internal combustion engine

Abstract

To suppress abrasion of a cam journal resulting from deformation of a crankshaft while maintaining lubricating performance in a bearing portion of the cam journal.SOLUTION: An engine 1 includes: an engine body 10 having an intake valve 25A and an exhaust valve 25B opening and closing an intake and exhaust opening of a cylinder 2; camshafts 21A, 21B having a cam crest 231 pressing down the intake valve 25A, the exhaust valve 25B so as to open the opening; and a bearing member 30 pivotally supporting the crankshafts 21A, 21B via lubricating oil. The camshafts 21A, 21B include a cam journal 24 arranged in a region close to the cam crest 231 and pivotally supported by the bearing member 30, and a recession 4 formed at a position facing the cam crest 231 in a circumferential direction and recessed to a radial inner side of the cam journal 24. The recession 4 is deeper in axial ends 241, 242 than in an axial center 243 of the cam journal 24.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to an internal combustion engine having a structure in which a cam journal of a camshaft is journaled by a bearing member via lubricating oil. [Background technology]

[0002] An internal combustion engine is equipped with a camshaft that operates intake valves that open and close intake ports of cylinders and exhaust valves that open and close exhaust ports. The camshaft has cam lobes that press down the stem ends of the intake valves or exhaust valves, and a cam journal that is journaled on a bearing member of the cylinder head. The cam journal is journaled on a plain bearing via lubricating oil. Patent Document 1 discloses an internal combustion engine in which the crank journal, which is the journal-supported part of the crankshaft, is provided with multiple recesses on its outer surface to enhance its ability to retain lubricating oil. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-25653 Summary of the Invention [Problem to be solved by the invention]

[0004] Improving the fuel efficiency of internal combustion engines requires reducing various mechanical losses. Regarding the above-mentioned lubricating oil, it is desirable to use low-viscosity oil from the perspective of suppressing friction loss on the sliding surfaces. However, using low-viscosity oil can cause poor lubrication at the bearing portion of the cam journal, raising concerns about wear on the cam journal. Furthermore, when the cam lobe presses the intake or exhaust valve, a load is applied to the camshaft in a direction intersecting the axial direction, causing deformation forces. Therefore, wear due to deformation of the cam journal itself can also be a problem.

[0005] An object of the present invention is to provide an internal combustion engine that can suppress wear of the cam journal due to deformation of the camshaft while maintaining lubrication at the bearing portion of the cam journal. [Means for solving the problem]

[0006] An internal combustion engine according to one aspect of the present invention comprises an engine body having a cylinder with an intake and exhaust opening and a valve body that opens and closes the opening, a camshaft having a cam lobe that presses down on the valve body to open the opening, and a bearing member that supports the camshaft via lubricating oil, wherein the camshaft includes a cam journal supported by the bearing member, and a recess formed on the cam journal at a position circumferentially opposite the cam lobe and recessed radially inward of the cam journal, the recess being deeper at the axial end of the cam journal than at the axial center.

[0007] When the cam lobe presses the valve disc, a pressing load from the valve disc acts on the camshaft. The pressing load is a load in a direction intersecting the axial direction of the camshaft, and is a load that deforms the cam lobe in the direction opposite to the pressing direction of the valve disc. The camshaft includes a cam journal for supporting the camshaft. Therefore, the deformation force based on the pressing load acts in a direction that moves the circumferential surface of the cam journal closer to the bearing member. In other words, a state is created in which the circumferential surface of the cam journal is more likely to come into contact with the bearing member at a position circumferentially opposite the cam lobe.

[0008] According to the above internal combustion engine, a recess is formed at a position circumferentially facing the cam lobe, and the recess is deeper at the axial end of the cam journal than at the axial center. Therefore, even when a pressure load from the valve disc is applied to the camshaft, the recess ensures clearance between the circumferential surface of the cam journal and the bearing member at the position circumferentially facing the cam lobe, preventing contact between the two. Meanwhile, in areas where the recess is not provided, the clearance between the circumferential surface of the cam journal and the bearing member can be set small. Therefore, even when a low-viscosity oil is used as a lubricant, oil leakage is unlikely to occur, ensuring lubrication. This allows for both maintaining lubrication at the bearing portion of the cam journal and preventing wear on the cam journal.

[0009] In the above internal combustion engine, it is preferable that the recessed portion is a recessed portion whose depth gradually increases from the axial center side of the cam journal toward the axial end portion.

[0010] When a pressing load of the valve disc is applied to the camshaft, the axial end of the cam journal at the position circumferentially opposite the cam lobe deforms in a direction closest to the bearing member, and the amount of deformation decreases toward the axial center. According to the above internal combustion engine, the recesses can be formed with a depth distribution that matches this type of deformation of the cam journal, thereby more effectively ensuring lubrication and preventing wear.

[0011] In the above-described internal combustion engine, it is desirable that the recess has a predetermined axial width and a predetermined circumferential width in the axial direction and the circumferential direction of the cam journal, and that the axial width of the recess is wider on the upstream side in the rotation direction of the camshaft than on the downstream side.

[0012] In particular, it is more desirable that the planar shape of the recess in the planar shape obtained by expanding the cam journal in the circumferential direction has a bulging portion that bulges toward the axial center in a steep curve near the upstream end of the circumferential width in the rotational direction, and a gently curved portion that extends from the bulging portion to the downstream end of the circumferential width in the rotational direction in a gentle curve.

[0013] According to the inventors' analysis, it was found that the pressing load of the valve disc tends to be greater in the upstream portion of the camshaft in the rotational direction than in the downstream portion at a position circumferentially opposite the cam lobe. More specifically, it was found that the greatest load is applied near the upstream end of the camshaft in the rotational direction and the load gradually decreases toward the downstream end of the rotational direction. In the above internal combustion engine, the recess can be formed with an axial width that follows this load tendency, thereby more reliably preventing contact between the cam journal and the bearing member.

[0014] In the above internal combustion engine, each cylinder may be provided with two intake and two exhaust openings, and as the valve bodies, an intake camshaft and an exhaust camshaft each may be provided with a first valve body and a second valve body that respectively open and close the two openings, the camshafts may include a first cam lobe and a second cam lobe that press down the first valve body and the second valve body, respectively, and the cam journal may be arranged at a position sandwiched between the first cam lobe and the second cam lobe.

