Patent Publication Number: US-7717100-B2

Title: Breather structure of engine

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a breather structure of an engine, and more particularly to a breather chamber separating an air including an oil mist within the crank case into oil and gas. 
   2. Prior Art 
   In general, a breather chamber of the engine is formed in an upper wall portion of the crank case away from the cylinder. Particularly, in an engine provided with a transmission chamber in a rear portion of the crank case, as described in Japanese Unexamined Patent Publication No. 8-218842, the structure is made such that the breather chamber for separating oil mist from the blow-by gas is formed in an upper wall of the transmission chamber in the rear portion of the crank case, the oil component after being separated in the breather chamber is returned to a bottom portion of the transmission chamber or an oil pan. 
   In an engine mounted to a vehicle such as a motor cycle or the like, in order to respond to a high power requirement in recent years, a cylinder is improved by making a bore diameter large and making a thickness of the cylinder large, or forming a reinforcing rib, thereby reinforcing the cylinder, however, in the case that the thickness is increased or the reinforcing rib is independently formed, a size of the engine is increased and a weight thereof is increased. 
   Further, in the structure in which the oil component separated in the breather chamber is returned to an oil reservoir via an oil return oil path, in the case that an oil level of the oil reservoir is high, the oil in the oil reservoir flows back through the oil return oil path so as to be pushed back to the breather chamber, on the basis of an oil level fluctuation of the oil reservoir at a time of a rapid acceleration or a rapid deceleration, for example, a back and forth movement of the oil, or a ruffling phenomenon generated by the air pushed down by a vertical motion of the piston pushing the oil within the oil reservoir, so that there is a risk that a breather function is affected. 
   SUMMARY OF THE INVENTION 
   The present invention is made by taking the problem mentioned above into consideration, and there is provided a breather structure of an engine having a crank case and a cylinder fastened to an upper side of the crank case, comprising a breather chamber integrally formed with the cylinder in an outer peripheral wall of the cylinder, and the breather chamber separates an air including an oil mist within the crank case into oil component and gas component, returns the oil component after being separated to an oil reservoir of the engine, and discharges the gas component out of the crank case. 
   In accordance with the structure mentioned above, a peripheral wall of the breather chamber serves as a reinforcing member of the cylinder, can improve a rigidity of the cylinder, can improve a strength of the cylinder, and can respond to a high power requirement while maintaining a light weight and a compactness of the engine. 
   In the present invention, it is preferable that the breather chamber may be integrally formed with the cylinder and the crank case in such a manner as to come over the upper surface of the crank case from the outer peripheral wall of the cylinder. 
   In accordance with the structure, the peripheral wall of the breather chamber serves as a reinforcing member coupling the cylinder and the crank case, can effectively increase a rigidity of the cylinder and the crank case, and can improve a strength thereof. 
   In the present invention, it is preferable that a rotating member accommodating chamber may be integrally formed with the cylinder together with the breather chamber, in the outer peripheral wall of the cylinder. 
   In accordance with the structure, it is possible to effectively improve the strength of the cylinder. 
   In the present invention, it is preferable that the rotating member accommodating chamber may be integrally formed with the cylinder and the crank case in such a manner as to come over the upper surface of the crank case from the outer peripheral wall of the cylinder. 
   In accordance with the structure, the peripheral wall of the rotating member accommodating chamber serves as the reinforcing member coupling the cylinder and the crank case, together with the peripheral wall of the breather chamber, can further increase the rigidity of the cylinder and the crank case, and can improve the strength thereof. 
   In the present invention, it is preferable that upper surfaces of the breather chamber and the rotating member accommodating chamber may be open, upper surface openings of the breather chamber and the rotating member accommodating chamber may be closed by a common lid to both the chambers. 
   In accordance with the structure, in the case of integrally casting the cylinder and the crank case, the opening directions of the breather chamber and the rotating member accommodating chamber come to a vertical direction which is approximately equal to a die drawing direction (a vertical direction) of a casting mold for forming the crank case. Accordingly, the openings of the breather chamber and the rotating member accommodating chamber can be simultaneously formed by the casting mold for the crank case, and even if a die drawing direction of a core for forming the breather chamber and the rotating member accommodating chamber is a lateral direction, it is possible to easily assemble the core within the casting mold by utilizing the opening, and a casting work can be easily executed. Further, since the common lid to both the chambers is attached over the openings of both the chambers, it is possible to improve the rigidity of both the chambers. 
