JPWO2016052533A1 - Decompression mechanism of internal combustion engine - Google Patents

Abstract

The decompression mechanism of the internal combustion engine has a cam lobe 32 on a camshaft 31, and a decompression member 51 having one end pivotally attached to a side surface of the cam lobe 32 by a support shaft 52 is provided. The decompression cam surface Sc of the decompression cam portion 51c projecting radially outward of the decompression member 51 has an arc Rs that is a swinging locus about the support shaft 52 of the decompression member 51 at the point P on the decompression cam surface Sc. The decompression cam surface Sc which is positioned with respect to the cam lobe 32 so as to be radially inward of the arc Rr of the virtual circle around the rotation center Cx of the camshaft 31 and projects radially outward is a point P on the decompression cam surface Sc. Is formed in a plane shape that passes through the swing center 52 or a position shifted from the swing center 52 in the rotational direction of the camshaft 31. Thereby, a compact and inexpensive decompression mechanism can be obtained with a simple structure with a small number of parts, and the internal combustion engine can be downsized.

Description

  The present invention relates to a decompression mechanism for an internal combustion engine.

  In an internal combustion engine, a decompression member is provided on a cam lobe of a camshaft that acts on and swings on an exhaust rocker arm. When the engine speed is small, the decompression member protrudes radially outward and acts on the exhaust rocker arm to act as an exhaust valve. The internal combustion engine is designed so that when the engine speed increases, the decompression member is submerged radially inward using the centrifugal force so that it does not act on the exhaust rocker arm. The decompression mechanism is known.

  When the engine speed is small, the decompression mechanism prevents the decompression member from sinking inward in the radial direction by the reaction force when the decompression member projecting radially outward due to the urging force acts on the exhaust rocker arm. When the height increases, the decompression member must be sunk radially inward by centrifugal force.

  Therefore, conventionally, when the camshaft has a decompression weight that is separate from the decompression member and the engine speed is small, the decompression weight that has received the urging force causes the decompression member to protrude radially outward, and the exhaust rocker arm The reaction force is applied to the decompression member, and as the engine speed increases, the decompression weight swings against the urging force due to centrifugal force, and the rotation of the decompression weight acts on the decompression member. Thus, a decompression mechanism in which the decompression member is retracted radially inward is common (see, for example, Patent Documents 1 and 2).

Japanese Patent No. 5171521 Japanese Unexamined Patent Publication No. 7-293217

  In the decompression mechanism disclosed in Patent Document 1, the decompression weight pivotally supported by the cam lobe of the camshaft is provided so as to be swingable along the side face of the cam lobe, and the connecting pin at the distal end of the decompression weight is provided on the side face of the cam lobe. Engaging with a guide groove of a decompression cam (decompression member) provided in a freely rotatable manner, the swing of the decompression weight causes the decompression cam to pivot, and the cam portion of the decompression cam is projected and retracted in the radial direction.

  In the decompression mechanism disclosed in Patent Document 2, a decompression weight is swingably provided on a camshaft, and a pressing piece at the tip of the decompression weight is attached to one end of a decompression pin (decompression member) that obliquely penetrates the camshaft. The other end of the compression pin is brought into contact with the exhaust cam, and the pressure piece moves the other end of the compression pin in the radial direction when the compression weight swings. Let

  Thus, the conventional decompression mechanism described in Patent Literature 1 and Patent Literature 2 includes a decompression weight that is separate from the decompression member, and the decompression member and the decompression weight are combined to form a complicated structure. In addition to the large number of parts, a large space is required and the valve operating mechanism is increased, and the internal combustion engine is increased in size and cost.

  The present invention has been made in view of the above points, and an object of the present invention is to reduce the size of the internal combustion engine by making the valve mechanism more compact by configuring a decompression mechanism having a simple structure with a small number of parts. Therefore, the decompression mechanism of the internal combustion engine that can perform the operation is provided at a low cost.

In order to achieve the above object, the present invention provides:
The intake rocker arm and exhaust rocker arm that swing by the cam lobe of the camshaft rotating in synchronization with the crankshaft actuate the intake valve and the exhaust valve, respectively, to open the intake port opening and the exhaust port opening facing the combustion chamber, respectively. It has a valve mechanism that opens and closes at a predetermined timing, a decompression cam portion that acts on the exhaust rocker arm, and the decompression cam portion appears and disappears in the radial direction with respect to the central axis of the camshaft according to the centrifugal force accompanying the rotation of the engine When the engine speed is low, the compression member configured as described above is provided on the cam lobe so that the decompression cam portion protrudes radially outward to act on the exhaust rocker arm to swing the exhaust valve to If the opening of the port is opened and the engine speed is increased, the decompression cam part is immersed inward in the radial direction. In the decompression mechanism of the institutions,
The decompression member includes a decompression weight portion that extends in a direction opposite to the rotational direction of the camshaft from a base end portion pivotally supported by the cam lobe at a swing center, and the decompression cam is provided at a distal end portion of the decompression weight portion. And the decompression member is swingable along a side surface of the cam lobe from a stop position that is taken by a biasing means that biases the decompression weight portion radially inward. Has a decompression cam surface that protrudes radially outward and acts on the exhaust rocker arm, and the decompression cam surface is a swing locus around the swing center of the decompression member at any point on the decompression cam surface. Is an arc of a virtual circle around the center of rotation of the camshaft passing through the point at the stop position of the decompression member. In other words, the decompression cam surface of the decompression cam portion, which is positioned relative to the cam lobe and protrudes radially outward, has a normal line at the point on the decompression cam surface that is the center of oscillation or oscillation of the decompression member. A decompression mechanism for an internal combustion engine is provided, wherein the decompression mechanism is formed in a surface shape passing through a position shifted in a rotation direction of the camshaft from a moving center.

  In a preferred embodiment of the present invention, the decompression member is a plate-like member having a certain thickness in the axial direction of the camshaft.

  According to a preferred embodiment of the present invention, the decompression weight portion of the decompression member is rotated around the shaft portion of the camshaft from the base end portion pivotally supported on the side surface of the cam lobe. It extends along the side surface of the cam lobe in a circular arc shape in the opposite direction.

  Preferably, the decompression member has a base end portion pivotally supported on a side surface of the cam lobe of the cam lobe, and a cam lobe of the cam lobe and a shaft portion of the camshaft formed at a distal end portion of the decompression weight portion. Located on the opposite side.

  In a preferred embodiment of the present invention, the swing range of the decompression member is restricted at the stop position where a part of the decompression weight portion is in contact with the shaft portion of the camshaft.

