Patent Publication Number: US-7588003-B2

Title: Valve train for internal combustion engine

Description:
This application is a divisional of U.S. patent application Ser. No. 10/589,244 filed Aug. 14, 2006, now U.S. Pat. No. 7,367,297 which is a continuation of International Application PCT/JP2005/002965 filed Feb. 17, 2005, and claim priority from Japanese patent Application No. 2004-040246 filed Feb. 17, 2004 and Japanese Application No. 2004-040247 filed Feb. 17, 2004. The entire contents of these applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a valve train for an internal combustion engine, and more particularly to a valve train which can change the valve operating properties including opening and closing timings and maximum lift amount of an engine valve made up of at least one of an inlet valve and an exhaust valve. 
     BACKGROUND ART 
     A valve train for an internal combustion engine which can change the valve operating properties of engine valves is disclosed in, for example, Japanese Patent Unexamined Publication No. JP-A-58-214610. The valve train so disclosed includes a rocker arm (hereinafter, referred to as a primary rocker arm) supported in an oscillatory fashion on a fixed point or fulcrum which is eccentric to a rocker shaft and adapted to be oscillated by a primary cam which rotates in synchronism with the rotation of the engine and a oscillating cam which is rotatably supported on a camshaft which is in parallel with the rocker shaft. A cam profile made up of a base circle portion where an inlet valve remains not lifted and a lifting lobe portion where the inlet valve is lifted and a contact surface with which the primary rocker arm is brought into abutment are formed on the oscillating cam which opens and closes an inlet valve provided in a cylinder head. The inlet valve is opened and closed in accordance with rotational positions of the primary cam when the valve drive force of the primary cam is transmitted to the oscillating cam via the primary rocker arm. Then, opening and closing timings and maximum lift amount of the inlet valve are changed by displacing the fulcrum. Here, it is understood that the camshaft, which supports the oscillating cam, is not displaced relative to the cylinder head. 
     For other conventional apparatuses for changing the valve operating properties of engine valves of internal combustion engines, there are apparatuses which are disclosed, for example, in Japanese Patent Unexamined Publications Nos. JP-A-7-91217, and JP-A-5-71321. An apparatus disclosed in the JP-A-7-91217 includes a drive shaft which is driven to rotate by an internal combustion engine, a camshaft which is provided on an outer circumference of the drive shaft in such a manner as to rotate freely relative to the drive shaft and which has a cam for actuating an inlet valve to be opened and closed, a disk housing provided so as to oscillate freely about a pivot support pin as a fulcrum in a radial direction relative to the drive shaft, an annular disk rotatably supported on an inner circumferential surface of the disk housing, a drive mechanism for oscillating the disk housing and a rocker arm which is pivot supported in an oscillatory fashion on a rocker shaft which is supported on the disk housing at one end portion thereof and which abuts with the cam and the inlet valve. Then, when the disk housing is cause to oscillate by the drive mechanism, the center of the annular disk becomes eccentric to the axial center of the drive shaft, whereby the angular velocity of the camshaft is changed, and then the operation angle of the inlet valve is changed. At the same time, due to the displacement of the rocker shaft which oscillates together with the disk housing, the pivot support point of the rocker arm is changed, and the other end portion of the rocker arm shifts in a diametrical direction on an upper surface of a valve lifter, whereby a rocker ratio relative to the inlet valve is changed, the valve lift amount being thereby changed. 
     In addition, a variable valve train disclosed in the JP-A-5-71321 includes a rocker arm which is brought into contact with a rotating cam and an inlet valve, a lever which is rotatably supported on a fulcrum shaft so as to be joined to a back side of the rocker arm in an oscillatory fashion, a link which connects the fulcrum shaft to the rocker arm and a controller cam which changes over the position of the lever from a high lift position where the position of the lever approaches the cam to a low lift position where the position of the lever moves apart from the cam. In a state where the rocker arm contacts a base circle of the cam, a distal end of a joint portion of the lever which connects a point where the lever contacts the rocker arm at a low lift position to a point where the lever contacts the rocker arm at a high lift position is formed into a concentric arc-like sectional shape which is formed about the fulcrum shaft, and a joint portion of the rocker arm which contacts the inlet valve is formed into a concentric arc-like sectional shape. Then, by changing over the lever position to the low lift position or high lift position, the valve lift amount of the inlet valve is changed. 
     In valve trains of internal combustion engines, a clearance is provided, for example, between an engine valve and a rocker arm which abuts with the engine valve or between a cam and a rocker arm which abuts with the cam and an engine valve. 
     In the conventional valve train that has been described in the JP-A-58-214610, the cam profile of the oscillating cam abuts with a valve lifter, which is a member on the inlet valve side. This is because the cam profile of the oscillating cam cannot be brought into abutment with the inlet valve as the shift amount of an abutment position where the cam profile abuts with the member becomes large between the cam profile and the member which abuts with the cam profile, when the operating angle and lift amount (valve operating properties) of the inlet valve are changed. Thus, in the conventional valve train, since the cylindrical valve lifter with which the oscillating cam is brought into abutment and a holding portion for holding the valve lifter slidably need to be provided in the cylinder head, the cylinder head is enlarged. Due to this, in an internal combustion engine in which the width of the cylinder head is narrow in a direction which intersects at right angles with a plane which includes cylinder axes of the internal combustion engine and which is in parallel with the rotational center line of the primary cam, it is difficult to install such a valve train while maintaining the compactness of the internal combustion engine. 
     In addition, a consideration is given to a valve train in which a separate rocker arm is adopted in place of the oscillating cam in the aforesaid conventional valve train for abutment with the inlet valve, and the separate rocker arm is made to be oscillated by the primary rocker arm. In this case, since the necessity of the valve lifter is obviated, it becomes possible for the valve train to be applied to the internal combustion engine which is narrow in the direction which intersects at right angles with the plane. However, since the fulcrum of the separate rocker arm is not displaced in contrast to the primary rocker arm whose fulcrum is displaced, it becomes difficult to maintain a clearance between the abutment portion of the primary rocker arm and the abutment portion of the separate rocker arm or the abutment state therebetween when the valve operating properties of the inlet valve are changed, thereby making it difficult to maintain an appropriate valve clearance. As a result, for example, due to an increase in valve clearance, noise is increased due to striking noise generated when the inlet valve starts to be opened, and noise is also increased due to collision of the rocker arms with each other when the internal combustion engine vibrates. In addition, irrespective of a change in the valve operating properties, when attempting to maintain the clearance between the abutment portions or abutment state therebetween, the configurations of the abutment portions become complicated, leading to an increase in costs. 
     Furthermore, in the event that the fulcrum of the separate rocker arm is not displaced, the control range of valve operating properties is determined solely by the displace amount and displacement direction of the fulcrum of the primary rocker arm, and therefore, for example, when attempting to expand the control range of the opening and closing timings of the inlet valve, since the displacement amount of the primary rocker arm needs to be increased, the aforesaid maintenance of the appropriate valve clearance becomes more difficult, and therefore, the control range of valve operating properties cannot be actually set large. 
     Then, in the technique disclosed in the JP-A-7-91217, since the rocker arm abuts with the cam and the valve lifter, when the disk housing is caused to oscillate so that the rocker shaft oscillates together with the disk housing in order to change the operating angle and the valve lift amount (valve operating properties), while an abutment state is maintained between the rocker arm and the valve lifter, the clearance between the cam and the rocker are changes, and as a result, the valve clearance changes. In addition, in the technique disclosed in the JP-A-5-71321, since the rocker arm abuts with the cam and the inlet valve, when the position of the lever is changed over so that the rocker arm pivot supported by the link rotates about the fulcrum shaft in order to change the valve lift amount (valve operating properties), while the clearance or the abutment state is maintained between the joint portion of the rocker arm and the inlet valve, the clearance between the rocker arm and the cam changes, and as a result, the valve clearance changes. 
     Thus, in the valve train in which when the valve operating properties are changed, the oscillating center line of the rocker arm which abuts with the engine valve changes, when the valve operating properties are changed, the valve clearance changes. In this case, even in case the valve clearance is an appropriate value for a specific valve operating property, the valve clearance does not become an appropriate value in another valve operating property. Then, for example, when the valve clearance becomes larger than the appropriate value, noise is increased which results from striking noise generated when inlet and exhaust valves start to be opened. 
     SUMMARY OF THE INVENTION 
     The present invention is such as to have been made in view of these situations. An object of present invention is to reduce the size of the valve train for an internal combustion engine which can change valve operating properties of an engine valve Further, the present invention also aims to make is ease to maintain the proper valve clearance at the time of changing the valve operating properties. 
     According to a first aspect of the invention, there is provided a valve train for an internal combustion engine, including: 
     a valve operating cam rotating around a rotational center line in synchronism with a rotation of an engine; 
     an engine valve including at least one of an inlet valve and an exhaust valve; 
     a transmission mechanism which transmits a valve drive force of the valve operating cam to the engine valve so as to operate the engine valve in open and close states, and includes:
         a oscillating member which oscillates about a main oscillating center line in accordance with a rotation of the valve operating cam and transmits the valve driving force to the engine valve; and   a holder oscillatably supporting the oscillating member and including:
           a pair of side walls on which fulcrum portions oscillatably supporting the holder on a cylinder head of the internal combustion engine are provided; and   a connecting wall which connects the pair of the side walls each other; and   
               

     a driving mechanism which applies the driving force to an operation portion provided on the connection wall of the holder to thereby oscillate the holder about the holder oscillating center line so as to control valve properties including opening and closing timings and maximum lift amount of the engine valve in accordance with a position of the holder, wherein 
     the holder is oscillated around the holder oscillating center line which differs from the rotation center line of the valve operating cam, and 
     the main oscillating center line oscillates together with the holder. 
