Abstract:
A four-wheel drive vehicle provides a torque distribution clutch mechanism which directly transmits the driving force generated from the engine to one of two front wheels or two rear wheels, and which transmits it to the other wheels therethrough in order to control the engaging force in accordance with a traveling status of the four-wheel drive vehicle. The vehicle further comprises a tight corner judging structure, a normal mode setting structure, a low friction road judging structure and a tight mode setting structure. The tight corner judging structure judges whether the vehicle travels at a large turning angle or not, and the normal mode setting structure sets a normal mode to control the engaging force when it is judged by the tight corner judging structure that the vehicle does not travel at the large turning angle. Further, the low friction road judging structure judges whether the vehicle travels on a low friction road or not, and the tight mode setting structure for setting a tight mode to control the engaging force in a condition that it is judged by the tight corner judging structure that the vehicle travels at the large turning angle when it is judged by the low friction road judging structure that the vehicle dose not travel on the low friction road.

Description:
INCORPORATION BY REFERENCE 
   The present application claims priority under 35 U.S.C. Section 119 to Japanese Patent Application No. 2002-121056 filed on Apr. 23, 2002. The content of this application is incorporated herein by reference in their entirety. 
   FIELD OF THE INVENTION 
   The present invention relates to a four-wheel drive vehicle. More particularly, the present invention relates to such a four-wheel drive vehicle in which a driving force generated in an engine is directly transmitted to one of front wheels or rear wheels, and is transmitted to the other wheels through a torque distribution clutch mechanism, an engaging force of which is controlled in accordance with a traveling status of the vehicle to distribute the driving force from the engine to the front and rear wheels. 
   BACKGROUND OF THE INVENTION 
   Conventionally, it is known such a four-wheel drive (referred to as “4WD” hereinafter) vehicle wherein an engaging force of a torque distribution clutch mechanism is adjustably controlled based upon a rotational difference between front wheels and rear wheels.  FIG. 7  presents an example of a control map used in such a conventional 4WD vehicle, in which “T” of an axis of ordinates represents an engaging force of the torque distribution clutch mechanism, and “ΔN” of an axis of abscissas represents the rotational difference between the front wheels and the rear wheels. 
   Herein, in a case that the vehicle acceleratingly starts on a low friction road such as a snow road or crust road, the engaging force T of the torque distribution clutch mechanism is increased by using the control map shown by a one-dotted line A of FIG.  7 . Therefore, steady acceleration and start can be performed in the 4WD vehicle. 
   However, in a case that the 4WD vehicle travels at a low speed at a large turning angle under condition that the engaging force of the torque distribution clutch mechanism is increased, the rotational difference between the front wheels and the rear wheel cannot be absorbed. As a result, this unabsorption of the rotational difference causes to generate a tight corner braking phenomenon (the vehicle difficulty turns so as to have activated the brakes). By this phenomenon, it is adopted such a map B that the gradient of the engaging force is easy in addition to the steep gradient of the map A. In such a situation, when a large steering angle is detected by a steering angle sensor and the like, it is judged to be on a tight corner state, so that the generation of the tight corner braking phenomenon can be prevented by using of the map B. 
   However, even if the 4WD vehicle travels on the snow road or the crust road that the 4WD vehicle easily slips, the engaging force of the torque distribution mechanism is diminished to change the control map to the map B when it is judged to be on the tight corner state. Therefore, the stability and traveling ability of the 4WD vehicle is decreased. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a four-wheel drive vehicle that is capable of improving the stability and traveling ability thereof even if it travels on a low friction road with prevention of the tight corner braking phenomenon. 
