Abstract:
In an internal combustion engine for a vehicle having a drive clutch, a blowby control system includes a fresh air introducing channel, for introducing fresh air into a crankcase, and a blowby gas return channel for returning blowby gas from the crankcase to an air-intake system. A blowby control valve is provided in the fresh air introducing channel or the blowby gas return channel, for selectively restricting gas flow through the blowby system. A controller is also provided on the vehicle for controlling operation of the blowby control valve. The controller is programmed to switch the blowby control valve from the closed state to the open state, with the drive clutch connected, when the vehicle starts traveling. The blowby control system and method hereof can reduce variations in the engine speed occurring during engine idling and before clutch engagement, to improve ride quality of the vehicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present invention claims priority under 35 USC 119 based on Japanese patent application No. 2003-346155, filed Oct. 3, 2003.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to internal combustion engines. More particularly, the present invention relates to a blowby control system and method for an internal combustion engine on a vehicle.  
         [0004]     2. Description of the Background Art  
         [0005]     Internal combustion engines are used for many applications today, and are particularly useful as vehicular power sources. A number of known systems have been proposed to cope with blowby gases, caused primarily by air bypassing the compression rings. Many vehicles are equipped with positive crankcase ventilation (PCV) systems as standard equipment, to help manage blowby gases.  
         [0006]     In an internal combustion engine in which blowby gas from a crankcase is not discharged outside of the engine, but instead, is returned to a combustion chamber from an air-intake channel via a breather channel and is then burned, there is an example of a known approach in which a solenoid valve is interposed in the breather channel. (see, for example, Japanese Published Patent Document JP-A-9-68028).  
         [0007]     In the system disclosed in this reference, the breather channel is closed during deceleration when the intake vacuum increases to an abnormally high level, for limiting suction of the blowby gas into the combustion chamber.  
         [0008]     Therefore, since the blowby gas is drawn into the combustion chamber by a significant air-intake negative pressure with the throttle valve closed or throttled, when a vehicle starts traveling, the engine speed varies due to the combustion of HC mixed in the blowby gas. In particular, in a vehicle having a drive clutch, variations in the engine speed affect the feeling of engagement of the clutch before the clutch is fully engaged.  
         [0009]     Although the known blowby control systems are usable for their intended purposes, a need still exists in the art for an improved blowby control system and method. In particular, there is a need for an improved blowby control system and method which will overcome the difficulties encountered with the known art.  
       SUMMARY OF THE INVENTION  
       [0010]     In view of such points, it is an object of the present invention to provide a blowby control system and method for an internal combustion engine in which variations in engine speed before connecting the clutch are reduced, to improve ride quality.  
         [0011]     In order to achieve the aforementioned object, a first embodiment of the invention provides a blowby control system for an internal combustion engine to be mounted to a vehicle having a drive clutch, where the engine includes a crankcase and an air-intake system. The blowby control system according to the first embodiment includes a fresh air introducing channel for introducing fresh air into the crankcase, and a blowby gas return channel for returning blowby gas in the crankcase to an air-intake system. The blowby control system according to the first embodiment also includes a blowby control valve provided in the fresh air introducing channel or in the blowby gas returning channel for opening and closing the channel. The blowby control system according to the first embodiment also includes an electronic controller for controlling operation of the blowby control valve, wherein the controller is operable to switch the blowby control valve from the closed state to the opened state, with the drive clutch connected, when the vehicle starts traveling.  
         [0012]     Since the blowby control valve is switched from the closed state to the opened state with the drive clutch connected when the vehicle starts traveling, the blowby control valve is closed and the channel is blocked before the drive clutch is connected, so that the blowby gas is not drawn into the combustion chamber more than necessary, even when the intake vacuum is high. As a result, combustion is hardly affected by the blowby gas, and hence variations in the engine speed can be reduced to improve ride quality.  
         [0013]     Since the channel is closed in the idling state after having started the engine, variations in the idle speed are minimized.  
         [0014]     Since the blowby control valve is opened with the drive clutch connected, when the vehicle starts traveling thereafter, combustion is hardly affected by the blowby gas, and hence, the engine speed can be controlled without significant variation.  
         [0015]     In addition to the blowby control system for an internal combustion engine according to the first embodiment hereof, the invention according to a second embodiment is characterized in that the controller determines the operational state of the drive clutch based on the engine speed.  
         [0016]     Since the drive clutch is connected when the engine speed exceeds a certain level, the connecting state of the drive clutch, for controlling the blowby control valve, is determined based on the rotary speed of the internal combustion engine.  
