Abstract:
A valve unit of an internal combustion engine is accommodated in a space between a cylinder head and a rocker cover. The valve unit comprises a camshaft, a variable valve operating mechanism, a sensor, and a retaining member. The camshaft is provided with a cam for each cylinder. The variable valve operating mechanism receives a displacement of the cam, outputs a valve drive output, and continuously variable-controls the valve drive output in accordance with a rotational displacement of a control shaft provided substantially in parallel with the camshaft. The sensor detects the rotational displacement of the control shaft. The retaining member retains the camshaft, the variable valve operating mechanism, and the sensor. The retaining member expose the sensor to the out side of the rocker cover thereby to fix the camshaft, the variable valve operating mechanism, and the sensor to the cylinder head.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2006-297044, filed Oct. 31, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a valve unit of an internal combustion engine in which a valve characteristic of an intake valve or an exhaust valve is continuously controlled. 
         [0004]    2. Description of the Related Art 
         [0005]    In a valve unit of a multicylinder reciprocating engine (internal combustion engine) mounted on an automobile, in order to reduce fuel consumption by exhaust gas measures or by improving pumping loss, a variable valve in which a characteristic of an intake valve (or an exhaust valve) is continuously and variably controlled is incorporated in a head part of a cylinder head covered with a rocker cover. 
         [0006]    In most variable valve units, a structure is used in which a characteristic of an intake valve, e.g., an opening/closing timing or a valve lift amount is continuously varied by a rotational displacement of a control shaft received from a cam. A variable valve unit of this type is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-299536. 
         [0007]    In most methods of installing such a variable valve unit, a method is used in which, a cylinder head is mounted on a cylinder block on a main line for assembling engines, and each part of the variable valve unit is attached to each corresponding section of the cylinder head, thereby assembling the entire variable valve unit. 
         [0008]    In recent times, in order to increase production efficiency of the main line, on the main line, work in which only camshafts and valves are attached to a cylinder head is performed. On a sub-line separate from the main line, a method is used in which a variable valve unit constituting a part of a cylinder head from a camshaft to a valve is modularized. 
         [0009]    That is, only the variable valve unit, which is troublesome in assembly, is modularized on the sub-line, the modularized variable valve unit is returned to the main line, and the variable valve unit is attached to a cylinder head (which is already equipped with camshafts and valves). By doing so, a measure is used in which a working process taking much time is reduced on the main line. Assembling methods of such a type are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-299536 and Jpn. Pat. Appln. KOKAI Publication No. 2005-299538. 
         [0010]    Incidentally, the variable valve unit is required to continuously control valves of cylinders in accordance with the same valve characteristic so that a set performance can be exhibited in any operational state of an engine. For that purpose, the variable valve unit is required to undergo adjustment work for adjusting a valve drive output in accordance with a cam profile of each cam for each cylinder, thereby eliminating variation between cylinders. 
         [0011]    However, in the above adjustment for eliminating variation between cylinders, troublesome and considerably time-consuming fine adjustment work for making relationships between cams and parts of the variable valve unit for receiving the cams with respect to the respective cylinders so that the continuously variable valve characteristic can be appropriately exhibited is required. 
         [0012]    Particularly, in the technique disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2005-299536, a contrivance is employed in which an adjustment mechanism is incorporated in the variable valve unit, the adjustment mechanism having a structure in which a position of a part for receiving a cam is made adjustable, thereby facilitating the adjustment work. For this reason, the adjustment work can be performed only after the variable valve unit provided with parts for receiving cams of a camshaft is attached to the cylinder head provided with camshafts. Therefore, on the main line for assembling engines, considerably time-consuming adjustment work (adjustment for eliminating variation between cylinders) is still required, which is a factor for causing stagnation of the main line. 
         [0013]    Furthermore, in the adjustment for eliminating variation between cylinders, not only simply making positional relationships between cams and parts for receiving the cams uniform, but also making uniform the valve characteristics on the basis of the continuously variable control shaft is needed. Accordingly, work for attaching a sensor for detecting a rotational displacement of the control shaft, and work for adjusting the sensor is required on the main line. Such work is also a factor causing stagnation of the main line. Particularly, the sensor is an important part for continuously and variably controlling the valve characteristic. Therefore, the attaching of the sensor must be performed in consideration of maintenance because maintenance of the sensor is required in a state where the assembly of the engine is finished or after the engine is completed as a product. Considering these requirements, considerably difficult problems must be solved to eliminate the stagnation of the main line. 
       BRIEF SUMMARY OF THE INVENTION 
       [0014]    An object of the present invention is to provide a valve unit of an internal combustion engine that can improve the productivity of the internal combustion engine. 
         [0015]    The valve unit of the present invention comprises: a camshaft provided with a cam for each cylinder; a variable valve operating mechanism for receiving a displacement of the cam, outputting a valve drive output, and continuously variable-controlling the valve drive output in accordance with a rotational displacement of a control shaft provided substantially in parallel with the camshaft; a sensor for detecting the rotational displacement of the control shaft; and a retaining member for retaining the camshaft, the variable valve operating mechanism, and the sensor, wherein the sensor is exposed to the outside of the rocker cover, thereby fixing the camshaft, the variable valve operating mechanism, and the sensor to the cylinder head through the retaining member. 
