Abstract:
A variable valve drive mechanism of an internal combustion engine is provided which includes a camshaft that is operatively connected to a crankshaft of the engine such that the camshaft is rotated by the crankshaft, a rotating cam provided on the camshaft, and an intermediate drive mechanism disposed between the camshaft and an intake or exhaust valve of the engine. The intermediate drive mechanism is supported rockably on a shaft that is different from the camshaft, and includes an input portion operable to be driven by the rotating cam of the camshaft, and an output portion operable to drive the valve when the input portion is driven by the rotating cam. The variable valve drive mechanism further includes an intermediate phase-difference varying device for varying a relative phase difference between the input portion and the output portion of the intermediate drive mechanism.  
     INCORPORATION BY REFERENCE  
     The disclosure of Japanese Patent Application No.  2000 - 078134  filed on Mar.  21, 2000  including the specification, drawings and abstract is incorporated herein by reference in its entirety.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a variable valve drive mechanism of an internal combustion engine capable of varying valve characteristics of intake valves or exhaust valves of the engine, and also relates to an intake air amount control apparatus of an internal combustion engine that employs the variable valve drive mechanism.  
           [0003]    2. Description of Related Art  
           [0004]    Variable valve drive mechanisms adapted to vary the amount of lift or the operating angle of intake valves or exhaust valves of an internal combustion engine in accordance with the operating state or conditions of the engine are known in the art. An example of such mechanisms is disclosed in Japanese laid-open Patent Publication (Kokai) No. 11-324625, in which a rocking cam is provided coaxially with a rotating cam that rotates or moves in accordance with a crankshaft, and the rotating cam and the rocking cam are connected to each other by a complicated link mechanism. The variable valve drive mechanism further includes a control shaft disposed midway in the complicated link mechanism. The phase of the rocking cam may be changed by causing the control shaft to displace or offset the center of rocking of an arm that forms a portion of the link mechanism. By changing the phase of the rocking cam in this manner, the amount of lift or the operating angle of the intake or exhaust valves can be varied. This makes it possible to improve the fuel economy and achieve stable operating characteristics of the engine during, for example, low-speed and low-load operations, and to improve the intake air charging efficiency to thereby ensure sufficiently large outputs during, for example, high-speed and high-load operations.  
           [0005]    However, the link mechanism, which links the rotating cam and the rocking cam that are disposed on the same axis, is likely to be long and complicated.  
           [0006]    This may result in reduced certainty or reliability in the operations of the variable valve drive mechanism.  
         SUMMARY OF THE INVENTION  
         [0007]    It is therefore an object of the invention to provide a variable valve drive mechanism of an internal combustion engine that operates with sufficient certainty or reliability, without requiring a long and complicated link mechanism as employed in the conventional engine. It is another object of the invention to provide an intake air amount control apparatus that utilizes the variable valve drive mechanism.  
           [0008]    To accomplish the above object and/or other object(s), a first aspect of the invention provides a variable valve drive mechanism of an internal combustion engine, which is capable of varying a valve characteristic of an intake valve or an exhaust valve of the internal combustion engine, comprising: (a) a camshaft that is operatively connected with a crankshaft of the engine such that the camshaft is rotated by the crankshaft; (b) a rotating cam provided on the camshaft; (c) an intermediate drive mechanism disposed between the camshaft and the valve and supported rockably on a shaft that is different from the camshaft, the intermediate drive mechanism including an input portion operable to be driven by the rotating cam of the camshaft, and an output portion operable to drive the valve when the input portion is driven by the rotating cam; and (d) an intermediate phase-difference varying device positioned and configured to vary a relative phase difference between the input portion and the output portion of the intermediate drive mechanism.  
           [0009]    The intermediate drive mechanism having the input portion adapted to be driven by the rotating cam and the output portion that drives the valve when the input portion is driven by the rotating cam is rockably supported by the shaft that is different from the camshaft on which the rotating cam is provided. With this arrangement, there is no need to provide a long, complicated link mechanism for connecting the rotating cam with the intermediate drive mechanism (or rocking cam). Thus, when the rotating cam drives the input portion of the intermediate drive mechanism, the driving force is readily transmitted from the input portion to the output portion within the drive mechanism, so that the output portion drives the intake or exhaust valve in accordance with the driving state of the rotating cam.  
           [0010]    The intermediate phase-difference varying device is capable of varying a relative phase difference between the input and output portions of the intermediate drive mechanism. It is thus possible to advance or retard the start of lifting of the intake or exhaust valve that occurs in accordance with the driving state (or rotational phase) of the rotating cam, thus making it possible to adjust the amount of lift or operating angle of the valve that varies with the driving state or rotational phase of the rotating cam.  
           [0011]    As described above, the amount of lift or operating angle of the intake or exhaust valve may be changed with a relatively simple construction in which the relative phase difference between the input and output portions is changed, without requiring the conventional long and complicated link mechanism. It is thus possible to provide a variable valve drive mechanism of an internal combustion engine that operates with improved certainty and reliability.  
           [0012]    In one preferred embodiment of the invention, the output portion comprises a rocking cam that includes a nose, and the intermediate phase-difference varying device is operable to vary the relative phase difference between the nose of the rocking cam and the input portion.  
           [0013]    In the above-described variable valve drive mechanism in which the output portion principally consists of the rocking cam, the intermediate phase-difference varying device is able to vary the relative phase difference between the nose formed on the rocking cam and the input portion, thereby to advance or retard (or delay) the start of lifting of the intake or exhaust valve that occurs in accordance with the driving state (or rotational phase) of the rotating cam provided on the camshaft. Since the amount of lift or operating angle of the intake or exhaust valve can be varied with such a simple construction, the variable valve drive mechanism can operate with improved certainty and reliability. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The foregoing and further objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings in which like numerals are used to represent like elements and wherein:  
         [0015]    [0015]FIG. 1 is a schematic block diagram illustrating the construction of an internal combustion engine and a control system thereof according to a first embodiment of the invention;  
         [0016]    [0016]FIG. 2 is a vertical cross-sectional view of the engine of FIG. 1;  
         [0017]    [0017]FIG. 3 is a cross-sectional view taken along line Y-Y of FIG. 2;  
         [0018]    [0018]FIG. 4 is a view showing a portion of the cylinder head of the engine of FIG. 1, including intake and exhaust camshafts and a variable valve drive mechanism;  
         [0019]    [0019]FIG. 5 is a perspective view showing an intermediate drive mechanism included in the first embodiment of the invention;  
         [0020]    [0020]FIGS. 6A, 6B and  6 C are a plan view, a front elevational view, and a right-hand side view, respectively, of the intermediate drive mechanism of FIG. 5;  
         [0021]    [0021]FIG. 7 is a perspective view showing an input portion included in the first embodiment of the invention;  
         [0022]    [0022]FIGS. 8A, 8B and  8 C are a plan view, a front elevational view, and a right-hand side view, respectively, of the input portion of FIG. 7;  
         [0023]    [0023]FIG. 9 is a perspective view showing a first rocking cam included in the first embodiment of the invention;  
         [0024]    [0024]FIGS. 10A, 10B,  10 C,  10 D and  10 E are a plan view, a front elevational view, a bottom plan view, and a right-hand side view, respectively, of the first rocking cam of FIG. 9;  
         [0025]    [0025]FIG. 11 is a perspective view showing a second rocking cam included in the first embodiment of the invention;  
         [0026]    [0026]FIGS. 12A, 12B,  12 C,  12 D and  12 E are a plan view, a front elevational view, a bottom plan view, a right-hand side view, and a left-hand side view, respectively, of the second rocking cam of FIG. 11;  
         [0027]    [0027]FIG. 13 is a perspective view showing a slider gear included in the first embodiment of the invention;  
         [0028]    [0028]FIGS. 14A, 14B and  14 C are a plan view, a front elevational view, and a right-hand side view, respectively, of the slider gear of FIG. 13;  
         [0029]    [0029]FIGS. 15A, 15B,  15 C and  15 D are a perspective view, a plan view, a front elevational view, and a right-hand side view, respectively, of a support pipe included in the first embodiment of the invention;  
         [0030]    [0030]FIGS. 16A, 16B,  16 C and  16 D are a perspective view, a plan view, a front elevational view, and a right-hand side view, respectively, of a control shaft included in the first embodiment of the invention;  
         [0031]    [0031]FIG. 17 is a perspective view showing an assembly of the support pipe and the control pipe of the first embodiment;  
         [0032]    [0032]FIGS. 18A, 18B and  18 C are a plan view, a front elevational view, and a right-hand side view, respectively, of the assembly of the support pipe and the control pipe of FIG. 17;  
         [0033]    [0033]FIG. 19 is a perspective view of an assembly of the support pipe, the control shaft and the slider gear of the first embodiment;  
         [0034]    [0034]FIGS. 20A, 20B and  20 C are a plan view, a front elevational view, and a right-hand side view, respectively, of the assembly of the support pipe, the control shaft and the slider gear of FIG. 19;  
         [0035]    [0035]FIG. 21 is a partially cutaway perspective view showing the internal construction of the intermediate drive mechanism according to the first embodiment of the invention;  
         [0036]    [0036]FIG. 22 is a vertical cross-sectional view showing a lift-varying actuator included in the first embodiment of the invention;  
         [0037]    [0037]FIG. 23 is a view showing a driving state of the intermediate drive mechanism of the first embodiment;  
         [0038]    [0038]FIGS. 24A and 24B are views for explaining the operation of the variable valve drive mechanism of the first embodiment that is shown in cross section;  
         [0039]    [0039]FIGS. 25A and 25B are views for explaining the operation of the variable valve drive mechanism of the first embodiment that is shown in cross section;  
         [0040]    [0040]FIGS. 26A and 26B are views for explaining the operation of the variable valve drive mechanism of the first embodiment that is shown in cross section;  
         [0041]    [0041]FIGS. 27A and 27B are views for explaining the operation of the variable valve drive mechanism of the first embodiment that is shown in cross section;  
         [0042]    [0042]FIG. 28 is a graph indicating changes in the amount of lift of an intake valve adjusted by the variable valve drive mechanism of the first embodiment;  
         [0043]    [0043]FIG. 29 is a vertical cross-sectional view showing a rotational-phase-difference-varying actuator according to the first embodiment of the invention;  
         [0044]    [0044]FIG. 30 is a cross-sectional view taken along line A-A of FIG. 29;  
         [0045]    [0045]FIG. 31 is a view for explaining the operation of the rotational-phase-difference-varying actuator of the first embodiment;  
         [0046]    [0046]FIG. 32 is a flowchart illustrating a valve drive control routine that is executed by an ECU included in the first embodiment;  
         [0047]    [0047]FIG. 33 is a one-dimensional map used for determining a target displacement Lt of the control shaft in the axial direction based on the accelerator operation amount ACCP in the first embodiment;  
         [0048]    [0048]FIG. 34 are two-dimensional maps used for determining a target timing advance value θt based on the engine speed NE and the amount of intake air GA in the first embodiment;  
         [0049]    [0049]FIG. 35 is a graph indicating various operating regions of the engine for use in the two-dimensional maps shown in FIG. 34;  
         [0050]    [0050]FIG. 36 is a flowchart illustrating a lift amount varying control routine that is executed by the ECU in the first embodiment;  
         [0051]    [0051]FIG. 37 is a flowchart illustrating a rotational phase difference varying control routine that is executed by the ECU in the first embodiment;  
         [0052]    [0052]FIG. 38 is a view illustrating a variable valve drive mechanism according to a first modified example of the first embodiment of the invention;  
         [0053]    [0053]FIGS. 39A and 39B are views showing an intermediate drive mechanism according to a second modified example of the first embodiment of the invention;  
         [0054]    [0054]FIG. 40 is a view showing an intermediate drive mechanism according to a third modified example of the first embodiment;  
         [0055]    [0055]FIGS. 41A and 41B are views showing an intermediate drive mechanism according to a fourth modified example of the first embodiment of the invention;  
         [0056]    [0056]FIGS. 42A and 42B are views for explaining the operation of the intermediate drive mechanism of the fourth modified example of FIGS. 41A and 41B;  
         [0057]    [0057]FIGS. 43A and 43B are views for explaining the operation of the intermediate drive mechanism of the fourth modified example of FIGS. 41A and 41B;  
         [0058]    [0058]FIGS. 44A and 44B are views for explaining the operation of the intermediate drive mechanism of the fourth modified example of FIGS. 41A and 41B;  
         [0059]    [0059]FIGS. 45A and 45B are views showing an intermediate drive mechanism according to a fifth modified example of the first embodiment of the invention;  
         [0060]    [0060]FIGS. 46A and 46B are views for explaining the operation of the intermediate drive mechanism of the fifth modified example of FIGS. 45A and 45B;  
         [0061]    [0061]FIGS. 47A and 47B are views for explaining the operation of the intermediate drive mechanism of the fifth modified example of FIGS. 45A and 45B;  
         [0062]    [0062]FIGS. 48A and 48B are views for explaining the operation of the intermediate drive mechanism of the fifth modified example of FIGS. 45A and 45B;  
         [0063]    [0063]FIGS. 49A and 49B are views showing an intermediate drive mechanism according to a sixth modified example of the first embodiment of the invention;  
         [0064]    [0064]FIGS. 50A and 50B are views for explaining the operation of the intermediate drive mechanism of the sixth modified example of FIGS. 49A and 49B;  
         [0065]    [0065]FIGS. 50A and 51B are views for explaining the operation of the intermediate drive mechanism of the sixth modified example of FIGS. 49A and 49B; and  
         [0066]    [0066]FIGS. 52A and 52B are views for explaining the operation of the intermediate drive mechanism of the sixth modified example of FIGS. 49A and 49B.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0067]    First Embodiment  
         [0068]    [0068]FIG. 1 is a block diagram schematically illustrating a gasoline engine (hereinafter simply referred to as “engine”)  2  as one type of internal combustion engine to which the invention is applied, and a control system for controlling the engine  2 . FIG. 2 is a vertical cross-sectional view of the engine  2  (which is taken along line X-X indicated in FIG. 3). FIG. 3 is a cross-sectional view taken along line Y-Y indicated in FIG. 2.  
