EP3363999A1

EP3363999A1 - Engine

Info

Publication number: EP3363999A1
Application number: EP16831610.7A
Authority: EP
Inventors: Masayuki Aoyama
Original assignee: Yamaha Motor Co Ltd
Current assignee: Yamaha Motor Co Ltd
Priority date: 2015-10-15
Filing date: 2017-04-20
Publication date: 2018-08-22
Anticipated expiration: 2036-10-12
Also published as: EP3363999A4; EP3363999B1; BR112017003899A2; BR112017003899B1; TW201814146A; TWI652403B; EP3156618B1; TWI626366B; ES2675287T3; TW201713846A; EP3156618A1; WO2017065168A1; JP2018197498A

Abstract

A first rocker arm includes a roller and a pressing member. The pressing member presses a valve. A second rocker arm includes a slipper. When the engine rotation speed is in a predetermined low-speed region, the pressing member presses the valve according to the rotation of the first rocker arm. When the engine rotation speed is in a predetermined high-speed region, the pressing member presses the valve according to the rotation of the second rocker arm. The roller comes into rolling contact with the camshaft. The slipper comes into sliding contact with the camshaft. The tip end of the slipper is closer to the axis of the rocker shaft than the tip end of the roller as seen from the axial direction of the rocker shaft. The maximum width of the slipper is greater than the width of the roller in the axial direction of the rocker shaft.

Description

Field of the Invention

The present invention relates to an engine.

Description of the Related Art

An engine is provided with a variable valve mechanism. The variable valve mechanism includes a low-speed rocker arm used in a low-speed region of the engine rotation speed, and a high-speed rocker arm used in a high-speed region of the engine rotation speed.
For example, the low-speed rocker arm and the high-speed rocker arm are attached to a rocker shaft side-by-side in the axial direction of the rocker shaft. The low-speed rocker arm includes a first roller which rolls and comes into contact with a low-speed cam of a camshaft. The high-speed rocker arm includes a second roller which rolls and comes into contact with a high-speed cam of the camshaft.
The low-speed rocker arm is driven in the low-speed region of the engine rotation speed by the low-speed cam thereby opening and closing a valve. The low-speed rocker arm and the high-speed rocker arm are coupled in the high-speed region of the engine rotation speed. Specifically, a coupling pin inserted into a hole in the low-speed rocker arm is moved by an actuator and is inserted into a hole in the high-speed rocker arm. As a result, the low-speed rocker arm and the high-speed rocker arm are coupled together. In this state, the low-speed rocker arm is not driven by the low-speed cam and the high-speed rocker arm is driven by the high-speed cam thereby opening and closing a valve.

Prior Art Documents

References

Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2015-010554

Summary of the Invention

Technical Problem

Rollers are used in the low-speed rocker arm and the high-speed rocker arm and therefore the weight is increased in the abovementioned engine. As a result, the effect of the equivalent inertia weight of the rocker arms is increased when the engine rotation speed is in the high-speed region. For example, the behavior of the rocker arms in the high-speed region is affected when the equivalent inertia weight of the rocker arms is large. As a result, there is a problem that the upper limit of the engine rotation speed needs to be lowered in order to suppress any disturbance in the behavior of the rocker arm.
Accordingly, the inventor of the present application considered changing both of the rollers in the low-speed rocker arm and the high-speed rocker arm to slippers in order to reduce the equivalent inertia weight. By using slippers, the weight of the rocker arms can be reduced more in comparison to a case when rollers are used, thereby reducing the equivalent inertia weight.
However, an oil film produced on the contact surfaces of the slippers becomes a thin boundary lubrication because the frictional speed of the slippers with respect to the cam is low in the low-speed region of the engine rotation speed. As a result, there is a problem that the frictional resistance between the slippers and the cam increases and the mechanical loss of the engine increases when a slipper is used in the low-speed rocker arm.
Conversely, the frictional speed of the slipper with respect to the cam is greater in the high-speed region of the engine rotation speed and therefore the mechanical loss is relatively low. However, there is a problem that the allowable surface pressure of the slipper is low in comparison to the roller. In particular, it is difficult to increase the stiffness of the two rocker arms because the rocker arms are coupled each other in the high-speed region of the engine rotation speed as discussed above. As a result, partial contact between the slipper and the cam may occur due to the deformation of the rocker arms. Localized surface pressure on the slipper increases when partial contact occurs. This type of partial contact occurs more easily in the high-speed region than in the low-speed region of the engine rotation speed. Specifically, there is a problem that the partial contact must also be considered along with the low allowable surface pressure of the slipper in the high-speed region of the engine rotation speed.
Increasing the curvature radius of the contact surface of the slipper may be considered as one means for reducing the surface pressure. However, the length of the slipper increases if the curvature radius is increased. Moreover, the length of the arm section for supporting the slipper also tends to increase if the length of the slipper is increased. The equivalent inertia weight of the rocker arm may increase due to the aforementioned factors and thus the effect of reducing the equivalent inertia weight is limited even when a slipper is used in place of a roller.
An object of the present invention is to reduce mechanical loss in the low-speed region of the engine rotation speed and increase the upper limit of the engine rotation speed.

