KR100813746B1 - Decompressor for 4 stroke cycle internal combustion engines - Google Patents

Abstract

In a four-stroke cycle internal combustion engine having an intake valve and an exhaust valve 9, 10 which are opened and closed driven by an intake cam and an exhaust cam 27, 28 integral with the cam shaft 13, the internal combustion engine is a cam shaft. The one-way clutch 29 and the decompression cam 38 which can transmit the reverse rotation torque to the decompression cam 38 only when the decompression cam 38 and the camshaft 13 rotate in the reverse direction are provided in the rotatable manner. And the torque limiter 40 inserted between the one-way clutch 29 and the fixed portion engaged with the engaging portion 41 formed on the decompression cam, so as to rotate the decompression cam 38 in the respective forward and reverse rotation directions. The pressure reduction cam stoppers 42 and 43 which stop the process are provided. The lift of the cam portion 38a of the decompression cam 38 is higher than the base circle of the exhaust cam 28 and lower than the maximum lift of the cam portion 28a of the exhaust cam 28. The exhaust valve 10 is not opened by the decompression cam when the cam shaft 13 rotates forward and the decompression cam stopper 43 engages with the engaging portion. The structure is simple and a compact and low cost pressure reducing device is provided.

Description

Decompressor for 4 stroke cycle internal combustion engine {DECOMPRESSOR FOR 4 STROKE CYCLE INTERNAL COMBUSTION ENGINES}

The present invention relates to a pressure reducing device for starting up a four stroke cycle internal combustion engine using a starter motor.

Mechanical pressure reducing devices are known (see Japanese Patent Laid-Open No. 11-107727, Japanese Patent Laid-Open No. 2000-199412, and Japanese Patent No. 2890218). When the internal combustion engine is started by the starter motor, the mechanical pressure reducing device is operated to open the intake valve or the exhaust valve when the rotation speed of the camshaft is less than the predetermined value, and when the rotation speed of the camshaft exceeds the predetermined value, The non-operating state is set by the centrifugal force of the weight of the pressure reducing device.

In addition, an electric pressure reducing device is also known (Japanese Patent Laid-Open No. 11-324744). When the internal combustion engine is started by the starter motor, the electric decompression device is activated by the excitation of the electromagnetic solenoid to promote the rotation of the crankshaft of the internal combustion engine, and the crankshaft does not depend on the starter motor for the internal combustion engine. After reaching the rotational speed sufficient to start the rotation of the furnace, the excitation of the electromagnetic solenoid is released to deactivate the decompression device.

In the above-mentioned mechanical decompression device, since the decompression device is converted to actuation and non-operation using the centrifugal force, the weight for generating the centrifugal force is essential. Therefore, since the inertia of the valve actuating cam system is increased and a large output starter motor is required, miniaturization of the starter motor has been difficult.

In the above-mentioned electric pressure reducing device, the pressure reducing device can be miniaturized because it is necessary to excite the electromagnetic solenoid only when the operation of the pressure reducing device is required. However, since an electronic judging device for determining the operation and non-operation of the decompression device is required, it is difficult to apply this decompression device in a small four-stroke cycle internal combustion engine without an electronic control device. In addition, even in the case of an internal combustion engine having an electronic controller, this electronic controller has a problem of becoming complicated.

The present invention relates to an improvement of the pressure reducing device of a four stroke cycle internal combustion engine which solves the above-mentioned problems. The present invention has an intake valve and an exhaust valve which are opened and closed by an intake cam and an exhaust cam integral with the camshaft, respectively, and in a four-stroke cycle internal combustion engine started by a starter motor, the decompression cam is relatively relatively to the camshaft. A one-way clutch is installed in the rotatable position and adjacent to the decompression cam and capable of transmitting the reverse rotation torque to the decompression cam only during the reverse rotation of the cam shaft. A torque limiter for releasing the transmission of reverse torque above the torque is interposed, and a decompression cam stopper section is installed in the fixed section to engage with the engaging portion formed in the decompression cam and to stop the decompression cam in two forward and reverse rotation directions of the decompression cam. The cam portion lift of the cam is higher than the base circle of the exhaust cam, and also the maximum of the cam portion of the exhaust cam. A pressure reducing device for a four stroke cycle internal combustion engine is provided, wherein the exhaust valve is configured not to open by the decompression cam when it is set lower than the large lift, and the decompression cam stopper portion and the engaging portion of the cam shaft in the forward rotation are stopped. .