         [0069]    The engine  2  is installed in an automobile for driving the automobile. The engine  2  includes a cylinder block  4 , pistons  6  provided for reciprocating movements in the cylinder block  4 , a cylinder head  8  mounted on the cylinder block  4 , etc. Four cylinders  2   a  are formed in the cylinder block  4 . In each cylinder  2   a , a combustion chamber  10  is defined by the cylinder block  4 , the corresponding piston  6  and the cylinder head  8 .  
         [0070]    As shown in FIG. 1, a first intake valve  12   a , a second intake valve  12   b , a first exhaust valve  16   a  and a second exhaust valve  16   b  are disposed so as to face each combustion chamber  10 . These valves are arranged such that the first intake valve  12   a  opens and closes a first intake port  14   a , the second intake valve  12   b  opens and closes a second intake port  14   b , the first exhaust valve  16   a  opens and closes a first exhaust port  18   a , and the second exhaust valve  16   b  opens and closes a second exhaust port  18   b.    
         [0071]    The first intake port  14   a  and the second intake port  14   b  of each cylinder  2   a  are connected to a surge tank  32  via a corresponding one of intake channels  30   a  formed in an intake manifold  30 . Each intake channel  30   a  is provided with a fuel injector  34 , so that a required amount of fuel can be injected into the first intake port  14   a  and the second intake port  14   b.    
         [0072]    The surge tank  32  is connected to an air cleaner  42  via an intake duct  40 . A throttle valve is not provided in the intake duct  40 . Control of the amount of intake air in accordance with the operation of an accelerator pedal  74  and the engine speed NE during idle speed control is accomplished by adjusting the amount of lift of the first and second intake valves  12   a ,  12   b.  The amount of lift of the intake valves  12   a ,  12   b  is adjusted by causing a lift-varying actuator  100  (FIG. 1) to drive intermediate drive mechanisms  120  (which will be described later) disposed between rocker arms  13  and intake cams  45   a  (corresponding to “rotating cam”) provided on an intake camshaft  45 . The valve timing of the intake valves  12   a ,  12   b  is adjusted by a rotational-phase-difference-varying actuator  104  (FIG. 1) (which will be simply referred to as “phase-different-varying actuator  104 ) in accordance with the operation state or conditions of the engine  2 .  
         [0073]    The first exhaust valve  16   a  for opening and closing the first exhaust port  18   a  of each cylinder  2   a  and the second exhaust valve  16   b  for opening and closing the second exhaust port  18   b  are opened and closed by means of rocker arms  14  with a constant amount of lift while exhaust cams  46   a  provided on an exhaust camshaft  46  are being rotated in accordance with the operation of the engine  2 . The first exhaust port  18   a  and the second exhaust port  18   b  of each cylinder  2   a  are connected to an exhaust manifold  48 . With this arrangement, exhaust gases are discharged to the outside through a catalytic converter  50 .  
         [0074]    An electronic control unit (hereinafter referred to as “ECU”)  60 , which is in the form of a digital computer, includes a RAM (random access memory))  64 , a ROM (read-only memory)  66 , a CPU (microprocessor)  68 , an input port  70 , and an output port  72  that are interconnected by a bidirectional bus  62 .  
         [0075]    An accelerator operation amount sensor  76  is attached to the accelerator pedal  74 , and produces an output voltage signal that is proportional to the amount of depression of the accelerator pedal  74  (hereinafter referred to as “accelerator operating amount ACCP”). The output voltage signal is transmitted to the input port  70  through an A/D converter  73 . A top dead center sensor  80  generates an output pulse when, for example, the number  1  cylinder of the cylinders  2   a  reaches the top dead center during the intake stroke. The output pulses thus generated by the top dead center sensor  80  are transmitted to the input port  70 . A crank angle sensor  82  generates an output pulse at every 30° rotation of the crankshaft. The output pulses thus generated by the crank angle sensor  82  are transmitted to the input port  70 . The CPU  68  calculates a current crank angle based on the output pulses received from the top dead center sensor  80  and the output pulses received from the crank angle sensor  82 , and calculates an engine speed NE based on the frequency of the output pulses received from the crank angle sensor  82 .  
         [0076]    The intake duct  40  is provided with an intake air amount sensor  84  that produces an output voltage signal corresponding to the amount of intake air GA flowing in the intake duct  40 . The output voltage signal is transmitted from the sensor  84  to the input port  70  via an A/D converter  73 . The cylinder block  4  of the engine  2  is provided with a water temperature sensor  86  that detects the temperature THW of cooling water of the engine  2  and produces an output voltage signal in accordance with the cooling water temperature THW. The output voltage signal is transmitted from the sensor  86  to the input port  70  via an A/D converter  73 . Furthermore, the exhaust manifold  48  is provided with an air-fuel ratio sensor  88  that produces an output voltage signal indicative of the air-fuel ratio of exhaust gases flowing through the manifold  48 . The output voltage signal is transmitted from the sensor  88  to the input port  70  via an A/D converter  73 .  
         [0077]    Furthermore, a shaft position sensor  90  is provided for detecting the displacement of a control shaft  132  in the axial direction thereof when the shaft  132  is moved by the lift-varying actuator  100 . The shaft position sensor  90  generates an output voltage signal indicative of the axial displacement of the shaft to the input port  70  via an A/D converter  73 . A cam angle sensor  92  is provided for detecting the cam angle of the intake cams  45   a  that drive the intake valves  12   a ,  12   b  via an intermediate drive mechanisms  120 . The cam angle sensor  92  generates output pulses to the input port  70  as the intake camshaft  45  rotates. The input port  70  also receives various other signals, which are not essential to the first embodiment of the invention and are thus not illustrated in FIG. 1.  
         [0078]    The output port  72  is connected to each fuel injector  34  via a corresponding drive circuit  94 . The ECU  60  performs valve opening control on each fuel injector  34  in accordance with the operating state of the engine  2 , to thereby control the fuel injection timing and the fuel injection amount.  
         [0079]    The output port  72  is also connected to a first oil control valve  98  via a drive circuit  96 , so that the ECU  60  controls the lift-varying actuator  100  in accordance with the operating state of the engine  2 , such as a required amount of intake air. The output port  72  is further connected to a second oil control valve  102  via a drive circuit  96 , so that the ECU  60  controls the phase-difference-varying actuator  104  in accordance with the operating state of the engine  2 . With this arrangement, the ECU  60  controls the valve timing and the amount of lift of the intake valves  12   a ,  12   b , so as to implement the intake air amount control and other controls (such as those for improving the volumetric efficiency or controlling an EGR amount).  
         [0080]    The variable valve drive mechanism for the intake valves  12   a ,  12   b  will be now described. FIG. 4 shows in detail a portion of the cylinder head  8  including the intake camshaft  45 , a variable valve drive mechanism attached to the intake camshaft  45 , and other components.  
         [0081]    The variable valve drive mechanism includes a total of four intermediate drive mechanisms  120  provided for the respective cylinders  2   a , the lift-varying actuator  100  attached to one end of the cylinder head  8 , and the phase-difference-varying actuator  104  attached to the other end of the cylinder head  8 .  
         [0082]    One of the intermediate drive mechanisms  120  is illustrated in FIGS. 5 and 6A to  6 C. FIG. 5 is a perspective view of the intermediate drive mechanism  120 . FIGS. 6A, 6B and  6 C are a plan view, a front elevational view, and a right-hand side view of the drive mechanism  120 , respectively. The intermediate drive mechanism  120  has an input portion  122  formed in a central portion thereof, a first rocking cam  124  formed to the left of the input portion  122 , and a second rocking cam  126  formed to the right of the input portion  122 . A housing  122   a  of the input portion  122 , and housings  124   a ,  126   a  of the rocking cams  124 ,  126  have cylindrical shapes with equal outside diameters.  
