JP2020153254A - Exhaust purification device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust purification device capable of appropriately purifying NOx in an exhaust gas with a downstream catalyst. An exhaust purification device includes an upstream catalyst, a downstream catalyst, and a control device for controlling the air-fuel ratio of exhaust gas discharged from an engine body. The control device has a normal air-fuel ratio control that controls the time-average air-fuel ratio of the exhaust gas flowing into the downstream catalyst so that it becomes a rich air-fuel ratio richer than the theoretical air-fuel ratio, and an empty exhaust gas discharged from the engine body. It is possible to execute lean spike control that controls the fuel ratio to a lean air-fuel ratio that is leaner than the theoretical air-fuel ratio. When the execution condition of lean spike control is satisfied, S22, the execution period of lean spike control is calculated based on the estimated value of the amount of oxygen that can be occluded on the upstream side, which is the amount of oxygen that can be occluded by the upstream catalyst, and S25, and then calculated. S28 The lean spike control is executed over the executed execution period. [Selection diagram] FIG. 8

Description

The present disclosure relates to an exhaust gas purification device for an internal combustion engine.

Conventionally, two exhaust purification catalysts, an upstream catalyst and a downstream catalyst, are provided in the exhaust passage of the internal combustion engine, and when the oxygen storage amount of the downstream catalyst becomes low, the internal combustion engine supplies oxygen to the downstream catalyst. Exhaust gas purification devices are known (for example, Patent Documents 1 and 2).

In particular, in the control device described in Patent Document 1, a downstream air-fuel ratio sensor that detects the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst and flowing into the downstream catalyst between the upstream catalyst and the downstream catalyst. Is provided. The oxygen storage amount of the downstream catalyst is estimated based on the air-fuel ratio detected by this downstream air-fuel ratio sensor, and when the estimated oxygen storage amount falls below a certain amount, the storage amount recovery control that supplies oxygen to the downstream catalyst Is done.

In this occlusion recovery control, the oxygen storage amount of the downstream catalyst is estimated based on the air-fuel ratio detected by the downstream air-fuel ratio sensor, and when the estimated oxygen storage amount reaches a predetermined reference amount, the occlusion is stored. The amount recovery control is terminated.

International Publication No. 2014/118890 Japanese Unexamined Patent Publication No. 2013-217266

By the way, since there is a certain distance from the engine body to the downstream catalyst, the air-fuel ratio of the exhaust gas flowing into the downstream catalyst changes when the air-fuel ratio of the exhaust gas discharged from the engine body is changed. It takes some time to change accordingly. Therefore, even if the end time of the occlusion amount recovery control is determined based on the air-fuel ratio detected by the downstream air-fuel ratio sensor as described in Patent Document 1, a large amount is applied to the downstream catalyst after the end of the occlusion amount recovery control. Oxygen (and NOx) will flow in. As a result, oxygen that has an oxygen occlusal amount exceeding the maximum occlusable oxygen amount (the maximum value of the oxygen occlusal amount that the downstream side catalyst can occlude) flows into the downstream side catalyst, and NOx that flows in together with the oxygen flows to the downstream side. It will not be possible to sufficiently purify with a catalyst.

Therefore, in order to sufficiently purify NOx in the exhaust gas with the downstream catalyst near the end time of the occlusion recovery control, the control is controlled by a control concept different from the control device described in Patent Document 1. Equipment is required.

In view of the above problems, an object of the present disclosure is to provide an exhaust gas purification device for an internal combustion engine having a mode different from the conventional one, which can appropriately purify NOx in the exhaust gas with a downstream catalyst. is there.

The gist of this disclosure is as follows.

(1) The upstream side catalyst provided in the exhaust passage of the internal combustion engine, the downstream side catalyst provided in the exhaust passage on the downstream side in the exhaust flow direction from the upstream side catalyst, and the exhaust gas discharged from the engine body. An exhaust gas purification device for an internal combustion engine including a control device for controlling the air-fuel ratio. The control device is a rich air in which the time average air-fuel ratio of the exhaust gas flowing into the downstream catalyst is richer than the theoretical air-fuel ratio. It is possible to execute normal air-fuel ratio control that controls the fuel ratio and lean spike control that controls the air-fuel ratio of the exhaust gas discharged from the engine body to a lean air-fuel ratio that is leaner than the theoretical air-fuel ratio. When the execution condition of the lean spike control is satisfied, the execution period of the lean spike control is calculated based on the estimated value of the amount of oxygen that can be stored on the upstream side, which is the amount of oxygen that the upstream catalyst can currently store, and then calculated. An exhaust gas purification device for an internal combustion engine that executes the lean spike control over an execution period.

(2) An upstream air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas flowing into the upstream catalyst is further provided, and the control device is based on the air-fuel ratio detected by the upstream air-fuel ratio sensor. The exhaust gas purification device for an internal combustion engine according to (1) above, which estimates the amount of oxygen that can be stored.

(3) The control device has an estimated value of the current oxygen occlusion amount of the upstream side catalyst estimated based on the air fuel ratio detected by the upstream side air fuel ratio sensor and the amount of oxygen that the upstream side catalyst can occlude. The exhaust purification device for an internal combustion engine according to (2) above, which calculates the amount of oxygen that can be occluded on the upstream side as a difference from the maximum amount of oxygen that can be occluded, which is the maximum value of.

(4) The control device calculates the maximum occlusable oxygen amount of the downstream catalyst, which is the maximum value of the occlusable oxygen amount of the downstream catalyst, based on the maximum occlusable oxygen amount of the upstream catalyst. The exhaust purification device for an internal combustion engine according to (3) above.

(5) The control device has the lean calculated based on the amount of oxygen that can be occluded on the upstream side and the maximum amount of oxygen that can be occluded by the downstream catalyst, which is the maximum value of the amount of oxygen that can be occluded by the downstream catalyst. The execution period is calculated so that oxygen corresponding to the total value of the sum of the amount of oxygen that can be occluded on the downstream side at the start of the spike control flows into the upstream catalyst during the execution of the lean spike control (2). ) To the exhaust purification device for the internal combustion engine according to any one of (4).

(6) In the control device, the air-fuel ratio of the exhaust gas flowing into the upstream catalyst during the execution of the lean spike control is at least temporarily, the upstream catalyst during the execution of the normal air-fuel ratio control. One of (1) to (5) above, which controls the air-fuel ratio of the exhaust gas discharged from the engine body so that the exhaust gas flowing into the engine is leaner than the leanest air-fuel ratio reached. The exhaust purification device for an internal combustion engine described.

(7) The internal combustion engine has a plurality of cylinders and has a plurality of cylinders.
During the execution of the lean spike control, fuel cut control is performed for at least a part of the plurality of cylinders to stop the supply of fuel during the operation of the internal combustion engine. The exhaust gas purification device for an internal combustion engine according to any one of (6).

(8) The exhaust gas purification device for an internal combustion engine according to (7) above, wherein the lean spike control is executed after the estimated value of the oxygen storage amount of the upstream catalyst reaches a predetermined storage amount.

(9) In the control device, the degree of leanness of the air-fuel ratio of the exhaust gas discharged from the engine body at the start of the lean spike control is the degree of leanness of the exhaust gas discharged from the engine body at the end of the lean spike control. To one of the above (1) to (8), the air-fuel ratio of the exhaust gas discharged from the engine body is changed during the execution of the lean spike control so as to be smaller than the lean degree of the air-fuel ratio. The exhaust purification device for an internal combustion engine described.

(10) The control device has an air-fuel ratio of exhaust gas discharged from the engine body from the start of the lean spike control until an amount of oxygen corresponding to the amount of occlusable oxygen on the upstream side flows into the upstream catalyst. The degree of leanness is the air-fuel ratio of the exhaust gas discharged from the engine body from the time when the amount of oxygen corresponding to the amount of occlusable oxygen on the upstream side flows into the upstream side catalyst to the end of the lean spike control. The exhaust gas purification device for an internal combustion engine according to (5) above, wherein the air-fuel ratio of the exhaust gas discharged from the engine body is changed during the execution of the lean spike control so as to be smaller than the degree.

(11) The method according to any one of (1) to (10) above, further comprising a downstream air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas flowing out of the upstream catalyst and flowing into the downstream catalyst. Exhaust purification device for internal combustion engine.

(12) The internal combustion engine according to (11) above, wherein the control device does not utilize the output of the downstream air-fuel ratio sensor during the execution period of the lean spike control when calculating the execution period of the lean spike control. Engine exhaust purification device.

(13) The control device controls the lean spike so that the lean spike control is terminated before the downstream air-fuel ratio sensor detects that the air-fuel ratio of the exhaust gas has reached the lean air-fuel ratio. The exhaust gas purification device for an internal combustion engine according to (12) above, wherein a target air-fuel ratio of exhaust gas discharged from the engine body is set during execution.

According to the present disclosure, there is provided an exhaust gas purification device for an internal combustion engine having a mode different from the conventional one, in which NOx in the exhaust gas can be appropriately purified by a downstream catalyst.

