Abstract:
This invention relates to a cam mechanism with a decompression device formed by integrating a centrifugal weight and a decompression cam lobe. The decompression device is formed by preparing a cylindrical shaft having a decompression cam lobe at one end of the shaft and a centrifugal weight at the other end. The device is constructed to be disposed on a camshaft by inserting the decompression cam lobe into a grove portion of a second cam, and pivotally supporting the shaft receiving hole on the camshaft.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on Japanese Patent Application No. 2003-071175 filed on Mar. 17, 2003. 
   FIELD OF THE INVENTION 
   The present invention relates to a cam mechanism with a decompression device which reduces the compression pressure in a combustion chamber of a reciprocating internal combustion engine to facilitate starting. 
   BACKGROUND OF THE INVENTION 
   In a reciprocating internal combustion engine, an air/fuel mixture, introduced into a combustion chamber by opening an intake valve, is compressed by a piston in a cylinder and burnt. The piston reciprocates due to the energy of this burning thereby resulting in motive power. Pressure in the cylinder can make engine starting difficult. However, pressure in the cylinder during operation is desirable to maximize performance and engine efficiency. Therefore, when the revolution of the engine is lower than a given speed, such as during starting, a decompression device can be provided to open the exhaust valve to displace the compressed air/fuel mixture. In particular, auxiliary cam lobes that open the exhaust valve to reduce the pressure of the air/fuel mixture in the combustion chamber before the air/fuel mixture is compressed and burnt can be provided. (For an example, refer to Japanese Patent Publication No. S64-46409.) Decompression devices can include a centrifugal weight which rocks due to the centrifugal force caused by the rotation of a camshaft, and a decompression pin which protrudes from and can be inserted into the cam with the centrifugal weight. 
   The problem with the prior art systems is that the centrifugal weights and the decompression pins are configured as separated pieces of the decompression device. Accordingly, the overall number of parts of the camshaft system is increased and it is therefore more difficult to combine the parts. Moreover, in the prior art systems, it is necessary to enlarge the cam lobe in order to attach the decompression device thereto. 
   In view of such a problem, an object of the present invention is to provide a cam mechanism with a decompression device wherein the centrifugal weight and the decompression cam lobe are integrally configured, improving the combining characteristic of the decompression device and achieving a small sized decompression device. 
   BRIEF SUMMARY OF THE INVENTION 
   In order to solve the above-described problems, a cam mechanism with a decompression device according to the present invention includes: a camshaft driven to rotate in conjunction with a crankshaft including at least one and a guide part formed in vicinity of the cam; a flange member disposed on the camshaft, facing the cam with the guide part interposed therebetween; and a decompression cam including a cylindrical shaft part, a decompression cam lobe formed on a circumferential surface side at one end of the shaft part, and a centrifugal weight part extending in a direction orthogonal to an axis of this shaft part at the other end of the shaft part. 
   A groove portion in the cam is provided opposite the cam lobe with the camshaft interposed therebetween. A shaft receiving hole, penetrating through the guide part in parallel with the camshaft, is formed at a position facing the groove portion, in the guide part. 
   The decompression cam is disposed in such a manner that the shaft part of the decompression cam is inserted into the shaft receiving hole and pivotally supported. The decompression cam lobe is inserted into the groove portion and the centrifugal weight part is positioned between the guide part and the flange member. 
   Therefore, the cam mechanism with a decompression device is configured so that when the camshaft rotates at a given rotational speed or less, the centrifugal weight part is positioned in vicinity of the camshaft, whereby the decompression cam lobe is protruded outward from the groove portion. When the camshaft rotates faster than the given rotational speed, the centrifugal weight part is separated from the camshaft by centrifugal force causing the shaft part to rotate. Accordingly, the decompression cam lobe is positioned inside the groove portion. 
   According to this configuration, since the centrifugal weight and the decompression cam lobe are integrally configured as a decompression cam, the number of parts is reduced. The reduced number of parts results in a small sized decompression device having improved combining characteristic. 
   Note that the cam mechanism with the decompression device according to the present invention preferably further includes: a spring attached portion formed in such a manner that the decompression cam is extended along the axis of the shaft part; and a return spring wound around the spring attached portion and having elasticity, wherein a latching portion for latching the return spring is formed in the centrifugal weight part, in the vicinity of the shaft part, and one end of the return spring is latched with the latching portion, the other end is latched onto the camshaft, and the centrifugal weight part is energized toward the camshaft by energizing force of the return spring. 
   According to this configuration, the combining characteristic of the return spring is improved. At the same time, since the return spring is supported on the axis of the shaft part, which is the center of the rocking movement of the decompression cam, it becomes possible to favorably maintain an operational characteristic by energizing the centrifugal weight toward a camshaft side against the centrifugal force imposed on the centrifugal weight, thus improving the starting characteristic of an internal combustion engine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional view (taken along the I—I line of  FIG. 3 ) showing a cam mechanism with a decompression device according to the present invention. 
