Patent Publication Number: US-9850790-B2

Title: Internal combustion engine equipped with decompression mechanism

Description:
FIELD OF THE INVENTION 
     The present invention relates to an internal combustion engine equipped with a decompression mechanism for securing appropriate activation of the internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Among the conventionally-known internal combustion engines equipped with a decompression mechanism is one disclosed, for example, in Japanese Patent Application Laid-Open Publication No. 2002-235516 (hereinafter referred to as “the relevant patent literature”), in which a pair of supporting protrusions each having an insertion hole, is formed on a gear of a camshaft and in which a decompression member and a biasing spring are supported on a support shaft inserted through the respective insertion holes of the supporting protrusions. The decompression member is pivotably supported on a portion of the support shaft located between the pair of supporting protrusions. Further, the camshaft has a guide recess formed in a side of its peripheral surface opposed to the decompression member. The decompression member can move appropriately by an actuating section of the decompression member being moved along the guide recess. 
     Further, in the internal combustion engine disclosed in the relevant literature, a biasing spring, which is provided on a portion of the support shaft between the pair of supporting protrusions, normally biases the actuating section of the decompression member toward an actuating position. The actuating section is kept in a state where it adjoins a cam of the cam shaft and slightly projects beyond the cam, so that a push rod is slightly raised by the actuating section to keep an exhaust valve in a slightly-opened position so as to allow a starting or activating operation of the internal combustion engine to be performed in an appropriate manner. Once the internal combustion engine reaches a predetermined number of rotations, the decompression member moves, by centrifugal force, to a retracted position remote from the cam so that the push rod is no longer raised by the actuating section. In this way, the exhaust valve and an intake valve of the internal combustion engine can be opened and closed appropriately, with the result that the internal combustion engine can be driven in an appropriate manner. 
     However, because the biasing spring of the decompression mechanism extends to be located outside the pair of supporting protrusions, it is difficult to reduce the size of, or downsize, the decompression mechanism disclosed in the relevant patent literature; in this respect, the internal combustion engine disclosed in the relevant patent literature has a room for improvement. Further, in the decompression mechanism disclosed in the relevant patent literature, the guide recess is formed in one side of the outer peripheral surface of the camshaft for permitting appropriate movement of the decompression member. The presence of such a guide recess in the one side of the camshaft would make it difficult to keep smooth rotation of the camshaft; in this respect, the internal combustion engine disclosed in the relevant patent literature has another room for improvement. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing prior art problems, it is an object of the present invention to provide an improved internal combustion engine equipped with a decompression mechanism which has a reduced size and can keep smooth rotation of the camshaft. 
     In order to accomplish the above-mentioned object, the present invention provide an improved internal combustion engine, which comprises: a camshaft having a metal core rod insert formed centrally in the camshaft, the camshaft also having a gear section and a cam section both formed integrally with the camshaft; and a decompression mechanism for securing activation of the internal combustion engine, the decompression mechanism including: a pair of supporting protrusions provided on the gear section and each having an insertion hole formed therethrough; a support shaft inserted through the insertion holes of the pair of supporting protrusions; a decompression member pivotably supported on the support shaft and having a pair of projections disposed between the pair of supporting protrusions, the decompression member being movable toward and away from the cam section; and a biasing spring supported on a portion of the support shaft between the pair of projections for normally biasing the decompression member toward the cam section, the camshaft also having: a guide recess formed in a side of the camshaft opposed to the decompression member and between the gear section and the cam section so that the decompression member is slidable along the guide recess; and a balancing recess formed in another side of the camshaft opposite the guide recess, the metal core rod being exposed through the guide recess and the balancing recess. 
     According to the present invention, the pair of the projections provided on the decompression member is located between the pair of supporting protrusions, and the biasing spring is disposed in a space between the pair of the projections. Namely, the biasing spring is disposed in the space that has never been used in the conventionally-known counterparts. Thus, in the present invention, there is no need to provide the biasing spring outside the pair of supporting protrusions, and thus, downsizing (reduction in size) of the decompression mechanism can be achieved. 
