JP2022157783A - Cooling structure of internal combustion engine - Google Patents

Abstract

To provide a cooling structure of an internal combustion engine which cools an oil with a simple structure to improve cooling efficiency of the internal combustion engine.SOLUTION: A cooling structure of an internal combustion engine is mounted on a vehicle in which oil passages allowing communication between a crank case 21 and a cylinder head 23 are provided at a cylinder block 22. At least one of the oil passages is formed in a cylindrical wall of an oil passage cylindrical wall 22t which is formed expanding to the outer side as a protrusion on a side wall 22F of the cylinder block 22. A ventilation flue W through which travel wind passes is formed between a rib 22r, which is formed protruding in a plate-like shape on the side wall of the cylinder block 22, and the oil passage cylindrical wall 22t.SELECTED DRAWING: Figure 6

Description

The present invention relates to a cooling structure for an internal combustion engine.

There are two types of cooling methods for internal combustion engines: air cooling and water cooling. Oil that lubricates various sliding parts of the engine can also be expected to have a cooling effect by circulating inside the engine.

For example, in the case of an air-cooled internal combustion engine disclosed in Patent Literature 1, cooling fins are provided in the engine body to flow running air around the engine body for cooling. Lubricating oil contributes to the cooling of the internal combustion engine by circulating through the cylinder head and cylinder block to lubricate the valve train of the cylinder head in particular.

Japanese Patent No. 4209440

In Patent Document 1, the oil only circulates in the engine, and when the output of the internal combustion engine increases, the temperature of the engine oil itself rises due to friction and does not contribute to the cooling of the internal combustion engine.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object of the present invention is to provide a cooling structure for an internal combustion engine that cools oil with a simple structure to improve the cooling efficiency of the internal combustion engine.

In order to achieve the above object, the present invention
Mounted on a vehicle in which an engine body is formed by stacking a cylinder block and a cylinder head in order on a crankcase that houses a crankshaft, and an oil passage communicating between the crankcase and the cylinder head is provided in the cylinder block. In the cooling structure of the internal combustion engine,
at least one of the oil passages is formed in a cylindrical wall of an oil passage cylindrical wall formed on a side wall of the cylinder block so as to protrude outward;
Provided is a cooling structure for an internal combustion engine, characterized in that a ventilation passage through which running air passes is formed between a rib formed projecting in a plate shape on a side wall of the cylinder block and the oil passage cylindrical wall.

According to this configuration, at least one of the oil passages that communicate between the crankcase and the cylinder head is located in the cylindrical wall of the oil passage cylindrical wall that is formed in the side wall of the cylinder block so as to protrude outward. A ventilation passage through which running air passes is formed between a rib formed on a side wall of the cylinder block so as to protrude in a plate shape and the oil passage cylindrical wall. It has a simple structure that forms a ventilation passage for running wind, and the running wind, which is rectified in the ventilation passage and increased in flow speed, takes heat from the oil flowing through the internal oil passage via the oil passage cylindrical wall, and the oil is released. It can be cooled to improve the cooling efficiency of the internal combustion engine.

In a preferred embodiment of the invention,
In the engine body, a cylinder axis, which is a center axis of a cylinder bore of the cylinder block, is tilted forward, and the cylinder block and the cylinder head project obliquely forward and upward from the crankcase,
A cam chain chamber that houses a cam chain that transmits the rotational power of the crankshaft to a valve mechanism provided in the cylinder head to drive it is formed inside either the left or right side wall of the cylinder block,
The oil passage cylindrical wall is formed inside the cam chain chamber in the vehicle width direction of the front side wall of the cylinder block facing obliquely forward and downward,
The inside of the cylinder wall of the oil passage cylinder wall is defined as a first oil return passage for returning oil from the cylinder head side of the oil passage to the crankcase side,
The cam chain chamber is used as a second oil return passage,
The rib is formed in a portion of the front side wall of the cylinder block on the front side of the chain chamber in front of the cam chain chamber so as to face the cylindrical wall of the oil passage.

