WO2008068574A1

WO2008068574A1 - Egr system for internal combustion engine and method for controlling the same

Info

Publication number: WO2008068574A1
Application number: PCT/IB2007/003696
Authority: WO
Inventors: Takeshi Hashizume; Hiroyuki Haga; Isao Matsumoto; Masahiro Nagae; Tomoyuki Ono; Hiroki Murata; Akira Yamashita; Teruhiko Miyake; Hajime Shimizu
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2006-12-01
Filing date: 2007-11-30
Publication date: 2008-06-12
Also published as: JP2008138598A

Abstract

An EGR system for an internal combustion engine includes a high-pressure EGR unit and a low-pressure EGR unit. With the EGR system, the ratio of the amount of exhaust gas recirculated back to the internal combustion engine by the high-pressure EGR unit to the entire amount of exhaust gas recirculated back to the internal combustion engine is made higher when the temperature of a coolant in the internal combustion engine is lower than the reference coolant temperature than when the temperature of the coolant is equal to or higher than the reference coolant temperature.

Description

EGR SYSTEM FOR

INTERNAL COMBUSTION ENGINE AND

METHOD FOR CONTROLLING THE SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to an EGR (exhaust gas recirculation) system for an internal combustion engine, and a method for controlling the same.

2. Description of the Related Art [0002] EGR, i.e., a technology for recirculating a portion of the exhaust gas back to an internal combustion engine has been suggested in order to reduce the amount of nitrogen oxide (NOx) generated in the process of burning fuel in the internal combustion engine.

[0003] A technology that makes it possible to perform EGR in a broader operating range of an internal combustion engine has been suggested. According to this technology, a high-pressure EGR unit and a low-pressure EGR unit are provided, and EGR is performed while the EGR unit used for EGR is changed between the high-pressure EGR unit and the low-pressure EGR unit or both the high-pressure EGR unit and the low-pressure EGR unit are used in combination, based on the operation mode of the internal combustion engine. The high-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between an exhaust passage, at a portion upstream of a turbine of a turbocharger, and an intake passage, at a portion downstream of a compressor of the turbocharger. The low-pressure EGR unit recirculates a portion of the exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor.

[0004] For example, a technology for preventing the intake air temperature from becoming excessively high when EGR is performed is described in Japanese Patent Application Publication No. 07-233761 (JP-A-07-233761). According to this technology, when the exhaust gas temperature is low, for example, when an internal combustion engine is operating at low load, EGR is performed using a high-pressure EGR unit. On the other hand, when the exhaust gas temperature is high, for example, when the internal combustion engine is operating at high load, EGR is performed using a low-pressure EGR unit. In addition, technologies related to EGR are described in Japanese Patent Application Publication No. 2004-162674 (JP-A-2004- 162674) and Japanese Patent Application Publication No. 2004-197634 (JP-A-2004-197634).

[0005] A high-pressure EGR passage is relatively short. Accordingly, the temperature of the exhaust gas that is recirculated back to an internal combustion engine by a high-pressure EGR unit (hereinafter, referred to as the "high-pressure EGR gas") tends to be relatively high. On the other hand, a low-pressure EGR passage is relatively long, and the exhaust gas that is recirculated back to the internal combustion engine by a low-pressure EGR unit (hereinafter, referred to as the "low-pressure EGR gas") is cooled by an EGR cooler provided in the low-pressure EGR passage and an intercooler provided in an intake passage. Accordingly, the temperature of the low-pressure EGR gas tends to be relatively low.

[0006] Recirculating the low-pressure EGR gas having a low temperature back to the internal combustion engine enhances the fuel efficiency and the effect of reducing the amount of NOx. However, when the intake air temperature is likely to decrease, for example, when the temperature of the air around the internal combustion engine is low (for example, in winter) or when the temperature of the internal combustion engine itself is low (for example, immediately aftei the internal combustion engine is started), if the low-pressure EGR gas having a low-temperature is recirculated back to the internal combustion engine, the intake air temperature may abruptly decrease, resulting in incomplete combustion, for example, a misfire. SUMMARY OF THE INVENTION

[0007] The invention provides a technology for suppressing unstable combustion and an increase in the amount of toxic substance in the exhaust gas that may be caused especially when the ambient air temperature or the temperature of a coolant is low, in an internal combustion engine including an EGR system that performs EGR using a low-pressure EGR unit and a high-pressure EGR unit in combination.

[0008] A first aspect of the invention relates to an EGR system for an internal combustion engine including: a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor, a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor; an EGR control unit that performs EGR while changing the EGR unit used for EGR between the high-pressure EGR unit and the low-pressure EGR unit, or using the high-pressure EGR unit and the low-pressure EGR unit in combination, based on the operation mode of the internal combustion engine; and a determination unit that determines whether there is a possibility that incomplete combustion will take place in the internal combustion engine. In a case where the operation mode of the internal combustion engine is in a predetermined operating range in which EGR is performed using at least the low-pressure EGR unit, the EGR control unit makes the ratio of the amount of exhaust gas, which is recirculated back to the internal combustion engine by the high-pressure EGR unit, to the entire amount of exhaust gas, which is recirculated back to the internal combustion engine, higher when the determination unit determines that there is a possibility that incomplete combustion will take place than when the determination unit does not determine that there is a possibility that incomplete combustion will take place.

[0009] In this case, the "predetermined operating range in which EGR is performed using at least the low-pressure EGR unit" is the operating range that includes the operating range in which EGR is performed using only the low-pressure EGR unit (hereinafter, referred to as the "range LPL'^*) and the operating range in which EGR is performed using the low-pressure EGR unit and the high-pressure EGR unit in combination (hereinafter, referred to as the "range MIX^"). The "predetermined operating range in which EGR is performed using at least the low-pressure EGR unit" is defined in advance.

[0010] The determination unit may determine that "there is a possibility that incomplete combustion will take place in the internal combustion engine", when the intake air temperature is likely to decrease, for example, when the temperature of the air around the internal combustion engine is low (for example, in winter) or when the temperature of the internal combustion engine itself is low (for example, immediately after the internal combustion engine is started).

