Power unit

A small center distance is provided between a crankshaft and a gearshift shaft disposed in parallel. A power unit is built compactly in a direction of arrangement of the crankshaft and the gearshift shaft for supporting reliably the gearshift shaft on which heavy, first and second gearshift clutches are supported. A transmission includes a main shaft, disposed in parallel with a crankshaft and rotatably supported in a crankcase, and first and second gearshift clutches. The first and second gearshift clutches change a gearshift position in a first gearshift portion and a gearshift position in a second gearshift portion, respectively. The main shaft includes an outer shaft portion that extends from the crankcase and supports the first and second gearshift clutches. The outer shaft portion is supported by a bearing portion disposed in a front cover on a side opposite to the crankcase across the first and second gearshift clutches.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2006-050508 filed on Feb. 27, 2006 the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a power unit that includes an engine and a transmission, in which gears are changed using first and second gearshift clutches.

2. Description of Background Art

A power unit is known that includes a transmission having first and second gearshift clutches. The first and second gearshift clutches transmit or disconnect power from an engine to first and second gearshift portions, respectively. These gearshift clutches achieve a change of gears. See, for example, Japanese Patent Laid-Open No. 2002-357267.

In the power unit having the transmission that includes the first and the second gearshift clutches, the transmission may be arranged to have a gearshift shaft disposed in parallel with an output shaft (e.g., a crankshaft) of the engine in order to build a smaller power unit in an axial direction of the engine output shaft. If, at this time, the gearshift clutches are disposed inside a crankcase that rotatably supports the crankshaft, the gearshift shaft that supports the heavy gearshift clutches is reliably supported by the crankcase at a position of sandwiching the gearshift clutches. Contrary to this advantage of the known art, a center distance between the crankshaft and the gearshift shaft becomes greater so that interference can be avoided between the crankshaft and the gearshift clutches having a relatively large diameter. This results in the power unit becoming larger in a direction of arranging the crankshaft and the gearshift shaft.

SUMMARY AND OBJECTS OF THE INVENTION

The present invention has been made to solve the foregoing problems. In accordance with embodiments of the present invention, it is an object of the embodiments of the present invention to allow a center distance between a crankshaft and a gearshift shaft, that are disposed in parallel with each other, to be kept small. Thus, a power unit can be built compactly in a direction of the arrangement of the crankshaft and the gearshift shaft and the gearshift shaft. In addition, the gearshift is reliably supported on which heavy, first and second gearshift clutches are supported. According to another embodiment of the present invention, operational stability of the first and the second gearshift clutches is improved.

A power unit according to an embodiment of the present invention includes an engine having a crankcase rotatably supporting a crankshaft and a transmission changing the speed of the power of the crankshaft. The transmission includes a gearshift shaft, first and second gearshift portions, a first gearshift clutch, and a second gearshift clutch. The gearshift shaft is disposed in parallel with the crankshaft and rotatably supported in the crankcase. The first and second gearshift portions change a speed of the power from the crankshaft. The first gearshift clutch transmits and disconnects the power relative to the first gearshift portion. The second gearshift clutch transmits and disconnects the power relative to the second gearshift portion. The first and second gearshift clutches change a gearshift position in the first gearshift portion and the second gearshift portion. The gearshift shaft includes an extending shaft portion that extends in one axial direction from the crankcase, which accommodates a crank portion of the crankshaft. The first and second gearshift clutches are supported by the extending shaft portion. Further, the extending shaft portion is supported by a bearing portion on a side opposite to the crankcase across the first and second gearshift clutches.

According to the foregoing arrangements, the first and second gearshift clutches are disposed on an outside of the crankcase. This helps to reduce a center distance between the crankshaft and the gearshift shaft, which are disposed in parallel with each other, while avoiding interference between the first and second gearshift clutches and the crank portion. Moreover, the extending shaft portion, by which the heavy first and second gearshift clutches are supported, is supported by the crankcase and the bearing portion across the first and second gearshift clutches. Thus, shaft runout of a portion of the extending shaft portion on the side of a shaft end portion can be suppressed.

According to an embodiment of the present invention, the bearing portion is disposed at a projecting portion of a cover that cooperates with the crankcase in defining an accommodation chamber, in which the first and second gearshift clutches are accommodated. The projecting portion projects towards the first and second gearshift clutches.

According to this arrangement, the bearing portion is disposed at the projecting portion. Accordingly, the extending shaft portion is supported by the bearing portion at a position closer to the first and second gearshift clutches. As a result, shaft runout of the extending shaft portion on the side near the shaft end portion can be suppressed even more effectively.

According to an embodiment of the present invention, the bearing portion includes an oil path of a hydraulic oil and the hydraulic oil in the oil path is supplied to the first and second gearshift clutches formed from hydraulic clutches through an in-shaft oil path disposed inside the extending shaft portion.

