Abstract:
An oil pan structure includes a plurality of reinforcement beads of several kinds. An engine utilizes the oil pan, whereby the strength and vibration-rigidity (rigidity against vibration) of the oil pan and the engine are sufficiently enhanced so that a natural frequency of the oil pan and the engine therewith becomes adjustable through a selection as to the numbers, locations, and sizes of the beads.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oil pan structure and an engine therewith whereby the structure comprises: an oil pan flange; a bottom wall that forms a bottom part of an engine; two side walls (i.e. a left side wall and a right side wall) along a longitudinal direction of the engine, each wall that communicates a longitudinal edge of the bottom wall with the oil pan flange; and two side walls (i.e. a front side wall and a rear side wall) along a transverse direction of the engine, each wall that communicates a transverse edge of the bottom wall with the oil pan flange, thereby, the structure forms a boxy space, being capable of storing engine-internal oil therein. 
     2. Description of the Related Art 
     Conventionally known oil pan structures comprise: an oil pan flange that is fastened with bolts to the undersurface of the crankcase; a bottom wall that forms a bottom part of an engine; two side walls along a longitudinal direction of the engine, each wall that communicates a longitudinal edge of the bottom wall with the oil pan flange; and two side walls along a transverse direction of the engine, each wall that communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space so that the space can store engine-internal oil therein. In many cases, such an oil pan is manufactured by means of welding thin sheets of metal; thus, the stiffness of the structure is not sufficient. Further, the engine vibration level is high in general. Therefore, various measures to secure the strength as well as to control the vibration have been proposed. 
     A patent reference 1 (JP1999-280548) discloses an oil pan structure whereby the bottom part is reinforced with a bellow type plate, that is, with a plurality of reinforcement ribs  27 , while the left- and right-side walls are provided with a plurality of reinforcement ribs  26 , inside the oil pan. 
     A patent reference 2 (JP 2002-227626) discloses an oil pan structure whereby the bottom part is reinforced with a plurality of reinforcement channels  58 , whereby the channels give the bottom part a bellow surface. 
     A patent reference 3 (JP2002-364324), i.e. a reference 4 (JP 2007 Patent 3925614) discloses an oil pan structure whereby the bottom part is provided with a plurality of reinforcement beads  1   k ′, while the left- and right-side walls, along a longitudinal direction of the engine, are provided with a plurality of reinforcement beads  1   a′.    
     A patent reference 5 (JP1999-270408) discloses an oil pan structure whereby the bottom part is provided with a plurality of reinforcement beads along the engine longitudinal direction as well as the engine transverse direction. 
     In an oil pan structure comprising: an oil pan flange that is fastened with bolts to the undersurface of the crankcase; a bottom wall that forms a bottom part of an engine; two side walls along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange; and two side walls along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space so that the space can store engine-internal oil therein, 
     the mentioned oil pan is, in many cases, manufactured by means of welding thin sheets of metal. 
     Thus, the stiffness of the structure is not sufficient; further, the engine vibration level is high in general. As the mentioned patent references 1 to 5 disclose, mainstream measures to secure the strength as well as to control the vibration are based on reinforcement beads which are provided on the bottom part of the oil pan, the left/right side wall along the engine longitudinal direction, or the front/rear side wall along the engine transverse direction. 
     However, in the mentioned measures according to conventional manners to provide an engine oil pan with reinforcement beads, improvements on strength as well as vibration-reduction are limited in spite of the reinforcement beads. The reason is that the beads are provided independently on the bottom part of the oil pan, on the left/right side wall along the engine longitudinal direction, or on the front/rear side wall along the engine longitudinal direction,
         whereby the oil pan structure comprises: an oil pan flange that is fastened with bolts to the undersurface of the crankcase, a bottom wall that forms a bottom part of an engine, two side walls along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange, and two side walls along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space.       

