Air-fuel ratio control device

Abstract

To provide an air-fuel ratio control device for an internal combustion engine that suppresses deterioration of emissions.SOLUTION: An upstream sensor that detects an air-fuel ratio, a front stage catalyst that is a three-way catalyst having oxygen storage capacity, a downstream side sensor that detects an air-fuel ratio, and a rear stage catalyst that is a three-way catalyst having oxygen storage capacity are disposed in an exhaust passage of an engine, in this order. An air-fuel ratio control device for the internal combustion engine includes: a calculation section that calculates integrated oxygen excess / deficiency amounts of the front stage catalyst and the rear stage catalyst; and a setting section that sets a target air-fuel ratio of the internal combustion engine in accordance with the integrated oxygen excess / deficiency amounts. The calculation section calculates the integrated oxygen excess / deficiency amounts by using output of a downstream side sensor when the target air-fuel ratio is set to be lean and the downstream side sensor indicates a theoretical air fuel ratio or a lean air-fuel ratio, calculates the integrated oxygen excess / deficiency amounts by using output of an upstream side sensor when the target air-fuel ratio is set to be lean and the downstream side sensor indicates a rich air-fuel ratio, and calculates the integrated oxygen excess / deficiency amounts by using the output of the upstream side sensor when the target air-fuel ratio is set to a rich air-fuel ratio.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an air-fuel ratio control device. [Background technology]

[0002] When the air-fuel ratio of the exhaust gas flowing into a three-way catalyst is rich, the output air-fuel ratio of a sensor downstream of the three-way catalyst may deviate richer than the actual air-fuel ratio of the exhaust gas due to H2 generated in the three-way catalyst (see Patent Document 1). Also, the cumulative oxygen excess / deficiency of the three-way catalyst is calculated based on the air-fuel ratio of the exhaust gas flowing into the three-way catalyst, and the target air-fuel ratio of the internal combustion engine is set to a rich or lean air-fuel ratio based on this cumulative oxygen excess / deficiency (see Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5871009 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-132190 Summary of the Invention [Problem to be solved by the invention]

[0004] For example, there are cases where an upstream catalyst and a downstream catalyst are arranged in the exhaust passage. In such cases, it is conceivable to switch the target air-fuel ratio of the internal combustion engine based on the cumulative oxygen excess / deficiency between the upstream catalyst and the downstream catalyst. The cumulative oxygen excess / deficiency can be calculated based on the air-fuel ratio of the exhaust gas flowing into the upstream catalyst and the air-fuel ratio of the exhaust gas flowing into the downstream catalyst. However, as described above, when a rich deviation occurs, the accuracy of calculating the cumulative oxygen excess / deficiency decreases, and it may be impossible to set the target air-fuel ratio at an appropriate time. This may result in a deterioration in emissions.

[0005] Therefore, an object of the present invention is to provide an air-fuel ratio control device that suppresses the deterioration of emissions. [Means for solving the problem]

[0006] The object of the present invention is to provide an air-fuel ratio control device for an internal combustion engine in which an upstream sensor for detecting the air-fuel ratio of exhaust gas, a front-stage catalyst which is a three-way catalyst having oxygen storage capacity, a downstream sensor for detecting the air-fuel ratio of exhaust gas, and a rear-stage catalyst which is also a three-way catalyst having oxygen storage capacity are arranged in this order from upstream to downstream of an exhaust passage connected to an engine body, the air-fuel ratio control device comprising: a calculation unit for calculating an integrated oxygen excess / deficiency of the front-stage catalyst and the rear-stage catalyst; and a setting unit for setting a target air-fuel ratio of the internal combustion engine to a rich air-fuel ratio or a lean air-fuel ratio in accordance with the integrated oxygen excess / deficiency; This can be achieved by an air-fuel ratio control device that calculates the cumulative oxygen excess / deficiency using the output air-fuel ratio of the downstream sensor when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a stoichiometric air-fuel ratio or a lean air-fuel ratio, calculates the cumulative oxygen excess / deficiency using the output air-fuel ratio of the upstream sensor when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a rich air-fuel ratio, and calculates the cumulative oxygen excess / deficiency using the output air-fuel ratio of the upstream sensor when the target air-fuel ratio is set to a rich air-fuel ratio.

