Abstract:
In a variable valve timing mechanism, a valve-lifting cam member is fitted, slidably in the circumferential direction, onto a camshaft that is driven to rotate in synchronization with a crankshaft of a four-stroke cycle internal combustion engine. An eccentric collar is set between a driving collar fixed on the camshaft and the valve-lifting cam member. A driving projection is formed in the driving collar and engages with one of sandwiching portions of the eccentric collar. A driven protrusion is formed in the valve-lifting cam member and engages with another one of the sandwiching portions of the eccentric collar. A linkage mechanism includes the eccentric collar, the drive, and the driven protrusions. The variable valve timing mechanism adjusts the timing of opening and closing of the valve while the rotational phase of the valve-lifting cam member is cyclically varied relative to the camshaft by the eccentricity of the eccentric collar.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application Nos. 2007-043938 filed on Feb. 23, 2007; 2007-044914 filed on Feb. 26, 2007 and 2007-052244 filed on Mar. 2, 2007 the entire contents thereof are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable valve timing mechanism that varies the timing of the opening and closing the valves in a four-cycle internal combustion engine. 
     2. Description of Background Art 
     A variable valve timing mechanism has the following structure. An eccentric shaft that rotates independently from the rotation of a camshaft is disposed inside the camshaft. An eccentric collar having two sandwiching portions located on two sides across the center thereof is supported by the outer circumference of an eccentric portion of the eccentric shaft with rollers, as clearance-securing members, interposed in between. The eccentric collar thus supported is capable of rotating eccentrically. A driving collar having a driving projection that engages with a first one of the sandwiching portions of the eccentric collar is attached to the outer circumference of the camshaft and thus is integrated with the camshaft. A valve-lifting cam member having a driven projection that engages with a second one of the sandwiching portions of the eccentric collar is attached onto the outer circumference of the camshaft. The valve-lifting cam member thus attached is capable of sliding in the circumferential direction. The torque of the driving projection of the driving collar that rotates integrally with the camshaft is transmitted to the driven projection of the valve-lifting cam member via the pair of the sandwiching portions of the eccentric collar. The valve-lifting cam member is driven while the rotational phase thereof is cyclically varied. The eccentric shaft is provided to adjust and set the center position of the eccentric collar by use of the eccentric portion. See, for example, Japanese Unexamined Patent Application Laid-open Publication No. Sho63-1707 (FIG. 4 and FIG. 6)). 
     In a variable valve timing mechanism a pin is conventionally used to fix a driving collar onto a camshaft so as to rotate integrally with the camshaft. In the variable valve timing mechanism in which the pin is used, the pin has to be pressed to fit, so that the assembling operation of the driving collar to the camshaft and the maintenance operation that needs the detaching of the pin become complicated. In addition, a space for a pin hole has to be secured, so that the cylindrical portion of the driving collar becomes larger in size in the axial direction. As a consequence, the variable valve timing mechanism becomes larger in size. Moreover, forming the hole for this purpose needs a larger driving collar so as to secure a sufficient strength of the driving collar. 
     A conventional structure of eccentric shaft includes a shaft portion and an eccentric portion. The eccentric portion has a smaller diameter than that of the shaft portion, and has its center that is offset from the center of the shaft portion. When a large force is applied on a first one of the control driving portion of the eccentric shaft and the valve-lifting cam member, the large force is transmitted to a second one of the two members. The two members, thus, have to be made stronger. This leads to an increase in weight of the eccentric shaft. 
     In a conventional variable valve timing mechanism, the eccentric collar eccentrically rotates in a state where a plurality of rollers are disposed to secure a certain clearance between the eccentric collar and the outer circumference of the eccentric portion. The rollers are held respectively in the retaining windows formed in the camshaft. When the internal combustion engine runs normally, the eccentric portion of the eccentric shaft is stopped, the camshaft rotates, and the eccentric collar eccentrically rotates along with the rotation of the camshaft. Accordingly, the relative amount of rotation of the camshaft to the eccentric collar is small, while the relative amount of rotation of the camshaft to the eccentric portion of the eccentric shaft is large. Accordingly, each of the rollers does not actually roll, but slides on a particular portion thereof that is in contact with the eccentric portion of the eccentric shaft. In this particular portion, the convex surface of the roller is in contact with the convex surface of the eccentric portion, so that the surface pressure is high. As a consequence, securing the durability of these rollers is difficult. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     An object of an embodiment of the present invention is to provide fixation means of a driving collar by which means the operations for assembling and disassembling of the driving collar are made easier. In addition, by the fixation means, a cylindrical portion of the driving collar is made more compact in size in the axial direction while a sufficient strength of the driving collar is secured. 
     According to an embodiment of the present invention, a variable valve timing mechanism, in which a valve-lifting cam member is fitted, slidably in the circumferential direction, onto a camshaft that is driven to rotate in synchronization with a crankshaft of a four-cycle internal combustion engine. In addition, in the variable valve timing mechanism an eccentric collar is set between a driving collar fixed on the camshaft and the valve-lifting cam member. A linkage mechanism includes the eccentric collar, a driving projection formed in the driving collar and engaging with one of sandwiching portions of the eccentric collar, and a driven projection formed in the valve-lifting cam member and engaging with another one of the sandwiching portions of the eccentric collar. With the linkage mechanism, the torque of the driving collar is transmitted to the valve-lifting cam member. The timing of the opening and closing of the valve is adjusted by making the rotational phase of the valve-lifting cam member to be cyclically varied relative to the camshaft by the eccentricity of the eccentric collar. The variable valve timing mechanism is characterized by a key provided between the camshaft and the driving collar and used to fix the driving collar onto the camshaft. 
     According to an embodiment of the present invention, the variable valve timing mechanism includes the driving collar that includes a cylindrical portion and the driving projection protruding from the cylindrical portion. The key is in a position partially overlapping the driving projection in the axial direction of the camshaft. 
     According to an embodiment of the present invention, the variable valve timing mechanism further includes a bearing for the camshaft disposed between two flanges provided on the camshaft. In the variable valve timing mechanism, the driving projection protrudes from one of the flanges to the opposite side of the flange from the side where the bearing is located. 
     According to an embodiment of the present invention, the variable valve timing mechanism includes the driving-projection sandwiching portion of the eccentric collar. The driven-projection sandwiching portions are disposed as being offset from each other in the axial direction so as to make each of the sandwiching portions closer to the corresponding one of the projections that engage with the sandwiching portion. 
     According to an embodiment of the present invention, the variable valve timing mechanism further includes clearance-securing members installed respectively in a plurality of retaining windows formed in the circumference of the camshaft. Each clearance-securing member is in contact with the outer circumferential surface of an eccentric portion of an eccentric shaft fitted to a central hole of the camshaft with the inner circumferential surface of the eccentric collar. Accordingly, a clearance is secured between the two surfaces. In the cross section of each clearance-securing member, each of the two side-end portions of the clearance-securing member, the portions being in contact with the corresponding retaining window, is formed by a part of an outer circumferential circle. The central portion of the clearance-securing member is formed by a section that is in contact with the outer circumference of the eccentric portion and with the inner circumference of the eccentric collar. 
     According to an embodiment of the present invention, fixing the driving collar to the camshaft with the key allows an easy operation in assembling the driving collars to the other members and an easy maintenance operation that requires the detaching of the driving collars. In addition, no space for holes is required. Thus, the driving collar can be made smaller in size. Moreover, no holes are actually formed in the circumference of the driving collar, so that the strength can be secured easily. This contributes further to an even more compact construction of the driving collar. Furthermore, the variable valve timing mechanism as a whole can also be made more compact in size. 
     According to an embodiment of the present invention, the driving collar is composed of the cylindrical portion and the driving projection that protrudes from the cylindrical portion. The key is in a position partially overlapping the driving projection. Accordingly, the driving collar can be made compact in size in the axial direction. 