[0011] With the configuration described above, when the operation mode of the internal combustion engine is within the range LPL, if the determination unit determines that there is a possibility that incomplete combustion will take place, the ratio of the high-pressure EGR gas amount to the entire EGR gas amount is changed from 0% to a value higher than 0% and EGR is performed with this ratio. In other words, a part or the entirety of the operating range, which is defined as the operating range that belongs to the range LPL when the determination unit does not determine that there is a possibility that incomplete combustion will take place, is redefined as the operating range that temporarily belongs to the range MIX, and EGR is performed in this state.

[0012] In this case, the ratio of the high-pressure EGR gas amount to the entire EGR gas amount may be changed from 0% to 100%. In other words, a part or the entirety of the operating range, which is defined as the operating range that belongs to the range LPL when the determination unit does not determine that there is a possibility that incomplete combustion will take place, may be redefined as the operating range that temporarily belongs to the range HPL, and EGR may be performed in this state. In this case, the range ''HPL" indicates the operating range in which EGR is performed using only the high-pressure EGR unit. [0013] When the operation mode of the internal combustion engine is within the range MIX, if the determination unit determines that there is a possibility that incomplete combustion will take place, the ratio of the high-pressure EGR gas amount to the entire EGR gas amount is made higher than that when the determination unit does not determine that there is a possibility that incomplete combustion will take place. [0014] In this case, the ratio of the high-pressure EGR gas amount to the entire EGR gas amount may be changed from a predetermined value, which is used when the determination unit does not determine that there is a possibility that incomplete combustion will take place, to 100%. In other words, a part or the entirety of the operating range, which is defined as the operating range that belongs to the range MIX when the determination unit does not determine that there is a possibility that incomplete combustion will take place, may be redefined as the operating range that temporarily belongs to the range HPL, and EGR may be performed in this state.

[0015] According to the first aspect of the invention described above, when the intake air temperature is likely to decrease, for example, when the ambient air temperature is low or when the coolant temperature is low, a larger amount of high-pressure EGR gas having a high temperature flows into the internal combustion engine than when the determination unit does not determine that there is a possibility that incomplete combustion will take place. Therefore, an excessive decrease in the intake air is suppressed. As a result, it is possible to suppress occurrence of incomplete combustion, for example, a misfire, and generation of a large amount of unbumed fuel components, even when the ambient air temperature is low or when the coolant temperature is low.

[0016] In the first aspect of the invention, when the operation mode of the internal combustion engine is within a predetermined high load-side range in the predetermined operating range in which EGR is performed using at least the low-pressure EGR unit, even if the determination unit determines that there is a possibility that incomplete combustion will take place, the ratio of the high-pressure EGR gas amount to the entire EGR gas amount may remain unchanged, and EGR may be performed with the ratio used when the determination unit does not determine that there is a possibility that incomplete combustion will take place.

[0017] In this case, the '"predetermined high load-side range" is the operating range in which there is a possibility that fuel efficiency will be reduced or the amount of toxic substance in the exhaust gas will increase if the ratio of the high-pressure EGR gas amount to the entire EGR gas amount is made high and EGR is performed with this ratio.

[0018] In the first aspect of the invention, the ambient air temperature may be used as the parameter that indicates the temperature of the ambient environmental temperature of the internal combustion engine. Also, the temperature of the coolant in the internal combustion engine may be used as the parameter that indicates the temperature of the internal combustion engine itself. Namely, the determination unit may determine whether there is a possibility that incomplete combustion will take place based on the ambient air temperature or the coolant temperature.

[0019] For example, when the ambient air temperature is lower than the predetermined reference ambient air temperature or when the coolant temperature is lower than the reference coolant temperature, the determination unit may determine that there is a possibility that incomplete combustion will take place. In this case, the predetermined reference ambient air temperature is the lower limit value of the ambient air temperature range in which the intake air temperature does not excessively decrease even when EGR control, which is executed when the determination unit does not determine that there is a possibility that incomplete combustion will take place, is executed. Also, the predetermined reference coolant temperature is the lower limit value of the coolant temperature range in which the intake air temperature does not excessively decrease even when EGR control, which is executed when the determination unit does not determine that there is a possibility that incomplete combustion will take place, is executed.

[0020] A second aspect of the invention relates to an EGR system for an internal combustion engine including: a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor; a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor; an EGR control unit that performs EGR while changing the EGR unit used for EGR between the high-pressure EGR unit and the low-pressure EGR unit, or using the high-pressure EGR unit and the low-pressure EGR unit in combination, based on the operation mode of the internal combustion engine; and a coolant temperature detection unit that detects the temperature of a coolant in the internal combustion engine. The EGR control unit changes the ratio of the amount of exhaust gas, which is recirculated back to the internal combustion engine by the low-pressure EGR unit, to the entire amount of exhaust gas, which is recirculated back to the internal combustion engine, based on the temperature of the coolant detected by the coolant temperature detection unit.

[0021] The temperature of the high-pressure EGR gas is relatively high, and the temperature of the low-pressure EGR gas is relatively low. Accordingly, as the ratio of the low-pressure EGR gas amount to the entire EGR gas amount increases (as the ratio of the high-pressure EGR gas amount to the entire EGR gas amount decreases), the intake air temperature achieved when EGR is performed decreases. Conversely, as the ratio of the low-pressure EGR gas amount to the entire EGR gas amount decreases (as the ratio of the high-pressure EGR gas amount to the entire EGR gas amount increases), the intake air temperature achieved when EGR is performed increases. [0022] With the configuration described above, the ratio of the low-pressure EGR gas amount to the entire EGR gas amount is changed based on the temperature of the coolant in the internal combustion engine. Accordingly, the intake air temperature achieved when EGR is performed is adjusted to the optimum temperature independently of the temperature of the internal combustion engine. In addition, it is possible to enhance the fuel efficiency characteristics and the property of the exhaust gas.

[0023] For example, when the coolant temperature is low, the temperature of the internal combustion engine is low. If the low-pressure EGR gas having a low temperature is recirculated back to the internal combustion engine in this state, the intake air temperature may excessively decrease. Therefore, the ratio of the low-pressure EGR gas amount to the entire EGR gas amount is decreased to cause the high-pressure EGR gas to make up most or the entirety of the EGR gas. In this way, it is possible to suppress a decrease in the intake air temperature that may be caused when EGR is performed. It is also possible to suppress occurrence of inconveniences such as incomplete combustion, for example, a misfire and discharge of a large amount of unburned hydrocarbon.

