Patent Publication Number: US-2015075480-A1

Title: Engine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to engines, and more specifically to an Over Head Valve or OHV engine configured to use splash lubrication. 
     2. Description of the Related Art 
     An example of conventional techniques in this field is disclosed in JP-A 2004-218488. 
     JP-A 2004-218488 discloses an engine provided by an air-cooled, single-cylinder, four-cycle OHV engine, in which a big end portion of a connecting rod is formed with a splash section so as to splash oil stored inside a crank case bottom portion. The splashed oil, together with a mist of oil, finds ways into an oil supply path, to inlet/exhaust valves and a surrounding area inside a valve chamber. This arrangement makes it possible to lubricate the inlet/exhaust valves and a surrounding area without using an oil pump. 
     Such an engine as described above is capable of lubricating the inlet/exhaust valves and a surrounding area inside the valve chamber with oil, but is not capable of sufficiently lubricating a piston underside area including pin holes in pin boss regions, inner surfaces of a small end portion of the connecting rod, and outer circumferential surfaces of piston pin. This poses a risk that the connecting rod will not move smoothly with respect to the piston. The risk becomes especially high in cases where the connecting rod is designed not to have a conventional bearing metal commonly fitted into the small end portion. 
     SUMMARY OF THE INVENTION 
     Therefore, preferred embodiments of the present invention provide an engine capable of keeping smooth movement of a connecting rod with respect to a piston. 
     According to an aspect of various preferred embodiments of the present invention, an engine includes a cylinder; a piston provided in the cylinder and including pin boss regions; a crank shaft configured to convert reciprocating movement of the piston into rotating movement; a connecting rod including a small end portion connected to the pin boss regions of the piston, and a big end portion connected to the crank shaft; an oil dipper provided in the big end portion of the connecting rod; a piston pin configured to connect the pin boss regions and the small end portion of the connecting rod; and a crank case provided at the cylinder and accommodating the crank shaft and the oil dipper. In this engine, the piston pin is rotatably movable with respect to the pin boss regions and the connecting rod. Further, counter regions between the small end portion and the piston pin include a first counter region which is closer to a tip portion of the connecting rod, and a second counter region which is closer to the big end portion of the connecting rod, and when the connecting rod is viewed from a direction which is perpendicular or substantially perpendicular to both an axial direction of the piston pin and an axial direction of the connecting rod, the first counter region has its two ends at positions located farther inward in the axial direction of the piston pin than two ends of the second counter region in the axial direction of the piston pin. 
     In a preferred embodiment of the present invention, the oil dipper, which is provided on the connecting rod, splashes oil which is stored inside the crank case, to supply the oil to an underside of the piston. In this process, the oil is supplied to an outer circumferential surface of the piston pin from between the pin boss regions and the small end portion of the connecting rod. In regard to the counter regions between the small end portion and the piston pin, the two ends of the first counter region are at positions located farther inward than the two ends of the second counter region in the axial direction of the piston pin. Therefore, oil which has arrived at the outer circumferential surface of the piston pin on the side closer to the first counter region is drawn as the piston pin rotates, and makes its way to a location between the piston pin and the small end portion of the connecting rod, and between the piston pin and the pin boss regions. Specifically, it is possible to sufficiently lubricate the underside area of the piston including pin holes in the pin boss regions, an inner surface of the small end portion of the connecting rod, and the outer circumferential surface of the piston pin. Therefore, the arrangement makes the movement of the connecting rod smooth with respect to the piston. The arrangement is particularly advantageous in configurations where the connecting rod is designed not to have a conventional bearing metal which is commonly fitted into the small end portion of the connecting rod. 