     According to the construction, by taking advantage of the connection wall which connects the pair of the side walls, on which the support portion oscillatably supporting the holder on the cylinder head, each other, the fulcrum portion on which the driving force for oscillating the holder is provided. 
     According to a second aspect of the invention as set forth in the first aspect of the present invention, it is preferable that the operation portion is a gear portion of which shape in a plane perpendicular to the holder oscillating center line is an arc of which center is the holder oscillating center. 
     According to the construction, a line of action of the driving force applied to the gear portion is tangential direction of the arc of which center is the holder oscillating center line in the plane perpendicular to the holder oscillating center line. 
     According to a third aspect of the invention as set forth in the first aspect of the present invention, it is more preferable that the operation portion is located on the holder at a position which is farthest from the holder oscillating center line in a plane perpendicular to the holder oscillating center line. 
     According to the construction, the distance in the holder defined between the holder oscillating center line and the operation position of the driving force can be made substantially maximum. Thus, since the magnitude of the driving force of the driving mechanism can be reduced and the driving mechanism can be made compact. 
     According to a fourth aspect of the invention as set forth in the first aspect of the present invention, it is more preferable that the oscillating member is a primary oscillating member having a cam abutment portion which abuts with the valve operating cam, 
     the holder oscillating center line is an oscillating center line of the primary oscillating member, 
     the transmission mechanism includes a secondary oscillating member having a valve abutment portion which abuts with the engine valve, 
     the secondary oscillating member oscillates about a oscillating center line of the secondary oscillating member by an abutment of the primary oscillating member so as to transmit the valve driving force transmitted via the primary oscillating member to the engine valve, 
     among a driving abutment portion of the primary oscillating member and a follower abutment portion of the secondary oscillating member, one abutment portion is provided with a cam surface including:
             a lost motion profile for maintaining the engine valve in a closed state by not transmitting the valve driving force transmitted via the primary oscillating member to the secondary oscillating member while in a state that the other of the abutment portion abutting with; and   a drive profile for driving the engine valve in the open state by abutting with the other abutment portion, and           

     in a sectional shape of the lost motion profile in a plane which intersects at right angles with the primary oscillating center line is an arc-like shape of which center is the primary oscillating center line. 
     According to the construction, because sectional shape of the lost motion profile of the cam surface formed on the one abutment portion is the arc shape of which center is the primary oscillating center line, the clearance between the lost motion profile and the other abutment portion or abutment condition between the lost motion profile and the other abutment portion can be easily maintained. Thus, it becomes easy to maintain the proper valve clearance at the time of changing the operation property. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a main part of an internal combustion engine having a valve train of the invention, which shows a first embodiment of the invention; 
         FIG. 2  is an enlarged view of the main part in  FIG. 1 , which is a sectional view taken along the line indicated by arrows IIa-IIa and as viewed in a direction indicated by the same arrows in  FIG. 3  as to a cylinder head, and which is a sectional view taken along the line indicated by arrows IIb-IIb and as viewed in a direction indicated by the same arrows in  FIG. 3  as to a transmission mechanism; 
         FIG. 3  is a view of the valve train with a cylinder head cover of the internal combustion engine being removed, as viewed in a direction indicated by an arrow III in  FIG. 1 ; 
         FIG. 4  is a sectional view taken along the line indicated by arrows IV-IV and as viewed in a direction indicated by the same arrows in  FIG. 3 ; 
         FIG. 5  is a graph showing valve operating properties of the valve train shown in  FIG. 1 ; 
         FIG. 6  is a drawing explaining the operation of an inlet operation mechanism when a maximum valve operating property of the valve train shown in  FIG. 1  is obtained; 
         FIG. 7  is a drawing explaining the operation of the inlet operation mechanism when a minimum valve operating property of the valve train shown in  FIG. 1  is obtained; 
         FIG. 8  is a drawing explaining the operation of the inlet operation mechanism when an intermediate valve operating property of the valve train shown in  FIG. 1  is obtained; and 
         FIG. 9  is a drawing showing a second embodiment of the invention, which corresponds to  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the invention will be described below by reference to  FIGS. 1 to 9 . 
       FIGS. 1 to 8  are drawings which describe a first embodiment of the invention. Referring to  FIG. 1 , an internal combustion engine E provided with a valve train of the invention is an overhead camshaft, water-cooled, in-line four-cylinder, four-stroke internal combustion engine, and is installed transversely in a vehicle in such a manner that a crankshaft thereof extends in a transverse direction of the vehicle. The internal combustion engine E includes a cylinder block  2  in which four cylinders  1  are formed integrally, a cylinder head  3  connected to an upper end portion of the cylinder block  2  and a cylinder head cover  4  connected to an upper end portion of the cylinder head  3 , the cylinder block  2 , the cylinder head  3  and the cylinder head cover  4  making up an engine main body of the internal combustion engine E. 
     Note that in this specification, it is understood that a vertical direction denotes a direction which coincides with a cylinder axis direction A 1  of the cylinder  1  and that upward denotes a direction in which the cylinder head  3  is disposed relative to the cylinders  1  in the cylinder axis direction A 1 . In addition, a sectional shape means a sectional shape in a plane (hereinafter, simply referred to as an orthogonal plane) which intersects at right angles with a holder oscillating center line L 3 , a primary oscillating center line L 4 , a secondary oscillating center line L 5  or a rotational center line L 2 , all of which will be described later on. Then, this orthogonal plane also constitutes an oscillating plane which is a plane parallel to an oscillating direction of a holder  30 , a primary rocker arm  50  or a secondary rocker arm  60 , all of which will be described later on. 
     A cylinder bore is formed in each cylinder  1  in which a piston  5  connected to the crankshaft by a connecting rod  6  fits in such a manner as to reciprocate freely therein. In the cylinder head  3 , a combustion chamber  7  is formed in a surface which faces the cylinder bores in the cylinder axis direction A 1  in such a manner as to correspond to each cylinder  1 , respectively, and an inlet port  8  having a pair inlet openings and an exhaust port  9  having a pair of exhaust openings are also formed in the cylinder head  3  in such a manner as to open to each combustion chamber  7 . A spark plug  10  is installed in the cylinder head  3  in such a manner as to be inserted into an insertion hole formed in the cylinder  3  on an exhaust side thereof together with an ignition coil  11  connected to the spark plug  10 . 
     Here, the inlet side of the internal combustion engine E means a side where an inlet valve  14  or an entrance  8   a  to the inlet port  8  is disposed relative to a reference plane H 1  which includes cylinder axes L 1  and which is parallel to a rotational center line L 2  of an inlet cam  21  and an exhaust cam  22  which also constitutes a rotational center line L 2  of a camshaft  20 , and the exhaust side of the internal combustion engine E means a side where an exhaust valve  15  or an exit  9   a  from the exhaust port  9  is disposed. Then, the inlet side is one of one side and the other side relative to the reference plane H 1 , whereas the exhaust side is the other of the one side and the other side. 
     In the cylinder head  3 , a pair of inlet valves  14  functioning as primary engine valves and a pair of exhaust valves  15  functioning as secondary engine valves are provided for each cylinder  1 , the inlet valves  14  and the exhaust valves  15  each being made up of a poppet valve which is supported in a valve guide  12  in such a manner as to reciprocate therein and is biased in a normally closed direction. The pair of inlet valves  14  and the pair of exhaust valves  15  which belong to each cylinder  1  are operated to be opened and closed by a valve train V so as to open and close the pair of inlet openings and the pair of exhaust openings, respectively. The valve train V, excluding an electric motor  28  for driving a drive shaft  29 , which will be described later on, is disposed within a valve chamber  16  defined by the cylinder head  3  and the cylinder head cover  4 . 
     The internal combustion engine E includes further inlet system  17  and an exhaust system  18 . The inlet system  17 , which includes an air cleaner, a throttle valve and an inlet manifold  17   a  for induction of air for combustion into the inlet port  8 , is mounted on a side on the inlet side of the cylinder head  3  to which the openings  8   a  of each port  8  are made to open, whereas the exhaust system  18 , which includes an exhaust manifold  18   a  for guiding exhaust gases flowing thereinto from the combustion chambers  7  via the exhaust ports  9  to the outside, is mounted on a side on the exhaust side of the cylinder head  3  to which the openings  9   a  of each exhaust port  9  are made to open. In addition, a fuel injection valve  19 , which is a fuel supply system for supplying fuel for intake air, is installed in the cylinder head  3  in such a manner as to be inserted into an insertion hole provided on the inlet side of the cylinder head  3  so as to face the inlet port  8  of each cylinder  1 . 
     Then, air drawn in through the inlet system  17  is drawn further into the combustion chamber  7  from the inlet port  8  via the inlet valves  14  which are opened in an induction stroke where the piston  5  descends and is compressed in a compression stroke where the piston  5  ascends in a state in which the air is mixed with fuel. The air/fuel mixture is ignited by the spark plug  10  in a final stage of the compression stroke for combustion, and the piston  5 , which is driven by virtue of the pressure of combustion gases in a power stroke where the piston descends, drives and rotates the crankshaft via the connecting rod  6 . Combustion gases are discharged from the combustion chamber  7  into the exhaust port  9  as exhaust gases via the exhaust valves  15  which are opened in an exhaust stroke where the piston  5  ascends. 
