Control device for internal combustion engine

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

[Technical field]

[0001] The present invention relates to a control device for an internal combustion engine. [Background technology]

[0002] 2. Description of the Related Art There is known an internal combustion engine in which an upstream catalyst and a downstream catalyst having an oxygen storage capacity are provided on the upstream and downstream sides, respectively, of an exhaust passage (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2018-3777 A Summary of the Invention [Problem to be solved by the invention]

[0004] The air-fuel ratio of the exhaust gas from an 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 catalyst, oxygen is stored due to the oxygen storage capacity of the upstream catalyst, and exhaust gas with an air-fuel ratio richer than the air-fuel ratio of the exhaust gas that flows into the upstream catalyst flows into the downstream catalyst. In this way, there are few opportunities for exhaust gas with a lean air-fuel ratio that contains a sufficient amount of oxygen to flow into the downstream catalyst. This may cause HC poisoning, in which hydrocarbons in the exhaust gas adhere to the downstream catalyst, to progress. If HC poisoning progresses in the downstream catalyst, exhaust emissions may deteriorate.

[0005] Therefore, an object of the present invention is to provide a control device for an internal combustion engine that suppresses deterioration of exhaust emissions. [Means for solving the problem]

[0006] The object of the present invention is to provide an engine having an exhaust passage connected to the engine body, an upstream catalyst and a downstream catalyst, each of which has an oxygen storage capacity and is provided on the upstream side and downstream side of an exhaust passage connected to the engine body, an upstream sensor that is provided in the exhaust passage upstream of the upstream catalyst and detects an air-fuel ratio of the exhaust; and a midstream sensor that is provided in the exhaust passage downstream of the upstream catalyst and upstream of the downstream catalyst and detects an air-fuel ratio of the exhaust.and a downstream sensor that is provided in the exhaust passage downstream of the downstream catalyst and detects an air-fuel ratio of the exhaust gas, When the air-fuel ratio detected by the midstream side sensor becomes a lean judged air-fuel ratio which is higher than the stoichiometric air-fuel ratio, the target air-fuel ratio is set to a rich target air-fuel ratio which is lower than the stoichiometric air-fuel ratio, and when the air-fuel ratio detected by the midstream side sensor becomes a rich judged air-fuel ratio which is lower than the stoichiometric air-fuel ratio, the target air-fuel ratio is set to a lean target air-fuel ratio which is higher than the stoichiometric air-fuel ratio, thereby controlling the air-fuel ratio of the exhaust gas discharged from the engine body so that the air-fuel ratio detected by the upstream side sensor becomes the target air-fuel ratio. A control device for an internal combustion engine that controls an air-fuel ratio of the internal combustion engine to be alternately switched between a rich air-fuel ratio lower than a stoichiometric air-fuel ratio and a lean air-fuel ratio higher than the stoichiometric air-fuel ratio, the control device comprising: a determination unit that determines whether or not a HC poisoning condition that may cause HC poisoning of the downstream catalyst is established based on the detected air-fuel ratio of the downstream sensor; and a control unit that controls the internal combustion engine to increase the amount of oxygen flowing into the downstream catalyst when a positive determination is made by the determination unit compared to when a negative determination is made by the determination unit. When the determination unit makes a positive determination, the control unit switches the lean determined air-fuel ratio to a strong lean determined air-fuel ratio on the leaner side, thereby controlling the air-fuel ratio of the internal combustion engine so that the air-fuel ratio of the exhaust gas having the lean air-fuel ratio flowing into the downstream catalyst becomes leaner than when the determination unit makes a negative determination, thereby increasing the amount of oxygen flowing into the downstream catalyst. This can be achieved by a control device for the internal combustion engine.

[0007] The above object can be achieved by applying the above object to an internal combustion engine having an engine body, an upstream catalyst and a downstream catalyst which are respectively provided on the upstream and downstream sides of an exhaust passage connected to the engine body and have an oxygen storage capacity, an upstream sensor which is provided in the exhaust passage upstream of the upstream catalyst and detects an air-fuel ratio of exhaust gas, a midstream sensor which is provided in the exhaust passage downstream of the upstream catalyst and upstream of the downstream catalyst and detects an air-fuel ratio of exhaust gas, and a downstream sensor which is provided in the exhaust passage downstream of the downstream catalyst and detects the air-fuel ratio of exhaust gas, When the detected air-fuel ratio of the midstream sensor becomes a lean judged air-fuel ratio higher than the stoichiometric air-fuel ratio, the target air-fuel ratio is set to a rich target air-fuel ratio lower than the stoichiometric air-fuel ratio, and when the detected air-fuel ratio of the midstream sensor becomes a rich judged air-fuel ratio lower than the stoichiometric air-fuel ratio, the target air-fuel ratio is set to a lean target air-fuel ratio higher than the stoichiometric air-fuel ratio, and the air-fuel ratio of the exhaust gas discharged from the engine body is controlled so that the detected air-fuel ratio of the upstream sensor becomes the target air-fuel ratio. The control device for an internal combustion engine controls the rich target air-fuel ratio and the lean target air-fuel ratio to be alternately switched between the rich target air-fuel ratio and the lean target air-fuel ratio, the control device comprising: a determination unit that determines whether or not a HC poisoning condition that may cause the downstream catalyst to be HC poisoned is established based on the detected air-fuel ratio of the downstream sensor; and a control unit that controls the internal combustion engine to increase the amount of oxygen flowing into the downstream catalyst when a positive determination is made by the determination unit, compared to when a negative determination is made by the determination unit. This can also be achieved by a control device for an internal combustion engine, which switches the rich target air-fuel ratio to a strong rich target air-fuel ratio on the richer side which is even smaller than the rich target air-fuel ratio, and switches the lean target air-fuel ratio to a weak lean target air-fuel ratio on the richer side which is even smaller than the lean target air-fuel ratio, by switching to a rich side control center on the richer side than the stoichiometric air-fuel ratio, and controls the air-fuel ratio of the internal combustion engine so that the inflow time of exhaust gas with the lean air-fuel ratio flowing into the downstream catalyst is longer than when a negative judgment is made by the judgment unit, thereby increasing the amount of oxygen flowing into the downstream catalyst.

