Patent Publication Number: US-6662771-B2

Title: Timing chain lubricating system for engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an engine in which a sprocket is fixed to one end of a camshaft and a timing chain is wrapped around the sprocket and, in particular, to a timing chain lubricating system therefor. 
     2. Description of the Related Art 
     A camshaft of an overhead camshaft type engine is driven by an arrangement in which a sprocket fixed to a shaft end of the camshaft is linked to a sprocket fixed to a shaft end of a crankshaft via a timing chain. A lubricating system for such a timing chain is known in Japanese Patent Application Laid-open No, 6-146838. The timing chain lubricating system disclosed in the above-mentioned application has an arrangement in which a relief valve is provided in an oil passage for supplying oil to a hydraulic tappet, and the section where the sprocket is meshed with the chain is lubricated with a jet of oil that issues from an oil jet that is integral with the relief valve, 
     In the above-mentioned conventional arrangement, since the oil jet can lubricate only one position of the timing chain, when the amount of oil required and the position that is to be lubricated change according to the operational state of the engine, it is difficult to carry out appropriate lubrication according to the changes. 
     SUMMARY OF THE INVENTION 
     The present invention has been carried out in view of the above-mentioned circumstances, and it is an object of the present invention to enable a timing chain wrapped around a sprocket of a camshaft to be reliably lubricated according to the operational state of an engine. 
     In order to achieve the above-mentioned object, in accordance with a first aspect of the present invention, there is proposed a timing chain lubricating system for an engine in which a sprocket is fixed to an end of a camshaft and a timing chain is wrapped around the sprocket, comprising a plurality of oil supply means for supplying oil to the timing chains the operation of the plurality of oil supply means being changed according to the operational state of the engine. 
     In accordance with the above-mentioned arrangement since the operation of the plurality of oil supply means for supplying oil to the timing chain is changed according to the operational state of the engine, lubrication can be carried out according to the operational state of the engine thus reducing the wear of the timing chain. 
     Furthermore, in accordance with a second aspect of the present invention, in addition to the above-mentioned first aspect, there is proposed a timing chain lubricating system for an engine, wherein the number of oil supply means that are in operation is increased as the rotational speed of the engine or the engine load increases. 
     In accordance with the above-mentioned arrangement, since the number of oil supply means that are in operation is increased as the rotational speed of the engine or the engine load increases, the number of positions that are lubricated can be increased according to the rotational speed of the engine or the engine load thus further effectively reducing the wear of the timing chain, 
     Furthermore, in accordance with a third aspect or a sixth aspect of the present invention, in addition to the above-mentioned first aspect or second aspect, there is proposed a timing chain lubricating system for an engine, further comprising variable valve lift means for changing the relationship between the size of the valve lift of an intake valve and the size of the valve lift of an exhaust valve according to the operational state of the engine, the amount of oil that is supplied to the section where the sprocket that drives the valve having a large valve left is meshed with the timing chain being made larger than the amount of oil that is supplied to the section where the sprocket that drives the valve having a small valve lift is meshed with the timing chain. 
     In accordance with the above-mentioned arrangement, since the relationship between the size of the valve lift of the intake valve and the size of the valve lift of the exhaust valve is changed by the variable valve lift means so that the amount of oil that is supplied to the section where the sprocket that drives the valve having a large valve lift is meshed with the timing chain is larger than the amount of oil that is supplied to the section where the sprocket that drives the valve having a small valve lift is meshed with the timing chain, a larger amount of oil can be supplied to the sprocket having a larger valve operating load so prolonging the life span of the timing chain. 
     Furthermore, in accordance with a fourth aspect, a fifth aspect or a seventh aspect of the present invention, in addition to any one of the above-mentioned first aspect to third aspect, there is proposed a timing chain lubricating system for an engine, further comprising a hydraulic control valve for switching over between a low speed valve lift and a high speed valve lift, the low speed valve lift being used when the rotational speed of the engine or the engine load is lower than a predetermined value, the high speed valve lift being used when the rotational speed of the engine or the engine load is higher than a predetermined value, the timing chain being lubricated with relief oil from the hydraulic control valve when the low speed valve lift is established and the timing chain being lubricated with valve lift control oil from the hydraulic control valve when the high speed valve lift is established. 
     In accordance with the above-mentioned arrangement in which the hydraulic control valve establishes a low speed valve lift when the rotational speed of the engine or the engine load is low and a high speed valve lift when the rotational speed of the engine or the engine load is high, when the low speed valve lift, which imposes a low load on the timing chain, is established the timing chain is lubricated with relief oil from the hydraulic control valve; when the high speed valve lift, which imposes a high load on the timing chain, is established the timing chain is lubricated with valve lift control oil from the hydraulic control valve, aid an amount of oil that is appropriate for the state of the load can thus be supplied to the timing chain so effectively preventing wear of the timing chain. 
     Furthermore, in accordance with an eighth aspect of the present invention, in addition to the above-mentioned first aspect there is proposed a timing chain lubricating system for an engine, further comprising an oil jet that issues a jet of chain lubricating oil when the rotational speed of the engine or the engine load is higher than a predetermined value. 
     In accordance with the above-mentioned arrangement, since the oil jet issues a jet of chain lubricating oil when the rotational speed of the engine or the, engine load is higher than the predetermined value, it becomes easy to supply oil to the timing chain. 
     Furthermore, in accordance with a ninth aspect of the present invention, in addition to the above-mentioned first aspect, there is proposed a timing chain lubricating system for an engine, further comprising an oil jet and a hydraulic control valve for switching over between a low speed valve lift and a high speed valve lift, the low speed valve lift being used when the rotational speed of the engine or the engine load is lower than a predetermined value, the high speed valve lift being used when the rotational speed of the engine or the engine load is higher than the predetermined value, the timing chain being lubricated with relief oil from the hydraulic control valve when the low speed valve lift is established and the timing chain being lubricated with valve lift control oil from the; hydraulic control valve via the oil jet when the high speed valve lift is established. 
