Abstract:
An engine includes engine valves disposed on intake and exhaust sides of a combustion chambers with a valve operating device for opening and closing the engine valves and an engine valve disabling mechanism for disabling opening and closing movement of the engine valves in response to hydraulic pressure. An oil pump is disposed in a lower portion of the crankcase, for supplying hydraulic pressure to the engine valve disabling mechanism. A hydraulic pressure control valve controls the supply of hydraulic pressure to the engine valve disabling mechanism depending on a vehicle operating state. An oil passage extends to the hydraulic pressure control valve and can easily be formed without affecting mounts for functional components. A pipe for delivering working oil from the lower portion of the crankcase to the hydraulic pressure control valve is disposed on a front surface of the engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2007-224989 filed on Aug. 31, 2007 the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an engine with a cylinder disabling mechanism, and more particularly to a hydraulic pressure supply oil passage for supplying hydraulic pressure to hydraulic pressure control valves for controlling the supply of the hydraulic pressure to the cylinder disabling mechanism. 
         [0004]    2. Description of Background Art 
         [0005]    Engines are known wherein the engines have a valve disabling mechanism associated with engine valves. According to the valve disabling mechanism, oil is delivered from an oil pump disposed in a lower portion of the engine to hydraulic pressure control valves, which controls hydraulic pressure delivered to the valve disabling mechanism to disable cylinders depending on a vehicle operating state. The oil is supplied from the oil pump to the hydraulic pressure control valves through oil passages defined in a crankcase and a cylinder block. See, for example, Japanese Patent Laid-Open No. 2000-204917. 
         [0006]    Heretofore, it has been difficult to define the oil passages in the crankcase and the cylinder block. More particularly, since functional components such as a starter motor, etc., are mounted on a rear surface of the engine, design considerations have been required to form the oil passages in bypassing relation to attachment areas for the functional components. 
       SUMMARY AND OBJECTS OF THE INVENTION 
       [0007]    An embodiment of the present invention proposes a hydraulic pressure supply oil passage for supplying hydraulic pressure to hydraulic pressure control valves, which can simply be formed without affecting attachment areas of a rear surface of an engine for functional components. 
         [0008]    According to an embodiment of the present invention, an engine is provided with a cylinder disabling mechanism that includes a crankcase of a motorcycle engine with a cylinder block mounted on an upper portion of the crankcase and having a plurality of cylinders with pistons disposed respectively therein. A cylinder head is mounted to an upper portion of the cylinder block with a cylinder head cover being mounted to an upper portion of the cylinder head. Combustion chambers are defined between a lower surface of the cylinder head and upper surfaces of the pistons. Engine valves are disposed in the cylinder head on intake and exhaust sides of the combustion chambers with a valve operating device disposed between the cylinder head and the cylinder head cover, for opening and closing the engine valves. An engine valve disabling mechanism is provided for disabling opening and closing movement of the engine valves in response to hydraulic pressure. An oil pump is disposed in a lower portion of the crankcase, for supplying the hydraulic pressure to the engine valve disabling mechanism. A hydraulic pressure control valve is provided for controlling the supply of hydraulic pressure to the engine valve disabling mechanism depending on a vehicle operating state. A pipe is provided for delivering working oil from the lower portion of the crankcase to the hydraulic pressure control valve. The pipe is disposed on a front surface of the engine. 
         [0009]    According to an embodiment of the present invention, the cylinder disabling mechanism is associated with the cylinders. The hydraulic pressure control valve includes a plurality of hydraulic pressure control valves corresponding to the cylinders associated with the cylinder disabling mechanism. The pipe is disposed on the front surface of the engine and includes an upper portion connected to a working oil supply passage defined in an upper portion of the cylinder head cover. The working oil supply passage extends to a region intermediate between adjacent ones of the hydraulic pressure control valves and is branched and connected to inlet ports of the respective hydraulic pressure control valves. 
         [0010]    According to an embodiment of the present invention, since the oil passage to the hydraulic pressure control valve includes an externally mounted pipe, the oil passage can be installed more easily than if it is defined in the crankcase and the cylinder block as is the case with the background art. 
         [0011]    If the oil passage is disposed on a rear surface of the engine, the pipe is complex and long because of functional components including an intake system, a starter motor, a knock sensor, a coolant hose, etc. The oil passage disposed on the front surface of the engine allows the pipe to be short and easily installed. 
