Patent Publication Number: US-7717073-B2

Title: Collar and variable valve actuation mechanism

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2004-140022, filed on May 10, 2004, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a collar for receiving a shaft for a variable valve lift mechanism in a multiple-cylinder internal combustion engine. 
   Japanese Laid-Open Patent Publication No. 2001-263015 describes a variable valve actuation mechanism for an internal combustion engine. The variable valve actuation mechanism includes a variable valve lift mechanism, which is arranged for each cylinder to adjust the lift amount of intake and exhaust valves. A support pipe (rocker shaft) extends through the center of the variable valve lift mechanism. A control shaft is arranged in the support pipe. The variable valve lift mechanism is pivoted in a state supported by the support pipe. The lift amount of the valve is adjusted by moving the control shaft in the axial direction. 
   The support pipes are supported by a plurality of supports arranged on a cylinder head between the variable valve lift mechanisms. The supports position the variable valve lift mechanisms in the axial direction. The valve lift mechanisms are positioned in the axial direction with high accuracy so that the movement of the control shaft adjusts the valve lift amount to be the same in every cylinder. 
   In an internal combustion engine, the cylinder block, cylinder head, and cam carrier are formed from a light alloy or a light metal, such as aluminum, to reduce weight. However, shafts included in the variable valve actuation mechanism, such as the control shaft, are not formed from a light alloy or a light metal and formed from a steel material, such as cast steel or cast iron, to meet the high strength requirements. 
   The coefficient of thermal expansion differs greatly between light alloy and steel. Thus, when comparing a state in which the engine is cool and a state in which the engine is warm, the control shaft becomes shorter and changes the interval between the supports located closer to the cylinder head and cam carrier. This produces a difference in the relative positions of the control shaft and the variable valve lift mechanism between cylinders close to the basal end of the control shaft and cylinders close to the distal end of the control shaft. Accordingly, the lift amount differs between cylinders. Such difference causes difficulties for adjusting the combustion state of each cylinder with high accuracy. This may generate vibrations or deteriorate emission and cause an undesirable engine operation state. 
   The rocker shaft, which supports the variable valve lift mechanism, is arranged at the outer side of the control shaft. When the rocker shaft, which receives the control shaft, has a large diameter, the variable valve lift mechanism that receives the rocker shaft is enlarged. This enlarges and increases the weight of the variable valve actuation mechanism, which would contradict the demand for a smaller and lighter internal combustion engine. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a variable valve actuation mechanism that substantially equally adjusts the valve lift amount in each cylinder. Another object of the present invention is to provide a compact and light variable valve actuation mechanism. A further aspect of the present invention is to provide a collar for such a variable valve actuation mechanism. 
   One aspect of the present invention is a collar for receiving a shaft of a multiple cylinder engine. The shaft supports a plurality of variable valve lift mechanisms respectively arranged in correspondence with a plurality of cylinders. Each variable valve lift mechanism has an end face, and the engine includes a plurality of supports for supporting the shaft. The collar includes a sleeve extending in an axial direction and end portions formed integrally with the sleeve. In use, a plurality of said collars are fastened to the shaft, with the sleeve of each collar being arranged between the shaft and a corresponding one of the supports so that at least one of the end portions directly or indirectly contacts or engages the end face of one of the variable valve lift mechanisms to determine the positions of the variable valve lift mechanisms. 
   Another aspect of the present invention is a variable valve actuation mechanism for use in a multiple cylinder engine. The variable valve actuation mechanism includes a plurality of variable valve lift mechanisms respectively arranged in association with the cylinders of the engine. Each variable valve lift mechanism includes an end face. A control shaft extends through the variable valve lift mechanisms in an axial direction. An actuator moves the control shaft in the axial direction and drives the variable valve lift mechanisms. A plurality of collars are arranged alternately with the variable valve lift mechanisms for determining the positions of the variable valve lift mechanisms with respect to one another in the axial direction. Each collar includes a sleeve extending in the axial direction and end portions formed integrally with the sleeve. At least one of the end portions directly or indirectly contacts the end face of one of the variable valve lift mechanisms. 
   A further aspect of the present invention is a variable valve actuation mechanism for use in a multiple cylinder engine. The variable valve actuation mechanism includes a plurality of variable valve lift mechanisms respectively arranged in association with the cylinders of the engine. Each variable valve lift mechanism includes an end face. A control shaft extends through the variable valve lift mechanisms in an axial direction. An actuator moves the control shaft in the axial direction and drives the variable valve lift mechanisms. A plurality of collars are arranged alternately with the variable valve lift mechanisms for determining the positions of the variable valve lift mechanisms with respect to one another in the axial direction. Each collar includes a sleeve extending in the axial direction and end portions formed integrally with the sleeve. At least one of the end portions engages the end face of an adjacent one of the variable valve lift mechanisms and includes a shaft projection functioning as part of a pivot shaft of the variable valve lift mechanisms. 
