[0001] The present invention relates to an engine equipped with a decompression mechanism,
and to a vehicle.
[0002] When starting an engine, the engine needs to be rotated with an external force until
the start is completed. For example, the engine may be rotated using a starter motor
or using a kick starter. Conversely, resistance to the rotation increases because
air inside the cylinder is compressed during the compression stroke of the engine.
In order to reduce this resistance, a decompression mechanism is known that reduces
the pressure inside the cylinders during the compression stroke while the engine is
being rotated using the external force.
[0003] For example, the decompression mechanism disclosed in Japanese Laid-Open Patent Publication
No.
2008-128171 includes a decompression cam that alternates between an active state and a release
state due to the rotation of a weight. This decompression mechanism is supported by
a sprocket on a cam chain. As a result, there is a problem that the cam shaft that
includes the decompression mechanism needs to be longer in the axial direction.
[0004] An engine disclosed in Japanese Laid-Open Patent Publication No.
2008-64083 includes a decompression mechanism that is disposed at a location between both end
portions of a cam shaft. The decompression mechanism includes a weight and a decompression
cam, and the weight is supported by a support shaft in a rotatable manner around the
cam shaft. The decompression cam and the weight are connected by a pin and the pin
allows the decompression cam to rotate due to the weight rotating around the support
shaft.
[0005] The weight in the decompression mechanism in Japanese Laid-Open Patent Publication
No.
2008-64083 is held in a closed state due to the action of a return spring when the cam shaft
is not rotating. The decompression cam enters a state that allows an action on an
exhaust valve while the weight is in the closed state. Therefore, the pressure inside
the cylinder is reduced due to the decompression cam acting on the exhaust valve and
opening the exhaust valve when starting the engine.
[0006] When the cam shaft rotates, the weight rotates around the support shaft due to centrifugal
force. When the rotation speed of the cam shaft meets or exceeds a set rotation speed,
the centrifugal force exceeds the spring force of the return spring and the weight
enters an open state. The decompression cam does not act upon the exhaust valve while
the weight is in the open state.
[0007] The decompression mechanism described in Japanese Laid-Open Patent Publication No.
2008-64083 is able to effectively use the space between both ends of the cam shaft. As a result,
the cam shaft that includes the decompression mechanism can be made more compact in
the axial direction in comparison to a case in which the decompression mechanism is
disposed outside of the cam shaft. However, according to a study by the inventors
of the present application, it can be seen that the decompression mechanism does not
work during starting and the improvement in startability is insufficient in the engine
according to the prior art as described in Japanese Laid-Open Patent Publication No.
2008-64083.
[0008] An object of the present invention is to provide an engine that is able to improve
the startability of the decompression mechanism, and a vehicle.
[0009] This object is solved by an engine according to claim 1 and a vehicle according to
claim 11. Advantageous further developments of the invention are specified in the
dependent claims and described in the specification.
[0010] The inventors of the present application studied the cause of the decompression not
working during starting in the engine according to the prior art. As a result, it
can be seen that the weight may already be in an open state when starting the engine.
Specifically, it can be seen that the weight may be kept in the open state without
closing when the engine is stopped.
[0011] According to studies by the inventors of the present application, it is thought that
the aforementioned state arises due to weakness of the spring force of the return
spring. The cause of the weakness of the spring force is attributed to the weight
being reduced due to the disposition of the decompression mechanism between both ends
of the cam shaft. That is, because the decompression mechanism is disposed between
both ends of the cam shaft as in the decompression mechanism described in Japanese
Laid-Open Patent Publication No.
2008-64083, the limitation of the disposition is greater than that of the decompression mechanism
described in Japanese Laid-Open Patent Publication No.
2008-128171. As a result, there is a need to make the weight in the decompression mechanism as
described in Japanese Laid-Open Patent Publication No.
2008-64083 smaller than the weight as described in Japanese Laid-Open Patent Publication No.
2008-128171.
[0012] If the weight is reduced in size, the mass of the weight is reduced. The magnitude
of the centrifugal force acting on the weight becomes smaller in correspondence to
a reduction in the mass of the weight if the rotation speed is the same. As a result,
the centrifugal force acting on the weight at the set rotation speed for initiating
the open state of the weight is reduced. Therefore, there is a need to reduce the
spring force.
[0013] However, the location of the weight changes in accordance with the phase of the cam
shaft rotation. The cam shaft may stop when the opening direction of the weight and
the gravitational force direction are in correspondence. In this case, the inventors
of the present application came to the conclusion that the moment due to gravitational
force acting on the weight becomes greater than the spring force and consequently
the weight enters the open state.
[0014] While it may be thought that increasing the spring force would prevent the above
phenomenon from occurring, the set rotation speed for opening the weight would rise
if the spring force is increased. A rise in the set rotation speed would lead to the
occurrence of noise which is not desirable.
[0015] An engine according to the present invention is equipped with a cylinder head, an
exhaust valve, a valve mechanism, a cam shaft, a bearing, and a decompression mechanism.
The exhaust valve is housed inside the cylinder head. The valve mechanism opens and
closes the exhaust valve. The cam shaft drives the valve mechanism by coming into
contact with the valve mechanism. The bearing supports the cam shaft in a rotatable
manner on the cylinder head. The decompression mechanism is disposed between both
ends of the cam shaft in the axial direction.
[0016] The decompression mechanism includes a weight, a return spring, and a decompression
cam. The weight is supported on the cam shaft in a rotatable manner between a closed
state and an open state. The return spring urges the weight to return from the open
state to the closed state. The decompression cam is provided so as to come into contact
with the valve mechanism when the weight is in the closed state, and so as not to
come into contact with the valve mechanism when the weight is in the open state.
[0017] A straight line that passes through the center of rotation of the cam shaft and the
center of rotation of the weight is assumed to be a vertical axis as seen from the
axial direction of the cam shaft. A straight line that is orthogonal to the vertical
axis and that passes through the center of rotation of the cam shaft is assumed to
be a horizontal axis. A direction from the center of rotation of the cam shaft toward
the center of rotation of the weight among directions parallel to the vertical axis
is assumed to be a first vertical direction. One direction among the directions parallel
to the horizontal axis is assumed to be a first horizontal direction.
[0018] The center of gravity of the weight is disposed in a first region as seen from the
axial direction of the cam shaft. The first region is located in the first vertical
direction from the horizontal axis and in the first horizontal direction from the
vertical axis. The weight includes a first portion that is disposed in the first region
as seen from the axial direction of the cam shaft. In a closed state, the first portion
includes a first protruding portion that protrudes to the outside of an external peripheral
surface of the bearing as seen from the axial direction of the cam shaft.
[0019] In the engine according to the present invention, the first protruding portion of
the first portion located in the first region of the weight protrudes to the outside
of the external peripheral surface of the bearing. As a result, the center of gravity
of the weight can be kept further away from the center of rotation of the cam shaft
in comparison to when the first portion does not protrude to the outside of the external
peripheral surface. Therefore, the centrifugal force acting on the weight increases
if the rotation speed of the cam shaft is the same. As a result, the spring force
can be increased whereby the opening of the weight due to gravitational force can
be suppressed without raising the set rotation speed.