         [0016]    That is, in the valve unit, the camshaft and the sensor are also combined with the valve unit, and hence the valve unit becomes a structure in which cylinder-to-cylinder variation can be adjusted singly. In other words, unlike the conventional case, it is possible not only to complete the assembly of the valve unit on a line separate from the line for assembling internal combustion engines but also to perform adjustment of cylinder-to-cylinder variation, e.g., adjustment of cylinder-to-cylinder variation using a simulation system in which a cylinder head of an internal combustion engine is simulated. Accordingly, the work required on the main line is only work for attaching a valve unit, for which adjustment has already been finished, to a cylinder head on the main line. The cylinder-to-cylinder variation adjustment work and the troublesome work for attaching the sensor and adjusting the sensor, which become factors causing stagnation on the main line, are made unnecessary. Furthermore, the sensor is attached to the cylinder head in a state where it is arranged outside the rocker cover, and hence maintenance thereof can be facilitated. 
         [0017]    In a desirable aspect of the present invention, a configuration including an adjustment mechanism capable of adjusting the valve drive output for each cylinder is employed in the variable valve operating mechanism. 
         [0018]    In another desirable aspect of the present invention, the configuration is made such that a sensor for detecting the rotational displacement is arranged at an axial end of the control shaft, and the other end of the control shaft is coupled to an actuator mechanism for rotationally displacing the control shaft. 
         [0019]    In another desirable aspect of the present invention, the retaining member comprises a holder member for holding one side of the camshaft in the diametric direction, the variable valve operating mechanism, and the sensor, a cap member for holding remaining one side of the camshaft, and a fixing bolt member which penetrate the holder member and the cap member, and can be screwed into the cylinder head. 
         [0020]    In a further desirable aspect of the present invention, the Plurality of retaining members are provided so as to axis-support at least both ends of the camshaft and the control shaft, and the retaining members are connected to each other by the camshaft and the control shaft. 
         [0021]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0022]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
           [0023]      FIG. 1  is a partial cutaway perspective view showing a cylinder head of an internal combustion engine according to an embodiment of the present invention together with a valve unit mounted on the cylinder head. 
           [0024]      FIG. 2  is an exploded perspective view showing the modularized variable valve unit together with peripheral units and devices. 
           [0025]      FIG. 3  is an exploded perspective view for explaining structures of parts of the variable valve unit. 
           [0026]      FIG. 4  is a cross-sectional view around a sensor taken along a line indicated by an arrow A in  FIG. 1 . 
           [0027]      FIG. 5  is a cross-sectional view around the cylinder head taken along line B indicated by an arrow B in  FIG. 1 . 
           [0028]      FIG. 6  is a cross-sectional view around the cylinder head taken along line C indicated by an arrow B in  FIG. 1 . 
           [0029]      FIG. 7  is a cross-sectional view taken along line D-D in  FIG. 2 . 
           [0030]      FIG. 8  is a cross-sectional view showing an engine equipped with a valve unit of an internal combustion engine according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    A variable valve unit of an internal combustion engine according to a first embodiment of the present invention will be described below with reference to  FIGS. 1 to 7 .  FIG. 1  shows a perspective view of a head part of an engine main body in a reciprocating inline four-cylinder petrol engine, which is an example of a multicylinder internal combustion engine.  FIG. 2  is a perspective view showing a state where the head part is disassembled.  FIGS. 4 to 7  are cross-sectional views respectively showing states where respective parts (cross sections taken along lines A, B, and-C in  FIG. 1 , and line D-D in  FIG. 2 ) are cross-sectioned. 
         [0032]    A reference numeral  1  in  FIG. 1  denotes a cylinder head mounted on a head part of a cylinder block  2  (shown in only  FIG. 5  by two-dot chain lines). A reference numeral  3  denotes a rocker cover covering an upper part of the cylinder head  1 . A reference numeral  4  denotes SOHC type variable valve unit which is in a space between the cylinder head  2  and the rocker cover  3 . The variable valve unit  4  is an example of a valve unit of the present invention. 
         [0033]    The cylinder head  1  is provided with a head main body  1   x.  As shown in  FIGS. 1 ,  2 , and  5 , the head main body  1   x  is surrounded by a peripheral wall  1   a  at an upper part thereof. A top surface  1   y  of the head main body  1   x  is made lower than a rocker cover attaching seat  1   b  formed at an upper end part of the peripheral wall  1   a  as shown in  FIGS. 5 and 6 . 
         [0034]    Combustion chambers  7  (shown in only  FIG. 5 ) are formed on an undersurface of the head main body  1   x  so as to correspond to four cylinders  6  (shown by two-dot chain lines in only  FIG. 5 ) formed in the cylinder block  2 . A pair of intake ports  8  and a pair of exhaust ports  9  (both of which are shown in a part of  FIG. 5 ) which extend from the combustion chamber  7  are formed on both sides (in the width direction) of the head main body  1   x.    
