EXHAUST GAS RECIRCULATION 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 exhaust gas recirculation system for an internal combustion engine, and a method for controlling the same.

2. Description of the Related Art [0002] In order to reduce the amount of toxic substance contained in the exhaust gas discharged from a vehicle, a technology for reducing the amount of toxic substance produced in the process of burning the fuel in an internal combustion engine, and a technology for removing the toxic substance contained in the exhaust gas discharged from the internal combustion engine by an exhaust-gas after-treatment have been developed.

[0003] Examples of the technology for reducing the amount of toxic substance include an exhaust gas recirculation (EGR) system that recirculates a portion of the exhaust gas back to an intake system of an internal combustion engine. The amount of nitrogen oxide (NOx) produced in the process of burning the fuel in the. internal combustion engine is reduced by performing EGR. In order to make it possible to perform EGR in a broader operating range of an internal combustion engine, an EGR system that uses a high-pressure EGR unit and a low-pressure EGR unit in combination has been suggested (refer to, for example, Japanese Patent Application Publication No. 2004-150319 (JP-A-2004-150319)). 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] Examples of the technology for removing the toxic substance include a technology in which an exhaust gas control apparatus or an exhaust gas catalyst, for example, a particulate filter that traps particulate matter (PM) in the exhaust gas, a NOx catalyst that reduces NOx, and an oxidation catalyst that removes unburned fuel components such as hydrocarbon (HC) and carbon monoxide (CO), is arranged in a middle portion of an exhaust passage.

[0005] When an exhaust gas catalyst is in the inactive state, PM and HC in the exhaust gas are not sufficiently removed by the exhaust gas catalyst and are allowed to pass through the exhaust gas catalyst in some cases. In these cases, in an exhaust gas recirculation system including a low-pressure EGR unit in which a low-pressure EGR passage is formed in a manner such that the exhaust gas is introduced from an exhaust passage, at a portion downstream of a turbine and the exhaust gas catalyst, PM and HC that have passed through the exhaust gas catalyst partly flow into the low-pressure EGR passage. Then, the PM and HC that flow into the low-pressure EGR passage may adhere to an inside portion of an EGR cooler provided in the low-pressure EGR passage, resulting in clogging of the EGR cooler and deterioration of the cooling performance of the EGR cooler. If the unburned fuel components, such as HC, flow through the low-pressure EGR passage and then flow into a cylinder of an internal combustion engine along with the exhaust gas, the air-fuel ratio of the intake air may change because of the unburned fuel components, resulting in unstable fuel combustion and an increase in the amount of toxic substance in the exhaust gas.

SUMMARY OF THE INVENTION

[0006] The invention provides a technology for minimizing inconveniences that are caused by PM and HC that flow into an EGR system and an intake system of an internal

combustion engine when an exhaust gas catalyst is in the inactive state, in an exhaust gas recirculation system for the internal combustion engine, which includes the exhaust gas catalyst in an EGR gas flow path.

[0007] A first aspect of the invention relates to an exhaust gas recirculation system for an internal combustion engine, including: a turbocharger that includes a turbine arranged in an exhaust passage of the internal combustion engine, and a compressor arranged in an intake passage of the internal combustion engine; an exhaust gas catalyst that is provided in the exhaust passage and that purifies exhaust gas; a low-pressure EGR unit that recirculates a portion of the exhaust gas, discharged from the internal combustion engine, 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 exhaust gas catalyst, and the intake passage, at a portion upstream of the compressor; an EGR cooler that is provided in the low-pressure

EGR passage and that cools the exhaust gas flowing through the low-pressure EGR passage; a bypass unit that causes the exhaust gas flowing through the low-pressure EGR passage to bypass the EGR cooler; a catalyst activation-inactivation determination unit that determines whether the exhaust gas catalyst is in a predetermined active state; and a control unit that controls the bypass unit to cause the exhaust gas flowing through the low-pressure EGR passage to bypass the EGR cooler, when the catalyst activation-inactivation determination unit determines that the exhaust gas catalyst is not in the predetermined active state.

