Patent Publication Number: US-10306828-B2

Title: Lawn mower powered by cylinder deactivation engine

Description:
TECHNICAL FIELD 
     The present invention relates to a lawn mower provided with a multiple cylinder internal engine configured to selectively deactivate at least one of a plurality of cylinders. 
     BACKGROUND ART 
     JP2009-273386A discloses power equipment in the form of a ride-on mower which is provided with cutting blades housed in a cutter deck. The cutter deck can be adjusted vertically by manually operating a lever which is connected to the cutter deck via a linkage mechanism so that grass can be cut at a desired height. The cutting blades are rotated by the power of the engine. 
     JP2007-315455A discloses a lawn mower provided with a cutting blade housed in a blade housing. In this lawn mower, the blade is connected to the output shaft of the engine via an electromagnetic clutch. When a clutch switch is turned on by the operator, the control unit of the lawn mower engages the clutch if an increase in the engine rotational speed is detected, and disengages the clutch if a decrease in the engine rotational speed is detected. This operation is cyclically repeated so that the clutch can be finally engaged without stalling the engine. 
     JP2013-022987A discloses a ride-on mower provided with a cutting blade housed in a blade housing. The mower is provided with driven wheels which are powered by an internal combustion engine via a hydraulic continuous transmission system. The control system of the mower detects a steering operation of the mower, and reduces the speed of the driven wheels when a steering action of the mower is detected. 
     In power equipment having a propelling device (such as wheels and crawlers) and a work implement which are both powered by a same internal combustion, it is desirable to appropriately allocate the power to the propelling device and the work implement in such a way that the energy efficiency of the power equipment is optimized. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, and the recognition by the inventors, a primary object of the present invention is to provide power equipment which can minimize fuel consumption. 
     To achieve such an object, the present invention provides a lawn mower, comprising: a lawn mower main body ( 15 ) provided with a propelling device; a power source ( 100 ) for selectively powering the propelling device; a traveling state sensor ( 40 ) for detecting a traveling state of the lawn mower; an internal combustion engine ( 100 ) having a plurality of cylinders and supported by the lawn mower main body; an engine control unit ( 110 ) for selectively deactivating at least one of the cylinders; a cutting blade ( 70 ) rotatably supported by the lawn mower main body via a cutter deck ( 75 ) and connected to the engine in a power transmitting relationship; a blade clutch ( 80 ) provided in a power transmission path between the engine and the cutting blade; a height adjustment mechanism ( 90 ) provided on the cutter deck for adjusting a height of the cutting blade; and a central control unit ( 50 ) for controlling an operation of the blade clutch, the height adjustment mechanism and the engine control unit; wherein the central control unit is configured to cause the engine control unit to deactivate at least one of the cylinders when a prescribed operating condition is met, and to raise the cutting blade to a prescribed elevated height before deactivating the at least one of the cylinders. 
     Preferably, at least one of the cylinders of the engine may be deactivated when the engine load is light so that the fuel economy of the lawn mower may be improved. However, it is possible that the cutting blade may be operated at any time if the blade clutch is engaged, and this may suddenly increase the engine load. The central control unit should take such factors into account in determining the favorable timing for deactivating at least one of the cylinders of the engine. 
     On the other hand, the central control unit may also positively increase the likelihood of light engine load. For instance, the probability of encountering a heavy engine load can be positively decreased by raising the height of the cutting blade. 
     Based on such a consideration, according to the present invention, the central control unit is configured to cause the engine control unit to deactivate at least one of the cylinders when a prescribed operating condition is met, and to raise the cutting blade to a prescribed elevated height before deactivating the at least one of the cylinders. The probability of encountering a heavy engine load can be positively decreased by raising the height of the cutting blade. 
     According to a certain aspect of the present invention, the central control unit is configured to deactivate at least one of the cylinders when the lawn mower is not traveling forward. 
     When the lawn mower is not traveling forward, it is highly likely that the engine load is light. Therefore, it is advantageous to deactivate at least one of the cylinders when the lawn mower is not traveling forward in order to save fuel without any detrimental effect. 
     According to another aspect of the present invention, the central control unit is configured to maintain the cutting blade at the prescribed elevated height when the lawn mower is commanded to travel rearward or is traveling rearward. 
     When the lawn mower is commanded to travel rearward or is traveling rearward, no grass cutting is typically intended. Therefore, it is advantageous to maintain the cutting blade at the prescribed elevated height, and positively decrease the probability of a high engine load at such a time. 
     According to yet another aspect of the present invention, the lawn mower further comprises a steering angle sensor, and the central control unit is configured to deactivate at least one of the cylinders when the lawn mower is traveling forward, and a steering angle greater than a prescribed value is detected. 
     When the steering angle is great, it is highly probable that the lawn mower is being displaced from one place to another or making a U turn at an end of a mowing pass, and the engine load is light. Therefore it is advantageous to deactivate one of the cylinders to improve the fuel economy of the lawn mower at such a time. 
     According to yet another aspect of the present invention, when the lawn mower is traveling forward, and a steering angle greater than the prescribed value is detected, the central control unit further raises the cutting blade to a prescribed elevated height and decelerates the lawn mower. 
     By decelerating the lawn mower when a steering angle greater than a prescribed value is detected, the precision in the turning maneuver of the lawn mower can be improved and/or the turning radius of the lawn mower can be minimized. 