     Referring to  FIG. 2 , the valve train V provided on the cylinder head  3  includes a single camshaft  20  which is rotatably supported on the cylinder head  3  in such a manner as to have a rotational center line L 2  which is parallel to the rotational center line of the crankshaft, and further includes an inlet cam  21  which is a primary valve operating cam provided on the camshaft  20  so as to rotate together with the camshaft  20  and exhaust cams  22  (refer to  FIG. 3 ) which constitutes a pair of secondary valve operating cams, an inlet operation mechanism for actuating the inlet valves  14  to be opened and closed in response to the rotation of the inlet cam  21 , and an exhaust operation mechanism for actuating the exhaust valves  15  to be opened and closed in response to the rotation of the exhaust cams. Then, in this embodiment, the inlet operation mechanism is made up of variable properties mechanism which can control the valve operating properties including opening and closing timings and maximum lift of the inlet valves  14  in accordance with the operating state of the internal combustion engine E. 
     Referring to  FIGS. 2 to 4 , the camshaft  20 , which is situated between the inlet valves  14  and the exhaust valves  15  in an orthogonal direction A 2  relative to the reference plane H 1 , which intersects at right angles with the reference plane H 1  and which is situated closer to a lower wall of the valve chamber  16 , is supported rotatably on a camshaft holder which is provided integrally on the cylinder head  3 . The camshaft holder has a plurality of, here, five, bearing portions  23  which are provided on the cylinder head  3  at certain intervals in a rotational center line direction A 3 . Each bearing portion  23  is made up of a bearing wall  23   a  which is formed integrally on the cylinder head  3  and a bearing cap  23   b  which is connected to the bearing wall  23   a . The camshaft  20  is driven to rotate at half crankshaft rotational speed, while interlocked therewith, by virtue of the power of the crankshaft which is transmitted via a valve operating transmission mechanism including a chain which is an endless transmission belt extended between a shaft end portion of the crankshaft and a shaft end portion of the camshaft  20 . Consequently, the camshaft  20 , the inlet cams  21  and the exhaust cams  22  rotate in synchronism with the rotation of the crankshaft, which is the rotation of the engine. In addition, the single inlet cam  21  is disposed between the pair of exhaust cams  22  in the rotational center line direction A 3 . 
     The exhaust operation mechanism includes a transmission mechanism Me which transmits a valve drive force of the exhaust cam  22  to each exhaust valve  15  so as to actuate the exhaust vale  15  to be opened and closed. The transmission mechanism Me includes a rocker shaft  24  as a single support shaft which is disposed directly above the camshaft  20  so as to be in parallel with the camshaft  20  and to intersect at right angles with the reference plane H 1  and which is fixedly supported on each bearing cap  23   b  and exhaust rocker arms  25  which are tertiary rocker arms as a pair of tertiary oscillating members. Each rocker arm  25 , which is supported in an oscillatory fashion at a fulcrum portion  25   c  on the rocker shaft  24  functioning as a pivot support portion, abuts with the exhaust cam  22  via a roller  26  possessed by a cam abutment portion  25   a  which is made up of an end portion of the exhaust rocker arm  25  and abuts with a valve stem  15   a  as a valve shaft of the exhaust valve  15  via an adjustment screw  27  possessed by a valve abutment portion  25   b  which is made up of the other end portion the exhaust rocker arm  25 . Here, in the exhaust rocker arm  25 , the valve abutment portion  25   b  is a location positioned closer to the exhaust valve  15  and is also a location positioned on an extension of a valve spring  13  in a direction in which the valve spring  13  extends and contracts (a direction in parallel with an axis L 8 , which will be described later on). Then, in the exhaust rocker arm  25 , the fulcrum portion  25   c  is provided at an intermediate portion, which is a location between the cam abutment portion  25   a  and the cam abutment portion  25   b . The adjustment screw  27  and an adjustment screw  65 , which will be described later on, are such as to adjust the valve clearance to an appropriate value. 
     The inlet operation mechanism includes a transmission mechanism Mi for transmitting a valve drive force F 1  (refer to  FIG. 6 ) of the inlet cam  21  to each inlet valve  14  so as to actuate the inlet valve  14  to be opened and closed and a drive mechanism Md having an electric motor  28  as an actuator for driving a movable holder  30  provided on the transmission mechanism Mi, whereby the valve operating properties of the inlet valve  14  are controlled in accordance with the shift position of the holder  30  which is driven to shift by the drive mechanism Md. 
     The transmission mechanism Mi includes the holder  30  which is supported in such a manner as to oscillate about the holder oscillating center line L 3  which is parallel to the rotational center line L 2  relative to the cylinder head  3  so as to oscillate in response to the operation of the electric motor  28 , a primary rocker arm  50  as a primary oscillating member which is supported in such a manner as to oscillate about the primary oscillating center line L 4  so as to oscillate in response to the rotation of the inlet cam  21  and a secondary rocker arm  60  as a secondary oscillating member which is supported on the holder in such a manner as to oscillate about the secondary oscillating center line L 5  so as to oscillate in response to the oscillation of the primary rocker arm  50 . The secondary rocker arm  60  transmits the valve drive force F 1  transmitted thereto via the primary rocker arm  50  to the inlet valve  14 . Therefore, in this embodiment, an inlet rocker arm for actuating the inlet valve  14  to be opened and closed is made up of a plurality of rocker arms, here, a group of rocker arms which is made up of the primary and secondary rocker arms  50 ,  60 . 
     The drive mechanism Md includes the electric motor  28 , which is mounted on the cylinder head cover  4  outside the valve chamber  16 , and the drive shaft  29  which is supported in such a manner as to oscillate relative to the cylinder head  3  so as to be driven to rotate by the reversible electric motor  28  to thereby oscillate the holder  30 . 
     Here, the primary and secondary oscillating center lines L 4 , L 5  and a rotational center line L 6  of the drive shaft  29  are parallel to the holder oscillating center line L 3 , which differs from the rotational center line L 2  of the inlet cam  21  and the exhaust cam  22 . In addition, the holder oscillating center line L 3  and the rotational center line L 2  are situated on the inlet side, whereas the rotational center line L 6  is situated on the exhaust side. 
     Referring to  FIGS. 2 ,  3 , the holder  30 , which is disposed between the pair of bearing portions  23  which are adjacent to each other in the rotational center line direction A 3  above the camshaft  20  for each cylinder  1 , includes a fulcrum portion  31  which is situated on the inlet side of the cylinder head  3  and is pivot supported on the bearing cap  23   b , a gear portion  32  as an acting portion which is situated on the exhaust side of the cylinder head  3  and on which the drive force of the electric motor  28  acts via the drive shaft  29  and primary and secondary support portions  33 ,  34  which are disposed between the holder oscillating center line L 3  and the gear portion  32  in the orthogonal direction A 2  and which support the primary and secondary rocker arms  50 ,  60 , respectively. In addition, almost the whole of the transmission mechanism Mi is disposed within an triangle having the rotational center line L 2 , the holder oscillating center line L 3  and the rotational center line L 6  as three vertexes thereof (refer to  FIG. 2 ) when viewed from the rotational center line direction A 3  (hereinafter, referred to as when viewed sideways). 
     The holder  30 , which appears something like an L-shape which bends downwardly toward the inlet cam  21  when viewed sideways, has an arm-like base portion  41  which extends linearly from the holder oscillating center line L 3  toward the gear portion  32  and a projecting portion  42  which projects from the base portion  41  in a direction to approach the inlet cam  21 . The base portion  41  is made up of a pair of side walls  43  which face each other in the rotational center line L 3  and a part  44   a  of a connecting wall  44  which connects the two side walls  43  together and which makes up an outermost end portion of the holder  30  in a radial direction which radiates from the holder oscillating center line L 3  as a center. In addition, the projecting portion  42  is made up of a pair of projecting walls  45  extending downwardly from the respective side walls  43  and the remaining part  44   b  of the connecting wall  44  which connects the pair of projecting walls  45  at portions thereof which are situated closer to the base portion  41 . 
     The base portion  41  is disposed above the camshaft  20 , the inlet cam  21  and the rocker shaft  24  in such a manner as to extend substantially in the orthogonal direction A 2  from the inlet side to the exhaust side, the fulcrum portion  31  is disposed substantially at the same position as a valve abutment portion, which will be described later on, in the orthogonal direction A 2 , and the holder oscillating center line L 3  is disposed on an extension (in  FIG. 2 , the extension is shown by chain double-dashed lines) of a valve stem  14   a  as a valve shaft of the inlet valve  14  which extends along an axis L 7  of the valve stem  14   a . By adopting this construction, a distance between the holder oscillating center line L 3  and a line of action of a reaction force F 2  (refer to  FIG. 6 ) from the inlet valve  14  is maintained small within the range of the valve stem  14   a  as a maximum limit. On the other hand, the projecting portion  42 , which is disposed to extend substantially in the cylinder axis direction A 1 , is always situated on the exhaust side within the oscillating range of the holder  30 . 
     The fulcrum portion  31  and the secondary support portion  34  are provided on each side wall  43 , the gear portion  32  is provided on the connecting wall  44  in such a manner as to extend from the base portion  41  to the projecting portion  42 , and the primary support portion  33  is provided on each projecting wall  45 . As shown in  FIG. 4 , the fulcrum portion  31  is pivot supported on a support portion  23   c  formed on the bearing cap  23   b . The support portion  23   c  defines a hole  71  having a circular section in cooperation with a holding cap  70  connected to an upper end portion of the bearing cap  23   b  with a bolt, so that a support shaft  31   a  formed on the fulcrum portion  31  is inserted into the hole  71  in such a manner as to slide therein. Then, a support shaft  31   a  of a holder  30  belonging to the adjacent cylinder  1  is supported on the common gearing cap  23   b.    
