Patent Publication Number: US-6708658-B2

Title: Internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement of an internal combustion engine in which bearing sections rotatably carrying a camshaft are provided on a cylinder head, and rocker arms moved with the rotation of the camshaft and swingably carried on rocker shafts. 
     2. Description of the Related Art 
     Such internal combustion engines are conventionally known, for example, from Japanese Patent Application Laid-open Nos.6-17613 and 10-148113. 
     In the prior arts disclosed in the above Patent Applications, the rocker shafts are fixed to the bearing sections by bolts passed through the rocker shafts on one diametrical line and fastened to the bearing sections. Therefore, when the fastening of the bolts is only conducted, there is a possibility that the support rigidity of the rocker shafts is sufficient. 
     There is also an internal combustion engine already known, for example, from Japanese Patent No.2688732 and the like, in which rocker shafts are supported on a rocker shaft holder which includes a plurality of shaft-supporting sections fastened respectively to a plurality of bearing sections of a cylinder head for rotatably carrying a camshaft, and a connecting section connecting the shaft-supporting sections to one another. In this internal combustion engine, it is doubtful that the support rigidity of the rocker shafts is sufficient, because the rocker shafts are supported only by rocker shaft holder. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an internal combustion engine, wherein the support rigidity of the rocker shafts is enhanced sufficiently. 
     To achieve the above object, according to a first aspect and feature of the present invention, there is provided an internal combustion engine including bearing sections rotatably carrying a camshaft and provided integrally or separately on a cylinder head, and rocker arms moved with the rotation of the camshaft and swingably carried on rocker shafts, wherein a rocker shaft holder fastened and fixed to the bearing sections to support the rocker shafts is provided with accommodating recesses into each of which a portion of each of the bearing sections protrudes. 
     With such arrangement of the first feature, the rocker shaft holder is fastened and fixed to the bearing sections provided on the cylinder head to rotatably carry the camshaft, and the rocker shafts are supported on the rocker shaft holder, and hence, support rigidity of the rocker shafts can be enhanced. Moreover, the accommodating recesses are provided in the rocker shaft holder, and a portion of each of the bearing section protrudes into each of the recesses. Therefore, the fastening structure for the bearing sections and the rocker shaft holder can be formed compactly in a direction of axes of the cylinders, thereby avoiding an increase in size of the engine. 
     According to a second aspect and feature of the present invention, in addition to the first feature, a portion of each of the bearing sections protrudes into each of the accommodating recesses to avoid the contact with the rocker shaft holder. With such arrangement, it is possible to inhibit the vibration of the camshaft to utmost from being transmitted to the rocker shafts through the rocker shaft holder. 
     According to a third aspect and feature of the present invention, in addition to the second feature, the plurality of rocker shafts parallel to one another are supported on the rocker shaft holder, and the accommodating recesses are provided in the rocker shaft holder between the rocker shafts. With such arrangement, the fastening structure for the bearing sections and the rocker shaft holder can be formed further compactly in a direction of axes of the cylinders in such a manner that the distance between each of the rocker shafts and the camshaft can be shortened. 
     According to a fourth aspect and feature of the present invention, in addition to the third feature, the rocker shaft holder includes a plurality of shaft-supporting sections fastened to the plurality of bearing sections and provided with the accommodating recesses, respectively, and a connecting section which integrally connects at least portions of the shaft-supporting sections corresponding to the accommodating recesses to one another. According to a fifth aspect and feature of the present invention, in addition to the second feature, the rocker shaft holder includes a plurality of shaft-supporting sections fastened to the plurality of bearing sections and provided with the accommodating recesses, respectively, and a connecting section which integrally connects at least portions of the shaft-supporting sections corresponding to the accommodating recesses to one another. 
     With the arrangement of the fourth and fifth features, it is possible to avoid a reduction in rigidity of the rocker shaft holder, in spite of the provision of the accommodating recesses in the shaft-supporting sections. 
     According to a sixth aspect and feature of the present invention, in addition to the first feature, the plurality of rocker shafts parallel to the one another are supported on the rocker shaft holder, and the accommodating recesses are provided in the rocker shaft holder between the rocker shafts. With such arrangement, the fastening structure for the bearing sections and the rocker shaft holder can be formed further compactly in a direction of axes of the cylinders in such a manner that the distance between each of the rocker shafts and the camshaft can be shortened. 
     According to a seventh aspect and feature of the present invention, in addition to the sixth feature, the rocker shaft holder includes a plurality of shaft-supporting sections fastened to the plurality of bearing sections and provided with the accommodating recesses, respectively, and a connecting section which integrally connects at least portions of the shaft-supporting sections corresponding to the accommodating recesses to one another. According to an eighth aspect and feature of the present invention, in addition to the first feature, the rocker shaft holder includes a plurality of shaft-supporting sections fastened to the plurality of bearing sections and provided with the accommodating recesses, respectively, and a connecting section which integrally connects at least portions of the shaft-supporting sections corresponding to the accommodating recesses to one another. 
     With the arrangement of the seventh and eighth features, it is possible to avoid a reduction in rigidity of the rocker shaft holder, in spite of the provision of the accommodating recesses in the shaft-supporting sections. 
     To achieve the above object, according to a ninth aspect and feature of the present invention, there is provided an internal combustion engine including bearing sections rotatably carrying a camshaft and provided integrally or separately on a cylinder head; and a rocker shaft holder which includes a plurality of shaft-supporting sections fastened to the plurality of bearing sections, respectively, and a connecting section integrally connecting the shaft-supporting sections to one another and which supports a rocker shaft swingably carrying thereon rocker arms and moved with the rotation of the camshaft, wherein the rocker shaft is supported on the shaft-supporting sections and also on shaft-supporting intermediate portions which are integrally provided on the connecting section and each disposed between the adjacent shaft-supporting sections. 