     Further, it is noted that conventional oil pans are often of a welded structure. The present invention, however, is based on the premise that the oil pan structure is basically a one-piece press-work product. 
     Moreover, it is noted that the mentioned conventional arts do not disclose how a natural frequency of an oil pan can be adjustable in connection with the shape of the reinforcement beads. 
     In this specification, a term “bead” is fundamentally used for explanation, so as to clarify that the oil pan walls are provided with corrugation reinforcement. Namely, this invention clearly recognizes a bead as a groove that is provided on a sheet metal by means of press working. A groove may forms a module of corrugation. A plurality of grooves may form continuation of corrugation. 
     Further, a closed end of a bead is defined as a groove end where the groove disappears toward the original sheet metal, with a trace curve of a U-shape, while an open end or end-opening of the bead means a groove end where the groove disappears toward the original sheet metal, without a trace curve. Sometimes, in this open end or end-opening, the groove disappears three-dimensionally with a shape of a round egg-cone. For this reason, a bead both ends of which are the open end or end-opening is named a substantially egg-shaped bead, in this specification. On the other hand, when one end of a bead is a closed end, the bead is named a substantially U-shaped bead in this specification. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the subjects to be solved in the conventional arts, the present invention is aiming at: 
     improving strength as well as vibration reduction according to reinforcement beads provided on oil pan structures; 
     making it possible to adjust natural frequencies of an oil pan in connection with a shape of the reinforcement beads; and 
     realizing an engine and an oil pan thereof, whereby improved strength and vibration-rigidity are secured. 
     In order to accomplish the mentioned purposes, the present invention discloses an oil pan structure and an engine therewith whereby the structure comprises: 
     an oil pan flange that is fastened with bolts to the undersurface of an engine crankcase; 
     a bottom wall that forms a bottom part of an engine; 
     two side walls (i.e. a left side wall and a right side wall) along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange; and 
     two side walls (i.e. a front side wall and a rear side wall) along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space, being capable of storing engine-internal oil therein, 
     wherein, the bottom wall as well as the two side walls in the longitudinal direction as to the engine and the two side walls in the transverse direction as to the engine, namely, a bottom wall and four side walls comprise a plurality of reinforcement beads which swell with a corrugation shape, toward the inside space of the oil pan; the reinforcement beads stretch on from the bottom wall to the left- or right-side wall or to the front- or rear-side wall. 
     In a preferable example (1) according to the present invention, a mentioned reinforcement bead, that is, a first type reinforcement bead, comprises: 
     a first end-opening which communicates an surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part the surface of which swells toward the inside space of the oil pan; 
     a second end-opening which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     whereby the first end-opening, the tunnel-shaped part, and the second end-opening are connected and arranged so as to form an outer appearance of a substantially egg-shape; in addition, the curved surface of the tunnel-shaped part forms a wave up-rise module; further, by means of adjusting the length of the tunnel-shaped part, a natural frequency becomes adjustable. 
     In another preferable example (2) according to the present invention, a mentioned reinforcement bead, that is, a second type reinforcement bead, comprises: 
     a closed end which shifts, with a trace line yet with a geometrically-smooth continuation, a surface of the bead swelling toward the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part the surface of which swells toward the inside space of the oil pan; 
     an open end which communicates a surface of the bead swelling geometrically-smoothly with the an inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     whereby the closed end, the tunnel-shaped part, and the open end are connected and arranged so as to form an outer appearance of a substantially U-shaped; in addition, the curved surface of the tunnel-shaped part forms a shape like a wave up-rise module; further, by means of adjusting the length of the tunnel-shaped part, a natural frequency becomes adjustable. 
     In another preferable example (3) according to the present invention, a drain hole is provided on the bottom wall with the mentioned first and second type beads, whereby a drain cock is fitted to the drain hole as a reinforcement element for the bottom wall. 
