Patent Publication Number: US-2023151775-A1

Title: Egr estimation method for internal combustion engine and egr estimation device for internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to EGR estimation of an internal combustion engine. 
     BACKGROUND ART 
     JP2016-211456A discloses a technique in which when an exhaust gas accumulation amount of exhaust gas leaked into a downstream EGR passage when an EGR valve is fully closed exceeds a predetermined threshold value, fresh air introduction control is performed to introduce fresh air into the downstream EGR passage. 
     SUMMARY OF INVENTION 
     Even if the EGR valve is fully closed, a gap may occur due to a structure thereof. In this case, even if fuel cut of an internal combustion engine is started when the EGR valve is fully closed, the exhaust gas may leak from the EGR valve to an intake passage side through the gap. As a result, an inside of an upstream EGR passage, that is, a portion between the EGR valve and an exhaust passage, is replaced with fresh air from an upstream side according to the leakage of the exhaust gas during the fuel cut. 
     In this case, even if the EGR valve is opened by restarting EGR after fuel injection of the internal combustion engine is restarted, the fresh air remaining in the upstream EGR passage first flows into the EGR valve. As a result, the EGR gas, which is the exhaust gas recirculated, may not reach the EGR valve immediately. Therefore, in light of such a flow mode of the gas, it is desired to estimate a more accurate EGR rate when the fuel cut is started when the EGR valve is fully closed. 
     The present invention is made in view of such a problem, and an object of the present invention is to estimate a more accurate EGR rate when the fuel cut is started when the EGR valve is fully closed. 
     An EGR estimation method for internal combustion engines according to one embodiment of this invention is an EGR estimation method for an internal combustion engine that estimates an EGR rate in an intake and exhaust system of an internal combustion engine, the intake and exhaust system of an internal combustion engine including: an intake system including an intake passage that is connected to an internal combustion engine, an exhaust system including an exhaust passage that is connected to the internal combustion engine, and the intake and exhaust system being provided with an EGR device including an EGR passage that connects the intake passage and the exhaust passage and an EGR valve that is provided in the EGR passage, the EGR estimation method comprising: determining a gas replacement state by exhaust gas and fresh air in an upstream EGR passage, which is a portion of the EGR passage between the EGR valve and the exhaust passage, when fuel cut of the internal combustion engine is started and the EGR valve is fully closed; and estimating the EGR rate based on a result of the determination. 
     According to another embodiment of the invention, there is provided an EGR estimation device for internal combustion engines corresponding to the above EGR estimation method for an internal combustion engine. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic configuration diagram showing a main part of a vehicle. 
         FIG.  2 A  is a first diagram for explaining a flow mode of gas. 
         FIG.  2 B  is a second diagram for explaining the flow mode of the gas. 
         FIG.  2 C  is a third diagram for explaining the flow mode of the gas. 
         FIG.  2 D  is a fourth diagram for explaining the flow mode of the gas. 
         FIG.  2 E  is a fifth diagram for explaining the flow mode of the gas. 
         FIG.  3    is a control block diagram showing an EGR rate estimation process. 
         FIG.  4    is a flowchart showing the EGR rate estimation process. 
         FIG.  5    is a diagram showing an example of a timing chart. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. 
       FIG.  1    is a diagram showing a main part of a vehicle. The vehicle includes an internal combustion engine  1 , an intake system  10 , an exhaust system  20 , a supercharger  30 , an EGR device  40 , and a controller  50 . 
     The intake system  10  includes an intake passage  11 , an air cleaner  12 , an air flow meter  13 , an intake throttle valve  14 , an intercooler  15 , a throttle valve  16 , a collector  17 , a compressor  31 , an intake bypass passage  18 , and a recirculation valve  19 . The intake passage  11  connects the air cleaner  12  and the internal combustion engine  1  and circulates intake air introduced into the internal combustion engine  1 . The intake passage  11  is provided with the air cleaner  12 , the air flow meter  13 , the intake throttle valve  14 , the compressor  31 , the intercooler  15 , the throttle valve  16 , and the collector  17  in this order from an upstream side. 