   In the present invention, it is preferable that the breather structure may be provided with an oil return passage returning the oil component separated in the breather chamber to the oil reservoir in the bottom portion of the engine from the breather chamber, and an upper side of the oil reservoir in the bottom portion of the engine may be covered by an oil level fluctuation preventing plate. 
   In accordance with the structure, it is possible to prevent an oil ruffling phenomenon generated by the air being pushed down on the basis of the vertical motion of the piston, by the oil level fluctuation preventing plate, and it is possible to prevent an air biting by the oil pump. Further, it is possible to prevent a back flow of the oil to the breather chamber from the oil reservoir and it is possible to maintain improved oil pump function and breather function. Particularly, in the case that the engine in accordance with the present invention is mounted to a motor cycle or the like, it is possible to prevent the movement of the oil in the oil reservoir at a time of a rapid acceleration or a rapid deceleration. 
   In the present invention, it is preferable that the air including oil mist in the crank case may be introduced into the breather chamber through the rotary member accommodating chamber and a breather passage formed between the rotary member accommodating chamber and the breather chamber. 
   In accordance with the structure, it is possible to make the breather passage compact. 
   In the present invention, it is preferable that the rotary member accommodating chamber may be a balancer chamber for accommodating a balancer of the engine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be become more apparent from the following description taken in connection with the accompanying drawings. 
       FIG. 1  is a left side elevational view of an embodiment of a 4-cylinder engine for a motor cycle to which the present invention is applied. 
       FIG. 2  is a front elevational view of the engine shown in  FIG. 1 . 
       FIG. 3  is a left side elevational view of a cylinder and a crank case of the engine shown in  FIG. 1 . 
       FIG. 4  is a perspective view of an integrally formed product of the cylinder, an upper crank case member, a breather chamber and a balancer chamber of the engine in  FIG. 1  as seen from the above in a backward left side. 
       FIG. 5  is a perspective view of a common lid to the breather chamber and the balancer chamber as seen from the below in the backward left side. 
       FIG. 6  is an enlarged view of a cross section along a line VI-VI in  FIG. 3 . 
       FIG. 7  is a vertical cross sectional side elevational view of a lower crank case member and an oil pan of the engine shown in  FIG. 1 . 
       FIG. 8  is a perspective view of the oil pan of the engine shown in  FIG. 1  from the above in a forward left side; 
       FIG. 9  is a plan view of the oil pan of the engine shown in  FIG. 1 . 
       FIG. 10  is a cross section along a line X-X in  FIG. 9 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiment 
   Attached  FIGS. 1 to 10  show an example obtained by applying the present invention to an inline 4-cylinder engine for a motor cycle, and a description will be given of an embodiment in accordance with the present invention on the basis of these drawings. 
   [Outline of Engine] 
     FIG. 1  is a left side elevational view of the engine mentioned above, and the following description will be given by setting a direction of an axial length of a crank shaft  7  to a lateral (right and left) direction, and setting a cylinder arranged side in a horizontal direction orthogonal to the axial length direction of the crank shaft  7  to a front side, for convenience of explanation. 
   In  FIG. 1 , an outer shell of an engine is mainly constituted by a crank case  1  having a vertically divided two-piece structure comprising an upper crank case member  2  and a lower crank case member  3 , a cylinder (a cylinder block)  4  integrally formed with the upper crank case member  2  in an upper surface of a front end portion of the upper crank case member  2 , a cylinder head  5  fastened to an upper surface of the cylinder  4 , a cylinder head cover  6  fastened to an upper surface of the cylinder head  5 , and an oil pan  8  fastened to a lower surface of the lower crank case member  3 . A generator cover  10 , an output chain cover (an output sprocket cover)  11  and the like are attached to a left end portion of the crank case  1 , a water pump  13  is provided in a backward lower side of the generator cover  10 , an oil cooler  15  is attached to a front end surface of the lower crank case member  3  so as to protrude forward, and a secondary oil filter  16  is attached to a front end surface of the oil pan  8  so as to protrude forward. 
