Patent Publication Number: US-9850801-B2

Title: Piston cooling structure in combustion engine

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a piston cooling structure in a combustion engine of a kind in which a piston back face is cooled with the utilization of a cooling liquid such as, for example, cooling oil. 
     Description of Related Art 
     The JP Laid-open Patent Publication No. 2013-130129, for example, discloses a structure in which the cooling liquid is jetted towards a piston back face in a direction substantially parallel to the cylinder axis line. According to this patent document, a to-be-cooled portion of the piston back face, which the cooling liquid is brought into contact with, is effectively cooled, but it is difficult to suppress the temperature rise that occurs in any other portion of the piston back face, which departs from the to-be-cooled portion of the piston back face. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, the present invention has been devised to provide a piston cooling structure which is effective to suppress the temperature rise occurring in a wide range of portions of the piston back face. 
     In order to accomplish the above described object of the present invention, the present invention provides a piston cooling structure for a combustion engine which includes first and second nozzles configured to inject a cooling liquid towards a back face of a piston which reciprocatingly move along a cylinder axis line within a cylinder bore. The first and second nozzles have respective axes that are inclined at first and second angles relative to the cylinder axis line. Also, the first angle of inclination relative to the cylinder axis line is set to a value smaller than the second angle of inclination relative to the cylinder axis line. It is to be noted that the “back face of the piston or piston back face” referred to above and hereinafter is intended to mean a face of the piston opposite to a top face forming a bottom face of a combustion chamber. Also, the first angle of inclination may be zero degree (0°), that is, the axis of the first nozzle may extend parallel to the cylinder axis line. 
     According to the present invention, the first angle of inclination is set to a value smaller than the second angle of inclination of the second nozzle. Accordingly, the first nozzle, as compared with the second nozzle, can continue injecting the cooling liquid towards a specific site of a back face of the piston regardless of the position of the piston which undergoes a reciprocating movement. On the other hand, the second nozzle, as compared with the first nozzle, can inject the cooling liquid towards a wide range of the back face of the piston while the position of the cooling liquid, which is blown towards the back face of the piston, changes in dependence on the position of the piston that undergoes the reciprocating movement. Thus, of the back face of the piston, while the cooling liquid is kept being injected intensively towards the specific site, the cooling liquid is injected to the wide range of the back face of the piston. Therefore, the temperature rise occurring in a wide range of the back face of the piston can be suppressed. 
     Any combination of at least two constructions, disclosed in the appended claims and/or the specification and/or the accompanying drawings should be construed as included within the scope of the present invention. In particular, any combination of two or more of the appended claims should be equally construed as included within the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In any event, the present invention will become more clearly understood from the following description of preferred embodiments thereof, when taken in conjunction with the accompanying drawings. However, the embodiments and the drawings are given only for the purpose of illustration and explanation, and are not to be taken as limiting the scope of the present invention in any way whatsoever, which scope is to be determined by the appended claims. In the accompanying drawings, like reference numerals are used to denote like parts throughout the several views, and: 
         FIG. 1  is a schematic side view of a motorcycle having a combustion engine equipped with a piston cooling structure designed in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a schematic longitudinal sectional view showing, on an enlarged scale, an important portion of the piston cooling structure of the combustion engine; 
         FIG. 3  is a view similar to that shown in  FIG. 2 , but showing the section taken at a location different from that shown in  FIG. 2 ; 
         FIG. 4  is a diagram showing a portion of the engine cooling structure; 
         FIG. 5  is a diagram showing the engine cooling structure as viewed in a direction diagonally forwardly and laterally; 
         FIG. 6  is a diagram showing the engine cooling structure as viewed in a direction diagonally rearwardly and laterally; 
         FIG. 7  is a schematic longitudinal sectional view showing the important portion of the combustion engine; 
         FIG. 8  is a schematic longitudinal sectional view showing a first nozzle of the piston cooling structure; 
         FIG. 9  is a schematic longitudinal sectional view showing a second nozzle of the piston cooling structure; and 
         FIG. 10  is a schematic top plan view of a reciprocating piston as viewed from top. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described in detail in connection with a preferred embodiment thereof with reference to the accompanying drawings. Before the description proceeds, it is to be noted that the term “left and right” used hereinabove and hereinafter are to be understood as relative terms descriptive of positions and/or direction as viewed from a motorcycle rider occupying the seat during the forward travel of the motorcycle. 