   Another aspect of the present invention is a variable valve actuation mechanism for use in a multiple cylinder engine. The variable valve actuation mechanism includes a plurality of variable valve lift mechanisms respectively arranged in association with the cylinders of the engine. Each variable valve lift mechanism includes an end face. A control shaft extends through the variable valve lift mechanisms in an axial direction. An actuator moves the control shaft in the axial direction and drives the variable valve lift mechanisms. A plurality of collars are arranged alternately with the variable valve lift mechanisms for determining the positions of the variable valve lift mechanisms with respect to one another in the axial direction. Each collar includes a sleeve extending in the axial direction and end portions formed integrally with the sleeve. At least one of the end portions directly or indirectly contacts the end face of an adjacent one of the variable valve lift mechanisms to determine the positional relationship between the variable valve lift mechanisms in the axial direction, and includes a shaft projection for engaging the end face of the one of the variable valve lift mechanisms to function as part of a pivot shaft of the variable valve lift mechanism. 
   A further aspect of the present invention is a variable valve actuation mechanism for use in a multiple cylinder engine. The variable valve actuation mechanism includes a plurality of variable valve lift mechanisms respectively arranged in association with the cylinders of the engine. Each variable valve lift mechanism has an end face. A control shaft extends through the variable valve lift mechanisms in an axial direction. A hollow shaft receives the control shaft. The hollow shaft is formed from a metal material having a first coefficient of thermal expansion. An actuator moves the control shaft in the axial direction and drives the variable valve lift mechanisms. A plurality of collars are fastened to the hollow shaft and arranged alternately with the variable valve lift mechanisms for determining the positions of the variable valve lift mechanisms with respect to one another in the axial direction. A plurality of supports respectively support the collars. Each collar includes a sleeve extending in the axial direction and end portions formed integrally with the sleeve. The sleeve and the at least one end portion is formed from a material having a coefficient of thermal expansion that is equal to or approximate to the first coefficient of thermal expansion. Each collar is supported by the corresponding support such that a clearance is formed between the end face of an adjacent one of the variable valve lift mechanisms and the corresponding support. The contact shaft and the support holding the collar such as to restrict the sleeve from becoming eccentric while enabling movement of the collar in the axial direction. 
   Another aspect of the present invention is a variable valve actuation mechanism for use in a multiple cylinder engine. The variable valve actuation mechanism includes a plurality of variable valve lift mechanisms respectively arranged in association with the cylinders of the engine. Each variable valve lift mechanism has an end face. A control shaft extends in an axial direction. The variable valve lift mechanisms are fastened to a hollow shaft, which receives the control shaft. An actuator moves the control shaft in the axial direction and drives the variable valve lift mechanisms. A plurality of supports support the variable valve lift mechanisms via the hollow shaft. The variable valve lift mechanisms are fastened to the hollow shaft in a state in which movement in the axial direction is restricted in order to determine the positions of the variable valve lift mechanisms with respect to one another in the axial direction. 
   Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
       FIG. 1  is a cross-sectional diagram showing an engine and a variable valve actuation mechanism according to a first embodiment of the present invention; 
       FIG. 2  is a plan view showing the engine of  FIG. 1 ; 
       FIG. 3  is a plan view showing a cam carrier of the first embodiment; 
       FIGS. 4A ,  4 B, and  4 C are respectively plan, front, and perspective views showing an intermediate collar of the first embodiment; 
       FIGS. 5A ,  5 B, and  5 C are respectively plan, front, and perspective views showing an end collar of the first embodiment; 
       FIG. 6  is a perspective view showing a variable valve lift mechanism of the first embodiment; 
       FIGS. 7A and 7B  are partially cutaway perspective views showing the variable valve lift mechanism of the first embodiment; 
       FIG. 8  is an exploded perspective view showing the variable valve lift mechanism of  FIG. 6 ; 
       FIGS. 9A and 9B  are partially cutaway perspective views showing the variable valve lift mechanism of  FIG. 6 ; 
       FIGS. 10A ,  10 B, and  10 C are plan views and a front view showing a slider gear of the first embodiment; 
       FIG. 11  is a perspective view showing the slider gear of  FIGS. 10A ,  10 B, and  10 C; 
       FIG. 12  is a partial cutaway perspective view showing the slider gear of  FIG. 11 ; 
       FIG. 13A  is a perspective view showing a rocker shaft of the first embodiment; 
       FIG. 13B  is a perspective view showing a control shaft of the first embodiment; 
       FIG. 13C  is a perspective view showing the rocker shaft retained in the control shaft of  FIG. 13B ; 
       FIG. 14  is a partial cutaway perspective view showing the variable valve lift mechanism of  FIG. 6 ; 
       FIG. 15  is a perspective view showing a plurality of collars arranged between variable valve lift mechanisms; 
       FIG. 16  is a perspective view showing the collars fastened to the control shaft and the variable valve lift mechanisms; 
       FIG. 17  is a diagram showing clearances formed between the collars and the cam carriers; 
       FIGS. 18A ,  18 B,  19 A, and  19 B show the operation of the variable valve lift mechanism of  FIG. 6 ; 
       FIG. 20  is a plan view showing a cam carrier according to a second embodiment of the present invention; 
       FIGS. 21A ,  21 B, and  21 C are respectively front, side, and perspective views showing a collar of the second embodiment; 
       FIG. 22  is a perspective view showing a plurality of collars fastened to a control shaft; 
       FIGS. 23 and 24  are enlarged cross-sectional diagrams showing the joint between a collar and variable valve lift mechanism; 
       FIG. 25  is a perspective view showing the collars fastened to the control shaft and the variable valve lift mechanisms; and 
       FIG. 26  is a plan view showing a cam carrier according to a third embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a schematic cross-sectional diagram showing a cylinder of a multiple-cylinder gasoline engine  2 , which is installed in a vehicle.  FIG. 2  is a plan view showing a cam carrier  150  arranged on the upper portion of the engine  2 . 