   Intake passage inlets  18  which open toward a backward upper side are formed in a rear surface of the cylinder head  5  per cylinders, and a throttle body  19  is connected to each of the intake passage inlets  18 . Exhaust passage outlets  20  which open forward are formed per cylinders, and an exhaust pipe  21  is connected to each of the exhaust passage outlets  20 . 
     FIG. 2  is a front elevational view of the engine shown in  FIG. 1 . The exhaust pipes  21  extend downward in a front side of the engine, are collected at one position as well as being curved backward in a front lower end portion of the engine, and are connected to one thick catalyst pipe  22  in a lower side of the engine. Further, two right and left exhaust pipes  23  are connected to a rear end portion of the catalyst pipe  22 , and these two exhaust pipes  23  respectively extend to both right and left sides of a rear wheel (not shown) and are respectively connected to an exhaust muffler (not shown). 
     FIG. 3  is a left side elevational view of the cylinder  4  and the crank case  1 , a transmission case  1   a  is integrally formed in a rear portion of the crank case  1 , and respective shafts for changing gears such as an input shaft  25 , an output shaft  26  and the like are accommodated within the transmission case  1   a . The upper crank case member  2  and the lower crank case member  3  are divided into upper and lower pieces by a split surface (a mating surface) B passing through an axis O 1  of the crank shaft  7  and an axis O 3  of the output shaft  26 , and the crank case member  2  is formed in an approximately L shape in a side view so as to be integral with the cylinder  4  in the front end portion as mentioned above. Further, the input shaft  25  is arranged in such a manner that an axis O 2  thereof is positioned in a lower side than the split surface B. 
   [Breather Chamber and Balancer Chamber] 
   In  FIG. 3 , a breather chamber  30  and a balancer chamber  31 , in which a balancer (not shown) of the engine is accommodated corresponding to one example of the rotating member accommodating chamber are integrally formed with the cylinder  4  and the upper crank case member  2  in such a manner as to come over the upper surface of the upper crank case member  2 , from a back surface of an outer peripheral wall of the cylinder  4 , respective upper ends of both the chambers  30  and  31  are open, and both the openings of the chambers  30  and  31  are closed by a common lid  32  having a lid portion  32   a  for the breather chamber  30  and a lid portion  32   b  for the balancer chamber  31 . A starter motor mounting portion  33  is integrally formed with the balancer chamber  31  in a rear side of the balancer chamber  31 , and a starter motor  34  is attached to the starter motor mounting portion  33  as shown in  FIG. 1 . 
     FIG. 4  is a perspective view of an integrally formed product of the cylinder  4 , the upper crank case member  2 , the breather chamber  30  and the balancer chamber  31  as seen from the above in a backward left side. In this  FIG. 4 , the engine is provided with first, second, third and fourth cylinders C 1 , C 2 , C 3  and C 4  in this order from the left side, the breather chamber  30  is positioned approximately at a back surface of the second cylinder C 2 , and the balancer chamber  31  is formed in a right side of the breather chamber  30  via a partition wall  37  and is positioned approximately at a back surface of the third cylinder C 3 . Lid mounting surfaces  30   a ,  31   a  and  37   a  formed in upper ends of peripheral walls  30   b  and  31   b  of both the chambers  30  and  31  and the partition wall  37  are inclined in a backward downward direction in such a manner as to surround the openings of both the chambers  30  and  31 , whereby the peripheral walls  30   b  and  31   b  of the breather chamber  30  and the balancer chamber  31  and the partition wall  37  serve as a reinforcing rib connecting the back surface of the cylinder  4  and the upper surface of the upper crank case member  2  in an approximately triangular shape in a side view. In this case, a common lid mounting surface  37   a  formed in an upper surface of the partition wall  37  is formed so as to be wider than the other lid mounting surfaces  30   a  and  31   a.    
