JP2015071985A - Control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control device for an internal combustion engine, for suppressing the outflow of NOx.SOLUTION: The control device for the internal combustion engine performs normal operation control including first lean control to set the air-fuel ratio of exhaust gas flowing into an exhaust emission control catalyst to be a lean air-fuel ratio and first rich control to set the air-fuel ratio of exhaust gas flowing into the exhaust emission control catalyst to be a rich air-fuel ratio. During a period for the normal operation control, sequentially after finishing fuel cut control, the control device performs second rich control to set the air-fuel ratio of the exhaust gas to be a rich air-fuel ratio and second lean control to set the air-fuel ratio of the exhaust gas to be a lean air-fuel ratio. The control device performs control so that the air-fuel ratio of the exhaust gas flowing into the exhaust emission control catalyst in the second lean control is leaner than the air-fuel ratio of the exhaust gas flowing into the exhaust emission control catalyst in the first lean control.

Description

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

  The exhaust gas discharged from the combustion chamber contains unburned gas, NOx, and the like, and an exhaust purification catalyst is disposed in the engine exhaust passage in order to purify the components of the exhaust gas. A three-way catalyst is known as an exhaust purification catalyst capable of simultaneously purifying components such as unburned gas and NOx. The three-way catalyst can purify unburned gas, NOx and the like with a high purification rate when the air-fuel ratio of the exhaust gas is close to the stoichiometric air-fuel ratio. For this reason, conventionally, there has been known a control device that is provided with an air-fuel ratio sensor in the exhaust passage of the internal combustion engine and controls the amount of fuel supplied to the internal combustion engine based on the output value of the air-fuel ratio sensor.

  As the exhaust purification catalyst, one having an oxygen storage capacity can be used. When the oxygen storage amount is an appropriate amount between the upper limit storage amount and the lower limit storage amount, the exhaust purification catalyst having an oxygen storage capacity has a richer air / fuel ratio of the exhaust gas flowing into the exhaust purification catalyst than the target air / fuel ratio. Even so, unburned gas (HC, CO, etc.), NOx, etc. can be purified. When an exhaust gas having an air-fuel ratio richer than the stoichiometric air-fuel ratio (hereinafter also referred to as “rich air-fuel ratio”) flows into the exhaust purification catalyst, unburned gas in the exhaust gas is absorbed by oxygen stored in the exhaust purification catalyst. It is oxidized and purified.

  Conversely, when an exhaust gas having an air-fuel ratio leaner than the stoichiometric air-fuel ratio (hereinafter also referred to as “lean air-fuel ratio”) flows into the exhaust purification catalyst, oxygen in the exhaust gas is stored in the exhaust purification catalyst. As a result, an oxygen-deficient state occurs on the exhaust purification catalyst surface, and NOx in the exhaust gas is reduced and purified accordingly. As described above, the exhaust purification catalyst can purify the exhaust gas regardless of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst as long as the oxygen storage amount is an appropriate amount.

  Therefore, in such a control device, an air-fuel ratio sensor is provided upstream of the exhaust purification catalyst in the exhaust flow direction and an oxygen sensor is provided downstream of the exhaust flow direction in order to maintain an appropriate amount of oxygen stored in the exhaust purification catalyst. I am doing so. Using these sensors, the control device performs feedback control based on the output of the upstream air-fuel ratio sensor so that the output of the air-fuel ratio sensor becomes a target value corresponding to the target air-fuel ratio. In addition, the target value of the upstream air-fuel ratio sensor is corrected based on the output of the downstream oxygen sensor.

  For example, in the control device described in Japanese Patent Application Laid-Open No. 2011-069337, when the output voltage of the downstream oxygen sensor is equal to or higher than the high-side threshold and the exhaust purification catalyst is in an oxygen-deficient state, The target air-fuel ratio of the inflowing exhaust gas is set to the lean air-fuel ratio. Conversely, when the output voltage of the downstream oxygen sensor is equal to or lower than the low threshold value and the exhaust purification catalyst is in the oxygen excess state, the target air-fuel ratio is set to the rich air-fuel ratio. With this control, when the oxygen purification state is in an oxygen deficient state or an oxygen excess state, the state of the exhaust purification catalyst is quickly changed to an intermediate state between these two states, that is, a state where an appropriate amount of oxygen is occluded in the exhaust purification catalyst. It can be returned.

  Further, in the control device described in Japanese Patent Laid-Open No. 2001-234787, the oxygen storage amount of the exhaust purification catalyst is calculated based on the outputs of the air flow meter and the air-fuel ratio sensor upstream of the exhaust purification catalyst. In addition, when the calculated oxygen storage amount is larger than the target oxygen storage amount, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is set to a rich air-fuel ratio, and the calculated oxygen storage amount is larger than the target oxygen storage amount. When it is low, the target air-fuel ratio is set to the lean air-fuel ratio. With this control, the oxygen storage amount of the exhaust purification catalyst can be kept constant at the target oxygen storage amount.

JP 2011-069337 A JP 2001-234787 A JP-A-8-232723 JP 2009-162139 A

  An exhaust purification catalyst having oxygen storage capacity stores oxygen in the exhaust gas when the oxygen storage amount is close to the maximum oxygen storage amount when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a lean air-fuel ratio. It becomes difficult to do. The exhaust purification catalyst is in an oxygen-excess state, and NOx contained in the exhaust gas is difficult to be reduced and purified. For this reason, when the oxygen storage amount becomes close to the maximum oxygen storage amount, the NOx concentration of the exhaust gas flowing out from the exhaust purification catalyst rapidly increases.

  For this reason, as disclosed in the above Japanese Patent Application Laid-Open No. 2011-069337, when the output voltage of the downstream oxygen sensor becomes equal to or lower than the low threshold value, the target air-fuel ratio is set to the rich air-fuel ratio. However, there is a problem that a certain amount of NOx flows out from the exhaust purification catalyst.

  FIG. 15 is a time chart for explaining the relationship between the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst and the NOx concentration flowing out from the exhaust purification catalyst. FIG. 15 shows the oxygen storage amount of the exhaust purification catalyst, the air-fuel ratio of the exhaust gas detected by the downstream oxygen sensor, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst, and the upstream air-fuel ratio sensor. 3 is a time chart of the air-fuel ratio of exhaust gas and the NOx concentration in exhaust gas flowing out from the exhaust purification catalyst.

In time t ₁ prior state, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a lean air-fuel ratio. For this reason, the oxygen storage amount of the exhaust purification catalyst gradually increases. On the other hand, since all the oxygen in the exhaust gas flowing into the exhaust purification catalyst is occluded in the exhaust purification catalyst, the exhaust gas flowing out from the exhaust purification catalyst contains almost no oxygen. For this reason, the air-fuel ratio of the exhaust gas detected by the downstream oxygen sensor is substantially the stoichiometric air-fuel ratio. Similarly, since all NOx in the exhaust gas flowing into the exhaust purification catalyst is reduced and purified by the exhaust purification catalyst, almost no NOx is contained in the exhaust gas flowing out from the exhaust purification catalyst.

When the oxygen storage amount of the exhaust purification catalyst gradually increases and approaches the maximum oxygen storage amount Cmax, part of the oxygen in the exhaust gas flowing into the exhaust purification catalyst is not stored in the exhaust purification catalyst, and as a result, the time t _{From 1} , oxygen is contained in the exhaust gas flowing out from the exhaust purification catalyst. For this reason, the air-fuel ratio of the exhaust gas detected by the downstream oxygen sensor becomes a lean air-fuel ratio. Thereafter, when the oxygen storage amount of the exhaust purification catalyst further increases, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst reaches a predetermined upper limit air-fuel ratio AFhighref (corresponding to a low threshold), and the target air-fuel ratio is rich. Switch to air-fuel ratio.

When the target air-fuel ratio is switched to the rich air-fuel ratio, the fuel injection amount in the internal combustion engine is increased in accordance with the switched target air-fuel ratio. Even if the fuel injection amount is increased in this way, since there is a certain distance from the internal combustion engine body to the exhaust purification catalyst, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is immediately changed to the rich air-fuel ratio. Without delay. Therefore, even if the target air-fuel ratio is switched to the rich air-fuel ratio at time t ₂ Note that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a time t ₃ will remain in the lean air-fuel ratio. Therefore, during the period from time t ₂ to t ₃ , the oxygen storage amount of the exhaust purification catalyst reaches the maximum oxygen storage amount Cmax or becomes a value near the maximum oxygen storage amount Cmax. As a result, from the exhaust purification catalyst, Oxygen and NOx will flow out. Thereafter, at time t ₃ , the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes a rich air-fuel ratio, and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst converges to the stoichiometric air-fuel ratio.

As described above, there is a delay from when the target air-fuel ratio is switched from the lean air-fuel ratio to the rich air-fuel ratio until the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes the rich air-fuel ratio. As a result, NOx had flowed out of the exhaust purification catalyst during the period from time t ₁ to t ₄ .

  An object of the present invention is to provide a control device for an internal combustion engine that suppresses the outflow of NOx in an internal combustion engine having an exhaust purification catalyst having an oxygen storage capacity.

  A first internal combustion engine control device according to the present invention is an internal combustion engine control device including an exhaust purification catalyst having oxygen storage capacity in an engine exhaust passage, and is disposed upstream of the exhaust purification catalyst and flows into the exhaust purification catalyst. An upstream air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas to be discharged, and a downstream air-fuel ratio sensor that is disposed downstream of the exhaust purification catalyst and detects the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst. The control device sets the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio, and increases the oxygen storage amount in the exhaust purification catalyst to a determination reference storage amount that is smaller than the maximum oxygen storage amount. 1 until the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made richer than the stoichiometric air-fuel ratio, and the output of the downstream air-fuel ratio sensor becomes a rich judgment air-fuel ratio less than the stoichiometric air-fuel ratio. The normal operation control that alternately performs the first rich control that continuously or intermittently continues the air-fuel ratio richer than the air-fuel ratio, and the fuel cut control that blocks the fuel supply to the combustion chamber are performed. . The control device continuously performs the second rich control and the second rich control so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made richer than the first rich control continuously after the end of the fuel cut control. After the control, the second lean control is performed so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is leaner than the first lean control, and the normal operation control is performed following the second lean control. To do.

  In the above invention, the second lean control can be terminated when the output value of the downstream air-fuel ratio sensor becomes leaner than the rich determination air-fuel ratio.

  In the above invention, the second lean control is performed such that the richer the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst during the execution period of the second lean control, the more the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst. Control can be included to set the value to lean.

  In the above invention, the amount of oxygen stored in the exhaust purification catalyst in the second lean control can be equal to the amount of oxygen stored in the exhaust purification catalyst in the first lean control.

  In the above-described invention, the normal operation control performed following the second lean control can be started from the first rich control.