     In accordance with the above-mentioned arrangement in which the hydraulic control valve establishes a low speed valve lift when the rotational speed of the engine or the engine load is low and a high speed valve lift when the rotational speed of the engine or the engine load is high, when the low speed valve lift, which imposes a low load on the timing chain, is established the timing chain is lubricated with relief oil from the hydraulic control valve; in the high speed valve lift, which imposes a high load on the timing chain, is established the timing chain is lubricated with valve lift control oil from the hydraulic control valve via the oil jet, an amount of oil that is appropriate for the state of the load can thus be supplied to the timing chain so effectively preventing wear of the timing chain and, moreover, it becomes easy to supply oil to the timing chain. 
     Furthermore, in accordance with a tenth aspect of the present invention, in addition to the above-mentioned eighth aspect, there is proposed a timing chain lubricating system for an engines wherein the camshaft is supported by an upper camshaft holder and a lower camshaft holder and the oil jet is fastened to the lower camshaft holder. 
     In accordance with the above-mentioned arrangement, since the camshaft is supported by the upper camshaft holder and the lower camshaft holder and the oil jet is fastened to the lower camshaft holder, the rigidity with which the camshaft and the oil jet are supported can be enhanced, 
     Furthermore, in accordance with an eleventh aspect of the present invention, in addition to the above-mentioned eighth aspect, there is proposed a timing chain lubricating system for an engine, further comprising a relief oil discharge hole, the relief oil discharge hole and the oil jet being disposed on the side of the sprocket that is close to the hydraulic control valve. 
     In accordance with the above-mentioned arrangement, since the relief oil discharge hole and the oil jet are disposed on the side of the sprocket that is close to the hydraulic control valve, the oil passage can be shortened, 
     Furthermore, in accordance with a twelfth aspect of the present inventions in addition to the above-mentioned eleventh aspect, there is proposed a timing chain lubricating system for an engine, wherein the oil jet lubricates the section where the sprocket is meshed with the chain on the side that is far from the hydraulic control valve 
     In accordance with the above-mentioned arrangement, since the oil jet lubricates the section where the sprocket is meshed with the chain on the side that is far from the hydraulic control valve, it becomes easy to supply oil to the section to be lubricated. 
     An intake camshaft  12  and an exhaust camshaft  13  of the embodiments correspond to the camshafts of the present invention, an intake camshaft sprocket  15  an exhaust camshaft sprocket  16  of the embodiments correspond to the sprockets of the present invention, an oil drain hole  25   b  and an oil jet  36  of the embodiments correspond to the oil supply means of the present invention, a first hydraulic control valve  34  of the embodiments corresponds to the hydraulic control valve of the present invention, and a fist variable valve operating characteristic mechanism V 1  of the embodiments corresponds to the variable lift means of the present invention. 
     Furthermore, the above-mentioned predetermined value for the engine rotational speed is, for example, 2500 rpm when the valve lift is switched over on the low speed side and, for example, 5000 rpm when the valve lift is switched over on the high speed side, but it is not limited thereby, 
    
    
     The above-mentioned objects, other objects, characteristics and advantages of the present invention will become apparent from explanation of preferred embodiments that will be described in detail below by reference to the attached drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to  13  illustrate a first embodiment of the present invention. 
     FIG. 1 is a perspective view of an engine. 
     FIG. 2 is a magnified view from arrow  2  in FIG.  1 . 
     FIG. 3 is a magnified view from arrow  3  in FIG.  1 . 
     FIG. 4 is a cross section at line  4 — 4  in FIG.  3 . 
     FIG. 5 is a magnified view of an essential part of FIG.  4 . 
     FIG. 6 is a diagram for explaining the action corresponding to FIG.  5 . 
     FIG. 7 is a view from line  7 — 7  in FIG.  3 . 
     FIG. 8 is a magnified cross section at line  8 — 8  in FIG.  3 . 
     FIG. 9 is a magnified cross section of an essential part of FIG.  3 . 
     FIG. 10 is a magnified cross section at line  10 — 10  in FIG  2 . 
     FIG. 11 is a cross section at line  11 — 11  in FIG. 3, 
     FIG. 12 is a cross section at line  12 — 12  in FIG.  11 . 
     FIG. 13 is a diagram for explaining a state in which a measurement apparatus is used. 
     FIG. 14 is a diagram corresponding to FIG. 8 relating to a second embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     A first embodiment of the present invention is explained below by reference to FIGS. 1 to  13 . 
     As shown in FIG. 1, a DOHC type in-line four cylinder engine E has a crankshaft  11 , an intake camshaft  12  and an exhaust camshaft  13 . A timing chain  17  is wrapped around a crankshaft sprocket  14  provided on a shaft end of the crankshaft  11 , an intake camshaft sprocket  15  provided on a shaft end of the intake camshaft  12  and an exhaust camshaft sprocket  16  provided on a shaft end of the exhaust camshaft  13 . The timing chain  17  is driven in the direction of the arrow a by the crankshaft  11 . The intake camshaft  12  and the exhaust camshaft  13  rotate at a speed that is half that of the crankshaft  11 . Each of the cylinders has two intake valves  18  driven by the intake camshaft  12  and to exhaust valves  19  driven by the exhaust camshaft  13 . The amount of valve lift and the duration for which the valve is open for the two intake valves  18  can be controlled by a first variable valve operating characteristic mechanism V 1  provided on each of the cylinders. The valve timing can be controlled by a second variable valve operating characteristic mechanism V 2  provided on the shaft end of the intake camshaft  12 . 
