WO2016097843A1

WO2016097843A1 - Exhaust gas recirculation device for internal combustion engine

Info

Publication number: WO2016097843A1
Application number: PCT/IB2015/002361
Authority: WO
Inventors: Akira Kato; Yusuke HOUKI
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2014-12-18
Filing date: 2015-12-17
Publication date: 2016-06-23
Also published as: JP2016118102A

Abstract

A first EGR passage (60) circulates a CO₂-enriched gas such that the CO₂-enriched gas obtained by converting an exhaust gas using a CO₂enrichment module (52) is returned to an intake passage (30) of an engine (10). A second EGR passage (70) circulates the exhaust gas such that the exhaust gas flowing in an exhaust passage (40) bypasses a CO₂ enrichment portion (50) and is returned to the intake passage (30) of the engine (10). A gas amount adjustment portion (81, 82) adjusts the amount of the CO₂-enriched gas returned to the intake passage (30) through the first EGR passage (60) and the amount of the exhaust gas returned to the intake passage (30) through the second EGR passage (70). With this, an increase in the generation amount of an unburned gas resulting from the circulation of the CO₂-enriched gas is avoided while reductions in the generation amounts of NOx and soot resulting from the circulation of the CO₂- enriched gas are utilized.

Description

EXHAUST GAS RECIRCULATION DEVICE FOR INTERNAL COMBUSTION

ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The invention relates to an exhaust gas recirculation (hereinafter sometimes referred to as "EGR") device for an internal combustion engine that performs EGR. 2. Description of Related Art

[0002] An EGR device that extracts part of an exhaust gas discharged from a combustion chamber of an internal combustion engine from an exhaust passage, guides the exhaust gas to an intake passage via an EGR passage that provides communication between the exhaust passage and the intake passage, and causes the combustion chamber to take in the exhaust gas again is widely used. The exhaust gas recirculated to the combustion engine in this manner (i.e., an EGR gas, hereinafter sometimes referred to as "the EGR gas") has a low concentration of oxygen and a high concentration of each of carbon dioxide and water (water vapor) as compared with those of fresh air newly sucked into the combustion chamber from the outside. Further, the specific heat ratio of each of carbon dioxide and water is smaller than the specific heat ratio of each of oxygen and nitrogen as main components of the fresh air.

[0003] Accordingly, the oxygen concentration and the specific heat ratio of the gas sucked into the combustion chamber when the EGR gas is recirculated (i.e., during execution of the EGR) are lower than the oxygen concentration and the specific heat ratio of the gas (the fresh air) sucked into the combustion chamber when the EGR is not executed. As a result, a peak combustion temperature (the maximum value of a combustion temperature in the combustion chamber) is reduced, and hence the generation amount of nitrogen oxides (NOx) is reduced.

[0004] Incidentally, when an EGR ratio (a ratio of the amount of "the EGR gas" to the total amount of "a gas mixture of the fresh air and the EGR gas" as the gas sucked into the combustion chamber) is excessively increased in order to further reduce the generation amount of NOx, an oxygen amount in the combustion chamber becomes significantly insufficient for the oxygen amount required for ideal combustion. As a result, the combustion becomes unstable, and the generation amount of soot is increased.

[0005] To cope with this, there is proposed the EGR device in which a carbon dioxide (C0₂) enrichment module is disposed in the exhaust passage, a gas having a high carbon dioxide concentration is generated from the exhaust gas discharged from the combustion chamber, and the generated gas is recirculated to the combustion chamber (see, e.g., Japanese Patent Application Publication No. 2011-001944). Note that, in the following description, the exhaust gas of which the carbon dioxide concentration is increased by the C0₂ enrichment module (i.e., the gas that mainly contains only carbon dioxide contained in the exhaust gas) is sometimes simply referred to as "a C0₂-enriched gas".

[0006] According to the conventional EGR device described above, the

C0₂-enriched gas having the specific heat ratio smaller than the specific heat ratio of "the gas discharged from the combustion chamber" having nitrogen, water, and carbon dioxide as main components is recirculated as the EGR gas. As a result, as compared with the case where the exhaust gas discharged from the combustion chamber is recirculated as the EGR gas, it is possible to reduce the peak combustion temperature with a smaller amount of the EGR gas, and thereby reduce the generation amount of NOx. Consequently, in the conventional EGR device described above, it is possible to effectively reduce the generation amount of NOx without excessively reducing the oxygen concentration of the gas sucked into the combustion chamber, and hence it is possible to avoid an increase in the generation amount of not only NOx but also soot.

[0007] However, the C0₂ enrichment module has a relatively large heat capacity, and hence the temperature of the C0₂-enriched gas generated by the C0₂ enrichment module is considerably lower than the temperature of the exhaust gas flowing into the C0₂ enrichment module. Accordingly, when the amount of "the C0₂-enriched gas having a relatively low temperature" flowing into the combustion chamber is increased, there are cases where the peak combustion temperature is excessively reduced. As a result, there is a possibility that a problem that an unburned gas (e.g., carbon monoxide (CO), hydrocarbon (HC), or the like) contained in exhaust is increased occurs.

SUMMARY OF THE INVENTION

[0008] The invention has been made in view of the above-described problem. That is, the invention provides the EGR device for the internal combustion engine capable of reducing the amount of emission of NOx by recirculating the C0₂-enriched gas to the combustion chamber and preventing an increase in the amount of emission of the unburned gas resulting from the recirculation of the C0₂-enriched gas. Hereinafter, carbon dioxide is sometimes abbreviated as "C0₂", as in the C0₂-enriched gas.

[0009] According to an aspect of the invention, there is provided an exhaust gas recirculation (i.e. EGR) device (hereinafter sometimes referred to as "a device of the invention") for an internal combustion engine that includes a C0₂ enrichment portion, a first EGR passage, a second EGR passage, and a gas amount adjustment portion.

[0010] The C0₂ enrichment portion includes a C0₂ enrichment module, and the C0₂ enrichment portion is configured to take in an exhaust gas flowing in an exhaust passage of the internal combustion engine (i.e., an usual exhaust gas discharged from a combustion chamber) from a gas inlet portion connected to the exhaust passage. The C0₂ enrichment portion is configured to convert the exhaust gas, which has been taken in the C0₂ enrichment portion, into "a C0₂-enriched gas, a carbon dioxide concentration of which is increased to be higher than a carbon dioxide concentration of the exhaust gas flowing in the exhaust passage" by using "the C0₂ enrichment module". As will be described later, the C0₂ enrichment module may be a module that uses a C0₂ facilitated transport phenomenon or a module that uses an absorbent such as active carbon, and is not particularly limited.

[0011] The first EGR passage is configured to circulate (return) the C0₂-enriched gas resulting from the conversion such that the C0₂-enriched gas is returned to an intake passage of the internal combustion engine. The second EGR passage is configured to circulate (return) the exhaust gas flowing in the exhaust passage such that the exhaust gas flowing in the exhaust passage is caused to bypass the C0₂ enrichment portion and is returned to the intake passage without being converted into the C0₂-enriched gas.

[0012] The gas amount adjustment portion is configured to adjust a first gas amount and a second gas amount. The first gas amount is an amount of the C0₂-enriched gas returned to the intake passage through the first EGR passage. The second gas amount is an amount of the exhaust gas returned to the intake passage through the second EGR passage.

[0013] According to the device of the invention, "the C0₂-enriched gas of which a specific heat ratio is made smaller than that of the usual exhaust gas by increasing the carbon dioxide concentration" is returned to the intake passage through the first EGR passage. That is, part of the exhaust gas is converted into the C0₂-enriched gas, and the C0₂-enriched gas is recirculated to the combustion chamber. As a result, as compared with the case where the usual exhaust gas is recirculated as an EGR gas without the conversion, it is possible to reduce a peak combustion temperature with a smaller amount of the EGR gas, and hence it is possible to reduce the generation amount of NOx with a smaller amount of the EGR gas.

[0014] In addition, the device of the invention is also capable of returning the usual exhaust gas (a gas having a relatively high temperature) to the intake passage through the second EGR passage. Consequently, in the case where there is a possibility that the peak combustion temperature is excessively reduced and an unburned gas component is increased when only the C0₂-enriched gas having a relatively low temperature is recirculated, it is possible to recirculate the usual exhaust gas having a relatively high temperature in addition to or instead of the C0₂-enriched gas. Further, it is possible to adjust the amount of the C0₂-enriched gas and the amount of the usual exhaust gas using the gas amount adjustment portion.

[0015] Consequently, the device of the invention is capable of preventing the temperature of the entire EGR gas recirculated to the combustion chamber from being excessively reduced, and avoid an excessive reduction in peak combustion temperature. As a result, it is possible to avoid a disadvantage resulting from the recirculation of the C0 -enriched gas (i.e., an increase in the generation amount of the unburned gas) while utilizing an advantage resulting from the recirculation of the C0₂-enriched gas (i.e., it is possible to reduce the generation amount of NOx with a smaller amount of the EGR gas, and therefore also reduce the generation amount of soot).

[0016] In addition, in the device of the invention, the C0₂ enrichment portion may include a branched passage branched from the exhaust passage. One end portion of the branched passage constitutes the gas inlet portion of the C0₂ enrichment portion. Further, the C0₂ enrichment module may include a C0₂ facilitated transport membrane that allows passage of the exhaust gas flowing into the C0₂ enrichment module through the branched passage.

