Abstract:
An exhaust manifold is constructed of material varying in thickness. Auxiliary cooling, such as from an air flow, to a portion of the exhaust manifold, permits making this portion thinner than a portion shielded from the auxiliary cooling. This lowers the heat capacity of the thinner material, allowing the exhaust manifold to heat rapidly to the activation temperature of a catalyst. Thus, the catalyst is capable of removing harmful elements from the exhaust gases of an internal combustion engine more quickly, thereby reducing pollution to the atmosphere. Furthermore, an exhaust manifold having this structure is lighter and requires less material than conventional exhaust manifolds, thereby making production easier and less costly.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an exhaust manifold for internal combustion engines. More specifically, the present invention relates to an exhaust manifold for internal combustion engines that allows early activation of a catalyst immediately after starting, reduces weight and costs, increases joining precision between plate materials, and improves welding precision. 
     Internal combustion engines have an exhaust manifold to collect exhaust gas discharged from gas columns. A catalyst is disposed following the exhaust manifold to purge harmful components of the exhaust gas from the exhaust manifold. 
     Exhaust manifolds for internal combustion engines include exhaust manifolds integrally formed by casting, as well as exhaust manifolds formed by joining a plurality of plate materials. 
     Examples of these exhaust manifolds for internal combustion engines are disclosed in Japanese examined utility model publication number 8-7055, Japanese laid-open patent publication number 10-89064, Japanese laid-open patent publication number 8-260958, Japanese laid-open patent publication number 9-317462, and Japanese laid-open patent publication number 10-89060. 
     Japanese examined utility model publication number 8-7055 discloses two plate materials joined to form an exhaust pipe collecting section. Of the two plate materials, the one that faces the main engine unit is formed thinner than the plate material positioned on the other side of the main engine unit. This difference in thickness generates a difference in vibration frequencies between the two plate materials, thereby reducing vibration noise. Also, the thicker plate material on the opposite side from the main engine unit restricts the transmission of exhaust noise. 
     Japanese laid-open patent publication number 10-89064 discloses the joining together of a front half, a partitioning body, and a rear half, each formed as plates. A plurality of exhaust pipes and confluence sections between two exhaust pipes are formed from the partitioning body and either the front half or the rear half. This allows for the reduced thickness and weight of the exhaust manifold. 
     Japanese laid-open patent publication number 8-260958 discloses junctures between the branching pipes that are made thicker than the other sections. The thickness is made greatest at the juncture disposed at the longitudinal center of the cylinder head. This increases the compressive stress generated at the junctures. 
     Japanese laid-open patent publication number 9-317462 discloses an outer pipe and an inner pipe, supported in the outer pipe, so that the two are separated by a gap. The outer pipe is formed so that the gap is larger near the engine attachment flange. This allows the thermal transfer from the inner pipe to the outer pipe to be reduced while allowing the exhaust temperature guided to the catalyst to quickly rise when the engine is started. 
     Japanese laid-open patent publication number 10-89060 discloses a vertically oriented engine having an exhaust port with an exit-side opening positioned higher toward the front or the rear of the automobile. The branching pipes, continuous with the exit-side opening, are positioned outward. This reduces the overlap between the branching pipes, when seen from the front of the automobile. As a result, the variations in the running airstreams that come into contact with the branching pipes are reduced and thermal warping is prevented. 
     Internal combustion engines use a catalyst to reduce harmful elements in the exhaust gas. The catalyst efficiently purges harmful elements when the catalyst temperature reaches its activation temperature. 
     In recent years, there has been an increasing demand for reducing harmful elements in the exhaust gas. In particular, there has been a demand to reduce the harmful elements in the exhaust gas that is discharged immediately after an internal combustion engine is started, since, at this time, the catalyst temperature is too low for the catalyst to be effective in removing the harmful elements. 
     For this reason, an exhaust manifold for internal combustion engines is formed by joining plate material having a smaller heat capacity than that of an exhaust manifold formed by casting. This allows the catalyst temperature to rise to the activation temperature quickly after the engine is started, thus providing early activation of the purging effect. 
