Patent Publication Number: US-6655134-B2

Title: Exhaust control valve

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an exhaust control valve for controlling the flow of exhaust gas, and more particularly to an exhaust control valve for a motor vehicle exhaust gas system. 
     2. Background Art 
     In an exhaust control valve according to the background art as shown in Japanese Pre-examination Patent Publication (KOKAI) No. 63-212728 (1988), a cast valve having a crank-like shape has the flow of exhaust gas controlled by a crank portion. 
     However, this crank-like valve body suffers from some disadvantages. Since the shape of the valve body is asymmetrical with respect to the axis line of the valve shaft, distribution of melt is often defective at the time of casting from an end of the valve shaft, and thermal deformation due to partial material thickness can easily occur. In addition, since the crank portion serving as a valve portion makes contact with the valve housing over a small area, it is difficult to maintain high sealing properties. Further, since the weight balance about the axis line of the valve body is offset/poor, response to driving torque is relatively poor. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the shortcomings associated with the background art and achieves other advantages not realized by the background art. 
     An aspect of the present invention is to provide an exhaust control valve which solves the afore-mentioned drawbacks of the background art, specifically to provide high sealing properties, improve thermal properties through improved casting integrity, and desirable weight distribution. 
     These and other aspects of the present invention are accomplished by an exhaust control valve comprising a valve housing having a valve chamber; a valve body having a cylindrical shape rotatably contained in the valve chamber and cooperatively engaged with the valve housing for controlling a flow of exhaust gas; a transmission member rotationally driving at least one valve shaft of the valve body; and a plurality of bearing bushings mounted in the valve housing and rotatably supporting each valve shaft, wherein the valve body is coaxially arranged with an axial centerline of each valve shaft. 
     Since the valve body and the valve shaft have a coaxial cylindrical shape, it is possible to obtain good distribution of melt from a central portion of an end of the valve shaft at the time of casting, and to prevent thermal deformation from occurring due to partial material thickness. Accordingly, the external peripheral surfaces of the valve body and the valve shaft can be finished by cutting continuously after casting. Therefore, the valve body can be manufactured with high precision efficiently. 
     The high-precision valve body can have its external peripheral surface in thorough contact with the internal surface of the valve housing, so that it is possible to effectively restrain leakage of exhaust gas at the contact area and to carry out appropriate exhaust control. Further, since the cylindrical valve body has good weight balance about the axis line, it is possible to achieve a reduction of driving torque for the valve body and, hence, enhancement of response to the driving torque. Also, it is possible to minimize non-uniform loading on the bearing bushes, thereby contriving improved durability of the bearing bushes. 
     These and other aspects of the present invention are further accomplished by an exhaust control system for an internal combustion engine of a vehicle comprising a plurality of exhaust pipes from the internal combustion engine containing an exhaust gas flow, the exhaust pipes each having an intermediate portion; a common valve housing interposed in the intermediate portions of the exhaust pipes, the valve housing having at least one pair of inlet ports, at least one pair of outlet ports, and a valve chamber; a valve body having a cylindrical shape rotatably mounted within the valve chamber, and cooperatively engaged with the valve housing for controlling a flow of exhaust gas; a transmission member rotationally driving at least one valve shaft of the valve body; and a plurality of bearing bushings mounted in said valve housing and rotatably supporting each valve shaft, wherein the valve body is coaxially arranged with an axial centerline of each valve shaft. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a side view of a motorcycle with an engine having an intake control device and an exhaust control device; 
     FIG. 2 is a vertical sectional side view of a portion of an intake control device according to an embodiment of the present invention; 
     FIG. 3 is a vertical sectional side view of a portion of an intake control device according to an embodiment of the present invention and corresponding to a different operational position than that of FIG. 2; 
     FIG. 4 is a sectional view taken along line  4 — 4  of FIG. 2 according to an embodiment of the present invention; 
     FIG. 5 is a sectional view taken along line  5 — 5  of FIG. 4 according to an embodiment of the present invention; 
     FIG. 6 is a sectional view taken along line  6 — 6  of FIG. 4 according to an embodiment of the present invention; 
     FIG. 7 is a perspective view of an exhaust system according to an embodiment of the present invention; 
     FIG. 8 is a side view of an exhaust control device according to an embodiment of the present invention; 
     FIG. 9 is a sectional view taken along line  9 — 9  of FIG. 8 showing an exhaust control valve in its low-speed control position according to an embodiment of the present invention; 
     FIG. 10 is a sectional view taken along line  10 — 10  of FIG. 9 according to an embodiment of the present invention; 
     FIG. 11 is a sectional view taken along line  9 — 9  of FIG. 8 showing an exhaust control valve in its medium-speed control position according to an embodiment of the present invention; 
     FIG. 12 is a sectional view taken along line  9 — 9  of FIG. 8 showing an exhaust control valve in its high-speed control position according to an embodiment of the present invention; 
     FIG. 13 is an enlarged plan view of a portion of the exhaust system according to an embodiment of the present invention; 
     FIG. 14 is a sectional view taken along line  14 — 14  of FIG. 13 according to an embodiment of the present invention; 
     FIG. 15 is a sectional view taken along line  15 — 15  of FIG. 14 according to an embodiment of the present invention; 
     FIG. 16 is a sectional view taken along line  16 — 16  of FIG. 13 according to an embodiment of the present invention; 
     FIG. 17 is a sectional view taken along line  17 — 17  of FIG. 16 according to an embodiment of the present invention; 
     FIG. 18 is a plan view of a driving device for an intake control valve and an exhaust control valve according to an embodiment of the present invention; 
     FIG. 19 is a sectional view taken along line  19 — 19  of FIG. 18 according to an embodiment of the present invention; and 
     FIG. 20 is a sectional view taken along line  20 — 20  of FIG. 18 according to an embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the embodiments illustrated by the accompanying drawings. 
