Engine starter mechanism arrangement for snow vehicles

An engine starter mechanism arrangement for snow vehicles, in which a starter mechanism is disposed so as to maintain excellent traveling stability of the snow vehicle. A crankshaft (7) extends through an engine (2) in a transverse direction of a body frame. A rear balancer shaft (62) is disposed in a rear part of the engine at a location rearward of the crankshaft in a manner extending substantially parallel to the crankshaft. A starter motor (65) is disposed in the engine at a location above the rear balancer shaft.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an engine starter mechanism arrangement for snow vehicles with a four-cycle engine installed thereon.

2. Description of the Related Art

In recent years, more and more four-cycle engines have come to be used for snow vehicles, and in snow vehicles with a four-cycle engine installed thereon, it is desired to lay out such that the overall height of the engine is made low so as to avoid interference between an engine hood or the optical path of a head light and the engine, and that the center of gravity of the engine is as closest as possible to the center of the vehicle body so as to improve traveling stability of the vehicle. For this reason, the engine is tilted rearward, for example.

Further, in the case where the engine is provided with a starter mechanism including a starter motor large in weight and volume, it is desired to lay out the starter mechanism in consideration of weight distribution of component parts thereof, and so as to avoid interference thereof with other component parts, such as a steering post, and to make effective use of space.

For example, in Japanese Laid-Open Patent Publication (Kokai) No. 2001-199380, a forward-tilted engine has been proposed in which a starter motor is disposed on a flywheel side of the engine opposite from an alternator to suppress the overall height of the engine. Also in Japanese Laid-Open Patent Publications (Kokai) No. 2002-201954, No. 2001-280111, and No. H05-149220, various designs have been proposed as to the disposition of a starter motor in the engine.

However, when a starter motor is disposed together with a clutch mechanism of a continuously variable transmission or the like, an oil pump, a balancer shaft, a magnet device, or other like components, it is important to dispose the starter motor with the relationship of the starter motor with these component parts, including weight distribution and positional relationship comprehensively taken into consideration, so as to improve traveling stability of the vehicle and suppress an increase in the size of the engine.

Further, if a starter-related component part, such as a starter limiter or a starter gear associated with the starter motor largely protrudes outward from the engine, not only space saving but also installment of the engine is hindered, and the degree of freedom in the layout of engine components within the engine room is limited. Therefore, it is necessary to consider the disposition of the starter motor together with the arrangement of the starter-related components.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide an engine starter mechanism arrangement for snow vehicles, in which a starter mechanism is disposed so as to maintain excellent traveling stability of the snow vehicle.

It is a second object of the present invention to provide an engine starter mechanism arrangement for snow vehicles, in which a starter mechanism is disposed so as to suppress an increase in the size of the engine.

To attain the above first object, in a first aspect of the present invention, there is provided an engine starter mechanism arrangement for a snow vehicle having a vehicle body, comprising an engine (2) having a rear part, a crankshaft (7) extending through the engine in a transverse direction of the vehicle body, a balancer shaft (62) disposed in the rear part of the engine at a location rearward of the crankshaft and extending substantially parallel to the crankshaft, and a starter motor (65) disposed in the engine at a location above the balancer shaft.

With this arrangement of the engine starter mechanism arrangement according to the first aspect of the present invention, the starter mechanism is disposed so as to maintain excellent traveling stability of the snow vehicle with the starter motor disposed close to the center of the vehicle body. Further, it is possible to suppress the influence of traveling wind on the starter mechanism.

Preferably, the engine starter mechanism arrangement further comprises an engine room that accommodates the engine, and a clutch mechanism (38) disposed in the engine room on one side thereof with respect to a transverse center (CL) of the vehicle body, and the starter motor is disposed on an opposite side of the engine from a side thereof on which the clutch mechanism is disposed, with respect to a transverse center (ECL) of the engine.

