Abstract:
An oil passage of an internal combustion engine including a crankcase, a cylinder block integrally connected to the crankcase, a piston operatively mounted within the cylinder block, a piston jet for providing oil to the piston within the cylinder block and a breather device integrally formed on the cylinder block wherein the breather device is in communication with the inside of the crankcase. An elevation of temperature of the oil which flows in an oil supply path is prevented thus allowing oil in the oil supply path to maintain a low temperature wherein the oil in the supply path is supplied to a piston jet formed on the internal combustion engine. An oil supply path through which oil is supplied to the piston jet is integrally formed on a wall body exposed to ambient air on a peripheral wall which forms the breather device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2013-072911 filed Mar. 29, 2013 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an oil passage formed in a crankcase of an internal combustion engine. 
     2. Description of Background Art 
     A V-type internal combustion engine is known wherein an oil supply path to a piston jet for injecting oil into a piston is formed in a bottom portion inside a V-shaped bank. In this prior art, a breather device is arranged such that the breather device covers an upper side of the oil supply path. Thus, oil which passes through the oil supply path is brought into a state where the temperature of the oil is liable to be elevated by heat transferred from a cylinder. At the same time, the oil supply path is brought into a state where the oil supply path is covered with the breather so that the temperature of oil is maintained. As a result, high temperature oil is supplied to the piston jet. See, for example, JP-A-2003-106132. In view of the cooling the piston, it is desirable that oil supplied to the piston jet is supplied such that the elevation of the oil temperature is suppressed as much as possible. Also in the case where a breather and an oil supply path are provided in the vicinity of the cylinder, there has been a demand for a structure that can maintain oil in the oil supply path at a low temperature. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is an object of an embodiment of the present invention to provide a structure for maintaining oil inside an oil supply path at a low temperature by preventing the elevation of the temperature of oil which flows in the oil supply path for supplying oil to a piston jet mounted in an internal combustion engine. 
     According to an embodiment of the present invention, an oil passage of an internal combustion engine ( 1 ), the internal combustion engine ( 1 ) includes: 
     a crankcase ( 2 ); 
     a cylinder block ( 3 F, 3 R) that is integrally connected to the crankcase ( 2 ); 
     a piston ( 12 ) that is housed in the cylinder block ( 3 F, 3 R); 
     a piston jet ( 53 ) that is provided to the cylinder block ( 3 F, 3 R) for supplying oil in the internal combustion engine ( 1 ) to the piston ( 12 ); and 
     a breather device ( 30 ) that is integrally formed on the cylinder block ( 3 F, 3 R) such that the breather device ( 30 ) is in communication with the inside of the crankcase ( 2 ), wherein an oil supply path ( 50 ) through which oil is supplied to the piston jet ( 53 ) is integrally formed on a wall body exposed to outside air of a peripheral wall which forms the breather device ( 30 ). 
     According to an embodiment of the present invention, a plurality of cylinders ( 4 ) mounted in the cylinder blocks ( 3 F, 3 R) are arranged such that the cylinders ( 4 ) form a pair in the longitudinal direction and form a V-shaped bank, 
     the breather device ( 30 ) is formed in a valley portion of the V-shaped bank, and 
     the oil supply path ( 50 ) to the piston jet ( 53 ) is formed in a ceiling wall ( 33 ) of the breather device ( 30 ). 
     According to an embodiment of the present invention, a recessed portion ( 36 ) is formed on an outer surface of the ceiling wall ( 33 ) of the breather device ( 30 ), and the oil supply path ( 50 ) is integrally formed on a lower side of the recessed portion ( 36 ). 
     According to an embodiment of the present invention, a plurality of piston jet branched oil passage ( 54 ) each having a small diameter for supplying oil to a respective piston jet ( 53 ) are formed traversing the inside of the breather device ( 30 ) from the oil supply path ( 50 ) in a downward direction. 
