Patent Publication Number: US-11639704-B2

Title: Controller and control method for internal combustion engine

Description:
This is a continuation of U.S. patent application Ser. No. 16/504,468 filed Jul. 8, 2019 (now U.S. Pat. No. 10,890,143), which claims priority from Japanese Application No. 2018-148059 filed in Japan on Aug. 7, 2018. The disclosure of each of the prior applications is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure relates to a controller and control method for an internal combustion engine. 
     2. Description of Related Art 
     US Patent Application Publication No. 2014/0041362 discloses an internal combustion engine fueled by gasoline. The internal combustion engine includes a three-way catalyst for purifying exhaust gas in the exhaust passage. A particulate filter for trapping particulate matter (PM) is arranged in a section of the exhaust passage at the downstream side of the three-way catalyst. 
     In some cases, the internal combustion engine disclosed in the document stops combustion in the cylinder when the load on the internal combustion engine is low in a case in which the required torque of the internal combustion engine is reduced, for example, due to cancellation of accelerator operation. In the combustion stop period, a fuel introduction process for regenerating the particulate filter is executed. That is, the fuel introduction process causes the fuel injection valve to inject fuel and causes the fuel to flow out unburned from inside the cylinder to the exhaust passage. When introduced into the three-way catalyst, the fuel is burned to increase the temperature of the three-way catalyst. Then, high temperature gas flows into the particulate filter, increasing the temperature of the particulate filter. This burns particulate matter trapped by the particulate filter. 
     SUMMARY 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     Examples of the present disclosure will now be described. 
     Example 1: A controller for an internal combustion engine is provided. The internal combustion engine includes a three-way catalyst that is arranged in an exhaust passage and purifies exhaust gas, a particulate filter that is arranged in a section of the exhaust passage at a downstream side of the three-way catalyst to trap particulate matter contained in the exhaust gas, an exhaust gas recirculation passage that extends from a section of the exhaust passage at an upstream side of the particulate filter and communicates with an intake passage to recirculate the exhaust gas to the intake passage, and an exhaust gas recirculation valve that opens and closes a flow path of the exhaust gas recirculation passage. The internal combustion engine is configured to burn, in a cylinder, air-fuel mixture containing fuel injected from an fuel injection valve by spark discharge of an ignition device. The controller is configured to execute a fuel introduction process when stopping combustion in the cylinder under a situation in which a crankshaft of the internal combustion engine is rotating. The fuel introduction process causes fuel to be injected from the fuel injection valve and causes the fuel to flow out unburned from inside the cylinder to the exhaust passage. The controller includes a recirculation valve controlling section that controls opening and closing of the exhaust gas recirculation valve. An opening degree of the exhaust gas recirculation valve at a point in time when a state in which an execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition of the fuel introduction process is satisfied is a preliminary opening degree. The recirculation valve controlling section is configured to cause an opening degree of the exhaust gas recirculation valve to be smaller than the preliminary opening degree during execution of the fuel introduction process. 
     The above-described configuration limits the opening degree of the exhaust gas recirculation valve during the execution of the fuel introduction process. This restricts returning of the fuel that has been introduced into the exhaust passage and the gas that has been heated in the three-catalyst to the intake passage via the exhaust gas recirculation passage. Therefore, the particulate filter is effectively heated without wasting the fuel that has been introduced into the exhaust passage and the gas that has been heated in the three-catalyst. 
     Technologies such as that disclosed in the above-mentioned document may employ an exhaust gas recirculation device to recirculate exhaust gas to the intake passage. An exhaust gas recirculation device includes an exhaust gas recirculation passage that connects a section of the exhaust passage at the upstream side of the particulate filter to the intake passage. In an internal combustion engine equipped with an exhaust gas recirculation device, execution of the above-mentioned fuel introduction process causes some the fuel that has flowed out to the exhaust passage and some of the heated gas to flow out to the intake passage. The particulate filter thus may not be effectively heated. The above-described configuration decreases the possibility of such a drawback. 
     Example 2: The recirculation valve controlling section is configured to set the opening degree of the exhaust gas recirculation valve to 0 during the execution of the fuel introduction process. 
     With the above-described configuration, the amount of fuel or gas returned to the intake passage via the exhaust gas recirculation passage is almost 0. This allows the particulate filter to be promptly heated to an intended target temperature that corresponds to the fuel injection amount from the fuel injection valve. 
     Example 3: The recirculation valve controlling section is configured to cause, after the execution condition of the fuel introduction process is satisfied, the opening degree of the exhaust gas recirculation valve to be smaller than the preliminary opening degree in a period from a point in time before fuel is injected from the fuel injection valve to a point in time during the execution of the fuel introduction process. 
     With the above-described configuration, the opening degree of the exhaust gas recirculation valve is already limited at the point in time when the fuel introduction process is started. This prevents fuel or high-temperature gas from being returned to the intake passage during the period from when the opening degree of the exhaust gas recirculation valve starts being controlled to a small opening degree to when the control of the opening degree of the exhaust gas recirculation valve is completed. 
     Example 4: A control method for an internal combustion engine is provided that performs the various processes described in Examples 1 to 3. 
     Example 5: A non-transitory computer readable memory medium is provided that stores a program that causes a processing device to perform the various processes described in Examples 1 to 3. 
     Other features and aspects will be apparent from the following detailed description, the drawings, and the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic diagram of a hybrid system including a controller for an internal combustion engine according to an embodiment of the present disclosure. 
         FIG.  2    is a schematic diagram of then internal combustion engine of  FIG.  1   . 
         FIG.  3    is a flowchart showing the procedure of an injection valve controlling process in the internal combustion engine of  FIG.  1   . 
         FIG.  4    is a flowchart showing the procedure of a recirculation valve controlling process in the internal combustion engine of  FIG.  1   . 
         FIG.  5    is a flowchart showing a modification to the recirculation valve controlling process. 
     
    
    
     Throughout the drawings and the detailed description, the same reference numerals refer to the same elements. 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 
     This description provides a comprehensive understanding of the methods, apparatuses, and/or systems described. Modifications and equivalents of the methods, apparatuses, and/or systems described are apparent to one of ordinary skill in the art. Sequences of operations are exemplary, and may be changed as apparent to one of ordinary skill in the art, with the exception of operations necessarily occurring in a certain order. Descriptions of functions and constructions that are well known to one of ordinary skill in the art may be omitted. 
     Exemplary embodiments may have different forms, and are not limited to the examples described. However, the examples described are thorough and complete, and convey the full scope of the disclosure to one of ordinary skill in the art. 
     A controller for an internal combustion engine according to an embodiment of the present disclosure will now be described with reference to  FIGS.  1  to  4   . 
     First, the schematic configuration of the hybrid system in a hybrid vehicle will be described. 