To attain the above first object, in a second aspect of the present invention, there is provided an engine starter mechanism arrangement for a snow vehicle having a vehicle body, comprising a crankshaft (7) extending in a transverse direction of the vehicle body, at least one crankcase (5,6) having a rear part and rotatably supporting the crankshaft, at least one cylinder head (4) disposed above the crankcase, and a starter motor (65) disposed in the rear part of the crankcase, wherein the starter motor is disposed at a location such that when a first imaginary plane (PL1) parallel to the transverse direction of the vehicle body is brought into contact with the cylinder head and the crankcase from a rear side thereof, the starter motor does not project rearward from the first imaginary plane.

With this arrangement of the engine starter mechanism arrangement according to the second aspect of the present invention, the starter mechanism is disposed so as to maintain excellent traveling stability of the snow vehicle with the starter motor disposed close to the center of the vehicle body. Further, it is possible to suppress an increase in the size of the engine.

Preferably, the crankshaft has one end (7b), and the engine starter mechanism arrangement further comprises a starter clutch section (70) disposed on a side of the crankshaft toward the one end thereof, and a transmission mechanism (68,69) disposed between the starter motor and the starter clutch section, for transmitting rotation of the starter motor to the starter clutch section, the transmission mechanism being disposed at a location such that when a second imaginary plane (PL2) parallel to the transverse direction of the vehicle body is brought into contact with the starter motor and the crankcase from the rear side thereof, the transmission mechanism does not project rearward from the second imaginary plane.

Preferably, the crankshaft has one end (7b), and the engine starter mechanism arrangement further comprises a starter clutch section (70) disposed on a side of the crankshaft toward the one end thereof, a transmission mechanism (68,69) disposed between the starter motor and the starter clutch section, for transmitting rotation of the starter motor to the starter clutch section, a magnet cover (60) fixed to the crankcase on the side of the crankshaft toward the one end thereof, and at least one oil pump (FEP, SCP) disposed within the magnet cover, the transmission mechanism being disposed in the magnet cover on a substantially opposite side thereof with respect to the crankshaft, from the oil pump, as viewed in an axial direction of the crankshaft.

To attain the above second object, in a third aspect of the present invention, there is provided an engine starter mechanism arrangement for a snow vehicle having a vehicle body, comprising an engine (2) having a side part, an engine room (30) that accommodates the engine, a crankshaft (7) having opposite ends and extending through the engine in a transverse direction of the vehicle body, a clutch mechanism (38) disposed in the engine room on one side thereof with respect to a transverse center of the vehicle body, a magnet device (MG) disposed in the side part of the engine on an opposite side of the engine room from the one side thereof with respect to the transverse center of the vehicle body, a battery (51) disposed in the engine room at a location close to the magnet device, on the opposite side of the engine room with respect to the transverse center of the vehicle body, a starter motor (65) disposed on an opposite side of the engine from a side thereof on which the clutch mechanism is disposed, with respect to a transverse center of the engine, and a starter clutch section (70) mounted at a location toward one of the opposite ends of the crankshaft on the opposite side of the engine room with respect to the transverse center of the vehicle body, substantially in concentricity with the crankshaft, the starter clutch section comprising a starter one-way clutch (72), and a starter one-way gear (71) that transmits a driving force from the starter motor to the starter one-way clutch, wherein the starter one-way clutch is secured to the magnet device, and the driving force from the starter motor is transmitted to the crankshaft via the starter one-way gear, the starter one-way clutch, and the magnet device.

With this arrangement of the engine starter mechanism arrangement according to the third aspect of the present invention, the starter mechanism is disposed so as to maintain excellent traveling stability of the vehicle, with the starter motor, the battery, and the magnet device being arranged in a concentrated manner on an opposite side of the engine from a side thereof on which the clutch mechanism is disposed, to thereby distribute the weights of the component elements in a well-balanced manner. Further, it is also possible to achieve more simplified wiring in the engine room to thereby simplify the construction of the engine and save space within the engine room.