     The air cleaner  12  removes foreign matters contained in the intake air. The air flow meter  13  measures a flow rate of the intake air. The intake throttle valve  14  is provided in a portion of the intake passage  11 , which is on an upstream side of an EGR convergence portion  11   a  to which an EGR passage  41  is connected, which will be described later. The intake throttle valve  14  increases a recirculation amount of exhaust gas through the EGR passage  41  by reducing an opening degree. 
     The intercooler  15  cools the supercharged intake air. The throttle valve  16  adjusts an amount of the intake air introduced into the internal combustion engine  1 . The collector  17  temporarily stores the intake air. The compressor  31  is a compressor of the supercharger  30  and compresses the intake air. 
     The exhaust system  20  includes an exhaust passage  21 , an upstream catalyst  22 , a downstream catalyst  23 , and a turbine  32 . The exhaust passage  21  is connected to the internal combustion engine  1  and circulates exhaust gas discharged from the internal combustion engine  1 . The exhaust passage  21  is provided with the turbine  32 , the upstream catalyst  22 , and the downstream catalyst  23  in this order from an upstream side. The upstream catalyst  22  and the downstream catalyst  23  purify the exhaust gas. The turbine  32  is the turbine of the supercharger  30  and recovers energy from the exhaust gas. 
     The supercharger  30  compresses the intake air and then supplies to the internal combustion engine  1 . The supercharger  30  is a turbocharger and includes the compressor  31 , the turbine  32 , and a shaft  33 . The supercharger  30  is provided in the intake passage  11  and the exhaust passage  21  by providing the compressor  31  in the intake passage  11  and the turbine  32  in the exhaust passage  21 . In the supercharger  30 , when the turbine  32  is rotated by the exhaust gas, the compressor  31  is rotated via the shaft  33  to compress the intake air. In the compressor  31 , a pair of compressor wheels arranged in a back-to-back direction are provided on the shaft  33 , and the intake air is compressed by the pair of compressor wheels. The turbine  32  is provided with an exhaust bypass passage, and the exhaust bypass passage is provided with a waste gate valve (not shown) that adjusts a flow rate of the flowing exhaust gas. 
     The EGR device  40  includes the EGR passage  41 , an EGR cooler  42 , and an EGR valve  43 . The EGR device  40  recirculates the exhaust gas from the exhaust passage  21  to the intake passage  11 . 
     The EGR passage  41  connects the exhaust passage  21  and the intake passage  11 . The EGR passage  41  recirculates a part of the exhaust gas flowing through the exhaust passage  21  to the intake passage  11  as EGR gas. The EGR passage  41  is provided with the EGR cooler  42  and the EGR valve  43 . The EGR cooler  42  cools the EGR gas flowing through the EGR passage  41 . The EGR valve  43  adjusts a flow rate of the EGR gas flowing through the EGR passage  41 . The EGR valve  43  includes, for example, a butterfly valve. 
     The EGR passage  41  includes an upstream EGR passage  41   a , which is a portion between the EGR valve  43  and the exhaust passage  21 , and a downstream EGR passage  41   b , which is a portion between the EGR valve  43  and the intake passage  11 . It can be understood that the upstream EGR passage  41   a  includes the EGR cooler  42 . 
     The EGR device  40 , specifically, the EGR passage  41  connects a portion downstream of the supercharger  30 , that is, the turbine  32  in the exhaust passage  21 , and a portion upstream of the supercharger  30 , that is, the compressor  31  in the intake passage  11 . In this way, the EGR passage  41  connecting the intake passage  11  and the exhaust passage  21  forms an EGR path of a low pressure loop, that is, an LPL. More specifically, the EGR passage  41  connects a portion of the exhaust passage  21  between the upstream catalyst  22  and the downstream catalyst  23  and a portion of the intake passage  11  between the intake throttle valve  14  and the compressor  31 . 
     The intake bypass passage  18  connects an upstream pressure portion and a downstream pressure portion of the supercharger  30  in the intake system  10 . The upstream pressure portion is a portion of the intake passage  11  on an upstream side of the supercharger  30  and on a downstream side of the EGR convergence portion  11   a . The downstream pressure portion is a portion of the intake passage  11  on a downstream side of the supercharger  30  and on an upstream side of the intercooler  15 . Connecting to the downstream pressure portion of the supercharger  30  in the intake system  10  includes connecting to the compressor  31  so that the compressed intake air can flow into the intake bypass passage  18 . 
