Patent Publication Number: US-2017350284-A1

Title: Valve gear for engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a valve gear for an engine, which switches between a plurality of cams with different cam profiles. 
     2. Description of the Related Art 
     Some recent engines mounted in vehicles are able to switch operation modes during operation. The operation modes to be switched include two operation modes having different fuel consumptions or output characteristics. Switching of the operation mode is often done using a valve gear that drives an intake valve and an exhaust valve. 
     A conventional valve gear capable of switching the operation mode is described in, for example, Japanese Patent Laid-Open No. 2010-249123. The valve gear disclosed in Japanese Patent Laid-Open No. 2010-249123 includes a camshaft, a rocker arm that transmits a driving force between the camshaft and an intake valve or an exhaust valve, and a driving device that switches the operation mode. The camshaft is provided with first and second cams that drive the intake valve or the exhaust valve, and an advancing and retreating cams that switch the operation mode. 
     The first cam and the second cam have shapes of different cam profiles. For example, the first cam has a shape with a cam nose projecting from a base circle, and the second cam has a shape of a perfect circle (for cylinder deactivation). The first and second cams or the rocker arm is movable in the axial direction of the camshaft. The first and second cams movable in the axial direction rotate integrally with the camshaft. 
     The valve gear described in Japanese Patent Laid-Open No. 2010-249123 presses the first and second cams or the rocker arm in the axial direction of the camshaft using the above-described advancing and retreating cams. That is, switching is performed between a first operation mode in which the rocker arm is pressed by the first cam and a second operation mode in which the rocker arm is pressed by the second cam. 
     The advancing cam and the retreating cam are constituted by spirally formed cam grooves and disposed side by side in the axial direction of the camshaft. The spiral of the advancing cam extends along the outer surface of the camshaft in one axial direction and the rotation direction. The spiral of the retreating cam extends along the outer surface of the camshaft in the other axial direction and the rotation direction. That is, the advancing cam and the retreating cam have shapes with spirals extending in opposite directions. This valve gear includes an advancing cam follower that selectively comes into contact with the advancing cam, and a retreating cam follower that selectively comes into contact with the retreating cam. 
     If the first and second cams are able to move in the axial direction, an arrangement to move the advancing cam and the retreating cam in the axial direction integrally with the first and second cams is used. In this case, the advancing cam follower, the retreating cam follower, and the rocker arm are supported by a cylinder head in a state in which they cannot move in the axial direction of the camshaft. 
     On the other hand, if the rocker arm is able to move in the axial direction, the advancing cam follower and the retreating cam follower are supported by a slide member that moves in the axial direction integrally with the rocker arm. 
     Another conventional valve gear of this type moves the rocker arm by the spring force of a helical compression spring without using the above-described advancing and retreating cams. In this valve gear, a timing of switching between the first operation mode and the second operation mode is defined by a switching timing control cam that rotates integrally with the first and second cams. 
     In the valve gear for an engine described in Japanese Patent Laid-Open No. 2010-249123, since the advancing cam and the retreating cam are needed on the camshaft, the total length of the camshaft increases. Recent camshafts have many functions to implement a 4-valve engine or expand capabilities. For example, the camshaft is provided with members such as gears and cams used to drive auxiliary machinery such as a high pressure fuel pump and a vacuum pump, and a rotation angle detection rotor. For this reason, to provide the advancing cam and the retreating cam on such a camshaft, the total length of the camshaft needs to be increased. 
     In the valve gear that moves the rocker arm in the axial direction by the spring force of a helical compression spring, a problem arises because the switching speed depends on only the spring load of the helical compression spring. In this valve gear, to correctly perform switching in a state in which the operation range of the engine is the high rotation range, a high spring load is necessary to increase the switching speed. However, if the spring load is high, a high impact load is applied to the switching portion at the time of switching, resulting in abnormal noise. The abnormal noise is not problematic in a high rotation mode with a loud engine sound. In a low rotation mode with a small engine sound, however, the abnormal noise may be unpleasant. 
     SUMMARY OF THE INVENTION 
     Preferred embodiments of the present invention provide a valve gear for an engine which provides a compact camshaft and also increases the reliability of a switching operation and reduces a switching operation sound. 
     According to a preferred embodiment of the present invention, a valve gear for an engine includes a camshaft rotatably supported by a cylinder head, a first cam provided on the camshaft and that drives one of an intake valve and an exhaust valve, a second cam provided on the camshaft spaced apart from the first cam in an axial direction, and that drives one of the intake valve and the exhaust valve, the second cam having a shape with a cam profile different from a cam profile of the first cam, a synchronous cam provided on the camshaft that rotates in synchronism with the first cam and the second cam, a rocker shaft parallel or substantially parallel to the camshaft, a rocker arm supported by the rocker shaft that swings and moves in the axial direction and converts a rotation of one of the first cam and the second cam into a reciprocal motion and transmits the reciprocal motion to one of the intake valve and the exhaust valve, a cam follower swingably supported by the rocker shaft and that comes into contact with the synchronous cam, and a thruster that converts the swing motion of the cam follower into a thrust in the axial direction and moves the rocker arm to one of a first side and a second side in the axial direction. 
     According to a preferred embodiment of the present invention, in the valve gear for the engine, the thruster preferably includes a slide portion that swings integrally with the cam follower and moves in the axial direction integrally with the rocker arm, and a switching portion supported by the cylinder head and including a first switch and a second switch, wherein the first switch and the second switch selectively come into contact with the slide portion, and the slide portion preferably includes a first inclined cam surface that receives a force in a first side thereof in the axial direction, wherein the force is generated by one of the first switch and the second switch in contact with the first inclined cam surface, and a second inclined cam surface that receives a force in a second side thereof in the axial direction, wherein the force is generated by the other of the first switch and the second switch in contact with the second inclined cam surface. 
     According to a preferred embodiment of the present invention, in the valve gear for the engine, a movement of the cam follower in the axial direction is preferably regulated or controlled, and the slide portion is separate from the cam follower and movable in the axial direction relative to the cam follower. 
     According to a preferred embodiment of the present invention, in the valve gear for the engine, each of the first switch and the second switch preferably include a pin that moves between an advancing position at which a first end comes into contact with the slide portion and a retreating position at which the first end separates from the slide portion, the second end of the pin preferably abuts against a pin cam of a moving member that moves in a direction perpendicular or substantially perpendicular to a direction in which the pin moves, and the pin cam has a shape such that when the moving member moves to a first side, the first switch moves to the advancing position and the second switch moves to the retreating position, and when the moving member moves to a second side, the first switch moves to the retreating position and the second switch moves to the advancing position. 
