Exhaust manifold for internal combustion engine

Abstract: An exhaust manifold provides improved output by suppressing exhaust interference between cylinders at the exhaust manifold. An air-fuel ratio sensor (5) is installed in position where it can uniformly detect the exhaust gas of each cylinder. The exhaust manifold has a plurality of exhaust tubes (1a, 1b, 1c, 1d), one per cylinder, that connect with a collector case (3). Each of exhaust tubes (1a, 1b, 1c, 1d) has a linear portion (4) located directly above the section where it merges with the collector case (3). The exhaust tubes (1a, 1b, 1c, 1d) are connected to the collector case (3) such that the center axes of the linear portions (4) intersect at intersection point (G) inside the collector case (3) or downstream thereof. The air-fuel ratio sensor (5) is arranged such that its detecting part (5a) is positioned in the vicinity of the intersection point (G). Depending upon whether the exhaust tubes (1a, 1b, 1c, 1d) of cylinders have firing orders that are successive or not determines whether the exhaust tubes (1a, 1b, 1c, 1d) are slanted or parallel with respect to each other.

Description: The present invention relates to an exhaust manifold of an internal combustion engine according to the preamble of the independent claim 1. Many internal combustion engines have an exhaust manifold with a single exhaust tube extending from each cylinder. The exhaust tubes are typically merged together. This merger of the exhaust tubes can result in exhaust interference and reduced output depending on the exhaust order of the merged cylinders and the position where the exhaust tubes are merged. JP 59-188022 discloses an engine exhaust manifold for a four cylinder in line engine. The exhaust manifold disclosed in this publication merges the exhaust tubes of cylinders having non-successive firing order first, i.e., cylinders #1 and #4 and cylinders #2 and #3. However, in recent years, the demand for improved exhaust performance of internal combustion engines has created a need for the catalyst to be held directly below the exhaust manifold. In such an arrangement, the distance from the exhaust ports of the internal combustion engine to the catalyst can be very shorten. Consequently, when it is attempted to merge the exhaust gases from cylinders #1 and #4 together and the exhaust gases from cylinders #2 and #3 together before merging with the collector case, the exhaust tubes must be merged immediately downstream of the exhaust port outlet. This arrangement leads to the problem of reduced output caused by exhaust interference. JP 7-63092 discloses an engine exhaust manifold having two manifold catalytic converters. One of the manifold catalytic converters is provided for the exhaust tubes extending from cylinders #1 and #4. The other manifold catalytic converter is provided for the exhaust tubes extending from cylinders #2 and #3. Thus, the exhaust tubes of cylinders #1 and #4 are merged into a separate manifold catalytic converter from the exhaust tubes of cylinders #2 and #3. With this arrangement, there is little exhaust interference and no reduction of output, but there is the problem of increased cost resulting from using two manifold catalytic converters. It is also necessary to install an air-fuel ratio sensor, typically an oxygen sensor, in the exhaust manifold in order to utilize the catalyst effectively. The air-fuel ratio sensor needs to be installed in a position where it can uniformly detect the exhaust gas from all of the cylinders. However, in the case of a manifold catalytic converter that is disposed directly below the exhaust manifold, it is becoming difficult to install the air-fuel ratio sensor such that it can uniformly detect the exhaust gas from each cylinder. JP 6-241040 discloses an engine exhaust manifold with a collector case that is divided into two chambers by a partitioning wall such that at the exhaust gases from cylinders #1 and #4 are merged into one chamber and the exhaust gases from cylinders #2 and #3 are merged into the other of the chambers. The air-fuel ratio sensor is then arranged in an air communication passageway provided through the partitioning wall. The problem with this arrangement is that, under high load conditions in which the exhaust gas flows at a high speed, the mainstream of the exhaust gas passes through the collector case without much flow toward the communication passageway. Thus it is difficult for the air-fuel ratio sensor to uniformly detect the exhaust gas from each cylinder. JP 11-13468 discloses an engine exhaust manifold that uses ribs in the exhaust tubes in order to direct exhaust gas toward the air-fuel ratio sensor. The problem with this arrangement is that the output declines because of these ribs in the exhaust tubes. From EP-A-1 028 236 an exhaust manifold as indicated above is known, wherein, in particular, four exhaust tubes are respectively connected at one end to a cylinder head and at an other end to a mixing pipe. Here, the exhaust tubes are introduced into the mixing pipe in almost a parallel fashion, such that the exhaust gases of the exhaust tubes can pass through the mixing pipe without colliding in the wall of the mixing pipe. It is an objective of the present application to improve an exhaust manifold of an internal combustion engine as indicated above, so as to be capable of reducing a backflow of the exhaust gas and to increase the mixing of the exhaust gas in front of a catalyst. The objective is solved according to the present invention by an exhaust manifold of an internal combustion engine, comprising a collector case having an upstream end and a downstream end; and a plurality of exhaust tubes having inlet ends adapted to be connected to exhaust ports of the internal combustion engine and outlet ends connected to said upstream end of said collector case by merging portions, wherein said outlet ends include linear portions disposed contiguously with said merging portions, where said exhaust tubes merge with said collector case, said linear portions are arranged in pairs of parallel linear portions, and wherein one pair of parallel linear portions is slanted with respect to a second pair of parallel linear portions, and comprising center axes intersecting at an intersection point inside said collector case or downstream thereof, and said exhaust tubes with said inlet ends arranged to receive exhaust gas from cylinders, whose firing orders are successive, constitute one pair of parallel linear portions. Thus, the exhaust manifold can suppress exhaust interference and improve output even in cases where a manifold catalytic converter is used close to the collector case. According to an embodiment, a positioning point of a detecting part of an air-fuel ratio sensor is arranged at said intersection point or adjacent thereto. Therefore, it is possible to install an air-fuel ratio sensor in a position where it can uniformly detect the exhaust gas from each cylinder so that the catalyst can be utilized effectively and emissions can be reduced. Further preferred embodiments of the present invention are laid down in the further subclaims. In the following, the present invention is explained in greater detail by means of several embodiments thereof in conjunction with the accompanying drawings, wherein: Figure 1 is a simplified side elevational view of an exhaust manifold in accordance with a first embodiment of the present invention; Figure 2 is a cross sectional view of the exhaust manifold illustrated in Figure 1 as viewed along line A-A of Figure 1; Figure 3 is an explanatory cross sectional view illustrating the cross sectional shape of one of the exhaust tubes of the exhaust manifold illustrated in Figure 1; Figure 4 is a simplified schematic view showing the direction of the center axes of the linear portions of the exhaust tubes of the exhaust manifold in accordance with the first embodiment of the present invention illustrated in Figure 1; Figure 5 is a first explanatory cross sectional view illustrating the flow of exhaust gas from the first exhaust tube into the collector case of the exhaust manifold illustrated in Figure 1 as viewed along line B-B of Figure 7; Figure 6 is a second explanatory cross sectional view illustrating the flow of exhaust gas from the second exhaust tube into the collector case of the exhaust manifold illustrated in Figure 1 as viewed along line B-B of Figure 7; Figure 7 is a explanatory cross sectional view illustrating the flow of exhaust gas from the four exhaust tubes into the catalytic converter of the exhaust manifold illustrated in Figure 1 as viewed along line C-C of Figure 5; Figure 8 is a simplified side elevational view of an exhaust manifold in accordance with a second embodiment not falling under the scope of the present invention; Figure 9 is a explanatory