[0010] The present invention may further include a counter calculation unit that counts up a judgment counter when the detected air-fuel ratio of the downstream sensor is equal to or lower than an upper limit value at which the downstream catalyst may be poisoned by HC, and the judgment unit may determine that the HC poisoning condition is established when the judgment counter becomes equal to or higher than a threshold value.

[0011] The counter calculation unit may execute one of counting down and resetting the determination counter 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 a stoichiometric air-fuel ratio.

[0012] The counter calculation portion may execute one of counting down and resetting the determination counter when the process of increasing the amount of oxygen flowing into the downstream catalyst by the control portion is stopped.

[0013] The counter calculation unit may increase the count-up amount of the determination counter at least in one of the cases where the detected air-fuel ratio of the downstream sensor is smaller and / or the flow rate of the exhaust gas flowing into the downstream catalyst is greater.

[0014] The counter calculation unit is configured to calculate the air-fuel ratio detected by the downstream sensor so as to increase the air-fuel ratio. Downstream the greater the flow rate of the exhaust gas flowing into the catalyst, the greater the countdown amount of the determination counter may be. Effect of the Invention

[0015] It is possible to provide a control device for an internal combustion engine that suppresses deterioration of exhaust emissions. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram of an internal combustion engine. [Diagram 2] FIG. 2 is a timing chart showing a comparative example in which the downstream catalyst is poisoned by HC. [Diagram 3] FIG. 3 is a timing chart showing an example of the poisoning suppression control in this embodiment. [Figure 4] FIG. 4 is a flow chart showing an example of the poisoning suppression control of this embodiment. [Diagram 5] FIG. 5 is a timing chart showing an example of the poisoning suppression process of this embodiment. [Figure 6] FIG. 6 is a flowchart showing an example of the poisoning suppression process of this embodiment. [Figure 7] FIG. 7 is a timing chart showing the poisoning suppression process of the first modified example. [Figure 8] FIG. 8 is a flowchart showing the poisoning suppression process of the first modified example. [Figure 9] FIG. 9 is a timing chart showing the poisoning suppression process of the second modified example. [Figure 10] FIG. 10 is a flowchart showing the poisoning suppression process of the second modified example. [Figure 11] FIG. 11A is a flowchart showing calculation control of the count-up amount of the determination counter in the first modified example, and FIG. 11B is a map that defines the relationship between the detected air-fuel ratio AFc and the count-up amount. [Figure 12] FIG. 12A is a flowchart showing calculation control of the count-up amount of the determination counter in the second modified example, and FIG. 12B is a map that defines the relationship between the exhaust flow rate Fr and the count-up amount. [Figure 13] FIG. 13A is a flowchart showing calculation control of the countdown amount of the determination counter in the first modified example, and FIG. 13B is a map that defines the relationship between the detected air-fuel ratio AFc and the countdown amount. [Figure 14] FIG. 14A is a flowchart showing calculation control of the countdown amount of the determination counter in the second modified example, and FIG. 14B is a map that defines the relationship between the exhaust flow rate Fr and the countdown amount. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] [General configuration of an internal combustion engine] FIG. 1 is a schematic diagram of an internal combustion engine 1. The internal combustion engine 1 is mounted on, for example, a vehicle, but is not limited thereto, 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 a combustion chamber 11, a piston 12, an ignition plug 16, and the like are provided in each cylinder. In addition, a connecting rod 13 and a crankshaft 14 are arranged inside the engine body 10. The piston 12 is connected to the crankshaft 14 by the connecting rod 13. The engine body 10 is provided with a rotation speed sensor 15, and an in-cylinder injection valve 17 is provided for each cylinder. The rotation speed sensor 15 detects the rotation speed of the crankshaft 14 to detect the rotation speed of the engine body 10. The in-cylinder injection valve 17 directly injects fuel into the combustion chamber 11. Incidentally, a port injection valve that injects fuel toward an 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. 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 the intake port and exhaust port of the engine body 10, respectively. An intake valve 18a and an exhaust valve 18b open and close the intake port and exhaust port of the engine body 10, respectively.

[0018] In the intake passage 20, an air cleaner 21, an air flow meter 22, and a throttle valve 23 are provided in this order from upstream to downstream. The air cleaner 21 removes dust and other particles from the air flowing in from the outside. The air flow meter 22 acquires the amount of intake air. The throttle valve 23 is driven, for example, by an actuator (not shown) and adjusts the amount of intake air. When the opening of the throttle valve 23 becomes larger, the amount of intake air increases, and when the opening becomes smaller, the amount of intake air decreases.

[0019] When intake valve 18a opens, air is introduced from intake passage 20 into combustion chamber 11. A mixture of air and fuel injected from in-cylinder injection valve 17 is compressed by piston 12 and ignited by spark plug 16. Ignition of the mixture causes piston 12 to reciprocate up and down within combustion chamber 11, rotating crankshaft 14. Exhaust gas after combustion is discharged from exhaust passage 30.

[0020] 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 the upstream side to the downstream side. 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, and at least one of them may be an oxygen concentration sensor that can detect 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 gas discharged from the engine body 10 and flowing into the upstream catalyst 32a. The midstream sensor 31b detects the air-fuel ratio of the exhaust gas discharged from the upstream catalyst 32a and flowing into the downstream catalyst 32b. The downstream sensor 31c detects the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b.

[0021] The upstream catalyst 32a and the downstream catalyst 32b are three-way catalysts that contain catalytic metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) and have oxygen storage capacity. The three-way catalyst has a catalytic action and an oxygen storage capacity, and therefore has a purification action of NOx and HC according to the amount of oxygen stored. That is, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is lean, when the amount of oxygen stored in the three-way catalyst is small, the oxygen in the exhaust gas is stored by the three-way catalyst, and the NOx in the exhaust gas is reduced and purified accordingly. When the amount of oxygen stored in the three-way catalyst increases, the concentration of oxygen and NOx in the exhaust gas flowing out from the three-way catalyst increases. When the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is rich, when the amount of oxygen stored in the three-way catalyst is large, the oxygen stored in the three-way catalyst is released, and the HC in the exhaust gas is oxidized and purified. 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 of the exhaust gas flowing into the three-way catalyst and the amount of oxygen stored. At least one of the upstream catalyst 32a and the downstream catalyst 32b may be a catalyst other than the three-way catalyst as long as it has a catalytic action and an oxygen storage capacity.

