JP6003710B2 - Cooling structure of forced air cooling engine - Google Patents

Description

  The present invention relates to a cooling structure for an engine in a vehicle such as a motorcycle, particularly a forced air cooling type engine.

  There is a structure in which a cylinder and a cylinder head, which are heat radiation parts of an internal combustion engine, are covered with a shroud, cooling air is introduced into the shroud with a cooling fan provided at the tip of the crankshaft, and the cooling air is applied around the cylinder head to cool it. . In this case, in order to further enhance the cooling effect, those related to the shape of the cooling rib provided on the side surface of the cylinder so as to smoothly flow the introduced cooling air are known (Patent Documents 1 and 2).

  In both cases, the cooling fins on the cylinder wall are inclined downward toward the cooling fan, and the cooling air introduced into the shroud is smoothly conducted along the inclination direction of the cooling fins to cool uniformly and efficiently. Yes.

Japanese Patent No. 3439824 Japanese Utility Model Publication No. 56-48817

  When a cam chain tensioner adjuster as shown in Patent Document 1 is arranged on the opposite side of the engine center line (cylinder axis) from the cooling fan, it is effective as a cooling structure. However, when the cam chain tensioner adjuster is arranged on the same side as the cooling fan, it is impossible to form an opening on the cooling fan side (cooling air intake side) of the cooling fin on the upper surface of the cylinder. For this reason, the cooling air cannot be smoothly flown, and a good cooling effect cannot be exhibited. Further, even in the case described in Patent Document 2, similarly, there is a problem that the cooling performance is inevitably lowered.

  In view of such circumstances, an object of the present invention is to provide a cooling structure for a forced air cooling engine that effectively and effectively improves cooling performance.

  The cooling structure of the forced air-cooled engine of the present invention includes a plurality of cooling fins provided on a cylinder and a cylinder head, a centrifugal cooling fan fitted on the end of a crankshaft, the cylinder, the cylinder head, and the A forced air-cooled engine having a shroud covering a centrifugal cooling fan, and a cam chain tensioner adjuster arranged on the centrifugal cooling fan side of the cylinder upper surface than the cylinder axis on the cylinder upper surface. An end portion of the provided cooling fin is opened, an end portion of a part of the cooling fin is bent and extended forward, and the extended end portion overlaps with the cam chain tensioner adjuster. To do.

  In the cooling structure of the forced air-cooled engine according to the present invention, the upper and lower reservoirs are formed by a smooth curved surface that is concave inwardly on the substantially cylinder axis at the tip of the shroud in a side view. The side surfaces of the shroud corresponding to the upper reservoir portion and the lower reservoir portion are narrowed in the vehicle width direction from the air introduction portion toward the tip portion in plan view, and the center of curvature of the tip portion is the cylinder. It is characterized by being positioned outside in the vehicle width direction from the side surface of the head.

  In the cooling structure for a forced air-cooled engine according to the present invention, a stepped portion projecting upward is provided at a front portion of the upper surface of the cylinder head, and cooling is performed in a gap between the upper surface of the cylinder head and the shroud at the stepped portion. A flow path through which wind is circulated is formed.

  Further, in the cooling structure of the forced air cooling engine according to the present invention, the cooling air exhaust port provided in the shroud is in a plan view, on the side opposite to the cooling fan in the vehicle width direction with respect to the cylinder axis, and on the bottom surface of the cylinder The cooling fin is provided below.

  According to the present invention, the cooling air taken from the periphery of the opening at the end of the cooling fin flows smoothly along the inclination direction of the cooling fin, and is efficiently and efficiently cooled both in the cylinder axial direction and in the circumferential direction. Therefore, the cooling performance is significantly improved. By increasing the cooling efficiency, the size and quantity of the cooling fins can be suppressed, and the weight reduction and compactness of the cylinder can be effectively realized.

1 is a side view showing an overall configuration of a motorcycle according to the present invention. 1 is a plan view showing a configuration example of a power unit of a motorcycle according to the present invention. 1 is a side view showing a configuration example of a power unit of a motorcycle according to the present invention. FIG. 4 is a bottom view of the engine as viewed in the direction of arrow X in FIG. 3. FIG. 4 is a perspective view showing the periphery of a crankcase in the motorcycle according to the present invention. It is sectional drawing which follows the II line | wire of FIG. (A) is a principal part top view which shows the state which removed the shroud in the engine which concerns on this invention, (B) is the Y section enlarged view of (A). It is a perspective view of the cylinder of the engine concerning the present invention. It is a principal part side view which shows the state which removed the shroud in the engine which concerns on this invention. It is a top view which shows the distribution | circulation state of the cooling air in a shroud in the engine which concerns on this invention. It is a side view which shows the distribution | circulation state of the cooling air in a shroud in the engine which concerns on this invention. It is sectional drawing which follows the II-II line | wire of FIG. 11 which shows the distribution | circulation state of the cooling air in a shroud in the engine which concerns on this invention.

