JP2022191663A - Control device for internal combustion engine - Google Patents

Abstract

To provide a control device for an internal combustion engine enabling suppression of deterioration of exhaust emissions.SOLUTION: A control device for an internal combustion engine is applied to the internal combustion engine including an engine body, an upstream side catalyst and a downstream side catalyst which are provided on the upstream side and downstream side of an exhaust passage connected to the engine body, respectively and have an oxygen occlusion capacity, and a downstream side sensor provided on the downstream side of the downstream side catalyst in the exhaust passage to detect an air-fuel ratio of exhaust gas, and controls the air-fuel ratio of the internal combustion engine so as to switch the air-fuel ratio alternately between a rich air-fuel ratio smaller than a theoretical air-fuel ratio and a lean air-fuel ratio larger than the theoretical air-fuel ratio. The control device for the internal combustion engine includes: a determination section for determining whether or not an HC poisoning condition enabling HC poisoning of the downstream side catalyst is satisfied on the basis of the detected air-fuel ratio obtained by the downstream side sensor; and a control section for controlling the internal combustion engine so as to increase oxygen amount flowing into the downstream side catalyst when the determination section makes an affirmative determination, compared to when the determination section makes a negative determination.SELECTED DRAWING: Figure 3

Description

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

2. Description of the Related Art An internal combustion engine is known in which an upstream side catalyst and a downstream side catalyst having an oxygen storage capacity are provided on the upstream side and the downstream side of an exhaust passage, respectively (see Patent Document 1).

JP 2018-3777 A

The air-fuel ratio of the exhaust of the internal combustion engine is alternately switched between a rich air-fuel ratio and a lean air-fuel ratio. Therefore, when exhaust gas with a lean air-fuel ratio flows into the upstream side catalyst, oxygen is occluded by the oxygen storage capacity of the upstream side catalyst, and the air-fuel ratio of the exhaust gas flowing into the upstream side catalyst becomes richer than the air-fuel ratio of the exhaust gas flowing into the upstream side catalyst. of air-fuel ratio flows in. In this way, there are few opportunities for lean air-fuel ratio exhaust gas having a sufficient amount of oxygen to flow into the downstream side catalyst. As a result, HC poisoning, in which hydrocarbons in the exhaust adhere to the downstream side catalyst, may progress. If HC poisoning progresses in the downstream side catalyst, exhaust emissions may deteriorate.

Accordingly, it is an object of the present invention to provide a control device for an internal combustion engine that suppresses deterioration of exhaust emissions.

The above object is provided by an engine body, an upstream side catalyst and a downstream side catalyst provided respectively on the upstream side and the downstream side of an exhaust passage connected to the engine body and having an oxygen storage capacity, and the downstream side of the exhaust passage. A downstream sensor that detects the air-fuel ratio of the exhaust gas is provided downstream of the side catalyst, and the air-fuel ratio of the internal combustion engine is set to a rich air-fuel ratio that is smaller than the stoichiometric air-fuel ratio and a rich air-fuel ratio that is smaller than the stoichiometric air-fuel ratio. In a control device for an internal combustion engine that alternately switches between a lean air-fuel ratio and a large lean air-fuel ratio, an HC poisoning condition that can cause HC poisoning of the downstream side catalyst has been established based on the air-fuel ratio detected by the downstream sensor. and a determination unit for determining whether or not the above-mentioned determination unit makes an affirmative determination, the amount of oxygen flowing into the downstream side catalyst is increased more than when the determination unit makes a negative determination. and a control unit for controlling the internal combustion engine.

When the determination unit makes an affirmative determination, the control unit makes the air-fuel ratio of the lean air-fuel ratio exhaust flowing into the downstream side catalyst even leaner than when the determination unit makes a negative determination. The amount of oxygen flowing into the downstream side catalyst may be increased by controlling the air-fuel ratio of the internal combustion engine so as to increase the amount of oxygen flowing into the downstream side catalyst.

When the determination unit makes an affirmative determination, the control unit makes the inflow time of the lean air-fuel ratio exhaust gas flowing into the downstream side catalyst longer than when the determination unit makes a negative determination. The amount of oxygen flowing into the downstream side catalyst may be increased by controlling the air-fuel ratio of the internal combustion engine as described above.

When the determination unit makes an affirmative determination, the control unit cuts fuel in at least one of a plurality of cylinders of the internal combustion engine to reduce oxygen flowing into the downstream side catalyst. You can increase the amount.

A counter calculation unit that counts up a determination counter when the air-fuel ratio detected by the downstream sensor is equal to or lower than an upper limit value at which the downstream catalyst can be poisoned by HC, wherein the determination unit It may be determined that the HC poisoning condition is met when the threshold value is exceeded.

When the air-fuel ratio detected by the downstream sensor is a lean air-fuel ratio that is greater than the poisoning upper limit value and greater than the stoichiometric air-fuel ratio, the counter calculator counts down or resets the determination counter. good too.

The counter calculation section may execute one of counting down and resetting the determination counter when the processing for increasing the amount of oxygen flowing into the downstream side catalyst by the control section is stopped.

The counter calculation unit increases the count-up amount of the determination counter in at least one of the smaller the air-fuel ratio detected by the downstream sensor and the larger the flow rate of the exhaust gas flowing into the downstream side catalyst. may

The counter calculation unit increases the countdown amount of the determination counter when at least one of the larger the air-fuel ratio detected by the downstream sensor and the smaller the flow rate of the exhaust gas flowing into the two catalysts. good.

It is possible to provide a control device for an internal combustion engine that suppresses deterioration of exhaust emissions.