[0022] [Outline of ECU configuration] An 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, and performs various controls by executing programs stored in the ROM or the storage device. The ECU 100 controls the spark plug 16, the in-cylinder injection valve 17, and the throttle valve 23 based on the amount of operation of the accelerator pedal or the brake pedal operated by the driver, the rotation speed and the load of the engine body 10, etc. The ECU 100 receives inputs of 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 ratios AFa, AFb, and AFc detected by the upstream sensor 31a, the midstream sensor 31b, and the downstream sensor 31c, respectively.

[0023] Although the details will be described later, the ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 so as to alternate between a rich air-fuel ratio lower than the stoichiometric air-fuel ratio ST and a lean air-fuel ratio higher than the stoichiometric air-fuel ratio ST. Specifically, the ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 as follows.

[0024] The ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 so that the detected air-fuel ratio AFa of the upstream sensor 31a becomes the target air-fuel ratio TAF. Specifically, the ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 to the target air-fuel ratio TAF by feedback-controlling the fuel injection amount from the in-cylinder injection valve 17 and the opening degree of the throttle valve 23 based on the detected air-fuel ratio AFa of the upstream sensor 31a so that the detected air-fuel ratio of the upstream sensor 31a becomes the target air-fuel ratio TAF. In addition, when the detected air-fuel ratio AFb of the midstream sensor 31b becomes a lean judged air-fuel ratio LD that is greater 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 that is smaller than the stoichiometric air-fuel ratio ST. When the detected air-fuel ratio AFb of the midstream sensor 31b becomes a rich judged air-fuel ratio RD which is smaller than the stoichiometric air-fuel ratio, the ECU 100 sets the target air-fuel ratio TAF to a lean target air-fuel ratio TL which is larger than the stoichiometric air-fuel ratio. As a result, the detected air-fuel ratio AFb of 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. In addition, the ECU 100 functionally realizes a judgement unit, a control unit, and a counter calculation unit which will be described later, by the above-mentioned CPU, RAM, ROM, storage device, etc.

[0025] [HC poisoning of downstream catalyst] 2 is a timing chart showing a comparative example in which the downstream catalyst 32b is poisoned with HC. FIG. 2 shows the changes in the detected air-fuel ratios AFb and AFc of the midstream sensor 31b and the downstream sensor 31c, and the changes in the total amount of HC and NOx emissions from the upstream catalyst 32a and the downstream catalyst 32b.

[0026] As described above, the ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 so as to alternate between a rich air-fuel ratio and a lean air-fuel ratio. As a result, the detected air-fuel ratios AFb and AFc also switch from a rich air-fuel ratio (time t0 to time t1) to a lean air-fuel ratio (time t1 to time t2), and thereafter alternate between 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 catalyst 32a, oxygen is stored in the upstream catalyst 32a, and the air-fuel ratio of the exhaust gas discharged from the upstream catalyst 32a and flowing into the downstream catalyst 32b becomes a leaner air-fuel ratio that is richer than the air-fuel ratio of the exhaust gas that flowed into the upstream catalyst 32a. On the other hand, when exhaust gas with a rich air-fuel ratio flows into the upstream catalyst 32a, the oxygen stored in the upstream catalyst 32a is released, and the air-fuel ratio of the exhaust gas discharged from the upstream catalyst 32a and flowing into the downstream catalyst 32b becomes a rich air-fuel ratio that is leaner than the air-fuel ratio of the exhaust gas that flowed into the upstream catalyst 32a.

[0027] In this way, the downstream catalyst 32b rarely receives exhaust gas with a lean air-fuel ratio, but rich exhaust gas flows in continuously. If the internal combustion engine 1 is operated continuously, the surface of the downstream catalyst 32b may be covered with hydrocarbons, and the HC poisoning of the downstream catalyst 32b may progress. If the HC poisoning of the downstream catalyst 32b progresses, the contact between the downstream catalyst 32b and the exhaust gas is hindered, so that the downstream catalyst 32b cannot sufficiently purify the HC and NOx emitted from the upstream catalyst 32a, and the total amount of HC and NOx emitted from the downstream catalyst 32b increases (time t3). In this way, the exhaust emission is deteriorated.

[0028] [Poisoning Suppression Control in this Example] FIG. 3 is a timing chart showing an example of the 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 the upstream catalyst 32a and the downstream catalyst 32b, and the transition of the value of the judgment counter. The judgment counter is a counter that counts up by a predetermined count-up amount when the detected air-fuel ratio AFc is equal to or less 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 catalyst 32b progresses when the detected air-fuel ratio AFc is equal to or less than the poisoning upper limit value PU. In this embodiment, the poisoning upper limit value PU is set to the theoretical air-fuel ratio ST, specifically, 14.6, but is not limited to this, and may be, for example, a value of 14.65 or less.

[0029] When the detected air-fuel ratio AFc is equal to or less than the poisoning upper limit value PU (time t10 to t11), the judgment counter is counted up, and when the detected air-fuel ratio AFc is greater than the poisoning upper limit value PU (time t11 to t12), the judgment counter is maintained. The judgment counter increases stepwise as the detected air-fuel ratio AFc alternates between a rich air-fuel ratio and a lean air-fuel ratio. When the judgment counter becomes equal to or greater than the threshold value α (time t13), the poisoning suppression process is executed. The threshold value α is a lower limit value at which it is considered that the HC poisoning condition in which the downstream catalyst 32b may be poisoned by HC is established. The poisoning suppression process is a process for suppressing the progress of HC poisoning of the downstream catalyst 32b by controlling the internal combustion engine 1 so as to increase the amount of oxygen flowing into the downstream catalyst 32b compared to before the judgment counter becomes equal to or greater than the threshold value α. When the amount of oxygen flowing into the downstream catalyst 32b is large, the oxidation of the hydrocarbons attached to the surface of the downstream catalyst 32b is promoted, and the amount of CO and CO 2 ,H 2 This is because it can be removed as O. A specific control method for the poisoning suppression process will be described later.

