Patent Publication Number: US-2018030876-A1

Title: Egr device for internal combustion engine

Description:
TECHNICAL FIELD 
     The present invention relates to an EGR (exhaust gas recirculation) device for an internal combustion engine equipped with a supercharger such as a turbocharger. 
     BACKGROUND ART 
     In a known EGR device for an internal combustion engine, an EGR valve is attached to a cylinder row end of a cylinder head, and internally defines a part of an exhaust gas passage that extends from an exhaust system. The upstream end of the exhaust gas passage is connected to a converging part of an exhaust manifold via an exhaust gas introduction pipe, and the downstream end of the exhaust gas passage is connected to an intake passage of an intake manifold via an exhaust gas injection pipe. See JP2000-87807A, for instance. 
     An engine is often equipped with a supercharger such as a turbocharger for the purpose of improving fuel economy. A turbocharger supplies compressed air into the combustion chambers of the engine so that the volumetric efficiency is improved and a high engine input can be obtained for the given engine displacement of the engine. In an engine equipped with a turbocharger, both a high-pressure EGR device that recirculates the high-pressure exhaust gas in an upstream part of the exhaust system to the intake air under high pressure, and a low-pressure EGR device that recirculates the low-pressure exhaust gas in a downstream part of the exhaust system to the intake air under negative pressure are employed at the same time. As a result, the structure of the intake system and the exhaust system tends to be highly complex owing to the presence of these EGR devices. To allow a compact design of the intake system and the exhaust system, it is a common practice to place at least one of the EGR valves adjacent to the turbocharger. 
     When an EGR valve is positioned adjacent to a turbocharger, because the associated piping and other components bunch up around the turbocharger, it becomes difficult to remove the EGR valve for maintenance purpose. Oftentimes, it is necessary to remove the turbocharger itself to remove or replace the EGR valve. 
     The present invention was made in view of such problems of the prior art, and has a primary object to provide an EGR device that allows an EGR valve to be removed with ease even when the EGR valve is provided adjacent to a supercharger. 
     To achieve such an object, the present invention provides a An EGR device for a multiple-cylinder engine equipped with a supercharger ( 41 ) for returning a part of exhaust gas expelled from an exhaust system of the engine to an intake system of the engine, comprising: a first connecting pipe portion ( 41   b ) extending from a compressor of the supercharger and having a first annular shoulder surface ( 41   c ) at a base end thereof; an EGR valve ( 65 ) fixedly attached to a part of the engine; a joint member ( 71 ) including a flange ( 71   a ) attached to the EGR valve ( 65 ) and a second connecting pipe portion ( 71   b ) defining a passage communicating with an inlet end of the EGR valve, and opposing the first connecting pipe portion in a coaxial relationship, the second connecting pipe portion being provided with a second annular shoulder surface ( 71   c ) at a base end thereof; and a flexible pipe member ( 72 ) having a first end fitted onto the first connecting pipe portion and a second end fitted onto the second connecting pipe portion; wherein a distance (L 2 ) between the two annular shoulder surfaces is greater than a length (L 1 ) of the flexible pipe member by a prescribed distance. 
     According to this arrangement, by moving the joint member (optionally along with the flexible pipe member) in the axial direction away from the EGR valve, the EGR valve can be removed without requiring any major components such as the supercharger to be removed. The axial movement of the joint member is permitted until the two ends of the flexible pipe member abut the respective annular shoulder surfaces. 
     In a preferred embodiment of the present invention, the supercharger consists of a turbocharger including a turbine for powering the compressor and attached to a part of the engine such that the compressor projects from an exhaust side part of the engine beyond a cylinder row end part of the engine, and the first connecting pipe portion ( 41   b ), the EGR valve ( 65 ), the flexible pipe member ( 72 ) and the second connecting pipe portion ( 71   b ) extend from the compressor along the cylinder row end part of the engine. 
     Thereby, the EGR device can be installed in an area adjoining the main body of the engine in a highly compact manner. 
     The EGR device may further comprises an upstream EGR pipe assembly ( 62 - 64 ) communicating a part of the exhaust system to the inlet end of the EGR valve, the upstream EGR pipe assembly ( 62 - 64 ) including a part generally extending vertically along the cylinder row end part of the engine toward a downstream part of the exhaust system of the engine. 
     This also contributes to the compact arrangement of the EGR device. 
     Preferably, an exhaust gas purification device is positioned on an exhaust side of the engine under the turbine, and an upstream end of the upstream EGR pipe assembly is connected to a downstream part of the exhaust gas purification device. 
     Thereby, the low-pressure exhaust gas having a relatively low temperature is returned to the intake system of the engine, and is mixed with the intake air under negative pressure. The exhaust gas contains moisture of a certain acidity, but the acidity of the exhaust gas is reduced by the catalytic converter before being returned to the intake system. Therefore, the thermal degradation of the flexible pipe member can be minimized. 
     According to a preferred embodiment of the present invention, the upstream EGR pipe assembly extends generally upward from the upstream end thereof between the engine and the exhaust gas purification device, and along the cylinder row end part of the engine toward the intake side of the engine before doubling back toward the exhaust side of the engine and being connected to the inlet end of the EGR valve. 
     This also contributes to the compact arrangement of the EGR device. 
     The upstream EGR pipe assembly may comprise an EGR cooler ( 63 ). 
     Thereby, the temperature of the exhaust gas conducted to the flexible pipe member can be lowered by the EGR cooler so that the thermal degradation of the flexible pipe member can be minimized. 
     The upstream EGR pipe assembly may include a rigid pipe member ( 64 ) connected between the EGR cooler and the EGR valve. 
     The rigid pipe member may be fixedly attached to a suitable part of the engine so that the EGR valve (on the downstream end of the rigid pipe member) and the EGR cooler (on the upstream end of the rigid pipe member) can be fixedly secured to the engine in a highly stable manner without requiring additional brackets. 