     According to an embodiment of the present invention, a journal lubrication oil passage ( 56 ) extends toward a journal ( 55 ) for a crankshaft ( 10 ) of the internal combustion engine ( 1 ) that is branched from the oil supply path ( 50 ), and traverses the inside of the breather device ( 30 ) in a downward direction. 
     According to an embodiment of the present invention, a plurality of journal lubrication oil passages ( 56 ) extending toward the plurality of journals ( 55 ) for the crankshaft ( 10 ) are provided in a branched manner from the oil supply path ( 50 ). A piston jet branched oil passage ( 54 ) extending toward the piston jet ( 53 ) is also branched from the oil supply path ( 50 ) in a spaced-apart manner from the journal lubrication oil passage ( 56 ). 
     According to an embodiment of the present invention, a first oil supply path ( 51 ) through which oil is supplied from one end side of the oil supply path ( 50 ) is connected to the oil supply path ( 50 ). A second oil supply path ( 52 ) having a smaller diameter than the first oil supply path ( 51 ) is connected to a portion of the oil supply path ( 50 ) close to the other end of the oil supply path ( 50 ). 
     According to an embodiment of the present invention, between positions where the plurality of journal lubrication oil passages ( 56 ) extending toward the journal ( 55 ) from the oil supply path ( 50 ) are branched, the second oil supply path ( 52 ) is connected in a form that the second oil supply path ( 52 ) intersects a side portion of the oil supply path ( 50 ). 
     According to an embodiment of the present invention, the breather device ( 30 ) is integrally formed on the cylinder block ( 3 F, 3 R), and the oil supply path ( 50 ) through which oil is supplied to the piston jet ( 53 ) is integrally formed in the wall body exposed to outside air of the peripheral wall of the breather device ( 30 ). Accordingly, not only the oil supply path ( 50 ) can be separated from cylinder ( 4 ), but also there is no possibility that the oil supply path ( 50 ) is surrounded by the cylinder ( 4 ) and the breather device ( 30 ). Thus, a temperature maintaining action by the breather device ( 30 ) can be lowered and hence, the elevation of the temperature of oil in the oil supply path ( 50 ) can be lowered whereby oil having an oil temperature lower than conventional oils can be supplied to the piston jet ( 53 ) whereby cooling performance can be enhanced. 
     According to an embodiment of the present invention, the oil supply path ( 50 ) and the breather device ( 30 ) are integrally formed on the valley portion of the V-shaped bank, and the oil supply path ( 50 ) is formed in the ceiling wall ( 33 ) of the breather device ( 30 ). Thus, the oil supply path ( 50 ) can be spaced apart from the cylinder ( 4 ) whereby the thermal effect from the cylinder ( 4 ) on the oil supply path ( 50 ) can be lowered, and at the same time, oil supply path ( 50 ) is exposed to outside air thus suppressing the elevation of the oil temperature by cooling. 
     According to an embodiment of the present invention, the oil supply path ( 50 ) is integrally formed directly below the recessed portion ( 36 ) formed on an outer surface of the ceiling wall ( 33 ). Thus, a surface area which is exposed to outside air is increased thus enhancing heat radiation property. 
     According to an embodiment of the present invention, a plurality of piston jet branched oil passages ( 54 ) are provided having a small diameter and are formed such that the piston jet branched oil passages ( 54 ) extend toward the piston jet ( 53 ) while traversing the inside of the breather device ( 30 ) downward from the oil supply path ( 50 ). Accordingly, by branching the flow of oil from the oil supply path ( 50 ), the elevation of the oil temperature in the oil supply path ( 50 ) can be suppressed. 
     According to an embodiment of the present invention, the journal lubrication oil passage ( 56 ) through which oil is supplied to the journal ( 55 ) for the crankshaft ( 10 ) from the oil supply path ( 50 ) arranged in the ceiling wall ( 33 ) of the breather device ( 30 ) is formed such that journal lubrication oil passage ( 56 ) traverses the breather device ( 30 ). Thus, the journal lubrication oil passage ( 56 ) can be provided with a shortest path. 