[0008] When the exhaust gas catalyst is in the "predetermined active state", the exhaust gas catalyst is able to remove PM and the unburned fuel components contained in the exhaust gas to such a degree that clogging of the EGR cooler does not occur when the exhaust gas discharged from the exhaust gas catalyst passes through the low-pressure EGR passage. Whether the exhaust gas catalyst is in the predetermined active state is determined based on, for example, the temperature of the exhaust gas catalyst.

[0009] The bypass passage includes, for example, a bypass passage that connects a portion of the low-pressure EGR passage, at a position upstream of the EGR cooler, to

another portion of the low-pressure EGR passage, at a position downstream of the EGR cooler, and a selector valve that changes the exhaust gas flow path between the path through which the exhaust gas flowing through the low-pressure EGR passage passes through the EGR cooler and the path through which the exhaust gas passes through the bypass passage.

[0010] With the configuration described above, when it is determined that the exhaust gas catalyst is not in the predetermined active state (i.e. the exhaust gas catalyst is in the inactive state), the exhaust gas flowing through the low-pressure EGR passage bypasses the EGR cooler. Accordingly, it is possible to suppress occurrence of the situation in which clogging of the EGR cooler occurs because the exhaust gas containing a high-concentration of PM and unburned fuel component, which is discharged from the exhaust gas catalyst when the exhaust gas catalyst is in the inactive state, passes through the EGR cooler.

[0011] As a result, it is possible to suppress the inconveniences such as an excessive increase in pressure loss in the low-pressure EGR passage, occurrence of a malfunction in the EGR cooler due to high heat generated when abrupt oxidation reaction of the accumulated PM occurs, and deterioration of the cooling performance of the EGR cooler.

[0012] In addition, because the exhaust gas that is not cooled by the EGR cooler is recirculated back to the internal combustion engine through the low-pressure EGR passage, the temperature of the intake air increases and the concentration of the unburned fuel component in the exhaust gas decreases. Thus, the amount of unbumed fuel component flowing into the exhaust gas catalyst in the inactive state decreases. As a result, it is possible to decrease the amount of unburned fuel component in the exhaust gas flowing into the low-pressure EGR passage. [0013] If EGR is performed using the low-pressure EGR passage when the exhaust gas catalyst is in the inactive state, the exhaust gas containing a high-concentration of unburned fuel component is recirculated back to the internal combustion engine through the low-pressure EGR passage. The air-fuel ratio of the intake air may become richer due to the unburned fuel component, resulting in unstable combustion and an increase in

the amount of toxic substance in the exhaust gas.

[0014] The exhaust gas recirculation system according to the first aspect of the invention may further include a fuel injection correction unit that corrects a fuel injection amount based on the concentration of the unburned fuel component in the exhaust gas that is recirculated back to the internal combustion engine through the low-pressure EGR passage, when the catalyst activation-inactivation determination unit determines that the exhaust gas catalyst is not in the predetermined active state.

[0015] With this configuration, the fuel injection amount is corrected by a correction amount obtained by subtracting the amount, which corresponds to the amount of the unburned fuel component recirculated back to the internal combustion engine through the low-pressure EGR passage, from the predetermined fuel injection amount, and the corrected amount of fuel is injected. In this way, even when the exhaust gas catalyst is in the inactive state, the air-fuel ratio of the intake air is made substantially equal to an appropriate value. Thus, the inconveniences described above are minimized. [0016] The concentration of the unburned fuel component in the exhaust gas may be estimated according to a known calculation model, based on the EGR rate and the engine operation mode such as the engine speed and the fuel injection amount. Alternatively, a sensor that detects the concentration of the unburned fuel component in the exhaust gas may be provided in the exhaust passage or the low-pressure EGR passage, and the concentration may be directly detected.