     Referring to  FIG. 2 , in a lower side portion of each side wall  43  which constitutes a lower side portion of the base portion  41 , a portion on the camshaft  20  side where the projecting wall  45  projects downwardly from the side wall  43  forms an accommodating portion  39  which defines an accommodating space  39   a  for accommodating therein the holder  30  and the rocker shaft  24  which is a member disposed on the periphery of the primary rocker arm  50  in cooperation with a portion of the projecting wall  45  which is closer to the side wall  43 . The accommodating space  39   a  opens downwardly toward the rocker shaft  24 . Then, a ratio at which the rocker shaft  24  is accommodated in the accommodating space  39  becomes maximum when the rocker shaft  24  occupies a primary limit position as a predetermined position which is an oscillation position resulting when the holder  30  oscillates most downwardly (a state shown in  FIG. 2  or  FIG. 6 ). 
     Referring to  FIG. 3 , as well, in the base portion  41 , a portion excluding the fulcrum portion  31  is disposed between the pair of exhaust rocker arms  25  in the rotational center line direction A 3 , and the primary and secondary rocker arms  50 ,  60  are disposed between the pair of side walls  43  in the rotational center line direction A 3 . The primary support portion  33  and the primary oscillating center line L 4  are situated on the exhaust side, whereas the secondary support portion  34  and the secondary oscillating center line L 5  are situated on the inlet side. Then, the distance to the holder oscillating center line L 3  gets longer in the order of the secondary oscillating center line L 5 , the rotational center line L 2 , the primary oscillating center line L 4  and the rotational center line L 6 . Therefore, as shown in  FIG. 2 , with a primary intersection point C 1  between the orthogonal plane and the primary oscillating center line L 4  and a secondary intersection point C 2  between the orthogonal plane and the secondary oscillating center line L 5 , a distance between the holder oscillating center line L 3  and the primary intersection point C 1  is longer than a distance between the holder oscillating center line L 3  and the secondary intersection point C 2 . 
     In addition, in the oscillating range of the holder  30 , the primary oscillating center line L 4  includes the holder oscillating center line L 3  and is situated on a camshaft side where the camshaft  20  is situated or a lower side relative to a specific plane H 2  which intersects at right angles with the reference plane H 1 , whereas the secondary oscillating center line L 5  is situated on an opposite side to the camshaft side or an upper side. In this embodiment, when the holder  30  occupies a secondary limit position as a predetermined position which is an oscillation position resulting when the holder  30  oscillates most upwardly (a state shown in chain double-dashed lines in  FIG. 1 , or a state shown in  FIG. 7 ), the primary oscillating center line L 4  is situated substantially on the specific plane H 2  and is situated below the specific plane H 2  when the holder  30  occupies any other position than the secondary limit position. 
     The primary support portion  33 , which regulates the primary oscillating center line L 4 , is provided on a lower end portion of the projecting portion  42  which constitutes a location closer to the inlet cam  21  and has a cylindrical support shaft  35  which is press fitted into a hole formed in each side wall  45 . The primary rocker arm  50 , which is supported by the support shaft  35  at a fulcrum portion  51  in an oscillatory fashion via a multiplicity of needles  36 , abuts with the inlet cam  21  at a roller  53  possessed by a cam abutment portion  52  made up of one end portion of the primary rocker arm  50  and abuts with the secondary rocker arm  60  at a drive abutment portion  54  made up of the other end portion thereof. In the primary rocker arm  50 , the fulcrum portion  51  is provided at an intermediate portion which is a location between the cam abutment portion  52  and the drive abutment portion  54 . Then, the primary rocker arm  50  is biased by virtue of a biasing force of a biasing device (not shown) such as a spring held by the holder  30  such that the roller  53  is pressed against the inlet cam  24  at all times. In addition, an accommodation space  57  for accommodating therein the roller  53  is provided in the primary rocker arm  50  in such a manner as to extend from the fulcrum portion  51  to the cam abutment portion  52 , and the accommodation space  57  constitutes an escape space which allows the passage of a cam lobe portion  21   b  of the rotating inlet cam  21 . Then, the primary rocker arm  50  and the inlet cam  24  can be disposed close to each other, while the interference of the primary rocker arm  50  with the inlet cam  24  is avoided by the accommodation space  57 . 
     The secondary support portion  34 , which regulates the primary oscillating center line L 5 , is provided on the base portion  41  so as to be situated between the primary support portion  33  and the holder oscillating center line L 3  in the orthogonal direction A 2  and has a support shaft  37  which is press fitted into a hole formed in each side wall  43 . The secondary rocker arm  60 , which is supported by the support shaft  37  at a fulcrum portion  61  in an oscillatory fashion via a multiplicity of needles  38 , abuts with the drive abutment portion  54  of the primary rocker arm  50  at a roller  63  possessed by a follower abutment portion  62  made up of one end portion of the secondary rocker arm  60  and abuts with the valve stems  14   a  as the abutment portions of the pair of inlet valves  14 , respectively, at adjustment screws  65  possessed by a pair of valve abutment portions  64  made up of the other end portion thereof. Here, in the secondary rocker arm  60 , the valve abutment portion  64  is a location which is situated closer to the inlet valve  14  and is also a location which is situated on an extension of the valve spring  13  in a direction (a direction parallel to the axis L 7 ) in which the valve spring  13  extends and contracts. Then, in the secondary rocker arm  60 , the fulcrum portion  61  is provided on an intermediate portion which is a location between the follower abutment portion  62  and the valve abutment portion  64 . In addition, since the sectional shape of the roller  63  is of a circular shape, the sectional shape of an abutment surface of the follower abutment portion  62 , which is brought into abutment with a cum profile  55 , which will be described later, is of an arc-like shape, as well. 
     On the drive abutment portion  54  acting as one of the drive abutment portion  54  and the follower abutment portion  62  which are brought into abutment with each other, the cam profile  55  is formed, which cam profile  55  has a lost motion profile  55   a  which maintains the inlet valve  14  in a closed state and a drive profile  55   b  which puts the inlet valve  14  in an opened state through the abutment with the roller  63  of the follower abutment portion  62  which acts as the other abutment portion. Then, an arm abutment position P 2 , which is an abutment position where the cam profile  55  and the roller  63  abut with each other, resides above the camshaft  20  and the rocker shaft  24  and is situated at a position which is superposed above the camshaft  20  and the rocker shaft when viewed from the cylinder axis direction A 1  (hereinafter, referred to as when viewed from, the top). 
     The lost motion profile  55   a  is formed so as to have an arc-like sectional shape which is formed about the primary oscillating center line L 4  and is designed such that the valve drive force F 1  of the inlet valve  21  which is transmitted via the primary rocker arm  50  is not transmitted to the secondary arm  60  in a state in which a clearance is formed between the lost motion profile  55   a  and the roller  63 , as well as in a state in which the roller  63  is in abutment with the lost motion profile  55   a . As this occurs, the primary rocker arm  50  is in a rest state where the secondary rocker arm  60  is not oscillated by the inlet cam  21  via the primary rocker arm  50 . Then, when the primary rocker arm  50  and the secondary rocker arm  60  are brought into abutment with each other in a state where the roller  53  of the primary rocker arm  50  is in abutment with a base circle portion  21   a  of the inlet cam  21 , the roller  63  abuts with the lost motion profile  55   a  at all times. Consequently, when the arm abutment position P 2  is located at an arbitrary position on the lost motion profile  55   a , the inlet valve  14  is maintained in the closed state by virtue of the spring force of the valve spring  13 , and a valve clearance is formed between a valve abutment surface  65   a  of the adjustment screw  65  which acts as a valve abutment surface of the valve abutment portion  64  and a distal end surface  14   b  of the valve stem  14   a  which acts as an abutment surface of the inlet valve  14 . 
     The drive profile  55   b  transmits the valve drive force F 1  of the inlet cam  21  which is transmitted thereto via the primary rocker arm  50  to the secondary rocker arm  60  so as to oscillate the secondary rocker arm  60 , and when the adjustment screw  65  is in abutment with the valve stem  14   a , the secondary rocker arm  60  which is oscillating transmits the valve drive force F 1  to the inlet valve  14  to thereby put the inlet valve  14  into an opened state with a predetermined lift amount being provided. 
     Consequently, the oscillating position of the secondary rocker arm  60  relative to the holder  30  is regulated by the primary rocker arm  50 . 
     In addition, the drive abutment portion  54  has a pent roof-like thin portion  54   a  which projects diagonally downwardly toward the inlet cam  24  or the inlet valve  14 , and the lost motion profile  55   a  is formed on the thin portion  54   a . Then, an accommodation portion  56  in which the rocker shaft  24  is accommodated in accordance with the oscillating position thereof is formed by making use of the thin portion  54   a  in the primary rocker arm  50  between the primary oscillating center line L 4  and the lost motion profile  55   a  in a radial direction which radiates from the primary oscillating center line L 4  as a center. Then, as the holder  30  approaches the primary limit position and the primary rocker arm  50  oscillates in a direction in which the lift amount of the inlet valve  14  is increased, the ratio at which the rocker shaft  24  is accommodated in the accommodation portion  56  is increased. 