A preferred embodiment of the cooling structure for a forced air cooling engine according to the present invention will be described below with reference to the drawings.
FIG. 1 is a side view of a motorcycle according to the present invention. First, the overall configuration of the motorcycle will be described with reference to FIG. In the drawings used in the following description, including FIG. 1, the front of the vehicle is indicated by an arrow Fr, the rear of the vehicle is indicated by an arrow Rr, and the lateral right side of the vehicle is indicated by an arrow R as necessary. The left side is indicated by an arrow L.

  The vehicle 100 has a vehicle body skeleton formed by a plurality of vehicle body frames made of steel or aluminum alloy, and is configured through various components mounted on the vehicle body frame. Although not shown in detail, the down tube, which is a part of the vehicle body frame, has a front end coupled to the steering head pipe, extends substantially downward from the steering head pipe, and has an under frame near its lower end. 101 is connected. The under frame 101 extends substantially rearward. A pair of left and right rear frames 102 as a part of the vehicle body frame is coupled to the rear side of the under frame 101, and each of the pair extends generally inclining rearward and upward.

  Further, the steering head pipe supports the front fork 103 so that the front fork 103 can be rotated in the left-right direction. A handlebar 104 is fixed above the front fork 103 and a front wheel 105 is rotatably supported on the lower end side. . The front wheel 105 is equipped with a brake disk 106 that rotates integrally therewith.

  Further, a bracket for supporting the power unit 10 including the engine 11 is attached to the rear end of the under frame 101, and swings in the vertical direction around the swing shaft 108 via a stay 107 provided on the bracket. The swing type power unit 10 is supported as possible. The power unit 10 is a unit that includes a cylinder assembly (described later) of the engine 11 and a power transmission device that includes a continuously variable transmission using a crankcase and a belt / pulley. The rear wheel 109 is rotatably supported by a power transmission device on the rear side of the vehicle. The axle side of the rear wheel 109 and the rear frame 102 are connected by a shock absorber 110, and the power unit 10 as a whole functions as a swing arm.

  As shown in FIG. 1, a seat 111 for a rider to sit on is installed above the power unit 10, and a step board 112 on which the rider seated on the seat 111 places his feet above the underframe 101, Supported by a step frame. The step board 112 is formed integrally with the leg frame cover 113 that constitutes the exterior of the vehicle. Further, between the power unit 10 and the seat 111, there is a luggage box configured as a storage space for articles and the like opened and closed by the seat 111.

  As a vehicle exterior, various vehicle body covers are supported and attached to appropriate positions such as a vehicle body frame. The front leg shield 114 covers the front side of the vehicle and is mounted with a winker or the like, and is continuous with the above-described step board 112. The rear frame cover 115 covers the lower side of the seat 111 or the rear side of the vehicle, and a winker and a brake lamp are mounted. Upper portions of the front wheel 105 and the rear wheel 109 are covered with a front fender 116 and a rear fender 117, respectively.

  Next, the power unit 10 will be described with reference to FIGS. The basic configuration of the power unit 10 includes an air-cooled four-cycle single-cylinder engine 11, which is an SOHC engine in this example. The engine 11 includes a crankcase 13 that rotatably supports and accommodates a crankshaft 12 (abbreviated by a one-dot chain line in FIG. 2) that is horizontally arranged in the left-right direction, and a cylinder 14 and a cylinder sequentially forward with respect to the crankcase 13. The head 15 and the cylinder head cover 16 are coupled, and the cylinder axis Z is set in a substantially horizontal direction. In this case, the cylinder axis Z of the cylinder 14 is tilted considerably forward, that is, approximately in the form of substantially horizontal so that the cylinder head 15 side is appropriately raised.

  A centrifugal cooling fan 17 for cooling the engine is fitted to the shaft end of the crankshaft 12, and this cooling fan 17 generates cooling air as will be described later. In this case, the cylinder 14, the cylinder head 15, and the centrifugal cooling fan 17 are covered with the shroud 18 so as to form a predetermined cooling air flow path. For this reason, the part covered with the shroud 18 in the cylinder 14 and the cylinder head 15 does not appear in the appearance as shown in FIG.