[0017] A second aspect of the invention relates to a method for controlling an exhaust gas recirculation system for an internal combustion engine, which includes: a turbocharger that includes a turbine arranged in an exhaust passage of the internal combustion engine, and a compressor arranged in an intake passage of the internal combustion engine; an exhaust gas catalyst that is provided in the exhaust passage and that purifies exhaust gas; a low-pressure EGR unit that recirculates a portion of the exhaust gas, discharged from the internal combustion engine, 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 exhaust gas

catalyst, and the intake passage, at a portion upstream of the compressor; and an EGR cooler that is provided in the low-pressure EGR passage and that cools the exhaust gas flowing through the low-pressure EGR passage. According to the method, whether the exhaust gas catalyst is in a predetermined active state is determined. When it is determined that the exhaust gas catalyst is not in the predetermined active state, the exhaust gas flowing through the low-pressure EGR passage is caused to bypass the EGR cooler.

[0018] According to the aspects of the invention described above, it is possible to minimize the inconveniences caused by PM and HC that flow into the EGR system and the intake system of the internal combustion engine when the exhaust gas catalyst is in the inactive state, in the exhaust gas recirculation system for the internal combustion engine, which includes the exhaust gas catalyst in the EGR gas flow path.

BRIEF DESCRIPTION OF THE DRAWINGS [0019] 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 exhaust gas recirculation system according to an embodiment of the invention;

FIG 2 is a graph showing the EGR modes that are changed based on the operation mode of an internal combustion engine by the EGR control according to the embodiment of the invention; FIG. 3 is a flowchart showing the changeover-to-bypass passage control routine according to the embodiment of the invention; and

FIG 4 is a flowchart showing the fuel injection amount correction control routine according to the embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

[0020] 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.

[0021] 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 exhaust gas recirculation 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.

[0022] 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 by changing the flow passage area of the intake pipe 3, is provided in the intake pipe 3 at a position upstream of the intake manifold 17. The second intake throttle valve 9 is opened/closed by an electric actuator.

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 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 by changing the flow passage area of the intake pipe 3, is provided in the intake pipe 3, at a position upstream of the compressor 11. The first intake throttle valve 6 is opened/closed by an electric actuator.

[0023] 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 is provided in a middle portion of the exhaust pipe 4. The turbocharger is a variable capacity tuibocharger

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 catalyst 10 is provided in the exhaust pipe 4, at a position downstream of the turbine 12, The exhaust gas catalyst 10 includes a paniculate filter (hereinafter, referred to as a "filter"). The filter supports a NOx storage reduction catalyst (hereinafter, referred to as a "NOx catalyst"). When the exhaust gas passes through the filter, particulate matter contained in the exhaust gas is trapped in the filter. NOx contained in the exhaust gas is stored in the filter, when the exhaust gas is in the oxidant atmosphere. Then, the NOx stored in the filter is reduced, when the exhaust gas is in the reducing atmosphere. An exhaust throttle valve 19 is provided in the exhaust pipe 4, at a position downstream of the exhaust gas catalyst 10 and regulates the flow rate of the exhaust gas flowing through the exhaust pipe 4 by changing the flow passage area of the exhaust pipe 4. The exhaust throttle valve 19 is opened/closed by an electric actuator. In the embodiment of the invention, the exhaust throttle valve 19 is provided in the exhaust pipe 4, at a position immediately downstream of the exhaust gas catalyst 10. Alternatively, 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. The temperature of the filter is increased by reducing the opening amount of the exhaust throttle valve 19, and the filter recovery process for removing the particulate matter trapped in the filter by oxidizing the particulate matter is performed. Also, the sulfur removing process for removing sulfur oxide stored in the NOx catalyst may be performed.

[0024] 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, at high pressure, 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". [0025] 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 by changing the flow passage area of the high-pressure EGR passage 41. That is, 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 by a method other than adjustment of the opening amount of high-pressure EGR valve 42. For example, 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.

[0026] 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. The exhaust gas is introduced to the intake pipe 3, at low pressure, 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".

[0027] 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 by

changing the flow passage area of 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 by a method other than adjustment of the opening amount of the low-pressure EGR valve 32. For example, 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.

