Abstract:
An exhaust system of an internal combustion engine having at least one cylinder comprises a main exhaust passage connected to the cylinder; a main catalytic converter disposed in the main exhaust passage; a bypass exhaust passage that diverges from the main exhaust passage, the bypass exhaust passage having a gas flow resistance larger than that of the main exhaust passage and having a downstream end connected to the main exhaust passage at a position upstream of the main catalytic converter; an auxiliary catalytic converter disposed in the bypass exhaust passage; and a gas flow switching device that is capable of forcing exhaust gas from the cylinder of the engine to flow toward the bypass exhaust passage when assuming a given operation position.

Description:
[0001]     This application is C-I-P of U.S. patent application Ser. No. 11/145,962, filed Jun. 7, 2005, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates in general to exhaust systems of a multi-cylinder internal combustion engine, that have a catalytic converter for purifying the exhaust gas from the engine, and more particularly to the exhaust systems of a double converter type that has main and auxiliary catalytic converters wherein when, like in a condition just after cold starting of the engine, the main catalytic converter is not sufficiently activated, the exhaust gas is enforcedly led to the auxiliary catalytic converter that is easily activated.  
         [0004]     2. Description of the Related Art  
         [0005]     Usually, in motor vehicles powered by an internal combustion engine, the exhaust system of the engine is arranged beneath a floor of the vehicle and has a catalytic converter installed therein. If the catalytic converter is arranged in a relatively downstream position of the system, the converter takes a long time to be heated to a sufficient level for the gas purifying operation thereof particularly in a condition just after cold starting of the engine. That is, for such long time, the catalytic converter fails to exhibit its normal gas purifying work. However, if, for solving the above-mentioned drawback, the catalytic converter is arranged in a relatively upstream position of the system, that is, a position near the engine, another drawback tends to arise wherein due to attack of heat of the engine and the highly heated exhaust gas from the engine, the durability of the catalytic converter is lowered.  
         [0006]     In order to solve the above-mentioned drawbacks, various measures have been proposed, one of which is disclosed in Japanese Laid-open Patent Application (Tokkaihei) 5-321644. In this measure, a main passage extends from an exhaust manifold of the engine to a main catalytic converter. A bypass passage having an auxiliary catalytic converter installed therein extends from an upstream part of the main passage to a downstream part of the same. A switch valve is arranged at the upstream part of the main passage to open and close the main and bypass passages selectively, and a controller is connected to the switch valve. In operation, just after cold starting of the engine, the controller controls the switch valve to take a bypass position to introduce the exhaust gas from the engine into the bypass passage.  
         [0007]     Since, in this measure, the auxiliary catalytic converter is positioned at a relatively upstream portion of the exhaust system, earlier activation of the auxiliary catalytic converter is expected, which induces earlier exhaust gas purifying work by the exhaust system.  
       SUMMARY OF THE INVENTION  
       [0008]     In the measure of the above-mentioned patent application publication, the gas inlet port of the bypass passage is positioned downstream of a branch junction portion of the exhaust manifold. That is, from a portion that is downstream of the junction portion where exhaust gas paths of all cylinders of the engine are joined, there extend the main and bypass passages in parallel. Thus, even though the auxiliary catalytic converter can take a more upstream position than the main catalytic converter, the distance from the exhaust port of each cylinder to the auxiliary catalytic converter can not be so short, and thus actually, the earlier exhaust gas purifying work in the cold starting of the engine is not effectively carried out.  
         [0009]     Because of branching of the bypass passage from the downstream position of the exhaust manifold, the exhaust manifold that has a marked thermal capacity causes the temperature of the exhaust gas led into the bypass passage to be lowered, which obstructs the earlier exhaust gas purifying work of the auxiliary catalytic converter.  
         [0010]     Furthermore, since the modernized exhaust manifolds are designed and constructed to avoid or at least minimize an exhaust interference, it is difficult to shorten the distance to the auxiliary catalytic converter from the inlet of the bypass passage. That is, in case of an exhaust manifold for in-line four cylinder engines, a so-called “4-2-1” connection type is currently employed wherein branches for #1 and #4 cylinders form one unit and braches for #2 and #3 cylinders form the other unit, and these two units are united at an outlet portion of the manifold. As is easily known, in this type exhaust manifold, it is difficult to reduce the overall length. The above-mentioned “4-2-1” connection type tends to have a complicated construction of the manifold, which increases the thermal capacity of the same.  
         [0011]     Furthermore, it has been difficult to operate the switch valve accurately. Actually, even when the switch valve is controlled to take the bypass position, part of the exhaust gas is led to the main passage due to a poor sealing of the switch valve to the main passage. Of course, in such case, the early activation of the auxiliary catalytic converter is adversely affected by a certain degree that corresponds to the amount of the exhaust gas led to the main passage. This undesired phenomenon becomes much severe when the pulsation of the exhaust gas is marked.  
         [0012]     It is therefore a main object of the present invention to provide an exhaust system of a multi-cylinder internal combustion engine, which is free of the above-mentioned drawbacks.  
         [0013]     In accordance with a first aspect of the present invention, there is provided an exhaust system of an internal combustion engine having at least one cylinder, which comprises a main exhaust passage connected to the cylinder; a main catalytic converter disposed in the main exhaust passage; a bypass exhaust passage that diverges from the main exhaust passage, the bypass exhaust passage having a gas flow resistance larger than that of the main exhaust passage and having a downstream end connected to the main exhaust passage at a position upstream of the main catalytic converter; an auxiliary catalytic converter disposed in the bypass exhaust passage; and a gas flow switching device that is capable of forcing exhaust gas from the cylinder of the engine to flow toward the bypass exhaust passage when assuming a given operation position.  
