Patent Publication Number: US-11041425-B1

Title: Air-cooled engine

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an air-cooled engine that is cooled by cooling air generated by a cooling fan. 
     Description of Related Art 
     Air-cooled engines used for e.g. mowers have been known in which a disk-shaped screen rotates integrally with a cooling fan and a screen cover covers the screen. In such engines, foreign matters (e.g., grass clippings) are sucked in together with air when the cooling fan operates. If the foreign matters get into the engine, they may stick to cooling fins of a cylinder, leading to deterioration in cooling performance of the engine. Thus, the screen cover is provided so as to suppress entry of the foreign matters, and the screen is used to finely shred the foreign matters. 
     However, since it is difficult to completely shred foreign matters to a desired size or smaller, it is required to improve shredding capability. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a cover structure for an air-cooled engine, which is capable of improving the capability to shred foreign matters. 
     In order to achieve the above object, an air-cooled engine of the present invention includes:
         a cooling fan configured to rotate in conjunction with an engine rotation shaft;   a fan housing covering the cooling fan;   a screen having a disk shape and configured to rotate integrally with the cooling fan so as to shred foreign matters; and   a screen cover having a plurality of openings defined therein and attached to the fan housing, the screen cover suppressing entry, into the fan housing, of foreign matters of a specific size or larger which are drawn by the cooling fan, wherein   the screen cover has a polygonal outer shape when viewed from an axial direction of the engine rotation shaft.       

     According to this configuration, a non-constant radial distance (gap) is defined between an outer edge of the screen and an inner surface of the screen cover in a circumferential direction of the screen. Specifically, the outer edge of the screen and the inner surface of the screen cover define a gap therebetween such that large-gap regions where the gap is larger and small-gap regions where the gap is smaller are alternately arranged. Thus, in the small-gap regions where the gap is smaller between the outer edge of the screen and the inner surface of the screen cover, a foreign matter rotates having one end caught in the gap of the screen and the other end hit against the screen cover so that the capability to shred the foreign matters is improved. As a result, it is possible to reduce the amount of the foreign matters sticking to the cooling fins of the cylinders so as to prevent deterioration of the cooling performance of the engine. 
     In the present invention, the screen cover may be inclined so as to have a decreasing outer diameter with an increasing distance from the cooling fan in the axial direction of the engine rotation shaft on a cross section along a plane including an axis of the engine rotation shaft. According to this configuration, the screen cover is formed in a pyramid shape, and therefore, it is easier to suppress deformation due to a force applied in the axial direction of the engine rotation shaft when compared to the case where the screen cover has a cylindrical shape. The rigidity of the screen cover is thus improved. 
     In the present invention, a shield part configured to prevent entry of foreign matters into the fan housing may be formed at a central part of the screen cover having the polygonal shape. The screen has a higher shredding capability in an outer edge part thereof thanks to a higher peripheral speed of rotation at this part, whereas the screen has a lower shredding capability at the central part. This configuration makes it possible to prevent introduction of the foreign matters to the central part where the shredding effect is lower. As a result, the capability to shred the foreign matters is improved. 
     In the present invention, on the cross section along the plane including the axis of the engine rotation shaft, the screen cover may include: a first inclined part inclined so as to have a decreasing outer diameter from a first proximal end portion that is in contact with the fan housing toward a first distal end portion; and a second inclined part inclined at an inclination angle larger than an inclination angle of the first inclined part so as to have a decreasing outer diameter from a second proximal end portion connected to the first distal end portion of the first inclined part toward a second distal end portion. As used herein, the term “inclination angle” refers to an inclination angle with respect to the axial direction of the engine rotation shaft. This configuration makes it possible to enhance the rigidity of the screen cover while providing a sufficient axial distance between the screen and the screen cover. Since the screen and the screen cover are separated in the axial direction, for example, it is possible to prevent the screen cover from coming into contact with the screen due to an external impact. 
     In the present invention, the screen cover may include: a first rib having a main part, at least the main part extending in a radial direction of an inscribed circle of the polygonal shape when viewed from the axial direction of the engine rotation shaft; and a second rib extending in tangential directions of circles concentric with the inscribed circle when viewed from the axial direction. In such a case, the first rib may be thicker than the second rib. According to this configuration, the two types of the ribs enhance the rigidity of the screen cover. Also, the two types of the ribs having different thicknesses can effectively reinforce the screen cover. 