[0007] The calculation unit may calculate the integrated oxygen excess / deficiency using a value obtained by subtracting the stoichiometric air-fuel ratio from the output air-fuel ratio of the downstream sensor when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a stoichiometric air-fuel ratio or a lean air-fuel ratio; when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a rich air-fuel ratio, the calculation unit may calculate the integrated oxygen excess / deficiency using a value obtained by subtracting the stoichiometric air-fuel ratio from the output air-fuel ratio of the upstream sensor; and when the target air-fuel ratio is set to a rich air-fuel ratio, the calculation unit may calculate the integrated oxygen excess / deficiency using a value obtained by subtracting the stoichiometric air-fuel ratio from the output air-fuel ratio of the upstream sensor.

[0008] The setting unit may set the target air-fuel ratio to a rich air-fuel ratio when the target air-fuel ratio is set to a lean air-fuel ratio and the cumulative oxygen excess / deficiency amount is equal to or greater than an upper limit reference value, and may set the target air-fuel ratio to a lean air-fuel ratio when the target air-fuel ratio is set to a rich air-fuel ratio and the cumulative oxygen excess / deficiency amount is less than a lower limit reference value that is smaller than the upper limit reference value. [Effects of the Invention]

[0009] It is possible to provide an air-fuel ratio control device in which deterioration of emissions is suppressed. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic diagram of an internal combustion engine. [Figure 2] 3 is a flowchart illustrating an example of air-fuel ratio control. [Figure 3] 10 is a flowchart illustrating calculation control of an integrated oxygen excess / deficiency ΣOED in a comparative example. [Figure 4] 6 is a timing chart illustrating an example of air-fuel ratio control according to a comparative example. [Figure 5] 10 is a flowchart illustrating an example of calculation control of an integrated oxygen excess / deficiency ΣOED in this embodiment. [Figure 6] 4 is a timing chart illustrating the air-fuel ratio control of the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] [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 also be mounted on a vessel or other 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 multiple cylinders, each of which 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 also disposed 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 each cylinder is provided with an in-cylinder injection valve 17. The rotation speed sensor 15 detects the rotation speed of the crankshaft 14, thereby detecting the rotation speed of the engine body 10. The in-cylinder injection valve 17 directly injects fuel into the combustion chamber 11. Note that 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.

[0012] 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 Ga. The throttle valve 23 is driven by, for example, an actuator (not shown) to adjust the amount of intake air Ga.

[0013] When intake valve 18a opens, air is introduced from intake passage 20 into combustion chamber 11. A mixture of fuel and air 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.

[0014] The exhaust passage 30 is provided with an upstream sensor 31a, a front-stage catalyst 32a, a downstream sensor 31b, and a rear-stage catalyst 32b, in this order from upstream to downstream. The upstream sensor 31a and the downstream sensor 31b 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 to this. At least one of these sensors may be an oxygen concentration sensor that can detect the air-fuel ratio of the exhaust gas by detecting the oxygen concentration in the exhaust gas. The upstream sensor 31a detects the air-fuel ratio of the exhaust gas that is discharged from the engine body 10 and flows into the front-stage catalyst 32a. The downstream sensor 31b detects the air-fuel ratio of the exhaust gas that is discharged from the front-stage catalyst 32a and flows into the rear-stage catalyst 32b.

[0015] The front-stage catalyst 32a and rear-stage catalyst 32b are three-way catalysts containing catalytic metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) and possessing oxygen storage capacity. The three-way catalyst, with its catalytic activity and oxygen storage capacity, purifies NOx and HC according to the amount of oxygen stored. Specifically, when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is lean, the three-way catalyst stores oxygen in the exhaust gas when the oxygen storage capacity of the three-way catalyst is low. This reduces and purifies NOx in the exhaust gas. When the oxygen storage capacity of the three-way catalyst increases, the concentrations of oxygen and NOx in the exhaust gas flowing out of the three-way catalyst increase. When the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is rich, the oxygen stored in the three-way catalyst is released when the oxygen storage capacity of the three-way catalyst is high, and HC in the exhaust gas is oxidized and purified. When the oxygen storage capacity of the three-way catalyst decreases, the concentration of HC in the exhaust gas flowing out of 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 depending on the air-fuel ratio and the amount of oxygen stored in the exhaust gas flowing into the three-way catalyst.