         [0014]     In accordance with a second aspect of the present invention, there is provided an exhaust system of an in-line four cylinder internal combustion engine, which comprises first, second, third and fourth upstream main exhaust passages extending from first, second, third and fourth cylinders of the engine, the first and fourth cylinders being those whose firing order is not successive and the second and third cylinders being those whose firing order is not successive; a first intermediate main exhaust passage that is provided by joining downstream ends of the first and fourth upstream main exhaust passages; a second intermediate main exhaust passage that is provided by joining downstream ends of the second and third upstream main exhaust passages; a downstream main exhaust passage that is provided by joining downstream ends of the first and second intermediate main exhaust passages; a main catalytic converter mounted in the downstream main exhaust passage; first and second upstream bypass exhaust passages respectively extending from upstream portions of the first and second upstream main exhaust passages; third and fourth upstream bypass exhaust passages respectively extending from upstream portions of the third and fourth upstream main exhaust passages; a first intermediate bypass exhaust passage that is provided by jointing downstream ends of the first and second upstream bypass exhaust passages; a second intermediate bypass exhaust passage that is provided by joining downstream ends of the third and fourth upstream bypass exhaust passages; a downstream bypass exhaust passage that is provided by joining downstream ends of the first and second intermediate bypass exhaust passages, the downstream bypass exhaust passage having a downstream end connected to the downstream main exhaust passage at a position upstream of the main catalytic converter; and an auxiliary catalytic converter mounted in the downstream bypass exhaust passage.  
         [0015]     In accordance with a third aspect of the present invention, there is provided an exhaust system of an in-line four cylinder internal combustion engine, which comprises first, second, third and fourth upstream main exhaust passages extending from first, second, third and fourth cylinders of the engine, the first and fourth cylinders being those whose firing order is not successive and the second and third cylinders being those whose firing order is not successive; a first intermediate main exhaust passage that is provided by joining downstream ends of the first and fourth upstream main exhaust passages; a second intermediate main exhaust passage that is provided by joining downstream ends of the second and third upstream main exhaust passages; a downstream main exhaust passage that is provided by joining downstream ends of the first and second intermediate main exhaust passages; a main catalytic converter mounted in the downstream main exhaust passage; first and second upstream bypass exhaust passages respectively extending from upstream portions of the first and second upstream main exhaust passages; third and fourth upstream bypass exhaust passages respectively extending from upstream portions of the third and fourth upstream main exhaust passages; a first intermediate bypass exhaust passage that is provided by jointing downstream ends of the first and second upstream bypass exhaust passages; a second intermediate bypass exhaust passage that is provided by joining downstream ends of the third and fourth upstream bypass exhaust passages; a downstream bypass exhaust passage that is provided by joining downstream ends of the first and second intermediate bypass exhaust passages, the downstream bypass exhaust passage having a downstream end connected to the downstream main exhaust passage at a position upstream of the main catalytic converter; an auxiliary catalytic converter mounted in the downstream bypass exhaust passage; and switch valves respectively mounted in the first, second, third and fourth upstream main exhaust passages. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Other objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings, in which:  
         [0017]      FIG. 1  is a conceptual drawing showing an exhaust system of a first embodiment of the present invention;  
         [0018]      FIG. 2  is a side view of the exhaust system of the first embodiment;  
         [0019]      FIG. 3  is a bottom view of an exhaust manifold employed in the first embodiment;  
         [0020]      FIG. 4  is a top view of the exhaust manifold employed in the first embodiment;  
         [0021]      FIG. 5  is an enlarged sectional view of a switch valve unit and its nearby portion;  
         [0022]      FIG. 6  is a front view of the switch valve unit;  
         [0023]      FIG. 7  is a view of a link mechanism for synchronously actuating two switch valves that constitute the switch valve unit;  
         [0024]      FIG. 8  is a sectional view of a switch valve unit employed in an exhaust system of a second embodiment of the present invention;  
         [0025]      FIG. 9  is a sectional view of a switch valve unit employed in an exhaust system of a third embodiment of the present invention;  
         [0026]      FIG. 10  is a sectional view of a switch valve unit employed in an exhaust system of a fourth embodiment of the present invention;  
         [0027]      FIG. 11  is a sectional view of a switch valve unit employed in an exhaust system of a first embodiment of the present invention;  
         [0028]      FIG. 12  is a view similar to  FIG. 1 , but showing an exhaust system of a sixth embodiment of the present invention;  
         [0029]      FIG. 13  is a sectional view of an essential portion of an exhaust system of a seventh embodiment of the present invention, in which an auxiliary catalytic converter is directly connected to a cylinder head of an engine;  
         [0030]      FIG. 14  is a conceptional drawing showing an exhaust system of a seventh embodiment of the present invention;  
         [0031]      FIG. 15  is a plan view of a switch valve unit installed in an exhaust system of an eighth embodiment of the present invention;  
         [0032]      FIG. 16  is a sectional view of one switch valve of the switch valve unit of  FIG. 15 , that is, a sectional view of the portion indicated by the arrow “XVI” of  FIG. 15 ;  
         [0033]      FIG. 17  is a schematic illustration showing operation of the switch valve unit in case of the eighth embodiment;  
         [0034]      FIG. 18 a  sectional view similar to  FIG. 16 , but showing a switch valve unit that is to be installed in an exhaust system of a ninth embodiment of the present invention; and  
         [0035]      FIG. 19  is a sectional view also similar to  FIG. 16 , but showing a switch valve unit that is to be installed in an exhaust system of a tenth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0036]     In the following, various embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
         [0037]     For ease of understanding, various directional terms, such as, right, left, upper, lower, rightward and the like are used in the following description. However, these terms are to be understood with respect to only a drawing or drawings on which the corresponding part or portion is shown.  
         [0038]     Referring to  FIG. 1 , there is shown a conceptual drawing of an exhaust system  100  of a first embodiment of the present invention. In this embodiment, an in-line four-cylinder internal combustion engine is employed for embodying the invention.  