[0024] On the other hand, when the coolant temperature is high, the intake air temperature does not excessively decrease even if the low-pressure EGR gas is recirculated back to the internal combustion engine. Accordingly, it is preferably to make the ratio of the low-pressure EGR gas amount to the entire EGR gas amount high and perform EGR using both the high-pressure EGR unit and the low-pressure EGR unit. In this way, the efficiency of supercharging performed by the turbocharger is enhanced because EGR is performed using the low-pressure EGR unit. In addition, the pumping loss is reduced as compared with the case where the entire EGR gas is recirculated back to the internal combustion engine by the high-pressure EGR unit. As a result, it is possible to enhance the fuel efficiency characteristics due to performance of EGR.

[0025] As described above, when the ratio of the low-pressure EGR gas amount to the entire EGR gas amount is decreased (the ratio of the high-pressure EGR gas amount to the entire EGR gas amount is increased) as the coolant temperature decreases, it is possible to achieve the optimum combustion state, fuel efficiency characteristics, and the exhaust gas property based on the temperature of the internal combustion engine. The ratio of the low-pressure EGR gas amount to the entire EGR gas amount may be changed continuously or in a stepwise manner based on the coolant temperature. The case in δ which the determination unit determines, based on the coolant temperature, whether there is a possibility that incomplete combustion will take place in the internal combustion engine in the first aspect of the invention may correspond to the case in which the ratio of the low-pressure EGR gas amount to the entire EGR gas amount is changed in two stages based on the coolant temperature in the second aspect of the invention.

10 [0026] The temperature of the internal combustion engine may be estimated based on the temperature of the wall face of the intake passage, the temperature of the engine oil, or the like, instead of the coolant temperature.

[0027] A third aspect of the invention relates to a method for controlling an EGR system for an internal combustion engine. The EGR system includes: a turbocharger lδ that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage,

20 at a portion downstream of the compressor, and a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor. According to the method, EGR is performed while changing the EGR unit

25 used for EGR between the high-pressure EGR unit and the low-pressure EGR unit, or using the high-pressure EGR unit and the low-pressure EGR unit in combination, based on the operation mode of the internal combustion engine. It is determined whether there is a possibility that incomplete combusu^'on will take place in the internal combustion engine. Then, the ratio of the amount of exhaust gas, which is recirculated back to the internal combustion engine by the high-pressure EGR unit, to the entire amount of exhaust gas, which is recirculated back to the internal combustion engine, is made higher when it is determined that there is a possibility that incomplete combustion will take place than when it is not determined that there is a possibility that incomplete combustion will take place, in a case where the operation mode of the internal combustion engine is in a predetermined operating range in which EGR is performed using at least the low-pressure EGR unit.

[0028] The configurations described above may be appropriately combined with each other. [0029] The aspects of the invention described above make it possible to perform

EGR while suppressing occurrence of incomplete combustion and an increase in the toxic substance in the exhaust gas, even when the ambient air temperature is low or when the coolant temperature is low, in the internal combustion engine provided with the EGR system that performs EGR using the low-pressure EGR unit and the high-pressure EGR unit in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of an example embodiment with reference to the accompanying drawings, wherein the same or corresponding portions will be denoted by the same reference numerals and wherein:

FIG 1 is a view schematically showing the structure of an intake system, an exhaust system and a control system of an internal combustion engine provided with an EGR system according to an embodiment of the invention; FIG 2 illustrates an example of a reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range HPL, the range MIX and the range LPL;

FIG 3 illustrates another example of a reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range HPL and the range LPL;

FIG. 4 illustrates yet another example of a reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range MIX and the range LPL;

FIG. 5 illustrates an example of correction made, at low coolant temperature times, to the reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range HPL, the range MIX and the range LPL;

FIG 6 illustrates an example of correction made, at low coolant temperature times, to the reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range HPL and the range LPL; FIG 7 illustrates an example of correction made, at low coolant temperature times, to the reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range HPL and the range LPL;

FIG 8 illustrates an example of correction made, at low coolant temperature times, to the reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range MIX and the range LPL;

FIG. 9 illustrates an example of correction made, at low coolant temperature times, to the reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range MIX and the range LPL;

FlG 10 illustrates an example of correction made, at low temperature times, to the reference EGR control map which is used in the EGR system according to the embodiment of the invention, and in which the operating range of the internal combustion engine is divided into the range MIX and the range LPL;

FIG. 11 is a flowchart showing the EGR control map correction control routine executed by the EGR system according to the embodiment of the invention; and

FIG. 12 is a graph showing the relationship between the coolant temperature and the ratio of the low-pressure EGR gas amount to the entire EGR gas amount, when the ratio of the low-pressure EGR gas amount to the entire EGR gas amount is changed based on the coolant temperature in the EGR system according to the embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENT [0031] Hereafter, an example embodiment of the invention will be described in detail with reference to the accompanying drawings. Unless otherwise noted, the sizes, materials, shapes, relative arrangements, etc. of the components described in the embodiment do not limit the technical scope of the invention.

[0032] FIG 1 is a view schematically showing an intake system, an exhaust system, and a control system of an internal combustion engine provided with an EGR system for an internal combustion engine according to the embodiment of the invention. An internal combustion engine 1 shown in FIG. 1 is a water-cooled four-cycle diesel engine having four cylinders 2.

