Controller and control method for an internal combustion engine

Abstract

A controller for an internal combustion engine, the internal combustion engine comprising cylinders, a catalyst configured to purify exhaust gas and configured to store oxygen, and an exhaust gas sensor disposed on a downstream side of the catalyst and configured to detect oxygen, the controller comprising: a processing circuit, wherein: the processing circuit is configured to perform a richening process that supplies exhaust gas with a rich air-fuel ratio to the catalyst until the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio; the processing circuit is configured such that, after the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio in the enrichment process, it executes an air supply process which supplies air to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean air-fuel ratio; the processing circuit is configured to perform an oxygen storage capacity estimation process which estimates an oxygen storage capacity of the catalyst by accumulating an amount of air supplied to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean fuel ratio in the air supply process; and the air supply process comprises stopping the fuel supply to one or more of the cylinders and performing combustion at an air-fuel ratio less than or equal to a stoichiometric air-fuel ratio in one or more remaining cylinders, such that the cylinders supply an exhaust gas to the catalyst which is controlled overall to a lean air-fuel ratio.

Description

Background area

[0001] The following description relates to a controller for an internal combustion engine and a method for controlling an internal combustion engine. Description of the related prior art

[0002] Japanese patent application JP 2010-174 805 A describes a controller for an internal combustion engine having an exhaust passage provided with a catalyst that purifies exhaust gas, an upstream air-fuel ratio sensor arranged on an upstream side of the catalyst, and a downstream air-fuel ratio sensor arranged on a downstream side of the catalyst.

[0003] A process for calculating the stored oxygen capacity of the catalyst is known from the prior art. Specifically, a controller first sets a target air-fuel ratio to an air-fuel ratio that is richer than the stoichiometric air-fuel ratio. This causes the air-fuel ratio of the exhaust gas on the downstream side of the catalyst to become rich after a certain time delay. This indicates that the oxygen stored in the catalyst has been completely released from the catalyst. Then, the controller sets a target air-fuel ratio to an air-fuel ratio that is leaner than the stoichiometric air-fuel ratio. This causes the air-fuel ratio of the exhaust gas on the upstream side of the catalyst to become lean after a certain time delay.This indicates that lean combustion has begun, supplying oxygen to the catalyst. After oxygen supply to the catalyst begins, the amount of oxygen stored in the catalyst increases. When oxygen is stored in the catalyst and the amount of stored oxygen increases, the amount of oxygen released from the catalyst to the downstream side is small. As the amount of stored oxygen in the catalyst increases, the downstream air-fuel ratio sensor continues to detect a rich air-fuel ratio. When the amount of stored oxygen in the catalyst reaches an oxygen storage capacity, no more oxygen can be stored. This causes the oxygen to flow to the downstream side of the catalyst.Therefore, when the amount of oxygen stored by the catalyst reaches the oxygen storage capacity, the air-fuel ratio detected by the downstream air-fuel ratio sensor will become lean.

[0004] In this manner, the controller continues rich combustion until the downstream air-fuel ratio sensor detects a rich air-fuel ratio, and then continues lean combustion until the downstream sensor detects a lean air-fuel ratio. The controller calculates the oxygen storage capacity by cumulating the amount of oxygen flowing into the catalyst from the time the upstream air-fuel ratio sensor detects a lean air-fuel ratio as a result of lean combustion to the time the downstream air-fuel ratio sensor detects a lean air-fuel ratio.

[0005] The controller supplies oxygen to the catalyst by performing lean combustion to calculate the catalyst's oxygen storage capacity. Such lean combustion adversely affects exhaust gas characteristics and is therefore not preferred.

[0006] Accordingly, instead of lean combustion, engine control can be implemented to supply the catalyst with oxygen. The engine control interrupts the fuel supply to all cylinders and drives the output shaft of the internal combustion engine with a motor generator, causing the engine to idle.

[0007] However, the engine control system consumes battery power when supplying oxygen to the catalyst and calculating the catalyst's oxygen storage capacity. This battery power consumption requires the internal combustion engine to generate electrical power. Therefore, when the engine control system is running to calculate the catalyst's oxygen storage capacity, the subsequent electrical power generation by the internal combustion engine results in lower fuel economy.

[0008] Furthermore, DE 10 2014 114 784 A1 discloses a system comprising a storage capacity module and at least one of an engine speed control module and an ignition spark control module. The storage capacity module determines a capacity of a catalytic converter for storing oxygen. The engine speed control module controls a speed of an engine based on the oxygen storage capacity of the catalytic converter. The ignition spark control module controls an ignition timing of the engine based on the oxygen storage capacity of the catalytic converter. Summary

[0009] One aspect of the present disclosure corresponds to a controller for an internal combustion engine. The internal combustion engine includes cylinders, a catalyst configured to purify exhaust gas and to store oxygen, and an exhaust gas sensor disposed downstream of the catalyst and configured to detect oxygen. The controller includes a processing circuit. The processing circuit is configured to perform a rich-fuel process that supplies exhaust gas with a rich air-fuel ratio to the catalyst until the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio.The processing circuit is configured to execute an air supply process that supplies air to the catalyst after the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio in the enrichment process. The processing circuit is configured to execute an oxygen storage capacity estimation process that estimates an oxygen storage capacity of the catalyst by accumulating an amount of air supplied to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean air-fuel ratio in the air supply process.The air supply process includes stopping fuel supply to one or more of the cylinders and performing combustion at an air-fuel ratio less than or equal to a stoichiometric air-fuel ratio in one or more remaining cylinders such that the cylinders supply exhaust gas to the catalyst that is controlled to a lean air-fuel ratio overall.

[0010] Another aspect of the present disclosure corresponds to a method for controlling an internal combustion engine. The internal combustion engine includes cylinders, a catalyst configured to purify exhaust gas and configured to store oxygen, and an exhaust gas sensor disposed downstream of the catalyst and configured to detect oxygen. The method includes performing a rich-fuel process that supplies exhaust gas with a rich air-fuel ratio to the catalyst until the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio.The method further includes performing an air supply process that supplies air to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean air-fuel ratio after the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio in the enrichment process. Furthermore, the method includes performing an oxygen storage capacity estimation process that estimates an oxygen storage capacity of the catalyst by accumulating an amount of air supplied to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean fuel ratio in the air supply process.The air supply process includes stopping the fuel supply to one or more of the cylinders and performing combustion at an air-fuel ratio less than or equal to a stoichiometric air-fuel ratio in one or more remaining cylinders such that the cylinders supply exhaust gas to the catalyst which is controlled to an overall lean air-fuel ratio.