     With such arrangement of the ninth feature, the shaft-supporting sections of the rocker shaft holder are fastened to the plurality of bearing sections provided integrally or separately on the cylinder head to rotatably carry the camshaft, respectively, and the rocker shaft is supported on the shaft-supporting sections and also on the shaft-supporting intermediate portions integrally provided on the connecting section of the rocker shaft holder and disposed between the plurality of shaft-supporting sections. Therefore, it is possible to sufficiently enhance the support rigidity of the rocker shaft. 
     According to a tenth aspect and feature of the present invention, in addition to the ninth feature, the rocker shaft supported on the shaft-supporting intermediate portions and another rocker shaft having an axis parallel to such rocker shaft are supported on the rocker shaft holder, and a roller in rolling contact with a cam provided on the camshaft is supported on each of rocker arms swingably carried on the other rocker shaft at locations corresponding to the shaft-supporting intermediate portions, so that they are opposed to the shaft-supporting intermediate portions, and each of the shaft-supporting intermediate portions is provided with a notch for avoiding the interference with the roller. With such arrangement, the rocker arms each having the roller can be disposed sufficiently in proximity to the rocker shaft in spite of the existence of the shaft-supporting intermediate portions, whereby a valve-operating device including the rocker arms and the rocker shafts can be constructed compactly. 
     According to an eleventh aspect and feature of the present invention, in addition to the tenth feature, rocker arms disposed adjacent the shaft-supporting intermediate portions are swingably carried on the rocker shaft supported on the shaft-supporting intermediate portions. With such arrangement, the axial movement of the rocker arms can be restricted by each of the shaft-supporting intermediate portions, and a part exclusively for restricting the axial movement of the rocker arms is not required and hence, the number of parts can be reduced. 
     According to a twelfth aspect and feature of the present invention, in addition to the eleventh feature, the rocker shaft holder is integrally provided with ribs which extend in a direction parallel to the axis of the rocker shaft and connects the shaft-supporting sections and the shaft-supporting intermediate portions to each other. With such arrangement, it is possible to enhance the rigidity of the rocker shaft holder, particularly, the rigidity of the shaft-supporting intermediate portions and in turn, to further enhance the support rigidity of the rocker shafts. 
     According to a thirteenth aspect and feature of the present invention, in addition to the eleventh feature, the rocker shaft holder has an oil sump defined in its upper surface, and lubricating oil passages provided therein for guiding a lubricating oil from the oil sump to slide portions between the shaft-supporting intermediate portions and the rocker arms adjoining the shaft-supporting intermediate portions. With such arrangement, the lubricating oil can be supplied to the slide portions between shaft-supporting intermediate portions and the rocker arms without need of a part other than the rocker shaft holder. 
     According to a fourteenth aspect and feature of the present invention, in addition to the tenth feature, the rocker shaft holder is integrally provided with ribs which extend in a direction parallel to the axis of the rocker shaft and connects the shaft-supporting sections and the shaft-supporting intermediate portions to each other. With such arrangement, it is possible to enhance the rigidity of the rocker shaft holder, particularly, the rigidity of the shaft-supporting intermediate portions and in turn, to further enhance the support rigidity of the rocker shafts. 
     According to a fifteenth aspect and feature of the present invention, in addition to the ninth feature, the rocker arms disposed adjacent the shaft-supporting intermediate portions are swingably carried on the rocker shaft supported on the shaft-supporting intermediate portions. With such arrangement, the axial movement of the rocker arms can be restricted by the shaft-supporting intermediate portions, and a part exclusively for restricting the axial movement of the rocker arm is not required and hence, the number of parts can be reduced. 
     According to a sixteenth aspect and feature of the present invention, in addition to the fifteenth feature, the rocker shaft holder is integrally provided with ribs which extend in a direction parallel to the axis of the rocker shaft and connects the shaft-supporting sections and the shaft-supporting intermediate portions to each other. With such arrangement, it is possible to enhance the rigidity of the rocker shaft holder, particularly, the rigidity of the shaft-supporting intermediate portions and in turn, to further enhance the support rigidity of the rocker shafts. 
     According to a seventeenth aspect and feature of the present invention, in addition to the fifteenth feature, the rocker shaft holder has an oil sump defined in its upper surface, and lubricating oil passages provided therein for guiding a lubricating oil from the oil sump to slide portions between the shaft-supporting intermediate portions and the rocker arms adjoining the shaft-supporting intermediate portions. With such arrangement, the lubricating oil can be supplied to the slide portions between shaft-supporting intermediate portions and the rocker arms without need of a part other than the rocker shaft holder. 
     According to an eighteenth aspect and feature of the present invention, in addition to the ninth feature, the rocker shaft holder is integrally provided with ribs which extend in a direction parallel to the axis of the rocker shaft and connects the shaft-supporting sections and the shaft-supporting intermediate portions to each other. With such arrangement, it is possible to enhance the rigidity of the rocker shaft holder, particularly, the rigidity of the shaft-supporting intermediate portions and in turn, to further enhance the support rigidity of the rocker shafts. 
     The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  12  show an embodiment of the present invention, wherein 
     FIG. 1 is a front view of a V-shaped multi-cylinder internal combustion engine to which the present invention is applied; 
     FIG. 2 is a plan view taken along a line  2 — 2  in FIG. 1 with a head cover removed; 
     FIG. 3 is a plan view similar to FIG. 2, but showing the internal combustion engine with a rocker shaft holder and a camshaft being partially cut away; 
     FIG. 4 is a sectional view taken along a line  4 — 4  in FIG. 2; 
     FIG. 5 is a sectional view taken along a line  5 — 5  in FIG. 2; 
     FIG. 6 is a sectional view taken along a line  6 — 6  in FIG. 2; 
     FIG. 7 is an enlarged sectional view taken along a line  7 — 7  in FIG. 6; 
     FIG. 8 is an enlarged sectional view taken along a line  8 — 8  in FIG. 4; 
     FIG. 9 is a sectional view taken along a line  9 — 9  in FIG. 8; 
     FIG. 10 is an enlarged sectional view taken along a line  10 — 10  in FIG. 2; 
     FIG. 11 is a view of a passage-defining member, taken in a direction of an arrow  11  in FIG. 10; and 
     FIG. 12 is an enlarged sectional view taken along a line  12 — 12  in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described by way of an embodiment of the present invention with reference to FIGS. 1 to  12 . Referring first to FIG. 1, a V-shaped multi-cylinder internal combustion engine E carried on a vehicle includes an engine block  15  having first and second cylinder arrays  14 A and  14 B disposed in a V-shape, cylinder heads  16 ,  16  coupled to upper ends of the first and second cylinder arrays  14 A and  14 B, and head covers  17 ,  17  coupled to the cylinder heads  16 ,  16 . Three cylinder bores  18  are provided in a vertical arrangement as viewed on a paper sheet surface of FIG. 1 in each of the first and second cylinder arrays  14 A and  14 B, and pistons  19  slidably received in the cylinder bores  18  are commonly connected to a single crankshaft  21  by connecting rods  20 , respectively. 