     Referring to FIG. 12, there is shown an example of this type of exhaust manifold for internal combustion engines. Referring to FIG. 12, there is shown an engine compartment  102  for an automobile (not shown in the figure). An internal combustion engine  104 , a cylinder head  106 , and an exhaust manifold  108  are mounted in engine compartment  102 . Exhaust manifold  108  is formed by welding an upper case  110  and a lower case  112 . Upper case  110  and lower case  112  are formed as two metal sheets. 
     Exhaust manifold  108  attaches to cylinder head  106  with a head attachment flange  114 . A catalyst (not shown in the figure) is attached to a catalyst attachment flange  116 . When internal combustion engine  104  is mounted sideways in engine compartment  102  toward the front of the automobile (not shown in the figure), exhaust manifold  108  is disposed to the front of internal combustion engine  104 . 
     Exhaust manifold  108  is generally formed so that a sheet thickness t 1  of upper case  110  and a sheet thickness t 2  of lower case  112  are identical (t 1 =t 2 ). Exhaust manifold  108  is cooled by air currents flowing through engine compartment  110  such as cooling air from a radiator fan (not shown in the figure) and running airflow. 
     Since upper case  110  is positioned further toward the front than lower case  112 , relative to the direction of the air currents, upper case  110  is cooled more than lower case  112 . 
     The stress tolerance of the metal sheets forming upper case  110  and lower case  112  increases for lower temperatures. Also, the heat capacity of the metal sheets is smaller if the thickness of the sheets is smaller. 
     Upper case  110  and lower case  112  are formed with the same sheet thickness (t 1 =t 2 ) based on the stress tolerance of lower case  112 , which receives less cooling from air flows. As a result, there is excess strength in upper case  110 , which is cooled more than lower case  112 , and therefore has a larger stress tolerance. This increases the amount of required materials, the weight, and the production costs. Furthermore, upper case  110  has a higher heat capacity. In particular, the temperature of the exhaust gas sent to the catalyst immediately after internal combustion engine  102  is started is reduced, thus lengthening the time required for the catalyst to be heated to its activation temperature. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an exhaust manifold for an internal combustion engine which overcomes the foregoing problems. 
     It is a further object of the present invention to provide an exhaust manifold for an internal combustion engine which allows for activation of a catalyst immediately after starting the internal combustion engine. 
     It is another object of the present invention to provide an exhaust manifold for an internal combustion engine which reduces weight and costs, increases joining precision between plate materials, and improves welding precision. 
     The present invention provides an exhaust manifold disposed to collect exhaust gas from gas columns of an internal combustion engine mounted in an engine compartment of an automobile. The exhaust manifold is formed by joining at least two sheet materials. One of the sheet materials, positioned toward the front, relative to a direction of an air current flowing through the engine compartment, is formed with a thickness less than a thickness of an other sheet material, positioned toward the rear, relative to the current flow direction 
     Briefly stated, the present invention provides an exhaust manifold constructed of material varying in thickness. Auxiliary cooling, such as from an air flow, to a portion of the exhaust manifold, permits making this portion thinner than a portion shielded from the auxiliary cooling. This lowers the heat capacity of the thinner material, allowing the exhaust manifold to heat rapidly to the activation temperature of a catalyst. Thus, the catalyst is capable of removing harmful elements from the exhaust gases of an internal combustion engine more quickly, thereby reducing pollution to the atmosphere. Furthermore, an exhaust manifold having this structure is lighter and requires less material than conventional exhaust manifolds, thereby making production easier and less costly. 
     According to an embodiment of the present invention, there is provided an exhaust manifold comprising: at least one structure for receiving exhaust gas from an engine; the at least one structure having a first section, having a first thickness, which receives auxiliary cooling during operation of the engine, and a second section, having a second thickness; the second section receiving less auxiliary cooling than said first section; and the first thickness being less than the second thickness. 
     According to another embodiment of the present invention, there is provided an exhaust manifold for a V-shaped internal combustion engine, comprising: a first, front manifold structure receiving exhaust gas from a first side of the V-shaped internal combustion engine; a second, rear manifold structure receiving exhaust gas from a second side of the V-shaped internal combustion engine; the first, front manifold structure receiving auxiliary cooling from operation of the V-shaped internal combustion engine; and the first, front manifold structure being made from a material having a thickness less than the second, rear manifold structure, thereby reducing the overall heat capacity of the exhaust manifold. 
