Abstract:
A piston is constructed to have improved rigidity yet be relatively light weight. The piston includes a head section having a surface on one side thereof and a rear or bottom surface on the opposite side thereof. At lease one piston pin inserting section is provided on the rear side of the head section and is configured to receive at least a portion of a piston pin of the engine. At least one rib joins together the rear surface of the head section and the piston pin inserting section to increase the rigidity of the piston.

Description:
RELATED CASES  
       [0001]     The present application is based on and claims priority under 35 U.S.C. § 119(a-d) to Japanese Patent Application No. 2004-031420, filed on Feb. 6, 2004, and to Japanese Patent Applications No. 2005-011029, filed on Jan. 19, 2005, the entire contents of which are expressly incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an internal combustion engine and, in particular, to an internal combustion engine including a piston.  
         [0004]     2. Description of Background Art  
         [0005]     Piston rigidity have been improved using known techniques over the years, including the one disclosed, for example, in, for example, Japanese patent publication number JP-A-4-244673.  
         [0006]     Japanese patent publication number JP-A-4-244673 discloses a technique for providing a reinforcing beam section in a piston, which also includes a skirt section and a piston pin insertion section (a pin boss section) on a rear surface side of a head section. The beam section connects the skirt section and the pin boss section to improve rigidity of the piston.  
         [0007]     However, since the head section constituting the piston is not reinforced, when the piston is reduced in weight by, for example, reducing a thickness of the head section, design and performance complications typically arise as the rigidity of the head section decreases. If the head thickness were to be reduced on pistons used in a high power engine, the high loads produced by such an engine would tend to damage the head sections of the pistons. Consequently, the piston head design disclosed in Japanese Patent Publication No. JP-A-4-244673 can not be used in high power engines to obtain fuel economy through a reduction in weight of the pistons.  
       SUMMARY OF THE INVENTION  
       [0008]     A need therefore exists for a piston that is of relatively light weight yet posses the rigidity necessary for operation in high horsepower engine.  
         [0009]     Accordingly, one aspect of the present invention involves a piston for an internal combustion engine that includes: a head section; at least one piston pin inserting section that is provided in an area of a rear surface of the head section and in which a piston pin is inserted; and at least one rib that is provided to connect the rear surface of the head section and the piston pin inserting section.  
         [0010]     In the internal combustion engine according to the aspect, as described above, by providing at least one rib connecting the rear surface of the head section and the piston pin inserting section, the head section and the piston pin inserting section are reinforced by the rib. Thus, it is possible to improve rigidity of the piston. Consequently, even if a load applied to the piston increases because of high power of the internal combustion engine, it is possible to prevent the piston (the head section and the piston pin inserting section) from being damaged. In addition, for example, even when the piston is reduced in weigh by reducing a thickness of the head section, since the head section is reinforced by the rib, it is possible to prevent rigidity of the piston (the head section) from decreasing by a meaningful degree. As a result, it is possible to realize high power of the internal combustion engine while improving fuel efficiency through a reduction in weight of the piston. In addition, by improving rigidity of the piston with the at least one rib connecting the rear surface of the head section and the piston pin inserting section, compared with the case in which rigidity of the piston is improved by increasing the thickness of the head section and a thickness of the piston pin inserting section, it is possible to prevent a weight of the piston from increasing. Consequently, the rigidity of the piston head can be maintained (or increased) while reducing the weigh of the piston.  
         [0011]     In a preferred mode, a pair of ribs are connected to the piston pin inserting section. It is further preferred that the pair of ribs are arranged such that the piston pin inserting section is disposed between the pair of ribs. With such a structure, it is possible to achieve reinforcement of the head section and the piston pin inserting section more surely.  