[0011] Further features and aspects will become apparent from the following detailed description, illustrations and claims. Short description of the figures Fig. 1 is a schematic diagram illustrating a controller and a hybrid electric vehicle including an internal combustion engine subject to control by the controller. Fig. Figure 2 is a graph showing how a catalytic converter deteriorates with increasing driving distance. Fig. 3 is a flowchart illustrating an exhaust system normality determination process executed by the controller according to the embodiment. Fig. 4 is a flowchart illustrating a cumulative air quantity calculation process performed during the exhaust system normality determination process of Fig. 3 is executed. Detailed description

[0012] A controller 39 serving as a controller for an internal combustion engine according to an embodiment and corresponding to a hybrid electric vehicle controller will now be described with reference to the Fig. 1 to 4 described. Vehicle configuration

[0013] As in Fig. 1, a hybrid electric vehicle 10 of the present embodiment includes an internal combustion engine (hereinafter simply referred to as engine) 11. The hybrid electric vehicle 10 will be simply referred to as the vehicle 10 hereinafter. The vehicle 10 includes a battery 28. The vehicle 10 includes a first motor 12 and a second motor 13. The first motor 12 and the second motor 13 are each operated in a motor mode and a generator mode. That is, the first motor 12 and the second motor 13 each function as a motor and a generator. In the motor mode, at least one of the first motor 12 and the second motor 13 is supplied with electric power from the battery 28, and the supplied power is converted into driving force. That is, the first motor 12 and / or the second motor 13 can drive the vehicle 10.In generator mode, the first motor 12 and / or the second motor 13 utilize driving force supplied by an external device to generate electrical power. The battery 28 is charged with the electrical power generated by the first motor 12 and / or the second motor 13.

[0014] The vehicle 10 includes a planetary gear mechanism 17. The planetary gear mechanism 17 includes three rotating elements. Specifically, the planetary gear mechanism 17 includes a sun gear 14, a planetary gear 15, and a ring gear 16. A crankshaft 30, which corresponds to the output shaft of the engine 11, is coupled to the planetary gear 15 via a transaxle damper 18. The sun gear 14 is coupled to the first motor 12. A countershaft drive gear 19 is integrated into the ring gear 16. A countershaft driven gear 20 meshes with the countershaft drive gear 19. The second motor 13 is coupled to a reduction gear 21, which meshes with the countershaft driven gear 20.

[0015] A final drive gear 22 is coupled to the intermediate driven gear 20 in such a way that the final drive gear 22 can rotate integrally with the intermediate driven gear 20. The final drive gear 22 meshes with a final driven gear 23. The final driven gear 23 is coupled to drive axles 26 of the wheels 25 via a differential mechanism 24.

[0016] The first motor 12 and the second motor 13 are electrically connected to the battery 28 through a power control unit 27 (hereinafter referred to as the PCU 27). The PCU 27 regulates the amount of electric power supplied from the battery 28 to the first motor 12 and the second motor 13. The PCU 27 also regulates the amount of electric power supplied from the first motor 12 and the second motor 13 to the battery 28. That is, the PCU 27 regulates the amount of discharge and the amount of charge.

[0017] The engine 11 comprises cylinders 31, an intake or suction passage 32 and an exhaust passage 33. In the Fig. 1, the engine 11 is a four-cylinder engine with four cylinders 31. Intake air flowing through the intake passage 32 enters the cylinders 31. An air-fuel mixture is combusted in each cylinder 31. The exhaust gas resulting from the combustion in each cylinder 31 flows into the exhaust passage 33. The intake passage 32 includes a throttle valve 34 for regulating the flow rate of the intake air flowing through the intake passage 32. The cylinders 31 each include a fuel injector 35 that injects fuel into the intake air. Each cylinder 31 may be provided with more than one fuel injector 35, and each cylinder 31 may be provided with a different number of fuel injectors 35. Each cylinder 31 is provided with a spark plug 36 that ignites the air-fuel mixture with an electric spark.Each cylinder 31 may be provided with more than one spark plug 36, and each cylinder 31 may be provided with a different number of spark plugs 36. A catalyst 37 is disposed in the exhaust passage 33 to store oxygen and react the stored oxygen with unburned fuel so that the exhaust gas can be purified. The catalyst 37 removes unburned fuel from the exhaust gas. A three-way catalyst is supported on the surface of a porous material forming the catalyst 37. The catalyst 37 may further capture particulate matter (PM) emitted into the exhaust gas. Thus, the catalyst 37 may be a gasoline particulate filter (GPF) supporting a three-way catalyst. Controller

[0018] The vehicle 10 includes an engine control unit 38. The engine control unit 38 is an electronic control unit that controls the engine 11. The vehicle 10 further includes the controller 39, which centrally controls the engine control unit 38 and the PCU 27. The controller 39 corresponds to a controller for an internal combustion engine and controls the engine 11 by controlling the engine control unit 38. Furthermore, the controller 39 controls the first motor 12 and the second motor 13 by controlling the PCU 27 to regulate the discharge amount and the charge amount. That is, the controller 39 controls the engine 11, the first motor 12, and the second motor 13 to control the vehicle 10. The engine control unit 38 and the controller 39 are each formed by a computer unit. The computer unit includes a read-only memory (ROM), a central processing unit (CPU), and a random access memory (RAM).The ROM stores programs and data for control purposes. The CPU executes the programs stored in the ROM. The RAM serves as a working area when the CPU executes a program.

