WO2008152491A1

WO2008152491A1 - Exhaust gas recirculation system for internal combustion engine and exhaust gas recirculation system control method

Info

Publication number: WO2008152491A1
Application number: PCT/IB2008/001524
Authority: WO
Inventors: Koichiro Nakatani; Terutoshi Tomoda; Shinobu Ishiyama; Michio Furuhashi; Tomoyuki Ono; Tomoyuki Kogo; Yoshihiro Hisataka
Original assignee: Toyota Motor Corp
Current assignee: Toyota Motor Corp
Priority date: 2007-06-13
Filing date: 2008-06-13
Publication date: 2008-12-18
Anticipated expiration: 2009-12-13
Also published as: JP2008309030A

Abstract

An exhaust gas recirculation system for an internal combustion engine includes a turbocharger; a low-pressure EGR passage through which the low-pressure EGR gas is recirculated back to the intake passage; a high-pressure EGR passage through which the high-pressure EGR gas is recirculated back to the intake passage; and valve timing changing means for adjusting an amount of internal EGR gas. In the exhaust gas recirculation system, when an amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below a first predetermined amount, a feedback control over an EGR rate, which indicates a proportion of EGR gas to intake air, is executed by adjusting the amount of internal EGR gas using the valve timing changing means.

Description

EXHAUST GAS RECIRCULATION SYSTEM

FOR INTERNAL COMBUSTION ENGINE AND

EXHAUST GAS RECIRCULATION SYSTEM CONTROL METHOD

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 an exhaust gas recirculation system control method.

2. Description of the Related Art [0002] Japanese Patent Application Publication No. 2004-150319 describes a technology in which the amount of low-pressure EGR gas that flows through a low-pressure EGR passage and the amount of high-pressure EGR gas that flows through a high-pressure EGR passage are adjusted, and the low-pressure EGR gas and the high-pressure EGR gas are effectively used in combination to reduce exhaust emission over a broader engine operating region without reducing the dynamics of a vehicle, efficiency of an EGR control and response to the EGR control.

[0003] A conventional internal combustion engine generally includes a low-pressure EGR passage and a high-pressure EGR passage. A portion of the exhaust gas is taken into the low-pressure EGR passage as low-pressure EGR gas from an exhaust passage at a portion downstream of a turbine, and the low-pressure EGR gas is recirculated back to an intake passage at a portion upstream of a compressor. A portion of the exhaust gas is taken into the high-pressure EGR passage as high-pressure EGR gas from the exhaust passage at a portion upstream of the turbine, and the high-pressure EGR gas is recirculated back to the intake passage at a portion downstream of the compressor. When the internal combustion engine operates at low load and low rotational speed, only the high-pressure EGR gas is recirculated back to the intake passage through the high-pressure EGR passage. When the internal combustion engine operates at medium load and medium rotational speed, the low-pressure EGR gas and the high-pressure EGR gas are recirculated back to the intake passage through the low-pressure EGR passage and the high-pressure EGR passage, respectively. When the internal combustion engine operates at high load and high rotational speed, only the low-pressure EGR gas is recirculated back to the intake passage through the low-pressure EGR passage.

[0004] A feedback control over an EGR rate, which indicates the proportion of the EGR gas to the intake air, is executed by adjusting the amount of low-pressure EGR gas or the amount of high-pressure EGR gas that is recirculated back to the intake passage. When the low-pressure EGR gas and the high-pressure EGR gas are recirculated back to the intake passage at the same time, for example, when the internal combustion engine operates at medium load and medium rotational speed, the feedback control over the EGR rate is executed by adjusting the amount of high-pressure EGR gas so that the response to the control and the efficiency of the control are appropriately maintained.

[0005] However, if the amount of high-pressure EGR gas is decreased and the opening amount of the high-pressure EGR gas is decreased, the amount of change in the EGR rate for a given amount of change in the opening amount of the high-pressure EGR valve increases. Therefore, if a feedback gain is large, the high-pressure EGR valve opens and closes frequently, which causes fluctuations in torque output. On the other hand, if a feedback gain is small, it takes a long time to achieve the target EGR rate. As described above, when the amount of high-pressure EGR gas is decreased, execution of the feedback control over the EGR rate by adjusting the amount of high-pressure EGR gas is not very efficient. [0006] Therefore, when the amount of high-pressure EGR gas is decreased, the feedback control over the EGR rate may be executed by adjusting the amount of low-pressure EGR gas. However, if the feedback control over the EGR rate is executed by adjusting the amount of low-pressure EGR gas, an increase in the amount of low-pressure EGR gas causes the intake air temperature to deviate further from a target intake air temperature because the temperature of the low-pressure EGR gas is low. In addition, a low-pressure EGR valve, which is used to adjust the amount of low-pressure EGR gas, is nearly fully open, because the amount of high-pressure EGR gas is decreased. Therefore, the range over which the amount of EGR gas may be changed by opening and closing the low-pressure EGR valve is small. As a result, sometimes it is not possible to achieve the target EGR rate. More specifically, because the low-pressure EGR valve is nearly fully open and the range over which the amount of EGR gas may be changed by increasing the opening amount of the low-pressure EGR valve is small, the amount of EGR gas is increased only to the amount corresponding to the largest opening amount of the low-pressure EGR valve. Therefore, if more EGR gas is required, a shortage of the EGR gas may occur.