[0020] The weight preferably further includes a second portion. The second portion is disposed
in a second region as seen from the axial direction of the cam shaft. The second region
is located in a second vertical direction from the horizontal axis and in the first
horizontal direction from the vertical axis. The second vertical direction is the
direction opposite the first vertical direction. The external peripheral surface of
the second portion is located on the inside of the external peripheral surface of
the bearing as seen from the axial direction of the cam shaft.
[0021] In this case, the center of gravity of the weight can be disposed further away from
the center of rotation of the cam shaft. The location of the center of gravity of
the weight can be nearer the center of rotation of the weight in comparison to when
the external peripheral surface of the second portion is located to the outside of
the external peripheral surface of the bearing. Therefore, the moment caused by the
gravitational force acting on the weight can be reduced. As a result, the opening
of the weight due to the gravitational force can be suppressed. Consequently, the
startability of the engine can be improved because the open state of the weight when
starting the engine can be avoided more easily.
[0022] The second portion may include a second protruding portion that protrudes to the
outside of the external peripheral surface of the bearing as seen from the axial direction
of the cam shaft. The volume of the first protruding portion is greater than the volume
of the second protruding portion.
[0023] In this case, the center of gravity of the weight can be disposed further away from
the center of rotation of the cam shaft. The location of the center of gravity of
the weight can be provided nearer the center of rotation of the weight in comparison
to when the volume of the second protruding portion is greater than the volume of
the first protruding portion. Therefore, the moment caused by the gravitational force
acting on the weight can be reduced. As a result, the opening of the weight due to
the gravitational force can be suppressed. Consequently, the startability of the engine
can be improved because the open state of the weight when starting the engine can
be avoided more easily.
[0024] The maximum thickness of the first protruding portion in the axial direction of the
cam shaft is preferably greater than the maximum thickness of the second portion in
the axial direction of the cam shaft. In this case, the center of gravity of the weight
can be provided further away from the center of rotation of the cam shaft.
[0025] The weight preferably further includes a third portion. The third portion is disposed
in a third region as seen from the axial direction of the cam shaft. The third region
is located in the second vertical direction from the horizontal axis and in a second
horizontal direction from the vertical axis. The second horizontal direction is the
direction opposite the first horizontal direction. The external peripheral surface
of the third portion is located on the inside of the external peripheral surface of
the bearing as seen from the axial direction of the cam shaft. In this case, the center
of gravity of the weight can be provided further away from the center of rotation
of the cam shaft in comparison to when the external peripheral surface of the third
portion is located to the outside of the external peripheral surface of the bearing
as seen from the axial direction of the cam shaft.
[0026] The third portion may include a third protruding portion that protrudes to the outside
of the external peripheral surface of the bearing as seen from the axial direction
of the cam shaft. The volume of the first protruding portion is greater than the volume
of the third protruding portion. In this case, the center of gravity of the weight
can be provided further away from the center of rotation of the cam shaft in comparison
to when the volume of the third protruding portion is greater than the volume of the
first protruding portion.
[0027] The maximum thickness of the first protruding portion in the axial direction of the
cam shaft is preferably greater than the maximum thickness of the third portion in
the axial direction of the cam shaft. In this case, the center of gravity of the weight
can be provided further away from the center of rotation of the cam shaft in comparison
to when the maximum thickness of the first protruding portion is less than the maximum
thickness of the third protruding portion.
[0028] The first protruding portion preferably includes a near portion and a distant portion
which are respectively nearer to and further away from the center of rotation of the
weight in the circumferential direction of the bearing than the location of the center
of gravity of the weight if there were no first protruding portion, and the near portion
is larger than the distant portion. In this case, the center of gravity of the weight
can be provided nearer the center of rotation of the weight in comparison to when
the distant portion is larger than the near portion.
[0029] The weight preferably includes a pivot pin support portion. The decompression mechanism
further includes a pivot pin to be attached to the pivot pin support portion. The
weight is supported in a rotatable manner on the cam shaft by the pivot pin. The thickness
of the first protruding portion in the axial direction of the cam shaft is greater
than the thickness of the pivot pin support portion in the axial direction of the
cam shaft.
[0030] In this case, the center of gravity of the weight can be provided further away from
the center of rotation of the cam shaft in comparison to when the thickness of the
first protruding portion is less than the thickness of the pivot pin support portion.
[0031] The engine is preferably further equipped with a crankshaft and a cam chain. The
cylinder head includes a bearing support hole for supporting the bearing. The bearing
support hole includes a recessed portion that allows passage of the first protruding
portion. At least one portion of the recessed portion is located on the opposite side
of the crankshaft from the center of the bearing support hole.
[0032] In this case, the first protruding portion is able to pass through the recessed portion
when the cam shaft, the decompression mechanism, and the bearing are attached in an
integrated manner to the cylinder head. As a result, interference of the first protruding
portion on the bearing support hole can be avoided. Conversely, when the recessed
portion is provided in the bearing support hole, there is a concern that the bearing
strength of the bearing may be reduced because the bearing is not supported by the
recessed portion. However, at least one portion of the recessed portion is located
on the opposite side of the crankshaft from the center of the bearing support hole.
As a result, a reduction in the bearing strength of the portion on the crankshaft
side of the bearing support hole that receives a large portion of the load from the
crankshaft can be suppressed.
[0033] A vehicle according to the present invention includes the above engine.
[0034] According to the present invention, an engine that allows for an improvement in the
startability of the decompression mechanism, and a vehicle can be provided.
FIG. 1 is a side view of a vehicle.
FIG. 2 is a partial cross-sectional view of an engine.
FIG. 3 is a cross-sectional view of a cylinder head on a plane perpendicular to a
cam shaft.
FIG. 4 is an enlarged cross-sectional view of a cam shaft assembly.
FIG. 5 is a perspective view of the cam shaft assembly.
FIG. 6 is an exploded view of the cam shaft assembly.
FIG. 7 illustrates a weight in a closed state.
FIG. 8 illustrates the weight in an open state.
FIGS. 9 depicts enlargements of an exhaust cam.
FIG. 10 is side view of the cam shaft assembly.
FIG. 11 illustrates the cam shaft assembly as seen from the axial direction of the
cam shaft.
FIG. 12 illustrates the weight as seen from the axial direction of the cam shaft.
FIG. 13 is a perspective view of the weight.
FIG. 14 is side view of the cam shaft assembly as seen from the direction of arrow
XIV in FIG. 10.
FIG. 15 illustrates a flange, the weight, and a return spring as seen from the axial
direction of the cam shaft.
FIG. 16 illustrates a cylinder head while a head cover is removed.
FIG. 17 illustrates a weight according to a first modified example.
FIG. 18 illustrates a weight according to a second modified example.
FIG. 19 illustrates a weight according to a third modified example.
FIG. 20 illustrates a weight according to a fourth modified example.
FIG. 21 illustrates a weight according to a fifth modified example.
FIG. 22 illustrates a weight according to a sixth modified example.
[0035] The following is an explanation of a vehicle 1 according to the embodiments with
reference to the drawings. FIG. 1 is a side view of the vehicle 1. The vehicle 1 is
a scooter-type motorcycle. The vehicle 1 includes a vehicle body 2, a front wheel
3, a rear wheel 4, a handlebar 5, and a seat 6. The vehicle body 2 includes a flat
foot board 2a. The vehicle body 2 supports the front wheel 3 and the rear wheel 4.
