Patent Publication Number: US-9885312-B2

Title: Piston for internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a piston for an internal combustion engine, and more particularly, to a piston for a cylinder injection (direct injection) type internal combustion engine such as a diesel engine or a gasoline engine that directly injects fuel into cylinders. 
     A conventional example of a piston will now be described.  FIG. 11  is a cross-sectional view showing a combustion chamber of a diesel engine  100 .  FIG. 12  is a plan view of a piston body  102 . As shown in  FIG. 11 , the diesel engine  100  includes a piston  101  that includes the piston body  102 . The piston body  102  includes a head  104 , two side walls  109 , and two skirts  112 . The crown of the head  104  includes a recess  105 . Each side wall  109  includes a pin boss  110 . The two skirts  112  are respectively located at a thrust side (Th side) and an anti-thrust side (ATh side) with respect to the axis of a piston pin  107 . The thrust side (Th side) is the side of the piston  101  forced against the wall of the cylinder immediately after the piston  101  reaches the top dead center. In  FIG. 11 , the left side of the piston  101  is the thrust side (Th side), and the right side of the piston  101  is the anti-thrust side. The pin bosses  110  support the piston pin  107 . An injector  114  is located above the piston  101  to inject fuel toward the recess  105 . Japanese Laid-Open Utility Model Publication No. 6-4348 describes an example of such a piston including a piston body with a recessed crown. 
     The piston body  102  includes a lip  116  defined by the rim of the recess  105  in the head  104 . In the power stroke of the diesel engine, the piston  101  receives combustion gas having a high temperature and a high pressure. Thus, referring to  FIG. 12 , a large tensile stress that acts in the sideward direction of the engine (directions indicated by arrows y 1  in  FIG. 12 ) is applied to the lip  116  at the end located toward the engine front (Fr direction) and the end located toward the engine rear (Rr direction). A large pressure stress that acts in the front and rear directions of the engine (directions indicated by arrows y 2  in  FIG. 12 ) is applied to the lip  116  at the end located toward the thrust direction (Th direction) and the end located toward the anti-thrust direction (ATh direction). Further, the peripheral portion of the head  104  including the lip  116  is where the temperature of the piston  101  becomes the highest. Thus, the material strength is apt to decrease at this portion. This may form cracks  118  in the crown of the piston body  102  that extend from the Fr direction end and Rr direction end of the lip  116 . 
     In Japanese Laid-Open Utility Model Publication No. 6-4348, a plate formed from copper alloy is coupled to an inner top surface of the piston body. The lower surface of the plate includes fins. The plate, which includes the fins, cools the piston body to prevent the formation of cracks in the crown of the piston body. However, the plate is merely coupled to the inner top surface of the piston body. Thus, the plate does not effectively increase the flexural rigidity of the head in the thrust direction and the anti-thrust direction. Further, the plate does not effectively prevent the formation of cracks in the crown of the piston body. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a piston for an internal combustion engine that is able to reduce the formation of cracks in the crown of the piston body. 
     To achieve the above object, a piston for an internal combustion engine includes a piston body. The piston body includes a head, two side walls, two skirts, and a reinforcement member. The head includes a recessed crown. Each side wall includes a pin boss configured to support a piston pin. The two skirts are respectively located at a thrust side with respect to an axis of the piston pin and an anti-thrust side with respect to the axis of the piston pin. The reinforcement member includes two legs and a connecting portion connecting upper ends of the two legs. The two legs are respectively insert-casted in the two skirts, and the connecting portion is insert-casted in the head. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view showing a combustion chamber of a diesel engine in a first embodiment; 
         FIG. 2  is a cross-sectional view of a piston shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line III-III in  FIG. 2 ; 
         FIG. 4  is a bottom view of the piston shown in  FIG. 3 ; 
         FIG. 5  is a perspective view showing a high-rigidity member; 
         FIG. 6  is a partial perspective view of a high-rigidity member in a second embodiment; 
         FIG. 7  is a partial perspective view of a high-rigidity member in a third embodiment; 
         FIG. 8  is a side view of a piston in a fourth embodiment; 
         FIG. 9  is a diagram showing the rise of oil in a compression stroke of a diesel engine; 
         FIG. 10  is an enlarged view of the portion marked by X in  FIG. 9 ; 
         FIG. 11  is a cross-sectional view showing a prior art example of a diesel engine combustion chamber; and 
         FIG. 12  is a plan view showing a prior art example of a piston body. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the drawings. 