         [0039]     Four cylinders  1 , viz., #1, #2, #3 and #4 are aligned in a cylinder block of the engine. From each cylinder  1 , there extends an upstream main exhaust passage  2 .  
         [0040]     It is to be noted that upstream main exhaust passage  2  is a passage that extends from an exhaust valve (not shown) of the engine to a downstream part where an after-mentioned junction portion with the other upstream main exhaust passage  2  is provided.  
         [0041]     As shown, upstream main exhaust passages  2  from cylinders #1 and #4 whose firing order is not successive are joined by a first intermediate main exhaust passage  3 A, and upstream main exhaust passages  2  from cylinders #2 and #3 whose firing order is not successive are joined by a second intermediate main exhaust passage  3 B.  
         [0042]     In each of the joined portions, there is installed a switch valve  4 A or  4 B. As will be described in detail hereinafter, switch valves  4 A and  4 B are controlled by a single actuator.  
         [0043]     That is, when the engine is in a condition just after cold staring, switch valves  4 A and  4 B are controlled by the actuator to take their closed position blocking a fluid communication between each main exhaust passage  2  and corresponding intermediate main exhaust passage  3 A or  3 B, and at the same time, blocking a fluid communication between the two upstream main exhaust passages  2  which are joined.  
         [0044]     As will be described in detail hereinafter, the two switch valves  4 A and  4 B constitute a switch valve unit  5 A (see  FIGS. 2 and 5 ).  
         [0045]     As is seen from  FIG. 1 , first and second intermediate main exhaust passages  3 A and  3 B extending from switch valve unit  5  are joined at a junction portion  6  and a downstream main exhaust passage  7  extends downstream from the junction portion  6 .  
         [0046]     In downstream main exhaust passage  7 , there is installed a main catalytic converter  8  that includes a three-way catalyst and a HC (hydrocarbon) trapping catalyst. This main catalytic converter  8  is arranged beneath a vehicle floor and has a sufficient capacity.  
         [0047]     Thus, the four upstream main exhaust passages  2 , the two intermediate main exhaust passages  3 A and  3 B, the downstream main exhaust passage  7  and the main catalytic converter  8  constitute a main exhaust passage structure through which the exhaust gas from the engine is permitted to flow in a normal operation condition of the engine. That is, in such normal operation condition, the “4-2-1” connection type passage arrangement of the exhaust system is defined, and thus, the charging efficiency of the cylinders is increased with the aid of the exhaust dynamic effect.  
         [0048]     As is seen from  FIG. 1 , from a branched portion  12  of each upstream main exhaust passage  2 , there extends an upstream bypass exhaust passage  11 .  
         [0049]     It is to be noted that the branched portion  12  is provided in the passage  2  as upstream as possible. More specifically, the branched portion  12  should be provided at least in a range that is upstream to a ½ point of the entire length of upstream main exhaust passage  2 .  
         [0050]     Each upstream bypass exhaust passage  11  has a cross-sectional area that is sufficiently smaller than that of the corresponding upstream main exhaust passage  2 .  
         [0051]     As shown, upstream bypass exhaust passages  11  from the main exhaust passages  2  from cylinders #1 and #2 are united at a junction portion  13 , and a first intermediate bypass exhaust passage  14 A extends downstream from the junction portion  13 . Similarly, upstream bypass exhaust passages  11  from the main exhaust passages  2  from cylinders #3 and #4 are joined at a junction portion  13 , and a second intermediate bypass exhaust passage  14 B extends downstream from the junction portion  13 . Each bypass exhaust passage  11  is made as short as possible.  
         [0052]     As shown, the two intermediate bypass exhaust passages  14 A and  14 B are united at a junction portion  15 , and a downstream bypass exhaust passage  16  extends downstream from junction portion  15 . The downstream bypass exhaust passage  16  is led to a junction portion  17  provided on the downstream main exhaust passage  7  at a position upstream of main catalytic converter  8 , as shown.  
         [0053]     In downstream bypass exhaust passage  16 , there is installed an auxiliary catalytic converter  18  that includes a three-way catalyst. This auxiliary catalytic converter  18  is arranged in the bypass exhaust passage as upstream as possible. In other words, the length of each bypass exhaust passage between branched portion  12  and junction portion  15  should be made as short as possible.  
         [0054]     If desired, the four upstream bypass exhaust passages  11  may be joined at a position just upstream of auxiliary catalytic converter  18  without the aid of the above-mentioned two intermediate bypass exhaust passages  14 A and  14 B. However, when considering a fixed positioning between each branched portion  12  and auxiliary catalytic converter  18 , the above-mentioned “4-2-1” united type passage arrangement is preferable in the present invention. That is, in this type, the entire length of the passages practically used can be reduced, and thus, the piping used can have a reduced thermal capacity and a reduced heat radiation area to the atmosphere.  
         [0055]     As shown in  FIG. 1 , auxiliary catalytic converter  18  comprises a first catalyst part  18   a  and a second catalyst part  18   b  which are arranged in tandem. Between these two parts  18   a  and  18   b , there is defined a certain clearance  19  to which an inlet part of EGR (viz., exhaust gas recirculation) passage  20  is exposed. Although not shown in the drawing, an outlet part of this EGR passage  20  is exposed to a part of an air intake system of the engine through an EGR control valve. That is, during operation of the engine, part of the exhaust gas is picked up from the clearance  19  and led to the air intake system.  
         [0056]     It is to be noted that auxiliary catalytic converter  18  has a small capacity as compared with main catalytic converter  8 , and is of a type that can exhibits a sufficient activation even in a relatively cold condition.  
         [0057]     When, in operation, the engine is in a condition just after a cold staring, that is, when the temperature of the exhaust gas from the engine is not sufficiently high, the actuator (not shown) causes the two switch valves  4 A and  4 B to take a closed position, thereby blocking the main exhaust passage. Under this condition, the exhaust gas from the engine is entirely led to the four upstream bypass exhaust passages  11  from the branched portions  12  and then led to auxiliary catalytic converter  18  through the two intermediate bypass exhaust passages  14 A and  14 B.  
