JP4732375B2 - Forced air-cooled internal combustion engine - Google Patents

Description

  The present invention provides a cylinder block that is coupled to a crankcase that rotatably supports a crankshaft and is provided with a cooling fin, and a cylinder head that is coupled to the cylinder block and to which a spark plug is attached. Surrounded by a shroud that forms a cooling air passage between the block and the cylinder head, and connected to the crankshaft is a cooling fan that blows cooling air sucked from the outside of the shroud to the cooling air passage side The shroud relates to a forced air-cooled internal combustion engine in which an exhaust port for discharging cooling air flowing through a cooling air passage to the outside is formed.

Patent Document 1 discloses a forced air-cooling type internal combustion engine in which exhaust ports are provided in the vicinity of the cylinder block and the cylinder head of the shroud covering the cylinder block and the cylinder head so that the cylinder block and the cylinder head are uniformly cooled. .
Japanese Patent Laid-Open No. 7-332084

  By the way, the periphery of the spark plug is a part that needs to be cooled. However, in the one disclosed in Patent Document 1, the exhaust outlet is disposed at a position away from the spark plug, and the spark plug is cooled more effectively. It is desirable.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a forced air-cooling type internal combustion engine that can efficiently cool a spark plug.

  In order to achieve the above object, a first aspect of the present invention is a cylinder block coupled to a crankcase for rotatably supporting a crankshaft and having a cooling fin projecting thereon, and coupled to the cylinder block. A cylinder head to which a spark plug is attached is surrounded by a shroud that forms a cooling air passage between the cylinder block and the cylinder head, and cooling air sucked from outside the shroud is cooled on the crankshaft. In a forced air-cooling internal combustion engine in which a cooling fan that blows air to the air passage side is linked and connected, and an exhaust port that discharges cooling air that has flowed through the cooling air passage to the outside is formed in the shroud, the outer periphery of the cylinder block The cooling fin protruding from the outer periphery of the cylinder block is partly provided A notch space is formed by shrinking or missing, and the shroud includes a first protrusion projecting inward along the notch space, and the spark plug attached to one side surface of the cylinder head. A second protrusion protruding inward is formed at a position sandwiched between the protrusion and the first protrusion.

  According to a second aspect of the invention, in addition to the configuration of the first aspect of the invention, the exhaust port is adjacent to an exhaust pipe connected to the cylinder head through an exhaust port provided in the cylinder head. It is provided in the shroud.

  According to a third aspect of the invention, in addition to the configuration of the first or second aspect of the invention, the notch space is formed on the outer periphery of the cylinder block on the side corresponding to the intake port provided in the cylinder head. It is characterized by.

  According to a fourth aspect of the present invention, in addition to the configuration of the first aspect, the pair of side surfaces disposed on both sides of a plane including the axis of the spark plug among the side surfaces of the shroud. On the other hand, the exhaust port and the second protrusion are formed so as to sandwich a portion corresponding to the spark plug.

  According to a fifth aspect of the invention, in addition to the configuration of the first aspect of the invention, the first plug and the second protrusion may have the spark plug as viewed from a direction along the cylinder axis. It is characterized by being arranged offset from each other in a direction along a plane including the axis.

  Furthermore, the invention according to claim 6 is characterized in that, in addition to the configuration of the invention according to any one of claims 1 to 5, the spark plug is attached to the cylinder head so as to be inclined toward the cooling fan. To do.

  According to the first aspect of the present invention, the cooling air blown by the cooling fan is discharged from the exhaust port, and the first and second protrusions are sandwiched between the spark plugs on the side surfaces of the shroud. Therefore, the cooling air flowing to the exhaust port side is effectively guided to the periphery of the spark plug by the first and second protrusions, and the spark plug can be efficiently cooled. Moreover, since the 1st protrusion protrudes inward along the notch space formed in the outer periphery of a cylinder block, the amount of cooling air which distribute | circulates the periphery of a cylinder head with a higher temperature can be increased.

  According to the second aspect of the present invention, since the exhaust port is disposed adjacent to the exhaust pipe, it is possible to effectively cool the vicinity of the exhaust port that is likely to become high temperature.