     Where the first ribs and the second ribs are provided, the first rib may extend from corners of the screen cover having the polygonal shape. This configuration makes it possible to enhance the rigidity of portions of the screen cover at which stress is concentrated. 
     In such a case, the screen cover may include a frame part constituting an outer peripheral edge of the polygonal shape, and a plurality of the first ribs, two adjacent ones of the first ribs and the frame part being located on three sides of a triangle when viewed from the axial direction of the engine rotation shaft. For example, two of the first ribs and the frame part define an isosceles triangle having the apex on the axis of engine rotation shaft. This configuration makes it possible to enhance the rigidity of the portions of the screen cover at which stress is concentrated. 
     Where the first ribs and the second ribs are provided, the screen cover may be removably attached to the fan housing by a fastener; the screen cover may be provided with an attachment part through which the fastener is inserted; and the first rib may extend from the attachment part in the radial direction. According to this configuration, the first ribs improve the rigidity of the attachment part. 
     In such a case, the attachment part may be provided so as to protrude radially outward from a side of the polygonal shape of the screen cover when viewed from the axial direction of the engine rotation shaft. According to this configuration, the attachment part provided at the side of the polygonal shape makes it possible to reduce the space for disposing the entire screen cover. 
     Where the first ribs and the second ribs are provided, the screen cover may include a plurality of the second ribs aligned in the radial direction, and a part of the plurality of second ribs may be longer than the rest of the second ribs in the axial direction of the engine rotation shaft. According to this configuration, the part of the second ribs which is longer than the rest of the second ribs in the axial direction of the engine rotation shaft makes it possible to effectively reinforce the portions of the screen cover at which stress is concentrated while keeping sufficient opening areas between the adjacent second ribs in other sections so as to secure a necessary amount of air. 
     In the present invention, the polygonal shape may be, for example, a hexagon. The screen cover is attached to the fan housing typically at three or more points. A hexagonal structure enables a well-balanced attachment when the cover is fixed at three points. 
     In the present invention, the screen cover may include a plurality of the first ribs and third ribs each located at an intermediate position between the two adjacent ones of the first ribs. In such a case, each of the third ribs may extend from a side of the polygonal shape in a direction including an axial component of the engine rotation shaft. Also, each of the third ribs may extend in a direction including an axial component of the engine rotation shaft from a position in the vicinity of a point at which the inscribed circle of the screen cover having the polygonal shape is in contact with the polygonal shape of the screen cover. According to this configuration, since the third ribs are provided in the areas where the distance between the outer edge of the screen and the inner surface of the screen cover is smaller, foreign matters are more likely to be caught by the third ribs so as to be shredded. 
     In the present invention, the screen cover may be removably attached to the fan housing by a fastener; the screen cover may be provided with an attachment part through which the fastener is inserted; and the attachment part may be provided so as to protrude radially outward from a side of the polygonal shape of the screen cover. In such a case, the attachment part may be provided so as to protrude radially outward from a position in the vicinity of a point at which the inscribed circle of the screen cover having the polygonal shape is in contact with the polygonal shape of the screen cover. According to this configuration, the attachment part provided at the side of the polygonal shape makes it possible to reduce the space for disposing the entire screen cover. 
     In the present invention, the screen cover may have a regular-polygonal outer shape when viewed from the axial direction of the engine rotation shaft. This configuration makes it possible to provide areas where the distance between the outer edge of the screen and the inner surface of the screen cover is small, at equal intervals in the circumferential direction. 
