Abstract:
To protect a plastic intake manifold of an internal combstion engine from heat possessed by exhaust gas recirculation gas, a cooling device is arranged between the plastic intake manifold and an exhaust gas recirculation valve. The cooling device cools the exhaust gas recirculation gas by means of a coolant. A gas discharge part of the cooling device constitutes a pipe portion which penetrates through an exhaust gas inlet hole of the intake manifold keeping a given space between an outer wall of the pipe portion and an inner wall of the exhaust gas inlet hole. The pipe portion may be a leading end portion of an exhaust gas recirculation pipe extending from an exhaust system of the engine.

Description:
This application is a continuation of application Ser. No. 08/931,497 filed Sep. 16, 1997, now U.S. Pat. No. 5,970,960. 
    
    
     The contents of Patent Applications Nos. 8-245793 and 8-245794, with a filing date of Sep. 18, 1996 in Japan, are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to emission control systems for an internal combustion engine, and particularly to an exhaust gas recirculation (EGR) system of the engine. More specifically, the present invention relates to an improvement in connecting an EGR valve or an EGR pipe to a plastic intake manifold of an internal combustion engine. 
     2. Description of the Prior Art 
     Hitherto, in motor vehicles powered by an internal combustion engine, an exhaust gas recirculation (EGR) system has been commonly installed for reducing NOx emissions produced by the engine. As is known, the EGR system is designed to recirculate a metered amount of exhaust gas into the air-fuel mixture in the combustion chambers to reduce the temperature in the combustion chambers and thus NOx emissions. In the EGR systems, an EGR valve is installed in an EGR passage for regulating the amount of EGR. Usually, the EGR valve is connected to an intake manifold of the engine. Under operation of the EGR system, the EGR valve which is constructed of a metal is highly heated by absorbing heat of the recirculating exhaust gas. 
     Thus, if the intake manifold is constructed of a plastic (viz., glass fiber-reinforced plastic) for reducing the weight of the engine system or for other reasons, it is necessary to take any measure for protecting the plastic intake manifold from the heat of the EGR valve. 
     Hitherto, various measures have been proposed and put into practical use for protection of the plastic intake manifold from the heat of the EGR valve, some of which are shown in Japanese Patent First Provisional Publications 5-256217 and 6-101587 and Japanese Utility Model First Provisional Publication 63-164554. In the Publication 5-256217, the EGR valve is mounted to the plastic intake manifold through a mounting bracket of corrugated stainless steel plate. In the Publication 6-101587, the EGR valve is connected to the plastic intake manifold with an interposal of a heat insulator therebetween, first bolts are used to secure the heat insulator to the manifold and second bolts are used to secure the valve to the heat insulator. In the publication 63-164554, a junction portion between the EGR valve and the plastic intake manifold is formed with an annular groove through which a coolant flows for cooling the junction portion. 
     In addition to the above-mentioned measures, a measure for protection of the plastic intake manifold from the heat of exhaust gas is described in Japanese Utility Model First Provisional Publication 1-102465. In this measure, a fresh air from an air cleaner is fed into an EGR pipe to reduce the temperature of the EGR gas led into the plastic intake manifold. Furthermore, for suppressing or minimizing direct contact of the highly heated exhaust gas with an inner wall of the plastic intake manifold, a leading end of the EGR pipe is projected into the interior of the intake manifold through a pipe passing opening formed in the same. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an exhaust gas recirculation system of an internal combustion engine, which is provided in view of the disclosure of the above-mentioned publications. 
     According to a first aspect of the present invention, there is provided an exhaust gas recirculation system for use with an internal combustion engine having a plastic intake passage. The system comprises a connecting base formed on the plastic intake passage, the connecting base having an exhaust gas inlet hole connected with the interior of the intake passage; an exhaust gas recirculation valve through which a metered amount of exhaust gas produced by the engine is fed back to the interior of the plastic intake passage; and a cooling device arranged between the connecting base and the exhaust gas recirculation valve, the cooling device including mutually separated first and second passages, the first passage connecting an outlet opening of the exhaust gas recirculation valve to the exhaust gas inlet hole of said connecting base, the second passage being shaped to surround the first passage and adapted to flow therein a coolant. The first passage of the cooling device includes a pipe portion which penetrates through the exhaust gas inlet hole keeping a given space between an outer wall of the pipe portion and an inner wall of the exhaust gas inlet hole. 