         [0012]     In the structure in which the pair of ribs are connected to the piston pin inserting section, the piston preferably further includes a pair of skirt sections that extend from the vicinity of an outer periphery of the rear surface of the head section to an opposite side of the surface of the head section and are spaced apart so as to be opposed to each other. The pair of ribs are formed to extend from the piston pin inserting section to the vicinity of one and the other of the pair of skirt sections. With such a structure, it is possible to reinforce not only a portion of the head section located near the piston pin inserting section but also a portion of the head section located near the skirt sections.  
         [0013]     In a further preferred mode, the piston includes a first piston pin inserting section and a second piston pin inserting section that are arranged to be opposed to each other and to be disposed apart from each other. The piston further includes (i) a first rib that is connected to a portion of the first piston pin inserting section opposed to the second piston pin inserting section and (ii) a second rib that is connected to a portion of the second piston pin inserting section opposed to the first piston pin inserting section. With such a structure, in which the piston includes the first piston pin inserting section and the second piston pin inserting section, it is possible to reinforce the head section, the first piston pin inserting section, and the second piston pin inserting section.  
         [0014]     In a structure that includes the first rib and the second rib, the first rib and the second rib preferably are skewed (that is, as viewed in plan view, formed to extend in directions inclined at an angle) with respect to both a central axis of the piston pin inserting section and an orthogonal axis that is orthogonal to the central axis of the piston pin inserting section. With such a structure, it is possible to prevent the head section from bending along the central axis of the piston pin inserting section and to prevent the head section from bending along the orthogonal axis orthogonal to the central axis of the piston pin inserting section with the first rib and the second rib.  
         [0015]     It is further preferred in such a piston construction for a distance, which is measured parallel to the central axis of the piston pin inserting section and between an end of the first rib on the opposite side of the first piston pin inserting section and an end of the second rib on the opposite side of the second piston pin inserting section, to be smaller than a second distance, which is measured parallel to the central axis and between an end of the first rib on the first piston pin inserting section side and an end of the second rib on the second piston pin inserting section side. With such a structure, it is possible to inhibit the head section from bending along the central axis of the piston pin inserting section and prevent the head section from bending along the orthogonal axis.  
         [0016]     In a more preferred mode, the end of the first rib on the opposite side of the first piston pin inserting section is arranged to extend to the vicinity of the outer periphery of the rear surface of the head section and the end of the second rib on the opposite side of the second piston pin inserting section is arranged to extend to the vicinity of the outer periphery of the rear surface of the head section. With such a structure, it is possible to reinforce the entire head section with the first rib and the second rib.  
         [0017]     The piston also can include at least one skirt section that extends from an area of the outer periphery of the rear surface of the head section in a direction on the opposite side of the surface of the head section and at least one wall that is provided to connect the rear surface of the head section, the piston pin inserting section, and the skirt section. With such a structure, since the head section, the piston pin inserting section, and the skirt section are reinforced by the wall, it is possible to further improve rigidity of the piston. Consequently, even if a load applied to the piston increases, it is possible for the piston to withstand such loads.  
         [0018]     In the structure in which the at least one wall is connected to the skirt section, the piston preferably includes a first piston pin inserting section and a second piston pin inserting section that are arranged to oppose and be deposed apart from each other. The piston also preferably includes a first pair of first that are connected to the first piston pin inserting section and a second pair of walls that are connected to the second piston pin inserting section. With such a structure, it is possible to reinforce the head section, the first piston pin inserting section and the second piston pin inserting section, and the skirt section with the pair of first walls and the pair of second walls.  
         [0019]     In the structure in which the wall includes the pair of first walls and the pair of second walls, the piston preferably also includes a first pair of ribs that are connected to a side surface side on an inner side of the first piston pin inserting section and a second pair of ribs that are connected to a side surface side on an inner side of the second piston pin inserting section. The first pair of walls are connected to a side surface side on an outer side of the first piston pin inserting section, and the second pair of walls are connected to a side surface side on an outer side of the second piston pin inserting section. With such a structure, since the inner side of the head section is reinforced by the first rib and the second rib and the outer side of the head section is reinforced by the first wall and the second wall, it is possible to reinforce the entire head section uniformly.  