     As shown in  FIG.  1   , the hybrid vehicle includes an internal combustion engine  10 , a driving force distribution-integration mechanism  40  connected to a crankshaft  14  of the internal combustion engine  10 , and a first motor generator  71  connected to the driving force distribution-integration mechanism  40 . The driving force distribution-integration mechanism  40  is coupled to a second motor generator  72  via a reduction gear  50  and to driven wheels  62  via a speed reduction mechanism  60  and a differential  61 . 
     The driving force distribution-integration mechanism  40  is a planetary gear mechanism and includes a sun gear  41 , which is an external gear, and a ring gear  42 , which is an internal gear coaxially arranged with the sun gear  41 . Pinion gears  43  meshing with the sun gear  41  and the ring gear  42  are provided between the sun gear  41  and the ring gear  42 . The pinion gears  43  are supported by a carrier  44  to be allowed to rotate and orbit. The sun gear  41  is coupled to the first motor generator  71 . The carrier  44  is coupled to the crankshaft  14 . The ring gear  42  is connected to a ring gear shaft  45 . The ring gear shaft  45  is coupled to both of the reduction gear  50  and the speed reduction mechanism  60 . 
     When the output torque of the internal combustion engine  10  is input to the carrier  44 , the output torque is distributed to the sun gear  41  and the ring gear  42 . That is, the output torque of the internal combustion engine  10  is input to the first motor generator  71  to cause the first motor generator to generate power. 
     In contrast, when the first motor generator  71  is caused to perform as an electric motor, the output torque of the first motor generator  71  is input to the sun gear  41 . The output torque of the first motor generator  71  input to the sun gear  41  is distributed to the carrier  44  and the ring gear  42 . The output torque of the first motor generator  71  is input to the crankshaft  14  via the carrier  44  to rotate the crankshaft  14 . This process, in which the crankshaft  14  is rotated by operation of the first motor generator  71 , is referred to as “motoring” in the present embodiment. 
     The reduction gear  50  is a planetary gear mechanism and includes a sun gear  51  and a ring gear  52 . The sun gear  51  is an external gear coupled to the second motor generator  72 . The ring gear  52  is an internal gear coaxially arranged with the sun gear  51 . The ring gear  52  is connected to the ring gear shaft  45 . Pinion gears  53  meshing with the sun gear  51  and the ring gear  52  are provided between the sun gear  51  and the ring gear  52 . Each pinion gear  53  is rotational but is not allowed to orbit. 
     By causing the second motor generator  72  to perform as a generator when decelerating the vehicle, regenerative braking force is generated in the vehicle in accordance with the amount of power generated by the second motor generator  72 . Also, when the second motor generator  72  is caused to perform as an electric motor, the output torque of the second motor generator  72  is input to the driven wheels  62  via the reduction gear  50 , the ring gear shaft  45 , the speed reduction mechanism  60 , and the differential  61 . The driven wheels  62  are thus rotated to drive the vehicle. 
     The first motor generator  71  exchanges electric power with a battery  77  through a first inverter  75 . The second motor generator  72  exchanges electric power with the battery  77  through a second inverter  76 . 
     As shown in  FIG.  2   , a piston  12  is accommodated and reciprocates in a cylinder  11  of the internal combustion engine  10 . The piston  12  is coupled to a crankshaft  14  via a connecting rod  13 . An air flowmeter  80  for detecting an intake air amount is arranged in an intake passage  15  of the internal combustion engine  10 . A throttle valve  16  is arranged in a section of the intake passage  15  at the downstream side of the air flowmeter  80 . The throttle valve  16  opens or closes the flow path of the intake passage  15  to regulate the intake air amount to the cylinder  11 . The internal combustion engine  10  also includes a fuel injection valve  17 , which injects fuel into a section of the intake passage  15  at the downstream side of the throttle valve  16 . When an intake valve  18  is open, fuel and air are introduced into the cylinder  11  via the intake passage  15 . Then, in the cylinder  11 , mixture of the air introduced through the intake passage  15  and the fuel injected from the fuel injection valve  17  is burned by spark discharge of an ignition device  19 . Exhaust gas generated by burning the air-fuel mixture in the cylinder  11  is discharged to an exhaust passage  21  when an exhaust valve  20  is opened. The exhaust passage  21  is provided with a three-way catalyst  22 , which purifies exhaust gas, and a particulate filter  23 , which is arranged at the downstream side of the three-way catalyst  22 . The particulate filter  23  has a function of collecting particulate matter contained in the exhaust gas flowing through the exhaust passage  21 . An air-fuel ratio sensor  81  is arranged at the upstream side of the three-way catalyst  22  in the exhaust passage  21  to detect the oxygen concentration of the gas flowing through the exhaust passage  21 , that is, the air-fuel ratio of the air-fuel mixture. 
     An exhaust gas recirculation passage  25  extends from a section of the exhaust passage  21  between the three-way catalyst  22  and the particulate filter  23 . The exhaust gas recirculation passage  25  recirculates exhaust gas to the intake passage  15 . The exhaust gas recirculation passage  25  is connected to a section of the intake passage  15  at the upstream side of the throttle valve  16 . An exhaust gas recirculation (EGR) valve  26  is provided in the middle of the exhaust gas recirculation passage  25 . The exhaust gas recirculation valve  26  opens and closes the flow path of the exhaust gas recirculation passage  25 . The flow rate of the gas returned from the exhaust passage  21  to the intake passage  15  is regulated by adjusting the opening degree of the exhaust gas recirculation valve  26 . Although not illustrated, the exhaust gas recirculation valve  26  has an actuator that actuates the valve element. A valve opening degree sensor  27 , which detects the opening degree of the exhaust gas recirculation valve  26 , is arranged in the vicinity of the exhaust gas recirculation valve  26 . 
     In the internal combustion engine  10 , combustion of air-fuel mixture in the cylinder  11  may be stopped while the crankshaft  14  is rotating. The period during which combustion of air-fuel mixture in the cylinder  11  is stopped while the crankshaft  14  is rotating will be referred to as a combustion stop period. In the combustion stop period, the piston  12  reciprocates in synchronization with rotation of the crankshaft  14 . Thus, the air introduced into the cylinder  11  via the intake passage  15  flows out to the exhaust passage  21  without being used for combustion. 
     In the combustion stop period, either a fuel cutoff process or a fuel introduction process is selected and executed. The fuel cutoff process stops fuel injection of the fuel injection valve  17 . The fuel introduction process causes the fuel injection valve  17  to inject fuel and causes the fuel to flow out unburned from inside the cylinder  11  to the exhaust passage  21 . When the fuel introduction process is executed, the fuel injected from the fuel injection valve  17  flows through the exhaust passage  21  together with air. The fuel is introduced into the three-way catalyst  22 . At this time, if the temperature of the three-way catalyst  22  is higher than or equal to the activation temperature and a sufficient amount of oxygen is present in the three-way catalyst  22  to burn the fuel, the fuel is burned in the three-way catalyst  22 . This increases the temperature of the three-way catalyst  22 . Then, high temperature gas flows into the particulate filter  23 , increasing the temperature of the particulate filter  23 . If the temperature of the particulate filter  23  becomes higher than or equal to the temperature at which the particulate matter can be burned when oxygen is supplied to the particulate filter  23 , the particulate matter trapped in the particulate filter  23  is burned. 