     According to a second aspect of the present invention, there is provided an exhaust gas recirculation system for use with an internal combustion engine having a plastic intake passage. The system includes a connecting base formed on the plastic intake passage, the connecting base having an exhaust gas inlet hole connected with the interior of the intake passage; an exhaust gas recirculation valve through which a metered amount of exhaust gas produced by the engine is fed back to the interior of the plastic intake passage; and an exhaust gas recirculation pipe having first and second end portions, the first end portion penetrating through the exhaust gas inlet hole and the second end portion being connected to an outlet opening of the exhaust gas recirculation valve. An opening defined by an inner end of the exhaust gas inlet hole is larger than that defined by the other portion of the exhaust gas inlet hole. 
     According to a third aspect of the present invention, there is provided a cooling device for use in an exhaust gas recirculation system. The device comprises a front housing member; a rear housing member; a seal member; and bolts for coupling the front and rear housing members having the seal member interposed therebetween thereby to constitute a housing unit. The housing unit includes mutually separated first and second passages, the first passage being adapted to pass therethrough exhaust gas for the exhaust gas recirculation, the second passage being shaped to surround the first passage and adapted to flow therethrough a coolant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view of an essential portion of an exhaust gas recirculation system which is a first embodiment of the present invention; 
     FIG. 2 is a front view of a cooling housing installed in the first embodiment; 
     FIG. 3 is a side view of the cooling housing; 
     FIG. 4 is a sectional view of the cooling housing connected to a plastic intake manifold; 
     FIG. 5 is a front view of a rear housing member of the cooling housing; 
     FIG. 6 is a partially cut plan view of a plastic intake manifold to which an exhaust gas recirculation system of a second embodiment of the present invention is practically applied; 
     FIG. 7 is an enlarged sectional view of a portion indicated by an arrow “VII” of FIG. 6; 
     FIG. 8 is an illustration showing a positional relation between a throttle valve and a pipe inserting opening formed in the plastic intake manifold; 
     FIG. 9 is an illustration showing a test device for recognizing a cooling effect of an annular groove possessed by the second embodiment; 
     FIG. 10 is a graph showing the result of the experiment; and 
     FIG. 11 is an illustration showing vortexes produced by a throttle valve of a throttle chamber. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Referring to FIGS. 1 to  5 , particularly FIG. 1, there is shown an exhaust gas recirculation (or EGR) system which is a first embodiment of the present invention. 
     In FIG. 1, denoted by numeral  1  is a plastic intake manifold which is secured to a cylinder head (not shown) of an internal combustion engine in a known manner. The intake manifold  1  generally comprises an elongated collector portion  2  which extends along the row of engine cylinders (not shown), a plurality of branches (not shown) which extend from one side of the collector portion  2  to respective intake ports of the cylinder head and an inlet flange  3  which is formed on an upstream end of the collector portion  2  to mount thereto a throttle chamber (not shown). The entire structure of the plastic intake manifold  1  may be well understood when referring to FIG.  6 . The intake manifold  1  is molded from glass fiber-reinforced Nylon-6, 6 or the like. 
     As is seen from FIG. 1, the collector portion  2  is integrally formed near the inlet flange  3  with a mounting seat  4  which is rectangular in shape. The mounting seat  4  has at a center thereof a cylindrical hole  5  connected with the interior of the collector portion  2 . 
     To the mounting seat  4 , there is fixed a cooling housing  6  of metal. To the cooling housing  6 , there is connected an EGR valve  7  which is of a diaphragm type. One end of an EGR pipe  8  is connected to the EGR valve  7  and the other end of the EGR pipe  8  is connected to an exhaust manifold (not shown) of the engine, so that part of exhaust gas in the exhaust manifold is led to the EGR valve  7  through the EGR pipe  8 . 
     The cooling housing  6  is constructed of an aluminum die-cast. The cooling housing  6  comprises generally a rear housing member  11  which is placed on the mounting seat  4  of the intake manifold  1  and a front housing member  12  to which the EGR valve  7  is connected. As is seen from FIGS. 3 and 4, these two housing members  11  and  12  are united through three bolts  13  with an interposal of a seal member  34  therebetween. The seal member  34  may be a liquid gasket or the like. 