     As shown in FIGS. 2 to  4 , on the upper face of a cylinder block  21  is superimposed a cylinder head  23  via a gasket  22 , and it is fastened by a plurality of bolts  24 . On the upper face of the cylinder head  23  are superimposed a lower camshaft holder  25 , which also functions as a rocker arm shaft holder, and an upper camshaft holder  26 , and they are together fastened to the cylinder head  23  by four bolts  27 ,  28 ,  29  and  30 . Upper parts of the lower camshaft holder  25  and the upper camshaft holder  26  are covered with a head cover  31 . In the lower camshaft holder  25  are fixed an intake rocker arm shaft  32  and an exhaust rocker arm shaft  33 . The intake camshaft  12  and the exhaust camshaft  13  are rotatably supported in the plane in which the lower camshaft holder  25  and the upper camshaft holder  26  are joined together. 
     As is clear from referring to FIGS. 5 and 7 together, an oil passage P 1  connected to an oil pump (not illustrated) driven by the crankshaft  11  is formed in the cylinder head  23 , and an oil passage P 2  branching from the oil passage P 1  communicates with a first hydraulic control valve  34  mounted on the side of the cylinder head  23 . An oil passage P 6  that originates from the first hydraulic control valve  34  and goes through the inside of the cylinder head  23  further extends upward and communicates with an oil passage P 7  formed on the lower face of a protruding expanded part  25   a  (the plane in which the protruding expanded part  25   a  and the cylinder head  23  are joined together), which is integral with the lower camshaft holder  26 . At the downstream end of the oil passage P 7  is formed an oil drain hole  25   b  which is opposite the start of the section where the exhaust camshaft sprocket  16  is meshed with the timing chain  17 . The oil drain hole  25   b  is slightly constricted in comparison with the cross section of the flow passage of the oil passage P 7  so that the oil can reliably be supplied to the above-mentioned start of the meshed section. A blind cap  35  is provided on the upper face of the protruding expanded part  25   a  of the lower camshaft holder  25  at a position that is an extension of the oil passage P 6  that extends upward within the cylinder head  23 . 
     An oil passage P 9  that originates from the first hydraulic control valve  34  and extends horizontally within the cylinder head  23  communicates with an oil passage P 10  that extends upward. The oil passage P 10  opens on the upper face of the cylinder head  23  and communicates with an oil passage P 11  formed on the lower face of the lower camshaft holder  25 . The oil passage P 11  of the lower camshaft holder  25  communicates with oil passages P 12  and P 13  formed on the outer peripheries of the two bolts  28  and  29  of the four bolts  27  to  30  that fasten both the lower camshaft holder  25  and the upper camshaft holder  26  to the cylinder head  23 . The oil passage P 12  formed on the outer periphery of the bolt  28  communicates with both an oil passage  33   a  formed within the exhaust rocker arm shaft  33  in the axial direction and an oil let  36  provided in the lower camshaft holder  25 . The oil passage P 13  formed on the outer periphery of the bolt  29  communicates with an oil passage  32   a  formed within the intake rocker arm shaft  32  in the axial direction. 
     As is clear from FIG. 8, the oil jet  36  includes an oil jet main body  37  having a nozzle hole  37   a  and a mounting bolt  39  for fixing the oil lot main body  37  to the lower camshaft holder  25  via a sealing member  38 . Within the mounting bolt  39  is housed a relief valve  40 , the upstream side of the relief valve  40  communicating with the oil passage P 12  formed on the outer periphery of the bolt  28  and the downstream side of the relief valve  40  communicating with the nozzle hole  37   a  of the oil jet main body  37 . Fitting a positioning projection  37   b  formed on the oil jet main body  37  in a positioning hole  25   c  formed in the lower camshaft holder  25  positions the nozzle hole  37   a  so that it is directed toward the start of the section where the intake camshaft sprocket  15  is meshed with the timing chain  17 . 
     The oil jet  38  is placed in a dead space interposed between the lower camshaft holder  25  and the exhaust camshaft sprocket  16  so as to be housed within the outer diameter of the exhaust camshaft sprocket  16 . It is therefore possible to minimize the influence an other members from mounting the oil jet  36 . In particular, since the oil jet  36  is placed by effectively utilizing the dead space behind the exhaust camshaft sprocket  16 , which is not where the second variable valve operating characteristic mechanism V 2  is provided, it is possible to minimize any increase in the dimensions of the engine E and any interference with the mounting of other members from mounting the oil jet  36 . As shown in FIG. 2, the oil jet  36  is opposite a cut-out hole  16   a  that is formed in the exhaust camshaft sprocket  16  in order to reduce the weight of the exhaust camshaft sprocket  16 . That is to say, since the oil jet  36  faces the cut-out hole  16   a  formed in the exhaust camshaft sprocket  16 , it is possible to easily check through the cut-out hole  16   a  the presence of the oil jet  36  and the state in which it is mounted. 
     If the entire mounting bolt  39  of the oil jet  36  is placed within the cut-out hole  16   a  of the exhaust camshaft sprocket  16 , the mounting bolt  39  can be attached/detached through the cut-out hole  16   a  thus enhancing the ease of maintenance. If the entire oil jet  36  is placed within the cut-out hole  16   a  of the exhaust camshaft sprocket  18 , the oil jet  36  can be attached/detached through the cut-out hole  16   a  thus enhancing the ease of maintenance. 
     As is clear from FIGS. 3,  4  and  8 , a chain guide  41  is fastened by the two bolts  28  and  29  (inner bolts placed inside the intake camshaft  12  and the exhaust camshaft  13 ) that fasten the upper camshaft holder  26 . The above-mentioned two bolts  28  and  29  that fasten the upper camshaft holder  26  are offset relative to the two bolts  27  and  30  (outer bolts placed outside the intake camshaft  12  and the exhaust camshaft  13 ) that are placed outside the two bolts  28  and  29  by a distance  6  in a direction away from the oil jet  36 . This allows a mounting space for the oil jet  36  to be secured while avoiding any interference with the bolts  28  and  29  and, moreover, the rigidity with which the oil jet  36  is supported can be enhanced. 
