Abstract:
Downstream fitting parts of exhaust manifold branch pipes which fit into a manifold container each has a sector or fan-like cross section which is formed so that the cross-section area of the downstream end of each of the branch pipes is substantially equal to that of the corresponding upstream end. During manufacture, the downstream ends of each pipe member are expanded by a predetermined amount and then shaped to have the fan or sector-like shaped cross section. This maintains the cross-sectional area of the shaped portions substantially equal to the original cross-sectional areas and therefore the cross-sectional area of the upstream ends.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to branch pipes mounted to an exhaust manifold of an internal combustion engine, and a method of manufacturing the same. 
     This type of the exhaust manifold is known as disclosed in the Unexamined Japanese Patent Application Publication No. Hei9-236012. 
     An example of the exhaust manifold is shown in FIGS. 6 and 7. 
     An exhaust manifold  1  is made up of a flange  2  fastened to the exhaust side of an engine, four branch pipes  3  coupled to the flange  2 , and a manifold container  4  with a branch-pipe mounting part  4   a  into which those branch pipes  3  are fit in a state that those are arranged in parallel. The fitting part  3   a  of each branch pipe  3  at which the branch pipe is fit into the branch-pipe mounting part  4   a  is shaped like a fan in cross section. 
     An O 2  sensor  5  is attached to the manifold container  4  at a position near the base parts of those branch pipes  3 . 
     A manifold converter  6  as a cylindrical container, elliptical in cross section, is coupled to a downstream side of the manifold container  4 . 
     A metal catalyst carrier  7  for purifying exhaust gas is contained in the manifold converter  6 . 
     In the exhaust manifold  1 , exhaust gas discharged from the engine is introduced from a plurality of branch pipes  3  into the manifold container  4 , and an oxygen concentration in the exhaust gas is measured by the O 2  sensor  5  and introduced into the manifold converter  6 . 
     Thereafter, the exhaust gas is purified by the metal catalyst carrier  7 , and discharged out through an exhaust gas discharging port  8 . Then, it is guided into a muffler (not shown) for deadening the sound of exhausting gases, and then is discharged into the air. 
     The branch pipes  3  are shaped to have their fitting parts  3   a  whose cross section is fan in shape. 
     In the shaping process, the peripheral length adjustment is used. Accordingly, a cross section area of the resultant fitting part is smaller than that of an original pipe  9 , as shown in FIG.  8 . 
     Specifically, a cross section area S 1  of the fitting part  3   a  is equal to the result of subtracting cross section areas S 2  and S 3  of two crescent-like parts from the cross section area S of the original pipe  9 . 
     The reduction of the cross section area will adversely affect the pressure loss. In other words, this possibly leads to the engine output reduction. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide branch pipes for an exhaust manifold which are able to reduce the pressure loss and a method of manufacturing the same. 
     The above object is achieved by branch pipes for an exhaust manifold in which a fitting part to be fit into a manifold container comprises a group of pipe members, wherein a cross section of the fitting part of each branch pipe is shaped into a fan-like shape, and the cross sectional area of the branch pipe is substantially equal to that of the branch pipe before the branch pipe is worked. 
     To achieve the above object, there is also provided a method of manufacturing the exhaust manifold branch pipes as mentioned in aspect 1. The manufacturing method comprises a step of expanding the end of each pipe member, and a step of shaping the expanded end of the pipe member into a fan-like shape in cross section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     [FIG.  1 ] 
     FIG. 1 is a front view showing an exhaust manifold using exhaust manifold branch pipes, which form an embodiment of the present invention. 
     [FIG.  2 ] 
     FIG. 2 is a bottom view showing a key portion of the exhaust manifold of FIG.  1 . 
     [FIG.  3 ] 
     FIG. 3 is a cross sectional view showing a key portion of the FIG. 1 exhaust manifold. 
     [FIG.  4 ] 
     FIG. 4 is an explanatory diagram for explaining a manufacturing process of the FIG. 1 branch pipe. 
     [FIG.  5 ] 
     FIG. 5 is another explanatory diagram for explaining a manufacturing process of the FIG. 1 branch pipe. 
     [FIG.  6 ] 
     FIG. 6 is a front view showing a conventional exhaust manifold. 
     [FIG.  7 ] 
     FIG. 7 is a cross sectional view showing the FIG. 6 exhaust manifold. 
     [FIG.  8 ] 
     FIG. 8 is an explanatory diagram for explaining a manufacturing process of the FIG. 6 branch pipe. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIGS. 1 to  5  show an exhaust manifold using branch pipes for an exhaust manifold (referred to as branch pipes), which form an embodiment of the present invention (corresponding to aspect 1). 
     In an exhaust manifold of the embodiment, as shown in FIGS. 1 to  3 , four branch pipes  10 ,  11 ,  12 ,  13 , which are each circular in cross section, are fastened at the upstream side to a flange  20  fastened to the exhaust side of an engine, and are coupled at the downstream side of those pipes to a manifold container  30 . 