     FIG. 5  is a perspective view of the common lid  32  as seen from below in the forward left side. A groove-shaped breather passage  40  extending from a left rear end portion  40   a  of the lid portion  32   b  for the balancer chamber  30  to a front end of a partition wall  38  formed between both the lid portions  32   a  and  32   b  is formed in a back surface  32   c  of the common lid  32 , and a plurality of communication holes (notches)  41  communicating the groove-shaped breather passage  40  and an inner side  32   aa  of the breather chamber lid portion  32   a  are formed in the partition wall  38 , whereby it is possible to circulate the air including the oil mist from an inner side  32   ba  of the balancer chamber lid portion  32   b  to the inner side  32   aa  of the breather chamber lid portion  32   a . The groove-shaped breather passage  40  and the communication hole  41  are completed as a tunnel-shaped passage by being lapped over the upper end lid mounting surface  37   a  of the partition wall  37  between both the chambers  30  and  31  in  FIG. 4 . In a state in which the common lid  32  is attached on the balancer chamber  31 , the rear end portion  40   a  of the breather passage  40  is communicated with the inner side  32   ba  of the balancer chamber lid portion  32   b.    
     FIG. 6  is an enlarged view of a cross section along a line VI-VI in  FIG. 3 . An oil gas separating wall  44  is formed in the breather chamber  30  at a position spaced at a predetermined distance from the breather chamber lid portion  32   a  to a lower side, a lower breather chamber  43  is formed in a lower side of the oil gas separating wall  44 , and the upper and lower breather chambers  30  and  43  are communicated with a gap  45  formed in a left end of the oil gas separating wall  44 . An oil return passage  51  extending from a bottom surface of the lower breather chamber  43  to a lower side is formed within the upper crank case member  2 , the oil return passage  51  is communicated with an oil return passage  52  formed in the lower crank case member  3 , the oil return passage  52  extends to a lower side, and an oil return hose  53  is connected to a lower end opening portion of the oil return passage  52 . On the other hand, an air vent passage  55  extending to an upper side while communicating with the lower breather chamber  43  is formed within the partition wall  37  between both the chambers  30  and  31 , an upper end opening of the air vent passage  55  is communicated with an air vent passage  56  formed within the common lid  32 , an air vent hose  58  is connected to the air vent passage  56  via an L-shaped joint pipe  57 , and the air vent hose  58  is connected to an air cleaner  59 . 
   [Oil Pan and Oil Path] 
     FIG. 9  is a plan view of the oil pan  8 . The oil pan  8  is formed approximately in a rectangular shape in a plan view, and a left end portion in a front half portion protrudes leftward a little. About a left half or less region (a region having about a quarter or a fifth part of an entire area of the oil pan) is formed as a concave portion  61  having a deep bottom in a front half portion of the oil pan  8 , and the remaining region is formed as a flat shape having a shallow bottom (see  FIG. 8 ). 
   In a bottom surface of the oil pan  8 , an oil passage  75  and an oil passage  70  are formed within an upwardly protruding tubular wall and are open upward, in left and right end portions of a front end portion of the concave portion  61 . The oil passage  75  is communicated with a filtered side space portion of a secondary oil filter  16 , and the oil passage  70  is communicated with an unfiltered side space portion of the secondary oil filter  16 . A fitting concave portion  69  for connecting an oil passage pipe  67  is formed coaxially with the oil passage  70  in an upper end portion of the latter oil passage  70 , a fitting concave portion  71  for connecting the oil passage pipe  67  is formed at a position spaced at a predetermined distance to a right rear side from the fitting concave portion  69  (near an approximately center position of the oil pan  8 ), and the fitting concave portion  71  is also formed within the upward protruding tubular wall. 
   A plurality of guide ribs  74  are formed in the flat portion having the shallow bottom in the oil pan  8  for guiding the oil within the oil pan  8  into the concave portion  61 , and a plurality of (three) support projections  92  is formed for supporting a primary oil filter  81  mentioned below in a contact manner from below. 
   A plurality of boss portions  91  for attaching the oil level fluctuation preventing plate  62  are formed at an interval around the concave portion  61  so as to protrude upward, and the flat oil level fluctuation preventing plate  62  covering an upper side of the concave portion  61  is mounted on the boss portion  91 , and is fixed to the oil pan  8  by a plurality of bolts  63 . 
