Exhaust gas recirculation system for an internal combustion engine

Abstract: Disclosed is an exhaust gas recirculation system for an engine having a high compression ratio or with a supercharger. An outside EGR duct connecting the exhaust system (4) to the intake system (3) is provided with a first control valve (22), which has a second outside EGR duct (23) bypassing the first control valve. The second EGR duct has a second EGR control valve (24) and a cooler (25). When the engine exists in an extremely low load state, the first and second control valves are closed to inhibit the outside EGR. In the light load state, the first control valve is opened while the second control valve is closed, thereby allowing recirculation of the EGR gases having high temperature and reducing the pumping loss. In the high load state, the first control valve is being closed while opening the second control valve, thereby increasing a recirculation ratio of EGR gases cooled by the cooler and performing both an improvement of anti-knocking performance and a decrease in pumping loss, as well as reducing the heat load of the exhaust system and within the engine.

Description: The present invention relates to an internal combustion engine and, more particularly, to an exhaust gas recirculation (EGR) system so adapted as to recirculate a portion of exhaust gases into an intake system of the internal combustion engine. Internal combustion engines, particularly for automotive vehicles, are designed such that a portion of exhaust gases is recirculated into the intake system. In other words, for example, as shown in Japanese Patent Publication No. 160,052/1984, the internal combustion engines adopt a so-called EGR system that has been employed as measures against NOx (measures to curtail NOx as hazardous ingredients in exhaust gases) in a light load region. As effective means for enhancing heat efficiency of the internal combustion engine, it is known to set a compression ratio of the internal combustion engine to a high value. A supercharged engine is further known, which has a supercharger mounted to an intake system of the internal combustion engine so as to produce a large output at a small displacement. For the internal combustion engines of the type having a high compression ratio or the supercharged engines, the problems exist, first, in the fact that knocking is likely to occur in a high load region and, secondly, in the fact that heat load within the internal combustion engine or within the exhaust system thereof increases in the high load region. These problems make it difficult to set a high compression ratio. This is particularly so for the supercharged engines. An internal combustion engine according to the preamble of claim 1 is known from JP-A-62-7943. However, from this document it is not known to dispose a catalyst converter in the exhaust system. Furthermore, according to this document, EGR control valves are used, which constitute an "all or nothing valves", meaning that EGR is performed only via a first duct of a low load, and via a second duct at a high load, this system not being flexible during a continuous increase of the load. From FR-A-2 553 471 it is known to forcibly cool at least a part of the exhaust gases. However, this document also does not relate to the use of a catalyst converter. From JP-A-62-41941 and exhaust gas recirculation system is known, which controls exhaust gas recirculation only before a steady state condition is reached. The object of the present invention is to provide an exhaust gas recirculation (EGR) system for an internal combustion engine so adapted to improve anti-knocking performance in a high load driving region as well as to reduce a heat load of the exhaust system or within the internal combustion engine, when the compression ratio of the internal combustion engine is made high or when the internal combustion engine is provided with a supercharger. This object is achieved with an internal combustion engine with the features of claim 1. This arrangement permits the EGR gases forcibly cooled by the EGR cooler to be recirculated into the intake system, thereby allowing the cooled EGR gases to lower combustion temperature. Hence, the recirculation of the cooled EGR gases into the intake system in the high load region can reduce NOx during the high load driving region by recirculating the cooled EGR gases to the intake system in the high load region, as well as the anti-knocking performance can be improved while reducing the heat load of the exhaust system or within the internal combustion engine. By applying the exhaust gas recirculation according to the present invention to the internal combustion engine having a high compression ratio or to the supercharged internal combustion engine, the heat load of the exhaust system or within the internal combustion engine can be reduced while improving the anti-knocking performance. A plurality of pipes may be juxtaposed with each other, in place of the EGR cooler, thereby forming the outside EGR duct. This arrangement can cool down the EGR gases during passage through the outside EGR duct and recirculate the cooled EGR gases in a manner similar to the EGR cooler. Other objects, features and advantages of the present invention will become apparent in the course of the description of the preferred embodiments, which follows, with reference to the