     In a particularly preferred embodiment of the present invention, a plurality of threaded bolts ( 81 ,  82 ) are passed through a flange ( 64   b ) provided on a downstream end of the rigid pipe member of the upstream EGR pipe assembly, the EGR valve ( 65 ) and the joint member ( 71 ) to fasten these components to one another. 
     Thereby, the EGR valve and the joint member can be fixedly secured to the rigid pipe member of the upstream EGR pipe assembly in a both simple and stable manner. 
     Preferably, the threaded bolts include at least a pair of stud bolts each having a base end threaded into one of the flange ( 64   b ) of the rigid pipe member and the flange ( 71   a ) of the joint member, and a free end having a threaded portion having a nut ( 83 ) fastened thereon. 
     Thereby, the rigid pipe member, the EGR valve and the joint member can be assembled together in a proper alignment with one another with the aid of the stud bolts so that the assembly work can be facilitated. Typically, a gasket is required to be placed between each interface, but this arrangement allows the assembly process to be carried out in a highly efficient manner. 
     Typically, the free end of each stud bolt is provided with a tool engagement feature ( 81   a ). Thereby, each stud bolt can be installed and removed with ease by engaging a suitable tool with the tool engagement feature. 
     Alternatively or additionally, the threaded portion on the free end of each stud bolt may be provided with a length at least twice as long as a thickness of the nut. Thereby, by threading an additional nut ( 84 ) until the additional nut abuts the original nut (a double nut arrangement), the stud bolt can be unscrewed from the flange of the rigid pipe member or the flange of the joint member (as the case may be) by engaging the original nut with a suitable tool. 
     In a preferred embodiment of the present invention, a first gasket ( 68 ) is interposed between the flange of the rigid pipe member of the upstream EGR pipe assembly and the EGR valve, and a second gasket ( 69 ) is interposed between the EGR valve and the joint member, and wherein the distance (L 2 ) between the two annular shoulder surfaces is greater than the length of the flexible pipe member (L 1 ) at least by a combined thickness of the two gaskets in an unused state. 
     According to this arrangement, adequate spaces can be created between the flange of the rigid pipe member of the upstream EGR pipe assembly and the EGR valve, and between the EGR valve and the joint member so that the replacement of the EGR valve can be accomplished without causing any difficulty. 
     Each end of the flexible pipe member may be secured onto the corresponding connecting pipe portion with a hose band. 
     Thereby, the flexible pipe member can be installed in a both simple and economical manner. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         FIG. 1  is a plan view of an engine of a motor vehicle equipped with an EGR device embodying the present invention; 
         FIG. 2  is a block diagram of an intake system and an exhaust system of the engine; 
         FIG. 3  is a fragmentary perspective view of a low-pressure EGR device shown in  FIG. 2 ; 
         FIG. 4  is a fragmentary sectional view of a part of the low-pressure EGR device; 
         FIG. 5  is an exploded perspective view of a part of the low-pressure EGR device; and 
         FIG. 6  is a view similar to  FIG. 4  when the low-pressure EGR device is being disassembled. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     An embodiment of the present invention is described in the following with reference to the appended drawings. An engine  4  consisting of an in-line four-cylinder diesel engine is positioned in an engine room  3  formed in a front part of a vehicle body  2  of a motor vehicle  1 . 
     The engine  4  is laterally mounted in the engine room  3 , and slightly offset in the rightward direction. The engine  4  is supported by the vehicle body  2  via an engine mount (not shown in the drawings) with a slight rearward slant. A transmission system is connected to a lower side of a left end part of the engine  4 . A pair of front side frames  6  (only one of them is shown in  FIG. 1 ) extend along either side of the engine room  3 , and a pair of damper bases  7  are positioned on either side of a rear end part of the engine room  3 . 
     A rectangular battery  8  is positioned on an inboard side of the left damper base  7  with the long side of the battery  8  extending in the fore and aft direction. An ECU unit  9  for controlling various parts of the vehicle  1  is provided immediately ahead of the battery  8 , and an air cleaner  10  is positioned immediately ahead of the ECU unit  9 . A relay box  11  which is elongate in the fore and aft direction is positioned on the outboard side of the battery  8 . 
     A front bulkhead (not shown in the drawings) supporting a radiator (not shown in the drawings) is provided in a front end part of the engine room  3 . A cover member  12  is positioned on top of the front bulkhead. The engine  4  is provided with an intake/exhaust system  18  consisting of an intake system  20  for supplying air to the engine  4  and an exhaust system  30  for expelling exhaust gas from the engine  4 . The exhaust system  30  is provided with a turbocharger  40  which is powered by the flow of the exhaust gas and compresses the intake air supplied to the engine  4 . A high-pressure EGR device  50  is provided in a downstream part of the turbocharger  40 , and a low-pressure EGR device  60  is provided in an upstream part of the turbocharger  40 , each for returning a controlled amount of the exhaust gas to the intake system  20 . 
       FIG. 2  is a block diagram illustrating the overall structure of the intake/exhaust system  18 . In the intake system  20 , air drawn from the atmosphere is introduced into a first intake duct  21  via an intake inlet  21   a , and is then conducted to a throttle valve  23  via an air cleaner  10  and a second intake duct  22 . Then, after being compressed by a compressor  41  of the turbocharger  40 , the intake air is forwarded to an intercooler  25  via a third intake duct  24 , and to an intake manifold  29  via a fourth intake duct  26 , an intake shutter valve  27  and a fifth intake duct  28 . 