     According to an embodiment of the present invention, in each of the camshafts, the bearing has a simpler structure. In addition, the driving projection is formed in the flange. Accordingly, no driving collar is needed in this portion, so that the variable valve timing mechanism can be made more compact in size in the axial direction. In addition, a reduction in the number of component parts can be accomplished. 
     According to an embodiment of the present invention, the driving-projection sandwiching portion and the driven-projection sandwiching portion are disposed as being offset from each other in the axial direction so that the sandwiching portions are made closer to the respective projections that engage with the corresponding sandwiching portions. This allows the drive projections and the driven protrusion to be made shorter in dimension. As a result the variable valve timing mechanism can be made lighter in weight. 
     According to an embodiment of the present invention, though a columnar clearance-securing member makes the clearance-securing member to be in contact with the eccentric portion of the eccentric shaft with a convex surface being against another convex surface, a sectorial clearance-securing member allows the clearance-securing member to be in contact with the eccentric portion of the eccentric shaft with a convex surface being against a concave surface. Accordingly, the surface pressure between the outer surface of the eccentric portion of the eccentric shaft and each of the clearance-securing members is reduced, so that each of the clearance-securing members can be made more compact in size in the axial direction of the clearance-securing member. 
     An object of an embodiment of the present invention is to provide a lighter eccentric shaft. To this end, a large force that is applied on a first one of the control driving portion (gear train and servo motor) and the valve-lifting cam portion has to be prevented from transmitting to a second one of the two members, and an appropriate strength has to be set for each member. 
     According to an embodiment of the present invention, a variable valve timing mechanism is provided with the following features. In the variable valve timing mechanism, an eccentric shaft having an eccentric portion is inserted into a central hole of a camshaft that is driven to rotate in synchronization with a crankshaft of a four-cycle internal combustion engine. The eccentric shaft is thus made capable of rotating relative to the camshaft. A valve-lifting cam member is fitted, slidably in the circumferential direction, onto the outer circumference of the camshaft. An eccentric collar that is made eccentric in accordance with the position of the center of the eccentric portion is set between a driving collar fixed on the camshaft and the valve-lifting cam member. A linkage mechanism is composed of the eccentric collar, a driving protrusion formed in the driving collar and engaging with one of sandwiching portions of the eccentric collar, and a driven protrusion formed in the valve-lifting cam member and engaging with another one of the sandwiching portions of the eccentric collar. With the linkage mechanism, the torque of the driving collar is transmitted to the valve-lifting cam member. The timing of opening and closing the valve is adjusted by making the rotational phase of the valve-lifting cam member be cyclically varied relative to the camshaft by the eccentricity of the eccentric collar. The variable valve timing mechanism includes a breakable portion breakable by an occurrence of an abnormally excessive input and formed in the eccentric shaft between the eccentric portion and a power-for-control inputting portion. 
     According to an embodiment of the present invention, the variable valve timing mechanism further includes an oil passage for supplying oil to lubricate components, such as cams, formed inside the eccentric shaft in the axial direction. The breakable portion that is breakable by an occurrence of an abnormally excessive input is formed outside of a section where the oil passage exists. 
     According to an embodiment of the present invention, the variable valve timing mechanism includes the breakable portion that is breakable by an occurrence of an abnormally excessive input and has a shape having at least two parallel faces formed by cutting away portions of a shaft portion of the eccentric shaft. The breakable portion is formed as being exposed out of the camshaft. 
     According to an embodiment of the present invention, the variable valve timing mechanism includes the breakable portion that is breakable by an occurrence of an abnormally excessive input has a polygonal cross-sectional shape. 
     According to an embodiment of the present invention, the variable valve timing mechanism includes the breakable portion that is breakable by an occurrence of an abnormally excessive input is formed at an end of the eccentric shaft. 
     According to an embodiment of the present invention, when an especially large force is applied on either the valve-lifting cam portion or the control driving portion of the eccentric shaft of the engine that is running, the breakable portion that can be broken by an occurrence of an abnormally excessive input is broken to protect the component parts. Accordingly, the component parts have to have less strength, thereby preventing the increase in weight. The breakable portion that can be broken by a large force is formed not in the camshaft but in the eccentric shaft. Accordingly, even with the breaking of the breakable portion, the drive of the camshaft and that of the cam continue over the breakage of the breakable portion just like before the breakage. In this case, the variable valve timing mechanism is reduced to a simple valve timing mechanism without any function that will give a name of “variable” to the mechanism. The internal combustion engine continues to run over the loss of the above-mentioned function. 
     According to an embodiment of the present invention, the breakable portion that can be broken by an occurrence of an abnormally excessive input is formed outside of the section where the oil passage exists. Accordingly, the breakage of the breakable portion does not damage the oil passage. 
     According to an embodiment of the present invention, the faces formed by cutting away parts of the eccentric shaft are used as a guide for the initial setting of the eccentric shaft, and the tools can be used by taking advantage of these faces at the assembling. 
     According to an embodiment of the present invention, the breakable portion is made thinner, and thereby the eccentric shaft can be made still lighter in weight. 
     According to an embodiment of the present invention, the assembling of the driven gear for inputting the power for control to the eccentric shaft can be done easily by use of the breakable portion that can be broken by an occurrence of an abnormally excessive input. Particularly, in the case of the breakable portion with the two parallel faces, the dimensional accuracy can be managed easily, and there is less looseness between the gear and the eccentric shaft. As a result, an accurate control can be accomplished. 
     An object of an embodiment of the present invention is providing a variable valve timing valve-lifting system equipped with clearance-securing members which can replace the rollers and which can lower the surface pressure in the contacting portion. 
     According to an embodiment of the present invention, a variable valve timing valve-lifting system includes a camshaft having a central hole and rotating in synchronization with rotations of a crankshaft, an eccentric shaft having an eccentric portion and being inserted into the central hole of the camshaft, a driving collar fixed onto the camshaft and rotating together with the camshaft, an eccentric collar rotating, in response to the rotation of the driving collar, on a rotating center that is offset from a rotating center of the camshaft, the eccentric portion of the eccentric shaft, the eccentric portion positioned on the inner circumferential side of the eccentric collar and changing the position of the rotating center of the eccentric collar when the eccentric shaft moves rotationally. A valve-lifting cam member is provided rotating in response to the rotation of the eccentric collar with a plurality of retaining windows formed in a part, located between the eccentric collar and the eccentric portion, of the camshaft, and formed so as to allow the communication between an eccentric-collar side and an eccentric-portion side to be accomplished therethrough. Clearance-securing members are disposed respectively in the retaining windows, each clearance-securing member being in contact both with the eccentric collar and with the eccentric portion, thereby securing clearance between the eccentric collar and the eccentric portion. Here, the clearance-securing members are sliding spacers. In each of the sliding spacers, an inner-side and an outer-side contact surfaces are formed by curved lines that are considered, substantially, to be parts of concentric circles when viewed in the axial direction of the camshaft. The sliding spacers slide both on the eccentric collar and on the eccentric portion. 
     According to an embodiment of the present invention, the variable valve timing valve-lifting system as recited in the first aspect with the following additional features. When viewed in the axial direction of the camshaft, the curvature radius of a sliding surface of the sliding spacer on the eccentric-collar side is larger than the radius of an inner-side surface of the eccentric collar, and the curvature radius of the sliding surface of the sliding spacer on a side facing the eccentric-portion of the eccentric shaft is larger than the radius of an outer-side surface of the eccentric portion. 
     According to an embodiment of the present invention, the variable valve timing valve-lifting system as recited in the first aspect with the following additional features. When viewed in the axial direction of the camshaft, the curvature radius of a sliding surface of the sliding spacer on the eccentric-collar side is smaller than the radius of an inner-side surface of the eccentric collar, and the curvature radius of the sliding surface of the sliding spacer on a side facing the eccentric portion of the eccentric shaft is smaller than the radius of an outer-side surface of the eccentric portion. In addition, a curved surface is formed in the edge portion of a sliding contact portion of the sliding spacer with an outer circumference of the eccentric portion. 
