive unburned gas, etc.) when the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34 is set to the stoichiometric air-fuel ratio. Amount). In the present embodiment, the electronic control unit 200, as shown in the following formula (1), outputs the air / fuel ratio AFup of the first air / fuel ratio sensor 213, the control center air / fuel ratio AFcen, and the fuel supply amount Qi ( Alternatively, the oxygen excess / deficiency amount OEDsc is calculated at any time based on the estimated value of the intake air amount into the combustion chamber 6 calculated based on the output of the air flow meter 211 or the like. In formula (1), 0.23 represents the oxygen concentration in the air.
OEDsc = 0.23 × Qi × (AFup-AFcen) (1)

  When the estimated value of the first oxygen storage amount OSAsc becomes equal to or less than a predetermined first switching reference amount Crefup1 during the period when the target air-fuel ratio is set to the first rich air-fuel ratio AFR1, the electronic control unit 200 The fuel ratio is switched from the first rich air-fuel ratio AFR1 to a predetermined first lean air-fuel ratio AFL1 that is larger than the lean determination air-fuel ratio AFref1, and then maintained at the first lean air-fuel ratio AFL1.

  The first lean air-fuel ratio AFL1 is a predetermined air-fuel ratio that is somewhat larger than the stoichiometric air-fuel ratio, and is, for example, 14.65 to 20, preferably 14.65 to 18, and more preferably 14.65 to 16 It is said to be about. The first lean air-fuel ratio AFL1 can also be expressed as an air-fuel ratio obtained by adding a lean correction amount (positive value) to the control center air-fuel ratio (the stoichiometric air-fuel ratio in this embodiment) AFcen. In the present embodiment, the difference (lean degree) from the stoichiometric air-fuel ratio of the first lean air-fuel ratio AFL1 is made equal to or less than the difference (lean degree) from the stoichiometric air-fuel ratio of the first rich air-fuel ratio AFR1.

  When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or higher than the lean determination air-fuel ratio AFref during the period when the target air-fuel ratio is set to the first lean air-fuel ratio AFL1, the electronic control unit 200 sets the target air-fuel ratio again. The operation is switched to the first rich air-fuel ratio AFR1, and then the same operation is repeated.

  However, even when lean failure control is performed, the estimated value of the first oxygen storage amount OSAsc is less than or equal to the first switching reference amount Crefup1 during the period in which the target air-fuel ratio is set to the first rich air-fuel ratio AFR1. Before the actual value, the actual value of the first oxygen storage amount OSAsc may decrease to near zero. As the cause, for example, the oxygen storage capacity of the first three-way catalyst 34 is reduced due to deterioration over time, and the maximum storage amount of the second three-way catalyst 34 is reduced to less than the first switching reference amount Crefup1 or temporarily. In particular, the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34 is instantaneously greatly shifted from the target air-fuel ratio to the rich side. Thus, when the actual value of the first oxygen storage amount OSAsc decreases to near zero, the target air-fuel ratio is set to the first rich air-fuel ratio AFR1, so that the first three-way catalyst 34 is rich including unburned gas. Air-fuel ratio exhaust will flow out. Therefore, in the present embodiment, when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes a rich air-fuel ratio during lean failure control (during normal control), the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1. ing.

  In this embodiment, when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri (for example, 14.55) that is slightly smaller than the theoretical air-fuel ratio, the electronic control unit 200 It is determined that the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 has become a rich air-fuel ratio.

<Explanation of lean failure control using time chart>
With reference to FIG. 6, the operation of lean failure control will be described. FIG. 6 shows the air-fuel ratio correction amount AFC, the output air-fuel ratio AFup of the first air-fuel ratio sensor 213, and the oxygen storage amount (first oxygen storage amount) of the first three-way catalyst 34 when lean failure control is performed as normal control. ) OSAsc, output air-fuel ratio AFmid of the second air-fuel ratio sensor 214, oxygen storage amount of the second three-way catalyst 35 (hereinafter referred to as “second oxygen storage amount”) OSAufc, and output air-fuel ratio AFdwn of the third air-fuel ratio sensor It is the time chart which showed each.

  In FIG. 6, an air-fuel ratio correction amount AFC represents a correction amount related to the target air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34. When the air-fuel ratio correction amount AFC is 0, the target air-fuel ratio becomes the control center air-fuel ratio (the stoichiometric air-fuel ratio in this embodiment) AFcen. When the air-fuel ratio correction amount AFC is a positive value, the target air-fuel ratio becomes an air-fuel ratio larger than the control center air-fuel ratio AFcen. When the air-fuel ratio correction amount AFC is a negative value, the target air-fuel ratio becomes an air-fuel ratio smaller than the control center air-fuel ratio AFcen. The control center air-fuel ratio AFcen is 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 used as a reference when changing the target air-fuel ratio according to the air-fuel ratio correction amount AFC. means.

  In the time chart shown in FIG. 6, before the time t1, the air-fuel ratio correction amount AFC is set to a predetermined first lean correction amount AFCL1, and thereby the predetermined first lean air-fuel ratio AFL1 where the target air-fuel ratio is larger than the theoretical air-fuel ratio. Set to Therefore, before the time t1, the output air-fuel ratio AFup of the first air-fuel ratio sensor 213 is the first lean air-fuel ratio AFL1, and the lean air-fuel ratio exhaust gas flows into the first three-way catalyst 34. NOx contained in the exhaust gas flowing into the first three-way catalyst 34 is reduced and purified by the first three-way catalyst 34. Therefore, the first oxygen storage amount OSAsc gradually increases before time t1. Further, since the exhaust gas flowing out from the first three-way catalyst 34 does not contain oxygen due to reduction purification in the first three-way catalyst 34, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is substantially equal to the stoichiometric air-fuel ratio. Become.

  When the first oxygen storage amount OSAsc gradually increases and at time t1, the first oxygen storage amount OSAsc increases to near the maximum storage amount Cmaxup, a part of the oxygen in the exhaust gas flowing into the first three-way catalyst 34 is obtained. The first three-way catalyst 34 starts to flow without being occluded. As a result, the lean air-fuel ratio exhaust gas containing oxygen flows through the exhaust pipe 32 between the first three-way catalyst 34 and the second three-way catalyst 35 and flows into the second three-way catalyst 35.

  Thereby, after the time t1, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 gradually increases. As a result, at time t2, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 reaches the lean determination air-fuel ratio AFrefle. Further, after time t1, NOx contained in the exhaust gas flowing out from the first three-way catalyst 34 and flowing into the second three-way catalyst 35 is reduced and purified by the second three-way catalyst 35. 2. The oxygen storage amount OSAufc gradually increases. Since the exhaust gas flowing out from the second three-way catalyst 35 by the reduction purification in the second three-way catalyst 35 does not contain oxygen, the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes substantially the stoichiometric air-fuel ratio. .

  When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or higher than the lean determination air-fuel ratio AFref at time t2, the air-fuel ratio correction amount AFC is changed from the first lean correction amount AFCL1 to reduce the first oxygen storage amount OSAsc. The target air-fuel ratio is switched from the first lean air-fuel ratio AFR1 to the first rich air-fuel ratio AFR1.

  In the present embodiment, the target air-fuel ratio is switched after the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or higher than the lean determination air-fuel ratio AFrefle. This is because when the first three-way catalyst 34 is in a state where it can sufficiently store oxygen, that is, when the first oxygen storage amount OSAsc is sufficiently small, the first three-way catalyst 34 has a lean air-fuel ratio. This is because when the exhaust gas flows in, the air-fuel ratio of the exhaust gas flowing out from the first three-way catalyst 34 may deviate from the stoichiometric air-fuel ratio. Conversely, when the lean air-fuel ratio exhaust gas flows into the first three-way catalyst 34, the lean determination air-fuel ratio AFrefle is the first three-way catalyst 34 when the first three-way catalyst 34 is in a state where it can sufficiently store oxygen. The air-fuel ratio is such that the air-fuel ratio of the exhaust gas flowing out from the original catalyst 34 does not reach.