     Next, the control configuration of the hybrid vehicle will be described with reference to  FIGS.  1  and  2   . 
     As shown in  FIG.  1   , the vehicle controller  100  of the hybrid vehicle calculates a required torque TQR, which is the torque to be output to the ring gear shaft  45 , based on an accelerator operation amount ACC and a vehicle speed VS. The accelerator operation amount ACC is the operation amount of an accelerator pedal AP by the driver of the vehicle and is detected by an accelerator operation amount 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 vehicle controller  100  controls the internal combustion engine  10  and the motor generators  71  and  72  based on the calculated required torque TQR. 
     The vehicle controller  100  includes an engine controlling unit (internal combustion engine controlling unit)  110 , which controls the internal combustion engine  10 , and a motor controlling unit  120 , which controls the motor generators  71  and  72 . The engine controlling unit  110  is one example of a controller for an internal combustion engine in the present embodiment. When the fuel introduction process is executed during the combustion stop period, the motor controlling unit  120  controls operation of the first motor generator  71  to perform the motoring. That is, the rotation speed of the crankshaft  14  in the combustion stop period can be controlled through the performance of the motoring. 
     As shown in  FIG.  2   , the engine controlling unit  110  receives an air-fuel ratio detection value AF, which is an air-fuel ratio detected by the air-fuel ratio sensor  81 . The engine controlling unit  110  also receives an air amount detection value DR, which is an intake air amount detected by the air flowmeter  80 . Further, the engine controlling unit  110  receives a crank position detection value 0, which is the rotational position of the crankshaft  14  detected by a crank angle sensor  82 . The engine controlling unit  110  receives an opening degree detection value PN, which is the opening degree of the exhaust gas recirculation valve  26  detected by the valve opening degree sensor  27 . The engine controlling unit  110  also receives detection values from other sensors attached to various sections of the internal combustion engine  10 . 
     As shown in  FIG.  2   , the engine controlling unit  110  includes, as functional components, an ignition controlling section  111 , which controls the ignition device  19 , an injection valve controlling section  112 , which controls the fuel injection valve  17 , and a recirculation valve controlling section  114 , which controls opening and closing of the exhaust gas recirculation valve  26 . 
     The ignition controlling section  111  causes the ignition device  19  to perform spark discharge at point in time when the piston reaches the vicinity of the compression top dead center while a spark discharge permission flag is ON. The ignition controlling section  111  does not cause the ignition device  19  to perform spark discharge while the spark discharge permission flag is OFF, that is, during the combustion stop period. 
     The injection valve controlling section  112  executes a fuel combustion process for burning fuel in the cylinder  11 , the above-described fuel cutoff process, and the above-described fuel introduction process. When executing various processes, the injection valve controlling section  112  calculates a required value QPR of the fuel injection amount of the fuel injection valve  17  and controls the operation of the fuel injection valve  17  based on the required value QPR. The injection valve controlling section  112  calculates the required value QPR of the fuel injection amount that corresponds to the operational state of the internal combustion engine  10  in each process. 
     The injection valve controlling section  112  executes the fuel introduction process under a situation in which a condition for executing the fuel introduction process (hereinafter referred to as the “execution condition of the fuel introduction process”) is satisfied. This execution condition is satisfied when the following two conditions are satisfied in addition to a combustion stop condition in the cylinder  11  of the internal combustion engine  10  (condition 1). One of the conditions (condition 2) is that the temperature of the three-way catalyst  22  is determined to be higher than or equal to a specified temperature. The specified temperature is set to the activation temperature of the three-way catalyst  22  or a temperature slightly higher than the activation temperature. The other one of the conditions (condition 3) is that an estimated value of the amount of trapped particulate matter in the particulate filter  23  is greater than or equal to a determination trapped amount. The combustion stop condition in the cylinder  11  of the internal combustion engine  10  is, for example, that the required value of the output torque of the internal combustion engine  10  is less than 0. 
     The recirculation valve controlling section  114  basically controls the opening degree of the exhaust gas recirculation valve  26  based on the operational state of the internal combustion engine  10 . Specifically, the recirculation valve controlling section  114  basically controls a target opening degree EGV of the exhaust gas recirculation valve  26  based on the operational state of the internal combustion engine  10 . The recirculation valve controlling section  114  controls the exhaust gas recirculation valve  26  such that the opening degree of the exhaust gas recirculation valve  26  becomes the target opening degree EGV. That is, the recirculation valve controlling section  114  delivers a drive signal to the actuator of the exhaust gas recirculation valve  26 . These processes are normal processes through which the recirculation valve controlling section  114  controls the opening degree of the exhaust gas recirculation valve  26  based on the operational state of the internal combustion engine  10 . 
     The recirculation valve controlling section  114  controls the opening degree of the exhaust gas recirculation valve  26  through a limiting process, which is different from the normal processes, in a period from a point in time that is after the execution condition of the fuel introduction process is satisfied and before fuel is injected from the fuel injection valve  17  in the fuel introduction process to when the execution of the fuel introduction process ends. Specifically, the opening degree of the exhaust gas recirculation valve  26  at the point in time when a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied will be referred to as a preliminary opening degree WGV. The recirculation valve controlling section  114  controls the exhaust gas recirculation valve  26  such that the opening degree of the exhaust gas recirculation valve  26  becomes smaller than the preliminary opening degree WGV in the limiting process. 
     The preliminary opening degree WGV will now be described. When the condition that the required value of the output torque is 0, which is the combustion stop condition in the cylinder  11  of the internal combustion engine  10 , is satisfied, and the opening degree of the throttle valve  16  is small, the pumping loss for introducing intake air into the cylinder  11  of the internal combustion engine  10  increases. Therefore, at the point in time when the required value of the output torque of the internal combustion engine  10  is set to 0, the opening degree of the exhaust gas recirculation valve  26  is increased in order to eliminate the pumping loss. The rotation of the crankshaft  14  of the internal combustion engine  10  remains stopped after the combustion stop condition is satisfied and until the execution condition of the fuel introduction process is satisfied. Thus, no pumping loss occurs in the internal combustion engine  10 , and there is no need to adjust the opening degree of the exhaust gas recirculation valve  26  to eliminate pumping loss. Under these circumstances, the opening degree of the exhaust gas recirculation valve  26  after the combustion stop condition is satisfied and until the execution condition of the fuel introduction process is satisfied is maintained at the value when the required value of the output torque of the internal combustion engine  10  is 0 or is slightly smaller than that value. Therefore, the preliminary opening degree WGV, which is the opening degree of the exhaust gas recirculation valve  26  when a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied, is at least a positive value. 
