Patent Publication Number: US-11378041-B2

Title: Air intake device for engine

Description:
TECHNICAL FIELD 
     The present invention relates to an air intake device for an engine. 
     BACKGROUND ART 
     Patent Literature 1 discloses that a supercharger raising a pressure of air introduced into engine combustion chambers is arranged in an intake passage of a multi-cylinder engine, a bypass passage bypassing the supercharger is provided in the intake passage, a bypass valve adjusting an opening of the bypass passage is provided in the bypass passage, and an exhaust gas recirculation (EGR) valve is provided in an EGR passage connecting the intake passage with an exhaust passage. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Laid-Open No. 2003-322039 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a case of a multi-cylinder engine causing EGR gas to return, when EGR amounts become non-uniform among cylinders, stable combustion may not be performed in all of the cylinders. The inventor has investigated non-uniformity of the EGR amounts among the cylinders. Then, it has been found that one factor in the non-uniformity is that the EGR gas returned to an intake passage and fresh air flowing through the intake passage are not sufficiently mixed together before those are distributed from the intake passage to each of the cylinders. 
     In  FIG. 7 , a reference numeral  25  denotes a bypass passage configuring an intake passage, and a reference numeral  69  denotes a connection port of an EGR passage in which an EGR valve  62  is provided. In this example, the fresh air flows on an upper side in the bypass passage  25  as indicated by broken lines, and the EGR gas flows from the connection port  69  mainly into a lower side in the bypass passage  25  as indicated by solid lines. In the simplest of terms, the fresh air and the EGR gas thus flow to a downstream side of the bypass passage  25  in two separate layers. This can be understood from the EGR concentration distribution in each portion in the bypass passage  25 , which is illustrated in  FIG. 8 . 
     In  FIG. 8 , on an immediately downstream side of the connection port  69 , an inside of the bypass passage  25  is split into a region A in which a concentration related to the EGR gas is high and a region B in which the concentration is low. Toward the downstream side of the bypass passage  25 , the high concentration region A and the low concentration region B decrease, and a medium concentration region C expands. However, although the bypass passage  25  branches to branch portions  25   a  and  25   b  and is connected with a surge tank  75 , even in the branch portion.  25   a , the high concentration region A and the low concentration region B remain. That is, it can be understood that the fresh air and the EGR gas flow into the surge tank  75  without being completely mixed together. Thus, non-uniformity likely to occur to the EGR amounts among cylinders. 
     An important problem is that in accordance with an operation state of an engine (for example, an engine speed), not only an uneven state of a fresh air flow in the intake passage (the bypass passage in the example of  FIGS. 7 and 8 ) changes, but also an uneven state of a flow in a case where the EGR gas flows from the connection port  69  into the intake passage changes. As a result, in accordance with the operation state of the engine, the non-uniformity of the EGR amounts among the cylinders becomes different, and it thus becomes difficult to secure combustion stability. 
     Accordingly, an object of the present invention is to efficiently mix fresh air and EGR gas. 
     Solution to Problem 
     To solve the above problem, in the present invention, an expanding portion in which a passage cross-sectional area expands and which lowers a flow speed of EGR gas flowing into an intake passage is provided in a position close to a connection port of an EGR passage to the intake passage. 
     An air intake device for an engine disclosed herein, includes: 
     an intake passage leading intake air to combustion chambers of a multi-cylinder engine; 
     an exhaust passage discharging exhaust gas from the combustion chambers; and 
     an EGR passage connecting the intake passage with the exhaust passage and returning a portion of the exhaust gas as EGR gas from the exhaust passage to the intake passage, and is characterized in that 
     the EGR passage includes, in a position close to a connection port to the intake passage, an expanding portion in which a passage cross-sectional area expands and which lowers a flow speed of the EGR gas so as to reduce an uneven flow, in the connection port, of the EGR gas flowing, into the intake passage. 