[0017] The C0₂ facilitated transport membrane (hereinafter sometimes simply referred to as "a facilitated transport membrane") is a membrane obtained by, e.g., forming a polymer compound layer containing a carbon dioxide carrier and water on a surface of a carrier such as a porous membrane. In general, when high pressure is applied to such a C0₂ facilitated transport membrane, there is a possibility that the C0₂ facilitated transport membrane is damaged. Accordingly, when the C0₂ enrichment module including the C0₂ facilitated transport membrane is disposed in the exhaust passage simply (in series), all of the exhaust gas discharged from the combustion chamber flows into the C0₂ enrichment module, and hence an internal pressure of the C0₂ enrichment module becomes excessive, and there is a possibility that the C0₂ facilitated transport membrane is damaged.

[0018] In contrast to this, according to the configuration of the device of the invention described above, since the exhaust gas flows into the C0₂ enrichment module through "the branched passage branched from the exhaust passage", the internal pressure of the C0₂ enrichment module is less likely to become excessive. As a result, it is possible to reduce the possibility that the C0₂ facilitated transport membrane is damaged.

[0019] Incidentally, in general, the C0₂ facilitated transport membrane is degraded (deteriorated or damaged) when a high-temperature gas comes in contact with the C0₂ facilitated transport membrane, and there is a possibility that the C0₂ facilitated transport membrane cannot exert its C0₂ enrichment function adequately due to the degradation.

[0020] To cope with this, the C0₂ enrichment portion of the device of the invention may include an exhaust gas cooling device. The exhaust gas cooling device is disposed in the branched passage and is configured to cool the exhaust gas flowing into the C0₂ enrichment module by using a refrigerant.

[0021] According to the configuration of the device of the invention described above, since it is possible to reduce the temperature of the exhaust gas flowing into the C0₂ enrichment module by using the refrigerant, it is possible to increase the life of the C0₂ enrichment module having the C0₂ facilitated transport membrane.

[0022] In this case, the EGR device may further include a first controller. The first controller may be configured to (i) acquire a refrigerant temperature as a temperature of the refrigerant, (ii) determine an upper limit value of the first gas amount such that the upper limit value is smaller as the acquired refrigerant temperature is higher, and (iii) issue an instruction to the gas amount adjustment portion such that the first gas amount is not more than the determined upper limit value.

[0023] According to the configuration of the device of the invention described above, the upper limit value of the amount of the exhaust gas flowing into the C0₂ enrichment module can be made smaller as the temperature of the refrigerant (i.e., the refrigerant temperature) of the exhaust gas cooling device is higher. Consequently, it is possible to prevent the temperature in the C0₂ enrichment module from becoming excessively high. As a result, it is possible to return the C0₂-enriched gas in an amount that is as close to "the amount of the C0₂-enriched gas required to improve the emission of the internal combustion engine" as possible to the intake passage within a range that does not cause the degradation of the C0₂ enrichment module.

[0024] In addition, in the device of the invention, one end portion of the second EGR passage may be connected to the exhaust passage at a portion of the exhaust passage "on an upstream side of the gas inlet portion of the C0₂ enrichment portion".

[0025] According to the device of the invention described above, the connection portion between the exhaust passage and the second EGR passage is close to the combustion chamber, and hence it is possible to cause the exhaust gas having a high temperature to flow into the second EGR passage and return to the intake passage. Consequently, it is possible to reduce the amount of the exhaust gas required to avoid a reduction in peak combustion temperature resulting from the recirculation of the C0₂-enriched gas. As a result, the oxygen concentration of the gas flowing into the combustion chamber is less likely to be excessively reduced, and hence it is possible to reduce the generation amount of soot more reliably. Further, the connection portion between the exhaust passage and the gas inlet portion of the C0₂ enrichment portion is away from the combustion chamber, and hence it is possible to cause the exhaust gas having a relatively low temperature that is preferable for the C0₂ enrichment module from the viewpoint of avoiding the degradation of the C0₂ enrichment module in general to flow into the C0₂ enrichment module. Consequently, it is possible to delay the degradation of the C0₂ enrichment module.

[0026] In addition, in the device of the invention, the first EGR passage, the second EGR passage, and the gas amount adjustment portion may be configured in the following manner. (1) The first EGR passage is a passage that connects (provides communication between) a C0₂-enriched gas outlet of the C0₂ enrichment module and the intake passage. (2) The second EGR passage includes a bypass passage and a downstream portion of the first EGR passage. The bypass passage connects (provides communication between) the portion of the exhaust passage on the upstream side of the gas inlet portion of the C0₂ enrichment portion in a direction of flow of the exhaust gas flowing in the exhaust passage and the first EGR passage. The downstream portion of the first EGR passage extends from a portion of the first EGR passage to which the bypass passage is connected to a portion of the first EGR passage connected to the intake passage. (3) The gas amount adjustment portion includes a three-way valve and a flow rate adjustment valve, the three-way valve is disposed at a portion at which the first EGR passage and the bypass passage are connected to each other, and the flow rate adjustment valve is disposed in the downstream portion of the first EGR passage.

[0027] According to the configuration of the device of the invention described above, the downstream portion of the first EGR passage is used as part of the second EGR passage, and hence it is possible to reduce the total length of members constituting the first EGR passage and the second EGR passage. Further, it is possible to adjust an EGR ratio using the flow rate adjustment valve, and adjust "a ratio of the C0₂-enriched gas to the EGR gas returned to the intake passage (a C0₂-enriched gas ratio as a ratio of the first gas amount to a total of the first gas amount and the second gas amount)" using the three-way valve.

[0028] In addition, the EGR device may further include a second controller. The second controller may be configured to (i) acquire a load of the internal combustion engine, and (ii) issue an instruction to the gas amount adjustment portion such that a C0₂-enriched gas ratio in a case where the acquired load is a first load is smaller than a C0₂-enriched gas ratio in a case where the acquired load is a second load. Here, the second load is higher than the first load. The C0₂-enriched gas ratio is the ratio of the first gas amount to the total of the first gas amount and the second gas amount.

[0029] According to the configuration of the device of the invention described above, in the case where the load of the internal combustion engine is small and an in-cylinder temperature is low (consequently, the peak combustion temperature is low), it is possible to increase the ratio of "the usual exhaust gas having a relatively high temperature" to the entire EGR gas. As a result, it is possible to prevent the peak combustion temperature from being excessively reduced, and hence it is possible to prevent an increase in the generation amount of the unburned gas.

[0030] In addition, the EGR device may further include a third controller. The third controller may be configured to (i) acquire an engine correlation temperature having a correlation with a temperature of the internal combustion engine, and (ii) issue an instruction to the gas amount adjustment portion such that the C0₂-enriched gas ratio in a case where the acquired engine correlation temperature is a first temperature is smaller than the C0₂-enriched gas ratio in a case where the acquired engine correlation temperature is a second temperature. Here, the second temperature is higher than the first temperature.

[0031] According to the configuration of the device of the invention described above, the ratio of "the usual exhaust gas having a relatively high temperature" to the entire EGR gas can be made higher as the temperature of the internal combustion engine is lower and the in-cylinder temperature is lower (consequently, the peak combustion temperature is lower). As a result, it is possible to prevent the peak combustion temperature from being excessively reduced, and hence it is possible to prevent an increase in the generation amount of the unburned gas.

[0032] In addition, the EGR device may further include a fourth controller. The fourth controller may be configured to (i) acquire a load of the internal combustion engine, a rotational speed of the internal combustion engine, and an engine correlation temperature having a correlation with a temperature of the internal combustion engine, and (ii) issue an instruction to the gas amount adjustment portion such that the first gas amount and the second gas amount are adjusted based on the acquired load, the acquired rotational speed, and the acquired engine correlation temperature.

[0033] The peak combustion temperature has strong correlations with the load of the internal combustion engine, the rotational speed of the internal combustion engine, and the engine correlation temperature (e.g., a coolant temperature) having the correlation with the temperature of the internal combustion engine. Consequently, as in the above configuration, when the first gas amount (i.e., the amount of the C0₂-enriched gas having a relatively low temperature) and the second gas amount (i.e., the amount of the usual exhaust gas having a relatively high temperature) are adjusted based on these parameters, it is possible to set the peak combustion temperature to a proper value corresponding to an operation state of the engine. Consequently, it is possible to effectively reduce the generation amount of each of NOx, soot, and the unburned component.

[0034] In addition, the EGR device may further include a fifth controller. The fifth controller may be configured to (i) acquire a load of the internal combustion engine, a rotational speed of the internal combustion engine, and an engine correlation temperature having a correlation with a temperature of the internal combustion engine, (ii) determine a threshold engine temperature, based on the acquired load and the acquired rotational speed, (iii) issue an instruction to the gas amount adjustment portion such that the first gas amount matches a first predetermined amount corresponding to the acquired load and the acquired rotational speed and the second gas amount is 0 in a case where the acquired engine correlation temperature is higher than or equal to the threshold engine temperature, and (iv) issue an instruction to the gas amount adjustment portion such that the second gas amount matches a second predetermined amount corresponding to the acquired load and the acquired rotational speed and the first gas amount is 0 in a case where the acquired engine correlation temperature is lower than the threshold engine temperature.