     In a preferable example according to the present invention, a plurality of substantially triangular wave-grooves run on the right- and left-side walls of the oil pan in the engine longitudinal direction, whereby the distance between the two walls is broadened as the location goes upward, and the two walls are communicated through a cross section of the bottom wall; further, the cross section of the substantially triangular wave-grooves forms a continuation of triangle zigzag shape. 
     From a different view point, in a preferable embodiment (1) according to the present invention, an oil pan structure and an engine therewith whereby the structure comprises: 
     an oil pan flange that is fastened with bolts to the undersurface of an engine crankcase; 
     a bottom wall that forms a bottom part of an engine; 
     two side walls (i.e. a left side wall and a right side wall) along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange; and 
     two side walls (i.e. a front side wall and a rear side wall) along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space, being capable of storing engine-internal oil therein, 
     wherein, the bottom wall as well as the two side walls in the longitudinal direction as to the engine and the two side walls in the transverse direction as to the engine, namely, a bottom wall and four side walls comprise a plurality of reinforcement beads which swell with a corrugation shape, toward the inside space of the oil pan; the reinforcement beads stretch over on from the bottom wall to the left- or right-side wall or to the front- or rear-side wall. 
     Further, in this embodiment, a mentioned reinforcement bead, that is, a first type reinforcement bead, comprises: 
     a first end-opening which communicates an surface of the bead swelling geometrically-smoothly with the an inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part the surface of which swells toward the inside space of the oil pan; 
     a second end-opening which communicates a surface of the bead swelling geometrically-smoothly with the an inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     whereby the first end-opening, the tunnel-shaped part, and the second end-opening are connected and arranged so as to form an outer appearance of a substantially egg-shape; in addition, the curved surface of the tunnel-shaped part forms a shape like a wave up-rise module; further, by means of adjusting the length of the tunnel-shaped part, a natural frequency of the oil pan becomes adjustable so that the frequency can be apart from an engine explosion frequency, i.e. an engine first order revolution, or a high order frequency thereof. 
     Besides the above preferable embodiment (1), in another preferable embodiment (2), an oil pan structure and an engine therewith whereby the structure comprises: 
     an oil pan flange that is fastened with bolts to the undersurface of an engine crankcase; 
     a bottom wall that forms a bottom part of an engine; 
     two side walls (i.e. a left side wall and a right side wall) along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange; and 
     two side walls (i.e. a front side wall and a rear side wall) along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space, being capable of storing engine-internal oil therein, 
     whereby, the bottom wall as well as the two side walls in the longitudinal direction as to the engine and the two side walls in the transverse direction as to the engine, namely, a bottom wall and four side walls comprise a plurality of reinforcement beads which swell with a corrugation shape, toward the inside space of the oil pan; the reinforcement beads stretch on from the bottom wall to the left- or right-side wall or to the front- or rear-side wall, wherein the mentioned reinforcement bead, that is, a second type reinforcement bead, comprises: 
     a closed end which shifts, with a trace line yet with a geometrically-smooth continuation, a surface of the bead swelling toward the an inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part the surface of which swells toward the inside space of the oil pan; 
     an open end which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     wherein the closed end, the tunnel-shaped part, and the open end are connected and arranged so as to form an outer appearance of a substantially U-shaped; in addition, the curved surface of the tunnel-shaped part forms a shape like a wave up-rise module; further, by means of adjusting the length of the tunnel-shaped part, a natural frequency becomes adjustable so that the frequency can be apart from an engine explosion frequency, i.e. an engine first order revolution, or a high order frequency thereof. 
     A reinforcement bead in the present invention is defined as a groove which is formed with a metal sheet that configures an oil pan of an engine, wherein the groove is a strip swelling from the outer periphery surface (a box outer side) of the oil pan toward the inner periphery surface (a box inner side) of the oil pan or from the inner periphery surface (a box inner side) of the oil pan toward the outer periphery surface (a box outer side) of the oil pan. 
