Abstract:
An oil tank uses centrifugal movement of oil to separate blow-by gases. The oil tank has a tank body with an internal oil chamber. The oil chamber is spaced from the walls of the oil tank. The oil is delivered to the oil chamber and the oil swirls along the inner wall of the oil chamber in a helical pattern thereby allowing separation between the oil and the blow-by gases. The oil settles in the bottom of the oil chamber, which is in fluid communication with the region defined between the tank body and the oil chamber. The oil chamber is placed in an off-center location relative to the bottom of the tank body.

Description:
RELATED APPLICATIONS  
       [0001]     This application claims the priority benefit under 35 U.S.C. § 119 of Japanese Patent Application No. 2004-271359, filed on Sep. 17, 2004, which is hereby incorporated by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to an oil tank for an engine-driven vehicle that separates oil from blow-by gas. More particular, the present invention relates to such an oil tank in which blow-by gas is separated from the oil by centrifugal action.  
         [0004]     2. Description of the Related Art  
         [0005]     In oil tanks, such as that disclosed in United States Published Patent Application No. 2003/0045187, published on Mar. 6, 2003, which claimed priority to Japanese Patent Application No. 2001-233362, filed on Aug. 1, 2001, there often is a mixture of oil and so-called blow-by gases. The oil tank disclosed in the &#39;187 publication comprises an outer cylinder that extends in a vertical direction. An upper cover and a lower cover close off the top and the bottom of the outer cylinder. An inner cylinder is positioned along the axial centerline of the outer cylinder. A plurality of annular partition plates are positioned along the inner cylinder and extend between the inner cylinder and the outer cylinder. These partition plates divide the annular space between the inner cylinder and the outer cylinder into multiple oil chambers in the vertical direction. The inner peripheral edges of the partition plates are fixed to the outer peripheral surface of the inner cylinder while the outer peripheral edges of the partition plates are spaced from the inner peripheral surface of the outer cylinder.  
         [0006]     The inlet of the oil tank is in the upper end of the outer cylinder. The inlet is positioned such that the oil flows into the annular space between the outer cylinder and the inner cylinder. The oil inlet also is positioned such that, when seen in plan view, the oil flows in along the inner peripheral surface of the inner peripheral wall of the outer cylinder. The oil outlet of the tank is formed at the lower end of the outer cylinder such that it opens to the lower end of the annular space defined between the inner and outer cylinders.  
         [0007]     The annular space is partitioned by the plural partition plates into plural oil chambers arranged in the vertical direction. The oil chambers are connected by the gap formed between the inner peripheral surface of the outer cylinder and the outer peripheral edges of the partition plates. The upper portion of the uppermost oil chamber of the plural oil chambers is connected to the atmosphere by a blow-by gas discharge pipe. One end of the blow-by gas discharge pipe opens to the upper end portion of the annular space and the pipe then extends through the inside cylinder such that the other end is positioned outside of the oil tank.  
         [0008]     In an oil tank constructed in this manner, oil mixed with blow-by gas is pressure-fed into the uppermost annular oil chamber. The mixed oil flows along the inner peripheral surface of the outer cylinder and it spins around inside the oil chamber. The oil and the blow-by gas are separated with the oil going to the outer side and blow-by gas moving to a more central location due to centrifugal forces. The spinning of the oil causes these forces and the differences of the specific gravities of oil and blow-by gas causes the movement. The oil flows down into the lower oil chamber through the gap formed between the outer cylinder and the partition plates, and is discharged to the outside of the oil tank (is supplied to the engine) from an oil discharge port positioned in the lowermost portion of the oil tank. The blow-by gas is dispersed into the atmosphere through the blow-by gas discharge pipe from the uppermost oil chamber inside the oil tank.  
         [0009]     Because the oil must flow downward through the gaps formed between the outer cylinder and each of the partition plates, and there has been a limit on increasing the flow volume of oil through the tank. For this reason, it has not been possible to use such an oil tank in an engine requiring a large supply of oil.  
         [0010]     Sometimes the conventional oil tank cannot separate the blow-by gas from the oil in the upper oil chamber, and blow-by gas remains in the oil. The blow-by gas cannot rise counter to the oil flowing downward. For this reason, the ability of the conventional oil tank to separate gas and liquid is poor and some of the blow-by gas ends up being supplied to the engine together with the oil.  