         [0017]     In addition to the blowby control system for an internal combustion engine according to the second embodiment hereof, the invention according to a third embodiment is characterized in that the controller performs hysteresis control by setting the engine speed for closing the blowby control valve at a lower value than the engine speed for opening the valve.  
         [0018]     By opening the blowby control valve at a higher engine speed, switching operation with the drive clutch connected is reliably performed, and by closing the blowby control valve at a lower engine speed, a natural reduction of the engine speed by the inertia force of a crankshaft can be utilized after having disconnected the drive clutch, to prevent the vehicle ride quality from deteriorating. In addition, air ventilation of the crankcase is delayed until the engine speed is reduced to a low level, whereby the reduction capability of the blowby gas is improved.  
         [0019]     In addition to the blowby control system for an internal combustion engine according to the first embodiment hereof, the invention according to a fourth embodiment is characterized in that the blowby control valve is a solenoid valve which opens when energized.  
         [0020]     Since it is the solenoid valve which opens when energized, and since it is closed when no electricity is distributed thereto in the idling state after having started the engine, an electric load of the solenoid is prevented, and engine friction in association with power generation is also reduced.  
         [0021]     In addition to the blowby control system for an internal combustion engine according to the fourth embodiment hereof, the invention according to a fifth embodiment is characterized in that the controller shuts off power to the blowby conrol valve and closes the solenoid thereof when the engine speed is high.  
         [0022]     In the state in which the engine speed is high, increase in blowby gas at high rotation can be prevented by stopping power to, and closing the solenoid valve.  
         [0023]     In addition to the blowby control system for an internal combustion engine according to the first embodiment hereof, the invention according to a sixth embodiment is characterized in that a one-way valve is provided in the fresh air introducing channel to ventilate within the crankcase by variations in pressure in the crankcase in association with the reciprocation of the piston. This valve may be a reed valve.  
         [0024]     With the provision of the one-way valve in the fresh air introducing channel to ventilate within the crankcase compulsorily by vibrations in pressure in the crankcase in association with the reciprocation of the piston, reduction capability of the blowby gas can be improved.  
         [0025]     For a more complete understanding of the present invention, the reader is referred to the following detailed description section, which should be read in conjunction with the accompanying drawings. Throughout the following detailed description and in the drawings, like numbers refer to like parts. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a side view of a scooter-type vehicle, incorporating a blowby control system according to an illustrative embodiment of the present invention.  
         [0027]      FIG. 2  is a cross-sectional view of a powertrain unit of the scooter-type vehicle of  FIG. 1 , taken along a horizontal plane.  
         [0028]      FIG. 3  is a schematic block diagram showing an internal combustion engine and the blowby control system according to the present invention.  
         [0029]      FIG. 4  is a control map of a solenoid valve.  
         [0030]      FIG. 5  is a flowchart showing a control procedure of the solenoid valve. 
     
    
     DETAILED DESCRIPTION  
       [0031]     It should be understood that only structures considered necessary for clarifying the present invention are described herein. Other conventional structures, and those of ancillary and auxiliary components of the system, are assumed to be known and understood by those skilled in the art.  
         [0032]     Referring now to  FIG. 1  to  FIG. 5 , a selected illustrative embodiment of the present invention will be described.  
         [0033]     As best seen in  FIG. 1 , an internal combustion engine  30 , to which a selected illustrative embodiment of the present invention is applied, is a four-stroke, single-cylinder spark-ignition engine. The engine  30  is designed and configured to be mounted, as one part of a powertrain unit P, on a two-wheeled scooter-type vehicle V.  
         [0034]     The scooter-type vehicle V includes a vehicle body frame F, including a front frame  1  and a rear frame  2 . The vehicle V also includes a front fork  3  pivotally supported by a head pipe  1   a  of the front frame  1 , and a front wheel  6  rotatably supported by an arm  4 , which is pivotally supported at the lower end of the front fork  3 . The vehicle V also includes a front shock absorber  5  connecting the front fork  3  and the arm  4 , a handlebar  7  fixed to the upper end of the front fork  3 . The powertrain unit P is a swing-type powertrain unit, pivotally supported by a pivot mount  8  provided on the rear frame  2  of the vehicle body frame F.  
         [0035]     The vehicle V also includes a rear wheel  10 , rotatably supported at the rear end of the powertrain unit P. A rear shock absorber  9  connects a rear portion of the powertrain unit P to the rear frame  2 . A storage box  12  is attached to the rear frame  2 , for accommodating an article, such as a helmet  11 . A seat  13  is pivotally attached to the top of the storage box  12 , for covering the opening of the storage box  12  in a manner so as to be capable of opening and closing. A fuel tank  14  is mounted to the front frame  1 , and a vehicle body cover  15  is provided for covering the front vehicle frame F.  