[0033] An intake manifold 17 is connected to the cylinders 2 of the internal combustion engine 1 via an intake port (not shown). An intake pipe 3 is connected to the intake manifold 17. A second intake throttle valve 9, which regulates the flow rate of the intake air flowing through the intake pipe 3, is provided in the intake pipe 3 at a position upstream of the intake manifold 17. An intercooler 8 that cools the intake air is provided in the intake pipe 3, at a position upstream of the second intake throttle valve 9. A compressor 11 of a turbocharger 13 that operates using the energy of the exhaust gas as a driving source is provided in the intake pipe 3, at a position upstream of the intercooler 8. A first intake throttle valve 6, which regulates the flow rate of the intake air flowing through the intake pipe 3, is provided in the intake pipe 3, at a position upstream of the compressor 11. [0034] An exhaust manifold 18 is connected to the cylinders 2 of the internal combustion engine 1 via an exhaust port (not shown). An exhaust pipe 4 is connected to the exhaust manifold 18. A turbine 12 of the turbocharger 13 is provided in the exhaust pipe 4. The turbocharger 13 is a variable capacity turbocharger provided with a nozzle vane 5 that is able to change the flow characteristics of the exhaust gas that flows into the turbine 12. An exhaust gas control apparatus 10 is provided in the exhaust pipe 4, at a position downstream of the turbine 12. The exhaust gas control apparatus 10 includes a particulate filter (hereinafter, referred to as a "filter") that traps particulate matter in the exhaust gas, and a storage reduction NOx catalyst. The storage reduction NOx catalyst is supported on the filter. When the exhaust gas is in the oxidant atmosphere, the storage reduction NOx catalyst stores NOx in the exhaust gas. When the exhaust gas is in the reduction atmosphere, the storage reduction NOx catalyst reduces NOx in the exhaust gas to remove it. An exhaust throttle valve 19, which regulates the flow rate of the exhaust gas flowing through the exhaust pipe 4, is provided in the exhaust pipe 4, at a position downstream of the exhaust gas control apparatus 10. The exhaust gas throttle valve 19 may be provided in the exhaust pipe 4, at a position downstream of the connection portion at which a low-pressure EGR passage 31, which will be described later in detail, is connected to the exhaust pipe 4.

[0035] The internal combustion engine 1 is provided with a high-pressure EGR unit 40 that introduces a portion of the exhaust gas flowing through the exhaust pipe 4 to the intake pipe 3, at high pressure, to recirculate it back to the cylinders 2. The high-pressure EGR unit 40 includes a high-pressure EGR passage 41, and a high-pressure EGR valve 42. The high-pressure EGR passage 41 provides communication between the exhaust pipe 4, at a portion upstream of the turbine 12, and the intake pipe 3, at a portion downstream of the second intake throttle valve 9. The exhaust gas is introduced to the intake pipe 3 through the high-pressure EGR passage 41. In the following description concerning the embodiment of the invention, the exhaust gas that is recirculated back to the cylinders 2 through the high-pressure EGR passage 41 will be referred to as the "high-pressure EGR gas". [0036] The high-pressure EGR valve 42 is a flow-rate regulating valve that regulates the flow rate of the exhaust gas flowing through the high-pressure EGR passage 41. The flow rate of the high-pressure EGR gas is regulated by adjusting the opening amount of high-pressure EGR valve 42. The flow rate of the high-pressure EGR gas may be regulated in a method in which the pressure difference between the upstream side and the downstream side of the high-pressure EGR passage 41 is changed by adjusting the opening amount of the second intake throttle valve 9. The flow rate of the high-pressure EGR gas may be regulated by adjusting the opening amount of the nozzle vane 5.

[0037] The internal combustion engine 1 is provided with a low-pressure EGR unit 30 that introduces a portion of the exhaust gas, flowing through the exhaust pipe 4, to the intake pipe 3, at low pressure, to recirculate it back to the cylinders 2. The low-pressure EGR unit 30 includes the low-pressure EGR passage 31, a low-pressure EGR valve 32 and a low-pressure EGR cooler 33. The low-pressure EGR passage 31 provides communication between the exhaust pipe 4, at a portion downstream of the exhaust throttle valve 19, and the intake pipe 3, at a portion upstream of the compressor 11 and downstream of the first intake throttle valve 6. A portion of the exhaust gas is introduced to the intake pipe 3 through the low-pressure EGR passage 31. In the following description concerning the embodiment of the invention, the exhaust gas that is recirculated back to the cylinders 2 through the low-pressure EGR passage 31 will be referred to as the "low-pressure EGR gas".

[0038] The low-pressure EGR valve 32 is a flow-rate regulating valve that regulates the flow rate of the exhaust gas flowing through the low-pressure EGR passage 31. The flow rate of the low-pressure EGR gas is regulated by adjusting the opening amount of the low-pressure EGR valve 32. The flow rate of the low-pressure EGR gas may be regulated in a method in which the pressure difference between the upstream side and the downstream side of the low-pressure EGR passage 31 is changed by adjusting the opening amount of the first intake throttle valve 6. The low-pressure EGR cooler 33 cools the low-pressure EGR gas that flows through the low-pressure EGR passage 31.

[0039] The internal combustion engine 1 is provided with an electronic control unit (ECU) 20 that controls the internal combustion engine 1. The ECU 20 is a microcomputer that has a known structure in which read only memory (ROM), random access memory (RAM), a central processing unit (CPU), an input port, an output port, a digital-analog converter (DA converter), an analog-digital converter (AD converter), etc. are connected to each other via a bi-directional bus.

[0040] The ECU 20 executes various known basic controls for a diesel engine, such as the fuel injection control, based on the operation mode of the internal combustion engine 1 and an instruction from a driver. Therefore, the internal combustion engine 1 in the embodiment of the invention is provided with an airflow meter 7 that detects the flow rate of the newly-taken air flowing through the intake pipe 3, a coolant temperature sensor 14 that detects the temperature of the coolant in the internal combustion engine 1, an accelerator angle sensor 15 that detects the amount by which an accelerator pedal (not shown) is depressed by the driver (accelerator angle), a crank position sensor 16 that detects the rotational phase (crank angle) of a crankshaft (not shown) of the internal combustion engine 1, and various sensors (not shown) that are usually provided to a diesel engine.

[0041] These sensors are connected to the ECU 20 via electric wiring, and signals output from these sensors are transmitted to the ECU 20. Devices such as drive units that drive the first intake throttle valve 6, the second intake throttle valve 9, the exhaust throttle valve 19, the low-pressure EGR valve 32, and the high-pressure EGR valve 42 are connected to the ECU 20 via electric wiring. These devices are controlled according to control signals transmitted from the ECU 20.