  According to a preferred embodiment of the present invention, the intake rocker arm and the exhaust rocker arm are pivotally supported on a common rocker arm shaft.

  In a preferred embodiment of the present invention, the intake rocker arm and the exhaust rocker arm swing with the common camshaft acting.

  Preferably, the intake rocker arm and the exhaust rocker arm swing with the cam lobe common to the camshaft acting.

  According to a preferred embodiment of the present invention, each of the intake rocker arm and the exhaust rocker arm includes a cam contact portion that the cam lobe contacts and acts, and each cam contact portion has an axial width of the cam lobe. The decompression cam contact portion that is accommodated in the exhaust rocker arm and acts by contacting the decompression cam portion of the exhaust rocker arm is located adjacent to the cam contact portion of the exhaust rocker arm.

  In a preferred embodiment of the present invention, each of the cam contact portions is an intake side roller and an exhaust side roller that are respectively supported by roller bearing portions of the intake rocker arm and the exhaust rocker arm, and the decompression cam contact portion The portion is a protrusion formed to protrude from the roller bearing portion of the exhaust rocker arm.

  In a preferred embodiment of the present invention, an intake side roller and an exhaust side roller are pivotally supported at each end of the intake rocker arm and the exhaust rocker arm, respectively, and the intake side roller contacts the cam lobe, and the exhaust side roller Are in contact with the cam lobe and the decompression cam portion projecting radially from the base circle of the cam lobe.

  According to the decompression mechanism of the internal combustion engine of the present invention, the decompression member has a decompression weight portion extending from a base end portion pivotally supported by the cam lobe in a direction opposite to the rotation direction of the camshaft, and the decompression cam portion is disposed at a distal end portion thereof. Since it is formed and swings along the side surface of the cam lobe, the decompression member swings integrally with the decompression cam portion and the decompression weight portion.

  The decompression cam surface acting on the exhaust rocker arm of the decompression cam portion projecting radially outward is an arc that is a rocking locus centering on the rocking center of the decompression member at an arbitrary point on the decompression cam surface. The decompression member is located at a position relative to the cam lobe, which is located in the radial direction inside the circular arc of the virtual circle around the rotation center of the camshaft at an arbitrary point in the vicinity of the rotation direction side of the point. When the decompression weight part receives the centrifugal force and swings against the urging force, the decompression cam surface of the decompression cam part can be sunk radially inward along the arc that is the swinging locus.

  Further, the decompression cam surface of the decompression cam portion protruding radially outward has a position where the normal line at any point of the decompression cam surface is shifted in the rotation direction of the camshaft from the center of the decompression member or the center of swinging of the decompression member. The reaction force that the decompression cam surface receives when the decompression cam surface of the decompression member is in contact with the exhaust rocker arm is formed by the curved shape that passes through the rotation center of the decompression member or the rotation direction of the camshaft from the oscillation center. Therefore, the exhaust cam rocker arm can be reliably swung, while the decompression cam portion is kept projecting radially outward without being swung.

  Therefore, in this decompression mechanism, the decompression member has a decompression cam part and a decompression weight part, and the decompression mechanism and the decompression weight are separated and combined to form a decompression mechanism. Despite this, it has a simple structure that does not require complicated machining, and despite this, the decompression cam surface of the decompression cam part can be sunk inward in the radial direction by centrifugal force due to the rotation of the camshaft, while the exhaust rocker arm The reaction force received by the decompression cam surface of the decompression cam part when it is in contact with the cylinder can be firmly received without submerging the decompression cam surface inward in the radial direction, and the exhaust rocker arm can be swung securely to perform decompression. it can.

  Therefore, this decompression mechanism greatly reduces the number of parts, simplifies the structure and reduces the number of processing steps, thereby reducing costs and reducing the space occupied by the decompression mechanism and making the valve mechanism compact. Thus, the internal combustion engine can be reduced in size.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, since the decompression member is a plate-like member having a constant thickness in the axial direction of the camshaft, the decompression member falls within a narrow axial width in the axial direction of the camshaft. The mechanism can be accommodated, and the valve operating mechanism can be made more compact, and the internal combustion engine can be downsized.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, the decompression member extends from the base end portion pivotally supported on the side surface of the cam lobe around the shaft portion of the camshaft in a direction opposite to the rotation direction of the camshaft. Since the decompression weight portion extends along the side surface of the cam lobe in an arc shape, the decompression member is arranged in an arc shape around the shaft portion of the camshaft, and most of the decompression weight portion excluding the decompression cam portion and the base end portion However, it is always within the side surface of the cam lobe, the periphery of the camshaft of the valve mechanism can be made compact, and the internal combustion engine can be downsized.

According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, since the decompression member is pivotally supported on the side surface of the cam lobe of the cam lobe, it is easy to ensure the rigidity of the cam lobe pivot support portion.
And since the decompression cam part formed in the tip part of the decompression weight part is located on the substantially opposite side with respect to the cam crest of the cam lobe and the shaft part of the camshaft, the arcuate decompression weight part goes around the shaft part of the camshaft. The weight necessary to oscillate approximately half a circle and swing by centrifugal force can be easily secured, and the decompression cam portion is positioned on the substantially opposite side with respect to the cam mountain and the shaft portion of the camshaft to ensure decompression. be able to.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, the swing range of the decompression member is restricted by a part of the decompression weight portion coming into contact with the shaft portion of the camshaft. There is no need to provide a member whose range is restricted, and the number of parts can be reduced and the structure can be simplified.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, the intake rocker arm and the exhaust rocker arm are pivotally supported by the common rocker arm shaft, so that the intake rocker arm and the exhaust rocker arm have the same rocker arm shaft. Thus, the number of parts can be reduced, the periphery of the rocker arm shaft can be kept small, the cylinder head can be made smaller, and the internal combustion engine can be made smaller.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, since the intake rocker arm and the exhaust rocker arm swing due to the common camshaft acting, the number of camshafts is reduced, the periphery of the camshaft is kept small, and the cylinder head The internal combustion engine can be downsized.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, the intake rocker arm and the exhaust rocker arm are swung by the common cam lobe of the camshaft, so that one cam lobe is formed on one camshaft, The number of machining steps for the camshaft can be reduced, and the cam lobe periphery of the camshaft can be kept small in the axial direction, and the internal combustion engine can be downsized.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, each cam contact portion on which the cam lobes contact and act on the intake rocker arm and the exhaust rocker arm is accommodated within the axial width of the cam lobe, and the decompression cam portion in the exhaust rocker arm is Since the decompression cam contact portion acting in contact is located adjacent to the cam contact portion of the exhaust rocker arm, it is possible to reduce the axial width of the valve mechanism to reduce the size of the cylinder head of the internal combustion engine. it can.