     FIG. 1 is a side view of a motorcycle with an engine having an intake control device and an exhaust control device. FIG. 2 is a vertical sectional side view of a portion of an intake control device according to an embodiment of the present invention. FIG. 3 is a vertical sectional side view of a portion of an intake control device according to an embodiment of the present invention and corresponding to a different operational position than that of FIG.  2 . FIG. 4 is a sectional view taken along line  4 — 4  of FIG. 2 according to an embodiment of the present invention. FIG. 5 is a sectional view taken along line  5 — 5  of FIG. 4 according to an embodiment of the present invention. FIG. 6 is a sectional view taken along line  6 — 6  of FIG. 4 according to an embodiment of the present invention. FIG. 7 is a perspective view of an exhaust system according to an embodiment of the present invention. 
     FIG. 8 is a side view of an exhaust control device according to an embodiment of the present invention. FIG. 9 is a sectional view taken along line  9 — 9  of FIG. 8 showing an exhaust control valve in its low-speed control position according to an embodiment of the present invention. FIG. 10 is a sectional view taken along line  10 — 10  of FIG. 9 according to an embodiment of the present invention. FIG. 11 is a sectional view taken along line  9 — 9  of FIG. 8 showing an exhaust control valve in its medium-speed control position according to an embodiment of the present invention. FIG. 12 is a sectional view taken along line  9 — 9  of FIG. 8 showing an exhaust control valve in its high-speed control position according to an embodiment of the present invention. 
     FIG. 13 is an enlarged plan view of a portion of the exhaust system according to an embodiment of the present invention. FIG. 14 is a sectional view taken along line  14 — 14  of FIG. 13 according to an embodiment of the present invention. FIG. 15 is a sectional view taken along line  15 — 15  of FIG. 14 according to an embodiment of the present invention. FIG. 16 is a sectional view taken along line  16 — 16  of FIG. 13 according to an embodiment of the present invention. FIG. 17 is a sectional view taken along line  17 — 17  of FIG. 16 according to an embodiment of the present invention. FIG. 18 is a plan view of a driving device for an intake control valve and an exhaust control valve according to an embodiment of the present invention. FIG. 19 is a sectional view taken along line  19 — 19  of FIG. 18 according to an embodiment of the present invention. FIG. 20 is a sectional view taken along line  20 — 20  of FIG. 18 according to an embodiment of the present invention. 
     In FIG. 1, a vehicle body frame  2  of a motorcycle  1  includes left and right pairs of main frames  4 ,  4  having a head pipe  3  at their front ends. The left and right pairs of main frames  4 , 4  slope downward and rearward, and have their rear ends coupled to each other. A seat rail  5  is connected to the rear ends of the main frames  4 ,  4  and sloping up rearwards, and a parallel four-cylinder engine En is mounted on the pair of main frames  4 ,  4 . 
     The engine En is mounted within the frame  2  by sloping a cylinder block  8  and a cylinder head  9  a slightly forward with respect to a vehicle longitudinal centerline and inserting the cylinder head  9  between the main frames  4 ,  4 . 
     A front fork  6   f  for supporting a front wheel  7   f  through a shaft is steerably connected to the head pipe  3 . A rear fork  6   r  supporting a rear wheel  7   r  is vertically connected to a rear portion of a crankcase  10  of the engine En through a pivotal shaft  11 . A rear cushion unit  12  is inserted between the rear fork  6   r  and the main frames  4 ,  4  thereby permitting oscillatory movement. An output shaft  13  of the engine En mounted on a front side of the pivotal shaft  11  drives the rear wheel  7   r  through a chain transmission device  14 . 
     A fuel tank  15  is mounted on the main frames  4 ,  4 , and a tandem main seat  16  is fitted to the seat rail  5 . 
     An intake system In of the engine En includes an air cleaner  17  and a throttle body  18  disposed on an upper side of the cylinder head  9  in such a fashion as to be covered with the fuel tank  15 . An exhaust system Ex of the engine En includes exhaust pipes  51   a  to  51   d  and an exhaust muffler  54  disposed so as to extend from a front side of the cylinder head  9  and the cylinder block  8  through the lower side of the crankcase  10  and slanting in an upward direction. 
     The intake system In of the engine En according to an embodiment of the present invention will now be described with reference to FIG. 1 to FIG.  6 . 
     As shown in FIG. 1 to FIG. 4, four throttle bodies  18 ,  18  each corresponding to each of four cylinders are connected to the cylinder head  9  of the engine En. Air funnels  21 ,  21  are connected to an inlet of an intake path  18   a  of the throttle bodies  18 ,  18 . A cleaner case  22  of the air cleaner  17  for containing all the air funnels  21 ,  21  is fitted to the four throttle bodies  18 ,  18 . 
     The cleaner case  22  includes a lower case half  22   b  attached to the throttle bodies  18 ,  18  and an upper case half  22   a  separately and removably joined to the lower case half  22   b  by small screws  27 . An element fitting plate  25  for partitioning the interior of the cleaner case  22  into a lower dirty air chamber  23  and an upper clean chamber  24  is sandwiched between the case halves  22   a  and  22   b.  A cleaner element  26  is fitted within a fitting hole  25   a  provided in the element fitting plate  25 . 
     An air intake port  28  for opening the dirty air chamber  23  to the atmosphere is provided on one side of the lower case half  22   b.  The air funnels  21 ,  21  are arranged to penetrate a bottom wall of the lower case half  22   b  and their respective inlets open into the clean chamber  24 . 
     Therefore, during operation of the engine En, air flowing through the air intake port  28  into the dirty air chamber  23  is filtered by the cleaner element  26  before passing into the cleaning air chamber  24 . Inlet air then flows into the air funnels  21  and throttle bodies  18 , and is taken into the engine En at a flow rate controlled by throttle valves  29  positioned within the throttle bodies  18 . 