         [0083]    The construction of the input portion  122  is illustrated in FIGS. 7 and 8A to  8 C. FIG. 7 is a perspective view of the input portion  122 . FIGS. 8A, 8B and  8 C are a plan view, a front elevational view, and a right-hand side view of the input portion  122 , respectively. The housing  122   a  of the input portion  122  defines an internal space that extends in the direction of the axis of the housing  122   a.  An inner circumferential surface of the housing  122   a  defining the internal space has helical splines  122   b  that are formed in the axial direction in a helical fashion of a right-hand thread. Two parallel arms  122   c ,  122   d  protrude from an outer circumferential surface of the housing  122   a.  Distal end portions of the arms  122   c ,  122   d  support a shaft  122   e  extending between the arms  122   c,    122   d.  The shaft  122   e  extends in parallel with the axis of the housing  122   a . A roller  122   f  is rotatably mounted on the shaft  122   e.    
         [0084]    The construction of the first rocking cam  124  is illustrated in FIGS. 9 and 10A to  10 E. FIGS. 9, 10A,  10 B,  10 C,  10 D and  10 E are a perspective view, a plan view, a front elevational view, a bottom plan view, a right-hand side view, and a left-hand side view, respectively. The housing  124   a  of the first rocking cam  124  defines an internal space that extends in the axial direction of the housing  124   a . An inner circumferential surface of the housing  124   a  defining the internal space has helical splines  124   b  that are formed in the axial direction in a helical fashion of a left-hand thread. A left-side end of the internal space is covered with a ring-like bearing  124   c  having a small-diameter central hole. A generally triangular nose  124   d  protrudes from an outer circumferential surface of the housing  124   a.  One side of the nose  124   d  forms a cam face  124   e  that is a concavely curved face.  
         [0085]    The construction of the second rocking cam  126  is illustrated in FIGS. 11 and 12A to  12 E. FIGS. 11, 12A,  12 B,  12 C,  12 D and  12 E are a perspective view, a plan view, a front elevational view, a bottom plan view, a right-hand side view, and a left-hand side view, respectively. The housing  126   a  of the second rocking cam  126  defines an internal space that extends in the axial direction of the housing  126   a . An inner circumferential surface of the housing  126   a  defining the internal space has helical splines  126   b  that are formed in the axial direction in a helical form of a left-hand thread. A right-side end of the internal space is covered with a ring-like bearing  126   c  having a small-diameter central hole. A generally triangular nose  126   d  protrudes from an outer circumferential surface of the housing  126   a.  One side of the nose  126   d  forms a cam face  126   e  that is a concavely curved face.  
         [0086]    The first rocking cam  124  and the second rocking cam  126  are disposed on the opposite sides of the input portion  122  such that the bearings  124   c ,  126   c  face axially outward, and such that corresponding end faces of the cams and input portion contact with each other. Thus, the assembly of the cams  124 ,  126  and the input portion  122  that are arranged on the same axis has a generally cylindrical shape with an internal space as shown in FIG. 5.  
         [0087]    A slider gear  128  as shown in FIGS. 13 and 14A to  14 C is disposed in the internal space defined by the input portion  122  and the two rocking cams  124 ,  126 . FIGS. 13, 14A,  14 B and  14 C are a perspective view, a plan view, a front elevational view, and a right-hand side view of the slider gear  128 , respectively. The slider gear  128  has a generally cylindrical shape. A central portion of an outer circumferential surface of the slider gear  128  has input helical splines  128   a  that are formed in a helical fashion of a right-hand thread. First output helical splines  128   c  that are formed in a helical fashion of a left-hand thread are located on the left-hand side of the input helical splines  128   a . A small-diameter portion  128   b  is interposed between the input helical splines  128   a  and the first output helical splines  128   c . Second output helical splines  128   e  that are formed in a helical fashion of a left-hand thread are located on the right-hand side of the input helical splines  128   a.  A small-diameter portion  128   d  is interposed between the input helical splines  128   a  and the second output helical splines  128   e.  The first and second output helical splines  128   c ,  128   e  have a smaller outside diameter than the input helical splines  128   a.  When the input portion  122  is mounted onto the input helical splines  128   a,  therefore, the first output helical splines  128   c,    128   e  are allowed to pass through the internal space of the input portion  122 .  
         [0088]    A through-hole  128   f  is formed through the slider gear  128  in the direction of the center axis of the gear  128 . The small-diameter portion  128   d  has an elongate hole  128   g  through which the through-hole  128   f  is open onto the outer circumferential surface of the slider gear  128 . The elongate hole  128   g  has a larger dimension in the circumferential direction of the slider gear  128 .  
         [0089]    A support pipe  130  that is partially shown in FIGS. 15A to  15 D is disposed within the through-hole  128   f  of the slider gear  128  such that the support pipe  130  is slidable in the circumferential direction. FIGS. 15A, 15B,  15 C and  15 D are a perspective view, a plan view, a front elevational view, and a right-hand side view, respectively. The support pipe  130  is a single support pipe that is shared by all the intermediate drive mechanisms  120  as shown in FIG. 4. The support pipe  130  has an elongate hole  130   a  for each intermediate drive mechanism  120 . Each elongate hole  130   a  has a larger dimension in the axial direction of the support pipe  130 .  
         [0090]    The control shaft  132  extends through an interior of the support pipe  130  such that the control shaft  132  is slidable in the axial direction. FIGS. 16A, 16B,  16 C and  16 D are a perspective view, a plan view, a front elevational view and a right-hand side view each showing a part of the control shaft  132 . Like the support pipe  130 , the single control shaft  132  is shared or commonly used by all the intermediate drive mechanisms  120 . A stopper pin  132   a , which protrudes from the control shaft  132 , is provided for each intermediate drive mechanism  120 . Each stopper pin  132   a  extends through a corresponding one of the axially elongated holes  130   a  of the support pipe  130 . A sub-assembly of the support pipe  130  and the control shaft  132  is illustrated in FIGS. 17 and 18A to  18 C. FIGS. 17, 18A,  18 B and  18 C are a perspective view, a plan view, a front elevational view, and a right-hand side view of the assembly, respectively.  
         [0091]    An assembly in which the slider gear  128  is assembled with the support pipe  130  and the control shaft  132  is shown in FIGS. 19 and 20A to  20 C. FIGS. 19, 20A,  20 B and  20 C are a perspective view, a plan view, a front elevational view, and a right-hand side view, respectively.  
         [0092]    Each stopper pin  132   a  of the control shaft  132  extends through a corresponding one of the axially elongated holes  130   a  of the support pipe  130  having a larger dimension in the axial direction. Furthermore, a distal end of each stopper pin  132   a  is inserted through the circumferentially elongated hole  128   g  of a corresponding one of the slider gears  128 . To provide the arrangement of FIGS. 19 and 20A to  20 C, it is possible to form the stopper pin  132   a  on the control shaft  132  by passing the pin  132  through the elongated holes  128   g  and  130   a  while the control shaft  132 , the support pipe  130  and the slider gear  128  are assembled together as shown in FIGS. 19 and 20A to  20 C.  
         [0093]    With the axially elongated holes  130   a  thus formed in the support pipe  130 , it is possible to move the stopper pins  132  of the control shaft  132  in the axial direction so as to move the slider gears  128  in the axial direction even though the support pipe  130  is fixed to the cylinder head  8 . Each slider gear  128  engages, at its circumferentially elongated hole  128   g , with the corresponding one of the stopper pins  132   a , so that the axial position of each slider gear  128  is determined by the corresponding stopper pin  132   a . Since the stopper pin  132  is movable in the circumferentially elongated hole  128   g , the slider gear  128  is rockable about the axis.  
         [0094]    The structure as shown in FIGS. 19 and 20A to  20 C is disposed within the combination of the input portion  122  and the rocking cams  124 ,  126  as shown in FIGS. 5 and 6, so as to construct each intermediate drive mechanism  120 . The inner structure of the intermediate drive mechanism  120  is shown in the perspective view of FIG. 21. In FIG. 21, the inner structure of the intermediate drive mechanism  120  is shown by horizontally cutting the input portion  122  and the rocking cams  124 ,  126  and removing the upper halves of these portion and cams  122 ,  124 ,  126 .  
         [0095]    As shown in FIG. 21, the input helical splines  128   a  of the slider gear  128  mesh with the helical splines  122   b  formed in the input portion  122 . The first output helical splines  128   c  mesh with the helical splines  124   b  formed in the first rocking cam  124 . The second output helical splines  128   e  mesh with the helical splines  126   b  formed within the second rocking cam  126 .  
         [0096]    As shown in FIG. 4, each intermediate drive mechanism  120  constructed as described above is sandwiched, at the sides of the bearings  124   c ,  126   c  of the rocking cams  124 ,  126 , between vertical wall portions  136 ,  138  formed on the cylinder head  8 , so that each intermediate drive mechanism  120  is allowed to rock about the axis but is inhibited from moving in the axial direction. Each of the vertical wall portions  136 ,  138  has a hole that is aligned with the central hole of the corresponding one of the bearings  124   c,    126   c.  The support pipe  130  is inserted through the holes of the wall portions  136 ,  138  and is fixed to these portions. Thus, the support pipe  130  is fixed to the cylinder head  8 , and is therefore inhibited from moving in the axial direction or rotating about the axis.  
         [0097]    The control shaft  132  disposed in the support pipe  130  extends through the support pipe  130  slidably in the axial direction, and is connected at its one end to the lift-varying actuator  100 . The displacement of the control shaft  132  in the axial direction can be adjusted by the lift-varying actuator  100 .  
         [0098]    The construction of the lift-varying actuator  100  is illustrated in FIG. 22. FIGS.  22  shows a vertical cross section of the lift-varying actuator  100 , and also shows the first oil control valve  98 .  
         [0099]    The lift-varying actuator  100  principally consists of a cylinder tube  100   a , a piston  100   b  disposed in the cylinder tube  100   a , a pair of end covers  100   c ,  100   d  for closing the opposite openings of the cylinder tube  100   a , and a coil spring  100   e  disposed in a compressed state between the piston  100   b  and the outer end cover  100   c  that is located remote from the cylinder head  8 . The lift-varying actuator  100  is fixed at the inner end cover  100   d  to a vertical wall portion  140  as part of the cylinder head  8 .  
         [0100]    The control shaft  132 , which extends through the inner end cover  100   d  and the vertical wall portion  140  of the cylinder head  8 , is connected at one end thereof to the piston  100   b . Therefore, the control shaft  132  is moved in accordance with movements of the piston  100   b.    
         [0101]    An internal space of the cylinder tube  100   a  is divided by the piston  100   b  into a first pressure chamber  100   f  and a second pressure chamber  100   g . A first oil passage  100   h  that is formed in the inner end cover  100   d  is connected to the first pressure chamber  100   f . A second oil passage  100   i  that is formed in the outer end cover  100   c  is connected to the second pressure chamber  100   g.    
         [0102]    When hydraulic oil is supplied selectively to the first pressure chamber  100   f  and the second pressure chamber  100   g  through the first oil passage  100   h  or the second oil passage  100   i , the piston  100   b  is moved in the axially opposite directions (as indicated by arrow S in FIG. 22) of the control shaft  132 . With the piston  100   b  thus moved, the control shaft  132  is also moved in the axial direction.  