According to this configuration, oil is returned from the cylinder head side of the oil passage to the crankcase side within the cylindrical wall of the oil passage cylindrical wall formed in the front side wall of the cylinder block that is inclined forward and obliquely downward. 1 oil return passage, and the cam chain chamber is used as the second oil return passage, and a rib is formed on the front side wall portion of the chain chamber in front of the cam chain chamber so as to face the oil passage cylindrical wall. The running wind passing through the ventilation passage between the and takes heat from the return oil flowing through the first oil return passage in the cylinder wall via the oil passage cylinder wall, and the front side wall portion of the chain chamber in front of the cam chain chamber and Heat can also be taken from the return oil flowing through the second oil return passage of the cam chain chamber via the rib, and the cooling effect of the oil on the engine body can be enhanced.

In a preferred embodiment of the invention,
The rib formed on the front side wall portion of the chain chamber is formed outside the center of the cam chain chamber in the vehicle width direction.

According to this configuration, the rib formed on the front side wall portion of the chain chamber is formed outside the center of the cam chain chamber in the vehicle width direction, so that the area of the front side wall portion of the chain chamber in contact with the ventilation passage is increased. , more heat can be removed from the return oil in the second oil return passage, and the oil cooling effect can be further enhanced.

In a preferred embodiment of the invention,
In the cylinder head, an air guide passage for guiding running air to the air passage is formed by an air guide frame wall.

According to this configuration, the air guide passage for guiding the traveling air to the air passage is formed in the cylinder head by the air guide passage frame wall. can increase the cooling effect.

In a preferred embodiment of the invention,
The rib has a positional relationship in which it overlaps with the air guide passage frame wall when viewed in the axial direction of the cylinder axis.

According to this configuration, since the ribs are positioned so as to overlap the frame wall of the air guide passage when viewed in the axial direction of the cylinder axis, the running wind is rectified by the frame wall of the air guide passage and guided from the air guide passage to the air passage. In addition, the ribs rectify the flow, allowing the flow to flow without being disturbed, thereby increasing the flow velocity and enhancing the cooling effect.

According to the present invention, at least one of the oil passages that communicate between the crankcase and the cylinder head is formed in the cylindrical wall of the oil passage cylindrical wall that is formed on the side wall of the cylinder block so as to protrude outward. A ventilation passage through which traveling air flows is formed between the rib formed on the side wall of the cylinder block to protrude in a plate shape and the oil passage cylindrical wall. It is a simple structure that forms a ventilation passage through which the traveling wind, which is rectified and increased in flow speed, takes heat from the oil flowing through the internal oil passage via the oil passage cylinder wall, cooling the oil. Therefore, the cooling efficiency of the internal combustion engine can be improved.

1 is an overall side view of a motorcycle equipped with an internal combustion engine according to an embodiment of the present invention; FIG. Fig. 2 is a developed cross-sectional view of the internal combustion engine; It is a partial bottom view of the same internal combustion engine. It is a perspective view of the same internal combustion engine. It is a figure of the axial direction view of the cylinder axis line of the same internal combustion engine. It is the figure seen from the side of the cylinder head side mating surface of the cylinder block. It is the figure seen from the side of the cylinder block side mating surface of the cylinder head. It is a cross-sectional view of the same internal combustion engine showing a power transmission mechanism using a cam chain.

An embodiment according to the present invention will be described below with reference to FIGS. 1 to 8. FIG.
FIG. 1 is a side view of a motorcycle 1 equipped with an internal combustion engine E having a cooling structure according to the present invention.
In the description of this specification, the front, rear, left, and right directions are based on the normal standard that the straight traveling direction of the motorcycle 1 according to the present embodiment is the front. In the drawings, FR indicates the front, and RR indicates the rear. , LH indicates left and RH indicates right.

Although not shown, the vehicle body frame extends rearward from the head pipe 2 in the front part of the vehicle and is vertically provided downward in the central part of the vehicle.
A bar handle 9 is attached to the upper end of a steering shaft 8 pivotally supported by the head pipe 2, the lower part of the steering shaft 8 is connected to a front fork 10, and a front wheel 11 is pivotally supported to the tip of the front fork 10. .