     In this process, a fuel is injected toward an intake port of the engine En from a fuel injection valve  32  fitted at a side wall of each of the throttle bodies  18 . 
     The throttle valves  29  of all the throttle bodies  18  have valve shafts  29   a  connected with each other for conjunctive operation. The throttle valves are opened and closed by a throttle grip fitted to a steering handle of the motorcycle  1  through a pulley  30  attached to the valve shaft  29   a  on its exterior and an operating wire  31  connected to the pulley  30 . 
     The lower case half  22   b  is provided integrally with a partition wall  34  for partitioning an intermediate portion of the dirty air chamber  23  into a lower small-section passage  33   a  and an upper large-section passage  33   b.  An intake control valve  35  for opening and closing the large-section passage  33   b  is supported by the partition wall  34  through a shaft. 
     The intake control valve  35  includes a valve plate  36  and a valve shaft  37  formed integrally with a side end of the valve plate  36 . The partition wall  34  is provided with one bearing  38  for rotatably supporting one end portion of the valve shaft  37  and a left-right pair of bearings  39 ,  39  for rotatably supporting the other end portion of the valve shaft  37 . 
     As shown in FIG. 3, the intake control valve  35  is turned between a first intake control position A (See FIG. 2) where the tip end of the valve plate  36  is put into contact with a ceiling surface of the large-section passage  33   b  to fully close the large-section passage  33   b,  and a second intake control position B where the valve plate  36  is put in parallel with the partition wall  34  to fully open the passage  33   b.    
     In the case illustrated, the angle of turning is about 45 degrees. In the second intake control position B of the intake control valve  35 , the valve plate  36  is in a slanted position with its tip end directed to the upstream side of the large-section passage  33   b,  and the valve plate  36  is urged toward a closing direction by the intake negative pressure of the engine En. 
     A return spring  41  is connected to an arm  40  for urging the valve plate  36  in a closing direction, specifically, toward the first intake control position A through the arm  40 . The arm  40  is formed integrally with a first end portion of the valve shaft  37 . A driven pulley  46  connected through a first transmission wire  75   a  to a driving pulley  73  of an actuator  71  (described later) between the pair of bearings  39 ,  39  is fitted to a second end portion of the valve shaft  37 . 
     A lost motion mechanism  42  for coupling the driven pulley  46  and the valve shaft  37  is provided between the driven pulley  46  and the valve shaft  37 . The lost motion mechanism  42  includes a transmission pin  43  projecting from a side surface of the valve shaft  37 , an arc groove  44  formed in an internal circumferential surface of the driven pulley  46  and extending in the circumferential direction for engaging the transmission pin  43 , and a lost motion spring  45  urging the driven pulley  46  toward the first intake control position A of the intake control valve  35 . 
     A center angle of the arc groove  44  is set larger than the angle of opening and closing of the intake control valve  35  so that, when the driven pulley  46  is rotated from a retracted position in the opening direction of the intake control valve  35 , namely, toward the second intake control position B, an end surface of the arc groove  44  comes into contact with the transmission pin  43 . This action starts moving the intake control valve  35  toward the second intake control position B after a predetermined play angle α is passed. 
     Next, the exhaust system Ex of the engine En will be described in detail with reference to FIG.  1  and FIG.  7  through FIG.  17 . 
     First, in FIG.  1  and FIG. 7, four parallel cylinders of the engine En will be called No. 1 to No. 4 cylinders  50   a  to  50   d,  respectively, as seen from the left side of the vehicle. An ignition sequence for each of the cylinders is carried out according to the sequence of No. 1 cylinder  50   a,  No. 2 cylinder  50   b,  No. 4 cylinder  50   d  and No. 3 cylinder  50   c.    
     No. 1 to No. 4 exhaust pipes  51   a  to  51   d  corresponding respectively to the No. 1 to No. 4 cylinders  50   a  to  50   d  are connected to a front surface of the cylinder head  9 . The exhaust pipes  51   a  to  51   d  extend downward from a front surface of the engine En and then bend rearwards at a lower location. 
     Below the engine En, the No. 1 and No. 4 exhaust pipes  51   a  and  51   d  are adjacently disposed on the left and right sides, and the No. 2 and No. 3 exhaust pipes  51   b  and  51   c  are adjacently disposed beneath the No. 1 and No. 4 exhaust pipes, respectively. An exhaust control valve  55  is provided at an intermediate portion of the exhaust pipes  51   a  to  51   d.    
     As shown in FIG. 8 to FIG. 12, the exhaust control valve  55  includes a common valve housing  56  interposed in an intermediate portion of the No. 1 to No. 4 exhaust pipes  51   a  to  51   d,  and a valve body  57  mounted in the valve housing  56 . The upstream side and the downstream side of the No. 1 to No. 4 exhaust pipes  51   a  to  51   d  are connected respectively to front and rear flanges  56 A,  56 B provided at front and rear ends of the valve housing  56 . 
     The valve housing  56  is provided with pairs of inlet ports  56   a,    56   a  and outlet ports  56   b,    56   b  opening to each end face of the front and rear flanges  56 A,  56 B and coinciding with the upstream-side and downstream-side pipes of the No. 1 and No. 4 exhaust pipes  51   a,    51   d.  A cylindrical valve chamber  56   c  is provided between the inlet ports  56   a,    56   a  and the outlet ports  56   b,    56   b  and extending in a direction orthogonal to the axis line of each port. 
     A pair of communication ports  56   d,    56   d  formed between the front and rear flanges  56 A,  56 B and coinciding with-the upstream-side and downstream-side pipes of the No. 2 and No. 3 exhaust pipes  51   b,    51   c  is also provided in the valve housing  56 . A pair of communication holes  56   e,    56   e  for communicating the communication ports  56   d,    56   d  to the valve chamber  56   c  are provided on the upper side of the communication ports  56   d,    56   d.    