[0016] [ECU schematic configuration] The ECU (Electric Control Unit) 100 includes a CPU (Central Processing Unit), RAM (Random Access Memory), ROM (Read Only Memory), flash memory, and other storage devices, and performs various controls by executing programs stored in the ROM and storage devices. The ECU 100 controls the spark plugs 16, the in-cylinder injection valves 17, and the throttle valve 23 based on the amount of operation of the accelerator pedal or brake pedal operated by the driver, the engine speed and load of the engine body 10, and other factors. The ECU 100 receives inputs of the engine speed detected by the engine speed sensor 15, the intake air amount detected by the air flow meter 22, and the output air-fuel ratios AFup and AFdown detected by the upstream sensor 31a and downstream sensor 31b, respectively.

[0017] As will be described in detail later, the ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 so that it alternates between a rich air-fuel ratio lower than the stoichiometric air-fuel ratio ST (for example, 14.6) 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.

[0018] The ECU 100 controls the air-fuel ratio of the exhaust gas discharged from the engine body 10 so that the output air-fuel ratio AFup of the upstream sensor 31a becomes the target air-fuel ratio AFT. Specifically, the ECU 100 feedback-controls the fuel injection amount from the direct injection valve 17 and the opening degree of the throttle valve 23 based on the output air-fuel ratio AFup of the upstream sensor 31a so that the output air-fuel ratio of the upstream sensor 31a becomes the target air-fuel ratio AFT. As a result, the air-fuel ratio of the exhaust gas discharged from the engine body 10 is controlled to the target air-fuel ratio AFT. Furthermore, the ECU 100 sets the target air-fuel ratio AFT to a rich air-fuel ratio or a lean air-fuel ratio based on the cumulative oxygen excess / deficiency ΣOED, as will be described in detail later. The rich air-fuel ratio is a predetermined air-fuel ratio, for example, a value between 12.00 and 14.58. The lean air-fuel ratio is a predetermined air-fuel ratio, for example, a value between 14.65 and 20.00. The ECU 100 is an example of an air-fuel ratio control device. The ECU 100 also includes the CPU, RAM, ROM, storage device, and the like, which functionally implement a calculation unit and a setting unit, which will be described later.

[0019] [Air-fuel ratio control] FIG. 2 is a flowchart illustrating the air-fuel ratio control. This control is repeatedly executed while the internal combustion engine 1 is operating. The ECU 100 determines whether or not the conditions for executing the air-fuel ratio control are met (step S1). The execution conditions include, for example, that fuel cut control is not being executed. If the answer is No in step S1, this control ends.

[0020] If the answer is Yes in step S1, the ECU 100 calculates the cumulative oxygen excess / deficiency ΣOED (step S2). The cumulative oxygen excess / deficiency ΣOED is the sum of the cumulative value of the oxygen excess / deficiency in the upstream catalyst 32a and the cumulative value of the oxygen excess / deficiency in the downstream catalyst 32b. The oxygen excess / deficiency in the upstream catalyst 32a refers to the excess or deficiency of oxygen when the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 32a is set to the stoichiometric air-fuel ratio. Similarly, the oxygen excess / deficiency in the downstream catalyst 32b refers to the excess or deficiency of oxygen when the air-fuel ratio of the exhaust gas flowing into the downstream catalyst 32b is set to the stoichiometric air-fuel ratio. For example, when the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 32a is lean and oxygen is stored in the upstream catalyst 32a, the value of the oxygen excess / deficiency will be positive. When the air-fuel ratio of the exhaust gas flowing into the front-stage catalyst 32a is a rich air-fuel ratio and oxygen is released from the front-stage catalyst 32a, the value of the oxygen excess / deficiency becomes negative.