[0015] In this internal combustion engine, the cam journal is disposed between the first cam lobe and the second cam lobe, and therefore recesses are provided on the cam journal at positions circumferentially facing the first cam lobe and at positions circumferentially facing the second cam lobe. Therefore, even if the pressing loads received by the first cam lobe from the first valve body and the second cam lobe from the second valve body are applied to the camshaft, the recesses can ensure clearance between the peripheral surface of the cam journal and the bearing member.

[0016] In the above internal combustion engine, each cylinder may be provided with two intake and two exhaust openings, and as the valve bodies, an intake camshaft and an exhaust camshaft each may be provided with a first valve body and a second valve body that open and close the two openings, respectively, and the camshafts may include a first cam lobe and a second cam lobe that press down the first valve body and the second valve body, respectively, and as the cam journals, a pair of cam journals may be provided that are arranged to sandwich the first cam lobe and the second cam lobe.

[0017] In this internal combustion engine, one of the pair of cam journals is subjected to a pressing load from the first valve body to the first cam lobe, and the other is subjected to a pressing load from the second valve body to the second cam lobe. Even when these pressing loads are applied, the recesses of each cam journal can ensure a clearance between the circumferential surface of each cam journal and the bearing member.

[0018] In the above-described internal combustion engine, the engine body has a plurality of cylinders aligned in a predetermined arrangement direction, the camshaft is arranged to extend in the arrangement direction, and among the cam journals provided on the camshaft, the cam journal located at one end side or the other end side in the arrangement direction can be configured so that the recess is provided only on the side facing the first cam lobe or the second cam lobe.

[0019] In an embodiment in which a pair of cam journals are arranged to sandwich a first cam lobe and a second cam lobe, the cam journals located at one end or the other end of the arrangement direction have cam lobes only on their inner axial sides. According to the above internal combustion engine, the cam journals located at the ends of the camshaft have the recesses only on the sides facing the first cam lobe or the second cam lobe. Therefore, no unnecessary clearance is formed between the cam journals and the bearing member, ensuring lubrication and preventing wear of the cam journals at the same time. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide an internal combustion engine that can suppress wear of the cam journal due to deformation of the camshaft while maintaining lubrication at the bearing portion of the cam journal. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a perspective view showing the appearance of an engine, which is an example of an internal combustion engine according to the present invention. [Figure 2] FIG. 2 is a longitudinal cross-sectional view of the engine taken along the direction of cylinder alignment, including a cross-section of a valve mechanism provided in the engine. [Figure 3] FIG. 3 is a perspective view of the valve mechanism. [Figure 4] FIG. 4 is a schematic diagram for explaining the pressing action of the valve body by the cam. [Figure 5] 5(A) to 5(C) are diagrams showing the pressing action of the cam on the valve disc over time, and FIG. 5(D) is a graph showing the pressing load applied to the cam. [Figure 6] FIG. 6 is a diagram showing an example of a camshaft, and is a diagram showing the relationship between the rotation phase of the cam and the position of the pressing load of the valve element applied to the cam journal. [Figure 7] FIG. 7 is a schematic diagram showing the deformation of the cam journal when a pressing load is applied to the valve disc. [Figure 8] FIG. 8(A) is a simplified cross-sectional view showing an example of a recess provided in a cam journal, and FIG. 8(B) is a diagram showing the function of the recess. [Figure 9] FIG. 9 is a cross-sectional view showing a camshaft according to the first embodiment of the present invention. [Figure 10A] FIG. 10A is a developed view of the cam journal surface showing the axial profile of the recess. [Figure 10B] FIG. 10B is a development of the cam journal surface showing the depth profile of the recess. [Figure 11] FIG. 11 is a diagram showing an example of a camshaft, and is a diagram showing the relationship between the rotation phase of the cam and the position of the pressing load of the valve disc applied to the cam journal. [Figure 12] FIG. 12 is a cross-sectional view showing a camshaft according to a second embodiment of the present invention. [Figure 13] 13(A) to 13(C) are developments of the surface of a cam journal showing the axial profile of recesses formed in the camshaft of the second embodiment. [Figure 14] 14(A) to 14(C) are schematic cross-sectional views showing modified examples of the recess. DETAILED DESCRIPTION OF THE INVENTION

[0022] An internal combustion engine according to an embodiment of the present invention will be described in detail below with reference to the drawings. In this embodiment, an engine mounted on a vehicle such as an automobile as a power source for driving the vehicle will be described as an example of the internal combustion engine.

[0023] [Engine structure] FIG. 1 is a perspective view showing the exterior of an engine 1 according to this embodiment. The engine 1 is a four-stroke inline four-cylinder engine. FIG. 1 and several other figures are marked with directional symbols F and R, indicating the front and rear of the engine 1, respectively. The engine 1 includes an engine body 10 and a valve train 20 incorporated into an upper portion of the engine body 10. FIG. 2 is a longitudinal cross-sectional view of the engine 1 taken along the cylinder row direction, including a cross-section of the valve train 20. FIG. 3 is a perspective view of the valve train 20.

[0024] The engine body 10 includes a cylinder block 11 and a cylinder head 12. The cylinder block 11 has four cylinders 2 aligned in a line along the longitudinal direction FR of the engine (a predetermined arrangement direction). A piston is housed inside each cylinder 2 so that it can slide back and forth. The cylinder block 11 may include more cylinders 2, and may be for an in-line six-cylinder engine 1, for example. A crankshaft 16 is disposed inside and below the engine body 10, converting the reciprocating motion of the pistons into rotational motion.

[0025] The cylinder head 12 is attached to the top surface of the cylinder block 11 and closes the upper openings of the cylinders 2. The cylinder head 12 is formed with intake ports 14, which are openings for taking intake air into the cylinders 2, and exhaust ports, which are exhaust openings not shown in FIGS. 1 and 2. Each cylinder 2 is connected to the intake system and exhaust system in a four-valve configuration (two intake valves and two exhaust valves). FIGS. 1 and 2 show four sets of intake ports 14, each consisting of a pair of a first intake port 14A and a second intake port 14B, lined up in the direction of cylinder arrangement.

[0026] The cylinder head 12 is equipped with an intake valve 25A (valve body) that opens and closes the intake port 14, and an exhaust valve 25B (valve body) that opens and closes the exhaust port. The valve mechanism 20 is mounted on the top surface of the cylinder head 12. A cylinder head cover (not shown) is attached to the top surface of the cylinder head 12 so as to cover the valve mechanism 20.