     According to an embodiment of the present invention, the variable valve timing valve-lifting system as recited in any of the second and third aspects with the following additional features. When viewed in the axial direction of the camshaft, the sliding surface of the sliding spacer on the eccentric-collar side and the sliding surface of the sliding spacer on the eccentric-portion side are formed by parts of concentric circles. 
     According to an embodiment of the present invention, the variable valve timing valve-lifting system as recited in any of the first and fourth aspects with the following additional features. An inner-side surface of the retaining window, which surface is in contact with a side surface of the sliding spacer, is formed to be flat. In addition, the side surface of the sliding spacer, which side surface is in contact with the inner-side surface of the retaining window, is formed to be in an arc when viewed in the axial direction of the camshaft. 
     According to an embodiment of the present invention, in comparison with a roller, the sliding spacer of this form has a large sliding area between the sliding spacer and the eccentric collar as well as between the sliding spacer and the eccentric portion of the eccentric shaft. Accordingly, the surface pressure is lowered while the durability is improved. 
     According to an embodiment of the present invention, each sliding spacer thus formed slides on three points in total, two outside and one inside. This renders the sliding surfaces stabilized. In addition, the rotation amount of the eccentric collar relative to the sliding spacer is small so that the sliding by line-contact at end portions of the sliding spacer causes no problems at all. Moreover, the rotation amount of the eccentric portion of the eccentric shaft relative to the sliding spacer is large, but the angle formed by the tangential line of the eccentric portion and the tangential line of the sliding spacer is extremely small in the vicinity of each contact surface. Accordingly, the surface pressure can be lowered and favorable lubrication can be achieved. 
     According to an embodiment of the present invention, each sliding spacer thus formed slides on three points in total—one outside and two inside. This renders the sliding surfaces stabilized. 
     In addition, the rotation amount of the eccentric collar relative to the sliding spacer is small, so that the eccentric collar is in contact with the sliding spacer at one point. Meanwhile, the rotation amount of the eccentric portion of the eccentric shaft relative to the sliding spacer is large, so that the eccentric portion of the eccentric shaft is in contact with the sliding spacer at two points, thereby reducing the surface pressure of the sliding surface. In addition, forming the edge portion of the slidingly contact portion with a curved surface allows favorable lubrication to be achieved. 
     According to an embodiment of the present invention, when viewed in the axial direction of the camshaft, the inner and the outer sliding surfaces of the sliding spacer with this form are formed by parts of concentric circles. Accordingly, the sliding spacer can be fabricated by cutting a hollow pipe and then scraping a part thereof. This leads to a higher productivity. 
     According to an embodiment of the present invention, when the eccentric portion of the eccentric shaft is located at a certain position, the sliding spacer slides within the retaining window in the radial direction. In such a situation, an improvement in operation is achieved. In addition, the side surface of the sliding spacer is formed to be in an arc when viewed in the axial direction of the camshaft. Accordingly, when the position of the eccentric portion of the eccentric shaft in the circumferential direction thereof differs, the sliding surface also differs. In such a situation, uniform sliding characteristics can be accomplished. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a plan view showing a cylinder head of a four-cycle two-cylinder internal combustion engine according to a first embodiment of the present invention, wherein the shown cylinder head is cut by a plane including the center line of each of camshafts; 
         FIG. 2  is a longitudinal cross-sectional view of the camshaft; 
         FIG. 3  is an enlarged view of an eccentric collar viewed from the axial direction; 
         FIG. 4  is a cross-sectional view taken along the line IV-IV in  FIG. 3 ; 
         FIG. 5  is a cross-sectional view of a driving collar taken along the axis thereof; 
         FIG. 6  is a cross-sectional view of a central portion of the eccentric collar taken along the line VI-VI in  FIG. 2  and showing a low-speed state; 
         FIG. 7  is a cross-sectional view of the same position that is shown in  FIG. 6 , but showing a high-speed state; 
         FIG. 8  is a cross-sectional view of a central portion of an eccentric collar according to a second embodiment of the present invention; 
         FIG. 9(   a ) is an end-face view of the clearance-securing member;  FIG. 9(   b ) is a view showing an outside appearance the clearance-securing member; and  FIG. 9(   c ) is a side elevational view of the clearance-securing member. 
         FIG. 10  is a cross-sectional view taken along the line X-X in  FIG. 8 ; 
         FIG. 11  is a cross-sectional view taken along the line XI-XI in  FIG. 10 . 
         FIG. 12  is a cross-sectional view taken along the line XII-XII in  FIG. 2  and showing an example of a breakable portion which has a circular cross section and which can be broken by an occurrence of an abnormally excessive input; 
         FIG. 13  is a cross-sectional view of the same position that is shown in  FIG. 12 , but showing an example of a breakable portion with a cross section having two parallel faces; 
         FIG. 14  is a cross-sectional view of of the same position that is shown in  FIG. 12 , but showing an example of a polygonal cross section; 
         FIG. 15  is a vertical cross sectional view of an example where a gear where the power for control is inputted is coupled onto the breakable portion with two parallel faces formed at an end portion of the eccentric shaft; 
         FIG. 16  is a cross-sectional view taken along the line XIII-XIII in  FIG. 15 ; 
         FIGS. 17(   a ) to  17 ( d ) are four-side views of a sliding spacer  17 ; 
         FIG. 18  is an enlarged cross-sectional view showing a sliding spacer  17  and its peripheral members according to an embodiment of the variable valve timing valve-lifting system of the present invention; 
         FIG. 19  is an enlarged cross-sectional view showing a sliding spacer  25  and its peripheral members according to another embodiment of the variable valve timing valve-lifting system of the present invention; 
         FIG. 20  is an enlarged cross-sectional view showing a sliding spacer  30  and its peripheral members according to a third embodiment of the variable valve timing valve-lifting system of the present invention; 
         FIG. 21  is a vertical cross-sectional view of the eccentric collar  18 ; and 
         FIG. 22  is a cross-sectional view of the same position that is shown in  FIG. 8  and showing a high-speed state. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a plan view showing a cylinder head  2  of a four-cycle two-cylinder internal combustion engine  1  according to a first embodiment of the present invention. The shown cylinder head  2  is cut by a plane including the center line of each of camshafts  3  and  4 . The camshaft  3  for the inlet system and the camshaft  4  for the exhaust system are disposed in parallel with each other on the top surface of the cylinder head  2 . Each of the cylinders has two inlet valves and two exhaust valves. In the camshaft  3  for the inlet system, two identical valve-lifting cam members  5 ,  5  are provided to open and close the inlet valves of each cylinder, and are arranged along the camshaft  3 . Each of the two valve-lifting cam members  5 ,  5  has two cam lobes  5   a  and  5   b . Likewise, onto the camshaft  4  for the exhaust system, two valve-lifting cam members  5 ,  5  that are identical to the above-mentioned valve-lifting cam members  5 ,  5  are attached to open and close the exhaust valves of each cylinder. Each of these two valve-lifting cam members  5 ,  5  also has two cam lobes  5   a  and  5   b.    
     Two flanges  6  and  7  are formed at an end of each of the camshafts  3  and  4 . Each of the outer flanges  6  is integrally formed with the corresponding one of the camshafts  3  and  4 . Meanwhile, each of the inner flanges is separately formed from the corresponding one of the camshafts  3  and  4 , but is integrated with the camshaft by being pressed to fit onto the camshaft. Each of the camshafts  3  and  4  is rotatably supported by bearings  8 ,  9 , and  10 . The bearings  8  and  9  are respectively provided to the two valve-lifting cam members. Each of the bearings  8  and  9  is disposed between the two cam lobes  5   a  and  5   b  formed in each of the two valve-lifting cam members, which are fitted and thus attached onto each of the camshafts  3  and  4 . The bearing  10  is disposed between the two above-mentioned flanges  6  and  7 . 