         [0103]    The first oil passage  100   h  and the second oil passage  100   i  are connected to the first oil control valve  98 . A supply passage  98   a  and a discharge passage  98   b  are connected to the first oil control valve  98 . The supply passage  98   a  is connected to an oil pan  144  via an oil pump P that is driven in accordance with rotation of a crankshaft  142  (FIG. 4). The discharge passage  98   b  is directly connected to the oil pan  144 .  
         [0104]    The first oil control valve  98  includes a casing  98   c , which has a first supply/discharge port  98   d , a second supply/discharge port  98   e,  a first discharge port  98   f , a second discharge port  98   g,  and a supply port  98   h . The first oil passage  100   h  is connected to the first supply/discharge port  98   d . The second oil passage  100   i  is connected to the second supply/discharge port  98   e . Furthermore, the supply passage  98   a  is connected to the supply port  98   h.  The discharge passage  98   b  is connected to the first discharge port  98   f  and the second discharge port  98   g . The casing  98   c  receives a spool  98   m  that has four valve portions  98   i.  The spool  98   m  is urged by a coil spring  98   j  in one of the axially opposite directions, and is moved in the other direction by means of an electromagnetic solenoid  98   k.    
         [0105]    When the electromagnetic solenoid  98   k  is in a non-energized state in the first oil control valve  98  constructed as described above, the spool  98   m  is biased toward the electromagnetic solenoid  98   k  in the casing  98   c  under the bias force of the coil spring  98   j . In this state, the first supply/discharge port  98   d  communicates with the first discharge port  98   f , and the second supply/discharge port  98   e  communicates with the supply port  98   h . When the first oil control valve  98  is in this state, hydraulic oil is supplied from the oil pan  144  into the second pressure chamber  100   g  through the supply passage  98   a , the first oil control valve  98  and the second oil passage  100   i . At the same time, hydraulic oil is returned from the first pressure chamber  100   f  into the oil pan  144  through the first oil passage  100   h , the first oil control valve  98  and the discharge passage  98   b . As a result, the piston  100   b  is moved toward the cylinder head  8 . With the piston  100   b  thus moved, the control shaft  132  is moved in the direction F as one of the directions indicated by the arrows S.  
         [0106]    For example, an operating state of each intermediate drive mechanism  120  when the piston  100   b  is moved closest to the cylinder head  8  is illustrated in FIG. 21. In this state, the phase difference between the roller  122   f  of the input portion  122  and the noses  124   d ,  126   d  of the rocking cams  124 ,  126  is maximized. It is to be noted that this state is also established by the urging or bias force of the coil spring  100   e  when the engine  2  is not operated and thus no hydraulic pressure is generated by the oil pump P.  
         [0107]    When the electromagnetic solenoid  98   k  is energized, the spool  98   m  is moved toward the coil spring  98   j  in the casing  98   c  against the bias force of the coil spring  98   j , so that the second supply/discharge port  98   e  communicates with the second discharge port  98   g  and the first supply/discharge port  98   d  communicates with the supply port  98   h.  In this state, hydraulic oil is supplied from the oil pan  144  to the first pressure chamber  100   f  through the supply passage  98   a , the first oil control valve  98  and the first oil passage  100   h . At the same time, hydraulic oil is returned from the second pressure chamber  100   g  into the oil pan  144  through the second oil passage  100   i , the first oil control valve  98  and the discharge passage  98   b . As a result, the piston  100   b  is moved away from the cylinder head  8 . In accordance with the movement of the piston  100   b , the control shaft  132  is moved in the direction R as one of the directions indicated by the arrows S.  
         [0108]    For example, an operating state of each intermediate drive mechanism  120  when the piston  100   b  is moved farthest from the cylinder head  8  is illustrated in FIG. 23. In this state, the phase difference between the roller  122   f  of the input portion  122  and the noses  124   d ,  126   d  of the rocking cams  124 ,  126  is minimized.  
         [0109]    When the spool  98   m  is positioned at an intermediate position in the casing  98   c  by controlling electric current applied to the electromagnetic solenoid  98   k,  the first supply/discharge port  98   d  and the second supply/discharge port  98   e  are closed, and hydraulic oil is inhibited from moving through the supply/discharge ports  98   d ,  98   e . In this state, no hydraulic oil is supplied to or discharged from either the first pressure chamber  100   f  or the second pressure chamber  100   g , and hydraulic oil is held within the first pressure chamber  100   f  and the second pressure chamber  100   g . Therefore, the piston  100   b  and the control shaft  132  are fixed in position in the axial direction thereof. This state in which the piston  100   b  and the control shaft  132  are fixed in position is illustrated in FIG. 22. By fixing the piston  100   b  and the control shaft  132  to an intermediate state between the states indicated in FIG. 21 and FIG. 23, for example, the phase difference between the roller  122   f  of the input portion  122  and the noses  124   d ,  126   d  of the rocking cams  124 ,  126  can be fixed to an intermediate state.  
         [0110]    Furthermore, by controlling the duty cycle with which the electromagnetic solenoid  98   k  is energized, the degree of opening of the first supply/discharge port  98   d  and the degree of opening of the second supply/discharge port  98   e  may be adjusted so as to control the rate of supply of hydraulic oil from the supply port  98   h  to the first pressure chamber  100   f  or to the second pressure chamber  100   g.    
         [0111]    As shown in FIG. 2, the roller  122   f  provided in the input portion  122  of each intermediate drive mechanism  120  is held in contact with the corresponding intake cam  45   a.  Therefore, the input portion  122  of each intermediate drive mechanism  120  rocks about the axis of the support pipe  130  in accordance with the profile of the cam face of the intake cam  45   a.  Compressed springs  122   g  are provided between the cylinder head  8  and the arms  122   c,    122   d  supporting the roller  122   f  such that the roller  122   f  is urged by the compressed springs  122   g  toward the corresponding intake cam  45   a.  Therefore, each roller  122   f  is always held in contact with the corresponding intake cam  45   a.    
         [0112]    A base circular portion of each of the rocking cams  124 ,  126  (i.e., a portion that excludes the nose  124   d  or  126   d ) is in contact with a roller  13   a  that is provided at a center of a corresponding one of two rocker arms  13 . Each rocker arm  13  is rockably supported by an adjuster  13   b  at a proximal end portion  13   c  thereof located close to the center of the cylinder head  8 , while a distal end portion  13   d  of the rocker arm  13  located outwardly of the cylinder head  8  is in contact with a stem end  12   c  of a corresponding intake valve  12   a  or  12   b.    
         [0113]    As described above, the phase difference between the roller  122   f  of the input portion  122  and the noses  124   d ,  126   d  of the rocking cams  124 ,  126  can be adjusted via the control shaft  132  and slider gear  128 , by adjusting the position of the piston  100   b  of the lift-varying actuator  100 . With the position of the piston  100   b  of the lift-varying actuator  100  thus adjusted, the amount of lift of the intake valves  12   a ,  12   b  can be continuously varied in the manner as described below and as shown in FIGS. 24A to  27 B.  
         [0114]    [0114]FIGS. 24A and 24B are vertical cross-sectional views corresponding to that of FIG. 21. FIGS. 24A and 24B illustrate operating states of an intermediate drive mechanism  120  after the piston  100   b  of the lift-varying actuator  100  is moved to the most advanced position (closest to the cylinder block  8 ) in the direction F as viewed in FIG. 22. While FIGS. 24A to  27 B illustrate only a mechanism in which the second rocking cam  126  drives the first intake valve  12   a , a mechanism in which the first rocking cam  124  drives the second intake valve  12   b  is substantially the same as the mechanism shown in the drawings. In the following description, therefore, reference numerals denoting the first rocking cam  124  and the second intake valve  12   b  as well as those denoting the second rocking cam  126  and the first intake valve  12   a  will be provided.  
         [0115]    In FIG. 24A, a base circular portion of the intake cam  45   a  (which excludes a nose  45   b ) is in contact with the roller  122   f  of the input portion  122  of the intermediate drive mechanism  120 . In this condition, the nose  124   d ,  126   d  of the rocking cam  124 ,  126  is not in contact with the roller  13   a  of the rocker arm  13 , but a base circular portion of the rocking cam  124 ,  126  adjacent to the nose  124   d ,  126   d  is in contact with the roller  13   a . As a result, the intake valve  12   a ,  12   b  is in a closed state or position.  
         [0116]    When the nose  45   b  of the intake cam  45   a  pushes down the roller  122   f  of the input portion  122  as the intake camshaft  45  turns, the rocking motion is transmitted from the input portion  122  to the rocking cam  124 ,  126  via the slider gear  128  in the intermediate drive mechanism  120 , so that the rocking cam  124 ,  126  rocks or swivels in such a direction that the nose  124   d ,  126   d  moves downward. As a result, the curved cam face  124   e ,  126   e  formed on the nose  124   d ,  126   d  immediately contacts the roller  13   a  of the rocker arm  13 , and pushes down the roller  13   a  of the rocker arm  13  with the entire area of the cam face  124   e ,  126   e  being in contact with the roller  13   a , as shown in FIG. 24B. As a result, the rocker arm  13  pivots about the proximal end portion  13   c  so that the distal end portion  13   d  of the rocker arm  13  pushes down the stem end  12   c  to a great extent. In this manner, the intake valve  12   a ,  12   b  is lifted the greatest distance away from the valve seat to thus open the intake port  14   a ,  14   b.  Thus, the maximum amount of lift is provided.  
         [0117]    [0117]FIGS. 25A and 25B illustrate operating states of the intermediate drive mechanism  120  after the piston  100   b  of the lift-varying actuator  100  is slightly moved in the direction R from the most advanced position as established in FIGS. 24A and 24B. In FIG. 25A, the base circular portion of the intake cam  45   a  is in contact with the roller  122   f  of the input portion  122  of the intermediate drive mechanism  120 . In this condition, the nose  124   d ,  126   d  of the rocking cam  124 ,  126  is not in contact with the roller  13   a  of the rocker arm  13 , but a base circular portion of the rocking cam  124 ,  126  is in contact with the roller  13   a.  Therefore, the intake valve  12   a,    12   b  is in the closed state or position. The base circular portion of the rocking cam  124 ,  126  contacting the roller  13   a  in FIG. 25A is slightly remote from the nose  124   d ,  126   d  as compared with the case of FIG. 24A. This is because the slider gear  128  has been slightly moved in the direction R within the intermediate drive mechanism  120 , so that the phase difference between the roller  122   f  of the input portion  122  and the nose  124   d,    126   d  of the rocking cam  124 ,  126  has been reduced.  
         [0118]    When the nose  45   b  of the intake cam  45   a  pushes down the roller  122   f  of the input portion  122  as the intake camshaft  45  turns, the rocking motion is transmitted from the input portion  122  to the rocking cam  124 ,  126  via the slider gear  128  in the intermediate drive mechanism  120 , so that the rocking cam  124 ,  126  rocks in such a direction that the nose  124   d,    126   d  moves downward.  