[0027] The valve mechanism 20 is a mechanism that drives the intake valve 25A and the exhaust valve 25B to open and close the intake port 14 and the exhaust port. The intake valve 25A and the exhaust valve 25B are driven by the valve mechanism 20 so as to be linked to the rotation of the crankshaft. This drive causes the valve head 251 of the intake valve 25A to open and close the port opening 14H (Figure 4) of the intake port 14. The same is true for the exhaust valve 25B.

[0028] The intake valve 25A and the exhaust valve 25B are poppet-type valves and include a valve head 251 that actually opens and closes the intake port 14 and the exhaust port, a stem 252 that extends upward from the valve head 251, and a stem end 253 that is the upper end of the stem 252 and receives a pressing force from the valve mechanism 20. A valve spring 254 is inserted into the stem 252. One end of the valve spring 254 abuts against a spring seat 255 that is fixed to the stem 252.

[0029] [Valve train details] Next, we will explain the detailed structure and operation of the valve train 20. The valve train 20 includes an intake valve camshaft 21A and an exhaust valve camshaft 21B, a roller rocker arm 26, a lash adjuster 27, and a bearing member 30 that supports the camshafts 21A and 21B via lubricating oil. The intake valve camshaft 21A and the exhaust valve camshaft 21B are connected to the crankshaft 16 by a chain or belt, and are driven to rotate about their own axes in conjunction with the rotation of the crankshaft 16.

[0030] The intake valve camshaft 21A is disposed above eight intake valves 25A arranged in series. Similarly, the exhaust valve camshaft 21B is disposed above eight exhaust valves 25B arranged in series. The intake valve camshaft 21A and the exhaust valve camshaft 21B each include a shaft body 22, a cam 23, and a cam journal 24. The shaft body 22 has a length corresponding to the arrangement length of the intake valves 25A or the exhaust valves 25B, and extends linearly in the front-to-rear direction FR of the engine. A hollow hole 22H extending in the axial direction of the camshafts 21A and 21B is formed inside the shaft body 22 for purposes such as the circulation of cooling oil and weight reduction.

[0031] The cam 23 is disposed on the shaft body 22 at locations corresponding to the positions of the eight intake valves 25A or the eight exhaust valves 25B. The cam 23 includes a cam lobe 231 and a base circle 232. The cam lobe 231 is the long diameter portion of the cam 23 and presses down the intake valve 25A or the exhaust valve 25B via the roller rocker arm 26 to open the intake port 14 or the exhaust port. Note that a direct acting configuration may be used in which the cam lobe 231 directly presses down the intake valve 25A or the exhaust valve 25B without using the roller rocker arm 26. The base circle 232 is the short diameter portion of the cam 23 and has a diameter larger than that of the shaft body 22.

[0032] The cam journals 24 are portions where the camshafts 21A, 21B are journaled by the bearing members 30. The cam journals 24 are formed to have a diameter slightly larger than that of the shaft body 22, and are disposed in an area close to the cams 23. In this embodiment, one cam journal 24 is disposed between a pair of cams 23 disposed for one cylinder 13.

[0033] The roller rocker arm 26 is a member that transmits the pressing force of the cam 23 to the intake valve 25A or the exhaust valve 25B by using the action of a lever, and is disposed for each of the eight cams 23. The roller rocker arm 26 includes a roller 261 that contacts the peripheral surface of the cam 23, and a swing arm 262 that pivotally supports the roller 261. A contact portion 263 that presses down on the stem end 253 of the intake valve 25A or the exhaust valve 25B is formed on one end of the swing arm 262. A pivot portion 264 that serves as a pivot point for the swing arm 262 to swing is formed on the other end of the swing arm 262.

[0034] The lash adjuster 27 automatically adjusts the valve clearance between the stem end 253 and the contact portion 263. A hydraulic lash adjuster that uses the hydraulic pressure of engine oil can be used as the lash adjuster 27. If the valve clearance increases due to wear or the like, the lash adjuster 27 increases the amount of oil stored inside to reduce the valve clearance.

[0035] The bearing member 30 supports each cam journal 24 of the camshafts 21A and 21B via lubricating oil. The bearing member 30 includes a head-side bearing 31 and a cam cap 32. The cam journal 24 is held by a support body formed by the engagement of the head-side bearing 31 and the cam cap 32. The head-side bearing 31 is a bearing portion formed integrally with the cylinder head 12 and supports the annular circumferential surface of the lower half of the cam journal 24. The cam cap 32 is a member with a semicircular bearing portion that supports the annular circumferential surface of the upper half of the cam journal 24 and is fixed to the head-side bearing 31 by screws or the like. Lubricating oil is supplied between the inner circumferential surfaces of the head-side bearing 31 and cam cap 32 and the outer circumferential surface of the cam journal 24. When the camshafts 21A and 21B rotate about their axes, oil film pressure of the lubricating oil is generated, and this oil film supports the rotation of the cam journal 24.

[0036] FIG. 4 is a schematic diagram illustrating the pressing action of the cam 23 on the intake valve 25A. Note that the same action is performed on the exhaust valve 25B as described below. The circumferential surface of the cam 23 is constantly in contact with the circumferential surface of the roller 261 of the roller rocker arm 26 due to the spring force of a valve spring 254 (not shown in FIG. 4). In FIG. 4, the solid line shows the state in which the base circle 232 of the cam 23 is in contact with the roller 261. In this state, the contact portion 263 of the swing arm 262 does not substantially press down on the stem end 253 of the intake valve 25A. Therefore, the valve head 251 of the intake valve 25A is in contact with the valve seat 15, and the port opening 14H of the intake port 14 is closed.

[0037] As the cam 23 continues to rotate clockwise from the state shown in FIG. 4, the cam lobe 231 of the cam 23 comes into contact with the roller 261, as indicated by the dotted line in the figure. In this state, the roller 261 is pressed downward by the amount of cam lift, and the swing arm 262 tilts downward with the pivot portion 264 as the swing point. This tilting motion causes the contact portion 263 to press the stem end 253 downward. As a result, the valve head 251 moves downward away from the valve seat 15 and enters the cylinder 13, opening the port opening 14H. At this time, a pressing load F of the intake valve 25A acts on the cam 23 at a position circumferentially facing the cam lobe 231, as indicated by the dotted arrow in FIG. 4. This pressing load F will now be further explained.