     One end of the valve chamber  56   c  is closed by an end wall integral with the valve housing  56 , and a bearing bushing  59  is mounted on the end wall. The other end of the valve chamber  56   c  is open, and a bearing bracket  58  for closing the other end is fixed to the valve housing  56  by bolts  64 . The bearing bracket  58  has a bearing bushing  60  arranged coaxial with the bearing bush  59 . 
     On the other hand, the valve body  57  is rotatably mounted in the valve chamber  56   c  and is generally cylindrical in shape. At both ends in the axial direction, the valve body  57  is provided integrally with valve shafts  61 ,  62  that are coaxial with the valve body  57 . The valve body  57  having its valve shafts  61 ,  62  rotatably supported by the bearing bushes  59 ,  60 , is rotated between a low-speed control position C, a medium-speed control position D and a high-speed control position E. 
     In this case particularly, the bearing bush  60  in the bearing bracket  58  protrudes a little from an internal end face of the bearing bracket  58  so as to support an end face of the valve body  57  also. 
     In a preferred embodiment, the valve housing  56  is cast from a titanium material, and the valve body  57  also is cast from a titanium material together with the valve shafts  61 ,  62 . On the other hand, the bearing bushes  59 ,  60  for supporting the valve shafts  61 ,  62  are formed from a nonmetallic material having excellent bearing properties and excellent sealing properties as well. For example, a carbon material such as carbon graphite is utilized in a preferred embodiment. 
     A driven pulley  67  is attached by a nut  65  to a tip end portion of the valve shaft  62  protruding to the outside of the bearing bracket  58 . The driven pulley  67  is driven by a driving pulley  73  of the actuator  71  (described later), through second and third transmission wires  75   b,    75   c.    
     The driven pulley  67  is provided integrally with a flange portion  80  having an annular retaining recess portion  80   a  opening to the side of the bearing bracket  58 . An annular retainer  81  and two thrust washers  82 ,  82 ′ are retained rotatably relative to the retainer  81  in the retaining recess portion  80   a.  A thrust spring  83  is compressed and disposed between the thrust washers  82 ,  82 ′ in a compressed/stored energy position. 
     The bearing bracket  58  having a certain load from the thrust spring  83  ensures that an end face of the valve body  57  and an end face of the bearing bush  60  are maintained in a pressure contact seal condition. A gap g is formed between opposed end faces of an end wall of the valve housing  56  on the opposite side of the bearing bracket  58  and the valve body  57 . Thermal expansion of the valve body  57  in the axial direction is thereby absorbed by the gap g. 
     The valve body  57  is provided with a pair of through-holes  57   a  capable of coinciding with the inlet port  56   a  and the outlet port  56   b  crossing the axis line of the valve body  57 . Communication holes  57   b  for opening one side face of the through-holes  57   a  in a radial direction of the valve body  57  are also provided therein. 
     In a low-speed control position C of the valve body  57  (See FIG.  9  and FIG.  10 ), the communication hole  57   b  overlaps with the inlet port  56   a  of the valve housing  56 , while one end side of the through-hole  57   a  overlaps with the communication hole  56   e  of the valve housing  56 . A valve wall  57 A of the valve body  57  opposed to the communication hole  57   b  closes up the outlet port  56   b.    
     In a medium-speed control position D (See FIG.  11 ), the through-holes  57   a  coincide with the inlet and outlet ports  56   a,    56   b,  and the valve wall  57 A closes the communication hole  56   e.  An outer surface of the valve wall  57 A is provided with an arc-shaped recess portion  57   c  connected to an internal circumferential surface of the communication port  56   d  in the medium-speed control position D (See FIG.  12 ). 
     In a high-speed control position E, the through-holes  57   a  coincide with the inlet and outlet ports  56   a,    56   b,  and the communication hole  57   b  coincides with the communication hole  56   e.  Therefore, the medium-speed control position D and the high-speed control position E of the valve body  57  are spaced from each other by about 180 degrees, and the low-speed control position C occurs at a middle point between the control positions D and E. 
     In FIG. 1, FIG.  7  and FIG. 13, where the No. 1 to No. 4 exhaust pipes  51   a  to  51   d  have passed by the exhaust control valve  55 , the No. 1 and No. 4 exhaust pipes  51   a,    51   d  are connected to an upper first exhaust collecting pipe  52   a  for collecting exhaust gases from these pipes. Similarly, the No. 2 and No. 3 exhaust pipes  51   b,    51   c  are connected to a lower first exhaust collecting pipe  52   b  for collecting exhaust gases therefrom. 
     Thereafter, the exhaust collecting pipes  52   a,    52   b  are connected to a second exhaust collecting pipe  53  for collecting exhaust gases therefrom, and an exhaust muffler  54  is connected to the rear end of the second exhaust collecting pipe  53 . The upper and lower first exhaust collecting pipes  52   a,    52   b  each are provided with exhaust cleaners. The lower first exhaust collecting pipe  52   b  in communication with the communication port  56   d  of the exhaust control valve  55  is provided therein with a primary exhaust cleaner  84 . The second exhaust collecting pipe  53  is provided therein with a secondary exhaust cleaner  85 . 
     As shown in FIG.  14  and FIG. 15, the primary exhaust cleaner  84  is not limited in its type. In the embodiment shown, the cleaner  84  includes a ternary catalyst converter having a cylindrical catalyst carrier  87  having innumerable through-pores  88  in its peripheral wall. One end portion of the catalyst carrier  87  is fixed by welding to the inner wall of the lower first exhaust collecting pipe  52   b.  The other end portion is slidably retained on the inner wall through a heat insulating member  89  made from glass wool, steel wool or the like. A cylindrical adiabatic space  90  is formed between the rest intermediate portion of the catalyst carrier  87  and the lower first exhaust collecting pipe  52   b.    