FIG. 1 is a schematic configuration diagram of an internal combustion engine. FIG. 2 is a timing chart showing a comparative example in which the downstream side catalyst is poisoned with HC. FIG. 3 is a timing chart showing an example of poisoning suppression control in this embodiment. FIG. 4 is a flowchart showing an example of poisoning suppression control according to this embodiment. FIG. 5 is a timing chart showing an example of the poisoning suppression process of this embodiment. FIG. 6 is a flowchart showing an example of the poisoning suppression process of this embodiment. FIG. 7 is a timing chart showing the poisoning suppression process of the first modified example. FIG. 8 is a flowchart showing the poisoning suppression process of the first modified example. FIG. 9 is a timing chart showing the poisoning suppression process of the second modified example. FIG. 10 is a flowchart showing the poisoning suppression process of the second modified example. FIG. 11A is a flowchart showing control for calculating the count-up amount of the determination counter of the first modification, and FIG. 11B is a map that defines the relationship between the detected air-fuel ratio AFc and the count-up amount. FIG. 12A is a flowchart showing control for calculating the count-up amount of the determination counter of the second modification, and FIG. 12B is a map that defines the relationship between the exhaust gas flow rate Fr and the count-up amount. FIG. 13A is a flowchart showing control for calculating the countdown amount of the determination counter of the first modification, and FIG. 13B is a map that defines the relationship between the detected air-fuel ratio AFc and the countdown amount. FIG. 14A is a flowchart showing the calculation control of the countdown amount of the determination counter of the second modification, and FIG. 14B is a map that defines the relationship between the exhaust gas flow rate Fr and the countdown amount.

[Schematic Configuration of Internal Combustion Engine]
FIG. 1 is a schematic configuration diagram of an internal combustion engine 1. As shown in FIG. The internal combustion engine 1 is mounted, for example, on a vehicle, but is not limited to this, and may be mounted on a vessel or the like other than a vehicle. The internal combustion engine 1 has an engine body 10 , an intake passage 20 and an exhaust passage 30 . The engine body 10 is a multi-cylinder engine having a plurality of cylinders, and each cylinder is provided with a combustion chamber 11, a piston 12, a spark plug 16, and the like. A connecting rod 13 and a crankshaft 14 are arranged inside the engine body 10 . Piston 12 is connected to crankshaft 14 by connecting rod 13 . The engine body 10 is provided with a rotation speed sensor 15 and an in-cylinder injection valve 17 for each cylinder. The rotation speed sensor 15 detects the rotation speed of the engine body 10 by detecting the rotation speed of the crankshaft 14 . In-cylinder injection valve 17 directly injects fuel into combustion chamber 11 . A port injection valve that injects fuel toward the intake port of the engine body 10 may be provided instead of the in-cylinder injection valve 17, or a port injection valve may be provided in addition to the in-cylinder injection valve 17. may be The spark plug 16 ignites the air-fuel mixture in the combustion chamber 11 . An intake passage 20 and an exhaust passage 30 are connected to an intake port and an exhaust port of the engine body 10, respectively. The intake valve 18a and the exhaust valve 18b open and close an intake port and an exhaust port of the engine body 10, respectively.

The intake passage 20 is provided with an air cleaner 21, an air flow meter 22, and a throttle valve 23 in order from the upstream side to the downstream side. The air cleaner 21 removes dust and the like from the air that flows in from the outside. The airflow meter 22 acquires the amount of intake air. The throttle valve 23 is driven by, for example, an actuator (not shown) to adjust the amount of intake air. As the opening of the throttle valve 23 increases, the amount of intake air increases, and as the opening decreases, the amount of intake air decreases.

Air is introduced from the intake passage 20 into the combustion chamber 11 by opening the intake valve 18a. A mixture of fuel and air injected from the in-cylinder injection valve 17 is compressed by the piston 12 and ignited by the ignition plug 16 . Ignition of the air-fuel mixture causes the piston 12 to reciprocate up and down in the combustion chamber 11, and the crankshaft 14 rotates. The exhaust after combustion is discharged from the exhaust passage 30 .

The exhaust passage 30 is provided with an upstream sensor 31a, an upstream catalyst 32a, a midstream sensor 31b, a downstream catalyst 32b, and a downstream sensor 31c in this order from upstream to downstream. The upstream sensor 31a, the midstream sensor 31b, and the downstream sensor 31c are air-fuel ratio sensors that detect the air-fuel ratio of the exhaust gas flowing through the exhaust passage 30, but are not limited thereto. It may be an oxygen concentration sensor capable of detecting the air-fuel ratio of the exhaust gas by detecting the oxygen concentration of the exhaust gas. The upstream sensor 31a detects the air-fuel ratio of the exhaust discharged from the engine body 10 and flowing into the upstream catalyst 32a. The midstream side sensor 31b detects the air-fuel ratio of the exhaust discharged from the upstream side catalyst 32a and flowing into the downstream side catalyst 32b. The downstream sensor 31c detects the air-fuel ratio of the exhaust discharged from the downstream catalyst 32b.

The upstream side catalyst 32a and the downstream side catalyst 32b are three-way catalysts containing catalytic metals such as platinum (Pt), palladium (Pd), rhodium (Rh), etc., and having an oxygen storage capacity. A three-way catalyst has a catalytic action and an oxygen storage capacity, and thus has a NOx and HC purifying action in accordance with the amount of oxygen stored. That is, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is a lean air-fuel ratio, the oxygen in the exhaust gas is stored by the three-way catalyst when the amount of oxygen stored in the three-way catalyst is small. is reduced and purified. As the amount of oxygen stored in the three-way catalyst increases, the concentrations of oxygen and NOx in the exhaust gas flowing out from the three-way catalyst increase. When the air-fuel ratio of the exhaust flowing into the three-way catalyst is a rich air-fuel ratio, the oxygen stored in the three-way catalyst is released when the amount of oxygen stored in the three-way catalyst is large, and the HC in the exhaust is oxidized and purified. be. When the amount of oxygen stored in the three-way catalyst decreases, the concentration of HC in the exhaust gas flowing out from the three-way catalyst increases. According to the three-way catalyst of this embodiment, the purification characteristics of NOx and HC in the exhaust gas change according to the air-fuel ratio and the oxygen storage amount of the exhaust gas flowing into the three-way catalyst. At least one of the upstream side catalyst 32a and the downstream side catalyst 32b may be a catalyst different from the three-way catalyst as long as it has catalytic action and oxygen storage capacity.

[Schematic configuration of ECU]
The ECU (Electric Control Unit) 100 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), a ROM (Read Only Memory), and a storage device such as a flash memory. Various controls are performed by executing The ECU 100 controls the spark plug 16, the in-cylinder injection valve 17, and the throttle valve 23 based on the operation amount of the accelerator pedal and the brake pedal operated by the driver, the rotation speed of the engine body 10, the load, and the like. The ECU 100 includes the rotation speed detected by the rotation speed sensor 15, the intake air amount detected by the air flow meter 22, and the detected air-fuel ratio AFa detected by the upstream sensor 31a, the midstream sensor 31b, and the downstream sensor 31c. AFb and AFc are input.