To attain the above second object, in a fourth aspect of the present invention, there is provided an engine starter mechanism arrangement for a snow vehicle, comprising a starter motor (65), a crankshaft (7) having one end, a magnet device (MG) disposed at a location toward the one end of the crankshaft, a cam sprocket (73) mounted on the one end of the crankshaft, a cam chain (74) passed over the cam sprocket, and a starter clutch section (70) mounted at a location toward the one end of the crankshaft, substantially in concentricity with the crankshaft, the starter clutch section comprising a starter one-way clutch (72), and a starter one-way gear (71) that transmits a driving force from the starter motor to the starter one-way clutch, wherein the starter clutch section is disposed between the cam sprocket and the magnet device, the starter one-way clutch is secured to the magnet device, and the driving force from the starter motor is transmitted to the crankshaft via the starter one-way gear, the starter one-way clutch, and the magnet device.

With this arrangement of the engine starter mechanism arrangement according to the fourth aspect of the present invention, the starter mechanism is disposed so as to suppress an increase in the size of the engine with a simplified construction thereof, such that the driving force from the starter motor is transmitted to the crankshaft while achieving reduced engine space.

To attain the above second object, in a fifth aspect of the present invention, there is provided an engine starter mechanism arrangement for a snow vehicle having a vehicle body, comprising at least one cylinder (94), a crankshaft (7) having one end and extending in a transverse direction of the vehicle body, at a location below the cylinder, a starter motor (65), at least one balancer shaft (62) disposed at least one of locations forward and rearward of the crankshaft and extending substantially parallel to the crankshaft, a starter clutch section (70) mounted at a location toward the one end of the crankshaft, substantially in concentricity therewith, and a starter limiter (68) and a starter gear (69) that cooperate with each other to transmit a driving force from the starter motor to the starter clutch section, wherein the starter gear, the starter limiter, and the starter motor are arranged substantially linearly, as viewed in an axial direction of the crankshaft, in a mentioned order from below, substantially along a central axis of the cylinder at a location above one of the at least one balancer shaft.

With this arrangement of the engine starter mechanism arrangement according to the fifth aspect of the present invention, the engine starter mechanism is disposed so as to suppress an increase in the size of the engine by making effective use of space on a side of the cylinders toward the balancer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described in detail below with reference to the accompanying drawings showing a preferred embodiment thereof.

FIG. 1is a side view showing a snow vehicle with an engine starter mechanism arrangement according to an embodiment of the present invention.FIG. 2is a perspective view showing the interior of an engine room as viewed from a lateral side of the vehicle.FIG. 3is a perspective view showing the interior of the engine room as viewed from the top side of the vehicle.

The snow vehicle1has a two-cylinder four-cycle engine (hereinafter simply referred to as “the engine”)2installed in the engine room30, referred to hereinafter. In the following description, the terms “front”, “rear”, “left”, “right”, “forward”, “rearward”, and “transverse” related to the snow vehicle1will be used as referring to respective directions defined with reference to the position of a driver on a driver's seat.

First, a description will be given of the whole construction of the snow vehicle1.

Referring first toFIG. 1, the snow vehicle1includes a body frame10extending in forward and rearward directions or running direction of the vehicle1, a pair of left and right steering sleds13horizontally movably disposed under a front part (hereinafter referred to as “the front frame section”)10aof the body frame10, and a driving crawler16for circulating a track belt15disposed under a rear part (hereinafter referred to as “the rear frame section”)10bof the body frame10. The crawler16includes a pair of left and right drive wheels17disposed in the vicinity of a front end of the rear frame section10b, a pair of left and right driven wheels18disposed in the vicinity of a rear end of the rear frame section10b, a pair of left and right intermediate wheels19, a suspension mechanism20that suspends and cushions these pairs of wheels, and the track belt15stretched over the wheels for turning around them.

The body frame10has a monocock structure. The front frame section10ain which the engine2is installed is shaped generally like a boat bottom which progressively narrows toward the front in plan view, with an open top, and the front frame section10ais covered with an engine hood29from above.

The front frame section10ahas a front part thereof formed as a sled housing section41protruding upward. The sled housing section41accommodates a suspension and steering mechanism42. Further, a track housing11, not shown, that accommodates a front part (above the drive wheel17) of the crawler16is formed continuously and integrally with the rear frame section10b.

