Abstract:
An object of the present invention is to provide a muffler mounting structure configured to be able to prevent fastening parts from slackening by reducing the thermal conductivity from an exhaust port to the muffler mounting bosses and an engine including the muffler mounting structure. An engine of the present invention  100  includes a cylinder block  101  or the cylinder head  10  constitutes a combustion chamber  105  with a piston  104 , an exhaust port configured to communicate with the combustion chamber, and a muffler mounted to an outlet of the exhaust port, wherein the muffler is mounted with a muffler mounting boss into which a fixing part is fitted, and the muffler mounting boss is connected to a fin formed in the cylinder.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an engine, and more particularly, relates to an engine configured to be able to prevent muffler mounting bolts to fix a muffler to an engine body, from slackening. 
         [0003]    2. Description of the Related Art 
         [0004]    Conventionally, as a muffler mounting structure for fixing a muffler to the engine body, a structure in which a muffler is fixed to the engine body with bolts, is generally known. 
         [0005]    In a muffler mounting structure disclosed, for example, in Japanese Patent Application Laid-Open No. 2007-2730, fastening parts are fitted into muffler mounting bosses, and they are fixed with a fixing member placed on them to mount the muffler to the engine body. 
         [0006]    In the muffler mounting structure of the engine described in the above-mentioned patent document, those muffler mounting bosses are provided nearby an exhaust port. 
         [0007]    Accordingly, there is a technical problem that the muffler mounting bosses tend to increase in the temperature, and therefore the fastening parts fitted into the muffler mounting bosses slacken by thermal expansion of the muffler mounting bosses. 
       SUMMARY 
       [0008]    In view of the above-described problems, an object of the present invention is to provide an engine configured to be able to prevent fastening parts from slackening by reducing the thermal conductivity from a combustion chamber and an exhaust port to the muffler mounting bosses. 
         [0009]    An engine of the present invention includes a cylinder constituting a combustion chamber with a piston, an exhaust port configured to communicate with the combustion chamber, and a muffler mounted to an outlet of the exhaust port. The muffler is mounted with a muffler mounting boss into which a fixing part is fitted, and the muffler mounting boss is connected to a fin formed in the cylinder. 
         [0010]    The fin of the engine serves as a cooling fin for the cylinder. 
         [0011]    In the engine, a plurality of fins are formed in the cylinder, and the muffler mounting boss bridges between the plurality of fins. 
         [0012]    In the engine, the fin serves as a bearing surface to which the muffler is attached. 
         [0013]    The engine further includes a cooling fan configured to send cooling air. The muffler mounting boss includes a first muffler mounting boss and a second mounting boss, the first muffler mounting boss is positioned in a more upstream side of the cooling air than the exhaust port, and the second muffler mounting boss is positioned in a more downstream side of the cooling air than the exhaust port, and the first muffler mounting boss is connected directly to the cylinder, and the second muffler mounting boss is connected to the cylinder via the fin. 
         [0014]    The engine further includes a cooling fan configured to send cooling air. The muffler mounting boss includes a first muffler mounting boss and a second muffler mounting boss, the first muffler mounting boss is positioned in a more upstream side of the cooling air than the exhaust port, and the second muffler mounting boss is positioned in a more downstream side of the cooling air than the exhaust port, and a distance between the second muffler mounting boss and the cylinder is greater than a distance between the first muffler mounting boss and the cylinder. 
         [0015]    In the engine, the cylinder is configured to be able to be separated into a cylinder head and a cylinder block. 
         [0016]    With the engine according to the present invention, the muffler mounting bosses are connected to the cylinder via the fins, and therefore the thermal conductivity from the combustion chamber and the exhaust port to the muffler mounting bosses can be reduced. By this means, it is possible to prevent the fastening parts fitted into the muffler mounting bosses from slackening. 