[0030] When the poisoning suppression process is executed, the detected air-fuel ratio AFb corresponding to the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b becomes leaner than the lean air-fuel ratio before time t13, and similarly, the detected air-fuel ratio AFc corresponding to the air-fuel ratio of the exhaust gas flowing out from the downstream catalyst 32b becomes leaner than the lean air-fuel ratio before time t13. In this way, the amount of oxygen flowing into the downstream catalyst 32b increases. When the poisoning suppression process is stopped after that (time t14), the judgment counter is reset to "0". After time t14, the judgment counter similarly increases, and when the judgment counter becomes equal to or greater than the threshold value α, the poisoning suppression process is executed again, and then the poisoning suppression process is stopped and the judgment counter is reset to "0".

[0031] By repeatedly executing the poisoning suppression process in this manner, the progression of HC poisoning in the downstream catalyst 32b can be suppressed, and the purification performance of the downstream catalyst 32b can be maintained. Therefore, the total amount of HC and NOx discharged from the downstream catalyst 32b can be suppressed. Note that a poisoning recovery lower limit value LL is shown in FIG. 3, but will be described later.

[0032] FIG. 4 is a flowchart showing an example of the poisoning suppression control of this embodiment. This poisoning suppression control is repeatedly executed while the internal combustion engine 1 is operating. The ECU 100 judges whether the detected air-fuel ratio AFc is equal to or lower than the poisoning upper limit value PU (step S1). If the answer is Yes in step S1, the ECU 100 counts up the judgment counter by a predetermined count-up amount (step S2). For example, in this embodiment, the count-up amount is "1", but is not limited to this. The processing of steps S1 and S2 is an example of processing executed by the counter calculation unit.

[0033] Next, the ECU 100 judges whether the judgment counter is equal to or greater than the threshold value α (step S3). If the result in step S3 is No, it is assumed that the HC poisoning condition in which the downstream catalyst 32b may be HC poisoned is not satisfied, and the control ends. If the result in step S3 is Yes, it is assumed that the HC poisoning condition in which the downstream catalyst 32b may be HC poisoned is satisfied, and the ECU 100 starts the poisoning suppression process (step S4). The ECU 100 stops the poisoning suppression process after executing the poisoning suppression process for a predetermined period (step S5), and resets the judgment counter to the initial value "0" (step S6). In this way, the progress of HC poisoning of the downstream catalyst 32b can be suppressed. Step S3 is an example of a process executed by the judgment unit. Steps S4 and S5 are an example of a process executed by the control unit. The process in step S6 is an example of a process executed by the counter calculation unit.

[0034] If the answer is No in step S1, the ECU 100 judges whether the detected air-fuel ratio AFc is equal to or greater than the poisoning recovery lower limit LL (step S7). The poisoning recovery lower limit LL is a lower limit at which the HC poisoning of the downstream catalyst 32b can be recovered when the detected air-fuel ratio AFc is equal to or greater than the poisoning recovery lower limit LL, and the poisoning recovery lower limit LL is, for example, 15.0, but is not limited thereto. If the answer is No in step S7, this control ends. If the answer is Yes in step S7, the ECU 100 counts down the judgment counter by a predetermined countdown amount (step S8). For example, in this embodiment, the countdown amount is "1", but is not limited thereto. Steps S7 and S8 are an example of processing executed by the counter calculation unit. Note that, if the detected air-fuel ratio AFc is greater than the poisoning upper limit PU and less than the poisoning recovery lower limit LL, the HC poisoning of the downstream catalyst 32b does not progress, but is not recovered either, so the judgment counter is maintained at the current value. Steps S7 and S8 are an example of a process executed by the determination unit.

[0035] In this way, since the value of the judgment counter corresponds to the progress degree of HC poisoning of the downstream side catalyst 32b, the poisoning suppression process can be executed only when necessary by executing the poisoning suppression process based on the value of the judgment counter. This makes it possible to avoid the poisoning suppression process being repeatedly executed when it is not necessary, thereby preventing the stability of the operation of the internal combustion engine 1 from being impaired.

[0036] Also, instead of using the detected air-fuel ratio AFb corresponding to the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b, the determination counter is counted up or down based on the detected air-fuel ratio AFc corresponding to the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b. For example, when the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b is a relatively small rich air-fuel ratio (strong rich air-fuel ratio), if sufficient oxygen is stored in the downstream catalyst 32b, the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b will be a relatively large rich air-fuel ratio (weak rich air-fuel ratio), but if sufficient oxygen is not stored in the downstream catalyst 32b, the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b will be a moderately rich air-fuel ratio (moderate rich air-fuel ratio). In addition, when the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b is a relatively large lean air-fuel ratio (strong lean air-fuel ratio), if the downstream catalyst 32b has sufficient oxygen storage capacity, the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b will be a relatively small lean air-fuel ratio (weak lean air-fuel ratio), but if the downstream catalyst 32b does not have sufficient oxygen storage capacity, the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b will be a moderately lean air-fuel ratio (moderate lean air-fuel ratio). Thus, even if the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b is the same, the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b will vary depending on the amount of oxygen stored and the oxygen storage capacity of the downstream catalyst 32b. Here, if the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b is a rich air-fuel ratio, it means that the exhaust gas with the rich air-fuel ratio has passed through the downstream catalyst 32b, and as described above, the progress degree of HC poisoning of the downstream catalyst 32b depends on the richness of the rich air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b. Also, if the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b is a lean air-fuel ratio, it means that the exhaust gas with the lean air-fuel ratio has passed through the downstream catalyst 32b, and the elimination degree of HC poisoning of the downstream catalyst 32b depends on the leanness of the lean air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b. Therefore, the progress degree of HC poisoning of the downstream catalyst 32b can be accurately grasped by counting up or down the judgment counter based on the detected air-fuel ratio AFc corresponding to the air-fuel ratio of the exhaust gas discharged from the downstream catalyst 32b.