     Therefore, thermal elongation of the primary exhaust cleaner  84  is allowed by slippage between the primary exhaust cleaner  84  and the heat insulating member  89 . It is possible to suppress generation of thermal strain in the primary exhaust cleaner  84  and the lower first exhaust collecting pipe  52   b.  With the heat insulating member  89  and the adiabatic space  90 , it is possible to sustain the temperature of the primary exhaust cleaner  84  and to prevent overheating of the lower first exhaust collecting pipe  52   b.    
     As shown in FIG.  16  and FIG. 17, the second exhaust collecting pipe  53  includes an outer pipe  92  connected to an upstream side and an inner pipe  93  connected to the downstream side. The inner pipe  93  is disposed in the inside of the outer pipe  92  with a cylindrical adiabatic space  94  therebetween. 
     The downstream end of the outer pipe  92  is welded to the outer circumference of the inner pipe  93 , and the upstream end of the inner pipe  93  is relatively slidably supported by the outer pipe  92  via a heat insulating member  95  composed of glass wool, steel wool or the like. The second exhaust collecting pipe  53  is slightly bent at its intermediate portion, and a guide ring  96  surrounding the inner pipe  93  is welded to the inner circumferential surface of the outer pipe  92  at the bend portion. 
     The secondary exhaust cleaner  85  also is not limited to the embodiment shown in the accompanying figures. The preferred embodiment shows a secondary exhaust cleaner  85  having a ternary catalyst converter with a cylindrical catalyst carrier  98  having innumerable through-pores  99  in its circumferential wall. The catalyst carrier  98  is fitted to the inner pipe  93  through a heat insulating member  100  and a retaining ring  101  at a central portion in the axial direction. 
     The heat insulating member  100  is made of glass wool, steel wool or the like. The retaining ring  101  is formed by overlap welding of opposed end portions of a pair of half-rings  101   a,    101   b.  In this case, a compression force is applied to the heat insulating member  100 , whereby a frictional force for slidably retaining the catalyst carrier  98  is produced between the heat insulating member  100  and the catalyst carrier  98 . 
     The inner pipe  93  is provided with a pair of protuberances  93   a  projecting radially inward and opposed to each other diametrically. The outer circumferential surface of the retaining ring  101  is welded to the protuberances  93   a.  An adiabatic space  102  is formed at the rest portion other than the welded portions between the retaining ring  101  and the inner pipe  93 . 
     Portions of the catalyst carrier  98  other than the central portion retained by the retaining ring  101  are sufficiently parted away from the inner circumferential surface of the inner pipe  93 , so that exhaust gas can be freely circulated in the interior and exterior of the catalyst carrier  98  through the innumerable through-pores  99 . 
     Therefore, a central portion of the secondary exhaust cleaner is slidably supported on the inner pipe  93  through the heat insulating member  100  and the retaining ring  101 . Thermal elongation of the secondary exhaust cleaner  85  is allowed by slippage between the secondary exhaust cleaner  85  and the heat insulating member  100 . Accordingly, it is possible to suppress generation of thermal strain in the secondary exhaust cleaner  85  and the inner pipe  93 . 
     With this arrangement of the heat insulating member  100 , the adiabatic space  102 , the inner pipe  93  and the outside adiabatic space  94 , it is possible to effectively sustain the temperature of the secondary exhaust cleaner  85  and to prevent overheating of the outer pipe  92 . In addition, the secondary exhaust cleaner  85  is supported in one stable position. At portions other than the supported portion, exhaust gas can be circulated in the inside and outside of the catalyst carrier  98  through the through-pores  88 , so that cleaning of the exhaust gas can be achieved effectively. 
     Further, any difference between thermal elongations of the outer pipe  92  and the inner pipe  93  constituting the second exhaust collecting pipe  53  is permitted by slippages between the inner pipe  93 , the heat insulating member  95  and the outer pipe  92 . In addition, the adiabatic spaces  94 ,  102  existing doubly between the secondary exhaust cleaner  85  and the outer pipe  92  promise an effective prevention of thermal damage relating to the secondary exhaust cleaner  85 . 
     Next, a driving device for the intake control valve  35  and the exhaust control valve  55  will be described with reference to FIG.  1  and FIG. 18 to FIG.  20 . 
     As shown in FIG.  1  and FIG. 18, on the upper side of the crankcase  10  of the engine En, a common actuator  71  between a pair of brackets  70 ,  70  is fixed to inside surfaces of the main frame  4  and mounted by a bolt  78  through an elastic member  77 . The actuator  71  is disposed in a manner so that a first distance from the actuator  71  to the intake control valve  35  and a second distance from the actuator  71  to the exhaust control valve  55  are nearly equal to each other. 
     In the embodiment shown, the actuator  71  includes a normally and reversibly rotatable electric motor. The driving pulley  73  attached to an output shaft  72  of the motor is provided with a first wire groove  73   a  having a small diameter and second and third transmission wire grooves  73   b,    73   c  having a large diameter. 
     A first transmission wire  75   a  is engaged with the first wire groove  73   a  and a wire groove  46   a  of the driven pulley  46  (See FIG. 6) on the side of the intake control valve  35 . End terminals of the first transmission wire  75   a  are connected to the driving and driven pulleys  73 ,  46 , respectively. Second and third transmission wires  75   b,    75   c  are engaged with the second and third wire grooves  73   b,    73   c  and a pair of wire grooves  67   b,    67   c  of the driven pulley  67  (See FIG. 9) on the side of the exhaust control valve  55  in opposite wrap-around directions. End terminals of the second and third transmission wires  75   b,    75   c  are connected to the driving pulley  73  and the driven pulley  67 . 
     An electronic control unit  76  connected to the actuator  71  determines and detects a low-speed rotation range, an intermediate-speed rotation range and a high-speed rotation range of the engine En based on the revolution rate of the engine En, boost negative pressure and the like inputted from sensors (not shown). The ECU  76  controls the actuator  71  on the basis of the monitored results. 