     Preferably, the small end portion of the connecting rod includes a first portion defining a portion closer to a tip portion of the connecting rod, and a second portion defining a portion closer to the big end portion of the connecting rod; and when the connecting rod is viewed from the direction which is perpendicular or substantially perpendicular to both the axial direction of the piston pin and the axial direction of the connecting rod, the first portion includes side surfaces at positions located farther inward than those of the second portion in the axial direction of the piston pin. The arrangement that the side surfaces of the first portion which is on the side closer to the tip portion of the small end portion of the connecting rod are at positions located farther inward than the side surfaces of the second portion which is on the side closer to the big end portion in the axial direction of the piston pin means that the portion closer to the tip portion is narrower than the portion closer to the big end portion, in the small end portion of the connecting rod. In this case, it becomes possible to shorten the length of the piston pin, and simultaneously to move the pin boss regions toward the center of the piston, in the piston underside region. This arrangement makes it possible to increase an oil contact area on the radially outer side than the pin boss regions, in the underside region of the piston, and therefore makes it possible to enhance cooling of the piston and to significantly reduce or prevent a temperature increase in the piston. Especially in cases where the connecting rod is designed not to have a conventional bearing metal which is commonly fitted into the small end portion of the connecting rod, the small end portion is often given an increased thickness. Even in such cases, various preferred embodiments of the present invention make it possible to have a sufficient amount of area for oil contact in the underside region of the piston, and to provide sufficient cooling to the piston. 
     Further preferably, the connecting rod is preferably made of an aluminum alloy, whereas the piston pin is preferably made of a ferrous alloy, for example. As described above, the connecting rod is preferably made of an aluminum alloy but the piston pin is preferably made of a ferrous alloy. By making the two members of materials that are mutually dissimilar from each other, good adhesion resistance is provided in the counter regions between the connecting rod and the piston pin. This eliminates the need for an expensive bearing metal, bushing or the like which must otherwise be fitted to the small end portion of the connecting rod and further, and the need for surface treatment to the small end portion of the connecting rod, thus making it possible to reduce cost of manufacture of the engine. Also, the pin boss regions which are closer to the center of the piston shortens a heat transfer path from a center region of the piston, via the pin boss regions and the piston pin, to the connecting rod. Further, by using an aluminum alloy, i.e., a material having a high thermal conductivity, for the connecting rod, the arrangement makes it possible to conductively remove heat from the center region of the piston and then effectively release it from the connecting rod, thus enhancing cooling of the piston. 
     Further, preferably, the pin boss regions overlap the big end portion side of the small end portion when viewed from a direction of a center axis of the cylinder. If the pin boss regions do not overlap the small end portion of the connecting rod when viewed from the direction of the center axis of the cylinder like in the conventional engines, the explosion in the engine exerts a bending load to the piston pin to act in a direction from the tip portion of the small end portion of the connecting rod to the big end portion. This can deform the piston pin, which in turn increases localized surface pressure from the piston pin to the pin boss regions and the connecting rod. However, if the pin boss regions overlap a portion of the small end portion which is closer to the big end portion when viewed from the direction of the center axis of the cylinder, the load exerted by the explosion in the engine to the piston pin in the direction from the tip portion of the small end portion of the connecting rod to the big end portion is received by the portion closer to the big end portion, of the small end portion of the connecting rod, and therefore the piston pin is less prone to deformation. Therefore, the arrangement improves durability of the piston, the piston pin and the connecting rod. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view from a front left view point, of an engine generator which includes an engine according to a preferred embodiment of the present invention. 
         FIG. 2  is a perspective view from a rear right view point, of the engine generator which includes the engine according to a preferred embodiment of the present invention. 
         FIG. 3  is an illustrative drawing, showing a longitudinal section of the engine. 
         FIG. 4A  is a sectional view of a piston, whereas  FIG. 4B  is a bottom view thereof. 
         FIG. 5A  is a side view of a connecting rod, whereas  FIG. 5B  is a partially unillustrated sectional view showing a small end portion of the connecting rod. 
         FIG. 6  is a side view, showing a state where the connecting rod is assembled to the piston. 
