Abstract:
An automatic engine shutdown apparatus for automatically shutting down an engine under a predetermined running or operating condition. The apparatus includes a vehicle speed sensor for detecting a vehicle speed, a throttle sensor for detecting a throttle opening, a travel history memory for storing a travel history of a vehicle, and an engine shutdown controller for automatically shutting down the engine depending on the vehicle speed, the throttle opening, and the travel history. The engine shutdown controller shuts down the engine after elapse of a standby time depending on the travel history since a predetermined engine shutdown condition has been satisfied. As a result, the automatic engine shutdown time is optimized depending on predetermined running or operating conditions of the vehicle.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2004-229417, filed Aug. 5, 2004, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an automatic engine shutdown apparatus, and more particularly to an automatic engine shutdown apparatus for automatically shutting down an engine in view of the travel history of a vehicle. 
     2. Description of Background Art 
     Electric vehicles having a motor as a power source are environment-friendly and have good acceleration and deceleration responses. However, they have disadvantages in that their cruising distance is presently short due to a limited battery capacity and the unit cost of energy per traveled distance is high, making the electric vehicles less economic. Hybrid vehicles carrying a motor and an engine are put to practical use as eliminating the disadvantages and taking the advantages of the electric vehicles. 
     Generally known hybrid vehicles are classified into the series hybrid type, wherein only a motor is used as a power source of the vehicle and an engine is used as a drive source for a generator for charging a battery; the parallel hybrid type, wherein a motor and an engine are used as power sources of the vehicle and selectively operated depending on running conditions, etc.; and the series parallel type wherein the above two types are selectively used depending on running conditions. 
     Japanese Patent Laid-Open No. 2000-115908 discloses a hybrid vehicle technology for shutting down the engine when the accelerator pedal is not depressed, the vehicle speed is of a predetermined value or less, and the remaining charged capacity of the battery is sufficient, in order to reduce wasteful fuel consumption while the hybrid vehicle is at rest and also to reduce the emission of exhaust gases. 
     According to the related art described above, if engine shutdown conditions are satisfied based on the accelerator pedal state and the vehicle speed, the engine is automatically shut down unless remaining charged capacity of the battery is insufficient. In the related art described above, the travel history of the vehicle is not taken into account in shutting down the engine. When the hybrid vehicle runs on a jammed street, the engine tends to be repeatedly automatically shut down and restarted. If a standby time after an automatic engine shutdown condition is satisfied until the engine is actually shut down is set to a long period depending on the travel on the jammed street, then when the hybrid vehicle stops at a traffic signal, or the like, while running on a street with less traffic, the engine is not immediately shut down. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     It is an object of the present invention to provide an automatic engine shutdown apparatus for automatically shutting down an engine under a predetermined condition, which will solve the above related-art problems and allows an automatic engine shutdown time to be optimized depending on the running state of a vehicle. 
     In order to achieve the above object, in accordance with the present invention provides an automatic engine shutdown apparatus for automatically shutting down an engine under a predetermined running or operating condition. The apparatus includes: 
     (1) a vehicle speed sensor for detecting a vehicle speed; and a throttle sensor for detecting a throttle opening, travel history memory means for storing a travel history of a vehicle, and engine shutdown control means for automatically shutting down the engine depending on the vehicle speed, the throttle opening, and the travel history. 
     (2) The engine shutdown control means shuts down the engine after elapse of a standby time depending on the travel history since a predetermined engine shutdown condition has been satisfied. 
     (3) The engine shutdown control means increases the standby time as the vehicle stops more frequently. 
     (4) The automatic engine shutdown apparatus further includes means for storing an initial value with respect to the standby time, wherein the engine shutdown control means shuts down the engine after elapse of the initial value since detected results of the vehicle speed and the throttle opening have satisfied predetermined conditions if information relative to the travel history is short. 
     (5) In the automatic engine shutdown apparatus, the travel history represents the number of times that the vehicle stops within a predetermined period of time. 
     (6) According to the present invention, there is also provided an automatic engine shutdown apparatus for automatically shutting down an engine under a predetermined running or operating condition, the automatic engine shutdown apparatus including power transmitting means for transmitting power of the engine to a drive wheel. This apparatus includes a starting clutch disposed between the engine and the power transmitting means, for transmitting the power of the engine to the power transmitting means when the engine reaches a predetermined rotational speed, a vehicle speed sensor for detecting a vehicle speed, a throttle sensor for detecting a throttle opening, travel history memory means for storing a travel history of a vehicle, and engine shutdown control means for automatically shutting down the engine depending on the vehicle speed, the throttle opening, and the travel history. 