FIG. 1 is a diagram schematically showing an internal combustion engine in which the exhaust gas purification device according to one embodiment is used. FIG. 2 is a diagram showing the relationship between the air-fuel ratio of the exhaust gas around the air-fuel ratio sensor and the output current of the air-fuel ratio sensor. FIG. 3 is a time chart of the target air-fuel ratio and the like when the normal air-fuel ratio control is performed. FIG. 4 is a flowchart showing a control routine for normal air-fuel ratio control. FIG. 5 is a time chart of the target air-fuel ratio and the like in the normal air-fuel ratio control according to one modification. FIG. 6 is a diagram showing the relationship between the maximum occlusable oxygen amount of the upstream catalyst and the maximum occlusable oxygen amount of the downstream catalyst. FIG. 7 is a time chart of the target air-fuel ratio and the like when lean spike control is performed. FIG. 8 is a flowchart showing a control routine related to lean spike control. FIG. 9 is a flowchart showing a control routine related to lean spike control. FIG. 10 is a time chart similar to that of FIG. 7, showing the target air-fuel ratio and the like when the lean spike control according to the third embodiment is performed.

Hereinafter, embodiments will be described in detail with reference to the drawings. In the following description, similar components are given the same reference number.

<First Embodiment>
≪Explanation of the entire internal combustion engine≫
FIG. 1 is a diagram schematically showing an internal combustion engine in which the exhaust gas purification device according to the first embodiment is used. Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is a piston 3 and a cylinder head 4. Combustion chambers formed between them, 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, and 9 is an exhaust port. The intake valve 6 opens and closes the intake port 7, and the exhaust valve 8 opens and closes the exhaust port 9. In the present embodiment, a plurality of cylinders are formed in the cylinder block 2, and one piston 3 reciprocates in each cylinder.

As shown in FIG. 1, the spark plug 10 is arranged at the center of the inner wall surface of the cylinder head 4, and the fuel injection valve 11 is arranged around the inner wall surface of the cylinder head 4. The spark plug 10 is configured to generate a spark in response to an ignition signal. Further, the fuel injection valve 11 injects a predetermined amount of fuel into the combustion chamber 5 in response to the injection signal. The fuel injection valve 11 may be arranged so as to inject fuel into the intake port 7. Further, in the present embodiment, gasoline having a stoichiometric air-fuel ratio of 14.6 is used as the fuel. However, the internal combustion engine may use a fuel other than gasoline or a mixed fuel with gasoline.

The intake port 7 of each cylinder is connected to the surge tank 14 via the corresponding intake branch pipe 13, and the surge tank 14 is connected to the air cleaner 16 via the 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. Further, a throttle valve 18 driven by a throttle valve drive actuator 17 is arranged in the intake pipe 15. The throttle valve 18 can be rotated by the 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 the exhaust manifold 19. The exhaust manifold 19 has a plurality of branches connected to each exhaust port 9 and an aggregate portion in which these branches are aggregated. The collecting portion of the exhaust manifold 19 is connected to the upstream casing 21 containing the upstream exhaust purification catalyst (hereinafter, referred to as “upstream catalyst”) 20. The upstream casing 21 is connected to the downstream casing 23 containing the downstream exhaust purification catalyst (hereinafter, referred to as “downstream catalyst”) 24 via the 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 electronic control unit (ECU) 31 is composed of a digital computer, and has a RAM (random access memory) 33, a ROM (read-only memory) 34, a CPU (microprocessor) 35, and inputs connected to each other via a bidirectional bus 32. It includes a port 36 and an output port 37. An air flow meter 39 for detecting the flow rate of air flowing in the intake pipe 15 is arranged in the intake pipe 15, and the output of the air flow meter 39 is input to the input port 36 via the corresponding AD converter 38. Further, an upstream air-fuel ratio sensor 40 for detecting the air-fuel ratio of the exhaust gas flowing in the exhaust manifold 19 (that is, the exhaust gas flowing into the upstream catalyst 20) is arranged at the gathering portion of the exhaust manifold 19. In addition, the downstream air-fuel ratio sensor 41 that detects the air-fuel ratio of the exhaust gas flowing in the exhaust pipe 22 (that is, the exhaust gas flowing out of the upstream catalyst 20 and flowing into the downstream catalyst 24) in the exhaust pipe 22 Is placed. The outputs of the air-fuel ratio sensors 40 and 41 are also input to the input port 36 via the corresponding AD converter 38.

In the present embodiment, the limit current type air-fuel ratio sensor is used as the air-fuel ratio sensors 40 and 41. Therefore, as shown in FIG. 2, the air-fuel ratio sensors 40 and 41 output from the air-fuel ratio sensors 40 and 41 as the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40 and 41 becomes higher (that is, leaner). It is configured to increase the current. In particular, the air-fuel ratio sensors 40 and 41 of the present embodiment are configured so that the output current changes linearly (proportionally) with respect to the air-fuel ratio of the exhaust gas around the air-fuel ratio sensors 40 and 41. In the present embodiment, the limit current type air-fuel ratio sensor is used as the air-fuel ratio sensors 40 and 41, but if the sensor has an output that changes according to the air-fuel ratio of the exhaust gas, the limit current type air-fuel ratio sensor An air-fuel ratio sensor other than the above may be used. Examples of such an air-fuel ratio sensor include an oxygen sensor whose output suddenly changes in the vicinity of the stoichiometric air-fuel ratio without applying a voltage between the electrodes constituting the sensor.

Further, 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. To. For example, the crank angle sensor 44 generates an output pulse every time the crankshaft rotates 15 degrees, and this output pulse is input to the input port 36. The CPU 35 calculates the engine speed from the output pulse of the crank angle sensor 44. On the other hand, the output port 37 is connected to the spark plug 10, the fuel injection valve 11, and the throttle valve drive actuator 17 via the corresponding drive circuit 45. The ECU 31 functions as a control device for controlling the air-fuel ratio of the exhaust gas discharged from the engine body 1.

The upstream side catalyst 20 and the downstream side catalyst 24 are three-way catalysts having an oxygen storage capacity. Specifically, in the exhaust purification catalysts 20 and 24, a catalytic noble metal having a catalytic action (for example, platinum (Pt)) and a substance having an oxygen storage capacity (for example, ceria (CeO ₂ )) are added to a carrier made of ceramic. It is a supported three-way catalyst. The three-way catalyst has a function of simultaneously purifying unburned HC, CO and NOx when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is maintained at the stoichiometric air-fuel ratio. In addition, when a certain amount of oxygen is occluded in the exhaust purification catalysts 20 and 24, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20 and 24 is on the rich side or lean side with respect to the theoretical air-fuel ratio. Unburned HC, CO and NOx are purified at the same time even if there is a slight deviation.

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

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

In this way, when a certain amount of oxygen is stored in the exhaust purification catalysts 20 and 24, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalysts 20 and 24 is rich or lean with respect to the stoichiometric air-fuel ratio. Even if it is slightly displaced to the side, unburned HC, CO and NOx are purified at the same time, and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalysts 20 and 24 becomes the stoichiometric air-fuel ratio.

≪Normal air-fuel ratio control≫
Next, an outline of the normal air-fuel ratio control usually performed in the control device of the internal combustion engine according to the present embodiment will be described. In the normal air-fuel ratio control of the present embodiment, the fuel injection amount from the fuel injection valve 11 is set so that the output air-fuel ratio of the upstream air-fuel ratio sensor 40 becomes the target air-fuel ratio based on the output air-fuel ratio of the upstream air-fuel ratio sensor 40. Feedback control is performed to control. That is, in the normal air-fuel ratio control of the present embodiment, feedback control is performed so that the air-fuel ratio of the exhaust gas discharged from the engine body 1 becomes the target air-fuel ratio based on the output air-fuel ratio of the upstream air-fuel ratio sensor 40. .. The "output air-fuel ratio" means an air-fuel ratio corresponding to the output value of the air-fuel ratio sensor.

Further, in the normal air-fuel ratio control of the present embodiment, the target air-fuel ratio is set based on the output air-fuel ratio of the downstream air-fuel ratio sensor 41 and the like. Specifically, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes a richer air-fuel ratio than the theoretical air-fuel ratio (hereinafter referred to as "rich air-fuel ratio"), the target air-fuel ratio becomes the lean set air-fuel ratio. Set. As a result, the air-fuel ratio of the exhaust gas discharged from the engine body 1 also becomes the lean set air-fuel ratio. Here, the lean set air-fuel ratio is a predetermined constant value air-fuel ratio that is lean to some extent from the theoretical air-fuel ratio (air-fuel ratio that is the control center), and is, for example, about 14.65 to 16. To. In addition, in the present embodiment, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes a rich determination air-fuel ratio (for example, 14.55) or less, which is slightly richer than the theoretical air-fuel ratio, the downstream air-fuel ratio is empty. It is determined that the output air-fuel ratio of the fuel ratio sensor 41 has reached the rich air-fuel ratio.

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

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

When the integrated oxygen excess / deficiency amount, which is the sum of the oxygen excess / deficiency amounts calculated in this way, exceeds the predetermined switching reference value (corresponding to the predetermined switching standard storage amount Cref), the lean setting is empty until then. The target air-fuel ratio, which was the fuel ratio, is set to the rich set air-fuel ratio. The rich set air-fuel ratio is a predetermined air-fuel ratio that is somewhat richer than the theoretical air-fuel ratio (air-fuel ratio that is the control center), and is, for example, about 14 to 14.55.

