Patent Publication Number: US-2022213846-A1

Title: Piston of internal combustion engine and internal combustion engine

Description:
TECHNICAL FIELD 
     The present disclosure relates to a piston reciprocatingly housed in a cylinder, and an internal combustion engine including the above-described piston. 
     BACKGROUND 
     Conventionally, as an internal combustion engine, a reciprocating engine is known which includes a cylinder having a cylindrical hole portion, a piston reciprocatingly housed in the above-described hole portion of the cylinder, and an injection nozzle configured to inject a fuel to a combustion chamber divided by the cylinder and the piston. 
     As the combustion chamber of the reciprocating engine, a so-called reentrant type combustion chamber is known which has a cavity formed to be recessed in a center of a piston top surface, and includes a narrowing portion narrowing an opening of the cavity and a protruding portion disposed in the center of the cavity (see Patent Document 1, for example). 
     The cavity described in Patent Document 1 includes the protruding portion disposed in the center of the cavity, a curved portion disposed so as to encompass the above-described protruding portion, and a separating step portion disposed in the above-described protruding portion. The above-described curved portion is configured to guide, toward the protruding portion, a non-combusted fuel (fuel spray) which is injected from the injection nozzle disposed in the center of the combustion chamber toward a wall surface of the curved portion. The non-combusted fuel injected from the injection nozzle flows toward the protruding portion along the wall surface of the curved portion after reaching the wall surface of the curved portion. The non-combusted fuel injected from the injection nozzle loses kinetic energy by the injection and is decreased in fluidity as the non-combusted fuel flows along the wall surface of the curved portion, making it difficult to take in ambient oxygen. If the non-combusted fuel is combusted in the state where sufficient oxygen is not taken in the non-combusted fuel, soot may be generated. 
     The cavity described in Patent Document 1 separates the non-combusted fuel flowing along the wall surface of the curved portion and reaching the protruding portion from the wall surface of the protruding portion by the above-described separating step portion, thereby facilitating mixing of the non-combusted fuel decreased in fluidity and the ambient oxygen of the non-combusted fuel. The above-described cavity can combust the non-combusted fuel reaching the protruding portion in the state where sufficient oxygen is taken in the non-combusted fuel, making it possible to reduce a region having a high fuel concentration in the periphery of the protruding portion, and to suppress generation of soot in the periphery of the protruding portion. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP2018-131942A 
     SUMMARY 
     Technical Problem 
     The separating step portion in Patent Document 1 is disposed on a radially inner side of a deepest portion of the cavity and above the above-described deepest portion in a cross-section including the axis of the piston. Thus, the wall surface of the curved portion includes, between the deepest portion of the cavity and the separating step portion, an inclined surface which is inclined to gradually be raised radially inward from the deepest portion of the cavity. In the cavity described in Patent Document 1, the non-combusted fuel injected from the injection nozzle and flowing along the wall surface of the curved portion may be accumulated between the deepest portion of the cavity and the separating step portion without being able to move beyond the above-described inclined surface, and the region having the high fuel concentration may occur between the deepest portion of the cavity and the separating step portion. If the region having the high fuel concentration described above widely occurs, generation of soot may be accelerated or combustion efficiency may be decreased. 
     In view of the above issues, an object of at least one embodiment of the present invention is to provide a piston of an internal combustion engine capable of suppressing generation of soot and capable of improving combustion efficiency. 
     Solution to Problem 
     (1) A piston of an internal combustion engine according to at least one embodiment of the present invention is a piston of an internal combustion engine configured to be reciprocable along an axial direction in a cylinder, the piston of the internal combustion engine including a cavity formed to be recessed in a center of a piston top surface, and an outer circumferential edge portion located on a radially outer side of the above-described cavity in the above-described piston top surface. The above-described cavity includes a lip portion which has an inclined surface extending obliquely downward from the above-described outer circumferential edge portion toward a radially inner side, a raised portion protruding upward from a bottom of the above-described cavity, and a curved portion connecting the above-described raised portion and the above-described lip portion. The above-described curved portion includes an outer circumference-side curved surface which includes a curved surface connected to the above-described lip portion, an inner circumference-side concave surface which is located on the radially inner side of the above-described outer circumference-side curved surface and includes a surface connected to the above-described raised portion, the inner circumference-side concave surface including a deepest portion of the above-described cavity, and a convex surface formed between the above-described outer circumference-side curved surface and the above-described inner circumference-side concave surface, as well as protruding upward. 