     In a preferred embodiment of the present invention, when the cam follower is pressed by the synchronous cam and swings, the thruster moves the rocker arm to a first side or a second side in the axial direction. When the rocker arm moves in the axial direction, switching is performed between a first operation mode in which the rocker arm is driven by the first cam and a second operation mode in which the rocker arm is driven by the second cam. 
     The synchronous cam is preferably short in the axial direction, as compared to conventional advancing and retreating cams including helical grooves. 
     In the valve gear, the switching speed when switching the operation mode depends on the profile (shape) and the cam rotational speed of the synchronous cam. For this reason, the switching speed changes in proportion to the cam rotational speed. As compared to a case in which the spring load of a spring member is increased when increasing the switching speed, reliability in switching at high rotations becomes high, and the operation sound at low rotations becomes small. 
     According to various preferred embodiments of the present invention, it is possible to provide a valve gear for an engine which provides a compact camshaft and also increases operation reliability and reduces the operation sound. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view showing an arrangement of a valve gear for an engine according to a first preferred embodiment of the present invention. 
         FIG. 2  is a front view showing a main portion of the valve gear according to the first preferred embodiment of the present invention in a state in which a cylinder head and a portion of a thruster are cut away. 
         FIG. 3  is a sectional view showing the main portion of the valve gear according to the first preferred embodiment of the present invention taken along a line III-III in  FIG. 2 . 
         FIG. 4  is a rear view showing the main portion of the valve gear according to the first preferred embodiment of the present invention in a state in which the cylinder head and a portion of the thruster are cut away. 
         FIG. 5A  is a plan view of a cam follower of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 5B  is a left side view of the cam follower of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 5C  is a front view of the cam follower of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 5D  is a right side view of the cam follower of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 5E  is a rear view of the cam follower of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 5F  is a bottom view of the cam follower of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 5G  is a perspective view of the cam follower of the valve gear according to the first preferred embodiment of the present invention viewed obliquely from the lower left side. 
         FIG. 6A  is a sectional view showing the main portion viewed from the axial direction of a camshaft so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 6B  is a front view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 6C  is a rear view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 7A  is a sectional view showing the main portion viewed from the axial direction of the camshaft so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 7B  is a front view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 7C  is a rear view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 8A  is a sectional view showing the main portion viewed from the axial direction of the camshaft so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 8B  is a front view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 8C  is a rear view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 9A  is a sectional view showing the main portion viewed from the axial direction of the camshaft so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention. 
         FIG. 9B  is a front view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 9C  is a rear view showing the main portion so as to explain the operation of the valve gear according to the first preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 10  is a perspective view of the cam follower and the slide portion of a valve gear according to a second preferred embodiment of the present invention. 
         FIG. 11  is a rear view showing the main portion of the valve gear according to the second preferred embodiment of the present invention in a state in which a portion of a thruster is cut away. 
         FIG. 12  is a rear view showing the main portion of the valve gear according to the second preferred embodiment of the present invention in a state in which a portion of the thruster is cut away. 
         FIG. 13  is an exploded perspective view of the main portion of a valve gear according to a third preferred embodiment of the present invention. 
         FIG. 14  is a sectional view of the main portion of the valve gear according to the third preferred embodiment of the present invention in an operation pause state. 
         FIG. 15  is a front view of the valve gear according to the third preferred embodiment of the present invention in an operation pause state. 
         FIG. 16  is a rear view of the valve gear according to the third preferred embodiment of the present invention in an operation pause state in which the cutaway position in  FIG. 14  is indicated by a line XIV-XIV. 
         FIG. 17  is a sectional view of the valve gear according to the third preferred embodiment of the present invention in a normal operation state. 
         FIG. 18  is a front view of the valve gear according to the third preferred embodiment of the present invention in a normal operation state. 
         FIG. 19  is a rear view of the valve gear according to the third preferred embodiment of the present invention in a normal operation state. 
         FIG. 20  is a perspective view of the main portion of a valve gear according to a fourth preferred embodiment of the present invention. 
         FIG. 21  is a side view of the valve gear according to the fourth preferred embodiment of the present invention in which the shaft main body of a camshaft is not illustrated. 
         FIG. 22  is a plan view of the valve gear according to the fourth preferred embodiment of the present invention in which the shaft main body of the camshaft is not illustrated. 
         FIG. 23  is an exploded perspective view of the main portion of the valve gear according to the fourth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Preferred Embodiment 
     A valve gear for an engine according to a first preferred embodiment of the present invention will now be described in detail with reference to  FIGS. 1 to 9C . 
     A valve gear  1  for an engine shown in  FIG. 1  includes a camshaft  3  in a cylinder head  2 , and a rocker arm  5  between the camshaft  3  and an intake valve  4 . The rocker arm  5  is supported by a rocker shaft  6  to be swingable and movable in the axial direction. 
     The rocker shaft  6  is supported by the cylinder head  2  to be parallel or substantially parallel to the camshaft  3 . The position of the rocker arm  5  in the axial direction is controlled by a thruster  11  to be described below. 
     The present preferred embodiment is applicable to both the valve gear  1  for an intake valve shown in  FIG. 1  and a valve gear for an exhaust valve (not shown) that drives an exhaust valve  12 . Note that the valve gear for an exhaust valve preferably has the same or similar structure as the valve gear  1  for an intake valve. Hence, in the present preferred embodiment, illustration and explanation of the valve gear for an exhaust valve are omitted. 
     Two intake valves  4  are provided for each cylinder. Each intake valve  4  includes a valve body  4   a  that opens/closes an intake port  13  in the cylinder head  2 , and a valve stem  4   b  extending from the valve body  4   a  into a valve gear chamber  14  in the cylinder head  2 . The valve stem  4   b  is movably supported on the cylinder head  2  via a valve stem guide  15 . A valve spring  16  that biases the intake valve  4  in a closing direction is provided between the cylinder head  2  and the distal end of the valve stem  4   b . A cap-shaped shim  17  is provided at the distal end of the valve stem  4   b.    
     The intake port  13  preferably has a fork shape branching in the cylinder head  2 . The upstream end of the intake port  13  opens to a side of the cylinder head  2 , and the downstream end of the intake port  13  opens to a combustion chamber  18 . A spark plug  19  is provided at the center of the combustion chamber  18 . As shown in  FIG. 1 , the spark plug  19  is provided at a position different from a cylinder axis C when viewed from the axial direction of the camshaft  3 . 