Although details will be described later, the ECU 100 alternately switches the air-fuel ratio of the exhaust gas discharged from the engine body 10 between a rich air-fuel ratio smaller than the stoichiometric air-fuel ratio ST and a lean air-fuel ratio larger than the stoichiometric air-fuel ratio ST. Control. Specifically, the ECU 100 controls the air-fuel ratio of the exhaust discharged from the engine body 10 as follows.

The ECU 100 controls the air-fuel ratio of the exhaust discharged from the engine body 10 so that the air-fuel ratio AFa detected by the upstream sensor 31a becomes the target air-fuel ratio TAF. Specifically, the ECU 100 adjusts the fuel injection amount from the in-cylinder injection valve 17 and the throttle valve based on the air-fuel ratio AFa detected by the upstream sensor 31a so that the air-fuel ratio detected by the upstream sensor 31a becomes the target air-fuel ratio TAF. By feedback-controlling the opening of 23, the air-fuel ratio of the exhaust gas discharged from the engine body 10 is controlled to the target air-fuel ratio TAF. Further, when the air-fuel ratio AFb detected by the midstream side sensor 31b becomes the lean judged air-fuel ratio LD larger than the stoichiometric air-fuel ratio ST, the ECU 100 sets the target air-fuel ratio TAF to a rich target air-fuel ratio TR which is smaller than the stoichiometric air-fuel ratio ST. set to The ECU 100 sets the target air-fuel ratio TAF to a lean target air-fuel ratio TL that is higher than the stoichiometric air-fuel ratio when the air-fuel ratio AFb detected by the midstream sensor 31b becomes the rich judged air-fuel ratio RD that is lower than the stoichiometric air-fuel ratio. As a result, the air-fuel ratio AFb detected by the midstream sensor 31b periodically fluctuates between the lean judged air-fuel ratio LD and the rich judged air-fuel ratio RD. The ECU 100 is an example of a control device for the internal combustion engine 1 . Further, the ECU 100 functionally realizes a determination unit, a control unit, and a counter calculation unit, which will be described later, by the above-described CPU, RAM, ROM, storage device, and the like.

[HC Poisoning of Downstream Catalyst]
FIG. 2 is a timing chart showing a comparative example in which the downstream side catalyst 32b is poisoned with HC. FIG. 2 shows transitions in the air-fuel ratios AFb and AFc detected by the midstream side sensor 31b and the downstream side sensor 31c, respectively, and the total emissions of HC and NOx from the upstream side catalyst 32a and the downstream side catalyst 32b, respectively.

As described above, the ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 so as to alternately switch between the rich air-fuel ratio and the lean air-fuel ratio. As a result, the detected air-fuel ratios AFb and AFc are also switched from the rich air-fuel ratio (time t0 to time t1) to the lean air-fuel ratio (time t1 to time t2), and thereafter alternately switched to the rich air-fuel ratio and the lean air-fuel ratio. Here, when exhaust gas with a lean air-fuel ratio flows into the upstream side catalyst 32a, oxygen is occluded in the upstream side catalyst 32a, and the air-fuel ratio of the exhaust gas discharged from the upstream side catalyst 32a and flowing into the downstream side catalyst 32b is changed to the upstream side catalyst 32a. The air-fuel ratio becomes leaner on the rich side than the air-fuel ratio of the exhaust gas that has flowed into the catalyst 32a. On the other hand, when rich air-fuel ratio exhaust gas flows into the upstream side catalyst 32a, the oxygen stored in the upstream side catalyst 32a is released, and the air-fuel ratio of the exhaust gas discharged from the upstream side catalyst 32a and flowing into the downstream side catalyst 32b becomes , the air-fuel ratio becomes leaner than the air-fuel ratio of the exhaust gas flowing into the upstream side catalyst 32a.

In this way, the downstream side catalyst 32b has few opportunities for the lean air-fuel ratio exhaust gas to flow into the downstream side catalyst 32b, but the rich air-fuel ratio exhaust gas continuously flows, so the operation of the internal combustion engine 1 continues. As a result, the surface of the downstream side catalyst 32b may be covered with hydrocarbons and the HC poisoning of the downstream side catalyst 32b may progress. As the HC poisoning of the downstream side catalyst 32b progresses, the contact between the downstream side catalyst 32b and the exhaust gas is inhibited. As a result, the total amount of HC and NOx discharged from the downstream side catalyst 32b increases (time t3). Exhaust emissions are thus degraded.

[Poisoning suppression control in this embodiment]
FIG. 3 is a timing chart showing an example of poisoning suppression control in this embodiment. FIG. 3 shows the detected air-fuel ratios AFb and AFc, the total amount of HC and NOx emitted from each of the upstream side catalyst 32a and the downstream side catalyst 32b, and the transition of the determination counter value. The determination counter is a counter that is counted up by a predetermined count-up amount when the detected air-fuel ratio AFc is equal to or lower than the poisoning upper limit value PU. The predetermined count-up amount is, for example, "1", but is not limited to this. The poisoning upper limit value PU is an upper limit value at which HC poisoning of the downstream side catalyst 32b progresses when the detected air-fuel ratio AFc is equal to or lower than the poisoning upper limit value PU. In this embodiment, the poisoning upper limit PU is set to the theoretical air-fuel ratio ST, specifically 14.6, but is not limited to this. may be

When the detected air-fuel ratio AFc is equal to or less than the poisoning upper limit PU (time t10-t11), the determination counter is counted up, and when the detected air-fuel ratio AFc is greater than the poisoning upper limit PU (time t11-t12), determination is made. A counter is maintained. By alternately switching the detected air-fuel ratio AFc between the rich air-fuel ratio and the lean air-fuel ratio, the determination counter increases stepwise. When the determination counter reaches or exceeds the threshold α (time t13), poisoning suppression processing is executed. The threshold value α is a lower limit value under which it can be assumed that the HC poisoning condition under which the downstream side catalyst 32b can be poisoned by HC is satisfied. The poisoning suppression process controls the internal combustion engine 1 to increase the amount of oxygen flowing into the downstream side catalyst 32b before the determination counter reaches or exceeds the threshold value α, thereby suppressing the progression of HC poisoning of the downstream side catalyst 32b. This is a process for suppressing This is because if the amount of oxygen flowing into the downstream side catalyst 32b is large, the oxidation of the hydrocarbons adhering to the surface of the downstream side catalyst 32b is accelerated and can be removed as CO, CO ₂ and H ₂ O. A specific control method for the poisoning suppression process will be described later.

