FOUR-WHEEL DRIVE VEHICLE

A four-wheel drive vehicle includes a drive-power distribution device including (a) a clutch for distributing an engine drive power, between main and auxiliary drive wheels, (b) an electric motor, (c) a press mechanism for pressing the clutch by converting a rotary motion of the electric motor into a linear motion. The drive-power distribution device adjusts a torque capacity of the clutch to adjust a drive-power distribution ratio between the main and auxiliary drive wheels. The vehicle further includes a control apparatus for executing a drive-power distribution control for adjusting the drive-power distribution ratio, and an automatic-stop control for causing the engine to be automatically stopped upon satisfaction of an engine-stop condition. When the engine is in a stop state by execution of the automatic-stop control, the control apparatus inhibits change of the drive-power distribution ratio which is to be made by change of a rotational direction of the electric motor.

This application claims priority from Japanese Patent Application No. 2020-085538 filed on May 14, 2020, the disclosure of which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a four-wheel drive vehicle in which a ratio of distribution of a drive power between main and auxiliary drive wheels is adjustable.

BACKGROUND OF THE INVENTION

There is well-known a four-wheel drive vehicle including: main drive wheels and auxiliary drive wheels; a drive-power distribution device capable of transmitting a drive power of a drive power source to the main drive wheels and the auxiliary drive wheels and adjusting a drive-power distribution ratio that is a ratio of distribution of the drive power between the main drive wheels and the auxiliary drive wheels; an engine serving as the drive power source and configured to output the drive power; and a control apparatus configured to execute a drive-power distribution control for adjusting the drive-power distribution ratio, and configured to execute an engine automatic-stop control for causing the engine to be automatically stopped upon satisfaction of an engine-stop condition. A four-wheel drive vehicle is disclosed in WO/2011/042951 is an example of such a vehicle. Further, JP-2010-151309A discloses a drive-power distribution device corresponding to the above-described drive-power distribution device. The drive-power distribution device disclosed in this Japanese Patent application Publication includes (a) a drive-power distribution clutch configured to distribute the drive power between the main drive wheels and the auxiliary drive wheels, (b) an electric motor, (c) a press mechanism configured to press the drive-power distribution clutch by converting a rotary motion of the electric motor into a linear motion in an axial direction of the drive-power distribution clutch, wherein the drive-power distribution device is configured to adjust a torque capacity of the drive-power distribution clutch so as to adjust the drive-power distribution ratio.

SUMMARY OF THE INVENTION

By the way, in the drive-power distribution device described in the above-identified JP-2010-151309A, when the drive-power distribution ratio is changed by change of a rotational direction of the electric motor, a rattling noise could be generated due to play or backlash between members constituting the press mechanism, more precisely, due to inversion of a direction in which the backlash is to be eliminated. Meanwhile, when the engine is in a stop state by execution of the engine automatic-stop control, a background noise is smaller than when the engine is operated. Thus, in a four-wheel drive vehicle provided with the drive-power distribution device, there is a problematic reduction of NV performance if the rattling noise is generated when the engine is in the stop state by execution of the engine automatic-stop control. The NV is a generic term including noise generated in the vehicle and vibration sensible by a driver and passengers in the vehicle. The NV performance is, for example, a performance for suppressing or preventing generation of the NV and/or making the driver and passengers being hardly influenced by the NV.

The present invention was made in view of the background art described above. It is therefore an object of the present invention to provide a four-wheel drive vehicle capable of improving the NV performance when the engine is in the stop state by execution of the engine automatic-stop control.

The object indicated above is achieved according to the following aspects of the present invention.

According to a first aspect of the invention, there is provided a four-wheel drive vehicle comprising: main drive wheels and auxiliary drive wheels; a drive-power distribution device including (a) a drive-power distribution clutch configured to distribute a drive power of a drive power source, between the main drive wheels and the auxiliary drive wheels, (b) an electric motor, (c) a press mechanism configured to press the drive-power distribution clutch by converting a rotary motion of the electric motor into a linear motion in an axial direction of the drive-power distribution clutch, the drive-power distribution device being configured to adjust a torque capacity of the drive-power distribution clutch so as to adjust a drive-power distribution ratio that is a ratio of distribution of the drive power between the main drive wheels and the auxiliary drive wheels; an engine serving as the drive power source and configured to output the drive power; and a control apparatus configured to execute a drive-power distribution control for adjusting the drive-power distribution ratio, and configured to execute an engine automatic-stop control for causing the engine to be automatically stopped upon satisfaction of an engine-stop condition, wherein the control apparatus is configured, when the engine is in a stop state by execution of the engine automatic-stop control, to inhibit change of the drive-power distribution ratio which is to be made by switch or change of a rotational direction of the electric motor. For example, the engine is configured to output the drive power directly and/or indirectly through conversion between a mechanical power and an electric power. It is also noted that the term “change of the drive-power distribution ratio which is to be made by change of a rotational direction of the electric motor” may be interpreted to mean “the change of the drive-power distribution ratio made by the rotation of the electric motor in a direction that is opposite to a direction in which the electric motor is rotated last time before the engine is stopped”.

According to a second aspect of the invention, in the four-wheel drive vehicle according to the first aspect of the invention, the control apparatus is configured, when a running speed of the four-wheel drive vehicle is lower than a threshold value, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, wherein the control apparatus is configured, when the running speed is not lower than the threshold value, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to a third aspect of the invention, in the four-wheel drive vehicle according to the first or second aspect of the invention, the control apparatus is configured, when a yaw rate of the four-wheel drive vehicle is lower than a threshold value, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, wherein the control apparatus is configured, when the yaw rate is not lower than the threshold value, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to a fourth aspect of the invention, in the four-wheel drive vehicle according to any one of the first through third aspects of the invention, the control apparatus is configured, when a steering angle of the four-wheel drive vehicle is smaller than a threshold value, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, wherein the control apparatus is configured, when the steering angle is not smaller than the threshold value, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to a fifth aspect of the invention, in the four-wheel drive vehicle according to any one of the first through fourth aspects of the invention, the control apparatus is configured, when the four-wheel drive vehicle is running straight, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, wherein the control apparatus is configured, when the four-wheel drive vehicle is turning, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to a sixth aspect of the invention, in the four-wheel drive vehicle according to any one of the first through fifth aspects of the invention, the control apparatus is configured to execute a vehicle attitude control for assuring a running stability of the four-wheel drive vehicle, wherein the control apparatus is configured, when not executing the vehicle attitude control, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, and wherein the control apparatus is configured, when executing the vehicle attitude control, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to a seventh aspect of the invention, in the four-wheel drive vehicle according to any one of the first through sixth aspects of the invention, the control apparatus is configured, when an outside temperature that is a temperature outside the four-wheel drive vehicle is not lower than a threshold value, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, wherein the control apparatus is configured, when the outside temperature is lower than the threshold value, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to an eighth aspect of the invention, in the four-wheel drive vehicle according to any one of the first through seventh aspects of the invention, the control apparatus is configured, when a braking operation amount or a requested braking amount in the four-wheel drive vehicle is smaller than a threshold value, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, wherein the control apparatus is configured, when the braking operation amount or the requested braking amount is not smaller than the threshold value, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to a ninth aspect of the invention, in the four-wheel drive vehicle according to any one of the first through eighth aspects of the invention, the control apparatus is configured, when an accelerating operation amount or a requested driving amount in the four-wheel drive vehicle is smaller than a threshold value, with the engine being in the stop state, to inhibit the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor, wherein the control apparatus is configured, when the accelerating operation amount or the requested driving amount is not smaller than the threshold value, to allow the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

According to a tenth aspect of the invention, in the four-wheel drive vehicle according to the first or second aspect of the invention, when the engine is in the stop state by the execution of the engine automatic-stop control, the control apparatus is configured to inhibit the execution of the engine automatic-stop control and to restart the engine, in a case in which the control apparatus predicts a situation that requires a higher priority to be given to suppression of change of attitude of the four-wheel drive vehicle, rather than to inhibition of the change of the drive-power distribution ratio which is to be made by the change of the rotational direction of the electric motor.

In the four-wheel drive vehicle according to the first aspect of the invention, when the engine is in the stop state by execution of the engine automatic-stop control, the change of the drive-power distribution ratio, which is to be made by change of the rotational direction of the electric motor, is inhibited, so that it is possible to prevent a rattling noise from being generated due to play or backlash between members constituting the press mechanism when a background noise is small, by avoiding inversion of a direction in which the backlash is to be eliminated. Therefore, when the engine is in the stop state by execution of the engine automatic-stop control, the NV performance can be improved.

In the four-wheel drive vehicle according to the second aspect of the invention, when the running speed of the four-wheel drive vehicle is lower than the threshold value, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the running speed is not lower than the threshold value, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Therefore, when the running speed is not lower than the threshold value, namely, when the background noise is large, it is possible to assure a vehicle controllability owing to execution of the drive-power distribution control, and accordingly to improve the NV performance while suppressing influence to the vehicle controllability.

In the four-wheel drive vehicle according to the third aspect of the invention, when the yaw rate of the four-wheel drive vehicle is lower than the threshold value, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the yaw rate is not lower than the threshold value, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Thus, in a situation with a steering operation being large, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to improvement of the NV performance. Therefore, it is possible to suppress change of an attitude of the vehicle and also to improve the NV performance.

In the four-wheel drive vehicle according to the fourth aspect of the invention, when the steering angle of the four-wheel drive vehicle is smaller than the threshold value, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the steering angle is not smaller than the threshold value, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Thus, in a situation with the steering operation being large, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to the improvement of the NV performance. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

In the four-wheel drive vehicle according to the fifth aspect of the invention, when the four-wheel drive vehicle is running straight, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the four-wheel drive vehicle is turning, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Thus, in a situation with presence of the steering operation, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to the improvement of the NV performance. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

In the four-wheel drive vehicle according to the sixth aspect of the invention, when the vehicle attitude control is not executed, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the vehicle attitude control is executed, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Thus, when the vehicle attitude control is executed, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to the improvement of the NV performance. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

In the four-wheel drive vehicle according to the seventh aspect of the invention, when the outside temperature is not lower than the threshold value, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the outside temperature is lower than the threshold value, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Thus, when a road surface is likely to be frozen, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to the improvement of the NV performance. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

In the four-wheel drive vehicle according to the eighth aspect of the invention, when the braking operation amount or the requested braking amount in the four-wheel drive vehicle is smaller than the threshold value, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the braking operation amount or the requested braking amount is not smaller than the threshold value, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Thus, in a situation with presence of a sudden braking operation, for example, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to the improvement of the NV performance. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

In the four-wheel drive vehicle according to the ninth aspect of the invention, when the accelerating operation amount or the requested driving amount in the four-wheel drive vehicle is smaller than the threshold value, with the engine being in the stop state, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is inhibited. When the accelerating operation amount or the requested driving amount is not smaller than the threshold value, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is allowed. Thus, in a situation with presence of a sudden starting operation or a sudden accelerating operation, for example, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to the improvement of the NV performance. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

In the four-wheel drive vehicle according to the tenth aspect of the invention, when the engine is in the stop state by the execution of the engine automatic-stop control, the execution of the engine automatic-stop control is inhibited and the engine is restarted, in the case in which the situation that requires a higher priority to be given to suppression of the vehicle attitude change is predicted. Thus, in the event of the situation that requires the higher priority to be given to suppression of the vehicle attitude change, the change of the drive-power distribution ratio, which is to be made by the change of the rotational direction of the electric motor, is not inhibited. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a view schematically showing a construction of a four-wheel drive vehicle10to which the present invention is applied, for explaining major portions of control functions and control systems that are provided to perform various control operations in the vehicle10. As shown inFIG. 1, the vehicle10is a hybrid vehicle including drive power sources in the form of an engine12(see “ENG” inFIG. 1), a first rotating machine MG1and a second rotating machine MG2. Thus, the vehicle10includes at least one drive power source including the engine12. The vehicle10further includes right and left front wheels14R,14L, right and left rear wheels16R,16L and a power transmission apparatus18that is configured to transmit a drive power of the engine12to the right and left front wheels14R,14L and the right and left rear wheels16R,16L. The rear wheels16R,16L are main drive wheels that serve as drive wheels during a four-wheel drive running of the vehicle10but also during a two-wheel drive running of the vehicle10. The front wheels14R,14L are auxiliary drive wheels that serve as driven wheels during the two-wheel drive running and serve as the drive wheel during the four-wheel drive running. The vehicle10is a four-wheel drive vehicle based on a vehicle of FR (front engine and rear drive) system. In the following description, the front wheels14R,14L will be referred to as “front wheels14” and the rear wheels16R,16L will be referred to as “rear wheels16”, unless they are to be distinguished from each other. Further, the engine12, first rotating machine MG1and second rotating machine MG2will be referred to as “drive power source PU”, unless they are to be distinguished from one another.