     The camshaft  3  rotates when the rotation of a crankshaft (not shown) is transmitted via a transmission mechanism. The camshaft  3  according to the present preferred embodiment includes a camshaft main body  21  preferably with a rod shape, and a plurality of cams provided on the camshaft main body  21 , as shown in  FIG. 2 . The plurality of cams includes a first cam  22  and a second cam  23  which are provided for each intake valve  4 , and a synchronous cam  24  located between the two sets of first cams  22  and second cams  23 . 
     The first cam  22  and the second cam  23  drive the intake valve  4 . The second cam  23  has a cam profile different from that of the first cam  22 , and has a shape the provides a different valve lift amount in the present preferred embodiment. In addition, the second cam  23  is provided on the camshaft  3  at a position spaced apart from the first cam  22  in the axial direction. As shown in  FIG. 3 , the first cam  22  and the second cam  23  include base circle portions  22   a  and  23   a  and nose portions  22   b  and  23   b , respectively. Each of the base circle portions  22   a  and  23   a  preferably has a columnar shape and is located on the same axis as the camshaft main body  21 , and has a size in which the valve lift amount of the intake valve  4  becomes 0. 
     Each of the nose portions  22   b  and  23   b  has a shape projecting from a corresponding one of the base circle portions  22   a  and  23   a  outward in the radial direction by a predetermined projecting amount so as to have a mountain-shaped section. The projecting amount of the nose portion  22   b  of the first cam  22  is larger than the projecting amount of the nose portion  23   b  of the second cam  23 . 
     The synchronous cam  24  drives the thruster  11  (to be described below), and includes a base circle portion  24   a  and a nose portion  24   b . The synchronous cam  24  rotates in synchronism with valve driving cams including the first cams  22  and the second cams  23 . The nose portion  24   b  of the synchronous cam  24  is located at a position different from the positions of the nose portions  22   b  and  23   b  of the first cam  22  and the second cam  23  in the rotation direction of the camshaft  3 . 
     The rocker arm  5  is substantially U-shaped in a plan view including two arm main bodies  25  that each convert the rotation of the first cam  22  or the second cam  23  into a reciprocal motion and transmit it to the intake valve  4 , and a connector  26  that connects the swing ends of the arm main bodies  25  to each other. The rocker shaft  6  extends through the proximal portions of the two arm main bodies  25 . 
     A presser  27  that presses the intake valve  4  is provided at each swing end of the rocker arm  5 , as shown in  FIG. 2 . The presser  27  is larger than the shim  17  in the axial direction of the rocker shaft  6 . For this reason, the presser  27  of the rocker arm  5  never disengages from the shim  17  even if the rocker arm  5  moves in the axial direction of the rocker shaft  6 . 
     As shown in  FIG. 4 , the two arm main bodies  25  are spaced apart at a predetermined interval in the axial direction of the rocker shaft  6 . A slider  31  that defines a portion of the thruster  11  is inserted between the two arm main bodies  25 . 
     As shown in  FIG. 3 , the thruster  11  includes a slide portion  32  with the above-described slider  31 , and a switching portion  33  provided at a position adjacent to the slide portion  32 . 
     The slide portion  32  includes the slider  31  through which the rocker shaft  6  extends, and a plurality of functional portions (to be described below in detail) provided on the slider  31 . As shown in  FIG. 4 , the slider  31  is inserted between the two proximal portions of the two arm main bodies  25  in a state in which it is in slidable contact with the proximal portions, and is also supported by the rocker shaft  6  to be pivotal and movable in the axial direction. When the slider  31  moves in the axial direction of the rocker shaft  6 , the rocker arm  5  integrally moves in the same direction as the slider  31 . 
     A cam follower  34  that contacts the synchronous cam  24  is integral with the slider  31  in the present preferred embodiment. As shown in  FIG. 3 , the cam follower  34  preferably has a lever shape extending in a direction crossing the longitudinal direction of the rocker arm  5  as viewed from the axial direction of the rocker shaft  6 . The distal end of the cam follower  34  extends up to a position adjacent to the camshaft  3 . When the camshaft  3  rotates in a state in which the cam follower  34  is close to the camshaft  3 , the synchronous cam  24  presses the cam follower  34 , and the cam follower  34  and the slider  31  swing about the rocker shaft  6  in a swinging direction indicated by an arrow A in  FIGS. 5B to 5D . 
     As shown in  FIG. 4 , an axial length of the synchronous cam  24  according to the present preferred embodiment is larger than the width (the width in the horizontal direction in  FIG. 4 , or the width in the axial direction of the rocker shaft  6 ) of the cam follower  34 . This prevents the cam follower  34  from disengaging from the synchronous cam  24  when the cam follower  34  moves in the axial direction together with the slider  31 . 
     The synchronous cam  24  has a shape that presses the cam follower  34  when the rocker arm  5  contacts the base circle portion  22   a  of the first cam  22  or the base circle portion  23   a  of the second cam  23 , as shown in  FIG. 9A . In other words, when the intake valve  4  is closed, the cam follower  34  is pressed by the synchronous cam  24  and swings. 
     The plurality of functional portions provided on the slider  31  include a first inclined cam surface  35  (see  FIG. 4 ) and a second inclined cam surface  36 , which are located on the slider  31  on the opposite side of the cam follower  34 , and a first concave groove  37  (see  FIG. 2 ) and a second concave groove  38 . 
     As shown in  FIGS. 5B and 5D , the first inclined cam surface  35  and the second inclined cam surface  36  are provided on a convex portion  39  of the slider  31 . The convex portion  39  projects in a direction different from the direction in which the cam follower  34  projects from the slider  31 . In the assembled state shown in  FIG. 3 , the convex portion  39  according to the present preferred embodiment projects in a direction opposite to the direction in which the rocker arm  5  extends. As shown in  FIG. 5C , the convex portion  39  has a mountain-shaped section projecting to the opposite side of the cam follower  34 . The first inclined cam surface  35  and the second inclined cam surface  36  are provided on the surface (lower surface) of the convex portion  39  on the opposite side of the cam follower  34 . 
     The first inclined cam surface  35  and the second inclined cam surface  36  according to the present preferred embodiment are preferably flat surfaces that are inclined in directions opposite to each other in the axial direction of the rocker shaft  6 , as shown in  FIGS. 4 and 5B to 5G . As shown in  FIG. 5C , the first inclined cam surface  35  and the second inclined cam surface  36  extend from the center of the convex portion  39  in the axial direction of the rocker shaft  6  to a first end and a second end. The first inclined cam surface  35  is inclined to gradually lower from the center of the convex portion  39  to the first end. 