     When the oil pan is manufactured by means of sheet metal press working, the reinforcement bead is formed as a narrow strip swelling from an outer periphery surface of an oil pan side walls or an oil pan bottom wall toward the inner side of the oil pan; or the reinforcement bead is formed as a narrow strip swelling from an inner periphery surface of an oil pan side walls or an oil pan bottom wall toward the outer side of the oil pan. 
     In an embodiment of the present invention, an oil pan structure (and an engine therewith whereby the structure) comprises: 
     an oil pan flange that is fastened with bolts to the undersurface of an engine crankcase; 
     a bottom wall that forms a bottom part of an engine; 
     two side walls (i.e. a left side wall and a right side wall) along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange; and 
     two side walls (i.e. a front side wall and a rear side wall) along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space, (being capable of storing engine-internal oil therein), 
     wherein, the bottom wall as well as the two side walls in the longitudinal direction as to the engine and the two side walls in the transverse direction as to the engine, namely, a bottom wall and four side walls comprise a plurality of reinforcement beads which swell with a corrugation shape, toward the inside space of the oil pan; the reinforcement beads stretch on from the bottom wall to the left- or right-side wall or to the front- or rear-side wall. 
     Further, the reinforcement beads are fitted to all the walls, namely, the bottom wall, the first and second longitudinal side walls, and the first or second transverse side walls; what is more, the reinforcement beads stretch over the bottom wall and the transverse side walls, or the beads stretch over the bottom wall and the longitudinal side walls; namely, each of the beads is fitted so as to always stretch over two walls; thus, the beads are so remarkable to not only improve strength and vibration-rigidity (rigidity against vibration) but also enhance natural frequencies in comparison with conventional manners whereby reinforcement beads are placed independently on a bottom wall, longitudinal side walls, or transverse side walls. 
     Moreover, since the reinforcement beads stretch over the bottom wall and the transverse side walls, or the beads stretch over the bottom wall and the longitudinal side walls, the beads are effective to not only improve strength and vibration-rigidity but also enhance natural frequencies as to the structure of the oil pan and the engine; thus, a baffle plate welding used in conventional arts to improve strength can be dispensed with; that is, simply with sheet metal drawing (sheet metal press working) an oil pan structure with improved strength and enhanced vibration-rigidity, namely, enhanced natural frequencies as to engine deformations such as engine bending deformation. 
     Moreover, the substantially egg-shaped bead (a first type reinforcement bead) comprises: 
     a first end-opening which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part the surface of which swells toward the inside space of the oil pan; 
     a second end-opening which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     whereby the substantially egg-shaped bead (a first type reinforcement bead) forms an outer appearance of a substantially egg-shape. 
     Similarly, on the other hand, the substantially U-shaped bead (a second type reinforcement bead) comprises: 
     a closed end which shifts, with a trace line yet with a geometrically-smooth continuation, a surface of the bead swelling toward the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part the surface of which swells toward the inside space of the oil pan; 
     an open end which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     whereby the substantially U-shaped bead (a second type reinforcement bead) forms an outer appearance of a substantially U-shape. 
     According to the above, by means of making changes in the length or the width of the tunnel-shaped part of the substantially egg-shaped bead or in a layout of the beads thereof, or 
     by means of making changes in the length or the width of the tunnel-shaped part of the substantially U-shaped bead or in a layout of the beads thereof, 
     a natural frequency of the bead (either the former bead or the latter bead), by extension of the oil pan or the engine, can be adjustable so that the natural frequency of the structure can be easily apart from a resonance frequency due to engine-operation. Hereby, it is noted that a crankshaft torsional vibration, for example, brings such a resonance frequency. 