The engine12is one of the drive power sources for driving the four-wheel drive vehicle10to run, and is a known internal combustion engine such as gasoline engine and diesel engine. The vehicle10is provided with an engine control device20that includes a throttle actuator, a fuel injection device and an ignition device. With the engine control device20being controlled by an electronic control apparatus130that is described below, an engine torque Te, which is an output torque of the engine12, is controlled.

Each of the first and second rotating machines MG1, MG2is a rotating electric machine having a function serving as an electric motor and a function serving as a generator. That is, each of the first and second rotating machines MG1, MG2is a so-called “motor generator”. Each of the first and second rotating machines MG1, MG2is a rotating machine that can serve as the drive power source for driving the four-wheel drive vehicle10to run. The first and second rotating machines MG1, MG2are connected to a battery24provided in the vehicle10, through an inverter22provided in the vehicle10. The inverter22is controlled by the electronic control apparatus130whereby an MG1torque Tg and an MG2torque Tm as output torques of the respective first and second rotating machines MG1, MG2are controlled. The output torque of each of the first and second rotating machines MG1, MG2serves as a power running torque when acting as a positive torque for acceleration of the vehicle10, with the each of the first and second rotating machines MG1, MG2being rotated in a forward direction. The output torque of each of the first and second rotating machines MG1, MG2serves as a regenerative torque when acting as a negative torque for deceleration of the vehicle10, with the each of the first and second rotating machines MG1, MG2being rotated in the forward direction. The battery24is the electric storage device to and from which an electric power is supplied from and to the first rotating machine MG1and the second rotating machine MG2. The output torque of each of the first and second rotating machines MG1, MG2serves as a power running torque when acting as a positive torque for acceleration of the vehicle10, with the each of the first and second rotating machines MG1, MG2being rotated in a forward direction. The output torque of each of the first and second rotating machines MG1, MG2serves as a regenerative torque when acting as a negative torque for deceleration of the vehicle10, with the each of the first and second rotating machines MG1, MG2being rotated in the forward direction. The battery24is an electric storage device to and from which an electric power is supplied from and to the first rotating machine MG1and the second rotating machine MG2. The first and second rotating machines MG1, MG2are disposed inside a casing26as a non-rotary member that is attached to a body of the vehicle10.

The power transmission apparatus18includes an automatic transmission28(see “T/M FOR HV” inFIG. 1) that is a transmission for hybrid system, a transfer30(see “T/F” inFIG. 1), a front propeller shaft32, a rear propeller shaft34, a front-wheel-side differential gear device36(see “FDiff” inFIG. 1), a rear-wheel-side differential gear device38(see “RDiff” inFIG. 1), right and left front axles40R,40L and right and left rear axles42R,42L, so that the drive power of the engine12, example, is to be transmitted to the rear wheels16R,16L sequentially through the transfer30, rear propeller shaft34, rear-wheel-side differential gear device38and right and left rear axles42R,42L, for example. When a part of the drive power transmitted to the transfer30from the engine12is distributed toward the front wheels14R,14L in the power transmission apparatus18, the distributed part of the drive power is transmitted to the front wheels14R,14L sequentially through the front propeller shaft32, front-wheel-side differential gear device36and right and left front axles40R,40L, for example.

FIG. 2is a view schematically showing a construction of the automatic transmission28. As shown inFIG. 2, the automatic transmission28includes an electrically-operated continuously-variable transmission portion44and a mechanically-operated step-variable transmission portion46that are disposed in series on a rotary axis CL1that are common to the transmission portions44,46within the casing26. The electrically-operated continuously-variable transmission portion44is connected to the engine12directly or indirectly through, for example, a damper (not shown). The mechanically-operated step-variable transmission portion46is connected to an output rotary member of the electrically-operated continuously-variable transmission portion44. The transfer30is connected to an output rotary member of the mechanically-operated step-variable transmission portion46. In the automatic transmission28, the drive power outputted from the engine12or the second rotating machine MG2, for example, is transmitted to the mechanically-operated step-variable transmission portion46, and is then transmitted from the mechanically-operated step-variable transmission portion46to the transfer30. In the following description, the electrically-operated continuously-variable transmission portion44and the mechanically-operated step-variable transmission portion46will be referred simply to as “continuously-variable transmission portion44” and “step-variable transmission portion46”, respectively. The power corresponds to a torque and a force unless they are to be distinguished from one another. Each of the continuously-variable transmission portion44and the step-variable transmission portion46is constructed substantially symmetrically about the rotary axis CL1, so that a lower half of each of the transmission portions44,46is not shown inFIG. 2. The rotary axis CL1corresponds to an axis of a crank shaft of the engine12, an axis of a connection shaft48which is an input rotary member of the automatic transmission28and which is connected to the crank shaft of the engine12, and an axis of an output shaft50which is an output rotary member of the automatic transmission28. The connection shaft48serves also as an input rotary member of the continuously-variable transmission portion44. The output shaft50serves also as an output rotary member of the step-variable transmission portion46.

The continuously-variable transmission portion44is provided with: the above-described first rotating machine MG1; and a differential mechanism54serving as a drive-power distribution mechanism to mechanically distribute the power of the engine12to the first rotating machine MG1and to an intermediate transmission member52that is an output rotary member of the continuously-variable transmission portion44. The above-described second rotating machine is MG2connected to the intermediate transmission member52in a power transmittable manner. The continuously-variable transmission portion44is an electrically-operated continuously-variable transmission wherein a differential state of the differential mechanism54is controllable by controlling an operation state of the first rotating machine MG1. The continuously-variable transmission portion44is operated as the electrically-operated continuously-variable transmission whose gear ratio (may be referred also to as “speed ratio”) γ0(=engine rotational speed Ne/MG2rotational speed Nm) is to be changed. The engine rotational speed Ne is a rotational speed of the engine12, and is equal to an input rotational speed of the continuously-variable transmission portion44, i.e., a rotational speed of the connection shaft48. The engine rotational speed Ne is also an input rotational speed of the automatic transmission28that is constituted mainly by the continuously-variable transmission portion44and the step-variable transmission portion46. The MG2rotational speed Nm is a rotational speed of the second rotating machine MG; and is equal to an output rotational speed of the continuously-variable transmission portion44, i.e., a rotational speed of the intermediate transmission member52. The first rotating machine MG1is a rotating machine capable of controlling the engine rotational speed Ne. It is noted that controlling an operation state of the first rotating machine MG1is controlling the operation of the first rotating machine MG1.

The differential mechanism54is a planetary gear device of a single-pinion type having a sun gear S0, a carrier CA0and a ring gear R0. The carrier CA0is connected to the engine12through the connection shaft48in a drive-power transmittable manner, and the sun gear S0is connected to the first rotating machine MG1in a drive-power transmittable manner, and the ring gear R0is connected to the second rotating machine MG2in a drive-power transmittable manner. In the differential mechanism54, the carrier CA0serves as an input element, the sun gear S0serves as a reaction element, and the ring gear R0serves as an output element.

The step-variable transmission portion46is a step-variable transmission that constitutes a power transmission path between the intermediate transmission member52and the transfer30. The intermediate transmission member52also serves as an input rotary member of the step-variable transmission portion46. The second rotating machine MG2is connected to the intermediate transmission member52, so as to be rotated integrally with the intermediate transmission member52. The step-variable transmission portion46is an automatic transmission that constitutes a part of a power transmission path between the drive power source PU (for driving the vehicle10to run) and the drive wheels (front and rear wheels14,16). The step-variable transmission portion46is a known automatic transmission of a planetary gear type provided with a plurality of planetary gear devices including first and second planetary gear devices56,58and a plurality of engagement devices including a one-way clutch F1, a clutch C1, a clutch C2, a brake B1and a brake B2. Hereinafter, the clutch C1, clutch C2, brake B1and brake B2will be referred to as “engagement devices CB” unless they are to be distinguished from one another.

Each of the engagement devices CB is a hydraulically-operated frictional engagement device constituted by, for example, a wet-type multiple-disc clutch including a plurality of friction plates which are superposed on each other and which are forced against each other by a hydraulic actuator, or a band brake including a rotary drum and one band or two bands which is/are wound on an outer circumferential surface of the rotary drum and tightened a hydraulic actuator. Each of the engagement devices CB receives a regulated hydraulic pressure supplied from a hydraulic control unit (hydraulic control circuit)60(seeFIG. 1) that is provided in the four-wheel drive vehicle10, whereby its operation state is switched between an engaged state and a released state, for example.

In the step-variable transmission portion46, selected ones of rotary elements of the first and second planetary gear devices56,58are connected to each other or to the intermediate transmitting member52, casing26or output shaft50, either directly or indirectly through the engagement devices CB or the one-way clutch F1. The rotary elements of the first planetary gear device56are a sun gear S1, a carrier CA1and a ring gear R1. The rotary elements of the second planetary gear device58are a sun gear S2, a carrier CA2and a ring gear R2.

The step-variable transmission portion46is shifted to a selected one of a plurality of gear positions (speed positions) by engaging actions of selected ones of the engagement devices CB. The plurality of AT gear positions have respective different gear ratios (speed ratios) γat (=AT input rotational speed Ni/output rotational speed No). Namely, the step-variable transmission portion46is shifted up and down from one gear position to another by placing selected ones of the engagement devices in the engaged state. The step-variable transmission portion46is a step-variable automatic transmission configured to establish a selected one of the plurality of gear positions. In the following description of the present embodiment, the gear position established in the step-variable transmission portion46will be referred to as AT gear position. The AT input rotational speed Ni is an input rotational speed of the step-variable transmission portion46that is a rotational speed of the input rotary member of the step-variable transmission portion46, which is equal to a rotational speed of the intermediate transmission member52, and which is equal to the MG2rotational speed Nm. Thus, the AT input rotational speed Ni can be represented by the MG2rotational speed Nm. The output rotational speed No is a rotational speed of the output shaft50that is an output rotational speed of the step-variable transmission portion46, which is considered to be an output rotational speed of the automatic transmission28.

As shown in a table ofFIG. 3, the step-variable transmission portion46is configured to establish a selected one of a plurality of AT gear positions in the form of four forward AT gear positions and a reverse AT gear position. The four forward AT gear positions consist of a first speed AT gear position, a second speed AT gear position, a third speed AT gear position and a fourth speed AT gear position, which are represented by “1st”, “2nd”, “3rd” and “4th” in the table ofFIG. 3. The first speed AT gear position is the lowest-speed gear position having a highest gear ratio γat, while the fourth speed AT gear position is the highest-speed gear position having a lowest gear ratio γat. The reverse AT gear position is represented by “Rev” in the table ofFIG. 3, and is established by, for example, engagements of the clutch C1and the brake B2. That is, when the vehicle10is to run in reverse direction, the first speed AT gear position is established, for example. The table ofFIG. 3indicates a relationship between each of the AT gear positions of the step-variable transmission portion46and operation states of the respective engagement devices CB of the step-variable transmission portion46, namely, a relationship between each of the AT gear positions and a combination of ones of the engagement devices CB, which are to be placed in theirs engaged states to establish the each of the AT gear positions. In the table ofFIG. 3, “O” indicates the engaged state of the engagement devices CB, “Δ” indicates the engaged state of the brake B2during application of an engine brake to the vehicle10or during a coasting shift-down action of the step-variable transmission portion46, and the blank indicates the released state of the engagement devices CB.

The step-variable transmission portion46is configured to switch from one of the AT gear positions to another one of the AT gear positions, namely, to establish one of the AT gear positions which is selected, by the electronic control apparatus130, according to, for example, an accelerating operation made by a vehicle driver (operator) and the vehicle running speed Vv. The step-variable transmission portion46is shifted up or down from one of the AT gear positions to another, for example, by so-called “clutch-to-clutch” shifting operation that is made by releasing and engaging actions of selected two of the engagement devices CB, namely, by a releasing action of one of the engagement devices CB and an engaging action of another one of the engagement devices CB.