         [0011]     The conventional oil tank has also had the problem that oil mist floating above the liquid surface in the uppermost oil chamber also ends up being discharged into the atmosphere through the discharge pipe together with the blow-by gas.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, there is a need for an oil tank with improved ability to separate out blow-by gas and/or to separate out oil mist.  
         [0013]     One aspect of the present invention involves an oil tank for an engine-driven vehicle. The oil tank comprises a tank body comprising a generally cylindrical inner wall, a top end and a bottom end. The tank body inner wall is joined to the tank body top end and the tank body bottom end. An oil chamber is positioned within the tank body. The oil chamber comprises a generally cylindrical inner wall, a top end and a bottom end. The oil chamber inner wall is joined to the oil chamber top end and the oil chamber bottom end. The oil chamber inner wall is radially spaced from the tank body inner wall. A passage is formed through a lower portion of the oil chamber inner wall such that an oil chamber volume defined within the oil chamber is in fluid communication with a tank body volume defined between the oil chamber and the tank body. A tank oil inlet communicates with the oil chamber volume through an upper portion of the oil chamber wall and a tank oil outlet communicates with the tank body chamber through a lower portion of the tank body. A blow-by gas chamber comprises a blow-by gas inlet that is in fluid communication with an upper portion of tank body and a blow-by gas outlet. The blow-by gas inlet is connected to the blow-by gas outlet by a curved air path. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     These and other features, aspects and advantages of the present invention will now be described with reference to the drawings of a preferred embodiment, which embodiment is intended to illustrate and not to limit the invention, and in which figures:  
         [0015]      FIG. 1  is side view of a snowmobile engine having an oil tank that is arranged and configured in accordance with certain features, aspects and advantages of the present invention;  
         [0016]      FIG. 2  is a plan view of the engine of  FIG. 1 ;  
         [0017]      FIG. 3  is a sectioned view of the oil tank of  FIG. 1  taken along the line  3 - 3  in  FIG. 2 ;  
         [0018]      FIG. 4  is a sectioned view of the oil tank of  FIG. 1  taken along the line  4 - 4  in  FIG. 2 ;  
         [0019]      FIG. 5  is a sectioned view taken along the line V-V in  FIG. 4 ;  
         [0020]      FIG. 6  is a sectioned view taken along the line VI-VI in  FIG. 4 ;  
         [0021]      FIG. 7  is a sectioned view taken along the line VII-VII in  FIG. 4 ; and,  
         [0022]      FIG. 8  is a schematic view of a lubricating system of the engine of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]     With reference now to  FIG. 1 , a snowmobile  1  is shown that has an engine  2  equipped with an oil tank  12  that is arranged and configured in accordance with certain features, aspects and advantages of the present invention. While the oil tank  12  will be described in the context of the snowmobile  1 , certain features, aspects and advantages of the oil tank  12  can be utilized in other vehicles, such as, for example but without limitation, four wheeled vehicles, including automobiles, two wheeled vehicles, including motorcycles and watercraft, including jet-propelled boats and personal watercraft.  
         [0024]     With reference to  FIG. 1  and with additional reference to  FIG. 2 , the illustrated snowmobile  1  comprises a seat  3  upon which a user and, in some configurations, a passenger are positioned during operation. The seat is generally positioned in the center portion of the vehicle body. A steering handle  4  is positioned forward of at least a portion of the seat and is used to control the direction in which the snowmobile will travel. In some configurations, a throttle control also is mounted to the steering handle  4 .  
         [0025]     In the illustrated configuration, the engine  2  is a 4-cycle multi-cylinder engine. The illustrated engine  2  is installed with the crankshaft (not shown) extending in a transverse direction. In addition, the engine  2  preferably is installed in a forward portion of the vehicle body and is generally centered relative to the width of the vehicle body. With continued reference to  FIGS. 1 and 2 , the illustrated engine  2  is generally inclined with the axial centerline of the cylinders being slanted rearward and upward. A carburetor  6  preferably is connected to the front surface of a cylinder head  5  of the engine  2 . In the illustrated engine, the engine has one carburetor  6  for each cylinder and the carburetors  6  receive air collectively from a single air cleaner  7 . In the illustrated configuration, the air cleaner  7  is disposed in front of and above the engine  2 . Other engine configurations also can be used. For instance, some features, aspects and advantages of the present invention may be utility with two-stroke engines, engines having less than four cylinders or more than four cylinders, and engines having differing cylinder configurations and/or differing air supply configurations.  
