Abstract:
A vehicle has a transmission apparatus that changes the distribution of power transmitted from a power source to a plurality of wheels. The method for distributing power of the vehicle includes detecting the temperature of a part located on a power transmission path between the power source and a wheel to which power is transmitted from the power source through the transmission apparatus, the heat of the part being increased as the power distribution ratio is increased; determining that the current state is a specific state in which the detected temperature reaches a previously set first reference temperature; and controlling the transmission apparatus to lower the power distribution ratio to the wheel from the transmission apparatus when the current state is determined to the specified state.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a power distribution method and a power distribution control apparatus for vehicles. 
     A standby four-wheel drive system is known in the art. The standby four-wheel drive system controls distribution of power to the front wheels and the rear wheels according to the driving state of a vehicle. When all the four wheels of a four-wheel drive vehicle are being driven, some of the power generated by the engine is transmitted to the rear wheels by a transmission apparatus, which is capable of changing the power distribution ratio to the front wheels and the rear wheels. 
     A typical transmission apparatus includes an electromagnetic clutch mechanism of a multi-plate wet type. The frictional force among the clutch disks of the electromagnetic clutch mechanism is varied in accordance with the amount of the current supplied to a magnet coil. As the frictional force is increased, the ratio of the power distribution to the rear wheels is increased. 
     When the power distribution to the rear wheels is increased, the load on the transfer located between the engine and the transmission apparatus, and the load on the rear differential located between the transmission apparatus and the rear wheels are increased. Accordingly, the temperature of the oil lubricating the transfer and the temperature of the oil lubricating the rear differential are likely to increase. When excessively heated, the lubrication performance of the lubricating oil deteriorates. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to prevent excessive temperature increases at a part that is heated as the ratio of power distribution to vehicle rear wheels is increased. 
     To achieve the above objects, one aspect of the present invention provides a method for distributing power of a vehicle. The vehicle has a transmission apparatus that changes the distribution ratio of power transmitted from a power source to a plurality of wheels. The method includes detecting the temperature of a part located on a power transmission path between the power source and a wheel to which power is transmitted from the power source through the transmission apparatus, the heat of the part being increased as the power distribution ratio is increased; determining that the current state is a specific state in which the detected temperature reaches a previously set first reference temperature; and controlling the transmission apparatus to lower the power distribution ratio to the wheel from the transmission apparatus when the current state is determined to the specified state. 
     Another aspect of the present invention provides a power distribution control apparatus for a vehicle. The control apparatus includes a transmission apparatus that changes the distribution ratio of power transmitted from a power source to a plurality of wheels. The control apparatus has a detector and an electronic control unit. The detector detects the temperature of a part located on a power transmission path between the power source and a wheel to which power is transmitted from the power source through the transmission apparatus, the heat of the part being increased as the power distribution ratio at the transmission apparatus is increased. The control unit controls the power distribution ratio at the transmission apparatus thereby controlling the distribution ratio of power to the wheels. When the current state is a specific state in which the temperature detected by the detector is equal to or higher that a previously set first reference temperature, the control unit controls the transmission apparatus to decrease the power distribution ratio. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a schematic plan view showing a four-wheel drive vehicle according to a first embodiment of the present invention; 
     FIG. 2 is a diagram showing a control circuit of the first embodiment; 
     FIG. 3 is a flowchart showing a power distribution control of the first embodiment; 
     FIG.  4 ( a ) is a map of data used in the power distribution of a normal control according to the first embodiment; 
     FIG.  4 ( b ) is a map of data used in the power distribution of a specified control according to the first embodiment; and 
     FIG. 5 is a flowchart showing a power distribution control according to a second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A four-wheel drive vehicle according to a first embodiment of the present invention will now be described with reference to FIGS. 1 to  4 ( b ). The vehicle is normally in two-wheel drive. Specifically, the engine drives the front wheels in normal circumstances. 
     As shown in FIG. 1, the four-wheel drive vehicle has a drive source, which is an internal combustion engine  11  in this embodiment, and a transaxle  12 . The transaxle  12  includes a transmission  121 , a front differential  122 , and a transfer  123 . First and second front axles  13 ,  14  are coupled to the front differential  122 . A left front wheel  16  is coupled to the first front axle  13 . A right front wheel  17  is coupled to the second front axle  14 . Power of the engine  11  is transmitted to the front wheels  16 ,  17  by the transmission  121 , the front differential  122 , and the front axles  13 ,  14 . 