The four-wheel drive vehicle10further includes an MOP62that is a mechanically-operated oil pump, and an electrically-operated oil pump (not shown).

The above-described one-way clutch F0is a locking mechanism by which the carrier CA0can be fixed to be unrotatable. That is, the one-way clutch F0is the lock mechanism capable of fixing the connection shaft48(which is connected to the crank shaft of the engine12and is to be rotated integrally with the carrier CA0) relative to the casing26. The one-way clutch F0includes two members that are rotatable relative to each other, wherein one of the two members is connected integrally to the connection shaft48, and the other member is connected integrally to the casing26. The other member of the one-way clutch F0is to be rotated in a positive direction (that corresponds to a direction of rotation of the engine12during operation of the engine12), with the one-way clutch F0being in its released state. However, the other member of the one-way clutch F0is not rotatable in a negative direction (that is opposite to the above-describe positive direction), with the one-way clutch F0being automatically placed in its engaged. Thus, the engine12is rotatable relative to the casing26when the one-way clutch F0is in the released state, and is unrotatable relative to the casing26when the one-way clutch F0is the engaged state. That is, the engine12is fixed to the casing26by the engagement of the one-way clutch F0. Thus, the one-way clutch F0allows the carrier CA0to be rotated in the above-described positive direction corresponding to the direction of rotation of the engine12, and inhibits the carrier CA0from being rotated in the above-described negative direction. That is, the one-way clutch F0is the locking mechanism which allows rotation of the engine12in the positive direction and which inhibits rotation of the engine12in the negative direction.

The MOP62is connected to the connection shaft48so as to be rotated together with rotation the engine12and to discharge a working fluid OIL that is be used in the power transmission apparatus18. Further, the electrically-operated oil pump (not shown) is operated, for example, when the engine12is stopped, namely, when the MOP62is not operated. The working fluid OIL discharged from the MOP62and the electrically-operated oil pump is supplied to the hydraulic control unit60. The working fluid OIL is regulated by the hydraulic control unit60, and the regulated hydraulic pressure is supplied to each of the engagement devices CB of the power transmission apparatus18(seeFIG. 1).

FIG. 4is a collinear chart indicating a relationship among rotational speeds of the rotary elements of the continuously-variable transmission portion44and the step-variable transmission portion46. InFIG. 4, three vertical lines Y1, Y2, Y3corresponding to the three rotary elements of the differential mechanism54constituting the continuously variable transmission portion44are a g-axis representing the rotational speed of the sun gear S0corresponding to a second rotary element RE2, an e-axis representing the rotational speed of the carrier CA0corresponding to a first rotary element RE1, and an m-axis representing the rotational speed of the ring gear R0corresponding to a third rotary element RE3(i.e., the input rotational speed of the step-variable transmission portion46) in order from the left side to the right. Four vertical lines Y4, Y5, Y6, Y7of the step-variable transmission portion46are axes representing a rotational speed of the sun gear S2corresponding to a fourth rotary element RE4, a rotational speed of the ring gear R1and the carrier CA2connected to each other and corresponding to a fifth rotary element RE5(i.e., the rotational speed of the output shaft50), a rotational speed of the carrier CA1and the ring gear R2connected to each other and corresponding to a sixth rotary element RE6, and a rotational speed of the sun gear S1corresponding to a seventh rotary element RE7, respectively, in order from the left side to the right. An interval between the vertical lines Y1, Y2, Y3is determined in accordance with a gear ratio ρ0of the differential mechanism54. An interval between the vertical lines Y4, Y5, Y6, Y7is determined in accordance with gear ratios ρ1, ρ2of the first and second planetary gear devices56,58. Where an interval between the sun gear and the carrier is set to an interval corresponding to “1” in the relationship between the vertical axes of the collinear chart, an interval between the carrier and the ring gear is set to an interval corresponding to the gear ratio ρ (=number of teeth of the sun gear/number of teeth of the ring gear) of the planetary gear device.

As shown in the collinear chart ofFIG. 4, in the differential mechanism54of the continuously-variable transmission portion44, the engine12(see “ENG” inFIG. 4) is connected to the first rotary element RE1, the first rotating machine MG1(see “MG1” inFIG. 4) is connected to the second rotary element RE2, and the second rotating machine MG2(see “MG2” inFIG. 4) is connected to the third rotary element RE3that is to be rotated integrally with the intermediate transmission member52, such that rotation of the engine12is to be transmitted to the step-variable transmission portion46through the intermediate transmission member52. The relationship between the rational speeds of the sun gear S0and the ring gear R0in the continuously-variable transmission portion44is represented by straight lines L0e, L0m, L0R that pass through the vertical line Y2.

In the step-variable transmission portion46, the fourth rotary element RE4is selectively connected to the intermediate transmission member52through the clutch C1, the fifth rotary element RE5is connected to the output shaft50, the sixth rotary element RE6is selectively connected to the intermediate transmission member52through the clutch C2and is selectively connected to the casing26through the brake B2, and the seventh rotary element RE7is selectively connected to the casing26through the brake B1. In the step-variable transmission portion46, the gear positions “1st”, “2nd”, “3rd”, “4th”, “Rev” are selectively established by engagement/release controls of the engagement devices CB, and the rotational speed of the output shaft50when each of the gear positions is established is indicated by an intersection of a corresponding one of straight lines L1, L2, L3, L4, LR with the vertical line Y5.

InFIG. 4, a straight line L0eand the straight lines L1, L2, L3, L4, which are represented by respective solid lines, indicate the relationship among the rotational speeds of the rotary elements in forward running of the vehicle10in HV running mode in which the vehicle10is enabled to perform hybrid running (=HV running) with at least the engine12being operated as the drive power source. In this hybrid running mode, when a reaction torque, i.e., a negative torque from the first rotating machine MG1, is inputted in positive rotation to the sun gear S0with respect to the engine torque Te inputted to the carrier CA0in the differential mechanism54, an engine direct transmission torque Td [=Te/(1−ρ0)=−(1/ρ0)×Tg] appears in the ring gear R0as a positive torque in positive rotation. A combined torque of the engine direct transmission torque Td and the MG2torque Tm is transmitted as a drive torque of the vehicle10acting in the forward direction depending on a requested drive power to the transfer30through the step-variable transmission portion46in which one of the AT first to fourth gear positions is established. The first rotating machine MG1functions as the generator when generating a negative torque with its rotation in positive direction. An electric power Wg generated by the first rotating machine MG1is stored in the battery24or consumed by the second rotating machine MG2. The second rotating machine MG2outputs the MG2torque Tm by using all or a part of the generated electric power Wg or using the electric power supplied from the battery24in addition to the generated electric power Wg. Thus, the engine12is configured to directly output, as the engine direct transmission torque Td, the drive power transmittable toward the front and rear wheels14,16. Further, the engine12is configured to indirectly output the drive power transmittable toward the front and rear wheels14,16, via the first rotating machine MG1serving as the generator and the second rotating machine MG2serving as the electric motor.

InFIG. 4, a straight line L0mrepresented by one-dot chain line and the straight lines L1, L2, L3, L4represented by the respective solid lines indicate the relationship among the rotational speeds of the rotary elements in forward running of the vehicle10in EV running mode in which the vehicle10is enabled to perform motor running (=EV running) with at least one of the first and second rotating machines MG1, MG2being operated as the drive power source in a state in which the engine12is stopped. As the EV running in forward direction in the EV running mode, there are a one-motor-drive EV running and a two-motor-drive EV running, for example. In the one-motor-drive EV running, the vehicle10is caused to run with only the second rotating machine MG2being operated as the drive power source. In the two-motor-drive EV running, the vehicle10is caused to run with both of the first and second rotating machines MG1, MG2being operated as the drive power sources. In the one-motor-drive EV running, the carrier CA0is not rotated, and the MG2torque Tm acting as a positive torque is inputted to the ring gear R0whereby the ring gear R0is rotated in positive direction. In this instance, the first rotating machine MG1, which is connected to the sun gear S0, is placed in non-load state and is idled in negative direction. In the one-motor-drive EV running, the one-way clutch F0is released so that the connection shaft48is not fixed to the casing26.

In the two-motor-drive EV running, in a state in which the carrier CA0is not rotated, when the MG1torque Tg acting as a negative torque is inputted to the sun gear S0whereby the sun gear S0is rotated in negative direction, the one-way clutch F0is automatically engaged so as to inhibit the carrier CA0from being rotated in negative direction. While the carrier CA0is fixed to be unrotatable by engagement of the one-way clutch F0, the MG1torque Tg acts as a reaction torque on the ring gear R0. Further, in the two-motor-drive EV running, the MG2torque Tm is inputted to the ring gear R0as in the one-motor-drive EV running. In the state in which the carrier CA0is not rotated, if the MG2torque Tm is not inputted to the ring gear R0when the MG1torque Tg acting as the negative torque is inputted to the sun gear S0, the one-motor-drive EV running is performed with the MG1torque Tg. In the forward running in the EV running mode, the engine rotational speed Ne is zeroed with the engine12being not operated, and the torque of at least one of the MG1torque Tg and the MG2torque Tm is transmitted, as a drive torque for driving the four-wheel drive vehicle10to run in forward direction, to the transfer30through the step-variable transmission portion46in which one of the AT first to fourth gear positions is established. In the forward running in the EV running mode, the MG1torque Tg acts as a negative torque in negative direction and serves as a power running torque, while the MG2torque Tm acts as a positive torque in positive direction and serves as a power running torque.

InFIG. 4, the straight lines L0R, LR represented by respective broken lines indicate the relationship among the rotational speeds of the rotary elements in reverse running of the four-wheel drive vehicle10in the EV running mode. In this reverse running in the EV running mode, the MG2torque Tm acting as the negative torque in the negative direction is inputted to the ring gear R0, and is transmitted, as a drive torque for driving the vehicle10to run in reverse direction, to the transfer30through the step-variable transmission portion46in which the AT first gear position is established. In the vehicle10, under controls executed by the electronic control apparatus130, in a state in which the AT first gear position or other low-speed gear position among the plurality of AT gear positions is established, the MG2torque Tm acting in the negative direction that is opposite to when the vehicle10runs in the forward direction, is outputted from the second rotating machine MG2whereby the reverse running of the vehicle10can be performed. In the reverse running in the EV running mode, the MG2torque Tm acts as a negative torque in the negative direction and serves as a power running torque. It is noted that, in the HV running mode, too, since the second rotating machine MG2can be rotated in the negative direction as indicated by the straight line L0R, the reverse running of the vehicle10can be performed as in the EV running mode.

FIG. 5is a view schematically showing a construction of the transfer30. The transfer30includes a transfer casing64as a non-rotary member, a rear-wheel-side output shaft66, a front-wheel driving gear68and a front-wheel drive clutch70. The rear-wheel-side output shaft66, front-wheel driving gear68and front-wheel drive clutch70are provided inside the transfer casing64, and are disposed on a rotary axis CL1that is common to the output shaft66, driving gear68and drive clutch70. The transfer30further includes a front-wheel-side output shaft72, a front-wheel driven gear74and a front-wheel idler gear76that are provided inside the transfer casing64, such that the front-wheel-side output shaft72and the front-wheel driven gear74are disposed on a rotary axis CL2that is common to the output shaft72and driven gear74. The rotary axis CL2corresponds to axes of the front propeller shaft32and the front-wheel-side output shaft72, for example.

The rear-wheel-side output shaft66is connected to the output shaft50in a power transmittable manner, and is connected to the rear propeller shaft34in a power transmittable manner, so that the drive power transmitted from the drive power source PU to the output shaft50though the automatic transmission28is to be outputted toward the rear wheels16by the rear-wheel-side output shaft66. The output shaft50serves also as an input rotary member of the transfer30, which is configured to input the drive power transmitted from the drive power source PU, to the rear-wheel-side output shaft66of the transfer30, namely, serves as a drive-power transmission shaft configured to transmit the drive power transmitted from the drive power source PU, to the transfer30. The automatic transmission28is an automatic transmission configured to transmit the drive power of the drive power source PU to the output shaft50.