According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, the intake side roller and the exhaust side roller that are respectively supported by the roller bearing portions of the intake rocker arm and the exhaust rocker arm are the cam contact portions. The sliding resistance due to contact with can be reduced.
Since the protrusion formed on the roller bearing portion of the exhaust rocker arm is the decompression cam contact portion, the decompression cam contact portion is provided with a simple structure in which the protrusion is formed on the roller bearing holding portion of the exhaust rocker arm. be able to.

  According to the embodiment of the decompression mechanism of the internal combustion engine of the present invention, the intake side roller and the exhaust side roller are pivotally supported at the end portions of the intake rocker arm and the exhaust rocker arm, respectively, and the intake side roller contacts only the cam lobe, Since the side roller contacts the cam lobe and the decompression cam portion protruding in the radial direction from the base circle of the cam lobe, it is possible to reduce the sliding resistance due to the contact with the claw and the sliding resistance due to the contact with the decompression cam portion.

It is the left view made into the partial cross section of the internal combustion engine provided with the decompression mechanism which concerns on one embodiment of this invention. FIG. 2 is an enlarged sectional view of a cylinder head and a valve mechanism in FIG. 1. FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2. It is a top view of a cylinder head and a valve mechanism. It is a top view of the valve mechanism which abbreviate | omitted the cylinder head, the rocker arm, etc. It is an enlarged view of the camshaft periphery seen from the axial direction. It is sectional drawing of a cylinder head and a valve mechanism at the time of decompression execution. It is a top view of the valve mechanism and cylinder head concerning another embodiment. It is a top view of the valve mechanism and cylinder head concerning another embodiment. It is a top view of the same valve mechanism and a cylinder head.

Hereinafter, an embodiment according to the present invention will be described with reference to FIGS.
An internal combustion engine 10 having a decompression mechanism of the present embodiment is an SOHC type two-valve single-cylinder four-stroke internal combustion engine mounted on a motorcycle, and a crankshaft 12 is disposed in a vehicle width direction with respect to a vehicle body (not shown). The cylinder is tilted slightly forward and suspended from the vehicle body in a standing posture.
In the description of the present specification, the directions of front, rear, left and right are based on a normal standard in which the straight traveling direction of the motorcycle according to the present embodiment is the forward direction.
In the figure, arrow FR indicates the front side of the vehicle, LH indicates the left side of the vehicle, RH indicates the right side of the vehicle, and RR indicates the rear side of the vehicle.

FIG. 1 is a left side view of a partial cross section of an internal combustion engine 10 having a decompression mechanism according to an embodiment to which the present invention is applied.
A crankcase 11 that rotatably supports a crankshaft 12 of the internal combustion engine 10 includes a transmission gear mechanism 15 between a main shaft 13 and a countershaft 14 that are disposed behind the crankshaft 12, and the counter The shaft 14 is an output shaft.

A cylinder block 16 having one cylinder bore 16b formed on the crankcase 11 and a cylinder head 17 are stacked on the cylinder block 16 via a gasket. The cylinder block 16 and the cylinder head 17 are fastened together. The cylinder head cover 18 covers the cylinder head 17 from above.
The cylinder block 16, the cylinder head 17, and the cylinder head cover 18 stacked on the crankcase 11 extend upward from the crankcase 11 in a slightly inclined posture (see FIG. 1).

  The piston 20 is fitted in the cylinder bore 16b of the cylinder block 16 so as to be slidable back and forth, and the connecting rod 21 is connected between the piston pin 20p of the piston 20 and the crank pin 12p of the crankshaft 12 to constitute a crank mechanism. .

A combustion chamber 22 is formed between the top surface of the piston 20 sliding inside the cylinder bore 16b of the cylinder block 16 and the ceiling surface of the cylinder head 17 facing the top surface.
In the cylinder head 17, an intake valve port 23 and an exhaust valve port 24 are opened on the ceiling surface facing the combustion chamber 22 one by one at positions opposite to each other with respect to the cylinder axis Cy that is the central axis of the cylinder bore 16b. The intake port 23p and the exhaust port 24p are formed so as to extend from the port 23 and the exhaust valve port 24 while being curved away from each other.

  Referring to FIG. 2, an intake valve 25 and an exhaust valve 26 that are slidably supported by valve guides 25g and 26g integrally fitted to the cylinder head 17 are substantially symmetrical with respect to the cylinder axis Cy. The intake port 23p and the exhaust port 24p that are provided obliquely toward the opening 22 open and close the intake valve port 23 and the exhaust valve port 24 that face the combustion chamber 22, respectively.

  The intake valve 25 and the exhaust valve 26 are urged upward by valve springs 25 s and 26 s so as to close the intake valve port 23 and the exhaust valve port 24 facing the combustion chamber 22, and are provided on the cylinder head 17. Driven by the valve operating mechanism 30, the intake valve port 23 of the intake port 23p and the exhaust valve port 24 of the exhaust port 24p are opened and closed in synchronization with the rotation of the crankshaft 12.

  The valve operating mechanism 30 is a valve operating mechanism of a SOHC type internal combustion engine in which one camshaft 31 is axially supported on the cylinder head 17 in the left-right direction. As shown in FIG. The shaft 31 is rotatably supported by bearings 33L and 33R on the left and right sides, and a cam lobe 32 having a predetermined cam profile on the left side of the cylinder axis Cy is expanded between the left and right bearings 33L and 33R of the cam shaft 31. One diameter is formed.

  A cam chain sprocket 34 is fitted to the left end portion of the cam shaft 31 that passes through the left bearing 33L, and a cam chain is inserted between the cam chain sprocket 34 and a cam chain sprocket (not shown) fitted to the crankshaft 12. 35 is laid over and transmits the rotation of the crankshaft 12 to the camshaft 31 at half the rotational speed.

Above this camshaft 31, a single rocker arm shaft 41 is installed in parallel with both ends supported by bearing portions 42R and 42L.
An intake rocker arm 43 and an exhaust rocker arm 44 are pivotally supported adjacent to each other on this one rocker arm shaft 41.