         [0012]    According to an embodiment of the present invention, the working oil supply passage defined in the upper portion of the cylinder head cover extends to the region intermediate between adjacent ones of the hydraulic pressure control valves and is branched and connected to the inlet ports of the respective hydraulic pressure control valves. Therefore, the working oil supply passage in the upper portion of the cylinder head cover is short. 
         [0013]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
           [0015]      FIG. 1  is a side elevational view of a motorcycle  1  according to an embodiment of the present invention; 
           [0016]      FIG. 2  is a plan view of the motorcycle  1 ; 
           [0017]      FIG. 3  is a vertical cross-sectional view of an engine including ignition plugs; 
           [0018]      FIG. 4  is an enlarged view of a portion of  FIG. 3 ; 
           [0019]      FIG. 5  is an enlarged view of a valve lifter of an intake valve and associated parts; 
           [0020]      FIG. 6  is an enlarged view of the valve lifter and associated parts at the time the valve is disabled; 
           [0021]      FIG. 7  is a longitudinal cross-sectional view of an upper portion of the engine; 
           [0022]      FIG. 8  is a cross-sectional view of a hydraulic pressure control valve; 
           [0023]      FIG. 9  is a plan view of a cylinder head cover; and 
           [0024]      FIG. 10  is a vertical cross-sectional view of the engine including intake and exhaust valves. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  is a side elevational view of a motorcycle  1  according to an embodiment of the present invention. In  FIG. 1 , the motorcycle  1  includes a body frame  2  including a head pipe  3 , main frames  4 ,  4  extending obliquely rearwardly from the head pipe  3 , central frames  5 ,  5  extending downwardly from the rear ends of the main frames  4 ,  4 , down pipes  6 ,  6 , and a seat stay  7  extending rearwardly from the main frames  4 ,  4  and the down pipes  6 ,  6 . A front wheel WF is supported by a front fork  8  which is steerably supported by the head pipe  3 . A steering handle  9  is mounted to an upper portion of the front fork  8 . A rear wheel WR is supported by a rear fork  10  that is vertically swingably supported by a rear portion of one of the central frames  5 . A shock absorber  11  is disposed between the seat stay  7  and the rear fork  10 . An engine E is supported by the main frames  4 ,  4  and the central frames  5 ,  5 . Power from the engine E is transmitted to the rear wheel WR through a transmission combined with the engine E and a rear wheel drive chain  12 . A fuel tank  13  that is positioned above the engine E is mounted on the main frames  4 ,  4  and the central frames  5 ,  5 . A tandem seat  14  for the driver and passenger to be seated thereon is mounted on the seat stay  7 . A radiator  15  is disposed in front of the engine E. An exhaust pipe  16  which extends from a front surface of the engine E extends below the engine E and is connected to a muffler  17  disposed in a rear portion of the body frame  2 . 
         [0026]      FIG. 2  is a plan view of the motorcycle  1 . In  FIG. 2 , the engine E includes a four-cylinder engine including a cylinder block  20  with four cylinders  21  ( 21   a,    21   b,    21   c,    21   d ) defined therein which are arrayed in a lateral direction. 
         [0027]      FIG. 3  is a vertical cross-sectional view of the engine E including ignition plugs according to the above embodiment. The engine E is a four-cylinder engine, and the cylinders  21  defined in the cylinder block  20  are inclined forwardly in the direction in which the motorcycle  10  is propelled. As shown in  FIG. 2 , the four cylinders  21  are arrayed widthwise across the motorcycle  10 . A crankcase  22  houses a crankshaft  23  extending widthwise across the motorcycle  10 . Pistons  24  are slidably fitted in the respective cylinders  21  and connected to the crankshaft  23  by connecting rods  25 . 
         [0028]      FIG. 4  is an enlarged view of a portion of  FIG. 3 . In  FIG. 4 , a cylinder head  26  is mounted to an upper portion of the cylinder block  20  and has recesses defined in a lower surface thereof in alignment with the cylinders  21 , providing combustion chambers  27  between themselves and the upper ends of the pistons  24 . The cylinder head  26  has intake ports  28  and exhaust ports  29  which communicate with the combustion chambers  27 . Each of the intake ports  28  is connected to the combustion chamber  27  through a pair of intake openings  30 , and each of the exhaust ports  29  is connected to the combustion chamber  27  through a pair of exhaust openings  31 . The intake openings  30  and the exhaust openings  31  can be opened and closed by intake valves  32  and exhaust valves  33  disposed respectively therein. A cylinder head cover  35  is mounted to an upper portion of the cylinder head  26  with a cylinder head extension member  34  interposed therebetween. A valve operating device  36  is disposed between the cylinder head extension member  34  and the cylinder head cover  35 . An intake system  37  is connected to the intake ports  28 . 