     The recirculation valve  19  is provided in the intake bypass passage  18 . The recirculation valve  19  includes an on-off valve. During supercharging, a downstream pressure of the supercharger  30  is higher than an upstream pressure thereof. Therefore, when the recirculation valve  19  is opened during the supercharging, the intake air compressed by the compressor  31  is returned to the intake passage  11  on the portion upstream of the supercharger  30  via the intake bypass passage  18 . 
     The controller  50  is an electronic control device, and in addition to the air flow meter  13 , signals from various sensors and switches such as a crank angle sensor  71  and an accelerator position sensor  72  are input to the controller  50 . The crank angle sensor  71  generates a crank angle signal for each predetermined crank angle. The crank angle signal is used as a signal representing a rotation speed NE of the internal combustion engine  1 . The accelerator position sensor  72  detects an amount of depression of an accelerator pedal of the vehicle. The amount of depression of the accelerator pedal is used as a signal representing a load of the internal combustion engine  1 . 
     The controller  50  controls the intake throttle valve  14 , the throttle valve  16 , the recirculation valve  19 , and the EGR valve  43  in addition to the internal combustion engine  1  based on the above input signals from the various sensors and switches. The controller  50  controls the internal combustion engine  1  by controlling an ignition timing and a fuel injection amount according to an engine operating state. The engine operating state is, for example, the rotation speed NE or the load. 
     Even if the EGR valve  43  is fully closed, a gap may occur due to a structure thereof. In this case, even if fuel cut of the internal combustion engine  1  is started when the EGR valve  43  is fully closed, the exhaust gas may leak from the EGR valve  43  to the intake passage  11  side through the gap. As a result, an inside of the upstream EGR passage  41   a  is replaced with fresh air from an upstream side according to the leakage of the exhaust gas during the fuel cut. 
     In this case, even if the EGR valve  43  is opened by restarting EGR after fuel injection of the internal combustion engine  1  is restarted, the fresh air remaining in the upstream EGR passage  41   a  first flows into the EGR valve  43 . As a result, the EGR gas may not reach the EGR valve  43  immediately. Such a flow mode of the gas is described in detail as follows. 
       FIGS.  2 A to  2 E  are diagrams for explaining the flow mode of the gas.  FIG.  2 A  shows a state during EGR operation. At this time, the exhaust gas is recirculated from the exhaust passage  21  to the intake passage  11 . Therefore, as shown by hatching, the exhaust gas or the mixed gas of the exhaust gas and the intake gas flows through the exhaust passage  21 , the EGR passage  41 , and a portion of the intake passage  11  downstream of the EGR convergence portion  11   a . 
       FIG.  2 B  shows a state at a start of the fuel cut. The fuel cut is started when the EGR valve  43  is fully closed. As a result, the fresh air flows through the intake passage  11  and the exhaust passage  21 . In this case, the exhaust gas is left in the EGR passage  41 . 
       FIG.  2 C  shows a state during the fuel cut. During the fuel cut, the exhaust gas leaks from the EGR valve  43  to the intake passage  11  side through the gap. As a result, the exhaust gas is replaced with the fresh air in the upstream EGR passage  41   a , and in this case, finally, the entire EGR passage  41  is filled with the fresh air. 
       FIG.  2 D  shows a state after the fuel injection is restarted and before the EGR valve  43  is opened. After the fuel injection is restarted, the fresh air flows through the intake passage  11  and the exhaust gas flows through the exhaust passage  21 . After the fuel injection is restarted and before the EGR valve  43  is opened, the fresh air leaks through the gap of the EGR valve  43 . Therefore, the exhaust gas is filled in the upstream EGR passage  41   a  by an amount of the fresh air leaking. The exhaust gas is gradually filled from an upstream portion of the upstream EGR passage  41   a . 
       FIG.  2 E  shows a state in which the upstream EGR passage  41   a  is filled with the exhaust gas after the EGR valve  43  is opened. When the EGR valve  43  is opened, the fresh air remaining in the upstream EGR passage  41   a  first flows into the EGR valve  43 , and then the exhaust gas flows into the EGR valve  43 . 