cross sectional view, similar to Figure 7, illustrating the flow of exhaust gas from the four exhaust tubes into the catalytic converter of the exhaust manifold of the second embodiment illustrated in Figure 8; Figure 10 is a simplified lateral cross sectional view of a modified exhaust manifold in accordance with a third embodiment, not falling under the scope of the present invention, in which the exhaust tubes of the cylinders are all angled; Figure 11 is a simplified schematic view showing the direction of the center axes of the linear portions of the exhaust tubes of the exhaust manifold illustrated in Figure 10 in accordance with the third embodiment; Figure 12 is a explanatory cross sectional view, similar to Figures 7 and 9, illustrating the flow of exhaust gas from the four exhaust tubes into the catalytic converter of the exhaust manifold of the third embodiment illustrated in Figures 10 and 11; Figure 13 is a simplified side elevational view of a modified exhaust manifold in accordance with a fourth embodiment of the present invention; Figure 14 is a simplified side elevational view of a modified exhaust manifold in accordance with a fifth embodiment of the present invention; Figure 15 is a simplified side elevational view of a modified exhaust manifold in accordance with a sixth embodiment of the present invention; Figure 16 is a simplified side elevational view of a modified exhaust manifold in accordance with a seventh embodiment of the present invention; Figure 17 is a simplified side elevational view of a modified exhaust manifold in accordance with an eighth embodiment of the present invention; Figure 18 is a simplified side elevational view of a modified exhaust manifold in accordance with a ninth embodiment not falling under the scope of the present invention; Figure 19 is a cross sectional view of the exhaust manifold illustrated in Figure 18 as viewed along line D-D of Figure 18; Figure 20 is a simplified side elevational view of a modified exhaust manifold in accordance with a tenth embodiment of the present invention in which the intersection point is downstream of the collector case; Figure 21 is a simplified side elevational view of an exhaust manifold in accordance with an eleventh embodiment, not falling under the scope of the present invention, in which the exhaust tubes of the cylinders are all angled; Figure 22 is a simplified lateral cross sectional view of the exhaust manifold in accordance with the third embodiment in which the exhaust tubes of the cylinders are all angled; and Figure 23 is a cross sectional view, similar to Figure 2, of a modified exhaust manifold that can be used in any of the preceding embodiments of the present invention when only three cylinders are merged together. Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following description of the embodiments of the present invention is provided for illustration purposes only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Referring initially to Figure 1, an internal combustion engine exhaust manifold 10 is illustrated to explain a first embodiment of the present invention. The exhaust manifold 10 comprises a plurality of exhaust tubes 1a to 1d, a mounting flange 2 coupled to the upstream ends of the exhaust tubes 1a to 1d, and a collector case 3 coupled to the downstream ends of the exhaust tubes 1a to 1d. Each of the exhaust tubes 1a to 1d has an upstream end connected to an exhaust port of one of the cylinders of an internal combustion engine via the mounting flange 2 and the downstream end connected to the collector case 3. Preferably, a catalyst unit or catalytic converter 6 is attached to the outlet or downstream end of the collector case 3. Since catalytic converters are well known in the art, the structure and function of the catalytic converter 6 will not be discussed or illustrated herein. Accordingly, it will further be apparent to those skilled in the art that the catalytic converter 6 can have structure and use any catalyst that will carry out the present invention. Figure 2 shows the cross sectional shape of the portion (section A-A in Figure 1) where the exhaust tubes 1a to 1d from the cylinders are merged at the point of connection with the collector case 3. Also, as shown in FIG. 3, the exhaust tubes 1a to 1d have a substantially circular cross section at one end (i.e., the end that connects to the flange 2) and a substantially fan-shaped cross section at the other end (i.e., the end that connects to the collector case 3). The cross sections of the exhaust tubes 1a to 1d gradually changes in the section between the two ends. Each of the exhaust tubes 1a to 1d is substantially fan-shaped and substantially the same size and shape. In other words, the cross sectional shapes of the exhaust tubes 1a to 1d at the merging portion are substantially fan-shaped and have substantially equal sizes. Therefore, the exhaust tubes 1a to 1d can be connected to the collector case 3 using sheet metal and welding and the manufacturing cost can be reduced. The exhaust streams from the exhaust tubes 1a to 1d of the cylinders do not merge with the exhaust tubes of the other cylinders until they merge within the collector case 3. In a four cylinder inline engine, the firing order of the cylinder is as follows: cylinder #1, cylinder #3, cylinder #4 and then cylinder #2. Thus, cylinders #1 and #4 do not have successive firing orders, and cylinders #2 and #3 do not have successive firing orders. Additionally, each of the exhaust tubes 1a to 1d has a linear portion 4 that is linearly shaped with a length L1. The linear portions 4 are located directly above the position where the exhaust tubes 1a to 1d merge with the collector case 3. In other words, the linear portions 4 form the downstream ends of the exhaust tubes 1a to 1d that are directly or contiguously connected to the collector case 3. The exhaust gas streams of the exhaust tubes 1a to 1d are directed by the linear portions 4 and flow downstream into the collector case 3. Therefore, there is little back-flow of the exhaust gas streams from one of the exhaust tubes 1a to 1d of one of the cylinders into one or more of the exhaust tube from the other cylinders. In other words, exhaust interference is reduced while output is improved. Cylinders #1 and #4 do not have successive firing orders. The linear portions 4 of the exhaust tube 1a of cylinder #1 and the exhaust tube 1d of cylinder #4 are slanted with respect to each other such that their centerlines or axes C1 and C2 intersect with angle 1 inside the collector case 3 to form intersection point G. Cylinders #2 and #3 do not have successive firing orders. The linear portions 4 of the exhaust tube 1b of cylinder #2 and the exhaust tube 1c of cylinder #3 are also slanted in the same manner as cylinders #1 and #4. In other words, the center axes C1 and C2 of the exhaust tubes 1b and 1c of cylinder #2 and cylinder #3, respectively, are slanted to form an angle 1 inside the collector case 3 at the intersection point G. Of course, the intersection point G of the center axes C1 and C2 of the exhaust tubes 1b and 1c are located directly behind the intersection point G of the center axes C1 and C2 of the exhaust tubes 1a and 1d. The centerlines C1, C2, C3 and C4 of the linear portions 4 are lines that are oriented in the flow direction and pass through the center of gravity of the substantially fan-shaped cross section each linear portion 4. The linear portions 4 of the exhaust tube 1a of cylinder #1 and the exhaust tube 1c of cylinder #3, which cylinders have successive firing orders, are substantially parallel to each other. Similarly, the exhaust tube 1d of cylinder #4 and the exhaust tube 1b of cylinder #2 have successive firing orders with the linear portions 4 being substantially parallel. Thus, in this first embodiment, the centerlines C1, C2, C3 and C4 of the linear portions 4 of the exhaust tubes 1a to 1d are disposed as shown in Figure 4 and have two intersection points G (G1, G2). The collector case 3 is preferably substantially a partial sphere at the connection to the downstream ends of the exhaust tubes 1a to 1d. Thus, the transverse cross section of the collector case 3 is preferably a substantially circular shape that envelops the exhaust tubes 1a to 1d of each cylinder. Therefore, the process of connecting the exhaust tubes 1a to 1d, which are made of pipe, to the collector case 3 can be accomplished using sheet metal and welding steps. This arrangement results in reducing the cost of manufacturing the exhaust manifold 10 in comparison with cast molding the manifold as a single unit. Since the collector case 3 has a diffuser shape whose cross sectional area is sufficiently large with respect to the exhaust tubes 1a to 1d, the exhaust gas streams from the exhaust tubes 1a to 1d spreads inside the collector case 3 