     As is seen from FIG. 3, the rear housing member  11  has at a rear side thereof a flat contact surface  11   a  which is intimately put on the above-mentioned mounting seat  4 , and as is seen from FIGS. 1 and 3, the front housing member  12  has at a front side thereof a flat contact surface  12   a  to which a body  7   a  of the EGR valve  7  is mounted through a gasket  16  (see FIG.  1 ). The flat contact surface  12   a  is raised from a major flat portion  12   b  of the front side of the front housing member  12 . 
     As is best seen from FIG. 4, the cooling housing  6  has therein an exhaust gas passage  14  which straightly passes through the front and rear housing members  12  and  11 . The exhaust gas passage  14  is enclosed or surrounded by a water jacket  15  defined in the cooling housing  6 . As will be described in detail hereinafter, in the water jacket  15 , there flows cooling water. 
     As is seen from FIG. 1, a front end of the exhaust gas passage  14  is exposed to the flat contact surface  12   a  and connected to an outlet port of the EGR valve  7 . While, as is seen from FIG. 4, a rear end of the exhaust gas passage  14  is defined by an integral pipe portion  17  which is projected from the flat contact surface  11   a . The outer diameter of the pipe portion  17  is slightly smaller than the diameter of the cylindrical hole  5  of the mounting seat  4  of the intake manifold  1 . Upon assembly, the pipe portion  17  is received in the cylindrical hole  5  leaving a small annular clearance therebetween. Preferably, the clearance is about 1 mm to 2 mm in thickness. With the clearance, a certain heat insulation is obtained. If desired, a separate pipe member of metal (such as stainless steel or the like) may be used in place of the integral pipe portion  17 . In this case, the separate pipe member is press-fitted into the exhaust gas passage  14  of the cooling housing  6 . 
     As is well seen from FIGS. 1 and 2, the flat contact surface  12   a  of the front housing member  12  is formed at both sides of the exhaust gas passage  14  with threaded bolt holes  18 . 
     As will be seen from FIG. 4, two threaded bolts extending from the EGR valve  7  are engaged with the bolt holes  18  for securing the EGR valve  7  to the flat contact surface  12   a.    
     As is seen from FIG. 4, each bolt hole  18  extends in the direction of the thickness of the front housing member  12  and has a counter bore part  18   a  at an open side thereof. As shown, each bolt hole  18  is formed in a boss portion whose outer surface is exposed to the water jacket  15 . The length of the counter bore part  18   a  of each bolt hole  18  is equal to or greater than the thickness of a wall of the water jacket  15 , that is, the distance from the flat contact surface  12   a  to the water jacket  15 . In the illustrated embodiment, the length of the counter bore part  18   a  is equal to the thickness of the wall of the water jacket  15 . This means that the threaded part of each bolt hole  18  is entirely surrounded or enclosed by the water jacket  15 . As will become apparent as the description proceeds, this entire enclosure by the water jacket  15  brings about an assured cooling of the boss portions for the bolt holes  18 . Due to provision of the counter bore part  18   a , the actually engaged portion of the bolt with the threaded part of each bolt hole  18  is positioned closer to the water jacket  15  and thus effectively cooled by the cooling water in the water jacket  15 . Thus, undesired looseness of the bolt is suppressed. 
     As is seen from FIGS. 1 to  3 , the front housing member  12  is provided at a lower portion thereof with an inlet pipe  19  and at a side portion thereof with an outlet pipe  20 , these pipes  19  and  20  being connected to the water jacket  15  in the cooling housing  6 . Although not shown in the drawings, water pipes are connected to the inlet and outlet pipes  19  and  20 , so that part of engine cooling water driven by a water pump (not shown) is forced to flow in the water jacket  15 . 
     As is understood from FIGS. 1,  2  and  4 , the cooling housing  6  is provided with three through bolt holes  22  each extending through both the front and rear housing members  12  and  11 . Each bolt hole  22  has a front end exposed to the major flat portion  12   b  of the front housing member  12 . 
     As is understood from FIG. 4, threaded bolts  21  pass through respective through bolt holes  22  and engage with respective metal nuts  23  embedded in the mounting seat  4  of the intake manifold  1 . With this, the cooling housing  6  is tightly secured to the mounting seat  4  of the intake manifold  1 . Each bolt  21  has an enlarged head  21   a  seated on the major flat portion  12   b  of the front side of the front housing member  12 . As shown, each nut  23  has a trapezoidal cross section to increase an area which intimately contacts with the rear housing member  11 . If desired, stud bolts extending from the mounting seat  4  may be used in place of the above-mentioned threaded bolts  21 . That is, in this case, each stud bolt passes through the bolt hole  22  and engages with a nut placed on the major flat portion  12   b.    