       FIG. 1  shows a schematic side view of a motorcycle having mounted thereon a combustion engine equipped with a piston cooling structure designed in accordance with the preferred embodiment of the present invention. The illustrated motorcycle includes a motorcycle frame structure FR made up of a main frame  1 , forming a front half portion of the motorcycle frame structure FR, and a seat rail  2  connected rigidly with a rear portion of the main frame  1  and forming a rear half portion of the motorcycle frame structure FR. A front fork  8  is rotatably supported by a head pipe  4 , provided at a front end of the main frame  1 , through a steering shaft (not shown), and a front wheel  10  is fitted to this front fork  8  in any known manner. A handlebar  6  for steering purpose is fixed to an upper end portion of the front fork  8  also in any known manner. 
     On the other hand, a swingarm  12  is supported at a rear end portion of the main frame  1 , which is a lower intermediate portion of the motorcycle frame structure FR, through a pivot pin  16  for movement up and down, and a rear wheel  14  is rotatably supported at a rear end portion of the swingarm  12 . The main frame  1  has a lower portion to which a combustion engine E is fitted. Rotation of the combustion engine E is transmitted through a transmission  13  to a power transmitting mechanism  11  such as, for example, a drive chain, which is disposed on a left side of the motorcycle body. The rear wheel  14  is driven through this power transmitting mechanism  11 . 
     A fuel tank  15  is disposed at an upper portion of the main frame  1  and a seat assembly comprised of a driver&#39;s or motorcyclist&#39;s seat  18  and a fellow passenger&#39;s set  20  is supported by the seat rail  2 . Also, a front fairing  22  made of a resinous material is mounted on a front portion of the motorcycle body so as to cover forwardly of the head pipe  4 . The front fairing  22  is formed with an air intake opening  24  defined therein for introducing an intake air A therethrough into the combustion engine E. 
     The combustion engine E referred to above is in the form of a four cylinder, four stroke parallel multi-cylinder engine having a crankshaft  26  which is a rotary shaft and extends in a motorcycle widthwise direction. It is, however, noted that the type of the combustion engine E is not necessarily limited to that described above. The combustion engine E includes a crankcase  28  for supporting the crankshaft  26 , a cylinder block  30  connected with an upper portion of the crankcase  28 , a cylinder head  32  connected with an upper portion of the cylinder block  30 , and an oil pan  34  fitted to a lower portion of the crankcase  28 . The oil pan  34  accommodates therein a quantity of lubricant oil OL that concurrently serves as a cooling liquid. 
     The crankcase  28  has a rear portion forming a transmission casing for accommodating the transmission  13  therein. This crankcase  28  is of a split type casing having a split interface  31  and made up of a casing upper half body  280  and a casing lower half body  282  that are positioned on respective opposite sides of the split interface  31 . The cylinder block  30  and the cylinder head  32  cooperate with each other to define an engine cylinder CY of the combustion engine E. Each of the crankcase  28 , the cylinder block  30  and the cylinder head  32  is in the form of a molded product formed by die casting of aluminum or aluminum alloy. In the practice of this embodiment of the present invention, the casing upper half body  280  of the crankcase  28  and the cylinder block  30  are formed integrally with each other by the use of any known die forming. 
     The engine cylinder CY is somewhat tilted. Specifically, the engine cylinder CY has an axis line C 0  extending upwardly and tilted forwardly. The cylinder head  32  has a rear portion provided with air intake ports  47 . Four exhaust pipes  36  fluid connected with exhaust ports  35  defined in a front surface of the cylinder head  32  are merged together at a location beneath the combustion engine E and is then fluid connected with an exhaust muffler  38  that is disposed on a right side of the rear wheel  14 . A supercharger  42  for sucking an external air as an intake air I and then supplying it into the combustion engine E is disposed rearwardly of the cylinder block  30  and above a rear portion of the crankcase  28 . 
     The supercharger  42  serves to compress the external air sucked through an air suction port  46 , and then to discharge it via a discharge port  48  after having increased the pressure of the air, thereby to finally supply it into the combustion engine E. Accordingly, the amount of the intake air to be supplied to the combustion engine E is increased to enhance the engine output. 
     The supercharger  42  employed in the practice of this embodiment of the present invention is a centrifugal supercharger and has a centrifugal impeller (not shown) fixed to a supercharger rotary shaft (not shown) that extends in the motorcycle widthwise direction. However, the specific supercharger  42  is not necessarily limited to that shown and described above, and any known supercharger can be employed. 
     The air suction port  46  of the supercharger  42  is fluid connected with an outlet of an air cleaner  40  that is disposed on an upstream side of the supercharger  42 , and an air intake duct  50  for introducing an incoming air A into the supercharger  42  is fluid connected with an inlet of the air cleaner  40 . The air intake duct  50  has a front end opening  50   a  defined therein and is supported by the main frame  1  with the front end opening  50   a  positioned in face to face relation with the air intake opening  24 . Accordingly, the incoming wind A introduced through the front end opening  50   a  can be increased in pressure by the known ram effect before it is introduced into the air cleaner  40  as the intake air I. The air cleaner  40  serves to substantially purify the intake air I that is introduced through the air intake duct  50 . The intake air I which has been substantially purified by the air cleaner  40  is sucked into the supercharger  42 . 