but also maintains the directivity as it flows downstream. Meanwhile, in order to detect the air-fuel ratio of the exhaust gas, an air-fuel ratio sensor 5 is installed so that it faces inside the case from a wall of the collector case 3. The air-fuel ratio sensor 5 is typically an oxygen sensor. The air-fuel ratio sensor 5 is a conventional component that is well known in the art. Since air-fuel ratio sensors are well known in the art, the construction of the air-fuel ratio sensor 5 will not be discussed or illustrated herein. A detecting part 5a at the tip of the air-fuel ratio sensor 5 is positioned in the vicinity of the intersection point G. As a result, the air-fuel ratio sensor 5 can detect the concentration of the exhaust gas of each cylinder uniformly. Since there are actually two intersection points G in the four cylinder inline engine, the detecting part 5a at the tip of the air-fuel ratio sensor 5 should be positioned at an intermediate position between the two intersection points G. In particular, the detecting part 5a should be positioned close to a midpoint M of a line segment that joins the two intersection points G1 and G2 as shown in Figure 4. In other words, the detecting part 5a of an air-fuel ratio sensor is positioned close to the intersection points G so that the air-fuel ratio sensor 5 can uniformly detect the concentration of the exhaust gas streams of each cylinder. As a result, the catalyst unit 6 can be used effectively and emissions can be reduced because the air-fuel ratio can be controlled with good precision. Thus, in this first embodiment of the present invention, the linear portions 4 of the exhaust tubes 1a to 1d are connected separately to the collector case 3 so that the exhaust gas streams of each cylinder does not interfere with the exhaust gas streams of the other cylinders until it enters the collector case 3. Also, since the exhaust gas streams of each cylinder flows directly into the collector case 3, the amount of back-flow into the exhaust tubes 1a to 1d of the other cylinders caused by exhaust pulsation is small. As a result, exhaust gas interference can be reduced and output can be improved. Additionally, since the center axes C1 and C2 of the linear portions 4 of at least the exhaust tubes 1a to 1d connected to cylinders of non-successive firing order are slanted with respect to each other and intersect downstream, the exhaust gas streams from the cylinders can be mixed to some degree inside the collector case 3 before being directed to the catalytic converter 6, while, at the same time, exhaust gas interference between cylinders of successive firing order can be prevented. Furthermore, this arrangement makes it easy to position the air-fuel ratio sensor 5 such that it can detect the concentration of the exhaust gas from each cylinder uniformly. When the linear portions 4 of the exhaust tubes 1a to 1d connected to cylinders with successive firing orders are substantially parallel to each other, exhaust interference between these exhaust tubes can be reduced with certainty. Now, the advantages and disadvantages of the first embodiment (i.e., where the exhaust tubes of the cylinders of non-successive firing order have intersection point G1 or G2, while those of cylinders of successive firing order are parallel) will be discussed with reference to Figures 5-7. Specifically, the advantages and disadvantages of the first embodiment will be discussed from the standpoint of: (1) the effect of reducing exhaust gas interference; (2) the effect of making the flow rate distribution inside the catalyst more uniform; and (3) the effect of improving the sensitivity of the air-fuel ratio sensor. The exhaust gas interference is reduced in the first embodiment since two pairs of cylinders are parallel. Also, since exhaust gas interference occurs more readily between the cylinders whose firing orders are successive, the exhaust gas interference can be reduced further in the first embodiment because the two pairs of cylinders whose firing orders are successive are arranged so as to be parallel. The intra-catalyst flow rate distribution unifying effect will discussed using Figures 5-7. The flow patterns of the exhaust gas from the linear portions 4 of the exhaust tubes 1a to 1d into the collector case 3 is shown in Figures 5 and 6. Figure 7 illustrates the main flow area (shown by the shaded ovals) and the entire flow area (shown by the larger ovals) of the exhaust gas from each cylinder in a front-end plane of the catalytic converter 6 corresponding to section C-C in Figure 5. The exhaust gas that flows into the collector case 3 (the catalyst diffuser section) from the exhaust tubes 1a to 1d diffuses inside the collector case 3 as it flows into the catalytic converter 6 (the catalyst carrier section). When two cylinders are parallel as in the first embodiment, the concentration of the main flow of gas into the catalytic converter 6 can be reduced. When the flow of exhaust gas is concentrated, the performance of the catalytic converter 6 deteriorates more rapidly in that area and the durability performance of the catalytic converter 6 declines. The exhaust gas that enters the collector case 3 (catalyst diffuser section) from the exhaust tubes 1a to 1d forms a large vortex (vertical vortex) in the collector case 3 as it flows into the catalytic converter 6 (the catalyst carrier section). Thus, the exhaust gas flows gradually into the catalytic converter 6 (the catalyst carrier section), while forming a vortex as seen in Figures 5 and 6. In this arrangement, the diameter of the vortex is largest and the vortex is the most stable when the vortex forms so as to be parallel with plane B-B shown in Figure 7. When four cylinders are concentrated on a single point as shown in the third embodiment, not falling under the scope of the present invention, illustrated in the Figures 10 and 11, it is difficult to form a large vortex because the gas flows in at an angle with respect to the plane B-B of Figure 7. Furthermore, the exhaust gas from each cylinder interferes with the vortexes formed by exhaust gases from all the other cylinders. Conversely, when two cylinders are arranged to have parallel flow as in the first embodiment, a vortex forms more readily because the exhaust gas flows in such that it is parallel to the plane B-B. Furthermore, a stable vortex is formed because the vortexes of the two parallel cylinders do not interfere with each other. Consequently, the flow distribution of not only the exhaust gas in the main flow area but also of the exhaust gas that diffused as it formed a vortex spreads more widely as the exhaust gas passes through the catalytic converter 6 and the flow of gas inside the catalytic converter 6 becomes more uniform (see Figure 7). In the first embodiment, in which cylinders whose firing orders are successive are arranged in pairs having parallel flow, there is little interference between the respective vortexes formed by the exhaust gases from the two cylinders making up each pair (because the vortexes form so as to be parallel to the plane B-B). Consequently, it is easier for the vortexes to form than in the second embodiment, not falling under the scope of the present invention, illustrated in the Figure 8 and discussed below, in which cylinders whose firing orders are not successive are arranged in pairs having parallel flow. As a result, as discussed previously, the exhaust gases diffuse as they form vortexes and consequently the flow distribution spreads more widely as the exhaust gases pass through the catalytic converter 6 and the flow of the exhaust gases inside the catalytic converter 6 becomes more uniform. The air-fuel ratio sensor sensitivity improvement effect will now be discussed. The sensitivity of the air-fuel ratio sensor 5 can be improved the most by concentrating all four cylinders on a single point, and positioning the air-fuel ratio sensor 5 at that point. Since the exhaust gases from the linear portions 4 of the exhaust tubes 1a to 1d converge at the air-fuel ratio sensor 5, the air-fuel ratio sensor sensitivity is improved. Referring now to Figures 8 and 9, an internal combustion engine exhaust manifold 20 is illustrated in accordance with a second embodiment not falling under the scope of the present invention. Basically, the first and second embodiments are identical, except that the arrangement of the exhaust tubes 21a to 21c has been modified in this second embodiment as explained below. In view of the similarity between the first and second embodiments, the parts of the second embodiment that are identical to the parts of the first embodiment will be given the same reference numerals increased by twenty as the parts of the first embodiment. Moreover, the descriptions of the parts of the second embodiment that are identical to the parts of the