[0019] A detection signal from an air flow meter 40, which detects the intake air quantity of the engine 11, is input to the engine control unit 38. A detection signal from a crank angle sensor 41, which detects the rotation angle of the crankshaft 30, is input to the engine control unit 38. A detection signal from a coolant temperature sensor 42, which detects the coolant temperature of the engine 11, is input to the engine control unit 38. A detection signal from an exhaust gas sensor 43, which detects the temperature of the exhaust gas entering the catalyst 37, is input to the engine control unit 38. A detection signal from an upstream air-fuel ratio sensor 46 is input to the engine control unit 38.The upstream air-fuel ratio sensor 46 is located in the exhaust passage 33 on the upstream side of the catalyst 37 and detects the oxygen concentration of the gas flowing through the exhaust passage 33. That is, the upstream air-fuel ratio sensor 46 detects the air-fuel ratio. A detection signal from a downstream air-fuel ratio sensor 47 is input to the engine control unit 38. The downstream air-fuel ratio sensor 47 corresponds to an exhaust gas sensor configured to detect oxygen. The downstream air-fuel ratio sensor 47 is located in the exhaust passage 33 on the downstream side of the catalyst 37 and detects the oxygen concentration of the gas flowing through the exhaust passage 33.The downstream air-fuel ratio sensor 47 is thus the same type of air-fuel ratio sensor as the upstream air-fuel ratio sensor 46. The upstream air-fuel ratio sensor 46 and the downstream air-fuel ratio sensor 47 may each be a sensor that sharply changes its output once the stoichiometric air-fuel ratio is reached. Therefore, the upstream air-fuel ratio sensor 46 and the downstream air-fuel ratio sensor 47 may each be an oxygen sensor that produces a rich output when the air-fuel ratio is richer than the stoichiometric air-fuel ratio and a lean output when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio.The engine control unit 38 calculates the rotational speed of the crankshaft 30 (hereinafter referred to as the engine speed) from the detection signal of the crank angle sensor 41. Furthermore, the engine control unit 38 calculates an engine load ratio KL from the engine speed and the intake air amount. The engine load ratio KL will now be described. The amount of air drawn into each cylinder 31 during the intake stroke is referred to as a cylinder intake air amount. The cylinder intake air amount when the engine 11 is stably running in a state where the throttle valve 34 is fully open at the current engine speed is referred to as the full-open air amount. The engine load ratio KL corresponds to the ratio of the current cylinder intake air amount to the full-open air amount.The engine control unit 38 performs air-fuel ratio feedback control based on the detection signals of the upstream air-fuel ratio sensor 46 and the downstream air-fuel ratio sensor 47 to regulate the fuel injection amount so that the air-fuel ratio approaches a target air-fuel ratio. For example, in a rich-fuel process described later, an air-fuel ratio richer than the stoichiometric air-fuel ratio is set as the target air-fuel ratio. In such a case, the air-fuel ratio is controlled by the air-fuel ratio feedback control to approach a rich air-fuel ratio. As a result, exhaust gas having a rich air-fuel ratio is supplied to the catalyst 37.

[0020] The current IB, the voltage VB, and the temperature TB of the battery 28 are input to the controller 39. The controller 39 calculates the charging rate or state of charge (SOC) of the battery 28 from the current IB, the voltage VB, and the temperature TB. Furthermore, the controller 39 receives a detection signal from an accelerator pedal sensor 44, which detects the accelerator pedal opening degree ACCP, which corresponds to the amount by which the accelerator pedal is depressed by the driver.

[0021] A detection signal from a vehicle speed sensor 45, which detects the vehicle speed V corresponding to the traveling speed of the vehicle 10, is input to the controller 39. The controller 39 calculates a required vehicle driving force, which represents the value of the driving force required for the vehicle 10, from the accelerator pedal opening degree ACCP and the vehicle speed V. The controller 39 calculates the required engine output, which corresponds to the value of the required engine output, from the required vehicle driving force, the charging rate (SOC), and the like. The controller 39 calculates the required MG1 torque, which corresponds to the value of the power / regenerative torque required for the first motor 12, from the required vehicle driving force, the charging rate (SOC), and the like.The controller 39 calculates the required MG2 torque, which corresponds to the power / regenerative torque value required for the second motor 13, from the required vehicle driving force, the charging rate (SOC), and the like. Then, travel control is performed on the vehicle 10. Specifically, the engine control unit 38 performs output control of the engine 11 according to the required engine output. The PCU 27 performs torque control of the first motor 12 and the second motor 13 according to the required MG1 torque and the required MG2 torque. Estimation process for oxygen storage capacity

[0022] The controller 39 executes an oxygen storage capacity estimation process to estimate the oxygen storage capacity of the catalyst 37. The oxygen storage capacity can be estimated by accumulating the amount of air supplied to the catalyst 37 from the time the stored oxygen amount becomes zero until the stored oxygen amount reaches the oxygen storage capacity. Before executing the oxygen storage capacity estimation process, the controller 39 first executes a rich-air-fuel ratio enrichment process that supplies exhaust gas with a rich air-fuel ratio to the catalyst 37 until the downstream air-fuel ratio sensor 47 detects that the exhaust gas has a rich air-fuel ratio.When the downstream air-fuel ratio sensor 47 detects a rich air-fuel ratio while exhaust gas with a rich air-fuel ratio is supplied to the catalyst 37, the stored oxygen amount is assumed to be zero. In detail, it is assumed that the catalyst 37 becomes oxygen deficient, and the exhaust gas with a rich air-fuel ratio flows downstream of the catalyst 37 without being purified by the catalyst 37. Subsequently, after the downstream air-fuel ratio sensor 47 detects that the exhaust gas has a rich air-fuel ratio in the enrichment process, the controller 39 executes an air supply process that supplies air to the catalyst 37 until the downstream air-fuel ratio sensor 47 detects that the exhaust gas has a lean air-fuel ratio.When the downstream air-fuel ratio sensor 47 detects a lean air-fuel ratio while exhaust gas with a lean air-fuel ratio is supplied to the catalyst 37, it is assumed that the stored oxygen amount has reached the oxygen storage capacity. In detail, it is assumed that the oxygen in the exhaust gas with a lean air-fuel ratio flows downstream of the catalyst 37 without being stored in the catalyst 37 when the stored oxygen amount reaches the oxygen storage capacity. Therefore, the controller 39 estimates the oxygen storage capacity of the catalyst 37 in the oxygen storage capacity estimation process by accumulating the amount of air supplied to the catalyst 37 until the downstream air-fuel ratio sensor 47 detects that the exhaust gas has a lean air-fuel ratio in the air supply process.In this manner, the oxygen storage capacity is estimated by accumulating the amount of air supplied to the catalyst 37 from the time the downstream air-fuel ratio sensor 47 detects that the air-fuel ratio is rich to the time the downstream air-fuel ratio sensor 47 detects that the air-fuel ratio is lean. The oxygen storage capacity estimation process will be described later with reference to FIG. Fig. 3 and Fig. 4 described in detail. Deterioration of the catalyst

[0023] With reference to Fig. 2, a deterioration of the catalyst 37 will now be described, which occurs starting from a state in which the catalyst 37 is new.

[0024] When the vehicle 10 is running and an operation of the engine 11 is repeated, a thermal stress accumulates in the catalyst 37 and deteriorates the catalyst 37. Thus, deterioration of the catalyst 37 progresses with increasing driving distance and deteriorates the oxygen storage capacity of the catalyst 37, as shown in Fig. 2. In the diagram of Fig. 2, the vertical axis indicates the oxygen storage capacity of the catalyst 37, and the horizontal axis indicates the traveled distance or driving distance. If the catalyst 37 is defective, the catalyst 37 cannot store a sufficient amount of oxygen. Specifically, the catalyst 37 is deemed to be defective when the oxygen storage capacity of the catalyst 37 decreases and becomes lower than a first threshold value OSCTh1. The first threshold value OSCTh1 corresponds to a threshold value set in advance taking into account the allowable deterioration of the catalyst 37. Specifically, when the oxygen storage capacity is greater than or equal to the first threshold value OSCTh1, the catalyst is deemed to be functioning normally. If the oxygen storage capacity is less than the first threshold value OSCTh1, the catalyst is deemed to be defective.