     In the limiting process, the recirculation valve controlling section  114  sets the target opening degree EGV of the exhaust gas recirculation valve  26  to an introduction opening degree EGV 1 , which is dedicated to the limiting process. In the present embodiment, the introduction opening degree EGV 1  is set to 0 in consideration of the fact that the preliminary opening degree WGV is at least a positive value. The recirculation valve controlling section  114  controls the exhaust gas recirculation valve  26  such that the opening degree of the exhaust gas recirculation valve  26  becomes the introduction opening degree EGV 1 . Therefore, during the period in which the limiting process is executed, that is, during the period from a point in time before fuel is injected in the fuel introduction process to point in time during the execution of the fuel introduction process, the opening degree of the exhaust gas recirculation valve  26  is smaller than the preliminary opening degree WGV. The recirculation valve controlling section  114  executes the limiting process when a valve opening degree limitation flag, which is a flag for permitting execution of the limiting process, is set to ON. 
     Next, the procedure of an injection valve controlling process, through which the injection valve controlling section  112  controls the operation of the fuel injection valve  17 , will be described with reference to  FIG.  3   . The injection valve controlling section  112  executes the following process at every predetermined control cycle (for example, every several milliseconds) while the vehicle controller  100  (engine controlling unit  110 ) of the hybrid vehicle is operating. At the point in time when the vehicle controller  100  of the hybrid vehicle is activated, the valve opening degree limitation flag is OFF. 
     The injection valve controlling section  112  executes the process of step S 10  when the injection valve controlling process is started. In step S 10 , the injection valve controlling section  112  determines whether the combustion stop condition of air-fuel mixture in the cylinders  11  is satisfied. As described above, the combustion stop condition of air-fuel mixture in the cylinder  11  is, for example, that the required value of the output torque of the internal combustion engine  10  is less than 0. The injection valve controlling section  112  determines that the combustion stop condition of air-fuel mixture in the cylinder  11  is not satisfied when the required value of the output torque of the internal combustion engine  10  is greater than 0 (step S 10 : NO). In this case, the injection valve controlling section  112  advances the process to step S 200 . When the process proceeds to step S 200 , the motor controlling unit  120  stops the motoring if the motoring is being executed at that point in time. 
     In step S 200 , the injection valve controlling section  112  sets the spark discharge permission flag to ON and advances the process to step S 210 . Then, in step S 210 , the injection valve controlling section  112  sets the valve opening degree limitation flag to OFF. Thereafter, the injection valve controlling section  112  advances the process to step S 220 . 
     In step S 220 , the injection valve controlling section  112  calculates the required value QPR of the fuel injection amount of the fuel injection valve  17 . The injection valve controlling section  112  calculates the required value QPR such that the air-fuel ratio detection value AF becomes a target air-fuel ratio of the intake air amount that corresponds to the required value of the output torque of the internal combustion engine  10 . The target air-fuel ratio is set to, for example, the stoichiometric air-fuel ratio or a value near the stoichiometric air-fuel ratio. After the process of step S 220 , the injection valve controlling section  112  advances the process to step S 230 . 
     In step S 230 , the injection valve controlling section  112  controls the operation of the fuel injection valve  17  based on the calculated required value QPR. Then, the injection valve controlling section  112  temporarily ends the series of processes. The process of step S 220  and step S 230  is a fuel combustion process in which air-fuel mixture containing fuel injected from the fuel injection valve  17  is burned in the cylinder  11  of the internal combustion engine  10 . 
     If it is determined in step S 10  that the required value of the output torque of the internal combustion engine  10  is less than or equal to 0, the injection valve controlling section  112  determines that the combustion stop condition of air-fuel mixture in the cylinder  11  is satisfied (step S 10 : YES). In this case, the injection valve controlling section  112  advances the process to step S 15 . In step S 15 , the injection valve controlling section  112  sets the spark discharge permission flag to OFF and advances the process to step S 20 . The internal combustion engine  10  is in the combustion stop period while the spark discharge permission flag is OFFf. 
     In step S 20 , the injection valve controlling section  112  determines whether the execution condition of the fuel introduction process is satisfied. As described above, one of the conditions (condition 2) for satisfying the execution condition is that the temperature of the three-way catalyst  22  is determined to be higher than or equal to the specified temperature. The temperature of the three-way catalyst  22  can be calculated based on the operating state of the internal combustion engine  10 . The other one of the conditions (condition 3) is that an estimated value of the amount of trapped particulate matter in the particulate filter  23  is greater than or equal to a determination trapped amount. When the trapped amount increases, the pressure difference between the section in the exhaust passage  21  between the three-way catalyst  22  and the particulate filter  23  and the section in the exhaust passage  21  that is at the downstream side of the particulate filter  23  is likely to increase. Therefore, the pressure difference can be used to calculate the estimated value of the trapped amount. 
     When determining that at least one of the above two conditions (condition 2 and condition 3) is not satisfied in step S 20  (step S 20 : NO), the injection valve controlling section  112  advances the process to step S 300 . When the process proceeds to step S 300 , the motor controlling unit  120  stops the motoring if the motoring is being executed at that point in time. 
     In step S 300 , the injection valve controlling section  112  sets the valve opening degree limitation flag to OFF. Thereafter, the injection valve controlling section  112  advances the process to step S 310 . 
     In step S 310 , the injection valve controlling section  112  sets the required value QPR of the fuel injection amount of the fuel injection valve  17  to 0. In the subsequent step S 320 , the injection valve controlling section  112  controls the operation of the fuel injection valve  17  based on the required value QPR. That is, fuel is not injected from the fuel injection valve  17  in this case. After executing the process of step S 320 , the injection valve controlling section  112  temporarily ends the series of processes. The process of step S 310  and step S 320  is the fuel cutoff process, in which combustion in the cylinder  11  is stopped, and no fuel is introduced into the cylinder  11  under a situation in which the crankshaft  14  of the internal combustion engine  10  is rotating. 
     When determining that the two conditions (condition 2 and condition 3) for executing the fuel introduction process are both satisfied in step S 20  (step S 20 : YES), the injection valve controlling section  112  advances the process to step S 30 . As the process proceeds to step S 30 , the motor controlling unit  120  performs the motoring. 
     In step S 30 , the injection valve controlling section  112  determines whether the valve opening degree limitation flag is ON. When determining that the valve opening degree limitation flag is OFF (step S 30 : NO), the injection valve controlling section  112  advances the process to step S 35  and sets the valve opening degree limitation flag to ON, and then advances the process to step S 40 . In contrast, when determining that the valve opening degree limitation flag is ON in step S 30  (step S 30 : YES), the injection valve controlling section  112  advances the process directly to step S 40 . 