accompanying drawings. Figs. 1 to 4 are directed to the first embodiment of the present invention, in which: Fig. 1 is a schematic representation showing an outline of the internal combustion engine; Fig. 2 is a graphic representation showing contents of control over the EGR system; Fig. 3 is a graphic representation showing the relationship between the pumping loss and the temperature of EGR gases; and Fig. 4 is a graphic representation showing the cylinder pressure and the temperature of fuel in an equal volume cycle. Fig. 5 is a schematic representation showing an outline of the internal combustion engine according to the second embodiment of the present invention. Fig. 6 is a schematic representation showing an outline of the internal combustion engine according to the third embodiment of the present invention. The present invention will be described more in detail by way of examples with reference to the accompanying drawings. First Embodiment (Figs. 1 to 4) As shown in Fig. 1, reference numeral 1 denotes an engine body of the internal combustion engine, and the engine body 1 includes an in-line 4-cylinder engine of natural air intake type having four cylinders 2 disposed in a row. The internal combustion engine to be employed herein has a compression ratio of 12.5. Reference numeral 3 stands for an intake system of the internal combustion engine and reference numeral 5 for a common intake passage. To the common intake passage 5 are disposed an air cleaner 6, an air flowmeter 7 for detecting a quantity of intake air, and a throttle valve 8 in this order from its upstream side to its downstream side. Connected to a downstream end of the common intake passage 5 is a surge tank 9 which in turn branches into four independent intake passages 10, and a downstream end portion of each of the independent intake passages 10 further branches into two intake ports 11 and 12 for respective cylinders 2. On the other hand, each of the cylinders 2 has two exhaust ports 13 and 14, to each of which is connected the independent exhaust passage 15. A downstream end of the independent exhaust passage 15 connected to each of the cylinders 2 is combined into a common exhaust passage 16 which in turn is provided with a catalyst converter 17. Reference numeral 20 denotes an exhaust gas recirculation unit (an EGR unit) through which the intake system 3 is communicated with the exhaust system 4, thereby allowing a portion of exhaust gases to be recirculated into the intake system 3. The EGR unit 20 has an EGR duct 21 disposed outside the engine body 1 of the internal combustion engine. One end of the EGR duct 21 is connected to an immediate downstream side of the catalyst converter 17 and the other end thereof is connected to the independent intake passage 10. Further, the EGR duct 21 has a first EGR control valve 22 disposed in its intermediate position. Further, the EGR unit 20 has a second EGR duct 23 bypassing the first EGR control valve 22. The second EGR duct 23 has a second EGR control valve 24 and an EGR cooler 25. The EGR cooler 25 is of a water cooled type and is disposed in an intermediate position of a coolant passage 26 for cooling the internal combustion engine. The first EGR control valve 22 is connected to a first actuator 27 of a diaphragm type, while the second EGR control valve 24 is connected to a second actuator 28 of a diaphragm type. A vacuum chamber of the first actuator 27 is communicated with the surge tank 9 through a first pressure pipe 29 which in turn is provided with a first solenoid valve 30 of an electromagnetic type. A vacuum chamber of the second actuator 28 is communicated with the surge tank 9 through a second pressure pipe 31 which in turn is provided with a second solenoid valve 32. The opening angles of the first and second solenoid valves 30 and 32 are controlled by a duty ratio. The first EGR control valve 22 is opened when the first solenoid valve 30 is opened and the negative pressure is introduced into the first actuator 27. On the other hand, the second EGR control valve 24 is closed when the second solenoid valve 32 is opened and the negative pressure is introduced into the second actuator 28. As shown in Fig. 1, reference numeral 40 denotes a control unit which is composed of, for example, a microcomputer with CPU, ROM, RAM, etc. built therein in conventional manner. Entered into the control unit 40 are a signal indicative of a quantity of intake air from the air flowmeter 7 as well as signals from sensors 41 and 42. The sensor 41 is to sense an opening angle of the throttle valve 8, that is, a load. The sensor 42 is to sense the number of rotation of the engine. Control signals are generated from the control unit 40 to the first solenoid valve 30 and the second solenoid valve 32, thereby controlling the opening angles of the first EGR control valve 22 and the second EGR control valve 24. Fig. 2 shows the contents of the control of the first EGR control valve 22 and the second EGR control valve 24. In Fig. 2, the broken line represents the first EGR control valve 22, while the solid line represents the second EGR control valve 24. As is apparent from Fig. 2, the first EGR control valve 22 and the second EGR control valve 24 are both closed in an extremely low load region. In other words, the outside exhaust gas recirculation (the outside EGR) through the first EGR duct 21 and the second EGR duct 23 