  According to the third aspect of the present invention, since the first protrusion is formed on the side surface of the shroud on the side corresponding to the intake port, the amount of cooling air flowing around the intake port having a relatively low temperature is reduced. Only the amount of cooling air flowing around the spark plug and the exhaust port can be increased.

  According to the fourth aspect of the present invention, the exhaust port and the exhaust port are arranged such that the portion corresponding to the spark plug is sandwiched between one of the pair of side surfaces disposed on both sides of the plane including the axis of the spark plug among the side surfaces of the shroud. Since the second protrusion is formed, it is possible to reliably provide a path for cooling air that flows in the vicinity of the spark plug to the exhaust port and a path for cooling air that flows through the other side of the both side surfaces to the exhaust port. Can be formed.

  According to the fifth aspect of the present invention, the first and second protrusions are offset from each other in a direction along the plane including the axis of the spark plug, and therefore flow from the periphery of the cylinder block to the periphery of the cylinder head. The passage area of the cooling air can be ensured, and the cooling air can be easily collected around the spark plug while avoiding a decrease in the cooling air volume as a whole.

  According to the sixth aspect of the present invention, the spark plug can be cooled with cooling air having a lower temperature by bringing the spark plug closer to the cooling fan side.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below based on one embodiment of the present invention shown in the accompanying drawings.

  1 to 9 show an embodiment of the present invention. FIG. 1 is a left side view of a scooter type motorcycle, FIG. 2 is an enlarged sectional view taken along line 2-2 of FIG. 1, and FIG. 4 is an enlarged sectional view taken along line 3-3, FIG. 4 is a view taken in the direction of the arrow 4 in FIG. 2, FIG. 5 is a view taken in the direction of the arrow 5 in FIG. 6 is a view seen from the direction of arrow 7 in FIG. 6, FIG. 8 is a view of the upper cover member of the shroud seen from the direction of arrow 8 in FIG. 6, and FIG. 9 is a sectional view taken along line 9-9 in FIG.

  First, in FIG. 1, a body frame 15 of a scooter type motorcycle includes a front fork 16 that pivotally supports a front wheel WF and a head pipe 18 that supports a steering handle 17 connected to the front fork 16 so as to be steerable. The down tube 19 extending rearwardly downward from the head pipe 18, a pair of left and right lower frame pipes 20... Which are fixed to the lower sides of the down tube 19 and extending rearward, and a rear end of the lower frame pipe 20. A pair of left and right rear frame pipes 21. The rear frame pipe 21 extends rearward from the rear end of the lower frame pipe 20 and the rear frame pipe 21 is substantially horizontal from the rear end of the rise frame part 21a. The rear frame pipe 21 is composed of an extended upper frame portion 21b, and the rear portion of both rear frame pipes 21 ... They are connected to each other.

  A bracket 22 is provided at an integrally connected portion of both the lower frame pipes 20 and the rear frame pipes 21 in the body frame 15, and the front portion of the power unit P is vertically moved via the link mechanism 23 to the bracket 22. The rear wheel WR disposed on the right side of the rear portion of the power unit P is pivotally supported on the rear portion of the power unit P. A rear cushion unit 24 is provided between the rear part of the left rear frame pipe 21 and the rear part of the power unit P among the rear frame pipes 21.

  A fuel tank 25 supported by the lower frame pipes 20 is disposed between the lower frame pipes 20 in front of the power unit P, and a storage box 26 is disposed above the power unit P. The storage box 26 is supported by the rear frame pipes 21.

  The vehicle body frame 15 is covered with a vehicle body cover 27 made of synthetic resin. The vehicle body cover 27 is provided under the leg shield 28 for covering the front of the driver's feet and the leg shield 28 for placing the rider's feet. A tandem type that includes a step floor 29 that continuously covers the fuel tank 25 and a side cover 30 that continues to the step floor 29 and covers the rear portion of the vehicle body from both sides. The storage box 26 can be opened and closed on the side cover 30 from above. Is provided.