     A situation in which the determination in step S 30  is NO and a situation in which the determination in step S 30  is YES will be described. If the determination in step S 20  was NO in the previous cycle of the injection valve controlling process and the determination in step S 20  is YES in the current cycle, that is, if a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied, the valve opening degree limitation flag is OFF at that point in time. Therefore, the determination of step S 30  is NO in this case. If the determination in step S 30  is NO, the process proceeds to step S 35  as described above, and the valve opening degree limitation flag is set to ON. Once the valve opening degree limitation flag is set to ON, the determination of step S 30  will be YES when the process in the subsequent cycle proceeds to step S 30  if, in the subsequent cycle, the combustion stop condition in the cylinder  11  is still satisfied (step S 10 : YES) and the execution condition of the fuel introduction process is still satisfied (step S 20 : YES). Thereafter, the determination of step S 30  will continuously be YES as long as the state continues in which the combustion stop condition in the cylinder  11  is satisfied and the execution condition of the fuel introduction process is satisfied. When, as time elapses thereafter, the combustion stop condition in the cylinder  11  or the execution condition of the fuel introduction process becomes no longer satisfied, the valve opening degree limitation flag is set to OFF in step S 210  or step S 300 . Therefore, when both the combustion stop condition in the cylinder  11  and the execution condition of the fuel introduction process are both satisfied and the process proceeds to step S 30  after the combustion stop condition becomes unsatisfied or the execution condition of the fuel introduction process becomes unsatisfied, the determination of step S 30  is NO. In this case, the process proceeds to step S 40  via step S 35  as described above. 
     In step S 40 , the injection valve controlling section  112  determines whether the opening degree of the exhaust gas recirculation valve  26  is the introduction opening degree EGV 1 . Specifically, the injection valve controlling section  112  compares the opening degree detection value PN detected by the valve opening degree sensor  27  with the introduction opening degree EGV 1  set by the recirculation valve controlling section  114 . As described above, the introduction opening degree EGV 1  is 0 in the present embodiment. The injection valve controlling section  112  determines that the opening degree of the exhaust gas recirculation valve  26  has not reached the introduction opening degree EGV 1  (step S 40 : NO) if the opening degree detection value PN does not match the introduction opening degree EGV 1 . In this case, the injection valve controlling section  112  advances the process to step S 150 . In contrast, the injection valve controlling section  112  determines that the opening degree of the exhaust gas recirculation valve  26  has reached the introduction opening degree EGV 1  (step S 40 : YES) if the opening degree detection value PN matches the introduction opening degree EGV 1 . In this case, the injection valve controlling section  112  advances the process to step S 50 . 
     A situation in which the determination in step S 40  is NO and a situation in which the determination in step S 40  is YES will be described. It takes a certain period of time from when the exhaust gas recirculation valve  26  starts operating in connection with the limiting process executed by the recirculation valve controlling section  114  until the exhaust gas recirculation valve  26  moves to the position corresponding to the introduction opening degree EGV 1 . Therefore, the determination of step S 40  is NO when the exhaust gas recirculation valve  26  is operating and the opening degree of the exhaust gas recirculation valve  26  has no reached the introduction opening degree EGV 1  although the limiting process is being executed in association with the valve opening degree limitation flag being set to ON in step S 30  or step S 35 . In contrast, when the opening degree of the exhaust gas recirculation valve  26  reaches the introduction opening degree EGV 1  in association with the execution of the limiting process, the determination of step S 40  is YES. 
     If the determination in step S 40  is NO and the process proceeds to step S 150 , the injection valve controlling section  112  sets the required value QPR of the fuel injection amount in the fuel injection valve  17  to 0. In the subsequent step S 160 , the injection valve controlling section  112  controls the operation of the fuel injection valve  17  based on the required value QPR. Therefore, fuel is not injected from the fuel injection valve  17  when the opening degree of the exhaust gas recirculation valve  26  has not reached the introduction opening degree EGV 1 . After executing the process of step S 160 , the injection valve controlling section  112  temporarily ends the series of processes. The process of step S 150  and step S 160  is the same process as the above-described fuel cutoff process. However, since the motoring is being performed in the process of step S 150  and S 160 , the situation is different from that in which the process of step S 310  and S 320  is executed (the case in which the motoring is not executed). 
     If the determination in step S 40  is YES and the process proceeds to step S 50 , the injection valve controlling section  112  calculates the required value QPR of the fuel injection amount for the fuel introduction process. The injection valve controlling section  112  calculates the required value QPR based on the operating state of the internal combustion engine  10 . The fuel injection amount for causing fuel to flow out unburned from inside the cylinder  11  to the exhaust passage  21  in the fuel introduction process is smaller than the fuel injection amount when burning air-fuel mixture in the cylinder  11  in the fuel combustion process. Thus, the required value QPR calculated in step S 50  is less than the required value QPR calculated in step S 220 . After step S 50 , the injection valve controlling section  112  advances the process to step S 60 . 
     In step S 60 , the injection valve controlling section  112  controls the operation of the fuel injection valve  17  based on the calculated required value QPR. As described above, when the process proceeds to step S 50 , the opening degree of the exhaust gas recirculation valve  26  has reached the introduction opening degree EGV 1 . That is, fuel is injected from the fuel injection valve  17  when the opening degree of the exhaust gas recirculation valve  26  has reached the introduction opening degree EGV 1 . After executing the process of step S 60 , the injection valve controlling section  112  temporarily ends the series of processes. The process of step S 50  and step S 60  is the fuel introduction process, in which, when combustion in the cylinder  11  is stopped under a situation in which the crankshaft  14  of the internal combustion engine  10  is rotating, fuel is injected from the fuel injection valve  17  to cause the fuel to flow out unburned from inside the cylinder  11  to the exhaust passage  21 . 
     Next, the procedure of a recirculation valve controlling process, through which the recirculation valve controlling section  114  controls opening and closing of the exhaust gas recirculation valve  26 , will be described with reference to  FIG.  4   . The recirculation valve controlling section  114  executes the following process at every predetermined control cycle (for example, every several milliseconds) while the vehicle controller  100  (engine controlling unit  110 ) of the hybrid vehicle is operating. 
     The recirculation valve controlling section  114  executes the process of step S 500  when starting the recirculation valve controlling process. In step S 500 , the recirculation valve controlling section  114  determines whether the valve opening degree limitation flag, which is set by the injection valve controlling section  112 , is ON. When determining that the valve opening degree limitation flag is OFF (step S 500 : NO), the recirculation valve controlling section  114  advances the process to step S 610 . 
     In step S 610 , the recirculation valve controlling section  114  basically controls a target opening degree EGV of the exhaust gas recirculation valve  26  based on the operational state of the internal combustion engine  10 . Specifically, the recirculation valve controlling section  114  stores a map that defines the relationship between the engine rotation speed NE, the engine load KL, and the opening degree of the exhaust gas recirculation valve  26 . The recirculation valve controlling section  114  refers to this map to calculate, as a target opening degree EGV, the opening degree of the exhaust gas recirculation valve  26  that corresponds to the current engine rotation speed NE, which is calculated based on the crank position detection value 0, and the current engine load KL, which is calculated based on the crank position detection value 0 and the air amount detection value DR. Thereafter, the recirculation valve controlling section  114  advances the process to step S 620 . 