         FIG. 7  is a partially unillustrated sectional view of a region including the small end portion, showing the state where the connecting rod is assembled to the piston. 
         FIG. 8A  is a sectional view of a conventional piston, whereas  FIG. 8B  is a bottom view thereof. 
         FIG. 9  is a partially unillustrated sectional view of a conventional example, showing a region including a small end portion where a connecting rod is assembled to the piston. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. 
       FIG. 1  and  FIG. 2  show an engine generator  10  which includes an engine  24  (which will be described later) according to a preferred embodiment of the present invention. In the present specification, a “fore-aft direction” and a “left-right direction” in the engine generator  10  are defined as shown in  FIG. 1  and  FIG. 2  for the sake of descriptive convenience. Thus, a side on which the engine  24  is provided is a “front side”, a side on which a generator  26  (to be described later) is provided is a “rear side”, and a side on which an operation panel  48  (to be described later) is provided is a “left side”. 
     The engine generator  10  is a portable generator, including a generator frame  12 . The generator frame  12  includes a front frame  14 , a rear frame  16 , an upper frame  18 , and a pair of lower frames  20 ,  22 . The front frame  14  is provided by a pipe-shaped member which is preferably formed into a general shape of inverted letter of U in a front view, whereas the rear frame  16  is provided by a pipe-shaped member which is preferably formed into a general shape of inverted letter of U in a rear view, for example. The front frame  14  and the rear frame  16  are connected with each other at both of their end portions. The upper frame  18  is provided by a pipe-shaped member and extends in the fore-aft direction, connecting upper left end portions of the front frame  14  and the rear frame  16 , respectively. The upper frame  18  defines and serves as a grip. The lower frame  20  is a platy member extending in the left-right direction, connecting left and right lower portions of the front frame  14  with each other. The lower frame  22  is a platy member extending in the left-right direction, connecting left and right lower portions of the rear frame  16  with each other. 
     The engine  24  is installed on the lower frame  20 , whereas the generator  26  is installed on the lower frame  22 . The engine  24  and the generator  26  are arranged in the fore-aft direction, with the engine  24  being on the front side and the generator  26  being on the rear side. The engine  24  includes a crank shaft  66  (to be described later), which is connected with a rotating shaft (not illustrated) of the generator  26 . 
     The engine  24  includes, on its front side, an air intake section  28  to introduce outside air. The air intake section  28  incorporates a cooling fan (not illustrated). An air cleaner  30  is provided on the right side of the air intake section  28 . As the cooling fan is driven, outside air introduced from the air intake section  28  cools the engine  24 . Near the air intake section  28 , a recoil starter  32  is provided. 
     A muffler  34  is provided behind the engine  24 , on the right side of the generator  26 . Exhaust gas from the engine  24  is discharged to outside via the muffler  34 . A canister  36  is provided below the engine  24 . A fuel tank  38  is connected to the air cleaner  30  via the canister  36 . Gasoline vapor from the fuel tank  38  is adsorbed in the canister  36 . 
     The fuel tank  38  is provided to cover the engine  24  and the generator  26  from above. The fuel tank  38  stores fuel (for example, gasoline, in the present preferred embodiment) which is to be supplied to the engine  24 . The fuel tank  38  has its right side portion attached to a support frame  40  which connects an upper right end portion of the front frame  14  and an upper right end portion of the rear frame  16  to each other. The fuel tank  38  has its left front portion and left rear portion connected to the front frame  14  and the rear frame  16  respectively via brackets  42  and  44 . 
     An operation box  46  is provided on the left side of the fuel tank  38 . The operation box  46  includes the operation panel  48 , and a case  50  which is provided on the right side of the operation panel  48  and incorporates an operation section (not illustrated), etc. A battery  52  is provided below the case  50 . 
