Abstract:
In a power-transmission control mechanism for controlling transmission of power from an internal combustion engine of a lawn mower to mowing blades, a controller for an electromagnetic clutch periodically repeats: reading detected engine rotational speed during a predetermined time, when an operating switch is turned on; comparing present engine rotational speed read in the present cycle with previous engine rotational speed read in the previous cycle; engaging the electromagnetic clutch when the present engine rotational speed is equal to or higher than the previous engine rotational speed; and disengaging the electromagnetic clutch when the present engine rotational speed N is lower than the previous engine rotational speed. Thus, the power of the engine is reliably transmitted to the mowing blades without occurrence of engine stall and without being influenced by the load following characteristic of the engine, the clutch, and load fluctuation.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a power-transmission control mechanism of a lawn mower wherein the power of an internal combustion engine is transmitted to a mowing blade through an electromagnetic clutch. 
   2. Description of the Related Art 
   A mechanical clutch such as a friction clutch is used in lawn mowers in order to control the transmission of power from the internal combustion engine to the mowing blades. Rapid load fluctuation in the internal combustion engine, which is caused by the load such as inertia of the blades, has been prevented by gradually engaging the clutch (going through a half clutch state). Heretofore, running stop (engine stall) of the internal combustion engine under the load has thus been prevented. 
   In a lawn mower using an electromagnetic clutch in its power transmission mechanism (for example, refer to JP-A-9-36), power is transmitted through the engagement between rotating disks. Thus, in this type of electromagnetic clutch, it is not possible to avoid rapid load fluctuation because there are no damping elements, and therefore engine stall tends to occur when load is applied. 
   In general, there has been known a method of pulse-driving the electromagnetic clutch in lawn mowers in which a large load fluctuation is apt to be imposed to the internal combustion engine when the electromagnetic clutch is engaged (for example, refer to JP-A-58-191326). 
   The power transmission control disclosed in JP-A-58-191326 is a control for pulse-driving the electromagnetic clutch by gradually increasing the pulse width. 
   A complex control circuit is required to pulse-drive the electromagnetic clutch while the pulse width is gradually increased. Further, load following characteristic of the internal combustion engine, the characteristic of the clutch, and load fluctuation need to be considered to set the pulse width. For this reason, it is not easy to set the pulse width. Even though the pulse width is set once, the set pulse width cannot cope with characteristic changes with time of the clutch. 
   In addition, when the electromagnetic clutch is pulse-driven, abrasion of the electromagnetic clutch is expedited because the clutch is maintained under a “half clutch” condition. 
   SUMMARY OF THE INVENTION 
   The invention has been made to solve the above problems, and it is an object of the invention to provide a power-transmission control mechanism of a lawn mower that is not influenced by the load following characteristic of the internal combustion engine, the characteristic of the clutch, and load fluctuation, and can reliably transmit power of the internal combustion engine to the mowing blades by using an electromagnetic clutch without the occurrence of the engine stall. 
   In order to achieve the abject, according to an aspect of the invention, there is provided a power-transmission control mechanism for a lawn mower wherein power of an internal combustion engine is transmitted to a mowing blade through an electromagnetic clutch, wherein the power-transmission control mechanism comprises: an operating switch that is turned on or off by an operator; an engine rotational speed detector that detects engine rotational speed of the internal combustion engine; and an electromagnetic clutch controller that controls operation of the electromagnetic clutch on the basis of an operation signal of the operating switch and the engine rotational speed detected by the engine rotational speed detector; wherein the electromagnetic clutch controller is configured to periodically repeats: reading the engine rotational speed detected by the engine rotational speed detector, during a predetermined time, when the operating switch is turned on, comparing a present engine rotational speed read in present cycle with a previous engine rotational speed read in a previous cycle, engaging the electromagnetic clutch when the present engine rotational speed is equal to or higher than the previous engine rotational speed, and disengaging the electromagnetic clutch when the present engine rotational speed is lower than the previous engine rotational speed. 
   According to this aspect of the invention, when the operating switch is turned on, the electromagnetic clutch controller periodically repeats: engaging the electromagnetic clutch when present engine rotational speed is equal to or higher than previous engine rotational speed, and disengaging the electromagnetic clutch when present engine rotational speed is lower than previous engine rotational speed. Therefore, the electromagnetic clutch is engaged when the engine rotational speed is increasing, and the electromagnetic clutch is disengaged when the engine rotational speed is decreasing, for a predetermined time, whereby engine stall due to continuous engagement of the electromagnetic clutch is prevented. It is thus possible to reliably engage the electromagnetic clutch without the occurrence of the engine stall. 
   Since the periodical control of the electromagnetic clutch is performed only within a predetermined time, abrasion of the electromagnetic clutch is suppressed. Further, since the electromagnetic clutch is controlled on the basis of engine rotational speed, it is possible to reliably transmit the power of the internal combustion engine to the mowing blade by using the electromagnetic clutch without the occurrence of the engine stall and without influence on the load following characteristic of the internal combustion engine, the characteristic of the clutch, and load fluctuation. 
   In order to achieve the abject, according to another aspect of the invention, there is provided a power-transmission control mechanism for a lawn mower wherein power of an internal combustion engine is transmitted to a mowing blade through an electromagnetic clutch, wherein the power-transmission control mechanism comprises: an operating switch that is turned on or off by an operator; an engine rotational speed detector that detects engine rotational speed of the internal combustion engine; and an electromagnetic clutch controller that controls operation of the electromagnetic clutch on the basis of an operation signal of the operating switch and the engine rotational speed detected by the engine rotational speed detector, wherein the electromagnetic clutch controller is configured to periodically repeats: reading the engine rotational speed detected by the engine rotational speed detector, during a predetermined time, when the operating switch is turned on, disengaging the electromagnetic clutch when the read engine rotational speed is lower than a lower limit engine rotational speed, which is lower than a specified engine rotational speed by a first predetermined rotational speed, engaging the electromagnetic clutch when the read engine rotational speed is equal to or higher than an upper limit engine rotational speed, which is lower than the specified engine rotational speed by second predetermined rotational speed, comparing the present engine rotational speed read in present cycle with a previous engine rotational speed read in a previous cycle, when the read engine rotational speed in the present cycle is equal to or higher than the lower limit engine rotational speed and is lower than the upper limit engine rotational speed, engaging the electromagnetic clutch when the present engine rotational speed is equal to or higher than the previous engine rotational speed, and disengaging the electromagnetic clutch when the present engine rotational speed is lower than the previous engine rotational speed. 