         [0020]     In the structure in which the piston includes first and second pairs of walls, the piston preferably also includes a first skirt section and a second skirt section that are arranged to be opposed to each other and be spaced apart from each other. One of the first pair of walls is provided to connect the first piston pin inserting section and one side end of the first skirt section and the other of the first pair of walls is provided to connect the first piston pin inserting section and one side end of the second skirt section. Additionally, one of the second pair of walls is provided to connect the second piston pin inserting section and the other side end of the first skirt section and the other of the second pair of walls is provided to connect the second piston pin inserting section and the other side end of the second skirt section. With such a structure, it is possible to reinforce the head section, the first piston inserting section and the second piston inserting section, and the first skirt section and the second skirt section with the first and second pairs of walls.  
         [0021]     In the structure that includes first and second pairs of walls, the first pair of walls and the second pair of walls are skewed with respect to both a central axis of the piston pin inserting section and an orthogonal axis that is orthogonal to the central axis of the piston pin inserting section. With such a structure, it is possible to inhibit the head section from bending along the central axis of the piston pin inserting section and to inhibit the head section from bending along the orthogonal axis orthogonal to the central axis of the piston pin inserting section with the first walls and the second walls. In addition, in a plan view, since the first walls and the second walls are formed to extend in the directions inclined at an angle with respect to both the central axis of the piston pin inserting section and the orthogonal axis, when an external force is applied to the first skirt section and the second skirt section in a central direction of the piston, the first walls and the second walls tend to bend. Thus, it is possible to withstand the external force applied to the first skirt section and the second skirt section with the first walls and the second walls.  
         [0022]     In the structure in which the first walls and the second walls are skewed with respect to both the central axis of the piston pin inserting section and the orthogonal axis, preferably a distance, which is measured parallel to the central axis of the piston pin inserting section and between ends of the first walls on the opposite side of the first piston pin inserting section and ends of the second walls on the opposite side of the second piston pin inserting section, is larger than a second distance that is measured parallel to the central axis of the piston pin inserting section and between ends of the first walls on the first piston pin inserting section side and ends of the second walls on the second piston pin inserting section side. With such a structure, it is possible to inhibit the head section from bending along the central axis of the piston pin inserting section and to inhibit the head section from bending along the orthogonal axis orthogonal to the central axis of the piston pin inserting section with the first walls and the second walls. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     These and other features, aspects and advantages of the present invention are now described with reference to the drawings of a preferred embodiment, which is intended to illustrate and not to limit the present invention, and in which:  
         [0024]      FIG. 1  is a sectional view along a pin boss central axis around a piston of an internal combustion engine according to a preferred embodiment of the invention;  
         [0025]      FIG. 2  is a sectional view along an axis orthogonal to the pin boss central axis around the piston of  FIG. 1 ;  
         [0026]      FIG. 3  is a perspective view of the piston of  FIG. 1  taken from an upper side view thereof;  
         [0027]     FIGS.  4  is a perspective view of the piston of  FIG. 3  taken from an lower side view thereof;  
         [0028]      FIG. 5  is a bottom perspective view of the piston of  FIG. 4 ;  
         [0029]      FIG. 6  is a side view of the piston of  FIG. 3  as viewed from a skirt section side thereof;  
         [0030]      FIG. 7  is a side view of the piston of  FIG. 3  as viewed from a pin boss section side thereof;  
         [0031]      FIG. 8  is a top plan view of the piston of  FIG. 3  showing a surface side thereof; and  
         [0032]      FIG. 9  is a bottom plan view of the piston of  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0033]     An embodiment of the invention will be hereinafter explained on the basis of the drawings.  
         [0034]     First, a structure of a four-cycle internal combustion engine  1  in which a piston  3  in the form of a preferred embodiment can be used. All or some of the aspects and features of the present piston design, however, can be used with other types of internal combustion engines; for example, some or all of the aspects and features of the present piston design can be used with engines that have different numbers of cylinders, have different cylinder arrangements, use different ignition schemes, employ different fuel charging devices (e.g., direct fuel injection) and different valve arrangements and operation (e.g., VVT and VVL), and/or operate on different combustion principles (e.g., a two-stroke internal combustion engine).  