Solution to Problem

An engine according to a first aspect includes a cylinder head, a valve, a rocker unit, a camshaft, and an open/close timing changing unit. The valve is attached to the cylinder head. The rocker unit presses the valve and opens and closes the valve. The camshaft drives the rocker unit. The open/close timing changing unit changes the opening and closing timing of the valve.
The rocker unit includes a rocker shaft, a first rocker arm, a second rocker arm, and a coupling pin. The rocker shaft is supported by the cylinder head. The first rocker arm includes a roller and a pressing member. The roller is provided in a manner to allow contact with the camshaft. The pressing member presses the valve. The first rocker arm rotates around the axis of the rocker shaft due to the roller coming into contact with the camshaft. A second rocker arm includes a slipper. The slipper is arranged to be contact with the camshaft. The second rocker arm is aligned with the first rocker arm in the axial direction of the rocker shaft. The second rocker arm rotates around the axis of the rocker shaft due to the slipper coming into contact with the camshaft. The coupling pin is configured to move between a coupling position and a release position due to the open/close timing changing unit. The coupling pin couples the second rocker arm to the pressing member in the coupling position. The coupling pin releases the second rocker arm from the pressing member in the release position.
When the engine rotation speed is in a predetermined low-speed region, the open/close timing changing unit positions the coupling pin in the release position whereby the pressing member presses the valve according to the rotation of the first rocker arm. When the engine rotation speed is in a predetermined high-speed region, the open/close timing changing unit positions the coupling pin in the coupling position whereby the pressing member presses the valve according to the rotation of the second rocker arm.
The roller comes into rolling contact with the camshaft. The slipper comes into sliding contact with the camshaft. The tip end of the slipper is closer to the axis of the rocker shaft than the tip end of the roller as seen from the axial direction of the rocker shaft. The maximum width of the slipper is greater than the width of the roller in the axial direction of the rocker shaft.
The roller is used in the first rocker arm for low speeds in the engine according to the present aspect. As a result, the frictional resistance between the roller and the camshaft can be reduced in the low-speed region of the engine rotation speed. The mechanical loss can thereby be reduced in the low-speed region.
Furthermore, the slipper is used in the second rocker arm for high speeds. As a result, the equivalent inertia weight of the second rocker arm can be reduced. Moreover, because the frictional speed of the slipper with respect to the camshaft in the high-speed region of the engine rotation speed is high, a thick oil film can be produced on the contact surface of the slipper. As a result, mechanical loss can be reduced even when the slipper is used in the second rocker arm for high speeds. By using the roller in the first rocker arm for low speeds and the slipper in the second rocker arm for high speeds in this way, the equivalent inertia weight can be reduced while limiting mechanical loss in all regions of the engine rotation speed.
Furthermore, the maximum width of the slipper is greater than the width of the roller in the axial direction of the rocker shaft. As a result, the surface pressure of the slipper can be limited and the generation of partial contact can be suppressed. Moreover, the tip end of the slipper is closer to the axis of the rocker shaft than the tip end of the roller as seen from the axial direction of the rocker shaft. That is, the surface pressure of the slipper can be reduced by making the maximum width of the slipper greater than the width of the roller, whereby the need to increase the curvature radius in order to reduce the surface pressure is reduced. As a result, a slipper with a shorter configuration is possible. Consequently, an increase in the equivalent inertia weight can be suppressed in comparison to when the length of the slipper is increased and the curvature radius of the slipper is increased. As a result, the upper limit of the engine rotation speed can be increased.
The slipper may include a curved contact surface for contact with the camshaft. The curvature radius of the curved surface may be greater than the curvature radius of the roller. In this case, the surface pressure of the slipper can be reduced.
The center of gravity of the second rocker arm is closer to the axis of the rocker shaft than the center of gravity of the first rocker arm. In this case, the equivalent inertia weight of the second rocker arm can be reduced even further.
The weight of a portion of the second rocker arm positioned further on the tip end side of the slipper than an imaginary plane including the axis of the camshaft and extending in the cylinder axial direction of the cylinder head may be less than the weight of a portion of the first rocker arm positioned further on the tip end side of the roller than the imaginary plane. In this case, the equivalent inertia weight of the second rocker arm can be reduced even further.
The second rocker arm may include a boss portion and an arm portion. The boss portion may include a hole through which the rocker shaft passes. The arm portion may extend from the boss portion to the slipper. The slipper may include a contact surface for contact with the camshaft.
The maximum width of the contact surface of the slipper may be less than the width of the boss portion in the axial direction of the rocker shaft. In this case, the weight of the slipper can be reduced while limiting the surface pressure of the slipper, and consequently the equivalent inertia weight of the second rocker arm can be further reduced.
The arm portion may include a recessed portion positioned between the contact surface and the boss portion. In this case, the weight can be further reduced than if the contact surface continued as far as the boss portion and the equivalent inertia weight of the second rocker arm can be reduced even further. Moreover, interference with a jig for machining when machining the contact surface can be avoided due to the recessed portion.
The arm portion includes a protruding portion that extends from the slipper up to the boss portion and protrudes from the surface opposite the contact surface of the slipper. In this case, the stiffness of the arm portion can be assured by the protruding portion while reducing the weight of the arm portion.
The width of the protruding portion may be less than the width of the contact surface in the axial direction of the rocker shaft. In this case, the weight of the arm portion can be reduced and consequently the equivalent inertia weight of the second rocker arm can be further reduced.
The surface of the arm portion opposite the contact surface may have a shape that is recessed toward the contact surface as seen from the axial direction of the rocker shaft. In this case, the weight of the arm portion can be reduced and consequently the equivalent inertia weight of the second rocker arm can be further reduced.
The slipper may include a hardened layer. The hardened layer may come into contact with the camshaft and may have a coefficient of friction less than that of the base material of the slipper and may have a hardness greater than that of the base material of the slipper. In this case, the abrasion resistance of the slipper can be improved.

Effects of Invention

According to the present invention, mechanical loss in the low-speed region of the engine rotation speed is reduced and the upper limit of the engine rotation speed can be increased.

Brief Description of Drawings

FIG. 1 is a side view of the straddle-type vehicle according to an embodiment.
FIG. 2 is a cross-sectional view of a portion of an engine for a straddle-type vehicle according to the embodiment.
FIG. 3 is a cross-sectional view of a cylinder head and a head cover as seen from a direction perpendicular to the cylinder axis and the cam axis.
FIG. 4 is a perspective view of the inside of the cylinder head.
FIG. 5 is a perspective view of the inside of the cylinder head.
FIG. 6 is a view of the inside of the cylinder head as seen from the cylinder axial direction.
FIG. 7 is a cross-sectional view of the inside of the cylinder head as seen from the cam axial direction.
FIG. 8 is a perspective view of an intake rocker unit.
FIG. 9 is a view of the intake rocker unit as seen from the direction perpendicular to the cam axis.
FIG. 10 is a view of the intake rocker unit as seen from the cam axial direction.
FIG. 11 is a view of the second rocker arm as seen from below.
FIG. 12 is a view of the second rocker arm as seen from the cam axial direction.
FIG. 13 is a cross-sectional view in the vicinity of a second shaft supporting portion and an arm urging member.
FIG. 14 is a cross-sectional view of the inside of the cylinder head as seen from the cam axial direction.
FIG. 15 is a perspective view of the intake rocker shaft.
FIG. 16 illustrates changes in loss torque with respect to the engine rotation speed when a roller is used and when a slipper is used in the rocker arm.
FIG. 17 is a view of the intake rocker unit as seen from the cylinder axial direction according to a first modified example.
FIG. 18 is a view of the intake rocker unit as seen from the cylinder axial direction according to a second modified example.
FIG. 19 is a view of the inside of the cylinder head according to a third modified example as seen from the cylinder axial direction.