     The arrangement of the engine E on the side of the first cylinder array  14 A will be described with reference to FIGS. 2 to  6 . Combustion chambers  22  are defined between the cylinder head  16  and the pistons  19  in the cylinder bores  18 . Provided in the cylinder head  16  at locations corresponding to the combustion chambers  22  are a pair of intake valve bores  23  capable of leading to the combustion chamber  22 , an intake port  24  leading commonly to the intake valve bores  23  and opening into one side of the cylinder head  16 , a pair of exhaust valve bores  25  capable of leading to the combustion chamber  22 , and an exhaust port  26  leading commonly to the exhaust valve bores  25  and opening into the other side of the cylinder head  16 . On the basis of the first and second cylinder arrays  14 A and  14 B forming the V-shape in combination with each other, the cylinder head  16  is disposed, inclined to one of the left and the right in a direction of arrangement of the cylinders, i.e., inclined so that the exhaust port  26  is at a location lower than the intake port  24 . 
     Stems of intake valve VI, VI as engine valves capable of individually opening and closing the intake valve bores  23  are slidably received in guide tubes  28  provided in the cylinder head  16 . Valve springs  30  for biasing the intake valves VI, VI in a closing direction are mounted between retainers  29  mounted at upper ends of the intake valves VI, VI and the cylinder head  16 . Stems of exhaust valves VE, VE as engine valves capable of individually opening and closing the exhaust valve bores  25  are slidably received in guide tubes  31  provided in the cylinder head  16 . Valve springs  33  for biasing the exhaust valves VE, VE in a closing direction are mounted between retainers  32  mounted at upper ends of the exhaust valves VE, VE and the cylinder head  16 . 
     The intake valves VI, VI and the exhaust valves VE, VE for every cylinder are opened and closed by a valve operating device  34 A. The valve operating device  34 A includes a camshaft  35  having an axis extending in the direction of arrangement of the cylinders, a pair of rocker shafts  36  and  37  having axes parallel to the camshaft  35 , driving rocker arms  38  and  39  and a free rocker arm  40 , which are swingably carried on one of the rocker shaft  36  for every cylinder, and driving rocker arms  41 ,  41  and free rocker arms  42 ,  42 , which are swingably carried on the other rocker shaft  37  for every cylinder. 
     A plurality of (four in the present embodiment) bearing sections  43  are integrally projectingly provided on the cylinder head  16  at distances in the direction of arrangement of the cylinders, so that the adjacent bearing sections sandwich each of the combustion chamber  22  therebetween, and the camshaft  35  is rotatably carried by the bearing sections  43 . Moreover, the camshaft  35  is operatively connected to the crankshaft  21  at a reduction ratio of 1/2. 
     The pair of the rocker shafts  36  and  37  are fixedly disposed above the camshaft  35 , and a rocker shaft holder  44  is fastened and fixed to upper surfaces of the bearing sections  43  to support the rocker shafts  36  and  37 . 
     Referring also to FIG. 7, the rocker shaft holder  44  includes shaft-supporting sections  44   a  corresponding to the plurality of bearing sections  43 , and a connecting section  44   b  for integrally connecting the shaft-supporting sections  44   a  together. The shaft-supporting sections  44   a  are fastened to the bearing sections  43  on opposite sides of the camshaft  35  by bolts  45 , respectively. 
     The pair of rocker shafts  36  and  37  are supported on the shaft-supporting sections  44   a  at locations where the camshaft  35  is disposed below and between the rocker shafts  36  and  37 . The rotation of the rocker shafts  36  and  37  about axes are inhibited by engagement of the bolts  45  for fastening the shaft-supporting sections  44   a  to the bearing sections  43  with a portion of an outer surface of each of the rocker shafts  36  and  37 , whereby the rocker shafts  36  and  37  are fixedly supported on the rocker shaft holder  44 . 
     A flat fastening seats  46  and  47  for fastening the rocker shaft holder  44  is formed on each of the bearing sections  43  on opposite sides of a portion which rotatably supports the camshaft  35 , and the bearing sections  43  are formed to bulge above and between the fastening seats  46  and  47 . 
     On the other hand, accommodating recesses  48  are provided in lower surfaces of the shaft-supporting sections  44   a  of the rocker shaft holder  44  at locations between the rocker shafts  36  and  37 , so that a portion of a central bulge of the bearing section  43  protrudes into each of the accommodating recesses  48 . The portion of the central bulge of the bearing section  43  is mounted to protrude into each of the accommodating recesses  48  in such a manner that the contact with the rocker shaft holder  44  is avoided, i.e., a gap  49  is defined between the accommodating recess  48  and the bearing section  43 . 
     Moreover, the connecting section  44   b  of the rocker shaft holder  44  for connecting the shaft-supporting portions  44   a  together is formed to integrally connect at least portions of the shaft-supporting sections  44   a  corresponding to the accommodating recesses  48  to one another. 