     In the exhaust system  30 , the exhaust gas collected from the engine  4  by an exhaust manifold  31  is forwarded to a turbine  42  of the turbocharger  40 , and is expelled to the atmosphere via a first exhaust pipe  32 , a catalytic converter  33 , a DPF  34 , and a second exhaust pipe  35 . The high-pressure EGR device  50  includes a first high-pressure EGR pipe  51  directly connected to the exhaust manifold  31 , a high-pressure EGR valve  52 , and a second high-pressure EGR pipe  53  connected to the downstream side of the intake shutter valve  27 , in this order from the side of the exhaust system  30 . The low-pressure EGR device  60  includes an EGR filter device  61  connected to the DPF  34 , a first low-pressure EGR pipe  62 , a low-pressure EGR cooler  63 , a second low-pressure EGR pipe  64 , a low-pressure EGR valve  65 , and a third low-pressure EGR pipe  66  connected to the downstream side of the throttle valve  23 , in this order from the side of the exhaust system  30 . 
     The throttle valve  23  controls the intake air amount and the intake pressure of the intake air supplied into the cylinders of the engine  4 . The intake shutter valve  27  is configured to selectively reduce the intake air amount by narrowing the intake passage at the valve body to raise the temperature of the exhaust gas when the DPF is required to be regenerated by burning off the particulate matter (PM) collected by the DPF  34 , but is otherwise kept fully open. 
     The first low-pressure EGR pipe  62 , the low-pressure EGR cooler  63 , and the second low-pressure EGR pipe  64  on the upstream side of the low-pressure EGR valve  65  may be collectively referred to as an upstream EGR pipe assembly. In the illustrated embodiment, the second low-pressure EGR pipe  64  forms a part of the upstream EGR pipe assembly. 
     Referring to  FIG. 1  once again, exhaust ports of the engine  4  are provided on the front side of the engine  4 . An exhaust converging pipe  36  is attached to a planar mounting surface defined on the front side of the engine  4 , and communicates with the exhaust manifold  31  which, in the illustrated embodiment, is internally defined in the cylinder head of the engine  4 . The exhaust manifold may also be provided separately from the engine and attached to the front side of the engine  4 . The outlet end of the exhaust converging pipe  36  is positioned on the left end side of the engine  4 , and is fitted with the turbine  42  of the turbocharger  40 . 
     The turbine  42  is positioned on the front side of the exhaust converging pipe  36 , and is provided with a turbine housing and a turbine wheel rotatably supported by the turbine housing around a rotational center line extending in the lateral direction of the vehicle body. The turbine housing defines a turbine inlet extending circumferentially along a tangential direction of the turbine housing, and a turbine inlet extending in the axial direction from a central part of the turbine housing in the rightward direction. The turbine inlet is connected to the outlet end of the exhaust converging pipe  36 , and the turbine outlet is connected to the first exhaust pipe  32  connected to the right side wall of the turbine housing. 
     The first exhaust pipe  32  curves downward as it extends rightward, and is connected to an upper part of the catalytic converter  33  provided under the turbine  42  in a forwardly spaced apart relationship to the exhaust side (the front side) of the engine  4 . The catalytic converter  33  removes HC, CO and NOx from the exhaust gas. The DPF  34  ( FIG. 2 ) for trapping particulates from the exhaust gas is provided under the catalytic converter  33 . The second exhaust pipe  35  ( FIG. 2 ) is connected to the lower side of the DPF  34 , and extends under the engine  4  in the rearward direction. The second exhaust pipe  35  further extends under the floor to a rear end part of the vehicle. 
     The air cleaner  10  is connected to the downstream end of the first intake duct  21  ( FIG. 2 ) so that the air drawn into the engine room  3  via a front grill is forwarded to the air cleaner  10  via the intake inlet  21   a . An intake outlet is formed on the right side of the air cleaner  10 , and is connected to the upstream end of the second intake duct  22 . The downstream end of the second intake duct  22  is connected to the left end of the throttle valve  23  which internally defines a laterally extending intake passage. 
     The compressor  41  of the turbocharger  40  is positioned between the throttle valve  23  and the turbine  42  so as to be coaxial with the turbine  42  and projects beyond the left end of the engine  4 . The compressor  41  includes a compressor housing  41   a  ( FIG. 4 ) and a compressor wheel rotatably supported by the compressor housing  41   a  around a laterally extending rotational center line. The compressor housing  41   a  is provided with a compressor inlet provided centrally on the left side wall of the compressor housing  41   a , and a low-pressure EGR introduction port opens out and projects from a rear side of the peripheral wall of the compressor housing  41   a . The compressor housing  41   a  is further provided with a compressor outlet extending tangentially from a lower part of the outer peripheral wall of the compressor housing  41   a . The compressor inlet is connected to the intake passage of the throttle valve  23 , and the low-pressure EGR introduction port is connected to the EGR gas passage of the low-pressure EGR device  60 . The compressor outlet is connected to the intake passage of the third intake duct  24  connected to the lower wall of the compressor housing  41   a.    
     The turbocharger  40  is provided with a drive shaft  40   a  ( FIG. 2 ) connecting the turbine wheel on the side of the exhaust system  30  to the compressor wheel on the side of the intake system  20 . The rotational power of the turbine wheel is transmitted to the compressor wheel via the drive shaft  40   a . Thereby, the turbocharger  40  compresses the intake air supplied from the throttle valve  23  and the exhaust gas supplied from the low-pressure EGR device  60 , and forwards the mixture at a pressure higher than the atmospheric pressure to the engine  4 . 
     The third intake duct  24  connected to the lower part of the outer periphery of the compressor  41  is connected to the intercooler  25  ( FIG. 2 ) positioned under the cover member  12 . The fourth intake duct  26  ( FIG. 2 ) is passed behind the engine  4  via the intercooler  25 , and is connected to the intake manifold  29  ( FIG. 2 ) attached to the rear side of the engine  4 . 