SUMMARY OF THE INVENTION [0007] The invention provides a technology for an exhaust gas recirculation system for an internal combustion engine and an exhaust gas recirculation system control method. The technology makes it possible to minimize the deviation of the intake air temperature from a target intake air temperature and to appropriately achieve a target EGR rate. [0008] A first aspect of the invention relates to an exhaust gas recirculation system for an internal combustion engine, which includes: a turbocharger that has a turbine which is provided in an exhaust passage for an internal combustion engine and a compressor which is provided in an intake passage for the internal combustion engine; a low-pressure EGR passage into which a portion of exhaust gas is taken, as low-pressure EGR gas, from the exhaust passage at a portion downstream of the turbine and through which the low-pressure EGR gas is recirculated back to the intake passage at a portion upstream of the compressor; a high-pressure EGR passage into which a portion of the exhaust gas is taken, as high-pressure EGR gas, from the exhaust passage at a portion upstream of the turbine and through which the high-pressure EGR gas is recirculated back to the intake passage at a portion downstream of the compressor; and valve timing changing means for adjusting an amount of internal EGR gas (i.e., a portion of the gas that has been burned in a combustion chamber of the internal combustion engine remains within a cylinder, or a portion of the burned gas that is discharged to the exhaust passage is drawn back into the cylinder). In a period in which an amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below a first predetermined amount, a feedback control over an EGR rate, which indicates a proportion of the EGR gas to the intake air, is executed by adjusting the amount of internal EGR gas using the valve timing changing means.

[0009] According to the first aspect of the invention, in the period in which the amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below the first predetermined amount, the feedback control over the EGR rate, which indicates the proportion of the EGR gas to the intake air, is executed by adjusting the amount of internal EGR gas using the valve timing changing means.

[0010] The first predetermined amount may be a threshold value, and if the amount of high-pressure EGR gas is equal to or below the first predetermined amount, it is not possible to execute the feedback control over the EGR rate only by adjusting the amount of high-pressure EGR gas.

[0011] The valve timing changing means may retard, for example, at least the closing timing of an exhaust valve. As a result, a portion of the gas that has been burned in a combustion chamber of the internal combustion engine remains within a cylinder, or a portion of the burned gas that is discharged to the exhaust passage is drawn back into the cylinder. In this way, the internal EGR gas is supplied into the cylinder. Then, the amount of internal EGR gas is adjusted using the valve timing changing means, and the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas. Because the temperature of the internal EGR gas is close to the temperature of the high-pressure EGR gas, it is possible to minimize the deviation of the temperature of the intake air from the target intake air temperature even if the amount of internal EGR gas increases. In addition, the range over which the amount of EGR gas may be changed is large because the entire amount of the internal EGR gas may be adjusted through the feedback control. Further, the internal EGR gas does not need to flow through circulation passages outside of the internal combustion engine, and responds quickly to a control for increasing or decreasing the amount of internal EGR gas. Therefore, it is possible to appropriately achieve the target EGR rate. [0012] The period in which the amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below the first predetermined amount may be a period of transition from an EGR control state in which the low-pressure EGR gas and the high-pressure EGR gas are recirculated back to the intake passage through the low-pressure EGR passage and the high-pressure EGR passage, respectively, to an EGR control state in which only the low-pressure EGR gas is recirculated back to the intake passage through the low-pressure EGR passage.

[0013] In this period, the amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below the first predetermined amount. Therefore, the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas.

[0014] In a high-pressure gas decrease region that includes a region in which the amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below the first predetermined amount, a second predetermined amount of internal EGR gas may be reserved using the valve timing changing means. The second predetermined amount is used as a reference amount for the feedback control over the EGR rate.

[0015] The second predetermined amount may be equal to or larger than a maximum amount by which the amount of internal EGR gas is decreased when the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas.

[0016] When the EGR rate is decreased through the feedback control over the EGR rate, namely, when the amount of EGR gas is excessive, the amount of EGR gas needs to be decreased. In this case, if the second predetermined amount of internal EGR gas, that is, the reference amount of internal EGR gas, is reserved, the amount of EGR gas can be decreased by decreasing the amount of internal EGR gas. As a result, the feedback control over the EGR rate may be executed by adjusting only the amount of internal EGR gas. [0017] When it is not feasible to adjust the EGR rate within a predetermined adjustment range by adjusting the amount of internal EGR gas using the valve timing changing mechanism, the feedback control over the EGR rate may be executed by adjusting the amount of high-pressure EGR gas that flows through the high-pressure EGR passage and then adjusting the amount of internal EGR gas using the valve timing changing means again.

[0018] The predetermined adjustment range may be, for example, a range in which an amount of change from an immediately preceding valve timing to the current valve timing is equal to or below a predetermined amount. If the amount of change from the immediately preceding valve timing to the current valve timing is larger than the predetermined amount, changing the valve timing alone will not make it possible to achieve the target EGR rate.

[0019] With this configuration, because the amount of high-pressure EGR gas that flows through the high-pressure EGR passage is adjusted and then the amount of internal EGR gas is adjusted, it is possible to make the intake air temperature closer to the target intake air temperature, and to further appropriately achieve the target EGR rate.