[0035] According to the configuration of the device of the invention described above, in the case where there is no possibility that the peak combustion temperature is excessively reduced, it is possible to recirculate only the C0₂-enriched gas to the combustion chamber, and hence it is possible to reduce the generation amount of each of NOx and soot. On the other hand, in the case where there is a possibility that the peak combustion temperature is excessively reduced, it is possible to recirculate only the exhaust gas to the combustion chamber, and avoid an increase in the generation amount of the unburned gas.

[0036] Other features and associated advantages of the invention will be readily understood from the following description of embodiments of the invention described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic view showing a configuration of an EGR device (first device) for an internal combustion engine according to a first embodiment of the invention; FIG 2 is a flowchart showing a routine executed by an electric control device shown in FIG 1 ;

FIG 3 is a schematic view showing a configuration of an EGR device (second device) for an internal combustion engine according to a second embodiment of the invention;

FIG. 4 is a flowchart showing a routine executed by an electric control device shown in FIG 3;

FIG. 5 is a flowchart showing a routine executed by an electric control device of an EGR device (third device) for an interval combustion engine according to a third embodiment of the invention;

FIG. 6 is a flowchart showing a routine executed by an electric control device of an EGR device (fourth device) for an internal combustion engine according to a fourth embodiment of the invention;

FIG. 7 is a schematic view showing a configuration of an EGR device for an internal combustion engine according to a first modification of the invention;

FIG. 8 is a schematic view showing a configuration of a C0₂ enrichment portion of the EGR device for an internal combustion engine according to the first modification of the invention; and

FIG. 9 is a schematic view showing a configuration of an EGR device for an internal combustion engine according to a second modification of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0038] Hereinbelow, an EGR device (hereinafter sometimes referred to as "a first device") for an internal combustion engine according to a first embodiment of the invention will be described.

[0039] (configuration)

The first device is applied to an internal combustion engine (engine) 10 shown in FIG 1. The engine 10 is a spark-ignition multi-cylinder gasoline engine. The engine 10 has a main body portion 20, an intake passage 30, and an exhaust passage 40. [0040] The main body portion 20 includes a combustion chamber 21. To the combustion chamber 21, an air- fuel mixture containing fuel injected from a fuel injection valve 22 is supplied. The air- fuel mixture supplied to the combustion chamber 21 is ignited by a spark generated by an ignition plug 23 and is combusted. As a result, an exhaust gas is generated.

[0041] The intake passage 30 is a gas passage formed of an intake pipe and an intake port. A throttle valve 31 is disposed in the intake passage 30. The throttle valve 31 is rotated (driven) by a throttle motor 32. The throttle motor 32 changes an opening degree of the throttle valve 31 (throttle valve opening degree) in response to a signal from an electric control device 90 described later. Air (hereinafter sometimes referred to as "fresh air") is introduced into the intake passage 30 via an air filter that is not shown, and the fresh air is sucked into the combustion chamber 21 as the air- fuel mixture (see an open thick arrow in FIG 1).

[0042] The exhaust passage 40 is a gas passage formed of an exhaust port and an exhaust pipe. The exhaust gas generated in the combustion chamber 21 is circulated in the exhaust passage 40, and is released into the air after passing through a catalyst device that is not shown (see a hatched thick arrow in FIG. 1).

[0043] The first device includes a carbon dioxide enrichment portion (C0₂ enrichment portion) 50, a first EGR passage 60, an unnecessary gas discharge passage 65, a bypass passage 70, an EGR control valve 81, a three-way valve 82, and the electric control device 90.

[0044] The C0₂ enrichment portion 50 includes a branched passage 51 and a carbon dioxide enrichment module (C0₂ enrichment module) 52. One end portion (first end portion) of the branched passage 51 is connected to the exhaust passage 40 at a predetermined portion PI of the exhaust passage 40, and the other end portion (second end portion) of the branched passage 51 is connected to an exhaust gas inlet 52a of the C0₂ enrichment module 52. With this, the branched passage 51 causes part of the exhaust gas flowing in the exhaust passage 40 to flow into the C0₂ enrichment module 52. Consequently, the predetermined portion PI also serves as a gas inlet portion PI of the C0₂ enrichment portion 50.

[0045] The C0₂ enrichment module 52 (its case) has the exhaust gas inlet 52a, a C0₂-enriched gas outlet 52b, and an unnecessary gas discharge port 52c. Further, the C0₂ enrichment module 52 has a carbon dioxide facilitated transport membrane (sometimes referred to as a C0₂ facilitated transport membrane or a facilitated transport membrane) 52d. The C0₂-enriched gas outlet 52b is an outlet of a C0₂-enriched gas of the C0₂ enrichment portion 50. The unnecessary gas discharge port 52c is a discharge port discharging an unnecessary gas from the C0₂ enrichment portion 50.

[0046] The C0₂ facilitated transport membrane 52d has carbon dioxide permselectivity, and is a membrane capable of selectively separating carbon dioxide from a gas in which various gases are mixed. The C0₂ facilitated transport membrane 52d is obtained by forming a polymer compound layer containing a carbon dioxide carrier and water on a surface of a carrier such as a porous membrane. Specifically, the C0₂ facilitated transport membrane 52d is a facilitated transport membrane obtained by impregnating a hydrous gel of a polyvinyl alcohol (PVA)-based polymer or the like with a solution of the carbon dioxide carrier selected from carbonate, amine, and the like. In the embodiment, the C0₂ facilitated transport membrane 52d is formed by causing a hydrophilic porous membrane to support a gel layer in which cesium carbonate is added to a gel membrane of a polyvinyl alcohol-polyacrylic acid copolymer. Note that the detail of the C0₂ facilitated transport membrane is also disclosed in Japanese Patent Application Publication No. 2013-27841 (JP 2013-27841 A), and the disclosure of JP 2013-27841 A is incorporated in the application by citation.

[0047] The exhaust gas having flown into the C0₂ enrichment module 52 through the branched passage 51 and the exhaust gas inlet 52a is separated into the C0₂-enriched gas and the other gas (hereinafter sometimes referred to as "an unnecessary gas") by the C0₂ facilitated transport membrane 52d. The C0₂ enrichment module 52 generates the C0₂-enriched gas in this manner, and causes the C0₂-enriched gas to flow out from the C0₂-enriched gas outlet 52b and the unnecessary gas to flow out from the unnecessary gas discharge port 52c. [0048] The first EGR passage 60 is an EGR passage that circulates the C0₂-enriched gas such that the C0₂-enriched gas generated by the C0₂ enrichment module 52 is returned to the intake passage 30. More specifically, one end portion (first end portion) of the first EGR passage 60 is connected to the C0₂-enriched gas outlet 52b, and the other end portion (second end portion) of the first EGR passage 60 is connected to "a predetermined portion P2 of the intake passage 30 on the downstream side of the throttle valve 31". That is, the first EGR passage 60 provides communication between the C0₂-enriched gas outlet 52b of the C0₂ enrichment portion 50 and the intake passage 30 (the predetermined portion P2). Note that the predetermined portion P2 is sometimes referred to as "an EGR gas confluence portion P2".

[0049] The unnecessary gas discharge passage 65 is a passage that circulates the unnecessary gas such that the unnecessary gas generated by the C0₂ enrichment module 52 is returned to the exhaust passage 40. More specifically, one end portion (first end portion) of the unnecessary gas discharge passage 65 is connected to the unnecessary gas discharge port 52c, and the other end portion (second end portion) of the unnecessary gas discharge passage 65 is connected to "a predetermined portion P3 of the exhaust passage 40 on the downstream side of the portion to which the branched passage 51 is connected (i.e., the gas inlet portion PI of the C0₂ enrichment portion 50)". That is, the unnecessary gas discharge passage 65 provides communication between the unnecessary gas discharge port 52c of the C0₂ enrichment portion 50 and the exhaust passage 40 (the predetermined portion P3).

[0050] The bypass passage 70 constitutes part of a second EGR passage that circulates the exhaust gas flowing in the discharge passage such that the exhaust gas flowing in the exhaust passage 40 bypasses the C0₂ enrichment portion 50 and is returned to the intake passage 30 without being converted into the C0₂-enriched gas.

[0051] More specifically, one end portion (first end portion) of the bypass passage 70 is connected to "a predetermined portion P4 of the exhaust passage 40 on the upstream side of the portion to which the branched passage 51 is connected (i.e., the gas inlet portion PI of the C0₂ enrichment portion 50)". The other end portion (second end portion) of the bypass passage 70 is connected to a predetermined portion P5 of the first EGR passage 60. That is, the bypass passage 70 provides communication between the exhaust passage 40 (the predetermined portion P4) and the first EGR passage 60 (the predetermined portion P5). Note that a portion between one end portion (first end portion) of the first EGR passage 60 connected to the C0₂-enriched gas outlet 52b and the predetermined portion P5 is sometimes referred to as "an upstream portion 61 of the first EGR passage 60". Further, a portion between the predetermined portion P5 of the first EGR passage 60 and the predetermined portion P2 (the EGR gas confluence portion P2) of the intake passage 30 is sometimes referred to as "a downstream portion 62 of the first EGR passage 60".

[0052] The EGR control valve 81 is a flow rate adjustment valve that is disposed in the first EGR passage 60. More specifically, the EGR control valve 81 is disposed in the portion of the first EGR passage 60 "between the other end portion (the second end portion, the predetermined portion P2)" and the portion to which the bypass passage 70 is connected (the predetermined portion P5)" (i.e., the downstream portion 62 of the first EGR passage 60). An opening degree D of the EGR control valve 81 is changed in response to a signal (e.g., a duty signal) from the electric control device 90, and an EGR ratio described later can be thereby adjusted.