     Each of the outer flanges  6  provided at an end of each of the camshafts  3  and  4  has one of two driven sprocket  11 ,  11 . A timing belt is looped around the driven sprockets  11 ,  11  and a drive sprocket provided on the crankshaft (not illustrated), and, with the timing belt, the camshafts  3  and  4  are driven to rotate by the crankshaft at a half revolving speed of the crankshaft. 
     The camshafts  3  and  4  are hollow and cylindrical. Into the central hole of each of the camshafts  3  and  4 , one of eccentric shafts  12 ,  12  is inserted, and thus is made rotatable relative to the corresponding one of the camshafts  3  and  4 . Each of the eccentric shafts  12 ,  12  has two identical eccentric portions  12   a ,  12   a . One of identical driven gears  13  for control is attached onto and thus integrated with an end of each of the eccentric shafts  12 ,  12 . The driven gears  13 ,  13  for control are driven and controlled by a control unit via a gear train and a servo motor (not illustrated). The driven gears  13 ,  13  for control thus controlled moves the central position of each eccentric portion  12   a  of each eccentric shaft  12  rotationally to a position such as to meet a predetermined purpose. At the steady driving, the eccentric shaft is stopped and the camshaft is rotating around the eccentric shaft. 
       FIG. 2  is a longitudinal cross-sectional view of the camshaft  3  for the inlet system. The following description is based on the camshaft  3  for the inlet system taken as an example of the two camshafts  3  and  4  for the inlet and exhaust systems, which have substantially identical structures to each other. In the description that follows, the “camshaft for the inlet system” is simply called the “camshaft” while the “inlet valve” is simply called the “valve.” 
     In the camshaft  3 , five rectangular retaining windows  16  are formed as arranged in the circumferential direction at each of the positions located at outer side of and corresponding to the two eccentric portions  12   a  and  12   a  of the eccentric shaft  12 . A roller  17  is disposed and held inside each of the retaining windows  16  so as to be in contact with the outer circumferential surface of the eccentric portion  12   a . Each of eccentric collars  18 ,  18  is attached at the outer side of each group of rollers. Here, each roller  17  is in contact with the inner surface of a cylindrical portion  18   a  of each of the eccentric collars  18 ,  18  while the rollers  17  are allowed to move in the circumferential direction relative to the cylindrical portion  18   a.    
     A driving collar  19  is fitted onto the outer circumference of the camshaft  3 , and is adjacent to one of the two eccentric collars  18 ,  18 . The one that is adjacent to the driving collar  19  is located farther from the inner flange  7  than the other one is. The driving collar  19  is fixed to the camshaft  3  with a key  20 , and thus is capable of rotating together with the camshaft  3 . A driving projection  19   b  is formed integrally with a cylindrical portion  19   a , and protrudes from the outer circumference of an end portion of the cylindrical portion  19   a  of the driving collar  19  towards the eccentric collar  18 . The other eccentric collar  18  is adjacent to the inner flange  7 . A driving projection  7   a  is formed integrally with the inner flange  7  and protrudes from the outer circumferential portion of the inner flange  7  towards the eccentric collar  18 . 
     The valve-lifting cam members  5 ,  5  are provided at positions that are respectively adjacent to the eccentric collars  18 ,  18 . Each of driven protrusions  5   c ,  5   c  is formed integrally with the corresponding one of the cam lobes  5   b ,  5   b  adjacent to the respective eccentric collars  18 ,  18 . Each driven protrusion protrudes from an end face of each of the cam lobes  5   b ,  5   b  to the eccentric collar  18 .  FIG. 2  shows four valve top portions  21  that are brought into contact with the cam lobes  5   a  and  5   b . An oil passage  12   b  is formed in the center portion of the eccentric shaft  12  to supply lubricating oil to various portions of the cam, the bearings, and the like. 
       FIG. 3  is an enlarged view of the eccentric collar  18  that is viewed from the axial direction.  FIG. 4  is a cross-sectional view taken along the line IV-IV of  FIG. 3 . The eccentric collar  18  includes the cylindrical portion  18   a , a pair of protrusion sandwiching portions, a drive-projection sandwiching portion  18   b  and a driven-projection sandwiching portion  18   c , which are provided with the center of the cylindrical portion  18   a  disposed in between. In each of the sandwiching portions, grooves  18   d  and  18   e  are formed. 
     In  FIG. 4 , the sandwiching portions  18   b  and  18   c  are formed as being offset from each other to the shaft-end side. To put it other way, as  FIG. 2  shows, the drive-projection sandwiching portion  18   b  is provided so as to get closer to the drive protrusion  19   b  or  7   a  while the driven-projection sandwiching portion  18   b  is provided to get closer to the driven protrusion  5   c . This is because such provision makes the drive protrusions  19   b  and  7   a  as well as the driven protrusion  5   c  shorter in dimension. Accordingly, the bending stress acting on each protrusion can be made smaller. 
       FIG. 4  is a cross-sectional view of the eccentric collar  18  taken along the axis thereof. The eccentric collar  18  includes a cylindrical portion  18   a , a pair or protrusion sandwiching portions—a drive-protrusion sandwiching portion  18   b  and a driven-protrusion sandwiching portion  18   c  which are provided with the center of the cylindrical portion  18   a  disposed in between. 
     In  FIG. 2 , a driving collar  19  is fitted onto the outer circumference of the camshaft  3 , and is adjacent to one of the two eccentric collars  18 ,  18 . The one that is adjacent to the driving collar  19  is located farther from the inner flange  7  than the other one is. The driving collar  19  is fixed to the camshaft  3  with a key  20 , and thus is capable of rotating together with the camshaft  3 . A driving protrusion  19   b  is formed integrally with a cylindrical portion  19   a , and protrudes from the outer circumference of an end portion of the cylindrical portion  19   a  of the driving collar  19  towards the eccentric collar  18 . The other eccentric collar  18  is adjacent to the inner flange  7 . A driving protrusion  7   a  is formed integrally with the inner flange  7  and protrudes from the outer circumferential portion of the inner flange  7  towards the eccentric collar  18 . 
     In  FIG. 2 , the valve-lifting cam members  5 ,  5  are provided at positions that are respectively adjacent to the eccentric collars  18 ,  18 . Each of driven protrusions  5   c ,  5   c  is formed integrally with the corresponding one of the cam lobes  5   b ,  5   b  adjacent to the respective eccentric collars  18 ,  18 . Each driven protrusion protrudes from an end face of each of the cam lobes  5   b ,  5   b  to the eccentric collar  18 .  FIG. 2  shows four valve top portions  21  that are brought into contact with the cam lobes  5   a  and  5   b.    
       FIG. 5  is a cross-sectional view of the driving collar  19  taken along the axis thereof, and shows a state in which the driving collar is fixed to the camshaft  3  with the key  20  so as to rotate integrally with the camshaft  3 . The driving collar  19  includes the cylindrical portion  19   a  and the driving projection  19   b , which is formed integrally with the cylindrical portion  19   a , and protrudes from the outer circumference of an end portion, on the eccentric-collar  18  side, of a cylindrical portion  19   a  towards the eccentric collar  18 . The key  20  is provided so as to partially overlap the driving projection  19   b  (the overlapping portion is indicated by reference numeral A in the drawing). With this structure, the key  20  can receive the moment that acts on a base portion  19   c  of the driving projection  19   b . Consequently, the cylindrical portion  19   a  can be made in a smaller thickness. In addition, while enough contact area between the key  20  and the driving collar  19  is secured to transmit the torque of the camshaft  3  from the key  20  to the driving collar  19 , the cylindrical portion  19   a  of the driving collar  19  can be made compact in the axial direction. 
