Driving force control apparatus for automotive vehicles

In a driving force control apparatus for an automotive vehicle employing an engine that drives a main drive wheel, a generator driven by the engine, and a motor driven by an electric power output generated by the generator to drive a subsidiary drive wheel, a subsidiary-drive-wheel acceleration slip estimation circuitry is provided to estimate a subsidiary-drive-wheel acceleration slip rate. An electric power output suppression circuitry is provided to suppress the electric power output of the generator when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate. Also provided is a subsidiary-drive-wheel acceleration-slip period engine output torque reduction circuitry that reduces an engine output torque responsively to suppressing the electric power output.

TECHNICAL FIELD

The present invention relates to a driving force control apparatus for automotive vehicles, and specifically to a vehicle driving force control apparatus capable of driving a generator by an internal combustion engine that drives main drive wheels and supplying electric power generated by the generator to a motor that drives subsidiary drive wheels.

BACKGROUND ART

In recent years, there have been proposed and developed various vehicle driving force control apparatus in which main drive wheels (either front or rear road wheels) are driven by an engine and subsidiary drive wheels (the remaining road wheels) are driven by a motor. One such vehicle driving force control apparatus has been disclosed in Japanese Patent Provisional Publication No. 7-231508 (hereinafter is referred to as “JP7-231508”). In the vehicle driving force control apparatus disclosed in JP7-231508, a generator is driven by an engine, whereas a motor is driven by electric energy generated by the generator. The electric energy, which is supplied from the generator to the motor, is controlled depending on a state of the vehicle, estimated based on, for example, a deviation between a standard wheel speed based on an accelerator opening and a front wheel speed, a deviation between the standard wheel speed and a rear wheel speed, and a deviation between the front and rear wheel speeds.

SUMMARY OF THE INVENTION

In the vehicle driving force control apparatus as disclosed in JP7-231508, suppose that a control system is designed to suppress or reduce electric power output of the generator for reducing a driving force of each individual subsidiary drive wheel exceeding a grip force limit and for recovering the grip force of the subsidiary drive wheel on the road, when acceleration slip greater than a predetermined slip rate takes place at the subsidiary drive wheels. Under such an acceleration slip condition of the subsidiary drive wheels, the control system operates to suppress or reduce the generator's electric power output depending on the detected subsidiary-drive-wheel slip rate. On the one hand, suppressing the generator's electric power output contributes to the enhanced or improved convergence performance of acceleration-slip suppression control for the subsidiary drive wheel side. On the other hand, suppressing the generator's electric power output means that the generator's load carried on the engine is rapidly reduced or released, thus deteriorating the convergence performance of acceleration-slip suppression control for the main drive wheel side.

Accordingly, it is an object of the invention to provide a vehicle driving force control apparatus capable of effectively suppressing acceleration slip of a subsidiary drive wheel side without deteriorating a convergence performance of acceleration-slip suppression for a main drive wheel side.

In order to accomplish the aforementioned and other objects of the present invention, a driving force control apparatus for an automotive vehicle employing an engine that drives a main drive wheel, a generator driven by the engine, and a motor driven by an electric power output generated by the generator to drive a subsidiary drive wheel, comprises a subsidiary-drive-wheel acceleration slip estimation circuitry that estimates an acceleration slip rate of the subsidiary drive wheel, an electric power output suppression circuitry that suppresses the electric power output of the generator when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate, and a subsidiary-drive-wheel acceleration-slip period engine output torque reduction circuitry that reduces an engine output torque responsively to suppressing the electric power output.

According to another aspect of the invention, a driving force control apparatus for an automotive vehicle employing an engine that drives a main drive wheel, a generator driven by the engine, and a motor driven by an electric power output generated by the generator to drive a subsidiary drive wheel, comprises a subsidiary-drive-wheel acceleration slip estimation circuitry that estimates an acceleration slip rate of the subsidiary drive wheel, an electric power output suppression circuitry that suppresses the electric power output of the generator by a reduced value when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate, and a target engine output torque reduction circuitry that reduces a target engine output torque by a load torque value corresponding to the reduced value of the electric power output responsively to suppressing the electric power output.

According to a further aspect of the invention, a driving force control apparatus for an automotive vehicle employing an engine that drives a main drive wheel, a generator driven by the engine, and a motor driven by an electric power output generated by the generator to drive a subsidiary drive wheel, comprises a subsidiary-drive-wheel acceleration slip estimation circuitry that estimates an acceleration slip rate of the subsidiary drive wheel, an electric power output suppression circuitry that suppresses the electric power output of the generator when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate, a main-drive-wheel acceleration slip estimation circuitry that estimates an acceleration slip rate of the main drive wheel, a main-drive-wheel acceleration-slip period engine output torque suppression circuitry that suppresses the engine output torque by a reduced value determined based on the estimated main-drive-wheel acceleration slip rate regardless of a driver-required vehicle acceleration when the estimated main-drive-wheel acceleration slip rate exceeds an engine traction-control-system (TCS) intervention threshold value, and an acceleration slip threshold value alteration circuitry that alters the TCS intervention threshold value to a predetermined low threshold value lower than an initial threshold value, when the estimated subsidiary-drive-wheel acceleration slip rate exceeds the predetermined slip rate.

According to a still further aspect of the invention, an automotive vehicle comprises an engine that drives a main drive wheel, a generator driven by the engine, a motor driven by an electric power output generated by the generator for driving a subsidiary drive wheel, sensors that detect slip conditions of the main drive wheel and the subsidiary drive wheel, and a controller being configured to be electrically connected to the engine, the motor, the generator, and the sensors, for controlling driving forces applied to the main drive wheel and the subsidiary drive wheel, the controller comprising a subsidiary-drive-wheel acceleration slip estimation circuitry that estimates, based on the slip condition of the subsidiary drive wheel, a subsidiary-drive-wheel acceleration slip rate, an electric power output suppression circuitry that suppresses the electric power output of the generator when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate, and a subsidiary-drive-wheel acceleration-slip period engine output torque reduction circuitry that reduces an engine output torque responsively to suppressing the electric power output.

According to another aspect of the invention, a driving force control apparatus for an automotive vehicle employing an engine that drives a main drive wheel, a generator driven by the engine, and a motor driven by an electric power output generated by the generator to drive a subsidiary drive wheel, comprises subsidiary-drive-wheel acceleration slip estimation means for estimating an acceleration slip rate of the subsidiary drive wheel, electric power output suppression means for suppressing the electric power output of the generator when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate, and subsidiary-drive-wheel acceleration-slip period engine output torque reduction means for reducing an engine output torque responsively to suppressing the electric power output.

According another aspect of the invention, a method of controlling driving forces applied to a main drive wheel and a subsidiary drive wheel of an automotive vehicle employing an engine that drives the main drive wheel, a generator driven by the engine, and a motor driven by an electric power output generated by the generator to drive the subsidiary drive wheel, the method comprises estimating an acceleration slip rate of the subsidiary drive wheel, suppressing the electric power output of the generator when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate, and reducing an engine output torque responsively to suppressing the electric power output.

According to another aspect of the invention, a method of controlling driving forces applied to a main drive wheel and a subsidiary drive wheel of an automotive vehicle employing an engine that drives the main drive wheel, a generator driven by the engine, a motor driven by an electric power output generated by the generator to drive the subsidiary drive wheel, and sensors that detect slip velocities of the main drive wheel and the subsidiary drive wheel and a driver-required vehicle acceleration, the method comprises estimating, based on the slip velocity of the main drive wheel, a main-drive-wheel acceleration slip rate, estimating, based on the slip velocity of the subsidiary drive wheel, a subsidiary-drive-wheel acceleration slip rate, calculating a first target motor torque based on the slip velocity of the main drive wheel, calculating a second target motor torque based on the driver-required vehicle acceleration, selecting a higher one of the first and second target motor torques as a target motor torque, reducing the target motor torque by a reduced torque value determined based on the estimated subsidiary-drive-wheel acceleration slip rate when the estimated subsidiary-drive-wheel acceleration slip rate exceeds a predetermined slip rate, suppressing the electric power output of the generator by a reduced value corresponding to the reduced torque value when the estimated subsidiary-drive-wheel acceleration slip rate exceeds the predetermined slip rate, and reducing an engine output torque responsively to suppressing the electric power output.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, particularly toFIG. 1, the vehicle driving force control apparatus of the embodiment is exemplified in a four-wheel-drive (4WD) vehicle in which front-left and front-right road wheels1L and1R are main drive wheels driven by an engine2, and rear-left and rear-right road wheels3L and3R are subsidiary drive wheels driven by a motor4. An output torque Te produced by engine2is transmitted through a transmission30and a front differential gear31to front-left and front-right road wheels1L and1R.