According to the present invention, in the reverse rotation of the starter motor, the reverse rotation torque is transmitted from the reverse rotation camshaft through the one-way clutch and the torque limiter to the decompression cam so that the decompression cam reversely rotates, and the engagement portion of the decompression cam is depressurized. The cam stopper is engaged with any one of the cam stoppers. In this stationary state, even if the cam shaft rotates in reverse, since there is a torque limiter, a large reverse rotational force of the starter motor does not act on the cam shaft, the decompression cam, the engaging portion and the decompression cam stopper.

If the starter motor is rotated forward from the state in which the reverse rotation is stopped, the exhaust valve or the intake valve is opened by the decompression cam having a lift larger than the base circle of the exhaust cam or the intake cam, so that the large forward rotation on the crankshaft in the compression stroke The occurrence of resistance torque is avoided, and the crankshaft can rotate with large rotational acceleration.

Then, when the exhaust valve or the intake valve is largely opened by the cam mount of the exhaust cam or the intake cam, the rotational resistance of the decompression cam which acts to open the exhaust valve or the intake valve is released, and the decompression cam is slightly with the camshaft. Rotate Next, when the action of the exhaust cam or the intake cam that opens the exhaust valve or the intake valve is released, the decompression cam is stopped. When the exhaust valve or the intake valve is opened by the next cam mount, the motor rotates forward again. After this process is repeated several times, the engaging portion of the decompression cam is engaged with another decompression cam stopper portion of the internal combustion engine body, and the decompression cam is stopped in the inoperative position. In this state, the action of the decompression cam for opening the exhaust valve or the intake valve is automatically released, and the internal combustion engine can switch to normal operation.

According to the present invention, since the centrifugal force is not used, the inertia of the valve actuation system becomes small, and the starter motor can be miniaturized. Since the conversion from the starting operating state to the normal operating state is carried out mechanically without using electronic control, a control system of a complicated internal combustion engine can be avoided, and cost reduction is possible.

The torque transmitted through the torque limiter may be small at the forward rotation of the camshaft and may be formed to be large at the reverse rotation of the camshaft. When the cam shaft is reversely rotated to engage the engaging portion of the decompression cam with one of the stopper portions of the fixed portion, the decompression cam can be reliably reversed by overcoming the reverse rotation resistance imparted to the decompression cam by the exhaust valve. The state can be easily set.

The torque limiter includes a contact member disposed on one side of an adjacent side of the one-way clutch and the decompression cam, a recess formed on the other side of the adjacent side, and a spring for pressing the contact member against the recess, the recess being the camshaft. When the forward rotation of the small, the reverse rotation of the camshaft may be formed in a shape in which a large torque is transmitted. Alternatively, the torque limiter includes a contact member disposed on one side of the adjacent circumferential surface of the one-way clutch and the decompression cam, a recess formed on the other side of the adjacent outer circumferential surface, and a spring that presses the contact member against the recess. The concave portion may be formed to have a small shape when the cam shaft is rotated forward and a large torque is transmitted when the cam shaft is rotated reversely. By this structure, the torque limiter can be formed compactly and simply, and miniaturization and light weight of the pressure reducing device can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a longitudinal sectional view of a four stroke cycle internal combustion engine including a decompression device according to an embodiment of the present invention.

2 is a cross-sectional view of the internal combustion engine.

3 is an enlarged cross-sectional view of the main portion of the internal combustion engine.

4 is a cross-sectional view taken along line IV-IV of FIG. 3.

FIG. 5 is a cross-sectional view taken along the line VV of FIG. 3.

6A and 6B are front and sectional views of an egg-shaped recess formed on a decompression cam, respectively.

7 is an explanatory view of the operation of the above embodiment.

8 is an explanatory view of the operation of the above embodiment.

9 is an explanatory view of the operation of the above embodiment.

10 is an explanatory view of the operation of the above embodiment.

11 is a diagram showing a relationship between the crank angle and the cam lift for the exhaust cam, the intake cam, and the decompression cam.

12 is an enlarged cross-sectional view of a main portion of an internal combustion engine according to another embodiment of the present invention.                 

FIG. 13 is a cross-sectional view taken along the line III-III of FIG. 12.

14 is a cross-sectional view taken along the line IV-XIV of FIG. 12.

15 is an exploded view of a portion of the inner surface of the decompression cam.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described with reference to FIGS.