     According to a feature of the present invention, there is provided an exhaust manifold for an internal combustion engine comprising: a plurality of branching pipes, each receiving an exhaust gas from the internal combustion engine; the plurality of branching pipes connecting to a connecting pipe; each of the plurality of branching pipes having at least a first thickness and a second thickness; the first thickness, receiving auxiliary cooling during operation of the internal combustion engine, being thinner than the second thickness, thereby reducing the overall heat capacity of the exhaust manifold. 
    
    
     The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-section detail drawing along the I—I line from FIG. 4 of an exhaust manifold for internal combustion engines according to an embodiment of the present invention. 
     FIG. 2 is a cross-section drawing of an exhaust manifold. 
     FIG. 3 is a plan drawing of an exhaust manifold. 
     FIG. 4 is a front-view drawing of an exhaust manifold. 
     FIG. 5 is a perspective drawing of an exhaust manifold. 
     FIG. 6 is a transparent side-view drawing of an automobile. 
     FIG. 7 is a cross-section detail drawing of an exhaust manifold according to another embodiment. 
     FIG. 8 is a perspective drawing of an exhaust manifold according to a first alternative embodiment. 
     FIG. 9 is an enlarged cross-section drawing of an exhaust manifold according to a first alternative embodiment. 
     FIG. 10 is a perspective drawing of an exhaust manifold according to a second alternative embodiment. 
     FIG. 11 is an enlarged cross-section drawing of an exhaust manifold according to the second alternative embodiment. 
     FIG. 12 is a schematic side-view drawing of a conventional exhaust manifold. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An exhaust manifold for internal combustion engines, according to the present invention, is formed by joining at least two sheets. These sheets are formed so that the thickness of the sheet positioned toward the front, relative to the air current flow in an engine compartment, is less than the thickness of the sheet positioned toward the rear, relative to the flow. Since the sheet positioned toward the front, relative to the air current flow, is efficiently cooled, there is greater stress tolerance. Thus, the front sheet can be made thinner, without resulting in reduced strength. This provides decreased heat capacity and reduces the amount of required material. The decrease in heat capacity permits more rapid heating, and thus earlier effectiveness of the catalyst. 
     Referring to FIG. 6, automobile  2  has an engine compartment  4  which contains an internal combustion engine  6 . A transmission  8  attaches internal combustion engine  6  to a front wheel  10  and a rear wheel  92 . Automobile  2  acquires drive force by using transmission  8  to convert the drive force from internal combustion engine  6  mounted toward the front of automobile  2 . 
     A radiator  12  is in front of internal combustion engine  6 . A radiator fan  14  is positioned to cool radiator  12 . 
     An exhaust manifold  18  attaches to a cylinder head  16  of internal combustion engine  6  in order to collect the exhaust gas discharged from the gas columns (not shown in the figure) toward the front of automobile  2 . Exhaust manifold  18  connects, in sequence, to a catalyst  20 , an exhaust gas pipe  22 , and a muffler  24 . The exhaust gas, collected by exhaust manifold  18 , is purged of harmful elements by catalyst  20  and discharged to the atmosphere by exhaust gas pipe  22  through muffler  24 . Exhaust manifold  18  is formed by the joining together of at least two sheets. 
     Referring to FIGS. 2 through 5, exhaust manifold  18 , according to this embodiment of the present invention, includes branching pipes  26 , preferably formed from cylindrical sheets, and an upper case  28  and a lower case  30 , preferably formed from bent sheets. 
     Referring to FIG. 1, exhaust manifold  18  is formed so that a thickness t 1  of upper case  28  and a thickness t 2  of lower case  30  are different. Thickness t 1  of upper case  28 , which is positioned to the front, relative to the air current flow, in engine compartment  4 , is thinner than thickness t 2  of lower case  30 , which is positioned to the rear, relative to the air current flow, (t 1 &lt;t 2 ). 
     Each of the plurality of branching pipes  26  is cylindrically formed from sheet material, with one end fixed to a heat attachment flange  32 . A head attachment opening  34  (see FIG. 5) is disposed on heat attachment flange  32 . 