     The sectional shape of the valve abutment surface  65   a  of the adjustment screw  65  which abuts with the distal end surface  14   b  of the inlet valve  14  is an arc that is formed about the holder oscillating center line L 3  when in a state where the cam profile  55  of the primary rocker arm  50  and the roller  63  of the secondary rocker arm  60  are in abutment with each other and a state where the secondary rocker  60  is in the rest state, that is, a state where the roller  63  abuts with the lost motion profile  55   a . Due to this, the valve abutment surface  65   a  is made up of a partially cylindrical surface which is part of a cylindrical surface that is formed about the holder oscillating center line L 3  or a partially spherical surface which is part of a spherical surface that is formed about a point on the holder oscillating center line  3  when in a state the secondary rocker arm  60 , which is in the rest state, abuts with the lost motion profile  55   a . Then, the secondary rocker arm  60 , when in the rest state, does not oscillate relative to the holder  30  irrespective of the oscillating position of the holder  30  in the state where the roller  63  of the secondary rocker arm  60  does not abut with the lost motion profile  55   a  of the primary rocker arm  50 . 
     The pair of fulcrum portions  31  on the base portion constitutes an accommodation space in which the pair of valve abutment portions  64  provided in series in the rotational center line direction A 3  and the pair of adjustment screws  65  are accommodated. 
     Furthermore, when the primary rocker arm  60  is in the rest state so as to maintain the inlet valve  14  in the closed state, the fulcrum portion  31  is situated at a position where the fulcrum portion  31  is superposed on the valve abutment portion  64  and the adjustment screw  65  when viewed sideways, and the holder oscillating center line L 3  is situated at a position where the holder oscillating center line L 3  intersects at right angles with the valve abutment portion  64  and, furthermore, the adjustment screw  65 , and more precisely, the holder oscillating center line L 3  is situated at a position where it intersects at right angles with the center axis of the adjustment screw  65 . 
     In addition, the primary rocker arm  50  is disposed in such a manner as to extend long in the cylinder axis direction A 1  and is situated on the exhaust side except for the drive abutment portion  54  within the oscillating range of the holder, the cam abutment position P 1  which is the abutment position where the roller  53  abuts with the inlet cam  21  is situated on the exhaust side, and the arm abutment position P 2  is situated on the inlet side. Then, the roller  53  abuts with the inlet cam  21  at a portion which is closer to the exhaust valve  15  in the orthogonal direction A 2 , and when the holder  30  oscillates, the cam abutment position P 1  shifts mainly in the cylinder axis direction A 1 . On the other hand, the secondary rocker arm  60  is disposed in such a manner as to extend long in the orthogonal direction A 2  and along the base portion  41  and is situated at on the inlet side within the oscillating range of the holder  30 . 
     Referring to  FIG. 4 , as well, the drive shaft  29  is a single rotating shaft which is common to all the cylinders  1  in the orthogonal direction A 2  and is rotatably supported on the bearing caps  23   b  at journal portions  29   a  thereof by means of holding caps  72  which are connected to the bearing caps  23   a  with bolts to thereby be rotatably supported on the cylinder head  3 . Drive gears  29   b  are provided on the drive shaft  29  at certain intervals in the rotational center line direction A 3  for each cylinder  1 , and the drive gear  29   b  meshes with the gear portion  32  formed in the connecting wall  44  so as to oscillate the holder  30  about the holder oscillating center line L 3  by virtue of the torque of the electric motor  28 . 
     The gear portion  32  is a surface on the connecting wall  44  constituting part of the base portion  41  and the projecting portion  42  which surface faces the drive shaft  29  and is formed to extend between the base portion  41  and the projecting portion  42  on an outer circumferential surface  44   c  in a radial direction which radiates from the holder oscillating center line L 3  as a center. This outer circumferential surface  44   c  constitutes a location of the holder  30  which is farthest apart from the holder oscillating center line L 3 . The gear portion  32  is formed such that the shape thereof on the orthogonal plane becomes an arc-like shape which is formed about the holder oscillating center line L 3  and has a number of teeth which are arranged in an arc-like fashion on the orthogonal plane. Then, a line of action of a drive force exerted from the drive shaft  29  so as to act on the gear portion  32  is directed in a tangential direction to an arc that is formed about the holder oscillating center line L 3  on the orthogonal plane. 
     In addition, the drive shaft  29  is situated on an extension of a valve stem  15   a  of the exhaust valve  15  which extends along an axis L 8  of the valve stem  15   a , and most of the whole of drive shaft  29  is situated closer to the reference plane H 1  than the extension of the valve stem  15   a . In addition, in the orthogonal direction A 2 , the drive shaft  29  is situated substantially at the same position as those of the valve abutment portion  25   b  of the exhaust rocker arm and a distal end face  15   b  of the valve stem  15   a . Due to this, as shown in  FIG. 4 , when viewed from the top, the drive shaft  29  is situated at a position which is superposed above the valve abutment portion  25   b  and the distal end face  15   b . Here, in the exhaust valve  15 , the valve stem  15   a  is an abutment portion with which the valve abutment portion  25  is brought into abutment, and the distal end face  15   b  is an abutment surface of the abutment portion. 
     The electric motor  28  is controlled by an electronic control unit (hereinafter, referred to as ECU) into which detection signals from operating conditions detecting units for detecting operating conditions of the internal combustion engine E are inputted. The operating conditions detecting units include a rotational speed detecting unit for detecting the engine rotational speed of the internal combustion engine E, a load detecting unit for detecting the load of the internal combustion engine E and the like. Then, by controlling the rotational direction and rotational speed of the electric motor  28  according to the operating conditions by the ECU, the rotational direction and rotational amount of the drive shaft  29  are controlled, whereby the holder  30  is driven to oscillate within the oscillating range which is regulated between the primary limit position and the secondary limit position by the electric motor  28 , irrespective of the rotational position of the inlet cam  21  or the camshaft  20 . Then, the primary rocker arm  50  having the primary center line L 4  which oscillates together with the holder  30  and the secondary rocker arm  60  having the secondary oscillating center line L 5  shift, respectively, in accordance with the oscillating position of the holder that is controlled in accordance with the operating conditions, whereby the opening and closing timings, maximum lift amount and maximum lift timing are changed continuously. 
     In addition, as shown in  FIG. 3 , the holder  30 , the primary and secondary rocker arms  50 ,  60  and the drive gear  29   b  are formed so as to be substantially symmetrical with respect to plane relative to a plane H 3  which contains a central point which bisects the width of the primary rocker arm  50  in the rotational center line direction A 3  and intersects at right angles with the holder oscillating center line L 3 . Consequently, since in the transmission mechanism Mi, there is generated no moment acting around a straight line which intersects at right angles with the reference plane H 1  based on the valve drive force F 1 , the reaction force F 2  from the inlet valve  14  and the drive force of the drive shaft  29 , an increase in abutment pressure that is generated locally at a sliding portion by the moment is prevented, thereby the durability of the transmission mechanism Mi being increased. 
     Next, referring to  FIGS. 5 to 8 , the valve operating properties will be described below that can be obtained by the inlet operation mechanism. 
     Referring to  FIG. 5 , the valve operating properties are changed between a maximum valve operating property Ka and a minimum valve operating property Kb continuously with the maximum valve operating property Ka and the minimum valve operating property Kb acting as limit properties, whereby a countless number of intermediate valve operating properties Kc ban be obtained between both the valve operating properties Ka, Kb. For example, the opening and closing timings and maximum valve lift amount of the inlet valve  14  changes as will be described below from the maximum valve operating property Ka which is a valve operating property resulting when the internal combustion engine E is operated in a high rotational speed region or high load region to the minimum valve operating property Kb via the intermediate valve operating properties Kc which are valve operating properties resulting when the internal combustion engine E is operated in a low rotational speed region or low load region via. The valve opening timing is delayed continuously, whereas the valve closing timing is advanced continuously in a large changing amount when compared with the opening timing so that the valve opening period becomes short continuously, and furthermore, the maximum lift timing where the maximum lift amount can be obtained is advanced continuously, and the maximum lift amount becomes small continuously. Note that the maximum lift timing is introduced to a timing which bisects the valve timing period. 
     In addition, in this embodiment, the minimum valve operating property is a valve operating property where a valve rest state can be obtained where the maximum lift amount becomes zero and the opening and closing operation of the inlet valve  14  comes to rest. 
     In the valve operating properties that can be obtained by the inlet operation mechanism, in the maximum valve operating property Ka, the valve opening period and the maximum lift amount become maximum, and the valve closing timing is introduced to a timing where it is most delayed. The maximum valve operating property Ka can be obtained when the holder  30  occupies the primary limit position as shown in  FIGS. 2 ,  6 . Note that in  FIGS. 6 to 8 , the transmission mechanism Mi is shown in solid lines which results when the inlet valve  14  is in the closed state, whereas the transmission mechanism Mi is shown in chain double-dashed lines which results when the inlet valve  14  is opened in the maximum lift amount. 
     Referring to  FIG. 6 , when situated at the primary limit position, the holder  30  occupies an oscillating position which is closest to the rotational center line L 2  or the inlet cam  21  within the oscillating range, and the primary support portion  33  is situated so as to be superposed above the cam lobe portion  21   b  of the inlet cam  21  in the cylinder axis direction A 1 . The roller  63  of the secondary rocker arm  60  is in a state where the roller  63  abuts with the lost motion profile  55   a  of the cam profile  55  in a state where the roller  53  of the primary rocker arm  50  abuts with the base circle portion  21   a  of the inlet cam  21 . As this occurs, the rocker shaft  24  is accommodated in the accommodation space  56   a  at a relatively small ratio. When the primary rocker arm  50  is brought into abutment with the cam lobe portion  21   b  to thereby be caused to oscillate in a counter-rotational direction R 2  (a direction opposite to the rotational direction R 1  of the inlet cam  21 ) by virtue of the valve drive force F 1 , the drive profile  55   b  abuts with the roller  63 , so that the secondary rocker arm  60  is caused to oscillate in the counter-rotational direction R 2 , whereby the secondary rocker arm  60  opens the inlet valve  14  against the spring force of the valve spring  13 . Then, the rocker shaft  24  is accommodated in the accommodation space  56   a  at a maximum ratio. 