1 and 2 show a single cylinder OHC four stroke cycle internal combustion engine 1 of a motorcycle with a pressure reduction device according to the invention. The main body of the internal combustion engine 1 includes a cylinder block 2, a cylinder head 3 detachably installed at an upper end of the cylinder block 2, and a crank case (not shown) provided at a lower portion of the cylinder block 2. Equipped. The cylinder bore 4 of the cylinder block 2 is provided so that the piston 5 may slide up and down. When the piston reciprocates up and down, the piston 5 is connected to a crankshaft (not shown) via a connecting rod so that the crankshaft is driven to rotate.

The cylinder head 3 is provided with an intake passage 7 and an exhaust passage 8 in communication with the combustion chamber 8 at the upper position of the cylinder bore 4. The intake valve 9 and the exhaust valve 10 which open and close the intake passage 7 and the exhaust passage 8 are arranged in a V shape. By the valve spring 12 inserted between the retainer 11 integrally mounted on the top of the intake valve 9 and the exhaust valve 10 and the cylinder head 3, the intake valve and the exhaust valve 10 are always Pressurized to close. A throttle valve and a carburetor are disposed upstream of the intake passage 7.

The camshaft 13 is rotatably supported by the pair of bearings 14 at the central position of the space above the intake valve 9 and the exhaust valve 10. At one end of the camshaft 13, the driven sprocket 16 is integrally mounted by bolts 15, and the drive sprocket (not shown) and the driven sprocket 16 fixed to the crankshaft (not shown) are mounted. An endless chain 17 is covered. The camshaft 13 is driven to rotate at a rotational speed of 1/2 of the rotational speed of the crankshaft.

At each intermediate position between the intake valve 9 and the camshaft 13 and between the exhaust valve 10 and the camshaft 13, the rocker shaft 18 is parallel to the camshaft 13 and the cylinder head. (3) is supported. Each rocker shaft 18 has an intake rocker arm 19 or an exhaust rocker arm 20 that rotates to oscillate. Each rocker arm 19, 20 has an end that is secured to the tappet screw 21 by a lock nut 22 and a bifurcated other end. The roller support shaft 23 is provided at the bisected end, and the intake roller 25 or the exhaust roller 26 is rotatably supported on the roller support shaft 23 through the needle bearing 24.

The camshaft 13 is formed with an intake cam 27 and an exhaust cam 28 adapted to contact the intake roller 25 and the exhaust roller 26 of the intake rocker arm 19 and the exhaust rocker arm 20, respectively. .

As shown in FIG. 2, a one-way clutch 29 is located outside the exhaust cam 28 and installed in the camshaft 13. The pressure reduction cam 38 is rotatably provided in the small diameter cylindrical part 30 of the one-way clutch 29.

FIG. 3 is an enlarged view of the one-way clutch 29, the decompression cam 38 and their adjacent parts shown in FIG. 2. 4 is a cross-sectional view taken along line IV-IV of FIG. 3, and FIG. 5 is a cross-sectional view taken along line V-V of FIG. 3. As shown in Fig. 4, the large-diameter cylindrical portion 31 of the one-way clutch has three cam cut portions 32 formed at equal intervals in the circumferential direction. The cam roller 33 is loosely provided in each cam cut part 32, and the coil spring 34 is inserted between the cam cut part 32 and the cam roller 33. As shown in FIG. When the camshaft 13 rotates forward as shown by arrow N, the one-way clutch 29 is in a shut off state, and when the camshaft 13 rotates back as shown by arrow R. , The one-way clutch 29 is brought into a connected state.

In addition, the large-diameter cylindrical portion 31 of the one-way clutch 29 has respective blind holes 35 extending from the inner side toward the outer side. Between the adjacent cam cuts 32, three blind holes 35 are respectively disposed. As shown in FIG. 3, a coil spring 36 and a ball 37 are put in each blind hole 35. As shown in FIG. 5, twelve oval recesses 39 are formed on the outer surface of the decompression cam 38 at equal intervals in the circumferential direction. The ball 37 pushed toward the decompression cam 38 by the coil spring 36 opposes the egg-shaped recess 39 and fits into the recess (FIG. 3). The blind hole 35, the coil spring 36 and the ball 37 and the egg-shaped recess 39 of the decompression cam 38 constitute the torque limiter 40 of the one-way clutch 29. 6A is a front view of the egg-shaped recess 39, and FIG. 6B is a sectional view thereof. The recessed part 39 is provided with the steep inclination part 39a and the gentle inclination part 39b.