     Further, between the gear section and the cam section of the camshaft, the guide recess is formed in the side of the cam shaft opposed to the decompression member, and the balancing recess is formed in the other side of the camshaft opposite the guide recess. With the guide recess and the balancing recess provided on the opposite sides of the camshaft, a position of the center of gravity of the camshaft between the gear section and the cam section is maintained at the center of the camshaft. In this way, the rotation of the camshaft can be kept smooth during driving of the internal combustion engine. 
     Further, the metal core rod is insert formed centrally in the camshaft, which can secure sufficient rigidity and strength of the camshaft despite formation of the guide recess and the balancing recess in the opposite sides of the camshaft. 
     Furthermore, the metal core rod is exposed through the guide recess. Thus, when the decompression member is to be moved toward or away from the cam section, the decompression member can be slid along the core rod. Thus, the sliding movement of the decompression member can effectively prevent abrasion of the camshaft, so that durability of the decompression mechanism can be increased. In addition, with the guide recess and the balancing recess provided on the opposite sides of the camshaft, the camshaft can be reduced in weight. 
     In a preferred implementation, the pair of supporting protrusions is molded of resin, the decompression member is formed of a metal material, and the projections of the decompression member project outwardly beyond the supporting protrusions. More specifically, the projections each have an outer peripheral protruding portion. Thus, by the projections being extended outwardly of the supporting protrusions, the outer peripheral protruding portions can be located outwardly of the supporting protrusions. In this way, the present invention can prevent respective end corner portions of the projections from contacting and abrading the supporting protrusions. 
     The following will describe embodiments of the present invention, but it should be appreciated that the present invention is not limited to the described embodiments and various modifications of the invention are possible without departing from the basic principles. The scope of the present invention is therefore to be determined solely by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain preferred embodiments of the present invention will hereinafter be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a sectional view showing an internal combustion engine equipped with a decompression mechanism according to an embodiment of the present invention; 
         FIG. 2  is a perspective view showing a concept of the internal combustion engine of  FIG. 1 ; 
         FIG. 3  is a perspective view showing the decompression mechanism of  FIG. 2 ; 
         FIG. 4  is an exploded perspective view showing the decompression mechanism of  FIG. 3 ; 
         FIG. 5  is a view taken in the direction of arrow  5  of  FIG. 4 ; 
         FIG. 6  is a sectional view taken along line  6 - 6  of  FIG. 4 ; 
         FIG. 7  is an enlarged view of a section encircled at  7  of  FIG. 3 ; 
         FIG. 8  is a sectional view taken along line  8 - 8  of  FIG. 3 ; 
         FIG. 9  is a perspective view showing a decompression member of  FIG. 4 ; 
         FIG. 10  is a view taken in the direction of arrow  10  of  FIG. 3 ; and 
         FIGS. 11A and 11B  are views showing an example manner in which a camshaft, a driven gear, an exhaust cam and an intake cam are molded. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a sectional view showing an internal combustion engine  10  equipped with a decompression mechanism according to an embodiment of the present invention, and  FIG. 2  is a perspective view showing a concept of the internal combustion engine  10 . The internal combustion engine  10  is, for example, an internal combustion engine for an electric power generator 
     As shown in  FIGS. 1 and 2 , the internal combustion engine  10  includes: a cylinder block  11  having a cylinder  12  formed therein; a cylinderhead  14  provided at the top of the cylinder block  11 ; a con rod  16  interconnecting a piston  13  and a crankshaft; a valve mechanism  18  connected to the crankshaft; and a decompression mechanism  20  connected to the valve mechanism  18 . 
     The valve mechanism  18  includes: a driven gear (gear section)  23  held in meshing engagement with a driving gear of the crankshaft; a camshaft  24  supporting the driven gear  13 ; an exhaust cam (cam section)  25  and an intake cam  26  provided on the camshaft  24 ; an exhaust lifter  28  held in contact with the cam surface  25   a  of the exhaust cam  25 ; and an intake lifter  29  held in contact with the cam surface  26   a  of the intake cam  26 . The valve mechanism  18  further includes: an exhaust rocker arm  32  to which the exhaust lifter  28  is connected via an exhaust push rod  31 ; an intake rocker arm  34  to which the intake lifter  29  is connected via an intake push rod  33 ; an exhaust valve  35  connected to the exhaust rocker arm  32 ; and an intake valve  36  connected to the intake rocker arm  34 . 