When the poisoning suppression process is executed, the detected air-fuel ratio AFb, which corresponds to the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b, becomes a leaner value than the lean air-fuel ratio before time t13. The detected air-fuel ratio AFc, which corresponds to the air-fuel ratio of the exhaust gas flowing out of the catalyst 32b, also becomes a leaner value than the lean air-fuel ratio before time t13. Thus, the amount of oxygen flowing into the downstream side catalyst 32b increases. After that, when the poisoning suppression process stops (time t14), the determination counter is reset to "0". After time t14, the determination counter similarly increases, and when the determination counter reaches or exceeds the threshold α, the poisoning suppression process is executed again, after which the poisoning suppression process is stopped and the determination counter is reset to "0".

By repeatedly performing the poisoning suppression process in this manner, the progress of HC poisoning of the downstream side catalyst 32b can be suppressed, and the purification performance of the downstream side catalyst 32b can be maintained. Therefore, the total amount of HC and NOx discharged from the downstream side catalyst 32b can be suppressed. FIG. 3 shows the poison recovery lower limit LL, which will be described later.

FIG. 4 is a flowchart showing an example of poisoning suppression control according to this embodiment. This poisoning suppression control is repeatedly executed while the internal combustion engine 1 is running. The ECU 100 determines whether or not the detected air-fuel ratio AFc is equal to or less than the poisoning upper limit value PU (step S1). If Yes in step S1, the ECU 100 counts up the determination counter by a predetermined count-up amount (step S2). For example, in this embodiment, the count-up amount is "1", but it is not limited to this. The processing of steps S1 and S2 is an example of processing executed by the counter calculator.

Next, the ECU 100 determines whether or not the determination counter is greater than or equal to the threshold value α (step S3). In the case of No in step S3, it is assumed that the HC poisoning condition for causing HC poisoning of the downstream side catalyst 32b is not established, and this control ends. If Yes in step S3, the ECU 100 assumes that the HC poisoning condition under which the downstream side catalyst 32b can be poisoned by HC is satisfied, and the ECU 100 starts the poisoning suppression process (step S4). After executing the poisoning suppression process for a predetermined period of time, the ECU 100 stops the poisoning suppression process (step S5), and resets the determination counter to the initial value of "0" (step S6). In this manner, progress of HC poisoning of the downstream side catalyst 32b can be suppressed. Step S3 is an example of processing executed by the determination unit. Steps S4 and S5 are an example of processing executed by the control unit. The process of step S6 is an example of the process executed by the counter calculator.

If No in step S1, the ECU 100 determines whether or not the detected air-fuel ratio AFc is equal to or higher than the poisoning recovery lower limit LL (step S7). The poisoning recovery lower limit value LL is a lower limit value at which HC poisoning of the downstream side catalyst 32b can be recovered when the detected air-fuel ratio AFc is equal to or higher than the poisoning recovery lower limit value LL. .0, but is not limited to this. If No in step S7, this control ends. If Yes in step S7, the ECU 100 counts down the determination counter by a predetermined countdown amount (step S8). For example, in this embodiment, the countdown amount is "1", but it is not limited to this. Steps S7 and S8 are an example of processing executed by the counter calculator. When the detected air-fuel ratio AFc is greater than the poisoning upper limit value PU and less than the poisoning recovery lower limit value LL, the HC poisoning of the downstream side catalyst 32b does not progress but does not recover, so the determination counter is set at the current value. maintained at Steps S7 and S8 are an example of processing executed by the determination unit.

Thus, the value of the determination counter corresponds to the progress of HC poisoning of the downstream side catalyst 32b. Processing can be performed. As a result, it is possible to prevent the poisoning suppression process from being repeatedly executed when it is not necessary and destabilizing the operation of the internal combustion engine 1 .

Further, instead of using the detected air-fuel ratio AFb corresponding to the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b, the determination is made based on the detected air-fuel ratio AFc corresponding to the air-fuel ratio of the exhaust gas discharged from the downstream side catalyst 32b. Count up or count down a counter. For example, when the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b is a relatively small rich air-fuel ratio (strongly rich air-fuel ratio), if sufficient oxygen is stored in the downstream side catalyst 32b, the downstream side catalyst 32b The air-fuel ratio of the exhaust gas discharged from the side catalyst 32b becomes a relatively large rich air-fuel ratio (weakly rich air-fuel ratio). The air-fuel ratio of the exhaust gas discharged from is a medium rich air-fuel ratio (middle rich air-fuel ratio). Further, when the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b is a relatively large lean air-fuel ratio (strongly lean air-fuel ratio), when the downstream side catalyst 32b has a sufficient oxygen storage capacity, , the air-fuel ratio of the exhaust gas discharged from the downstream side catalyst 32b becomes a relatively small lean air-fuel ratio (weakly lean air-fuel ratio). The air-fuel ratio of the exhaust gas discharged from the downstream side catalyst 32b becomes a medium lean air-fuel ratio (intermediate lean air-fuel ratio). As described above, even if the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b is the same, the air-fuel ratio of the exhaust gas discharged from the downstream side catalyst 32b varies depending on the oxygen storage amount and oxygen storage capacity of the downstream side catalyst 32b. fluctuates. Here, if the air-fuel ratio of the exhaust discharged from the downstream side catalyst 32b is a rich air-fuel ratio, it means that the exhaust with the rich air-fuel ratio has passed through the downstream side catalyst 32b. The progress of HC poisoning depends on the richness of the rich air-fuel ratio of the exhaust discharged from the downstream side catalyst 32b. If the air-fuel ratio of the exhaust gas discharged from the downstream side catalyst 32b is a lean air-fuel ratio, it means that the lean air-fuel ratio exhaust gas has passed through the downstream side catalyst 32b. The degree of elimination depends on the lean degree of the lean air-fuel ratio of the exhaust gas discharged from the downstream side catalyst 32b. Therefore, by counting up or down the determination counter based on the detected air-fuel ratio AFc corresponding to the air-fuel ratio of the exhaust gas discharged from the downstream side catalyst 32b, the degree of progress of HC poisoning of the downstream side catalyst 32b can be accurately determined. can grasp.