     The present invention encompasses any combination of at least two features disclosed in the claims and/or the specification and/or the drawings. In particular, any combination of two or more of the appended claims should be equally construed as included within the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more clearly understood from the following description of a preferred embodiment thereof, when taken in conjunction with the accompanying drawings. However, the embodiment and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views: 
         FIG. 1  is a front view of an air-cooled engine according to a first embodiment of the present invention; 
         FIG. 2  is a plan view of the engine; 
         FIG. 3  is a plan view of the engine, with a fan housing removed; 
         FIG. 4  is a plan view of the engine, with a screen cover partially cut; 
         FIG. 5  is a plan view of a screen of the engine; 
         FIG. 6  is a front view of the screen; 
         FIG. 7  is a plan view of the screen cover of the engine; 
         FIG. 8  is a longitudinal cross-sectional view of a part of the screen cover; 
         FIG. 9A  is a schematic view of the screen and the screen cover; and 
         FIG. 9B  is a developed view of the screen cover shown in  FIG. 9A , in which the sides of the screen cover are developed in a linear manner. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the drawings. In  FIG. 1 , the engine E including a cover structure according to a first embodiment of the present invention is a so-called V-type engine in which cylinder axes A 1  of two cylinder units  6  extend in a V shape. The engine E of the present embodiment is an air-cooled vertical-twin engine having a rotation shaft extending in a vertical direction and may be mounted in e.g. a riding mower such that the rotation shaft extends in the vertical direction. In the following description, the term “front” and the like refer to a V-bank side, i.e., a direction in which the V shape is opened in a state where the engine is mounted in a machine (e.g., a mower or an agricultural machine), and the term “rear” and the like refer to the opposite side. Also, the “vertical direction” and the like refer to an axial direction of a rotation shaft of the machine, and the “widthwise direction” and the like refer to a direction perpendicular to both of the vertical direction and the front/rear direction. However, the type and application of the engine E are not limited to these. 
     The engine E of the present embodiment includes: a crankshaft  2  (one example of the engine rotation shaft) having an axis AX extending in the vertical direction in a state where the engine is mounted in the machine; a crankcase  4  supporting the crankshaft  2 ; and a pair of cylinder units  6 ,  6  protruding frontward from a front part of the crankcase  4 . The crankshaft  2  has a lower end portion to which a power transmission member for transmitting power to a work tool (such as a mower blade) is attached. 
     Each cylinder unit  6  includes: a cylinder  8  having a base end portion coupled to the crankcase  4 ; and a cylinder head  10  coupled to a protruding end portion of the cylinder  8 . As shown in  FIG. 3 , each cylinder unit  6  has a cylinder axis A 1  extending frontward and outwardly in a widthwise direction of the engine in a slant manner. The cylinder axes A 1  of the two cylinder units  6  define a V shape opened frontward. 
     A cooling fin  12  is formed on an outer periphery of each cylinder unit  6 . The cooling fin  12  provides an increased surface area so that the cooling effect of the air-cooled engine is improved. A head cover  14  is attached to a front end of each cylinder unit  6 . An intake port  10   a  is formed on one side (on an upper side in the present embodiment) of each cylinder head  10  in the axial direction AX of the engine rotation shaft  2 , and an exhaust port (not illustrated) is formed on the other side (on a lower side in the present embodiment). Intake system components such as an air cleaner AC and a throttle body TB are connected to the intake ports  10   a , and exhaust system components (not illustrated) such as an exhaust pipe and an exhaust muffler are connected to the exhaust ports. 
     A cooling fan  16  is attached to an upper end of the crankshaft  2 . The cooling fan  16  rotates in conjunction with the engine rotation shaft (crankshaft  2 ). The cylinder units  6  are cooled by cooling air W generated by the cooling fan  16 . The cooling fan  16  is disposed on one side with respect to the cylinder units  6  (above the cylinder units in the present embodiment) in the axial direction AX of the engine rotation shaft  2 . The cooling fan  16  of the present embodiment is a sirocco fan configured to feed the cooling air W in a radial fashion from the engine rotation shaft  2  toward a radially outer side. The cooling fan  16  is not limited to this example. 
     A screen  17  having a disk shape as shown in  FIG. 4  is attached to an upper end of the cooling fan  16 . The screen  17  is configured to rotate integrally with the cooling fan  16  and shred foreign matters G such as grass clippings that are sucked in by a suction force of the cooling fan  16 . Details of the screen  17  will be described later. 
     As shown in  FIG. 1 , a fan housing  18  is attached to the crankcase  4 . The fan housing  18  may be formed of, for example, a sheet metal. The material of the fan housing  18 , however, is not limited to this example. The fan housing  18  covers an outer periphery and an upper side of the cooling fan  16  shown in  FIG. 3 , except for a front side of the cooling fan  16 , and guides the cooling air W into the cylinder units  6 . 