         [0026]     With reference now to  FIG. 8 , the engine  2  includes a lubrication system  11 . The illustrated lubrication system  11  has a configuration which causes oil to circulate through the engine  2  and an oil tank  12 . In one configuration, the oil tank  12  can be disposed at the right side of the engine  2 . Other positions also are possible. The oil tank  12  is connected by a first oil pipe  14  to an oil discharge port (not shown) of a scavenge pump  13  disposed inside the engine  2 , and is connected by a second oil pipe  15  to an oil feed pump (not shown) inside the engine  2 . Other suitable configurations also can be used to supply oil to the engine  2  from the tank  12 . In addition, as used herein, oil is intended to be broadly defined as a lubricant that is circulated within an engine for reducing friction and/or cooling components of the engine.  
         [0027]     The scavenge pump  13  supplies oil from the bottom of the engine  2  to the oil tank  12 , and the oil feed pump supplies oil from inside the oil tank  12  to lubricated portions of the engine  2 . Any suitable oil delivery system can be used. A breather box  16  can be connected to an upper portion of the oil tank  12 . In one configuration, the breather box  16  is connected to the air cleaner  7  by a blow-by gas pipe  17 . In another configuration, the breather box  16  is formed integrally with the rest of the oil tank  12  while, in one other configuration, the breather box  16  can be a separate component that is in fluid communication with the oil tank  12 .  
         [0028]     With reference now to FIGS.  3  to  7 , the illustrated oil tank  12  has a tank body  21 . Preferably, the tank body  21  generally comprises a closed container. The tank body  21  can have any suitable configuration. An inner cylinder  24  is supported inside the tank body  21  by two partition plates (e.g., an upper partition plate  22  and a lower partition plate  23  in the illustrated arrangement). The inner cylinder  24  can have any suitable configuration keeping in mind the goal of generating a suitable swirl of oil, as described below. The breather box  16  in the illustrated configuration extends upward from the upper portion of the illustrated tank body  21 .  
         [0029]     In the illustrated configuration, the tank body  21  is formed of a cylinder  25  with a cover plate  26  that closes off one end of the cylinder  25  and a bottom plate  27  that closes off the other end of the cylinder  25 . In one configuration, the tank body  21  is disposed at the right side of the engine  2  and a center axis of the tank body  25  is oriented in a substantially vertical direction. The cover plate  26  preferably is positioned generally directly vertically above the bottom plate  27 . More preferably, a substantially closed space  28  is defined within the tank body  21  and the closed space preferably is in fluid communication with the inside of the inner cylinder  24  and, even more preferably, the substantially closed space  28  generally envelopes the inner cylinder  24 , which is positioned within the tank body  21  in the illustrated configuration.  
         [0030]     With reference to FIGS.  5  to  7 , the cylinder  25  that defines the illustrated tank body  21  is formed such that its transverse sectional shape is substantially circular and generally constant from its upper end to its lower end. Other suitable configurations can be used so long as the purposes of the tank body  21  are accomplished. In the illustrated oil tank  12 , the transverse sectional shape of the tank body  21  is substantially constant from its upper end to its lower end. Thus, the speed at which the oil level drops becomes uniform when the oil inside the tank body  21  is supplied to the engine  2  and the oil level drops. For this reason, the oil can be prevented from undulating unnecessarily when it flows inside the tank body  21 . Moreover, because the illustrated oil tank  12  has a generally uniform transverse sectional shape, the plate-like members (e.g., the cover plate  26 , the bottom plate  27 , the upper partition plate  22  and the lower partition plate  23 ) can be formed from a single common blank.  
         [0031]     In the illustrated configuration, the cover plate  26  is formed in a disk shape. The cover plate  26  can be welded to the cylinder  25  such that the outer peripheral portion of the cover plate  26  is sealed with the cylinder  25 . In one preferred configuration, the joint between the cover plate  26  and the cylinder  25  is liquid-tight.  