         [0119]    In the state of FIG. 25A, the roller  13   a  of the rocker arm  13  is in contact with the base circular portion of the rocking cam  124 ,  126  that is located slightly remote from the nose  124   d,    126   d,  as described above. Therefore, after the rocking cam  124 ,  126  starts rocking, the roller  13   a  of the rocker arm  13  is not immediately brought into contact with the curved cam face  124   e ,  126   e  formed on the nose  124   d ,  126   d,  but remains in contact with the base circular portion for a while. After a while, the curved cam face  124   e,    126   e  comes into contact with the roller  13   a,  and pushes down the roller  13   a  of the rocker arm  13  as shown in FIG. 25B. As a result, the rocker arm  13  pivots about its proximal end portion  13   c.  Since the roller  13   a  of the rocker arm  13  is initially located slightly remote from the nose  124   d,    126   d,  the area of the cam face  124   e,    126   e  that contacts with the roller  13   a  is correspondingly reduced, and the pivot angle of the rocker arm  13  is also reduced. As a result, the amount by which the distal end portion  13   d  of the rocker arm  13  pushes down the stem end  12   c  of the intake valve  12   a,    12   b  is reduced, which means that the amount of lift of the intake valve  12   a,    12   b  is reduced. Thus, the intake valve  12   a,    12   b  opens the intake port  14   a,    14   b  while providing an amount of lift that is smaller than the above-indicated maximum amount.  
         [0120]    [0120]FIGS. 26A and 26B illustrate operating states of the intermediate drive mechanism  120  after the piston  100   b  of the lift-varying actuator  100  is further moved in the direction R from the position established in FIGS. 25A and 25B.  
         [0121]    In FIG. 26A, the base circular portion of the intake cam  45   a  is in contact with the roller  122   f  of the input portion  122  of the intermediate drive mechanism  120 . At this moment, the nose  124   d,    126   d  of the rocking cam  124 ,  126  is not in contact with the roller  13   a  of the rocker arm  13 , but a base circular portion of the rocking cam  124 ,  126  is in contact with the roller  13   a.  Therefore, the intake valve  12   a,    12   b  is in the closed state. The base circular portion of the rocking cam  124 ,  126  that is in contact with the roller  13   a  in FIG. 26A is located further remote from the nose  124   d,    126   d  as compared with the case of FIG. 25A. This is because the slider gear  128  has been moved in the direction R within the intermediate drive mechanism  120  as mentioned above, so that the phase difference between the roller  122   f  of the input portion  122  and the nose  124   d,    126   d  of the rocking cam  124 ,  126  has been further reduced.  
         [0122]    When the nose  45   b  of the intake cam  45   a  pushes down the roller  122   f  of the input portion  122  as the intake camshaft  45  turns, the rocking motion is transmitted from the input portion  122  to the rocking cam  124 ,  126  via the slider gear  128  in the intermediate drive mechanism  120 , so that the rocking cam  124 ,  126  rocks in such a direction that the nose  124   d,    126   d  moves downward.  
         [0123]    In the state of FIG. 26A, the roller  13   a  of the rocker arm  13  is in contact with the base circular portion of the rocking cam  124 ,  126  that is located considerably remote from the nose  124   d ,  126   d , as described above. Therefore, after the rocking cam  124 ,  126  starts rocking, the roller  13   a  of the rocker arm  13  is not immediately brought into contact with the curved cam face  124   e ,  126   e  formed on the nose  124   d,    126   d , but remains in contact with the base circular portion for a while. After a while, the curved cam face  124   e,    126   e  comes into contact with the roller  13   a , and pushes down the roller  13   a  of the rocker arm  13  as shown in FIG. 26B. Thus, the rocker arm  13  pivots about its proximal end portion  13   c.  Since the roller  13   a  of the rocker arm  13  is initially located significantly remote from the nose  124   d,    126   d , the area of the cam face  124   e ,  126   e  that contacts with the roller  13   a  is further reduced, and the pivot angle of the rocker arm  13  is also further reduced. Consequently, the amount by which the distal end portion  13   d  of the rocker arm  13  pushes down the stem end  12   c  of the intake valve  12   a,    12   b  is considerably reduced, which means that the amount of lift of the intake valve  12   a ,  12   b  is considerably reduced. Thus, the intake valve  12   a,    12   b  slightly opens the intake port  14   a,    14   b  while providing an amount of lift that is far smaller than the above-indicated maximum amount.  
         [0124]    [0124]FIGS. 27A and 27B are vertical cross-sectional views corresponding to that of FIG. 23. FIGS. 27A and 27B illustrate operating states of the intermediate drive mechanism  120  after the piston  100   b  of the lift-varying actuator  100  is moved in the direction R to the most retracted position (that is farthest from the cylinder block  8  in FIG. 22).  
         [0125]    In FIG. 27A, the base circular portion of the intake cam  45   a  is in contact with the roller  122   f  of the input portion  122  of the intermediate drive mechanism  120 . At this moment, the nose  124   d,    126   d  of the rocking cam  124 ,  126  is not in contact with the roller  13   a  of the rocker arm  13 , but a base circular portion of the rocking cam  124 ,  126  is in contact with the roller  13   a . Therefore, the intake valve  12   a,    12   b  is in the closed state. The base circular portion of the rocking cam  124 ,  126  that is in contact with the roller  13   a  in FIG. 27A is greatly remote from the nose  124   d,    126   d.  This is because the slider gear  128  has been moved to the maximum extent in the direction R within the intermediate drive mechanism  120  as mentioned above, so that the phase difference between the roller  122   f  of the input portion  122  and the nose  124   d,    126   d  of the rocking cam  124 ,  126  is minimized.  
         [0126]    When the nose  45   b  of the intake cam  45   a  pushes down the roller  122   f  of the input portion  122  as the intake camshaft  45  turns, the rocking motion is transmitted from the input portion  122  to the rocking cam  124 ,  126  via the slider gear  128  in the intermediate drive mechanism  120 , so that the rocking cam  124 ,  126  rocks in such a direction that the nose  124   d,    126   d  moves downward.  
         [0127]    In the state of FIG. 27A, the roller  13   a  of the rocker arm  13  is in contact with the base circular portion of the rocking cam  124 ,  126  that is greatly remote from the nose  124   d,    126   d,  as described above. Therefore, during the entire period of the rocking action of the rocking cam  124 ,  126 , the roller  13   a  of the rocker arm  13  remains in contact with the base circular portion of the rocking cam  124 ,  126  without contacting with the curved cam face  124   e,    126   e  formed on the nose  124   d ,  126   d.  That is, even when the nose  45   b  of the intake cam  45   a  pushes down the roller  122   f  of the input portion  122  to the maximum extent, the curved cam face  124   e,    126   e  is not used for pushing down the roller  13   a  of the rocker arm  13 .  
         [0128]    Therefore, the rocker arm  13  does not pivot about its proximal end portion  13   c , and the amount by which the distal end portion  13   d  of the rocker arm  13  pushes down the stem end  12   c  of the intake valve  12   a ,  12   b  becomes equal to zero, which means that the amount of lift of the intake valve  12   a ,  12   b  becomes zero. Thus, the intake port  14   a,    14   b  is kept closed by the intake valve  12   a,    12   b.    
         [0129]    By adjusting the position of the piston  100   b  of the lift-varying actuator  100  as described above, the amount of lift of the intake valves  12   a,    12   b  can be continuously adjusted so as to vary in accordance with a selected one of lift patterns as indicated in FIG. 28. That is, the lift-varying actuator  100 , the control shaft  132 , the slider gear  128 , the helical splines  122   b  of the input portion  122 , and the helical splines  124   b,    126   b  of the rocking cams  124 ,  126  constitute an intermediate phase-difference-varying device adapted for varying the phase difference between the roller  122   f  of the input portion  122  and the nose  124   d,    126   d  of the rocking cam  124 ,  126 .  
         [0130]    The rotational-phase-difference-varying actuator  104  will be now described with reference to FIGS. 29 and 30. The phase-difference-varying actuator  104  is disposed such that that toque can be transmitted from the crankshaft  142  to the intake camshaft  45  via the actuator  104 . The phase-difference-varying actuator  104  is capable of varying the rotational phase difference between the intake camshaft  45  and the crankshaft  142 .  
         [0131]    [0131]FIG. 29 is a vertical cross-sectional view, and FIG. 30 is a cross-sectional view taken along line A-A of FIG. 29. Furthermore, the cross-sectional view of FIG. 29 illustrating an internal rotor  234  and its associated components is taken along line B-B in FIG. 30.  
         [0132]    The vertical wall portions  136 ,  138 ,  139  of the cylinder head  8  as shown in FIG. 4 serve as journal bearings for the intake camshaft  45 . Thus, the vertical wall portion  139  of the cylinder head  8  and a bearing cap  230  rotatably support a journal  45   c  of the intake camshaft  45 , as shown in FIG. 29. The internal rotor  234  that is secured to a distal end face of the intake camshaft  45  by a bolt  232  is prevented from rotating relative to the intake camshaft  45  by a knock pin (not shown), so that the internal rotor  234  rotates together with the intake camshaft  45 . The internal rotor  234  has a plurality of vanes  236  formed on its outer circumferential surface.  
         [0133]    A timing sprocket  224   a  is provided on a distal end portion of the intake camshaft  45  such that the timing sprocket  224   a  is rotatable relative to the intake camshaft  45 . The timing sprocket  224   a  has a plurality of outer teeth  224   b  formed on its outer periphery. A side plate  238 , a main body  240  and a cover  242 , all of which form parts of a housing, are mounted in this order on a distal end face of the timing sprocket  224   a , and are fixed to the timing sprocket  224   a  by bolts  244  such that the side plate  238 , the main body  240  and the cover  242  rotate together with the timing sprocket  224   a.    
         [0134]    The cover  242  is provided for covering distal end faces of the housing body  240  and the internal rotor  234 . The main body  240  is arranged to receive the internal rotor  234 , and has a plurality of projections  246  formed on its inner circumferential surface.  
         [0135]    One of the vanes  236  of the internal rotor  234  has a through-hole  248  that extends in the direction of the axis of the intake camshaft  45 . A lock pin  250  that is movably disposed within the through-hole  248  has a receiving hole  250   a  formed therein. A spring  254  is provided in the receiving hole  250   a  for urging the lock pin  250  toward the side plate  238 . When the lock pin  250  faces a stopper hole  252  formed in the side plate  238 , the lock pin  250  enters and engages with the stopper hole  252  under the bias force of the spring  254  so as to fix or lock the position of the internal rotor  234  relative to the side plate  238  in the circumferential direction. As a result, rotation of the internal rotor  234  relative to the main body  240  of the housing is restricted or inhibited, and therefore the intake camshaft  45  fixed to the internal rotor  234  and the timing sprocket  224   a  fixed to the housing are adapted to rotate together as a unit while maintaining the relative positional relationship therebetween.  
         [0136]    The internal rotor  234  has an oil groove  256  formed in a distal end face thereof. The oil groove  256  communicates an elongate hole  258  formed in the cover  242  with the through-hole  248 . The oil groove  256  and the elongate hole  258  function to discharge the air or oil present at around the distal end portion of the lock pin  250  in the through-hole  248  to the outside of the actuator  104 .  