         [0035]     As shown in  FIGS. 1 and 2 , the four-cycle internal combustion engine  1  according to this embodiment includes a cylinder  2 , a piston  3  preferably of aluminum cast, two spark plugs  4 , an intake valve  5 , and an exhaust valve  6  (see  FIG. 2 ). The cylinder  2  includes a cylinder body  7  set on a crank case (not shown) side and a cylinder head  8  attached to the cylinder body  7 . A cylinder bore  7   a  is formed in the cylinder body  7 , and a recess  8   a  constituting a combustion chamber  9  is formed in the cylinder head  8 . The recess  8   a  of the cylinder head  8  is arranged to correspond to a position of the cylinder bore  7   a . In other words, an opening on the cylinder head  8  side of the cylinder bore  7   a  is closed by the recess  8   a  of the cylinder head  8 . In addition, as shown in  FIG. 1 , two ignition element attaching sections  8   b  preferably are formed in the cylinder head  8   b . Ignition elements (e.g., spark plugs  4 ) are attached to the attaching sections  8   b . Electric discharge sections  4   a  (i.e., the spark gap) of the spark plugs  4  project into the recess  8   a  of the cylinder head  8 .  
         [0036]     As shown in  FIG. 2 , the cylinder head  8  includes at least one intake passage  8   c  for supplying a mixture of gasoline and air to the combustion chamber  9  and at least one exhaust passage  8   d  for discharging a burnt charge (i.e., the exhaust gas) from the combustion chamber  9 . Although one intake passage  8   c  and one exhaust passage  8   d  are shown in  FIG. 2 , respectively, actually, a pair of intake passages  8   c  and a pair of exhaust passages  8   d  are preferably formed in the cylinder head  8 . The pair of intake passages  8   c  and the pair of exhaust passages  8   d  are combined to be one intake passage  8   c  and one exhaust passage  8   d , respectively, in the cylinder head  8 . A valve seat  10  is defined to an opening (an intake port)  8   e  on the combustion chamber  9  (the recess  8   a ) of the intake passage  8   c  and a valve seat  11  is defined to an opening (an exhaust port)  8   f  on the combustion chamber  9  side of the exhaust passage  8   d . The valve seats  10 ,  11  preferably consist of a material having satisfactory abrasion resistance, heat resistance, and heat conductivity. The valve seats  10 ,  11  are provided to control abrasion of the intake port  8   e  and the exhaust port  8   f  and to dissipate heat from the intake valve  5  and the exhaust valve  6 . In addition, guide members  12 ,  13  are attached to the intake passage  8   c  side and the exhaust passage  8   d  side of the cylinder head  8 , respectively. The intake valve  5  is arranged in a portion corresponding to the intake port  8   e  and the exhaust valve  6  is arranged in a portion corresponding to the exhaust port  8   f . The intake valve  5  and the exhaust valve  6  have functions of opening and closing the intake port  8   e  and the exhaust port  8   f , respectively. In addition, the intake valve  5  and the exhaust valve  6  are guided by the guide members  12 ,  13 , respectively. The description of the engine thus far is believed to be conventional and further description thereof is not necessary for an understand of the present piston construction.  
         [0037]     As shown in FIGS.  3  to  5 , the piston  3  has a structure including a head section  31 , a pair of pin boss sections  32   a ,  32   b , and a pair of skirt sections  33   a ,  33   b . The pin boss section  32   a  is an example of the “piston pin inserting section” and the “first piston pin inserting section” of a piston and the pin boss section  32   b  is an example of the “piston pin inserting section” and the “second piston pin inserting section” of a piston. The skirt sections  33   a ,  33   b  are examples of the “first skirt section” and the “second skirt section” of a piston, respectively.  