     One end of a propeller shaft  18  is coupled to the transfer  123 . The other end of the propeller shaft  18  is coupled to an electromagnetic clutch mechanism  19  of a multi-plate wet type. Power of the engine  11  is transmitted to the clutch mechanism  19  by the transmission  121 , the transfer  123 , and the propeller shaft  18 . The clutch mechanism  19  is coupled to a rear differential  20  with a drive pinion  21 . First and second rear axles  22 ,  23  are coupled to the rear differential  20 . A left rear wheel  24  is coupled to the first rear axle  22 . A right rear wheel  25  is coupled to the second rear axle  23 . In this embodiment, the transfer  123 , the propeller shaft  18 , the clutch mechanism  19 , the drive pinion  21 , the rear differential  20 , and the rear axles  22 ,  23  form a power transmission system, which extends from the engine  11  to the rear wheels  24 ,  25 . 
     The clutch mechanism  19  includes an electromagnetic coil  191  and clutch disks (not shown), which are coupled to and separated from one another. When the electromagnetic coil  191  is excited, the clutch disks are engaged, and power is transmitted from the propeller shaft  18  to the drive pinion  21 . The power transmitted to the drive pinion  21  is transmitted to the rear wheels  24 ,  25  through the rear differential  20  and the rear axles  22 ,  23 . 
     The distribution ratio of the power transmitted from the engine  11  to the drive pinion  21  through the propeller shaft  18  is determined by the frictional force among the clutch disks. The power transmitted to the drive pinion  21  is increased as the frictional force among the clutch disks is increased. The frictional force among the clutch disks is determined by the amount of the current supplied to the electromagnetic coil  191 . Therefore, if the current supplied to the electromagnetic coil  191  is controlled, the power distribution ratio to the front wheels  16 ,  17  and the rear wheels  24 ,  25  will be controlled. The clutch mechanism  19  is a transmission apparatus capable of varying the ratio of the power transmitted from the engine  11  to the front wheels  16 ,  17  and the rear wheels  24 ,  25 . 
     A temperature detecting device, which is a temperature sensor  26  in this embodiment, is attached to the rear differential. The temperature sensor  26  detects the temperature of lubricating oil that is stored in and lubricates the rear differential  20 . The temperature sensor  26  is connected an electronic control unit (ECU)  27  for distributing power. The information detected by the temperature sensor  26  is sent to the ECU  27 . The ECU  27  is connected to a vehicle speed sensor  28  and a throttle opening sensor  29 . The value detected by the vehicle speed sensor  28  and the value detected by the throttle opening sensor  29  are sent to the ECU  27 . 
     As shown in FIG. 2, the ECU  27  includes a CPU  271 , a ROM  272 , a RAM  273 , and input-output circuit  274 . The electromagnetic coil  191  of the clutch mechanism  19  is electrically connected to a drive circuit  15 . The drive circuit  15 , the temperature sensor  26 , the vehicle speed sensor  28 , and the throttle opening sensor  29  are connected to the input-output circuit  274 . The ECU  27  controls the current to the electromagnetic coil  191  by means of the drive circuit  15 . The current is duty controlled. 
     As the duty ratio is increased, the frictional force of the clutch mechanism  19  is increased. Accordingly, ratio of power transmission at the clutch  19  is increased. In other words, the power distribution ratio to the rear wheels  24 ,  25  is increased. When the duty ratio is 100%, the power distribution ratio is maximized. When the duty ratio is 0%, the power distribution ratio is zero. As the power distribution ratio to the rear wheels  24 ,  25  is increased, the load on the rear differential  20  is increased. As a result, the temperature of oil lubricating the rear differential  20  is increased. As the power distribution ratio to the rear wheels  24 ,  25  is decreased, the load on the rear differential  20  is decreased. As a result, the temperature of oil lubricating the rear differential  20  is decreased. 
     The ROM  272  stores a control program and map data for controlling the current to the electromagnetic coil  191  of the clutch mechanism  19 . A flowchart of FIG. 3 schematically shows a control program stored in the ROM  272 . The CPU  271  executes various computations for controlling the current to the clutch mechanism  19  based on a control program and map data stored in the ROM  272 . The RAM  273  temporarily stores computation results of the CPU  271  and stores various data. 
