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

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

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, 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, 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, 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 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.

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, 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, 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, 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 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.