[0037] In step S6, the judgment counter is reset to "0", but is not limited to this, and may be counted down by a predetermined value, for example. In this case, for example, the countdown amount in step S6 may be increased to be greater than the countdown amount in step S8. Also, in step S8, the judgment counter is counted down, but is not limited to this, and may be reset to "0". In this case, it is desirable to set the poisoning recovery lower limit value LL to a high value so that it is equivalent to the suppression of HC poisoning by the poisoning suppression process.

[0038] [Poisoning Suppression Treatment in this Example] 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 gas flowing into the downstream 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 the transition of the target air-fuel ratio TAF and the detected air-fuel ratios AFb and AFc. First, before explaining the poisoning suppression process, the switching of the target air-fuel ratio TAF before the poisoning suppression process is executed will be explained.

[0039] When the detected air-fuel ratio AFb falls to the rich judged air-fuel ratio RD (time t21), the target air-fuel ratio TAF is switched from a predetermined rich target air-fuel ratio TR to a predetermined lean target air-fuel ratio TL. As a result, the oxygen in the exhaust gas flowing into the upstream catalyst 32a is stored in the upstream catalyst 32a, and the NOx in the exhaust gas is reduced and purified. The NOx in the exhaust gas that cannot be purified by the upstream catalyst 32a is similarly purified by the downstream 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 amount of oxygen stored in the upstream catalyst 32a increases, the oxygen concentration of the exhaust gas discharged from the upstream catalyst 32a increases, and the detected air-fuel ratio AFb rises to the lean side, and accordingly the oxygen concentration of the exhaust gas discharged from the downstream catalyst 32b also rises slightly, and the detected air-fuel ratio AFc also rises slightly to the lean side.

[0040] 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 catalyst 32a is purified by oxidation using the oxygen stored in the upstream catalyst 32a. HC in the exhaust gas that cannot be purified by the upstream catalyst 32a is similarly purified by the downstream 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 catalyst 32a approaches zero, the oxygen concentration of the exhaust gas discharged from the upstream catalyst 32a decreases, and the detected air-fuel ratio AFb decreases to the rich side. Accordingly, the oxygen concentration of the exhaust gas discharged from the downstream catalyst 32b also decreases slightly, and the detected air-fuel ratio AFc also decreases slightly to the rich side. In this way, 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.

[0041] For example, when the judgment counter becomes equal to or greater than the threshold value α at time t23, the poisoning suppression process is executed. Specifically, when the judgment counter becomes equal to or greater than the threshold value α, the lean judged air-fuel ratio LD is switched to the leaner strong lean judged air-fuel ratio LDA. As a result, exhaust gas having an air-fuel ratio leaner than the lean judged air-fuel ratio LD can be supplied to the downstream catalyst 32b, and the amount of oxygen flowing into the downstream catalyst 32b increases. In addition, the target air-fuel ratio TAF is maintained at the lean target air-fuel ratio TL from the time t24 when the target air-fuel ratio TAF is switched to the lean target air-fuel ratio TL until the detected air-fuel ratio AFb reaches the lean judged air-fuel ratio LDA at time t25. 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 longer period than the time t21 to t22. Therefore, the poisoning suppression process of this embodiment also lengthens the inflow time of the lean air-fuel ratio exhaust gas flowing into the downstream catalyst 32b, which also increases the amount of oxygen flowing into the downstream catalyst 32b. As a result, the HC attached to the downstream catalyst 32b can be oxidized and removed by a large amount of oxygen, and the HC poisoning of the downstream 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.

[0042] FIG. 6 is a flow chart showing an example of the poisoning suppression processing of this embodiment. In the poisoning suppression processing, 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 judges whether the detected air-fuel ratio AFb has become equal to or greater than the strong lean judged air-fuel ratio LDA (step S4-2). If the answer is No in step S4-2, step S4-1 is executed again. That is, the first example of the poisoning suppression processing is continued. If the answer is 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 processing.

[0043] In the poisoning suppression process described above, the poisoning suppression process is executed from when the detected air-fuel ratio AFb reaches the rich judged air-fuel ratio RD after the judgment counter becomes equal to or greater than the threshold value α until the detected air-fuel ratio AFb reaches the strong lean judged air-fuel ratio LDA the next time, but is not limited thereto. For example, the poisoning suppression process may be executed from when the detected air-fuel ratio AFb reaches the rich judged air-fuel ratio RD after the judgment counter becomes equal to or greater than the threshold value α until the detected air-fuel ratio AFb reaches the strong lean judged air-fuel ratio LDA for the second time or a predetermined number of times thereafter.

[0044] [Poisoning Suppression Treatment of the First Modified Example] Next, the poisoning suppression process of the first modification will be described. In the poisoning suppression process of the first modification, the air-fuel ratio of the internal combustion engine 1 is controlled so that the inflow time of the exhaust gas with a lean air-fuel ratio flowing into the downstream catalyst 32b becomes longer. FIG. 7 is a timing chart showing the poisoning suppression process of the first modification. FIG. 7 shows the transition of the target air-fuel ratio TAF, the detected air-fuel ratio 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, and 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 judgment counter becomes equal to or greater than 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 on the rich side of the stoichiometric air-fuel ratio ST at time t34. That is, the target air-fuel ratio TAF is switched from a state in which it is alternately switched between a lean target air-fuel ratio TL and a rich target air-fuel ratio TR to a weak lean target air-fuel ratio TLa or a strong rich target air-fuel ratio TRa centered on the rich side control center TCa. 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 weak 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.