         [0058]     Since auxiliary catalytic converter  18  is positioned at the upstream side of the exhaust system, that is, near the cylinders #1, #2, #3 and #4, and a smaller capacity, the converter  18  is heated quickly and thus activated quickly, and thus a sufficient exhaust purifying operation of the auxiliary catalytic converter  18  can start early.  
         [0059]     Because, under this condition, each switch valve  4 A or  4 B isolates the paired upstream main exhaust passages  2 , undesired exhaust gas temperature reduction, that would be caused by a reciprocating movement of the exhaust gas between the paired upstream main exhaust passages  2  through the switch valve  4 A or  4 B, is suppressed or at least minimized. This promotes the quick activation of the auxiliary catalytic converter  18 .  
         [0060]     Furthermore, since, under this condition, the exhaust gas led into EGR passage  20  is the gas that has been purified or cleaned by first catalyst part  18   a  of the auxiliary converter  18 , the EGR system, particularly, the EGR gas control valve of the same can be protected from solid deposit and contamination.  
         [0061]     While, when, due to continuous operation of the engine, the exhaust gas from the engine shows a sufficiently high temperature, the actuator causes the two switch valves  4 A and  4 B to take an open position, thereby establishing an open condition of the main exhaust passage.  
         [0062]     With this, the exhaust gas from the engine is mainly led into the four upstream main exhaust passages  2  and into the main catalytic converter  8  through the paired intermediate main exhaust passages  3 A and  3 B. Because each upstream bypass exhaust passage  11  has a cross-sectional area that is smaller than that of the corresponding upstream main exhaust passage  2 , and because of presence of the auxiliary catalytic converter  18  in the bypass exhaust system, almost all of the exhaust gas from the engine is forced to flow in the main exhaust system. Thus, in this case, the auxiliary catalytic converter  18  is free of a thermal degradation. Furthermore, since the bypass exhaust passage is opened but in a small degree, a part of the exhaust gas is permitted to enter the bypass exhaust passage when, under a high speed and high load operation condition, a larger amount of exhaust gas is produced by the engine. This prevents lowering of charging efficiency of the cylinders caused by a back pressure.  
         [0063]     As is described hereinabove, the main exhaust passage system is in the form of “4-2-1” united type arrangement, and thus, increase in the charging efficiency of the cylinders is achieved with the aid of the exhaust dynamic effect.  
         [0064]     While, the bypass exhaust passage system is arranged in the above-mentioned manner without taking the exhaust interference avoidance into a consideration. However, since each upstream bypass exhaust passage  11  used has a sufficiently smaller cross-sectional area, the exhaust interference inevitably induced by a communication with the corresponding cylinder can be reduced to a very small degree. If the cross-sectional area of upstream bypass exhaust passage  11  is made larger than a predetermined high degree, a marked lowering of the charting efficiency would take place due to the exhaust interference. While, if the cross-sectional area is made smaller than a predetermined low degree, the amount of exhaust gas during the time when switch valves  4 A and  4 B are kept in their closed position is excessively reduced, and thus, the operation range in which the bypass exhaust system can be practically operated is excessively reduced.  
         [0065]     Thus, actually, the cross-sectional area of upstream bypass exhaust passage  11  should have a range that varies in accordance with an engine displacement. Examination has revealed that when the engine has a displacement of 2000 cc, a sufficient performance is obtained from the bypass exhaust system with an inner diameter of upstream bypass exhaust passage  11  that ranges from 5 mm to 15 mm.  
         [0066]     If operation of the EGR system is carried out under the open position of switch valves  4 A and  4 B, the exhaust gas for the recirculation is picked up from auxiliary catalytic converter  18 . If, under this EGR operation, a part of the exhaust gas flowing in downstream main exhaust passage  7  flows backward in downstream bypass exhaust passage  16 , the exhaust gas in the passage  16  is forced to pass through second catalyst part  18   b  of the converter  18 , and thus, the exhaust gas used for the EGR system is not contaminated and thus, the EGR system, particularly, the EGR gas control valve is protected from solid deposit and contamination. The exhaust gas flowing backward in downward bypass exhaust passage  16  is relatively slow in flow speed, and thus, the exhaust gas can stay in second catalyst part  18   b  for a sufficient time. Thus, second catalyst part  18   b  may have a size or length smaller or shorter than first catalyst part  18   a , as shown.  
         [0067]     In the following, detailed explanation on the exhaust system  100  of the first embodiment will be made with reference to  FIG. 2 .  
         [0068]     In the drawing, denoted by numeral  31  is an internal combustion engine, which comprises a cylinder block  32  and a cylinder head  33  mounted on cylinder block  32 . The engine  31  is transversely mounted in an engine room of a motor vehicle. In the drawing, a right side of engine  31  faces rearward of the vehicle, and thus, the right side will be referred to a back side in the following description.  
         [0069]     An exhaust manifold  34  is mounted to the back side of cylinder head  33 , that defines therein the above-mentioned four upstream main exhaust passages  2 . To a rear end of exhaust manifold  34 , there is mounted the switch valve unit  5 A that is equipped with the two switch valves  4 A and  4 B. Extending downstream from switch valve unit  5 A is a front exhaust tube  35  that constitutes the above-mentioned downstream main exhaust passage  7 . An upstream portion of front exhaust tube  35  has therein two parallel passages that constitute the above-mentioned intermediate main exhaust passages  3 A and  3 B. Main catalytic converter  8  is mounted on the front exhaust tube  35  at a position downstream of the parallel passages  3 A and  3 B.  
