Control system for an internal combustion engine and method for controlling an internal combustion engine

Abstract

Control (110) for a spark-ignition type internal combustion engine (10), wherein: the internal combustion engine (10) comprises a fuel injection valve (17) configured to inject fuel and a three-way catalyst (22) provided in an exhaust passage (21), and wherein the internal combustion engine (10) is configured to combust an air-fuel mixture containing fuel injected into a cylinder (11) from the fuel injection valve (17); the controller (110) is configured to selectively execute one of the following: a fuel cut-off process or a fuel introduction process when the combustion in the cylinder (11) is stopped in a situation in which a crankshaft of the internal combustion engine (10) is rotating; and the fuel cut-off process comprises stopping the fuel injection of the fuel injection valve (17), and the fuel introduction process comprises injecting fuel from the fuel injection valve (17) and discharging the fuel in an unburned state from the cylinder (11) into the exhaust port (21); and the controller (110) comprises an injection valve control unit (111), wherein the injection valve control unit (111) is designed to carry out an enrichment process that controls the fuel injection valve (17) when combustion is resumed in the cylinder (11) in which combustion has been stopped, so that an air-fuel ratio is set to be richer than a stoichiometric air-fuel ratio, and the injection valve control unit (111) is configured to execute the enrichment process when the combustion in the cylinder (11) in which the combustion has been stopped is resumed, so that in the case that the fuel introduction process is executed in a period in which the combustion in the cylinder (11) is stopped, a reduction amount of the oxygen storage amount of the three-way catalyst (22) is smaller than in the case that the fuel introduction process is not executed in the period in which the combustion in the cylinder (11) is stopped.

Description

Background1. Field of invention

[0001] The present disclosure relates to a controller for an internal combustion engine and a method for controlling an internal combustion engine. 2. Description of related technology

[0002] US patent application US 2014 / 0041362 A1 discloses an example of an internal combustion engine that uses gasoline as fuel. An exhaust gas purification device of the internal combustion engine includes a three-way catalyst provided in an exhaust passage and a particulate filter arranged downstream of the three-way catalyst in the exhaust passage.

[0003] In the internal combustion engine described in the above-mentioned document, combustion in a cylinder can be stopped when a demand torque of the internal combustion engine decreases, for example, due to the release of an accelerator pedal operation, and a light load is applied to the internal combustion engine. During such a combustion stop period, one of a fuel cut process that stops fuel injection from a fuel injection valve or a fuel introduction process is selectively executed. The fuel introduction process injects fuel from the fuel injection valve and discharges the fuel in an unburned state from the cylinder to the exhaust port.

[0004] According to the above publication, the fuel introduction process is executed when the particulate filter is regenerated. If regeneration is not performed, the fuel cutoff process is executed.

[0005] During the fuel injection process, the fuel injected from the fuel injector flows with air through the exhaust port. As the fuel is drawn into the three-way catalyst, the fuel combusts, and the temperature of the three-way catalyst rises. This allows high-temperature gas to flow into the particulate filter, increasing the temperature of the particulate filter. As a result, particulate matter trapped in the particulate filter is burned.

[0006] During the combustion stall period, air drawn into the cylinder from the intake port flows out of the exhaust port without being burned, and thus the oxygen storage amount of the three-way catalyst increases compared to when in-cylinder combustion is performed. In this regard, as described in Japanese Patent Laid-Open Publication JP 2014-066154 A, for example, when the combustion stall period ends and in-cylinder combustion resumes, an enrichment process is performed to control the air-fuel ratio to be richer than a stoichiometric air-fuel ratio, thereby reducing the oxygen storage amount of the three-way catalyst to a proper level.

[0007] When the fuel introduction process is performed during the combustion pause period, the oxygen storage amount of the three-way catalyst at the time when combustion resumes in the cylinder may be different from that when the fuel introduction process is not performed during the combustion pause period. Specifically, when the fuel introduction process is performed during the combustion pause period, the unburned fuel in the three-way catalyst is combusted, consuming the oxygen present in the three-way catalyst.As a result, when the fuel introduction process is executed during the combustion stop period, the oxygen storage amount of the three-way catalyst at the time when combustion in the cylinder is resumed may not be increased compared to when the fuel introduction process is not executed during the combustion stop period.

[0008] Thus, when the fuel introduction process is executed during the combustion stop period and the combustion in the cylinder is resumed, if the enrichment process is executed in the same manner as when the fuel introduction process is not executed during the combustion stop period, the oxygen storage amount of the three-way catalyst may be less than the proper amount. Brief description

[0009] This Summary is provided to introduce, in a simplified form, a selection of concepts that are further described below in the "Detailed Description." This Summary is not intended to precisely identify key features or essential features of the claimed subject matter, nor is it intended to be used as a tool for determining the scope of the claimed subject matter.

[0010] One aspect of the present disclosure provides a controller for a spark-ignition type internal combustion engine having the features of claim 1. The internal combustion engine includes a fuel injection valve configured to inject fuel and a three-way catalyst provided in an exhaust passage. The internal combustion engine is configured to combust an air-fuel mixture including fuel injected from a fuel injection valve into a cylinder. The controller is configured to selectively execute one of a fuel cutoff process and a fuel introduction process when combustion in the cylinder is stopped in a situation where a crankshaft of the internal combustion engine is rotating. The fuel cutoff process includes stopping fuel injection from the fuel injection valve.The fuel introduction process includes injecting fuel from the fuel injector and discharging the fuel in an unburned state from the cylinder into the exhaust port. The controller includes an injector control unit. The injector control unit is configured to execute an enrichment process that controls the fuel injector when combustion is resumed in the cylinder in which combustion was stopped, so that an air-fuel ratio is adjusted to be richer than a stoichiometric air-fuel ratio.The injection valve control unit is configured to execute the enrichment process when the combustion in the cylinder in which the combustion has been stopped is resumed, so that in the case that the fuel introduction process is executed in a period in which the combustion in the cylinder is stopped, a decrease amount of the oxygen storage amount of the three-way catalyst is smaller than in the case that the fuel introduction process is not executed in the period in which the combustion in the cylinder is stopped.

[0011] When the fuel introduction process is executed during the combustion stop period of the cylinder, the resumption oxygen storage amount tends to be smaller than when the fuel introduction process is not executed during the combustion stop period of the cylinder. In this regard, in the above-mentioned embodiment, the injection valve control unit may be configured to, when combustion is resumed in the cylinder in which combustion was stopped, execute the enrichment process such that, in the case where the fuel introduction process is executed during the combustion stop period of the cylinder, a reduction amount of the oxygen storage amount of the three-way catalyst is smaller than in the case where the fuel introduction process is not executed during the combustion stop period of the cylinder.Thus, even if the fuel introduction process is performed during the combustion pause period of the cylinder, the enrichment process is performed in accordance with the resumption oxygen storage amount at the time of combustion resumption in the cylinder. This limits the deviation of the oxygen storage amount of the three-way catalyst from the correct amount, which would otherwise be caused by the enrichment process.

[0012] The controller for an internal combustion engine may include a storage amount calculator configured to calculate an estimated value of the oxygen storage amount of the three-way catalyst.

[0013] During the execution of the fuel introduction process, fuel injected from the fuel injection valve is sucked into the three-way catalyst in an unburned state, and the fuel is burned in the three-way catalyst. At this time, oxygen present in the three-way catalyst is consumed. Thus, the oxygen storage amount of the three-way catalyst may not be increased compared to during the execution of the fuel cut-off process. Thus, the storage amount calculator may be configured to calculate the estimated value of the oxygen storage amount such that an increase rate of the oxygen storage amount of the three-way catalyst is lower during the execution of the fuel introduction process than during the execution of the fuel cut-off process.

[0014] When the controller for an internal combustion engine includes such a storage amount calculator, the enrichment process may include a first enrichment process and a second enrichment process such that, when the second enrichment process is executed, the amount of reduction in the oxygen storage amount of the three-way catalyst is smaller than when the first enrichment process is executed. The injection valve control unit may be configured to selectively execute one of the following: the first enrichment process or the second enrichment process.

[0015] Thus, during the fuel injection process, the oxygen storage capacity of the three-way catalyst increases more gradually than during the fuel shutoff process. Specifically, the oxygen storage capacity of the three-way catalyst increases even during the fuel injection process. Thus, even when the fuel injection process is performed during the combustion pause period of the cylinder, the oxygen storage capacity of the three-way catalyst can reach the maximum value of the oxygen storage capacity during the pause period.

[0016] The injector control unit may be configured to execute the second enrichment process when combustion in the cylinder resumes, and the estimated value of the oxygen storage amount at a time when combustion in the cylinder resumes is less than the maximum value of the oxygen storage amount of the three-way catalyst. The injector control unit may be configured to execute the first enrichment process when combustion in the cylinder resumes, and the estimated value of the oxygen storage amount at a time when combustion in the cylinder resumes is the maximum value of the oxygen storage amount of the three-way catalyst.

[0017] According to the above-described embodiment, even if the fuel introduction process is executed during the combustion pause period of the cylinder, the second enrichment process is not executed, and the first enrichment process is executed when the oxygen storage amount of the three-way catalyst is maximum at a time when combustion resumes in the cylinder. Thus, the execution of the enrichment process reduces the oxygen storage amount of the three-way catalyst to the proper amount.

