Abstract:
An all-terrain vehicle, which limits a speed of backward movement of the vehicle to a predetermined speed and which assures a necessary power output while limiting the backward speed, without a driver&#39;s taking any manual trouble to operate a switch to control an engine of the vehicle, even though a high load is exerted upon the vehicle. The all-terrain vehicle includes a vehicle speed sensor device, a backward movement positional sensor device, and an engine controller for limiting the engine speed of the vehicle when the backward movement positional sensor device detects that the vehicle is running backward and when the vehicle speed sensor device detects that the vehicle speed reaches a predetermined value, thereby limiting the speed of the vehicle running backward.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to an all-terrain vehicle, and particularly relates to an all-terrain four-wheeled vehicle provided with an automatic transmission type belt converter. 
   2. Description of the Related Art 
   In a case of an all-terrain four-wheeled vehicle which is provided with a gear transmission, but not provided with a belt converter (i.e. automatic V-belt transmission), the speed reduction ratio of the gear transmission is fixed. Therefore, when the gear transmission is shifted to a backward position by which the vehicle is moved backward, and when an engine speed of the vehicle is increased by operating a throttle of the engine, the speed of the vehicle is proportional to the engine speed. In other words, when a maximum power region of the engine is used, the increment of the vehicle speed is limited within a predetermined region set when the gear transmission has been shifted to the backward position. 
   On the other hand, in a case of an all-terrain four-wheeled vehicle which is provided with a belt converter and a gear type sub-transmission, when the gear transmission (gear type sub-transmission) of the vehicle is shifted to the backward position, the speed reduction ratio by the belt converter automatically changes in compliance with the engine speed and a load exerted upon the belt converter. In other words, the vehicle speed is changeable from a practical speed region to an unnecessary high speed region. 
   As the all-terrain four-wheeled vehicle which is provided with the belt converter, there has been proposed a vehicle which has a controller for controlling the engine speed so as to prevent the vehicle speed from reaching such an unnecessary high speed, when the vehicle moves backwards. For example, the controller includes an engine speed detector, a backward movement detector, an ignition limiter for cutting ignition of the engine when the engine speed reaches a predetermined value during the backward movement of the vehicle, and an ignition cut release switch which is of a manual button type, or of a manual lever type. 
     FIG. 11  is a flowchart showing steps for controlling the engine by the conventional controller. That is, at step S 100 , it is determined whether the position of the gear transmission (gear position) is a reversion position (i.e., position for the backward movement of the vehicle) or not. If YES, it proceeds to step S 200  in which it is determined whether the ignition cut release switch is open or not. If YES, it proceeds to step S 300  in which it is determined whether the engine speed is over and above a set value (predetermined value) or not. If YES, it proceeds to step S 400  in which the ignition of the engine is cut so as to decrease the engine speed to a speed up to the set value. 
   Namely, when the engine speed becomes more than the predetermined value during backward movement of the vehicle, the ignition of the engine is cut so as to limit the engine speed to under the predetermined value. On the other hand, when the driver of the vehicle desires, the ignition cut of the engine can be released by operating (handling) the ignition cut release switch manually, in order to increase the engine speed. 
   Incidentally, as prior art relating to the present invention, Japanese Laid-Open Patent Publication No. 61-129473 discloses a straddle type all-terrain four-wheeled vehicle having no belt converter, in which when the engine speed is more than a predetermined value during backward movement of the vehicle, the engine speed is limited to a value under the predetermined value. 
   Concerning the all-terrain four-wheeled vehicle provided with the belt converter in which the engine speed is detected while the vehicle is running backwards and in which the engine speed is controlled to be smaller than the predetermined value, there are some problems as follows. That is, 
   (1) when a high load is exerted upon the belt converter during backward movement of the vehicle, a large power output (i.e. large drive power) is necessary. At this time, the driver must take the trouble to operate the ignition cut release switch manually inconveniently, in order to increase the engine speed,
 
(2) it is necessary to arrange the ignition cut release switch near the driver, and there must be an arrangement of wiring which connects the ignition cut release switch to a controller, etc., thus creating additional cost, and
 
(3) when the load exerted upon the belt converter frequently varies during backward movement of the vehicle, the operation of the ignition cut release switch is troublesome.
 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the present invention to provide an all-terrain vehicle, which prevents a speed of the vehicle from being over and above a predetermined speed (i.e. from being unnecessarily high) when the vehicle moves backward and which secures a necessary power output while limiting the speed of the vehicle, without taking the trouble to operate a switch to control the engine manually, even though a high load is exerted upon the belt converter when the vehicle moves backward and/or even though the load exerted upon the belt converter frequently varies when the vehicle moves backward. 
   It is another object of the present invention to provide the all-terrain vehicle having an engine control mechanism which is assembled from a small number of parts, thus realizing a low cost. 