     Since one bolt  28  of the two offset bolts  28  and  29  overlaps the oil jet  36  in the axial direction of the exhaust camshaft  13 , not only can the dimensions of the lower camshaft holder  25  be reduced, but also the rigidity with which the exhaust camshaft  13  is supported can be enhanced. This is because placing the oil jet  36  in a position closer to the bolt  29  than to the bolt  28  (on the side away from the exhaust camshaft  13 ) would increase the dimensions of the lower camshaft holder  25  by a proportion corresponding to the space required for the oil jet  36 , if, on the other hand, the oil jet  36  were placed closer to the exhaust camshaft  13  side rather than to the bolt  28 , it would be necessary to form a mounting hole for the oil jet  36  close to the face of the lower camshaft holder  25  that supports the exhaust camshaft  13  and there would, therefore, be a possibility that the rigidity with which the exhaust camshaft  13  is supported might be degraded. Furthermore, since the oil passage P 12  extending to the oil jet  36  is formed around the above-mentioned bolt  28 , the oil passages for supplying oil to the oil jet  36  can be arranged simply and at the same time the oil passages can be shortened. 
     The chain guide  41  has a chain guide main body  42  made of a metal sheet. The lower face of a sliding member  43  made of a synthetic resin provided at the extremity of the chain guide main body  42  is in sliding contact with the upper face of the timing chain  17 . The sliding member  43  can guide the timing chain  17  while restricting its vibration so as to suppress wear of the timing chain  17 , and the sliding resistance between the chain guide  41  and the timing chain  17  can thus be reduced. A pair of tooth skipping prevention plates  42   a  and  42   b  are formed integrally at both ends of the chain guide main body  42  in the longitudinal direction. One tooth skipping prevention plate  42   a  covers the start of the section where the intake camshaft sprocket  15  is meshed with the timing chain  17  and prevents tooth skipping of the timing chain  17 . The other tooth skipping prevention plate  42   b  covers the end of the section where the exhaust camshaft sprocket  16  is meshed with the timing chain  17  and prevents tooth skipping of the timing chain  17 . Since the rigidity of the chain guide  41  increases due to the presence of the two tooth skipping prevention plates  42   a  and  42   b,  the rigidity with which the intake camshaft  12  and the exhaust camshaft  13  are supported is further enhanced. 
     Since the tooth skipping prevention plates  42   a  and  42   b  are formed at the two ends of the sliding member  43  made of a synthetic resin, even though the sliding member  43  is made of a synthetic resin its durability is enhanced. 
     The upper camshaft holder  26  includes a cam cap  26   a  for restraining the intake camshaft  12 , a cam cap  26   b  for restraining the exhaust camshaft  13  and a connecting wall  26   c  for providing a connection between the two caps  26   a  and  26   b.  Between the two bolts  28  and  29  and the connecting wall  26   c,  that is to say, on a face of the connecting wall  26   c  opposite the chain guide  41  is formed a U-shaped recess  26   d  for reducing the weight of the upper camshaft holder  26 . In addition to the lower ends of the two cam caps  26   a  and  26   b  being connected to each other through the connecting wall  26   c,  the upper ends thereof are connected to each other by the chain guide  41 . That is to say, since the chain guide  41  is mounted so as to bridge the recess  26   d  formed between the two cam caps  26   a  and  26   b  and the connecting wall  2   c,  the two cam caps  26   a  and  26   b  can be connected by means of both the connecting wall  26   c  and the chain guide  41  while reducing the weight of the upper camshaft holder  26  and maintaining an adequate rigidity and the rigidity with which the intake camshaft  12  and the exhaust camshaft  13  are supported can be enhanced. 
     As hereinbefore described, since the chain guide  41  is fastened by means of the two bolts  28  and  29  among the four bolts  27  to  30  that also fasten both the lower camshaft holder  25  and the upper camshaft holder  26  to the cylinder head  23 , the number of parts is reduced and the rigidity with which the chain guide  41  is mounted is enhanced. Although the height of the seats for the two inner bolts  28  and  29  fixing the chain guide  41 , among the above-mentioned four bolts  27  to  30 , is restricted by the height of the timing chain  17 , the height of the seats for the two outer bolts  27  and  30  that are not involved in the fixing of the chain guide  41  is not restricted by the height of the timing chain  17  and can be made low. It is thereby possible to lower the two ends of the upper camshaft holder  26  relative to the seat, for the bolts  28  and  29  thus achieving a reduction in the dimensions of the head cover  31 . 
     Referring again to FIG. 4, a filter housing  45  is fixed to a side of the cylinder head  23  by means of bolts  44 . An oil passage P 14  branching from the oil passage P 1  of the cylinder head  23  extends in a direction away from the first variable valve operating characteristic mechanism V 1  and communicates with an oil passage P 16  of the cylinder head  23  via a filter  46  within the filter housing  45 , and an oil passage P 15 . The oil passage P 16  communicates with a second hydraulic control valve  47  housed within the cylinder head  23  (an end wall of the cylinder head  23  on the timing chain  17  side). The second hydraulic control valve  47  communicates with the outer periphery of the intake camshaft  12  via oil passages  17   a  and  17   b  formed in the cylinder head  23  and oil passages  18   a  and  18   b  formed in the lower camshaft holder  26 . The filter housing  45  is mounted utilizing a space on the side of the cylinder head  23  that is opposite the side of the cylinder head  23  on which the first hydraulic control valve  34  is mounted. 
     Next, the structure of the first hydraulic control valve  34  is explained by reference to FIG.  5 . 
