Patent Publication Number: US-6702062-B2

Title: Exhaust system for automobile engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust system for an internal combustion engine that is equipped in an automotive vehicle, and, more particular, to an engine exhaust system that has a plurality of discrete exhaust pipes branching off from an exhaust pipe and connected to a cylinder head and an exhaust pipe. 
     2. Description of Related Art 
     Typically, a catalytic converter, that is disposed in an engine exhaust line, is desirably activated in a short period of time after a start of an engine in order to improve a catalytic conversion efficiency. In recent years, for quick activation of the catalytic converter, there has been proposed a heat insulated exhaust manifold operative to hold a temperature of an exhaust gas discharged from the engine as high as possible and then to direct the exhaust gas to the catalytic converter. Such heat insulated exhaust manifolds are known from, for example, Japanese Unexamined Patent Publication No. 11-303630 and Japanese Unexamined Utility Model Publication No. 63-54816. 
     The prior art heat insulated exhaust manifold disclosed in Japanese Unexamined Patent Publication No. 11-303630 comprises four double-walled discrete exhaust pipes connected at upstream ends to a cylinder head and joined at downstream ends to a double-walled chamber for collecting exhaust gases passing through the discrete exhaust pipes. This double-walled structure is somewhat troublesome to manufacture the heat insulated exhaust manifold and exerts various restraints on the configuration of the heat insulated exhaust manifold. Specifically, a bent portion of the heat insulated exhaust manifold must be large in bending radius and long in length. In other words, due to layout requirements of the engine in an engine compartment, a straight portion of the double-walled heat insulated exhaust manifold must be short in length. As a result, the double-walled heat insulated exhaust manifold provides an exhaust gas with a high resistance, so as to have an adverse influence on the output property of the engine. 
     The prior art heat insulated exhaust manifold disclosed in Japanese Unexamined Utility Model No. 63-54816 comprises single wall discrete exhaust pipes and a double-walled chamber for collecting exhaust gases passing through the discrete exhaust pipes. This heat insulated exhaust manifold having the double-walled chamber is focused only on lowering noises of an exhaust gas flow passing therethrough and preventing pulsations of the exhaust gas from propagating into exhaust pipes before and after the double-walled chamber. 
     While the heat insulated exhaust manifold is advantageous to protecting parts around the exhaust manifold from heat damage and activating quickly a catalytic converter in an exhaust line, the heat insulated exhaust manifold is apt to be heated by a hot exhaust gas passing therethrough and therefore to be filled with heat. Accordingly, the heat insulated exhaust manifold is necessary to have a heat releasing mechanism. If releasing heat through a fitting flange of the heat insulated exhaust manifold through which the heat insulated exhaust manifold is fitted to the cylinder head, a rubber gasket (a sealing member) between the cylinder head and the cylinder cover and supplementary devices around the engine are apt to encounter heat deterioration. 
     There has been proposed in Japanese Unexamined Utility Model 4-91224 an exhaust manifold having a generally U-shaped fitting bracket. Specifically, the exhaust manifold for an in-line four cylinder engine comprises four discrete exhaust pipes made of steel pipes that are connected to a cylinder block through a fitting flange and a collective chambered pipe made of a casting pipe, that are divided by a casting partition into two collective chambers and connected to downstream ends of the discrete exhaust pipes so that exhaust gas streams passing through the discrete exhaust pipes merge together in the collective chambered pipe. The exhaust manifold is fitted to the engine by securing the U-shaped fitting bracket bolted to the collective chambered pipe to a fitting boss of a cylinder head of the engine. As the partition is also made of a casting as well as the collective chambered pipe and is thick, it does not make noises resulting from engine vibrations. 
     When using a steel pipe in place of the casting pipe for the collective chambered pipe in order to reduce the weight of the collective chambered pipe, the partition makes vibrations resulting from engine vibrations, so as to make noises. In addition, as the U-shaped fitting bracket is fitted to the collective chambered pipe by fastening bolts into bolt holes formed in bosses of the collective chambered pipe and also to the boss of the cylinder block, it is hard to absorb thermal expansion of the exhaust manifold due to a hot exhaust gas. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an exhaust system having a heat insulated exhaust manifold that can provide an engine with an improved output characteristic and gain improved heat insulation effectiveness. 
     It is another object of the present invention to provide an exhaust system having a heat insulated exhaust manifold is adapted to radiate heat from a space formed between a fitting flange of an exhaust manifold and a cylinder head and has a heat guide member operative to conduct heat toward far from the engine so as thereby to prevent a sealing member disposed between a cylinder head and a cylinder cover and supplementary devices around the engine from encountering heat deterioration. 
     It is still another object of the present invention to provide an exhaust system having an exhaust manifold that is improved in structural rigidity by securing a partition of the collective chambered pipe to a welded joint structure of high rigidity through which a plurality of discrete exhaust pipes are joined to a collective chambered pipe. 
     It is still a further object of the present invention to provide an exhaust system having an exhaust manifold that is further improved in structural rigidity by supporting the exhaust manifold to the engine through a supporting bracket that is attached at or in close proximity to a welded joint structure between discrete exhaust pipes and a collective chambered pipe. 
     The exhaust manifold according to an embodiment of the present invention comprises a plurality of single-shell discrete exhaust pipes connected to exhaust ports formed in the cylinder head, respectively, a collective chambered pipe joined to the single-shell discrete exhaust pipes that is made up of an internal pipe shell into which exhaust gas streams passing through the single-shell discrete exhaust pipes are introduced and merge together and an external pipe shell surrounding the internal pipe shell, and spherical joint means connecting the collective chambered pipe to the exhaust pipe disposed downstream the collective chambered pipe and supported to the vehicle body. Each of the single-shell discrete exhaust pipes has a bent portion and a straight portion continuously extending from the bent portion to the collective chambered pipe. The bent portions of the single-shell discrete exhaust pipes are oriented toward a center of a straight row of cylinders so as to lay the straight portions of the single-shell discrete exhaust pipes nearly parallel to one another, and the single-shell discrete exhaust pipes and the collective exhaust pipe are joined to each other at least partly at a height of a plane including the interface between the cylinder block and the cylinder head. 
     The exhaust manifold thus structured can be provided with a long straight path of exhaust gas by the straight portions of the single-shell discrete exhaust pipes and the collective chambered pipe that is straight, so as thereby to reduce the resistance of exhaust gas. This results in helping the engine to raise output. In addition, the collective chambered pipe can be provided with high heat insulation effectiveness by having a length as long as possible. This results in quick activation of the catalytic converter installed in the exhaust system. The discrete exhaust pipe that is made of a single shell is easily bent. 
     The spherical joint between the collective chambered pipe and the exhaust pipe allows relative movement between the exhaust manifold and the exhaust pipe. This eases undesirable transmission of rolling vibrations of the engine to the exhaust pipe through the exhaust manifold. 
     The engine is preferably of a rear exhaust type that draws in intake air at the front side thereof and discharges exhaust gases at the rear side thereof. Owing to this engine position, the exhaust manifold is protected from exposure to the wind of speeding vehicle. This improves the high heat insulation effectiveness of the intake manifold. In addition, this makes it possible to shorten the length of the exhaust path from the upstream ends of the single-shell discrete exhaust pipes to the catalytic converter. 
     The exhaust manifold may be provided with a fitting flange connected to the upstream ends of the single-shell discrete exhaust pipes. The bolt holes formed in the fitting flange are arranged alternately along opposite upper and under sides of the fitting flange in a direction of the straight row of cylinders. At least one of the bolt holes that is in close proximity to the single-shell discrete exhaust pipes, specifically the bolt hole in close proximity to the single-shell discrete exhaust pipes, is located above a horizontal plane including centers of openings of the upstream ends of the single-shell discrete exhaust pipes. This arrangement of the bolt holes provides the straight portion of each of the single-shell discrete exhaust pipes with a long path of exhaust gas a sufficient space for installation work using, for example, an impact wrench. In this connection, if the bolt hole in close proximity to the single-shell discrete exhaust pipes is located below horizontal plane including centers of openings of the upstream ends of the single-shell discrete exhaust pipes, the bent portions of the single-shell discrete exhaust pipes must be long in consideration of a space for installation work using an impact wrench and the straight portions of the single-shell discrete exhaust pipes must be correspondingly shortened. 