     The map data stored in the ROM  272  is related to the duty ratio of the current supplied to the electromagnetic coil  191 . The map data is defined by using the throttle opening degree and the vehicle speed as parameters. The map data includes a first map D 1  shown in FIG.  4 ( a ) and a second map D 2  shown in FIG.  4 ( b ). The first map D 1  is used in normal conditions and the second map D 2  is used in a specific state. As shown in FIGS.  4 ( a ) and  4 ( b ), V 1 , V 2 , V 3 , V 4 , . . . show the vehicle speed, and θ 1 , θ 2 , θ 3 , θ 4 , . . . show the throttle opening degree. R 00 , R 10 , R 20  . . . , R 01 , R 11 , R 21  . . . , (hereinafter referred to as Rmn, in which m and n are integers equal to or greater than zero), and K 00 , K 10 , K 20  . . . , K 01 , K 11 , K 21  (hereinafter referred to as Kmn, in which m and n are integers equal to or greater than zero) represent duty ratios (see FIG.  4 ( b )). When Rmn is not zero, a formula Rmn&gt;Kmn is satisfied. 
     The ECU  27  selects one of the first and second maps D 1 , D 2  based on the information detected by the temperature sensor  26 . The ECU  27  controls the current to the electromagnetic coil  191  based on the information detected by the vehicle speed sensor  28 , the information detected by the throttle opening sensor  29 , and the map data. 
     The power distribution control will now be described with reference to the flowchart of FIG.  3 . The ECU  27  samples the detected information from the temperature sensor  26 , the vehicle speed sensor  28 , and the throttle opening sensor  29  at predetermined time intervals. 
     In step S 1 , the ECU  27  compares the temperature Tx detected by the temperature sensor  26  with a first reference temperature T 1 . The first reference temperature T 1  is an index to indicate that the temperatures of the oil lubricating the rear differential  20  above it are undesirable. The first reference temperature T 1  is obtained through experiments or is theoretically computed. The first reference temperature T 1  is stored in the ROM  272 . If a formula Tx&lt;T 1  is satisfied, the ECU  27  proceeds to step So and performs a normal control by using the first map D 1 . If a formula Tx≧T 1  is satisfied, the ECU  27  proceeds to step S 2  and compares the detected temperature Tx with a predetermined second reference temperature T 2  (T 2 &gt;T 1 ). The second reference temperature T 2  is an index to indicate that a temperature increase of the oil lubricating the rear differential  20  above it likely to cause a lubrication failure. The second reference temperature T 2  is obtained through experiments or is theoretically computed. The second reference temperature T 2  is stored in the ROM  272 . In step S 3 , the ECU  27  compares a first duration t 1  in which the formula T 2 &gt;Tx≧T 1  is satisfied with a first reference duration α. The first reference duration α is stored in the ROM  272 . If a formula t 1 &lt;α is satisfied, the ECU  27  returns to step S 1 . If a formula t 1 ≧α is satisfied, the ECU  27  proceeds to step S 4  and performs the specified control by using the second map D 2 . 
     During the specified control using the second map D 2 , the ECU  27  compares the detected temperature Tx with a first return permission temperature To in step S 5 . The first return permission temperature To is an index to indicate that the control is permitted to return to the normal control after the temperature of the lubricating oil drops from the state represented by the formula T 2 &gt;Tx≧T 1 . The first return permission temperature To is obtained through experiments or is theoretically computed. The first return permission temperature To is stored in the ROM  272 . If a formula To≧Tx is satisfied, the ECU  27  proceeds to step So and performs the normal control by using the first map D 1 . 
     If a formula Tx≧T 2  is satisfied, the ECU  27  proceeds to step S 6 . In step S 6 , the ECU  27  compares a second duration t 2  in which the formula Tx≧T 2  is satisfied with a second reference duration γ. If a formula t 2 &lt;γ is satisfied, the ECU  2  returns to step S 2 . If a formula t 2 ≧γ is satisfied, the ECU  27  proceeds to step S 7 . In step S 7 , the ECU  27  stops the current to the electromagnetic coil  191  to start the two-wheel drive, in which only the front wheels  16 ,  17  are driven. During the two-wheel drive, the ECU  27  compares the detected temperature Tx with a predetermined second return permission temperature T 3  in step S 8 . The second return permission temperature T 3  is an index to indicate that the control is permitted to return to the normal control when the temperature of the lubricating oil drops from the state represented by the formula Tx≧T 2 . The second return permission temperature T 3  is obtained through experiments or is theoretically computed. The second return permission temperature T 3  is stored in the ROM  272 . If a formula T 3 ≧Tx is satisfied, the ECU  27  proceeds to step So and performs the normal control by using the first map D 1 . 