         [0070]     As shown in  FIG. 2 , auxiliary catalytic converter  18  and its associated parts  11 ,  14 A,  14 B,  16  and  20  are arranged below the main exhaust system that extends rearward from cylinder head  33  of the engine  31 . Auxiliary catalytic converter  18  is placed in the engine room in front of front exhaust tube  35 . Thus, under running of the associated motor vehicle, auxiliary catalytic converter  18  is effectively cooled by air flow that is produced when the vehicle runs, and thus, overheating of this converter  18  is suppressed.  
         [0071]     As shown, each upstream bypass exhaust passage  11  is branched at the branched portion  12  from the corresponding upstream main exhaust passage  2  in such a manner as to define an acute angle therebetween, which smoothes the gas flow from the main passage  2  to the bypass passage  11  at the time when switch valves  4 A and  4 B take their closed position. It is to be noted that downstream portions of the two bypass exhaust passages  11  are united to constitute the fist intermediate bypass exhaust passage  14 A, and downstream portions of the other two bypass exhaust passages  11  are united to constitute the second intermediate bypass exhaust passage  14 B.  
         [0072]     The arrangement of the exhaust system  100  of the first embodiment will be much clearly understood from  FIGS. 3 and 4  of the accompanying drawings.  
         [0073]      FIG. 3  shows a bottom view of exhaust manifold  34 , and  FIG. 4  shows a top view of the same.  
         [0074]     As is seen from these drawings, exhaust manifold  34  comprises four branches  41 ,  42 ,  43  and  44  that constitute upstream main exhaust passages  2  respectively, two mounting flanges  45  and  46  that are to be fixed to the cylinder head of the engine and have openings (no numerals) to which upstream ends of branches  41 ,  42 ,  43  and  44  are exposed respectively, and four bypass tubes  11  that extend from the upstream portions ( 12 ) of corresponding branches  41 ,  42 ,  43  and  44  to constitute the upstream bypass exhaust passages  11  respectively.  
         [0075]     As shown, the two bypass tubes  11  extending from branches  41  and  42  from cylinders # 1  and # 2  are united at their downstream portions to constitute a united passage portion that corresponds to the first intermediate bypass passage  14 A, and the other two bypass tubes  11  extending from branches  43  and  43  from cylinders #3 and #4 are united at their downstream portions to constitute another united passage portion that corresponds to the second intermediate bypass passage  14 B.  
         [0076]     These two united passage portions  14 A and  14 B have a mounting flange  47  that has an opening (no numeral) to which downstream ends of the passage portions  14 A and  14 B are exposed. Although not shown in these drawings, an inlet portion of the above-mentioned auxiliary catalytic converter  18  is secured to mounting flange  47 .  
         [0077]     As will be understood from  FIGS. 3 and 4 , at a downstream portion of four branches  41 ,  42 ,  43  and  44  of exhaust manifold  34 , there are provided two mounting flanges  48 A and  48 B which are united.  
         [0078]     As is seen from  FIG. 3 , mounting flange  48 A has two openings to which branches  41  and  44  are connected respectively, and the other mounting flange  48 B has two openings to which the other branches  42  and  43  are connected respectively. If desired, in place of using the two mounting flanges  48 A and  48 B,  30  a single mounting flange may be used. In this case, the mounting flange has four isolated openings to which the four branches  41 ,  44 ,  42  and  43  are connected respectively in the above-mentioned manner.  
         [0079]     Referring to  FIGS. 5 and 6 , there is shown the detail of switch valve unit  5 A that is operatively connected to the two mounting flanges  48 A and  48 B.  
         [0080]     As is well seen from  FIG. 5 , switch valve unit  5 A comprises a casing  51  that has four openings, which are a first pair of openings  52  and  55  and a second pair of openings  53  and  54 . Upon connection of the casing  51  with the two mounting flanges  48 A and  48 B, the two openings of mounting flange  48 A are connected to the paired openings  52  and  55 , and the two openings of the other mounting flange  48 B are connected to the other paired openings  53  and  54 .  
         [0081]     As is seen from  FIG. 5  and as has been mentioned hereinabove, from switch valve unit  5 A, there extends the front exhaust tube  35  that has at its upstream portion two parallel passages  3 A and  3 B (which are the above-mentioned first and second intermediate main exhaust passages  3 A and  3 B) partitioned by an axially extending partition wall  59 . One of the two parallel passages  3 A and  3 B is communicated with the paired openings  52  and  55 , and the other one of the parallel passages  3 A and  3 B is communicated with the other paired openings  53  and  54 .  
         [0082]     Behind the paired openings  52  and  55 , there is pivotally arranged a first switch valve  4 A that comprises a pivot shaft  56  actuated by an after-mentioned actuator, a body holder  57  integral with pivot shaft  56  and a flat rectangular valve body  58  fitted to body holder  57 . Similarly, behind the other paired openings  53  and  55 , there is pivotally arranged a second switch valve  4 B that also comprises a pivot shaft  56  actuated by the actuator, a body holder  57  integral with pivot shaft  56  and a flat rectangular valve body  58  fitted to body holder  57 .  
         [0083]     Thus, first switch valve  4 A functions to selectively open and close the paired openings  52  and  55  at the same time, and second switch valve  4 B functions to selectively open and close the other paired openings  53  and  54  at the same time. That is, first switch valve  4 A functions to selectively open and close a communication between a group of cylinders #1 and #4 and one passage  3 A, and second switch valve  4 B functions to selectively open and close a communication between the other group of cylinders #2 and #3 and the other passage  3 B, as is understood from the drawing.  
         [0084]     When first switch valve  4 A assumes the closed position, a communication between the paired openings  52  and  55  is also blocked and thus a communication between main exhaust passage  2  for cylinder #1 and main exhaust passage  2  for cylinder #4 is blocked, and when second switch valve  4 B assumes the closed position, a communication between the paired openings  53  and  54  is also blocked and thus a communication between main exhaust passage  2  for cylinder #2 and main exhaust passage  2  for cylinder #3 is blocked.  