     The exhaust manifold may include a catalytic converter disposed downstream from the collective chambered pipe and under the vehicle body in close proximity to the collective chambered pipe. This arrangement of the catalytic converter can get rid of the necessity of installing a catalytic converter immediately after an exhaust manifold like an exhaust system of a front exhaust engine and, as a result, can be located as close to the collective chambered pipe as possible so as to be quickly activated due to a hot exhaust gas. 
     The exhaust pipe may be of a double shell type that is made up of internal and external pipe shells. The double shell exhaust pipe provides improvement of hear insulation effectiveness in cooperation with the double shell collective chambered pipe. 
     The collective chambered pipe may be divided into two collective chambers, the first collective chamber and a second collective chamber located closer to the engine than the first collective chamber, by the partition. Exhaust gas streams passing through the single-shell discrete exhaust pipes, respectively, for the cylinders at opposite ends of the straight row of cylinders enter into the first collective chamber and merge together. Exhaust gas streams passing through the single-shell discrete exhaust pipes and for the remaining cylinders enter into the second collective chamber and merge together. The internal pipe shell of the collective chambered pipe shell extends straight in a direction of exhaust gas streams and gradually decreases in a cross-sectional area from the upstream end to the downstream end. 
     The collective chambered pipe having the second collective chamber located closer to the first collective chamber makes it possible to employ the single-shell discrete exhaust pipes that have the bent portions shortened in length. The use of the partition installed in the internal chambered pipe shell makes the collective chambered pipe compact as compared with using two independent internal pipe shells for dividing the interior of the collective chambered pipe into the first and second collective chambers and makes a surface area of the collective chambered pipe small. This structure of the collective chambered pipe reduces a thermal capacity, and hence radiation of heat, of the collective chambered pipe and, in addition, gradually constricts an exhaust gas stream. In the case of installing the exhaust system to, in particular, a four-cylinder engine, a compact 4-2-1 type exhaust system can be realized. The 4-2-1 type exhaust system is referred to the exhaust system, in which four exhaust gas streams passing through the four discrete exhaust pipes merge together into two exhaust gas streams and thereafter into one exhaust gas stream, is hardly affected by back pressure and exhaust gas pulsations. 
     The exhaust manifold may further comprise an insulation shell made up of two mating shell halves, i.e. the upper insulation shell half and the under insulation shell half, that covers the exhaust manifold, in particular at least the single-shell discrete exhaust pipes that are apt to radiate a comparatively large amount of heat as compared with the double-shell collective chambered pipe. This prevents the exhaust manifold from losing heat to the atmosphere and, therefore, protects peripheral devices and parts from heat damage. Further, this provides the exhaust manifold with high heat insulation effectiveness. 
     The exhaust manifold may be provided with spacer means installed in a space between the internal and external pipe shells and of the double-shell collective chambered pipe. This spacer means separates the internal pipe shell from the external pipe shell mechanically so as to allow longitudinal expansion of the internal pipe shell due to a difference of thermal expansion between the internal and external pipe shells. Further, the spacer means has supporting rigidity greater at a specified extent of its lower portion than at the remaining portion. The difference in supporting rigidity can be realized by differing at least one of material, width in the lengthwise direction of the internal or external pipe, thickness, and mesh size for the lower portion from that for the remaining portion. The exhaust manifold with the spacer means installed in the collective chambered pipe prevents the internal pipe shell, in particular the lower portion of the internal pipe shell, from causing mechanical interference with the external pipe shell due to vibrations. 
     According to another embodiment of the present invention, the exhaust manifold has a collective chambered pipe divided into a first collective chamber into which exhaust gas streams passing through single-shell discrete exhaust pipes for cylinders not sequentially adjoin in firing order and a second collective chambers into which exhaust gas streams passing through single-shell discrete exhaust pipes for the remaining cylinders not sequentially adjoin in firing order by a partition so that the first collective chamber is positioned farther away from the engine than the second collective chamber. The supporting bracket is secured to the exhaust manifold at a welded joint structure between the discrete exhaust pipes and the collective chambered pipe, or in close proximity to the welded joint structure at a side of the second collective chamber, so as to extend toward the engine. The partition at its top end is preferably secured to the welded joint structure that is high in structural rigidity. The partition and the supporting bracket are separately are separately located far away from each other. This structure of the collective chambered pipe prevents transmission of engine vibrations to the partition and, accordingly, from producing noises resulting from vibrations. 
     The supporting bracket may comprise a bracket arm and a flange tongue connected as one right-angle piece by a curved portion. The supporting bracket at the flange tongue is secured to the collective chambered pipe at the welded joint structure or in close proximity to the welded joint structure at a side of the second collective chamber. This right angle configuration of the supporting bracket is helpful in easing thermal expansion of the exhaust manifold and in absorption of engine vibrations. In addition, 
     The collective chambered pipe comprises the internal and external pipe shells joined and welded at their upstream ends only so as to form a space between the internal and external pipe shells and the supporting bracket is secured to the outer pipe shell in a position downstream from the welded joint structure where the external pipe shell is separated from the internal pipe shell. This joint structure of the collective chambered pipe is advantageous, on one hand, to preventing transmission of engine vibrations to the partition in the collective chambered pipe and, on the other hand, to improving structural rigidity and weld strength in addition to realizing improved heat insulation effectiveness of the collective chambered pipe that is advantageous to quick activation of the catalytic converter. 
     The single-shell discrete exhaust pipe may have a downstream end portion that comprises a rounded shell portion configured and a right-angle shell portion. The four discrete exhaust pipes thus configured and connected to the collective chambered pipe form four quadrant downstream end portions arranged so as to meet in configuration the internal pipe shell of the collective chambered pipe and form a cruciform space among the right-angle shell portions in which a cruciform reinforcement is welded to the right-angle portions of the discrete exhaust pipes. The cruciform reinforcement may be made up of an upper extension of the partition of the collective chambered pipe  28  and a reinforcement strip assembled crosswise. This cruciform reinforcement welded to the discrete exhaust pipes avoids a presence of a joint gap among the discrete exhaust pipes, so as to increase structural rigidity and weld strength of the welded joint structure and, in addition, prevent the discrete exhaust pipes from making mechanical interference with one another. 
     According to another embodiment, the exhaust manifold covered by the insulation shell made up of the upper and under insulation shell halves configured so as to leave a specified length of heat radiating clearance or space between the insulation shell and the fitting flange. The exhaust manifold thus structured may be provided with a heat guide member that conducts and releases exhaust heat from the heat radiating space toward far from the engine. The exhaust manifold provided with the heat guide member is advantageous to preventing a sealing member disposed between the cylinder head and the cylinder cover and supplementary devices around the engine from encountering heat deterioration. The heat guide member may be formed integrally with or separately from a gasket through which the exhaust manifold  1  is installed to the cylinder head of the engine. 
     The insulation shell halves that are joined to one another so as to partly overlap along their lengthwise sides. The joint gap between the overlapped sides of the insulation shell halves is desirably as small as possible and, more desirably, smaller than the space distance of the heat radiation space as possible. The insulation shell thus structured is advantageous to, on one hand, preventing exhaust heat from escaping from the insulation shell through the joint gap and, on the other hand, radiating the exhaust heat into the atmosphere through the heat radiating space between the insulation shell and the fitting flange. In addition, the insulation shell thus structured enhances the heat releasing effect of the heat guide member. The exhaust manifold is suitably installed to a rear exhaust type in-line multiple cylinder engine installed in the transverse direction in the engine compartment. This engine arrangement is advantageous to protecting the engine and supplementary. 