     First Embodiment 
     The present embodiment is applied to a piston  14  for a four-cycle direction injection diesel engine  10  (hereafter referred to as the “diesel engine  10 ”).  FIG. 1  is a cross-sectional view showing a combustion chamber  22  of the engine  10 .  FIG. 2  is a cross-sectional view showing the piston  14 .  FIG. 3  is a cross-sectional view taken along line III-III in  FIG. 2 .  FIG. 4  is a bottom view of the piston  14 .  FIG. 1  shows the piston  14 , which is located in one of a plurality of cylinders  12  of the engine  10 . In the present specification, the directions in which a piston pin  16  and a crankshaft (not shown) extend are referred to as the engine-front direction (Fr direction) and the engine-rear direction (Rr direction). The directions that are orthogonal to the Fr direction and the Rr direction are referred to as the thrust direction (Th direction) and the anti-thrust direction (ATh direction). 
     Referring to  FIG. 1 , the cylinder  12  accommodates the piston  14  in a manner allowing the piston  14  to reciprocate in the vertical direction. The piston pin  16  rotationally couples the piston  14  to a connecting rod  18 . A crank pin (not shown) rotationally couples the connecting rod  18  to the crankshaft. A cylinder head  20  is coupled above the cylinder  12 . The cylinder  12 , the piston  14 , and the cylinder head  20  form a combustion chamber  22 . An injector  24  is mounted on the cylinder head  20  to inject fuel into the combustion chamber  22 . The cylinder head  20  includes an intake port  26  and an exhaust port  28 . An intake valve  30  is arranged in the intake port  26  at the end connected to the combustion chamber  22 . An exhaust valve  32  is arranged in the exhaust port  28  at the end connected to the combustion chamber  22 . 
     As shown in  FIGS. 2 to 4 , the piston  14  includes a piston body  34 , piston rings  36 , and a high-rigidity member  38 . The piston body  34  is formed from, for example, an aluminum alloy. The piston body  34  includes a head  40 , two side walls  42 , and two skirts  44 . The peripheral portion of the head  40  extends along a land  46 . A plurality of (three in the drawing) piston rings  36  are coupled to the outer circumferential surface of the land  46 . The upper two piston rings  36  each function as a compression ring, and the remaining piston ring  36  functions as an oil ring. 
     As shown in  FIGS. 2 and 3 , the crown of the head  40  includes a cylindrical recess  48 , which has a closed bottom surface. The inside of the recess  48  defines a cavity that is in communication with the combustion chamber  22  (refer to  FIG. 1 ). A side wall surface  48   a  defining the recess  48  has the form of a truncated cone so that the diameter increases toward the bottom surface. The bottom surface of the recess  48  is defined by a gradual and conical projection  50 . The side wall surface  48   a  and the projection  50  (bottom surface) are gradually continuous and form an inwardly bulged surface. 
     As shown in  FIGS. 3 and 4 , the two side walls  42  are located at the lower side of the head  40  and aligned with each other in the front and rear directions of the engine  10  (Fr and Rr directions). The two side walls  42  are parallel to each other. Each side wall  42  includes a pin boss  52  that supports the piston pin  16  (refer to  FIG. 1 ). 
     As shown in  FIG. 2 , the two skirts  44  are respectively located at the thrust side (Th side) and the anti-thrust side (ATh side) with respect to the axis of the piston pin  16 . Each skirt  44 , which has an arcuate cross-section, is located at the lower side of the land  46  of the head  40 . Each skirt  44  includes two ends in the circumferential direction. Each side wall  42  includes two ends in the sideward direction of the engine  10 , that is, the Th direction and the ATh direction. Each end of the skirt  44  located at the Th side is continuously connected to the Th end of the adjacent side wall  42 , and each end of the skirt  44  located at the ATh side is continuously connected to the ATh end of the adjacent side wall  42  (refer to  FIG. 4 ). When the piston  14  moves in the vertical direction, the two skirts  44  slide along the wall surface of the cylinder  12  (refer to  FIG. 1 ). The head  40 , the two side walls  42 , and the two skirts  44  define an inner void  54  that is open to the lower side of the piston body  34 . 