         [0029]    In  FIG. 3 , the intake system  37  mainly includes a throttle valve  38  and a fuel injector  39 . An air cleaner  40  shown in  FIG. 1  is connected to a rear portion of the throttle valve  38 . A starter motor  41  is disposed on an upper surface of the crankcase  22  behind the cylinder block  20 . For starting the engine E, drive power from the starter motor  41  is transmitted through a pinion  42  mounted on the rotational shaft of the starter motor  41 , a first idle gear  43 , and a second idle gear  44  to a large-diameter driven gear  45  mounted on the crankshaft  23 , thereby rotating the crankshaft  23 . The crankcase  22  houses therein a full-time-mesh gear transmission (not shown) including a main shaft and a countershaft. The countershaft has a shaft end portion projecting out of the crankcase  22  and supporting a sprocket  46  mounted thereon. The rear wheel drive chain  12  shown in  FIG. 1  is held in mesh with the sprocket  46 . A water pump  47  which is driven by the main shaft through a chain is mounted on a side surface of the crankcase  22 . The water pump  47  supplies a coolant through a water hose  48  to a water jacket  49  defined in the cylinder block  20  and the cylinder head  26  to cool the engine E. After having cooled the water jacket  49  the water is cooled by the radiator  15  shown in  FIG. 1  and retrieved for circulation. An oil pan  50  is mounted on a lower portion of the crankcase  22 , and an oil filter  51  is mounted on a front portion of the crankcase  22 . A knock sensor  55  is positioned between the intake system  37  and the cylinder block  20 . 
         [0030]    In  FIG. 4 , an ignition plug  56  is disposed centrally in each of the combustion chambers  27  and surrounded by a pair of intake valves  32  and a pair of exhaust valves  33 . The four cylinders  21  include two cylinders  21   a,    21   d  on the left and right ends of the cylinder array. Each of the cylinders  21   a,    21   d  is associated with an intake valve lifter  64  and an exhaust valve lifter  65  each combined with a valve disabling mechanism  66 .  FIG. 4  shows a cross section of the cylinder  21   a  or  21   d  combined with the valve disabling mechanism  66 . The intake valves  32  and the exhaust valves  33  are normally urged to be closed by valve springs  57  disposed in the cylinder head  26 . These valves are operated by the valve operating device  36  to open and close the intake openings  30  and the exhaust openings  31 , respectively. The intake valves  32  and the exhaust valves  33  are reciprocally movable along the directions in which they are opened and closed because they have valve stems  58  slidably fitted in tubular valve guides  59  fixedly mounted in the cylinder head  26 . 
         [0031]    The valve operating device  36 , which is disposed between the cylinder head  26  and the cylinder head cover  35 , opens and closes the intake valves  32  and the exhaust valves  33  at given timings depending on the angular position of the crankshaft  23 . The valve operating device  36  includes an intake cam shaft  60 , an exhaust cam shaft  61 , intake cams  62 , exhaust cams  63 , intake valve lifters  64 , exhaust valve lifters  65 , and the valve disabling mechanisms  66  for disabling the intake valves  32  and the exhaust valves  33 . The valve disabling mechanisms  66  are disposed in the respective valve lifters  64 ,  65  of the intake valves  32  and the exhaust valves  33  in the cylinders  21   a,    21   d  on the left and right ends of the cylinder array. The valve lifters  64 ,  65  are fitted in respective lifter supports  67  disposed in the cylinder head extension member  34  and supported for sliding movement in the directions along the valve stems  58 . 
         [0032]    Since the valve disabling mechanism  66  for the intake valve  32  and the valve disabling mechanism  66  for the exhaust valve  33  are identical in structure to each other, the valve disabling mechanism  66  for the intake valve  32  will be described below. The valve disabling mechanism  66  is a mechanism for selectively transmitting the valve drive power which has been transmitted from the intake cam  62  to the valve lifter  64 , to the intake valve  32 . The valve disabling mechanism  66  is controlled by the working oil of a hydraulic pressure control system. When the engine E operates at a low speed or under a low load, the valve disabling mechanism  66  does not transmit the reciprocating motion of the valve lifter  64  to the intake valve  32 , keeping the intake valve  32  closed. 