     The second inclined cam surface  36  is inclined to gradually lower from the center of the convex portion  39  to the second end. Note that the first inclined cam surface  35  and the second inclined cam surface  36  may have concave curved surfaces, although not illustrated. 
     As shown in  FIG. 5F , the first concave groove  37  and the second concave groove  38  are located at an end of the slider  31  on the opposite side of the cam follower  34  at positions adjacent to the first inclined cam surface  35  and the second inclined cam surface  36  in the longitudinal direction of the convex portion  39 . The first concave groove  37  and the second concave groove  38  are side by side in the axial direction of the rocker shaft  6 , and extend in a direction perpendicular or substantially perpendicular to the axial direction of the rocker shaft  6 . 
     As shown in  FIGS. 3 and 4 , the switching portion  33  of the thruster  11  includes a first pin  41  facing the first inclined cam surface  35 , a second pin  42  facing the second inclined cam surface  36 , a moving member  43  in contact with the pins  41  and  42 , and a third pin  44  to be engageably inserted in the first concave groove  37  or second concave groove  38 . In the present preferred embodiment, the first pin  41  corresponds to a first switch, and the second pin  42  corresponds to a second switch. 
     As shown in  FIG. 3 , the first pin  41  and the second pin  42  are supported by the cylinder head  2  to be movable in the longitudinal direction in a state in which they are parallel or substantially parallel to the valve stem  4   b  of the intake valve  4 . As shown in  FIG. 4 , the first pin  41  and the second pin  42  are provided at predetermined positions spaced apart from each other at a predetermined interval in the axial direction of the rocker shaft  6 . The predetermined positions are positions associated with the first inclined cam surface  35  and the second inclined cam surface  36 . 
     As shown in  FIG. 6C , the first pin  41  is provided at a position facing the projecting end of the first inclined cam surface  35  in a state in which the slider  31  has moved to the first end with the first inclined cam surface  35  in the axial direction of the rocker shaft  6 . The projecting end is a portion near the top defined by the first inclined cam surface  35  and the second inclined cam surface  36 . 
     On the other hand, as shown in  FIG. 4 , the second pin  42  is provided at a position facing the projecting end of the second inclined cam surface  36  in a state in which the slider  31  has moved to the second end with the second inclined cam surface  36  in the axial direction of the rocker shaft  6 . 
     The first pin  41  and the second pin  42  move between an advancing position to advance toward the slider  31  and a retreating position to retreat in a direction opposite to the slider  31 . When the slider  31  swings integrally with the cam follower  34 , the first pin  41  and the second pin  42  that advance to the advancing position are brought into contact with the first inclined cam surface  35  or the second inclined cam surface  36 . In a state in which the first pin  41  and the second pin  42  move to the retreating position, the movement of the first inclined cam surface  35  or the second inclined cam surface  36  is not impeded even if the slider  31  swings.  FIG. 4  shows a state in which the first pin  41  is located at the advancing position, and the second pin  42  is located at the retreating position. The advancing position and the retreating position are controlled by the moving member  43  that comes into contact with the first pin  41  and the second pin  42 . 
     The moving member  43  preferably has a columnar shape and is movably fitted in an oil hole  45  of the cylinder head  2 . The oil hole  45  is parallel or substantially parallel to the rocker shaft  6 . For this reason, the moving member  43  moves in a direction perpendicular or substantially perpendicular to the direction in which the first pin  41  and the second pin  42  move. 
     The moving member  43  according to the present preferred embodiment includes a piston that moves in the oil hole  45 . A helical compression spring  46  is inserted on a first end (the left side in  FIG. 4 ) of the oil hole  45 . The helical compression spring  46  biases the moving member  43  to the second end of the oil hole  45 . Note that both the spring force of the helical compression spring  46  and an oil pressure may be applied to one end of the moving member  43 . The end of the moving member  43  close to the helical compression spring  46  will simply be referred to as a first end and the end on the opposite side as the second end hereinafter. 
     The second end of the oil hole  45  is connected to an oil pressure supply device (not shown). Hence, an oil pressure propagated from the oil pressure supply device is applied to the second end (the end on the right side in  FIG. 4 ) of the moving member  43 . 
     A first pin cam  47  that moves the first pin  41  between the advancing position and the retreating position and a second pin cam  48  that moves the second pin  42  between the advancing position and the retreating position are provided in the moving member  43 . The cams  47  and  48  preferably symmetrical to each other with respect to a plane of symmetry defined by a virtual plane perpendicular or substantially perpendicular to the axis of the moving member  43 . 
     The first pin cam  47  and the second pin cam  48  include curved surfaces extending from concave portions  49  and  50  in which the ends of the first pin  41  and the second pin  42  are inserted to the outer surface of the moving member  43 . The first pin  41  and the second pin  42  are inserted in the concave portions  49  and  50  and thus located at the retreating position. 
     The first pin cam  47  is provided at a first end of the moving member  43 . When the moving member  43  moves to a first end (the left side in  FIG. 6C ) of the oil hole  45  from a state in which the first pin  41  is located in the concave portion  49  and at the retreating position (see  FIG. 6C ), the first pin cam  47  pushes the first pin  41  out of the concave portion  49  and places the first pin  41  on the outer surface of the moving member  43 , as shown in  FIG. 7C . The first pin  41  that has moved to the advancing position comes into contact with the first inclined cam surface  35  when the slider  31  swings. 
     The second pin cam  48  is provided at the second end of the moving member  43 . The second pin cam  48  has a shape that moves the second pin  42  to the advancing position (see  FIG. 6C ) when the moving member  43  moves to the second end (the right side in  FIG. 4 ) from a state in which the second pin  42  is located in the concave portion  50  and at the retreating position (see  FIG. 4 ). The second pin  42  that has moved to the advancing position comes into contact with the second inclined cam surface  36  when the slider  31  swings. 
     That is, the first pin  41  and the second pin  42  selectively come into contact with the slide portion  32  (slider  31 ) when the moving member  43  moves to the first end or the second end. 
     The first pin cam  47  and the second pin cam  48  are arranged such that, when one of the first pin  41  and the second pin  42  is located at the advancing position, the other of the first pin  41  and the second pin  42  moves to the retreating position. That is, when the moving member  43  moves to a first end, the first pin  41  moves to the advancing position, and the second pin  42  returns to the retreating position, as shown in  FIG. 7C . In addition, when the moving member  43  moves to the second end that is the other side in the longitudinal direction, the first pin  41  returns to the retreating position, and the second pin  42  moves to the advancing position, as shown in  FIG. 6C . 