As shown in FIG. 3, the exhaust rocker arm 44 is disposed on the left side of the intake rocker arm 43, and one cam lob 32 of the camshaft 31 is positioned below the exhaust rocker arm 44. That is, in the cam lobe 32, the axial position (left-right direction position) of the cam axis Cx of the camshaft 31 is biased toward the exhaust rocker arm 44 side.
In the figure, the cam lobe 32 has a dotted pattern.

The intake rocker arm 43 and the exhaust rocker arm 44 are pivotally supported by a common rocker arm shaft 41 at the center.
2 and 4, the intake rocker arm 43 includes an intermediate swing shaft support 43a, a valve side arm 43v, and a cam side arm 43c, and the valve side arm 43v includes a swing shaft support 43a. The cam side arm 43c extends forward from the swing shaft support 43a and bends toward the exhaust rocker arm 44 while extending rearward toward the upper end of the intake valve 25 located behind the cylinder axis Cy. It bends at the portion 43e (FIG. 4) and extends toward the cam peripheral surface of the cam lobe 32 at a position biased toward the exhaust rocker arm 44 in the axial direction of the camshaft 31.

  Similarly, referring to FIGS. 2 and 4, the exhaust rocker arm 44 includes an intermediate swing shaft support portion 44a, a valve side arm portion 44v, and a cam side arm portion 44c. The cam side arm portion 44c has a swing shaft. Extending rearward from the support portion 44a toward the cam circumferential surface of the cam lobe 32 that is biased toward the exhaust rocker arm 44 in the axial direction of the camshaft 31, and extending forward from the swing shaft support portion 44a. The valve side arm portion 44v is bent toward the intake rocker arm 43 side by a bent portion 44e (FIG. 4) and extends toward the upper end of the exhaust valve 26 positioned in front of the cylinder axis Cy.

  Therefore, the intake rocker arm 43 and the exhaust rocker arm 44 are arranged in the middle swing shaft support 43a, as viewed in the cam axis direction (FIG. 2) as viewed in the direction of the cam axis Cx (or as viewed in the axis direction of the rocker arm shaft 41). 44a overlaps, and the cam side arm portion 43c of the intake rocker arm 43 and the valve side arm portion 44v of the exhaust rocker arm 44 overlap each other when viewed in the cylinder axis direction (FIG. 4) as viewed in the direction in which the cylinder axis Cy is directed.

  The intake rocker arm 43 is in contact with the upper end of the valve stem of the intake valve 25, which is biased upward by the valve spring 25 s, with the adjusting screw 43 t screwed to the tip of the valve side arm portion 43 v, and the cam side arm portion 43 c. An intake side roller 43r is rotatably supported by a pair of roller bearing portions 43cc and 43cc branched at the front end via a support shaft 43ra, and the intake side roller 43r is supported by the cam periphery of the cam lobe 32 of the camshaft 31. Roll to contact the surface.

  On the other hand, the exhaust rocker arm 44 is in contact with the upper end of the valve stem of the exhaust valve 26, which is biased upward by the valve spring 26 s, with the adjusting screw 44 t screwed to the tip of the valve side arm portion 44 v, and the cam side arm portion 44 c. An exhaust-side roller 44r is rotatably supported by a pair of roller bearings 44cc and 44cc branched at the front end via a support shaft 44ra. The exhaust-side roller 44r is supported by the cam periphery of the cam lobe 32 of the camshaft 31. Roll to contact the surface.

  Referring to FIG. 4, the cam side arm portion 43c of the intake rocker arm 43 is bent toward the exhaust rocker arm 44 side (left side) in the cam axis Cx direction, and an intake side roller 43r which is a tip contact portion of the cam side arm portion 43c. Is shifted to the same position in the cam shaft direction as that of the swinging shaft support 44a of the exhaust rocker arm 44.

As shown in FIGS. 4 and 5, the intake side roller 43r at the tip of the cam side arm portion 43c of the intake rocker arm 43 and the exhaust side roller 44r at the tip of the cam side arm portion 44c of the exhaust rocker arm 44 are connected to the camshaft 31. The cam lobes of the camshaft 31 are in the same axial position, and are in rolling contact with the cam peripheral surface of the common cam lobe 32. That is, in order to act on the intake rocker arm 43 and the exhaust rocker arm 44, conventionally, the cam lobes provided separately are cam lobes 32 integrated into a single body.
The left side surface of the intake side roller 43r and the left side surface of the exhaust side roller 44r are substantially flush with the left side surface of the cam lobe 32.

  Further, the valve side arm portion 44v of the exhaust rocker arm 44 is bent toward the intake rocker arm 43 side (right side) in the cam axis direction, and the adjustment screw 44t, which is the tip acting portion of the valve side arm portion 44v, swings the intake rocker arm 43. It is shifted to the same axial position as the shaft support 43a.

  2 and 4, the bent portion 43e of the cam side arm portion 43c of the intake rocker arm 43 and the bent portion 44e of the valve side arm portion 44v of the exhaust rocker arm 44 are adjacent to each other.

  As described above, the main valve mechanism 30 is configured, and when the camshaft 31 rotates in synchronization with the crankshaft 12, one cam side arm portion that is in rolling contact with the common cam lobe 32 of the common camshaft 31. The intake rocker arm 43 and the exhaust rocker arm 44 swing at predetermined timings via rollers 43r and 44r at the tips of 43c and 44c, respectively, and adjusting screws 43t and 44t at the tips of the other valve side arm portions 43v and 44v that swing. Operates the intake valve 25 and the exhaust valve 26 to open and close the intake valve port 23 of the intake port 23p facing the combustion chamber 22 and the exhaust valve port 24 of the exhaust port 24p, respectively, at predetermined timings.

In the internal combustion engine 10, a valve mechanism 30 is provided with a decompression mechanism 50 (FIG. 6).
A decompression member 51 is provided on the left side surface of the cam lobe 32 of the camshaft 31 that swings both the intake rocker arm 43 and the exhaust rocker arm 44, as shown in FIGS. The left roller bearing portion 44cc that supports the roller 44r of the side arm portion 44c has a protrusion 44p that is the same position as the decompression member 51 in the camshaft axial direction and protrudes toward the camshaft 31. Is formed.

  As shown in FIG. 6, the protrusion 44p of the cam side arm portion 44c has a diameter larger than the base circle Bc along the left side surface of the cam lobe 32 when the exhaust side roller 44r is in contact with the base circle Bc of the cam lobe 32. It protrudes slightly inward.