     The first hydraulic control valve  34  provided on the side of the cylinder head  23  has a valve hole  51   a  formed within a valve housing  51 . The two ends of an oil passage P 3  passing through a lower part of the valve hole  51   a  communicate with the oil passage P 2  and an oil passage P 4  respectively. The two ends of an oil passage P 5  passing through a middle part of the valve hole  51   a  communicate with the oil passage P 9  and the oil passage P 4  respectively. An upper part of the valve hole  51   a  communicates with the oil passage P 6  via a drain port  51   b.  A filter  52  is attached to the entrance of the oil passage P 3 . On a spool  53  housed within the valve hole  51   a  are formed a pair of lands  53   a  and  53   b,  a groove  53   c  interposed between the two lands  53   a  and  53   b,  an inner hole  53   d  extending in the axial directions an orifice  53   e  passing through the upper end of the inner hole  53   d,  and a groove  53   f  providing communication between the inner hole  53   d  and the drain port  51   b.  The spool  53  is forced upward by a spring  54  housed in the lower end of the inner hole  53   d  and is in contact with a cap  55  blocking the upper end of the valve hole  51   a.  The oil passage P 4  and the oil passage P 5  communicate with each other via an orifice  51   c.  An ON/OFF solenoid  56  is provided between the oil passage P 4  and an oil passage P 8  so as to allow or block communication therebetween. 
     Next, the structure of the first variable valve operating characteristic mechanism V 1  is explained by reference to FIG.  9 . 
     The first variable valve operating characteristic mechanism V 1  for driving the intake valves  18  includes first and second low speed rocker arms  57  and  58  pivotally supported on the intake rocker arm shaft  32  in a rockable manner and a high speed rocker arm  59  interposed between the two low speed rocker arms  57  and  58 . Sleeves  60 ,  61  and  62  are press-fitted into the middle sections of the corresponding rocker arms  57 ,  58  and.  59 . A roller  63  that is rotatably supported around the sleeve  60  is in contact with a low speed intake cam  64  provided on the intake camshaft  12 . A roller  65  that is rotatably supported around the sleeve  61  is in contact with a high speed intake cam  66  provided on the intake camshaft  12 . A roller  67  that is rotatably supported around the sleeve  62  is in contact with a low speed intake cam  68  provided on the intake camshaft  12 . The cam lobe of the high speed intake cam  66  is made higher than the cam lobes of the pair of low speed intake cams  64  and  68 , which have an identical profile. 
     A first switch-over pin  69 , a second switch-over pin  70  and a third switch-over pin  71  are slidably supported within the three sleeves  60 ,  61  and  62 . The first switch-over pin  69  is forced toward the second switch-over pin  70  by a spring  73  disposed in a compressed manner between the first switch-over pin  69  and the spring seat  72  fixed to the sleeve  60  and stops in a position in which the first switch-over pin  69  is in contact with a clip  74  fixed to the sleeve  60 . At this point, the plane in which the first switch-over pin  69  and the second switch-over pin  70  are in contact with each other is positioned between the first low speed rocker arm  57  and the high speed rocker arm  59 , and the plane in which the second switch-over pin  70  and the third switch-over pin  71  are in contact with each other is positioned between the high speed rocker arm  59  and the second low speed rocker arm  58 . An oil chamber  58   a  formed within the second low speed rocker arm  58  communicates with the oil passage  32   a  formed within the intake rocker arm shaft  32 . 
     When no hydraulic pressure acts on the oil passage  32   a  of the intake rocker arm shaft  32 , the first to third switch-over pins  69  to  71  are in the positions shown in FIG.  9 . The first and second low speed rocker arms  57  and  58  and the high speed rocker arm  59  can rock freely. The pair of intake valves  18  are therefore driven with a low valve lift by the first low speed rocker arm  57  and the second low speed rocker arm  58  respectively. At this point, the high speed rocker arm  59  is detached from the first low speed rocker arm  57  and the second low speed rocker arm  58  and rotates without effect on the action of the pair of intake valves  18 . 
     When a hydraulic pressure acts on the oil chamber  58   a  through the oil passage  32   a  of the intake rocker arm shaft  32 , the first to third switch-over pins  69  to  71  move against the spring  73 , and the first and second low speed rocker arms  57  and  58  and the high speed rocker arm  59  are united. As a result, the first and second low speed rocker arms  57  and  58  and the high speed rocker arm  59  are driven as a unit by the high speed intake cam  66  having the high cam lobe, and the pair of intake valves  18  connected to the first low speed rocker arm  57  and the second low speed rocker arm  58  are driven with a high valve lift. At this point, the pair of low speed intake cams  64  and  68  are detached from the first and second low speed rocker arms  57  and  58  and rotate without effect. 
     Next, the structure of the second hydraulic control valve  47  is explained by reference to FIG.  10 . 
     Five ports  82  to  86  are formed in a cylindrical valve housing  81  fitted in a valve hole  23   a  formed in the cylinder head  23 . The central port  84  communicates with the oil passage P 16 , the ports  83  and  85  that are on either side of the central port  84  communicate with the pair of oil passages P 17   a  and P 17   b  respectively, and the ports  82  and  86  that are outside the ports  83  and  85  communicate with a pair of oil drainage passages P 19   a  and P 19   b  respectively. Three grooves  87 ,  88  and  89  are formed on the outer periphery of a spool  90 . The spool  90  is slidably fitted in the valve housing  81 , and forced by the resilient force of a spring  91  toward a linear solenoid  92 , the spring being disposed on one end of the spool  90  and the solenoid  92  being disposed on the other end thereof. 
     When the spool  90  is in a neutral position as shown in the figures all of the oil passages P 16 , P 17   a  and P 17   b  are blocked. When the spool  90  is moved leftward from the neutral position by duty control of the linear solenoid  92 , the oil passage P 16  communicates with the oil passage P 17   a  via the port  84 , the groove  88  and the port  83  and the oil passage P 17   b  communicates with the oil passage  19   b  via the port  85 , the groove  89  and the port  86 . When the spool  90  is moved rightward from the neutral position by duty control of the linear solenoid  92 , the oil passage P 16  communicates with the oil passage P 17   b  via the port  84 , the groove  88  and the port  85 , and the oil passage P 17   a  communicates with the oil passage  19   a  via the port  83 , the groove  87  and the port  82 . 