     With the above configuration (1), the curved portion of the piston includes the outer circumference-side curved surface which includes the curved surface connected to the lip portion, the inner circumference-side concave surface which is located on the radially inner side of the outer circumference-side curved surface, includes the surface connected to the raised portion, and includes the deepest portion of the cavity, and the convex surface formed between the outer circumference-side curved surface and the inner circumference-side concave surface , as well as protruding upward. That is, the convex surface is disposed on the radially outer side of the raised portion and the deepest portion of the cavity, in the cross-section including the axis of the piston. Thus, the piston including the cavity with the above-described curved portion can cause a non-combusted fuel injected from an injection nozzle and flowing toward the raised portion along the outer circumference-side curved surface to reach the convex surface before kinetic energy by the injection is lost, making it possible to prevent accumulation of the non-combusted fuel in the space facing the inner circumference-side concave surface located on the radially outer side of the convex portion. By preventing accumulation of the non-combusted fuel in the space facing the inner circumference-side concave surface, it is possible to suppress occurrence of a region having a high fuel concentration. Thus, it is possible to suppress generation of soot, and to improve combustion efficiency in a combustion chamber. 
     Further, with the above configuration (1), since the inner circumference-side concave surface includes the surface connected to the raised portion, it is possible to facilitate introduction of a combustion gas into the space facing the inner circumference-side concave surface from the center of the combustion chamber along the raised portion. The curved portion including the inner circumference-side concave surface described above can cause the non-combusted fuel separated from the curved portion by the convex surface to sufficiently take in the combustion gas in the space facing the above-described inner circumference-side concave surface, making it possible to effectively suppress generation of soot. 
     (2) In some embodiments, in the piston of the internal combustion engine according to the above configuration (1), the above-described inner circumference-side concave surface includes an inner circumference-side curved surface curved downward into a concave shape. 
     With the above configuration (2), since the inner circumference-side concave surface includes the inner circumference-side curved surface curved downward into the concave shape, it is possible to encourage the combustion gas flowing from the center of the combustion chamber along the raised portion to smoothly flow along the inner circumference-side curved surface. Since the curved portion having the inner circumference-side curved surface described above can introduce, into the non-combusted fuel separated from the curved portion by the convex surface, the combustion gas along a direction intersecting with a flow direction of the above-described separated non-combusted fuel, it is possible to cause the above-described separated non-combusted fuel to efficiently take in the combustion gas, and to effectively suppress generation of soot. 
     (3) In some embodiments, in the piston of the internal combustion engine according to the above configuration (1) or (2), the above-described lip portion has a lip-side convex curved surface which includes a surface connected to the above-described inclined surface, the lip-side convex curved surface projecting radially inward relative to the above-described inclined surface. 
     With the above configuration (3), the lip-side convex curved surface includes the surface connected to the inclined surface and projects radially inward relative to the inclined surface. The lip portion having the above-described lip-side convex curved surface can divide the flow of the non-combusted fuel injected from the injection nozzle without decreasing the flow velocity of the non-combusted fuel very much, making it possible to efficiently propagate the non-combusted fuel into the combustion chamber. Further, the lip portion having the above-described lip-side convex curved surface can form a section including the above-described lip-side convex curved surface into a thick shape, making it possible to ensure a strength capable of withstanding a heat load applied by the non-combusted fuel injected from the injection nozzle. 
     (4) In some embodiments, in the piston of the internal combustion engine according to any one of the above configurations (1) to (3), the piston of the internal combustion engine is configured such that the above-described convex surface is located on the radially inner side of an inner circumferential end of the above-described lip portion. 
     With the above configuration (4), since the piston of the internal combustion engine is configured such that the convex surface is located on the radially inner side of the inner circumferential end of the lip portion, as compared with a case where the convex surface is located on the radially outer side of the inner circumferential end of the lip portion, the non-combusted fuel injected from the injection nozzle can flow along the outer circumference-side curved surface over a long distance. Thus, it is possible to effectively use the combustion gas in a space facing the outer circumference-side curved surface in the combustion chamber for combustion. 
     (5) In some embodiments, in the piston of the internal combustion engine according to any one of the above configuration (1) to (4), the above-described outer circumference-side curved surface has a maximum inner diameter D 2  which is not greater than 0.7D 1 , where D 1  is an outer diameter of the above-described piston. 
     With the above configuration (5), the maximum inner diameter D 2  of the outer circumference-side curved surface is not greater than 0.7D 1 , where D 1  is the outer diameter of the piston. Thus, it is possible to provide a cooling passage, where a cooling liquid flows, on the radially outer side of the outer circumference-side curved surface in the piston. Providing the above-described cooling passage for the piston, it is possible to cool a slide portion with the piston, in particular, the cylinder by the liquid in the cooling passage. Thus, a higher output of the internal combustion engine is possible. 