   According to this aspect of the invention, if the detected engine rotational speed is lower than the lower limit engine rotational speed and is far lower than the specified engine rotational speed, the electromagnetic clutch is maintained in the disengaged state. If the detected engine rotational speed is equal to or higher than the upper limit engine rotational speed and is close to the specified engine rotational speed, the electromagnetic clutch is maintained in the engaged state. If the detected engine rotational speed is an intermediate value between the lower and upper limit engine rotational speed and if the engine rotational speed is increasing, then the electromagnetic clutch is engaged, while if the engine rotational speed is decreasing, the electromagnetic clutch is disengaged. Thus the number of repetition of the engagement and disengagement of the electromagnetic clutch is reduced as much as possible, thereby further suppressing the abrasion of the electromagnetic clutch, whereby it is possible to reliably engage the electromagnetic clutch without the occurrence of the engine stall. 
   Further, when output deteriorates due to the decrease of the engine rotational speed caused by the characteristic of the internal combustion engine, it is possible that the engine rotational speed is repeatedly increased and decreased at a low level of engine rotational speed due to the load applied by the electromagnetic clutch. However, the electromagnetic clutch is forcibly disengaged in the case of the low engine rotational speed lower than the lower limit engine rotational speed. It is thus possible to prevent the repetition of the engagement and disengagement of the electromagnetic clutch at a low engine rotational speed. 
   In addition, the periodical control of the electromagnetic clutch is performed only within a predetermined time, and the number of the repetition of the engagement and disengagement of the electromagnetic clutch can be reduced as much as possible even within the predetermined time. Therefore, the abrasion of the electromagnetic clutch is further suppressed. 
   Furthermore, since the electromagnetic clutch is controlled on the basis of engine rotational speed, it is possible to reliably transmit the power of the internal combustion engine to the mowing blade by using the electromagnetic clutch without the occurrence of the engine stall and without influence on load following characteristic of the internal combustion engine, the characteristic of the clutch, and load fluctuation. 
   In order to achieve the object, according to another aspect of the invention, there is provided a power-transmission control mechanism for a lawn mower wherein power of an internal combustion engine is transmitted to a mowing blade through an electromagnetic clutch, wherein the power-transmission control mechanism comprises: an operating switch that is turned on or off by an operator; a throttle opening detector that detects a throttle opening of a throttle valve provided in an inlet system of the internal combustion engine; and an electromagnetic clutch controller that controls operation of the electromagnetic clutch on the basis of an operation signal of the operating switch and the throttle opening detected by the throttle opening detector, wherein the electromagnetic clutch controller is configured to periodically repeats: reading the throttle opening detected by the throttle opening detector, during a predetermined time, when the operating switch is turned on, engaging the electromagnetic clutch when the read throttle opening is smaller than an upper limit throttle opening, and disengaging the electromagnetic clutch when the read throttle opening is equal to or larger than the upper limit throttle opening. 
   According to this aspect of the invention, the electromagnetic clutch controller periodically repeats: engaging the electromagnetic clutch when the read throttle opening is smaller than an upper limit throttle opening; and disengaging the electromagnetic clutch when the read throttle opening is equal to or larger than the upper limit throttle opening. Therefore, when the throttle opening is equal to or larger than the upper limit throttle opening and the internal combustion engine does not have a margin in the output thereof, the electromagnetic clutch is disengaged. It is thus possible to reliably engage the electromagnetic clutch without the occurrence of the engine stall. 
   Further, since the periodical control of the electromagnetic clutch is performed only within the predetermined time, the abrasion of the electromagnetic clutch is suppressed. 
   In addition, since the electromagnetic clutch is controlled on the basis of throttle opening, it is possible to reliably transmit the power of the internal combustion engine to the mowing blade by using the electromagnetic clutch without the occurrence of the engine stall and without influence on the load following characteristic of the internal combustion engine, the characteristic of the clutch, and load fluctuation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing an entire lawn mower to which a power-transmission control mechanism according to the invention is applied; 
       FIG. 2  is a side view of a body of the lawn mower; 
       FIG. 3  is a plan view of the body of the lawn mower; 
       FIG. 4  is a rear view of the body of the lawn mower; 
       FIG. 5  is a side view, partly in section, of the lawn mower in which a part of the lawn mower is omitted; 
       FIG. 6  is a rear view, partly in section, of the lawn mower in which a part of the lawn mower is omitted; 
       FIG. 7  is a sectional view of a travel DC motor and a speed reduction mechanism, taken along line VII-VII of  FIG. 5 ; 
       FIG. 8  is a perspective view showing the structure near a grip part of an operation handle; 
       FIG. 9  is a schematic block diagram of a control system of the lawn mower; 
       FIG. 10  is a flow chart showing a control procedure of an electromagnetic clutch in a first embodiment; 
       FIG. 11  is a flow chart showing a control procedure of an electromagnetic clutch in a second embodiment; 
       FIG. 12  is a flow chart showing a control procedure of an electromagnetic clutch in a third embodiment; and 
       FIG. 13  is a flow chart showing a control procedure of an electromagnetic clutch in a fourth embodiment. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment according to the invention will be described below with reference to  FIGS. 1 to 13 . 
