Abstract:
A clutch actuator structure provided for an internal combustion engine and a hydraulic clutch mechanism for transmitting a rotational driving force of a crankshaft of said engine includes a clutch actuator for controlling an oil pressure for engaging and disengaging the hydraulic clutch mechanism. The clutch actuator structure ensures that noises generated at the times of operation of the clutch actuator for actuating a hydraulic clutch are prevented from being transmitted to the exterior. The clutch actuator structure includes an oil sump part for reserving oil provided in the periphery of a clutch actuator. The clutch actuator is disposed in the oil sump part.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a clutch actuator for actuating a hydraulic clutch of an internal combustion engine. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventionally, there has been an exemplary structure in which a clutch actuator is mounted to the outside of a crankcase of an internal combustion engine. In this structure, however, during the operation of the clutch actuator, the operating sounds generated at valve opening and closing times are transmitted to the exterior, which naturally is undesirable (see, for example, FIG. 3 of Japanese Patent Laid-open No. 2008-057620). 
         [0003]    There has been another exemplary structure in which a clutch actuator is mounted to the inside of a crankcase. Also in this structure, however, sounds are echoed onto a thin clutch cover, again leading to leakage of operating sounds to the exterior (see, for example, FIG. 6 of Japanese Patent Laid-open No. 2008-138541). 
       SUMMARY OF THE INVENTION 
       [0004]    The clutch actuator structure ensures that the operating sounds generated at the time of operation of a clutch actuator for actuating a hydraulic clutch of an internal combustion engine are prevented from being transmitted to the exterior. 
         [0005]    The first aspect of the structure pertains to a clutch actuator structure provided for an internal combustion engine and a hydraulic clutch mechanism for transmitting a rotational driving force of a crankshaft of the engine. The clutch actuator structure controls an oil pressure for engaging and disengaging the hydraulic clutch mechanism. An oil sump part for reserving the oil is provided in the periphery of the clutch actuator, and the clutch actuator is disposed in the oil sump part. 
         [0006]    The second aspect of the structure is characterized in that the clutch actuator is mounted to the inside of the oil sump part provided in a crankcase or a crankcase cover, and a cover member is fixed so as to cover the oil sump part. 
         [0007]    The third aspect of the structure is characterized in that the clutch mechanism includes a plurality of clutches, and a plurality of oil passages are formed to extend from the clutch actuator to the clutch mechanism substantially in parallel to each other. 
         [0008]    The fourth aspect of the structure is characterized in that an oil discharge hole is provided in the clutch actuator and opens into the oil sump part. 
         [0009]    The fifth aspect of the structure is characterized in that the clutch actuator includes a tubular member to which the plurality of oil passages are connected and a sliding member operating in the tubular member so as to switch between communicating conditions of the oil passages; wherein the tubular member and the sliding member are made to slide by an operation of the clutch actuator, whereby a return oil from a clutch is discharged from the discharge hole. 
         [0010]    The sixth aspect of the structure is characterized in that a supply oil passage for supplying the oil from an oil pump to the tubular member and the sliding member is branched and extends to form a clutch lubricating oil passage. 
         [0011]    The seventh aspect of the structure is characterized, in that the cover member includes an oil return hole above the clutch actuator. 
         [0012]    The eighth aspect of the structure is characterized in that the clutch actuator is located on a front lower side of the clutch mechanism in side view of the internal combustion engine, and is disposed on the inside of the oil sump part provided in the crankcase cover. 
         [0013]    The ninth aspect of the structure is characterized in that the clutch mechanism is disposed on a rear upper side of the crankshaft in side view of the internal combustion engine, and the clutch actuator is disposed on the lower side of the crankshaft in side view of the internal combustion engine. 
         [0014]    The tenth aspect of the structure is characterized in that a main shaft coaxial with the clutch mechanism is disposed on a rear upper side of the crankshaft, an upper wall of the crankcase forms a slant surface facing toward a front upper side between the crankshaft and the main shaft, and an engine hanger is projectingly formed on the slant surface of the upper wall of the crankcase. 
         [0015]    In the first aspect of the clutch actuator structure, the oil sump part is provided, and the clutch actuator is disposed in the oil sump part. Therefore, the valve opening and closing sounds generated during the operation of the clutch actuator can be attenuated, and the sound leaking to the exterior is reduced. 
         [0016]    In the second aspect of the structure, the oil sump part can be formed in a simple structure. 
         [0017]    In the third aspect of the structure, the plurality of oil passages can be formed substantially in parallel to each other. Therefore, notwithstanding that the oil passages are provided in plurality, they can be arranged efficiently. In addition, since the directions of machining can be unified, machinability can be enhanced. 
         [0018]    In the fourth aspect of the structure, the oil discharged from the clutch actuator is reserved. Therefore, the need for other oil supply means is eliminated, and the number of component parts is reduced. 
         [0019]    In the fifth aspect of the structure, the return oil is discharged from the discharge hole by sliding of the sliding member. Therefore, the sliding part can be lubricated utilizing the oil thus discharged. 
         [0020]    In the sixth aspect of the structure, the supply oil passage is branched and extended to form the clutch lubricating oil passage, which is easier to carry out than the formation of a clutch lubricating oil passage communicating with other lubricating oil circuit. Accordingly, the number of machining steps is reduced. 
         [0021]    In the seventh aspect of the structure, the oil return hole is formed above the clutch actuator by the cover member. Therefore, it is ensured that the clutch actuator is sufficiently immersed in the oil, so that surplus oil can be speedily recovered into the internal combustion engine while producing the operating sound reducing effect. 
