Control apparatus for vehicular drive system

[0001] The present application is based on Japanese Patent Application No. 2005-154749.

[0002] 1. Field of the Invention

[0003] The present invention relates to a vehicular drive system including a differential mechanism having a differential function, and electric motors, and more particularly to techniques for reducing the size of the electric motors.

[0004] 2. Discussion of Prior Art

[0005] There is known a drive system for a vehicle, which includes a differential mechanism operable to distribute an output of an engine to a first electric motor and its output shaft, and a second electric motor disposed between the output shaft of the differential mechanism and a drive wheel of the vehicle. Patent Document 1 discloses an example of such a vehicular drive system, which is a hybrid vehicle drive system. In this hybrid vehicle drive system, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of a drive force of the engine is mechanically transmitted to the drive wheels through the differential function of the differential mechanism, while the rest of the drive force is electrically transmitted from the first electric motor to the second electric motor, through an electric path therebetween, so that the differential mechanism functions as a transmission the speed ratio of which is continuously variable, for example, as an electrically controlled continuously variable transmission, thereby making it possible to drive the vehicle under the control of a control device, with the engine kept in an optimum operating state with an improved fuel economy.

[0006] Patent Document 1 JP-2003-301731A

[0007] Generally, a continuously variable transmission is known as a transmission which permits an improved fuel economy of the vehicle, while on the other hand a gear type transmission such as a step-variable automatic transmission is known as a transmission having a high power transmitting efficiency. However, there is not available any power transmitting mechanism having the advantages of those two types of transmission. For example, the hybrid vehicle drive system disclosed in the above-identified publication JP-2003-301731A includes the electric path for transmitting an electric energy from the first electric motor to the second electric motor, namely, a power transmitting path for transmitting a portion of the vehicle drive force as an electric energy, the first electric motor is required to be large-sized to meet a need for an increased output of the engine, and the second electric motor driven by the electric energy generated by the first electric motor is also required to be accordingly large-sized, so that the overall size of the hybrid vehicle drive system tends to be large-sized. It is also noted that a portion of the output of the engine is once converted into an electric energy which is subsequently converted into a mechanical energy to be transmitted to the drive wheels, whereby the fuel economy of the vehicle may possibly be deteriorated under some running condition of the vehicle, for instance, during a high-speed running of the vehicle. Where the above-described differential mechanism is a transmission the speed ratio of which is electrically variable, for example, a continuously variable transmission so-called an “electric CVT”, the vehicular drive system suffers from a similar problem.

[0008] In the hybrid vehicle drive system described above, it is well known to provide a step-variable transmission in a power transmitting path between the output member of the differential mechanism (electrically controlled continuously variable transmission) and the vehicle drive wheel, for the purpose of reducing the required capacity of the second electric motor when a high vehicle drive torque is required, for thereby reducing the size of the second electric motor.

[0009] As an example of the step-variable transmission, there is well known a step-variable automatic transmission which is shifted by a releasing action of a coupling device and an engaging action of another coupling device. Generally, this type of step-variable automatic transmission is controlled to effect a shift-down action so called “a power-on shift-down action” for increasing the vehicle drive force, when an operation to accelerate the vehicle, such as a rapid operation of an accelerator pedal by a large amount is performed by the vehicle operator.

[0010] When the power-on shift-down action of the step-variable automatic transmission is effected as described above, the speed ratio of the automatic transmission is changed in steps after the power-on shift-down action, so that the vehicle drive force is increased in steps. On the other hand, the step-variable automatic transmission does not have a continuous change of its speed ratio in the process of the shift-down action, unlike a continuously variable transmission, and therefore requires a relatively long time before it becomes capable of providing a vehicle drive force with completion of the shift-down action. Accordingly, the step-variable automatic transmission has a risk of a low response to a drive torque increase upon the shift-down action, so that the vehicle tends to have a low degree of acceleration performance upon a vehicle accelerating operation by the vehicle operator, for instance, a low accelerating response, leading to deterioration of the vehicle acceleration drivability as felt by the vehicle operator.

[0011] A vehicular drive system may include two transmission mechanisms in the form of the differential mechanism and step-variable automatic transmission described above. The overall speed ratio of this vehicular drive system is determined by the speed ratios of the two transmission mechanisms. Like the step-variable automatic transmission, the vehicular drive system of this type suffers from a low response to the drive torque increase upon the shift-down action of the step-variable transmission, leading to a low acceleration performance of the vehicle and deterioration of the vehicle acceleration drivability as felt by the vehicle operator.

[0012] A vehicular drive system arranged to achieve the object of the hybrid vehicle drive system described in the above-identified Patent Document 1 also suffers from a low response to the drive torque increase upon a shift-down action of the step-variable transmission disposed between the output member of the differential mechanism and the vehicle drive wheel, leading to a low acceleration performance of the vehicle and deterioration of the vehicle acceleration drivability as felt by the vehicle operator.

[0013] The present invention was made in view of the background art described above. It is therefore an object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism operable to distribute an output of an engine to a first electric motor and its output shaft, a second electric motor disposed in a power transmitting path between the differential mechanism and a drive wheel of a vehicle, and a step-variable transmission constituting a part of the power transmitting path, which control apparatus permits size reduction or improvement of fuel economy of the vehicular drive system, and improvement of vehicle acceleration performance upon a shifting action of the step-variable transmission.

[0014] The present invention according to claim 1 provides a control apparatus for (a) a vehicular drive system including a continuously-variable transmission portion operable as an electrically controlled continuously variable transmission, and a step-variable transmission portion, the continuously variable transmission portion having a differential mechanism operable to distribute an output of an engine to a first electric motor and a power transmitting member, and a second electric motor disposed in a power transmitting path between the power transmitting member and a drive wheel of a vehicle, the step-variable transmission portion constituting a part of the power transmitting path, the control apparatus being characterized by comprising (b) a differential-state limiting device provided in the differential mechanism, and operable to limit a differential function of the differential mechanism, for limiting an operation of the continuously-variable transmission portion as the electrically controlled continuously variable transmission, and (c) electric-motor control means for controlling a drive-force assisting operation to generate an assisting drive force to drive the vehicle during a shifting action of the step-variable transmission portion, in different manners depending upon whether the continuously-variable transmission portion is placed in a continuously-variable shifting state in which the continuously-variable transmission portion is operable as the electrically controlled continuously variable transmission, or a non-continuously-variable shifting state in which the continuously-variable transmission portion is not operable as the electrically controlled continuously variable transmission.

[0015] In the control apparatus constructed as described above, the continuously-variable transmission portion of the vehicular drive system is switchable by the differential limiting device, between the continuously-variable shifting state in which the differential mechanism has the differential function without a limitation, to permit the continuously-variable transmission portion to be operable as the electrically controlled continuously variable transmission, and the non-continuously-variable shifting state in which the differential function of the differential mechanism is limited to the operation of the continuously-variable transmission portion. When the differential mechanism is placed in a non-differential state, for example, in a locked state, the continuously-variable transmission portion is placed in the non-continuously-variable shifting state, for example in a step-variable shifting state in which the continuously-variable transmission portion is not operable as the electrically controlled continuously variable transmission. Accordingly, the present drive system has both an advantage of an improvement of the fuel economy of a transmission the speed ratio of which is electrically variable, and an advantage of a high power transmitting efficiency of a gear type power transmitting device constructed for mechanical transmission of power.