         [0035]    To the intake ports  8  of these ports, a pair of normally-closed intake valves  8   a  are attached. A pair of normally-closed exhaust valves  9   a  are attached to the exhaust ports  9 . Stem ends of the valves  8   a  and  9   a  protrude upwardly from the top surface  1   y  of the head main body  1   x.  Incidentally, for example, an ignition plug is attached to each combustion chamber  7 , and an injector is attached to each cylinder (both are not shown). 
         [0036]    In the variable valve unit  4 , a modularized structure in which various parts are assembled is employed. To specifically describe the modularization, as shown in, for example,  FIGS. 2 and 5 , a variable valve operating mechanism  13  of the intake side having an adjusting function, a control shaft  14  (serving also as a rocker shaft for intake) for controlling the variable valve operating mechanism  13 , a camshaft  15 , a shaft displacement detection sensor  16  (corresponding to the sensor of the present application) for detecting a rotational displacement of the control shaft  14 , and a rocker arm mechanism  17  (only a part thereof is shown in  FIG. 5 ) of the exhaust side are assembled by using a plurality of (five) retaining members  11   a  to  11   c  (only three representative ones are shown). 
         [0037]    Structures of respective parts will be described below. The retaining members  11   a  to  11   c  are, as shown in  FIGS. 1 and 2 , parts each having a wall-shape divided in accordance with an arrangement of each of the cylinders  6  (four), and arranged at the foremost part of the cylinder array, between the cylinders, and at the backmost part in parallel with each other. Incidentally, the retaining members may be only the foremost and backmost members in the case of modularization. However, it is desirable that the retaining member be provided between the cylinders in consideration of the rigidity and the like. 
         [0038]    As shown in  FIG. 3 , a two-piece structure provided with a wall-shaped holder member  18   a  extending in the width direction (direction perpendicular to the cylinder array direction) of the cylinder head  1 , and a cap member  18   b  to be combined with the holder member  18   a  at a lower end part thereof, and a structure in which a holder member  18   a,  a cap member  18   b,  and a plurality of fixing bolt members  18   c  to be attached to the members  18   a  and  18   b  so as to penetrate the members  18   a  and  18   b  are combined with each other are used for these retaining members  11   a  to  11   c.    
         [0039]    Of the above members, each of the holder members  18   a  has the same structure, and as shown in  FIG. 3 , an intake rocker shaft retaining hole  20   a  and an exhaust rocker shaft retaining hole  20   b  arranged in the lateral direction with a predetermined interval between them are formed in the middle stage on both sides of each holder member  18   a.  On a top surface of the holder member  18   a,  an arcuate attaching seat  21  is formed at a position between the intake rocker shaft retaining hole  20   a  and the exhaust rocker shaft retaining hole  20   b  and closer to the hole  20   b.  On an undersurface of the holder member  18   a,  a semicircular journal bearing surface  22  is formed at a position between the intake rocker shaft retaining hole  20   a  and the exhaust rocker shaft retaining hole  20   b  and closer to the hole  20   b.  The entire undersurface of the holder member  18   a  except for the bearing surface  22  is used as a cap attaching seat  23 . 
         [0040]    For example, a plate-like member having an arcuate recession at a central part thereof is used as the cap member  18   b.  A semicircular journal bearing surface  25  is formed at the central part on a top surface of the cap member  18   b,  and the entire top surface except for the bearing surface  25  is used as a cap attaching surface  26 . Incidentally, flat undersurface parts on both sides on the undersurface of the cap member  18   b  between which the journal bearing surface  25  is interposed are used as a module installation seat surface  27 . 
         [0041]    Each of the foremost holder member  18   a  and cap member  18   b  has, unlike the other members, a pair of leg parts  29  formed so as to externally extend on both sides thereof. A journal bearing surface  22 , a cap attaching seat  23 , a journal bearing surface  25 , a cap-attaching surface  26 , and a seat surface  27  are also formed on the pairs of the leg parts  29 . 
         [0042]    Incidentally, through holes  28  in which head bolts (not shown) are inserted are formed in the leg parts  29 . A sensor attaching part  30  is formed on the holder member  18   a  arranged at the backmost position. In the sensor attaching part  30 , as shown in  FIGS. 3 and 4 , a structure in which a cylinder part  31   a  extending from the intake rocker shaft retaining hole  20   a  toward the backmost position is formed, and a fan-shaped sensor attaching boss  31   b  is formed at a distal end of the cylinder part  31   a  is employed. 
         [0043]    In the respective intake rocker shaft retaining holes  20   a,  as shown in  FIGS. 2 and 3 , a control shaft  14  (constituted of a hollow member) serving also as the intake side rocker shaft is rotatably inserted so as to allow the shaft  14  to extend from the foremost retaining member  11   a  to the backmost retaining member  11   c.  The exhaust side rocker shaft  34  (constituted of a hollow member) is inserted in the respective exhaust rocker shaft retaining holes  20   b  so as to allow the shaft  34  to extend from the foremost retaining member  11   a  to the backmost retaining member  11   c.  Likewise, a support shaft  35  (constituted of a hollow member) is fitted in the respective attaching seats  21  so as to allow the shaft  35  to extend from the foremost retaining member  11   a  to the backmost retaining member  11   c.    