The rear frame section10balso plays the role of a cover accommodating the whole crawler16as viewed from above. A cradle-shaped seat22is disposed above the rear frame section10b, and on opposite lateral sides of the seat22in a transverse direction of the vehicle body (body frame10), there are provided running boards23(left and right running boards23L and23R) which are one step lower than the seat22(refer toFIG. 3). At an approximately central location in the transverse direction of the vehicle body between the seat22and the front frame section10a, a steering post25is erected in a fashion extending rearwardly aslant, and a steering bar or handle bar26horizontally extends from the upper end of the steering post25in the transverse direction. The steering sleds13are operated via the steering post25by the steering bar26.

In the vicinity of the steering bar26and in front thereof, an instrument panel27is provided. Further, a wind shield28extends aslant in front of the instrument panel27with its upper edge rearwardly located so as to cover the front side of the instrument panel27over the entire transverse size thereof. The engine hood29gently slopes down in generally streamlined fashion and is shaped generally like a boat bottom upside down. In the vicinity of a stepped border between the engine hood29and the instrument panel27, a headlight31is disposed for illuminating ahead of the vehicle. The engine room30is defined under the instrument panel27and the engine hood29thus arranged.

Next, a description will be given of the construction of the engine2installed in the engine room30.

Referring toFIG. 2, the engine2is a two-cylinder four-cycle engine with cylinder heads4thereof located on a top side thereof, and is disposed close to the steering post25. The engine2has a crankshaft7extending substantially parallel to the transverse direction of the vehicle body (refer toFIG. 3), and is disposed with the cylinder heads4tilted rearward. With this disposition, the overall height of the engine is made so low that the engine hood29does not block an illumination light path from the headlight31.

As shown inFIG. 3, a clutch mechanism38is disposed at a location leftward of the crankshaft7and in a left side part of the engine room30. The clutch mechanism38is implemented e.g. by a V-belt continuously variable transmission with a centrifugal clutch incorporated therein, and is configured to transmit a driving force to the track belt15via its gears on the driven side.

As shown inFIG. 2, at a location in the upper part of the engine2and slightly rearward of the head cover8, there is disposed a throttle body57. An exhaust passage36extends in a front part of the engine2, and an exhaust muffler, not shown, is connected to the exhaust passage36. Thus, the snow vehicle1has the front side of the engine2as an exhaust side and the rear side of the engine2as an intake side.

As shown inFIGS. 2 and 3, a battery51is disposed on the right side of the engine2. Further, in a rear part of the engine room30, there is disposed a fuel tank55at a location rearward of the steering post25. An oil tank59is disposed in a rear right-hand part of the engine room30.

The snow vehicle1employs a dry sump lubrication system, and, as described in detail hereinafter, the engine2is provided with a feed oil pump (hereinafter referred to as “the oil pump”) FEP for feeding lubricating oil, and a scavenging oil pump (hereinafter referred to as “the oil pump”) SCP for recovering lubricating oil. As shown inFIG. 2, there are provided an oil pipe53for returning lubricating oil from the oil pump SCP to the oil tank59, and an oil pipe54for feeding lubricating oil to the oil pump FEP.

FIG. 4is a front view showing the engine2.FIG. 5is a rear view showing the same.FIG. 6is a right-side view showing the engine2, andFIG. 7a left-side view showing the same.

As shown inFIGS. 4 and 5, the crankshaft7is rotatably supported between an upper crankcase5, which serves as a cylinder block, and a lower crankcase6, and disposed such that an output shaft (PTO)7aof the crankshaft7extends leftward. The upper crankcase5is disposed under the cylinder heads4, and an oil pan49is disposed under the lower crankcase6. Further, on the right side of the engine2, there are provided a water pump58and a magnet cover60. The magnet cover60is fixed to right-side parts of the upper crankcase5and the lower crankcase6. The head cover8has a right part (left part as viewed inFIG. 4) thereof forming a cam chain chamber66.

As shown inFIG. 7, a front balancer shaft61and a rear balancer shaft62are disposed at respective locations forward and rearward of the crankshaft7, and extend substantially parallel to the crankshaft7. The front and rear balancer shafts61and62are rotatably supported between the upper crankcase5and the lower crankcase6, together with the crankshaft7. Further, as shown inFIGS. 5 and 7, a starter motor65is provided above the rear balancer shaft62in a rear part of the engine2.