     Therefore, immediately after the EGR valve  43  is opened, the EGR rate in the EGR valve  43  remains zero. The EGR rate then begins to rise after the upstream EGR passage  41   a  is filled with the exhaust gas as shown in  FIG.  4 E . 
     In light of such a flow mode of the gas, in the present embodiment, the EGR rate (actual EGR rate) in the EGR valve  43  is estimated as described below. 
       FIG.  3    is a control block diagram showing an EGR rate estimation process performed by the controller  50 . A target EGR mass flow rate calculation unit  51  calculates a target EGR mass flow rate. The target EGR mass flow rate is a mass flow rate of the EGR gas according to a target EGR rate in the upstream EGR passage  41   a , and is set in advance according to an engine operating state. The target EGR mass flow rate indicates a flow rate of the EGR gas flowing through the upstream EGR passage  41   a  according to a valve opening state of the EGR valve  43 . 
     An EGR mass calculation unit  52  calculates an EGR mass M1. The EGR mass M1 is a mass of the EGR gas in the upstream EGR passage  41   a , and becomes an EGR mass M2 corresponding to a volume of the upstream EGR passage  41   a  when the upstream EGR passage  41   a  is filled with the exhaust gas. The EGR mass M1 is calculated as follows. 
     During the fuel cut, the exhaust gas is replaced with the fresh air in the upstream EGR passage  41   a  as described above with reference to  FIG.  2 C . In this case, the EGR mass calculation unit  52  subtracts a fresh air replacement mass (an EGR mass corresponding to the fresh air replacement) from the EGR mass M1 for each JOB period of the EGR rate estimation process, thereby calculating the EGR mass M1. That is, a new EGR mass M1 is calculated by subtracting the fresh air replacement mass from the latest value of the EGR mass M1. 
     The fresh air replacement mass is a mass of the exhaust gas newly replaced by the fresh air in the upstream EGR passage  41   a , and is set based on the target EGR mass flow rate and the gap of the EGR valve  43 . However, the EGR is stopped during the fuel cut, and the target EGR mass flow rate becomes zero. Therefore, the fresh air replacement mass can be set in advance based on the gap of the EGR valve  43  on the premise that the engine operating state is in the fuel cut. In this case, it can be understood that it is premised that the fuel is being cut based on the target EGR mass flow rate. 
     When the fuel cut is stopped and the fuel injection is restarted, the EGR mass calculation unit  52  adds an EGR filling mass to the EGR mass M1 for each JOB period of the EGR rate estimation process, thereby calculating the EGR mass M1. That is, a new EGR mass M1 is calculated by adding the EGR filling mass to the latest value of the EGR mass M1. 
     The EGR filling mass is a mass of the exhaust gas newly filled in the upstream EGR passage  41   a , and is set based on at least the target EGR mass flow rate between the target EGR mass flow rate and the gap of the EGR valve  43 . The EGR filling mass is set to different values after the fuel injection is restarted when the target EGR mass flow rate is zero and when it is not zero, that is, before and after the EGR is restarted. 
     After the fuel injection is restarted and before the EGR is restarted, the EGR filling mass is a value set in advance based on the gap of the EGR valve  43 , similarly to the fresh air replacement mass. When the EGR is restarted, the EGR filling mass is set based on the target EGR mass flow rate. The EGR filling mass will be further described later. The EGR mass M1 calculated by the EGR mass calculation unit  52  is input to a determination unit  53 . 
     The determination unit  53  determines whether the EGR mass M2 corresponding to the volume of the upstream EGR passage  41   a  is equal to or less than the EGR mass M1 (whether the EGR mass M1 is equal to or more than the EGR mass M2). The EGR mass M2 is a determination value for determining whether the upstream EGR passage  41   a  is filled with the exhaust gas, and if an affirmative determination is made, it is determined that the upstream EGR passage  41   a  is filled with the exhaust gas. In this case, a signal is input from the determination unit  53  to a selection unit  54 . 
     The selection unit  54  selects an estimated EGR rate as the EGR rate in the EGR valve  43 . As described above with reference to  FIG.  2 E , immediately after the EGR valve  43  is opened after the fuel injection is restarted, fresh air is introduced into the EGR valve  43  from the upstream EGR passage  41   a , so that the EGR rate remains zero. 