       FIG. 2  is a sectional view showing an internal combustion engine in which the cam mechanism with the decompression device according to the present invention is installed. 
       FIG. 3  is a front view of the cam mechanism with the decompression device according to the present invention, viewed from an axis direction of a camshaft. 
       FIG. 4  is a sectional view including an axis of the camshaft. 
       FIG. 5  is a side view of essential part of the camshaft. 
       FIG. 6  is a front view of the camshaft, viewed in a direction from a guide part. 
       FIG. 7  is a sectional view taken along the VII—VII line of FIG.  4 . 
       FIG. 8  is a front view of a decompression cam. 
       FIG. 9  is a sectional view taken along the IX—IX line of FIG.  8 . 
       FIG. 10  is a rear view of the decompression cam. 
       FIG. 11  is a front view of a flange member. 
       FIG. 12  is a sectional view taken along the XII—XII line of FIG.  11 . 
       FIG. 13  is a sectional view including an axis of a sprocket. 
       FIG. 14  is a sectional view including an axis of another embodiment of the sprocket. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. First, referring to  FIG. 2 , a description will be given of an internal combustion engine in which a cam mechanism with a decompression device according to the present invention is installed.  FIG. 2  shows a cylinder head  1  of an internal combustion engine E. A combustion chamber  2  formed in the cylinder head  1  communicates with intake ports (not shown) and exhaust ports (not shown) via intake inlets  3  and exhaust outlets (not shown), respectively. Mushroom-shaped intake valves and exhaust valves (neither shown) are attached to these intake inlets  3  and exhaust outlets and are energized by springs to normally close the intake inlets  3  and the exhaust outlets. 
   On an upper portion of the cylinder head  1 , a cam mechanism  10  with a decompression device according to the present invention is disposed. The cam mechanism  10  is rotatably installed with its ends supported by the cylinder head  1  with bearings  5  and  6 , and includes a camshaft  20  having thereon an intake cam  22 , an exhaust cam  23 , a decompression cam  30 , and a sprocket  40 , which are attached to this camshaft  20 . The rotation of a crankshaft (not shown) of the internal combustion engine E is transmitted to the camshaft  20  by the sprocket  40  and a timing chain  7  looped around this sprocket  40 , thus causing the intake cam  22  and the exhaust cam  23 , formed on the camshaft  20 , to rotate. Cam lobes are formed in the intake cam  22  and the exhaust cam  23 , and these cam lobes push down the intake valves and the exhaust valves directly, or by use of swing arms or rocker arms. Accordingly, the intake inlets  3  and the exhaust outlets are opened at respective timings determined by angles with which the respective cam lobes are formed relative to the axis of the camshaft  20 . After an air/fuel mixture introduced in the combustion chamber  2  from the intake inlets  3  is compressed by an unillustrated piston, the air/fuel mixture is ignited by an ignition plug  8  and burnt to become energy which displaces the piston thereby rotating the crankshaft. Thereafter, the exhaust gas is forced out of the exhaust outlets through the exhaust ports. 
   The cam mechanism  10  thus configured will be described in further detail with reference to the drawings.  FIGS. 1 and 3  show the cam mechanism  10  with the decompression device according to the present invention, where sprocket members  42  shown in  FIG. 2  are omitted. First, referring to  FIGS. 4  to  7 , the camshaft  20  will be described. On the camshaft  20 , the intake cam  22 , the exhaust cam  23  and a guide part  24  are formed side by side in this order so as to protrude on the circumferential surface of a cylindrical shaft part  21 . Note that cam lobes  22   a  and  23   a  for respectively pushing down the intake valves and the exhaust valves are formed in the intake cam  22  and the exhaust cam  23 , respectively. 
   A groove portion  23   b  is formed at a portion in the exhaust cam  23  opposite the cam lobe  23   a  with the shaft part  21  interposed therebetween. This groove portion  23   b  is formed with a side face penetrated on the guide part  24  side. On the other hand, a shaft receiving hole  24   a , penetrating through in parallel with the shaft part  21 , is formed at a portion of the guide post  24  that faces the groove portion  23   b.    
   Next, referring to  FIGS. 8  to  10 , a description will be given of the decompression cam  30  to be fit in the groove portion  23   b  and shaft receiving hole  24   a  of the camshaft  20 . On the decompression cam  30 , a centrifugal weight part  32 , extending in a direction orthogonal to the axis of a shaft part  31 , is formed at one end of the cylindrical shaft part  31 . A decompression cam lobe  34  is formed at the other end of the shaft part  31 , and further, a cylindrical spring attached portion  35 , extending from the centrifugal weight part  32  along the axis of the shaft part  31 , is formed. The decompression cam lobe  34  has such a shape that part  34   a  (two spots in this embodiment) of the circumferential surface at the other end of the cylindrical shaft part  31  is cut away, and the remaining portion is used as the decompression cam lobe  34  (FIG.  10 ). 