  A second internal combustion engine control apparatus according to the present invention is an internal combustion engine control apparatus including an exhaust purification catalyst having oxygen storage capacity in an engine exhaust passage, and is disposed upstream of the exhaust purification catalyst and flows into the exhaust purification catalyst. An upstream sensor for detecting the air-fuel ratio of the exhaust gas to be detected, and a downstream sensor for detecting the air-fuel ratio of the exhaust gas that is disposed downstream of the exhaust purification catalyst and flows out of the exhaust purification catalyst. The upstream sensor is composed of an upstream air-fuel ratio sensor, and the downstream sensor is composed of a downstream oxygen sensor or a downstream air-fuel ratio sensor. The control device makes the air-fuel ratio of the exhaust gas flowing into the first lean control and exhaust purification catalyst to increase the oxygen storage amount in the exhaust purification catalyst by setting the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to be a lean air-fuel ratio. The normal operation control including the first rich control for setting the air-fuel ratio and the fuel cut control for cutting off the fuel supplied to the combustion chamber are performed. The lean determination air-fuel ratio is predetermined in a region where the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is leaner than the stoichiometric air-fuel ratio, and the region where the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is richer than the stoichiometric air-fuel ratio In FIG. 5, the rich determination air-fuel ratio is predetermined. The normal operation control includes control for switching to the first rich control when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst by the first lean control becomes a lean air-fuel ratio equal to or higher than the lean determination air-fuel ratio, And control for switching to the first lean control when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst by the rich control becomes a rich air-fuel ratio equal to or lower than the rich determination air-fuel ratio. After the end of the fuel cut control, the exhaust gas is exhausted after the second rich control in which the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made richer than the first rich control, and the second rich control. The second lean control is performed so that the air-fuel ratio of the exhaust gas flowing into the purification catalyst is leaner than the first lean control, and the normal operation control is performed following the second lean control. Has been.

  In the above-described invention, the normal operation control performed following the second lean control can be started from the first rich control.

  ADVANTAGE OF THE INVENTION According to this invention, the control apparatus of the internal combustion engine which suppresses the outflow of NOx can be provided.

1 is a schematic view of an internal combustion engine in an embodiment. It is a figure which shows the relationship between the oxygen storage amount of an exhaust purification catalyst, and the density | concentration of NOx and unburned gas in the exhaust gas which flows out from an exhaust purification catalyst. It is a schematic sectional drawing of an air fuel ratio sensor. It is the figure which showed the operation | movement of the air fuel ratio sensor roughly. It is a figure which shows the relationship between the exhaust air fuel ratio in an air fuel ratio sensor, and an output current. It is a figure which shows an example of the specific circuit which comprises a voltage application apparatus and a current detection apparatus. 4 is a time chart of an oxygen storage amount of an upstream side exhaust purification catalyst. 4 is a time chart of the oxygen storage amount of the downstream side exhaust purification catalyst. It is a functional block diagram of a control device. It is a flowchart which shows the control routine which calculates the air fuel ratio correction amount in normal driving | operation control. It is a time chart explaining the fuel cut control in embodiment and control after fuel cut control. It is a time chart explaining control after fuel cut control in a comparative example. It is a flowchart explaining the fuel cut control in embodiment, and the control after fuel cut control. It is the schematic explaining the other form of the 2nd lean control in embodiment. It is a time chart of control of conventional technology.

  With reference to FIGS. 1 to 14, a control apparatus for an internal combustion engine in the embodiment will be described. The internal combustion engine in the present embodiment includes an engine body that outputs rotational force, and an exhaust treatment device that purifies exhaust gas flowing out from the combustion chamber.

<Description of the internal combustion engine as a whole>
FIG. 1 schematically shows an internal combustion engine in the present embodiment. The internal combustion engine includes an engine body 1, and the engine body 1 includes a cylinder block 2 and a cylinder head 4 fixed to the cylinder block 2. A hole is formed in the cylinder block 2, and a piston 3 that reciprocates inside the hole is disposed. The combustion chamber 5 is configured by a space surrounded by the hole of the cylinder block 2, the piston 3, and the cylinder head 4. An intake port 7 and an exhaust port 9 are formed in the cylinder head 4. The intake valve 6 opens and closes the intake port 7, and the exhaust valve 8 is formed to open and close the exhaust port 9.

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

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

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

  The control device for the internal combustion engine of the present embodiment includes an electronic control unit (ECU) 31. The electronic control unit 31 in the present embodiment is composed of a digital computer, and includes a RAM (random access memory) 33, a ROM (read only memory) 34, and a CPU (microprocessor) 35 which are connected to each other via a bidirectional bus 32. Input port 36 and output port 37.

  An air flow meter 39 for detecting the flow rate of air flowing through the intake pipe 15 is disposed in the intake pipe 15, and the output of the air flow meter 39 is input to the input port 36 via the corresponding AD converter 38.

  Further, an upstream air-fuel ratio sensor as an upstream sensor that detects an air-fuel ratio of exhaust gas flowing through the exhaust manifold 19 (that is, exhaust gas flowing into the upstream exhaust purification catalyst 20) is provided at a collecting portion of the exhaust manifold 19. 40 is arranged. In addition, the air-fuel ratio of the exhaust gas flowing in the exhaust pipe 22 (that is, the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 and flowing into the downstream side exhaust purification catalyst 24) is detected in the exhaust pipe 22. A downstream air-fuel ratio sensor 41 is disposed as a downstream sensor. The outputs of these air-fuel ratio sensors are also input to the input port 36 via the corresponding AD converters 38. The configuration of these air-fuel ratio sensors will be described later.

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

<Description of exhaust purification catalyst>
The internal combustion engine exhaust treatment apparatus of the present embodiment includes a plurality of exhaust purification catalysts. The exhaust treatment apparatus of the present embodiment includes an upstream side exhaust purification catalyst 20 and a downstream side exhaust purification catalyst 24 disposed downstream of the exhaust purification catalyst 20. The upstream side exhaust purification catalyst 20 and the downstream side exhaust purification catalyst 24 have the same configuration. Hereinafter, only the upstream side exhaust purification catalyst 20 will be described, but the downstream side exhaust purification catalyst 24 has the same configuration and operation.

The upstream side exhaust purification catalyst 20 is a three-way catalyst having oxygen storage capacity. Specifically, the upstream side exhaust purification catalyst 20 has a catalytic noble metal (for example, platinum (Pt), palladium (Pd), and rhodium (Rh)) and oxygen storage capacity on a ceramic support. A substance (for example, ceria (CeO ₂ )) is supported. When the exhaust gas purification catalyst 20 on the upstream side reaches a predetermined activation temperature, in addition to the catalytic action of simultaneously purifying unburned gas (HC, CO, etc.) and nitrogen oxides (NOx), the exhaust gas purification catalyst 20 exhibits oxygen storage capacity. .

  According to the oxygen storage capacity of the upstream side exhaust purification catalyst 20, the upstream side exhaust purification catalyst 20 is such that the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is leaner than the stoichiometric air-fuel ratio (lean air-fuel ratio). ) Occludes oxygen in the exhaust gas. On the other hand, the upstream side exhaust purification catalyst 20 releases oxygen stored in the upstream side exhaust purification catalyst 20 when the air-fuel ratio of the inflowing exhaust gas is richer than the stoichiometric air-fuel ratio (rich air-fuel ratio). . Note that the “air-fuel ratio of exhaust gas” means the ratio of the mass of fuel to the mass of air supplied until the exhaust gas is generated. Normally, combustion is performed when the exhaust gas is generated. It means the ratio of the mass of fuel to the mass of air supplied into the chamber 5. In the present specification, the air-fuel ratio of the exhaust gas may be referred to as “exhaust air-fuel ratio”. Next, the relationship between the oxygen storage amount and the purification capacity of the exhaust purification catalyst in the present embodiment will be described.

  FIG. 2 shows the relationship between the oxygen storage amount of the exhaust purification catalyst and the concentrations of NOx and unburned gas (HC, CO, etc.) in the exhaust gas flowing out from the exhaust purification catalyst. FIG. 2A shows the relationship between the oxygen storage amount and the NOx concentration in the exhaust gas flowing out from the exhaust purification catalyst when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a lean air-fuel ratio. On the other hand, FIG. 2B shows the oxygen storage amount and the concentration of unburned gas in the exhaust gas flowing out from the exhaust purification catalyst when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a rich air-fuel ratio. Show the relationship.

  As can be seen from FIG. 2A, when the oxygen storage amount of the exhaust purification catalyst is small, there is a margin up to the maximum oxygen storage amount. For this reason, even if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a lean air-fuel ratio (that is, the exhaust gas contains NOx and oxygen), oxygen in the exhaust gas is occluded in the exhaust purification catalyst. As a result, NOx is also reduced and purified. As a result, the exhaust gas flowing out from the exhaust purification catalyst contains almost no NOx.

  However, when the amount of oxygen stored in the exhaust purification catalyst increases, if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a lean air-fuel ratio, it becomes difficult for the exhaust purification catalyst to store oxygen in the exhaust gas. Thus, NOx in the exhaust gas is also difficult to be reduced and purified. Therefore, as can be seen from FIG. 2A, when the oxygen storage amount increases beyond the upper limit storage amount Cuplim in the vicinity of the maximum oxygen storage amount Cmax, the NOx concentration in the exhaust gas flowing out from the exhaust purification catalyst increases rapidly. .

  On the other hand, when the oxygen storage amount of the exhaust purification catalyst is large, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a rich air-fuel ratio (that is, the exhaust gas includes unburned gas such as HC and CO). Oxygen stored in the exhaust purification catalyst is released. For this reason, the unburned gas in the exhaust gas flowing into the exhaust purification catalyst is oxidized and purified. As a result, as can be seen from FIG. 2B, the exhaust gas flowing out from the exhaust purification catalyst contains almost no unburned gas.

  However, when the oxygen storage amount of the exhaust purification catalyst decreases and becomes close to 0, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a rich air-fuel ratio, less oxygen is released from the exhaust purification catalyst, As a result, the unburned gas in the exhaust gas is also hardly oxidized and purified. Therefore, as can be seen from FIG. 2B, when the oxygen storage amount decreases beyond a certain lower limit storage amount Clowlim, the concentration of unburned gas in the exhaust gas flowing out from the exhaust purification catalyst increases rapidly.

  As described above, according to the exhaust purification catalysts 20 and 24 used in the present embodiment, the NOx in the exhaust gas and the unexposed amount in accordance with the air-fuel ratio and oxygen storage amount of the exhaust gas flowing into the exhaust purification catalyst 20 and 24 are determined. The purification characteristics of the fuel gas change. The exhaust purification catalysts 20 and 24 may be different from the three-way catalyst as long as they have a catalytic action and an oxygen storage capacity.

<Configuration of air-fuel ratio sensor>
Next, the structures of the upstream air-fuel ratio sensor 40 and the downstream air-fuel ratio sensor 41 in the present embodiment will be described with reference to FIG. FIG. 3 is a schematic cross-sectional view of the air-fuel ratio sensor. The air-fuel ratio sensor in the present embodiment is a one-cell type air-fuel ratio sensor having one cell composed of a solid electrolyte layer and a pair of electrodes. The air-fuel ratio sensor is not limited to this form, and another form of sensor in which the output continuously changes according to the air-fuel ratio of the exhaust gas may be adopted. For example, a 2-cell type air-fuel ratio sensor may be employed.

  The air-fuel ratio sensor in the present embodiment includes a solid electrolyte layer 51, an exhaust side electrode (first electrode) 52 disposed on one side surface of the solid electrolyte layer 51, and the other side surface of the solid electrolyte layer 51. Arranged atmosphere-side electrode (second electrode) 53, diffusion-controlling layer 54 that performs diffusion-controlling the passing exhaust gas, protective layer 55 that protects diffusion-controlling layer 54, and heater unit that heats the air-fuel ratio sensor 56.

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

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

  The heater unit 56 is provided with a plurality of heaters 59, and these heaters 59 can control the temperature of the air-fuel ratio sensor, particularly the temperature of the solid electrolyte layer 51. The heater unit 56 has a heat generation capacity sufficient to heat the solid electrolyte layer 51 until it is activated.

The 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 a voltage application device 60 mounted on the electronic control unit 31. In addition, the electronic control unit 31 detects a current that flows between the exhaust-side electrode 52 and the atmosphere-side electrode 53 through the solid electrolyte layer 51 when the sensor application voltage Vr is applied by the voltage application device 60. A detection device 61 is provided. The current detected by the current detector 61 is the output current of the air-fuel ratio sensor.