[0020] According to the first aspect of the invention, it is possible to minimize the deviation of the intake air temperature from the target intake air temperature, and to appropriately achieve the target EGR rate in the exhaust gas recirculation system for an internal combustion engine.

[0021] 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 has a turbine which is provided in an exhaust passage for an internal combustion engine and a compressor which is provided in an intake passage for the internal combustion engine; a low-pressure EGR passage into which a portion of exhaust gas is taken, as low-pressure EGR gas, from the exhaust passage at a portion downstream of the turbine and through which the low-pressure EGR gas is recirculated back to the intake passage at a portion upstream of the compressor; a high-pressure EGR passage into which a portion of the exhaust gas is taken, as high-pressure EGR gas, from the exhaust passage at a portion upstream of the turbine and through which the high-pressure EGR gas is recirculated back to the intake passage at a portion downstream of the compressor; and valve timing changing means for adjusting an amount of internal EGR gas. According to the method, a feedback control over an EGR rate, which indicates a proportion of the EGR gas to the intake air, is executed by adjusting the amount of internal EGR gas using the valve timing changing means in a period in which an amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below a first predetermined amount.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments 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 internal combustion engine and an intake system and an exhaust system for the internal combustion engine according to a first embodiment of the invention; FIG. 2 is a cross-sectional view schematically showing the internal combustion engine according to the first embodiment of the invention;

FIG. 3 is a graph showing the EGR control states corresponding to the respective operating states of the internal combustion engine according to the first embodiment of the invention;

FIG. 4 is a graph showing changes in the amount of low-pressure EGR gas and the amount of high-pressure EGR gas in each of a region MIX and a region LPL according to the first embodiment of the invention;

FIG. 5 is a flowchart showing the routine of a feedback control executed over an EGR rate in the region MIX according to the first embodiment of the invention; and

FIGs. 6A and 6B illustrate a flowchart showing the routine of a feedback control executed over the EGR rate in the region MIX according to a second embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0022] Hereafter, example embodiments of the invention will be described in detail.

[0023] <First embodiment of the invention> FIG. 1 is a view schematically showing the structure of an internal combustion engine 1 and an intake system and an exhaust system for the internal combustion engine, to which an exhaust gas recirculation system for an internal combustion engine according to a first embodiment of the invention is applied. The internal combustion engine 1 shown in FIG. 1 is a water-cooled four-stroke cycle diesel engine having four cylinders 2. An intake passage 3 and an exhaust passage 4 are connected to the internal combustion engine 1. [0024] A compressor 5a of a turbocharger that operates using the energy of exhaust gas as a drive power source is provided at a middle portion of the intake passage 3 that is connected to the internal combustion engine 1. A first throttle valve 6 that adjusts the flow rate of the intake air that flows through the intake passage 3 is provided in the intake passage 3 at a position upstream of the compressor 5a. The first throttle valve 6 is opened and closed by an electric actuator. An airflow meter 7 that outputs a signal indicating the flow rate of the newly-taken air which flows through the intake passage 3 is provided in the intake passage 3 at a position upstream of the first throttle valve 6. The airflow meter 7 measures the amount of newly-taken air that is introduced into the internal combustion engine 1.

[0025] An intercooler 8 that promotes heat exchange between the intake air and the outside air is provided in the intake passage 3 at a position downstream of the compressor 5a. A second throttle valve 9, which adjusts the flow rate of the intake air that flows through the intake passage 3, is provided in the intake passage 3 at a position downstream of the intercooler 8. The second throttle valve 9 is opened and closed by an electric actuator.

[0026] Meanwhile, a turbine 5b of the turbocharger is provided at a middle portion of the exhaust passage 4 that is connected to the internal combustion engine 1. An exhaust gas control unit 10 is provided in the exhaust passage 4 at a position downstream of the turbine 5b.

[0027] The exhaust gas control unit 10 includes an oxidation catalyst and a particulate filter (hereinafter, simply referred to as "filter") that is provided downstream of the oxidation catalyst. The filter supports a storage-reduction NOx catalyst (hereinafter, simply referred to as "NOx catalyst").

[0028] An exhaust gas throttle valve 11, which adjusts the flow rate of the exhaust gas that flows through the exhaust passage 4, is provided in the exhaust passage 4 at a position downstream of the exhaust gas control unit 10. The exhaust gas throttle valve 11 is opened and closed by an electric actuator. [0029] The internal combustion engine 1 is provided with a low-pressure EGR unit 30 that recirculates a portion of the exhaust gas which flows through the exhaust passage 4 back to the intake passage 3 at low pressure (performs exhaust gas recirculation). The low-pressure EGR unit 30 includes a low-pressure EGR passage 31, a low-pressure EGR valve 32 and a low-pressure EGR cooler 33.

[0030] The low-pressure EGR passage 31 connects the exhaust passage 4, at a portion downstream of the exhaust throttle valve 11, to the intake passage 3, at a portion upstream of the compressor 5a and downstream of the first throttle valve 6. The exhaust gas is introduced, at low pressure, into the internal combustion engine 1 through the low-pressure EGR passage 31. In the first embodiment of the invention, the exhaust gas that is recirculated back to the intake passage 3 through the low-pressure EGR passage 31 will be referred to as "low-pressure EGR gas".