According to this arrangement, thanks to shaft runout being suppressed at the extending shaft portion, a good sealing performance can be easily achieved in the oil path between the bearing portion as a fixing portion and the extending shaft portion, allowing the hydraulic oil to be reliably supplied to the first and second gearshift clutches.

The following effect can be achieved according to an embodiment of the present invention. More specifically, since the center distance between the crankshaft and the gearshift shaft can be made small, the power unit can be built compactly in a direction of the arrangement of the crankshaft and the gearshift shaft. Thus, the extending shaft portion of the gearshift shaft, on which the heavy first and second gearshift clutches are supported, can be reliably supported.

According to an embodiment of the present invention, the extending shaft portion, at which the first and second gearshift clutches are supported, can be even more reliably supported.

According to an embodiment of the present invention, the hydraulic oil can be reliably supplied to the first and second gearshift clutches. This contributes to an improved operational stability of the first and second gearshift clutches.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A specific embodiment to which the present invention is applied will be described below with reference toFIGS. 1 through 6.

Referring toFIGS. 1 and 2(A), a power unit P to which the present invention is applied includes an internal combustion engine E as an engine and a power transmission apparatus. The power transmission apparatus includes a gear-type transmission M as an automatic transmission. The power unit P is mounted on a saddle-riding type vehicle for offroad use as a vehicle. The power transmission apparatus includes the gear-type transmission M (hereinafter referred to as “transmission M”), a centrifugal clutch C, and a drive shaft D. The transmission M is a constant-mesh type. The clutch C forms a starting clutch that transmits or cuts off power generated by the internal combustion engine E relative to the transmission M. The drive shaft D receives power wherein the speed is changed by the transmission M. The power reaching the drive shaft D is transmitted to a front wheel and a rear wheel via a front propeller shaft and a rear propeller shaft, respectively. The wheels are thereby rotatably driven.

The internal combustion engine E is a water-cooled, single-cylinder, four-stroke engine. The engine E has an engine body that includes a cylinder1, a cylinder head and a head cover, and a crankcase2. The cylinder1has a piston3reciprocatingly fitted therewithin. The cylinder head and the head cover are connected, the latter on top of the former, to the cylinder1. The crankcase2serves as a casing connected to a lower end portion of the cylinder1. A crankshaft5has a centerline of rotation L1that is oriented toward a longitudinal direction of a vehicle body. The crankcase2rotatably supports the crankshaft5via a pair of main bearings6,7. The crankcase2includes two case half bodies2a,2bthat are divided into two axially in a direction of the centerline of rotation L1. More specifically, a front case half body2aand a rear case half body2bare connected together to form the crankcase2. The crankcase2forms a crank chamber8, in which a crank portion of the crankshaft5is accommodated. The crank portion contains a crankpin5eand a crank web5f.

The internal combustion engine E includes an intake valve, an exhaust valve, and a valve actuating mechanism. The intake valve and the exhaust valve open and close an intake port and an exhaust port, respectively, disposed in the cylinder head. The valve actuating mechanism, of an overhead valve type, opens and closes, in synchronism with the rotation of the crankshaft5, the intake valve and the exhaust valve using a pushrod driven by a valve actuating cam on a camshaft9. The piston3is driven by a combustion pressure generated when a mixture drawn in through the intake port is burned in a combustion chamber formed between the piston3and the cylinder head. The piston3thereby rotatably drives the crankshaft5via a connecting rod4.

The crankshaft5as an output shaft of the internal combustion engine E includes a front extending portion5aand a rear extending portion5b. The front extending portion5aand the rear extending portion5bextend forwardly and rearwardly, respectively, from the crank chamber8. It should be noted that “front” and “rear” refer to first and second axial directions of the crankshaft5. Accordingly, the “front” and “rear” extending portions may be referred, instead, to as “first” and “second” extending portions, respectively.

A front cover10, which covers the front case half body2afrom a forward direction, is connected to the front case half body2a. The front case half body2aand the front cover10thus cooperate with each other in defining a front accommodation chamber12. The front cover10includes an annular intermediate cover portion10aand a front end cover portion10b. The intermediate cover portion10aforms a peripheral wall of the front accommodation chamber12by being connected to a front end portion of the front case half body2a. The front end cover portion10bforms a peripheral wall of the front accommodation chamber12by being connected to a front end portion which is an open end portion of the intermediate cover portion10a.