     In an example of this invention, the substantially egg-shaped bead (a first type reinforcement bead) and/or the substantially egg-shaped bead (a first type reinforcement bead) stretch over the bottom wall and the right side wall, or over the bottom wall and the left side wall. In this way, a baffle plate welding used in conventional arts to improve strength/rigidity can be dispensed with; that is, only with sheet metal drawing (press-working), is realized an oil pan structure with improved strength and enhanced vibration-rigidity, namely, enhanced natural frequencies as to engine deformations such as engine bending deformation; further, by means of making changes in the length or the arrangement of the beads, a potential resonance vibration during engine operation can be avoidable; namely, an oil pan, as a whole, with enhanced rigidity can be realized. 
     In a preferable example according to the present invention, the right and left side walls are placed so that the distance of the two side walls comes nearer as the location goes downward, and a transverse cross section of the walls forms a large egg-shaped curve (a downwardly cone-shaped curve); thus, the oil pan can have side walls of enhanced rigidity and natural frequencies. Further, by means of adjusting the numbers of the beads, the locations of the beads, and the sizes of the beads, a natural frequency can be apart from a resonance frequency. 
     In a preferable example according to the present invention, a plurality of substantially triangular wave-grooves run on the right- and left-side walls of the oil pan in the engine longitudinal direction, whereby the distance between the two walls is broadened as the location goes upward, and the two walls are communicated through a cross section of the bottom wall; further, the cross section of the substantially triangular wave-grooves forms a continuation of triangle zigzag shape; in addition, the substantially triangular wave-grooves of the zigzag shape are manufactured by means of sheet metal press working. 
     Therefore, a natural frequency of the engine as to engine bending deformation can be enhanced because of the bead reinforcement effect and press-work hardening. Further, by means of adjusting the amplitude and period of the substantially triangular wave-grooves, a potential resonance vibration during engine operation can be avoidable; in particular, the rigidity of the right-side and left-side walls, by extension, the whole oil pan rigidity is enhanced. 
     In a preferable example according to the present invention, a drain hole is provided on a broader area of the bottom wall so as to be fitted a drain cock as a reinforcement member. Further, toward a broader area and a narrower area of the bottom wall from the front side wall and the rear side wall, a plurality of the reinforcement beads, hereby the substantially U-shaped beads, are elongated in parallel to the engine longitudinal direction. Thus, the rigidity of the bottom wall including both the broader area with the drain cock and the narrower area with the U-shaped beads is enhanced. 
     The above-explained oil pan structure can be preferably applied to an oil pan of reciprocating engines with crankshafts, the structure comprising: 
     an oil pan flange that is fastened with bolts to the undersurface of an engine crankcase and the like; 
     a bottom wall that forms a bottom part of an engine; 
     two side walls, i.e. a left side wall and a right side wall, along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall with the oil pan flange; and 
     two side walls, i.e. a front side wall and a rear side wall, along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall with the oil pan flange, whereby, the structure forms a boxy space, being capable of storing engine-internal oil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in greater detail with reference to the preferred embodiments of the invention and the accompanying drawings, wherein: 
         FIG. 1  shows a top view of an oil-pan for small general-purpose engines as to embodiments of the present invention; 
         FIG. 2  shows a sectional view along the A-A section in  FIG. 1  as to the first embodiment; 
         FIG. 3  shows a sectional view along the B-B section in  FIG. 1  as to the first embodiment; 
         FIG. 4  shows an enlarged front view of a substantially egg-shaped bead as to the first embodiment; 
         FIG. 5  shows an enlarged front view of a substantially U-shaped bead as to the first embodiment; and 
         FIG. 6  relates to the second embodiment of the present invention and shows a schematic section structure of an oil-pan for small general-purpose engines. 