[0025] As described above, the oxygen storage capacity of the catalyst 37 is estimated based on the accumulated value of the air quantity until the downstream air-fuel ratio sensor 47 indicates a lean air-fuel ratio. Therefore, if the estimated oxygen storage capacity is excessively larger than the specifications of the catalyst 37, the downstream air-fuel ratio sensor 47 is deemed to be defective. A second threshold value OSCTh2 is set to determine whether the estimated oxygen storage capacity is excessively larger than the specifications of the catalyst 37. The second threshold value OSCTh2 corresponds to a value that is, for example, 1.1 times larger than the oxygen storage capacity of a new catalyst 37.Adopting such a configuration enables a determination that the downstream air-fuel ratio sensor 47 is defective when the estimated oxygen storage capacity is greater than the second threshold OSCTh2. The downstream air-fuel ratio sensor 47 is defective when the downstream air-fuel ratio sensor 47 cannot output a value reflecting the actual air-fuel ratio, for example, due to extremely poor responsiveness of the downstream air-fuel ratio sensor 47. When the estimated oxygen storage capacity is less than or equal to the second threshold OSCTh2, the downstream air-fuel ratio sensor 47 is determined to be functioning normally. Exhaust system normality determination process

[0026] With reference to Fig. 3, an exhaust system normality determination process for determining whether the catalyst 37 or the downstream air-fuel ratio sensor 47 is functioning normally will now be described. The exhaust system normality determination process is executed once under the condition that the main switch of the vehicle 10 is turned on.

[0027] In S300, the controller 39 determines whether a precondition for executing subsequent processes is met. The precondition will be described later. If the precondition is not met (S300: No), the controller 39 repeats S300. If the precondition is met (S300: Yes), the controller 39 proceeds to S302. The precondition may include, for example, a condition where the temperature of the downstream air-fuel ratio sensor 47 is estimated to be greater than or equal to an enabled temperature. The detection of the downstream air-fuel ratio sensor 47 is accurate when the temperature of the downstream air-fuel ratio sensor 47 is greater than or equal to the enabled temperature. This ensures the accuracy of the exhaust system normality determination process that uses the detection value of the downstream air-fuel ratio sensor 47.

[0028] In S302, the controller 39 executes the enrichment process. Specifically, the controller 39 sets the target air-fuel ratio to an air-fuel ratio richer than the stoichiometric air-fuel ratio and supplies exhaust gas with a rich air-fuel ratio to the catalyst 37. Then, the controller 39 proceeds to S304.

[0029] In S304, the controller 39 determines whether the downstream air-fuel ratio sensor 47 detects a rich air-fuel ratio. If the downstream air-fuel ratio sensor 47 does not detect a rich air-fuel ratio (S304: No), the controller 39 returns to S300. If the downstream air-fuel ratio sensor 47 detects a rich air-fuel ratio (S304: Yes), the controller 39 proceeds to S306. If the downstream air-fuel ratio sensor 47 detects a rich air-fuel ratio, the stored oxygen amount of the catalyst 37 is assumed to be zero.

[0030] In S306, the controller 39 executes cylinder-specific fuel cutoff control (hereinafter referred to as the cylinder-specific F / C control). The cylinder-specific F / C control stops the supply of fuel to one or more of the cylinders 31 and performs combustion at the stoichiometric air-fuel ratio in the remaining one or more of the cylinders 31. For example, in S306, the controller 39 causes the engine control unit 38 to stop the supply of fuel to one of the cylinders 31 and perform combustion at the stoichiometric air-fuel ratio in the remaining three cylinders 31.By executing the cylinder-specific F / C control in such a manner, the energy generated by combustion at the stoichiometric air-fuel ratio drives the crankshaft 30 and supplies air from the cylinder 31 where the fuel supply is stopped to the catalyst 37. Therefore, S306 corresponds to an air supply process that supplies air to the catalyst 37. In this way, the controller 39 executes the air supply process that supplies air to the catalyst 37. This supplies air to the catalyst 37 and allows the output of the engine 11 to be used to drive the wheels 25 or charge the battery 28. Therefore, the cylinder-specific F / C control is executed when a load is operated (for example, when driving force or charging is required). Then, the controller 39 proceeds to S308.

[0031] In S308, the controller 39 executes a cumulative air quantity calculation process. The cumulative air quantity corresponds to a value obtained by accumulating the air quantity supplied to the catalyst 37 from the time the downstream air-fuel ratio sensor 47 detects a rich air-fuel ratio to the time the downstream air-fuel ratio sensor 47 detects a lean air-fuel ratio. Here, the cumulative air quantity corresponds to the cumulative air quantity until the downstream air-fuel ratio sensor 47 detects that the air-fuel ratio of the exhaust gas is lean in the air supply process. The cumulative air quantity calculation process is performed in the manner described below.

[0032] As in Fig. 4, at the start of the cumulative air amount calculation process, the controller 39 obtains the previous cumulative air amount in S400. As described above, the cumulative air amount corresponds to a taken cumulative value after the start of the exhaust system normality determination process and from the time when an affirmative determination is made for the first time in S304 and the cylinder-specific F / C control is started. Therefore, the initial value of the cumulative air amount at the start of the exhaust system normality determination process is zero. Then, at S402, the controller 39 obtains the cylinder intake air amount of the present F / C cylinder 31 based on the intake air amount. The F / C cylinder 31 corresponds to the cylinder 31 subject to fuel cutoff. The intake air amount is obtained from the detection value of the air flow meter 40.Then, in S404, the controller 39 adds the cylinder intake air quantity of the current F / C cylinder 31 to the previous cumulative air quantity and updates the cumulative air quantity. After the cumulative air quantity calculation process, the controller 39 proceeds to S310.

[0033] In S310, the controller 39 determines whether the downstream air-fuel ratio sensor 47 detects a lean air-fuel ratio. If the downstream air-fuel ratio sensor 47 detects a lean air-fuel ratio, it is assumed that the stored oxygen amount has reached the oxygen storage capacity. If a lean air-fuel ratio is detected by the downstream air-fuel ratio sensor 47 in S310 (S310: Yes), the controller 39 proceeds to S312.