[0042] The ECU 20 determines the operation mode of the internal combustion engine 1 and the instruction from the driver based on the values detected by these sensors. For example, the ECU 20 detects the operation mode of the internal combustion engine 1 based on the engine speed, which is determined based on the crank angle indicated by a signal from the crank position sensor 16, and the engine load, which is determined based on the accelerator angle indicated by a signal from the accelerator angle sensor 15. Then, the ECU 20 controls the low-pressure EGR valve 32, the high-pressure EGR valve 42, etc. based on the detected engine operation mode and instruction from the driver, thereby controlling the EGR gas amount, the intake air amount, etc.

[0043] Next, the EGR control executed by the ECU 20 will be described. [0044] FIG 2 is an EGR control map showing the manner in which the EGR unit used for EGR is selected from among the high-pressure EGR unit 40 and the low-pressure EGR unit 30, and which is set for each operating range of the internal combustion engine 1. In FIG. 2, the abscissa axis of the graph represents the rotational speed of the internal combustion engine 1, and the ordinate axis of the graph represents the load placed on the internal combustion engine 1. [0045] As shown in FIG 2, when the internal combustion engine 1 is operating at low load, EGR is performed using only the high-pressure EGR unit 40. This operating range in which only the high-pressure EGR unit 40 is used will be referred to as the '"range HPL". When the internal combustion engine 1 is operating at medium load, EGR is performed using the high-pressure EGR unit 40 and the low-pressure EGR unit 30 in combination. This operating range in which the high-pressure EGR unit 40 and the low-pressure EGR unit 30 are used in combination will be referred to as the "range MIX". When the internal combustion engine 1 is operating at high load, EGR is performed using only the low-pressure EGR unit 30. This operating range in which only the low-pressure EGR unit 30 is used will be referred to as the "range LPL". [0046] The EGR control map in FIG 2 shows the case where the operating range of the internal combustion engine is divided into three operating ranges, namely, the range HPL, the range MIX, and the range LPL. However, the operating range of the internal combustion engine may be divided in another manner. For example, as shown in an EGR control map in FIG. 3, the operating range in which the internal combustion engine 1 is operating at low load may be defined as the range HPL, and the other operating range in which the internal combustion engine 1 is operating at higher load may be defined as the range LPL. Alternatively, as shown in an EGR control map in FIG. 4, the operating range in which the internal combustion engine 1 is operating at low load may be defined as the range MIX, and the other operating range in which the internal combustion engine 1 is operating at higher load may be defined as the range LPL.

[0047] The concrete operation modes of the internal combustion engine 1 , which define the range LPL, the range MIX, and the range HPL, and the target values of the various parameters related to the EGR control such as the high-pressure EGR gas amount, the low-pressure EGR gas amount, and the ratio of each of the high-pressure EGR gas amount and the low-pressure EGR gas amount to the entire amount of the exhaust gas that is recirculated back to the internal combustion engine 1 by the EGR system in each operating range are determined in advance through, for example, experiments. The above-mentioned concrete operation modes and target values are determined in a manner such that the EGR rate of the intake air matches the target EGR rate in each operation mode and the combustion characteristics in the internal combustion engine 1, the property of the exhaust gas, the fuel efficiency characteristics due to performance of EGR, etc. satisfy predetermined requirements. The target value of the thus determined high-pressure EGR gas amount will be referred to as the "reference high-pressure EGR gas amount". The target value of the thus determined low-pressure EGR gas amount will be referred to as the "reference low-pressure EGR gas amount". The ratio of the reference high-pressure EGR gas amount to the sum of the reference high-pressure EGR gas amount and the reference low-pressure EGR gas amount will be referred to as the "reference high-pressure EGR gas ratio". The ratio of the reference low-pressure EGR gas amount to the sum of the reference high-pressure EGR gas amount and the reference low-pressure EGR gas amount will be referred to as the "reference low-pressure EGR gas ratio". The EGR control map that is defined by the reference high-pressure EGR gas amount and the reference low-pressure EGR gas amount for each operation mode will be referred to as the "reference EGR control map". [0048] Then, the opening amount of the low-pressure EGR valve 32, at which the low-pressure EGR gas amount matches the reference low-pressure EGR gas amount when the internal combustion engine 1 performs the steady operation, is determined and used as the reference low-pressure EGR valve opening amount. The opening amount of the high-pressure EGR valve 42, at which the high-pressure EGR gas amount matches the reference high-pressure EGR gas amount when the internal combustion engine 1 performs the steady operation, is determined and used as the reference high-pressure EGR valve opening amount. The reference low-pressure EGR valve opening amount and the reference high-pressure EGR valve opening amount are stored in the ROM of the ECU 20.

[0049] The ECU 20 reads the reference low-pressure EGR valve opening amount and the reference high-pressure EGR valve opening amount from the ROM based on the operation mode of the internal combustion engine 1. The ECU 20 controls the low-pressure EGR valve 32 so that the opening amount of the low-pressure EGR valve 32 matches the reference low-pressure EGR valve opening amount. The ECU 20 also controls the high-pressure EGR valve 42 so that the opening amount of the high-pressure EGR valve 42 matches the reference high-pressure EGR valve opening amount.

[0050] The high-pressure EGR gas passage 41 is relatively short and the high-pressure EGR gas is unlikely to be cooled while flowing through the high-pressure EGR gas passage 41. Accordingly, the exhaust gas having a relatively high temperature is recirculated back to the internal combustion engine 1 by performing EGR using the high-pressure EGR unit 40. On the other hand, the low-pressure EGR passage 31 is relatively long, and the low-pressure EGR cooler 33, the intercooler 8, etc. are provided on the path through which the low-pressure EGR gas flows. Accordingly, the low-pressure EGR gas is cooled while flowing through the low-pressure EGR passage 31. Accordingly, the exhaust gas having a relatively low temperature is recirculated back to the internal combustion engine 1 by performing EGR using the low-pressure EGR unit 30.