[0018] Even if the fuel introduction process is not performed during the combustion pause period of the cylinder, the resumption oxygen storage amount may not be increased so much, for example, if the pause period is short. Even if the fuel introduction process is performed during the combustion pause period of the cylinder, the resumption oxygen storage amount may be increased, for example, if the fuel introduction process is performed for a long period. In this regard, according to the embodiment described above, the reduction amount of the oxygen storage amount of the three-way catalyst according to the enrichment process is adjusted according to the resumption oxygen storage amount, regardless of whether the fuel introduction process is performed during the combustion pause period of the cylinder.This limits the deviation of the oxygen storage amount of the three-way catalyst from the correct amount, which would otherwise be caused by carrying out the enrichment process.

[0019] During the combustion shutdown period of the cylinder, as the amount of air flowing through the exhaust passage increases, the oxygen storage amount of the three-way catalyst tends to increase. Thus, the storage amount calculator may be configured to calculate the estimated value of the oxygen storage amount during the execution of the fuel cut-off process, so that the rate of increase of the oxygen storage amount of the three-way catalyst increases as the gas flow rate in the exhaust passage increases.

[0020] During the execution of the fuel introduction process, the fuel drawn into the three-way catalyst is burned in the three-way catalyst. As the amount of fuel increases, a larger amount of oxygen tends to be consumed at that time. Thus, the storage amount calculator may be configured to calculate the estimated value of the oxygen storage amount during the execution of the fuel introduction process such that the rate of increase of the oxygen storage amount of the three-way catalyst becomes higher as the flow rate of gas in the exhaust passage increases, and such that the rate of increase becomes lower as the fuel injection amount of the fuel injection valve increases.

[0021] The rate of reduction of the oxygen storage amount of the three-way catalyst corresponding to the enrichment process depends on the length of the enrichment process execution period and the air-fuel ratio during the enrichment process execution. Specifically, as the enrichment process execution period shortens, a period in which the air-fuel ratio is richer than the stoichiometric air-fuel ratio shortens. This reduces the rate of reduction of the oxygen storage amount of the three-way catalyst corresponding to the enrichment process. Furthermore, when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, as the air-fuel ratio approaches the stoichiometric air-fuel ratio, the rate of reduction of the oxygen storage amount of the three-way catalyst during the enrichment process execution decreases.More specifically, when the air-fuel ratio is approached to the stoichiometric air-fuel ratio, the reduction amount of the oxygen storage amount of the three-way catalyst corresponding to the enrichment process decreases.

[0022] The injection valve control unit may be configured to adjust the amount of reduction in the oxygen storage capacity of the three-way catalyst during the enrichment process, for example, by adjusting the length of the enrichment process execution period. When the enrichment process is executed in this manner, the length of the enrichment process execution period is adjusted. Executing the enrichment process reduces the oxygen storage capacity of the three-way catalyst to a proper amount.

[0023] The injection valve control unit may be configured to adjust the amount of reduction in the oxygen storage amount of the three-way catalyst during the enrichment process, for example, by adjusting the air-fuel ratio during the execution of the enrichment process. When the enrichment process is executed in this manner, the air-fuel ratio is adjusted during the execution of the enrichment process. Thus, the execution of the enrichment process reduces the oxygen storage amount of the three-way catalyst to a proper amount.

[0024] Another aspect of the present disclosure provides a method for controlling a spark-ignition type internal combustion engine having the features of claim 6. The internal combustion engine includes a fuel injection valve configured to inject fuel and a three-way catalyst provided in an exhaust port. The internal combustion engine is configured to combust an air-fuel mixture containing fuel injected into a cylinder from a fuel injection valve. The method includes selectively executing one of a fuel cut-off process and a fuel introduction process when combustion in the cylinder is stopped in a situation where a crankshaft of the internal combustion engine is rotating, wherein the fuel cut-off process includes stopping fuel injection from the fuel injection valve, and wherein the fuel introduction process includesInject fuel from the fuel injection valve and discharge the fuel in an unburned state from the cylinder into the exhaust port; execute an enrichment process that controls the fuel injection valve when combustion is resumed in the cylinder in which combustion has been stopped, so that an air-fuel ratio is set to be richer than a stoichiometric air-fuel ratio; and execute the enrichment process when combustion is resumed in the cylinder in which combustion has been stopped, so that in the case where the fuel introduction process is executed during a period in which combustion in the cylinder is stopped, a reduction amount of an oxygen storage amount of the three-way catalyst is smaller than in the case where the fuel introduction process is not executed during the period,in which combustion in the cylinder is stopped.,

[0025] Other features and aspects will become apparent from the following detailed description, drawings and claims. Short description of the drawings Fig. 1 is a schematic diagram according to a first embodiment showing a controller including an engine control unit and a hybrid vehicle in which the controller is mounted. Fig. 2 is a diagram showing a functional configuration of the engine control unit and a schematic configuration of an internal combustion engine mounted in the hybrid vehicle according to the first embodiment. Fig. 3 is a flowchart according to the first embodiment, showing a procedure for calculating an estimated value of an oxygen storage amount of a three-way catalyst during the combustion stop period. Fig. 4 is a flowchart according to the first embodiment, showing a procedure for calculating an estimated value of the oxygen storage amount of the three-way catalyst during execution of an enrichment process. Fig. 5 is a flowchart according to the first embodiment, showing a procedure for controlling a fuel injection valve during the combustion stop period. Fig. 6 is a flowchart according to the first embodiment, showing a procedure when the enrichment process is performed when the combustion of the air-fuel mixture in a cylinder is resumed. Fig. 7 is a timing chart according to the first embodiment when the combustion stop period ends and the combustion of the air-fuel mixture in the cylinder is resumed. Fig. 8 is a flowchart according to a second embodiment, showing a procedure when the enrichment process is executed when the combustion of the air-fuel mixture in the cylinder is resumed. Fig. 9 is a timing chart according to the second embodiment when the combustion stop period ends and the combustion of the air-fuel mixture in the cylinder is resumed. Fig. 10 is a flowchart according to a third embodiment, showing a procedure when the enrichment process is executed when the combustion of the air-fuel mixture in the cylinder is resumed. Fig. 11 is a flowchart according to a modified example showing a part of a procedure when the enrichment process is executed when the combustion of the air-fuel mixture in the cylinder is resumed.

[0026] The same reference numerals refer to the same elements throughout the drawings and the detailed description. The drawings may not be to scale, and the relative size, proportions, and depiction of elements in the drawings may be exaggerated for clarity, illustration, and convenience. Detailed description

[0027] This description provides a comprehensive overview of the methods, devices, and / or systems described. Modifications and equivalents of the described methods, devices, and / or systems will be apparent to those skilled in the art. The sequences of operations are exemplary and may be changed as would be apparent to one skilled in the art, except for operations that necessarily occur in a particular order. Descriptions of functions and systems commonly known to those skilled in the art may be omitted.

[0028] Example embodiments may take various forms and are not limited to the described examples. However, the described examples are thorough and complete, and will fully convey the scope of the disclosure to those of ordinary skill in the art.

[0029] When an element is described as being “on” or “connected to” another element, this includes cases where the element is directly “on” or “connected to” the other element and cases where the element is indirectly “on” or “connected to” the other element. First embodiment

[0030] Hereinafter, a controller for an internal combustion engine according to a first embodiment will be described with reference to FIG. Fig. 1 to 7 described.

[0031] The Fig. Figure 1 shows a schematic design of a hybrid vehicle. As shown in the Fig. 1, a hybrid vehicle includes a spark-ignition type internal combustion engine 10, a power distribution integration mechanism 40 connected to a crankshaft 14 of the internal combustion engine 10, and a first motor generator 71 connected to the power distribution integration mechanism 40. A second motor generator 72 is connected to the power distribution integration mechanism 40 via a reduction gear 50, and drive wheels 62 are connected to the power distribution integration mechanism 40 via a speed reduction mechanism 60 and a differential 61.

[0032] The power distribution and integration mechanism 40 is a planetary gear mechanism and includes a sun gear 41 of an external gear and a ring gear 42 of an internal gear arranged coaxially with the sun gear 41. The planetary gears 43 are arranged between the sun gear 41 and the ring gear 42 and mesh with the sun gear 41 and the ring gear 42. Each planetary gear 43 is supported by a carrier 44, allowing the planetary gear 43 to rotate and revolve. The first motor generator 71 is connected to the sun gear 41. The crankshaft 14 is connected to the carrier 44. A ring gear shaft 45 is connected to the ring gear 42, and the reduction gear 50 and the speed reduction mechanism 60 are connected to the ring gear shaft 45.

[0033] When an output torque of the engine 10 is applied to the carrier 44, the output torque is split into a component for the sun gear 41 and a component for the ring gear 42. Specifically, when the output torque of the engine 10 is applied to the first motor generator 71, the first motor generator 71 generates power.

[0034] When the first motor generator 71 is used as an electric motor, the output torque of the first motor generator 71 is applied to the sun gear 41. The output torque of the first motor generator 71 applied to the sun gear 41 is divided into a component for the carrier 44 and a component for the ring gear 42. When the output torque of the first motor generator 71 is applied to the crankshaft 14 via the carrier 44, the crankshaft 14 is rotated. In the first embodiment, such rotation of the crankshaft 14 caused by driving the first motor generator 71 is referred to as "motoring."

[0035] The reduction gear 50 is a planetary gear mechanism and includes a sun gear 51 of an external gear and a ring gear 52 of an internal gear arranged coaxially with the sun gear 51. The sun gear 51 is connected to the second motor generator 72. A ring gear shaft 45 is connected to the ring gear 52. The planetary gears 53 are arranged between the sun gear 51 and the ring gear 52 and mesh with the sun gear 51 and the ring gear 52. Although each planetary gear 53 can rotate freely, the planetary gear 53 cannot rotate.