  When the target air-fuel ratio is switched to the first rich air-fuel ratio AFR1 at time t2, the output air-fuel ratio AFup of the first air-fuel ratio sensor 213 becomes the first rich air-fuel ratio AFR1. In practice, there is a delay between the change of the target air-fuel ratio and the change of the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34, but in the illustrated example, it is assumed to change simultaneously for convenience.

  When the output air-fuel ratio AFup of the first air-fuel ratio sensor 213 becomes the first rich air-fuel ratio AFR1 at time t2, and the air-fuel ratio of the exhaust flowing into the first three-way catalyst 34 changes from the lean air-fuel ratio to the rich air-fuel ratio, Thereafter, oxygen stored in the first three-way catalyst 34 is consumed in order to oxidize and purify unburned gas contained in the exhaust gas flowing into the first three-way catalyst 34. Therefore, the first oxygen storage amount OSAsc gradually decreases. As a result, the air-fuel ratio of the exhaust gas flowing out from the first three-way catalyst 34 changes to the stoichiometric air-fuel ratio, and the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 converges to the stoichiometric air-fuel ratio at time t3. .

  When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 converges to the stoichiometric air-fuel ratio at time t3, the second oxygen storage amount OSAufc is maintained constant without increasing thereafter.

  After time t2, the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34 is the first rich air-fuel ratio AFR1, but since the first three-way catalyst 34 stores sufficient oxygen, Unburned gas in the exhaust gas flowing into the first three-way catalyst 34 is oxidized and purified by the first three-way catalyst 34. For this reason, the amount of unburned gas discharged from the first three-way catalyst 34 becomes substantially zero.

  When the first oxygen storage amount OSAsc decreases to the first switching reference amount Crefup1 at time t4, the air-fuel ratio correction amount AFC is switched to the first lean correction amount AFCL1, and the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1.

  In the example shown in FIG. 6, the first oxygen storage amount OSAsc increases at the same time as the target air-fuel ratio is switched at time t4. However, the first oxygen storage amount OSAsc is actually changed after the target air-fuel ratio is switched. There is a delay before it increases. Further, during the operation of the internal combustion engine 100, it is conceivable that the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34 unintentionally deviates greatly to the rich side with respect to the target air-fuel ratio for some reason.

  On the other hand, the first switching reference amount Crefup1 is set to a value sufficiently higher than zero. For this reason, even when the above-described delay occurs or the actual air-fuel ratio of the exhaust gas deviates from the target air-fuel ratio unintentionally, the first oxygen storage amount OSAsc decreases to zero. do not do. In other words, the first switching reference amount Crefup1 is set to a high value so that the first oxygen storage amount OSAsc does not decrease to zero even when the above-described delay or unintended air-fuel ratio shift occurs. For example, the first switching reference amount Crefup1 is set to 1/4 or more, preferably 1/2 or more, more preferably 4/5 or more of the maximum storage amount Cmaxup of the first three-way catalyst 34.

  When the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1 at time t4 and the output air-fuel ratio AFup of the first air-fuel ratio sensor 213 becomes the first lean air-fuel ratio AFL1, the first three-way catalyst 34 has a lean air-fuel ratio. Exhaust flows in. Therefore, the first oxygen storage amount OSAsc gradually increases, and at time t5, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 begins to increase, and at the time t5, the second oxygen storage amount OSAufc decreases. Start to increase. Further, after the time t4, the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1, and the exhaust of the lean air-fuel ratio flows into the first three-way catalyst 34. Therefore, the first three-way catalyst 34 is also after the time t4. The amount of unburned gas discharged from is almost zero.

  Next, at time t6, similarly to time t2, when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or greater than the lean determination air-fuel ratio AFrefle, the air-fuel ratio correction amount AFC is switched to the first lean correction amount AFCL1. As a result, the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1, and thereafter the same operation is repeated.

  As can be seen from the above description, as long as lean failure control is performed, the amount of unburned gas discharged from the first three-way catalyst 34 can be constantly suppressed. The amount of unburned gas emissions can be made almost zero. In general, the three-way catalyst has a reduced oxygen storage capacity when its oxygen storage amount is kept constant. That is, in order to keep the oxygen storage capacity of the three-way catalyst high, it is necessary to vary the oxygen storage amount of the three-way catalyst. On the other hand, according to the present embodiment, as shown in FIG. 6, the first occlusion amount OSAsc and the second occlusion amount OSAufc are constantly fluctuating, and therefore the first three-way catalyst 34 and the second three-way catalyst. A decrease in the oxygen storage capacity of the catalyst 35 can be suppressed.

  In the present embodiment, the air-fuel ratio correction amount AFC is maintained at the first rich correction amount AFCR1 at times t2 to t4. However, in such a period, the air-fuel ratio correction amount AFC does not necessarily need to be kept constant, and may be set so as to fluctuate, for example, gradually increase. Alternatively, during the period from the time t2 to the time t4, the air-fuel ratio correction amount AFC may be temporarily set to a value larger than 0 (for example, the first lean correction amount AFCL1). That is, during the period from time t2 to t4, the target air-fuel ratio may be temporarily set to the lean air-fuel ratio (for example, the first lean air-fuel ratio AFL1).

  Similarly, in the present embodiment, the air-fuel ratio correction amount AFC is maintained at the first lean correction amount AFCL1 from time t4 to t6. However, in such a period, the air-fuel ratio correction amount AFC does not necessarily need to be kept constant, and may be set so as to fluctuate, for example, gradually decrease. Alternatively, during the period from time t4 to time t6, the air-fuel ratio correction amount AFC may be temporarily set to a value smaller than 0 (for example, the first rich correction amount AFCR1). That is, during the period from time t4 to t6, the target air-fuel ratio may be temporarily set to the rich air-fuel ratio (for example, the first rich air-fuel ratio AFR1).

  The setting of the air-fuel ratio correction amount AFC in this embodiment, that is, the setting of the target air-fuel ratio is performed by the electronic control unit 200. Therefore, during the lean failure control, the electronic control unit 200 determines that the estimated value of the first oxygen storage amount OSAsc is the first value when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is equal to or greater than the lean determination air-fuel ratio AFrefle. The target air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34 is continuously or intermittently changed to the rich air-fuel ratio until the switching reference amount Crefup1 or less, and the estimated value of the first oxygen storage amount OSAsc is changed to the switching reference amount. It can be said that the target air-fuel ratio is continuously or intermittently set to the lean air-fuel ratio until the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or higher than the lean determination air-fuel ratio AFref when it becomes Crefup1 or less.

  More simply, in the present embodiment, during the lean failure control, the electronic control unit 200 sets the target air-fuel ratio when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or higher than the lean determination air-fuel ratio AFrefle. In addition to switching to the rich air-fuel ratio, it can be said that the target air-fuel ratio is switched to the lean air-fuel ratio when the estimated value of the first oxygen storage amount OSAsc becomes equal to or less than the first switching reference amount Creupup1.

  In the present embodiment, the target air-fuel ratio is switched from the first rich air-fuel ratio AFR1 to the first lean air-fuel ratio AFL1 when the first oxygen storage amount OSAsc becomes equal to or less than the first switching reference value Crefup1. However, the timing at which the target air-fuel ratio is switched from the first rich air-fuel ratio AFR1 to the first lean air-fuel ratio AFL1 is, for example, the engine operation time after switching the target air-fuel ratio to the first rich air-fuel ratio AFR1, the integrated intake air amount, etc. Other parameters may be used as a reference. However, even in this case, it is necessary to switch the target air-fuel ratio from the first rich air-fuel ratio AFR1 to the first lean air-fuel ratio AFL1 until the first oxygen storage amount OSAsc is estimated to be zero. Become.

<Problems that arise during the implementation of lean failure control>
As described above, when the lean failure control is performed as the target air-fuel ratio setting control, after the target air-fuel ratio is switched to the first rich air-fuel ratio AFR1, the first oxygen storage amount OSAsc becomes equal to or less than the first switching reference amount Crefup1. The target air-fuel ratio is switched to the first lean air-fuel ratio AFL1. That is, before the lean air-fuel ratio exhaust gas flows out from the first three-way catalyst 34, the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1. Therefore, the amount of unburned gas discharged from the first three-way catalyst 34 can always be suppressed, and basically the amount of unburned gas discharged from the first three-way catalyst 34 can be made substantially zero. .