The front-wheel driving gear68is provided to be rotatable relative to the rear-wheel-side output shaft66. The front-wheel drive clutch70is a multi-plate friction clutch configured to adjust a torque transmitted from the rear-wheel-side output shaft66to the front-wheel driving gear68, namely, adjust a torque transmitted from the rear-wheel-side output shaft66to the front-wheel-side output shaft72.

The front-wheel driven gear74is provided to be integral with the front-wheel-side output shaft72, so as to be connected to the front-wheel-side output shaft72in a power transmittable manner. The front-wheel idler gear76is provided to mesh with the front-wheel driving gear68and the front-wheel driven gear74, so as to connect between the front-wheel driving gear68and the front-wheel driven gear74in a power transmittable manner.

The front-wheel-side output shaft72is connected to the front-wheel driving gear68through the front-wheel driven gear74and the front-wheel idler gear76to the front-wheel driving gear68in a power transmittable manner, and is connected also to the front propeller shaft32in a power transmittable manner. The front-wheel-side output shaft72is configured to output a part of the drive power of the drive power source PU, which part is transmitted to the front-wheel driving gear68through the front-wheel drive clutch70, so that the outputted part of the drive power is to be transmitted toward the front wheels14.

The front-wheel drive clutch70includes a clutch hub78, a clutch drum80, a frictional engagement elements82and a piston84. The clutch hub78is connected to the rear-wheel-side output shaft66in a power transmittable manner. The clutch drum80is connected to the front-wheel driving gear68in a power transmittable manner. The frictional engagement elements82include a plurality of first friction plates82aand a plurality of second friction plates82b. The first friction plates82aare provided to be movable in the direction of the rotary axis CL1relative to the clutch hub78and to be unrotatable relative to the clutch hub78. The second friction plates82bare provided to be movable in the direction of the rotary axis CL1relative to the clutch drum80and to be unrotatable relative to the clutch drum80. The first and second friction plates82a,82bare alternately arranged and supposed on each other in the direction of the rotary axis CL1. The piston84is provided to be movable in the direction of the rotary axis CL1, so as to be brought into contact with the frictional engagement elements82and press the first and second friction plates82a,82b, thereby adjusting a torque capacity of the front-wheel drive clutch70. When the frictional engagement elements82are not pressed by the piston84, the torque capacity of the front-wheel drive clutch70is zeroed whereby the front-wheel drive clutch70is released.

With the torque capacity of the front-wheel drive clutch70being adjusted, the transfer30distributes the drive power of the drive power source PU transmitted through the automatic transmission28, toward the rear-wheel-side output shaft66and the front-wheel-side output shaft72. When the front-wheel drive clutch70is in its released state, namely, when a power transmission path between the rear-wheel-side output shaft66and the front-wheel driving gear68is cut off, the drive power of the drive power source PU transmitted to the transfer30through the automatic transmission28is transmitted toward the rear wheels16through, for example, the rear propeller shaft34. When the front-wheel drive clutch70is in its slip-engaged state or fully engaged state, namely, when the power transmission path between the rear-wheel-side output shaft66and the front-wheel driving gear68is not cut off, a part of the drive power of the drive power source PU transmitted to the transfer30is transmitted toward the front wheels14through, for example, the front propeller shaft32, and the remainder of the drive power of the drive power source PU transmitted to the transfer30is transmitted toward the rear wheels16through, for example, the rear propeller shaft34. The front-wheel drive clutch70is a drive-power distribution clutch configured to distribute the drive power of the drive power source PU, between the pair of front wheels14and the pair of rear wheels16. The transfer30is a drive-power distribution device capable of transmitting the drive power of the drive power source PU toward the front wheels14and the rear wheels16.

The transfer30includes an electric motor86, a worm gear88and a cam mechanism90that cooperate with one another to constitute a device configured to operate the front-wheel drive clutch70.

The worm gear88is a pair of gears consisting of a worm92integrally formed on a shaft of the electric motor86and a worm wheel94provided with teeth that mesh with the worm92. The worm wheel94is provided to be rotatable about the rotary axis CL1, so as to be rotated about the rotary axis CL1when the electric motor86is rotated.

The cam mechanism90is provided between the worm wheel94and the piston84of the front-wheel drive clutch70. The cam mechanism90includes a first member96connected to the worm wheel94, a second member98connected to the piston84, and a plurality of balls99interposed between the first and second members96,98, and is a mechanism configured to convert a rotary motion of the electric motor86into a linear motion.

The plurality of balls99are arranged equi-angularly in a circumferential direction about the rotary axis CL1. Each of first and second members96,98has a cam groove provided in its contact surface that is in contact with the balls99. The cam groove provided in the contact surface of each of the first and second members96,98has a shape by which the first and second members96,98are moved away from each other in the direction of the rotary axis CL1when the first and second members96,98are rotated relative to each other. Therefore, when the first and second members96,98are rotated relative to each other, the first and second members96,98are moved away from each other in the direction of the rotary axis CL1whereby the piston84connected to the second member98is caused to press the frictional engagement elements82. When the worm wheel94is rotated by the electric motor86, a rotary motion of the worm wheel94is converted by the cam mechanism90into a liner motion in the direction of the rotary axis CL1, which is transmitted to the piston84, and the frictional engagement elements82are pressed by the piston84. A pressing force by which the piston84presses the frictional engagement elements82is adjusted whereby the torque capacity of the front-wheel drive clutch70is adjusted.

The worm gear88and the cam mechanism90cooperate with each other to constitute a press mechanism configured to press the front-wheel drive clutch70, by converting a rotary motion of the electric motor86into a linear motion acting in an axial direction of the front-wheel drive clutch70, i.e., in the direction of the rotary axis CL1. In the transfer30, with the torque capacity of the front-wheel drive clutch70being adjusted, it is possible to adjust a drive-power distribution ratio Rx that is a ratio of distribution of the drive power of the drive power source PU, between the pair of front wheels14and the pair of rear wheels16.

The drive-power distribution ratio Rx is, for example, a rear-wheel-side drive-power distribution ratio Xr that is a ratio of the drive power transmitted from the drive power source PU to the rear wheels16, to all of the drive power transmitted from the drive power source PU to the rear and front wheels16,14. Alternatively, the drive-power distribution ratio Rx is, for example, a front-wheel-side drive-power distribution ratio Xf (=1−Xr) that is a ratio of the drive power transmitted from the drive power source PU to the front wheels14, to all of the drive power transmitted from the drive power source PU to the rear and front wheels16,14. In the present embodiment in which the rear wheels16are the main drive wheels, the rear-wheel-side drive-power distribution ratio Xr, which is a ratio of the drive power transmitted to the main drive wheels, is used as the drive-power distribution ratio Rx.

When the piston84does not press the frictional engagement elements82, the torque capacity of the front-wheel drive clutch70is zeroed. In this instance, the front-wheel drive clutch70is released whereby the rear-wheel-side drive-power distribution ratio Xr becomes 1.0. In other words, the drive-power distribution ratio Rx, which is the ratio of distribution of the drive power between the pair of front wheels14and the pair of rear wheels16, i.e., (drive power transmitted to front wheels14): (drive power transmitted to rear wheels16), is 0:100 where 100 represents all of the drive power of the drive power source PU transmitted to the transfer30. On the other hand, when the piston84presses the frictional engagement elements82, the torque capacity of the front-wheel drive clutch70is made larger than 0, and the rear-wheel-side drive-power distribution ratio Xr is reduced with increase of the torque capacity of the front-wheel drive clutch70. When the torque capacity of the front-wheel drive clutch70is maximized, namely, when the front-wheel drive clutch70is fully engaged, the rear-wheel-side drive-power distribution ratio Xr becomes 0.5, namely, the drive-power distribution ratio Rx becomes 50:50 that is an equilibrium state. Thus, the transfer30is capable of adjusting the rear-wheel-side drive-power distribution ratio Xr within a range from 1.0 to 0.5, namely, adjusting the drive-power distribution ratio Rx within a range from 0:100 to 50:50, by adjusting the torque capacity of the front-wheel drive clutch70. That is, the transfer30is capable of selectively establishing its two-wheel drive state and four-wheel drive state, such that the drive power of the drive power source PU is transmitted only toward the rear wheels16when the two-wheel drive state is established, and such that the drive power of the drive power source PU is transmitted toward the rear and front wheels16,14when the four-wheel drive state is established.

Referring back toFIG. 1, the four-wheel drive vehicle10is provided with a wheel brake device100which includes a brake master cylinder (not shown) and wheel brakes101that are provided for respective wheels14,16. The wheel brake device100is configured to apply braking forces generated by the respective wheel brakes101, to the respective wheels14,16. The wheel brakes101consist of front brakes101FR,101FL provided for the respective front wheels14R,14L and rear brakes101RR,101RL provided for the respective rear wheels16R,16L. The wheel brake device100is configured to supply a brake hydraulic pressure to a wheel cylinder (not shown) provided in each of the wheel brakes101, in accordance with, for example, an operation for depressing a brake pedal by the vehicle driver. In the wheel brake device100, normally, the brake master cylinder is configured to generate a master-cylinder hydraulic pressure whose magnitude corresponds to a braking operation amount Bra, and the generated master-cylinder hydraulic pressure is supplied as the brake hydraulic pressure to the wheel cylinder. On the other hand, in the wheel brake device100, for example, during execution of an ABS control, an anti-skid control or a vehicle-running-speed control, the brake hydraulic pressure required for execution of such a control is supplied to the wheel cylinder for enabling the wheel brakes101to generate the braking forces. The braking operation amount Bra is an operation amount of the brake pedal operated by the vehicle driver, which corresponds to a depressing force applied to the brake pedal. Thus, the wheel brake device100is capable of adjusting the braking forces generated by the wheel brakes101and applied to the wheels14,16.

Further, the four-wheel drive vehicle10is provided with the electronic control apparatus130as a controller that includes a control apparatus configured to control, for example, the drive power source PU and the transfer30.FIG. 1is a view showing an input/output system of the electronic control apparatus130, and is a functional block diagram for explaining major control functions and control portions of the electronic control apparatus130. For example, the electronic control apparatus130includes a so-called microcomputer incorporating a CPU, a ROM, a RAM and an input-output interface. The CPU performs control operations of the vehicle10, by processing various input signals, according to control programs stored in the ROM, while utilizing a temporary data storage function of the RAM. The electronic control apparatus130may be constituted by two or more control units exclusively assigned to perform different control operations such as an engine control operation and a shift control operation.

The electronic control apparatus130receives various input signals based on values detected by respective sensors provided in the four-wheel drive vehicle10. Specifically, the electronic control apparatus130receives: an output signal of an engine speed sensor102indicative of an engine rotational speed Ne which is a rotational speed of the engine12; an output signal of an output speed sensor104indicative of an output rotational speed No which corresponds to the running speed Vv of the vehicle10; an output signal of a MG1speed sensor106indicative of an MG1rotational speed Ng which is a rotational speed of the first rotating machine MG1; an output signal of a MG2speed sensor108indicative of an MG2rotational speed Nm which is a rotational speed of the second rotating machine MG2and which is equal to an AT input rotational speed Ni; an output signal of a wheel speed sensor110indicative of a wheel rotational speed Nr of each of the wheels14,16; an output signal of an accelerator-opening degree sensor112indicative of an accelerator opening degree θacc representing an amount of accelerating operation made by the vehicle driver; an output signal of a throttle-opening degree sensor114indicative of a throttle opening degree θth that is an opening degree of an electronic throttle valve; an output signal of a brake pedal sensor116indicative of a brake-ON signal Bon representing a state of depression of the brake pedal by the vehicle driver to operate the wheel brakes101and also a braking operation amount Bra representing an amount of depression of the brake pedal by the vehicle driver corresponding to the depressing force applied to the brake pedal; an output signal of a G senor118indicative of a longitudinal acceleration Gx and a lateral acceleration Gy of the vehicle10; an output signal of a shift position sensor120indicative of an operation position POSsh of a shift lever provided in the vehicle10; an output signal of a yaw rate sensor122indicative of a yaw rate Vyaw that is a rate of change of a vehicle rotational angle about a vertical axis passing through a center of gravity of the vehicle10; an output signal of a steering sensor124indicative of a steering angle θsw and a steering direction Dsw of a steering wheel provided in the vehicle10; an output signal of a battery sensor126indicative of a battery temperature THbat, a battery charging/discharging electric current Ibat and a battery voltage Vbat of the battery24; an output signal of a fluid temperature sensor128indicative of a working fluid temperature THoil that is a temperature of the working fluid OIL; and an output signal of an outside temperature sensor129indicative of an outside temperature THair that is a temperature around or outside the vehicle10.