     The rocker shafts  36  and  37  are supported by the shaft-supporting sections  44   a  of the rocker shaft holder  44 , but the rocker shaft  37  on the side of the exhaust valves VE, VE is also supported by shaft-supporting intermediate portions  44   c  integrally provided on the connecting section  44   b  and disposed between the shaft-supporting sections  44   a.    
     Referring to FIG. 8, the driving rocker arms  38  and  39  and the free rocker arm  40  swingably carried on the rocker shaft  36  are disposed in such a manner that the free rocker shaft  40  is interposed between the driving rocker arms  38  and  39 . Tappet screws  54 ,  54  are threadedly engaged into the rocker arms  38  and  39 , so that the advanced and retracted positions thereof can be adjusted, and the driving rocker arms  38  and  39  are operatively connected to the intake valves VI, VI by putting the tappet screws  54 ,  54  into abutment against upper ends of the intake valves VI, VI. 
     The driving rocker arms  41 ,  41  and the free rocker arms  42 ,  42  swingably carried on the rocker shaft  37  are disposed in such a manner that they form pairs at locations spaced apart from each other in an axial direction of the rocker shaft  37 . Tappet screws  55 ,  55  are threadedly engaged into the rocker arms  41 ,  41 , so that the advanced and retracted positions thereof can be adjusted, and the rocker arms  41 ,  41  are operatively connected to the exhaust valves VE, VE by putting the tappet screws  55 ,  55  into abutment against upper ends of the exhaust valves VE, VE. 
     Moreover, the shaft-supporting intermediate portion  44   c  of the rocker shaft holder  44  is disposed between the driving rocker arms  41 ,  41 , and the driving rocker arms  41 ,  41  are disposed adjacent opposite sides of the shaft-supporting intermediate portion  44   c  in the axial direction of the rocker shaft  37 . 
     Plug insertion tubes  56  are mounted in the cylinder head  16 , so that they are disposed between the driving rocker arms  41 ,  41 . Spark plugs  57  are inserted into the plug insertion tubes  56  and threadedly fitted in the cylinder head  16  to face the combustion chambers  22 . 
     Referring also to FIG. 9, the camshaft  35  is provided with a cam  62  with which a roller  58  supported on the free rocker arm  40  on the side of the intake valves VI, VI is brought into rolling contact, cams  63 ,  63  with which rollers  59 ,  59  supported on the free rocker arms  42 ,  42  on the side of the exhaust valves VE, VE are brought into rolling contact, and a pair of cams  64 ,  64  with which cam slippers  60 ,  60  provided on the driving rocker arms  38  and  39  on the intake valves VI, VI and cam slippers  61 ,  61  provided on the driving rocker arms  41 ,  41  on the exhaust valves VE, VE are brought into sliding contact. The cams  62 ,  63 ,  63 ,  64 ,  64  are disposed, so that the cams  64 ,  64  are interposed between the cam  62  central in the axial direction of the camshaft  35  and the cams  63 ,  63  on opposite sides in the axial direction of the camshaft  35 . 
     Moreover, the cams  62  are provided at locations corresponding to the shaft-supporting intermediate portions  44   c  of the rocker shaft holder  44 , and the rollers  58  in rolling contact with the cams  62  are supported on the free rocker arms  40  on the intake valves VI, VI, so that they are opposed to the shaft-supporting intermediate portions  44   c . On the other hand, the shaft-supporting intermediate portions  44   c  are provided with notches  53  for avoiding the interference with the rollers  58 , whereby each of the shaft-supporting intermediate portions  44   c  is formed into a substantially J-shape. 
     The cam  62  is formed to have a cam profile for opening and closing the intake valves VI, VI, and each of the cams  63 ,  63  is formed to have a cam profile for opening and closing the exhaust valves VE, VE, but the cams  64 ,  64  are formed, so that they substantially close the intake valves VI, VI and the exhaust valves VE, VE to bring them out of operation. Therefore, in a state in which the driving rocker arms  38  and  39  have been connected to the free rocker arm  40 , the intake valves VI, VI can be opened and closed, but in a state in which the connection of the driving rocker arms  38  and  39  to the free rocker arm  40  has been released, the intake valves VI, VI are in substantially closed states and out of operation. In a state in which the driving rocker arms  41 ,  41  have been connected to the free rocker arms  42 ,  42 , the exhaust valves VE, VE can be opened and closed, but in a state in which the connection of the driving rocker arms  41 ,  41  to the free rocker arms  42 ,  42  has been released, the exhaust valves VE, VE are in substantially closed states and out of operation. 
     A valve-operating characteristic changing mechanism  65  is provided in the driving rocker arms  38  and  39  and the free rocker arm  40  on the side of the intake valves VI, VI for changing the connection and disconnection of the driving rocker arms  38  and  39  to and from the free rocker arm  40 . 
     The valve-operating characteristic changing mechanism  65  includes a connecting pin  67  slidably received in the driving rocker arm  38  and the free rocker arm  40  with one end facing a first hydraulic pressure chamber  66  defined in the driving rocker arm  38 , a connecting pin  68  slidably received in the free rocker arm  40  and the driving rocker arm  39  with one end being in sliding contact with the other end of the connecting pin  67 , a pin  69  with one end being in sliding contact with the other end of the connecting pin  68  and with the other end facing a second hydraulic pressure chamber  70  defined in the driving rocker arm  39 , and a return spring  71  mounted between the driving rocker arm  38  and the connecting pin  67  and accommodated in the first hydraulic pressure chamber  66 . 