The front extending portion5aextending forwardly from the main bearing6retained by the front case half body2aextends in the front accommodation chamber12. A front shaft end portion5cof the front extending portion5ais rotatably supported on the front end cover portion10bvia a bearing14. A rear cover11, covering the rear case half body2bfrom a rearward direction, is, on the other hand, connected to the rear case half body2b. The rear case half body2band the rear cover11cooperate with each other in defining a rear accommodation chamber13. The rear extending portion5bextending rearwardly from the main bearing7retained by the rear case half body2bextends in the rear accommodation chamber13. It is to be noted that the crankcase2and the front and rear covers10,11form a housing of the power unit P.

Referring toFIGS. 1,3, and4, an oil filter140is disposed on a side portion of the power unit P on the right of the front end cover portion10b. Lubricant, which is discharged from an oil pump130and flows past a delivery oil path131, flows through the oil filter140. The oil pump130is disposed at the intermediate cover portion10aand includes a trochoid pump rotatably driven by the power of the crankshaft5. More specifically, the oil filter140includes a filter case141, a cap142, and a filter portion143. The filter case141, of a cylindrical shape, is molded integrally with the front end cover portion10b. The cap142is removably attached to the filter case141so as to close an open end portion141aof the filter case141opening to the right of the filter case141. The filter portion143is fitted to the cap142and accommodated in an accommodation chamber144formed in the filter case141. The filter portion143includes a cylindrically formed filter element143aand a holder143b. The holder143b, which retains the filter element143a, is connected to the cap142.

The lubricant flows into the accommodation chamber144via the delivery oil path131that is disposed across the intermediate cover portion10aand the front end cover portion10band opens toward the accommodation chamber144at a peripheral wall141cof the filter case141. The lubricant is then purified as the lubricant flows past the filter element143a. The lubricant thereafter flows into a main oil path132that opens at a center of a bottom wall141bof the filter case141. Lubricant in the main oil path132is supplied to different lubrication points of the internal combustion engine E and the transmission M via a large number of oil paths. The lubricant is further supplied to a control valve unit71to be described later via an oil path133.

A plurality of projecting ribs145is disposed on an inner peripheral surface having a substantially circular transverse cross section of the peripheral wall141c. The ribs, spaced apart from each other in a circumferential direction, serve as a control portion restricting oscillations of the filter portion143in the accommodation chamber144arising from vibration transmitted to the oil filter140or the like. Each of the plurality of ribs145extends substantially in parallel with a center axis L4of the cylindrical filter case141from a point near the open end portion141ato the bottom wall141b.

There may be cases where the filter portion143is removed together with the cap142from the filter case141for replacement of the filter element143aor the like. If there is not enough space available in a direction parallel with the center axis L4because of the construction surrounding the oil filter140at this time, it is necessary that the filter portion143be removed and inserted in a position inclined to a specific direction relative to the filter case141. If there is a wall or other obstacle present in part of a peripheral direction of the filter case141in this case, the direction in which the filter portion143is inclined is also restricted.

To solve such a problem as that noted above, the ribs145for suppressing vibration at the filter portion143are adapted to include a facilitation means that facilitates removal and insertion of the filter portion143in a specific inclined direction (seeFIG. 4). The facilitation means is formed by eliminating part of the ribs145disposed on a side opposite to a side (or a direction) in which the oil filter140is inclined in the filter case141or keeping an amount of projection of the part of the ribs145small. The facilitation means helps lessen restriction on inclination imposed by abutment between the holder143band the ribs145, allowing the filter portion143to be inclined at a greater angle. In accordance with the embodiment of the present invention, part of the plurality of ribs145to be equally spaced apart from each other, more specifically one of the ribs145, is eliminated. As a result, a spacing between a specific pair of ribs145aadjacent circumferentially is adapted to be greater than a spacing between each of the other pairs of ribs145that are adjacent circumferentially.

The foregoing arrangement allows the filter portion143to be inclined largely during removal and insertion thereof (chain double-dashed lines ofFIG. 4show two conditions during removal.) The arrangement allows for easy removal of the filter portion143even with a small space available in the direction of the center axis L4. Moreover, it is easy to check the position of the filter portion143relative to the filter case141during insertion, allowing the filter portion143to be inserted smoothly in the filter case141. This makes for easy replacement of the filter element143aand other service jobs performed for the oil filter140.

Referring toFIG. 2(A), the centrifugal clutch C, a primary reduction mechanism R, and a drive sprocket15are disposed in that order from the front shaft end portion5con the front extending portion5ain the front accommodation chamber12. The drive sprocket15forms part of the valve actuating transmission mechanism that rotatably drives the camshaft9. An ac generator17and a starting driven gear19are disposed in that order from a rear shaft end portion5d, to which a recoil starter16is connected, on the rear extending portion5bin the rear accommodation chamber13. The driven gear19forms part of a starting reduction mechanism that transmits rotation of a starter motor18mounted to the rear cover11to the crankshaft5. The driven gear19is connected to a rotor17aof the ac generator17via a one-way clutch20.