A swing arm 12 is pivotally supported at its front end by a pivot shaft 13 and is connected to a portion of the vehicle body frame which is vertically provided at the center of the vehicle so as to be capable of swinging up and down. supported.
A rear cushion 15 is interposed between the swing arm 12 and the vehicle body frame above.

A crankcase 21 is supported by a vertical portion of the vehicle body frame at the center of the vehicle, and an internal combustion engine E is mounted.
The internal combustion engine E is a four-stroke cycle, single-cylinder, air-cooled internal combustion engine, and is mounted on the vehicle body frame in a horizontal orientation with the crankshaft 40 oriented in the lateral direction of the vehicle.

The internal combustion engine E comprises an engine body 20 in which a cylinder block 22 and a cylinder head 23 are stacked in order on a crankcase 21 that rotatably supports a crankshaft 40 .
In the engine body 20, the cylinder axis Lc, which is the central axis of the cylinder bores 22b of the cylinder block 22, is tilted forward to nearly horizontal, and the cylinder block 22 projects obliquely forward and upward from the crankcase 21. The cylinder block 22 and the cylinder head 23 are tilted forward by overlapping the cylinder head 23 .
A head cover 24 covers the opening at the tip of the cylinder head 23 .

A fuel tank 17 is supported above the engine body 20 on the vehicle body frame, and a passenger seat 18 is supported behind the fuel tank 17 and disposed.
An intake pipe 30 extends upward from a side wall surface of the cylinder head 23 which is tilted forward, and is connected to an air cleaner (not shown) arranged behind it via a carburetor 31. be done.
An exhaust pipe 35 extending downward from a side wall surface of the cylinder head 23 facing obliquely downward to the front bends and extends rearward along the lower surface of the internal combustion engine E, and is connected to a muffler 36 arranged on the right side of the vehicle body. there is

FIG. 2 is a developed cross-sectional view of the internal combustion engine E. Referring to FIG. 2, the crankshaft 40 of the internal combustion engine E is connected to a pair of left crankcase 21L and right crankcase 21R of the crankcases 21 split left and right. It is pivotally supported via main bearings 41, 41 and is rotatably supported.

The piston pin 42p of the piston 42 slidably fitted in the cylinder bore 22b of the cylinder block 22 and the crankpin 40p of the crankshaft 40 are connected by a connecting rod 43. It is converted into rotational motion of the crankshaft 40 .
A combustion chamber 23n formed between the top surface of the piston 42 and the ceiling surface of the cylinder head 23 has an electrode at the tip of an ignition plug 44 inserted into the ceiling wall of the cylinder head 23 facing the combustion chamber 23n.

A left shaft portion 40L of the crankshaft 40 extending leftward from the left main bearing 41 is provided with a drive sprocket 45, a starter driven gear 46, and an AC generator 48 in order leftward from the main bearing 41 side.
The AC generator 48 is covered from the left side by the left crankcase cover 25L.

In the valve chamber 23v inside the cylinder head 23, a cam shaft 70 of the valve system is rotatably supported in the left-right direction.
An intake rocker arm 74i and an exhaust rocker arm 74e swinging in contact with an intake cam 70i and an exhaust cam 70e of the camshaft 70 respectively open and close an intake valve and an exhaust valve (not shown) at a predetermined timing to perform intake and exhaust of the internal combustion engine E. .

A cam sprocket 71 is integrally fitted to the left end of the cam shaft 70 .
A cam chain 72 is wound between the cam sprocket 71 and a drive sprocket 45 integrally connected to the left shaft portion 40L of the crankshaft 40, so that the camshaft 70 rotates at half the rotation speed of the crankshaft 40. rotationally driven.

In the cylinder block 22, a cam chain chamber 22c having a rectangular cross-section and being flat in the left and right vehicle width direction is formed inside the left side wall 22L and penetrates in the axial direction of the cylinder axis Lc. The head 23 side communicates with the cam chain chamber 23c of the cylinder head 23, and the crankcase 21 side communicates with the inside of the crankcase 21.