   To perform the above-mentioned object, a four-wheel drive vehicle according to the present invention provides a torque distribution clutch mechanism which directly transmits the driving force generated from the engine to one of tow front wheels or two rear wheels, and which transmits it to the other wheels therethrough in order to control the engaging force in accordance with a traveling status of the four-wheel drive vehicle. The vehicle further comprises a tight corner judging means, a normal mode setting means, a low friction road judging means and a tight mode setting means. 
   In thus-configured vehicle, the tight corner judging means judges whether the vehicle travels at a large turning angle or not, and the normal mode setting means sets a normal mode to control the engaging force when it is judged by the tight corner judging means that the vehicle does not travel at the large turning angle. Further, the low friction road judging means judges whether the vehicle travels on a low friction road or not, and the tight mode setting means for setting a tight mode to control the engaging force in a condition that it is judged by the tight corner judging means that the vehicle travels at the large turning angle when it is judged by the low friction road judging means that the vehicle dose not travel on the low friction road. 
   With this configuration, the engaging force of the torque distribution clutch mechanism is set to the normal mode when it is judged that the vehicle travels on the low friction road. In other words, the engaging force of the torque distribution clutch mechanism is set to the normal mode even when the vehicle travels at the large turning angle. As a result, the vehicle is controlled with the normal mode, so that the driving force from the engine is adjustably distributed to the four wheels even through the torque distribution clutch mechanism if the vehicle travels at the large turning angle on the low friction road such snow road, crust road, sandy road and the like. Accordingly, the improved stability and traveling ability can be realized. 
   The four-wheel drive vehicle according to the present invention further comprises a wheel speed sensor provided at each of the four wheels for detecting wheel speed of the each wheel. With the provision of the wheel speed sensors, a rotational radius of the vehicle is calculated from the wheel speeds detected by the speed sensors is larger than a predetermined radius, and also a rotational difference between the front wheels and the rear wheels is calculated from the wheel speeds detected by the wheel speed sensors. Accordingly, whether the vehicle travels or not on the low friction road can be judged precisely. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and many of the attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is an explanatory view showing a general construction of a four-wheel drive vehicle according to a first embodiment of the present invention; 
       FIG. 2  is a flowchart showing a processing flow executed in a CPU  52  to control an engaging force T according to the first embodiment of the present invention; 
     FIG.  3 (A) is an explanatory view of a normal pre-torque map and a normal mode map used to organize an engaging force control map referred to in the CPU  52  according to the first embodiment of the present invention, and FIG.  3 (B) is an explanatory view of a tight pre-torque map and a tight mode map used to organize an engaging force control map referred to in the CPU  52  according to the first embodiment of the present invention; 
       FIG. 4  an explanatory view a contexture of a low friction road judging map referred in the CPU  52  according to the first embodiment of the present invention; 
       FIG. 5  is a flowchart showing a processing flow executed in the CPU  52  to control an engaging force T according to a second embodiment of the present invention; 
       FIG. 6  is a flowchart showing a processing flow executed in the CPU  52  to control an engaging force T according to a third embodiment of the present invention; and 
       FIG. 7  is an explanatory view showing an example of a control map used in a conventional four-wheel drive vehicle. 
   