         [0022]    In the eighth aspect of the structure, the clutch actuator is disposed on the inside of the oil sump part provided in the crankcase cover, so that the clutch actuator projects on a lateral side of the internal combustion engine together with the clutch mechanism. However, since the clutch actuator is located on the front lower side of the clutch mechanism in side view of the internal combustion engine, a foot rest space for the rider can be secured at an optimum position on the rear side of the clutch actuator and on the lower side of the clutch mechanism. 
         [0023]    Incidentally, since the oil sump part to which the clutch actuator is mounted is provided in the crankcase cover, a situation that the space inside the crankcase is limited to be narrower by the oil sump part can be obviated. 
         [0024]    In the ninth aspect of the structure, the clutch mechanism is disposed on a rear upper side of the crankshaft in side view of the internal combustion engine, and the clutch actuator is disposed on the lower side of the crankshaft in side view of the internal combustion engine. Therefore, a position on the rear lower side of the crankshaft becomes a foot rest position for the rider, and the position is an optimum position as a foot rest position for the rider in the case of a motorcycle in which the internal combustion engine is horizontally mounted on a vehicle body. 
         [0025]    In the tenth aspect of the structure, the main shaft coaxial with the clutch mechanism is disposed on a rear upper side of the crankshaft, the upper wall of the crankcase forms the slant surface facing toward a front upper side between the crankshaft and the main shaft, and the engine hanger is projectingly formed on the slant surface. Therefore, it is unnecessary to provide the engine hanger projecting upward beyond a highest part of the crankcase, so that the internal combustion engine can be mounted on the vehicle body in a compact fashion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The advantages of the invention will become apparent in the following description taken in conjunction with the drawings, wherein: 
           [0027]      FIG. 1  is a right side view of an internal combustion engine according to an embodiment of the present invention; 
           [0028]      FIG. 2  is a sectional view of a transmission for the internal combustion engine; 
           [0029]      FIG. 3  is an enlarged view of the vicinity of a clutch; 
           [0030]      FIG. 4  is an outside view of a right crankcase cover; 
           [0031]      FIG. 5  is an inside view of the right crankcase cover; 
           [0032]      FIG. 6  is a view showing a condition where a clutch actuator accommodating case is covered with a cover member; 
           [0033]      FIG. 7  illustrates the structure and operation of a clutch actuator; 
           [0034]      FIG. 8  is a sectional view of a supply oil passage for supplying oil to the clutch actuator; 
           [0035]      FIG. 9  is a sectional view of a control oil passage communicating with a pressurization chamber in a first clutch; 
           [0036]      FIG. 10  is a sectional view of a control oil passage communicating with a pressurization chamber in a second clutch; 
           [0037]      FIG. 11  is a sectional view of a lubricating oil passage communicating with a pressure regulation chamber in the first clutch; 
           [0038]      FIG. 12  is a view showing a condition where the clutch actuator accommodating case is covered with a cover member according to a second embodiment of the present invention; 
           [0039]      FIG. 13  is a sectional view showing a condition where the cover member according to the second embodiment is mounted to the clutch actuator accommodating case; 
           [0040]      FIG. 14  is a right side view of an internal combustion engine according to a third embodiment of the invention; 
           [0041]      FIG. 15  is a right side view of a motorcycle on which the internal combustion engine is mounted; 
           [0042]      FIG. 16  is an enlarged view of a major part of  FIG. 15 ; and 
           [0043]      FIG. 17  is a partial front view of the motorcycle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]      FIG. 1  is a right side view of an internal combustion engine  1  according to an embodiment of the present invention. The internal combustion engine  1  is a parallel two-cylinder internal combustion engine. Arrow F indicates the front side corresponding to the front side of a vehicle on which the internal combustion engine  1  is mounted. 
         [0045]    A main outer shell of the internal combustion engine  1  includes a crankcase  2  which is composed of an upper crankcase  2 A and a lower crankcase  2 B, a cylinder block  3 , a cylinder head  4 , a cylinder head cover  5 , and an oil pan  6 . A transmission  7  is integrally incorporated in the crankcase  2 . 
         [0046]    A crankshaft  8 , and a main shaft  9  and a counter shaft  10  of the transmission are rotatably borne on bearings at mating surfaces of the crankcase  2  which is bisected to the upper and lower components. 
         [0047]    The oil pan  6  connected to the lower end of the lower crankcase  2 B is provided with an oil suction pipe  12  having a strainer  11 , and a control oil pump  13  and a control oil filter  14  connected thereto are connected to an upper portion of the oil suction pipe  12 . 
         [0048]    The internal combustion engine  1  is provided also with a lubricating oil pump, which is omitted in the drawing. 
         [0049]    A lubricating oil filter  15  is shown in the figure. The discharge pressure of the control oil pump  13  is set high for clutch actuator use, and the discharge pressure of the lubricating oil pump is lower than the discharge pressure of the control oil pump. 
         [0050]    The transmission  7  accommodated in a rear portion of the crankcase  2  of the internal combustion engine  1  is a constant-mesh type twin-clutch transmission. 
         [0051]    In addition, a change mechanism including a shift drum  16  and the like for changing the transmission gear speed is accommodated in the rear portion of the crankcase  2 . 
         [0052]    A crankcase right side portion is covered with a right crankcase cover  17 . Outside portions of various oil passages are bulging in the form of folds on the outer surface of the right crankcase cover  17 . 
         [0053]    Incidentally, an output shaft  18  for driving the vehicle by obtaining power from the counter shaft  10  is provided below the crankshaft  8 . 
         [0054]      FIG. 2  is a sectional view of the transmission  7 . 