     According to an embodiment of the present invention, the journal lubrication oil passage ( 56 ) which extends toward the journal ( 55 ) and the piston jet branched oil passage ( 54 ) which extends toward the piston jet ( 53 ) are branched from the common oil supply path ( 50 ) in a spaced-apart manner from each other and hence, portions of the oil passages can be used in common whereby the oil passages can be shortened and simplified. 
     According to an embodiment of the present invention, the plurality of oil supply paths are connected to the oil supply path ( 50 ). Thus, when the supply of oil from the first oil supply path ( 51 ) becomes short, oil can be supplied also from the second oil supply path ( 52 ). Accordingly, it is possible to avoid a situation where the oil supply to the journal ( 55 ) for the crankshaft ( 10 ) and the piston jet ( 53 ) becomes short. 
     According to an embodiment of the present invention, the second oil supply path ( 52 ) is connected to the oil supply path ( 50 ) between the branching positions of the plurality of lubrication oil passages ( 56 ) extending toward the journals ( 55 ). Accordingly, while it is necessary to increase an amount of oil toward the journal ( 55 ) on the downstream side of the oil supply path ( 50 ), such oil can be also supplied from the second oil supply paths ( 52 ). Thus, the oil shortage can be avoided. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a left side view of a V-type 4-cylinder 4-cycle internal combustion engine for mounting on a motorcycle according to one embodiment the invention; 
         FIG. 2  is an enlarged cross-sectional view of an essential part of the internal combustion engine; 
         FIG. 3  is a view of an upper surface of a breather device as viewed from above the internal combustion engine; 
         FIG. 4  is a cross-sectional view taken along a line IV-IV in  FIG. 2  and  FIG. 3 ; 
         FIG. 5  is a cross-sectional view taken along a line V-V in  FIG. 2 ; 
         FIG. 6  is an arrangement view of an oil supply path and various oil passages connected with the oil supply path as a prospective view as viewed from behind the internal combustion engine; 
         FIG. 7  is an external appearance view of a right surface of an essential part of the internal combustion engine; 
         FIG. 8  is an external appearance view of a left surface of an essential part of the internal combustion engine; and 
         FIG. 9  is a cross-sectional view taken along a line IX-IX in  FIG. 2 , and is a view where a piston jet is viewed from a lower surface side. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a left side view of a V-type 4-cylinder 4-cycle internal combustion engine for mounting on a motorcycle according to one embodiment the invention. An arrow F indicates a front side in a state where the internal combustion engine  1  is mounted on a vehicle, and an arrow Rear indicates a rear side in a state where the internal combustion engine  1  is mounted on the vehicle. A center portion of the internal combustion engine  1  is occupied by a crankcase  2 . The crankcase  2  is constituted of an upper crankcase  2 A and a lower crankcase  2 B. The upper crankcase  2 A includes a front cylinder block  3 F and a rear cylinder block  3 R which are integrally formed. The front cylinder block  3 F and the rear cylinder block  3 R include two cylinders  4  respectively. A cylinder head  5  is fastened to upper end surfaces of the front cylinder block  3 F and the rear cylinder block  3 R, and a cylinder head cover  6  is fastened to upper end surfaces of the cylinder heads  5 , respectively. An upper end surface of the lower crankcase  2 B is fastened to a lower end surface of the upper crankcase  2 A thus forming the integral crankcase  2 . In the inside of the cylinder head  5  and the cylinder head cover  6 , a valve operating mechanism  7  and an ignition plug  8  are arranged corresponding to the respective cylinders  4 . 
     A front half portion of the inside of the crankcase  2  defines a crank chamber  9 . A crankshaft  10  is rotatably supported in a state where the axis of rotation is positioned on a mating surface between the upper and lower crankcases  2 A,  2 B. Two left and right crank pins  11  are mounted on the crankshaft  10 . Two pistons  12  on a front side and two pistons  12  on a rear side are connected to the crank pins  11  by way of connecting rods  13 , respectively. 