The amount of accelerating operation made by the vehicle driver is, for example, an accelerating operation amount that is an amount of operation of an acceleration operating member such as an accelerator pedal, and corresponds to a requested output amount that is an amount of output of the four-wheel drive vehicle10requested by the vehicle driver. As the requested output amount requested by the vehicle driver, the throttle opening degree θth can be used in addition to or in place of the accelerator opening degree θacc, for example.

The electronic control apparatus130generates various command signals to the various devices provided in the four-wheel drive vehicle10, such as: an engine control command signal Se that is to be supplied to the engine control device20for controlling the engine12; a rotating-machine control command signal Smg that is to be supplied to the inverter22for controlling the first and second rotating machines MG1, MG2; a hydraulic-pressure control command signal Sat that is to be supplied to the hydraulic control unit60for controlling the operation states of the engagement devices CB; an electric-motor control command signal Sw that is to be supplied to the electric motor86for controlling the electric motor86; and a brake control command signal Sb that is to be supplied to the wheel brake device100for controlling the braking forces generated by the wheel brakes101.

For performing various control operations in the four-wheel drive vehicle10, the electronic control apparatus130includes an AT-shift control means in the form of an AT-shift control portion132, a hybrid control means in the form of a hybrid control portion134, a four-wheel-drive control means in the form of a four-wheel-drive control portion136, and a braking-force control means in the form of a braking-force control portion138.

The AT-shift control portion132is configured to determine whether a shifting action of the step-variable transmission portion46is to be executed, by using, for example, an AT gear position shift map as shown inFIG. 6, which is a relationship obtained by experimentation or determined by an appropriate design theory, and to output the hydraulic-pressure control command signal Sat supplied to the hydraulic control unit60, so as to execute the shift control operation in the step-variable transmission portion46as needed. The AT gear position shifting map represents a predetermined relationship between two variables in the form of the vehicle running speed Vv and a requested drive force Frdem, for example, which relationship is used to determine need of the shifting action of the step-variable transmission portion46and is represented by shifting lines in two-dimensional coordinates in which the running speed Vv and the requested drive force Frdem are taken along respective two axes. It is noted that one of the two variables may be the output rotational speed No in place of the vehicle running speed Vv and that the other of the two variables may be a requested drive torque Trdem, accelerator opening degree θacc or throttle valve opening degree θth in place of the requested drive force Frdem. The shifting lines in the AT gear position shifting map consist of shift-up lines (indicated by solid lines inFIG. 6) for determining need of a shift-up action of the step-variable transmission portion46, and shift-down lines (indicated by broken lines inFIG. 6) for determining need of a shift-down action of the step-variable transmission portion46.

The hybrid control portion134has a function serving as an engine control means in the form of an engine control portion134afor controlling the operation of the engine12and a function serving as a rotating-machine control means or a rotating-machine control portion134bfor controlling the operations of the first rotating machine MG1and the second rotating machine MG2via the inverter22, and executes a hybrid drive control, for example, using the engine12, the first rotating machine MG1and the second rotating machine MG2through these control functions.

The hybrid control portion134calculates a requested driving amount in the form of the requested drive force Frdem, by applying the accelerator opening degree θacc and the vehicle running speed Vv to, for example, a requested driving amount map that represents a predetermined relationship. The requested drive torque Trdem [Nm] applied to the drive wheels (front and rear wheels14,16), a requested drive power Prdem [W] applied to the drive wheels, a requested AT output torque applied to the output shaft50, etc can be used as the requested driving amount, in addition to the requested drive force Frdem [N]. The hybrid control portion134outputs the engine control command signal Se for controlling the engine12and the rotating-machine control command signal Smg for controlling the first and second rotating machines MG1, MG2, by taking account of a maximum chargeable amount Win of electric power that can be charged to the battery24, and a maximum discharging amount Wout of electric power that can be discharged from the battery24, such that the requested drive power Prdem based on the requested drive torque Trdem and the vehicle running speed Vv is obtained. The engine control command signal Se is, for example, a command value of an engine power Pe that is the power of the engine12outputting the engine torque Te at the current engine rotation speed Ne. The rotating-machine control command signal Smg is, for example, a command value of the generated electric power Wg of the first rotating machine MG1outputting the MG1torque Tg as the reaction torque of the engine torque Te at the MG1rotation speed Ng which is the MG1rotation speed Ng at the time of the command signal Smg output, and is a command value of a consumed electric power Wm of the second rotating machine MG2outputting the MG2torque Tm at the MG2rotation speed Nm which is the MG2rotation speed Nm at the time of the command signal Smg output.

The maximum chargeable amount Win of the battery24is a maximum amount of the electric power that can be charged to the battery24, and indicates an input limit of the battery24. The maximum dischargeable amount Wout of the battery24is a maximum amount of the electric power that can be discharged from the battery24, and indicates an output limit of the battery24. The maximum chargeable and dischargeable amounts Win, Wout are calculated by the electronic control apparatus130, for example, based on a battery temperature THbat and a charged state value SOC [%] of the battery24. The charged state value SOC of the battery24is a value indicative of a charged state of the battery24, i.e., an amount of the electric power stored in the battery24, and is calculated by the electronic control apparatus130, for example, based on the charging/discharging electric current Ibat and the voltage Vbat of the battery24.

For example, when the automatic transmission28is operated as a continuously variable transmission as a whole by operating the continuously variable transmission portion44as a continuously variable transmission, the hybrid control portion134controls the engine12and controls the generated electric power Wg of the first rotating machine MG1so as to attain the engine rotational speed Ne and the engine torque Te at which the engine power Pe achieving the requested drive power Prdem is acquired in consideration of an engine optimum fuel consumption point etc., and thereby provides the continuously variable shift control of the continuously variable transmission portion44to change the gear ratio γ0of the continuously variable transmission portion44. As a result of this control, the gear ratio γt (=γ0×γat=Ne/No) of the automatic transmission28is controlled in the case of operating the automatic transmission28as a continuously variable transmission. The above-described engine optimum fuel consumption point is predetermined as an optimum engine operation point, i.e., an engine operation point that maximizes a total fuel efficiency in the four-wheel drive vehicle10including not only a fuel efficiency of the engine12but also a charge/discharge efficiency of the battery24, for example, when the requested engine power Pedem is to be acquired. The engine operation point is an operation point of the engine12which is defined by a combination of the engine rotational speed Ne and the engine torque Te. The engine rotational speed Ne at the optimum engine operation point is an optimum engine rotational speed Neb that maximizes the energy efficiency in the vehicle10.

For example, when the automatic transmission28is operated as a step-variable transmission as a whole by operating the continuously variable transmission portion44as in a step-variable transmission, the hybrid control portion134uses a predetermined relationship, for example, a step-variable gear position shift map, to determine need of a shifting action of the automatic transmission28and provides the shift control of the continuously variable transmission portion44so as to selectively establish the plurality of gear positions in coordination with the shift control of the AT gear position of the step-variable transmission portion46by the AT-shift control portion132. The plurality of gear positions can be established by controlling the engine rotational speed Ne by the first rotating machine MG1depending on the output rotational speed No so as to maintain the respective gear ratios γt.

The hybrid control portion134selectively establishes the motor running mode or the hybrid running mode as the running mode depending on a running state, so as to cause the vehicle10to run in a selected one of the running modes. For example, the hybrid control portion134establishes the EV running mode when the requested drive power Prdem is in an EV running region smaller than a predetermined threshold value, and establishes the HV running mode when the requested drive power Prdem is in an HV running region equal to or greater than the predetermined threshold value. InFIG. 6, one-dot chain line A is a boundary line between the HV running region and the EV running region, for switching between the HV running mode and the EV running mode. A predetermined relationship having the boundary line as indicated by the one-dot chain line A ofFIG. 6is an example of a running-mode switching map defined by the two-dimensional coordinates of variables in the form of the vehicle running speed Vv and the requested drive force Frdem. It is noted that, inFIG. 6, the running-mode switching map is shown together with AT gear position shift map, for convenience of the description.

In the EV running mode, when the requested drive power Prdem can be obtained only by the second rotating machine MG2, the hybrid control portion134causes the four-wheel drive vehicle10to run in the one-motor-drive EV running with only the second rotating machine MG2being operated as the drive power source PU. On the other hand, when the requested drive power Prdem cannot be obtained only by the second rotating machine MG2in the EV running mode, the hybrid control portion134causes the vehicle10to run in the two-motor-drive EV running. However, even when the requested drive power Prdem can be obtained only by the second rotating machine MG2, the vehicle10may be caused to run in the two-motor-drive EV running, if the use of both of the first rotating machine MG1and second rotating machine MG2provides better efficiency than the use of only the second rotating machine MG2.

Even when the requested drive power Prdem is in the EV running region, the hybrid control portion134establishes the HV running mode, for example, in a case in which the state-of-charge value SOC of the battery24becomes less than a predetermined engine-start threshold value or in a case in which the engine12needs to be warmed up. The engine-start threshold value is a predetermined threshold value for determining that the state-of-charge value SOC reaches a level at which the battery24needs to be charged by automatically starting the engine12.

The hybrid control portion134functionally includes an engine-start control means in the form of an engine-start control portion134cthat is configured, upon satisfaction of a predetermined engine-start condition RMst, to execute an engine automatic-start control CTst for causing the engine12to be automatically started. The predetermined engine-start condition RMst is, for example, that the HV running mode is established when operation of the engine12has been stopped, and/or that a known idle-stop control (by which the engine12is temporarily stopped in response to stop of running of the four-wheel drive vehicle10when the engine12has been operated in the HV running mode) is cancelled. The engine-start control portion134cdetermines whether the engine-start condition RMst is satisfied or not, and determines that the start of the engine12is requested when determining that the engine-start condition RMst is satisfied. When determining that the start of the engine12is requested, the engine-start control portion134cexecutes the engine automatic-start control CTst.

When executing the engine automatic-start control CTst, the engine-start control portion134ccauses the engine rotational speed Ne to be increased by, for example, the first rotating machine MG1, and then causes the engine12to be rotated by itself by supplying fuel to the engine12and igniting the engine12when the engine rotational speed Ne has been increased to a predetermined ignitable rotational speed Neigf or higher. The predetermined ignitable rotational speed Neigf is, for example, a predetermined speed value of the engine rotational speed Ne at which a complete combustion can be made in the engine12that is being self-rotated after an initial combustion of the engine12. After the combustion of the engine12has been stabilized as a result of the complete combustion, the engine-start control portion134ccompletes a series of steps of the engine automatic-start control CTst, by controlling the engine rotational speed Ne to a target engine rotational speed Netgt that is a target speed value of the engine rotational speed Ne. The target engine rotational speed Netgt, which is the target speed value of the engine rotational speed Ne after the complete combustion of the engine12in the engine automatic-start control CTst, is a predetermined engine-start rotational speed Nestf such as the above-described optimum engine rotational speed Neb and an idling rotational speed Neidl.

The hybrid control portion134functionally includes an engine-stop control means in the form of an engine-stop control portion134dthat is configured, upon satisfaction of a predetermined engine-stop condition RMsp, to execute an engine automatic-stop control CTsp for causing the engine12to be automatically stopped. The predetermined engine-stop condition RMsp is, for example, that the EV running mode is established when operation of the engine12has been operated, and/or that the idle-stop control is executed in response to stop of running of the four-wheel drive vehicle10when the engine12has been operated in the HV running mode. The engine-stop control portion134ddetermines whether the engine-stop condition RMsp is satisfied or not, and determines that stop of the engine12is requested when determining that the engine-stop condition RMsp is satisfied. When determining that the stop of the engine12is requested, the engine-stop control portion134dexecutes the engine automatic-stop control CTsp.

When the engine automatic-stop control CTsp is executed, the engine-stop control portion134dstops the fuel supply to the engine12. In this instance, the engine-stop control portion134dmay control the MG1torque Tg, for example, such that the MG1torque Tg provides the engine12with a torque that reduces the engine rotational speed Ne, so as to quickly reduce the engine rotational speed Ne and quickly stop rotation of the engine12.