         [0032]     With reference again to  FIGS. 3 and 4 , a convex portion  26   a  can be formed in the cover plate  26  near the radial center of the cover plate  26 . The convex portion  26   a  protrudes upward. In some configurations, the convex portion  26   a  can be formed of a member that is secured to an upper surface of the cover plate  26 . Regardless of how the convex portion  26   a  is formed, the convex portion  26   a  should protrude upward from the surrounding portion of the cover plate  26 . In the illustrated arrangement, the convex portion  26   a  is formed in a circular shape when seen in plan view at a position that is slightly eccentric or off-center relative to the axial center of the cylinder  25 . Other positions also can be used. In the illustrated embodiment, the direction in which the convex portion  26   a  is eccentric with respect to the cylinder  25  is toward the rear of the vehicle body (the upper side in  FIG. 5 ).  
         [0033]     With reference now to  FIGS. 4 and 5 , a blow-by gas inlet  29  is formed through the cover plate  26 . In the illustrated arrangement, the inlet  29  comprises a hole that is positioned toward the right side of the vehicle body (the left side in the drawings). Other placements also can be used. In the illustrated oil tank  12 , the blow-by gas inlet  29  is at the vehicle body right side. For this reason, when the vehicle body is tilted sideways such that the engine  2  is positioned below the oil tank  12 , the blow-by gas inlet  29  is positioned above the oil level indicated by the two-dot chain line L 2  in  FIG. 5 . Thus, the oil inside the tank body  21  does not pass through the blow-by gas inlet  29  and flow into the first blow-by gas chamber  66 . As a result, when the vehicle body is tilted such that the engine  2  is positioned below the oil tank  12 , the likelihood of oil passing through the blow-by gas pipe  17  and flowing out into the air cleaner  7  can be greatly reduced or eliminated.  
         [0034]     With reference now to  FIGS. 3 and 5 , a threaded insert  31  for supporting an oil level sensor  30  is secured to the cover plate  26 . The insert  31  can have any suitable configuration and preferably provides a female threaded surface. In the illustrated arrangement, the insert  31  is positioned on the vehicle body front side of the cover plate  26  (i.e., the right side in  FIG. 3  and the lower side in  FIG. 5 ). The oil level sensor  30  is used to detect the level of oil contained within the tank body  21 . In the illustrated oil tank  12 , the oil level sensor  30  is housed effectively using the space formed at the side of the inner cylinder  24 . Thus, the size of the tank body  21  does not increase when it is equipped with the oil level sensor  30 .  
         [0035]     The bottom plate  27  of the tank body  21  is coupled with the cylinder  25  in any suitable manner. In one configuration, the bottom plate  27  and the cylinder  25  are welded together and, in a preferred configuration, the bottom plate  27  and the cylinder  25  are joined in a fluid-tight manner.  
         [0036]     An oil discharge port  32  extends through the bottom plate  27 . The oil discharge port  32  preferably comprises a hole through the bottom plate  27 . In some configurations, the bottom plate  27  can define a sloping surface with the discharge port  32  being positioned in a lowermost location. The oil discharge port  32  allows oil to drain from the closed space  28  formed inside the tank body  21 . In the illustrated oil tank  12 , the inner cylinder  24  and the oil discharge port  32  are disposed at positions that are offset toward the vehicle body&#39;s rear side with respect to the tank body  21 , which causes them to be off-center. Thus, oil can be supplied to the engine  2  from the lowest location when the snowmobile  1  equipped with the illustrated oil tank  12  travels up a slope. For this reason, the oil can be reliably supplied to the lubricated parts of the engine  2  when the load of the engine  2  increases due to the slope.  
         [0037]     A pipe coupling  34  connects a pipe member  33  to the oil discharge port  32 . The pipe coupling  34  can have any suitable configuration and can be welded to the undersurface of the bottom plate  27  in one configuration. The pipe member  33  connects with the end of the second oil pipe  15 . Any suitable coupling can be used to join the pipe member  33  and the second oil pipe  15 .  
         [0038]     In the illustrated embodiment, an O-ring  35  is positioned where the pipe member  33  and the pipe coupling  34  are connected. The O-ring preferably reduces the likelihood of oil leakage in the region of the pipe coupling  34 . A strainer or filter  36  can be positioned within the closed space  28 . In some configurations, the filter  36  can be disposed in the pipe connection member  34 .  