         [0055]    The change mechanism including the shift drum and a shift fork is omitted in the drawing. 
         [0056]    Arrows L and R indicate leftward and rightward directions corresponding to the left and right sides of the vehicle on which the internal combustion engine  1  is mounted. 
         [0057]    The transmission  7  includes the main shaft  9 , the counter shaft  10 , a primary driven gear  20 , and a pair of clutches  21  consisting of a first clutch  21 A and a second clutch  21 B. 
         [0058]    Of the main shaft  9 , the left end is rotatably borne on the crankcase  2  through a ball bearing  22 , and a central portion on the crankcase  2  through a ball bearing  23 . 
         [0059]    The right end of the main shaft  9  is rotatably borne on the right crankcase cover  17  through a ball bearing  24 . 
         [0060]    Of the counter shaft, the left end is rotatably borne on the crankcase  2  through a ball bearing  25 , and the right end on the crankcase  2  through a needle bearing  26 . 
         [0061]    The main shaft  9  includes a long main shaft inner shaft  9 A, a main shaft outer shaft  9 B, and a clutch part outer shaft  9 C. 
         [0062]    The main shaft outer shaft  9 B covers a part of the main shaft inner shaft  9 A in a relatively rotatable manner, with a needle bearing  27  therebetween. 
         [0063]    The left end of the main shaft outer shaft  9 B is restrained by a C-shaped snap ring  28  from leftward movement. 
         [0064]    The right end of the main shaft outer shaft  9 B abuts on the clutch part outer shaft  9 C, with an annular spacer  29  therebetween, and is restrained from rightward movement. Six gears M 1  to M 6  are provided on the main shaft  9 , while six gears C 1  to C 6  constantly meshing with these gears M 1  to M 6  are provided on the counter shaft  10 , correspondingly to the gears M 1  to M 6 . 
         [0065]    Here, M indicates that the gear belongs to the main shaft, while C indicates that the gear belongs to the counter shaft, and suffixes  1  to  6  indicate that the gears are for determining the first-speed to sixth-speed gear ratios, respectively. 
         [0066]    The odd-numbered speed gears M 1 , M 5  and M 3  are provided on the main shaft inner shaft  9 A, while the even-numbered speed gears M 4 , M 6  and M 2  are provided on the main shaft outer shaft  9 B. 
         [0067]    In  FIG. 2 , suffix x attached to the gear symbol indicates a fixed gear which is formed integrally with a shaft, suffix w indicates an idle gear which is held on a shaft and which can be rotated relative to the shaft at a predetermined position on the shaft, and suffix s indicates a slide gear which is slidable in the axial direction. 
         [0068]    The slide gear is held on the shaft by splines  30  so that it is incapable of relative rotation to the shaft in the circumferential direction but is capable of sliding in the axial direction. 
         [0069]    A gear meshed with the fixed gear (suffix x) or the slide gear (suffix s) is necessarily an idle gear (suffix w). 
         [0070]    The idle gear (suffix w) cannot function as a gear by itself. For the idle gear (suffix w) to function as a gear, it must be fixed to the shaft by the slide gear (suffix s) provided adjacent thereto. 
         [0071]    The slide gear is provided with an engaging groove G for engagement with the shift fork of the change mechanism (not shown). The slide gear (suffix s) is moved in the axial direction by the shift fork engaged therewith. 
         [0072]      FIG. 3  is an enlarged view of the vicinity of the clutches  21 . 
         [0073]    The clutch part outer shaft  9 C is provided on a right half of the main shaft inner shaft  9 A. 
         [0074]    The clutch part outer shaft  9 C covers the peripheries of end portions of the main shaft inner shaft  9 A in a rotatable manner, with needle bearings  31 A and  31 B therebetween. 
         [0075]    The right end of the clutch part outer shaft  9 C abuts on another member, with an annular spacer  32  therebetween, and is restrained together with the member from movement, by a washer  33  and a nut  34  provided at an end of the main shaft inner shaft  9 A. 
         [0076]    The left end of the clutch part outer shaft  9 C abuts on the right end of the main shaft outer shaft  9 B, with the annular spacer  29  therebetween. The left end of the main shaft outer shaft  9 B is restrained by the C-shaped snap ring  28  from movement ( FIG. 2 ). 
         [0077]    The primary driven gear  20  and clutch outers  37 A and  37 B of the first clutch  21 A and the second clutch  21 B are fixed to the clutch part outer shaft  9 C by splines  38  and C-shaped snap rings  39 , respectively. 
         [0078]    The primary driven gear  20  is non-rotatably fitted to the clutch part outer shaft through splines  19 . 
         [0079]    Movements of the primary driven gear  20  in the left-right direction are restrained by the clutch outers  37 A and  37 B. 
         [0080]    The primary driven gear  20  is a gear in constant mesh with a primary drive gear (not shown) provided on the crankshaft  8 , and, by receiving a rotational driving force from the crankshaft  8 , drives the clutch part outer shaft  9 C to rotate. 
         [0081]    According to this, the clutch outers  37 A and  37 B of the pair of clutches  21  are rotated together. 
         [0082]    Clutch inners  40 A and  40 B of these clutches  21  are connected respectively to separate members. 
         [0083]    The clutch inner  40 A of the first clutch  21 A is fitted to splines  41  at the right end of the main shaft inner shaft  9 A, and is fixed to the main shaft inner shaft  9 A by the washer  33  and the nut  34 . 
         [0084]    The clutch inner of the second clutch  21 B is fixed by being fitted to splines  42  at the right end of the main shaft outer shaft  9 B. 
         [0085]    The pair of clutches  21 A and  21 B are both multiple disc clutches. 