[0051] When EGR is performed using the low-pressure EGR unit 30, the fuel efficiency and the effect of reducing the amount of NOx are enhanced because the low-pressure EGR gas having a low temperature is supplied to the internal combustion engine 1 by the tυrbocharger 13. However, when the intake air temperature is likely to decrease, for example, when the temperature of the air around the internal combustion engine 1 is low (for example, in winter) or when the temperature of the internal combustion engine 1 itself is low (for example, immediately after the internal combustion engine 1 is started), if the low-pressure EGR gas having a low-temperature flows into the internal combustion engine 1, the intake air temperature may abruptly decrease, resulting in incomplete combustion, for example, a misfire. [0052] Therefore, according to the embodiment of the invention, when there' is a possibility that the intake air temperature will excessively decrease and incomplete combustion will take place, the EGR control map is corrected in such a manner that the ratio of the high-pressure EGR gas amount to the entire EGR gas amount is increased in a part or the entirety of operating range in which EGR is performed using the low-pressure EGR unit 30, namely, the range MIX or the range LPL. Then, EGR is performed according to the corrected EGR control map.

[0053] Thus, even when the intake air temperature is likely to decrease, the high-pressure EGR gas having a high temperature, of which the amount is larger than the reference high-pressure EGR gas amount, flows into the internal combustion engine 1. Accordingly, it is possible to appropriately suppress occurrence of the situation in which the intake air temperature excessively decreases and incomplete combustion, for example, a misfire takes place when EGR is performed.

[0054] According to the embodiment of the invention, it is determined whether there is a possibility that the intake air temperature will excessively decrease and incomplete combustion will take place, based on the coolant temperature detected by the coolant temperature sensor 14 and/or the ambient air temperature. More specifically, when the coolant temperature Tw is lower than the reference coolant temperature Twth and/or the ambient air temperature Ta is lower than the reference ambient air temperature Tath, it is determined that there is a possibility that incomplete combustion will take place. Hereinafter, the situation where it is determined that there is a possibility that incomplete combustion will take place will be referred to as "low coolant temperature times". On the other hand, when the coolant temperature is equal to or higher than the reference coolant temperature and the ambient air temperature is equal to or higher than the reference ambient air temperature, it is not determined that there is a possibility that incomplete combustion will take place. Hereinafter, the situation where it is not determined that there is a possibility that incomplete combustion will take place will be referred to as '"normal temperature times.

[0055] Hereafter, some examples of correction on the EGR control map will be described below.

[0056] FIG. 5 shows an example of correction made to the reference EGR control map in which the operating- range of the internal combustion engine is divided into the range HPL, the range MIX and the range LPL as in the case shown in FIG 2. In the example shown in FIG. 5, in a part of the low load-side range in the LPL range (the range in the LPL range, which is close to the range MIX), the ratio of the high-pressure EGR gas amount to the entire EGR gas amount (hereinafter, referred to as the "high-pressure EGR gas ratio") is increased from 0% to a predetermined ratio higher than 0% at low coolant temperature times. In addition, the high-pressure EGR gas ratio in the range MIX is made higher than the reference high-pressure EGR gas ratio at low coolant temperature times. Especially, in a part of the low load-side range in the range MIX (a range in the range MIX, which is close to the range HPL), the high-pressure EGR gas ratio is increased from a predetermined ratio, lower than 100%, to 100% .

[0057] Namely, at low coolant temperature times, the operating range that corresponds to the shaded area A, which is defined as the operating range that belongs to the range LPL at normal temperature times, is redefined as the operating range that belongs to the range MEX, and therefore the range LPL is narrowed. In addition, the operating range that corresponds to the shaded area B, which is defined as the operating range that belongs to the range MIX at normal temperature times, is redefined as the operating range that belongs to the range HPL, and therefore the range HPL is enlarged. [0058] Correcting the reference EGR control map in the above-described manner makes it possible to perform EGR using the high-pressure EGR unit 40 and the low-pressure EGR unit 30 in combination at low coolant temperature times, even in the operating range that corresponds to the shaded area A in which EGR is performed using only the low-pressure EGR unit 30 at normal temperature times. Accordingly, the high-pressure EGR gas having a high temperature flows into the internal combustion engine 1 , which suppresses an excessive decrease in the intake air temperature. In the operating range that belongs to the range MIX, a larger amount of EGR gas flows into the internal combustion engine 1 at low coolant temperature times than at normal temperature times. Accordingly, it is possible to suppress an excessive decrease in the intake air temperature.

[0059] FIG. 6 shows an example of correction made to the reference EGR control map in which the operating range of the internal combustion engine 1 is divided into the range HPL and the range LPL as in the case shown in FIG. 3. In this example, in a part of the low load-side range in the range LPL (a range in the range LPL, which is closeclose to the range HPL), the high-pressure EGR gas ratio is increased from 0% to 100% at low coolant temperature times. Namely, at low coolant temperature times, the operating range that corresponds to the shaded area C, which is defined as the operating range that belongs to the range LPL at normal temperature times, is redefined as the operating range that belongs to the range HPL, and therefore the range LPL is narrowed.

[0060] Correcting the reference EGR control map in the above-described manner makes it possible to perform EGR using only the high-pressure EGR unit 40 at low coolant temperature times, even in the operating range that corresponds to the shaded area C in which EGR is performed using only the low-pressure EGR unit 30 at normal temperature times. Therefore, the high-pressure EGR gas having a high temperature flows into the internal combustion engine 1, which suppresses an excessive decrease in the intake air temperature.

[0061] FIG. 7 is another example of correction made to the reference EGR control map shown in FIG 3. In this example, in a part of the low load-side range in the range LPL (a range in the range LPL, which is close to the range HPL), the high-pressure EGR gas ratio is increased from 0% to a predetermined ratio higher than 0% at low coolant temperature times. Namely, the operating range that corresponds to the shaded area D, which is defined as the operating range that belongs to the range LPL at normal temperature times, is redefined as the operating range that belongs to the range MIX, and therefore the range LPL is narrowed.

[0062] Correcting the reference EGR control map in the above-described manner makes it possible to perform EGR using the high-pressure EGR unit 40 and the low-pressure EGR unit 30 in combination at low coolant temperature times, even in the operating range that corresponds to the shaded area D in which EGR is performed using only the low-pressure EGR unit 30 at normal temperature times. Accordingly, the high-pressure EGR gas having a high temperature flows into the internal combustion engine 1, which suppresses an excessive decrease in the intake air temperature.