         [0044]    Likewise, the camshaft  15  is arranged between the respective journal bearing surfaces  22  and the journal bearing surfaces  25  so as to allow the shaft  15  to extend from the foremost retaining member  11   a  to the backmost retaining member  11   c.  A plurality of journals  37  (shown in  FIG. 6 ) formed on the shaft part of the camshaft  15  are received between the journal bearing surfaces  22  and the journal bearing surfaces  25 , thereby rotatably supporting the camshaft  15 . 
         [0045]    Incidentally, each of parts of the camshaft  15  between the respective journals  37  (between the cylinders) includes a cam group constituted of an intake cam  38   a  arranged in the center and (two) exhaust cams  38   b  arranged on both sides. 
         [0046]    The variable valve operating mechanism  13  (intake side) is attached to parts of the support shaft and the control shaft between the above-mentioned holder members  18   a,  and the rocker arm mechanism  17  (exhaust side) is attached to parts of the exhaust rocker shaft  34  (for each cylinder). 
         [0047]    Here, the respective mechanisms will be described below. As shown in  FIGS. 3 and 5 , a valve drive mechanism of a type called a swing cam type in which a swing cam  50  is used, for example, a mechanism in which a rocker arm  40 , a swing cam  50 , and a center rocker arm  60  are combined with each other is used as the variable valve operating mechanism  13 . 
         [0048]    The above elements will be described below. As the rocker arm  40 , the one having a bifurcate arm shape is used. Specifically, the rocker arm  40  is provided with a pair of L-shaped rocker arm pieces  43  having needle rollers  41  rotatably provided between one ends of the pieces  43  and having adjust screw sections  42  serving as valve drive sections provided at the other ends of the pieces  43 . 
         [0049]    Further, a part of the control shaft  14  between the holder members  18   a  is swingably inserted in a pair of support holes  44  formed in intermediate parts of the respective rocker arm pieces  43 . Further, the needle rollers  41  are arranged on the support shaft  35  side, and the pair of adjust screw sections  42  are arranged on the opposite side of the support shaft  35 . 
         [0050]    As shown in  FIGS. 3 and 5 , a structure in which a supporting boss  52  having a cylindrical shape is provided at one end of an arm section  51 , a cam surface  53  extending in the vertical direction is provided at the other end of the arm section  51 , and a slide roller  54  is rotatably embedded in the lower part of the arm section  51  in such a manner that the outer circumferential surface thereof is exposed from the lower side is used for the swing cam  50 . 
         [0051]    Incidentally, reference numeral  54   a  denotes a shaft member for supporting the slide roller  54 . A part of the support shaft  35  between the holder members  18   a  is swingably fitted in the supporting boss  52 . As a result of this, the cam surface  53  at the distal end of the arm section  51  is in rolling contact with the needle rollers  41 . 
         [0052]    A pusher receiving rib  56  is protruded from an upper part of the supporting boss  52 . A pusher  57  having, for example, a piston structure is combined with the rib  56  at a lower position of the rib  56  in an inclined posture. This pusher  57  is supported by fitting a C-shaped leg section  58  formed on the side part thereof on a part of the exhaust side rocker shaft  34 . 
         [0053]    Incidentally, an installation seat  59  is formed at a lower part of the pusher  57 . A structure is made such that when the variable valve unit  4  is attached to the cylinder head  1  by means of the installation seat  59 , an energizing force is imparted to the swing cam  50  (this is because when the installation seat  59  is provided on the cylinder head  1 , the pusher is rotationally displaced using the rocker shaft  34  as a fulcrum). 
         [0054]    The center rocker arm  60  is, as shown in  FIGS. 3 and 5 , constituted of an L-shaped part arranged at a position surrounded by the intake cam  38   a,  slide roller  54 , and control shaft  14 . 
         [0055]    The center rocker arm  60  includes a relaying arm section  61  extending toward the slide roller  54  above, and a fulcrum arm section  62  extending toward a part immediately below a part of the control shaft  14  located at a lateral position. 
         [0056]    An inclined surface  65  for controlling the movement of the swing cam  50  is formed on a distal end surface of the relaying arm section  61 . This inclined surface is a flat surface having a lower part on the control shaft  14  side and a higher part on the rocker shaft  34  side. Further, a slide roller  63  is supported at an intermediate part at which both the arm sections  61  and  62  intersect each other so as to be rotatable in the same direction as the intake cam  38   a.    
         [0057]    Further, in the relaying arm section  61  interposed between the intake cam  38   a  and the swing cam  50 , the slide roller  63  is in rolling contact with the cam surface of the intake cam  38   a,  and the inclined surface  65  of the relaying arm section  61  is bumped against an outer circumferential surface of the slide roller  54  of the swing cam  50 . As a result of this, the displacement of the intake cam  38  is transmitted to the swing arm  50  through the relaying arm section  61 . 
         [0058]    Further, a support pin  66  is flexibly supported on the fulcrum arm section  62  by means of a pin  67 . A distal end of the support pin  66  is rotatably inserted in a through hole  68  formed on the lower side of the control shaft  14  in a direction perpendicular to the axial direction, whereby the control shaft  14  is caused to support the center rocker arm  60 . 