         [0017]    In addition, with the engine according to the present invention, the plurality of fins are disposed in the cylinder, and the muffler mounting bosses bridge between the plurality of fins, and therefore are supported by the plurality of fins. As a result, the rigidity of the muffler mounting bosses can be improved. By this means, it is possible to prevent the muffler mounting bosses from being damaged. 
         [0018]    Moreover, with the engine according to the present invention, the fin serves as a bearing surface to which the muffler is attached, and therefore the muffler can be mounted without a separate seat plate. Hence, the number of parts can be reduced. 
         [0019]    Furthermore, with the engine according to the present invention, the first muffler mounting boss is connected directly to the cylinder, and the second muffler mounting boss is connected to the cylinder via the cooling fin which is cooled by cooling air where the first muffler mounting boss is positioned in the more upstream side of the cooling air than the exhaust port, and the second muffler mounting boss is positioned in the more downstream side of the cooling air than the exhaust port. As a result, the cooling efficiency of the second muffler mounting boss is improved. Therefore, the difference in temperature between the first muffler mounting boss and the second muffler mounting boss can be reduced. By this means, it is possible to equalize the thermal expansion rate of the fastening part fitted into the first muffler mounting boss and the thermal expansion rate of the fastening part fitted into the second muffler mounting boss. Therefore, it is possible to prevent the fastening parts from slackening. 
         [0020]    Furthermore, with the engine according to the present invention, the first muffler mounting boss and the second muffler mounting boss bridge between the plurality of fins which serve as cooling fins to cool the cylinder. Hence, the first muffler mounting boss and the second muffler mounting boss are cooled by the cooling air flowing through the plurality of cooling fins. In addition, the distance between the second muffler mounting boss and the cylinder is greater than the distance between the first muffler mounting boss and the cylinder. Therefore, in particular, cooling air flows smoothly around the second muffler mounting boss which tends to rise in temperature, so that the cooling efficiency is improved. Therefore, the difference in temperature between the first muffler mounting boss and the second muffler mounting boss can be reduced. By this means, it is possible to equalize the thermal expansion rate of the fastening part fitted into the first muffler mounting boss and the thermal expansion rate of the fastening part fitted into the second muffler mounting boss. Therefore, it is possible to prevent the fastening parts from slackening. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a perspective view schematically showing the configuration of an engine according to one embodiment of the present invention; 
           [0022]      FIG. 2  is a plan view schematically showing the configuration of the engine according to one embodiment of the present invention; 
           [0023]      FIG. 3  schematically shows a cross section taken along line I-I′ in  FIG. 2 ; 
           [0024]      FIG. 4  schematically shows a cross section taken along line II-II′ in  FIG. 2 ; 
           [0025]      FIG. 5  is a perspective view schematically showing the configuration of a cylinder head of the engine according to one embodiment of the present invention; 
           [0026]      FIG. 6  is a perspective view schematically showing the configuration of the cylinder head of the engine according to one embodiment of the present invention; 
           [0027]      FIG. 7  is a plan view schematically showing the configuration of the cylinder head of the engine according to one embodiment of the present invention; 
           [0028]      FIG. 8  schematically shows a cross section taken along line III-III′ in  FIG. 7 ; and 
           [0029]      FIG. 9  is a cross-sectional view schematically showing the cylinder head of the engine according to another embodiment of the present invention, taken along the line III-III′ in  FIG. 7 . 
       
    
    
     DETAILED DESCRIPTION 
     Embodiment 1  
       [0030]      FIG. 1  is a perspective view schematically showing the configuration of an engine  100  according to one embodiment of the present invention.  FIG. 2  is a plan view schematically showing the configuration of the engine  100  according to one embodiment of the present invention.  FIG. 3  schematically shows a cross section taken along line I-I′ in  FIG. 2 .  FIG. 4  schematically shows a cross section taken along line II-II′ in  FIG. 2 . 
         [0031]    As shown in  FIG. 1  to  FIG. 4 , an engine  100  of the present embodiment mainly includes a cylinder block  101 , a crankcase  102 , an oil case  103 , and a cylinder head  10 . These components are detachably mounted by coupling with each other with bolts. 