     In the valve mechanism  18 , the cam shaft  24  rotates together with the driven gear  23  as the driving gear is rotated by the crankshaft. Such rotation of the cam shaft  24  rotates the exhaust cam  25  and the intake cam  26 . In response to the rotation of the exhaust cam  25 , the exhaust lifter  28  is pivoted, by the cam surface  25   a  of the exhaust cam  25 , about a lifter shaft  37  vertically or in an up-down direction. Such vertical pivoting movement of the exhaust lifter  28  is transmitted to the exhaust rocker arm  32  via the exhaust push rod  31 . Thus, the exhaust rocker arm  32  is actuated, in response to which the exhaust valve  35  is actuated so that an exhaust opening  38  is opened and closed through cooperation between the exhaust valve  35  and an exhaust valve spring  41 . 
     Further, in response to the rotation of the intake cam  26 , the intake lifter  29  is pivoted about the lifter shaft  37  vertically by the cam surface  65   a  of the intake cam  26 . Such vertical pivoting movement of the intake lifter  29  is transmitted to the intake rocker arm  34  via the intake push rod  33 . Thus, the intake rocker arm  34  is actuated, in response to which the intake valve  36  is actuated so that an intake opening  39  is opened and closed through cooperation between the intake valve  36  and an intake valve spring  44 . 
     As shown in  FIG. 3 , the decompression mechanism  20  is provided on the driven gear  23 , and the decompression mechanism  20  is held in contact with the camshaft  24  and the exhaust cam  25 . The decompression mechanism  20  includes: a pair of supporting protrusions  41  and  42  provided on a wall portion  23   a  of the driven gear  23  and spaced apart from each other by a predetermined interval L 1  (see  FIG. 8 ); a support shaft  44  supported by the supporting protrusions  41  and  42 ; a decompression member  45  pivotably supported on the support shaft  44 ; and a biasing sprint  47  normally biasing the decompression member  45 . An opening portion  49  is formed in the wall portion  23   a  of the driven gear  23 . 
     As shown in  FIG. 4 , the camshaft  24  has a metal core rod  52  insert formed centrally therein and having a circular cross-sectional shape. The core rod  52  is covered with an outer shaft portion  51  formed of resin. 
     The driven gear  23 , the exhaust cam  25  and the intake cam  26  are formed of resin integrally with the outer shaft portion  51  when the core rod  52  is covered with the outer shaft portion  51 . Namely, the camshaft  24  are formed integrally with the driven gear  23 , the exhaust cam  25  and the intake cam  26 . 
     Further, as shown in  FIGS. 5 and 6 , the cam shaft  24  has a guide recess  54  and a balancing recess  55  formed therein between the driven gear  23  and the exhaust cam  25  and in symmetric relation to each other with respect to the axis line  53  of the cam shaft  24 . 
     More specifically, the guide recess  54  is formed in an outer peripheral surface of the cam shaft  24  opposed to the decompression member  45  ( FIG. 3 ), so that one outer peripheral side portion  52   a  of the core rod  52  is exposed to the outside through the guide recess  54 . The decompression member  45  is slidable along the guide recess  54  and the exposed one outer peripheral side portion  52   a  of the core rod  52 . The balancing recess  55  is formed in another outer peripheral surface of the camshaft  24  opposite the guide recess  54 , so that another outer peripheral side portion  52   b  of the core rod  54  is exposed to the outside through the balancing recess  55 . Hereinafter, the one outer peripheral side portion  52   a  will be referred to as “guide side portion”, and the other outer peripheral side portion of the core rod  52   b  will be referred to as “balancing side portion  52   b”.    
     With the guide recess  54  and the balancing recess  55  provided in the opposite sides of the camshaft  24 , a position of the center of gravity of the camshaft  24  between the driven gear  23  and the exhaust cam  25  is maintained at the center (i.e., at the axis line  53 ) of the camshaft  24 . In this way, the rotation of the camshaft  24  can be kept smooth during driving of the internal combustion engine  10 . Further, with the guide recess  54  and the balancing recess  55  provided in the opposite sides of the camshaft  24 , the camshaft  24  can be reduced in weight. 