     The first high-pressure EGR pipe  51  of the high-pressure EGR device  50  is connected to the right end of the exhaust converging pipe  36  and receives the exhaust gas from the exhaust converging pipe  36 . The high-pressure EGR valve  52  is attached to the front side of the cylinder head, and the rear end of the first high-pressure EGR pipe  51  is attached to the front end of the high-pressure EGR valve  52 . In the cylinder head of the engine  4 , an upstream side passage portion of the second high-pressure EGR pipe  53  ( FIG. 2 ) communicating with the EGR passage of the high-pressure EGR valve  52  is formed so as to extend in the fore and aft direction. The downstream side passage portion of the second high-pressure EGR pipe  53  connected to the rear surface of the engine  4  is connected to the intake introduction portion of the intake manifold  29  and causes the exhaust gas having passed through the high-pressure EGR valve  52  to return to the intake system  20 . The downstream side passage portion of the second high-pressure EGR pipe  53  may be connected to a portion other than the intake manifold  29  as long as it is on the downstream side of the compressor  41  in the intake system  20  and on the upstream side of the intake introduction portion. 
     As shown in  FIG. 3 , the first low-pressure EGR pipe  62  that extends vertically includes a first upstream flange  62   a  disposed at the lower end thereof and fastened to the lower end of the DPF  34  with threaded bolts, and a first downstream flange  62   b  disposed at the upper end thereof. The first low-pressure EGR pipe  62  extends rightward from the first upstream flange  62   a , and then curves upward to extend along the right side of the DPF  34  before extending obliquely upward and rearward. The first upstream flange  62   a  generally faces to the left, and the first downstream flange  62   b  faces obliquely upward and rearward. The lower portion of the vertical section of the first low-pressure EGR pipe  62  is formed as a flexible pipe or a bellows  62   c . As a result, the first upstream flange  62   a  and the first downstream flange  62   b  can be displaced relative to each other, and stress concentration in the first low-pressure EGR pipe  62  due to thermal expansion of the exhaust system  30  can be avoided. 
     The EGR filter device  61  consists of a metallic mesh interposed between the first upstream flange  62   a  of the first low-pressure EGR pipe  62  and a connecting flange (not shown in the drawings) of the DPF  34 . The EGR filter device  61  captures fragments of the DPF  34  and other metallic pieces that might be introduced into the first low-pressure EGR pipe  62 . 
     The low-pressure EGR cooler  63  is provided with a rectangular cooler main body portion  63   a  having a laterally elongated cross section and a smaller fore and aft dimension than a vertical dimension. A right end part of the cooler main body portion  63   a  is integrally provided with an upstream side connecting pipe portion  63   b  extending obliquely downward and forward, and a left end part of the cooler main body portion  63   a  is integrally provided with a downstream side connecting pipe portion  63   c  extending to the front. Connecting flanges are integrally formed at the free ends of the upstream side connecting pipe portion  63   b  and the downstream side connecting pipe portion  63   c , respectively. The cooler main body portion  63   a  is provided with a plurality of mounting pieces  63   d  for attachment to the engine  4 . The cooler main body portion  63   a  is positioned between the engine  4  and the catalytic converter  33  which is spaced from the front side of the engine  4 , and is attached to the front side of the engine  4  via the mounting pieces  63   d  and associated threaded bolts. The cooler main body portion  63   a  is configured to circulate the cooling water therein, and cool the exhaust gas by heat exchange between the cooling water flowing through the cooler main body portion  63   a  and the exhaust gas. 
     The second low-pressure EGR pipe  64  is made of a rigid pipe member, and includes a second upstream flange  64   a  positioned at the lower end thereof and fastened to the downstream side connecting pipe portion  63   c  of the low-pressure EGR cooler  63  by threaded bolts, and a second downstream flange  64   b  positioned at the upper end thereof. The second low-pressure EGR pipe  64  extends from the second upstream flange  64   a  forward and then toward the left end side of the engine  4  in an upward and leftward direction along a curved path. The second low-pressure EGR pipe  64  is bent again along the left end side of the engine  4  in the rearward direction (toward the intake side). The second low-pressure EGR pipe  64  is then bent upward and forward (toward the exhaust side) making a U turn. In other words, the downstream part of the second low-pressure EGR pipe  64  extends rearward along the cylinder row end part of the engine toward the intake side of the engine, and then doubles back toward the exhaust side of the engine. The second low-pressure EGR pipe  64  is made of metal, and is therefore highly rigid (as opposed to a flexible tube). A connecting piece  64   d  for attachment to the engine  4  is provided at an appropriate position of the second low-pressure EGR pipe  64 . The second low-pressure EGR pipe  64  is fixed to the engine  4  via a pipe stay  64   e  (which is attached to a left end surface of the engine  4 ) by fastening the connecting piece  64   d  to the pipe stay  64   e  with a threaded bolt. 
       FIG. 4  is a longitudinal sectional view of the third low-pressure EGR pipe  66  and the EGR valve  65  which form an essential part of the low-pressure EGR device  60 , and  FIG. 5  is an exploded perspective view of the third low-pressure EGR pipe  66  and associated parts. As shown in  FIGS. 3 and 4 , a compressor connecting pipe portion  41   b  extends rearward from the rear surface of the rear wall portion of the compressor housing  41   a  of the compressor  41 . The base end of the compressor connecting pipe portion  41   b  is formed with an annular shoulder surface  41   c  facing rearward. The annular shoulder surface  41   c  protrudes rearward from the surrounding surface of the rear wall portion of the compressor housing  41   a  by a prescribed distance in the illustrated embodiment, but may also be defined by a part of the surface of the rear wall portion itself surrounding the compressor connecting pipe portion  41   b.    
     The third low-pressure EGR pipe  66  includes a joint member  71  positioned on the downstream side of the low-pressure EGR valve  65 , a flexible pipe member  72 , and a pair of hose bands  73  provided on either axial end of the flexible pipe member  72 . As shown in  FIGS. 4 and 5 , the joint member  71  includes a joint flange portion  71   a  and a joint connecting pipe portion  71   b  extending forward toward the free end of the compressor connecting pipe portion  41   b . The joint connecting pipe portion  71   b  is provided with an annular shoulder surface  71   c  facing forward so as to face the annular shoulder surface  41   c  in a coaxial relationship. The annular shoulder surface  71   c  protrudes forward from the forwardly facing surface of the joint flange portion  71   a  by a certain distance in the illustrated embodiment, but may also be defined by a part of the forwardly facing surface of the joint flange portion  71   a  surrounding the joint connecting pipe portion  71   b . The joint flange portion  71   a  is provided with three bolt holes  70 , one centrally in an upper part and two on either lower side part of the joint flange portion  71   a.    
