JP7798061B2 - Air-fuel ratio control device - Google Patents

Description

The present invention relates to an air-fuel ratio control device.

When the air-fuel ratio of the exhaust gas flowing into a three-way catalyst is rich, _H2 generated in the three-way catalyst can cause the output air-fuel ratio of a sensor downstream of the three-way catalyst to deviate richer than the actual air-fuel ratio of the exhaust gas (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 the cumulative oxygen excess/deficiency (see Patent Document 2).

Patent No. 5871009 JP 2015-132190 A

For example, there may be cases where an upstream catalyst and a downstream catalyst are arranged in the exhaust passage. In such cases, it is possible 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 mentioned above, if a rich deviation occurs, the accuracy of calculating the cumulative oxygen excess/deficiency may decrease, and the target air-fuel ratio may not be set at the appropriate time. This may result in a deterioration in emissions.

The objective is to provide an air-fuel ratio control device that suppresses the deterioration of emissions.

The above object is to provide an air-fuel ratio control device for controlling the air-fuel ratio of an internal combustion engine, which has an upstream sensor for detecting the air-fuel ratio of the exhaust gas, a front-stage catalyst which is a three-way catalyst with oxygen storage capacity, a downstream sensor for detecting the air-fuel ratio of the exhaust gas, and a rear-stage catalyst which is also a three-way catalyst with oxygen storage capacity, arranged in this order from upstream to downstream of an exhaust passage connected to an engine body, and which is equipped with a calculation unit that calculates the cumulative oxygen excess/deficiency of the front-stage catalyst and the rear-stage catalyst, and a setting unit that sets the 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 cumulative oxygen excess/deficiency, and 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; that 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 that 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.

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 cumulative 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 cumulative 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 cumulative oxygen excess/deficiency using a value obtained by subtracting the stoichiometric air-fuel ratio from the output air-fuel ratio of the upstream sensor.

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 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 is less than a lower limit reference value that is smaller than the upper limit reference value.

It is possible to provide an air-fuel ratio control device that suppresses deterioration of emissions.

1 is a schematic diagram of an internal combustion engine. 3 is a flowchart illustrating an example of air-fuel ratio control. 10 is a flowchart illustrating an example of calculation control of an integrated oxygen excess/deficiency ΣOED in a comparative example. 6 is a timing chart illustrating an example of air-fuel ratio control according to a comparative example. 4 is a flowchart illustrating an example of calculation control of an integrated oxygen excess/deficiency ΣOED in this embodiment. 4 is a timing chart illustrating the air-fuel ratio control of the present embodiment.

[General configuration of internal combustion engine]
FIG. 1 is a schematic diagram of an internal combustion engine 1. The internal combustion engine 1 is mounted, for example, on a vehicle, but is not limited thereto and may also be mounted on a vessel other than a vehicle, such as a ship. The internal combustion engine 1 includes 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 includes 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 within 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.

In the intake passage 20, an air cleaner 21, an air flow meter 22, and a throttle valve 23 are provided, 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 intake air volume Ga. The throttle valve 23 is driven, for example, by an actuator (not shown) to adjust the intake air volume Ga.

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

The exhaust passage 30 is provided with, from upstream to downstream, an upstream sensor 31a, a front-stage catalyst 32a, a downstream sensor 31b, and a rear-stage catalyst 32b. The upstream sensor 31a and 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 discharged from the engine body 10 and flowing into the front-stage catalyst 32a. The downstream sensor 31b detects the air-fuel ratio of the exhaust gas discharged from the front-stage catalyst 32a and flowing into the rear-stage catalyst 32b.

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's catalytic activity and oxygen storage capacity enable it to purify 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. As 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. As 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. With the three-way catalyst in this embodiment, the purification characteristics of NOx and HC in the exhaust gas change depending on the air-fuel ratio and oxygen storage amount of the exhaust gas flowing into the three-way catalyst.

[Outline of ECU Configuration]
The ECU (Electronic Control Unit) 100 includes a CPU (Central Processing Unit), RAM (Random Access Memory), ROM (Read Only Memory), and a storage device such as a flash memory, and performs various controls by executing programs stored in the ROM and the storage device. 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 and the brake pedal operated by the driver, the engine speed and load of the engine body 10, etc. 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 output air-fuel ratios AFup and AFdown detected by the upstream sensor 31a and the downstream sensor 31b, respectively.

As will be described in more detail below, ECU 100 controls the air-fuel ratio of the exhaust gas discharged from engine body 10 so that it alternates between a rich air-fuel ratio that is lower than the stoichiometric air-fuel ratio ST (e.g., 14.6) and a lean air-fuel ratio that is higher than the stoichiometric air-fuel ratio ST. Specifically, ECU 100 controls the air-fuel ratio of the exhaust gas discharged from engine body 10 as follows.

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 in-cylinder 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 more detail below. 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. ECU 100 is an example of an air-fuel ratio control device. Furthermore, ECU 100 functionally realizes the calculation unit and setting unit described below using the above-mentioned CPU, RAM, ROM, storage device, etc.

[Air-fuel ratio control]
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.

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 oxygen excess/deficiency values of the upstream catalyst 32a and the downstream catalyst 32b. The oxygen excess/deficiency value of 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 value of 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, if 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 rich and oxygen is released from the front-stage catalyst 32a, the oxygen excess/deficiency value will be negative.

Next, 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, ECU 100 determines whether the cumulative oxygen excess/deficiency ΣOED is less than the lower limit reference value OEDrefr (step S4). Note that the lower limit reference value OEDrefr is a negative value. If the answer is Yes in step S4, 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, 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 a rich air-fuel ratio continues.