         [0038]     The piston  3  preferably includes at least one recess on an upper surface thereof (i.e., the surface that faces into the combustion chamber  9 ). In the illustrated embodiment, as shown in  FIGS. 3 and 8 , two recesses  31   a ,  31   b  having a designed depth are formed in the surface of the head section  31 . As shown in  FIG. 8 , in a plan view, the recesses  31   a ,  31   b  preferably have substantially a semicircular shape. In addition, the recesses  31   a  and  31   b  are arranged such that linear portions thereof (that is, the straight edges thereof) are opposed to each other. The piston, however, need not include one or more recesses, nor do the recesses need to be of the same shape, be of the same size, or be symmetrically positioned relative to a central plane of the piston.  
         [0039]     The piston preferably carries at least one sealing element to inhibit gases within the chamber from blowing by the piston into the crankcase. As shown in FIGS.  1  to  4 ,  6 , and  7 , three ring grooves  31   c  having a designed depth are preferably formed in an outer peripheral surface (a side) of the head section  31 . The three ring grooves  31   c  are provided in order to fix three piston rings  36  to be described later to the head section  31 . Fewer or greater rings, however, can be used.  
         [0040]     As shown in FIGS.  1  to  5  and  9 , the pair of pin boss sections  32   a ,  32   b  are provided integrally on the rear surface of the head section  31 . The pin boss sections  32   a ,  32   b  are spaced apart from the outer periphery of the head section  31  by equal distances. The pin boss sections  32   a ,  32   b  are arranged to oppose each other at a generally precise distance. In addition, holes  32   c ,  32   d , in which a piston pin  37  is inserted, are formed in the pin boss sections  32   a ,  32   b , respectively.  
         [0041]     In the illustrated embodiment, as shown in  FIGS. 4, 5 , and  9 , the piston includes on a rear or lower surface of the head section  31 , a pair of ribs  34   a . The ribs  34   a  are connected to and integrated with the rear surface of the head section  31  and the pin boss section  32   a . The piston  3  similarly includes a pair of ribs  34   b  that are connected to and integrated with the rear surface of the head section  31  and the pin boss section  32   b . The ribs  34   a ,  34   b  are examples of the “first rib” and the “second rib” of a piston, respectively. The present embodiment illustrates a further preferred arrangement of the ribs  324 ,  34   b . As shown, the pair of ribs  34   a  preferably are connected to a side surface  32   e  side on an inner side of the pin boss section  32   a  so as to dispose the pin boss section  32   a  between the pair of ribs  34   a , and the pair of ribs  34   b  are preferably connected to a side surface  32   f  side on an inner side of the pin boss section  32   b  so as to dispose the pin boss section  32   b  between the pair of ribs  34   b.    
         [0042]     With reference to  FIG. 9 , the ribs  34   a ,  34   b  are arranged to extend in directions skewed at an angle with respect to both a central axis  110  of the pin boss sections  32   a ,  32   b  (hereinafter referred to as pin boss central axis  110 ) and an orthogonal axis  120  that lies normal to the pin boss central axis  110 . More specifically, a distance W 1 , which is measured parallel to the pin boss central axis  110  and between ends  34   c  of the ribs  34   a  on the opposite side of the pin boss section  32   a  and ends  34   d  of the ribs  34   b  on the opposite side of the pin boss section  32   b , is smaller than a second distance W 2 , which is measured in a direction parallel to the pin boss central axis  110  and between ends  34   e  on the pin boss section  32   a  side of the ribs  34   a  and ends  34   f  on the pin boss section  32   b  side of the ribs  34   b.    