     According to yet another aspect of the present invention, the prescribed elevated height corresponds to a highest possible position of the cutter deck. 
     When the cutter deck is placed at the highest possible position, the likelihood for the cutting blade to encounter a high load can be minimized. 
     Thus, the present invention provides a lawn mower which can minimize fuel consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         FIG. 1  is a functional block diagram of a ride-on lawn mower embodying the present invention; 
         FIG. 2  is a time chart showing a first mode of operation of the mower; 
         FIG. 3  is a flowchart showing the first mode of operation of the mower; 
         FIG. 4  is an additional part of the flowchart showing the first mode of operation of the mower; 
         FIG. 5  is a time chart showing a second mode of operation of the mower; 
         FIG. 6  is a flowchart showing the second mode of operation of the mower; 
         FIG. 7  is an additional part of the flowchart showing the second mode of operation of the mower; 
         FIG. 8  is a time chart showing a third mode of operation of the mower; 
         FIG. 9  is a flowchart showing the third mode of operation of the mower; and 
         FIG. 10  is an additional part of the flowchart showing the third mode of operation of the mower. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     A preferred embodiment of the present invention is described in the following with reference to the appended drawings. 
     Referring to  FIG. 1 , a ride-on lawn mower  10  embodying the present invention is provided with an internal combustion engine  100  consisting of a multiple cylinder engine (two-cylinder engine in the illustrated embodiment) supported in a front part of a lawn mower main body  15 . A cutting blade  70  that can be rotatively actuated by the engine  100  is supported by a central lower part of the lawn mower main body  15 . In the case of a lawn mower, as is the case with other forms of power equipment, the engine  100  is required to power not only the propelling device consisting of driven wheels but also the work implement consisting of a cutting blade  70 ; namely, in the illustrated embodiment, the engine  100  embodies both a power source for powering the propelling device and an internal combustion engine connected to the work implement in a power transmitting relationship. Therefore, in view of increasing the operation efficiency, high output engines typically consisting of multiple cylinder engines are preferred in most situations. 
     In the illustrated embodiment, the engine  100  is provided with a cylinder deactivation control device so that at least one of the cylinders may be selectively deactivated under a prescribed condition. The cylinder deactivation control device includes a central control unit  50  consisting of an electronic control unit, and an engine control unit  110  for controlling the operation of the engine  100 . The engine control unit  110  may be either separately provided from the central control unit  50  or internally incorporated with the central control unit  50 . 
     The central control unit  50  typically but not exclusively consists of a microcomputer including a CPU, ROM and RAM. The ROM stores a computer program (cylinder deactivation control program) for instructing the CPU to execute a series of actions (cylinder deactivation control process), and the RAM provides a work area for the CPU. The ROM also stores various data that are required for the CPU to operate. 
     The engine control unit  110  is configured to adjust the rotational speed of the engine  100  which may be incorporated with a throttle by wire system, and may be configured to perform the functions of an engine governor. The rotational speed of the engine may be given by revolutions per minute, “rpm”, but may also be given by an inverse of time (in minutes) required for each revolution, “min −1 ”. The engine  100  is provided with a sensor unit  120  that includes an rpm sensor for detecting the rotational speed of the engine  100 . The engine control unit  110  receives a target rotational speed from the central control unit  50 , and controls the throttle valve  100   a  of the engine  100  so that the engine speed detected by the rpm sensor coincides with the target rotational speed. 
     The traveling speed of the lawn mower  10  can be controlled by the depression stroke of an accelerator pedal (not shown in the drawings). The depression stroke may be detected by a detection unit  40  equipped in the lawn mower  10 . In the illustrated embodiment, the lawn mower  10  is equipped with a hydraulic transmission system which can be controlled by the accelerator pedal. However, other forms of transmission systems may also be used in the present invention. At any event, the traveling speed of the lawn mower  10  can be controlled by the depression stroke of the accelerator pedal under normal situations, but the central control unit  50  is able to change the traveling speed of the lawn mower  10  without regard to the depression stroke of the accelerator pedal. 
     The detection unit  40  further includes a speed sensor (which may consist of a wheel speed sensor) for detecting the traveling speed of the lawn mower  10 . The lawn mower  10  is fitted with an operation console  60  which includes a start switch for starting the engine and a blade switch for selectively connecting the cutting blade with the output shaft of the engine in a torque transmitting relationship. 
     The cutting blade  70  is connected to the output shaft of the engine  100  via a blade clutch  80  consisting of an electromagnetic clutch. 
     The cutting blade  70  is rotatably supported by the blade deck  75  which is in turn attached to the lawn mower main body  15  via a height adjustment mechanism  90  including a linkage mechanism and an actuator (such as an electric motor). The blade deck  75  can be vertically adjusted by actuating the electric motor of the height adjustment mechanism  90  via the central control unit  50 . This action may be performed both automatically and manually as required. 
       FIG. 2  is a time chart showing a first mode of operation of the lawn mower  10 . The operator starts the engine  100  at time ta 1  by operating the start switch. The two cylinders are both in the activated condition at this time, and the throttle valve is opened gradually from a fully closed state to Tha 1  (F in  FIG. 3 ). 