Although the judgment counter is reset to "0" in step S6, the present invention is not limited to this, and the judgment counter may be counted down by a predetermined value, for example. For example, in this case, the countdown amount in step S6 may be increased more than the countdown amount in step S8. Also, in step S8, the determination counter is counted down, but the present invention is not limited to this, and the determination counter may be reset to "0". In this case, it is desirable to set the lower limit of poisoning recovery LL to a high value so that HC poisoning is suppressed by the poisoning suppression process to the same extent.

[Poisoning suppression treatment of the present embodiment]
Next, the poisoning suppression process of this embodiment will be specifically described. In the poisoning suppression process of this embodiment, the air-fuel ratio of the internal combustion engine 1 is controlled so that the air-fuel ratio of the lean air-fuel ratio exhaust flowing into the downstream side catalyst 32b becomes even leaner. FIG. 5 is a timing chart showing an example of the poisoning suppression process of this embodiment. FIG. 5 shows transitions of the target air-fuel ratio TAF and the detected air-fuel ratios AFb and AFc. First, before the poisoning suppression process is described, switching of the target air-fuel ratio TAF before the poisoning suppression process is executed will be described.

When the detected air-fuel ratio AFb decreases to the rich judged air-fuel ratio RD (time t21), the target air-fuel ratio TAF is switched from the predetermined rich target air-fuel ratio TR to the predetermined lean target air-fuel ratio TL. As a result, the oxygen in the exhaust flowing into the upstream side catalyst 32a is occluded in the upstream side catalyst 32a, whereby NOx in the exhaust is reduced and purified. NOx in the exhaust that has not been purified by the upstream side catalyst 32a is likewise purified by the downstream side catalyst 32b. As a result, the detected air-fuel ratios AFb and AFc rise from the rich side toward the stoichiometric air-fuel ratio ST. When the oxygen storage amount of the upstream side catalyst 32a increases, the oxygen concentration in the exhaust gas discharged from the upstream side catalyst 32a rises, and the detected air-fuel ratio AFb rises to the lean side. The oxygen concentration of the exhaust gas also slightly increases, and the detected air-fuel ratio AFc also slightly increases to the lean side.

When the detected air-fuel ratio AFb rises to the lean judged air-fuel ratio LD (time t22), the target air-fuel ratio TAF is switched from the lean target air-fuel ratio TL to the rich target air-fuel ratio TR. As a result, HC in the exhaust gas flowing into the upstream side catalyst 32a is oxidized and purified by the oxygen stored in the upstream side catalyst 32a. HC in the exhaust that has not been purified by the upstream side catalyst 32a is likewise purified by the downstream side catalyst 32b. As a result, the detected air-fuel ratios AFb and AFc decrease from the lean side toward the stoichiometric air-fuel ratio ST. When the oxygen storage amount of the upstream side catalyst 32a approaches zero, the oxygen concentration in the exhaust gas discharged from the upstream side catalyst 32a decreases and the detected air-fuel ratio AFb decreases to the rich side. The oxygen concentration of the discharged exhaust gas also decreases slightly, and the detected air-fuel ratio AFc also decreases slightly to the rich side. In this manner, the target air-fuel ratio TAF is alternately switched to the lean target air-fuel ratio TL or the rich target air-fuel ratio TR.

For example, when the determination counter reaches or exceeds the threshold α at time t23, poisoning suppression processing is executed. Specifically, when the determination counter reaches or exceeds the threshold α, the lean judged air-fuel ratio LD is further switched to the strong lean judged air-fuel ratio LDa on the lean side. As a result, the downstream side catalyst 32b can be supplied with exhaust gas having an air-fuel ratio leaner than the lean judged air-fuel ratio LD, and the amount of oxygen flowing into the downstream side catalyst 32b is increased. Further, after the target air-fuel ratio TAF is switched to the lean target air-fuel ratio TL at time t24, until the detected air-fuel ratio AFb reaches the strong lean judged air-fuel ratio LDa at time t25, the target air-fuel ratio TAF is set to the lean target air-fuel ratio. The fuel ratio is maintained at TL. That is, during the period from time t24 to t25, the target air-fuel ratio TAF is set to the lean target air-fuel ratio TL for a period longer than from time t21 to t22. Therefore, due to the poisoning suppression process of this embodiment, the inflow time of the lean air-fuel ratio exhaust gas flowing into the downstream side catalyst 32b is also lengthened, and this also increases the amount of oxygen flowing into the downstream side catalyst 32b. ing. As described above, the HC adhering to the downstream side catalyst 32b can be oxidized and removed by a large amount of oxygen, and the HC poisoning of the downstream side catalyst 32b can be suppressed. When the detected air-fuel ratio AFb reaches the strong lean judged air-fuel ratio LDa at time t25, the strong lean judged air-fuel ratio LDa is switched to the original lean judged air-fuel ratio LD, and the poisoning suppression process is stopped.

FIG. 6 is a flowchart showing an example of the poisoning suppression process of this embodiment. In the poisoning process, the ECU 100 switches the lean judged air-fuel ratio LD to the strong lean judged air-fuel ratio LDa (step S4-1). Next, the ECU 100 determines whether or not the detected air-fuel ratio AFb has become equal to or greater than the strong lean determination air-fuel ratio LDa (step S4-2). If No in step S4-2, step S4-1 is executed again. That is, the first example of the poisoning suppression process is continued. If Yes in step S4-2, the ECU 100 switches the strong lean judged air-fuel ratio LDa to the original lean judged air-fuel ratio LD (step S5-1). This stops the poisoning suppression process.

In the poisoning suppression process, after the determination counter reaches the threshold α or more, the detected air-fuel ratio AFb reaches the rich determined air-fuel ratio RD, and then the detected air-fuel ratio AFb reaches the strong lean determined air-fuel ratio LDa next time. is executed between, but not limited to. For example, when the detected air-fuel ratio AFb reaches the rich judged air-fuel ratio RD after the judgment counter reaches or exceeds the threshold α, the detected air-fuel ratio AFb changes to the strong lean judged air-fuel ratio LDa for the second time or a predetermined number of times after the second time. The poisoning suppression process may be executed until reaching .

