Patent Publication Number: US-9885400-B2

Title: Balancer device of engine and motorcycle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The disclosure of Japanese Patent Application No. 2015-210356 filed on Oct. 27, 2015, including specification, drawings and claims is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The disclosure relates to a balancer device of an engine and a motorcycle. 
     BACKGROUND 
     An engine of a motorcycle has been known which has a balancer device so as to reduce vibrations, which are to be generated in association with reciprocal movement of a piston and rotation of a crankshaft (for example, refer to Patent Document 1 or Patent Document 2). In a balancer device disclosed in Patent Document 1 and Patent Document 2, a pair of balancer shafts having weights is disposed to be opposite to each other with sandwiching a crankshaft in a front-rear direction. One end of the crankshaft is provided with drive gears for driving the balancer shafts, and the drive gears are meshed with driven gears provided for the respective balancer shafts. Thereby, the rotation of the crankshaft is transmitted to each of the balancer shafts, so that periodical rotation vibrations of the crankshaft are cancelled. 
     Patent Document 1: Japanese Patent Application Publication No. H02-113145A 
     Patent Document 2: Japanese Patent Application Publication No. 2014-95405A 
     However, according to the engine disclosed in Patent Document 1 and Patent Document 2, while one end of the crankshaft is provided with the drive gears for driving the balancer shafts, the other end of the crankshaft is provided with a primary drive gear for driving a countershaft. In this way, since the drive gears for driving the balancer shafts and the primary drive gear for driving the countershaft are separately provided, the engine is enlarged in a vehicle width direction. For example, when the crankshaft is provided with the drive gears corresponding to the number of the balancer shafts, the engine is further enlarged in the vehicle width direction by the drive gears, which causes a severer problem. 
     SUMMARY 
     It is therefore an object of the disclosure to provide an engine balancer device and a motorcycle, in which balancer shafts can be disposed without enlarging an engine. 
     According to an aspect of the embodiments of the present invention, there is provided a balancer device of an engine, configured to reduce rotation vibrations of the engine, the balancer device comprising: a crankshaft provided with a primary drive gear; a countershaft provided with a primary driven gear meshed with the primary drive gear; and a plurality of balancer shafts provided with balancer driven gears meshed with the primary drive gear. 
     According to the above configuration, since it is possible to rotate the countershaft and the plurality of balancer shafts by the existing primary drive gear, it is not necessary to provide the crankshaft with a gear for driving the balancer shaft. Therefore, as compared to a configuration where a gear for driving the balancer shaft is separately provided from the primary drive gear, it is possible to shorten a length of the crankshaft, thereby reducing a width of the engine in a right-left direction. In this way, it is possible to dispose the balancer shafts without enlarging the engine. 
     In the balancer device, the plurality of balancer shafts may comprise a first balancer shaft disposed in front of the crankshaft and a second balancer shaft disposed below the crankshaft. According to the above configuration, since the second balancer shaft is disposed below the crankshaft, it is possible to reduce a width in a front-rear direction of the engine, as compared to a configuration where the first balancer shaft and the second balancer shaft are disposed in the front-rear direction. 
     In the balancer device, the primary drive gear may be provided at an end portion of the crankshaft, the second balancer shaft may be disposed at one side of the engine in a right-left direction, and an oil passage extending in a front-rear direction of the engine may be provided at another side of the engine in the right-left direction. According to the above configuration, since the second balancer shaft is disposed toward one side of the engine in the right-left direction and an empty space of the other side of the engine is provided with the oil passage, it is possible to efficiently utilize a space in the engine and to form an optimal oil passage. As a result, it is possible to make the engine compact. 
     In the balancer device, the second balancer shaft may be disposed at an opposite side to a side stand configured to support a vehicle body with respect to the right-left direction. 