[0016] When the continuously-variable transmission portion is placed in the continuously-variable shifting state in a normal output state of the engine during a low-speed or medium-speed running or a low-output or medium-output running of the vehicle, for example, the fuel economy of the vehicle is improved. When the continuously-variable transmission portion is placed in the non-continuously-variable shifting state during a high-speed running of the vehicle, the output of the engine is transmitted to the drive wheel primarily through the mechanical power transmitting path, the fuel economy is improved owing to reduction of a loss of conversion of a mechanical energy into an electric energy, which would take place when the continuously-variable transmission portion is operated as the electrically controlled continuously variable transmission. Where the continuously-variable transmission portion is placed in the non-continuously-variable shifting state during a high-output running state of the vehicle, the continuously-variable transmission portion is operated as a transmission the speed ratio of which is electrically variable, only when the vehicle speed or output is relatively low or medium, so that the required amount of electric energy generated by the electric motor, that is, the maximum amount of electric energy that must be transmitted from the electric motor can be reduced, making it possible to minimize the required size of the electric motor, and the required size of the vehicular drive system including the electric motor.

[0017] During a shifting action of the step-variable transmission portion, the drive-force assisting operation to generate the assisting drive force to drive the vehicle is performed in the respective different manners depending upon whether the continuously-variable transmission portion is placed in the continuously-variable shifting state in which the continuously-variable transmission portion is operable as the electrically controlled continuously variable transmission, or in the non-continuously-variable shifting state in which the continuously-variable transmission portion is not operable as the electrically controlled continuously variable transmission. Accordingly, the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator are improved, not only in the continuously-variable shifting state in which the vehicle drive force can be rapidly increased immediately after the moment of initiation of the shifting action of the step-variable transmission portion, by controlling the speed of the engine as desired, but also in the non-continuously-variable shifting state in which a response to an increase of the vehicle drive force during the shifting action of the step-variable transmission portion is lower than in the continuously-variable shifting state of the continuously-variable transmission portion.

[0018] In the invention according to claim 2, the electric-motor control means controls the drive-force assisting operation such that an amount of the assisting drive force is larger in the non-continuously-variable shifting state of the continuously-variable transmission portion than in the continuously-variable shifting state. In this case, the vehicle drive force during a power-on shift-down action of the step-variable transmission portion is increased by a larger amount in the non-continuously-variable shifting state in which the response to an increase of the vehicle drive torque during the shifting action of the step-variable transmission portion tends to be lower than in the continuously-variable shifting state, whereby the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator are improved in the non-continuously-variable shifting state.

[0019] In the invention according to claim 3, the electric-motor control means initiates the drive-force assisting operation at an earlier point of time when the continuously-variable transmission portion is placed in the non-continuously-variable shifting state than when the continuously-variable transmission portion is placed in the continuously-variable shifting state. Accordingly, the vehicle drive force is more rapidly increased during the power-on shift-down action of the step-variable transmission portion, in the non-continuously-variable shifting state in which the response to an increase of the vehicle drive torque during the shifting action of the step-variable transmission portion tends to be lower than in the continuously-variable shifting state, whereby the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator are improved in the non-continuously-variable shifting state.

[0020] Preferably, the electric-motor control means is arranged to determine the amount of the assisting drive force to be generated by the drive-force assisting operation, and the moment of initiation of the drive-force assisting operation, so as to obtain substantially the same shifting response in the continuously-variable shifting state and the non-continuously-variable shifting state of the continuously-variable transmission portion, that is, to obtain substantially the same amount of increase of the vehicle drive force in the continuously-variable and non-continuously-variable shifting states, so that substantially the same vehicle acceleration performance (vehicle acceleration drivability as felt by the vehicle operator) is obtained in the continuously-variable and non-continuously-variable shifting states of the continuously-variable transmission portion.

[0021] Preferably, the differential-state limiting device is arranged to place the differential mechanism in a differential state in which the differential mechanism has the differential function, for thereby placing the continuously-variable transmission portion in the continuously-variable shifting state in which the continuously-variable transmission portion is operable as the electrically controlled continuously variable transmission, and to place the differential mechanism in a non-differential state (for example, a locked state) in which the differential function of the differential mechanism is limited, for thereby placing the continuously-variable transmission portion in the non-continuously-variable shifting state (for example, a step-variable shifting state) in which the operation of the continuously-variable transmission portion as the electrically controlled continuously variable transmission is limited. In this case, the continuously-variable transmission portion is switchable between the continuously-variable shifting state and the non-continuously-variable shifting state.

[0022] Preferably, the differential mechanism has a first element connected to the engine, a second element connected to the first electric motor and a third element connected to the power transmitting member, and the differential limiting device is operable to place the differential mechanism in a differential state in which the first, second and third elements of the differential mechanism are rotatable relative to each other, and to place the differential mechanism in a non-differential state (for example, locked state) in which at least the second and third elements are not rotatable at different speeds. For example, the second and third elements are rotatable at respective different speeds in the differential state of the differential mechanism, and the first, second and third elements are rotated as a unit or the second element is held stationary in the non-differential or locked state of the differential mechanism. Thus, the differential mechanism is switchable between the differential and non-differential states.

[0023] Preferably, the differential limiting device includes a clutch operable to connect any two of the first, second and third elements of the differential mechanism to each other for rotating the first, second and third elements as a unit, and/or a brake operable to fix the second element to a stationary member for holding the second element stationary. This arrangement permits the differential mechanism to be easily switched between the differential and non-differential states.

[0024] Preferably, the clutch and brake are released to place the differential mechanism in the differential state in which at least the second and third elements are rotatable at respective different speeds, and in which the differential mechanism is operable as an electrically controlled differential device. In this case, the clutch is engaged to permit the differential mechanism to be operable as a transmission having a speed ratio of 1, or the brake is engaged to permit the differential mechanism to be operable as a speed-increasing transmission having a speed ratio lower than 1. In this arrangement, the differential mechanism is switchable between the differential state and the non-differential state, and is operable as a transmission having a single gear position with a single fixed speed ratio or a plurality of gear positions having respective fixed speed ratios.

[0025] Preferably the differential mechanism is a planetary gear set, and the first element is a carrier of the planetary gear set, and the second element is a sun gear of the planetary gear set, while said third element is a ring gear of the planetary gear set. In this arrangement, the axial dimension of the differential mechanism can be reduced, and is simply constituted by one planetary gear device.

[0026] Preferably, the planetary gear set is of a single-pinion type. In this case, the axial dimension of the differential mechanism can be reduced, and the differential mechanism is simply constituted by one planetary gear set.