As shown in FIG. 5, the decompression cam 38 has an engagement protrusion 41 projecting in the radial direction. As shown in FIG. 1, the cylinder head 3 is provided with a reverse rotation stopper 42 which engages with the engaging projection 41 to stop the decompression cam 38 when the decompression cam 38 reverses. do. The intake rocker arm 19 is formed with a forward rotation stopper 43 that engages with the engagement protrusion 41 to stop the pressure reduction cam 38 when the pressure reduction cam 38 rotates forward.

The outer circumferential surface of the small diameter cylindrical portion 30 of the one-way clutch 29 is provided with a plastic stopper ring 44 adjacent to the inner edge of the decompression cam 38. As shown in FIG. 2, the spark plug 45 is screwed into the cylinder head through the cylinder head such that the electrode 46 of the spark plug 45 protrudes into the combustion chamber 6.

In the embodiment configured as described above, if the crankshaft (not shown) is reversed by the starter motor when the single cylinder OHC four stroke cycle internal combustion engine 1 is stopped, the camshaft 13 also counterclockwise. To reverse.

When the cam shaft 13 rotates counterclockwise as shown in FIG. 4, the one-way clutch 29 also rotates counterclockwise with the cam shaft, and the blind hole 35 of the one-way clutch 29 is rotated. The ball 37, which is pressed by the spring force of the coil spring 36 provided on the shaft, engages with the steep inclined portion 39a of the egg-shaped recess 39 of the decompression cam 38 so that the torque limiter 40 rotates in the reverse direction. Generates a large transmission torque. Therefore, the reverse rotation torque is transmitted so that the decompression cam 38 reverses from the one-way clutch 29 to the decompression cam 38. Although the rotational resistance torque is imparted to the decompression cam 38 by the divided end 20a of the exhaust rocker arm 20 in contact with the cam peak 38a of the decompression cam 38, the decompression from the one-way clutch 29 is reduced. Since the reverse rotation torque transmitted to the cam 38 is large, the decompression cam 38 can rotate in the counterclockwise direction. Since the engaging projection 41 of the decompression cam 38 is stopped by the reverse rotation stopper 42 as shown in FIG. 1, the decompression cam 38 is stopped. In this state, the bisected end 20a of the exhaust rocker arm 20 contacts the cam peak 38a of the decompression cam 38, and the exhaust valve 10 is slightly opened.

If the crankshaft is rotated forward by the starter motor from the state of FIG. 1, the camshaft 13 also rotates forward clockwise, and the exhaust cam 28 also rotates in the same direction. However, the bisected end 20a of the exhaust rocker arm 20 contacts the cam peak 38a of the decompression cam 38 to add rotational resistance torque to the decompression cam 38, and the one-way clutch 29 Since the camshaft 13 is disconnected from the camshaft 13 to prevent torque transmission, the camshaft 13 remains in the stopped state shown in FIG. On the other hand, the exhaust cam and the intake cam rotate together with the cam shaft 13.

When the cam peak 28a of the exhaust cam 28 contacts the exhaust roller 26, the exhaust roller 26 starts to rise. Then, as shown in FIG. 8, the bisected end 20a of the exhaust roller 26 and the exhaust rocker arm 20 is raised, and the exhaust valve 10 is greatly opened. At the same time, since the divided end 20a of the exhaust rocker arm 20 is separated from the cam peak 38a of the decompression cam 38, the rotational resistance torque generated in the decompression cam 38 is eliminated. Further, the decompression cam 38 is rotated by a very small rotational torque obtained by friction between the camshaft 13 and the one-way clutch 29. That is, the decompression cam 38 rotates in conjunction with the cam shaft 13. Only when the bisected end 20a of the exhaust rocker arm 20 is separated from the cam peak 38a of the decompression cam 38, ie only by an angle corresponding to the cam angle of the exhaust cam 28 ), The decompression cam 38 rotates. Thereafter, the cam peak 28a of the exhaust cam 28 passes through the exhaust roller 26, and the divided roller 20 and the bisected end 20a of the exhaust rocker arm 20 are lowered, and the decompression cam ( 38 is stopped by the rotational resistance torque, and the crankshaft 13, the exhaust cam 28, and the intake cam 27 continue to rotate forward.