<Operation of air-fuel ratio sensor>
Next, a basic concept of the operation of the air-fuel ratio sensor configured as described above will be described with reference to FIG. FIG. 4 is a diagram schematically showing the operation of the air-fuel ratio sensor. In use, the air-fuel ratio sensor is arranged so that the outer peripheral surfaces of the protective layer 55 and the diffusion-controlling layer 54 are exposed to the exhaust gas. In addition, air is introduced into the reference gas chamber 58 of the air-fuel ratio sensor.

  As described above, the solid electrolyte layer 51 is formed of a sintered body of an oxygen ion conductive oxide. Therefore, when a difference in oxygen concentration occurs between both side surfaces of the solid electrolyte layer 51 in a state activated by high temperature, an electromotive force E that attempts to move oxygen ions from the high concentration side surface to the low concentration side surface. Has a property (oxygen battery characteristics).

  Conversely, when a potential difference is applied between both side surfaces of the solid electrolyte layer 51, oxygen ions move so that an oxygen concentration ratio is generated between both side surfaces of the solid electrolyte layer according to the potential difference. Characteristics (oxygen pump characteristics). Specifically, when a potential difference is applied between both side surfaces, the oxygen concentration on the side surface provided with positive polarity is a ratio corresponding to the potential difference with respect to the oxygen concentration on the side surface provided with negative polarity. The movement of oxygen ions is caused to increase. Further, as shown in FIGS. 3 and 4, in the air-fuel ratio sensor, between the exhaust side electrode 52 and the atmosphere side electrode 53 so that the atmosphere side electrode 53 is positive and the exhaust side electrode 52 is negative. A constant sensor applied voltage Vr is applied to the. In the present embodiment, the sensor applied voltage Vr in the air-fuel ratio sensor is the same voltage.

  When the exhaust air-fuel ratio around the air-fuel ratio sensor is leaner than the stoichiometric air-fuel ratio, the ratio of oxygen concentration between both side surfaces of the solid electrolyte layer 51 is not so large. For this reason, if the sensor applied voltage Vr is set to an appropriate value, the actual oxygen concentration ratio becomes smaller between the both side surfaces of the solid electrolyte layer 51 than the oxygen concentration ratio corresponding to the sensor applied voltage Vr. Therefore, as shown in FIG. 4A, the oxygen concentration ratio between the both side surfaces of the solid electrolyte layer 51 increases from the exhaust side electrode 52 to the atmosphere so as to increase toward the oxygen concentration ratio corresponding to the sensor applied voltage Vr. Oxygen ions move toward the side electrode 53. As a result, a current flows from the positive electrode of the voltage application device 60 that applies the sensor application voltage Vr to the negative electrode of the voltage application device 60 via the atmosphere side electrode 53, the solid electrolyte layer 51, and the exhaust side electrode 52.

  The magnitude of the current (output current) Ir flowing at this time is the amount of oxygen flowing into the measured gas chamber 57 from the exhaust gas through the diffusion rate controlling layer 54 if the sensor applied voltage Vr is set to an appropriate value. Is proportional to Therefore, by detecting the magnitude of the current Ir by the current detector 61, it is possible to know the oxygen concentration and thus the air-fuel ratio in the lean region.

  On the other hand, when the exhaust air-fuel ratio around the air-fuel ratio sensor is richer than the stoichiometric air-fuel ratio, unburned gas flows from the exhaust gas through the diffusion-controlled layer 54 into the measured gas chamber 57, and therefore the exhaust-side electrode 52 Any oxygen present above is removed by reacting with unburned gas. For this reason, the oxygen concentration in the measured gas chamber 57 becomes extremely low, and as a result, the ratio of the oxygen concentration between both side surfaces of the solid electrolyte layer 51 becomes large. For this reason, if the sensor applied voltage Vr is set to an appropriate value, the actual oxygen concentration ratio between the both side surfaces of the solid electrolyte layer 51 becomes larger than the oxygen concentration ratio corresponding to the sensor applied voltage Vr. For this reason, as shown in FIG. 4B, the exhaust gas is exhausted from the atmosphere side electrode 53 so that the oxygen concentration ratio between the both side surfaces of the solid electrolyte layer 51 decreases toward the oxygen concentration ratio corresponding to the sensor applied voltage Vr. Oxygen ions move toward the side electrode 52. As a result, a current flows from the atmosphere side electrode 53 to the exhaust side electrode 52 through the voltage application device 60 that applies the sensor application voltage Vr.

  The current flowing at this time is the output current Ir. The magnitude of the output current is determined by the flow rate of oxygen ions that can be moved from the atmosphere-side electrode 53 to the exhaust-side electrode 52 in the solid electrolyte layer 51 if the sensor applied voltage Vr is set to an appropriate value. The oxygen ions react (combust) on the exhaust-side electrode 52 with the unburned gas that flows into the measured gas chamber 57 from the exhaust gas through the diffusion-controlling layer 54 by diffusion. Therefore, the moving flow rate of oxygen ions corresponds to the concentration of unburned gas in the exhaust gas flowing into the measured gas chamber 57. Therefore, by detecting the magnitude of the current Ir by the current detection device 61, it is possible to know the unburned gas concentration and thus the air-fuel ratio in the rich region.

  When the exhaust air-fuel ratio around the air-fuel ratio sensor is the stoichiometric air-fuel ratio, the amount of oxygen and unburned gas flowing into the measured gas chamber 57 is the chemical equivalent ratio. For this reason, both of them are completely combusted by the catalytic action of the exhaust side electrode 52, and the concentration of oxygen and unburned gas in the measured gas chamber 57 does not change. As a result, the oxygen concentration ratio between the both side surfaces of the solid electrolyte layer 51 is not changed and is maintained as the oxygen concentration ratio corresponding to the sensor applied voltage Vr. For this reason, as shown in FIG. 4C, oxygen ions do not move due to the oxygen pump characteristics, and as a result, no current flows through the circuit.

  The air-fuel ratio sensor configured in this way has the output characteristics shown in FIG. That is, in the air-fuel ratio sensor, the output current Ir of the air-fuel ratio sensor increases as the exhaust air-fuel ratio increases (that is, the leaner the exhaust air-fuel ratio). In addition, the air-fuel ratio sensor is configured such that the output current Ir becomes zero when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio.

<Circuit of voltage application device and current detection device>
FIG. 6 shows an example of a specific circuit constituting the voltage application device 60 and the current detection device 61. In the illustrated example, E is an electromotive force generated by oxygen battery characteristics, Ri is an internal resistance of the solid electrolyte layer 51, and Vs is a potential difference between the exhaust side electrode 52 and the atmosphere side electrode 53.

  As can be seen from FIG. 6, the voltage application device 60 basically performs negative feedback control so that the electromotive force E generated by the oxygen battery characteristics matches the sensor applied voltage Vr. In other words, when the potential difference Vs between the exhaust-side electrode 52 and the atmosphere-side electrode 53 changes due to the change in the oxygen concentration ratio between the both side surfaces of the solid electrolyte layer 51, the voltage application device 60 also has this potential difference Vs. Negative feedback control is performed so that the sensor applied voltage Vr is obtained.

  Therefore, when the exhaust air-fuel ratio is the stoichiometric air-fuel ratio and the change in the oxygen concentration ratio does not occur between the both side surfaces of the solid electrolyte layer 51, the oxygen concentration ratio between the both side surfaces of the solid electrolyte layer 51 is The oxygen concentration ratio corresponds to the voltage Vr. In this case, the electromotive force E coincides with the sensor applied voltage Vr, and the potential difference Vs between the exhaust side electrode 52 and the atmosphere side electrode 53 is also the sensor applied voltage Vr. As a result, the current Ir does not flow.

  On the other hand, when the exhaust air-fuel ratio is different from the stoichiometric air-fuel ratio and the oxygen concentration ratio changes between both side surfaces of the solid electrolyte layer 51, the oxygen concentration between both side surfaces of the solid electrolyte layer 51 The ratio is not the oxygen concentration ratio corresponding to the sensor applied voltage Vr. In this case, the electromotive force E has a value different from the sensor applied voltage Vr. For this reason, between the exhaust side electrode 52 and the atmosphere side electrode 53 in order to move oxygen ions between both side surfaces of the solid electrolyte layer 51 so that the electromotive force E matches the sensor applied voltage Vr by negative feedback control. Is given a potential difference Vs. And current Ir flows with the movement of oxygen ions at this time. As a result, the electromotive force E converges on the sensor applied voltage Vr, and when the electromotive force E converges on the sensor applied voltage Vr, the potential difference Vs eventually converges on the sensor applied voltage Vr.

  Therefore, it can be said that the voltage applying device 60 substantially applies the sensor applied voltage Vr between the exhaust side electrode 52 and the atmosphere side electrode 53. Note that the electric circuit of the voltage application device 60 is not necessarily as shown in FIG. 6, and the sensor application voltage Vr can be substantially applied between the exhaust side electrode 52 and the atmosphere side electrode 53. As long as it is possible, the apparatus of any aspect may be sufficient.

The current detector 61 is actually a current rather than detecting, and calculates the current from the voltage E ₀ by detecting the voltage E _0. Here, E ₀ can be expressed as the following formula (1).

E ₀ = Vr + V ₀ + IrR (1)

Here, V ₀ is an offset voltage (a voltage applied so that E ₀ does not become a negative value, for example, 3 V), and R is a value of the resistance shown in FIG.

In the equation (1), the sensor applied voltage Vr, the offset voltage V ₀ and the resistance value R are constant, so that the voltage E ₀ changes according to the current Ir. Therefore, if the voltage E ₀ is detected, the current Ir can be calculated from the voltage E ₀ .

  Therefore, it can be said that the current detection device 61 substantially detects the current Ir flowing between the exhaust side electrode 52 and the atmosphere side electrode 53. The electric circuit of the current detection device 61 is not necessarily as shown in FIG. 6, and any mode can be used as long as the current Ir flowing between the exhaust side electrode 52 and the atmosphere side electrode 53 can be detected. The apparatus may be used.

<Outline of normal operation control>
Next, an outline of air-fuel ratio control in the control apparatus for an internal combustion engine of the present embodiment will be described. First, normal operation control for determining the fuel injection amount so that the gas air-fuel ratio matches the target air-fuel ratio in the internal combustion engine will be described. The control device for an internal combustion engine includes inflow air-fuel ratio control means for adjusting the air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst. The inflow air-fuel ratio control means of the present embodiment adjusts the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst by adjusting the amount of fuel supplied to the combustion chamber. The inflow air-fuel ratio control means is not limited to this form, and any device capable of adjusting the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst can be employed. For example, the inflow air-fuel ratio control means may include an EGR (Exhaust Gas Recirculation) device that recirculates exhaust gas to the engine intake passage, and may be formed so as to adjust the amount of recirculation gas.

  In the internal combustion engine of the present embodiment, the output current of the upstream air-fuel ratio sensor 40 (that is, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst) Irup is based on the output current Irup of the upstream air-fuel ratio sensor 40. Feedback control is performed so that the value corresponds to the fuel ratio.

  The target air-fuel ratio is set based on the output current of the downstream air-fuel ratio sensor 41. Specifically, when the output current Irdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination reference value Iref, the target air-fuel ratio is set to the lean set air-fuel ratio and is maintained at that air-fuel ratio. Here, as the rich determination reference value Iref, a value corresponding to a predetermined rich determination air-fuel ratio (for example, 14.55) that is slightly richer than the theoretical air-fuel ratio can be adopted. The lean set air-fuel ratio is a predetermined air-fuel ratio that is somewhat leaner than the stoichiometric air-fuel ratio, and is, for example, 14.65 to 20, preferably 14.65 to 18, and more preferably 14.65. ˜16.