The handlebar 5 and the seat 6 are attached to the vehicle body 2. The flat foot board
2a is disposed in front of and under the seat 6.
[0036] The vehicle 1 includes an engine 7 according to the embodiment. FIG. 2 is a partial
cross-sectional view of the engine 7. As illustrated in FIG. 2, the engine 7 includes
a crankshaft 11, a crankcase 12, a cylinder body 13, a cylinder head 14, and a head
cover 19. The cylinder body 13 is connected to the crankcase 12. The cylinder body
13 may be integrated with the crankcase 12 or may be a separate body. The cylinder
body 13 houses a piston 15. The piston 15 is coupled to the crankshaft 11 via a connecting
rod 16. The crankshaft 11 is connected to a transmission 8.
[0037] The cylinder head 14 is connected to the cylinder body 13. The cylinder head 14 includes
a combustion chamber 17. A spark plug 18 is attached to the cylinder head 14. A distal
end portion of the spark plug 18 is disposed so as to face the combustion chamber
17. The head cover 19 is attached to the cylinder head 14.
[0038] The engine 7 includes a valve mechanism 25 and a cam shaft 26. The valve mechanism
25 and the cam shaft 26 are housed in the cylinder head 14. The cam shaft 26 drives
the valve mechanism 25 by coming into contact with the valve mechanism 25.
[0039] The cam shaft 26 is supported on the cylinder head 14. The cylinder head 14 includes
a first supporting wall 141 and a second supporting wall 142. The first supporting
wall 141 and the second supporting wall 142 are disposed so as to be aligned in the
axial direction of the cam shaft 26 (referred to below as "cam shaft direction").
The first supporting wall 141 supports the cam shaft 26. The first supporting wall
141 supports the cam shaft 26 via a first bearing 27. The second supporting wall 142
supports the cam shaft 26. The second supporting wall 142 supports the cam shaft 26
via a second bearing 28. The first bearing 27 and the second bearing 28 are supported
in the cylinder head 14 in a manner that allows the cam shaft 26 to rotate. The outer
diameter of the first bearing 27 is larger than the outer diameter of the second bearing
28. The first supporting wall 141 may support the cam shaft 26 without the first bearing
27. The second supporting wall 142 may support the cam shaft 26 without the second
bearing 28.
[0040] The cam shaft 26 includes a first cam shaft end portion 261 and a second cam shaft
end portion 262. The first bearing 27 is disposed nearer the first cam shaft end portion
261 in the cam shaft direction than the second cam shaft end portion 262. The second
bearing 28 is disposed nearer the second cam shaft end portion 262 in the cam shaft
direction than the first cam shaft end portion 261.
[0041] A cam chain 29 is wound around the cam shaft 26 and the crankshaft 11. Specifically,
a first sprocket 31 is attached to the cam shaft 26. The first sprocket 31 is attached
to the first cam shaft end portion 261. A second sprocket 32 is attached to the crankshaft
11. The cam chain 29 is wound around the first sprocket 31 and the second sprocket
32.
[0042] The rotation of the crankshaft 11 is transmitted to the cam shaft 26 via the cam
chain 29 whereby the cam shaft 26 rotates. The cam shaft 26 includes a suction cam
263 and an exhaust cam 264. The suction cam 263 and the exhaust cam 264 are disposed
in a line in the cam shaft direction. The cam shaft 26 rotates whereby the suction
cam 263 and the exhaust cam 264 rotate. The suction cam 263 and the exhaust cam 264
come into contact with the valve mechanism 25 and the valve mechanism 25 is driven
by the rotation of the suction cam 263 and the exhaust cam 264.
[0043] FIG. 3 is a cross-sectional view of the cylinder head 14 on a plane perpendicular
to the cam shaft 26. As illustrated in FIG. 3, the engine 7 includes an exhaust valve
23 and a suction valve 24. The cylinder head 14 includes a suction port 21 and an
exhaust port 22 that communicate with the combustion chamber 17. The exhaust valve
23 and the suction valve 24 are housed in the cylinder head 14. The suction valve
24 opens and closes the suction port 21. The exhaust valve 23 opens and closes the
exhaust port 22. The valve mechanism 25 opens and closes the suction valve 24 and
the exhaust valve 23.
[0044] A suction valve spring 241 is attached to the suction valve 24. The suction valve
spring 241 urges the suction valve 24 in a direction that causes the suction valve
24 to close the suction port 21. An exhaust valve spring 231 is attached to the exhaust
valve 23. The exhaust valve spring 231 urges the exhaust valve 23 in a direction that
causes the exhaust valve 23 to close the exhaust port 22.
[0045] The valve mechanism 25 includes an exhaust rocker shaft 33 and an exhaust rocker
arm 34. The exhaust rocker shaft 33 is disposed parallel to the cam shaft 26. The
exhaust rocker shaft 33 is supported on the cylinder head 14. The exhaust rocker arm
34 is supported on the exhaust rocker shaft 33 in a manner that enables swinging around
the exhaust rocker shaft 33. The exhaust rocker arm 34 is provided in a manner that
allows the exhaust valve 23 to operate. The exhaust rocker arm 34 includes an arm
body 341, an exhaust roller 342, and an exhaust valve compressing portion 343.
[0046] The arm body 341 is supported on the exhaust rocker shaft 33 in a manner that enables
swinging. One end of the arm body 341 supports the exhaust roller 342 in a rotatable
manner. The other end of the arm body 341 supports the exhaust valve compressing portion
343. The exhaust roller 342 comes into contact with the exhaust cam 264 and rotates
due to the rotation of the exhaust cam 264. A distal end of the exhaust valve compressing
portion 343 faces a stem end 232 of the exhaust valve 23.
[0047] When the exhaust roller 342 is pushed upward due to the exhaust cam 264, the exhaust
valve compressing portion 343 presses down on the stem end 232 of the exhaust valve
23 due to the swinging of the exhaust rocker arm 34. As a result, the exhaust valve
23 is pressed down and the exhaust port 22 is opened. When the exhaust roller 342
is not pushed upward by the exhaust cam 264, the exhaust valve 23 is pressed upward
by the exhaust valve spring 231 and the exhaust port 22 is closed.
[0048] The valve mechanism 25 includes a suction rocker shaft 35 and a suction rocker arm
36. The suction rocker shaft 35 is disposed parallel to the cam shaft 26. The suction
rocker shaft 35 is supported on the cylinder head 14. The suction rocker arm 36 is
supported on the suction rocker shaft 35 in a manner that enables swinging around
the suction rocker shaft 35. The suction rocker arm 36 is provided in a manner that
allows the suction valve 24 to operate. The suction rocker arm 36 includes an arm
body 361, a suction roller 362, and a suction valve compressing portion 363.
[0049] The arm body 361 is supported on the suction rocker shaft 35 in a manner that enables
swinging. One end of the arm body 361 supports the suction roller 362 in a rotatable
manner. The other end of the arm body 361 supports the suction valve compressing portion
363. The suction roller 362 comes into contact with the suction cam 263 and rotates
due to the rotation of the suction cam 263. A distal end of the suction valve compressing
portion 363 faces a stem end 242 of the suction valve 24.