     As shown in  FIG. 2 , the fan housing  18  has an upper side on which opening parts  25  for maintenance of the cylinder units  6  are formed. There are two such opening parts  25  arranged so as to face the cylinder units  6 ,  6  of the respective cylinders. When carrying out maintenance, for example, an air duster can be inserted from the opening parts  25  to remove grass clippings etc. caught in the cylinder units  6 . 
     Each of the opening parts  25  is closed by a debris cover  26 . The debris cover  26  is removably attached to the fan housing  18  by a fastener  27  such as a bolt. The opening part  25  is closed when the debris cover  26  is attached to the fan housing  18 , and the opening part  25  is opened when the debris cover  26  is removed from the fan housing  18 . 
     As shown in  FIG. 4 , the screen  17  includes a cutter part  17   a  that constitutes an outer peripheral edge of the screen  17  and a screen part  17   b  that covers the cooling fan  16  from one side (from above in the present embodiment) in the axial direction AX of the engine rotation shaft  2 . The screen  17  is attached to the cooling fan  16  by a plurality of (four in the present embodiment) fasteners  22 . 
     The screen  17  is made of, for example, a metal plate material. As shown in  FIG. 6 , the screen part  17   b  of the screen  17  is smoothly bulged from the cutter part  17   a  at the outer edge to a central part in a direction away from the cooling fan  16  (upwardly in the present embodiment). The screen part  17   b  is formed with a plurality of punch holes  17   c  ( FIG. 5 ) over a substantially entire surface thereof. That is, the air sucked in by the cooling fan  16  can pass through the screen part  17   b , whereas foreign matters G such as grass clippings do not pass through the screen part  17   b . The foreign matters G captured by the screen part  17   b  are guided along the smooth surface of the screen part  17   b  to the cutter part  17   a.    
     The cutter part  17   a  of the screen  17  includes a plurality of protrusions  17   d  arranged in a circumferential direction as shown in  FIG. 6 . That is, the cutter part  17   a  has the protrusions  17   d  and recesses  17   e  that are alternately arranged in the circumferential direction. The cutter part  17   a  is configured to shred the foreign matters G passing through the recesses  17   e  by the protrusions  17   d . More specifically, the protrusions  17   d  and recesses  17   e  and an end portion of the fan housing  18  define a gap therebetween, and the foreign matters G are shredded between the moving protrusions  17   d  and the end portion of the fan housing  18  as they pass through such a gap. 
     A screen cover  20  is attached to the upper side of the fan housing  18  as shown in  FIG. 2 . The screen cover  20  covers the screen  17  from above and is fixed to the fan housing  18 . The screen cover  20  can suppress entry, into the fan housing  18 , of foreign matters G drawn by the cooling fan  16  ( FIG. 3 ), where the foreign matters are of a specific size or larger. 
     The screen cover  20  shown in  FIG. 2  is made of a resin and integrally formed by die molding. The screen cover  20  of the present embodiment has a non-texture processed, smooth surface. The material and the production method for the screen cover  20  are not limited to these. 
     In the present embodiment, the screen cover  20  has a hexagonal outer shape when viewed from the axial direction AX of the engine rotation shaft  2 . However, as long as the screen cover  20  has a polygonal outer shape when viewed from the axial direction AX of the engine rotation shaft  2 , the shape is not limited to a hexagonal shape. The screen cover  20  preferably has a regular-polygonal outer shape and more preferably a polygonal outer shape with four or more sides when viewed from the axial direction AX of the engine rotation shaft  2 . 
     The screen cover  20  includes: a frame part  20   a  constituting an outer peripheral edge of the screen cover  20 ; a central shield part  20   b  having a plate shape; first ribs  20   c  connecting the frame part  20   a  and the shield part  20   b ; and second ribs  20   d  intersecting with the first ribs  20   c . The first ribs  20   c  and the second ribs  20   d  define a plurality of slit-like openings  24 . Air A can passes through the openings  24  and flows into the fan housing  18 , whereas foreign matters G larger than the openings  24  cannot pass through the screen cover  20 . The plate-like shield part  20   b  is constituted by a face perpendicular to the axial direction AX of the engine rotation shaft  2 . The frame part  20   a  constitutes an outer peripheral edge of the polygonal (hexagonal) screen cover  20 . 