     If the temperature detected by the temperature sensor  26  reaches a predetermined reference temperature, the ECU  27  decreases the power distribution ratio at the clutch mechanism  19 . 
     This embodiment provides the following advantages. 
     (1-1) If the temperature of the oil lubricating the rear differential  20  has not reached the first reference temperature T 1 , the normal control using the first map D 1  is executed. If the temperature of the oil lubricating the rear differential  20  reaches the first reference temperature T 1  and this temperature state has continued over the first reference duration α, that is, if the temperature is in a specific state, the normal control in which the four wheels are driven, likely to further increase the temperature of the oil lubricating the rear differential  20 . 
     When the specific state occurs, the control using the second map D 2  is executed. When the vehicle speed and the throttle opening degree are constant, the duty ratio Kmn on the second map D 2 , which is not zero, is less than the duty ratio Rmn on the first map D 1 . Therefore, if the control is shifted from the one using the first map D 1  to the one using the second map D 2 , the power transmission distribution ratio at the clutch mechanism  19  is lowered to the power transmission distribution ratio for the specific state. Accordingly, the power distribution ratio to the rear wheels  24 ,  25  is decreased. As a result, the load on the rear differential  20  is decreased, and the temperature of the oil lubricating the rear differential  20  is lowered. Such decrease in the power transmission distribution ratio prevents the oil lubricating the rear differential  20  from being excessively heated, that is, prevents the rear differential  20  from being excessively heated. 
     (1-2) If the temperature of the oil lubricating the rear differential  20  reaches the second reference temperature T 2  and this temperature state has continued over the second reference duration γ, that is, if the temperature is in the specific state, the normal control using the first map D 1  is likely to excessively heat the oil lubricating the rear differential  20 . 
     However, in this embodiment, the current to the electromagnetic coil  191  is stopped in the specific state. Accordingly, the power transmission distribution ratio at the clutch mechanism  19  is lowered to zero, and the power distribution ratio to the rear wheels  24 ,  25  is lowered to zero. As a result, the load on the rear differential  20  is decreased. The state in which the power transmission distribution ratio to the clutch mechanism  19  is zero readily decreases the temperature of the rear differential  20  to the second reference temperature T 2 . Such decrease in the power transmission distribution ratio to zero prevents the rear differential  20  from being excessively heated, that is, prevents the oil lubricating the rear differential  20  from being excessively heated. 
     (1-3) The first return permission temperature To is an index to indicate that the control is permitted to return to the normal control after the temperature of the lubricating oil drops from the state represented by the formula Tx≧T 1 . If the control is returned to the normal control when the temperature of the lubricating oil drops below the first reference temperature T 1 , hunting of the lubricating oil temperature is likely to occur. That is, the lubricating oil temperature is likely to repeatedly surpass and drop blow the first reference temperature T 1  in a short time. However, since the first return permission temperature To, which is lower than the first reference temperature T 1 , is used as an index to return the control to the normal control, such hunting is prevented. 
     (1-4) The second return permission temperature T 3  is an index to indicate that the control is permitted to return to the normal control after the temperature of the lubricating oil drops from the state represented by the formula Tx≧T 2 . If the control is returned to the normal control when the temperature of the lubricating oil drops below the second reference temperature T 2 , hunting of the lubricating oil temperature is likely to occur. That is, the lubricating oil temperature is likely to repeatedly surpass and drop blow the second reference temperature T 2  in a short time. However, since the second return permission temperature T 3 , which is lower than the second reference temperature T 2 , is used as an index to return the control to the normal control, such hunting is prevented. 
     (1-5) If the state in which the temperature Tx of the lubricating oil is equal to or higher than T 1  and lower than T 2  continues over the first reference duration α, the control is shifted from the normal control using the first map D 1  to the specified control using the second map D 2 . If the state in which a formula T 2 &gt;Tx≧T 1  is satisfied continues over the first reference duration α (for example, several tens of seconds), the temperature of the lubricating oil is highly likely to increase further. Even if the temperature Tx of the lubricating oil momentarily surpasses T 1 , it is not certain that the lubricating oil temperature will increase further. Therefore, if the control is shifted from the normal control to the specified control as soon as the lubricating oil temperature Tx reaches T 1 , the shifting of the control may be meaningless. Meaningless shifting hinders the power distribution ratio from being optimized. Therefore, using the first reference duration α is effective to determine whether the lubricating oil temperature will continue to increase. 