     Accordingly, the flow speed of the EGR gas in the EGR passage is lowered in a position close to the connection port to the intake passage, and the uneven flow of the EGR gas in the connection port is thereby reduced. That is, the extent of the uneven flow becomes low, and the EGR gas easily flows into the intake passage along a whole circumference of the connection port. As a result, even if a flow of fresh air flowing through the intake passage is slightly uneven, the EGR gas is likely to collide with the fresh air, that is, mixing of the fresh air and the EGR gas easily progresses, and non-uniformity of EGR amounts among the cylinders is reduced. Consequently, an advantage in securing combustion stability of the engine is obtained. 
     In one embodiment, the EGR passage includes a passage portion extending toward the connection port and in a direction intersecting with the intake passage and intersecting with a center line of the connection port and a direction-changing portion starting from the passage portion, changing a direction to the direction of the center line of the connection port, and reaching the connection port, and the expanding portion is provided in the direction-changing portion. 
     In a case where the direction-changing portion in which a flow direction of the EGR gas changes is present in a position close to the connection port to the intake passage in the EGR passage, unevenness of the flow of the EGR gas is likely to occur, but the expanding portion is provided in the direction-changing portion, and the unevenness is thereby reduced. 
     In one embodiment, the intake passage includes a supercharging passage in which a supercharger raising a pressure of the intake air introduced into the combustion chambers is arranged and a bypass passage connecting an upstream side with a downstream side of the supercharger and leading the intake air to the combustion chambers while bypassing the supercharger, and the EGR passage is connected with the bypass passage of the intake passage. 
     When the fresh air is led from the bypass passage to the combustion chambers without going through the supercharger, mixing of the fresh air and the EGR gas by the supercharger is not expected. However, even in this case, as described above, the expanding portion is provided in the EGR passage, mixing of the fresh air and the EGR gas thereby easily progresses in the bypass passage, and non-uniformity of the EGR amounts among the cylinders is reduced. 
     In one embodiment, an EGR valve of a poppet type is included, the EGR valve being provided to the connection port and adjusting a returning amount of the EGR gas, and a valve shaft of the EGR valve passes through the bypass passage. Accordingly, in a portion around the valve shaft, collision between the fresh air flowing while bypassing the valve shaft and the EGR gas flowing along the valve shaft is caused, and mixing of the fresh air and the EGR gas thereby easily progresses. 
     In one embodiment, a bypass valve is included, the bypass valve being provided in the bypass passage and adjusting a supercharging pressure of the intake air by the supercharger, and the connection port opens on an upstream side of the bypass valve in the bypass passage. Accordingly, because the flows of the fresh air and the EGR gas are disturbed when those pass through the bypass valve, mixing of the fresh air and the EGR gas easily progresses. 
     In one embodiment, the expanding portion includes a divergent portion in which a passage cross-sectional area gradually expands toward the connection port. Accordingly, when the EGR gas passes through the divergent portion, the EGR gas is easily spread in the whole expanding portion while its flow speed is gradually lowered toward the connection port. Thus, unevenness of the flow of the EGR gas can be reduced without excessively disturbing the flow of the EGR gas. 
     Advantageous Effects of Invention 
     In the present invention, an EGR passage includes, in a position close to a connection port to an intake passage, an expanding portion in which a passage cross-sectional area expands and which lowers a flow speed of EGR gas so as to reduce an uneven flow of the EGR gas in the connection port. Thus, the EGR gas flowing into the intake passage is likely to collide with fresh air, and consequently, mixing of the fresh air and the EGR gas easily progresses. As a result, non-uniformity of EGR amounts among cylinders reduced, and an advantage in securing combustion stability of an engine is thus obtained. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of an engine system. 
         FIG. 2  is a front view of an engine. 
         FIG. 3  is a cross-sectional view of an intake system of the engine. 
         FIG. 4  is a perspective view or the intake system of the engine. 
         FIG. 5  is a front view of the intake system of the engine. 
         FIG. 6  is a cross sectional view of a connection portion between a bypass passage and an EGR passage. 
         FIG. 7  is a side view illustrating flows of fresh air and EGR gas. 
