JP2005083228A - Oil strainer and oil reservoir structure of four cycle engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a strainer of a four cycle engine, reducing overall weight of an oil reservoir structure including an oil pan. <P>SOLUTION: Restraining structure 8 restraining movement of oil in the oil pan 2 is disposed to a strainer main body 4 housing an oil filter 3, in the oil strainer 2 sucking the oil in the oil pan 1 under the engine E. The restraining structure 8 has a lateral wall 13 approximately horizontally extending, and vertical walls 11, 12 extending in a longitudinal direction of the engine E and approximately vertical. Flow passages 24, 29 through which the oil flows are formed between the lower parts of the vertical walls 11, 12 and an inner bottom surface 19a of the oil pan 1. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an oil strainer that can be suitably used for a four-cycle engine mounted on a vehicle such as a motorcycle.

  Conventionally, as an example of an oil sump structure for a motorcycle, an oil strainer is housed in a recess provided in an oil pan below the engine, and a guide fin for guiding oil to the recess is provided on a high oil pan bottom surface around the recess. In some cases, the oil on the inner bottom surface is returned to the recess when decelerating or rapidly accelerating, so that the oil is smoothly sucked up by an oil strainer (see Patent Document 1). However, in this configuration, since the concave portion and the guide fin are formed in the oil pan, the weight of the oil pan is increased.

  As another conventional example, as shown in FIG. 9A, an oil strainer 52 is placed in an oil reservoir recess 51 of an oil pan 50, and the front and rear positions of the oil strainer 52 in the oil reservoir recess 51 (shown in the figure). The front vertical wall 53 and the rear vertical wall 54 for suppressing the flow of oil in the oil reservoir recess 51 are provided in the vertical position). The front and rear vertical walls 53 and 54 suppress the rapid forward or backward flow of oil in the oil sump recess 51 when the vehicle body is suddenly decelerated or accelerated, and the oil strainer shown in FIG. This is provided in order to ensure a sufficient amount of oil in the vicinity of the oil suction port 52a at the lower end of 52.

  The oil in the oil pan 50 is sucked up from the oil suction port 52a of the oil strainer 52 by driving an oil pump (not shown) and supplied to each lubrication point of the engine. The oil for circulating this oil A relief valve for keeping the hydraulic pressure below a certain value is provided in the passage. Although not shown, this relief valve is held in a state where it is prevented from coming off by a valve receiving portion formed integrally with the oil pan 50, and a locking claw provided on the relief valve is used as a protrusion on the crankcase. Engagement prevents rotation.

Japanese Patent Laid-Open No. 9-256830

  However, the front and rear vertical walls 53 and 54 need to have a certain height, but are formed by integral molding with the oil pan 50. Since no gap can be formed between the bottom surface and the bottom surface, the height dimension is increased, and accordingly, the thickness and the thickness are increased. As a result, the weight of the oil pan 50 is increased. Further, since the front and rear vertical walls 53, 54 in FIG. 9A cannot be provided with a bottom surface of the oil reservoir recess 51, the inner wall on either side of the oil reservoir recess 51 The gaps 57 and 58 for introducing the oil into the oil suction port 52a of the oil strainer 52 need to be provided between them. Therefore, the oil strainer 52 has a labyrinth structure before and after the oil strainer 52, and the oil existing between the front vertical wall 53 and the rear vertical wall 54, that is, in the suction space 55 where the oil strainer 52 is installed, A larger amount of oil flows out from the suction space 55 through the rear gap 58 to the rear than flows from the gap 57 into the suction space 55, and on the other hand, during sudden deceleration, the oil is sucked from the rear gap 58. Since a larger amount of oil than flowing into the raising space 55 flows out from the suction space 55 through the front gap 57, oil shortage may occur in the vicinity of the oil suction port 52a of the oil strainer 52. .

  Further, the valve receiving portion formed integrally with the oil pan 50 has a shape that protrudes greatly upward from the bottom surface of the oil reservoir recess 51 in order to hold the relief valve with the lower portion of the engine case. Increase weight.

  The present invention has been made in view of the above-described conventional problems, and has an object to provide a strainer for a four-cycle engine that can reduce the overall weight of an oil reservoir structure including an oil pan, and an oil reservoir structure including the same. Yes.