[0028] The low-pressure EGR cooler 33 promotes heat exchange between the low-pressure EGR gas flowing through the low-pressure EGR cooler 33 and the coolant that cools the internal combustion engine 1 to cool the low-pressure EGR gas. The low-pressure EGR unit 30 according to the embodiment of the invention is provided with a bypass passage 35 that connects a portion of the EGR passage 31, which is positioned upstream of the low-pressure EGR cooler 33, to another portion of the EGR passage 31, which is positioned downstream of the low-pressure EGR cooler 33. A selector valve 34 is provided at the connection portion, which is positioned upstream of the low-pressure EGR cooler 33 and at which the bypass passage 35 is connected to the low-pressure EGR passage 31. The selector valve 34 changes the communication state between the state where the portion of the low-pressure EGR passage 31, at a position upstream of the low-pressure EGR cooler 33, is communicated with the low-pressure EGR cooler 33 and the state where this portion of the low-pressure EGR passage 31 is communicated with the bypass passage 35. When the selector valve 35 is operated to permit communication between the low-pressure EGR passage 31 and the low-pressure EGR cooler 33, the exhaust gas that flows from the exhaust pipe 4 into the low-pressure EGR passage 31 flows through the low-pressure EGR passage 31 via the low-pressure EGR cooler 33. On the other hand, when the selector valve 35 is operated to permit communication between the low-pressure EGR passage 31 and the bypass passage 35, the exhaust gas that flows from the exhaust pipe 4 into the low-pressure EGR passage 31 flows through the low-pressure EGR passage 31 via the bypass passage 35 without

passing through the low-pressure EGR cooler 33. In this embodiment, the bypass passage 35 and the selector valve 34 function as a bypass unit according to the invention. [0029] 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.

[0030] 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 into the intake pipe 3, 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.

[0031] 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.

[0032] 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 and the intake air amount.

[0033] Next, the EGR control executed by the ECU 20 will be described. [0034] In the exhaust gas recirculation system according to the embodiment of the invention, the target EGR rate is set for each operation mode of the internal combustion engine 1 so that the NOx discharge amount matches a predetermined target value. Then, the target EGR rate is achieved by performing EGR using the high-pressure EGR unit 40 and the low-pressure EGR unit 30 in combination, and the combination of the high-pressure EGR gas amount and the low-pressure EGR gas amount (or the proportion of each of the high-pressure EGR gas amount and the low-pressure EGR gas amount to the entire EGR gas amount), which is optimum for achieving the predetermined engine characteristics such as the specific fuel consumption characteristics, the combustion characteristics and the exhaust gas quality due to performance of EGR, is obtained. The EGR rate is the proportion of the entire EGR gas amount (the sum of the high-pressure EGR gas amount and the low-pressure EGR gas amount), which is recirculated back to the internal combustion engine 1 by the exhaust gas recirculation system, to the intake air amount. When the intake air amount is denoted by Gcyl and the amount of newly-taken air detected by the airflow meter 7 is denoted by Gn, the EGR rate is expressed by the equation, EGR rate = (Gcyl - Gn) / Gcyl.

[0035] 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.

[0036] The ECU 20 reads the reference tow-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.

[0037] FIG. 2 is a graph showing the EGR modes that are set for the respective operating ranges of the internal combustion engine 1. In this case, the EGR mode indicates the EGR unit that is selected from among the high-pressure EGR unit 40 and the low-pressure EGR unit 30 when EGR is performed by the ECU 20. 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.

[0038] With the exhaust gas recirculation system according to the embodiment of the invention, in the operating range in which the internal combustion engine 1 is operating at low load, the reference high-pressure EGR valve opening amount is set to a value other than zero, the reference low-pressure EGR valve opening amount is set to zero, and EGR is performed in the HPL mode in which only the high-pressure EGR unit 40 is used. The "range HPL" in FIG. 2 corresponds to this operating range. In the operating range in which the internal combustion engine 1 is operating at medium load, each of the reference high-pressure EGR valve opening amount and the reference low-pressure EGR valve opening amount is set to a value other than zero, and EGR is performed in the MIX mode in which both the high-pressure EGR unit 40 and the low-pressure EGR unit 30 are used. The "range MIX" in FIG 2 corresponds to this operating range. In the operating range in which the internal combustion engine 1 is operating at high load, the reference high-pressure EGR valve opening amount is set to zero, the reference low-pressure EGR valve opening amount is set to a value other than zero, and EGR is performed in the LPL

mode in which only the low-pressure EGR unit 30 is used. The '"range LPL" in FIG. 2 corresponds to this operating range.