       FIG. 6  is a cross-sectional view of a central portion of the eccentric collar  18  taken along the line VI-VI in  FIG. 2 , and shows a low-speed state. In the drawing, reference numeral O denotes the center of the camshaft  3  while reference numeral E denotes the center of the eccentric portion  12   a  of the eccentric shaft  12 . The inner surface of the cylindrical portion  18   a  of the eccentric collar  18  is supported by the plural rollers  17  at a uniform distance from the outer circumference of the eccentric portion  12   a . Accordingly, the center of the eccentric collar  18  is aligned with the center E of the eccentric portion  12   a  of the eccentric collar  12 . The rollers  17  serve as the clearance-securing members to keep a constant clearance between the outer circumference of the eccentric portion  12   a  and the inner circumference of the cylindrical portion  18   a  of the eccentric collar  18 . In the eccentric collar  18 , the driving-projection sandwiching portion  18   b  and the driven-projection sandwiching portion  18   c  are formed integrally the eccentric collar  18  at positions that are symmetrical with respect to the center of the eccentric collar  18 . The holding grooves  18   d  and  18   e  of the sandwiching portions hold the driving projection  19   b  and the driven protrusion  5   c  respectively. In the case of the eccentric collar  18  that is closer to the inner flange  7 , what engages with the sandwiching groove  18   d  is not the driving projection  19   b  of the driving collar  19  but the driving projection  7   a  protruding from the inner flange  7 . 
     With the above-described structure according to this embodiment, the following effects can be obtained. In the case of a low-revolution state of the internal combustion engine  1 , by a controlling signal from the control unit, the eccentric portion  12   a  of the eccentric shaft  12  is turned and kept at a position farthest away from the valve top portion  21 .  FIG. 6  shows such a state. In addition, in accordance with a principle that is similar to the principle described in Japanese Unexamined Patent Application Laid-open Publication Sho63-1707, the timing of the starting of the opening of the valve is retarded and the timing of the closing of the valve is advanced. Accordingly, a retarded opening-start timing of the inlet valve and an advanced closing timing of the exhaust valve render the valve overlapping period shorter. 
       FIG. 7  is a cross-sectional view of the same position that is shown in  FIG. 6 , but showing a high-speed state. When the revolution of the internal combustion engine  1  is increased up to the maximum revolution, the eccentric portion  12   a  of the eccentric shaft  12  is turned and kept at a position closest to the valve top portion  21 .  FIG. 7  shows such a state. In addition, in accordance with a principle that is similar to the principle described above, the timing of the starting of the opening of the valve is advanced and the timing of the closing of the valve is retarded. Accordingly, an advanced opening-start timing of the inlet valve and a retarded closing timing of the exhaust valve render the valve overlapping period longer. As a result, a state suitable for the high-speed running performance is achieved. 
     In  FIG. 2 , an oil passage  12   b  is formed in the center portion of the eccentric shaft  12  to supply lubricating oil to various portions of the cam, the bearings, and the like. A breakable portion  12   c  is formed near the end portion of the eccentric shaft  12 . The smaller diameter of the breakable portion  12   c  than that of the eccentric portion  12   a  allows the breakable portion  12   c  to be broken by an occurrence of an abnormally excessive input. The breakable portion  12   c  is formed between the eccentric portion  12   a  and the position of the driven gear  13  for control where the driving power for controlling the eccentric shaft is inputted. Within the section, the breakable portion  12   c  has the smallest cross-sectional area, so that the breakable portion  12   c  is the weakest against the torsion moment. The breakable portion  12   c  is formed at the outer side of the section where the oil passage is formed. More specifically, the part of the shaft portion of the eccentric shaft  12 , in which part the breakable portion  12   c  is formed, is the portion sticking out of the camshaft  3  and exposed to the outside. 
       FIG. 8  is a cross-sectional view of a central portion of an eccentric collar of a variable valve timing mechanism according to a second embodiment of the present invention.  FIG. 8  shows a low-speed running state. This embodiment differs from the first embodiment in a camshaft  25 , retaining windows  26 , and clearance-securing members  27 . The rest of the components shown in  FIG. 8 , an eccentric portion  12   a  of an eccentric shaft  12 , an eccentric collar  18  and each of the components thereof, a driving projection  19   b , a cam lobe  5   b , a driven protrusion  5   c , a valve top portion  21 , is the same as in the first embodiment, so that the same reference numerals are used. As illustrated in  FIG. 8 , O and E denote respectively the centers of the camshaft  25  and of the eccentric portion  12   a.    
     Each of the clearance-securing members  27  of this embodiment has a sectorial cross section. In the cross section, each of the two side portions are formed by a part of an outer circumferential circle  28  while the central portion is formed by a part of a sector that is in contact with the outer circumference of the eccentric portion  12   a  and with the inner circumference of the eccentric collar  18 . Each of the clearance-securing members  27  is cut out from a single pipe material with a part thereof being gouged off and thus the outer circumference circle of the clearance-securing member  27  has a larger diameter than the diameter of the roller of the first embodiment. For this reason, each of the retaining windows  26  formed in the camshaft  25  has a larger width than that of the first embodiment, and a reduced number of the clearance-securing members  27  are formed. 
     Each clearance-securing member  27  of the above-described structure has its inner surface in contact with the outer surface of the eccentric portion  12   a  and its outer surface in contact with the inner surface of the eccentric collar  18 . The above-described use of the clearance-securing members  27 , each with the sectorial cross section, reduces the surface pressure on the inner surface of the clearance-securing members  27  from the corresponding surface pressure in the case of the columnar clearance-securing members. As a consequence, each of the clearance-securing members  27  has a shorter dimension and the member can be made more compact in size. 
       FIGS. 9(   a ) to  9 ( c ) is a three-side view of the clearance-securing member  27 .  FIG. 9(   a ) is an end-face view,  FIG. 9(   b ) is a view showing an outside appearance thereof (a diagram viewed as indicated by the arrow B in  FIG. 9(   a )), and  FIG. 9(   c ) is a side elevational view (a diagram viewed as indicated by the arrow C in  FIG. 9(   a )). The cross section of the clearance-securing member  27  has each of the two side portions formed by a part of the outer circumferential circle  28 . Meanwhile, the inner and the outer sides of the cross section of the clearance-securing member  27  are made to be in contact respectively with the outer circumference of the eccentric portion  12   a  and with the inner circumference of the eccentric collar  18 . In addition, each of the end portions of each clearance-securing member  27  is cut, as shown in  FIGS. 9(   b ) and  9 ( c ) to be made into the shape of a circular truncated cone. Each of end faces  29  is thus formed in a circle with a smaller diameter. 
       FIG. 10  is a cross-sectional view taken along the line X-X in  FIG. 8 .  FIG. 11  is a cross-sectional view taken along the line XI-XI in  FIG. 10 . The cutting of the end portions of each clearance-securing member  27  into the shape of a circular truncated cone makes the end portion have a smaller contact area with the inner surface of each end portion of the retaining window  26 . As a result, the friction in this part of the mechanism can be reduced. 
     The embodiments that have been described in detail will have the following effects. 
     Fixing the driving collar  19  to the camshaft  3  with the key  20  allows an easy operation in assembling the driving collars to the other members and an easy maintenance operation that requires the detaching of the driving collar  19 . In addition, no space for holes is required, and thus the driving collar  19  can be made smaller in size. Moreover, no holes are actually formed in the circumference of the driving collar  19 , so that the strength can be secured easily. This contributes further to an even more compact construction of the driving collar  19 . Furthermore, the variable valve timing mechanism as a whole can also be made more compact in size. 