     
    
    
     The items with numerals in the figures are explained as follows:
       1  an oil pan flange;     2  bolts;     3 , 4  a side wall along a longitudinal direction,
       (a first or second longitudinal side wall), or   (a right or left side wall);   
         5 , 6  a side wall along a transverse direction,
       (a first or second transverse side wall), or   (a front or rear side wall);   
         7  a bottom wall;     8  an upper side opening;     10  a first bead;     11  a second bead;     12  a third bead;     13  a fourth bead;     14  a fifth bead;     10 - 14  reinforcement beads;   a substantially egg-shaped bead/a first type reinforcement bead;     15   a  a first end-opening of the bead  15 ;     15   b  a tunnel-shaped part (a corrugation module) of the bead  15 ;     15   c  a second end-opening of the bead  15 ;     16  a substantially U-shaped bead/a second type reinforcement bead;     16   a  a closed end of the bead  16 ;     16   b  a tunnel-shaped part (a corrugation module) of the bead  16 ;     16   c  an open end (a first end-opening) of the bead  16 ;     20  a drain port (a drain hole);     21  an oil outlet;     41  a transverse cross-section of a right or left side-wall of an oil pan as an embodiment of this invention; and     42  a transverse cross-section of a right or left side-wall of an oil pan as an embodiment of this invention.   

     DETAILED DESCRIPTION OF THE INVENTION 
     Hereafter, the present invention will be described in detail with reference to the embodiments shown in the figures. However, the dimensions, materials, shape, the relative placement, and so on of a component described in these embodiments shall not be construed as limiting the scope of the invention thereto, unless especially specific mention is placed. 
       FIG. 1  shows a top view of an oil pan for small general-purpose engines as to embodiments of the present invention;  FIG. 2  shows a sectional view along the A-A section in  FIG. 1  as to the first embodiment; and  FIG. 3  shows a sectional view along the B-B section in  FIG. 1  as to the first embodiment. 
     In  FIGS. 1 to 3 , the numeral  100  denotes an oil pan; the structure thereof is as follows. 
     The oil pan  100  comprises: 
     an oil pan flange  1  that is fastened with a plurality of bolts  2  to the undersurface of an engine crankcase (not shown); 
     a bottom wall  7  that forms a bottom part of the engine; 
     two side walls  3  and  4  (i.e. a left side wall and a right side wall) along a longitudinal direction of the engine, thereby each wall communicates a longitudinal edge of the bottom wall  7  with the oil pan flange; and 
     two side walls  5  and  6  (i.e. a front side wall and a rear side wall) along a transverse direction of the engine, thereby each wall communicates a transverse edge of the bottom wall  7  with the oil pan flange, whereby, the structure with an upper opening  8  forms a boxy space, being capable of storing engine-internal oil which falls down out of the engine into the boxy space. In addition, the symbol S in  FIGS. 1 and 2  denotes a center plane, i.e. a longitudinal median plane. 
     The mentioned oil pan is provided with a plurality of reinforcement beads. 
     As shown in  FIG. 3 , a first bead  10  stretches on from the bottom wall  7  to a first transverse side wall  5 , and a second bead  11  stretches on from the bottom wall  7  to a second transverse side wall  6 . 
     As shown in  FIG. 2 , a third bead  12  stretches on from the bottom wall  7  to a first longitudinal side wall  3 , and a fourth bead  13  stretches on from the bottom wall  7  to a second longitudinal side wall  4 . 
     As shown in  FIG. 1 , a fifth bead  14  goes across the bottom wall  7  in the transverse direction so as to reach both the first longitudinal side wall  3  and the second longitudinal side wall  4 . 
     Further, the mentioned beads  10  to  14  are provided so that each of the beads  10  to  14  is either a substantially egg-shaped bead  15  or a substantially U-shaped bead, as occasion demands. 
     As shown in  FIG. 4 , the substantially egg-shaped bead  15  comprises: 
     a first end-opening  15   a  which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part  15   b  the surface of which swells toward the inside space of the oil pan; 
     a second end-opening  15   c  which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     whereby the substantially egg-shaped bead  15  (a first type reinforcement bead) forms an outer appearance of a substantially egg-shape. In addition, the surface of the tunnel-shaped part  15   b  forms a shape like a wave up-rise module. Namely, the part  15   b  forms a module of metal sheet corrugation. 