  In order to achieve the above object, an oil strainer according to a first configuration of the present invention is an oil strainer that sucks oil in an oil pan below an engine, and the oil pan of the oil is placed in a strainer body that houses an oil filter. A deterrent structure is provided to suppress movement within the interior.

  In this first configuration, the restraining structure that is integrally formed with the conventional oil pan is provided in the oil strainer, so that a restraint is provided by providing a gap between the restraining structure and the inner bottom surface of the oil pan. Since the vertical dimension of the structure can be made smaller than that provided in the oil pan, the overall weight of the oil sump structure including the oil pan can be reduced. For example, the restraining structure can be formed integrally with a strainer body such as a resin, or can be joined to a metal strainer body by welding. Further, the restraining structure does not need to be a maze structure in which a gap for oil flow is provided between any one of the inner walls as in the case of being integrally formed with a conventional oil pan, and the inner bottom surface of the oil pan is not required. Unlike the maze structure, a sufficient amount of oil can be secured around the strainer's oil suction port even during sudden acceleration or sudden deceleration.

  In a preferred embodiment of this first configuration, the restraining structure has a lateral wall extending substantially horizontally. According to this configuration, the lateral wall effectively suppresses the oil surface from moving up and down when the oil suddenly accelerates or decelerates, so that a sufficient amount of oil is secured around the oil suction port. Can do.

  In a preferred embodiment of this first configuration, the restraining structure has a substantially vertical vertical wall extending in the left-right direction of the engine. According to this configuration, since the vertical wall is provided so as to extend in the left-right direction of the engine, the oil that tries to flow rapidly from the periphery of the oil suction port in the front-rear direction during sudden deceleration or sudden acceleration is prevented. Therefore, a sufficient amount of oil can be secured around the oil suction port. In particular, if the configuration has vertical walls in the vicinity of the front and rear sides of the oil suction port, a sufficient amount of oil can be secured more easily around the oil suction port.

  It is preferable that a flow path for allowing oil to flow is formed between the lower portion of the vertical wall having the above-described configuration and the inner bottom surface of the oil pan. According to this configuration, the oil can be smoothly guided to the periphery of the oil suction port through the flow path between the lower part of the vertical wall and the inner bottom surface of the oil pan, and the front and rear are The oil around the oil suction port that tends to flow suddenly in the direction can be effectively blocked by the vertical wall located above the flow path, and a sufficient amount of oil can always be secured around the oil suction port. it can.

  An oil strainer for an engine according to a second configuration of the present invention is an oil strainer that sucks oil in an oil pan below the engine, and a valve receiving portion that holds a relief valve for an oil passage in a strainer body that houses an oil filter. Is provided.

  In this second configuration, unlike the conventional oil sump structure in which the valve receiving part is integrally formed with the oil pan, the valve receiving part can be spaced apart from the bottom surface of the oil pan and disposed at a position close to the engine case. Since the vertical dimension of the valve receiving portion is reduced, the entire oil sump structure including the oil pan can be reduced in weight.

  In a preferred embodiment of the second configuration, the oil outlet portion and the valve receiving portion open upward in the same direction at the upper portion of the strainer body. According to this configuration, when the oil pan is coupled to the engine case when the engine is assembled, the oil outlet is connected to the oil introduction part at the lower part of the engine case, and the valve receiving part is connected to the oil extraction part at the lower part of the engine case. By being connected, the oil strainer is prevented from rotating, that is, positioned in a horizontal plane. Therefore, the anti-rotation claw provided on the conventional oil strainer and engaged with the protrusion of the engine case becomes unnecessary, and the assemblability of the oil strainer is improved.

  An oil sump structure for a four-cycle engine according to the present invention includes an oil strainer having the vertical wall, and a plurality of the vertical walls are provided. An outlet of a breather return passage is provided between a pair of adjacent vertical walls. Has been placed. According to this configuration, the oil returned through the breather return passage can be guided between the pair of vertical walls that suppress the rapid flow of the oil in the front-rear direction, and the oil between the pair of vertical walls is Since the sufficient amount of oil is always secured by controlling the fluctuation amount of the oil, the outlet of the breather return passage can always be kept immersed in the oil, preventing air from being caught in the oil in the oil pan. It becomes possible to do.