Description of Embodiment

The following is an explanation of a straddle-type vehicle and an engine for a straddle-type vehicle according to an embodiment with reference to the drawings. FIG. 1 is a side view of a straddle-type vehicle 100. The straddle-type vehicle 100 is a so-called scooter-type motorcycle. As illustrated in FIG. 1, the straddle-type vehicle 100 includes a front wheel 101, a seat 102, a rear wheel 103, a power unit 104, a steering device 105, and a vehicle body cover 106.
The front wheel 101 is rotatably supported on the steering device 105. A handle 113 is attached to the upper end of the steering device 105. The seat 102 is disposed to the rear of the steering device 105. The power unit 104 is disposed below the seat 102. The power unit 104 includes an engine 1 and a transmission 107. The power unit 104 rotatably supports the rear wheel 103.
The vehicle body cover 106 includes a rear cover 108, a lower cover 109, and a front cover 110. The rear cover 108 is disposed under the seat 102. The front cover 110 covers the vicinity of the steering device 105. The lower cover 109 is disposed between the front cover 110 and the rear cover 108. The upper surface of the lower cover 109 includes a foot board 111 and a tunnel portion 112.
The tunnel part 112 is arranged in the middle portion in the vehicle width direction on the upper surface of the lower cover 109. The tunnel portion 112 protrudes upward higher than the foot board 111. The foot board 111 is disposed on the right and left of the tunnel portion 112. The foot board 111 is provided for a rider to place his or her feet. The tunnel portion 112 may be omitted. That is, the upper surface of the lower cover 109 may have a flat foot board that extends in the left-right direction.
FIG. 2 is a cross-sectional view of a portion of an engine 1 for a straddle-type vehicle according to the embodiment. The engine 1 according to the present embodiment in a water-cooling type single-cylinder engine. As illustrated in FIG. 2, the engine 1 includes a crankcase 2, a cylinder body 3, a cylinder head 4, and a head cover 5.
The crankcase 2 houses a crankshaft 6. The cylinder body 3 is connected to the crankcase 2. The cylinder body 3 may be integrated with the crankcase 2 or may be a separate body. The cylinder body 3 houses a piston 7. The piston 7 is coupled to the crankshaft 6 via a connecting rod 8.
The direction from the cylinder head 4 toward the head cover 5 in the cylinder axis Ax1 direction of the cylinder body 3 is referred to as the "head cover side" in the present embodiment. Moreover, the direction from the cylinder head 4 to the cylinder body 3 in the cylinder axis Ax1 direction is referred to as the "cylinder body side."
The cylinder head 4 is disposed on the head cover side of the cylinder body 3. The cylinder head 4 is attached to the cylinder body 3. The head cover 5 is disposed on the head cover side of the cylinder head 4. The head cover 5 is attached to the cylinder head 4.
FIG. 3 is a cross-sectional view of the cylinder head 4 and the head cover 5 as seen from a direction perpendicular to the cylinder axis Ax1 and a cam axis Ax3. As illustrated in FIG. 3, the cylinder head 4 includes a side wall 4a that extends in the cylinder axis Ax1 direction. The head cover 5 includes a side wall 5a that extends in the cylinder axis Ax1 direction. An end portion 4b (referred to hereinbelow as "side wall end 4b") of the side wall 4a of the cylinder head 4 is disposed face to face with an end portion 5b (referred to hereinbelow as "side wall end 5b") of the side wall 5a of the head cover 5. Specifically, the side wall end 4b of the cylinder head 4 is disposed face to face with the side wall end 5b of the head cover 5 with a seal member 9 disposed therebetween. The cylinder head 4 may be integrated with the cylinder body 3 or may be a separate body.
As illustrated in FIG. 2, the cylinder axis Ax1 is perpendicular to a central axis Ax2 (referred to hereinbelow as "crankshaft axis Ax2") of the crankshaft 6. The cylinder head 4 includes a combustion chamber 11. A spark plug 12 is attached to the cylinder head 4. The tip end portion of the spark plug 12 is disposed so as to face the combustion chamber 11. The base end portion of the spark plug 12 is disposed outside of the engine 1. A valve mechanism 13 is housed in the cylinder head 4 and the head cover 5.
The valve mechanism 13 is a mechanism for opening and closing belowmentioned exhaust valves 25 and 26 and intake valves 27 and 28. A single overhead camshaft (SOHC) mechanism is used in the valve mechanism 13. A so-called variable valve mechanism which switches the timing for opening and closing the intake valves 27 and 28 is used in the valve mechanism 13.
The valve mechanism 13 includes a camshaft 14. The camshaft 14 is supported on the cylinder head 4. The center axis Ax3 (referred to hereinbelow as "cam axis Ax3") of the camshaft 14 runs perpendicular to the cylinder axis Ax1. The cam axis Ax3 runs parallel to the crankshaft axis Ax2.
As illustrated in FIG. 3, the camshaft 14 includes a first camshaft end portion 141 and a second camshaft end portion 142.
A sprocket 29 is attached to the first camshaft end portion 141. A cam chain 15 illustrated in FIG. 2 is wound onto the sprocket 29. As illustrated in FIG. 2, a cam chain chamber 16 is provided in the cylinder head 4 and the cylinder body 3. The cam chain 15 is disposed in the cam chain chamber 16. The camshaft 14 is coupled to the crankshaft 6 via the cam chain 15. The rotation of the crankshaft 6 is transferred through the cam chain 15 to the camshaft 14 thereby rotating the camshaft 14.
A water pump 17 is coupled to the first camshaft end portion 141. The water pump 17 is connected to a liquid coolant passageway (not illustrated) and a radiator 19 in the engine 1 through a liquid coolant hose 18. The water pump 17 is driven by the rotation of the camshaft 14 thereby circulating a liquid coolant for the engine 1.
As illustrated in FIG. 3, the camshaft 14 includes a rod portion 143, a first intake cam portion 144, a second intake cam portion 145, and an exhaust cam 146. The rod portion 143 is rotatably supported by a first shaft support portion 21 and a second shaft support portion 22 of the cylinder head 4. The first intake cam portion 144, the second intake cam portion 145, and the exhaust cam 146 are disposed on the outer circumference of the rod portion 143. The first intake cam portion 144, the second intake cam portion 145, and the exhaust cam 146 are aligned in the cam axis Ax3 direction.
FIGS. 4 and 5 are perspective views of the inside of the cylinder head 4. FIG. 6 is a view of the inside of the cylinder head 4 as seen from the cylinder axis Ax1 direction. As illustrated in FIGS. 3 to 6, the cylinder head 4 includes the first shaft support portion 21 and the second shaft support portion 22. The first shaft support portion 21 and the second shaft support portion 22 are formed integrally with the cylinder head 4. The first shaft support portion 21 and the second shaft support portion 22 are aligned in the cam axis Ax3.
The first shaft support portion 21 and the second shaft support portion 22 rotatably support the camshaft 14. As illustrated in FIG. 3, the first shaft support portion 21 includes a first camshaft hole 211 into which the camshaft 14 is inserted. A first shaft bearing 23 is attached to the first camshaft hole 211. The first shaft support portion 21 supports the camshaft 14 via the first shaft bearing 23. The second shaft support portion 22 includes a second camshaft hole 221 into which the camshaft 14 is inserted. A second shaft bearing 24 is attached to the second camshaft hole 221. The second shaft support portion 22 supports the camshaft 14 via the second shaft bearing 24.
An end portion 21a on the head cover side of the first shaft support portion 21 is positioned further to the head cover side than the side wall end 4b of the cylinder head 4. That is, the first shaft support portion 21 protrudes further to the head cover side than the side wall end 4b of the cylinder head 4. An end portion 22a on the head cover side of the second shaft support portion 22 is positioned further to the head cover side than the side wall end 4b of the cylinder head 4. That is, the second shaft support portion 22 protrudes further to the head cover side than the side wall end 4b of the cylinder head 4.
As illustrated in FIG. 6, the intake valves 27 and 28 and the exhaust valves 25 and 26 are attached to the cylinder head 4. FIG. 7 is a cross-sectional view of the inside of the cylinder head 4 as seen from the cam axis Ax3 direction. As illustrated in FIG. 7, the cylinder head 4 includes an intake port 31 and an exhaust port 32 that communicate with the combustion chamber 11.
The intake valves 27 and 28 open and close the intake port 31. As illustrated in FIG. 6, the intake valves 27 and 28 include a first intake valve 27 and a second intake valve 28. The first intake valve 27 and the second intake valve 28 are aligned in the cam axis Ax3 direction.
As illustrated in FIG. 4, an intake valve spring 271 is attached to the first intake valve 27. The intake valve spring 271 urges the first intake valve 27 in the direction in which the first intake valve 27 closes the intake port 31. An intake valve spring 281 is also attached to the second intake valve 28 in the same way and urges the second intake valve 28 in the direction in which the second intake valve 28 closes the intake port 31.