     As shown in  FIGS. 2 and 3 , the third pin  44  is faces the first concave groove  37  or the second concave groove  38  of the slider  31  and is movably supported by the cylinder head  2  parallel or substantially parallel to the valve stem  4   b  of the intake valve  4 . The direction in which the third pin  44  moves is the direction parallel or substantially parallel to the valve stem  4   b  of the intake valve  4 . The distal end of the third pin  44  preferably has a hemispherical shape. 
     In addition, the third pin  44  is pressed against the first concave groove  37  or the second concave groove  38  by the spring force of a helical compression spring  51  provided between the third pin  44  and the cylinder head  2 . For this reason, the slider  31  is biased by the spring force of the helical compression spring  51  in a direction in which the cam follower  34  separates from the camshaft  3  about the rocker shaft  6 . When biased by the spring force of the helical compression spring  51 , the slider  31  swings in the swinging direction A about the rocker shaft  6  until the first inclined cam surface  35  or the second inclined cam surface  36  comes into contact with the first pin  41  or the second pin  42 . For this reason, the slider  31  and the cam follower  34  are maintained in a state in which the first inclined cam surface  35  or the second inclined cam surface  36  is in contact with the first pin  41  or the second pin  42  when the cam follower  34  is not pressed by the synchronous cam  24 . 
     The first concave groove  37  and the second concave groove  38  each preferably include a V-shaped section, as shown in  FIG. 2 . For this reason, for example, if the slider  31  moves in a direction (the right side in  FIG. 2 ) opposite to the second concave groove  38  in a state in which the third pin  44  engages with the first concave groove  37 , as shown in  FIG. 2 , the inclined side wall of the first concave groove  37  pushes the third pin  44 , and the third pin  44  moves in a direction opposite to the slider  31  against the spring force of the helical compression spring  51 . 
     Then, the third pin  44  moves across the top that defines the boundary between the first concave groove  37  and the second concave groove  38  and enters the second concave groove  38 . The third pin  44  that has entered the second concave groove  38  presses the side wall of the second concave groove  38  by the spring force of the helical compression spring  51 . Since this side wall is inclined as well, the movement of the slider  31  is assisted by the spring force of the helical compression spring  51 . The slider  31  stops when the third pin  44  advances to the deepest point of the second concave groove  38 . The operation of the third pin  44  is performed similarly even if the slider  31  moves in a direction opposite to the above-described direction. 
     In a state in which the third pin  44  is inserted in the first concave groove  37 , as shown in  FIG. 2 , the slider  31  and the rocker arm  5  according to the present preferred embodiment are located at a first position at which the rocker arm  5  contacts the first cams  22 . When the rocker arm  5  is located at the first position, a first operation mode in which the intake valve  4  is driven by the first cams  22  is performed. 
     In a state in which the third pin  44  is inserted in the second concave groove  38 , as shown in  FIG. 6B , the slider  31  and the rocker arm  5  are located at a second position at which the rocker arm  5  contacts the second cams  23 . When the rocker arm  5  is located at the second position, a second operation mode in which the intake valve  4  is driven by the second cams  23  is performed. 
     The operation of the valve gear  1  will be described next with reference to  FIGS. 6A to 9C . An operation performed when shifting from the second operation mode in which the intake valve  4  is driven by the second cams  23  to the first operation mode will be explained here. 
     When the second operation mode is performed, the rocker arm  5  is located at a position where it is pressed by the second cams  23 , as shown in  FIG. 6A , and the third pin  44  is inserted in the second concave groove  38 , as shown in  FIG. 6B . The moving member  43  moves to the second end, as shown in  FIG. 6C . The first pin  41  is located at the retreating position, and the second pin  42  is located at the advancing position. 
     When switching from the second operation mode to the first operation mode, the moving member  43  is moved from the second end to the first end, as shown in  FIG. 7C . When the moving member  43  moves from the second end to the first end, the first pin  41  is located on the outer surface of the moving member  43  and moves to the advancing position to press the first inclined cam surface  35 . When the first inclined cam surface  35  is pressed by the first pin  41 , as shown in  FIG. 7A , the slider  31  and the cam follower  34  swing in a direction (counterclockwise in  FIG. 7A ) opposite to the swinging direction A, and the cam follower  34  approaches the camshaft  3 . At this time, the movement (movement in the axial direction of the rocker shaft  6 ) of the slider  31  is controlled by the third pin  44 . Additionally, at this time, the concave portion  50  of the moving member  43  is located at a position facing the second pin  42 . 
     When the camshaft  3  rotates in this state, the cam follower  34  is pressed by the synchronous cam  24  in a state in which the rocker arm  5  is in contact with the base circle portions  23   a  of the second cams  23 , and the slider  31  integrally swings in the swinging direction A with the cam follower  34 , as shown in  FIG. 8A . When the slider  31  swings, the projecting end of the first inclined cam surface  35  is pressed against the first pin  41 , as shown in  FIG. 8C . In a state in which the first pin  41  is located at the advancing position, the first pin  41  cannot move (retreat) even if the slider  31  swings to bring the first inclined cam surface  35  into contact with the first pin  41 . 
     As described above, when the projecting end of the first inclined cam surface  35  is pressed against the first pin  41 , the first inclined cam surface  35  receives a thrust. The direction in which the thrust acts is the direction in which the low portion of the first inclined cam surface  35  approaches the first pin  41 . As a result, the slider  31  integrally moves to the second end (the right side in  FIG. 8C ) with the rocker arm  5 . When the slider  31  starts moving, the third pin  44  is pressed by the side wall of the second concave groove  38  and retreats against the spring force of the helical compression spring  51 , as shown in  FIG. 8B . 
     As shown in  FIGS. 9A and 9B , the third pin  44  moves from the second concave groove  38  into the first concave groove  37  during a time until the top (the distal end portion where the nose portion  24   b  projects most) of the synchronous cam  24  presses the cam follower  34 . When the top of the synchronous cam  24  passes through the cam follower  34 , the thrust disappears because the cam follower  34  is not pressed by the synchronous cam  24 . Note that when the slider  31  moves in accordance with the swing motion of the cam follower  34 , the second pin  42  is pressed by the second inclined cam surface  36  and returns to the retreating position. 
     When the top of the synchronous cam  24  passes through the cam follower  34 , the third pin  44  is in a state in which it presses the side wall of the first concave groove  37 . For this reason, although the first inclined cam surface  35  separates from the first pin  41 , the side wall of the first concave groove  37  is pressed by the third pin  44  according to the spring force of the helical compression spring  51 , and the slider  31  further moves to the second end. The slider  31  stops when the third pin  44  advances to the deepest point of the first concave groove  37 . When the slider  31  stops in this way, the rocker arm  5  is located at the first position at which the rocker arm  5  contacts the first cams  22 , as shown in  FIGS. 9B and 9C , and the operation mode shifts to the first operation mode in which the intake valve  4  is driven by the first cams  22 . 