     The insulation shell may be formed with a plurality of ribs or beads as reinforcement mean extending in the transverse direction thereof. The insulation shell formed with the ribs or beads is preferably installed to the exhaust manifold by fastening bolts through bolt holes formed in the ribs or beads. The exhaust manifold covered by the insulation shell thus installed improves structural rigidity of the exhaust manifold. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will be clearly understood from the following detailed description when read with reference to the accompanying drawings, wherein the same numeral numbers have been used to denote same or similar parts or mechanisms throughout the drawings and in which: 
     FIG. 1 is a side view of an exhaust system according to an embodiment of the present invention; 
     FIG. 2 is an enlarged view of an exhaust manifold of the exhaust system; 
     FIG. 3 is a rear view of the exhaust manifold; 
     FIG. 4 is an exploded view of the exhaust manifold; 
     FIG. 5 is a plan view of the exhaust manifold with an insulation shell removed; 
     FIG. 6 is a front perspective view of the exhaust manifold with an insulation shell removed; 
     FIG. 7 is a side view of he exhaust manifold with an insulation shell removed; 
     FIG. 8 is a front view of a collective chambered pipe of the exhaust manifold; 
     FIG. 9 is a longitudinal cross-sectional view of the exhaust manifold taken along line IX—IX of FIG. 8; 
     FIG. 10 is a transverse cross-sectional view of the exhaust manifold taken along line X—X of FIG. 8; 
     FIG. 11 is a transverse cross-sectional view of the exhaust manifold taken along line XI—XI of FIG. 8; 
     FIG. 12 is a longitudinal cross-sectional view of a spherical joint; 
     FIG. 13 is a plan view of an upper insulation shell half of the insulation shell; 
     FIG. 14 is a front view of the upper insulation shell half of the insulation shell; 
     FIG. 15 is a side view of the upper insulation shell half of the insulation shell; 
     FIG. 16 is a partial longitudinal cross-sectional view of the upper insulation shell half of the insulation shell; 
     FIG. 17 is a plan view of an under insulation shell half of the insulation shell; 
     FIG. 18 is a front view of the under insulation shell half of the insulation shell; 
     FIG. 19 is a side view of the under insulation shell half of the insulation shell; 
     FIG. 20 is a partial longitudinal cross-sectional view of the under insulation shell half of the insulation shell; 
     FIG. 21 is a front view of a gasket; 
     FIG. 22 is an enlarged cross-sectional view of the gasket taken along line XXII—XXII of FIG. 21; 
     FIG. 23 is a longitudinal cross-sectional view of an exhaust manifold according to another embodiment of the present invention; 
     FIG. 24 is a front view, partly in cross-section, of a collective chambered pipe of an exhaust manifold according to another embodiment of the present invention; 
     FIG. 25 is a front view, partly in cross-section, of an collective chambered pipe of an exhaust manifold according to still another embodiment of the present invention; and 
     FIG. 26 is a partial side view of an exhaust manifold according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings in detail and, in particular, to FIG. 1 showing a structure of an exhaust system for an automotive vehicle according to an embodiment of the present invention, an in-line four cylinder internal combustion engine (which is hereafter referred to as an engine for simplicity)  11  comprises a cylinder block  12  and a cylinder head  13  with a head cover  14  attached thereto. The engine  11  is provided with a crankshaft  16  mounted to the cylinder block  12  and an oil pun  15  below the cylinder block  12 . Designated by a reference numeral  17  is an interface between the cylinder block  12  and the cylinder head  13 . The engine  12  is provided with an intake system (not shown) directed to the front of the vehicle body and an exhaust manifold  18  directed to the back of the vehicle body (rear exhaust type engine) and is mounted in a transverse direction in an engine compartment (not shown). This arrangement of the exhaust manifold  18  is intended to prevent the exhaust manifold  18  from being exposed to the wind of the speeding vehicle so as thereby to prevent the exhaust gas from lowering its temperature. The exhaust system, that will be described in detail later, includes an exhaust manifold  18  comprising four, namely first to fourth, single-shell discrete exhaust pipes  24 - 27  and a double-shell collective chambered pipe  28 . These first to fourth single-shell discrete exhaust pipes  24 - 27  are in communication with first to fourth cylinders (# 1 , # 2 , # 3  and # 4 ), respectively. The exhaust manifold  18  at its downstream end is connected to an exhaust pipe  19  that extends below a front floor panel  20  of the vehicle body and is equipped with a catalytic converter  21 . The exhaust manifold  18  is provided with a fitting flange  23  and is bolted to the cylinder head  13  through a gasket  22  disposed between the fitting flange  23  and the cylinder head  13  (see FIGS.  2  and  3 ). 
     Referring to FIGS. 2 to  3  showing the exhaust manifold  18  in detail, the collective chambered pipe  28  is connected to downstream ends of the respective single-shell discrete exhaust pipes  24 - 27  so as to collect exhaust gases passing through the respective single-shell discrete exhaust pipes  24 - 27  therein. The collective chambered pipe  28  at its downstream side is provided with a spherical joint  29  (which will be described in connection with FIG. 12) through which the collective chambered pipe  28  is connected to the exhaust pipe  19  so as to restrain undesirable transmission of rolling vibrations of the engine  11  to the exhaust pipe  19  through the exhaust manifold  18 . The exhaust manifold  18  is covered from the fitting flange  23  to the downstream end of the collective chambered pipe  28  by an insulation shell  30  made up of two mating shell halves, namely an upper insulation shell half  30   a  and an under insulation shell half  30   b,  for securely keeping an exhaust gas as high as possible and preventing heat damage to parts around the exhaust manifold  18 . The catalytic converter  21  is disposed below the front floor panel  20  at a location downstream from and close to the collective chambered pipe  28 . The insulation shell  30  may be made up of more than two parts. 
     FIGS. 5 to  7  show the exhaust manifold  18  and, in particular, the single-shell discrete exhaust pipes  24 - 27  with the upper and under insulation shell halves  30   a  and  30   b  removed. These single-shell discrete exhaust pipes  24 - 27 , each of which is made of a round steel pipe, are formed separately from one another in consideration of bending workability. As seen in FIG. 7, the single-shell discrete exhaust pipes  24 - 27  have bent portions  32  and straight portions  33 , respectively, The bent portions  32  are located at their upstream ends in close proximity to the exhaust ports of the corresponding cylinders, respectively, and are directed spatially toward the middle of a straight row of the four cylinders so that center lines of the respective single-shell discrete exhaust pipes  24 - 27  are substantially parallel to one another. The straight portion  33  of each single-shell discrete exhaust pipe  24 - 27  extends from the bent portion  32  to the collective chambered pipe  28 . In this instance, the first and fourth single-shell discrete exhaust pipes  24  and  27 , that are for the foremost and rearmost or first and fourth cylinders (# 1  and # 4 ), are substantially the same in overall length. Similarly, the second and third single-shell discrete exhaust pipes  25  and  26 , that are for two center, or second and third cylinders (# 2  and # 3 ) located between the first and fourth cylinders (# 1  and # 4 ), are substantially the same in overall length but shorter than the first and fourth single-shell discrete exhaust pipes  24  and  27 . The straight portions  33  of the respective single-shell discrete exhaust pipes  24 - 27  and the collective chambered pipe  28  are in a straight line as one whole. As shown in FIG. 9, a welded joint structure  28   a  between each of the single-shell discrete exhaust pipe  24 - 27  and the collective chambered pipe  28  is at a height of a interface  17  between the cylinder block  12  and the cylinder head  13 . More specifically, in this embodiment, the welded joint structure  28   a  inclines so as to intersects a horizontal plane  17   a  including the interface  17  between the cylinder block  12  and the cylinder head  13 . Further, as shown in FIG. 7, the spherical joint  29  between the collective chambered pipe  28  and the exhaust pipe  19  is located closely to a horizontal plane  16   a  including an axis of rotation of the crankshaft  16 . 
