Abstract:
A lubricating device for a power unit includes an oil pan, an oil tank being separated from the oil pan, a scavenging pump for circulating the received oil of the oil pan to the oil tank, and a feed pump to feeding oil stored in the oil tank to the engine and the transmission, wherein an oil reservoir for storing a certain amount of oil to immerse a lower part of a clutch that connects and disconnects a transmission route between the engine and the transmission is provided in a power unit case, and a first overflow passage is provided in the oil tank such that when the stored oil in the oil tank is increased beyond a first level, the first overflow passage allows the increased oil to overflow into the oil reservoir side.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2012-134494, filed Jun. 14, 2012, the contents of which is incorporated herein, by reference, in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to a lubricating device for a power unit, which comprises an oil pan for receiving oil that has lubricated an engine and a transmission, an oil tank separated from the oil pan, a scavenging pump for circulating or returning the received oil of the oil pan to the oil tank, and a feed pump for feeding the stored oil in the oil tank to the engine and the transmission, and more particularly, to the lubricating device configured to use the lubricating oil of the power unit to perform the cooling of a clutch that connects and disconnects the transmission route between the engine and the transmission. 
       BACKGROUND OF THE INVENTION 
       [0003]    A lubricating device for a power unit has already been known as disclosed in Japanese patent laid-open publication No. 2010-208602, and there is also known a lubricating device configured to use the lubricating oil of the power unit to perform the cooling of the clutch, as disclosed in Japanese patent laid open publication No 2006-105132. 
       SUMMARY OF THE INVENTION 
       [0004]    Hitherto, in the lubricating device which cools the clutch by using the lubricating oil of the power unit, the lubricating oil splashed up by a crank weight of the engine is stored in an oil reservoir which accommodates a lower part of the clutch, and the lower part of the clutch is immersed in the stored oil, as disclosed in Japanese patent laid open publication No. 2006-105132. 
         [0005]    In the lubricating device of this type, variations exist in the amount of oil splashed up by the crank weight, depending upon operating conditions of the engine, so that there may be cases where the stable oil feeding to the oil reservoir becomes difficult. 
         [0006]    A lubricating device for a power unit is provided that allows a clutch to be always cooled during operation of an engine by using a surplus part of discharge oil of a scavenging pump thereby to increase the durability thereof. In the lubricating device for the power unit having the scavenging pump and a feed pump, a pump capacity of the scavenging pump is set greater than that of the feed pump so as to securely perform the oil circulation from an oil pan to an oil tank. 
         [0007]    According to a first feature, there is provided a lubricating device for a power unit, comprising an oil pan for receiving oil that has lubricated an engine and a transmission, an oil tank being separated from the oil pan, a scavenging pump for circulating or returning the received oil of the oil pan to the oil tank, and a feed pump for feeding stored oil in the oil tank to the engine and the transmission, wherein an oil reservoir for storing a certain amount of oil to immerse a lower part of a clutch that connects and disconnects a transmission route between the engine and the transmission is provided in a power unit case, and a first overflow passage is provided in the oil tank such that when the stored oil in the oil tank is increased beyond a first level, the first overflow passage allows the increased oil to overflow into the oil reservoir side. 
         [0008]    According to a second feature in addition to the first feature, an outlet passage is provided in a bottom portion of the oil reservoir for allowing the oil to flow from the oil reservoir to the oil pan, wherein an outflow amount of the oil passing the outlet passage is less than an outflow amount of the oil flowing from the oil tank to the oil reservoir side. 
         [0009]    According to a third feature in addition to the first or second feature, a second overflow passage is provided in the oil tank such that when the stored oil in the oil tank is increased beyond a second level which is higher than the first level, the second overflow passage allows the increased oil to overflow into the oil pan side, wherein flow passage resistance of the second overflow passage is set smaller than that of the first overflow passage. 
         [0010]    According to a fourth feature in addition to the first feature, the power unit case is divided into a case cover for covering the clutch and a case block adjacent to the case cover, wherein the oil tank and the oil reservoir are formed between joining faces of the case cover and the case block. By the way, the case cover and the case block each correspond to a front case cover  10   f  and an intermediate case  10   m  in an embodiment of the present invention as described later. 
         [0011]    Further, according to a fifth feature in addition to the third feature, an oil filler hole which is open to the oil tank above a lower edge of the second overflow passage and is normally closed by a filler cap is provided on an upper part of the oil tank. 