   A lawn mower  1  according to this embodiment is a hybrid self-propelled lawn mower that can rotate mowing blades  12  (see  FIG. 2 ) by a four-stroke cycle internal combustion engine  10  to perform the mowing operation and can self-travel by a travel DC motor  30 . 
     FIG. 1  shows a perspective view showing the entire lawn mower  1 ,  FIG. 2  shows a side view of a body of the lawn mower,  FIG. 3  is a plan view of the body of the lawn mower, and  FIG. 4  is a rear view of the body of the lawn mower. 
   Referring to  FIG. 1 , a blade housing  2 , which supports the mowing blades  12  (see  FIG. 2 ) rotating above the ground and covers the blades from above, is supported by a pair of (left and right) front wheels  6  and  6  and rear wheels  7  and  7  so as to freely travel on the ground. 
   A direction in which the lawn mower  1  moves forward will be referred as a forward direction in the description, and the front, the rear, the left, and the right are determined on the basis of the above-mentioned direction. 
   Bearing portions  2   f ,  2   f ,  2   r , and  2   r , which support four shafts of the front and rear wheels  6 ,  6 ,  7 , and  7 , are provided at four corners of the blade housing  2 . Further, the lower portion of a central portion  2   c , which is surrounded by the bearing portions  2   f ,  2   f ,  2   r , and  2   r , of the blade housing  2  is formed to have the shape of a flat bowl, thereby forming a blade receiving portion  2   b  covering the blades  12 . The rear half portion of the central portion  2   c  is expanded upward toward the rear side thereof, thereby forming an expansion portion  2   e  that is continuous to the rear side and expanded upward. 
   An internal combustion engine  10  is provided in the central portion  2   c  of the blade housing  2  so that a crankshaft  11  (see  FIG. 2 ) is oriented in a vertical direction. In the internal combustion engine  10 , cylinders  10   cy  are oriented toward the front side, and the crankshaft  11  protrudes downward from within a crank case  10   c.    
   As shown in  FIG. 5 , an electromagnetic clutch  20  is provided between the crankshaft  11  and the blades  12 . Accordingly, if the electromagnetic clutch  20  is engaged during the operation of the internal combustion engine  10 , the blades  12  are rotated. As a result, it is possible to perform the mowing operation. 
   A vertical partition plate  3  (see  FIG. 3 ) is obliquely provided throughout from the right side of the central portion  2   c  to the expansion portion  2   e  at the posterior half of the blade housing  2 . Further, the inside of the blade housing  2  is partitioned by the vertical partition plate  3 , so that a lawn conveying passage  4  is formed. 
   The lawn conveying passage  4  is a passage, which is formed by partitioning the inside of the blade housing  2 . The front end of the passage is opened to the blade receiving portion  2   b , and the cross-sectional area of the passage is gradually increased from a front opening toward the rear side thereof. For this reason, a large rear opening  4   b  (see  FIGS. 3 and 4 ) is formed in the rear wall, which is slightly inclined, of the expansion portion  2   e.    
   The rear opening  4   b  of the lawn conveying passage  4  is largely opened to occupy an area larger than the right half portion of the rear wall  2   d  of the expansion portion  2   e , and the front opening is connected to the rear opening  4   b . A lawn collecting bag  5  shown in  FIG. 1  is connected to the rear opening  4   b  to extend toward the rear. 
   The inside of the blade housing  2  is partitioned by the inclined vertical partition plate  3 , so that the lawn conveying passage  4  is formed at the right portion in the blade housing. Further, a travel DC motor  30  and a speed reduction mechanism  40  are provided in a lower half of a left-side space, which is partitioned by the vertical partition plate  3 . 
   As shown in  FIG. 4 , a motor driving shaft  31  of the travel DC motor  30  is disposed in the upper portion of the speed reduction mechanism  40 , as an input shaft of the speed reduction mechanism  40 . Further, the torque of the motor driving shaft  31  is transmitted to a driving shaft  50 , which serves as an output shaft provided in the lower portion of the speed reduction mechanism  40 , through the engagement of reduction gears at a reduced speed. 
   As shown in  FIGS. 4 and 6 , the driving shaft  50  extends in the left-and-right or transverse direction and is rotatably provided on the rear side of rear axles  7   a  and  7   a  by which the rear wheels  7  and  7  are rotatably supported. Further, driving gears  61  and  61 , which are fitted to both ends of the driving shaft  50  with two-way or bi-directional clutches  55  interposed therebetween, are engaged with driven gears  62  and  62 , which are integrally fixed to the rear wheels  7  and  7 . 
   Accordingly, the torque of the motor driving shaft  31  of the travel DC motor  30  is transmitted to the driving shaft  50  through the speed reduction mechanism  40  at a reduced speed, and the torque of the driving shaft  50  is transmitted to the rear wheels  7  and  7  through the two-way or bi-directional clutches  55  and the engagement between the driving and driven gears  61 ,  61 ,  62 , and  62 . Therefore, the lawn mower  1  travels. 
   The two-way or bi-directional clutch is a clutch in which only forward directional power of the driving shaft of a driving source is transmitted to the driving wheels and both the forward and backward torques of the driving wheels are not transmitted to the driving shaft if the clutch is not engaged and in the disengagement state. 
   The operation control of the travel DC motor  30 , the operation control of the internal combustion engine  10 , and the engagement and disengagement control of the electromagnetic clutch  20 , which transmits the power of the internal combustion engine  10  to the mowing blades  12 , are performed by an ECU  70  (see  FIGS. 2 and 4 ), which is an electronic control unit using a computer. 
   The ECU  70  is provided in an upper half of the left space of the lawn conveying passage  4 , which left space is partitioned by the vertical partition plate  3  at the upper portion of the rear expansion portion  2   e  of the blade housing  2 . The travel DC motor  30  is provided on the lower side of the ECU  70 . The ECU  70  is received in a case having a rectangular parallelepiped shape, and a plurality of cooling fins  71  protrude in line from the upper surface of the case. 
   An inclined upper wall of the rear expansion portion  2   e  of the blade housing  2  is partially opened so that a rectangular opening is formed, and the rectangular opening is slightly smaller than the rectangular upper surface of the case of the ECU  70 . As shown in  FIG. 3 , the cooling fins  71  are inserted into the rectangular opening from below so as to be exposed to the upper side. Further, the outer edges of the upper surface of the case of the ECU  70  come in contact with the edges of the rectangular opening, and are fixed to the edges of the rectangular opening with screws  72 . Accordingly, the ECU  70  is supported on the upper wall of the blade housing  2 . 
   A power transmission system will be described below with reference to  FIGS. 5 to 7 . 
   First, the structure of the electromagnetic clutch  20 , which transmits the power of the internal combustion engine  10  to the blades  12 , will be described with reference to a cross-sectional view of  FIG. 5 . 
   A rotary disk  21  is serration-fitted from below to the crankshaft  11 , which protrudes downward from the internal combustion engine  10 . In addition, a cylindrical collar  22  is fitted to the crankshaft  11  and then integrally fixed to the crankshaft  11  by using a flange bolt  23  with a washer  23   w  interposed therebetween. Accordingly, the crankshaft  11  and the rotary disk  21  rotate as a single body. 
   The rotary disk  21  is composed of a cylindrical portion  21   a  supported by a bearing  19  and a disk portion  21   b  formed at the lower end of the cylindrical portion. Further, the upper portion of an annular electromagnetic coil  24  is held, so that an annular electromagnetic coil  24  is suspended close to the upper surface of the disk portion  21   b.    
   An annular blade supporting member  26  is provided on the outer peripheral surface of the collar  22  with a bearing  25  interposed therebetween so as to freely rotate relative to the crankshaft  11 . Further, an annular base end  12   a  of the blades  12  comes in contact with the lower surface of the annular supporting member  26 , and integrally fixed to the lower surface with a flange bolt  26   b . Therefore, the blades  12  are supported to freely rotate with respect to the crankshaft  11 . 
   A hollow disk-shaped clutch disk  27  is supported on the blade supporting member  26  so as to move up and down. That is, a plurality of pins  26   p , which stands on the upper surface of the blade supporting member  26 , passes through the clutch disk  27 . The clutch disk  27  moves up and down with respect to the blade supporting member  26 , but has the structure in which the rotation of the clutch disk  27  with respect to the blade supporting member  26  is limited. 
   The clutch disk  27  is close to the disk portion  21   b  of the rotary disk  21  and faces the disk portion. When moved upward, the clutch disk  27  comes in contact with the disk portion  21   b . A friction member is attached to the portion, which comes in contact with the disk portion  21   b  of the rotary disk  21 , of the upper surface of the clutch disk  27 . 
   Further, an annular locking plate  28 , which is fixed to the blade housing  2  with a bolt  29 , is supported below the outer edge of the lower surface of the clutch disk  27 . Friction members  28   a  are attached in an annular shape on the upper surface of the annular locking plate  28 . 
   The electromagnetic clutch  20  has the above-mentioned structure. When current is not supplied to the electromagnetic coil  24  and the electromagnetic coil is demagnetized, the clutch disk  27  moves downward to be separated from the rotary disk  21 . For this reason, although the crankshaft  11  and the rotary disk  21  rotate due to the driving of the internal combustion engine  10 , power is not transmitted to the blade supporting member  26  and the blades  12  thus do not rotate. 
   Meanwhile, when current is supplied to the electromagnetic coil  24  and the electromagnetic coil is energized, the clutch disk  27  moves upward to be attached to the rotary disk  21  due to the magnetic force. For this reason, the torque of the crankshaft  11  causes the rotary disk  21  and the clutch disk  27  to rotate as a single body, and the torque of the clutch disk  27  is transmitted to the blade supporting member  26  through the pins  26   p . Therefore, the blades  12  rotate. 
   In this case, when the electromagnetic coil  24  is deenergized, the clutch disk  27  is separated from the rotary disk  21  and moves downward to be placed on the friction members  28   a  of the annular locking plate  28 . For this reason, the rotation of the clutch disk  27  and the blade  12  is limited due to inertia, so that the clutch disk  27  and the blades  12  stop. 
   Next, a travel driving system using the travel DC motor  30  will be described with reference to  FIGS. 5 to 10 . 
   As described above, the travel DC motor  30  and the speed reduction mechanism  40  are provided in the lower half of the left space, which is partitioned by the vertical partition plate  3 , in the rear expansion portion  2   e  of the blade housing  2 . Further, as shown in  FIG. 7 , the motor driving shaft  31  protruding from the right side of the travel DC motor  30  is inserted into the upper portion of a reduction gear case  41 , and a motor driving gear  32  is fitted to the end of the motor driving shaft  31 . 
   The driving shaft  50  passes through the lower portion of the reduction gear case  41  in a right-and-left or transverse direction. Further, two gear shafts  42  and  43  are provided between the motor driving shaft  31  and the driving shaft  50  in the reduction gear case  41 . The gear shafts  42  and  43  are oriented in the right-and-left direction. 
   A large diameter gear  44 , which is integrally fitted to a small diameter gear  45  rotatably supported by the gear shaft  42 , is engaged with the driving gear  32 . 
   A large diameter gear  46  and a small diameter  47 , which are integrally formed with each other, are rotatably supported by the gear shaft  43 . Further, the large diameter gear  46  is engaged with the small diameter gear  45 , and the small diameter  47  is engaged with the large diameter gear  48  fitted to the driving gear  50 . 
   The speed reduction mechanism  40  has the above-mentioned structure. The torque of the motor driving shaft  31  is transmitted to the driving shaft  50  at a reduced speed through the engagement of gears provided between the small diameter gear and the large gear. 
   The driving gears  61  and  61  are fitted to the both ends of the driving shaft  50  with the two-way or bi-directional clutches  55  and  55  interposed therebetween, and the driving gears  61  and  61  are engaged with the driven gears  62  and  62 , which are integrally fixed to the rear wheels  7  and  7 , respectively. 
   Accordingly, the bi-directional clutches  55  and  55  are engaged due to the operation of the travel DC motor  30 , so that the rear wheels  7  and  7  rotate and the lawn mower  1  can travel. 
   If the travel DC motor  30  is stopped (with short-circuit) while stop electric power is being supplied to the motor, the two-way or bi-directional clutches  55  and  55  are retained in the disengagement state. If the bi-directional clutches  55  and  55  are in the disengagement state, the forward and backward (bi-directional) torque of the driving wheels  7  is not transmitted to the driving shaft  50 . For this reason, the operator can easily push and pull the lawn mower  1  and easily change the direction of the lawn mower. 
   In the body of the lawn mower  1  having the above-mentioned structure, an operation handle  80  extends rearward from the upper portion of the rear expansion portion  2   e  of the blade housing  2 . 
   The operation handle  80  is a member, which is obtained by bending a tubular member in a U shape. Left and right long handgrips  81 L and  81 R extend rearward in an obliquely upwardly sloping manner from the left and right sides of the rear expansion portion  2   e  of the blade housing  2 , and the rear ends of the handgrips  81 L and  81 R are connected with each other through a grip part  82 , thereby forming the operation handle  80 . 
   The operation handle  80  is provided with various operation members, which are operated by the operator. 
   Referring to  FIG. 8 , a first operating switch case  83  having a rectangular parallelepiped shape is fixed to a central portion of the grip part  82 , which is convexly bent upward, so as to be depend downward from the central portion of the grip part. A push button  84  as a first operation member is provided on a rear surface of the first operating switch case  83 . 
   A blade lever  85  as a second operation member is provided on the front side of the bent grip part  82  so as to be movable toward and away from the grip part  82 . 
   A swing central shaft passes through right and left sidewalls of the first operating switch case  83 . The swing central shaft has both ends thereof protruding outward. Base ends of left and right travel levers  86  and  86  are fitted to the both ends of the swing central shaft. Therefore, the left and right travel levers  86  and  86  can swing toward the rear side of the grip part  82 . 
   Each of the travel levers  86  is composed of a swing arm  86   a  of which base end is fitted to the swing central shaft, and an operation portion  86   b  bent to the right or left from the end of the swing arm  86   a.    
   When the left and right travel levers  86  and  86  swing forward, the operation portions  86   b  and  86   b  come into contact with the grip part  82 . When the left and right travel levers swing rearward, the operation portions  86   a  are separated from the grip part  82 . 
   Each of the operation portion  86   b  has a circular arc shape in cross-section and has the same shape as the grip part  82  so as to be fittable on the outer peripheral surface of the circular-tube-shaped grip part  82 . 
   A second operating switch case  87  is attached to the inner portion of the right long handgrip  81 R at a position near the grip part  82 . Furthermore, a speed control lever  88  is provided on the left side surface of the second operating switch case  87 , which has a triangular shape in side view. The speed control lever  88  is able to swing forward and rearward. 
   In addition, an ignition knob  89  is rotatably provided on the rear surface (facing the operator) of the second operating switch case  87 . 
   Further, as shown in  FIG. 1 , a starting grip  96  is supported by a grip receiver  95 , which protrudes upward from the right long handgrip  81 R. A starting cable  97  extends forward from the starting grip  96 , and is connected to a recoil starter (not shown) provided on the upper portion of the internal combustion engine  10 . 
   As shown in  FIG. 9 , a push button switch  84   s , a blade lever switch  85   s , a travel lever switch  86   s , a speed control knob  88   v , and an ignition switch  89   s  are provided which are operated by the push button  84 , the blade lever  85 , the travel lever  86 , the speed control lever  88 , and the ignition knob  89 . Signals from the push button switch  84   s , the blade lever switch  85   s , the travel lever switch  86   s , the speed control knob  88   v , and the ignition switch  89   s  are input to the ECU  70 . 
   A schematic block diagram of a control system of the lawn mower  1  is shown in  FIG. 9 . 
   The internal combustion engine  10  is provided with an electronic governor mechanism that maintains a constant engine rotational speed, and the ECU  70  controls an electronic governor motor  75  for driving a throttle valve of the internal combustion engine  10 . 
   The ECU  70  controls the operation of the electromagnetic clutch  20  and the travel DC motor  30 . 
   The internal combustion engine  10  is provided with an AC generator  76 , which generates electric current by using the rotation of the crankshaft  11 . Travel electric power of electric power generated by the AC generator  76  is supplied to the travel DC motor, so that the lawn mower travels. Further, electric power for control is supplied to the control system such as the electronic governor motor  75  and the ECU  70 . 
   The ECU  70  is provided with an engine rotational speed sensor  77 , which detects the rotation of the internal combustion engine  10 , and a throttle opening sensor  78  in order to control the operation of the internal combustion engine  10  and the travel DC motor  30 . Data signals of the engine rotational speed detected by the engine rotational speed sensor  77  and the throttle opening detected by the throttle opening sensor  78  are input to the ECU  70 . 
   When the ignition knob  89  is operated, the ignition switch  89   s  is turned on. When the travel lever  86  is operated to swing toward the front grip part  82 , the travel lever switch  86   s  is turned on, and travel electric power generated by the AC generator  76  is supplied to the travel DC motor  30  whereby the travel DC motor thus begins to be operated. As a result, the lawn mower begins to travel. 
   If the blade lever  85  is operated to swing toward the rear grip part  82  after the push button  84  is depressed, the push button switch  84   s  and the blade lever switch  85   s  are sequentially turned on. Thus current is supplied to the electromagnetic clutch  20  and the electromagnetic coil  24  is energized. Accordingly, the clutch  20  is brought into engagement and the blades  12  rotate. As a result, the operator can perform the mowing operation. 
   A first embodiment of the invention will be described with reference to  FIG. 10 , with respect to a control procedure of the electromagnetic clutch  20  controlled by the ECU  70  when the electromagnetic clutch  20  is engaged and the blades  12  are rotated by the power of the internal combustion engine  10 . 
   First, when the push button switch  84   s  is turn on, it is determined whether a flag F indicating “1” rises (step S 1 ). When the flag indicating “1” does not rise, the control procedure proceeds to step S 2  and it is determined whether the push button switch  84   s  is turned on, that is, whether the push button switch  84   s  is depressed. 
   When the push button switch  84   s  is turned off, the control procedure proceeds to step S 5 , and a specified engine rotational speed of the internal combustion engine  10  is set to a predetermined engine rotational speed Na when the mowing operation is not performed. Then, a control time counter value T is set in step S 6 . 
   The control time counter value T corresponds to a maximum time in which the electromagnetic clutch  20  is controlled. 
   When the push button  84  is depressed and the push button switch  84   s  is turned on, the control procedure proceeds to step S 3  from step S 2  and the flag F indicating “1” rises. Then, the control procedure proceeds to step S 5 . 
   Accordingly, when the push button switch  84   s  is turned on, the flag F indicating “1” rises through the determination of the flag F in step S 1 . As a result, the control procedure proceeds to step S 4 . 
   In step S 4 , it is determined whether the blade lever switch  85   s  is turned on, that is, whether the blade lever  85  is operated. 
   Until the blade lever switch  85   s  is turned on, the control procedure proceeds to step S 5  from step S 4 . When the blade lever  85  is operated and the blade lever switch  85   s  is turned on, the control procedure proceeds to step S 7 . Then, a flag F indicating “0” rises and the control procedure proceeds to step S 8 . 
   In step S 8 , the specified engine rotational speed Ns is set to a predetermined engine rotational speed Nb when the mowing operation is performed. The predetermined engine rotational speed Nb is higher than the engine rotational speed Na specified when the mowing operation is not performed. 
   When the push button  84  and the blade lever  85  are sequentially operated and the push button switch  84   s  and the blade lever switch  85   s  are sequentially turned on as described above, the operation control of the electromagnetic clutch  20  is performed. 
   Even though the blade lever  85  is operated to swing without depression of the push button  84 , the operation control of the electromagnetic clutch  20  is not performed and it is not possible to perform the mowing operation. 
   When the push button switch  84   s  and the blade lever switch  85   s  are sequentially turned on in this order and the operation control of the electromagnetic clutch  20  is started, the control procedure proceeds to step S 9  from steps S 7  and S 8 . Then, it is determined whether the control time counter value T is positive. When the control time counter value T is positive, the control procedure proceeds to step S 10 . When the control time counter value T is 0 or less, the control procedure proceeds to step S 19 . 
   While the control time counter value T is positive, the control procedure proceeds to step S 10  from step S 9 . Then, the control time counter value T is decremented, and the control procedure proceeds to step S 11 . 
   When control time reaches a predetermined control time so that the control time counter value T is repeatedly decremented and becomes thus 0 or less, the control procedure proceeds to step S 19 . Current is thus supplied to the electromagnetic clutch  20  so that the electromagnetic clutch is engaged to rotate the blades  12 . As a result, the mowing operation is forcibly performed. 
   The reason for this is to prevent the operation control of the electromagnetic clutch  20  from being indefinitely repeated, and to stabilize the rotation of the blades  12 . 
   When the control time counter value T is positive and the control procedure thus proceeds to steps S 10  and S 11  from step S 9 , it is determined whether a control cycle counter value t is positive. 
   The control cycle counter value t corresponds to a cycle in which the operation control of the electromagnetic clutch  20  is repeated. 
   While the control cycle counter value t is positive, the control procedure proceeds to step S 12 . Then, the control cycle counter value t is decremented. 
   When the control cycle counter value t is repeatedly decremented and becomes thus 0 or less, the control procedure proceeds to step S 13  and the control cycle counter value t is set to a predetermined period. 
   The above-mentioned predetermined cycle is close to one cycle obtained from the specified engine rotational speed. Since the internal combustion engine  10  is a four-stroke cycle internal combustion engine, two revolutions of the engine is one cycle thereof. Therefore, it is considered that the predetermined cycle is set to an intermediate value between the period in the engine rotational speed when the mowing operation is not performed and the period in the engine rotational speed when the mowing operation is performed. 
   When the control cycle counter value t is set to a predetermined period in step S 13 , the control procedure proceeds to step S 14  and engine rotational speed N detected by the engine rotational speed sensor  77  is read. Then, in step  15 , present engine rotational speed N read in the present cycle is compared with previous engine rotational speed No read in the previous cycle (before one cycle). When the present engine rotational speed N is lower than the previous engine rotational speed No, the control procedure proceeds to step S 16 . When the present engine rotational speed N is equal to or higher than the previous engine rotational speed No, the control procedure proceeds to step S 18 . 
   When the present engine rotational speed N is lower than the previous engine rotational speed No and the engine rotational speed is decreasing as compared to the previous cycle (before one cycle), the control procedure proceeds to step S 16 . Then, the previous engine rotational speed No is replaced with the present engine rotational speed N, and current is not supplied to the electromagnetic clutch  20  in step S 17 . Thus the electromagnetic clutch  20  is disengaged and the blades  12  stop. 
   That is, when the engine rotational speed is decreasing, load applied to the rotating blades  12  is too large. When the electromagnetic clutch  20  is engaged, there is a possibility that the internal combustion engine  10  stops (engine stall occurs). For this reason, the electromagnetic clutch  20  is disengaged. 