first embodiment may be omitted for the sake of brevity. The second embodiment differs from the first embodiment in that the respective linear portions 24 of the exhaust tube 21a of the cylinder #1 and the exhaust tube 21d of the cylinder #4 are located lateral adjacent one another, and the respective linear portions 24 of the exhaust tube 21b of the cylinder #2 and the exhaust tube 21c of the cylinder #3 are located lateral adjacent one another. Thus, the respective linear portions 24 of the exhaust tube 21a of the cylinder #1 and the exhaust tube 21b of the cylinder #2 (which cylinders have successive firing orders) are slanted with respect to each other such that the centerlines C1 and C2 thereof intersect at an angle 2 inside a collector case 23 and form the intersection point G. Also, the respective linear portions 24 of the exhaust tube 21c of the cylinder #3 and the exhaust tube 21d of the cylinder #4 (which cylinders have successive firing orders) are also slanted in the same manner. Meanwhile, the respective linear portions 24 of the exhaust tubes of cylinders whose firing orders are not successive, i.e., the respective linear portions 24 of the exhaust tube 21a of cylinder #1 and the exhaust tube 21d of cylinder #4 and the respective linear portions 24 of exhaust tube 21b of cylinder #2 and exhaust tube 21 c of cylinder #3, are substantially parallel to each other. Thus, a certain degree of effect can be obtained by arranging the exhaust tubes of cylinders whose firing orders are successive so as to be slanted with respect to each other and arranging the exhaust tubes of cylinders whose firing orders are not successive so as to be parallel to each other. The effect of reducing exhaust gas interference declines somewhat in comparison with the first embodiment because intersection points have been established for the exhaust tubes of cylinders whose firing orders are successive, between which exhaust gas interference occurs more readily. In the present embodiment, the exhaust tubes each have a linear portion 24 that are connected separately to the collector case 23. Consequently, the exhaust gas of each cylinder does not interfere with the exhaust gas of the other cylinders until it enters the collector case 23 and, since the exhaust gas of each cylinder flows into the collector case 23 with directivity, the amount of back-flow into the exhaust tubes of the other cylinders caused by exhaust pulsation is small. As a result, exhaust gas interference can be reduced and output can be improved. Additionally, since the centerlines C1 and C2 of the linear portions 24 of the portion of the exhaust tubes, i.e., those connected to cylinders of successive firing order, are slanted with respect to each other and intersect downstream, the exhaust gas from the cylinders can be mixed to some degree inside the collector case 23 before being directed to the catalytic converter. Furthermore, this arrangement makes it easy to position an air-fuel ratio sensor 25 such that it can detect the concentration of the exhaust gas from each cylinder uniformly. In the embodiment, the linear portions 24 of a portion of the exhaust tubes, i.e., those connected to cylinders whose firing orders are not successive, are generally parallel to each other. Therefore, exhaust interference between these exhaust tubes can be reduced as discussed below. Now, the advantages and disadvantages of the second embodiment (i.e., where exhaust tubes of cylinders of successive firing order have an intersection point G, while those of cylinders of non-successive firing order are parallel) will be discussed with reference to Figures 5, 6 and 9. The exhaust gas interference is reduced in the second embodiment, since two pairs of cylinders are parallel in a similar manner to the first embodiment. The intra-catalyst flow rate distribution unifying effect will discussed using Figures 5, 6 and 9. The flow patterns of the exhaust gas from the linear portions 24 of the exhaust tubes 21a to 21d into the collector case 23 is the same as the first embodiment shown in Figures 5 and 6. Figure 9 illustrates the main flow area (shown by the shaded ovals) and the entire flow area (shown by the larger ovals) of the exhaust gas from exhaust tubes 21a to 21d in a front-end plane of the catalytic converter that corresponds to section C-C in Figure 5. The exhaust gas that flows into the collector case 23 (the catalyst diffuser section) from the exhaust tubes 21a to 21d diffuses inside the collector case 3 as it flows into the catalytic converter (the catalyst carrier section). When two cylinders are parallel as in the second embodiment, the concentration of the main flow of gas into the catalytic converter can be reduced. When the flow of exhaust gas is concentrated, the performance of the catalytic converter deteriorates more rapidly in that area and the durability performance of the catalytic converter declines. The exhaust gas that enters the collector case 23 (catalyst diffuser section) from the exhaust tubes 21a to 21d forms a large vortex (vertical vortex) in the collector case 23 as it flows into the catalytic converter (the catalyst carrier section). Thus, the exhaust gas flows gradually into the catalytic converter (the catalyst carrier section), while forming a vortex as seen in Figures 5 and 6. In this arrangement, the diameter of the vortex is smaller than the first embodiment. As mentioned above, when four cylinders are concentrated on a single point as shown in the third embodiment, not falling under the scope of the present invention and illustrated in the Figures 10 and 11, it is difficult to form a large vortex because the gas flows in at an angle with respect to the plane B-B of Figure 7. Furthermore, the exhaust gas from each cylinder interferes with the vortexes formed by exhaust gases from all the other cylinders. Conversely, when two cylinders are arranged to have parallel flow as in the second embodiment, a vortex forms more readily because the exhaust gas flows in such that it is parallel to the plane B-B. Furthermore, a stable vortex is formed because the vortexes of the two parallel cylinders do not interfere with each other. Consequently, the flow distribution of not only the exhaust gas in the main flow area but also of the exhaust gas that diffused as it formed a vortex spreads more widely as the exhaust gas passes through the catalytic converter and the flow of gas inside the catalytic converter becomes more uniform (see Figure 9). In the second embodiment, not falling under the scope of the present invention, in which cylinders whose firing orders are not successive are arranged in pairs having parallel flow, there is more interference between the respective vortexes formed by the exhaust gases from the two cylinders making up each pair than in the first embodiment. Consequently, it is harder for the vortexes to form in the second embodiment illustrated in the Figure 8, than in the first embodiment, in which cylinders whose firing orders are successive are arranged in pairs having parallel flow. The air-fuel ratio sensor sensitivity improvement effect will now be discussed. The sensitivity of the air-fuel ratio sensor 25 can be improved the most by concentrating all four cylinders on a single point, as in the third embodiment, and positioning the air-fuel ratio sensor at that point. In this second embodiment, the exhaust gases from the cylinders are more concentrated at the air-fuel ratio sensor 25, than the first embodiment as seen by comparing Figures 7 and 9. Thus, the sensitivity of the air-fuel ratio sensor 25 is improved in the second embodiment over the first embodiment. Referring now to Figures 10-12, an internal combustion engine exhaust manifold 30 is illustrated in accordance with a third embodiment not falling under the scope of the present invention. Basically, the first and third embodiments are identical, except that angles of the exhaust tube 31a to 31d have been changed as explained below. In view of the similarity between the first and third embodiments, the parts of the third embodiment that are identical to the parts of the first embodiment will be given the same reference numerals increased by thirty as the parts of the first embodiment. Moreover, the descriptions of the parts of the third embodiment that are identical to the parts of the first embodiment have been omitted for the sake of brevity. In the first embodiment, exhaust interference is reduced because the exhaust tube 1a of the cylinder #1 and the exhaust tube 1c of the cylinder #3, which cylinders have successive firing orders, are substantially parallel to each other, and the exhaust tube 1d of the cylinder #4 and the exhaust tube 1b of the cylinder #2, which cylinders have successive firing orders, are substantially parallel to each other. In the third embodiment, however, the