     On the other hand, the minimum valve operating property Kb can be obtained when the holder  30  occupies the secondary limit position as shown in  FIG. 7 . In the minimum valve operating property Kb, irrespective of the fact that the primary rocker arm  50  is caused to oscillate by virtue of the valve drive force F 1  of the inlet cam  21 , the roller  63  is in the state where the roller  63  abuts with the lost motion profile  55   a , and the secondary rocker arm  60  is in the rest stage. The holder  30 , which is situated at the secondary limit position, occupies a farthest oscillating position from the rotational center line L 2  or the inlet cam  21  within the oscillating range. 
     In addition, when the holder  30  occupies a central position which is substantially the center of the oscillating range, as shown in  FIG. 8 , as an oscillating position between the primary limit position and the secondary limit position, an intermediate valve operating property Kc 1  can be obtained as one of a countless number of intermediate valve operating properties Kc between the maximum valve operating property Ka and the minimum valve operating property Kb, as shown in  FIG. 5 . In the intermediate valve operating properties Kc, when compared with the maximum valve operating property Ka, the valve opening period and maximum lift amount become small, and the opening timing is introduced to timing where it is delayed, whereas the closing timing and the maximum lift timing are introduced to timing where they are advanced. 
     Thus, in the valve train V, as the maximum lift amount becomes smaller, while the opening timing is delayed in a relatively small changing amount, the closing timing and the maximum lift timing are advanced in a relative large changing amount when compared with the opening timing, whereby the inlet valve  14  is closed earlier. Due to this, when the internal combustion engine E is operated in the low rotational speed region or low load region, the inlet valve  14  is operated to be opened and closed in a small lift amount region where the maximum lift amount is small, and the valve operating properties are controlled so that the closing timing of the inlet valve  14  is advanced, whereby a pumping loss is reduced to thereby increase the fuel consumption performance by implementing an earlier closing of the inlet valve  14 . 
     Next, referring to  FIGS. 5 ,  6 ,  7 , the operation of the transmission mechanism Mi will be described below which results when the holder  30  oscillates from the primary limit position to the secondary limit position. 
     When the drive force of the drive shaft  29  driven by the electric motor  28  acts on the gear portion  32 , whereby the holder  30  oscillates upwardly from the primary limit position in an oscillating direction (in the counter-rotational direction R 2 ) in which the holder  30  moves apart from the rotational center line L 2 , the cam abutment position P 1  shifts in the counter-rotational direction R 2 , and at the same time the primary and secondary oscillating center lines L 4 , L 5  oscillate together with the holder  30  so that the arm abutment position P 2  shifts in a direction in which the maximum lift amount of the inlet valve  14  is decreased and in a direction to move apart from the rotational center line L 2 , whereby the primary and secondary rocker arms  50 ,  60  oscillate around the primary and secondary oscillating center lines L 4 , L 5 , respectively. In  FIG. 7 , L 4   a , L 5   a , P 1   a  and P 2   a  denote, respectively, primary and secondary oscillating center lines, a cam abutment position and an arm abutment position when the holder occupies the primary limit position. 
     When the primary oscillating center line L 4  oscillates, the cam abutment position P 1  shifts in the counter-rotational direction R 2 , and the timing when the roller  53  is brought into abutment with the cam lobe portion  21   b  is advanced, while the drive abutment portion  54  shifts in a direction in which a shift range of the arm abutment position P 2  on the lost motion profile  55   a  (a range of the rotational angle of the camshaft  20  or a range of the crank angle of the crankshaft) is increased in a state where the roller  53  is in abutment with the base circle portion  21   a . Then, even in the event that the shift range of the arm abutment position P 2  on the lost motion profile  55   a  is expanded, so that the arm abutment position R 2  is brought into abutment with the cam lobe portion  21   b , whereby the primary rocker arm  50  starts to oscillate, since the roller  63  stays on the lost motion profile  55   a , the secondary rocker arm  60  is in the rest state, and when the inlet cam  21  rotates further so that the primary rocker arm  50  is caused to oscillate more largely, whereby the roller  63  is brought into abutment with the drive profile  55   b , the secondary rocker arm  60  oscillates largely, whereby the inlet valve  14  is opened. Due to this, even with the roller  63  being in abutment with an apex  21   b   1  of the cam lobe portion  21 , the oscillating amount of the secondary rocker arm  60  that is caused to oscillate by the drive profile  55   b  is reduced when compared with when at the primary limit position, whereby the maximum lift amount of the inlet valve  14  is reduced. Then, in this embodiment, the shape of the inlet cam  21 , the shape of the cam profile  55 , and the positions of the primary and secondary oscillating center lines L 4 , L 5  are set such that when the holder oscillates from the primary limit position toward the secondary limit position, while the opening timing of the inlet valve  14  is, as shown in  FIG. 5 , delayed in a relatively small changing amount, the closing timing and maximum lift amount of the inlet valve  14  are advanced in a larger changing amount than the changing amount of the opening timing. 
     In addition, the valve operating properties are controlled such that when the holder  30  oscillates from the secondary limit position toward the primary limit position in such a manner as to approach the rotational center line L 2 , the opening timing of the inlet valve  14  advances continuously from the minimum valve operating property Kb to the maximum valve operating property Ka, whereas the closing timing is delayed continuously, so that the valve opening period is extended continuously, and furthermore, the maximum lift amount timing is delayed continuously and the maximum lift amount is increased continuously. 
     In addition, as is clear from  FIGS. 6 ,  7 , since, when the oscillating position of the holder  30  is situated at the primary limit position where the maximum valve operating property Ka can be obtained where the maximum lift amount becomes maximum, the cam abutment position P 1  where the roller  53  of the cam abutment portion  52  abuts with the cam lobe portion  21   b  of the inlet cam  21  is situated at a position close to a specific straight line L 10  which passes through the holder oscillating center line L 3  and the rotational center line L 2  on the orthogonal plane which intersects at right angles with the holder oscillating center line L 3  when compared with when the holder  30  occupies the secondary limit position where the minimum valve operating property Kb can be obtained where the maximum lift amount becomes smallest, as the holder  30  approaches the primary limit position where the valve drive force is increased, the cam abutment position P 1  where the roller  53  abuts with the cam lobe portion  21   b  approaches the specific straight line L 10  on the orthogonal plane. 
     Next, referring to  FIG. 7 , the operation of the primary and secondary rocker arms  50 ,  60  will be described below which results when the holder  30  oscillates within the oscillating range. 
     Since the primary and secondary rocker arms  50 ,  60  shift in accordance with the oscillating positions of the primary and secondary oscillating center lines L 4 , L 5  which oscillate together with the holder, the relative position of the primary and secondary oscillating center lines L 4 , L 5  on the holder  30  remains unchanged, and moreover, since the sectional shape of the lost motion profile  55   a  is the arc-like shape which is formed about the primary oscillating center line L 4 , the positional relationship among the three members such as the primary and secondary oscillating center lines L 4 , L 5  and the arm abutment position P 2  remains unchanged irrespective of the oscillating position of the holder  30  when the lost motion profile  55   a  and the roller  63  are in the abutment state where the two members abut with each other. 
     In addition, since the primary and secondary oscillating center lines L 4 , L 5  oscillate together with the holder  30 , the control range of the valve operating properties can be set large by increasing the shift amount of the cam abutment position P 1 . For example, in order to obtain the same abutment position as the arm abutment position relative to the lost motion profile  55   a , as with primary and secondary rocker arms n 1 , n 2  shown in chain triple-dashed lines in  FIG. 7 , a primary oscillating center line N 3  shifts, and when compared with a case where while a primary oscillating center line n 3  shifts, a secondary oscillating center line n 4  does not shift, in this transmission mechanism Mi, the shift amount of the cam abutment position P 1  can be increased. As a result, when compared with the conventional example, the opening and closing timings of the inlet valve  14  can be changed in a large oscillating amount. Then, even in the event that the holder oscillates in a large oscillating amount so that the control range of the valve operating properties is set large, the relative shift amount of the arm abutment position P 2  with the roller on the cam profile  55   a  can be suppressed to a small level. 
     Next, the function and advantage of the embodiment constructed as has been described heretofore will be described below. 