In addition, when the camshaft 13 rotates approximately once, the cam peak 28a of the exhaust cam 28 again contacts the exhaust roller 26 and pushes it up, thereby dividing the end 20a of the exhaust rocker arm 20. ) Is separated from the cam peak 38a of the decompression cam 38. At the same time, the decompression cam 38 rotates again with the camshaft 13 by the cam angle of the exhaust cam 28. When the camshaft 13 makes two or more turns, the divided end 20a of the exhaust rocker arm 20 is separated from the cam peak 38a of the decompression cam 38 to end the decompression operation. At the same time that normal operation is started, the engaging projection 41 of the decompression cam 38 is stopped by the forward rotation stopper 43, and the decompression cam 38 is kept in the inoperative state. The gentle inclined portion 39b of the egg-shaped recess 39 of the decompression cam 38 is formed to alleviate the impact when the decompression cam 38 is stopped by the forward rotation stopper 43.

11 is a graph showing the relationship between the crank angle and the cam lift for the intake cam, the exhaust cam and the decompression cam. In Fig. 11, IN is a lift of the intake cam, EX is a lift of the exhaust cam and DC is a lift of the decompression cam. The exhaust valve is opened by the influence of the larger one of the lifts of the exhaust cam and the decompression cam. After the forward rotation is started, the exhaust valve is opened along the curve of A-B-C-D-E-F-G-H-I-J. In the decompression cam, the above mentioned continuous rotation proceeds to some extent, so that the lift becomes zero when the cam peak is released from the divided end of the exhaust rocker arm.

As mentioned above, in the embodiment shown in Figs. 1 to 11, weights having a large moment of inertia are not installed in the starting system, and the crankshaft is rotated at a relatively small rotational speed by manual rotation. Therefore, the start of the internal combustion engine is greatly improved, and the output of the starter motor is reduced, so that the starter motor can be miniaturized and reduced in weight.

12-14 illustrate another embodiment of the present invention. This embodiment differs from the above-mentioned embodiment only in the shapes of the one-way clutch and the decompression cam. In other words, since both embodiments have the same structure, the same code | symbol is attached | subjected to the same part.

It is an enlarged view of the principal part of embodiment. As shown in FIG. 12, the one-way clutch 50 located outside the exhaust cam 28 is provided on the camshaft 13, and the decompression cam 58 is the outer circumferential portion 51 of the one-way clutch to rotate. To be equipped.

FIG. 13 is a cross-sectional view of XIII-XIII of FIG. 12, and FIG. 14 is a cross-sectional view of XIV-XIV of FIG. 12. As shown in FIG. 13, three cam cutouts 52 are formed at equal intervals in the circumferential direction on the inner surface of the outer half of the one-way clutch 50. Each cam cut portion 52 is loosely provided with a cam roller 53 so that a coil spring 54 is inserted between the cam cut portion 52 and the cam roller 53. When the camshaft 13 rotates forward, the one-way clutch 50 is in a shut-off state, and when the camshaft 13 reverses rotation, the one-way clutch 50 is in a connected state. This configuration is the same as that of the above-mentioned embodiment.                 

12 and 14, twelve blind holes 55 opened outwardly are formed in the outer circumferential portion 51 of the one-way clutch 50 at equal intervals. A coil spring 56 and a ball 57 are put into each blind hole 55. On the inner circumferential surface of the decompression cam 58, twelve egg-shaped recesses 59 are formed at equal intervals in the circumferential direction. 15 is an exploded view of a portion of the inner surface of the decompression cam. The recessed part 59 has the same steep inclination part 59a and gentle inclination part 59b as the above-mentioned embodiment. The blind hole 55 of the one-way clutch 50, the coil spring 56, and the egg-shaped recess 59 of the ball 57 and the decompression cam 58 constitute the torque limiter 60.

The decompression cam 58 has the same cam peak 58a and the engaging projection 61 as in the above-mentioned embodiment. The engaging projection 61 is stopped by the reverse rotation stopper 42 of the cylinder head 3 when the decompression cam 58 rotates in reverse, and the intake rocker arm 19 when the decompression cam 58 rotates forward. ) Is stopped by the forward rotation stopper 43. A plastic stopper ring 62 is provided in an annular recess formed in the outer circumferential surface of the one-way clutch 50 adjacent to the edge of the decompression cam 58.

This embodiment shows that a torque limiter can be formed between the outer circumferential surface of the one-way clutch and the inner circumferential surface of the decompression cam. When the camshaft reverses, the decompression cam also reverses. When the camshaft rotates forward and the bisected end 20a of the exhaust rocker arm is separated from the cam peak 58a of the decompression cam, the engagement of a very small accompanying rotating torque and torque limiter generated between the camshaft and the one-way clutch Due to this, the decompression cam rotates forward. This action and the action of the parts not shown are the same as those of the above-mentioned embodiment, and thus detailed description thereof will be omitted.