The cam chain 72 is stretched over the drive sprocket 45 and the cam sprocket 71 through the cam chain chambers 22c and 23c.
A guide roller 73 for guiding the cam chain 72 on the inner peripheral side of the cam chain 72 is provided in the cam chain chamber 22 c of the cylinder block 22 .

On the other hand, an outer cylinder member 54 is rotatably fitted around the right shaft portion 40R extending rightward from the right main bearing 41 of the crankshaft 40, and the right end portion of the outer cylinder member 54 and the outer cylinder member 54 A centrifugal clutch 60 and the like are provided between the right end portion of the right shaft portion 40R passing through.
The centrifugal clutch 60 is constructed between a drive plate 61 that rotates integrally with the right shaft portion 40R of the crankshaft 40 and a bowl-shaped clutch outer 62 integral with the outer cylinder member 54 .

When the crankshaft 40 exceeds a predetermined number of revolutions, the centrifugal clutch 60 is engaged and the rotation of the crankshaft 40 is transmitted to the outer cylindrical member 54 .
A primary drive gear 53 is integrally formed with the outer cylindrical member 54 .

As shown in FIG. 2, the main internal combustion engine E has a multi-stage transmission 80 with gears in constant mesh behind a crankshaft 40. It is rotatably supported by the crankcases 21L and 21R.
In the left and right crankcases 21L and 21R, the gears corresponding to the main gear group 81g on the main shaft 81 and the counter gear group 82g on the counter shaft 82 are meshed with each other.

Referring to FIG. 2, a main shaft 81 of a multi-speed transmission 80 is provided with a transmission clutch 85 at a portion that protrudes rightward through the right crankcase 21R.
A clutch outer 86 of a variable speed clutch 85 is supported via a damper on a primary driven gear 84 rotatably supported on a main shaft 81 . It meshes with the primary drive gear 53 which is connected.

The variable speed clutch 85 has a large number of clutch plates interposed between a clutch inner 87 integrally connected to the main shaft 81 and a clutch outer 86, and is frictionally engaged or disengaged by a driver-operated release mechanism. is done.
The centrifugal clutch 60, the shift clutch 85, etc. are covered from the right side by the right crankcase cover 25R.

A countershaft 82 of the multi-stage transmission 80 is an output shaft, and a drive sprocket 88 is fitted to the left end of the countershaft 82 that penetrates leftward through the left crankcase 21L.
A drive chain 89 wound around the drive sprocket 88 is wound around the other driven sprocket (not shown) on the rear wheel 14 side, and the driving force of the internal combustion engine E is transmitted to the rear wheel 14 via the drive chain 89. The motorcycle 1 runs.

6 (and FIGS. 2 and 3), the cylinder block 22 projecting obliquely forward and upward from the crankcase 21 is formed by the wall surface of the rear side wall 22B and the right side wall 22R. A plurality of cooling fins 22f are formed in the axial direction of the cylinder axis Lc on the wall surface and part of the wall surface of the front side wall 22F facing forward and downward.
No cooling fins are formed on the left wall surface of the left side wall 22L of the cylinder block 22, and as shown in FIG. there is

A front side wall 22F of the cylinder block 22 facing obliquely downward forward is formed with an oil passage tube wall 22t that bulges outward (toward the front side).
The oil passage cylindrical wall 22t is located on the left side of the cylinder axis Lc of the front side wall 22F and on the right side of the cam chain chamber 22c, and extends through the cylinder block 22 in the axial direction of the cylinder axis Lc.
A cooling fin 22f is formed on the right side of the oil passage cylindrical wall 22t of the front side wall 22F.

As shown in FIG. 6, on the wall surface of the front side wall (referred to as the chain chamber front side wall portion 22FL) of the left side cam chain chamber 22c of the front side wall _22F of the cylinder block 22 facing obliquely forward and downward, a plate-like A protruding rib 22r is formed facing the oil passage cylindrical wall 22t.
A rib 22r formed on the front side wall portion _22FL of the chain chamber protrudes outward (to the left) from the center C of the cam chain chamber 22c in the vehicle width direction.