  The control device for an internal combustion engine according to the present embodiment includes an oxygen storage amount acquisition means for acquiring the amount of oxygen stored in the exhaust purification catalyst. When the target air-fuel ratio is a lean set air-fuel ratio, the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 is estimated. In the present embodiment, the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 is estimated even when the target air-fuel ratio is the rich set air-fuel ratio. The oxygen storage amount OSAsc is estimated by estimating the intake air amount into the combustion chamber 5 calculated based on the output current Irup of the upstream air-fuel ratio sensor 40, the air flow meter 39, and the like, and the fuel from the fuel injection valve 11. This is performed based on the injection amount. If the estimated value of the oxygen storage amount OSAsc becomes equal to or greater than a predetermined determination reference storage amount Cref during the period in which the control is performed so that the target air-fuel ratio is set to the lean set air-fuel ratio, the lean set air-fuel ratio until then is reached. The target air-fuel ratio that was the fuel ratio is made the rich set air-fuel ratio and maintained at that air-fuel ratio. In the present embodiment, a weak rich set air-fuel ratio is adopted. The weakly rich set air-fuel ratio is slightly richer than the stoichiometric air-fuel ratio, and is, for example, about 13.5 to 14.58, preferably 14 to 14.57, and more preferably about 14.3 to 14.55. . Thereafter, when the output current Irdwn of the downstream air-fuel ratio sensor 41 again becomes equal to or less than the rich determination reference value Iref, the target air-fuel ratio is again set to the lean set air-fuel ratio, and thereafter the same operation is repeated.

  As described above, in the present embodiment, the target air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is alternately set to the lean set air-fuel ratio and the weak rich set air-fuel ratio. In particular, in the present embodiment, the difference between the lean set air-fuel ratio and the stoichiometric air-fuel ratio is larger than the difference between the weak rich set air-fuel ratio and the stoichiometric air-fuel ratio. Therefore, in the present embodiment, the target air-fuel ratio is alternately set to a short-term lean set air-fuel ratio and a long-term weak rich set air-fuel ratio.

  The difference between the lean set air-fuel ratio and the stoichiometric air-fuel ratio may be substantially the same as the difference between the rich set air-fuel ratio and the stoichiometric air-fuel ratio. That is, the depth of the rich set air-fuel ratio and the depth of the lean set air-fuel ratio may be substantially equal. In such a case, the lean set air-fuel ratio period and the rich set air-fuel ratio period have substantially the same length.

<Description of control using time chart>
With reference to FIG. 7, the operation as described above will be specifically described. FIG. 7 shows the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20, the output current Irdwn of the downstream side air-fuel ratio sensor 41, and the air-fuel ratio correction amount when air-fuel ratio control is performed in the control apparatus for an internal combustion engine of the present invention. 4 is a time chart of AFC, output current Irup of an upstream air-fuel ratio sensor 40, and NOx concentration in exhaust gas flowing out from an upstream side exhaust purification catalyst 20.

  The output current Irup of the upstream air-fuel ratio sensor 40 becomes zero when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas is rich. It becomes a negative value when it is a fuel ratio, and becomes a positive value when the air-fuel ratio of the exhaust gas is a lean air-fuel ratio. Further, when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio or a lean air-fuel ratio, the output current Irup of the upstream side air-fuel ratio sensor 40 increases as the difference from the stoichiometric air-fuel ratio increases. The absolute value increases. The output current Irdwn of the downstream air-fuel ratio sensor 41 also changes similarly to the output current Irup of the upstream air-fuel ratio sensor 40 according to the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20. The air-fuel ratio correction amount AFC is a correction amount related to the target air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20. When the air-fuel ratio correction amount AFC is 0, the target air-fuel ratio is the stoichiometric air-fuel ratio. When the air-fuel ratio correction amount AFC is a positive value, the target air-fuel ratio is a lean air-fuel ratio, and the air-fuel ratio correction amount AFC is a negative value. In some cases, the target air-fuel ratio becomes a rich air-fuel ratio.

In the illustrated example, before the time t ₁ , the air-fuel ratio correction amount AFC is set to the weak rich set correction amount AFCrich. The weak rich set correction amount AFCrich is a value corresponding to the weak rich set air-fuel ratio, and is a value smaller than zero. Accordingly, the target air-fuel ratio is set to a rich air-fuel ratio, and accordingly, the output current Irup of the upstream air-fuel ratio sensor 40 becomes a negative value. Since the exhaust gas flowing into the upstream side exhaust purification catalyst 20 contains unburned gas, the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 gradually decreases. However, since the unburned gas contained in the exhaust gas is purified by the upstream side exhaust purification catalyst 20, the output current Irdwn of the downstream side air-fuel ratio sensor becomes substantially 0 (corresponding to the theoretical air-fuel ratio). At this time, since the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio, the NOx emission amount from the upstream side exhaust purification catalyst 20 is suppressed.

When the oxygen storage amount OSAsc of the exhaust gas purification catalyst 20 on the upstream side gradually decreases, the oxygen storage amount OSAsc decreases beyond the lower limit storage amount (see Crowlim in FIG. 2) at time t ₁ . When the oxygen storage amount OSAsc decreases below the lower limit storage amount, a part of the unburned gas that has flowed into the upstream side exhaust purification catalyst 20 flows out without being purified by the upstream side exhaust purification catalyst 20. Therefore, after time t ₁ , the output current Irdwn of the downstream air-fuel ratio sensor 41 gradually decreases as the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 decreases. Also at this time, since the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio, the NOx emission amount from the upstream side exhaust purification catalyst 20 is suppressed.

Thereafter, at time t ₂ , the output current Irdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination reference value Iref corresponding to the rich determination air-fuel ratio. In the present embodiment, when the output current Irdwn of the downstream side air-fuel ratio sensor 41 reaches the rich determination reference value Iref, the air-fuel ratio correction amount AFC is set to suppress the decrease in the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20. The lean setting correction amount AFClean is switched to. The lean set correction amount AFClean is a value corresponding to the lean set air-fuel ratio, and is a value larger than zero. Therefore, the target air-fuel ratio is a lean air-fuel ratio.

  In the present embodiment, after the output current Irdwn of the downstream air-fuel ratio sensor 41 reaches the rich judgment reference value Iref, that is, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is rich judgment air. After reaching the fuel ratio, the air-fuel ratio correction amount AFC is switched. This is because even if the oxygen storage amount of the upstream side exhaust purification catalyst 20 is sufficient, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 may slightly deviate from the stoichiometric air-fuel ratio. Because. That is, even if the output current Irdwn is slightly deviated from zero (corresponding to the stoichiometric air-fuel ratio), if it is determined that the oxygen storage amount has decreased beyond the lower limit storage amount, sufficient oxygen Even if there is an occlusion amount, it may be determined that the oxygen occlusion amount has decreased beyond the lower limit occlusion amount. Therefore, in the present embodiment, it is determined that the oxygen storage amount decreases beyond the lower limit storage amount only after the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 reaches the rich determination air-fuel ratio. Yes. In other words, the rich determination air-fuel ratio is such that the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 does not reach when the oxygen storage amount of the upstream side exhaust purification catalyst 20 is sufficient. The air-fuel ratio is assumed.

Even when the target air-fuel ratio is switched to the lean air-fuel ratio at time t ₂ , the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 does not immediately become the lean air-fuel ratio, and some delay occurs. . As a result, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes from the rich air-fuel ratio to the lean air-fuel ratio at time t ₃ . At times t _{2 to} t ₃ , since the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio, this exhaust gas contains unburned gas. Become. However, the NOx emission amount from the upstream side exhaust purification catalyst 20 is suppressed.

When the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes to the lean air-fuel ratio at time t ₃ , the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 increases. As a result, the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 changes to the stoichiometric air-fuel ratio, and the output current Irdwn of the downstream side air-fuel ratio sensor 41 also converges to zero. At this time, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a lean air-fuel ratio. However, since the oxygen storage capacity of the upstream side exhaust purification catalyst 20 has a sufficient margin, it flows in. Oxygen in the exhaust gas is stored in the upstream side exhaust purification catalyst 20, and NOx is reduced and purified. For this reason, the NOx emission amount from the upstream side exhaust purification catalyst 20 is suppressed.

Thereafter, when the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 increases, the oxygen storage amount OSAsc reaches the determination reference storage amount Cref at time t ₄ . The determination reference storage amount Cref is set to be equal to or less than the maximum oxygen storage amount Cmax. In the present embodiment, when the oxygen storage amount OSAsc reaches the determination reference storage amount Cref, the air-fuel ratio correction amount AFC is set to the weak rich set correction amount AFCrich (0) in order to stop storing oxygen in the upstream side exhaust purification catalyst 20. Smaller value). Therefore, the target air-fuel ratio is set to a rich air-fuel ratio.

However, as described above, there is a delay from when the target air-fuel ratio is switched to when the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 actually changes. For this reason, even if switching is performed at time t ₄ , the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 changes from the lean air-fuel ratio to the rich air-fuel ratio at time t ₅ when a certain amount of time has elapsed. To do. From time t _{4 to} t ₅ , the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a lean air-fuel ratio, so the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 increases.

However, the criterion storage amount Cref is the maximum oxygen storage amount Cmax and upper storage amount since it is set sufficiently lower than (see Cuplim in FIG. 2), the oxygen storage amount OSAsc even at time t ₅ is the maximum oxygen storage amount Cmax And the upper limit occlusion amount is not reached. In other words, the judgment reference storage amount Cref is not changed even if a delay occurs until the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 actually changes after switching the target air-fuel ratio. Is made sufficiently small so as not to reach the maximum oxygen storage amount Cmax or the upper limit storage amount. For example, the criterion storage amount Cref is set to 3/4 or less, preferably 1/2 or less, more preferably 1/5 or less of the maximum oxygen storage amount Cmax. Therefore, even at time t ₄ ~t _5, NOx emissions from the exhaust purification catalyst 20 on the upstream side is suppressed.

At time t ₅ or later, the air-fuel ratio correction amount AFC there is a weak rich set correction amount AFCrich. Accordingly, the target air-fuel ratio is set to a rich air-fuel ratio, and accordingly, the output current Irup of the upstream air-fuel ratio sensor 40 becomes a negative value. Since the exhaust gas flowing into the upstream side of the exhaust purification catalyst 20 will include unburned gas, the oxygen storage amount OSAsc the upstream side of the exhaust purification catalyst 20 is gradually decreased, at time t ₆ Similarly to the time t ₁ , the oxygen storage amount OSAsc decreases beyond the lower limit storage amount. Also at this time, since the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio, the NOx emission amount from the upstream side exhaust purification catalyst 20 is suppressed.

Next, at time t ₇ , similarly to time t ₂ , the output current Irdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination reference value Iref corresponding to the rich determination air-fuel ratio. As a result, the air-fuel ratio correction amount AFC is switched to the lean set correction amount AFClean that corresponds to the lean set air-fuel ratio. Thereafter, the cycle from the time t _{1 to} t ₆ described above is repeated.