linear portions 34 of the pair of exhaust tubes 31a and 31c and the pair of exhaust tubes 31b and 31d are slanted with respect to each other as shown in Figures 10 and 11. Thus, the centerlines of these linear portions of the pairs of exhaust tubes 31a, 31c and 31b, 31d intersect with an angle 3 inside the collector case 33 (or downstream thereof with a short collector case) so as to form the intersection point G, as shown in Figures 10 and 11 (which are, respectively, a view from the left side of and an oblique view of Figure 1). In other words, the pairs of cylinders have successive firing orders are slanted with respect to each other instead of being parallel to each other as in the prior embodiments. As a result, the linear portions 34 of all exhaust tubes 31a to 31d are slanted such that their centerlines intersect at an intersection point G located downstream. Stated differently, the center axes of the linear portions 34 of all exhaust tubes 31a to 31d, including those connected to cylinders whose firing orders are successive, are slanted with respect to each other and intersect downstream. Preferably, the center axes of the exhaust tubes 31a to 31d intersect at a single intersection point G in this embodiment. When this arrangement is used, there is a slight drop in output caused by exhaust gas interference, but an air-fuel ratio sensor 35 can detect the concentration of the exhaust gas of each cylinder more uniformly because the detecting part 35a of the air-fuel ratio sensor 35 can be positioned at one intersection point G. As a result, there is a higher probability that interference will occur than in the prior embodiment of the invention. However, it is easier to arrange the air-fuel ratio sensor 35 such that it can detect the concentration of the exhaust gas streams of each cylinder uniformly because the exhaust gas streams from all cylinders can be made to merge at a single intersection point G. The sensitivity of the air-fuel ratio sensor 35 can be improved the most by concentrating all four cylinders on a single point G, as in this third embodiment, and positioning the air-fuel ratio sensor at that point. The intra-catalyst flow rate distribution unifying effect will discussed using Figures 5, 6 and 12. The flow patterns of the exhaust gas from the linear portions 34 of the exhaust tubes 31a to 31d into the collector case 33 is similar to the first embodiment as shown in Figures 5 and 6. Figure 12 illustrates the main flow area (shown by the shaded ovals) and the entire flow area (shown by the larger ovals) of the exhaust gas from each cylinder in a front-end plane of the catalytic converter that corresponds to section C-C in Figure 5. The exhaust gas that flows into the collector case 33 (the catalyst diffuser section) from the exhaust tubes 31a to 31d diffuses inside the collector case 33 as it flows into the catalytic converter (the catalyst carrier section). When two cylinders are all slanted as in the third embodiment, the main flow of exhaust gas is concentrated. Thus, the performance of the catalytic converter deteriorates more rapidly in that area and the durability performance of the catalytic converter declines as compared to the first embodiment. In other words, when all four cylinders are concentrated on a single point as in the third embodiment, the main flow areas of the exhaust gas from the cylinders are concentrated in a single region (see Figure 12). The exhaust gas that enters the collector case 33 (catalyst diffuser section) from the exhaust tubes 31a to 31d forms a smaller vortex (vertical vortex) in the merging portion as it flows into the catalytic converter. The gas flows gradually into the catalyst section while forming a vortex (see Figure 12). In this arrangement, the diameter of the vortex is the smallest and the vortex is the least stable. When four cylinders are concentrated on a single point as in the third embodiment, not falling under the scope of the present invention, it is difficult to form a large vortex because the gas flows in at an angle with respect to plane B-B of Figure 12. Furthermore, the exhaust gas from each cylinder interferes with the vortexes formed by exhaust gases from all the other cylinders. Conversely, when two cylinders are arranged to have parallel flow as in the first and second embodiments, a vortex forms more readily because the gas flows in such that it is parallel to plane B-B. Furthermore, a stable vortex is formed because the vortexes of the two parallel cylinders do not interfere with each other. Consequently, the flow distribution of not only the gas in the main flow area but also of the gas that diffused as it formed a vortex spreads more widely as the gas passes through the catalyst and the flow of gas inside the catalyst becomes more uniform (see Figure 12). Referring now to Figure 13, an internal combustion engine exhaust manifold 40 is illustrated in accordance with a fourth embodiment of the present invention. Basically, the first and fourth embodiments are identical, except that the intersection point G as explained below. In view of the similarity between the first and fourth embodiments, the parts of the fourth embodiment that are identical to the parts of the first embodiment will be given the same reference numerals increased by forty as the parts of the first embodiment. Moreover, the descriptions of the parts of the fourth embodiment that are identical to the parts of the first embodiment have been omitted for the sake of brevity. The fourth embodiment is similar to the first embodiment (Figure 1) in that the linear portions 44 of the exhaust tube 41a of cylinder # 1 and the exhaust tube 41d of cylinder #4 are slanted with respect to each other such that their center axes C1 and C2 intersect with angle 4 inside the collector case 43 to form intersection point G. The linear portions 44 of the exhaust tube 41b of cylinder #2 and the exhaust tube 41c of cylinder #3 are also slanted in the same manner as cylinders #1 and #4. The fourth embodiment is different from the first embodiment (Figure 1) in that the lengths L2 of the linear portions 44, which are located directly above the portion where the exhaust tubes 41a to 41d merge with the collector case 43, are longer. By making the linear portions 44 longer, the flow of exhaust gases that are directed by the linear portions 44 becomes stronger and the amount of exhaust gas that flows backward into the exhaust tubes of the other cylinders is reduced even further. As a result, exhaust interference is reduced and output is improved. However, when the linear portions 44 are made longer, the distance from the exhaust port to the collector case 43 becomes longer. As a result, the distance to the catalytic converter installed downstream of the collector case 43 becomes longer and the temperature rise characteristic of the catalyst worsens. Consequently, the lengths L2 of the linear portions 44 are determined by the balancing the desired output against the desired emissions, which are determined by the temperature rise characteristic of the catalyst. As indicated by broken line 47, it is also acceptable to expand the form of the portion where the exhaust gas flows into the catalyst so that the exhaust gas is directed downstream in a more uniform manner. This feature can be applied to the other embodiments of the present invention shown and described herein. Referring now to Figure 14, an internal combustion engine exhaust manifold 50 is illustrated in accordance with a fifth embodiment of the present invention. Basically, the fourth and fifth embodiments are identical, except that orientation of the air-fuel ratio sensor 55 has been changed as explained below. In view of the similarity between the fifth embodiment and the prior embodiments, the parts of the fifth embodiment that are identical to the parts of the first embodiment will be given the same reference numerals increased by fifty as the parts of the first embodiment. Moreover, the descriptions of the parts of the fifth embodiment that are identical to the parts of the prior embodiments have been omitted for the sake of brevity. The fifth embodiment differs from the fourth embodiment (Figure 13) in that the angle of the air-fuel ratio sensor 55 has been changed relative to the center axes C1 and C2 of the exhaust tube linear portions 54. The detecting part 55a of the air-fuel ratio sensor 55 is positioned at intersection point G of the center axes C1 and C2 of the exhaust tube linear portions 54. Also the air-fuel ratio sensor 55 is arranged such that the center axes C1 of the linear portions 54 of the exhaust tubes 51a and 51c form an angle with a center axis m of the air-fuel ratio sensor 55 that is different from the angle form between the center axes C2 of the linear portions 54 of the exhaust tubes 51d and 51b and the center