[0050] When the suction roller 362 is pushed upward due to the suction cam 263, the suction
valve compressing portion 363 presses down on the stem end 242 of the suction valve
24 due to the swinging of the suction rocker arm 36. As a result, the suction valve
24 is pressed down and the suction port 21 is opened. When the suction roller 362
is not pushed upward by the suction cam 263, the suction valve 24 is pressed upward
by the suction valve spring 241 and the suction port 21 is closed.
[0051] As illustrated in FIG. 2, the engine 7 includes a decompression mechanism 40. FIG.
4 is an enlargement of an assembly (referred to as "cam shaft assembly" below) including
the cam shaft 26, the decompression mechanism 40, and the first bearing 27. The decompression
mechanism 40 is disposed between the first cam shaft end portion 261 and the second
cam shaft end portion 262 in the cam shaft direction. The decompression mechanism
40 is disposed between the first supporting wall 141 and a second supporting wall
142 of the cylinder head 14.
[0052] FIG. 5 is a perspective view of the cam shaft assembly. FIG. 6 is an exploded view
of the cam shaft assembly. As illustrated in FIGS. 5 and 6, the decompression mechanism
40 includes a flange 41, a weight 42, a decompression cam 43, a decompression pin
44, and a return spring 45.
[0053] As illustrated in FIG. 6, the flange 41 is separate from the cam shaft 26 and is
fixed to the cam shaft 26. Specifically, the flange 41 includes a hole 411. The cam
shaft 26 is inserted into the hole 411 of the flange 41 and the flange 41 is fixed
to the cam shaft 26 by press-fitting. The flange 41 is disposed between the weight
42 and the exhaust cam 264 in the cam shaft direction.
[0054] The flange 41 includes a first convex portion 412 and a second convex portion 413.
A pivot pin 46 is attached to the first convex portion 412. A hole 414 is provided
in the second convex portion 413. The decompression cam 43 is inserted into the hole
414 of the second convex portion 413.
[0055] The weight 42 is disposed between the first bearing 27 and the flange 41 in the cam
shaft direction. The weight 42 is supported on the cam shaft 26 in a rotatable manner
between a closed state and an open state.
[0056] FIGS. 7 and 8 are cross-sectional views along line A-A in FIG. 4. FIG. 7 illustrates
the weight 42 in the closed state. FIG. 8 illustrates the weight 42 in the open state.
[0057] The decompression cam 43 is supported in a rotatable manner on the flange 41. Specifically,
the weight 42 is supported in a rotatable manner on the flange 41 via the pivot pin
46. The weight 42 switches between the closed state and the open state by rotating
around the pivot pin 46.
[0058] The decompression cam 43 is connected to the weight 42 via the decompression pin
44. As a result, the decompression cam 43 rotates in response to the rotation of the
weight 42.
[0059] Specifically as illustrated in FIGS. 4 and 6, the decompression cam 43 includes a
head portion 431 and a shaft portion 432. The shaft portion 432 is inserted into the
hole 414 of the flange 41. The head portion 431 is disposed between the flange 41
and the weight 42. The outer diameter of the head portion 431 is larger than the inner
diameter of the hole 414 of the flange 41. The head portion 431 includes a groove
portion 433. The groove portion 433 has a shape that is recessed from the end surface
of the head portion 431. The groove portion 433 extends from the external peripheral
surface of the head portion 431 toward the inside of the head portion 431. An end
portion of the decompression pin 44 is disposed inside the groove portion 433. In
the present embodiment, inside signifies the inside in the radial direction. Further,
outside signifies outside in the radial direction.
[0060] The shaft portion 432 includes a cam portion 434. The exhaust cam 264 includes a
recessed portion 265, and the recessed portion 265 has a shape that is recessed from
the external peripheral surface of the exhaust cam 264 toward the inside of the exhaust
cam 264. FIGS. 9 depicts enlargements of the exhaust cam 264. FIG. 10 is side view
of the cam shaft assembly.
[0061] The cam portion 434 is disposed inside the recessed portion 265 of the exhaust cam
264. A cross-section of the cam portion 434 has a shape that is circular with a portion
cut out. As mentioned above, the decompression cam 43 rotates in response to the rotation
of the weight 42. FIG. 9A illustrates the decompression cam 43 when the weight 42
is in the open state. FIG. 9B illustrates the decompression cam 43 when the weight
42 is in the closed state. The decompression cam 43 switches between a state of coming
into contact with the exhaust roller 342 of the valve mechanism 25 and a state of
not coming into contact with the exhaust roller 342, in response to the rotation of
the weight 42.
[0062] Specifically as illustrated in FIG. 9A, the entire cam portion 434 of the decompression
cam 43 is disposed inside the recessed portion 265 when the weight 42 is in the open
state. That is, the cam portion 434 is in a state of not protruding to the outside
from the external peripheral surface of the exhaust cam 264 when the weight 42 is
in the open state. As a result, the decompression cam 43 does not come into contact
with the exhaust roller 342 when the weight 42 is in the open state.
[0063] When the weight 42 is in the closed state as illustrated in FIG. 9B, a portion of
the cam portion 434 of the decompression cam 43 is disposed outside of the recessed
portion 265. That is, a portion of the cam portion 434 is in a state of protruding
to the outside from the external peripheral surface of the exhaust cam 264 when the
weight 42 is in the closed state. As a result, the decompression cam 43 comes into
contact with the exhaust roller 342 when the weight 42 is in the closed state.
[0064] The return spring 45 urges the weight 42 to return to the closed state from the open
state. In the present embodiment, the return spring 45 is a coil spring. However,
the return spring 45 may be another type of spring. As illustrated in FIG. 6, the
return spring 45 includes a first spring end portion 451 and a second spring end portion
452. The first spring end portion 451 extends in the cam shaft direction. The second
spring end portion 452 extends in a direction that is orthogonal to the cam shaft
direction. The second spring end portion 452 extends in the circumferential direction
of the return spring 45. The first spring end portion 451 is locked to the flange
41. The second spring end portion 452 is locked to the weight 42.
[0065] The following is a detailed description of the structure of the weight 42. As illustrated
in FIG. 7, a straight line that passes through the center of rotation C1 of the cam
shaft 26 and the center of rotation C2 of the weight 42 is assumed to a vertical axis
Y as seen from the axial direction of the cam shaft. A straight line that is orthogonal
to the vertical axis Y and passes through the center of rotation C1 of the cam shaft
26 is assumed to be a horizontal axis X. The direction that extends from the center
of rotation C1 of the cam shaft 26 toward the center of rotation C2 of the weight
42 among directions parallel to the vertical axis Y is assumed to be a first vertical
direction y1. The direction opposite the first vertical direction y1 is assumed to
be a second vertical direction y2. One direction among the directions parallel to
the horizontal axis X is assumed to be a first horizontal direction x1. The direction
opposite the first horizontal direction x1 is assumed to be a second horizontal direction
x2.
[0066] A region located in the first vertical direction y1 from the horizontal axis X and
in the first horizontal direction x1 from the vertical axis Y is assumed to be a first
region A1. A region located in the second vertical direction y2 from the horizontal
axis X and in the first horizontal direction x1 from the vertical axis Y is assumed
to be a second region A2. A region located in the second vertical direction y2 from
the horizontal axis X and in the second horizontal direction x2 from the vertical
axis Y as seen from the cam shaft direction is assumed to be a third region A3. A
region located in the first vertical direction y1 from the horizontal axis X and in
the second horizontal direction x2 from the vertical axis Y is assumed to be a fourth
region A4.