  In FIG. 2, the power unit P includes an OHC type internal combustion engine E that is a four-cycle forced air-cooled single cylinder disposed in front of the rear wheel WR, and a continuously variable engine provided between the internal combustion engine E and the rear wheel WR. And a transmission M.

  The engine body 34 of the internal combustion engine E slides on a crankcase 35 that rotatably supports a crankshaft 45 having a rotation axis parallel to the rotation axis of the rear wheel WR, and a piston 41 connected to the crankshaft 45. A sleeve 39 having a cylinder bore 40 to be freely fitted is provided, a cylinder block 36 coupled to the crankcase 35, a cylinder head 37 coupled to the cylinder block 36 on the opposite side of the crankcase 35, and the cylinder block 36 And a head cover 38 coupled to the cylinder head 37 on the opposite side.

  The cylinder block 36, the cylinder head 37, and the head cover 38 are disposed between the rear frame pipes 21 of the vehicle body frame 15, and the cylinder block 36 slightly rises forward until the axis of the cylinder bore 40 becomes substantially horizontal. And is coupled to the crankcase 35.

  A combustion chamber 42 that faces the top of the piston 41 is formed between the cylinder block 36 and the cylinder head 37, and the piston 41 is connected to the crankshaft 45 via a connecting rod 43 and a crankpin 44.

  The crankcase 35 includes a pair of case halves 35a and 35b that include an axis of the cylinder bore 40 and are coupled to each other in a plane perpendicular to the axis of the crankshaft 45. The crankshaft 45 includes both case halves. It is rotatably supported between 35a and 35b.

  Referring also to FIG. 3, an intake port 47 that can communicate with the combustion chamber 42 is provided on the upper side surface of the cylinder head 37, and an exhaust port 48 that can communicate with the combustion chamber 42 on the lower side surface of the cylinder head 37. Provided. An intake valve 49 and an exhaust valve 50 that open and close the intake port 47 and the exhaust port 48, respectively, are arranged in the cylinder head 37 so as to be arranged in a substantially V shape on a projection view orthogonal to the rotation axis of the crankshaft 45. Provided, and spring-biased in the direction of closing the intake port 47 and the exhaust port 48, respectively. A spark plug 51 facing the combustion chamber 42 is attached to the right side surface of the cylinder head 37 with the motorcycle facing forward in the traveling direction.

  A valve operating mechanism 52 that opens and closes the intake valve 49 and the exhaust valve 50 is accommodated between the cylinder head 37 and the head cover 38, and the camshaft 53 provided in the valve operating mechanism 52 includes the crankshaft 45. Rotational power is transmitted at a reduction ratio of 1/2 through a timing transmission mechanism 55 including a cam chain 54. The cam chain 54 is a cam formed over the cylinder block 36, the cylinder head 37, and the head cover 38. The chain chamber 56 is stored so as to be able to run.

  The valve mechanism 52 has an axis parallel to the axis of the crankshaft 45 and is rotatably supported by the cylinder head 37, and includes a camshaft 53 having intake and exhaust cams 101 and 102, and a camshaft 53. Intake-side and exhaust-side rocker shafts 103 and 104 supported by the cylinder head 37 with axes parallel to each other, and the intake-side rocker shaft 103 driven by the intake cam 101 to drive the intake valve 49. And an exhaust-side rocker arm 106 swingably supported by the exhaust-side rocker shaft 104 so as to open and close the exhaust valve 50 by following the exhaust cam 102. With.

  Moreover, the camshaft 53 has a decompression cam 33 that protrudes from the base circle 102a of the exhaust cam 102 so as to protrude from the base circle 102a of the exhaust cam 102 when the rotational speed of the internal combustion engine E is lower than a predetermined value. The decompression device 32 is attached to the exhaust valve 50 at the closing timing determined by the base circle 102a of the exhaust cam 102 when the rotational speed of the internal combustion engine E is less than a predetermined value. Can be opened, whereby the exhaust valve 50 is slightly opened, and the cranking torque is reduced by slightly reducing the pressure in the combustion chamber 42.