         [0085]     When first and second switch valves  4 A and  4 B are opened, the exhaust gas from cylinders #1 and #4 is led into first intermediate main exhaust passage  3 A, and at the same tirrie, the exhaust gas from cylinders #2 and #3 is led into second intermediate main exhaust passage  3 B.  
         [0086]     As will be described in detail hereinafter, a link mechanism actuated by the single actuator is arranged between the respective pivot shafts  56  of first and second switch valves  4 A and  4 B, so that these two valves  4 A and  4 B assume their open and closed positions synchronously.  
         [0087]      FIG. 7  shows the detail of the link mechanism actuated by the single actuator  64 . The link mechanism comprises a first link plate  61 A fixed to pivot shaft  56  of first switch valve  4 A, a second link plate  61 B fixed to pivot shaft  56  of second switch valve  4 B, a rod  62  having one end pivotally connected to first link plate  61 A and the other end pivotally connected to second link plate  61 B, and a third link plate  63  fixed to pivot shaft  56  of first switch valve  4 A. As shown, first and second link plates  61 A and  61 B are arranged to define therebetween an angle of 90 degrees. The actuator  64  may be of a vacuum power type or an electromagnetic type, which has an output member connected to third link plate  63  through a rod  65 . When the actuator  64  is energized, the output member of the same pushes rod  65 . With this, pivot shaft  56  of first switch valve  4 A is turned clockwise in  FIG. 7  and at the same time pivot shaft  56  of second switch valve  4 B is turned counterclockwise, and thus, first and second switch valves  4 A and  4 B assume their closed position.  
         [0088]     Referring to  FIG. 8 , there is shown a switch valve unit  5 B that is installed in an exhaust system  200  of a second embodiment of the present invention. For easy understanding of this embodiment, the drawing is schematically illustrated with some illustration contradiction.  
         [0089]     Valve unit  5 B employed in this embodiment  200  has two butterfly valves  70  for the respective switch valves  4 A and  4 B.  
         [0090]     Each butterfly valve  70  comprises a pivot shaft  71  that is arranged at an upstream end portion of the corresponding passage  3 A or  3 B. Each pivot shaft  71  has two flat valve bodies  72   a  and  72   b  that extend radially outward therefrom. As shown, flat valve bodies  72   a  and  72   b  are somewhat offset with respect to an axis of the pivot shaft  71 .  
         [0091]     A partition wall  73  extends axially in the upstream end portion of each passage  3 A or  3 B to a position near the corresponding pivot shaft  71 , and thus the upstream end portion is divided into two passages  52 A and  55 A (or  53 A and  54 A) which are merged with openings  52  and  55  respectively (or openings  53  and  54 ).  
         [0092]     Seal members  74  are secured to inner wall of each passage  3 A or  3 B. As shown, when switch valve  4 A or  4 B takes its closed position, the two flat valve bodies  72   a  and  72   b  close downstream ends of the two passages  52 A and  55 A (or  53 A and  54 A), respectively. Under this condition, each valve body  72   a  or  72   b  is in contact with the corresponding seal member  74 .  
         [0093]     That is, when first switch valve  4 A assumes the closed position, the communication between each of the paired openings  52  and  55  and the passage  3 A is blocked and at the same time, the communication between the paired openings  52  and  55  is also blocked. Similarly, when second switch valve  4 B assumes the closed position, the communication between each of the paired openings  53  and  54  and the other passage  3 B is blocked and at the same time, the communication between the paired openings  53  and  54  is blocked. Although not shown in the drawing, a link mechanism actuated by an actuator is incorporated with both pivot shafts  71  to synchronously actuate the same.  
         [0094]     Referring to  FIG. 9 , there is shown a switch valve unit  5 C that is installed in an exhaust system  300  of a third embodiment of the present invention. Also, this drawing is schematically illustrated with some illustration contradiction for easy understanding of the embodiment.  
         [0095]     Valve unit  5 C employed in this embodiment  300  is substantially the same as switch valve unit  5 B of the above-mentioned embodiment  200 , except that in this third embodiment  300 , there is no means corresponding to partition walls  73  and in this third embodiment  300 , two flat valve bodies  72   a  and  72   b  extend radially outward from pivot shaft  71 . Thus, in this embodiment  300 , even when each switch valve  4 A or  4 B takes the closed position as shown, the fluid communication between the paired openings  52  and  55  (or  53  and  54 ) is kept unlike the case of the above-mentioned first and second embodiments  100  and  200 .  
         [0096]     Referring to  FIG. 10 , there is shown a switch valve unit  5 D that is installed in an exhaust system  400  of a fourth embodiment of the present invention. Also, this drawing is schematically illustrated with some illustration contradition for easy understanding.  
         [0097]     Valve unit  5 D employed in this embodiment  400  has only one butterfly valve  80  that serves as both switch valves  4 A and  4 B for controlling the fluid communication between the four upstream main exhaust passages  2  and the two intermediate main exhaust passages  3 A and  3 B.  
         [0098]     Butterfly valve  80  comprises a pivot shaft  81  that is arranged at an upstream portion of the two intermediate main exhaust passages  3 A and  3 B. In the illustrated embodiment, pivot shaft  81  is rotatably supported by the axially extending partition wall  59 . Pivot shaft  81  has two flat valve bodies  82   a  and  82   b  that extend radially outward therefrom. However, actually, flat valve bodies  82   a  and  82   b  are somewhat offset with respect to an axis of pivot shaft  81 , as shown.  
         [0099]     Seal members  84  are secured to inner walls of the passages  3 A and  3 B. As shown, when butterfly valve  80  takes its closed position, the outside ends of two flat valve bodies  82   a  and  82   b  are in contact with seal members  84 . When turning in the direction of the arrows, butterfly valve  80  takes an open position.  