     Accordingly, by means of making changes in the length L 1  or the width of the part  15   b  or in a layout of the part  15   b , a natural frequency of the bead  15 , by extension to a natural frequency of the oil pan can be adjustable. 
     On the other hand, as shown in  FIG. 5 , the substantially U-shaped bead  16  comprises: 
     a closed end  16   a  which shifts, with a trace line yet with a geometrically-smooth continuation, a surface of the bead swelling toward the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction; 
     a tunnel-shaped part  16   b  the surface of which swells toward the inside space of the oil pan; 
     an open end  16   c  which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction, 
     whereby the substantially U-shaped bead  16  (a second type reinforcement bead) forms an outer appearance of a substantially U-shape. In addition, the surface of the tunnel-shaped part  16   b  forms a shape like a wave up-rise module. Namely, the part  15   b  forms a module of metal sheet corrugation. 
     Accordingly, by means of making changes in the length L 2  or the width of the part  16   b  or in a layout of the parts  16 , a natural frequency of the bead  16 , by extension to a natural frequency of the oil pan can be adjustable. 
     As shown in  FIG. 1 , in the bottom wall  7  where the mentioned reinforcement beads such as a bead  10  are placed, are provided a drain port  20  and an oil outlet  21 . 
     A drain cock (not shown) which is fitted to the drain port  20  also acts as a reinforced element for the bottom wall  7 . 
     Therefore, by means of providing a plurality of reinforcement beads  10  and  11  so that the beads stretch on, in parallel to the longitudinal direction i.e. in parallel to the first and second longitudinal side walls  3  and  4 , from the first transverse side wall  5  as well as from the second transverse side wall  6  toward the bottom wall  7 , the oil pan of the present invention is reinforced not only a broader part of the oil pan, the part comprising of the bottom wall  7  where the drain cock is fitted, and the first or second transverse side walls  5  or  6 , but also a narrower part of the oil pan, the part comprising of the substantially U-shaped beads  10  and  11 , and the first or second transverse side walls  5  or  6 . 
     Namely, the rigidity of the oil pan can be enhanced. 
     According to the embodiment described thus far, the reinforcement beads such as the bead  10  are fitted to all the walls, namely, the bottom wall  7 , the first and second longitudinal side walls  3  and  4 , and the first or second transverse side walls  5  and  6 ; what is more, the reinforcement beads such as the bead  10  stretch over the bottom wall  7  and the transverse side walls  5  and  6 , or the beads stretch over the bottom wall  7  and the longitudinal side walls  3  and  4 ; namely, each of the beads is fitted so as to always stretch over two walls; thus, the beads such as the bead  10  are effective to not only improve strength and vibration-rigidity but also enhance natural frequencies; moreover, an oil pan structure according to the embodiment can be formed easily through sheet metal press working rather than according to conventional manners; furthermore, according to the embodiment, the improvement on strength and vibration-rigidity is remarkable in comparison with a conventional structure whereby reinforcement beads are placed independently on a bottom wall, longitudinal side walls, or transverse side walls. 
     As already described, since the reinforcement beads such as the bead  10  stretch over the bottom wall  7  and the transverse side walls  5  and  6 , or the beads stretch over the bottom wall  7  and the longitudinal side walls  3  and  4 , the beads such as the bead  10  are effective to not only improve strength and vibration-rigidity but also enhance natural frequencies. Accordingly, a baffle plate welding used in conventional arts to improve strength can be dispensed with; that is, only with sheet metal drawing, is realized an oil pan structure with improved strength and enhanced vibration-rigidity, namely, enhanced natural frequencies as to engine deformations such as engine bending deformation. 