         FIG. 8  is a diagram illustrating an EGR concentration distribution in each portion in the bypass passage. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     A form for carrying out the present invention will hereinafter be described based on drawings. The description of a preferable embodiment is substantially only exemplification and is not intended to restrict the present invention, applications thereof, or uses thereof. 
     &lt;General Configuration of Engine&gt; 
     In a vehicle-installed engine system illustrated in  FIG. 1 , a reference numeral  1  denotes an engine, a reference numeral  2  denotes an intake passage of the engine  1 , a reference numeral  3  denotes an exhaust passage of the engine  1 , and a reference numeral  4  denotes a fuel tank. The system includes an evaporated fuel treatment device  5  leading evaporated fuel produced in the fuel tank  4  to the intake passage of the engine  1 . 
     The engine  1  is an in-line four-cylinder compression ignition engine.  FIG. 1  illustrates only one cylinder of the engine  1 . The engine  1  described in this embodiment is merely one example, and in the present invention, types and specific configurations of an engine are not limited. The engine  1  includes a direct injection fuel injection valve  11 , a spark plug  12 , and a cylinder inner pressure sensor  13 , which face a combustion chamber  10  of each cylinder. In the engine  1 , an intake valve  14  is provided to an intake port, and an exhaust valve  15  is provide to an exhaust port. The engine  1  includes variable valve mechanisms  16  and  17  for respectively driving the intake valve  14  and the exhaust valve  15  to open and close. A reference numeral  18  denotes a piston of the engine  1 . 
     The intake passage  2  includes an intake manifold (not illustrated) for introducing intake air into the combustion chambers  10  of the cylinders in a branched manner. In the intake passage  2 , in order from an upstream side to a downstream side, an air cleaner  21 , a throttle valve  22  adjusting an introduction amount of fresh air into the combustion chambers  10 , a supercharger raising a pressure of gas introduced into the combustion chambers  10 , and an intercooler  24  cooling the gas introduced into the combustion chambers  10  by a supercharger  3  are disposed. Further, in the intake passage  2 , a bypass passage  25  connecting an upstream side of the supercharger  23  with a downstream side of the intercooler  24  is provided on a downstream side of the throttle valve  22 . 
     That is, the intake passage  2  includes a supercharging passage in which the supercharger  23  raising a pressure of the intake air introduced into the combustion chambers  10  is arranged and the bypass passage  25  leading the intake air to the combustion chambers  10  while bypassing the supercharger  23 . In the bypass passage  25 , a bypass valve  26  is provided which adjusts a flow amount of gas flowing through the bypass passage  25 . 
     The supercharger  23  of this embodiment is a mechanical supercharger driven via a belt by a crankshaft of the engine  1 . A supercharger  44  of a mechanical type may be of a Roots type, a Lysholm type, a vane type, or a centrifugal type, for example. Note that instead of a mechanical supercharger, an electric supercharger, or a turbocharger driven by exhaust energy may be employed. 
     The supercharger  23  is connected with the crankshaft of the engine  1  via an electromagnetic clutch  27 . Transmission and disconnection of motive power from the engine  1  to the supercharger are performed by connection and disconnection of the electromagnetic clutch  27 . 
     When the electromagnetic clutch  27  is set to a disconnected state (when the supercharger  23  is not acting), the bypass valve  26  is fully opened. Accordingly, the intake air is introduced into the combustion chambers  10  of the engine  1  by the bypass passage  25  without going through the supercharger  23 . That is, the engine  1  is operated in a naturally aspirated (non-supercharging) state. 
     When the electromagnetic clutch  27  is set to a connected state (when the supercharger  23  is acting), a supercharging pressure is adjusted to a desired pressure by control of the bypass valve  26 . That is, when the bypass valve  26  is opened, a portion of the intake air passing through the supercharger  23  goes through the bypass passage  25  and reversely flows to an upstream side of the supercharger  23 . Because a reverse flow amount of the intake air changes in accordance with the opening of the bypass valve  26 , the supercharging pressure of the intake air introduced into the combustion chambers  10  can be controlled. 