   The shaft part  31  is inserted into the shaft receiving hole  24   a  formed in the guide part  24  of the camshaft  20 , whereby this decompression cam  30  is rotatably supported, and installed so that the portion where the decompression cam lobe  34  is formed is positioned inside the groove portion  23   b  formed in the exhaust cam  23 . In addition, the centrifugal weight part  32  is positioned at the opposite side to the exhaust cam  23  with the guide part  24  interposed therebetween. Therefore, the centrifugal weight part  32  freely rocks relative to the guide part  24 , centering around the shaft part  31  supported by the shaft receiving hole  24   a.    
   As shown in  FIGS. 1  to  3 , a return spring  50  is wound around the circumferential surface of the spring attached portion  35  of the decompression cam  30 . This return spring  50  is attached so as to energize the centrifugal weight part  32  of the decompression cam  30  toward the camshaft  20 . One end of the return spring  50  is latched with a latching portion  33  formed in the vicinity of the shaft part  31 , in the centrifugal weight part  32  of the decompression cam  30 , and the other end thereof is latched with the shaft part  21  of the camshaft  20 . When the return spring  50  is thus configured, combining of the return spring  50  becomes easy since the spring attached portion  35  is positioned so as to protrude from the decompression cam  30  attached to the camshaft  20 . 
   In a state where the decompression cam  30  is attached to the camshaft  20 , the decompression cam lobe  34  is formed so as to be positioned protruding outward from the groove portion  23   b  formed in the exhaust cam  23  when the centrifugal weight part  32  is energized by the return spring  50  toward the camshaft  20 . When the centrifugal weight part  32  rocks centering around the shaft part  31  to separate from the camshaft  20 , the decompression cam lobe  34  is rotated around the shaft part  31 , and the decompression cam lobe  34  is positioned inside the groove portion  23   b  of the exhaust cam  23  to be housed therein. Accordingly, the portion protruding outward from the exhaust cam  23  disappears. 
   Moreover, on the camshaft  20  outside the decompression cam  30 , the sprocket  40  includes a flange member  41  and the sprocket members  42 . As shown in  FIGS. 11 and 12 , in the flange member  41 , formed are two flange portions  41   c  extending, in a flange shape, outward from the circumferential surface of a cylindrical attachment portion  41   a  at one end thereof in a direction of its cylinder axis, opposite to each other with the axis interposed therebetween. A camshaft fit hole  41   b  penetrating along the axis is formed in the attachment portion  41   a , and sprocket attaching holes  41   d  for attaching the sprocket members  42  as shown in  FIG. 13  are formed in the respective two flange portions  41   c . Note that the sprocket members  42  are attached in such a manner that fastening members  43 , such as bolts, are fastened into the sprocket attaching holes  41   d.    
   The flange member  41  is fixed in such a manner that the shaft part  21  of the camshaft  20  is forcibly inserted into the camshaft fit hole  41   b  from a face on the side where the flange portions  41   c  extend. In a state where the flange member  41  is attached to the camshaft  20 , as shown  FIG. 1 , the movement of the decompression cam  30  along the shaft receiving hole  24   a  of the guide part  24  is controlled with the flange member  41 . Note that adequate clearance is provided between the centrifugal weight part  32  and the guide part  24 , and between the centrifugal weight part  32  and the flange member  41  in order not to prevent the freely rocking movement of the centrifugal weight part  32 . 
   The decompression cam  30  is combined to the camshaft  20  as described above. As for a combining method thereof, first, the decompression cam lobe  34  of the decompression cam  30  is inserted into the shaft receiving hole  24   a  formed in the guide part  24  of the camshaft  20 , from the opposite side to the exhaust cam  23 , and further inserted up to a position where the decompression cam lobe  34  is positioned in the groove portion  23   b  of the exhaust cam  23  and the shaft part  31  is pivotally supported by the shaft receiving hole  24   a . Thereafter, the return spring  50  is attached to the spring attached portion  35  as described above to energize the decompression cam  30  toward the camshaft  20 , and lastly, the sprocket  40  (flange member  41 ) is pressed onto the camshaft  20  to be attached. 