The oil passage cylindrical wall 22t and the rib 22r both extend in the axial direction of the cylinder axis Lc and face each other in parallel. A path W is formed (see FIGS. 3 and 6).

5 and 7, the cylinder head 23, which is superimposed on the front side of the cylinder head 23, has a wall surface of the rear side wall 23B that faces upward rearward, a wall surface of the right side wall 23R, and a front side wall 23F that faces downward forward. A plurality of cooling fins 23f are formed on a part of the wall surface of the cylinder to extend in the axial direction of the cylinder axis Lc.

No cooling fins are formed on the left wall surface of the left side wall 23L of the cylinder head 23, and as shown in FIG. is formed corresponding to the cam chain chamber 22c.

A front side wall 23F of the cylinder head 23 facing obliquely downward is formed with an oil passage cylinder wall 23t that bulges outward (toward the front side) as a protrusion.
The oil passage tubular wall 23t is located on the left side of the cylinder axis Lc of the front side wall 23F and on the right side of the cam chain chamber 23c, and corresponds to the oil passage tubular wall 22t of the cylinder block 22. As shown in FIG.

When the cylinder block side mating surface 23a of the cylinder head 23 is aligned with the cylinder block side mating surface 22a of the cylinder block 22 and the cylinder head 23 is placed on the cylinder block 22, the oil passage cylindrical wall 22t of the cylinder block 22 and the cylinder head 23 are aligned. The oil passage cylindrical wall 23t continues to define the first oil return passage P1 in the cylindrical wall, and the cam chain chamber 22c of the cylinder block 22 and the cam chain chamber 23c of the cylinder head 23 are connected to each other. A second oil return passage P2 is formed inside.

The inclined first oil return passage P1 formed by the forwardly inclined cylinder head 23 and continuous oil passage cylindrical walls 23t, 22t of the cylinder block 22 communicates with the valve gear chamber 23v on the cylinder head 23 side, and communicates with the crankcase 21 side. It communicates with the inside of the crankcase 21, and can return the oil that has lubricated the valve mechanism of the valve chamber 23v of the cylinder head 23 back into the crankcase 21 behind.

The inclined second oil return passage P2 constituted by the continuous cam chain chambers 23c, 22c of the cylinder head 23 and the cylinder block 22 is driven by the inclined bottoms of the continuous cam chain chambers 23c, 22c. The oil lubricating the cam sprocket 71 and the guide roller 73 can be returned into the rear crankcase 21 through the .

As shown in FIGS. 4 and 7, the cylinder head 23 has, on the left side wall surface of the front side wall 23F facing diagonally downward, an oil passage cylindrical wall 22t of the cylinder block 22 and a rib 22r. An air guide frame wall 23g that constitutes an air guide G that guides the running air to the air guide W is projected forward.

The air guide frame wall 23g has a front wall portion 23gf that is bent to the same side on both left and right sides to form a left wall portion 23gl and a right wall portion 23gr. It is formed by extending from the front obliquely downward.
Therefore, the frame wall 23g of the air guide has a left wall portion 23gl, a front wall portion 23gf, and a right wall portion 23gr, which are bent in a U-shape, and protrudes from the left wall surface of the front side wall 23F of the cylinder head 23, and extends along the cylinder axis Lc. It constitutes an air guide passage G that penetrates in the axial direction of.

4, 5 and 8, the outlet Ge of the air guide passage G is formed by an opening biased toward the front side (lower side) of the bottom wall portion 23gb.
Cooling fins 23f are formed on the outer surface of the front wall portion 23gf.