   In accomplishing the above objects of the present invention, there is provided an all-terrain vehicle having a belt converter, comprising: a vehicle speed detection mechanism for detecting a vehicle speed of the vehicle; a backward movement detection mechanism for detecting a backward movement of the vehicle; and an engine control mechanism for automatically controlling an engine speed of an engine of the vehicle, so as to make the vehicle speed lower than a predetermined vehicle speed, when the backward movement detection mechanism detects the backward movement of the vehicle and when the vehicle speed detection mechanism detects that the vehicle speed reaches the predetermined vehicle speed. In the arrangement, the engine control mechanism can control the engine speed, for example, by limiting an ignition of the engine, or by limiting a supply of fuel to the engine. 
   According to the arrangement, the engine speed (i.e. the number of revolutions of the engine) is controlled with reference to the vehicle speed, as a reference (i.e. standard), upon the backward movement of the vehicle. Therefore, when a high load is exerted upon the belt converter during backward movement of the vehicle, it is possible to keep the vehicle speed low while increasing the engine speed (the number of revolutions of the engine) in order to increase the power output of the engine, without performing a switching operation to cancel a control of the engine speed such as a cut of ignition of the engine for example, thus making it possible to cope with the high load. 
   Also, according to the arrangement, even though the load exerted upon the belt converter frequently varies when the vehicle moves backward, the driver does not have to perform the switching operation frequently to cancel the control of the engine speed such as the ignition cut for example, thus making it possible to cope with the change of the load, as well. 
   In the arrangement, preferably, there is further provided a gear transmission, wherein the backward movement detection mechanism is a mechanism for detecting that the gear transmission is shifted to a reverse position (i.e., a position for backward movement of the vehicle). 
   According to the arrangement, the mounting of the backward movement detection mechanism to the vehicle becomes easy. Also, as the backward movement detection mechanism, it is possible to employ a backward movement detection mechanism for a conventional indicator which indicates a position of speed change of the vehicle. In the arrangement, for example, the backward movement detection mechanism can detect a backward position of a shift rod of the gear transmission, and the backward movement detection mechanism can have an approximate switch arranged so as to oppose an edge surface of the shift rod. 
   Alternatively, in a case that the all-terrain vehicle has a rotation member in which a rotational direction of the rotation member when the all-terrain vehicle moves forwards is opposite to a rotational direction of the rotation member when the all-terrain vehicle moves backward, the backward movement detection mechanism can be a mechanism for detecting the rotational direction of the rotation member when the all-terrain vehicle moves backward. 
   According to the arrangement, of all the rotation members in which a rotational direction of the rotation members when the all-terrain vehicle moves forwards is opposite to a rotational direction of the rotation members when the all-terrain vehicle moves backward, in a drive power transmission system of the vehicle, it is possible to optionally select one rotation member with respect to which the backward movement detection mechanism is easy to mount. That is, according to the arrangement, it is possible to expand a degree of freedom for positioning the backward movement detection mechanism relative to the vehicle. 
   Preferably, the predetermined vehicle speed, as a reference to execution of the control of the engine speed by the engine control mechanism, is set to be one of a first speed at which the belt converter starts an automatic shift from a state of a generally maximum reduction ratio in speed when the all-terrain vehicle accelerates backward with a throttle of the engine opening wide, and a second speed in the vicinity of the first speed. 
   According to the arrangement, the vehicle speed upon the backward movement of the vehicle, is limited to a region in speed which is able to be realized when a state of the belt converter is fixed to be a low state (i.e. state of maximum reduction ratio in speed). Therefore, even though the engine speed (the number of revolutions of the engine) is increased, both of the speed of the vehicle moving backward and the power output of the engine are maintained within a practical degree. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and features of the present invention will become clear from the following description taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings. 
       FIG. 1  is a plan view showing a straddle type all-terrain four-wheeled vehicle according to an embodiment of the present invention. 
       FIG. 2  is a right side view of the straddle type all-terrain four-wheeled vehicle of  FIG. 1 . 
       FIG. 3  is a view showing a gear transmission and a driven pulley of an automatic belt converter, in a cross section taken along a plane passing through respective axes of the gear. 
       FIG. 4  is a longitudinal cross sectional view of a drive pulley of the automatic belt converter. 
       FIG. 5  is a schematic diagram showing the engine control mechanism according to a first embodiment of the present invention. 
       FIG. 6  shows a relation between an engine speed and a vehicle speed of the straddle type all-terrain four-wheeled vehicle of  FIG. 1 . 
       FIG. 7  is a flowchart showing steps taken by the engine control mechanism of  FIG. 5 . 
       FIG. 8  is a schematic diagram showing the engine control mechanism according to a second embodiment of the present invention. 
       FIG. 9  is a flowchart showing steps taken by the engine control mechanism of  FIG. 8 . 
       FIG. 10  is a cross sectional view showing detection means of the vehicle speed and of the backward movement according to a third embodiment of the present invention. 
       FIG. 11  is a flowchart showing steps taken by an engine control mechanism according to prior art. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before a description of preferred embodiments of the present invention proceeds, it is to be noted that like or corresponding parts are designated by like reference numerals throughout the accompanying drawing. 