   The engine  2  includes a cylinder block  4 , pistons  6 , and a cylinder head  8  mounted on the cylinder block  4 . The cylinder block  4  and the cylinder head  8  are formed from an aluminum alloy material. 
   A plurality of (four) cylinders  2   a  are defined in the cylinder block  4 . A combustion chamber  10  is defined in each cylinder  2   a  between the cylinder block  4 , the corresponding piston  6 , and the cylinder head  8 . Two intake valves  12  and two exhaust valves  16  are arranged in each cylinder  2   a . The intake valves  12  and the exhaust valves  16  respectively open and close associated intake ports  14  and exhaust ports  18 . 
   Each intake port  14  is connected to a surge tank via an intake passage formed in an intake manifold. Each cylinder  2   a  is supplied with air from the surge tank. A fuel injector is arranged in each intake passage to inject fuel into the intake port  14  of the corresponding cylinder  2   a . In this manner, fuel is supplied to a position upstream from the intake valve  12 . Fuel may be directly supplied into each combustion chamber  10  as in an in-cylinder injection type gasoline engine. 
   The lift amount of the intake valve  12  is varied to adjust the intake air amount. The engine  2  of the first embodiment does not include a throttle valve that would be arranged in an intake passage upstream from the surge tank in a normal engine. However, the engine  2  of the first embodiment may include an auxiliary throttle valve. When an auxiliary throttle valve is employed, the auxiliary throttle valve is, for example, fully opened when the engine  2  is started and fully closed when the engine  2  is stopped. The open amount of the auxiliary throttle valve may be adjusted to control the intake air amount when lift amount adjustment of the intake valves  12  with valve lift mechanisms  120  is disabled. 
   Referring to  FIG. 1 , rotation of an intake camshaft  45  rotates an intake cam  45   a . A variable valve lift mechanism  120  arranged on the cylinder head  8  converts the rotation of the intake cam  45   a  to a pivoting action of a roller rocker arm  52 . Movement of the roller rocker arm  52  drives the intake valve  12 . In this manner, the drive force of the intake camshaft  45  is transmitted to the intake valve  12 . 
   Referring to  FIG. 2 , a slide actuator  100  adjusts the transmission state of the variable valve lift mechanisms  120  to adjust the lift amount of the intake valves  12 . 
   A variable valve timing mechanism  140  is arranged at the front end of the intake camshaft  45 . The intake camshaft  45  rotates in cooperation with the rotation of a crankshaft  49  of the engine  2  by means of a timing sprocket of the variable valve timing mechanism  140  and a timing chain  47 . 
   An exhaust camshaft  46  is rotated in cooperation with rotation produced by the engine  2 . Exhaust cams  46   a  arranged on the exhaust camshaft  46  open and close corresponding exhaust valves  16  with a constant lift amount by means of roller rocker arms  54 . Each exhaust port  18  is connected to an exhaust manifold. Exhaust passes through a purification catalyst converter before being discharged. 
   The intake camshaft  45 , the exhaust camshaft  46 , the slide actuator  100 , the variable valve lift mechanisms  120 , and the variable valve timing mechanism  140  are incorporated as a single unit in the cam carrier  150 .  FIG. 3  shows a state in which five cam caps  152  are removed from the cam carrier  150 . 
   The cam carrier  150  includes a front wall  154 , a rear wall  156 , and two side walls  158  and  160 . In the internal space defined by the walls  154 ,  156 ,  158 , and  160 , four parallel bearings  162  extends so as to connect the side walls  158  and  160 . The walls  154  to  160  and the bearing  162  are formed integrally. The front wall  154  also functions as a bearing. The cam carrier  150  is formed from the same aluminum alloy material as the cylinder block  4  and the cylinder head  8 . 