[0034] In S312, the controller 39 determines whether the cumulative air amount is less than a first threshold IAATh1. If the cumulative air amount is less than the first threshold IAATh1 (S312: Yes), the controller 39 proceeds to S314 and determines that the catalyst 37 is defective. The first threshold IAATh1 corresponds to a value obtained by converting the first threshold OSCTh1 into an air amount. The cumulative air amount mentioned in S312 is converted into an oxygen amount to obtain the oxygen storage capacity. Comparing the cumulative air amount with the first threshold IAATh1 is equivalent to comparing the oxygen storage capacity with the first threshold OSCTh1. As described above with reference to Fig. As described in Figure 2, the catalyst is assumed to be defective if the oxygen storage capacity is less than the first threshold OSCTh1. If the cumulative air quantity is less than the first threshold IAATh1, the controller 39 determines that the catalyst 37 is defective.

[0035] If the cumulative air quantity is greater than or equal to the first threshold IAATh1 (S312: No), the controller 39 proceeds to S316 and determines that the catalyst 37 is operating normally. Subsequently, in S318, the controller 39 determines that the downstream air-fuel ratio sensor 47 is operating normally.

[0036] If no lean air-fuel ratio is detected by the downstream air-fuel ratio sensor 47 in S310 (S310: No), the controller 39 proceeds to S320. In S320, the controller 39 determines whether the accumulated air amount is greater than a second threshold IAATh2. If the accumulated air amount is greater than the second threshold IAATh2 (S320: Yes), the controller 39 proceeds to S322 and determines that the downstream air-fuel ratio sensor 47 is faulty. The second threshold IAATh2 corresponds to a value obtained by converting the second threshold OSCTh2 into an air amount. The accumulated air amount mentioned in S320 is converted into an oxygen amount to obtain the stored oxygen amount. A comparison of the cumulative air quantity with the second threshold IAATh2 is equivalent to a comparison of the stored oxygen quantity with the second threshold OSCTh2.As mentioned above with reference to . Fig. As described in Figure 2, the downstream air-fuel ratio sensor 47 is considered to be defective when the oxygen storage capacity is greater than the second threshold OSCTh2. If the accumulated air amount is greater than the second threshold IAATh2, the controller 39 determines that the downstream air-fuel ratio sensor 47 is defective. In this case, the accumulated air amount significantly deviates from a predetermined range corresponding to the oxygen storage capacity according to the specifications of the catalyst 37. If the accumulated air amount is less than or equal to the second threshold IAATh2 (S320: No), the controller 39 proceeds to S306 and continues processing.

[0037] The exhaust system normality determination process ends when the controller 39 performs S314, S318 or S322. Operation of the present embodiment

[0038] The exhaust system normality determination process first sets the stored oxygen amount of the catalyst 37 to zero through the enrichment process (S300 to S304). When the stored oxygen amount of the catalyst 37 is zero (S304: Yes), the air supply process is executed by performing cylinder-specific F / C control (S306), and the accumulated air amount is calculated (S308). When the downstream air-fuel ratio sensor 47 detects a lean air-fuel ratio (S310: Yes) and oxygen reaches the downstream side of the catalyst 37, the calculation of the accumulated air amount is stopped. The cumulative air quantity up to this point in time corresponds to the air quantity supplied to the catalyst 37 from the point in time at which the stored oxygen quantity is zero until the point in time at which the catalyst 37 reaches its capacity and can no longer store any more oxygen.Therefore, the cumulative air quantity at this time indicates the oxygen storage capacity of the catalyst 37. The process of calculating the cumulative air quantity until the controller 39 makes an affirmative determination in S310 corresponds to the estimation process for an oxygen storage capacity.

[0039] When the oxygen storage capacity estimation process is completed, the exhaust system normality determination process determines from the cumulative air amount serving as an index value of the oxygen storage capacity whether the catalyst 37 or the downstream air-fuel ratio sensor 47 is normal or defective (S314, S316, S318, S322). Advantages of the present embodiment

[0040] (1) The air supply process includes stopping the fuel supply to one or more of the cylinders 31 and performing combustion at the stoichiometric air-fuel ratio in the remaining one or more of the cylinders 31. The energy generated by the combustion at the stoichiometric air-fuel ratio drives the crankshaft 30 and supplies air from the one or more cylinders 31 to which the fuel supply is stopped to the catalyst 37. Therefore, unlike performing lean combustion, the oxygen storage capacity can be estimated without adversely affecting the exhaust gas characteristics. Furthermore, the air supply process performed in the present embodiment does not adversely affect fuel economy because there is no need to perform engine control that supplies air to the catalyst 37 and reduces fuel economy.

[0041] (2) When the catalyst 37 is defective, the amount of oxygen that the catalyst 37 can store is insufficient. Therefore, when the catalyst 37 is defective, the accumulated air amount is small when a lean air-fuel ratio is detected. Accordingly, in the above-described configuration, the first threshold value IAATh1 is set as a threshold value that is compared with the accumulated air amount to determine whether the catalyst 37 is defective. This configuration enables a determination that the catalyst 37 is defective when the accumulated air amount is less than the first threshold value IAATh1.

[0042] (3) When the catalyst 37 is functioning normally, the catalyst 37 can store a sufficient amount of oxygen. Therefore, when the catalyst 37 is functioning normally, the accumulated air amount is relatively large when a lean air-fuel ratio is detected. Accordingly, in the above-described configuration, the first threshold value IAATh1 is set as a threshold value that is compared with the accumulated air amount to determine whether the catalyst 37 is functioning normally. This configuration enables a determination that the catalyst 37 is functioning normally when the accumulated air amount is greater than or equal to the first threshold value IAATh1.

[0043] (4) The oxygen storage capacity is determined according to the specifications of the catalyst 37. This enables a determination that the downstream air-fuel ratio sensor 47 is defective when the cumulative air amount is outside a predetermined range corresponding to the oxygen storage capacity according to the specifications of the catalyst 37. Accordingly, in the above-described configuration, the second threshold value IAATh2 is set as a threshold value, which is compared with the cumulative air amount to determine whether the downstream air-fuel ratio sensor 47 is defective. This configuration enables a determination that the downstream air-fuel ratio sensor 47 is defective when the cumulative air amount is greater than the second threshold value IAATh2.