         [0036]     The vehicle body frame F is a two-piece frame, including the front frame  1  and the rear frame  2  connected to each other with bolts. The front frame  1  includes the head pipe  1   a , a down frame  1   b  extending downward from the head pipe  1   a , and a pair of left and right floor supporting frames  1   c  extending rearward and substantially horizontally from the lower end of the down frame  1   b , all formed integrally by casting.  
         [0037]     The rear frame  2  includes a pair of left and right main frames  2   a , and a plurality of cross members (not shown) connected to both of the main frames  2   a , formed integrally with each other by casting. The front ends of the left and right main frames  2   a  are connected to the rear ends of the left and right floor supporting frames  1   c  by bolts.  
         [0038]     The vehicle body cover  15  includes a front cover  15   a  for covering the front portion of the head pipe  1   a , and a leg shield  15   b , for covering the rear portion of the head pipe  1   a  and the down frame  1   b . The leg shield  15   b  is positioned to protect the front of legs of a driver. The vehicle body cover  15  also includes a step floor  15   c  for supporting the driver&#39;s feet, and an under cover  15   d  disposed below the step floor  15   c . The under cover  15   d  is provided for covering the left and right floor supporting frames  1   c . The vehicle body cover  15  also includes a rear side cover  15   e  for covering the left and right main frame  2   a.    
         [0039]     The fuel tank  14  is disposed between the left and right floor supporting frames  1   c  and in a storage space defined by the step floor  15   c  and the under cover  15   d . Fuel in the fuel tank  14  is pumped by a fuel pump, and is supplied to a carburetor  122  of the internal combustion engine  30  (to be described later).  
         [0040]     The powertrain unit P includes a laterally disposed internal combustion engine  30  including a laterally extending crankshaft, and a belt-type stepless transmission  35 , for transmitting power from the internal combustion engine  30  to the rear wheel.  
         [0041]      FIG. 2  is a cross-sectional view of a powertrain unit of the scooter-type vehicle of  FIG. 1 , taken along a horizontal plane. The arrow drawn above the capital letter F in  FIG. 2  is provided for reference, and points toward the front of the vehicle.  
         [0042]     As seen in  FIG. 2 , a unit swing case  31  of the powertrain unit P includes a left unit case  31 L and a right unit case  31 R, joined together. The right unit case  31 R corresponds to a right half of a crankcase C. The left unit case  31 L includes a left front crankcase unit  31   a , which is elongated in the fore-and-aft direction, a transmission case unit  31   b  at the center, and a speed reducer case unit  31   c  at the rear. The crankcase C has a left protruded portion defined inside of the left front crankcase unit  31   a , and a right protruded portion inside of the right unit case  31 R. A crankshaft  40  is rotatably supported by left and right main bearings  41 ,  41  in the crankcase C, including the left front crankcase unit  31   a  and the right unit case  31 R.  
         [0043]     An opening formed in the left side of the left unit case  31 L is covered by a transmission case cover  36 , and accommodates the belt-type stepless transmission  35  therein. An opening formed in the right side of the rear speed reducer case unit  31   c  accommodates a speed-reducing mechanism  38  stored therein, and is covered by a speed reducer case cover  37 .  
         [0044]     An alternating current (AC) generator  60  is provided in the right protruded portion of the crankcase C, extending horizontally in the lateral direction. A cam-chain drive sprocket  55 , and a belt-drive pulley  76  of the belt-type stepless transmission  35 , are provided in the left protruded portion of the crankcase C.  
         [0045]     The internal combustion engine  30  includes a piston  42  reciprocating inside of a cylinder liner  44  of a cylinder block  32 . The piston  42  is connected to a crankpin  40   a  of the crankshaft  40  by a connecting rod  43 .  
         [0046]     This four-cycle internal combustion engine  30  employs an OHC type valve system, and includes a valve mechanism  50  in a cylinder head cover  34 , and a timing chain  51  extended between a camshaft  53  and the crankshaft  40 , for transmitting drive force to the valve mechanism  50 . A timing chain chamber  52  is provided therefor, to communicate between the left front crankcase unit  31   a , the cylinder block  32 , and a cylinder head  33 .  
         [0047]     In other words, the timing chain  51  is extended, through the timing chain chamber  52 , between a driven sprocket  54  fitted on the left end of the camshaft  53  oriented horizontally in the lateral direction and the drive sprocket  55  fitted to the crankshaft  40 .  
         [0048]     A spark plug  45  is fitted in the cylinder head  33  from the opposite side (right side) of the timing chain chamber  52  toward the combustion chamber.  