  An intake pipe 57 leading to the intake port 47 is fastened to the upper side surface of the cylinder head 37, and the intake pipe 57 is formed to bend in an arc shape toward the rear side of the motorcycle. Further, as shown in FIG. 1, the upstream end of the intake pipe 57 is connected to the downstream end of the carburetor 58 disposed behind the intake pipe 57 via a connecting tube 59, and the upstream end of the carburetor 58 is upstream. The end is connected to an air cleaner 60 disposed above the power unit P on the left side of the rear wheel WR.

  An exhaust pipe 61 communicating with the exhaust port 48 is connected to the lower side surface of the cylinder head 37, and a downstream end of the exhaust pipe 61 is connected to an exhaust muffler 62 disposed on the right side of the rear wheel WR.

  4 to 5, the crankcase 35 is connected to a transmission case 80 extending to the left side of the rear wheel WR while facing the front in the running direction of the motorcycle. The transmission case 80 includes a case main body 81 integrally connected to the case half 35 b of the crankcase 35 and extending rearward, and a first transmission chamber 84 formed between the case main body 81 and the case main body 81. The left cover 82 is fastened to the case main body 81 from the left side, and the right cover 83 is fastened to the right rear portion of the case main body 81 by forming a second transmission chamber 85 between the case main body 81.

  A support arm portion 86 protrudes from a front portion of the case main body 81 in the transmission case 80 so as to be disposed on the side of the cylinder block 36 in the internal combustion engine E. The support arm portion 86 is formed on the body frame. Fifteen brackets 22 are swingably supported via a link mechanism 23.

  The axle 87 of the rear wheel WR is rotatably supported by the rear portion of the case main body 81 and the right cover 83 in the transmission case 80. A rear cushion unit 24 is provided between the rear portion of the case main body 81 and the upper frame portion 21 a of the left rear frame pipe 21 of the vehicle body frame 15.

  The continuously variable transmission M is a V-belt type housed in the first transmission chamber 84, and a reduction gear train 88 is provided between the continuously variable transmission M and the axle 87.

  The continuously variable transmission M includes a drive pulley 89 connected to the other end of the crankshaft 45 outside the crankcase 35, a rear portion of the case main body 81, and a right cover 83 having an axis parallel to the crankshaft 45. And a driven pulley 92 that is mounted on a driven shaft 90 rotatably supported by a centrifugal clutch 91, and a drive pulley 89 and an endless V-belt 93 that is wound around the driven pulley 92. is there.

  Thus, the drive pulley 89 includes a ramp plate 94 fixed to the crankshaft 45 and a plurality of weight rollers 95 housed in a floating state between the drive pulleys 89 according to an increase in the rotational speed of the crankshaft 45. It operates to increase the contact radius to the V-belt 93 as it moves radially outward 45.

  On the other hand, the driven pulley 92 operates so that the contact radius of the V-belt 93 becomes smaller as the contact radius of the V-belt 93 to the drive pulley 89 becomes larger, whereby the driven pulley 92 is connected between the crankshaft 45 and the driven shaft 90. A continuously variable transmission according to the rotation of the crankshaft 45 is performed.

  The reduction gear train 88 is provided between the driven shaft 90 and the axle 87 and is housed in the second transmission chamber 85. The rotational power of the driven shaft 90 in the continuously variable transmission M is reduced by the reduction gear train 88. To the axle 87 of the rear wheel WR.

  A kick shaft 97 is rotatably supported on the left cover 82 of the transmission case 80, and a kick pedal 98 (see FIG. 1) is provided on the outer end of the kick shaft 97. Further, on the inner surface side of the left cover 82, a kick-type starting device 99 is provided between the kick shaft 97 and the crankshaft 45 so that the power of the kick shaft 97 according to the depression operation of the kick pedal 98 can be transmitted to the crankshaft 45. Provided.

  Referring again to FIG. 2, the outer rotor 66 of the generator 65 is fixed to one end of the crankshaft 45 outside the crankcase 35, and the stator 67 surrounded by the outer rotor 66 is a crankcase. 35. A cooling fan 68 is fixed to the crankshaft 45 outside the generator 65. The cooling fan 68 is configured such that a plurality of blades 68b, 68b,... Are integrally provided on the outer peripheral portion of a base portion 68a fastened to the outer rotor 66 of the generator 65.