[0031] The low-pressure EGR valve 32 is provided in the low-pressure EGR passage 31, and adjusts the amount of low-pressure EGR gas that flows through the low-pressure EGR passage 31 by adjusting the flow passage area of the low-pressure

EGR passage 31. The amount of low-pressure EGR gas may be adjusted by a method other than adjustment of the opening amount of the low-pressure EGR valve 32. For example, the amount of low-pressure EGR gas is adjusted by adjusting the opening amount of the first throttle valve 6 to change the pressure difference between the upstream side and the downstream side in the low-pressure EGR passage 31.

[0032] The low-pressure EGR cooler 33 is provided in the low-pressure EGR passage 31, and promotes heat exchange between the low-pressure EGR gas passing through the low-pressure EGR cooler 33 and an engine coolant in the internal combustion engine 1 to decrease the temperature of the low-pressure EGR gas.

[0033] In addition, the internal combustion engine 1 is provided with a high-pressure EGR unit 40 that recirculates a portion of the exhaust gas that flows through the exhaust passage 4 back to the intake passage 3 at high pressure. The high-pressure EGR unit 40 includes a high-pressure EGR passage 41 and a high-pressure EGR valve 42.

[0034] The high-pressure EGR passage 41 connects the exhaust passage 4, at a portion upstream of the turbine 5b, to the intake passage 3, at a portion downstream of the compressor 5a. The exhaust gas is introduced, at high pressure, into the internal combustion engine 1 through the high-pressure EGR passage 41. In the first embodiment of the invention, the exhaust gas that is recirculated back to the intake passage 3 through the high-pressure EGR passage 41 will be referred to as "high-pressure EGR gas". [0035] The high-pressure EGR valve 42 is provided in the high-pressure EGR passage 41, and adjusts the amount of high-pressure EGR gas that flows through the high-pressure EGR passage 41 by adjusting the flow passage area of the high-pressure EGR passage 41. The amount of high-pressure EGR gas may be adjusted by a method other than adjustment of the opening amount of the high-pressure EGR valve 42. For example, the amount of high-pressure EGR gas is adjusted by adjusting the opening amount of the second throttle valve 9 to change the pressure difference between the upstream side and the downstream side in the high-pressure EGR passage 41. If the turbine 5b of the turbocharger is a variable capacity turbine, the amount of high-pressure EGR gas may be changed by adjusting the opening amount of a nozzle vane that changes the flow characteristics of the turbine 5b.

[0036] As shown in FIG. 2, a piston 12 is slidably arranged in each cylinder 2 of the internal combustion engine 1. An intake port 13 that leads to the intake passage 3 and an exhaust port 14 that leads to the exhaust passage 4 are connected to a combustion chamber formed at an upper portion within the cylinder 2. The intake port 13 and the exhaust port 14 have openings to the combustion chamber. The opening of the intake port 13 and the opening of the exhaust port 14 are opened and closed by an intake valve 15 and an exhaust valve 16, respectively. The intake valve 15 is provided with an intake VVT (valve timing) mechanism 17 that controls the valve timing of the intake valve 15. The exhaust valve 16 is provided with an exhaust VVT mechanism 18 that controls the valve timing of the exhaust valve 16. A fuel injection valve 19, which injects fuel directly into the combustion chamber, is fitted to the upper face of the cylinder 2.

[0037] The internal combustion engine 1 structured as described above is provided with an ECU 20, which is an electronic control unit that controls the internal combustion engine 1. The ECU 20 controls the operating state of the internal combustion engine 1 based on the operating conditions of the internal combustion engine 1 and a request from a driver. [0038] Various sensors such as the airflow meter 7 are connected to the ECU 20 via electric wires, and signals output from these sensors are transmitted to the ECU 20.

[0039] The actuators for the first throttle valve 6, the second throttle valve 9, the exhaust throttle valve 11 , the low-pressure EGR valve 32, and the high-pressure EGR valve 42, and the intake VVT mechanism 17, the exhaust VVT mechanism 18, and the fuel injection valve 19 are connected to the ECU 20 via electric wires. The ECU 20 controls these devices.

[0040] Next, an EGR control according to the first embodiment of the invention will be described. FIG. 3 is a graph schematically showing the operating region in which only the low-pressure EGR unit 30 is used, the operating region in which the low-pressure EGR unit 30 and the high-pressure EGR unit 40 are used in combination, and the operating region in which only the high-pressure EGR unit 40 is used. The EGR unit that is used is selected based on the operating state of the internal combustion engine, as shown in FIG. 3. In FIG. 3, the abscissa axis represents the rotational speed NE of the internal combustion engine, and the ordinate axis represents the amount Qf of fuel that is injected into the internal combustion engine (hereinafter, referred to as "fuel injection amount Qf). The fuel injection amount Qf is a parameter that is typically used to indicate the load placed on the internal combustion engine.