         [0137]    As shown in FIG. 30, the internal rotor  234  has a cylindrical boss  260  located in a central portion of the rotor  234 , and vanes  236 , for example, four vanes  236  that are formed at equal intervals of 90° to extend radially outwards from the boss  260 .  
         [0138]    The main body  240  of the housing four projections  246  formed on its inner circumferential surface at substantially equal intervals, like the vanes  236 . The vanes  236  are respectively inserted in four recesses  262  formed between the projections  246 . An outer circumferential surface of each vane  236  is in contact with an inner circumferential surface of a corresponding one of the recesses  262 . Also, a distal end face of each projection  246  is in contact with an outer circumferential surface of the boss  260 . With this arrangement, each recess  262  is divided by the corresponding vane  236  so that a first oil pressure chamber  264  and a second oil pressure chamber  266  are formed on the opposite sides of each vane  236  in the rotating direction. Each of these vanes  236  is movable between two adjacent projections  246 . Thus, the internal rotor  234  is allowed to rotate relative to the housing  240  within a range or region that is defined by two limit positions at which each vane  236  abuts on the corresponding opposite projections  24 .  
         [0139]    When the valve timing is to be advanced, hydraulic oil is supplied to each of the first oil pressure chambers  264  that is located on one side of each vane  236  that is behind the vane  236  as viewed in the rotating direction of the timing sprocket  224   a  (as indicated by an arrow in FIG. 30). When the valve timing is to be retarded, on the other hand, hydraulic oils is supplied to each of the second oil pressure chambers  266  that is located on the other side of each vane  236  that is ahead of the vane  236  as viewed in the rotating direction. The above-indicated rotating direction of the timing sprocket  224   a  will be hereinafter referred to as “timing advancing direction”, and the direction opposite to this rotating direction will be referred to as “timing retarding direction”.  
         [0140]    A groove  268  is formed in a distal end portion of each of the vanes  236 , and a groove  270  is formed in a distal end portion of each of the projections  246 . A seal plate  272  and a sheet spring  274  for urging the seal plate  272  are disposed within the groove  268  of each vane  236 . Likewise, a seal plate  276  and a sheet spring  278  for urging the seal plate  276  are disposed within the groove  270  of each projection  246 .  
         [0141]    The lock pin  250  functions to inhibit relative rotation between the internal rotor  234  and the housing  240 , for example, when the engine is started, or when the ECU  60  has not initiated hydraulic pressure control. That is, when the hydraulic pressure in the first oil pressure chambers  264  is zero or has not been sufficiently elevated, a cranking operation for starting the engine causes the lock pin  250  to reach a position at which the lock pin  250  can enter the stopper hole  252 , so that the lock pin  250  enters and engages with the stopper hole  252  as shown in FIG. 29. When the lock pin  250  is in engagement with the stopper hole  252 , the rotation of the internal rotor  234  relative to the housing  240  is prohibited, and the internal rotor  234  and the housing  240  can rotate together as a unit.  
         [0142]    The lock pin  250  engaging with the stopper hole  252  is released when the hydraulic pressure supplied to the actuator  104  is sufficiently raised so that hydraulic pressure is supplied from the second oil pressure chamber  266  to an annular oil space  282  via an oil passage  280 . That is, when the hydraulic pressure supplied to the annular oil space  282  is elevated, the lock pin  250  is forced out of the stopper hole  252  against the bias force of the spring  254 , and is thus disengaged from the stopper hole  252 . Hydraulic pressure is also supplied from the first oil pressure chamber  264  to the stopper hole  252  via another oil passage  284 , so as to surely hold the lock pin  250  in the disengaged or released state. With the lock pin  250  thus disengaged from the stopper hole  252 , the housing  240  and the internal rotor  234  are allowed to rotate relative to each other, so that the rotational phase of the internal rotor  234  relative to the housing  240  can be adjusted by controlling the hydraulic pressure supplied to the first oil pressure chambers  264  and the second oil pressure chambers  266 .  
         [0143]    Next, an oil supply/discharge structure for supplying or discharging hydraulic oil to or from each of the first oil pressure chambers  264  and second oil pressure chambers  266  will be now described with reference to FIGS. 29.  
         [0144]    The vertical wall portion  139  of the cylinder head  8  formed as a journal bearing has a first oil passage  286  and a second oil passage  288  formed therein. The first oil passage  286  is connected to an oil channel  294  formed within the intake camshaft  45 , via an oil hole  292  and an oil groove  290  that extends over the entire circumference of the intake camshaft  45 . One end of the oil channel  294  remote from the oil hole  292  is open to an annular space  296 . Four oil holes  298  that generally radially extend through the boss  260  connect the annular space  296  to the corresponding first oil pressure chambers  264 , and permit hydraulic oil in the annular space  296  to be supplied to the first oil pressure chambers  264 .  
         [0145]    The second oil passage  288  communicates with an oil groove  300  that is formed over the entire circumference of the intake camshaft  45 . The oil groove  300  is connected to an annular oil groove  310  formed in the timing sprocket  224   a , via an oil hole  302 , an oil channel  304 , an oil hole  306  and an oil groove  308  formed in the intake camshaft  45  The side plate  238  has four oil holes  312 , each of which is open at a location adjacent to a side face of a corresponding one of the projections  246  as shown in FIGS. 29 and 30. Each of the oil holes  312  connects the oil groove  310  to a corresponding one of the second oil pressure chambers  266 , and allows hydraulic oil to be supplied from the oil groove  310  to the corresponding second oil pressure chamber  266 .  
         [0146]    The first oil passage  286 , the oil groove  290 , the oil hole  292 , the oil channel  294 , the annular space  296  and each of the oil holes  298  form an oil passage for supplying oil into a corresponding one of the first oil pressure chambers  264 . The second oil passage  288 , the oil groove  300 , the oil hole  302 , the oil channel  304 , the oil hole  306 , the oil groove  308 , the oil groove  310  and each of the oil holes  312  form an oil passage for supplying hydraulic oil into a corresponding one of the second oil pressure chambers  266 . The ECU  60  drives the second oil control valve  102  so as to control hydraulic pressures applied to the first oil pressure chambers  264  and to the second oil pressure chambers  266  via these oil passages.  
         [0147]    The vane  236  having the through-hole  248  is formed with the oil passage  284  as shown in FIG. 30. The oil passage  284  communicates the first oil pressure chamber  264  with the stopper hole  252 , and allows hydraulic pressure supplied to the first oil pressure chamber  264  to be also supplied to the stopper hole  252 , so as to maintain the released state of the lock pin  250  as described above.  
         [0148]    In the through-hole  248 , the annular oil space  282  is formed between the lock pin  250  and the vane  236 . The annular oil space  282  communicates with the second oil pressure chamber  266  via the oil passage  280  as shown in FIG. 30, and allows hydraulic pressure supplied to the second oil pressure chamber  266  to be also supplied to the annular oil space  282 , so as to disengage or release the lock pin  250  from the stopper hole  252  as described above.  
         [0149]    As shown in FIG. 29, the second oil control valve  102  is substantially the same in basic construction as the first oil control valve  98  as described above.  
         [0150]    When an electromagnetic solenoid  102   k  of the second oil control valve  102  is in a non-energized state, hydraulic oil is supplied from the oil pan  144  to the second oil pressure chambers  266  via the second oil passage  288 , the oil groove  300 , the oil hole  302 , the oil channel  304 , the oil hole  306 , the oil groove  308 , the oil groove  310 , and the respective oil holes  312 . Furthermore, hydraulic oil is returned from the first oil pressure chambers  264  to the oil pan  144  via the respective oil holes  298 , the annular space  296 , the oil channel  294 , the oil hole  292 , the oil passage  290 , and the first oil passage  286 . As a result, the internal rotor  234  and the intake camshaft  45  are rotated or turned relative to the timing sprocket  224   a  in a direction opposite to the rotating direction. That is, the intake camshaft  45  is retarded in timing.  
         [0151]    Conversely, when the electromagnetic solenoid  102   k  is energized, hydraulic oil is supplied from the oil pan  144  to the first oil pressure chambers  264  via the first oil passage  286 , the oil passage  290 , the oil hole  292 , the oil channel  294 , the annular space  296 , and the respective oil holes  298 . Furthermore, hydraulic oil is returned from the second oil pressure chambers  266  to the oil pan  144  via the respective oil holes  312 , the oil groove  310 , the oil groove  308 , the oil hole  306 , the oil channel  304 , the oil hole  302 , the oil groove  300 , and the second oil passage  288 . As a result, the internal rotor  234  and the intake camshaft  45  are rotated relative to the timing sprocket  224   a  in the same direction as the rotating direction. That is, the intake camshaft  45  is advanced in timing. If the intake camshaft  45  is advanced in timing from the state as shown in FIG. 30, the intake camshaft  45  and the internal rotor  234  are brought into, for example, a state as shown in FIG. 31.  
         [0152]    If the electric current applied to the electromagnetic solenoid  102   k  is controlled so as to inhibit movement of hydraulic oil, hydraulic oil is not supplied to nor discharged from the first oil pressure chambers  264  and the second oil pressure chambers  266 , and hydraulic oil currently present in the first oil pressure chambers  264  and the second oil pressure chambers  266  is maintained. As a result, the positions of the internal rotor  234  and the intake camshaft  45  relative to the timing sprocket  224   a  are fixed. For example, the operating state as shown in FIG. 30 or  31  is fixed, and the intake camshaft  45  held in this state is rotated by receiving torque from the crankshaft  142 .  
         [0153]    The manner of controlling the valve timing of the intake valves differs depending upon the type of the engine. For example, the intake camshaft  45  is retarded in timing to thereby retard the opening and closing timing of the intake valves  12   a,    12   b  during low-speed operations and high-load and high-speed operations of the engine  2 . The intake camshaft  45  is advanced in timing to thereby advance the opening and closing timing of the intake valves  12   a ,  12   b  during high-load and middle-speed operations and medium-load operation of the engine  2 . This manner of valve timing control is intended to achieve stable engine operations by reducing the valve overlap during the low-speed operations of the engine  2 , and to improve the efficiency with which an air/fuel mixture is sucked into the combustion chambers  10  by delaying the closing timing of the intake valves  12   a ,  12   b  during the high-load and high-speed operations of the engine  2 . Furthermore, during the high-load and middle-speed operations or medium-load operations of the engine  2 , the opening timing of the intake valves  12   a ,  12   b  is advanced so as to increase the valve overlap, thereby reducing the pumping loss and improving the fuel economy.  
         [0154]    Next, valve drive control executed by the ECU  60  for controlling the intake valves  12   a ,  12   b  will be described. FIG. 32 shows a flowchart of a valve drive control routine according to which the valve drive control is performed. This control routine is repeatedly executed at certain time intervals.  