     Further, the camshaft  24  is accommodated within a crankcase and immersed in lubricant oil within the crankcase. Thus, the lubricating oil can be effectively stirred and spread by the guide recess  54  and the balancing recess  55  provided on the opposite sides of the camshaft  24 . 
     Sufficient rigidity and strength of the camshaft  24  can be secured by the metal core rod  52  insert formed centrally in the camshaft  24  although the guide recess  54  and the balancing recess  55  are provided in the opposite sides of the camshaft  24 . 
     Further, as shown in  FIGS. 7 and 8 , the pair of supporting protrusions  41  and  42  is formed of resin integrally with the wall portion  23   a  of the driven gear  23  above the camshaft  24 . The supporting protrusions  41  and  42  are spaced apart from each other by the predetermined interval L 1  and project toward the exhaust cam  25 . Insertion holes  57  are formed coaxially through respective ones of the supporting protrusions  41  and  42 , and the support shaft  44  is inserted through the insertion holes  57  of the supporting protrusions  41  and  42 . Of the pair of the supporting protrusions  41  and  42 , the left supporting protrusion  41  in  FIG. 8  will be referred to as “the one supporting protrusion  41 ” while the right supporting protrusion  42  in  FIG. 8  will be referred to as “the other supporting protrusion  42 ”. 
     The support shaft  44  includes a horizontal shaft section  61  held in a horizontal posture by being inserted through the insertion holes  57  of the supporting protrusions  41  and  42 , and a vertical shaft section  62  extending vertically downward from a proximal end portion  61   a  of the horizontal shaft section  61 . With such horizontal and vertical shaft sections  61  and  62 , the support shaft  44  has a generally L shape. 
     The horizontal shaft section  61  has a distal end portion  61   b  inserted through the insertion hole  57  of the other supporting protrusion  42  to project in a direction away from the other supporting protrusion  42 . Further, the proximal end portion  61   a  of the horizontal shaft section  61  is inserted through the insertion hole  57  of the one supporting protrusion  41  to project in a direction away from the one supporting protrusion  41 . In this manner, the horizontal shaft section  61  is supported in a horizontal posture by the supporting protrusions  41  and  42 . 
     The vertical shaft section  62 , extending vertically downward from the proximal end portion  61   a  of the horizontal shaft section  61 , has an upper end portion  62   a  located near the pair of supporting protrusions  41  and  42 . Thus, by the upper end portion  62   a  of the vertical shaft section  62  by being interfered with by the pair of supporting protrusions  41  and  42 , the horizontal shaft section  61  can be prevented from slipping out in a direction of arrow A from the one supporting protrusion  41  toward the other supporting protrusion  42 . 
     Further, the vertical shaft section  62  has a lower end portion  62   b  fitting in an engaging groove  65  of an engagement section  64 . The engaging groove  65  opens toward the camshaft  24 . With the horizontal shaft section  61  inserted through the insertion holes  57  of the supporting protrusions  41  and  42 , the lower end portion  62   b  of the vertical shaft section  62  is brought into fitting engagement with the engaging groove  65  from the side of the camshaft  24 . 
     In this state, the lower end portion  62   b  of the vertical shaft section  62  can be prevented by the engagement section  64  from moving away from the side of the camshaft  24 . Thus, by the lower end portion  62   b  of the vertical shaft section  62 , the horizontal shaft section  61  can be prevented from slipping out in a direction of arrow B from the other supporting protrusion  42  toward the one supporting protrusion  41 . 
     Thus, there is no need to crush, bend or otherwise deform the distal end portion  61   b  of the horizontal shaft section  61  so as to prevent the horizontal shaft section  61  from slipping out in the direction of arrow B, as a result of which it is possible to reduce time and labor in assembling the decompression mechanism  20 . 