The exhaust valves 25 and 26 open and close the exhaust port 32. The exhaust valves 25 and 26 includes a first exhaust valve 25 and a second exhaust valve 26. The first exhaust valve 25 and the second exhaust valve 26 are aligned in the cam axis Ax3 direction.
As illustrated in FIG. 5, an exhaust valve spring 251 is attached to the first exhaust valve 25. The exhaust valve spring 251 urges the first exhaust valve 25 in the direction in which the first exhaust valve 25 closes the exhaust port 32. An exhaust valve spring 261 is also attached to the second exhaust valve 26 in the same way and urges the second exhaust valve 26 in the direction in which the second exhaust valve 26 closes the exhaust port 32.
As illustrated in FIG. 7, the valve mechanism 13 includes an exhaust rocker unit 33 and an intake rocker unit 34. The exhaust rocker unit 33 presses the exhaust valves 25 and 26 and opens and closes the exhaust valves 25 and 26. The intake rocker unit 34 presses the intake valves 27 and 28 and opens and closes the intake valves 27 and 28. The exhaust rocker unit 33 and the intake rocker unit 34 are driven by the camshaft 14.
The exhaust rocker unit 33 includes an exhaust rocker shaft 35, an exhaust rocker arm 36, and a pressing member 38. The exhaust rocker shaft 35 is disposed parallel to the camshaft 14. The exhaust rocker shaft 35 is supported on the cylinder head 4. Specifically, the exhaust rocker shaft 35 is supported on the first shaft support portion 21 and the second shaft support portion 22.
The exhaust rocker arm 36 is supported on the exhaust rocker shaft 35 in a swingable manner centered on the exhaust rocker shaft 35. The exhaust rocker arm 36 is configured to act on the exhaust valves 25 and 26. The exhaust rocker arm 36 includes a roller 37 and an arm portion 39.
The arm portion 39 includes a through-hole 364 and the exhaust rocker shaft 35 passes through the through-hole 364. As illustrated in FIG. 6, the arm portion 39 rotatably supports the roller 37. The rotational center axis of the roller 37 runs parallel to the cam axis Ax3. The roller 37 comes into contact with the exhaust cam 146 and rotates due to the rotation of the exhaust cam 146.
The pressing member 38 is formed integrally with the arm portion 39. As illustrated in FIGS. 5 and 6, a first adjuster screw 365 and a second adjuster screw 366 are provided on the tip end of the pressing member 38. The tip end of the first adjuster screw 365 faces the stem end of the first exhaust valve 25. As illustrated in FIG. 7, the tip end of the second adjuster screw 366 faces the stem end of the second exhaust valve 26.
When the roller 37 is lifted upward by the exhaust cam 146, the exhaust rocker arm 36 swings whereby the pressing member 38 presses the first exhaust valve 25 and the second exhaust valve 26 downward. As a result, the exhaust port 32 is opened. When the roller 37 is not pressed upward by the exhaust cam 146, the first exhaust valve 25 and the second exhaust valve 26 are respectively lifted upward by the exhaust valve springs 251 and 261 and the exhaust port 32 is closed.
FIG. 8 is a perspective view of the intake rocker unit 34. FIG. 9 is a view of the intake rocker unit 34 as seen from the direction perpendicular to the cam axis. FIG. 10 is a view of the intake rocker unit 34 as seen from the cam axial direction. As illustrated in FIGS. 8 to 10, the intake rocker unit 34 includes an intake rocker shaft 41, a first rocker arm 42, a second rocker arm 43, a pressing member 44 (see FIG. 6), and a coupling pin 45. While the intake rocker shaft 41 is omitted in FIG. 10, the position of the axis of the intake rocker shaft 41 is indicated by the reference numeral Ax4.
The intake rocker shaft 41 is disposed parallel to the camshaft 14. The intake rocker shaft 41 is supported on the cylinder head 4. Specifically, the intake rocker shaft 41 is supported on the first shaft support portion 21 and the second shaft support portion 22.
The first rocker arm 42 is supported on the intake rocker shaft 41 in a swingable manner centered on the intake rocker shaft 41. The first rocker arm 42 is configured to act on the intake valves 27 and 28. As illustrated in FIG. 3, the first rocker arm 42 includes a first attachment portion 421. The first attachment portion 421 is a hole provided in the first rocker arm 42. The intake rocker shaft 41 passes through the first attachment portion 421.
The first rocker arm 42 includes a first coupling hole 422. The first coupling hole 422 is positioned further to the head cover side than the intake rocker shaft 41. The first coupling hole 422 extends in the cam axis Ax3 direction. The coupling pin 45 is inserted into the first coupling hole 422.
As illustrated in FIG. 8, the first rocker arm 42 includes a first arm portion 420 and a roller 423. The roller 423 is arranged to come into contact with the first intake cam portion 144. The roller 423 is rotatably supported by the first arm portion 420. The roller 423 comes into rolling contact with the first intake cam portion 144. The roller 423 rotates due to the rotation of the first intake cam portion 144. The rotational center axis of the roller 423 runs parallel to the cam axis Ax3. The roller 423 comes into contact with the first intake cam portion 144 whereby the first rocker arm 42 rotates around the axis Ax4 of the intake rocker shaft 41.
As illustrated in FIG. 7, the second rocker arm 43 is supported in a swingable manner centered on the intake rocker shaft 41. The second rocker arm 43 is aligned with the first rocker arm 42 in the cam axis Ax3 direction. The second rocker arm 43 is disposed on the cam chain chamber 16 side of the first rocker arm 42. As illustrated in FIG. 3, the second rocker arm 43 includes a second attachment portion 431. The second attachment portion 431 is a hole provided in the second rocker arm 43. The intake rocker shaft 41 passes through the second attachment portion 431.
The second rocker arm 43 includes a second coupling hole 432. The second coupling hole 432 is positioned further to the head cover side than the intake rocker shaft 41. The second coupling hole 432 extends in the cam axis Ax3 direction. The second coupling hole 432 is disposed so as to overlap the first coupling hole 422 in the cam axis Ax3 direction. Therefore, the coupling pin 45 can be inserted into the second coupling hole 432 of the second rocker arm 43.
As illustrated in FIGS. 8 and 10, the second rocker arm 43 includes a boss portion 430, a slipper 433, and a second arm portion 434. The boss portion 430 includes the abovementioned second attachment portion 431. The second arm portion 434 extends from the boss portion 430 to the slipper 433. The second arm portion 434 supports the slipper 433. The slipper 433 comes into contact with the second intake cam portion 145 and is provided in a slidable manner with the second intake cam portion 145. The boss portion 430, the second arm portion 434, and the slipper 433 are formed integrally. The slipper 433 comes into slide contact with the second intake cam portion 145 whereby the second rocker arm 43 rotates around the axis Ax4 of the intake rocker shaft 41.
As illustrated in FIG. 9, the maximum width of the slipper 433 is greater than the width of the roller 423 in the axial direction of the intake rocker shaft 41. FIG. 11 is a view of the second rocker arm 43 in FIG. 10 as seen from below. As illustrated in FIGS. 10 and 11, the slipper 433 includes a contact surface 435 that comes into contact with the second intake cam portion 145. The maximum width of the contact surface 435 of the slipper 433 is less than the width of the boss portion 430 in the axial direction of the intake rocker shaft 41. The second arm portion 434 includes a protruding portion 439 that protrudes from the surface opposite the contact surface 435 in the slipper 433. The protruding portion 439 extends from the slipper 433 through the second arm portion 434 to the boss portion 430. The width of the protruding portion 439 is less than the width of the contact surface 435 in the axial direction of the intake rocker shaft 41.
As illustrated in FIG. 10, the second arm portion 434 includes a recessed portion 436. The recessed portion 436 is positioned between the contact surface 435 and the boss portion 430. More specifically, the recessed portion 436 has a shape that is recessed toward the head cover side from an imaginary plane Q that extends along the contact surface 435 as far as the boss portion 430. The recessed portion 436 has a shape that is recessed toward the head cover side from the contact surface 435 as seen in the axial direction of the intake rocker shaft 41. The recessed portion 436 has a shape that is curved in a circular arc. The recessed portion 436 extends in the axial direction of the intake rocker shaft 41. The tip end of the slipper 433 is closer to the axis Ax4 of the intake rocker shaft 41 than the tip end of the roller 423 as seen from the axial direction of the intake rocker shaft 41.