         [0033]      FIG. 5  is an enlarged view of the valve lifter  64  of the intake valve  32  and associated parts. The valve disabling mechanism  66  includes a hollow cylindrical holder  68  slidably fitted in the valve lifter  64  for sliding movement in the direction along the valve stem  58 . A slide pin  69  is slidably fitted in the holder  68  for sliding movement in a direction perpendicular to the direction along the valve stem  58  with a helical spring  70  disposed between the holder  68  and the slide pin  69 . A stopper pin  71  is mounted on the holder  68 . The holder  68  is an integral component which includes a ring  72 , a joint  73  diametrically joining the ring  72 , and a presser  75  projecting upwardly centrally from the joint  73  and pressing a ceiling wall  74  of the valve lifter  64 . The ring  72  has an inner annular oil passage  76  defined fully circumferentially in an outer circumferential surface thereof. The joint  73  has a cylindrical slide pin housing hole  77  defined therein which has an open end  77 A and a closed end  77 B. The joint  73  also has a lower through hole  78  defined in a lower portion thereof and receiving the tip end of the valve stem  5   8  inserted therein. The presser  75  has an upper through hole  79  defined therein coaxially with the lower through hole  78  for receiving the tip end of the valve stem  58  inserted therein. The slide pin housing hole  77  receives therein the slide pin  69  which is reciprocally movable in its axial directions. The slide pin  69  has a valve stem insertion hole  80  defined centrally therein which has an axis parallel to the valve stem  58 . The valve spring  57  is surrounded by a support spring  81  for pushing the holder  68  upwardly to press the presser  75  into abutment against the ceiling wall  74  and urge the valve lifter  64  upwardly to hold the ceiling wall  74  in contact with the intake cam  62 . 
         [0034]    The lifter support  67  has an outer annular oil passage  82  defined in an inner circumferential surface thereof. The inner annular oil passage  76  and the outer annular oil passage  82  are held in communication with each other through lifter side through holes  83  defined in the side of the valve lifter  64 . The outer annular oil passage  82  is supplied with hydraulic pressure from a hydraulic pressure supply passage  84  (also see  FIG. 4 ). The hydraulic pressure acts in a hydraulic pressure chamber  85  on the end of the slide pin  69  through the lifter side through holes  83  and the inner annular oil passage  76 . The hydraulic pressure can switch between a low hydraulic pressure level and a high hydraulic pressure level depending on how the valve disabling mechanism  66  should operate. When the high hydraulic pressure level acts in the hydraulic pressure chamber  85 , the slide pin  69  is pushed to compress the helical spring  70 . 
         [0035]      FIG. 5  shows the position of the parts in which the low hydraulic pressure level acts in the hydraulic pressure chamber  85 . When the helical spring  70  is extended, the slide pin  69  is forced to a stop in abutment against the stopper pin  71 . The support spring  81  holds the presser  75  of the holder  68  in abutment against the ceiling wall  74  of the valve lifter  64 , holding the valve lifter  64  in contact with the cam  60 . The valve spring  57  causes a cotter  87  and a retainer  88  to press the valve stem  58 , holding the tip end of the valve stem  58  in abutment against an abutting surface  86  on a lower portion of the slide pin  69 . When the cam  62  is rotated, the valve lifter  64  vertically moves in unison with the valve stem  58 , operating the intake valve  32  ( FIG. 4 ). In this state, the intake valve  32  is operated, operating the cylinder  21   a  or  21   d.    