[0067] FIG. 7 illustrates the weight 42 as seen from the first cam shaft end portion 261
side in the cam shaft direction. Therefore, the aforementioned directions x1, x2,
y1, and y2 and the regions A1 to A4 are defined when seen from the first cam shaft
end portion 261 side in the cam shaft direction, but the aforementioned directions
x1, x2, y1, and y2 and the regions A1 to A4 may also be defined when seen from the
second cam shaft end portion 262 side in the cam shaft direction.
[0068] As illustrated in FIG. 7, the weight 42 has a shape that extends along the circumferential
direction of the cam shaft 26. The weight 42 is disposed around the cam shaft 26 in
the first region A1, the second region A2, and the fourth region A4. The weight 42
has a shape that straddles a plurality of regions among the first to fourth regions
A1 to A4 in the circumferential direction of the cam shaft 26. The weight 42 does
not include a portion that is disposed in the third region A3 as seen from the cam
shaft direction.
[0069] Specifically, the weight 42 includes a first weight portion 47 and a second weight
portion 48. The first weight portion 47 extends from a center of rotation C2 of the
weight 42 in the circumferential direction of the cam shaft 26 and in the first horizontal
direction x1. An end portion 471 in the circumferential direction of the first weight
portion 47 is located in the first horizontal direction x1 from the vertical axis
Y. That is, the entire first weight portion 47 is located in the first horizontal
direction x1 from the vertical axis Y. The end portion 471 of the first weight portion
47 is disposed in the second region A2 as seen from the cam shaft direction.
[0070] The second weight portion 48 extends from a center of rotation C2 of the weight 42
in the circumferential direction of the cam shaft 26 and in the second horizontal
direction x2. An end portion 481 in the circumferential direction of the second weight
portion 48 is located in the second horizontal direction x2 from the vertical axis
Y That is, the entire second weight portion 48 is located in the second horizontal
direction x2 from the vertical axis Y. The end portion 481 of the second weight portion
48 is disposed in the fourth region A4 as seen from the cam shaft direction.
[0071] The first weight portion 47 is longer than the second weight portion 48 in the circumferential
direction of the cam shaft 26. That is, an angle from the center of rotation C2 of
the weight 42 to the end portion 471 of the first weight portion 47 around the center
of rotation C1 of the cam shaft 26 is greater than an angle from the center of rotation
C2 of the weight 42 to the end portion 481 of the second weight portion 48.
[0072] The first weight portion 47 includes a first portion 421 and a second portion 422.
The first portion 421 is disposed in the first region A1 as seen from the cam shaft
direction. The second portion 422 is disposed in the second region A2 as seen from
the cam shaft direction. The second weight portion 48 is disposed in the fourth region
A4.
[0073] The weight 42 includes a pivot pin support portion 423. The pivot pin support portion
423 is disposed across the first portion 421 and the second portion 422. The pivot
pin 46 is attached to the pivot pin support portion 423.
[0074] The exhaust cam 264 includes a cam lobe 267 that protrudes further to the outside
than a base circle 266. A portion of the pivot pin 46 does not overlap the cam lobe
267 as seen from the cam shaft direction. That is, a portion of the pivot pin 46 is
located to the outside of the external peripheral surface of the exhaust cam 264 as
seen from the cam shaft direction. The pivot pin 46 further includes a portion located
on the inside of the base circle 266 as seen from the cam shaft direction.
[0075] The decompression pin 44 is connected to the first weight portion 47. Specifically,
the decompression pin 44 is connected to the second portion 422. The decompression
pin 44 is disposed in the second region A2 as seen from the cam shaft direction. A
distance between the center of rotation C2 of the weight 42 and the decompression
pin 44 as seen from the cam shaft direction is equal to or greater than a distance
between the center of rotation C2 of the weight 42 and the center of rotation C1 of
the cam shaft 26.
[0076] FIG. 11 illustrates the cam shaft assembly as seen from the cam shaft direction.
As illustrated in FIG. 11, the contour of the flange 41 as seen from the cam shaft
direction includes a portion larger than the contour of the first bearing 27. Specifically,
the first convex portion 412 protrudes to the outside of the external peripheral surface
of the first bearing 27.
[0077] The first portion 421 of the weight 42 in the closed state includes a first protruding
portion 424. The first protruding portion 424 protrudes to the outside of the external
peripheral surface of the first bearing 27 as seen from the cam shaft direction. The
external peripheral surface of the second portion 422 is located on the inside of
the external peripheral surface of the first bearing 27 as seen from the cam shaft
direction. The external peripheral surface of the second weight portion 48 is located
on the inside of the external peripheral surface of the first bearing 27 as seen from
the cam shaft direction.
[0078] The pivot pin support portion 423 includes a protruding portion 425 that protrudes
to the outside of the external peripheral surface of the first bearing 27 as seen
from the cam shaft direction. The maximum value of the protrusion length of the protruding
portion 425 is greater than the maximum value of the protrusion length of the first
protruding portion 424. That is, the protruding portion 425 protrudes more than the
first protruding portion 424 in the radial direction of the first bearing 27. The
protrusion length signifies the length of protrusion from the external peripheral
surface of the first bearing 27 in the radial direction of the first bearing 27.
[0079] As illustrated in FIG. 11, the first bearing 27 includes an inner ring 271 and an
outer ring 272. The inner ring 271 is in contact with the cam shaft 26. The outer
ring 272 is in contact with the first supporting wall 141 of the cylinder head 14.
As illustrated in FIG. 7, the weight 42 includes an inner ring contact portion 426.
The inner ring contact portion 426 is disposed in line with the inner ring 271 in
the cam shaft direction. The inner ring contact portion 426 protrudes toward the inner
ring 271 from the surface of the weight 42 adjacent to the first bearing 27.
[0080] As illustrated in FIG. 11, the inner ring contact portion 426 is located further
to the inside of the inner peripheral surface of the outer ring 272. The inner ring
contact portion 426 is located further to the inside than the inner peripheral surface
of the outer ring 272 regardless of whether the weight 42 is in the closed state or
the open state. At least a portion of the inner ring contact portion 426 is disposed
nearer the center of rotation C2 of the weight 42 than the center of rotation C1 of
the cam shaft 26 as seen from the cam shaft direction. The inner ring contact portion
426 is located between the center of rotation C2 of the weight 42 and the cam shaft
26 as seen from the cam shaft direction. As illustrated in FIG. 4, the other portions
of the weight 42 do not come into contact with the outer ring 272 in a state in which
the inner ring contact portion 426 is in contact with the inner ring 271.
[0081] Specifically, the inner ring contact portion 426 is disposed across the fourth region
A4, the first region A1, and the second region A2 when the weight 42 is in the closed
state. The inner ring contact portion 426 includes a first contact portion 426a, a
second contact portion 426b, and a third contact portion 426c. The first contact portion
426a is disposed in the first region A1 when the weight 42 is in the closed state.
The second contact portion 426b is disposed in the second region A2 when the weight
42 is in the closed state. The third contact portion 426c is disposed in the fourth
region A4 when the weight 42 is in the closed state. The surface area of the first
contact portion 426a is larger than the surface area of the second contact portion
426b as seen from the cam shaft direction. The surface area of the first contact portion
426a is larger than the surface area of the third contact portion 426c as seen from
the cam shaft direction.