     In the engine generator  10  described as above, the recoil starter  32  is pulled to rotate the crank shaft  66  and start the engine  24 . As the engine  24  starts, the generator  26  starts its power generating operation. The electric power from the generator  26  can be taken out of the operation panel  48  or stored in the battery  52 . 
     Reference will now be made to  FIG. 3  to describe the engine  24 . 
     The engine  24  preferably is an air-cooled, single-cylinder, four-cycle OHV engine (Over Head Valve Engine), for example, of a slanted type in which a cylinder center axis A is slanted obliquely. The engine  24  includes a cylinder  54 . The cylinder  54  includes a cylinder body  56  and a cylinder head  58  which is attached to an upper end portion of the cylinder body  56 . A cylinder head cover  60  is attached to an upper end portion of the cylinder head  58 . A crank case  62  is provided in a lower portion of the cylinder body  56 . 
     The cylinder body  56  includes an inner circumferential surface provided with a cylinder liner  56   a . Inside the cylinder body  56 , a piston  64  is provided slidably with respect to the cylinder liner  56   a . The crank case  62  accommodates the crank shaft  66  and a cam shaft  68  which moves in association with the crank shaft  66 . The crank shaft  66  is disposed horizontally. The crank shaft  66  and the cam shaft  68  are parallel or substantially parallel to each other. The cam shaft  68  is disposed not to interfere (contact) with crank webs  70  of the crank shaft  66 . The piston  64  and the crank shaft  66  are connected to each other by a connecting rod  72 , such that reciprocating movement of the piston  64  is converted into rotating movement by the crank shaft  66 . The crank shaft  66  is provided with a drive gear  74 , whereas the cam shaft  68  is provided with a driven gear  76  which rotates in association with rotation of the drive gear  74 . The crank case  62  also accommodates a balancer  78 . The balancer  78  is in engagement with a gear  80  provided in the crank shaft  66 , to reduce vibration. As shown in  FIG. 3 , when the engine  24  is viewed from a position where the crank shaft  66  is located on the left side and the cam shaft  68  is located on the right side, rotation direction of the crank shaft  66  is counterclockwise as indicated by Arrow B. 
     From the cylinder body  56  to the cylinder head  58 , there is provided a communication path  84  which provides communication between inside of the crank case  62  and inside of a rocker arm chamber  82  in the cylinder head cover  60 . A pushrod  86 , and a tappet  88  provided on an end portion of the pushrod  86  are inserted through the communication path  84 . Inside the crank case  62 , the tappet  88  has its tip portion contacted to a cam  90  of the cam shaft  68 . The pushrod  86  includes another end portion contacted to a rocker arm  92  which is provided inside the rocker arm chamber  82 . The rocker arm  92  drives an exhaust valve  94 . Additionally, though not illustrated in  FIG. 3 , the engine  24  accommodates a pushrod, a tappet and a rocker arm to drive an inlet valve, in parallel or substantially in parallel to the pushrod  86 , the tappet  88  and the rocker arm  92  to drive the exhaust valve  94 . 
     Next, description will be made for an arrangement which includes the piston  64 , the connecting rod  72  and their surroundings. 
     Referring to  FIG. 4A  and  FIG. 4B , the piston  64  includes a piston head  96 . The piston head  96  includes a lower surface including pin boss regions  98   a ,  98   b  to oppose to each other. The pin boss regions  98   a ,  98   b  include pin holes  100   a ,  100   b  respectively. The pin holes  100   a ,  100   b  are on a straight line so that a piston pin  118  (to be described later) can be inserted thereto. The pin holes  100   a ,  100   b  preferably include grooves  102 ,  102   b  respectively for circlips  120  (to be described later). The piston head  96  includes lower surface provided with a piston skirt  104 . Further, the lower surface of the piston head  96  includes a plurality of recesses  106   a ,  106   b . The recesses  106   a ,  106   b  preferably are I-shaped in a bottom view. The recesses  106   a ,  106   b  are on an outer side of the pin boss regions  98   a ,  98   b , radially of the piston head  96 , respectively. The recesses  106   a ,  106   b  increase a surface area of the piston  64 , and help cooling of the piston  64 . 