     The transmission mechanism Mi includes the primary and secondary rocker arms  50 ,  60  which have, respectively, the drive abutment portion  54  and the follower abutment portion  62  which abut with each other and the holder  30  which is caused to oscillate around the holder oscillating center line L 3  by the electric motor  28  and which support the primary and secondary rocker arms  50 ,  60  in an oscillatory fashion so that the primary and secondary oscillating center lines L 4 , L 5  oscillate together. The cam profile  55  having the lost motion profile  55   a  and the drive profile  55   b  is formed on the drive abutment portion  54 , and since the sectional shape of the lost motion profile  55   a  on the orthogonal plane which intersects at right angles with the primary oscillating center line L 4  is the arc-like shape which is formed about the primary oscillating center line L 4 , the relative position of the primary and secondary oscillating center lines L 4 , L 5  in the holder  30  remains unchanged, when the valve operating properties are changed through the shift of the primary and secondary rocker arms  50 ,  60  in accordance with the oscillating positions of the primary and secondary oscillating center lines L 4 , L 5  which rotate together with the holder  30 . Moreover, since the sectional shape of the lost motion profile  55   a  is the arc-like shape which is formed about the primary oscillating center line L 4 , it becomes easy to maintain the clearance formed between the lost motion profile  55   a  and the roller  63  or the abutment state between the lost motion profile  55   a  and the roller  63 , thereby making it possible to maintain an appropriate valve clearance even at the time of changing the valve operating properties. Due to this, the increase in noise can be prevented which would otherwise result, for example, from the valve striking noise by virtue of an increase in valve clearance and collision of both the rocker arms  50 ,  60  with each other. In addition, even in the event that the holder  30 , which supports the primary and secondary rocker arms  50 ,  60 , oscillates in a large oscillating amount in order to increase the control range of the valve operating properties, since the primary and secondary oscillating center lines L 4 , L 5  oscillate together with the holder  30 , when compared with the case where while one of the primary and secondary oscillating center lines shifts, the other does not, the relative shift amount of the arm abutment position P 2  can be suppressed to a small level, and therefore, also in this case, it becomes easy to maintain the clearance between the cam profile  55   a  and the roller  63  or the abutment state therebetween, thereby making it possible to set large the control range of the valve operating properties. 
     The secondary rocker arm  60  has the valve abutment portion  64  which has, in turn, the valve abutment surface  65   a  which is brought into abutment with the inlet valve  14 , and the distance between the primary oscillating center line L 4  and the holder oscillating center line L 3  is longer than the distance between the secondary oscillating center line L 5  and the holder oscillating center line L 3 , whereby since the valve drive force F 1  of the inlet cam  21  is transmitted to the inlet valve  14  only through the primary and secondary rocker arms  50 ,  60 , the transmission mechanism Mi is made compact in size, and hence the valve train V itself is made compact in size. Due to this, the cylinder head  3  on which the valve train V is provided becomes compact in size. In addition, when the holder  3  oscillates, since the shift amount of the primary oscillating center line L 4  becomes larger than that of the secondary oscillating center line L 5 , the shift amount of the cam abutment position P 1  can be increased, and therefore, the control range of the opening closing timings of the inlet valve  14  can be set large. Moreover, since the shift amount of the valve abutment position which is the abutment position where the valve abutment portion  64  of the secondary rocker arm  60  abuts with the inlet valve  14  can be reduced, the wear of the valve abutment portion  64  can be suppressed, thereby making it possible to extend a period of time when the proper valve clearance is maintained. 
     In the holder  30  having the base portion  41  which extends from the holder oscillating center line L 3  toward the gear portion  32  substantially in the orthogonal direction A 2  and the projecting portion  42  which projects from the base portion  41  in the direction to approach the inlet cam  21  substantially in the cylinder axis direction A 1 , the primary support portion  33  is provided on the projecting portion  42  for supporting the primary rocker arm  50  in an oscillatory fashion, and the secondary support portion  34  is provided on the base portion  41  for supporting the secondary rocker arm  60  in an oscillatory fashion. Since the primary and secondary support portions  33 ,  34  are disposed between the holder oscillating center line L 3  and the gear portion  32 , the gear portion  32  is situated farther than the primary and secondary support portions  33 ,  34  relative to the holder oscillating center line L 3 , and therefore, the drive force of the electric motor  28  can be reduced, whereby the electric motor  28  is made compact in size. Moreover, since the primary support portion  33  and the secondary support portion  34  are provided on the projecting portion and the base portion separately, the space between the holder oscillating center line L 3  and the gear portion  32  can be reduced, whereby the holder  30  is made compact in size between the holder oscillating center line L 3  and the gear portion  32 . Due to this, the cylinder head  3  on which the valve train V is provided can be made compact in size in the orthogonal direction A 2 . In addition, since the primary support portion  33  which is provided on the projecting portion  41  is situated closer to the inlet cam  21  than to the base portion  41 , in the primary rocker arm  50 , when compared with a case where the primary support portion is provided on the base portion  41 , the distance between the primary oscillating center line L 4  and the cam abutment portion  52  becomes short, a required rigidity against the valve drive force F 1  is ensured, while the primary rocker arm  50  is made light in weight. 
     The accommodation space  39   a  for accommodating the rocker shaft  24  which supports the exhaust rocker arm  25  is formed in the holder  30 , whereby the holder  30  and the rocker shaft  24  can be disposed close to each other, while the interference of the holder  30  with the rocker shaft  24  is avoided, and therefore, the valve train V is made compact in size, and moreover, the oscillating range of the holder  30  can be increased within the limited space, and therefore, the control range of the valve operating properties can be increased. 
     In the primary rocker arm  50 , the accommodation space  56   a  for accommodating the rocker shaft  24  which supports the exhaust rocker arm  25  in an oscillatory fashion is formed between the primary oscillating center line L 4  and the lost motion profile  55   a  in the radial direction which radiates from the primary oscillating center line L 4  as a center, whereby almost no valve drive force F 1  or reaction force F 2  from the inlet valve  14  is transmitted to the lost motion profile  55   a , and therefore, the rigidity required for the portion of the drive abutment portion  54  where the lost motion profile  55   a  is formed only has to be small, and the portion can be made thin, and therefore, the primary rocker arm  50  is made light in weight. In addition, the accommodation space  56   a  is formed by making used of the thin portion  54   a . Then, since, by allowing the rocker shaft  24  to be accommodated in the accommodation space  56   a , the primary rocker arm  50  and the rocker shaft  24  can be disposed close to each other, while the interference of the primary rocker arm  50  with the rocker shaft  24  is avoided, the valve train V is made compact in size. Furthermore, by allowing the rocker shaft to also be accommodated in the accommodation space  39   a , the primary rocker arm  50  and the rocker shaft  24  can be disposed close to each other, while the interference of the primary rocker arm  50  with the rocker shaft  24  is avoided, and therefore, the valve train V is made compact in size. In addition, since the oscillating range of the holder  30  which supports the primary rocker arm  50  within the space in the limited valve chamber  16  can be increased, the control range of the valve operating properties can be set large. 
     Due to the primary rocker arm  50  which is in abutment with the inlet cam  24  and the secondary rocker arm  60  being in the state where the primary rocker arm  50  and the secondary rocker arm  60  are in abutment with each other at the abutment portions  54 ,  63 , respectively, the sectional shape of the valve abutment surface  65   a  of the valve abutment portion  64  provided on the secondary rocker arm  60  having the secondary oscillating center line L 5  which oscillates together with the holder  30  on the orthogonal plane which intersects at right angles with the holder oscillating center line L 3  is the arc-like shape which is formed about the holder oscillating center line L 3  in the state where there exists no clearance in the transmission path of the valve drive force which extends from the inlet cam  21  to the secondary rocker arm  60  via the primary rocker arm  50 , and with the secondary rocker arm  60  being in the rest state where the secondary rocker arm  60  is not caused to oscillate by the inlet cam  21  via the primary rocker arm  50 , and therefore, even in the event that the holder  30  oscillates about the holder oscillating center line L 3  in order to change the valve operating properties, the secondary rocker arm  60  having the secondary oscillating center line L 5  which oscillates together with the holder  30  oscillates together with the holder  30 , and the clearance between the valve abutment surface  65   a  and the distal end face  14   b  of the inlet valve  14  is maintained constant, whereby the valve clearance from the inlet cam  21  to the inlet valve  14  is maintained constant. 
     The valve abutment portion  64  having the valve abutment surface  65   a  which is brought into abutment with the distal end face  14   b  of the inlet valve  14  is provided on the secondary rocker arm  60  at the position which intersects at right angles with the holder oscillating center line L 3 , whereby the valve abutment surface  65   a  is allowed to be close to the holder oscillating center line L 3 , and therefore, even in the event that the secondary oscillating center line L 5  oscillates due to the oscillation of the holder  30 , whereby the valve abutment position where valve abutment surface  65   a  abuts with the distal end face  14   b  is caused to shift, the shift amount is made to be small, and in this respect, as well, the progress in wear of the valve abutment surface  65   a  attributed to the oscillation of the holder  30  is suppressed, and then, the period of time when the appropriate valve clearance is maintained is extended. In addition, the valve abutment surface  65   a  resides close to the holder oscillating center line L 3 , whereby the valve abutment portion  64  can be reduced, and therefore, the secondary rocker arm  60  is made small in size. 
     The gear portion  32  on which the drive force of the drive shaft  29  acts is provided on the holder  30  on the outer circumference  44   c  which is the location of the holder  30  which is farthest apart from the holder oscillating center line L 3  on the orthogonal plane, whereby on the holder  30 , the distance from the holder oscillating center line L 3  to the acting position of the drive force can be made substantially maximum, and therefore, the drive force of the electric motor  28  can be reduced, the electric motor  28  being thereby made compact in size. In addition, the gear portion  32  is provided so as to extend from the base portion  41  to the projecting portion  42 , whereby the forming range of the gear portion  32  can be increased, and therefore, the oscillating range of the holder  30  can be increased. 
     When the holder  30  oscillates in the oscillating direction to move away from the rotational center line L 2 , the cam abutment position P 1  shift in the counter-rotational direction R 2 , and at the same time the arm abutment position P 2  shifts in the direction in which the maximum lift amount of the inlet valve  14  is reduced and in the direction to move away from the rotational center line L 2 , whereby the closing timing and the maximum lift timing are advanced, and at the same time the valve operating property can be obtained where the maximum lift amount is reduced. As this occurs, although the secondary rocker arm  60  shifts together with the holder in the direction to move away from the rotational center line L 2 , since at the same time the maximum lift amount of the inlet valve  14  which is actuated to be opened and closed by the secondary rocker arm  60  is reduced, the oscillating amount of the secondary rocker arm  60  is reduced, and therefore, the operating space occupied by the secondary rocker arm  60  is made compact by that extent, thereby making it possible to disposed the valve train V in a relatively compact space. 