  On the other hand, after the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1, the lean air-fuel ratio exhaust starts to flow out from the first three-way catalyst 34, and the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes the lean determination air-fuel ratio. When it becomes AFref or more, the target air-fuel ratio is switched to the first rich air-fuel ratio AFR1. Therefore, in the period before and after switching the target air-fuel ratio from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1 (in the time chart of FIG. 6, for example, the period from time t1 to time t3), the first three-way catalyst 34 Lean air-fuel ratio exhaust will flow out of the air.

  NOx contained in the exhaust of the lean air-fuel ratio that flows out from the first three-way catalyst 34 during the period before and after switching the target air-fuel ratio from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1 is the second three-way catalyst. If there is an allowance in the oxygen storage amount of 35, it is reduced and purified by the second three-way catalyst 35.

  Therefore, when the lean failure control is performed as the target air-fuel ratio switching control, every time the target air-fuel ratio is switched from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1, the amount is reduced from the first three-way catalyst 34. However, NOx temporarily flows out, and oxygen in the exhaust gas is stored in the second three-way catalyst 35 to reduce and purify this NOx, so that the second oxygen storage amount OSAufc increases.

  Therefore, every time the target air-fuel ratio is switched from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1, the second oxygen storage amount OSAufc continues to increase, and finally the second oxygen storage amount OSAufc. Increases to the maximum storage amount Cmaxdwn of the second three-way catalyst 35. As a result, NOx cannot be reduced and purified by the second three-way catalyst 35, and the lean air-fuel ratio exhaust gas containing NOx flows out from the second three-way catalyst 35.

  Therefore, in the present embodiment, the lean air-fuel ratio exhaust gas starts to flow out from the second three-way catalyst 35 during the execution of the lean failure control, and the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes equal to or greater than the lean determination air-fuel ratio AFrefle. In some cases, the lean failure control (normal control) is stopped, and the storage amount reduction control for decreasing the second oxygen storage amount OSAufc is performed as the target air-fuel ratio setting control.

<Occlusion amount reduction control according to the first embodiment>
FIG. 7 is a time chart for explaining the operation of the occlusion amount reduction control according to this embodiment.

  In the time chart of FIG. 7, before time t4, lean failure control is performed as described above with reference to FIG. 6, and when the first oxygen storage amount OSAsc approaches the maximum storage amount Cmaxup at time t1, The lean air-fuel ratio exhaust gas containing NOx flowing into the first three-way catalyst 34 starts to flow out from the first three-way catalyst 34.

  Thus, after time t1, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 starts increasing, and when the lean determination air-fuel ratio AFrefle is reached at time t2, the electronic control unit 200 sets the air-fuel ratio correction amount AFC to the first air-fuel ratio correction amount AFC. The lean correction amount AFCL1 is switched to the first rich correction amount AFCR1. That is, the target air-fuel ratio is switched from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1.

  Further, in order to reduce and purify NOx flowing out from the first three-way catalyst 34, oxygen in the exhaust gas is occluded in the second three-way catalyst 35, so that the second oxygen occlusion amount OSAufc gradually increases after time t1. It will increase.

  Although the target air-fuel ratio is switched from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1 at time t2, there is a time delay until the rich air-fuel ratio exhaust gas actually flows into the exhaust pipe 32. The output air-fuel ratio AFmid of the two air-fuel ratio sensor 214 is still larger than the theoretical air-fuel ratio. Therefore, the second oxygen storage amount OSAufc increases after time t2.

  When the second oxygen storage amount OSAufc reaches the maximum storage amount Cmaxdwn at time t3, oxygen cannot be stored by the second three-way catalyst 35, and the exhaust of lean air-fuel ratio including NOx is started immediately before time t3. Begins to flow out of the second three-way catalyst 35. As a result, the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 starts to increase immediately before time t3, and the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 reaches the lean determination air-fuel ratio AFrefle at time t4.

  When the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes equal to or higher than the lean determination air-fuel ratio AFrefri at time t4, the electronic control unit 200 stops lean failure control (normal control) and stores the amount as target air-fuel ratio setting control. Start decreasing control.

  At the start of the occlusion amount reduction control, the electronic control unit 200 changes the air-fuel ratio correction amount AFC to a predetermined second rich correction amount AFCR2 having an absolute value larger than the first rich correction amount AFCR1 set during lean failure control. Switch. That is, the target air-fuel ratio is switched to a predetermined second rich air-fuel ratio AFR2 that is smaller than the first rich air-fuel ratio AFR1 set during the lean failure control (that is, the rich degree is large).

  Here, in order to decrease the second oxygen storage amount OSAufc, the first oxygen storage amount OSAsc is reduced to zero so that the first three-way catalyst 34 cannot oxidize and purify the unburned gas. It is necessary to exhaust the rich air-fuel ratio exhaust gas containing unburned gas from the original catalyst 34.

  At this time, the target air-fuel ratio is switched to the second rich air-fuel ratio AFR2, which is richer than the first rich air-fuel ratio AFR1, as in the storage amount reduction control according to the present embodiment, so that the target air-fuel ratio is changed to the first rich air-fuel ratio. Compared with the case where the fuel ratio is set to AFR1, a large amount of unburned gas can be supplied to the first three-way catalyst 34, so that the first oxygen storage amount OSAsc can be quickly reduced. That is, since the first oxygen storage amount OSAsc can be reduced to zero in a short period, the rich air-fuel ratio exhaust gas containing unburned gas can be quickly discharged from the first three-way catalyst 34. As a result, the unburned gas can be quickly supplied to the second three-way catalyst 35 to reduce the second oxygen storage amount OSAufc.

  When the first oxygen storage amount OSAsc approaches zero at time t5, the rich air-fuel ratio exhaust gas including unburned gas starts to flow out from the first three-way catalyst 34. Thereby, after time t5, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 starts to decrease, and at time t6, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 reaches the rich determination air-fuel ratio AFrefri. Thus, after time t5, the rich air-fuel ratio exhaust gas containing unburned gas flows into the second three-way catalyst 35, and is stored in the second three-way catalyst 35 to oxidize and purify the unburned gas. As the remaining oxygen is consumed, the second oxygen storage amount OSAufc decreases.

  In the present embodiment, when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri at time t6, the air-fuel ratio correction amount AFC is changed from the second rich correction amount AFCR2 to the second rich correction amount. Switching is made to a predetermined rich correction amount having an absolute value smaller than that of AFCR2. In particular, in the present embodiment, the air-fuel ratio correction amount AFC is switched to a predetermined third rich correction amount AFCR3 having an absolute value smaller than the first rich correction amount AFCR1, thereby the target air-fuel ratio becomes the first air-fuel ratio at time t6. The second rich air-fuel ratio AFR2 is switched to a predetermined third rich air-fuel ratio AFR3 that is larger than the first rich air-fuel ratio AFR1 (that is, the degree of richness is small). The reason why the target air-fuel ratio is switched to the air-fuel ratio with a small rich degree when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or lower than the rich determination air-fuel ratio AFrefri will be described later.