     Referring to  FIG. 5A  and  FIG. 5B , the connecting rod  72  is preferably made of an aluminum alloy, for example, and includes a small end portion  108  connected to the pin boss regions  98   a ,  98   b  of the piston  64 , and a big end portion  110  connected to the crank shaft  66 . The small end portion  108  of the connecting rod  72  includes a first portion  112  which is closer to a tip portion of the connecting rod  72 ; a second portion  114  which is closer to the big end portion  110  of the connecting rod  72 ; and a curved portion  116  which connects the first portion  112  and the second portion  114  with each other. As shown in  FIG. 5B , the curved portion  116  includes a region of a smaller thickness extending from the second portion  114  toward the first portion  112 , so that the first portion  112  has a smaller thickness than the second portion  114 . 
     Referring to  FIG. 6  and  FIG. 7 , the pin boss regions  98   a ,  98   b  and the small end portion  108  of the connecting rod  72  are connected by the piston pin  118 . The piston pin  118  is inserted into the pin holes  100   a ,  100   b  of the pin boss regions  98   a ,  98   b , and through the small end portion  108  of the connecting rod  72 . The circlips  120  are fitted into the grooves  102   a ,  102   b  in the respective pin holes  100   a ,  100   b . The circlips  120  support two ends of the piston pin  118 , such that the piston pin  118  is fixed. The piston pin  118  is preferably made of a ferrous alloy, for example, and is rotatably movable with respect to the pin boss regions  98   a ,  98   b  and the connecting rod  72 . 
     Referring to  FIG. 5A ,  FIG. 5B  and  FIG. 7 , when the connecting rod  72  is viewed from a direction E which is perpendicular to both of an axial direction C of the piston pin  118  and an axial direction D of the connecting rod  72 , side surfaces  112   a ,  112   b  of the first portion  112  are located farther inward (closer to the axis of the connecting rod  72 ) than side surfaces  114   a ,  114   b  of the second portion  114 , in the axial direction C of the piston pin  118 . There are counter regions between the small end portion  108  and the piston pin  118 ; a first counter region  122  on a side closer to a tip portion of the connecting rod  72  and a second counter region  124  on a side closer to the big end portion  110  of the connecting rod  72 . In the axial direction C of the piston pin  118 , two ends  122   a ,  122   b  of the first counter region  122  are located farther inward (closer to the axis of the connecting rod  72 ) than two ends  124   a ,  124   b  of the second counter region  124  in the axial direction C of the piston pin  118 . 
     Further, referring to  FIG. 3 ,  FIG. 4A  and  FIG. 4B , when viewed from a direction of the center axis A of the cylinder  54  (from a direction of sliding action of the piston  64 ), bulged portions  126   a ,  126   b  (upper portions of the pin boss regions  98   a ,  98   b ), i.e., portions of the pin boss regions  98   a ,  98   b  closer to the piston head  96  overlap a portion of the small end portion  108  (the second portion  114 ) which is a portion closer to the big end portion  110 . 
     Referring to  FIG. 5A  and  FIG. 6 , the big end portion  110  of the connecting rod  72  is provided with an oil dipper  128 . The oil dipper  128  is preferably made of an aluminum alloy for example. In the present preferred embodiment, the oil dipper  128  is preferably formed by die-casting integrally with a connecting rod cap  111  of the big end portion  110 , for example. Referring to  FIG. 3 , the oil dipper  128  is in the crank case  62 , and the crank case  62  stores oil  130  therein. 
     According to the engine  24  as described, the oil  130  is splashed by the oil dipper  128  provided on the connecting rod  72 , to the cylinder body  56 , the cylinder head  58 , the cylinder head cover  60  and so on, directly or indirectly after spattering on the crank shaft  66 , the cam shaft  68 , etc., such that lubrication of the crank shaft  66 , the cam shaft  68 , the cylinder body  56 , the rocker arm  92 , etc. is achieved. 