     The screen cover  20  is bulged from the frame part  20   a  at the outer edge to the shield part  20   b  at the center in a direction away from the screen  17  (upwardly in the present embodiment). Specifically, as shown in  FIG. 8 , the screen cover  20  is inclined so as to have a decreasing outer diameter (toward the top of  FIG. 8 ) with an increasing distance from the screen  17  (cooling fan  16 ) in the axial direction AX of the engine rotation shaft  2  on a cross section along a plane including the axis of the engine rotation shaft  2 . 
     More specifically, the screen cover  20  includes a first inclined part  32  and a second inclined part  34  having a different inclination angle from that of the first inclined part. As used herein, the term “inclination angle” refers to an inclination angle with respect to the axial direction AX of the engine rotation shaft  2 . On the cross-section along a plane including the axis AX of the engine rotation shaft  2 , the first inclined part  32  of the screen cover  20  is inclined so as to have a decreasing outer diameter from a first proximal end portion  32   a  that is in contact with the fan housing  18  toward a first distal end portion  32   b.    
     The second inclined part  34  of the screen cover  20  is inclined at an inclination angle θ 2  larger than an inclination angle θ 1  of the first inclined part  32  so as to have a decreasing outer diameter from a second proximal end portion  34   a  connected to the first distal end portion  32   b  of the first inclined part  32  toward a second distal end portion  34   b . The second distal end portion  34   b  of the second inclined part  34  is connected to the shield part  20   b  at the center of the screen cover  20 . 
     As shown in  FIG. 7 , the first ribs  20   c  extend in a radial direction of an inscribed circle C of the polygon (hexagon). It is only necessary that at least main parts of the first ribs  20   c  extend in the radial direction when viewed from the axial direction. AX of the engine rotation shaft  2 . In the present embodiment, the first ribs  20   c  extend from corners of the polygonal (hexagonal) screen cover  20 . In other words, two adjacent ones of the first ribs  20   c  and the frame part  20   b  are located on three sides of a triangle when viewed from the axial direction AX of the engine rotation shaft  2 . In the present embodiment, two of the first ribs  20   c ,  20   c  and the frame part  20   b  define an isosceles triangle having the apex on the axis AX of engine rotation shaft  2 . 
     On the other hand, the second ribs  20   d  extend parallel to tangential directions of circles concentric with the inscribed circle C of the polygonal screen cover  20 . It is only necessary that the second ribs  20   d  extend in the tangential directions of the concentric circles when viewed from the axial direction AX of the engine rotation shaft  2 . There are a plurality of such second ribs  20   d  aligned on a radially outer side with respect to the axial direction AX. In the present embodiment, as shown in  FIG. 8 , a part (denoted by  20   d ( 1 ) in  FIG. 8 ) of the plurality of second ribs  20   d  is longer than the rest of the second ribs in the axial direction AX of the engine rotation shaft  2 . That is, a part  20   d ( 1 ) of the second ribs  20   d  is thicker and, therefore, has a larger cross section than the rest of the second ribs  20   d  in the axial direction AX. However, all the second ribs  20   d  may have a same thickness. 
     As shown in  FIG. 7 , in the present embodiment, the first ribs  20   c  are thicker than the second ribs  20   d , i.e., wider in the circumferential direction. However, the first ribs  20   c  and the second ribs  20   d  may have a same thickness, or the second ribs  20   d  may be thicker than the first ribs  20   c.    
     The screen cover  20  further includes third ribs  36  each located at an intermediate position between two adjacent ones of the first ribs  20   c ,  20   c . Each of the third ribs  36  extends from a side of the polygonal shape of the screen cover  20  in a direction including a component of the axial direction AX of the engine rotation shaft  2 . The third ribs  36  are formed in the first inclined part  32  and extend from the sides of the frame part  20   a  of the polygonal screen cover  20  along the first inclined part  32 . Specifically, each of the third ribs  36  is formed at a position in the vicinity of a point at which the inscribed circle C of the screen cover  20  is closest to (as used herein, expressed as “in contact with”) the polygonal screen cover  20 . The third ribs  36  may be omitted. 