As shown inFIGS. 4,6, and7, an oil filter56and a water-cooled oil cooler64are provided in a front part of the engine2. The oil filter56is disposed substantially above the front balancer shaft61and mounted to the upper crankcase5via the oil cooler64.

FIG. 8is a right-side view showing the upper crankcase5and the lower crankcase6, with the magnet cover60and the oil pump SCP removed therefrom.FIG. 9is a left-side view showing the magnet cover60.FIG. 10is a cross-sectional view taken on line A—A ofFIG. 8.FIG. 11is a cross-sectional view taken on line B—B ofFIG. 8.

As shown inFIGS. 10 and 11, the magnet cover60is mounted on the right-side parts5cand6cof the crankcases5and6. When the magnet cover60is fixed onto the crankcases5and6, a joint surface5a(refer toFIGS. 8 and 10) of the upper crankcase5and a joint surface6a(refer toFIGS. 8 and 11) of the lower crankcase6come into contact with a joint surface60a(refer toFIGS. 9,10, and11) of the magnet cover60in facing relation.

Referring toFIG. 10, the crankshaft7is rotatably supported at its first to third journals CJ1to CJ3. The front balancer shaft61is rotatably supported at its first and second journals BJ1and BJ2, and although not shown, the rear balancer shaft62is supported similarly to the front balancer shaft61.

The front balancer shaft61has first and second balancer shaft driven gears78and79rigidly fitted on a left end61bthereof, and an oil pump drive gear77rigidly fitted on a right end61athereof. The crankshaft7has a balancer shaft drive gear80fitted thereon at a location rightward of the third journal CJ3so that rotation of the crankshaft7is transmitted to the front balancer shaft61via the balancer shaft drive gear80and the first and second balancer shaft driven gears78and79.

The crankshaft7has a cam sprocket73rigidly mounted on a right end7bthereof opposite from the output shaft7a, and a cam chain74is passed over the cam sprocket73to transmit the driving force to camshafts95, as shown inFIG. 13. Also mounted on the right end7bof the crankshaft7substantially in concentricity with the crankshaft7is a starter clutch section70which is comprised of a starter one-way gear71and a starter one-way clutch72.

At a location rightward of the starter clutch section70, there is provided a magnet (magnet device) MG for power generation. The magnet MG is disposed between the two crankcases5and6and the magnet cover60. The magnet MG is comprised of a magnet flywheel76, a magnet stator75, and a pulser coil106(refer toFIG. 9). The magnet flywheel76is mounted on the right end7bof the crankshaft7, for rotation in unison with the crankshaft7. The magnet stator75and the pulser coil106are fixed to the magnet cover60.

The starter one-way clutch72is disposed between the cam sprocket73and the magnet MG, and fixed to the magnet flywheel76. The starter one-way clutch72has a ratchet structure that transmits rotation of the starter one-way gear71to the crankshaft7, but inhibits transmission of rotation of the crankshaft7to the starter one-way gear71.

Further, as shown inFIG. 8, above the rear balancer shaft62, there are disposed a starter gear69, a starter limiter68, and a starter motor65in the mentioned order from below and in a substantially linear arrangement, as viewed from a lateral side of the engine (or as viewed in an axial direction of the crankshaft7). More specifically, respective axial centers62z,69z,68z, and65zof the rear balancer shaft62, the starter gear69, the starter limiter68, and the starter motor65are arranged substantially linearly and substantially along a central axis CCL (refer toFIG. 8) of two cylinders94(refer toFIG. 13), referred to hereinafter. With this layout of these components, it is possible to make effective use of space on a side of the cylinders94toward the rear balancer shaft62. The starter motor65, the starter limiter68, the starter gear69, and the starter clutch section70form a “starter mechanism”.