     In the valve-operating characteristic changing mechanism  65 , when a hydraulic pressure is applied to the first hydraulic pressure chamber  66 , the connecting pins  67  and  68  and the pin  69  connected together are moved to a position where the volume of the second hydraulic pressure chamber  70  is smallest, whereby the driving rocker arm  38  and the free rocker arm  40  are connected to each other by the connecting pin  67 , and the free rocker arm  40  and the driving rocker arm  39  are connected to each other by the connecting pin  68 , as shown in FIG.  8 . When a hydraulic pressure is applied to the second hydraulic pressure chamber  70 , the connecting pins  67  and  68  and the pin  69  connected together are moved to a position where the volume of the first hydraulic pressure chamber  66  is smallest, whereby the connection of the driving rocker arm  38  and the free rocker arm  40  is released, because contact faces of the connecting pins  67  and  68  exist between the driving rocker arm  38  and the free rocker arm  40 , and the connection of the free rocker arm  40  and the driving-rocker arm  39  is released, because contact faces of the connecting pin  68  and the pin  69  exist between the free rocker arm  40  and the driving rocker arm  39 . 
     In this manner, the valve-operating characteristic changing mechanism  65  changes the operating characteristic for the intake valves VI, VI by switching the connection and disconnection of the free rocker arm  40  to and from the driving rocker arms  38  and  39  by the alternative application of the hydraulic pressure to the first and second hydraulic pressure chambers  66  and  70 . The return spring  71  may merely exhibit a spring force enough to be able to avoid the chattering of each of the pins  67 ,  68  and  69  in a state in which no hydraulic pressure is applied to any of the first and second hydraulic pressure chambers  66  and  70  in response to the stoppage of the operation of the engine E. 
     A dividing member  72  is received in the rocker shaft  36  for dividing the inside of the rocker shaft  36  into two portions, so that a first working oil passage  73  leading to the first hydraulic pressure chamber  66  and a second working oil passage  74  leading to the second hydraulic pressure chamber  70  are defined independently in the rocker shaft  36  by the dividing member  72 . 
     Valve-operating characteristic changing mechanisms  75 ,  75  are provided in the driving rocker arms  41 ,  41  and the free rocker arms  42 ,  42  disposed adjacently to form pairs on the side of the exhaust valves VE, VE for changing the connection and disconnection of the driving rocker arms  41 ,  41  to and from the free rocker arms  42 ,  42 . 
     Each of the valve-operating characteristic changing mechanism  75  includes a connecting pin  77  slidably received in the driving rocker arm  41  and the free rocker arm  42  with one end facing a first hydraulic pressure chamber  76  defined in the driving rocker arm  41 , a pin  78  slidably received in the free rocker arm  42  with one end being in sliding contact with the other end of the connecting pin  77  and with other end facing a hydraulic pressure chamber  79  defined in the free rocker arm  42 , and a return spring  80  mounted between the driving rocker arm  41  and the connecting pin  77  and accommodated in the first hydraulic pressure chamber  76 . 
     In the valve-operating characteristic changing mechanism  75 , when a hydraulic pressure is applied to the first hydraulic pressure chamber  76 , the connecting pin  77  and the pin  78  connected to each other are moved to a position where the volume of the second hydraulic pressure chamber  79  is smallest, whereby the driving rocker arm  41  and the free rocker arm  42 , as shown in FIG.  8 . When a hydraulic pressure is applied to the second hydraulic pressure chamber  79 , the connecting pin  77  and the pin  78  connected to each other are moved to a position where the volume of the first hydraulic pressure chamber  76  is smallest, whereby the connection of the free rocker arm  42  and the driving rocker arm  41  is released, because contact faces of the connecting pin  77  and the pin  78  exist between the free rocker arm  42  and the driving rocker arm  41 . 
     In this manner, the valve-operating characteristic changing mechanism  75  changes the operating characteristic for the exhaust valves VE, VE by switching the connection and disconnection of the driving rocker arm  41  and the free rocker arm to and from each other by the alternative application of the hydraulic pressure to the first and second hydraulic pressure chambers  76  and  79 . The return spring  80  may merely exhibit a spring force enough to be able to avoid the chattering of each of the pins  77  and  78  in a state in which no hydraulic pressure is applied to any of the first and second hydraulic pressure chambers  76  and  79  in response to the stoppage of the operation of the engine E. 
     A dividing member  82  is received in the rocker shaft  37  for dividing the inside of the rocker shaft  37  into two portions, so that a first working oil passage  83  leading to the first hydraulic pressure chamber  76  and a second working oil passage  84  leading to the second hydraulic pressure chamber  79  are defined independently in the rocker shaft  37  by the dividing member  82 . 
     Lost motion springs  85  are mounted between the cylinder head  16  and the free rocker arms  40  for exhibiting a spring force for urging the free rocker arm  40  to the cam  62  of the camshaft  35  in a state in which the valve-operating characteristic changing mechanism  65  on the side of the intake valves VI, VI has released the connection of the free rocker arm  40  to the driving rocker arms  38  and  39 . A portion of each of the spring  85  is accommodated in each of bottomed holes  86  provided in the upper surface of the cylinder head  16 . 
     Lost motion springs  87  are mounted between the rocker shaft holder  44  and the free rocker arms  42  for exhibiting a spring force for urging the free rocker arms  42 ,  42  to the cams  63 ,  63  of the camshaft  35  in a state in which the valve-operating characteristic changing mechanisms  75 ,  75  on the side of the exhaust valves VE, VE have released the connection of the free rocker arms  42 ,  42  to the driving rocker arms  41 ,  41 . Bottomed cylindrical tubes  88  are integrally provided on the connecting section  44   b  of the rocker shaft holder  44  to protrude upwards from the rocker shaft holder  44 , so that a portion of each of the springs  87  is accommodated in each of the bottomed cylindrical tubes  88 . 
     The lost motion springs  87  each having the portion accommodated in each of the bottomed cylindrical tubes  88  cannot be fallen from the rocker shaft holder  44  in a state in which the rocker shaft  37  swingably carrying the driving rocker arms  41  and the free rocker arms  42  has been supported in the rocker shaft holder  44 . Thus, the lost motion springs  87  can be retained on the rocker shaft holder  44  in an extremely simple structure in which they are only partially accommodated in the bottomed cylindrical tubes  88 . 