   On the other hand, when the present engine rotational speed N is equal to or higher than the previous engine rotational speed No and the engine rotational speed is increasing as compared to the previous cycle (before one cycle), the control procedure proceeds to step S 18 . Then, the previous engine rotational speed No is replaced with the present engine rotational speed N, and current is supplied to the electromagnetic clutch  20  in step S 19 . Accordingly, the electromagnetic clutch  20  is engaged and the blades  12  rotate. 
   That is, when the engine rotational speed is increasing, there is no possibility that engine stall occurs. Therefore, the electromagnetic clutch  20  is engaged to rotate the blades  12 . 
   As described above, until the control time (control time counter value T) has elapsed, the present engine rotational speed N is compared with the previous engine rotational speed No in every control cycle (control cycle counter value t). When the engine rotational speed is decreasing, the electromagnetic clutch  20  is disengaged. When the engine rotational speed is increasing, the electromagnetic clutch  20  is engaged. The electromagnetic clutch  20  is engaged depending on the repetition of the above-mentioned engagement and disengagement of the electromagnetic clutch, whereby engine stall is prevented. It is thus possible to reliably engage the electromagnetic clutch without the occurrence of the engine stall. 
   After the control time has elapsed, the internal combustion engine  10  can generally run at the specified engine rotational speed. Accordingly, the electromagnetic clutch  20  is maintained in the engaged state, so that the mowing operation is performed in the ordinary manner. 
   The electromagnetic clutch  20  is controlled on the basis of engine rotational speed. Therefore, it is possible to reliably transmit the power of the internal combustion engine  10  to the blades  12  by using the electromagnetic clutch  20  without the occurrence of the engine stall and without influence on the load following characteristic of the internal combustion engine, the characteristic of the clutch, and load fluctuation. 
   Next, a second embodiment of the invention will be described with reference to  FIG. 11 , with respect to a control procedure of the electromagnetic clutch  20 . 
   According to the first embodiment, the control cycle is a constant cycle close to one cycle, which is previously obtained from the specified engine rotational speed Ns. However, according to the second embodiment, one cycle used as the control cycle is obtained from present engine rotational speed N, which is detected by the engine rotational speed sensor  77 . 
   Steps S 21  to S 30  of the second embodiment are the same as steps S 1  to S 10  of the first embodiment. Further, steps S 32  to S 37  of the second embodiment are the same as steps S 14  to S 19  of the first embodiment. 
   With respect to the determination in step S 31  of whether one cycle of the internal combustion engine has elapsed, the elapse of the initial cycle is determined by using one cycle obtained from the specified engine rotational speed when the mowing operation is not performed. Further, the elapse of the next cycle is determined by using one cycle obtained on the basis of the detected engine rotational speed N read in step S 32 . 
   Accordingly, until the control time (control time counter value T) has elapsed, present engine rotational speed N is compared with previous engine rotational speed No in every cycle of the internal combustion engine  10 . When the engine rotational speed is decreasing, the electromagnetic clutch  20  is disengaged. When the engine rotational speed is increasing, the electromagnetic clutch  20  is engaged. The electromagnetic clutch  20  is engaged depending on the repetition of the above-mentioned engagement and disengagement of the electromagnetic clutch, whereby engine stall is prevented. Therefore, it is possible to reliably engage the electromagnetic clutch without the occurrence of the engine stall. 
   Since the periodical control of the electromagnetic clutch  20  is performed only within the control time, the abrasion of the electromagnetic clutch is suppressed. 
   Next, a third embodiment of the invention will be described with reference to  FIG. 12 , with respect to a control procedure of the electromagnetic clutch  20 . 
   Steps S 41  to S 54  of the third embodiment are the same as steps S 1  to S 14  of the first embodiment. 
   Further, when the control cycle counter value t is repeatedly decremented and becomes thus 0 or less, the control procedure proceeds to step S 53  and the control cycle counter value t is set to a predetermined period. The detected engine rotational speed N is read in step S 54 . Then, in step S 55 , it is determined whether the detected engine rotational speed N is lower than a lower limit engine rotational speed. In this case, the lower limit engine rotational speed is lower than the specified engine rotational speed Ns by a first predetermined rotational speed n 1 . 
   When the detected engine rotational speed N is lower than the lower limit engine rotational speed, the control procedure proceeds to step S 58 . Then, the previous engine rotational speed No is replaced with the present engine rotational speed N, and current is not supplied to the electromagnetic clutch  20  in step S 59 . Accordingly, the electromagnetic clutch  20  is disengaged and the blades  12  stop. 
   When output deteriorates due to decrease of the engine rotational speed caused by the characteristic of the internal combustion engine, there is a possibility that the engine rotational speed is repeatedly increased and decreased at a low level of the engine rotational speed due to a load, which is applied through the electromagnetic clutch  20 . However, the electromagnetic clutch  20  is forcibly disengaged in the case of the low engine rotational speed lower than the lower limit engine rotational speed. Accordingly, after step S 57  to be described below, it is possible to prevent the repetition of the engagement and disengagement of the electromagnetic clutch  20  at a low level of the engine rotational speed. 
   In the case of a low engine rotational speed lower than the lower limit engine rotational speed, there is a possibility that engine stall occurs. Since the finishing of the mowing operation in such a case is not excellent and the low engine rotational speed is not suitable for the mowing operation. For this reason, the electromagnetic clutch  20  is forcibly disengaged and the mowing operation is stopped. 
   If it is determined in step S 55  that the detected engine rotational speed N exceeds the lower limit engine rotational speed, the control procedure proceeds to step S 56  and it is determined whether the detected engine rotational speed N is equal to or lower than an upper limit engine rotational speed. In this case, the upper limit engine rotational speed is lower than the specified engine rotational speed Ns by a second predetermined rotational speed n 2 . 
   Meanwhile, the first predetermined rotational speed n 1  is higher than the second predetermined rotational speed n 2 . 