is inhibited in the extremely low load region, thereby ensuring stability in combustion in this range. On the other hand, in a light load region, the first EGR control valve 22 is fully opened while the second EGR control valve 24 is fully closed. In other words, the exhaust gas recirculation is implemented using the first EGR duct 21 in the light load region where the problem with a pumping loss may arise. Since no EGR cooler is mounted to the first EGR duct 21, the EGR gases at relatively high temperature are recirculated into the intake system 3. Fig. 3 is a graphical representation showing the relationship between the temperature of EGR gases and an extent to which the pumping loss is reduced, by using an EGR ratio as a parameter. As the extent of restriction of intake air by the throttle valve 8 is large in the light load region, the pumping loss becomes likely to occur at a higher rate. As is apparent from Fig. 3, however, the pumping loss can be reduced to a larger extent by a slight quantity of exhaust gas recirculation as the temperature of the EGR gases becomes higher. It is thus to be noted that it is desired to use the EGR gases at a higher temperature in the light load region. As shown in Fig. 2, in the high load region, on the other hand, the opening angle of the first EGR control valve 22 is made smaller as the load becomes larger. Conversely, the opening angle of the second EGR control valve 24 is made larger. In other words, in the high load region, the rate of the EGR by using the first EGR duct 21 is made smaller while the rate of the EGR by using the second EGR duct 23 is made higher, as the load becomes higher. This means that the rate of recirculation of the EGR gases cooled by the EGR cooler 25 is increased to a larger extent as the load becomes larger. It is further to be noted that, as the temperature of the EGR gases is lower or as the quantity of the EGR gases is larger, the temperature of combustion becomes lower. Fig. 4 is a graph representation showing the relationship between the cylinder pressure P and the combustion temperature T. Reference symbols T A , T TDC , ΔT and Tb are represented by the following formulas: T A = C pa G a T a + C pe G e T e C pa T a + C pe T e T TDC = T A ×ε k -1 Δ T = Q C v [ G a (1+1/ AF ) + G e ] Tb = T TDC +Δ T where T A is the temperature at the moment of start of compression; T TDC is the temperature of the top dead center of compression; Tb is the temperature after combustion; ε is an effective compression ratio; Cp is an equal pressure specific heat; Cv is an equal volume specific heat; AF is an air-to-fuel ratio; suffix a represents fresh air; suffix e represents exhaust gases (EGR); Q is the total calorific value, Q ∝ Ga (constant AF); and G is the gas weight. As is apparent from the above formulas, the temperature of combustion becomes lower as the temperature of the EGR gases is made lower or as the quantity of the EGR gases is made larger. When the temperature of combustion is lowered, the temperature on the wall of the combustion chamber becomes lower, too, thereby reducing the heat load within the internal combustion engine. Further, the temperature of burned gases is so low in the course of combustion that the radiation heat against unburned gases is reduced and that the temperature of the unburned gases can be suppressed. Hence, the anti-knocking performance can be improved. Furthermore, the reduction in the temperature of combustion leads to the decrease in the temperature of the exhaust gases, so that it is advantageous from the heat-resistance point of view for parts of the exhaust system and the heat load of the exhaust system is made lower. Hence, in the high load region, as the load is made greater, the adverse effect of the pumping loss becomes smaller, while the problems may arise with the heat load within the internal combustion engine, etc. In order to solve the aforesaid problems, the recirculation ratio of the EGR gases having relatively high temperature is reduced, while the recirculation ratio of the EGR gases having relatively low temperature is increased, as the load becomes larger. In this embodiment, the inlet of the EGR gases, that is, a site connecting the EGR duct 21 to the exhaust system 4, is disposed on the downstream side of the catalyst converter 17, so that the exhaust gases are introduced into the EGR duct 21 after they have been cooled to some extent by the exhaust system 4. This arrangement enables the EGR gases cooled by the EGR cooler 25 to become lower in temperature, thereby improving the effects sought to be achieved in the high load region. The EGR cooler 25 may be of an air cooled type as well as of a water cooled type. The duct length of the second EGR duct 23 may be prolonged to thereby cool the EGR gases during passage through the prolonged EGR duct 23, without disposition of the EGR cooler to the second EGR duct 23. Further, the EGR ducts 23 may be composed of plural ducts juxtaposed with each other, thereby cooling the EGR gases. Figs. 5 et seq are directed to the other embodiments according to the present invention and, in those embodiments, the same parts or substantially the same parts are provided with the same reference numerals employed for the first embodiment as described hereinabove and description thereof will be omitted for brevity of explanation. The characteristic portion of the present invention