   LIST OF DESIGNATORS 
   
     
       
             
             
             
           
         
             
                 
                 
             
           
           
             
                 
               10 
               four-wheel drive vehicle 
             
             
                 
               12 
               engine (motor) 
             
             
                 
               14 
               transmission 
             
             
                 
               15 
               front differential 
             
             
                 
               18 
               propeller shaft 
             
             
                 
               19 
               clutch mechanism 
             
             
                 
               20 
               coupling (torque distribution clutch mechanism) 
             
             
                 
               25 
               rear differential 
             
             
                 
               50 
               electric control circuit 
             
             
                 
               52 
               CPU (Central Processing Unit) 
             
             
                 
               64a 
               acceleration pedal sensor 
             
             
                 
                 
             
           
        
       
     
   
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments according to the present invention will be explained hereinafter with reference to the drawings. 
   [First Embodiment] 
     FIG. 1  is an explanatory view showing a general construction of a four-wheel drive vehicle according to a first embodiment of the present invention. In the four-wheel drive vehicle  10 , a driving torque from-an engine  12  applies to front wheels FT 1  and FT 2 , and is transmitted to rear wheels RT 3  and RT 4  in a condition that the driving torque is adjusted in accordance with a traveling status of the vehicle  10 . In a transmission  14  assembled aside of the engine  12 , there is installed a front differential  15  in which a driving force from the engine  12  is outputted to an axle shaft  16  to drive the front wheels FT 1  and FT 2 , and is transmitted to a propeller shaft  18 . The propeller shaft  18  is connected to a rear differential  25  through a coupling  20 . The coupling  20  provides a clutch mechanism  19  and is configured so as to be able to adjust the transmission of torque. The clutch mechanism  19  is controlled by a signal from an electric control circuit  50 , so that the transmission torque can be adjusted. Besides, a torque distribution clutch mechanism consist of the clutch mechanism  19  and the coupling  20 . 
   The driving force from the coupling  20  drives the rear wheels RT 3  and RT 4  through the rear differential  25  and an axle shaft  26 . At the front and rear wheels FT 1 , FT 2 , TR 3  and RT 4 , there are arranged wheel speed sensors S 1 , S 2 , S 3  and S 4  for detecting a wheel speed of the wheels, respectively. 
   The electric control circuit  50 , as mentioned above, controls the coupling  20 . The electric control circuit  50  is composed of a CPU  52  for executing a variety of calculation and control, a ROM  54  for storing control programs, a RAM  56  for serving as work areas of the CPU  52 , and an input/output circuit  58 . The electric control circuit  50  controls the torque transmitting force of the coupling  20  by detecting a slipping state between the front and rear wheels based upon output signals from the wheel speed sensors S 1 , S 2 , S 3  and S 4 . 
   Into the input/output circuit  58  of the electric control circuit  50 , there is inputted an accelerator operation signal from an acceleration pedal sensor  64   a  attached on an acceleration pedal  64 . 
   Next, an engaging force control map will be explained with reference to FIG.  3 (A) and FIG.  3 (B) showing its contexture that is referred to when the CPU  52  executes the computer program for controlling the coupling  20 . The engaging force control map is organized from a normal pre-torque map and a normal mode map represented in FIG.  3 (A) and from a tight pre-torque map and a tight mode map represented in FIG.  3 (B). Each of the normal and tight pre-torque maps has a torque T (referred to as torque T 1  in the pre-torque maps and T 2  in the mode maps), a throttle opening degree and a vehicle speed as parameters. Namely, the torque T 1  is led from the throttle opening degree (%) and the vehicle speed. Herein, the throttle opening degree represents the degree of depress amount of the acceleration pedal  64  detected by the acceleration pedal sensor  64   a , changes from 0% to 100%, and is obtained by the acceleration operation signal from the acceleration pedal sensor  64   a . Further, the vehicle speed is calculated from an average of wheel speeds of the rear wheels RT 3  and RT 4  detected by the wheel speed sensors S 3  and S 4 . In each of the normal and tight mode maps, an axis of ordinates represents an engaging force T 2  (torque: N·m), and an axis of abscissas represents a rotational difference ΔN (rpm) that is calculated from a difference between the average of the wheel speeds of the front wheels FT 1  and FT 2  and the average of the wheel speeds of the rear wheels RT 3  and RT 4 . In such a contexture, a normal state control is performed by a combination of the normal pre-torque map and the normal mode map, and a tight state control is performed by a combination of the tight pre-torque map and the tight mode map. Besides, the normal and tight mode maps may be so called as an engaging force control map. 