     FIGS. 8 to  11  show partly the exhaust manifold  18  in detail. The collective chambered pipe  28  into which exhaust gases are collectively introduced through the single-shell discrete exhaust pipes  24 - 27  is of a double-shell type. Specifically, the collective chambered pipe  28 , which is desirable to have a length as large as possible, comprises an internal pipe shell  34  through which an exhaust gas stream enters and an external pipe shell  35  surrounding the internal pipe shell  34  with a cylindrical space between the internal pipe shell  34 . As shown in FIGS. 10 and 11, the external pipe shell  35  is made up two mating shell halves  35   a  and  35   b.  These pipe shells  34  and  35  are often constructed of heat-resisting cast iron and are welded at their upstream ends using MIG inert-gas arc welding or laser welding so as to form an upstream end of the collective chambered pipe  28 . As shown in FIGS. 9 to  11 , the collective chambered pipe  28  has a cross-section gradually reducing in area from the upstream end to the downstream end and has a partition  36  made of a heat-resisting iron which divides an upstream half portion of the interior of the collective chambered pipe  28  into two chambers, namely a first collective chamber  41  and a second collective chamber  42 . The partition  36  is disposed in and welded to the internal pipe shell  34  so as to position the first collective chamber  41  farther away from the engine  11  than the second collective chamber  42 . Specifically, the single-shell discrete exhaust pipes  24  and  27  for the first and fourth cylinders (# 1  and # 4 ) that do not sequentially adjoin in firing order are in communication with the first collective chamber  41 . Similarly, the single-shell discrete exhaust pipes  25  and  26  for the second and cylinders (# 2  and # 3 ) that do not sequentially adjoin in firing order are in communication with the second collective chamber  42 . Accordingly, exhaust gas streams passing through the first and forth exhaust pipes  24  and  27  merge together in the first collective chamber  41  and exhaust gas streams passing through the second and third single-shell discrete exhaust pipes  25  and  26  merge together in the second collective chamber  42 . Further, the exhaust gas streams passing through the first and second collective chambers  41  and  42  merge together in the downstream half portion of the interior of the collective chambered pipe  28 . The exhaust system thus constructed is what is called 4-2-1 type for joining exhaust four gas streams step by step into one exhaust gas stream. This 4-2-1 type exhaust system is hardly affected by back pressure and exhaust gas pulsations. In this instance, the firing order in the in-line four cylinder internal combustion engine  11  may be # 1 , # 3 , # 4 , # 2  or # 1 , # 2 , # 4 , # 3 . 
     Referring to FIG. 10 showing the welded joint structure between the collective chambered pipe  28  and the single-shell discrete exhaust pipes  24 - 27  in detail, the single-shell discrete exhaust pipes  24 - 27  have downstream end portions, respectively, each of which comprises a rounded shell portion  24   a - 27   a  formed so as to meet partly the inner wall of the internal pipe shell  34  of the collective chambered pipe  28  and a right-angle shell portion  24   b - 27   b.  The first and forth exhaust pipes  24  and  27  are arranged so as to provide a cruciform space  37  between the adjacent right-angle shell portions  27   b - 27   b  in which a cruciform reinforcement  38  is welded, or otherwise fixed, to the lower ends of right-angle shell portions  27   b - 27   b  of the first and forth exhaust pipes  24  and  27 . The cruciform reinforcement  38  is made up of an upper extension  36   a  of the partition  36  of the collective chambered pipe  28  and a reinforcement strip  38   a.  This cruciform arrangement of the first and forth exhaust pipes  24  and  27  that are welded to the cruciform reinforcement  38  avoids a presence of a joint gap among the first and forth exhaust pipes  24  and  27 , so as to improve structural rigidity and weld strength of the welded joint structure. 
     In order to support the exhaust manifold  18  to the engine  11 , there is provided a supporting bracket  43  such as shown in detail in FIG.  9 . The supporting bracket  43  is welded, or otherwise fixedly attached, to the collective chambered pipe  28  at or in close proximity to the welded joint structure  28   a  between the collective chambered pipe  28  and the single-shell discrete exhaust pipes  24 - 27  on the side of the second collective chamber  42  so as to extend toward the engine  11 . The supporting bracket  43  is fixedly connected to a bracket  45  secured to the cylinder block  12  of the engine  11  as shown in FIG. 1 by bolt and nut fastening means  44  (see FIG.  2 ). The supporting bracket  43  has a bracket arm  43   a  and a flange tongue  43   b  connected as one right-angle piece by a curved portion  43   c.  The flange tongue  43   b  is bent downward at an approximately right angle in this embodiment. In particular, the supporting bracket  43  at its lower end of the flange tongue  43   b  is welded to the collective chambered pipe  28  at the welded joint structure  28   a  where the internal pipe shell  34  and the external pipe shell  35  at their upstream ends are welded to each other. The right angle configuration of the supporting bracket  43  is helpful in easing thermal expansion of the exhaust manifold  18 . 
     The exhaust manifold  18  is further provided with brackets  46 - 51  for securing the insulation shell  30 , specifically the upper and under insulation shell halves  30   a  and  30   b  as shown in detail in FIGS. 7 and 8. One of these brackets  46 - 51 , namely the bracket  46  in this embodiment, is welded, or otherwise fixedly attached, to the upper side of the straight portion  33  of the single-shell discrete exhaust pipe  27  and another one of these brackets  46 - 51 , namely the bracket  49  in this embodiment, is welded, or otherwise fixedly attached, to the under side of the straight portion  33  of the single-shell discrete exhaust pipe  25 . Two of the remaining brackets  47 ,  48 ,  50  and  51 , namely the brackets  47  and  48  are welded, or otherwise fixedly attached, to the under side of the external pipe shell  35  of the collective chambered pipe  28  near opposite end portions so as to cross over an elongated recess  35   c.  Similarly, other two of the remaining brackets  47 ,  48 ,  50  and  51 , namely the brackets  50  and  51  are welded, or otherwise fixedly attached, to the upper side of the external pipe shell  35  of the collective chambered pipe  28  near opposite end portions so as to cross over an elongated recess  35   c.  Each of the bracket  46 - 51  is provided with a nut (not shown) into which a fitting bolt is fastened so as to fix the insulation shell  30 , namely the upper insulation shell half  30   a  and the under insulation shell half  30   b,  to the single-shell discrete exhaust pipes  25  and  27  and the external pipe shell  35  of the collective chambered pipe  28 . This structure enables the use of small sized brackets  47 ,  48 ,  50  and  51 , a compact external pipe shell  35  that is less bulgy, and fastening means, such as bolts and nuts, having an increased fastening length for fixedly attaching the insulation shell  30 , i.e. the upper and under insulation shell halves  30   a  and  30   b,  to the exhaust manifold  18 . 
     As was previously described, the exhaust manifold  18  is provided with the fitting flange  23  through which the exhaust manifold  18  is installed to the cylinder head  12  of the engine  11 . As shown in detail in FIG. 6, the fitting flange  23  is formed with a plurality of bolt holes  23   a - 23   g  arranged alternately along opposite sides thereof in a direction of said straight row of cylinders. The bolt hole  23   c  at a location close to the single-shell discrete exhaust pipes  25  and  26  for the center cylinders (# 2  and # 3 ) is positioned above a straight row of the first to fourth cylinders (# 1  to # 4 ), more specifically above a horizontal plane including centers of end openings of the four single-shell discrete exhaust pipes  24 - 27 , in consideration of providing the straight portion  33  of each of the single-shell discrete exhaust pipes  24 - 27  with a long length and forming a sufficient space for installation work using, for example, an impact wrench. One of the bolt holes  23   a  and  23   f  at opposite extreme ends of the fitting flange  23 , namely the bolt hole  23   f,  is formed in the shape of oval in consideration of easy positional adjustment and installation of the fitting flange  23  to the cylinder head  12  of the engine  11 . One or more of the remaining bolt holes may be of course formed in the shape of oval. 
     Referring to FIG. 11 showing a structure for supporting the internal pipe shell  34  in the collective chambered pipe  28 , while the internal pipe shell  34  at its upstream end is welded to the external pipe shell  35  as shown in FIG. 9, the internal pipe shell  34  at its downstream end is mechanically separated from the external pipe shell  35  so as to be expandable in a lengthwise direction due to a difference of thermal expansion between the internal and external pipe shells  34  and  35 . In order to support the internal pipe shell  34  to the external pipe shell  35  or vise versa, spacer means  52  made of, for example, a stainless wire mesh is installed in the cylindrical space formed between the internal and external pipe shells  34  and  35 . In particular, the spacer means  52  is fitted in the external pipe shell  34  or on the internal pipe shell  35 . The spacer means  52  may be a cylindrical spacer tube filling the cylindrical space or a plurality of annular spacer rings arranged with separations in the cylindrical space. The spacer means  52  is configured so as to have supporting rigidity greater at its lower portion on which the weight of the internal pipe shell  34  acts than at the remaining portion. This difference in supporting rigidity may be provided by differing at least one of material, width in the lengthwise direction, thickness, and mesh size for the lower portion from that for the remaining portion. While, in this embodiment shown in FIG. 11, the spacer means  52  is configured so as to lie entirely in the circumferential direction in the cylindrical space between the internal and eternal pipe shells  34  and  35 , the spacer means  52  may be configured so a to lie partly or intermittently in the circumferential direction in the cylindrical space. Further, the spacer means may be a cylindrical member or may comprise a plurality of annular ring members arranged with separations in the lengthwise direction in the cylindrical space. 