     A transmission chamber  14  is defined in a rear half portion of the inside of the crankcase  2 , and a constant-mesh-gear transmission  15  is housed in the transmission chamber  14 . An oil pan  16  is fastened to a lower end surface of the lower crankcase  2 B. An oil pump  17  and the like are mounted on a lower portion of the lower crankcase  2 B, and lubrication oil is supplied to respective parts of the internal combustion engine  1 . 
       FIG. 2  is an enlarged cross-sectional view of an essential part of the internal combustion engine  1 . 
     The piston  12  is slidably fitted into the respective front and rear cylinders  4 . The crankshaft  10  is pivotally supported on the mating surface between the upper crankcase  2 A and the lower crankcase  2 B. Both ends of the connecting rod  13  are pivotally mounted on the crankpin  11  and the piston  12 , and the crankshaft  10  is rotatably driven corresponding to the upward and downward movement of the piston  12 . A piston jet which injects oil to a slide portion of the piston is formed on a lower portion of each cylinder block. Oil to be injected is supplied from an oil supply path. 
     An intake port  18  is arranged on respective sides of the cylinder heads  5  where lower portions of the front and rear cylinder heads  5  approach to each other respectively, and an exhaust port  19  is arranged on front and rear outer sides of the front and rear cylinder heads  5 , respectively. A throttle body connection member  20  is mounted on an upper portion of the intake port  18 . An intake valve  21  is mounted on the intake port  18  in an openable and closable manner, and an exhaust valve  22  is mounted on the exhaust port  19  in an openable and closable manner. 
     A breather device  30  is arranged at a valley portion of a V-shaped bank sandwiched between the front and rear cylinder blocks  3 F,  3 R which is also a portion positioned on an upper surface of the crankcase  2 . A breather chamber  31  is formed in the inside of the breather device  30 . A peripheral wall of the breather chamber  31  excluding a ceiling wall  33  is formed commonly with portions of the front and rear cylinder blocks  3 F,  3 R and a portion of the upper crankcase  2 A. 
     More specifically, the breather chamber  31  is formed by the bottom wall  32 , the ceiling wall  33  and side walls  34  ( FIG. 5 ), wherein the bottom wall  32  is integrally and commonly formed of lower end portions of the plurality of cylinders  4  formed in a V shape, the ceiling wall  33  is integrally formed above the bottom wall  32  with the plurality of cylinders formed into a V shape in a state where the ceiling wall  33  straddles over the plurality of cylinders, and the side walls  34  which form the breather chamber  31  by closing both end portions of a space defined in the crankshaft  10  direction between the plurality of cylinders  4  formed in a V shape. 
     A breathing gas discharge pipe  35  is mounted in an erected manner on the ceiling wall  33  of the breather chamber  31 . A water drain groove  36  is formed on the ceiling wall  33  adjacent to the breathing gas discharge pipe  35 . An oil supply path  50  is provided directly below the water drain groove  36 . A piston jet  53  which injects oil to the slide portion of the piston  12  is formed on a lower portion of each cylinder  4 . Oil to be injected is supplied from the above-mentioned oil supply path  50 . 
       FIG. 3  is a view showing an upper surface of the breather device  30  as viewed from above the internal combustion engine  1 . In  FIG. 3 , two intake ports  18  corresponding to two cylinders  4  formed on the front cylinder block  3 F and two intake ports  18  corresponding to two cylinders  4  formed on the rear cylinder block  3 R are shown. Although one cylinder is provided with two intake valves  21  and two exhaust valves  22 , only two intake valves  21  arc shown in the drawing with respect to each intake port. The breathing gas discharge pipe  35 , the water drain groove  36 , a drain hole  37  and closure plugs  39  are mounted on an upper surface of the ceiling wall  33 . A cooling water pipe  38  for cooling the cylinder  4  is provided on a left side of the internal combustion engine. The breathing gas discharge pipe  35  and the water drain groove  36  are also shown in  FIG. 2 . A groove bottom of the water drain groove  36  is formed such that the groove bottom is high on a right side and is gradually lowered toward a left side. The drain hole  37  is formed adjacent to the lowest position of the groove bottom. 