[0027] Preferably, an overall speed ratio of the vehicular drive system is defined by a speed ratio of the continuously-variable transmission portion and a speed ratio of the step-variable transmission portion. In this case, the vehicle drive force can be obtained over a relatively wide range of the overall speed ratio, by utilizing the speed ratio of the step-variable transmission portion, so that the efficiency of the continuously-variable shifting control of the continuously-variable transmission portion is further improved. The continuously-variable transmission portion placed in its continuously-variable shifting state and the step-variable transmission portion cooperate to constitute a continuously-variable transmission, while the continuously-variable transmission portion placed in the non-continuously-variable shifting state and the step-variable transmission portion cooperate to constitute a step-variable transmission.

[0028] Preferably, the step-variable transmission portion is a step-variable automatic transmission. In this case, the overall speed ratio of the drive system is changed in steps when the step-variable transmission portion is shifted. The change of the overall speed ratio in steps is more rapid than when the overall speed ratio is continuously changed. Accordingly, the drive system functions as a continuously variable transmission capable of smoothly changing the vehicle drive torque, and also capable of changing the speed ratio in steps for rapidly obtaining the vehicle drive torque.

[0029] FIG. 1 is a schematic view showing an arrangement of a drive system of a hybrid vehicle to which the present invention is applicable.

[0030] FIG. 2 is a table indicating shifting actions of the hybrid vehicle drive system of FIG. 1, which is operable in a selected one of a continuously-variable shifting state and a step-variable shifting state, in relation to different combinations of operating states of hydraulically operated frictional coupling devices to effect the respective shifting actions.

[0031] FIG. 3 is a collinear chart indicating relative rotating speeds of the hybrid vehicle drive system of FIG. 1 operated in the step-variable shifting state, in different gear positions of the drive system.

[0032] FIG. 4 is a view indicating input and output signals of an electronic control device according to one embodiment of this invention to control the drive system of FIG. 1.

[0033] FIG. 5 is a functional block diagram illustrating major control functions of the electronic control device of FIG. 4.

[0034] FIG. 6 is a view illustrating an example of a stored shifting boundary line map used for determining a shifting action of an automatic transmission portion, an example of a stored switching boundary line map used for switching the shifting state of a transmission mechanism, and an example of a stored drive-power-source switching boundary line map defining boundary lines between an engine drive region and a motor drive region for switching between an engine drive mode and a motor drive mode, in the same two-dimensional coordinate system defined by control parameters in the form of a running speed and an output torque of the vehicle, such that those maps are related to each other.

[0035] FIG. 7 is a view illustrating an example of a fuel consumption map defining a highest-fuel-economy curve of an engine, and explaining a difference between an operation of the engine in a continuously-variable shifting state (indicated by broken line) of the transmission mechanism and an operation of the engine in a step-variable shifting state (indicated by one-dot chain line) of the transmission mechanism.

[0036] FIG. 8 is a view illustrating a stored relationship defining boundary lines between a continuously-variable shifting region and a step-variable shifting region, which relationship is used to map boundary lines defining the continuously-variable and step-variable shifting regions which are indicated by broken lines in FIG. 6.

[0037] FIG. 9 is a view indicating an example of a change of the engine speed as a result of a shift-up action of the step-variable transmission.

[0038] FIG. 10 is a view showing an example of a manually operated shifting device including a shift lever and operable to select one of a plurality of shift positions.

[0039] FIG. 11 is a flow chart illustrating a control operation of the electronic control device of FIG. 4, that is, a control routine to improve a vehicle acceleration response of an automatic transmission portion upon a shifting action thereof.

[0040] FIG. 12 is a time chart for explaining the control operation when the automatic transmission portion is commanded to effect a power-on shift-down action from a third gear position to a second gear position while a differential portion is placed in a locked state (step-variable shifting state).

[0041] FIG. 13 is a time chart for explaining the control operation when the automatic transmission portion is commanded to effect the power-on shift-down action from the third gear position to the second gear position while the differential portion is placed in a continuously-variable shifting state.

[0042] FIG. 14 is a schematic view corresponding to that of FIG. 1, showing an arrangement of another hybrid vehicle drive system to which the present invention is also applicable.

[0043] FIG. 15 is a table corresponding to that of FIG. 2, indicating shifting actions of the hybrid vehicle drive system of FIG. 14, which is operable in a selected one of the continuously-variable and step-variable shifting states, in relation to different combinations of operating states of hydraulically operated frictional coupling devices to effect the respective shifting actions.

[0044] FIG. 16 is a collinear chart corresponding to that of FIG. 3, indicating relative rotating speeds of the rotary elements of the hybrid vehicle drive system of FIG. 14 in the step-variable shifting state, in the different gear positions.

[0045] FIG. 17 is a perspective view showing an example of a manually operable shifting-state selecting device in the form of a seesaw switch operated by a user to select the shifting state.

[0046] The embodiments of this invention will be described in detail by reference to the drawings.

[0047] Referring to the schematic view of FIG. 1, there is shown a transmission mechanism 10 constituting a part of a drive system for a hybrid vehicle, which drive system is controlled by a control apparatus according to one embodiment of this invention. In FIG. 1, the transmission mechanism 10 includes: an input rotary member in the form of an input shaft 14; a continuously-variable transmission portion in the form of a differential portion 11 connected to the input shaft 14 either directly, or indirectly via a pulsation absorbing damper (vibration damping device) not shown; a step-variable or multiple-step transmission portion in the form of an automatic transmission portion 20 disposed between the differential portion 11 and drive wheels 38 of the vehicle, and connected in series via a power transmitting member 18 (power transmitting shaft) to the transmission portion 11 and the drive wheels 38; and an output rotary member in the form of an output shaft 22 connected to the automatic transmission portion 20. The input shaft 12, differential portion 11, automatic transmission portion 20 and output shaft 22 are coaxially disposed on a common axis in a casing 12 (hereinafter referred to as casing 12) functioning as a stationary member attached to a body of the vehicle, and are connected in series with each other. This transmission mechanism 10 is suitably used for a transverse FR vehicle (front-engine, rear-drive vehicle), and is disposed between a drive power source in the form of an internal combustion engine 8 and the pair of drive wheels 38, to transmit a vehicle drive force from the engine 8 to the pair of drive wheels 38 through a differential gear device 36 (final speed reduction gear) and a pair of drive axles, as shown in FIG. 5. The engine 8 may be a gasoline engine or diesel engine and functions as a vehicle drive power source directly connected to the input shaft 14 or indirectly via a pulsation absorbing damper.

[0048] In the present transmission mechanism 10, the engine 8 and the differential portion 11 are directly connected to each other. This direct connection means that the engine 8 and the transmission portion 11 are connected to each other, without a fluid-operated power transmitting device such as a torque converter or a fluid coupling being disposed therebetween, but may be connected to each other through the pulsation absorbing damper as described above. It is noted that a lower half of the transmission mechanism 10, which is constructed symmetrically with respect to its axis, is omitted in FIG. 1. This is also true to the other embodiments of the invention described below.

[0049] The differential portion 11 is provided with: a first electric motor M1; a power distributing mechanism 16 functioning as a differential mechanism operable to mechanically distribute an output of the engine 8 received by the input shaft 14, to the first electric motor M1 and the power transmitting member 18; and a second electric motor M2 which is rotated with the output shaft 22. The second electric motor M2 may be disposed at any portion of the power transmitting path between the power transmitting member 18 and the drive wheels 38. Each of the first and second electric motors M1 and M2 used in the present embodiment is a so-called motor/generator having a function of an electric motor and a function of an electric generator. However, the first electric motor M1 should function at least as an electric generator operable to generate an electric energy and a reaction force, while the second electric motor M2 should function at least as a drive power source operable to produce a vehicle drive force.