         [0012]    Furthermore, according to a sixth feature in addition to the fifth feature, an oil strainer which partitions an internal part of the oil tank into an upper chamber and a lower chamber is arranged within the oil tank, wherein the oil filler hole, the first overflow passage and the second overflow passage are opened into the upper chamber, and wherein the first overflow passage and the second overflow passage are offset with respect to the oil filler hole, in a plan view from above. 
         [0013]    According to the first feature, the oil reservoir for storing a certain amount of oil to immerse the lower part of the clutch is provided in the power unit case, and the first overflow passage is provided in the oil tank such that when the stored oil in the oil tank is increased beyond the first level, the first overflow passage allows the increased oil to overflow into the oil reservoir side. Therefore, during the operation of the engine, the surplus oil which overflows from the oil tank is stored a certain amount in the oil reservoir, and the lower part of the clutch can be immersed in the stored oil of the oil reservoir, so that, during the operation of the engine the cooling and lubrication of the clutch can be always ensured, and a power loss due to the oil stirring by a starting clutch can be controlled to the most possible extent. 
         [0014]    According to the second feature, the outlet passage is provided in the bottom portion of the oil reservoir for allowing the oil to flow from the oil reservoir to the oil pan side such that an outflow amount of the oil passing the outlet passage is less than an outflow amount of the oil flowing from the oil tank to the oil reservoir side. Therefore, at the time of the operation of the engine, when the operation of the scavenging pump and the feed pump stops, the overflow of the oil from the oil tank to the oil reservoir side stops. On the other hand, the stored oil in the oil reservoir flows out through the outlet passage to the oil pan, whereby it is possible to empty the oil reservoir after a lapse of a predetermined time. Accordingly, when starting the engine again, since the oil reservoir is in an empty condition, the oil stirring due to the rotation of the clutch does not occur, thereby reducing the starting load, so that the engine starting can be smoothly performed. 
         [0015]    According to the third feature, the second overflow passage is provided in the oil tank such that when the stored oil in the oil tank is increased beyond the second level which is higher than the first level, the second overflow passage allows the increased oil to overflow into the oil pan, and the flow passage resistance of the second overflow passage is set smaller than that of the first overflow passage. Therefore, after the oil tank is filled with the oil discharged from the scavenging pump, the surplus oil is allowed to overflow from the second overflow passage to the oil pan, whereby the increase in load of the scavenging pump can be restrained. 
         [0016]    According to the fourth feature, the power unit case is divided into the case cover for covering the clutch and the case block adjacent to the case cover, wherein the oil tank and the oil reservoir are formed between the joining faces of the case cover and the case block. Therefore, there is no need for provision of a special oil tank and a special oil reservoir separate from the power unit case, whereby the power unit can be simplified in construction and lightened in weight. 
         [0017]    According to the fifth feature, the oil filler hole which is open to the oil tank above the lower edge of the second overflow passage and is normally closed by the filler cap is provided on the upper part of the oil tank. Therefore, when the oil has been poured from the oil filler hole of the oil tank, firstly the oil fills the oil tank. Thereafter, when the oil within the oil tank is increased beyond the first and the second levels, the increased oil overflows through the first and second overflow passages so as to be stored finally in the oil pan. Accordingly, the oil feed of a predetermined amount to the oil tank and the oil pan can be preformed rapidly by the oil feeding from the single oil filler hole, and the oil feeding operation can be facilitated. 
         [0018]    According to the sixth feature, the oil strainer having an internal part of the oil tank partitioned into the upper chamber and the lower chamber is arranged within the oil tank, wherein the oil filler hole, the first overflow passage and the second overflow passage are opened into the upper chamber, and wherein the first overflow passage and the second overflow passage are offset with respect to the oil filler hole in a plan view from above. Therefore, at the time of the oil feeding through the oil filler hole, the oil fills the oil tank firstly and, thereafter, overflows through the first and second overflow passages so as to be stored in the oil pan. Accordingly, when monitoring an oil level of the oil within the oil pan with an oil gauge, excessive feed to the oil pan can be prevented. Moreover, the oil filling the oil tank can be filtrated by the oil strainer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The advantages of the invention will become apparent in the following description taken in conjunction with the drawings, wherein: 
           [0020]      FIG. 1  is a left side view of a straddle type four-wheeled all-terrain vehicle; 
           [0021]      FIG. 2  is a front view of a power unit shown in  FIG. 1 ; 
           [0022]      FIG. 3  is a cross sectional view taken along line  3 - 3  of  FIG. 2 ; 
           [0023]      FIG. 4  is a cross sectional view taken along line  4 - 4  of  FIG. 2 ; 
           [0024]      FIG. 5  is a cross sectional view taken along line  5 - 5  of  FIG. 4 ; 
           [0025]      FIG. 6  is a cross sectional view taken along line  6 - 6  of  FIGS. 4 ; and 
           [0026]      FIG. 7  is an oil route map of the above power unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Hereinafter, the embodiment applying the present invention to a straddle type four-wheeled all-terrain vehicle will be described with reference to accompanying drawings. In the description hereunder, “front”, “rear”, “left” and “right” are referred to in accordance with the orientation of the straddle type four-wheeled all-terrain vehicle. 