     The exhaust passage  3  includes an exhaust manifold  31  for gathering and discharging exhaust gas of the cylinders. In the exhaust passage  3  on a downstream side of the exhaust manifold  31 , two catalytic converters purifying the exhaust gas are provided. The catalytic converter on an upstream side has a three-way catalyst  32  and a GPF (gasoline particulate filter)  33  and is disposed in an engine room of a vehicle. The catalytic converter on a downstream side has a three-way catalyst  34  and is disposed on the outside of the engine room. An exhaust shutter valve  35  is provided to each branch pipe of the exhaust manifold  31 . 
     The intake passage  2  and the exhaust passage  3  are connected together by an exhaust gas recirculation (EGR) passage  6  returning a portion of the exhaust gas as EGR gas to the intake passage  2 . An upstream end of the EGR passage  6  is connected with a portion in the exhaust passage  3  between the upstream catalytic converter and the downstream catalytic converter. A downstream end of the EGR passage  6  is connected with an intermediate portion of the bypass passage  25  so as to supply the EGR gas to a portion in the intake passage  2  on a downstream side of the throttle valve  22  and on an upstream side of the supercharger  23 . The EGR gas enters an upstream side of the supercharger  23  in the intake passage  2  without going through the bypass valve  26  of the bypass passage  25 . In the EGR passage  6 , an EGR cooler  61  cooling the EGR gas and an EGR valve  62  adjusting a returning amount of the EGR gas are disposed. 
     Note that although  FIG. 1  depicts the EGR valve  62  as provided in an intermediate portion of the EGR passage  6 , in this embodiment, the EGR valve  62  provided to a connection port of the EGR passage  6  to the bypass passage  25 . 
     The fuel tank  4  is connected with the fuel injection valves  11  by a fuel supply passage  41 . An upstream end of the fuel supply passage  41  is connected with a fuel strainer  40  in the fuel tank  4 . In the fuel supply passage  41 , a fuel pump  42  and a common rail  43  are provided. The fuel pump  42  pumps fuel into the common rail  43 . The common rail  43  stores the fuel pumped from the fuel pump  42  at a high fuel pressure. When the fuel injection valve  11  is opened, the fuel stored in the common rail  43  is injected from an injection hole of the fuel injection valve  11  into the combustion chamber  10 . 
     The evaporated fuel treatment device  5  includes canisters  51  causing the evaporated fuel produced in the fuel tank  4  to be adsorbed onto activated carbon. The fuel tank  4  and the canisters are connected together by a tank-side passage  52 , and the canisters  51  and the intake passage  2  are connected together by a purge passage  53 . An outside air introduction passage  54  having an atmospheric opening is connected with the canisters  51 . A purge valve  55  opening and closing the purge passage  53  is provided to the purge passage  53 . The purge valve  55  opens when a predetermined purge condition is satisfied, for example, in a state where an air-fuel ratio of the engine  1  can properly be controlled by control of a fuel injection amount by the fuel injection valves  11 . 
     When a negative pressure is generated on a downstream side of the throttle valve  22  in the intake passage  2  in a state where the purge valve  55  is open, the evaporated fuel collected in the canisters  51  is purged. That is, together with air introduced from the outside air introduction passage  54  into the canisters  51 , the evaporated fuel is purged from the purge passage  53  to a downstream side of the throttle valve  22  in an intake passage  21 . The purged evaporated fuel is supplied to the combustion chambers of the engine  1  through the supercharger  23  or the bypass passage  25  and is combusted together with the fuel supplied from the fuel injection valves  11 . 
     The engine system includes a blowby gas returning device. The blowby gas returning device includes a blowby passage  57  and an air introduction passage  58 . One end of the blowby passage  57  is connected with a crankcase  1   a  of the engine  1 , and the other end is connected with a portion of the intake passage  2  on a downstream side of the throttle valve  22  and on an upstream side of the supercharger  23 . A PCV (positive crankcase ventilation) valve  59  is provided to the blowby passage  57 . 