     An intake air chamber  52  is disposed between the discharge port  48  of the supercharger  42  and the air intake port  47  of the combustion engine E. This intake air chamber  52  reserves therein a quantity of the intake air I to be supplied to the air intake port  47 . A throttle body  54  is disposed between the intake air chamber  52  and the cylinder head  32 . In this throttle body  54 , fuel is injected into the intake air so as to form an air-fuel mixture which is in turn supplied into the engine cylinder CY in any known manner. The fuel tank  15  referred to previously is disposed above the intake air chamber  52  and the throttle body  54 . 
     As shown in  FIG. 4 , the combustion engine E also includes an oil pump  56  for supplying oil OL within the oil pan  34  (shown in  FIG. 1 ) under pressure to various parts of the combustion engine E, an oil filter  58  disposed on a downstream side of the oil pump  56  for substantially purifying the oil OL, and an oil cooler  60  disposed on a downstream side of the oil filter  58  for cooling the oil OL. The oil filter  58  and the oil cooler  60  are disposed in a front surface  28   a  of the crankcase  28  while juxtaposed relative to each other in a motorcycle widthwise direction (leftward and rightward direction). 
     As shown in  FIG. 2 , the cylinder CY has a cylinder bore  60  defined therein, and a reciprocating piston  62  is movably accommodated within the cylinder bore  61  with a lower end thereof drivingly connected with the crankshaft  26  through a connecting rod  64 . As a matter of course, the piston  62  undergoes a reciprocating movement within the cylinder bore  61  in a direction parallel to the cylinder axis line C 0 . 
     As shown in  FIG. 4 , the oil filter  58  has an inflow passage  66  to which a discharge passage  68  of the oil pump  56  is fluid connected, and also has an outflow passage  70  communicated with an inflow passage  72  of the oil cooler  60  through a filter-cooler communicating passage  74 . An outflow passage  76  on a downstream side of the oil cooler  60  is communicated with a cooling passage  78  through which the oil OL is supplied to an engine body. 
     Between the oil filter  58  and the oil cooler  60 , particularly in the filter-cooler communicating passage  74 , a lubricant passage  80  for supplying the oil OL to, for example, the transmission  13  and the supercharger  42  is fluid connected. In other words, the oil pump  56  is operable to supply the oil OL commonly to both of the cooling passage  78  and the lubricant passage  80 . 
     The cooling passage  78  includes a first branch passage  82 , which is ramified in one way from a point of ramification BP, and a second branch passage  84  which is ramified in the other way from the point of ramification BP. The first branch passage  82  extends forwards (towards an oil filter side) from the point of ramification BP and then extends upwards. On the other hand, the second branch passage  84  extends from the point of ramification BP in a leftward and rightward direction (motorcycle widthwise direction). 
       FIGS. 5 and 6  illustrate the lubricant passage formed internally within a wall of the crankcase  28  and a wall of the cylinder block  30 . As shown in  FIG. 5 , the first branch passage  82  extends in the motorcycle widthwise direction after having extended diagonally forwardly and upwardly. Specifically, as shown in  FIG. 3 , the first branch passage  82  extends diagonally forwardly and upwardly within a wall of the crankcase  28  until reaching the split interface  31 , and then extends within a front wall W of the cylinder CY in a leftward and rightward direction. 
     As shown in  FIG. 6 , a portion  82   a  of the first branch passage  82 , which extends in the leftward and rightward direction, is formed with four outlet passage portions  82   b  which are oriented downwards within the wall of the crankcase  28 . A first nozzle  85  for spraying oil as shown in  FIG. 2  is fluid connected with an outlet end at a lower end of the outlet passage portion  82   b . The first nozzle  85  serves to jet the oil OL upwardly from a front surface side of the cylinder CY towards a back face  69  of the piston  62 . In other words, the first branch passage  82  forms a cooling passage for piston jetting purpose that is dedicated to jet the oil toward the piston  62 . It is to be noted that the “back face  69  of the piston  62  or piston back face  69 ” referred to above and hereinafter is intended to mean a face of the piston  62  opposite to a top face  62   a  forming a bottom face of a combustion chamber  63 . The detail of piston jetting will be discussed later. 