         [0086]    On the inside of an outer circumferential portion of each of the clutch outers  37 A and  37 B of the pair of clutches  21 , a plurality of drive friction discs  43  are provided which are engaged with the clutch outer  37  so as to be incapable of relative rotation but capable of axial movement. 
         [0087]    On the outside of each of the clutch inners  40  of the pair of clutches  21 , a plurality of driven friction discs  44  are provided which are engaged with the clutch inner  40  so as to be incapable of relative rotation but capable of axial movement. 
         [0088]    The drive friction discs  43  and the driven friction discs  44  are alternately disposed to constitute a friction disc group  45 . 
         [0089]    A pressure plate  46  is provided between an end plate part  37   x  of the clutch outer  37 A,  37 B and the friction disc group  45  in each of the clutches  21 . An end portion of an outer circumferential portion of the pressure plate  46  abuts on the drive friction disc  43  at one end of the friction disc group  45 . 
         [0090]    The drive friction disc  43  at the other end of the friction disc group  45  is restrained by a C-shaped snap ring  47  from movement. 
         [0091]    A spring bearing member  48  is provided between the pressure plate  46  and the clutch inner  40 . 
         [0092]    The inner circumferential end of the spring bearing member  48  is restrained from movement, by a C-shaped snap ring  49  provided on a clutch outer boss part  37   y.    
         [0093]    The outer circumferential end of the spring bearing member  48  is in sliding contact with the inside of an outer circumferential portion of the pressure plate  46 , with a seal member  50  therebetween. 
         [0094]    The pressure plate  46  is pressed toward the clutch outer end plate part  37   x , by a coil spring  51  abutting on the spring bearing member  48  at its one end. 
         [0095]    A pressurization chamber  52  ( 52 A,  52 B) is formed between the clutch outer end plate part  37   x  and the pressure plate  46 . 
         [0096]    A pressure regulation chamber  53  ( 53 A,  53 B) is formed between the spring bearing member  48  and the pressure plate  46 . 
         [0097]    The pressure regulation chamber  53  is a chamber such that a pressure increase in the pressure chamber  52  by a centrifugal force is canceled by a pressure increase due to a centrifugal force exerted on the oil in the pressure regulation chamber  53 , whereby the clutch is disengaged. 
         [0098]    The main shaft inner shaft  9 A is provided with a main shaft left-side center hole  56  opening to the left side, and a main shaft right-side center hole  57  opening to the right side. 
         [0099]    The right-side center hole  57  is a stepped hole having a plurality of stages of inside diameter. 
         [0100]    Two coaxial pipes, namely, an inner pipe  58  and an outer pipe  59  are inserted in the right-side center hole  57 . 
         [0101]    A left end portion of the inner pipe  58  is fitted in the small-diameter portion of the right-side center hole  57 , with a seal member  60 A therebetween. A right end portion of the inner pipe  58  is supported on the right crankcase  17 , with a seal member  60 B therebetween. The inside of the inner pipe  58  and the outside of the inner pipe  58  are partitioned from each other through the seal members  60 A and  60 B. 
         [0102]    A left end portion of the outer pipe  59  is fitted in the large-diameter portion of the right-side center hole  57 , with a seal member  61 A therebetween. A right end portion of the outer pipe  59  is supported on the right crankcase  17 , with a seal member  61 B therebetween. The inside of the outer pipe  59  and the outside of the outer pipe  59  are partitioned from each other by the seal members  61 A and  61 B. 
         [0103]    An oil passage formed between the outside of the inner pipe  58  and the inside of the outer pipe  59  and the inside of the middle-diameter portion of the right-side center hole  57  is referred to as main shaft end first oil passage  62 A; an oil passage connecting the inner bore of the inner pipe  58  with the small-diameter portion of the right-side center hole  57  is referred to as main shaft end second oil passage  62 B; and an oil passage formed between the outside of the outer pipe  59  and the inside of the large-diameter portion of the right-side center hole  57  is referred to as main shaft end third oil passage  62 C. 
         [0104]    The main shaft end first oil passage  62 A communicates with the pressurization chamber  52 A of the first clutch  21 A through oil passages  63  which radially penetrate the main shaft inner shaft  9 A and the clutch part outer shaft  9 C for the communication. 
         [0105]    The main shaft end second oil passage  62 B communicates with the pressurization chamber  52 B of the second clutch  21 B through oil passages  64  which radially penetrate the main shaft inner shaft  9 A and the clutch part outer shaft  9 C for the communication. 
         [0106]    The main shaft end third oil passage  62 C communicates with the pressure regulation chamber  53 A of the first clutch  21 A through an oil passage  65  which radially penetrates the main shaft inner shaft  9 A, the needle bearing  31 A, a main shaft inner shaft outer circumferential gap oil passage  66 , and an oil passage  67  which radially penetrates the clutch part outer shaft  9 C. 
         [0107]    Incidentally, the main shaft left-side center hole  56  communicates with the pressure regulation chamber  53 B of the second clutch  21 B through an oil passage  68  which radially penetrates the main shaft inner shaft  9 A, the needle bearing  31 B, a main shaft inner shaft outer circumferential gap oil passage  69 , and an oil passage  70  which radially penetrates the clutch part outer shaft  9 C. 
         [0108]      FIG. 4  is an outside view of the right crankcase cover  17 . 
         [0109]    A pair of clutch actuators  74 , namely, an upper-side clutch actuator  74 A and a lower-side clutch actuator  74 B are accommodated in the clutch actuator accommodating case  73  provided at the inside surface of the right crankcase cover  17 . 