   Since the sprocket  40  including the flange member  41  and the sprocket members  42  can be fabricated separately from the main body of the camshaft  20  as described above, the forming and processing thereof is facilitated. Moreover, the decompression cam  30  in which the decompression cam lobe  34  and the centrifugal weight part  32  are integrally formed, makes it possible to reduce the number of parts of the decompression device, whereby the combining characteristic thereof to the camshaft  20  is improved. Further, since the decompression cam  30  can be attached to the camshaft  20  such that the decompression cam  30  is pivotally supported by the guide part  24 , it is possible to achieve the small sized decompression device. Furthermore, after the decompression cam  30  is attached to the camshaft  20 , the assembly of the cam mechanism  10  is completed if the flange member  41  with the sprocket members  42  attached thereto is pressed onto the camshaft  20  to be fixed. Accordingly, the fabrication and assembly of the whole cam mechanism  10  with the decompression device, as well as combining thereof to the engine, are facilitated. 
   Incidentally, in the above description, the flange member  41  and the sprocket members  42  are separately configured as the sprocket  40  which transmits the rotation of the crankshaft to the camshaft  20 . As shown in  FIG. 14 , however, a sprocket  41 ′ in which a flange member and sprocket members are integrally configured would make it possible to further reduce the number of parts and also to obtain similar effects. 
   Lastly, a description will be given of the operation of the cam mechanism  10  with the decompression device thus configured. Before the internal combustion engine E is started, the camshaft  20  is not rotated, and the centrifugal weight part  32  of the decompression cam  30  is energized toward the camshaft  20  by the return spring  50 . Therefore, the decompression cam lobe  34  is protruded outward from the groove portion  23   b  of the exhaust cam  23 . In this state, when the internal combustion engine E is started, the rotation of the crankshaft is transmitted to the sprocket  40  through the timing belt  7 , and the camshaft  20  is rotated. With this rotation of the camshaft  20 , the intake cam  22  and the exhaust cam  23  are rotated to open the intake inlets and the exhaust outlets, respectively, and in synchronization with the respective timings, the air/fuel mixture and the exhaust gas are taken in and displaced from the combustion chamber, respectively. At this time, since the decompression cam lobe  34  is protruded, as described above, at the opposite portion in the exhaust cam  23  to the cam lobe  23   a , the exhaust outlets are slightly opened with the decompression cam lobe  34  at the last moment of a compression stroke apart from a normal exhaust stroke, thus reducing the pressure in the combustion chamber  2 . 
   On the other hand, when the internal combustion engine E is started and the rotation of the camshaft  20  exceeds a given rotational frequency, the centrifugal weight part  32  is swung outward by the centrifugal force against the energizing force of the return spring  50 . When the centrifugal weight part  32  is swung outward and rocks, the decompression cam lobe  34  is rotated around the shaft part  31  relative to the guide part  24  to be housed in the groove portion  23   b , and the exhaust cam  23  comes to have the cam lobe  23   a  only. Accordingly, with the cam lobe  23   a  of the exhaust cam  23 , the exhaust outlets are opened only during a normal exhaust stroke. 
   As described above, in the case of using the cam mechanism  10  with the decompression device according to the present invention for the internal combustion engine E, when the rotation of the camshaft  20  (crankshaft) is at a given speed or less during starting time or the like, the exhaust outlets are slightly opened with the decompression cam lobe  34  at the last moment of a compression stroke to reduce the pressure of the air/fuel mixture in the combustion chamber  2 , thereby facilitating combustion. When the internal combustion engine E is started and the rotational speed of the camshaft  20  (crankshaft) becomes faster than a given speed, the decompression cam lobe  34  is housed in the groove portion  23   b , and the exhaust outlets are not opened with the decompression cam lobe  34 . Consequently, the air/fuel mixture is satisfactorily compressed and then burnt, whereby it becomes possible to derive the maximum power of the internal combustion engine E. 
   Incidentally, since the return spring  50  is attached around the center of the rocking movement of the decompression cam  30  (the spring attached portion  35  located on the axis of the shaft part  31 ) as described above, it is possible to favorably maintain an operational characteristic to energize the centrifugal weight part  32  toward the camshaft  20  against the centrifugal force imposed on the centrifugal weight part  32 , and therefore the starting characteristic of the internal combustion engine E is improved. 
   As described above, according to the cam mechanism with the decompression device of the present invention, the decompression cam is configured as a decompression cam in which the decompression cam lobe of the decompression device to be attached to the camshaft, and the centrifugal weight for causing the decompression cam lobe to protrude from and be housed in the cam are integrated, whereby the number of parts is reduced, and it is possible to improve the combining characteristic and also to achieve the small sized decompression device. 
   Moreover, the return spring for energizing the centrifugal weight of the decompression cam toward the camshaft is attached by being wound around the center of the rocking movement of the decompression cam when the decompression cam is attached to the camshaft, whereby it is possible to improve the attaching characteristic of the return spring and also to favorably maintain the operational characteristic to energize, with this return spring, the centrifugal weight toward the camshaft side against the centrifugal force imposed on the centrifugal weight, and therefore, the starting characteristic of the internal combustion engine is improved.