The four-wheel-drive control portion136executes a drive-power distribution control CTx for adjusting the rear-wheel-side drive-power distribution ratio Xr. The four-wheel-drive control portion136determines a target ratio value of the rear-wheel-side drive-power distribution ratio Xr, which is dependent on the running state of the four-wheel drive vehicle10that is obtained through, for example, the output speed sensor104and the G sensor118. Then, the four-wheel-drive control portion136outputs the electric-motor control command signal Sw for controlling the electric motor86such that the rear-wheel-side drive-power distribution ratio Xr is adjusted to the target ratio value with the torque capacity of the front-wheel drive clutch70being adjusted.

When the four-wheel drive vehicle10is running straight, for example, the four-wheel-drive control portion136controls the rear-wheel-side drive-power distribution ratio Xr to 1.0, namely, controls the drive-power distribution ratio Rx to 0:100, by releasing the front-wheel drive clutch70. Further, when the vehicle10is turning, the four-wheel-drive control portion136calculates a target yaw rate Vyawtgt, based on, for example, the steering angle θsw and the vehicle running speed Vv during turning of the vehicle10, and adjusts the rear-wheel-side drive-power distribution ratio Xr such that the yaw rate Vyaw, which is constantly detected by the yaw rate sensor122, follows the target yaw rate Vyawtgt.

The braking-force control portion138calculates a target deceleration, for example, based on the running speed Vv, gradient of downhill road and braking operation (such as a rate of increase of the braking operation amount Bra or braking operation amount Bra) made by the vehicle driver, and determines a requested braking force Bdem as a requested braking amount requested by the vehicle driver for obtaining the target deceleration, by using a predetermined relationship. During deceleration of the four-wheel drive vehicle10, the braking-force control portion138causes the wheel brake device100to generate the braking force such that the generated braking force corresponds to the requested braking force Bdem in the vehicle10.

The braking force, which is to be applied to the four-wheel drive vehicle10, is constituted by, for example, a braking force generated by each of the wheel brakes101and/or a regenerative braking force, i.e., a braking force generated by the second rotating machine MG2that is subjected to a regenerative control, such that a higher priority is given to generation of the regenerative braking force, for example, from the point of view of improvement of energy efficiency. The braking-force control portion138outputs a command for executing the regenerative control by which the second rotating machine MG2is to be controlled to provide a regenerative torque required for the regenerative braking force, and the outputted command is supplied to the hybrid control portion134. The regenerative control of the second rotating machine MG2is a control in which the second rotating machine MG2is to be rotated and driven by a driven torque transmitted from the wheels14,16, so as to be operated as the generator for generating the electric power by which the battery24is to be charged through the inverter22.

When the requested braking force Bdem is relatively small, for example, the braking-force control portion138realizes the requested braking force Bdem by only the regenerative braking force. When the requested braking force Bdem is relatively large, for example, the braking-force control portion138realizes the requested braking force Bdem by not only the regenerative braking force but also the braking force generated by each of the wheel brakes101. When the four-wheel drive vehicle10is to be stopped, for example, the braking-force control portion138realizes the requested braking force Bdem, by replacing the regenerative braking force with the braking force generated by each of the wheel brakes101shortly before the vehicle10is stopped. The braking-force control portion138outputs the brake control command signal Sb for obtaining the braking force of each of the wheel brakes101, which is required for realizing the requested braking force Bdem, and the outputted brake control command signal Sb is supplied to the wheel brake device100.

In addition to a basic braking-force control for realizing the braking force based on the braking operation made by the vehicle driver, the braking-force control portion138executes an attitude-controlling braking-force control for realizing the braking force required for a vehicle attitude control CTvs that is to be executed for assuring a running stability of the four-wheel drive vehicle10. The vehicle attitude control CTvs is a known control for stabilizing the vehicle10. As the vehicle attitude control CTvs, there are, for example, a control for activating an ABS function, a braking-force distribution control, a control for activating a brake assist function, a control for activating a TRC function, a lateral-slip suppress control and a control for activating an automatic brake function. The braking-force control portion138outputs the brake control command signal Sb for obtaining the braking force of each of the wheel brakes101, which is required for realizing the vehicle attitude control CTvs, and the outputted brake control command signal Sb is supplied to the wheel brake device100.

By the way, there is a case in which the rear-wheel-side drive-power distribution ratio Xr is changed by change of an electric-motor rotational direction that is a rotational direction of the electric motor86, in the transfer30. That is, when the rear-wheel-side drive-power distribution ratio Xr is to be changed, there is a case in which the electric-motor rotational direction is switched from a 4WD direction to a 2WD direction and also a case in which the electric-motor rotational direction is switched from the 2WD direction to the 4WD direction. The 4WD direction is a direction in which the piston84is caused to press the frictional engagement elements82. The 2WD direction is a direction in which the piston84is separated from the frictional engagement elements82. When the electric-motor rotational direction is switched between the 4WD direction and the 2WD direction in the transfer30, a rattling noise could be generated due to play or backlash between members (such as the worm92and worm wheel94) constituting the worm gear88and the cam mechanism90, more precisely, due to inversion of a direction in which the backlash is to be eliminated. When the engine12is in a stop state by execution of the engine automatic-stop control, namely, when a background noise is smaller than when the engine12is operated, there is a risk of reduction of a NV performance if the rattling noise is generated. The stop state of the engine12by execution of the engine automatic-stop control is, for example, a state in which the four-wheel drive vehicle10is running in the EV running mode that has been established after the HV running mode with operation of the engine12, a state in which the vehicle10is running in the EV running mode from start of the vehicle10, or a state in which an idle-stop control is executed in the HV running mode during stop of the vehicle10.

Therefore, for improving the NV performance, the electronic control apparatus130inhibits change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the rotational direction of the electric motor86, when the engine12is in the stop state by the execution of the engine automatic-stop control CTsp.

Specifically described, the electronic control apparatus130further includes a distribution-ratio-change-inhibition determining means in the form of a distribution-ratio-change-inhibition determining portion140, for realizing the four-wheel drive vehicle10capable of improving the NV performance when the engine12is in the stop state by the execution of the engine automatic-stop control CTsp.

The distribution-ratio-change-inhibition determining portion140determines whether the engine12is in the stop state by the execution of the engine automatic-stop control CTsp or not. That is, the distribution-ratio-change-inhibition determining portion140determines whether the engine12is in a state in which the engine12has been automatically stopped.

When determining that the engine12is in the state in which the engine12has been automatically stopped, the distribution-ratio-change-inhibition determining portion140executes a distribution-ratio-change inhibition control CTpx for inhibiting the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the electric-motor rotational direction in the transfer30, and outputs a command for inhibiting the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the electric-motor rotational direction in the transfer30, such that the outputted command is supplied to the four-wheel-drive control portion136. Further, even when executing the distribution-ratio-change inhibition control CTpx, the distribution-ratio-change-inhibition determining portion140outputs a command for allowing a change of the rear-wheel-side drive-power distribution ratio Xr which is to be made without the change of the electric-motor rotational direction in the transfer30, such that the outputted command is supplied to the four-wheel-drive control portion136. On the other hand, when determining that the engine12is not in the state in which the engine12has been automatically stopped, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx, and does not output the command for inhibiting the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the electric-motor rotational direction in the transfer30. Thus, when the engine12is not in the state in which the engine12has been automatically stopped, the four-wheel-drive control portion136can make any change the rear-wheel-side drive-power distribution ratio Xr irrespective of whether the change of the rear-wheel-side drive-power distribution ratio Xr is to be made with or without the change of the electric-motor rotational direction in the transfer30.

FIG. 7is a flow chart showing a main part of a control routine executed by the electronic control apparatus130, namely, a control routine that is executed for realizing the four-wheel drive vehicle10that is capable of improving the NV performance when the engine12is in the stop state by the execution of the engine automatic-stop control CTsp. This control routine is executed, for example, in a repeated manner.FIG. 8is a time chart showing, by way of example, a case in which the control routine shown by the flow chart ofFIG. 7is executed.

As shown inFIG. 7, the control routine is initiated with step S10corresponding to function of the four-wheel-drive control portion136, which is implemented to determine a target value of the rear-wheel-side drive-power distribution ratio Xr, which is dependent on a running state of the four-wheel drive vehicle10. Step S10is followed by step S20corresponding to function of the distribution-ratio-change-inhibition determining portion140, which is implemented to determine whether the engine12is in the state in which the engine12has been automatically stopped, or not. When a negative determination is made at step S20, one cycle of execution of the control routine is terminated. When an affirmative determination is made at step S20, step S30corresponding to function of the distribution-ratio-change-inhibition determining portion140is implemented to execute the distribution-ratio-change inhibition control CTpx for inhibiting a change of the rear-wheel-side drive-power distribution ratio Xr, which requires change of the electric-motor rotational direction in the transfer30, namely, which is to be made by change of the electric-motor rotational direction in the transfer30.

FIG. 8shows, by way of example, a case in which the engine12is automatically stopped by execution of the engine automatic-stop control CTsp when the four-wheel drive vehicle10has been running in the HV running mode with the rear-wheel-side drive-power distribution ratio Xr being changed, as needed, depending on the running state of the vehicle10. InFIG. 8, arrow D4wdindicates a state in which the electric-motor rotational direction as a control direction of the transfer30becomes a 4WD direction, while arrow D2wdindicates a state in which the electric-motor rotational direction becomes a 2WD direction. Further, inFIG. 8, a zero (0) point in the control direction of the transfer30indicates a state in which the piston84is positioned in a position that causes the torque capacity of the front-wheel drive clutch70to become zero. When the piston84is moved in the 4WD direction from the zero point, the torque capacity of the front-wheel drive clutch70is generated and increase. When the piston84is moved in the 2WD direction from the zero point, the torque capacity of the front-wheel drive clutch70stays in zero (0). As shown inFIG. 8, at a time point t1, the engine12is automatically stopped so that the HV running mode is switched to the EV running mode. In a period before the time point t1in which the engine12is operated, any change of the rear-wheel-side drive-power distribution ratio Xr is allowed irrespective of whether the change of the rear-wheel-side drive-power distribution ratio Xr is made with or without the change of the electric-motor rotational direction in the transfer30(see solid line CD inFIG. 8). However, at the time point t1at which the engine12is automatically stopped, the distribution-ratio-change inhibition control CTpx, i.e., a control for inhibiting an inversion motion of the electric motor86that causes change of the electric-motor rotational direction, starts to be executed. A change of the rear-wheel-side drive-power distribution ratio Xr, which is indicated by solid line CD1, is made after the time point t1, but is allowed because this change is made without change of the electric-motor rotational direction, more precisely, because this change is made by the rotation of the electric motor86in a direction that is the same as a direction in which the electric motor86is rotated last time before the engine12is stopped at the time point t1. Further, solid line CD2indicates a case in which an operation state (i.e., angular position) of the electric motor86is kept the same as at the time point t1, and the electric-motor rotational direction is not changed in the transfer30, so that the rear-wheel-side drive-power distribution ratio Xr can remain unchanged. On the other hand, a change of the rear-wheel-side drive-power distribution ratio Xr, which is indicated by solid line CD3, is made after the time point t1, and is inhibited because this change is to be made by change of the electric-motor rotational direction, more precisely, because this change is to be made by the rotation of the electric motor86in a direction that is opposite to the direction in which the electric motor86is rotated last time before the engine12is stopped at the time point t1. When the change of the rear-wheel-side drive-power distribution ratio Xr is inhibited, a ratio value of the rear-wheel-side drive-power distribution ratio Xr at the time point t1is kept after the time point t1as indicated by the solid line CD2, for example.

As described above, in the present embodiment, when the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the change of the rear-wheel-side drive-power distribution ratio Xr (i.e., drive-power distribution ratio), which is to be made by change of the rotational direction of the electric motor86, is inhibited, so that it is possible to prevent the rattling noise from being generated due to play or backlash between members constituting the worm gear88and the cam mechanism90when the background noise is small, by avoiding inversion of a direction in which the backlash is to be eliminated. Therefore, when the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the NV performance can be improved.

There will be described other embodiment of this invention. The same reference signs as used in the above-described first embodiment will be used in the following embodiments, to identify the functionally corresponding elements, and descriptions thereof are not provided.