         [0110]    Pipe parts are bulging in the form of folds on the outside surface of the right crankcase cover  17 . A single supply oil passage  75  which extends from the control oil pump  13  toward the clutch actuators  74  through the control oil filter  14 , and control oil passages  76 A and  76 B and a lubricating oil passage  77  which extend from the clutch actuators  74  toward the clutches  21 , are provided in the pipe parts. 
         [0111]    The control oil passage  76 A extending from the upper-side clutch actuator  74 A reaches the main shaft end first oil passage  62 A in  FIG. 3 . A control oil is fed from the upper-side clutch actuator  74 A to the pressurization chamber  52 A of the first clutch  21 A. 
         [0112]    The control oil passage  76 B extending from the lower-side clutch actuator  74 B reaches the main shaft end second oil passage  62 B. The control oil is fed from the lower-side clutch actuator  74 B to the pressurization chamber  52 B of the second clutch  21 B. 
         [0113]    The lubricating oil passage  77  branched and extended from the supply oil passage  75  through a communicating oil passage  80  reaches the main shaft end third oil passage  62 C. The control oil is constantly fed from the supply oil passage  75  to the pressure regulation chamber  53 A of the first clutch  21 A, as a lubricating oil. 
         [0114]    Incidentally, the pressure regulation chamber  53 B of the second clutch  21 B is fed with the lubricating oil from the main shaft left-side center hole  56  communicating with the lubricating oil pump. 
         [0115]    In each of the oil passages mentioned above, the oil reciprocates according to engagement and disengagement of the relevant clutch. 
         [0116]    It should be noted here, however, that the oil in the supply oil passage and the lubricating oil in the main shaft left-side center hole  56  do not reciprocate. 
         [0117]      FIG. 5  is an inside view of the right crankcase cover  17 . The lubricating oil passage  77  communicates with the supply oil passage  75  through the communicating oil passage  80 . The clutch actuator accommodating case  73  in which the clutch actuators  74  are accommodated is covered with a cover member  78  shown in  FIG. 6 . 
         [0118]      FIG. 6  is an inside view of the right crankcase cover  17 , showing a condition where the clutch actuator accommodating case  73  is covered with the cover member  78 . 
         [0119]    The inside of this member functions as an oil sump part  79 , and the return oil from the clutches  21  is discharged into the oil sump part  79 . 
         [0120]    A gap  95  is provided at an upper portion of the clutch actuator accommodating case  73 , so that surplus oil is discharged through the gap  95  into the inside of the crankcase  2 . 
         [0121]      FIG. 7  illustrates the structure and operation of the clutch actuator  74 . 
         [0122]    Two clutch actuators  74  having this configuration are provided, as above-mentioned. 
         [0123]    In  FIG. 7 , the clutch actuator  74  includes: a magnet coil  83  accommodated in a solenoid case  82 ; an iron piece  84  receiving an electromagnetic force of the magnet coil  83 ; a valve case  85 ; a valve element  86  accommodated in the valve case  85 ; a connecting rod  87  for connecting the iron piece  84  with the valve element  86 ; a return spring  88 ; and a spring bearing cover  89  for bearing the outer end of the return spring  88 . 
         [0124]    The iron piece  84  and the connecting rod  87  and the valve element  86  are moved rectilinearly in the valve case  85 , as one body. The valve case  85  is provided therein with the supply oil passage  75 , the control oil passage  76 , the lubricating oil passage  77  ( FIG. 11 ), the communicating oil passage  80  ( FIG. 11 ), a pressure regulation oil passage  90  which is branched from the control oil passage to reach an end portion of the valve element, and a return oil discharge oil passage  91 . 
         [0125]      FIG. 7(   a ) shows the position of the valve element  86  when the clutch actuator  74  is OFF. 
         [0126]    In  FIG. 7 , the densely crosshatched parts H indicate a high-pressure oil, while coarsely crosshatched parts L indicate a low-pressure oil. An end portion on the valve side of the supply oil passage  75  supplied with the high-pressure oil from the oil pump  13  is shut off by the valve element  86 , whereby the oil is stopped. 
         [0127]      FIG. 7(   b ) shows the position of the valve element  86  at the moment the magnet coil  83  is energized to turn ON the clutch actuator  74 . 
         [0128]    The valve element  86  has been moved to the right end of an iron piece moving space  92  in the figure, against the biasing force of the return spring  88 . There is shown a condition where the supply oil passage  75  and space at a small-diameter portion  86   a  of the valve element  86  communicate with each other, the high-pressure oil has started flowing into the control oil passage  76 , and the oil pressure is rising. 
         [0129]      FIG. 7(   c ) shows a condition where the pressure rise is completed. 
         [0130]    The pressure in the pressure regulation oil passage  90  branched from the control oil passage  76  has also been raised, and the high pressure in the pressure regulation oil passage  90  acts on the end portion side of the valve element  86 , so that the valve element  86  is pushed back a little. 
         [0131]    As a result, a valve-side end portion of the supply oil passage  75  is shut off by the valve element  86 , so that the supply of the high-pressure oil into the control oil passage  76  is stopped. In addition, since an actuator-side end portion of the control oil passage  76  is also shut off, the inside of the pressurization chamber  52  ( FIG. 3 ) of the clutch  21  is maintained at a high pressure, the pressure plate is pushed, and the multiple disc clutch  21  is put into an engaged state. 
         [0132]    When an instruction to turn OFF the clutch actuator  74  is given, the energizing of the magnet coil  83  is stopped, so that the electromagnetic force for pushing the valve element  86  is lost, and the valve element  86  is returned to the position in  FIG. 7(   a ) by the biasing force of the return spring  88 . 