     The central control unit  50  monitors the rotational speed of the engine  100 , and monitors if the engine rotational speed is maintained at a prescribe speed Na 1  in a stable manner (A in  FIG. 2 ). Once the engine rotational speed has established a stable condition at speed Na 1  at time ta 2 , the central control unit  50  deactivates one of the cylinders (C in  FIG. 2 ) by controlling the engine control unit  110 . At this time, in order to compensate for the deactivation of one of the cylinders, the throttle opening is increased to Tha 2  (F in  FIG. 2 ). At this time, the blade switch is still turned off (D in  FIG. 2 ). 
     It is possible to start the engine with one of the cylinders deactivated. However, if the engine is started with only one of the cylinders activated, it would take longer for the engine rotational speed to reach a stable condition. 
     It is also possible to deactivate one of the cylinders as soon as the engine is started. However, again, if the engine is warmed up with only one of the cylinders activated, it would take longer for the engine rotational speed to reach a stable condition, and may impair the fuel economy of the engine owing to the prolonged period of instability in the engine rotational speed. 
     Based on such considerations, once the engine rotational speed is placed in a stable condition at speed Na 1 , the central control unit  50  deactivates one of the cylinders at time ta 2  (C in  FIG. 2 ). Thereby, the fuel economy of the engine can be improved. In other words, once the rotational speed of the engine  100  is placed in a stable condition, even with one of the cylinders deactivated, the rotational speed of the engine  100  can be continued to be in a stable condition at speed Na 1 . 
     Furthermore, as the engine  100  is started with the two cylinders activated at ta 1 , the time period required for the rotational speed of the engine  100  to reach a stable condition (the time interval between ta 1  and ta 2 ) can be minimized. 
     As the throttle opening is increased from zero to Tha 1  by the central control unit  50  via the engine control unit  110 , the rotational speed of the engine  100  quickly increases from zero, and initially overshoots speed Na 1  to a significant extent (A in  FIG. 2 ). The rotational speed then oscillates around speed Na 1  for a certain time period. The central control unit  50  determines that the engine rotational speed has reached a stable condition once this oscillation has subsided to an acceptable level. 
     More specifically, the central control unit  50  determines if the rotational speed of the engine  100  is held within a tolerable range or determines if the rotational speeds of the engine  100  sampled at a prescribed time interval have converged into a tolerable range. The central control unit  50  determines that the rotational speed of the engine  100  is placed in a stable condition at time ta 2 , for instance, and starts the cylinder deactivation process at time ta 2  (C in  FIG. 2 ). This may be accomplished by defining a cylinder deactivation flag, and initiating the cylinder deactivation process when the cylinder deactivation flag is set to “1” instead of “0”. At this time, the throttle opening is increased from Tha 1  to Tha 2  to maintain the same rotational speed Na 1  with only one of the cylinders activated (F in  FIG. 2 ). 
     During the time interval in which one of the cylinders is deactivated, the central control unit  50  monitors if the rotation of the cutting blade  70  is commanded. When the operator turns on the blade switch at time ta 3 , the central control unit  50  activates all of the cylinders (C and D in  FIG. 2 ). This may be accomplished by setting the cylinder deactivation flag to “0” to terminate the cylinder deactivation process and activate all of the cylinders at time ta 3 . 
     Thus, the central control unit  50  keeps one of the cylinders deactivated until the operation of the cutting blade  70  is commanded, and activates all of the cylinders once data or a signal indicating the need for the operation of the cutting blade  70  is received from the operation console  60 . Therefore, the consumption of fuel is reduced during the time period between time ta 2  and time ta 3 . 
     In particular, when one of the cylinders is deactivated, the rotational speed of the engine is Na 1 . When the blade switch is turned on by the operator at time ta 3 , the central control unit  50  activates all of the cylinders, and increases the rotational speed of the engine to Na 2  at the same time. Immediately following this, the central control unit  50  engages the blade clutch  80  at ta 4 . 
     According to this arrangement, when the cutting blade  70  is required to be operated, not only all of the cylinders are activated but also the rotational speed of the engine  100  is increased so that the sudden increase in the engine load caused by the engagement of the blade clutch  80  is prevented from causing the stalling of the engine  100  or otherwise placing the engine in an unstable condition. Therefore, during the operation of the mower  10 , the possibility of stalling the engine  100  can be reduced without regard to the operating condition of the mower  10 . 
     Once the blade clutch  80  is engaged, the central control unit  50  maintains all of the cylinders activated, and maintains the rotational speed of the engine at the normal rotation speed of Na 1 . Thus, by reducing the rotational speed of the engine from the high level of Na 2  suitable for preventing the stalling of the engine to the normal level of Na 1  once the blade clutch  80  is engaged, the engine  100  is made resistant against stalling while minimizing the fuel consumption. 
     The engine rotational speed sharply drops when the blade clutch  80  is engaged at time ta 4  owing to the sudden increase in the engine load, but returns to the normal rotational speed of Na 1  in a short period of time as the rotational speed of the cutting blade  70  stabilizes. 
     During the time interval between time ta 2  and time ta 3 , the blade clutch  80  is disengaged while the mower  10  is stationary or traveling. Therefore, the engine load is so light that the engine may be operated with only one of the cylinders in an adequately stable manner. As a result, the fuel consumption can be minimized. 
     Preferably, when one of the cylinders is deactivated or during the time interval between time ta 2  and time ta 3 , a highly lean mixture may be supplied to the combustion chamber of the active cylinder of the engine at a relatively wide throttle opening. In the illustrated embodiment, the throttle opening is increased from Th 1  to Th 2  at time ta 2 . 
     Thus, during the time interval between time ta 2  and time ta 3 , the engine can be operated in a stable manner in spite of a highly lean mixture owing to the light load, and the increasing of the throttle opening reduces the pumping loss of the engine. As a result, the engine can be operated in a highly fuel efficient manner. 
     Furthermore, during the time interval between time ta 2  and time ta 3  or when one of the cylinders is deactivated, the ignition timing of the engine may be advanced and/or the duration of the spark of the spark plug may be increased so that a highly stable combustion may be maintained. 
     When the deactivated cylinder is activated once again at time ta 3 , the central control unit  50  returns the air fuel ratio back to the normal value, and reduces the throttle opening back to Tha 1 . In other words, the lean burn control is maintained during the time interval between time ta 2  and time ta 3 , and is turned off at time ta 3 . 
       FIGS. 3 and 4  show a flowchart of the mode of operation of the cylinder deactivation control from the time of engine start up. When the engine  100  is started, the central control unit  50  determines if the rotational speed of the engine  100  has stabilized at the rotational speed of Na 1  in step ST 01 . If the rotational speed of the engine  100  has stabilized at the rotational speed of Na 1 , the central control unit  50  monitors if the blade switch on the operation console  60  has been turned on in step ST 02 . 
     If the blade switch is turned off in step ST 02 , the central control unit  50  determines if the cylinder deactivation process is to be started according to the cylinder deactivation flag in step ST 03 . The cylinder deactivation flag is initially set to “0”. If the cylinder deactivation flag is “0” in step ST 03 , the central control unit  50  determines if the load of the engine  100  is heavy in step ST 04 . 
     More specifically, if the throttle opening is constant or in a stable condition, and additionally or alternatively, if the throttle opening is below a prescribed threshold value, the determination result of step ST 04  is No. The sensor unit  120  is provided with a throttle sensor for this purpose. 
     If the engine load is light in step ST 04 , the cylinder deactivation flag is set to or maintained at “1” in step ST 05 . Conversely if the engine load is heavy in step ST 04 , the cylinder deactivation flag is set to or maintained at “0” in step ST 06 . Then, in either case, the program flow returns to step ST 01 . 
     If the cylinder deactivation flag is “1” in step ST 03 , the central control unit  50  determines if the load of the engine  100  is heavy in step ST 07 . This determination step is similar to step ST 04 , and it is determined that the engine load is light if the throttle opening is constant or in a stable condition, and additionally or alternatively, if the throttle opening is below a prescribed threshold value. 
     If the engine load is light in step ST 07 , the cylinder deactivation flag is maintained at “1”. Conversely if the engine load is heavy in step ST 07 , the cylinder deactivation flag is set to “0” in step ST 08 , and the program flow returns to step ST 01 . 
     If the rotational speed Ne of the engine  100  is not stable in step ST 01 , the cylinder deactivation flag is set to or maintained at “0” in step ST 09 , and the target rotational speed of the engine  100  is set to a prescribed rotational speed Na 1  in step ST 10 . The target rotational speed of the engine  100  is initially (when the engine  100  is started) set to the prescribed rotational speed Na 1 . The blade clutch  80  is then disengaged (OFF) or maintained in the disengaged state in step ST 11 , and the program flow returns to step ST 01 . The blade clutch  80  is initially (when the engine is started) disengaged. 
     If the blade switch is turned on in step ST 02 , the central control unit  50  determines if the blade clutch  80  is engaged in step ST 12 . If the blade clutch  80  is disengaged, step ST 13  similar to step ST 03  is executed to determine if the cylinder deactivation process is to be started. 
     If the cylinder deactivation flag is “1” in step ST 13 , the cylinder deactivation flag is set to “0” in step ST 14 , and the target rotational speed of the engine  100  is set to Na 2  which is higher than the prescribed rotational speed Na 1  in step ST 15 . If the cylinder deactivation flag is “0” in step ST 13 , the central control unit  50  automatically engages the blade clutch  80  in step ST 16 . Step ST 17  similar to step ST 15  is then executed to set the target rotational speed Ne of the engine  100  to Na 2 . Following steps ST 15  and ST 17 , the program flow returns to step ST 01 . 
     If the blade clutch  80  is engaged in step ST 12 , the central control unit  50  determines if the rotational speed Ne of the engine  100  is maintained at Ne 2  in a stable manner in step ST 18 . If the rotational speed Ne of the engine  100  is maintained at Ne 2  in a stable manner in step ST 18 , the central control unit  50  then sets the target rotational speed to the prescribed rotational speed Na 1  in step ST 19 , and the program flow returns to step ST 01 . If the rotational speed is not stable in step ST 18 , the program flow returns to step ST 01 . 
       FIG. 5  is a time chart showing the mode of operation of the lawn mower  10  including the time point of starting a forward travel with the blade clutch  80  already placed in the engaged state and the engine  100  running. The throttle opening was initially Thb 1 , and the operator increases the throttle opening to Thb 2  at time tb 1  (E in  FIG. 5 ). This causes the lawn mower  10  to start traveling forward at time tb 1  (F in  FIG. 5 ). 
     The detection unit  40  detects the traveling speed of the lawn mower  10 , and the central control unit  50  maintains the traveling speed of the lawn mower  10  at Vb 1  which is greater than zero. At this time, because of the high loading placed on the engine  100 , the two cylinders are kept activated so that the rotational speed of the engine  100  may be maintained at Nb 2  in spite of the added loading required for propelling the lawn mower  10 . 
     If the cutting blade  70  does not cut grass while the lawn mower  10  is traveling forward, it is possible to deactivate one of the cylinders. However, as the blade clutch  80  is engaged, it is very possible that the cutting blade  70  starts cutting grass. Therefore, it is advantageous to keep both of the two cylinders activated when the lawn mower  10  has started traveling forward, and to continue to keep the two cylinders activated until the operating condition of the engine  100  has stabilized (B in  FIG. 5 ). 
     While the two cylinders are kept activated, following the time point tb 1 , the central control unit  50  monitors if the throttle opening is smaller than a prescribed level Thb 3 . If a throttle opening smaller than Thb 3  has persisted for more than a prescribed time period, the central control unit  50  deactivates one of the cylinders at time tb 2  so that the fuel consumption may be saved (B and E in  FIG. 5 ). 
     The fact that the throttle opening is smaller than Thb 3  means that the loading of the engine  100  is light even though the cutting blade  70  is rotating. Therefore, the central control unit  50  can safely deactivate one of the cylinders at time tb 2 . Thus, at this time, the central control unit  50  sets the cylinder deactivation flag to “1” at time tb 2  to initiate the cylinder deactivation control, and the engine  100  is operated with only one of the cylinders activated. At the same time, the throttle opening is increased to Thb 4  which is greater than Thb 3  so that the rotational speed of Nb 2  may be maintained with only one of the cylinders activated. 
     Following the time point tb 2 , the central control unit  50  continues to monitor the throttle opening if the throttle opening is greater than a prescribed opening Thb 5  which is higher than Thb 4 . If the throttle opening exceeds this threshold level Thb 5 , the central control unit  50  causes the engine  100  to operate with both of the cylinders activated at tb 3  (B and E in  FIG. 5 ). Thus, at this time, the central control unit  50  sets the cylinder deactivation flag to “0” at time tb 3  to interrupt the cylinder deactivation control, and the engine  100  is operated with both of the cylinders activated. 
     Thus, when the engine loading is high (as detected by the throttle opening exceeding Thb 5 ), both of the cylinders are activated so that the engine  100  is prevented from stalling or otherwise being placed in an unstable condition. In other words, both of the cylinders are activated when the power requirement of the engine is high (such as when the cutting blade  70  is cutting grass while the lawn mower  10  is traveling forward) but until such a time, the engine is allowed to operate with only one of the cylinders activated so that the fuel consumption can be minimized. 
     Alternatively, following the time point tb 2 , the central control unit  50  monitors if the rotational speed of the engine  100  is maintained at Nb 2  in a stable manner. If the throttle opening exceeds Thb 5  while the rotational speed of the engine  100  is not stable at Nb 2 , the central control unit  50  interrupts the cylinder deactivation control, and causes both of the cylinders to be activated at time tb 3 . 
     More specifically, the central control unit  50  determines if the rotational speed of the engine  100  remains in a prescribed tolerance range, or if the rotational speed of the engine  100  is within the tolerance range such as Nb 2 ±ΔN at each sampling time Δt. If the central control unit  50  determines that the engine rotational speed is not stable at time tb 3 , the cylinder deactivation control is interrupted at time tb 3 . 
     Thus, the loading of the engine is determined from the fluctuations in the engine rotational speed and the throttle opening in a highly exact manner so that the cylinder deactivation control is interrupted only when required. As a result, the cylinder deactivation control is kept turned on whenever possible, and the fuel consumption of the engine can be minimized. 
     Following the time point tb 3 , the central control unit  50  determines if the lawn mower  10  is still traveling forward. When a zero speed of the lawn mower  10  is detected by the detection unit  40  at time tb 4 , the central control unit  50  can determine that the lawn mower  10  has come to a stop at time tb 4  (F in  FIG. 5 ). At this time point, the central control unit  50  may continue to activate both of the cylinders from this time point tb 4  onward so that the lower rotational speed Nb 1  (which is lower than Nb 2 ) of the engine  100  may be regained in stable manner. 
     Once the lawn mower  10  has come to a stop, even though the blade clutch  80  may be engaged, it can be assumed that the engine load is light. Therefore, the central control unit  50  lowers the rotational speed of the engine to Nb 1  so that the fuel consumption may be minimized. 
     During the time interval between tb 2  and tb 3 , only one of the cylinders is activated while the rotational speed of the engine is maintained at Nb 2  so that the fuel consumption may be minimized. 
     Preferably, when one of the cylinders is deactivated or during the time interval between time tb 2  and time tb 3 , a highly lean mixture may be supplied to the combustion chamber of the active cylinder of the engine at a relatively wide throttle opening. In the illustrated embodiment, the throttle opening is increased from Thb 2  to Thb 4  at time tb 2 . More specifically, the central control unit  50  makes the fuel/air ratio leaner by adjusting the amount of fuel injection for the given air intake via the engine control unit  110  while the throttle opening is increased from Thb 2  to Thb 4  at time tb 2 . 
     Thus, during the time interval between time tb 2  and time tb 3 , the engine  100  can be operated in a stable manner in spite of a highly lean mixture owing to the light load, and the increasing of the throttle opening reduces the pumping loss of the engine. As a result, the engine can be operated in a highly fuel efficient manner. 
     Furthermore, during the time interval between time tb 2  and time tb 3  or when one of the cylinders is deactivated, the ignition timing of the engine may be advanced and/or the duration of the spark of the spark plug may be increased so that a highly stable combustion may be maintained. 
     When the cylinder deactivation control is interrupted, and both of the cylinders are activated, the central control unit  50  changes the air fuel ratio from a lean value to a normal or stoichiometric value while automatically closing the throttle opening from Thb 5  to Thb 3 . In other words, the central control unit  50  terminates the lean burn control at time tb 3 . 
       FIGS. 6 and 7  show a flowchart of the cylinder deactivation control process including the starting of the forward travel of the lawn mower  10 . It should be noted that this is only exemplary, and does not limit the scope of the present invention. After the blade switch is turned on by the operator, and the blade clutch  80  is engaged as a result, the central control unit  50  determines if the lawn mower  10  is moving forward or not according to the data or signal supplied by the detection unit  40  which may include a wheel speed sensor (step ST 21 ). 
     When the lawn mower  10  is traveling forward, the central control unit  50  determines if a target rotational speed of the engine  100  is set at a target rotational speed Nb 2  for a grass cutting condition according to a grass cutting mode Ne set flag (step ST 22 ). Initially or once the blade clutch  80  is engaged, the grass cutting mode Ne set flag is set to “0”. 
     When the grass cutting mode Ne set flag is set to “0”, the central control unit  50  sets or raises the target rotational speed to Nb 2  (step ST 23 ). Initially or once the blade clutch  80  is engaged, the target rotational speed is set to an idling target rotational speed Nb 1  (a non-grass cutting condition). However, as the mass of the lawn mower is significant particularly in the case of a ride-on mower, the target rotational speed should be increased to the higher value of Nb 2  in order to avoid the stalling of the engine at the time of start off. Then, the central control unit  50  sets the grass cutting mode Ne set flag to “1” (step ST 24 ), and the program flow returns to step ST 21 . 
     If the grass cutting mode Ne set flag is to “1” in step ST 22 , and the rotational speed Ne of the engine  100  is therefore set to the prescribed rotational speed Nb 2 , the central control unit  50  determines if the cylinder deactivation control is to be initiated by referring to the cylinder deactivation flag indicating whether the cylinder deactivation control is to be initiated or not (step ST 25 ). The cylinder deactivation flag is initially set to “0”. 
     If the cylinder deactivation flag is “0” in step ST 25 , the central control unit  50  determines if the load on the engine  100  is high (step ST 26 ). More specifically, the load on the engine  100  is determined to be not high when the throttle opening persists to be below the prescribed opening Thb 3  for a prescribed time period. 
     If the load on the engine  100  is not high in step ST 26 , the central control unit  50  sets the cylinder deactivation flag to “1” (step ST 27 ). On the other hand, if the load on the engine  100  is high in step ST 26 , the central control unit  50  sets the cylinder deactivation flag to “0” or maintains the cylinder deactivation flag at “0” (step ST 28 ). 
     When the cylinder deactivation flag is set to “1” in step ST 25 , the central control unit  50  determines if the cylinder deactivation control is to be terminated (step ST 29 ). More specifically, the central control unit  50  terminates the cylinder deactivation control if the throttle opening is greater than a value Thb 5  which is higher than the prescribed opening value Thb 3 . Alternatively, the central control unit  50  may terminate the cylinder deactivation control on the condition that the throttle opening is greater than the high value Thb 5 , and the rotationally speed of the engine does not stay stable at the prescribed rotational speed Nb 2 . 
     When the cylinder deactivation control is terminated in step ST 29 , the central control unit  50  sets the cylinder deactivation flag to “0” (step ST 30 ). Otherwise, the program flow returns to step ST 21 . If the traveling speed of the lawn mower is zero or if the lawn mower is stationary in step ST 21 , the central control unit  50  executes step ST 31  in a similar manner as in step ST 25 . 
     When the cylinder deactivation flag is “1” in step ST 31 , the central control unit  50  sets the cylinder deactivation flag to “0” (step ST 32 ). Typically, when the traveling speed of the lawn mower  10  is zero, both of the cylinders are activated as shown in B and F of  FIG. 5 . However, the lawn mower  10  may come to a stop (the traveling speed of the lawn mower may become zero) while one of the cylinders is deactivated. It may also be configured such that, under such a circumstance, the cylinder deactivation flag may be set to “1” in step ST 31 . 
     When the cylinder deactivation control is not detected in step ST 31  or when cylinder deactivation flag is set to “0” step ST 32 , the central control unit  50  sets the target rotational speed of the engine to the idling rotational speed Nb 1  or maintains the idling rotational speed Nb 1  (step ST 33 ). The central control unit  50  then sets the grass cutting mode Ne set flag to “0” or maintains this flag at “0” (step ST 34 ). Then, the program flow eventually returns to step ST 21 . 
       FIG. 8  is a time chart showing the mode of operation of the lawn mower  10  including time periods where the lawn mower is traveling forward, stationary and traveling rearward. While the blade clutch  80  is engaged by the operator by turning on the blade switch provided on the operation console  60 , the lawn mower  10  which has been traveling forward at speed Vc 3  is brought to a stop at time tc 2 . The detection unit  40  shown in  FIG. 1  detects the traveling speed of the lawn mower  10 , and then sends a signal or data indicating the stationary condition of the lawn mower  10  to the central control unit  50  at time tc 2 . 