         [0028]    First in  FIG. 1 , a vehicle body frame F of the straddle type four-wheeled all-terrain vehicle B comprises a pair of left and right main frames  1 L,  1 R having an upper side frame  2 , a lower side frame  3 , a front side frame  4  and a rear side frame  5  combined into a quadrilateral shape, a plurality of cross members (not shown) establishing an integral connection between the left and right main frames  1 L,  1 R, and a seat rail  7  connected to rear portions of the upper side frames  2  of each of the main frames  1 L,  1 R. From front and rear portions of the left and right main frames  1 L,  1 R there are suspended a pair of left and right front wheels Wf and a pair of left and right rear wheels Wr respectively. A power unit P for driving the front wheels Wf and the rear wheels Wr is mounted on the main frames  1 L,  1 R in such a manner as to be surrounded with the main frames  1 L,  1 R. Further, on upper parts of the main frames  1 L,  1 R a steering handle  8  and a fuel tank  9  are arranged in order from the front. A straddle type seat  21  extends from the main frames  1 L,  1 R to the seat rail  7  to be arranged on the upper sides of them. To an output shaft  43  (which will be described later) of the power unit P there are connected to front propeller shaft  32  for driving the front wheels Wf and a rear propeller shaft  33  for driving the rear wheels Wr. 
         [0029]    Referring to  FIGS. 2 and 3 , the power unit P is formed of a water-cooled single-cylinder engine E and a transmission T which is arranged within a crankcase  10   c  of the engine E. A cylinder block  11  connected to and extending upward from an upper surface of the crankcase  10   c  is arranged in such a condition as to be tilted a little to one side in a left and right direction. A cylinder head  12  connected to an upper end surface of the cylinder block  11  is provided with an intake port  14   i  and an exhaust port  14   e  each of which is opened and closed by an intake valve  13   i  and an exhaust valve  13   e.  A throttle body  16  is connected to the intake port  14   i.  To the exhaust port  14   e  there is connected an exhaust pipe  22  a downstream end of which is connected to a silencer  2  (see  FIG. 1 ). 
         [0030]    As shown in  FIG. 3 , in the power unit P, a crankshaft  25  supported by the crankcase  10   c  is arranged so as to extend substantially horizontally in the forward and rearward direction. To a front end surface of the crankcase  10   c  there are connected an annular intermediate case  10   m  and a front case cover  10   f  in turn. A primary transmission chamber  30  is defined between the front case cover  10   f  and the crankcase  10   c.  In the primary transmission chamber  30  there is accommodated a centrifugal starting clutch  31 . This starting clutch  31  comprises a driving plate  31   a  fixedly secured to the crankshaft  25  in splined engagement, a clutch drum  31   b  surrounding the driving plate  31   a  and supported on the crankshaft  25  in a relatively rotatable manner, a plurality of centrifugal shoes  31   c  pivotally supported on the driving plate  31   a  and swingable between a clutch-on position where they come in pressure contact with an inner circumferential surface of the clutch drum  31   b  and a clutch-off position where they move away from the inner circumferential surface of the clutch drum  31   b , and a return spring  31   d  for spring-biasing the centrifugal shoes  31   c  to the clutch-off position side. The centrifugal shoes  31   c  swing to the clutch-on position in opposition to the set load of the return spring  31   d  when the rotational frequency of the crankshaft  25  becomes more than the predetermined rotational frequency. 