[0053] The three-way valve (confluence three-way valve) 82 is disposed at the portion (the predetermined portion P5) of the first EGR passage 60 to which the bypass passage 70 is connected. More specifically, as will be described below, the three-way valve 82 is disposed in the first EGR passage 60.

(1) "The C0₂-enriched gas having flown from the C0₂-enriched gas outlet 52b to the upstream portion 61 of the first EGR passage 60" flows into a first gas entrance of the three-way valve 82.

(2) The exhaust gas circulated in the bypass passage 70 flows into a second gas entrance of the three-way valve 82.

(3) The gas having flown into the three-way valve 82 from the first gas entrance or the second gas entrance flows to the downstream portion 62 of the first EGR passage 60 (toward the EGR control valve 81) from a third gas entrance of the three-way valve 82. [0054] The three-way valve 82 is capable of selectively implementing a first state in which the first gas entrance communicates with the third gas entrance and a second state in which the second gas entrance communicates with the third gas entrance in response to the signal from the electric control device 90. Consequently, when the three-way valve 82 is brought into the first state, the C0₂-enriched gas is returned to the intake passage 30 via the first EGR passage 60. When the three-way valve 82 is brought into the second state, the exhaust gas flowing in the exhaust passage 40 is returned to the intake passage 30 through the bypass passage 70 and the downstream portion 62 of the first EGR passage 60 (without being changed to the C0₂-enriched gas by bypassing the C0₂ enrichment portion 50). Note that it can be said that "the bypass passage 70 and the downstream portion 62 of the first EGR passage 60" constitute "the second EGR passage".

[0055] The electric control device 90 sets the three-way valve 82 to the first state for a first time Tl in a specific time (period) T and sets the three-way valve 82 to the second state for the remaining time (second time) T2 (= T - Tl) of the specific time T, and repeatedly performs such an operation. As will be described later, the electric control device 90 is capable of changing the first time Tl and the second time T2 in accordance with the operation state of the engine 10 or the like.

[0056] Herein, a C0₂-enriched gas ratio R will be described. In the following description, the amount of the C0₂-enriched gas flowing into the intake passage 30 (i.e., the gas flowing in the upstream portion 61 and the downstream portion 62 of the first EGR passage 60) per unit time is referred to as a first gas amount Gl, and the amount of the usual exhaust gas flowing into the intake passage 30 (i.e., the gas flowing in the second EGR passage constituted by the bypass passage 70 and the downstream portion 62 of the first EGR passage 60) per unit time is referred to as a second gas amount G2. At this point, the C0₂-enriched gas ratio R is defined as a ratio of "the first gas amount Gl" to "the total of the first gas amount Gl and the second gas amount G2" (R = Gl / (Gl + G2)).

[0057] The electric control device 90 is an electric circuit that includes a conventional microcomputer including a CPU, a ROM, a RAM, and an interface. The electric control device 90 is connected to sensors described below, and receives detection signals from the sensors. · An air flow meter 91 that detects an intake air amount (mass flow rate) Ga. · A rotational speed sensor 92 that detects a rotational speed (engine rotational speed) NE of the engine 10. - A coolant temperature sensor 93 that detects a temperature of a coolant (coolant temperature) THW of the engine 10. Further, the electric control device 90 sends signals for driving the fuel injection valve 22, the ignition plug 23, the throttle motor 32, the EGR control valve 81, and the three-way valve 82 to them.

[0058] (specific operation)

A CPU of the electric control device 90 (hereinafter simply referred to as "a CPU") executes a routine shown in a flowchart in FIG. 2 every time a predetermined time elapses. Consequently, at a predetermined timing, the CPU starts processes from step 200 in FIG 2, performs processes from step 210 to step 260 described below sequentially, and proceeds to step 295 to end the routine temporarily.

[0059] Step 210: The CPU acquires the engine rotational speed NE. Step 220: The CPU calculates an engine load KL according to the following Expression (1). In Expression (1), Mc is "an amount of air sucked into a given cylinder in one intake stroke of the cylinder (in-cylinder intake air amount)" determined from the detected intake air amount Ga and the detected engine rotational speed NE, p is an air density (its unit is (g / 1)), L is a displacement of the engine 10 (its unit is (1)), and "4" is the number of cylinders of the engine 10.

[Mathematical Expression 1 ]

KL = (Mc / (p - L / 4)) - 100% ... (1)

[0060] Step 230: The CPU determines the C0₂-enriched gas ratio R (target value) based on the engine load KL. More specifically, the CPU determines the C0₂-enriched gas ratio R by applying the engine load KL to a look-up table MapR (KL) shown in a block Bl . The table MapR (KL) is created based on data determined by a preliminary experiment, and is pre-stored in the ROM. According to the table MapR (KL), the CCVenriched gas ratio R is determined so as to be larger as the engine load KL is larger. This is because the peak combustion temperature is higher as the engine load KL is larger and hence, even when a larger amount of the C0₂-enriched gas having a relatively low temperature is recirculated, the peak combustion temperature is not reduced excessively.

[0061] Step 240: The CPU determines the first time Tl and the second time T2 described above based on the determined C0₂-enriched gas ratio R. More specifically, the CPU determines the time Tl by applying the determined C0₂-enriched gas ratio R to a look-up table MapTl (R), and determines the time T2 by applying the determined C0₂-enriched gas ratio R to a look-up table MapT2 (R). The table MapTl (R) and the table MapT2 (R) are created based on data determined by a preliminary experiment, and are pre-stored in the ROM. Note that the CPU may determine the first time Tl by using the following Expression (2), and may determine the second time T2 by using the following Expression (3).

[Mathematical Expression 2]

Tl = Τ · Gl / (Gl + G2) ... (2)

Τ2 = Τ · G2 / (G1 + G2) ... (3)

[0062] Step 250: The CPU determines the opening degree D (target value) of the

EGR control valve 81 based on the engine rotational speed NE and the engine load KL. More specifically, the CPU determines the opening degree D by applying the engine rotational speed NE and the engine load KL to a look-up table MapD (NE, KL) shown in a block B2. The table MapD (NE, KL) is created based on data determined by a preliminary experiment, and is pre-stored in the ROM.

[0063] Step 260: The CPU controls the state of the three-way valve 82 in accordance with the first time Tl and the second time T2. Further, the CPU drives the EGR control valve 81 such that the actual opening degree of the EGR control valve 81 matches the opening degree D determined in step 250.

[0064] As described thus far, the first device has the C0₂ enrichment portion 50 including the C0₂ enrichment module 52. The C0₂ enrichment portion 50 takes in the exhaust gas flowing in the exhaust passage 40 of the internal combustion engine 10 (i.e., the usual exhaust gas discharged from the combustion chamber 21) from the gas inlet portion PI connected to the exhaust passage 40. The C0₂ enrichment portion 50 converts the exhaust gas taken in the C0₂ enrichment portion 50 into "the C0₂-enriched gas of which the carbon dioxide concentration is increased to be higher than that of the exhaust gas flowing in the exhaust passage 40" by using "the C0₂ enrichment module 52".

[0065] Further, the first device includes the first EGR passage 60 that circulates the C0₂-enriched gas resulting from the conversion such that the C0₂-enriched gas is returned to the intake passage 30, the second EGR passage (70 and 62) that circulates the exhaust gas flowing in the exhaust passage 40 such that the exhaust gas flowing in the exhaust passage 40 bypasses the C0₂ enrichment portion 50 and is returned to the intake passage 30 without being converted into the C0₂-enriched gas, and a gas amount adjustment portion (81 and 82) that adjusts the first gas amount as the amount of the C0₂-enriched gas that is returned to the intake passage 30 through the first EGR passage 60 and the second gas amount as the amount of the exhaust gas that is returned to the intake passage 30 through the second EGR passage (70 and 62).

[0066] Therefore, according to the first device, "the C0₂-enriched gas of which the specific heat ratio is made smaller than that of the usual exhaust gas by increasing the carbon dioxide concentration" is returned to the intake passage 30 through the first EGR passage 60 as the EGR gas. As a result, as compared with the case where the usual exhaust gas is recirculated as the EGR gas without the conversion, it is possible to reduce the peak combustion temperature with a smaller amount of the EGR gas, and hence it is possible to reduce the generation amount of NOx with a smaller amount of the EGR gas. Further, the oxygen concentration in the combustion chamber 21 is not reduced excessively, and hence it is possible to reduce the generation amount of soot.

[0067] In addition, the first device is also capable of returning the usual exhaust gas (a gas having a relatively high temperature) to the intake passage 30 through the second EGR passage (70, 62) as the EGR gas. Consequently, it is possible to recirculate the usual exhaust gas having a relatively high temperature in addition to the C0₂-enriched gas having a relatively low temperature. Further, it is possible to adjust the amount of the C0₂-enriched gas to be recirculated and the amount of the usual exhaust gas to be recirculated using the gas amount adjustment portion (81, 82). Consequently, it is possible to prevent the temperature of the entire EGR gas that is recirculated to the combustion chamber 21 from being excessively reduced, and avoid an excessive reduction in peak combustion temperature. As a result, it is possible to avoid an increase in the generation amount of an unburned gas.