After that, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes equal to or less than the rich determination air-fuel ratio again, the target air-fuel ratio is set to the lean set air-fuel ratio again, and then the same operation is repeated. As described above, in the present embodiment, the target air-fuel ratio of the exhaust gas discharged from the engine main body 1 is alternately and repeatedly set to the lean set air-fuel ratio and the rich set air-fuel ratio. In other words, in the normal air-fuel ratio control in the present embodiment, the air-fuel ratio of the exhaust gas discharged from the engine body 1 is a rich air-fuel ratio and an air-fuel ratio leaner than the theoretical air-fuel ratio (hereinafter referred to as "lean air-fuel ratio"). It can be said that it can be switched alternately with.

Further, in the present embodiment, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes the rich air-fuel ratio, the target air-fuel ratio is switched from the rich set air-fuel ratio to the lean set air-fuel ratio. Therefore, when the target air-fuel ratio is switched, the exhaust gas having a rich air-fuel ratio temporarily flows out from the upstream catalyst 20. On the other hand, in the present embodiment, the target air-fuel ratio is switched from the lean set air-fuel ratio to the rich set air-fuel ratio before the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes the lean air-fuel ratio. Therefore, when the target air-fuel ratio is switched, the exhaust gas having the lean air-fuel ratio does not flow out from the upstream catalyst 20. Therefore, the exhaust gas having a rich air-fuel ratio flows out from the upstream catalyst 20 on a regular basis, but the exhaust gas having a lean air-fuel ratio does not flow out. Therefore, in the present embodiment, the time average air-fuel ratio of the exhaust gas flowing out of the upstream catalyst 20 and flowing into the downstream catalyst 24 is a rich air-fuel ratio.

≪Explanation of normal air-fuel ratio control using time chart≫
The above-mentioned operation will be specifically described with reference to FIG. FIG. 3 shows the target air-fuel ratio AFT, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40, the oxygen occlusion amount OSUp of the upstream catalyst 20, and the upstream catalyst 20 when the normal air-fuel ratio control according to the present embodiment is performed. It is a time chart of the integrated oxygen excess / deficiency amount ΣOED, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41, and the oxygen storage amount OSAdwn of the downstream side catalyst 24.

In the illustrated example, in the state before the time t ₁ , the target air-fuel ratio AFT is set to the rich set air-fuel ratio AFTrich. The unburned gas and the like contained in the exhaust gas flowing into the upstream catalyst 20 react with the oxygen occluded in the upstream catalyst 20 and are purified by the upstream catalyst 20, and accordingly, the upstream side is purified. The oxygen occlusion amount OSUp of the catalyst 20 gradually decreases. By purifying the unburned gas and the like in the upstream catalyst 20, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 becomes substantially the theoretical air-fuel ratio.

When the oxygen storage amount OSUp of the upstream catalyst 20 gradually decreases and approaches zero, a part of the unburned gas or the like that has flowed into the upstream catalyst 20 begins to flow out without being purified by the upstream catalyst 20. As a result, the exhaust gas having a rich air-fuel ratio containing the unburned gas flows into the downstream side catalyst 24, and the unburned gas reacts with the oxygen occluded in the downstream side catalyst 24 to be purified. Along with this, the oxygen occlusion amount OSAdwn of the downstream side catalyst 24 decreases.

Upon reaching the rich determination air-fuel ratio AFrich at time t ₁ the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is gradually decreased, in the present embodiment, in order to increase the oxygen storage amount OSAup of the upstream catalyst 20, the target The air-fuel ratio AFT is switched to the lean set air-fuel ratio AFTlean. At this time, the accumulated oxygen excess / deficiency amount ΣOED is reset to 0.

When the target air-fuel ratio is switched to the lean air-fuel ratio at time t ₁ , the air-fuel ratio of the exhaust gas discharged from the engine body 1 changes from the rich air-fuel ratio to the lean air-fuel ratio. As a result, after time t ₁ , the oxygen occluded amount OSUp of the upstream catalyst 20 increases. Similarly, the cumulative oxygen excess / deficiency amount ΣOED also gradually increases.

At this time, the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 20 is the lean air-fuel ratio, but since the upstream catalyst 20 has a sufficient oxygen storage capacity, the oxygen in the inflowing exhaust gas is It is occluded in the upstream catalyst 20 and NOx is reduced and purified. As a result, the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst 20 changes to the stoichiometric air-fuel ratio.

Thereafter, the oxygen storage amount OSAup of the upstream catalyst 20 is increased and reaches the switching reference occlusion amount Cref at eventually time t _2. At this time, the accumulated oxygen excess / deficiency amount ΣOED reaches the switching reference value OEDref corresponding to the switching reference storage amount Clef. In the present embodiment, when the integrated oxygen excess / deficiency amount ΣOED becomes equal to or higher than the switching reference value OEDref, the target air-fuel ratio AFT is switched to the rich set air-fuel ratio AFTrich in order to stop the occlusion of oxygen in the upstream catalyst 20. At this time, the accumulated oxygen excess / deficiency amount ΣOED is reset to 0.

Switching the target air-fuel ratio to the rich air-fuel ratio at time t _2, the air-fuel ratio of the exhaust gas discharged from the engine body 1 is changed from a lean air-fuel ratio to a rich air-fuel ratio. Since the exhaust gas discharged from the engine body 1 contains unburned gas and the like, the oxygen storage amount OSA of the upstream catalyst 20 gradually decreases.

Then, at time t _3, as with time t _1, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the rich determination air-fuel ratio AFrich. As a result, the target air-fuel ratio AFT is switched to the lean set air-fuel ratio AFTlean. After that, the cycle from time t _{1 to} t ₃ described above is repeated.

≪Flow chart of normal air-fuel ratio control≫
FIG. 4 is a flowchart showing a control routine for normal air-fuel ratio control. The illustrated control routine is executed by the control device at regular time intervals (for example, several msec).

As shown in FIG. 4, first, in step S11, it is determined whether or not the execution condition of the normal air-fuel ratio control is satisfied. The execution condition of the normal air-fuel ratio control is, for example, that special control such as fuel cut control is not being executed. If it is determined in step S11 that the execution condition for the normal air-fuel ratio control is satisfied, the control routine proceeds to step S12.

In step S12, it is determined whether or not the lean setting flag Fl is set to OFF. The lean setting flag Fl is turned ON when the target air-fuel ratio AFT is set to the lean air-fuel ratio, and is turned OFF in other cases. If it is determined in step S12 that the lean setting flag Fl is set to OFF, the control routine proceeds to step S13. In step S13, it is determined whether or not the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is equal to or less than the rich determination air-fuel ratio AFrich. When it is determined that the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is larger than the rich determination air-fuel ratio AFrich, the target air-fuel ratio AFT is maintained and controlled while being set to the rich set air-fuel ratio AFrich in step S14. The routine is terminated.

On the other hand, when the oxygen storage amount OSA of the upstream catalyst 20 decreases and the air-fuel ratio of the exhaust gas flowing out from the upstream catalyst 20 decreases, the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 is determined to be rich in step S13. It is determined that the air-fuel ratio is AFrich or less. In this case, the control routine proceeds to step S15, and the target air-fuel ratio AFT is switched to the lean set air-fuel ratio AFTlean. Next, in step S16, the lean setting flag Fl is set to ON, and then the control routine is terminated.

When the lean setting flag Fl is set to ON, the next control routine proceeds from step S12 to step S17. In step S17, it is determined whether or not the cumulative oxygen excess / deficiency amount ΣOED after the target air-fuel ratio AFT is switched to the lean set air-fuel ratio AFTlean is equal to or greater than the switching reference value OEDref. If it is determined that the accumulated oxygen excess / deficiency amount ΣOED is less than the switching reference value OEDref, the control routine proceeds to step S18. In step S18, the target air-fuel ratio AFT is continuously maintained at the lean set air-fuel ratio AFTlean, after which the control routine is terminated. On the other hand, when the oxygen storage amount of the upstream catalyst 20 increases, it is determined in step S17 that the integrated oxygen excess / deficiency amount ΣOED is equal to or greater than the switching reference value OEDref, and the control routine proceeds to step S19. In step S19, the target air-fuel ratio AFT is switched to the rich set air-fuel ratio AFTrich. In step S20, the lean setting flag Fl is reset to OFF, and then the control routine is terminated.

≪Modified example of normal air-fuel ratio control≫
It should be noted that it is not always necessary to perform the above-mentioned control as the normal air-fuel ratio control performed when the fuel cut control or the fuel increase control for temporarily increasing the fuel supply amount is not performed. If the time average air-fuel ratio of the exhaust gas flowing into the downstream catalyst 24 is controlled to be a rich air-fuel ratio, various controls can be performed as normal air-fuel ratio control.

In the normal air-fuel ratio control according to the above embodiment, when the integrated oxygen excess / deficiency amount ΣOED reaches the switching reference amount OEDref, the target air-fuel ratio is switched from the lean set air-fuel ratio to the rich set air-fuel ratio. On the other hand, in this modification, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes the lean air-fuel ratio (specifically, when it becomes equal to or higher than the lean determination air-fuel ratio), the target air-fuel ratio becomes lean. The set air-fuel ratio can be switched to the rich set air-fuel ratio.

The above-mentioned operation will be specifically described with reference to FIG. FIG. 5 is a time chart similar to that of FIG. 3 such as a target air-fuel ratio AFT in the normal air-fuel ratio control according to one modification. In the illustrated example, the target air-fuel ratio AFT is switched to a lean set air-fuel ratio AFTlean at time t _1. After time t ₁ , the oxygen occlusal amount OSUp of the upstream catalyst 20 gradually increases, and finally at time t ₂ , the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the lean determined air-fuel ratio AFlean.