axis m. More specifically, the center axes C1 of the linear portions 54 of the exhaust tubes 51a and 51c (which are farther from the air-fuel ratio sensor 5) are closer to being perpendicular to the center axis m of the air-fuel ratio sensor 55 that are the center axes C2 of the linear portions 54 of the exhaust tubes 51d and 51b (which are closer to the air-fuel ratio sensor 55). In other words, the center axes C2 of the linear portions 54 of the exhaust tubes 51d and 51b (which are closer to the air-fuel ratio sensor 55) are angled so that they are closer to being parallel to the center axis m of the air-fuel ratio sensor 55. With the angle shown in Figure 6, the angle γ2 between the center axes C2 of the linear portions 54 of the exhaust tubes 51d and 51b (which are closer to the air-fuel ratio sensor 55) and the center axis m of the air-fuel ratio sensor 55 is more acute than the angle γ1 between the center axes C1 of the linear portions 54 of the exhaust tubes 51a and 51c (which are farther from the air-fuel ratio sensor 55) and the center axis m of the air-fuel ratio sensor 55. That is, γ2 < γ1. If angle γ2 equals angle γ1, then the exhaust gas streams from the exhaust tubes 51d and 51b, which are closer to the air-fuel ratio sensor 55, will strike air-fuel ratio sensor 55 more strongly. This will cause thermal degradation of the air-fuel sensor 55. Making angle γ2 less than angle γ1 suppresses excessive striking of exhaust gas streams from the exhaust tubes 51d and 51b against the oxygen sensor 55. Thus, in this embodiment, the center axes of the linear portions 54 are angled such that the center axis C1 of the linear portion 54 that is farther from the air-fuel ratio sensor 55 is closer to being perpendicular to the center axis m of the air-fuel ratio sensor 55 than is the center axis C2 of the linear portion 54 that is closer to the air-fuel ratio sensor 55. In other words, the center axis C1 of the linear portion 54 that is closer to the air-fuel ratio sensor 55 is angled so that it is closer to being parallel to the center axis m of the air-fuel ratio sensor 55. Therefore, exhaust gas stream from the cylinder that is closer to the air-fuel ratio sensor 55 can be prevented from striking the air-fuel ratio sensor 55 too strongly when the load is high and thermal degradation of the air-fuel ratio sensor 55 can be prevented. As a result, the air-fuel ratio can be controlled more precisely and emissions can be reduced because degradation of the air-fuel ratio sensor 55 over time can be reduced. Referring now to Figure 15, an internal combustion engine exhaust manifold 60 is illustrated in accordance with a sixth embodiment of the present invention. Basically, the sixth embodiment differs from the fourth embodiment (Figure 13) in that the positions where the linear portions 64 of the exhaust tubes 61a to 61d merge with the collector case 63 are different as explained below. In view of the similarity between the sixth embodiment and the prior embodiments, the parts of the sixth embodiment that are identical to the parts of the prior embodiments will be given the same reference numerals increased by sixty as the parts of the first embodiment. Moreover, the descriptions of the parts of the sixth embodiment that are identical to the parts of the prior embodiments have been omitted for the sake of brevity. As shown in Figure 15, the positions where the exhaust tubes 61d and 61b (which are closer to the air-fuel ratio sensor 65 and whose exhaust gas streams strike more strongly against the air-fuel ratio sensor 65) merge with the collector case 63 are upstream with respect to the positions where the exhaust tubes 61a and 61c (which are farther from the air-fuel ratio sensor 65) merge with the collector case 63. As a result, the expansion of the exhaust gas streams from the exhaust tubes 61d and 61b inside the collector case 63 begins sooner and excessive striking of the exhaust gas against the air-fuel ratio sensor 65 can be prevented. In this embodiment of the present invention, each exhaust tube 61a-61d has a linear portion 64 directly and separately connected with the collector case 63. Consequently, the exhaust gas streams of each cylinder does not interfere with the exhaust gas streams of the other cylinders until it enters the collector case 63. Also, since the exhaust gas streams of each cylinder flows directly into the collector case 63, the amount of back-flow into the exhaust tubes of the other cylinders caused by exhaust pulsation is small. As a result, exhaust gas interference can be reduced and output can be improved. Also in this embodiment of the present invention, the linear portions 64 of the exhaust tubes 61a to 61d with center axis collector case 63 is different for each exhaust tube 61a to 61d. Consequently, the connections of the exhaust tubes 61a to 61d with the collector case 63 can be laid out with a higher degree of freedom. Moreover, the linear portions 64 of the exhaust tubes 61a to 61d with center axis collector case are such that the merging positions of the exhaust tubes 61b and 61d that are closer to the air-fuel ratio sensor 65 are farther upstream. Thus, the exhaust gas streams from exhaust tubes 61b and 61d that are close to the air-fuel ratio sensor 65 spread inside the collector case 63 by the time they reach the air-fuel ratio sensor 65. Therefore, exhaust gas can be prevented from striking the air-fuel ratio sensor 65 too strongly and thermal degradation of the air-fuel ratio sensor 65 can be prevented. As a result, the air-fuel ratio can be controlled more precisely and emissions can be reduced because degradation of the air-fuel ratio sensor 65 over time can be reduced. Referring now to Figure 16, an internal combustion engine exhaust manifold 70 is illustrated in accordance with a seventh embodiment of the present invention. Basically, the fourth (Figure 13) and seventh embodiments are identical, except that the outlet or downstream end of the collector case 73 has been changed as explained below. In view of the similarity between the seventh embodiment and the prior embodiments, the parts of the seventh embodiment that are identical to the parts of the prior embodiments will be given the same reference numerals increased by seventy as the parts of the first embodiment. Moreover, the descriptions of the parts of the seventh embodiment that are identical to the parts of the prior embodiments have been omitted for the sake of brevity. The seventh embodiment differs from the fourth embodiment (Figure 13) in that the front or upstream end face 76a of the catalytic converter 76 is provided with tilt angle when the catalytic converter 76 is mounted to the outlet of the collector case 73. In the present invention, an axial line or centerline C constituting the center of the center axes of the linear portions 74 of the exhaust tubes 71a to 71d is offset from a center axis n of the catalyst converter 76 and the front end face 76a of the catalyst converter 76 is angled such that the distance from the merging portion where the exhaust tubes 71a to 71d merge with the collector case 73 to the front end face 76a of the catalyst converter 76 becomes longer. Therefore, the catalyst converter 76 can be utilized effectively and emissions can be reduced because the exhaust gas flows more uniformly through the inside of the catalyst converter 76. More specifically, the center axis n of the catalytic converter 76 is positioned so as to be offset by an offset distance OF from the centerline C that represents an axial line of the center axes C1 and C2 of the linear portions 74 of the exhaust tubes 71a to 71d. In the illustrated embodiment, the centerline C bisects the angles between the center axes C1 and C2 of the linear portions 74 of the exhaust tubes 71a to 71d. Using perpendicular plane P, which is perpendicular to the centerline C, as a reference, the catalytic converter 76 is arranged so that the front end face 76a has a slant / tilt angle β. With this arrangement, the exhaust gas streams can form a flow that moves away from intersection point G of the exhaust tubes of the linear portions 74 at the front end face 76a of the catalytic converter 76. As a result, the flow of exhaust gas inside the catalytic converter 76 can be made more uniform. Referring now to Figure 17, an internal combustion engine exhaust manifold 80 is illustrated in accordance with an eighth embodiment of the present invention. Basically, the seventh and eighth embodiments are identical, except that the connection between the catalytic converter 86 and the outlet or downstream end of the collector case 83 has been changed as explained below. In view of the similarity between the seventh and eighth embodiments, the parts of the eighth embodiment that are identical to the parts of the first embodiment will be given the same