     Also, the oil dipper  128  splashes the oil  130  stored inside the crank case  62  to underside surface of the piston  64 . In this process, as will be clear from  FIG. 7 , the oil  130  passes through between the pin boss regions  98   a ,  98   b  and the small end portion  108  of the connecting rod  72 , and reaches an outer circumferential surface of the piston pin  118 . To elaborate on this arrangement about the counter regions regarding the small end portion  108  and the piston pin  118 , the two ends  122   a ,  122   b  of the first counter region  122  are located farther inward than the two ends  124   a ,  124   b  of the second counter region  124  in the axial direction C of the piston pin  118 . Therefore, oil  130  which has arrived at the outer circumferential surface of the piston pin  118  on the side closer to the first counter region  122  is drawn as the piston pin  118  rotates, and then finds its way to areas between the piston pin  118  and the small end portion  108  of the connecting rod  72 , and between the piston pin  118  and the pin boss regions  98   a ,  98   b . Specifically, it is possible with this arrangement to sufficiently lubricate and cool an underside region of the piston  64  such as the pin holes  100   a ,  100   b  of the pin boss regions  98   a ,  98   b , an inner surface of the small end portion  108  of the connecting rod  72 , the outer circumferential surface of the piston pin  118 , etc. Therefore, the arrangement makes it possible to make smooth the movement of the connecting rod  72  with respect to the piston  64 . The arrangement is particularly advantageous in configurations where the connecting rod is designed not to have a conventional bearing metal which is commonly fitted into the small end portion. 
     The arrangement that the side surfaces  112   a ,  112   b  of the first portion  112  which are provided on the side closer to the tip portion of the small end portion  108  of the connecting rod  72  are located farther inward than the side surfaces  114   a ,  114   b  of the second portion  114  which are provided on the side closer to the big end portion  110  in the axial direction C of the piston pin  118  means that the portion closer to the tip portion is narrower than the portion closer to the big end portion  110 , in the small end portion  108  of the connecting rod  72 . In this case, it becomes possible to shorten the length of the piston pin  118 , and simultaneously to move the pin boss regions  98   a ,  98   b  toward the center of the piston  64 , in the underside region of the piston  64 . 
     This will be elaborated hereinafter. In the preferred embodiment shown in  FIG. 7 , a distance from a tip of the piston pin  118  to a top portion of the pin boss region  98   a ( 98   b ) in the axial direction C of the piston pin  118  will be called F; a distance from the tip of the piston pin  118  to a bottom portion of the pin boss region  98   a ( 98   b ) in the axial direction C of the piston pin  118  will be called G; a thickness of a tip portion side (the first portion  112 ) of the small end portion  108  of the connecting rod  72  will be called H; and a thickness of the big end portion  110  side (the second portion  114 ) of the small end portion  108  of the connecting rod  72  will be called I. Likewise, in a conventional example shown in  FIG. 9 , a distance from a tip of a piston pin  1  to a top portion of a pin boss region  2   a ( 2   b ) in an axial direction of the piston pin  1  will be called f; a distance from the tip of the piston pin  1  to a bottom portion of the pin boss region  2   a ( 2   b ) in the axial direction of the piston pin  1  will be called g; a thickness of a tip portion side of a small end portion  4  of a connecting rod  3  will be called h; and a thickness of a big end portion side of the small end portion  4  of the connecting rod  3  will be called i. In this case, even if there is a relationship of F=f and I=i between the preferred embodiment shown in  FIG. 7  and the conventional example shown in  FIG. 9 , there is still a relationship of G&lt;g and H&lt;h, so it is possible in the preferred embodiment in  FIG. 7  to shorten the overall length of the piston pin to be less than that of the conventional example in  FIG. 9 , and therefore it is possible to move the pin boss regions closer to the center of the piston. 