         [0036]      FIG. 6  is an enlarged view of the valve lifter  64  and associated parts at the time the intake valve  32  is disabled.  FIG. 6  shows the position of the parts at the instant the high hydraulic pressure level acts in the hydraulic pressure chamber  85  and the cam  62  is rotated to lower the valve lifter  64 . The slide pin  69  is pressed to the right in  FIG. 6  against the spring force of the helical spring  70 . Since the valve stem insertion hole  80  in the slide pin  69  and the valve stem  58  are axially aligned with each other, the valve stem  58  which is pressed upwardly by the valve spring  57  is insertable into the valve stem insertion hole  80  and the upper through hole  79 . As the valve stem  58  is pressed upwardly, the intake valve  32  is closed. The support spring  81  presses the holder  68  upwardly together with the slide pin  69 , bringing the valve lifter  64  into abutment against the cam  62 . When the cam  62  is rotated, the valve lifter  64 , the holder  68 , and the slide pin  69  vertically move in unison with each other. Since the tip end of the valve stem  58  is in a free state within the lower through hole  78 , the valve stem insertion hole  80 , and the upper through hole  79 , the intake valve  32  remains closed even though the valve lifter  64  moves. In this state, the intake valve  32  is disabled in the closed position, disabling the cylinder  21   a  or  21   d.    
         [0037]      FIG. 7  is a longitudinal cross-sectional view of an upper portion of the engine E. The cylinder block  20 , the cylinder head  26 , the cylinder head extension member  34 , and the cylinder head cover  35  are arranged successively upwardly in the order named in  FIG. 7 . The four cylinders  21  are arrayed parallel in the lateral direction. The intake cam shaft  60  is supported between the cylinder head cover  35  and the cylinder head extension member  34 . A cam shaft driven sprocket  89  is mounted on the right end of the intake cam shaft  60 , which is driven by the crankshaft  23  through a cam shaft drive chain  90  trained around the cam shaft driven sprocket  89 . Two intake valves  32  are provided in association with each cylinder  21 , and the intake valve lifter  64  is disposed above each of the intake valves  32 . The intake cams  62  on the intake cam shaft  60  are held against the upper ends of the intake valve lifters  64 . The intake valve lifters  64  are slidably supported by the lifter supports  67 . Though not shown, the exhaust cam shaft  61 , the exhaust cams  63 , the exhaust valves  33 , the exhaust valve lifters  65 , and the cam shaft driven sprocket  89  are also similarly provided. 
         [0038]    Each cylinder  21  has a total of four valves, i.e., two intake valves  32  and two exhaust valves  33 . The two cylinders  21   a,    21   d  on the left and right ends of the cylinder array are cylinders with cylinder disabling mechanisms. More specifically, all the valve lifters of the four intake and exhaust valves of each of the cylinders  21   a,    21   d  on the left and right ends of the cylinder array are associated with respective valve disabling mechanisms  66 . The hydraulic pressure supply passages  84  ( FIG. 4 ) which extend and are branched from hydraulic pressure control valves  91 , to be described later, apply hydraulic pressure simultaneously to the four valve disabling mechanisms  66 . When the hydraulic pressure applied simultaneously to the four valve disabling mechanisms  66  is of a high hydraulic pressure level, the four valve disabling mechanisms  66  are simultaneously operated to disable the valves, disabling the corresponding cylinder. The four valve disabling mechanisms  66  as they are combined together are referred to as the cylinder disabling mechanism. The two central cylinders of the cylinder array are not associated with valve disabling mechanisms. 
         [0039]    Two hydraulic pressure control valves  91  of an identical type are disposed on an upper surface of the cylinder head cover  35 . The hydraulic pressure control valves  91  serve to control the hydraulic pressures of the valve disabling mechanisms for the cylinder at the left end of the cylinder array and the cylinder at the right end of the cylinder array. As a breather chamber  92  is provided on the right end portion of the upper surface of the cylinder head cover  35 , the right hydraulic pressure control valve  91  is positioned closely to the center of the engine E, but serves to control the hydraulic pressure of the valve disabling mechanism  66  for the cylinder  21  at the right end of the cylinder array. The working oil discharged from the two hydraulic pressure control valves  91  is branched at positions near the corresponding cylinders  21 , and supplied simultaneously to the four valve disabling mechanisms  66 . 