[0039] When the exhaust gas catalyst 10 is in the inactive state, PM and HC in the exhaust gas are not sufficiently removed by the exhaust gas catalyst 10. The PM and HC pass through the exhaust gas catalyst 10, and flow into a portion of the exhaust pipe 4, which is positioned downstream of the exhaust gas catalyst 10, in some cases. In the exhaust gas recirculation system according to the embodiment of the invention, because the low-pressure EGR passage 31 is formed in a manner such that the exhaust gas is introduced from the exhaust pipe 4, at a portion downstream of the turbine 12 and the exhaust gas catalyst 10, PM and HC that have passed through the exhaust gas catalyst 10 may partly flow into the low-pressure EGR passage 31. Then, the PM and HC that flow into the low-pressure EGR passage 31 adhere to the inside portion of the EGR cooler 33, resulting in clogging of the EGR cooler 33 or deterioration of the cooling performance of the EGR cooler 33. [0040] Therefore, according to the embodiment of the invention, when the exhaust catalyst 10 is in the inactive state, the selector valve 34 is operated so that the low-pressure EGR passage 31 is communicated with the bypass passage 35. Thus, the exhaust gas containing a high-concentration of PM and HC, which is discharged from the exhaust gas catalyst 10 in the inactive state, is introduced to the intake pipe 3 through the bypass passage 35 without flowing into the low-pressure EGR cooler 33. Accordingly, it is possible to suppress adhesion of the PM and HC to the inside portion of the low-pressure EGR cooler 33. As a result, it is possible to suppress clogging of the low-pressure EGR cooler 33, an excessive increase in pressure loss in the low-pressure EGR cooler 33, and deterioration of the cooling performance of the low-pressure EGR cooler 33.

[0041] Hereafter, the changeover-to-bypass passage control according to the embodiment of the invention will be described with reference to FIG 3. FIG. 3 is a flowchart showing the changeover-to-bypass passage control routine according to the embodiment of the invention.

[0042] When the changeover-to-bypass passage control is executed, the ECU 20 first obtains the temperature Tcf of the exhaust gas catalyst 10 (step SlOl). More specifically, the ECU 20 estimates the catalyst temperature Tcf according to a known calculation model based on the operation mode (engine speed, engine load, etc.) of the internal combustion engine 1. Alternatively, a temperature sensor may be provided to the exhaust gas catalyst 10, and the catalyst temperature Tcf may be directly detected.

[0043] Then, the ECU 20 determines whether the exhaust gas catalyst 10 is in the inactive state. More specifically, the ECU 20 compares the obtained catalyst temperature Tcf with the reference temperature Tcfs. When the catalyst temperature Tcf is lower than the reference temperature Tcfs, the ECU 20 determines that the exhaust gas catalyst 10 is in the inactive state (step S102). The reference temperature Tcfs is the lower limit value of the catalyst temperature range in which the exhaust gas catalyst 10 is determined to be in the active state. The reference temperature Tcfs is determined in advance, for example, through experiments. In the case where the exhaust gas catalyst 10 is in the active state, the exhaust gas catalyst 10 is able to remove PM and HC in the exhaust gas to such a degree that clogging of the low-pressure EGR cooler 33 does not occur when the exhaust gas discharged from the exhaust gas catalyst 10 passes through the low-pressure EGR cooler 33. In this embodiment, the ECU 20 when executing step S 102 functions as a catalyst activation-inactivation determination unit. [0044] When it is determined in step S 102 that the exhaust gas catalyst 10 is in the inactive state, the ECU 20 controls the selector valve 34 so that the low-pressure EGR passage 31 is communicated with the bypass passage 35 (step S 103). In this embodiment, the ECU 20 when executing step S103 functions as a control unit according to the invention. [0045] On the other hand, when it is determined in step S 102 that the exhaust gas catalyst 10 is not in the inactive state, the ECU 20 ends the routine while maintaining the state of the selector valve 34 so that communication between the low-pressure EGR passage 31 and the low-pressure EGR cooler 33 is maintained.