[0050] The power distributing mechanism 16 includes, as major components, a first planetary gear set 24 of a single pinion type having a gear ratio ρ1 of about 0.418, for example, a switching clutch C0 and a switching brake B1. The first planetary gear set 24 has rotary elements consisting of: a first sun gear S1, a first planetary gear P1; a first carrier CA1 supporting the first planetary gear P1 such that the first planetary gear P1 is rotatable about its axis and about the axis of the first sun gear S1; and a first ring gear R1 meshing with the first sun gear S1 through the first planetary gear P1. Where the numbers of teeth of the first sun gear S1 and the first ring gear R1 are represented by ZS1 and ZR1, respectively, the above-indicated gear ratio ρ1 is represented by ZS1/ZR1.

[0051] In the power distributing mechanism 16, the first carrier CA1 is connected to the input shaft 14, that is, to the engine 8, and the first sun gear S1 is connected to the first electric motor M1, while the first ring gear R1 is connected to the power transmitting member 18. The switching brake B0 is disposed between the first sun gear S1 and the casing 12, and the switching clutch C0 is disposed between the first sun gear S1 and the first carrier CA1. When the switching clutch C0 and brake B0 are both released, the power distributing mechanism 16 is placed in a differential state in which three elements of the first planetary gear set 24 consisting of the first sun gear S1, first carrier CA1 and first ring gear R1 are rotatable relative to each other, so as to perform a differential function, so that the output of the engine 8 is distributed to the first electric motor M1 and the power transmitting member 18, whereby a portion of the output of the engine 8 is used to drive the first electric motor M1 to generate an electric energy which is stored or used to drive the second electric motor M2. Accordingly, the power distributing mechanism 16 is placed in the continuously-variable shifting state (electrically established CVT state), in which the rotating speed of the power transmitting member 18 is continuously variable, irrespective of the rotating speed of the engine 8, namely, placed in the differential state in which a speed ratio γ0 (rotating speed of the input shaft 14/rotating speed of the power transmitting member 18) of the power distributing mechanism 16 is continuously changed from a minimum value γ0min to a maximum value γ0max, that is, in the continuously-variable shifting state in which the power distributing mechanism 16 functions as an electrically controlled continuously variable transmission the speed ratio γ0 of which is continuously variable from the minimum value γ0min to the maximum value γ0max.

[0052] When the switching clutch C0 or brake B0 is engaged while the power distributing mechanism 16 is placed in the continuously-variable shifting state, the power distributing mechanism 16 is brought into a locked state or non-differential state in which the differential function is not available. Described in detail, when the switching clutch C0 is engaged, the first sun gear S1 and the first carrier CA1 are connected together, so that the power distributing mechanism 16 is placed in the locked state in which the three rotary elements of the first planetary gear set 24 consisting of the first sun gear S1, first carrier CA1 and first ring gear R1 are rotatable as a unit, namely, placed in a first non-differential state in which the differential function is not available, so that the differential portion 11 is also placed in a non-differential state. In this non-differential state, the rotating speed of the engine 8 and the rotating speed of the power transmitting member 18 are made equal to each other, so that the differential portion 11 (power distributing mechanism 16) is placed in a fixed-speed-ratio shifting state or step-variable shifting state in which the mechanism 16 functions as a transmission having a fixed speed ratio γ0 equal to 1.

[0053] When the switching brake B0 is engaged in place of the switching clutch C0, the first sun gear S1 is fixed to the casing 12, so that the power distributing mechanism 16 is placed in the locked state in which the first sun gear S1 is not rotatable, namely, placed in a second non-differential state in which the differential function is not available, so that the differential portion 11 is also placed in the non-differential state. Since the rotating speed of the first ring gear R1 is made higher than that of the first carrier CA1, the differential portion 11 is placed in the fixed-speed-ratio shifting state or step-variable shifting state in which differential portion 11 (the power distributing mechanism 16) functions as a speed-increasing transmission having a fixed speed ratio γ0 smaller than 1, for example, about 0.7.

[0054] Thus, the frictional coupling devices in the form of the switching clutch C0 and brake B0 function as a differential-state switching device operable to selectively switch the differential portion 11 (power distributing mechanism 16) between the differential state (namely, non-locked state) and the non-differential state (namely, locked state), that is, between the continuously-variable shifting state in which the differential portion 11 (the power distributing mechanism 16) is operable as an electrically controlled continuously variable transmission the speed ratio of which is continuously variable, and the locked state in which the differential portion 11 is not operable as the electrically controlled continuously variable transmission capable of performing a continuously-variable shifting operation, and in which the speed ratio of the transmission portion 11 is held fixed, namely, the fixed-speed-ratio shifting state (non-differential state) in which the transmission portion 11 is operable as a transmission having a single gear position with one speed ratio or a plurality of gear positions with respective speed ratios, namely, the fixed-speed-ratio shifting state in which the transmission portion 11 is operated as a transmission having a single gear position with one speed ratio or a plurality of gear positions with respective speed ratios.

[0055] In other words, the switching clutch C0 and switching brake B0 function as a differential limiting device operable to limit the differential function of the power distributing mechanism 16 for limiting the electric differential function of the differential portion 11, namely, the function of the differential portion 11 as the electrically controlled continuously variable transmission, by placing the power distributing mechanism 16 in its non-differential state to place the differential portion 11 in its step-variable shifting state. The switching clutch C0 and switching brake B0 are also operable to place the power distributing mechanism 16 in its differential state, for placing the differential portion 11 in its continuously-variable shifting state, in which the differential function of the power distributing mechanism 16 and the electric differential function of the differential portion 11 are not limited, namely, the function of the differential portion as the electrically controlled continuously variable transmission is not limited.

[0056] The automatic transmission portion 20 includes a single-pinion type second planetary gear set 26, a single-pinion type third planetary gear set 28 and a single-pinion type fourth planetary gear set 30. The second planetary gear set 26 has: a second sun gear S2; a second planetary gear P2; a second carrier CA2 supporting the second planetary gear P2 such that the second planetary gear P2 is rotatable about its axis and about the axis of the second sun gear S2; and a second ring gear R2 meshing with the second sun gear S2 through the second planetary gear P2. For example, the second planetary gear set 26 has a gear ratio ρ2 of about 0.562. The third planetary gear set 28 has: a third sun gear S3; a third planetary gear P3; a third carrier CA3 supporting the third planetary gear P3 such that the third planetary gear P3 is rotatable about its axis and about the axis of the third sun gear S3; and a third ring gear R3 meshing with the third sun gear S3 through the third planetary gear P3. For example, the third planetary gear set 28 has a gear ratio ρ3 of about 0.425. The fourth planetary gear set 30 has: a fourth sun gear S4; a fourth planetary gear P4; a fourth carrier CA4 supporting the fourth planetary gear P4 such that the fourth planetary gear P4 is rotatable about its axis and about the axis of the fourth sun gear S4; and a fourth ring gear R4 meshing with the fourth sun gear S4 through the fourth planetary gear P4. For example, the fourth planetary gear set 30 has a gear ratio ρ4 of about 0.421. Where the numbers of teeth of the second sun gear S2, second ring gear R2, third sun gear S3, third ring gear R3, fourth sun gear S4 and fourth ring gear R4 are represented by ZS2, ZR2, ZS3, ZR3, ZS4 and ZR4, respectively, the above-indicated gear ratios ρ2, ρ3 and ρ4 are represented by ZS2/ZR2. ZS3/ZR3, and ZS4/ZR4, respectively.