     As shown in  FIG. 5 , five crankshaft bearing cooling passages  86  extend upwardly from the second branch passage  84  that extends in the leftward and rightward direction. The crankshaft bearing cooling passage  86  is formed internally within a bearing portion  88  (shown in  FIG. 7 ) in the crankcase  28  shown in  FIG. 6  and is used to cool a bearing face of the crankshaft  26 . 
     Of the five crankshaft bearing cooling passages  86 , the four crankshaft bearing cooling passages  86  on the left side are formed with respective outlet passages  89  that are oriented upwardly. As shown in  FIG. 7 , each of the outlet passages  89  is fluid connected with a second nozzle  90 . This second nozzle  90  serves to jet the oil OL from a side surface side of the cylinder CY upwardly towards the back face  69  of the piston  62 . The detail of the piston jetting will be discussed later. 
     Also, a cylinder cooling passage  92  extends upwards from the crankshaft bearing cooling passage  86  on the rightmost side as shown in  FIG. 6 . This cylinder cooling passage  92  serves to supply the oil OL to a wall surface of the cylinder CY and a cam chain (not shown) for driving a cam shaft. This cylinder cooling passage  92  is formed within the wall of the crankcase  28  and the wall of the cylinder block  30 . 
     The oil supplied from the cylinder cooling passage  92  to the wall surface of the cylinder CY is returned to an upstream side of the oil cooler  60  on a downstream side of the oil filter  58  after having flown through an oil return passage  94  shown in  FIG. 5 . Specifically, the oil return passage  94  extends, as best shown in  FIG. 3 , diagonally forwards and downwards within a front wall of the cylinder block  30 , and further extends diagonally rearwardly and downwardly after having passed across the split interface  31  of the crankcase  28 . The oil returned from the oil return passage  94  to the upstream side of the oil cooler  60  is, after having been cooled by the oil cooler  60 , supplied again to the cooling passage  78 . 
     The lubricating passage  80  referred to previously extends diagonally rearwardly and upwardly within the wall of the crankcase  28  and serves to lubricate the transmission  13 , the supercharger  42  and others. Specifically, the lubricating passage  80  extends, as shown in  FIG. 5 , rearwardly after having been divided into a transmission input shaft lubricating passage  96 , a transmission output shaft lubricating passage  98  and a supercharger lubricating passage  100 , and serves to supply the oil OL to an input shaft  13   a  of the transmission  13 , an output shaft  13   b  of the transmission  13  and the supercharger  42 . 
     The piston jetting will now be described. As shown in  FIG. 7 , a first angle of inclination θ 1  of the axis C 1  of the first nozzle  85  relative to the cylinder axis line C 0  is so chosen as to be smaller than a second angle of inclination θ 2  of the axis C 2  of the second nozzle  90  relative to the cylinder axis line C 0 , that is, θ 1 &lt;θ 2 . The first angle of inclination θ 1  may be 0° (zero degree), that is, the axis C 1  of the first nozzle  85  and the cylinder axis line C 0  may be parallel to each other. However, the first angle of inclination θ 1  is preferably so chosen as to be within the range of 3 to 11° and, more preferably, within the range of 5 to 9°, but in the practice of this embodiment, the first angle of inclination θ 1  is chosen to be about 7°. The second angle of inclination θ 2  is preferably so chosen as to be within the range of 15 to 25° and, more preferably, within the range of about 18 to 22°, but in the practice of this embodiment, the second angle of inclination θ 2  is chosen to be about 20°. 
     As shown in  FIG. 8 , the first nozzle  85  is fitted to the cylinder block  30  by means of a banjo bolt  101 . Specifically, the first nozzle  85  includes a hollow disc shaped mounting portion  95  and a pipe  97  fitted to an outer peripheral surface of the mounting portion  95 . A tip end of this pipe  97  forms a first injection port  85   a  of the first nozzle  85 . 
     As shown in  FIG. 10 , the first injection port  85   a  is preferably spaced in a radial direction from the cylinder axis line C 0  in the vicinity of the exhaust port  35  a distance within the range of about 0.8 r to 1.0 r and, more preferably, within the range of about 0.85 r to 0.95 r relative to the radius r of the piston  62 , but in the practice of this embodiment, the first injection port  85   a  is radially spaced from the cylinder axis line C 0  a distance of about 0.9 r. 