         [0043]     The ends  34   c  of the ribs  34   a  and the ends  34   d  of the ribs  34   b  are arranged near an outer periphery (the skirt sections  33   a  and  33   b ) on the rear surface of the head section  31 . As shown in  FIGS. 2, 4 , and  5 , the ribs  34   a  preferably are formed such that a height H from the rear surface of the head section  31  gradually increases from the ends  34   c  on the opposite side of the pin boss section  32   a  towards the ends  34   e  on the pin boss section  32   a  side. The ribs  34   b  are formed such that a height from the rear surface of the head section  31  gradually increases from the ends  34   d  on the opposite side of the pin boss section  32   b  toward the ends  34   f  on the pin boss section  32   b  side. A height H 1  of the ends  34   e  of the ribs  34   a  and the ends  34   f  of the ribs  34   b  from the rear surface of the head section  31  preferably is made to substantially the same as a height H 1  of the pin boss central axis  110  from the rear surface of the head section  31  (see  FIG. 2 ).  
         [0044]     As shown in FIGS.  1  to  7  and  9 , the pair of skirt sections  33   a ,  33   b  are formed to extend in the same direction on the opposite side of the surface of the head section  31  from the outer periphery of the rear surface of the head section  31 . In addition, the skirt sections  33   a ,  33   b  are arranged to oppose each other at a designed spacing (i.e., a predetermined interval) in a direction parallel to the orthogonal axis  120 .  
         [0045]     In the illustrated embodiment, as shown in  FIGS. 4, 5 , and  9 , on the rear surface of the head section  31 , a pair of walls  35   a  are integrally provided with the juxtaposed elements of the piston. The walls  35   a  connect together the rear surface of the head section  31 , the pin boss section  32   a , and the skirt sections  33   a ,  33   b . Similarly, a second pair of walls  35   b  are integrally provided with the juxtaposed elements of the piston. The second pair of walls  35   b  connect together the rear surface of the head section  31 , the pin boss section  32   b , and the skirt sections  33   a  and  33   b . The walls  35   a ,  35   b  in the illustrated embodiment are examples of the “first walls” and the “second walls” of a piston, respectively. In a preferred form, one of the pair of walls  35   a  is provided to connect the pin boss section  32   a  and one side end  33   c  of the skirt section  33   a  and the other of the pair of walls  35   a  is provided to connect the pin boss section  32   a  and one side end  33   d  of the skirt section  33   b . One of the pair of walls  35   b  is provided to connect the pin boss section  32   b  and the other side end  33   e  of the skirt section  33   a  and the other of the pair of walls  35   b  is provided to connect the pin boss section  32   b  and the other side end  33   f  of the skirt section  33   b . The pair of walls  35   a  are connected to a side of a side surface  32   g  on an outer side of the pin boss section  32   a  and the pair of walls  35   b  are connected to a side of a side surface  32   h  on an outer side of the pin boss section  32   b.    
         [0046]     As best seen in  FIG. 9 , the walls  35   a ,  35   b  are arranged to extend in directions skewed at an angle with respect to both the pin boss central axis  110  and the orthogonal axis  120 . More specifically, a distance W 3 , which is measured in a direction parallel to the pin boss central axis  110  and between ends  35   c  of the walls  35   a  on the opposite side of the pin boss section  32   a  and ends  35   d  of the walls  35   b  on the opposite side of the pin boss section  32   b , is larger than a distance W 4 , which is measured in a direction parallel to the pin boss central axis  110  and between ends  35   e  on the pin boss section  32   a  side of the walls  35   a  and ends  35   f  on the pin boss section  32   b  side of the walls  35   b.    
         [0047]     As shown in  FIGS. 1 and 2 , piston rings  36  are attached to the respective three ring grooves  31   c  of the head section  31  of the piston  3 . In a state in which the piston rings  36  are attached to the ring grooves  31   c  of the head section  31 , the piston  3  is fitted into the cylinder bore  7   a  of the cylinder body  7 . Therefore, general air tightness of the combustion chamber  9 , which is formed in part by the surface of the head section  31  of the piston  3 , is achieved by the piston rings  36 . In addition, as shown in  FIG. 8 , the spark plugs  4 , the intake ports  8   e , and the exhaust ports  8   f  preferably are arranged in areas corresponding to the recesses  31   a ,  31   b  of the head section  31  of the piston  3 , respectively.  