     (6) In some embodiments, in the piston of the internal combustion engine according to any one of the above configuration (1) to (5), the outer diameter D 1  of the above-described piston is not less than 160 mm and not greater than 190 mm. 
     With the above configuration (6), the outer diameter D 1  of the piston is not less than 160 mm and not greater than 190 mm. The piston with the outer diameter D 1  satisfying the above-described condition is generally mounted on an internal combustion engine which is larger than an internal combustion engine for bicycle. As the combustion chamber of the internal combustion engine is large, a possibility for the non-combusted fuel injected from a combustion nozzle and flowing along the curved portion to reach the raised portion is low, and a possibility of occurrence of the region having the high fuel concentration in the space facing the curved portion is high. Even the piston with the outer diameter D 1  satisfying the above-described condition, with the piston having the configuration according to the above configuration (1), it is possible to suppress occurrence of the region having the high fuel concentration in the space facing the curved portion. Thus, it is possible to suppress generation of soot, and to improve combustion efficiency in the combustion chamber. 
     (7) In some embodiments, in the piston of the internal combustion engine according to any one of the above configurations (1) to (6), the above-described inner circumference-side concave surface includes an inner circumference-side curved surface curved downward into a concave shape, and the above-described inner circumference-side curved surface is configured to have a larger curvature than the above-described outer circumference-side curved surface. 
     With the above configuration (7), since the inner circumference-side curved surface is configured to have the larger curvature than the outer circumference-side curved surface, as compared with a case where the inner circumference-side curved surface is configured to have a smaller curvature than the outer circumference-side curved surface, it is possible to increase the volume of the space facing the inner circumference-side curved surface, and to fill the above-described space with a large amount of combustion gas. Filling the above-described space with the large amount of combustion gas, it is possible to facilitate mixing of the combustion gas filling the above-described space with the non-combusted fuel separated from the curved portion by the convex surface. Thus, the inner circumference-side curved surface having the larger curvature than the outer circumference-side curved surface can cause the above-described separated non-combusted fuel to sufficiently take in the combustion gas, making it possible to effectively suppress generation of soot. 
     (8) An internal combustion engine according to at least one embodiment of the present invention includes a cylinder, the piston according to any one of the above configurations (1) to (7), and an injection nozzle configured to inject a fuel to a combustion chamber divided by the above-described cylinder and the above-described piston. 
     With the above configuration (8), since the internal combustion engine includes the piston having the above configuration (1), it is possible to suppress occurrence of the region having the high fuel concentration in the space facing the curved portion. Thus, it is possible to suppress generation of soot, and to improve combustion efficiency. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, provided is a piston of an internal combustion engine capable of suppressing generation of soot and capable of improving combustion efficiency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view of an internal combustion engine including a piston according to an embodiment of the present invention. 
         FIG. 2  is a partially enlarged view of the internal combustion engine shown in  FIG. 1 . 
         FIG. 3  is a schematic perspective view of the piston according to an embodiment of the present invention. 
         FIG. 4  is a partially enlarged view corresponding to  FIG. 2  of the internal combustion engine including the piston according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Some embodiments of the present invention will be described below with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described or shown in the drawings as the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same”, “equal”, and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. 
     Further, for instance, an expression of a shape such as a rectangular shape or a tubular shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved. 
     On the other hand, the expressions “comprising”, “including” or “having” one constitutional element is not an exclusive expression that excludes the presence of other constitutional elements. 
     The same configurations are indicated by the same reference characters and may not be described again in detail. 
       FIG. 1  is a schematic cross-sectional view of an internal combustion engine including a piston according to an embodiment of the present invention.  FIG. 1  shows a state in which the piston is located in the vicinity of a compression top dead center. 
     As shown in  FIG. 1 , an internal combustion engine  1  according to some embodiments includes a cylinder  2  which has a cylindrical hole portion  21  extending along the axial direction (an extension direction of an axis LA of a piston  3 , the vertical direction in  FIG. 1 ), a piston  3  disposed in the hole portion  21  of the cylinder  2  and configured to be reciprocable along the above-described axial direction, and an injection nozzle  4  configured to inject a fuel to a combustion chamber  10  divided by the cylinder  2  and the piston  3 . That is, the internal combustion engine  1  includes a reciprocating engine. 
     In the illustrated embodiment, as shown in  FIG. 1 , the cylinder  2  includes a cylinder block  22  having at least one cylindrical hole portion  21  described above, and a cylinder head  23  disposed on the cylinder block  22  so as to fill the top of the cylindrical hole portion  21 . 