  The power unit 10 further includes a power transmission device 19 that is integrally coupled to the crankcase 13, and the power transmission device 19 includes a continuously variable transmission that uses a belt, a pulley, and the like. In this example, the crankcase 13 is divided into left and right parts (crankcase 13R, crankcase 13L), and the casing 20 of the power transmission device 19 is coupled to the crankcase 13L. A continuously variable transmission including a belt, a pulley, and the like is disposed in the casing 20, and the crankshaft 12 and the axle of the rear wheel 109 are connected by a power transmission device 19.

  Although not shown in FIG. 2 and the like, an intake system including an air cleaner and the like is arranged on the upper surface side of the power transmission device 19. In this intake system, an intake hose, a throttle body, an intake pipe (intake pipe), and the like are sequentially connected in front of the air cleaner. This intake pipe communicates with an intake port (intake port) 21 of the cylinder head 15 shown in FIG. 2, and is cleaned from the air cleaner to the intake port 21 of the cylinder head 15 through an intake passage constituted by these members. Supplied air is supplied. In addition, a fuel injection device is erected at an appropriate position in the middle of the intake pipe so as to inject fuel toward the intake port.

  On the other hand, in the exhaust system for exhausting exhaust gas from the engine 11, in this example, an exhaust port 22 (exhaust port) is provided on the lower surface side of the cylinder head 15 as schematically shown in FIG. An exhaust pipe 23 (exhaust pipe) is coupled to the exhaust port 22. The exhaust pipe 23 temporarily extends rightward from the connection portion with the exhaust port 22, and then curves rearward and further extends rearward through the vicinity of the lower portion of the cooling fan 17. The exhaust pipe 23 is further connected to the muffler 24 at the rear of the vehicle. The combustion gas generated in the engine 11 passes through the exhaust pipe 23 from the exhaust port 22 and is finally exhausted from the muffler 24 as exhaust gas.

  Here, FIGS. 5 and 6 show the periphery of the crankcase 13. As described above, the crankcase 13 is divided into left and right parts, and the crankshaft 12 is rotatably supported by the crankcase 13R and the crankcase 13L via a bearing. In this example, as shown in FIG. 6, a cooling fan 17 is integrally coupled to the right shaft end of the crankshaft 12, and a plurality of blades 25 are arranged radially around the crankshaft 12. . A generator 26 is disposed adjacent to the inside of the engine 11 of the cooling fan 17, and a rotor 27 of the generator 26 is coupled to rotate integrally with the crankshaft 12.

  The cylinder head 15 is provided with an intake valve and an exhaust valve for opening and closing the intake port 21 and the exhaust port 22 described above, and a valve operating device for driving the intake valve and the exhaust valve in the cylinder head 15. Have Although a detailed description of the power unit is omitted, the main members thereof include cams and camshaft mechanisms that drive and control intake valves and exhaust valves, and cam chains are attached to the sprockets respectively attached to the camshaft and crankshaft 12. It is wound and both are connected via this cam chain. Then, the rotation of the crankshaft 12 drives the valve operating device via the cam chain, thereby opening and closing the intake valve and the exhaust valve.

  In this example, as shown in FIGS. 5 and 6, a cam chain chamber 29 for accommodating the cam chain 28 is formed in the right crankcase 13R. More specifically, a cam chain chamber 29 is arranged in the vicinity of the right side of the cylinder bore of the cylinder 14 on the inner side of the engine 11 further than the generator 26. Since the cam chain 28 extends from the crankcase 13 to the cylinder head 15 so as to extend, the cam chain 28 and the cam chain chamber 29 are substantially along the cylinder axis Z in a side view, that is, somewhat forward. Although it inclines up, it is arrange | positioned so that it may become substantially horizontal along the vehicle front-back direction.