will be described. Second Embodiment (Fig. 5) This embodiment according to the present invention is directed to the supercharged internal combustion engine having a supercharger of a mechanical type, which has the compression ratio of 10. To this embodiment is applied the control of the exhaust gas recirculation as described hereinabove in the first embodiment. In this embodiment, the common intake passage 5 of the intake system 3 is provided with a supercharger 50 on its upstream side and an inter cooler 51 on its downstream side. An input shaft 50a of the supercharger 50 is connected to an output shaft 1a of the internal combustion engine through a belt 52, and the supercharger 50 is driven by the engine output. The intake system 3 has a bypass passage 53 bypassing the supercharger 50 and the inter cooler 51. The bypass passage 53 contains a relief valve 55 drivable by the actuator 54 of a diaphragm type. The pressure chamber of the actuator 54 is communicated with the surge tank 9 and the relief valve 55 is so disposed as to open the bypass passage 53 when the pressure becomes higher than a predetermined value. The EGR unit 20 has the second EGR duct 23 connected to the common intake passage 5 interposed between the throttle valve 8 and the supercharger 50. The site where the second EGR duct 23 is connected may be the one interposed between the supercharger 50 and the inter cooler 51. The first EGR control valve 22 is arranged to be opened when the first solenoid valve 30 is opened and the negative pressure is introduced into the first actuator 27, in the manner as in the first embodiment according to the present invention. On the other hand, the second EGR control valve 24 is arranged to be opened when the second solenoid valve 32 is opened and the positive pressure (supercharging pressure) is introduced into the second actuator 28. The practices for controlling the first EGR control valve 22 and the second EGR control valve 24 are the same as in the first embodiment, so that description thereof will be omitted from the following description. Further, the EGR gases cooled by the EGR cooler 25 are recirculated toward the upstream side of the inter cooler 51 of the intake system 3, thereby supplying the EGR gases having lower temperature to the internal combustion engine. Third Embodiment (Fig. 6) This embodiment according to the present invention is directed to the internal combustion engine with a turbo charger and the way of controlling the EGR as employed in the first embodiment is applied to this embodiment. A turbo charger 57 comprises a turbine 58 disposed at the common exhaust passage 16, a supercharger 59 disposed at the common intake passage 5, and a connecting shaft 60 for connecting the turbine 58 to the supercharger 59. The exhaust system 4 is provided with a bypass passage 61 bypassing the turbine 58, and the bypass passage 61 in turn is provided with a gate valve 63 drivable by an actuator 62 of a diaphragm type. A pressure chamber of the actuator 62 is communicated with the common intake passage 5 on the downstream side of the supercharger 59 and the gate valve 63 is opened to withdraw the exhaust gases when the supercharging pressure reaches a value higher than a predetermined value. The EGR unit 20 has the first EGR duct 21 connected to the independent exhaust passage 15, thereby permitting the exhaust gases immediately after withdrawn from the engine to be introduced thereinto. The second EGR duct 23 is independent from the first EGR duct 21, and one end of the second EGR duct 23 is connected to the common exhaust passage 16 on the downstream side of the turbine 58 while the other end thereof is connected to the common intake passage 5 on the upstream side of the supercharger 59. The first EGR control valve 22 is opened when the negative pressure of intake air is introduced into the first actuator 27, like in the first embodiment according to the present invention. The second EGR control valve 24 is opened when the supercharging pressure is introduced into the second actuator 28. The practices for controlling the first EGR control valve 22 and the second EGR control valve 24 are the same as in the first embodiment, so that description thereof will be omitted from the following description. In this embodiment, as described hereinabove, the first EGR duct 21 to be opened in the light load region is connected to the independent exhaust passage 15, thereby allowing the exhaust gases having high temperature, immediately after withdrawn from the engine body 1, to be recirculated into the intake system 3. This arrangement can suppress an increase in work of the supercharger 59 by the EGR gases having high temperature in the light load region, so that the speed of rotation of the supercharger 59 is increased and the turbine efficiency is elevated. On the other hand, in the high load region, the exhaust gases having relatively low temperature is introduced into the second EGR duct 23, like in the second embodiment as described hereinabove, and the exhaust gases are then cooled by the EGR cooler 25, followed by recirculation into the intake system 3. Hence, in the low-rotation and high-load region where particularly the knocking is likely to occur, this embodiment has the advantages that the EGR gases having low temperature prevents the knocking from occurring and the turbine efficiency from decreasing.