[0041] In a region HPL in FIG. 3, the internal combustion engine 1 operates at low load and low rotational speed, only the high-pressure EGR unit 40 performs EGR, and only the high-pressure EGR gas is recirculated back to the intake passage 3. In a region MIX, the internal combustion engine operates at medium load and medium rotational speed, the high-pressure EGR unit 40 and the low-pressure EGR unit 30 perform EGR in cooperation, and both the low-pressure EGR gas and the high-pressure EGR gas are recirculated back to the intake passage 3. In a region LPL, the internal combustion engine 1 operates at high load and high rotational speed, only the low-pressure EGR unit 30 performs EGR, and only the low-pressure EGR gas is recirculated back to the intake passage 3.

[0042] As described above, the EGR unit that is used is selected from among the low-pressure EGR unit 30 and the high-pressure EGR unit 40 or the low-pressure EGR unit 30 and the high-pressure EGR unit 40 are used in combination. This makes it possible to perform EGR over a broader engine operating region. As a result, it is possible to reduce the amount of NOx discharged from the internal combustion engine 1.

[0043] A feedback control over an EGR rate, which indicates the proportion of the EGR gas to the intake air, is executed by adjusting the amounts of low-pressure EGR gas and high-pressure EGR gas that are recirculated back to the intake passage 3. When the low-pressure EGR gas and the high-pressure EGR gas are recirculated back to the intake passage 3 at the same time, for example, when the operating state of the internal combustion engine 1 is in the region MIX in which the internal combustion engine operates at low load and low rotational speed, the feedback control over the EGR rate is executed by adjusting the amount of high-pressure EGR gas so that the response to the control and the efficiency of the control are appropriately maintained.

[0044] However, if the amount of high-pressure EGR gas is decreased and the opening amount of the high-pressure EGR valve 42 is decreased, the amount of change in the EGR rate for a given amount of change in the opening amount of the high-pressure EGR valve increases. Therefore, if a feedback gain is large, the high-pressure EGR valve 42 is frequently opened and closed, which causes fluctuations in torque output. On the other hand, if the feedback gain is small, it takes a long time to achieve the target EGR rate. When the amount of high-pressure EGR gas is decreased, execution of the feedback control over the EGR rate by adjusting the amount of high-pressure EGR gas is not very efficient.

[0045] Therefore, when the amount of high-pressure EGR gas is decreased, the feedback control over the EGR rate may be executed by adjusting the amount of low-pressure EGR gas. However, if the feedback control over the EGR rate is executed by adjusting the amount of low-pressure EGR gas, an increase in the amount of low-pressure EGR gas causes the intake air temperature to deviate further from a target intake air temperature because the temperature of the low-pressure EGR gas is low. In addition, the low-pressure EGR valve 32, which is used to adjust the amount of low-pressure EGR gas, is nearly fully open, because the amount of high-pressure EGR gas is decreased. Therefore, the range over which the amount of EGR gas may be changed by opening and closing the low-pressure EGR valve is small. As a result, sometimes it is not possible to achieve the target EGR rate. More specifically, because the low-pressure EGR valve 32 is nearly fully open and the range over which the opening amount of the low-pressure EGR valve 32 may be increased is small, the amount of EGR gas is increased only to the amount corresponding to the largest opening amount of the low-pressure EGR valve 32. Therefore, if more EGR gas is required, a shortage of the EGR gas may occur.

[0046] Therefore, during the transition from the EGR control state in the region MIX to the EGR control state in the region LPL, if the opening amount of the high-pressure EGR valve 42 becomes equal to or below a predetermined opening amount in the region MIX, commands are transmitted to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to control the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively, whereby the amount of internal EGR gas is adjusted. Thus, as shown in FIG. 4, the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas.

[0047] If the opening amount of the high-pressure EGR valve 42 is equal to or below the predetermined opening amount, the amount of high-pressure EGR gas that flows through the high-pressure EGR passage 41 is made equal to or below a first predetermined amount. If the amount of high-pressure EGR gas is equal to or below the first predetermined amount, which is used as a threshold value, the feedback control over the EGR rate cannot be executed only by adjusting the amount of high-pressure EGR gas. [0048] The ECU 20, which transmits commands to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to control the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively, functions as valve timing changing means according to the invention. The valve timing changing means is not limited to the ECU 20, as long as it is able to adjust the amount of internal EGR gas. For example, the valve timing changing means may be a unit that transmits a command at least to the exhaust VVT mechanism 18 to control the closing timing of the exhaust valve 16, thereby adjusting the amount of internal EGR gas.

[0049] Commands are transmitted to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to retard the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively. As a result, a portion of the gas that has been burned in the combustion chamber of the internal combustion engine 1 remains within the cylinder 2, or a portion of the burned gas that is discharged to the exhaust passage 4 is drawn back into the cylinder 2. In this way, the internal EGR gas is supplied into the cylinder 2. Then, commands are transmitted to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to control the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively, whereby the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas. Because the temperature of the internal EGR gas is close to the temperature of the high-pressure EGR gas, it is possible to minimize the deviation of the temperature of the intake air from the target intake air temperature even if the amount of internal EGR gas increases. In addition, the range over which the amount of EGR gas may be changed is large because the entire amount of the internal EGR gas may be adjusted through the feedback control. Further, the internal EGR gas does not need to flow through circulation passages outside of the internal combustion engine 1 and responds quickly to a control for increasing or decreasing the amount of internal EGR gas. Therefore, it is possible to appropriately achieve the target EGR rate.