     In the medium-speed rotation range of the engine En, the actuator  71  holds the driving pulley  73  in an initial position a. In the low-speed rotation range, the actuator  71  drives the driving pulley  73  to a first driving position b spaced from the initial position a by a predetermined angle along a reverse rotation direction R. In the high-speed rotation range, the actuator  71  drives the driving pulley  73  to a second driving position c spaced from the first driving position b by a predetermined angle in a forward rotation direction F via the initial position a. 
     Next, operation of a preferred embodiment will be described with reference to the accompanying drawings. 
     When the driving pulley  73  is driven by the actuator  71  to the first driving position b in the low-speed rotation range of the engine En, the driving pulley  73  pulls the first and second transmission wires  75   a,    75   b,  whereby the driven pulley  46  on the side of the intake control valve  35  is rotated by a predetermined angle in a valve-opening direction (in FIG. 6, counterclockwise). The driven pulley  67  on the side of the exhaust control valve  35  is rotated by a predetermined angle counterclockwise in FIG. 8, thereby resulting in the valve body  57  of the exhaust valve  35  being brought into the low-speed control position C of FIG.  9  and FIG.  10 . 
     However, the rotation by the predetermined angle of the driven pulley  46  is carried out within the range of the play angle α between the driven pulley  73  and the intake control valve  35  in the lost motion mechanism  42 . Therefore, the valve plate  36  of the intake control valve  35  is maintained in the first intake control position A by the urging force of the return spring  41 . 
     In this condition of the intake control valve  35 , as shown in FIG. 2, the large-section passage  33   b  is fully closed by the valve plate  36 . Therefore, air taken into the engine En is forced to flow through the small-section passage  33   a  when passing through the air cleaner  17 . Therefore, even at the time of an accelerating operation in this low-speed rotation range (when the throttle valve  29  is opened abruptly), dilution of the mixture gas is suppressed, and an appropriately rich mixture gas can be supplied to the engine En. Accordingly, favorable acceleration performance is achieved even during rapid accelerations/starts. 
     However, when the valve body  57  of the exhaust control valve  55  comes to the low-speed control position C of FIG.  9  and FIG. 10, as has been described hereinabove, the communication hole  57   b  of the valve body  57  overlaps with the inlet port  56   a  of the valve housing  56 . Further, while one end side of the through-hole  57   a  of the valve body overlaps with the communication hole  56   e  of the valve housing  56 , the valve wall  57 A of the valve body  57  closes up the outlet port  56   b.    
     Therefore, the exhaust gas flowing from the upstream side of the first and fourth exhaust pipes  51   a,    51   d  through the inlet port  56   a  of the valve housing  56  into the valve chamber  56   c  is blocked by the valve wall  57 A of the valve body  57 . Instead, the exhaust gas flowing through the upstream side of the first and fourth exhaust pipes  51   a,    51   d  is turned to the side of the communication port  56   d,  and joins the exhaust gas flowing from the upstream side of the No. 2 and No. 3 exhaust pipes  51   b,    51   c  and passing through the communication port  56   d.    
     Due to a resulting, increased exhaust resistance, an exhaust pressure suitable for the low-speed rotation range is applied from the exhaust pipes  51   a  to  51   d  to the engine En. Therefore, during a valve overlap period, blow-off of fresh gas from the cylinders  50   a  to  50   d  to the exhaust system is restrained, and enhancement of low-speed output performance can be achieved. 
     The exhaust gas passing through the communication port  56   d  of the valve housing  56  flows through the downstream side of the No. 2 and No. 3 exhaust pipes  51   b,    51   c  into the lower first exhaust collecting pipe  52   b.  Here, this exhaust gas joins another portion of exhaust gas flow, and is cleaned by the primary exhaust cleaner  84 . Therefore, the entire amount of exhaust gas from the engine En flows through the primary exhaust cleaner  84 . 
     Since the primary exhaust cleaner  84  is kept warm as described hereinabove, the primary exhaust cleaner  84  is quickly activated by exhaust heat and reaction heat even immediately after the engine En is started. The exhaust gas which has passed through the lower first exhaust collecting pipe  52   b  flows into the second exhaust collecting pipe  53 , where it is further clarified by the secondary exhaust cleaner  85 . Since the secondary exhaust cleaner  85  also is kept warm, activation thereof can be accelerated as well. 
     Thus, in the low-speed operation range of the engine En, all of the engine&#39;s exhaust gas is clarified by the primary and secondary exhaust cleaners  84 ,  85 , so that clarification efficiency can be enhanced even when the exhaust gas temperature is comparatively low. 
     Meanwhile, the downstream side of the No. 1 and No. 4 exhaust pipes  51   a,    51   d  is closed up by the valve wall  57 A of the valve body  57 , and the exhaust gas is prevented from flowing into the upper first exhaust collecting pipe  52   a,  so that it is unnecessary to provide an exhaust cleaner in the upper first exhaust collecting pipe  52   a.    
     Then, when the engine En operates within the medium-speed rotation range and the driving pulley  73  is returned to the initial position a by the actuator  71 , the driving pulley  73  relieves the first transmission wire  75   a  and pulls the third transmission wire  75   c.  By the relaxation of the first transmission wire  75   a,  the driven pulley  46  on the side of the intake control valve  35  is only returned to the initial position of FIG. 6 in the range of the play angle α under the urging force of the lost motion spring  45 . Therefore, there is no change in the first intake control position A of the intake control valve  35 . 
     However, by the rotation of the driven pulley  67  on the side of the exhaust control valve  35  due to pulling of the third transmission wire  75   c , the valve body  57  is brought to the medium-speed control position D of FIG.  11 . As a result, as has been described hereinabove, the through-holes  57   a  of the valve body  57  coincide with the inlet and outlet ports  56   a ,  56   b , and the valve wall  57 A closes up the communication hole  56   e , so that the No. 1 to No. 4 exhaust pipes  51   a ,  51   d  are in an individually conducting condition. 