     The low-pressure EGR valve  65  includes a valve housing  65   a  defining a low-pressure EGR passage extending in the front and aft direction, and a disk-shaped butterfly valve  65   d  rotatably supported in the valve housing  65   a  for opening and closing the low-pressure EGR passage defined in the valve housing  65   a . The two axial ends of the valve housing  65   a  define mutually parallel mating surfaces facing in the fore and aft direction. Three bolt holes  65   b  extending in the axial direction are passed through respective thick-walled portions  65   c  formed on the outer peripheral parts of the valve housing  65   a  at a regular angular interval in such a manner that the three bolt holes  65   b  align with the respective bolt holes  70  of the joint flange portion  71   a.    
     The second downstream flange  64   b  of the second low-pressure EGR pipe  64  opposes the compressor connecting pipe portion  41   b  in a coaxial relationship from the rear and at a certain distance. The second downstream flange  64   b  is provided with three bolt holes  64   c , one centrally in an upper part and two on either lower side part of the second downstream flange  64   b . One of the axial end surfaces (the rear axial end surface) of the valve housing  65   a  is joined to the second downstream flange  64   b  of the second low-pressure EGR pipe  64  via a first gasket  68 , and the other axial end surface (the front axial end surface) of the valve housing  65   a  is joined to the joint flange portion  71   a  of the joint member  71 . 
     The bolt holes  70  of the joint flange portion  71   a  consist of female threaded holes passed through the joint flange portion  71   a . A threaded bolt  82  having a threaded part at one end and a hexagonal head at the other end is passed through one of the bolt holes  64   c  (the lower right bolt hole  64   c ) of the second downstream flange  64   b  and the corresponding bolt hole  65   b  formed in the thick-walled portions  65   c  of the valve housing  65   a , and threaded into the corresponding bolt hole  70  of the joint flange portion  71   a . A stud bolt  81  (threaded bolt) having a threaded part at each end is passed through each of the remaining two bolt holes  64   c  (the upper bolt hole  64   c  and the lower left bolt hole  64   c ) of the second downstream flange  64   b  and the corresponding bolt hole  65   b  formed in the thick-walled portion  65   c  of the valve housing  65   a , and threaded into the corresponding bolt hole  70  of the joint flange portion  71   a . A nut  83  is threaded onto the threaded portion of each stud bolt  82  projecting rearward from second downstream flange  64   b.    
     As a result, the second downstream flange  64   b  of the second low-pressure EGR pipe  64 , the low-pressure EGR valve  65  and the joint flange portion  71   a  of the joint member  71  are fastened together with the first gasket  68  and the second gasket  69  placed in the interfaces between these three parts. Thus, the fasteners consisting of the threaded bolt  82 , the stud bolts  81  and the nuts  83  detachably join the second low-pressure EGR pipe  64 , the low-pressure EGR valve  65  and the joint member  71  to one another. 
     When the second downstream flange  64   b  and the low-pressure EGR valve  65  are jointly fastened to the joint flange portion  71   a , the length of the threaded part of each stud bolt  81  protruding from the outer (rear) surface of the second downstream flange  64   b  is at least twice the thickness of the nut  83  so that an additional nut  84  (indicated by imaginary lines), in addition to the nut  83 , can be threaded onto the protruding part of the stud bolt  81 . A tool engaging feature  81   a  for engaging a tool is formed at the free end of the stud bolt  81 . In the illustrated embodiment, the tool engaging feature  81   a  is formed as a projection having a hexagonal cross section. Alternatively, the tool engaging feature  81   a  may consist of a projection having any other non-circular cross section, or a non-circular recess as long as it can be used for turning the stud bolt  81  around the central axial line thereof. 
     The flexible pipe member  72  is made of an elastic material such as synthetic rubber, natural rubber and elastomer (such as urethane rubber and silicone rubber), and is elastically deformable in the lateral direction and the axial direction. The flexible pipe member  72  is disposed between the joint member  71  and the compressor  41 , and has an upstream side end portion  72   a  fitted over the joint connecting pipe portion  71   b  and a downstream side end portion  72   b  fitted over the compressor connecting pipe portion  41   b . A pair of annular protrusions  72   c  are formed on the outer peripheral surfaces of the upstream side end portion  72   a  and the downstream side end portion  72   b  of the flexible pipe member  72 , respectively, for preventing the axial misalignment of the hose bands  73 . Each pair of annular protrusions  72   c  are spaced apart from each other by a distance slightly greater than the width of the hose band  73 . 
     Each hose band  73  may consist of a per se known hose band, and the circumferential length thereof can be adjusted, for example, by using a fastening arrangement (not shown) such as a screw. The hose bands  73  clamp the upstream side end portion  72   a  and the downstream side end portion  72   b  of the flexible pipe member  72  onto the compressor connecting pipe portion  41   b  and the joint connecting pipe portion  71   b , respectively, to achieve an air tight connection at these two parts. 
     When exhaust gas flows through the low-pressure EGR device  60 , the second low-pressure EGR pipe  64  and the low-pressure EGR valve  65  are heated, and thermally expand. The flexible pipe member  72  absorbs expansion and contraction of these members, and prevents stress concentration owing to the thermal expansion. 