     Upper case  28  is formed from a sheet material bent roughly in the shape of a crescent. An upper joining section  36  is disposed at the edge of one lateral side. On the other lateral side is disposed a plurality of upper fixing sections  38 , which are formed as half-cylinders fixed to semicircular perimeter sections of branching pipes  26 . At the edges of this other lateral side, between upper fixing sections  38 , are disposed upper offset joining sections  40 . 
     Lower case  30  is formed from a sheet material bent roughly in the shape of a crescent. A lower joining section  42 , which joins with upper joining section  36 , is disposed at an edge of one lateral side. On the other lateral side, a plurality of lower fixing sections  44 , which are formed as half-cylinders that fit against upper fixing sections  38 , are positioned. Flat lower joining sections  46  are disposed at the edges of this other lateral side between lower fixing sections  44 . The lower fixing sections  44  engage with and are joined to upper offset joining sections  40 . 
     A collecting section  48 , extending in a downward direction in FIG. 1, is positioned on lower case  30 . A catalyst attachment flange  50  is fixed to the open end of collecting section  48 . A catalyst attachment opening  52  is formed on catalyst attachment flange  50 . 
     Upper case  28  and lower case  30  are joined by abutting the lateral sides of upper joining section  36  and lower joining section  42 . On the other lateral side, upper fixing section  38  is fitted to lower fixing section  44 . The ends of branching pipes  26  are fixed, and upper offset joining sections  40  and lower joining sections  46  on the other lateral side are engaged and joined to form exhaust manifold  18 . 
     Exhaust manifold  18  attaches to cylinder head  16  by inserting a head-side attachment bolt (not shown in the figure) disposed on cylinder head  16  through head attachment opening  34  on head attachment flange  32  and screwing an attachment nut (not shown in the figure) to the bolt. 
     Referring to FIGS. 2 and 3, catalyst  20  attaches to exhaust manifold  18  by inserting an attachment bolt (not shown in the figure), disposed on catalyst  20 , through catalyst attachment opening  52 , formed on catalyst attachment flange  50 , and screwing on an attachment nut  54 . An O 2  sensor attachment boss  56 , a flange-side cover attachment bracket  58 , a case-side cover attachment bracket  60 , and an EGR pipe  62  are each attached to exhaust manifold  18 . 
     The following is a description of the operations performed by the structure described above. 
     Exhaust manifold  18  collects exhaust gas from the gas columns of internal combustion engine  6 , which is mounted in engine compartment  4  of automobile  2 . Harmful elements are purged by catalyst  20 . The exhaust gas is then discharged to the outside atmosphere through exhaust gas pipe  22  via muffler  24 . 
     Referring to FIG. 1, exhaust manifold  18 , which collects the exhaust gas from the gas cylinders of internal combustion engine  6 , is formed with thickness t 1  of upper case  28  being different from thickness t 2  of lower case  30 . Upper case  28  and lower case  30  are formed so that thickness t 1  of upper case  28 , positioned toward the front, relative to the air current flow in engine compartment  4 , is less than thickness t 2  of lower case  30 , positioned toward the rear, relative to the air current flow (t 1 &lt;t 2 ). 
     As a result, exhaust manifold  18  is cooled efficiently if it is positioned toward the front, relative to the air current flow from radiator fan  14 , air currents flowing through engine compartment  4 , or the like. This increases the stress tolerance of exhaust manifold  18 , thus allowing upper case  28 , positioned toward the front, relative to the air current flow, to be formed with thinner sheets without a loss of strength. This allows the heat capacity of upper case  28  to be reduced in addition to reducing the amount of materials required. 
     Since the above exhaust manifold  18  allows the heat capacity to be reduced, the exhaust gas can be guided to catalyst  20  immediately after internal combustion engine  2  is started, with reduced drop in the exhaust gas temperature. This allows catalyst  20  to be heated to its activation temperature in a shorter period of time, reducing the time, after starting the engine, for catalyst  20  to be activated to purge harmful elements. Also, by reducing the amount of required materials, the structure is made lighter and less expensive. 
     Referring to FIG. 2, in exhaust manifold  18 , attachment nut  54  is screwed onto the attachment bolt (not shown in the figure) of catalyst  20  inserted into catalyst attachment opening  52  of catalyst attachment flange  50 . Attachment nut  54  is tightened using a tool  64 . 