Claims: An internal combustion engine (1) having a first outside exhaust gas recirculation duct (21) connected to the exhaust system (4) of the internal combustion engine (1) and an intake system (3) thereof for recirculating a portion of exhaust gases into the intake system (3) thereof, a second outside exhaust gas recirculation duct (23) connected to the exhaust system (4) of the internal combustion engine (1) and an intake system (3) thereof for recirculating a portion of exhaust gases into the intake system (3) thereof, while forcibly cooling that portion of the exhaust gases, a first exhaust gas recirculation control valve (22) disposed at said first outside exhaust gas recirculation duct (21), a second exhaust gas recirculation control valve (24) disposed at said second outside exhaust gas recirculation duct (23), and load detecting means (8, 41) for detecting the load of the engine, characterized in that the internal combustion engine further comprises a catalyst converter (17) disposed at an exhaust system of the internal combustion engine, said second outside exhaust gas recirculation duct (23) being connected to the exhaust system (4) at the downstream side of the catalyst converter (17), exhaust gas recirculation control means (27, 28) for controlling said first exhaust gas recirculation control valve (22) and said second exhaust gas recirculation control valve (24) based on the load condition of the engine in such a way as to reduce the recirculation ratio of the exhaust gases having relatively high temperature while increasing the recirculation ratio of the exhaust gases having relatively low temperature as the load becomes larger. An internal combustion engine as claimed in claim 1, wherein said first exhaust gas recirculation duct (21) is connected to the exhaust system (4) at the downstream side of the catalyst converter (17). An internal combustion engine according to claim 1 or claim 2, wherein said second outside exhaust gas recirculation duct (23) is provided with an EGR cooler. An internal combustion engine as claimed in claim 3, wherein said exhaust gas recirculation cooler (25) is of water cooled type. An internal combustion engine as claimed in claim 3, wherein said exhaust gas recirculation cooler (25) is of air cooled type. An internal combustion engine as claimed in any one of the claims 1 to 5, wherein said first outside exhaust gas recirculation duct (21) is connected with a downstream portion of the exhaust system (4). An internal combustion engine as claimed in any one of the preceding claims, wherein said internal combustion engine (1) is an engine having a high compression ratio. An internal combustion engine as claimed in claim 7, wherein said internal comustion engine is an engine having a supercharger (50) disposed in the intake system. An internal combustion engine as claimed in claim 8, wherein said supercharger (50) is of a mechanical type as drivable by output of the engine (1). An internal combustion engine as claimed in claim 8, wherein said supercharger (50) is a turbo charger of a type as drivable by exhaust gases from the engine (1). An internal combustion engine as claimed in claim 10, wherein said second outside exhaust gas duct (23) is connected on the downstream side of a turbine of said turbo charger (57). An internal combustion engine as claimed in claim 8, wherein an inter cooler (51) is disposed in the intake system of said engine on the downstream side of said supercharger (50). An internal combustion engine as claimed in any one of the preceding claims, wherein said first outside exhaust gas recirculation duct (21) is composed of pipes juxtaposed with each other. An internal combustion engine as claimed in any one of the preceding claims, wherein said second outside exhaust gas recirculation duct (23) has a duct length longer than said first outside exhaust gas recirculation duct (21). An internal combustion engine as claimed in any one of the preceding claims, wherein said second outside exhaust gas recirculation duct (23) is composed of pipes juxtaposed with each other. An internal combustion engine as claimed in any one of the claims 12 to 15, wherein said second outside exhaust gas recirculation duct (23) is connected on the upstream side of said inter cooler (51).

Inventors: HATTORI TOSHIHIKO, HITOMI MITSUO, IWATA NORIYUKI, KAIDE TADAYOSHI, KASHIYAMA KENJI, NOMURA KAZUMASA, SASAKI JUNSOU, UMEZAWA KAZUAKI, YAMAGATA NAOYUKI, HATTORI, TOSHIHIKO, HITOMI, MITSUO, IWATA, NORIYUKI, KAIDE, TADAYOSHI, KASHIYAMA, KENJI, NOMURA, KAZUMASA, SASAKI, JUNSOU, UMEZAWA, KAZUAKI, YAMAGATA, NAOYUKI, HATTORI, TOSHIHIKO, C/O MAZDA MOTOR CORPORATION, HITOMI, MITSUO, C/O MAZDA MOTOR CORPORATION, Iwata, Noriyuki, c/o Mazda Motor Corporation, Kaide, Tadayoshi, c/o Mazda Motor Corporation, KASHIYAMA, KENJI, C/O MAZDA MOTOR CORPORATION, Nomura, Kazumasa, c/o Mazda Motor Corporation, SASAKI, JUNSOU, C/O MAZDA MOTOR CORPORATION, Umezawa, Kazuaki, c/o Mazda Motor Corporation, Yamagata, Naoyuki, c/o Mazda Motor Corporation
Applicants: MAZDA MOTOR, MAZDA MOTOR CORPORATION
Application year: 1991
Main classification: F02D21
IPC codes: F02D21, F02M25