Second Embodiment

In the above-described first embodiment, when the engine12is in the stop state in which the background noise is smaller than when the engine12is operated, the distribution-ratio-change inhibition control CTpx is always executed. The background noise is made larger when a vehicle runs at a high speed than when the vehicle runs at a low speed or is stopped. Therefore, during running of the four-wheel drive vehicle10at a high speed, the rattling noise generated upon change of the electric-motor rotational direction in the transfer30is likely to be drowned out by the background noise. Further, it is preferable that a vehicle controllability owing to execution of the drive-power distribution control CTx is assured during the running at a high speed since it is preferable that an influence to the vehicle controllability owing to execution of the drive-power distribution control CTx is suppressed during the high-speed running. Therefore, when the running speed Vv is lower than a threshold value, i.e., a threshold speed value Vvf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86. The threshold speed value Vvf is, for example, a predetermined value for determining that the running state is in a state in which the background noise is so large that the rattling noise generated upon change of the electric-motor rotational direction in the transfer30is not problematic, or a predetermined value for improving the NV performance owing to execution of the distribution-ratio-change inhibition control CTpx while suppressing the influence to the vehicle controllability owing to execution of the drive-power distribution control CTx.

The distribution-ratio-change-inhibition determining portion140determines whether the running speed Vv is lower than the threshold speed value Vvf or not. When determining that the running speed Vv is lower than the threshold speed value Vvf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the running speed Vv is not lower than the threshold speed value Vvf, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as shown inFIG. 9, when determining that the running speed Vv is lower than the threshold speed value Vvf and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as shown inFIG. 9, when determining that the running speed Vv is not lower than the threshold speed value Vvf, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp.

According to another control arrangement, when a steering operation made by the vehicle driver is large, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control CTx rather than to the improvement of the NV performance, because it is preferable that change of attitude of the four-wheel drive vehicle10is suppressed by execution of the drive-power distribution control CTx. Thus, when the yaw rate Vyaw as a parameter presenting an amount of the steering operation is smaller than a threshold value, i.e., a threshold rate value Vyawf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86. The threshold rate value Vyawf is, for example, a predetermined value for determining that the running state is in a state in which the steering operation made by the vehicle driver is so large that the vehicle attitude change needs to be suppressed by execution of the drive-power distribution control CTx, or a predetermined value for improving the NV performance owing to execution of the distribution-ratio-change inhibition control CTpx while suppressing the vehicle attitude change.

The distribution-ratio-change-inhibition determining portion140determines whether the yaw rate Vyaw is lower than the threshold rate value Vyawf or not. When determining that the yaw rate Vyaw is lower than the threshold rate value Vyawf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the yaw rate Vyaw is not lower than the threshold rate value Vyawf, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as shown inFIG. 10, when determining that the yaw rate Vyaw is lower than the threshold rate value Vyawf and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as shown inFIG. 10, when determining that the yaw rate Vyaw is not lower than the threshold rate value Vyawf, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp.

It is also possible to use the steering angle θsw as another parameter representing the amount of the steering operation made by the vehicle driver. In this case, when the steering angle θsw is smaller than a threshold value, i.e., the threshold angle value θswf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86. The threshold angle value θswf is, for example, a predetermined value for determining that the running state is in a state in which the steering operation made by the vehicle driver is so large that the vehicle attitude change needs to be suppressed by execution of the drive-power distribution control CTx, or a predetermined value for improving the NV performance owing to execution of the distribution-ratio-change inhibition control CTpx while suppressing the vehicle attitude change.

The distribution-ratio-change-inhibition determining portion140determines whether the steering angle θsw is smaller than the threshold angle value θswf or not. When determining that the steering angle θsw is smaller than the threshold angle value θswf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the steering angle θsw is not smaller than the threshold angle value θswf, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as shown inFIG. 11, when determining that the steering angle θsw is smaller than the threshold angle value θswf and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as shown inFIG. 11, when determining that the steering angle θsw is not smaller than the threshold angle value θswf, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp.

According to still another control arrangement, when the steering operation is made by the vehicle driver, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control CTx rather than to the improvement of the NV performance, because it is preferable that change of attitude of the four-wheel drive vehicle10is suppressed by execution of the drive-power distribution control CTx. A parameter representing a situation with presence of the steering operation made by the vehicle driver is, for example, a parameter representing whether the four-wheel drive vehicle10is turning or running straight. Thus, when the vehicle10is running straight with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86.

The distribution-ratio-change-inhibition determining portion140determines whether the four-wheel drive vehicle10is turning or running straight. When determining that the vehicle10is running straight, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the vehicle10is turning, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as shown inFIG. 12, when determining that the vehicle10is running straight and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as shown inFIG. 12, when determining that the vehicle10is turning, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp. This control arrangement, in which the distribution-ratio-change inhibition control CTpx is executed on condition that the vehicle10is running straight, can be considered to correspond to an example of the control arrangement ofFIG. 10in which the threshold rate value Vyawf is set to zero or a value in the vicinity of zero, and to an example of the control arrangement ofFIG. 11in which the threshold angle value θswf is set to zero or a value in the vicinity of zero.

According to still another control arrangement, when the vehicle attitude control CTvs is executed, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control CTx, rather than to the improvement of the NV performance, because it is preferable that change of attitude of the four-wheel drive vehicle10is suppressed by execution of the drive-power distribution control CTx, in addition to assurance of the running stability of the vehicle10owing to execution of the vehicle attitude control CTvs. Thus, when the vehicle attitude control CTvs is not executed with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86.

The distribution-ratio-change-inhibition determining portion140determines whether the vehicle attitude control CTvs is being executed or not. When determining that the vehicle attitude control CTvs is not being executed, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the vehicle attitude control CTvs is being executed, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as indicated by broken line CD3bin a period from a time point t1bto a time point t2b, when determining that the vehicle attitude control CTvs is not being executed and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as indicated by solid line CD4bin a period after the time point t2b, when determining that the vehicle attitude control CTvs is being executed, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp. It is noted thatFIG. 13is a time chart corresponding to the time chart ofFIG. 8in which indication of the execution of the vehicle attitude control CTvs initiated at the time point t2b, is added. The time point t1binFIG. 13corresponds to the time point t1inFIG. 8. The solids lines CDb, CD1b, CD2binFIG. 13correspond to the solid lines CD, CD1, CD2inFIG. 8, respectively. The broken line CD3binFIG. 13corresponds to the broken line CD3inFIG. 8.

According to still another control arrangement, when a road surface is likely to be frozen, for example, when the outside temperature is low, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control CTx, rather than to the improvement of the NV performance, because it is preferable that change of attitude of the four-wheel drive vehicle10is suppressed by execution of the drive-power distribution control CTx. Thus, when the outside temperature THair is not lower than a threshold temperature value THairf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86. The threshold temperature value THairf is, for example, a predetermined value for determining that the outside temperature THair is so low that the road surface is likely to be frozen, or a predetermined value for improving the NV performance owing to execution of the distribution-ratio-change inhibition control CTpx while suppressing the vehicle attitude change.

The distribution-ratio-change-inhibition determining portion140determines whether the outside temperature THair is not lower than the threshold temperature value THairf. When determining that the outside temperature THair is not lower than the threshold temperature value THairf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the outside temperature THair is lower than the threshold temperature value THairf, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as shown inFIG. 14, when determining that the outside temperature THair is not lower than the threshold temperature value THairf and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as shown inFIG. 14, when determining that the outside temperature THair is lower than the threshold temperature value THairf, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp.

According to still another control arrangement, when a braking operation made by the vehicle driver is large, for example, in a sudden braking operation, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control CTx rather than to the improvement of the NV performance, because it is preferable that change of attitude of the four-wheel drive vehicle10is suppressed by execution of the drive-power distribution control CTx. Thus, when the braking operation amount Bra as a parameter presenting an amount of the braking operation is smaller than a threshold value, i.e., a threshold amount value Braf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86. The threshold amount value Braf is, for example, a predetermined value for determining that the running state is in a state in which the braking operation made by the vehicle driver is so large that the vehicle attitude change needs to be suppressed by execution of the drive-power distribution control CTx, or a predetermined value for improving the NV performance owing to execution of the distribution-ratio-change inhibition control CTpx while suppressing the vehicle attitude change.

The distribution-ratio-change-inhibition determining portion140determines whether the braking operation amount Bra is smaller than the threshold amount value Braf or not. When determining that the braking operation amount Bra is smaller than the threshold amount value Braf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the braking operation amount Bra is not smaller than the threshold amount value Braf, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as shown inFIG. 15, when determining that the braking operation amount Bra is smaller than the threshold amount value Braf and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as shown inFIG. 15, when determining that the braking operation amount Bra is not smaller than the threshold amount value Braf, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp.

According to still another control arrangement, when an accelerating operation made by the vehicle driver is large, for example, in a sudden starting operation or a sudden accelerating operation, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control CTx rather than to the improvement of the NV performance, because it is preferable that change of attitude of the four-wheel drive vehicle10is suppressed by execution of the drive-power distribution control CTx. Thus, when the accelerator opening degree θacc as a parameter presenting an amount of the accelerating operation is smaller than a threshold value, i.e., a threshold degree value θaccf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the rotational direction of the electric motor86. The threshold degree value θaccf is, for example, a predetermined value for determining that the running state is in a state in which the accelerating operation made by the vehicle driver is so large that the vehicle attitude change needs to be suppressed by execution of the drive-power distribution control CTx, or a predetermined value for improving the NV performance owing to execution of the distribution-ratio-change inhibition control CTpx while suppressing the vehicle attitude change.

The distribution-ratio-change-inhibition determining portion140determines whether the accelerator opening degree θacc is smaller than the threshold degree value θaccf or not. When determining that the accelerator opening degree θacc is smaller than the threshold degree value θaccf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the contrary, when determining that the accelerator opening degree θacc is not smaller than the threshold degree value θaccf, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx. Specifically, as shown inFIG. 16, when determining that the accelerator opening degree θacc is smaller than the threshold degree value θaccf and that the engine12is in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx. On the other hand, as shown inFIG. 16, when determining that the accelerator opening degree θacc is not smaller than the threshold degree value θaccf, the distribution-ratio-change-inhibition determining portion140allows the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by change of the electric-motor rotational direction in the transfer30, even if determining that the engine12is in the stop state by execution of the engine automatic-stop control CTsp.

It is noted that control arrangements shown in respectiveFIGS. 9, 10, 11, 12, 13, 14, 15 and 16do not all have to be provided in this second embodiment, as long as at least one of them is provided.

As described above, in the present second embodiment, when the running speed Vv is lower than the threshold speed value Vvf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change inhibition control CTpx is executed. When the running speed Vv is not lower than the threshold speed value Vvf, the change of the rear-wheel-side drive-power distribution ratio Xr, which is to be made by the change of the electric-motor rotational direction in the transfer30, is allowed. Therefore, when the running speed Vv is not lower than the threshold speed value Vvf, namely, when the background noise is large, it is possible to assure the vehicle controllability owing to execution of the drive-power distribution control CTx, and accordingly to improve the NV performance while suppressing influence to the vehicle controllability owing to execution of the drive-power distribution control CTx.

In the present second embodiment, the distribution-ratio-change inhibition control CTpx is executed, when the yaw rate Vyaw is lower than the threshold rate value Vyawf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, when the steering angle θsw is smaller than the threshold angle value θswf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, when the four-wheel drive vehicle10is running straight, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, when the vehicle attitude control CTvs is not being executed, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, when the outside temperature THair is not lower than the threshold temperature value THairf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, when the braking operation amount Bra is smaller than the threshold amount value Braf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp, and/or when the accelerator opening degree θacc is smaller than the threshold degree value θaccf, with the engine12being in the stop state by execution of the engine automatic-stop control CTsp. On the other hand, the change of the rear-wheel-side drive-power distribution ratio Xr, which is to be made by change of the electric-motor rotational direction in the transfer30, is allowed, when the yaw rate Vyaw is not lower than the threshold rate value Vyawf, when the steering angle θsw is not smaller than the threshold angle value θswf, when the four-wheel drive vehicle10is turning, when the vehicle attitude control CTvs is being executed, when the outside temperature THair is lower than the threshold temperature value THairf, when the braking operation amount Bra is not smaller than the threshold amount value Braf, and/or when the accelerator opening degree θacc is not smaller than the threshold degree value θaccf. Thus, a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control, rather than to the improvement of the NV performance, in a situation with the steering operation being large, in a situation with presence of the steering operation, in a situation with execution of the vehicle attitude control, in a situation in which the road surface is likely to be frozen, in a situation with presence of the sudden braking operation, and/or in a situation with presence of the sudden starting operation or the sudden accelerating operation. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

Third Embodiment

In the above-described second embodiment, when a higher priority is given to the vehicle controllability owing to execution of the drive-power distribution control CTx, rather than to the improvement of the NV performance, the distribution-ratio-change inhibition control CTpx is not executed. Further, in the above-described first and second embodiments, when the engine12is not in the stop state by execution of the engine automatic-stop control CTsp, the distribution-ratio-change inhibition control CTpx is not executed. Thus, when the engine12is being operated in a situation in which the a higher priority is to be given to the vehicle controllability owing to execution of the drive-power distribution control CTx, rather than to the improvement of the NV performance, the distribution-ratio-change inhibition control CTpx is not executed. Therefore, in a case in which a situation that requires a higher priority to be given to the vehicle controllability owing to execution of the drive-power distribution control CTx, rather than to the improvement of the NV performance, is predicated, when such a predicted situation actually occurs with the engine12being operated, the distribution-ratio-change inhibition control CTpx is not executed, so that any change of the rear-wheel-side drive-power distribution ratio Xr including the change made by the change of the electric-motor rotational direction in the transfer30can be made.