A shift position detector (or a shift position detecting device)32is provided in or attached to transmission30for detecting a selected operating range of the transmission and for generating a shift position signal indicative of the selected operating range to a 4WD controller8. The automatic shift sequence of transmission30is performed responsively to an automatic shift command from an electronic transmission controller (not shown). Memories (RAM, ROM) of the transmission controller store the information program concerning a shift schedule based on a vehicle speed Vv and an accelerator opening θ as a preprogrammed look-up table or a preprogrammed map. The transmission controller determines, based on the latest up-to-date information about vehicle speed Vv and accelerator opening θ, a timing of shifting action (an upshift or a downshift) passing through a shifting point, from the information program, and generates a shift command to transmission30.

As clearly shown inFIG. 1, a main throttle valve15and a sub-throttle valve16are provided in an intake pipe (or an intake manifold)14of engine2. The main throttle opening of main throttle valve15is adjusted or controlled depending on the amount of depression of an accelerator pedal17that serves as an accelerator-opening indicating device or a driver-required vehicle-acceleration-indication operating component part. Main throttle valve15is mechanically linked to accelerator pedal17such that the main throttle valve opening can be adjusted or controlled in synchronism with the amount of depression of accelerator pedal17. Alternatively, main throttle valve15may be electronically controlled by means of an engine controller18such that the main throttle valve opening is adjusted or controlled responsively to an accelerator position signal from an accelerator sensor40that detects the amount of depression of accelerator pedal17, in other words, the accelerator-pedal angular position. The accelerator position signal (indicative of accelerator opening θ) from accelerator sensor40is also output into the input/output interface (I/O) of 4WD controller8. On the other hand, sub-throttle valve16is electronically controlled by means of a step motor controller20electrically connected to 4WD controller8. In more detail, sub-throttle valve16is driven by a step motor19, serving as a sub-throttle actuator. Step motor19rotates in short and essentially uniform angular movements rather than continuously. Angular steps of step motor19are obtained electromagnetically. A sub-throttle valve opening α of sub-throttle valve16is controlled depending on a rotation angle (e.g., 30°, 45°, 90°, and the like) corresponding to the number of angular steps of step motor19. The rotation angle of step motor19is adjusted or controlled in response to a drive signal from step motor controller20. Although it is not clearly shown inFIG. 1, a sub-throttle position sensor, simply, a throttle sensor (seeFIG. 2) is located at sub-throttle valve16. The number of angular steps of step motor19is feedback-controlled based on the detected sub-throttle valve opening α sensed by the throttle sensor. Actually, the sub-throttle valve opening α of sub-throttle valve16is adjusted or controlled to a valve opening less than or equal to the main throttle valve opening of main throttle valve15, so that the output torque Te of engine2can be controlled independently of the manipulated variable of accelerator pedal17depressed by the driver.

An engine speed sensor21is provided to detect engine speed Ne of engine2. The sensor signal from engine speed sensor21is output to engine controller18as well as 4WD controller8. Engine controller18is electrically connected to 4WD controller8to communicate with the 4WD controller8through a data link (a plurality of signal lines). For instance, information about at least wheel speeds of four road wheels1L,1R,3L and3R and a state of a rear-wheel acceleration-slip indicative flag (simply, a rear-wheel slip flag) Rslip (described later) is sent from 4WD controller8to engine controller18.

A brake pedal stroke sensor (simply, a brake stroke sensor)35is provided to detect a brake-pedal stroke of a brake pedal34. The sensor signal from brake pedal stroke sensor35is output to a brake controller36as well as 4WD controller8.

The central processing unit (CPU) of brake controller36allows the access by the I/O interface of input information data signal from brake pedal stroke sensor35so as to control a braking force applied to the vehicle, exactly, four braking torques (braking forces) applied to road wheels1L,1R,3L, and3R, via respective braking devices, namely a front-left braking device (e.g., a FL disc brake)37FL, a front-right braking device (e.g., a FR disc brake)37FR, a rear-left braking device (e.g., a RL disc brake)37RL, and a rear-right braking device (e.g., a RR disc brake)37RR.

A drive mode selector switch (simply, a drive mode switch)39is also provided to generate a 2WD-to-4WD mode switching signal (simply, a 4WD mode signal) or a 4WD-to-2WD mode switching signal (simply, a 2WD mode signal) to the input interface circuitry of 4WD controller8.

As can be seen from the system diagram ofFIG. 1, part of engine output torque Te is transmitted through an endless belt6, wound on an engine-side pulley and a generator-side pulley, to generator7, such that generator7is driven or rotated at a generator speed Nh, which is obtained by multiplying engine speed Ne with a pulley ratio between the engine-side pulley and the generator-side pulley. Therefore, generator speed Nh of generator7is arithmetically calculated based on engine speed Ne as a product of the pulley ratio and engine speed Ne.

Referring now toFIG. 2, there is shown the control system wiring diagram of 4WD controller8of the vehicle driving force control apparatus of the embodiment ofFIG. 1. As can be seen from the circuit diagram ofFIG. 2, generator7is equipped with a voltage regulator22that regulates an output voltage Vg of generator7. A field current Ifh of generator7is adjusted or controlled responsively to a generator control command (a given duty ratio) C1generated from a generator control section8F (described later in reference to the block diagram shown inFIG. 3) of 4WD controller8to voltage regulator22of generator7. That is, the generator's load carried on engine2, in other words, a surplus engine torque Th (described later), and generator output voltage Vg are both controlled or regulated by adjusting generator field current Ifh. In more detail, voltage regulator22receives a generator control command (a given duty ratio) C1from generator control section8F, and acts to regulate generator field current Ifh responsively to the generator control command C1(the given duty ratio). Voltage regulator22is configured to be electrically connected to generator7, in a manner so as to output the regulated generator output voltage Vg to 4WD controller8, while detecting the regulated generator output voltage Vg. As can be seen from the system diagram ofFIG. 1, electric power generated by generator7is supplied via an electric cable (or an electric wire harness)9to motor4. As clearly shown inFIGS. 1-2, a junction box10is disposed in a middle of electric cable9. Motor torque, which is output from the drive shaft of motor4, flows through a reduction gear device11and a clutch12to a rear differential gear13, and then flows via rear differential gear13to rear-left and rear-right road wheels3L and3R. An electric current sensor23is provided in junction box10, for detecting or monitoring an electric current value Ia (i.e., an armature current of motor4) of the electric power supplied from generator7to motor4. The sensor signal from current sensor23, indicative of armature current Ia, is output to 4WD controller8. Additionally, 4WD controller8has a voltage detector that detects or monitors a motor voltage value of motor4(motor induced voltage E). A relay24is also provided to block or establish the electric power supply (the voltage supply and the current supply) from generator7to motor4in response to a control command from 4WD controller8.

A field current Ifm of motor4is adjusted or controlled responsively to a motor control command from 4WD controller8. That is, the motor torque (the driving torque) of motor4is adjusted or brought closer to a target motor torque Tm by adjusting motor field current Ifm.

A thermistor25is provided to detect or measure a motor temperature value of motor4. A motor revolution sensor26is provided to detect or monitor a rotational speed of the drive shaft of motor4, that is, motor speed Nm. The sensor signal from motor revolution sensor26, indicative of motor speed Nm, is output to 4WD controller8.

Also located at road wheels1L,1R,3L, and3R are four road wheel speed sensors27FL,27FR,27RL, and27RR. Wheel speed sensors27FL,27FR,27RL, and27RR are provided for detecting front-left, front-right, rear-left, and rear-right wheel speeds VwFL, VwFR, VwRL, and VwRR, which are collectively referred to as “Vw”, and for generating four pulse signals, respectively indicating the respective wheel speeds VwFL, VwFR, VwRL, and VwRR, to 4WD controller8.