[0050] The high-pressure EGR gas decrease region includes an internal EGR gas amount adjustment region in which the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas. In a region which is within the high-pressure EGR gas decrease region and which is close to the internal EGR gas amount adjustment region, a second predetermined amount of internal EGR gas is reserved. The second predetermined amount is used as a reference amount for the feedback control over the EGR rate.

[0051] As shown in FIG. 4, the high-pressure EGR gas decrease region is a portion of the region MIX, in which the amount of high-pressure EGR gas is decreasing. The high-pressure EGR gas decrease region includes a region in which the opening amount of the high-pressure EGR valve is equal to or below the predetermined opening amount. The second predetermined amount is an amount that is equal to or larger than the maximum amount by which the amount of internal EGR gas is decreased when the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas.

[0052] When the EGR rate is decreased through the feedback control over the EGR rate, namely, when the amount of EGR gas is excessive, the amount of EGR gas needs to be decreased. In this case, if the second predetermined amount of internal EGR gas, that is, the reference amount of internal EGR gas, is reserved, the amount of EGR gas is decreased by decreasing the amount of internal EGR gas. As a result, the feedback control over the EGR rate may be executed by adjusting only the amount of internal EGR gas.

[0053] In the first embodiment of the invention, the second predetermined amount of internal EGR gas is reserved only in the high-pressure EGR gas decrease region. Alternatively, the second predetermined amount of internal EGR gas may be reserved in the entire region MIX.

[0054] Next, the routine of the feedback control over the EGR rate which is executed when the engine operating state is in the region MIX according to the first embodiment of the invention will be described. FIG. 5 is a flowchart showing the routine of the feedback control over the EGR rate that is executed when the engine operating state is in the region MIX according to the first embodiment of the invention. The routine is executed at predetermined time intervals.

[0055] In step (hereinafter, referred to as "S") 101, the ECU 20 determines whether the internal combustion engine 1 operates at medium load and medium rotational speed and the EGR control state is within the region MIX.

[0056] If it is determined in SlOl that the EGR control state is not within the region MIX, the routine ends. On the other hand, if it is determined in SlOl that the EGR control state is within the region MIX, S 102 is executed.

[0057] In S 102, the ECU 20 determines whether the opening amount of the high-pressure EGR valve is larger than the predetermined opening amount.

[0058] If it is determined in S 102 that the opening amount of the high-pressure EGR valve is larger than the predetermined value, S 103 is executed. On the other hand, if it is determined in S 102 that the opening amount of the high-pressure EGR valve is equal to or below the predetermined amount, S 107 is executed. [0059] If it is determined by another process that the EGR control state is within the high-pressure EGR gas decrease region, the ECU 20 transmits commands to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to retard the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively, thereby supplying the second predetermined amount of internal EGR gas to the internal combustion engine 1. In this case, a control is executed to decrease the amount of high-pressure EGR gas by an amount corresponding to an increase in the EGR gas due to supply of the internal EGR gas.

[0060] In S 103, the ECU 20 subtracts the target EGR rate from the actual EGR rate, and determines whether the difference between the target EGR rate and the actual EGR rate is a positive value.

[0061] If it is determined that the difference between the target EGR rate and the actual EGR rate is a positive value (> 0), S 104 is executed. On the other hand, if it is determined that the difference between the target EGR rate and the actual EGR rate is a negative value (< 0), S 105 is executed.

[0062] In S 104, the ECU 20 decreases the opening amount of the high-pressure EGR valve. Thus, the amount of high-pressure EGR gas is decreased to decrease the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S 104, S 106 is executed.

[0063] In S 105, the ECU 20 increases the opening amount of the high-pressure EGR valve. Thus, the amount of high-pressure EGR gas is increased to increase the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S 105, S 106 is executed. [0064] In S 106, the ECU 20 subtracts the target EGR rate from the actual EGR rate, and determines whether the absolute value of the difference between the actual EGR rate and the target EGR rate is equal to or below a predetermined value.

[0065] It is preferable that the predetermined value be substantially equal to zero. However, a value other than zero may be used as long as it is possible to determine whether the actual EGR rate substantially matches the target EGR rate using the predetermined value.

[0066] If it is determined in S 106 that the absolute value of the difference between the actual EGR rate and the target EGR rate is larger than the predetermined value, S 102 is executed. On the other hand, if it is determined in S 106 that the absolute value of the difference between the actual EGR rate and the target EGR rate is equal to or below the predetermined value, it is determined that the actual EGR rate substantially matches the target EGR rate and the routine ends.

[0067] In S 107, the ECU 20 subtracts the target EGR rate from the actual EGR rate, and determines whether the difference between the target EGR rate and the actual EGR rate is a positive value.

[0068] If it is determined in S 107 that the difference between the target EGR rate and the actual EGR rate is a positive value (> 0), S 108 is executed. On the other hand, if it is determined in S 107 that the difference between the target EGR rate and the actual EGR rate is a negative value, S 109 is executed.

[0069] In S 108, the ECU 20 transmits commands to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to advance the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively. Thus, the amount of internal EGR gas is decreased to decrease the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S 108, SIlO is executed.

[0070] In S 109, the ECU 20 transmits commands to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to retard the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively. Thus, the amount of internal EGR gas is increased to increase the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S 109, SIlO is executed.