     The driving collar  19  is composed of the cylindrical portion  19   a  and the driving projection  19   b , and is fixed onto the camshaft  3  with the key  20 . The key  20  is disposed as partially overlapping the driving projection  19   b  in the axial direction of the shaft. With this structure, the key  20  can receive the moment acting on the base portion  19   c  of the driving projection  19   b . Consequently the cylindrical portion  19   a  can be made in a smaller thickness. In addition, while an enough contact area between the key  20  and the driving collar  19  is secured to transmit the torque of the camshaft  3  from the key  20  to the driving collar  19 , the cylindrical portion  19   a  of the driving collar  19  can be made compact in the axial direction. 
     In each of the camshafts  3  and  4 , the bearing  10  is disposed between the two flanges  6  and  7  that are provided on each of the camshafts  3  and  4 . Accordingly, the positioning of the bearing  10  in the axial direction of the corresponding camshaft is done by the help of the two flanges sandwiching the bearing  10 . Such a way of positioning needs a simpler structure for positioning than in the case where grooves for fitting to the flanges provided onto the camshaft are formed in the bearing for the purpose of positioning in the axial direction of the camshaft. In addition, the driving projection  7   a  protrudes from the inner flange  7  to the opposite side of the inner flange  7  from the side where the bearing  10  is located. Accordingly, no independent driving collar is needed in this portion, so that the variable valve timing mechanism can be made more compact in size in the axial direction. In addition, the reduction in the number of component parts can be accomplished. 
     The driving-projection sandwiching portion  18   b  and the driven-projection sandwiching portion  18   c  are disposed as being offset from each other in the axial direction onto the eccentric collar  18  so that the driving-projection sandwiching portion  18   b  is made to get closer to the driving projection  19   b  and  7   a , and that the driven-projection sandwiching portion  18   c  is made to get closer to the driven protrusion  5   c . Such provision makes the drive protrusions  19   b  and  7   a  as well as the driven protrusion  5   c  shorter in dimension. As a result the variable valve timing mechanism can be made lighter in weight. 
     In the second embodiment, each of the two side portions of the cross section of the clearance-securing member  27  is formed by a part of the outer circumferential circle, while the central portion is formed by a part of a section that is in contact with the outer circumference of the eccentric portion and with the inner circumference of the eccentric collar. Accordingly, the surface pressure between the outer surface of the eccentric portion of the eccentric shaft and each of the clearance-securing members is reduced from the corresponding surface pressure in the case of the columnar clearance-securing members, so that each of the clearance-securing members can be made more compact in size in the axial direction of the clearance-securing member. In addition, each of the end portions of each clearance-securing member  27  is cut to be made into the shape of a circular truncated cone. Such cutting of the end portion makes the end portion have a smaller contact area with the inner surface of each end portion of the retaining window  26 . As a result, the friction in this part of the mechanism can be reduced. 
       FIG. 12  is an enlarged cross-sectional view taken along the line XII-XII in  FIG. 2 , and shows a cross section of the breakable portion  12   c  which has a circular cross section and can be broken by an occurrence of an abnormally excessive input. The breakable portion  12   c  is thus formed in the eccentric shaft. Accordingly, when a large force is applied on a first one of the valve-lifting cam portion and the control driving portion of the eccentric shaft, the breakable portion is broken before the large force is transmitted to the second one of the two portions. Consequently, each one of the two component parts can be protected from the other, each of the two members can be formed with a modest strength, and thus, the increase in the weight of each member can be reduced. The role of the breakable portion is exactly the same that a fuse in an electrical apparatus plays. The formation of the breakable portion  12   c  outside the section where the oil passage  12   b  is formed prevents oil from leaking out even when the breakable portion is actually broken. Accordingly, the breaking of the breakable portion will never negatively affect the supplying of oil to the various parts of the apparatus. The drive of the camshaft and that of the cam continue over the breakage of the breakable portion just like before the breakage. In this case, the variable valve timing mechanism is reduced to a simple valve timing mechanism without any function that will give a name of “variable” to the mechanism. The internal combustion engine continues to run over the loss of the above-mentioned function. 
       FIG. 13  is a second example of the cross-sectional shapes of the breakable portion that can be broken by an occurrence of an abnormally excessive input. The cross section shown in  FIG. 13  is of the same position that the cross section of  FIG. 12  is taken at. Shown in this example is a breakable portion  12   d  that has two side faces being in parallel with each other. To form such a shape, the surface of the eccentric shaft  12  is cut out at two positions. A part of the shaft portion of the eccentric shaft  12  is exposed to the outside from the end of the camshaft, and the faces formed by cutting out portions of the eccentric shaft  12  are located in this part of the shaft portion. Accordingly, the faces are used as a guide for the initial setting of the eccentric shaft. In addition, tools can be used at the assembling by taking advantage of these faces. 
       FIG. 14  shows a third example of the cross-sectional shapes of the breakable portion that can be broken by an occurrence of an abnormally excessive input. The cross section shown in  FIG. 14  is of the same position that the cross section of  FIG. 12  is taken at. This is an example of a breakable portion  12   e  of a hexagonal cross-section, which represents polygonal cross-sections. The breakable portion  12   e  is made thinner so that the eccentric shaft can be made still lighter in weight. 
       FIG. 15  is a vertical cross-sectional view of an example where a breakable portion  12   f  with two parallel faces formed at an end portion of the eccentric shaft. In the example, the driven gear  13  for control where the power for control is inputted is coupled onto the breakable portion  12   f  by taking advantage of this breakable portion and is fastened with a nut  14 .  FIG. 16  is a cross-sectional view taken along the line XIII-XIII in  FIG. 15 . An easy assembling of the driven gear  13  for control onto the eccentric shaft is accomplished by using the breakable portion that can be broken by an occurrence of an abnormally excessive input. More particularly, in the case of the breakable portion with the two parallel faces, the dimensional accuracy can be managed easily, and there is less looseness between the gear and the eccentric shaft. As a result, an accurate control can be accomplished. 
     The embodiments that have been described in detail will have the following effects. 
     The breakable portion which has a smaller diameter than the eccentric portion and which can be broken by an occurrence of an abnormally excessive input is formed in the eccentric shaft between the eccentric portion  12   a  and the power-for-control input portion (the position of the driven gear  13  for control). Accordingly, when an especially large force is applied on either the valve-lifting cam portion or the control driving portion (gear train and servo motor) of the eccentric shaft of the engine that is running, the breakable portion is broken to protect the component parts. Accordingly, the component parts have to have less strength, thereby preventing the increase in weight. 
     The oil passage for supplying oil to lubricate the component parts, such as the cams, is formed inside the eccentric shaft in the axial direction. Meanwhile, the breakable portion that can be broken by an occurrence of an abnormally excessive input is formed outside of the section where the oil passage exists. Accordingly, the breakage of the breakable portion does not damage the oil passage. 
     The breakable portion that can be broken by an occurrence of an abnormally excessive input has a shape with at least two parallel faces that are formed by cutting away parts of the shaft portion of the eccentric shaft. In addition, the breakable portion is formed as being exposed out of the camshaft. In this case, the faces formed by cutting away are used as a guide for the initial setting of the eccentric shaft, and the tools can be used by taking advantage of these faces at the assembling. 
     When the breakable portion that can be broken by an occurrence of an abnormally excessive input has a polygonal shape, the breakable portion is made thinner. Thereby, the eccentric shaft can be made still lighter in weight. 
     When the breakable portion that can be broken by an occurrence of an abnormally excessive input is formed at an end of the eccentric shaft, the assembling of the driven gear for control to the eccentric shaft can be done easily by use of the breakable portion. Particularly, in the case of the breakable portion with two parallel faces, the dimensional accuracy can be managed easily, and there is less looseness between the gear and the eccentric shaft. As a result, an accurate control can be accomplished. 