When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri at time t6, the electronic control unit 200 estimates the second oxygen storage amount OSAufc in parallel with the storage amount reduction control. The second oxygen storage amount estimation control is started. Specifically, the electronic control unit 200 calculates the oxygen excess / deficiency OEDufc of the exhaust gas flowing into the second three-way catalyst 35 from time to time based on the output air / fuel ratio AFmid of the second air / fuel ratio sensor 214, and this oxygen excess / deficiency is calculated. The second oxygen storage amount OSAufc is estimated by integrating the amount OEDufc. The oxygen excess / deficiency amount OEDufc is the amount of oxygen that becomes excessive when the air-fuel ratio of the exhaust gas flowing into the second three-way catalyst 35 is set to the stoichiometric air-fuel ratio or the amount of oxygen that is insufficient (excessive unburned gas, etc. Amount). In the present embodiment, the electronic control unit 200, as shown in the following equation (2), outputs the air / fuel ratio AFmid of the second air / fuel ratio sensor 214, the control center air / fuel ratio (theoretical air / fuel ratio in this embodiment) AFcen, and the fuel injection. Based on the fuel supply amount Qi from the valve 13 (or the estimated value of the intake air amount into the combustion chamber 6 calculated based on the output of the air flow meter 211), the oxygen excess / deficiency OEDufc is calculated as needed. . In Equation (2), 0.23 represents the oxygen concentration in the air.
OEDufc = 0.23 × Qi × (AFmid−AFcen) (2)

  Here, since the internal combustion engine 100 according to the present embodiment includes the third air-fuel ratio sensor 215, the second air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 reaches the lean determination air-fuel ratio AFrefle at the time t4. It can be estimated that the oxygen storage amount OSAsc is the maximum storage amount Cmaxdwn. That is, at the start of the second oxygen storage amount estimation control, the second oxygen storage amount OSAufc can be regarded as the maximum storage amount Cmaxdwn. Therefore, the second oxygen storage amount OSAufc can be accurately estimated.

  When the second oxygen storage amount OSAufc becomes equal to or less than the predetermined restart reference amount Crefdwn at time t7, the electronic control unit 200 ends the storage amount recovery control and resumes normal control.

  When the normal control is resumed, the electronic control unit 200 switches the air-fuel ratio correction amount AFC from the third rich correction amount AFCR3 to the first lean correction amount AFCL1. That is, the target air-fuel ratio is switched from the third rich air-fuel ratio AFR3 to the first lean air-fuel ratio AFL1. Since the first oxygen storage amount OSAsc is zero after the storage amount decrease control, it is necessary to increase the first oxygen storage amount OSAsc using the exhaust gas flowing into the first three-way catalyst 34 as a lean air-fuel ratio. Because there is.

  After time t7, the electronic control unit 200 performs the lean failure control as the target air-fuel ratio setting control until the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes equal to or higher than the lean determination air-fuel ratio AFref again.

  Note that the second three-way catalyst 35 is more than the second three-way catalyst 35 for a while (approximately until time t8) after the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1 at time t7. The rich air-fuel ratio exhaust gas existing in the upstream exhaust passage flows in. Therefore, the restart reference amount Crefdwn is set to such a value that the second oxygen storage amount OSAufc does not decrease to zero even when all the rich air-fuel ratio exhaust gas existing in the exhaust passage flows into the second three-way catalyst 35. .

  By the way, after the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri at time t6, unburned gas in the exhaust gas flowing into the first three-way catalyst 34 becomes the first three-way catalyst. It flows out from the first three-way catalyst 34 without being oxidized and purified at 34 and flows into the second three-way catalyst 35 as it is.

  Therefore, if the target air-fuel ratio is maintained at the second rich air-fuel ratio AFR2 having a large rich degree after time t6, a relatively large amount of unburned gas flows into the second three-way catalyst 35 after time t6. Become. At time t6, since the second oxygen storage amount OSAufc is a value near the maximum storage amount Cmadwn, the unburned gas flowing into the second three-way catalyst 35 is basically oxidized by the second three-way catalyst 35. It will be purified. However, when a large amount of unburned gas flows into the second three-way catalyst 35, a part of the unburned gas blows through the second three-way catalyst 35 and is not oxidized and purified by the second three-way catalyst 35. It is also conceivable that the original catalyst 35 flows out.

  At this time, as in the occlusion amount reduction control according to the present embodiment, when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri, the target air-fuel ratio is set to the third rich air with a small rich degree. By switching to the fuel ratio AFR3, it is possible to suppress a large amount of unburned gas from flowing into the second three-way catalyst 35. Therefore, a part of the unburned gas flowing into the second three-way catalyst 35 blows through the second three-way catalyst 35 and flows out of the second three-way catalyst 35 without being oxidized and purified by the second three-way catalyst 35. Can be suppressed.

  Further, in the present embodiment, when the estimated value of the second oxygen storage amount OSAufc becomes equal to or less than the restart reference amount Crefdwn, the storage amount reduction control is terminated. Therefore, the estimated value and the actual value of the second oxygen storage amount OSAufc may deviate.

  For example, if the oxygen storage capacity of the second three-way catalyst 35 is reduced due to deterioration over time and the maximum storage amount of the second three-way catalyst 35 is reduced to less than the restart reference amount Crefdwn, the storage amount During the reduction control, the actual value of the second oxygen storage amount OSAufc is reduced to zero, the second three-way catalyst 35 is richly broken, and the rich air-fuel ratio exhaust gas containing unburned gas flows out from the second three-way catalyst 35. There is a fear.

  It is also conceivable that the actual air-fuel ratio of the exhaust gas is instantaneously greatly shifted from the target air-fuel ratio to the rich side during the occlusion amount reduction control. Even in such a case, the estimated value of the second oxygen storage amount OSAufc deviates from the actual value, and the actual value of the second oxygen storage amount OSAufc is reduced to zero, and the second three-way catalyst 35 may be richly broken. There is. As a result, the rich air-fuel ratio exhaust gas containing unburned gas may flow out from the second three-way catalyst 35.

  Thus, when the rich air-fuel ratio exhaust gas containing unburned gas flows out from the first three-way catalyst 34, the second oxygen storage amount OSAufc decreases to zero due to some factor, and the second three-way catalyst 35. If the engine fails, a large amount of unburned gas may temporarily flow out of the second three-way catalyst 35.

  Even in such a case, as in the occlusion amount reduction control according to the present embodiment, when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri, the target air-fuel ratio is reduced to a low degree. By switching to the three-rich air-fuel ratio AFR3, even if the second three-way catalyst 35 is richly broken, the amount of unburned gas that temporarily flows out of the second three-way catalyst 35 can be suppressed.

<Modification of timing to end occlusion amount reduction control>
In the example shown in FIG. 7, the occlusion amount reduction control is terminated when the second oxygen occlusion amount OSAufc is equal to or less than the predetermined resumption reference amount Crefdwn. It is not limited to timing. In the storage amount reduction control, after the second oxygen storage amount OSAufc is reduced to some extent, the second oxygen storage amount OSAufc reaches zero (that is, the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 is rich). Until the following).

  Note that whether or not the second oxygen storage amount OSAufc has decreased to a certain degree is determined by, for example, reducing the first oxygen storage amount OSAsc to zero during the storage amount reduction control and causing the second three-way catalyst 35 to exhaust the rich air-fuel ratio. And the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes the stoichiometric air-fuel ratio. That is, when the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes the stoichiometric air-fuel ratio (or less than the lean determination air-fuel ratio AFrefle), it may be determined that the second oxygen storage amount OSAufc has decreased to some extent.

  Further, the electronic control unit 200 estimates the unburned gas amount (oxygen deficient amount) in the exhaust existing in the exhaust passage upstream of the second three-way catalyst 35 based on, for example, the engine operating state. Until the value obtained by subtracting the estimated value of the unburned gas amount from the estimated value of the second oxygen storage amount OSAufc becomes zero, the storage amount reduction control is terminated and the normal control is resumed. Also good.

<Flowchart of target air-fuel ratio setting control>
FIG. 8 is a flowchart illustrating target air-fuel ratio setting control according to this embodiment. The electronic control unit 200 repeatedly executes this routine at a predetermined calculation cycle during engine operation.

  In step S1, the electronic control unit 200 determines whether or not the flag F1 is set to 1. The flag F1 is a flag that is set to 1 when the occlusion amount reduction control is performed, and the initial value is set to 0. If the flag F1 is set to 0, the electronic control unit 200 proceeds to the process of step S2. On the other hand, if the flag F1 is set to 1, the electronic control unit 200 proceeds to the process of step S5.

  In step S <b> 2, the electronic control unit 200 determines whether or not exhaust gas having a lean air-fuel ratio flows out from the second three-way catalyst 35. Specifically, the electronic control unit 200 determines whether or not the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 is greater than or equal to the lean determination air-fuel ratio AFrefle. If the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 is less than the lean determination air-fuel ratio AFrefle, the electronic control unit 200 proceeds to the process of step S3. On the other hand, if the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 is equal to or greater than the lean determination air-fuel ratio AFrefle, the electronic control unit 200 proceeds to the process of step S4.