     Therefore, the selection unit  54  selects zero when no signal is input from the determination unit  53 , that is, when the upstream EGR passage  41   a  is not filled with the exhaust gas. The selected EGR rate is input to an EGR rate determination unit  55 . 
     The EGR rate determination unit  55  determines the EGR rate input from the selection unit  54  as the EGR rate in the EGR valve  43 . In this way, the estimated EGR rate is determined. The determined EGR rate is input to a change amount limiting unit  56 . 
     The change amount limiting unit  56  limits a change amount of the EGR rate. The change amount limiting unit  56  calculates a limited change amount of the EGR rate based on the target EGR mass flow rate, and the calculated limited change amount is added to the EGR rate input from the EGR rate determination unit  55 . In this way, a limited EGR rate is calculated in which a change amount from the latest estimated EGR rate is the limited change amount. The limited change amount will be further described later. 
     The change amount limiting unit  56  further selects the smaller EGR rate from the calculated limited EGR rate and the target EGR rate. As a result, the limited EGR rate is selected when the limited EGR rate is smaller than the target EGR rate, and the target EGR rate is not used. When the limited EGR rate and the target EGR rate are the same, either one may be selected. The selected EGR rate is input to the selection unit  54 . 
     As described above with reference to  FIG.  2 E , the EGR rate in the EGR valve  43  begins to rise after the upstream EGR passage  41   a  is filled with the exhaust gas. Therefore, when the signal from the determination unit  53  is input, that is, when the upstream EGR passage  41   a  is filled with the exhaust gas, the selection unit  54  does not select zero and selects the EGR rate input from the change amount limiting unit  56 . 
     As a result, when the upstream EGR passage  41   a  is filled with the exhaust gas and the limited EGR rate is lower than the target EGR rate, the selection unit  54  selects the limited EGR rate that gradually increases from zero by an amount of the limited change amount. When the limited EGR rate exceeds the target EGR rate, the target EGR rate input from the change amount limiting unit  56  is selected. 
       FIG.  4    is a flowchart showing the EGR rate estimation process performed by the controller  50 . The controller  50  includes a determination control unit and an estimation control unit by being programmed to execute the process shown in this flowchart. 
     In step S 1 , the controller  50  determines whether the fuel is being cut. Whether the fuel is being cut can be determined by whether a predetermined fuel cut execution condition is satisfied. If an affirmative determination is made in step S 1 , the process proceeds to step S 2 . 
     In step S 2 , the controller  50  performs a reduction process to the EGR mass M1 in the upstream EGR passage  41   a . The reduction process to the EGR mass M1 is a process for reducing the EGR mass M1 by the above fresh air replacement mass, and since the exhaust gas is replaced with the fresh air in the upstream EGR passage  41   a  during the fuel cut, the EGR mass M1 is reduced. After step S 2 , the process proceeds to step S 4 . 
     In step S 4 , the controller  50  determines whether the EGR mass M1 is equal to or greater than the EGR mass M2. If an affirmative determination is made in step S 4 , it is determined that the upstream EGR passage  41   a  is filled with the exhaust gas. When the process proceeds from step S 2  to step S 4 , since the upstream EGR passage  41   a  is not filled with the exhaust gas, a negative determination is made, and the process proceeds to step S 9 . 
     In step S 9 , the controller  50  makes a fresh air replacement determination. The fresh air replacement determination is a determination that the upstream EGR passage  41   a  is being replaced with the fresh air, in other words, a determination that the upstream EGR passage  41   a  is not filled with the exhaust gas. After step S 9 , the process proceeds to step S 10 . 
     In step S 10 , the controller  50  determines the EGR rate in the EGR valve  43  to zero. As a result, when the upstream EGR passage  41   a  is not filled with the exhaust gas, the EGR rate in the EGR valve  43  is estimated to be zero. After step S 10 , the process ends temporarily. 
     In a subsequent routine, the same process is repeated during the fuel cut, and the EGR rate is estimated to be zero. In this case, the EGR mass M1 is reduced in step S 2  according to an execution time of the fuel cut. Then, when the fuel injection is restarted and the fuel is not being cut, a negative determination is made in step S 1 , and the process proceeds to step S 3 . 