The contact surface 435 has a curved shape. The curvature radius of the contact surface 435 is greater than the curvature radius of the roller 423. As illustrated in FIG. 10, the contact surface 435 has a shape that is curved around a center of curvature C1. The center of curvature C1 extends in the axial direction of the intake rocker shaft 41. The center of curvature C1 is positioned on the head cover side with respect to the contact surface 435. The center of curvature C1 is positioned so as not to overlap the intake rocker shaft 41 as seen from the axial direction of the intake rocker shaft 41. The center of curvature C1 is positioned further to the head cover side than the axis Ax4 of the intake rocker shaft 41 as seen from the axial direction of the intake rocker shaft 41.
The surface 438 opposite the contact surface 435 of the second arm portion 434 has a shape that is recessed toward the contact surface 435 as seen from the axial direction of the intake rocker shaft 41. Specifically, the opposing surface 438 includes a first surface 438a and a second surface 438b. The first surface 438a extends in a direction that approximately follows the contact surface 435. The second surface 438b extends in a direction from the first surface 438a toward the head cover side. The opposing surface 438 has a curved shape between the first surface 438a and the second surface 438b.
The weight of a portion of the second rocker arm 43 positioned further on the tip end side of the slipper 433 than an imaginary plane P1, is less than the weight of a portion of the first rocker arm 42 positioned further on the tip end side of the roller 423 than the imaginary plane P1. The imaginary plane P1 extends in the cylinder axial direction and includes the axis Ax4 of the intake rocker shaft 41.
FIG. 12 is a view of the second rocker arm 43 as seen from the cam axial direction. G1 in FIG. 12 indicates the location of the center of gravity of the first rocker arm 42. G2 indicates the location of the center of gravity of the second rocker arm 43. As illustrated in FIG. 12, the center of gravity G2 of the second rocker arm 43 is closer to the axis Ax4 of the intake rocker shaft 41 than the center of gravity G1 of the first rocker arm 42.
The contact surface 435 of the slipper 433 includes a hardened layer 437 formed with a surface treatment. The hardened layer 437 has a coefficient of friction less than the base material of the slipper 433 and a hardness greater than the base material of the slipper 433. The coefficient of friction of the hardened layer 437 is lower than the coefficient of friction of a chromium nitride coating or of the surface of a sintered material. In other words, the hardened layer 437 has a high seize resistance. Specifically, the hardened layer 437 is preferably a carbon-based hard coating, or more specifically, is preferably a diamond-like carbon (DLC). DLC exhibits self-lubrication, which is a property of a graphite structure, and therefore has a low coefficient of friction and a high seize resistance. Moreover, DLC has a diamond structure and therefore has a higher maximum hardness and higher abrasion resistance than the chromium nitride coating. The base material is, for example, a chrome molybdenum steel.
As illustrated in FIG. 6, the pressing member 44 is connected to the first rocker arm 42. The pressing member 44 is formed integrally with the first rocker arm 42. A first adjuster screw 441 and a second adjuster screw 442 are provided on the tip end of the pressing member 44. The tip end of the first adjuster screw 441 faces the stem end of the first intake valve 27. The tip end of the second adjuster screw 442 faces the stem end of the second intake valve 28. The pressing member 44 rotates around the axial direction of the intake rocker shaft 41 and presses the first intake valve 27 and the second intake valve 28.
The intake rocker unit 34 includes an arm urging member 46, a first supporting member 47, and a second supporting member 48. The arm urging member 46 urges the second rocker arm 43 in the direction for pressing slipper 433 against the camshaft 14. The arm urging member 46 in the present embodiment is a coil spring and the intake rocker shaft 41 passes through the arm urging member 46.
The first supporting member 47 supports one end of the arm urging member 46. The first supporting member 47 has a pin-like shape and protrudes from the second rocker arm 43 in the cam axis Ax3 direction.
The second supporting member 48 supports the other end of the arm urging member 46. The second supporting member 48 is configured as a curved plate. FIG. 13 is a cross-sectional view in the vicinity of the second shaft support portion 22 and the arm urging member 46. As illustrated in FIG. 13, a step portion 222 is provided on the second shaft support portion 22, and the second supporting member 48 is supported on the step portion 222.
As illustrated in FIG. 3, the coupling pin 45 is movable in the axial direction of the camshaft 14 and is configured to move between a coupling position and a release position. The coupling pin 45 is disposed across the first coupling hole 422 and the second coupling hole 432 in the coupling position. As a result, the coupling pin 45 couples the first rocker arm 42 and the second rocker arm 43. That is, the coupling pin 45 couples the pressing member 44 to the second rocker arm 43 via the first rocker arm 42 in the coupling position. As a result, the pressing member 44 swings integrally with the first rocker arm 42 and the second rocker arm 43.
The coupling pin 45 is disposed in the first coupling hole 422 and is not disposed in the second coupling hole 432 of the second rocker arm 43 in the release position. As a result, the coupling pin 45 does not couple the first rocker arm 42 and the second rocker arm 43 in the release position. That is, the coupling pin 45 releases the second rocker arm 43 from the pressing member 44 in the release position. As a result, the pressing member 44 and the first rocker arm 42 swing independently of the second rocker arm 43.
The valve mechanism 13 includes an open/close timing changing unit 49. The open/close timing changing unit 49 changes the opening and closing timing of the first intake valve 27 and the second intake valve 28. The open/close timing changing unit 49 is attached to the head cover 5.
The open/close timing changing unit 49 is an electromagnetic solenoid and switches the position of the coupling pin 45 from the release position to the coupling position by pressing the coupling pin 45 in the axial direction of the camshaft 14 when the open/close timing changing unit 49 is energized. When the energization of the open/close timing changing unit 49 is stopped, the position of the coupling pin 45 is returned from the coupling position to the release position due to the elastic force of a belowmentioned pin urging member 59.
The open/close timing changing unit 49 includes a rod 491 for pressing the coupling pin 45, and a body portion 492 for driving the rod 491. The center axis of the rod 491 runs parallel to the cam axis Ax3. The rod 491 is disposed so as to overlap the coupling pin 45 as seen from the cam axis Ax3 direction in the swing range of the coupling pin 45. The rod 491 presses the coupling pin 45 due to being driven by the body portion 492.
As illustrated in FIG. 3, the intake rocker unit 34 includes the pin urging member 59. The pin urging member 59 is disposed inside the first coupling hole 422. The pin urging member 59 urges the coupling pin 45 from the coupling position toward the release position. Therefore, the coupling pin 45 is held in the release position by the pin urging member 59 when the coupling pin 45 is not pressed by the open/close timing changing unit 49. When the coupling pin 45 is pressed by the open/close timing changing unit 49, the coupling pin 45 resists the urging force of the pin urging member 59 and moves from the release position to the coupling position.
FIG. 14 illustrates the state while the slipper 433 is being pressed upward by the second intake cam portion 145 when the coupling pin 45 is positioned in the coupling position. When the coupling pin 45 is positioned in the coupling position, the first rocker arm 42 is coupled to the second rocker arm 43 and swings integrally with the second rocker arm 43. As a result, when the slipper 433 is pressed upward by the second intake cam portion 145, the second rocker arm 43 swings around the intake rocker shaft 41 whereby the first rocker arm 42 also swings in the direction for lowering the pressing member 44.
As a result, the tip end of the first adjuster screw 441 presses the first intake valve 27 down and the tip end of the second adjuster screw 442 presses the second intake valve 28 down. Consequently, the first intake valve 27 and the second intake valve 28 open the intake port 31. In this way, the pressing member 44 presses the first intake valve 27 and the second intake valve 28 according to the rotation of the second rocker arm 43 while the coupling pin 45 is in the coupling position. When the slipper 433 is not being lifted upward by the second intake cam portion 145, the first intake valve 27 and the second intake valve 28 are lifted upward respectively by the intake valve springs 271 and 281 and the intake port 31 is closed.
When the coupling pin 45 is positioned in the release position, the first rocker arm 42 swings independently of the second rocker arm 43. As a result, when the roller 423 is lifted upward by the first intake cam portion 144, the first rocker arm 42 swings around the intake rocker shaft 41 in the direction for lowering the pressing member 44.