[0082] G1 in FIG. 12 indicates the location of the center of gravity of the weight 42. G2
indicates the location of the center of gravity of the weight 42 when there is no
first protruding portion 424. Hatching is provided for the first protruding portion
424 in FIG. 12. The phrase "when there is no first protruding portion 424" signifies
a state in which the hatched portions in FIG. 12 are removed. The chain double-dashed
line in FIG. 12 indicates the external peripheral surface of the first bearing 27.
As illustrated in FIG. 12, the center of gravity G1 of the weight 42 is disposed in
the first region A1 as seen from the cam shaft direction. The distance between the
center of gravity G1 of the weight 42 and the center of rotation C1 of the cam shaft
26 is greater than the distance between the center of gravity G1 of the weight 42
and the center of rotation C2 of the weight 42. The first protruding portion 424 includes
a near portion 424a and a distant portion 424b that are, as seen from the cam shaft
direction, respectively nearer to and further away from the center of rotation C2
of the weight 42 in the circumferential direction of the first bearing 27 than the
location of the center of gravity G2 of the weight 42 if there were no first protruding
portion 424. The amount of protrusion outward from the external peripheral surface
of the first bearing 27 is greater in the near portion 424a than in the distant portion
424b.
[0083] As seen from the cam shaft direction, a portion of the inner ring contact portion
426 nearer the center of rotation C2 of the weight 42 than the center of gravity G1
of the weight 42 is larger than a portion of the inner ring contact portion 426 further
away from the center of rotation C2 of the weight 42 than the center of gravity G1
of the weight 42. For example, the maximum width of the first contact portion 426a
in the radial direction of the cam shaft 26 is greater than the maximum width of the
second contact portion 426b in the radial direction of the cam shaft 26.
[0084] FIG. 13 is a perspective view illustrating the surface on the second cam shaft end
portion 262 side of the weight 42. FIG. 14 is side view of the cam shaft assembly
as seen from the direction of arrow XIV in FIG. 10. As illustrated in FIG. 13, the
maximum thickness of the first portion 421 in the cam shaft direction is greater than
the maximum thickness of the second portion 422 in the cam shaft direction. The maximum
thickness of the second weight portion 48 in the cam shaft direction is greater than
the maximum thickness of the second portion 422 in the cam shaft direction.
[0085] As illustrated in FIG. 13, the first portion 421 includes an inner diameter portion
421a and an outer diameter portion 421b. The inner diameter portion 421a is located
on the inside of the outer diameter portion 421b. The thickness of the outer diameter
portion 421b in the cam shaft direction is greater than the thickness of the inner
diameter portion 421a in the cam shaft direction. The outer diameter portion 421b
includes the aforementioned first protruding portion 424. Therefore, the maximum thickness
of the first protruding portion 424 in the cam shaft direction is greater than the
maximum thickness of the second portion 422 in the cam shaft direction. The maximum
thickness of the first protruding portion 424 in the cam shaft direction is greater
than the maximum thickness of the second weight portion 48 in the cam shaft direction.
The thickness of the first protruding portion 424 in the cam shaft direction is greater
than the thickness of the pivot pin support portion 423 in the cam shaft direction.
[0086] As illustrated in FIG. 10, a portion of the weight 42 overlaps the flange 41 as seen
from the radial direction of the cam shaft 26. Specifically, the outer diameter portion
421b of the first portion 421 overlaps the flange 41 as seen from the radial direction
of the cam shaft 26. The surface of the second weight portion 48 and the surface of
the inner diameter portion 421a on the second cam shaft end portion 262 side face
the surface of the flange 41 on the first cam shaft end portion 261 side. The aforementioned
head portion 431 of the decompression cam 43 is disposed between the second portion
422 and the flange 41.
[0087] As illustrated in FIG. 13, the pivot pin support portion 423 includes a housing portion
423a and a boss portion 423b. The boss portion 423b protrudes from the housing portion
423a in the cam shaft direction. The thickness of the housing portion 423a in the
cam shaft direction is less than the thicknesses of the first portion 421 and the
second weight portion 48 in the cam shaft direction. Therefore, the housing portion
423a has a shape that is recessed in the cam shaft direction from the surface of the
weight 42.
[0088] FIG. 15 is a view of the flange 41, the weight 42, and the return spring 45 as seen
from the second cam shaft end portion 262 side. As illustrated in FIG. 15, the housing
portion 423a houses the return spring 45. The boss portion 423b is inserted into the
aforementioned return spring 45. A hole 423c is provided in the boss portion 423b.
The pivot pin 46 is inserted into the hole 423c of the boss portion 423b.
[0089] As illustrated in FIGS. 13 and 15, the weight 42 includes a second locking portion
42b. The second locking portion 42b locks the second spring end portion 452 of the
return spring 45. The second locking portion 42b is included in the first portion
421. Specifically, the second locking portion 42b is a stepped portion shaped with
regard to the pivot pin support portion 423 in the first portion 421.
[0090] As illustrated in FIGS. 14 and 15, the flange 41 includes a first locking portion
42a. The first locking portion 42a locks the first spring end portion 451 of the return
spring 45. Specifically, the first locking portion 42a is a portion of the first convex
portion 412. The first locking portion 42a is formed integrally with the flange 41.
For example, the flange 41 is formed integrally to include the first locking portion
42a using a manufacturing method such as sintering, forging, or casting.
[0091] FIG. 16 illustrates the cylinder head 14 in a state in which the head cover 19 is
removed. As illustrated in FIG. 16, the cylinder head 14 includes a first bearing
support hole 143. The first bearing support hole 143 supports the first bearing 27.
The first bearing support hole 143 is provided in the first supporting wall 141. The
first bearing support hole 143 includes a first recessed portion 144, a second recessed
portion 145, and a third recessed portion 146. The first recessed portion 144, the
second recessed portion 145, and the third recessed portion 146 are located on the
side opposite the crankshaft 11 from the center of the first bearing support hole
143. The first recessed portion 144 has a shape that allows the passage of the first
protruding portion 424 and the suction cam 263. The second recessed portion 145 has
a shape that allows the passage of the exhaust cam 264. The third recessed portion
146 has a shape that allows the passage of the pivot pin support portion 423. A portion
of the first recessed portion 144 may be located on the opposite side of the crankshaft
11 from the center of the first bearing support hole 143, and the other portion of
the first recessed portion 144 may be located on the same side as the crankshaft 11
from the center of the first bearing support hole 143. Alternatively, a portion of
the second recessed portion 145 may be located on the opposite side of the crankshaft
11 from the center of the first bearing support hole 143, and the other portion of
the second recessed portion 145 may be located on the same side as the crankshaft
11 from the center of the first bearing support hole 143.
[0092] The first protruding portion 424 of the first portion 421 located in the first region
A1 within the weight 42 protrudes outward from the external peripheral surface of
the first bearing 27 in the engine 7 according to the present embodiment. As a result,
the center of gravity G1 of the weight 42 can be provided further away from the center
of rotation C1 of the cam shaft 26 in comparison to when the first portion 421 does
not protrude (see center of gravity G2 in FIG. 12). Therefore, the centrifugal force
acting on the weight 42 increases if the rotation speed of the cam shaft 26 is the
same. As a result, opening of the weight 42 due to gravitational force can be suppressed
without raising the set rotation speed because the spring force of the return spring
45 can be increased.