         [0040]      FIG. 8  is a cross-sectional view of each of the hydraulic pressure control valves  91 . The hydraulic pressure control valve  91  serves to control the level of the hydraulic pressure delivered to the valve disabling mechanism  66 . The hydraulic pressure control valve  91  serves to selectively connect and disconnect a working oil supply passage  93  and a working oil discharge passage  94  which are defined in the upper surface of the cylinder head cover  35 . The hydraulic pressure control valve  91  includes a housing  95  having an inlet port  96  communicating with the working oil supply passage  93  and an outlet port  97  communicating with the working oil discharge passage  94 . The housing  95  also has a valve body housing hole  98  defined centrally therein and having a closed end and an open end. The valve body housing hole  98  houses a spool valve body  99  slidably fitted therein. The open end of the valve body housing hole  98  is closed by a cap  100 . A first helical spring  102  for normally pushing the spool valve body  99  toward the cap  100  is housed in the valve body housing hole  98  near the close end thereof. The housing  95  has a communication passage  101  defined therein parallel to the valve body housing hole  98 . The communication passage  101  has an end connected through a small hole  101   a  to the input port  96  and another end connected to a solenoid-operated on-off valve  105  through a valve seat  104  having a connecting hole  103 . The solenoid-operated on-off valve  105  includes a valve body  106 , a second helical spring  107 , a solenoid coil  108 , and a power supply connector  109 . The second helical spring  107  normally pushes the valve body  106  in a direction to close the connecting hole  103 . 
         [0041]    A pilot oil chamber  110  is defined between the spool valve body  99  and the cap  100 , and is connected to the connecting hole  103  by a pressurization passage  111  defined in the housing  95 . The spool valve body  99  has a small-diameter central portion  112 . An oil filter  113  is held in place between the working oil supply passage  93  and the inlet port  96 . The housing  95  has an orifice  114  defined therein which provides fluid communication between the communication passage  101  and the outlet port  97 . Even when the spool valve body  99  is in a closed position, the inlet port  96  and the outlet port  97  communicate with each other through the small hole  101   a,  the communication passage  101 , and the orifice  114 , so that a low hydraulic pressure restricted by the orifice  114  is supplied at all times to the working oil discharge passage  94 . The housing  95  has an open port  115  defined in a lower portion thereof. When the spool valve body  99  is in a position to close the inlet port  96 , the open port  115  communicates with the outlet port  97  through the space around the small-diameter central portion  112  of the spool valve body  99 , allowing the hydraulic pressure in the working oil discharge passage  94  to be released into an inner space  35   a  in the cylinder head cover  35 . 
         [0042]    When the solenoid-operated on-off valve  105  of the hydraulic pressure control valves  91  is opened, the hydraulic pressure in the working oil supply passage  93  is supplied through the inlet port  96 , the small hole  101   a,  the communication passage  101 , the connecting hole  103 , and the pressurization passage  111  to the pilot oil chamber  110 , moving the spool valve body  99  in a direction to compress the first helical spring  102 . The inlet port  96  and the outlet port  97  are now brought into fluid communication with each other through the space around the small-diameter central portion  112  of the spool valve body  99 , and the outlet port  97  and the open port  115  are brought out of fluid communication with each other. Since the inlet port  96  and the outlet port  97  communicate with each other, the high hydraulic pressure level acts in the hydraulic pressure chamber  85  ( FIG. 6 ) of the valve disabling mechanism  66  through the working oil discharge passage  94  and the hydraulic pressure supply passage  84  ( FIG. 4 ), causing the valve disabling mechanism  66  to disable the intake valves  32  and the exhaust valves  33  in a closed state. 
         [0043]    When the solenoid-operated on-off valve  105  of the hydraulic pressure control valves  91  is closed, the spool valve body  99  is pushed to its original position by the first helical spring  102 , bringing the inlet port  96  and the outlet port  97  out of fluid communication with each other and also bringing the outlet port  97  and the open port  115  into fluid communication with each other. The hydraulic pressure in the hydraulic pressure chamber  85  of the valve disabling mechanism  66  is released, and the slide pin  69  of the valve disabling mechanism  66  moves to the position ( FIG. 5 ) to open and close the valves. The intake valves  32  and the exhaust valves  33  are now normally opened and closed as the cams  62 ,  63  are rotated. 
         [0044]      FIG. 9  is a plan view of the cylinder head cover  35 . On the upper surface of the cylinder head cover  35  are provided the breather chamber  92 , two secondary air chambers  117  for delivering air to the exhaust ports  29 , power supply connectors  109  for the four ignition plugs, and the two hydraulic pressure control valves  91 . The working oil supply passage  93  extends rearwardly from a working oil supply passage connector  120  on a front surface of the upper portion of the cylinder head cover  35 , extends to a region intermediate between the two hydraulic pressure control valves  91 , and is branched to the inlet ports  96  of the respective hydraulic pressure control valves  91 . The branched working oil supply passage  93  is also shown in  FIG. 7 . The power supply connectors  109  for the ignition pugs are also shown in  FIG. 7 . The working oil supply passage connector  120  is a member for interconnecting an externally mounted pipe oil passage  118 , to be described later, and the working oil supply passage  93 . 