The centrifugal clutch C includes a plate-like clutch inner21, a cup-like clutch outer22, and a clutch shoe23. The clutch inner21serves as an input member rotating integrally with the crankshaft5. The clutch outer22serves as an output member surrounding the clutch inner21radially outwardly. The clutch shoe23, which is pivotally supported on the clutch inner21, serves as a centrifugal weight that controls a connected or disconnected condition of the centrifugal clutch C using a centrifugal force generated according to a speed of the crankshaft5that is an engine speed. When the engine speed exceeds an idling speed, the clutch shoe23opposes a snapping force of a clutch spring24. Then, the centrifugal force causes the clutch shoe23to swing radially outwardly of the crankshaft5, thus to start to contact the clutch outer22. Power of the internal combustion engine E is thus transmitted from the clutch inner21to the clutch outer22. As the engine speed builds up, the centrifugal clutch C undergoes the following operating stages. More specifically, the clutch C first undergoes a partial engagement stage, in which the clutch outer22rotates with a slight slip over the clutch shoe23. The clutch C then enters a complete engagement stage, in which the clutch inner21and the clutch outer22rotate integrally.

The primary reduction mechanism R includes a drive gear25and a driven gear26in mesh with the drive gear25. The drive gear25is relatively rotatably mounted on the front extending portion5a. Further, the drive gear25is a spline fit in a boss portion of the clutch outer22, being integrally rotatable with the clutch outer22. The driven gear26is relatively rotatably mounted on a first main shaft31. A second main shaft32of the transmission M has a pair of front and rear connection portions26a,26bthat include boss portions sandwiching a disc-like disc portion26con an outer periphery of a first main shaft31and extending in a longitudinal direction. Power is transmitted to first and second gearshift clutches41,42of the transmission M via the front connection portion26aand rear connection portion26b, respectively. Accordingly, the primary reduction mechanism R serves as a transmission mechanism that transmits power from the centrifugal clutch C to the first and the second gearshift clutches41,42.

Referring also toFIG. 5, the transmission M includes a main shaft30, a counter shaft33, an intermediate shaft34, a transmission gear train group M1, a gearshift position selector mechanism M2, and the first and second gearshift clutches41,42. The main shaft30serves as an input shaft, while the counter shaft33serves as an output shaft. The main shaft30and the counter shaft33are rotatably supported by the front case half body2aand the rear case half body2bvia bearings35,36and37,38, respectively. Further the main shaft30and the counter shaft33have centerlines of rotation L2and L3, respectively, each extending in parallel with the centerline of rotation L1. The intermediate shaft34has both ends thereof fixed to the front case half body2aand the rear case half body2b. Further, the intermediate shaft34has a center axis that extends in parallel with the centerline of rotation L1. The transmission gear train group M1is a group of gear trains G1to G5, and GR that set a gearshift position. The gearshift position selector mechanism M2(seeFIG. 6) selects a specific gearshift position to realize a gearshift at a gear ratio a driver desires. The first and second gearshift clutches41,42transmit or disconnect power from the internal combustion engine E relative to the transmission gear train group M1. In the transmission M, the main shaft30, the counter shaft33, the intermediate shaft34, and the drive shaft D are supported by the crankcase2that functions also as a transmission case by being disposed in parallel with the crankshaft5. The main shaft30and the counter shaft33constitute gearshift shafts of the transmission M.

The main shaft30includes the first main shaft31and the second main shaft32, both being disposed across the crank chamber8and the front accommodation chamber12. The first main shaft31is relatively rotatably and coaxially passed through the second main shaft32which is a hollow shaft. The first main shaft31, includes a shaft length longer than the second main shaft32for defining the shaft length of the main shaft30. The first main shaft31includes an inner shaft portion31aand an outer shaft portion31b. The inner shaft portion31ais accommodated in the crank chamber8, while the outer shaft portion31bis accommodated in the front accommodation chamber12. The outer shaft portion31bforms an extending shaft portion that extends through the front accommodation chamber12forward in a first direction in an axial direction from the crankcase2. The outer shaft portion31bis supported rotatably on the front cover10via a bearing39at a front shaft end portion31cas a shaft end portion of the outer shaft portion31b.

The first gearshift clutch41, the driven gear26, and the second gearshift clutch42are disposed sequentially in that order from the front shaft end portion31ctoward the crankcase2and are supported on the outer shaft portion31b. Accordingly, the driven gear26is disposed between the first and second gearshift clutches41,42in an axial direction of the main shaft30. The first and second gearshift clutches41,42are mounted on the outer shaft portion31bvia the boss portions of the driven gear26. A rear shaft end portion31dof the first main shaft31is supported by the rear case half body2b.