     The position of the first injection port  85   a , shown in  FIG. 7 , in the direction parallel to the cylinder axis line C 0  is preferably disposed spaced a distance of about 0.05 r to 0.22 r (about 2 to 8 mm) and, more preferably, about 0.08 r to 0.19 r (about 3 to 7 mm) downwardly from a small end portion  67  of the piston  62 , then held at the bottom dead center, but in the practice of this embodiment the first injection port  85   a  is spaced a distance of about 0.13 r (about 5 mm) downwardly from the small end portion  67  of the piston  62  then held at the bottom dead center. If this position in the direction parallel to the axis line is chosen as discussed above, the first injection port  85   a  is disposed near the piston  62  and, therefore, the oil OL can be intensively injected onto an injection target portion of the piston  62  without being diffused. In order to maintain the preferred first angle of inclination θ 1  and the preferred axis line direction position, the first injection port  85   a  is preferably spaced a distance of about 0.8 r to 1.0 r and, more preferably, about 0.85 r to 0.95 r in the radial direction from the cylinder axis line C 0 . 
     With the banjo bolt  101  inserted from below onto a hollow portion  95   a  of the mounting portion  95 , the bolt  101  is fastened to a female threaded portion  83 , which is formed in the outlet passage portion  82   b  of the first branch passage  82 . By so doing, the first nozzle  85  is fitted to the cylinder block  30  with the first injection portion  85   a  oriented substantially upwardly. 
     As shown in  FIG. 2 , even though the piston  62  moves along the cylinder axis line C 0 , the first nozzle  85 , as compared with the second nozzle  90 , continues to inject the oil OL towards a portion A 1  of the back face  69  of the piston  62  adjacent (forwardly adjacent) the exhaust port  35 . Where the number of exhaust ports is one, the first nozzle  85  injects the oil towards the geometric center of the exhaust port, but where a plurality of exhaust ports are juxtaposed relative to each other in a lateral direction on a front side, the first nozzle  85  injects the oil to a point intermediate between the geometric center of the one exhaust port and the geometric center of the other exhaust port. In the practice of the embodiment now under discussion, as shown in  FIG. 7 , two exhaust ports  35  are shown as employed, and the first nozzle  85  injects the oil OL towards the point intermediate between respective centers of the two exhaust ports  35  and  35 . In other words, the first nozzle  85  injects the oil OL towards a center portion of the back face  69  of the piston  62 , while accommodated within the cylinder bore  61 , regardless of the position of the piston  62  then moving up and down within the cylinder bore  61 . 
     The second nozzle  90  has a second injection port  90   a  formed in the wall surface of the crankcase  28 . Specifically, as shown in  FIG. 9 , the second nozzle  90  is in the form of a tubular body having its outer peripheral surface formed with a male threaded portion  91   a . With the second nozzle  90  of the tubular body inserted from below into the outlet passage  89  of the crankshaft bearing cooling passage  86  (best shown in  FIG. 6 ), the male threaded portion  91  is fastened to a female threaded portion  89   a  formed in the outlet passage  89 . By so doing, the second nozzle  90  is fitted to the crankcase  28 . The outlet passage  89  has an outlet end (upper end) forming the second injection port  90   a  of the second nozzle  90 . 
     The second injection port  90   a  shown in  FIG. 10  is preferably spaced from the cylinder axis line C 0  in the radial direction a distance within the range of about 0.85 r to 1.05 r and, more preferably, within the range of about 0.90 r to 1.00 r relative to the radius r of the piston  62 , but in the practice of the embodiment now under discussion, the distance of such spacing is chosen to be about 0.95 r. The position of the second injection port  90   a , shown in  FIG. 7 , in the direction parallel to the cylinder axis line C 0  is preferably spaced downwardly (adjacent to the axis of the crankshaft) from a top face of a crank web  106   a , then held at the bottom dead center, a distance within the range of 0.26 r to 0.80 r (about 10 to 30 mm) and, more preferably, within the range of 0.40 r to 0.66 r (about 15 to 25 mm), but in the practice of the embodiment now under discussion, the distance of such spacing is chosen to be about 0.53 r (about 20 mm). 
     If the position in the direction parallel to the axis line is such as discussed above, the second injection port  90   a  is close to the piston  62  and, accordingly, the oil OL can be intensively injected onto an injection target portion of the piston  62  without being diffused, and also the oil OL can be smoothly injected without being disturbed by the crank web  106 . In order to maintain the preferred second angle of inclination θ 2  and the preferred axis line direction position, the second injection port  90   a  is preferably spaced a distance of about 0.85 r to 1.05 r and, more preferably, about 0.90 to 1.00 r in the radial direction from the cylinder axis line C 0 . 
     The second nozzle  90  shown in  FIG. 7  injects the oil OL towards a portion of the back face  69  of the piston adjacent to the exhaust port  35  when the piston  62  is brought to the top dead center. On the other hand, when the piston  62  is at the bottom dead center, the second nozzle  90  injects the oil OL towards a side portion A 2  of the back face  69  of the piston  62 . In the practice of the embodiment now under discussion, when the piston  62  is at the top dead center, the second nozzle  90  injects the oil OL towards a region (the portion A 1  referred to previously) intermediate between a pair of the exhaust ports  35 . 