     The arrangement makes it possible to increase an oil contact area (hatched regions in  FIG. 4B ) which is on the radially outer side than the pin boss regions  98   a ,  98   b , of the piston  64 , in the underside region of the piston  64 , and therefore makes it possible to enhance cooling of the piston  64  and to reduce temperature increase in the piston  64 . Especially in cases where the connecting rod is designed not to have a conventional bearing metal which is commonly fitted into the small end portion of the connecting rod, the small end portion is often given an increased thickness. Even in such cases, various preferred embodiments of the present invention make it possible to have a sufficient area for oil contact in the underside region of the piston, and to provide sufficient cooling to the piston. 
     Also, in the preferred embodiment in  FIG. 7 , the length of the pin boss region along the axial direction C can be shortened more than in the conventional example in  FIG. 9 , so as to decrease the weight of the pin boss region. 
     While the connecting rod  72  is preferably made of an aluminum alloy, the piston pin  118  is preferably made of a ferrous alloy, for example. By making the two members of materials that are mutually dissimilar from each other, good adhesion resistance is provided in the counter regions between the connecting rod  72  and the piston pin  118 . This eliminates need for an expensive bearing metal, bushing or the like which must otherwise be fitted to the small end portion  108  of the connecting rod  72  and further, need for surface treatment to the small end portion  108  of the connecting rod  72 , making it possible to reduce cost of manufacture of the engine  24 . Also, the pin boss regions  98   a ,  98   b  which are closer to the center of the piston  64  shortens a heat transfer path from a center region of upper surface in the piston head  96 , via the pin boss regions  98   a ,  98   b  and the piston pin  118 , to the connecting rod  72 . Further, by using an aluminum alloy, i.e., a material having a high thermal conductivity, for the connecting rod  72 , the arrangement makes it possible to conductively remove heat from the center region of the upper surface in the piston head  96  and then effectively release it from the connecting rod  72 , thus enhancing cooling of the piston  64 . 
       FIG. 8A ,  FIG. 8B  and  FIG. 9  shows a conventional example. As exemplified in these drawings, if the pin boss regions  2   a ,  2   b  do not overlap the small end portion  4  of the connecting rod  3  when viewed from the center axis of the cylinder (from the sliding direction of the piston  5 ), the explosion in the engine exerts a bending load to the piston pin  1  to act in a direction from the tip portion of the small end portion  4  to the big end portion of the connecting rod  3 . Specifically, as shown in  FIG. 9 , as the piston  5  receives an explosion load P, two sides of the piston pin  1  come under a load of P/2, or in other words, load is concentrated onto specific regions in the piston pin  1  indicated as X1, X2. This can deform the piston pin  1 , which in turn increases localized surface pressure from the piston pin  1  to the pin boss regions  2   a ,  2   b  and the connecting rod  3 . However, referring now to  FIG. 3 ,  FIG. 4B  and  FIG. 7 , in the engine  24  the upper portion (the bulged portions  126   a ,  126   b ) of the pin boss regions  98   a ,  98   b  overlap the portion closer to the big end portion  110  (the second portion  114 ), of the small end portion  108  of the connecting rod  72  when viewed from the center axial direction A of the cylinder  54  (the sliding direction of the piston  64 ). According to this arrangement, the load exerted by the explosion in the engine  24  to the piston pin  118  in the direction from the tip portion of the small end portion  108  of the connecting rod  72  to the big end portion  110  is received by the portion closer to the big end portion  110  (the second portion  114 ) of the small end portion  108  of the connecting rod  72 , and therefore the piston pin  118  is less prone to deformation. Specifically, the bending load caused by the explosion in the engine  24  onto the piston pin  118  is reduced. Therefore, this arrangement improves durability of the piston  64 , the piston pin  118  and the connecting rod  72 . 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.