The outer shaft portion31bis supported in a bearing portion150via a bearing39on a side opposite axially to the front case half body2aacross the first and second gearshift clutches41,42and at the front shaft end portion31c. The bearing portion150is disposed in an inner projecting portion151bthat axially projects toward the first and second gearshift clutches41,42from an inner surface of the front end cover portion10bat a projection portion151integrally formed at the front end cover portion10b.

The projection portion151includes an outer projecting portion151athat projects forwardly from an outer surface of the front end cover portion10band the inner projecting portion151b. The outer projecting portion151aincludes oil paths67a,68ato be described later. The bearing39, including a needle bearing supporting the front shaft end portion31c, is retained in the inner projecting portion151bthat axially projects to a position overlapping the centrifugal clutch C.

Referring to the counter shaft33having a front shaft end portion33asupported by a bearing37, an output drive gear29ais disposed on a rear shaft end portion33bprojecting rearwardly from a bearing38and extending in the rear accommodation chamber13. The output drive gear29adrives the drive shaft D that is rotatably supported by the front case half body2aand the rear case half body2bvia bearings27,28. The output drive gear29aand an output driven gear29bdisposed on the drive shaft D constitute a secondary reduction mechanism29. The secondary reduction mechanism29serves as a transmission mechanism transmitting power from the transmission M, with a speed thereof reduced, to the drive shaft D. The secondary reduction mechanism29is accommodated in the rear accommodation chamber13.

Referring toFIGS. 2(A) and 5, the transmission gear train group M1as a group of gearshift elements accommodated in the crank chamber8function also as a transmission chamber and include a group of gearshift gears including a predetermined plurality of gearshift gear trains G1to G5, and GR. The group of gearshift gears includes those gears mounted on the main shaft30, the counter shaft33, and the intermediate shaft34. More specifically, the predetermined plurality of drive gears are disposed on the main shaft30, that is, drive gears43to48according to the embodiment of the present invention, the predetermined plurality of driven gears are disposed on the counter shaft33, that is, driven gears53to58and intermediate gears49,50as reduction gears are disposed on the intermediate shaft34.

A first speed gear train G1that sets a first speed gearshift position includes the drive gear43and the driven gear53. The drive gear43is disposed relatively rotatably on the inner shaft portion31a. The driven gear53, which meshes with the drive gear43, is disposed integrally rotatably on the counter shaft33. A second speed gear train G2that sets a second speed gearshift position includes the drive gear44and the driven gear54. The drive gear44is formed integrally with and disposed integrally rotatably on the second main shaft32. The driven gear54, which meshes with the drive gear44, is disposed relatively rotatably on the counter shaft33. A third speed gear train G3that sets a third speed gearshift position includes the drive gear45and the driven gear55. The drive gear45is disposed relatively rotatably on the inner shaft portion31a. The driven gear55, which meshes with the drive gear45, is disposed integrally rotatably on the counter shaft33. A fourth speed gear train G4that sets a fourth speed gearshift position includes the drive gear46and the driven gear56. The drive gear46is formed integrally with and disposed integrally rotatably on the second main shaft32. The driven gear56, which meshes with the drive gear46, is disposed relatively rotatably on the counter shaft33. A fifth speed gear train G5that sets a fifth speed gearshift position includes the drive gear47and the driven gear57. The drive gear47is disposed integrally rotatably on the inner shaft portion31a. The driven gear57, which meshes with the drive gear47, is disposed relatively rotatably on the counter shaft33. A reverse gear train GR that sets a reverse gearshift position includes the drive gear48, the driven gear58, a first intermediate gear49, and a second intermediate gear50. The drive gear48is formed integrally with the drive gear44. The driven gear58is disposed relatively rotatably on the counter shaft33. The first intermediate gear49, which meshes with the drive gear48, is disposed relatively rotatably on the intermediate shaft34. The second intermediate gear50, which meshes with the driven gear58, is integrally formed and rotatable with the first intermediate gear49.

The gear trains G1, G3, and G5having the drive gears43,45, and47, respectively, disposed on the first main shaft31constitute a first transmission portion for changing the speed of power of the internal combustion engine E. The first gearshift clutch41transmits or disconnects the power to the first transmission portion. The gear trains G2, G4, and GR having the drive gears44,46,48, respectively, disposed on the second main shaft32constitute a second transmission portion that changes the speed of power of the internal combustion engine E. The second gearshift clutch42transmits or disconnects the power to the second transmission portion.

The first gearshift clutch41is disposed adjacent to the centrifugal clutch C and is closer axially to the front case half body2athan the centrifugal clutch C. The first gearshift clutch41is integrally rotatably connected through a spline fit to the front connection portion26aon an input side of the power of the internal combustion engine E transmitted via the centrifugal clutch C and the primary reduction mechanism R. Further, the first gearshift clutch41is integrally rotatably connected through a spline fit to the outer shaft portion31bon an output side of the power to the first main shaft31.