         [0045]    The working oil discharge passages  94  for the working oil discharged from the hydraulic pressure control valves  91  extend from the respective outlet ports  97  of the hydraulic pressure control valves  91 . The working oil discharge passages  94  are also shown in  FIG. 10 . The working oil discharge passages  94  are connected to the hydraulic pressure supply passage  84  shown in  FIG. 4  for supplying the hydraulic pressure to the valve disabling mechanisms  66 . 
         [0046]      FIG. 10  is a vertical cross-sectional view of the engine E including the intake and exhaust valves. The externally mounted pipe oil passage  118  extends from a front surface of a lower portion of the crankcase  22  toward an upper portion of the cylinder head cover  35 . The crankcase  22  houses therein two oil pumps (not shown) that are actuated by the main shaft of the transmission through chains. One of the oil pumps serves to deliver a lubricating oil, and the other to operate the cylinder disabling mechanism. The working oil discharged from the lubricating oil pump is supplied through the oil filter  51  ( FIG. 3 ) to components to be lubricated of the engine E. The working oil discharged from the oil pump for the cylinder disabling mechanism flows through an oil filter, not shown, and is delivered from a working oil outlet connector  119  on the front surface of the lower portion of the crankcase  22  through the externally mounted pipe oil passage  118  to the front surface of the upper portion of the cylinder head cover  35 . The externally mounted pipe oil passage  118  has an upper end connected to the working oil supply passage connector  120  (see also  FIG. 9 ) on the front surface of the cylinder head cover  35 . As shown in  FIG. 9 , the working oil supply passage  93  extends from the working oil supply passage connector  120  rearwardly on the upper surface of the cylinder head cover  35 , extends to the region intermediate between the two hydraulic pressure control valves  91 , is then branched and connected to the inlet ports  96  of the respective hydraulic pressure control valves  91  for supplying the working oil. The working oil discharge passages  94  that are connected to the outlet ports  97  of the hydraulic pressure control valves  91  extend downwardly, as shown in  FIG. 10 , and are connected to the hydraulic pressure supply passage  84  shown in  FIG. 4  for supplying the hydraulic pressure to the valve disabling mechanisms  66 . 
         [0047]    The oil for disabling the cylinders flow from the front portion of the crankcase  22  to the hydraulic pressure chamber  85  of each valve disabling mechanism along the following route: 
         [0048]    the working oil outlet connector  119  ( FIG. 10 ), 
         [0049]    the working oil supply passage connector  120  ( FIGS. 9 ,  10 ), 
         [0050]    the working oil supply passage  93  ( FIGS. 8 ,  9 ), 
         [0051]    the inlet port  96  of the hydraulic pressure control valve  91  ( FIGS. 8 ,  9 ), 
         [0052]    the hydraulic pressure control valve  91  ( FIGS. 7 ,  8 ,  9 ), 
         [0053]    the outlet port  97  of the hydraulic pressure control valve  91  ( FIGS. 8 ,  9 ), 
         [0054]    the working oil discharge passage  94  ( FIGS. 8 ,  9 ,  10 ), 
         [0055]    the hydraulic pressure supply passage  84  ( FIGS. 4 ,  5 ,  6 ), and 
         [0056]    the valve disabling mechanism  66  ( FIGS. 4 ,  5 ,  6 ). 
         [0057]    The embodiment of the present invention described in detail above offers the following advantages: 
         [0058]    Since the oil passage extending to the hydraulic pressure control valve includes an externally mounted pipe, the oil passage can be installed more easily than if it is defined in the crankcase and the cylinder block as is the case with the background art. If the oil passage is disposed on a rear surface of the engine, the pipe is complex and long because of functional components including the intake system  37 , the starter motor  41 , the knock sensor  55 , the coolant hose  48 , etc. The oil passage disposed on the front surface of the engine allows the pipe to be easily installed and short. 
         [0059]    Since the working oil supply passage disposed on the upper portion of the cylinder head cover extends to a region intermediate between adjacent hydraulic pressure control valves and is then branched and connected to the inlet ports of the hydraulic pressure control valves, the working oil supply passage disposed on the upper portion of the cylinder head cover can be shortened. 
         [0060]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.