[0063] FIG. 8 shows an example of correction made to the reference EGR control map in which the operating range of the internal combustion engine 1 is divided into the range MIX and the range LPL as in the case shown in FIG. 4. In this example, in a part of the low load-side range in the range LPL (a range in the range LPL, which is close to the range MIX), the high-pressure EGR gas ratio is increased from 0% to a predetermined ratio higher than 0% at low coolant temperature times. Namely, the operating range that corresponds to the shaded area E, which is defined as the operating range that belongs to the range LPL at normal temperature times, is redefined as the operating range that belongs to the range MDC, and therefore the range LPL is narrowed.

[0064] Correcting the reference EGR control map in the above-described manner makes it possible to perform EGR using the high-pressure EGR unit 40 and the low-pressure EGR unit 30 in combination at low coolant temperature times, even in the operating range that corresponds to the shaded area E in which EGR is performed using only the low-pressure unit 30 at normal temperature times. Accordingly, the high-pressure EGR gas having a high-temperature flows into the internal combustion engine 1, which suppresses an excessive decrease in the intake air temperature. [0065] FIG 9 shows another example of correction made to the reference EGR control map shown in FIG 4. In this example, the high-pressure EGR gas ratio in the range MIX is made higher than the reference high-pressure EGR gas ratio at low coolant temperature times. Correcting the reference EGR control map in the above-described manner allows a larger amount of high-pressure EGR gas to flow into the internal combustion engine 1 at low coolant temperature times than at normal temperature times.

Accordingly, it is possible to suppress an excessive decrease in the intake air temperature.

[0066] FIG. 10 shows a modified example of the correction shown in FIG 9. In this modified example, the high-pressure EGR gas ratio is increased from a predetermined ratio, lower than 100%, to 100% in a part of the low load-side range in the range MIX.

In this modified example, the operating range that corresponds to the shaded area F, which is defined as the operating range that belongs to the range MIX at normal temperature times, is redefined as the operating range that belongs to the range HPL, and therefore the range HPL is enlarged. [0067] Correcting the reference EGR control map in the above-described manner makes it possible to perform EGR using only the high-pressure EGR unit 40 at low coolant temperature times, in the operating range that coσesponds to the shaded area F in which EGR is performed using the high-pressure EGR unit 40 and the low-pressure EGR unit 30 in combination at normal temperature times. Accordingly, the high-pressure EGR gas having a high temperature flows into the internal combustion engine 1, which suppresses an excessive decrease in the intake air temperature.

[0068] Hereafter, the routine of the EGR control map correction control according to the embodiment of the invention, which is executed at low coolant temperature times, will be described with reference to FIG 11. FIG. 11 is a flowchart showing the routine of the EGR control map correction control. The routine is executed at predetermined time intervals while the internal combustion engine 1 is operating.

[0069] In step S lOI, the ECU 20 obtains the operation mode of the internal combustion engine 1. More specifically, the ECU 20 calculates the engine load based on the accelerator angle detected by the accelerator angle sensor 15, and calculates the engine speed based on the crank angle detected by the crank position sensor 16.

[0070] In step S 102, the ECU 20 obtains the coolant temperature Tw in the internal combustion engine 1. More specifically, the coolant temperature is directly detected by the coolant temperature sensor 14.

[0071] In step S 103, the ECU 20 determines whether there is a possibility that incomplete combustion will take place in the internal combustion engine 1 , namely, whether the intake air temperature is likely to decrease. More specifically, the ECU 20 determines whether the coolant temperature Tw obtained in step S 102 is lower than the reference coolant temperature Twth. If an affirmative determination is made in step S 103, the ECU 20 executes step S 104. On the other hand, if a negative determination is made in step S103, the ECU 20 executes step S106.

[0072] In step S 104, the ECU 20 corrects the reference EGR control map in the manner described above.

[0073] In step S 105, the ECU 20 executes EGR according to the EGC control map corrected in step S 104.

[0074] In step S 106, the ECU 20 executes EGR according to the reference EGR control map.

[0075] After executing step S 105 or step S 106, the ECU 20 ends the routine.

[0076] While the invention has been described with reference to an example embodiment thereof, it is to be understood that the invention is not limited to the example embodiment. To the contrary, the invention is intended to cover various modifications and equivalent arrangements within the scope of the invention. For example, according to the embodiment of the invention, EGR is performed while the EGR control map is changed between the corrected EGR control map used at low coolant temperature times and the reference EGR control map used at normal temperature times, based on the coolant temperature. Alternatively, the ratio of the low-pressure EGR gas amount to the entire EGR gas amount may be changed continuously or in a stepwise manner based on the coolant temperature. In this case, as in the case shown in FIG. 12, it is preferable to control the low-pressure EGR gas ratio in a manner such that the low-pressure EGR gas ratio decreases as the coolant temperature decreases. Thus, when the coolant temperature is low, EGR is performed using mainly the high-pressure EGR unit 40. Accordingly, a larger amount of high-pressure EGR gas flows into the internal combustion engine 1, which suppresses an excessive decrease in the intake air temperature. As a result, it is possible to suppress occurrence of incomplete combustion and generation of a large amount of unburned fuel component. In addition, when the coolant temperature is high, EGR is performed using the low-pressure EGR unit 30 and the high-pressure EGR unit 40 in combination. Accordingly, the efficiency of supercharging performed by the turbocharger 13 is enhanced by using the low-pressure EGR unit 30. In addition, the pumping loss is reduced by decreasing the high-pressure EGR gas ratio. As a result, it is possible to enhance the fuel efficiency. In the embodiment of the invention, when the internal combustion engine 1 is operating in a predetermined high-load range within the operating range that belongs to the range LPL, even if it is determined that there is a possibility that incomplete combustion will take place, the high-pressure EGR gas ratio may be prevented from increasing.

[0077] The temperature of the internal combustion engine may be estimated based on the temperature of the wall face of the intake passage, the temperature of the engine oil, or the like, instead of the coolant temperature.