The second gearshift clutch42, which is disposed on a side opposite axially to the first gearshift clutch41across the driven gear26, is integrally rotatably connected through a spline fit to the rear connection portion26bon an input side of the power of the internal combustion engine E transmitted via the centrifugal clutch C and the primary reduction mechanism R. Further, the second gearshift clutch42is integrally rotatably connected through a spline fit to a front shaft end portion32athat projects forwardly from a bearing35and extends in the front accommodation chamber12on an output side of the power to the second main shaft32.

The two gearshift clutches41,42are a hydraulic type multiple disc friction clutch having the same construction. Each of the gearshift clutches41,42includes a cup-like clutch outer60, a plurality of first clutch plates62, a plurality of second clutch plates63, a clutch inner61, and a piston64. The clutch outer60serves as an input member disposed integrally rotatably on an outer periphery of the front connection portion26aor the rear connection portion26bthrough a spline fit. The first clutch plates62mesh integrally rotatably with the clutch outer60. Each of the plurality of second clutch plates63is disposed alternately between two adjacent ones of the pluralities of first clutch plates62. The clutch inner61serves as an output member that meshes integrally rotatably with the second clutch plates63. The piston64is slidably fit in the clutch outer60so as to press such that the first clutch plates62and the second clutch plates63contact each other.

The clutch outer60and the piston64form each of hydraulic chambers65,66for the gearshift clutches41,42. The hydraulic chamber65of the first gearshift clutch41is disposed closer axially to the second gearshift clutch42. The hydraulic chamber66of the second gearshift clutch42is disposed closer axially to the first gearshift clutch41. The hydraulic chambers65,66are controlled for pressure through supply and discharge of hydraulic oil relative to the hydraulic chambers65,66through the oil paths67a,68adisposed in the front end cover portion10band in-shaft oil paths67c,68cdisposed in the outer shaft portion31b. When a pressure in the hydraulic chambers65,66builds up to a high level, the gearshift clutches41,42are brought to a connected state. Specifically, the piston opposes a snapping force of a return spring69to press the first and second clutch plates62,63. Then, friction between the first and second clutch plates62,63causes the clutch outer60and the clutch inner61to rotate integrally with each other. When the pressure in the hydraulic chambers65,66becomes low, the gearshift clutches are brought to a disconnected state. More specifically, the snapping force of the return spring69separates the clutch plates62,63. This disconnects transmission of power between the clutch outer60and the clutch inner61.

A hydraulic pressure control apparatus controls the pressure of the hydraulic oil in the hydraulic chambers65,66. The hydraulic pressure control apparatus includes the control valve unit71that controls the pressure of the hydraulic oil serving as a lubricant introduced through the oil path133, of the lubricant delivered from the oil pump130(seeFIG. 4) as a hydraulic oil source, thereby controlling the pressure of each of the hydraulic chambers65,66. The control valve unit71includes a valve housing71a(seeFIG. 1) and a plurality of hydraulic pressure control valves. The valve housing71ais mounted on the front end cover portion10b. The hydraulic pressure control valves are housed in the valve housing71a. An electronic control unit70controls the hydraulic pressure control valves such that the supply and discharge of the hydraulic oil relative to the hydraulic chambers65,66is controlled through an oil supply path having the in-shaft oil paths67c,68cdisposed in the first main shaft31. Disconnection and connection, that is, disconnected and connected states of each of the gearshift clutches41,42are thereby controlled.

Referring also toFIG. 2(B), the oil supply path disposed across the inner projecting portion151bas a fixing member and the outer shaft portion31bas a rotational member includes the oil paths67a,68a, oil paths67b,68b, and the in-shaft oil paths67c,68c. More specifically, the oil paths67a,68aare disposed in the outer projecting portion151a, into which the hydraulic oil controlled by the hydraulic pressure control valves is guided. The oil paths67b,68bare formed by guide pipes152,153inserted and fixed in the outer projecting portion151aand the inner projecting portion151b. The in-shaft oil paths67c,68care disposed inside the outer shaft portion31b. Each of the guide pipes152,153is disposed coaxially about the centerline of rotation L2and inserted in the outer shaft portion31b. Annular sealing members154,155are mounted between each of the guide pipes152,153and the outer shaft portion31b.

In the first gearshift clutch41, the power from the primary reduction mechanism R is transmitted to the clutch outer60, while the clutch inner61transmits the power to the first main shaft31. In the second gearshift clutch42, on the other hand, the power from the primary reduction mechanism R is transmitted to the clutch outer60, while the clutch inner61transmits the power to the second main shaft32.