     In the event that the abutment state where the inlet cam  21  abuts with the inlet valve  14  can be set by the separate rocker arms due to the primary and secondary rocker arms  50 ,  60  abutting with the inlet cam  21  and the inlet valve  14 , respectively, and since the primary and secondary oscillating center lines L 4 , L 5  oscillate together with the holder  30 , even in case the shift amount of the primary rocker arm  50  is increased by virtue of the oscillation of the holder  30  in order to set the control range of the valve operating properties large, when compared with the case where while one of the primary and secondary oscillating center lines shits, the other does not, the relative shift amount of the primary and secondary rocker arms  50 ,  60  can be suppressed to a small amount. As a result, the degree of freedom in arrangement of the transmission mechanism Mi is increased, and the application range thereof is expanded, and moreover, since the relative shift amount of the primary and secondary rocker arms  50 ,  60  can be suppressed to a small amount, the control range of the valve operating properties can be set large. 
     As the oscillating position of the holder  30  approaches the primary limit position where the maximum valve operating property Ka can be obtained, the cam abutment position P 1  between the cam abutment portion  52  and the cam lobe portion  21   b  approaches the specific straight line L 10  on the orthogonal plane which intersects at right angles with the holder oscillating center line L 3 , whereby when the cam abutment position P 1  is situated on the specific straight line L 10 , since the line of action of the valve drive force is positioned on the specific straight line L 10 , the moment generated around the holder oscillating center line L 3  to act on the holder  30  based on the valve drive force acting via the primary rocker arm  50  becomes zero. From this fact, while since the maximum lift amount is increased as the holder  30  approaches the primary limit position where the valve operating property can be obtained where the maximum lift amount of the inlet valve  14  becomes maximum, the valve drive force is also increased, the moment acting on the holder  30  can be reduced by allowing the cam abutment position P 1  on the cam lobe portion  21   b  to approach the specific straight line L 10 , and the drive force of the electric motor  28  which oscillates the holder  30  against the moment, whereby the electric motor  28  is made compact. 
     The valve abutment portion  64  abuts with the valve stem  14   a  of the inlet valve  14 , and the holder oscillating center line L 3  is disposed on the extension of the valve stem  14   a  which extends along the axis L 7  of the valve stem  14   a , whereby the distance between the holder oscillating center line L 3  and the line of action of the reaction force F 2  from the inlet valve  14  is maintained small within the range of the valve stem  14   a , and therefore, the moment acting on the holder  30  can be reduced based on the reaction force F 2 , and in this respect, too, the embodiment can contribute to the reduction in driving force of the electric motor  28 . 
     Next, referring to  FIG. 9 , a second embodiment of the invention will be described below. The second embodiment differs from the first embodiment mainly as to a primary rocker arm  50  and a holder oscillating center line, and the former is constructed basically the same as the latter as to the other features, and therefore, while the description of the same features will be omitted or briefly made, the description will be made as to different features of the second embodiment. Note that like reference numerals are given to members, as required, which are like or correspond to those described in the first embodiment. 
     In the second embodiment, a roller  53  is disposed such that a cam abutment portion  52  of a primary rocker arm  50  may be positioned on a specific straight line  10  where a cam abutment position P 1  passes through a holder oscillating center line L 3  and a rotational center line L 2  on an orthogonal plane. 
     To be specific, as shown in  FIG. 9 , when a holder  30  occupies a primary limit position, the cam abutment position P 1  situated on an apex  21   b   1  of a cam lobe portion  21   b  is situated on the specific straight line L 10 . Therefore, the roller  53  is disposed such that as the oscillating position of the holder  30  approaches a predetermined position where a maximum valve operating property can be obtained where a maximum lift amount of an inlet valve  14  becomes maximum, the cam abutment position P 1  residing at the apex  21   b   1  approaches the specific straight line L 10 . 
     Then, since when the cam abutment position P 1  residing at the apex  21   b   1  is situated on the specific straight line L 10 , the line of action of a valve drive force F 1  is situated on the specific straight line L 10 , a moment generated around the holder oscillating center line L 3  to act on the holder  30  based on the valve drive force F 1  becomes zero. 
     According to the second embodiment, similar functions and advantages to those in the first embodiment are provided, except for the fact that the valve operating properties are different, and in addition to the similar functions and advantages, the following function and advantage will also be provided. 
     By adopting the construction in which in a primary rocker arm, a cam abutment position  52  is disposed such that when the holder occupies the primary limit position, the cam abutment position P 1  may be situated on the specific straight line L 10 , since when the cam abutment position P 1  is situated on the specific straight line L 10 , the line of action of the valve drive force F 1  is situated on the specific straight line L 10 , the moment generated around the holder oscillating center line L 3  to act on the holder  30  based on the valve drive force F 1  which acts via the primary rocker arm  50  becomes zero. Due to this, in the state where the cam abutment position P 1  on the cam lobe portion  21   b  is situated on the specific straight line L 10  and in the vicinity thereof, since the drive force of an electric motor  28  which causes the holder  30  to oscillate against the moment can be reduced, the electric motor  28  is made compact. 
     Then, by adopting the construction in which the cam abutment position P 1  is situated on the specific straight line L 10  when the cam abutment position P 1  resides at the apex  21   b   1  of the cam lobe portion  21   b , since the moment acting on the holder  30  based on the maximum valve drive force F 1  becomes zero at the specific oscillating position of the holder  30 , the drive force of the electric motor  28  can be reduced further. 
     As to embodiments in which part of the constructions of the embodiments that have been described heretofore are changed, the changed constructions will be described below. 
     Instead of the inlet operation mechanism, the exhaust operation mechanism may be made up of the variable property mechanism, and both the inlet operation mechanism and the exhaust operation mechanism may be made up of the variable property mechanism. In addition, the valve train may be such as to include a pair of camshafts including, in turn, an inlet camshaft on which an inlet cam is provided and an exhaust camshaft on which an exhaust cam is provided. In the aforesaid embodiments, while the primary member which regulates the oscillating position of the secondary rocker arm  60  relative to the holder  30  is the primary oscillating member (the primary rocker arm  50 ) which is the oscillating member, the primary member may be a member which performs other movements than oscillation. 
     In stead of being formed on the drive abutment  54  of the primary rocker arm  50 , the cam profile may be formed on the follower abutment portion  62  of the secondary rocker arm  60 , and as this occurs, the portion, for example, a roller of the drive abutment portion of the primary rocker arm  50  is brought into abutment with the cam profile. The abutment surface such as the cam abutment portion or the follower abutment portion  62  may be made up of other sliding surfaces, whose sectional shape is something like an arc, than the roller. The primary and secondary rocker arms may be such as of a swing type. In addition, in the secondary rocker arm  60 , the valve abutment portion having the valve abutment surface may be such as to have no adjustment screw. 
     The drive mechanism Md may be such as to include, instead of the drive gear  29   b , a member or a link mechanism which is caused to oscillate by the drive shaft  29 . In addition, the drive mechanism Md may be such as not to have the common drive shaft to all the cylinders and may be such as to have a drive shaft that is driven by a separate actuator for a specific cylinder. By adopting this construction, the operation of part of the cylinders can be brought to rest in accordance with the operating conditions. 
     The holder oscillating center line L 3  may be set at a position where the center line L 3  intersects at right angles with the axis L 7  of the valve stem  14   a . In addition, the position of the holder oscillating center line L 3  may be set such that the reaction force F 2  from the inlet valve  14  generates moment acting in a direction in which the moment based on the valve drive force F 1  is cancelled thereby. 
     While the minimum valve operating property Kb is such that the maximum lift amount becomes zero, the minimum valve operating property Kb may be a valve operating property where the maximum lift amount has a value other than zero. 
     The inlet cam  14  relative to the crankshaft or a variable phase mechanism which can change the phase of the camshaft  20  may be provided on the camshaft  20  or the valve transmission mechanism. 
     The holder  30  does not have to be made up of a separate member for each cylinder so as to be separate from one another but may be such that separate members are connected together by a connecting means or the holder  30  may be formed integrally for all the cylinders. 
     When the cam abutment position P 1  is situated at the base circle portion  21   a , by adopting the construction in which the cam abutment portion is disposed such that the cam abutment position P 1  is situated on the specific straight line L 10 , a valve operating property can be obtained which has longer valve opening period and larger maximum valve properties than the valve operating properties obtained by the first embodiment. 
     In addition, while, in the second embodiment, in the state where the holder  30  is situated at the primary limit position, when the cam abutment position resides at the apex of the cam lobe portion, the cam abutment portion is disposed such that the cam abutment position is situated on the specific straight line, in a state where the holder is situated at any other oscillating positions than the primary limit position, the cam abutment portion may be disposed such that the cam abutment position situated at the apex of the cam lobe portion is positioned on the specific straight line or the cam abutment position situated at any other locations on the cam lobe portion than the apex is situated on the specific straight line. 
     The internal combustion engine may be a single-cylinder one and may be applied to equipment other than vehicles, for example, to a marine propelling apparatus such as outboard engines having a crankshaft which is directed in a perpendicular direction. 
     While there has been described in connection with the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modification may be made therein without departing from the present invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.