         [0059]    By virtue of this support, when the control shaft is  14  rotationally moved, the rocker arm  60  that swings around the pin  67  (end of the support pin  6 ) serving as a fulcrum can move in a direction intersecting the camshaft  15  (in the lead angle direction or the lag angle direction) while changing the position at which the rocker arm  60  is in rolling contact with the center intake cam  38   a.    
         [0060]    In this movement, the opening/closing timing and the valve lift amount of the intake valve  8   a  can be simultaneously and continuously varied. That is, the upper part of the cam surface  53  is a base circle section (formed by, for example, an arcuate surface having the axis of the support shaft  35  as a center thereof), and the lower part of the cam surface  53  is a lift section (formed by, for example, an arcuate surface having the same shape as the cam shape of the lift region of the intake cam  38   a ) continuing from the base circle section. 
         [0061]    When the slide roller  63  of the center rocker arm  60  moves in the lead angle direction or the lag angle direction of the intake cam  38   a,  the posture of the swing cam  50  is changed, and the region of the cam surface  53  in which the needle rollers  41  move is changed. 
         [0062]    In other words, the ratio of the base section to the lift section in which the needle rollers  41  travel is changed. A change in the ratio of the base section to the lift section accompanied by a change in the phase in the lead angle direction and a change in the phase in the lag angle direction continuously changes the valve lift amount of the intake valve  8   a  while largely changing the opening/closing timing of the intake valve  8   a.  In this case, the valve-closed period is more changed than the valve-opened period. This is output from the rocker arm  40  as the valve drive output. At this time, in order to prevent the alignment of the slide roller  54  with the inclined surface  65  from being shifted, a pair of guide walls  51   b  extending from wall sections  51   a  sandwiching the slide roller  54  from both sides (in the width direction) to both sides of the distal end of the relaying arm section  61  bumping against the slide roller  54  are formed on the wall sections  51   a  as shown in  FIGS. 3 and 5 . 
         [0063]    Specifically, the guide walls  51   b  are provided in such a manner that they cover the contact point at which the slide roller  54  of the swinging swing cam  50  and the inclined surface  65  of the center rocker arm  60  are in contact with each other. As a result of this, the center rocker arm  60  is prevented from being shaken around the support pin  66  serving as a fulcrum. To the part of the control shaft  14  in which the support pin  66  is inserted, an adjustment mechanism  70  is attached as shown in  FIGS. 3 and 5 . In the adjustment mechanism  70 , a structure is employed in which a threaded hole  71  continuing from the through hole  68  and opening upwardly is formed at, for example, the part of the control shaft  14 , and, for example, a screw member  73  having a slot  72  for screw-driving at a head part thereof is screwed into the threaded hole  71  so that it can be advanced or retreated. 
         [0064]    In other words, the adjustment mechanism  70  has a structure in which the protrusion amount of the support pin  66  is changed by a rotating operation of the screw member  73 , whereby the rolling contact position of the slide roller  63  is changed. Further, a change in the rolling contact position of the slide roller  63  changes the posture of the center rocker arm  60  and the posture of the swing cam  50 , thereby adjusting the valve opening/closing timing and the valve lift amount (each of which is a valve characteristic). The screw member  73  is locked by a locknut  74 . Incidentally, a reference numeral  75  denotes a notch forming a seat surface of the lock nut  74 . 
         [0065]    A proximal end part of an arm member  78  extending in the radial direction of the control shaft  14 , i.e., in this case, extending upwardly is fixed (by a screw) to an end of the control shaft  14  protruding from the foremost holder member  18   a  by means of, for example, a screw member  77  as shown in  FIGS. 2 and 3 . Rotational movement necessary for continuous control of the valve characteristic is input from the end of the arm member  78 . 
         [0066]    In the rocker arm mechanism  17  (exhaust side), a structure is employed in which a pair of rocker arms  80  are rotatably assembled on both sides of the leg section  58  of the pusher  57  at a part of the rocker shaft  34 , as shown in  FIGS. 3 and 5 . 
         [0067]    Specifically, each of the rocker arms  80  has a support hole  81  at an intermediate part thereof, has a roller member  82  serving as a contact piece at an end thereof, and has an adjusting screw section  83  serving as a valve drive section at the other end thereof. 
         [0068]    Then, the part of the rocker shaft  34  between the holder member  18   a  and the leg section  58  (pusher  57 ) is swingably inserted in the support holes  81  of the rocker arms  80 . Each of the roller members  82  is arranged on the exhaust cam  38   b  side and the adjust screw section  83  is arranged on the opposite side. That is, each rocker arm  80  is in a state where it can be combined with the exhaust valve  9   a.    
         [0069]    As shown in  FIGS. 3 ,  6 , and  7 , the fixing bolt member  18   c  is inserted from the seat surface  90  formed on the top surface of each holder member  18   a  directly above the rocker shaft  34 . The fixing bolt member  18   c  linearly penetrates (skewers) a central part of the rocker shaft  34  in the radial direction, a wall part on the support shaft  35  side (one side of the camshaft  15 ), the support shaft  35  being adjacent to the camshaft  15 , and the cap part of the cap member  18   b  on the rocker shaft  34  side. 