[0068] The C0₂ enrichment portion 50 includes the branched passage 51 branched from the exhaust passage 40, and the C0₂ enrichment module 52 includes the C0₂ facilitated transport membrane 52d that allows passage of the exhaust gas flowing in to the C0₂ enrichment module 52 through the branched passage 51.

[0069] Consequently, in the first device, the exhaust gas flows into the C0₂ enrichment module 52 through "the branched passage 51 branched from the exhaust passage 40", and hence it is possible to reduce the amount of the exhaust gas flowing into the C0₂ enrichment module 52 as compared with the case where the C0₂ enrichment module 52 is disposed in the exhaust passage 40. As a result, an internal pressure of the C0₂ enrichment module 52 is less likely to become excessive, and hence it is possible to reduce a possibility that the C0₂ facilitated transport membrane 52d is damaged.

[0070] Further, one end portion (first end portion) of the bypass passage 70 as one end portion of the second EGR passage (70, 62) is connected to the exhaust passage 40 at the portion (the predetermined portion P4) of the exhaust passage 40 "on the upstream side of the gas inlet portion PI of the C0₂ enrichment portion 50". The temperature of the exhaust gas at the predetermined portion P4 is naturally higher than the temperature of the exhaust gas at the gas inlet portion P 1.

[0071] Consequently, the first device is capable of returning the exhaust gas having a relatively high temperature to the intake passage 30, and hence it is possible to reduce the amount of "the exhaust gas to be recirculated" required to avoid a reduction in peak combustion temperature resulting from the recirculation of the C0₂-enriched gas. As a result, the oxygen concentration of the gas flowing into the combustion chamber 21 is less likely to be excessively reduced, and hence it is possible to reduce the generation amount of soot more reliably.

[0072] Further, the first device includes the following features. (1) The first EGR passage 60 is a passage that provides communication between the C0₂-enriched gas outlet 52b of the C0₂ enrichment portion 50 and the intake passage 30. (2) The second EGR passage (62, 70) is formed of the bypass passage 70 that provides communication between the portion (the predetermined portion P4) of the exhaust passage 40 on the upstream side of the gas inlet portion PI of the C0₂ enrichment portion 50 in a direction of flow of the exhaust gas flowing in the exhaust passage 40 and the first EGR passage 60, and the downstream portion 62 of the first EGR passage extending from the portion (the predetermined portion P5) of the first EGR passage 60 to which the bypass passage 70 is connected to the portion (the predetermined portion P2) of the first EGR passage 60 connected to the intake passage 30. (3) The gas amount adjustment portion includes the three-way valve 82 that is disposed at the portion (the predetermined portion P5) at which the first EGR passage 60 and the bypass passage 70 are connected to each other, and the flow rate adjustment valve (the EGR control valve 81) that is disposed in the downstream portion 62 of the first EGR passage 60.

[0073] Consequently, since the first device uses the downstream portion 62 of the first EGR passage 60 as part of the second EGR passage (62, 70), it is possible to reduce the total length of members constituting the first EGR passage and the second EGR passage. Further, it is possible to adjust the EGR ratio using the EGR control valve 81 as the flow rate adjustment valve and adjust the C0₂-enriched gas ratio using the three-way valve 82. Note that the EGR ratio in the embodiment is a ratio of "the sum of the first gas amount Gl and the second gas amount G2" to the amount of the gas flowing into the combustion chamber 21 (i.e., the sum total of the first gas amount Gl, the second gas amount G2, and the amount of the sucked fresh air per unit time).

[0074] Further, the first device includes a control portion (the electric control device 90, see step 230 and the block Bl in FIG. 2) that acquires the load KL of the internal combustion engine 10, and issues an instruction to the gas amount adjustment portion (the three-way valve 82) such that the C0₂-enriched gas ratio R is smaller as the acquired load KL is lower. That is, according to the first device, the C0₂-enriched gas ratio in the case where the acquired load KL is a first load is smaller than the C0₂-enriched gas ratio in the case where the acquired load KL is a second load, which is higher than the first load.

[0075] Consequently, in the case where the engine load KL is small and an in-cylinder temperature is low (consequently, the peak combustion temperature is low), the first device is capable of increasing the ratio of "the usual exhaust gas having a relatively high temperature" to the entire EGR gas (the gas returned to the intake passage 30). As a result, it is possible to prevent the peak combustion temperature from being excessively reduced, and hence it is possible to prevent an increase in the generation amount of the unburned gas.

[0076] Next, an EGR device (hereinafter sometimes referred to as "a second device") for an internal combustion engine according to a second embodiment of the invention will be described.

[0077] As shown in FIG 3, the second device is different from the first device only in that the C0₂ enrichment portion 50 includes an exhaust gas cooling device 53 and the CPU of the electric control device 90 executes a routine shown in a flowchart in FIG. 4 instead of FIG. 2. Consequently, hereinbelow, the difference points will be mainly described.

[0078] The exhaust gas cooling device 53 is disposed in the branched passage 51. The exhaust gas cooling device 53 cools the branched passage 51 by using a coolant WT as a refrigerant, and thereby cools the exhaust gas circulated in the branched passage 51. That is, the exhaust gas cooling device 53 is configured to cool the exhaust gas flowing into the C0₂ enrichment module 52. A low-temperature coolant WT is supplied to the exhaust gas cooling device 53 from a heat exchanger that is not shown, a coolant WT' of which temperature is increased in the exhaust gas cooling device 53 is returned to the heat exchanger, and the coolant WT' becomes the low-temperature coolant WT again in the heat exchanger.

[0079] The second device includes a refrigerant temperature sensor 94 that detects the temperature of the refrigerant (i.e., the coolant) supplied to the exhaust gas cooling device 53. The electric control device 90 is configured to receive (acquire) the refrigerant temperature detected by the refrigerant temperature sensor 94 as a refrigerant temperature Tempi .

[0080] The CPU of the second device is configured to execute the routine shown in FIG 4 every time a predetermined time elapses. Note that, in FIG. 4, steps for performing the same processes as those of steps shown in FIG. 2 are designated by the same reference numerals as those of such steps in FIG. 2. The description of these steps will be appropriately omitted.

[0081] At a predetermined timing, the CPU starts processes from step 400 in FIG 4, and performs the processes in "step 210 and step 220" described above sequentially. With the above processes, the engine rotational speed NE and the engine load KL are acquired. Next, the CPU performs processes in step 405 to step 425 described below sequentially, and proceeds to step 430.

[0082] Step 405: The CPU determines a temporary first gas amount Glz based on the engine load KL and the engine rotational speed NE. More specifically, the CPU determines the temporary first gas amount Glz by applying the engine load KL and the engine rotational speed NE to a look-up table MapGlz (KL, NE). The temporary first gas amount Glz is a temporary amount of the first gas amount Gl described above.

[0083] Step 410: The CPU determines a temporary second gas amount G2z based on the engine load KL and the engine rotational speed NE. More specifically, the CPU determines the temporary second gas amount G2z by applying the engine load KL and the engine rotational speed NE to a look-up table MapG2z (KL, NE). The temporary second gas amount G2z is a temporary amount of the second gas amount G2 described above.

[0084] Note that, with regard to the table MapGlz (KL, NE) and the table MapG2z (KL, NE), the first gas amount Gl and the second gas amount G2 are determined by a preliminary experiment such that the amount of emission of NOx is not more than a desired value and a situation in which "the peak combustion temperature is excessively reduced due to the inflow of the C0₂-enriched gas into the combustion chamber 21 and an unburned component in an amount not less than a predetermined value is generated" is avoided for any "engine load KL and engine rotational speed NE", and the table MapGlz (KL, NE) and the table MapG2z (KL, NE) are created based on the data. These tables are pre-stored in the ROM.

[0085] Step 415: The CPU acquires the refrigerant temperature Tempi of the exhaust gas cooling device 53.

[0086] Step 420: The CPU determines an upper limit value Gmax of the amount of the exhaust gas flowing into the C0₂ enrichment module 52 based on the acquired refrigerant temperature Tempi . More specifically, the CPU determines the upper limit value Gmax by applying the refrigerant temperature Tempi to a look-up table MapGmax (Tempi) shown in a block B3 in FIG. 4.

[0087] , According to the table MapGmax (Tempi), the upper limit value Gmax is determined so as to be smaller as the refrigerant temperature Tempi is higher. The table MapGmax (Tempi) is created in advance based on the value Gmax determined in the following manner by a preliminary experiment, and is stored in the ROM. That is, "the amount of the exhaust gas flowing into the C0₂ enrichment module 52" that does not allow the temperature in the C0₂ enrichment module 52 to have a value not less than a predetermined value Tup when the refrigerant temperature of the exhaust gas cooling device 53 is the temperature Tempi is determined as Gmax.

[0088] Step 425: The CPU acquires an upper limit value Glmax of the first gas amount corresponding to the upper limit value Gmax by applying the upper limit value Gmax to a look-up table MapGlmax (Gmax). That is, the CPU determines the amount of the C0₂-enriched gas obtained in the case where the exhaust gas having the upper limit value Gmax flows into the C0₂ enrichment module 52 as the upper limit value Glmax. The table MapGlmax (Gmax) is created based on data determined by a preliminary experiment, and is pre-stored in the ROM. According to the table MapGlmax (Gmax), the upper limit value Glmax of the first gas amount is determined as a value that is larger as the upper limit value Gmax is larger. Note that the CPU may acquire the upper limit value Glmax of the first gas amount by applying the engine load KL, the engine rotational speed NE, and the upper limit value Gmax to a look-up table MapGlmax (KL, NE, Gmax).