  In step S3, the electronic control unit 200 performs normal control as target air-fuel ratio setting control. In the present embodiment, the electronic control unit 200 performs lean failure control. Detailed processing contents of the lean failure control will be described later with reference to FIG.

  In step S4, the electronic control unit 200 sets the flag F1 to 1.

  In step S5, the electronic control unit 200 performs the occlusion amount reduction control as the target air-fuel ratio setting control. Detailed processing contents of the occlusion amount reduction control will be described later with reference to FIG.

<Normal control flowchart>
FIG. 9 is a flowchart for explaining detailed processing contents of lean failure control as normal control.

  In step S11, the electronic control unit 200 determines whether or not the normal control is resumed after the occlusion amount reduction control is performed. Specifically, the electronic control unit 200 determines whether or not the flag F1 at the time of the previous process was 1. If the normal control is resumed, the electronic control unit 200 proceeds to the process of step S12. On the other hand, if the normal control is not resumed, the electronic control unit 200 proceeds to the process of step S13.

  In step S12, the electronic control unit 200 returns the flag F2 and the flag F3 to 0. The flag F2 is a flag that is set to 1 when the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1 during normal control, and the initial value is set to 0. The flag F3 is a flag that is set to 1 when the target air-fuel ratio is switched to the first rich air-fuel ratio AFR1 during normal control, and the initial value is set to 0.

  In step S13, the electronic control unit 200 determines whether or not the flag F2 is set to 1. If the flag F2 is set to 0, the electronic control unit 200 proceeds to the process of step S14. On the other hand, if the flag F2 is set to 1, the electronic control unit 200 proceeds to the process of step S17.

  In step S14, the electronic control unit 200 determines whether or not the flag F3 is set to 1. If the flag F3 is set to 0, the electronic control unit 200 proceeds to the process of step S15. On the other hand, if the flag F3 is set to 1, the electronic control unit 200 proceeds to the process of step S21.

  In step S15, the electronic control unit 200 sets the target air-fuel ratio to the first lean air-fuel ratio AFL1.

  In step S16, the electronic control unit 200 sets the flag F2 to 1.

  In step S17, the electronic control unit 200 determines whether the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is greater than or equal to the lean determination air-fuel ratio AFrefle. If the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is less than the lean determination air-fuel ratio AFrefle, the electronic control unit 200 ends the current process. On the other hand, if the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is equal to or greater than the lean determination air-fuel ratio AFrefle, the electronic control unit 200 proceeds to the process of step S18.

  In step S18, the electronic control unit 200 returns the flag F2 to 0.

  In step S19, the electronic control unit 200 sets the target air-fuel ratio to the first rich air-fuel ratio AFR1.

  In step S20, the electronic control unit 200 sets a flag F3 to 1.

  In step S21, the electronic control unit 200 reads the first oxygen storage amount OSAsc estimated by the first oxygen storage amount estimation control, and determines whether or not the first oxygen storage amount OSAsc is equal to or less than the first switching reference amount Crefup1. To do. The electronic control unit 200 ends the current process if the first oxygen storage amount OSAsc is larger than the first switching reference amount Crefup1. On the other hand, if the first oxygen storage amount OSAsc is equal to or smaller than the first switching reference amount Crefup1, the electronic control unit 200 proceeds to the process of step S22.

  In step S22, the electronic control unit 200 returns the flag F3 to 0.

<Flow chart of occlusion amount reduction control according to the first embodiment>
FIG. 10 is a flowchart illustrating the detailed processing content of the occlusion amount reduction control according to the present embodiment.

  In step S31, the electronic control unit 200 determines whether or not the first oxygen storage amount OSAsc has become zero. Specifically, the electronic control unit 200 determines whether or not the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is equal to or less than the rich determination air-fuel ratio AFrefri. If the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is larger than the rich determination air-fuel ratio AFrefri, the electronic control unit 200 proceeds to the process of step S32. On the other hand, if the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is equal to or less than the rich determination air-fuel ratio AFrefri, the electronic control unit 200 proceeds to the process of step S33.

  In step S32, the electronic control unit 200 sets the target air-fuel ratio to the second rich air-fuel ratio AFR2.

  In step S33, the electronic control unit 200 sets the target air-fuel ratio to the third rich air-fuel ratio AFR3.

  In step S34, the electronic control unit 200 reads the second oxygen storage amount OSAufc estimated by the second oxygen storage amount estimation control, and determines whether or not the second oxygen storage amount OSAufc is equal to or less than the restart reference amount Crefdwn. To do. If the second oxygen storage amount OSAufc is greater than the resumption reference value Crefdwn, the electronic control unit 200 ends the current process. In this case, the occlusion amount reduction control is continuously performed. On the other hand, if the second oxygen storage amount OSAufc is less than or equal to the restart reference value Crefdwn, the electronic control unit 200 proceeds to the process of step S35.

  In step S35, the electronic control unit 200 returns the flag F1 to 0 in order to end the occlusion amount reduction control and resume the normal control.

<Effect>
According to the present embodiment described above, the engine main body 1, the first three-way catalyst 34 (first catalyst) having an oxygen storage capacity disposed in the exhaust passage of the engine main body 1, and the first three-way catalyst 34 The second three-way catalyst 35 (second catalyst) having an oxygen storage capacity disposed in the exhaust passage downstream of the exhaust flow direction and the exhaust passage upstream of the first three-way catalyst 34 are disposed. The first air-fuel ratio sensor 213 for detecting the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34, and the exhaust passage between the first three-way catalyst 34 and the second three-way catalyst 35, A second air-fuel ratio sensor 214 for detecting the air-fuel ratio of the exhaust gas flowing out from the first three-way catalyst 34, and an exhaust passage downstream of the second three-way catalyst 35 in the exhaust flow direction, Third air-fuel ratio sensor 2 for detecting the air-fuel ratio of the exhaust gas flowing out from the catalyst 35 5, the electronic control unit 200 (control device) for controlling the internal combustion engine 100 is discharged from the engine body 1 so that the output air-fuel ratio AFup of the first air-fuel ratio sensor 213 becomes the target air-fuel ratio. An air-fuel ratio control unit that controls the air-fuel ratio of the exhaust; and a target air-fuel ratio setting unit that sets the target air-fuel ratio.

  The target air-fuel ratio setting unit performs normal control to alternately switch the target air-fuel ratio between a predetermined first lean air-fuel ratio AFL1 larger than the stoichiometric air-fuel ratio and a predetermined first rich air-fuel ratio AFR1 smaller than the stoichiometric air-fuel ratio. When the output air-fuel ratio AFdwn of the first setting control unit and the third air-fuel ratio sensor 215 is greater than the theoretical air-fuel ratio and smaller than the first lean air-fuel ratio AFL1 is greater than or equal to a predetermined lean determination air-fuel ratio AFrefri And a second setting control unit that performs the occlusion amount reduction control for reducing the oxygen occlusion amount of the second three-way catalyst 35 by stopping the normal control.

  The second setting control unit sets the target air-fuel ratio to a predetermined second rich air-fuel ratio AFR2 smaller than the first rich air-fuel ratio AFR1 at the start of the storage amount reduction control, and sets the target air-fuel ratio to the second rich air-fuel ratio. During a period set to AFR2, a predetermined third rich air whose target air-fuel ratio is larger than the first rich air-fuel ratio AFR1 after exhaust having an air-fuel ratio smaller than the stoichiometric air-fuel ratio flows out from the first three-way catalyst 34. The fuel ratio is set to the fuel ratio AFR3.