     The fact that the lawn mower  10  is stationary typically indicates that the load on the engine  100  is light. However, it is also possible that the engine load is high because the blade clutch  80  is engaged, and the cutting blade  70  may be mowing grass. Therefore, the central control unit  50  still keeps both of the cylinders activated at time tc 2 . 
     Then, the central control unit  50  evaluates the probability of the engine encountering a high load after time tc 2 . If the probability of a light engine load is high following time tc 2 , the central control unit  50  deactivates one of the cylinders. 
     The probability of the engine load being light can be positively increased according to the present invention. For instance, the height of the cutting blade  70  can be automatically increased from a normal height H 2  to a highest possible height H 3  at time tc 2  by using the height adjustment mechanism  90  for adjusting the height of the blade deck. This is accomplished by setting a blade height control flag to “1”. 
     The higher the cutting blade  70  is, the smaller the cutting load of the cutting blade  70  becomes. When the blade deck  75  is placed in the highest position, the cutting blade  70  typically does not cut grass at all. When the cutting blade  70  is higher than the expected height of the grass, the cutting blade  70  consumes substantially no power of the engine or, in other words, the probability of a light engine load is high. 
     Thus, the central control unit  50  initiates the cylinder deactivation control at time tc 3 , and deactivates one of the cylinders. In other words, when the blade clutch  80  is engaged, and both of the cylinders are activated, the central control unit  50  determines that the cylinder deactivation control should be initiated, optionally, after raising the height of the cutting blade to the highest possible position H 3 . 
     More specifically, the central control unit  50  determines at time tc 2  if the lawn mower  10  continues to travel forward or has come to a stop. If the lawn mower  10  has come to a stop, the central control unit  50  automatically raises the cutting blade  70  from height H 2  to height H 3  during the time period of from tc 2  to tc 3 , and initiates the cylinder deactivation control at time tc 3  (A, D and E in  FIG. 8 ). 
     Then, suppose that the operator operates a transmission lever (forward/reverse switch) on the operation console  60  to command the central control unit  50  to put the gear in the reverse. The central control unit  50  detects the selection made by the transmission lever at time tc 4  (F in  FIG. 8 ). 
     As a result, the lawn mower  10  starts traveling rearward, and a signal or data indicating the rearward movement is forwarded from the detection unit  40  to the central control unit  50  (D in  FIG. 8 ). During the time interval between tc 2  and tc 6  where the lawn mower  10  is not traveling forward, or, in other words, where the lawn mower  10  is stationary or traveling rearward, the central control unit  50  maintains the cutting blade  70  at height H 3  (E in  FIG. 8 ). 
     The operator then decides to drive the lawn mower  10  forward at time tc 6 , and the central control unit  50  receives a signal or data indicating a forward speed greater than zero from the detection unit  40  at time tc 6  (D in  FIG. 8 ). At this point or time tc 6 , the central control unit  50  terminates the cylinder deactivation control, and activates both of the cylinders (A in  FIG. 8 ). At the same time, the central control unit  50  sets the blade height control flag to “0” so that the height of the cutting blade  70  is lowered from H 3  to H 2  (E in  FIG. 8 ). 
     The central control unit  50  sets the blade height control flag to “0” at time tc 6 , and automatically lowers the height of the blade  70  from H 3  to H 2  (E in  FIG. 8 ). The central control unit  50  then determines that the lawn mower  10  is steered to the right at time tc 7  with a steering angle exceeding a prescribed threshold value S 2  (F in  FIG. 8 ). The central control unit  50  detects the right turn of the lawn mower  10  via a steering angle sensor included in the detection unit  40 . 
     When a large steering angle is detected, it is highly likely that the lawn mower  10  is being relocated from one location to another or making a U turn at a perimeter of the lawn so that the probability of the engine load being light is high. Therefore, when a large steering angle (exceeding S 2 ) is detected, the central control unit  50  raises the height of the cutting blade  70  from H 2  to H 3  at time tc 7 , and, upon the cutting blade  70  reaching the height H 3  at time tc 8 , initiates the cylinder deactivation control at time tc 8  (A, E and F in  FIG. 8 ). 
     Also, upon detecting that the steering angle has exceeded the threshold value S 2  at tc 7 , not only the height of the cutting blade  70  is raised to H 3  but also the traveling speed of the lawn mower  10  is automatically reduced from Vc 3  to Vc 2  (D in  FIG. 8 ) by setting a steering speed reduction flag to “1” at time tc 7 . By thus automatically reducing the traveling speed of the lawn mower  10 , the precision of the turning maneuver of the lawn mower  10  can be improved, and/or the turning radius of the lawn mower  10  can be minimized. 
     In the illustrated embodiment in which a throttle by wire system is incorporated in the engine  100 , when the steering speed reduction flag is set to “1”, the central control unit  50  causes the traveling speed of the lawn mower  10  to be automatically reduced from Vc 3  to Vc 2  via the engine control unit  110 . Alternatively or additionally, the central control unit  50  may apply a brake to the lawn mower  10  to achieve the required speed reduction. 
     If the lawn mower  10  consists of a hybrid lawn mower  10  which uses an electric motor as a power source for powering the propelling device of the lawn mower and an internal combustion engine for driving the cutting blade  70 , the central control unit  50  can reduce the traveling speed of the lawn mower by suitably controlling the electric power supplied to the electric motor. 