     An O 2  sensor  40  is attached to the manifold container  30 . A catalytic converter  50  is mounted on the manifold container  30 . The catalytic converter  50  is consecutively coupled to a lower side container  60  and a flange  70  on which the container  60  is mounted. 
     In the embodiment, the branch pipe  10  is communicatively coupled to a first cylinder of the engine; the branch pipe  11 , to a second cylinder; the branch pipe  12 , to a third cylinder; and the branch pipe  13 , to a fourth cylinder. 
     Parts (fitting parts)  10   a ,  11   a ,  12   a ,  13   a  of the ends of the branch pipes  10 ,  11 ,  12 ,  13 , which are fit into a branch pipe mounting part  31 , are each shaped like a fan in cross section. 
     A cross section area SX of the parts (fitting parts)  10   a ,  11   a ,  12   a ,  13   a , which are fit into a branch pipe mounting part  31 , is substantially equal to across section area S of non-worked parts (original pipes) of the branch pipes  10 ,  11 ,  12 ,  13 , which are each circular in shape. 
     The branch pipe mounting part  31 , as shown in FIGS. 1 and 2, is curved toward the outside of the manifold container  30 . 
     The parts (fitting parts)  10   a ,  11   a ,  12   a ,  13   a  to be fit into the branch pipe mounting part  31 , as shown in FIGS. 2 and 3, are put in such a state that the walls  10   c ,  12   b ,  12   c ,  13   b ,  13   c ,  11   a ,  11   b ,  10   b  are brought into surface contact with one another. In this state, those fitting parts are arranged about the center line of the branch pipe mounting part  31  of the manifold container  30 , while being slanted with respect to the center line at a given angle. At the branch pipe mounting part  31 , those branch pipes  10  to  13  are consecutively arranged in the order of the branch pipes  10 ,  13 ,  11 ,  12 . 
     The other portions of the branch pipes  10 ,  11 ,  12 ,  13  than the parts (fitting parts)  10   a ,  11   a ,  12   a ,  13   a  to be fit into the branch pipe mounting part  31  are each circular in cross section, like the normal branch pipes, and the coupling parts of those portions to the bracket  20  are each rectangular in cross section. 
     The walls  10   c ,  12   b ,  12   c ,  13   b ,  13   c ,  11   a ,  11   b ,  10   b  are welded together by welding applied onto the inside of the manifold container  30 , and reliably fastened together. 
     As described above, in the embodiment, the parts (fitting parts)  10   a ,  11   a ,  12   a ,  13   a  to be fit into the branch pipe mounting part  31  are formed in a manner that a pipe having a circular cross section is worked into a pipe having a fan-shaped cross section, and the cross section area SX of it is substantially equal to the cross section area S of the non-worked part (original pipe) of each of the branch pipes  10 ,  11 ,  12 ,  13 , which are each circular in shape. Accordingly, the branch pipes of the invention succeeds in reducing the pressure loss to be smaller than that of the conventional branch pipes. In other words, there is no case of reducing the engine power. 
     FIGS. 4 and 5 show a method of manufacturing the branch pipes  10 ,  11 ,  12 ,  13  (corresponding to aspect  2 ). 
     To start with, a pipe  100 , which is circular in cross section, is prepared for working. 
     A cross section of the pipe  100  is S mm 2 . 
     A part  101  corresponding to the parts (fitting parts)  10   a ,  11   a ,  12   a ,  13   a  to be fit into the branch pipe mounting part  31  is expanded in diameter. 
     As a result, a cross section area of the expanded part is (S+α)mm 2 . 
     Next, the expanded part is shaped into a fan-shaped part. 
     In the shaping process, crescent parts  102  and  103  as defined by a radius of the fan are crushed to be flat, and hence the resultant fan-shaped part is α mm 2  in cross section. 
     Accordingly, the cross section area SX mm 2  of the fan-shaped part (parts (fitting parts)  10   a ,  11   a ,  12   a ,  13   a  to be fit into the branch pipe mounting part  31 ) is equal to S mm 2 : SX mm 2 =S mm 2 . That is to say, it is substantially equal to the cross section area of the ripe  100 . 
     As described above, in the embodiment, the pipe having a circular cross section is expanded in diameter, and worked to have squeezed parts, and then, the resultant is shaped into the pipe part whose cross section is fan in shape. Therefore, the cross section area of the resultant part is substantially equal to that of the original pipe. 
     Concurrently with the progression of the above working process, the end parts of the branch pipes  10 ,  11 ,  12 ,  13  at which the branch pipes are mounted to the bracket  20  are each worked to have a rectangular cross section area. 
     As seen from the foregoing description, in the invention, a cross section area of the fitting part of the branch pipes is substantially equal to that of the original pipe of the branch pipe. Therefore, there is eliminated the decrease of the pressure loss caused by a configuration variation of the fitting part.