     The present invention offers the following advantages: 
     (1) Since an engine shutdown time is determined in view of not only the vehicle speed and the throttle opening, but also the running state, the engine can automatically be shut down at an optimum time depending on the running state. 
     (2) Since the engine is shut down after elapse of the standby time depending on the travel history since the predetermined engine shutdown condition has been satisfied, the engine can automatically be shut down at an optimum time depending on the running state. 
     (3) Since the standby time after the engine shutdown condition is satisfied until the engine is automatically shut down is increased as the vehicle stops more frequently, the engine is prevented from being frequency shut down and started while the vehicle is running on a jammed street. When the vehicle stops less frequently while running on a street with less traffic, the standby time after the engine shutdown condition is satisfied until the engine is shut down is reduced, thereby preventing the engine from idling uselessly. Therefore, the engine can be optimally controlled for shutdown. 
     (4) If information relative to the travel history is short, the engine is shut down after elapse of the initial value that has been pre-registered since the engine shutdown condition has been satisfied. Therefore, even during a period shortly after the vehicle has started running, the engine can be optimally controlled for shutdown. 
     (5) Further, the frequency with which the vehicle stops can easily be determined. 
     (6) In addition, the power of the engine is blocked by the starting clutch and is not transmitted to a power transmitting mechanism, and hence the power transmitting mechanism does not operate. Therefore, the fuel consumption rate is increased. Inasmuch as the engine shutdown time is determined in view of not only the vehicle speed and the throttle opening, but also the running state, the engine can automatically be shut down at an optimum time depending on the running state. As a result, the fuel consumption rate is further increased. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a side elevational view of a two-wheeled vehicle as an embodiment of a hybrid vehicle according to the present invention; 
         FIG. 2  is a block diagram of a system arrangement of the two-wheeled vehicle shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view of a power unit of the two-wheeled vehicle shown in  FIG. 1 ; 
         FIG. 4  is an enlarged fragmentary view of  FIG. 3 ; 
         FIG. 5  is a flowchart of an engine shutdown control process; 
         FIG. 6  is a diagram showing an engine shutdown condition established using a throttle opening θth and a vehicle speed V as parameters; 
         FIG. 7  is a diagram showing the corresponding relationship between a vehicle stop frequency Mstop and a standby time Tk; and 
         FIG. 8  is a diagram showing another engine shutdown condition established using a throttle opening θth and a vehicle speed V as parameters. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a side elevational view of an embodiment of a hybrid vehicle to which the present invention is applied. 
     A hybrid vehicle has a front fork  1  on which a front wheel WF is supported by a shaft forwardly of a vehicle body. The front fork  1  is pivotally supported on a head pipe  2  and can be steered by a handle  3 . A down pipe  4  is mounted on and extends rearwardly and downwardly from the head pipe  2 . An intermediate frame  5  extends substantially horizontally from the lower end of the down pipe  4 . A rear frame  6  extends rearwardly and upwardly from the rear end of the intermediate frame  5 . 
     A power unit  11  including a power source has an end pivotally mounted on a vehicle frame  10  that is constructed as described above. A rear wheel WR is rotatably mounted on the other rear end of the power unit  11 , and suspended by a rear cushion mounted on the rear frame  6 . 
     The vehicle frame  10  is covered with a vehicle cover  13 , and a seat  14  for the rider to be seated thereon is fixed to a rear upper surface of the vehicle cover  13 . A step floor  15  for the rider to place its feet thereon is disposed forwardly of the seat  14 . A storage box  100  functioning as a utility space for storing a helmet, a cargo, etc. is disposed below the seat  14 . 
       FIG. 2  is a block diagram of a system arrangement of the hybrid vehicle. The power unit  11  comprises an engine  20 , an ACG starter motor  21  a functioning as an engine starter and generator, a continuously variable transmission (power transmitting means)  23  coupled to a crankshaft  22  for transmitting power of the engine  20  to the rear wheel WR, a starting clutch  40  for selectively transmitting power between the crankshaft  22  and the input shaft of the continuously variable transmission  23 , a drive motor  21   b  functioning as a motor or a generator, a one-way clutch (one-way power transmitting means)  44  for transmitting power from the engine  20  and the drive motor  21   b  to the rear wheel WR, but preventing power from being transmitted from the rear wheel WR to the engine  20 , and a speed reducer mechanism  69  for transmitting output power from the continuously variable transmission  23  at a certain speed reduction ratio to the rear wheel WR. The rotational speed Ne of the engine  20  is detected by an engine rotational speed sensor  36 . 
     Power from the engine  20  is transmitted from the crankshaft  22  through the starting clutch  40 , the continuously variable transmission  23 , the one-way clutch  44 , a drive shaft  60 , and the speed reducer mechanism  69  to the rear wheel WR. Power from the drive motor  21   b  is transmitted through the drive shaft  60  and the speed reducer mechanism  69  to the rear wheel WR. In the present embodiment, the driver shaft  60  doubles as the output shaft of the drive motor  21   b.    
     A battery  74  is connected to the ACG starter motor  21   a  and the drive motor  21   b . The battery  74  supplies electric power to the drive motor  21   b  and the ACG starter motor  21   a  when the drive motor  21   b  functions as a motor and the ACG starter motor  21   a  functions as a starter. The battery  74  is charged with electric power regenerated by the ACG starter motor  21   a  and the drive motor  21   b  when the ACG starter motor  21   a  and the drive motor  21   b  function as generators. 
     The engine  20  has an intake pipe  16  housing a throttle valve  17  angularly movably disposed therein for controlling the rate of intake air flowing through the intake pipe  16 . The throttle valve  17  is angularly moved depending on the movement of a throttle grip (not shown) that is operated by the rider of the hybrid vehicle. Between the throttle valve  17  and the engine  20 , there are disposed an injector  18  for injecting fuel and a negative pressure sensor  19  for detecting a negative pressure in the intake pipe  16 . The throttle valve  17  has its opening θth detected by a throttle sensor  39 . An ignition unit  38  supplies ignition energy at a predetermined ignition time to a spark plug  45 . The vehicle speed of the hybrid vehicle is detected by a vehicle speed sensor  37 . 
     A control unit  7  has an engine shutdown controller  7   a  for automatically shutting down the engine  20  when the hybrid vehicle stops, a running state monitor unit  7   b  for monitoring the running state of the hybrid vehicle, and a charge controller  7   c  for monitoring the remaining charged capacity of the battery  74  based on a battery voltage detected by a voltage sensor  47  and controlling the charging of the battery  74  when the hybrid vehicle stops. 
     A ROM  42  includes a θth/V table  42   a  containing an engine shutdown condition, to be described in detail later, determined using the throttle opening θth and the vehicle speed V as parameters, and a Tk/Mstop table  42   b  containing the corresponding relationship between a standby time Tk after an engine shutdown condition is satisfied until the engine is actually shut down, and a vehicle stop frequency Mstop. A RAM  43  includes a travel history memory  43   a  for storing the history of the running state detected by the running state monitor unit  7   b.    
     Structural details of the power unit  11  including the engine  20  and the drive motor  21   b  will be described below with reference to  FIG. 3 . 
     The engine  20  has a piston  25  operatively coupled to the crankshaft  22  by a connecting rod  24 . The piston  25  is slidable in a cylinder  27  disposed in a cylinder block  26 . The cylinder block  26  is arranged such that the cylinder  27  has a substantially horizontal axis. A cylinder head  28  is fixed to a front surface of the cylinder block  26 . The cylinder head  28 , the cylinder  27 , and the piston  25  jointly define a combustion chamber  20   a  for combusting an air-fuel mixture therein. 
     Valves (not shown) for controlling the intake of an air-fuel mixture into the combustion chamber  20   a  and the discharge of exhaust gases from the combustion chamber  20   a , and a spark plug  29  are mounted in the cylinder head  28 . The valves are opened and closed by the rotation of a camshaft  30  which is rotatably supported in the cylinder head  28 . The camshaft  30  has a driven sprocket  31  mounted on an end thereof, and an endless cam chain  33  is trained around the driven sprocket  31  and a drive sprocket  32  mounted on an end of the crankshaft  22 . A water pump  34  for cooling the engine  20  is connected to the end of the camshaft  30 . The water pump  34  has a rotatable shaft  35  attached for rotation with the camshaft  30 . Therefore, the water pump  34  operates when the camshaft  30  rotates. 
     A stator case  49  is coupled to a transversely right side of a crankcase  48  by which the crankshaft  22  is rotatably supported, and houses the ACG starter motor  21   a  therein. The ACG starter motor  21   a  is a so-called outer-rotor motor and has a stator comprising a coil  51  in the form of a conductive wire wound around teeth  50  fixed to the stator case  49 . An outer rotor  52  is fixed to the crankshaft  22  and has a substantially cylindrical shape covering the stator. Magnets  53  are disposed on the inner circumferential surface of the outer rotor  52 . 
     A fan  54   a  for cooling the ACG starter motor  21   a  is mounted on the outer rotor  52 . When the fan  54   a  rotates in synchronism with the crankshaft  22 , cooling air is introduced from a cooling air inlet defined in a side wall  55   a  of a cover  55  of the stator case  49 . 
     A transmission case  59  is coupled to a transversely left side of the crankcase  48 , and houses therein a fan  54   b  fixed to the left end of the crankshaft  22 , the continuously variable transmission  23  whose drive side is coupled to the crankshaft  22  through the starting clutch  40 , and the drive motor  21   b  coupled to the driven side of the continuously variable transmission  23 . The fan  54   b  serves to cool the continuously variable transmission  23  and the drive motor  21   b  which are housed in the transmission case  59 . The fan  54   b  is disposed on the same side of the continuously variable transmission  23  as that of the drive motor  21   b , i.e., on the transversely left side of the hybrid vehicle according to the present embodiment. 
     A cooling air inlet  59   a  is disposed in a front left side of the transmission case  59 . When the fan  54   b  rotates in synchronism with the crankshaft  22 , ambient air is introduced from the cooling air inlet  59   a  positioned near the fan  54   b  into the transmission case  59 , forcibly cooling the drive motor  21   b  and the continuously variable transmission  23 . 
     The continuously variable transmission  23  is a belt converter including a drive transmission pulley  58  mounted on the left end portion of the crankshaft  22  that projects transversely from the crankcase  48  with the starting clutch  40  interposed therebetween, a driven transmission pulley  62  mounted on the drive shaft  60  parallel to the crankshaft  22  with the one-way clutch  44  interposed therebetween, and an endless V-belt (endless belt)  63  trained around the drive transmission pulley  58  and the driven transmission pulley  62 . 
     As shown fragmentarily at an enlarged scale in  FIG. 4 , the drive transmission pulley  58  is mounted circumferentially rotatably on the crankshaft  22  with a sleeve  58   d  interposed therebetween. The drive transmission pulley  58  comprises a fixed drive pulley member  58   a  fixedly mounted on the sleeve  58   d  and a movable drive pulley member  58   c  axially slidably, but circumferentially nonrotatably, mounted on the sleeve  58   d.    
     The driven transmission pulley  62  comprises a fixed driven pulley member  62   a  mounted axially nonslidably, but circumferentially rotatably, on the drive shaft  60 , and a movable driven pulley member (movable driven pulley)  62   b  mounted axially slidably on a boss  62   c  of the fixed driven pulley member  62   a.    
     The endless V-belt  63  is trained around belt grooves of substantially V-shaped cross section which are defined between the fixed drive pulley member  58   a  and the movable drive pulley member  58   c  and between the fixed driven pulley member  62   a  and the movable driven pulley member  62   b.    
     A spring (resilient member)  64  for normally biasing the movable driven pulley member  62   b  toward the fixed driven pulley member  62   a  is disposed behind the movable driven pulley member  62   b , i.e., on the transversely left side of the movable driven pulley member  62   b.    
     When the rotational speed of the crankshaft  22  increases, a weight roller  58   b  of the drive transmission pulley  58  is subjected to a centrifugal force, causing the movable drive pulley member  58   c  to slide toward the fixed drive pulley member  58   a . The movable drive pulley member  58   c  moves toward the fixed drive pulley member  58   a , reducing the width of the groove of the drive transmission pulley  58 . The position where the drive transmission pulley  58  and the V-belt  63  contact each other is shifted radially outwardly, increasing the radius of the circle around which the V-belt  63  is trained. Accordingly, the width of the groove defined between the fixed driven pulley member  62   a  and the movable driven pulley member  62   b  of the driven transmission pulley  62  is increased. Thus, depending on the rotational speed of the crankshaft  22 , the radius of the circle around which the V-belt  63  is trained, i.e., the transmission pitch circle diameter, changes continuously, causing the speed reduction ratio to change automatically and continuously. 
     The starting clutch  40  is disposed outwardly of the continuously variable transmission  23 , i.e., on the transversely left side in the present embodiment, i.e., between the fixed drive pulley member  58   a  and the fan  54   b  closely to the cooling air inlet  59   a  on the transmission case  59 . 
     The starting clutch  40  comprises a cup-shaped outer case  40   a  fixedly mounted on the sleeve  58   d , an outer plate  40   b  fixed to the left end of the crankshaft  22 , a shoe  40   d  mounted on an outer circumferential portion of the outer plate  40   b  by a weight  40   c  and facing radially outwardly, and a spring  40   e  for biasing the shoe  40   d  radially inwardly. 
     When the rotational speed of the engine, i.e., the rotational speed of the crankshaft  22 , is equal to or lower than a predetermined value, e.g., 3000 rpm, no power is transmitted between the crankshaft  22  and the continuously variable transmission  23 . As the rotational speed of the engine increases and hence the rotational speed of the crankshaft  22  increases to a value in excess of the predetermined value, the centrifugal force acting on the weight  40   c  counteracts the resilient force applied radially inwardly by the spring  40   e , causing the weight  40   c  to move radially outwardly, whereupon the shoe  40   d  presses the inner circumferential surface of the outer case  40   a  with a force equal to or greater than a predetermined value. The rotation of the crankshaft  22  is now transmitted through the outer case  40   a  to the sleeve  58   d , driving the drive transmission pulley  58  fixed to the sleeve  58   d.    
     The one-way clutch  44  comprises a cup-shaped outer clutch member  44   a , an inner clutch member  44   b  inserted coaxially in the outer clutch member  44   a , and a roller  44   c  for transmission power unidirectionally from the inner clutch member  44   b  to the outer clutch member  44   a . The outer clutch member  44   a  doubles as the inner rotor body of the drive pulley  21   b  and is constructed as a member identical to the inner rotor body. 
     Power from the engine  20  which is transmitted to the driven transmission pulley  62  of the continuously variable transmission  23  is transmitted through the fixed driven pulley member  62   a , the inner clutch member  44   b , the outer clutch member  44   a , i.e., the inner rotor body, the drive shaft  60 , and the speed reducer mechanism  69  to the rear wheel WR. When the hybrid vehicle is pushed by the rider or is in a regenerative mode, power from the rear wheel WR is transmitted through the speed reducer mechanism  69  and the drive shaft  60  to the inner rotor body, i.e., the outer clutch member  44   a . Since the outer clutch member  44   a  rotates idly with respect to the inner clutch member  44   b , the power from the rear wheel WR is not transmitted to the continuously variable transmission  23  and the engine  20 . 
     The inner-rotor-type drive motor  21   b  with the drive shaft  60  serving as a motor output shaft is disposed in a rear portion of the transmission case  59 . 
     The drive motor  21   b  has an inner rotor  80  comprising the drive shaft  60  which also serves as the output shaft of the continuously variable transmission  23 , the cup-shaped inner rotor body, i.e., the outer clutch member  44   a , splined to the drive shaft  60  by a central boss  80   b  thereof, and magnets  80   c  disposed on an outer circumferential surface of the outer clutch member  44   a  near the open end thereof. A plurality of detectable elements  82  for being detected by a rotor sensor  81  mounted on an inner wall surface  59 A of the transmission case  59  are mounted on an outer circumferential surface of the outer clutch member  44   a  near the bottom end thereof. The drive motor  21   b  has a stator  83  comprising a coil  83   c  in the form of a conductive wire wound around teeth  83   b  fixed to a stator case  83   a  in the transmission case  59 . 
     The drive motor  21   b  functions as a motor for assisting in the output power of the engine  20  and also functions as a generator for converting the rotation of the drive shaft  60  into electric energy to charge the battery  74  not shown in  FIG. 4  in the regenerative mode. The drive motor  21   b  is directly mounted on the inner wall surface  59 A of the transmission case  59 , which is made of metal, by the stator case  83   a . A plurality of cooling fins  59   b  extending longitudinally of the hybrid vehicle and spaced at intervals are mounted on an outer wall surface  59 B of the transmission case  59  at locations where the drive motor  21   b  is directly mounted on the inner wall surface  59 A. 