     Particularly, the through-holes  57   a  of the valve body  57  coincide with the No. 1 and No. 4 exhaust pipes  51   a,    51   d  via the inlet port  56   a  and the outlet port  56   b,  so that the conduits of the No. 1 and No. 4 exhaust pipes  51   a,    51   d  can be provided with a uniform cross section over the entire length thereof. The arc-shaped recess portions  57   c  of the outer surface of the valve wall  57 A of the valve body  57  fronting on the communication holes  56   e  of the valve housing  56  are in continuation with the internal circumferential surfaces of the communication ports  56   d.    
     The communication ports  56   d  are originally made to coincide with the conduits of the No. 2 and No. 3 exhaust pipes  51   b,    51   c.  Therefore, the conduits of the No. 2 and No. 3 exhaust pipes  51   b,    51   c  can be provided with a uniform cross section over the entire length thereof. Accordingly, in the No. 1 to No. 4 exhaust pipes  51   a  to  51   d,  it is possible to obtain an effective exhaust inertial effect and/or exhaust pulsation effect by utilizing the entire lengths of the exhaust pipes. 
     Namely, the effective pipe length of each of the exhaust pipes  51   a  to  51   d  is a maximum from the engine En to the upper and lower first exhaust collecting pipes  52   a,    52   b.  The maximum pipe lengths are set so that the exhaust inertia effect and/or exhaust pulsation effect enhances the volumetric efficiency of the engine En in the medium-speed rotation range. Therefore, it is possible to enhance medium-speed output performance of the engine En. 
     Further, when the engine En operates within the high-speed rotation range and the driving pulley  73  is driven to the second driving position c by the actuator  71 , the driving pulley  73  pulls the first and second transmission wires  75   a,    75   b  with a greater force than the remaining operating ranges. By this relatively larger tensile force of the first transmission wire  75   a,  the driven pulley  46  on the side of the intake control valve  35  is rotated in a valve-opening direction in large excess of the play angle α. This action brings one end wall of the arc groove  44  into contact with the transmission pin  43  of the intake control valve  35 , and brings the valve plate  36  of the intake control valve  35  to the second intake control position B of FIG.  3 . 
     Due to the larger tensile force of the second transmission wire  75   b,  the driven pulley  67  on the side of the exhaust control valve  35  is rotated by about 180 degrees from the medium-speed control position D via the low-speed control position C. This final position is shown as the valve body&#39;s  57  high-speed control position E of FIG.  12 . 
     When the valve plate  36  of the intake control valve  35  reaches the second intake control position B, as shown in FIG. 3, the valve plate  36  fully opens the large-section passage  33   b,  so that air taken into the engine En can flow through both the large-section passage  33   b  and the small-section passage  33   a  when passing through the air cleaner  17 . Therefore, intake resistance is reduced, and volumetric efficiency of the engine En is enhanced, thereby contributing to enhancement of high-speed output performance. 
     However, when the valve body  57  of the exhaust control valve  55  reaches the high-speed control position E of FIG. 12, the through-holes  57   a  of the valve body  57  coincide with the inlet and outlet ports  56   a,    56   b  of the valve housing  56 , and the communication holes  57   b  of the valve body  57  coincide with the communication holes  56   e  of the valve housing  56 , as has been described hereinabove. 
     Although the communication conditions of the No. 1 to No. 4 exhaust pipes  51   a  to  51   d  are not changed, intermediate portions of the No. 1 and No. 4 exhaust pipes  51   a,    51   d  and the No. 2 and No. 3 exhaust pipes  51   b,    51   c  are respectively communicated via the through-holes  56   e,    56   e  and  57   b,    57   b.    
     Accordingly, the effective pipe length of each of the exhaust pipes  51   a  to  51   d  is minimized from the engine En to the exhaust control valve  55 . The minimum effective pipe lengths are set so that the exhaust inertial effect and/or exhaust pulsation effect enhances the volumetric efficiency of the engine En in the high-speed rotation range. Accordingly, it is possible to enhance high-speed output performance of the engine En. 
     In the medium-speed to high-speed operation ranges of the engine En, the exhaust gases having passed through the No. 1 and No. 4 exhaust pipes  51   a,    51   d  join each other in the upper first exhaust collecting pipe  52   a  and flow toward the second exhaust collecting pipe  53 . Concurrently, the exhaust gases having passed through the No. 2 and No. 3 exhaust pipes  51   b,    51   c  join each other in the lower first exhaust collecting pipe  52   b  and are cleaned by the primary exhaust cleaner  84 , before flowing toward the second exhaust collecting pipe  53 . 
     All the exhaust gases join one another in the second exhaust collecting pipe  53 , before being cleaned by the secondary exhaust cleaner  85 . Therefore, the exhaust gases having passed through the No. 1 and No. 4 exhaust pipes  51   a,    51   d  are cleaned only by the secondary exhaust cleaner  85 . However, this is not problematic since the flow rate of exhaust gas in the medium-speed to high-speed operation ranges is comparatively high, and the cleaning function of the secondary exhaust cleaner  85  is sufficiently enhanced by large quantities of exhaust heat and reaction heat that ensure effective cleaning of the exhaust gas. 
     The engine&#39;s En intake system In and exhaust system Ex are arranged with various functional requirements dependent upon the engine operating speed. Therefore, output performance of the engine En can be effectively enhanced over low-speed to high-speed rotation ranges of the engine En. 
     When the actuator  71  returns the driving pulley  73  from the second driving position c to the first driving position b again, the driven pulley  46  and the valve plate  36  of the intake control valve  35  are returned to the first intake control position A of FIG.  2 . This is accomplished by urging forces of the lost motion spring  45  and the return spring  41  at around the time when the exhaust control valve  35  is brought from the high-speed control position E to the low-speed control position located at an intermediate point. Thereafter, the driven pulley  46  can continue a returning rotation in the range of the play angle α of the lost motion mechanism  42 , and the exhaust control valve  35  can rotate past the low-speed control position to the medium-speed control position D. 