[0044] (5) To supply air to the catalyst 37, fuel cutoff control (hereinafter referred to as the all-cylinder F / C control) may be performed to stop combustion in all the cylinders 31. However, the all-cylinder F / C control is executed during a no-load state. That is, the all-cylinder F / C control is executed under the condition that no driving force and no charging are required. When the vehicle is driven, driving force and charging are often required. Therefore, when executing the all-cylinder F / C control to perform the air supply process, the all-cylinder F / C control is terminated before the stored oxygen amount of the catalyst 37 reaches the oxygen storage capacity. This will lengthen the time required to complete the estimation of the oxygen storage capacity.In contrast, the cylinder-specific F / C control is executed when driving force or charging is required. Therefore, the present embodiment provides more opportunities to calculate the cumulative air amount to estimate the oxygen storage capacity than in a case where air is supplied to the catalyst 37 only through the all-cylinder F / C control.

[0045] (6) Instead of lean combustion, air is supplied to the catalyst 37 via cylinder-specific F / C control when estimating the oxygen storage capacity. Cylinder-specific F / C control supplies air to the catalyst 37 more efficiently than lean combustion. Therefore, cylinder-specific F / C control allows the oxygen storage capacity to be estimated more quickly than in a configuration that estimates the oxygen storage capacity via lean combustion.

[0046] (7) When the cylinder-specific F / C control is executed, combustion is performed at the stoichiometric air-fuel ratio in the cylinders 31 other than the F / C cylinder 31. This prevents a situation in which the unburned fuel supplied to the catalyst 37 from the cylinders 31 other than the F / C cylinder 31 reacts with the oxygen in the catalyst 37. Thus, the oxygen storage capacity can be estimated more accurately.

[0047] Furthermore, there is no need for the cumulative air amount to include the air amount supplied to the catalyst 37 from cylinders other than the F / C cylinder 31. If lean combustion were to be performed in the cylinders 31 other than the F / C cylinder 31, the air amount supplied to the catalyst 37 from the cylinders 31 other than the F / C cylinder 31 would be calculated from the output of the upstream air-fuel ratio sensor 46. In the present embodiment, combustion is performed at the stoichiometric air-fuel ratio in the cylinders 31 other than the F / C cylinder 31. Therefore, the upstream air-fuel ratio sensor 46 is not necessary. This eliminates the possibility that the gain or response of the upstream air-fuel ratio sensor 46 may adversely affect the calculation of the cumulative air quantity. Modified examples

[0048] The present embodiment may be modified as described below. The present embodiment and the following modifications may be combined as long as no technical contradiction occurs.

[0049] In the above embodiment, the air supply process includes stopping the fuel supply to one or more of the cylinders 31 and performing combustion at the stoichiometric air-fuel ratio in the remaining one or more of the cylinders 31. The air supply process may instead include stopping the fuel supply to one or more of the cylinders 31 and performing combustion in the remaining one or more of the cylinders 31 at an air-fuel ratio less than the stoichiometric air-fuel ratio, so that the cylinders 31 supply exhaust gas to the catalyst 37 that is controlled to a lean air-fuel ratio as a whole. In such a case, the combustion-performing cylinders 31 do not perform lean combustion.Therefore, unlike performing lean combustion, the oxygen storage capacity can be estimated without adversely affecting the exhaust gas characteristics. To perform the exhaust system normality determination process even more accurately, the cumulative air amount in S312 or S320 can be obtained by subtracting the air amount that reacts with the unburned fuel supplied to the catalyst 37 from cylinders 31 that perform combustion at an air-fuel ratio lower than the stoichiometric air-fuel ratio.

[0050] The upstream air-fuel ratio sensor 46 may be omitted.

[0051] The above embodiment corresponds to an example in which the air supply process is performed only by the cylinder-specific F / C control. However, the air supply process may also be combined with all-cylinder F / C control. Specifically, air may be supplied via the cylinder-specific F / C control during a load operation, and air may be supplied via the all-cylinder F / C control during a no-load state. Such a configuration enables air supply regardless of whether a no-load operation is performed or a load operation is performed. Thus, air can be supplied to the catalyst 37 in a continuous and seamless manner. This enables rapid completion of the stored oxygen amount estimation process.

[0052] In the air supply process of the above embodiment, the engine control unit 38 stops the fuel supply to one of the cylinders 31 and performs combustion at the stoichiometric air-fuel ratio in the remaining three cylinders 31. In the air supply process, the engine control unit 38 may instead stop the fuel supply to two of the cylinders 31 and perform combustion at the stoichiometric air-fuel ratio in the remaining two cylinders 31. That is, the number of cylinders 31 to which the supply of fuel is stopped in the air supply process is not limited to one. The cylinders 31 subject to fuel supply cutoff may be changed. Further, the fuel supply cutoff may be performed to one or more specific ones of the cylinders 31. In a specific one of the cylinders 31, fuel supply cutoff may be performed at a frequency orFrequency of once for several combustion cycles.

[0053] When cylinder-specific F / C control is executed, a momentary torque loss occurs. A momentary torque loss may result in insufficient driving force and an increase in noise and vibration. When cylinder-specific F / C control is executed, a process may be executed to prevent such insufficient driving force and / or increased noise and vibration. For example, a process may be executed that increases the output value required for the engine 11 to compensate for a decrease in the output of the engine 11 so that the driving force does not become insufficient. A process may be executed that compensates for a decrease in the output of the engine 11 with the first motor 12 and / or the second motor 13 so that the driving force does not become insufficient.A process may be performed to cyclically compensate torque fluctuations of the machine 11 with a motor torque and reduce noise and vibration.

[0054] The precondition may include, for example, that the execution of a process is permitted to avoid insufficient driving force and / or increased noise and vibration caused by a momentary torque loss. The precondition may include, for example, that the battery 28 is in a predetermined state. This avoids a situation in which the above-described process using the battery 28 cannot be executed due to a low temperature or a low charging rate of the battery 28. The precondition may include, for example, that the first motor 12 and / or the second motor 13 are in a specific state.For example, if the first motor 12 and / or the second motor 13 includes a component (e.g., a coil or an inverter) that has a high temperature, execution of the above-described process using the first motor 12 and / or the second motor 13 may be avoided, and the torque of the first motor 12 and / or the second motor 13 is limited to protect the component. The precondition may include, for example, that the communication state is in a predetermined state (e.g., no communication interruption and no communication delay). This ensures the reliability of communication performed between ECUs for executing the above-described process.

[0055] If the cylinder-specific F / C control is interrupted during the exhaust system normality determination process, the exhaust system normality determination process is interrupted. To avoid interruption of the cylinder-specific F / C control, a process may be performed. For example, after a prohibition of intermittent stopping or a prohibition of all-cylinder F / C control in a hybrid electric vehicle, control may be executed to maintain or increase the required output for the engine, and the battery 28 may be charged or discharged to correct a lack or excess output of the engine 11.