In the present embodiment, when the output air-fuel ratio AFdwn of the downstream air-fuel ratio sensor 41 reaches the lean determination air-fuel ratio AFlean, the target air-fuel ratio AFT is switched to the rich set air-fuel ratio AFTrich in order to reduce the oxygen storage amount OSAfr. At this time, the exhaust gas having a lean air-fuel ratio temporarily flows out from the upstream catalyst 20, and the exhaust gas having this lean air-fuel ratio flows into the downstream catalyst 24. As a result, in the vicinity of time t ₂ , the oxygen occluded amount OSAdwn of the downstream catalyst 24 is increased. After that, the cycle of t ₁ to t ₂ described above is repeated.

In this modification, in the target air-fuel ratio AFT, the total amount of unburned gas (total oxygen deficiency) flowing into the downstream catalyst 24 near time t ₁ flows into the downstream catalyst 24 near time t _2. It is controlled to be larger than the total amount of oxygen (total excess amount of oxygen). Specifically, for example, the lean-determined air-fuel ratio AFlean and the rich-determined air-fuel ratio AFrich so that the difference between the lean-determined air-fuel ratio AFlean and the theoretical air-fuel ratio becomes larger than the difference between the rich-determined air-fuel ratio AFrich and the theoretical air-fuel ratio. Is set. Alternatively, the time from when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the rich judgment air-fuel ratio until the target air-fuel ratio is switched to the lean air-fuel ratio is the time from when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 reaches the lean judgment air-fuel ratio. It may be set to be longer than the time from reaching to to switching the target air-fuel ratio to the rich air-fuel ratio. In any case, also in this modification, the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 24 is controlled so that the time average air-fuel ratio becomes the rich air-fuel ratio. As a result, during normal air-fuel ratio control, the inflow of exhaust gas having a lean air-fuel ratio containing NOx into the downstream catalyst 24 is suppressed, and thus the outflow of NOx from the downstream catalyst 24 is suppressed. To.

≪Lean spike control≫
By the way, in the internal combustion engine according to the present embodiment, fuel cut control is performed during deceleration of a vehicle equipped with the internal combustion engine. The fuel cut control is a control for stopping the injection of fuel from the fuel injection valve 11 even when the crankshaft and the piston 3 are in motion. When such fuel cut control is performed, air is discharged from the engine body 1, so that a large amount of air flows into both the exhaust purification catalysts 20 and 24. As a result, the oxygen storage amounts of both the exhaust gas purification catalysts 20 and 24 become the maximum oxygen storage amount Cmax.

Therefore, even if the air-fuel ratio control is performed so that the time-average air-fuel ratio of the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 24 becomes the rich air-fuel ratio, if the fuel cut control is performed regularly, The oxygen storage amount of the downstream catalyst 24 does not become close to zero. However, since the fuel cut control is performed during deceleration of the vehicle or the like, it is not always performed at regular time intervals. Therefore, in some cases, the oxygen storage amount of the downstream catalyst 24 may reach near zero.

Therefore, in the present embodiment, when the oxygen storage amount of the downstream side catalyst 24 becomes small, lean spike control for controlling the air-fuel ratio of the exhaust gas discharged from the engine body 1 to the lean air-fuel ratio is executed.

In particular, in the lean spike control according to the present embodiment, the amount of oxygen to be supplied to both the exhaust gas purification catalysts 20 and 24 is calculated by the lean spike control at the start time of the lean spike control. Then, the execution period of the lean spike control is calculated at the start time of the lean spike control so that the calculated amount of oxygen is supplied to the exhaust gas purification catalysts 20 and 24. The lean spike control is executed after being executed for the execution period calculated at this time, and then terminated. Therefore, the lean spike control according to the present embodiment is performed as a feed forward control. Hereinafter, this lean spike control will be specifically described.

(1) Execution conditions for lean spike control First, lean spike control is executed when a predetermined execution condition is satisfied. The execution condition of the lean spike control is a condition that is satisfied when the oxygen storage amount of the downstream side catalyst 24 decreases to near zero or becomes almost zero.

Specifically, in the first example, the oxygen excess / deficiency of the downstream catalyst 24 is based on the output air-fuel ratio of the downstream air-fuel ratio sensor 41 and the like, similar to the integrated oxygen excess / deficiency amount ΣOED of the upstream catalyst 20 described above. The amount is calculated. The oxygen occlusion amount of the downstream side catalyst 24 is estimated based on the oxygen excess / deficiency amount of the downstream side catalyst 24 calculated in this way. The execution condition of the lean spike control is satisfied when the estimated oxygen storage amount becomes less than the predetermined lower limit amount.

In the second example, the execution condition of the lean spike control is the time when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 has become the rich air-fuel ratio since the last time the lean spike control or the fuel cut control was performed. It is established when it exceeds the time.

In the third example, the execution condition of the lean spike control is equal to or more than the total execution time of the normal air-fuel ratio control or the total operating time of the internal combustion engine since the last time the lean spike control or the fuel cut control was performed. It is established when it becomes.

(2) Calculation of target oxygen supply amount When the execution conditions for lean spike control are satisfied, the amount of oxygen to be supplied to both exhaust gas purification catalysts 20 and 24 by lean spike control (hereinafter, also referred to as "target oxygen supply amount") is determined. It is calculated. In the present embodiment, the target oxygen supply amount O2sp is calculated by the following formula (2).
O2sp = (Cmaxupes-OSAupes) + (Cmaxdwnes ・ K)… (2)
Here, in the formula (2), Cmaxupes is an estimated value of the maximum occlusion amount of the upstream catalyst 20, OSAppes is an estimated value of the current oxygen occlusion of the upstream catalyst 20, and Cmaxdwn is the maximum occlusion of the downstream catalyst 24. The estimated value of the amount of possible oxygen, K represents a coefficient of 0 or more and 1 or less (for example, 0.7).

In the formula (2), the first term is the difference between the maximum occluded oxygen amount of the upstream catalyst 20 and the estimated value of the oxygen occluded amount of the upstream catalyst 20, that is, the currently occluded oxygen amount of the upstream catalyst 20 (hereinafter). , Also referred to as "upstream occlusal oxygen content").

On the other hand, in the formula (2), the second term is a value obtained by multiplying the maximum possible oxygen storage amount of the downstream catalyst 24 by a predetermined coefficient. Here, as described above, the lean spike control is executed when the oxygen storage amount of the downstream catalyst 24 is reduced to a predetermined amount near zero. Therefore, at the start of lean spike control, the upstream catalyst 20 always occludes the same amount of oxygen. That is, each time the lean spike control is started, the amount of oxygen that can be occluded by the downstream catalyst 24 (hereinafter, also referred to as "the amount of oxygen that can be occluded on the downstream side") is about the same as the maximum amount of occluded oxygen. It has become. Therefore, the second term of the equation (2) represents an estimated value of the amount of oxygen that can be occluded on the downstream side at the start of lean spike control.

Therefore, in the present embodiment, the target oxygen supply amount in the lean spike control is the total value of the estimated value of the upstream side occluded oxygen amount and the estimated value of the downstream side occluded oxygen amount at the start of the lean spike control. , Calculated based on these parameters. In the present embodiment, the amount of oxygen that can be occluded on the upstream side is that of the upstream catalyst 20 that is estimated based on the maximum amount of oxygen that can be occluded by the upstream catalyst 20 and the air-fuel ratio detected by the upstream air-fuel ratio sensor 40. It is calculated as the difference from the estimated value of oxygen storage. In other words, the amount of oxygen that can be occluded on the upstream side is estimated based on the air-fuel ratio detected by the air-fuel ratio sensor 40 on the upstream side.

Here, the estimated value OSAppes of the current oxygen occlusion amount of the upstream catalyst 20 in the formula (2) is estimated based on the above-mentioned integrated oxygen excess / deficiency amount ΣOED. The oxygen occlusion amount of the upstream catalyst 20 may be estimated by any other known method.

Further, the estimated value Cmaxupes of the maximum occluded oxygen amount of the upstream catalyst 20 in the formula (2) is estimated, for example, by performing the air-fuel ratio control as shown in FIG. 5 at least temporarily. Here, in the air-fuel ratio control shown in FIG. 5, at time t ₁ and the oxygen storage amount OSAup of the upstream catalyst 20 becomes zero at time t ₂ oxygen storage amount OSAup of the upstream catalyst 20 is the maximum possible storage The amount of oxygen Cmax has been reached. Therefore, the total amount of oxygen flowing into the upstream catalyst 20 between time t ₁ and time t ₂ , that is, the cumulative oxygen excess / deficiency amount ΣOED (Q1 in FIG. 5) at time t ₂ is the maximum of the upstream catalyst 20. It represents the amount of oxygen that can be stored Cmax. Similarly, the cumulative oxygen excess / deficiency amount ΣOED (Q2 in FIG. 5) at time t ₃ also represents the maximum occlusable oxygen amount Cmax of the upstream catalyst 20. Therefore, the maximum amount of oxygen that can be stored in the upstream catalyst 20 is estimated based on the accumulated oxygen excess / deficiency amount ΣOED at time t ₂ and time t ₃ when the air-fuel ratio control as shown in FIG. 5 is performed. The maximum amount of oxygen that can be occluded by the upstream catalyst 20 may be estimated by any other known method.