Referring now toFIG. 3, there is shown the block diagram explaining the detailed structure of 4WD controller8. As seen from the block diagram ofFIG. 3, 4WD controller8is comprised of a target motor torque calculation section8A, a motor variable adjustment section8B (described later in reference to the flow chart ofFIG. 6), a motor control section8C, a relay control section8D, a clutch control section8E, a generator control section8F, and a motor TCS section (a motor traction-control-system section)8G (described later in reference to the flow chart ofFIG. 7). 4WD controller8comes into operation, when the 4WD mode is selected by drive mode switch39.

Relay control section8D controls blocking and establishing operations of the electric power supply from generator7to motor4. When the 4WD mode has been selected by drive mode switch39and therefore target motor torque Tm is greater than “0”, that is, Tm>0, relay control section8D controls relay24to keep relay24at its closed state (or an electric circuit connection state) where a pair of relay contacts are closed. Conversely when target motor torque Tm is equal to “0”, that is, Tm=0, relay control section8D controls relay24to keep relay24at its opened state (or an electric circuit disconnection state) where the pair of contacts are opened.

Clutch control section8E controls engagement and disengagement of clutch12. When the 4WD mode has been selected by drive mode switch39and therefore target motor torque Tm is greater than “0”, that is, Tm>0, clutch control section8E controls clutch12to keep clutch12at its engaged state. Conversely when target motor torque Tm is equal to “0”, that is, Tm=0, clutch control section8E controls clutch12to keep clutch12at its disengaged state. Target motor torque calculation section8A includes a surplus torque calculation section8Aa (hereunder described in detail in reference to the flow chart ofFIG. 4), a vehicle acceleration assist-torque calculation section (simply, an acceleration assist-torque calculation section)8Ab (hereunder described in detail in reference to the characteristics shown inFIGS. 5A-5B), and a motor torque determination section8Ac.

Surplus torque calculation section8Aa serves to calculate a surplus engine torque (simply, a surplus torque) Th corresponding to an acceleration slip rate of front road wheels (main drive wheels)1L and1R. Actually, surplus torque calculation section8Aa executes the surplus torque Th arithmetic-calculation routine shown inFIG. 4. The surplus torque Th calculation routine ofFIG. 4is executed as time-triggered interrupt routines to be triggered every predetermined sampling time intervals such as 10 milliseconds.

At step S10ofFIG. 4, first, front-left, front-right, rear-left, and rear-right wheel speeds VwFL, VwFR, VwRL, and VwRR, detected by wheel speed sensors27FL,27FR,27RL, and27RR, are read. Then, on the basis of the latest up-to-date information about wheel speeds VwFL, VwFR, VwRL, and VwRR, an acceleration slip velocity (simply, a slip velocity) ΔVF corresponding to an acceleration slip rate of the front road wheel side (main drive wheels1L,1R) is calculated by subtracting a rear wheel speed VwR (i.e., a subsidiary drive wheel speed) from a front wheel speed VwF (i.e., a main drive wheel speed). More concretely, slip velocity ΔVF of the front road wheel side is calculated as follows.

An average front wheel speed Vwf of front-left and front-right wheel speeds VwFL and VwFR is calculated as a simple average (VwFL+VwFR)/2. At the same time, an average rear wheel speed Vwr of rear-left and rear-right wheel speeds VwRL and VwRR is calculated as a simple average (VwRL+VwRR)/2. Thereafter, slip velocity ΔVF corresponding to an acceleration slip rate of the front road wheel side (main drive wheels1L,1R) is calculated as a deviation (Vwf−Vwr) between average front wheel speed Vwf (=(VwFL+VwFR)/2) and average rear wheel speed Vwr (=(VwRL+VwRR)/2), that is, ΔVF=(Vwf−Vwr). After step S10, step S20occurs.

At step S20, a check is made to determine whether slip velocity ΔVF, calculated at step S10, is greater than a predetermined value, in other words, a predetermined power-generation threshold value Tpg, such as “0”. When the answer to step S20is in the negative (NO), that is, in case of ΔVF≦0, the processor (surplus torque calculation section8Aa) of 4WD controller8determines or estimates that there is no acceleration slip at the front road wheel side (main drive wheels1L.1R), and thus the routine proceeds from step S20to step S30. Conversely when the answer to step S20is in the affirmative (YES), that is, in case of ΔVF>0, the processor (surplus torque calculation section8Aa) of 4WD controller8determines or estimates that acceleration slip occurs at the front road wheel side (main drive wheels1L.1R), and thus the routine proceeds from step S20to step S40. Step S20ofFIG. 4, step S610ofFIG. 8(described later) and step S610ofFIG. 10(described later), and wheel speed sensors27FL-27RR serve as a main-drive-wheel acceleration slip estimation circuitry (or a main-drive-wheel acceleration slip detector or main-drive-wheel acceleration slip estimation means).

At step S30, zero is substituted for a first target motor torque Tm1, that is, Tm1=0 (or Tm1←0). Thereafter, the surplus torque Th calculation routine returns to the main program.

At step S40, an absorption torque TΔVF, needed to suppress acceleration slip of the front road wheel side (main drive wheels1L,1R), is arithmetically calculated from the expression TΔVF=K1×ΔVF, where K1denotes an experimentally-determined proportional gain. As can be appreciated from the above expression TΔVF=K1×ΔVF, absorption torque TΔVF is a variable that varies in direct proportion to slip velocity ΔVF, that is, the acceleration slip rate of the front road wheel side (main drive wheels1L,1R). After step S40, step S50occurs.

At step S50, a current load torque TG of generator7(or an actual generator load torque) is arithmetically calculated from the expression TG=K2·(Vg×Ia)/(K3×Nh), where Vg denotes an output voltage of generator7, Ia denotes an armature current of motor4, Nh denotes a generator speed, K3denotes an efficiency of generator7, and K2denotes a coefficient. After step S50, step S60occurs.

At step S60, the surplus engine torque Th, in other words, a target generator load torque Th is calculated as a sum (TG+TΔVF) of the current value of load torque TG of generator7calculated at step S50and absorption torque TΔVF (=K1×ΔVF) calculated at step S40. After step S60, step S70occurs.

At step S70, a check is made to determine whether target generator load torque (surplus engine torque) Th is greater than a generator's maximum load capacity HQ, which is determined by specifications of generator7. When the answer to step S70is negative (NO), that is, in case of Th≦HQ, the routine jumps from step S70to step S90. Conversely when the answer to step S70is affirmative (YES), that is, in case of Th>HQ, the routine proceeds from step S70to step S80.

At step S80, a limiter processing is made. That is, the upper limit of target generator load torque Th is limited to the previously-noted maximum load capacity HQ of generator7. Thereafter, the routine flows from step S80to step S90.

At step S90, the first target motor torque Tm1corresponding to target generator load torque Th calculated at step S60is calculated. That is to say, the calculated target generator load torque Th is substituted for the first target motor torque Tm1, that is, Tm1←Th. In this manner, one cycle of the surplus torque Th calculation routine ofFIG. 4terminates.

As can be appreciated from the flow from step S10through steps S20, S40-S80to step S90, the first target motor torque Tm1can be set as a desired motor torque value substantially corresponding to the acceleration slip rate of the front road wheel side (main drive wheels1L,1R).

In the shown embodiment (in the surplus torque Th arithmetic-calculation routine shown inFIG. 4), the first target motor torque Tm1is calculated after target generator load torque (surplus engine torque) Th has been calculated (see steps S40-S60ofFIG. 4). In lieu thereof, the first target motor torque Tm1may be calculated, directly based on an acceleration slip rate of the front road wheel side (main drive wheels1L,1R), that is, front-road-wheel acceleration slip velocity ΔVF, as a function f(ΔVF) of slip velocity ΔVF.

Hereunder explained is the arithmetic processing of acceleration assist-torque calculation section8Ab.

Acceleration assist-torque calculation section8Ab arithmetically calculates or retrieves a second target motor torque Tm2, based on both of vehicle speed Vv and accelerator opening θ from a preprogrammed accelerator-opening θ versus second target motor torque Tm2characteristic map shown inFIG. 5Aand a preprogrammed vehicle speed Vv versus second target motor torque Tm2characteristic map shown inFIG. 5B. Accelerator opening θ means a vehicle-acceleration indicated value. As can be seen from the accelerator-opening θ versus second target motor torque Tm2characteristic map ofFIG. 5A, the second target motor torque Tm2increases, as accelerator opening θ increases, and additionally the second target motor torque Tm2decreases, as vehicle speed Vv decreases. Also, as can be seen from the vehicle speed Vv versus second target motor torque Tm2characteristic map shown inFIG. 5B, the second target motor torque Tm2is set to “0”, when vehicle speed Vv is greater than or equal to a predetermined vehicle speed value V1, corresponding to a predetermined low speed value above which the vehicle may be assumed to have escaped from the starting period.