     The breathing gas discharge pipe  35  is mounted in a breathing gas discharge pipe mounting hole formed in the ceiling wall  33 . The breathing gas discharge pipe  35  is a sleeve provided for feeding an unburned gas separated by the breather device  30  toward an air cleaner (not shown in the drawing) through a hose (not shown in the drawing). The fed unburned gas is again supplied to the internal combustion engine  1  together with air and is burned. 
     The closure plugs  39  are provided for closing core takeout through holes formed in the ceiling wall  33  of the breather chamber  31  for taking out core sands used for forming the breather chamber  31  at the time of forming the breather chamber  31  integrally with the upper crankcase  2 A by casting. 
       FIG. 4  is a cross-sectional view taken along a line IV-IV in  FIG. 2  and  FIG. 3 . In a cross section of the left and right cylinders  4  included in the front cylinder block  3 F, a cross section of the front connecting rod  13  and a large end portion  13   a  of the rear connecting rod  13  are shown. The oil supply path  50  is integrally formed with the ceiling wall  33  of the breather device  30  adjacent to a lower side of the water drain groove  36  (recessed portion) formed on an outer surface of the ceiling wall  33 . The groove bottom of the water drain groove  36  is formed such that the groove bottom is low on a left side. Accordingly, the oil supply path  50  is also formed such that the oil supply path  50  is low on a left side. The oil supply path  50  is an oil passage for supplying oil to a plurality of journals  55  for a plurality of piston jets  53  and the crankshaft  10 . The oil supply path  50  is formed integrally with the ceiling wall  33  directly below the water drain groove  36  formed on the outer surface of the ceiling wall  33 . Thus, a surface area exposed to the outside air is increased. Accordingly, heat radiation property can be enhanced. Thus, oil can be maintained at a low temperature. The piston jets  53  are shown in the cross section of the front cylinder block  3 F in  FIG. 4 . 
       FIG. 5  is a cross-sectional view taken along a line V-V in  FIG. 2 . In the drawing, a lower portion and the bottom wall  32  of the breather chamber  31 , a cross section of the front and rear cylinder blocks  3 F,  3 R, and a cross section of the front piston  12  are shown. Distal end portions of the piston jets  53  are shown in the cross section of the rear cylinder block  3 R. A plurality of labyrinth walls  40  for forming a labyrinth in the inside of the breather chamber  31  are formed in an erected manner on the bottom wall  32  of the breather chamber  31 . 
     A gear train chamber  41  which houses a gear train is formed in a right end portion of the internal combustion engine  1 . The gear train is provided for driving a cam shaft of the valve operating mechanism  7  by making use of the rotation of the crankshaft  10 . A drive gear  42  is illustrated which is mounted on the crankshaft  10  and constitutes a start point of the gear train. The gear train chamber  41  is communicated with the crank chamber  9 . 
     The inside of the crank chamber  9  is filled with a blow-by gas. A main component of the blow-by gas is an unburned gas. The inside of the crank chamber  9  is also filled with oil supplied to a rotation part in the form of an oil mist. The oil mist and the blow-by gas are mixed together thus forming an oil mixed blow-by gas. The oil mixed blow-by gas is introduced into the breather chamber  31  from the crank chamber  9  through the gear train chamber  41  and a breather chamber inlet port  43  and is subjected to gas/liquid separation. A separated unburned gas is discharged from the breathing gas discharge pipe  35  ( FIG. 3 ,  FIG. 4 ) and, as described previously, is returned to the internal combustion engine  1  through the air cleaner and is burned in the internal combustion engine  1 . The separated oil is discharged to an AC generator chamber (not shown in the drawing) from the oil discharge port  44  and is returned to the oil pan  16  through the AC generator chamber. 