[0093] The external peripheral surface of the second portion 422 of the weight 42 is located
on the inside of the external peripheral surface of the first bearing 27 as seen from
the axial direction of the cam shaft 26. As a result, the center of gravity G1 of
the weight 42 can be provided further away from the center of rotation C1 of the cam
shaft 26. Furthermore, the location of the center of gravity G1 of the weight 42 can
be provided nearer the center of rotation C2 of the weight 42 in comparison to when
the external peripheral surface of the second portion 422 is located on the outside
of the external peripheral surface of the first bearing 27. As a result, the moment
caused by the gravitational force acting on the weight 42 can be reduced. The opening
of the weight 42 due to the gravitational force can be suppressed. Consequently, the
startability of the engine 7 can be improved because the open state of the weight
42 when starting the engine 7 can be avoided more easily.
[0094] The maximum thickness of the first protruding portion 424 in the axial direction
of the cam shaft 26 is greater than the maximum thickness of the second portion 422
in the axial direction of the cam shaft 26. As a result, the center of gravity G1
of the weight 42 can be provided further away from the center of rotation C1 of the
cam shaft 26.
[0095] The near portion 424a that is nearer the center of rotation C2 of the weight 42 in
the circumferential direction of the first bearing 27 than the location of the center
of gravity G2 of the weight 42 if there were no first protruding portion 424 is larger
than the distant portion 424b in the first protruding portion 424 as seen from the
axial direction of the cam shaft 26. As a result, the center of gravity G1 of the
weight 42 can be provided nearer the center of rotation C2 of the weight 42 in comparison
to a case in which the distant portion 424b is larger than the near portion 424a.
[0096] The thickness of the first protruding portion 424 in the axial direction of the cam
shaft 26 is greater than the thickness of the pivot pin support portion 423 in the
axial direction of the cam shaft 26. As a result, the center of gravity G1 of the
weight 42 can be provided further away from the center of rotation C1 of the cam shaft
26 in comparison to a case in which the thickness of the first protruding portion
424 is less than the thickness of the pivot pin support portion 423.
[0097] The first bearing support hole 143 of the cylinder head 14 includes the first recessed
portion 144, the second recessed portion 145, and the third recessed portion 146.
As a result, the first protruding portion 424 and the suction cam 263 can pass through
the first recessed portion 144, the exhaust cam 264 can pass through the second recessed
portion 145, and the pivot pin support portion 423 can pass through the third recessed
portion 146 when the cam shaft 26, the decompression mechanism 40, and the first bearing
27 are integrally attached to the cylinder head 14. As a result, interference of the
first protruding portion 424, the suction cam 263, the exhaust cam 264, and the pivot
pin support portion 423 with the first bearing support hole 143 can be avoided.
[0098] The first recessed portion 144, the second recessed portion 145, and the third recessed
portion 146 are located on the side opposite the crankshaft 11 from the center of
the first bearing support hole 143. As a result, a reduction in the bearing strength
of the portion on the crankshaft 11 side of the first bearing support hole 143 that
receives a large portion of the load from the crankshaft 11 can be suppressed.
[0099] Although an embodiment of the present invention has been described so far, the present
invention is not limited to the above embodiments and various modifications may be
made within the scope of the invention.
[0100] The shape of the weight is not limited to the shape of the above embodiment and may
be changed. FIG. 17 illustrates the weight 42 according to a first modified example.
As illustrated in FIG. 17, the second portion 422 may include a second protruding
portion 427. The second protruding portion 427 protrudes to the outside of the external
peripheral surface of the first bearing 27 as seen from the cam shaft direction. The
volume of the first protruding portion 424 is greater than the volume of the second
protruding portion 427. In this case, the center of gravity G1 of the weight 42 can
be provided further away from the center of rotation C1 of the cam shaft 26 in the
same way as the above embodiment. The location of the center of gravity G1 of the
weight 42 can be provided nearer the center of rotation C2 of the weight 42 in comparison
to when the volume of the second protruding portion 427 is greater than the volume
of the first protruding portion 424.
[0101] FIG. 18 illustrates the weight 42 according to a second modified example. As illustrated
in FIG. 18, the weight 42 may include a third portion 428 disposed in the third region
A3. The external peripheral surface of the third portion 428 is located on the inside
of the external peripheral surface of the first bearing 27 as seen from the axial
direction of the cam shaft 26. The maximum thickness of the first protruding portion
424 in the axial direction of the cam shaft 26 is greater than the maximum thickness
of the second weight portion 48 in the axial direction of the cam shaft 26. In this
case, the location of the center of gravity G1 of the weight 42 can be provided further
away from the center of rotation C1 of the cam shaft 26 in comparison to when the
external peripheral surface of the third portion 428 is located on the outside of
the external peripheral surface of the first bearing 27 as seen from the axial direction
of the cam shaft 26.
[0102] Alternatively, the length in the circumferential direction of the weight can be made
shorter than that of the weight 42 of the above embodiment. For example, a circumferential
direction end portion 471 of the first weight portion 47 may be disposed in the first
region A1.
[0103] FIG. 19 illustrates the weight 42 according to a third modified example. As illustrated
in FIG. 19, the third portion 428 may include a third protruding portion 429. The
third protruding portion 429 protrudes to the outside of the external peripheral surface
of the first bearing 27 as seen from the axial direction of the cam shaft 26. The
volume of the first protruding portion 424 is greater than the volume of the third
protruding portion 429. In this case, the center of gravity G1 of the weight 42 can
be provided further away from the center of rotation C1 of the cam shaft 26 in comparison
to a case in which the volume of the third protruding portion 429 is greater than
the volume of the first protruding portion 424.
[0104] FIG. 20 illustrates the weight 42 according to a fourth modified example. As illustrated
in FIG. 20, the pivot pin support portion 423 may be located on the inside of the
external peripheral surface of the first bearing 27 as seen from the cam shaft direction.
[0105] FIG. 21 illustrates the weight 42 according to a fifth modified example. As illustrated
in FIG. 21, the pivot pin 46 may be located on the inside of the base circle 266 of
the exhaust cam 264 as seen from the cam shaft direction. That is, the center of rotation
C2 of the weight 42 may be located on the inside of the base circle 266 of the exhaust
cam 264 as seen from the cam shaft direction.
[0106] FIG. 22 illustrates the weight 42 according to a sixth modified example. As illustrated
in FIG. 22, the center of rotation C2 of the weight 42 may be located on the outside
of the cam lobe 267 of the exhaust cam 264 as seen from the cam shaft direction. The
entire pivot pin 46 may be located on the outside of the external peripheral surface
of the exhaust cam 264.
[0107] The weight 42 in the above embodiment is supported by the cam shaft 26 via the flange
41, but the weight may also be supported directly by the cam shaft. The flange 41
is separate from the cam shaft in the above embodiment and is fixed to the cam shaft
by press-fitting, but may be fixed with a fixing means other than press-fitting. Alternatively,
the flange may be formed integrally with the cam shaft.
[0108] The inner ring contact portion of the weight may be omitted. The housing portion
of the weight may be omitted. That is, the return spring may be disposed in a location
other than on the weight.