[0089] Next, the CPU proceeds to step 430, and determines whether or not the temporary first gas amount Glz determined in step 405 is not less than the upper limit value Glmax determined in step 425. When the temporary first gas amount Glz is not less than the upper limit value Glmax, the CPU determines "Yes" in step 430, performs processes in step 435 and step 440 described below sequentially, and then proceeds to step 455.

[0090] Step 435: The CPU adopts the upper limit value Glmax as the final first gas amount Gl . That is, the CPU sets the first gas amount Gl to the upper limit value Glmax such that the first gas amount Gl does not become equal to or more than the upper limit value Glmax.

[0091] Step 440: The CPU calculates the final second gas amount G2 according to the following Expression (4). That is, the CPU corrects the temporary second gas amount G2z such that the portion of the first gas amount (Glz - Glmax) by which the first gas amount is reduced as the result of setting the first gas amount Gl to the upper limit value Glmax as compared with the case where the temporary first gas amount Glz is supplied is compensated by the second gas amount, and adopts the corrected value as the second gas amount G2. In Expression (4), k is a predetermined constant.

[Mathematical Expression 3]

G2 = G2z + k (Glz - Glmax) ... (4)

[0092] In contrast to this, when the temporary first gas amount Glz is less than the upper limit value Glmax, the CPU determines "No" is step 430, performs processes in step 445 and step 450 described below sequentially, and then proceeds to step 455.

[0093] Step 455: The CPU adopts the temporary first gas amount Glz as the final first gas amount Gl . Step 450: The CPU adopts the temporary second gas amount G2z as the final second gas amount G2.

[0094] Next, the CPU performs processes in step 455 to step 465 described below sequentially, and proceeds to step 495.

[0095] Step 455: The CPU determines the control times (the first time Tl and the second time T2) of the three-way valve described above based on the final first gas amount Gl and the final second gas amount G2. For example, the CPU sets a value T · Gl / (Gl + G2) as the first time Tl, and sets a value T · G2 / (Gl + G2) as the second time T2.

[0096] Step 460: The CPU determines the opening degree D (target value) of the EGR control valve 81 based on the final first gas amount Gl , the final second gas amount G2, the engine load KL, and the engine rotational speed NE. More specifically, the CPU determines the opening degree D by applying a sum Gall of the final first gas amount Gl and the final second gas amount G2, the engine load KL, and the engine rotational speed NE to a look-up table MapD (KL, NE, Gall) that is not shown. With regard to the table MapD (KL, NE, Gall), the opening degree D is determined by a preliminary experiment such that the gas having the EGR total gas amount Gall flows into the intake passage 30 for any "engine load KL and engine rotational speed NE", and the table MapD (KL, NE, Gall) is a table created based on the determined data. The table is pre-stored in the ROM.

[0097] Step 465: The CPU controls the state of the three-way valve 82 in accordance with the first time Tl and the second time T2. Further, the CPU drives the EGR control valve 81 such that the actual opening degree of the EGR control valve 81 matches the opening degree D determined in step 460.

[0098] As the result of the processes described above, the first gas amount Gl is maintained at a value not more than the upper limit value Glmax, and hence the amount of the exhaust gas flowing into the C0₂ enrichment module 52 is also maintained at a value not more than the upper limit value Gmax.

[0099] As described thus far, the second device has the configuration similar to that of the first device, and the C0₂ enrichment portion 50 includes the exhaust gas cooling device 53 disposed in the branched passage 51. The exhaust gas cooling device 53 cools the exhaust gas flowing into the C0₂ enrichment module 52 by using the refrigerant (coolant). Consequently, the second device is capable of reducing the temperature of the exhaust gas flowing into the C0₂ enrichment module 52 including the C0₂ facilitated transport membrane 52d, and hence it is possible to increase the life of the C0₂ enrichment module 52.

[0100] Further, the second device includes the control portion that acquires the refrigerant temperature Tempi, determines the upper limit value Glmax such that the upper limit value Glmax of the first gas amount Gl is smaller as the acquired refrigerant temperature Tempi is higher, and issues an instruction to the gas amount adjustment portion (81 , 82) such that the first gas amount Gl is not more than the determined upper limit value Glmax (see step 415 to step 435, step 455 to step 465, and the block B3 in FIG. 4).

[0101] Consequently, the second device is capable of setting the amount of the exhaust gas flowing into the C0₂ enrichment module 52 to a value not more than "the upper limit value Gmax determined according to the refrigerant temperature Tempi", and hence it is possible to prevent the temperature in the C0₂ enrichment module 52 from becoming excessively high. As a result, it is possible to return the C0₂-enriched gas in an amount that is as close to "the amount of the C0₂-enriched gas required to improve the emission of the engine 10" as possible to the intake passage 30 within a range that does not cause the degradation of the C0₂ enrichment module 52 (the degradation of the C0₂ facilitated transport membrane 52d).

[0102] Hereinbelow, an EGR device (hereinafter sometimes referred to as "a third device") for an internal combustion engine according to a third embodiment of the invention will be described. The third device is different from the first device only in that the CPU of the electric control device 90 executes a routine shown in a flowchart in "FIG. 5 instead of FIG. 2" every time a predetermined time elapses. Consequently, hereinbelow, the difference point will be mainly described. Steps of performing the same processes as those of steps already described in FIG 5 are designated by the same reference numerals as those of such steps. The description of these steps will be appropriately omitted.

[0103] At a predetermined timing, the CPU starts processes from step 500 in FIG 5, and performs the processes in "step 210 and step 220" described above sequentially. With the above processes, the engine rotational speed NE and the engine load KL are acquired. Next, the CPU performs processes in step 510 to step 530 described below sequentially, and proceeds to step 455.

[0104] Step 510: The CPU acquires the coolant temperature THW of the engine 10 as an engine correlation temperature Temp2. [0105] Step 520: The CPU determines the first gas amount Gl based on the acquired engine correlation temperature Temp2. More specifically, the CPU determines the first gas amount Gl by applying the engine correlation temperature Temp2 to a look-up table MapGl (KL, NE, Temp2) shown in a block B4 in FIG 5. According to the table MapGl (KL, NE, Temp2), when the engine load KL is a specific load KLx and the engine rotational speed NE is a specific rotational speed NEx, the first gas amount Gl is determined so as to be larger as the engine correlation temperature Temp2 is higher.

[0106] Step 530: The CPU determines the second gas amount G2 based on the acquired engine correlation temperature Temp2. More specifically, the CPU determines the second gas amount G2 by applying the engine correlation temperature Temp2 to a look-up table MapG2 (KL, NE, Temp2) shown in a block B5 in FIG 5. According to the table MapG2 (KL, NE, Temp2), when the engine load KL is the specific load KLx and the engine rotational speed NE is the specific rotational speed NEx, the second gas amount G2 is determined so as to be smaller as the engine correlation temperature Temp2 is higher.

[0107] As a result, the C0₂-enriched gas ratio R (= Gl / (Gl + G2)) is larger as the engine correlation temperature Temp 2 is higher.

[0108] Thereafter, the CPU performs the processes "in step 455 to step 465" described above, and proceeds to step 595 to end the routine temporarily.

[0109] As described thus far, the third device further includes the control portion that acquires the engine correlation temperature Temp2 having a correlation with the temperature of the internal combustion engine 10, and issues an instruction to the gas amount adjustment portion such that the C0₂-enriched gas ratio R is smaller as the acquired engine correlation temperature Temp2 is lower (see step 510 to step 530, the blocks B4 and B5, and step 455 to step 465 in FIG 5). That is, according to the third device, the C0₂-enriched gas ratio in the case where the acquired engine correlation temperature Temp2 is a first temperature is smaller than the C0₂-enriched gas ratio in the case where the engine correlation temperature Temp2 is a second temperature, which is higher than the first temperature.

[0110] Consequently, in the third device, the ratio of "the usual exhaust gas having a relatively high temperature" to the entire EGR gas can be made higher as the temperature of the engine 10 is lower and the in-cylinder temperature is lower (consequently, the peak combustion temperature is lower). As a result, it is possible to prevent the peak combustion temperature from being excessively reduced, and hence it is possible to prevent an increase in the generation amount of the unburned gas.

[0111] Hereinbelow, an EGR device (hereinafter sometimes referred to as "a fourth device") for an internal combustion engine according to a fourth embodiment of the invention will be described. The fourth device is different from the first device only in that the CPU of the electric control device 90 executes a routine shown in a flowchart in "FIG. 6 instead of FIG. 2" every time a predetermined time elapses. Consequently, hereinbelow, the difference point will be mainly described. Note that steps for performing the same processes as those of steps already described in FIG 6 are designated by the same reference numerals as those of such steps. The description of these steps will be appropriately omitted.

[0112] At a predetermined timing, the CPU starts processes from step 600 in FIG.

6, and performs the processes in "step 210 and step 220" and the process in step 510 described above sequentially. With the above processes, the engine rotational speed NE, the engine load KL, and the engine correlation temperature Temp2 (= the coolant temperature THW) are acquired.