         [0155]    The valve drive control routine of FIG. 32 is initiated with step S 110  to read an accelerator operating amount or position ACCP obtained based on a signal from the accelerator operation amount sensor  76 , an amount of intake air GA obtained based on a signal from the intake air amount sensor  84 , and an engine speed NE obtained based on a signal from the crank angle sensor  82 , and store them into a work area of the RAM  64 . The control flow proceeds to step S 120  to set a target displacement Lt of the control shaft  132  in the axial direction thereof, based on the accelerator operating amount ACCP read in step S 110 . In the first embodiment, the target displacement Lt is determined by using a one-dimensional map as indicated in FIG. 33, in which appropriate values are empirically determined and are stored in advance in the ROM  66 . That is, the target displacement Lt of the surge tank  32  is set to a smaller value as the accelerator operating amount ACCP increases. As described above, the amount of lift of the intake valves  12   a ,  12   b  decreases with an increase in the displacement of the control shaft  132 . Thus, the map of FIG. 33 indicates that as the accelerator operating amount ACCP increases, the amount of lift of the intake valves  12   a,    12   b  is set to a greater value, resulting in an increase in the amount of intake air GA.  
         [0156]    Next, the control flow proceeds to step S 130  to select an appropriate map from a plurality of target timing advance value θt maps stored in the ROM  66 , in accordance with the target displacement Lt of the control shaft  132 , as shown in FIG. 34. The target timing advance value θt maps may be prepared in advance by empirically determining appropriate target timing advance values θt in relation to the amount of intake air GA and the engine speed NE for each range or region of the target displacement Lt. The resulting maps are stored in the ROM  66 .  
         [0157]    These maps for one type of engine are different from those for another type of engine. In general, however, the valve overlap may be adjusted differently in respective operating regions of the engine as shown in FIG. 35 by way of example. Namely, (1) when the engine operates in an idling region (i.e., during idling of the engine), the valve overlap is eliminated to thereby prevent exhaust gases from returning to combustion chambers, so that the engine operation is stabilized due to stable or reliable combustion achieved in the combustion chambers. (2) When the engine operates in a light-load region, the valve overlap is minimized to thereby prevent exhaust gases from returning to the combustion chambers, so that the engine operation is stabilized with stable combustion. (3) When the engine operates in a middle-load region, the valve overlap is slightly increased so as to increase the internal EGR rate and reduce the pumping loss. (4) When the engine operates in a high-load and middle-speed region, the valve overlap is maximized so as to improve the volumetric efficiency and increase the torque. (5) When the engine operates in a high-load and high-speed region, the valve overlap is controlled to be medium to large so as to improve volumetric efficiency.  
         [0158]    After an appropriate target timing advance value θt map corresponding to the target displacement Lt set in step S 120  is selected, the control flow proceeds to step S 140  to set a target timing advance value θt of the rotational-phase-difference-varying actuator  104  based on the amount of intake air GA and the engine speed NE, and based on the selected two-dimensional map. Thus, the valve drive control routine is once finished with execution of step S 140 . Thereafter, the steps S 110  to S 140  are repeatedly executed in subsequent control cycles, so that the appropriate target displacement Lt and target timing advance value θt are repeatedly updated and established.  
         [0159]    Using the target displacement Lt determined in the above control, the ECU  60  executes a valve lift varying control routine as illustrated in FIG. 36. This control routine is repeatedly executed at certain time intervals.  
         [0160]    The routine of FIG. 36 is initiated with step S 210  to read an actual displacement Ls of the control shaft  132  as represented by a signal from the shaft position sensor  90 , and store it in a work area of the RAM  64 .  
         [0161]    Next, the control flow proceeds to step S 220  to calculate a deviation ΔL of the actual displacement Ls from the target displacement Lt according to an expression (1) as follows:  
         Δ L←Lt−Ls   (1)  
         [0162]    The control flow then proceeds to step S 230  to perform PID control calculation based on the deviation ΔL determined as described above, to calculate a duty Lduty of a signal applied to the electromagnetic solenoid  98   k  of the first oil control valve  98  so that the actual displacement Ls approaches the target displacement Lt. The control flow proceeds to step S 240  to output the duty Lduty to the drive circuit  96 , so that a signal having the duty Lduty is applied to the electromagnetic solenoid  98   k  of the first oil control valve  98 . The control routine is once finished with execution of step S 240 . Then, the above-described steps S 210  to S 240  are again repeatedly executed in subsequent cycles. In this manner, hydraulic oil is supplied to the lift-varying actuator  100  via the first oil control valve  98  so that the target displacement Lt is achieved.  
         [0163]    Furthermore, using the target timing advance value θt, the ECU  60  controls a rotational phase difference between the crankshaft  142  and the intake camshaft  45 , in accordance with a control routine as illustrated in the flowchart of FIG. 37. This control routine is repeatedly executed at certain time intervals.  
         [0164]    The control routine is initiated with step S 310  to read an actual timing advance value θs of the intake camshaft  45  that is determined from the relationship between a signal from the cam angle sensor  92  and a signal from the crank angle sensor  82 , and store it in a work area of the RAM  64 .  
         [0165]    Next, step S 320  is executed to calculate a deviation Δθ between the target timing advance value θt and the actual timing advance value θs according to an expression (2) as follows:  
         Δθ←θ t−θs   (2)  
         [0166]    Then, the control flow proceeds to step S 330  to perform PID control calculation based on the deviation Δθ obtained in step S 320 , to thus calculate a duty θduty of a signal applied to the electromagnetic solenoid  102   k  of the second oil control valve  102  such that the actual timing advance value θs approaches the target timing advance value θt. Step S 340  is then executed to output the duty θduty to the drive circuit  96 , so that a signal having the duty θduty is applied to the electromagnetic solenoid  102   k  of the second oil control valve  102 . The control routine is once finished with execution of step S 340 . Then, the above-indicated steps S 310  to S 340  are again repeatedly executed in subsequent cycles. In this manner, hydraulic oil is supplied to the phase-difference-varying actuator  104  via the second oil control valve  102  so as to achieve the target timing advance value θt.  
         [0167]    The first embodiment of the invention as described above yields advantages or effects as follows.  
         [0168]    (1) Each intermediate drive mechanism  120  has the input portion  122  and the rocking cams  124 ,  126  as output portions. When the input portion  122  is driven by the intake cam  45   a , the rocking cams  124 ,  126  drive the intake valves  12   a ,  12   b  via the rocker arms  13 .  
         [0169]    The intermediate drive mechanism  120  is rockably supported by the support pipe  130 , which is a different shaft from the intake camshaft  45  provided with the intake cams  45   a . Therefore, with the intake cam  45   a  contacting with and driving the input portion  122 , the amount of lift and the operating angle of the intake valves  12   a ,  12   b  can be made in accordance with the operating state of the intake cam  45   a , via the rocking cams  124 ,  126  and the rocker arms  13 , without requiring a long and complicated link mechanism for connecting the intake cam  45   a  to the intermediate drive mechanism  120 .  
         [0170]    The relative phase difference between the input portion  122  and the rocking cams  124 ,  126  of each intermediate drive mechanism  120  can be varied by the lift-varying actuator  100 , the control shaft  132 , the slider gear  128 , the helical splines  122   b  of the input portion  122 , and the helical splines  124   b,    126   b  of the rocking cams  124 ,  126 . More specifically, the relative phase difference between the noses  124   d ,  126   d  formed on the rocking cams  124 ,  126  and the roller  122   f  of the input portion  122  is made variable. Therefore, the start of lifting of the intake valves  12   a ,  12   b  that occurs in accordance with the operating state of the intake cam  45   a  can be advanced or retarded in timing. Hence, the amount of lift and the operating angle of the intake valves  12   a ,  12   b  that accords with the operation or driving of the intake cam  45   a  can be suitably adjusted.  
         [0171]    Thus, the amount of lift and the operating angle of the valves can be varied by a relatively simple arrangement adapted to change the relative phase difference of the rocking cams  124 ,  126  with respect to the input portion  122 , without employing a long and complicated link mechanism. It is thus possible to provide a variable valve drive mechanism that operates with improved reliability.  
         [0172]    (2) The rocking cams  124 ,  126  of each intermediate drive mechanism  120  drive the valves via the rollers  13   a  of the rocker arms  13 . With this arrangement, the friction resistance that arises when the intake cam  45   a  drives the intake valves  12   a,    12   b  via the intermediate drive mechanism  120  is reduced, and therefore the fuel economy can be improved.  
         [0173]    (3) The input portion  122  of each intermediate drive mechanism  120  is provided with a roller  122   f  disposed between the distal end portions of the arms  122   c ,  122   d . Since the roller  122   f  contacts with the intake cam  45   a , the friction resistance that arises when the intake cam  45   a  drives the intake valves  12   a ,  12   b  via the intermediate drive mechanism  120  is further reduced, and the fuel economy can be further improved.  
         [0174]    (4) The intermediate drive mechanism  120  is provided with the slider gear  128 , which is moved in the axial direction by the lift-varying actuator  100 . With this arrangement, the input portion  122  is rocked by a spline mechanism formed by the input helical splines  128   a  of the slider gear  128  and the helical splines  122   b  of the input portion  122 . Furthermore, the rocking cams  124 ,  126  are rocked by a spline mechanism formed by the output helical splines  128   c ,  128   e  of the slider gear  128  and the helical splines  124   b ,  126   b  of the rocking cams  124 ,  126 . Thus, relative rocking motion between the input portion  122  and the rocking cams  124 ,  126  is realized.  
         [0175]    Since the relative phase difference between the input portion  122  and the rocking cams  124 ,  126  can be varied or changed by means of the spline mechanisms, the amount of lift and the operating angle of the valves can be varied without requiring a complicated arrangement. Accordingly, the variable valve drive mechanism ensures sufficiently high operating reliability.  
         [0176]    (5) Each intermediate drive mechanism  120  has a single input portion  122  and a plurality of rocking cams (two cams  124 ,  126 ) in this embodiment). The rocking cams  124 ,  126  drive the same number of intake valves  12   a ,  12   b  provided for the same cylinder  2   a.  Thus, only one intake cam  45   a  is required for driving a plurality of intake valves  12   a ,  12   b  provided for each cylinder  2   a,  which leads to a simplified structure of the intake camshaft  45 .  
         [0177]    (6) The lift-varying actuator  100  is able to continuously vary the relative phase difference between the input portion  122  and the rocking cams  124 ,  126  of the intermediate drive mechanism  120 . Since the relative phase difference can be continuously or steplessly changed, the amount of lift and operating angle of the intake valves  12   a ,  12   b  can be set to any desired values that are more precisely suited for the operating state of the engine  2 . Thus, the intake air amount can be controlled with improved accuracy.  
         [0178]    (7) The intake camshaft  45  is provided with the phase-difference-varying actuator  104  capable of continuously varying the phase difference of the intake camshaft  45  relative to the crankshaft  15 . Therefore, it becomes possible to advance and retard the valve timing of the intake valves  12   a ,  12   b  with high accuracy in accordance with the operating state of the engine  2 , as well as varying the amount of lift and the operating angle as described above. Accordingly, the engine drive control is performed with further enhanced accuracy.  