[0021] Next, the ECU 100 determines whether the target air-fuel ratio AFT is set to a rich air-fuel ratio (step S3). If the answer is Yes in step S3, the ECU 100 determines whether the cumulative oxygen excess / deficiency ΣOED is less than a lower limit reference value OEDrefr (step S4). The lower limit reference value OEDrefr is a negative value. If the answer is Yes in step S4, the ECU 100 sets the target air-fuel ratio AFT to a lean air-fuel ratio (step S5). In this case, the target air-fuel ratio AFT is switched from a rich air-fuel ratio to a lean air-fuel ratio. If the answer is No in step S4, the ECU 100 sets the target air-fuel ratio AFT to a rich air-fuel ratio (step S6). In this case, the state in which the target air-fuel ratio AFT is set to the rich air-fuel ratio is continued.

[0022] If the answer is No in step S3, the ECU 100 determines whether the cumulative oxygen excess / deficiency ΣOED is equal to or greater than the upper limit reference value OEDrefl (step S7). The upper limit reference value OEDrefl is a positive value and is greater than the lower limit reference value OEDrefr. If the answer is Yes in step S7, the ECU 100 sets the target air-fuel ratio AFT to a rich air-fuel ratio (step S6). In this case, the target air-fuel ratio AFT is switched from a lean air-fuel ratio to a rich air-fuel ratio. If the answer is No in step S7, the ECU 100 sets the target air-fuel ratio AFT to a lean air-fuel ratio (step S5). In this case, the state in which the target air-fuel ratio AFT is set to the lean air-fuel ratio is maintained. Steps S5 and S6 are an example of processing executed by the setting unit.

[0023] [Calculation control of cumulative oxygen excess / deficiency ΣOED in comparative example] 3 is a flowchart illustrating calculation control of the cumulative oxygen excess / deficiency ΣOED in a comparative example. The ECU 100 determines whether or not the execution condition for the calculation control is met (step S11). If the answer is Yes in step S11, the ECU 100 determines whether or not the previous target air-fuel ratio AFT in this calculation control is the same as the current target air-fuel ratio AFT (step S12). For example, if the previous target air-fuel ratio AFT was a rich air-fuel ratio and the current target air-fuel ratio AFT is a lean air-fuel ratio, or if the previous target air-fuel ratio AFT was a lean air-fuel ratio and the current target air-fuel ratio AFT is a rich air-fuel ratio, the result is No in step S12. If the previous target air-fuel ratio AFT and the current target air-fuel ratio AFT are rich air-fuel ratios, or if the previous target air-fuel ratio AFT and the current target air-fuel ratio AFT are lean air-fuel ratios, the result is Yes in step S12. If the answer is No in step S12, the ECU 100 resets the cumulative oxygen excess / deficiency ΣOED to 0 (step S13).

[0024] If the answer is Yes in step S12, the ECU 100 determines whether the target air-fuel ratio AFT is a lean air-fuel ratio (step S14). If the answer is Yes in step S14, the ECU 100 determines whether the output air-fuel ratio AFdown indicates a lean air-fuel ratio (step S15). For example, if the output air-fuel ratio AFdown is equal to or greater than a predetermined determination value, it is determined that the output air-fuel ratio AFdown indicates a lean air-fuel ratio. The predetermined determination value is a value equal to or greater than the stoichiometric air-fuel ratio ST. If the answer is Yes in step S15, the ECU 100 sets the output air-fuel ratio AFdown (step S16). AFoed will be described later. If the answer is No in step S15, the ECU 100 sets the output air-fuel ratio AFup to AFoed (step S17).

[0025] If the answer is No in step S14, the ECU 100 determines whether or not the output air-fuel ratio AFdown indicates a rich air-fuel ratio (step S18). For example, if the output air-fuel ratio AFdown is less than a predetermined determination value, it is determined that the output air-fuel ratio AFdown indicates a rich air-fuel ratio. The predetermined determination value is a value equal to or less than the determination value in step S15 described above and equal to or less than the stoichiometric air-fuel ratio ST. If the answer is Yes in step S18, step S16 is executed. If the answer is No in step S18, the above-mentioned step S17 is executed.