         [0049]     The AC generator  60 , provided inside the right unit case  31 R, includes a bowl-shaped outer rotor  62  fixed to the end of the crankshaft  40  projecting from the center cylindrical portion  31   d  of the right unit case  31 R via an ACG boss  61 , and a stator  64  having a stator coil  65  wound thereon is fixed to the center cylindrical portion  31   d  inside a magnet  63  disposed circumferentially on the inner peripheral surface.  
         [0050]     A compulsory air-cooling fan  66  is attached to the right side surface of the outer rotor  62 , and a fan cover  67  covers the fan, as shown. A fan shroud  68  is provided so as to continue from the fan cover  67  and to cover the periphery of the cylinder block  32  and the cylinder head  33 .  
         [0051]     On the other hand, the left front crankcase unit  31   a  of the unit swing case  31  includes the timing chain chamber  52  therein defined by being partitioned by the main bearing  41  from the crank chamber. The left wall of the timing chain chamber  52  corresponds to a partitioning wall  71 , which partitions between the belt-type stepless transmission chamber  70  and the timing chain chamber  52  on the left side thereof. The partitioning wall  71  is formed with a circular through-hole  71   a , formed into a flat cylindrical shape of a large diameter, through which the crankshaft  40  passes. An annular sealing member  72  is press-fitted into the through-hole  71   a , and the crankshaft  40  passes through a hollow portion of the annular sealing member  72 .  
         [0052]     The drive sprocket  55  is fitted to the crankshaft  40  between the sealing member  72  and the main shaft  41 , and the outer diameter of the annular sealing member  72  and the inner diameter of the through-hole  71   a  are larger than the maximum diameter thereof in a state in which the timing chain  51  is wound around the drive sprocket  55 .  
         [0053]     Therefore, since the through-hole  71   a  of a large diameter is opened if the annular sealing member  72  is removed from the through-hole  71   a , the timing chain  51  can be removed form the drive sprocket  55  or wound around the same by utilizing the opening.  
         [0054]     The belt-type stepless transmission chamber  70  is sealably partitioned from the timing chain chamber  52  by the sealing member  72 , so that oil is prevented from leaking into the belt-type stepless transmission chamber  70 .  
         [0055]     The crankshaft  40  passing through the sealing member  72  and extending therefrom is provided with the belt-drive pulley  76 , so as to be capable of rotating.  
         [0056]     The belt-drive pulley  76  includes a fixed pulley half  77  and a movable pulley half  78 , and the fixed pulley half  77  is fixed to the left end of the crankshaft  40  via a boss  79 , and the movable pulley half  78  is spline-fitted to the crankshaft  40  on the right side thereof. The movable pulley half  78  rotates with the crankshaft  40  and slides in the axial direction to move toward and away from the fixed pulley half  77 . A V-belt  75  is clamped between the both pulley halves  77 ,  78  and wound around.  
         [0057]     A cam plate  80  is provided at a fixed position which is on the right side of the movable pulley half  78  and close to the annular sealing member  72 , and a slide piece  80   a  provided on the outer peripheral edge thereof is slidably engaging a cam plate sliding boss  78   a  formed at the outer peripheral edge of the movable pulley half  78  in the axial direction.  
         [0058]     The side surface of the movable pulley half  78  on the side of the cam plate  80  is tapered toward the cam plate  80 , and a dry weight roller  81  is stored on the inner side of the tapered surface so as to be clamped by the cam plate  80 .  
         [0059]     Therefore, when the rotational speed of the crankshaft  40  increases, the dry weight roller  81 , which is disposed between the movable pulley half  78  and the cam plate  80  and rotates therewith, moves toward the centrifugal direction by a centrifugal force, and the movable pulley half  78  is pressed by the dry weight roller  81  and hence moved leftward toward the fixed pulley half  77 , so that the V-belt  75  clamped between the both pulley halves  77 ,  78  is moved in the centrifugal direction to increase the winding diameter.  
         [0060]     A belt-driven pulley  86  on the rear side corresponding to the belt-drive pulley  76  includes both pulley halves  87 ,  88  and configured in such a manner that the fixed pulley half  87  is fitted to an inner sleeve  89  supported so as to be capable of relative rotation with respect to a speed reducer input shaft  92  of the speed-reducing mechanism  38 , and the movable pulley half  88  is fitted to an outer sleeve  90  supported on the left side of the fixed pulley half  87  so as to be capable of sliding in the direction of the axis of the inner sleeve  89 .  
         [0061]     The V-belt  75  is clamped between the both pulley halves  87 ,  88 .  