In addition, the estimated value Cmaxdwn of the maximum occluded oxygen amount of the downstream side catalyst 24 in the formula (2) is estimated based on, for example, the estimated value of the maximum occluded oxygen amount of the upstream side catalyst 20. Here, the oxygen occlusion amounts of the upstream catalyst 20 and the downstream catalyst 24 decrease as the catalysts 20 and 24 deteriorate over time. The degree of deterioration of the downstream catalyst 24 changes in proportion to the degree of deterioration of the upstream catalyst 20. Therefore, in the present embodiment, for example, the maximum occlusable oxygen amount of the downstream side catalyst 24 is estimated based on the estimated value of the maximum occlusable oxygen amount of the upstream side catalyst 20 by using the map as shown in FIG. To. The maximum amount of oxygen that can be occluded by the downstream catalyst 24 may be estimated by any other known method.

For the amount of oxygen that can be occluded on the upstream side represented by the first term of the above formula (2), the estimated value of the maximum amount of oxygen that can be occluded on the upstream side catalyst 20 is Cmaxupes and the estimated value of the amount of oxygen occluded on the upstream side catalyst 20 is OSAppes. Instead, it may be estimated directly from the accumulated oxygen excess / deficiency amount ΣOED. Further, the amount of oxygen that can be occluded on the downstream side represented by the second term of the above formula (2) is calculated by calculating the accumulated oxygen excess / deficiency amount ΣOED in the downstream side catalyst 24 based on the downstream side air fuel ratio sensor 41 and the calculated integrated amount. It may be calculated in the same manner as the amount of oxygen that can be occluded on the upstream side using the oxygen excess / deficiency amount ΣOED.

(3) Calculation of execution time When the target oxygen supply amount in the lean spike control is calculated, the execution time of the lean spike control is calculated. The execution time Tsp of the lean spike control is calculated by the following equation (3).
Tsp = O2sp ・ AFTslean / (AFTslean-AFST) / (Ga ・ 0.23)… (3)
In the formula (3), O2sp is the target oxygen supply amount calculated in the above formula (2), AFTslean is the strong lean set air-fuel ratio set as the target air-fuel ratio during execution of the lean spike control, and AFST is the theoretical air-fuel ratio. Ga represents the intake air flow rate (g / sec), respectively. In the present embodiment, the strong lean set air-fuel ratio AFTslean is leaner than the lean set air-fuel ratio AFTlean. Further, the intake air flow rate is detected by, for example, an air flow meter 39.

That is, in the present embodiment, the execution time of the lean spike control is calculated based on the target oxygen supply amount in the lean spike control, the strong lean set air-fuel ratio AFTslean, and the intake air flow rate. Considering that the target oxygen supply amount in the lean spike control is calculated based on the estimated value of the upstream side occluded oxygen amount and the estimated value of the downstream side occluded oxygen amount, the execution time of the lean spike control is set in this embodiment. It can also be said that it is calculated based on the estimated value of the amount of oxygen that can be occluded on the upstream side and the estimated value of the amount of oxygen that can be occluded on the downstream side. In particular, in the present embodiment, the execution time of the lean spike control is the execution of the lean spike control by oxygen corresponding to the total value of the estimated value of the upstream side occluded oxygen amount and the estimated value of the downstream side occluded oxygen amount. It is calculated so that it flows into the upstream catalyst 20. Further, in the present embodiment, as can be seen from the above, the execution period of the lean spike control is calculated without using the output of the downstream air-fuel ratio sensor 41 during the execution period of the lean spike control.

When the execution time of the lean spike control is calculated, the normal air-fuel ratio control is stopped and the lean spike control is started. When the lean spike control is started, the target air-fuel ratio is set to the strong lean set air-fuel ratio AFTslean, and therefore the exhaust gas of the strong lean set air-fuel ratio AFTslean is discharged from the engine main body 1.

Lean spike control is executed for the execution time calculated as described above, and then terminated. When the lean spike control is completed, the normal air-fuel ratio control is restarted. At this time, the target air-fuel ratio is switched from the strong lean set air-fuel ratio AFTslean to the rich set air-fuel ratio AFTrich.

≪Explanation of lean spike control using time chart≫
FIG. 7 is a time chart of the target air-fuel ratio AFT and the like when lean spike control is performed. The integrated oxygen supply amount in FIG. 7 indicates an integrated value of oxygen supplied to the upstream catalyst 20 during execution of the lean spike control.

In the example shown in FIG. 7, it is considered that a certain delay occurs even before the exhaust gas flowing out from the upstream catalyst 20 flows into the downstream catalyst 24. On the other hand, in order to make the explanation easy to understand, the delay that occurs from the switching of the target air-fuel ratio AFT to the arrival of the exhaust gas of the air-fuel ratio after switching reaches the upstream catalyst 20 is not taken into consideration.

In the illustrated example, the time t ₁ has previously been conducted is normally air-fuel ratio control shown in FIG. 3, also, the oxygen storage amount OSAdwn of the downstream catalyst 24 is relatively decreased from time t ₁ earlier ..

In the example shown in FIG. 7, at time t _1, the execution condition of the lean spike control are satisfied. Thus, at time t _1, with the execution time Tsp of the lean spike control is calculated, the target air-fuel ratio AFT is switched to the strong lean set air-fuel ratio AFTslean from a rich set air-fuel ratio AFTrich.

When the target air-fuel ratio is switched to the strong lean set air-fuel ratio AFTslean at time t ₁ , the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 changes to the lean air-fuel ratio, and the integrated oxygen supply amount to the upstream catalyst 20 increases. I will do it. As a result, it increases the oxygen storage amount OSAup of the upstream catalyst 20 gradually reaches the maximum storable oxygen amount Cmax at time t _2. Therefore, after time t ₂ , the exhaust gas having a lean air-fuel ratio containing oxygen flows out from the upstream catalyst 20. However, because of the distance between the upstream catalyst 20 and the downstream catalyst 24, immediately a time t ₂ later it does not reach the exhaust gas of a lean air-fuel ratio on the downstream side catalyst 24. Therefore, the output air-fuel ratio AFdwn time t ₂ later also the downstream air-fuel ratio sensor 41 is maintained substantially remain stoichiometric air-fuel ratio.

After that, at time t ₃ , the elapsed time from time t ₁ reaches the execution time Tsp. At this time, the integrated oxygen supply amount to the upstream catalyst 20 during the execution of the lean spike control is an amount corresponding to the target oxygen supply amount O2sp. Elapsed time lean spike control at time t ₃ has reached the execution time Tsp is terminated, usually the air-fuel ratio control is resumed. For this reason, the target air-fuel ratio at time t ₃ is switched from the strong lean set air-fuel ratio AFTslean to a rich set air-fuel ratio AFTrich. Thus, at time t _3, the output air-fuel ratio AFup of the upstream air-fuel ratio sensor 40 changes to a rich air-fuel ratio, the oxygen storage amount OSA of the upstream catalyst 20 gradually decreases.

In the example shown in FIG. 7, then, at time t _4, the exhaust gas of a lean air-fuel ratio flowing out of the upstream catalyst 20 at time t ₂ later reaches the downstream catalyst 24. Thus, at time t ₄ later, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 is lean, the oxygen storage amount OSAdwn of the downstream catalyst 24 gradually increases. Then, at time t _5, the target air-fuel ratio AFT at time t ₃ in association with the switching to the rich set air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 24 returns substantially to the stoichiometric air-fuel ratio. In the present embodiment, at time t _5, the oxygen storage amount OSAdwn of the downstream catalyst 24 has become the amount of the vicinity of the maximum storable amount of oxygen Cmax, therefore the oxygen storage amount OSAdwn of the downstream catalyst 24 is caused to recover ..

≪Flowchart≫
FIG. 8 is a flowchart showing a control routine related to lean spike control. The illustrated control routine is executed by the control device at regular time intervals.

First, in step S21, it is determined whether or not the execution flag of the lean spike control is OFF. The execution flag is a flag that is turned ON during the execution of lean spike control and turned OFF at other times. If it is determined in step S21 that the execution flag is OFF, the control routine proceeds to step S22.

In step S22, it is determined whether or not the execution condition of the lean spike control is satisfied. The execution condition is satisfied, for example, when the estimated value of the oxygen occlusion amount OSAdwn of the downstream side catalyst 24 becomes less than the lower limit amount as described above. If it is determined that the execution condition is not satisfied, the control routine proceeds to step S23. In step S23, the air-fuel ratio control is normally executed, and the control routine is terminated.

On the other hand, if it is determined in step S22 that the execution condition for lean spike control is satisfied, the control routine proceeds to step S24. In step S24, the target oxygen supply amount O2sp is calculated based on the above formula (2). Next, in step S25, the execution time Tsp is calculated using the above formula (3) based on the target oxygen supply amount O2sp calculated in step S24. Next, in step S26, lean spike control is started, so that the target air-fuel ratio is set to the strong lean set air-fuel ratio AFTslean. Next, in step S27, the execution flag of the lean spike control is set to ON, and the control routine is terminated.