     In step S 620 , the recirculation valve controlling section  114  controls the exhaust gas recirculation valve  26  such that the opening degree of the exhaust gas recirculation valve  26  becomes the target opening degree EGV. After executing the process of step S 620 , the recirculation valve controlling section  114  temporarily ends the series of processes. The process of step S 610  and step S 620  is a normal process through which the opening degree of the exhaust gas recirculation valve  26  is controlled based on the operating state of the internal combustion engine  10 . 
     When determining that the valve opening degree limitation flag is ON in step S 500  (step S 500 : YES), the recirculation valve controlling section  114  advances the process to step S 510 . 
     In step S 510 , the recirculation valve controlling section  114  sets the target opening degree EGV of the exhaust gas recirculation valve  26  to 0, which is the introduction opening degree EGV 1 . Then, in the subsequent step S 520 , the recirculation valve controlling section  114  controls the exhaust gas recirculation valve  26  such that the opening degree of the exhaust gas recirculation valve  26  becomes the target opening degree EGV. Thereafter, the recirculation valve controlling section  114  temporarily ends the series of processes. The process of step S 510  and step S 520  is the limiting process for causing the opening degree of the exhaust gas recirculation valve  26  to be smaller than the preliminary opening degree WGV. 
     The operation and advantages of the present embodiment will now be described. 
     In the above-described configuration, when the execution condition of the fuel introduction process is satisfied, the recirculation valve controlling section  114  executes the limiting process, so that the opening degree of the exhaust gas recirculation valve  26  is limited. Specifically, when the execution condition of the fuel introduction process is satisfied, the process of step S 510  and step S 520  in the recirculation valve controlling process is repeated to control the exhaust gas recirculation valve  26  such that the opening degree of the exhaust gas recirculation valve  26  becomes 0. After the opening degree of the exhaust gas recirculation valve  26  reaches 0, the fuel introduction process is started. The opening degree of the exhaust gas recirculation valve  26  is controlled to be 0 over the entire period of time during which the fuel introduction process is executed, that is, over the entire period of during which the process of step S 50  and S 60  in the injection valve controlling process is repeated. Then, when the fuel introduction process ends, that is, when the period during which the process of step S 50  and step S 60  in the injection valve controlling process is repeated ends, the control to set the opening degree of the exhaust gas recirculation valve  26  to 0 is cancelled. 
     It is now assumed that during the execution of the fuel introduction process, the opening degree of the exhaust gas recirculation valve  26  is not limited but remains relatively large. In this case, the gas heated in the three-way catalyst  22  is returned to the intake passage  15  via the exhaust gas recirculation passage  25  in connection with fuel injection in the fuel introduction process. When the gas thus heated is returned to the intake passage  15 , the temperature increase of the particulate filter  23  is delayed in accordance with the amount of heat of the gas returned to the intake passage  15 . 
     In this respect, limiting the opening degree of the exhaust gas recirculation valve  26  during the execution of the fuel introduction process as described above prevents the gas heated in the three-way catalyst  22  from being returned to the intake passage  15  through the exhaust gas recirculation passage  25 . In particular, in the present embodiment, the opening degree of the exhaust gas recirculation valve  26  is set to 0 during the execution of the fuel introduction process. Therefore, the amount of gas returned to the intake passage  15  via the exhaust gas recirculation passage  25  is substantially 0. Therefore, the gas heated in the three-way catalyst  22  can be delivered to the particulate filter  23  almost without being wasted. 
     Moreover, in the present embodiment, the opening degree of the exhaust gas recirculation valve  26  is limited over the entire period of the execution of the fuel introduction process. That is, the gas heated in the three-way catalyst  22  reaches the particulate filter  23  almost without being wasted during the period in which the fuel injection for heating the particulate filter  23  continues. As described above, more or less the intended amount of heat is provided to the particulate filter  23  over the intended period for heating the particulate filter  23 . The temperature of the particulate filter  23  does not deviate significantly from the intended temperature. 
     It takes a certain period of time from when the control of the exhaust gas recirculation valve  26  to reduce the opening degree of the exhaust gas recirculation valve  26  is started to when the opening degree of the exhaust gas recirculation valve  26  reaches 0, which is the target opening degree EGV. Therefore, in a case in which the limiting process is started at the point in time of starting the fuel introduction process, that is, in a case in which the determination of step S 500  in the recirculation valve controlling process is switched from NO to YES at the point of time of starting the fuel introduction process and the process of step S 510  and step S 520  is repeated thereafter, fuel may be introduced into the three-way catalyst  22  and a temperature increase may occur in the three-way catalyst  22 , and the heated gas may be returned to the intake passage  15  via the exhaust gas recirculation passage  25  even during the period from the start of the limiting process to when the opening degree of the exhaust gas recirculation valve  26  reaches 0. In this respect, in the above-described configuration, the opening degree of the exhaust gas recirculation valve  26  has already become 0 by that point in when the fuel introduction process is started. Therefore, the high temperature gas for heating the particulate filter  23  will not be returned to the intake passage  15  while the exhaust gas recirculation valve  26  is operating. 
     The present embodiment may be modified as follows. The above-described embodiment and the following modifications can be combined as long as the combined modifications remain technically consistent with each other. 
     The introduction opening degree EGV 1 , which is defined as the target opening degree EGV of the exhaust gas recirculation valve  26  in the limiting process, is not necessarily required to be 0 as long as it is less than the preliminary opening degree WGV, which is the opening degree of the exhaust gas recirculation valve  26  at the point in time when the execution condition of the fuel introduction process becomes satisfied. Even if the opening degree of the exhaust gas recirculation valve  26  is not 0, the gas heated in the three-way catalyst  22  is somewhat prevented from being returned to the intake passage  15  through the exhaust gas recirculation passage  25  as long as the opening degree of the exhaust gas recirculation valve  26  is limited to a small value. 
     An example of a process executed by the recirculation valve controlling section  114  in a case in which the introduction opening degree EGV 1  is set to a value other than 0 will be described. In this case, each time a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied, the recirculation valve controlling section  114  stores the opening degree of the exhaust gas recirculation valve  26  at the point in time when the state is switched to the state in which the execution condition is satisfied. Specifically, in a case in which the determination of step S 20  in the previous cycle of the injection valve controlling process performed by the injection valve controlling section  112  is NO and the determination of step S 20  in the current cycle is YES, the recirculation valve controlling section  114  stores, at that point in time, the opening degree detection value PN detected by the valve opening degree sensor  27  as the preliminary opening degree WGV. Each time a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied, the recirculation valve controlling section  114  overwrites the previous preliminary opening degree WGV with the current preliminary opening degree WGV. Therefore, the preliminary opening degree WGV stored in the recirculation valve controlling section  114  is always the latest preliminary opening degree WGV. 