         [0032]    Note that, in the engine  100  of the present embodiment, the cylinder block  101  and the cylinder head  10  are separated from one another to help to achieve high output, and an exhaust port  110  (described later) is provided in the cylinder head  10 . 
         [0033]    The main body of the engine  100  is formed by the cylinder block  101 , the crankcase  102 , the oil case  103 , and the cylinder head  10 . A piston  104  is inserted in the cylinder block  101  to reciprocably move. 
         [0034]    The piston  104  can reciprocate in the cylinder block  101  by explosive power generated due to the combustion of mixed air in the combustion chamber  105  described later. A connecting rod  106  is pivotably connected to the piston  104 . 
         [0035]    The connecting rod  106  converts the reciprocating motion of the piston  104  in the cylinder block  101  into the rotating motion of a crank shaft  107  described later. One end of the connecting rod  106  is connected pivotably to the piston  104 . The other end of the connecting rod  106  is connected to a crank shaft  107 . 
         [0036]    As described above, the crank shaft  107  is connected to the piston  104  via the connecting rod  106 . Accordingly, the reciprocating motion of the piston  104  is converted into the rotating motion of the crank shaft  107  via the connecting rod  106 , and then transmitted to an output shaft. 
         [0037]    The crank shaft  107  is rotatably supported by the cylinder block  101  and the crankcase  102  which is provided on one end side (the lower side in  FIG. 3 ) in the longitudinal direction of the cylinder block  101 . 
         [0038]    Specifically, a crank chamber is formed in the crankcase  102 . Both ends of the crank shaft  107  projecting from the crank chamber are sandwiched and rotatably supported between the cylinder block  101  and the crankcase  102 . 
         [0039]    In addition to the output shaft, the cylinder block  101  constituting the main body of the engine  100  and a cooling fan  108  for sending cooling air to the cylinder head  10 , are connected to the crank shaft  107 . 
         [0040]    That is, the cooling fan  108  rotates in conjunction with the rotation of the crank shaft  107 , and sends cooling air in the radial direction and the axial direction of the crank shaft  107 . In other words, the cooling fan  108  sends cooling air to the cylinder block  101  and the cylinder head  10  constituting the main body of the engine  100 . 
         [0041]    In addition, the oil case  103  to store oil to be supplied to the crank chamber, is provided in the other end side of the crankcase  102  (the lower side in  FIG. 1  to  FIG. 4 ). The oil case  103  is formed as a housing which is surrounded on all four sides and the bottom and has an opening on the top. 
         [0042]    The inside of the oil case  103  surrounded on all four sides and the bottom serves as an oil reservoir. The semicircular portion of the crankcase  102  accommodating the crank shaft  107  where the crank shaft  107  can rotate, serves as a partition between the crank chamber and the oil reservoir. 
         [0043]    On the other hand, the cylinder head  10  is provided on the other end side of the cylinder block  101  (the upper part in  FIG. 1 ). The cylinder block  101 , the upper surface of the piston  104  and the cylinder head  10  form a combustion chamber  105 . An intake port  109  is formed on the cylinder head  10 . The intake port  109  communicates with a carburetor (not shown). 
         [0044]    An exhaust port  110  is formed on the cylinder head  10 . The exhaust port  110  is in communication with a muffler  115 . An intake valve  111  which opens and closes the intake port  109 , is provided on the intake port  109 . Additionally, an exhaust valve  112  which opens and closes the exhaust port  110  is provided on the exhaust port  110 . 
         [0045]    The intake valve  111  opens in an intake stroke over which the piston  104  moves from the top dead center to the bottom dead center. In the intake stroke, mixed air from the intake port  109  is mixed in the combustion chamber  105  due to the effect of the negative pressure generated by increasing the volume of the combustion chamber  105 . 