On the other hand, as shown inFIGS. 8 and 9, the oil pump FEP and the oil pump SCP are disposed above and below the front balancer shaft61, respectively, and mounted between the two crankcases5and6and the magnet cover60. Therefore, within the magnet cover60, the starter gear69and the starter limiter68(hereinafter also referred to as “the transmission mechanism”) having the function of transmitting rotation of the starter motor65to the starter clutch section70are located on an opposite side of the crankshaft7from the two pumps FEP and SCP, as viewed in the axial direction of the crankshaft7. This arrangement makes it possible to accommodate the transmission mechanism in a space saving manner.

As shown inFIG. 5, the starter motor65is disposed on an opposite side of a transverse center ECL of the engine2from the clutch mechanism38, i.e. at a location rightward of the transverse center ECL. This arrangement makes it possible to achieve appropriate weight distribution between the starter motor65and the weighty clutch mechanism38.

Now, let it be assumed that a first imaginary plane PL1parallel to the transverse direction of the vehicle body is moved toward the cylinder heads4, the upper crankcase5, and the lower crankcase6, for contact therewith from the rear side. In this case, as shown inFIG. 7, the imaginary plane PL1comes into contact with a contact point P2on the cylinder heads4and a contact point P1on a balancer chamber RM2of the upper crankcase5. Further, when it is assumed that a second imaginary plane PL2parallel to the transverse direction of the vehicle body is moved toward the starter motor65, the upper crankcase5, and the lower crankcase6, for contact therewith from the rear side, as shown inFIG. 8, the imaginary plane PL2comes into contact with a contact point P4on the starter motor65and a contact point P3on the balancer chamber RM2of the upper crankcase5.

As shown inFIG. 7, the starter motor65is disposed such that it does not project rearward from the first imaginary plane PL1. Further, as shown inFIG. 8, the “transmission mechanism” formed by the starter gear69and the starter limiter68is disposed such that it does not project rearward from the second imaginary plane PL2. Thus, the starter motor65and the transmission mechanism are disposed close to the center of the vehicle body in a compact layout, with the weight thereof concentrated toward the center of the vehicle.

The starter limiter68transmits the driving force within a predetermined range. As the starter motor65rotates to start the engine2, the rotation of the starter motor65is transmitted from a pinion gear43of the starter motor65(refer toFIG. 10) to the starter one-way gear71via the starter limiter68, a drive gear44formed integrally with the starter limiter68, and the starter gear69. Since the starter one-way gear71and the starter one-way clutch72are formed integrally with each other, the rotation of the starter one-way gear71is transmitted to the right end7bof the crankshaft7via the starter one-way clutch72and the magnet flywheel76, whereby the crankshaft7is driven for rotation to start the engine2.

As shown inFIG. 11, the oil pump SCP is comprised of an SC pump body81and an SC pump driven gear82, and is fixed to the lower crankcase6by a plurality of bolts85. The SC pump driven gear82is in mesh with the oil pump drive gear77, so that the oil pump SCP is driven by the front balancer shaft61via the oil pump drive gear77.

As shown inFIG. 8, the lower crankcase6has an oil suction hole6bformed therein at a location below the oil pump SCP, and an oil suction hole60bassociated with the oil suction hole6bis formed through the magnet cover60(refer toFIG. 9). Lubricating oil stored in the oil pan49(refer toFIGS. 4 and 5) is returned by the oil pump SCP through the oil suction hole6band the oil suction hole60b, to be collected in the oil tank59through the oil pipe53. Subsequently, the lubricating oil is guided from the oil tank59to the oil pump FEP through the oil pipe54(refer toFIGS. 2 and 3).

FIG. 12is a cross-sectional view taken on line C—C ofFIG. 8. The oil pump FEP is comprised of an FE pump body83and an FE pump driven gear84, as shown inFIG. 12, and is fixed to the magnet cover60by a plurality of bolts86(refer toFIG. 9). The FE pump driven gear84is in mesh with the oil pump drive gear77, so that the oil pump FEP is driven by the front balancer shaft61via the oil pump drive gear77.