         [0133]    As a result, the control oil passage  76  communicates with the return oil discharge oil passage  91 , and the oil is returned from the clutch  21  by the action of the coil spring  51  in the pressure regulation chamber  53  of the clutch  21 , to be discharged from the return oil discharge oil passage  91 . Consequently, the multiple disc clutch  21  is disengaged. 
         [0134]    The oil thus discharged collects in the oil sump part  79  in the clutch actuator accommodating case  73 . 
         [0135]    The clutch actuators  74  are immersed in this oil. 
         [0136]    The oil flows through the gap at the spring bearing cover  89  into the gap between the valve case  85  and the valve element  86 , to lubricate the sliding surfaces. Besides, since the clutch actuators  74  are immersed in the oil, the operating sounds generated from the clutch actuators  74  are intercepted, and transmission of the sounds to the exterior is suppressed. 
         [0137]      FIG. 8  is a sectional view of the supply oil passage  75  for supplying the oil from the control oil pump  13  to the clutch actuators  74  through the control oil filter  14 . 
         [0138]    The supply oil passage  75  is formed in a wall body of the right crankcase cover  17 . 
         [0139]    Two clutch actuators  74  are accommodated, at upper and lower positions, in the clutch actuator accommodating case  73 . 
         [0140]    The oil is supplied into an intermediate area between the clutch actuators  74 , to be supplied respectively to the upper-side clutch actuator  74 A and the lower-side clutch actuator  74 B. 
         [0141]    A gap  95  is provided at an upper joint portion between the clutch actuator accommodating case  73  and the cover member  78  for covering an opening of the clutch actuator accommodating case  73 . Therefore, surplus oil is discharged into the inside of the crankcase  2 . 
         [0142]      FIG. 9  is a sectional view of the control oil passage  76 A extending from the upper-side clutch actuator  74 A to the pressurization chamber  52 A of the first clutch  21 A. 
         [0143]    This oil passage extends through a right crankcase cover inside space  93 A and the main shaft end first oil passage  62 A, and through the oil passages  63 , to reach the pressurization chamber  52 A of the first clutch  21 A. 
         [0144]    When the high-pressure oil is supplied into the pressurization chamber  52 A of the first clutch  21 A, the pressure plate  46  is moved toward the friction disc group  45  against the biasing force of the coil spring  51 . As a result, the friction discs are pressed against one another, so that the rotation of the clutch outer  37 A is transmitted to the clutch inner  40 A, and the main shaft inner shaft  9 A is driven to rotate. 
         [0145]      FIG. 10  is a sectional view of the control oil passage  76 B extending from the lower-side clutch actuator  74 B to the pressurization chamber  52 B of the second clutch  21 B. 
         [0146]    This oil passage extends through a right crankcase cover inside space  93 B and the main shaft end second oil passage  62 B, and through the oil passages  64 , to reach the pressurization chamber  52 B of the second clutch  21 B. 
         [0147]    When the high-pressure oil is supplied into the pressurization chamber  52 B of the second clutch  21 B, the pressure plate  46  is moved toward the friction disc group  45  against the biasing force of the coil spring  51 . Consequently, the friction discs are pressed against one another, so that the rotation of the clutch outer  37 B is transmitted to the clutch inner  40 B, and the main shaft outer shaft  9 B is driven to rotate. 
         [0148]      FIG. 11  is a sectional view of the lubricating oil passage  77  extending from the lower-side clutch actuator  74 B to the pressure regulation chamber  53 A of the first clutch  21 A. 
         [0149]    This oil passage includes the communicating oil passage  80  which is formed, between the valve case  85  and a plate  94  in the lower-side clutch actuator  74 B, so as to make the supply passage  75  and the lubricating oil passage  77  communicate with each other. The high-pressure oil supplied through the supply oil passage  75  is constantly supplied to the lubricating oil passage  77  through the communicating oil passage  80 . 
         [0150]    The plate  94  is provided with an orifice  81 , to contrive a lowering in the pressure of the oil supplied into the lubricating oil passage  77 . 
         [0151]    The lubricating oil passage  77  extends through a right crankcase cover inside space  93 C and the main shaft end third oil passage  62 C, then through the oil passage  65 , the needle bearing  31 A, and the oil passages  66  and  67 , to reach the pressure regulation chamber  53 A of the first clutch  21 A. The high-pressure oil is supplied through the lubricating oil passage  77  into the pressure regulation chamber  53 A. 
         [0152]    The oil thus supplied lubricates the needle bearing  31 A. 
         [0153]    Incidentally, the pressure regulation chamber  53  of the second clutch  21 B is supplied with a low-pressure lubricating oil from the main shaft left-side center hole  56  through the oil passage  68 , the needle bearing  31 B, and the oil passages  69 ,  70 . 
         [0154]    The pair of clutch actuators are so operated that when one of them is ON, the other of them is OFF. 
         [0155]    Which one of the pair of clutch actuators is to be turned ON is automatically decided by an electronic control unit (not shown). 
         [0156]    At the time of starting the engine, the clutch actuator operation is carried out in order to confirm the operating conditions of the clutch actuators. The oil discharged in this operation can be reserved in the oil sump part  79 . 
         [0157]    Therefore, even in the case where the vehicle has been left non-operated for a long time, the condition where the oil is reserved in the oil sump part  79  is attained at the time of operating the engine. This contributes to prevention of leakage of operating sounds and to lubrication. 
         [0158]    While the embodiment of the present invention has been described presuming its application to a clutch mechanism, the technology of preventing leakage of actuator operating sounds can also be applied to other oil pressure control mechanisms. 