[Poisoning suppression process of the first modification]
Next, poisoning suppression processing of the first modified example will be described. In the poisoning suppression process of the first modified example, the air-fuel ratio of the internal combustion engine 1 is controlled such that the inflow time of lean air-fuel ratio exhaust gas flowing into the downstream side catalyst 32b is lengthened. FIG. 7 is a timing chart showing the poisoning suppression process of the first modified example. FIG. 7 shows transitions of the target air-fuel ratio TAF, the detected air-fuel ratios AFb, and AFc. When the detected air-fuel ratio AFb becomes the rich judged air-fuel ratio RD at time t31, the target air-fuel ratio TAF is switched to the lean target air-fuel ratio TL. , when the detected air-fuel ratio AFb becomes the lean judged air-fuel ratio LD at time t32, the target air-fuel ratio TAF is switched to the rich target air-fuel ratio TR. After that, for example, when the determination counter reaches or exceeds the threshold value α at time t33, the control center of the target air-fuel ratio TAF is switched from the stoichiometric air-fuel ratio ST to the rich side control center TCa, which is on the richer side than the stoichiometric air-fuel ratio ST, at time t34. That is, the state in which the target air-fuel ratio TAF is alternately switched to the lean target air-fuel ratio TL or the rich target air-fuel ratio TR is switched to the slightly lean target air-fuel ratio TLa or the strong rich target air-fuel ratio TRa centered on the rich side control center TCa. be done. The rich side control center TCa is set to a value smaller than the stoichiometric air-fuel ratio ST but larger than the rich target air-fuel ratio TR. The difference between the slightly lean target air-fuel ratio TLa and the strong rich target air-fuel ratio TRa is set to the same value as the difference between the lean target air-fuel ratio TL and the rich target air-fuel ratio TR.

When the target air-fuel ratio TAF is switched from the lean target air-fuel ratio TL to the strong rich target air-fuel ratio TRa at time t34, the detected air-fuel ratio AFb quickly decreases from the lean judged air-fuel ratio LD to the rich judged air-fuel ratio RD at time t35. . This is because the strong rich target air-fuel ratio TRa is a value on the rich side that is even smaller than the rich target air-fuel ratio TR, so the detected air-fuel ratio AFb decreases to the rich side in a short period of time. When the detected air-fuel ratio AFb decreases to the rich judged air-fuel ratio RD at time t35, the target air-fuel ratio TAF is switched from the strong rich target air-fuel ratio TRa to the slightly lean target air-fuel ratio TLa. Here, since the slightly lean target air-fuel ratio TLa is a value on the rich side smaller than the lean target air-fuel ratio TL, it takes time from time t35 to time t36 when the detected air-fuel ratio AFb reaches the lean judged air-fuel ratio LD. Therefore, the detected air-fuel ratio AFb can be maintained at a lean air-fuel ratio for a long period of time, and the amount of oxygen flowing into the downstream side catalyst 32b can be increased. As a result, HC poisoning of the downstream side catalyst 32b can be suppressed. When the detected air-fuel ratio AFb reaches the lean judged air-fuel ratio LD at time t36, the control center of the target air-fuel ratio TAF is switched from the rich side control center TCa to the original theoretical air-fuel ratio ST, and the poisoning suppression process is stopped. .

FIG. 8 is a flowchart showing the poisoning suppression process of the first modified example. In the first modification of the poisoning process, the ECU 100 switches the control center of the target air-fuel ratio TAF from the stoichiometric air-fuel ratio ST to the rich side control center TCa (step S4-1a). Next, the ECU 100 determines whether or not the detected air-fuel ratio AFb has become equal to or greater than the lean determined air-fuel ratio LD (step S4-2a). If No in step S4-2a, step S4-1a is executed again. That is, the poisoning suppression process is continued. If Yes in step S4-2a, the ECU 100 switches the control center of the target air-fuel ratio TAF from the rich side control center TCa to the original stoichiometric air-fuel ratio ST (step S5-1a). This stops the poisoning suppression process.

Also in the poisoning suppression process of the first modified example, after the control center of the target air-fuel ratio TAF is switched to the rich side control center TCa after the determination counter becomes equal to or greater than the threshold value α, the second time or a predetermined number of times after the second time The poisoning suppression process may be executed until the detected air-fuel ratio AFb reaches the lean judged air-fuel ratio LD.

In the poisoning suppression process of the first modified example, the control center of the target air-fuel ratio TAF is switched to the rich side control center TCa, but it is not limited to this. It is also possible to switch to the slightly lean target air-fuel ratio TLa and use the rich target air-fuel ratio TR as it is as the rich target air-fuel ratio TR. In this case as well, it takes time from when the detected air-fuel ratio AFb reaches the rich judged air-fuel ratio RD until it reaches the lean judged air-fuel ratio LD. The progression of HC poisoning of the downstream side catalyst 32b can be suppressed.

The poisoning suppression process of the present embodiment and the poisoning suppression process of the first modified example described above may be performed at the same time. That is, the lean judged air-fuel ratio LD may be switched to the strongly lean judged air-fuel ratio LDa, and the control center of the target air-fuel ratio TAF may be switched from the stoichiometric air-fuel ratio ST to the rich side control center TCa. As a result, a larger amount of oxygen can flow into the downstream side catalyst 32b, and HC poisoning of the downstream side catalyst 32b can be suppressed.

[Poisoning suppression process of the second modification]
Next, the poisoning suppression process of the second modified example will be described. In the poisoning suppression process of the second modified example, fuel cut is executed for one of the plurality of cylinders of the internal combustion engine 1 . FIG. 9 is a timing chart showing the poisoning suppression process of the second modified example. FIG. 9 shows the transition of the target air-fuel ratio TAF, the detected air-fuel ratios AFb and AFc, and the state of the execution flag of the one-cylinder fuel cut control. The one-cylinder fuel cut control is to perform fuel cut control for stopping fuel injection to one of the plurality of cylinders of the internal combustion engine 1 as described above.