  Referring to FIG. 6, decompression member 51 has a cam shaft 31 rotating around a shaft portion of a camshaft 31, that is, a cylindrical outer surface, from a base end portion 51 a that is pivotally supported by support shaft 52 on the left side surface of cam lobe 32. A decompression weight 51w extends in the arc direction in the opposite direction to that shown by the arrow in FIG. 6 along the left side surface of the cam lobe 32, and a decompression cam 51c is formed at the tip of the decompression weight 51w.

  The decompression member 51 has a base end portion 51a pivotally supported on the side surface of the cam crest 32a of the cam lobe 32 by a support shaft 52 on the radially inner side of the base circle Bc. The decompression weight portion 51w of the decompression member 51 extends in a circular arc shape around the shaft portion of the camshaft 31 in a direction opposite to the rotation direction of the camshaft 31 from the base end portion 51a over a half circumference. A decompression cam portion 51c formed at the distal end portion of the weight portion 51w is positioned on the substantially opposite side with respect to the cam crest 32a of the cam lobe 32 and the shaft portion of the camshaft 31.

As shown in FIGS. 2 and 3, the decompression member 51 is a plate-like member having a certain thickness in the axial direction of the cam axis Cx of the camshaft 31.
As shown in FIG. 6, the decompression member 51 is entirely inside the cam profile of the cam lobe 32 and further inside the base circle Bc of the cam lob 32 in the radial direction.

The decompression weight portion 51w that forms the arc shape of the decompression member 51 has an inner circumferential surface that extends slightly over the circumference substantially along the shaft portion of the camshaft 31, and an outer circumferential surface of the camshaft 31 at the center of the inner circumferential surface. A central contact surface 51wc having the same curvature is formed, and a front end contact surface 51ws having the same curvature as that of the outer peripheral surface of the camshaft 31 is also formed at the front end portion of the inner peripheral surface.
The decompression member 51 that swings around the support shaft 52 is controlled in its swing range by the central contact surface 51wc and the tip contact surface 51ws coming into contact with the shaft portion of the camshaft 31.

  A coiled torsion spring (biasing means) 55 is wound around a portion of the support shaft 52 protruding from the base end portion 51a of the decompression member 51, and one end thereof is inserted into a hole drilled in the camshaft 31 in the radial direction. The other end is locked to the outer peripheral surface of the decompression weight portion 51w of the decompression member 51.

The decompression member 51 is urged clockwise in FIG. 6 by the torsion spring 55, and the decompression member 51 is restricted and stopped where the central contact surface 51wc of the decompression weight portion 51w contacts the shaft portion of the camshaft 31.
The decompression member 51 shown by the solid line in FIG. 6 shows a state when the central contact surface 51wc contacts the shaft portion of the camshaft 31.

When the camshaft 31 rotates from this state and a centrifugal force of a predetermined level or more acts on the decompression weight portion 51w of the decompression member 51, the decompression member 51 swings counterclockwise against the urging force of the torsion spring 55, The leading end abutment surface 51ws of the decompression weight portion 51w is regulated and stops when it abuts against the shaft portion of the camshaft 31.
The decompression member 51 indicated by a two-dot chain line in FIG. 6 shows a state when the tip contact surface 51ws is in contact with the shaft portion of the camshaft 31.

The arcuate decompression weight portion 51w of the decompression member 51 has an outer peripheral surface protruding radially outward at the tip portion to form a decompression cam portion 51c.
When the centrifugal force due to the rotation of the camshaft 31 is small and the decompression member 51 abuts the central contact surface 51wc of the decompression weight portion 51w against the shaft portion of the camshaft 31 by the urging force of the torsion spring 55, that is, the decompression member 51 6, the decompression cam portion 51 c protruding outward in the radial direction is in a position in contact with the base circle Bc of the cam lobe 32, and radially outward of the decompression member 51 that rotates together with the cam lobe 32. Only the protruding decompression cam portion 51c is in sliding contact with the protruding portion 44p protruding slightly inward from the base circle Bc along the left side surface of the cam lobe 32 of the cam side arm portion 44c of the exhaust rocker arm 44.

A decompression cam surface Sc (indicated by a thick line in FIG. 6) of the decompression cam portion 51c that is in sliding contact with the projection 44p of the exhaust rocker arm 44 forms a smooth curved surface.
Referring to FIG. 6, the decompression cam surface Sc indicated by a thick line in FIG. 6 acting in sliding contact with the protrusion 44 p of the exhaust rocker arm 44 in the decompression cam portion 51 c projecting radially outward is as follows with respect to the cam lobe 32. In relative position. A swing locus around the swing center (support shaft 52) of the decompression member 51 at an arbitrary point P on the decompression cam surface Sc is an arc Rs. On the other hand, the decompression member 51 is decompressed under the biasing force of the torsion spring 55. The rotation center of the camshaft 31 passing through the arbitrary point P in the vicinity of the rotation direction of the arbitrary point P at the stop position where the central contact surface 51wc of the portion 51w is in contact with the shaft portion of the camshaft 31 ( When the arc of the virtual circle around the cam axis Cx) is the arc Rr, the relative position of the decompression cam surface Sc with respect to the cam lobe 32 is determined such that the arc Rs is radially inward of the arc Rr.

With this configuration, when the decompression member 51 swings against the urging force of the torsion spring 55 when the decompression weight portion 51w receives a centrifugal force due to the rotation of the camshaft 31, the decompression cam portion 51c The decompression cam surface Sc is displaced along the arc Rs that is the swing locus of the point P. Therefore, the decompression cam surface Sc is from the arc Rr that is the rotation locus around the cam shaft rotation center (cam axis Cx) at the point P. It is possible to take a trajectory on the radially inner side and sunk inward in the radial direction.
When the decompression cam surface Sc of the decompression cam portion 51c is sunk inward in the radial direction, the projection 44p of the exhaust rocker arm 44 does not contact the decompression cam surface Sc as shown by a two-dot chain line in FIG. There is no movement.

  Referring to FIG. 6, the curved surface shape of the decompression cam surface Sc of the decompression cam portion 51c projecting radially outward is such that the normal line L at an arbitrary point P of the decompression cam surface Sc is the center of oscillation of the decompression member 51 ( It is formed in a curved surface shape passing through a position shifted in the rotation direction of the camshaft 31 from the support shaft 52) or the center of swing (support shaft 52).

