Exhaust gas control system for internal combustion engine and control method for internal combustion engine

1. An exhaust gas control system for an internal combustion engine having a plurality of cylinders, the exhaust gas control system comprising:

a three-way catalyst provided in an exhaust passage on a downstream side from a collector of exhaust branch pipes, the exhaust branch pipes being connected to the plurality of cylinders respectively,

an upstream sensor provided in the exhaust passage on an upstream side from the three-way catalyst, the upstream sensor being configured to detect a physical quantity correlated with an air-fuel ratio of exhaust gas,

a downstream sensor provided in the exhaust passage on a downstream side from the three-way catalyst, the downstream sensor being configured to detect a physical quantity correlated with the air-fuel ratio of exhaust gas, and

an electronic control unit configured to:

execute a first air-fuel ratio control;

control an air-fuel ratio of an air-fuel mixture in a part of the plurality of cylinders to a lean air-fuel ratio and control an air-fuel ratio of an air-fuel mixture in the other part of the plurality of cylinders to a rich air-fuel ratio when the electronic control unit determines that a predetermined temperature increase condition for increasing the temperature of the three-way catalyst is established in the first air-fuel ratio control, the lean air-fuel ratio being an air-fuel ratio that is leaner than a stoichiometric air-fuel ratio, and the rich air-fuel ratio being an air-fuel ratio that is richer than the stoichiometric air-fuel ratio;

execute a malfunction diagnosis for diagnosing at least one of the three-way catalyst, the upstream sensor, and the downstream sensor;

execute a second air-fuel ratio control to perform the malfunction diagnosis;

switch the air-fuel ratio of the air-fuel mixture in all of the plurality of cylinders between the lean air-fuel ratio and the rich air-fuel ratio when the temperature of the three-way catalyst is equal to or higher than a predetermined diagnosis temperature in the second air-fuel ratio control;

estimate reference catalyst temperature while the electronic control unit executes the first air-fuel ratio control, the reference catalyst temperature being the temperature of the three-way catalyst not including an increase in temperature with the execution of the first air-fuel ratio control; and

perform the malfunction diagnosis by executing the second air-fuel ratio control in response to when the electronic control unit interrupts the execution of the first air-fuel ratio control after the estimated reference catalyst temperature becomes equal to or higher than the diagnosis temperature while the first air-fuel ratio control is being executed.

2. The exhaust gas control system according to claim 1 ,

wherein the electronic control unit is configured to perform the malfunction diagnosis by executing the second air-fuel ratio control in response to when the electronic control unit interrupts the execution of the first air-fuel ratio control after a state in which the estimated reference catalyst temperature is equal to or higher than the diagnosis temperature is continued for a first predetermined period or longer.

3. An exhaust gas control system for an internal combustion engine having a plurality of cylinders, the exhaust gas control system comprising:

a three-way catalyst provided in an exhaust passage on a downstream side from a collector of exhaust branch pipes, the exhaust branch pipes being connected to the plurality of cylinders respectively,

an upstream sensor provided in the exhaust passage on an upstream side from the three-way catalyst, the upstream sensor being configured to detect a physical quantity correlated with an air-fuel ratio of exhaust gas,

a downstream sensor provided in the exhaust passage on a downstream side from the three-way catalyst, the downstream sensor being configured to detect a physical quantity correlated with the air-fuel ratio of exhaust gas; and

an electronic control unit configured to:

execute a first air-fuel ratio control;

control an air-fuel ratio of an air-fuel mixture in a part of the plurality of cylinders to a lean air-fuel ratio and control an air-fuel ratio of an air-fuel mixture in the other part of the plurality of cylinders to a rich air-fuel ratio when the electronic control unit determines that a predetermined temperature increase condition for increasing the temperature of the three-way catalyst is established in the first air-fuel ratio control, the lean air-fuel ratio being an air-fuel ratio that is leaner than a stoichiometric air-fuel ratio, and the rich air-fuel ratio being an air-fuel ratio that is richer than the stoichiometric air-fuel ratio;

execute a malfunction diagnosis for diagnosing at least one of the three-way catalyst, the upstream sensor, and the downstream sensor;

execute a second air-fuel ratio control to perform the malfunction diagnosis;

switch the air-fuel ratio of the air-fuel mixture in all of the plurality of cylinders between the lean air-fuel ratio and the rich air-fuel ratio when the temperature of the three-way catalyst is equal to or higher than a predetermined diagnosis temperature in the second air-fuel ratio control; and

perform the malfunction diagnosis by executing the second air-fuel ratio control in response to when the electronic control unit interrupts the execution of the first air-fuel ratio control after the temperature of the three-way catalyst becomes equal to or higher than a predetermined interruption temperature while the first air-fuel ratio control is being executed, the interruption temperature being a temperature higher than the diagnosis temperature and a temperature at which the temperature of the three-way catalyst is maintained to be equal to or higher than the diagnosis temperature even when the electronic control unit interrupts the execution of the first air-fuel ratio control.

4. The exhaust gas control system according to claim 3 ,

wherein the electronic control unit is configured to perform the malfunction diagnosis by executing the second air-fuel ratio control in response to when the electronic control unit interrupts the execution of the first air-fuel ratio control after a state in which the temperature of the three-way catalyst is equal to or higher than the interruption temperature while the first air-fuel ratio control is being executed is continued for a second predetermined period or longer.