[0057] In the automatic transmission portion 20, the second sun gear S2 and the third sun gear S3 are integrally fixed to each other as a unit, selectively connected to the power transmitting member 18 through a second clutch C2, and selectively fixed to the casing 12 through a first brake B1. The second carrier CA2 is selectively fixed to the casing 12 through a second brake B2, and the fourth ring gear R4 is selectively fixed to the casing 12 through a third brake B3. The second ring gear R2, third carrier CA3 and fourth carrier CA4 are integrally fixed to each other and fixed to the output shaft 22. The third ring gear R3 and the fourth sun gear S4 are integrally fixed to each other and selectively connected to the power transmitting member 18 through a first clutch C1. Thus, the automatic transmission portion 20 and the power transmitting member 18 are selectively connected to each other through the first clutch C1 or the second clutch C2, which is provided to shift the automatic transmission portion 20. In other words, the first clutch C1 and the second clutch C2 function as a coupling device operable to place a power transmitting path between the power transmitting member 18 and the automatic transmission portion 20, that is, between the differential portion 11 (power transmitting member 18) and the drive wheels 38, selectively in one of a power transmitting state in which a vehicle drive force can be transmitted through the power transmitting path, and a power cut-off state in which the vehicle drive force cannot be transmitted through the power transmitting path. Described more specifically, the above-indicated power transmitting path is placed in the power transmitting state when at least one of the first clutch C1 and the second clutch C2 is placed in the engaged state, and is placed in the power cut-off state when the first clutch C1 and the second clutch C2 are placed in the released state. The automatic transmission portion 20 is a step-variable transmission which is operable to perform so-called “clutch-to-clutch shifting actions” each of which is effected by concurrent engaging and releasing actions of the appropriate two frictional coupling devices.

[0058] The above-described switching clutch C0, first clutch C1, second clutch C2, switching brake B0, first brake B1, second brake B2 and third brake B3 (hereinafter collectively referred to as clutches C and brakes B, unless otherwise specified) are hydraulically operated frictional coupling devices used in a conventional vehicular automatic transmission. Each of these frictional coupling devices is constituted by a wet-type multiple-disc clutch including a plurality of friction plates 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 the outer circumferential surface of the rotary drum and tightened at one end by a hydraulic actuator. Each of the clutches C0-C2 and brakes B0-B3 is selectively engaged for connecting two members between which each clutch or brake is interposed.

[0059] In the transmission mechanism 10 constructed as described above, the power distributing mechanism 16 is provided with the switching clutch C0 and the switching brake B0 one of which is engaged to place the differential portion 11 in the continuously-variable shifting state in which the differential portion 11 is operable as a continuously variable transmission, or in the step-variable shifting state (fixed-speed-ratio shifting state) in which the differential portion 11 is operable as a step-variable transmission having a fixed speed ratio or ratios. In the present transmission mechanism 10, therefore, the differential portion 11 placed in the fixed-speed-ratio shifting state by the engaging action of one of the switching clutch C0 and switching brake B0 cooperates with the automatic transmission portion 20 to constitute a step-variable transmission device, while the differential portion 11 placed in the continuously-variable shifting state with the switching clutch C0 and switching brake B0 being both held in the released state cooperates with the automatic transmission portion 20 to constitute an electrically controlled continuously variable transmission device. In other words, the transmission mechanism 10 is placed in its step-variable shifting state by engaging one of the switching clutch C0 and switching brake B9, and in its continuously-variable shifting state by releasing both of the switching clutch C0 and switching brake B0. Similarly, the differential portion 11 is selectively placed in one of its step-variable and continuously-variable shifting states.

[0060] Described in detail, when the transmission mechanism 10 is placed in its step-variable shifting state with the differential portion 11 placed in its step-variable shifting state with one of the switching clutch C0 and switching brake B0 held in the engaged state, one of a first gear position (first speed position) through a fifth gear position (fifth speed position), a reverse gear position (rear drive position) and a neural position is selectively established by engaging actions of a corresponding combination of the two frictional coupling devices selected from the above-described first clutch C1, second clutch C2, first brake B1, second brake B2 and third brake B3, as indicated in the table of FIG. 2. The two frictional coupling devices may consist of a frictional coupling device to be released, and a frictional coupling device to be engaged. The above-indicated positions have respective speed ratios γT (input shaft speed N_IN/output shaft speed N_OUT) which change as geometric series. The speed ratios γT are overall speed ratios of the transmission mechanism 10 determined by a speed ratio γ0 of the differential portion 11 and a speed ratio γof the automatic transmission portion 20.

[0061] Where the transmission mechanism 10 functions as the step-variable transmission, for example, the first gear position having the highest speed ratio γ1 of about 3.357, for example, is established by engaging actions of the switching clutch C0, first clutch C1 and third brake B3, and the second gear position having the speed ratio γ2 of about 2.180, for example, which is lower than the speed ratio γ1, is established by engaging actions of the switching clutch C0, first clutch C1 and second brake B2, as indicated in FIG. 2. Further, the third gear position having the speed ratio γ3 of about 1.424, for example, which is lower than the speed ratio γ2, is established by engaging actions of the switching clutch C0, first clutch C1 and first brake B1, and the fourth gear position having the speed ratio γ4 of about 1.000, for example, which is lower than the speed ratio γ3, is established by engaging actions of the switching clutch C0, first clutch C1 and second clutch C2. The fifth gear position having the speed ratio γ5 of about 0.705, for example, which is smaller than the speed ratio γ4, is established by engaging actions of the first clutch C1, second clutch C2 and switching brake B0. Further, the reverse gear position having the speed ratio γR of about 3.209, for example, which is intermediate between the speed ratios γ1 andγ2, is established by engaging actions of the second clutch C2 and the third brake B3. The neutral position N is established by engaging only the switching clutch C0.

[0062] Where the transmission mechanism 10 functions as the continuously-variable transmission with the differential portion 11 placed in its continuously-variable shifting state, on the other hand, the switching clutch C0 and the switching brake B0 indicated in FIG. 2 are both released, so that the differential portion 11 functions as the continuously variable transmission, while the automatic transmission portion 20 connected in series to the differential portion 11 functions as the step-variable transmission, whereby the speed of the rotary motion transmitted to the automatic transmission portion 20 placed in a selected gear position M, namely, the rotating speed of the power transmitting member 18 is continuously changed, so that the speed ratio of the drive system when the automatic transmission portion 20 is placed in the selected gear position M is continuously variable over a predetermined range. Accordingly, the overall speed ratio γT of the transmission mechanism 10 determined by the speed ratio γ0 of the differential portion 11 and the speed ratio γ of the automatic transmission portion 20 is continuously variable.