As shown in FIG. 9, when the detected air-fuel ratio AFb becomes the rich judged air-fuel ratio RD at time t41, the target air-fuel ratio TAF is switched to the lean target air-fuel ratio TL, and at time t42 the detected air-fuel ratio AFb changes to the lean judged air-fuel ratio LD. Then, the target air-fuel ratio TAF is switched to the rich target air-fuel ratio TR. After that, when the determination counter reaches or exceeds the threshold α at time t43, for example, the detected air-fuel ratio AFb becomes the lean determined air-fuel ratio LD and the target air-fuel ratio TAF is switched to the rich target air-fuel ratio TR at time t44. control is executed. It should be noted that the conventional control of fuel injection and air-fuel ratio is continued for cylinders that are not subject to fuel cut. As a result, a large amount of oxygen is supplied to the downstream side catalyst 32b from the cylinder targeted for fuel cut. Further, the speed at which the air-fuel ratio of the exhaust gas of the internal combustion engine 1 advances toward the rich side decreases, and accordingly, the speed at which the detected air-fuel ratio AFb, which indicates the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b, advances toward the rich side decreases. descend. As a result, exhaust gas with a lean air-fuel ratio flows into the downstream side catalyst 32b over a long period of time, and the amount of oxygen flowing into the downstream side catalyst 32b can be increased. As described above, HC poisoning of the downstream side catalyst 32b can be suppressed. After that, when the detected air-fuel ratio AFb becomes the rich judged air-fuel ratio RD at time t45, fuel injection is resumed in the cylinder for which the fuel cut was performed, and the 1-cylinder fuel cut control is stopped.

FIG. 10 is a flowchart showing the poisoning suppression process of the second modified example. In the second modification of the poisoning process, the ECU 100 determines whether or not the detected air-fuel ratio AFb has become equal to or greater than the lean determination air-fuel ratio LD (step S4-0b). If No in step S4-0b, this control ends. If Yes in step S4-0b, the ECU 100 executes a 1-cylinder fuel cut process (step S4-1b). Next, the ECU 100 determines whether or not the detected air-fuel ratio AFb has become equal to or less than the rich determined air-fuel ratio RD (step S4-2b). If No in step S4-2b, step S4-1b is executed again. That is, the poisoning suppression process is continued. If Yes in step S4-2b, the ECU 100 resumes fuel injection in the cylinders for which fuel was cut (step S5-1b). This stops the poisoning suppression process.

Also in the poisoning suppression process of the second modification, after the one-cylinder fuel cut process is started after the determination counter reaches or exceeds the threshold value α, the detected air-fuel ratio AFb becomes rich at the second time or a predetermined number of times after the second time. The one-cylinder fuel cut process may be continued until the determined air-fuel ratio RD is reached. In addition, in the poisoning suppression process of the second modification, only one cylinder is subject to fuel cut. However, the present invention is not limited to this. It may be a target. Thereby, the amount of oxygen flowing into the downstream side catalyst 32b can be further increased. Further, the poisoning suppression process of the present embodiment described above and at least one of the poisoning suppression processes of the second and third modifications may be performed at the same time.

[Calculation control of count-up amount in the first modification]
Calculation control of the count-up amount of the determination counter according to the first modification will be described. FIG. 11A is a flowchart showing control for calculating the count-up amount of the determination counter according to the first modification. This calculation control is performed, for example, when the process of step S2 is performed.

First, the ECU 100 acquires the detected air-fuel ratio AFc (step S11). Next, the ECU 100 calculates the count-up amount of the determination counter based on the acquired detected air-fuel ratio AFc (step S12). FIG. 11B is a map that defines the relationship between the detected air-fuel ratio AFc and the count-up amount. The vertical axis indicates the detected air-fuel ratio AFc, and the horizontal axis indicates the count-up amount. This map is stored in advance in the storage device of the ECU 100 or the like. In this map, the more the detected air-fuel ratio AFc decreases from the poisoning upper limit value PU, which is the theoretical air-fuel ratio ST, that is, the more the detected air-fuel ratio AFc moves to the rich side from the poisoning upper limit value PU, the more the count-up amount is "1". is defined to increase from This is because the rate at which HC poisoning of the downstream side catalyst 32b progresses is likely to increase as the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b is on the richer side from the poisoning upper limit value PU. In this manner, the count-up amount of the determination counter can be calculated in accordance with the actual progression speed of HC poisoning of the downstream side catalyst 32b, so the poisoning suppression process can be executed at appropriate timing.

[Calculation control of count-up amount in second modification]
FIG. 12A is a flowchart showing control for calculating the count-up amount of the determination counter according to the second modification. The ECU 100 calculates the exhaust flow rate Fr flowing into the downstream side catalyst 32b based on the amount of intake air detected by the airflow meter 22 (step S11a). Specifically, the ECU 100 calculates the exhaust flow rate Fr flowing into the downstream side catalyst 32b based on the intake air amount. If an EGR (Exhaust Gas Recirculation) device for extracting exhaust gas from the upstream side of the downstream side catalyst 32b is provided, the ECU 100 calculates the exhaust flow rate Fr based on the EGR rate in addition to the intake air amount.

Next, the ECU 100 calculates a count-up amount based on the calculated exhaust gas flow rate Fr (step S12a). FIG. 12B is a map that defines the relationship between the exhaust gas flow rate Fr and the count-up amount. The vertical axis indicates the exhaust flow rate Fr, and the horizontal axis indicates the count-up amount. This map is stored in advance in the storage device of the ECU 100 or the like. This map specifies that the count-up amount increases from "1" as the exhaust gas flow rate Fr increases from a predetermined value. This is because, as the exhaust flow rate Fr to the downstream side catalyst 32b increases, the flow rate of rich air-fuel ratio exhaust gas flowing into the downstream side catalyst 32b also increases, and the HC poisoning rate of the downstream side catalyst 32b tends to increase. In this manner, the count-up amount of the determination counter can be calculated in accordance with the actual progression speed of HC poisoning of the downstream side catalyst 32b, so the poisoning suppression process can be executed at appropriate timing. Note that the count-up amount may be calculated based on both the detected air-fuel ratio AFc and the exhaust gas flow rate Fr.

[Calculation Control of Countdown Amount in First Modification]
Calculation control of the countdown amount of the determination counter of the first modified example will be described. FIG. 13A is a flowchart showing control for calculating the countdown amount of the determination counter according to the first modification. This calculation control is performed, for example, when the process of step S7 is performed. The ECU 100 acquires the detected air-fuel ratio AFc (step S11b).

Next, the ECU 100 calculates the countdown amount of the determination counter based on the acquired detected air-fuel ratio AFc (step S12b). FIG. 13B is a map that defines the relationship between the detected air-fuel ratio AFc and the countdown amount. The vertical axis indicates the detected air-fuel ratio AFc, and the horizontal axis indicates the countdown amount. This map is stored in advance in the storage device of the ECU 100 or the like. In this map, the countdown amount increases from "1" as the detected air-fuel ratio AFc increases from the poisoning recovery lower limit LL, that is, as the detected air-fuel ratio AFc moves from the poisoning recovery lower limit LL to the rich side. stipulated. This is because the degree of elimination of HC poisoning of the downstream side catalyst 32b tends to increase as the air-fuel ratio of the exhaust gas flowing into the downstream side catalyst 32b is leaner than the poisoning recovery lower limit LL. In this manner, the countdown amount of the determination counter can be calculated in accordance with the actual degree of elimination of HC poisoning of the downstream side catalyst 32b, so the poisoning suppression process can be executed only when necessary.