     The above-described combustion chamber  10  is divided by a top surface  31  of the piston  3  (piston top surface), a bottom surface  231  of the cylinder head  23  disposed opposite to the above-described top surface  31 , and an inner wall surface  211  of the hole portion  21  of the cylinder block  22 . 
     Hereinafter, in the axial direction, a side (an upper side in the drawing) where the bottom surface  231  of the cylinder head  23  is located relative to the top surface  31  of the piston  3  will be referred to as an upside, and a side (a lower side in the drawing) where the top surface  31  of the piston  3  is located relative to the bottom surface  231  of the cylinder head  23  will be referred to as a downside. 
     As with a general piston of an internal combustion engine, the piston  3  is coupled to a crank shaft located below the piston  3  via a con rod. As shown in  FIG. 1 , the piston  3  includes at least one (three in the drawing) ring groove  33  annularly formed in an outer circumferential surface  32  along the circumferential direction around the axis LA. The ring groove  33  is mounted with a piston ring  34 . 
     In the illustrated embodiment, the injection nozzle  4  includes an injector configured to inject a liquid fuel into a spray. The internal combustion engine  1  is configured such that the liquid fuel (non-combusted fuel) is injected from an injection hole  41  of the injection nozzle  4  to a combustion gas filling the combustion chamber  10 , thereby causing the liquid fuel to be mixed with the combustion gas and to have self-ignition in the combustion chamber  10 . That is, the internal combustion engine  1  in the illustrated embodiment includes a diesel engine. 
     In some other embodiments, the internal combustion engine  1  may be configured to inject a fuel gas (gas fuel) from the injection hole  41  of the injection nozzle  4 , or may be configured to ignite (light) the non-combusted fuel by an ignition device such as a spark plug. 
     In the illustrated embodiment, the injection nozzle  4  is mounted in the cylinder block  22 . The injection nozzle  4  is inserted in a nozzle insertion hole  24  formed in the cylinder block  22 , and one end portion  42  having the at least one injection hole  41  protrudes downward from the bottom surface  231  of the cylinder head  23 . The injection nozzle  4  is disposed at the center of the combustion chamber  10  in the radial direction. In the embodiment shown in  FIG. 1 , the injection nozzle  4  is disposed on a straight line obtained by extending the axis LA of the piston  3 . 
     In the illustrated embodiment, as shown in  FIG. 1 , the internal combustion engine  1  further includes an intake port  11  (intake passage) for sending the combustion gas from the outside of the combustion chamber  10  to the combustion chamber  10 , an exhaust port  12  (exhaust passage) for discharging an exhaust gas from the combustion chamber  10  to the outside of the combustion chamber  10 , an intake valve  13  for opening/closing the intake port  11 , and an exhaust valve  14  for opening/closing the exhaust port  12 . As the combustion gas, air containing oxygen can be given as an example. 
     The internal combustion engine  1  may be configured to introduce, as the above-described air (combustion gas), compressed air compressed by a compressor to have a pressure lower than an atmospheric pressure to the combustion chamber  10 , or may be configured to introduce atmospheric pressure air. 
     In the embodiment shown in  FIG. 1 , the intake port  11  and the exhaust port  12  are each disposed in the cylinder head  23 . The intake port  11  communicates with the combustion chamber  10  via an intake hole  111  opening to the bottom surface  231  of the cylinder head  23 . The exhaust port  12  communicates with the combustion chamber  10  via an exhaust hole  121  opening to the bottom surface  231  of the cylinder head  23 . 
     Opening/closing the intake valve  13  and the exhaust valve  14  before the non-combusted fuel is injected from the injection nozzle  4 , the combustion gas sent to the combustion chamber  10  through the intake port  11  fills the combustion chamber  10 . The non-combusted fuel injected from the injection nozzle  4  is mixed with the combustion gas filling the combustion chamber  10 , and then combusted in the combustion chamber  10 . The exhaust gas having been combusted in the combustion chamber  10  is discharged to the outside of the internal combustion engine  1  through the exhaust port  12 . 
       FIG. 2  is a partially enlarged view of the internal combustion engine shown in  FIG. 1 .  FIG. 3  is a schematic perspective view of the piston according to an embodiment of the present invention.  FIG. 4  is a partially enlarged view corresponding to  FIG. 2  of the internal combustion engine including the piston according to another embodiment of the present invention. As shown in  FIGS. 1 to 4 , the piston  3  includes a cavity  5  formed to be recessed in the center of the top surface  31  of the piston  3 , and an outer circumferential edge portion  6  located on the radially outer side of the cavity  5  in the top surface  31  of the piston  3 . 