     The PCV valve  59  allows only gas in a direction from the crankcase  1   a  side to the intake passage  2  side to pass through. In a negative pressure state where the pressure on the downstream side of the throttle valve  22  in the intake passage  2  is lower than the pressure of the crankcase  1   a , the opening of the PCV valve  59  changes in accordance with the extent of the negative pressure. That is, a blowby gas flow amount from the crankcase  1   a  to the intake passage  2  is adjusted to an appropriate amount in accordance with the negative pressure. 
     One end of the air introduction passage  58  is connected with the crankcase  1   a  via a cylinder head  1   b  of the engine  1 , and the other end is connected with a portion of the intake passage  2  between the air cleaner  21  and the throttle valve  22 . In the air introduction passage  58 , a check valve  60  is provided which allows only air in a direction from the intake passage  2  side to the crankcase  1   a  side to pass through. 
     When blowby gas is released from the crankcase  1   a  to the intake passage  2  through the blowby passage  57 , air filtered by the air cleaner  21  is introduced from the air introduction passage  58  into the crankcase  1   a . Accordingly, the crankcase  1   a  is ventilated. 
     In the intake passage  2 , an air flow sensor  63  detecting an intake air amount, a pressure sensor  64  detecting an intake pressure on a downstream side of the throttle valve  22  (an upstream side of the supercharger  23 ), a temperature sensor  65  detecting the temperature of the intake air ejected from the supercharger  23 , and a pressure sensor  66  detecting the intake pressure on a downstream side of the intercooler  24  are provided, the sensors being for controlling the engine  1 . In the exhaust passage  3 , a linear O 2  sensor  67  detecting an oxygen concentration in the exhaust gas on an upstream side of the three-way catalyst  32  and a lambda O 2  sensor  68  detecting the oxygen concentration in the exhaust gas on a downstream side of the three-way catalyst  32  are provided. 
     &lt;Structures of Engine System Configuration Elements&gt; 
     As illustrated in  FIG. 2 , the supercharger  23  is provided in a state where an axis extends in a cylinder array direction in a portion above the engine  1 . An upstream intake pipe  71  configuring the intake passage  2  extending in the cylinder array direction is coupled with this supercharger  23 . A drive part housing  72  of the supercharger  23  protrudes toward the opposite side, in the supercharger  23 , to the upstream intake pipe  71 . The electromagnetic clutch  27  and a driving shaft for driving the supercharger  23  by the crankshaft of the engine  1  are housed in this drive part housing  72 . A transmission belt  74  is wound around a pulley  73  coupled with the driving shaft. 
     An upstream end of an ejection duct  76  for leading pressurized intake air to a surge tank (reference sign  75  in  FIG. 4 ) extending in the cylinder array direction is connected with a side surface of the supercharger  23 . The ejection duct  76  extends to a lower side of the supercharger  23 , and a lower end thereof is connected with the intercooler  24  arranged below the supercharger  23 . 
     As illustrated in  FIG. 3 , a throttle body  77  including the throttle valve  22  is provided to an upstream end portion of the upstream intake pipe  71 . The throttle valve  22  is a butterfly valve, and a valve shaft  22   a  thereof is horizontally provided. On a downstream side of the throttle body  77  (an upstream side of the supercharger  23 ), a bypass pipe  78  forming the bypass passage  25  obliquely rises from an upper surface of the upstream intake pipe  71  toward an upstream side of the upstream intake pipe  71 . That is, on a downstream side of the throttle valve  22 , a connection port  79  of the bypass passage  25  opens in a top portion of an upper half circumferential portion of the intake passage  2  formed with the upstream intake pipe  71 . 
     On a downstream side of the connection port  79  of the bypass passage  25 , the upstream intake pipe  71  forms a passage expanding portion  2   b  in which a passage cross-sectional area expands toward the supercharger  3 , and an expanding end thereof is connected with the supercharger  3 . 