     The high-rigidity member  38  is formed from a metal material having a higher Young&#39;s modulus than the material of the piston body  34  (aluminum alloy). The metal material may be steel, such as high-tensile steel or stainless steel.  FIG. 5  is a perspective view showing the high-rigidity member  38 . As shown in  FIG. 5 , the high-rigidity member  38  is formed by bending a strip of metal into a U-shaped form. The high-rigidity member  38  includes two legs  56  and a connecting portion  58  connecting the upper ends of the two legs  56 . The connecting portion  58  has the form of a reversed V that extends gradually in conformance with the inner surface, or top surface, in the head  40  of the piston body  34 . The connecting portion  58  extends in a direction orthogonal to the axis of the piston pin  16  (axis of pin bosses  52 ). 
     Referring to  FIGS. 2 to 4 , the two legs of the high-rigidity member  38  are insert-casted to the inner surfaces of the two skirts  44  in the piston body  34 . Further, the connecting portion  58  is insert-casted to the inner surface, or the top surface, in the head  40  of the piston body  34 . The inner surface of the high-rigidity member  38 , that is, the inner surfaces of the two legs  56  and the lower surface (inner bent surface) of the connecting portion  58  are exposed to the inner void  54  of the piston body  34 . The high-rigidity member  38  corresponds to the “reinforcement member” in the present specification. The inner surfaces of two legs  56  of the high-rigidity member  38  and the lower surface of the connecting portion  58  correspond to the “exposed surface” in the present specification. In this manner, the high-rigidity member  38  includes the two legs  56 , which are respectively embedded in the two skirts  44 , and the connecting portion  58 , which is embedded in the head  40 . Further, the high-rigidity member  38  includes the exposed surface, which is exposed to the inner void of the piston body  34 . 
     In the piston  14 , the high-rigidity member  38  in the piston body  34  increases the flexural rigidity of the head  40  in the thrust direction (Th direction) and the anti-thrust direction (ATh direction). Thus, the deformation amount of the head  40  is reduced in the thrust direction (Th direction) and the anti-thrust direction (ATh direction) at the crown of the piston body  34 . This reduces the formation of cracks in the crown of the piston body  34  (refer to  118  in  FIG. 12 ). 
     Second Embodiment 
     The following description will focus on differences from the first embodiment.  FIG. 6  is a perspective view showing a portion of the high-rigidity member  38 . As shown in  FIG. 6 , in the present embodiment, the lower surface of the connecting portion  58  in the high-rigidity member  38  of the first embodiment includes a plurality of (e.g., three) fins  60 . Each fin  60  has the form of a rib and extends in the longitudinal direction of the connecting portion  58 . Further, each fin  60  has a cross-section in the form of, for example, a reversed triangle. In the piston  14  of the present embodiment, the fins  60  on the lower surface of the connecting portion  58  increase the heat radiation surface and improve the cooling performance of the high-rigidity member  38 . This limits decreases in the fatigue strength of the piston body  34  under high temperatures and reduces the formation of cracks in the crown of the piston body  34 . The fins  60  of the high-rigidity member  38  increase the section modulus of the high-rigidity member  38  and improve the rigidity of the high-rigidity member  38 . 
     Third Embodiment 
     The following description will focus on differences from the second embodiment.  FIG. 7  is a perspective view showing a portion of the high-rigidity member  38 . As shown in  FIG. 7 , in the present embodiment, the lower surface of the connecting portion  58  in the high-rigidity member  38  includes a vast number of dimples  62  instead of the fins  60  of the second embodiment. Each dimple  62  is concave and has a semispherical wall surface. In the piston  14  of the present embodiment, the dimples  62  in the lower surface of the connecting portion  58  increase the amount of held oil and improve the cooling performance of the high-rigidity member  38 . This limits decreases in the fatigue strength of the piston body  34  under high temperatures and reduces the formation of cracks in the crown of the piston body  34 . 
     Fourth Embodiment 
     The following description will focus on differences from the first embodiment.  FIG. 8  is a side view showing the piston  14 . As shown in  FIG. 8 , in the present embodiment, the outer surface of each skirt  44  of the piston body  34  includes multiple (e.g., six) strips of resin-coated portions  64  extending in the vertical direction, that is, the axial direction of the piston body  34 . Each resin-coated portion  64  includes a lower end  64   a  having the shape of a reversed triangle. The resin-coated portion  64  is formed by, for example, a resin film having a low coefficient of friction and containing molybdenum. Further, the resin-coated portion  64  has a thickness of, for example, approximately 10 μm. 
