Abstract:
An engine decompressor includes a decompressor pin provided through a pin hole on the surface of a valve cam, a decompressor pin operating shaft in an oil passage provided in a valve cam shaft of the valve cam for vertically moving the decompressor pin by its rotation, a fly weight rotatable by rotation of the valve cam shaft so as to rotate the decompressor pin operating shaft, and a thrust receiving plate separate from the fly weight for restricting the axial movement of the decompressor pin operating shaft in the oil passage. The engine decompressor is capable of operating a decompressor mechanism normally even if the lubricating oil increases in pressure.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a decompressor and a vehicle, and in particular, it relates to an improvement in a decompressor of the type having a decompressor pin operating shaft in an oil passage provided in a valve cam shaft. 
   2. Description of the Related Art 
   Decompressors of engines force exhaust valves to open slightly in the process of compression at engine start-up or at engine stopping, thereby reducing the cranking torque of the engine to improve engine start-up performance. In other words, the decompressor releases a portion of compressed gas from a combustion chamber by slightly opening exhaust valves in the process of compression, thereby reducing cranking torque. Also, the decompressor slightly opens exhaust valves to reduce cranking torque during engine starting. The provision of such a decompressor to an engine allows an engine starter motor to be decreased in size and a battery for driving the starter motor to be reduced in capacity. 
   The patent document JP-A-2001-173421 discloses a structure of an engine decompressor, in which a valve cam has a pin hole on the surface, through which a decompressor pin is provided. A decompressor pin operating shaft is disposed in an oil passage provided in a valve cam shaft. The decompressor pin moves vertically with the rotation of the decompressor pin operating shaft. The decompressor pin operating shaft is rotated by a fly weight that rotates against the biasing force of a return spring, according to the rotation speed of the valve cam shaft. With the decompressor, when the fly weight rotates as the rotation speed of the valve cam shaft increases, the decompressor pin, whose head projects to the surface of the valve cam during engine stopping, extends into the deep pin hole thus achieving the above described decompressing function. 
   The decompressor disclosed in JP-A-2001-173421 has a decompressor pin operating shaft in an oil passage, thus allowing a smooth decompressing operation in normal environments, so that it may be advantageous. However, under very low temperatures, the viscosity of lubricating oil becomes extremely high, and the pressure of the lubricating oil significantly increases. In that case, the decompressor pin operating shaft is pushed axially in the valve cam shaft to bring the end surface into contact with another member, thus preventing smooth rotation of the decompressor pin operating shaft. Particularly, the fly weight is journaled at its rim, and the biasing force of the return spring is a relatively small value to operate the fly weight normally at a low rotation speed. Accordingly, when the decompressor pin operating shaft is pushed strongly against the center of the fly weight, even the motion of the fly weight may be locked. 
   SUMMARY OF THE INVENTION 
   In order to overcome the problems described above, preferred embodiments of the present invention provide an engine decompressor capable of operating a decompressor mechanism normally even if lubricating oil increases in pressure, and also provide a vehicle including such a novel engine decompressor. 
   In order to solve the above problems, an engine decompressor according to a preferred embodiment of the present invention includes a decompressor pin provided through a pin hole on a surface of a valve cam, a decompressor pin operating shaft provided through an oil passage provided in the valve cam shaft of the valve cam for vertically moving the decompressor pin by its rotation, a fly weight rotatable with the rotation of the valve cam shaft so as to rotate the decompressor pin operating shaft, and a restricting member separate from the fly weight and arranged to restrict the axial movement of the decompressor pin operating shaft in the oil passage. 
   According to the present preferred embodiment, a restricting member separate from the fly weight is provided to limit the axial movement of the decompressor pin operating shaft. Accordingly, even if the pressure of the lubricating oil increases, the decompressor mechanism can be operated normally. 
   According to another preferred embodiment of the invention, the restricting member is disposed between the fly weight and the decompressor pin operating shaft, and prevents the end surface of the decompressor pin operating shaft from coming into contact with the fly weight. This structure can reliably prevent the decompressor pin operating shaft from pushing the fly weight and locking the motion of the fly weight. 
   According to another preferred embodiment of the present invention, the restricting member is a plate that is in contact with one end of the decompressor pin operating shaft. This provides a decompressor mechanism that can operate normally even if the pressure of the lubricating oil increases with a compact structure that is easy to manufacture and mount. 