  Therefore, referring to FIG. 7, the reaction force F (indicated by an arrow in FIG. 7) received by the decompression cam surface Sc when the decompression cam surface Sc of the decompression member 51 contacts and acts on the projection 44p of the exhaust rocker arm 44. The decompression member 51 is not swung toward the swing center (support shaft 52) of the decompression member 51. On the other hand, as shown in FIG. 7, the reaction force F (FIG. 7) received by the decompression cam surface Sc acts toward a position shifted in the rotational direction of the camshaft 31 from the center of oscillation (support shaft 52). When the decompression cam surface Sc is in contact with and acts on the protrusion 44p of the exhaust rocker arm 44, the decompression cam portion 51c swings the decompression member 51 in the radial outward direction (clockwise in FIG. 7). Force) works. The decompression member 51 is in a state where the central abutment surface 51wc of the decompression weight portion 51w abuts against the shaft portion of the camshaft 31, and thus the decompression member 51 maintains this stop position firmly.

Therefore, the reaction force F received by the decompression cam surface Sc by contacting the projection 44p of the exhaust rocker arm 44 does not cause the decompression cam portion 51c to swing, and the state where the decompression cam portion 51c protrudes radially outward is maintained. Therefore, when the centrifugal force due to the rotation of the camshaft 31 is small, the exhaust rocker arm 44 can be reliably swung for decompression.
On the other hand, when the centrifugal force due to the rotation of the camshaft 31 is increased and the decompression member 51 is rotated clockwise around the support shaft 52 in FIG. 6 against the urging force of the torsion spring 55, the decompression member 51 on the decompression cam surface Sc. An arc Rs that is a swing locus around the swing center (support shaft 52) of the decompression member 51 at the point P is a virtual circle around the camshaft rotation center (cam axis Cx) at the point P on the decompression cam surface Sc. By taking a trajectory radially inward from the arc Rr, the decompression cam surface Sc sinks inward in the radial direction, the decompression cam surface Sc does not contact the projection 44p of the exhaust rocker arm 44, and no decompression action occurs.

  Thus, in this decompression mechanism 50, the decompression member 51 is integrally provided with the decompression cam portion 51c together with the decompression weight portion 51w, and the decompression mechanism is configured by combining the conventional decompression cam and decompression weight separately. Compared with, the number of parts is greatly reduced, and it has a simple structure that does not require complicated machining. Nevertheless, the decompression cam portion 51c can be swung against the urging force of the torsion spring 55 by the centrifugal force generated by the rotation of the camshaft 31, and the decompression cam surface Sc can be sunk inward in the radial direction. The reaction force F received by the decompression cam surface Sc of the decompression cam portion 51c when it comes into contact with the protrusion 44p of the rocker arm 44 is firmly received without sacrificing the decompression cam surface Sc inward in the radial direction, and the exhaust rocker arm 44 is reliably swung. And decompression can be executed.

  Therefore, when the engine speed of the internal combustion engine 10 is low, that is, when the rotational speed of the camshaft 31 is low, the decompression cam portion 51c of the decompression member 51 protrudes radially outward by the urging force of the torsion spring 55, and the exhaust rocker arm 44 The decompression cam surface Sc is brought into sliding contact with the projection 44p, and at this time, the decompression cam portion 51c is protruded radially outward and stopped, so that the exhaust rocker arm 44 is reliably swung as shown in FIG. The adjustment screw 44t at the tip of the valve side arm portion 44v pushes the exhaust valve 26 to open the exhaust valve port 24, and decompression is executed so that the pressure in the combustion chamber 22 is released, thereby facilitating starting.

  When the engine speed of the internal combustion engine 10 increases, a centrifugal force acts on the decompression weight portion 51w of the decompression member 51, and the decompression member 51 swings with respect to the cam lobe 32 against the biasing force of the torsion spring 55, the decompression member 51 stops at the position indicated by the two-dot chain line in FIG. 6. At this time, the decompression cam portion 51c sunk inward in the radial direction and does not contact the projection 44p of the exhaust rocker arm 44, so that decompression is not executed.

  As described above, the decompression mechanism 50 greatly reduces the number of parts, simplifies the structure and reduces the number of processing steps, thereby reducing costs and reducing the space occupied by the decompression mechanism 50. The mechanism 30 can be configured compactly, and the internal combustion engine 10 can be downsized.

  As shown in FIGS. 2 and 3, the decompression member 51 is a plate-like member having a constant thickness in the axial direction of the cam axis Cx of the camshaft 31. The decompression mechanism 50 can be accommodated therein, and the valve operating mechanism 30 can be configured more compactly, so that the internal combustion engine 10 can be downsized.

  As shown in FIGS. 2 and 6, the decompression member 51 is configured to rotate around the shaft portion of the camshaft 31 from the base end portion 51 a pivotally supported by the support shaft 52 on the left side surface of the cam lobe 32. Since the decompression weight 51w extends along the left side surface of the cam lobe 32 in a circular arc shape in the opposite direction, the decompression member 51 is disposed in a circular arc around the shaft portion of the camshaft 31, and the decompression weight 51w Most parts except the decompression cam part and the base end part 51a are always contained in the side surface of the cam lobe 32, and the periphery of the camshaft 31 of the valve operating mechanism 30 can be made compact, and the internal combustion engine 10 can be downsized. Can be achieved.

In the decompression member 51, the base end portion 51a is pivotally supported by the support shaft 52 inside the base circle Bc on the side surface of the cam crest 32a of the cam lobe 32, so that it is easy to ensure the rigidity of the cam lobe shaft support portion.
Since the decompression cam portion 51c formed at the distal end portion of the decompression weight portion 51w is located on the substantially opposite side with respect to the cam crest 32a of the cam lobe 32 and the shaft portion of the camshaft 31, the arcuate decompression weight portion 51w is formed on the camshaft 31. The weight necessary to oscillate about half a circle around the shaft portion and swing by centrifugal force can be easily secured, and the decompression cam portion 51c is located on the opposite side with respect to the cam portion 32a and the shaft portion of the camshaft 31. So that decompression can be performed reliably.

  Since the center contact surface 51wc and the tip contact surface 51ws, which are a part of the decompression weight portion 51w, are in contact with the shaft portion of the camshaft 31, the swing range of the decompression member 51 is restricted. There is no need to provide a member for regulating the swing range, and the number of parts can be reduced and the structure can be simplified.

  In this manner, the intake rocker arm 43 and the exhaust rocker arm 44 are pivotally supported by the common rocker arm shaft 41 disposed between the intake valve 25 and the exhaust valve 26, and are respectively pivoted from the swing shaft support portions 43a and 44a. Since the cam side arm portions 43c, 44c contacting the cam lobe 32 and the valve side arm portions 43v, 44v acting on the intake valve 25 or the exhaust valve 26 extend on both sides, the intake rocker arm 43 and the exhaust rocker arm 44 are connected to the rocker arm shaft 41. By making it common, the number of parts can be reduced, the periphery of the rocker arm shaft 41 can be kept small, the cylinder head 17 can be downsized, and the internal combustion engine 10 can be downsized.