     Thus, if there is a shift in the joining between upper case  28  and lower case  30 , a distance L 1  from a tightening center C to an outer edge of tool  64  may be less than a distance L 2  to an outer edge of upper case  28 . This would obstruct the use of tool  64 . 
     To prevent this, exhaust manifold  18  is formed so that an upper offset joining section  40  is disposed as an offset at the edge of upper case  28 , having thickness t 1 . This ensures that distance L 2 , from tightening center C to the outer edge of upper case  28 , is larger than distance L 1 , from tightening center C to the outer edge of tool  64  (L 1 &lt;L 2 ). Upper offset joining section  40  is engaged and joined to lower joining section  46  at the edge of thicker lower case  30 , having thickness t 2 . 
     By having upper offset joining section  40 , which is disposed at the edge of thinner upper case  28 , having thickness t 1 , engaged with lower joining section  46  of the edge of thicker lower case  30 , having thickness t 2 , upper case  28  and lower case  30  are accurately positioned when they are joined. Thus, the joining precision and the welding precision of exhaust manifold  18  is improved. Furthermore, shifting between upper case  28  and lower case  30  is prevented. This prevents the obstructions to the operation of tool  64 . Also, by forming upper offset joining section  40  at the edge of thinner upper case  28 , with thickness t 1 , the structure is easily formed. 
     Referring to FIG. 7, in this alternate embodiment of the present invention, upper case  28  and lower case  30  of exhaust manifold  18  are formed so that the sections positioned toward the front, relative to the direction of the air current flow in engine compartment  4 , are formed thinner than the sections positioned toward the rear, relative to the current flow. 
     In exhaust manifold  18 , if the thickness of the section of upper case  28  toward the front, relative to the air current flow, is t 1 , the thickness of the section of upper case  28  toward the rear, relative to the air current flow, is t 2 , the thickness of the section of lower case  30  toward the front, relative to the air current flow, is t 3 , and the thickness of the section of lower case  30  toward the rear, relative to the air current flow, is t 4 , then the thicknesses are formed at least so that t 1 &lt;t 2  or at least so that t 3 &lt;t 4 , with the relation between t 2  and t 3  being unimportant. Furthermore, it is also possible for the relationship between the thicknesses to be t 1 &lt;t 2 &lt;=t 3 &lt;=t 4 , t 1 &lt;=t 2 &lt;t 3 &lt;=t 4 , or t 1 &lt;t 2 &lt;t 3 &lt;t 4 . 
     Thus, in exhaust manifold  18  according to this alternate embodiment of the present invention, the differences in cooling states, depending on the position relative to the direction of air current flow, is reflected in the thicknesses of upper case  28  and lower case  30  so that they are thinly formed without reducing their strength. This provides reduced heat capacity and requires less materials. 
     Thus, as with the previous embodiment, exhaust manifold  18 , according to this alternate embodiment of the present invention, guides exhaust gas to catalyst  20  immediately after internal combustion engine  2  is started, without resulting in a drop in the exhaust gas temperature. This reduces the time it takes for the temperature of catalyst  20  to rise to its activation temperature, thus allowing catalyst  20  to be activated quickly, once the engine is started, so that it can purge harmful elements. Also, the resulting structure is made lighter and less expensive. 
     The present invention is not restricted to the embodiments described above, and various modifications may be made. 
     Referring to FIG. 8, there is shown a second alternative embodiment of the present invention. In this embodiment, a V-shaped internal combustion engine  68  is mounted horizontally in an engine compartment  66  of an automobile (not shown in the figure). Side exhaust manifolds  70  and  72  collect the exhaust gas from the gas columns of internal combustion engine  68 . A first exhaust manifold  70  is formed from a first upper case  74  and a first lower case  76 . A second exhaust manifold  72  is formed from a second upper case  78  and a second lower case  80 . 