[0063] For example, the input speed N_IN of the automatic transmission portion 20 placed in one of the first through fourth gear positions (or the fifth gear position established by the engaging actions of the same frictional coupling devices C1, C2 as used to establish the fourth gear position) is continuously variable in the released state of both of the switching clutch C0 and switching brake B0, as indicated in the table of FIG. 2, while the transmission mechanism 10 functions as the continuously variable transmission, so that the overall speed ratio γT of the transmission mechanism 10 is continuously variable across the adjacent gear positions.

[0064] The collinear chart of FIG. 3 indicates, by straight lines, a relationship among the rotating speeds of the rotary elements in each of the gear positions of the transmission mechanism 10, which is constituted by the differential portion 11 functioning as the continuously-variable shifting portion or first shifting portion, and the automatic transmission portion 20 functioning as the step-variable shifting portion (automatic transmission portion) or second shifting portion. The collinear chart of FIG. 3 is a rectangular two-dimensional coordinate system in which the gear ratios ρ of the planetary gear sets 24, 26, 28, 30 are taken along the horizontal axis, while the relative rotating speeds of the rotary elements are taken along the vertical axis. A lower one of three horizontal lines, that is, the horizontal line X1 indicates the rotating speed of 0, while an upper one of the three horizontal lines, that is, the horizontal line X2 indicates the rotating speed of 1.0, that is, an operating speed N_E of the engine 8 connected to the input shaft 14. The horizontal line XG indicates the rotating speed of the power transmitting member 18.

[0065] Three vertical lines Y1, Y2 and Y3 corresponding to the power distributing mechanism 16 of the differential portion 11 respectively represent the relative rotating speeds of a second rotary element (second element) RE2 in the form of the first sun gear S1, a first rotary element (first element) RE1 in the form of the first carrier CA1, and a third rotary element (third element) RE3 in the form of the first ring gear R1. The distances between the adjacent ones of the vertical lines Y1, Y2 and Y3 are determined by the gear ratio ρ1 of the first planetary gear set 24. That is, the distance between the vertical lines Y1 and Y2 corresponds to “1”, while the distance between the vertical lines Y2 and Y3 corresponds to the gear ratio ρ1. Further, five vertical lines Y4, Y5, Y6, Y7 and Y8 corresponding to the transmission portion 20 respectively represent the relative rotating speeds of a fourth rotary element (fourth element) RE4 in the form of the second and third sun gears S2, S3 integrally fixed to each other, a fifth rotary element (fifth element) RE5 in the form of the second carrier CA2, a sixth rotary element (sixth element) RE6 in the form of the fourth ring gear R4, a seventh rotary element (seventh element) RE7 in the form of the second ring gear R2 and third and fourth carriers CA3, CA4 that are integrally fixed to each other, and an eighth rotary element (eighth element) RE8 in the form of the third ring gear R3 and fourth sun gear S4 integrally fixed to each other. The distances between the adjacent ones of the vertical lines are determined by the gear ratios ρ2, ρ3 and ρ4 of the second, third and fourth planetary gear sets 26, 28, 30. In the relationship among the vertical lines of the collinear chart, the distances between the sun gear and carrier of each planetary gear set corresponds to “1”, while the distances between the carrier and ring gear of each planetary gear set corresponds to the gear ratio ρ. In the differential portion 11, the distance between the vertical lines Y1 and Y2 corresponds to “1”, while the distance between the vertical lines Y2 and Y3 corresponds to the gear ratio ρ. In the automatic transmission portion 20, the distance between the sun gear and carrier of each of the second, third and fourth planetary gear sets 26, 28, 30 corresponds to “1”, while the distance between the carrier and ring gear of each planetary gear set 26, 28, 30 corresponds to the gear ratio ρ.

[0066] Referring to the collinear chart of FIG. 3, the power distributing mechanism 16 (differential portion 11) of the transmission mechanism 10 is arranged such that the first rotary element RE1 (first carrier CA1) of the first planetary gear set 24 is integrally fixed to the input shaft 14 (engine 8) and selectively connected to the second rotary element RE2 (first sun gear S1) through the switching clutch C0, and this second rotary element RE2 is fixed to the first electric motor M1 and selectively fixed to the casing 12 through the switching brake B0, while the third rotary element RE3 (first ring gear R1) is fixed to the power transmitting member 18 and the second electric motor M2, so that a rotary motion of the input shaft 14 is transmitted (input) to the automatic transmission portion 20 through the power transmitting member 18. A relationship between the rotating speeds of the first sun gear S1 and the first ring gear R1 is represented by an inclined straight line L0 which passes a point of intersection between the lines Y2 and X2.

[0067] When the transmission mechanism 10 is brought into the continuously-variable shifting state (differential state) by releasing actions of the switching clutch C0 and brake B0, for instance, the first through third rotary elements RE1-RE3 are rotatable relative to each other, for example, at least the second rotary element RE2 and the third rotary element RE3 are rotatable relative to each other. In this case, the rotating speed of the first sun gear S1 represented by a point of intersection between the straight line L0 and the vertical line Y1 is raised or lowered by controlling the operating speed of the first electric motor M1, so that the rotating speed of the first carrier CA1 represented by the straight line L0 and the vertical line Y2, that is, the engine speed N_E is raised or lowered, if the rotating speed of the first ring gear R1 determined by the vehicle speed V and represented by a point of intersection between the straight line L0 and the vertical line Y3 is substantially held constant.

[0068] When the switching clutch C0 is engaged, the first sun gear S1 and the first carrier CA1 are connected to each other, and the power distributing mechanism 16 is placed in the first non-differential state in which the above-indicated three rotary elements RE1, RE2, RE3 are rotated as a unit and the relative rotation of the second and third rotary elements RE2, RE3 is prevented, so that the straight line L0 is aligned with the horizontal line X2, so that the power transmitting member 18 is rotated at a speed equal to the engine speed N_E. When the switching brake B0 is engaged, on the other hand, the first sun gear S1 is fixed to the casing 12, and the power distributing mechanism 16 is placed in the second non-differential state in which the second rotary element RE2 is stopped and the relative rotation of the second and third rotary elements RE2, RE3 is prevented, so that the straight line L0 is inclined in the state indicated in FIG. 3, whereby the differential portion 11 functions as a speed increasing mechanism. Accordingly, the rotating speed of the first ring gear R1 represented by a point of intersection between the straight lines L0 and Y3, that is, the rotating speed of the power transmitting member 18 is made higher than the engine speed NE and transmitted to the automatic transmission portion 20.

[0069] In the automatic transmission portion 20, the fourth rotary element RE4 is selectively connected to the power transmitting member 18 through the second clutch C2, and selectively fixed to the casing 12 through the first brake B1, and the fifth rotary element RE5 is selectively fixed to the casing 12 through the second brake B2, while the sixth rotary element RE6 is selectively fixed to the casing 12 through the third brake B3. The seventh rotary element RE7 is fixed to the output shaft 22 and the second electric motor M2, while the eighth rotary element RE8 is selectively connected to the power transmitting member 18 through the first clutch C1.