     In particular, the length L 1  of the flexible pipe member  72  is shorter than the distance L 2  between the annular shoulder surface  41   c  on the side of the compressor  41  and the annular shoulder surface  71   c  on the side of the joint member  71  opposing each other in a state where the low-pressure EGR device  60  is not at a high temperature (a state where maintenance work can be performed). As a result, the axial end surfaces of the flexible pipe member  72  are spaced from the corresponding annular shoulder surfaces  41   c  and  71   c  by a combined spacing G. In the illustrated example, the corresponding end of the flexible pipe member  72  abuts the annular shoulder surface  71   c  so that a space L 3  (=L 2 −L 1 ) is created between the annular shoulder surface  41   c  and the front end surface of the flexible pipe member  72 . 
     The dimension L 3  of the gap G is selected such that the first gasket  68  and the second gasket  69  both in an unused state (yet to be compressed) can be inserted between the low-pressure EGR valve  65  and the second low-pressure EGR pipe  64 , and between the low-pressure EGR valve  65  and the joint member  71 , respectively. More specifically, if the thickness of the first gasket  68  is t 1  and the thickness of the second gasket  68  is t 2 , the dimension L 3  is greater than the sum of these thicknesses by a certain margin a corresponding to the combined amount of compression of the two gaskets  68  and  69  (L 3 &gt;t 1 +t 2 +α). The thickness of the gasket in an unused state accounts for the thickness of the beads and the warping of the gasket. Therefore, when the stud bolts  81  and the threaded bolt  82  are unfastened, and the hose bands  73  for the flexible pipe member  72  are loosened, it is possible to insert the two gaskets  68  and  69  between the low-pressure EGR valve  65  and the second low-pressure EGR pipe  64 , and between the low-pressure EGR valve  65  and the joint member  71 , respectively. 
     The assembling process for the low-pressure EGR device  60  is described in the following. 
     As shown in  FIG. 3 , when installing the low-pressure EGR device  60 , the low-pressure EGR cooler  63  to which the first low-pressure EGR pipe  62  is connected is attached to the engine  4  via the mounting pieces  63   d . Next, the second upstream flange  64   a  of the second low-pressure EGR pipe  64  is connected to the downstream side connecting pipe portion  63   c  of the low-pressure EGR cooler  63 , and the second low-pressure EGR pipe  64  is attached to the engine  4  via the connecting piece  64   d . Since the second low-pressure EGR pipe  64  is fixed to the engine  4 , the supporting rigidity thereof is high and the two stud bolts  81  are also held in a stable condition. As shown in  FIG. 5 , the two stud bolts  81  are fastened to the joint member  71  in advance so that the two stud bolts  81  extend rearward from the joint flange portion  71   a  of the joint member  71 . 
     Thereafter, the two stud bolts  81  are passed into the through holes of the second gasket  69  to temporarily assemble the first gaskets  68  to the joint member  71 . Subsequently, the two stud bolts  81  are passed into the bolt holes  65   b  of the low-pressure EGR valve  65  to temporarily assemble the low-pressure EGR valve  65  to the joint member  71 . Since the two stud bolts  81  are provided in the upper portion and the lower left portion of the second upstream flange  64   a , the position of the low-pressure EGR valve  65  in the directions perpendicular to the axial line of the joint member  71  is correctly determined. 
     Thereafter, the two stud bolts  81  are passed into the through holes of the first gasket  68  to temporarily assemble the second gasket  69  to the joint member  71 . As a result, the first gasket  68 , the low-pressure EGR valve  65 , the second gasket  69 , and the joint member  71  are in a temporarily assembled state so that the relative position in the direction perpendicular to the axial line is correctly determined in a stable manner. Subsequently, while holding this assembly together, the two stud bolts  81  are passed into the bolt holes  64   c  of the second low-pressure EGR pipe  64 , and the two nuts  83  are threaded onto the stud bolts  81  so that the assembly is pressed onto the second downstream flange  64   b . The nuts  83  are only loosely tightened at this time. In this state, the low-pressure EGR valve  65  and the joint member  71  are suspended by the second low-pressure EGR pipe  64  via the stud bolts  81  in a stable condition. Thereafter, the threaded bolt  82  having the hexagonal head is passed through the bolt holes of these members from behind, and threaded into the bolt hole  70  of the joint member  71 . The two nuts  83  and the bolt  82  with a hexagonal head are tightened one after the other so that the second low-pressure EGR pipe  64 , the low-pressure EGR valve  65  and the joint member  71  are finally assembled together. 
     Thereafter, as shown in  FIGS. 3 and 4 , the upstream side end portion  72   a  of the flexible pipe member  72  having the hose band  73  loosely thereof is fitted onto the joint connecting pipe portion  71   b , and the corresponding axial end of the flexible pipe member  72  is brought into contact with the annular shoulder surface  71   c . The hose band  73  is tightened at this position so that the flexible pipe member  72  is connected to the joint connecting pipe portion  71   b  in an air tight manner. 
     Thereafter, to assemble the turbocharger  40  to the engine  4 , the compressor connecting pipe portion  41   b  is inserted into the downstream side end portion  72   b  of the flexible pipe member  72 , and the turbocharger  40  is fixedly attached to the engine  4  at appropriate positions. As a result, a gap G having a dimension L 3  is created between the annular shoulder surface  41   c  and the front end surface of the joint member  71 . Finally, the hose band  73  on the downstream side is tightened to connect the downstream side end portion  72   b  of the flexible pipe member  72  to the compressor connecting pipe portion  41   b  in an air tight manner. This concludes the assembly of the low-pressure EGR device  60  to the engine  4 . 
     Thereafter, the catalytic converter  33  and the DPF  34  are connected to the downstream side of the turbine  42 . This can be accomplished without being obstructed by the low-pressure EGR device  60 . Further, the first upstream flange  62   a  of the first low-pressure EGR pipe  62  is connected to the lower end of the DPF  34 . Since the lower portion of the first low-pressure EGR pipe  62  is configured as a bellows  62   c , stress concentration in the first low-pressure EGR pipe  62  due to thermal expansion of the exhaust system  30  can be avoided. 