         [0062]     A centrifugal clutch  91 , which corresponds to a drive clutch, is provided on the left side of the speed reducer input shaft  92  and the inner sleeve  89 . When the rotational speed thereof increases, the centrifugal clutch  91  joins, and thus a power transmitted via the V-belt  75  to the inner sleeve  89  is transmitted to the speed reducer input shaft  92 .  
         [0063]     The speed-reducing mechanism  38  reduces the speed of the power transmitted to an output shaft  94  by the speed reducer input shaft  92  by engagement of gears via the intermediate shaft  93 , and the output shaft  94  rotates the rear wheel  10  via the axle of the rear wheel  21 .  
         [0064]     A drum brake  95  is provided at the hub portion of the rear wheel  10 , and a drum brake arm  97 , to which a brake cable is connected, is fitted to the drum brake shaft  96 . When the brake cable is pulled, the drum brake arm  97  swings, and the drum brake shaft  96  is rotated so that the drum brake  95  works.  
         [0065]     The power transmission case cover  36 , which covers the belt-type stepless transmission chamber  70  from the left side, covers the portion extending from the belt-drive pulley  76  at the front to the centrifugal clutch  91  at the rear. The kick shaft  27  is pivotally passed through and supported by the position thereof slightly forward of the center, and a drive helical gear  20  is fitted on the inner end of the kick shaft  27 , and is urged by a return spring  21 .  
         [0066]     On the front inner surface of the power transmitting case cover  36 , a sliding shaft  22  is supported in the crankcase in a manner so as to be rotatable, axially slidable, and coaxially disposed with respect to the crankshaft  40 . The sliding shaft  22  is formed with a driven helical gear  23 , which engages the drive helical gear  20 , a ratchet wheel  24  is fixed to the right end thereof, and a friction spring  25  urges them leftward entirely.  
         [0067]     On the other hand, the boss  79  on the side of the crankshaft  40  includes a ratchet  79   a  formed so as to oppose the ratchet wheel  24 , and both of them can come into and out of contact with each other by sliding movement of the sliding shaft  22 .  
         [0068]     Therefore, when a kick pedal (not shown) is pressed down, and the kick shaft  27  rotates against the return spring  21 , the drive helical gear  20  rotates integrally with the kick shaft  27 , and the driven helical gear  23  engaging therewith rotates integrally with the sliding shaft  22  and simultaneously slides rightward against the friction spring  25 , so that the ratchet wheel  24  engages the ratchet  79   a  of the boss  79  to force the crankshaft  40  to rotate, thereby starting the internal combustion engine  30 .  
         [0069]     On the other hand, the internal combustion engine  30  includes a starter motor disposed above the left front crankcase unit  31   a  of the left unit case  31 L, although it is not shown in the drawing.  
         [0070]      FIG. 3  is a schematic block diagram showing the internal combustion engine  30  and a blowby control system  100  according to the present invention.  
         [0071]     The internal combustion engine  30  has the cylinder head  33  formed with an air-intake port  110  and an exhaust port  111  opening into a combustion chamber  109 . The cylinder head  33  is also provided with an air-intake valve  112  for controlling air flow through the air-intake port  110 , and an exhaust valve  113  for controlling air flow through the exhaust port  111 .  
         [0072]     In a cylinder head cover  34 , rocker arms  114 ,  115  for driving the air-intake valve  112  and the exhaust valve  113 , respectively, are pivotally provided in contact with a corresponding cam of the camshaft  53 .  
         [0073]     As previously noted, the camshaft  53  and the crankshaft  40  are connected by a timing chain  51  provided in the timing chain chamber  52 , which communicates between the valve chamber  34   a  and the crankshaft  40 . The camshaft  53  is rotated at a rotatary speed half that of the crankshaft  6 , so that the air-intake valve  112  and the exhaust valve  113  are opened and closed by a predetermined timing.  
         [0074]     An air-intake pipe  120  extending from the air-intake port  110  of the cylinder head  33  of the internal combustion engine  30  is connected to the carburetor  122  provided with a throttle valve  121 , and the carburetor  122  and an air cleaner  124  are connected by an intake air duct  123 .  
         [0075]     It is also possible to employ a fuel injection unit (not shown) instead of the carburetor  122 . In that case, the throttle valve is positioned upstream of the fuel injection valve.  
         [0076]     An exhaust pipe  28  extends downwardly from the exhaust port  111  and, as shown in  FIG. 1 , the exhaust pipe  28  extends along the lower surface of the crankcase C rearward and rightward to a muffler  29  disposed on the right side of the rear wheel  10 .  