[0036] When the vehicle decelerates, the second motor generator 72 is used as a power generator, so the vehicle generates regenerative braking force equal to the power generated by the second motor generator 72. When the second motor generator 72 is used as an electric motor, the output torque of the second motor generator 72 is transferred to the drive wheels 62 via the reduction gear 50, the ring gear axle 45, the speed reduction mechanism 60, and the differential 61. This allows the drive wheels 62 to rotate, causing the vehicle to move forward.

[0037] The first motor generator 71 exchanges power with a battery 77 via a first inverter 75. The second motor generator 72 exchanges power with the battery 77 via a second inverter 76.

[0038] As in the Fig. As shown in Figure 2, the internal combustion engine 10 includes a cylinder 11 that houses a piston 12 configured to reciprocate. The piston 12 is coupled to the crankshaft 14 by a piston rod 13. A crank angle sensor 86 detects an engine speed NE, which is the speed of the crankshaft 14.

[0039] A throttle valve 16 is provided in an intake passage 15 of the internal combustion engine 10 and rotates to adjust an intake air quantity GA of the cylinder 11. The intake air quantity GA is detected by an air flow meter 87. The air flow meter 87 is arranged in the intake passage 15 on the upstream side of the throttle valve 16.

[0040] The internal combustion engine 10 further includes a fuel injection valve 17 disposed in the intake passage 15 downstream of the throttle valve 16 to inject fuel. When an intake valve 18 is open, fuel and air are drawn into the cylinder 11 via the intake passage 15. In the cylinder 11, the air-fuel mixture, which includes the air drawn through the intake passage 15 and the fuel injected by the fuel injection valve 17, is combusted by a spark discharge from an ignition device 19. The combustion of the air-fuel mixture generates exhaust gas in the cylinder 11, and the exhaust gas is discharged into an exhaust passage 21 when an exhaust valve 20 is open. The exhaust passage 21 comprises a three-way catalyst 22 and a particulate filter 23 arranged on the downstream side of the three-way catalyst 22.The particulate filter 23 is capable of capturing particulate matter contained in the exhaust gas flowing through the exhaust passage 21.

[0041] An air-fuel ratio sensor 81 is arranged upstream of the three-way catalyst 22 in the exhaust passage 21 to detect the oxygen concentration in the gas flowing through the exhaust passage 21, that is, the air-fuel ratio of the air-fuel mixture. Furthermore, a temperature sensor 82 is arranged between the three-way catalyst 22 and the particulate filter 23 in the exhaust passage 21 to detect the temperature of the gas flowing through the exhaust passage 21. The internal combustion engine 10 further includes a differential pressure sensor 88 that detects a differential pressure ΔPex between a portion of the exhaust passage 21 between the particulate filter 23 and the three-way catalyst 22 and a portion of the exhaust passage 21 located downstream of the particulate filter 23.

[0042] In the internal combustion engine 10, the combustion of the air-fuel mixture in the cylinder 11 can be stopped while the vehicle is moving and the crankshaft 14 is rotating. The period during which the combustion of the air-fuel mixture in the cylinder 11 is stopped when the crankshaft 14 is rotating is referred to as a "combustion stall period (CSP). During the combustion stall period (CSP), the piston 12 reciprocates in synchronization with the rotation of the crankshaft 14. Thus, the air drawn into the cylinder 11 via the intake port 15 flows out to the exhaust port 21 without being used for combustion.

[0043] During the combustion stop period CSP, one of the following: a fuel cutoff process or a fuel introduction process is selectively executed. In the fuel cutoff process, the fuel injection of the fuel injection valve 17 is stopped. In the fuel introduction process, fuel is injected from the fuel injection valve 17, and the fuel flows in an unburned state from the cylinder 11 into the exhaust passage 21. When the fuel introduction process is executed, the fuel injected from the fuel injection valve 17 flows with air through the exhaust passage 21. The fuel is sucked into the three-way catalyst 22.When the fuel is sucked into the three-way catalyst 22, if the temperature of the three-way catalyst 22 is higher than or equal to an activation temperature and the amount of oxygen in the three-way catalyst 22 is sufficient to combust fuel, the fuel is combusted in the three-way catalyst 22. As a result, the temperature of the three-way catalyst 22 increases. When the heat generated by the three-way catalyst 22 is transferred to the particulate filter 23 via the gas flowing through the exhaust passage 21, the temperature of the particulate filter 23 increases. Then, when oxygen is supplied to the particulate filter 23 and the temperature of the particulate filter 23 is higher than or equal to a combustion temperature, the particulate matter captured by the particulate filter 23 is combusted.

[0044] Next, a control training form of the hybrid vehicle will be described with reference to the Fig. 1 and Fig. 2 described.

[0045] As in the Fig. As shown in FIG. 1, a controller 100 for the hybrid vehicle calculates a request torque TQR, which is a torque to be output to the ring gear axle 45, based on an accelerator pedal opening degree ACC and a vehicle speed VS. The accelerator pedal opening degree ACC is a degree of an accelerator pedal AP depressed by the driver of the vehicle and is a value detected by an accelerator pedal opening degree sensor 84. The vehicle speed VS is a value corresponding to the traveling speed of the vehicle and is detected by a vehicle speed sensor 85. The controller 100 controls the engine 10 and the motor generators 71 and 72 based on the calculated request torque TQR.The controller 100 or an element of the controller 100 may be embodied as circuitry comprising: 1) one or more processors operating according to a computer program (software); 2) one or more dedicated hardware circuits, such as an application-specific integrated circuit (ASIC), that execute at least one or more different processes; or 3) a combination thereof. The processor includes a CPU as well as a memory, such as RAM or ROM. The memory stores program code or instructions configured to cause the CPU to execute the processes. The memory, i.e., a computer-readable medium, includes various usable media accessible by a general-purpose or dedicated computer.

[0046] The controller 100 includes an engine control unit 110 that controls the internal combustion engine 10, and a motor control unit 120 that controls each of the motor generators 71 and 72. The engine control unit 110 corresponds to an example of a "controller for the internal combustion engine" in the first embodiment. When the fuel introduction process is executed during the combustion pause period CSP, the engine control unit 120 controls the driving of the first motor generator 71 so that motoring is performed. More specifically, the rotational speed of the crankshaft 14 during the combustion pause period CSP is controlled by motoring.

[0047] The Fig. 2 shows a functional embodiment of the engine control unit 110. The engine control unit 110 comprises an injection valve control unit 111, an ignition control unit 112, a storage quantity calculator 113 and a catalyst temperature calculator 114 as functional units.

[0048] The injector control unit 111 controls the fuel injection valve 17. More specifically, when the air-fuel mixture in the cylinder 11 is combusted, the injector control unit 111 calculates a request value QPR of the fuel injection amount so that an air-fuel ratio detection value AFS is a target air-fuel ratio AFTr. The air-fuel ratio detection value AFS is an air-fuel ratio detected by the air-fuel ratio sensor 81. When the air-fuel mixture in the cylinder 11 is combusted, the target air-fuel ratio AFTr is set, for example, to a stoichiometric air-fuel ratio or a value close to the stoichiometric air-fuel ratio. The injector control unit 111 controls the actuation of the fuel injection valve 17 based on the calculated request value QPR.When the combustion pause period CSP ends and combustion of the air-fuel mixture in cylinder 11 resumes, the fuel injector control unit 111 executes the enrichment process, which controls the fuel injector 17 to adjust the air-fuel ratio to be richer than the stoichiometric air-fuel ratio. The enrichment process will be described later.

[0049] In addition, the injector control unit 111 can control the fuel injection valve 17 even during the combustion stall period CSP. Controlling the fuel injection valve 17 during the combustion stall period CSP will be described later.

[0050] The ignition control unit 112 controls the ignition device 19. Specifically, when the air-fuel mixture is combusted in the cylinder 11, the ignition control unit 112 causes the ignition device 19 to perform spark discharge when the piston 12 approaches the compression top dead center. During the combustion standstill period CSP, the ignition control unit 112 does not cause the ignition device 19 to perform spark discharge.

[0051] The storage amount calculator 113 calculates an estimated value Ce of the oxygen storage amount in the three-way catalyst 22. The process of calculating the estimated value Ce of the oxygen storage amount will be described later.

[0052] The catalyst temperature calculator 114 calculates a catalyst temperature TPSC, which is an estimated value of the temperature of the three-way catalyst 22. It can be assumed that as the temperature of the gas passing through the three-way catalyst 22 to the particulate filter 23 increases, the temperature of the three-way catalyst 22 increases. Therefore, the catalyst temperature calculator 114 calculates the catalyst temperature TPSC, for example, such that the temperature of the three-way catalyst 22 increases as the gas temperature detected by the temperature sensor 82 increases.

[0053] Next, with reference to the Fig. 3 describes a process flow executed by the storage amount calculator 113 to calculate the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 during the combustion standstill period CSP. The sequence of processes executed in the Fig. 3 is repeated during the combustion standstill period CSP.

[0054] In the sequence of processes that take place in the Fig. As shown in Figure 3, in the first step, designated S12, the coefficient N is incremented by "1." Subsequently, in the next step, designated S13, the maximum value Cmax of the oxygen storage amount of the three-way catalyst 22 is derived.

[0055] The maximum value Cmax of the oxygen storage amount of the three-way catalyst 22 changes with the temperature of the three-way catalyst 22. Thus, the storage amount calculator 113 calculates the maximum value Cmax based on the catalyst temperature TPSC calculated by the catalyst temperature calculator 114.