[0071] In SIlO, the ECU 20 subtracts the target EGR rate from the actual EGR rate, and determines whether the absolute value of the difference between the actual EGR rate and the target EGR rate is equal to or below a predetermined value.

[0072] If it is determined in Sl 10 that the absolute value of the difference between the actual EGR rate and the target EGR rate is larger than the predetermined value, S 107 is executed. On the other hand, if it is determined in SI lO that the absolute value of the difference between the actual EGR rate and the target EGR rate is equal to or below the predetermined value, it is determined that the actual EGR rate substantially matches the target EGR rate and the routine ends.

[0073] Executing the above-described routine makes it possible to execute the feedback control over the EGR rate by adjusting the amount of high-pressure EGR gas and the amount of internal EGR gas when the EGR control state is within the region MIX.

[0074] <Second embodiment of the invention> FIGs. 6A and 6B illustrate the flowchart showing the routine of the feedback control over the EGR rate in the region MIX according to a second embodiment of the invention. The routine shown in FIGs. 6A and 6B is substantially the same as the routine shown in FIG. 5 in the first embodiment of the invention except that if the amount of change in each of the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16 exceeds a predetermined amount, the opening amount of the high-pressure EGR valve is adjusted again. The other configurations are same as those in the first embodiment of the invention. Therefore, the same step numbers are assigned and the detailed description will not be provided below.

[0075] In S 108, the ECU 20 transmits commands to the intake VVT mechanism 17 and the exhaust VVT mechanism 18 to advance the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16, respectively. Thus, the amount of internal EGR gas is decreased to decrease the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S 108, S201 is executed. [0076] In S 109, the ECU 20 transmits commands to the intake VVT mechanism

17 and the exhaust VVT mechanism 18 to retard the opening timing of the intake valve

15 and the closing timing of the exhaust valve 16, respectively. Thus, the amount of internal EGR gas is increased to increase the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S 109, S201 is executed.

[0077] In S201, the ECU 20 determines whether the amount of change in each of the opening timing of the intake valve 15 and the closing timing of the exhaust valve

16 is equal to or below a predetermined amount. [0078] If the amount of change exceeds the predetermined amount, which is used as a threshold value, the target EGR rate is not achieved only by changing the opening timing of the intake valve 15 and closing timing of the exhaust valve 16.

[0079] If it is determined in S201 that the amount of change is equal to or below the predetermined amount, SIlO is executed. After completion of SIlO, the same process as that in the first embodiment of the invention is executed. On the other hand, if it is determined that the amount of change exceeds the predetermined amount,

S202 is executed.

[0080] In S202, the ECU 20 subtracts the target EGR rate from the actual EGR rate, and determines whether the difference between the target EGR rate and the actual EGR rate is a positive value.

[0081] If it is determined in S202 that the difference between the target EGR rate and the actual EGR rate is a positive value (> 0), S203 is executed. On the other hand, if it is determined in S202 that the difference between the target EGR rate and the actual EGR rate is a negative value (< 0), S204 is executed.

[0082] In S203, the ECU 20 decreases the opening amount of the high-pressure EGR valve. Thus, the amount of high-pressure EGR gas is decreased to decrease the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S203, S 107 is executed. [0083] In S204, the ECU 20 increases the opening amount of the high-pressure

EGR valve. Thus, the amount of high-pressure EGR gas is increased to increase the actual EGR rate, whereby the actual EGR rate is brought closer to the target EGR rate. After completion of S204, S 107 is executed.

[0084] According to the routine described above, the amount of EGR gas is readjusted by adjusting the opening amount of the high-pressure EGR valve, when it is not possible to achieve the target EGR rate only by changing the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16. Therefore, it is possible to bring the intake air temperature closer to the target intake air temperature. In addition, it is possible to more appropriately achieve the target EGR rate.

[0085] According to the second embodiment of the invention, to determine whether the amount of EGR gas should be readjusted by adjusting the opening amount of the high-pressure EGR valve, it is determined whether the amount of change in each of the opening timing of the intake valve 15 and the closing timing of the exhaust valve 16 is equal to or below the predetermined amount. However, various other determination methods may be employed. For example, it may be determined whether the amount by which the internal EGR gas amount is adjusted exceeds the maximum control amount. [0086] The exhaust gas recirculation system for an internal combustion engine according to the invention is not limited to the embodiments described above, and may be implemented in various other embodiments that are within the scope of the invention.

Claims

CLAIMS:

1. An exhaust gas recirculation system for an internal combustion engine (1), which includes: a turbocharger (5) that has a turbine (5b) which is provided in an exhaust passage (4) for an internal combustion engine (1) and a compressor (5 a) which is provided in an intake passage (3) for the internal combustion engine (1); a low-pressure EGR passage (31) into which a portion of exhaust gas is taken, as low-pressure EGR gas, from the exhaust passage (4) at a portion downstream of the turbine (5b) and through which the low-pressure EGR gas is recirculated back to the intake passage (3) at a portion upstream of the compressor (5a); a high-pressure EGR passage (41) into which a portion of the exhaust gas is taken, as high-pressure EGR gas, from the exhaust passage (4) at a portion upstream of the turbine (5b) and through which the high-pressure EGR gas is recirculated back to the intake passage (3) at a portion downstream of the compressor (5 a); and valve timing changing means (20) for adjusting an amount of internal EGR gas, the exhaust gas recirculation system characterized in that in a period in which an amount of high-pressure EGR gas that flows through the high-pressure EGR passage (41) is equal to or below a first predetermined amount, a feedback control over an EGR rate, which indicates a proportion of EGR gas to intake air, is executed by adjusting the amount of internal EGR gas using the valve timing changing means (20).