     FIG. 12 shows the spherical joint  29  in detail. The spherical joint  29  comprises a connecting pipe  53  that is fixedly attached to the lower end of the external pipe shell  35  of the collective chambered pipe  28 , a substantially elliptical flange  54  welded, or otherwise fixedly attached, to the middle portion of the connecting pipe  53  and a joint  55  that is fixedly attached to the lower end of the connecting pipe  53 . The joint  55  has a quadric surface  55   a  as a rolling guide surface. The spherical joint  29  includes a pair of connecting rods  56  with springs  59  mounted thereon. Each of the connecting rod  56  has a threaded head  56   a  and an annular shoulder  56   b  that are formed as spring retainer. The connecting rods  56  are fixedly attached to the elliptical flange  54  by fastening nuts  58  to the threaded heads  56   a  through washers  57 , respectively. As shown also in FIG. 8, in connecting the exhaust pipe  19  to the spherical joint  29 , after mounting the connecting rods  56  to an annular flange  19   b  of the exhaust pipe  19 , the connecting rods  56  are fixedly attached to the elliptical flange  54  by fastening the nuts  58  so that a quadric surface  19   a  formed as a raceway of the annular flange  19   b  of the exhaust pipe  19  is remained in contact with the quadric rolling guide surface  55   a  by the springs  59 . By connecting the exhaust pipe  19  to the collective chambered pipe  28  through the spherical joint  29  thus structured, the collective chambered pipe  28  is prevented from receiving adverse influence of rolling vibrations of the engine  11 . 
     FIGS. 13 through 16 show a configuration of the upper insulation shell half  30   a  forming part of the insulation shell  30 . The upper insulation shell half  30   a  that has a T-shaped external configuration and a gate-shaped cross section covers the entire upper half section of the exhaust line from the fitting flange  23  of the exhaust manifold  18  to the downstream end of the collective chambered pipe  28 . As clearly shown in FIG. 16, the upper insulation shell half  30   a  comprises external and internal metal members  60  and  61  made of, for example, aluminum-plated metal plates and a heat insulation and sound absorption member  62  put between the external and internal wall metal members  60  and  61 . The upper insulation shell half  30   a  is entirely hemmed around by a folded flange  63 . As clearly shown in FIG. 13, the upper insulation shell half  30   a  having a T-shaped external configuration is formed as integral parts a plurality of projected ribs or beads  64 - 66  in a portion covering the collective chambered pipe  28 . Each of the beads  64 - 66  extends in a transverse direction perpendicular to a direction of exhaust gas flow between opposite sides of the upper insulation shell half  30   a.  The upper insulation shell half  30   a  at the beads  64 - 66  are formed with bolt holes  67 - 69 , respectively, for receiving fastening bolts for securing the upper insulation shell half  30   a  to the exhaust manifold  18 . Further, the upper insulation shell half  30   a  at a portion covering the upstream end portions of the single-shell discrete exhaust pipes  24 - 27  is formed with bolt holes  70  and  71  for receiving fastening bolts for securing the upper insulation shell half  30   a  to the exhaust manifold  18 . One of the two bolt holes  70  and  71 , namely the bolt hole  71  in this embodiment, is formed in the shape of oval in consideration of easy positional adjustment and installation relative to the exhaust manifold  18  and allowing thermal expansion of the upper insulation shell half  30   a.    
     FIGS. 17 through 20 show a configuration of the under insulation shell half  30   b  forming part of the insulation shell  30 . Similarly to the upper insulation shell half  30   a,  the under insulation shell half  30   b  that has a T-shaped external configuration and a gate-shaped cross section covers the entire under half section of the exhaust line from the fitting flange  23  of the exhaust manifold  18  to the downstream end of the collective chambered pipe  28 . As clearly shown in FIG. 20, the under insulation shell half  30   b  comprises external and internal wall metal members  72  and  73  made of, for example, aluminum-plated metal plates and a heat insulation and sound absorption member  62  put between the outer and inner wall metal members  60  and  61 . The under insulation shell half  30   b  is entirely hemmed around by a folded flange  65 . As clearly shown in FIG. 17, the under insulation shell half  30   b  having a T-shaped external configuration is formed as integral parts a plurality of projected beads or ribs  76 - 78  in a portion covering the collective chambered pipe  28 . Each of the beads  64 - 66  extends in a transverse direction perpendicular to a direction of exhaust gas flow between opposite sides of the under insulation shell half  30   b.  The under insulation shell half  30   b  are formed at the beads  76 - 78  with bolt holes  79 - 81 , respectively, and formed at a portion covering the upstream end portions of the single-shell discrete exhaust pipes  24 - 27  with bolt holes  82  and  83  for receiving fastening bolts for securing the under insulation shell half  30   b  to the exhaust manifold  18 . One of the two bolt holes  82  and  83 , namely the bolt hole  83  in this embodiment, is formed in the shape of oval in consideration of easy positional adjustment and installation relative to the exhaust manifold  18  and allowing thermal expansion of the under insulation shell half  30   b.  Further, the under insulation shell half  30   b  is formed with an aperture  84  between the beads  76  and  77  for allowing the supporting bracket  43  welded to the collective chambered pipe  28  to passes and extends to the bracket  45  fixedly attached to the cylinder block  12 . 
     The beads  64 - 66  of the upper insulation shell half  30   a  and the beads  76 - 78  of the under insulation shell half  30   b  form mating halves of three beads entirely surrounding the insulation shell  30  and serve as a reinforcement. 
     Referring back to FIG. 6, the fitting flange  23  of the exhaust manifold  18  is provided with two brackets secured thereto, namely a front bracket  85  disposed between the first and second single-shell discrete exhaust pipes  24  and  25  and a rear bracket  86  disposed between the third and fourth single-shell discrete exhaust pipes  26  and  27 . The front bracket  85  has upper and under fitting tongues  85   a  and  85   b.  Similarly, the rear bracket  86  has upper and under fitting tongues  86   a  and  86   b.    
     The insulation shell  30  is installed to the exhaust manifold  18  by bolting the upper and under insulation shell halves  30   a  and  30   b  separately to the exhaust line. Specifically, the upper insulation shell half  30   a  is bolted to the upper fitting tongues  85   a  and  86   a  of the brackets  85  and  86  secured to the fitting flange  23  through the bolt holes  70  and  71  of the upper insulation shell half  30   a,  respectively, and to the brackets  46 - 48  (see FIG. 6) secured to the external pipe shell  35  of the collective chambered pipe  28  through the bolt holes  67 - 69 , respectively. Similarly, the under insulation shell half  30   b  is bolted to the under fitting tongues  85   b  and  86   b  of the brackets  85  and  86  secured to the fitting flange  23  through the bolt holes  82  and  83  of the under insulation shell half  30   a,  respectively, and to the brackets  49 - 51  (see FIG. 7) secured to the external pipe shell  35  of the collective chambered pipe  28  through the bolt holes  70 - 81 , respectively. 