  The cam chain 28 travels along a predetermined traveling track in the cam chain chamber 29. A cam chain tensioner (not shown) for applying an appropriate tension to the cam chain 28 is provided at an appropriate position along the traveling track. Be placed. In addition, a cam chain tensioner adjuster is provided for adjusting the cam chain tensioner. As shown in FIGS. 7 to 9, the cam chain tensioner adjuster 30 is accommodated in a housing in which the mount portion 31 and the cap 32 are coupled to the outer wall surface of the upper surface of the cylinder 14 corresponding to the cam chain chamber 29. The The cam chain tensioner adjuster 30 protrudes upward from the upper surface of the cylinder 14 as shown in the figure. The cam chain tensioner adjuster 30 is disposed in the cylinder 14 so as to be offset from the cylinder axis Z toward the cooling fan 17 side, that is, the right side. As shown in FIG. 7, a cylinder liner or a sleeve 14 </ b> A is inserted into the cylinder 14, and a downwardly extending portion thereof is fitted inside the crankcase 13.

  Next, the cooling structure of the forced air cooling type engine 11 according to the present invention will be described more specifically. The power unit 10, particularly around the engine 11, has the basic configuration as described above. Here, referring to FIG. 7 and FIG. 8 and the like, the cylinder 14 and the cylinder head 15 are protruded into fins at substantially predetermined positions in the cylinder bore direction (cylinder radial direction) at predetermined positions on the outer peripheral surface. A plurality of cooling fins 33 are provided. These cooling fins 33 are formed at a predetermined pitch and spaced apart from each other in the cylinder axis Z direction, and have a rectangular shape or a nearly circular shape with a rounded corner or an R (R) when viewed in the cylinder axis Z direction. .

  In this case, the region around the cam chain chamber 29 is as shown in FIG. 8 and the like, and particularly in the cylinder head 15, such as the upper surface and the lower surface of the cylinder head 15 where the intake port 21 and the exhaust port 22 are disposed. The part does not have the cooling fin 33. Moreover, the shape of the front part in the upper surface of the cylinder head 15 is the level | step-difference part 34 which protruded upwards like FIG.

  In particular, the end of the cooling fin 33 provided on the upper surface of the cylinder 15, in this example, the right end is opened as shown in FIG. 7 or FIG. 8, that is, in the vicinity of the cam chain tensioner adjuster 30. 33 is in an open state. Further, end portions of some cooling fins 33 </ b> A are bent and extended forward, and the extended end portions overlap the cam chain tensioner adjuster 30.

  As described above, the cylinder 14, the cylinder head 15, and the cooling fan 17 are covered with the shroud 18 (see FIGS. 2 to 4). That is, the cylinder 14 and the cylinder head 15 including the cooling fan 17 are surrounded by the shroud 18 as shown in FIGS. The shroud 18 is integrally formed of a synthetic resin or a plastic material, and is tightly fitted by a fixing means such as a screw at a suitable place on the outer wall of the crankcase 13 or the cylinder block. The mounting is fixed so that there is no gap. As shown in FIG. 12, the top of each cooling fin 33 protruding from the outer peripheral surface of the cylinder 14 and the like and the inner surface of the shroud 18 are spaced so as to form a fixed gap.

  Regarding the shape of the shroud 18 and the like, the right side portion of the shroud 18 corresponding to the cooling fan 17 has the air introduction portion 1, and an air introduction opening 1 a concentric with the rotating shaft (crankshaft 12) of the cooling fan 17 is opened. . The shroud 18 further extends forward from the air introduction portion 1 to a portion corresponding to the right side surface of the cylinder head 15 (referred to as an extension portion 2 as shown in FIG. 10 and the like), and in a side view as shown in FIG. The front end portion has a recessed portion 3 that is recessed inward (rearward) on the substantially cylinder axis Z. The concave portion 3 having a smooth curved surface of the concave portion 3 forms an upper reservoir portion 4 and a lower reservoir portion 5 on the upper side and the lower side, respectively. Since the camshaft of the valve operating device is disposed corresponding to the concave portion 3 (arranged in the direction orthogonal to the paper surface in FIG. 11), it is necessary not to interfere with the shroud 18 for maintenance or the like. There is.

  Further, as shown in FIG. 10 in plan view, the side surface of the shroud 18 corresponding to the upper reservoir portion 4 and the lower reservoir portion 5 has a lateral width that narrows from the air introduction portion 1 toward the distal end, and the distal end thereof. Is located outside in the width direction with respect to the side surface of the cylinder head 15. That is, the extension part 2 is inclined so as to approach the cylinder axis Z toward the front in the plan view of FIG. 10, and the tip part 2a is curved with a large curvature radius. As described above, the center of curvature is located on the outer side in the width direction with respect to the side surface of the cylinder head 15, that is, shifted to the right side, so that the front portion of the shroud 18 has the flat portion 6.