     In step S 3 , the controller  50  performs an increase process to the EGR mass M1 in the upstream EGR passage  41   a . The increase process to the EGR mass M1 is a process for increasing the EGR mass M1 by the above EGR filling mass. After step S 3 , the process proceeds to step S 4 . 
     In step S 4 , when the EGR mass M1 is smaller than the EGR mass M2, a negative determination is made, and the process proceeds to step S 9  and further to step S 10 . That is, even when the fuel injection is restarted, as long as the upstream EGR passage  41   a  is not filled with the exhaust gas, the fresh air remaining in the upstream EGR passage  41   a  passes through the EGR valve  43 , so that the EGR rate is estimated to be zero. After step S 10 , the process ends temporarily. 
     In the subsequent routine, the same process is repeated until the upstream EGR passage  41   a  is not filled with the exhaust gas. When the upstream EGR passage  41   a  is filled with the exhaust gas, an affirmative determination is made in step S 4 , and the process proceeds to step S 5 . 
     In step S 5 , the controller  50  makes an exhaust gas filling determination. The exhaust gas filling determination is a determination that the upstream EGR passage  41   a  is filled with the exhaust gas, and the EGR rate in the EGR valve  43  increases after the upstream EGR passage  41   a  is filled with the exhaust gas after the EGR is restarted. After step S 5 , the process proceeds to step S 6 . 
     In step S 6 , the controller  50  determines whether the limited EGR rate is higher than the target EGR rate. As described above, the limited EGR rate is obtained by adding the change limited amount to the EGR rate, and is smaller than the target EGR rate immediately after the negative determination is switched to an affirmative determination in step S 4 . Therefore, in this case, a negative determination is made in step S 6 , and the process proceeds to step S 8 . 
     In step S 8 , the controller  50  determines the EGR rate as the limited EGR rate. As a result, when the limited EGR rate is equal to or less than the target EGR rate, the EGR rate in the EGR valve  43  is estimated to be the limited EGR rate. After step S 8 , the process ends temporarily. 
     In the subsequent routine, the same process is repeated when the limited EGR rate is equal to or less than the target EGR rate. Then, when the limited EGR rate becomes higher than the target EGR rate, an affirmative determination is made in step S 6 , and the process proceeds to step S 7 . 
     In step S 7 , the controller  50  determines the EGR rate as the target EGR rate. As a result, when the limited EGR rate becomes higher than the target EGR rate, the EGR rate in the EGR valve  43  is estimated to be the target EGR rate. After step S 7 , the process ends temporarily. 
     In this flowchart, in step S 4 , step S 5 , and step S 8 , a gas replacement state by the exhaust gas and the fresh air in the upstream EGR passage  41   a  is determined. Further, in step S 7 , step S 8  and step S 10 , the EGR rate is estimated based on a result of the determination. Further, by repeatedly executing the processes of step S 1 , step S 2 , step S 4 , and step S 9 , it is determined whether fresh air replacement is being executed according to the execution time of the fuel cut. When a negative determination in made in step S 1  and then the process proceeds to step S 10 , the EGR rate after the EGR is restarted is estimated to be zero. 
       FIG.  5    is a diagram showing an example of a timing chart corresponding to the flowchart shown in  FIG.  4   . An EGR rate shown by a broken line indicates the target EGR rate. The EGR is stopped at a timing T1. As a result, the EGR valve  43  is fully closed from the timing T1. 
     At a timing T2, the fuel cut is started. As a result, from the timing T2, the inside of the upstream EGR passage  41   a  begins to be replaced with the fresh air due to the leakage of the exhaust gas from the gap of the EGR valve  43 . In other words, the upstream EGR passage  41   a  is being replaced with the fresh air. From the timing T2, the EGR mass M1 begins to decrease and becomes smaller than the EGR mass M2. That is, the inside of the upstream EGR passage  41   a  is not filled with the exhaust gas due to inflow of the fresh air. 
     The EGR mass M1 that starts to decrease from the timing T2 decreases with the same inclination until a timing T3 when the fuel injection is restarted. The fresh air replacement mass is set in advance based on the gap of the EGR valve  43  as described above. Therefore, a magnitude of the inclination of the change in the EGR mass M1 between the timing T2 and the timing T3 indicates the fresh air replacement mass. 