       FIG. 10  is an enlarged longitudinal cross-sectional view showing the vicinity of one of the eccentric collars  18  that is located farther away from the inner flange  7  in  FIG. 2 .  FIG. 21  is a cross-sectional view of the eccentric collar  18  that appears at the center of  FIG. 10 . The eccentric collar  18  in  FIG. 21  includes the cylindrical portion  18   a , a drive-protrusion sandwiching portion  18   b  and a driven-protrusion sandwiching portion  18   c . In  FIG. 10 , the driving protrusion  19   b  that protrudes from the driving collar  19  is held by the driving-protrusion sandwiching portion  18   b  while the driven protrusion  5   c  that protrudes from the cam lobe  5   b  of the valve-lifting cam member  5  is held by the driven-protrusion sandwiching portion  18   c.    
     The configuration in the vicinity of the eccentric collar  18  that is located nearer the inner flange  7  is the same as the one shown in  FIG. 10  except that the driving protrusion held by the driving-protrusion sandwiching portion  18   b  is the driving protrusion  7   a  that protrudes from the inner flange  7 . 
       FIG. 8  is a cross-sectional view of a central portion of the eccentric collar  18  in  FIG. 10  that illustrates a low-speed state. In the drawing, reference numeral O denotes the center of the camshaft  3  while reference numeral E denotes the center of the eccentric portion  12   a  of the eccentric shaft  12 . The inner surface of the cylindrical portion  18   a  of the eccentric collar  18  is supported by the four sliding spacers  17  at a uniform distance from the outer circumference of the eccentric portion  12   a . Accordingly, the center of the eccentric collar  18  is aligned with the center E of the eccentric portion  12   a  of the eccentric collar  12 . The sliding spacers  17  serves as clearance-securing members to keep a constant clearance between the outer circumference of the eccentric portion  12   a  and the inner circumference of the cylindrical portion  18   a  of the eccentric collar  18 . In the eccentric collar  18 , the driving-protrusion sandwiching portion  18   b  and the driven-protrusion sandwiching portion  18   c  are formed integrally the eccentric collar  18  at positions that are symmetrical with respect to the center of the eccentric collar  18 . The holding grooves  18   d  and  18   e  of the sandwiching portions hold the driving protrusion  19   b  and the driven protrusion  5   c  respectively. In the case of the eccentric collar  18  that is closer to the inner flange  7 , what engages with the sandwiching groove  18   d  is not the driving protrusion  19   b  of the driving collar  19  but the driving protrusion  7   a  protruding from the inner flange  7 . 
     With the above-described structure according to this embodiment, the following effects can be obtained. In the case of a low-revolution state of the internal combustion engine  1 , by a controlling signal from the control unit, the eccentric portion  12   a  of the eccentric shaft  12  is turned and kept at a position farthest away from the valve top portion  21 .  FIG. 5  shows such a state. In addition, in accordance with a principle that is similar to the principle described in Japanese Unexamined Patent Application Laid-open Publication Sho63-1707, the timing of the starting of the opening of the valve is retarded and the timing of the closing of the valve is advanced. Accordingly, a retarded opening-start timing of the inlet valve and an advanced closing timing of the exhaust valve render the valve overlapping period shorter. 
       FIG. 22  is a longitudinal cross-sectional view of the same position that is shown in  FIG. 8 , but showing a high-speed state. When the revolution of the internal combustion engine  1  is increased up to the maximum revolution, the eccentric portion  12   a  of the eccentric shaft  12  is turned and kept at a position closest to the valve top portion  21 .  FIG. 22  shows such a state. In addition, in accordance with a principle that is similar to the principle described above, the timing of the starting of the opening of the valve is advanced and the timing of the closing of the valve is retarded. Accordingly, an advanced opening-start timing of the inlet valve and a retarded closing timing of the exhaust valve render the valve overlapping period longer. As a result, a state suitable for the high-speed running performance is achieved. 
       FIGS. 17(   a ) to  17 ( d ) are four-side views of the sliding spacer  17 .  FIG. 7(   a ) is an end-face view,  FIG. 7(   b ) is a top view seen from the direction as indicated by the arrow b in  FIG. 7(   a ),  FIG. 7(   c ) is a side elevational view seen from the direction as indicated by the arrow c in  FIG. 7(   a ), and  FIG. 7(   d ) is a cross-sectional view taken along the line d-d in  FIG. 7(   c ). In the longitudinal cross section, as shown in  FIGS. 7(   b ) and  7 ( c ), each of the two end portions of the sliding spacer  17  is cut into the shape of a circular truncated cone, and each of end faces  17   a  is thus formed in a circle with a smaller diameter. As shown in  FIG. 7(   d ), a cross section across the axis of the sliding spacer  17  is made up of an arc A, another arc B, and still another arc S. The arc A forms an outer-side surface of the cross section while the arc B forms an inner-side surface thereof. The circle S forms two side portions of the cross section. While the arcs A and B are parts of concentric circles that share the same center, the arc S is a part of an outer circumferential circle  22 . 
       FIG. 18  is an enlarged cross-sectional view showing the sliding spacer  17  according to a first embodiment of the variable valve timing valve-lifting system of the present invention. Peripheral members related to the sliding spacer  17  are also shown in  FIG. 18 . Three embodiments of the present invention will be described, and differ from one another in the way of setting the curvature radius of each of the arcs that form the contour of the cross-sectional shape of the sliding spacer  17 . For the following descriptions of the embodiments, definitions are given to the names and the reference numerals of the surfaces of the sliding spacer  17 , to the names and the reference numerals of the surfaces of the eccentric portion  12   a  of the eccentric shaft  12 , and to the names and the reference numerals of the eccentric collar  18 . The definitions are as follows. 
     A: the sliding surface on the eccentric-collar  18  side of the sliding spacer  17 . 
     B: the sliding surface on the eccentric-portion  12   a  side of the sliding spacer  17 . 
     S: the side surface of the sliding spacer  17  (=the surface of the sliding spacer on a side thereof that is in contact with the retaining window  16 ). 
     H: the inner-side surface of the cylindrical portion  18   a  of the eccentric collar  18 . 
     K: the outer-side surface of the eccentric portion  12   a  of the eccentric shaft  12 . 
     In addition, the curvature radius or the radius of these surfaces are given the following reference numerals. 
     Ra: the curvature radius of the sliding surface A on the eccentric-collar  18  side of the sliding spacer  17 . 
     Rb: the curvature radius of the sliding surface B on the eccentric-portion  12   a  side of the sliding spacer  17 . 
     (8) Rs: the curvature radius of the side surface S of the sliding spacer  17  (=the radius of the outer circumferential circle  22 ). 
     Rh: the radius of the inner-side surface H of the cylindrical portion  18   a  of the eccentric collar  18 . 
     Rk: the radius of the outer-side surface K of the eccentric portion  12   a  of the eccentric shaft  12 . 
     Note that the reference numerals shown in  FIG. 7(   d ) are also based on the above definitions. 
     In the sliding spacer  17  of the first embodiment shown in  FIG. 18 , the curvature radius Ra of the sliding surface A on the eccentric-collar side is made equal to the radius Rh of the inner-side surface H of the eccentric collar, that is, Ra=Rh. Meanwhile, the curvature radius Rb of the sliding surface B on the eccentric-portion side of the eccentric shaft is made equal to the radius Rk of the outer-side surface K of the eccentric portion, that is Rb=Rk. Accordingly, each of Ra, Rb, Rh, and Rk is a radius with the center being the center E of the eccentric portion  12   a.    
     The sliding spacer  17  is formed as having been described above. Accordingly, the sliding surface A on the eccentric-collar  18  side of the sliding spacer  17  is in surface-contact with the inner-side surface H of the cylindrical portion  18   a  of the eccentric collar  18 . In addition, the sliding surface B on the eccentric-portion  12   a  side of the sliding spacer  17  is in surface-contact with the outer-side surface K of the eccentric portion  12   a  of the eccentric shaft  12 . The sliding spacer with this form can significantly lower the surface pressure on the sliding surfaces. As a consequence, the lubricant oil is supplied from the oil passage  12   b  formed in the center of the eccentric shaft  12  to the sliding surfaces in an amount that is enough to reduce the sliding resistance. 