  Further, the shroud 18 has a cooling air exhaust port 7 (see FIG. 4) for discharging the cooling air introduced from the air introduction unit 1 and flowing through the inside to the outside. The cooling air exhaust port 7 is opened on the opposite side of the cooling fan 17 with respect to the cylinder axis Z in a plan view, that is, on the left side, and is provided below the cooling fins 33 </ b> B on the lower surface of the cylinder 15.

Next, the operation of the engine cooling structure having the above configuration will be described. Note that FIG. 10 to FIG. When the engine 11 is started and the crankshaft 12 rotates, the blades 25 of the cooling fan 17 attached to the shaft end of the crankshaft 12 rotate as indicated by an arrow A in FIG. As a result, the air around the cooling fan 17 is entrained and introduced into the shroud 18 as shown by the arrow _AIN in FIG. Mainstream of the cooling air flows to the cylinder head 15 along the upper surface of the shroud 18 as shown by an arrow A _1.

  In the present invention, first, a part of the plurality of cooling fins 33 provided on the upper surface of the cylinder 14 wall surface, that is, the cooling fins 33 </ b> A are bent and extended forward on the mounting side (right side) of the cooling fan 17. Thereby, even if the cam chain tensioner adjuster 30 or a sensor or the like is attached to the upper surface of the cylinder 14 on the right side as in this example, the cooling fin 33 is opened or opened. By securing the openings to the cooling fins 33 in this way, the cooling air can be reliably taken from the openings. Even when the cam chain tensioner adjuster 30 or the like is arranged on the opposite side, that is, on the left side, the cooling air can be reliably taken in.

The cooling air taken in from the periphery of the opening of the cooling fin 33 flows smoothly along the inclination direction of the cooling fin 33A. As a result, the cylinder 14 is not uniformly distributed in the cylinder axis Z direction or the circumferential direction, and efficiently. Since it is cooled, the cooling performance is remarkably improved.
Further, since the cooling efficiency is high in this way, the area of the cooling fins 33 provided in the cylinder 14 can be reduced, that is, the height of the cooling fins 33 is reduced, the number thereof is reduced, and the length is further reduced. As a result, the degree of freedom in layout with peripheral components can be greatly increased. Further, by suppressing the size and quantity of the cooling fins 33, it is possible to effectively realize the weight reduction, cost reduction, compactness, and the like of the engine 11.

Furthermore, characteristic actions including the flow of cooling air in the engine cooling structure of the present invention will be described. The main flow of the cooling air blown into the shroud 18 by the cooling fan 17 from the air introduction opening 1a for introducing the cooling air provided on the side surface of the shroud 18 at the right outer side of the crankshaft 12 is as described above. It flows in the direction of the cylinder head 15 along the upper surface of the shroud 18 as indicated by an arrow A _{1 in} FIG. Thereafter, the tip 2a of the shroud 18, toward the downward right wall surface of the cylinder head 15 as indicated by arrow A ₂ and arrow A ₃ in FIG. 11 near the bottom of the concave portion 3 which is recessed inwardly of the shroud 18 cooled Branches into wind and upward cooling air. Among these, the cooling air (arrow A ₃ ) branched downward passes through the lower surfaces of the cylinder 14 and the cylinder head 15 and is discharged from the cooling air exhaust port 7 (see FIG. 4).

The cooling air (arrow A ₂ ) directed to the upper surface of the cylinder 14 has an increased flow velocity due to a synergistic effect of the shape in which the shroud 18 becomes narrower in the width direction toward the tip portion 2 a and the shape of the upper reservoir portion 4. 10, the large curvature of the tip 2 a of the shroud 18 and the center of curvature are outside the side surface of the cylinder head in the plan view of FIG. 10, and the step shape of the step 34 (see FIG. 9 etc.) of the cylinder head 15. There were, wind back as to act as a wall arrow a ₄ is generated. Some of the return air is cooled around the cylinder head 15 around the upper surface of the cylinder head 15 along the shape of the distal end portion 2a of the shroud 18 as shown by arrow A _5, through the lower surface from the side of the cylinder head 15 Then, the air is discharged from the cooling air exhaust port 7 as indicated by the arrow A _OUT .