     The first injection port  85   a  is disposed radially inwardly of the second injection port  90  with respect to the cylinder CY. This first injection port  85   a  is also disposed at a position close to the piston  62 , rather than to the second injection port  90   a , with respect to the direction of the cylinder axis line C 0 . On the other hand, the second injection port  90   a  is disposed adjacent the second crank web  106  on one side opposite to a first crank web  104 , where a crank gear  102  is formed, with respect to the connecting rod  64 . The crank gear  102  serves to transmit a rotational force to a clutch (not shown). 
     Whereas the oil OL is directly supplied to the first nozzle from the oil cooler  60  (best shown in  FIG. 4 ), the oil, which has been used to cool the bearing portion  88  for the crankshaft  26 , is supplied to the second nozzle  90 . Accordingly, the oil OL supplied to the first nozzle  85  has a temperature lower than that of the oil OL that is supplied to the second nozzle  90 . 
     The first injection port  85   a  of the first nozzle  85  shown in  FIG. 8  has a first bore size D 1  so chosen as to be smaller than a second bore size D 2  of the second injection port  90   a  of the second nozzle  90 , that is, D 1 &lt;D 2 . With the injection port so throttled, a first supply pressure P 1  of the oil OL supplied to the first nozzle  85  is so chosen as to be higher than a second supply pressure P 2  of the oil OL supplied to the second nozzle  90 , that is P 1 &gt;P 2 . Accordingly, a second injection amount Q 2  of the second nozzle  90  is chosen so as to easily become larger than a first injection amount Q 11  of the first nozzle  85 , that is, Q 1 &lt;Q 2 . It is noted that the wording “the first and second injection amount Q 1  and Q 2 ” means an injection amount per unit time 
     When the combustion engine E rotates, the oil pump  56  shown in  FIG. 5  is also driven in operative association therewith. The oil OL discharged from the oil pump  56  is, after having been purified by the oil filter  58 , supplied into the oil cooler  60 . 
     A portion of the oil OL having been purified by the oil filter  58  is supplied to the input and output shafts  13   a  and  13   b  of the transmission  13 , shown in  FIG. 3 , the supercharger  42  and others after having passed through the lubricating passage  80  and without passing through the oil cooler  60 . 
     Also, the cooled oil OL is supplied from the downstream side of the oil cooler  60 , shown in  FIG. 4 , to the engine body through the cooling passage  78 . Specifically, the oil OL flowing through the first branch passage  82  of the cooling passage  78  is used for blowing onto the piston  62  shown in  FIG. 7 . Also, the oil OL flowing through the second branch passage  84  (shown in  FIG. 4 ) of the cooling passage  78  is, after having been used for lubrication of the bearing portion  88  for the crankshaft  26  in the crankcase  28 , used for blowing onto the piston  62  and cooling of an inner wall surface of the cylinder CY shown in  FIG. 3 . 
     According to the construction hereinbefore described, the first nozzle  85  shown in  FIG. 7  is such that the first angle of inclination θ 1  thereof is so set as to be smaller than the second angle of inclination θ 2  of the second nozzle  90 , and injects the oil OL in substantially parallel relation with the cylinder axis line C 0 . Accordingly, regardless of the positon of the piston  62  then reciprocatingly moving up and down, the oil OL can be intensively injected to a specific site of the back face  69  of the piston  62  and, more specifically, a portion P 1  of the back face  69  of the piston  62  which is adjacent to the exhaust port  35 . 
     On the other hand, the second nozzle  90  can inject the oil OL towards a wide range of the back face  69  of the piston  62  while the position of the oil OL to be blown onto the back face  69  of the piston  62  changes in dependence on the position of the piston  62  then reciprocatingly moving up and down. In this way, while the oil OL is intensively injected by the first nozzle  85  onto the portion of the back face  69  of the piston, then heated to a high temperature, adjacent to the exhaust port  35 , the oil OL is injected by the second nozzle  90  onto a wide range of the back face  69  of the piston  62 . As a result, the piston  62  can be efficiently cooled. 
     The first injection port  85   a  of the first nozzle  85  is disposed radially inwardly of the second injection port  90   a  of the second nozzle  90  with respect to the cylinder CY. Accordingly, it is easy to position the first injection port  85   a  of the first nozzle  85  so that the first angle of inclination θ 1  of the first nozzle  85  may become smaller than the second angle of inclination θ 2  of the second nozzle  90 . 