As a result, the tip end of the first adjuster screw 441 presses the first intake valve 27 down and the tip end of the second adjuster screw 442 presses the second intake valve 28 down. Consequently, the first intake valve 27 and the second intake valve 28 open the intake port 31. In this way, the pressing member 44 presses the first intake valve 27 and the second intake valve 28 according to the rotation of the first rocker arm 42 while the coupling pin 45 is in the release position. When the roller 423 is not being pressed upward by the first intake cam portion 144, the first intake valve 27 and the second intake valve 28 are lifted upward respectively by the intake valve springs 271 and 281 and the intake port 31 is closed.
The shapes of the first intake cam portion 144 and the second intake cam portion 145 are set such that the second intake cam portion 145 lifts the slipper 433 upward before the tip end of the first intake cam portion 144 reaches the roller 423. As a result, when the coupling pin 45 is positioned in the coupling position, the first rocker arm 42 is acted upon due to the rotation of the second intake cam portion 145 whereby the rotation of the first intake cam portion 144 is not transmitted to the first rocker arm 42.
Therefore, when the coupling pin 45 is positioned in the coupling position, the first intake valve 27 and the second intake valve 28 are opened and closed in response to the rotation of the second intake cam portion 145. Conversely, when the coupling pin 45 is positioned in the release position, the rotation of the second intake cam portion 145 is not transmitted the first rocker arm 42. As a result, when the coupling pin 45 is positioned in the release position, the first intake valve 27 and the second intake valve 28 are opened and closed in response to the rotation of the first intake cam portion 144.
When the engine rotation speed is in the predetermined low-speed region, the open/close timing changing unit 49 positions the coupling pin 45 in the release position. For example, the open/close timing changing unit 49 positions the coupling pin 45 in the release position when the engine rotation speed is less than a predetermined switching threshold. As a result, the pressing member 44 presses the first intake valve 27 and the second intake valve 28 according to the rotation of the first rocker arm 42. Consequently, the first intake valve 27 and the second intake valve 28 are opened and closed in response to the rotation of the first intake cam portion 144.
When the engine rotation speed is in the predetermined high-speed region, the open/close timing changing unit 49 positions the coupling pin 45 in the coupling position. For example, the open/close timing changing unit 49 positions the coupling pin 45 in the coupling position when the engine rotation speed is equal to or greater than a predetermined switching threshold. As a result, the pressing member 44 presses the first intake valve 27 and the second intake valve 28 according to the rotation of the second rocker arm 43. Consequently, the first intake valve 27 and the second intake valve 28 are opened and closed in response to the rotation of the second intake cam portion 145.
The following is a detailed description of the structure of the intake rocker shaft 41. FIG. 15 is a perspective view of the intake rocker shaft 41. As illustrated in FIG. 15, the intake rocker shaft 41 includes a shaft member 51 and a collar member 52. The shaft member 51 and the collar member 52 are separate from each other. The collar member 52 has a tube-like shape. The shaft member 51 is inserted into a hole 521 of the collar member 52. The shaft member 51 is not fixed to the collar member 52. Therefore, the collar member 52 is able to rotate with respect to the shaft member 51.
The shaft member 51 includes a first end portion 511 and a second end portion 512. The first end portion 511 is one end portion in the axial direction of the intake rocker shaft 41. The second end portion 512 is the other end portion in the axial direction of the intake rocker shaft 41. The first end portion 511 protrudes one way from the collar member 52 in the axial direction of the intake rocker shaft 41. The second end portion 512 protrudes the other way from the collar member 52 in the axial direction of the intake rocker shaft 41.
As illustrated in FIG. 3, the first end portion 511 is supported on the first shaft support portion 21. The first shaft support portion 21 includes a first rocker shaft hole 212. The first rocker shaft hole 212 is disposed adjacent to the first camshaft hole 211. The first rocker shaft hole 212 penetrates the first shaft support portion 21 in the cam axis Ax3 direction. The first end portion 511 is inserted into the first rocker shaft hole 212. The end surface of the first end portion 511 is disposed facing toward the cam chain chamber 16.
The second end portion 512 is supported on the second shaft support portion 22. The second shaft support portion 22 includes a second rocker shaft hole 223. The second rocker shaft hole 223 is disposed adjacent to the second camshaft hole 221. The second rocker shaft hole 223 does not penetrate the second shaft support portion 22. However, the second rocker shaft hole 223 may penetrate the second shaft support portion 22. The second end portion 512 is inserted into the second rocker shaft hole 223.
As illustrated in FIG. 8, a boundary B between the first coupling hole 422 of the first rocker arm 42 and the second coupling hole 432 of the second rocker arm 43 is closer to the second end portion 512 than an intermediate position M of the interval L between the first end portion 511 and the second end portion 512. More specifically, a distance L2 from the boundary B to the second end portion 512 is less than a distance L1 from the boundary B to the first end portion 511 (L2 < L1).
As illustrated in FIG. 15, a locking groove 513 is provided on the end surface of the first end portion 511. A tool is locked to the locking groove 513 whereby the shaft member 51 can be attached or removed to and from the first rocker shaft hole 212.
A locking hole 514 is formed in the second end portion 512. The locking hole 514 penetrates the second end portion 512 in the direction perpendicular to the axis of the shaft member 51. As illustrated in FIG. 5, a hole 224 which extends perpendicular to the axial direction of the second rocker shaft hole 223 is provided in the second shaft support portion 22. The hole 224 opens on the upper surface of the second shaft support portion 22. A fastening member 53 illustrated in FIG. 6 is inserted into the hole 224 of the second shaft support portion 22 and the locking hole 514 of the second end portion 512 whereby the shaft member 51 is locked to the second shaft support portion 22.
The collar member 52 is separate from the shaft member 51. The collar member 52 is disposed between the first end portion 511 and the second end portion 512 in the axial direction of the intake rocker shaft 41. The collar member 52 is disposed between the first shaft support portion 21 and the second shaft support portion 22. The first rocker arm 42 and the second rocker arm 43 are attached to the collar member 52. That is, the first attachment portion 421 of the first rocker arm 42 and the second attachment portion 431 of the second rocker arm 43 are inserted into the collar member 52. The arm urging member 46 and the second supporting member 48 are also attached to the collar member 52.
The outer diameter of the collar member 52 is larger than the outer diameter of the shaft member 51. The outer diameter of the collar member 52 is larger than the outer diameter of the exhaust rocker shaft 35. The outer diameter of the collar member 52 is larger than the outer diameter of the first end portion 511 and larger than the outer diameter of the second end portion 512. The inner diameter of the first rocker shaft hole 212 is smaller than the outer diameter of the collar member 52. The inner diameter of the second rocker shaft hole 223 is smaller than the outer diameter of the collar member 52.
The roller 423 is used in the first rocker arm 42 for low speeds in the engine 1 according to the present embodiment as explained above. Furthermore, the slipper 433 is used in the second rocker arm 43 for high speeds. The weight of the slipper 433 and the portion supporting the slipper 433 is less than the weight of the roller 423 and the portion supporting the roller 423. Consequently, the equivalent inertia weight of the second rocker arm 43 can be reduced. Therefore, by using the slipper 433 in the second rocker arm 43 for high speeds, the effect of the equivalent inertia weight of the second rocker arm 43 when the engine rotation speed is in a high-speed region is smaller in comparison to when a roller is used in the second rocker arm 43 is for high speeds. As a result, the upper limit of the engine rotation speed can be increased.