     In the illustrated embodiment, as shown in  FIG. 2, 4 , the outer circumferential edge portion  6  has a plane surface  61  extending along a direction intersecting with (orthogonal to) the axis LA and extending along the circumferential direction. In the embodiment shown in  FIG. 3 , the plane surface  61  (outer circumferential edge portion  6 ) is annularly formed. 
     As shown in  FIGS. 1 to 4 , the cavity  5  includes a lip portion  7  which has an inclined surface  71  extending obliquely downward from the outer circumferential edge portion  6  toward the radially inner side, a raised portion  8  protruding upward from a bottom  51  of the cavity  5 , and a curved portion  9  connecting the raised portion  8  and the lip portion  7 . The inclined surface  71  (lip portion  7 ) extends along the circumferential direction. In the embodiment shown in  FIG. 3 , the inclined surface  71  (lip portion  7 ) is annularly formed. 
     In the illustrated embodiment, as shown in  FIG. 2, 4 , the raised portion  8  is formed into a cone shape. The raised portion  8  includes, in a cross-section including the axis LA, an inclined surface  81  linearly extending radially inward from the curved portion  9  along the direction intersecting with the axis LA, and a ceiling surface  82  extending at an upper end of the inclined surface  81  along the direction intersecting with (orthogonal to) the axis LA. The ceiling surface  82  is disposed on the axis of the axis LA and is opposite to the one end portion  42  of the injection nozzle  4 . 
     As shown in  FIG. 2, 4 , the above-described injection nozzle  4  is configured to inject the non-combusted fuel from the injection hole  41  toward the lip portion  7  of the piston  3 . In the illustrated embodiment, an axis LB of the injection hole  41  of the injection nozzle  4  extends toward the lip portion  7 . A plurality of injection holes  41  are disposed along the circumferential direction. 
     The flow of the non-combusted fuel injected from the injection hole  41  of the injection nozzle  4  is vertically divided into two directions by the lip portion  7 . A part of the non-combusted fuel flows toward a space facing the plane surface  61  of the outer circumferential edge portion  6  along the inclined surface  71  of the lip portion  7 . A remaining part of the non-combusted fuel flows toward the raised portion  8  along the curved portion  9 . 
     As shown in  FIGS. 1 to 4 , the piston  3  according to some embodiments includes the above-described cavity  5  including the lip portion  7 , the raised portion  8 , and the curved portion  9 , and the above-described outer circumferential edge portion  6 . As shown in  FIG. 2, 4 , the above-described curved portion  9  includes an outer circumference-side curved surface  91 , an inner circumference-side concave surface  92  located on the radially inner side of the outer circumference-side curved surface  91 , and a convex surface  93  formed between the outer circumference-side curved surface  91  and the inner circumference-side concave surface  92 , as well as protruding upward. 
     In the illustrated embodiment, as shown in  FIG. 3 , the outer circumference-side curved surface  91 , the inner circumference-side concave surface  92 , and the convex surface  93  each extend along the circumferential direction. In the embodiment shown in  FIG. 3 , the outer circumference-side curved surface  91 , the inner circumference-side concave surface  92 , and the convex surface  93  are each annularly formed. In some other embodiments, the outer circumference-side curved surface  91 , the inner circumference-side concave surface  92 , and the convex surface  93  may each be formed into an arc shape. 
     In the embodiment shown in  FIG. 2 , the outer circumference-side curved surface  91  includes a curved surface  911  connected to the lip portion  7 . The curved surface  911  (lip portion-side curved surface) is curved obliquely downward into a concave shape from the lip portion  7  toward the radially outer side. The outer circumference-side curved surface  91  further includes a curved surface  912  connected to the convex surface  93 . The curved surface  912  (convex surface-side curved surface) is curved obliquely upward into a concave shape from the convex surface  93  toward the radially outer side. 
     In the embodiment shown in  FIG. 4 , the outer circumference-side curved surface  91  includes a curved surface  914  connected to the lip portion  7  and the convex surface  93 . The curved surface  914  is curved obliquely downward into a concave shape from the lip portion  7  toward the radially inner side. In other words, the curved surface  914  is curved obliquely upward into the concave shape from the convex surface  93  toward the radially outer side. 
     In the embodiment shown in  FIG. 2, 4 , the inner circumference-side concave surface  92  includes a deepest portion  52  of the cavity  5 , and includes a surface  921  connected to the raised portion  8 . The surface  921  (raised portion-side surface) is curved into a concave shape from the raised portion  8  toward the radially outer side. The inner circumference-side concave surface  92  further includes a surface  922  connected to the convex surface  93 . The surface  922  (convex surface-side surface) is curved obliquely upward into a concave shape from the deepest portion  52  of the surface  921  toward the radially outer side. The deepest portion  52  of the cavity  5  is disposed at a boundary between the surface  921  and the surface  922 . 