     The second nozzle injects the oil OL towards the portion of the back face  69  of the piston  62  adjacent the exhaust port  35  when the piston  62  is at the top dead center (at a position shown on left and right end sides of  FIG. 7 ). Accordingly, even with the second nozzle  90 , in addition to the piston  62  in its entirety, in the vicinity of the top dead center at which a high temperature is attained, the portion of the piston  62  adjacent the exhaust port  35  can be effectively cooled. 
     The first injection port  85   a  of the first nozzle  85  is disposed close to the piston  62  rather than to the second injection port  90   a  of the second nozzle  90  with respect to the direction parallel to the cylinder axis line C 0 . Accordingly, even when the distance from the first injection port  85   a  to the piston  62  is large as a result of the piston  62  arriving at the top dead center, the intensive cooling by means of the first nozzle  85  can be easily continued. 
     The first injection port  85   a  of the first nozzle  85  is constituted by a pipe protruding from the cylinder wall surface in a direction radially inwardly of the cylinder. Accordingly, the first injection port  85   a  of the first nozzle  85  can be disposed in proximity to the piston  62 . 
     The oil OL is supplied to the first nozzle  85  through the first branch passage  82  ramified in one way from the cooling passage  78  shown in  FIG. 4 , and the oil OL is supplied to the second nozzle  90  (shown in  FIG. 7 ) through the second branch passage  84  ramified in the other way from the cooling passage  78 . The entire amount of the oil OL flowing in the first branch passage  82  is supplied to the first nozzle  85  (shown in  FIG. 7 ) and a portion of the oil OL flowing in the second branch passage  84  is supplied to the second nozzle  90  (shown in  FIG. 7 ). Accordingly, it is easy to set the first supply pressure P 1  of the oil OL supplied to the first nozzle  85  shown in  FIG. 7  to a value higher than the second supply pressure P 2  of the oil OL supplied to the second nozzle  90 . As a result thereof, even when the piston  62  is at a separated positon (top dead center), a high temperature portion of the piston  62  can be effectively cooled by the first nozzle  85 . 
     Also, the first branch passage  82  is preferably short as compared with the second branch passage  84 . Accordingly, since the friction loss in the passage is reduced to a small value, the first supply pressure P 1  of the oil OL supplied to the first nozzle  85  can be easily set to a high pressure as compared with the second supply pressure P 2 . 
     The oil OL, which has been used to cool the bearing portion  88  for the crankshaft  26 , is supplied to the second nozzle  90 . Accordingly, the passage through which the oil OL is supplied to the second nozzle  90  can be concurrently used as the crankshaft bearing cooling passage  86  and, therefore, the structure can be simplified. Also, since the temperature of the oil OL supplied to the first nozzle  85  becomes lower than the temperature of the oil OL supplied to the second nozzle  90 , the portion adjacent to the exhaust port  35 , which is apt to be heated to a high temperature, can be effectively cooled. 
     The second injection port  90   a  of the second nozzle  90  is disposed in the neighborhood of the second crank web  106  on one side opposite to the first crank web  104  where the crank gear  102  is formed. Accordingly, the oil OL flowing towards the piston  62  can be prevented from interfering with the crank gear  102 . 
     The first bore size D 1  (shown in  FIG. 8 ) of the first injection port  85   a  of the first nozzle  85  is set to a value smaller than the second bore size D 2  (shown in  FIG. 9 ) of the second injection port  90   a  of the second nozzle  90 . Accordingly, the injection pressure of the oil OL injected from the first injection port  85   a  becomes higher than the injection pressure of the oil OL injected from the second injection port  90   a . Therefore, even when the piston  62  is held at a separated position (top dead center), the high temperature portion of the piston  62  can be effectively cooled by the oil OL from the first nozzle  85  and throttled thin. Also, by a different method, the first supply pressure P 1  of the oil OL to be supplied to the first nozzle  85  may be set to a value higher than the second supply pressure P 2  of the oil OL to be supplied to the second nozzle  90 . 
     The second injection amount Q 2  of the second nozzle  90  is set to a value larger than the first injection amount Q 1  of the first nozzle  85 . Therefore, the back face  69  of the piston  62  and the cylinder bore  61  can be cooled in a wide range by the second nozzle  90 . 
     As hereinabove discussed, in the practice of the present invention, the oil OL is injected intensively onto the particular site, and also the injection is possible in a wide range. Accordingly, even when a bias occurs in the temperature distribution of the back face  69  of the piston, the oil OL can be injected by the first and second nozzles  85  and  90  towards both of a high temperature portion and a low temperature portion, respectively. As a result, the piston  62  can be cooled efficiently with a minimized liquid amount so as to suppress the temperature rise of the piston  62 . 