2. The exhaust gas recirculation system according to claim 1, wherein the period in which the amount of high-pressure EGR gas that flows through the high-pressure EGR passage (41) is equal to or below the first predetermined amount is a period of transition from an EGR control state in which the low-pressure EGR gas and the high-pressure EGR gas are recirculated back to the intake passage through the low-pressure EGR passage (31) and the high-pressure EGR passage (41), respectively, to an EGR control state in which only the low-pressure EGR gas is recirculated back to the intake passage through the low-pressure EGR passage (31).

3. The exhaust gas recirculation system according to claim 1 or 2, wherein in a high-pressure gas decrease region which includes a region in which the amount of high-pressure EGR gas that flows through the high-pressure EGR passage (41) is equal to or below the first predetermined amount, a second predetermined amount of internal EGR gas is reserved using the valve timing changing means (20), and the second predetermined amount is used as a reference amount for the feedback control over the EGR rate.

4. The exhaust gas recirculation system according to any one of claims 1 to 3, wherein when it is not feasible to adjust the EGR rate within a predetermined adjustment range by adjusting the amount of internal EGR gas using the valve timing changing mechanism, the feedback control over the EGR rate is executed by adjusting the amount of high-pressure EGR gas that flows through the high-pressure EGR passage (41) and then adjusting the amount of internal EGR gas using the valve timing changing means (20) again.

5. The exhaust gas recirculation system according to any one of claims 1 to 4, wherein the first predetermined amount is a threshold value, and if the amount of high-pressure EGR gas is equal to or below the first predetermined amount, it is not possible to execute the feedback control over the EGR rate only by adjusting the amount of high-pressure EGR gas.

6. The exhaust gas recirculation system according to any one of claims 1 to 5, wherein the valve timing changing means (20) controls closing timing of an exhaust valve to adjust the amount of internal EGR gas.

7. The exhaust gas recirculation system according to claim 3, wherein the second predetermined amount is equal to or larger than a maximum amount by which the amount of internal EGR gas is decreased when the feedback control over the EGR rate is executed by adjusting the amount of internal EGR gas.

8. The exhaust gas recirculation system according to claim 3, wherein the second predetermined amount is an amount at which the feedback control over the EGR rate is executed by adjusting only the amount of internal EGR gas.

9. The exhaust gas recirculation system according to claim 4, wherein the predetermined adjustment range is a range in which an amount of change from an immediately preceding valve timing to a current valve timing is equal to or below a predetermined amount.

10. A method for controlling an exhaust gas recirculation system for an internal combustion engine (1), which includes: a turbocharger (5) that has a turbine (5b) which is provided in an exhaust passage (4) for an internal combustion engine (1) and a compressor (5a) which is provided in an intake passage (3) for the internal combustion engine (1); a low-pressure EGR passage (31) into which a portion of exhaust gas is taken, as low-pressure EGR gas, from the exhaust passage (4) at a portion downstream of the turbine (5b) and through which the low-pressure EGR gas is recirculated back to the intake passage (3) at a portion upstream of the compressor (5 a); a high-pressure EGR passage (41) into which a portion of the exhaust gas is taken, as high-pressure EGR gas, from the exhaust passage (4) at a portion upstream of the turbine (5b) and through which the high-pressure EGR gas is recirculated back to the intake passage (3) at a portion downstream of the compressor (5a); and valve timing changing means (20) for adjusting an amount of internal EGR gas, the method characterized by comprising: executing a feedback control over an EGR rate, which indicates a proportion of

EGR gas to intake air, by adjusting the amount of internal EGR gas using the valve timing changing means (20) in a period in which an amount of high-pressure EGR gas that flows through the high-pressure EGR passage (41) is equal to or below a first predetermined amount.

11. An exhaust gas recirculation system for an internal combustion engine, comprising: a turbocharger that has a turbine which is provided in an exhaust passage for an internal combustion engine and a compressor which is provided in an intake passage for the internal combustion engine; a low-pressure EGR passage into which a portion of exhaust gas is taken, as low-pressure EGR gas, from the exhaust passage at a portion downstream of the turbine and through which the low-pressure EGR gas is recirculated back to the intake passage at a portion upstream of the compressor; a high-pressure EGR passage into which a portion of the exhaust gas is taken, as high-pressure EGR gas, from the exhaust passage at a portion upstream of the turbine and through which the high-pressure EGR gas is recirculated back to the intake passage at a portion downstream of the compressor; and valve timing changing unit that adjusts an amount of internal EGR gas, wherein in a period in which an amount of high-pressure EGR gas that flows through the high-pressure EGR passage is equal to or below a first predetermined amount, a feedback control over an EGR rate, which indicates a proportion of EGR gas to intake air, is executed by adjusting the amount of internal EGR gas using the valve timing changing unit.