   A notch  64  and a notch  65  are formed as a potbellied shape in a plan view in a rear end portion of the oil level fluctuation preventing plate  62 , and a notch  68  is formed in a right end portion in a front side of the oil level fluctuation preventing plate  62 . The notch  64  is provided for inserting the oil return hose  53  therethrough, the notch  65  is provided for inserting an insertion pipe  66  formed in the primary oil filter  81  therethrough, and the notch  68  is provided for arranging the oil passage pipe  67 . 
     FIG. 10  is a cross sectional view along a line X-X in  FIG. 9 . The concave portion  61  of the oil pan  8  is positioned approximately in a lower side of the pistons  78  for the first and second cylinders C 1  and C 2 , and the air in the lower side of the pistons  78  are pushed down toward the concave portion  61  side as shown by an outline arrow, on the basis of a vertical motion of the pistons  78  of the first and second cylinders C 1  and C 2 . 
   In  FIG. 2 , as mentioned above, the concave portion  61  of the oil pan  8  is formed approximately in a left half of the oil pan  8 , and the catalyst pipe  22  is arranged in a space portion formed by the right side surface of the concave portion  61  and the lower surface of the flat portion having the shallow bottom in the oil pan  8 . 
     FIG. 8  is a perspective view of the oil pan  8  as seen from above in the forward left side. The primary oil filter  81  is arranged so as to extend to a right backward direction from the upper position of the rear end portion of the concave portion  61 , and a lower end portion of the oil return hose  53  is arranged next to the primary oil filter  81  in the left, and the oil return hose  53  is retained by a pair of circular arc shaped clamps  82  integrally formed in a left end of the primary oil filter  81 , and is opened toward the inner side of the concave portion  61 . 
   A tubular oil outlet portion  67   b  open to a lower side is formed in a front end of the oil passage pipe  67 , the oil outlet portion  67   b  is fitted to the fitting concave portion  69  of the oil pan  8 , a tubular oil inlet portion  67   a  open to an upper side is formed in a rear end portion of the oil passage pipe  67 , and the oil inlet portion  67   a  is fitted to the fitting concave portion  71  of the oil pan  8 . A relief valve  73  facing to the lower side concave portion  61  is provided in the middle of the oil passage pipe  67 . 
     FIG. 7  is a vertical cross sectional side elevational view of the lower crank case member  3  and the oil pan  8 . The oil passage pipe  67  is structured such that the oil outlet portion  67   b  is pinched between a fitting concave portion  95  formed in the lower crank case member  3  and the fitting concave portion  69  of the oil pan  8  from the above and the below, and the rear end oil inlet portion  67   a  is pinched between the discharge port  86   b  of the oil pump  86  and the fitting concave portion  71  of the oil pan  8  from the above and the below, by assembling the oil pan  8  in the lower surface of the lower crank case member  3  as shown in  FIG. 7 . 
   An outer shell of the primary oil filter  81  is structured as a vertically divided two-piece structure, and a net plate shaped filter element  83  is pinched between upper and lower case members  81   a  and  81   b . The suction pipe  66  integrally formed in a front end portion of the primary oil filter  81  extends to a lower side within the concave portion  61 , and is open near a bottom surface of the concave portion  61 . An upper end oil outlet  85  of the primary oil filter  81  is connected to a suction port  86   a  of the oil pump  86  provided within the lower crank case member  3 . The primary oil filter  81  is pinched between the oil suction port  86   a  of the oil pump  86  and the support projection  91  of the oil pan  8  from the above and the below by assembling the oil pan  8  in the lower surface of the lower crank case member  3 . 
   The discharge port  86   b  of the oil pump  86  is open to a lower side, and is connected to the rear end oil inlet portion  67   a  of the oil passage pipe  67  as mentioned above. The oil outlet portion  67   b  of the oil passage pipe  67  is communicated with the unfiltered side space portion of the secondary oil filter  16  via the oil passage  70  mentioned above. The filtered side space portion in the inner portion of the annular element  16   a  arranged within the secondary oil filter  16  is communicated with an inlet of the oil cooler  15  via a sideways oil passage  88  within the oil pan  8  and the oil passage  75  ( FIG. 8 ), and an oil outlet of the oil cooler  15  is communicated with a main gallery  90 . 