     FIGS. 21 and 22 show the gasket  22  through which the exhaust manifold  18  is installed to the cylinder head  13  of the engine  11 . The gasket  22  comprises a first gasket member  87  desirably formed from a stainless steel plate and a second gasket member  88  desirably formed from a steel plate. The first gasket member  87  is attached to under portion of the second gasket member  88 . The gasket  22  id formed with four apertures  89 - 92  corresponding in position to exhaust ports of the engine  11  in communication with the first to fourth cylinders (# 144 ), respectively, and a plurality of bolt holes  22   a - 22   g  corresponding in position to the bolt holes  23   a - 23   g  (see FIG. 6) formed in the fitting flange  23  of the exhaust manifold  18 . The upper portion of the second gasket member  88  extending beyond above the first gasket member  87  is bent back toward the exhaust manifold  18  so as to form a canopy fin  94  operative as a heat guide member to conduct and release exhaust heat therethrough. The canopy fin  94  extending toward the exhaust manifold  18  from the bend line  93 . The gasket  22  is formed with a plurality of reinforcing beads  95  arranged at specified locations between the canopy fin  94  and the middle portion of the second gasket  88  that is not backed by the first-gasket member  87 . Further, the second gasket member  88  at its upper edge of the canopy fin  94  is formed with recesses  94   a  and  94   b  engageable with the under fitting tongues  85   a  and  86   a  of the front and rear brackets  85  and  86 . The canopy fin  94  operates as a heat guide member to conduct and release exhaust heat from between the exhaust manifold  18  and the insulation shell  30  toward far from the engine  11 . 
     The gasket  22  thus structured prevents a rubber seal (not shown) for the gasket disposed between the cylinder head  13  and the head cover  14  and supplementary devices disposed around the engine  11  from encountering thermal deterioration. 
     In this instance, as shown in FIG. 2, the upper and under insulation shell halves  30   a  and  30   b  are adapted so that they are joined partly overlapping each other with a specified depth β along lengthwise sides. The joint gap of the overlapped sides of the upper and under insulation shell halves  30   a  and  30   b  is made as small as possible and desirably zero. Further, the upper and under insulation shell halves  30   a  and  30   b  are configured so as to provide a heat radiating space  31  having a specified distance L between the insulation shell  30  and the fitting flange  23  when the insulation shell  30  has been installed to the exhaust manifold  18 . This configuration of the insulation shell  30  that provides the heat radiating space  31  is advantageous to releasing exhaust heat from the heat radiating space  31  provided between the fitting flange  23  and the insulation shell  30  and causing the canopy fin  94  to conduct and release exhaust heat from the heat radiating space  31  toward far from the engine  11 . The space distance L is made greater than the joint gap between the overlapped sides β of the upper and under insulation shell halves  30   a  and  30   b  so as thereby to prevent exhaust heat from escaping from the insulation shell  30  through the joint gap but to radiate the exhaust heat to the exterior through the heat radiating space  31  between the insulation shell  30  and the fitting flange  23 . In other words, the insulation shell  30  has a heat emission-resistance greater at the heat radiating space  31  than at the joint gap. 
     According to the exhaust system shown by way of example and described in detail with reference to FIG. 1 through 22, the exhaust manifold  18  of the exhaust system for an automobile engine comprises a plurality of single-shell discrete exhaust pipes  24 - 27  connected to exhaust ports formed in the cylinder head  13 , respectively, the collective chambered pipe  28  joined to the single-shell discrete exhaust pipes  24 - 27  that is made up of an internal pipe shell  34  into which exhaust gas streams passing through the single-shell discrete exhaust pipes  24 - 27  are introduced and merge together and an external pipe shell  35  surrounding the internal pipe shell  34 , and the spherical joint  29  connecting the collective chambered pipe  28  to the exhaust pipe  19  disposed downstream the collective chambered pipe  28  and supported to the vehicle body so as to allow relative movement between said exhaust manifold and said exhaust pipe. Each of the single-shell discrete exhaust pipes  24 - 27  has a bent portion  32  and a straight portion  33  continuously extending from the bent portion  32  to the collective chambered pipe  28 . The bent portions of the single-shell discrete exhaust pipes  24 - 27  are bent toward a center of a straight row of cylinders so as to lay the straight portions of the single-shell discrete exhaust pipes nearly parallel to one another, and the single-shell discrete exhaust pipes  24 - 27  and the collective exhaust pipe  28  are joined to each other at least partly at a height of a plane including the interface  17   a  between the cylinder block  12  and the cylinder head  13 . 
     The exhaust manifold  18  thus structured can be provided with a long straight path of exhaust gas by the straight portions  33  of the single-shell discrete exhaust pipes  24 - 27  and the collective chambered pipe  28  that is straight, so as thereby to reduce the resistance of exhaust gas and, as a result, help the engine to raise output. In addition, the collective chambered pipe  28  can be provided with high heat insulation effectiveness by having a length as long as possible. This results in quick activation of the catalytic converter installed in the exhaust system. The discrete exhaust pipe  24 - 27  that is made of a single shell is easily bent. 
     The spherical joint  29  between the collective chambered pipe  28  and the exhaust pipe  19  allows relative movement between the exhaust manifold and the exhaust pipe. This eases undesirable transmission of rolling vibrations of the engine  11  to the exhaust pipe  19  through the exhaust manifold  18 . 
     The engine  11  is positioned so as to draw in intake air at the front side thereof and to discharge exhaust gases at the rear side thereof. Owing to this engine position, the exhaust manifold  18  is protected from exposure to the wind of speeding vehicle. This improves the high heat insulation effectiveness of the intake manifold  18 . In addition, this makes it possible to shorten the length of the exhaust path from the upstream ends of the single-shell discrete exhaust pipes  24 - 27  to the catalytic converter  21 . 
     The exhaust manifold  18  is provided with the fitting flange  23  connected to the upstream ends of the single-shell discrete exhaust pipes  24 - 27 . The bolt holes  23   a - 23   g  formed in the fitting flange  23  are arranged alternately along opposite upper and under sides of the fitting flange in a direction of the straight row of cylinders. At least one of the bolt holes  23   a - 23   g  that is in close proximity to the single-shell discrete exhaust pipes, specifically the bolt hole  23   c  in close proximity to the single-shell discrete exhaust pipes  25  and  26 , is located above a horizontal plane including centers of openings of the upstream ends of the single-shell discrete exhaust pipes  24 - 27 . This arrangement of the bolt holes  23   a - 23   g  provides the straight portion  33  of each of the single-shell discrete exhaust pipes  24 - 27  with a long path of exhaust gas a sufficient space for installation work using, for example, an impact wrench. In this connection, if the bolt hole  23   c  in close proximity to the single-shell discrete exhaust pipes  25  and  26  is located below horizontal plane including centers of openings of the upstream ends of the single-shell discrete exhaust pipes  24 - 27 , the bent portions  32  of the single-shell discrete exhaust pipes  24 - 27  must be long in consideration of a space for installation work using an impact wrench and the straight portions  33  of the single-shell discrete exhaust pipes  24 - 27  must be correspondingly shortened. 
     The exhaust manifold  18  includes the catalytic converter  21  disposed downstream from the collective chambered pipe  28  and under the vehicle body in close proximity to the collective chambered pipe  28 . This arrangement of the catalytic converter  21  can get rid of the necessity of installing a catalytic converter immediately after an exhaust manifold like an exhaust system of a front exhaust engine and, as a result, can be located as close to the collective chambered pipe  28  as possible so as to be quickly activated due to a hot exhaust gas. 
     The interior of the collective chambered pipe  28  is divided into two collective chambers, the first collective chamber  41  and a second collective chamber  42  located closer to the engine than the first collective chamber  41 , by the partition  36 . Exhaust gas streams passing through the single-shell discrete exhaust pipes  24  and  27 , respectively, for the cylinders at opposite ends of the straight row of cylinders enter into the first collective chamber and merge together. Exhaust gas streams passing through the single-shell discrete exhaust pipes  25  and  26  for the remaining cylinders enter into the second collective chamber  42  and merge together. The internal pipe shell  34  of the collective chambered pipe shell  34  extends straight in a direction of exhaust gas streams and gradually decreases in a cross-sectional area from the upstream end to the downstream end. 