  Since the intake rocker arm 43 and the exhaust rocker arm 44 swing when the common camshaft 31 acts, the camshaft 31 can be reduced to one, the periphery of the camshaft 31 can be kept small, and the cylinder head 17 can be made compact. The internal combustion engine can be downsized.

  The intake rocker arm 43 and the exhaust rocker arm 44 are swung by the common cam lobe 32 of the common cam shaft 31, so that one cam lobe 32 is formed on one cam shaft 31, and the machining man-hour of the cam shaft 31 is reduced. In addition to the reduction, the periphery of the cam lobe 32 of the camshaft 31 can be kept small in the axial direction, and the internal combustion engine can be downsized.

  As shown in FIG. 5, the rollers 43r and 44r that are acted upon by the cam lobe 32 in the intake rocker arm 43 and the exhaust rocker arm 44 are in contact with each other within the axial width of the cam lobe 32, and the decompression cam portion 51c in the exhaust rocker arm 44 is in contact. The protruding portion 44p, which is the decompression cam contact portion that acts in this manner, is positioned adjacent to the roller 44r, which is the cam contact portion of the exhaust rocker arm 44, so that the axial width of the valve mechanism 30 is reduced to reduce the internal combustion engine. Ten cylinder heads 17 can be reduced in size.

Since the intake-side roller 43r and the exhaust-side roller 44r that are pivotally supported at the end portions of the intake rocker arm 43 and the exhaust rocker arm 44 are used as the cam contact portions, the sliding resistance due to contact with the cam lobe 32 can be reduced. it can.
Since the protrusion 44p formed on the roller bearing portion 44cc of the exhaust rocker arm 44 is used as the decompression cam contact portion, a simple structure for forming the projection 44p on the roller bearing portion 44cc of the exhaust rocker arm 44 is used. A contact portion can be provided.

In the valve mechanism described above, the exhaust side roller 44r is supported at both ends by a pair of roller bearing portions 44cc and 44cc branched at the tip of the cam side arm portion 44c. Only the part 44cc may be cantilevered.
FIG. 8 shows another form in which the intake side roller 43r and the exhaust side roller 44r are both cantilevered by the left roller bearing portions 43cc and 44cc.

By omitting the roller bearings 43cc and 44cc on the right side of the intake side roller 43r and the exhaust side roller 44r, the axial width of the intake rocker arm 43 and the exhaust rocker arm 44 can be reduced, and the valve mechanism can be made smaller. Can be achieved.
The left roller bearing portion 44cc that supports the exhaust side roller 44r is provided with a projection 44p that contacts the decompression cam portion 51c of the decompression member 51.

Next, still another embodiment will be described with reference to FIGS.
The intake side roller 43r and the exhaust side roller 44R are cantilevered by the right side roller bearings 43cc and 44cc, and the exhaust side roller 44R has a large axial width.

The exhaust-side roller 44R having a larger axial width protrudes to the left in the axial direction from the cam lobe 32 and is positioned on the outer periphery of the decompression member 51.
And the decompression cam part 51C of the decompression member 51 protrudes more largely on the radially outer side than the decompression cam part 51c of the above embodiment.
As shown in FIG. 10, when the centrifugal force due to the rotation of the camshaft 31 is small and the decompression weight portion 51w of the decompression member 51 comes into contact with the shaft portion of the camshaft 31 by the urging force of the torsion spring 55 and stops. The decompression cam portion 51C protruding outward in the radial direction protrudes outside the base circle Bc of the cam lobe 32.

  Therefore, when the engine speed is small, the decompression cam portion 51C that protrudes outward from the base circle Bc of the cam lobe 32 contacts the exhaust roller 44R that protrudes to the left in the axial direction from the cam lobe 32 and swings the exhaust rocker arm 44. The exhaust valve 26 is pushed to open the exhaust valve port 24, and decompression is executed so that the pressure in the combustion chamber 22 is released, thereby facilitating starting.

  When the engine speed increases, centrifugal force acts on the decompression weight portion 51w of the decompression member 51, and the decompression member 51 swings against the cam lobe 32 against the biasing force of the torsion spring 55, the decompression cam portion 51C The cam lobe 32 is submerged inside the base circle Bc in the radial direction and does not contact the exhaust side roller 44R, so that decompression is not performed.

In the valve operating mechanism used in the present embodiment, the left and right roller bearings 43cc and 44cc of the intake side roller 43r and the exhaust side roller 44R are omitted, so that the intake rocker arm 43 and the exhaust rocker arm 44 are axially moved. The width can be kept small, and the valve mechanism can be downsized.
Further, since the exhaust side roller 44R that contacts the cam lobe 32 also rolls into contact with the decompression cam portion 51C of the decompression member 51 and acts on the exhaust rocker arm 44, the sliding resistance due to contact with the decompression cam portion 51C can be reduced. Further, there is no need to provide a dedicated cam contact portion that contacts the decompression cam portion 51, and the number of processing steps can be reduced.

Although the decompression mechanism of the internal combustion engine according to the embodiment of the present invention has been described above, the aspect of the present invention is not limited to the above embodiment, and may be implemented in various aspects within the scope of the gist of the present invention. Is included.
For example, the present invention can also be applied to an internal combustion engine provided with a valve operating mechanism in which a camshaft is provided on the crankcase side and the rotation of the camshaft is transmitted to and operated by a push rod to a cylinder head side valve.
Note that the left and right arrangement of each device has been described specifically for illustration for convenience of explanation, but the arrangement may be reversed to that shown in the above embodiment, and is included in the present invention. .