         [0077]     The interior of an air cleaner case  125  of the air cleaner  124  is partitioned by an air cleaner element  126  into a clean side and a dirty side, and the intake air duct  123  is connected to the clean side downstream of the air cleaner element  126 .  
         [0078]     In the internal combustion engine  30  configured as described above, the crankcase C and the air cleaner case  125  are connected by a fresh air introducing channel  131 , and this allows a crank chamber Ca and the clean side of the air cleaner  124  to be brought into communication.  
         [0079]     The portion of the crankcase C where the fresh air introducing channel  131  is connected is formed with a narrowed portion  132  where the introducing area is narrowed.  
         [0080]     A reed valve  133 , which is a one-way valve for allowing air to be introduced from the air cleaner  124  to the crank chamber  2   a  and preventing air flow in the reverse direction, is provided at the narrowed portion  132  on the side of the fresh air introducing channel  131 .  
         [0081]     On the other hand, the cylinder head cover  34  and the upstream side of the intake air duct  123  on the downstream side of the air cleaner  124  are connected by a blowby gas return channel  135 , so that the valve chamber  34   a  and the interior of intake air duct  123  are brought into communication with each other.  
         [0082]     A solenoid valve  140  is interposed at a midsection of the fresh air introducing channel  131 , and an electronic control unit ECU  141  of a microcomputer programmably controls operation of the solenoid valve  140 .  
         [0083]     The ECU  141  controls the solenoid valve  140  based on the instantaneous rotary speed NE of the internal combustion engine  30  and the throttle opening TH being supplied thereto by appropriately situated sensors in electronic communication therewith.  
         [0084]     The solenoid valve  140  opens when energized, to allow air to flow through the fresh air introducing channel  131 .  
         [0085]     As described above, the blowby control system  100  is configured with the fresh air introducing channel  131  and the blowby gas return channel  135 .  
         [0086]     When the solenoid valve  140  is opened, variations in pressure of the crank chamber Ca brought about by pumping of the piston  42  of the internal combustion engine  30  draws fresh air (see hollowed outline arrow in  FIG. 3 ) from the air cleaner  124  into the crank chamber Ca, passing through the fresh air introducing channel  131  via the narrowed portion  132  and the reed valve  133 .  
         [0087]     Blowby gas (see a solid arrow in  FIG. 3 ) in the crank chamber Ca is moved from the timing chain chamber  52  to the valve chamber  34   a  by being pushed out by drawn fresh air, then is moved from the valve chamber  34   a  to a breather chamber  34   b , where it is separated into air and liquid. Then, it passes through the blowby gas return channel  135 , is discharged to the downstream side of the air cleaner  124 , and is drawn into the intake air duct  123 , and mixed with incoming combustion air, by the negative pressure on the downstream side of the air cleaner  124 , so that the crank chamber Ca is effectively ventilated.  
         [0088]     When the solenoid valve  140  is closed, fresh air is blocked from entering into the crank chamber Ca via the fresh air introducing channel  131 . Therefore, the interior of the crank chamber Ca is not ventilated compulsorily, and hence discharge of blowby gas is restrained.  
         [0089]     Referring now to a control map in  FIG. 4  and a flowchart in  FIG. 5 , opening and closing control of the solenoid valve  140  done by the ECU  41  will be described below.  
         [0090]      FIG. 4  is a control map stored in a memory of the ECU  41 , which is a two-dimensional coordinate map with the lateral axis representing the engine speed NE and the vertical axis representing the throttle opening TH.  
         [0091]     A cross-hatched area at the center is a valve-open area in which the solenoid valve  140  receives power distribution and hence is opened, and areas in which the values of the engine speed NE and the throttle opening TH are small, and the areas in which they are large are the valve-closed area in which the solenoid valve  140  does not receive the power distribution and hence is closed.  
         [0092]     There are boundary areas (hatched area in  FIG. 4 ) interposed between two boundaries in the valve-open area and the valve-close area, and the hysteresis control of switching between open and closed states of the solenoid valve  140  in these areas is implemented.  
         [0093]     In other words, when the state of the engine speed NE and the throttle opening TH moves through the boundary area, switching between open and closed states is performed by the boundary, which is further in the direction of movement.  
         [0094]     The valve-close area in which the engine speed NE and the throttle opening TH are small is defined by the first boundary rotation number CN 1  and the first boundary opening CT 1 , and the first boundary rotation number CN 1  and the first boundary opening CT 1  include two boundary values with hysteresis, respectively.  