     The bypass pipe  78  has a folded portion  78   a  that is continuous with the above-described oblique rising portion and is folded, in a curved manner, toward a downstream side of the upstream intake pipe  71 . The bypass pipe  78  is continuous with the folded portion  78   a  and extends toward a central side of the surge tank  75  in the cylinder array direction above the supercharger  23 . An EGR pipe (not illustrated in  FIG. 3 ) forming the EGR passage  6  is connected with a downstream side of the folded portion  78   a  in the bypass pipe  78 , and the EGR valve  62  is provided to a connection port  69  of the EGR passage  6  to the bypass passage  25 . The connection port  69  opens in a side surface of the bypass passage  25 . The bypass pipe  78  branches to a first branch pipe  78   b  extending in one direction of the cylinder array direction and a second branch pipe  78   c  extending in the other direction of the cylinder array direction. 
     As illustrated in  FIG. 4 , branch portions  25   a  and  25   b  of the bypass passage  25  respectively formed with both of the branch pipes  78   b  and  78   c  are connected with the surge tank  75 . 
     As illustrated in  FIG. 3 , the bypass valve  26  is provided in the bypass pipe  78  on a downstream side of the EGR valve  62 . That is, the connection port  69  of the EGR passage  6  opens in the bypass passage  25  on an upstream side of the bypass valve  26 . The bypass valve  26  is a butterfly valve, and a valve shaft  26   a  thereof is horizontally provided. 
     As illustrated in  FIG. 5 , an intake air introduction passage  80  is integrally provided to the surge tank  75 . The intake air introduction passage  80  extends to a lower side of the surge tank  75  and is connected with the intercooler  24 . Further, as illustrated in  FIG. 5 , an EGR pipe  81  extending from the exhaust passage  3  includes a rising portion  91  rising from a lower position than the bypass pipe  78  toward a side surface of the bypass pipe  78 , an upper end portion of the rising portion  91  is connected with the side surface of the bypass pipe  78 . 
     As illustrated in  FIG. 6 , the rising portion  91  of the EGR pipe  81  forms a passage portion  92  extending toward the connection port  69  of the EGR passage  6  in the bypass passage  25  and in a direction intersecting with the bypass passage  25  and intersecting with a center line D of the connection port  69 . In a middle portion of this rising portion  91 , a flexible portion (bellows portion)  93  is provided which absorbs displacement between an upstream portion and a downstream portion of the middle portion. The upper end portion of the rising portion  91  forms a direction-changing portion  94  in a position close to the connection port  69 , the direction-changing portion  94  being continuous with the passage portion  92 , changing a direction to the direction of the center line D of the connection port  69 , and reaching the connection port  69 . 
     An expanding portion  95  in which a passage cross-sectional area expands compared to the passage portion  92  (a passage portion with a circular cross section on a downstream side of the flexible portion  84 ) is formed in the direction-changing portion  94 . The expanding portion  95  includes a divergent portion  96  in which the passage cross-sectional area gradually expands from a downstream end of the passage portion  92  toward the connection port  69 . The passage cross-sectional area of the expanding portion  95  is larger than the passage cross-sectional area of the connection port  69 . The direction-changing portion  94  includes a portion in which the passage cross-sectional area shrinks and which is continuous with the expanding portion  95  and reaches the connection port  69 , and a valve seat  97  of the EGR valve  62  opening and closing the connection port  69  is formed in the shrinking portion. 
     The EGR valve  62  of a poppet type, a valve shaft  98  thereof passes through the bypass passage  25  and extends in the direction of the center line  8 ) of the connection port  69 . That is, the valve shaft  98  crosses an inside of the bypass passage  25  in the direction of the center line D of the connection port  69 . The valve shaft  98  moves forward and backward by being driven by a solenoid-type EGR valve drive part  85  illustrated in  FIG. 2 , and the connection port  69  opens by movement of the EGR valve  62  to the expanding portion  95  side. 
     Note that in  FIG. 2 , a reference numeral  83  denotes a drive part of the throttle valve  22 , and a reference numeral  84  denotes a drive part of the bypass valve  26 . 