Cx: cam axis line, Cy: cylinder axis line, Sc: decompression cam surface, Bc: base circle, Rr: arc as a rotation locus, Rs: arc as a swing locus,
DESCRIPTION OF SYMBOLS 10 ... Internal combustion engine, 11 ... Crank case, 12 ... Crank shaft, 13 ... Main shaft, 14 ... Counter shaft, 15 ... Transmission gear mechanism, 16 ... Cylinder block, 17 ... Cylinder head, 18 ... Cylinder head cover, 20 ... Piston, 21 ... Connecting rod, 22 ... Combustion chamber, 23 ... Intake valve port, 24 ... Exhaust valve port, 25 ... Intake valve, 26 ... Exhaust valve,
30 ... Valve mechanism, 31 ... Cam shaft, 32 ... Cam lobe, 33 ..., 34 ... Cam chain sprocket, 35 ... Cam chain, 41 ... Rocker arm shaft,
43 ... Intake rocker arm, 43v ... Valve side arm part, 43t ... Adjustment screw, 43c ... Cam side arm part, 43cc ... Roller bearing part, 43r ... Intake side roller,
44 ... Exhaust rocker arm, 44v ... Valve side arm, 44t ... Adjustment screw, 44c ... Cam side arm, 44cc ... Roller bearing, 44p ... Protrusion, 44r, 44R ... Exhaust side roller,
50 ... Decompression mechanism, 51 ... Decompression member, 51a ... Base end part, 51w ... Decompression weight part, 51c, 51C ... Decompression cam part, 52 ... Support shaft, 55 ... Torsion spring.

Claims (11)

The intake rocker arm (43) and the exhaust rocker arm (44) swinging by the action of the cam lobe (32) of the camshaft (31) rotating in synchronization with the crankshaft (12) are respectively connected to the intake valve (25) and the exhaust valve ( It is equipped with a valve mechanism (30) that opens and closes the opening (23) of the intake port (23p) facing the combustion chamber (22) and the opening (24) of the exhaust port (24p) at a predetermined timing. ,
A decompression cam portion (51c) that acts on the exhaust rocker arm (44) is provided, and the decompression cam portion (51c) is in the radial direction with respect to the central axis (Cx) of the camshaft (31) according to the centrifugal force accompanying the rotation of the engine. A decompression member (51) configured to appear and disappear in the cam lobe (32),
When the engine speed is low, the decompression cam portion (51c) protrudes radially outward to act on the exhaust rocker arm (44) to swing the exhaust valve (26) to open the exhaust port (24p). When the engine speed increases, in the decompression mechanism (50) of the internal combustion engine (10) that sunk the decompression cam portion (51c) radially inward,
The decompression member (51) extends in a direction opposite to the rotational direction of the camshaft (31) from a base end portion (51a) pivotally supported by the cam lobe (32) at a swing center (52). The decompression cam portion (51c) is formed at the distal end of the decompression weight portion (51w), and the decompression member (51) has the decompression weight portion (51w) attached to the inside in the radial direction. It can be swung along the side surface of the cam lobe (32) from the stop position that is biased by the biasing means (55) to be biased,
The decompression cam portion (51c) protrudes radially outward and has a decompression cam surface (Sc) that acts on the exhaust rocker arm (44), and the decompression cam surface (Sc) is on the decompression cam surface (Sc). An arc (Rs) that is a swing locus around the swing center (52) of the decompression member (51) at an arbitrary point (P) is the point (P) at the stop position of the decompression member (51). ) Through the camshaft (31), the relative position relative to the cam lobe (32) is determined so as to be radially inward of the arc (Rr) of the virtual circle around the rotation center (Cx) of the camshaft (31),
The decompression cam surface (Sc) of the decompression cam portion (51c) protruding outward in the radial direction has a normal line (L) at the point (P) on the decompression cam surface (Sc), and the fluctuation of the decompression member (51c). A decompression mechanism for an internal combustion engine, characterized in that it is formed in a surface shape that passes through a position shifted in the rotational direction of the camshaft (31) from the moving center (52) or the swing center (52).   The decompression mechanism for an internal combustion engine according to claim 1, wherein the decompression member (51) is a plate-like member having a constant thickness in the axial direction of the camshaft (31).   The decompression weight portion (51w) of the decompression member (51) is formed around the shaft portion of the camshaft (31) from the base end portion (51a) pivotally supported on the side surface of the cam lobe (32). 3. The decompression mechanism for an internal combustion engine according to claim 2, wherein the decompression mechanism extends along the side surface of the cam lobe (32) in an arc shape in a direction opposite to the rotation direction of the shaft (31).   The decompression member (51) has the base end portion (51a) pivotally supported on the side surface of the cam crest (32a) of the cam lobe (32), and the decompression cam formed on the distal end portion of the decompression weight portion (51w). 4. A decompression mechanism for an internal combustion engine according to claim 3, wherein the portion (51c) is located on a substantially opposite side with respect to the cam crest (32a) of the cam lobe (32) and the shaft portion of the cam shaft (31).   The swing range of the decompression member (51) is restricted at the stop position where a part of the decompression weight portion (51w) contacts the shaft portion of the camshaft (31). 5. A decompression mechanism for an internal combustion engine according to claim 4.   The said intake rocker arm (43) and the said exhaust rocker arm (44) are pivotally supported by the common rocker arm shaft (41) so that rocking | fluctuation is possible, The any one of Claim 1 thru | or 5 characterized by the above-mentioned. Decompression mechanism for internal combustion engines.   The internal combustion engine according to any one of claims 1 to 6, wherein the intake rocker arm (43) and the exhaust rocker arm (44) are swung by the action of the common camshaft (31). Institution decompression mechanism.   The internal combustion engine according to claim 7, wherein the intake rocker arm (43) and the exhaust rocker arm (44) are swung by the cam lobe (32) common to the camshaft (31). Decompression mechanism.   The intake rocker arm (43) and the exhaust rocker arm (44) each include a cam contact portion (43r, 44r) that the cam lobe (32) contacts and acts, and each cam contact portion (43r, 44r) Is within the axial width of the cam lobe (32), and the decompression cam contact portion (44p) acting on the decompression cam portion (51c) of the exhaust rocker arm (44) is in contact with the exhaust rocker arm (44). 9. The decompression mechanism for an internal combustion engine according to claim 8, wherein the decompression mechanism is located adjacent to the cam contact portion (44r). The cam contact portions include an intake side roller (43r) and an exhaust side roller (44r) that are respectively supported by the roller bearing portions (43cc, 44cc) of the intake rocker arm (43) and the exhaust rocker arm (44). And
10. The decompression mechanism for an internal combustion engine according to claim 9, wherein the decompression cam contacted portion is a projection (44p) formed to project from the roller bearing portion (44cc) of the exhaust rocker arm (44). An intake side roller (43r) and an exhaust side roller (44r) are pivotally supported at each end of the intake rocker arm (43) and the exhaust rocker arm (44), respectively.
The intake side roller (43r) is in contact with the cam lobe (32),
9. The exhaust roller (44r) contacts the cam lobe (32) and the decompression cam portion (51c) protruding in a radial direction from a base circle (Bc) of the cam lobe (32). The internal combustion engine decompression mechanism.

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