         [0095]     The valve-close area in which both of the engine speed NE and the throttle opening TH are large is specified by the second boundary rotation number CN 2 , the third boundary rotation number CN 3  (&gt;CN 2 ), and the second boundary opening CT 2 , the third boundary opening CT 3  (&gt;CT 2 ), and the second boundary rotation number CN 2 , the third boundary rotation number CN 3 , the second boundary opening CT 2 , and the third boundary opening CT 3  include two boundary values with hysteresis, respectively.  
         [0096]     The first boundary rotation number CN 1  of the engine speed NE is substantially the same as the engine speed n, at which the centrifugal clutch  91  as a drive clutch is connected or disconnected, the boundary value CN 1   u  on the higher side of the first boundary rotation number CN 1  is larger than the engine speed n, and the boundary value CN  1   l  on the lower side of the first boundary rotation number CN 1  is smaller than the engine speed NE.  
         [0097]     In other words, those values are set so that the relation of CN  1   l &lt;n&lt;CN 1   u  is satisfied.  
         [0098]     The first boundary opening CT 1  of the throttle opening TH is a throttle opening which prevents the solenoid valve  140  from opening until the output of the engine increases by the throttle operation after starting and the engine speed NE reaches the engine speed n at which the centrifugal clutch  91  is connected.  
         [0099]     The second boundary rotation number CN 2 , the third boundary rotation number CN 3 , the second boundary opening CT 2 , and the third boundary opening CT 3  define the boundaries at which increase in blowby gas due to the high engine speed NE is promoted more than necessary.  
         [0100]     The second boundary rotation number CN 2  is a engine speed corresponding to the shift transmission area when the belt-type stepless transmission  35  is in the accelerating state, and the third boundary rotation number CN 3  is a engine speed at which the maximum output can be obtained.  
         [0101]     The second boundary opening CT 2  corresponds to the opening which is about 70% of the fully opened throttle opening, and the third boundary opening CT 3  corresponds to substantially the fully opened state.  
         [0102]     Referring now to a flowchart in  FIG. 5 , a control procedure implemented based on the control map described above will be described.  
         [0103]     In a first place, whether or not the engine speed NE is equal to or exceeds the first boundary rotation number CN 1  is determined (Step  1 ). When NE≦CN 1 , the procedure goes to Step  2 , and when NE&gt;CN 1 , the procedure jumps to Step  3 .  
         [0104]     The first boundary rotation number CN 1  in this case is with hysteresis having the lower boundary value CN 1   l  and the higher boundary value CN 1   u , and the boundary value at which the previous state of the solenoid valve  140  is maintained for a longer time becomes a reference value.  
         [0105]     In other words, when the solenoid valve  140  was in the valve-close state previously, the higher boundary value CN 1   u  becomes a reference value, and when it was in the valve-open state previously, the lower boundary value CN1l becomes a reference value.  
         [0106]     In the determination steps  2 ,  3 ,  4 ,  5  and  6 , described later, the hysteresis control is implemented in the same manner, and hence detailed description for each step is omitted.  
         [0107]     When it is determined that NE≦CN 1  in Step  1  and the procedure goes to Step  2 , whether or not the throttle opening TH is equal to or exceeds the first boundary opening CT  1  is determined.  
         [0108]     When it is determined that TH≦CT 1 , the procedure goes to Step  10 , where the solenoid valve  140  does not receive the power distribution and hence is brought into the closed state.  
         [0109]     In other words, when the engine speed NE does not exceed the first boundary revolving number CN 1  (NE≦CN 1 ), and the throttle opening TH does not exceed the first boundary opening CT 1  (TH≦CT 1 ), the solenoid valve  140  is closed and the fresh air introducing channel  131  is blocked.  
         [0110]     After having started the internal combustion engine  30  and before the centrifugal clutch  91  is connected and hence the vehicle V starts traveling, the solenoid valve  140  is closed to block the fresh air introducing channel  131 . Therefore, the blowby gas is prevented from being drawn into the combustion chamber  109  more than necessary even when the air-intake negative pressure is large, and hence combustion is hardly affected by the blowby gas. Consequently, variations in the engine speed are avoided and the ride quality may be improved.  
         [0111]     The valve-close area of the solenoid valve  140  includes an idling state after having started the internal combustion engine  30 , and hence variations in the number of idling rotation are prevented by closing the solenoid valve  140 .  
         [0112]     When it is determined to be NE&gt;CN 1  in Step  1  or when it is determined to be TH&gt;CT 1  in step  2 , the procedure jumps to Step  3 , where whether or not the engine speed NE is equal to or exceeds the second boundary rotation number CN 2  is determined. When it is determined to be NE≧CN 2 , the procedure goes to Step  4 . When it is determined to be NE&lt;CN 2 , the procedure jumps to Step  14 , where power is distributed to the solenoid valve  140  to bring it to the opened state.  