     The disassembling process for the low-pressure EGR device  60  for maintenance and other purposes is described in the following. 
     First of all, as shown in  FIGS. 4 and 5 , the hose band  73  on the downstream side is loosened, and the two nuts  83  and the bolt  82  are loosened. As a result, the joint member  71  and the flexible pipe member  72  can be moved toward the side of the compressor  41  so that the gap G is created. Subsequently, the bolt  82  is pulled out rearward. The stud bolts  81  are loosened each by engaging the tool engaging feature  81   a  with a suitable tool, and are pulled out rearward. If any of the stud bolts  81  is firmly lodged in the threaded hole of the joint flange portion  71   a  and cannot be turned, the additional nut  84  may be threaded onto the stud bolt  81  in addition to the original nut  83  so that the two nuts  83  and  84  become locked onto the stud bolt  81 . Then, the original nut  83  may be engaged by a spanner or any suitable tool, and is turned in the counter-clockwise direction. Since the effective diameter of the nut  83  is larger than that of the tool engaging feature  81   a , a larger torque can be applied to the stud bolt  81 . 
     Once the stud bolts  81  and the bolt  82  are removed, the low-pressure EGR valve  65  can be removed by sliding the low-pressure EGR valve  65  upward or leftward. If a high frictional resistance is encountered when sliding the low-pressure EGR valve  65  upward or leftward, the joint member  71  and the flexible pipe member  72  may be moved until the flexible pipe member  72  abuts against the annular shoulder surface  41   c . As a result, the distance between the second downstream flange  64   b  and the joint member  71  is increased so that the low-pressure EGR valve  65  can be relatively easily slid off. Therefore, the subsequent work of reinstalling the low-pressure EGR valve  65  or installing a new low-pressure EGR valve  65  can be facilitated. 
     As described above, the dimension L 3  of the gap G is selected such that the first gasket  68  and the second gasket  69  in an unused state can be inserted between the low-pressure EGR valve  65  and the second low-pressure EGR pipe  64  and between the low-pressure EGR valve  65  and the joint member  71 . More specifically, as shown in  FIG. 6 , with the low-pressure EGR valve  65  installed in the designated position, a gap t 3  greater than the thickness of the first gasket  68  in the unused state is created between the low-pressure EGR valve  65  and the second low-pressure EGR pipe  64 , and a gap t 4  greater than the second gasket  69  in the unused state is created between the low-pressure EGR valve  65  and the joint member  71 . Therefore, the first gasket  68  and the second gasket  69  can be installed in the designated positions after the low-pressure EGR valve  65  is installed without any difficulty. Thereafter, the remaining parts of the low-pressure EGR valve  65  and the associated parts can be assembled by reversing the order of disassembly discussed earlier. 
     According to the low-pressure EGR device  60  constructed as described above, the following advantages can be obtained. As shown in  FIG. 4 , the inlet end of the low-pressure EGR valve  65  is detachably connected to the second downstream flange  64   b  of the second low-pressure EGR pipe  64 , and the free end of the compressor connecting pipe portion  41   b  is detachably connected to the outlet end of the low-pressure EGR valve  65 . The flexible pipe member  72  connected to the compressor connecting pipe portion  41   b  and the joint connecting pipe portion  71   b  by the hose bands  73  has a length L 1  smaller than the distance L 2  between the annular shoulder surface  41   c  and the annular shoulder surface  71   c . Therefore, a gap G is created between the flexible pipe member  72  and the annular shoulder surface  41   c . As a result, the flexible pipe member  72  and the joint member  71  can be axially moved toward the side of the compressor  41  until the flexible pipe member  72  abuts against the annular shoulder surface  41   c  so that the low-pressure EGR valve  65  can be easily removed without removing the compressor  41 . 
     As shown in  FIGS. 1, 3 and 4 , the compressor  41  is provided so as to protrude beyond the left end of the engine  4 , and the second downstream flange  64   b  of the second low-pressure EGR pipe  64 , the low-pressure EGR valve  65 , the joint connecting pipe portion  71   b , the flexible pipe member  72  and the compressor connecting pipe portion  41   b  are arranged in the fore and aft direction along the left side of the engine  4 . Therefore, the low-pressure EGR device  60  can be arranged in a compact manner adjacent to the engine  4 . 
     As shown in  FIGS. 2 and 3 , the first upstream flange  62   a  of the first low-pressure EGR pipe  62  is connected to a portion of the exhaust system  30  on the downstream side of the catalytic converter  33 , and the compressor connecting pipe portion  41   b  is connected to the upstream side of the compressor  41 . Therefore, the exhaust gas contains moisture with a certain acidity, but the acidity of the exhaust gas is weakened as the exhaust gas passes through the catalytic converter  33  before the exhaust gas is recirculated to the intake air of the intake system  20  under negative pressure via the low-pressure EGR device  60 . Therefore, deterioration of the flexible pipe member  72  that could be caused by the acidity can be minimized. 
     As shown in  FIG. 3 , the low-pressure EGR device  60  includes the low-pressure EGR cooler  63  fixedly attached to the engine  4  and disposed between the engine  4  and the catalytic converter  33 . In terms of the flow of the exhaust gas, the low-pressure EGR cooler  63  is provided with an intermediate part of the path of the exhaust gas or between the first low-pressure EGR pipe  62  and the second low-pressure EGR pipe  64 . The low-pressure EGR device  60  extends from the first upstream flange  62   a  to the left side of the engine  4  through a space defined between the engine  4  and the catalytic converter  33 . The low-pressure EGR device  60  then extends toward the intake side of the engine  4  along the left side of the engine, and curves upward to be bent back toward the exhaust side of the engine  4 . Therefore, the temperature of the exhaust gas flowing through the flexible pipe member  72  is lowered as the exhaust gas flows along this path so that the thermal degradation of the flexible pipe member  72  is minimized. Furthermore, the low-pressure EGR device  60  can be arranged in a highly compact manner by making use of the space defined between the engine  4  and the catalytic converter  33 . 