     At the timing T3, the fuel cut is stopped and the fuel injection is restarted. Therefore, from the timing T3, the exhaust gas starts to be filled in the upstream EGR passage  41   a  due to the leakage of the fresh air from the gap of the EGR valve  43 . As a result, the EGR mass M1 begins to increase from the timing T3. 
     The EGR mass M1 that starts to increase from the timing T3 increases with the same inclination until a timing T4 when the EGR is restarted. After the fuel injection is restarted, the EGR filling mass before the EGR is restarted is set in advance based on the gap of the EGR valve  43  as described above. Therefore, a magnitude of the inclination of the change in the EGR mass M1 between the timing T3 and the timing T4 indicates such an EGR filling mass. 
     The EGR is restarted at the timing T4. As a result, the EGR valve  43  is opened, and the EGR mass M1 begins to increase in a larger degree than that before the EGR valve  43  is opened. The EGR mass M1 that starts to increase from the timing T4 increases with the same inclination until a timing T5 where the exhaust gas filling determination is made. 
     The EGR filling mass after the EGR is restarted is set based on the above target EGR mass flow rate, which therefore is a flow rate of the EGR gas flowing through the upstream EGR passage  41   a . A magnitude of the inclination of the change in the EGR mass M1 between the timing T4 and the timing T5 indicates such an EGR filling mass. 
     A period between the timing T4 and the timing T5 indicates a period until the EGR rate starts to increase after the EGR is restarted. The period is set to be shorter when the flow rate of the EGR gas flowing through the upstream EGR passage  41   a  is large than when the flow rate is small. This is because the larger the flow rate of the EGR gas after the EGR is restarted, the faster the upstream EGR passage  41   a  is filled with the exhaust gas. 
     Such a period can be set by setting the EGR filling mass after the EGR is restarted based on the target EGR mass flow rate, which therefore is the flow rate of the EGR gas flowing through the upstream EGR passage  41   a . The EGR filling mass after the EGR is restarted can be set in advance using map data according to the flow rate of the EGR gas flowing through the upstream EGR passage  41   a . In the flowchart shown in  FIG.  4    described above, the process of step S 3  corresponds to the setting of such a period. 
     Since the EGR mass M1 is smaller than the EGR mass M2 from the timing T2 to the timing T5, the EGR rate is set to zero, and the EGR mass M1 becomes the EGR mass M2 at the timing T5. As a result, the exhaust gas filling determination of the upstream EGR passage  41   a  is made. 
     As described above, the EGR mass M1 is set based on the target EGR mass flow rate. Therefore, the above exhaust gas filling determination is made based on the flow rate of the EGR gas flowing through the upstream EGR passage  41   a . In the flowchart shown in  FIG.  4    described above, the process of step S 4  following step S 3  and the process of step S 5  correspond to making such an exhaust gas filling determination. 
     From the timing T5, the EGR rate is set as the limited EGR rate according to the exhaust gas filling determination, and the EGR rate starts to increase. The EGR rate gradually increases from the timing T5 by the limited change amount. Further, the EGR rate increases with the same inclination until a timing T6 when the target EGR rate is reached. Therefore, a magnitude of the inclination of the change in the EGR rate between the timing T5 and the timing T6 indicates the limited change amount. 
     A degree of increase in the EGR rate from the timing T5 after the EGR is restarted is set to be larger when the flow rate of the EGR gas is large than that when the flow rate is small. This is because the exhaust gas is actually diluted with the fresh air in the process of filling the upstream EGR passage  41   a  with the exhaust gas from the timing T4 when the EGR is restarted, and because the larger the flow rate of the EGR gas, the shorter a dilution time, and the more difficult it is for the exhaust gas to be diluted. 
     Such a degree of increase can be set by setting the limited change amount based on the target EGR mass flow rate, which therefore is the flow rate of the EGR gas flowing through the upstream EGR passage  41   a . The limited change amount can be set in advance using map data according to the flow rate of the EGR gas flowing through the upstream EGR passage  41   a . 
     At the timing T6, the EGR rate reaches the target EGR rate. Therefore, the EGR rate is estimated to be the target EGR rate from the timing T6. 
     Next, main functions and effects of the present embodiment will be described. 