[0046] If EGR is performed using the low-pressure ECU passage 31 when the

exhaust gas catalyst 10 is in the inactive state, the exhaust gas containing a high-concentration of HC is recirculated back to the internal combustion engine 1 through the low-pressure EGR passage 31. The air-fuel ratio of the intake air may become richer because of this HC, resulting in unstable combustion and an increase in the toxic substance in the exhaust gas.

[0047] Therefore, according to the embodiment of the invention, when the exhaust gas catalyst 10 is in the inactive state, the fuel injection amount is corrected based on the HC concentration in the exhaust gas that is recirculated back to the internal combustion engine 1 through the low-pressure EGR passage 31. More specifically, the ECU 20 estimates the concentration of HC that is recirculated back to the internal combustion engine 1 through the low-pressure EGR passage 31, and the timing at which the HC reaches the cylinders 2 of the internal combustion engine 1. Then, the ECU 20 decreases the amount of fuel that is injected at the fuel injection timing, at which the air-fuel ratio of the intake air is estimated to become richer due to the HC, by an amount corresponding to the amount of HC. In this way, the air-fuel ratio of the intake air becomes substantially equal to an appropriate value. As a result, it is possible to suppress unstable combustion and an increase in the toxic substance in the exhaust gas because of the HC recirculated back to the cylinders 2 through the low-pressure EGR passage 31. [0048] Hereafter, the fuel injection amount correction control according to the embodiment of the invention will be described with reference to FIG 4. FIG. 4 is a flowchart showing the fuel injection amount correction control routine according to the embodiment of the invention.

[0049] When the fuel injection amount correction control is executed, the ECU 20 first obtains the temperature Tcf of the exhaust gas catalyst 10 (step S201). Then, the ECU 20 determines whether the exhaust gas catalyst 10 is in the inactive state based on the obtained catalyst temperature (step S202). When it is determined that the exhaust gas catalyst 10 is in the inactive state, the ECU 20 estimates the HC concentration in the exhaust gas that is recirculated back to the cylinders 2 through the low-pressure EGR

passage 31 (step S2O3). More specifically, the ECU 20 may estimate the HC concentration based on the HC concentration in the exhaust gas, which is determined based on the operation mode (engine speed, engine load, etc.) of the internal combustion engine 1 and the EGR rate. Alternatively, an HC sensor may be provided to the intake system to directly detect the HC concentration.

[0050] Then, the ECU 20 estimates the timing at which the low-pressure EGR gas having the estimated HC concentration reaches the cylinders 2 based on the operation mode of the internal combustion engine 1, the passage capacity of the low-pressure EGR passage 31, the opening amount of the low-pressure EGR valve 32, etc. The ECU 20 also estimates the fuel injection timing at which the air-fuel ratio of the intake air is estimated to become richer due to the HC recirculated back to the cylinders 2 along with the low-pressure EGR gas. Then, the ECU 20 decreases the amount of fuel that is injected at the estimated fuel injection timing based on the estimated HC concentration (step S204). More specifically, when the predetermined fuel injection amount that is read based on the operation mode of the internal combustion engine 1 is qfin ₀ , and the amount of unburned fuel components in the intake air because of the HC recirculated back to the internal combustion engine 1 along with the low-pressure EGR gas is δq, the command fuel injection amount qfin derived through the correction is expressed by the equation, qfin = qfino - δq. In this embodiment, the ECU 20 when executing step S203 and step S204 functions as a fuel injection correction unit according to the invention.

[0051] 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.