[0045] 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 drops from the lean judged air-fuel ratio LD to the rich judged air-fuel ratio RD early at time t35. This is because the strong rich target air-fuel ratio TRa is a value on the rich side that is smaller than the rich target air-fuel ratio TR, so the detected air-fuel ratio AFb drops to the rich side in a short period of time. When the detected air-fuel ratio AFb drops 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 weak lean target air-fuel ratio TLa. Here, since the weak lean target air-fuel ratio TLa is a value on the rich side that is 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. For this reason, the detected air-fuel ratio AFb can be maintained at the lean air-fuel ratio for a long period of time, and the amount of oxygen flowing into the downstream catalyst 32b can be increased. This makes it possible to suppress HC poisoning of the downstream catalyst 32b. 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 stoichiometric air-fuel ratio ST, and the poisoning suppression process is stopped.

[0046] FIG. 8 is a flow chart showing the poisoning suppression process of the first modification. 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 judges whether the detected air-fuel ratio AFb has become equal to or greater than the lean judged air-fuel ratio LD (step S4-2a). If the answer is No in step S4-2a, step S4-1a is executed again. That is, the poisoning suppression process is continued. If the answer is 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.

[0047] In the poisoning suppression processing of the first modified example, the poisoning suppression processing may also be executed from the time when the control center of the target air-fuel ratio TAF is switched to the rich side control center TCa after the judgment counter becomes equal to or greater than the threshold value α, until the detected air-fuel ratio AFb reaches the lean judgment air-fuel ratio LD for the second time or a predetermined number of times thereafter.

[0048] 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 the present invention is not limited to this, and for example, only the lean target air-fuel ratio TL of the target air-fuel ratio TAF may be switched to the weak lean target air-fuel ratio TLa, and the rich target air-fuel ratio TR may be used as it is for the rich target air-fuel ratio TR. In this case, too, it takes time for the detected air-fuel ratio AFb to reach the lean judged air-fuel ratio LD after it becomes the rich judged air-fuel ratio RD, and during that time, a large amount of oxygen can be circulated to the downstream catalyst 32b, and the progress of HC poisoning of the downstream catalyst 32b can be suppressed.

[0049] The poisoning suppression process of the present embodiment and the poisoning suppression process of the first modified example may be performed simultaneously. That is, the lean judged air-fuel ratio LD may be switched to the strong 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. This allows a larger amount of oxygen to flow into the downstream catalyst 32b, and the HC poisoning of the downstream catalyst 32b may be suppressed.

[0050] [Poisoning Suppression Treatment of the Second Modified Example] Next, the poisoning suppression process of the second modification will be described. In the poisoning suppression process of the second modification, fuel cut is performed for one of the multiple cylinders of the internal combustion engine 1. FIG. 9 is a timing chart showing the poisoning suppression process of the second modification. FIG. 9 shows the state of the execution flag of the one-cylinder fuel cut control in addition to the transition of the target air-fuel ratio TAF and the detected air-fuel ratios AFb and AFc. The one-cylinder fuel cut control is to perform fuel cut control to stop fuel injection for one of the multiple cylinders of the internal combustion engine 1 as described above.

[0051] 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 when the detected air-fuel ratio AFb becomes the lean judged air-fuel ratio LD at time t42, the target air-fuel ratio TAF is switched to the rich target air-fuel ratio TR. After that, for example, when the judgment counter becomes equal to or greater than the threshold value α at time t43, the detected air-fuel ratio AFb becomes the lean judged air-fuel ratio LD and the target air-fuel ratio TAF is switched to the rich target air-fuel ratio TR at time t44, the one-stage fuel cut control is executed. Note that, for cylinders that are not the target of fuel cut, the fuel injection and air-fuel ratio control is continued as before. As a result, a large amount of oxygen is supplied to the downstream catalyst 32b from the cylinder that is the target of fuel cut. In addition, the progress speed of the air-fuel ratio of the exhaust gas of the internal combustion engine 1 toward the rich side is reduced, and accordingly, the progress speed of the detected air-fuel ratio AFb, which indicates the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b, toward the rich side is reduced. This also allows lean air-fuel ratio exhaust gas to flow into the downstream catalyst 32b for a long period of time, increasing the amount of oxygen flowing into the downstream catalyst 32b. In this manner, HC poisoning of the downstream 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 in which fuel was cut, and the one-cylinder fuel cut control is stopped.

[0052] FIG. 10 is a flowchart showing the poisoning suppression process of the second modification. In the second modification of the poisoning process, the ECU 100 judges whether the detected air-fuel ratio AFb is equal to or greater than the lean judged air-fuel ratio LD (step S4-0b). If the answer is No in step S4-0b, this control ends. If the answer is Yes in step S4-0b, the ECU 100 executes one-cylinder fuel cut process (step S4-1b). Next, the ECU 100 judges whether the detected air-fuel ratio AFb is equal to or less than the rich judged air-fuel ratio RD (step S4-2b). If the answer is No in step S4-2b, step S4-1b is executed again. That is, the poisoning suppression process continues. If the answer is Yes in step S4-2b, the ECU 100 resumes fuel injection for the cylinder that was the target of the fuel cut (step S5-1b). This stops the poisoning suppression process.

[0053] In the poisoning suppression process of the second modification, the one-cylinder fuel cut process may be continued from the start of the one-cylinder fuel cut process after the judgment counter becomes equal to or greater than the threshold value α until the detected air-fuel ratio AFb reaches the rich judged air-fuel ratio RD for the second or a predetermined number of times thereafter. In the poisoning suppression process of the second modification, only one cylinder is subject to fuel cut, but the present invention is not limited to this, and two or more cylinders may be subject to fuel cut within a range in which the operating state of the internal combustion engine 1 is stable. This can further increase the amount of oxygen flowing into the downstream catalyst 32b. In addition, the poisoning suppression process of the present embodiment and at least one of the poisoning suppression processes of the second and third modifications may be performed simultaneously.

[0054] [Calculation control of count-up amount in the first modified example] The calculation control of the count-up amount of the determination counter in the first modified example will be described. Fig. 11A is a flowchart showing the calculation control of the count-up amount of the determination counter in the first modified example. This calculation control is executed, for example, when the process of step S2 is executed.