     Moreover, the bottomed cylindrical tubes  88  are integrally provided on the rocker shaft holder  44  to protrude therefrom, so that they are connected to the shaft-supporting sections  44   a , and the rigidity of the shaft-supporting sections  44   a  and in turn the rigidity of the entire rocker shaft holder  44  can be increased by the bottomed cylindrical tubes  88 . 
     Referring also to FIG. 10, a hydraulic pressure control valve unit  90  is mounted to the cylinder head  16  at one end in the direction of arrangement of the cylinders for controlling the hydraulic pressure of the working oil supplied to the first working oil passages  73  and  83  and the second working oil passages  74  and  84  defined in the rocker shafts  36  and  37 . 
     The hydraulic pressure control valve unit  90  comprises a valve spool  91  mounted to the cylinder had  16 , and a solenoid valve  92  mounted to the spool valve  91  for controlling the switching operation of the spool valve  91 . A passage-defining member  93  is mounted on the cylinder head  16  between the spool valve  91  and the rocker shaft holder  44 , and one end of each of the rocker shafts  36  and  37  is fitted into the passage-defining member  93 . 
     Referring also to FIG. 11, the passage-defining member  93  is provided with a first communication oil passage  94  connecting the first working oil passages  73  and  83  in the rocker shafts  36  and  37  and the spool valve  91  to each other, and a second communication oil passage  95  connecting the second working oil passages  74  and  84  in the rocker shafts  36  and  37  and the spool valve  91  to each other. The spool valve  91  is capable of being switched between a first state in which it permits the working oil from a hydraulic pressure source (not shown) to be passed to the first communication oil passage  94  and thus to the first working oil passages  73  and  83 , and a second state in which it permits the working oil from the hydraulic pressure source to be passed to the second communication oil passage  95  and thus to the second working oil passages  74  and  84 . 
     Moreover, the spool valve  91  is designed to discharge a drain oil upon the switching thereof between the first and second states, and a drain oil passage  96  for guiding the drain oil is provided in an upper portion of the passage-defining member  93 . 
     On the other hand, an oil sump  97  is defined in the upper surface of the rocker shaft holder  44 , so that the oil scattered within the valve-operating chamber between the cylinder head  16  and the head cover  17  can be reserved in the oil sump. 
     The oil sump  97  includes a groove portion  98  extending in the direction of arrangement of the cylinders and substantially T-shaped groove branches  99  which are disposed between the pair of bottomed cylindrical tubes  88 ,  88  at locations corresponding to the centers of the combustion chambers  22  and which are connected to the groove portion  98 . The groove branches  99  are formed, so that they are connected to the side of the groove portion  98  adjacent the exhaust valves VE, VE in order to guide the oil in the groove portion  98  toward the groove branches  99 , on the basis of the cylinder  16  being disposed, inclined so that the exhaust port  26  is located below the intake port  23 . 
     Referring also to FIGS. 6 and 12, a rib  101  is projectingly provided on the upper surface of the rocker shaft holder  44  to extend in the direction of arrangement of the cylinders. The rib  101  has a side face located below the groove portion  98  and connected flush to peripheral walls of ends of the groove branches  99 . The rib  101  connects those portions of the bottomed cylindrical tubes  88 ,  88  disposed on opposite sides of the groove branches  99 , which are located at lower sides in a direction of inclination of the cylinder head  16 . The bottomed cylindrical rubes  88 ,  88  and the rib  101  are disposed in a substantially U-shape to constitute a portion of the oil sump  97 , and the rib  101  is a wall defining a lowermost portion of the oil sump  97 . 
     Lubricating oil passages  100  are provided in the rocker shaft holder  44  to vertically extend through the connecting section  44   b  along the side face of the rib  101  adjacent the oil sump  97 . Upper ends of the lubricating oil passages  100  communicate in pairs with ends of the groove branches  99  which are portions of the oil sump  97  closer to the ribs  101 , i.e., with the oil sump  97  in a region surrounded by the bottomed cylindrical tubes  88 ,  88  and the rib  101 . 
     The lubricating oil passages  100  forming each pair are disposed at locations where the shaft-supporting intermediate portion of the rocker shaft holder  44  is interposed between them. A lubricating oil is supplied from the lubricating oil passages  100 ,  100  for lubricating slide portions between the shaft-supporting intermediate portion  44   c  and those  41 ,  41  of the rocker arms  38  to  40 ,  41  and  42  adjoining the shaft-supporting intermediate portion  44   c , slide portions between the driving rocker arms  41 ,  41  and the rocker shaft  37  and slide portions between the free rocker arms  42 ,  42  adjoining the driving rocker arms  41 ,  41  and the rocker shaft  37 . 
     A rib  102  is projectingly provided on the upper surface of the rocker shaft holder  44  to extend in the direction of arrangement of the cylinders, so that the oil sump  97  is interposed between the rib  101  and the rib  102 . Moreover, the ribs  101  and  102  are provided over the longitudinal entire length of the rocker shaft holder  44 , and the shaft-supporting sections  44   a  and the shaft-supporting intermediate portions  44   c  of the rocker shaft holder  44  are interconnected by the ribs  101  and  102 . 
     The drain oil discharged from the hydraulic pressure control valve unit  90  is also guided to the oil sump  97 , and an upper portion of the passage-defining member  93  interposed between the hydraulic pressure control valve unit  90  and the rocker shaft holder  44  is formed to permit the drain oil discharged from the hydraulic pressure control valve unit  90  to be delivered to the oil sump  97 . 
     More specifically, the upper portion of the passage-defining member  93  is provided with a recess  103  with which the drain oil passage  96  for guiding the drain oil from the hydraulic pressure control valve unit  90  communicate, and a substantially U-shaped guide  104  which opens upwards to guide the drain oil accumulated in the recess  103  toward the oil sump  97 . The guide  104  is extended from the passage-defining member  93  to above one end of the groove portion  98  of the oil sump  97 . 