   The normal mode map shown in FIG.  3 (A) is of the engaging force control map used when the four-wheel drive vehicle  10  travels in a normal condition, and has a characteristic that the engaging force T 2  increases in accordance with the rotational difference ΔN. Namely, the engaging force T can be increased as the slip increases between the front and rear wheels, so that the driving force generated from the engine is distributed to the rear wheels in accordance with the engaging force T, whereby the slip of the front wheel is prevented and a steady acceleration of the vehicle is performed. 
   The tight mode map shown in FIG.  3 (B) is of the engaging force control map used when the four-wheel drive vehicle  10  is turned on a tight corner, and has a characteristic that the engaging force T is gradually increased relative to the increase of the rotational difference ΔN. Namely, the tight mode map is used when the vehicle  10  travels at a large turning angle, the engaging force T is diminished even if the rotational difference ΔN is large in a turning state of the vehicle  10 . Therefore, a tight corner braking phenomenon can be prevented. 
   Next, a contexture of a low friction road judgment map used in the CPU  52  will be explained hereinafter with reference to FIG.  4 . In this embodiment, whether a low friction road is or not is judged from three parameters of the rotational difference ΔN between the front and rear wheels, the vehicle speed and the throttle opening degree by referring to the low friction road judgment map. It is judged to be on the low friction road in a case that the rotational difference ΔN is relatively large when the vehicle speed is low relative to the throttle opening degree. On the other hand, it is judged to be on a high friction road in a case that the rotational difference ΔN is relatively small when the vehicle speed is high relative to the throttle opening degree 
   Next, process flows executed to control the engaging force T by the CPU  52  will be explained hereinafter with reference to a flow chart shown in FIG.  2 . 
   Into the CPU  52  of the electric control circuit  50 , there are inputted rotational speeds ω 1 , ω 2 , ω 3  and ω 4  of the front wheels FT 1  and FT 2  and of the rear wheels RT 3  and RT 4  from the wheel speed sensors S 1 , S 2 , S 3  and S 4  (step S 12 ). In step S 14 , a turning radius of the vehicle  10  is calculated from the rotational speeds ω 1 , ω 2 , ω 3  and ω 4 . Next, the rotational difference ΔN between the front and rear wheels is calculated from the rotational speeds ω 1 , ω 2 , ω 3  and ω 4  in step S 16 . In step S 18 , it is judged to be a large turning state when the turning radius calculated in step S 14  is smaller than predetermined turning radius (10 m, for example). Namely, it is judged whether the vehicle travels at the large tuning angel in step S 18 . 
   In a case that the judgment is not the large turning state (No in step S 18 ), the engaging force T is, in step S 24 , determined by using the aforementioned tight mode map with reference to FIG.  3 (B). After the determination in step S 24 , a control signal is outputted to control the clutch mechanism  19 . 
   On the other hand, in a case that the judgment is the large turning state (Yes in step S 18 ), the throttle opening degree is calculated in step S 20  after the accelerator operation signal from the accelerator pedal sensor  64   a  attached on the accelerator pedal  64  is inputted into the CPU  52  through the input/output circuit  58 . Herein, whether the low friction road is or not is, in step S 22 , judged from the aforementioned vehicle speed, throttle opening degree and rotational difference ΔN between the front and rear wheels with reference to the above-mentioned low friction road judgment map shown in FIG.  4 . 
   In a case that the judgment is not the low friction road (No in step S 22 ), the engaging force T is, in step S 26 , determined by using the aforementioned tight mode map with reference to FIG.  3 (B). After the determination in step S 26 , the control signal is outputted to control the clutch mechanism  19 . With this control process, the engaging force T of the clutch mechanism  19  is diminished, so that the tight corner braking phenomenon can be prevented. 
   On the other hand, in a case that the judgment is the low friction road (Yes in step S 22 ), the engaging force T is, in step S 24 , determined by using the aforementioned normal mode map with reference to FIG.  3 (A). After the determination in step S 24 , the control signal is outputted to control the clutch mechanism  19 . With this control process, the driving force is distributed to the four wheels (FT 1 , FT 2 , RT 3  and RT 4 ) by increasing the engaging force T of the clutch mechanism  19  corresponding to the rotational difference ΔN between the front wheels and the rear wheels. Accordingly, remarkable stability and traveling ability can be realized when the vehicle  10  travels at a low speed of the vehicle on the low friction road such snow road or sandy road. 
   [Second Embodiment] 
   Subsequently, a four-wheel drive vehicle according to a second embodiment of the present invention will be explained hereinafter. In the above-mentioned first embodiment, whether the low friction road is or not is judged in the CPU  52 . In contrast with the first embodiment, a traveling mode selection switch  30  is provided with the vehicle to select a gear (speed) change pattern of an automatic transmission ( 14 ) in the second embodiment. Further, a snow road traveling mode is provided in the traveling mode selection switch  30 . With this configuration, the CPU  52  for the driving force distribution operation judges whether the low friction road is or not with the selection of the snow road traveling mode in the traveling mode selection switch  30 . 