     Then, in addition to repeating the above-described process, the recirculation valve controlling section  114  repeatedly executes a recirculation valve controlling process (see  FIG.  5   ), which will be discussed below, instead of the recirculation valve controlling process of the above-described the above embodiment (see  FIG.  4   ). The recirculation valve controlling process of  FIG.  5    is constructed by adding the processes of step S 502 , step S 504 , step S 506 , step S 530 , and step S 630  to the recirculation valve controlling process shown in  FIG.  4    of the above described embodiment. In this recirculation valve controlling process, a reset flag, which is a flag for requesting calculation of the introduction opening degree EGV 1 , is used. At the point in time when the vehicle controller  100  for a hybrid vehicle is activated, the reset flag is ON. 
     As shown in  FIG.  5   , in this recirculation valve controlling process, when determining that the valve opening degree limitation flag is OFF in step S 500  (step S 500 : NO), the recirculation valve controlling section  114  controls the opening degree of the exhaust gas recirculation valve  26  based on the operating state of the internal combustion engine  10  in step S 610  and step S 620 . Then, the recirculation valve controlling section  114  advances the process to step S 630 . In step S 630 , the recirculation valve controlling section  114  sets the reset flag to ON. Thereafter, the recirculation valve controlling section  114  temporarily ends the series of processes. 
     When determining that the valve opening degree limitation flag is ON in step S 500  (step S 500 : YES), the recirculation valve controlling section  114  determines whether the reset flag is ON in step S 502 . When determining that the reset flag is ON (step S 502 : YES), the recirculation valve controlling section  114  advances the process to step S 504 . When determining that the reset flag is OFF (step S 502 : NO), the recirculation valve controlling section  114  advances the process to step S 506 . 
     A situation in which the determination in step S 502  is YES and a situation in which the determination in step S 502  is NO will be described. A situation in which the determination in step S 502  is YES may be, for example, a situation in which the determination in step S 500  was NO in the previous cycle of the recirculation valve controlling process, and the determination in step S 500  is YES in the current cycle. This is a situation in which the valve opening degree limitation flag is set to ON for the first time after a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied (step S 35  in the injection valve controlling process of  FIG.  3   ). If the process proceeds with a valve opening degree limitation flag set to ON (step S 500 : YES), the reset flag is set to OFF in step S 530 , which will be discussed below. Once the reset flag is set to OFF, the determination of step S 502  will be NO when the process proceeds to step S 502  if a state in which the valve opening degree limitation flag is ON (the state in which the execution condition of the fuel introduction process is satisfied) still continues (step S 500 : YES). Thereafter, the determination of step S 502  remains NO as long as the valve opening degree limitation flag continues to be ON. 
     If the determination in step S 502  is YES and the process proceeds to step S 504 , the recirculation valve controlling section  114  calculates the introduction opening degree EGV 1  as a value less than the preliminary opening degree WGV. After step S 504 , the recirculation valve controlling section  114  advances the process to step S 510 . 
     In contrast, if the determination in step S 502  is NO and the process proceeds to step S 506 , the recirculation valve controlling section  114  sets the introduction opening degree EGV 1  to the introduction opening degree EGV 1  of the previous cycle. After step S 506 , the recirculation valve controlling section  114  advances the process to step S 510 . 
     With the process of step S 502 , step S 504 , and step S 506 , the introduction opening degree EGV 1  is provisionally calculated when a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied (step S 504 ). Then, while the execution condition of the fuel introduction process is satisfied, the introduction opening degree EGV 1  is maintained at a certain value (step S 506 ). 
     When the process proceeds to step S 510 , the recirculation valve controlling section  114  controls the exhaust gas recirculation valve  26  such that the opening degree of the exhaust gas recirculation valve  26  becomes the introduction opening degree EGV 1  through step S 510  and step S 520 . Then, in step S 530 , the recirculation valve controlling section  114  sets the reset flag to OFF and temporarily ends the series of processes. In the recirculation valve controlling process of  FIG.  5    described above, the processes of step S 502 , step S 504 , step S 506 , step S 510 , and step S 520  correspond to the limiting process. 
     Each time a state in which the execution condition of the fuel introduction process is not satisfied is switched to a state in which the execution condition is satisfied, the preliminary opening degree WGV may vary. The above-described recirculation valve controlling process allows the introduction opening degree EGV 1  to be changed in step S 504  each time such variation of the preliminary opening degree WGV occurs. 
     In the recirculation valve controlling process of the above-described modification shown in  FIG.  5   , the introduction opening degree EGV 1  is a constant value while the execution condition of the fuel introduction process continues to be satisfied. However, the introduction opening degree EGV 1  may be changed while the execution condition of the fuel introduction process continues to be satisfied. In this case, in the flow of the process shown in  FIG.  5   , step S 502 , step S 530 , and step S 630 , which are processes related to the reset flag, are deleted, and step S 506  is deleted. Then, each time the determination whether the valve opening limitation flag is ON in step S 500  becomes YES, the introduction opening degree EGV 1  is simply recalculated in step S 504 . 
     The period in which the opening degree of the exhaust gas recirculation valve  26  is limited may be changed as long as it includes the period during which the fuel introduction process is executed. Specifically, the point in time at which the opening degree of the exhaust gas recirculation valve  26  starts being limited is not limited to that in the above-described embodiment. For example, the opening degree of the exhaust gas recirculation valve  26  may start being decreased at the same time as the fuel introduction process is started. Also, the opening degree of the exhaust gas recirculation valve  26  may start being decreased after the fuel introduction process is started. Furthermore, the point in time at which the limitation on the opening degree of the exhaust gas recirculation valve  26  is ended is not limited to that in the above-described embodiment. For example, the limitation on the opening degree of the exhaust gas recirculation valve  26  may be ended at a point in time in the middle of the execution of the fuel introduction process. If the opening degree of the exhaust gas recirculation valve  26  is limited even for a short period of time during the execution of the fuel introduction process, the gas heated in three-way catalyst  22  is prevented from being returned to the intake passage  15  for that period. 
     The opening degree of the exhaust gas recirculation valve  26  may be limited from a point in time during the execution of the fuel introduction process to when a predetermined time has elapsed after the end of the fuel introduction process. A temporal delay can occur when the gas heated in the three-way catalyst  22  reaches the inlet of the exhaust gas recirculation passage  25 , which is located at the downstream side of the three-way catalyst  22  (the section of the exhaust passage  21  to which the exhaust gas recirculation passage  25  is connected) or the particulate filter  23 , and that temporal delay corresponds to the distance between the three-way catalyst  22  and the section at the downstream side of the three-way catalyst  22 . Thus, even after the fuel introduction process is ended, the gas heated in the three-way catalyst  22  may flow to a section at the downstream side of the three-way catalyst  22 . Therefore, if the opening degree of the exhaust gas recirculation valve  26  is limited even after the fuel introduction process is ended, the high-temperature gas flowing downstream from the three-way catalyst  22  after the end of the fuel introduction process is prevented from being returned to the intake passage  15  via the exhaust gas recirculation passage  25 . 