When the execution flag of the lean spike control is set to ON, the next control routine proceeds from step S21 to step S28. In step S28, it is determined whether or not the elapsed time T from the start of the lean spike control becomes equal to or greater than the execution time Tsp calculated in step S25 before the start of the lean spike control. If it is determined in step S28 that the elapsed time T is less than the execution time Tsp, the lean spike control is continued and the control routine is terminated.

On the other hand, if it is determined in step S28 that the elapsed time T is equal to or greater than the execution time Tsp, the control routine proceeds to step S29. In step S29, lean spike control is terminated. Next, in step S30, the execution flag of the lean spike control is reset to OFF. Next, in step S23, the normal air-fuel ratio control is executed, so that the target air-fuel ratio is set to the rich set air-fuel ratio AFTrich.

≪Action / effect≫
By the way, when the lean degree of the target air-fuel ratio during execution of the lean spike control is small, the oxygen concentration with respect to the NOx concentration in the exhaust gas flowing into the downstream catalyst 24 is low. Therefore, even if the exhaust gas having a lean air-fuel ratio flows in, the oxygen storage amount of the downstream catalyst 24 is unlikely to increase. On the other hand, in the present embodiment, the target air-fuel ratio is set to the lean air-fuel ratio having a relatively large degree of lean during the execution of the lean spike control. Therefore, the oxygen concentration with respect to the NOx concentration in the exhaust gas flowing into the downstream side catalyst 24 is high, and therefore the oxygen storage amount of the downstream side catalyst 24 tends to increase during the execution of the lean spike control.

On the other hand, as described above, after switching the target air-fuel ratio, there is a delay between the time when the exhaust gas having the changed air-fuel ratio flows into the downstream catalyst 24. In particular, when the target air-fuel ratio during execution of lean spike control is set to the lean air-fuel ratio with a large degree of lean, as shown in FIG. 7, when the output air-fuel ratio of the downstream air-fuel ratio sensor 41 becomes the lean air-fuel ratio. In some cases, a sufficient amount of oxygen has already been discharged from the engine body 1 to restore the oxygen storage amount of the downstream catalyst 24.

Since there is a delay before flowing into the downstream catalyst 24 in this way, if the lean spike control is terminated after the output air-fuel ratio of the downstream air-fuel ratio sensor 41 changes to the lean air-fuel ratio, the downstream catalyst 24 is required. Oxygen flows in above. Then, in some cases, the oxygen storage amount OSAdwn of the downstream side catalyst 24 may reach the maximum amount of oxygen that can be stored, and oxygen and NOx may flow out from the downstream side catalyst 24.

On the other hand, in the present embodiment, the execution time of the lean spike control is set at the start of the lean spike control, and the lean spike control is terminated after the execution time has elapsed. That is, in the present embodiment, the end timing of the lean spike control is set regardless of the output air-fuel ratio of the downstream air-fuel ratio sensor 41 during the execution of the lean spike control. Therefore, it is possible to prevent oxygen from flowing into the downstream catalyst 24 more than necessary and NOx from flowing out from the downstream catalyst 24.

In particular, in the present embodiment, as shown in FIG. 7, the lean spike control is terminated before the output air-fuel ratio of the downstream air-fuel ratio sensor 41 changes to the lean air-fuel ratio. Therefore, in the present embodiment, during the execution of the lean spike control so that the lean spike control is terminated before the downstream air-fuel ratio sensor 41 detects that the air-fuel ratio of the exhaust gas has reached the lean air-fuel ratio. It can be said that the target air-fuel ratio of the exhaust gas discharged from the engine body 1 is set.

≪Modification example≫
In the present embodiment, the target air-fuel ratio during execution of the lean spike control is set to a strong lean set air-fuel ratio AFTslean that is leaner than the lean set air-fuel ratio AFTlean. However, the target air-fuel ratio during execution of lean spike control does not necessarily have to be leaner than the lean set air-fuel ratio AFTlean over the entire execution period of lean spike control. That is, the target air-fuel ratio during execution of lean spike control is preferably lean, at least temporarily, rather than the most lean air-fuel ratio during execution of normal air-fuel ratio control. In other words, the air-fuel ratio of the exhaust gas discharged from the engine body 1 reaches at least temporarily during the execution of the lean spike control, and the exhaust gas discharged from the engine body 1 reaches at least temporarily during the execution of the normal air-fuel ratio control. It is preferable that the air-fuel ratio of the exhaust gas is controlled so as to be lean rather than the leanest air-fuel ratio.

Further, in the above embodiment, the oxygen occlusion amount of the upstream catalyst 20 is estimated based on the output air-fuel ratio of the upstream air-fuel ratio sensor 40. However, the oxygen storage amount of the upstream side catalyst 20 is not based on the output air-fuel ratio of the upstream side air-fuel ratio sensor 40, but based on the intake air flow rate detected by the air flow meter 39 and the fuel supply amount from the fuel injection valve 11. It may be estimated.

In addition, in the above embodiment, the amount of oxygen that can be occluded on the downstream side is estimated based on the maximum amount of oxygen that can be occluded on the downstream side catalyst 24 (paragraph 2 of the above formula (2)). However, the amount of oxygen that can be occluded on the downstream side can be calculated based on the maximum amount of oxygen that can be occluded on the downstream side catalyst 24 and the estimated value of the amount of oxygen that can be occluded on the downstream side catalyst 24, similarly to the amount of oxygen that can be occluded on the upstream side. Good. In this case, the oxygen occlusion amount of the downstream side catalyst 24 is estimated based on the output air-fuel ratio of the downstream side air-fuel ratio sensor 41.

Further, in the above embodiment, the execution time of the lean spike control is calculated first, and the lean spike control is executed over this execution time. However, instead of the execution time, other parameters that change depending on the execution time may be used. Therefore, for example, instead of the execution time of the lean spike control, the number of cycles in which the lean spike control is executed may be calculated first, and the lean spike control may be executed over this number of cycles. Considering the above, it can be said that the control device first calculates the execution period of the lean spike control and executes the lean spike control over this execution period.

<Second embodiment>
Next, the exhaust gas purification device according to the second embodiment will be described. The configuration and control of the exhaust gas purification device according to the second embodiment are basically the same as the configuration and control of the exhaust gas purification device according to the first embodiment. Therefore, in the following, the parts different from the exhaust gas purification device according to the first embodiment will be mainly described.

In the first embodiment, in the lean spike control, the target air-fuel ratio is controlled to be leaner than the lean set air-fuel ratio in the normal air-fuel ratio control. On the other hand, in the present embodiment, in lean spike control, fuel cut control for stopping fuel injection from the fuel injection valve 11 during operation of the internal combustion engine in at least a part of the plurality of cylinders of the internal combustion engine. Is done.

In the present embodiment, the target oxygen supply amount in the lean spike control is calculated as in the first embodiment, and then the execution time of the lean spike control is calculated. At this time, the average air-fuel ratio AFTav of the exhaust gas discharged from the engine body 1 during the execution of the lean spike control is calculated by the following equation (4). Then, the execution time Tsp of the lean spike control is calculated by the following equation (5).
AFTav = AFTco ・ Nall / (Nall-Nfc)… (4)
Tsp = O2sp ・ AFTav / (AFTav-AFST) / (Ga ・ 0.23)… (5)
In equation (4), AFTav is the target air-fuel ratio (for example, the theoretical air-fuel ratio) of the exhaust gas emitted from the cylinders being burned during lean spike control, and All is the number of all cylinders of the internal combustion engine, Nfc. Represents the number of cylinders for which fuel cut control is performed.

That is, in the present embodiment, the execution time of the lean spike control is the target oxygen supply amount in the lean spike control, the target air-fuel ratio AFTco of the exhaust gas discharged from the cylinder in which combustion is performed, and the cylinder in which fuel cut control is performed. It is calculated based on the number Nfc of the intake air flow rate. Since the target oxygen supply amount in the lean spike control is also used for calculating the execution time of the lean spike control, the execution time of the lean spike control is the estimated value of the amount of oxygen that can be occluded on the upstream side and the downstream side in the present embodiment as well. It can be said that it is calculated based on the estimated value of the amount of oxygen that can be stored.

According to this embodiment, fuel cut control is performed in lean spike control. Since air is discharged from the cylinder in which the fuel cut control is performed, NOx is not contained in the exhaust gas from this cylinder. Therefore, during the execution of the lean spike control, the exhaust gas having a low NOx concentration flows into the downstream catalyst 24, and the outflow of NOx from the downstream catalyst 24 is suppressed.

When fuel cut control is performed in some cylinders, for example, in cylinders where fuel cut control is not performed, the amount of air and fuel supplied to the combustion chamber 5 is temporarily increased to start fuel cut control. The output fluctuation of the internal combustion engine may be suppressed with the termination.

≪Modification example≫
Next, the exhaust gas purification device according to the modified example of the second embodiment will be described with reference to FIG. By the way, if the fuel cut control is performed once (one cycle) in one cylinder, a relatively large amount of oxygen will flow out from the engine body 1. Therefore, the amount of oxygen supplied from the engine body 1 to the exhaust system cannot be finely adjusted during the lean spike control only by the fuel cut control. Therefore, in this modification, even if the execution condition of the lean spike control is satisfied, the lean spike control is not started immediately, and the lean spike control is started after the oxygen storage amount of the upstream catalyst 20 becomes an appropriate amount. I try to do it.