         [0100]     As is understood from the drawing, when butterfly valve  80  takes the closed position, the fluid communication between the paired openings  52  and  55  (or  53  and  54 ) and first or second intermediate main exhaust passage  3 A or  3 B is blocked while keeping the communication between the paired openings  52  and  55 , that is, the communication between the two upstream main exhaust passages  2  for cylinders #1 and #4. While, when butterfly valve  80  turns to the open position, the fluid communication between the paired openings  52  and  55  (or  53  and  54 ) and first or second intermediate main exhaust passage  3 A or  3 B becomes established.  
         [0101]     Referring to  FIG. 11 , there is shown a switch valve unit  5 E that is employed in an exhaust system  500  of a fifth embodiment of the present invention. Like the above-mentioned drawings of FIGS.  8  to  10 , the drawing of this embodiment is schematically illustrated with some illustration contraction for easy understanding.  
         [0102]     Valve unit  5 E employed in this embodiment  500  uses two slide door type valves  90  as switch valves  4 A and  4 B. Each slide door type valve  90  comprises a flat slide door  91  that is arranged behind the paired openings  52 ,  55 ,  53  and  54  to slide in a direction perpendicular to the axes of the intermediate main exhaust passages  3 A and  3 B.  
         [0103]     Referring to  FIG. 12 , there is shown an exhaust system  600  of a sixth embodiment of the present invention.  
         [0104]     As is seen from this drawing, exhaust system  600  of this embodiment is similar to exhaust system  100  of the above-mentioned first embodiment. Thus, only parts or portions that are different from those of the first embodiment  100  will be described in detail in the following.  
         [0105]     In this sixth embodiment  600 , a switch valve  4  is installed in each of upstream main exhaust passages  2 . Each switch valve  4  is arranged in the corresponding passage  2  as upstream as possible in order that the corresponding upstream bypass exhaust passage  11  can get a higher temperature exhaust gas after the cold starting of the engine. Actually, the four branches  41 ,  42 ,  43  and  44  (or  2 ) (see  FIGS. 3 and 4 ) of exhaust manifold  34  have a considerably higher thermal capacity, and thus, if the length of each branch  41 ,  42 ,  43  or  44  defined from an inlet end of the same to the corresponding switch valve  4  is large, the exhaust gas led to the corresponding bypass exhaust passage  11  is subjected to a marked temperature drop, which is undesirable to auxiliary catalytic converter  18 .  
         [0106]     Referring to  FIGS. 13 and 14 , there is schematically shown an exhaust system  700  of a seventh embodiment of the present invention. In this embodiment  700 , auxiliary catalytic converter  18  is directly connected to cylinder head  33  of the engine  31  in order to shorten the length of the four bypass exhaust passages  11 . As shown, in this case, four bypass exhaust passages  11  are defined in cylinder head  33 , and each bypass exhaust passage  11  is branched from the corresponding exhaust port  100  that constitutes part of upstream main exhaust passage  2 .  
         [0107]     As is seen from  FIG. 14 , four bypass exhaust passages  11  are joined at their downstream ends and directly connected to the inlet of auxiliary catalytic converter  18 . In this case, a much higher temperature exhaust gas can be led to the converter  18  even just after cold staring of the engine  31 .  
         [0108]     Referring to FIGS.  15  to  17 , particularly  FIGS. 15 and 16 , there is shown a switch valve unit  5 F that is employed in an exhaust system  800  of an eighth embodiment of the present invention.  
         [0109]     This eight embodiment  800  is substantially the same as the above-mentioned first embodiment  100  (see  FIG. 1 ) except for the followings.  
         [0110]     Each bypass exhaust passage  11  has a cross-sectional area sufficiently smaller than that of main exhaust passage  2 . Thus, a gas flow resistance shown by each bypass exhaust passage  11  is higher than that of main exhaust passage  2 . More specifically, the gas flow resistance of a bypass exhaust line that extends from the branched portion  12  to the junction portion  17  is sufficiently higher than that of a main exhaust line that extends from the branched portion  12  to the junction portion  17 , because of the reduced cross section and presence of auxiliary catalytic converter  18 .  
         [0111]     In the illustrated example of  FIG. 1 , the four upstream bypass exhaust passages  11  are joined at two junction portions  13  and make the two bypass exhaust passages  14 A and  14 B for shortening the entire length of the bypass exhaust line for the purpose of reducing the heat capacity of the bypass exhaust line and reducing the heat radiation surface of the same. However, in the eight embodiment  800 , the layout of the bypass exhaust line for such purposes is at will. If, for example, auxiliary catalytic converter  18  is arranged in one-sided position with respect to the row of the four cylinders of the engine, a layout may be employed wherein two upstream bypass exhaust passages  11  are joined at a generally right angle.  
         [0112]     Furthermore, in the eighth embodiment  800 , a modified switch valve unit  5 F is used in place of the switch valve unit  5 A of the first embodiment  100 .  
         [0113]     As will be described in detail hereinafter, the modified switch valve unit  5 F is so constructed as to effectively use the exhaust gas pressure as means for biasing valve plates thereof toward their closed position to assure a sealing of the valve plates being in the closed position.  
         [0114]     In the following, switch valve unit  5 F that includes switch valves  4 A and  4 B will be described in detail with reference to  FIGS. 15 and 16 .  FIG. 15  is a plan view of switch valve unit  5 F and  FIG. 16  is a sectional view of one switch valve of switch valve  4 A of the unit  5 F, that is, the sectional view of the portion indicated by an arrow “XVI” of  FIG. 15 .  
         [0115]     As is seen from  FIG. 15 , switch valve unit  5 F comprises switch valves  4 A and  4 B which are combined to constitute a single unit.  
         [0116]     As will be understood from  FIG. 1 , also in the eight embodiment  800 , like in the first embodiment  100 , each switch valve  4 A or  4 B is constructed to selectively take an open position wherein the paired upstream main exhaust passages  2  and  2  are communicated with the first or second intermediate main exhaust passage  3 A or  3 B and a closed position wherein the paired main exhaust passages  2  and  2  are not communicated with the first or second intermediate main exhaust passage  3 A or  3 B.  