     An inlet baffle wall  45  is formed in an erected manner inside the breather chamber inlet port  43  in a state where the inlet baffle wall  45  partially overlaps with the breather chamber inlet port  43 . Due to such a construction, it is possible to prevent oil supplied from the crankcase  2  from directly flowing into the breather chamber  31 . 
       FIG. 6  is a view of the oil supply path  50  and various oil passages connected with the oil supply path  50  as a perspective view as viewed from behind the internal combustion engine  1 . A first oil supply path  51  through which oil is supplied to the oil supply path  50  is formed on a right end of the oil supply path  50 . On the other hand, a second oil supply path  52  having a small diameter through which oil is supplied to the oil supply path  50  is connected to the oil supply path  50  in the vicinity of a left end of the oil supply path  50  in an obliquely intersecting manner. The main streams of oil in the oil passages are indicated by arrows. Since the first oil supply path  51  is the main oil supply path. Thus, a right side of the oil supply path  50  is an upstream side and a left side of the oil supply path  50  is a downstream side. As described previously, the oil supply path  50  is arranged adjacent to the position directly below the water drain groove  36 . The groove bottom of the water drain groove  36  is lowered toward the left side. Accordingly, the oil supply path  50  is also formed such that the left side of the oil supply path  50  is low. That is, the groove bottom is formed such that the downstream side (left side) of the groove bottom is low. 
     The piston jet branched oil passages  54  extending toward the piston jets  53  (see  FIG. 2 ) formed on lower ends of the cylinders  4  are arranged above the crank chambers  9  in such a manner that the piston jet branched oil passages  54  traverse the inside of the breather device  30  downwardly from the oil supply path  50 . These piston jet branched oil passages  54  are formed on the labyrinth walls  40  of the breather device  30 . In  FIG. 4 , piston jet branched oil passage inlets  54   a  at two positions are shown on a left side of the drawing. In  FIG. 5 , cross sections of the piston jet branched oil passages  54  at four positions in total are shown. 
     The plurality of piston jet branched oil passages  54  having a small diameter which extend toward the piston jets  53  are formed in such a manner that the piston jet branched oil passages  54  traverse the inside of the breather device  30  downward from the oil supply path  50 . Both the oil supply paths and the piston jet branched oil passages  54  are cooled by outside air. A surface area of the oil passages is increased by forming the plurality of piston jet branched oil passages  54 . Thus, oil is brought into contact with and is cooled by inner surfaces of the oil passages and therefore, the elevation of the oil temperature in the oil supply path  50  and the piston jet branched oil passages  54  can be suppressed. 
     Journal lubrication oil passages  56  which extend toward the journals  55  formed in the crankshaft  10  of the internal combustion engine  1  are formed such that the journal lubrication oil passages  56  are branched from three portions of the oil supply path  50  and traverse the inside of the breather device  30  downward respectively. Upper half portions of the journal lubrication oil passages  56  are formed in the left and right side walls  34  of the breather device  30  and in the labyrinth walls  40  at the center portion of the breather device  30 . In addition, lower half portions of the journal lubrication oil passages  56  are formed on an outer wall body  60  of the crankcase  2  and a partition wall  61  formed between the cylinders  4 . In  FIG. 4 , the journal  55  and the journal lubrication oil passage  56  through which oil is supplied to the journal  55  are shown on the right side of the drawing, and journal lubrication oil passage inlets  56   a  are shown at the center and on the left side of the drawing. In  FIG. 5 , the cross sections of the journal lubrication oil passages  56  provided at left and right sides and at the center are shown. 