     A shift from this operation mode to the second operation mode in which the intake valve  4  is driven by the second cams  23  is made by moving the moving member  43  to the second end (the right side in  FIG. 9C ) from a state shown in  FIG. 9C . When the moving member  43  moves in this way, the second pin  42  moves to the advancing position, and the cam follower  34  comes into contact with the synchronous cam  24 . The cam follower  34  swings, the second pin  42  comes into contact with the second inclined cam surface  36  to generate a thrust, and the slider  31  moves. At this time, the slider  31  moves to the left side in  FIG. 9C  from the position shown in  FIG. 9C  to the position shown in  FIG. 6C . In addition, the first inclined cam surface  35  presses the first pin  41  in accordance with the movement of the slider  31 , and the first pin  41  returns to the retreating position. 
     The synchronous cam  24  used in the valve gear  1  for an engine is preferably short in the axial direction, as compared to conventional advancing and retreating cams including helical grooves. This means that the camshaft  3  is short. In addition, the synchronous cam  24  is able to be made by the same manufacturing method as the first cam  22  and the second cam  23 . That is, the synchronous cam  24  is able to be made using a cam processing machine used to make the first cam  22  and the second cam  23 . 
     In the valve gear  1  according to the present preferred embodiment, the switching speed when switching the operation mode is determined depending on the profile (shape) and the cam rotational speed of the synchronous cam  24 . For this reason, the switching speed changes in proportion to the cam rotational speed. As compared to a case in which the spring load of a spring member is increased when increasing the switching speed, reliability in switching in a high rotation state becomes high, and the operation sound in a low rotation state becomes small. 
     In the valve gear  1  according to the present preferred embodiment, the main operation sound generated when switching the operation mode includes the sound of friction between the first inclined cam surface  35  or the second inclined cam surface  36  and the first pin  41  or the second pin  42 , and the sound of friction between the third pin  44  and the slider  31 . Such a sound is smaller than the sound of collision between metal members. 
     Hence, according to the present preferred embodiment, it is possible to provide a valve gear for an engine, in which the camshaft  3  is made compact and at low cost and also increases the reliability of the operation and reduces the operation sound. 
     The slide portion  32  of the thruster  11  according to the present preferred embodiment includes the first inclined cam surface  35  and the second inclined cam surface  36 , and moves in the axial direction of the rocker shaft  6  when the cam follower  34  swings to press the cam surface  35  or  36  against the first pin  41  or the second pin  42 . 
     For this reason, the thruster  11  according to the present preferred embodiment is small and has a simple structure, as compared to a case in which a link or gear is used to convert the swinging motion of the cam follower  34  into a thrust in the axial direction. Hence, according to the present preferred embodiment, it is possible to provide a valve gear for an engine that has a small size at a reduced cost. 
     As for the first pin  41  and the second pin  42  according to the present preferred embodiment, when one pin is located at the advancing position, the other pin moves to the retreating position. Hence, according to the present preferred embodiment, since the first pin  41  and the second pin  42  never simultaneously move to the advancing position, it is possible to provide a valve gear for an engine in which the thruster  11  has high operation reliability. 
     Second Preferred Embodiment 
     A valve gear for an engine according to a second preferred embodiment of the present invention will be described in detail with reference to  FIGS. 10 to 12 . The same reference numerals as in  FIGS. 1 to 9C  denote the same or similar members in  FIGS. 10 to 12 , and a detailed description thereof will appropriately be omitted. 
     A valve gear  61  (see  FIG. 11 ) for an engine according to the present preferred embodiment is different from the valve gear  1  described in the first preferred embodiment only in the structures of a cam follower  34  and a slider  31 . The rest of the arrangement of the valve gear  61  is preferably the same as in the valve gear  1  described in the first preferred embodiment. 
     As shown in  FIG. 10 , the cam follower  34  according to the present preferred embodiment is separate from the slider  31 . A proximal portion  34   a  of the cam follower  34  is inserted into a concave portion  62  of the slider  31 . A through hole  63  that receives a rocker shaft  6  (see  FIG. 11 ) is provided in the proximal portion  34   a . The rocker shaft  6  passes through the through hole  63  and two through holes  64  at the two ends of the slider  31 . 
     A swing end  34   b  of the cam follower  34  is swingably inserted into a concave groove  66  of a stopper  65  fixed to a cylinder head (not shown). Each side wall of the concave groove  66  contacts the cam follower  34  when the cam follower  34  moves in the axial direction of the rocker shaft  6 . That is, movement of the cam follower  34  according to the present preferred embodiment is regulated or controlled by the side walls of the concave groove  66  and, therefore, cannot move in the axial direction of the rocker shaft  6 . 
     To allow the slider  31  to move relative to the cam follower  34  in the axial direction of the rocker shaft  6 , the concave portion  62  of the slider  31  is longer than the cam follower  34  by a predetermined length in the axial direction of the rocker shaft  6 . The predetermined length is a length that allows the slider  31  to move relative to the cam follower  34  between a position at which a rocker arm  5  contacts first cams  22 , as shown in  FIG. 11 , and a position at which the rocker arm  5  contacts second cams  23 , as shown in  FIG. 12 . 
     The proximal portion  34   a  of the cam follower  34  is provided with a first convex portion  67  and a second convex portion  68  to regulate or control its swinging motion relative to the slider  31 . The first convex portion  67  and the second convex portion  68  are provided at positions spaced apart to one side and the other side in the radial direction of the rocker shaft  6 . The first convex portion  67  comes into contact with a pressure receiving portion  69  of the slider  31 , and the second convex portion  68  comes into contact with a transmitting portion  70  of the slider  31 . That is, when the cam follower  34  is pressed by a synchronous cam  24  and swings, the pressing force is transmitted from the cam follower  34  to the slider  31  via the contact portion between the first convex portion  67  and the pressure receiving portion  69 . When the slider  31  is pressed by a third pin  44  and swings, the pressing force is transmitted from the slider  31  to the cam follower  34  via the contact portion between the second convex portion  68  and the transmitting portion  70 . 
     In the valve gear  61  according to the present preferred embodiment, even if the slider  31  moves in the axial direction of the rocker shaft  6 , the position of the cam follower  34  does not change. For this reason, as compared to a case in which the cam follower  34  moves in the axial direction of the rocker shaft  6 , the synchronous cam  24  that presses the cam follower  34  is short in the axial direction. Hence, according to the present preferred embodiment, since the location of the synchronous cam  24  on a camshaft  3  is narrow, the camshaft  3  is able to be shorter. 