     In some other embodiments, at least one of the surface  921  and the surface  922  may be a flat surface linearly extending in the cross-section including the axis LA, or may be curved into a convex shape. 
     As a comparative example of the curved portion  9 , a curved portion  9 A without the above-described convex surface  93  is indicated by a dotted line in  FIG. 2, 4 . If the cavity  5  of the piston includes the curved portion  9 A without the above-described convex surface  93 , the non-combusted fuel flowing radially inward along the curved portion  9 A may be accumulated in a space, which corresponds to a space  10 A facing the inner circumference-side concave surface  92 , without being able to move beyond the raised portion  8 , and a region having a high fuel concentration may occur. In the region having the high fuel concentration, combustion is performed in a state where mixing of the non-combusted fuel and the combustion gas is insufficient, which may generate soot or decrease combustion efficiency. 
     According to the above configuration, as shown in  FIG. 2, 4 , the curved portion  9  of the piston  3  includes the above-described outer circumference-side curved surface  91  which includes the curved surface  911  or the curved surface  914  connected to the lip portion  7 , the above-described inner circumference-side concave surface  92  which is located on the radially inner side of the outer circumference-side curved surface  91 , includes the surface  921  connected to the raised portion  8 , and includes the deepest portion  52  of the cavity  5 , and the above-described convex surface  93  formed between the outer circumference-side curved surface  91  and the inner circumference-side concave surface  92 , as well as protruding upward. That is, as shown in  FIG. 2, 4 , the convex surface  93  is disposed on the radially outer side of the raised portion  8  and the deepest portion  52  of the cavity  5 , in the cross-section including the axis LA of the piston  3 . Thus, the piston  3  including the cavity  5  with the curved portion  9  can cause the non-combusted fuel injected from the injection nozzle  4  and flowing toward the raised portion  8  along the outer circumference-side curved surface  91  to reach the convex surface  93  before kinetic energy by the injection is lost, making it possible to prevent accumulation of the non-combusted fuel in the space  10 A facing the inner circumference-side concave surface  92  located on the radially outer side of the convex surface  93 . By preventing accumulation of the non-combusted fuel in the space  10 A facing the inner circumference-side concave surface  92 , it is possible to suppress occurrence of the region having the high fuel concentration. Thus, it is possible to suppress generation of soot, and to improve combustion efficiency in the combustion chamber  10 . 
     Further, according to the above configuration, since the inner circumference-side concave surface  92  includes the surface  921  connected to the raised portion  8 , it is possible to facilitate introduction of the combustion gas into the space  10 A facing the inner circumference-side concave surface  92  from the center of the combustion chamber  10  along the raised portion  8 . The curved portion  9  including the inner circumference-side concave surface  92  described above can cause the non-combusted fuel separated from the curved portion  9  by the convex surface  93  to sufficiently take in the combustion gas in the space  10 A facing the inner circumference-side concave surface  92 , making it possible to effectively suppress generation of soot. 
     In some embodiments, as shown in  FIG. 2, 4 , the above-described inner circumference-side concave surface  92  includes an inner circumference-side curved surface  920  curved downward into a concave shape. That is, the inner circumference-side curved surface  920  includes the above-described surface  921  (raised portion-side surface) curved into the concave shape from the above-described raised portion  8  toward the radially outer side, and the above-described surface  922  (convex surface-side surface) connected to the convex surface  93  and curved obliquely upward into the concave shape from the deepest portion  52  of the surface  921  toward the radially outer side. The surface  922  in the inner circumference-side curved surface  920  is connected to the surface  921 , and has the same curvature as the surface  921 . 
     According to the above configuration, since the inner circumference-side concave surface  92  includes the inner circumference-side curved surface  920  curved downward into the concave shape, it is possible to encourage the combustion gas flowing from the center of the combustion chamber  10  along the raised portion  8  to smoothly flow along the inner circumference-side curved surface  920 . Since the curved portion  9  having the inner circumference-side curved surface  920  described above can introduce, into the non-combusted fuel separated from the curved portion  9  by the convex surface  93 , the combustion gas along the direction intersecting with a flow direction of the above-described separated non-combusted fuel, it is possible to cause the above-described separated non-combusted fuel to efficiently take in the combustion gas, and to effectively suppress generation of soot. 