     The second low-pressure EGR pipe  64  connecting the low-pressure EGR cooler  63  to the low-pressure EGR valve  65  is made of a rigid member and fixed to the engine  4 . Therefore, the support rigidity of the second low-pressure EGR pipe  64  is very high. This facilitates the work of installing and removing the low-pressure EGR valve  65 , and also eliminates the need to remove or install the low-pressure EGR cooler  63  when removing the low-pressure EGR valve  65  so that the need for the work involved in removing and filling cooling water for the low-pressure EGR cooler  63  can be eliminated. 
     As shown in  FIGS. 4 and 5 , as a fastening means for detachably fastening the second low-pressure EGR pipe  64 , the low-pressure EGR valve  65 , and the joint member  71  having the joint connecting pipe portion  71   b , at least one stud bolt  81  (more preferably at least two stud bolts) each penetrating the EGR valve  65  having a base end threaded into the joint member  71  is used. The free end of the stud bolt  81  is provided with a threaded position and the nut  83  is threaded thereon. Therefore, the low-pressure EGR valve  65  and the joint member  71  can be temporarily assembled to the second low-pressure EGR pipe  64  making use of the stud bolt  81  so that the assembling of the the low-pressure EGR device  60  is facilitated. In the illustrated embodiment, two stud bolts  81  are used as fastening means. Therefore, the relative position of the low-pressure EGR valve  65  in the direction perpendicular to the joint member  71  with respect to the joint member  71  is correctly determined while the EGR valve  65  and the joint member  71  are being temporarily assembled so that the assembling the low-pressure EGR device  60  is facilitated even further. 
     In the illustrated embodiment, the base end of each stud bolt  81  is screwed into the joint flange portion  71   a  of the joint member  71 , but may also be threaded into or otherwise detachably fastened to the second downstream flange  64   b  of the second low-pressure EGR pipe  64 . 
     As shown in  FIG. 4 , each stud bolt  81  has a tool engaging feature  81   a  at the free end. Further, the stud bolt  81  protrudes from the second downstream flange  64   b  by more than twice the thickness of the nut  83  in the assembled state so that an additional nut  84  may be threaded onto the free end of the stud bolt  81 . Therefore, even when the stud bolt  81  is firmly lodged in the threaded hole  70  of the joint flange portion  71   a , the stud bolt  81  can be unscrewed from the threaded hole  70  by applying an unfastening torque to the additional nut  84  with a suitable tool. Since the additional nut  84  has a relatively large diameter, a large unfastening torque can be applied to the stud bolt  81 . 
     As shown in  FIGS. 4 and 6 , the dimension L 3  of the gap Gin the axial direction of the flexible pipe member  72  is selected such that the first gasket  68  and the second gasket  69  in an unused state can be fitted into the gap between the low-pressure EGR valve  65  and the second low-pressure EGR pipe  64  and the gap between the low-pressure EGR valve  65  and the joint member  71 , respectively. More specifically, when the flexible pipe member  72  is moved leftward until the corresponding end of the flexible pipe member  72  abuts the annular shoulder surface  41   c , the gap (t 3 ) created between the low-pressure EGR valve  65  and the second low-pressure EGR pipe  64  and the gap (t 4 ) created between the low-pressure EGR valve  65  and the joint member  71  are larger than the thicknesses of the first gasket  68  and the second gasket  69  in an unused state. Therefore, the removal and reinstalling of the low-pressure EGR valve  65  can be accomplished with ease. 
     Although the present invention has been described in terms of a preferred embodiment thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the spirit of the present invention. For instance, the present invention was applied to an automotive diesel engine, but may also be applied to gasoline engines as well, and may also be applied to engines for other applications, such as railways and other ground transportation vehicles, watercraft and aircraft. 
     Further, in the above-described embodiment, the engine  4  was laterally mounted on the motor vehicle  1 , but may be mounted longitudinally. The exhaust side of the engine may also be provided on the opposite side of the engine without departing from the spirit of the present invention. In the foregoing embodiment, the EGR device was applied to the low-pressure EGR device  60 , but may be applied to the high-pressure EGR device  50 . In the above embodiment, the turbocharger  40  may be replaced with a supercharger which may be powered either by an electric motor or the output of the engine. In the above embodiment, the compressor connecting pipe portion  41   b  was formed integrally with the compressor housing  41   a , but may also be formed separately from the compressor housing  41   a  and coupled to the compressor housing  41   a . In the above embodiment, two stud bolts  81  were implanted in the joint member  71 , but one or three or more stud bolts  81  may be used, and the stud bolts  81  may also be implanted to the second low-pressure EGR piping  64 . Alternatively, it is also possible to do away with the stud bolts  81  altogether, and use only normal threaded bolts. 
     In the above embodiment, the stud bolts  81  and the normal bolt  82  were passed through the valve housing  65   a  of the low-pressure EGR valve  65 , and the second low-pressure EGR pipe  64 , the low-pressure EGR valve  65  and the joint member  71  were fastened together by the nut  83 . However, a fastening means for fastening the second low-pressure EGR pipe  64  and the low-pressure EGR valve  65  to each other and a fastening means for fastening the low-pressure EGR valve  65  and the joint member  71  to each other may be separately provided. 
     Alternatively, fasteners other than bolts and nuts may be used for the fastening member. In the above embodiment, the gap G was formed between the annular shoulder surface  41   c  and the flexible pipe member  72 , but it may also be formed between the annular shoulder surface  71   c  and the corresponding end of the flexible pipe member  72 . Furthermore, the compressor connecting pipe portion  41   b  and the joint connecting pipe portion  71   b  may not be exactly aligned in a coaxial relationship, and/or the flexible pipe member  72  may be bent or otherwise curved.