     Therefore, even if there is a large difference between the rotation angle of the intake control valve  35  and that of the exhaust control valve  55 , the difference is absorbed by the lost motion mechanism  42 . Accordingly, both the control valves  35 ,  55  can be properly operated by the common actuator  71 . 
     The rotation of the driving pulley  73  which operates the exhaust control valve  35  between the low-speed control position and the medium-speed control position D is absorbed by the lost motion mechanism  42 , thereby eliminating deleterious effects on the intake control valve  35  located at the first intake control position A. 
     Therefore, the valve body  57  of the exhaust control valve  55  can be operated freely between the low-speed control position C, medium-speed control position D and high-speed control position E. By providing the common actuator  71  for both the intake and exhaust control valves  35  and  55 , the structure of a driving system for the control valves  35  and  55  is simplified. This further achieves enhancement of engine performance, reduction of cost, and reduction in weight. 
     Meanwhile, in the exhaust control valve  55 , the bearing bushing  60  on the side of the driven pulley  67  of the valve housing  56 , as has been described hereinabove, not only supports the valve shaft  62  on one side of the valve body  57 , but also receives one end face of the valve body  57  urged to the side of the bearing bushing  60  by the load of the thrust spring  83 . Therefore, the bearing bushing  60  and the valve body  57  are maintained in a pressure contact seal condition. 
     The portion between the valve body  57  and the bearing bushing  60  can be sealed without using any special seal member, and leakage of exhaust gas from the vicinity of the valve shaft  62  can be prevented. In addition, since expensive seal members are unnecessary, the number of component parts is reduced and cost reductions can be achieved. Furthermore, the absence of seal members allows a bearing bushing  60  longer in the axial direction to be mounted in the bearing bracket  58  in order to achieve a large bearing capacity for bearing the valve shaft  62  in a broad range. 
     Therefore, the bearing bushing  60  can firmly support the valve shaft  62  and can display excellent durability even though it directly receives load from the driven pulley  67  fitted to the valve shaft  62 . 
     Where the bearing bushing  60 , particularly on the side of pressure contact with one end face of the valve body  57 , is formed from a nonmetallic material such as carbon graphite, good sealing property can be attained. Further, vibrations in the thrust direction of the valve body  57  due to exhaust pulsation can be absorbed, whereby generation of abnormal noise can be suppressed. 
     Furthermore, the valve housing  56  and the valve body  57  provided integrally with the valve shafts  61 ,  62  are formed from a titanium material, which greatly contributes to reduction of weight of the exhaust control valve. In addition, though the titanium material forming the valve body  57  is an active metal and normally has a high tendency toward seizure, the adoption of the bearing bushings  59 ,  60  made of a carbon material ensure that good rotational movement can be provided between the valve shafts  61 ,  62  and the bearing bushes  59 ,  60  in even high-temperature conditions. This arrangement, in cooperation with the reduction of weight of the valve body  57 , permits an enhanced response to driving torque. 
     The coaxial cylindrical shape of the valve body  57  and the valve shafts  61 ,  62  ensures good distribution of melt from a central portion of the shaft end at the time of casting. At the same time, thermal deformation due to partial material thickness can be prevented. 
     In addition, finishing by cutting the external peripheral surfaces of the valve body  57  and the valve shafts  61 ,  62  can be carried out continuously after casting, so that the valve body  57  is produced with high precision. 
     The high-precision valve body  57  thus obtained can have its external peripheral surface in uniform contact with the internal surface of the valve housing  56 , so that leakage of exhaust gas at the contact area can be restrained effectively, and appropriate exhaust control can be achieved. 
     Since the cylindrical valve body  57  has good weight balance about its rotational axis line, it is possible to achieve a reduction in the driving torque for the valve body  57  and, eventually an enhancement of system response to the driving torque. Also, partial loading on the bearing bushings  59 ,  60  can be minimized, whereby durability of the bearing bushes  59 ,  60  can be enhanced. 
     The present invention is not limited to or by the embodiments above, and various design modifications can be made without departure from the spirit and scope of the invention. For example, the intake control valve  35  can be so constructed that the effective pipe length of the intake system In is changed according to the operating condition of the engine En. The invention can also be applied to a two-cylinder engine, where the two exhaust pipes are controlled by the exhaust control valve  55  in the same manner as the No. 1 and No. 4 exhaust pipes  51   a,    51   d  and the No. 2 and No. 3 exhaust pipes  51   b,    51   c  in the above embodiment. Naturally, the invention can be applied also to other multi-cylinder engines. 
     As has been described hereinabove, according to an embodiment of the present invention, an exhaust control valve having a valve housing, and a valve body rotatably contained in a valve chamber of the valve housing to cooperate with the valve housing for controlling the flow of exhaust gas, with a transmission member for rotationally driving a valve shaft of the valve body rotatably borne by bearing bushes mounted in the valve housing being fitted to one end of the valve shaft, the valve body is formed in a cylindrical shape coaxial with the axis line of the valve shaft, and the valve body and the valve shaft are formed as one body by casting. 
     With the present invention, a valve body with high precision can be produced efficiently. The high-precision valve body can have uniform contact with the internal surface of the valve housing, whereby leakage of exhaust gas at the contact area can be restrained effectively. Accordingly, appropriate exhaust control can be achieved. 
     In addition, since the cylindrical valve body has good weight balance about its rotational axis, it is possible to achieave a reduction in the driving torque for the valve body. Therefore, an improved system response to the driving torque is achieved. Also, partial loading on the bearing bushings can be minimized, whereby durability of the bearing bushes can be improved. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.