         [0046]    Additionally, the exhaust valve  112  opens during an exhaust stroke over which the piston  104  moves from the bottom dead center to the top dead center. In the exhaust stroke, exhaust gas generated in the combustion chamber  105  is discharged from the exhaust port  110  to the muffler  115 , due to the effect of the positive pressure generated by reducing the volume of the combustion chamber  105 . 
         [0047]    A valve operating mechanism is connected to the intake valve  111  and the exhaust valve  112 , which drives the intake valve  111  and the exhaust valve  112  to open and close the intake port  109  and the exhaust port  110 . This valve operating mechanism is a so-called OHV type valve operating mechanism. 
         [0048]    Specifically, the valve operating mechanism mainly includes a crank shaft gear, a cam shaft and a rocker arm  113 . These crank shaft gear and cam shaft are provided in a side chamber which is formed along the cylinder block  101  and the crankcase  102 . Meanwhile, the rocker arm  113  is provided in a valve operating chamber which is formed in the cylinder head  10 . 
         [0049]    The crank shaft gear is disposed in the side chamber to rotate together with the crank shaft  107 . The cam shaft gear is provided in the cam shaft. The cam shaft gear meshes with the crank shaft gear in the side chamber, and rotates the cam shaft with rotation which is half of the rotation of the crank shaft  107 . 
         [0050]    Moreover, a cam is provided on the cam shaft. The cam rotates together with the cam shaft. The one end of a push rod  114  contacts the cam, and the push rod  114  moves in the longitudinal direction along with the rotation of the cam. 
         [0051]    Meanwhile, the other end of the push rod  114  is connected to the rocker arm  113 , and the rocker arm  113  swings with the movement of the push rod  114 . Then, the intake valve  111  and the exhaust valve  112  reciprocate along with the swing of the rocker arm  113 . This allows the intake port  109  and the exhaust port  110  to open and close. 
         [0052]    As described above, with the present embodiment, the exhaust port  110  is formed on the cylinder head  10 , and exhaust gas generated from the exhaust port  110  is discharged from the cylinder head  10  to the muffler  115 . 
         [0053]    In the present embodiment, the muffler  115  is attached to the cylinder head  10  on which the exhaust port  110  opens. In this case, the muffler  115  is mounted by fitting fastening parts (not shown)into a first muffler mounting boss  16  and a second muffler mounting boss  17  which are muffler mounting bosses described later. 
         [0054]    Next, the cylinder head  10  will be explained with reference to  FIG. 5  to  FIG. 8 . The cylinder head  10  is a main component of the muffler mounting structure for mounting the muffler  115  to the engine  100  according to the present embodiment. 
         [0055]      FIG. 5  is a perspective view schematically showing from above the configuration of the cylinder head  10  of the engine  100  according to one embodiment of the present invention.  FIG. 6  is a perspective view schematically showing from below the configuration of the cylinder head  10  of the engine  100  according to one embodiment of the present invention.  FIG. 7  is a plan view schematically showing from above the configuration of the cylinder head  10  of the engine  100  according to one embodiment of the present invention.  FIG. 8  schematically shows a cross section taken along line III-III′ in  FIG. 7 . Here, the arrows shown in  FIG. 8  illustrate the flows of cooling air send by the cooling fan  108 . 
         [0056]    As shown in  FIG. 5  to  FIG. 8 , the intake port  109  is formed on the cylinder head  10 . The intake port  109  is disposed in communication with the combustion chamber  105  as described above, and sucks in from the carburetor (not shown) to the combustion chamber  105 . 
         [0057]    In addition, the exhaust port  110  is formed on the cylinder head  10 . The exhaust port  110  is disposed in communication with the combustion chamber  105  as described above, and discharges the exhaust gas generated in the combustion chamber  105  to the muffler  115 . 
         [0058]    A plurality of fins  20  are formed on the cylinder head  10 . These fins  20  are disposed so as to cover the intake port  109  and the exhaust port  110 , and send cooling air from the cooling fan  108  to the neighborhood of the intake port  109  and the exhaust port  110 . 