[0055] First, the ECU 100 acquires the detected air-fuel ratio AFc (step S11). Next, the ECU 100 calculates the count-up amount of the judgment counter based on the acquired detected air-fuel ratio AFc (step S12). FIG. 11B is a map that specifies 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 a storage device or the like of the ECU 100. In this map, the count-up amount is specified to increase from "1" as the detected air-fuel ratio AFc decreases from the poisoning upper limit value PU, which is the theoretical air-fuel ratio ST, that is, as the detected air-fuel ratio AFc becomes richer from the poisoning upper limit value PU. This is because the progress speed of HC poisoning of the downstream catalyst 32b becomes faster as the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b becomes richer from the poisoning upper limit value PU. In this way, since the count-up amount of the determination counter can be calculated in accordance with the actual progress speed of HC poisoning of the downstream side catalyst 32b, the poisoning suppression process can be executed at an appropriate timing.

[0056] [Calculation control of count-up amount in the second modified example] 12A is a flowchart showing calculation control of the count-up amount of the determination counter in the second modified example. The ECU 100 calculates the exhaust flow rate Fr flowing into the downstream catalyst 32b based on the intake air amount detected by the air flow meter 22 (step S11a). Specifically, the ECU 100 calculates the exhaust flow rate Fr flowing into the downstream catalyst 32b based on the intake air amount. If an EGR (Exhaust Gas Recirculation) device that extracts exhaust from the upstream side of the downstream 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.

[0057] Next, the ECU 100 calculates the count-up amount based on the calculated exhaust flow rate Fr (step S12a). FIG. 12B is a map that specifies the relationship between the exhaust 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 a storage device or the like of the ECU 100. In this map, it is specified that the count-up amount increases from "1" as the exhaust flow rate Fr increases from a predetermined value. This is because the larger the exhaust flow rate Fr to the downstream catalyst 32b, the larger the flow rate of the rich air-fuel ratio exhaust gas flowing into the downstream catalyst 32b, and the HC poisoning progress speed of the downstream catalyst 32b tends to be faster. In this way, the count-up amount of the judgment counter can be calculated according to the actual progress speed of HC poisoning of the downstream catalyst 32b, so that the poisoning suppression process can be executed at an appropriate timing. The count-up amount may be calculated based on both the detected air-fuel ratio AFc and the exhaust flow rate Fr.

[0058] [Calculation control of countdown amount in the first modified example] The calculation control of the countdown amount of the determination counter in the first modified example will be described. Fig. 13A is a flowchart showing the calculation control of the countdown amount of the determination counter in the first modified example. This calculation control is executed, for example, when the process of step S7 is executed. The ECU 100 acquires the detected air-fuel ratio AFc (step S11b).

[0059] Next, the ECU 100 calculates the countdown amount of the judgment counter based on the acquired detected air-fuel ratio AFc (step S12b). FIG. 13B is a map that specifies 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 a storage device or the like of the ECU 100. In this map, the more the detected air-fuel ratio AFc increases from the poisoning recovery lower limit value LL, that is, the more the detected air-fuel ratio AFc decreases from the poisoning recovery lower limit value LL, the more the countdown amount of the judgment counter is calculated. Lean The countdown amount is specified to increase from "1" as the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b becomes leaner than the poisoning recovery lower limit LL. This is because the degree to which HC poisoning in the downstream catalyst 32b is eliminated tends to increase as the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b becomes leaner than the poisoning recovery lower limit LL. In this way, the countdown amount of the determination counter can be calculated according to the actual degree to which HC poisoning in the downstream catalyst 32b is eliminated, so that the poisoning suppression process can be executed only when necessary.

[0060] [Calculation control of countdown amount in the second modified example] FIG. 14A is a flowchart showing calculation control of the countdown amount of the judgment counter of the second modified example. The ECU 100 calculates the exhaust flow rate Fr flowing into the downstream catalyst 32b in the same manner as in step S11a (step S11c). Next, the ECU 100 calculates the countdown amount based on the calculated exhaust flow rate Fr (step S12c). FIG. 14B is a map that specifies the relationship between the exhaust flow rate Fr and the countdown amount. The vertical axis indicates the exhaust flow rate Fr, and the horizontal axis indicates the countup amount. This map is stored in advance in a storage device or the like of the ECU 100. In this map, it is specified that the countdown amount increases from "1" as the exhaust flow rate Fr increases from a predetermined value. This is because the larger the exhaust flow rate Fr to the downstream catalyst 32b, the larger the flow rate of the lean air-fuel ratio exhaust gas flowing into the downstream catalyst 32b, and the degree of elimination of HC poisoning in the downstream catalyst 32b is likely to increase. In this way, since the countdown amount of the determination counter can be calculated according to the actual degree of elimination of HC poisoning in the downstream side catalyst 32b, 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 flow rate Fr.

[0061] In the maps of FIG. 11B, FIG. 12B, FIG. 13B, and FIG. 14B, the count-up amount and the count-down amount are defined as a straight line, but are not limited to this, and may be defined as a quadratic curve or a broken line. The calculation of the count-up amount and the count-down amount is not limited to the above-mentioned maps, and may be calculated by an arithmetic expression. For example, the final count-up amount may be calculated by multiplying the initial value "1" of the count-up amount by a coefficient K (>1) that increases as the detected air-fuel ratio AFc falls from the poisoning upper limit value PU or the exhaust flow rate Fr increases. The final count-down amount may be calculated by multiplying the initial value "1" of the count-down amount by a coefficient K (>1) that increases as the detected air-fuel ratio AFc increases from the poisoning recovery lower limit value LL or the exhaust flow rate Fr increases.