The first and second gearshift clutches41,42are disposed between the front case half body2aand the centrifugal clutch C in the axial direction. A connection portion22aof the clutch outer22of the centrifugal clutch C with the drive gear25overlaps substantially entirely with the second gearshift clutch42in the axial direction. The clutch inner61of the second gearshift clutch42is disposed axially near the front case half body2a, being in contact axially with the bearing35. Further, the clutch outer60, which is a member on the side of the driven gear26in the first and second gearshift clutches41,42, is disposed near the disc portion26cof the driven gear26in the axial direction. In addition, the first and second gearshift clutches41,42and the driven gear26overlap with the crankpin5eand the crank web5fin a radial direction of the main shaft30.

Referring also toFIG. 1, the first and second gearshift clutches41,42are disposed at a position, at which the clutches41,42overlap with the centrifugal clutch C as viewed from the axial direction. In a radial direction of the front extending portion5a, the clutch outer60, as a member having the largest outside diameter in the second gearshift clutch42, is disposed near the connection portion22a. The clutch outer22, as a member having the largest outside diameter in the centrifugal clutch C, is disposed near the outer shaft portion31b. Further, as viewed from the axial direction, the clutch outer22of the centrifugal clutch C is radially disposed at a position overlapping with an inner peripheral portion of the clutch outer60or an inner peripheral portion of the piston64.

Referring toFIGS. 2(A),5, and6, the gearshift position selector mechanism M2includes a plurality of shifters81to83, a plurality of shift forks84to86, a shift drum90, an intermittent feed mechanism100, and an electric motor120. More specifically, the plurality of shifters81to83(there are three according to the embodiment of the present invention) is axially movable on the main shaft30or the counter shaft33so as to set the gear trains G1to G5, GR in a connected state, in which the gear train is integrally rotatable with the main shaft30or the counter shaft33, or a disconnected state, in which the gear train is relatively rotatable with the main shaft30or the counter shaft33, thereby allowing a gearshift position to be selected. The plurality of shift forks84to86, which corresponds in number with the shifters81to83and there are three according to the embodiment of the present invention, is axially slidably supported on a support shaft87that is supported in the front case half body2aand the rear case half body2bso as to move the shifters81to83. The shift drum90includes cam grooves91to93formed on an outer peripheral surface thereof. The cam grooves91to93serve as guide portions for guiding and moving each of the shifters81to83by guiding and moving axially a corresponding one of the shift forks84to86. The intermittent feed mechanism100intermittently rotates the shift drum90. The electric motor120, capable of rotating backward, operates the intermittent feed mechanism100based on the operating conditions of the vehicle.

The three shifters81to83having projections to be engaged with gears making up the gear trains G1to G5, GR include a first shifter81, a second shifter82, and a third shifter83. The first shifter81is for selecting a first or third speed. The second shifter82is for selecting a fifth or reverse speed. The third shifter83is for selecting a second or fourth speed. The shift forks84to86include a first shift fork84engaged with the first shifter81, a second shift fork85engaged with the second shifter82, and a third shift fork86engaged with the third shifter83. Each of the shift forks84to86, which moves by being guided along a corresponding one of the cam grooves91to93according to rotation of the shift drum90, moves a corresponding one of the shifters81to83to a selected position achieving the connected state or a neutral position achieving the disconnected state.

The first shifter81, which includes the drive gear47functioning also as a shifter, is axially movably disposed in a spline fit in the inner shaft portion31abetween the drive gear43and the drive gear45. When the drive gear47(the first shifter81) moves rearwardly to a selected position and is engaged with the drive gear43, the first speed gear train G1(first speed gearshift position) is selected. When the drive gear47moves forwardly to another selected position and is engaged with the drive gear45, the third speed gear train G3(third speed gearshift position) is selected.

The second shifter82, which includes the driven gear55functioning also as a shifter, is axially movably disposed in a spline fit in the counter shaft33between the driven gear57and the driven gear58. When the driven gear55(the second shifter82) moves rearwardly to a selected position and is engaged with the driven gear57, the fifth speed gear train G5(fifth speed gearshift position) is selected. When the driven gear55moves forwardly to another selected position and is engaged with the driven gear58, the reverse gear train GR for reversing the vehicle is selected.

The third shifter83is axially movably disposed in a spline fit in the counter shaft33between the driven gear54and the driven gear56. When the third shifter83is engaged with the driven gear54, the second speed gear train G2is selected. When the third shifter83moves forwardly to another selected position and is engaged with the driven gear56, the fourth speed gear train G4(fourth speed gearshift position) is selected.

When each of the shifters81to83occupies a neutral position, none of the gear trains G1to G5, GR (gearshift position) is selected, leaving the transmission M in a neutral position.