     The collective chambered pipe  28  having the second collective chamber  42  located closer to the first collective chamber  41  makes it possible to employ the single-shell discrete exhaust pipes  24 - 27  that have the bent portions  32  shortened in length. The use of the partition  36  installed in the internal chambered pipe shell  34  makes the collective chambered pipe  28  compact as compared with using two independent internal pipe shells for dividing the interior of the collective chambered pipe  28  into the first and second collective chambers  41  and  42  and makes a surface area of the collective chambered pipe  28  small. This structure of the collective chambered pipe  28  reduces a thermal capacity, and hence heat radiation effectiveness, of the collective chambered pipe  28  and, in addition, gradually constricts an exhaust gas stream. In the case of installing the exhaust system to, in particular, a four-cylinder engine, a compact 4-2-1 type exhaust system, that is hardly affected by back pressure and exhaust gas pulsations, can be realized. 
     The exhaust manifold  18  further comprises the insulation shell  30  made up of two mating shell halves, i.e. the upper insulation shell half  30   a  and the under insulation shell half  30   b,  that covers the exhaust manifold  18 , in particular at least the single-shell discrete exhaust pipes  24 - 27  that are apt to radiate a comparatively large amount of heat as compared with the double-shell collective chambered pipe  18 . This prevents the exhaust manifold  18  from releasing heat to the atmosphere and, therefore, protects peripheral devices and parts from heat damage. Further, this provides the exhaust manifold  18  with high heat insulation effectiveness. 
     The spacer means  52  that is installed in a cylindrical space between the internal and external pipe shells  34  and  35  of the double-shell collective chambered pipe  28  separates the internal pipe shell  34  from the external pipe shell  35  mechanically so as to allow longitudinal expansion of the internal pipe shell  34  due to a difference of thermal expansion between the internal and external pipe shells  34  and  35 . Further, the spacer means  52  has supporting rigidity greater at a specified extent of its lower portion than at the remaining portion. The difference in supporting rigidity can be realized by differing at least at least one of material, width in the lengthwise direction, thickness, and mesh size for the lower portion from that for the remaining portion. 
     The exhaust manifold  18  in which the spacer means  52  is installed in a cylindrical space in the collective chambered pipe  28  prevents the internal pipe shell  34 , in particular the lower portion of the internal pipe shell  34 , from causing mechanical interference with the external pipe shell  35  due to vibrations. 
     The exhaust manifold  18 , that is structured suitably for the in-line four cylinder engine  11 , comprises the four discrete exhaust pipes  24 - 27  connected to the cylinder head  13  and the collective chambered pipe  28  that is provided with the partition  36  and is connected to the downstream ends of the respective discrete exhaust pipes  24 - 27 . The partition  36  divides the interior of the collective chambered pipe  28  into two collective chambers, the first collective chamber  41  into which exhaust gas streams passing through the discrete exhaust pipes  24  and  27  for the first and fourth cylinders at opposite extreme ends of the straight row of cylinders that do not sequentially adjoin in firing order and the second collective chamber  42  into which exhaust gas streams passing through the discrete exhaust pipes  25  and  26  for the second and third cylinders that do not sequentially adjoin in firing order. The partition  36  is installed in the collective chambered pipe  28  so as to position the first collective chamber  41  farther away from the engine  11  than the second collective chamber  42 . The partition  36  at its top end is fixedly attached to the joined discrete exhaust pipes  24 - 27  at the welded joint structure  28   a  between the collective chambered pipe  28  and the discrete exhaust pipes  24 - 27  that has high structural rigidity. In addition, the supporting bracket  43  for supporting the exhaust manifold to the engine  11  is fixedly attached to the collective chambered pipe  28  at or in close proximity to the welded joint structure  28   a  on the side of the second collective chamber  42 . The exhaust manifold  18  thus structured provides improved structural rigidity. The supporting bracket  43  is fixedly attached to the collective chambered pipe  28  far away from the partition  36 . This location of the supporting bracket  43  restrains transmission of vibrations of the engine  11  to the partition  36  and, as a result, prevents the partition  36  from casing noises due to vibrations. 
     The supporting bracket  43 , that comprises the bracket arm  43   a  and the flange tongue  43   b  connected as one right-angle piece by the curved portion  43   c,  is fixedly attached to the collective chambered pipe  28  at or in close proximity to a welded joint structure  28   a  between the collective chambered pipe  28  and the single-shell discrete exhaust pipes  24 - 27  on the side of the second collective chamber  42  and extends toward the engine  11 . The supporting bracket  43  having the curved portion  43   c  is helpful in easing thermal expansion of the exhaust manifold  18  and also in absorbing vibrations transmitted from the engine  11 . 
     The supporting bracket  43  is fixedly attached to the external pipe shell  35  downstream from the weld zone α (see FIG. 9) where the internal and external pipe shells  34  and  35  forming the double-shell collective chambered pipe  28  are welded to each other at their upstream ends. Because the supporting bracket  43  is fixedly attached to the double-shell collective chambered pipe  28  at a position where there is provided a space having a specified radial distance between the internal and external pipe shells  34  and  35 , the collective chambered pipe  28  is prevented from receiving adverse influence of vibrations of the engine  11 . In addition, the double-shell collective chambered pipe  28  can be provided with high heat insulation effectiveness, so that the catalytic converter installed in the exhaust system is quick activated. 
     As shown in FIG. 10, the single-shell discrete exhaust pipes  24 - 27  have downstream end portions, respectively, each of which comprises a rounded shell portion  24   a - 27   a  formed so as to meet partly the inner wall of the internal pipe shell  34  of the collective chambered pipe  28  and a right-angle shell portion  27   b - 27   b  and are arranged so as to form the cruciform space  37  among the adjacent right-angle shell portions  24   b - 27   b.  The cruciform space  37  is filled up with a cruciform reinforcement  38  made up of an upper extension  36   a  of the partition  36  of the collective chambered pipe  28  and a reinforcement strip  38   a  and welded to the inner right-angle shell portions  24   b - 27   b.  The cruciform reinforcement  38  avoids a presence of a joint gap among the first to forth exhaust pipes  24 - 27 . This structure increases weld strength of the welded joint structure  28   a.  Further, the partition  36  having the upper extension  36   a  forming part of the cruciform reinforcement  38  is welded to the internal pipe shell  34  of the collective chambered pipe  28 . This structure increases structural rigidity of the welded joint structure  28   a.    
     The insulation shell  30  is configured so as to cover the exhaust manifold  18  from the fitting flange  23  to the downstream end of the collective chambered pipe  28  and is made up of at least two mating shell halves, i.e. the upper insulation shell half  30   a  and the under insulation shell half  30   b.  The insulation shell  30  provides a heat radiating space having a specified distance L between the insulation shell  30  and the fitting flange  23  when installed to the exhaust manifold  18 . Further, the heat guide member extending toward far from the engine  11  is provided so as to conduct and release exhaust heat from the radiating space. The insulation shell  30  covers almost completely the exhaust manifold  18  and provides high heat insulation effectiveness. Further, the heat guide member conducts and releases exhaust heat from the radiating space toward far away from the engine  11  so as thereby to prevent the heat from going toward supplementary devices disposed around the engine  11  and a sealing gasket between the cylinder head  13  and the cylinder cover  14 . This structure keeps an exhaust gas passing through the exhaust manifold  18  as high as possible so as thereby to activate quickly the catalytic converter and prevents the supplementary devices and the sealing gasket supplementary devices from encountering thermal deterioration. The heat guide member is provided as an integral part of the gasket  22  disposed between the fitting flange  23  and the cylinder head  13 . This is helpful in conducting heat toward far from the engine even with a simple structure and contributive to reducing the number of parts and simplifying installation work. 
     The insulation shell  30  that comprises a plurality of shell members and can accordingly be formed in comparatively complicated configuration. The insulation shell  30  thus structured is suitable to cover an exhaust manifold formed in comparatively complicated configuration and is easily installed to the exhaust manifold. The insulation shell  30  has the overlapped sides β of the upper and under insulation shell halves  30   a  and  30   b  having a join gap between the overlapped sides β made as small as possible. On the other hand, the space distance L between the fitting flange  23  and the cylinder head  13  is made greater than the joint gap between the overlapped sides β of the upper and under insulation shell halves  30   a  and  30   b.  Accordingly, an exhaust heat is prevented from escaping from the insulation shell  30  through the joint gap of the overlapped sides of the upper and under insulation shell halves  30   a  and  30   b  but is allowed to be radiated to the exterior through the heat radiating space  31 . This enhances a heat radiating effect of the heat guide member. 