     The arrangement of the second cylinder array  14 B is basically the same as the arrangement of the first cylinder array  14 A, but a valve-operating device  34 B for driving the intake valves VI and the exhaust valves VE provided in the cylinder head  16  on the second cylinder array  14 B is different from the valve-operating device  34 A on the first cylinder array  14 A in that the valve-operating device  34 B does not close the intake valves VI and the exhaust valves VE to stop their operation during operation of the engine E. 
     The operation of this embodiment will be described below. The bearing sections  43  rotatably carrying the camshaft  35  are projectingly provided on the cylinder head  16  at distances spaced apart from one another in the direction of arrangement of cylinders in such a manner that each of the combustion chambers  22  is defined between the adjacent bearing sections  43 . On the other hand, the rocker shafts  36  and  37  disposed above the camshaft  35  to swingably carry the rocker arms  38 ,  39 ,  40 ,  41  and  42  rotated with the rotation of the camshaft  35  are fixedly supported by the rocker shaft holder  44  fastened to the bearing sections  43  and thus, the support rigidity of the rocker shaft  36  and  37  can be enhanced. 
     The rocker shaft holder  44  includes the shaft-supporting sections  44   a  fastened to the bearing sections  43 , and the connecting section  44   b  connecting the shaft-supporting sections  44   a  together, and the accommodating recesses  48  are provided in the lower surfaces of the shaft-supporting sections  44   a , so that the bearing sections  43  partially protrude into the accommodating recesses  48 . Thus, the structure of fastening of the bearing sections  43  and the rocker shaft holder  44  to each other can be constructed compactly in the direction along the axes of the cylinder bores  18  to avoid an increase in size of the engine E. Moreover, the connecting section  44   b  integrally connects at least portions of the, shaft-supporting sections  44   a  corresponding to the accommodating recesses  48  to one another and hence, a reduction in rigidity of the rocker shaft holder  44  can be avoided in spite of the provision of the accommodating recesses  48  in the shaft-supporting sections  44   a.    
     The bearing sections  43  partially protrude into the accommodating recesses  48  to avoid the contact with the rocker shaft holder  44  and hence, it is possible to inhibit the vibration of the camshaft  35  to the utmost from being transmitted through the rocker shaft holder  44  to the rocker shafts  36  and  37 . 
     In addition, the pair of rocker shafts  36  and  37  parallel to each other are supported on the rocker shaft holder  44 , and the accommodating recesses  48  are provided in the rocker shaft holder  44  between the rocker shafts  36  and  37 . Thus, the structure of fastening between the bearing sections  43  and the rocker shaft holder  44  to each other can be constructed further compactly in the direction along the axes of the cylinder bores  18  in such a manner that the distances between the rocker shafts  36  and  37  and the camshaft  35  can be shortened. 
     The connecting section  44   b  of the rocker shaft holder  44  is integrally provided with the shaft-supporting intermediate portions  44   c  disposed between the shaft-supporting sections  44   a , and one  37  of the rocker shafts  36  and  37  is supported not only by the shaft-supporting sections  44   a  but also by the shaft-supporting intermediate portions  44   c . Thus, the support rigidity of the rocker shaft  37  can be enhanced sufficiently. 
     Additionally, the shaft-supporting intermediate portions  44   c  are provided with the notches  53  adapted to avoid the interference with the rollers  58  supported on the free rocker arm  40  on the side of the intake valves VI, VI and opposed to the shaft-supporting intermediate portions  44   c , and the free rocker arm  40  provided with the roller  58  can be disposed in sufficient proximity to the rocker shaft  37  in spite of the existence of the shaft-supporting intermediate portions  44   c , whereby the valve-operating devices  34 A and  34 B including the free rocker arm  40  and the rocker shaft  37  can be constructed compactly. 
     Moreover, the driving rocker arms  41 ,  41  on the exhaust valves VE, VE are disposed adjacent the shaft-supporting intermediate portions  44   c  in the axial direction of the rocker shaft  37 . Therefore, it is possible to inhibit the axial movement of the driving rocker arm  41  by the shaft-supporting intermediate portions  44   c  and hence, a part exclusively for limiting the axial movement of the driving rocker arm  41  is nor required, leading to a reduction in number of parts. 
     The oil sump  97  is defined in the upper surface of the rocker shaft holder  44 , and the lubricating oil passages  100  are provided in the rocker shaft holder  44 , so that the lubricating oil can be supplied to the slide portions of the driving rocker arm  41  and the free rocker arm  42  which are two of the rocker arms  38  to  40 ,  41  and  42  relative to the rocker shaft  37  and the slide portions between the shaft-supporting intermediate portions  44   c  and the driving rocker arm  41 . Therefore, the oil accumulated in the oil sump  97  can be reliably supplied from the oil sump through the lubricating oil passages  100  to the slide portions of the driving rocker arm  41  and the free rocker arm  42  relative to the rocker shaft  37  and the slide portions between the shaft-supporting intermediate portions  44   c  and the driving rocker arm  41  to positively lubricate such slide portions. Thus, a part exclusively for lubricating the slide portions between the driving rocker arm  41  and the free rocker arm  42  and the slide portions between the shaft-supporting intermediate portions  44   c  and the driving rocker arm  41  is not required, and the slide portions can be lubricated, while avoiding an increase in number of parts. Moreover, the oil sump  97  is defined in the upper surface of the rocker shaft holder  44  and hence, can be formed easily. 
     The oil sump  97  is formed in the upper surface of the rocker shaft holder  44  and provided with the groove portion  98  extending the direction of arrangement of the cylinders, and the groove branches  99  connected to the groove portion  98 . The rib  101  is projectingly provided in the rocker shaft holder  44  to extend in the direction of arrangement of the cylinders, and has the side face connected flush to a portion of the peripheral wall of the oil sump  97 , i.e., the peripheral walls of the ends of the groove branches  99 . In addition, the lubricating oil passages  100  are provided in the rocker shaft holder  44  in such manner that their upper ends communicate with a portion of the oil sump  97  closer to the rib  101  (the ends of the groove branches  99  in the present embodiment) and they extend through the rocker shaft holder  44  along the side face of the rib  101  adjacent the oil sump  97 . Therefore, it is possible to compensate for a reduction in rigidity of the rocker shaft holder  44  due to the provision of the lubricating oil passages  100 , while enabling the supplying of the oil in the oil sump  97  to each of the cylinders. 