As can be seen from the block diagram ofFIG. 3, motor torque determination section8Ac receives input information (Tm1) from surplus torque calculation section8Aa and input information (Tm2) from acceleration assist-torque calculation section8Ab. Within the motor torque determination section8Ac, a so-called select-HIGH process is made to select a higher one of the first and second target motor torques Tm1and Tm2in accordance with the expression Tm=MAX(Tm1, Tm2), and to determine or set the higher one as a final target motor torque (simply, a target motor torque) Tm.

Additionally, motor torque determination section8Ac is connected to motor TCS section (motor traction-control-system section)8G, for receiving input information concerning a state of a rear-wheel acceleration-slip indicative flag (simply, a rear-wheel slip flag) Rslip from motor TCS section8G. When rear-wheel slip flag Rslip is set to “1”, that is, Rslip=1 (or Rslip is ON), and thus the processor (exactly, motor TCS section8G) of 4WD controller8determines or estimates that acceleration slip takes place at the rear road wheel side (subsidiary drive wheels3L,3R), target motor torque Tm, obtained through the select-HIGH process Tm=MAX(Tm1, Tm2) of motor torque determination section8Ac, is decreasingly compensated for or reduced by a reduced torque value ΔTm, that is, Tm=Tm−ΔTm. The reduced torque value ΔTm means a reduced torque value of target motor torque Tm, corresponding to the acceleration slip rate of the rear road wheel side (subsidiary drive wheels3L,3R). Therefore, in case of Rslip=1, i.e., in case Rslip is ON, that is, in presence of acceleration slip at the rear road wheel side (subsidiary drive wheels3L,3R), the decreasingly-compensated target motor torque (Tm−ΔTm) is output from motor torque determination section8Ac to motor variable adjustment section8B. In contrast, when rear-wheel slip flag Rslip is reset to “0”, that is, Rslip=0 (or Rslip is OFF), and thus the processor (exactly, motor TCS section8G) of 4WD controller8determines or estimates that there is no acceleration slip at the rear road wheel side (subsidiary drive wheels3L,3R), target motor torque Tm, obtained through the select-HIGH process Tm=MAX(Tm1, Tm2) of motor torque determination section8Ac, is output to motor variable adjustment section8B. Motor TCS section8G, motor torque determination section8Ac, motor variable adjustment section8B, and generator control section8F, (in particular, motor TCS section8G), capable of decreasingly compensating target motor torque Tm, in other words, electric power output of generator7, in presence of acceleration slip at the subsidiary drive wheel side (rear road wheels3L,3R), construct an electric power output suppression circuitry (electric power output suppression means).

The arithmetic and logic operation executed within motor variable adjustment section8B is hereunder described in detail in reference to the flow chart ofFIG. 6. The routine shown inFIG. 6is also executed as time-triggered interrupt routines to be triggered every predetermined sampling time intervals such as 10 milliseconds.

At step S200, a check is made to determine whether target motor torque Tm is greater than “0”. When the answer to step S200is affirmative (Tm>0), the processor (motor variable adjustment section8B) of 4WD controller8determines that there is a motor driving requirement for driving subsidiary drive wheels (rear road wheels3L,3R) by motor4and additionally the vehicle is conditioned in the 4WD mode at which acceleration slip may occur at main drive wheels (front road wheels1L,1R). In case of Tm>0, the routine proceeds from step S200to step S210. Conversely when the answer to step S200is negative (Tm≦0), the processor (motor variable adjustment section8B) of 4WD controller8determines that there is no motor driving requirement for driving subsidiary drive wheels (rear road wheels3L,3R) by motor4and additionally the vehicle is not conditioned in the 4WD mode. Thus, in case of Tm≦0, the routine proceeds from step S200to step S310.

At step S210, a check is made to determine whether a 4WD-to-2WD transition from the 4WD mode to the 2WD mode occurs. When the answer to step S210is affirmative, that is, in presence of the 4WD-to-2WD transition, the routine proceeds from step S210to step S310. Conversely when the answer to step S210is negative, that is, in absence of the 4WD-to-2WD transition, in other words, when the 4WD mode is continuously selected, the routine proceeds from step S210to step S220. For instance, when the processor of 4WD controller8determines that motor speed Nm is approaching closer to a permissible limit level, or when the selected operating range of transmission30is a non-drive range such as a parking (P) range or a neutral (N) range, the processor (motor variable adjustment section8B) of 4WD controller8determines that there is a 4WD-to-2WD transition from the 4WD mode to the 2WD mode.

At step S310, the 4WD mode termination processing (including the power-generation stop processing (Vm=0)) is executed, and then the routine ofFIG. 6returns to the main program. Concretely, according to the 4WD mode termination processing of step S310, a power-generation stop signal is generated from motor variable adjustment section8B to adjust or control a generated voltage Vm, needed to attain target motor torque Tm, to “0”, that is, Vm=0.

At step S230, a target value of armature current Ia of motor4is calculated or retrieved, based on the latest up-to-date information about the target value of motor field current Ifm and target motor torque Tm (see a preprogrammed Ifm−Tm−Ia characteristic map shown in the block of step S230shown inFIG. 6). After step S230, step S240occurs.

At step S240, generated voltage Vm, needed to attain target motor torque Tm, is arithmetically calculated based on the target value of armature current Ia, from the expression Vm=Ia×R+E, where E denotes the induced voltage of motor4, and R denotes a value of resistance between generator7and motor4. In this manner, one cycle of the routine ofFIG. 6terminates.

On the other hand, generator control section8F is comprised of a power generation variable adjustment section8Fa and a power generation control section8Fb. Power generation variable adjustment section8Fa substitutes the generated voltage Vm, needed to attain target motor torque Tm and determined by motor variable adjustment section8B, for a target generated voltage Vt, that is, Vt←Vm, and additionally outputs a signal indicative of target generated voltage Vt to power generation control section8Fb. Power generation control section8Fb calculates or computes a target generator field current value (a target value of generator field current Ifh), needed to attain the target generated voltage Vt, on the basis of the latest up-to-date information about target generated voltage Vt and generator output voltage Vg. Thereafter, power generation control section8Fb calculates or computes a generator control command (a given duty ratio) C1corresponding to the computed target value of generator field current Ifh. The generator control command (the given duty ratio) C1is output from power generation control section8Fb of generator control section8F to voltage regulator22of generator7, and as a result generator output voltage Vg can be properly controlled or regulated.

Referring now toFIG. 7, there is shown the arithmetic and logic operation executed within motor TCS section8G of 4WD controller8. The routine shown inFIG. 7is also executed as time-triggered interrupt routines to be triggered every predetermined sampling time intervals such as 10 milliseconds.

At step S410, a check is made to determine whether acceleration slip greater than or equal to a predetermined slip rate, such as 0.01 G, occurs at the rear road wheel side (subsidiary drive wheels3L,3R). When the answer to step S410is affirmative (YES), that is, when the acceleration slip rate of the rear road wheel side is greater than or equal to the predetermined slip rate, the routine proceeds from step S410to step S420. Conversely when the answer to step S410is negative (NO), that is, when the acceleration slip rate of the rear road wheel side is less than the predetermined slip rate, the routine proceeds from step S410to step S430. Concretely, the acceleration slip rate of the rear road wheel side can be estimated or detected or calculated by way of acceleration of each of rear road wheels3L and3R or by subtracting a front wheel speed (i.e., a main drive wheel speed) from a rear wheel speed (i.e., a subsidiary drive wheel speed). When utilizing the acceleration of each of rear road wheels3L and3R for estimation purposes of the rear-wheel acceleration slip rate, an actual acceleration rate of each of rear road wheels3L and3R may be detected by means of a wheel acceleration sensor. In lieu thereof, the acceleration of each of rear road wheels3L and3R may be obtained by differentiating rear-left and rear-right wheel speeds VwRL and VwRR detected by rear road wheel speed sensors27RL and27RR. When the detected or calculated acceleration of the rear road wheel side is greater than or equal to a predetermined value, the processor (motor TCS section8G) of 4WD controller8determines that acceleration slip of the predetermined slip rate or more takes place at the rear road wheel side (subsidiary drive wheels3L,3R). Alternatively, the difference (Vwr−Vwf) between the rear wheel speed (i.e., average rear wheel speed Vwr) and the front wheel speed (i.e., average front wheel speed Vwf) may be utilized for estimation purposes of the rear-wheel acceleration slip rate. In such a case, when the difference (ΔVR=Vwr−Vwf) between the rear wheel speed and the front wheel speed is greater than or equal to a predetermined value, the processor (motor TCS section8G) of 4WD controller8determines that acceleration slip of the predetermined slip rate or more takes place at the rear road wheel side (subsidiary drive wheels3L,3R). In the shown embodiment, the acceleration slip velocity (or the acceleration slip rate) ΔVF of the front road wheel side and the acceleration slip velocity (or the acceleration slip rate) ΔVR of the rear road wheel side are collectively referred to as “ΔV”.