   The bearings  162  and the front wall  154  support the intake camshaft  45  and the exhaust camshaft  46  in a manner that they are parallel to each other and rotatable. The four variable valve lift mechanisms  120 , which are respectively arranged in correspondence with the cylinders  2   a , three intermediate collars  164 , and two end collars  166 , are arranged between the intake camshaft  45  and the side wall  158 . The three intermediate collars  164  are arranged between the four variable valve lift mechanisms  120 . The two end collars  166  are arranged at the outer sides of the two outer variable valve lift mechanisms  120 . A rocker shaft  130 , which commonly extends through the four variable valve lift mechanisms  120 , supports the collars  164  and  166 . 
   Referring to  FIGS. 4A ,  4 B, and  4 C, each intermediate collar  164  includes a cylindrical sleeve  164   a  and two flanges  164   b  formed on the two ends of the sleeve  164   a . The intermediate collar  164  has an interior space  164   d  (center bore). A pin hole  164   c  formed in the sleeve  164   a  is connected to the interior space  164   d.    
   Referring to  FIGS. 5A ,  5 B, and  5 C, each end collar  166  includes a cylindrical sleeve  166   a  and a flange  166   b  formed on one end of the sleeve  166   a . The end collar  166  has an interior space  166   d  (center bore). A pin hole  166   c  formed in the sleeve  166   a  is connected to the interior space  166   d . The collars  164  and  166  are each formed integrally from a steel material. 
   The variable valve lift mechanisms  120  will now be discussed with reference to  FIGS. 6 to 9 . 
   Each variable valve lift mechanism  120  includes an input sleeve  122  (input portion), a first rocking cam  124  (output portion) arranged rearward from the input sleeve  122 , a second rocking cam  126  (output portion) arranged frontward from the input sleeve  122 , and a slider gear  128  arranged in the input sleeve  122 . 
   The input sleeve  122  includes a housing  122   a  defining a cylindrical hollow space. A helical spline  122   b  ( FIG. 9 ) is formed in the inner wall surface of the housing  122   a . Each groove of the helical spline  122   b  extends helically about the axis of the housing  122   a  in the direction of a right-hand thread. Two parallel arms  122   c  and  122   d  extend from the outer walls surface of the housing  122   a . A pin  122   e  extends between the distal ends of the arms  122   c  and  122   d . The pin  122   e  extends parallel to the axis of the housing  122   a . Further, the pin  122   e  rotatably supports a roller  122   f . Referring to  FIG. 1 , the force of an urging member, such as a spring, constantly pushes the roller  122   f  towards the intake cam  45   a . The urging member may be arranged, for example, between the input sleeve  122  and the cylinder head  8  or rocker shaft  130 . 
   The first rocking cam  124  includes a housing  124   a  that defines a cylindrical internal space. A helical spline  124   b  ( FIG. 9 ) is formed in the inner wall surface of the housing  124   a . Each groove of the helical spline  124   b  extends helically about the axis of the housing  124   a  in the direction of a left-hand thread. The housing  124   a  includes a bearing end  124   c  having an end face in which a small center hole is formed. A triangular nose  124   d  extends from the outer wall surface of the housing  124   a . The nose  124   d  includes a cam surface  124   e  curved in a concave manner. 
   The second rocking cam  126  includes a housing  126   a  that defines a cylindrical internal space. A helical spline  126   b  ( FIG. 9 ) is formed in the inner wall surface of the housing  126   a . Each groove of the helical spline  126   b  extends helically about the axis of the housing  126   a  in the direction of a left-hand thread. The housing  126   a  includes a bearing end  126   c  having an end face in which a small center hole is formed. A triangular nose  126   d  extends from the outer wall surface of the housing  126   a . The nose  126   d  includes a cam surface  126   e  curved in a concave manner. 
   Referring to  FIG. 8 , the first rocking cam  124 , the input sleeve  122 , and the second rocking cam  126  are coaxially aligned. The first rocking cam  124  and the second rocking cam  126  contact opposite ends of the input sleeve  122 . The housings  122   a ,  124   a , and  126   a  define a single internal space. 
     FIGS. 10 to 12  show the slider gear  128  retained in the housings  122   a ,  124   a , and  126   a . The slider gear  128  includes an input helical spline  128   a , a first output helical spline  128   c , and a second helical spline  128   e . Each groove of the input helical spline  126   b  extends helically about the axis of the slider gear  128  in the direction of a right-hand thread. A small diameter portion  128   b  is formed between the input helical spline  128   a  and the first output helical spline  128   c . A further small diameter portion  128   d  is formed between the input helical spline  128   a  and the second output helical spline  128   e . Each groove of the first output helical spline  128   c  and the second output helical spline  128   e  extend helically about the axis of the slider gear  128  in the direction of a left-hand thread. The diameter of the first output helical spline  128   c  and the diameter of the second output helical spline  128   e  are smaller than that of the input helical spline  128   a.    