[0056] When the calculation of the maximum value Cmax is completed, the process proceeds to the next step, designated S14. In step S14, the previous value Ce(N-1) of the estimated value Ce of the oxygen storage amount is retrieved. The previous value Ce(N-1) of the estimated value Ce of the oxygen storage amount is the estimated value Ce of the oxygen storage amount calculated when the coefficient is "N-1". Then, in the next step, designated S15, a unit increase amount ΔCe of the oxygen storage amount, which is an estimated value of an increase rate of the oxygen storage amount per unit time, is calculated.

[0057] When the crankshaft 14 rotates during the combustion stall period CSP, the air drawn from the intake passage 15 into the cylinder 11 is discharged into the exhaust passage 21 without undergoing any change. Then, the oxygen contained in the air flowing through the exhaust passage 21 is stored in the three-way catalyst 22. At this time, as the gas flow rate in the exhaust passage 21 increases, the amount of oxygen storage per unit time increases. During the combustion stall period CSP, the gas flow rate in the exhaust passage 21 depends on the engine speed NE and the intake air amount GA. More specifically, as the engine speed NE increases, the flow rate of gas in the exhaust passage 21 tends to increase. Additionally, as the opening degree of the throttle valve 16 increases and the intake air amount GA increases, the flow rate of gas in the exhaust passage 21 tends to increase.Therefore, in step S15, the unit increase amount ΔCe of the oxygen storage amount is calculated based on the engine speed NE and the intake air amount GA. Specifically, the calculated unit increase amount ΔCe increases as the engine speed increases. The calculated unit increase amount ΔCe also increases as the intake air amount GA increases.

[0058] When the calculation of the unit increase amount ΔCe is completed, the process proceeds to the next step, designated S16. In step S16, it is determined whether the fuel introduction process is being executed. If the fuel introduction process is being executed (S16: YES), the process proceeds to the next step, designated S17. In step S17, reduction correction is performed on the unit increase amount ΔCe calculated in step S15. When the fuel introduction process is being executed, the unburned fuel drawn into the three-way catalyst 22 is burned. Combustion of the unburned fuel consumes oxygen. Thus, during the execution of the fuel introduction process, the rate of increase in the oxygen storage amount of the three-way catalyst 22 is smaller than during the execution of the fuel cutoff process.Thus, reduction correction is performed on the unit increase amount ΔCe so that the unit increase amount ΔCe decreases as the amount of unburned fuel sucked into the three-way catalyst 22 increases. That is, the reduction correction is performed on the unit increase amount ΔCe so that the unit increase amount ΔCe decreases as the fuel injection amount of the fuel injection valve 17 increases.

[0059] Specifically, the reduction correction is performed on the unit increase amount ΔCe using the following comparison expression (Expression 1). In the comparison expression (Expression 1), "ERfc" is an equivalent ratio during the execution of the fuel injection process. The fuel injection amount of the fuel injection valve 17 required to control the air-fuel ratio to the stoichiometric air-fuel ratio is called the stoichiometric fuel injection amount. The equivalent ratio ERfc is a value obtained by dividing an actual fuel injection amount of the fuel injection valve 17 by the stoichiometric fuel injection amount. Thus, the equivalent ratio ERfc increases as the fuel injection amount increases during the execution of the fuel injection process.Therefore, the reduction correction amount of the unit increase amount ΔCe increases as the fuel injection amount increases during the execution of the fuel introduction process. ΔCe←ΔCe⋅(1−ERfc)

[0060] When the reduction correction of the unit increase amount ΔCe is completed, the process proceeds to the next step, designated S18. In step S18, an estimated value Ce(N) of the oxygen storage amount of the three-way catalyst 22 is calculated. More specifically, an estimated value Ce(N) of the oxygen storage amount is calculated by adding the unit increase amount ΔCe to the previous value Ce(N-1) of the estimated value of the oxygen storage amount. That is, the estimated value Ce of the oxygen storage amount is calculated by accumulating the unit increase amount ΔCe. Thus, the rate of increase of the estimated value Ce of the oxygen storage amount increases as the unit increase amount ΔCe increases. Therefore, in the first embodiment, the unit increase amount ΔCe corresponds to the rate of increase of the estimated value Ce of the oxygen storage amount during the combustion stop period CSP.Specifically, the estimated value Ce of the oxygen storage amount reflects the tendency of the increase rate of the oxygen storage amount of the three-way catalyst 22 to be lower during the execution of the fuel introduction process than during the execution of the fuel cut-off process. In other words, the increase rate of the estimated value Ce of the oxygen storage amount tends to be lower during the execution of the fuel introduction process than during the execution of the fuel cut-off process.

[0061] Subsequently, in the next step, designated S19, the smaller of the estimated value Ce(N) of the oxygen storage amount calculated in step S18 and the maximum value Cmax of the oxygen storage amount derived in step S13 is set to the estimated value Ce(N) of the oxygen storage amount. Once the calculated estimated value Ce(N) is stored in memory, the sequence of processes is temporarily terminated.

[0062] If the fuel introduction process is not executed (S16: NO), the fuel cutoff process is executed, and the process proceeds to the next step, designated S20. In step S20, an estimated value Ce(N) of the oxygen storage amount of the three-way catalyst 22 is calculated. More specifically, the estimated value Ce(N) of the oxygen storage amount is calculated by adding the unit increase amount ΔCe calculated in step S15 to the previous value Ce(N-1) of the estimated value of the oxygen storage amount. Then, in the next step, designated S21, the smaller of the estimated value Ce(N) of the oxygen storage amount calculated in step S20 and the maximum value Cmax of the oxygen storage amount derived in step S13 is set to the estimated value Ce(N) of the oxygen storage amount.When the calculated estimated value Ce(N) is stored in memory, the sequence of processes is then temporarily terminated.

[0063] In the first embodiment, during the execution of the fuel-cut process, the estimated value Ce of the oxygen storage amount is calculated by accumulating the unit increase amount ΔCe that has not undergone any reduction correction. That is, during the execution of the fuel-cut process, the estimated value Ce of the oxygen storage amount is calculated so that the rate of increase of the oxygen storage amount increases as the flow rate of the gas in the exhaust passage 21 increases. The unit increase amount ΔCe reflects the tendency of the rate of increase of the oxygen storage amount of the three-way catalyst 22 to increase as the flow rate of gas in the exhaust passage 21 increases. Thus, the estimated value Ce of the oxygen storage amount reflects the tendency of the rate of increase of the oxygen storage amount of the three-way catalyst 22 to increase as the flow rate of gas in the exhaust passage 21 increases.In other words, the increase rate of the estimated value Ce of the oxygen storage amount tends to increase as the flow rate of gas in the exhaust passage 21 increases. On the other hand, during the execution of the fuel introduction process, the estimated value Ce of the oxygen storage amount is calculated by accumulating the unit increase amount ΔCe that has undergone a reduction correction. More specifically, during the execution of the fuel introduction process, the estimated value Ce of the oxygen storage amount is calculated such that the increase rate of the oxygen storage amount increases as the flow rate of gas in the exhaust passage 21 increases. That is, the estimated value Ce of the oxygen storage amount reflects the tendency of the increase rate of the oxygen storage amount of the three-way catalyst 22 to increase as the flow rate of gas in the exhaust passage 21 increases.In other words, the increase rate of the estimated value Ce of the oxygen storage amount tends to increase as the flow rate of gas in the exhaust passage 21 increases. In addition, during the execution of the fuel introduction process, the estimated value Ce of the oxygen storage amount is calculated so that the increase rate of the oxygen storage amount decreases as the fuel injection amount of the fuel injection valve 17 increases. More specifically, the estimated value Ce of the oxygen storage amount reflects the tendency of the increase rate of the oxygen storage amount to decrease as the fuel injection amount of the fuel injection valve 17 increases. The increase rate of the estimated value Ce of the oxygen storage amount tends to decrease as the fuel injection amount of the fuel injection valve 17 increases.As described above, the estimated value Ce of the oxygen storage amount tends to be calculated such that the rate of increase of the oxygen storage amount of the three-way catalyst 22 during the execution of the fuel introduction process is lower than during the execution of the fuel cut-off process.

[0064] Next, with reference to the Fig. 4 describes a sequence of processes executed by the storage amount calculator 113 to calculate the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 during the execution of the enrichment process when the combustion of the air-fuel mixture in the cylinder 11 is resumed. The sequence of processes executed in the Fig. 4 is repeated during the execution of the enrichment process.

[0065] In the sequence of processes that take place in the Fig. As shown in Figure 4, in the first step, designated S112, the coefficient N is incremented by "1." Subsequently, in the next step, designated S113, a unit reduction amount ΔCed of the oxygen storage amount is calculated, which is an estimated value of the reduction amount of the oxygen storage amount per unit time. The unit reduction amount ΔCed is a value greater than or equal to zero.

[0066] Although details will be described later, the air-fuel ratio is richer than the stoichiometric air-fuel ratio during the enrichment process. Thus, the oxygen storage amount of the three-way catalyst 22 is reduced. At this time, the rate of reduction of the oxygen storage amount increases as the fuel injection amount of the fuel injection valve 17 increases. Thus, in the first embodiment, the unit reduction amount ΔCed is calculated so that the unit reduction amount ΔCed increases as the fuel injection amount request value QPR increases.

[0067] When the unit reduction amount ΔCed is calculated, the process proceeds to the next step, designated S114. In step S114, the previous value Ce(N-1) of the estimated value of the oxygen storage amount is retrieved. Then, in the next step, designated S115, an estimated value Ce(N) of the oxygen storage amount is calculated by subtracting the unit reduction amount ΔCed from the previous value Ce(N-1) of the estimated value of the oxygen storage amount. In step S116, the larger of the calculated estimated value Ce of the oxygen storage amount and the estimated value Ce is set to zero. When the thus calculated estimated value Ce is stored in the memory, the sequence of processes is temporarily terminated.