         [0070]    The fixing bolt  18   c  is obliquely inserted from each seat surface  21   a  formed in the upper surface of the support shaft  35  arranged at the highest position. As a result of this oblique insertion, the wall part between the rocker shaft  34  and the control shaft  14 , the wall part between the camshaft  15  and the control shaft  14 , and the cap part of the cap member  18   b  on the control shaft  14  side are obliquely and linearly penetrated (skewered) by the fixing bolt  18   c.    
         [0071]    However, reference numerals  92  and  93  denote bolt insertion holes (only a part of them is shown in  FIG. 3 ), which are formed linearly or obliquely linearly in the holder member  18   a  and the cap member  18   b.  Incidentally, as for the number of the fixing bolt members  18   c  to be inserted obliquely, one bolt member  18   c  is used in the holder member  18   a  and the cap member  18   b  arranged at the foremost part or the backmost part, and two bolt members  18   c  are used in the holder member  18   a  and the cap member  18   b  arranged between the cylinders to which a load heavier than that applied to the member  18   a  and  18   b  arranged at the foremost part or the backmost part is applied (because a load incidental to the variable valve motion is applied to the member  18   a  and  18   b  from both sides). 
         [0072]    Furthermore, the shaft displacement detection sensor  16  for detecting the rotational displacement of the control shaft  14  is detachably attached to the sensor attaching boss  31   b  provided on the backmost holder member  18   a  by means of, for example, screws. 
         [0073]    That is, in the variable valve unit  4 , the variable valve operating mechanism  13 , the shaft displacement detection sensor  16 , the rocker arm mechanism  17  of the exhaust side, the camshaft  15 , and the adjustment mechanism  70  are modularized into a structure by the method in which each part is attached to a frame-like structure having high rigidity constituted of the shafts  14 ,  34 , and  35  and the retaining members  11   a  to  11   c.    
         [0074]    Accordingly, each of the shafts  14 ,  34 , and  35  plays a role of the frame, and hence the retaining members  11   a  to  11   c  can be provided solely at positions where they are required without increasing the size, and the weight of the variable valve unit  4  itself can be minimized. 
         [0075]    Further, the shaft displacement detection sensor  16  is positioned so as to be protruded from the cylinder head  1  and the rocker cover  3  to the outside by appropriately setting in advance the cylinder part  31   a  and the sensor attaching boss  31   b.  Thus, when the variable valve unit  4  is accommodated in a space between the cylinder head  1  and the rocker cover  3 , the entire assembly other than the shaft displacement detection sensor  16  can be accommodated in the space between the cylinder head  1  and the rocker cover  3 , and only the shaft displacement detection sensor  16  is exposed to the outside. 
         [0076]    By virtue of such modularization, the variable valve unit  4  becomes a structure in which cylinder-to-cylinder variation can be adjusted singly. Thus, in the variable valve unit  4 , cylinder-to-cylinder variation and the sensor output can be adjusted before the mechanism  4  is attached to the cylinder head  1 . As a result, in the variable valve unit  4 , cylinder-to-cylinder variation and the sensor output is adjusted before the mechanism  4  is attached to the cylinder head  1 , and then the mechanism  4  is attached to the cylinder head  1  as shown in  FIGS. 2 and 7 . 
         [0077]    This point will be specifically described below. It is recommended for the variable valve unit  4 , modularized before it is attached to the cylinder head  1 , to be subjected to adjustment of cylinder-to-cylinder variation and the sensor output on a sub-line separate from the mainline for assembling engines by using a simulation system in which a cylinder head of an engine is simulated. 
         [0078]    For example, a modularized variable valve unit  4  is attached to a simulated cylinder head, and a simulated drive apparatus (not shown) is also attached thereto. It is only required to adjust the opening/closing timing and the valve lift amount so as to be uniform and appropriate in the respective cylinders with respect to the target lift by advancing or retreating the screw member  73  of the adjustment mechanism  70  of each cylinder, and attaching the shaft displacement detection sensor  16  so that a signal output conforming to the target lift can be obtained. 
         [0079]    The variable valve unit  4  that has been adjusted is transferred by using a jig or a transportation apparatus (both are not shown) as it is so that the adjustment can be maintained so as to be set at a regular position of an actual cylinder head  1  (assembly of a cylinder block is already finished) on the main line for assembling engines, i.e., at module installation seats  94  and  300  (seat surfaces for receiving the seat surface  27 : shown in  FIGS. 5 to 7 ), for example, already formed on the top surface  1   y.    
         [0080]    Specifically, both the side parts (including the leg parts  29 ) of the cap member  18   b  are placed on the module installation seats  94  and  300 , and the threaded part  18   d  (formed only at each distal end) of each of the fixing bolt members  18   c  on both sides protruding from parts near both sides of the camshaft  15  is screwed into each of the threaded holes  18   e  (shown in only  FIG. 7 ) formed in the module installation seats  94  and  300 . 