   In this case, the restricting member may have a projection that is in contact with the axial center of the decompressor pin operating shaft at the end of the decompressor pin operating shaft. This structure can reduce the friction between the decompressor pin operating shaft and the restricting member. 
   According to another preferred embodiment of the present invention, the fly weight is separate from a sprocket for rotating the valve cam shaft, and the sprocket is mountable to the decompressor pin operating shaft after the fly weight is mounted to the valve cam shaft. Thus, the decompressor mechanism can be assembled independently from the assembly of the cam chain and the sprocket. This can prevent assembly problems and improve productivity. 
   A vehicle according to another preferred embodiment of the present invention includes one of the above described decompressors. This can provide vehicles with a reliable decompressor mechanism. The vehicles may preferably include saddle-type vehicles and compact four wheel vehicles. The saddle-type vehicles include motor-bicycles (including motorbikes and motor scooters), four wheel buggies (all terrain vehicles), and snowmobiles. The compact four wheel vehicles include two seater or four seater four wheel buggies (all terrain vehicles). 
   Other features, elements, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an external side view of a saddle-type vehicle according to a preferred embodiment of the invention. 
       FIG. 2  is a cross-sectional view of a cylinder head having a decompressor according to a preferred embodiment of the invention in  FIG. 1 . 
       FIG. 3  is a side view of the cylinder head, as viewed from a cam chain chamber. 
       FIG. 4  is an enlarged side view of the cylinder head to which a sprocket is mounted, as viewed from the cam chain chamber. 
       FIG. 5  is a diagram of a valve cam shaft to which a fly weight is mounted, as viewed from the shaft. 
       FIG. 6  is an enlarged cross-sectional view of the valve cam shaft, as viewed from the side. 
       FIG. 7  is an enlarged cross-sectional view principally showing the fly weight. 
       FIG. 8  is a plan view of a thrust receiving plate. 
       FIG. 9  is an enlarged perspective view of an end of a decompressor pin operating shaft. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention is described in detail below based on preferred embodiments, with reference to the attached drawings. 
     FIG. 1  is a side view of a saddle-type vehicle (all terrain vehicle) according to a preferred embodiment of the present invention. As shown in the drawing, a vehicle  1  has front suspensions  3  and  3  on the right and left of the vehicle at the front in the direction of travel (indicated by arrow Fr in the drawing), and front wheels  4  and  4  journaled at the lower ends. Front fenders  11  and  11  are disposed above the front wheels  4  and  4 . The upper ends of the front suspensions  3  and  3  are supported by a body frame  8 . The front end of the body frame  8  in the traveling direction journals a steering shaft  17  such that the front end can be turned to the right and left. At the upper end of the steering shaft  17 , a handle  18  extending to the right and left is supported at the center thereof. Grips  19  and  19  are provided on both ends of the handle  18 . The right grip  19  has an accelerator which is operated by the pressure of a driver&#39;s fingers during driving. A radiator  33  is provided ahead of the steering shaft  17 . The radiator  33  circulates cooling water in the radiator  33  to cool the cooling water with air passing from the front, thereby releasing heat generated from an engine  25 . An intake system  40  including an intake pipe and an air cleaner is connected to the rear of the engine  25 , and an exhaust pipe  38  is connected to the front of the engine  25 . The exhaust pipe  38  extends forward from the engine  25  and is then bent in a U-shape toward the rear in the traveling direction. The rear end of the exhaust pipe  38  connects to a muffler  13 . Exhaust gas discharged from the engine  25  circulates through the exhaust pipe  38 , and is then discharged from the rear of the muffler  13 . The engine  25  includes an internal combustion engine  25   a  and a crank  25   b  disposed under the internal combustion engine. Driving force output from the engine  25  is transmitted through a transmission  30  and a chain mechanism  31  to rear wheels  6  and  6 . Rear fenders  12  and  12  are disposed on the right and left above the rear wheels  6  and  6 . The engine  25  is suspended by the body frame  8  at the upper portion thereof, and supported at the lower portion thereof. 
   The body frame  8  includes right and left frames that are joined together at the front ends. The frames extend from the front to the rear in the traveling direction at the lower portion of the vehicle  1 , and are bent upward at the center of the vehicle  1 . The frames include right and left seat frames  8   a , with which a seat  10  is supported. In front of the seat  10  is disposed a fuel tank  21 . The fuel tank  21  is supported by the body frame  8 . 