         [0159]    Besides, while the clutch actuators are of the horizontal type in the embodiment above, they may be of the vertical type. The clutch actuators may be disposed not only in the right crankcase cover but also in any other part in the crankcase. 
         [0160]      FIG. 12  is an inside view of the right crankcase cover  17 , showing a condition where a clutch actuator accommodating case  99  is covered with a cover member  97  according to a second embodiment of the present invention. 
         [0161]    The accommodating case  99  is not provided with such an upper-portion gap as provided in the first embodiment. Instead, the accommodating case  99  is provided with oil return holes  98  at positions on the upper side of the upper-side actuator  74 A, whereby surplus oil is discharged to the inside of the crankcase  2 . 
         [0162]      FIG. 13  is a sectional view showing a condition where the cover member  97  according to the second embodiment is mounted to the clutch actuator accommodating case  99 . The surplus oil is discharged through the oil return holes  98 . 
         [0163]    In the internal combustion engines  1  in the first and second embodiments described above, the crankshaft  8  is rotatably borne on the bearings at the mating surfaces of the upper crankcase  2 A and the lower crankcase  2 B of the crankcase bisected to the upper and lower components, while the main shaft  9  and the counter shaft  10  of the transmission are also rotatably borne on the same mating surfaces, and the crankshaft  8 , the main shaft  9  and the counter shaft  10  are arranged substantially at the same height and on the rear side in this order. 
         [0164]    In addition, the clutch actuator  74  disposed in the oil sump part  79  formed in the inner surface of the right crankcase cover  17  is located on the upper side of the crankshaft  8  in side view of the internal combustion engine shown in  FIG. 1 . 
         [0165]    In contrast to the first and second embodiments, a parallel 2-cylinder internal combustion engine  100  according to a third embodiment shown in  FIGS. 14 to 17  has a configuration wherein a crankshaft  108  and a counter shaft  110  are rotatably borne on bearings at mating surfaces of an upper crankcase  102 A and a lower crankcase  102 B of a crankcase bisected into the upper and lower components, but a main shaft  109  is disposed, between the crankshaft  108  and the counter shaft  110 , above the mating surfaces. 
         [0166]    Besides, a clutch actuator  122  disposed in an oil sump part  121  formed in an inner surface of a right crankcase cover  120  is located on the lower side of the crankshaft  108  in side view of the internal combustion engine shown in  FIG. 14 . 
         [0167]    A twin clutch  112  is provided at a right end portion of the main shaft  109 , and the cutch  112  is covered by the right crankcase cover  120  on the right side thereof, so that the right crankcase cover  120  is bulging to the right side at its portion corresponding to the clutch  112 . 
         [0168]    The right crankcase cover  120  is formed with an oil sump part  121  in which to dispose the clutch actuator  122  on the lower side of the crankshaft  108  in side view of the internal combustion engine, the oil sump part  121  being so formed as to bulge to the right side. 
         [0169]    Thus, of the right crankcase cover  120 , the portion corresponding to the clutch  112  which is disposed on the rear upper side of the crankshaft  108  in side view of the internal combustion engine and the oil sump part  121  for the clutch actuator  122  which is disposed on the lower side of the crankshaft  108  are bulging to the right side. 
         [0170]    Therefore, a recessed space S is formed on the rear side of the clutch actuator  122  with respect to the right crankcase cover  120  and on the lower side of the clutch  112 , and the space S can be used as a foot rest space for the rider. 
         [0171]    In this internal combustion engine  100 , an upper wall of the upper crankcase  102 A forms a slant surface  102   s  which is facing to a front upper side in positional relationship between the crankshaft  108  and the main shaft  109  located on a rear upper side of the crankshaft  108 , and a cylinder block  103 , a cylinder head  104 , and a cylinder head cover  105  are projected in a forwardly slanted posture from a front portion of the slant surface  102   s.    
         [0172]    An engine hanger  115  is projected upward from a rear portion of the slant surface  102   s  between the crankshaft  108  and the main shaft  109 . 
         [0173]    In addition, engine hangers  116  and  117  are projected rearwards also from rear walls of the upper crankcase  102 A and the lower crankcase  102 B. 
         [0174]    In a motorcycle  130  on which this internal combustion engine  100  is mounted, as shown in  FIG. 15 , a body frame  132  includes a head pipe  133 , main frames  134  extending skewly rearwards from the head pipe  133 , center frames  135  extending downwards from the rear ends of the main frames  134 , a down frame  136  extending downwards from the head pipe  133 , seat stays  137  extending rearwards from upper portions of the center frames  135 , and mid frames  138  bridgingly arranged between rear portions of the center frames  135  and rear portions of the seat stays  137 . 
         [0175]    A front fork  140  for supporting a front wheel  139  is steerably supported on the head pipe  133 , and a steering handlebar  141  is connected to an upper portion of the front fork  140 . 
         [0176]    In addition, a rear fork  143  for supporting a rear wheel  142  is vertically swingably supported through a pivot bolt  145  of the center frame  135 . 
         [0177]    A fuel tank  146  is mounted between the left and right main frames  134 , and a tandem type seat  147  on which the driver P and a pillion passenger can be seated is mounted onto the left and right seat stays  137  on the rear side of the fuel tank  146 . 
         [0178]    The internal combustion engine  100  is disposed in a space surrounded by the main frames  134 , the center frames  135  and the down frame  136 , on the lower side of the fuel tank  146 . 