  Therefore, at the time of storage amount reduction control, the target air-fuel ratio is smaller than the first rich air-fuel ratio AFR1 set at the time of normal control (that is, the degree of richness) until at least the exhaust of the rich air-fuel ratio flows out from the first three-way catalyst 34. (Large) is set to the second rich air-fuel ratio AFR2. Therefore, compared with the case where the target air-fuel ratio is set to the first rich air-fuel ratio AFR1, a large amount of unburned gas can be supplied to the first three-way catalyst 34, so that the first oxygen storage amount OSAsc is quickly reduced. Can be made. That is, since the first oxygen storage amount OSAsc can be reduced to zero in a short period, the rich air-fuel ratio exhaust gas containing unburned gas can be quickly discharged from the first three-way catalyst 34. As a result, the unburned gas can be quickly supplied to the second three-way catalyst 35 to reduce the second oxygen storage amount OSAufc.

  Here, even after the rich air-fuel ratio exhaust gas flows out from the first three-way catalyst 34, if the target air-fuel ratio is maintained at the second rich air-fuel ratio AFR2 having a large rich degree, a relatively large amount is obtained. Unburned gas flows into the second three-way catalyst 35. Therefore, it is conceivable that a part of the unburned gas flows out from the second three-way catalyst 35 without being oxidized and purified by the second three-way catalyst 35. In addition, when the rich air-fuel ratio exhaust gas flows out from the first three-way catalyst 34, for example, when the estimated value of the second oxygen storage amount OSAufc deviates from the actual value, the second oxygen If the occlusion amount OSAufc is reduced to zero and the second three-way catalyst 35 is richly broken, a large amount of unburned gas may temporarily flow out of the second three-way catalyst 35.

  On the other hand, according to the second setting control unit of the present embodiment, after the rich air-fuel ratio exhaust gas flows out from the first three-way catalyst 34, the target air-fuel ratio is larger than the second rich air-fuel ratio AFR2 (that is, The third rich air-fuel ratio AFR3 (with a small rich degree) is set. Therefore, since the amount of unburned gas flowing into the second three-way catalyst 35 can be suppressed, it is possible to suppress the unburned gas from flowing out from the second three-way catalyst 35. In particular, in the present embodiment, the third lean air-fuel ratio AFL3 is set to an air-fuel ratio smaller than the first lean air-fuel ratio AFL1. Therefore, it is possible to further suppress the unburned gas from flowing out of the second three-way catalyst 35, and even if the second three-way catalyst 35 is richly broken, temporarily from the second three-way catalyst 35. The amount of unburned gas flowing out can be suppressed.

  As described above, according to the second setting control unit of the present embodiment, the second oxygen storage amount OSAufc can be reduced while suppressing the outflow of unburned gas from the second three-way catalyst 35. Deterioration of exhaust emission that occurs when the storage amount OSAufc increases to the maximum storage amount Cmaxdwn can be suppressed.

  Further, the second setting control unit according to the present embodiment allows the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 to be equal to or less than the rich determination air-fuel ratio AFrefri after the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes the stoichiometric air-fuel ratio. In the meantime, the occlusion amount reduction control is terminated and the normal control is resumed. Specifically, when the second oxygen storage amount OSAufc becomes equal to or less than a predetermined restart reference amount Crefdwn which is smaller than the maximum storage amount Cmaxdwn of the second three-way catalyst 35, the storage amount reduction control is terminated and normal control is performed. Configured to resume.

  Thus, after the rich air-fuel ratio exhaust gas flowing out from the first three-way catalyst 34 flows into the second three-way catalyst 35, at least the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes the stoichiometric air-fuel ratio. Thus, the storage amount reduction control can be performed until the second oxygen storage amount OSAufc is reduced to a certain level. Therefore, even if the lean air-fuel ratio exhaust gas containing NOx flows into the second three-way catalyst 35 after the normal control is resumed, the second three-way catalyst 35 can reliably reduce and purify NOx. Therefore, it is possible to suppress the exhaust of the rich air-fuel ratio containing NOx from the second three-way catalyst 35 and suppress the deterioration of the exhaust emission.

  Further, the second setting control unit according to the modification of the present embodiment is based on the estimated value of the second oxygen storage amount OSAufc, and is not present in the exhaust passage upstream of the front end surface of the second three-way catalyst 35 in the exhaust flow direction. Until the value obtained by subtracting the estimated value of the amount of fuel gas (oxygen deficient amount) reaches zero, the occlusion amount reduction control is terminated and normal control is resumed.

  Thus, even if the rich air-fuel ratio exhaust gas present in the exhaust passage upstream of the second three-way catalyst 35 after the storage amount reduction control flows into the second three-way catalyst 35, the second oxygen storage amount OSAufc is It can suppress falling to zero. Therefore, it is possible to suppress the exhaust of the rich air-fuel ratio including unburned gas from the second three-way catalyst 35.

  The first setting control unit according to the present embodiment is configured to set the target air-fuel ratio to the first lean air-fuel ratio AFL1 when normal control is resumed.

  As a result, the first oxygen storage amount OSAsc becomes zero after the storage amount reduction control, but the exhaust gas flowing into the first three-way catalyst 34 can be made the lean air-fuel ratio at the start of the normal control. One oxygen storage amount OSAsc can be increased.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. This embodiment is different from the first embodiment in the contents of the occlusion amount reduction control. Specifically, the second embodiment is different from the first embodiment in that rich failure control is performed during the storage amount reduction control to decrease the second oxygen storage amount OSAufc stepwise. Hereinafter, this difference will be mainly described.

  In the first embodiment described above, after the target air-fuel ratio is switched to the third rich air-fuel ratio AFR3, the target air-fuel ratio is set to the third rich air-fuel ratio AFR3 until the second oxygen storage amount OSAufc becomes equal to or less than the restart reference amount Crefdwn. Was maintained. That is, in the first embodiment described above, the rich air-fuel ratio exhaust gas containing unburned gas is continuously flowed into the second three-way catalyst 35 in order to reduce the oxygen storage amount of the second three-way catalyst 35. . During the period in which the target air-fuel ratio is maintained at the third rich air-fuel ratio AFR3, basically, sufficient oxygen is occluded in the second three-way catalyst 35, so unburned gas is stored in the second three-way catalyst 35. Even when the rich air-fuel ratio exhaust gas flows in, the unburned gas is oxidized and purified by the second three-way catalyst 35, and the unburned gas does not flow out from the second three-way catalyst 35. However, when rich air-fuel ratio exhaust gas containing unburned gas is continuously flowed into the second three-way catalyst 35, there is no possibility that the unburned gas will blow through the second three-way catalyst 35 without being oxidized and purified. is not.

  Therefore, in the present embodiment, when the oxygen storage amount of the second three-way catalyst 35 is decreased, the rich air-fuel ratio exhaust gas is intermittently introduced into the second three-way catalyst 35 little by little. Specifically, after the lean air-fuel ratio exhaust no longer flows out from the second three-way catalyst 35, the first oxygen storage is performed such that the rich air-fuel ratio exhaust intermittently flows into the second three-way catalyst 35. The amount OSAsc is periodically set to zero, and rich failure control is performed to make the first three-way catalyst 34 richly fail. Hereinafter, the occlusion amount reduction control according to this embodiment will be described.

<Rich failure control as occlusion reduction control>
The electronic control unit 200 switches the target air-fuel ratio to the first lean air-fuel ratio AFL1 when the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or lower than the rich determination air-fuel ratio AFrefri during the execution of the rich failure control. Thereafter, the first lean air-fuel ratio AFL1 is maintained.

  When the estimated value of the first oxygen storage amount OSAsc becomes equal to or greater than the predetermined second switching reference amount Crefup2 during the period when the target air-fuel ratio is set to the first lean air-fuel ratio AFL1, the electronic control unit 200 The fuel ratio is switched from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1, and then maintained at the first rich air-fuel ratio AFR1.

  When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri during the period when the target air-fuel ratio is set to the first rich air-fuel ratio AFR1, the electronic control unit 200 again sets the target air-fuel ratio. The operation is switched to the first lean air-fuel ratio AFL1, and then the same operation is repeated.

<Occlusion amount reduction control according to the second embodiment>
FIG. 11 is a time chart for explaining the operation of the occlusion amount reduction control according to this embodiment.

  Since the operation during the normal control (during lean failure control) and the operation during the occlusion amount reduction control up to time t1 are the same as those in the first embodiment, the description thereof is omitted here.