     When the opening degree of the exhaust gas recirculation valve  26  is limited until a predetermined time has elapsed after the end of the fuel introduction process, the point in time to end the limitation on the opening degree of the exhaust gas recirculation valve  26  simply needs be defined in accordance with an index that indicates that the gas heated in the three-way catalyst  22  has entirely reached the inlet of the exhaust gas recirculation passage  25  after the fuel introduction process is ended. As such an index, an accumulated value of the intake air amount from the point in time when the fuel introduction process is ended (hereinafter referred to as a post-termination accumulated value) may be used. For example, based on the volume of a section of the exhaust passage  21  from the downstream end of the three-way catalyst  22  to the inlet of the exhaust gas recirculation passage  25 , the intake air amount required for the gas located in the three-way catalyst  22  to reach the inlet of the exhaust gas recirculation passage  25  is calculated in advance as a specified value. When the post-termination accumulated value reaches the specified value, the limitation on the opening degree of the exhaust gas recirculation valve  26  simply needs to be ended. 
     In the above-described embodiment, the fuel injection through the fuel introduction process is started (step S 50 , step S 60 ) immediately after the opening degree of the exhaust gas recirculation valve  26  reaches the introduction opening degree EGV 1  (step S 40 : YES). However, after the opening degree of the exhaust gas recirculation valve  26  reaches the introduction opening degree EGV 1 , the fuel injection may be started when a certain amount of time has elapsed in a state in which the opening degree of the exhaust gas recirculation valve  26  as the introduction opening degree EGV 1  is maintained. For example, through experiments and the like, a specified time is obtained in advance that is considered to be sufficient for the opening degree of the exhaust gas recirculation valve  26  to reach the introduction opening degree EGV 1 . Then, fuel injection may be started when the specified time has elapsed from the start of the limiting process. In this case, the shorter the period in which the opening degree of the exhaust gas recirculation valve  26  reaches the introduction opening degree EGV 1 , the longer becomes the period from the point in time when the opening degree of the exhaust gas recirculation valve  26  reaches the introduction opening degree EGV 1  until the fuel injection is started. 
     The types of the exhaust gas recirculation valve  26  include a type that changes the opening degree of the exhaust gas recirculation valve  26  in accordance with the pressure of gas flowing in the exhaust passage  21  when the exhaust gas recirculation valve  26  is not controlled. When this type of the exhaust gas recirculation valve  26  is employed, the exhaust gas recirculation valve  26  does not need to be controlled under a situation in which the combustion stop condition in the cylinder  11  of the internal combustion engine  10  is satisfied, and the valve opening degree limitation flag is OFF. Even under a situation in which the combustion stop condition is satisfied, it is unlikely that there is no gas flowing through the exhaust passage  21  at all. In a case in which employing the above-described type of the exhaust gas recirculation valve  26 , if gas flowing through the exhaust passage  21  is present, the opening degree of the exhaust gas recirculation valve  26  is greater than 0 due to the pressure of the gas flowing through the exhaust passage  21 . Therefore, the preliminary opening degree WGV is at least a positive value. When the exhaust gas recirculation valve  26  is not controlled, the opening degree of the exhaust gas recirculation valve  26  changes in accordance with the pressure of gas as described above. Thus, the preliminary opening degree WGV is unlikely to be fixed to a constant opening degree. Even in this case, if the recirculation valve controlling process described in  FIG.  5    is used, the introduction opening degree EGV 1  can be changed in step S 504  in accordance with change in the preliminary opening degree WGV. 
     Under a situation in which the combustion stop condition in the cylinder  11  of the internal combustion engine  10  is satisfied and the valve opening degree control flag is OFF, the exhaust gas recirculation valve  26  may be controlled such that the opening degree of the exhaust gas recirculation valve  26  becomes a predetermined opening degree greater than 0. Even in this configuration, the preliminary opening degree WGV is a positive value. 
     The position of the inlet of the exhaust gas recirculation passage  25  may be located in a section of the exhaust passage  21  at the upstream side of the three-way catalyst  22 . In a case in which this configuration is employed, if the opening degree of the exhaust gas recirculation valve  26  is limited in a period that includes the execution of the fuel introduction process, the fuel injected from the fuel injection valve  17  is prevented from being returned to the intake passage  15  via the exhaust gas recirculation passage  25  before reaching the three-way catalyst  22 . 
     In the above-described embodiment, the ignition device  19  does not perform spark discharge during the execution of the fuel introduction process. However, during the execution of the fuel introduction process, spark discharge of the ignition device  19  may be performed in a period in which air-fuel mixture is not burned in the cylinder  11 . For example, if spark discharge is performed when the piston  12  is located near the bottom dead center, air-fuel mixture is not burned in the cylinder  11  in which spark discharge has been performed. Therefore, even if spark discharge is performed during the execution of the fuel introduction process, the fuel injected from the fuel injection valve  17  can flow out unburned to the exhaust passage  21  from inside the cylinders  11 . 
     The internal combustion engine for which the controller for an internal combustion engine is employed may be an engine that includes a direct injection valve, which injects fuel directly into the cylinder  11 . In this case, during the execution of the fuel introduction process, fuel may be injected from the direct injection valve into the cylinder  11  and flow out unburned to the exhaust passage  21 . Unburned fuel is thus introduced into the three-way catalyst  22 . 
     The system of the hybrid vehicle may be a system different from the system shown in  FIG.  1    as long as the rotation speed of the crankshaft  14  is controlled through operation of a motor. 
     The controller for an internal combustion engine according the present disclosure may be used for an internal combustion engine mounted on a vehicle that does not have a power source other than the internal combustion engine. Even in the internal combustion engine installed in such a vehicle, combustion of air-fuel mixture in the cylinder may be stopped under a situation in which the crankshaft  14  is rotating by inertia. If the execution condition of the fuel introduction process is satisfied during the combustion stop period, the fuel introduction process will be executed. 
     The controller (engine controlling unit) can be constructed by a device that includes a CPU and a ROM and executes software processing, but is not limited to this configuration. For example, at least part of the processes executed by the software in the above-illustrated embodiment may be executed by hardware circuits dedicated to executing these processes (such as ASIC). That is, the controller may be modified as long as it has any one of the following configurations (a) to (c). (a) A configuration including a processor that executes all of the above-described processes according to programs and a program storage device such as a ROM (including a non-transitory computer readable medium) that stores the programs. (b) A configuration including a processor and a program storage device that execute part of the above-described processes according to the programs and a dedicated hardware circuit that executes the remaining processes. (c) A configuration including a dedicated hardware circuit that executes all of the above-described processes. Software processing circuits each including a processor and a program storage device and dedicated hardware circuits may be provided. That is, the above processes may be executed in any manner as long as the processes are executed by processing circuitry that includes at least one of a set of one or more software processing circuits and a set of one or more dedicated hardware circuits. 
     Various changes in form and details may be made to the examples above without departing from the spirit and scope of the claims and their equivalents. The examples are for the sake of description only, and not for purposes of limitation. Descriptions of features in each example are to be considered as being 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 by 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 the disclosure.