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 target air-fuel ratio AFT remains set to the lean air-fuel ratio. Steps S5 and S6 are an example of processing executed by the setting unit.

[Calculation control of cumulative oxygen excess/deficiency ΣOED in comparative example]
3 is a flowchart illustrating the calculation control of the cumulative oxygen excess/deficiency ΣOED in a comparative example. The ECU 100 determines whether the execution conditions for the calculation control are met (step S11). If the answer to step S11 is Yes, the ECU 100 determines whether 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 was 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 was a rich air-fuel ratio, the result of step S12 is No. 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 of step S12 is Yes. If the answer is No in step S12, the ECU 100 resets the cumulative oxygen excess/deficiency ΣOED to 0 (step S13).

If step S12 is Yes, ECU 100 determines whether the target air-fuel ratio AFT is a lean air-fuel ratio (step S14). If step S14 is Yes, 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 step S15 is Yes, ECU 100 sets AFoed to the output air-fuel ratio AFdown (step S16). AFoed will be described later. If step S15 is No, ECU 100 sets AFoed to the output air-fuel ratio AFup (step S17).

If the answer is No in step S14, the ECU 100 determines whether 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 judgment value, it is determined that the output air-fuel ratio AFdown indicates a rich air-fuel ratio. The predetermined judgment value is a value equal to or less than the judgment 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.

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 an integrated oxygen excess/deficiency ΣOED by integrating the OED calculated for each infinitesimal time Δt (step S20).

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 stored in the rear-stage catalyst 32b. In such a case, the cumulative oxygen excess/deficiency ΣOED is accurately calculated 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. If 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 cases, the cumulative oxygen excess/deficiency ΣOED is accurately calculated 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.

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 cases, the cumulative oxygen excess/deficiency ΣOED is accurately calculated based on the output air-fuel ratio AFup.

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 cases, the cumulative oxygen excess/deficiency ΣOED is accurately calculated based on the output air-fuel ratio AFdown.

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, which will be described below, and may result in a deterioration in emissions.

[Air-fuel ratio control of comparative example]
FIG. 4 is a timing chart illustrating an example of air-fuel ratio control in a comparative example. This figure shows the transitions of the target air-fuel ratio AFT, the output air-fuel ratio AFup, the oxygen storage amount OSAf of the front catalyst, the oxygen storage amount OSAr of the rear catalyst, the cumulative oxygen excess/deficiency ΣOED, and the output air-fuel ratio AFdown. When the target air-fuel ratio AFT is set to a rich air-fuel ratio, the oxygen storage amount OSAf of the front catalyst decreases, and the oxygen storage amount OSAr of the rear catalyst 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 oxygen storage amount OSAf of the front catalyst decreases to 0, the oxygen storage amount OSAr of the rear catalyst begins to decrease, and the output air-fuel ratio AFdown indicates a rich air-fuel ratio (time t1). Here, due to the rich deviation described above, the output air-fuel ratio AFdown deviates richer than the actual exhaust air-fuel ratio. As a result, before the upstream catalyst oxygen storage amount OSAf becomes less than the lower limit target value Crefr, the cumulative oxygen excess/deficiency ΣOED becomes less than the lower limit reference value OEDrefr early, and the target air-fuel ratio AFT is set to a lean air-fuel ratio (time t2). Note that in Figure 4, the dotted lines show the output air-fuel ratio AFdown and the cumulative oxygen excess/deficiency ΣOED in the case where such a rich deviation does not occur. When the target air-fuel ratio AFT is set to a lean air-fuel ratio, the upstream catalyst oxygen storage amount OSAf begins to increase, and the rear catalyst oxygen storage amount OSAr 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.

When the oxygen storage amount OSAf of the front catalyst increases to its 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 exceeds 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 rear-stage catalyst oxygen storage amount OSAr decreases sufficiently. Therefore, the rear-stage catalyst oxygen storage amount OSAr increases to the maximum amount Cmaxf (time t5). As a result, the oxygen storage capacity of the rear-stage catalyst 32b becomes saturated, and there is a risk that the NOx in the lean air-fuel ratio exhaust gas that flows into the rear-stage catalyst 32b will not be purified by the rear-stage catalyst 32b. This could result in a deterioration in emissions.

[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 those in FIG. 3 will be omitted. In this embodiment, unlike the comparative example, the process of step S18 is not executed, and step S17 is always executed when step S14 is No. That is, when 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.

[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 suppresses the deviation between the timing when the pre-catalyst oxygen storage amount OSAf falls below the lower limit target value Crefr and the timing when the cumulative oxygen excess/deficiency ΣOED falls below the lower limit reference value OEDrefr (time t2). Note that FIG. 5 uses dotted lines to show the output air-fuel ratio AFdown in the case where such a rich deviation does not occur and the cumulative oxygen excess/deficiency ΣOED calculated in the comparative example. In this way, the rear-catalyst oxygen storage amount OSAr is prevented from exceeding the maximum amount Cmaxf, and deterioration of emissions is suppressed.

Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such specific embodiment, and various modifications and variations are possible within the scope of the gist of the present invention as set forth in the claims.

REFERENCE SIGNS 1 Internal combustion engine 10 Engine body 30 Exhaust passage 31a Upstream sensor 31b Downstream sensor 32a Front catalyst 32b Rear catalyst 100 ECU (air-fuel ratio control device, calculation unit, setting unit)

Claims (3)

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. An air-fuel ratio control device according to claim 1, wherein the calculation unit calculates the cumulative 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 calculates the cumulative 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 cumulative 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. 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.