         [0039]     With continued reference to  FIGS. 3 and 4 , the inner cylinder  24  is configured by a cylinder  41  that extends generally parallel to the cylinder  25  of the tank body  21 . In one configuration, the cylinder  41  is generally circular in configuration. Other suitable shapes also can be used. The inner cylinder  24  also comprises a plate member  42  that is welded to the upper end portion of the cylinder  41  such that it closes off the upper end portion of the cylinder  41 . In one configuration, the plate member  42  can be generally annular in configuration. Other suitable shapes also can be used. The lower end of the cylinder  41  can be secured to the lower partition plate  23 . In one configuration, the lower end of the cylinder  41  is welded to the lower partition plate  23 . Preferably, the cylinder  41  and the partition plate  23  are secured in a fluid-tight manner. In the illustrated oil tank  12 , a member functioning exclusively as the bottom wall of the inner cylinder  24  becomes unnecessary because the bottom wall of the inner cylinder  24  is configured by the lower partition plate  23 .  
         [0040]     A tube body  43  can be welded to the plate member  42 . In one configuration, the tube body  43  is welded to the center of the plate member  42 . In the illustrated configuration, the tube body  43  is positioned on the axial centerline of the cylinder  41  and the tube body  43  preferably is attached to the plate member  42  such that its lower portion faces the inside of the cylinder  41  and is positioned within the cylinder  41 .  
         [0041]     In the illustrated embodiment, as shown in  FIG. 6  and  FIG. 7 , the inner cylinder  24  is positioned such that it is offset or off-center toward one side in the radial direction with respect to the tank body  21  when seen in plan view. The direction in which the illustrated inner cylinder  24  is offset with respect to the tank body  21  is toward the rear of the vehicle body (the upper side in  FIG. 6  and  FIG. 7 ). Other positions also are possible. In the illustrated oil tank  12 , however, the inner cylinder  24  is disposed at an eccentric or off-center position with respect to the tank body  21 . Thus, the inner cylinder  24  can be more securely fixed to the tank body  21  by the upper partition plate  22  and the lower partition plate  23  at a location where the gap between the inner cylinder  24  and the tank body  21  is relatively narrow.  
         [0042]     As shown in  FIG. 6 , at the upper portion of the cylinder  41  and at the side of the tube body  43 , a through hole  44  is formed and a pipe member  45  is inserted into the through hole  44 . In one configuration, the pipe member  45  is welded in position. The pipe member  45  can be connected to the first oil pipe  14  in any suitable manner and the pipe member  45  defines an oil inlet for the oil tank  12 .  
         [0043]     The pipe member  45  can have a tapering end such that it defines a slight nozzle to increase the velocity of the oil flow. In some arrangements, the end of the pipe member  45  does not taper. In addition, the illustrated pipe member  45  penetrates the cylinder  25  of the tank body  21  and extends into the inner cylinder  24 . Advantageously, the illustrated pipe member  45  extends into the inner cylinder  24  generally in a tangential direction (e.g., as shown in  FIG. 6 ). In other words, an extension of an axial centerline of the pipe member  45  preferably does not intersect the center of the inner cylinder  24 . In addition, in the illustrated arrangement, the pipe member  45  is positioned generally between the cylinder  41  and the tube body  43 . The tube body  43  preferably extends downward beyond the lowermost portion of the pipe member  45 .  
         [0044]     Thus, the oil tank  12  is configured such that the oil flies through the air when it flows into the inner cylinder  24  from the pipe member  45 . Thus, the oil tank  12  can directly disperse, into the air chamber inside the inner cylinder  24 , the blow-by gas included in the vicinity of the oil surface. Oil flowing at a predetermined flow rate into the inner cylinder  24  from the pipe member  45  flows along the inner peripheral surface of the cylinder  41  due to inertia. Preferably, the oil flows inside an oil chamber  46 , which is formed inside the inner cylinder  24 , such that it is generally circular in plan view and such that the oil becomes a spiral flow along the inner peripheral surface of the cylinder  41 .  