         [0179]    As shown in  FIG. 16 , an engine hanger  115  projected from the slant surface  102   s  of the upper wall of the upper crankcase  102 A of the internal combustion engine  100  is suspended from the main frames  134  through brackets  134   b , and the engine hangers  116  and  117  projecting from the rear walls of the upper crankcase  102 A and the lower crankcase  102 B are supported by the center frames  135 , whereby the internal combustion engine  100  is suspended from the vehicle frame  132 . 
         [0180]    As shown in  FIG. 15 , on the right side of the vehicle body, a step holder  150  is provided at a lower portion of the center frame  135  connected to the mid frame  138 , and a foot step  151  is projectingly provided on the step holders  150 . 
         [0181]    On the left side of the vehicle body, similarly, a foot step  151  is provided at a position in left-right symmetry with the above (see  FIG. 17 ). 
         [0182]    When the driver P is seated on the tandem type seat  147  and put his or her feet Pf on the foot steps  151 , as shown in  FIGS. 15 and 16 , the toe of the foot Pf is accommodated in the recessed space S on the rear side of the clutch actuator  122  at a position on the rear lower side of the crankshaft, namely, on the lower side of the clutch  112 . 
         [0183]    Of the right crankcase cover  120  covering the right side of the crankcases  102 A and  102 B of the internal combustion engine  100 , the portion corresponding to the clutch  112  and the oil sump part  121  for the clutch actuator  122  are bulging to the right side, and the foot Pf of the driver P is placed in the recessed space S on the lower side of the bulging portion corresponding to the clutch  112  and on the rear side of the oil sump part  121 . This ensures that, as shown in  FIG. 17 , the feet Pf of the driver P can be settled in optimum positions, without letting the feet Pf of the driver P protrude excessively to the left and right outer sides. 
         [0184]    As has been described in detail above, in the above-mentioned embodiments, the following effects are obtained. 
         [0000]    (1) In  FIGS. 5 ,  6  and  8 , the oil sump part  79  for reserving oil is provided in the periphery of the clutch actuators  74 , and the clutch actuators  74  are disposed in the oil sump part  79  (see  FIGS. 5 ,  6  and  8 ). Therefore, the valve opening and closing sounds generated during operation of the clutch actuators  74  can be attenuated, and the sound leaking to the exterior can be reduced.
 
(2) Since the clutch actuators  74  are mounted to the inside of the oil sump part  79  provided in the crankcase cover  17  and the cover member  78  (see  FIG. 6 ) is so fixed as to cover the oil sump part  79 , the oil sump part  79  can be formed in a simple structure.
 
(3) Since the clutch mechanism is composed of the pair of clutches  21 A and  21 B (see  FIG. 3 ) and the three oil passages  76 A,  76 B and  77  are formed to extend from the clutch actuators  74  to the clutch mechanism substantially in parallel to each other (see  FIG. 4 ), the plurality of oil passages can be arranged efficiently. In addition, since the directions of machining are unified, machinability can be enhanced.
 
(4) Since the oil discharge oil passage  91  provided in the clutch actuator  74  is opening into the oil sump part  79 , it is possible to reserve the oil discharged from the clutch actuator  74  and thereby to produce the effect of (1) above. Accordingly, the need for other oil supply means is eliminated, and the number of component parts can be reduced.
 
(5) The clutch actuator  74  includes the tubular valve case  85  to which a plurality of oil passages are connected, and the valve element  86  slid in the valve case  85  so as to switch over the communicating conditions of the oil passages (see  FIG. 7 ), and the valve element  86  is slid in the valve case  85  by an operation of the clutch actuator  74 , whereby the return oil from the clutch is discharged via the discharge oil passage  91 . Therefore, the sliding parts can be lubricated utilizing the oil thus discharged.
 
(6) The supply oil passage  75  for supplying the oil from the oil pump  13  to the valve part including the valve case  85  and the valve element  86  is branched and extended to form the clutch lubricating oil passage  77  and the communicating oil passage  80  (see  FIGS. 5 and 11 ). Thus, the clutch lubricating oil passage  77  is formed by branching and extending the supply oil passage  75 , which is easier to carry out than the formation of a clutch lubricating oil passage communicating with other lubricating oil circuit. Consequently, the number of machining steps is reduced.
 
(7) Since only a single supply oil passage  75  is provided for supplying the oil to the pair of actuators  74 A and  74 B, the structure is simple.
 
(8) Design quality is enhanced. In addition, there is the merit that the clutch actuators would not be damaged even upon overturn of the vehicle.
 
(9) Since the clutch actuators are also cooled by the oil in the oil sump part  79  which is cooled through the wall body of the crankcase cover  17 , cooling performance is enhanced.
 
(10) Since the clutch  112  is disposed on the rear upper side of the crankshaft  108  in side view of the internal combustion engine and the clutch actuator  122  is disposed on the lower side of the crankshaft  108  in side view of the internal combustion engine (see  FIG. 16 ), the recessed space S on the lower side of the clutch  112  and on the rear side of the clutch actuator  122  can be secured as an optimum foot rest position for the driver.
 
(11) Since the engine hanger  115  is projectingly provided on the slant surface  102   s  of the upper wall of the upper crankcase  102 A (see  FIG. 16 ), it is unnecessary to provide the engine hanger in the state of projecting upward beyond the uppermost part of the crankcase, so that the internal combustion engine  100  can be mounted on the vehicle body in a compact fashion.
 
         [0185]    Although a specific form of embodiment of the instant invention has been described above and illustrated in the accompanying drawings in order to be more clearly understood, the above description is made by way of example and not as a limitation to the scope of the instant invention. It is contemplated that various modifications apparent to one of ordinary skill in the art could be made without departing from the scope of the invention which is to be determined by the following claims.