         [0059]    These fins  20  stand in the same direction and are spaced from and parallel to each other. That is, the cooling air from the cooling fan  108  can cool the intake port  109  and the exhaust port  110  by flowing through the plurality of fins  20 . 
         [0060]    In the present embodiment, an exhaust opening  12  of the exhaust port  110  is formed on the outermost fin  20   a  among the plurality of fins  20 . The opening of the first muffler mounting boss  16  and the opening of the second muffler mounting boss  17  are formed on the fin  20   a . The fastening parts to mount the muffler are fitted into the first muffler mounting boss  16  and the second muffler mounting boss  17 . 
         [0061]    The fin  20   a  includes the opening of the first muffler mounting boss  16 , the opening of the second muffler mounting boss  17 , and the exhaust opening  12  on the same plane, and serves as a bearing surface to which the muffler is attached. 
         [0062]    In this way, the outermost fin  20   a  serves as a bearing surface to which the muffler is attached, and therefore there is no need to separately form a bearing surface on a place in the main body of the engine  100 . Hence, it is possible to simplify the configuration of the mold for the main body of the engine  100 . Moreover, the number of parts can be reduced because there is no need to separately form a bearing surface on a place in the main body of the engine  100 . 
         [0063]    The first muffler mounting boss  16 , the second muffler mounting boss  17 , and the exhaust opening  12  of the exhaust port  110  slightly protrude from the surface of the fin  20   a . By this means, it is possible to form a small gap between the bearing surface of the muffler and the fin  20   a  when the muffler is mounted. Therefore, it is possible to reduce the thermal conductivity from the muffler to the first muffler mounting boss  16  and the second muffler mounting boss  17 . 
         [0064]    As described above, the exhaust port  110  of the present embodiment bridges between the outermost fin  20   a  and a fin  20   b  which is next to the fin  20   a  and penetrate the fin  20   a  and the fin  20   b.    
         [0065]    Also, the first muffler mounting boss  16  and the second muffler mounting boss  17  bridge between the outermost fin  20   a  and the fin  20   b  which is next to the fin  20   a . That is, the exhaust port  110 , the first muffler mounting boss  16  and the second muffler mounting boss  17  bridge between the fin  20   a  and the fin  20   b  and are arranged in parallel with each other. 
         [0066]    In this way, the first muffler mounting boss  16  and the second muffler mounting boss  17  bridge between the fin  20   a  and the fin  20   b . Therefore, it is possible to improve the rigidity of the first muffler mounting boss  16  and the second muffler mounting boss  17 . 
         [0067]    In the present embodiment, the first muffler mounting boss  16  is positioned in the more upstream side of cooling air than the exhaust port  110 . Meanwhile, the second muffler mounting boss  17  is positioned in the more downstream side of cooling air than the exhaust port  110 . 
         [0068]    Here, the cooling air sent from the cooling fan  108  flows into the space between the fin  20   a  and the fin  20   b , and passes through the neighborhood of the first muffler mounting boss  16  to cool the first muffler mounting boss  16 . 
         [0069]    Then, the cooling air passes around the exhaust port  110  to cool the exhaust port  110 . At this time, heat is generated because the cooling air passes around the exhaust port  110 , and this heat increases the temperature of the cooling air. 
         [0070]    Then, the cooling air whose temperature having increased passes through the neighborhood of the second muffler mounting boss  17  to cool the second muffler mounting boss  17 . By this means, the first muffler mounting boss  16  and the second muffler mounting boss  17  are different in cooling efficiency with the cooling air from the cooling fan  108 . 
         [0071]    In this way, taking into account the first muffler mounting boss  16  and the second muffler mounting boss  17  are different in cooling efficiency, the distance between the second muffler mounting boss  17 , and a base part  10   a  of the cylinder head  10  and the exhaust port  110  is greater than the distance between the first muffler mounting boss  16  and those. 