     With exhaust manifolds  70  and  72 , first upper case  74 , which is positioned toward the front and top of the air current flow, has a thickness of t 1 , and first lower case  76 , which is positioned toward the front and the bottom of the air current flow, has a thickness of t 2 . Second upper case  78 , which is positioned toward the rear and the top of the air current flow, has a thickness of t 3 , and second lower case  80 , which is positioned toward the rear and the bottom of the air current flow, has a thickness of t 4 . In this case, the thicknesses are formed with t 1 &lt;t 2 &lt;=t 3 &lt;=t 4 , or t 1 &lt;=t 2 &lt;t 3 &lt;=t 4 , or t 1 &lt;=t 2 &lt;=t 3 &lt;=t 4 . 
     With this structure according to the second alternative embodiment of the present invention, the differences in cooling states based on the positions relative to the air current flow are reflected in exhaust manifolds  70  and  72 . This allows upper cases  74  and  78  and lower cases  76  and  80  to be formed appropriately thin without leading to a reduction in strength. As a result, the heat capacity is decreased and less materials are required. 
     As with the previous embodiment, exhaust manifolds  70  and  72 , according to this second alternative embodiment of the present invention, reduces the time required for catalyst  20  to reach its activation temperature, thus allowing catalyst  20  to be quickly activated after the engine is started so that it can purge harmful elements. Also, the structure is made lighter and less expensive. 
     As with the embodiment shown in FIG. 7, exhaust manifolds  70  and  72 , according to the second alternative embodiment of the present invention, are formed so that the thicknesses toward the front, relative to the air flow direction in engine compartment  66 , are less than the thicknesses toward the rear, relative to the air flow direction. 
     Referring to FIG. 9, in, for example, exhaust manifold  70 , the thickness toward the front, relative to the air flow direction, of upper case  74  is t 1 f, the thickness toward the rear, relative to the air flow direction, of upper case  74  is t 1 r, the thickness toward the front, relative to the air flow direction, of lower case  76  is t 2 f, and the thicken toward the rear, relative to the air flow direction of lower case  76  is t 2 r. The thicknesses are such that at least t 1 f&lt;t 1 r or at least t 2 f&lt;t 2 r, with the relative values of t 1 r and t 2 f being arbitrary. Furthermore, it is also possible to use thicknesses where t 1 f&lt;t 1 r&lt;=t 2 f&lt;=t 2 r, t 1 f&lt;=t 1 r&lt;t 2 f&lt;=t 2 r, or t 1 f&lt;t 1 r&lt;t 2 f&lt;t 2 r. 
     As with exhaust manifold  70 , exhaust manifold  72  is formed so that the thicknesses toward the front and rear, relative to the air flow direction, of upper case  78  are t 3 f and t 3 r, and the thicknesses toward the front and rear, relative to the air flow direction, of lower case  80  are t 4 f and t 4 r. The thicknesses are such that at least t 3 f&lt;t 3 r or at least t 4 f&lt;t 4 r, with the relative values of t 3 r and t 4 r being arbitrary. Furthermore, it is also possible to use thicknesses where t 3 f&lt;t 3 r&lt;=t 4 f&lt;=t 4 r, t 3 f&lt;=t 3 r&lt;t 4 f&lt;=t 4 r, or t 3 f&lt;t 3 r&lt;t 4 f&lt;t 4 r. 
     With this structure, exhaust manifolds  70  and  72 , according to the second alternative embodiment of the present invention, reflect the different cooling that takes place depending on the position relative to the air flow direction. This allows the thicknesses of upper case  74 , lower case  76 , upper case  78 , and lower case  80  to be formed appropriately thin without resulting in reduced strength. This provides reduced thermal capacity and further reduces the amount of required materials. 
     Thus, as with the embodiment described above, exhaust manifolds  70  and  72  of the second alternative embodiment of the present invention guide the exhaust gas to catalyst  20  immediately after internal combustion engine  2  is started, without reducing the exhaust gas temperature. This shortens the time required for catalyst  20  to rise to its activation temperature so that, after staring, catalyst  20  is quickly activated to eliminate harmful elements. Furthermore, the resulting structure is made lighter and costs are reduced. 
     Referring to FIG. 10, in a third alternative embodiment of the present invention, an exhaust manifold  86  is disposed to collect the exhaust gas from gas columns (not shown in the figure) of an internal combustion engine  84  mounted vertically in an engine compartment  82  of an automobile (not shown in the figure). Exhaust manifold  86  is formed from a collecting pipe  90 , formed from a sheet material in a cylindrical shape, and a plurality of branching pipes  88 - 1 - 88 - 4 , formed from sheet materials in cylindrical shapes. 