[0068] Next, with reference to the Fig. 5 describes a sequence of processes executed by the injection valve control unit 111 to control the actuation of the fuel injection valve 17 during the combustion standstill period CSP. The sequence of processes executed in the Fig. 5 is repeated during the combustion standstill period CSP.

[0069] In the in the Fig. 5, it is determined in the first step S31 whether the condition for executing the fuel introduction process is satisfied or not.

[0070] Here, the execution condition of the fuel injection process is described. In the first embodiment, it is determined that the execution condition is satisfied when the following two conditions are both satisfied. (Condition 1) It is determined that the temperature of the three-way catalyst 22 is higher than or equal to a specified temperature. (Condition 2) The estimated value of the amount of particulate matter captured in the particulate filter 23 is greater than or equal to a capture determination amount.

[0071] In the case of Condition 1: Even if unburned fuel is sucked into the three-way catalyst 22, the fuel may not be combusted if the temperature of the three-way catalyst 22 is low. In this regard, a specified temperature is set as a reference temperature to determine whether the unburned fuel sucked into the three-way catalyst 22 can be combusted. Specifically, the specified temperature is set to the activation temperature or a temperature slightly higher than the activation temperature of the three-way catalyst 22.

[0072] In the case of Condition 2: as the amount of particulate matter trapped in the filter 23 increases, clogging of the filter 23 progresses. In this regard, the detection determination amount is set to a reference amount to determine whether or not clogging has increased to a level requiring regeneration of the particulate filter 23. As the detected amount increases, the differential pressure ΔPex between the portion of the exhaust passage 21 between the three-way catalyst 22 and the particulate filter 23 and the portion of the exhaust passage 21 located downstream of the particulate filter 23 tends to increase. Thus, the estimated value of the detected amount can be calculated, for example, based on the differential pressure ΔPex.

[0073] When the execution condition for the fuel introduction process is satisfied and the fuel introduction process is started during the combustion stop period CSP, it is determined that the execution condition of the fuel introduction process is satisfied until the combustion stop period CSP ends.

[0074] If it is determined that the execution condition for the fuel introduction process is satisfied (step S31: YES), the fuel introduction process is performed, and the process proceeds to the next step, designated S32. In step S32, the request value QPR of the fuel injection amount of the fuel injection valve 17 is calculated. The request value QPR of the fuel injection amount determined when the fuel introduction process is performed is less than the request value QPR determined when the air-fuel mixture in the cylinder 11 is combusted.

[0075] Once the request value QPR is calculated in step S32, the process proceeds to the next step, designated S33. In step S33, the actuation of the fuel injection valve 17 is controlled based on the calculated request value QPR. In this case, fuel is injected from the fuel injection valve 17 even during the combustion standstill period CSP. Then, the sequence of processes is temporarily terminated.

[0076] If it is determined that the execution condition for the fuel injection process is not met (step S31: NO), the fuel cutoff process is performed, and the process proceeds to the next step, designated S34. In step S34, the request value QPR of the fuel injection amount is set to zero. Subsequently, in the next step, designated S35, the actuation of the fuel injection valve 17 is controlled based on the calculated request value QPR. In this case, no fuel is injected from the fuel injection valve 17. Then, the sequence of processes is temporarily terminated.

[0077] Next, with reference to the Fig. 6 describes a sequence of processes performed by the injection valve control unit 111 to carry out the enrichment process when the air-fuel mixture is burned in the cylinder 11. A sequence of processes described in the Fig. 6 is executed when the combustion of the air-fuel mixture in the cylinder 11 is resumed.

[0078] In the in the Fig. 6, the length of the execution period of the enrichment process is set in the first step S41.

[0079] When the oxygen storage amount of the three-way catalyst 22 is maintained at a specified amount CTh or a value close to the specified amount CTh, the three-way catalyst 22 can operate. The specified amount CTh is a value greater than zero and less than the maximum value Cmax of the oxygen storage amount. During the combustion stop period CSP, no air is used for combustion in the cylinder 11 and is sucked into the three-way catalyst 22. Thus, at a time when the combustion stop period CSP ends, that is, at a time when combustion of the air-fuel mixture in the cylinder 11 resumes, the oxygen storage amount of the three-way catalyst can be much larger than the specified amount CTh. Thus, the enrichment process is performed when combustion of the air-fuel mixture in the cylinder 11 resumes.

[0080] When the enrichment process is performed as described above, the oxygen storage amount of the three-way catalyst 22 decreases. The amount of reduction in the oxygen storage amount corresponding to the enrichment process increases as the execution period of the enrichment process lengthens. In the first embodiment, in order to adjust the oxygen storage amount of the three-way catalyst 22 to the specified amount CTh by executing the enrichment process, the length of the execution period of the enrichment process is adjusted when the enrichment process is executed.Thus, in step S41, the length of the execution period is set such that when the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 at the time when combustion of the air-fuel mixture in the cylinder 11 is resumed is small, the execution period of the enrichment process is shorter than when the estimated value Ce is large. Specifically, in the first embodiment, the amount of reduction in the oxygen storage amount of the three-way catalyst 22 is adjusted according to the enrichment process by adjusting the length of the execution period of the enrichment process.

[0081] If the length of the enrichment process execution period is set, the process then proceeds to the next step, designated S42. In step S42, it is determined whether the enrichment process end condition is satisfied or not. Here, the enrichment process end condition is that, after the combustion of the air-fuel mixture in cylinder 11 has resumed, the duration of the enrichment process is greater than or equal to the enrichment process execution period set in step S41. The length of the enrichment process execution period set in step S41 is referred to as the end determination time. After the combustion of the air-fuel mixture in cylinder 11 has resumed, if the duration of the enrichment process is less than the end determination time, it is determined that the end condition is not satisfied.If the duration of the enrichment process is longer than or equal to the completion determination time, it is determined that the end condition is met.

[0082] If it is determined that the end condition is not satisfied (S42: NO), the process proceeds to the next step, designated S43. In step S43, the target air-fuel ratio AFTr is set to the first air-fuel ratio AFr. The first air-fuel ratio AFr is richer than the stoichiometric air-fuel ratio. Subsequently, in step S44, the request value QPR of the fuel injection amount is calculated so that the detection value AFS of the air-fuel ratio becomes the target air-fuel ratio AFTr (=AFr). In step S45, the actuation of the fuel injection valve 17 is controlled based on the calculated request value QPR. As a result, the fuel injection valve 17 injects a larger amount of fuel than when the target air-fuel ratio AFTr is the stoichiometric air-fuel ratio. This means that the enrichment process is being carried out.The process then proceeds to step S42 described above. More specifically, the enrichment process continues until it is determined that the condition for terminating the enrichment process is met.

[0083] If the duration of the enrichment process is longer than or equal to the completion determination time and if it is determined that the condition for ending the enrichment process is met (step S42: YES), the sequence of processes is terminated. That is, the enrichment process is terminated. After the target air-fuel ratio AFTr is set to the stoichiometric air-fuel ratio, the actuation of the fuel injection valve 17 is controlled.

[0084] Next, the operation and effects of the first embodiment will be described with reference to the Fig. 7 described.

[0085] As in the Fig. As shown in Fig. 7, before time t11, fuel is injected from the fuel injection valve 17 with the target air-fuel ratio AFTr set to the stoichiometric air-fuel ratio, and the air-fuel mixture containing the fuel is burned in the cylinder 11. In this case, the oxygen storage amount of the three-way catalyst 22 is gradually changed. At time t11, the combustion stop period CSP starts when the condition for stopping the combustion of the air-fuel mixture in the cylinder 11 is satisfied. Since the estimated value of the detected amount of particulate matter in the particulate filter 23 calculated at time t11 is less than the detection determination amount, the condition for executing the fuel introduction process is not satisfied. Thus, the fuel cut process is performed from time t11.

[0086] During the fuel cutoff process, the air drawn into the cylinder 11 from the intake port 15 is not used for combustion and is discharged into the exhaust port 21. Specifically, oxygen contained in the air is stored in the three-way catalyst 22. Thus, the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 increases. At this time, the rate of increase of the estimated value Ce increases as the flow rate of air in the exhaust port 21 increases.

[0087] If the estimated value of the detected amount of particulate matter in the particulate filter 23, calculated at time t12, is greater than or equal to the detection determination amount during the combustion stop period CSP, the condition for executing the fuel injection process is met. Specifically, the process changes from the fuel cutoff process to the fuel injection process at time t12.

[0088] During the execution of the fuel introduction process, the unburned fuel drawn into the three-way catalyst 22 is burned. At this time, the combustion of the fuel consumes oxygen present in the three-way catalyst 22. Thus, during the execution of the fuel introduction process, the oxygen storage amount of the three-way catalyst 22 is increased more gradually than during the execution of the fuel cut-off process. Therefore, the reduction correction to the unit increase amount ΔCe derived based on the engine speed NE and the intake air amount GA is not performed during the execution of the fuel cut-off process, whereas the reduction correction to the unit increase amount ΔCe derived based on the engine speed NE and the intake air amount GA is performed during the execution of the fuel introduction process.As a result, during the execution of the fuel introduction process, the increase rate of the estimated value Ce of the oxygen storage amount is smaller than during the execution of the fuel cut-off process.

[0089] At time t13, the condition for stopping the combustion of the air-fuel mixture in cylinder 11 is not met, and the combustion of the air-fuel mixture is resumed in cylinder 11. Then, the length of the execution period of the enrichment process is set based on the estimated value Ce of the oxygen storage amount determined at time t13. In the Fig. In the example shown in Figure 7, the period from time t13 to time t14 is the execution period of the enrichment process. Thus, the enrichment process is executed until time t14.