In FIG. 5 showing the axial view of the cylinder axis Lc of the internal combustion engine E, the cylinder head side mating surface 22a and the rib 22r of the cylinder block 22 are indicated by dashed lines. A left wall portion _23gl of the air guide passage frame wall 23g of the head 23 is positioned to overlap a rib 22r formed on the chain chamber front side wall portion 22FL of the cylinder block 22 when viewed in the axial direction of the cylinder axis Lc. An outlet Ge of the air passage G in the air passage frame wall 23g of the head 23 continues to the air passage W formed between the oil passage cylindrical wall 22t of the cylinder block 22 and the rib 22r.

Therefore, as indicated by thick arrows in FIGS. 3 and 8, the running wind passing through the air guide passage G of the cylinder head 23 is rectified by the air guide passage frame wall 23g, and is blown through the cylinder block 22 at high speed from the outlet Ge. It is guided to the road W, and the traveling wind can be positively flowed into the ventilation road W. - 特許庁
The ribs 22r further enhance the rectifying effect of the running wind that has flowed into the air passage W, and the flow speed can be increased without being disturbed.

The embodiment of the cooling structure for an internal combustion engine according to the present invention, which has been described in detail above, has the following effects.
As shown in FIGS. 3 and 6, it has a simple structure in which a ventilation passage W is formed between the rib 22r and the oil passage cylindrical wall 22t, and the air passage W is rectified to increase the flow velocity. The running wind takes heat from the oil flowing through the first oil return passage P1 inside through the oil passage cylindrical wall 22t, cools the oil, and improves the cooling efficiency of the internal combustion engine.

As shown in FIGS. 3 and 6, oil flows from the cylinder head side to the crankcase side through the cylindrical wall of an oil passage cylindrical wall 22t formed in the front side wall 22F of the cylinder block 22 that is inclined forward and obliquely downward. The cam chain chamber 22c is used as a second oil return passage P2, and a rib 22r is provided on the chain chamber front side wall portion _22FL in front of the cam chain chamber 22c so as to face the oil passage cylindrical wall 22t. Therefore, traveling air passing through the ventilation passage W between the rib 22r and the oil passage cylindrical wall 22t is heated from the return oil flowing through the first oil return passage P1 inside the cylindrical wall via the oil passage cylindrical wall 22t. In addition, heat can also be taken from the return oil flowing through the second oil return passage P2 of the cam chain chamber 22c via the chain chamber front side wall portion _22FL and the rib 22r in front of the cam chain chamber 22c. The cooling effect of the engine body can be enhanced.

As shown in FIG. 6, the rib 22r formed on the chain chamber front side wall portion 22FL is formed outside the center _C of the cam chain chamber 22c in the vehicle width direction. The area of the front side wall portion _22FL is widened, so that more heat can be taken from the return oil in the second oil return passage P2, and the cooling effect of the oil can be further enhanced.

As shown in FIG. 3, in the cylinder head 23, an air passage G that guides the traveling air to the air passage W is formed by the air passage frame wall 23g. can be made to flow into the ventilation passage W, and the cooling effect can be enhanced.

As shown in FIGS. 3 and 5, the rib 22r overlaps the left wall portion 23gl of the air guide frame wall 23g when viewed in the axial direction of the cylinder axis Lc. The air is rectified by the wall 23g and guided from the air guide passage G to the air duct W, further rectified by the ribs 22r, and flows without being disturbed, thereby increasing the flow velocity and further enhancing the cooling effect.

The cooling structure for an internal combustion engine according to one embodiment of the present invention has been described above. It includes what is done.

The internal combustion engine according to the present invention is applicable not only to the air-cooled internal combustion engine of the embodiment, but also to a water-cooled internal combustion engine.
Further, some internal combustion engines are equipped with an oil cooler for cooling the oil, but the structure is complicated, piping is also required, and the cost is high. , low cost.
The cooling structure of the present invention may be added to an internal combustion engine having an oil cooler to improve the cooling effect.