   Referring to  FIG. 12 , a gear bore  128   f  extends through the slider gear  128  along the slider gear axis. A circumferential groove  128   g  is formed in the inner wall surface of the gear bore  128   f  in the input helical spline  128   a . A pin insertion hole  128   h  connects the circumferential groove  128   g  and the input helical spline  128   a.    
     FIG. 13A  shows part of the rocker shaft  130 . The gear bore  128   f  of the slider gear  128  rotatably receives the rocker shaft  130 . As shown in  FIG. 3 , the four variable valve lift mechanisms  120  are mounted on the single rocker shaft  130 . 
   The rocker shaft  130  is hollow and includes an interior space  130   b . Four elongated holes  130   a  are formed in the outer surface of the rocker shaft  130  at positions corresponding to the variable valve lift mechanisms  120 . 
     FIG. 13B  shows part of the control shaft  132 . The control shaft  132  has a round cross-section. Referring to  FIG. 13C , the control shaft  132  is received in the rocker shaft  130  and axially movable. 
   The control shaft  132  includes support holes  132   b  respectively located at positions corresponding to the variable valve lift mechanisms  120 . Each support hole  132   b  receives the basal portion of a control pin  132   a . Each control pin  132   a , which is supported by the corresponding support hole  132   b , extends perpendicular to the axis of the control shaft  132 . 
   When the control shaft  132  is received in the rocker shaft  130 , each control pin  132   a  projects from the corresponding elongated hole  130   a  of the rocker shaft  130 . Referring to  FIG. 14 , the distal end of each control pin  132   a  is located in the circumferential groove  128   g  of the slider gear  128 . 
   The rocker shaft  130 , the control shaft  132 , and the control pin  132   a  are formed from a steel material and have high strength. 
   Referring to  FIG. 16 , a ball screw shaft  174  is formed on one end of the control shaft  132 . The ball screw shaft  174  transmits the drive force of the slide actuator  100  to the control shaft  132 . 
   The assembly of the variable valve lift mechanisms  120 , the rocker shaft  130 , the control shaft  132 , and the collars  164  and  166  will now be described. 
   The control shaft  132  is first inserted through the rocker shaft  130 . Referring to  FIG. 15 , the variable valve lift mechanisms  120  and the collars  164  and  166  are alternately fastened to the rocker shaft  130 . Referring to  FIG. 8 , the control pins  132   a  are inserted in the pin insertion holes  128   h  of the corresponding slider gears  128  and the elongated holes  130   a  of the rocker shaft  130  and fastened to the control shaft  132  in the support holes  132   b . Referring to  FIG. 15 , fastening pins  168  are inserted through the pin holes  164   c  and  166   c  of the collars  164  and  166  and fastened to the rocker shaft  130  in pin holes  130   c  ( FIG. 13 ). This fastens the collars  164  and  166  to the rocker shaft  130 . 
   Among the five cam caps  152 , the distal end of a bolt  170  for fastening the cam cap  152  located near the slide actuator  100  is inserted through the pin hole  166   c  of the corresponding collar  166  and into the pin hole  130   c  of the rocker shaft  130 . Accordingly, the collar  166  located near the slide actuator  100  is fixed to the rocker shaft  130  by the bolt  170  when fastening the cam cap  152 . In this manner, as shown in the state of  FIG. 16 , the variable valve lift mechanisms  120 , the rocker shaft  130 , the control shaft  132 , and the collars  164  and  166  are assembled as a single unit. In this state, the flanges  164   b  and  166   b  of the collars  164  and  166  are in contact with the end faces of the adjacent variable valve lift mechanisms  120 . 
   During the assembly, shim plates  172 , which are formed from a steel material, are arranged between the variable valve lift mechanisms  120  and the collars  164  and  166  if necessary to adjust the position of each variable valve lift mechanism  120 . In this case, the flanges  164   b  and  166   b  of the collars  164  and  166  indirectly contact the end faces of the adjacent variable valve lift mechanisms  120 . 
   The shaft assembly shown in  FIG. 16  is formed from a steel material. Referring to  FIG. 2 , the shaft assembly is secured to the cam carrier  150  by five cam caps  152 . The bolt  170  for fastening the cam cap  152  that is close to the slide actuator  100  restricts movement of the shaft assembly in the axial direction. The bolts for fastening the other three cam caps  152  do not restrict movement of the collars  164  and  166  in the axial direction. 
   As shown in  FIGS. 2 and 3 , the lengths of the sleeves  164   a  and  166   a  of the collars  164  and  166  in the axial direction are greater than the thicknesses of the front wall  154 , the bearings  162 , and the cam caps  152 . Referring to  FIG. 17 , clearances CL are formed between the flanges  164   b  and  166   b  and the adjacent front wall  154  or bearing  162  and cam cap  152 . Even if there is a difference in the expansion and contraction amount of the control shaft  132  and the cam carrier  150  due to a difference in the coefficient of thermal expansion, the collars  164  and  166 , excluding the collar  166  fastened by the bolt  170 , move in the axial direction of the control shaft  132  and absorb the difference in the expansion and contraction amounts. Accordingly, the axial positions of the variable valve lift mechanisms  120  do not change, and the relative positions of the control shaft  132  and the variable valve lift mechanisms  120  in the axial direction do not change. 