[0090] In the time chart showing changes in the estimated value Ce of the oxygen storage amount, the double-dashed line shows a comparative example in which the estimated value Ce is calculated without performing reduction correction on the unit increase amount ΔCe even during the execution of the fuel introduction process. In the comparative example, the difference in the increase rate of the oxygen storage amount of the three-way catalyst 22 between the execution of the fuel introduction process and the execution of the fuel cutoff process is not considered. Therefore, when the fuel introduction process is executed during the combustion stop period CSP, the estimated value Ce calculated as in the comparative example deviates from the actual oxygen storage amount of the three-way catalyst 22.

[0091] In addition, when the estimated value Ce is calculated as in the comparative example, the estimated value Ce at the time point when combustion of the air-fuel mixture in the cylinder 11 resumes is larger than in the first embodiment. As a result, when the length of the execution period of the enrichment process is set based on the estimated value Ce calculated as in the comparative example, the enrichment process is executed until time point t15. In this case, the actual oxygen storage amount is significantly smaller than the specified amount CTh at the end of the enrichment process. This may reduce the accuracy of estimating the oxygen storage amount within a period in which the air-fuel mixture in the cylinder 11 is burned after the end of the enrichment process.

[0092] In this regard, in the first embodiment, the estimated value Ce of the oxygen storage amount is calculated taking into account the difference in the increase rate of the oxygen storage amount of the three-way catalyst 22 during the execution of the fuel introduction process and the execution of the fuel cut-off process. This limits the deviation of the estimated value Ce of the oxygen storage amount from the actual oxygen storage amount, even when the fuel introduction process is executed during the combustion stop period CSP. That is, the estimated value Ce of the oxygen storage amount can be calculated with high accuracy. The length of the execution period of the enrichment process is set based on the estimated value Ce determined as described above. In the Fig. In the example shown in Figure 7, the execution of the enrichment process is terminated at time t14, which is before time t15. This prevents the actual oxygen storage amount from being significantly smaller than the specified amount CTh at the end of the enrichment process. After the enrichment process is terminated, the oxygen storage amount of the three-way catalyst 22 is maintained at the specified amount CTh or a value close to the specified amount CTh.

[0093] In the first embodiment, when the enrichment process is executed, the amount of reduction in the oxygen storage amount of the three-way catalyst 22 in the enrichment process is adjusted by adjusting the length of the enrichment process execution period. Thus, the enrichment process is executed in accordance with the oxygen storage amount of the three-way catalyst 22 when the combustion of the air-fuel mixture in the cylinder 11 is resumed. This limits the deviation of the oxygen storage amount of the three-way catalyst 22 from the specified amount CTh, which would otherwise be caused by executing the enrichment process.

[0094] If the execution period of the enrichment process is excessively long relative to the oxygen storage amount of the three-way catalyst 22 at the time of resuming combustion of the air-fuel mixture in the cylinder 11, the oxygen stored in the three-way catalyst 22 may be depleted during the execution of the enrichment process. If the enrichment process continues even after the oxygen is depleted, the emission performance may be adversely affected. In this regard, in the first embodiment, the length of the execution period of the enrichment process is optimized so that the depletion of the oxygen stored in the three-way catalyst 22 during the execution of the enrichment process is prevented. Accordingly, adverse effects on the emission performance caused by the execution of the enrichment process can be limited.Furthermore, optimizing the length of the enrichment process execution period limits adverse effects on the fuel efficiency of the internal combustion engine 10.

[0095] The double-dashed line shown in the time chart, which shows changes in the estimated value Ce of the oxygen storage amount, may also show changes in the estimated value Ce when the fuel introduction process is not executed during the combustion stop period CSP. When the fuel introduction process is not executed during the combustion stop period CSP, the oxygen storage amount of the three-way catalyst 22 at the time the combustion of the air-fuel mixture in the cylinder 11 resumes is larger than when the fuel introduction process is executed during the combustion stop period CSP. Thus, when the fuel introduction process is not executed during the combustion stop period CSP, the execution period of the enrichment process is longer than when the fuel introduction process is executed during the combustion stop period CSP.Specifically, in the first embodiment, when the fuel introduction process is executed during the combustion stop period CSP and the stopping of the combustion of the air-fuel mixture in the cylinder 11 is interrupted and the combustion of the air-fuel mixture in the cylinder 11 is resumed, the enrichment process is executed so that the decrease amount of the oxygen storage amount of the three-way catalyst 22 according to the enrichment process is smaller than when the fuel introduction process is not executed during the combustion stop period CSP.

[0096] However, when the combustion stop period CSP is long, the oxygen storage amount of the three-way catalyst 22 may reach the maximum value Cmax even if the fuel introduction process is executed during the combustion stop period CSP. The enrichment process execution period set when the fuel introduction process is executed during the combustion stop period CSP and the estimated value Ce of the oxygen storage amount is the maximum value Cmax at the time the combustion of the air-fuel mixture in the cylinder 11 is resumed has the same length as the enrichment process execution period set when the fuel introduction process is not executed during the combustion stop period CSP.Specifically, an enrichment process performed when the fuel introduction process is not performed during the combustion stop period CSP and the oxygen storage amount of the three-way catalyst 22 reaches the maximum value Cmax is referred to as a first enrichment process. An enrichment process performed when the fuel introduction process is performed during the combustion stop period CSP and the oxygen storage amount of the three-way catalyst 22 does not reach the maximum value Cmax is referred to as a second enrichment process. In this case, an enrichment process performed when the fuel introduction process is not performed during the combustion stop period CSP but the oxygen storage amount of the three-way catalyst 22 reaches the maximum value Cmax is not the second enrichment process, but the first enrichment process.Thus, when the enrichment process is carried out, the oxygen storage amount of the three-way catalyst 22 has decreased to near the specified amount CTh. Second embodiment

[0097] Next, a second embodiment of a controller for an internal combustion engine will be described with reference to the Fig. 8 and Fig. 9. The second embodiment differs from the first embodiment in the process of adjusting the reduction amount of the oxygen storage amount according to the enrichment process. Therefore, in the following description, parts different from the first embodiment will be mainly described, and the same reference numerals will be assigned to the same or corresponding components as in the first embodiment to avoid redundant description.

[0098] A sequence of processes executed by the injection valve control unit 111 to carry out the enrichment process when the air-fuel mixture in the cylinder 11 is burned will be described with reference to FIG. Fig. 8. A sequence of processes that occur in the Fig. 8 is executed when the combustion of the air-fuel mixture in cylinder 11 is resumed. In the second embodiment, the length of the execution period is set to a predetermined value, which is different from the first embodiment.

[0099] In the in the Fig. In the sequence of processes shown in Figure 8, in the first step, designated S141, the target air-fuel ratio AFTr used during execution of the enrichment process is set.

[0100] When the enrichment process is executed, the oxygen storage amount of the three-way catalyst 22 decreases. The amount of reduction in the oxygen storage amount corresponding to the enrichment process increases as the air-fuel ratio of the air-fuel mixture in the cylinder 11 becomes richer during the enrichment process. In the second embodiment, when the enrichment process is executed, the target air-fuel ratio AFTr used during the execution of the enrichment process is set so that when the enrichment process is executed, the oxygen storage amount of the three-way catalyst 22 becomes the specified amount CTh.In step S141, the target air-fuel ratio used during the execution of the enrichment process is set so that when the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 decreases at a time when combustion of the air-fuel mixture in the cylinder 11 resumes, the target air-fuel ratio becomes closer to the stoichiometric air-fuel ratio. Specifically, in the second embodiment, the amount of reduction in the oxygen storage amount of the three-way catalyst 22 is adjusted according to the enrichment process by adjusting the target air-fuel ratio AFTr used during the execution of the enrichment process.

[0101] After the target air-fuel ratio AFTr is set, the process proceeds to the next step, designated S142. In step S142, it is determined whether the end condition for the enrichment process is satisfied or not. The length of the execution period of the enrichment process, which has been set in advance, is referred to as the end determination time. If the duration of the enrichment process after the combustion of the air-fuel mixture in the cylinder 11 has resumed is less than the end determination time, it is determined that the end condition is not satisfied. If the duration of the enrichment process is longer than or equal to the end determination time, it is determined that the end condition is satisfied.

[0102] If it is determined that the end condition is not met (S142: NO), the process proceeds to the next step, designated S144. In step S144, the fuel injection amount request value QPR is calculated so that the air-fuel ratio detection value AFS becomes the target air-fuel ratio AFTr set in step S141. In step S145, the timing of the fuel injection valve 17 is controlled based on the calculated request value QPR. As a result, the fuel injection valve 17 injects a larger amount of fuel than when the target air-fuel ratio AFTr is the stoichiometric air-fuel ratio, and an enrichment process is executed. In this case, when the target air-fuel ratio set in step S141 becomes closer to the stoichiometric air-fuel ratio, the fuel injection amount of the fuel injection valve 17 decreases.Then, the process proceeds to step S142 described above. That is, the enrichment process continues until it is determined that the condition for terminating the enrichment process is met.

[0103] If it is determined that the condition for ending the enrichment process is met (step S142: YES), the sequence of processes is ended. That is, the enrichment process is ended. After the target air-fuel ratio AFTr is set to the stoichiometric air-fuel ratio, the actuation of the fuel injection valve 17 is controlled.

[0104] Next, the operation and effects of the second embodiment will be described with reference to FIG. Fig. 9 described.