         [0045]     With reference to  FIG. 3 ,  FIG. 4  and  FIG. 7 , communication holes  47  that extend from the inside of the cylinder  41  to the inside of the closed space  28  preferably are formed in the peripheral wall that defines the lower portion of the cylinder  41 . The communication holes  47  can be formed in any number of locations. In the illustrated arrangement, the communication holes  47  are formed at three places in the circumferential direction of the cylinder  41  in a lower region of the cylinder  41 . In the illustrated oil tank  12 , the communication holes  47  are formed in the lower portion of the inner cylinder  24 . Thus, the blow-by gas has largely separated from the oil before it passes through the communication holes  47  and flows into the second space  53 . For this reason, it becomes difficult for bubbles to form when the oil flows into the second space  53 .  
         [0046]     In the illustrated arrangement, the upper partition plate  22 , which supports the upper portion of the inner cylinder  24 , is formed in an annular shape. The inner cylinder  24  extends through the upper partition plate  22 . The upper partition plate is joined the inside of the cylinder  25  of the tank body  21  in any suitable manner. In one configuration, the upper partition plate  22  is welded to the cylinder  25 . The upper portion of the inner cylinder  24  is suitably secured to the upper partition plate  22 . In the illustrated configuration, the inner cylinder  24  is welded to the upper partition plate  22 . Thus, the inner cylinder  24  is supported in the tank body  21  via the upper partition plate  22 .  
         [0047]     As shown in  FIG. 6 , through holes  48 ,  49  and  50  extend through the upper partition plate  22 . These holes  48 ,  49 ,  50  are disposed at three places in sites (sites at the vehicle body front side) in the upper partition plate  22  opposite of the offset inner cylinder  24 . The through hole  49  preferably has a larger diameter than the other two holes  48 ,  50  and the oil level sensor  30  preferably is inserted through the enlarged hole  49 .  
         [0048]     The lower partition plate  23  supporting the lower portion of the inner cylinder  24  is joined with the inside of the cylinder  25  of the tank body  21  and, in some configurations, is welded to the cylinder  25 . As shown in  FIG. 7 , plural through holes  51  are disposed at sites in the lower partition plate  23  on the outer side of the inner cylinder  24 . Thus, in the illustrated oil tank  12 , the oil can be prevented from undulating inside the closed space  28  using the upper partition plate  22  and the lower partition plate  23 , which are members for retaining the inner cylinder  24  inside the tank body  21 , are baffles. For this reason, the number of parts can be reduced in comparison to the case where the oil tank is equipped with a stay for exclusively retaining the inner cylinder  24  and a baffle member exclusively used for preventing the oil from undulating.  
         [0049]     Because the inner cylinder  24  is supported in the tank body  21  by the upper partition plate  22  and the lower partition plate  23 , the closed space  28  inside the tank body  21  is partitioned into a first space  52  positioned above the upper partition plate  22 , a second space  53  positioned between the partition plates  22  and  23 , and a third space  54  positioned below the lower partition plate  23 .  
         [0050]     The illustrated tank body  21  is configured such that during ordinary use, the oil level is positioned generally at the height indicated by the two-dot chain line L 1  in  FIG. 3  and in  FIG. 4 . Namely, the first space  52  is filled substantially exclusively with blow-by gas, the second space  53  is filled with oil in its lower portion and with blow-by gas in its upper portion, and the third space  54  is filled substantially exclusively with oil.  
         [0051]     As shown in FIGS.  3  to  5 , the breather box  16  is generally defined by a housing  61 , which protrudes upward from the cover plate  26  of the tank body  21 , and a cylinder  62 , which is disposed inside the housing  61 . In the illustrated oil tank  12 , the bottom of the breather box  16  is defined by the cover plate  26  of the tank body  21 . Thus, a part dedicated to being the bottom of the breather box  16  becomes unnecessary and the number of components can be reduced as can the weight of the oil tank  12 .  
         [0052]     In the illustrated embodiment, the housing  61  has the shape of a bottomed cylinder that opens downward. Other configurations also are possible. As shown in  FIG. 5 , the illustrated housing  61  is also formed such that it is elongated in the left-right direction when seen in plan view. The housing  61  according to this embodiment is formed such that it protrudes toward the vehicle body right side (the left side in  FIG. 4  and  FIG. 5 ) with respect to the inner cylinder  24  when seen in plan view. According to this embodiment, a space is formed in the area above the tank body  21  to the front and left of the housing  61 . The threaded insert  31  is disposed in this space. Other configurations are possible.  