[0026] Next, the ECU 100 calculates the OED based on the following equation (1) (step S19). OED=Δt×0.23××(AFoed-14.6)×Ga / AFup…(1) 0.23 is the oxygen concentration in the air. 14.6 is the stoichiometric air-fuel ratio ST. Ga is the intake air amount. Δt is an infinitesimal time. Next, the ECU 100 calculates the integrated oxygen excess / deficiency ΣOED by integrating the OED calculated for each infinitesimal time Δt (step S20).

[0027] As described above, in the comparative example, when the target air-fuel ratio AFT is a lean air-fuel ratio and the output air-fuel ratio AFdown is a lean air-fuel ratio (Yes in steps S14 and S15), the cumulative oxygen excess / deficiency ΣOED is calculated based on the value obtained by subtracting the stoichiometric air-fuel ratio ST from the output air-fuel ratio AFdown (steps S16, S19, and S20). In this case, it is estimated that oxygen is saturated in the front-stage catalyst 32a and oxygen is stored in the rear-stage catalyst 32b. In such a case, the cumulative oxygen excess / deficiency ΣOED is calculated with high accuracy based on the output air-fuel ratio AFdown, which is the air-fuel ratio of the exhaust gas flowing into the rear-stage catalyst 32b. When the target air-fuel ratio AFT is a lean air-fuel ratio and the output air-fuel ratio AFdown is a rich air-fuel ratio (Yes in step S14, No in step S15), the cumulative oxygen excess / deficiency ΣOED is calculated based on the value obtained by subtracting the stoichiometric air-fuel ratio ST from the output air-fuel ratio AFup (steps S17, S19, and S20). In this case, it is estimated that oxygen is being stored in the front-stage catalyst 32a. In such a case, the cumulative oxygen excess / deficiency ΣOED is calculated with high accuracy based on the output air-fuel ratio AFup, which is the air-fuel ratio of the exhaust gas flowing into the front-stage catalyst 32a.

[0028] When the target air-fuel ratio AFT is a rich air-fuel ratio and the output air-fuel ratio AFdown is a lean air-fuel ratio (No in steps S14 and S18), the cumulative oxygen excess / deficiency ΣOED is calculated based on the value obtained by subtracting the stoichiometric air-fuel ratio ST from the output air-fuel ratio AFup (steps S17, S19, and S20). In this case, it is estimated that oxygen is being released from the front-stage catalyst 32a. In such a case, the cumulative oxygen excess / deficiency ΣOED is accurately calculated based on the output air-fuel ratio AFup.

[0029] When the target air-fuel ratio AFT is a rich air-fuel ratio and the output air-fuel ratio AFdown is a rich air-fuel ratio (No in step S14, Yes in step S18), the cumulative oxygen excess / deficiency ΣOED is calculated based on the value obtained by subtracting the stoichiometric air-fuel ratio ST from the output air-fuel ratio AFdown (steps S16, S19, and S20). In this case, it is estimated that there is a shortage of oxygen in the front-stage catalyst 32a and oxygen is being released from the rear-stage catalyst 32b. In such a case, the cumulative oxygen excess / deficiency ΣOED is calculated with high accuracy based on the output air-fuel ratio AFdown.

[0030] However, when the target air-fuel ratio AFT is a rich air-fuel ratio and the output air-fuel ratio AFdown is a rich air-fuel ratio, the output air-fuel ratio AFdown may deviate richer than the actual exhaust air-fuel ratio. The rich deviation occurs when the output air-fuel ratio AFdown of the downstream sensor 31b deviates richer due to H2 generated by the water-gas shift reaction and the steam reforming reaction in the front-stage catalyst 32a. This may result in a decrease in the accuracy of calculation of the cumulative oxygen excess / deficiency ΣOED, as described below, and may result in a deterioration in emissions.