Specifically, in the present embodiment, the target oxygen supply amount O2sp in the lean spike control is calculated as in the first embodiment. After that, the amount of oxygen O2one flowing out from the engine body 1 due to the fuel cut control being performed once in one cylinder is calculated by, for example, the following equation (6).
O2one = Ga ・ 0.23 / (Nall / Nfc ・ RE / 60)… (6)
In formula (6), RE represents the engine speed (rpm).

After that, the target oxygen supply amount O2sp in the lean spike control is divided by the oxygen amount O2one flowing out in one fuel cut control, and the quotient obtained as a result is defined as Q and the remainder is defined as R. Here, the surplus R indicates an amount of oxygen that is insufficient with respect to the target oxygen supply amount O2sp in the lean spike control when the fuel cut control is performed Q times.

Therefore, in the present embodiment, when the execution condition of the lean spike control is satisfied, the target air-fuel ratio is set to the lean set air-fuel ratio (the air-fuel ratio set in the normal air-fuel ratio control). Then, the target air-fuel ratio is maintained at the lean set air-fuel ratio from the time when the execution condition of the lean spike control is satisfied until the amount of oxygen corresponding to the surplus R flows into the upstream catalyst 20. In other words, in the present embodiment, the lean spike control is started after the estimated value of the oxygen storage amount of the upstream catalyst 20 reaches an amount increased by the residual R from the value when the execution condition of the lean spike control is satisfied. Will be done.

FIG. 9 is a flowchart showing a control routine related to lean spike control according to this modification. The illustrated control routine is executed by the control device at regular time intervals. Since steps S41 to S44 and S52 to S53 are the same as steps S21 to S24 and S29 to S30 in FIG. 8, the description thereof will be omitted.

In step S45, the execution time Tsp of the lean spike control is calculated based on the target oxygen supply amount O2sp calculated in step S44. Specifically, the amount of oxygen O2one flowing out from the engine body 1 is calculated by performing the fuel cut control once in one cylinder using the above equation (6), and the target oxygen supply amount O2sp is set to this oxygen amount O2one. Divide by with to calculate the quotient Q and the surplus R. The value obtained by multiplying the quotient Q calculated in this way by the time interval at which the fuel cut control is performed is calculated as the execution time Tsp.

Next, in step S46, the delay time Td is calculated based on the remainder R. Specifically, it is calculated by dividing the surplus R by the amount of oxygen flowing into the upstream catalyst 20 in a unit time when the target air-fuel ratio is set to the lean set air-fuel ratio. Next, in step S48, the execution flag of the lean spike control is set to ON, and the control routine is terminated.

When the execution flag of the lean spike control is set to ON, the next control routine proceeds from step S41 to step S49. In step S41, it is determined whether or not the elapsed time T after the execution condition of the lean spike control is satisfied is equal to or greater than the delay time Td calculated in step S46. If it is determined in step S49 that the elapsed time T is less than the delay time Td, the control routine is terminated without starting the lean spike control.

On the other hand, if it is determined in step S49 that the elapsed time T is equal to or greater than the delay time Td, the control routine proceeds to step S50. Lean spike control is executed in step S50. Next, in step S51, it is determined whether or not the elapsed time T is equal to or greater than the sum of the delay time Td and the execution time Tsp. When it is determined that the elapsed time T is less than the sum of the delay time Td and the execution time Tsp, the lean spike control is continued and the control routine is terminated. On the other hand, if it is determined in step S51 that the elapsed time T is equal to or greater than the sum of the delay time Td and the execution time Tsp, the process proceeds to step S52.

According to the present embodiment, the target air-fuel ratio is set to the lean set air-fuel ratio having a relatively small lean degree or the rich set air-fuel ratio having a relatively small rich degree, and the oxygen occlusion amount OSUp of the upstream catalyst 20 is appropriately set. After adjustment, lean spike control is started. Therefore, the amount of oxygen flowing into the downstream catalyst 24 can be appropriately adjusted.

In this modification, the target air-fuel ratio is set to the lean set air-fuel ratio when the execution condition of the lean spike control is satisfied. However, when the target air-fuel ratio when the execution condition of the lean spike control is satisfied is the rich air-fuel ratio, the start of the lean spike control may be delayed while maintaining the target air-fuel ratio at the rich air-fuel ratio. In that case, assuming that the amount obtained by subtracting the remainder R from the amount of oxygen O2one that flows out in one fuel cut control is R', the amount of oxygen corresponding to the above R'is reduced from the time when the execution condition of the lean spike control is satisfied. The execution of lean spike control is delayed until the resulting unburned gas flows in. In other words, the lean spike control is started after the estimated value of the oxygen storage amount of the upstream catalyst 20 reaches an amount reduced by R'from the value when the execution condition of the lean spike control is satisfied.

<Third Embodiment>
Next, the exhaust gas purification device according to the third embodiment will be described with reference to FIG. The configuration and control of the exhaust gas purification device according to the third embodiment are basically the same as the configuration and control of the exhaust gas purification device according to the first embodiment and the second embodiment. Therefore, in the following, the parts different from the exhaust gas purification device according to the first embodiment and the second embodiment will be mainly described.

In the first and second embodiments, the average air-fuel ratio per cycle is maintained constant during the execution of lean spike control. For example, in the first embodiment, the target air-fuel ratio is maintained at the strong lean set air-fuel ratio AFTslean during the execution of lean spike control. On the other hand, in the present embodiment, the degree of leanness of the air-fuel ratio of the exhaust gas discharged from the engine body 1 at the start of the lean spike control is the lean degree of the exhaust gas discharged from the engine body 1 at the end of the lean spike control. The air-fuel ratio of the exhaust gas discharged from the engine body 1 is changed during the execution of the lean spike control so that the air-fuel ratio becomes smaller than the lean degree. In particular, in the present embodiment, the target air-fuel ratio at the start of lean spike control is set to the rich set air-fuel ratio AFTlean, and the target air-fuel ratio at the end of lean spike control is set to the strong lean set air-fuel ratio AFTslean. ..

Specifically, in the present embodiment, when the execution condition of the lean spike control is satisfied, the estimated value O2sp1es of the upstream side occluded oxygen amount and the estimated value O2sp2es of the downstream side occluded oxygen amount are calculated by the above-mentioned method. To. Then, the first execution time Tsp1 in which the target air-fuel ratio is set to the lean set air-fuel ratio is calculated by the following equation (7), and the second execution time Tsp2 in which the target air-fuel ratio is set to the strong lean set air-fuel ratio is calculated. It is calculated by the following formula (8).
Tsp1 = O2sp1es ・ AFTlean / (AFTlean-AFST) / (Ga ・ 0.23)… (7)
Tsp2 = O2sp2es ・ AFTslean / (AFTslean-AFST) / (Ga ・ 0.23)… (8)

That is, in the present embodiment, the first execution time Tsp1 is calculated based on the estimated value O2sp1es of the amount of oxygen that can be occluded on the upstream side. In addition, the second execution time Tsp2 is calculated based on the estimated value O2sp2es of the amount of oxygen that can be occluded on the downstream side. Then, these execution times Tsp1 and Tsp2 are calculated without using the output of the downstream air-fuel ratio sensor 41 during the execution period of the lean spike control.

When the first execution time and the second execution time are calculated, the normal air-fuel ratio control is stopped and the lean spike control is started. When the lean spike control is started, the target air-fuel ratio is first set to the lean set air-fuel ratio AFTlean, so that the exhaust gas of the lean set air-fuel ratio AFTrene flows into the upstream catalyst 20. The state in which the target air-fuel ratio is set to the lean set air-fuel ratio AFTlean is maintained for the first execution time Tsp1.

After that, when the first execution time elapses after the lean spike control is started, the target air-fuel ratio is changed to the strong lean set air-fuel ratio AFTslean, so that the exhaust gas of the strong lean set air-fuel ratio AFTslean flows into the upstream catalyst 20. It will be. The state in which the target air-fuel ratio is set to the strong lean set air-fuel ratio AFTslean is maintained for the second execution time Tsp2.

Therefore, in the present embodiment, the degree of leanness of the air-fuel ratio of the exhaust gas discharged from the engine body 1 from the start of the lean spike control to the inflow of the amount of oxygen corresponding to the amount of occlusable oxygen on the upstream side into the upstream catalyst 20. However, it is smaller than the lean degree of the air-fuel ratio of the exhaust gas discharged from the engine body 1 from the time when the amount of oxygen corresponding to the amount of occlusable oxygen on the upstream side flows into the upstream catalyst 20 until the end of the lean spike control. As described above, the air-fuel ratio of the exhaust gas discharged from the engine body 1 is changed during the execution of the lean spike control.

FIG. 10 is a time chart similar to that of FIG. 7, such as a target air-fuel ratio AFT when lean spike control according to the present embodiment is performed. In FIG. 10, as in FIG. 7, it is considered that a certain delay occurs even before the exhaust gas flowing out from the upstream catalyst 20 flows into the downstream catalyst 24.

In the illustrated example, at time t _1, the execution condition of the lean spike control is satisfied, the lean spike control is started. In the present embodiment, immediately after the start of the lean spike control, the target air-fuel ratio AFT is set to the lean set air-fuel ratio AFTlean similar to the normal air-fuel ratio control. As a result, it increases the oxygen storage amount OSAup of the upstream catalyst 20 gradually reaches from time t ₁ to the maximum storable oxygen amount Cmax at time t ₂ the first execution time Tsp1 has elapsed.