  The control of the air-fuel ratio correction amount AFC is performed by the electronic control unit 31. Therefore, when the air-fuel ratio of the exhaust gas detected by the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio, the electronic control unit 31 determines the oxygen storage amount OSAsc of the upstream exhaust purification catalyst 20 as the determination criterion. The oxygen storage amount increasing means for continuously setting the target air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 to the lean set air-fuel ratio until the storage amount Cref reaches the oxygen storage amount, and the oxygen storage amount of the upstream side exhaust purification catalyst 20 When the amount OSAsc becomes equal to or greater than the determination reference storage amount Cref, the target air-fuel ratio is continuously set to the slightly rich set air-fuel ratio so that the oxygen storage amount OSAsc decreases toward zero without reaching the maximum oxygen storage amount Cmax. It can be said that it comprises oxygen storage amount reducing means.

  As can be seen from the above description, according to the above embodiment, the NOx emission amount from the upstream side exhaust purification catalyst 20 can always be suppressed. That is, as long as the above-described control is performed, the NOx emission amount from the upstream side exhaust purification catalyst 20 can be basically reduced.

In general, when the oxygen storage amount OSAsc is estimated based on the output current Irup of the upstream air-fuel ratio sensor 40, the estimated value of the intake air amount, and the like, an error may occur. Also in the present embodiment, since the oxygen storage amount OSAsc is estimated over time t _{3 to} t ₄ , the estimated value of the oxygen storage amount OSAsc includes some errors. However, even if such an error is included, if the reference storage amount Cref is set sufficiently lower than the maximum oxygen storage amount Cmax or the upper limit storage amount, the actual oxygen storage amount OSAsc will be the maximum oxygen storage amount. The amount Cmax and the upper limit storage amount are hardly reached. Therefore, from this point of view as well, the NOx emission amount from the upstream side exhaust purification catalyst 20 can be suppressed.

  Further, when the oxygen storage amount of the exhaust purification catalyst is kept constant, the oxygen storage capacity of the exhaust purification catalyst is lowered. On the other hand, according to the present embodiment, since the oxygen storage amount OSAsc constantly fluctuates up and down, it is possible to suppress a decrease in the oxygen storage capacity.

In the above embodiment, the air-fuel ratio correction amount AFC is maintained at the lean set correction amount AFClean from time t _{2 to} t ₄ . However, in such a period, the air-fuel ratio correction amount AFC does not necessarily have to be kept constant, and may be set so as to fluctuate, for example, gradually decrease. Similarly, at time t ₄ ~t _7, the air-fuel ratio correction amount AFC is maintained at a slightly rich set correction amount AFCrich. However, in such a period, the air-fuel ratio correction amount AFC does not necessarily have to be kept constant, and may be set so as to fluctuate, for example, gradually decrease.

However, even in this case, the air-fuel ratio correction amount AFC is at time t ₂ ~t _4, the difference between the average value and the stoichiometric air-fuel ratio the target air-fuel ratio in the period, the target air at time t ₄ ~t ₇ It can be set to be larger than the difference between the average value of the fuel ratio and the stoichiometric air-fuel ratio.

  In the above embodiment, the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 is estimated based on the output current Irup of the upstream side air-fuel ratio sensor 40 and the estimated value of the intake air amount into the combustion chamber 5. ing. However, the oxygen storage amount OSAsc may be calculated based on other parameters in addition to these parameters, or may be estimated based on parameters different from these parameters. In the above embodiment, when the estimated value of the oxygen storage amount OSAsc is equal to or greater than the determination reference storage amount Cref, the target air-fuel ratio is switched from the lean set air-fuel ratio to the slightly rich set air-fuel ratio. However, the timing at which the target air-fuel ratio is switched from the lean set air-fuel ratio to the weakly rich set air-fuel ratio is determined by other parameters such as the engine operation time after the target air-fuel ratio is switched from the weak rich set air-fuel ratio to the lean set air-fuel ratio. May be used as a reference. However, even in this case, the target air-fuel ratio is changed from the lean set air-fuel ratio to the slightly rich set air-fuel ratio while the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 is estimated to be smaller than the maximum oxygen storage amount. It is necessary to switch to

<Description of control using downstream catalyst>
Further, in the present embodiment, in addition to the upstream side exhaust purification catalyst 20, a downstream side exhaust purification catalyst 24 is also provided. The oxygen storage amount OSAufc of the downstream side exhaust purification catalyst 24 is set to a value in the vicinity of the maximum oxygen storage amount Cmax by fuel cut (F / C) control performed every certain period. Therefore, even if exhaust gas containing unburned gas flows out from the upstream side exhaust purification catalyst 20, these unburned gas is oxidized and purified by the downstream side exhaust purification catalyst 24.

  Here, the fuel cut control is a control for stopping the fuel injection from the fuel injection valve 11 even when the crankshaft or the piston 3 is moving, for example, when the vehicle equipped with the internal combustion engine is decelerated. is there. When this control is performed, a large amount of air flows into the exhaust purification catalyst 20 and the exhaust purification catalyst 24.

  Hereinafter, the transition of the oxygen storage amount OSAufc in the downstream side exhaust purification catalyst 24 will be described with reference to FIG. FIG. 8 is a diagram similar to FIG. 7, and instead of the transition of the NOx concentration in FIG. 7, the oxygen storage amount OSAufc of the downstream exhaust purification catalyst 24 and the exhaust gas flowing out from the downstream exhaust purification catalyst 24 are shown. It shows the transition of the concentration of unburned gas (HC, CO, etc.). Moreover, in the example shown in FIG. 8, the same control as the example shown in FIG. 7 is performed.

In the example shown in FIG. 8, fuel cut control is performed before time t ₁ . Therefore, before time t ₁ , the oxygen storage amount OSAufc of the downstream side exhaust purification catalyst 24 is a value in the vicinity of the maximum oxygen storage amount Cmax. Further, before the time t ₁ , the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is kept substantially at the stoichiometric air-fuel ratio. For this reason, the oxygen storage amount OSAufc of the downstream side exhaust purification catalyst 24 is kept constant.

Thereafter, at times t _{1 to} t ₄ , the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 is a rich air-fuel ratio. For this reason, exhaust gas containing unburned gas flows into the exhaust purification catalyst 24 on the downstream side.

As described above, since a large amount of oxygen is stored in the downstream exhaust purification catalyst 24, if unburned gas is contained in the exhaust gas flowing into the downstream exhaust purification catalyst 24, it is stored. Unburned gas is oxidized and purified by the oxygen. Along with this, the oxygen storage amount OSAufc of the downstream side exhaust purification catalyst 24 decreases. However, since there is not so much unburned gas flowing out of the upstream side exhaust purification catalyst 20 at times t _{1 to} t ₄ , the amount of decrease in the oxygen storage amount OSAufc during this period is slight. For this reason, all the unburned gas flowing out from the upstream side exhaust purification catalyst 20 at the times t _{1 to} t ₄ is reduced and purified by the downstream side exhaust purification catalyst 24.

Also after time t _6, unburned gas flows out from the upstream side exhaust purification catalyst 20 at a certain time interval as in the case of time t ₁ to t ₄ . The unburned gas flowing out in this way is basically reduced and purified by oxygen stored in the exhaust purification catalyst 24 on the downstream side. Accordingly, the unburned gas hardly flows out from the downstream side exhaust purification catalyst 24. As described above, considering that the NOx emission amount from the upstream side exhaust purification catalyst 20 is small, according to the present embodiment, the unburned gas and NOx from the downstream side exhaust purification catalyst 24 are reduced. The amount of emissions is always small.

<Description of specific control>
Next, with reference to FIG. 9 and FIG. 10, the control apparatus in the said embodiment is demonstrated concretely. As shown in FIG. 9 which is a functional block diagram, the control device in the present embodiment is configured to include each functional block of A1 to A9. Hereinafter, each functional block will be described with reference to FIG.

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

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

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

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

<Calculation of target air-fuel ratio>
Next, calculation of the target air-fuel ratio will be described. In calculating the target air-fuel ratio, oxygen storage amount calculation means A4, target air-fuel ratio correction amount calculation means A5, and target air-fuel ratio setting means A6 that function as oxygen storage amount acquisition means are used.

The oxygen storage amount calculation means A4 is an estimated value of the oxygen storage amount of the upstream side exhaust purification catalyst 20 based on the fuel injection amount Qi calculated by the fuel injection amount calculation means A3 and the output current Irup of the upstream side air-fuel ratio sensor 40. OSAest is calculated. For example, the oxygen storage amount calculating means A4 multiplies the difference between the air-fuel ratio corresponding to the output current Irup of the upstream air-fuel ratio sensor 40 and the stoichiometric air-fuel ratio by the fuel injection amount Qi and integrates the obtained value. An estimated value OSAest of the oxygen storage amount is calculated. The estimation of the oxygen storage amount of the upstream side exhaust purification catalyst 20 by the oxygen storage amount calculation means A4 may not always be performed. For example, when the target air-fuel ratio is actually switched from the rich air-fuel ratio to the lean air-fuel ratio (time t ₃ in FIG. 7), the oxygen storage amount estimated value OSAest reaches the determination reference storage amount Cref (in FIG. 7). The oxygen storage amount may be estimated only until the time t ₄ ).

  In the target air-fuel ratio correction amount calculation means A5, the air-fuel ratio of the target air-fuel ratio is calculated based on the estimated value OSAest of the oxygen storage amount calculated by the oxygen storage amount calculation means A4 and the output current Irdwn of the downstream air-fuel ratio sensor 41. A correction amount AFC is calculated. Specifically, the air-fuel ratio correction amount AFC is the lean set correction amount AFClean when the output current Irdwn of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination reference value Iref (value corresponding to the rich determination air-fuel ratio). It is said. Thereafter, the air-fuel ratio correction amount AFC is maintained at the lean set correction amount AFClean until the estimated value OSAest of the oxygen storage amount reaches the determination reference storage amount Cref. When the estimated value OSAest of the oxygen storage amount reaches the determination reference storage amount Cref, the air-fuel ratio correction amount AFC is set to the weak rich set correction amount AFCrich. Thereafter, the air-fuel ratio correction amount AFC is maintained at the weak rich set correction amount AFCrich until the output current Irdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination reference value Iref (a value corresponding to the rich determination air-fuel ratio).

  The target air-fuel ratio setting means A6 adds the air-fuel ratio correction amount AFC calculated by the target air-fuel ratio correction amount calculation means A5 to the reference air-fuel ratio, in this embodiment, the theoretical air-fuel ratio AFR, so that the target air-fuel ratio setting means A6 is added. The fuel ratio AFT is calculated. Therefore, the target air-fuel ratio AFT is the weak rich set air-fuel ratio (when the air-fuel ratio correction amount AFC is the weak rich set correction amount AFCrich) or the lean set air-fuel ratio (when the air-fuel ratio correction amount AFC is the lean set correction amount AFClean). ) The target air-fuel ratio AFT calculated in this way is input to the basic fuel injection amount calculating means A2 and an air-fuel ratio difference calculating means A8 described later.

  FIG. 10 is a flowchart showing a control routine for calculation control of the air-fuel ratio correction amount AFC. The illustrated control routine is performed by interruption at regular time intervals.

  As shown in FIG. 10, first, in step S11, it is determined whether the calculation condition for the air-fuel ratio correction amount AFC is satisfied. The case where the calculation condition of the air-fuel ratio correction amount is satisfied includes, for example, that fuel cut control is not being performed. If it is determined in step S11 that the target air-fuel ratio calculation condition is satisfied, the process proceeds to step S12. In step S12, the output current Irup of the upstream air-fuel ratio sensor 40, the output current Irdwn of the downstream air-fuel ratio sensor 41, and the fuel injection amount Qi are acquired. Next, in step S13, the estimated value OSAest of the oxygen storage amount is calculated based on the output current Irup and the fuel injection amount Qi of the upstream air-fuel ratio sensor 40 acquired in step S12.