         [0113]     When the engine speed NE is transferred from the valve-close area of NE≦CN 1  to the valve-open area of CN 1 &lt;NE&lt;CN 2 , power is distributed to the solenoid valve  140  to open the same at the timing when the value exceeds the boundary value CN 1   u , which is the reference value on the higher side of the first boundary rotation number CN 1 , which is larger than the engine speed n at which the centrifugal clutch  91  is connected as described above.  
         [0114]     Therefore, power is distributed to the solenoid valve  140  to open the same in a state in which the centrifugal clutch  91  is connected at the timing when the vehicle V starts traveling. Therefore, as described above, the solenoid valve  140  is closed to block the fresh air introducing channel  131  before the centrifugal clutch  91  is connected, and hence blowby gas is prevented from being drawn into the combustion chamber  109  more than necessary, and variations in the engine speed can be reduced.  
         [0115]     After power is distributed to the solenoid valve  140  to open the same in the state of connecting centrifugal clutch  91 , the engine speed is increased into the traveling state, where combustion is hardly affected by the blowby gas, and hence the engine speed is controlled without variations.  
         [0116]     In contrast, when the engine speed NE is transferred from the valve-open area of CN 1 &lt;NE&lt;CN 2  to the valve-close area of NE≦CN 1 , power distribution to the solenoid valve  140  is stopped to close the same at the timing when the value underruns the boundary value CN 1   l , which is the reference value on the lower side of the first boundary rotation number CN 1 , which is smaller than the engine speed n at which the centrifugal clutch  91  is disconnected as described above.  
         [0117]     Therefore, by utilizing the reducing effect of variations in the engine speed by the inertia force of the crankshaft  40  after having disconnected the centrifugal clutch  91  and preventing the ride quality from deteriorating, and by performing air ventilation in the crankcase until the engine speed is reduced to the lower boundary value CN  1   l  of the first boundary rotation number CN 1 , the reduction capability of the blowby gas is improved.  
         [0118]     When the engine speed NE is determined to be the second boundary rotation number CN 2  (NE≧&gt;CN 2 ) in Step  3 , and the procedure goes to the step  4 , whether or not the engine speed NE is equal to or exceeds the third boundary rotation number CN 3  is determined. If NE≧CN 3  is satisfied, the procedure goes to Step  5 , where whether or not the throttle opening TH is equal to or exceeds the second boundary opening CT 2  is determined. If TH≧CT 2  is satisfied, the procedure goes to Step  11 , where power distribution to the solenoid valve  140  is stopped to close the solenoid valve  140 , and if TH&lt;CT 2  is satisfied, the procedure goes to Step  12 , where power is distributed to the solenoid valve  140  to open the same.  
         [0119]     When it is determined to be NE&lt;CN 3  in Step  4 , the procedure goes to Step  6 , and whether or not the throttle opening TH is equal to or exceeds the third boundary opening CT 3  is determined. If TH≧CT 3 , the procedure goes to Step  13  where power distribution to the solenoid valve  140  is stopped to close the same, and if TH&lt;CT 3 , the procedure goes to Step  14 , where power is distributed to the solenoid vlave  140  to open the same.  
         [0120]     In this manner, in the operating area in which the engine speed NE is large and the throttle opening TH is also large, it is controlled in such a manner that power distribution to the solenoid valve  140  is stopped to close the same so that the fresh air introducing channel  131  is blocked, whereby increase in blowby gas in the crankcase C due to high-rotation is prevented from being promoted.  
         [0121]     Since the solenoid valve  140  is adapted to open when power is distributed thereto, when in the idling state after having started the engine, it is closed without being distributed with power, and hence power load of the solenoid is prevented, thereby reducing the engine friction in association with power generation.  
         [0122]     In the present embodiment, the solenoid valve  140  is provided in the fresh air introducing channel  131 . However, it is also possible to provide the solenoid valve in the blowby gas return channel  135  and implement the same control.  
         [0123]     Also, the vehicle V of the present invention employs the centrifugal clutch  91  as the drive clutch so as to be connected and disconnected automatically, based on the engine speed. However, the blowby control system and method of the present invention can be applied to a vehicle in which the clutch is operated manually.  
         [0124]     Although the present invention has been described herein with respect to a number of specific illustrative embodiments, the foregoing description is intended to illustrate, rather than to limit the invention. Those skilled in the art will realize that many modifications of the preferred embodiment could be made which would be operable. All such modifications, which are within the scope of the claims, are intended to be within the scope and spirit of the present invention.