     The screen cover  20  is removably attached to the fan housing  18  by fasteners  30  such as bolts. Specifically, the screen cover  20  is provided with an attachment part  38  having an insertion hole  38   a  through which the fastener  30  is inserted. The screen cover  20  is attached to the fan housing  18  through the attachment part  38 . In the present embodiment, the screen cover  20  is attached to the fan housing  18  through three attachment parts  38  disposed approximately 120° apart in the circumferential direction. The number of the attachment parts  38  is not limited to this example. 
     The attachment part  38  is provided so as to protrude radially outward from a side of the polygonal screen cover  20  when viewed from the axial direction AX of the engine rotation shaft  2 . In other words, the attachment part  38  is provided so as to protrude radially outward at a position in the vicinity of a point at which the inscribed circle C of the polygonal screen cover  20  is in contact with the polygonal screen cover  20 . In the present embodiment, the third ribs  36  extend from the attachment parts  38  in the axial direction. 
     The fastener  30  is, for example, a stepped bolt and is screwed into a threaded hole (not illustrated) provided to the fan housing  18 . The threaded hole (not illustrated) is, for example, a weld nut. The fastening structure of the screen cover  20  is not limited to this example. 
     The flow of the cooling air W and foreign matters G in the present embodiment will be described. When the engine E shown in  FIG. 2  is started and causes the crankshaft  2  to rotate, the cooling fan  16  and the rotary screen  17  also rotate integrally with the crankshaft  2 . As the cooling fan  16  rotates, the air A is sucked into the fan housing  18  as the cooling air W. The cooling air W is guided in the fan housing  18  and flows downward so as to cool cooling target components such as the cylinder units  6 ,  6 . 
     During operation, foreign matters G such as grass clippings cut by a mower blade are blown up and move toward the upper side of the engine E. Such foreign matters G are sucked into the fan housing  18  due to a suction force of the cooling fan  16 . The foreign matters G of a specific size or larger (e.g., grass clippings longer than 10 cm) are blocked by the screen cover  20  and thus cannot enter the fan housing  18 . Although the foreign matters G such as grass cuttings smaller than the openings  24  of the screen cover  20  (e.g., grass clippings of about 5 to 6 cm long) may pass through the screen cover  20 , they will be shredded finely (e.g., into grass clippings shorter than 1 cm) by the screen  17  shown in  FIG. 4 . The foreign matters G shredded by the screen  17  is exhausted from the gap between the crankcase  4  and the fan housing  18  shown in  FIG. 1 . 
     Since, as shown in  FIG. 4 , the screen  17  has a disk shape and the screen cover  20  has a hexagonal shape, a radial distance (gap) between the cutter part  17   a  at the outer edge of the screen  17  and an inner surface of the screen cover  20  is not constant in the circumferential direction of the screen  17  that rotates in a rotation direction D 1 . Specifically, as shown in  FIGS. 9A and 9B , the cutter part  17   a  of the screen  17  and the inner surface of the screen cover  20  define a gap therebetween such that large-gap regions R 1  where the gap is larger and small-gap regions R 2  where the gap is smaller are alternately arranged. Thus, in the small-gap region R 2  where the gap is smaller between the outer edge of the screen and the inner surface of the screen cover, the foreign matter G rotates having one end caught in the gap of the screen and the other end hit against the screen cover  20  so that the capability to shred the foreign matters G is improved. 
     Also, elongated foreign matters G such as grass clippings may be caught (clogged) in the punch holes  17   c  of the screen part  17   b  of the screen  17 . Clogging of such foreign matters G is, in particular, likely to occur at the outer peripheral part of the screen part  17   b  in which a strong suction force is generated. In the present invention, thanks to the alternate arrangement of the large-gap regions R 1  where the gap between the cutter part  17   a  of the screen  17  and the inner surface of the screen cover  20  is larger and the small-gap regions R 2  where the gap is smaller, when the screen  17  rotates, one end of the foreign matter G is caught in the screen part  17   b  and the other end thereof is brought into contact with the screen cover  20  multiple times in the small-gap regions R 2  in such a manner that the other end thereof is hit against the screen cover  17 . Consequently, the foreign matters G caught in the screen part  17   b  are removed from the punch holes  17   c  and flow to the cutter part  17   a  so as to be shredded. 