       FIG. 7  is an external appearance view of a right surface of an essential part of the internal combustion engine  1 . The first oil supply path  51  is formed on a right outer surface of the crankcase  2 . An upper end of the first oil supply path  51  is connected to a right end of the oil supply path  50  as shown in  FIG. 6 . A portion of the first oil supply path  51  is shown in the left end of  FIG. 3 . 
       FIG. 8  is an external appearance view of a left surface of an essential part of the internal combustion engine  1 . The second oil supply path  52  having a smaller diameter than the first oil supply path  51  is formed on a left outer surface of the crankcase  2 . As shown in  FIG. 6 , an upper end of the second oil supply path  52  is connected to the oil supply path  50  in an obliquely intersecting manner between the journal lubrication oil passage  56  on a left end side of the oil supply path  50  and the journal lubrication oil passage  56  on the center of the oil supply path  50 .  FIG. 4  shows an opening end  52   a  of the second oil supply path at the above-mentioned intersecting portion. 
     The plurality of oil supply paths are connected to the oil supply path  50 . Thus, when oil supplied from the first oil supply path  51  becomes short, it is possible to supply oil also from the second oil supply path  52  whereby it is possible to obviate the situation where oil supplied to the journals  55  for the crankshaft  10  and the piston jets  53  becomes short. The second oil supply path  52  is connected to the oil supply path  50  between the branching positions of the plurality of lubrication oil passages extending toward the journals  55 . Accordingly, while it is necessary to increase an amount of oil toward the journal  55  on the downstream side of the oil supply path  50 , such oil can be also supplied from the second oil supply paths  52  and hence, the oil shortage can be avoided. 
       FIG. 9  is a cross-sectional view taken along a line IX-IX in  FIG. 2 , wherein the piston jet  53  is viewed from a lower surface side. In  FIG. 9 , four sets of piston jets  53  are shown. As shown in  FIG. 6 , oil is supplied to the piston jets  53  through four piston jet branched oil passages  54  branched from the oil supply path  50 . 
     As has been explained in detail heretofore, the above-mentioned embodiment can acquire the following advantageous effects. 
     (1) The breather device  30  is integrally formed on the cylinder blocks  3 F,  3 R, and the oil supply path  50  through which oil is supplied to the piston jet  53  is integrally formed in the wall body exposed to outside air of the peripheral wall of the breather device  30 . Accordingly, oil having a lower oil temperature than conventional oil can be supplied to the piston jet  53 . 
     (2) The oil supply path  50  is formed in the ceiling wall  33  of the breather device  30 . Thus, the elevation of the oil temperature can be suppressed by cooling. 
     (3) The oil supply path  50  is integrally formed directly below the water drain groove  36  constituting the recessed portion formed on the outer surface of the ceiling wall  33 . Thus, the surface area which is exposed to outside air is increased thus enhancing heat radiation property. 
     (4) The plurality of piston jet branched oil passages  54  having a small diameter are formed such that the piston jet branched oil passages  54  traverse the inside of the breather device  30  downward from the oil supply path  50 . Accordingly, the elevation of the oil temperature in the oil supply path  50  can be suppressed. 
     (5) The journal lubrication oil passage  56  from the oil supply path  50  is formed such that the journal lubrication oil passage  56  traverses the breather device  30 . Thus, the journal lubrication oil passage  56  can be provided with the shortest path. 
     (6) The journal lubrication oil passage  56  and the piston jet branched oil passage  54  which extends toward the piston jet  53  are branched from the common oil supply path  50  in a spaced-apart manner from each other. Thus, the oil passages can be shortened and simplified. 
     (7) The first oil supply paths  51  and the second oil supply paths  52  are connected to the oil supply path  50 . Thus, it is possible to avoid a situation where oil which is supplied to the journal  55  for the crankshaft  10  and the piston jet  53  becomes short. 
     (8) Oil can be supplied to a downstream side of the oil supply path  50  also from the second oil supply path  52  and hence, the shortage of oil toward the journal  55  can be avoided. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.