     In the EGR estimation method for the internal combustion engine  1  according to the present embodiment, the EGR rate in the intake and exhaust system  10 ,  20  of the internal combustion engine  1  is estimated. The intake and exhaust system  10 ,  20  of the internal combustion engine  1  includes the intake system  10 , the exhaust system  20 , and the EGR device  40  including the EGR passage  41  and the EGR valve  43 . The EGR estimation method for the internal combustion engine  1  includes determining the gas replacement state by the exhaust gas and the fresh air in the upstream EGR passage  41   a  when the fuel cut of the internal combustion engine  1  is started and the EGR valve  43  is fully closed, and estimating the EGR rate based on a result of the determination. 
     According to such a method, the gas replacement state in the upstream EGR passage  41   a  can be reflected in estimating the EGR rate. Therefore, when the fuel cut is started and the EGR valve  43  is fully closed, even when the exhaust gas inside of the upstream EGR passage  41   a  is replaced with the fresh air, a more accurate EGR rate can be estimated. 
     In the present embodiment, whether the fresh air replacement is being executed is determined according to the execution time of the fuel cut, that is, whether the upstream EGR passage  41   a  is not filled with the exhaust gas due to the inflow of the fresh air is determined. 
     According to such a method, the gas replacement state in the upstream EGR passage  41   a  where the exhaust gas is replaced with the fresh air by leaking the exhaust gas through the gap of the EGR valve  43  during the fuel cut can be appropriately determined. Therefore, it is possible to estimate the EGR rate more accurately. 
     In the present embodiment, when it is determined that the inside of the upstream EGR passage  41   a  is not filled with the exhaust gas due to the inflow of the fresh air, the EGR rate after the EGR is restarted is estimated to be zero. 
     According to such a method, the EGR rate is estimated to be zero in light of that the fresh air remaining in the upstream EGR passage  41   a  first flows into the EGR valve  43  after the EGR is restarted, so that the EGR rate can be estimated more accurately. 
     In the present embodiment, after the fuel injection of the internal combustion engine  1  is restarted, it is determined whether the inside of the upstream EGR passage  41   a  is replaced with the exhaust gas based on the flow rate of the EGR gas flowing through the upstream EGR passage  41   a . 
     According to such a method, it is possible to appropriately determine whether the inside of the upstream EGR passage  41   a  is replaced with the exhaust gas, so that the EGR rate after the fuel injection is restarted can be appropriately estimated. 
     In the present embodiment, the period until the start of increase in the EGR rate after the EGR is restarted is shorter when the flow rate of the EGR gas flowing through the upstream EGR passage  41   a  is large than that when the flow rate is small. 
     According to such a method, the EGR rate can start to increase at an appropriate timing after the EGR is restarted in light of that the larger the flow rate of the EGR gas after the EGR is restarted, the earlier the upstream EGR passage  41   a  is filled with the exhaust gas. 
     In the present embodiment, the degree of increase in the EGR rate after the EGR is restarted is larger when the flow rate of the EGR gas flowing through the upstream EGR passage  41   a  is large than that when the flow rate is small. 
     According to such a method, the EGR rate can be increased in an appropriate degree after the EGR is restarted in light of that the larger the flow rate of the EGR gas after the EGR is restarted, the more difficult it is for the exhaust gas to be diluted. 
     Although the embodiment of the present invention has been described above, the above-mentioned embodiment is merely a part of application examples of the present invention, and does not mean that the technical scope of the present invention is limited to the specific configurations of the above-mentioned embodiment. 
     For example, in the above embodiment, the case of estimating the EGR rate in the EGR valve  43  is described. However, the estimated EGR rate may be an EGR rate at a predetermined position from the downstream EGR passage  41   b  to the internal combustion engine  1 , such as the EGR rate of the gas flowing into the cylinder of the internal combustion engine  1 . Such an EGR rate can be estimated, for example, by correcting the timing at which the EGR gas arrives and the EGR rate according to the distance from the EGR valve  43 , the inflow, and the like. 
     For example, in the above embodiment, the EGR estimation method for the internal combustion engine  1  and the case where the EGR estimation device for the internal combustion engine  1  is implemented by the controller  50  is described. However, the EGR estimation method for the internal combustion engine  1  and the EGR estimation device for the internal combustion engine  1  may be implemented by a plurality of controllers instead of a single controller  50 .