     Referring back to  FIG. 3 , the speed reducer mechanism  69  is disposed in a transmission chamber  70  that is contiguous to a right side of the rear end of the transmission case  59 . The speed reducer mechanism  69  has an intermediate shaft  73  rotatably supported parallel to the drive shaft  60  and the axle  68  of the rear wheel WR, a pair of first speed reducer gears  71  mounted respectively on a right end portion of the drive shaft  60  and a central portion of the intermediate shaft  73 , and a pair of second speed reducer gears  72  mounted respectively on a left end portion of the intermediate shaft  73  and a left end portion of the axle  68 . The rotation of the drive shaft  60  is transmitted at a predetermined speed reduction ratio to the axle  68  of the rear wheel WR which is rotatably supported parallel to the drive shaft  60 . 
     For starting the engine  20 , the crankshaft  22  is rotated by the ACG starter motor  21   a  on the crankshaft  22 . At this time, the starting clutch  40  is not engaged, and no power is transmitted from the crankshaft  22  to the continuously variable transmission  23 . 
     When the rotational speed of the crankshaft  22  exceeds the predetermined value, e.g., 3000 rpm, depending on the movement of the throttle grip, the rotational power of the crankshaft  22  is transmitted through the starting clutch  40  to the continuously variable transmission  23 , the one-way clutch  44 , and the speed reducer mechanism  69 , driving the rear wheel WR. When the hybrid vehicle is thus started, the drive motor  21   b  may be energized by the electric power supplied from the battery  74  to assist in the rotation of the drive shaft  60  that is rotated by the power from the engine  20 . 
     The hybrid vehicle may be started by the drive motor  21   b  only, rather than by the engine  20 . In this case, since the rotation of the drive shaft  60  that is rotated by the drive motor  21   b  is not transmitted to the driven transmission pulley  62  by the one-way clutch  44 , the continuously variable transmission  23  is not driven. Therefore, when the rear wheel WR is driven by the drive motor  21   b  only, the energy transmitting efficiency is increased. 
     If the load on the engine  20  is large upon acceleration or high-speed running while the hybrid vehicle is being propelled by the engine  20  only, the engine-propelled travel may be assisted by the drive motor  21   b . At this time, the rotational power of the crankshaft  22  which is rotated by the reciprocating motion of the crankshaft  22  is transmitted through the starting clutch  40 , the continuously variable transmission  23 , and the one-way clutch  44  to the drive shaft  60 , and the power from the drive motor  21   b  is also transmitted to the drive shaft  60 . Therefore, the combination of the power from the engine  20  and the power from the drive motor  21   b  drives the rear wheel WR through the speed reducer mechanism  69 . Conversely, while the hybrid vehicle is being propelled by the drive motor  21   b  only, the motor-propelled travel may be assisted by the engine  20 . 
     While the hybrid vehicle is being driven at a constant speed in a cruise mode by the drive motor  21   b  only, when the engine  20  is operated, the continuously variable transmission  23  may not be driven, but the ACG starter motor  21   a  may generate electric power, if the rotational speed of the crankshaft  22  is equal to or lower than the rotational speed for engaging the starting clutch  40 , i.e., the predetermined value referred to above. 
     While the hybrid vehicle is being thus driven at a constant speed by the drive motor  21   b  only, the energy transmitting efficiency is better because the power is transmitted from the drive motor  21   b  to the rear wheel WR without the continuously variable transmission  23  being driven. 
     When the hybrid vehicle is decelerated, since the one-way clutch  44  does not transmit the rotation of the drive shaft  60  to the driven transmission pulley  62  of the continuously variable transmission  23 , the rotation of the axle  68  can be directly transmitted through the speed reducer mechanism  69 , to the drive motor  21   b  in the regenerative mode without the continuously variable transmission  23  being driven. 
     Specifically, when the rear wheel WR drives the drive motor  21   b  in the regenerative mode, since the power transmitted from the rear wheel WR to the drive motor  21   b  is not consumed to drive the continuously variable transmission  23 , the charging efficiency in the regenerative mode is increased. 
       FIG. 5  is a flowchart of an engine shutdown control process that is performed by the engine shutdown controller  7   a . The engine shutdown control process is repeatedly performed in predetermined cyclic periods. 
     In step S 1 , the throttle opening θth is detected based on an output signal from the throttle sensor  39 , and the vehicle speed V is detected based on an output signal from the vehicle speed sensor  37 . In step S 2 , it is determined whether an engine shutdown condition is satisfied or not. In the present embodiment, the engine shutdown condition using the throttle opening θth and the vehicle speed V as parameters established in the θth/V table  42   a , as shown in  FIG. 6 . If the relationship between the throttle opening θth and the vehicle speed V satisfies the engine shutdown condition, then control goes to step S 4 . 