     Next, the structure of the second variable valve operating characteristic mechanism V 2  is explained by reference to FIGS. 11 and 12. 
     The second variable valve operating characteristic mechanism V 2  includes an outer rotor  93  and an inner rotor  96  fixed to the intake camshaft  12  by means of a pin  94  and bolts  95 . The outer rotor  93  includes a cap-shaped housing  97 , the intake camshaft sprocket  15  being formed integrally on the outer periphery of the housing  97 , an outer rotor main body  98  fitted in the housing  97  and an annular cover plate  99  covering the opening of the housing  97 , and these are combined integrally by means of four bolts  100 . A support hole  97   a  is formed in the center of the housing  97 , and fitting the support hole  97   a  around the outer periphery of the intake camshaft  12  allows the outer rotor  93  to be supported on the intake camshaft  12  in a relatively rotatable manner. 
     On the inner periphery of the outer rotor main body  98  are alternately formed four recesses  98   a  and four projections  98   b.  Four vanes  96   a  formed radially on the outer periphery of the inner rotor  96  are fitted in the above-mentioned four recesses  98  respectively. Sealing members  101  provided on the extremities of the projections  98   b  of the outer rotor main body  98  are in contact with the inner rotor  96  and sealing members  102  provided on the extremities of the vanes  96   a  of the inner rotor  96  are in contact with the outer rotor main body  98  thus defining four advance chambers  103  and four retard chambers  104  between the outer rotor main body  98  and the inner rotor  96 . 
     A stopper pin  105  is slidably supported in a pin hole  96   b  formed in the inner rotor  96 . An arc-shaped long channel  97   b  with which the extremity of the stopper pin  105  can engage is formed in the housing  97  of the outer rotor  93 . The stopper pin  105  is forced by a spring  106  in the direction in which the stopper pin  105  becomes detached from the long channel  97   b.  An oil chamber  107  is formed at the back of the stopper pin  105 . When the stopper pin  105  becomes detached from the long channel  97   b  due to the resilient force of the spring  106 , the outer rotor  93  and the inner rotor  96  can rotate relative to each other within an angle α (e.g. 30°) in which each of the vanes  96   a  of the inner rotor  96  can move from one end of the corresponding recess  98   a  of the outer rotor  93  to the other end thereof. When a hydraulic pressure is supplied to the oil chamber  107  so making the stopper pin  105  engage with the long channel  97   b,  the outer rotor  93  and the inner rotor  96  can rotate relative to each other within an angle β (e.g. 20°) in which the stopper  105  can move from one end of the long channel  97   b  to the other end thereof. 
     A pair of oil passages P 18   a  and P 18   b  formed in the lower camshaft holder  26  communicate with the advance chambers  103  and the retard chambers  104  respectively via a pair of oil passages  12   a  and  12   b  formed within the intake camshaft  12  and oil passages  96   c  and  96   d  formed in the inner rotor  96 . When a hydraulic pressure is supplied to the advance chambers  103  via the second hydraulic control valve  47 , the low speed intake cams  64  and  68  and the high speed intake cam  66  advance in angle relative to the intake camshaft  12  so advancing the valve timing of the intake valves  18 . On the other hand, when a hydraulic pressure is supplied to the retard chambers  104  via the second hydraulic control valve  47 , the low speed intake cams  64  and  68  and the high speed intake cam  66  are retarded in angle relative to the intake camshaft  12  so retarding the valve timing of the intake valves  18 . 
     In the second lower camshaft holder  25  viewed from the second variable operating characteristic mechanism V 2  side, is formed an oil passage P 20  that communicates with the oil passage P 13  (FIG.  4 ). The oil passage P 20  further communicates with the oil chamber  107 , the top part of the stopper pin  105  facing the oil chamber  107 , via an oil passage  12   c  formed within the intake camshaft  12  and oil passages  95   a  and  95   b  formed within the bolt  95 . 
     In the present embodiment, no variable valve operating characteristic mechanism is provided on the exhaust camshaft  13  side, and the exhaust valves  19  are driven with an intermediate valve lift. That is to say, the valve lift of the exhaust valves  19  is midway between the valve lift (small lift) of the intake valves  18  at low speed and the valve lift (large lift) at high speed. 
     The action of the embodiment having the above-mentioned arrangement is now explained. 
     When the engine E rotates at a low speed, the solenoid  56  of the first hydraulic control valve  34  is in an OFF state, communication between the oil passage P 4  and the oil passage P 8  is blocked, and the spool  53  is in the raised position shown in FIG. 5 due to the resilient force of the spring  54 . In this state the oil pump communicates with the oil chamber  58   a  of the first variable valve operating characteristic mechanism V 1  via the oil passages P 1  and P 2  of the cylinder head  23 , the oil passages P 3  and P 4 , the orifice  53   c  and the oil passage P 5  of the valve housing  51 , the oil passages P 9  and P 10  of the cylinder head  23 , the oil passages P 11  and P 13  of the lower camshaft holder  25  and the oil passage  32   a  within the intake rocker arm shaft  32 . At this point, since the hydraulic pressure that is transmitted to the oil chamber  58   a  of the first variable valve operating characteristic mechanism V 1  is low due to the action of the orifice  53   c,  the first to third switch-over pins  69 ,  70  and  71  are retained in the positions shown in FIG. 9, the pair of intake valves  18  are driven with a low valve lift and the valve operation system (rocker arm support parts, camshaft support parts, etc.) can be lubricated with this low pressure oil. 