At step S430, rear-wheel slip flag Rslip is reset to “0”, that is, Rslip=0 (or Rslip is OFF), and then the routine ofFIG. 7returns to the main program.

At step S440, a reduced load torque value ΔTh of target generator load torque Th, corresponding to the reduced torque value ΔTm of target motor torque Tm, is arithmetically calculated, and therefore a signal indicative of the reduced load torque value ΔTh of target generator load torque Th is output from motor TCS section8G to engine controller18. In this manner, one execution cycle of the routine ofFIG. 7terminates. More concretely, the reduced load torque value ΔTh of target generator load torque Th is arithmetically calculated based on both of the reduced torque value ΔTm of target motor torque Tm) calculated based on the rear-wheel acceleration slip rate and a conversion value ΔNm converted from a wheel speed (exactly, a wheel-speed deviation) corresponding to the rear-wheel acceleration slip rate (e.g., a rear-wheel acceleration slip velocity ΔVR=Vwr−Vwf) into a motor speed of motor4, from the following three expressions.
ΔP=ΔTm×ΔNm÷ηm×(2π/60)
ΔW=ΔP÷ηg
ΔTh=ΔW÷Nh÷(2π/60)
where ΔP denotes a reduced value of electric power output of generator7, ΔTm denotes the reduced target motor torque value corresponding to the rear-wheel acceleration slip rate (e.g., ΔVR=Vwr−Vwf), in other words, the reduced torque value of target motor torque Tm to be subtracted in synchronism with switching to the operative state of motor TCS section8G, ΔNm denotes the conversion value converted from the wheel speed corresponding to the rear-wheel acceleration slip rate (e.g., a rear-wheel acceleration slip velocity ΔVR=Vwr−Vwf) into the motor speed of motor4, ηm denotes a motor efficiency (unit: %), ΔW denotes a reduced generated electric energy (or a reduced value of load torque for power generation of generator7, simply a reduced power-generation load), ηg denotes a generator efficiency (unit: %), and Nh denotes the generator speed.

The previously-noted expressions for calculation of the reduced load torque value ΔTh of target generator load torque Th, are determined based on the reasons discussed below.

A generator's power output Δp(−) produced by generator7before motor TCS section8G shifts to the operative state, is represented by the following expression. The generator's power output Δp(−) is referred to as “before-operation generator's power output Δp(−)”.
Δp(−)=Tm_before×Nm_before÷ηm×(2π/60)
where Tm_before denotes a motor torque value of motor torque produced by motor4before motor TCS section8G shifts to the operative state, Nm_before denotes a motor speed value of motor4before motor TCS section8G shifts to the operative state, and ηm denotes the motor efficiency.

In a similar manner, a generator's power output Δp(+) produced by generator7after motor TCS section8G has shifted to the operative state, is represented by the following expression. The generator's power output Δp(+) is referred to as “after-operation generator's power output Δp(+)”.
Δp(+)=Tm_after×Nm_after÷ηm×(2π/60)
where Tm_after denotes a motor torque value of motor torque produced by motor4after motor TCS section8G has shifted to the operative state, Nm_after denotes a motor speed value of motor4after motor TCS section8G has shifted to the operative state, and ηm denotes the motor efficiency.

Therefore, a reduced power-generation load torque output Δp corresponding to the reduced target motor torque value ΔTm is represented by the following equations.

From the above equations, the reduced value ΔP of electric power output of generator7can be represented by the expression ΔP=ΔTm×ΔNm÷ηm×(2π/60). The difference (Nm_before−Nm_after) of before-operation motor speed value Nm_before and after-operation motor speed value Nm_after can be estimated or determined based on rear wheel speeds VwRL and VwRR. The reduced torque value (in other words, torque-down value) ΔTm of target motor torque Tm is determined as a variable substantially corresponding to the rear-wheel acceleration slip rate (e.g., ΔVR=Vwr−Vwf). In lieu thereof, the reduced target motor torque value ΔTm may be fixed to a predetermined constant. Motor TCS section8G serves as a part of the electric power output suppression circuitry (the electric power output suppression means).

Referring now toFIG. 8, there is shown the arithmetic and logic operation executed within engine controller18. The routine shown inFIG. 8is also executed as time-triggered interrupt routines to be triggered every predetermined sampling time intervals such as 10 milliseconds.

At step S600, an acceleration slip rate ΔV of the front road wheel side (main drive wheels1L,1R) is calculated. The acceleration slip rate ΔV corresponds to the previously-noted acceleration slip velocity ΔVF of the front road wheel side (main drive wheels1L,1R). After step S600, step S610occurs.

At step S610, a check is made to determine whether acceleration slip rate ΔV of the front road wheel side (main drive wheels1L,1R) exceeds a target front-wheel acceleration slip rate Tslip. When the answer to step S610is affirmative (YES), that is, in case of ΔV>Tslip, the routine proceeds from step S610to step S660. Conversely when the answer to step S610is negative (NO), that is, in case of ΔV≦Tslip, the routine proceeds from step S610to step S620. Target acceleration slip rate Tslip is preset to a predetermined slip rate, concretely, a relatively higher level, such as 10%, as compared to the previously-described predetermined power-generation threshold value Tpg (see step S20ofFIG. 4), such as “0”. Setting target acceleration slip rate Tslip to a relatively higher level, such as 10%, contributes to prevention of insufficient motor torque. In other words, target acceleration slip rate Tslip functions as a predetermined engine-TCS-control enabling threshold value or a predetermined engine TCS initiation threshold value or a predetermined engine TCS intervention threshold value above which an engine traction-control-system (simply, an engine TCS system) comes into operation so as to effectively suppress or reduce engine power output (engine output torque), and thus to suppress acceleration slip of the front road wheel (main drive wheels1L,1R).

At step S620, a driver-required target engine output torque TeN, indicative of a target engine output torque required by the driver, is determined based on the sensor signal from accelerator sensor40, indicative of accelerator opening θ. After step S620, step S623occurs.

At step S623, a check is made to determine whether rear-wheel slip flag Rslip is set to “1” (Rslip is ON). When rear-wheel slip flag Rslip is set (=1) and thus the answer to step S623is affirmative (Rslip:ON), the routine proceeds from step S623to step S626. Conversely when rear-wheel slip flag Rslip is reset (=0) and thus the answer to step S623is negative (Rslip:OFF), the routine proceeds from step S623to step S630.

At step S626, target engine output torque TeN, calculated through step S620, is decreasingly compensated or suppressed by the previously-noted reduced target generator load torque value ΔTh (calculated through step S440of the routine shown inFIG. 7executed within motor TCS section8G), corresponding to the reduced torque value (torque-down value) ΔTm of target motor torque Tm, taking into account the rear-wheel acceleration slip. After step S626, step S630occurs. Steps S623and S626, provided for decreasingly compensating for target engine output torque TeN by the reduced load torque value ΔTh of target generator load torque Th in presence of acceleration slip at the subsidiary drive wheel side (rear road wheels3L,3R), constructs a target engine output torque reduction circuitry (target engine output torque reduction means). The target engine output torque reduction circuitry (target engine output torque reduction means) is regarded as a subsidiary-drive-wheel acceleration-slip period engine output torque reduction circuitry (subsidiary-drive-wheel acceleration-slip period engine output torque reduction means).