The SC pump driven gear82and the FE pump driven gear84are both generally identical in position in the axial direction of the front balancer shaft61, to the oil pump drive gear77. Insofar as the positional relationship between these components in the axial direction of the front balancer shaft61is concerned, the SC pump driven gear82is disposed closer to the axial center CP (refer toFIG. 10) of the front balancer shaft61than the SC pump body81is, and similarly, the FE pump driven gear84is disposed closer to the axial center CP than the FE pump body83is. Further, the oil pump drive gear77is offset to the axial center CP of the front balancer shaft61relative to the starter one-way gear71. With the arrangement described above, it is possible to suppress rightward protrusion of the magnet flywheel76, thereby contributing to reduction of the overall width of the engine2. Further, the offset between the oil pump drive gear77and the starter one-way gear71makes it possible to increase the degree of freedom in the gear diameters of the gears77and71.

As shown inFIG. 12, the water pump58is disposed in concentricity with the oil pump FEP at a location rightward of the same. A fitted part CON4of the water pump58is fitted in a water pump mounting part CON3of the magnet cover60, whereby the water pump58is held in a state rigidly secured to the magnet cover60, for being driven together with the oil pump FEP. A connecting part CON1of the magnet cover60is opposed to a connecting part CON2of the upper crankcase5.

As shown inFIG. 8, the connecting part CON2of the upper crankcase5is formed with a hole5bfor introducing lubricating oil into the engine2. On the other hand, as shown inFIG. 9, the connecting part CON1of the magnet cover60is formed with a hole60cas a lubricating oil discharge port of the oil pump FEP. Referring again toFIG. 12, the connecting part CON2is provided with a sealing resilient member87formed e.g. of rubber, a backflow-preventive ball88, and a spring89, which are arranged in the mentioned order from the magnet cover side. The sealing resilient member87has a ball-seating portion87aagainst which the backflow-preventive ball88can abut. The spring89constantly urges the backflow-preventive ball88toward the sealing resilient member87. The sealing resilient member87is sandwiched between the connecting part CON1and the connecting part CON2, so that no special fixing member is needed, which simplifies the construction of the sealing resilient member87.

When lubricating oil fed under pressure is delivered from the oil pump FEP into the connecting part CON2via the hole60cof the connecting part CON1, the backflow-preventive ball88moves away from the sealing resilient member87against the resilient force of the spring89. Then, the lubricating oil flows into the hole5bthrough a gap created between the backflow-preventive ball88and the sealing resilient member87, to be supplied into the engine2. On the other hand, when lubricating oil starts to flow from the hole5btoward the hole60c, the resilient force of the spring89urges the backflow-preventive ball88into contact with the ball-seating portion87aof the sealing resilient member87, to seal between the backflow-preventive ball88and the sealing resilient member87. This makes it possible to prevent backflow of lubricating oil during stoppage of the engine and entry of lubricating oil from the oil tank59side.

FIG. 13is a longitudinal cross-sectional view showing the engine2. As shown inFIGS. 7 and 13, the front balancer shaft61and the rear balancer shaft62are accommodated, respectively, in a balancer chamber RM1and the balancer chamber RM2, each defined by joining parts (connecting parts) of the upper and lower crankcases5and6. The oil filter56is disposed above the balancer chamber RM1in a manner facing obliquely upward. The engine2is tilted rearward such that a space is formed in the vicinity of the front part of the upper half of the engine2. Therefore, by disposing the oil filter56in this space, it is possible to prevent forward projection of the oil filter56and interference of the same with other component parts, and also save space. Further, the oil filter56is disposed above the front balancer shaft61positioned at a location elevated by the inclination of the engine2, which makes the oil filter56higher in position, and what is more, in a manner facing obliquely upward. This disposition of the oil filter56facilitates replacement operation e.g. for periodical replacement thereof.

As shown inFIG. 13, a main oil gallery90is formed in the upper crankcase5at a location forward of the cylinders94. The main oil gallery90is in communication with an oil passage92into which oil flows from the oil filter56. Further, a sub oil gallery91is formed in the upper crankcase5at a location rearward of the cylinders94and on an opposite side of the upper crankcase5from the main oil gallery90. The main oil gallery90and the sub oil gallery91both extend in the transverse direction. The main oil gallery90and the sub oil gallery91communicate with each other via an oil passage, not shown.