     (1-6) When the lubricating oil temperature Tx is equal to or higher than T 2  over the second reference duration γ, the two-wheel drive is started. If the state in which a formula Tx≧T 2  is satisfied continues over the second reference duration γ (for example, several tens of seconds), the temperature of the lubricating oil is highly likely to increase further. Even if the temperature Tx of the lubricating oil momentarily surpasses T 2 , it is not certain that the lubricating oil temperature will increase further. Therefore, if the control is shifted from the normal control to the two-wheel drive as soon as the lubricating oil temperature Tx reaches T 2 , the shifting of the control may be meaningless. Meaningless shifting hinders the power distribution ratio from being optimized. Therefore, using the second reference duration γ is effective to determine whether the lubricating oil temperature will continue to increase. 
     The second embodiment will now be described with reference to FIG.  5 . The structure of the power distribution control apparatus according to this embodiment is the same as that of the embodiment shown in FIGS. 1 to  4 ( b ). This embodiment is different from the embodiment of FIGS. 1 to  4 ( b ) in the control program for the power distribution. 
     In this embodiment, step S 5  of FIG. 3 is replaced by steps S 51  and S 52 , and step S 8  of FIG. 3 is replaced by steps S 81  and S 82 . After step S 4 , that is, during the specific control using the second map D 2 , the ECU  27  compares the detected temperature Tx with the first reference temperature T 1  in step S 51 . If the detected temperature Tx is lower than the first reference temperature T 1 , the ECU  27  proceeds to step S 52 . In step S 52 , the ECU  27  compares a duration s 1  of the detected temperature Tx with a predetermined third reference duration β. If a formula s 1 &lt;β is satisfied, the ECU  1  returns to step S 51 . If a formula s 1 ≧β is satisfied, the ECU  1  proceeds to step So. In step So, the ECU  27  performs the normal control by using the first map D 1 . 
     After step S 7 , that is, during the two-wheel drive, the ECU  27  compares the detected temperature Tx with the predetermined second reference temperature T 2  in step S 81 . If the detected temperature Tx is lower than the reference temperature T 2 , the ECU  27  proceeds to step S 82 . In step S 82 , the ECU  27  compares a duration s 2  of the detected temperature Tx with a predetermined fourth reference duration δ. If a formula s 2 &lt;δ is satisfied, the ECU  2  returns to step S 7 . If a formula S 2 ≧δ is satisfied, the ECU  27  proceeds to step So and shifts the control to the normal control. 
     The first reference duration β is determined such that hunting at the first reference temperature T 1  is avoided. The second reference duration δ is determined such that hunting at the second reference temperature T 2  is avoided. Therefore, in this embodiment, hunting at both reference temperatures T 1  and T 2  is prevented. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     The control may be immediately shifted from the normal control to the specific control when the detected temperature of the lubricating oil reaches the first reference temperature T 1 . 
     The control may be immediately shifted from the normal control to the two-wheel drive when the detected temperature of the lubricating oil reaches the second reference temperature T 2 . 
     During the specific state, in which the detected temperature of the lubricating oil is equal to or above the first reference temperature T 1 , the power transmission distribution ratio at the clutch mechanism  19  may be decreased to zero. That is, the current to the electromagnetic coil  191  of the clutch  19  may be stopped so that the control is shifted to the two-wheel drive. 
     The power distribution control may be performed based on a power distribution control program that includes steps S 5 , S 8  of the flowchart of the embodiment shown in FIGS. 1 to  4 ( b ) and steps S 51 , S 81  of the flowchart of the embodiment shown in FIG.  5 . 
     In this case, when the formula Tx≦To is satisfied or when formulas Tx&lt;T 1  and s 1 ≧β are satisfied, the control is shifted from the specified control to the normal control. Also, when the formula Tx≦T 3  is satisfied or when formulas Tx&lt;T 2  and s 2 ≧δ are satisfied, the control is shifted from the specified control to the two-wheel drive. 
     As a parameter for the first and second maps, the difference (differential rotation speed) between the average rotation speed of the front wheels  16 ,  17  and the average rotation speed of the rear wheels  24 ,  25  may be used. 
     In this case, wheel speed sensors are needed for separately detecting the rotation speed of the front wheels  16 ,  17  and the rotation speed of the rear wheels  24 ,  25 . 
     Temperature may be detected at the transfer  123 . Specifically, the temperature of oil that is stored in and lubricates the transfer  123  may be detected. 
     The present invention may be applied to a FR type or RR type four-wheel-drive vehicle, which drives the rear wheels during the two-wheel drive. 
     The present invention may be applied to vehicles other than four-wheel-drive vehicles. 
     Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.