[0070] When the first clutch C1 and the third brake B3 are engaged, the automatic transmission portion 20 is placed in the first gear position. The rotating speed of the output shaft 22 in the first gear position is represented by a point of intersection between the vertical line Y7 indicative of the rotating speed of the seventh rotary element RE7 fixed to the output shaft 22 and an inclined straight line L1 which passes a point of intersection between the vertical line Y8 indicative of the rotating speed of the eighth rotary element RE8 and the horizontal line X2, and a point of intersection between the vertical line Y6 indicative of the rotating speed of the sixth rotary element RE6 and the horizontal line X1, as indicated in FIG. 3. Similarly, the rotating speed of the output shaft 22 in the second gear position established by the engaging actions of the first clutch C1 and second brake B2 is represented by a point of intersection between an inclined straight line L2 determined by those engaging actions and the vertical line Y7 indicative of the rotating speed of the seventh rotary element RE7 fixed to the output shaft 22. The rotating speed of the output shaft 22 in the third gear position established by the engaging actions of the first clutch C1 and first brake B1 is represented by a point of intersection between an inclined straight line L3 determined by those engaging actions and the vertical line Y7 indicative of the rotating speed of the seventh rotary element RE7 fixed to the output shaft 22. The rotating speed of the output shaft 22 in the fourth gear position established by the engaging actions of the first clutch C1 and second clutch C2 is represented by a point of intersection between a horizontal line L4 determined by those engaging actions and the vertical line Y7 indicative of the rotating speed of the seventh rotary element RE7 fixed to the output shaft 22. In the first through fourth gear positions in which the switching clutch C0 is placed in the engaged state, the eighth rotary element RE8 is rotated at the same speed as the engine speed N_E, with the drive force received from the differential portion 11, that is, from the power distributing mechanism 16. When the switching clutch B0 is engaged in place of the switching clutch C0, the eighth rotary element RE8 is rotated at a speed higher than the engine speed N_E, with the drive force received from the power distributing mechanism 16. The rotating speed of the output shaft 22 in the fifth gear position established by the engaging actions of the first clutch C1, second clutch C2 and switching brake B0 is represented by a point of intersection between a horizontal line L5 determined by those engaging actions and the vertical line Y7 indicative of the rotating speed of the seventh rotary element RE7 fixed to the output shaft 22.

[0071] FIG. 4 illustrates signals received by an electronic control device 40 provided to control the transmission mechanism 10, and signals generated by the electronic control device 40. This electronic control device 40 includes a so-called microcomputer incorporating a CPU, a ROM, a RAM and an input/output interface, and is arranged to process the signals according to programs stored in the ROM while utilizing a temporary data storage function of the ROM, to implement hybrid drive controls of the engine 8 and electric motors M1 and M2, and drive controls such as shifting controls of the transmission portion 20.

[0072] The electronic control device 40 is arranged to receive various sensors and switches shown in FIG. 4, various signals such as: a signal indicative of a temperature TEMP_W of cooling water of the engine 8; a signal indicative of a selected operating position P_SH of a shift lever; a signal indicative of the operating speed N_E of the engine 8; a signal indicative of a value indicating a selected group of forward-drive positions of the transmission mechanism 10; a signal indicative of an M mode (motor drive mode); a signal indicative of an operated state of an air conditioner; a signal indicative of a vehicle speed V corresponding to the rotating speed N_OUT of the output shaft 22; a signal indicative of a temperature of a working oil of the automatic transmission portion 20; a signal indicative of an operated state of a side brake; a signal indicative of an operated state of a foot brake; a signal indicative of a temperature of a catalyst; a signal indicative of an amount of operation (an angle of operation) A_CC of a manually operable vehicle accelerating member in the form of an accelerator pedal 45 (shown in FIG. 5); a signal indicative of an angle of a cam; a signal indicative of the selection of a snow drive mode; a signal indicative of a longitudinal acceleration value G of the vehicle; a signal indicative of the selection of an auto-cruising drive mode; a signal indicative of a weight of the vehicle; signals indicative of speeds of the drive wheels of the vehicle; a signal indicative of an operating state of a step-variable shifting switch provided to place the differential portion 11 (power distributing mechanism 16) in the step-variable shifting state (locked state) in which the transmission mechanism 10 functions as a step-variable transmission; a signal indicative of a continuously-variable shifting switch provided to place the differential portion 11 in the continuously variable-shifting state (differential state) in which the transmission mechanism 10 functions as the continuously variable transmission; a signal indicative of a rotating speed N_M1 of the first electric motor M1 (hereinafter referred to as “first electric motor speed N_M1); a signal indicative of a rotating speed N_M2 of the second electric motor M2 (hereinafter referred to as “second electric motor speed N_M2); and a signal indicative of an amount of electric energy SOS stored in (a charging state of) an electric-energy storage device 60 (shown in FIG. 5).

[0073] The electronic control device 40 is further arranged to generate various signals such as: control signals to be applied to an engine output control device 43 (shown in FIG. 5) to control the output of the engine 8, such as a drive signal to drive a throttle actuator 97 for controlling an angle of opening θ_TH of an electronic throttle valve 96 disposed in a suction pipe 95 of the engine 8, a signal to control an amount of injection of a fuel by a fuel injecting device 98 into the suction pipe 95 or cylinders of the engine 8, a signal to be applied to an ignition device 99 to control the ignition timing of the engine 8, and a signal to adjust a supercharger pressure of the engine 8; a signal to operate the electric air conditioner; signals to operate the electric motors M1 and M2; a signal to operate a shift-range indicator for indicating the selected operating or shift position of the shift lever 46; a signal to operate a gear-ratio indicator for indicating the gear ratio; a signal to operate a snow-mode indicator for indicating the selection of the snow drive mode; a signal to operate an ABS actuator for anti-lock braking of the wheels; a signal to operate an M-mode indicator for indicating the selection of the M-mode; signals to operate solenoid-operated valves incorporated in a hydraulic control unit 42 (shown in FIG. 5) provided to control the hydraulic actuators of the hydraulically operated frictional coupling devices of the differential portion 11 and automatic transmission portion 20; a signal to operate an electric oil pump used as a hydraulic pressure source for the hydraulic control unit 42; a signal to drive an electric heater; and a signal to be applied to a cruise-control computer.

[0074] FIG. 5 is a functional block diagram of FIG. 5 for explaining major control functions of the electronic control device. A step-variable shifting control means 54 shown in FIG. 5 is arranged to determine whether a shifting action of the automatic transmission portion 20 should take place, that is, to determine the gear position to which the automatic transmission portion 20 should be shifted. This determination is made on the basis of a condition of the vehicle in the form of the vehicle speed V and an output torque T_OUT of the automatic transmission portion 20, and according to a shifting boundary line map (shifting control map or relation) which is stored in memory means 56 and which represents shift-up boundary lines indicated by solid lines in FIG. 5 and shift-down boundary lines indicated by one-dot chain lines in FIG. 5. The step-variable shifting control means 54 generates commands (shifting commands or hydraulic control command) to be applied to the hydraulic control unit 42, to selectively engage and release the respectively two hydraulically operated frictional coupling devices (except the switching clutch C0 and brake B0), for establishing the determined gear position of the automatic transmission portion 20 according to the table of FIG. 2. Described in detail, the step-variable shifting control means 54 commands the hydraulic control unit 42 to control the solenoid-operated valves incorporated in the hydraulic control unit 42, for activating the appropriate hydraulic actuators to concurrently engage one of the two frictional coupling device and release the other frictional coupling device, to effect the clutch-to-clutch shifting actions of the automatic transmission portion 20.