     In some embodiments, as shown in  FIG. 2 , the above-described inner circumference-side curved surface  920  is configured to have a larger curvature than the above-described outer circumference-side curved surface  91 . That is, a curvature R 1  of the inner circumference-side curved surface  920  is larger than a curvature R 2  of the outer circumference-side curved surface  91 . The outer circumference-side curved surface  91  includes the above-described curved surface  911 , and the above-described curved surface  912  connected to the curved surface  911  and having the same curvature as the curved surface  911 . 
     According to the above configuration, since the inner circumference-side curved surface  920  is configured to have the larger curvature than the outer circumference-side curved surface  91 , as compared with a case where the inner circumference-side curved surface  920  is configured to have a smaller curvature than the outer circumference-side curved surface  91 , it is possible to increase the volume of the space  10 A facing the inner circumference-side curved surface  920 , and to fill the above-described space  10 A with a large amount of combustion gas. Filling the above-described space  10 A with the large amount of combustion gas, it is possible to facilitate mixing of the combustion gas filling the space  10 A with the non-combusted fuel separated from the curved portion  9  by the convex surface  93 . Thus, the inner circumference-side curved surface  920  having the larger curvature than the outer circumference-side curved surface  91  can cause the above-described separated non-combusted fuel to sufficiently take in the combustion gas, making it possible to effectively suppress generation of soot. 
     In some embodiments, as shown in  FIG. 2 , the above-described inner circumference-side curved surface  920  is configured to have a shorter arc length in the cross-section including the axis LA than the above-described outer circumference-side curved surface  91 . In this case, it is possible to immediately mix the combustion gas flowing from the center of the combustion chamber  10  along the raised portion  8  with the non-combusted fuel separated from the curved portion  9  by the convex surface  93 , making it possible to facilitate mixing of the combustion gas filling the space  10 A with the non-combusted fuel separated from the curved portion  9  by the convex surface  93 . 
     In some embodiments, as shown in  FIG. 2 , the above-described lip portion  7  has a lip-side convex curved surface  72  which includes a surface connected to the above-described inclined surface  71  and projecting radially inward relative to the inclined surface  71 . 
     According to the above configuration, the lip-side convex curved surface  72  has the surface connected to the inclined surface  71  and projects radially inward relative to the inclined surface  71 . The lip portion  7  having the lip-side convex curved surface  72  can divide the flow of the non-combusted fuel injected from the injection nozzle  4  without decreasing the flow velocity of the non-combusted fuel very much, making it possible to efficiently propagate the non-combusted fuel into the combustion chamber  10 . Further, the lip portion  7  having the lip-side convex curved surface  72  can form a section  721  including the lip-side convex curved surface  72  into a thick shape, making it possible to ensure a strength capable of withstanding a heat load applied by the non-combusted fuel injected from the injection nozzle  4 . 
     In some embodiments, as shown in  FIG. 2 , the internal combustion engine  1  of the piston  3  is configured such that the above-described convex surface  93  is located on the radially inner side of an inner circumferential end  722  of the above-described lip portion  7 . The inner circumferential end  722  is a section where the inner diameter of the lip portion  7  is minimum. 
     In this case, since the internal combustion engine  1  of the piston  3  is configured such that the convex surface  93  is located on the radially inner side of the inner circumferential end  722  of the lip portion  7 , as compared with a case where the convex surface  93  is located on the radially outer side of the inner circumferential end  722  of the lip portion  7 , the non-combusted fuel injected from the injection nozzle  4  can flow along the outer circumference-side curved surface  91  over a long distance. Thus, it is possible to effectively use the combustion gas in a space  10 B facing the outer circumference-side curved surface  91  in the combustion chamber  10  for combustion. 
     In some embodiments, as shown in  FIG. 4 , the above-described lip portion  7  has an inner surface  73  which includes a surface connected to the above-described inclined surface  71  and extending along the direction intersecting with the inclined surface  71 . In the illustrated embodiment, as shown in  FIG. 4 , the above-described inner surface  73  extends downward from an inner circumferential end  711  of the inclined surface  71  along the axial direction, and has a lower end gently connected to an upper end of the curved surface  914 . In some other embodiments, the inner circumferential end  711  of the inclined surface  71  described above may be connected to the upper end of the curved surface  914 . 
     In some embodiments, as shown in  FIG. 1 , the above-described outer circumference-side curved surface  91  has a maximum inner diameter D 2  which is not greater than 0.7D 1 , where D 1  is an outer diameter of the above-described piston  3  (an outer diameter of the outer circumferential surface  32 ). 