  Of the engine body 34, a part of the crankcase 35, the cylinder block 36, and the cylinder head 37 are covered with a shroud 70, and a cooling air passage 71 formed between the engine body 34 and the shroud 70 is connected to the cooling fan 68. Discharged forced air cooling air flows. In addition, a plurality of cooling fins 36a, 37a,... Protrude from the outer surfaces of the cylinder block 36 and the cylinder head 37 in order to enable efficient cooling by the cooling air flowing through the cooling air passage 71. A cutout space 78 is formed in a part of the outer periphery of the cylinder block 36 by partially shortening or missing the cooling fins 36a projecting from the outer periphery of the cylinder block 36. The notch space 78 is formed in a portion corresponding to the upper side surface of the outer periphery of the cylinder block 36 on the side corresponding to the intake port 47 provided in the cylinder head 36, that is, the outer periphery of the cylinder block 36.

  The shroud 70 covers a part of the crankcase 35 and a pair of upper and lower upper and lower cover members 72 and 73 coupled to each other so as to cover the cylinder block 36 and the cylinder head 37 in cooperation. The upper cover member 72, the lower cover member 73, and the fan cover 74 are each formed of a synthetic resin.

  The upper cover member 72 is provided with a plurality of engaging claws 108 projecting from the mating surface 107 with the lower cover member 73 toward the lower cover member 73, and the lower cover member 73 has the engaging claws 108. Is provided with a locking portion 109. The upper and lower cover members 72, 73 are integrally provided with a plurality of fastened portions 110 ..., 111 ... that are in contact with each other at the mating surface 107, and the fastened portions 110 ..., 111 ... that are in contact with each other. Are fastened to each other by screw members 112.

  In this way, the upper and lower cover members 72 and 73 are mutually connected by the elastic engagement of the engaging claws 108 to the locking portions 109 and the fastening of the fastened portions 110. The upper and lower cover members 72 and 73 coupled to each other are fastened to the crankcase 35 of the engine body 34 by a plurality of bolts 113... And the head cover 38 is connected to the upper and lower cover members 72 and 73. Projects forward.

  The fan cover 74 is fastened to the case half 35 a so as to cover the case half 35 a and the cooling fan 68 that are part of the crankcase 35, and is fastened to the upper and lower cover members 72 and 73.

  The fan cover 74 is provided with a suction cylinder 76 that forms a suction port 75 for sucking air from the outside toward the cooling fan 68 so as to correspond to the cooling fan 68 on the outside. The suction port 76 is provided with a louver 77.

  In FIG. 7, a flange 114 facing the opening end of the exhaust port 48 is provided on the lower side surface of the cylinder head 37 so as to connect the exhaust pipe 61, but the lower cover member 73 of the shroud 70 is provided. Is provided with an opening 115 having a shape corresponding to the flange portion 114 in the front portion so as to spread rearward from a portion corresponding to the flange portion 114, and the exhaust pipe 61 is connected to the flange portion 114. In the state, an exhaust port 116 is formed between the outer surface of the exhaust pipe 61 and the periphery of the opening 115. That is, the lower cover member 73 in the shroud 70 is provided with the exhaust port 116 adjacent to the exhaust pipe 61.

  Moreover, the spark plug 51 is attached to the right side surface of the cylinder head 37 along the traveling direction of the motorcycle with an axis line arranged on a plane including the mating surfaces 107 of the upper and lower cover members 72 and 73 in the shroud 70. On the other hand, in one of the pair of side surfaces along the plane including the axis of the spark plug 51 among the side surfaces of the shroud 70, the exhaust port 116 is formed on the lower side surface of the lower cover member 73 in the shroud 70 in this embodiment.