     Between the mounting seat  4  and the rear housing member  11 , there is disposed a seal ring  24  which is held in an annular groove  25  formed in the mounting seat  4 . The mounting seat  4  has around the groove  25  a heat insulation groove  26 . That is, the heat insulation groove  26  effects a heat insulation between the cooling housing  6  and the intake manifold  1 . 
     As is seen from FIGS. 2 and 5, the rear housing member  11  of the cooling housing  6  is integrally formed at a lower part thereof with a bracket portion  28  which has a pair of threaded bolt holes  27 . 
     As is understood from FIGS. 3 and 5, the rear side of the rear housing member  11  has three depressions  29  which receive heads of the above-mentioned bolts  13  by which the rear and front housing members  11  and  12  are united. 
     As is seen from FIGS. 1 and 5, the rear housing member  11  is formed with an air discharging threaded hole  30  which is communicated with the water jacket  15 . The air discharging hole  30  is closed by an air discharging plug  31  (see FIG. 2) detachably engaged therewith. 
     As is seen from FIG. 5, the rear housing member  11  is formed near the air discharging hole  30  with a sensor mounting bore  32  which is exposed to the exhaust gas passage  14 . Although not shown in the drawing, a temperature sensor is received in the bore  32  for sensing the temperature of EGR gas flowing in the exhaust gas passage  14 . 
     Under operation of the associated engine, part of exhaust gas in the exhaust manifold is led into the plastic intake manifold  1  through the above-mentioned EGR system for reducing NOx emissions. Due to operation of the EGR valve  7 , the amount of EGR gas led into the intake manifold is adjusted. 
     It is now to be noted that during operation of the EGR system, part of cooling water driven by the water pump of the engine is forced to flow in the water jacket  15  in the cooling housing  6 . 
     In the following, advantages possessed by the EGR system of the first embodiment will be described. 
     First, the cooling housing  6  is effectively cooled by the cooling water. Thus, the amount of heat transmitted from the highly heated EGR valve  7  to the plastic intake manifold  1  is greatly reduced. 
     Second, due to provision of the pipe portion  17  (see FIG. 4) through which exhaust gas is led into the interior of the plastic intake manifold, it does not occur that the highly heated exhaust gas directly blows on the wall of the cylindrical hole  5  of the of the intake manifold  1 . 
     Third, since the threaded part of each bolt hole  18  (see FIG. 4) is entirely enclosed by the water jacket  15 , the threaded part is effectively cooled. Thus, undesired thermal deformation of the threaded part is suppressed, and thus undesired looseness of the corresponding bolt by which the EGR valve  7  is secured to the cooling housing  6  is suppressed or at least minimized. 
     Fourth, as is seen from FIG. 4, due to provision of a gap between the flat contact surface  12   a  and the major flat portion  12   b  of the front housing member  12 , the heat transferring pass from the EGR valve  7  to the bolts  21  is substantially increased. Accordingly, the heat transmission to the plastic intake manifold  1  through the bolts  21  is minimized. Cooling effect applied to the bolts  21  from cooling water in the water jacket  15  promotes the minimization of heat transmission to the plastic intake manifold  1 . 
     Fifth, due to provision of the seal member  34  interposed between the front and rear housing members  12  and  11 , heat transmission through the cooling housing  6  is obstructed by a certain degree. The split construction of the cooling housing  6  simplifies formation of the water jacket  15 . 
     Referring to FIGS. 6 to  10 , particularly FIG. 6, there is shown an EGR system which is a second embodiment of the present invention. 
     In FIG. 6, there is shown a plastic intake manifold  1  designed for an in-line 6 cylinder internal combustion engine (not shown), to which the second embodiment is practically applied. Like in the above-mentioned first embodiment, the intake manifold  1  is molded from a fiber-reinforced plastic material such as those described in the section of the first embodiment. 