   The slide actuator  100  drives a ball screw mechanism  210  ( FIGS. 2 and 3 ) to move the control shaft  132 , which includes the ball screw shaft  174 , in the axial movement. The movement adjusts the axial position of the slider gear  128  in each variable valve lift mechanism  120 . 
   Referring to  FIG. 14 , the control pin  132   a  is received in the circumferential groove  128   g  of the slider gear  128 . Thus, the slider gear  128  is rotatable relative to the control shaft  132  regardless of the position of the control pin  132   a.    
   The input helical spline  128   a  of the slider gear  128  meshes with the helical spline  122   b  of the input sleeve  122 . The first output helical spline  128   c  meshes with the helical spline  124   b  of the first rocking cam  124 . The second output helical spline  128   e  meshes with the helical spline  126   b  of the second rocking cam  126 . The input splines  122   b  and  128   a  differ from the splines  124   b ,  128   c ,  126   b , and  128   e  in the helical direction (helical angle) relative to the control shaft  132 . 
   Referring to  FIG. 16 , the collars  164  and  166  are arranged on opposite sides of each variable valve lift mechanism  120 . This restricts axial movement of the input sleeve  122  and the rocking cams  124  and  126  in each variable valve lift mechanism  120  relative to the rocker shaft  130 . Thus, even if the control shaft  132  axially moves the slider gears  128 , axial movement of the input sleeves  122  and the rocking cams  124  and  126  is restricted. 
   When the slide actuator  100  axially moves the control shaft  132 , the slider gear  128  axially moves in the internal space of the corresponding variable valve lift mechanism  120 . The helical splines  128   a ,  122   b ,  128   c ,  124   b ,  128   e , and  126   b  function to relatively rotate the input sleeve  122  and the rocking cams  124  and  126 . In this embodiment, the input sleeve  122  rotates in a direction opposite to that of the rocking cams  124  and  126 . The rotation angle of the input sleeve  122  and the rocking cams  124  and  126  are determined in accordance with the movement of the slider gear  128 . Accordingly, adjustment of the movement amount of the control shaft  132  changes the positions (angle along the circumferential direction of the rocker shaft  130 ) of the rollers  122   f  relative to the noses  124   d  and  126   d . This adjusts the lift amount of the intake valves  12 . 
     FIG. 18A  shows the intake valve  12  when it is closed and  FIG. 18B  shows the intake valve  12  when it is open in a state in which the control shaft  132  is moved by the maximum amount in direction L ( FIG. 16 ). In this state, the angle between the roller  122   f  and the nose  126   d  ( 124   d ) in each variable valve lift mechanism  120  is minimal. Thus, referring to  FIG. 18B , the amount the cam surfaces  124   e  and  126   e  of the noses  124   d  and  126   d  push the rocker roller  52   a  down, that is, the maximum lift amount of the intake valve  12  is relatively small. In this case, the amount of air supplied to each combustion chamber  10  from the corresponding intake port  14  is minimal. 
     FIG. 19A  shows the intake valve  12  when it is closed and  FIG. 19B  shows the intake valve  12  when it is open in a state in which the control shaft  132  is moved by the maximum amount in direction H ( FIG. 16 ). In this state, the angle between the roller  122   f  and the nose  126   d  ( 124   d ) in each variable valve lift mechanism  120  is maximal. Thus, referring to  FIG. 19B , the amount the cam surfaces  124   e  and  126   e  of the noses  124   d  and  126   d  push the rocker roller  52   a  down, that is, the minimum lift amount of the intake valve  12  is relatively large. In this case, the amount of air supplied to each combustion chamber  10  from the corresponding intake port  14  is maximal. 
   The control shaft  132  axially moves between the state of  FIG. 18  and the state of  FIG. 19  in a continuous (stepless) manner. Adjustment of the movement amount of the control shaft  132  adjusts the lift amount of each intake valve  12  in a continuous (stepless) manner. Accordingly, the intake air amount is adjustable in a stepless manner without using a throttle valve. 
   In the example of  FIG. 18B , when the lift amount of the intake valve  12  is minimal, the intake port  14  is slightly open. However, the intake port  14  may be closed when the lift amount of the intake valve  12  is minimal. This is a state in which the minimal lift amount of the intake valve  12  is zero and the intake air amount is zero. 
   In the first embodiment, the rocker shaft  130  functions as a shaft (hollow shaft). The front wall  154  and the bearings  162  of the cam carrier  150  function as supports. The flanges  164   b  and  166   b  formed on the ends of the sleeves  164   a  and  166   a  function to position the variable valve lift mechanisms  120 . The shaft assembly ( FIG. 16 ) including the variable valve lift mechanisms  120 , the ball screw mechanism  210 , and the slide actuator  100  form a variable valve actuation mechanism. 