     Once the cutting blade  70  has been raised to the height of H 3  at time tc 8 , the central control unit  50  initiates the cylinder deactivation control (A and E in  FIG. 8 ). When the steering angle is reduced to zero at tc 9 , and the lawn mower  10  has thereby started traveling forward, the central control unit  50  terminates the cylinder deactivation control, and activates both of the cylinders (A and F in  FIG. 8 ). This is advantageous because, typically, once the lawn mower  10  has started traveling straight forward, the cutting blade  70  is required to cut grass, and it is therefore highly likely that the engine load will increase. 
       FIGS. 9 and 10  show a flowchart illustrating the mode of operation of the lawn mower  10  during a time interval in which the traveling direction of the lawn mower  10  is reversed, and the lawn mower  10  is steered by a relatively large angle. Suppose that the blade clutch  80  is initially engaged. The central control unit  50  determines if the lawn mower  10  is steered by detecting a steering angle exceeding the prescribed threshold value S 2  according to a signal or data obtained from the detection unit  40  (step ST 41 ). 
     More specifically, when the absolute value of the steering angle is smaller than the threshold value S 2 , the central control unit  50  determines if the transmission system of the lawn mower  10  is placed in the state for the forward movement according to the data or signal from the FWD/RVS switch in the operation console  60  (step ST 42 ). 
     Upon detecting the state for the forward movement of the lawn mower  10 , the central control unit  50  determines if the lawn mower  10  is being accelerated according to the data or signal from the detection unit  40  which may include an accelerator pedal sensor (step ST 43 ). More specifically, the central control unit  50  determines that the lawn mower  10  is not accelerating when the depression stroke of the accelerator pedal is zero, and that the lawn mower  10  is accelerating when the depression stroke of the accelerator pedal is not zero. 
     Upon detecting that the lawn mower  10  is not accelerating, the central control unit  50  determines if the lawn mower  10  is traveling forward from the data or signal from the detection unit  40  which may include a wheel speed sensor (step ST 44 ). More specifically, the central control unit  50  determines that the lawn mower  10  is not traveling forward when the wheel speed is zero, and that the lawn mower  10  is traveling forward when the wheel speed is not zero. 
     When the traveling speed of the lawn mower is zero, the central control unit  50  sets the blade height control flag to “1” (step ST 45 ). In the initial condition or when the blade clutch  80  is engaged, the blade height control flag is set to “0”. 
     If the lawn mower  10  is accelerating in step ST 43 , the blade height control flag is set to or maintained at “0” (step ST 46 ). Similarly, if the traveling speed of the lawn mower  10  is not zero in step ST 44 , the blade height control flag is set to or maintained at “0” (step ST 46 ). 
     If the transmission system of the lawn mower  10  is not in the state for forward travel in step ST 42 , the blade height control flag is set to “1” (step ST 47 ). Similarly, if the absolute value of the steering angle is greater than the prescribed threshold value S 2 , the blade height control flag is set to “1” (step ST 48 ). 
     Then, the central control unit  50  may set the steering deceleration command flag to “1” (step ST 49 ). Initially or when the blade clutch  80  is engaged, the steering deceleration command flag is set to “0”. When the deceleration during the cornering maneuver is completed, the central control unit  50  sets the steering deceleration command flag back to “0”. 
     The central control unit  50  determines if the blade height control flag is “1” in step ST 50 . If the blade height control flag is “1”, the central control unit  50  sets the height of the blade  70  to the highest position H 3  or the eighth stage (step ST 51 ). More specifically, the central control unit  50  keeps moving the blade deck  75  upward while monitoring the data or signal from the height adjustment mechanism  90  until the highest position H 3  (eighth stage) is reached (step ST 52 ). Once the blade  70  has reached the highest position, the blade height control flag is set to “0”. 
     At the same time, the central control unit  50  sets the cylinder deactivation flag to “1” (step ST 53 ). In the initial condition or when the blade clutch  80  is engaged, the cylinder deactivation flag is set to “0”. 
     If the blade height has not reached the highest position or the signal received from the height adjustment mechanism  90  indicates that the blade height is lower than the highest position H 3  (in the seventh stage or lower) in step ST 51 , the central control unit  50  moves the blade deck  75  upward by one stage (step ST 54 ). 
     If the blade height control flag is not “1” in step ST 50 , the central control unit  50  determines if the height of the blade  70  should be set to a desired height (such as the fourth stage) according to the setting entered from the operation console  60  by the operator (step ST 55 ). 
     For instance, when the current blade height is in the highest position (the eighth stage), and the desired height is a certain number of stages lower than the highest stage (fourth stage, for instance), or in other words, the current blade height is higher than the desired height according to the signal from the height adjustment mechanism  90  for the blade deck  75 , the central control unit  50  lowers the height of the blade  70  by one stage (step ST 56 ). At the same time, the central control unit  50  sets the cylinder deactivation flag to “0” (step ST 57 ). 
     If the signal received from the height adjustment mechanism  90  indicates that the blade  70  has reached the desired height in step ST 55 , the central control unit  50  stops the downward movement of the blade  70  (step ST 58 ). At the same time, the central control unit  50  sets the cylinder deactivation flag to “0” (step ST 59 ). In each of the cases, the program flow eventually returns to step ST 41 . 
     Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims. The contents of the original Japanese patent application on which the Paris Convention priority claim is made for the present application as well as the contents of the prior art references mentioned in this application are incorporated in this application by reference.