   With reference to  FIGS. 1 through 10 , a description is made below upon a straddle type all-terrain four-wheeled vehicle according to each of first to third embodiments of the present invention. Hereinbelow, the straddle type all-terrain four-wheeled vehicle, is simply referred to as a “vehicle”. 
   First, with reference to  FIGS. 1 through 7 , the description is made of a vehicle according to the first embodiment of the present invention. 
     FIG. 1  is a plan view showing the vehicle. In the explanation below, a “right and left direction” is defined as such a direction that a driver is taken as a reference with respect to a body of the vehicle and with respect to a direction in which the vehicle moves forward. 
   The vehicle has right and left front wheels (also, hereinbelow referred to as a “front wheel pair”)  1 , right and left rear wheels (also, hereinbelow referred to as a “rear wheel pair”)  2 , an engine  3  between the front wheel pair  1  and the rear wheel pair  2 , and foot-steps  4  on both sides of the body of the vehicle. Further, as shown in  FIG. 1 , there is arranged a handlebar  5  for steering the vehicle in a front upper part of the body, and there is arranged a straddle type seat  6  in a rear upper part of the body. The handlebar  5  has a grip part provided with a brake lever  8  and so on. 
   A crank case  10  of the engine  3  extends rearward, and the crank case  10  has a transmission case  11  integrally at a rear part of the crank case  10 . There is provided a gear transmission (i.e. gear type sub-transmission)  13  inside the transmission case  11 , and there is provided an automatic belt converter (i.e. automatic V-belt transmission)  15  on a right side of the crank case  10 , which is located upstream of a transmitting path of a drive power from the engine for the vehicle with respect to the gear transmission  13 . 
   Under the transmission case  11 , a drive shaft  17  extends back and forth. A front part of the drive shaft  17  is connected to a gear mechanism accommodated inside a reduction gear case  21  for the front wheel pair  1 , via a propeller shaft  18  for the front wheel pair  1 . On the other hand, a rear part of the drive shaft  17  is connected to a gear mechanism accommodated inside a reduction gear case  22  for the rear wheel pair  2 , via a propeller shaft  19  for the rear wheel pair  2 . 
     FIG. 2  is a right side view of the vehicle of  FIG. 1 . As shown in the figure, the belt converter  15  has a drive pulley  26  which is mounted on a drive shaft  25 , a driven pulley  28  which is mounted on a driven shaft  27 . The drive pulley  26  is located forward of the driven pulley  28 , and a V-belt  29  is wound around both of the pulleys  26 ,  28 . The drive pulley  26 , the drive shaft  25 , the driven pulley  28 , the driven shaft  27 , and the V-belt  29  are all covered by a belt converter cover  30 . 
   As shown in  FIG. 2 , there is arranged a shift operation lever  32  in the vicinity of the handlebar  5 . The shift operation lever  32  is interlocked with an outer change lever  35  of the transmission case  11  through a connection rod  33 . 
   (Structure of Belt Converter) 
     FIG. 4  is a cross sectional view, taken along an axis O 0  of a crank shaft  36  of the engine, of the drive pulley  26 . The drive shaft  25  for the belt converter  15  is integrally connected to a right end part of the crank shaft  36 . The drive pulley  26  is composed of a fixed sheave  41  on a left side and of a movable sheave  42  on a right side. The fixed sheave  41  is mounted on the drive shaft  25  such that the former is not rotatable relative to the latter and such that the former is not movable relative to the latter in a direction of the crank shaft axis O 0 . 
   On the other hand, the movable sheave  42  is mounted on the drive shaft  25  such that the former can integrally rotate relative to the latter in a direction of rotation through a spider  44  and such that the former is movable relative to the latter in the direction of the crank shaft axis O 0 . 
   Behind the movable sheave  42 , there is provided a propulsion generating mechanism, for the drive pulley  26 , which is composed of the spider  44 , a plurality of flyweights  45 , a pressure adjustment spring  46 , a support disk  47 , and so on. Each of the flyweights  45  is pivoted rotatably about one of pins  48  which are mounted on a back side of the movable sheave  42 , and each of free ends of the flyweights  45  opens towards the spider  45  (i.e. toward a right side in  FIG. 4 ) due to a centrifugal force, as the number of revolutions of the crank shaft  36  increases. A connection arm  49  is formed on the back side of the movable sheave  42 . The connection arm  49  passes through the spider  44  and extends towards the support disc  47  (i.e. rightwards in  FIG. 4 ) such that the support disc  47  is connected to a right edge part of the connection arm  49 . The support disc  47  is movable along the drive shaft  25  in the direction of the crank shaft axis O 0 , and the support disc  47  is rotatable relative to the drive shaft  25 . 
   The spider  44  is screwed to the drive shaft  25 , and the spider  44  has pressure receiving rollers  52  with which the flyweights  45  contact respectively. The pressure adjustment spring  46  is mounted between the spider  44  and the support disk  47  in a state in which the pressure adjustment spring  46  is compressed between the spider  44  and the support disk  47 . 
   In the construction, the support disk  47  is biased rightwards, and the movable sheave  42  is also biased rightwards through the connection arm  49 . That is, the pressure adjustment spring  46  exerts a force in a direction in which the movable sheave  42  and the fixed sheave  41  are kept away from each other, namely, in a direction in which the movable sheave  42  and the fixed sheave  41  open relative to each other. When the flyweights  45  open as an engine speed of the engine  3  increases, the movable sheave  42  is moved leftwards together with the support disk  47  against the biasing force exerted by the pressure adjustment spring  46 , due to a reaction force relative to the pressure receiving roller  52 . As a result, the V-belt  29  is sandwiched (pinched or pressed) between the fixed sheave  41  and the movable sheave  42 . 
   As shown in  FIG. 3 , the driven shaft  27  of the belt converter  15  is integrated with an input shaft  62  of the gear transmission  13 . The driven pulley  28  has a fixed sheave  54  on a right side and a movable sheave  55  on a left side. The fixed sheave  54  is fixed to a cylindrical cam shaft  56  which is fixed to the driven shaft  27  such that the fixed sheave  54  is not movable relative to the driven shaft  27  in an axial direction of the driven shaft  27  and such that the fixed sheave  54  is not rotatable relative to the driven shaft  27 . The cam shaft  56  has a plurality of cam guide grooves  57  which are spiral-shaped. 
   On the other hand, the movable sheave  55  has a sleeve  58  which is fixed to an inner peripheral edge of the movable sheave  55 . The sleeve  58  engages with the cam shaft  56  such that the former  58  is movable relative to the latter  56  in an axial direction of the latter  56  and such that the former  58  is rotatable relative to the latter  56 . The movable sheave  55  is biased toward the fixed sheave  54  by a pressure adjustment spring  59 . The sleeve  58  supports a cam roller  60 . The cam roller  60  slidably engages with the cam guide grooves  57 . 
   That is, when a tension of the V-belt  29  on a pulled side thereof increases as the load of wheels of the vehicle increases upon the vehicle&#39;s running, the movable sheave  55  rotates relatively with respect to the fixed sheave  54  in its rotational direction. At this time, both of the sleeve  58  and the movable sheave  55  are spirally moved rightward (i.e. toward the fixed sheave  54 ) relative to the cam shaft  56  on the basis of the cam action exerted between the cam guide grooves  57  and the cam roller  60 , so that the radius of contact of the driven pulley  28  increases. 
   (Gear Transmission and Shift Mechanism) 
   The gear transmission  13 , housed inside the transmission case  11 , is constructed so as to be able to be switched over amongst a forward high speed position, a forward low speed position, a neutral position, and a backward position. The gear transmission  13  has the input shaft  62  which is integrated with the driven shaft  27 , an output shaft  63 , a counter shaft  64 , an idle shaft  65  for the backward movement of the vehicle, and all of which are parallel to each other and extend in the right and left direction. The gear transmission  13  also has a shift rod  72 , and the shift rod  72  is arranged in parallel with the aforementioned shafts  62 – 65  as well. 
   On a right edge part of the input shaft  62  inside the transmission case  11 , there are arranged a high speed gear  67  for the forward movement of the vehicle and a low speed gear  68  for the forward movement. The high speed gear  67  and the low speed gear  68  are juxtaposed with each other, as shown in  FIG. 3 . On the other hand, on a left edge part of the input shaft  62 , there is arranged a gear  69  for the backward movement of the vehicle, and on a middle part of the input shaft  62 , there is arranged a shift sleeve  70  which engages with the input shaft  62  through a spline such that the shift sleeve  70  is movable relative to the input shaft  62  in an axial direction of the input shaft  62 . 
   The gear  69  for the backward movement has a dog claw  69   a  on a right edge surface, and the gear  69  rotatably engages with the input shaft  62  through a needle bearing. 
   The low speed gear  68  has a boss extending leftward and a dog claw  68   a  on a left edge periphery of the boss, and the low speed gear  68  rotatably engages with the input shaft  62  through a needle bearing. 
   The high speed gear  67  has an arm extending leftward and a dog claw  67   a , extending inwardly radially, on a left edge part of the arm, and the high speed gear  67  rotatably engages with an outer periphery of the boss of the low speed gear  68  for the forward movement, through a needle bearing. 
   The shift sleeve  70  has a left edge part which is formed as a dog claw  70   b  for the backward movement, and has a right edge part which is formed as a dog claw  70   a  for the forward movement. 
   The axial distance between the dog claw  67   a  and the dog claw  68   a , is secured to such an extent that the dog claw  70   a  of the sleeve  70  can be once in a neutral state. 
   The shift sleeve  70  has an outer peripheral groove, and a shift fork  71  engages with the outer peripheral groove. The shift fork  71  is fixed to the aforementioned shift rod  72  which is supported by the transmission case  11  so as to be able to move in the right and left direction. 
     FIG. 3  shows that the shift rod  72  is in a neutral position. When the shift rod  72  is moved leftwards from the neutral position, the dog claw  70   b  of the shift sleeve  70  meshes with the dog claw  69   a  of the gear  69  for the backward movement, thus being set at the backward position. On the other hand, when the shift rod  72  is moved rightwards from the neutral position, the dog claw  70   a  of the shift sleeve  70  firstly meshes with the dog claw  67   a  of the high speed gear  67  for the forward movement, thus being set at the forward high speed position. Subsequently, when the shift rod  72  is further moved rightward, the dog claw  70   a  of the shift sleeve  70  meshes with the dog claw  68   a  of the gear  68  and is set at the forward low speed position after passing the neutral position. 
   The shift rod  72  has a right edge part which is provided with a change pin  85  projecting upward, and the change pin  85  engages with an inner change lever  86 . The inner change lever  86  is connected to an outer change lever  35 , through a change lever shaft  87 . 
   The counter shaft  64  has a right edge part to which a pair of intermediate gears  73 ,  74  for the forward movement of the vehicle are fixed. The intermediate gears  73 ,  74  mesh with the forward high speed gear  67  and with the forward low speed gear  68 , respectively. Meanwhile, the counter shaft  64  has a left edge part to which an intermediate output gear  75  is fixed. 
   The idle shaft  65  has a left edge part which has a first idle gear  77 , for the backward movement, meshing with the gear  69  for the backward movement, and which has a second idle gear  78 , for the backward movement, meshing with the intermediate output gear  75 . 
   The intermediate output gear  75  meshes with an output gear  80  which is fixed to a left edge part of the output shaft  63 , and a bevel gear  81  which is integrated with a right edge part of the output shaft  63  meshes with a bevel gear  82  which is fixed to the drive shaft  17 . 
   (Engine Control Mechanism) 
   As shown in  FIG. 1 , there is arranged a controller  105  for controlling the engine  3 , behind and near the transmission case  11 , and the transmission case  11  accommodates a vehicle speed detection mechanism and a backward movement detection mechanism. 
   That is, as shown in  FIG. 3  showing an inside of the transmission case  11 , there is provided a vehicle speed sensor  110 , as the vehicle speed detection mechanism, which faces (opposes) a peripheral surface of the bevel gear  82  fixed to the drive shaft  17 . The vehicle speed sensor  110  is screwed into a right wall of the transmission case  11 . The vehicle speed sensor  110  outputs an electric pulse by detecting a change in magnetic flux which passes through a detecting element provided at a tip part of the vehicle speed sensor  110 . Namely, the vehicle speed sensor  110  detects teeth surfaces of the bevel gear  82 , and the vehicle speed sensor  110  outputs a vehicle speed signal in a form of the number of pulses per unit time (or in a form of frequency of pulses) to an electric cable  112 . 
   On the other hand, there are provided a backward movement positional detecting switch  90  (refer to  FIG. 5 ) which is located so as to oppose a left edge surface of the shift rod  72 , a rotor  115  for detection fixed to the output gear  80 , and a backward movement detecting sensor  111 , as the backward movement detection mechanism. The backward movement positional detecting switch  90  is screwed into a left wall of the transmission case  11 , and the backward movement detecting sensor  111  is screwed into the right wall of the transmission case  11 . One of them can be employed for the backward movement detection mechanism. The first embodiment of the present invention is explained as having a construction in which the backward movement positional detecting switch  90  is employed as the backward movement detection mechanism. Incidentally, another embodiment is explained as having the backward movement detecting sensor  111 . 
     FIG. 5  is a diagram showing the engine control mechanism of the vehicle of  FIG. 1 . As shown in the figure, the controller  105 , mounted on the body of the vehicle, has an output part which is electrically connected to an engine speed control circuit  92  of an ignitor  91  for the engine  3 . Also, the controller  105  has an input part which is electrically connected to the vehicle speed sensor  110  via an electric cable  112  and which is electrically connected to the backward movement positional detecting switch  90  via an electric cable  113 . The vehicle speed sensor  110  outputs the vehicle speed signal via the electric cable  112 , as aforementioned. 
   The backward movement positional detecting switch  90  detects whether a free end of the shift rod  72  is close to the switch  90  or not. That is, when the free end of the shift rod  72  moves to a backward movement position (reverse position) “R” (i.e. left side position, shown by an imaginary line, in  FIG. 5 ) from a neutral position “N”, namely when the free end of the shift rod  72  moves into a detection region in which the backward movement positional detecting switch  90  can detect the free end of the shift rod  72 , the backward movement positional detecting switch  90  functions. Then, the backward movement positional detecting switch  90  outputs the backward movement detection signal to the controller  105  via the electric cable  113 . 
   The controller  105  operates in accordance with a program, on the basis of which the controller  105  outputs an ignition cut instruction signal to the engine speed control circuit  92  of the ignitor  91  in order to cancel ignition of the engine  3  when the backward movement positional detecting switch  90  outputs the backward movement detection signal and when the vehicle speed signal outputted from the vehicle speed sensor  110  reaches, or exceeds, a predetermined value. 
   (Setting of Predetermined Value of Vehicle Speed) 
   A predetermined value of vehicle speed, used as a reference to cut the ignition of the engine  3 , is set to be a vehicle speed Vc at which an automatic shift of the belt converter starts during acceleration of the vehicle with a throttle of the engine being open wide, or is set to be near the speed Vc. 
   Explaining it more in detail with reference to  FIG. 6 , a curve X 1  denotes the speed change in acceleration of the vehicle at the time of opening the throttle wide, a curve X 2  denotes the speed change in deceleration of the vehicle, a straight line XL denotes a theoretical change in speed when the belt converter  15  is set to be in a low state (i.e. in a maximum reduction ratio in speed), and a straight line XH denotes a theoretical change in speed when the belt converter  15  is set to be in a high state (i.e. in a minimum reduction ratio in speed). The vehicle speed Vc is set to be a vehicle speed of a point P 4  or its nearby speed in the curve X 1  of  FIG. 6  at the time of acceleration, at which the driven pulley  28  (refer to  FIG. 3 ) starts opening and at which the automatic shift of the belt converter begins, from the low state in which both of the engine speed and the vehicle speed increase along the straight line XL. 
   Incidentally, a region between a point P 1  and a point P 2  is the region in which the engine  3  idles. In the region, the vehicle speed is zero even though the number of revolutions of the engine  3  increases. On the other hand, a region between the point P 2  and a point P 3  is the region in which a clutch of the vehicle is half-disengaged. In the region, part of the drive power of the engine  3  is transmitted to the driven pulley  28 , and the vehicle speed increases with an inclination gentler than an inclination of the straight line XL. On the other hand, a region between the point P 3  and a point P 4  is a region in which the vehicle accelerates in the low state. 
   (Power Transmission System) 
   In  FIG. 1 , a rotation power of the engine  3  is transmitted to the front wheel pair  1  and the rear wheel pair  2 , via the belt converter  15 , the gear transmission  13 , the drive shaft  17 , and the propeller shafts  18 ,  19 , respectively. 
   (Upon Forward Movement at High Speed) 
   In  FIG. 3 , when the shift sleeve  70  is moved rightward from the neutral position, the dog claw  70   a  for forward movement meshes with the dog claw  67   a  of the high speed gear  67  for forward movement, resulting in the setting of the forward movement high speed position. 
   The rotational power of the input shaft  62  is transmitted to the output shaft  63 , via the high speed gear  67  for the forward movement, the intermediate gear  73 , the counter shaft  64 , the intermediate output gear  75  and the output gear  80 . As a result, the output shaft  63  is rotated in a direction of the forward movement of the vehicle, and the drive shaft  17  is rotated in the direction of the forward movement of the vehicle via the bevel gears  81 ,  82 . 
   (Upon Forward Movement at Low Speed) 
   When the shift sleeve  70  is moved further rightward from the forward movement high speed position, the dog claw  70   a  for forward movement meshes with the dog claw  68   a  of the low speed gear  68  for forward movement, resulting in the setting of the forward movement low speed position. 
   The rotational power of the input shaft  62  is transmitted to the output shaft  63 , via the low speed gear  68 , the intermediate gear  74 , the counter shaft  64 , the intermediate output gear  75  and the output gear  80 . As a result, the output shaft  63  is rotated in the direction of the forward movement of the vehicle, and the drive shaft  17  is rotated in the direction of the forward movement of the vehicle via the bevel gears  81 ,  82 . 
   (Upon Backward Movement) 
   In  FIG. 3 , when the shift sleeve  70  is moved leftward from the neutral position, the dog claw  70   b  for the backward movement meshes with the dog claw  69   a  of the gear  69  for the backward movement, resulting in the setting of the backward movement position. 
   The rotational power of the input shaft  62  is transmitted to the output shaft  63 , via the gear  69  for backward movement, the first idle gear  77 , the idle shaft  65 , the second idle gear  78 , the intermediate output gear  75  and the output gear  80 . As a result, the output shaft  63  is rotated in a direction of the backward movement of the vehicle, and the drive shaft  17  is rotated in the direction of the backward movement of the vehicle via the bevel gears  81 ,  82 . 
   (Power Transmission by Belt Converter) 
   When the engine  3  stops, each of the flyweights  45  is closed as shown by a solid line in  FIG. 4 . In the state, the movable sheave  42  is located at a position furthest from the fixed sheave  41  (i.e. located in the right extremity in  FIG. 4 ), and the drive power is cut between the V-belt  29  and the drive pulley  26  (i.e. a belt-clutch-off state). 