According to the present invention, since the centrifugal force is not used, the inertia of the valve movement system becomes small, and the starter motor can be miniaturized. Since the conversion from the starting operating state to the normal operating state is carried out mechanically without using electronic control, a control system of a complicated internal combustion engine can be avoided, and cost reduction is possible.

In addition, the torque limiter is simply formed, so that downsizing and weight reduction of the pressure reducing device can be achieved.

Claims (8)

In a four-stroke cycle internal combustion engine having an intake valve and an exhaust valve which are opened and closed by an intake cam and an exhaust cam integral with a cam shaft, respectively, and started by a starter motor, A one-way clutch is installed on the camshaft so as to be relatively rotatably inserted, and a one-way clutch is adjacent to the pressure reducing cam and capable of transmitting a reverse rotation torque to the decompression cam only when the camshaft reverses. Installed, Between the decompression cam and the one-way clutch is interposed a torque limiter for releasing the transmission of the reverse rotation torque of a predetermined torque or more, A decompression cam stopper portion which is engaged with the engaging portion formed in the decompression cam and engages and stops the decompression cam in two forward and reverse directions of the decompression cam, is provided in the fixed portion, In the state where the lift of the cam portion of the decompression cam is set higher than the base circle of the exhaust cam and lower than the maximum lift of the cam portion of the exhaust cam, in the state where the decompression cam stopper portion and the engaging portion of the camshaft forward rotation are engaged with and stopped. And said exhaust valve is not opened by a pressure reducing cam. The method according to claim 1, The torque limiter is formed so that the transmission torque at the time of forward rotation of the camshaft is small and the transmission torque at the time of reverse rotation of the camshaft is increased. The method according to claim 1 or 2, The torque limiter includes a contact member disposed on either side of the adjacent side of the one-way clutch and the decompression cam, a recess disposed on the other side of the adjacent side, and a spring for pressing the contact member to the recess. And The concave portion is formed in a shape such that the transmission torque at the time of forward rotation of the camshaft is small and the transmission torque at the reverse rotation of the camshaft is increased. The method according to claim 1 or 2, The torque limiter includes a contact member disposed on either of the one-way clutch and the adjacent inner and outer circumferential surfaces of the decompression cam, a recess disposed on the other side of the adjacent inner and outer circumferential surfaces, and the contact member is pressed against the recess. With spring to do, The concave portion is formed in a shape such that the transmission torque at the time of forward rotation of the camshaft is small and the transmission torque at the time of reverse rotation of the camshaft is increased. In a four-stroke cycle internal combustion engine having an intake valve and an exhaust valve which are opened and closed by an intake cam and an exhaust cam integral with a cam shaft, respectively, and started by a starter motor, A one-way clutch is installed on the camshaft so as to be relatively rotatably inserted, and a one-way clutch is adjacent to the pressure reducing cam and capable of transmitting a reverse rotation torque to the decompression cam only when the camshaft reverses. Installed, Between the decompression cam and the one-way clutch, a torque limiter for releasing the transmission of reverse rotation torque equal to or greater than a predetermined torque is interposed. A decompression cam stopper portion which is engaged with the engaging portion formed in the decompression cam and engages and stops the decompression cam in two forward and backward rotation directions of the decompression cam, is provided in the fixed portion, The cam portion of the decompression cam is lifted higher than the base circle of the intake cam and lower than the maximum lift of the cam portion of the intake cam, and in the state where the decompression cam stopper portion and the engaging portion stop at the cam shaft forward rotation, the stop is engaged. And said intake valve is not opened by a decompression cam. The method according to claim 5, The torque limiter is formed so that the transmission torque at the time of forward rotation of the camshaft is small and the transmission torque at the time of reverse rotation of the camshaft is increased. The method according to claim 5, The torque limiter includes a contact member disposed on either side of the adjacent side of the one-way clutch and the decompression cam, a recess disposed on the other side of the adjacent side, and a spring for pressing the contact member to the recess. And The concave portion is formed in a shape such that the transmission torque at the time of forward rotation of the camshaft is small and the transmission torque at the time of reverse rotation of the camshaft is increased. The method according to claim 5, The torque limiter includes a contact member disposed on one of the adjacent inner and outer peripheral surfaces of the one-way clutch and the decompression cam, a recess disposed on the other side of the adjacent inner and outer peripheral surfaces, and press the contact member to the recess. With spring to do, The concave portion is formed in a shape such that the transmission torque at the time of forward rotation of the camshaft is small and the transmission torque at the time of reverse rotation of the camshaft is increased.

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