     Also as already described, the substantially egg-shaped bead  15  comprises: 
     a first end-opening  15   a  which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction (such as walls  3 , 4 , 5  or  6  without bead provision); 
     a tunnel-shaped part  15   b  the surface of which swells toward the inside space of the oil pan; 
     a second end-opening  15   c  which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction (such as walls  3 , 4 , 5  or  6  without bead provision), 
     whereby the substantially egg-shaped bead (a first type reinforcement bead)  15  forms an outer appearance of a substantially egg-shape. In addition, the surface of the tunnel-shaped part  15   b  forms a surface like that of a bedpan 
     Similarly, the substantially U-shaped bead  16  comprises: 
     a closed end  16   a  which shifts, with a trace line yet with a geometrically-smooth continuation, a surface of the bead swelling toward the inherent side walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction (such as walls  3 , 4 , 5  and  6  without bead provision); 
     a tunnel-shaped part  16   b  the surface of which swells toward the inside space of the oil pan; 
     an open end  16   c  which communicates a surface of the bead swelling geometrically-smoothly with the inherent walls of the oil pan without bead provision, i.e. a bottom wall, a side wall along a longitudinal direction, or a side wall along a transverse direction (such as walls  3 , 4 , 5  and  6  without bead provision), 
     whereby the substantially U-shaped bead  16  forms an outer appearance of a substantially U-shape. In addition, the surface of the tunnel-shaped part  16   b  forms a surface like that of a bedpan. 
     Therefore, by means of making changes in the length L 1  or the width of the part  15   b  or in a layout of the parts  15 , or 
     by means of making changes in the length L 1  or the width of the part  16   b  or in a layout of the parts  16 , 
     a natural frequency of the bead (either bead  15  or bead  16 ) can be adjustable so that the natural frequency of the structure can be easily apart from a resonance frequency due to engine-operation. 
     Thus, the substantially egg-shaped bead (a first type reinforcement bead) and/or the substantially U-shaped bead (a second type reinforcement bead) stretch over the bottom wall and the right side wall, or over the bottom wall and the left side wall. In this way, a baffle plate welding used in conventional arts to improve strength/rigidity can be dispensed with; that is, only with sheet metal drawing (press-working), is realized an oil pan structure with improved strength and enhanced vibration-rigidity, namely, enhanced natural frequencies as to engine deformations such as engine bending deformation; further, by means of making changes in the length or the arrangement of the beads, a potential resonance vibration during engine operation can be avoidable; namely, an oil pan, as a whole, with enhanced rigidity can be realized. 
     As to an second embodiment of the present invention,  FIG. 6  shows a schematic transverse sectional view of an oil pan for small general-purpose engines. 
     In this embodiment, a plurality of substantially triangular wave-grooves run on the right- and left-side walls of the oil pan in the engine longitudinal direction, whereby the distance between the two walls is broadened as the location goes upward, and the two walls are communicated through a cross section of the bottom wall  7 , as shown in  FIG. 6 ; moreover, the mentioned reinforcement beads such as the substantially U-shaped beads or the substantially egg-shaped beads are provided on the bottom wall  7  and the front- and rear-side wall; further, the cross section  41  or  42  of the substantially triangular wave-grooves forms a continuation of triangle zigzag shape; in addition, the substantially triangular wave-grooves of the zigzag shape are manufactured by means of sheet metal press working. 
     According to the above embodiment, a natural frequency of the engine as to engine bending deformation can be enhanced because of the bead reinforcement effect and press work hardening. Further, by means of adjusting the amplitude and period of the substantially triangular wave-grooves, a potential resonance vibration during engine operation can be avoidable; in particular, the rigidity of the right-side and left-side walls, by extension, the whole oil pan rigidity is enhanced. 
     The present invention can be applied to all the oil pans which are able to store engine-oil therein. Thus, the invention is preferable especially to the oil pans for small general-purpose engines. 
     The present invention realizes an oil pan and an engine therewith whereby enhanced strength and vibration-resistance are secured so that a plurality of reinforcement beads are provided on the oil pan surfaces, the beads themselves being effectively strong for the structure and rigid enough for vibration; further, natural frequencies of the oil pan can be adjusted by means of changing the size or shape of the beads.