   [Oil Circulation During Engine Operation] 
   In  FIG. 7 , during the engine operation, the oil (for example, the oil level L 1 ) within the concave portion  61  of the oil pan  8  passes through the inner side of the suction pipe  66  and the primary oil filter  81 , and is sucked into the oil pump  86  from the suction port  86   a , and the oil pressurized by the oil pump  86  comes into the second oil filter  16  from the oil discharger port  86   b  through the oil passage pipe  67  and the oil passage  70 , and is filtered by the element  16   a . The filtered oil comes into the oil cooler  15  through the oil passage  88  or the like, and is pressure fed to each of the lubricating positions such as the bearing portion of the crank shaft  7  and the like from the main gallery  90  after being cooled. Further, the oil after lubricating each of the lubricating positions is returned to the oil pan  8 . 
   [Operation and Effect of Embodiment] 
   (1) In  FIG. 10 , during the engine operation, a pressure fluctuation is generated within the crank case  1  on the basis of the vertical motion of the pistons  78 , and the air in the lower space of the piston  78  is pushed down. 
   The air pushed down by the piston  78  for the first and second cylinders C 1  and C 2  positioned in the upper side of the concave portion  61  moves toward the inner side of the concave portion  61 , however, is shielded by the oil level fluctuation preventing plate  62  arranged in the upper side of the concave portion  61 . Accordingly, the pushed-down air does not act on the oil within the concave portion  61 , and the oil within the concave portion  61  is not ruffled. In other words, since the oil ruffling phenomenon is not generated, it is possible to prevent the oil within the oil return hose  53  in  FIG. 7  from flowing back due to the pushed-down air, and there is no risk that it affects the breather operation in the breather chamber  30  in  FIG. 6 . 
   (2) In  FIG. 7 , if the vehicle is rapidly accelerated during the engine operation, the oil within the concave portion  61  is moved backward, however, since the oil level fluctuation preventing plate  62  is arranged in the upper side of the concave portion  61  as mentioned above, an amount of the oil overflowing to the rear side from the concave portion  61  is small, and the oil level L 1  within the concave portion  61  is not widely lowered. Further, since the oil suction pipe  66  is arranged near the rear end portion of the concave portion  61  and is open near the bottom surface of the concave portion  61 , there is no fear that the air biting is generated in the oil suction operation by the oil pump  86 . On the other hand, if the vehicle is rapidly decelerated, the oil within the concave portion  61  is moved to the front side, however, since the oil level fluctuation preventing plate  62  is arranged in the upper side of the concave portion  61 , and the front end of the concave portion  61  forms the front end of the oil pan  8  itself, the oil hardly leaks from the concave portion  61 . In this case, there is no fear that the air biting is generated in the oil suction operation by the oil pump  86 . 
   (3) In  FIG. 6 , the pressure fluctuation is generated within the crank case  1  as mentioned above during the operation, however, when the pressure is increased, the air including the oil mist within the crank case  1  flows into the breather chamber  30  through the breather passage  40  and the communication hole  41  within the common lid  32  in  FIG. 5  from the inner side of the balancer chamber  31 , and comes into collision with the oil gas separating wall  44  and the inner peripheral surface of the breather chamber  30 . Accordingly, the air is separated into the oil component and the gas component, and the gas component after being separated is sucked into the air cleaner  59  through the air vent passage  55  within the partition wall  37 , the air vent passage  56  within the partition wall  38  of the common lid  32 , the joint pipe  57  and the air vent hose  58 . On the other hand, the oil component separated within the breather chamber  30  is directly returned to the concave portion  61  of the oil pan  8  in  FIG. 7  through the lower side oil return passages  51  and  52  and the oil return hose  53 . 