  The downstream portion of the breather return passage is preferably formed in the wall of the oil pan. According to this structure, the rubber pipe which has formed the breather return passage in the conventional oil sump structure is not required, and the weight reduction and the cost reduction are realized by the simplification of the structure accompanying the reduction in the number of parts. Further, the assemblability is improved as compared with the case of using a conventional rubber pipe.

  As described above, according to the present invention, it is possible to reduce the size of the restraining structure that suppresses the rapid flow of oil or the valve receiving portion that holds the relief valve, so the overall weight of the oil sump structure including the oil pan is reduced. can do.

  FIG. 1 is a side view showing a four-sail engine constructed using an oil strainer according to an embodiment of the present invention. In this embodiment, a four-cycle engine mounted on a motorcycle is illustrated. In the figure, an engine E mounted on a vehicle body frame FR is a parallel 4-cylinder 4-cycle engine with a crankshaft CS extending in the left-right direction of the vehicle body, and a crankcase CR, a cylinder case CY, and a gear case (mission case) of the engine E. CE forms the engine case EC. This engine case EC is composed of a case upper C1 and a case lower C2 which are divided into upper and lower parts, a cylinder head 41 is connected to the upper surface of the cylinder case CY of the case upper C1, and a head cover which forms a valve operating chamber on the upper surface of the cylinder head 41. 42 is attached. An oil pan 1 for storing lubricating oil is connected to the lower surface of the case lower C2, that is, the crankcase CR, and an oil strainer 2 according to an embodiment of the present invention is mounted inside the oil pan 1. ing. A sub tank 15 is mounted on the front surface of the oil pan 1 to lower the oil level while increasing the amount of oil stored. The engine case EC, the cylinder head 41, the head cover 42, and the oil pan 1 form an engine body 40.

  FIG. 2 is a perspective view of the oil strainer 2. The oil strainer 2 sucks and filters oil in the oil pan 1 by driving an oil pump, which will be described later, and is made of resin. The oil strainer 2 includes a filter main body 4 in which an oil filter 3 is housed, a suction cylinder portion 7 that communicates with the inside of the filter main body 4 and extends in a vertical direction from the lower surface thereof, and a filter main body 4. The deterrence structure 8 that is provided continuously and suppresses the movement of oil in the oil pan 1, the oil outlet portion 9, and a valve receiving portion 10 that holds a relief valve described later are integrally provided. An oil suction port 7 a is opened at the lower end of the suction cylinder 7. The oil strainer 2 is divided into upper and lower parts, and a dividing surface 4 a is set in the filter body 4.

  The restraining structure 8 includes a pair of vertical walls 11 and 12 that are substantially vertical, a horizontal wall 13 that extends horizontally from the upper end of one vertical wall 11 to the strainer body 4, and the upper ends of the horizontal wall 13 and the other vertical wall 12. And a pair of horizontally extending connecting walls 17, 18 that connect the upper end of the other vertical wall 12 and the strainer body 4. The lateral wall 13 extends substantially parallel to the mating surface 1a with respect to the crankcase CR of the oil pan 1 in FIG. 1, and the three connecting walls 16, 17, 18 in FIG. It extends in the orthogonal direction.

  FIG. 3 is a plan view of a state in which the oil strainer 2 is mounted in the oil pan 1. The lower side of the figure is the front F of the engine E, and the upper side of the figure is the rear R. As shown in the figure, the three connecting walls 16, 17, 18 extend in the front-rear direction, and the connecting walls 16, 18 on both sides are located on the left and right sides in FIG. 3 with respect to the oil suction port 7a. The pair of vertical walls 11 and 12 are disposed at the front and rear positions with respect to the oil suction port 7a in a state extending in the axial direction of the engine E, that is, in the left-right direction that is the axial direction of the crankshaft CS of FIG.

  The oil pan 1 has a generally rectangular shape when viewed from above, and an oil sump recess 19 that has a deep bottom portion that protrudes downward and extends in the front-rear direction is formed on the right side portion (left side portion of the motorcycle) in FIG. Is formed. The left portion of the oil reservoir recess 19 is a shallow bottom portion 20 that is shallower than the oil reservoir recess 19. An exhaust pipe P of the engine E is disposed below the shallow bottom portion 20. A plurality of fins 21 extending from the mating surface 1a toward the oil reservoir recess 19 are formed on the shallow bottom portion 20 shown in FIG. The oil recovered from the crankcase CR and the gear case GE of FIG. 1 by dropping onto the shallow rough portion 20 of FIG. 3 flows on the shallow bottom portion 20 along the fins 21 and passes through the oil introduction surface 23. Guided into the oil sump recess 19.