       FIG. 19  is an enlarged cross-sectional view showing the sliding spacer  25  according to a second embodiment of the variable valve timing valve-lifting system of the present invention. Peripheral members related to the sliding spacer  25  are also shown in  FIG. 9 . The component parts of the second embodiment are the same as those in the first embodiment, except for the sliding spacer  25 . In the second embodiment, the curvature radius Ra of the sliding surface A on the eccentric-collar side of the sliding spacer is made larger than the radius Rh of the inner-side surface H of the eccentric collar, that is, Ra&gt;Rh. Meanwhile, the curvature radius Rb of the sliding surface B of the sliding spacer on the eccentric-portion side of the eccentric shaft is made larger than the radius Rk of the outer-side surface K of the eccentric portion, that is Rb&gt;Rk. Accordingly, each of Ra and Rb is a radius with the center being the curvature center C that is far away from the sliding spacer  25 . 
     The sliding spacer  25  is formed as having been described above. Accordingly, the sliding surface A on the eccentric-collar  18  side of the sliding spacer  25  is in line-contact, at two positions X and Y, with the inner-side surface H of the cylindrical portion  18   a  of the eccentric collar  18 . In addition, the sliding surface B on the eccentric-portion  12   a  side of the sliding spacer  25  is in line-contact, at a point Z, with the outer-side surface K of the eccentric portion  12   a  of the eccentric shaft  12 . Each sliding spacer slides on three points in total—two points outside and a point inside. Accordingly, the sliding surfaces can be stabilized. In addition, the rotation amount of the eccentric collar  18  relative to the sliding spacer  25  is small so that the sliding by line-contact at end portions of the sliding spacer causes no problems at all. Moreover, the rotation amount of the eccentric portion of the eccentric shaft relative to the sliding spacer  25  is large, but the angle formed by the tangential line of the eccentric portion and the tangential line of the sliding spacer is extremely small in the vicinity of each contact surface. Accordingly, the surface pressure can be lowered and favorable lubrication can be achieved. 
       FIG. 20  is an enlarged cross-sectional view showing the sliding spacer  30  according to a third embodiment of the variable valve timing valve-lifting system of the present invention. Peripheral members related to the sliding spacer  30  are also shown in  FIG. 20 . The component parts of the third embodiment are the same as those in the first embodiment, except for the sliding spacer  30 . In the third embodiment, the curvature radius Ra of the sliding surface A on the eccentric-collar side of the sliding spacer is made smaller than the radius Rh of the inner-side surface H of the eccentric collar, that is, Ra&lt;Rh. Meanwhile, the curvature radius Rb of the sliding surface B of the sliding spacer on the eccentric-portion side of the eccentric shaft is made smaller than the radius Rk of the outer-side surface K of the eccentric portion, that is Rb&lt;Rk. Accordingly, each of Ra and Rb is a radius with the center being the curvature center D that is close to the sliding spacer  30 . In addition, the sliding contact portion of the sliding spacer with the outer circumference K of the eccentric portion is formed into a curved surface L. 
     The sliding spacer  30  is formed as having been described above. Accordingly, each sliding spacer slides on three points in total—a point U outside and two points V and W inside. Accordingly, the sliding surfaces can be stabilized. In addition, the rotation amount of the eccentric collar relative to the sliding spacer is small, so that the eccentric collar is in contact with the sliding spacer at one point. Meanwhile, the rotation amount of the eccentric portion of the eccentric shaft relative to the sliding spacer is large, so that the eccentric portion of the eccentric shaft is in contact with the sliding spacer at two points, thereby reducing the surface pressure of the slidingly contact surface. In addition, forming the slidingly contact portion with a curved surface allows favorable lubrication to be achieved. 
     The embodiments that have been described in detail will have the following effects. 
     The sliding surfaces in and outside of the sliding spacer are formed by parts of substantially concentric circles, which is concentric when viewed in the axial direction of the camshaft. Accordingly, the sliding area between the sliding spacer and the eccentric collar as well as the sliding area between the sliding spacer and the eccentric portion of the eccentric shaft are both increased from a conventional case of using rollers. This leads to a lower surface pressure while the durability of the sliding spacer is improved. 
     A possible sliding spacer has such a shape as follows. When viewed in the axial direction of the camshaft, the curvature radius of the sliding surface of the sliding spacer on the eccentric-collar side is made larger than the radius of the inner-side surface of the eccentric collar. When viewed in the axial direction of the camshaft, the curvature radius of the sliding surface of the sliding spacer on the side facing the eccentric-portion of the eccentric shaft is made larger than the radius of the outer-side surface of the eccentric portion. In this case, each sliding spacer slides on three points in total—two outside and one inside. Accordingly, the sliding surfaces are stabilized. In addition, the rotation amount of the eccentric collar relative to the sliding spacer is small so that the sliding by line-contact at end portions of the sliding spacer causes no problems at all. Moreover, the rotation amount of the eccentric portion of the eccentric shaft relative to the sliding spacer is large, but the angle formed by the tangential line of the eccentric portion and the tangential line of the sliding spacer is extremely small in the vicinity of each line-contact surface. Accordingly, the surface pressure can be lowered and favorable lubrication can be achieved. 
     Another possible sliding spacer has such a shape as follows. When viewed in the axial direction of the camshaft, the curvature radius of the sliding surface of the sliding spacer on the eccentric-collar side is made smaller than the radius of the inner-side surface of the eccentric collar. When viewed in the axial direction of the camshaft, the curvature radius of the sliding surface of the sliding spacer on the side facing the eccentric-portion of the eccentric shaft is made smaller than the radius of the outer-side surface of the eccentric portion. In addition, the sliding contact portion of the sliding spacer with the outer circumference of the eccentric portion is formed into a curved surface. In this case, each sliding spacer slides on three points in total—a point outside and two points inside. Accordingly, the sliding surfaces can be stabilized. In addition, the rotation amount of the eccentric collar relative to the sliding spacer is small, so that the eccentric collar is in contact with the sliding spacer at one point. Meanwhile, the rotation amount of the eccentric portion of the eccentric shaft relative to the sliding spacer is large, so that the eccentric portion of the eccentric shaft is in contact with the sliding spacer at two points, thereby reducing the surface pressure of the sliding surface. In addition, forming the slidingly contact portion with a curved surface allows favorable lubrication to be achieved. 
     When viewed in the axial direction of the camshaft, the inner and the outer sliding surfaces of the sliding spacer are formed by parts of concentric circles. Accordingly, the sliding spacer can be fabricated by cutting a hollow pipe and then scraping a part thereof. This leads to a higher productivity. 
     While the inner surface of the retaining window, which surface is in contact with a side surface of the sliding spacer, is formed to be flat, the side surface of the sliding spacer, which side surface is in contact with the inner-side surface of the retaining window, is formed to be in an arc when viewed in the axial direction of the camshaft. When the eccentric portion of the eccentric shaft is located at a certain position, the sliding spacer slides within the retaining window in the radial direction. In such a situation, an improvement in operation is achieved. In addition, when the position of the eccentric portion of the eccentric shaft in the circumferential direction thereof differs, the sliding surface also differs. In such a situation, uniform sliding characteristics can be accomplished. 
     In the examples described in the first to the third embodiments, the sliding surface A of the sliding spacer  17  on the side facing the eccentric collar  18  and the sliding surface B on the side facing the eccentric portion  12   a  are made up of parts of concentric circles. When the productivity can be ignored, the circles do not have to be perfectly concentric circles that have perfectly the same curvature center. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.