At step S630, the current value of engine output torque Te of engine2is calculated or determined based on at least one of the sub-throttle opening α of sub-throttle valve16and engine speed Ne. After step S630, step S640occurs.

At step S640, a deviation ΔTe between the calculated target engine output torque TeN and the current engine output torque Te is arithmetically calculated from the expression ΔTe=TeN−Te. After step S640, step S650occurs.

At step S650, a change Δα in sub-throttle opening α (exactly, repetition of an increase and a decrease in the sub-throttle opening), corresponding to engine-output-torque deviation ΔTe, is calculated or computed. A throttle-opening control command signal corresponding to the change Aa in sub-throttle opening α, is output to step motor19serving as a sub-throttle actuator. In explaining the embodiment, for the purpose of simplification of the disclosure, it is indicated that the throttle-opening control command signal corresponding to the sub-throttle opening change Δα is output to step motor19. Actually, in order to smoothly change or reduce or suppress the engine output torque and to avoid undesirable rapid torque change, the sub-throttle opening is cyclically changed (incremented or decremented) by a predetermined increment or a predetermined decrement every predetermined execution cycles of the engine output control executed within engine controller18.

In contrast, when the flow from step S610to step S660occurs, engine TCS control comes into operation. At step S660, for the purpose of suppression of acceleration slip of the front road wheel side (main road wheels1L,1R), a preprogrammed engine-TCS-control torque change ΔTtcs is substituted for engine-output-torque deviation ΔTe, that is, ΔTe=ΔTtcs (or ΔTe←ΔTtcs) in accordance with the engine TCS control. In the shown embodiment, preprogrammed engine-TCS-control torque change ΔTtcs is a variable, which is determined based on the front-wheel acceleration slip rate ΔVF. After step S660, step S650occurs. Step S660, capable of initiating the engine TCS control and substituting engine-TCS-control torque change ΔTtcs for engine-output-torque deviation ΔTe under the condition of ΔVF>Tslip, constructs a main-drive-wheel acceleration-slip period engine output torque suppression circuitry (main-drive-wheel acceleration-slip period engine output torque suppression means).

With the previously-described arrangement, under a particular condition where the 4WD mode is selected via drive mode switch39, the driving force control apparatus of the embodiment operates as follows.

Suppose that the magnitude of driving torque transmitted from engine2to the main drive wheel side (front road wheels1L,1R) is exceeding a grip limit (or a road-surface reaction limit) of the main drive wheel on the road owing to the low-μ road driving, or suppose that accelerator pedal17is heavily depressed by the driver. Acceleration slip may take place at the main drive wheel side (front road wheels1L,1R), that is, ΔVF>0. Under these conditions where the condition defined by the inequality ΔVF>0 is satisfied and the 4WD mode is selected by drive mode switch39, relay24becomes kept at its closed state where a pair of relay contacts are closed by means of relay control section8D. Additionally, clutch12becomes kept at its engaged state by means of clutch control section8E. Generator7performs generating operation based on the generator's power-generation load (exactly, target generator load torque Th, in other words, the first target motor torque Tm1) corresponding to the front-wheel acceleration slip rate ΔV (=ΔVF) and thus motor4is driven, so that shifting to the vehicle's 4WD mode is completed (see the flow from step S20through steps S50, S60, and S70to step S90ofFIG. 4). At this time, motor4is rotated by way of the surplus electric power generated by generator7such that subsidiary drive wheels (rear road wheels3L,3R) are driven by means of motor torque, thereby enhancing the vehicle's acceleration performance. Additionally, motor4is driven by way of the surplus driving torque exceeding the grip limit (or the road-surface reaction limit) of the main drive wheel1L,1R on the road, thereby enhancing the energy efficiency and ensuring reduced fuel consumption rate of the vehicle. After this, the magnitude of driving torque transmitted from engine2to the main drive wheel side (front road wheels1L,1R) is adjusted or converged or brought closer to the grip limit (or the road-surface reaction limit) of the main drive wheel on the road. Therefore, the operating state of the vehicle is gradually shifted from the 4WD mode to the 2WD mode, and whereby the main-drive-wheel acceleration slip can be effectively suppressed.

During the vehicle's starting period, an acceleration slip of the main drive wheel side (front road wheels1L,1R) does not develop sufficiently, and thus there is a less front-wheel acceleration slip rate. Under such a condition (Tm2>Tm1), motor4is driven so that motor torque is brought closer to the second target motor torque Tm2determined based on at least accelerator opening θ. This enhances the vehicle's acceleration performance during the starting period.

Thereafter, suppose that a front-wheel acceleration slip of an acceleration slip rate greater than the predetermined engine-TCS-control enabling threshold value Tslip, which is set to be relatively higher than the predetermined power-generation threshold value Tpg, takes place, that is, ΔVF>Tslip. The routine ofFIG. 8flows from step S610to step S660irrespective of whether accelerator pedal17is depressed or undepressed by the driver, exactly, regardless of the manipulated variable of accelerator pedal17depressed by the driver, and whereby the engine power output is effectively automatically forcibly suppressed by way of engine TCS control. As a result of this, the main-drive-wheel acceleration slip of front road wheels1L,1R can be effectively suppressed or converged.

As appreciated from the vehicle speed Vv versus second target motor torque Tm2characteristic map shown inFIG. 5B, when vehicle speed Vv is greater than or equal to the predetermined vehicle speed value V1above which the vehicle may be assumed to have escaped from the starting period, the second target motor torque Tm2is set to “0”. Under the conditions of Vv≧V1and ΔVF>0, by way of the select-HIGH process Tm=MAX(Tm1, Tm2) of motor torque determination section8Ac, the first target motor torque Tm1is selected as target motor torque Tm. Therefore, generator7is driven in such a manner as to attain the generator's power-generation load corresponding to the front-wheel acceleration slip rate ΔV (=ΔVF), in other words, absorption torque TΔVF needed to suppress the front-wheel acceleration slip, and thus the engine output torque transmitted from engine2to the main drive wheel side (front road wheels1L,1R) can be properly reduced or absorbed, thereby effectively suppressing the main-drive-wheel acceleration slip. Additionally, under the condition of ΔVF>Tslip, the engine TCS control comes into operation at once (see the flow from step S610to step S660inFIG. 8), so that the engine power output itself can be quickly timely suppressed, so as to rapidly suppress acceleration slip of the main drive wheel side (front road wheels1L,1R).

On the contrary, suppose that motor4is driven by electric power output of generator7to ensure the 4WD mode of the vehicle, and additionally an acceleration slip greater than or equal to the predetermined slip rate occurs at the rear road wheel side (subsidiary drive wheels3L,3R) owing to the low-μ road driving.

In a conventional manner, assuming that the electric power output of generator7is simply reduced without deliberation for subsidiary-drive-wheel acceleration slip suppression of rear road wheels3L,3R, the power-generation load, in other words, the generator load torque, is lightened by the reduced value of electric power output of generator7, thereby resulting in an undesirable increase in driving torque flow to front road wheels1L,1R. An acceleration slip of the main drive wheel side (front road wheels1L,1R) starts to develop, thus resulting in an undesirable engine speed rise. As a consequence, the power-generation load may be increased again, and therefore acceleration slip of the subsidiary drive wheel side (rear wheels3L,3R) may occur again. That is, there is an increased tendency for undesirable control hunting between subsidiary-drive-wheel acceleration-slip suppression control and main-drive-wheel acceleration-slip suppression control to occur.