     The cylinder head  16  is disposed in the inclined state with the exhaust port  26  located below the intake port, and the rib  101  is projectingly provided on the upper surface of the rocker shaft holder  44  below the groove portion  98 . Therefore, the rib  101  enhancing the rigidity of the rocker shaft holder  44  can function as the wall defining the lower portion of the oil sump  97 , and a larger amount of the oil can be positively stored on the upper surface of the rocker shaft holder  44 . 
     In addition, the rocker shaft holder  44  includes the bottomed cylindrical tubes  88  integrally provided on the upper surface thereof and having the lost motion springs  87  accommodated therein for biasing the free rocker arm  42  on the side of the exhaust valves VE, VE toward the cam  63  of the camshaft  35 , and the rib  101  also integrally provided on the upper surface thereof to connect the bottomed cylindrical tubes  88  to one another, and a portion of the oil sump  97  is defined by the bottomed cylindrical tubes  88  and the rib  101 . Therefore, it is possible to increase the rigidity of the rocker shaft holder  44  by the bottomed cylindrical tubes  88  and the rib  101 , and to define a portion of the oil sump  97  in the upper surface of the rocker shaft holder  44 . Particularly, as in the present embodiment, it is possible to further increase the rigidity of the rocker shaft holder  44  by interconnecting the bottomed cylindrical tubes  88  and the rib  101  to form a substantially U-shape. 
     The lubricating oil passages  100  are provided in the rocker shaft holder  44  in such manner that the upper ends of the lubricating oil passages  100  communicate with the oil sump  97  in the region surrounded by the bottomed cylindrical tubes  88  and the rib  101 , and hence, the upper ends of the lubricating oil passages  100  communicate with the portions having the increased rigidity. Therefore, it is possible to avoid a reduction in rigidity of the rocker shaft holder  44  due to the provision of the lubricating oil passages  100 . 
     The rib  102  is also projectingly provided on the upper surface of the rocker shaft holder  44  to extend in the direction of arrangement of the cylinders, so that the oil sump  97  is interposed between the ribs  101  and  102 . The rigidity of the rocker shaft holder  44 , and in turn the support rigidity of the rocker shafts  36  and  37  can be enhanced even by the rib  102 . 
     Further, since the ribs  101  and  102  connect the shaft-supporting sections  44   a  and the shaft-supporting intermediate portions  44   c  in the rocker holder  44  to one another, it is possible to enhance the rigidity of the rocker shaft holder  44 , particularly, the rigidity of the shaft-supporting intermediate portions  44   c  by the ribs  101  and  102 , and in turn to further enhance the support rigidity of the rocker shaft  37 . 
     The hydraulic pressure control valve unit  90  is mounted to the cylinder head  16  and capable of controlling the pressure of the working oil supplied to the valve-operating characteristic changing mechanism  65  provided in the rocker arms  38  to  40  on the intake valves VI, VI and the valve-operating characteristic changing mechanism  75  provided in the rocker arms  41  and  42  on the side of the exhaust valves VE, VE, and the drain oil discharged from the hydraulic pressure control valve unit  90  is guided to the oil sump  97 . Therefore, the drain oil discharged from the hydraulic pressure control valve unit  90  can be also passed to the slide portions of the rocker arms  41  and  42  on the side of the exhaust valves VE, VE and utilized to lubricate the slide portions, and hence, it is unnecessary to provide other special lubricating oil passages. Moreover, the hydraulic pressure control unit  90  discharges only the drain oil upon the switching operation of the valve-operating characteristic changing mechanisms  65  and  75 , and a reduction in pressure of the working oil supplied to the valve-operating characteristic changing mechanisms  65  and  75  cannot occur, and the responsiveness of the hydraulic pressure control valve unit  90  cannot be reduced, due to the utilization of the drain oil for the lubrication. 
     The passage-defining member  93  having the first and second communication passages  94  and  95  interconnecting the first working oil passages  73  and  83  and the second working oil passages  74  and  84  defined respectively in the rocker shafts  36  and  37  is interposed between the hydraulic pressure control valve unit  90  and the rocker shaft holder  44 , and the upper portion of the passage-defining member  93  is formed to enable the delivery of the drain oil to the oil sump  97 . Therefore, a part exclusively for delivering the drain oil between the hydraulic pressure control valve unit  90  and the oil sump  97  is not required. 
     Further, the upper portion of the passage-defining member  93  is provided with the recess  103  for receiving the drain oil from the hydraulic pressure control valve unit  90 , and the guide  104  for guiding the drain oil accumulated in the recess  103  toward the oil sump  97 . The guide  104  is extended from the passage-defining member  93  to above one end of the groove portion  98  of the oil sump  97 . Therefore, the drain oil can be reliably delivered from passage-defining member  93  to the oil sump  97  in the rocker shaft holder  44 , and a sufficient amount of the drain oil can be supplied to the oil sump  97  to perform the sufficient lubrication. 
     The bearing sections  43  are integrally provided on the cylinder head  16  in the above-described embodiment, but the present invention is also applicable to an internal combustion engine in which bearing sections separate from a cylinder head  16  are provided on the cylinder head  16 . In addition, the passage-defining member  93  separate from the hydraulic pressure control valve unit  90  is interposed between the hydraulic pressure control valve unit  90  and rocker shaft holder  44  in the embodiment, but the passage-defining member may be provided integrally on the hydraulic pressure control valve unit  90  and in this case, the number of parts can be reduced. 
     Although the embodiment of the present invention has been described in detail, it will be understood that the present invention is not limited to the above-described embodiments, and various modifications in design may be made without departing from the spirit and scope of the invention defined in the claims.