   When the engine  3  is started, the flyweights  45  start opening due to the centrifugal force, and the movable sheave  42  starts moving towards the fixed sheave  41  (i.e. leftward in  FIG. 4 ). When the engine speed starts exceeding the engine idling rotational region (i.e. exceeding the region between the points P 1  and P 2  in  FIG. 6 ), the V-belt  29  starts to be pinched between the movable sheave  42  and the fixed sheave  41 . As a result, part of the power from the engine  3  starts to be transmitted from the V-belt  29  to the drive pulley  26  under the state of the half-clutch (refer to the region from the point P 2  to the point P 3  in  FIG. 6 ). 
   As the engine speed further increases from the state of the half-clutch, the state of the half-clutch is changed into a belt-clutch-on state in which the power is transmitted from the drive pulley  26  shown in  FIG. 4  to the driven pulley  28  shown in  FIG. 3  under the low state having the maximum reduction ratio in speed (refer to the region from the point P 3  to the point P 4  in  FIG. 6 ). 
   When the engine speed further increases and exceeds the engine speed corresponding to the vehicle speed Vc, the automatic shift of the belt converter starts. That is, the width between the movable sheave  42  and the fixed sheave  41  becomes narrower (i.e. the movable sheave  42  moves more towards the fixed sheave  41 ) such that the radius of contact between the V-belt  29  and the drive pulley  26  increases, and width between the movable sheave  55  and the fixed sheave  54  of the driven pulley  28 , shown in  FIG. 3 , increases such that the radius of contact between the V-belt  29  and the driven pulley  28  decreases. In other words, the reduction ratio becomes smaller. 
   (Engine Control) 
   In  FIG. 5 , when the shift rod  72  is in the neutral position “N”, a free end (i.e. a left edge surface) of the shift rod  72  is kept away from the detection region by the backward movement positional detecting switch  90 . Namely, in the state, the backward movement positional detecting switch  90  does not detect any backward movement position. 
   When the shift rod  72  is moved rightward to a forward movement high speed position F 2  or to a forward movement low speed position F 1 , the shift rod  72  is moved in a direction in which the shift rod  72  is moved away from the backward movement positional detecting switch  90 . Therefore, the state in which the backward movement positional detecting switch  90  does not detect the backward movement position, is maintained. 
   Meanwhile, when the shift rod  72  is moved leftward to the backward movement position (reverse position) “R” from the neutral state shown in  FIG. 5 , and when the free end of the shift rod  72  enters the detection region by the backward movement positional detecting switch  90 , the switch  90  is in a state of detection of the shift rod  72 . Namely, in the state, the switch  90  outputs the backward movement detection signal to the controller  105 . 
   The vehicle speed sensor  110  continuously counts the teeth surfaces of the bevel gear  82 , and the sensor  110  outputs the signal of vehicle speed on the basis of the number of pulses, to the controller  105 . 
   When the controller  105  receives the backward movement detection signal outputted from the backward movement positional detecting switch  90 , and when the controller  105  determines that the vehicle speed signal outputted from the vehicle speed sensor  110  exceeds the predetermined value, the controller  105  outputs the ignition cut instruction signal to the engine speed control circuit  92  of the ignitor  91 , thus cutting the ignition of the engine  3 . 
   Based upon the cut of the ignition of the engine  3 , the engine speed decreases, and at the same time the vehicle speed decreases. And when the vehicle speed is below the speed Vc at which the automatic shift of the belt converter starts, the ignition cut is cancelled. 
     FIG. 7  is a flowchart showing steps taken by the engine control mechanism of the straddle type all-terrain four-wheeled vehicle of  FIG. 1 . Referring to the flowchart, it is determined at step S 1  whether the shift position is the backward movement position (i.e. backward movement gear position) or not. If it is determined at the same step that the shift position is the backward movement position, it proceeds to step S 2  where it is determined whether the vehicle speed has reached the predetermined value of “Vc” or not. If it is determined at the same step that the vehicle speed has reached the predetermined value of “Vc”, the ignition of the engine  3  is cut such that the engine speed is decreased. 
   According to the first embodiment of the present invention, when a high load is exerted upon the vehicle during the backward movement of the vehicle and the state of the belt converter  15  is changed into the low state such that the vehicle speed is decreased, it is possible to make the vehicle go backward (or rearward) by making use of a relatively greater power output region of the engine, only with the operation of the throttle of the engine to increase the engine speed. 
   Also, according to the first embodiment, when the aforementioned high load is changed into a low load, and when the vehicle speed increases, there is no need of hasty or sudden operation to close the throttle and/or hasty or sudden operation of the ignition cut release switch, etc., different from the aforementioned conventional vehicle, because the vehicle speed is controlled not to exceed the predetermined value. 
   Next, with reference to  FIGS. 8 and 9 , a vehicle is described according to the second embodiment of the present invention. 
   Namely, in the second embodiment, instead of employing the backward movement positional detecting switch  90  as shown in  FIGS. 3 and 5 , the rotor  115  for detection and the backward movement detecting sensor  111  which opposes a peripheral surface of the rotor  115  for detection are utilized as the backward movement detection mechanism. The rotor  115  is fixed to an edge surface of the output gear  80  which is fixed to the output shaft  63 . Similar to the aforementioned vehicle speed sensor  110 , the backward movement detecting sensor  111  outputs an electric pulse by detecting a change in magnetic flux which passes through a detection element provided at a tip part of the sensor  111 . 