         [0117]     Referring back to  FIG. 15 , each switch valve  4 A or  4 B comprises two circular valve plates  152   a  and  152   a  (or  152   b  and  152   b ) that are connected to a common pivot shaft  153   a  or  153   b  through respective arms  155   a  and  155   a  (or  155   b  and  155   b ), and two circular valve openings  151   a  and  151   a  (or  151   b  and  151   b ) that are provided in a base wall structure and have valve seat portions  154   a  (see  FIG. 16 ).  
         [0118]     As is seen from  FIG. 16 , each valve seat portion  154   a  is formed with a tapered seat surface  154   a ′ that can intimately receive a tapered periphery  152   a ′ of the corresponding valve plate  152   a  or  152   b.    
         [0119]     Each upstream main exhaust passage  2  is formed near the corresponding valve plate  152   a  or  152   b  with a recessed portion  156  for receiving the valve plate  152   a  or  152   b  being in an open position.  
         [0120]     As is easily understood from  FIG. 16 , in each switch valve  4 A or  4 B, circular valve plate  152   a  or  152   b  is arranged at an upstream position relative to the corresponding valve seat portion  154   a , so that a differential pressure produced between upstream and downstream positions relative to valve plate  152   a  or  152  functions to bias the valve plate in a valve closing direction, that is, in a clockwise direction in  FIG. 16 .  
         [0121]     Although not shown in the drawings, the two common pivot shafts  153   a  and  153   b  are controlled by a single actuator through respective link mechanisms. That is, when the actuator is operated to run in one direction, the four circular valve plates  152   a  and  152   b  are pivoted to their open position, and when the actuator is operated to run in the other direction, the valve plates  152   a  and  152   b  are pivoted to their closed position.  
         [0122]     As is understood from  FIG. 16 , for taking the closed position, each valve plate  152   a  or  152   b  pivots in a downstream direction, that is, in a direction from an upstream position to a downstream position with respect to the flow of the exhaust gas. Thus, when valve plate  152   a  or  152   b  takes the closed position, there is produced a certain differential pressure between the upstream and downstream zones, which biases valve plate  152   a  or  152   b  against the tapered seat surface  154   a ′ thereby to assure the sealing between valve plate  152   a  or  152   b  and the seat surface  154   a ′. This phenomenon will be much clearly understood from the following description when taken in conjunction with  FIG. 17 .  
         [0123]     As is seen from  FIG. 17 , the bypass passage line including upstream bypass exhaust passages  11 , first and second intermediate bypass exhaust passages  14 A and  14 B, auxiliary catalytic converter  18  and downstream bypass exhaust passage  16  has a relatively large gas flow resistance due to its inherent construction. Actually, auxiliary catalytic converter  18  shows a certain pressure loss which causes the increase of the gas flow resistance of the bypass passage line. Accordingly, the pressure “P1” exerted at a position just upstream of the valve plate  152   a  or  152   b  is larger than the pressure “P2” exerted at a position just downstream of valve plate  152   a  or  152   b . Thus, when valve plate  152   a  or  152   b  takes its closed position, the sealing between valve plate  152   a  or  152   b  and the tapered seat surface  154   a ′ is assured. It has been revealed that such desired sealing is not affected even when the pulsation of the exhaust gas is applied to valve plates  152   a  and  152   b.    
         [0124]     It is to be noted that due to provision of auxiliary catalytic converter  18 , the pulsation of the pressure “P2” is suitably attenuated. Furthermore, as is seen from  FIGS. 1 and 15 , the two valve plates  152   a  and  152   a  (or  152   b  and  152   b ) connected by the common pivot shaft  153   a  or  153   b  are respectively exposed to the paired upstream exhaust passages  2  that transport the exhaust gases whose pulsations are different from each other. This phenomenon promotes the sealing effected between each valve plate and valve seat.  
         [0125]     For the reasons as mentioned hereinabove, in the eighth embodiment  800 , when taking their closed position, the switch valves  4 A and  4 B can take an assured sealing condition, and thus, substantially all exhaust gas can be led to the bypass passage line and to the auxiliary catalytic converter  18 , which promotes the early activation of the converter  18 .  
         [0126]     Referring to  FIG. 18 , there is shown but partially a switch valve unit  5 G that is employed in an exhaust system  900  of a ninth embodiment of the present invention.  
         [0127]     In the switch valve  4 A or  4 B of the switch valve unit  5 G, each circular valve plate  152   a  or  152   b  has no tapered peripheral edge. That is, in this case, a line contact is established between valve plate  152   a  or  152   b  and the tapered seat surface  154   a ′ when valve plate takes its closed position. As shown, circular valve plate  152   a  or  152   b  has a convex outer surface.  
         [0128]     Referring to  FIG. 19 , there is shown but partially a switch valve unit  5 H that is employed in an exhaust system  1000  of a tenth embodiment of the present invention.  
         [0129]     In the switch valve  4 A or  4 B of the switch valve unit  5 H, an annular ring member  154   c  is used for producing a valve seat portion that has a seat surface  154   c ′. Upon taking a closed position, circular valve plate  152   a  or  152   b  abuts against the seat surface  154   c ′ at a peripheral edge thereof, as shown.  
         [0130]     In the foregoing description, the present invention is directed to the in-line four cylinder internal combustion engine  31 , the present invention is widely applicable to various types of multi-cylinder internal combustion engine including in-line type and V-type.  
         [0131]     The entire contents of Japanese Patent Applications 2004-169394 filed Jun. 8, 2004, 2004-205357 filed Jul. 13, 2004 and 2005-111478 filed Apr. 8, 2005 are incorporated herein by reference.  
         [0132]     Although the invention has been described above with reference to the embodiments of the invention, the invention is not limited to such embodiments as described above. Various modifications and variations of such embodiments may be carried out by those skilled in the art, in light of the above description.