[0031] [Air-fuel ratio control of comparative example] FIG. 4 is a timing chart illustrating air-fuel ratio control of a comparative example. FIG. 4 shows the transitions of the target air-fuel ratio AFT, output air-fuel ratio AFup, front-stage catalyst oxygen storage amount OSAf, rear-stage catalyst oxygen storage amount OSAr, cumulative oxygen excess / deficiency ΣOED, and output air-fuel ratio AFdown. When the target air-fuel ratio AFT is set to a rich air-fuel ratio, the front-stage catalyst oxygen storage amount OSAf decreases, and the rear-stage catalyst oxygen storage amount OSAr is maintained at the upper limit target value Crefl (t0). Furthermore, the output air-fuel ratio AFup indicates a rich air-fuel ratio, and the output air-fuel ratio AFdown indicates the stoichiometric air-fuel ratio ST (t0). When the front-stage catalyst oxygen storage amount OSAf decreases to 0, the rear-stage catalyst oxygen storage amount OSAr begins to decrease, and the output air-fuel ratio AFdown indicates a rich air-fuel ratio (time t1). Here, due to the above-mentioned rich deviation, the output air-fuel ratio AFdown deviates to the rich side from the actual exhaust air-fuel ratio. As a result, the cumulative oxygen excess / deficiency ΣOED becomes less than the lower limit reference value OEDrefr early before the oxygen storage amount OSAf of the front catalyst becomes less than the lower limit target value Crefr, and the target air-fuel ratio AFT is set to a lean air-fuel ratio (time t2). Note that in FIG. 4, the output air-fuel ratio AFdown and the cumulative oxygen excess / deficiency ΣOED in the case where such a rich deviation does not occur are shown by dotted lines. When the target air-fuel ratio AFT is set to a lean air-fuel ratio, the oxygen storage amount OSAf of the front catalyst begins to increase, and the oxygen storage amount OSAr of the rear catalyst is maintained at its current value. Furthermore, the cumulative oxygen excess / deficiency ΣOED is reset to 0 and increases, the output air-fuel ratio AFup indicates a lean air-fuel ratio, and the output air-fuel ratio AFdown indicates the stoichiometric air-fuel ratio ST.

[0032] When the oxygen storage amount OSAf of the front catalyst increases to the maximum amount Cmaxf, the oxygen storage amount OSAr of the rear catalyst begins to increase, and the output air-fuel ratio AFdown indicates a lean air-fuel ratio (time t3). When the oxygen storage amount OSAr of the rear catalyst increases beyond the upper limit target value Crefl and the cumulative oxygen excess / deficiency ΣOED becomes equal to or greater than the upper limit reference value OEDrefl, the target air-fuel ratio AFT is set to a rich air-fuel ratio (time t4). Furthermore, the oxygen storage amount OSAf of the front catalyst begins to decrease, the oxygen storage amount OSAr of the rear catalyst is maintained at its current value, the cumulative oxygen excess / deficiency ΣOED is reset to 0, and the output air-fuel ratio AFdown indicates the stoichiometric air-fuel ratio ST (time t4). By controlling the target air-fuel ratio AFT in this manner, the target air-fuel ratio AFT is switched from a rich air-fuel ratio to a lean air-fuel ratio before the oxygen storage amount OSAr of the rear catalyst decreases sufficiently. Therefore, the oxygen storage amount OSAr of the rear-end catalyst increases to the maximum amount Cmaxf (time t5). As a result, the oxygen storage capacity of the rear-end catalyst 32b becomes saturated, and NOx in the lean air-fuel ratio exhaust gas that flows into the rear-end catalyst 32b may not be purified by the rear-end catalyst 32b. This may result in a deterioration of emissions.

[0033] [Calculation control of cumulative oxygen excess / deficiency ΣOED in this embodiment] FIG. 5 is a flowchart illustrating the calculation control of the cumulative oxygen excess / deficiency ΣOED in this embodiment. FIG. 5 corresponds to FIG. 3. Note that duplicated explanations of the same processes as in FIG. 3 will be omitted. In this embodiment, unlike the comparative example, the process of step S18 is not executed, and if step S14 is No, step S17 is always executed. That is, if the target air-fuel ratio AFT is a rich air-fuel ratio, the cumulative oxygen excess / deficiency ΣOED is calculated based on the output air-fuel ratio AFup regardless of the output air-fuel ratio AFdown (steps S17, S19, S20). Steps S17, S19, and S20 are examples of processes executed by the calculation unit.