  Next, in step S14, it is determined whether or not the lean setting flag Fr is set to zero. The lean setting flag Fr is set to 1 when the air-fuel ratio correction amount AFC is set to the lean setting correction amount AFClean, and is set to 0 otherwise. If the lean setting flag Fr is set to 0 in step S14, the process proceeds to step S15. In step S15, it is determined whether or not the output current Irdwn of the downstream air-fuel ratio sensor 41 is equal to or less than the rich determination reference value Iref. When it is determined that the output current Irdwn of the downstream air-fuel ratio sensor 41 is larger than the rich determination reference value Iref, the control routine is ended.

  On the other hand, when the oxygen storage amount OSAsc of the upstream side exhaust purification catalyst 20 decreases and the air-fuel ratio of the exhaust gas flowing out from the upstream side exhaust purification catalyst 20 decreases, the output of the downstream side air-fuel ratio sensor 41 in step S15. It is determined that the current Irdwn is equal to or less than the rich determination reference value Iref. In this case, the process proceeds to step S16, and the air-fuel ratio correction amount AFC is set to the lean set correction amount AFClean. Next, at step S17, the lean setting flag Fr is set to 1, and the control routine is ended.

  In the next control routine, it is determined in step S14 that the lean setting flag Fr is not set to 0, and the process proceeds to step S18. In step S18, it is determined whether or not the estimated value OSAest of the oxygen storage amount calculated in step S13 is smaller than the determination reference storage amount Cref. When it is determined that the estimated value OSAest of the oxygen storage amount is smaller than the determination reference storage amount Cref, the routine proceeds to step S19, where the air-fuel ratio correction amount AFC is continuously set to the lean set correction amount AFClean. On the other hand, when the oxygen storage amount of the upstream side exhaust purification catalyst 20 increases, it is determined in step S18 that the estimated value OSAest of the oxygen storage amount is equal to or greater than the determination reference storage amount Cref, and the process proceeds to step S20. In step S20, the air-fuel ratio correction amount AFC is set to the weak rich setting correction amount AFCrich. Next, in step S21, the lean setting flag Fr is reset to 0, and the control routine is ended.

<Calculation of F / B correction amount>
Returning to FIG. 9 again, calculation of the F / B correction amount based on the output current Irup of the upstream air-fuel ratio sensor 40 will be described. In calculating the F / B correction amount, numerical value conversion means A7, air-fuel ratio difference calculation means A8, and F / B correction amount calculation means A9 are used.

  The numerical value conversion means A7 is a map or a calculation formula (for example, as shown in FIG. 5) that defines the output current Irup of the upstream air-fuel ratio sensor 40 and the relationship between the output current Irup of the upstream air-fuel ratio sensor 40 and the air-fuel ratio. The upstream exhaust air-fuel ratio AFup corresponding to the output current Irup is calculated. Therefore, the upstream side exhaust air-fuel ratio AFup corresponds to the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20.

  The air-fuel ratio difference calculating means A8 calculates the air-fuel ratio difference DAF by subtracting the target air-fuel ratio AFT calculated by the target air-fuel ratio setting means A6 from the upstream side exhaust air-fuel ratio AFup determined by the numerical value converting means A7 ( DAF = AFup-AFT). This air-fuel ratio difference DAF is a value that represents the excess or deficiency of the fuel supply amount with respect to the target air-fuel ratio AFT.

  The F / B correction amount calculating means A9 supplies fuel based on the following equation (2) by subjecting the air-fuel ratio difference DAF calculated by the air-fuel ratio difference calculating means A8 to proportional / integral / differential processing (PID processing). An F / B correction amount DFi for compensating for the excess or deficiency of the amount is calculated. The F / B correction amount DFi calculated in this way is input to the fuel injection amount calculation means A3.

  DFi = Kp / DAF + Ki / SDAF + Kd / DDAF (2)

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

  In the above embodiment, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is detected by the upstream side air-fuel ratio sensor 40. However, since the detection accuracy of the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 is not necessarily high, for example, this is based on the fuel injection amount from the fuel injection valve 11 and the output of the air flow meter 39. The air-fuel ratio of the exhaust gas may be estimated.

  As described above, in the normal operation control, the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst is repeatedly richer than the stoichiometric air-fuel ratio and leaner than the stoichiometric air-fuel ratio, and the oxygen storage amount is further increased. By performing control to avoid reaching the vicinity of the maximum oxygen storage amount, the outflow of NOx can be suppressed.

<Description of control after fuel cut control>
Next, control after the fuel cut control in the present embodiment is performed will be described. When fuel cut control is performed to reduce the amount of fuel supplied to the combustion chamber during operation of the internal combustion engine, air flows into the exhaust purification catalyst. That is, oxygen-excess gas flows into the exhaust purification catalyst. For this reason, the inside of the exhaust purification catalyst is in an oxidizing atmosphere, and if the operation shifts to normal operation control after fuel cut control, there is a possibility that NOx cannot be sufficiently purified. Therefore, after the fuel cut control is completed, in order to return the inside of the exhaust purification catalyst to the reducing atmosphere, control is performed to make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst a rich air-fuel ratio continuously after the fuel cut control. . By setting the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a rich air-fuel ratio, the inside of the exhaust purification device can be quickly returned to the reduced state. Further, in the operation control of the present embodiment, after the control of setting the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a rich air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is further set to the lean air-fuel ratio. Take control.

  In the present embodiment, in normal operation control, control for setting the air-fuel ratio of the exhaust gas flowing into the upstream side exhaust purification catalyst 20 to the rich air-fuel ratio is referred to as first rich control, and the exhaust gas flowing into the exhaust purification catalyst 20 Control for setting the air-fuel ratio of the gas to a lean air-fuel ratio is referred to as first lean control. That is, in the normal operation control, the first rich control and the first lean control are repeatedly performed.

  FIG. 11 shows a time chart of the control after performing the fuel cut control and the fuel cut control in the present embodiment. In the operation control of the present embodiment, after the fuel cut control is performed, the second rich control is performed in which the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is made rich. Further, after the second rich control, a second lean control is performed to make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 a lean air-fuel ratio. After the second lean control is performed, the normal operation control is restored.

Until the time t ₁₁ have done the normal operation control. In time t _2, the are switched to the first lean control of air-fuel ratio to a lean air-fuel ratio of the exhaust gas from the first rich control to make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 to the slightly rich. After this, at time t _4, it is switched from the first lean control to the first rich control. Note that when the air-fuel ratio correction amount is changed as described above, the output value of the upstream air-fuel ratio sensor changes with a time delay.

In the example of the operation control shown in FIG. 11, at time t _11, which starts the fuel cut control. In the fuel cut control, the supply of fuel to the combustion chamber 5 is shut off, and air flows into the exhaust purification catalyst 20. During the period when the fuel cut control is performed, the normal operation control is stopped. In this embodiment, the air-fuel ratio correction amount is set to zero during the fuel cut control period. Alternatively, the target air-fuel ratio correction amount calculating means for calculating the air-fuel ratio correction amount may be stopped during the fuel cut control period. That is, the output of the air-fuel ratio correction amount may be stopped.

  When the fuel cut control is performed, the upstream air-fuel ratio sensor 40 and the downstream air-fuel ratio sensor 41 output a large value on the lean side. The oxygen storage amount OSAsc of the exhaust purification catalyst 20 reaches the maximum oxygen storage amount Cmax in a short time.

At time t _12, the fuel cut control has ended. At time t _12, the oxygen storage amount OSAsc of the exhaust purification catalyst 20 has the maximum oxygen storage amount Cmax. When the fuel cut control period is short, the oxygen storage amount of the exhaust purification catalyst 20 may not reach the maximum oxygen storage amount.

  The control device of the present embodiment ends the fuel cut control and starts the second rich control. In the present embodiment, the air-fuel ratio correction amount is set to a predetermined value. That is, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is set in advance. The air-fuel ratio correction amount is set so that the air-fuel ratio of the exhaust gas becomes deep and rich. For example, the air-fuel ratio correction amount is controlled so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 becomes approximately 13.5.

When starting the second rich control at time t _12, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 becomes rich. The output value of the upstream air-fuel ratio sensor 40 becomes a small rich air-fuel ratio. The inside of the exhaust purification catalyst 20 changes from the oxidizing atmosphere to the reducing atmosphere, and the oxygen storage amount OSAsc decreases. The oxygen stored in the exhaust purification catalyst 20 is consumed. By performing the second rich control, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 rapidly decreases, and the output value of the downstream air-fuel ratio sensor 41 is the value at which the air-fuel ratio of the exhaust gas is rich. It decreases until it shows.

By the way, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst in the second rich control becomes richer than the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst in the first rich control. In the air-fuel ratio of the exhaust gas having a small difference from the stoichiometric air-fuel ratio as in the first rich control, oxygen is released only at a shallow portion from the surface of the exhaust purification catalyst. For example, when a noble metal having a catalytic action and a substance having an oxygen storage capacity are arranged in a coat layer formed on the surface of the carrier, oxygen is released only at a shallow portion from the surface of the coat layer. . However, when an air-fuel ratio exhaust gas having a large difference from the stoichiometric air-fuel ratio as in the second rich control flows into the exhaust purification catalyst, oxygen is released from the surface of the exhaust purification catalyst to a deep portion. In the graph shown in FIG. 11, at time t _13, but has the title of the oxygen storage amount of the exhaust purification catalyst 20 at zero, the second rich control, deep oxygen is released at the first rich control The oxygen stored in the deeper part is released.

  In the present embodiment, the second rich control is terminated when the output current of the downstream air-fuel ratio sensor 41 becomes 0 or less. That is, the second rich control is terminated when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 becomes the stoichiometric air-fuel ratio or rich.

The control device of the present embodiment, together with the second rich control is ended at time t _13, to start the second lean control. The second lean control is preferably started at a lean air-fuel ratio value at which the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is large. In the present embodiment, the second lean control is started at an air-fuel ratio (an air-fuel ratio) leaner than the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst in the first lean control of the normal operation control. . That is, the air-fuel ratio correction amount for the second lean control is set to a larger value than the air-fuel ratio correction amount for the first lean control.

  In the second lean control of the present embodiment, the exhaust gas flowing into the exhaust purification catalyst 20 becomes stronger as the richness of the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 during the period of execution of the second lean control becomes stronger. Control is performed to increase the lean degree of the air-fuel ratio. That is, control is performed to make the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 leaner (increase) as the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 becomes richer (smaller). . Proportional control is performed to increase the air-fuel ratio correction amount as the output value of the downstream air-fuel ratio sensor 41 decreases. For this reason, in the embodiment shown in FIG. 11, there is a period during which the air-fuel ratio correction amount is smaller than the air-fuel ratio correction amount in the first lean control during the second lean control period.

  Here, in the second lean control of the present embodiment, a guard value is provided so that the air-fuel ratio correction amount does not become too large. That is, the maximum value of the air-fuel ratio correction amount is determined in advance. When the air-fuel ratio correction amount is calculated based on the output value of the downstream side air-fuel ratio sensor 41, a value larger than the guard value may be calculated. In this case, control is performed to set the air-fuel ratio correction amount to the guard value. For example, the guard value can be set so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is 14.9 or less. When the air-fuel ratio correction amount for the first lean control is set in advance, the guard value is set larger than the air-fuel ratio correction amount for the first lean control.

From time t ₁₃ to the time t _14, the air-fuel ratio correction amount is set to the guard value, and is controlled so as not to exceed the guard value. Thus, from the time t ₁₃ to the time t _14, the air-fuel ratio correction amount is maintained at the preset guard value.