  By performing the occlusion amount reduction control, the first oxygen occlusion amount OSAsc decreases to zero, and the rich air-fuel ratio exhaust gas flows into the second three-way catalyst 35, and the third air-fuel ratio sensor 215 at time t1. When the output air-fuel ratio AFdwn becomes the stoichiometric air-fuel ratio, the electronic control unit 200 determines that the lean air-fuel ratio exhaust no longer flows out from the second three-way catalyst 35, and starts rich failure control as occlusion amount reduction control . Specifically, the electronic control unit 200 switches the target air-fuel ratio from the third rich air-fuel ratio AFR3 to the first lean air-fuel ratio AFL1. Thus, after time t1, the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34 becomes the lean air-fuel ratio, so the first oxygen storage amount OSAsc increases. Note that when the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes less than the lean determination air-fuel ratio AFrefle, it may be determined that the lean air-fuel ratio exhaust does not flow out from the second three-way catalyst 35.

  On the other hand, although the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1 at time t1, there is a time delay until the lean air-fuel ratio actually flows downstream of the first three-way catalyst 34. The output air-fuel ratio AFmid of the two air-fuel ratio sensor 214 is still smaller than the theoretical air-fuel ratio. Therefore, the second oxygen storage amount OSAufc decreases after time t1. When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 converges to the stoichiometric air-fuel ratio at time t2, the second oxygen storage amount OSAufc does not decrease and is maintained constant.

  When the first oxygen storage amount OSAsc increases to a predetermined second switching reference amount Crefup2 at time t3, the electronic control unit 200 switches the target air-fuel ratio from the first lean air-fuel ratio AFL1 to the first rich air-fuel ratio AFR1. Thereby, after time t3, the air-fuel ratio of the exhaust gas flowing into the first three-way catalyst 34 becomes a rich air-fuel ratio, so the first oxygen storage amount OSAsc decreases.

  The second switching reference amount Crefup2 is set sufficiently lower than the maximum storage amount Cmaxup when the first three-way catalyst 34 is new (unused). Therefore, there is a delay from when the target air-fuel ratio is actually switched to the first rich air-fuel ratio AFR1, until the first oxygen storage amount OSAsc decreases. Even if the air-fuel ratio unintentionally deviates from the target air-fuel ratio instantaneously to the lean side, the first oxygen storage amount OSAsc does not reach the maximum storage amount Cmaxup. In other words, the second switching reference amount Crefup2 is sufficiently small so that the first oxygen storage amount OSAsc does not reach the maximum storage amount Cmaxup even if the above-described delay or unintended air-fuel ratio shift occurs. It is said. For example, the second switching reference amount Crefup2 is set to 3/4 or less, preferably 1/2 or less, more preferably 1/5 or less of the maximum storage amount Cmaxup of the first three-way catalyst 34.

  At time t4, the first oxygen storage amount OSAsc approaches zero, and accordingly, the rich air-fuel ratio exhaust gas gradually starts to flow out from the first three-way catalyst 34. Thereby, after time t4, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 gradually decreases.

  When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes equal to or less than the rich determination air-fuel ratio AFrefri at time t5, the electronic control unit 200 sets the target air-fuel ratio to the first air-fuel ratio in order to increase the first oxygen storage amount OSAsc. The rich air-fuel ratio AFR1 is switched to the first lean air-fuel ratio AFL1 again. As a result, at time t6, the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 converges to the stoichiometric air-fuel ratio.

  At this time, during the period from time 4 to time t6, the output air-fuel ratio AFmid of the second air-fuel ratio sensor is smaller than the stoichiometric air-fuel ratio, and the rich air-fuel ratio exhaust gas flows out from the first three-way catalyst 34. Yes. In other words, rich air-fuel ratio exhaust gas containing unburned gas flows into the second three-way catalyst 35. Therefore, in the period from time 4 to time t6, the oxygen stored in the second three-way catalyst 35 is consumed to oxidize and purify the unburned gas, so the second oxygen storage amount OSAufc gradually decreases. Go. When the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 converges to the stoichiometric air-fuel ratio at time t6, the second oxygen storage amount OSAufc does not decrease and is maintained constant.

  When the rich failure control is performed in this way, the rich gas containing the unburned gas from the first three-way catalyst 34 temporarily in the period before and after the target air-fuel ratio is switched from the first rich air-fuel ratio AFR1 to the first lean air-fuel ratio AFL1. Air-fuel ratio exhaust will flow out. Therefore, by performing rich failure control, until the second oxygen storage amount OSAufc becomes equal to or less than the restart reference amount Crefdwn, the rich three-way catalyst 35 is caused to flow into the second three-way catalyst 35 intermittently and the second oxygen storage amount OSAufc flows. The amount OSAufc can be decreased gradually.

  When the first oxygen storage amount OSAsc increases to a predetermined second switching reference amount Crefup2 at time t7, the target air-fuel ratio is again switched to the first rich air-fuel ratio AFR1, and at time t8, the output air-fuel ratio AFmid of the second air-fuel ratio sensor. Becomes the rich determination air-fuel ratio AFrefri or more, the target air-fuel ratio is switched again to the first lean air-fuel ratio AFL1.

  At time t9, when the second oxygen storage amount OSAufc becomes equal to or less than the restart reference amount Crefdwn, the electronic control unit 200 ends the storage amount reduction control and resumes normal control.

  In the time chart of FIG. 11, for convenience, the target air-fuel ratio when the rich failure control is performed as the occlusion amount reduction control is set to the same target air-fuel ratio as that during the normal control (that is, the first rich air-fuel ratio AFR1 and the first lean). The air / fuel ratio is set to AFL1), but it may be set to a target air / fuel ratio different from that during normal control (that is, any rich air / fuel ratio and lean air / fuel ratio).

<Flow chart of occlusion amount reduction control according to the second embodiment>
FIG. 12 is a flowchart for explaining detailed processing contents of the occlusion amount reduction control according to the present embodiment.

  Since the processing from step S31 to step S33 is the same as that of the first embodiment, description thereof is omitted here.

  In step S41, the electronic control unit 200 determines whether or not the flag F4 is set to 1. The flag F4 is a flag that is set to 1 when the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1 when rich failure control is performed as the occlusion amount reduction control, and the initial value is set to 0 Is set. If the flag F4 is set to 0, the electronic control unit 200 proceeds to the process of step S42. On the other hand, if the flag F4 is set to 1, the electronic control unit 200 proceeds to the process of step S46.

  In step S42, the electronic control unit 200 determines whether or not the flag F5 is set to 1. The flag F5 is a flag that is set to 1 when the target air-fuel ratio is switched to the first rich air-fuel ratio AFR1 when rich failure control is performed as the occlusion amount reduction control, and the initial value is set to 0. Is set. If the flag F5 is set to 0, the electronic control unit 200 proceeds to the process of step S31. On the other hand, if the flag F5 is set to 1, the electronic control unit 200 proceeds to the process of step S50.

  In step S43, the electronic control unit 200 determines whether or not the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 has reached the stoichiometric air-fuel ratio. If the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 is not the stoichiometric air-fuel ratio, the electronic control unit 200 ends the current process. In this case, the target air-fuel ratio is maintained at the third rich air-fuel ratio AFR3 until the lean air-fuel ratio exhaust does not flow out from the second three-way catalyst 35. On the other hand, when the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes the stoichiometric air-fuel ratio, the electronic control unit 200 determines that the lean air-fuel ratio exhaust does not flow out from the second three-way catalyst 35. The process proceeds to S44.

  In step S44, the electronic control unit 200 sets the target air-fuel ratio to the first lean air-fuel ratio AFL1, and starts rich failure control as occlusion amount reduction control.

  In step S45, the electronic control unit 200 sets a flag F4 to 1.

  In step S46, the electronic control unit 200 determines whether or not the first oxygen storage amount OSAsc is greater than or equal to the second switching reference amount Crefup2. The electronic control unit 200 ends the current process if the first oxygen storage amount OSAsc is less than the second switching reference amount Crefup2. On the other hand, if the first oxygen storage amount OSAsc is greater than or equal to the second switching reference amount Crefup2, the electronic control unit 200 proceeds to the process of step S47.