     According to the above configuration, even when the side stand is used and the engine is thus inclined toward the side stand-side, the second balancer shaft is located at a higher position than a liquid surface of the oil at the side stand-side. For this reason, it is possible to prevent the second balancer shaft from being submerged in the oil, so that it is possible to reduce a mechanism loss upon start of the engine. 
     Also, the motorcycle of the disclosure may have the above-described balancer device of the engine. 
     According to the disclosure, the countershaft and the plurality of balancer shafts are rotated by the primary drive gear, so that it is possible to dispose the balancer shafts without enlarging the engine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a side view depicting a schematic configuration of an engine of a motorcycle in accordance with an illustrative embodiment; 
         FIG. 2  is a front view of the engine shown in  FIG. 1 ; 
         FIG. 3  is a side view depicting shaft arrangement in the engine in accordance with the illustrative embodiment; 
         FIG. 4  is a front view depicting the shaft arrangement in the engine in accordance with the illustrative embodiment; 
         FIG. 5  is a front view of a crank case in accordance with the illustrative embodiment; 
         FIG. 6  is a sectional view taken along a line A-A of the crank case shown in  FIG. 5 ; 
         FIG. 7  is a sectional view taken along a line B-B of the crank case shown in  FIG. 5 ; 
         FIG. 8  is a side view depicting a state where a clutch cover is detached from the engine in accordance with the illustrative embodiment; 
         FIG. 9  is a bottom view depicting a state where an oil pan is detached from the engine in accordance with the illustrative embodiment; and 
         FIG. 10  is a front view of the engine when a side stand is used for a motorcycle in accordance with an illustrative embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, an illustrative embodiment of the disclosure will be described in detail with reference to the accompanying drawings. Meanwhile, in the below, an example where a balancer device of an engine of the disclosure is applied to a motorcycle will be described. However, the disclosure is not limited thereto. For example, the balancer device of the engine of the disclosure may also be applied to a motorcycle of other type, a three-wheeled motor vehicle of a buggy type, a four-wheeled motor vehicle, or the like. Also, regarding directions, a vehicle front side is denoted with an arrow FR, a vehicle rear side is denoted with an arrow RE, a vehicle left side is denoted with an arrow L, and a vehicle right side is denoted with an arrow R, respectively. Also, in the respective drawings, some configurations are omitted for convenience of explanations. 
     A schematic configuration of an engine of a motorcycle in accordance with an illustrative embodiment is described with reference to  FIGS. 1 and 2 .  FIG. 1  is a side view depicting a schematic configuration of an engine of a motorcycle in accordance with an illustrative embodiment.  FIG. 2  is a front view of the engine shown in  FIG. 1 . 
     As shown in  FIGS. 1 and 2 , an engine  1  is a two-cylinder four-cycle engine and has constitutional components such as pistons  22  (refer to  FIG. 3 ) and the like accommodated in a cylinder assembly  12  configured by a cylinder block  10  and a cylinder head  11 , and a cylinder head cover  13  is attached to an upper end of the cylinder assembly  12  (cylinder head  11 ). A crank case  2  configured to accommodate therein a crankshaft  20  (refer to  FIG. 3 ) is attached to a rear-lower side of the cylinder assembly  12 . 
     The crank case  2  is configured to be vertically separable and has an upper case  3  and a lower case  4 . When the upper case  3  and the lower case  4  are combined, a space for accommodating a variety of shafts in the crank case  2  is formed. A front-upper part of the upper case  3  opens, and the cylinder block  10  is attached to the upper case  3  so as to block the opening. The lower case  4  opens downward, and an oil pan  5  is attached to the lower case  4  so as to block the opening. 
     Also, an oil cooler  14  configured to cool oil in the engine  1  and an oil filter  15  (which is not shown in  FIG. 1 ) configured to filter unclean oil are attached to a front part of the lower case  4 . As shown in  FIG. 2 , the oil cooler  14  is provided at a right side of the front part of the lower case  4  and the oil filter  15  is provided at a left side. 
     Both left and rights sides of the crank case  2  are formed with openings, respectively. A magneto cover  16  (which is not shown in  FIG. 1 ) configured to cover a magneto (not shown) is attached to the left opening, and a clutch cover  17  configured to cover a clutch (not shown) is attached to the right opening. A water pump  18  configured to supply cooling water into the engine  1  is provided in front of the clutch cover  17 . Also, although not shown, a side stand  19  (refer to  FIG. 10 ) configured to support a vehicle body (engine  1 ) is provided at a left-lower side of the crank case  2 . 
     Subsequently, shaft arrangement in the crank case and a balancer device in accordance with the illustrative embodiment are described with reference to  FIGS. 3 and 4 .  FIG. 3  is a side view depicting the shaft arrangement in the engine in accordance with the illustrative embodiment.  FIG. 4  is a front view depicting the shaft arrangement in the engine in accordance with the illustrative embodiment. In  FIGS. 3 and 4 , the crank case is not shown and only the shaft arrangement is shown for convenience of explanations. 
     As shown in  FIGS. 3 and 4 , not only the crankshaft  20  but also a variety of shafts for transmitting a driving force of the engine  1  (refer to  FIG. 1 ) are accommodated in the crank case  2  (refer to  FIG. 1 ). In the below, arrangement positions of the diverse shafts are described on the basis of a position of the crankshaft  20 . The crankshaft  20  is accommodated at the slight front of a center of the crank case  2  (refer to  FIG. 6 or 7 ). Two pistons  22  are axially side by side attached to the crankshaft  20  via connecting rods  21 . Meanwhile, a crank phase of the crankshaft  20  in accordance with the illustrative embodiment is set to 270°. Also, a right end portion of the crankshaft  20  is provided with a primary drive gear  20   a  for driving the diverse shafts in the engine  1  so that the primary drive gear can integrally rotate. 
     A countershaft  23  is provided at an oblique upper side of the rear of the crankshaft  20 . A right end of the countershaft  23  is provided with a clutch (not shown), and a primary driven gear  23   a  is provided close to the clutch at an inner side of the clutch in a vehicle width direction. 
     The primary driven gear  23   a  is attached to the countershaft  23  so that it can integrally rotate, and is meshed with the primary drive gear  20   a.  Also, the countershaft  23  is provided with a variety of gears for transmission at the left of the primary driven gear  23   a.    
     A drive shaft  24  is provided at an oblique lower side of the rear of the countershaft  23 . The drive shaft  24  is provided with a variety of gears for transmission. Also, a plurality of (two, in the illustrative embodiment) balancer shafts  25  configuring a part of the balancer device is provided around the crankshaft  20 . 
     The balancer shafts  25  include a first balancer shaft  26  disposed at the front of the crankshaft  20  and a second balancer shaft  27  disposed below (just below) the crankshaft  20 . The first balancer shaft  26  and the second balancer shaft  27  are disposed so that an angle between a line connecting the first balancer shaft  26  and the crankshaft  20  and a line connecting the second balancer shaft  27  and the crankshaft  20  is substantially a right angle. 
     The first balancer shaft  26  and the second balancer shaft  27  extend along an axial direction of the crankshaft  20 . A first balancer driven gear  26   a  configured to mesh with the primary drive gear  20   a  is provided at a right end portion of the first balancer shaft  26  so as to be integrally rotatable. The first balancer shaft  26  is provided with two balancer weights  26   b  aligned side by side in the axial direction in correspondence to the positions of the two pistons  22 . 
     A second balancer driven gear  27   a  configured to mesh with the primary drive gear  20   a  is provided at a right end portion of the second balancer shaft  27  so as to be integrally rotatable. The second balancer shaft  27  is provided with a balancer weight  27   b  at a position corresponding to the right piston  22 . The second balancer shaft  27  is disposed at one side (right side) of the engine  1 , i.e., the same side as the clutch. 
     Also, side surfaces of the first balancer driven gear  26   a  and the second balancer driven gear  27   a  are provided with reference marks  26   c,    27   c,  which are references when phase adjusting the two balancer shafts  25  relative to the crankshaft  20 . Likewise, a side surface of the primary drive gear  20   a  attached to the crankshaft  20  is also provided with two reference marks (not shown). 
     When mounting the two balancer shafts  25  to the crank case  2 , the reference marks  26   c ,  27   c  of the first balancer driven gear  26   a  and the second balancer driven gear  27   a  are matched with the respective reference marks of the crankshaft  20 , so that it is possible to adjust the two balancer shafts  25  to a predetermined phase with respect to the crankshaft  20 . 
     In the engine  1  configured as described above, rotation of the crankshaft  20  is transmitted to the countershaft  23  via the primary drive gear  20   a  and the primary driven gear  23   a . The rotation of the countershaft  23  is transmitted to the drive shaft  24  with a predetermined change gear ratio by combinations of the diverse gears. The rotation of the drive shaft  24  is transmitted to a rear wheel (not shown) via a transmission mechanism (not shown). 
     Also, the rotation of the crankshaft  20  is transmitted to the two balancer shafts  25  configuring the balancer device, too. Specifically, the rotation of the crankshaft  20  is transmitted to the first balancer shaft  26  via the primary drive gear  20   a  and the first balancer driven gear  26   a,  and the rotation of the crankshaft  20  is transmitted to the second balancer shaft  27  via the primary drive gear  20   a  and the second balancer driven gear  27   a.    
     The vibrations of the engine  1  generated in association with the rotation of the crankshaft  20  are cancelled by the rotations of the first balancer shaft  26  and the second balancer shaft  27 . The vibrations of the engine  1  are reduced in this way, so that it is possible to suppress an influence of the vibrations or noises on a passenger. 
     In the meantime, the balancer device of the motorcycle has been mainly configured by one balancer shaft. However, since the vibrations of the engine increase in a vehicle of large engine displacement or a vehicle required to have a supercharger, it is not possible to achieve the sufficient effect of attenuating the vibrations by one balancer shaft. Regarding this, a balancer device having two or more balancer shafts has been suggested. However, the balancer shafts cause an increase in weight of the engine and enlarge the engine due to the shaft arrangement in the engine. 
     For example, when the two balancer shafts are vertically disposed, the engine is enlarged in a height direction. In this case, the lower balancer shaft is submerged into the oil in the oil pan, so that a mechanism loss is generated. Therefore, it is necessary to dispose the balancer shaft at a predetermined height with respect to an oil surface of the oil pan. Also, when the two balancer shafts are disposed in a front-rear direction, the engine is enlarged in the front-rear direction. Particularly, in recent motorcycles, a wheelbase of a vehicle is reduced to secure a comfortable riding feeling. 
     Also, as described above, the drive gear for driving the balancer shaft and the primary drive gear for driving the countershaft are generally separately provided for the crankshaft. For this reason, it is necessary to further lengthen the crankshaft by a magnitude corresponding to the drive gear for driving the balancer shaft, so that the engine is enlarged in the vehicle width direction. Like this, there are many restraints on the arrangement of the diverse components in the engine, and a new problem may be caused due to the arrangement. 
     Therefore, in the illustrative embodiment, the shaft arrangement in the engine  1  is changed to drive the three shafts of the countershaft  23  and the two balancer shafts  25  (the first balancer shaft  26  and the second balancer shaft  27 ) with one primary drive gear  20   a.  Thereby, it is possible to drive the two balancer shafts  25  only with the existing primary drive gear  20   a  and it is not necessary to separately provide the gear for driving the balancer shaft. As a result, it is possible to reduce the width of the engine  1  in the right-left direction without extending the crankshaft  20 . Further, since it is possible to configure a gear, which is to be provided for the crankshaft  20 , only by the one primary drive gear  20   a,  it is possible to save the processing cost of the crankshaft  20 , thereby achieving the cost down. 
     Also, the first balancer shaft  26  is provided at the front of the crankshaft  20  and the second balancer shaft  27  is provided below the crankshaft  20 , so that it is possible to dispose the two balancer shafts  25  in the engine  1  without increasing the widths in the upper-lower direction and the front-rear direction of the engine  1 . By these configurations, it is possible to make the engine compact as a whole. 
     Also, as described above, the three shafts are driven by the one primary drive gear  20   a  and the respective gears (the primary driven gear  23   a,  the first balancer driven gear  26   a  and the second balancer driven gear  27   a ) are intensively disposed at the right side. Thereby, it is possible to check the reference marks  26   c,    27   c  of the first balancer driven gear  26   a  and the second balancer driven gear  27   a  at the same time, so that it is possible to improve the mounting ability of the two balancer shafts  25 . 
     Also, in the illustrative embodiment, since the first balancer shaft  26  and the water pump  18  (refer to  FIG. 1 ) are coaxially provided, it is possible to drive the water pump  18  by using the rotation of the first balancer shaft  26  as a driving source. Therefore, it is not necessary to provide a dedicated gear for the water pump  18  and it is possible to simplify the configuration. 
     Subsequently, a positional relation between the diverse shafts in the engine and the crank case and the internal configuration of the crank case are described with reference to  FIGS. 5 to 8 .  FIG. 5  is a front view of the crank case in accordance with the illustrative embodiment.  FIG. 6  is a sectional view taken along a line A-A of the crank case shown in  FIG. 5 .  FIG. 7  is a sectional view taken along a line B-B of the crank case shown in  FIG. 5 . In  FIG. 7 , the oil pan is not shown for convenience of explanations.  FIG. 8  is a side view depicting a state where the clutch cover and the oil pan are detached from the engine in accordance with the illustrative embodiment. 
     As shown in  FIGS. 5 and 6 , in the illustrative embodiment, the crankshaft  20  and the first balancer shaft  26  are disposed on mating surfaces of the upper case  3  and the lower case  4 . More specifically, the mating surfaces of the upper case  3  and the lower case  4  are formed with two bearings in the front-rear direction. The first balancer shaft  26  is disposed in the front bearing, and the crankshaft  20  is disposed in the rear bearing. Also, the second balancer shaft  27  is supported by the lower case  4  and the balancer housing  6  configured to accommodate therein the second balancer shaft  27 . Mating surfaces of the lower case  4  and the balancer housing  6  are formed with a bearing in which the second balancer shaft  27  is disposed. 
     The lower case  4  and the balancer housing  6  are respectively formed with a plurality of through-holes  40 ,  60  (only two through-holes are shown, respectively, in  FIG. 6 ) in which fastening bolts  7  are to be inserted. The plurality of through-holes  40 ,  60  is formed at positions between which the crankshaft  20  or the second balancer shaft  27  is sandwiched in the front-rear direction. Also, the upper case  3  is formed with screw holes (not shown) at positions corresponding to the plurality of through-holes  40 ,  60 . The fastening bolts  7  are inserted into the through-holes  40 ,  60  from the lower of the balancer housing  6  and are then screwed into the upper case  3 , so that the upper case  3 , the lower case  4  and the balancer housing  6  are integrally fixed. 
     In this way, the crankshaft  20 , the first balancer shaft  26  and the second balancer shaft  27  are arranged on the mating surfaces of the crank case  2  and the balancer housing  6 , so that it is possible to attach the corresponding shafts just by sandwiching the same with the upper case  3 , the lower case  4  and the balancer housing  6 . 
     Particularly, the first balancer shaft  26  can be mounted to the crank case  2  with the first balancer driven gear  26   a  and the balancer weight  26   b  being assembled in advance. The second balancer shaft  27  can also be mounted to the crank case  2  with the second balancer driven gear  27   a  and the balancer weight  27   b  being assembled in advance. As a result, the mounting ability of the engine  1  is improved. 
     Also, since the balancer housing  6  is formed to cover the lower side of the second balancer shaft  27 , it is possible to support the second balancer shaft  27  simply by covering the crank case  2  (lower case  4 ) with the balancer housing  6 . Thereby, the mounting ability of the engine  1  is improved. Also, for a vehicle where the second balancer shaft  27  is not used, for example, it is possible to easily change a design by omitting the balancer housing  6  to shorten a length of the fastening bolt  7 . For this reason, it is possible to save the design man-hour, too. 
     Further, since the upper case  3 , the lower case  4  and the balancer housing  6  are together fastened by the fastening bolts  7 , it is possible to reduce the number of the fastening bolts  7  to be used, as compared to a configuration where the bolt fastening is performed for each of the crankshaft  20  and the second balancer shaft  27 . Thereby, it is possible to reduce the mounting man-hour and an operator&#39;s burden on the bolt fastening. Also, even when combining the respective bearings (bearings of the crankshaft and the two balancer shafts  25 ), it is possible to reduce the number of times of the bolt fastening, so that it is possible to save the processing man-hour and to improve the operation efficiency. 
     Herein, an oil passage in the crank case  2  is described. As shown in  FIG. 6 , the lower case  4  is provided with a main oil gallery  41 , which configures a part of the oil passage in the engine  1 , below the first balancer shaft  26  and in front of the second balancer shaft  27 . The main oil gallery  41  extends in the right-left direction. 
     Also, the lower case  4  is formed with oil passages  42 ,  43  for supplying the oil from the main oil gallery  41  to the crankshaft  20  and the first balancer shaft  26  and an oil passage  44  for supplying the oil from the crankshaft  20  to the second balancer shaft  27 . Further, the oil passage  44  is formed with an oil passage  45  for supplying the oil to the drive shaft  24  (refer to  FIG. 3 ). The oil passages  42 ,  45  are formed to obliquely penetrate the plurality of through-holes  40 . Thereby, it is possible to utilize the through-hole  40  for the fastening bolt  7  as a part of the oil passage. 
     Meanwhile, the upper case  3  is provided with a sub-oil gallery  30  above the first balancer shaft  26 . The sub-oil gallery  30  extends in the right-left direction. Also, the upper case  3  is formed with an oil passage  31  for supplying the oil from the main oil gallery  41  to the sub-oil gallery  30  through the first balancer shaft  26 . 
     In the illustrative embodiment, the above-described shaft arrangement is adopted and the main oil gallery  41  and the sub-oil gallery  30  are provided in an empty space near the two balancer shafts  25 . Thereby, it is possible to form the oil passage to each shaft (bearing) as a linear through-hole. For this reason, it is possible to form the oil passage by simple hole processing, so that it is possible to save the processing man-hour. 
     Also, since the crankshaft  20  and the second balancer shaft  27  are disposed adjacent to each other in the lower case  4 , it is possible to easily form the oil passage between the crankshaft  20  and the second balancer shaft  27  by the hole processing. Therefore, it is possible to save a weight of the crank case  2 , as compared to a configuration where an independent oil passage is separately formed using an unnecessary thickness of the crank case  2 . 
     Also, as shown in  FIGS. 7 and 8 , in the engine  1 , the oil is reserved in the oil pan  5 . The oil is pumped by an oil pump (not shown) and is then supplied to each part in the engine  1  through the main oil gallery  41 , the sub-oil gallery  30  and the like. In the illustrative embodiment, a liquid surface of the oil is positioned at a height at which the second balancer driven gear  27   a  and the balancer weight  27   b  of the second balancer shaft  27  are not submerged. For this reason, it is possible to prevent the oil from becoming a rotation resistance (mechanism loss) of the second balancer shaft  27 . Also, it is possible to prevent the rotary bodies (the second balancer driven gear  27   a  and the balancer weight  27   b ) from stirring the oil to introduce air bubbles into the oil. 
     Also, as described above, the liquid surface height of the oil is set to the height at which the rotary bodies are not submerged. Thereby, even though the oil surface move in waves when the vehicle body is banked during the cornering, for example, it is possible to prevent the oil from directly colliding with the rotary bodies. 
     Further, the second balancer shaft  27  is provided just below the crankshaft  20 , so that it is possible to interrupt a descending air current, which is to be generated when the pistons  22  (refer to  FIG. 3 ) moves downward, by the balancer housing  6  (refer to  FIG. 6 ). That is, it is possible to achieve a windshield effect by the balancer housing  6  becoming a wall. For this reason, the liquid surface of the oil reserved in the oil pan  5  does not undulate by the descending air current. Therefore, it is possible to prevent a situation where the air bubbles are introduced into the oil and the lubrication performance is thus lowered. 
     Also, as shown in  FIG. 8 , the right side of the crank case  2  is formed with an opening  29  for accommodating the clutch (not shown) by an annular peripheral wall part  28 . The peripheral wall part  28  has a shape conforming to a profile of the clutch cover  17  (refer to  FIG. 1 or 2 ). Also, an end surface (side surface) of the peripheral wall part  28  configures a mating surface  28   a  with the clutch cover  17 . The clutch cover  17  is attached along the mating surface  28   a,  so that the opening  29  is blocked. 
     At a state where the clutch cover  17  is detached, parts of the crankshaft  20  and the second balancer shaft  27  are positioned in the opening  29 , as seen from a side of the engine  1 . More specifically, parts of the primary drive gear  20   a  and the second balancer driven gear  27   a  and the bearings of the crankshaft  20  and the second balancer shaft  27  are disposed in a space surrounded by the annular peripheral wall part  28 . 
     In this way, parts of the crankshaft  20  and the second balancer shaft  27  are exposed to an outside, so that it is possible to easily recognize the reference mark  27   c  for phase adjustment. That is, it is possible to utilize the opening  29  of the crank case  2 , which occupies most of the side surface of the engine  1 , as a viewing window for phase adjustment of the second balancer shaft  27 . Thereby, it is possible to phase-adjust the second balancer shaft  27  relative to the crankshaft  20  without providing a dedicated viewing window. 
     Also, it is not necessary to perform hole processing for forming a dedicated viewing window and to provide a blind plug for blocking the hole, so that it is possible to simplify the configuration of the crank case  2 . Further, the blind plug is not required, so that an outward appearance of the engine  1  is not influenced. 
     Also, the bearings of the crankshaft  20  and the second balancer shaft  27  are exposed from the opening  29  of the crank case  2 , so that it is possible to secure a wide space when processing the bearings. That is, since it is possible to secure a wide area for supporting a processing tool, it is possible to stably support the processing tool to the mating surface  28   a . Therefore, it is possible to suppress shaking upon the processing of the bearings (holes), so that it is possible to improve processing precision. 
     Also, since only parts (bearings) of the crankshaft  20  and the second balancer shaft  27  are exposed, instead of entirely exposing the same, it is possible to form the opening  29  of the crank case  2  to a minimal size. Therefore, it is possible to increase the rigidity of the crank case  2 , as compared to a configuration where the respective shafts are entirely exposed. 
     Subsequently, a positional relation between the shaft arrangement and the oil passage in the crank case is described with reference to  FIG. 9 .  FIG. 9  is a bottom view depicting a state where the oil pan is detached from the engine in accordance with the illustrative embodiment. 
     As shown in  FIG. 9 , the second balancer shaft  27  is disposed at the right side of the engine  1  (lower case  4 ). Also, a space of an opposite side (left side) of the second balancer shaft  27  is provided with a cylindrical oil passage  46  extending in the front-rear direction. The oil passage  46  configures a path for supplying the oil from the oil pump (not shown) to the oil filter  15 . In this way, the primary drive gear  20   a  (not shown in  FIG. 9 ) and the second balancer shaft  27  are disposed at one side (right side) of the engine  1  in the right-left direction and the left empty space of the engine  1  is provided with the oil passage  46 , so that it is possible to efficiently utilize the space in the engine  1 . As a result, it is possible to form an optimal oil passage without enlarging the engine  1 , so that it is possible to make the engine  1  compact. 
     Subsequently, the oil surface in the engine when a side stand is used is described with reference to  FIG. 10 .  FIG. 10  is a front view of the engine when a side stand is used for a motorcycle in accordance with an illustrative embodiment. 
     As shown in  FIG. 10 , a side stand  19  configured to support the vehicle body (engine  1 ) is provided at the left of the engine  1  and below the crank case  2  (lower case  4 ) (at the left of the oil pan  5 ). At a state where the vehicle body is supported by the side stand  19 , the vehicle is inclined toward the side stand-side (left side), so that the engine  1  is inclined. 
     As described above with reference to  FIG. 3 , the second balancer shaft  27  is disposed at the right side of the engine  1 . That is, the second balancer shaft  27  is disposed at an opposite side to the side stand  19 . Therefore, even when the side stand  19  is used and the engine  1  is thus inclined toward the side stand  19 -side, the second balancer shaft  27  is located at a higher position than the liquid surface of the oil at the side stand  19 -side. For this reason, it is possible to prevent the second balancer shaft  27  from being submerged in the oil, so that it is possible to reduce the mechanism loss upon the start of the engine. 
     Also, it is possible to prevent the rotary bodies such as the second balancer driven gear  27   a  and the balancer weight  27   b  provided for the second balancer shaft  27  from stirring the oil to introduce the air bubbles into the oil. 
     As described above, according to the illustrative embodiment, since it is possible to rotate the countershaft  23  and the two balancer shafts  25  (the first balancer shaft  26  and the second balancer shaft  27 ) by the existing primary drive gear  20   a,  it is not necessary to provide the crankshaft  20  with a gear for driving the balancer shaft. Therefore, as compared to a configuration where a gear for driving the balancer shaft is separately provided from the primary drive gear  20   a,  it is possible to shorten a length of the crankshaft  20 , thereby reducing the width of the engine  1  in the right-left direction. In this way, it is possible to dispose the two balancer shafts  25  without enlarging the engine  1 . 
     In the meantime, the disclosure is not limited to the illustrative embodiment and can be diversely changed and implemented. In the illustrative embodiment, the sizes, shapes and the like shown in the accompanying drawings are not limited thereto and can be appropriately changed within a scope of achieving the effects of the disclosure. In addition, the illustrative embodiment can be appropriately changed without departing from the object of the disclosure. 
     For example, in the above illustrative embodiment, the balancer device having the two balancer shafts  25  has been described. However, the disclosure is not limited thereto. For example, the balancer device may have one balancer shaft or three or more balancer shafts. 
     Also, in the above illustrative embodiment, the first balancer shaft  26  is provided with the two balancer weights  26   b.  However, the disclosure is not limited thereto. For example, only one balancer weight  26   b  may be provided for the first balancer shaft  26 . In this case, the balancer weight  26   b  is preferably provided in front of the left piston  22 . 
     Also, in the above illustrative embodiment, the first balancer shaft  26  and the balancer weight  26   b  are configured as separate components. However, the disclosure is not limited thereto. For example, the first balancer shaft  26  and the balancer weight  26   b  may be integrally configured. The second balancer shaft  27  and the balancer weight  27   b  may also be integrally configured without being limited to the configuration where they are configured as separate components. 
     As described above, the disclosure has the effect of disposing the balancer shafts without enlarging the engine, and is particularly useful for the balancer device of the engine and the motorcycle.