At time t ₁₄ after the value calculated by the proportional control is employed as the air-fuel ratio correction amount. As the output value of the downstream air-fuel ratio sensor 41 increases, the air-fuel ratio correction amount decreases. As the air-fuel ratio correction amount decreases, the output value of the upstream air-fuel ratio sensor 40 also decreases with time. As described above, in the control example shown in FIG. 11, the air-fuel ratio correction amount in the second lean control is gradually changed. However, the present invention is not limited to this mode, and even if the air-fuel ratio correction amount is changed discontinuously. I do not care. For example, the air-fuel ratio correction amount may be changed stepwise.

  In the second lean control, since the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 becomes the lean air-fuel ratio, the exhaust gas with excess air flows into the exhaust purification catalyst 20. The exhaust purification catalyst 20 stores oxygen, and the oxygen storage amount of the exhaust purification catalyst 20 gradually increases.

In the present embodiment, when the output current of the downstream air-fuel ratio sensor 41 becomes 0 or more, that is, when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 becomes the stoichiometric air-fuel ratio or lean, The second lean control is finished. At time t ₁₅ , the output current of the downstream air-fuel ratio sensor 41 becomes zero, and the second lean control is finished.

At time t _15, and resume normal operation control at the end of the second lean control. In the present embodiment, the first rich control is started out of the first lean control and the first rich control of the normal operation control. That is, the control is started from making the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 a weak rich air-fuel ratio. By performing the first rich control, the oxygen storage amount of the exhaust purification catalyst 20 decreases. At time t ₇ , the output value of the downstream air-fuel ratio sensor 41 becomes the rich determination reference value, and the first rich control is switched to the first lean control. After this, at time t _8, it is switched from the first lean control to the first rich control. Thus, normal operation control can be resumed.

  In FIG. 12, the time chart of the operation control of a comparative example is shown. Also in the operation control of the comparative example, the second rich control is performed after the fuel cut control is performed. However, in the operation control of the comparative example, the second rich control ends and the normal operation control is resumed.

Until time t ₁₃ is the same as the operation control of the present embodiment (see FIG. 11). At time t _13, the output value of the downstream air-fuel ratio sensor 41 has been completed the second rich control becomes 0 or less. In operation control of the comparative example, at time t _13, the second rich control is started normal operation control with ends. Since the output value of the downstream air-fuel ratio sensor 41 is less than the rich determination reference value, the first lean control is started. The air-fuel ratio correction amount is maintained at a predetermined lean setting correction amount for the first lean control.

In the exhaust purification catalyst 20, the oxygen storage amount increases with time. The oxygen storage amount calculation means calculates the oxygen storage amount OSAsc of the exhaust purification catalyst 20. However, in the first lean control of the normal operation control, the increasing speed of the output value of the downstream side air-fuel ratio sensor 41 is slow. This is because when an air-fuel ratio exhaust gas having a large difference from the stoichiometric air-fuel ratio as in the second rich control flows into the exhaust purification catalyst, oxygen is released from the surface of the exhaust purification catalyst to a deep portion. It is thought that the rising speed will be slow. In the comparative example, at time t ₁₅ , the oxygen storage amount reaches the predetermined criterion storage amount Cref. For this, at time t _15, it is switched from the first lean control to the first rich control.

In the operation control of the comparative example, it takes time for the output value of the downstream air-fuel ratio sensor 41 to return to the theoretical air-fuel ratio or more. For this, the output value of the downstream air-fuel ratio sensor 41 at time t ₁₅ in some cases is less than the rich determination reference value Iref. In this case, the first rich control is switched in a state where the output value of the downstream air-fuel ratio sensor 41 is smaller than the rich determination reference value. As a result, it is not possible to accurately determine the switching time from the next first rich control to the first lean control, and there is a possibility that normal operation control becomes impossible. That is, normal operation control may fail.

  Further, it is considered that when the second rich control is performed, the activity of the exhaust purification catalyst 20 is reduced. If the first lean control is performed for a long time in this state, NOx flowing out from the engine body cannot be sufficiently purified, and NOx may flow out.

  Referring to FIG. 11, in the operation control of the present embodiment, on the other hand, after the second rich control, exhaust gas having a high air-fuel ratio of exhaust gas is supplied to the exhaust purification catalyst 20, thereby The air-fuel ratio of the exhaust gas flowing out from the purification catalyst 20 can be quickly increased. In order to perform the second lean control after the second rich control, the output value of the downstream air-fuel ratio sensor 41 can be returned to the theoretical air-fuel ratio or more in a short time. Further, since the shift to the normal operation control is performed after the output value of the downstream side air-fuel ratio sensor 41 becomes larger than the rich determination air-fuel ratio, it is possible to avoid the normal operation control from failing. Further, since the second lean control is performed in a short time, it is possible to suppress a decrease in the NOx purification ability.

  Further, during the normal operation period, the air-fuel ratio sensor can be learned when the air-fuel ratio of the exhaust gas becomes the rich determination air-fuel ratio. For example, the amount of oxygen stored in the exhaust purification catalyst and the amount of oxygen released by the exhaust purification catalyst are estimated for the period from when the previous rich judgment reference value is reached to when this rich judgment reference value is reached. Therefore, it is possible to perform learning for correcting the output of the air-fuel ratio sensor based on these estimated values. However, in the operation control of the comparative example, since it takes time for the output value of the downstream air-fuel ratio sensor to return to the stoichiometric air-fuel ratio, if learning is performed at this time, the output value of the downstream air-fuel ratio sensor shifts. End up. For example, the output value of the downstream air-fuel ratio sensor shifts to the lean side. This deviation increases as the number of fuel cut controls increases.

  On the other hand, in the operation control of the present embodiment, the second lean control different from the first lean control of the normal operation control is performed. For this reason, learning of the air-fuel ratio sensor is not performed during the period of the second lean control. Alternatively, it is possible to perform control to prohibit learning of the air-fuel ratio sensor during the second lean control period. By not learning the air-fuel ratio sensor in the second lean control, it is possible to avoid a deviation in the output value of the downstream air-fuel ratio sensor. As the number of times of fuel cut control increases, it can be suppressed that the learning value of the downstream air-fuel ratio sensor gradually shifts to the lean side.

  The end time of the second lean control may be ended when the output current of the downstream air-fuel ratio sensor 41 becomes leaner than the rich determination reference value in order to avoid failure of normal operation control. preferable. That is, the process can be terminated when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 becomes leaner than the rich determination air-fuel ratio. Alternatively, when the output value of the downstream air-fuel ratio sensor 41 becomes equal to or higher than the theoretical air-fuel ratio, it is preferable to end the second lean control and shift to normal operation control. By this control, failure of normal operation control can be avoided more reliably.

  FIG. 13 shows a flowchart of the fuel cut control and the control after the fuel cut control in the present embodiment. The control shown in FIG. 13 can be performed during the period of normal operation control.

  In step S40, it is determined whether or not a fuel cut control execution condition is satisfied during the normal operation control period. For example, referring to FIG. 1, when the amount of depression of accelerator pedal 42 is zero and the engine speed exceeds a predetermined determination value, it is determined that a condition for performing fuel cut is satisfied. be able to. If the execution condition of the fuel cut control is not satisfied in step S40, this control is terminated. In step S40, when the fuel cut control execution condition is satisfied, the process proceeds to step S41.

  In step S41, the normal operation control is stopped. In step S42, fuel cut control is performed. That is, the normal operation control is stopped and the fuel cut control is started. In the fuel cut control, control is performed to reduce the amount of fuel supplied to the combustion chamber to zero.

  Next, in step S43, it is determined whether or not an end condition for fuel cut control is satisfied. As an end condition of the fuel cut control, for example, it can be determined that the condition is satisfied when the engine speed is less than a predetermined determination value. Alternatively, when the depression amount of the accelerator pedal 42 becomes larger than zero, it can be determined that the fuel cut control end condition is satisfied.

  In step S43, when the fuel cut control end condition is not satisfied, the process returns to step S42 and the fuel cut control is continued. In step S43, when the fuel cut control end condition is satisfied, the process proceeds to step S44. In step S44, the fuel cut control is terminated.

  Next, in step S45, the second rich control is started. In the present embodiment, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is determined in advance. In the present embodiment, the air-fuel ratio correction amount is set to a predetermined value. The air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 becomes a rich air-fuel ratio, and inside the exhaust purification catalyst 20, it changes from an oxidizing atmosphere to a reducing atmosphere.

  Next, in step S46, it is determined whether or not the output value of the downstream air-fuel ratio sensor 41 has become the stoichiometric air-fuel ratio or rich. That is, it is determined whether or not the output current Irdwn of the downstream air-fuel ratio sensor 41 has become 0 or less. If the output current of the downstream air-fuel ratio sensor 41 is greater than zero in step S46, the process returns to step S45 and the second rich control is continued. If the output current of the downstream air-fuel ratio sensor 41 is 0 or less in step S46, the process proceeds to step S47. In step S47, the second rich control is terminated. Next, the second lean control is started together with the end of the second rich control.

  In step S48, a second lean control air-fuel ratio correction amount is calculated. That is, the target air-fuel ratio of the exhaust gas flowing into the second lean control exhaust purification catalyst 20 is calculated. In the present embodiment, the air-fuel ratio correction amount is calculated based on the output value of the downstream air-fuel ratio sensor 41. Control is performed to increase the air-fuel ratio correction amount as the output value of the downstream air-fuel ratio sensor 41 is smaller. Control is performed to increase the air-fuel ratio correction amount to the lean side as the difference between the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 and the stoichiometric air-fuel ratio increases.

  Next, in step S49, it is determined whether or not the air-fuel ratio correction amount calculated in step S48 is equal to or less than a guard value. The guard value for the air-fuel ratio correction amount can be set in advance. If the air-fuel ratio correction amount is equal to or less than the guard value in step S49, the process proceeds to step S50. In this case, the air-fuel ratio of the exhaust gas supplied to the exhaust purification catalyst 20 is within a predetermined lean air-fuel ratio range. In step S50, in step S48, the air-fuel ratio correction amount calculated as the air-fuel ratio correction amount of the second lean control is selected.

  If the calculated air-fuel ratio correction amount is larger than the guard value in step S49, the process proceeds to step S51. In this case, it can be determined that the air-fuel ratio correction amount calculated by calculation is too large on the lean side. Therefore, the guard value is adopted as the air-fuel ratio correction amount for the second lean control.

  Next, in step S52, the second lean control is performed with the air-fuel ratio correction amount set in step S50 or step S51.

  Next, in step S53, it is determined whether or not the second lean control end condition is satisfied. In the present embodiment, it is determined whether or not the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is the stoichiometric air-fuel ratio or lean. That is, it is determined whether or not the output current Irdwn of the downstream air-fuel ratio sensor 41 is 0 or more.

  If the output current of the downstream air-fuel ratio sensor 41 is less than 0 in step S53, the process returns to step S48, and the air-fuel ratio correction amount for the second lean control is calculated again. In this way, by repeating step S48, the air-fuel ratio of the exhaust gas supplied to the exhaust purification catalyst 20 can be lowered along with the increase of the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20. Control based on the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 can be performed. In step S53, when the output current of the downstream air-fuel ratio sensor 41 becomes 0 or more, it can be determined that the second lean control end condition is satisfied. In this case, the process proceeds to step S54.

  In step S54, the second lean control is terminated. In step S55, normal operation control is started. The second lean control is terminated and the normal operation control is resumed. In the normal operation, the first rich control can be resumed as described above. Alternatively, the oxygen storage amount of the exhaust purification catalyst 20 may be estimated in the second lean control and restarted from the first lean control.