[0056] When cylinder-specific F / C control is executed, air-fuel ratio feedback control may be suspended. Alternatively, the feedback gain may be reduced when cylinder-specific F / C control is executed. This avoids a situation in which cylinder-specific F / C control causes a lean spark (the air-fuel ratio temporarily becomes lean) and the target air-fuel ratio in a combustion cylinder 31 performing combustion is corrected to rich.

[0057] Cylinder-specific F / C control may cause a lean spark, resulting in inappropriate updating of an air-fuel ratio learning value. This can be prevented by stopping air-fuel ratio learning control during cylinder-specific F / C control.

[0058] The ignition of the F / C cylinder 31 may be suspended during cylinder-specific F / C control. This avoids unintended combustion in the F / C cylinder 31. Additionally, methods that may be taken to avoid unintended combustion in the F / C cylinder 31 include purge shutdown, direct fuel injection into the combustion cylinder 31, fuel injection synchronized with an intake valve opening in a configuration including only one port injector, EGR shutdown, and advancing intake valve timing to restrict reverse flow of an air-fuel mixture into the intake system, and the like.

[0059] In the above embodiment, the number of cylinders 31 is four. The number of cylinders 31 can be changed.

[0060] In the above embodiment, a process for determining whether the catalyst 37 is faulty or normal and a process for determining whether the downstream air-fuel ratio sensor 47 is faulty or normal are performed. These processes may be omitted. Specifically, S312, S314, S316, S318, S320, and S322 may be omitted.

[0061] In the above embodiment, the exhaust system normality determination process is executed once under the condition that the main switch of the vehicle 10 is turned on. Instead, the exhaust system normality determination process may be executed more than once, for example, when the deviation between the cumulative air quantity and the first threshold value IAATh1 is small or when the deviation between the cumulative air quantity and the second threshold value IAATh2 is small. The determination of whether the exhaust system is operating normally is even more accurate when it is based on the results of the exhaust system normality determination process performed multiple times.

[0062] If it is determined in the exhaust system normality determination process that the catalyst 37 or the downstream air-fuel ratio sensor 47 is defective, the abnormality may be determined by another method such as that described in the background section.

[0063] In the above embodiment, the exhaust system normality determination process is executed once under the condition that the main switch of the vehicle 10 is turned on. Instead, the exhaust system normality determination process may be executed, for example, under the condition that the cylinder-specific F / C control is executed to reduce emissions during stable driving, or the cylinder-specific F / C control is executed for the purpose of GPF regeneration.

[0064] The precondition of the exhaust system normality determination process described in the above embodiment may be changed. For example, the precondition may include a condition that the components or sensors used to calculate the cumulative air quantity (e.g., throttle valve 34 and air flow meter 40) are functioning normally. This ensures the accuracy of the exhaust system normality determination process. For example, the precondition may include a condition that the engine coolant temperature and oil temperature are greater than or equal to 75 degrees Celsius, indicating that warm-up of the engine 11 has been completed. The precondition may include a condition that the engine 11 is running and not in a stopped state.The precondition may, for example, include a condition that control that can change the air-fuel ratio from the stoichiometric air-fuel ratio is not executed. This is, for example, a condition that special fuel increase control is not executed. The special fuel increase control can be performed, for example, to protect components. The component-protecting fuel increase control prevents deterioration of components that come into contact with the exhaust gas, the temperature of which is lowered by a fuel increase. The special fuel increase control is performed, for example, during a power increase, when the engine is cold, immediately after the engine is started, or after the fuel cut is ended.When no special fuel increase control is performed, combustion occurs at the stoichiometric air-fuel ratio in the cylinders 31 other than the F / C cylinder 31. In this way, the cumulative air amount can be accurately calculated based on the amount of air supplied from the F / C cylinder 31 to the catalyst 37. The precondition may include, for example, a condition that the temperature of the catalyst 37 is within a predetermined range (for example, 500 degrees Celsius to 800 degrees Celsius). The temperature of the catalyst 37 may affect the oxygen storage capacity. The lower limit of the predetermined range may correspond to the catalyst activation temperature, and the upper limit of the predetermined range may correspond to a component protection temperature. The predetermined condition may include, for example, a condition that the engine speed is low and the load fluctuations are small.That is, the precondition may include a condition that the engine 11 is not in a transient operating state. This avoids a situation in which a transient operating state of the engine 11 reduces the calculation accuracy of the cumulative air quantity and destabilizes the control of the air-fuel ratio. For example, the condition may be set for an engine with a port fuel injector to avoid a situation in which a port moisture quantity destabilizes the air-fuel ratio during a transient operating state of the engine. For example, the precondition may include a condition related to the ambient pressure, the intake air temperature, and the ambient temperature, which affect the calculation of the cumulative air quantity.The precondition may include, for example, a condition that the intake air amount is within a predetermined range (e.g., 5 to 30 g / s). The lower limit of the intake air amount is set to avoid a situation in which the exhaust system normality determination process based on the cumulative air amount takes time when the intake air amount is too small. The upper limit of the intake air amount is set to ensure the reliability of the exhaust system normality determination process based on the cumulative air amount. If the intake air amount were too large, the output of the downstream air-fuel ratio sensor 47 would be lean when S306 and S308 are first performed, and the output of the cumulative air amount might exceed the first threshold IAATh1.In such a case, the catalyst 37 is not determined to be defective even if the oxygen storage capacity becomes smaller than the first threshold OSCTh1. The precondition may include, for example, a condition that control that can supply fuel to the F / C cylinder 31 is not executed. This avoids a situation in which the air supplied from the F / C cylinder 31 to the catalyst 37 reacts with fuel and hinders the calculation of the cumulative air amount. For the same reason, the precondition may include, for example, a condition that the purge concentration (concentration of fuel vapor flowing from the fuel tank into the intake passage 32) is low (for example, zero) and / or the exhaust gas recirculation (EGR) amount is low (for example, zero).The precondition may include, for example, a condition that learning of the air-fuel ratio control is completed in the operating region of the engine 11 and near the operating region at the time the exhaust system normality determination process is executed. This ensures the accuracy for controlling the air-fuel ratio to the stoichiometric air-fuel ratio.

[0065] In the above embodiment, the controller 39 compares the cumulative air amount with the first threshold IAATh1 or the second threshold IAATh2. However, this is only an example. The controller 39 may convert the cumulative air amount into an oxygen amount and compare the converted oxygen amount with the first threshold OSCTh1 or the second threshold OSCTh2.