   The first embodiment has the advantages described below. 
   The ends of the collars  164  and  166 , or the flanges  164   b  and  166   b , directly contact the end faces of the rocking cams  124  and  126  or indirectly contact the end faces of the rocking cams  124  and  126  by means of the shim plates  172  in the variable valve lift mechanisms  120 . This contact determines the distance (positional relationship) between the variable valve lift mechanisms  120  in the axial direction. The flanges  164   b  and  166   b  are spaced from the front wall  154 , the bearings  162 , and the cam caps  152  by clearance C. Accordingly, changes in the interval of the supports (front wall  154  and bearings  162 ) in the cam carrier  150  does not affect the positional relationship between the variable valve lift mechanisms  120 . Even if a difference in coefficient of thermal expansion exists between the cam carrier  150  and the control shaft  132 , the coefficient of thermal expansion of the cam carrier  150  does not affect the positional relationship of the variable valve lift mechanisms  120 . 
   The coefficient of thermal expansion of the collars  164  and  166 , the input sleeves  122 , and the rocking cams  124  and  126  affect the positional relationship of the variable valve lift mechanisms  120 . However, the collars  164  and  166 , the input sleeves  122 , and the rocking cams  124  and  126  are formed from a steel material having a coefficient of thermal expansion that is the same or approximate to that of the material the control shaft  132  is formed from. Accordingly, even if temperature changes affect the collars  164  and  166 , the input sleeves  122 , and the rocking cams  124  and  126 , the change in the positional relationship of the slider gears  128 , which is determined by the control shaft  132 , is substantially the same as the change in the positions of the input sleeve  122  and the rocking cams  124  and  126 . Thus, the intake valves  12  have substantially the same lift amount in all of the cylinders. Since temperature changes do not cause differences between cylinders in the lift amount of the intake valves  12 , the accuracy of lift amount adjustment is improved. 
   A variable valve actuation mechanism according to a second embodiment of the present invention is similar to that of the first embodiment except in that the rocker shaft  130  is omitted. A plurality of collars  364  ( FIG. 21 ) are used in lieu of the collars  164  and  166  of the first embodiment. Referring to  FIG. 20 , the collars  364  function as pivot shafts of variable valve lift mechanisms  320 .  FIG. 20 , which corresponds to  FIG. 3  of the first embodiment, shows a cam carrier  350  from which cam caps are removed. In the second embodiment, the cylinder block, the cylinder head, and the cam carrier  350  are formed from a steel material. 
   Referring to  FIG. 21 , each collar  364  includes a cylindrical sleeve  364   a , two flanges formed on the two ends of the sleeve  364   a , and a shaft projection or pivot shaft portion  364   c  extending from each flange  364   b  along the axis of the sleeve  364   a . The collar  364  has a center bore  364   d . Further, the collar  364  is formed from a steel material. 
   Referring to  FIG. 22 , a control shaft  332  extends through the center bores  364   d  of the collars  364 . The control shaft  332  directly supports the collars  364 . Referring to  FIG. 23 , the pivot shaft portions  364   c  of each collar  364  are received by bearing ends  324   c  and  326   c  of the adjacent variable valve lift mechanism  320 . This rotably supports the rocking cams  324  and  326  of each variable valve lift mechanism  320  with the pivot shaft portions  364   c  of the two adjacent collars  364 . 
   Referring to  FIG. 24 , a control pin  332   a , which is fixed to the control shaft  332 , is engaged with a slider gear  328 . Movement of the control shaft  332  moves the slider gear  328  in the axial direction. The omission of a rocker shaft that would extend through the entire variable valve lift mechanism  320  reduces the diameter of each variable valve lift mechanism  320 . 
   In the shaft assembly shown in  FIG. 25 , shim plates  372  are arranged between the collars  364  and the rocking cams  324  and  326 . As shown in  FIG. 20 , the shaft assembly is mounted on the cam carrier  350 . The distance between the two flanges  364   b  in each collar  364  is substantially the same as the thicknesses of a front wall  354  and the bearings  362 . When the shaft assembly is arranged as shown in the state of  FIG. 20 , the front wall  354  and the bearings  362  support the collars  364  in a rotatable manner. The front wall  354  and the bearings  362  are held between the two flanges  364   b  of the corresponding collars  364 . This prevents each collar  364  from being moved in the axial direction and determines the position of each variable valve lift mechanism  320  (input shaft  322 , and rocking cams  324  and  326 ) in the axial direction. 
   In the second embodiment, the control shaft  332  functions as a shaft. The pivot shaft portions  364   c  formed on the ends of the sleeves  364   a  function to position the variable valve lift mechanisms  320 . 
   The second embodiment has the advantages described below. 