         [0053]     The end portion of the housing  61  at the vehicle body right side (the end portion at the left side in  FIG. 5 ) is formed such that covers, from above, the blow-by gas inlet  29  that extends through the cover plate  26 . A pipe member  63  extends through and, in some configurations, can be welded to an upper wall  61   a  of the housing  61  at a site that generally intersects the extension line of the axial centerline of the inner cylinder  24 . Other placements can also be used. The pipe member  63  can be connected to the blow-by gas pipe  17  in any suitable manner. The lower end of the pipe member  63  is positioned in the vicinity of the center of the housing  61  in the vertical direction. Again, other configurations are possible.  
         [0054]     The position of the pipe member  63  in the left-right direction is also positioned at the vehicle body right side (the left side in  FIG. 5 ) from the two-dot chain line L 2  shown in  FIG. 5 . The two-dot chain line L 2  represents the height of the oil level when the oil tank  12  is tilted to a worst case degree. Namely, as shown in  FIG. 5 , the opening in the lower end of the pipe member  63  will be positioned above the oil level L 2  in  FIG. 5  when the oil tank  12  reaches a worst-case scenario of tilting. For this reason, even when the vehicle body is tilted sideways such that the engine  2  is positioned below the oil tank  12 , the oil does not flow out toward the air cleaner  7  from the blow-by gas outlet. In particular, when the vehicle body is tilted sideways during maintenance, it becomes unnecessary to discharge the oil from the oil tank  12  so that maintenance can be easily conducted.  
         [0055]     Upper communication holes  64  extend through the cylinder  62  such that the inside and the outside of the cylinder  62  are placed in communication. In the illustrated arrangement, the holes  64  are disposed in the peripheral wall at the upper portion of the cylinder  62  of the breather box  16 . The cylinder  62  can be welded to, and/or supported on, the upper wall  61   a  of the housing  61 . As shown in  FIG. 3  and  FIG. 5 , the upper communication holes  64  can be formed in the end portion at the vehicle body front side and in the end portion at the vehicle body rear side of the cylinder  62 . In a preferred configuration, the upper communication holes  64  are formed at positions at about the same height and generally higher than the lower end of the pipe member  63 .  
         [0056]     With reference to  FIG. 3  and  FIG. 4 , the lower end portion of the cylinder  62  preferably receives the convex portion  26   a  of the cover plate  26 . Thus, the cylinder  62  preferably is positioned on the same axial line as the inner cylinder  24 . As shown in  FIG. 4 , a lower communication hole  65  that communicates the inside and the outside of the cylinder  62  can be disposed in the lower end portion of the cylinder  62 . Any lubricant that happens to make its way into the cylinder will drop from the air as it is drawn into the pipe member  63  and will spill out of the communication hole  65  into a first blow-by gas chamber  66 .  
         [0057]     The first blow-by gas chamber  66 , which is formed between the housing  61  and the cylinder  62 , and a second blow-by gas chamber  67 , which is formed inside the cylinder  62 , are formed inside the breather box  16  according to this embodiment. In this embodiment, what is called a curved air path in the present invention is configured by the first and second blow-by gas chambers  66  and  67 , the blow-by gas inlet  29 , the upper communication holes  64 , and the opening  68  in the lower end of the pipe member  63 . A blow-by gas outlet of the breather box  16  is defined by the opening  68  in the lower end of the pipe member  63 .  
         [0058]     In the oil tank  12  configured in this manner, the scavenge pump  13  is driven together with the engine  2 , whereby the oil flows at a predetermined flow speed into the inner cylinder  24  from the pipe member  45  disposed in the upper portion of the inner cylinder  24 . The oil flows into the inner cylinder  24  from a position higher than the oil level L 1 . Thus, the oil momentarily flies through the air before striking the inner peripheral surface of the inner cylinder  24 , and then flows along this inner peripheral surface. The oil flows in a spiral flow pattern inside the inner cylinder  24 . Thus, the oil spins around the inside of the inner cylinder  24  whereby the blow-by gas entrained in the oil is separated from the oil by centrifugal separation.  