     As described above, when the hydraulic pressure output by the first hydraulic control valve  34  is low, the hydraulic pressure that is transmitted to the oil chamber  107  of the second variable valve operating characteristic mechanism V 2  via the oil passage P 20  of the lower camshaft holder  25  and the oil passage  12   c  within the intake camshaft  12  shown in FIG. 11 is low, and the stopper pin  105  becomes detached from the long channel  97   b  due to the resilient force of the spring  106 . Controlling the duty ratio of the second hydraulic control valve  47  (FIG.  10 ), which is connected to the oil pump via the oil passages P 1  and P 14  of the cylinder head  23 , the oil passage P 15  within the filter housing  45  and the oil passage P 16  of the cylinder head  23 , generates a difference between the hydraulic pressures transmitted via the pair of oil passages  17   a  and  17   b  to the advance chambers  103  and the retard chambers  104  of the second variable valve operating characteristic mechanism V 2 . As a result, the phase of the inner rotor  96  relative to the outer rotor  93  can be varied in the range of the angle α (FIG. 12) so controlling the valve timing of the intake valves  18 . 
     When the engine E rotates at a low speed as described above, the oil (relief oil) that has passed through the orifice  53   c  of the first hydraulic control valve  34  and has a reduced pressure flows through the oil passage P 5 , the groove  53   c  of the spool  53 , the drain port  51   b,  the oil passage P 6  of the cylinder head  23  and the oil passage P 7  of the protruding expanded part  25   a  of the lower camshaft holder  25  and flows out of the oil drain hole  25   b  to the start of the section (or meshed section) where the exhaust camshaft sprocket  16  is meshed with the timing chain  17  so lubricating the timing chain  17  (FIG.  7 ). Since the rotational speed of the timing chain  17  is low when the engine E rotates at a low speeds only a small amount of the oil that has become attached to the timing chain  17  scatters due to centrifugal force. If oil is supplied to the start of the section where the exhaust camshaft sprocket  16  is meshed with the timing chain  17 , which is to the rear in the rotational direction of the timing chain  17 , since the engine E is rotating at a low speed with a small load imposed on the timing chain  17 , the section where the intake camshaft sprocket  15  is meshed with the timing chain  17 , which is to the front in the rotational direction of the timing chain  17 , can be lubricated well. 
     As hereinbefore described, since the timing chain  17  is lubricated with the relief oil of the first hydraulic control vale  34  flowing out through the oil drain hole  25   b,  it is unnecessary to employ an oil jet and secure a space for mounting it. Moreover, since the oil passage P 7  connected to the oil drain hole  25   b  is formed in the plane in which the cylinder head  23  and the lower camshaft holder  25  are joined together, the oil passage P 7  can be arranged simply. Furthermore, since the first hydraulic control valve  34  is mounted on the side wall of the cylinder head  23  that is close to the oil drain hole  26   b,  the length of the oil passage P 7  for the above-mentioned relief oil can be reduced and the rigidity with which the first hydraulic control valve  34  is mounted can be enhanced in comparison with a case where the first hydraulic control valve  34  is mounted on a side wall of the cylinder head that is far from the oil drain hole  25   b.    
     Furthermore, since the oil passage P 7  for the relief oil, which is formed in the plane in which the cylinder head  23  and the lower camshaft holder  25  are joined together, and the first hydraulic control valve  34  are placed in a same plane that is perpendicular to the camshafts  12  and  13 , the lengths of the oil passages P 6  and P 7  from the first hydraulic control valve  37  to the oil drain hole  25   b  can be further reduced. 
     As shown in FIG. 6, when the engine E rotates at a high speed and the solenoid  56  of the first hydraulic control valve  34  is in an ON state so providing communication between the oil passage P 4  and the oil passage P 8  and moving the spool  53  downward due to the hydraulic pressure acting on the land  53   b,  the oil passage P 3  and the oil passage P 5  communicate with each other via the groove  53   c.  As a result, a high hydraulic pressure is transmitted to the oil chamber  58   a  of the first variable valve operating characteristic mechanism V 1  via the oil passages P 9  and P 10  of the cylinder head  23 , the oil passages P 11  and P 13  of the lower camshaft holder  25  and the oil passage  32   a  within the intake rocker arm shaft  32 , the first to third switch-over pins  69 ,  70  and  71  move against the spring  73  and the pair of intake valves  18  are driven with a high valve lift. 
     As hereinbefore described, when the hydraulic pressure output by the first hydraulic control valve  34  is high, the hydraulic pressure that is transmitted to the oil chamber  107  of the second variable valve operating characteristic mechanism V 2  via the oil passage P 20  of the lower camshaft holder  25  and the oil passage  12   c  within the intake camshaft  12  shown in FIG. 11 also becomes high so engaging the stopper pin  105  with the long channel  97   b  against the spring  106 . It is therefore possible by controlling the duty ratio of the second hydraulic control valve  47 , which is connected to the oil pump via the oil passages P 1  and P 14  of the cylinder head  23 , the oil passage P 15  within the filter housing  45  and the oil passage P 16  of the cylinder head  23 , to generate a difference between the hydraulic pressures transmitted via the pair of oil passages P 17   a  and P 17   b  to the advance chambers  103  and the retard chambers  104  of the second variable valve operating characteristic mechanism V 2  thus varying the phase of the inner rotor  96  relative to the outer rotor  93  in the range of the angle β (FIG.  12 ), so as to control the valve timing of the intake valves  18 . 
     In FIG. 8, when the engine E rotates at high speed, oil at a high pressure supplied to the oil passage P 12  formed on the outer periphery of the bolt  28  pushes the relief valve  40  within the mounting bolt  39  of the oil jet  36  so as to open it and issues from the nozzle hole  37   a  of the oil jet main body  37  thus lubricating the start of the section (or meshed section) where the intake camshaft sprocket  15  is meshed with the timing chain  17 . In FIG,  6 , the oil supplied to the oil passage P 8  of the first hydraulic control valve  34  flows through the orifice  53   e,  the inner hole  53   d  and the groove  53   f  of the spool  53 , the drain port  51   b  of the valve housing  51 , the oil passage P 6  of the cylinder head  23  and the oil passage P 7  of the protruding expanded part  25   a  of the lower camshaft holder  25  and flows out from the oil drain hole  26   b  toward the start of the section (or meshed section) where the exhaust camshaft sprocket  16  is meshed with the timing chain  17  thus lubricating the timing chain  17  (FIG.  7 ). 