   A decompressor mounted to the engine  25  of the vehicle  1  will be specifically described hereinbelow.  FIG. 2  is a cross-sectional view of the cylinder head of the engine  25 , as viewed from a direction that is perpendicular to the valve cam shaft.  FIG. 3  is a side view of the cylinder head of the engine  25  without the cover of the cam chain chamber and the sprocket, as viewed from the cam chain chamber.  FIG. 4  is an enlarged side view of the cylinder head of the engine  25  equipped with a sprocket, as viewed from the cam chain chamber. 
     FIG. 5  is a plan view of a decompressor assembly.  FIG. 6  is a cross-sectional view of a valve cam shaft assembly in which the decompressor according to the present preferred embodiment is combined, wherein the left of break line X is a cross-sectional view taken along line VIA—VIA of  FIG. 5 , as viewed along the arrow; the right of break line X is a cross-sectional view taken along line VIB—VIB of  FIG. 5 , as viewed along the arrow.  FIG. 7  is a cross-sectional view taken long line VII—VII of  FIG. 5 , as viewed along the arrow. 
   In the drawings, a valve cam shaft  86  journaled by bearings  50  and  50  is disposed above a combustion chamber  80  where a gas mixture is ignited by a spark plug  78 . In the axial middle of the valve cam shaft  86 , an exhaust valve cam  52  and an intake valve cam  54  are provided. The torque of a crankshaft (not shown) is transmitted to the valve cam shaft  86  through a sprocket  64  and a chain (not shown) routed around the sprocket  64 , whereby the valve cams  52  and  54  lift up rocker arms  51  and  53  at the appropriate times. The rocker arms  51  and  53  connect to an exhaust valve and an intake valve (not shown), so that the valves can be opened or closed with the stroke of the engine. 
   The valve cam  52  that activates the exhaust valve has a pin hole  58  in which a decompressor pin  60  is accommodated. The axial center of the valve cam shaft  86  is hollow, which serves as an oil passage  96  for lubricating oil. The valve cam  54  also has an oil passage  56 . The oil passage  56  communicates with the oil passage  96 . Thus, the lubricating oil circulating in the oil passage  96  can be supplied to the surface of the valve cam  54 . Although not shown, the valve cam  52  also has an oil passage that communicates with the oil passage  96 , allowing the valve cam  52  to be supplied with lubricating oil. 
   A decompressor pin operating shaft  62  passes through a portion of the oil passage  96  adjacent to a cam chain chamber  82 . The decompressor pin operating shaft  62  includes a cylinder that is smaller in diameter than the oil passage  96  and sandwiched between opposite ends having the same diameter as that of the oil passage  96 . One end of the decompressor pin operating shaft  62  has an engaging pin  62   a  projecting from a position remote from an axial center of the decompressor pin operating shaft  62 . The engaging pin  62   a  is in engagement with a recess  60   a  provided in the axial middle of the decompressor pin  60 . Accordingly, when the decompressor pin operating shaft  62  rotates in the oil passage  96 , the decompressor pin  60  moves vertically in the pin hole  58 . Thus, decompression in which the head of the decompressor pin  60  projects from the pin hole  58  and non-decompression in which the head is completely accommodated in the depth of the pin hole  58 , can be achieved by the rotation of the decompressor pin operating shaft  62 . 
   A flange  84  is press-fitted in the end of the valve cam shaft  86  adjacent to the cam chain chamber  82 . The flange  84  rotates together with the valve cam shaft  86 . A plate  88  having an opening in the center is fixed to the flange  84 . To the opening of the plate  88 , a thrust receiving plate  66  is fixed with a rotation shaft  70  and a stopper pin  68 .  FIG. 8  is a plan view of the thrust receiving plate  66 . As shown in  FIG. 8 , the thrust receiving plate  66  has a semicircular opening  66   b  in the center. The center of the linear rim of the semicircular opening  66   b  is enlarged toward the semicircular opening  66   b , at which a projection  66   a  is provided. The projection  66   a  projects to the back of the thrust receiving plate  66  (toward the decompressor pin operating shaft  62 ), so that the projection  66   a  is in contact with the center of the end surface of the decompressor pin operating shaft  62  (a position through which the axial center of the decompressor pin operating shaft  62  passes) with the thrust receiving plate  66  mounted to the decompressor assembly. A pair of engaging projections  62   b  arranged at the end surface of the decompressor pin operating shaft  62  extends through a portion of the opening  66   b  located on the side of the projection  66   a  extending toward the cam chain chamber  82 .  FIG. 9  is a perspective view of the engaging projections  62   b  arranged at the end of the decompressor pin operating shaft  62 . As shown in  FIG. 9 , most of the end surface of the decompressor pin operating shaft  62  is in a plane that is substantially perpendicular to the axis of the decompressor pin operating shaft  62 , from which plane the pair of engaging projections  62   b  extend. Between the pair of engaging projections  62   b , a parallel space is formed, in which an operating shaft rotating pin  76  of a fly weight  72 , to be described later, is located. Referring back to  FIG. 8 , the thrust receiving plate  66  has openings  66   c  and  66   d  arranged at the rim. The opening  66   c  allows the stopper pin  68  to pass through and the opening  66   d  allows the rotation shaft  70  to pass through. As shown in  FIG. 7 , the stopper pin  68  and the rotation shaft  70  increase in diameter in a step-shape at a specified length from the ends. The thrust receiving plate  66  is pushed and fixed to the plate  88  at the step-shape portion. 