     According to the above configuration, the cutter part  17   a  of the screen  17  and the inner surface of the screen cover  20  define the gap therebetween such that the large-gap regions R 1  where the gap is larger and the small-gap regions R 2  where the gap is smaller are alternately arranged as discussed above. Thus, in the small-gap regions R 2 , the foreign matter G rotates having one end caught in the gap of the screen, and the other end thereof is hit against the screen cover  20  so that the capability to shred the foreign matters G is improved. As a result, it is possible to reduce the amount of the foreign matters G sticking to the cooling fins  12  of the cylinders  6  so as to prevent deterioration of the cooling performance of the engine E. 
     As shown in  FIG. 8 , the screen cover  20  is inclined so as to have a decreasing outer diameter with an increasing distance from the cooling fan  16  in the axial direction AX of the engine rotation shaft  2 . Thus, the screen cover  20  is formed in a pyramid shape, and therefore, it is easier to suppress deformation due to a force applied in the axial direction when compared to a cylindrical shape. The rigidity of the screen cover  20  is thus improved. 
     The screen cover  20  includes two inclined parts  32 ,  34  which have different inclination angles θ 1 , θ 2  from each other. This makes it possible to enhance the rigidity of the screen cover  20  while providing a sufficient distance between the screen  17  and the screen cover  20  in the axial direction AX. Where the screen  17  and the screen cover  20  are separated in the axial direction, for example, it is possible to prevent the screen cover  20  from coming into contact with the screen  17  due to an external impact. 
     As shown in  FIG. 7 , the flat shield part  20   b  is formed at a central part of the screen cover  20 . Although not playing a predominant role, the punch holes  17   c  of the screen part  17   b  also serve to shred the foreign matters due to the rotation of the screen part  17   b . When the screen part  17   b  rotates at a certain angular speed, the central part and the outer edge part make one revolution at a same angular speed. Since the punch holes  17   c  located in a radially outer portion of the screen part  17   b  move over a greater distance per unit time, the punch holes  17   c  in the radially outer portion move at a higher movement speed. Therefore, the punch holes  17   c  in the radially outer portion of the screen  17  provide a stronger shedding effect. In contrast, the holes in the central part provide a weaker shedding effect so that the foreign matters are more likely to cause clogging. Provision of the shield part  20   b  at the central part of the screen cover  20  as in the above feature can prevent introduction of the foreign matters G to the central part where the shredding effect is lower. As a result, the capability to shred the foreign matters G is improved. 
     The screen cover  20  includes the first ribs  20   c  and the second ribs  20   d , and the first ribs  20   c  are thicker than the second ribs  20   d . Therefore, the two types of the ribs  20   c ,  20   d  enhance the rigidity of the screen cover  20 , and the two types of the ribs  20   c ,  20   d  having different thicknesses can effectively reinforce the screen cover  20 . 
     The first ribs  20   c  extend from corners of the hexagonal screen cover  20 , and therefore, the portions of the screen cover  20  at which stress is concentrated can have the enhanced rigidity. 
     Two adjacent ones of the first ribs  20   c  and the frame part  20   a  are located on three sides of a triangle. Specifically, the two adjacent ones of the first ribs  20   c  and the frame part  20   a  define an isosceles triangle having the apex on the axis AX of engine rotation shaft  2 . This makes it possible to enhance the rigidity of the portions of the screen cover  20  at which stress is concentrated. 
     The screen cover  20  is provided with the third ribs  36 , and the third ribs  36  of the screen cover  20  extend from the attachment parts  38  in the axial direction. Thus, the third ribs  36  enhance the rigidity of the attachment parts  38 . Consequently, the screen cover  20  can be supported stably. The third ribs  36  are provided at positions in the vicinity of points at which the inscribed circle C of the screen cover  20  is in contact with the screen cover  20 . Thus, the third ribs  36  shown in  FIG. 7  are provided in the regions R 2  where the distance between the outer edge of the screen  17  and the inner surface of the screen cover  20  is smaller as shown in  FIG. 4 , so that foreign matters G are more likely to be caught by the third ribs  36  so as to be shredded. 