   (4) In the present embodiment, as shown in  FIG. 3 , since the breather chamber  30  and the balancer chamber  31  are integrally formed with the cylinder  4  and the upper crank case member  2  from the back surface of the cylinder  4  over the upper surface of the upper crank case member  2 , in the structure in which the cylinder  4  and the upper crank case member  2  are integrally formed in the L shape, the peripheral walls  30   b  and  31   b  of the breather chamber  30  and the balancer chamber  31  and the partition wall  37  serve as a reinforcing member. Accordingly, it is possible to increase a rigidity of the cylinder  4  and the crank case  1  and improve strength without making the thicknesses of the cylinder  4  and the crank case  1  large in correspondence to the high power requirement of the engine or forming the rib exclusively used for reinforcing as in the conventional structure. In other words, it is possible to improve the rigidity of the cylinder  4  and the crank case  1  in such a manner as to respond to the high power requirement while maintaining the compact structure and the reduced weight of the engine. 
   (5) In  FIG. 4 , since the peripheral walls  30   b  and  31   b  of the breather chamber  30  and the balancer chamber  31  and the partition wall  37  are formed such that the upper end lid mounting surfaces  30   a ,  31   a  and  37   a  are inclined in the backward downward direction, the peripheral walls of the breather chamber  30  and the balancer chamber  31  and the partition wall forming a cavity in a casting mold form a molten metal passing way in the case of casting the integral product of the cylinder  4  and the upper crank case member  2 . Accordingly, the molten metal smoothly flows from the cavity for forming the cylinder  4  to the cavity for forming the upper crank case member  2 , it is possible to easily execute casting work and a yield ratio of the product is improved. 
   (6) In  FIG. 4 , since the openings are formed in the upper ends of the breather chamber  30  and the balancer chamber  31 , the opening directions come to the vertical direction which is approximately equal to a die drawing direction (a vertical direction) of the casting mold for forming the upper crank case member  2  in the case of integrally casting the cylinder  4  and the upper crank case member  2 . Accordingly, the openings of the breather chamber  30  and the balancer chamber  31  can be simultaneously formed by the casting mold for the crank case, and even if a die drawing direction of a core for the breather chamber  30  and the balancer chamber  31  is a lateral direction, it is possible to easily assemble the core within the casting mold by utilizing the opening, and casting work can be easily executed. 
   (7) In  FIG. 3 , since the common lid  32  is attached to the open ends of both the chambers  30  and  31 , the common lid  32  itself serves as the reinforcing member of the cylinder  4  and the upper crank case member  2 , and contributes to an improvement of the rigidity of the cylinder  4  and the upper crank case member  2 . 
   (8) In  FIG. 6 , since the oil component after being separated into the oil and the gas in the breather chamber  30  is returned to the inner side of the concave portion  61  in  FIG. 8  from the oil return passages  51  and  52  formed within the crank case members  2  and  3  by utilizing the oil return hose  53 , and the oil return hose  53  is retained by the clamp  82  formed in the primary oil filter  81  arranged in the upper side of the concave portion  61 , it is possible to easily structure a part of the oil component return path to the concave portion  61  by the oil return hose  53 , and it is possible to prevent the vibration of the oil return hose  53  due to the vibration of the engine and the oscillation of the vehicle. 
   Other Embodiments 
   (1) In the embodiment mentioned above, the present invention is applied to the engine provided with the crank case having the vertical two-piece divided structure, however, the present invention can be applied to an engine provided with a crank case having a lateral two-piece divided structure. 
   (2) In the embodiment mentioned above, the breather chamber is formed in the back surface of the outer peripheral wall of the cylinder, however, the breather chamber can be formed near the right and left side surfaces, in the outer peripheral wall of the cylinder. 
   (3) As the rotating member accommodating chamber provided next to the breather chamber, the balancer chamber accommodating the balancer is formed in the embodiment mentioned above, however, another chamber accommodating the other rotating members, for example, various rotating members such as a gear for transmitting a starting power, an intermediate gear of the transmission and the like can be provided. 
   (4) The breather chamber can be integrally formed with the cylinder in the back surface of the cylinder, in a state in which the breather chamber is separated from the upper surface of the crank case. 
   The present invention is not limited to the engine for the motor cycle, but can be utilized in an engine for the other vehicles such as a saddle riding type four-wheeled traveling vehicle or the like. 
   Although the present invention has been described in its preferred embodiments with a certain degree of particularity, obviously many changes and variations are possible therein. It is therefore to be understood that the present invention may be practical otherwise than as specifically described herein without departing from the scope and spirit thereof.