     In the event that any bias occurs in the temperature distribution of the back face  69  of the piston, depending on the bias in the temperature distribution, either one of the intensive injection towards the specific site and a diffusive injection over the wide range will make it difficult to sufficiently lower the temperature of the piston  62  with a minimized liquid amount. In contrast thereto, since in the practice of the present invention, both of the intensive injection towards the specific site and the diffusive injection over the wide range are performed, the oil OL can be injected in dependence on the bias in the temperature distribution. As a result, while the amount of the oil OL to be injected is reduced, the temperature rise of the piston  62  can be efficiently suppressed. 
     In this case, of the back face  69  of the piston, the oil OL is preferably injected by the first nozzle  85  towards a portion tending to become high in temperature. For example, the oil OL is injected by the first nozzle  85  towards the portion adjacent the exhaust port. In this way, the oil OL can be continuously injected by the first nozzle  85  intensively towards the high temperature portion, and also the oil OL can be diffusively injected by the second nozzle  90  towards a low temperature portion around the high temperature portion. Accordingly, the temperature rise of the piston  62  can be suppressed while occurrence of deficiency in the liquid amount is prevented. 
     The first nozzle  85 , as compared with the second nozzle  90 , can continue injecting the oil OL towards the specific site of the piston back face  69  with no change occurring in position, at which the oil OL is applied, in the event of a change in position of the piston  62 . On the other hand, the second nozzle  90 , as compared with the first nozzle  85 , is susceptible to change in position, at which the oil OL is injected towards the piston back face  69 , in dependence on the position of the piston  62 . Accordingly, the second nozzle  90  can inject the oil OL in a wide range of the piston back face  69 . 
     In the meantime, it is preferred that while the oil OL is continuously injected by the first nozzle  85  onto a specific site of the piston back face  69  that is determined beforehand, the oil OL can also be injected by the second nozzle  90  onto such predetermined specific site. By so doing, the temperature rise of the specific site of the piston back face  69  during a high temperature time can further be suppressed. In the practice of the above described embodiment of the present invention, at the top dead center, the oil OL is injected onto such predetermined specific site by means of the first nozzle  85  and the second nozzles  90 , but at any position other than the top dead center, the second nozzle  90  injects the oil OL at a site different from the predetermined specific site. It is, however, to be noted that, at any position other than the top dead center, the injection target side of the first nozzle  85  and the injection target site of the second nozzle  90  may overlap with each other. 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings which are used only for the purpose of illustration, those skilled in the art will readily conceive numerous changes and modifications within the framework of obviousness upon the reading of the specification herein presented of the present invention. By way of example, although in describing the preferred embodiment of the present invention, reference has been made to the four cylinder, four stroke parallel multi-cylinder combustion engine, the present invention is not necessarily limited thereto and may be applied to an in-line cylinder combustion engine or a V-type twin cylinder combustion engine, or to a two cylinder combustion engine or a single cylinder combustion engine. Also, the cylinder axis line may not extend in a manner such as described in connection with the preferred embodiment of the present invention, may extend in a vertical direction, a horizontal direction or any direction relative to the vertical direction or relative to the horizontal direction. 
     Also, the piston cooling structure of the present invention can be equally applied not only to the motorcycle, but also to any automotive vehicle or a marine engine, but also to a ground installed engine. Moreover, although the cooling structure of the present invention is suitably employed in a high output engine having a supercharger mounted thereon, particularly the combustion engine having the displacement not smaller than 600 cc, it can be applied to any a vehicle having no supercharger mounted thereon. 
     Accordingly, such changes and modifications are, unless they depart from the scope of the present invention as delivered from the claims annexed hereto, to be construed as included therein. 
     REFERENCE NUMERALS 
     
         
         
           
               26  . . . Crankshaft 
               28  . . . Crankcase 
               35  . . . Exhaust port 
               61  . . . Cylinder bore 
               62  . . . Piston 
               69  . . . Piston back face 
               78  . . . Cooling passage 
               82  . . . First branch passage 
               84  . . . Second branch passage 
               85  . . . First nozzle 
               85   a  . . . First injection port 
               88  . . . Bearing 
               90  . . . Second nozzle 
               90   a  . . . Second injection port 
               102  . . . Crank gear 
               104  . . . First crank web 
               106  . . . Second crank web 
             θ 1  . . . First angle of inclination 
             θ 2  . . . Second angle of inclination 
             CO . . . Cylinder axis line 
             CY . . . Cylinder 
             D 1  . . . First bore size 
             D 2  . . . Second bore size 
             E . . . Combustion engine 
             OL . . . Oil (Cooling liquid)