On the other hand, the outlet or outlet of the return air coincides with the wall rising portion of the step 34 of the cylinder head 15. Therefore, when viewed from the side, the front surface is the rising portion of the wall surface of the stepped portion 34 of the cylinder head 15, the upper surface is the inner surface of the shroud 18, and the lower surface is the upper surface of the cylinder head 15 and the cylinder 14. Is formed, a return air with a high flow velocity is blown in the direction of the cylinder 14 as indicated by an arrow A ₆ . The blown cooling air is smoothly cooled from the upper surface to the side of the cylinder 14 along the cooling fins 33 by the cooling fins 33A that are bent and extended to the front of the engine 11, and the cooling air exhaust port as indicated by the arrow A _OUT. 7 is discharged.

  As described above, the cooling air taken into the shroud 18 by the cooling fan 17 is used to devise the shape of the shroud 18, the shape of the cylinder head 15, and the cooling fins 33 of the cylinder 14, particularly the shape of the cooling fins 33 </ b> A. ) The cooling air is circulated in a well-balanced manner on both the exhaust side and the exhaust side. And since the cooling air which flowed to any one of them does not wrap around again, the high cooling performance with fresh cooling air is always obtained. Further, since the cooling air sucked into the shroud 18 from the cooling fan 17 can be supplied to the cooling fins 33 of the cylinder very efficiently and smoothly, a necessary and sufficient cooling effect can be secured with a substantially small amount of air. As a result, there is an advantage that it contributes to an improvement in fuel consumption.

As mentioned above, although this invention was demonstrated with various embodiment, this invention is not limited only to these embodiment, A change etc. are possible within the scope of the present invention.
For example, even when the cam chain tensioner adjuster 30 and further the sensor or the like are arranged on the left side of the upper surface of the cylinder 14, the same effect as the above embodiment can be obtained.

DESCRIPTION OF SYMBOLS 1 Air introduction part, 2 Extension part, 3 Concave part, 4 Upper accumulation part, 5 Lower accumulation part, 6 Flat part, 7 Cooling air exhaust port, 10 Power unit, 11 Engine, 12 Crankshaft, 13 Crankcase, 14 Cylinder , 15 Cylinder head, 16 Cylinder head cover, 17 Centrifugal cooling fan, 18 shroud, 19 Power transmission device, 20 Casing, 21 Intake port, 22 Exhaust port, 23 Exhaust pipe, 24 Muffler, 25 blades, 26 Generator, 27 Rotor, 28 Cam chain, 29 Cam chain chamber, 30 Cam chain tensioner adjuster, 31 Mount part, 32 Cap, 33 Cooling fin, 34 Step part, 100 Vehicle, 101 Under frame, 102 Rear frame, 103 Front Oak, 104 Handlebar, 105 Front wheel, 106 Brake disc, 107 Stay, 108 Swing shaft, 109 Rear wheel, 110 Shock absorber, 111 Seat, 112 Step board, 113 Leg frame cover, 114 Front leg shield, 115 Rear frame cover, 116 Front fender, 117 Rear fender 117.

Claims (4)

A plurality of cooling fins provided on the cylinder and the cylinder head;
A centrifugal cooling fan fitted to the end of the crankshaft;
A forced air-cooled engine comprising: a shroud covering the cylinder, the cylinder head, and the centrifugal cooling fan;
A cam chain tensioner adjuster disposed on the upper surface of the cylinder on the centrifugal cooling fan side of the cylinder axis;
The ends of the cooling fins provided on the upper surface of the cylinder are opened, and the ends of some of the cooling fins are bent forward and the extended ends overlap the cam chain tensioner adjuster. A cooling structure for a forced air-cooled engine. The shroud has an upper reservoir portion and a lower reservoir portion formed by a smooth curved surface having a concave shape inwardly on a substantially cylinder axis at a tip portion thereof in a side view.
The shroud side surfaces corresponding to the upper reservoir portion and the lower reservoir portion are narrowed in the vehicle width direction from the air introduction portion toward the tip portion in plan view, and the center of curvature of the tip portion is the cylinder head. The cooling structure for a forced air-cooled engine according to claim 1, wherein the cooling structure is located on the outer side in the vehicle width direction with respect to the side surface.   It has a step portion projecting upward at the front portion of the upper surface of the cylinder head, and a flow path through which cooling air flows is formed in the gap between the upper surface of the cylinder head and the shroud at the step portion. The cooling structure for a forced air-cooled engine according to claim 1 or 2.   The cooling air exhaust port provided in the shroud is provided on the opposite side of the cooling fan in the vehicle width direction with respect to the cylinder axis in a plan view and below the cooling fin on the lower surface of the cylinder. The cooling structure for a forced air-cooled engine according to any one of claims 1 to 3.

Images