     The exhaust manifold  18  is suitably installed to a rear exhaust type in-line multiple cylinder engine having a straight row of cylinders arranged in a transverse direction. In this arrangement, the direction in which exhaust heat from the heat radiating space  31  is radiated through the heat guide member such (the canopy fin  94 ) is identical with the direction in which the wind of the speeding vehicle blows into the engine compartment. This results in protecting that the engine  11  and its associated supplementary devices from heat damage and prevents the engine compartment from being filled with heat. Further, the exhaust system  18 , in particular the discrete exhaust pipes  24 - 27  and the collective chambered pipe  28 , are prevented from being exposed to the wind of the speeding vehicle. This not only prevents the exhaust gas from lowering its temperature but makes the exhaust path from the engine  11  to the catalytic converter  21  shorter as compared with a front exhaust system. 
     The insulation shell  30  has a plurality of beads  64 - 66  and  76 - 78  arranged in the transverse direction as a reinforcement that are formed with bolt holes  67 - 69  and  79 - 81 , respectively. This insulation shell  30  thus structured is improved in its own structural rigidity. In addition, the insulation shell  30  is installed to the exhaust manifold  10  with high fitting rigidity by fastening bolts into the bolt holes  67 - 69  and  79 - 81 . The beads  64 - 66  and  76 - 78  also serves to absorb and ease thermal expansion of the insulation shell  30 . 
     FIG. 23 shows part of an exhaust manifold  28  of the exhaust system according to another embodiment of the invention that is almost the same in structure and operation as that of the previous embodiment except a welded joint structure  28   a.  Specifically, a collective chambered pipe  28  into which exhaust gases are collectively introduced through single-shell discrete exhaust pipes  24 - 27  (single-shell discrete exhaust pipes  26  and  27  are hidden behind single-shell discrete exhaust pipes  24  and  25 ) is of a double-shell type. The collective chambered pipe  28 , that is just the same in structure and operation as that of the previous embodiment, comprises an internal pipe shell  34  which an exhaust gas stream enters, an external pipe shell  35  surrounding the internal pipe shell  34  with an air space between the internal pipe shell  34  and a partition  36  that divides an upstream half portion of the interior of the collective chambered pipe  28  into two collective chambers, a first collective chamber  41  and a second collective chamber  42 . These internal pipe shell  34  and external pipe shell  35  are welded at their upstream ends to each other. 
     The exhaust manifold  18  is provided with a supporting bracket  43  is welded, or otherwise fixedly attached, to the collective chambered pipe  28  downstream from the welded zone α, more particularly, at or in close proximity to a welded joint structure  28   a  between the collective chambered pipe  28  and the single-shell discrete exhaust pipes  24 - 27  on the side of the second collective chamber  42  and extending toward the engine  11 . The supporting bracket  43  is fixedly connected to a bracket  45  secured to a cylinder block  12  of the engine  11  as shown in FIG. 1 by bolt and nut fastening means  44  (see FIG.  2 ). The supporting bracket  43  has a bracket arm  53   a  and a flange tongue  43   b  connected as one right-angle piece by a curved portion  43   c.  The flange tongue  43   b  is bent upward at an approximately right angle in this embodiment. The supporting bracket  43  at its upper end of the flange tongue  43   b  is welded to the upstream end of the external pipe shell  34 . 
     According to this embodiment, the supporting bracket  43  is welded at a welded joint structure  28   a  at which the internal pipe shell  34 , the external pipe shell  35  and the discrete exhaust pipes  25  and  26  are collectively joined, so that the weld strength at the welded joint structure  28   a  is improved and, in addition, the welding work is simplified as compared with the welded joint structure  28   a  of the exhaust manifold  28  of the previous embodiment shown in FIG.  9 . 
     FIG. 24 shows part of an exhaust manifold  28  of the exhaust system according to another embodiment of the invention that is almost the same in structure and operation as that of the previous embodiment, except a partition  36  that is installed in a collective chambered pipe  28  in order to divide the interior of the collective chambered pipe  28  into two collective chambers, namely a first collective chamber  41  and a second collective chamber  42 . The partition  36  has a lower extension  36   b  extending into the interior of the exhaust pipe  19 . The lower extension  36   b  may be provided as an independent additional partition separately from the partition  36 . In this case, the independent partition member is installed in the exhaust pipe  19  with a clearance between the partition  36  that is appropriate for the independent additional partition to prevent the independent additional partition from causing mechanical interference with the partition  36  due to relative movement between the collective chambered pipe  18  and the exhaust pipe  19  and from having no adverse effect on the exhaust gas flow. The exhaust manifold  28  provided with the partition  36  having the lower extension  36   b  into the interior of the exhaust pipe  19  can form the 4-2-1 type exhaust system so as to make the two-channel part of exhaust path long. 
     FIG. 25 shows part of an exhaust manifold  28  of the exhaust system according to still another embodiment of the invention that is almost the same in structure and operation as that of the previous embodiment, except an exhaust pipe  19 . The exhaust pipe  19 , that is of a double-shell type, comprises an internal pipe shell  96  through which an exhaust gas stream enters and an external pipe shell  97  surrounding the internal pipe shell  96  with an air space between the internal pipe shell  97 . The exhaust system provided with the double-shell exhaust pipe  19  in addition to the collective chambered pipe  28  has improved high heat insulation effectiveness. 
     FIG. 26 shows part of an exhaust manifold  28  of the exhaust system according to a further embodiment of the invention that is almost the same in structure and operation as those of the previous embodiment, except a gasket  22  through which the exhaust manifold  18  is installed to a cylinder head  13  of the engine  11 . The gasket  23  is different from those of the previous embodiment in that the gasket  23  has no canopy fin as a heat guide member. Specifically, the gasket  22  comprises a first gasket member and a second gasket member that however has no upper portion beyond above the first gasket member. In this embodiment, a heat guide member  98  is similar to the canopy fin  94  of the previous embodiment and is however formed separately from the gasket  22 . The heat guide member  98  comprises a mounting portion  98   a  through which the heat guide member  98  is fixedly mounted to the cylinder head  13  by means of bolts  99  and a heat guide portion  98   b  that is bent forward up so as to conduct and guide exhaust heat from the heat radiating space between the exhaust manifold  18  and the insulation shell  30  toward far from the engine  11 . This heat guide member  98  prevents a rubber seal (not shown) for the gasket disposed between the cylinder head  13  and the head cover  14  and supplementary devices disposed around the engine  11  from encountering thermal deterioration. 
     As described above, according to the exhaust system of the present invention, the exhaust manifold helps the engine to raise output due to a long straight path of exhaust gas that is provided by the straight portions  33  of the single-shell discrete exhaust pipes  24 - 27  and the collective chambered pipe  28  that is straight. In addition, the exhaust manifold provides improvement of heat insulation effectiveness by making the double-shell collective chambered pipe as long as possible. The discrete exhaust pipe that is of a single shell type makes bending work quite easy. 
     According to the structure of the exhaust manifold in which the partition is welded to the joint between the discrete exhaust pipes and the collective chambered pipe and the supporting bracket is welded to the collective chambered pipe  28  at or in close proximity to the joint that has a high structural rigidity, the exhaust manifold is provided with an improved high structural rigidity. Further, since these partition and supporting bracket are separately located as far away from each other as possible, the partition is prevented from receiving vibrations of the engine and, accordingly, from producing noises resulting from vibrations. 
     Further, according to the structure of the exhaust manifold in which there are provided the heat radiating space between the insulation shell and the fitting flange sufficient to radiate exhaust heat and the heat guide member extending toward far from the engine operative to conduct exhaust heat radiated from the heat radiating space toward far from the engine. This structure of the exhaust manifold prevents the gasket disposed between the cylinder head and the head cover and supplementary devices disposed around the engine from encountering thermal deterioration. 
     The present invention has been described with reference to preferred embodiments thereof. However, it will be appreciated that variants and other embodiments can be effected by person of ordinary skill in the art without departing from the scope of the invention.