         [0031]    Further, to a rear end surface of the crankcase  10   c  there is connected a rear case cover  10   r  in such a manner as to form an electric generating chamber  27  together with the crankcase  10   c,  and an electric power generator  28  is arranged in the power generating chamber  27 . 
         [0032]    As described above, the power unit case  10  is formed by the crankcase  10   c , the front case cover  10   f , the intermediate case  10   m  and the rear case cover  10   r.  In addition, the crankcase  10   c  is formed by a front case half body  10   c F and ear case half body  10   c R. Namely, the power unit case  10  is divided into a plurality of blocks along planes orthogonal to an axis of the crankshaft  25  and formed by joining these blocks together. 
         [0033]    In  FIGS. 2 and 3 , the transmission T accommodated in the crankcase  10   r  has a first and second main shafts  35 A,  35 B and a counter shaft  36  each of which has an axis parallel to the crank shaft  25  and is rotatably supported on the crankcase  10   c  so as to be rotated relative to each other, and gear trains of a plurality of speed stages, for example, gear trains G 1  through G 6  of from first speed stage to sixth speed stage, provided in a selectively establishable manner between the first and second main shafts  35 A,  35 B and the counter shaft  36 . 
         [0034]    In detail, odd-numbered gear trains such as the first speed stage gear train G 1 , a third speed stage gear train G 3  and a fifth speed stage gear train G 5  are provided between the first main shaft  35 A and the counter shaft  36 , while even-numbered gear trains such as a second speed stage gear train G 2 , a fourth speed stage gear train G 4  and the sixth speed stage gear train G 6  are provided between the second main shaft  35 B and the counter shaft  36 . 
         [0035]    In the primary transmission chamber  30 , a cylindrical transmission shaft  37  which is axially adjacent to the second main shaft  35 B is fitted on an intermediate portion of the first main shaft  35 A in a relatively rotatable manner, and a hub of the clutch drum  31   b  of the starting clutch  31  is connected through a primary transmission gear train  38  to the cylindrical transmission shaft  37 . A first speed change clutch  41 A is interposed between the cylindrical transmission shaft  37  and the first main shaft  35 A, and a second speed change clutch  41 B is interposed between the cylindrical transmission shaft  37  and the second main shaft  35 B, respectively. 
         [0036]    The first speed change clutch  41 A and the second speed change clutch  41 B are formed each in a hydraulic multiple disc type. When hydraulic pressure is not applied to each of hydraulic chambers, they turn to an ON state by the force of a spring. When the hydraulic pressure is applied to each of the hydraulic chambers, they turn to an OFF state. The ON state of the first speed change clutch  41 A allows the transmission between the cylindrical transmission shaft  37  and the first main shaft  35 A to be established, and the ON state of the second speed change clutch  41 B allows the transmission between the cylindrical transmission shaft  37  and the second main shaft  35 B to be established. A clutch control valve  42  is configured to control the feeding and blocking of the hydraulic pressure to each of the hydraulic chambers of the first and second speed change clutches  41 A,  41 B. 
         [0037]    Further, an output shaft  43  arranged parallel to the counter shaft  36  is pivotally supported on the crankcase  10   c.  This output shaft  43  has an intermediate portion thereof connected through an output gear train  44  to the counter shaft  36 . Both ends of the output shaft  43  are connected to the front and rear propeller shafts  32 ,  33  outside the crankcase  10   c.    
         [0038]    In  FIG. 4 , a balancer shaft  45  which is driven in reverse through a gear train of the same diameter from the crank shaft  25  so as to reduce an engine vibration is pivotally supported on the crankcase  10   c  in parallel with the crankshaft  25 . A scavenging pump  48  and a feed pump  49  driven by a common pump shaft  47 , which is connected to a rear end portion of the balancer shaft  45 , are mounted on the intermediate case  10   m . Moreover, on a lateral wall of the front case cover  10   f  an oil filter  52  is mounted. 
         [0039]    As shown in  FIG. 4  through  FIG. 6 , a bottom of the crankcase  10   c  forms an oil pan  50  for receiving the oil that has lubricated each part of the engine and the transmission T. Further, a substantially circular arc-shaped and vertically extending oil tank  51  is formed, adjacent to an outer circumferential surface of the starting clutch  31 , between joining faces of the intermediate case  10   m  and the front case cover  10   f.  In concrete terms, the oil tank  51  is formed such that substantially circular arc-shaped recesses  10   a ,  10   ma  each formed on the joining faces of the intermediate case  10   m  and the front case cover  10   f  are placed and joined in a face to face relationship. A meshed oil strainer  55  is arranged in the oil tank  51  so as to partition the same into a lower chamber  51   b  of a larger volume and an upper chamber  51   a  of small volume. The oil strainer  55  is inserted into a support groove  57  of a rib  56  projecting from the front case cover  10   f  to the oil tank  51  side, and is prevented from removal from the support groove  57  by a rib  56 ′ which projects from the intermediate case  10   m  to the oil tank  51  side. 