         [0072]    Accordingly, it is possible to make the thermal conductivity from the base part  10   a  of the cylinder head  10  and the exhaust port  110  to the second muffler mounting boss  17  lower than the thermal conductivity from the base part  10   a  of the cylinder head  10  and the exhaust port  110  to the first muffler mounting boss  16  to improve the cooling efficiency of the second muffler mounting boss  17 . Therefore, it is possible to prevent the fastening parts fitted into the second muffler boss  17  from slackening. 
         [0073]    Although the first muffler mounting boss  16  and the second muffler mounting boss  17  are different in cooling efficiency with the cooling air from the cooling fan  108 , the distance between the second muffler mounting boss  17  and the base part  10   a  of the cylinder head  10  is greater than the distance between the first muffler mounting boss  16  and the base part  10   a  of the cylinder head  10 . Therefore, the thermal conductivity from the combustion chamber  105  and the exhaust port  110  to the second muffler mounting boss is reduced. As a result of this, it is possible to reduce the difference in temperature between the first muffler mounting boss  16  and the second muffler mounting boss  17 . 
         [0074]    As described above, with the present embodiment, the thermal conductivity from the combustion chamber  105  and the exhaust port  110  to the second muffler mounting boss  17  can be reduced. Also, the difference in temperature between the first muffler mounting boss  16  and the second muffler mounting boss  17  can be reduced. Therefore, it is possible to prevent the fastening parts from slackening. 
       Embodiment 2  
       [0075]    Next, the engine of the second embodiment will be described with reference to  FIG. 9 .  FIG. 9  is a cross-sectional view schematically showing the configuration of the cylinder head  80  of the engine according to another embodiment of the present invention, taken along line III-III′ in  FIG. 7 . 
         [0076]    Here, the engine of the second embodiment is different from the engine of the above-described first embodiment in that, a first muffler mounting boss  86  is connected directly to a base part  80   a  of a cylinder head  80 , and a second muffler mounting boss  87  is connected to the base part  80   a  of the cylinder head  80  via a fin  90   a . The other components are the same as in the engine of the first embodiment. The same or equivalent components as in the first embodiment are assigned the same reference numerals, and overlapping descriptions will be omitted. 
         [0077]    As shown in  FIG. 9 , in the cylinder head  80  of the present embodiment, the first muffler mounting boss  86  is connected directly to the base part  80   a  of the cylinder head  80 , and the second muffler mounting boss  87  is connected to the base part  80   a  of the cylinder head  80  via the fin  90   a , as described above. 
         [0078]    Therefore, it is possible to reduce the difference in temperature between the first muffler mounting boss  86  and the second muffler mounting boss  87  where the first muffler mounting boss  86  is positioned in the more upstream side of cooling air than the exhaust port  110  and the second muffler mounting boss  87  is positioned in the more downstream side of cooling air than the exhaust port  110 . By this means, it is possible to equalize the thermal expansion rate of the fastening part fitted into the first muffler mounting boss  86  and the thermal expansion rate of the fastening part fitted into the second muffler mounting boss  87  to prevent the fastening parts from slackening. 
         [0079]    As described above, the engine  100  of the present embodiment has high output where the cylinder block  101  and the cylinder head  10  are separated from one another to help to achieve high output. This high-output engine  100  tends to increase in temperature at the combustion chamber  105  and the exhaust port  110 . Therefore, the cooling efficiency with the present invention is especially effective for the engine  100 . 
         [0080]    The engine  100  of the present embodiment is not limited to a configuration with high output where the cylinder block  101  and the cylinder head  10  are separated from one another, but another configuration is possible to produce the same effect where the cylinder block  101  and the cylinder head  10  are integrally formed. 
         [0081]    In addition, although the engine  100  of the present embodiment is a four-stroke engine here, the present invention is not limited to this, and, even when applied to a two-stroke engine, it is possible to produce the same effect, as long as the muffler mounting bosses to mount the muffler  115  to the main body of the engine  100  are apart from the exhaust port and there is a distance between the exhaust port and the muffler mounting bosses.