   When the detected engine rotational speed N is equal to or higher than the upper limit engine rotational speed, the control procedure proceeds to step S 60 . Then, the previous engine rotational speed No is replaced with the present engine rotational speed N, and current is supplied to the electromagnetic clutch  20  in step S 61 . Accordingly, the electromagnetic clutch  20  is engaged and the blades  12  rotate. 
   When the detected engine rotational speed is equal to or higher than the upper limit engine rotational speed and is close to the specified engine rotational speed, engine stall does not occur even though the engine rotational speed is decreased, and the finishing of the mowing operation is also excellent. Accordingly, the electromagnetic clutch is maintained in the engaged state, and the number of the repetition of the engagement and disengagement of the electromagnetic clutch  20  is reduced as much as possible. In addition, the electromagnetic clutch is quickly and reliably engaged without the occurrence of the engine stall. 
   If it is determined in step S 56  that the detected engine rotational speed N is lower than the upper limit engine rotational speed, the control procedure proceeds to step S 57 . 
   That is, when the detected engine rotational speed N is equal to or higher than the lower limit engine rotational speed and is lower than the upper limit engine rotational speed, the control procedure proceeds to step S 57  and present engine rotational speed N read in the present cycle is compared with previous engine rotational speed No read in the previous cycle (before one cycle). 
   When the present engine rotational speed N is lower than the previous engine rotational speed No, the control procedure proceeds to step S 58 . Then, the previous engine rotational speed No is replaced with the present engine rotational speed N, and current is not supplied to the electromagnetic clutch  20  in step S 59 . Accordingly, the electromagnetic clutch  20  is disengaged and the blades  12  stop. Meanwhile, when the present engine rotational speed N is equal to or higher than the previous engine rotational speed No, the control procedure proceeds to step S 60 . Then, the previous engine rotational speed No is replaced with the present engine rotational speed N, and current is supplied to the electromagnetic clutch  20  in step S 61 . Accordingly, the electromagnetic clutch  20  is engaged and the blades  12  rotate. 
   When the detected engine rotational speed N is lower than the lower limit engine rotational speed and is far lower than the specified engine rotational speed Ns, the electromagnetic clutch  20  is maintained in the disengaged state. When the detected engine rotational speed N is equal to or higher than the upper limit engine rotational speed and is close to the specified engine rotational speed Ns, the electromagnetic clutch  20  is maintained in the engaged state. When the detected engine rotational speed N is an intermediate value between the lower and upper limit engine rotational speeds, the electromagnetic clutch  20  is engaged if the engine rotational speed is increasing. When the engine rotational speed is decreasing, the electromagnetic clutch  20  is disengaged. Accordingly, the number of the repetition of the engagement and disengagement of the electromagnetic clutch  20  is reduced as much as possible. In addition, it is possible to quickly and reliably engage the electromagnetic clutch without the occurrence of engine stall. 
   The periodical control of the electromagnetic clutch  20  is performed only within the control time, and the number of the repetition of the engagement and disengagement of the electromagnetic clutch  20  can be reduced as much as possible even within the control time. Therefore, the abrasion of the electromagnetic clutch  20  is further suppressed. 
   Next, a fourth embodiment of the invention will be described with reference to  FIG. 13 , with respect to a control procedure of the electromagnetic clutch  20 . 
   According to the fourth embodiment, the operation of the electromagnetic clutch  20  is controlled on the basis of throttle opening θ that is a valve opening of a throttle valve of the air intake system of the internal combustion engine  10 , instead of the engine rotational speed N in the above-mentioned embodiments. 
   The throttle opening θ is detected by the throttle opening sensor  78 . 
   Steps S 71  to S 83  of the fourth embodiment are the same as steps S 1  to S 13  of the first embodiment. 
   When the control cycle counter value t is repeatedly decremented and becomes thus 0 or less, the control procedure proceeds to step S 83  and the control cycle counter value t is set to a predetermined period. Then, the throttle opening θ detected by the throttle opening sensor  78  is read in step S 84 , and it is determined whether detected throttle opening θ is smaller than an upper limit throttle opening θu (for example, opening of 80%) in step S 85 . 
   When the detected throttle opening θ is equal to or larger than the upper limit throttle opening θu, the control procedure proceeds to step S 86  and current is not supplied to the electromagnetic clutch  20 . Accordingly, the electromagnetic clutch  20  is disengaged. When the detected throttle opening θ is smaller than the upper limit throttle opening θu, current is supplied to the electromagnetic clutch  20 . Accordingly, the electromagnetic clutch  20  is engaged and the blades  12  rotate. 
   That is, when the detected throttle opening θ is equal to or larger than the upper limit throttle opening θu and the internal combustion engine  10  does not have a margin in output thereof, there is a possibility that engine stall occurs if there is any load fluctuation. Therefore, the electromagnetic clutch  20  is disengaged. When the detected throttle opening θ is smaller than the upper limit throttle opening θu and the internal combustion engine  10  has a margin in output thereof, the electromagnetic clutch  20  is engaged and the blades  12  rotate. 
   The engagement and disengagement control of the electromagnetic clutch  20 , which is performed by using the throttle opening θ in steps S 83  to S 87 , is performed in every control cycle t within the control time T. Therefore, it is possible to reliably engage the electromagnetic clutch  20  without the occurrence of engine stall. 
   Since the electromagnetic clutch  20  is controlled on the basis of throttle opening θ, it is possible to reliably transmit the power of the internal combustion engine  10  to the blades  12  by using the electromagnetic clutch  20  without the occurrence of engine stall and without an influence on the load following characteristic of the internal combustion engine  10 , the characteristic of the clutch, and load fluctuation. 
   Since the periodical control of the electromagnetic clutch  20  is performed only within the control time, the abrasion of the electromagnetic clutch  20  is suppressed. 
   The invention has been described as applied to a hybrid self-propelled lawn mower in the above-mentioned embodiments. However, the invention may be applied to a self-propelled lawn mower, which performs a mowing operation by using an internal combustion engine and also travels by using an internal combustion engine.