reference numerals increased by eighty as the parts of the first embodiment. Moreover, the descriptions of the parts of the eighth embodiment that are identical to the parts of the prior embodiments have been omitted for the sake of brevity. Figure 17 shows an example in which the catalytic converter 86 has a different length on its oxygen sensor side than on its opposite side. The catalyst length n2 on the side adjacent the oxygen sensor 85 in the vicinity of the intersection point G of the exhaust tube linear portions 84 is longer than the catalyst length n1 on the opposite side (n2 > n1). The same effect can be obtained with this arrangement as with the arrangement in Figure 16. In this embodiment of the present invention, a front end face 86a of the catalytic converter 86 is angled with respect to a plane that is perpendicular to an axial line constituting the center C of the center axes C1 and C2 of the linear portions 84 of the exhaust tubes. Consequently, when the load is high and the flow rate of the exhaust gas is high, the flow of the exhaust gas concentrates on a portion of the catalytic converter 86 and thermal degradation of the catalyst can be prevented. As a result, reduction of the emission conversion rate of the catalyst can be prevented because degradation of the catalytic converter 86 over time can be prevented. Referring now to Figures 18 and 19, an internal combustion engine exhaust manifold 90 is illustrated in accordance with a ninth embodiment of the present invention. Basically, the first and ninth embodiments are identical, except that angles of the exhaust tube 91a to 91d have been changed as explained below. In view of the similarity between the first and ninth embodiments, the parts of ninth embodiment that are identical to the parts of the first embodiment will be given the same reference numerals increased by ninety as the parts of the first embodiment. Moreover, the descriptions of the parts of the ninth embodiment that are identical to the parts of the first embodiment have been omitted for the sake of brevity. The ninth embodiment differs from the first embodiment (Figure 1) and the fourth embodiment (Figure 13) in that the linear portions 94 of the exhaust tubes 91a to 91d of all cylinders are substantially parallel to one another just prior to the points that they are connected to the collector case 93. Here, a length LC of the collector case 93 as measured from the portion where the exhaust tubes 91a to 91d join the collector case 93 to the catalyst 96 is sufficiently long in comparison with the lengths L of the linear portions 4 up to the portions where the exhaust tubes 91a to 91d merge with the collector case 93. This arrangement assumes that the collector case 93 is longer than situations where the linear portions 94 are angled to form intersection points within the collector case 93 or down stream thereof, as in the embodiments described previously. Therefore, the linear portions 94 of the exhaust tubes 91a to 91d cause the flow of the exhaust gas streams to be directed in the direction of the linear portions 94. Since the linear portions 94 of all cylinders are substantially parallel, it is even more difficult (in comparison with a case where the linear portions are angled) for the exhaust gas streams to flow backward into one of the exhaust tubes of one of the other cylinders. As a result, exhaust interference is reduced further and output can be improved. As shown in Figure 19, the cross section of the collector case 93, as viewed upwardly along section line B-B of Figure 18, is larger than the area circumscribing the linear portions 94 of the exhaust tubes 91a to 91d. Also as shown in Figure 19, the air-fuel ratio sensor 95 is arranged such that its detecting part 95a is positioned within the projected cross sectional shape of the area circumscribing the substantially parallel linear portions 94 of the exhaust tubes 91a to 91d. Also, in consideration of the flow direction of the exhaust gas, the air-fuel ratio sensor 95 should be arranged at a position some distance away from the merging portion of the exhaust tubes 91a to 91d. Thus, even if the air-fuel ratio sensor 95 is disposed on the side with the exhaust tubes 91d and 91b, the exhaust gas on the side with the exhaust tube 91a and 91c will diffuse and pass through the air-fuel ratio sensor 95 and the concentration of the exhaust gas of each cylinder can be detected more precisely. However, with such an arrangement, there is the possibility that the temperature rise characteristic of the catalyst will worsen because the distance from the exhaust ports of the internal combustion engine to the catalyst installed at the outlet side of the collector case 93 is longer. Consequently, it is necessary to install the air-fuel ratio sensor 95 in a position where balance is achieved between the cylinder sensitivity of the sensor and the temperature rise characteristic of the catalyst. Although the air-fuel ratio sensor 95 is illustrated in Figure 19 as being installed between the exhaust tubes 91d and 91b, the air-fuel ratio sensor 95 can be installed anywhere within the circle of the projection plane. In this arrangement, the detecting part of the air-fuel ratio sensor 95 is positioned within projected cross sectional shape of the area circumscribing the substantially parallel linear portions 94 of the exhaust tubes 91a to 91d so that the air-fuel ratio sensor 95 can detect the concentration of the exhaust gas streams of each cylinder uniformly. As a result, the catalyst unit 96 can be utilized effectively and emissions can be reduced because the air-fuel ratio can be controlled with good precision. Additionally, since the linear portions 94 of the exhaust tubes 91a to 91d for all cylinders are substantially parallel, exhaust interference within the collector case 93 is reduced even further and further improvement of the output performance can be expected. This arrangement is inferior to that of the first embodiment in that the exhaust gas streams from the cylinders are only mixed to a small degree inside the collector case 93. However, mixing of the exhaust gas streams is not a problem if the length of the collector case 93 (i.e., the distance from the merging position of the exhaust tubes to the catalytic converter 96) is made long enough to allow through mixing. Referring now to Figure 20, an internal combustion engine exhaust manifold 120 is illustrated in accordance with a tenth embodiment of the present invention. Basically, the first and tenth embodiments are identical, except that the intersection point G relative to the collector case 123 has been modified in this tenth embodiment as explained below. In view of the similarity between the first and tenth embodiments, the parts of the tenth embodiment that are identical to the parts of the first embodiment will be given the same reference numerals as the parts of the first embodiment, but increased by one hundred and twenty. Moreover, the descriptions of the parts of the tenth embodiment that are identical to the parts of the first embodiment may be omitted for the sake of brevity. In the first embodiment, the center axes C1 and C2 of the linear portions 4 intersect at the intersection point G inside the collector case 3. In this tenth embodiment, the intersection point G of the center axes C1 and C2 of the linear portions 124 intersect at a location downstream of the collector case 123, as shown in Figure 20, since the length of collector case 123 is shorter than in the first embodiment. In this tenth embodiment, the concentration of the exhaust gas of each cylinder can be detected uniformly by positioning the detecting part 125a of the air-fuel ratio sensor 125 in the close to intersection point G of the center axes C1 and C2 of the linear portions 124. Here, exhaust interference is reduced because the linear portions 124 of the exhaust tube 121a of cylinder #1 and the exhaust tube 121c of cylinder #3, which cylinders have successive firing orders, are substantially parallel to each other. Likewise, the linear portions 124 of the exhaust tube 121d of cylinder #4 and the exhaust tube 121b of cylinder #2 are substantially parallel. Referring now to Figures 21 and 22, an internal combustion engine exhaust manifold 130 is illustrated in accordance with an eleventh embodiment not falling under the scope of the present invention. Basically, the first and eleventh embodiments are identical, except that the intersection point G relative to the collector case 133 has been modified in this eleventh embodiment as explained below. In view of the similarity between the first and eleventh embodiments, the parts of the eleventh embodiment that are identical to the parts of the first embodiment will be given the same reference numerals as the parts of the first embodiment, but increased by one