  In step S47, the electronic control unit 200 sets the target air-fuel ratio to the first rich air-fuel ratio AFR1.

  In step S48, the electronic control unit 200 returns the flag F4 to 0 and sets the flag F5 to 1.

  In step S49, the electronic control unit 200 determines whether or not the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 has become equal to or less than the rich determination air-fuel ratio AFrefri. The electronic control unit 200 ends the current process if the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is greater than the rich determination air-fuel ratio AFrefri. On the other hand, if the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 is equal to or less than the rich determination air-fuel ratio AFrefri, the electronic control unit 200 proceeds to the process of step S50.

  In step S50, the electronic control unit 200 returns the flag F5 and the flag F1 to 0.

  According to the present embodiment described above, the same effect as that of the first embodiment can be obtained, and the rich air-fuel ratio exhaust gas can be intermittently introduced into the second three-way catalyst 35 little by little to thereby store the second oxygen storage amount. OSAufc can be decreased step by step. Therefore, the unburned gas contained in the rich air-fuel ratio exhaust gas can be reliably reduced and purified by the second three-way catalyst 35.

  The embodiment of the present invention has been described above. However, the above embodiment only shows a part of application examples of the present invention, and the technical scope of the present invention is limited to the specific configuration of the above embodiment. Absent.

  For example, in each of the above-described embodiments, the example in which the lean failure control is performed as the normal control in which the target air-fuel ratio is alternately switched between the first lean air-fuel ratio AFL1 and the first rich air-fuel ratio AFR1 has been described. However, such normal control is not limited to lean failure control. For example, after performing the above-described rich failure control as normal control, the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes the lean determination air-fuel ratio AFrefle. You may make it implement occlusion amount reduction | decrease control when it becomes above.

  When rich failure control is performed as normal control, the estimated value of the first oxygen storage amount OSAsc is sufficiently larger than the maximum storage amount Cmaxup after the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1 as described above. The target air-fuel ratio is switched to the first rich air-fuel ratio AFR1 when the second switching reference amount Crefup2 is exceeded.

  Therefore, during the period when the target air-fuel ratio is set to the first lean air-fuel ratio AFL1, the first oxygen storage amount OSAsc does not increase to the vicinity of the maximum storage amount Cmaxup, and basically the NOx from the first three-way catalyst 34. Lean air-fuel ratio exhaust gas containing no gas will not flow out. That is, when rich failure control is performed as normal control, the outflow of NOx from the first three-way catalyst 34 can always be suppressed, and basically the amount of NOx discharged from the first three-way catalyst 34 is substantially reduced. Can be zero.

  On the other hand, after the target air-fuel ratio is switched to the first rich air-fuel ratio AFR1, the rich air-fuel ratio exhaust begins to flow out from the first three-way catalyst 34, and the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 becomes the rich determination air-fuel ratio. When it becomes AFrefri or less, the target air-fuel ratio is switched to the first lean air-fuel ratio AFL1. Therefore, the rich air-fuel ratio exhaust gas flows out from the first three-way catalyst 34 before and after the target air-fuel ratio is switched from the first rich air-fuel ratio AFR1 to the first lean air-fuel ratio AFR1.

  As described above, when the rich failure control is performed as the normal control, only the rich air-fuel ratio exhaust gas basically flows out from the first three-way catalyst 35. Therefore, when rich failure control is performed as normal control, the second oxygen storage amount OSAufc gradually decreases, contrary to when lean failure control is performed as normal control. Therefore, when rich failure control is performed as normal control, basically, the second oxygen storage amount OSAufc does not increase to the maximum storage amount Cmaxdwn during normal control, and the second three-way catalyst 35 does not fail lean. .

  However, for example, the oxygen storage capacity of the first three-way catalyst 34 is reduced due to deterioration over time, and the maximum storage amount of the first three-way catalyst 34 is reduced to less than the second switching reference amount Crefup2. It is also conceivable that the air-fuel ratio of the exhaust gas deviates from the target air-fuel ratio momentarily to the lean side. In such a case, since the lean air-fuel ratio exhaust gas flows into the second three-way catalyst 35, the second oxygen storage amount OSAufc increases to the maximum storage amount Cmaxdwn during the normal control, and the second three-way catalyst 35 increases. The possibility that the catalyst 35 will fail in lean and the second air-fuel ratio exhaust gas containing NOx flows out from the second three-way catalyst 35 is not zero.

  Therefore, after the rich failure control is performed as the normal control, the occlusion amount decrease control is performed when the output air-fuel ratio AFdwn of the third air-fuel ratio sensor 215 becomes equal to or higher than the lean determination air-fuel ratio AFref, as described above. Even if the second three-way catalyst 35 unintentionally fails, the second oxygen storage amount OSAufc can be decreased by quickly flowing the rich air-fuel ratio into the second three-way catalyst 35.

  In each of the above embodiments, the first oxygen storage amount OSAsc is estimated based on the output air-fuel ratio AFup of the first air-fuel ratio sensor 213 and the like, and based on the output air-fuel ratio AFmid of the second air-fuel ratio sensor 214 and the like. The second oxygen storage amount OSAufc was estimated. However, the first oxygen storage amount OSAsc and the second oxygen storage amount OSAufc may be calculated based on other parameters in addition to these parameters, or may be estimated based on parameters different from these parameters. .

1 Engine body 34 First three-way catalyst (first catalyst)
35 Second three-way catalyst (second catalyst)
100 Internal combustion engine 200 Electronic control unit (control device)
213 First air-fuel ratio sensor 214 Second air-fuel ratio sensor 215 Third air-fuel ratio sensor

Claims (1)

The engine body,
A first catalyst having an oxygen storage capacity disposed in an exhaust passage of the engine body;
A second catalyst having an oxygen storage capacity disposed in the exhaust passage downstream of the first catalyst in the exhaust flow direction;
A first air-fuel ratio sensor which is disposed in the exhaust passage upstream of the first catalyst in the exhaust flow direction and detects the air-fuel ratio of the exhaust flowing into the first catalyst;
A second air-fuel ratio sensor disposed in the exhaust passage between the first catalyst and the second catalyst for detecting the air-fuel ratio of the exhaust gas flowing out from the first catalyst;
A third air-fuel ratio sensor that is disposed in the exhaust passage downstream of the second catalyst in the exhaust flow direction and detects the air-fuel ratio of the exhaust gas flowing out from the second catalyst;
An internal combustion engine control apparatus for controlling an internal combustion engine comprising:
An air-fuel ratio controller that controls the air-fuel ratio of the exhaust discharged from the engine body so that the output air-fuel ratio of the first air-fuel ratio sensor becomes a target air-fuel ratio;
A target air-fuel ratio setting unit for switching the target air-fuel ratio;
With
The target air-fuel ratio setting unit is
A first setting control unit that performs normal control for alternately switching the target air-fuel ratio between a predetermined lean air-fuel ratio that is larger than the theoretical air-fuel ratio and a predetermined first rich air-fuel ratio that is smaller than the theoretical air-fuel ratio;
When the output air-fuel ratio of the third air-fuel ratio sensor becomes equal to or greater than a predetermined lean determination air-fuel ratio that is larger than the stoichiometric air-fuel ratio and smaller than the first lean air-fuel ratio, the normal control is stopped, and the second air-fuel ratio sensor is stopped. A second setting control unit for performing storage amount reduction control for reducing the oxygen storage amount of the catalyst;
With
The second setting control unit
Setting the target air-fuel ratio to a predetermined second rich air-fuel ratio smaller than the first rich air-fuel ratio at the start of the occlusion amount reduction control;
During a period in which the target air-fuel ratio is set to the second rich air-fuel ratio, after exhaust having an air-fuel ratio smaller than the stoichiometric air-fuel ratio flows out from the first catalyst, the target air-fuel ratio is set to the first rich air-fuel ratio. Set to a predetermined third rich air-fuel ratio greater than the fuel ratio;
Control device for internal combustion engine.

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