In this third embodiment, when the engine12is in the stop state by the execution of the engine automatic-stop control CTsp, the electronic control apparatus130inhibits or suspend the execution of the engine automatic-stop control CTsp and restarts the engine12, in a case in which the electronic control apparatus130predicts a situation that requires a higher priority to be given to the change of the rear-wheel-side drive-power distribution ratio Xr made by change of the electric-motor rotational direction in the transfer30, rather than to inhibition of the change of the rear-wheel-side drive-power distribution ratio Xr. The situation that requires a higher priority to be given to the change of the rear-wheel-side drive-power distribution ratio Xr made by change of the electric-motor rotational direction in the transfer30, rather than to inhibition of the change of the rear-wheel-side drive-power distribution ratio Xr, is, for example, a situation that requires a higher priority to be given to the vehicle controllability owing to execution of the drive-power distribution control CTx rather than to the improvement of the NV performance, namely, requires a higher priority to be given to suppression of the vehicle attitude change.

Specifically, when determining that the engine12is in the stop state by the execution of the engine automatic-stop control CTsp, the distribution-ratio-change-inhibition determining portion140determines whether occurrence of a situation that requires a higher priority to be given to suppression of the vehicle attitude change, is predicted or not. The situation that requires the higher priority to be given to suppression of the vehicle attitude change is, for example, a situation in which the yaw rate Vyaw is not lower than the threshold rate value Vyawf, a situation in which the steering angle θsw is not smaller than the threshold angle value θswf, a situation in which the four-wheel drive vehicle10is turning, a situation in which the vehicle attitude control CTvs is being executed, a situation in which the outside temperature THair is lower than the threshold temperature value THairf, a situation in which the braking operation amount Bra is not smaller the threshold amount value Braf, and/or a situation in which the accelerator opening degree θacc is not smaller than the threshold degree value θaccf.

The distribution-ratio-change-inhibition determining portion140determines whether occurrence of the situation that requires the higher priority to be given to suppression of the vehicle attitude change, is predicted or not, for example, based on (i) a situation of a road on which the four-wheel drive vehicle10will run, which situation is obtained through a known navigation system (not shown), (ii) information obtained from a known vehicle-area information sensor (not shown) configured to directly obtain information relating to a road on which the vehicle10is running and information relating to an object or objects present around the vehicle10, (iii) information relating to weather and other vehicles present around the vehicle10, which information is obtained through a wireless communication, (iv) operation states of various devices installed in the vehicle1, (v) a change of the braking operation amount Bra and/or (vi) a change of the accelerator opening degree θacc. The vehicle-area information sensor includes a lidar (light detection and ranging), a radar (radio detection and ranging) and/or an onboard camera.

When determining that the engine12is in the stop state by the execution of the engine automatic-stop control CTsp, and that the occurrence of the situation that requires the higher priority to be given to suppression of the vehicle attitude change, is not predicted, the distribution-ratio-change-inhibition determining portion140executes the distribution-ratio-change inhibition control CTpx and outputs the command for inhibiting the change of the rear-wheel-side drive-power distribution ratio Xr which is to be made by the change of the electric-motor rotational direction in the transfer30, such that the outputted command is supplied to the four-wheel-drive control portion136. On the other hand, when determining that the engine12is in the stop state by the execution of the engine automatic-stop control CTsp, and that the occurrence of the situation that requires the higher priority to be given to suppression of the vehicle attitude change, is predicted, the distribution-ratio-change-inhibition determining portion140does not execute the distribution-ratio-change inhibition control CTpx, and suspends or inhibits execution of the engine automatic-stop control CTsp and outputs a command for restarting the engine12, such that the outputted command is supplied to the hybrid control portion134. Thus, when the situation that requires the higher priority to be given to suppression of the vehicle attitude change, is predicted, the four-wheel-drive control portion136can make any change of the rear-wheel-side drive-power distribution ratio Xr including the change made by the change of the electric-motor rotational direction in the transfer30.

FIG. 17is a flow chart showing a main part of a control routine executed by the electronic control apparatus130, namely, a control routine that is executed for realizing the four-wheel drive vehicle10that is capable of improving the NV performance when the engine12is in the stop state by the execution of the engine automatic-stop control CTsp. This control routine is executed, for example, in a repeated manner. The control routine shown inFIG. 17is to be executed in this third embodiment, and is different from the control routine shown inFIG. 7that is executed in the above-described first embodiment, in terms of some parts that will be described below.

As shown inFIG. 17, when an affirmative determination is made at step S20(corresponding to step S20in the above-described control routine shown inFIG. 7), step S25corresponding to function of the distribution-ratio-change-inhibition determining portion140is implemented to determine whether occurrence of the situation that requires the higher priority to be given to suppression of the vehicle attitude change, is predicted or not. When a negative determination is made at step S25, the control flow goes to step S30(corresponding to step S30in the above-described control routine shown inFIG. 7). When an affirmative determination is made at step S25, step S40corresponding to functions of the distribution-ratio-change-inhibition determining portion140and the hybrid control portion134is implemented to inhibit execution of the engine automatic-stop control CTsp and to restart the engine12.

As described above, in the present third embodiment, substantially the same effects as in the above-described first embodiment are obtained.

In the present third embodiment, when the engine12is in the stop state by the execution of the engine automatic-stop control CTsp, the execution of the engine automatic-stop control CTsp is inhibited and the engine12is restarted, in the case in which the situation that requires a higher priority to be given to suppression of the vehicle attitude change is predicted. Thus, in the event of the situation that requires the higher priority to be given to suppression of the vehicle attitude change, the change of the rear-wheel-side drive-power distribution ratio Xr, which is to be made by the change of the electric-motor rotational direction in the transfer30, is not inhibited. Therefore, it is possible to suppress the vehicle attitude change and also to improve the NV performance.

While the preferred embodiments of this invention have been described in detail by reference to the drawings, it is to be understood that the invention may be otherwise embodied.

For example, in the above-described second embodiment, the braking operation amount Bra is used as an example of the parameter representing the amount of the braking operation made by the vehicle driver. However, the parameter representing the amount of the braking operation made by the vehicle driver does not necessarily have to be the braking operation amount Bra, but may be, for example, the requested braking amount which is requested by the vehicle driver and which can be calculated, for example, based on the braking operation amount Bra.

Further, in the above-described second embodiment, the accelerating operation amount such as the accelerator opening degree θacc is used as an example of the parameter representing the amount of the accelerating operation made by the vehicle driver. However, the parameter representing the amount of the braking operation made by the vehicle driver does not necessarily have to be the accelerating operation amount, but may be, for example, the requested driving amount such as the requested drive force Frdem which can be calculated, for example, based on the accelerating operation amount. In an automatic drive control or an automatic running-speed control, for example, there is a case in which the requested driving amount is not dependent on the amount of the accelerating operation made by the vehicle drive. The requested driving amount is useful in a four-wheel drive vehicle including control functions for executing the automatic drive control or the automatic running-speed control, for example.

Further, in the above-described embodiments, the four-wheel drive vehicle10is a four-wheel drive vehicle based on a vehicle of FR (front engine and rear drive) system, and is a part-time four-wheel drive vehicle in which the two-wheel drive state and the four-wheel drive state are switchable to each other depending on the running state. Further, the four-wheel drive vehicle10in the above-described embodiments is a hybrid vehicle having the drive power sources in the form of the engine12and the first and second rotating machines MG1, MG2, and is a four-wheel drive vehicle provided with the automatic transmission28including the continuously-variable transmission portion44and the step-variable transmission portion46that are arranged in series. However, this arrangement is not essential. The present invention is applicable also to a four-wheel drive vehicle based on a vehicle of FF (front engine and front drive) system, a full-time four-wheel drive vehicle, a parallel-type hybrid vehicle in which drive powers of an engine and a rotating machine are to be transmitted to drive wheels, a series-type hybrid vehicle in which a drive power of a rotating machine, which is to be driven by an electric power of a battery and/or an electric power generated by a generator driven by a drive power of an engine, is to be transmitted to drive wheels, or a vehicle provided with a single drive power source in the form of an engine. Further, the present invention is applicable also to a four-wheel drive vehicle provided with an automatic transmission in the form of a known planetary-gear type automatic transmission, a known synchronous-meshing parallel-two-shaft-type transmission including DCT (dual clutch transmission), a known belt-type continuously variable transmission or an electrically-operated continuously variable transmission. Further, where the present invention is applied to the above-described series-type hybrid vehicle, such a series-type hybrid vehicle does not necessarily have to be provided with an automatic transmission.

It is noted that, in case of the four-wheel drive vehicle based on the vehicle of FF system, the front wheels serve as the main drive wheels while the rear wheels serve as the auxiliary drive wheels so that the front-wheel-side drive-power distribution ratio Xf is a ratio of the drive power transmitted to the main drive wheels. In case of the full-time four-wheel drive vehicle provided with a center differential gear device including a differential limiting clutch, for example, the drive-power distribution ratio Rx (that is the ratio of distribution of the drive power between the front wheels14and the rear wheels16) is 50:50 when the differential limiting clutch is operated to limit or inhibit a differential motion of the center differential gear device, and the drive-power distribution ratio Rx is a predetermined ratio such as 30:70 when the differential limiting clutch is not operated. In case of the above-described series-type hybrid vehicle, the engines is used as a drive power source configured to output the drive power indirectly through conversion between a mechanical power and an electric power. However, where the series-type hybrid vehicle is provided with a clutch through which the engine is mechanically connectable to the drive wheels in a power transmittable manner, the engine can be used as a drive power source configured to output the drive power directly. That is, the present invention is applicable to any four-wheel drive vehicle including: a drive-power distribution device including (a) a drive-power distribution clutch configured to distribute a drive power of a drive power source, between main drive wheels and auxiliary drive wheels, (b) an electric motor, (c) a press mechanism configured to press the drive-power distribution clutch by converting a rotary motion of the electric motor into a linear motion in an axial direction of the drive-power distribution clutch, and configured to adjust a torque capacity of the drive-power distribution clutch so as to adjust a drive-power distribution ratio that is a ratio of distribution of the drive power between the main drive wheels and the auxiliary drive wheels; an engine serving as the drive power source and configured to output the drive power; and a control apparatus configured to execute a drive-power distribution control for adjusting the drive-power distribution ratio, and configured to execute an engine automatic-stop control for causing the engine to be automatically stopped upon satisfaction of an engine-stop condition.

In the above-described embodiments, the front-wheel drive clutch70of the transfer30is constructed such that, when the electric motor86is rotated, the piston84is moved through the cam mechanism90in a direction toward the frictional engagement elements82, so as to press the frictional engagement elements82. However, this construction is not essential. For example, the front-wheel drive clutch70may include a ball screw configured to covert a rotation motion into a linear motion, such that the piston84is moved, upon rotation of the electric motor86, through the ball screw, in the direction toward the frictional engagement elements82, so as to press the frictional engagement elements82.

It is to be understood that the embodiments described above are given for illustrative purpose only, and that the present invention may be embodied with various modifications and improvements which may occur to those skilled in the art.

NOMENCLATURE OF ELEMENTS