In contrast to the above, according to the driving force control apparatus of the embodiment shown inFIGS. 7-8, in order to suppress the acceleration slip of the subsidiary drive wheel side (rear road wheels3L,3R), the engine power output (exactly, target engine output torque TeN) can be decreasingly compensated for or reduced down to the value TeN−ΔTh (see the flow from step S623to step S626) responsively to or in synchronism with a decrease in power-generation load, exactly, a reduction ΔTh in target generator load torque Th (see the flow from S410through step S420to step S440inFIG. 7). For the reasons discussed above, even if the electric power output of generator7is rapidly reduced for subsidiary-drive-wheel acceleration slip suppression of rear road wheels3L,3R, it is possible to prevent or avoid an excessive rise in engine speed Ne, thereby effectively suppressing or preventing a main-drive-wheel acceleration slip from starting to develop. Suppressing the engine power output (exactly, target engine output torque TeN) synchronously with a decrease in power-generation load (exactly, a reduction ΔTh in target generator load torque Th), made for subsidiary-drive-wheel acceleration slip suppression, means that an undesirable increase in main-drive-wheel acceleration slip can be suppressed beforehand in a similar manner to feedforward control. During the subsidiary-drive-wheel acceleration-slip suppression control, the driving force control apparatus of the embodiment, never performs such engine output torque suppression as to be executed by the engine TCS system (see step S660) under the condition of ΔVF>Tslip. That is to say, during the subsidiary-drive-wheel acceleration-slip suppression control, the driving force control apparatus of the embodiment, does not operate to suppress the engine output torque after engine traction-control-system (TCS) intervention threshold value Tslip has been reached, but quickly performs the feedforward-control like engine torque output suppression, while foreseeing or anticipating the acceleration-slip increase of the main drive wheel side (front road wheels1L,1R) in advance.

Referring now toFIGS. 9A-9E, there are shown the time charts explaining the operation of the vehicle driving force control apparatus employing motor TCS section8G of 4WD controller8executing the routine ofFIG. 7and engine controller18executing the routine ofFIG. 8.FIG. 9Ashows variations in rear wheel speed VwR.FIG. 9Bshows variations in the motor-torque command (corresponding to target motor torque Tm) for motor4by which the subsidiary drive wheels (rear road wheels3L,3R) are driven.FIG. 9Cshows variations in generator load torque carried on engine2, exactly, variations in target generator load torque Th.FIG. 9Dshows variations in engine output torque, exactly target engine output torque TeN.FIG. 9Eshows variations in engine speed Ne. As can be seen from the time charts ofFIGS. 9A-9E, when acceleration slip occur at the subsidiary drive wheels (rear road wheels3L,3R), target motor torque Tm is reduced by the reduced torque value ΔTm determined based on the rear-wheel acceleration slip rate (e.g., ΔVR=Vwr−Vwf). At the same time, as shown inFIG. 9C, target generator load torque value Th is reduced by the reduced target generator load torque value ΔTh corresponding to the reduced target motor torque value ΔTm. As shown inFIG. 9D, the engine power output (exactly, target engine output torque TeN) is decreasingly compensated for or reduced or suppressed in synchronism with the reduction ΔTh in target generator load torque Th. As can be seen from the time chart ofFIG. 9E, it is possible to avoid or suppress an undesirable rise in engine speed by reducing target engine output torque TeN in synchronism with the target generator load torque reduction ΔTh.

As previously described, according to the driving force control apparatus of the embodiment, generator7is operated based on the power-generation load corresponding to the front-wheel acceleration slip rate ΔV (=ΔVF), so that motor4drives the subsidiary drive wheels (rear road wheels3L,3R) depending on the front-wheel acceleration slip rate ΔV (=ΔVF). That is, the apparatus of the embodiment is so constructed or designed that, as a result of the enhanced vehicle acceleration performance, the acceleration slip of the main drive wheels (front road wheels1L,1R) can be properly suppressed. For the reasons discussed above, target front-wheel acceleration slip rate (predetermined engine TCS intervention threshold value) Tslip (e.g., 10%) is preset to be relatively higher than predetermined power-generation threshold value Tpg (e.g., “0”). In other words, only in case that, in order to suppress the front-wheel acceleration slip, part of engine torque flow to front road wheels1L,1R has been utilized for power generation and thereafter the front-wheel acceleration slip cannot yet be adequately suppressed or converged, the engine TCS function is engaged or enabled. Setting of target front-wheel acceleration slip rate (engine TCS intervention threshold value) Tslip (e.g., 10%) relatively higher than predetermined power-generation threshold value Tpg (e.g., “0”), prevents the engine TCS function from being undesirably engaged at an acceleration-slip point where a front-wheel acceleration slip would start to develop or at an excessively earlier timing of front-wheel acceleration slip occurrence. Inhibiting or disengaging the engine TCS function at the excessively earlier timing of front-wheel acceleration slip occurrence, prevents or avoids a lack of motor torque produced by motor4, in other words, insufficient power generation of generator7.

In the shown embodiment, target motor torque Tm is decreasingly compensated for or reduced by the reduced torque value ΔTm corresponding to the rear-wheel acceleration slip rate (e.g., ΔVR=Vwr−Vwf), depending on whether an acceleration slip of the subsidiary drive wheel (rear road wheels3L,3R) is greater than or equal to the predetermined slip rate, that is, depending on the state (Rslip:ON or Rslip:OFF) of rear-wheel slip flag Rslip. The reduced load torque value ΔTh of target generator load torque Th is calculated based on the reduced torque value ΔTm of target motor torque Tm, through step S440ofFIG. 7. On the other hand, target generator load torque Th itself is calculated by the surplus torque Th arithmetic-calculation routine shown inFIG. 4, separately from the routine ofFIG. 7for calculation of the reduced torque value ΔTm. Instead of using the reduced torque value ΔTm of target motor torque Tm, the reduced load torque value ΔTh of generator7may be calculated as a function f(Vv) of vehicle speed Vv. In lieu thereof, the reduced load torque value ΔTh of generator7may be preset as a predetermined fixed value. Alternatively, in the surplus torque Th arithmetic-calculation routine shown inFIG. 4, through which the first target motor torque Tm1can be calculated taking into account the rear-wheel acceleration slip rate (e.g., ΔVR=Vwr−Vwf), in presence of the rear-wheel acceleration slip (i.e., Rslip:ON), the surplus engine torque (target generator load torque Th) may be decreasingly compensated for or reduced by the reduced load torque value ΔTh just before step S90of the routine ofFIG. 4, executed within surplus torque calculation section8Aa of 4WD controller8.

Referring now toFIG. 10, there is shown the modified arithmetic and logic operation executed within engine controller18. The modified arithmetic and logic processing ofFIG. 10is similar to the processing ofFIG. 8, except that steps S623and S626included in the routine shown inFIG. 8are eliminated. Thus, the same step numbers used to designate steps in the routine shown inFIG. 8will be applied to the corresponding step numbers used in the modified arithmetic and logic processing shown inFIG. 10, for the purpose of comparison of the two different interrupt routines. Detailed description of steps S600, S610, S620, S630, S640, S650, and S660shown inFIG. 10will be omitted because the above description thereon seems to be self-explanatory.

Referring now toFIG. 11, there is shown the modified arithmetic and logic operation executed within motor TCS section8G of 4WD controller8.

The countermeasure for a main-drive-wheel acceleration slip which may occur due to the target generator load torque reduction executed in presence of a subsidiary-drive-wheel acceleration slip, achieved by the modified vehicle driving force control apparatus employing engine controller18executing the routine ofFIG. 10and motor TCS section8G executing the routine ofFIG. 11, is somewhat different from that of the vehicle driving force control apparatus of the embodiment employing engine controller18executing the routine ofFIG. 8and motor TCS section8G executing the routine ofFIG. 7. In particular, the processing (seeFIG. 11) executed within motor TCS section8G of 4WD controller incorporated in the modified vehicle driving force control apparatus shown inFIGS. 10-11considerably differs from the processing (seeFIG. 7) executed within motor TCS section8G of 4WD controller incorporated in the vehicle driving force control apparatus of the embodiment shown inFIGS. 7-8. Details of the modified arithmetic and logic operation executed within motor TCS section8G are hereunder described in reference to the flow chart ofFIG. 11.

At step S710ofFIG. 11, in a similar manner to step S410ofFIG. 7, a check is made to determine whether a rear-wheel acceleration slip rate is greater than or equal to a predetermined slip rate. When the answer to step S710is affirmative (YES), that is, when the rear-wheel acceleration slip rate is greater than or equal to the predetermined slip rate, the routine proceeds from step S710to step S720. Conversely when the answer to step S710is negative (NO), that is, when the rear-wheel acceleration slip rate is less than the predetermined slip rate, the routine proceeds from step S710to step S740. Step S710ofFIG. 11and step S410ofFIG. 7, and wheel speed sensors27FL-27RR serve as a subsidiary-drive-wheel acceleration slip estimation circuitry (a subsidiary-drive-wheel acceleration slip detector or subsidiary-drive-wheel acceleration slip estimation means).