   In other words, the ends of the rotation shaft  70  for journaling the flyweight  72  and the stopper pin  68  are press-fitted into the plate  88  and fixed by caulking, or the like. The flyweight  72  includes a planar first fly weight plate  72   a  and a second fly weight plate  72   b . The second fly weight plate  72   b  has an opening  74 , through which the stopper pin  68  passes. The stopper pin  68  also passes through the thrust receiving plate  66 . The end of the stopper pin  68  is press-fitted into the plate  88  and is fixed by caulking, or the like. Thus, the thrust receiving plate  66  is fixed to the plate  88  and the movable range of the fly weight  72  is limited. The fly weight  72  is biased to the axial center of the valve cam shaft  86  by a return spring  91  fixed to the rotation shaft  70 . Particularly, the operating shaft rotating pin  76  extends from the rim of the second fly weight plate  72   b  corresponding to the opening  66   b  of the thrust receiving plate  66  and extends to the decompressor pin operating shaft  62 . The operating shaft rotating pin  76  is located between the pair of engaging projections  62   b  provided on the end surface of the decompressor pin operating shaft  62 , as described above. Accordingly, when the fly weight  72  rotates (angles) in the direction separating from the axial center of the valve cam shaft  86  against the biasing force of the return spring  91  with the rotation of the valve cam shaft  86 , the side of the operating shaft rotating pin  76  is brought into contact with the side of the engaging projections  62   b , thereby applying torque to the decompressor pin operating shaft  62 . 
   As shown in  FIGS. 3 and 4 , the sprocket  64  is fixed to the flange  84  with bolts, and the flange  84  is fixed to the plate  88  with bolts  90  and  92 . The sprocket  64  is exposed by the opening  65  on the side of the cam chain chamber  82 . Thus the sprocket  64  can be easily mounted or dismounted by removing a cover. 
   With this structure, the projection  66   a  of the thrust receiving plate  66  comes in contact with the end surface of the decompressor pin operating shaft  62  at the axial center. Thus the friction between the thrust receiving plate  66  and the decompressor pin operating shaft  62  can be extremely small. Accordingly, even if the viscosity of the oil in the oil passage  96  increases due to very low temperatures so that the decompressor pin operating shaft  62  is pushed strongly toward the cam chain chamber  82 , the decompressor pin operating shaft  62  can be smoothly rotated in the oil passage  96 , allowing normal decompression. 
   Since the thrust receiving plate  66  has the opening  66   b  beside the projection  66   a  through which the operating shaft rotating pin  76  of the fly weight  72  and the engaging projections  62   b  of the decompressor pin operating shaft  62  are brought into engagement with each other, the decompressor pin operating shaft  62  is surely prevented from pushing the fly weight  72  in the direction of its axis. This prevents locking of the motion of the fly weight  72 . 
   The sprocket  64  in engagement with the cam chain and the fly weight  72  are separately constructed. The center of the sprocket  64  has a large opening so that after the entire decompressor mechanism including the fly weight  72  is attached to the engine  25 , the sprocket  64  can be mounted to the engine  25 . This prevents a decompressor mechanism mount failure when the sprocket  64  is integral with the fly weight  72 , thereby improving the reliability of the decompressor mechanism. 
   The present invention is not limited to the above preferred embodiments. Although an example applicable to a four wheel saddle-type vehicle has been described, the present invention may also be applied to two wheel or three wheel saddle-type vehicles. Also, the present invention may be applied to general vehicles of various sizes. 
   While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.