         [0081]    As a result, the already adjusted variable valve unit  4  is attached to the top surface  1   y  of the cylinder head  1  on the main line. Incidentally, each adjust screw section  42  of the intake side rocker arm  40  is arranged at a stem end of the intake valve  8   a.  Each adjust screw section  83  of the exhaust side rocker arm  80  is arranged at a stem end of the exhaust valve  9   a.    
         [0082]    The installation seat  59  bumps against the installation seat  1   c  (shown in  FIGS. 1 and 5 ) formed on the inner surface of the peripheral wall  1   a  of the cylinder block  1 , the entire pusher  57  is supported by the leg section  58 , and the swing cam  50  is energized in a direction in which a distal end thereof is forced down. 
         [0083]    On the other hand, as shown in  FIGS. 1 to 3 , a driving source apparatus for driving the variable valve operating mechanism  13 , for example, an electrically-driven actuator unit  95  (corresponding to an actuator) is installed at the foremost part of the cylinder head  1 . 
         [0084]    The electrically-driven actuator unit  95  includes a motor section  96  of a lateral (width direction of the cylinder head  1 ) type arranged outside the peripheral wall  1   a  of the cylinder head  1 , a reduction gear section  97  (for reducing the motor output) connected to the front part of the motor section  96 , and a screw shaft  99  connected to the output section of the reduction gear section  97  through a universal joint  98 . These are formed into one part as a driving unit. 
         [0085]    This electrically-driven actuator unit  95  is attached to the cylinder head  1  in a direction in which the axis thereof intersects the variable valve unit  4  by fixing the leg section  97   b  formed on the casing  97   a  of the reduction gear section  97  to the top surface  1   y  of the cylinder head  1  or the rocker cover attaching seat  1   b  by means of bolts. 
         [0086]    In this way, the motor section  96  is caused to protrude toward the outside of the cylinder head  1 , and the screw shaft  99  is caused to extend to the arm member  78  end (variable arm mechanism  13 ) side. That is, the screw shaft  99  extends to the opposite side of the motor section  96 . 
         [0087]    Incidentally, a part of the peripheral wall  1   a  or the rocker cover  3  at which the electrically-driven actuator unit penetrates the wall  1   a  or the cover  3  is formed into a fan-shaped opening. 
         [0088]    A nut member  100  is screw-fitted on the screw shaft  99  so that it can be advanced or retreated. The nut member  100  is constituted of a pin-shaped member having a flange part  100   c  at one end thereof, and having a threaded through hole  100   a  formed in the axial direction thereof at an axis part thereof. The thread hole  100   a  penetrates the nut member  100  in the diameter direction. The threaded hole  100   a  of the nut member is screw-fitted on the screw shaft  99  so that it can be advanced or retreated. This nut member  100  is attached to the distal end of the arm member  78 , and the control shaft  14  can be driven by the electrically-driven actuator unit  95 . 
         [0089]    That is, the nut member  100  is rotatably fitted in a support cylinder  78   a  formed at the distal end of the arm member  78  (variable valve unit  4 ) and, for example, a C-shaped clip member  100   b  is fitted on the distal end of the nut member  100  so as to allow it to prevent the nut member  100  from slipping off the support cylinder  78   a,  thereby attaching the nut member  100  to the arm member  78 . 
         [0090]    The part of the screw shaft on both sides of the nut member  100  penetrates a pair of elongated holes formed on both sides of the peripheral wall of the support cylinder  78   a  and extending in the circumferential direction. When the motor section  96  is operated, the screw shaft  99  is rotated, and the nut member  100  is moved along the screw shaft  99  which is swingable. As a result, the arm member  78  is swung, and the control shaft  14  is rotated. In other words, by the driving of the electrically-driven actuator unit  95 , the opening/closing timing of the intake valve  8   a  and the valve lift amount can be continuously controlled. 
         [0091]    As shown in  FIG. 2 , the rocker cover  3  is formed into a box-like shape in accordance with the shape of the cylinder head  1 . Further, at parts of the peripheral edge of the rocker cover corresponding to the penetration position of the shaft displacement detection sensor  16  and the motor section  96   a,  fan-shaped notch parts  3   a  (only a notch part for the sensor is shown in  FIG. 4 ) for allowing the shaft displacement detection sensor  16  or the motor section  96   a  to penetrate the rocker cover while sealing the penetration parts are formed. 
         [0092]    This rocker cover  3  is set on the rocker cover attaching seat  1   b  formed at the peripheral edge of the cylinder head  1  as shown in  FIGS. 1 and 4 . As a result, of the units and devices to be mounted on the cylinder head  1 , the shaft displacement detection sensor  16  and the motor section  96  are exposed to the outside of the rocker cover  3 , and the remaining variable valve unit  4 , and the greater part of the electrically-driven actuator unit  95  are accommodated in the closed space between the cylinder head  1  and the rocker cover  3 . 
         [0093]    The shaft displacement detection sensor  16  is exposed to the outside of the rocker cover  3 , and hence the shaft displacement detection sensor  16  can be replaced from outside while the rocker cover  3  is closed. 