     Similar to the case of the first embodiment of FIG. 1, the plastic intake manifold  1  to which the second embodiment is applied comprises generally an elongated collector portion  2  which extends along the row of the engine cylinders, six branches  2   a  which extend from one side of the collector portion  2  to respective intake ports of the cylinder head, an inlet portion  2   b  which defines an upstream part of the collector portion  2  and an inlet flange  3  which is integrally formed on the inlet portion  2   b  to mount thereto a throttle chamber “TC”. Denoted by numeral  2   c  is a circular inlet opening defined in the inlet flange  3 , which thus connects the interior of the inlet portion  2   b  and the throttle chamber “TC”. The branches  2   a  have at their leading ends an integral mounting flange  2   d  which is bolted to the cylinder head. 
     The inlet portion  2   b  has therein a passage  2   e  whose sectional area is substantially the same throughout the length thereof. The sectional form of the passage  2   e  gradually changes from a circle to a flat rectangular as a position moves from the inlet flange  3  to the collector portion  2 . 
     As is seen from FIG. 6, the inlet portion  2   b  of the intake manifold  1  is integrally formed with a mounting seat  4  which is slightly raised. The mounting seat  4  has at a center thereof a cylindrical hole  5  connected with the interior of the inlet portion  2   b . The cylindrical hole  5  extends in a direction perpendicular to a direction in which intake air in the inlet portion  2   b  flows. Into the cylindrical hole  5 , there is inserted a leading end  8   a  of an EGR pipe  8 . The other end of the EGR pipe  8  is connected to an exhaust manifold (not shown) of the engine, so that part of exhaust gas in the exhaust manifold is led into the inlet portion  2   b  through the EGR pipe  8 . Although not shown in the drawing, the EGR pipe  8  has an EGR valve operatively connected thereto. 
     FIG. 7 shows in detail a mounting structure through which the leading end  8   a  of the EGR pipe  8  is tightly supported in the cylindrical hole  5  of the intake manifold  1 . As shown in the drawing, within the cylindrical hole  5 , there is disposed a collar member  50  of metal which surrounds the leading end portion of the EGR pipe  8  to define therebetween a certain annular clearance  52 . The outer diameter of the collar member  50  is slightly smaller than the diameter of the cylindrical hole  5  thereby to define therebetween an annular clearance  54 . The collar member  50  has a diametrically reduced front end  50   a  intimately disposed on and welded to the leading end  8   a  of the EGR pipe  8 . Designated by numeral  50   b  is a stepped portion through which the reduced front end  50   a  is connected to a major portion of the collar member  50 . As shown, the leading end  8   a  of the EGR pipe  8  and that of the reduced front end  50   a  are flush with each other. The collar member  50  has at a rear end thereof a radially outwardly extending flange  50   c  which is welded to a mounting plate  56 . The mounting plate  56  is formed with a circular opening  56   a  through which the EGR pipe  8  passes. As shown, the diameter of the circular opening  56   a  is larger than that of the EGR pipe  8  thereby to define therebetween an annular gap. Due to provision of this annular gap, the annular clearance  52  defined between the EGR pipe  8  and the collar member  54  is communicated with the open air. 
     As is seen from FIG. 6, the mounting plate  56  is secured to the mounting seat  4  of the intake manifold  1  by means of two threaded bolts  58   a  and  58   b.    
     Referring back to FIG. 7, the flange  50   c  of the collar member  50  is thus intimately put between the mounting seat  4  and the mounting plate  56 . A seal ring  58  is disposed between the mounting seat  4  and the mounting plate  56  to isolate the annular gap  54 . 
     As is seen from FIGS. 6 and 7, upon assembly, the leading end  8   a  of the EGR pipe  8  is slightly projected into the interior of the inlet portion  2   b  beyond an inner wall  2   f  of the inlet portion  2   b.    
     As is seen from FIG. 7, the cylindrical hole  5  has a chamfered inner end  5   a  which surrounds the reduced front end  50   a  of the collar member  50 . Thus, an annular groove  60  is formed around the reduced front end  50   a  of the collar member  50 , which has a generally trapezoidal cross section, as shown. That is, in the illustrated example, the annular groove  60  is substantially defined by the chamfered inner end  5   a , the stepped portion  50   b  of the collar member  50  and the reduced front end  50   a  of the same. However, if desired, the annular groove  60  may take various shapes other than the above-mentioned one, which are, for example, a shape having a semi-circular cross section, a shape having a rectangular cross section, a shape having a zigzag cross section, etc.,. 