         [0040]    Moreover, between the intermediate case  10   m  and the front case cover  10   f  there is formed an oil reservoir  53  winch accommodates a lower part of the stating clutch  31 , in the vicinity Of a substantially horizontal upper wall of the lower part of the oil tank  51 . This oil reservoir  53  is arranged so as to occupy a position above the oil pan  50 . The oil reservoir  43  is defined by a dam  58  which is formed from the intermediate case  10  to the front case cover  10   f  in such a manner as to extend upward from an upper wall surface of the lower part of the oil tank  51 . Thus, the oil stored in the oil reservoir  53  overflows into the oil pan  50  side when the oil level goes beyond an upper end of the dam  58 . On a lowermost portion of the dam  58  there is provided an outlet passage  59  to open the oil reservoir  53  to the oil pan  50  side. 
         [0041]    As shown in  FIGS. 5 through 7 , an oil filler hole  60  opened into the upper chamber  51   a  of the oil tank  51  is provided on the upper end portion of the front case cover  10   f . On this oil filler hole  60  a filler cap  61  for normally closing this oil filler hole  60  is screwed. On the lateral wall of the upper chamber  51  of the oil tank  51  there are provided a first overflow passage  62  and a second overflow passage  63  which is arranged in a higher position than the first overflow passage  62  and which has flow passage resistance smaller than that of the first overflow passage  62 . When the stored oil in the oil tank  51  is increased beyond a predetermined first level L 1 , the increased oil overflows through the first overflow passage  62  into the oil reservoir  53  side. Also, when the oil level of the stored oil in the oil tank  51  is increased beyond a second level L 2 , which is higher than the first level L 1 , the increased oil overflows through the second overflow passage  63  into the oil pan  50  side. 
         [0042]    In the above construction, the flow passage resistance of the outlet passage  59  is set greater than that of the first overflow passage  61 . The oil filler hole  60  is positioned higher than the lower edge of the second overflow passage  63 . Further, in a plan view from above, the first overflow passage  62  and the second overflow passage  63  are offset with respect to the oil filler hole  60 . The outlet passage  59  is arranged at substantially the same level as a normal oil level L 3  of the stored oil in the oil pan  50 . 
         [0043]    When feeding the power unit P with the oil the filler cap  61  is removed and a predetermined amount of oil is poured into the oil tank  51  from the oil filler hole  60 . Since the oil filler hole  60  is offset from the first overflow passage  62  and the second overflow passage  63  in a plan view from above, the oil tank  51  is filled first with the poured oil. When the oil within the oil tank  51  is increased beyond the first and second levels L 1 , L 2 , the increased oil overflows through the first and second overflow passages  62 ,  63  and finally is stored in the oil pan  50 . Accordingly, by feeding the oil from the single oil filler hole  60 , the feeding of the oil of the predetermined amount can be rapidly performed, and the oil feeding operation can be simplified. In addition, an oil amount stored within the oil pan  50  can be checked with an of level gauge  64  provided on the lateral wall of the intermediate case  10   m.    
         [0044]    Referring again to  FIG. 4 , an upstream end  65   a  of a suction passage  65  communicating with a suction port of the scavenging pump  48  is opened into a bottom portion of the oil pan  51 , while a downstream end  66   a  of a discharge passage  66  communicating with a discharge port of the scavenging pump  48  is opened into a central portion of the oil tank  51 . Thus, the scavenging pump  48  is capable of sucking the oil stored in the bottom portion of the oil pan  50  and returning it into the oil tank  51 . 