[Calculation Control of Countdown Amount in Second Modification]
FIG. 14A is a flowchart showing control for calculating the countdown amount of the determination counter according to the second modification. As in step S11a, the ECU 100 calculates the exhaust gas flow rate Fr flowing into the downstream side catalyst 32b (step S11c). Next, the ECU 100 calculates a countdown amount based on the calculated exhaust gas flow rate Fr (step S12c). FIG. 14B is a map that defines the relationship between the exhaust gas flow rate Fr and the countdown amount. The vertical axis indicates the exhaust flow rate Fr, and the horizontal axis indicates the count-up amount. This map is stored in advance in the storage device of the ECU 100 or the like. This map specifies that the countdown amount increases from "1" as the exhaust gas flow rate Fr increases from a predetermined value. This is because, as the exhaust flow rate Fr to the downstream side catalyst 32b increases, the flow rate of lean air-fuel ratio exhaust gas flowing into the downstream side catalyst 32b also increases, and the degree of elimination of HC poisoning of the downstream side catalyst 32b tends to increase. In this manner, the countdown amount of the determination counter can be calculated in accordance with the actual degree of elimination of HC poisoning of the downstream side catalyst 32b, so the poisoning suppression process can be executed only when necessary. The countdown amount may be calculated based on both the detected air-fuel ratio AFc and the exhaust gas flow rate Fr.

In addition, in the maps of FIGS. 11B, 12B, 13B, and 14B, the count-up amount and the count-down amount are defined in a straight line, but are not limited to this, and are defined in a quadratic curve or polygonal line. may Further, the calculation of the count-up amount and the count-down amount is not limited to the map as described above, and may be calculated by an arithmetic expression. For example, the initial value of the count-up amount "1" is multiplied by a coefficient K (>1) that increases as the detected air-fuel ratio AFc decreases from the poisoning upper limit value PU or as the exhaust flow rate Fr increases. count-up amount may be calculated. The initial value "1" of the countdown amount is multiplied by a coefficient K (>1) that increases as the detected air-fuel ratio AFc increases from the poison recovery lower limit LL or as the exhaust flow rate Fr increases, resulting in a final countdown. amount may be calculated.

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to such specific embodiments, and various modifications and variations can be made within the scope of the gist of the present invention described in the scope of claims. Change is possible.

Reference Signs List 1 internal combustion engine 10 engine body 17 in-cylinder injection valve 20 intake passage 30 exhaust passage 31a upstream sensor 31b midstream sensor 31c downstream sensor 32a upstream catalyst 32b downstream catalyst 100 ECU (control device, determination unit, control unit)

Claims (9)

an engine body, an upstream side catalyst and a downstream side catalyst provided respectively on the upstream side and the downstream side of an exhaust passage connected to the engine body and having an oxygen storage capacity, and the downstream side catalyst of the exhaust passage. and a downstream sensor that detects the air-fuel ratio of the exhaust gas. In a control device for an internal combustion engine that controls to alternately switch to the fuel ratio,
a determination unit that determines, based on the air-fuel ratio detected by the downstream sensor, whether or not an HC poisoning condition that may cause HC poisoning of the downstream side catalyst is satisfied;
a control unit that controls the internal combustion engine such that, when the determination unit makes an affirmative determination, the amount of oxygen flowing into the downstream catalyst is increased more than when the determination unit makes a negative determination; A control device for an internal combustion engine. When the determination unit makes an affirmative determination, the control unit makes the air-fuel ratio of the lean air-fuel ratio exhaust flowing into the downstream side catalyst even leaner than when the determination unit makes a negative determination. 2. The control device for an internal combustion engine according to claim 1, wherein the amount of oxygen flowing into said downstream side catalyst is increased by controlling the air-fuel ratio of said internal combustion engine so as to increase the amount of oxygen flowing into said downstream side catalyst. When the determination unit makes an affirmative determination, the control unit makes the inflow time of the lean air-fuel ratio exhaust gas flowing into the downstream side catalyst longer than when the determination unit makes a negative determination. 3. The control device for an internal combustion engine according to claim 1, wherein the amount of oxygen flowing into said downstream side catalyst is increased by controlling the air-fuel ratio of said internal combustion engine so as to increase the amount of oxygen flowing into said downstream side catalyst. When the determination unit makes an affirmative determination, the control unit cuts fuel in at least one of a plurality of cylinders of the internal combustion engine to reduce oxygen flowing into the downstream side catalyst. 4. The control device for an internal combustion engine according to any one of claims 1 to 3, wherein the amount is increased. a counter calculation unit that counts up a determination counter when the air-fuel ratio detected by the downstream sensor is equal to or lower than an upper limit value at which the downstream catalyst can be poisoned by HC;
5. The control device for an internal combustion engine according to claim 1, wherein said determination unit determines that said HC poisoning condition is satisfied when said determination counter reaches a threshold value or more. When the air-fuel ratio detected by the downstream sensor is a lean air-fuel ratio that is greater than the poisoning upper limit value and greater than the theoretical air-fuel ratio, the counter calculation unit either counts down or resets the determination counter. The control device for an internal combustion engine according to claim 5. 7. The internal combustion engine according to claim 5, wherein said counter calculation unit executes one of counting down and resetting said determination counter when said control unit stops processing for increasing the amount of oxygen flowing into said downstream side catalyst. Engine control device. The counter calculator increases the count-up amount of the determination counter when at least one of the smaller the air-fuel ratio detected by the downstream sensor and the larger the flow rate of exhaust gas flowing into the downstream catalyst. 8. A control system for an internal combustion engine according to any one of claims 5 to 7. The counter calculation unit increases the countdown amount of the determination counter when at least one of the larger the air-fuel ratio detected by the downstream sensor and the smaller the flow rate of the exhaust gas flowing into the two catalysts. 9. A control device for an internal combustion engine according to any one of items 5 to 8.

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