         [0059]     The oil flows downward while spiraling inside the inner cylinder  24 , and passes through the communication holes  47  formed in the lower end portion of the inner cylinder  24 , whereby it flows out into the second space  53  from the inside of the inner cylinder  24 . At this time, the oil enters the communication holes  47  due to centrifugal force because the oil flows along the peripheral wall of the inner cylinder  24 . When the oil enters the second space  53  from the inside of the inner cylinder  24 , its flow speed drops and the direction in which it flows changes downward. Together with this, the blow-by gas that remains in the oil without having been separated inside the inner cylinder  24  rises and separates from the oil as a result of the change occurring in the flow of the oil inside the second space  53 . Thereafter, the oil passes through the through holes  51  in the lower partition plate  23 , flows into the third space  54  positioned therebelow, and is supplied from here to the engine  2  by the second oil pipe  15  including the pipe member  33 . The illustrated oil tank  12  supplies the oil to the engine from the bottom portion of the tank body  21 , into which the oil flows after the blow-by gas has been separated therefrom. Thus, just oil that is not mixed with blow-by gas, or oil mixed with a miniscule amount of blow-by gas, can be supplied to the engine  2 .  
         [0060]     The illustrated inner cylinder  24  of the oil tank  12  advantageously does not have any other members disposed in the axial center portion. For this reason, the blow-by gas collecting at the center portion due to the principle of centrifugal separation is not obstructed by another member when it moves upward. Thus, the blow-by gas can be efficiently separated. Intake air negative pressure acts inside the oil tank  12  including the inside of the breather box  16  while the engine  2  is running. Thus, the blow-by gas separated from the oil inside the inner cylinder  24  passes through the tube body  43  inside the tank body  21  and enters the first space  52 .  
         [0061]     The blow-by gas separated from the oil inside the second space  53  passes through the through holes  48  to  50  in the upper partition plate  22  and enters the first space  52 . The blow-by gas inside the first space  52  passes through the blow-by gas inlet  29  formed in the cover plate  26  and enters the first blow-by gas chamber  66  inside the breather box  16 .  
         [0062]     The blow-by gas flowing into the first blow-by gas chamber  66  flows upward as indicated by the arrow in  FIG. 4  and  FIG. 5  while separating the inside of the first blow-by gas chamber  66  into a vehicle body front side and a vehicle body rear side, passes through the upper communication holes  64  formed in the cylinder  62 , and flows into the second blow-by gas chamber  67  inside the cylinder  62 . Because the blow-by gas moves in this manner while curving in the horizontal direction and the vertical direction inside the first blow-by gas chamber  66 , oil mist included in the blow-by gas adheres to the housing  61  and the cylinder  62  and is separated from the blow-by gas.  
         [0063]     The blow-by gas flowing into the second blow-by gas chamber  67  similarly moves while curving in the horizontal direction and the vertical direction and is sucked into the pipe member  63 , because the upper communication holes  64  are positioned above the opening in the lower end of the pipe member  63 . For this reason, oil mist can be separated from the blow-by gas even in the second blow-by gas chamber  67 . The oil separated from the blow-by gas inside the second blow-by gas chamber  67  passes through the lower communication hole  65  formed in the lower end portion of the cylinder  41  and flows into the first blow-by gas chamber  66 . This oil, and the oil separated from the blow-by gas inside the first blow-by gas chamber  66 , passes through the blow-by gas inlet  29  opening to the bottom of the first blow-by gas chamber  66  and flows into the tank body  21 .  
         [0064]     The oil tank  12  is configured to accommodate a high rate of oil flow because the oil is forcibly discharged from the inner cylinder  24  into the second space  53  by centrifugal force. Also, because the oil tank  12  can separate the blow-by gas from the oil in at least two places (e.g., inside of the inner cylinder  24  and inside of the closed space  28 ) gas/liquid separation is sufficiently conducted, and oil mist included in the blow-by gas can be more effectively separated and removed by the first and second blow-by gas chambers  66  and  67 .  
         [0065]     Although the present invention has been described in terms of a certain embodiment, other embodiments apparent to those of ordinary skill in the art also are within the scope of this invention. Thus, various changes and modifications may be made without departing from the spirit and scope of the invention. For instance, various components may be repositioned as desired. Moreover, not all of the features, aspects and advantages are necessarily required to practice the present invention. Accordingly, the scope of the present invention is intended to be defined only by the claims that follow.