[0109] While a scooter-type motorcycle is mentioned as an example of the vehicle in the
above embodiment, the vehicle according to the present invention is not limited to
a scooter and may be another type of motorcycle such as a sports type, an off-road
type, or a moped. The motorcycle is not limited to two wheels and includes a vehicle
with three wheels. Moreover, while the vehicle according to the present invention
is preferably a saddle-riding vehicle such as a motorcycle, an all-terrain vehicle,
or a snowmobile, the vehicle may also be a vehicle other than a saddle-riding vehicle.
1. An engine (7) comprising:
a cylinder head (14);
an exhaust valve (23) housed inside the cylinder head (14);
a valve mechanism (25) for opening and closing the exhaust valve (23);
a cam shaft (26) for driving the valve mechanism (25) by coming into contact with
the valve mechanism (25);
a bearing (27) supporting the cam shaft (26) in a rotatable manner on the cylinder
head (14); and
a decompression mechanism (40) disposed between both ends of the cam shaft (26) in
an axial direction of the cam shaft (26);
wherein the decompression mechanism (40) includes:
a weight (42) supported on the cam shaft (26) in a rotatable manner between a closed
state and an open state;
a return spring (45) for urging the weight (42) to return from the open state to the
closed state; and
a decompression cam (43) provided so as to come into contact with the valve mechanism
(25) when the weight (42) is in the closed state and so as not to come into contact
with the valve mechanism (25) when the weight (42) is in the open state;
and when, as seen from the axial direction of the cam shaft (26), a straight line
that passes through a center of rotation (C1) of the cam shaft (26) and through a
center of rotation (C2) of the weight (42) is assumed to be a vertical axis (Y), and
a straight line that is orthogonal to the vertical axis (Y) and that passes through
the center of rotation (C1) of the cam shaft (26) is assumed to be a horizontal axis
(X), and when a direction from the center of rotation (C1) of the cam shaft (26) toward
the center of rotation (C2) of the weight (42) among directions parallel to the vertical
axis is assumed to be a first vertical direction (y1), and a direction parallel to
the horizontal axis (X) is assumed to be a first horizontal direction (x1),
the center of gravity (G1) of the weight (42) is disposed in a first region (A1) that
is located in the first vertical direction (y1) from the horizontal axis (X) and in
the first horizontal direction (x1) from the vertical axis (Y) as seen from the axial
direction of the cam shaft (26);
the weight (42) includes a first portion (421) that is disposed in the first region
(A1) as seen from the axial direction of the cam shaft (26); and
in a closed state, the first portion (421) includes a first protruding portion (424)
that protrudes to the outside of an external peripheral surface of the bearing (27)
as seen from the axial direction of the cam shaft (26).
2. An engine according to claim 1, wherein:
when a direction opposite the first vertical direction (y1) is assumed to be a second
vertical direction (y2), and
the weight (42) further includes a second portion (422); and
the second portion (422) is disposed in a second region (A2) that is located in the
second vertical direction (y2) from the horizontal axis (X) and in the first horizontal
direction (x1) from the vertical axis (Y) as seen from the axial direction of the
cam shaft (26);
an external peripheral surface of the second portion (422) is located on the inside
of the external peripheral surface of the bearing (27) as seen from the axial direction
of the cam shaft (26).
3. An engine according to claim 1, wherein:
when a direction opposite the first vertical direction (y1) is assumed to be a second
vertical direction (y2),
the weight (42) further includes a second portion (422); and
the second portion (422) is disposed in a second region (A2) that is located in the
second vertical direction (y2) from the horizontal axis (X) and in the first horizontal
direction (x1) from the vertical axis (Y) as seen from the axial direction of the
cam shaft (26);
the second portion (422) includes a second protruding portion (427) that protrudes
to the outside of the external peripheral surface of the bearing (27) as seen from
the axial direction of the cam shaft (26); and the volume of the first protruding
portion (424) is greater than the volume of the second protruding portion (427).
4. An engine according to claims 2 or 3, wherein:
a maximum thickness of the first protruding portion (424) in the axial direction of
the cam shaft (26) is greater than a maximum thickness of the second portion (422)
in the axial direction of the cam shaft (26).
5. An engine according to any one of claims 1 to 4, wherein:
when a direction opposite the first vertical direction (y1) is assumed to be the second
vertical direction (y2),
a direction opposite the first horizontal direction (x1) is assumed to be the second
horizontal direction (x2),
the weight (42) further includes a third portion (428), and
the third portion (428) is disposed in a third region (A3) that is located in the
second vertical direction (y2) from the horizontal axis (X) and in the second horizontal
direction (x2) from the vertical axis (Y) as seen from the axial direction of the
cam shaft (26); and
an external peripheral surface of the third portion (428) is located on the inside
of the external peripheral surface of the bearing (27) as seen from the axial direction
of the cam shaft (26).
6. An engine according to any one of claims 1 to 4, wherein:
when a direction opposite the first vertical direction (y1) is assumed to be the second
vertical direction (y2),
a direction opposite the first horizontal direction (x1) is assumed to be the second
horizontal direction (x2),
the weight (42) further includes a third portion (428), and
the third portion (428) is disposed in a third region (A3) that is located in the
second vertical direction (y2) from the horizontal axis (X) and in the second horizontal
direction (x2) from the vertical axis (Y) as seen from the axial direction of the
cam shaft (26); and
the third portion (428) includes a third protruding portion (429) that protrudes to
the outside of the external peripheral surface of the bearing (27) as seen from the
axial direction of the cam shaft (26); and
a volume of the first protruding portion (424) is greater than a volume of the third
protruding portion (429).
7. An engine according to claim 5 or claim 6, wherein
a maximum thickness of the first protruding portion (424) in the axial direction of
the cam shaft (26) is greater than a maximum thickness of the third portion (428)
in the axial direction of the cam shaft (26).
8. An engine according to any one of claims 1 to 7, wherein,
as seen from the axial direction of the cam shaft (26), the first protruding portion
(424) includes a near portion (424a) and a distant portion (424b) that are respectively
nearer to and further away from a center of rotation (C2) of the weight (42) in the
circumferential direction of the bearing (27) than a location of a center of gravity
(G2) of the weight (42) if there were no first protruding portion (424), and the near
portion (424a) is larger than the distant portion (424b).
9. An engine according to any one of claims 1 to 8, wherein,
the weight (42) includes a pivot pin support portion (423);
the decompression mechanism (40) further includes a pivot pin (46) attached to the
pivot pin support portion (423);
the weight (42) is supported in a rotatable manner on the cam shaft (26) via the pivot
pin (46); and
a thickness of the first protruding portion (424) in the axial direction of the cam
shaft (26) is greater than a thickness of the pivot pin support portion (423) in the
axial direction of the cam shaft (26).
10. An engine according to any one of claims 1 to 9, further comprising:
a crankshaft (11), and
a cam chain (29) wound around the crankshaft (11) and the cam shaft (26),
wherein,
the cylinder head (14) includes a bearing support hole (143) supporting the bearing
(27);
the bearing support hole (143) includes a recessed portion (144) that allows passage
of the first protruding portion (424); and
at least a portion of the recessed portion (144) is located on the side opposite the
crankshaft (11) from the center of the bearing support hole (143).
11. A vehicle (1) comprising an engine (7) according to any one of the claims 1 to 10.