Referring toFIG. 6, the shift drum90having a centerline of rotation that extends in parallel with the centerlines of rotation L1to L3is rotated forward or backward intermittently by the intermittent feed mechanism100. The intermittent feed mechanism100includes a shift spindle101rotatably driven by the electric motor120and a shifter plate102that is to be engaged with a plurality of feed pins103integrated with the shift drum90. The shift drum90thereby occupies the aforementioned predetermined number of, that is, six according to the embodiment of the present invention, angular positions that establish the gearshift positions of the transmission M. In response to rotation of the shift drum90, each of the shift forks84to86is guided along the corresponding one of the cam grooves91to93to move axially. This allows an alternative selection of each of the gear trains G1to G5, GR corresponding to each of the aforementioned predetermined angular positions to be made. Each of the predetermined angular positions is detected by an angular position detector72including a potentiometer. In addition, the rear cover11includes an angular position detector73including a potentiometer mounted thereon, the detector73detecting an angular position of the shift spindle101.

The electronic control unit70, which controls the connected and disconnected state of the first and second gearshift clutches41,42and the amount and direction of rotation of the electric motor120, receives inputs of signals from an operating state detection means74and the two angular position detectors72,73. The operating state detection means74detects the operating condition of the internal combustion engine E and the vehicle. The operating state detection means74includes a vehicle speed detection means74aand an accelerator opening detection means74bthat detects load on the internal combustion engine E. Based on the signal from the operating state detection means74, the electronic control unit70rotatably drives the shift spindle101, thereby automatically controlling the gearshift position of the transmission M according to the operating condition. Further, the electronic control unit70provides a feedback control of the angular position of the shift spindle101based on the angular position detected by the angular position detector73.

The operation and effects of the embodiment of the present invention as described heretofore is described below.

The main shaft30as the gearshift shaft of the transmission M includes the outer shaft portion31bthat extends from the crankcase2accommodating the crank portion of the crankshaft5. The first and second gearshift clutches41,42and the driven gear26are supported by the outer shaft portion31b. The outer shaft portion31bis supported by the bearing portion150on the side opposite to the crankcase2across the first and second gearshift clutches41,42. Accordingly, the first and second gearshift clutches41,42are disposed on an outside of the crankcase2. Consequently, the center distance between the crankshaft5and the main shaft30, which are disposed in parallel with each other, can be made small, while avoiding interference between the first and second gearshift clutches41,42and the crank portion. Moreover, the outer shaft portion31b, which supports the heavy first and second gearshift clutches41,42, and the driven gear26, is supported by the crankcase2and the bearing portion150across the first and second gearshift clutches41,42. This suppresses shaft runout at a portion of the outer shaft portion31bnear the front shaft end portion31c. As a result, the center distance between the crankshaft5and the main shaft30can be made small, which allows the power unit P to be built compactly in a direction of arrangement of the crankshaft5and the main shaft30. Further, the outer shaft portion31bof the main shaft30, on which the heavy first and second gearshift clutches41,42are supported, can be reliably supported.

It is to be noted herein that the direction of arrangement refers to a direction, in which the main shaft30is disposed relative to the crankshaft5. More specifically, the direction refers to a direction that is orthogonal to the centerlines of rotation L1, L2in a plane including the centerlines of rotation L1, L2.

The bearing portion150is disposed at the inner projecting portion151bof the front cover10that cooperates with the crankcase2in defining the front accommodation chamber12, in which the first and second gearshift clutches41,42are accommodated. The inner projecting portion151bprojects toward the first and second gearshift clutches41,42, by which the bearing portion150is disposed at the inner projecting portion151b. Accordingly, the outer shaft portion31bis supported by the bearing portion150at a position closer to the first and second gearshift clutches41,42. Shaft runout of the outer shaft portion31bat a portion on the side of the front shaft end portion31ccan therefore be suppressed even more effectively. Thus, the outer shaft portion31bcan be even more reliably supported.

The bearing portion150includes the oil paths for hydraulic oil. The hydraulic oil in the oil paths is supplied to the first and second gearshift clutches41,42through the in-shaft oil paths67c,68cdisposed in the outer shaft portion31b. Further, shaft runout is suppressed in the outer shaft portion31b. Accordingly, it becomes easier to ensure good sealing performance of the oil path between the bearing portion150as a fixing member and the outer shaft portion31bas a rotational member. The hydraulic oil is therefore reliably supplied to the first and second gearshift clutches41,42. This contributes to the improved operational stability of the first and second gearshift clutches41,42.

Another embodiment of the present invention having some parts of the first embodiment of the present invention modified will be described.

The transmission may be arranged such that the first and second gearshift clutches are supported by the counter shaft. The transmission may also be a manual type. The internal combustion engine may even be a multi-cylinder internal combustion engine.