[0105] As in the Fig. As shown in Fig. 9, before time t21, fuel is injected from the fuel injection valve 17 with the target air-fuel ratio AFTr set to the stoichiometric air-fuel ratio, and the air-fuel mixture containing the fuel is burned in the cylinder 11. At time t21, the combustion stop period CSP starts when the condition for stopping the combustion of the air-fuel mixture in the cylinder 11 is satisfied. Since the estimated value of the detected amount of particulate matter in the particulate filter 23 calculated at time t21 is less than the detection determination amount, the condition for executing the fuel introduction process is not satisfied. Thus, the fuel cut process is performed from time t21.If the estimated value of the detected amount of particulate matter in the particulate filter 23, calculated at time t22, is greater than or equal to the detection determination amount during the combustion stop period CSP, the condition for executing the fuel injection process is met. Specifically, the process changes from the fuel cutoff process to the fuel injection process at time t22.

[0106] At time t23, the condition for stopping the combustion of the air-fuel mixture in cylinder 11 is not met, and the combustion of the air-fuel mixture resumes in cylinder 11. The target air-fuel ratio AFTr used during the execution of the enrichment process is set based on the estimated value Ce of the oxygen storage amount obtained at time t23. The enrichment process is performed from time t23 to time t24. That is, the period from time t23 to time t24 corresponds to the preset execution period of the enrichment process.

[0107] In the time chart showing changes in the target air-fuel ratio AFTr, the double-dashed line shows a comparative example in which the target air-fuel ratio AFTr is maintained at a predetermined value without using the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 at the time when combustion of the air-fuel mixture in the cylinder 11 is resumed. More specifically, in the comparative example, the target air-fuel ratio is not changed in accordance with the estimated value Ce of the oxygen storage amount. In addition, the double-dashed line in the time chart showing changes in the estimated value Ce of the oxygen storage amount shows changes in the oxygen storage amount in the comparative example.In the time chart showing changes in the request value QPR of the fuel injection amount, the double-dashed line shows changes in the request value QPR in the case of the comparative example. In the comparative example, the target air-fuel ratio AFTr used during execution of the enrichment process is set without using the estimated value Ce obtained at time t23. The target air-fuel ratio AFTr in the comparative example is richer than the target air-fuel ratio AFTr in the second embodiment. Thus, when the request value QPR of the fuel injection amount is calculated based on the target air-fuel ratio AFTr of the comparative example, the fuel injection amount of the fuel injection valve 17 increases. As a result, execution of the enrichment process significantly decreases the oxygen storage amount of the three-way catalyst 22 from the specified amount CTh.

[0108] In this regard, in the second embodiment, the target air-fuel ratio AFTr used during the execution of the enrichment process is set based on the estimated value Ce obtained at time t23. Specifically, as the estimated value Ce decreases at time t23, the target air-fuel ratio AFTr is set to a value closer to the stoichiometric air-fuel ratio. As the estimated value Ce decreases at time t23, the rate of reduction of the oxygen storage amount of the three-way catalyst 22 decreases during the execution period of the enrichment process.Thus, when the enrichment process is carried out, the oxygen storage amount is set to a value close to the specified amount CTh regardless of the oxygen storage amount of the three-way catalyst 22 at a time when the combustion of the air-fuel mixture in the cylinder 11 is resumed. Third embodiment

[0109] Next, a third embodiment of a controller for an internal combustion engine will be described with reference to the Fig. 10. The third embodiment differs from the first and second embodiments in that the reduction amount of the oxygen storage amount corresponding to the enrichment process is adjusted without using the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 at a time when the combustion of the air-fuel mixture in the cylinder 11 is resumed. Therefore, in the following description, parts different from the first and second embodiments will be mainly described, and the same reference numerals will be assigned to the same or corresponding components as in the first and second embodiments to avoid redundant description.

[0110] In the third embodiment, the enrichment process includes the first enrichment process and the second enrichment process. The injection valve control unit 111 selectively executes one of the first enrichment process and the second enrichment process at a time when combustion of the air-fuel mixture in the cylinder 11 resumes. The second enrichment process is a process in which the amount of reduction in the oxygen storage amount of the three-way catalyst 22 becomes smaller than at the time of executing the first enrichment process.For example, the target air-fuel ratio AFTr used during the execution of the second enrichment process is the same as the target air-fuel ratio AFTr used during the execution of the first enrichment process, and the execution period of the second enrichment process is shorter than the execution period of the first enrichment process.

[0111] A sequence of processes executed by the injection valve control unit 111 to carry out the enrichment process at a time when the combustion of the air-fuel mixture in the cylinder 11 is resumed will be described with reference to FIG. Fig. 10. A sequence of processes that occur in the Fig. 10 is executed when the combustion of the air-fuel mixture in the cylinder 11 is resumed.

[0112] In the in the Fig. In the sequence of processes shown in FIG. 10, in the first step S51, it is determined whether or not the fuel introduction process is being executed during the combustion stop period CSP. It can be assumed that when the fuel introduction process is being executed during the combustion stop period CSP, the oxygen storage amount of the three-way catalyst 22 at the time when combustion of the air-fuel mixture in the cylinder 11 is resumed is smaller than when the fuel introduction process is not being executed during the combustion stop period CSP. If it is determined that the fuel introduction process is not being executed during the combustion stop period CSP (S51: NO), it can be assumed that the oxygen storage amount of the three-way catalyst 22 is large, and thus the process proceeds to the next step, designated S52.

[0113] In step S52, the first enrichment process is performed. Subsequently, in step S53, it is determined whether the condition for ending the first enrichment process is met. The length of the execution period of the first enrichment process is referred to as a first determination execution time. If the duration of the first enrichment process is less than the first determination execution time, it is determined that the end condition is not met. If the duration of the first enrichment process is greater than or equal to the first determination execution time, it is determined that the end condition is met.

[0114] If it is determined that the condition for ending the first enrichment process is not met (S53: NO), the process proceeds to step S52 described above. That is, the first enrichment process continues. If it is determined that the condition for ending the first enrichment process is met (S53: YES), the sequence of processes ends. That is, the first enrichment process ends. After the target air-fuel ratio AFTr is set to the stoichiometric air-fuel ratio, the control of the fuel injection valve 17 is controlled.

[0115] If it is determined that the fuel introduction process is being executed during the combustion stop period CSP (step S51: YES), it can be assumed that the oxygen storage amount of the three-way catalyst 22 has not increased much, and the process proceeds to the next step designated S54.

[0116] In step S54, the second enrichment process is performed. Subsequently, in step S55, it is determined whether the condition for ending the second enrichment process is met. The length of the execution period of the second enrichment process is referred to as a second determination execution time. If the duration of the second enrichment process is less than the second determination execution time, it is determined that the end condition is not met. If the duration of the second enrichment process is greater than or equal to the second determination execution time, it is determined that the end condition is met. The second determination execution time is less than the first determination execution time.

[0117] If it is determined that the condition for ending the second enrichment process is not met (S55: NO), the process proceeds to step S54 described above. That is, the second enrichment process continues. If it is determined that the condition for ending the second enrichment process is met (S55: YES), the sequence of processes ends. That is, the second enrichment process ends. After the target air-fuel ratio AFTr is set to the stoichiometric air-fuel ratio, the actuation of the fuel injection valve 17 is controlled.

[0118] In the third embodiment, when the enrichment process is performed, the reduction amount of the oxygen storage amount of the three-way catalyst 22 is adjusted according to the enrichment process depending on whether the fuel introduction process is performed during the combustion stop period CSP. Specifically, when the fuel introduction process is not performed during the combustion stop period CSP, it can be assumed that the oxygen storage amount of the three-way catalyst 22 at the time the combustion of the air-fuel mixture in the cylinder 11 is resumed is larger than when the fuel introduction process is performed during the combustion stop period CSP. Thus, the first enrichment process is performed.When the fuel introduction process is executed during the combustion pause period CSP, it can be assumed that the oxygen storage amount of the three-way catalyst 22 at the time when the combustion of the air-fuel mixture in the cylinder 11 is resumed is smaller than when the fuel introduction process is not executed during the combustion pause period CSP. Thus, in this case, the second enrichment process, which has a shorter execution period than the first enrichment process, is executed. As a result, when the combustion of the air-fuel mixture in the cylinder 11 is resumed, the enrichment process is executed in accordance with the oxygen storage amount of the three-way catalyst 22 at the time of combustion resumption.Thus, the execution of the enrichment process prevents the oxygen storage amount of the three-way catalyst 22 from significantly decreasing from the specified amount CTh. Modified examples

[0119] Each of the above embodiments can be modified and implemented as follows. Each of the above embodiments and the following modified examples can be implemented by combining them with each other within a scope in which they are technically non-contradictory.

[0120] In the first and second embodiments, when the estimated value Ce of the oxygen storage amount is small at the time when the combustion of the air-fuel mixture in the cylinder 11 is resumed, the enrichment process may be executed such that the execution period of the enrichment process is shorter than when the estimated value Ce is large, and the target air-fuel ratio AFTr is set to a value closer to the stoichiometric air-fuel ratio than when the estimated value Ce is large.

[0121] In the third embodiment, the length of the execution period of the second enrichment process may be set to be the same as the length of the execution period of the first enrichment process, and the target air-fuel ratio AFTr at the time of execution of the second enrichment process may be set to a value closer to the stoichiometric air-fuel ratio than the target air-fuel ratio AFTr at the time of execution of the first enrichment process.

[0122] In the third embodiment, the second enrichment process may be a process in which the execution period is shorter than the execution period of the first enrichment process, and the target air-fuel ratio AFTr is set to a value closer to the stoichiometric air-fuel ratio than that used during the execution of the first enrichment process.