     In step S 4 , it is determined whether a vehicle stop time timer Tstop for measuring a continuous period of time in which the engine shutdown condition is satisfied is measuring the continuous period of time, i.e., whether the vehicle stop time timer Tstop has started, or not. Since the vehicle stop time timer Tstop is initially not measuring the continuous period of time, control goes to step S 5  in which the vehicle stop time timer Tstop starts measuring the continuous period of time. In step S 6 , it is determined whether the travel history memory  43   a  of the RAM  43  has collected a sufficient travel history or not. 
     In the present embodiment, the vehicle running state after the ignition switch is turned on until it is turned off is monitored by the running state monitor unit  7   b , and is stored as a travel history in the travel history memory  43   a . If the period of time that has elapsed since the hybrid vehicle has started running is not short and a sufficient travel history has been collected, then control goes to step S 7  in which the vehicle stop frequency Mstop is detected based on the travel history. In the present invention, the moving average of the number of times that the hybrid vehicle has stopped in a predetermined unit time is determined as the vehicle stop frequency Mstop. 
     In step S 8 , a standby time Tk after the engine shutdown condition is satisfied until the engine is actually shut down is determined based on the vehicle stop frequency Mstop. In the present invention, the corresponding relationship between the vehicle stop frequency Mstop and the standby time Tk is pre-registered in the Tk/Mstop table  42   b , as shown in  FIG. 7 , such that the standby time Tk is longer as the vehicle stop frequency Mstop is higher. The standby time Tk corresponding to the vehicle stop frequency Mstop is determined. 
     If no sufficient travel history has been collected immediately after the hybrid vehicle has started running, then control goes from step S 6  to step S 9  in which a predetermined initial value Tint is registered as the standby time Tk. In step S 10 , the vehicle stop time timer Tstop is compared with the standby time Tk. If the vehicle stop time timer Tstop exceeds the standby time Tk, then control goes to step S 14  in which the engine  20  is automatically shut down. If the vehicle stop time timer Tstop does not exceed the standby time Tk, then control goes to step S 11  in which the remaining charged capacity of the battery  74  is detected based on the battery voltage that is periodically detected by the voltage sensor  47 . 
     If the remaining charged capacity of the battery  74  is sufficient, then control goes to step S 12  in which the engine rotational speed Ne while the hybrid vehicle is at rest is maintained at the level of an idling speed Nidle. If the remaining charged capacity of the battery  74  is not sufficient, then control goes to step S 13 . In step S 13 , the engine rotational speed Ne while the hybrid vehicle is at rest is maintained at the level of a charging speed Ncharge that is higher than the idling speed Nidle and slightly lower than the clutch engaging speed of the starting clutch  40 , by the charge controller  7   c.    
     Thereafter, if the vehicle stop time timer Tstop exceeds the standby time Tk as detected in step S 10 , then control goes to step S 14  in which the engine  20  is automatically shut down. If the hybrid vehicle starts running before the vehicle stop time timer Tstop exceeds the standby time Tk and it is judged in step S 2  that the engine shutdown condition is not satisfied, then control goes to step S 3  in which the vehicle stop time timer Tstop is reset. 
     According to the present embodiment, as described above, if the vehicle stop frequency is higher as when the hybrid vehicle is running on a jammed street, then the standby time after the engine shutdown condition is satisfied until the engine is shut down is increased to prevent the engine from being frequently shut down and started. If the vehicle stop frequency is lower as when the hybrid vehicle is running on a street with less traffic, then the standby time after the engine shutdown condition is satisfied until the engine is shut down is reduced to prevent the engine from idling uselessly. Therefore, the engine can be optimally controlled for shutdown depending on the running state of the vehicle. 
     In the above embodiment, the engine shutdown condition is established using the throttle opening θth and the vehicle speed V as parameters. Howvever, as shown in  FIG. 8 , the engine shutdown condition may be established using the throttle opening θth only as a parameter, irrespective of the vehicle speed V. 
     The present invention is not limited to the above embodiment, but various design changes may be made therein without departing from the scope of the invention. For example, the present invention is not limited to being applied to a two-wheeled vehicle, but may be applied to other movable vehicles such as a three-wheeled vehicle, a four-wheeled vehicle, etc. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.