     Third Preferred Embodiment 
     A valve gear according to a third preferred embodiment the present invention is shown in  FIGS. 13 to 19 . The same reference numerals as in  FIGS. 1 to 9C  denote the same or similar members in  FIGS. 13 to 19 , and a detailed description thereof will appropriately be omitted. 
     A valve gear  71  for an engine according to the present preferred embodiment is different from the valve gear  1  described in the first preferred embodiment in the structures of a camshaft  3 , a rocker arm  5 , a cam follower  34 , and a thruster  11 . As for the cam follower  34  according to the present preferred embodiment, the movement in the axial direction is regulated or controlled, as in a case of the second preferred embodiment. The rest of the arrangement of the valve gear  71  is preferably the same as in the valve gear  1  described in the first preferred embodiment. 
     As shown in  FIG. 15 , two first cams  22  of the camshaft  3  according to the present preferred embodiment are located at positions adjacent to a synchronous cam  24 . Second cams  23  are located at positions that sandwich the first cams  22  from both sides. As shown in  FIG. 14 , each second cam  23  has no nose portion and includes only a base circle portion  23   a . That is, the valve gear  71  according to the present preferred embodiment switches between a first operation mode in which an intake valve  4  is driven by the first cams  22  and a second operation mode in which the intake valve  4  does not open. 
     As shown in  FIG. 13 , the rocker arm  5  according to the present preferred embodiment is provided for each intake valve  4  (see  FIG. 15 ). That is, the rocker arm  5  according to the present preferred embodiment includes only an arm main body  25 , and includes no connector  26  included in the first preferred embodiment. 
     A slide portion  32  of the thruster  11  according to the present preferred embodiment includes a first slider  72  and a second slider  73 , which are separated from the cam follower  34 , and a plurality of functional portions provided on each of the sliders  72  and  73 . The first slider  72  and the second slider  73  are preferably symmetrical to each other with respect to a plane of symmetry defined by a virtual plane perpendicular or substantially perpendicular to the axis of the rocker arm  5 . Through holes  74  that receive the rocker shaft  6  (see  FIG. 15 ) are provided in the first slider  72  and the second slider  73 . The first slider  72  and the second slider  73  are supported by a rocker shaft  6  to be pivotal and movable in the axial direction. 
     The functional portions provided on the first slider  72  and the second slider  73  include a first inclined cam surface  35  and a second inclined cam surface  36  (see  FIG. 16 ), and a first concave groove  37  and a second concave groove  38  (see  FIG. 15 ). In the present preferred embodiment, inclined cam surfaces and concave grooves located on lateral portions of the first slider  72  and the second slider  73  close to each other will be referred to as the first inclined cam surfaces  35  and the first concave grooves  37  for convenience. In addition, inclined cam surfaces and concave grooves located on the other lateral portion of the first slider  72  and the second slider  73  will be referred to as the second inclined cam surfaces  36  and the second concave grooves  38 . 
     As shown in  FIG. 13 , an outer concave portion  75  that holds the rocker arm  5  and an inner concave portion  77  that receives a boss  76  of the cam follower  34  (to be described below) are provided in each of the first slider  72  and the second slider  73  according to the present preferred embodiment. 
     The outer concave portion  75  has a shape that allows the rocker arm  5  to swing and regulates or controls the movement of the rocker arm  5  in the axial direction relative to the first slider  72  and the second slider  73 . 
     The rocker arms  5  are swingably supported by the first slider  72  and the second slider  73  via the rocker shaft  6  by inserting the rocker shaft  6  into the through holes  74  of the sliders  72  and  73  and shaft holes  78  of the rocker arms  5  in a state in which the proximal portions are inserted in the outer concave portions  75 . The rocker arm  5  supported by the first slider  72  moves in the axial direction of the rocker shaft  6  together with the first slider  72 . The rocker arm  5  supported by the second slider  73  moves in the axial direction of the rocker shaft  6  together with the second slider  73 . 
     As shown in  FIG. 13 , the cam follower  34  according to the present preferred embodiment includes the cylindrical boss  76  through which the rocker shaft  6  passes, a lever  79  extending from the boss  76  in the radial direction of the rocker shaft  6 , and a first connector  80  and a second connector  81  which extend from the lever  79  in the axial direction of the rocker shaft  6 . The boss  76 , the lever  79 , the first connector  80 , and the second connector  81  are preferably integral, for example, and may be formed by integral molding. 
     The hollow portion of the boss  76  has a shape that allows the rocker shaft  6  to be rotatably fitted therein. The length of the boss  76  in the axial direction is larger than the width (the width in the axial direction of the rocker shaft  6 ) of the lever  79 . The lever  79  is located at the center of the boss  76  in the axial direction. For this reason, the two ends of the boss  76  project from the lever  79  in the axial direction. As shown in  FIG. 18 , the projecting portions are located in the inner concave portions  77  of the first slider  72  and the second slider  73  when the first slider  72  and the second slider  73  approach each other. 
     The first connector  80  and the second connector  81  regulate or control the swinging motion of the cam follower  34  relative to the sliders  72  and  73 , and are located at different positions in the swinging direction of the cam follower  34 . As shown in  FIG. 14 , the first connector  80  is located on the downstream side of the lever  79  in the swinging direction of the cam follower  34 . Here, the downstream side is the downstream side in a swinging direction A when the cam follower  34  is pressed by the synchronous cam  24  and swings. The first connector  80  comes into contact with pressure receiving portions  82  provided on the first slider  72  and the second slider  73  from the upstream side in the above-described swinging direction. That is, when the cam follower  34  is pressed by the synchronous cam  24  and swings, the pressing force is transmitted from the cam follower  34  to the first slider  72  and the second slider  73  via the contact portions between the first connector  80  and the pressure receiving portions  82 . 
     The second connector  81  is located on the upstream side of the first connector  80  in the swinging direction A. The second connector  81  comes into contact with transmitting portions  83  provided on the first slider  72  and the second slider  73  from the downstream side in the swinging direction A. That is, when the first slider  72  and the second slider  73  are pressed by third pins  44  (to be described below) and swing, the pressing force is transmitted from the first slider  72  and the second slider  73  to the cam follower  34  via the contact portions between the second connector  81  and the transmitting portions  83 . 
     As shown in  FIG. 16 , each of the first connector  80  and the second connector  81  has a length to contact the pressure receiving portion  82  or transmitting portion  83  in a state in which the first slider  72  and the second slider  73  move to maximum moving positions in a direction in which they are separated from each other. Hence, the cam follower  34 , the first slider  72 , and the second slider  73  always integrally swing. 