     By preventing the horizontal shaft section  61  from slipping out in the direction of arrow B as noted above, the proximal and distal end portions  61   a  and  61   b  of the horizontal shaft section  61  can be held supported by the pair of supporting protrusions  41  and  42 . The decompression member  45  is pivotably supported on the horizontal shaft section  61 . 
     Further, as shown in  FIGS. 9 and 10 , the decompression member  45  is formed by bending a metal plate. The decompression member  45  includes: a base section  67  formed in a generally rectangular shape; a weight provided at the upper end of the base section  67 ; a pair of projections  71  and  72  provided on opposite side edges of the base section  67 ; an arm  73  extending from one of the projections  71  and  72 ; and an actuating section  74  provided at the distal end of the arm  73 . 
     One of the pair of projections  71  extends from one side edge of the base section  67  along an inner wall  41   a  of the one supporting protrusion  41 , while the other of the pair of projections  72  extends from the other side edge of the base section  67  along an inner wall  42   a  of the other supporting protrusion  41 . Supporting holes  76  are formed through respective ones of the projections  71  and  72 . 
     With the supporting holes  76  of the projections  71  and  72  fitted over the horizontal shaft section  61 , the base section  67  and the pair of projections  71  and  72  are located in a space  78  between the supporting protrusions  41  and  42 . In this state, the weight  68  is located in the opening portion  49  ( FIG. 7 ) of the driven gear  23 . Further, the arm  73  extends from the one projection  71  toward the exhaust cam  25 , and the actuating section  74  is provided at the distal end of the arm  73 . 
     With the pair of projections  71  and  72  located in the space  78  between the supporting protrusions  41  and  42 , the one projection  71  is located adjacent to the inner wall  41  of the one supporting protrusion  41 . The peripheral edge  72   a  of the other projection  72  projects beyond the other supporting protrusion  42  by a length L 2  toward the exhaust cam  25 . 
     The reason why the projections  71  and  72  project outwardly beyond the corresponding supporting protrusions  41  and  42  is as follows. The one projection  71  has a convexly-shaped corner portion  71   b  formed on the peripheral edge  71   a.  Similarly, the other projection  72  has a convexly-shaped corner portion  72   b  formed on the peripheral edge  72   a.  Thus, it is likely that the corner portion  71   b  abuts against and undesirably abrades the inner wall  41   a  of the one supporting protrusion  41 , and similarly that the corner portion  72   b  abuts against and undesirably abrades the inner wall  42   a  of the other supporting protrusion  42 . Thus, in the instant embodiment, the peripheral edges  71   a  and  72   b  of the projections  71  and  72  are projected outwardly beyond the supporting protrusions  41  and  42 , respectively. In this way, the corner portion  71   b  can be prevented from contacting the one supporting protrusion  41  and thus prevented from abrading the inner wall  41   a  of the one supporting protrusion  41 . Similarly, the corner portion  72   b  can be prevented from contacting the other supporting protrusion  42  and thus prevented from abrading the inner wall  42   a  of the other supporting protrusion  42 . 
     Referring now back to  FIG. 7 , the decompression member  45  pivots in a direction of arrow C about the horizontal shaft section  61  so that the actuating section  74  moves between an actuating position P 1  and a retracted position P 2 . 
     The actuating position P 1  is where the actuating section  74  contacts (abuts against) a side wall  25   b  (see also  FIG. 10 ) of the exhaust cam  25  and adjoins a base surface  25   c  of the exhaust cam  25 . The actuating section  74  is formed in such a manner that, when in contact with the one outer peripheral side portion  52   a  of the core rod  52 , it slightly projects beyond the base surface  25   c  (see also  FIG. 8 ). Thus, the exhaust lifter  28  (see also  FIG. 2 ) is raised by a slight amount by the actuating section  74  being located in the actuating position P 1 . 
     Further, the retracted position P 2  is where the actuating section  74  is located away or remote from the side wall surface  25   b  of the exhaust cam  25 . Thus, the actuating section  74  is located remote from the exhaust lifter  28  (see also  FIG. 2 ) by being moved to the retracted position P 2 . 