[0113] Next, the CPU proceeds to step 610, and determines a threshold engine temperature (threshold coolant temperature) Tth based on the engine load KL and the engine rotational speed NE. More specifically, the CPU determines the threshold engine temperature Tth by applying the engine load KL and the engine rotational speed NE to a look-up table MapTth (KL, NE) shown in a block B6 in FIG. 6. According to the table MapTth (KL, NE), the threshold engine temperature Tth is determined so as to be lower as the engine load KL is larger, and lower as the engine rotational speed NE is higher. For example, in an example shown in the block B6, when KL = KL1 and NE = NE1 are satisfied, the threshold engine temperature Tth is determined to be 70°C.

[0114] Next, the CPU proceeds to step 620, and determines whether or not the engine correlation temperature Temp2 is not less than the threshold engine temperature Tth. When the engine correlation temperature Temp2 is not less than the threshold engine temperature Tth, the CPU performs processes in step 630 and step 640 described below sequentially, and proceeds to step 465.

[0115] Step 630: The CPU sets the first time Tl to the period T described above, and sets the second time T2 to "0". That is, the CPU sets the first time Tl and the second time T2 such that the three-way valve 82 is brought into the first state, and the C0₂-enriched gas is constantly returned to the intake passage 30 through the first EGR passage 60.

[0116] Step 640: The CPU determines the opening degree D (target value) of the

EGR control valve 81 based on the engine rotational speed NE and the engine load KL on the assumption that only the C0₂-enriched gas flows into the combustion chamber 21 as the EGR gas. More specifically, the CPU determines the opening degree D by applying the engine rotational speed NE and the engine load KL to a look-up table MapDC02 (NE, KL). With regard to the table MapDC02 (NE, KL), the opening degree D is determined such that the amounts of emission of NOx and the unburned gas have desired amounts in the case where only the C0₂-enriched gas flows into the combustion chamber 21 as the EGR gas for the engine rotational speed NE and the engine load KL, and the table MapDC02 (NE, KL) is created based on the determined data. The table is pre-stored in the ROM.

[0117] In contrast to this, when the engine correlation temperature Temp2 is less than the threshold engine temperature Tth, the CPU performs processes in step 650 and step 660 described below sequentially, and proceeds to step 465.

[0118] Step 650: The CPU sets the first time Tl to "0", and sets the second time T2 to the period T described above. That is, the CPU sets the first time Tl and the second time T2 such that the three-way valve 82 is brought into the second state, and the usual exhaust gas is constantly returned to the intake passage 30 through the second EGR passage (70, 62).

[0119] Step 660: The CPU determines the opening degree D (target value) of the EGR control valve 81 based on the engine rotational speed NE and the engine load KL on the assumption that only the usual exhaust gas flows into the combustion chamber 21 as the EGR gas. More specifically, the CPU determines the opening degree D by applying the engine rotational speed NE and the engine load KL to a look-up table MapDEXH (NE, KL). With regard to the table MapDEXH (NE, KL), the opening degree D is determined such that the amounts of emission of NOx and the unburned gas have desired amounts in the case where only the exhaust gas flows into the combustion chamber 21 as the EGR gas for the engine rotational speed NE and the engine load KL, and the table MapDEXH (NE, KL) is created based on the determined data. The table is pre-stored in the ROM.

[0120] Thereafter, the CPU proceeds to step 465, controls the state of the three-way valve 82 in accordance with the first time Tl and the second time T2, and drives the EGR control valve 81 such that the actual opening degree of the EGR control valve 81 matches the opening degree D determined in step 250.

[0121] As described thus far, the fourth device allows only the C0₂-enriched gas to flow into the combustion chamber 21 as the EGR gas in the case where the engine correlation temperature Temp2 is higher than "the threshold engine temperature Tth determined based on the engine rotational speed NE and the engine load KL", i.e., in the case where the peak combustion temperature is not reduced excessively (in a case where the generation amount of an unburned gas is not increased) even when only the C0₂-enriched gas flows into the combustion chamber 21 as the EGR gas.

[0122] In contrast to this, the fourth device allows only the exhaust gas to flow into the combustion chamber 21 as the EGR gas in the case where the engine correlation temperature Temp2 is lower than "the threshold engine temperature Tth determined based on the engine rotational speed NE and the engine load KL", i.e., in the case where the peak combustion temperature is excessively reduced (in a case where the generation amount of an unburned gas is increased) when only the C0₂-enriched gas flows into the combustion chamber 21 as the EGR gas.

[0123] Consequently, it is possible to avoid an increase in the generation amount of soot by utilizing the C0₂-enriched gas, and avoid an increase in the generation amount of the unburned gas by utilizing the exhaust gas.

[0124] As described thus far, the EGR device according to each of the embodiments of the invention is capable of avoiding a disadvantage resulting from the recirculation of the C0₂-enriched gas (i.e., an increase in the generation amount of the unburned gas) while utilizing an advantage resulting from the recirculation of the C0₂-enriched gas (i.e., it is possible to reduce the generation amount of NOx with a smaller amount of the EGR gas, and therefore also reduce the generation amount of soot) by properly recirculating the C0₂-enriched gas and the usual exhaust gas.

[0125] The invention is not limited to the above embodiments, and various modifications can be adopted within the scope of the invention. Hereinbelow, modifications will be briefly described.

[0126] As shown in FIG. 7, an EGR device for an internal combustion engine according to a first modification of the invention is different from the first device in that a C0₂ enrichment portion 50A is disposed in the exhaust passage 40. That is, in the C0₂ enrichment portion 50A, the gas inlet portion PI and an unnecessary gas discharge portion P7 are directly connected to the exhaust passage 40. Consequently, all of the exhaust gas discharged from the combustion chamber 21 flows into the C0₂ enrichment portion 50A, and all of the unnecessary gas is discharged from the unnecessary gas discharge portion P7 to the exhaust passage 40. The C0₂-enriched gas is supplied to the upstream portion 61 of the first EGR passage 60 via an enriched gas outlet portion P6.

[0127] As the C0₂ enrichment module of the C0₂ enrichment portion 50A described above, a module that uses an absorbent that absorbs carbon dioxide is more suitable than a module that uses a C0₂ facilitated transport phenomenon. Examples of the absorbent include absorbents using zeolite, active carbon, and alumina. An example of a method for enriching carbon dioxide by using the above absorbents includes a PSA method in which, after carbon dioxide is selected and absorbed under a predetermined pressure, carbon dioxide is separated and collected by changing the pressure.

[0128] Herein, the specific operation of the C0₂ enrichment portion 50A including the C0₂ enrichment module that uses the absorbent will be described in detail. The C0₂ enrichment portion 50A shown in FIG. 8 includes a first absorber 501 and a second absorber 502 each having an absorbent (including, e.g., specific aluminumsilicate). Further, the C0₂ enrichment portion 50A includes a switching valve 503 that switches a supply destination of the exhaust gas flowing into the C0₂ enrichment portion 50A from the gas inlet portion PI between the first absorber 501 and the second absorber 502, a switching valve 504 that switches a supply source of the exhaust gas flowing out from the C0₂-enriched gas outlet 52b (the enriched gas outlet portion P6) between the first absorber 501 and the second absorber 502, and a switching valve 505 that switches a supply source of the unnecessary gas discharged from the unnecessary gas discharge port 52c (the unnecessary gas discharge portion. P7) between the first absorber 501 and the second absorber 502. The C0₂ enrichment portion 50A returns the C0₂-enriched gas to the intake passage 30 by repeatedly performing a first mode and a second mode described below.

[0129] In the first mode, by operating the switching valve 503 as indicated by a solid-line arrow, the exhaust gas having flown into the C0₂ enrichment portion 50A from the exhaust passage 40 via the gas inlet portion PI is caused to flow into the first absorber 501, and the absorbent in the absorber 501 is caused to absorb carbon dioxide in the exhaust gas. In addition, by operating the switching valve 505 as indicated by a solid-line arrow, the unnecessary gas that has not been absorbed by the absorbent in the absorber 501 is discharged from the unnecessary gas discharge portion P7. On the other hand, by operating the switching valve 504 as indicated by a solid-line arrow, carbon dioxide separated from the absorbent in the absorber 502 is supplied to the upstream portion 61 of the first EGR passage 60 via the enriched gas outlet portion P6.

[0130] In the second mode, by operating the switching valve 503 as indicated by a dotted-line arrow, the exhaust gas having flown into the C0₂ enrichment portion 50A from the exhaust passage 40 via the gas inlet portion PI is caused to flow into the second absorber 502, and the absorbent in the absorber 502 is caused to absorb carbon dioxide in the exhaust gas. In addition, by operating the switching valve 505 as indicated by a dotted-line arrow, the unnecessary gas that has not been absorbed by the absorbent in the absorber 502 is discharged from the unnecessary gas discharge portion P7. On the other hand, by operating the switching valve 504 as indicated by a dotted-line arrow, carbon dioxide separated from the absorbent in the absorber 501 is discharged to the upstream portion 61 of the first EGR passage 60 via the predetermined portion P6.

[0131] As shown in FIG 9, an EGR device for an internal combustion engine according to a second modification of the invention is different from the first device in the following points. The other end portion (second end portion) of a bypass passage 70A that replaces the bypass passage 70 is connected to a predetermined portion P8 of the intake passage 30 on the downstream side of "the predetermined portion P2 on the downstream side of the throttle valve 31 to which the other end portion (second end portion) of the first EGR passage 60 is connected". A first gas amount adjustment valve 83 is disposed in a portion of the first EGR passage 60 "between the C0₂-enriched gas outlet 52b of the C0₂ enrichment portion 50 and the predetermined portion P2". A second gas amount adjustment valve 84 is disposed in a portion of the bypass passage 70A "between the predetermined portion P4 and the predetermined portion P8".