The lubricating oil fed under pressure from the oil pump FEP and having passed through the connecting part CON2(refer toFIGS. 8 and 12) flows into the oil filter56via a relief valve, not shown. Then, the lubricating oil flows from the oil filter56through the main oil gallery90, whereafter part of the lubricating oil lubricates the first to third journals CJ1to CJ3of the crankshaft7and the first and second journals BJ1and BJ2of the balancer shafts61and62, and the remaining part of the same flows through the sub oil gallery91, and is injected from a piston cooling jet93, to lubricate the interior of the cylinders94(including the associated pistons).

According to the present embodiment, the starter motor65is disposed above the rear balancer shaft62in the rear part of the engine2, whereby the relatively weighty starter motor65is positioned close to the center of the vehicle body. Thus, the starter mechanism is disposed so as to maintain excellent traveling stability of the vehicle. Further, due to disposition of the starter motor65in the rear part of the engine2, it is possible to suppress the influence of traveling wind upon the starter motor65, thereby protecting the starter motor65e.g. from rain, snow and dust, and preventing troubles caused by rust, a short circuit, and so forth.

Particularly, while in the engine room30, the clutch mechanism38is disposed leftward of a transverse center CL (refer toFIG. 3) of the vehicle body, the starter motor65is disposed rightward of the transverse center ECL of the engine2, so that the weight distribution between the starter motor65and the clutch mechanism38is well balanced, which makes it possible to further improve traveling stability.

Moreover, according to the present embodiment, the starter motor65is disposed forward of the first imaginary plane PL1, and the starter gear69and the starter limiter68are disposed forward of the second imaginary plane PL2. This makes it possible not only to save space within the engine, thereby suppressing an increase in the size of the engine, but also to concentrate weight load toward the center of the vehicle, thereby enhancing traveling stability of the vehicle. In addition, it is possible to enhance the degree of freedom in the layout of the interior of the engine room30and facilitate installment of the engine2.

Further, the transmission mechanism is disposed in the magnet cover60, on the substantially opposite side of the crankshaft7from the two pumps FEP and SCP. This makes it possible to dispose the transmission mechanism in a space saving manner, thereby suppressing an increase in the size of the engine.

Furthermore, according to the present embodiment, while in the engine room30, the clutch mechanism38is disposed leftward of the transverse center CL (refer toFIG. 3) of the vehicle body, the magnet MG, the starter motor65, and the battery51are arranged rightward of the transverse center CL, in a concentrated manner, and the battery51is disposed close to the magnet MG. This makes it possible not only to distribute the weights of the component elements in a more well-balanced manner to thereby maintain excellent traveling stability of the vehicle, but also to achieve more simplified wiring in the engine room30to thereby simplify the construction of the engine and save space within the engine room30.

Moreover, according to the present embodiment, the starter clutch section70is disposed between the cam sprocket73and the magnet MG, and the starter one-way clutch72is fixed to the magnet flywheel76so that the driving force of the starter motor65can be transmitted to the crankshaft7via the starter one-way gear71, the starter one-way clutch72, and the magnet MG. As a result, the space for accommodating the starter mechanism can be reduced, and the construction of the starter mechanism can be simplified. In addition, since the starter gear69, the starter limiter68, and the starter motor65are arranged substantially linearly, as viewed from the lateral side of the engine, above the rear balancer shaft62, it is possible to make effective use of space on the rear balancer shaft62side of the cylinders94, thereby saving space. Thus, the starter mechanism can be disposed so as to suppress an increase in the size of the engine. Insofar as the linear arrangement of the rear balancer shaft62, the starter gear69, the starter limiter68, and the starter motor65is secured, the engine2may be tilted forward, and at the same time the positional relationship in the longitudinal direction of the front balancer shaft61, the rear balancer shaft62, the starter gear69, the starter limiter68, and the starter motor65may be reversed to that in the above-described embodiment.

The above component elements within the engine room30may be arranged in a fashion symmetrically reverse to the arrangement in the above-described embodiment, which also makes it possible to obtain the advantageous effects of the present invention.