   A process flow executed to control the engaging force T by the CPU  52  in the second embodiment will be explained hereinafter with reference to a flow chart shown in FIG.  5 . In  FIG. 5 , the processing executed in steps S 12 -S 16  are same as that of the aforementioned first embodiment shown in  FIG. 2 , so that the explanations thereabout is omitted herein 
   In step S 18 , whether the large turning state (tight corner) is or not is judged by the CPU  52 . In a case that the large turning state is not judged (No in step S 18 ), the engaging force T is, in step S 24 , determined by using the aforementioned normal mode map shown in FIG.  3 (A). As a result of this determination, a control signal is outputted to control the clutch mechanism  19  in step S 28 . 
   On the other hand, in a case that the large turning state is judged (Yes in step S 18 ), it is in step S 23  judged whether the snow road traveling mode is selected or not. In a case that the snow road traveling mode is not selected (No in step S 23 ), the engaging force T is, in step S 26 , determined by using the aforementioned tight mode map shown in FIG.  3 (B), whereby a control signal is outputted to control the clutch mechanism  19  in step S 28 . As a result of this control, the engaging force T of the clutch mechanism  19  is reduced, so that the tight corner braking phenomenon can be prevented. 
   In a case of the selection of the snow road traveling mode in the traveling mode selection switch (Yes in step S 23 ), the engaging force T is, in step S 24 , determined by using the aforementioned normal mode map shown in FIG.  3 (A), whereby a control signal is outputted to control the clutch mechanism  19  in step S 28 . Namely, the driving force is distributed to the four wheels (FT 1 , FT 2 , RT 3  and RT 4 ) by increasing the engaging force T of the clutch mechanism  19  corresponding to the rotational difference ΔN between the front wheels and the rear wheels. Accordingly, remarkable stability and traveling ability can be realized when the vehicle  10  travels on the low friction road such snow road or sandy road in the large turning state. 
   [Third Embodiment] 
   Subsequently, a four-wheel drive vehicle according to a third embodiment of the present invention will be explained hereinafter. In the above-mentioned first embodiment, whether the low friction road is or not is judged in the CPU  52 . In contrast with the first embodiment, the electric control circuit  50  for the driving force distribution operation obtains such information corresponding to whether the vehicle travels or not on the low friction road from a control device for an ABS (Anti Break Skid) installed in the vehicle. 
   A process flow executed to control the engaging force T by the CPU  52  in the third embodiment will be explained hereinafter with reference to a flow chart shown in FIG.  5 . In  FIG. 5 , the processing executed in steps S 12 -S 16  are same as that of the aforementioned first embodiment shown in  FIG. 2 , so that the explanations thereabout is omitted herein. 
   In step S 18 , whether the large turning state (tight corner) is or not is judged by the CPU  52 . In a case that the large turning state is not judged (No in step S 18 ), the engaging force T is, in step S 24 , determined by using the aforementioned normal mode map shown in FIG.  3 (A). As a result of this determination, a control signal is outputted to control the clutch mechanism  19  in step S 28 . 
   On the other hand, in a case that the large turning state (tight corner) is judged (Yes in step S 18 ), a road surface information μ (friction between a road surface and the surface of the wheel) is inputted from the control device of the ABS to the CPU  52  through the input/output circuit  58  (in step S 19 ). When the road surface is high μ (the road surface information μ is high)(No in step S 22 ), the engaging force T is, in step S 26 , determined by using the aforementioned tight mode map shown in FIG.  3 (B), whereby a control signal is outputted to control the clutch mechanism  19  in step S 28 . As a result of this control, the engaging force T of the clutch mechanism  19  is reduced, so that the tight corner braking phenomenon can be prevented. 
   When the judgment is the low friction road (the road surface information μ is low)(Yes in step S 22 ), the engaging force T is, in step S 24 , determined by using the aforementioned normal mode map shown in FIG.  3 (A), whereby a control signal is outputted to control the clutches  19  in step S 28 . Namely, the driving force is distributed to the four wheels (FT 1 , FT 2 , RT 3  and RT 4 ) by increasing the engaging force T of the clutch mechanism  19  corresponding to the rotational difference ΔN between the front and rear wheels. Accordingly, remarkable stability and traveling ability can be realized when the vehicle  10  travels on the low friction road such snow road or sandy road in the large turning state. 
   Herein, the road surface information μ may be obtained from ECU (Electric Control Unit) for engine control, for automatic transmission control and the like, and a navigation controller through an electric communication interface such as “CAN”, “Been”, “J1850” and the like. Further, the snow road information may be obtained from the road information broadcast, whereby the snow road traveling mode is selected by its snow road information. 
   Having described embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.