     As shown in  FIGS. 15 and 16 , a switching portion  33  of the thruster  11  according to the present preferred embodiment includes a first pin  41  and a second pin  42  for each slider, one moving member  43  including first pin cams  47  and second pin cams  48  that drive the pins  41  and  42 , and a third pin  44  for each slider. 
     The first pin  41  faces the first inclined cam surface  35 , and the second pin  42  faces the second inclined cam surface  36 . 
     The first pin cam  47  and the second pin cam  48  of the moving member  43  are provided for each slider. The first pin cam  47  and the second pin cam  48  according to the present preferred embodiment include an arrangement to move the first slider  72  and the second slider  73  in directions opposite to each other. More specifically, when the moving member  43  moves from the position on the second end shown in  FIG. 19  to the position on the first end shown in  FIG. 16 , the first pin cam  47  moves the first pin  41  from the retreating position to the advancing position. 
     When the moving member  43  moves from the position on the first end shown in  FIG. 16  to the position on the second end shown in  FIG. 19 , the second pin cam  48  moves the second pin  42  from the retreating position to the advancing position. In the present preferred embodiment as well, the first pin cam  47  and the second pin cam  48  include an arrangement such that, when one of the first pin  41  and the second pin  42  is located at the advancing position, the other pin moves to the retreating position. 
     In the valve gear  71  for an engine according to the present preferred embodiment, when the moving member  43  moves from the position on the first end shown in  FIG. 16  to the position on the second end shown in  FIG. 19 , the second pin  42  moves from the retreating position to the advancing position, and the first slider  72  and the second slider  73  are moved to positions at which they are in contact with each other, as shown in  FIG. 19 , by a thrust acting on the second inclined cam surfaces  36 . At this time, the third pins  44  move from the first concave grooves  37  of the first slider  72  and the second slider  73  into the second concave grooves  38 . 
     When the first slider  72  and the second slider  73  move in this way, the rocker arms  5  contact the first cams  22 , and the intake valves  4  are driven by the first cams  22 , as shown in  FIGS. 17 and 18 . 
     On the other hand, when the moving member  43  moves from the position on the second end shown in  FIG. 19  to the position on the first end shown in  FIG. 16 , the first pin  41  moves to the advancing position, and the first slider  72  and the second slider  73  are moved in directions in which they are separated from each other, as shown in  FIG. 16 , by a thrust acting on the first inclined cam surfaces  35 . At this time, the third pins  44  move from the second concave grooves  38  of the sliders  72  and  73  into the first concave grooves  37 . When the first slider  72  and the second slider  73  move in this way, the rocker arms  5  contact the second cams  23 , and the intake valves  4  are maintained in the closed state, as shown in  FIGS. 14 and 15 . 
     For this reason, according to the present preferred embodiment, it is possible to provide a valve gear for an engine that is able to switch between the first operation mode in which the intake valves  4  operate and the second operation mode in which the intake valves  4  are at rest. 
     Fourth Preferred Embodiment 
     A slider and a cam follower in a valve gear according to a fourth preferred embodiment of the present invention is shown in  FIGS. 20 to 23 . The same reference numerals as in  FIGS. 1 to 12  denote the same or similar members in  FIGS. 20  to  23 , and a detailed description thereof will appropriately be omitted. 
     A valve gear  91  for an engine according to the present preferred embodiment is different from the valve gear  61  described in the second preferred embodiment ( FIGS. 10 to 12 ) in the structures of a cam follower  34  and a slider  31 . The rest of the arrangement of the valve gear  91  is preferably the same as in the valve gear  61  described in the second preferred embodiment. As for the cam follower  34  according to the present preferred embodiment, the movement in the axial direction is regulated or controlled by a stopper  65  (see  FIG. 21 ). Two rocker arms  5  per cylinder are located on both sides of a slider  92  (see  FIG. 23 ) according to the present preferred embodiment, and are swingably supported by one tubular shaft  93  together with the slider  92 . 
     The tubular shaft  93  is inserted into shaft holes  94  of the two rocker arms  5  and through holes  64  of the slider  92  and extends through these members. A rocker shaft  6  is fitted in the hollow portion of the tubular shaft  93 . The tubular shaft  93  is supported by the rocker shaft  6  to be rotatable and movable in the axial direction. The two rocker arms  5  and the slider  92  are mounted on the tubular shaft  93  in a state in which they are in contact with each other in the axial direction of the tubular shaft  93 . Circlips  95  are attached to the two ends of the tubular shaft  93  in a state in which the circlips  95  are in contact with the rocker arms  5 . That is, the two rocker arm  5 , the slider  92 , and the tubular shaft  93  integrally move relative to the rocker shaft  6  in the axial direction. 
     Each rocker arm  5  according to the present preferred embodiment includes a roller  96  that contacts a first cam  22  or a second cam  23 . 
     The slider  92  according to the present preferred embodiment is different from the slider  31  described in the second preferred embodiment in the location of a convex portion  39  including a first inclined cam surface  35 , a second inclined cam surface  36 , a first concave groove  37 , and a second concave groove  38 . The convex portion  39  extends almost parallel or substantially parallel to a cylinder axis C (see  FIG. 1 ) to the opposite side of a combustion chamber  18 , and has a shape conforming to the cam follower  34 . The first inclined cam surface  35 , the second inclined cam surface  36 , the first concave groove  37 , and the second concave groove  38  are provided on the lateral side of the convex portion  39  opposite to the cam follower  34 . For this reason, a switching portion  33  of a thruster  11  is disposed at the same position as the cam follower  34  in the axial direction (the vertical direction in  FIG. 21 ) of the cylinder. 
     The slider  92  includes a pressure receiving portion  97  (see  FIGS. 21 and 23 ) and a transmitting portion  98  to regulate or control a swinging motion relative to the cam follower  34 . The pressure receiving portion  97  contacts an intermediate portion  34   c  (see  FIG. 23 ) located between a swing end  34   b  and the swing center of the cam follower  34  (the axis of the rocker shaft  6 ). The transmitting portion  98  contacts the other swing end  34   d  (see  FIG. 23 ) located on the opposite side of the swing end  34   b  with respect to the swing center of the cam follower  34 . 
     In the valve gear  91  for an engine according to the present preferred embodiment, since the switching portion  33  of the thruster  11  is provided at the same position as the cam follower  34  in the axial direction of the cylinder, a wide space in which to locate other elements is provided between the rocker arm  5  and the combustion chamber  18 . 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.