[0038] [Valve body pressing load and its effects] 5(A) to 5(C) are diagrams showing the pressing action of intake valve 25A by cam lobe 231 of cam 23 over time, and FIG. 5(D) is a graph showing the pressing load F applied to cam 23. FIG. 5(A) shows the state at the initial stage of contact when cam lobe 231 begins to contact roller 261 (phase in the rotational direction of camshaft 21A=θ1). Pressing load F acts from the contact position between cam lobe 231 and roller 261 toward the opposite radial side of cam 23. At this initial stage of contact, as shown in FIG. 5(D), pressing load F increases rapidly. This is because cam 23 requires a relatively large pressing force when it starts to press intake valve 25A.

[0039] 5(B) shows the state in the first half of the middle contact period (rotational phase=θ2) when the contact of cam lobe 231 with roller 261 has progressed. Swing arm 262 swings downward relatively significantly with pivot portion 264 as the swing fulcrum, and contact portion 263 presses down intake valve 25A. In this state, the apex of cam lobe 231 has not yet come into contact with roller 261, but the pressing load F is at its maximum as shown in FIG. 5(D).

[0040] FIG. 5(C) shows a state in the latter stage of contact (rotational phase = θ3) when the contact between the cam lobe 231 and the roller 261 is nearing the end. After phase = θ2, the pressure load F gradually decreases. After passing the peak of the cam lobe 231, the intake valve 25A may move upward, causing the pressure load F to decrease more gradually. As the rotation progresses further and the cam lobe 231 and the roller 261 disengage, the pressure load F disappears.

[0041] 5(A) to 5(C) show cam high load points PA where a pressure load F acts on the cam 23 due to contact between the cam lobe 231 and the roller 261. The cam high load points PA occur at points on the cam 23 that face the cam lobe 231 in the circumferential direction, in other words, at points on the cam 23 on the opposite side of the axial center of the camshaft 21A from the cam lobe 231. In the drawings, the cam high load points PA are depicted as crescent-shaped. This is because, in order to schematically show the distribution of the pressure load F, the positions where the pressure load F is greater are depicted as having a greater radial thickness. However, the cam high load points PA do not actually have a load distribution that forms a simple crescent shape, but rather have a load distribution in which the center of gravity of the load is eccentric upstream in the direction of rotation, as shown in FIG. 5(D).

[0042] FIG. 6 is a simplified diagram of the intake valve camshaft 21A (exhaust valve camshaft 21B) shown in FIGS. 1 to 3, illustrating the relationship between the rotational phase of the cam 23 and the position of the pressure load on the intake valve 25A (exhaust valve 25B) applied to the cam journal 24. The reference numerals #1 to #4 in the diagram indicate the four cylinders 13 aligned in the longitudinal direction FR of the engine. As described above, the intake valve camshaft 21A has two cams 23 arranged for each of the four-valve cylinders #1 to #4, and the cam journal 24 is located midway between the two cams 23.

[0043] As a result of this positional relationship, the cam journal 24 is disposed in a region of the shaft body 22 that is close to the cam 23 (cam lobe 231). Here, the "close region" refers to a region where a deforming force acts on the shaft body 22 due to the pressing load F that the cam 23 receives. For example, as shown in FIG. 2, a typical example of a "close region" is when the axial distance between the cam journal 24 and the cam 23 is approximately the axial width of one cam 23.

[0044] 6 shows a state in which intake valve 25A corresponding to cylinder #4 is pressed down by cam lobe 231 via roller rocker arm 26, and cam lobes 231 for cylinders #1 to #3 are in a phase where they do not engage with roller 261. For cam 23 of cylinder #4, pressing load F is actually acting on the cam high load point PA described above. On the other hand, for cams 23 of cylinders #1 to #3, pressing load F is not acting on the cam high load point PA.

[0045] When a pressing load F acts on a cam high load point PA in the cam 23, a high load is also applied to the cam journal 24, resulting in a journal high load point PB. Like the cam high load point PA, the position at which the journal high load point PB occurs is a position circumferentially opposite the cam lobe 231. At this journal high load point PB, deformation of the cam journal 24 occurs due to the pressing load F applied to the cam 23. Figure 7 is a schematic diagram showing the deformation of the cam journal 24 when a pressing load F from the intake valve 25A is applied.

[0046] The cam journal 24 is rotatably supported by a sliding bearing support formed by the engagement between the head-side bearing 31 and the cam cap 32. A lubricating oil film LB is formed between the inner peripheral surfaces of the head-side bearing 31 and the cam cap 32 and the outer peripheral surface of the cam journal 24. When the cam lobe 231 presses down on the roller 261 of the roller rocker arm 26, a pressing load F acts toward the cam high load area PA circumferentially facing the cam lobe 231. This pressing load F generates a deformation force Fw that deforms the camshaft 21A (shaft main body 22) so as to lift the cam 23 upward, as shown by the dotted line in FIG. 7. Note that the deformation of the cam 23 is exaggerated in FIG. 7.

[0047] When the cam 23 deforms in this manner, a high journal load point PB is also generated in the cam journal 24 adjacent to the cam 23, causing the cam journal 24 to deform. In this embodiment, the cam journal 24 is positioned between a pair of cams 23, and the shaft body 22 deforms so that the pair of cams 23 are lifted upward. As a result, the cam journal 24 deforms in a bow shape, lifting both axial ends. This deformation brings the outer peripheral surface of the cam journal 24 near the F-side and R-side ends closer to the inner peripheral surface of the cam cap 32, which supports the annular peripheral surface of the upper half of the cam journal 24. In other words, a condition is created in which the cam journal 24 is likely to come into contact with the cam cap 32. For the #1 to #3 cylinders, when the rotational phase of the cam 23 is the same as that of the #4 cylinder, deformation occurs in the journal high load point PB of the cam journal 24.

[0048] To suppress mechanical resistance, it is desirable to reduce the gap between the inner peripheral surfaces of the head-side bearing 31 and cam cap 32 and the cam journal 24, thereby making the oil film LB as thin as possible. However, reducing this gap can cause contact between the cam journal 24 and the cam cap 32 due to deformation of the cam journal 24 caused by the application of the pressing load F to the cam 23, which could actually increase mechanical resistance and accelerate wear. In consideration of this problem, in this embodiment, the cam journal 24 is designed in a shape that prevents contact between the cam journal 24 and the cam cap 32 while maintaining the overall gap small. This design is described below.