         [0045]    Further, an upstream end  68   a  of a suction passage  68  communicating with a suction port of the feed pump  49  is opened into a bottom portion of the oil tank  51 , and an oil strainer  54  is attached to the suction passage  68 . A discharge passage  69  communicating with a discharge port of the feed pump  49  passes through the oil tank  51  and is connected to the oil filter  52 . Thus, the feed pump  49  sucks the oil within the oil tank  51  through the oil strainer  54  and forcibly feeds it through the discharge passage  69  to the oil filter  52 . A part of the oil filtrated by the oil filter  52  is selectively fed as operating oil through the clutch control valve  42  to the first and second speed change clutches  41 A,  41 B, while residual oil is fed as lubricating oil to each part of the engine E such as the crankshaft  25  and the environs thereof, the valve operating mechanism within the cylinder head  12 , the starting clutch  31  and the like, and to each part of the transmission T. Then, after finishing the predetermined job, the operating oil and the lubricating oil flow downward so as to be received or collected in the oil pan  50 . 
         [0046]    By the way, a pump capacity of the scavenging pump  48  is set greater than that of the feed pump  49 . Therefore, the oil received in the oil pan  50  is immediately pumped up to the oil tank  51  by the scavenging pump  48 , and the amount of the oil stored in the oil pan  50  is decreased to the most possible extent, so that it is possible to limit the power loss caused due to the stirring of the stored oil by the crankshaft  25  of the engine E and various kinds of gears of the transmission T. 
         [0047]    When the oil stored in the oil tank  51  is increased beyond the first level L 1 , the increased oil overflows through the first overflow passage  62  to the oil reservoir  53  side which accommodates the lower part of the starting clutch  31 , so that the starting clinch  31  is cooled and lubricated by immersing the lower part thereof in the oil, thereby making it possible to improve the durability. 
         [0048]    The surplus part of the oil that has filled the oil reservoir  53  flows over the dam  58  into the oil an  50  side. In addition, a part of the oil within the oil reservoir  53  flows through the outlet passage  59  into the oil pan  50  side. However, since the flow passage resistance of the outlet passage  59  is set greater than that of the first overflow passage  62 , an outflow amount of the oil from the outlet passage  59  is smaller than an inflow amount of the oil from the first overflow passage  62  to the oil reservoir  53  side. 
         [0049]    Like this, during the operation of the engine E, the surplus oil which overflows from the oil tank  51  is stored a certain amount in the oil reservoir  53 , and the lower part of the starting clutch  31  is immersed in the stored oil in the oil reservoir  53 . Therefore, during the operation of the engine E, the cooling and lubrication of the starting clutch  31  can be always ensured, and a power loss due to the oil stirring of the starting clutch  31  can be controlled to the most possible extent. 
         [0050]    When the stored oil amount in the oil tank  51  is increased further until the oil level thereof goes over the second level L 2 , the increased part of the oil overflows through the second overflow passage  63  into the oil pan  50  side. Since the flow passage resistance of the second overflow passage  63  is set smaller than that of the first overflow passage  62 , the surplus oil can overflow smoothly into the oil pan  50  side and it is possible to avoid an unnecessary rise in the load of the scavenging pump  48 . 
         [0051]    When the operation of the scavenging pump  48  and the feed pump  49  stops in response to the stop of operation of the engine E, the overflow of the oil from the oil tank  51  through the first overflow passage  62  to the oil reservoir  53  side stops after the oil level of the stored oil in the oil tank  51  goes below the first level L 1 . On the other hand, since the stored oil in the oil reservoir  53  continues flowing out through the outlet passage  59  into the oil pan  50  side, the oil reservoir  53  is substantially emptied of the oil after a lapse of a predetermined time. In this respect, when the oil reservoir  53  is substantially emptied of the oil, it means not that the oil reservoir  53  becomes completely empty but that the oil level of the oil reservoir  53  is lowered until the bottom surface of the starting clutch  31  is exposed. 
         [0052]    Accordingly, when starting the engine E again, since the oil reservoir  53  is in substantially an empty condition, the oil stirring due to the rotation of starting clutch  31  does not occur, thereby reducing the starting load, so that the starting of the engine E can be smoothly performed. 
         [0053]    Furthermore, since the oil tank  51  and the oil reservoir  53  are formed between the joining surfaces of the front case cover  10   f  and the intermediate case  10   m  which form a part of the power unit case  10 , the power unit P can be simplified in construction and lightened in weight, without providing special oil tank and oil reservoir separate from the power unit case  10 . 
         [0054]    While a preferred embodiment of the present invention has been described, it is to be understood that the present invention is not limited to the above described embodiment, and that various changes and modifications may be made without departing from the sprit of the present invention.