     &lt;Mixing of EGR Gas and Fresh Air&gt; 
     In the above embodiment, in a case where the supercharger  23  is not acting, the case being illustrated in  FIG. 3 , the fresh air passing through the throttle valve  22  of the intake passage  2  flows into the bypass passage  25  through the connection port  79 . The fresh air goes through a portion, in which the EGR valve  62  is provided, and a portion, in which the bypass valve  26  is provided, of the bypass passage  25  and is introduced from the branch portions  25   a  and  25   b  illustrated in  FIG. 4  into the surge tank  75 . 
     As illustrated in  FIG. 6 , when the EGR valve  62  opens (a valve open state is indicated by the chain lines), the EGR gas is led upward through the passage portion  92  of the EGR passage  6 . The flow direction of the EGR gas is changed from an upward direction to a lateral direction in the direction-changing portion  94  and flows from a portion around the EGR valve  62  into the bypass passage  25  through the connection port  69 . 
     As described above, when the flow direction of the EGR gas changes in the direction-changing portion  94 , in related art, in accordance with an operation state of the engine, that is, in accordance with the flow speed of the EGR gas, unevenness occurs to a flow of the EGR gas in the direction-changing portion  94 . For example, as the ow speed becomes higher, the EGR gas is more likely to flow unevenly along an upper half circumferential side of the direction-changing portion  94  and to flow from an upper side of the EGR valve  62  into the bypass passage  25  through the connection port  69 . In this case, because the EGR gas moves obliquely downward from the upper side of the EGR valve  62  toward the connection port  69  and as a result flows into a lower half circumferential side of the bypass passage  25 , as illustrated in  FIG. 7  and  FIG. 8 , the fresh air and the EGR gas are likely to flow in two separate layers in the bypass passage  25 . 
     On the other hand, in the above embodiment, because the expanding portion  95  of the passage cross-sectional area is formed in the direction-changing portion  94 , the flow speed of the EGR gas flowing through the passage portion  92  is lowered in the expanding, portion  95 . This lowering of the flow speed reduces unevenness of the EGR gas in the direction-changing portion  94 , and the EGR gas flows from the portion around the EGR valve  62  into the bypass passage  25  while comparatively evenly going through the connection port  69 . As a result, in the bypass passage  25 , the EGR gas is likely to contact with the flow of the fresh air from a lateral side, and the fresh air and the EGR gas are thus easily mixed together. 
     Furthermore, in the above embodiment, because an upstream side of the expanding portion  95  is formed as the divergent portion  96 , when the EGR gas passes through the divergent portion  96 , the EGR gas is easily spread in the whole expanding portion while its flow speed is gradually lowered. Thus, this is advantageous to reduction of unevenness of the flow of the EGR gas. 
     Further, in the above embodiment, because the valve shaft  98  of the EGR valve  62  crosses the bypass passage  25 , the fresh air moving while bypassing the valve shaft  98  collides with the EGR gas flowing along the valve shaft  98 , and the fresh air and the EGR gas are easily mixed together. In addition, because when the fresh air and the EGR gas pass through the bypass valve  26 , the flows of those are disturbed by the bypass valve  26 , mixing easily progresses. 
     As described above, unevenness of the flow of the EGR gas passing through the connection port  69  is reduced, and mixing of the fresh air and the EGR gas in the bypass passage  25  easily progresses. As a result, non-uniformity of EGR amounts among cylinders is reduced, and an advantage in securing combustion stability of the engine is consequently obtained. 
     Note that the EGR valve  62  of the above embodiment is of a poppet type; however, a butterfly type EGR valve can also reduce unevenness of the flow of the EGR gas passing through the connection port  69  by providing an expanding portion as described above in the vicinity of the connection port on a downstream side of the EGR valve. 
     REFERENCE CHARACTERS LIST 
     
         
         
           
               1  engine 
               2  intake passage 
               3  exhaust passage 
               6  EGR passage 
               10  combustion chamber 
               23  supercharger 
               25  bypass passage 
               26  bypass valve 
               62  EGR valve 
               69  connection port 
               92  passage portion 
               94  direction-changing portion 
               95  expanding portion 
               96  divergent portion 
               98  valve shaft