[0066] In the above embodiment, the intake air quantity of the F / C cylinder 31 is obtained from the detection value of the air flow meter 40. The intake air quantity may instead be calculated from a physical model of the intake system. For example, the intake air quantity may be calculated from specifications, a throttle opening degree, and an actuation amount of the variable valve timing (VVT), EGR, or the like. The intake air quantity may instead be obtained from an intake manifold pressure sensor.

[0067] In the above embodiment, the controller 39 determines in S318 that the downstream air-fuel ratio sensor 47 is functioning normally when the cumulative air amount is greater than or equal to the first threshold IAATh1. The controller 39 may determine that the downstream air-fuel ratio sensor 47 is functioning normally if the cumulative air amount taken when a lean air-fuel ratio is detected is less than or equal to the second threshold IAATh2. The oxygen storage capacity is determined according to the specifications of the catalyst 37. Therefore, when the cumulative air amount is included in a predetermined range corresponding to the oxygen storage capacity according to the specifications of the catalyst 37, the downstream air-fuel ratio sensor 47 may be determined to be functioning normally.Accordingly, in the described configuration, the second threshold IAATh2 is set as a threshold value that is compared with the cumulative air quantity to determine whether the downstream air-fuel ratio sensor 47 is functioning normally. This configuration enables the determination that the downstream air-fuel ratio sensor 47 is functioning normally when the cumulative air quantity is less than or equal to the second threshold IAATh2.

[0068] When the exhaust system normality determination process ends, the control state may be returned to the original control state by cylinder-specific F / C control and the like. However, if cylinder-specific F / C control is required to increase the temperature for GPF regeneration, cylinder-specific F / C control may be continued.

[0069] When the exhaust system normality determination process ends, the amount of oxygen supplied to the catalyst 37 may be too high. Therefore, after the exhaust system normality determination process ends, the fuel injection amount may be increased, for example, by setting a target air-fuel ratio that is richer than normal.

[0070] During the exhaust system normality determination process, the temperature of cylinder 31 undergoing fuel cutoff is lower than that of the other cylinders 31 undergoing combustion, and thus is in a state of insufficient port wetting. Therefore, after the exhaust system normality determination process, a larger amount of fuel can be injected into the cylinder 31 undergoing fuel cutoff than into the other cylinders 31 undergoing combustion, so that the torque generated by the cylinders 31 is uniform.

[0071] In the above embodiment, the controller 39 includes a CPU, a ROM, and a RAM, and processes software. However, this is only an example. The controller 39 may include, for example, a dedicated hardware circuit such as an application-specific integrated circuit (ASIC) that processes at least part of the processes executed by software in the present embodiment. That is, the controller 39 may have any of the following configurations (a) to (c). (a) The controller 39 includes a processor that executes all the processes according to programs, and a program storage device such as a ROM that stores the programs. That is, the controller 39 includes a software execution device. (b) The controller 39 includes a processor that executes part of the processes according to programs, and a program storage device.Furthermore, the controller 39 includes a dedicated hardware circuit that executes the remaining processes. (c) The controller 39 includes a dedicated hardware circuit that executes all processes. There may be more than one software processing device and / or dedicated hardware circuit. That is, the above processes may be executed by a processing circuit that includes a set of one or more software processing devices and / or a set of one or more dedicated hardware circuits. The processing circuit may include more than one software processing device and / or exclusive hardware circuit. The program storage device or computer-readable medium includes any available medium accessible by a general-purpose or dedicated computer.

Claims

[1] A controller for an internal combustion engine, the internal combustion engine comprising cylinders, a catalyst configured to purify exhaust gas and configured to store oxygen, and an exhaust gas sensor disposed on a downstream side of the catalyst and configured to detect oxygen, the controller comprising: a processing circuit, wherein: the processing circuit is configured to perform a rich-fuel process that supplies exhaust gas with a rich air-fuel ratio to the catalyst until the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio; the processing circuit is configured such that, after the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio in the enrichment process, it executes an air supply process which supplies air to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean air-fuel ratio; the processing circuit is configured to perform an oxygen storage capacity estimation process which estimates an oxygen storage capacity of the catalyst by accumulating an amount of air supplied to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean fuel ratio in the air supply process; and the air supply process comprises stopping the fuel supply to one or more of the cylinders and performing combustion at an air-fuel ratio less than or equal to a stoichiometric air-fuel ratio in one or more remaining cylinders, such that the cylinders supply an exhaust gas to the catalyst which is controlled overall to a lean air-fuel ratio. [2] The controller of claim 1, wherein the processing circuit is configured to determine that the catalyst is defective when a cumulative air amount is less than a first threshold, the cumulative air amount corresponding to the cumulative air amount until the exhaust gas sensor detects that the exhaust gas has a lean air-fuel ratio in the air supply process. [3] The controller of claim 1, wherein the processing circuit is configured to determine that the catalyst is functioning normally when a cumulative air amount is greater than or equal to a first threshold, the cumulative air amount corresponding to the cumulative air amount until the exhaust gas sensor detects that the exhaust gas has a lean air-fuel ratio in the air supply process. [4] The controller of claim 2 or 3, wherein the processing circuit is configured to determine that the exhaust gas sensor is functioning normally when the cumulative air quantity is less than or equal to a second threshold value which is greater than the first threshold value. [5] The controller of claim 2 or 3, wherein the processing circuit is configured to determine that the exhaust gas sensor is defective when the cumulative air quantity is greater than a second threshold which is greater than the first threshold. [6] The controller of any one of claims 1 to 5, wherein the air supply process comprises stopping fuel supply to the one or more cylinders and performing combustion at the stoichiometric air-fuel ratio in the one or more remaining cylinders. [7] A method for controlling an internal combustion engine, the internal combustion engine comprising cylinders, a catalyst configured to purify exhaust gas and configured to store oxygen, and an exhaust gas sensor disposed on a downstream side of the catalyst and configured to detect oxygen, the method comprising: Performing an enrichment process that supplies exhaust gas with a rich air-fuel ratio to the catalyst until the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio; Carrying out an air supply process which supplies air to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean air-fuel ratio after the exhaust gas sensor detects that the exhaust gas has a rich air-fuel ratio in the enrichment process; and Executing an oxygen storage capacity estimation process that estimates an oxygen storage capacity of the catalyst by accumulating an amount of air supplied to the catalyst until the exhaust gas sensor detects that the exhaust gas has a lean fuel ratio in the air supply process; wherein the air supply process comprises stopping the fuel supply to one or more of the cylinders and performing combustion at an air-fuel ratio less than or equal to a stoichiometric air-fuel ratio in one or more remaining cylinders such that the cylinders supply exhaust gas to the catalyst which is controlled to have an overall lean air-fuel ratio.

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