FIG. 16 illustrates changes in loss torque with respect to the engine rotation speed when the roller 423 is used and when the slipper 433 is used in a rocker arm. The loss torque indicates the size of the output torque of the engine 1 lost in the rocker arm. In FIG. 16, L_roller indicates a case when the roller 423 is used in the rocker arm. L_slipper indicates a case when the slipper 433 is used in the rocker arm. As illustrated in FIG. 16, the loss torque when the slipper 433 is used is greater than the loss torque when the roller 423 is used.
However, because the frictional speed of the slipper 433 with respect to the camshaft 14 in the high-speed region of the engine rotation speed is high, a thick oil film is produced on the contact surface 435 of the slipper 433. As a result, the frictional resistance between the slipper 433 and the camshaft 14 is reduced in the high-speed region. Consequently, the difference in loss torque decreases as the engine rotation speed increases as can be seen in FIG. 16. Therefore, by using the slipper 433 in the second rocker arm 43 for high speeds, the mechanical loss during high speeds can be limited while reducing the equivalent inertia weight.
Conversely, the difference in the loss torque increases in the low-speed region of the engine rotation speed. Accordingly, by using the roller 423 in the first rocker arm 42 for low speeds, the frictional resistance of the roller 423 and the camshaft 14 can be decreased. As a result, the mechanical loss can be limited in the low-speed region. Moreover, because the effect of the equivalent inertia weight is reduced more in the low-speed region in comparison to the high-speed region, the effect of the equivalent inertia weight of the second rocker arm 43 can be reduced even when the roller 423 is used.
As described above, by using the roller 423 in the first rocker arm 42 for low speeds and the slipper 433 in the second rocker arm 43 for high speeds, the equivalent inertia weight can be reduced while limiting the mechanical loss in all regions of the engine rotation speed.
Further, the maximum width of the slipper 433 is greater than the width of the roller 423 in the axial direction of the intake rocker shaft 41. As a result, the surface pressure of the slipper 433 can be limited and the generation of partial contact can be suppressed.
Further, the tip end of the slipper 433 is closer to the axis of the exhaust rocker shaft 35 than the tip end of the roller 423 as seen from the axial direction of the exhaust rocker shaft 35. That is, the surface pressure of the slipper 433 can be reduced by making the maximum width of the slipper 433 greater than the width of the roller 423, whereby the need to increase the curvature radius to reduce the surface pressure is reduced. As a result, the slipper 433 can have a shorter configuration. Consequently, an increase in the equivalent inertia weight can be suppressed in comparison to when the length of the slipper 433 is increased and the curvature radius of the slipper 433 is increased. As a result, the upper limit of the engine rotation speed can be increased.
Moreover, the slipper 433 includes the hardened layer 437. Consequently, the abrasion resistance of the slipper 433 can be improved.
The maximum width of the contact surface 435 of the slipper 433 is less than the width of the boss portion 430 in the axial direction of the exhaust rocker shaft 35. As a result, the weight of the slipper 433 can be reduced while limiting the surface pressure of the slipper 433, and consequently the equivalent inertia weight of the second rocker arm 43 can be further reduced.
The weight of the portion of the second rocker arm 43 positioned further toward the tip end side of the slipper 433 than the imaginary plane P1, is less than the weight of the portion of the first rocker arm 42 positioned further toward the tip end side of the roller 423 than the imaginary plane P1. The maximum width of the contact surface 435 of the slipper 433 is less than the width of the boss portion 430 in the axial direction of the exhaust rocker shaft 35. The width of the protruding portion 439 is less than the width of the contact surface 435 in the axial direction of the exhaust rocker shaft 35. Moreover, the surface opposite the contact surface 435 of the second arm portion 434 has a shape that is recessed toward the contact surface 435 as seen from the axial direction of the exhaust rocker shaft 35. The weights of the slipper 433 and the second arm portion 434 can be further reduced due to the aforementioned shapes of the slipper 433 and the second arm portion 434. Consequently, the weight of the slipper 433 side of the second rocker arm 43 is reduced. Consequently, the equivalent inertia weight of the second rocker arm 43 can be further reduced.
The second arm portion 434 includes the recessed portion 436 positioned between the contact surface 435 and the boss portion 430. The weight can be reduced in comparison to when the contact surface 435 is joined to the boss portion 430, and thus the equivalent inertia weight of the second rocker arm 43 can be further reduced. Consequently, interference with tools for machining can be avoided due to the recessed portion 436 when machining the contact surface 435. As described above, the slipper 433 can be brought closer to the boss portion 430 by bringing the tip end of the slipper 433 closer to the axis Ax4 of the intake rocker shaft 41 than the tip end of the roller 423. Due to the formation of the recessed portion 436 even with the above configuration, tools for machining are not hindered by the boss portion 430 and the machining (for example, polishing) of the curved contact surface 435 is possible.
The second arm portion 434 includes the protruding portion 439. Consequently, the weight of the second arm portion 434 can be reduced and the stiffness of the second arm portion 434 can be assured.
The center of gravity G2 of the second rocker arm 43 is closer to the axis of the rocker shaft 41 than the center of gravity G1 of the first rocker arm 42. As a result, the equivalent inertia weight of the second rocker arm 43 can be further reduced. Consequently, the spring load (urging force) of the arm urging member 46 can be reduced and wear of the arm urging member 46 can be limited. Moreover, mechanical loss due to the arm urging member 46 can be limited.
Although an embodiment of the present invention has been described so far, the present invention is not limited to the above embodiments and various modifications may be made within the scope of the invention.
The engine is not limited to a water-cooling type single-cylinder engine. For example, the engine may be an air-cooling type. The engine may be a multi-cylinder engine.
The number of exhaust valves is not limited to two and may be one or three or more. The number of intake valves is not limited to two and may be one or three or more.
While a mechanism for switching the timing for opening and closing the valves with the open/close timing changing unit 49 is used for the intake valves in the above embodiment, the switching mechanism may also be used for the exhaust valves. The structure of the rocker shafts including the shaft member 51 and the collar member 52 may also be used for the exhaust rocker shafts.
The collar member 52 may be attached to the shaft member 51 in a manner that does not allow rotation. The collar member 52 may be omitted.
The pressing member 44 may be separate from the first rocker arm 42 and the second rocker arm 43 as illustrated by the first modified example in FIG. 17. For example, the second rocker arm 43 and the pressing member 44 may be coupled by the coupling pin 45 when the abovementioned coupling pin 45 is in the coupling position, and the first rocker arm 42 and the pressing member 44 may be coupled by the coupling pin 45 when the coupling pin 45 is in the release position.
The coupling pin 45 may be driven by a hydraulic pump (open/close timing changing unit). For example, a first oil chamber 42r and an oil path 42m are formed in the first rocker arm 42 in the second modified example in FIG. 18. Oil in the first oil chamber 42r can be used to raise and lower the pressure via the oil path 42m. Similarly, a second oil chamber 44r and an oil path 43m are formed in the second rocker arm 43. Oil in the second oil chamber 43r can be used to raise and lower the pressure via the oil path 43m. A pin hole 45r is formed in the pressing member 44. The pin hole 45r communicates with the first oil chamber 42r and the second oil chamber 43r. The coupling pin 45 is housed in the pin hole 45r. By displacing the coupling pin 45 with oil pressure in the above configuration, the pressing member 44 can be selectively coupled to the first rocker arm 42 and the second rocker arm 43.
Pressing members 44a and 44b may be provided respectively on the first rocker arm 42 and the second rocker arm 43 as illustrated by the third modified example in FIG. 19. That is, a first pressing member 44a may be provided on the first rocker arm 42 and a second pressing member 44b may be provided on the second rocker arm 43. In this case, the pressing member 44a provided on the first rocker arm 42 may press the first intake valve 27 according to the rotation of the first rocker arm 42 while the coupling pin 45 is in the release position. Moreover, the second pressing member 44b provided on the second rocker arm 43 may press the second intake valve 28 according to the rotation of the second rocker arm 43 when the coupling pin 45 is in the coupling position.