         [0094]    As described above, the variable valve unit  4  becomes a structure in which cylinder-to-cylinder variation and the sensor output can be adjusted singly by modularizing the camshaft  15 , the shaft displacement detection sensor  16 , and the adjustment mechanism  70 . As a result of this, the cylinder-to-cylinder variation adjustment and the sensor output adjustment, which require troublesome operations, can be performed at a place separate from the main line for assembling engines. 
         [0095]    Accordingly, the only work required on the main line for engine assembly is that for attaching a variable valve unit  4  for which the cylinder-to-cylinder variation adjustment and the sensor output adjustment have been finished to a cylinder head  1  on the main line. The cylinder-to-cylinder variation adjustment work and the troublesome work for attaching the shaft displacement detection sensor  16 , which are regarded as factors in stagnation, are made unnecessary. 
         [0096]    Therefore, the productivity of engines can be improved. Moreover, the shaft displacement detection sensor  16  is exposed to the outside of the rocker cover ( FIG. 4 ), and hence, in a completely assembled engine or an engine completed as a product, even when maintenance of the shaft displacement detection sensor  16  is required, it is easily possible to cope with the requirement. 
         [0097]    Particularly, the shaft displacement detection sensor  16  can be replaced from outside the rocker cover  3 , and the replacement work of the sensor  16  can therefore be easily performed. Even when replacement of the shaft displacement detection sensor  16  is required after the engine is completed as a product, it is possible to quickly cope with the requirement. 
         [0098]    Furthermore, the shaft displacement detection sensor  16  for detecting the rotational displacement is arranged at one end of the control shaft  14 , whereby the rotational displacement can be directly detected, adjustment accuracy can be enhanced, and accurate control can be enabled. 
         [0099]    The other end of the control shaft  14  is coupled to the electrically-driven actuator unit  95 , whereby even the elastic torsion of the control shaft  14  caused by valve lift reaction force can be detected as the rotational displacement, and accurate control is enabled. 
         [0100]    Moreover, in the variable valve unit  4 , modularization including the shaft displacement detection sensor  16  and the arm member  78 , which is an actuator coupling member, is enabled, and hence the number of assembly man-hours can be reduced. 
         [0101]    Furthermore, by employing the structure in which the holder members  18   a  for holding the one side of the camshaft  15  in the diametric direction, the variable valve operating mechanism  13 , the adjustment mechanism  70 , and the shaft displacement detection sensor  16 , the cap members  18   b  for holding the remaining one side of the camshaft  15 , and the fixing bolt members  18   c  penetrating the holder members  18   a  and the cap members  18   b  are combined with each other as the retaining members  11   a  to  11   c,  the fixing bolt members  18   c  used for attachment to the cylinder head  1  can also be used as parts for modularization as they are, and the work for modularization and the adjustment work are performed on the basis of the fixing bolt members  18   c  set as the standard, and hence highly accurate modularization of the variable valve unit  4  and highly accurate adjustment can be performed. 
         [0102]    Next, a valve unit of an internal combustion engine according to a second embodiment of the present invention will be described below with reference to  FIG. 8 . Incidentally, a configuration having the same function as the first embodiment will be denoted by using the same reference symbols as those in the first embodiment, and explanation of them will be omitted. 
         [0103]    This embodiment differs from the first embodiment in including a variable valve operating mechanism  200  in place of the variable valve operating mechanism  13 . The other part of the structure may be identical to the first embodiment. The point of the second embodiment different from the first embodiment will be specifically described below. 
         [0104]      FIG. 8  is a cross-sectional view showing an engine  10  of this embodiment. As shown in  FIG. 8 , in this embodiment, the engine is provided with the variable valve operating mechanism  200  in place of the variable valve operating mechanism  13 . The variable valve operating mechanism  200  has a function of adjusting the opening/closing operation of an exhaust valve  9   a  and not that of an intake valve  8   a.    
         [0105]    The variable valve operating mechanism  200  has a structure in which the intake side and the exhaust side are replaced with each other in the structure of the variable valve operating mechanism  13  described in the first embodiment (accordingly, the configuration having the same function as the first embodiment is denoted by the same reference symbols). 
         [0106]    In the variable valve operating mechanism  200 , the control shaft  14  doubles as a rocker shaft of the exhaust side. Further, on the intake side, an intake rocker shaft  201  is provided in place of the control shaft  14 . 
         [0107]    An intake valve rocker arm (not shown) is attached to the intake rocker shaft  201 . The intake valve rocker arm drives (opens/closes) the intake valve  8   a.  A structure for driving the intake valve  8   a  in this embodiment may be a mirror image structure of the structure for driving the exhaust valve  9   a  in the first embodiment. 
         [0108]    Even when the variable valve operating mechanism  200  has a structure in which driving of the exhaust valve  9   a  can be adjusted as in this embodiment, the same advantage as in the first embodiment can be obtained. 
         [0109]    Incidentally, the present invention is not limited to the firs and second embodiments described above, and may be variously modified and implemented within the scope not deviating from the gist of the present invention. For example, the variable valve operating mechanism of the swing cam type is described as an example in the first and second embodiments. The present invention is not limited to this, and a variable valve operating mechanism of another structure may be used. 
         [0110]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.