     In the embodiment shown in  FIG. 1, 2 , the outer circumference-side curved surface  91  includes a maximum inner diameter portion  913  which is a boundary between the curved surface  911  and the curved surface  912 . In the embodiment shown in  FIG. 4 , the outer circumference-side curved surface  91  includes the maximum inner diameter portion  913  which is a boundary with the upper end of the curved surface  914 , that is, the inner surface  73 . The maximum inner diameter D 2  of the outer circumference-side curved surface  91  is the inner diameter of the maximum inner diameter portion  913 . 
     Further, in the illustrated embodiment, the piston  3  is provided with a cooling passage  35  where a cooling liquid flows between the outer circumferential surface  32  and the outer circumference-side curved surface  91 . 
     According to the above configuration, the maximum inner diameter D 2  of the outer circumference-side curved surface  91  is not greater than 0.7D 1 , where D 1  is the outer diameter of the piston  3 . Thus, it is possible to provide the cooling passage  35 , where the cooling liquid flows, on the radially outer side of the outer circumference-side curved surface  91  in the piston  3 . Providing the cooling passage  35  for the piston  3 , it is possible to cool a slide portion (a portion in the vicinity of the ring groove  33  shown in  FIG. 1 ) with the piston  3 , in particular, the cylinder  2  by the liquid in the cooling passage  35 . Thus, a higher output of the internal combustion engine  1  is possible. 
     In some embodiments, the outer diameter D 1  of the above-described piston  3  is not less than 160 mm and not greater than 190 mm. The piston  3  with the outer diameter D 1  satisfying the above-described condition is generally mounted on an internal combustion engine which is larger than an internal combustion engine for bicycle. As the combustion chamber  10  of the internal combustion engine  1  is large, a possibility for the non-combusted fuel injected from the injection nozzle  4  and flowing along the curved portion  9  to reach the raised portion  8  is low, and a possibility of occurrence of the region having the high fuel concentration in the space facing the curved portion  9  (the space  10 A facing the inner circumference-side concave surface  92 ) is high. Even the piston  3  with the outer diameter D 1  satisfying the above-described condition, if the curved portion  9  includes the above-described convex surface  93 , it is possible to suppress occurrence of the region having the high fuel concentration in the space facing the curved portion  9 . Thus, it is possible to suppress generation of soot, and to improve combustion efficiency in the combustion chamber  10 . 
     As described above, the internal combustion engine  1  according to some embodiments includes the above-described cylinder  2 , the above-described piston  3 , and the above-described injection nozzle  4  configured to inject a fuel to the combustion chamber  10  divided by the cylinder  2  and the piston  3 . In this case, in the internal combustion engine  1 , since the curved portion  9  of the above-described piston  3  includes the above-described convex surface  93 , it is possible to suppress occurrence of the region having the high fuel concentration in the space facing the curved portion  9  (the space  10 A facing the inner circumference-side concave surface  92 ). Thus, it is possible to suppress generation of soot, and to improve combustion efficiency. 
     The present invention is not limited to the above-described embodiments, and also includes an embodiment obtained by modifying the above-described embodiments and an embodiment obtained by combining these embodiments as appropriate. 
     Reference Signs List 
     
         
           1  Internal combustion engine 
           2  Cylinder 
           21  Hole portion 
           211  Inner wall surface 
           22  Cylinder block 
           23  Cylinder head 
           231  Bottom surface 
           24  Nozzle insertion hole 
           3  Piston 
           31  Top surface 
           32  Outer circumferential surface 
           33  Ring groove 
           34  Piston ring 
           35  Cooling passage 
           4  Injection nozzle 
           41  Injection hole 
           42  One end portion 
           5  Cavity 
           51  Bottom 
           52  Deepest portion 
           6  Outer circumferential edge portion 
           61  Plane surface 
           7  Lip portion 
           71  Inclined surface 
           711  Inner circumferential end 
           72  Lip-side convex curved surface 
           722  Inner circumferential end 
           73  Inner surface 
           8  Raised portion 
           81  Inclined surface 
           82  Ceiling surface 
           9  Curved portion 
           91  Outer circumference-side curved surface 
           913  Maximum inner diameter portion 
           92  Inner circumference-side concave surface 
           920  Inner circumference-side curved surface 
           93  Convex surface 
           10  Combustion chamber 
           10 A Space facing inner circumference-side curved surface 
           10 B Space facing outer circumference-side curved surface 
           11  Intake port 
           111  Intake hole 
           12  Exhaust port 
           121  Exhaust hole 
           13  Intake valve 
           14  Exhaust valve 
         D 1  Outer diameter 
         D 2  Maximum inner diameter 
         LA, LB Axis 
         R 1 , R 2 , Curvature