     As described above, when the engine E rotates at a low speed with a low load on the timing chain  17 , only the start of the section where the exhaust camshaft sprocket  16  is meshed with the timing chain  17  Is lubricated with the relief oil. When the engine E rotates at a high speed with a high load on the timing chain  17 , the start of the section where the intake camshaft sprocket  16  is meshed with the timing chain  17  is lubricated intensively with oil from the oil jet  36  and at the same time the start of the section where the exhaust camshaft sprocket  16  is meshed with the timing chain  17  receives auxiliary lubrication with the relief oil from the oil drain hole  25   b,  The timing chain  17  can thus be lubricated optimally according to the operational state of the engine E so enhancing the durability. 
     That is to say, since the operation of the oil drain hole  25   b  and the oil jet  36 , which form a plurality of oil supply means for supplying oil to the timing chain  17 , are controlled according to the operational state of the engine E, lubrication can be carried out according to the operational state of the engine E so suppressing the wear of the timing chain  17 . Moreover, since the number of oil supply means that are operated is increased as the rotational speed of the engine E increases, the number of parts that are lubricated is increased as the load increases and wear of the timing chain  17  can be suppressed yet more effectively. 
     In particular, when the engine E rotates at a low speed and the valve lift of the exhaust valves  19  (intermediate valve lift) is larger than the valve lift of the intake valves  18  (small valve lift), a comparatively large amount of oil is supplied to the exhaust camshaft sprocket  16 , the load on the exhaust camshaft sprocket  16  being larger than that on the intake camshaft sprocket  15 . On the other hand, when the engine E rotates at a high speed and the valve lift of the intake valves  18  (large valve lift) is larger than the valve lift of the exhaust valves  19  (intermediate valve lift), a comparatively large amount of oil is supplied to the intake camshaft sprocket  15 , the load on the intake camshaft sprocket  15  being larger than that on the exhaust camshaft sprocket  16 , a comparatively small amount of oil is supplied to the exhaust camshaft sprocket  16 , and supply of an optimal amount of oil can thus be guaranteed according to the operational state of the engine E. 
     That is to say, the first variable valve operating characteristic mechanism V 1  is provided for varying the relative amount of valve lift between the intake valves  18  and the exhaust valves  19  according to the operational state of the engine E, the amount of oil supplied to the section where the timing chain  17  is meshed with the sprocket that drives the valves having a larger lift being larger than the amount of oil supplied to the section where the timing chain  17  is meshed with the sprocket that drives the valves having a smaller lift, and a larger amount of oil can thus be supplied to the sprocket having a larger valve operating load so prolonging the life span of the timing chain  17 . Moreover, the first hydraulic control valve  34  is provided for switching over between a low speed valve lift and a high speed valve lift, the low speed valve lift being used when the rotational speed of the engine E is lower than a predetermined value and the high speed valve lift being used when the rotational speed of the engine E is higher than the predetermined value. The first hydraulic control valve  34  establishes the low speed valve lift when the engine E rotates at a low speed and the high speed valve lift when the engine E rotates at a high speed; when the low speed valve lift is established, the timing chain  17  is lubricated with low pressure relief oil from the first hydraulic control valve  34 , and when the high speed valve lift is established, the timing chain  17  is lubricated with high pressure valve lift control oil from the first hydraulic control valve  34 , and an amount of oil that is appropriate for the state of the load can thus be supplied to the timing chain  17  so effectively preventing wear thereof. 
     The operating conditions of the first variable valve operating characteristic mechanism V 1  can easily be checked by detaching the blind cap  35  provided on the protruding expanded part  25   a  of the lower camshaft holder  25  facing the downstream end of the oil passage P 6  of the cylinder head  23 , attaching a measurement apparatus  108  instead of the above-mentioned blind cap  36  as shown in FIG  13  and supplying a fluid pressure of, for example, air from the measurement apparatus  108 . As is dear from FIG. 5, since the seat for the blind cap  35  formed in the lower camshaft holder  25  is provided at a lower position than the place where it is joined to the upper camshaft holder, not only can the length of the blind cap  35  be shortened, but also the dimensions of the lower camshaft holder  25  can be reduced. 
     Merely fitting the extremity of the measurement apparatus  108  in the oil passage P 6  within the cylinder head  23  via a seating member allows the operating conditions of the first variable valve operating characteristic mechanism V 1  to be checked without receiving any influence (escape of fluid pressure) from the oil passage P 7  for the relief oil. 
     Next, a second embodiment of the present invention is explained by reference to FIG.  14 . 
     A chain guide  41  of the second embodiment does not have a sliding member  43  made of a synthetic resin; instead, the upstream side of an oil passage  41   a  formed within the chain guide  41  communicates with an oil passage P 12  formed on the outer periphery of a bolt  28  and the downstream side of the oil passage  41   a  communicates with an orifice  41   c  opening on a sliding face  41   b  facing a timing chain  17 . When an engine E rotates at a high speed, and oil at a high pressure is supplied to the oil passage P 12 , the oil issues toward the inner periphery of the timing chain  17  from an oil jet  36  as well as toward the outer periphery of the timing chain  17 , via the orifice  41   c,  from the oil passage  41   a  formed within the chain guide  41 . A sliding section between the sliding face  41   b  of the chain guide  41  and the timing chain  17  can be lubricated effectively with the oil issuing through the orifice  41   c.  It is also possible to make the above-mentioned orifice  41   c  open on tooth skipping prevention plates  42   a  and  42   b  (FIG. 3) of the chain guide  41 , and this arrangement allows the sections where the intake camshaft sprocket  15  and the exhaust camshaft sprocket  16  are meshed with the timing chain  17  to be lubricated effectively. 
     Although embodiments of the present invention have been explained in detail above, the present invention can be modified in a variety of ways without departing from the spirit and scope of the present invention.