At time t ₂ , the target air-fuel ratio AFT is changed to the strong lean set air-fuel ratio AFTslean. Further, the time t ₂ or later exhaust gas of a lean air-fuel ratio from the upstream side catalyst 20 flows out, immediately not reach the downstream catalyst 24.

After that, at time t ₃ , the elapsed time from time t ₂ reaches the second execution time Tsp 2. Thus, at time t _3, the lean spike control is ended, usually is air-fuel ratio control resumes, the target air-fuel ratio is switched to a rich set air-fuel ratio AFTrich.

After that, at time t ₄ , the exhaust gas having a lean air-fuel ratio that flows out from the upstream catalyst 20 after time t ₂ reaches the downstream catalyst 24, and the oxygen occlusion OSAdwn of the downstream catalyst 24 gradually increases. .. Then, at time t _5, the air-fuel ratio of the exhaust gas to the target air-fuel ratio AFT at time t ₃ in association with the switching to the rich set air-fuel ratio flowing into the downstream catalyst 24 is returned substantially to the stoichiometric air-fuel ratio. In the present embodiment, at time t _5, the oxygen storage amount OSAdwn of the downstream catalyst 24 is in an amount in the vicinity of its maximum storable oxygen amount Cmax.

≪Action / effect≫
By the way, when there is an error in the estimated value of the oxygen storage amount of the upstream catalyst 20, the actual oxygen storage amount of the upstream catalyst 20 has reached the maximum occluded oxygen amount Cmax, but the estimated value. May not reach the maximum occluded oxygen amount Cmax. If the target air-fuel ratio is set to the lean air-fuel ratio with a large degree of lean from the start of the lean spike control, the lean spike control will be executed to the end without noticing that such an error has occurred. Therefore, the target oxygen supply amount to both the exhaust gas purification catalysts 20 and 24 may become excessive.

On the other hand, according to the present embodiment, after the start of lean spike control, the target air-fuel ratio is lean with a low degree of lean until the oxygen occluded amount OSUp of the upstream catalyst 20 reaches the maximum occluded oxygen amount Cmax. Set to air-fuel ratio. Therefore, if the estimated value of the oxygen storage amount of the upstream catalyst 20 has an error as described above, the actual oxygen storage amount of the upstream catalyst 20 is lean after reaching the maximum occlusable oxygen amount Cmax. The exhaust gas of the fuel ratio reaches the air-fuel ratio sensor 41 on the downstream side. As a result, the output air-fuel ratio of the downstream air-fuel ratio sensor 41 changes to the lean air-fuel ratio before switching the target air-fuel ratio to the strong lean set air-fuel ratio AFTslean in order to recover the oxygen storage amount of the downstream catalyst 24.

Therefore, according to the present embodiment, it is possible to detect that an error has occurred in the estimated value of the oxygen occlusion amount of the upstream catalyst 20 before the lean spike control is executed to the end. When it is detected that the output air-fuel ratio of the downstream air-fuel ratio sensor 41 has changed to the lean air-fuel ratio during the execution of the lean spike control, the lean spike control is stopped or the execution time of the lean spike control is corrected. Therefore, it is possible to prevent the oxygen supply amount to both the exhaust gas purification catalysts 20 and 24 from becoming excessive.

In the present embodiment, the target air-fuel ratio is switched at the timing when an amount of oxygen corresponding to the amount of oxygen that can be occluded on the upstream side flows into the upstream catalyst 20. However, if there is an error in the estimated value of the oxygen storage amount of the upstream catalyst 20, and if the error can be detected before the lean spike control is executed to the end, the timing of switching the target air-fuel ratio is such a timing. The timing may be different from.

Although the preferred embodiments according to the present disclosure have been described above, the present disclosure is not limited to these embodiments, and various modifications and changes can be made within the scope of the claims.

1 Engine body 11 Fuel injection valve 20 Upstream catalyst 24 Downstream catalyst 31 ECU
40 Upstream air-fuel ratio sensor 41 Downstream air-fuel ratio sensor

Claims (13)

The air-fuel ratio of the exhaust gas discharged from the engine body is determined by the upstream catalyst provided in the exhaust passage of the internal combustion engine, the downstream catalyst provided in the exhaust passage on the downstream side in the exhaust flow direction from the upstream catalyst, and the exhaust gas discharged from the engine body. An exhaust gas purification device for an internal combustion engine provided with a control device for controlling.
The control device includes normal air-fuel ratio control for controlling the time-average air-fuel ratio of the exhaust gas flowing into the downstream catalyst so as to have a rich air-fuel ratio richer than the theoretical air-fuel ratio, and exhaust gas discharged from the engine body. It is possible to execute lean spike control that controls the air-fuel ratio of gas to a lean air-fuel ratio that is leaner than the theoretical air-fuel ratio.
When the execution condition of the lean spike control is satisfied, the execution period of the lean spike control is calculated based on the estimated value of the amount of oxygen that can be occluded on the upstream side, which is the amount of oxygen that the upstream catalyst can currently occlude, and then calculated. An exhaust gas purification device for an internal combustion engine that executes the lean spike control over a period of execution. An upstream air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas flowing into the upstream catalyst is further provided.
The exhaust gas purification device for an internal combustion engine according to claim 1, wherein the control device estimates the amount of oxygen that can be occluded on the upstream side based on the air-fuel ratio detected by the air-fuel ratio sensor on the upstream side. The control device has an estimated value of the current oxygen occlusion amount of the upstream side catalyst estimated based on the air fuel ratio detected by the upstream side air fuel ratio sensor and a maximum value of the oxygen amount that can be stored by the upstream side catalyst. The exhaust purification device for an internal combustion engine according to claim 2, wherein the amount of oxygen that can be stored on the upstream side is calculated as a difference from the maximum amount of oxygen that can be stored. 3. The control device calculates the maximum occlusable oxygen amount of the downstream catalyst, which is the maximum value of the oxygen occlusable by the downstream catalyst, based on the maximum occlusable oxygen amount of the upstream catalyst. The exhaust purification device for an internal combustion engine described in. The control device has a lean spike control calculated based on the amount of oxygen that can be occluded on the upstream side and the maximum amount of oxygen that can be occluded by the downstream catalyst, which is the maximum value of the amount of oxygen that can be occluded by the downstream catalyst. 2. 4. The execution period is calculated so that oxygen corresponding to the total value of the sum of the amount of oxygen that can be occluded on the downstream side at the start time flows into the upstream catalyst during the execution of the lean spike control. The exhaust purification device for an internal combustion engine according to any one item. In the control device, the air-fuel ratio of the exhaust gas flowing into the upstream catalyst flows into the upstream catalyst at least temporarily during the execution of the normal air-fuel ratio control. The exhaust gas of an internal combustion engine according to any one of claims 1 to 5, which controls the air-fuel ratio of the exhaust gas discharged from the engine body so as to be leaner than the most lean air-fuel ratio reached by the exhaust gas. Purification device. The internal combustion engine has a plurality of cylinders and has a plurality of cylinders.
While the lean spike control is being executed, fuel cut control is performed for at least a part of the plurality of cylinders so that the supply of fuel is stopped during the operation of the internal combustion engine. The exhaust gas purification device for an internal combustion engine according to any one of the above items. The exhaust gas purification device for an internal combustion engine according to claim 7, wherein the lean spike control is executed after the estimated value of the oxygen storage amount of the upstream catalyst reaches a predetermined storage amount. In the control device, the degree of leanness of the air-fuel ratio of the exhaust gas discharged from the engine body at the start of the lean spike control is the air-fuel ratio of the exhaust gas discharged from the engine body at the end of the lean spike control. The exhaust of an internal combustion engine according to any one of claims 1 to 8, wherein the air-fuel ratio of the exhaust gas discharged from the engine body is changed during the execution of the lean spike control so as to be smaller than the degree of lean. Purification device. The control device has a lean degree of the air-fuel ratio of the exhaust gas discharged from the engine body from the start of the lean spike control until the amount of oxygen corresponding to the amount of oxygen that can be stored on the upstream side flows into the upstream catalyst. However, the degree of leanness of the air-fuel ratio of the exhaust gas discharged from the engine body from the time when the amount of oxygen corresponding to the amount of oxygen that can be stored on the upstream side flows into the upstream catalyst to the end of the lean spike control The exhaust gas purification device for an internal combustion engine according to claim 5, wherein the air-fuel ratio of the exhaust gas discharged from the engine body is changed during the execution of the lean spike control so as to be smaller. The exhaust purification of an internal combustion engine according to any one of claims 1 to 10, further comprising a downstream air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas flowing out of the upstream catalyst and flowing into the downstream catalyst. apparatus. The exhaust gas purification of an internal combustion engine according to claim 11, wherein the control device does not utilize the output of the downstream air-fuel ratio sensor during the execution period of the lean spike control when calculating the execution period of the lean spike control. apparatus. The control device is performing the lean spike control so that the lean spike control is terminated before the downstream air-fuel ratio sensor detects that the air-fuel ratio of the exhaust gas has reached the lean air-fuel ratio. The exhaust gas purification device for an internal combustion engine according to claim 12, wherein a target air-fuel ratio of exhaust gas discharged from the engine body is set.

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