         [0179]    (8) By executing step S 120  in the valve drive control routine of FIG. 32 and executing the control routine of FIG. 36 for varying the lift amount, the amount of lift of the intake valves  12   a ,  12   b  is changed in accordance with the operation of the accelerator pedal  74  by the driver, so as to control the amount of intake air. Thus, the amount of intake air can be adjusted without using a throttle valve, and therefore the engine  2  is simplified in construction and is reduced in weight.  
         [0180]    In the first embodiment, the exhaust valves  16   a,    16   b  are driven by the exhaust cams  46   a  simply via the rocker arms  14  as shown in FIG. 2, so that neither the amount of lift nor the operating angle of the valves  16   a,    16   b  is adjusted. However, the amount of lift and the operating angle of the exhaust valves  16   a,    16   b  may also be adjusted so as to perform various control operations, such as exhaust flow control, and control of returning exhaust for internal EGR. That is, an intermediate drive mechanism  520  may be provided between each exhaust cam  46   a  and corresponding rocker arms  14  as shown in FIG. 38, and the amount of lift and the operating angle of the exhaust valves  16   a ,  16   b  may be adjusted in accordance with the operating state of the engine  2  by using a newly provided lift-varying actuator (not shown). Furthermore, a rotational-phase-difference-varying actuator may also be provided for the exhaust camshaft  46  so as to adjust the valve timing of the exhaust valves  16   a,    16   b.    
         [0181]    In the first embodiment, the control shaft  132  is received within the support pipe  130 , and the entire structure of the intermediate drive mechanism  120  is supported by the support pipe  130 . However, it is also possible to provide only a control shaft  532  without providing a support pipe such that the control shaft  532  serves also as a support pipe, as shown in FIG. 39A. Here, the control shaft  532  functions to displace or move a slider gear  528  in the axial direction and also functions to support the entire structure of the intermediate drive mechanism  520 , as shown in FIG. 39B. In this case, the control shaft  532  is supported via journal bearings on a cylinder head so as to be slidable in the axial direction.  
         [0182]    In the first embodiment, the input portion  122  and the rocking cams  124 ,  126  of the intermediate drive mechanism  120  are disposed side by side with their corresponding end faces being in contact with each other. Instead, the intermediate drive mechanism may be constructed as shown in FIG. 40, in order to more reliably prevent the entry of foreign matters into the intermediate drive mechanism. More specifically, recessed engaging portions  522   m  are formed in opposite end portions of an input portion  522 , and protruding engaging portions  524   m,    526   m  are formed in opening end portions of rocking cams  524 ,  526 , respectively. The protruding engaging portions  524   m,    526   m  are respectively fitted into the recessed engaging portions  522   m.  These engaging portions are slidable relatively to each other, so that the input portion  522  and the rocking cams  524 ,  526  are allowed to rock or turn relative to each other. The recessed and protruding engaging portions may be reversed.  
         [0183]    In the first embodiment, the first rocking cam  124  and the second rocking cam  126  are coupled to the slider gear  128  via the helical splines having equal helical angles, so that the amount of lift and the operating angle of the two intake valves  12   a ,  12   b  of each cylinder  2   a  are changed or varied by the same degrees. Alternatively, the helical splines of the first rocking cam  124  and the helical splines of the second rocking cam  126  may have different angles, and the first output helical splines  128   c  and second output helical splines  128   e  of the slider gear  128  may be formed in accordance with those splines of the first and second rocking cams  124 ,  126 , respectively, so that the two intake valves of the same cylinder operate with different amounts of lift and different operating angles. With this arrangement, different amounts of intake air can be introduced in different timings from the two intake valves into the corresponding combustion chamber, so that turn flow, such as swirl, can be formed in the combustion chamber. In this way, the combustion characteristic can be improved so as to enhance the engine performance.  
         [0184]    In the above arrangement, differences in the angles of the helical splines of the first and second rocking cams give rise to differences in the amount of lift and the operating angle between the two intake valves of the same cylinder. However, differences in the amount of lift and the operating angle between the valves may also be realized by providing differences in the phase between the noses  124   d ,  126   d  of the rocking cams  124 ,  126  or by providing differences in the shape of the cam faces  124   e,    126   e  of the noses  124   d ,  126   d.    
         [0185]    Also, in the intermediate drive mechanism  120  of the first embodiment, a relative phase difference between the input portion  122  and at least one of the noses  124   d,    126   d  of the rocking cams  124 ,  126  may be maintained at a constant value. In this case, a relative phase difference between the input portion  122  and the remaining output portion, if any, may be made variable.  
         [0186]    In the first embodiment, the amount of lift of the intake valves is controlled in order to adjust the amount of intake air in the engine having no throttle valve. However, the invention is also applicable to an engine equipped with a throttle valve. For example, the intermediate drive mechanism may be used for adjusting, for example, the valve timing, since the operating angle is changed by adjusting the intermediate drive mechanism, and the valve timing is adjusted by changing the operating angle.  
         [0187]    In the first embodiment, rocker arms  13  are interposed between each intermediate drive mechanism  120  and the corresponding intake valves  12   a ,  12   b . However, an arrangement as shown in FIGS. 41A to  44 B may be employed in which a rocking cam  626  of an intermediate drive mechanism  620  contacts with and drives a valve lifter  613  that opens or closes an intake valve  612 . FIGS. 41A, 42A,  43 A and  44 A show the operating states of the valve drive mechanism when the intake valve  612  is closed. FIGS. 41B, 42B,  43 B and  44 B show the operating states of the valve drive mechanism when the intake valve  612  is opened. Unlike the first embodiment, a nose  626   d  of the rocking cam  626  is curved in a convex shape, and a curved surface  626   e  of the nose  626   d  slidably contacts with a top face  613   a  of the valve lifter  613 . A slider gear and a spline mechanism within the intermediate drive mechanism  620  are substantially the same as those of the first embodiment. With this arrangement, the relative phase difference between an input portion  622  and the rocking cam  626  can be changed by moving the slider gear in the axial direction. The relative phase difference between the input portion  622  and the rocking cam  626  as shown in FIGS. 41A and 41B provides the maximum amount of lift and the greatest operating angle. As the relative phase difference decreases from the state of FIGS. 41A and 41B to the states of FIGS. 42A and 42B, FIGS. 43A and 43B and FIGS. 44A and 44B in this order, the amount of lift and the operating angle are reduced with the decrease in the relative phase difference. In the state of FIGS. 44A and 44B, the amount of lift and the operating angle become zero, and the intake valve  612  is kept closed even if an intake cam  645   a  provided on an intake shaft  645  rotates. This arrangement provides substantially the same advantages (1), and (3) to (8) as stated above in conjunction with the first embodiment.  
         [0188]    Furthermore, an arrangement as shown in FIGS. 45A to  48 B may be employed in which a rocking cam  726  of an intermediate drive mechanism  720  contacts at a roller  726   e  with a valve lifter  713  for opening and closing an intake valve  712 . FIGS. 45A, 46A,  47 A and  48 A show the operating states of the valve drive mechanism when the intake valve  712  is closed. FIGS. 45B, 46B,  47 B and  48 B show the operating states of the valve drive mechanism when the intake valve  712  is opened. Unlike the first embodiment, a nose  726   d  of the rocking cam  726  is provided at its distal end with the roller  726   e , and the rocking cam  726  abuts at the roller  726   e  upon a top face  713   a  of the valve lifter  713 . A slider gear and a spline mechanism within the intermediate drive mechanism  720  are substantially the same as those of the first embodiment. With this arrangement, the relative phase difference between an input portion  722  and the rocking cam  726  can be changed by moving the slider gear in the axial direction. The relative phase difference between the input portion  722  and the rocking cam  726  as shown in FIGS. 45A and 45B provides the maximum amount of lift and the greatest operating angle. As the relative phase difference decreases from the state of FIGS. 45A and 45B to the states of FIGS. 46A and 46B, FIGS. 47A and 47B and FIGS. 48A and 48B in this order, the amount of lift and the operating angle are reduced with the decrease in the relative phase difference. In the state of FIGS. 48A and 48B, the amount of lift and the operating angle become zero, and the intake valve  712  is kept closed even if an intake cam  745   a  provided on an intake shaft  745  rotates. This arrangement provides substantially the same advantages (1), and (3) to (8) as stated above in conjunction with the first embodiment. Furthermore, since the rocking cam  726  drives the intake valve  712  via the roller  726   e  provided on the distal end of the nose  726   d , the friction resistance that arises when the intake cam  745   a  drives the intake valve  712  via the intermediate drive mechanism  720  is further reduced, and therefore the fuel economy can be improved.  
         [0189]    Furthermore, an arrangement as shown in FIGS. 49A to  52 B may be employed in which a rocking cam  826  of an intermediate drive mechanism  820  drives an intake valve  812  by contacting with a roller  813   a  provided on a valve lifter  813  for opening and closing the intake valve  812 . FIGS. 49A, 50A,  51 A and  52 A show the operating states of the valve drive mechanism when the intake valve  812  is closed. FIGS. 49B, 50B,  51 B and  52 B show the operating states of the valve drive mechanism when the intake valve  812  is opened. The valve lifter  813  is provided at the top part thereof with the roller  813   a.  Unlike the first embodiment, a nose  826   d  of the rocking cam  826  is curved in a concave shape at its proximal portion and in a convex shape at its distal portion, and the curved surface  826   e  of the nose  826  abuts on the roller  813   a  of the valve lifter  813 . A slider gear and a spline mechanism within the intermediate drive mechanism  820  are substantially the same as those of the first embodiment. With this arrangement, the relative phase difference between an input portion  822  and the rocking cam  826  can be changed by moving the slider gear in the axial direction. The relative phase difference between the input portion  822  and the rocking cam  826  as shown in FIGS. 49A and 49B provides the maximum amount of lift and the greatest operating angle. As the relative phase difference decreases from the state of FIGS. 49A and 49B to the states of FIGS. 50A and 50B, FIGS. 51A and 51B and FIGS. 52A and 52B in this order, the amount of lift and the operating angle are reduced with the decrease in the relative phase difference. In the state of FIGS. 52A and 52B, the amount of lift and the operating angle become zero, and the intake valve  712  is kept closed even if an intake cam  845   a  provided on an intake shaft  845  rotates. This arrangement provides substantially the same advantages (1), and (3) to (8) as stated above in conjunction with the first embodiment.  
         [0190]    While the hydraulically operated lift-varying actuator  100  is employed to move the control shaft in the axial directions in the first embodiment, an electrically driven actuator, such as a stepping motor or the like, may be employed instead.  
         [0191]    In the first embodiment, the relative phase difference between the input portion and the rocking cams is changed by moving the control shaft in the axial direction. Alternatively, a hydraulically operated actuator may be provided in an intermediate drive mechanism, so that the relative phase difference between the input portion and the rocking cams is changed by supplying regulated hydraulic pressure to the intermediate drive mechanism. It is also possible to provide an electrically operated actuator in an intermediate drive mechanism so that the relative phase difference between the input portion and the rocking cams is changed by controlling an electric signal applied to the actuator.  
         [0192]    While each intermediate drive mechanism is provided with one input portion and two rocking cams in the illustrated embodiment, the number of cams may also be one or more than two.  
         [0193]    While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the preferred embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.