[0049] [Cam journal of this embodiment] This embodiment shows a specific example of a cam journal 24 that can prevent contact between the cam journal 24 and the cam cap 32 and does not impair the maintenance of lubricating oil even if the above-mentioned bowing deformation of the cam journal 24 occurs. Referring to Figure 8, the cam journal 24 of this embodiment has a recess 4 that is recessed radially inward of the cam journal 24. The recess 4 is formed on the circumferential surface of the cam journal 24 at a position that faces the cam lobe 231 in the circumferential direction, that is, at a position opposite the protruding position of the cam lobe 231. Furthermore, the recess 4 is deeper at the axial end of the cam journal 24 than at the axial center.

[0050] 8(A) is a simplified cross-sectional view showing an example of a recess 4 provided in the cam journal 24, and FIG. 8(B) is a diagram showing the action of the recess 4. Here, it is assumed that the cam lobe 231 of the cam 23 adjacent to the F side of the cam journal 24 is in a phase in which it receives a pressing load F. In this case, a journal high load spot PB as shown in FIG. 6 occurs in an area on the F side of the cam journal 24 opposite the cam lobe 231. The recess 4 is provided so as to recess a portion corresponding to such an area.

[0051] The recess 4 has a cross-sectional shape that gradually deepens from the axial center 243 of the cam journal 24 toward the F-side end 241 (axial end). That is, the recess 4 is recessed deepest toward the radially inward direction at the F-side end 241. As shown in FIG. 7, when a pressing load F is applied to the cam 23, the F-side end 241 of the cam journal 24 deforms in a direction closest to the cam cap 32 at a position circumferentially opposite the cam lobe 231, and the amount of deformation decreases toward the axial center. That is, the amount of deformation near the F-side end 241 is greatest. The recess 4 has a depth distribution that matches this deformation mode of the cam journal 24. Note that the depth of the recess 4 is exaggerated in FIG. 8; the actual depth of the deepest part of the recess 4 is on the order of several microns to several tens of microns.

[0052] 8(A) shows clearances G1 and G2 between the inner peripheral surface of the cam cap 32 and the outer peripheral surface of the cam journal 24. The clearance G1 at a location near the R-side end 242 where the recess 4 is not formed is set to a standard clearance that is set in consideration of the viscosity of the lubricating oil for the sliding bearing, etc. On the other hand, the clearance G2 at a location near the F-side end 241 where the recess 4 is formed is larger than G1, and is largest at the F-side end 241.

[0053] In Figure 8(B), the dotted lines show how the cam 23 and cam journal 24 (recess 4) deform when a pressing load F is applied. As described above, when pressing load F is applied to the cam 23, the shaft body 22 deforms so that the cam 23 is lifted upward. Following this deformation, the portion of the cam journal 24 near the F-side end 241 deforms in a direction approaching the cam cap 32. Even when such deformation occurs, the presence of recess 4 ensures clearance G3 between the cam journal 24 and the cam cap 32. Therefore, contact between the two can be avoided.

[0054] The recess 4 is not provided around the entire circumference of the portion of the cam journal 24 near the F-side end 241, but is provided so as to recess only the portion corresponding to the journal high load point PB. Forming the recess 4 on the cam journal 24 increases the clearance between the opposing cam cap 32, which can cause oil leakage, where lubricating oil escapes from the clearance. In this embodiment, the recess 4 is provided only at the portion corresponding to the journal high load point PB, and in the area where the recess 4 is not provided, the gap between the peripheral surface of the cam journal 24 and the cam cap 32 is set to the standard clearance G1. This minimizes oil leakage.

[0055] The recess 4 also has a profile that gradually deepens from the axial center 243 of the cam journal 24 toward the F-side end 241. This also prevents the clearance from unnecessarily expanding. Therefore, even if a low-viscosity oil such as 0W8 is used as the lubricant, oil leakage is unlikely to occur, ensuring sufficient lubrication. This allows the cam cap 32 (bearing member 30) to maintain lubrication while preventing wear on the cam journal 24.

[0056] [Location and specific shape of recessed portion / first embodiment] Next, the position of the recessed portions 4 relative to the camshafts 21A, 21B and the specific shape of the recessed portions 4 will be described. The recessed portions 4 are provided at positions corresponding to the journal high load areas PB of the cam journal 24 shown in FIG. 6. FIG. 9 is a cross-sectional view showing the camshafts 21A, 21B according to the first embodiment. FIG. 9 shows the cam journal 24 and bearing member 30 corresponding to the #4 cylinder in FIG. 6, the cam 23 adjacent thereto, and the formation mode of the recessed portions 4. Similar recessed portions 4 are formed at the journal high load areas PB for the #1 to #3 cylinders as well.

[0057] The cam journal 24 of the intake valve camshaft 21A shown in Figures 6 and 9 is positioned so as to be sandwiched between a pair of cams 23. The F-side cam lobe 231 (first cam lobe) presses down on the intake valve 25A (first valve body) that opens and closes the first intake port 14A (Figure 2), and the R-side cam lobe 231 (second cam lobe) presses down on the intake valve 25A (second valve body) that opens and closes the second intake port 14B. The same is true for the exhaust valve camshaft 21B. The cam journal 24 is positioned so as to be sandwiched between and close to both the F-side cam lobe 231 and the R-side cam lobe 231.

[0058] With this arrangement, journal high load points PB are generated at positions on the cam journal 24 that circumferentially face the F-side cam lobe 231 and the R-side cam lobe 231, respectively. Accordingly, as recesses 4, a first recess 4a is provided on the F-side of the cam journal 24, and a second recess 4b is provided on the R-side. The first recess 4a is deepest at the F-side end 241 of the cam journal 24 and gradually becomes shallower toward the axial center 243. The second recess 4b is deepest at the R-side end 242 and gradually becomes shallower toward the axial center 243. With this arrangement, even if a pressing load F is applied to the camshaft 21A from the F-side cam lobe 231 and the R-side cam lobe 231, the first recess 4a and the second recess 4b can ensure clearance between the circumferential surface of the cam journal 24 and the cam cap 32.