[0123] As in the Fig. As shown in Figure 11, in the third embodiment, even when the fuel introduction process is executed during the combustion stop period CSP (S51: YES), if it is determined that the oxygen storage amount of the three-way catalyst 22 has reached the maximum value Cmax at the time the combustion of the air-fuel mixture in the cylinder 11 is resumed (S511: YES), the first enrichment process may be executed instead of the second enrichment process. The determination that the oxygen storage amount of the three-way catalyst 22 has reached the maximum value Cmax may be made, for example, based on the execution period of the fuel introduction process being sufficiently long. When the estimated value Ce of the oxygen storage amount determined by the Fig. 3 has reached the maximum value Cmax, it can be determined that the oxygen storage amount of the three-way catalyst 22 has reached the maximum value Cmax at the time when combustion is resumed.

[0124] During the execution of the enrichment process, the rate of reduction of the oxygen storage amount of the three-way catalyst 22 increases as the fuel injection amount of the fuel injection valve 17 increases. Specifically, the rate of reduction of the oxygen storage amount corresponding to the enrichment process can be estimated based on the accumulated value of the fuel injection amount during the execution of the enrichment process. Therefore, in step S41 of the first embodiment and step S141 of the second embodiment, an accumulated determination value, which is a determination value of the accumulated value of the fuel injection amount, can be set.For example, an accumulated determination value is set to a value that increases as the estimated value Ce of the oxygen storage amount of the three-way catalyst 22 increases at the time when combustion of the air-fuel mixture in the cylinder 11 resumes. Then, the execution of the enrichment process can be terminated when the accumulated value of the fuel injection amount during the execution of the enrichment process is greater than or equal to the accumulated determination value. Thus, by setting the accumulated determination value, the amount of reduction in the oxygen storage amount is adjusted according to the enrichment process.

[0125] A flow rate sensor may be provided in a portion of the exhaust passage 21 on the upstream side of the three-way catalyst 22 to detect the flow rate of gas, and the unit increase amount ΔCe may be calculated based on the gas flow rate of the exhaust passage detected by the flow rate sensor.

[0126] In the embodiments described above, the ignition device 19 may not perform spark discharge during the execution of the fuel introduction process. However, during the execution of the fuel introduction process, the ignition device 19 may perform spark discharge at a time when the air-fuel mixture is not combusted in the cylinder 11. For example, when the piston 12 is near the bottom dead center and the spark discharge is performed, the air-fuel mixture is not combusted in the cylinder 11 in which the spark discharge is performed. Thus, during the execution of the fuel introduction process, even if the spark discharge is performed, the fuel injected from the fuel injection valve 17 can be discharged from the cylinder 11 into the exhaust port 21 in an unburned state.

[0127] The control for an internal combustion engine can be applied to an internal combustion engine that includes a direct fuel injection valve, which is a direct fuel injection valve that injects fuel directly into the cylinder 11. In this case, during the execution of the fuel introduction process, fuel is injected from the direct fuel injection valve into the cylinder 11, and the fuel is discharged into the exhaust port 21 in an unburned state. Thus, the unburned fuel is drawn into the three-way catalyst 22.

[0128] The system of a hybrid vehicle may be designed to control the speed of the crankshaft 14 by driving a motor and may be different from that described in the Fig. 1 shown system.

[0129] The control for an internal combustion engine may be implemented in a device for controlling an internal combustion engine mounted in a vehicle that has no power source other than the internal combustion engine. Even in an internal combustion engine mounted in such a vehicle, the combustion of the air-fuel mixture in the cylinder may be stopped in a situation where the crankshaft 14 rotates with inertia. When the execution condition of the fuel injection process is met during the combustion stop period CSP, the fuel injection process is executed, and the temperature of the three-way catalyst 22 increases.

[0130] Various changes in form and details may be made to the above examples without departing from the spirit and scope of the claims and their equivalents. The examples are for the purpose of description and not limitation. Descriptions of features in each example should be construed as applicable to similar features or aspects in other examples. Suitable results may be achieved if sequences are performed in a different order and / or if components in a described system, architecture, device, or circuit are combined differently and / or replaced or supplemented with other components or their equivalents. The scope of the disclosure is not defined by the detailed description, but by the claims and their equivalents. All variations within the scope of the claims and their equivalents are included in this disclosure.

Claims

[1] Control (110) for a spark-ignition type internal combustion engine (10), wherein: the internal combustion engine (10) comprises a fuel injection valve (17) configured to inject fuel and a three-way catalyst (22) provided in an exhaust passage (21), and wherein the internal combustion engine (10) is configured to combust an air-fuel mixture containing fuel injected from the fuel injection valve (17) into a cylinder (11); the controller (110) is configured to selectively execute one of the following: a fuel cut-off process or a fuel introduction process when the combustion in the cylinder (11) is stopped in a situation in which a crankshaft of the internal combustion engine (10) is rotating; and the fuel cut-off process comprises stopping the fuel injection of the fuel injection valve (17), and the fuel introduction process comprises injecting fuel from the fuel injection valve (17) and discharging the fuel in an unburned state from the cylinder (11) into the exhaust port (21); and the controller (110) comprises an injection valve control unit (111), wherein the injection valve control unit (111) is designed to carry out an enrichment process that controls the fuel injection valve (17) when combustion is resumed in the cylinder (11) in which combustion has been stopped, so that an air-fuel ratio is set to be richer than a stoichiometric air-fuel ratio, and the injection valve control unit (111) is configured to execute the enrichment process when the combustion in the cylinder (11) in which the combustion has been stopped is resumed, so that in the case that the fuel introduction process is executed in a period in which the combustion in the cylinder (11) is stopped, a reduction amount of the oxygen storage amount of the three-way catalyst (22) is smaller than in the case that the fuel introduction process is not executed in the period in which the combustion in the cylinder (11) is stopped. [2] The controller (110) according to claim 1, further comprising a storage amount calculator (113) configured to calculate an estimated value of the oxygen storage amount of the three-way catalyst (22), wherein the storage amount calculator (113) is designed to calculate the estimated value of the oxygen storage amount such that during the execution of the fuel introduction process, an increase rate of the oxygen storage amount of the three-way catalyst (22) is lower than during the execution of the fuel cut-off process, wherein the enrichment process comprises a first enrichment process and a second enrichment process, wherein, when the second enrichment process is carried out, the amount of reduction in the oxygen storage amount of the three-way catalyst (22) is smaller than when the first enrichment process is carried out, the injection valve control unit (111) is designed to selectively carry out one of the following, the first enrichment process or the second enrichment process, wherein the injection valve control unit (111) executes the second enrichment process when combustion in the cylinder (11) is resumed and the estimated value of the oxygen storage amount at a time when combustion in the cylinder (11) is resumed is less than the maximum value of the oxygen storage amount of the three-way catalyst (22), and the injection valve control unit (111) executes the first enrichment process when combustion in the cylinder (11) is resumed and the estimated value of the oxygen storage amount at a time when combustion in the cylinder (11) is resumed is the maximum value of the oxygen storage amount of the three-way catalyst (22). [3] Controller (110) according to claim 2, wherein the storage quantity calculator (113) is designed to: to calculate the estimated value of the oxygen storage amount during the execution of the fuel cut-off process so that the rate of increase of the oxygen storage amount of the three-way catalyst (22) becomes higher as the flow rate of gas in the exhaust passage (21) increases, and to calculate the estimated value of the oxygen storage amount during execution of the fuel introduction process such that the rate of increase of the oxygen storage amount of the three-way catalyst (22) becomes higher as the flow rate of gas in the exhaust passage (21) increases, and such that the rate of increase becomes lower as the fuel injection amount of the fuel injection valve (17) increases. [4] The controller (110) according to any one of claims 1 to 3, wherein the injection valve control unit (111) is configured to adjust a reduction amount of the oxygen storage amount of the three-way catalyst (22) in the enrichment process by adjusting a length of an execution period of the enrichment process. [5] The controller (110) according to any one of claims 1 to 4, wherein the injection valve control unit (111) is configured to adjust a reduction amount of the oxygen storage amount of the three-way catalyst (22) in the enrichment process by adjusting the air-fuel ratio during the execution of the enrichment process. [6] A method for controlling a spark-ignition type internal combustion engine (10), the internal combustion engine (10) comprising a fuel injection valve (17) configured to inject fuel and a three-way catalyst (22) provided in an exhaust passage (21), the internal combustion engine (10) being configured to combust an air-fuel mixture containing fuel injected into a cylinder (11) from the fuel injection valve (17), the method comprising: selectively execute one of the following, a fuel cut-off process or a fuel introduction process when the combustion in the cylinder (11) is stopped in a situation where a crankshaft of the internal combustion engine (10) is rotating, wherein the fuel cut-off process comprises stopping the fuel injection of the fuel injection valve (17), and wherein the fuel introduction process comprises injecting fuel from the fuel injection valve (17) and discharging the fuel in an unburned state from the cylinder (11) into the exhaust port (21); to carry out an enrichment process that controls the fuel injection valve (17) when combustion is resumed in the cylinder (11) in which combustion has been stopped, so that an air-fuel ratio is set to be richer than a stoichiometric air-fuel ratio; and to carry out the enrichment process when the combustion in the cylinder (11) in which the combustion has been stopped is resumed, so that in the case that the fuel introduction process is carried out in a period in which the combustion in the cylinder (11) is stopped, a decrease amount of an oxygen storage amount of the three-way catalyst (22) is smaller than in the case that the fuel introduction process is not carried out in the period in which the combustion in the cylinder (11) is stopped.

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