     Further, the biasing sprint  47  is disposed between the projections  71  and  72 . The biasing sprint  47  is a coil spring and supported on the support shaft  61  by its coil section being fitted over the horizontal shaft section  61 . In this state, one end  47   a  of the spring  47  abuts against and presses the wall portion  23   a,  while the other end  47   b  of the biasing sprint  47  abuts against and presses the weight  68 . 
     Thus, the arm  73  is normally biased about the horizontal shaft section  61  toward the actuating position P 1  by means of the biasing sprint  47 . By means of the spring or biasing force of the spring  47 , the actuating section  74  is normally held in contact with the side wall  25   b  of the exhaust cam  25  and adjoining the base surface  25   c  of the exhaust cam  25 . 
     The horizontal shaft section  61  is supported at the opposite end portions, i.e. the proximal end portion  61   a  and the distal end portion  61   b,  by the pair of supporting protrusions  41  and  42 . Thus, the biasing sprint  47  is supported stably on the horizontal shaft section  61  supported at the opposite end portions by the pair of supporting protrusions  41  and  42 . In this way, the spring or biasing force of the spring  47  can be applied appropriately to the arm  73 , so that the decompression member  45  can operate in an appropriate manner. 
     Further, because the coil section of the biasing spring  47  is disposed between the pair of projections  71  and  72 , the biasing spring  47  can be prevented, by the pair of projections  71  and  72 , from slipping out from the horizontal shaft section  61 . Thus, there is no need to provide collars or the like on the opposite ends of the spring  47  so as to prevent the biasing spring  47  from slipping out from the horizontal shaft section  61 , as a result of which it is possible to reduce the number of necessary component parts of the decompression mechanism  20  and reduce time and labor in assembling the decompression mechanism  20 . 
     Further, when the camshaft  24  rotates with less than a predetermined number of rotations at the time of activation of the internal combustion engine  10 , as shown in  FIGS. 2 and 7 , the exhaust lifter  28  is raised by a slight amount by the actuating section  74 , so that the exhaust rocker arm  32  is actuated via the exhaust push rod  31 . 
     Thus, the exhaust value  35  is actuated by the exhaust rocker arm  32 , so that the exhaust opening  38  ( FIG. 1 ) is opened slightly. In this way, cylinder compressing force in the internal combustion chamber  10  can be reduced, so that the starting or activating operation of the internal combustion chamber  10  can be performed in an appropriate manner. 
     As the number of rotations of the camshaft  24  exceeds the predetermined number following the activation of the internal combustion chamber  10 , the weight  68  moves by centrifugal force in a direction of arrow D against the biasing force of the spring  47 . Thus, the actuating section  74  moves from the actuating position P 1  to the retracted position P 2 , so that it gets away from the side wall  25   b  of the exhaust cam  25 , i.e. from the exhaust lifter  28 . In this way, it is possible to accurately open and close the exhaust valve  35  in accordance with regular operation of the internal combustion chamber  10  while preventing the exhaust lifter  28  from being raised by the actuating section  74 . 
     As shown in  FIGS. 7 and 8 , the one outer peripheral side portion  52   a  of the core rod  52  is exposed through the guide recess  54 , and the actuating section  74  and the distal end of the arm  73  are placed in contact with the one outer peripheral side portion  52   a.  Thus, as the decompression member  45  moves between the actuating position P 1  and the retracted position P 2 , the actuating section  74  and the distal end of the arm  73  can slide in the direction of arrow C in contact with the one outer peripheral side portion  52   a.  Thus, it is possible to prevent abrasion of the camshaft  24  by virtue of the sliding movement of the decompression member  45  and thereby increase durability of the decompression mechanism  20 . 
     Further, in the instant embodiment, the biasing spring  47  is disposed in the space  78  between the pair of the projections  71  and  72  and between the pair of supporting protrusions  41  and  42 . Namely, the biasing spring  47  is disposed in the space  78  that has never been used in the conventionally-known counterparts. Thus, in the instant embodiment, there is no need to provide the biasing spring  47  outside the pair of supporting protrusions  41  and  42 , and thus, downsizing (reduction in size) of the decompression mechanism  20  can be achieved, as a result of which the crankcase can have an increased inner space and such an increased inner space can be used efficiently. 