[0132] Further, a valve opening degree Dl of the first gas amount adjustment valve 83 is changed in response to a signal (e.g., a duty signal) from the electric control device 90, and the first gas amount adjustment valve 83 is thereby capable of adjusting the first gas amount. In addition, a valve opening degree D2 of the second gas amount adjustment valve 84 is changed in response to the signal (e.g., the duty signal) from the electric control device 90, and the second gas amount adjustment valve 84 is thereby capable of adjusting the second gas amount.

[0133] That is, the EGR device for an internal combustion engine according to the second modification of the invention is capable of adjusting the first gas amount Gl and the second gas amount G2 individually. Further, by adjusting the first gas amount Gl and the second gas amount G2 in this manner, the EGR device is capable of adjusting the EGR ratio and the C0₂-enriched gas ratio R.

[0134] Next, EGR devices for an internal combustion engine according to other modifications of the invention will be described. Note that, in each of the embodiments of the invention described above, the description has been given of the case where the EGR device according to the invention is applied to the spark-ignition multi-cylinder gasoline engine. However, the engine to which the EGR device according to the invention is applied may be a diesel engine. Further, the configuration of the engine such as, e.g., the number and disposition of cylinders of the engine, a fuel injection system, or the presence or absence of a turbocharger is not particularly limited.

[0135] In addition, the refrigerant of the exhaust gas cooling device that is disposed in the branched passage and cools the exhaust gas circulated in the branched passage and flowing into the C0₂ enrichment module is not limited to the above-described liquid (the coolant WT) and, for example, oil may also be used as the refrigerant. Further, the exhaust gas cooling device may also be an air-cooling cooling device that performs heat dissipation using a fin provided on, e.g., an outer surface of the branched passage.

[0136] In the second device described above, the amount of the exhaust gas flowing in the branched passage 51 and flowing into the C0₂ enrichment module 52 may be measured directly by a sensor, and the amount of the exhaust gas measured in this manner may be not more than "the upper limit value Gmax determined according to the refrigerant temperature Tempi ".

[0137] Incidentally, in the fourth device described above, in the case where the engine correlation temperature Temp2 is not less than the threshold engine temperature Tth, the first time Tl is set to the period T described above, and the second time T2 is set to "0". On the other hand, in the case where the engine correlation temperature Temp2 is less than the threshold engine temperature Tth, the first time Tl is set to "0", and the second time T2 is set to the period T described above.

[0138] However, in the modification of the fourth device, in the case where the engine correlation temperature Temp2 is not less than the threshold engine temperature Tth, the state of the three-way valve 82 may be controlled such that the first time Tl is gradually increased and the second time T2 is gradually decreased, and the C0₂-enriched gas ratio R (= Gl / (Gl + G2)) may be thereby increased. On the other hand, in the case where the engine correlation temperature Temp2 is less than the threshold engine temperature Tth, the state of the three-way valve 82 may be controlled such that the first time Tl is gradually decreased and the second time T2 is gradually increased, and the C0₂-enriched gas ratio R (= Gl / (Gl + G2)) may be thereby reduced.

[0139] Although some embodiments each having a specific configuration have been described with reference to the accompanying drawings in the description of the invention, the scope of the invention should not be interpreted to be limited to these exemplary embodiments, and it is needless to say that modifications can be suitably added within the range of matters described in claims and the specification.

Claims

CLAIMS:

1. An exhaust gas recirculation device for an internal combustion engine, comprising: a C0₂ enrichment portion including a C0₂ enrichment module, the C0₂ enrichment portion being configured to take in an exhaust gas flowing in an exhaust passage of the internal combustion engine from a gas inlet portion connected to the exhaust passage, and the C0₂ enrichment portion being configured to convert the exhaust gas, which has been taken in the C0₂ enrichment portion, into a C0₂-enriched gas, a carbon dioxide concentration of which is increased to be higher than a carbon dioxide concentration of the exhaust gas flowing in the exhaust passage by using the C0₂ enrichment module;

a first EGR passage configured to circulate the C0₂-enriched gas resulting from a conversion such that the C0₂-enriched gas is returned to an intake passage of the internal combustion engine;

a second EGR passage configured to circulate the exhaust gas flowing in the exhaust passage such that the exhaust gas flowing in the exhaust passage is caused to bypass the C0₂ enrichment portion and is returned to the intake passage without being converted into the C0₂-enriched gas; and

a gas amount adjustment portion configured to adjust a first gas amount and a second gas amount, the first gas amount being an amount of the C0₂-enriched gas returned to the intake passage through the first EGR passage, the second gas amount being an amount of the exhaust gas returned to the intake passage through the second EGR passage.

2. The exhaust gas recirculation device according to claim 1, wherein

the C0₂ enrichment portion is a passage branched from the exhaust passage, one end portion of the C0₂ enrichment portion includes a branched passage constituting the gas inlet portion, and the C0₂ enrichment module includes a C0₂ facilitated transport membrane that allows passage of the exhaust gas flowing into the C0₂ enrichment module through the branched passage.

3. The exhaust gas recirculation device according to claim 2, wherein

the C0₂ enrichment portion includes an exhaust gas cooling device, the exhaust gas cooling device being disposed in the branched passage and being configured to cool the exhaust gas flowing into the C0₂ enrichment module by using a refrigerant.

4. The exhaust gas recirculation device according to claim 3, further comprising: a first controller configured to:

(i) acquire a refrigerant temperature as a temperature of the refrigerant;

(ii) determine an upper limit value of the first gas amount such that the upper limit value is smaller as the acquired refrigerant temperature is higher; and

(iii) issue an instruction to the gas amount adjustment portion such that the first gas amount is not more than the determined upper limit value.

5. The exhaust gas recirculation device according to any one of claims 1 through 4, wherein

one end portion of the second EGR passage is connected to the exhaust passage at a portion of the exhaust passage on an upstream side of the gas inlet portion of the C0₂ enrichment portion.

6. The exhaust gas recirculation device according to any one of claims 1 through 4, wherein

the first EGR passage is a passage that connects a C0₂-enriched gas outlet of the C0₂ enrichment module and the intake passage,

the second EGR passage includes a bypass passage and a downstream portion of the first EGR passage, the bypass passage connects the portion of the exhaust passage on the upstream side of the gas inlet portion of the C0₂ enrichment portion and the first EGR passage, and the downstream portion extends from a portion of the first EGR passage to which the bypass passage is connected to a portion of the first EGR passage connected to the intake passage, the gas amount adjustment portion includes a three-way valve and a flow rate adjustment valve, the three-way valve is disposed at a portion at which the first EGR passage and the bypass passage are connected to each other, and the flow rate adjustment valve is disposed in the downstream portion of the first EGR passage.

7. The exhaust gas recirculation device according to any one of claims 1 through 6, further comprising:

a second controller configured to:

(i) acquire a load of the internal combustion engine; and

(ii) issue an instruction to the gas amount adjustment portion such that a C0₂-enriched gas ratio as a ratio of the first gas amount to a total of the first gas amount and the second gas amount in a case where the acquired load is a first load is smaller than a C0₂-enriched gas ratio in a case where the acquired load is a second load, the second load being higher than the first load.

8. The exhaust gas recirculation device according to any one of claims 1 through 6, further comprising:

a third controller configured to:

(i) acquire an engine correlation temperature having a correlation with a temperature of the internal combustion engine; and

(ii) issue an instruction to the gas amount adjustment portion such that a C0₂-enriched gas ratio as a ratio of the first gas amount to a total of the first gas amount and the second gas amount in a case where the acquired engine correlation temperature is a first temperature is smaller than a C0₂-enriched gas ratio in a case where the acquired engine correlation temperature is a second temperature, the second 'temperature being higher than the first temperature.

9. The exhaust gas recirculation device according to any one of claims 1 through 6, further comprising: a fourth controller configured to:

(i) acquire a load of the internal combustion engine, a rotational speed of the internal combustion engine, and an engine correlation temperature having a correlation with a temperature of the internal combustion engine; and

(ii) issue an instruction to the gas amount adjustment portion such that the first gas amount and the second gas amount are adjusted based on the acquired load, the acquired rotational speed, and the acquired engine correlation temperature.

10. The exhaust gas recirculation device according to any one of claims 1 through 6, further comprising:

a fifth controller configured to:

(i) acquire a load of the internal combustion engine, a rotational speed of the internal combustion engine, and an engine correlation temperature having a correlation with a temperature of the internal combustion engine;

(ii) determine a threshold engine temperature, based on the acquired load and the acquired rotational speed;

(iii) issue an instruction to the gas amount adjustment portion such that the first gas amount matches a first predetermined amount corresponding to the acquired load and the acquired rotational speed and the second gas amount is 0 in a case where the acquired engine correlation temperature is higher than or equal to the threshold engine temperature; and

(iv) issue an instruction to the gas amount adjustment portion such that the second gas amount matches a second predetermined amount corresponding to the acquired load and the acquired rotational speed and the first gas amount is 0 in a case where the acquired engine correlation temperature is lower than the threshold engine temperature.