1. Field of the Invention
The present invention relates to a merging pipe and a method of manufacturing the merging pipe.
2. Description of the Related Art
Heretofore, an engine exhaust merging pipe has been known which combines a plurality of exhaust pipes such that the exhaust pipes are obliquely arranged and joined together with their axes meeting into a common axis on the downstream side.
Such an exhaust merging pipe is constructed, by way of example, as shown in FIG. 10. Two exhaust pipes 1, 2 are cut at their downstream ends along lines X, Y crossing axes O.sub.1, O.sub.2 of the pipes. Cut edges 1a, 2a of the pipes are positioned to abut with each other and joined together by a weld 3 to form a joint portion, as shown in FIGS. 11 to 13. In the drawings, reference numeral 7 denotes a conjoint pipe. This exhaust merging pipe will be referred to as a first conventional merging pipe.
Another example of the exhaust merging pipe is disclosed in, e.g., JP-Y2-46-31288 and JP-U-02-141623. According to the disclosed example, as shown in FIGS. 14 to 16, two exhaust pipes 4, 5 are arranged obliquely with their axes O.sub.1, O.sub.2 meeting into a common axis on the downstream side. Circumferential walls of the pipes 4, 5 contiguous to each other at their downstream ends are formed into flat abutting walls 4a, 5a which are then positioned in close contact relation and joined together by a weld 6. This exhaust merging pipe will be referred to as a second conventional merging pipe.
In the above first conventional merging pipe, if the mating accuracy between the cut edges of both the exhaust pipes is poor, a welding failure, such as an unwelded hole, tends to occur. Therefore, the first conventional merging pipe requires not only high accuracy in cutting and mating of the exhaust pipes, but also a larger number of manufacturing steps. This results in the problem of pushing up the production cost.
Another problem is that because the joined edges of the pipes 1, 2 define intricate curved surfaces as shown in FIGS. 12 and 13, a difficulty is encountered in welding those joined edges and hence increases the probability of a welding failure. This problem is more serious as an intersect angle .theta., shown in FIG. 12, at which outer circumferential surfaces of the pipes 1, 2 cross each other is reduced.
The above problem is magnified due to the fact that the intersect angle e can be relatively large, as shown in FIG. 12, in a downstream joint end "a" in FIG. 11, but it becomes very small particularly in an upstream joint end "b". Further, an angle a formed between the pipes 1 and 2 in an upstream crotch portion "c" is very small as shown in FIG. 11, which also poses a difficulty in welding work and magnifies the above problem in the crotch portion "c". Of course, the severity of the problem is increased as the intersect angle .alpha. between both the pipe axes is reduced.
Meanwhile, in order to reduce merging resistance of exhaust gases from the pipes 1, 2 and hold down the occurrence of unusual sounds or noises upon confluence of two exhaust gas flows, it is required to make smaller the intersect angle .alpha. between the axes of the pipes 1, 2. Also, from the viewpoint of improving exhaust performance, there is a demand for reducing the intersect angle .alpha. between both the pipe axes.
Thus, the first conventional merging pipe has not succeeded in satisfying the above-stated contradictory demands for overcoming the problem in efficiency of welding work and reducing the intersect angle from the viewpoint of improving exhaust performance.
In the second conventional merging pipe, the pipes 4, 5 have the flat opposing walls 4a, 5a and therefore the problem of leaving an unwelded hole can be avoided. However, because the demand for reducing the intersect angle a between both the pipe axes in the crotch portion necessarily makes the intersect angle .alpha. and the intersect angle .theta. in the joint portion set to be small, the second conventional merging pipe cannot also satisfy the contradictory demands for reducing the intersect angle between both the pipe axes and facilitating welding work. Thus, the second conventional merging pipe has accompanied the problems of requiring a large number of steps to ensure a high degree of welding accuracy and pushing up the production cost.