  8 and 9, the notch formed on the outer periphery of the cylinder block 36 is formed on the side surface of the shroud 70 on the side corresponding to the intake port 47 of the cylinder head 36, that is, on the upper side surface of the upper cover member 72. A first protrusion 72 a that protrudes inward along the space 78 is formed. Further, a second protrusion 73 a that protrudes inward at a position sandwiching the spark plug 51 attached to the side surface of the cylinder head 37 with the first protrusion 72 a is formed in the lower cover member 73 in the shroud 70. Is done.

  Moreover, as clearly shown in FIG. 6, the first protrusion 72a and the second protrusion 73a are offset from each other in the direction along the axis of the spark plug 51 as viewed from the direction along the cylinder axis. One of a pair of side surfaces disposed on both sides of a plane including the axis of the spark plug 51 among the side surfaces of the shroud 70, and in this embodiment, on the lower side surface of the lower cover member 73 of the shroud 70, The air outlet 116 and the second protrusion 73a are formed so that the portion corresponding to 51 is sandwiched between them.

  By the way, the spark plug 51 is attached to the cylinder head 37 in a posture inclined toward the cooling fan 68, and the mating surface 107 between the upper and lower cover members 72, 73 in the shroud 70 is attached to the spark plug 51. A through-hole 117 is formed to allow the attachment operation and lead out the high tension cord connected to the spark plug 51 from the shroud 70 to the outside.

  Next, the operation of this embodiment will be described. The cylinder block 36 and the cylinder head 37 are surrounded by a shroud 70 that forms a cooling air passage 71 between the cylinder block 36 and the cylinder head 37, and suction is performed from the outside of the shroud 70. A cooling fan 68 that blows the cooled cooling air to the cooling air passage 71 side is interlocked and connected to the crankshaft 45, and the cooling fin 36a protruding from the outer periphery of the cylinder block 36 is partially shortened or missing in the shroud 70. At least the first protrusion 72 a that protrudes inward along a notch space 78 formed at a part of the outer periphery of the cylinder block 36 and the spark plug 51 that is attached to one side surface of the cylinder head 37 are provided. A second protrusion 73a that protrudes inward at a position sandwiched between the protrusion 72a and the protrusion 72a is formed.

  Therefore, the cooling air blown by the cooling fan 68 is discharged from the air exhaust port 116 on the lower side surface of the lower cover member 73, but the first and first sides are sandwiched between the spark plugs 51 on the side surfaces of the shroud 70. Since the two protrusions 72a and 73a are formed, the cooling air flowing toward the exhaust port 116 is effectively guided to the periphery of the spark plug 51 by the first and second protrusions 72a and 73a, and the spark plug 51 is efficiently Can be cooled. Moreover, since the 1st protrusion part 72a protrudes inward along the notch space 78 formed in the outer periphery of the cylinder block 36, it may increase the amount of cooling air which distribute | circulates the periphery of the cylinder head 37 with higher temperature. it can.

  Moreover, since the air exhaust port 116 is formed in the lower cover member 73 of the shroud 70 adjacent to the exhaust pipe 61 connected to the cylinder head 37 through the exhaust port 48 of the cylinder head 37, it tends to be hot. The vicinity of the exhaust port 48 can be effectively cooled.

  Further, since the first projecting portion 72a projecting inward is formed on the side surface of the shroud 70 corresponding to the intake port 47 of the cylinder head 36, that is, the upper side surface of the upper cover member 72, the intake port having a relatively low temperature. The amount of cooling air flowing around 47 can be reduced, and the amount of cooling air flowing around the spark plug 51 and the exhaust port 48 can be increased accordingly.

  Further, in this embodiment, a portion corresponding to the spark plug 51 is formed on one of the pair of side surfaces disposed on both sides of the plane including the axis of the spark plug 51 among the side surfaces of the shroud 70. Since the exhaust port 116 and the second projection 73a are formed so as to be sandwiched between them, the cooling air path that flows in the vicinity of the spark plug 51 and reaches the exhaust port 116 and the upper cover member 72 side are exhausted. The path of the cooling air reaching 116 can be reliably formed.