[0059] Next, the specific shape of recess 4 will be described. FIG. 10A is a development of the surface of cam journal 24, showing the axial profile of recess 4, i.e., a diagram illustrating the planar shape of cam journal 24 developed in the circumferential direction. Recess 4 has a predetermined axial width and circumferential width in the axial (width) and circumferential (rotational) directions of cam journal 24. The axial width of recess 4 is wider on the upstream side in the rotational direction of cam journal 24 (camshaft 21A, 21B) than on the downstream side. In other words, if the planar shape of recess 4 is divided into two parts, an upstream side and a downstream side in the rotational direction (synonymous with the first half and second half in the rotational direction), recess 4 has a relatively wider axial width on the upstream side than on the downstream side. The axial width is the length from F-side end 241 or R-side end 242 of cam journal 24 to the edge of recess 4 on the axial center side. The circumferential width is the width of the recess along the rotational direction.

[0060] More specifically, in a plan view of the cam journal 24 developed in the circumferential direction, the recess 4 has a teardrop shape including a bulging portion 41 on the upstream side in the rotational direction and a gently curved portion 42 on the downstream side in the rotational direction. The bulging portion 41 is a portion that bulges toward the axial center in a steep curve near the upstream end of the circumferential width of the recess 4 in the rotational direction. The gently curved portion 42 is a portion that extends in a gentle curve from the bulging portion 41 to the downstream end of the circumferential width in the rotational direction. In other words, the edge of the recess 4 on the axial center side rises steeply from the upstream end in the rotational direction toward the F-side end 241 or the R-side end 242, reaches a peak position where the width is maximum in the upstream region in the rotational direction, and then has a curved shape that gently approaches the F-side end 241 or the R-side end 242.

[0061] The planar shape of the recess 4 corresponds to the distribution of the pressure load F applied to the cam 23 shown in FIG. 5(D). The pressure load F distribution has a peak at phase θ2, which is upstream of the contact point between the apex of the cam lobe 231 and the roller 261 in the rotational direction, and has a teardrop-shaped distribution with the center of gravity of the load eccentric toward the upstream side in the rotational direction. In other words, the largest pressure load F is applied to the cam 23 near the upstream end in the rotational direction and the pressure load F tends to gradually decrease toward the downstream end in the rotational direction. This load tendency is evident at the high cam load point PA (FIG. 6) of the cam 23, and a similar load tendency is evident at the high journal load point PB of the cam journal 24. Therefore, the high journal load point PB has a teardrop-shaped distribution with the center of gravity of the load eccentric toward the upstream side in the rotational direction. In line with this load tendency of the high journal load point PB, the axial profile of the recess 4 also has a teardrop-shaped shape that is wider upstream in the rotational direction. This reliably prevents contact between the cam journal 24 and the cam cap 32.

[0062] The depth of recess 4 is also set to match the load tendency of the journal high load point PB. In other words, the deeper the recess 4 is set, the greater the load on the cam journal 24. FIG. 10B is an exploded side view of cam journal 24, showing the depth profile of recess 4 along the rotational direction. This profile is the depth profile of recess 4 at F-side end 241 or R-side end 242. Note that this profile also exaggerates the size in the depth direction.

[0063] The recess 4 has a recessed shape including an upstream inclined portion 43 on the upstream side in the rotational direction and a downstream inclined portion 44 on the downstream side in the rotational direction. The upstream inclined portion 43 has an inclined surface that deepens at a first inclination L1 in a direction from the upstream end of the circumferential width of the recess 4 in the rotational direction toward the center portion LC in the rotational direction. The deepest portion MD of the recess 4 is located upstream of the center portion LC in the rotational direction. The downstream inclined portion 44 has an inclined surface that shallows at a second inclination L2 from the deepest portion MD toward the downstream end in the rotational direction. The first inclination L1 and the second inclination L2 have a relationship of L1 > L2 when the inclination directions of both are aligned. In other words, the recess 4 has a recessed shape that steeply deepens on the upstream side in the rotational direction and gradually shallows downstream of the deepest portion MD. For example, when comparing L1 and L2 in terms of the angle formed with a tangent to the circumferential surface of the cam journal 24, L1 can be set to approximately 1.2 to 3 times L2.

[0064] According to the inventors' analysis, the energy loss due to direct contact between the cam journal 24 and the cam cap 32 caused by deformation of the camshafts 21A, 21B exhibits a characteristic that rises relatively steeply in the first half of the contact period and drops relatively gradually in the second half. Because the direct contact causes wear on the cam journal 24, the amount of wear is large in the first half of the contact period and small in the second half. Therefore, by providing the cam journal 24 with recesses 4 having a depth profile with first and second slopes L1 and L2, it is possible to implement a contact wear prevention measure that is in line with the above energy loss characteristics.

[0065] 10A, the relationship is such that the greater the axial width of the recess 4, the deeper the depth at the axial end (F-side end 241 or R-side end 242) of the recess 4. Note that the axial depth profile of the recess 4 is similar to the basic example shown in FIG. 8A in that the depth gradually increases from the axial center (the teardrop-shaped edge) toward the F-side end 241 or R-side end 242.

[0066] That is, the depth profile of recess 4 is set so that it is relatively deep where the load is heavy and relatively shallow where the load is light, in line with the load distribution of journal high load point PB. According to this embodiment, the portion of recess 4 that has a long axial width and a deep recess is located in the portion of cam journal 24 that receives the greatest pressing load F. Therefore, wear of cam journal 24 due to contact with cam cap 32 can be reliably avoided.

[0067] [Location and specific shape of recessed portion / Second embodiment] Next, an example in which the present invention is applied to camshafts 21A, 21B of a type different from that shown in FIG. 6 will be described. FIG. 11 is a simplified diagram of a different type of intake valve camshaft 21A (exhaust valve camshaft 21B), illustrating the relationship between the rotational phase of the cam 23 and the position of the pressing load F applied to the cam journal 24. The reference numerals #1 to #4 in the diagram indicate four cylinders 13 aligned in the longitudinal direction FR of the engine. As with the example shown in FIG. 6, two cams 23 are arranged for each of the four-valve cylinders #1 to #4. The difference from FIG. 6 is that the camshaft 21A shown in FIG. 11 has cam journals 24 (cam caps 32) arranged so as to sandwich the two cams 23.