   The pivot shaft portions  364   c  are formed on opposite ends of each collar  364 . The pivot shaft portions  354   c  pivotally support the adjacent variable valve lift mechanism  320  and function as a pivot shaft of the variable valve lift mechanisms  320 . This eliminates the need for a rocker shaft that extends through the variable valve lift mechanisms  320  and reduces the diameter of the variable valve lift mechanisms  320 . 
   A third embodiment of the present invention will now be discussed with reference to  FIG. 26 .  FIG. 26  shows a cam carrier  550  from which cam caps are removed. The third embodiment employs collars  564  that are similar to those of the second embodiment. However, the distance between two flanges  564   b  in each collar  564  is greater than the thicknesses of a front wall  554  and bearings  562  of the cam carrier  550 . This enables movement of the collars  564  in the axial direction with respect to the front wall  554  and the bearings  562 . 
   Each variable valve lift mechanism  520  is rotatably supported by pivot shaft portions  564   c  of the adjacent collar  564  without the use of a rocker shaft. The collar  564  located farthest from the slide actuator  500  is fixed to the front wall  554  by a pin  565  and does not move in the axial direction. The collar  564  located closest to the slide actuator  500  is pushed toward the corresponding variable valve lift mechanism  520  by a spring  567 . This keeps the collars  564  in a state directly contacting the variable valve lift mechanisms  520  or in a state indirectly contacting the variable valve lift mechanisms  520  by means of shim plates  572 . 
   In the third embodiment, the cylinder block, the cylinder head, and the cam carrier  550  are formed from an aluminum alloy material. The variable valve lift mechanisms  520 , the collars  564 , and the shim plates  572  are formed from a steel material. 
   The control shaft  532  functions as a shaft. The flanges  564   b  and pivot shaft portions  564   c  formed on the ends of the sleeves  564   a  function to position the variable valve lift mechanisms  520 . 
   The third embodiment has the advantages described below. 
   The flanges  564   b  of the collars  564  directly contact the end faces of the rocking cams  524  and  526  or indirectly contacts the end faces of the rocking cams  524  and  526  by means of the shim plates  572 . This contact determines the positions of the variable valve lift mechanisms  520  in the axial direction. The flanges  564   b  are spaced with a clearance from the adjacent bearings  562  and cam caps. The positional relationship of the variable valve lift mechanisms  520  is affected only by the coefficient of thermal expansion of the collars  564 , the input sleeves  522 , and the rocking cams  524  and  526 . However, the collars  564 , the input sleeves  522 , and the rocking cams  524  and  526  are formed from a steel material having a coefficient of thermal expansion that is the same or approximate to that of the material the control shaft  532  is formed from. Accordingly, even if temperature changes affect the collars  564 , the input sleeves  522 , and the rocking cams  524  and  526 , the change in the positions of the slider gears in the variable valve lift mechanisms  520 , which is determined by the control shaft  532 , is substantially the same as the change in the positions of the input sleeve  522  and the rocking cams  524  and  526 . Thus, the intake valves  12  have substantially the same lift amount in all of the cylinders. Since temperature changes do not cause differences between cylinders in the lift amount of the intake valves  12 , the accuracy of lift amount adjustment is improved. 
   The pivot shaft portions  564   c  are formed on the two ends of each collar  564 . The pivot shaft portions  564   c  rotatably support the adjacent variable valve lift mechanisms  520 . Since the pivot shaft portions  564   c  function as pivot shafts of the variable valve lift mechanisms  520 , the diameter of the variable valve lift mechanisms  520  is reduced. 
   It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
   In each of the above embodiments, the variable valve lift mechanisms and the camshafts may be directly mounted on the cylinder head without using a cam carrier. 
   The engine is not limited to a gasoline engine and may be any type of engine such as a diesel engine. Further, the engine is not limited to an engine used to drive vehicles and may be an engine used for other applications. In addition to lift amount adjustment of intake valves, the present invention may be applied to lift amount adjustment of exhaust valves or lift amount adjustment of both intake and exhaust valves. 
   In each of the above embodiments, the collars restrict movement of the variable valve lift mechanisms in the axial direction. When using a hollow shaft (rocker shaft) covering the control shaft as in the first embodiment, positioning members such as pins may be arranged on the rocker shaft. The positioning members may restrict movement of the variable valve lift mechanisms in the axial direction. This fixes the positional relationship of the variable valve lift mechanisms with respect to the rocker shaft. Thus, the distance between the bearings arranged on the cam carrier or cylinder head does not affect the positional relationship between the variable valve lift mechanisms. 
   Accordingly, even if the cylinder head or cam carrier is formed from a material other than steel, such as a light alloy, to reduce weight, a variable valve actuation mechanism may be formed from a material selected in accordance with the strength requirements. Further, even if a temperature change occurs, the valve lift adjustment amount is prevented from differing between cylinders. This improves the accuracy for adjusting the valve lift amount. 
   The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.