  As shown in FIG. 4, the oil suction port 7 a at the lower end of the suction cylinder portion 7 is located above the inner bottom surface 19 a of the oil reservoir recess 19 with a gap necessary for sucking oil OL. Yes. The oil reservoir recess (deep bottom portion) 19 of the oil pan 1 has a steep slope 22 whose right side portion rises to the right from the inner bottom surface 19a, and the left side portion from the inner bottom surface 19a becomes the curved oil introduction surface 23. The oil introduction surface 23 is continuous with the shallow bottom portion 20. On the other hand, the vertical wall 11 of the oil strainer 2 includes a pair of divided walls 11a and 11b divided into left and right, and one of the divided walls 11a has an outer edge shaped along the oil introduction surface 23, and the other The dividing wall 11 b has an outer edge along the steep slope 22.

  Both the dividing walls 11a and 11b of the vertical wall 11 are connected to the suction cylinder part 7, and the vertical wall 11 and the suction cylinder part 7 allow the oil 60 in the oil reservoir recess 19 to flow in the front-rear direction. Acts as a cough. Between the lower end of the vertical wall 11 and the inner bottom surface 19 a of the oil reservoir recess 19, a flow path 24 through which the oil 60 flows is formed. In this embodiment, since the lower end (oil suction port 7a) of the suction cylinder 7 is lower than the lower end of the vertical wall 11, the fluid part 24 is connected to the lower end surface of the suction cylinder 7 and the oil reservoir recess 19. It is formed between the inner bottom surface 19a. In FIG. 4, reference numeral 61 denotes the oil level of the oil 60.

  As shown in FIG. 5, the oil strainer 2 has the suction cylinder portion 7 fitted into the oil reservoir recess 19 of the oil pan 1, and the pedestal portion 26 that protrudes from the lower surface of the strainer body 4 is mounted on the shallow portion 20. The oil pan 1 is mounted in a state of being placed. Further, the oil strainer 2 has its pedestal portion 26 abutted against a protrusion 27 that protrudes on the shallow bottom portion 20, and movement within the oil pan 1 is restricted.

  The lower portion of the vertical wall 12 is formed in an outer shape that fits between the left and right inner wall surfaces of the oil reservoir recess 19 and acts to block the flow of the oil 60 in the oil reservoir recess 19 in the front-rear direction. Between the lower end of the vertical wall 12 and the inner bottom surface 19 a of the oil reservoir recess 19, a flow path 29 through which the oil 60 flows is formed. The vertical wall 12 of the oil strainer 2 has a curved outer edge corresponding to the oil introduction surface 23 of the oil pan 1.

  6 is a cross-sectional view taken along the line VI-VI in FIG. 3. As shown in FIG. 6, the oil outlet portion 9 and the valve receiving portion 10 provided at the upper portion of the oil strainer 2 are directed obliquely upward. Open in the same direction. On the other hand, a breather return passage downstream portion 30 that is opened in the mating surface 1a of the oil pan 1 and is inclined obliquely downward on the front side is formed in the rear wall of the oil sump recess 19 by casting or machining. An oil outlet 30 a opened at the lower end of the breather return passage downstream portion 30 is disposed so as to face a suction chamber 35 between a pair of adjacent vertical walls 11, 12 of the oil strainer 2.

  The upstream passage of the breather return passage downstream portion 30 is shown in FIG. In FIG. 1, a breather chamber 31 for separating oil from an atmosphere containing oil is provided at the upper portion of the transmission chamber in the gear case GE, and the oil separated in the breather chamber 31 is naturally dropped into the rear wall of the engine case EC. Thus, a breather return passage 32 leading out toward the oil pan 1 is formed. When the oil pan 1 is connected to the engine case EC, the breather return passage downstream portion 30 of the oil pan 1 is communicated with the lower end of the breather return passage 32 of the engine case EC. The breather chamber 43 of the head cover 42 communicates with the downstream side of the cleaner element of an air cleaner (not shown).

  As shown in FIG. 7, the oil outlet portion 9 of the oil strainer 2 is fitted and connected to an oil introduction portion 34 below the oil passage 33 provided in the engine case EC. A trochoid oil pump 37 that is linked to the crankshaft CS (FIG. 1) is attached to the oil passage 33 in the vicinity of the oil introduction portion 34. A relief valve 38 is held in a fitted state in the valve receiving portion 10 of the oil strainer 2, and the relief valve 38 is screwed to an oil extraction portion 39 branched from the oil passage 33 and connected to the relief passage 36. Has been.

  As shown in FIG. 8, the oil outlet portion 9 is connected to the oil introduction portion 34 in a sealed state with a rubber damper 44 interposed therebetween. The relief valve 38 is held by the valve receiving portion 10 in a state where the rubber damper 47 is interposed between the lower end portion of the relief valve 38 and the valve receiving portion 10 so as to be liquid-tightly sealed. An annular oil reservoir space 48 is formed between a portion of the valve receiving portion 10 above the rubber damper 47 and the outer peripheral surface of the relief valve 38. The relief valve 38 keeps the oil pressure in the oil passage 33 below a certain value, and when the oil pressure in the oil extraction unit 39 rises higher than the spring pressure of the valve spring 38a, the piston 38b is driven by the oil pressure. The oil spring is pushed down against the urging force of the valve spring 38a, whereby the oil discharge hole 38c is communicated with the relief passage 36, and excess oil in the oil passage 33 is discharged from the oil discharge hole 38c to the oil sump space 48. . The oil stored in the oil sump space 48 has a function to mute the vibration sound when the piston 38b is operated.

  Next, in the above configuration, when the oil pump 37 in FIG. 7 is operated, the oil 60 in the oil reservoir recess 19 of the oil pan 1 is sucked into the suction cylinder portion 7 from the oil suction port 7a, and then the strainer body. The foreign matter is removed by being filtered by the oil filter 3 in 4, enters the oil introduction portion 34 from the oil outlet portion 9, and is supplied to each lubrication location such as the crankshaft CS (FIG. 1) and the transmission (not shown). The After the oil 60 having been lubricated has dropped onto the shallow bottom portion 20 of the oil pan 1 in FIG. 4, it circulates so as to flow back into the oil sump recess 19.

  When the motorcycle equipped with the engine E is suddenly accelerated, the oil 60 in the oil reservoir recess 19 tends to flow rapidly toward the rear R. On the other hand, when the motorcycle suddenly decelerates, the oil in the oil reservoir recess 19 moves forward. Attempts to flow rapidly toward F. At this time, the pair of front and rear vertical walls 11 and 12 in the restraint structure 8 block the flow of the oil 60 in the vicinity of the front side and the rear side with respect to the oil suction port 7a. Therefore, a sufficient amount of oil can be secured around the oil suction port 7a.

  In addition, when the motorcycle turns sharply, the oil 60 in the oil reservoir recess 19 shown in FIG. 4 flows violently in the left-right direction and tends to ride on the shallow bottom portion 20 from the oil reservoir recess 19. In this case, the three connecting walls 16, 17, 18 of the body 8 suppress the flow of the oil in the left-right direction and the lateral wall 13 prevents the oil in the oil reservoir recess 19 from jumping upward. In addition, a sufficient amount of oil can be secured around the oil suction port 7a. Such a lateral wall 13 and connecting walls 16, 17, 18 (shown in FIG. 2) cannot be provided in a conventional restraining structure in which a flow restraining wall is integrally formed with the oil pan 1.

  Since the vertical walls 11 and 12 are provided in the oil strainer 2, the vertical walls 11 and 12 are vertically moved by the flow paths 24 and 29 in FIG. In addition to being able to reduce the size, the thickness can be reduced while maintaining the required strength, and in addition, the oil strainer 2 having the restraining structure 8 is an integrally formed product made of resin. The total weight of the oil storage structure composed of the oil strainer 2 and the oil pan 1 is reduced. Even if the oil strainer 2 has a structure in which a metal restraining structure 8 formed as a separate part is joined to the metal strainer body 4 by means of welding or the like, the vertical size and thickness can be increased as compared with the conventional case. The weight can be reduced by the amount that can be reduced.

  Further, as shown in FIG. 6, the oil that has returned through the breather return passage 31 (FIG. 1) varies in the oil amount between the pair of vertical walls 11 and 12, that is, the oil surface 61 varies. Since the oil outlet 30a of the downstream portion 30 of the breather return passage 30 can always be kept immersed in the oil 60, the oil 60 is returned from above the oil surface 61 because it is guided into the small suction chamber 35. As a result, foaming as in the case is eliminated, and it is possible to prevent air from being caught in the oil 60.

  Furthermore, since the breather return passage downstream portion 30 is formed on the wall of the oil pan 1, the rubber pipe forming the conventional breather return passage is not necessary, and the improvement in assembly performance, weight reduction, and reduction in the number of parts are eliminated. Cost reduction is realized.

  Further, as shown in FIG. 7, since the oil strainer 2 is integrally provided with a valve receiving portion 10 for holding the relief valve 38, a conventional oil sump structure in which the valve receiving portion is integrally formed with the oil pan. Unlike the oil receiving recess 19, the valve receiving portion 10 can be spaced upward from the inner bottom surface 19 a and can be disposed at a position close to the engine case EC. Therefore, the vertical dimension of the valve receiving portion 10 is reduced, so that the oil pan 1 And the oil reservoir structure including the oil strainer 2 can be further reduced in weight.

  Moreover, the oil strainer 2 is integrally formed with an oil outlet 9 and a valve receiver 9 that are open upward in the same direction, so that the oil pan 1 is connected to the lower part of the engine case EC. Sometimes, the oil outlet portion 9 is fitted to the oil introduction portion 34 below the engine case EC, and the valve receiving portion 10 is fitted to the oil extraction portion 39 below the engine case EC via the relief valve 38. Therefore, the rotation of the oil strainer 2 in the horizontal plane is prevented. Therefore, the anti-rotation claw provided on the conventional oil strainer and engaged with the protrusion of the engine case becomes unnecessary, and the assemblability of the oil strainer 2 is further improved.

It is a side view showing a 4 sill engine using an oil strainer of one embodiment of the present invention. It is a perspective view which shows the oil strainer. It is a top view which shows the oil pan with which the oil strainer was mounted | worn. It is the IV-IV sectional view taken on the line of FIG. FIG. 5 is a cross-sectional view taken along line V-V in FIG. 3. It is the VI-VI sectional view taken on the line of FIG. It is a side view of the oil strainer. It is a longitudinal cross-sectional view which shows a part of FIG. (A) is a schematic plan view which shows the conventional oil sump structure, (b) is the BB sectional drawing of (a).

Explanation of symbols

E Engine 1 Oil pan 2 Oil strainer 3 Oil filter 4 Strainer body 8 Deterrent structure 9 Oil outlet 10 Valve receiver 11, 12 Vertical wall 13 Horizontal wall 16, 17, 18 Connecting wall 24, 29 Flow path 30 Breather return passage Downstream part 30a Breather return passage outlet 32 Breather return passage 33 Oil passage 38 Relief valve

Claims (8)

An oil strainer that sucks oil in the oil pan below the engine,
An oil strainer for a four-cycle engine, wherein a strainer body that houses an oil filter is provided with a restraining structure that suppresses movement of the oil in an oil pan.   The oil strainer of a four-stroke engine according to claim 1, wherein the restraining structure has a lateral wall extending substantially horizontally.   3. The oil strainer for a four-cycle engine according to claim 1, wherein the restraining structure has a substantially vertical vertical wall extending in the left-right direction of the engine.   4. The oil strainer for a four-cycle engine according to claim 3, wherein a flow path for allowing oil to flow is formed between a lower portion of the vertical wall and an inner bottom surface of the oil pan. An oil strainer that sucks oil in the oil pan below the engine,
An oil strainer for a four-cycle engine, wherein a strainer body that houses an oil filter is provided with a valve receiving portion that holds a relief valve for an oil passage.   6. The oil strainer for a four-stroke engine according to claim 5, wherein an oil outlet portion and the valve receiving portion are opened upward in the same direction at an upper portion of the strainer body.   4. An oil sump structure for a four-stroke engine, comprising the oil strainer according to claim 3, wherein a plurality of said vertical walls are provided, and an outlet of a breather return passage is disposed between a pair of adjacent vertical walls. 8. The oil sump structure for a four-cycle engine according to claim 7, wherein a downstream portion of the breather return passage is formed in a wall of the oil pan.

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