[0068] The shaft body 22 of the camshaft 21A has four pairs of cams 23 (first and second cam lobes) that press down pairs of intake valves 25A provided for each of the #1 to #4 cylinders. The shaft body 22 also has first to fifth cam journals 24A, 24B, 24C, 24D, and 24E, which are arranged on either side of the pair of cams 23. The upper halves of these five cam journals 24A to 24E are journaled by cam caps 32A, 32B, 32C, 32D, and 32E, respectively. Figure 11 shows a state in which the intake valve 25A corresponding to the #4 cylinder is pressed down by the cam lobe 231 via the roller rocker arm 26, while the cam lobes 231 for the #1 to #3 cylinders are in a phase where they do not engage with the roller 261. In other words, the pressing load F is actually acting only on the high-load cam portion PA of the cam 23 for the #4 cylinder.

[0069] The camshaft 21A shown in Figure 6 has a structure in which one cam journal 24 is sandwiched between a pair of cams 23 of any one of cylinders #1 to #4. Therefore, the two journal high load points PB created by the cam high load points PA of the pair of cams 23 appear in the same phase on the circumferential surface of the cam journal 24. However, in the camshaft 21A of the type shown in Figure 11, the two journal high load points PB may appear in different phases on the circumferential surface of the cam journal 24, or only one journal high load point PB may appear.

[0070] With respect to the first cam journal 24A closest to the F-side in the arrangement direction of the #1 to #4 cylinders, a journal high load spot PB is generated only on the R-side of the first cam journal 24A at a location (180 degrees circumferentially) that faces circumferentially the cam lobe 231 (first cam lobe) of the F-side cam 23 of the #1 cylinder. On the other hand, with respect to the fifth cam journal 24E closest to the R-side, a journal high load spot PB is generated only on the F-side of the fifth cam journal 24E at a location (0 degrees circumferentially) that faces circumferentially the cam lobe 231 (second cam lobe) of the R-side cam 23 of the #4 cylinder.

[0071] On the other hand, the second cam journal 24B, which is second on the F-side, is affected by a pressing load F from the R-side cam 23 of the #1 cylinder and the F-side cam 23 of the #2 cylinder, whose cam lobes 231 have different protrusion phases. Therefore, journal high load points PB can occur on the F-side and R-side of the second cam journal 24B at locations circumferentially facing the cam lobes 231 of each cam 23. In the example of FIG. 11 , the journal high load points PB are expected to occur at a 180-degree position in the circumferential direction on the F-side of the second cam journal 24B and a 270-degree position in the circumferential direction on the R-side. As with the second cam journal 24B, the third and fourth cam journals 24C and 24D also have journal high load points PB on the F-side and R-side at different circumferential positions.

[0072] Fig. 12 is a cross-sectional view showing a camshaft 21A (21B) according to the second embodiment. Fig. 12 shows a pair of cams 23 corresponding to the #4 cylinder in Fig. 11, fourth and fifth cam journals 24D, 24E and their bearing members 30 arranged to sandwich these cams 23, and the manner in which recesses 4 are formed on the fourth and fifth cam journals 24D, 24E.

[0073] As described above, the fifth cam journal 24E has a high journal load point PB only at the location opposite the cam lobe 231 of the R-side cam 23 of the #4 cylinder. Therefore, the first recess 4A is provided as the recess 4 only at the high journal load point PB on the F-side of the fifth cam journal 24E. The first recess 4A is deepest at the F-side end of the fifth cam journal 24E and gradually becomes shallower toward the center in the axial direction.

[0074] The fourth cam journal 24D is affected by a pressing load F from the R-side cam 23 of the #3 cylinder and the F-side cam 23 of the #4 cylinder, whose cam lobes 231 have different protrusion phases. For this reason, a second recessed portion 4B is provided on the R-side of the fourth cam journal 24D at a position opposite the cam lobe 231 of the F-side cam 23 of the #4 cylinder. Meanwhile, a third recessed portion 4C is provided on the F-side of the fourth cam journal 24D at a position opposite the cam lobe 231 of the R-side cam 23 of the #3 cylinder. The second recessed portion 4B is provided at a position 0 degrees circumferentially of the fourth cam journal 24D, and the third recessed portion 4C is provided at a position 90 degrees circumferentially.

[0075] The recesses 4 are formed on the first cam journal 24A in a mirror configuration with the fifth cam journal 24E, and on the second and third cam journals 24B and 24C in a similar manner to the fourth cam journal 24D. With the recesses 4 arranged in this manner, even if a pressing load F is applied to the camshaft 21A from the cam lobes 231 of each cam 23, the recesses 4 can ensure a clearance between the peripheral surface of the cam journal 24 and the cam cap 32.

[0076] 13A-13C are developments of the cam journal surfaces, showing the axial profiles of the recesses 4 formed in the camshaft 21A of the second embodiment. FIG. 13A is a plan view showing the recesses 4 formed in the first cam journal 24A, the F-sidemost, as developed in the circumferential direction. The first cam journal 24A has one recess 4 formed at a 180-degree circumferential position, extending from the R-side end 242 toward the axial center. This recess 4 has a teardrop shape similar to that illustrated in FIG. 10A. That is, the recess 4 has a bulge 41 on the upstream side in the rotational direction and a gently curved portion 42 on the downstream side in the rotational direction. The recess 4 of the fifth cam journal 24E is formed symmetrically to the first cam journal 24A, with the F-side end 241 as the reference point. The depth profile of the recess 4 is set as illustrated in FIG. 10B.

[0077] 13(B) shows the recesses 4 provided on the second cam journal 24B. The second cam journal 24A has an F-side recess 4 extending from the F-side end 241 toward the center in the axial direction, and an R-side recess 4 from the R-side end 242 toward the center in the axial direction. These recesses 4 also have a teardrop shape with a bulge 41 and a gently curved portion 42. The F-side recess 4 is formed near the 180-degree position in the circumferential direction of the second cam journal 24B, and the R-side recess 4 is formed near the 270-degree position in the circumferential direction. The fourth cam journal 24D also has F-side and R-side recesses 4 provided in a similar phase relationship.

[0078] 13(C) shows the recesses 4 provided on the third cam journal 24C. The third cam journal 24C also has an F-side recess 4 extending from the F-side end 241 toward the center in the axial direction, and an R-side recess 4 from the R-side end 242 toward the center in the axial direction. These recesses 4 also have a teardrop shape with a bulge 41 and a gently curved portion 42, and are arranged in a positional relationship facing each other around the circumferential direction of the third cam journal 24C. The F-side recess 4 is formed near a position at 90 degrees around the circumferential direction of the third cam journal 24C, and the R-side recess 4 is formed near a position at 270 degrees around the circumferential direction.