At step S720, rear-wheel slip flag Rslip is set to “1”, that is, Rslip=1 (or Rslip is ON). After step S720, step S730occurs.

At step S730, target front-wheel acceleration slip rate (engine TCS intervention threshold value) Tslip, which is used as a decision criterion for determining the presence or absence of a main-drive-wheel acceleration slip of front road wheels1L,1R, and for determining the starting point of engine TCS control, is altered to a predetermined low threshold value Tslip1lower than an initial threshold value Tslip0.

At step S740, a check is made to determine whether rear-wheel slip flag Rslip is set to “1”, that is, Rslip=1 (or Rslip is ON). When the answer to step S740is affirmative (Rslip:ON), the routine proceeds from step S740to step S750. Conversely when the answer to step S740is negative (Rslip:OFF), the routine returns' to the main program. At the starting point of the interrupt routine ofFIG. 11, rear-wheel slip flag Rslip is initialized to “OFF”, in other words, Rslip=0.

At step S750, a check is made to determine whether the current value of engine TCS intervention threshold value Tslip is greater than or equal to the initial threshold value Tslip0. When the answer to step S750is negative (Tslip<Tslip0), the routine proceeds from step S750to step S760. Conversely when the answer to step S750is affirmative (Tslip≧Tslip0), the routine proceeds from step S750to step S770.

At step S760, engine TCS intervention threshold value Tslip is incremented by a predetermined increment ΔTslip.

At step S770, rear-wheel slip flag Rslip is reset to “0”, that is, Rslip=0 (or Rslip is OFF). By way of repeated executions of a series of steps S740, S750, S760, and S770, engine TCS intervention threshold value Tslip does not rapidly return to the initial threshold value Tslip0, but gradually increases and approaches to the initial threshold value Tslip0by the predetermined increment ΔTslip every execution cycles. Motor TCS section8G (in particular, steps S720-S730of the routine shown inFIG. 11), serves as an acceleration slip threshold value alteration circuitry (acceleration slip threshold value alteration means). The acceleration slip threshold value alteration circuitry (acceleration slip threshold value alteration means) is also regarded as the subsidiary-drive-wheel acceleration-slip period engine output torque reduction circuitry (subsidiary-drive-wheel acceleration-slip period engine output torque reduction means).

According to the modified driving force control apparatus shown inFIGS. 10-11, when a rear-wheel acceleration slip occurs, power-generation load (target generator load torque Th) is decreasingly compensated for or reduced depending on the degree of the rear-wheel slip rate. At the same time, target front-wheel acceleration slip rate (engine TCS intervention threshold value) Tslip, which is used as a criterion for determining the starting point of engine TCS control, is rapidly altered down to a low level (Tslip1), with the result that the engine TCS system comes into operation at an earlier timing of rear-wheel acceleration slip occurrence. As a result, as soon as the power-generation load, in other words, the generator load torque, is lightened in order to suppress the rear-wheel acceleration slip, a front-wheel acceleration slip rate, on the one hand, tends to increase owing to an increase in engine speed Ne, but the engine TCS function, on the other hand, is able to be quickly engaged at the earlier timing of rear-wheel acceleration slip occurrence, thus effectively timely suppressing the engine power output. As a consequence, even when the rear-wheel acceleration slip occurs, it is possible to effectively optimally converge or suppress the front-wheel acceleration slip as well as the rear-wheel acceleration slip. Additionally, from the point of time when the rear-wheel acceleration slip has been satisfactorily suppressed and converged, target front-wheel acceleration slip rate (engine TCS intervention threshold value) Tslip is not rapidly returned or recovered to the initial threshold value Tslip0, but gradually stepped up to the initial threshold value Tslip0by predetermined increment ΔTslip every execution cycles. Gradually stepping up the threshold value) Tslip avoids a rear-wheel acceleration slip from occurring again owing to a rapid increase in engine power output, thereby effectively avoiding or suppressing a risk of undesirable control hunting between subsidiary-drive-wheel acceleration-slip suppression control and main-drive-wheel acceleration-slip suppression control.

Referring now toFIGS. 12A-12F, there are shown the time charts explaining the operation of the modified vehicle driving force control apparatus employing motor TCS section8G of 4WD controller8executing the routine ofFIG. 11and engine controller18executing the routine ofFIG. 10.FIG. 12Ashows variations in rear wheel speed VwR.FIG. 12Bshows variations in the motor-torque command (corresponding to target motor torque Tm) for motor4by which the subsidiary drive wheels (rear road wheels3L,3R) are driven.FIG. 12Cshows variations in rear-wheel slip flag Rslip.FIG. 12Dshows alteration of target front-wheel acceleration slip rate (engine TCS intervention threshold value) Tslip.FIG. 12Eshows variations in each of front-wheel speed VwF and rear-wheel speed VwR.FIG. 12Fshows variations in engine speed Ne. As can be seen from the time charts ofFIGS. 12A-12F, when the rear-acceleration slip rate exceeds the predetermined slip rate, target motor torque Tm is reduced by the reduced torque value ΔTm determined based on the rear-wheel acceleration slip rate (e.g., ΔVR=Vwr−Vwf). At the time t1, rear-wheel slip flag Rslip becomes ON, that is, Rslip=1 (seeFIG. 12C). As shown inFIG. 12D, the target front-wheel acceleration slip rate (engine TCS intervention threshold value) Tslip is rapidly altered down to a low level Tslip1in synchronism with the reduction ΔTm in target motor torque for motor4, in other words, the reduction in power-generation load of generator7. Due to alteration of engine TCS intervention threshold value Tslip down to the low level Tslip1(lower than the initial threshold value Tslip0), immediately when the power-generation load is lightened to suppress the rear-wheel acceleration slip, a front-wheel acceleration slip rate, on the one hand, tends to increase owing to the engine-speed increase, but the engine TCS system, on the other hand, quickly comes into operation at the earlier timing of rear-wheel acceleration slip occurrence. As a result of this, it is possible to properly timely suppress the engine power output, thus effectively suppressing the front-wheel acceleration slip (see the moderate change in front wheel speed VwF ofFIG. 12E) as well as the rear-wheel acceleration slip (see the moderate change in rear wheel speed VwR ofFIG. 12E). As can be appreciated from the time period (t2-t2) of the time chart ofFIG. 12F, it is possible to avoid or suppress an undesirable engine speed rise by altering engine TCS intervention threshold value Tslip down to the low level Tslip1(<Tslip0) in synchronism with the power-generation load reduction. Additionally, as can be appreciated from the wheel-speed change (VwF, VwR) after the time t2of the time chart ofFIG. 12F, it is possible to avoid or suppress an undesirable engine speed rise by gradually increasing or recovering engine TCS intervention threshold value Tslip from the predetermined low threshold value Tslip1to the initial threshold value Tslip0even after the rear-wheel acceleration slip has been converged and suppressed.

Suppose that engine TCS intervention threshold value Tslip is maintained at the predetermined low threshold value Tslip1for a comparatively long time period. In such a case, the engine TCS system may be engaged always at an earlier timing of rear-wheel acceleration slip occurrence, and whereby engine power output is forcibly reduced by engine TCS control for front-wheel acceleration slip suppression. This leads to the problem of insufficient power generation of generator7, that is, an undesirably reduced motor torque. For the reasons discussed above, preferably, the time period (t2-t1), during which engine TCS intervention threshold value Tslip is held at the predetermined low threshold value Tslip1(<Tslip0), has to be set to a short time period.

In the modified vehicle driving force control apparatus shown inFIGS. 10-12F, engine TCS intervention threshold value Tslip is rapidly altered down to the predetermined low threshold value Tslip1(<Tslip0), regardless of the magnitude of subsidiary-drive-wheel acceleration slip of rear road wheels3L,3R. In lieu thereof, engine TCS intervention threshold value Tslip may be a variable. That is, in order to ensure a better starting point of the engine TCS, it is more preferable that engine TCS intervention threshold value Tslip decreases, as the rear-wheel acceleration slip rate increases.

The entire contents of Japanese Patent Application No. 2004-172588 (filed Jun. 10, 2004) are incorporated herein by reference.

While the foregoing is a description of the preferred embodiments carried out the invention, it will be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the scope or spirit of this invention as defined by the following claims.