[0034] [Air-fuel ratio control of this embodiment] FIG. 6 is a timing chart illustrating the air-fuel ratio control of this embodiment. FIG. 6 corresponds to FIG. 4. When the target air-fuel ratio AFT is set to a rich air-fuel ratio (time t0), even if the output air-fuel ratio AFdown indicates a rich air-fuel ratio (time t1), the output air-fuel ratio AFup is used to calculate the cumulative oxygen excess / deficiency ΣOED. Therefore, the cumulative oxygen excess / deficiency ΣOED is calculated without being affected by a rich deviation of the output air-fuel ratio AFdown. This makes it possible to suppress the deviation between the timing when the front-stage catalyst oxygen storage amount OSAf becomes less than the lower limit target value Crefr and the timing when the cumulative oxygen excess / deficiency ΣOED becomes less than the lower limit reference value OEDrefr (time t2). Note that FIG. 5 uses dotted lines to show the output air-fuel ratio AFdown when such a rich deviation does not occur and the cumulative oxygen excess / deficiency ΣOED calculated in the comparative example. In this way, the rear-stage catalyst oxygen storage amount OSAr is prevented from exceeding the maximum amount Cmaxf, thereby suppressing deterioration of emissions.

[0035] 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 are possible within the scope of the gist of the present invention as described in the claims. [Explanation of symbols]

[0036] 1. Internal combustion engine 10. Main body of the organization 30 Exhaust passage 31a Upstream sensor 31b Downstream sensor 32a Front stage catalyst 32b Post-catalyst 100 ECU (air-fuel ratio control unit, calculation unit, setting unit)

Claims

1. An air-fuel ratio control device for an internal combustion engine, which controls the air-fuel ratio of an internal combustion engine, comprising an upstream sensor for detecting the air-fuel ratio of exhaust gas, a front-stage catalyst which is a three-way catalyst having an oxygen storage capacity, a downstream sensor for detecting the air-fuel ratio of exhaust gas, and a rear-stage catalyst which is a three-way catalyst having an oxygen storage capacity, which are arranged in this order from upstream to downstream of an exhaust passage connected to an engine body, a calculation unit that calculates an accumulated oxygen excess / deficiency amount of the upstream catalyst and the downstream catalyst; a setting unit that sets a target air-fuel ratio of the internal combustion engine to a rich air-fuel ratio or a lean air-fuel ratio according to the cumulative oxygen excess / deficiency amount, an air-fuel ratio control device in which, when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a stoichiometric air-fuel ratio or a lean air-fuel ratio, the calculation unit calculates the integrated oxygen excess / deficiency using the output air-fuel ratio of the downstream sensor; when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a rich air-fuel ratio, the calculation unit calculates the integrated oxygen excess / deficiency using the output air-fuel ratio of the upstream sensor; and when the target air-fuel ratio is set to a rich air-fuel ratio, the calculation unit calculates the integrated oxygen excess / deficiency using the output air-fuel ratio of the upstream sensor.

2. 2. The air-fuel ratio control device of claim 1, wherein, when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a stoichiometric air-fuel ratio or a lean air-fuel ratio, the calculation unit calculates the integrated oxygen excess / deficiency using a value obtained by subtracting the stoichiometric air-fuel ratio from the output air-fuel ratio of the downstream sensor, when the target air-fuel ratio is set to a lean air-fuel ratio and the output air-fuel ratio of the downstream sensor indicates a rich air-fuel ratio, the calculation unit calculates the integrated oxygen excess / deficiency using a value obtained by subtracting the stoichiometric air-fuel ratio from the output air-fuel ratio of the upstream sensor, and when the target air-fuel ratio is set to a rich air-fuel ratio, the calculation unit calculates the integrated oxygen excess / deficiency using a value obtained by subtracting the stoichiometric air-fuel ratio from the output air-fuel ratio of the upstream sensor.

3. 3. The air-fuel ratio control device of claim 2, wherein the setting unit sets the target air-fuel ratio to a rich air-fuel ratio when the target air-fuel ratio is set to a lean air-fuel ratio and the cumulative oxygen excess / deficiency amount is equal to or greater than an upper limit reference value, and sets the target air-fuel ratio to a lean air-fuel ratio when the target air-fuel ratio is set to a rich air-fuel ratio and the cumulative oxygen excess / deficiency amount is less than a lower limit reference value that is smaller than the upper limit reference value.

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