   By the way, the output shaft  63  with the output gear  80  rotate in reverse directions, upon forward and backward movements of the vehicle. 
     FIG. 8  shows the engine control mechanism, according to the second embodiment, in which the rotor  115  for detection and the backward movement detection sensor  111  are employed. As shown in the figure, the rotor  115  has three detecting projections  120 ,  121 ,  122  which project radially outwardly from a main body  115   a  of the rotor  115 , in a state in which the three detecting projections  120 ,  121 ,  122  are arranged circumferentially along the main body at unequal intervals. More specifically, one  121  of the three detecting projections is constructed to be the longest in the circumferential direction, and the one  121  occupies approximately ¼ of the total circumference of the body of the rotor  115 . On the other hand, each of the other two  120 ,  122  of the detecting projections are constructed to be shorter in the circumferential direction than the above one  121 , as shown in  FIG. 8 . The longest projection  121  serves as a reference projection (i.e. standard projection), one projection  120  of the shorter projections is located on a forward side (i.e., in a direction of forward rotation) of the longest projection  121  with a long circumferential distance D 1  existing between adjacent edges of the longest projection  121  and the one projection  120  (i.e. approximately 90 degrees between the adjacent edges of the longest projection  121  and the one projection  120  relative to an axis of the output shaft  63 ), and the other projection  122  of the shorter projections is located on a backward side (i.e., in a direction of backward rotation) of the longest projection  121  with a short circumferential distance D 2 , shorter than D 1 , existing between adjacent edges of the longest projection  121  and the other projection  122  (i.e. approximately 45 degrees between the adjacent edges of the longest projection  121  and the other projection  122  relative to an axis of the output shaft  63 ). 
   According to the arrangement, the three detecting projections  120 ,  121 ,  122  are positioned circumferentially at unequal intervals as aforementioned. Therefore, it is possible to generate different pulse waveforms between a state in which the output shaft  63  rotates forwards and a state in which the output shaft  63  rotates backward, and thus possible to determine that the rotation is a backward rotation when the pulse waveform generated upon backward movement of the vehicle is inputted to the controller  105 . 
     FIG. 9  is a flowchart showing steps taken by the engine control mechanism of  FIG. 8 . That is, it is determined at step S 10  whether the vehicle is moving backward or not, by the backward movement detecting sensor  111 . If it is determined at the same step that the vehicle is moving backward, it proceeds to step S 20  where it is determined whether the vehicle speed reaches a predetermined speed or not. If it is determined at the same step that the vehicle speed reaches the predetermined speed, the ignition of the engine  3  is cut. 
   As a modification to each of the first and second embodiments in which the ignition of the engine  3  is cut or cancelled in order to slow down the engine speed, it is possible to employ a system in which the ignition of the engine is thinned out. 
   As another modification to each of the first and second embodiments in which the aforementioned ignition limitation mechanism is employed, it is possible to employ a mechanism in which fuel from a fuel injector of the engine is cut or limited. 
   As another modification to each of the first and second embodiments, instead of employing the construction, as shown in  FIG. 3 , in which the rotor  115 , detected by the backward movement detecting sensor  111 , is fixed to the output gear  80  of the output shaft  63 , it is possible to employ a construction in which the rotor  115  for detection is mounted on another rotation member which rotates in opposite rotational directions between a state in which the vehicle moves forward and a state in which the vehicle moves backward. For example, in the gear transmission  13  shown in  FIG. 3 , it is possible that the rotor  115  for detection is mounted on the bevel gear  81  of the output shaft  63 , the intermediate output gear  75  of the counter shaft  64 , or the bevel gear  82  of the drive shaft  17 . Alternatively, it is possible that the rotor  115  for detection is mounted on the front wheel  1  and/or the rear wheel  2 . 
   Next, with reference to  FIG. 10 , a vehicle is described according to the third embodiment of the present invention. 
   That is,  FIG. 10  is a cross sectional view showing detection means of the vehicle speed and of the backward movement according to the third embodiment of the present invention, and the figure shows a single sensor  149  which functions as a vehicle speed sensor and as a backward movement detection sensor. As shown in the figure, there are provided a pair of detection elements  151 ,  152  inside a single sensor case, in which the first detection element  151  and the second detection element  152  are arranged with a predetermined distance between the detection elements  151 ,  152  in a direction in which the bevel gear  82  rotates. 
   According to the third embodiment, at least one of the detection elements  151 ,  152  detects the vehicle speed, and it is determined whether the vehicle moves forward or backward, on the basis of a sequence of which detection element  151 ,  152  firstly detects the tooth of the bevel gear  81 . For example, when the first detection element  151  detects the gear tooth first, it is determined that the vehicle is running forward. On the contrary, when the second detection element  152  detects the gear tooth first, it is determined that the vehicle is running backward. 
   Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various other changes and modifications are also apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.