     FIG. 8 shows a positional relation between a throttle valve  62  in the throttle chamber “TC” and the cylindrical hole  5  of the mounting seat  4 . As is understood from this drawing, the throttle valve  62  is of a butterfly valve type which comprises two wings  62   a  and  62   b  and a pivot shaft  62   c  about which the wings  62   a  and  62   b  pivot. In the illustrated example, the two wings  62   a  and  62   b  are arranged to pivot clockwise by a certain angle from the illustrated position upon need of opening the valve  62 . That is, upon this need, the wing  62   a  moves upstream and the other wing  62   b  moves downstream. It is to be noted that assuming that the wings  62   a  and  62   b  are arranged in the above-mentioned manner, the cylindrical hole  5  is positioned at a position downstream of the wing  62   a . In other words, the cylindrical hole  5  is positioned downstream of one of the wings  62   a  and  62   b  which moves upstream during opening operation of the valve  62 . This is because such positioning provides the cylindrical hole  5  with a greater suction effect. In fact, as is seen from FIG. 11, since the vortexes produced behind the upwardly moving wing  62   a  are less than those produced behind the downwardly moving wing  62   b , larger air flow is obtained in the downstream position of the wing  62   a.    
     In the following, advantages possessed by the EGR system of the second embodiment will be described. 
     First, due to provision of the collar member  50  in the cylindrical hole  5  of the intake manifold  1 , the inner wall of the cylindrical hole  5  is effectively protected from the heat radiated from the EGR pipe  8 . That is, due to presence of the collar member  50 , two annular clearances  52  and  54  are defined between the inner wall of the cylindrical hole  5  and the EGR pipe  8 , the clearances  52  and  54  serving as excellent heat insulating means. Thus, undesired thermal deformation of the inner wall of the cylindrical hole  5  is suppressed or at least minimized. 
     Second, due to provision of the annular groove  60  (see FIG.  7 ), the chamfered inner end  5   a  of the cylindrical hole  5  is effectively protected from the heat radiated from the reduced front end  50   a  of the collar member  50 . In fact, the reduced front end  50   a  is heated very high because it is welded to the EGR pipe  8 . Provision of the chamfered inner end  5   a  can avoid formation of a sharpen edge of the cylindrical hole  5  where heat is collected. As is understood from FIG. 7, under flowing of air along the inner wall  2   f  in the direction of the arrow “A”, turbulent flows are produced near the annular groove  60  as is indicated by arrows “tf”, which can absorb heat from the wall of the groove  60  and the reduced front end  50   a  of the collar member  50 . 
     Third, since the leading end  8   a  of the EGR pipe  8  is projected into the interior of the intake manifold  1 , EGR gas discharged from the end  8   a  instantly and easily mixes with intake air flowing in the intake manifold  1 . The highly heated exhaust gas is suppressed from directly blowing on the inner wall  2   f  of the plastic intake manifold  1 . If, as is described hereinabove, the cylindrical hole  5  is positioned downstream of the wing  62   a  which moves upstream during opening operation of the throttle valve  62 , larger intake air flow is obtained in the area where the leading end  8   a  of the EGR pipe  8  is exposed. This promotes not only the cooling effect applied to the wall of the groove  60  by the turbulent flows “tf” but also the mixing of EGR gas and intake air in the intake manifold  1 . 
     In order to recognize the cooling effect of the above-mentioned annular groove  60 , an experiment has been carried out by the inventor. FIG. 9 shows a method of the experiment, and FIG. 10 shows the result of the experiment. 
     As shown in FIG. 9, in the experiment, a simple test device was provided, which comprises a plastic intake manifold  101  having a cylindrical hole  105  formed therethrough, and an EGR gas feeder  106  having a pipe portion  108  spacedly received in the cylindrical hole  105 . Like in the second embodiment, the leading end  108   a  of the pipe portion  108  is slightly projected into the interior of the plastic intake manifold  101 . As shown, the inner end of the cylindrical hole  105  is formed at diametrically opposed portions with a tapered part “a” and a non-tapered part “b” respectively. Denoted by reference “c” is a part near an outer end of the cylindrical hole  105 . The distance between the parts “b” and “c” was about 22 mm. For the experiment, intake air was forced to flow in the intake manifold  101  and EGR gas was led into the intake manifold  101  from the pipe portion  108 , and the temperature of the three parts “a”, “b” and “c” was measured. 
     The result of the experiment is shown in the graph of FIG.  10 . As is understood from this graph, the temperature (100° C.) of the part “a” was very low as compared with that (125° C.) of the part “b”. This proves the cooling effect possessed by the annular groove  60 .