Industrial Applicability

List of Reference Numerals

4:: Cylinder head
26:: Intake valve
34:: Intake rocker unit
14:: Camshaft
49:: Open/close timing changing unit
41:: Intake rocker shaft
423:: Roller
44:: Pressing member
42:: First rocker arm
433:: Slipper
43:: Second rocker arm
45:: Coupling pin
435:: Contact surface
430:: Boss portion
434:: Second arm portion
436:: Recessed portion
437:: Hardened layer

Claims

An engine comprising:
a cylinder head;

a valve attached to the cylinder head;

a rocker unit that presses the valve and opens and closes the valve;

a camshaft that drives the rocker unit; and

an open/close timing changing unit that changes opening and closing timing of the valve,
wherein

the rocker unit includes:
a rocker shaft supported by the cylinder head;

a first rocker arm including a roller that comes into contact with the camshaft, and a pressing member pressing the valve, first rocker arm rotating around an axis of the rocker shaft when the roller comes into contact with the camshaft;

a second rocker arm aligned with the first rocker arm in an axial direction of the camshaft, and including a slipper that comes into contact with the camshaft, the second rocker arm rotating around the axis of the rocker shaft when the slipper comes into contact with the camshaft; and

a coupling pin that moves between a coupling position and a release position due to the open/close timing changing unit, the coupling pin coupling the second rocker arm to the pressing member in the coupling position and releasing the second rocker arm from the pressing member in the release position; and,

when an engine rotation speed is in a predetermined low-speed region, the open/close timing changing unit positions the coupling pin in the release position whereby the pressing member presses the valve according to a rotation of the first rocker arm, and

when the engine rotation speed is in a predetermined high-speed region, the open/close timing changing unit positions the coupling pin in the coupling position whereby the pressing member presses the valve according to a rotation of the second rocker arm, and

the roller comes into rolling contact with the camshaft,

the slipper comes into sliding contact with the camshaft, and

a tip end of the slipper is closer to the axis of the rocker shaft than a tip end of the roller as seen from the axial direction of the rocker shaft, and

a maximum width of the slipper is greater than a width of the roller in the axial direction of the rocker shaft.
The engine according to claim 1, wherein
the slipper includes a curved contact surface that comes into contact with the camshaft, and

a curvature radius of the contact surface is greater than a curvature radius of the roller.
The engine according to claim 1 or 2, wherein
a center of gravity of the second rocker arm is closer to the axis of the rocker shaft than a center of gravity of the first rocker arm.
The engine according to any one of claims 1 to 3, wherein
a weight of a portion of the second rocker arm positioned further on the tip end side of the slipper than an imaginary line which includes the axis of the camshaft and extends in a cylinder axial direction of the cylinder head, is less than a weight of a portion of the first rocker arm positioned further on the tip end side of the roller than the imaginary line.
The engine according to any one of claims 1 to 4, wherein
the second rocker arm includes

a boss portion including a hole through which the rocker shaft passes, and

an arm portion extending from the boss portion to the slipper, and

the slipper includes a contact surface that comes into contact with the camshaft.
The engine according to claim 5, wherein
a maximum width of the contact surface of the slipper is greater than a width of the boss portion in the axial direction of the rocker shaft.
The engine according to claims 5 or 6, wherein
the arm portion includes a recessed portion positioned between the contact surface and the boss portion.
The engine according to any one of claims 5 to 7, wherein
the arm portion includes a protruding portion extending from the slipper to the boss portion and protrudes from a surface opposite the contact surface of the slipper.
The engine according to any one of claims 5 to 8, wherein
a width of the protruding portion is less than the width of the contact surface in the axial direction of the rocker shaft.
The engine according to any one of claims 5 to 9, wherein
the surface of the arm portion opposite the contact surface has a shape that is recessed toward the contact surface side as seen from the axial direction of the rocker shaft.
The engine according to any one of claims 1 to 10, wherein
the slipper includes a hardened layer that comes into contact with the camshaft, and

the hardened layer has a coefficient of friction less than that of a base material of the slipper and a hardness that is greater than that of the base material of the slipper.