  Further, since the first protrusion 72a and the second protrusion 73a are arranged so as to be offset from each other in the direction along the plane including the axis of the spark plug 51 as viewed from the direction along the cylinder axis, Therefore, the passage area of the cooling air flowing around the cylinder head 37 can be secured, and the cooling air can be easily collected around the spark plug 51 while avoiding a decrease in the cooling air volume as a whole.

  Further, since the spark plug 51 is inclined to the cooling fan 68 side and attached to the cylinder head 37, the spark plug 51 is cooled with cooler air having a lower temperature by bringing the spark plug 51 closer to the cooling fan 68 side. Can do.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various design changes can be made without departing from the present invention described in the claims. It is.

It is a left view of a scooter type motorcycle. FIG. 2 is an enlarged sectional view taken along line 2-2 of FIG. FIG. 3 is an enlarged sectional view taken along line 3-3 in FIG. 2. FIG. 4 is a view taken in the direction of arrow 4 in FIG. 2. FIG. 5 is a view taken in the direction of arrow 5 in FIG. 2. FIG. 6 is a view taken in the direction of arrow 6 in FIG. 4. It is the figure which looked at the lower cover member of the shroud from the direction of arrow 7 in FIG. It is the figure which looked at the upper cover member of the shroud from the direction of arrow 8 in FIG. FIG. 9 is a sectional view taken along line 9-9 in FIG. 8.

Explanation of symbols

35 ... Crankcase 36 ... Cylinder block 37 ... Cylinder head 45 ... Crankshaft 47 ... Intake port 48 ... Exhaust port 51 ... Spark plug 61 ... Exhaust pipe 68 .... Cooling fan 70 ... shroud 71 ... cooling air passage 72a ... first protrusion 73a ... second protrusion 78 ... notch space 116 ... air outlet

Claims (6)

  A cylinder block (36) that is coupled to a crankcase (35) that rotatably supports the crankshaft (45) and is provided with a cooling fin (36a), and is coupled to the cylinder block (36) and ignited. A cylinder head (37) to which a plug (51) is attached is surrounded by a shroud (70) that forms a cooling air passage (71) between the cylinder block (36) and the cylinder head (37), and A cooling fan (68) for blowing cooling air sucked from the outside of the shroud (70) to the cooling air passage (71) side is linked and connected to the crankshaft (45), and the shroud (70) is connected to the shroud (70). In the forced air-cooled internal combustion engine in which the exhaust port (116) for discharging the cooling air flowing through the cooling air passage (71) to the outside is formed A cutout space (78) is formed in a part of the outer periphery of the cylinder block (36) by partially shortening or missing the cooling fin (36a) protruding from the outer periphery of the cylinder block (36). The shroud (70) includes a first protrusion (72a) protruding inward along the notch space (78), and the spark plug (51) attached to one side surface of the cylinder head (37). A forced air-cooling type internal combustion engine, wherein a second protrusion (73a) that protrudes inward is formed at a position between the first protrusion (72a) and the first protrusion (72a).   The exhaust port (116) communicates with the shroud (70) adjacent to an exhaust pipe (61) connected to the cylinder head (37) through an exhaust port (48) provided in the cylinder head (37). The forced air-cooled internal combustion engine according to claim 1, wherein the forced air-cooled internal combustion engine is provided.   The notch space (78) is formed in the outer periphery of the cylinder block (36) on the side corresponding to the intake port (47) provided in the cylinder head (36). Forced air-cooled internal combustion engine.   Of the side surface of the shroud (70), a portion corresponding to the spark plug (51) is sandwiched between one of a pair of side surfaces disposed on both sides of a plane including the axis of the spark plug (51). The forced air-cooling type internal combustion engine according to any one of claims 1 to 3, wherein an exhaust port (116) and the second protrusion (73a) are formed.   The first protrusion (72a) and the second protrusion (73a) are arranged so as to be offset from each other in a direction along a plane including the axis of the spark plug (51) when viewed from the direction along the cylinder axis. The forced air-cooling type internal combustion engine according to any one of claims 1 to 4.   The forced air-cooling type internal combustion engine according to any one of claims 1 to 5, wherein the spark plug (51) is attached to the cylinder head (37) so as to be inclined toward the cooling fan (68).

Images