     In the piston  14  of the present embodiment, the resin-coated portions  64  on the outer surface of each skirt  44  decrease the coefficient of friction of the piston  14 . This reduces friction loss of the piston  14 . Further, the resin-coated portions  64  are formed as vertical strips. Thus, oil readily falls out from between adjacent resin-coated portions  64  (refer to arrows Y 1  in  FIG. 8 ). This reduces the amount of oil that rises along the skirts  44  and limits unnecessary oil consumption. 
     Further, the lower end  64   a  of each resin-coated portion  64  has the shape of a reversed triangle. Thus, oil may be readily discharged from between the adjacent resin-coated portions  64  (refer to arrows Y 2  in  FIG. 8 ). This further reduces the amount of oil that rises along the skirts  44  and effectively limits unnecessary oil consumption. 
     The operation and advantages of the piston  14  of the present embodiment will now be described in comparison with the piston  101  of the prior art example (refer to  FIG. 11 ).  FIG. 9  is a diagram showing the rise of oil in a compression stroke of a diesel engine  10 .  FIG. 10  is an enlarged view of the portion marked by X in  FIG. 9 . Referring to  FIG. 9 , during a compression stroke of the engine  10 , the piston body  34  is tilted, and the lower end of the skirt  44  located at the anti-thrust side (ATh side) is forced against the wall of the cylinder  12 . This elastically deforms the lower end of the skirt  44  located at the anti-thrust side (ATh side). 
     The piston body  34  of the present embodiment includes the high-rigidity member  38 . This decreases the elastically deformed amount of the skirt  44  as compared with the elastically deformed amount of the skirt  112  in the prior art example (refer to  FIG. 11 ). In the present embodiment, oil collects between the skirt  44  and the wall surface of the cylinder  12  where region R, which has a wedge-shaped cross-section, is defined (refer to hatched portion and meshed portion in  FIG. 10 ). In the prior art, oil collects between the skirt  112  and the wall surface of the cylinder  12  where region r, which has a wedge-shaped cross-section, is defined (refer to meshed portion in  FIG. 10 ). Region R where oil collects in the present embodiment is larger than region r where oil collects in the prior art example. Thus, it may be expected that the structure of the present embodiment increases the amount of rising oil and consumes unnecessary oil. It is assumed here that the piston  14  of the present embodiment is located at the same height as the piston  101  of the prior art example. Further, in  FIGS. 9 and 10 , the tilted amount of the piston  14  is shown in an exaggerated manner. 
     However, the piston  14  of the present embodiment includes the vertical strips of the resin-coated portions  64  so that oil readily falls out from region R (refer to arrows Y 1  in  FIG. 8 ). Further, the lower end  64   a  of each resin-coated portion  64  has the shape of a reversed triangle so that oil is readily discharged out of region R (refer to arrows Y 2  in  FIG. 8 ). Accordingly, the high-rigidity member  38  increases the rigidity of the piston body  34  and decreases the elastically deformed amount of the skirt  44 . Further, the amount of oil that rises due to the decrease in the elastically deformed amount is reduced. This limits unnecessary oil consumption. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
     The present invention is not limited to the diesel engine  10  and may be a cylinder injection type gasoline engine. Further, the high-rigidity member  38  may be formed through casting. The high-rigidity member  38  may be entirely insert-casted in the piston body  34  so that the high-rigidity member  38  is hidden in the piston body  34 . At least one leg  56  of the high-rigidity member  38  may be entirely insert-casted in the piston body  34  so that the leg  56  is hidden in the piston body  34 . In the high-rigidity member  38 , the width of the exposed surface of the connecting portion  58  may be changed. In addition or instead, the width of the exposed surface of at least one of the legs may be changed. In addition to or instead of the lower surface of the connecting portion  58 , the fins  60  or the dimples  62  of the high-rigidity member  38  may be arranged in or on the inner surface of at least one of the legs  56 . Further, reinforcement ribs may be formed on the upper surface of the connecting portion  58  of the high-rigidity member  38 . In addition or instead, reinforcement ribs may be formed on the outer surface of at least one of the legs  56 . The fins  60  do not need to be shaped to have a triangular cross-section. The fins  60  may be shaped to have a tetragonal cross-section, a semicircular cross-sectional, or the like. Further, the dimples  62  may be changed in shape. 
     The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.