DESCRIPTION OF SYMBOLS 1... Motorcycle, 2... Head pipe, 8... Steering shaft, 9... Bar handle, 10... Front fork, 11... Front wheel, 12... Swing arm, 13... Pivot shaft, 14... Rear wheel, 15... Rear cushion, 17 …fuel tank, 18…passenger seat,
E... internal combustion engine, 20... engine body, 21... crankcase, 21L...left crankcase, 21R...right crankcase,
22 Cylinder block 22F Front side wall 22F _L Chain chamber front side wall portion 22B Rear side wall 22R Right side wall 22L Left side wall 22f Cooling fin 22c Cam chain chamber 22t Oil passage Cylindrical wall 22r Rib P1 First oil return passage P2 Second oil return passage W Ventilation passage
23... Cylinder head 23F... Front side wall 23B... Rear side wall 23R... Right side wall 23L... Left side wall 23c... Cam chain chamber 23v... Valve train chamber 23t... Oil passage cylindrical wall 23g... Wind guide path frame wall, G... air guideway, Ge... outlet,
24...Head cover, 25L...Left crankcase cover, 25R...Right crankcase cover,
30... intake pipe, 31... carburetor, 35... exhaust pipe, 36... muffler,
40... Crankshaft, 41... Main bearing, 42... Piston, 43... Connecting rod, 44... Spark plug, 45... Drive sprocket, 46... Starter driven gear, 47..., 48... AC generator, 54... Outer cylinder member, 60... Centrifuge clutch,
70 Camshaft 71 Cam sprocket 72 Cam chain 73 Guide roller 74i Intake rocker arm 74e Exhaust rocker arm
80...multi-stage transmission, 81...main shaft, 82...countershaft, 84...primary driven gear, 85...speed change clutch, 86...clutch outer, 87...clutch inner, 88...drive sprocket, 89...drive chain.

Claims (5)

A cylinder block (22) and a cylinder head (23) are stacked in order on a crankcase (21) that houses a crankshaft (40) to form an engine body (20). 23), in a cooling structure for an internal combustion engine mounted on a vehicle in which the cylinder block (22) is provided with an oil passage that communicates with
At least one of the oil passages is formed in a cylindrical wall of an oil passage cylindrical wall (22t) formed on a side wall (22F) of the cylinder block (22) so as to protrude outward,
A ventilation passage (W) through which traveling air flows is formed between a plate-like rib (22r) formed on a side wall (22F) of the cylinder block (22) and the oil passage cylindrical wall (22t). A cooling structure for an internal combustion engine, characterized by: In the engine body (20), the cylinder axis (Lc), which is the center axis of the cylinder bores (22b) of the cylinder block (22), is tilted forward so that the cylinder block (22) and the cylinder block (22) move away from the crankcase (21). The cylinder head (23) protrudes obliquely forward and upward.
A cam chain chamber (22c) housing a cam chain (72) for transmitting and driving the rotational power of the crankshaft (40) to a valve mechanism provided in the cylinder head (23) is located in the cylinder block (22). formed inside either the left or right side wall of the
The oil passage tubular wall (22t) is formed inside the cam chain chamber (22c) in the vehicle width direction of the front side wall (22F) of the cylinder block (22) that faces diagonally downward,
a first oil return passage (P1) for returning oil from the cylinder head (23) side of the oil passage to the crankcase (21) side, and
The cam chain chamber (22c) is used as a second oil return passage (P2),
Of the front side wall (22F) of the cylinder block (22), the chain chamber front side wall portion (22F _L ) in front of the cam chain chamber (22c) has the rib (22r) attached to the oil passage cylindrical wall (22t). 2. The cooling structure for an internal combustion engine according to claim 1, wherein the cooling structure is formed opposite to the . The rib (22r) formed in the chain chamber front side wall portion (22F _L ) is formed outside the center of the cam chain chamber (22c) in the vehicle width direction. A cooling structure for an internal combustion engine as described. The cylinder head (23) is provided with an air guide passage (G) that guides running air to the air passage (W) and is formed by an air guide passage frame wall (23g). 4. A cooling structure for an internal combustion engine according to 3. 5. The cooling structure for an internal combustion engine according to claim 4, wherein the rib (22r) is positioned to overlap the air guide passage frame wall (23g) when viewed in the axial direction of the cylinder axis (Lc).

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