[0062] Although a preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such a specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention described in the claims. [Explanation of symbols]

[0063] 1. Internal combustion engine 10. Main Body of the Agency 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, judgment unit, control unit)

Claims

1. The present invention is applied to an internal combustion engine having an engine body, an upstream catalyst and a downstream catalyst which are respectively provided on the upstream and downstream sides of an exhaust passage connected to the engine body and have an oxygen storage capacity, an upstream sensor which is provided in the exhaust passage upstream of the upstream catalyst and detects an air-fuel ratio of exhaust gas, a midstream sensor which is provided in the exhaust passage downstream of the upstream catalyst and upstream of the downstream catalyst and detects an air-fuel ratio of exhaust gas, and a downstream sensor which is provided in the exhaust passage downstream of the downstream catalyst and detects an air-fuel ratio of exhaust gas, a control device for an internal combustion engine, which controls an air-fuel ratio of the internal combustion engine to alternately switch between a rich air-fuel ratio smaller than the stoichiometric air-fuel ratio and a lean air-fuel ratio larger than the stoichiometric air-fuel ratio by controlling an air-fuel ratio of exhaust gas discharged from an engine body so that the air-fuel ratio detected by the upstream sensor becomes the target air-fuel ratio when the lean judged air-fuel ratio is reached and by setting a target air-fuel ratio to a rich target air-fuel ratio smaller than the stoichiometric air-fuel ratio when the air-fuel ratio detected by the midstream sensor becomes a rich judged air-fuel ratio smaller than the stoichiometric air-fuel ratio, and by controlling the air-fuel ratio of exhaust gas discharged from the engine body so that the air-fuel ratio detected by the upstream sensor becomes the target air-fuel ratio, a determination unit that determines whether or not a HC poisoning condition that may cause HC poisoning of the downstream catalyst is established based on the detected air-fuel ratio of the downstream sensor; a control unit that controls the internal combustion engine so as to increase the amount of oxygen flowing into the downstream catalyst when a positive determination is made by the determination unit, compared to when a negative determination is made by the determination unit; a control device for an internal combustion engine, wherein, when a positive determination is made by the determination unit, the control unit switches the lean determined air-fuel ratio to a strong lean determined air-fuel ratio that is even leaner, thereby controlling the air-fuel ratio of the internal combustion engine so that the air-fuel ratio of the lean air-fuel ratio exhaust gas flowing into the downstream catalyst is even leaner than when a negative determination is made by the determination unit, thereby increasing the amount of oxygen flowing into the downstream catalyst.

2. The present invention is applied to an internal combustion engine having an engine body, an upstream catalyst and a downstream catalyst which are respectively provided on the upstream and downstream sides of an exhaust passage connected to the engine body and have an oxygen storage capacity, an upstream sensor which is provided in the exhaust passage upstream of the upstream catalyst and detects an air-fuel ratio of exhaust gas, a midstream sensor which is provided in the exhaust passage downstream of the upstream catalyst and upstream of the downstream catalyst and detects an air-fuel ratio of exhaust gas, and a downstream sensor which is provided in the exhaust passage downstream of the downstream catalyst and detects an air-fuel ratio of exhaust gas, a control device for an internal combustion engine, which controls an air-fuel ratio of the internal combustion engine to alternately switch between a rich air-fuel ratio smaller than the stoichiometric air-fuel ratio and a lean air-fuel ratio larger than the stoichiometric air-fuel ratio by controlling an air-fuel ratio of exhaust gas discharged from an engine body so that the air-fuel ratio detected by the upstream sensor becomes the target air-fuel ratio when the lean judged air-fuel ratio is reached and by setting a target air-fuel ratio to a rich target air-fuel ratio smaller than the stoichiometric air-fuel ratio when the air-fuel ratio detected by the midstream sensor becomes a rich judged air-fuel ratio smaller than the stoichiometric air-fuel ratio, and by controlling the air-fuel ratio of exhaust gas discharged from the engine body so that the air-fuel ratio detected by the upstream sensor becomes the target air-fuel ratio, a determination unit that determines whether or not a HC poisoning condition that may cause HC poisoning of the downstream catalyst is established based on the detected air-fuel ratio of the downstream sensor; a control unit that controls the internal combustion engine so as to increase the amount of oxygen flowing into the downstream catalyst when a positive determination is made by the determination unit, compared to when a negative determination is made by the determination unit; a control device for an internal combustion engine, when a positive determination is made by the determination unit, switching from the stoichiometric air-fuel ratio, which is a control center between the rich target air-fuel ratio and the lean target air-fuel ratio, to a rich side control center on the richer side than the stoichiometric air-fuel ratio, thereby switching the rich target air-fuel ratio to a strong rich target air-fuel ratio on the richer side that is even smaller than the rich target air-fuel ratio, and switching the lean target air-fuel ratio to a weak lean target air-fuel ratio on the richer side that is even smaller than the lean target air-fuel ratio, and controlling the air-fuel ratio of the internal combustion engine so that an inflow time of the exhaust gas having the lean air-fuel ratio flowing into the downstream catalyst is longer than when a negative determination is made by the determination unit, thereby increasing the amount of oxygen flowing into the downstream catalyst.

3. a counter calculation unit that counts up a determination counter when the detected air-fuel ratio of the downstream sensor is equal to or less than an upper limit value at which the downstream catalyst may be poisoned by HC; 3. The control device for an internal combustion engine according to claim 1, wherein the determination unit determines that the HC poisoning condition is established when the determination counter is equal to or greater than a threshold value.

4. 4. The control device for an internal combustion engine according to claim 3, wherein the counter calculation unit executes one of counting down and resetting the determination counter 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 a stoichiometric air-fuel ratio.

5. 5. The control device for an internal combustion engine according to claim 3, wherein the counter calculation unit either counts down or resets the determination counter when the process of increasing the amount of oxygen flowing into the downstream catalyst by the control unit is stopped.

6. 6. The control device for an internal combustion engine according to claim 3, wherein the counter calculation unit increases the count-up amount of the determination counter in at least one of the cases where the detected air-fuel ratio of the downstream sensor is smaller and / or the flow rate of the exhaust gas flowing into the downstream catalyst is larger.

7. 7. The control device for an internal combustion engine according to claim 3, wherein the counter calculation unit increases the countdown amount of the determination counter in at least one of the cases where the detected air-fuel ratio of the downstream sensor is larger and the flow rate of the exhaust gas flowing into the downstream catalyst is larger.

Images