     Branching pipe  88 - 1 , positioned at the very front relative to the direction of airflow, has a thickness t 1 , branching pipe  88 - 2 , positioned second from the front relative to the direction of airflow, has a thickness t 2 , branching pipe  88 - 3 , positioned third from the front relative to the direction of airflow, has a thickness t 3 , and branching pipe  88 - 4 , positioned at the very rear relative to the direction of airflow, has a thickness t 4 . The structure is formed so that the thicknesses are t 1 &lt;t 2 &lt;=t 3 &lt;=t 4 , t 1 &lt;=t 2 &lt;t 3 &lt;=t 4 , or t 1 &lt;=t 2 &lt;=t 3 &lt;=t 4 . 
     With this structure, exhaust manifold  86 , according to the third alternative embodiment of the present invention, reflects the differences in cooling states based on the position relative to the direction of airflow. This allows branching pipes  88 - 1 - 88 - 4  to be formed appropriately thin without reducing their strength. As a result, heat capacity is reduced and less materials are required. 
     Thus, as with the embodiments described above, exhaust manifold  86 , according to this third alternative embodiment of the present invention, reduces the time required for catalyst  20  to reach its activation temperature so that catalyst  20  is quickly activated to purge harmful elements after the engine is started. Also, the structure is made lighter and less expensive. 
     As with the embodiment shown in FIG. 7, exhaust manifold  86 , according to the third alternative embodiment of the present invention, is formed so that, for branching pipes  88 - 1 - 88 - 4 , the thicknesses toward the front, relative to the direction of airflow in engine compartment  4 , is smaller than the thicknesses toward the rear, relative to the direction of airflow. 
     Referring to FIG. 11, exhaust manifold  86  can, for example, be formed so that the thicknesses of branching pipe  88 - 1  toward the front and rear, relative to the direction of airflow, are t 1 f and t 1 r, respectively. The thicknesses of branching pipe  88 - 2  toward the front and rear, relative to the direction of airflow, are t 2 f and t 2 r, respectively. The thicknesses of branching pipe  88 - 3  toward the front and rear, relative to the direction of airflow, are t 3 f and t 3 r, respectively. The thicknesses of branching pipe  88 - 4  toward the front and rear, relative to the direction of airflow, are t 4 f and t 4 r, respectively. The structure is formed so that at least t 1 f&lt;t 1 r, or at least t 2 f&lt;t 2 r, or at least t 3 f&lt;t 3 r, or at least t 4 f&lt;t 4 r, where the relative sizes of t 1 r and t 2 f, t 2 r and t 3 f, t 3 r and t 4 f are unimportant. 
     With this structure, exhaust manifold  86 , according to this third alternative embodiment of the present invention, reflects the different cooling that takes place depending on the position relative to the direction of airflow. Thus, the thickness of the front and rear, relative to the direction of airflow, of branching pipes  88 - 1 - 88 - 4  is appropriately reduced without resulting in reduced strength. Furthermore, thermal capacity is lowered and the amount of materials required is reduced. 
     Thus, as with the embodiment described above, exhaust manifold  86  of the third alternative embodiment of the present invention, guides the exhaust gas to catalyst  20  immediately after internal combustion engine  2  is started, without reducing the exhaust gas temperature. This shortens the time required for catalyst  20  to rise to its activation temperature so that, after starting, catalyst  20  is quickly activated to eliminate harmful elements. Furthermore, the structure is made lighter and costs are reduced. 
     As described above, the exhaust manifold for internal combustion engines according to the present invention takes advantage of the fact that stress tolerance is increased if the manifold is cooled efficiently by being positioned toward the front, relative to the direction of airflow. Thus, the sheet material is formed thin without having the strength reduced. This reduces heat capacity and requires less material. 
     Since the heat capacity is reduced in this exhaust manifold, the exhaust gas is guided to the catalyst right after the internal combustion engine is started without having the exhaust temperature reduced. This allows the time required for the catalyst to reach the activation temperature to be reduced so that the catalyst is quickly activated right after the engine is started. Also, by reducing the amount of required materials, the structure is made lighter and less expensive. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.