[0079] According to the second embodiment described above, even in the camshaft 21A in which the journal high load points PB occur at different circumferential positions on the F side and the R side for one cam journal 24, clearance between the circumferential surface of each cam journal 24 and the bearing member 30 (cam cap 32) can be ensured by each recessed portion 4. Furthermore, for the first and fifth cam journals 24A and 24E located at the ends of the camshaft 21A, only one recessed portion 4 corresponding to one adjacent cam 23 is provided. Therefore, no unnecessary clearance is formed between the cam journals 24A and 24E and the bearing member 30, and it is possible to ensure lubrication and prevent wear of the cam journals at the same time.

[0080] [Variations] Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and the following modified embodiments can be adopted, for example.

[0081] (1) In the above embodiment, the camshafts 21A, 21B are illustrated as being compatible with the in-line four-cylinder engine 1. The camshafts 21A, 21B may be camshafts for other multi-cylinder engines, for example, an in-line six-cylinder engine.

[0082] (2) In the above embodiment, the recess 4 gradually becomes deeper from the axial center of the cam journal 24 toward the axial end (F-side end 241 or R-side end 242) thereof. However, various modifications of the recess 4 are possible as long as the axial end of the cam journal 24 is deeper than the axial center thereof. FIGS. 14(A) to 14(C) show modified recesses 4-1, 4-2, and 4-3.

[0083] FIG. 14(A) is a schematic cross-sectional view of a cam journal 24 showing a recess 4-1 with a stepped recess shape. The recess 4-1 has a recess shape consisting of alternating horizontal portions 45 with no inclination and inclined portions 46 with a slope. The recess is deeper at the axial end than at the axial center of the cam journal 24. FIG. 14(B) shows a recess 4-2 composed of one horizontal portion 47 and one inclined portion 48. The inclined portion 48 is located at the axial center of the cam journal 24, and the horizontal portion 47 extends from the deepest end of the inclined portion 48 to the axial end. FIG. 14(C) shows a recess 4-3 with a concave-convex inclined portion 49. The concave-convex inclined portion 49 is an inclined portion that repeatedly rises and falls, gradually becoming deeper overall from the axial center to the axial end of the cam journal 24. These recesses 4-1, 4-2, and 4-3 also provide the same effects as the recess 4 described above. [Explanation of symbols]

[0084] 1. Engine (internal combustion engine) 10 Engine body 13 cylinders 14 Intake port 14H port opening (opening for intake and exhaust) 21A Intake valve camshaft (camshaft) 21B Exhaust valve camshaft (camshaft) 23 Cam 231 Mount Kam 24 Cam Journal 241 F side end (axial end) 242 R side end (axial end) 243 Axial center 25A, 25B Intake valve, exhaust valve (valve body) 30 Bearing material 31 Head side bearing 32 Cam cap (bearing part) 4 recess 41 Bulge 42 Gentle bend section

Claims

1. an engine body including a cylinder having an intake / exhaust opening and a valve body for opening and closing the opening; a camshaft having a cam lobe that presses the valve body to open the opening; a bearing member that supports the camshaft via lubricating oil, The camshaft is a cam journal journal journaled by the bearing member; a recess formed on the cam journal at a position circumferentially opposed to the cam lobe and recessed radially inward of the cam journal, In an internal combustion engine, the recessed portion is a recessed portion that is deeper at an axial end portion than at an axial center portion of the cam journal, the recess has a predetermined axial width and a predetermined circumferential width in the axial direction and the circumferential direction of the cam journal, an axial width of the recessed portion is greater on an upstream side in a rotation direction of the camshaft than on a downstream side.

2. 2. The internal combustion engine according to claim 1, The recessed portion has a depth that gradually increases from an axial center side of the cam journal toward an axial end side of the cam journal.

3. In the internal combustion engine according to claim 1, The shape of the recess in a plan view in a circumferentially developed planar shape of the cam journal is as follows: a bulging portion that bulges toward the axial center in a steep curve near the upstream end of the circumferential width in the rotational direction, and a gently curved portion that extends from the bulging portion to the downstream end of the circumferential width in the rotational direction in a gentle curve; An internal combustion engine having a shape having:

4. In the internal combustion engine according to any one of claims 1 to 3, Each cylinder has two intake and two exhaust openings. As the valve bodies, the intake camshaft and the exhaust camshaft each include a first valve body and a second valve body that open and close the two openings, respectively; the camshaft includes a first cam lobe and a second cam lobe that press down the first valve body and the second valve body, respectively; The cam journal is disposed at a position sandwiched between the first cam lobe and the second cam lobe.

5. In the internal combustion engine according to any one of claims 1 to 3, Each cylinder has two intake and two exhaust openings. As the valve bodies, the intake camshaft and the exhaust camshaft each include a first valve body and a second valve body that open and close the two openings, respectively; the camshaft includes a first cam lobe and a second cam lobe that press down the first valve body and the second valve body, respectively; The internal combustion engine includes a pair of cam journals arranged to sandwich the first cam lobe and the second cam lobe.

6. An engine body having a cylinder with an intake and exhaust opening and a valve body that opens and closes the opening; a camshaft having a cam lobe that presses the valve body to open the opening; a bearing member that supports the camshaft via lubricating oil, The camshaft is a cam journal journal journaled by the bearing member; a recess formed on the cam journal at a position circumferentially opposed to the cam lobe and recessed radially inward of the cam journal, In an internal combustion engine, the recessed portion is a recessed portion that is deeper at an axial end portion than at an axial center portion of the cam journal, Each cylinder has two intake and two exhaust openings. As the valve bodies, the intake camshaft and the exhaust camshaft each include a first valve body and a second valve body that open and close the two openings, respectively; the camshaft includes a first cam lobe and a second cam lobe that press down the first valve body and the second valve body, respectively; The cam journals include a pair of cam journals arranged to sandwich the first cam lobe and the second cam lobe, The engine body has a plurality of cylinders aligned in a predetermined arrangement direction, The camshaft is disposed to extend in the arrangement direction, an internal combustion engine, wherein the cam journals provided on the camshaft and located at one end or the other end in the arrangement direction have the recessed portion provided only on the side facing the first cam lobe or the second cam lobe.

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