     The attachment parts  38  are provided so as to protrude radially outward from sides of the hexagonal screen cover  20 . Thus, since the attachment parts  38  is provided to the sides of the hexagon, it is possible to reduce a space for disposing the entire screen cover  20 . The screen cover  20  is attached to the fan housing  18  typically at three or more points. According to the above configuration, since the screen cover  20  has a hexagonal shape, the attachment parts  38  are arranged at equal intervals in the circumferential direction when the screen cover  20  is fixed at three points so that a well-balanced attachment is achieved. In addition, since the screen cover  20  is shaped in a regular hexagon, the regions R 2  ( FIG. 4 ) where the distance between the outer edge of the screen  17  and the inner surface of the screen cover  20  is small can also be arranged at equal intervals in the circumferential direction. 
     The attachment parts  38  are provided so as to protrude radially outward from the sides of the hexagonal screen cover  20 . Specifically, the attachment parts  38  are provided so as to protrude radially outward from the positions in the vicinity of the points at which the inscribed circle C of the screen cover  20  is in contact with the screen cover  20 . Thus, the attachment parts  38  provided to the sides of the hexagon make it possible to reduce a space for disposing the entire screen cover  20 . 
     There are a plurality of the second ribs  20   d  aligned in the radial direction, and a part of the plurality of second ribs  20   d  is longer than the rest of the second ribs  20   d  in the axial direction AX of the engine rotation shaft  2  as shown in  FIG. 8 . During operation, the air A is sucked into the first inclined part  32  from the horizontal direction, and the air A is sucked into the second inclined part  34  from diagonally above. On this account, if all the second ribs  20   d  of the second inclined part  34  are elongated in the axial direction AX (vertical direction), the areas of the openings  24  between the adjacent second ribs  20   d  would be reduced so that a smaller amount of air is introduced. In contrast, if all the second ribs  20   d  are thin, they would have a reduced reinforcing effect for the screen cover  20 . According to the above configuration, one second rib  20   d ( 1 ) of the second ribs  20   d  aligned in the radial direction, which is located in an intermediate part in the radial direction, is thicker than the rest of the second ribs, and therefore, the screen cover  20  can be effectively reinforced while keeping sufficient areas of the openings  24  defined between the second ribs  20   d  so that a necessary amount of air is secured. 
     In the above embodiment, the first ribs  20   c  shown in  FIG. 7  are formed at the corners of the polygonal screen cover  20 , and the third ribs  36  are formed at positions on the sides of the polygonal screen cover  20 . Alternatively, the first ribs  20   c  may be formed at positions on the sides of the screen cover  20 , and the third ribs  36  may be formed at the corners. This configuration also provides the same effects as those of the above embodiment. In addition, although the second ribs  20   d  are formed so as to linearly extend between the adjacent first ribs  20   c  in the above embodiment, they may be structured in such a way that the second ribs  20   d  are branched in the radial direction so as to define honeycomb-like openings  24  between the second ribs  20   d . In such a case, for example, a load applied from above the screen  17  is distributed on the upper side of the screen  17  due to the honeycomb-like bridge structure defining the opening  24 , so that the strength of the screen cover  20  is improved. 
     The present invention is not intended to be limited to the above embodiment, and various addition, changes, or deletions may be made without departing from the scope of the invention. For example, although the above embodiment has been described with reference to a V-twin engine, the cover structure of the present invention may be applied to engines other than V-twin engines. Accordingly, such variants should also be included within the scope of the present invention. 
     REFERENCE NUMERALS 
     
         
         
           
               2  . . . Crankshaft (engine rotation shaft) 
               16  . . . Cooling fan 
               17  . . . Screen 
               18  . . . Fan housing 
               20  . . . Screen cover 
               20   a  . . . Frame part 
               20   b  . . . Shield part 
               20   c  . . . First rib 
               20   d  . . . Second rib 
               24  . . . Opening 
               30  . . . Fastener 
               32  . . . First inclined part 
               32   a  . . . First proximal end portion 
               32   b  . . . First distal end portion 
               34  . . . Second inclined part 
               34   a  . . . Second proximal end portion 
               34   b  . . . Second distal end portion 
               36  . . . Third rib 
               38  . . . Attachment part 
             AX . . . Axial direction of the engine rotation shaft 
             E . . . Air-cooled engine 
             G . . . Foreign object