EP2949889B1

EP2949889B1 - Engine and vehicle

Info

Publication number: EP2949889B1
Application number: EP15163610.7A
Authority: EP
Inventors: Hideaki Hashimoto; Chihiro HARA
Original assignee: Yamaha Motor Co Ltd
Current assignee: Yamaha Motor Co Ltd
Priority date: 2014-05-27
Filing date: 2015-04-15
Publication date: 2017-03-15
Anticipated expiration: 2035-04-15
Also published as: TWI611094B; CN105134325B; TW201604383A; EP2949889A2; EP2949889A3; CN105134325A; JP2015224579A

Description

The present invention relates to an engine equipped with a decompression mechanism, and to a vehicle.
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.
For example, the decompression device disclosed in US 2012/199087 A1 (representing the closest prior art) is mounted on a left surface of an exhaust cam, and includes a support shaft which is fitted into and is mounted on an inner side of a cam nose of the exhaust cam, a decompression weight which is supported on the support shaft in a rotatable manner about the support shaft, a decompression cam which is supported in a rockable manner by way of a decompression shaft portion which is inserted into an inner side of a base circle portion of the exhaust cam on a side opposite to the cam nose, and an operation pin which is fixed to a distal end portion of the decompression weight and is engaged with a groove portion formed on the decompression cam.
Furthermore, 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.
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.
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.
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.
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 .
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.
This object is solved by an engine according to claim 1 and a vehicle according to claim 10. Advantageous further developments of the invention are specified in the dependent claims and described in the specification.
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.
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 .
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.
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.
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.
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.
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.
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.
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.
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.
The weight 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 maximum thickness of the first protruding portion in the axial direction of the cam shaft is 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.
The external peripheral surface of the second portion is preferably 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 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
A vehicle according to the present invention includes the above engine.
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.

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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.

Claims

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;

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);

and 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),
characterized in that
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).
An engine according to claim 1, wherein:
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).
An engine according to claim 1 or 2, wherein:
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).
An engine according to any one of claims 1 to 3, 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).
An engine according to any one of claims 1 to 3, 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);

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);

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).
An engine according to claim 4 or 5, 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).
An engine according to any one of claims 1 to 6, 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).
An engine according to any one of claims 1 to 7, 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).
An engine according to any one of claims 1 to 8, 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).
A vehicle (1) comprising an engine (7) according to any one of the claims 1 to 9.