         [0048]     As shown in  FIG. 1 , the piston pin  37  is inserted in the pin boss sections  32   a ,  32   b  (through the holes  32   c ,  32   d ) of the piston  3 . This piston pin  37  is preferably fixed within the holes  32   c ,  32   d  by a C ring  38 . The piston pin  37  pivotally attaches a connecting rod  39  to the piston  3 . This connecting rod  39  is movable in a direction in which the cylinder axis  130  extends in synchronization with a rotational movement of a crankshaft (not shown). Consequently, the piston  3  coupled via the piston pin  37  to the connecting rod  39  reciprocates in the direction in which the cylinder axis  130  extends in synchronization with the rotational movement of the crankshaft.  
         [0049]     In the illustrated embodiment, as described above, the ribs  34   a ,  34   b  reinforce the head section  31  and the pin boss sections  32   a ,  32   b  on the rear surface of the head section  31 . This construction increases the rigidity of the piston  3 . Consequently, even if a high load is applied to the piston  3  (for example, if the piston is used in a high powered engine) the piston  3  can withstand such load and not be damaged. In addition, by improving rigidity of the piston  3  with the ribs  34   a ,  34   b , it is possible to reduce the weight of the piston  3  without significantly sacrificing the rigidity of the piston  3  by increasing an overall thickness of the head section  31  and a thickness of the pin boss sections  32   a ,  32   b.    
         [0050]     Consequently, it is possible to increase the rigidity of the piston  3  without significantly increasing the weight of the piston  3  so as not to sacrifice fuel efficiency to any meaningful degree.  
         [0051]     In the illustrated embodiment, the pair of ribs  34   a  are arranged so as to dispose the pin boss section  32   a  between the pair of ribs  34   a  and the pair of ribs  34   b  are arranged so as to dispose the pin boss section  32   b  between the pair of ribs  34   b . This arrangement reinforces the head section  31  as well as the pin boss sections  32   a ,  32   b.    
         [0052]     The piston preferably is constructed such that the distance W 1  is smaller than the distance W 2  (as seen  FIG. 9 ). Consequently, it is possible to inhibit the head section  31  from bending along the pin boss central axis  110  and to inhibit the head section  31  from bending along the orthogonal axis  120 .  
         [0053]     Additionally, the ends  34   c  of the ribs  34   a , which are on the opposite side of the pin boss section  32   a , and the ends  34   d  of the ribs  34   b , which are on the opposite side of the pin boss section  32   b , are preferably arranged near the outer periphery (the skirt sections  33   a ,  33   b ) on the rear surface of the head section  31 . This arrangement reinforces not only portions of the head section  31  located near the pin boss sections  32   a ,  32   b  but also portions located near the skirt sections  33   a ,  33   b . The ribs  34   a ,  34   b  also reinforce the entire head section  31  when so arranged.  
         [0054]     The height H of the ribs  34   a , as measured from the rear surface of the head section  31 , preferably increases gradually from the ends  34   c  on the opposite side of the pin boss section  32   a  toward the ends  34   e  on the pin boss section  32   a  side. Similarly, the height of the ribs  34   b  from the rear surface of the head section  31  preferably increases gradually from the ends  34   d  on the opposite side of the pin boss section  32   b  toward the ends  34   f  on the pin boss section  32   b  side. Consequently, it is possible to substantially reinforce of the pin boss sections  32   a ,  32   b , to which a large external force is applied when the four-cycle internal combustion engine  1  is operated. In this case, the piston can be formed such that (1) the height H 1  of the ends  34   e  on the pin boss section  32   a  side of the ribs  34   a  is substantially the same as the height H 1  of the pin boss central axis  110  from the rear surface of the head section  31 , and (2) the height of the ends  34   f  on the pin boss section  32   b  side of the rib  34   b  is substantially the same as the height of the pin boss central axis  110  from the rear surface of the head section  31 . Consequently, it is possible to prevent the reinforcement for the pin boss sections  32   a ,  32   b  by the ribs  34   a ,  34   b  from becoming excessive. Thus, it is possible to prevent the weight of the piston  3  from significantly increases as a result of the reinforcement of the pin boss sections  32   a ,  32   b  by the ribs  34   a ,  34   b  becoming excessive.  