5. A control method for an internal combustion engine,

the internal combustion engine including a plurality of cylinders and provided with an exhaust gas control system, the exhaust gas control system including a three-way catalyst, an upstream sensor, a downstream sensor, and an electronic control unit,

the three-way catalyst being provided in an exhaust passage on a downstream side from a collector of exhaust branch pipes, the exhaust branch pipes being connected to the plurality of cylinders respectively,

the upstream sensor being provided in the exhaust passage on an upstream side from the three-way catalyst, and the upstream sensor being configured to detect a physical quantity correlated with an air-fuel ratio of exhaust gas, and

the downstream sensor being provided in the exhaust passage on a downstream side from the three-way catalyst, and the downstream sensor being configured to detect a physical quantity correlated with the air-fuel ratio of exhaust gas,

the control method comprising:

executing, by the electronic control unit, a first air-fuel ratio control;

controlling, by the electronic control unit, an air-fuel ratio of an air-fuel mixture in a part of the plurality of cylinders to a lean air-fuel ratio and controlling, by the electronic control unit, an air-fuel ratio of an air-fuel mixture in the other part of the plurality of cylinders to a rich air-fuel ratio when the electronic control unit determines that a predetermined temperature increase condition for increasing the temperature of the three-way catalyst is established in the first air-fuel ratio control, the lean air-fuel ratio being an air-fuel ratio that is leaner than a stoichiometric air-fuel ratio, and the rich air-fuel ratio being an air-fuel ratio that is richer than the stoichiometric air-fuel ratio;

performing, by the electronic control unit, a malfunction diagnosis for diagnosing at least one of the three-way catalyst, the upstream sensor, and the downstream sensor;

executing, by the electronic control unit, a second air-fuel ratio control to perform the malfunction diagnosis;

switching, by the electronic control unit, the air-fuel ratio of the air-fuel mixture in all of the plurality of cylinders between the lean air-fuel ratio and the rich air-fuel ratio, when the temperature of the three-way catalyst is equal to or higher than a predetermined diagnosis temperature in the second air-fuel ratio control;

estimating, by the electronic control unit, reference catalyst temperature while the electronic control unit executes the first air-fuel ratio control, the reference catalyst temperature being the temperature of the three-way catalyst not including an increase in temperature with the execution of the first air-fuel ratio control; and

performing, by the electronic control unit, the malfunction diagnosis by executing the second air-fuel ratio control in response to when the electronic control unit interrupts the execution of the first air-fuel ratio control after the estimated reference catalyst temperature becomes equal to or higher than the diagnosis temperature while the first air-fuel ratio control is being executed.

6. A control method for an internal combustion engine,

the internal combustion engine including a plurality of cylinders and provided with an exhaust gas control system, the exhaust gas control system including a three-way catalyst, an upstream sensor, a downstream sensor, and an electronic control unit,

the three-way catalyst being provided in an exhaust passage on a downstream side from a collector of exhaust branch pipes, the exhaust branch pipes being connected to the plurality of cylinders respectively,

the upstream sensor being provided in the exhaust passage on an upstream side from the three-way catalyst, and the upstream sensor being configured to detect a physical quantity correlated with an air-fuel ratio of exhaust gas, and

the downstream sensor being provided in the exhaust passage on a downstream side from the three-way catalyst, and the downstream sensor being configured to detect a physical quantity correlated with the air-fuel ratio of exhaust gas,

the control method comprising:

executing, by the electronic control unit, a first air-fuel ratio control;

controlling, by the electronic control unit, an air-fuel ratio of an air-fuel mixture in a part of the plurality of cylinders to a lean air-fuel ratio and controlling, by the electronic control unit, an air-fuel ratio of an air-fuel mixture in the other part of the plurality of cylinders to a rich air-fuel ratio when the electronic control unit determines that a predetermined temperature increase condition for increasing the temperature of the three-way catalyst is established in the first air-fuel ratio control, the lean air-fuel ratio being an air-fuel ratio that is leaner than a stoichiometric air-fuel ratio, and the rich air-fuel ratio being an air-fuel ratio that is richer than the stoichiometric air-fuel ratio;

performing, by the electronic control unit, a malfunction diagnosis for diagnosing at least one of the three-way catalyst, the upstream sensor, and the downstream sensor;

executing, by the electronic control unit, a second air-fuel ratio control to perform the malfunction diagnosis;

switching, by the electronic control unit, the air-fuel ratio of the air-fuel mixture in all of the plurality of cylinders between the lean air-fuel ratio and the rich air-fuel ratio when the temperature of the three-way catalyst is equal to or higher than a predetermined diagnosis temperature in the second air-fuel ratio control; and

performing, by the electronic control unit, the malfunction diagnosis by executing the second air-fuel ratio control in response to when the electronic control unit interrupts the execution of the first air-fuel ratio control after the temperature of the three-way catalyst becomes equal to or higher than a predetermined interruption temperature while the first air-fuel ratio control is being executed, the interruption temperature being a temperature higher than the diagnosis temperature and a temperature at which the temperature of the three-way catalyst is maintained to be equal to or higher than the diagnosis temperature even when the electronic control unit interrupts the execution of the first air-fuel ratio control.