  By increasing the air-fuel ratio of the exhaust gas in the second lean control to the lean side, that is, by increasing the difference from the stoichiometric air-fuel ratio, the output value of the downstream air-fuel ratio sensor is set to the stoichiometric air-fuel ratio in a short time. It can be returned to the vicinity. However, if the exhaust air-fuel ratio flowing into the exhaust purification catalyst 20 is kept lean for a long time (the exhaust air-fuel ratio is large) for a long time, NOx cannot be completely purified, and NOx may flow out. As in the present embodiment, by controlling the air-fuel ratio of the exhaust gas supplied to the exhaust purification catalyst based on the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst, the NOx outflow is suppressed. The value of the downstream air-fuel ratio sensor can be returned to the theoretical air-fuel ratio or near the theoretical air-fuel ratio in a short time.

  By the way, when the second rich control is performed, the activity of the exhaust purification catalyst 20 is lowered. If a large amount of exhaust gas having a high air-fuel ratio is supplied in the second lean control in a state where the activity is reduced, NOx cannot be completely purified inside the exhaust purification catalyst 20, and there is a possibility that NOx flows out from the exhaust purification catalyst 20. is there. For this reason, in the second lean control, when a predetermined amount of oxygen flows into the exhaust purification catalyst, a control for terminating the second lean control may be performed.

  Referring to FIG. 13, the second lean control of the present embodiment is continued until the air-fuel ratio of the exhaust gas flowing out from exhaust purification catalyst 20 becomes equal to or higher than the theoretical air-fuel ratio (see step S53). The present invention is not limited to this configuration, and when the amount of surplus oxygen that exceeds the stoichiometric air-fuel ratio flowing into the exhaust purification catalyst exceeds a predetermined determination value, control for terminating the second lean control may be performed. Absent. In this case, the amount of surplus oxygen exceeding the stoichiometric air-fuel ratio can be calculated by multiplying the difference between the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 and the stoichiometric air-fuel ratio by the fuel injection amount. Alternatively, when the oxygen storage amount of the exhaust purification catalyst is estimated during the period of performing the second lean control, and the oxygen storage amount becomes larger than a predetermined determination value, the second lean control is performed. You can end it. As the determination value of the oxygen storage amount in this case, for example, a determination value having the same amount as the determination reference storage amount in the first lean control can be employed. Alternatively, a determination value for the oxygen storage amount larger than the determination reference storage amount in the first lean control can be employed. For example, a determination value approximately 1.3 times the determination reference storage amount in the first lean control can be employed.

  In the second lean control of the present embodiment, control is performed to calculate the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst, that is, the air-fuel ratio correction amount, based on the output value of the downstream air-fuel ratio sensor. However, the present invention is not limited to this mode. For example, in the second lean control, the air-fuel ratio change may be set in advance so that the air-fuel ratio of the exhaust gas supplied to the exhaust purification catalyst gradually decreases. Alternatively, it may be set in advance so that the air-fuel ratio of the exhaust gas supplied to the exhaust purification catalyst becomes substantially constant.

FIG. 14 is a time chart illustrating another form of the second lean control of the present embodiment. A second lean control is started at time t _13, it has completed the second lean control at time t _15. After the second lean control is performed once, the operation shifts to the normal operation control. In another embodiment, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst 20 is maintained almost constant. That is, the air-fuel ratio correction amount is kept constant. The air-fuel ratio correction amount in the second lean control is made larger than the air-fuel ratio correction amount in the first lean control. That is, the air-fuel ratio of the exhaust gas in the second lean control is set higher than the air-fuel ratio of the exhaust gas in the first lean control. At this time, the air-fuel ratio correction amount AFCd in the second lean control can be increased, for example, within a range of about 1.3 to 1.5 times the air-fuel ratio correction amount in the first lean control. .

  As described above, in the second lean control, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst can be controlled to be a predetermined constant value. In this case, the second lean control can be continued until, for example, the output value of the downstream air-fuel ratio sensor becomes 0 or more, similarly to the above-described second lean control. Alternatively, the period (time length) for performing the second lean control may be determined in advance. In other words, the second lean control may be switched to the normal operation control after a predetermined duration. As the second lean control, the second lean control is performed by maintaining the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst at a predetermined constant value and performing the control for a predetermined duration. Therefore, the second lean control can be completed in a short time.

  Further, the second lean control can be performed in a period in which an amount of oxygen substantially equal to the amount of oxygen stored in the exhaust purification catalyst in the first lean control is stored. In this case, the amount of oxygen stored in the exhaust purification catalyst in the second lean control is equal to the amount of oxygen stored in the exhaust purification catalyst in the first lean control. Alternatively, in the second lean control, oxygen can be supplied to the exhaust purification catalyst so that an amount of oxygen larger than the amount of oxygen stored in the exhaust purification catalyst in the first lean control is occluded. For example, the duration of the second lean control can be set so as to supply a larger amount of oxygen than the amount of oxygen supplied to the exhaust purification catalyst in the first lean control.

  Further, in the control device for the internal combustion engine of the present embodiment, the upstream air-fuel ratio sensor 40 is disposed as the upstream sensor, and the downstream air-fuel ratio sensor 41 is disposed as the downstream sensor. However, a sensor capable of detecting the air-fuel ratio of the exhaust gas can be adopted as the upstream sensor and the downstream sensor. For example, while an upstream air-fuel ratio sensor is employed as the upstream sensor, the downstream sensor that detects the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst 20 can be constituted by a downstream oxygen sensor.

  The air-fuel ratio sensor is an all-zone sensor, and can output a continuous value according to the air-fuel ratio of the exhaust gas. The air-fuel ratio sensor is formed so that the output becomes larger as the air-fuel ratio of the exhaust gas becomes leaner (see FIG. 5). In contrast, the oxygen sensor generates a substantially step-like output based on the air-fuel ratio of the exhaust gas. For example, when changing from a lean air-fuel ratio to a rich air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio, it suddenly changes from a predetermined lean output value to a predetermined rich output value. Based on the output of the oxygen sensor, it can be determined whether the air-fuel ratio of the exhaust gas is a rich air-fuel ratio, a lean air-fuel ratio, or a stoichiometric air-fuel ratio.

  In the control device that employs the downstream oxygen sensor as the downstream sensor, the lean determination air-fuel ratio may be determined in advance in the region where the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is lean in addition to the rich determination air-fuel ratio. it can. In the normal operation control, control is performed to switch to the first rich control when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst becomes equal to or higher than the lean determination air-fuel ratio by the first lean control. In addition, when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst by the first rich control becomes equal to or lower than the rich determination air-fuel ratio, control for switching to the first lean control can be performed. Also in the control device for an internal combustion engine that employs a downstream oxygen sensor as such a downstream sensor, the above-described second rich control and second lean control can be performed.

  The control to switch to the first rich control when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst by the first lean control becomes equal to or higher than the lean determination air-fuel ratio is performed by the downstream sensor using the downstream air-fuel ratio sensor. It can also be implemented when configured.

  In the first lean control of the present embodiment, the air-fuel ratio of the exhaust gas that continuously flows into the exhaust purification catalyst is made leaner than the stoichiometric air-fuel ratio until the oxygen storage amount becomes equal to or greater than the determination reference storage amount. However, the present invention is not limited to this, and the air-fuel ratio of the exhaust gas that intermittently flows into the exhaust purification catalyst may be made leaner than the stoichiometric air-fuel ratio. Similarly, also in the first rich control, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst continuously or intermittently until the output of the downstream air-fuel ratio sensor becomes equal to or lower than the rich determination air-fuel ratio. The rich set air-fuel ratio can be made richer than the fuel ratio.

  In each of the above-described controls, the order of the steps can be appropriately changed within a range where the function and the action are not changed.

  In the respective drawings described above, the same or equivalent parts are denoted by the same reference numerals. In addition, said embodiment is an illustration and does not limit invention. Furthermore, in the embodiment, changes in the form shown in the claims are included.

5 Combustion chamber 11 Fuel injection valve 20, 24 Exhaust purification catalyst 31 Electronic control unit 39 Air flow meter 40 Upstream air-fuel ratio sensor 41 Downstream air-fuel ratio sensor 42 Accelerator pedal

Claims (7)

A control device for an internal combustion engine comprising an exhaust purification catalyst having oxygen storage capacity in an engine exhaust passage,
An upstream air-fuel ratio sensor that is disposed upstream of the exhaust purification catalyst and detects an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst;
A downstream air-fuel ratio sensor disposed downstream of the exhaust purification catalyst and detecting an air-fuel ratio of exhaust gas flowing out from the exhaust purification catalyst;
First, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made leaner than the stoichiometric air-fuel ratio, and the oxygen storage amount in the exhaust purification catalyst is increased to a determination reference storage amount smaller than the maximum oxygen storage amount. The lean air-fuel ratio of the exhaust gas flowing into the exhaust control catalyst is made richer than the stoichiometric air-fuel ratio, and the stoichiometric air-fuel ratio is output until the output of the downstream air-fuel ratio sensor becomes a rich judgment air-fuel ratio less than the stoichiometric air-fuel ratio. Performing normal operation control that alternately performs first rich control that continuously or intermittently continues the air-fuel ratio richer than the fuel ratio, and fuel cut control that shuts off the supply of fuel to the combustion chamber,
After the second rich control and the second rich control, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made richer than the first rich control continuously after the end of the fuel cut control. The second lean control is performed so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is leaner than the first lean control, and the normal operation control is performed following the second lean control. A control apparatus for an internal combustion engine, wherein   The control device for an internal combustion engine according to claim 1, wherein the second lean control is terminated when the output value of the downstream air-fuel ratio sensor becomes leaner than the rich determination air-fuel ratio.   In the second lean control, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst becomes leaner as the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst becomes richer during the execution period of the second lean control. The control apparatus for an internal combustion engine according to claim 1, comprising control for setting.   4. The oxygen amount stored in the exhaust purification catalyst in the second lean control is equal to the oxygen amount stored in the exhaust purification catalyst in the first lean control. 5. Control device for internal combustion engine.   The control apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the normal operation control that is performed following the second lean control starts from the first rich control. A control device for an internal combustion engine comprising an exhaust purification catalyst having oxygen storage capacity in an engine exhaust passage,
An upstream sensor that is disposed upstream of the exhaust purification catalyst and detects an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst;
A downstream sensor that is disposed downstream of the exhaust purification catalyst and detects an air-fuel ratio of exhaust gas flowing out of the exhaust purification catalyst;
The upstream sensor is composed of an upstream air-fuel ratio sensor,
The downstream sensor is composed of a downstream oxygen sensor or a downstream air-fuel ratio sensor,
First lean control for increasing the oxygen storage amount in the exhaust purification catalyst by setting the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst to a lean air-fuel ratio, and the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst as rich air It is formed so as to perform normal operation control including first rich control to make the fuel ratio and fuel cut control for cutting off fuel supplied to the combustion chamber,
A lean determination air-fuel ratio is predetermined in a region where the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is leaner than the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is richer than the stoichiometric air-fuel ratio. The rich determination air-fuel ratio is predetermined in the region of
The normal operation control includes control for switching to the first rich control when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst by the first lean control becomes a lean air-fuel ratio equal to or higher than the lean determination air-fuel ratio, Control to switch to the first lean control when the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst by the rich control becomes a rich air-fuel ratio equal to or lower than the rich determination air-fuel ratio,
After the second rich control and the second rich control, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is made richer than the first rich control continuously after the end of the fuel cut control. The second lean control is performed so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is leaner than the first lean control, and the normal operation control is performed following the second lean control. A control apparatus for an internal combustion engine, wherein   The control apparatus for an internal combustion engine according to claim 6, wherein the normal operation control performed following the second lean control starts from the first rich control.

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