         [0055]     In the illustrated embodiment, on the rear surface of the head section  31 , the pair of walls  35   a , which connect the rear surface of the head section  31 , the pin boss section  32   a , and the skirt sections  33   a  and  33   b , are provided and the pair of walls  35   b , which connect the rear surface of the head section  31 , the pin boss section  32   b , and the skirt sections  33   a  and  33   b , are provided. Consequently, the head section  31 , the pin boss section  32   a , and the skirt sections  33   a  and  33   b  are reinforced by the pair of walls  35   a  and the head section  31 , the pin boss section  32   b , and the skirt sections  33   a  and  33   b  are reinforced by the pair of walls  35   b . This construction further improves the rigidity of the piston  3 .  
         [0056]     In this embodiment, the piston  3  is constituted such that the distance W 3  is larger than the distance W 4  (as seen in  FIG. 9 ). This arrangement of the ribs  34   a ,  34   b  inhibits the head section  31  from bending along the pin boss central axis  110  and inhibits the head section  31  from bending along the orthogonal axis  120 . In addition, the walls  35   a ,  35   b  are formed to extend in the direction skewed with respect to both the pin boss central axis  110  and the orthogonal axis  120 . When an external force is applied to the skirt sections  33   a ,  33   b  generally along the cylinder axis  130 , the walls  35   a ,  35   b  tend to bend. Thus, the walls  35   a ,  35   b  absorb the external force applied to the skirt sections  33   a  and  33   b . Consequently, it is possible to inhibit damage to the piston  3  in cases where a large external force is applied to the skirt sections  33   a ,  33   b.    
         [0057]     In this embodiment, the ribs  34   a ,  34   b  are connected to the side of the side surface  32   e  on the inner side of the pin boss section  32   a  and the side of the side surface  32   f  on the inner side of the pin boss section  32   b . The walls  35   a ,  35   b  are connected to the side of the side surface  32   g  on the outer side of the pin boss section  32   a  and the side of the side surface  32   h  on the outer side of the pin boss section  32   b . In an embodiment so constructed, the inner side of the head section  31  is reinforced by the ribs  34   a ,  34   b  and the outer side of the head section  31  is reinforced by the walls  35   a ,  35   b . Thus, it is possible to reinforce the entire head section  31  substantially uniformly.  
         [0058]     In this embodiment, the two recesses  31   a  and  31   b  having the predetermined depth are formed on the surface of the head section  31 . Consequently, it is possible to easily obtain a reduction in weigh of the piston  3  while keeping rigidity of the piston  3  with the ribs  34   a  and  34   b.    
         [0059]     Although this invention has been disclosed in the context of certain preferred embodiments, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. For example, in the illustrated embodiment, the ribs are arranged to extend in the directions skewed with respect to both the pin boss central axis and the orthogonal axis. However, the ribs can be arranged to extend in a direction parallel to the orthogonal axis or in a direction parallel to the pin boss central axis. It is further understood that, while the walls are arranged in the illustrated embodiment to extend in the directions skewed with respect to both the pin boss axis and the orthogonal axis, the walls can be arranged to extend in a direction parallel to the orthogonal axis. Such variations are merely examples of the various embodiments of which the invention can take.  
         [0060]     It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiment may be made and still fall within the scope of the invention. For example, in the illustrated embodiment, the ribs connecting the rear surface of the head section and the pin boss section and the walls connecting the rear surface of the head section, the pin boss section, and the skirt sections are provided. However, in some applications only the ribs may be provided without providing the walls. It should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow.