Abstract:
A swinging type power unit for a two-wheeled vehicle where the direction of the cylinders or an engine can be freely set without interfering with the engine crankcase so as to give greater freedom of design even when it is intended to increase power output by adopting a two cylinder configuration. The crankcase is divided by a partitioning surface passing through a central part of a crankshaft in the axial direction thereof and into a first crankcase half and a second crankcase half. An inner transmission case half of a transmission case is integrally formed with the first crankcase half and a cylinder block. The cylinder block has two cylinders running parallel in a horizontal direction of the vehicle which are joined to a mounting block surface formed at the outer peripheral surface of the crankcase.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power unit for a two-wheeled vehicle coupled in a freely swingable manner to a vehicle frame. In particular, the present invention is directed to improving a power unit provided on an engine with a crankshaft arranged in a lateral direction of the vehicle, a transmission case extending rearward from one end in a horizontal direction of the crankcase of the engine and axially supporting a rear wheel at a rear end part thereof, and a transmission housed in the transmission case coupling the crankshaft of the engine and the rear wheel. 
     2. Background Art 
     Conventionally, this kind of power unit for a two-wheeled vehicle is constructed for single cylinder engines; for example, refer to Japanese Patent Laid-open Publication No. Hei. 4-163287. The main object of the conventional art is to prevent the occurrence of interference of a power unit with a road surface during banking or rocking of the two-wheeled vehicle. This is accomplished by arranging an exhaust purifying device adjoined to an exhaust pipe of the two-wheeled vehicle which is positioned above a central line extending in a longitudinal direction as seen from a side view of the two-wheeled vehicle. Positioned thereunder is the power unit, being of the rockable swing type, such that the lateral placement thereof is sufficiently drawn inward to the body of the two-wheeled vehicle in order to prevent contact with the road surface when the two-wheeled vehicle is banked. 
     However, due to the positioning of the exhaust purifying device adjoined to the exhaust pipe, to allow placement of the power unit close to the frame of the two-wheeled vehicle, the conventional art two-wheeled vehicle is limited to single cylinder engines. Therefore, it is not possible to increase the engine power output by readily adopting a two cylinder engine configuration. Due to the lack of positioning freedom of the cylinders of the two cylinder engine caused by the exhaust purifying device adjoined to the exhaust pipe, the cylinders of the two cylinder engine can not be freely set without interfering with a crankcase of the two cylinder engine. Thus, implementation of a two cylinder engine with the two wheeled vehicle of the conventional art complicates a manufacturing cycle of the vehicle and further complicates assembly of the crankcase and cylinder block of the two cylinder engine. 
     SUMMARY OF THE INVENTION 
     In order the overcome the clear disadvantages of the conventional art, it is the object of the present invention to provide a power unit for a two-wheeled vehicle where the direction of the cylinders can be freely set without interfering with a crankcase of an engine so as to provide greater freedom of design when there is an intent to increase the engine power output by adopting a two cylinder configuration, and where manufacture and assembly of the crankcase and cylinder block are straightforward. 
     In order to achieve the above object, in a first aspect of the present invention, a power unit for a two-wheeled vehicle coupled in a freely swinging manner to a vehicle frame comprises an engine with a crankshaft arranged in a lateral direction of the vehicle, a transmission case extending rearwards from one end in a horizontal direction of the crankcase of the engine and axially supporting a rear wheel at a rear end part thereof, and a transmission housed in the transmission case coupling the crankshaft of the engine and the rear wheel. The crankcase is divided by a partitioning surface passing esentially through a central part of the crankshaft in the axial direction in such a manner that the crankcase comprises a first crankcase half and a second crankcase half. The transmission case is integrally formed with the first crankcase half and a cylinder block having two cylinders running parallel with the horizontal direction of the vehicle, and the cylinder block is joined with a block mounting surface formed at the outer peripheral surface of the crankcase. At the outer peripheral surface of the crankcase, the region forming a mounting block surface for connecting the cylinder block can be made broad without interfering with the transmission case. A degree of freedom can therefore be obtained for designing the angle of attachment of the cylinder block with respect to the crankcase and the power units. 
     In a second aspect of the present invention, journals supported by the crankcase are integrally formed with the crankshaft at just the two left and right ends thereof According to this second aspect, the crankshaft is integrally formed and the simplification of this structure is therefore achieved. Support of the crankshaft can therefore be carried out simultaneously with the joining of the first and second crankcase halves. 
     In addition to the first and second aspects, in a third aspect of the present invention, the transmission case is constructed by joining an inner transmission case half, integrally formed so as to extend from the first crankcase half, and an outer transmission case half positioned opposite the first crankcase half The partitioning surface is offset from a central position between the two cylinders to the side of the inner transmission case half. The width of the connected body of the first crankcase half and the inner transmission case half and the width of the second crankcase half are the same or substantially similar regardless of the inner transmission case being integrally formed with the first crankcase. Casting and cutting processes can therefore be carried out easily because the casting depth and processing depth is not deeper for one or the other of the connected body and the second crankcase. 
     In addition to the first to third aspects, in a fourth aspect of the present invention, an axis of the cylinder positioned on the side of the transmission case coincides with or is in the vicinity of a plane of the central rotation of a rear wheel. According to this fourth aspect, balancing of the weight to the left and right of the power unit can be achieved with respect to the rear wheel. This contributes to improved handling. 
     In addition to any one of the first to fourth aspects, in a fifth aspect of the present invention, the cylinders are arranged in a substantially flat manner with each head thereof facing towards the front of the vehicle. A low center of gravity can be achieved for the power unit. Further, as a broad space can be ensured above the engine, the power unit is appropriate for use in motor scooters and other two-wheeled vehicles with an accessory compartment arranged above the engine. 
     In addition to the first to fifth aspects, in a sixth aspect of the present invention, a timing transmission chamber is formed at the outer side of the cylinder and positioned on the opposite side of the transmission case. The timing transmission chamber houses a timing transmission assembly coupling the crankshaft and a moving cam shaft which is provided at the heads of the cylinders and being housed there. The spacing of the journals at both ends of the crankshaft can be kept to a minimum and the amount of overhang of an output end of the crankshaft linking to a transmission assembly can also be kept to a minimum. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a side view of a motor scooter equipped with the power unit of the present invention; 
     FIG. 2 is a plan view of the power unit; 
     FIG. 3 is an enlarged vertical cross-section of the cylinder block and the crankcase in accordance with the power unit of the present invention; 
     FIG. 4 is an enlarged vertical cross-section of the transmission case in accordance with the power unit of the present invention; 
     FIG. 5 is a cross-section taken along line  5 — 5  of FIG. 3; and 
     FIG. 6 is a cross-section taken along line  6 — 6  of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred example of the present invention is now described based on an embodiment of the present invention shown in the appended drawings. However, the present invention is not limited to the embodiments shown in the figures. It is readily appreciated by those of ordinary skill in the art that the embodiments shown in the figures have been provided to merely illustrate the functionality of the present invention, and therefore should not be construed as limiting the scope of the claimed invention. 
     In the description hereinafter, front and rear, and left and right are taken with respect to the body of the motor scooter illustrated in the attached drawings. 
     As illustrated in FIG. 1, a front fork  4  supporting a front wheel  3   f  is coupled in a steerable manner at the front end of a vehicle frame  2  of a motor scooter  1 . A steering handle  5  is attached to a steering system for the front fork  4 . At the center of the vehicle frame  2 , a power unit  6  supporting a rear wheel  3   r  is coupled in such a manner as to be capable of vertically swinging via a swinging link  7 , with rear shock absorbers  8  being attached to the rear ends of the power unit  6  and the vehicle frame  2 . A step  11 , a seat  12  and a carrier  13  are provided on an outer vehicle plate  10  covering the vehicle frame  2 . An accessory compartment  14 , which is formed within the outer vehicle plate  10 , is positioned directly under the seat  12 . 
     As illustrated in FIG. 2 to FIG. 4, the power unit  6  is equipped with a four cycle two cylinder engine  15  and a transmission  16  for transmitting power from the engine  15  to the rear wheel  3   r . The transmission  16  is housed in a transmission case  18  extending forward from the left-side end of a crankcase  17  of the engine  15 , with the rear wheel  3   r  being supported at the rear end of the transmission case  18 . 
     The engine  15  is equipped with an integrated crankshaft  20  arranged with an axis thereof and extending across the vehicle body, a crankcase  17  for bearing the crankshaft  20 , a cylinder block  21  connected to the outer peripheral surface of the crankcase  17 , a cylinder head  22  connected to the upper surface of the cylinder block  21 , and a carburetor  23  (refer to FIG. 1) mounted on the outside surface of the cylinder head  22 . Two cylinders  24   a  and  24   b  are provided in parallel to the left and right of the cylinder block  21 . Two corresponding cranks  25   a  and  25   b  are formed left and right of the crankshaft  20 , and pistons  26   a  and  26   b  positioned in the cylinders  24   a  and  24   b  are connected to the corresponding cranks  25   a  and  25   b  via connecting rods  27   a  and  27   b . The crankshaft  20  has journals  28   a  and  28   b  at two neighboring places on the outside of the cranks  25   a  and  25   b . These journals  28   a  and  28   b  support the crankcase  17  via bearing members  29   a  and  29   b . Namely, an intermediate axis  20   a  of the crankshaft  20  coupled between the two cranks  25   a  and  25   b  is not supported at the crankcase  17  and therefore has a larger diameter than the journals  28   a  and  28   b  in order to maintain bending rigidity thereof. 
     The crankcase  17  comprises a first crankcase half  17   a , partitioned in a vertical direction by a partitioning surface  30  offset from a central position between the two cylinders  24   a  and  24   b  towards the side of the transmission case  18 , and is positioned at the side of the transmission case  18 . A second crankcase half  17   b  is positioned on the side of the transmission case  18  opposite the first crankcase  17   a . The transmission case  18  comprises an inner transmission case half  18   a , partitioned by a partitioning surface  31  parallel to the partitioning surface  30 , and an outer transmission case half  18   b  positioned on the opposite side thereof. The inner transmission case half  18   a  and the first crankcase half  17   a  are integrally cast together. 
     The first and second crankcase halves  17   a  and  17   b  are connected together by a plurality of bolts (not shown) at peripheral edges thereof, and the inner and outer transmission case halves  18   a  and  18   b  are connected together by a plurality of bolts  32  at the peripheral edges thereof 
     The assembly for the crankcase  17  and the transmission case  18  is divided into three simple steps. First, the assembly of the first half  17   a  with the inner transmission case half  18   a ; second the assembly of the second crankcase half  17   b  with the first half  17   a . Finally the attachment of the outer transmission case half  18   b  with the inner transmission case half  18   a . Subsequently, the number of parts for the assembly is therefore kept to a minimum and the casting of each of the parts is kept straightforward. 
     Since the journals  28   a  and  28   b  supported at the first and second crankcase halves  17   a  and  17   b  are formed with two end points of the integrated crankshaft  20 , supporting of the crankshaft  20  can be carried out simultaneously via the connectivity of the first and second crankcase halves  17   a  and  17   b , regardless of the crankshaft  20  being integrally formed. 
     Further, as illustrated in FIG. 2, the partitioning surface  30 , partitioning the inner transmission case half  18   a , and the first crankcase half  17   a  from the second crankcase half  17   b , is arranged in such a manner as to be offset from a central position between the two left and right cylinders  24   a  and  24   b  to the side of the inner transmission case half  18   a . The width S 1  of the connected body of the first crankcase half  17   a , and the inner transmission case half  18   a , and a width S 2  of the second crankcase half  17   b  are therefore the same or similar. Casting and cutting processes can therefore be carried out easily because the casting depth and processing depth is not deeper for one or the other of the connected body and the second crankcase  17   b.    
     The power unit  6  is arranged in such a manner that an axis Y of the cylinder  24   a , on the side of the transmission case  18  coincides with or is in the proximity of a plane of central rotation C of the rear wheel  3   r . As a result, weight balancing to the left and right of the power unit  6  with respect to the rear wheel  3   r  can be achieved and thus handling can be improved. 
     As illustrated in FIG. 3, FIG.  5  and FIG. 6, a mounting block  35  is formed at the outer peripheral surface of the crankcase  17 , in particular, the front part in the embodiment shown in the drawings. Six stud bolts  36  are embedded in the crankcase  17  in such a manner as to encompass the two cylinders  24   a  and  24   b . These stud bolts  36  are inserted into six bolt holes  37  passing through the cylinder block  21  and the cylinder head  22 . Nuts  38  are then screwed and tightened at the ends of the stud bolts  36  so that the cylinder block  21  and the cylinder head  22  are joined to the crankcase  17 . The head of the cylinder block  21  is arranged substantially horizontally facing towards the front of the motor scooter  1 . 
     Since the mounting block  35  is formed at the outer peripheral surface of the crankcase  17 , this forming region can be kept slightly broader at the transmission case  18 . The angle of attachment of the cylinder block  21  with respect to the crankcase  17  can therefore be selected with a greater degree of freedom, and various kinds of power units compatible with various types of two-wheeled vehicles of different forms can be easily implemented. 
     In particular, as shown in the drawings, the cylinder block  21  is connected at the mounting block  35  formed at the front surface of the crankcase  17 . A low center of gravity is obtained for the power unit  6  when this is arranged in a substantially horizontal manner. In addition, a broad space can be maintained above the engine  15 . It is therefore possible to form a large accessory compartment  14  for housing helmets, etc., above the engine  15 . 
     As illustrated in FIG.  3  and FIG. 5, a timing transmission chamber  40  is formed next to the outer side of the cylinder  24   b  on the opposite side to the transmission case  18  at the cylinder block  21  and the cylinder head  22 . A timing transmission assembly  45  comprising a drive sprocket  41  fixed to the crankshaft  20 , a driven sprocket  43  fixed to a valve cam shaft  42  supported at the cylinder head  22 , and a chain  44  wrapped around sprockets  42  and  43 , is arranged in the timing transmission chamber  40 . A head cover  46  covering a valve assembly including the moving cam shaft  42  is connected at the front surface of the cylinder head  22 . 
     As a result, the spacing of the left and right journals  28   a  and  28   b  of the crankshaft  20  can be kept to a minimum, and the arrangement of the timing transmission assembly  45  can be carried out in a straightforward manner while keeping the extent of overhang of the output end connected to the transmission  16  of the crankshaft  20  to a minimum. 
     An opening  47  for opening the timing transmission chamber  40  is provided at an outer side wall of the second crankcase half  17   b . A rotor  49  of a dynamo  48  is fixed at the right end of the crankshaft  20  protruding outside of the opening  47 . A stator of the dynamo  48  is fixed to an inner wall of a side cap  51  connected to the second crankcase half  17   b  for closing the opening  47 . A large diameter gear  52  driven from a starter motor (not shown) is provided in a freely rotatable manner between the timing transmission assembly  45  and the dynamo  48 . An overlying clutch  53  is interposed between the large diameter gear  52  and the rotor  49 . 
     The transmission  16  is equipped with a variable radius drive pulley  54  attached to the output end of the crankshaft  20 , a variable diameter driven pulley  55  attached to an intermediate transmission shaft  59  supported at the rear end of the inner transmission case half  18   a , and a belt  57  wrapped around the drive and driven pulleys  54  and  55 . A centrifugal transmission mechanism  56  for increasing the diameter of the pulley in response to increases in the rotational speed of the crankshaft  20  is provided at the drive pulley  54 , and a centrifugal clutch  58  which facilitates the connection state in response to increases in the rotational speed of the driven pulley  55  is interposed between the driven pulley  55  and the transmission  18 . 
     A partition wall  60  is joined to the inner wall at the rear end of the inner transmission case half  18   a , so as to define a deceleration chamber  61  therebetween. The intermediate transmission shaft  59  is also supported by this partition wall  60 . Further, a rear wheel drive shaft  63  is supported by the partition wall  60  and the inner transmission case half  18   a , and a gear reduction assembly  62  coupling the intermediate transmission shaft  59  and the rear wheel drive shaft  63  is provided in the deceleration chamber  61 . The rear wheel  3   r  is spline-fitted to the outer end of the rear wheel drive shaft  63  in a detachable manner. 
     When the engine  15  is operating, this power is transmitted to the rear wheel  3   r  from the crankshaft  20  sequentially via the drive pulley  54 , the belt  57 , the driven pulley  55 , the centrifugal clutch  58 , the intermediate transmission shaft  59 , the gear reduction assembly  62  and the rear wheel drive shaft  63 , so as to drive the rear wheel  3   r . During this time, the engine  15  can run at a high output by means of the two cylinders  24   a  and  24   b.    
     A cooling fan  64  is integrally formed at the outer surface of the drive pulley  54 . A cooling air inlet  65  facing the cooling fan  64  is provided at a sidewall of the outer transmission case half  18   b . An air cleaner  66  is installed at the cooling air inlet  65 . When the cooling fan  64  rotates, outside air filtered by the air cleaner  66  enters into the transmission case  18  from the cooling air inlet  65 , and is released to the outside from an exhaust outlet (not shown) after cooling each of the parts of the transmission  16 . 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.