hundred and thirty. Moreover, the descriptions of the parts of the eleventh embodiment that are identical to the parts of the first embodiment may be omitted for the sake of brevity. Figure 21 shows the basic features of the eleventh embodiment of the internal combustion engine exhaust manifold 130 while Figure 22 shows a schematic lateral cross sectional view of the internal combustion engine exhaust manifold 130 as viewed from the left. The eleventh embodiment is structured such that, when viewed from the side (Figure 21), the exhaust tubes of pairs of cylinders whose firing orders are not successive (i.e., the pair of cylinders #1 and # 4 coupled to the exhaust tubes 131a and 131d and the pair cylinders #2 and #34 coupled to the exhaust tubes 131b and 131c) are parallel. When viewed from the side (Figure 22), the exhaust tubes of pairs of cylinders whose firing orders are successive (i.e., the pair cylinders #1 and #3 coupled to the exhaust tubes 131a and 131c and the pair cylinders #2 and #4 coupled to the exhaust tubes 131b and 131d) are slanted to form a pair of intersection points G. This embodiment functions in the same manner as the first embodiment (Figure 1) and the fourth embodiment (Figure 13), except that the arrangement of the exhaust tubes 131a to 131d is different. Although the above examples illustrate engine exhaust manifolds for four cylinders, it will be apparent to those skilled in the art from this disclosure that each of the engine exhaust manifolds, discussed above, can be used with three cylinders. In examples with three cylinders (including examples with groups of three cylinders, such as a V-6 engine), the linear part of the exhaust tubes of all cylinders can be arranged such that their center axes intersect and form an intersection point. Figure 23 shows the cross sectional shape at the portion where the exhaust tube of each cylinder merges with the collector case in a situation where there are three cylinders (including V-6 engines). When there are three cylinders, the cross section of the exhaust tubes comprises fan shapes with 120-degree center angles therebetween. The present invention can also be applied to other numbers of cylinders by changing the center angle between the fan shapes. Based on this same principle, the present invention can also be applied to a six-cylinder inline engine. Moreover, terms that are expressed as "means-plus function" in the claims should include any structure that can be utilized to carry out the function of that part of the present invention. The terms of degree such as "substantially", "about" and "approximately" as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ± 5% of the modified term if this deviation would not negate the meaning of the word it modifies. While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. Thus, the scope of the invention is not limited to the disclosed embodiments. As described above, there is provided an exhaust manifold of an internal combustion engine is provided that comprises a collector case and a plurality of exhaust tubes. The collector case has an upstream end and a downstream end. The exhaust tubes have inlet ends that are adapted to be connected to exhaust ports of the internal combustion engine and outlet ends that are connected to the upstream end of the collector case by merging portions. The outlet ends of the exhaust tubes include linear portions disposed contiguously with the merging portions where the exhaust tubes merge with the collector case. The exhaust tubes have first exhaust tubes with the inlet ends of the first exhaust tubes arranged to receive exhaust gas from cylinders whose firing orders are not successive. The linear portions of the first exhaust tubes, whose firing orders are not successive, are slanted with respect to each other such that the linear portions of the first exhaust tubes, whose firing orders are not successive, have center axes intersecting at a point inside the collector case or downstream thereof.

Claims: An exhaust manifold of an internal combustion engine, comprising: a collector case (3) having an upstream end and a downstream end; and a plurality of exhaust tubes (1a-1d) having inlet ends adapted to be connected to exhaust ports of the internal combustion engine and outlet ends connected to said upstream end of said collector case (3) by merging portions, characterized in that said outlet ends include linear portions (4) disposed contiguously with said merging portions, where said exhaust tubes (1a-1d) merge with said collector case (3), wherein said linear portions (4) are arranged in pairs of parallel linear portions (4), and wherein one pair of parallel linear portions (4) is slanted with respect to a second pair of parallel linear portions (4), and comprising center axes (C1,C2) intersecting at an intersection point (G1, G2) inside said collector case (3) or downstream thereof, and said exhaust tubes (1a, 1c and 1b, 1d) with said inlet ends arranged to receive exhaust gas from cylinders, whose firing orders are successive, constitute one pair of parallel linear portions (4). An exhaust manifold according to claim 1, characterized in that a positioning point of a detecting part (5a) of an air-fuel ratio sensor (5) is arranged at said intersection point (G1, G2) or adjacent thereto. An exhaust manifold according to claim 2, characterized in that said detecting part (55a) of said air-fuel ratio sensor (55) has a center axis (m); and said center axes (C1 and C2) of said linear portions (54) of said exhaust tubes (51a-51d) and said center axis (m) of said air-fuel ratio sensor (55) form angles (γ1, γ2) therebetween with said angles (γ1) of said linear portions (54) of said exhaust tubes (51a-51d) farther from said air-fuel ratio sensor (55) being closer to perpendicular than said angles (γ2) of said linear portions (54) of said exhaust tubes (51a-51d) that are closer to said air-fuel ratio sensor (5). An exhaust manifold according to at least one of the preceding claims 1 to 3, characterized in that each of said merging portions of said exhaust tubes (61a-61d) has a longitudinal merging position with respect to said collector case (63) with at least some of longitudinal said merging positions being located at different distances as measure in an air stream direction of each of said exhaust tubes (61a-61d) into said collector case (63). An exhaust manifold according to claim 4, characterized in that said collector case (63) has said air-fuel ratio sensor (65) located in said collector case (63), and said longitudinal merging positions of said exhaust tubes (61a-61d) that are closer to said air-fuel ratio sensor (65), as measure in a transverse direction to the air stream directions, are spaced farther upstream from said air-fuel ratio sensor (65) as measure in a longitudinal direction of the air stream directions. An exhaust manifold according to at least one of the preceding claims 1 to 5, characterized in that said collector case (73 or 83) has a catalyst unit (76 or 86) with a front end face (76a or 86a) connected to said downstream end of said collector case (73 or 83), wherein said front end face (76a or 86a) is angled with respect to a reference plane (P) which is perpendicular to an axial line (C) that represents a centerline of said center axes (C1 and C2) of said linear portions (74 or 84) of said exhaust tubes (71a-71d or 81a-81d). An exhaust manifold according to claim 6, characterized in that said catalyst unit (76 or 86) has a center axis (n) that is offset from said centerline (C) of said center axes (C1 and C2) of said linear portions (74 or 84) of said exhaust tubes (71a-71d or 81a-81d), and said front end face (76a or 86a) of said catalyst unit (76 or 86) is angled such that distances of said merging portions to said front end face (76a or 86a) becomes longer across said front end face (76a or 86a) of said catalyst unit (76 or 86). An exhaust manifold according to at least one of the preceding claims 1 to 7, characterized in that said linear portions (4) of said exhaust tubes (1a-1d) have substantially fan-shaped cross sectional shapes that are substantially equal in size where said exhaust tubes (1a-1d) are connected to said collector case (3) by said merging portions. An exhaust manifold according to claim 8, characterized in that each of said center axes (C1 and C2) of said linear portions (4) of said exhaust tubes (1a-1d) is a line that is oriented in a direction of flow and passes through a center of gravity of one of said substantially fan-shaped cross sections of said linear portions (4).

Inventors: HASEGAWA KAZUYA, MORI KOUJI, YOSHIZAWA KOUDAI, HASEGAWA, KAZUYA, MORI, KOUJI, YOSHIZAWA, KOUDAI
Applicants: NISSAN MOTOR, NISSAN MOTOR COMPANY, LIMITED
Application year: 2001
Main classification: F01N7
IPC codes: F01N3, F02D41, F02D35, F01N13, F01N99