Claims

1. An EGR system for an internal combustion engine, characterized by comprising: a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor; a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor; an EGR control unit that performs EGR while changing the EGR unit used for EGR between the high-pressure EGR unit and the low-pressure EGR unit, or using the high-pressure EGR unit and the low-pressure EGR unit in combination, based on an operation mode of the internal combustion engine; and a determination unit that determines whether there is a possibility that incomplete combustion takes place in the internal combustion engine, wherein, in a case where the operation mode of the internal combustion engine is in a predetermined operating range in which EGR is performed using at least the low-pressure EGR unit, the EGR control unit makes a ratio of an amount of exhaust gas, which is recirculated back to the internal combustion engine by the high-pressure EGR unit, to an entire amount of exhaust gas, which is recirculated back to the internal combustion engine, higher when the determination unit determines that there is a possibility that incomplete combustion takes place than when the determination unit does not determine that there is a possibility that incomplete combustion takes place.

2. The EGR system according to claim 1, wherein, when the determination unit determines that there is a possibility that incomplete combustion takes place, the EGR control unit changes a part of an operating range, in which EGR is performed using only the low-pressure EGR unit when the determination unit does not determine that there is a possibility that incomplete combustion takes place, to an operating range, in which EGR is performed using the high-pressure EGR unit and the low-pressure EGR unit in combination, or an operating range, in which EGR is performed using only the high-pressure EGR unit.

3. The EGR system according to claim 1 or 2, wherein, when the determination unit determines that there is a possibility that incomplete combustion takes place, the EGR control unit changes a part of an operating range, in which EGR is performed using the high-pressure EGR unit and the low-pressure EGR unit in combination when the determination unit does not determine that incomplete combustion takes place, to an operating range, in which EGR is performed using only the high-pressure EGR unit.

4. The EGR system according to any one of claims 1 to 3, wherein, when the operation mode of the internal combustion engine is within a predetermined high load-side range in the predetermined operating range, even if the determination unit determines that there is a possibility that incomplete combustion takes place, the EGR control unit does not increase the ratio of the amount of exhaust gas, which is recirculated back to the internal combustion engine by the high-pressure EGR unit, to the entire amount of exhaust gas, which is recirculated back to the internal combustion engine.

5. The EGR system according to any one of claims 1 to 4, wherein the determination unit determines whether there is a possibility that incomplete combustion takes place based on at least one of an ambient air temperature and a temperature of a coolant in the internal combustion engine,

6. An EGR system for an internal combustion engine, characterized by comprising: a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor; a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor; an EGR control unit that performs EGR while changing the EGR unit used for EGR between the high-pressure EGR unit and the low-pressure EGR unit, or using the high-pressure EGR unit and the low-pressure EGR unit in combination, based on an operation mode of the internal combustion engine; and a coolant temperature detection unit that detects a temperature of a coolant in the internal combustion engine, wherein the EGR control unit changes a ratio of an amount of exhaust gas, which is recirculated back to the internal combustion engine by the low-pressure EGR unit, to an entire amount of exhaust gas, which is recirculated back to the internal combustion engine, based on the temperature of the coolant detected by the coolant temperature detection unit.

7. The EGR system according to claim 6, wherein the EGR control unit decreases the ratio of the amount of exhaust gas, which is recirculated back to the internal combustion engine by the low-pressure EGR unit, to the entire amount of exhaust gas, which is recirculated back to the internal combustion engine, as the temperature of the coolant decreases.

8. A method for controlling an EGR system for an internal combustion engine, the EGR system including: a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor; and a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor, the method characterized by comprising: performing EGR while changing the EGR unit used for EGR between the high-pressure EGR unit and the low-pressure EGR unit, or using the high-pressure EGR unit and the low-pressure EGR unit in combination, based on an operation mode of the internal combustion engine; determining whether there is a possibility that incomplete combustion takes place in the internal combustion engine, and making a ratio of an amount of exhaust gas, which is recirculated back to the internal combustion engine by the high-pressure EGR unit, to an entire amount of exhaust gas, which is recirculated back to the internal combustion engine, higher when it is determined that there is a possibility that incomplete combustion takes place than when it is not determined that there is a possibility that incomplete combustion takes place, Ln a case where the operation mode of the internal combustion engine is in a predetermined operating range in which EGR is performed using at least the low-pressure EGR unit.

9. The method according to claim 8, wherein, when it is determined that there is a possibility that incomplete combustion takes place, a part of an operating range, in which EGR is performed using only the low-pressure EGR when it is not determined that there is a possibility that incomplete combustion takes place, is changed to an operating range, in which EGR is performed using the high-pressure EGR unit and the low-pressure EGR unit in combination, or an operating range, in which EGR is performed using only the high-pressure EGR unit.

10. The method according to claim 8 or 9, wherein, when it is determined there is a possibility that incomplete combustion takes place, a part of an operating range, in which EGR is performed using the high-pressure EGR unit and the low-pressure EGR unit in combination when it is not determined that incomplete combustion takes place, is changed to an operating range, in which EGR is performed using only the high-pressure EGR unit.

11. The method according to any one of claims 8 to 10, wherein, when the operation mode of the internal combustion engine is within a predetermined high load-side range in the predetermined operating range, even if it is determined that there is a possibility that incomplete combustion takes place, the ratio of the amount of exhaust gas, which is recirculated back to the internal combustion engine by the high-pressure EGR unit, to the entire amount of exhaust gas, which is recirculated back to the internal combustion engine, is not increased.

12. The method according to any one of claims 8 to 11, wherein it is determined whether there is a possibility that incomplete combustion takes place based on at least one of an ambient air temperature and a temperature of a coolant in the internal combustion engine.

13. A method for controlling an EGR system for an internal combustion engine, the EGR system including: a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor; and a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor; the method characterized by comprising: performing EGR while changing the EGR unit used for EGR between the high-pressure EGR unit and the low-pressure EGR unit, or using the high-pressure EGR unit and the low-pressure EGR unit in combination, based on an operation mode of the internal combustion engine; detecting a temperature of a coolant in the internal combustion engine; and changing a ratio of an amount of exhaust gas, which is recirculated back to the internal combustion engine by the low-pressure EGR unit, to an entire amount of exhaust gas, which is recirculated back to the internal combustion engine, based on the temperature of the coolant detected by the coolant temperature detection unit.

14. The method according to claim 13, wherein the ratio of the amount of exhaust gas, which is recirculated back to the internal combustion engine by the low-pressure EGR unit, to the entire amount of exhaust gas, which is recirculated back to the internal combustion engine, is decreased as the temperature of the coolant decreases.