     The following describe, with reference to  FIGS. 11A and 11B , an example manner in which the camshaft  24 , the driven gear  23 , the exhaust cam  25  and the intake cam  26  are formed. 
     First, as shown in  FIG. 11A , the core rod  52  is placed in a cavity  84  of a forming mold unit  81  with fixed and movable molds  82  and  83  clamped together. To ease understanding of resin forming, portions of the cavity  84  corresponding to the driven gear  23  ( FIG. 5 ) will be referred to as “cavities  84   a ”, and portions of the cavity  84  corresponding to the intake cam  26  ( FIG. 5 ) will be referred to as “cavities  84   b”.    
     Further, the core rod  52  has an outer peripheral surface  52   c  formed as a rugged or knurled surface, having small ridges and grooves, through a knurling process. The outer peripheral surface portion  52   c  is a surface portion formed between the driven gear  23  and the exhaust cam  52  while avoiding the guide recess  54  and the balancing recess  55  ( FIG. 5 ). In this state, molten resin is injected into the cavities  84   b  through the cavities  84   a  as depicted by arrow E until the cavity  84  is filled with the molten resin. 
     As noted above, the camshaft  24  has the guide recess  54  and the balancing recess  55  provided on the opposite sides thereof (see  FIG. 5 ). Thus, the cavity  84  is formed substantially symmetrically with respect to an injected direction of the molten resin (i.e., arrow E direction). Thus, the molten resin is filled to opposite side portions of the cavity  84  substantially symmetrically with respect to the injected direction. 
     Further, by filling the molten resin to the opposite side portions of the cavity  84  substantially symmetrically with respect to the injected direction as noted above, the molten resin in the opposite side portions of the cavity can be solidified uniformly so that the camshaft  24 , the driven gear  23 , the exhaust cam  25  and the intake cam  26  are formed, as shown in  FIG. 11B . In this way, it is possible to enhance formability of the camshaft  24 . 
     Further, the outer peripheral surface portion  52   c  of the core rod  52  is formed as a rugged or knurled surface, having small ridges and grooves, through the knurling process, as noted above. Thus, the molten resin is filled into the grooves in the rugged or knurled surface  52   c,  so that sticking force between the solidified resin (i.e., part of the outer shaft portion  51 )  51   a  and the knurled surface  52   c  can be greatly enhanced. In this way, the guide recess  54  and the balancing recess  55  can be provided on the opposite sides of the camshaft  24 . 
     It should be appreciated that the internal combustion engine equipped with the decompression mechanism of the present invention is not limited to the above-described embodiment and may be modified variously. For example, whereas the internal combustion engine  10  has been described above as an internal combustion engine for an electric power generator, the present invention is not so limited, and the decompression mechanism  20  constructed in the above-described manner may be applied to internal combustion engines for use in any other apparatus, such as management machines and snow removal machines. 
     Furthermore, whereas the embodiment has been described above in relation to the case where the exhaust lifter  28  is actuated by the actuating section  74  of the decompression mechanism  20 , the present invention is not so limited, and the camshaft  24  may be provided above the cylinder  12  so that the exhaust rocker arm  32  is activated directly by the actuating section  74 . 
     Furthermore, whereas the embodiment has been described above in relation to the case where the decompression mechanism  20  is applied to the exhaust cam  25  (and hence the exhaust valve  35 ), the present invention is not so limited, and the decompression mechanism  20  may be applied to both the exhaust cam  25  and the intake cam  26 . 
     Furthermore, the shapes and constructions of the above-described internal combustion engine  10 , the decompression mechanism, the driven gear, camshaft, the exhaust cam, the pair of supporting protrusions, the support shaft, the biasing spring, the core rod, the guide recess, the balancing recess, the insertion holes, the pair of projections, etc. are modifiable as necessary without being limited to those shown and described in relation to the embodiment. 
     Finally, it should be appreciated that the basic principles of the present invention are well suited for application to internal combustion engines where a gear section and a cam section are formed of resin integrally with a camshaft having a metal core rod insert formed therein, and which are equipped with a decompression mechanism for securing appropriate starting performance.