[0075] A hybrid control means 52 functions as continuously-variable shifting control means and is arranged to control the engine 8 to be operated in an operating range of high efficiency, and control the first and second electric motors M1, M2 so as to optimize a proportion of drive forces generated by the engine 8 and the second electric motor M2, and a reaction force generated by the first electric motor M1 during its operation as the electric generator, for thereby controlling the speed ratio γ0 of the differential portion 11 operating as the electrically controlled continuously variable transmission, while the transmission mechanism 10 is placed in the continuously-variable shifting state, that is, while the differential portion 11 is placed in the differential state. For instance, the hybrid control means 52 calculates a target (required) vehicle output at the present running speed V of the vehicle, on the basis of the operating amount A_CC of the accelerator pedal 46 used as an operator's required vehicle output and the vehicle running speed V, and calculate a target total vehicle output on the basis of the calculated target vehicle output and a required amount of generation of an electric energy by the first electric motor M1. The hybrid control means 52 calculates a target output of the engine 8 to obtain the calculated target total vehicle output, while taking account of a power transmission loss, a load acting on various devices of the vehicle, an assisting torque generated by the second electric motor M2, etc. The hybrid control means 52 controls the speed N_E and torque T_E of the engine 8, so as to obtain the calculated target engine output, and the amount of generation of the electric energy by the first electric motor M1.

[0076] The hybrid control means 52 is arranged to implement the hybrid control while taking account of the presently selected gear position of the automatic transmission portion 20, so as to improve the drivability of the vehicle and the fuel economy of the engine 8. In the hybrid control, the differential portion 11 is controlled to function as the electrically controlled continuously-variable transmission, for optimum coordination of the engine speed N_E and vehicle speed V for efficient operation of the engine 8, and the rotating speed of the power transmitting member 18 determined by the selected gear position of the transmission portion 20. That is, the hybrid control means 52 determines a target value of the overall speed ratio γT of the transmission mechanism 10, so that the engine 8 is operated according to a stored highest-fuel-economy curve (fuel-economy map or relation) stored in memory means and indicated by broken line in FIG. 7. The target value of the overall speed ratio γT of the transmission mechanism 10 permits the engine torque T_E and speed N_E to be controlled so that the engine 8 provides an output necessary for obtaining the target vehicle output (target total vehicle output or required vehicle drive force). The highest-fuel-economy curve is obtained by experimentation so as to satisfy both of the desired operating efficiency and the highest fuel economy of the engine 8, and is defined in a two-dimensional coordinate system defined by an axis of the engine speed N_E and an axis of the engine torque T_E. The hybrid control means 52 controls the speed ratio γ0 of the differential portion 11, so as to obtain the target value of the overall speed ratio γT, so that the overall speed ratio γT can be controlled within a predetermined range, for example, between 13 and 0.5.

[0077] In the hybrid control, the hybrid control means 52 controls an inverter 58 such that the electric energy generated by the first electric motor M1 is supplied to an electric-energy storage device 60 and the second electric motor M2 through the inverter 58. That is, a major portion of the drive force produced by the engine 8 is mechanically transmitted to the power transmitting member 18, while the remaining portion of the drive force is consumed by the first electric motor M1 to convert this portion into the electric energy, which is supplied through the inverter 58 to the second electric motor M2, so that the second electric motor M2 is operated with the supplied electric energy, to produce a mechanical energy to be transmitted to the output shaft 22. Thus, the drive system is provided with an electric path through which an electric energy generated by conversion of a portion of a drive force of the engine 8 is converted into a mechanical energy.

[0078] It is noted in particular that the automatic transmission portion 20 is shifted under the control of the step-variable shifting control means 54, to change its speed ratio in steps, whereby the overall speed ratio γT of the transmission mechanism 10 is changed in steps. Namely, the overall speed ratio γT of the transmission mechanism 10 is changed during a shifting action of the automatic transmission portion 30, in steps or non-continuously, rather than continuously as in a continuously variation transmission the speed ratio of which is continuously changed. The change of the overall speed ratio γT in steps permits a more rapid change of the vehicle drive force than the continuous change of the overall speed ratio γT, but on the other hand has a risk of causing a shifting shock and deterioration of the fuel economy due to a failure of control of the engine speed N_E following the highest-fuel-economy curve.

[0079] In view of the facts indicated above, the hybrid control means 52 is arranged to change the speed ratio of the differential portion 11 during a shifting action of the automatic transmission portion 20, so as to reduce the change of the overall speed ratio γT in steps during the shifting action of the automatic transmission portion 20, that is, to cause a continuous transient change of the overall speed ratio γT during the shifting action. For example, the hybrid control means 52 is arranged to control the speed ratio of the differential portion 11 in synchronization with the shifting action of the automatic transmission portion 20, so as to reduce the amount of change of the engine speed N_E as a result of the shifting action, by utilizing the electric CVT function (differential function) of the differential portion 11.

[0080] Described in detail, the hybrid control means 52 is arranged to control the speed ratio of the differential portion 11 in synchronization with the shifting action of the automatic transmission portion 20, so as to control the engine speed N_E at a value not higher than a predetermined speed N_E′, irrespective of a change of the rotating speed of the power transmitting member 18 that is the input speed N_IN of the automatic transmission portion 20, which takes place due to the shifting action of the automatic transmission portion 20. The above-indicated predetermined speed N_E′ is a target value of the engine speed N_E value used for controlling the speed ratio γ0 of the differential portion 11 for reducing the change of the engine speed N_E so as to cause a continuous transient change of the overall speed ratio γT during the shifting action of the automatic transmission portion 20. This target value is obtained by experimentation and stored in memory.

[0081] For instance, the hybrid control means 52 is arranged to change the speed ratio γ0 of the differential portion 11 in a direction opposite to the direction of change of the speed ratio γ of the automatic transmission portion 20, by an amount equal to the amount of change of the speed ratio γ in steps, in synchronization with the shifting action of the automatic transmission portion 20, so as to prevent a non-continuous transient change of the overall speed ratio γT during the shifting action of the automatic transmission portion 20, that is, so as to hold the engine speed N_E substantially constant before and after the shifting action of the automatic transmission portion 20, for thereby permitting a continuous change of the overall speed ratio γT. This arrangement is effective to reduce the amount of change of the overall speed ratio γT in steps during the shifting action of the automatic transmission 20, for thereby reducing the shifting shock, in spite of the change of the speed ratio γ of the automatic transmission 20 due to the shifting action. Thus, the hybrid control means 52 functions as continuously-variable-shifting control means for controlling the speed ratio of the differential portion 11 by changing the first electric motor speed N_M1, so as to reduce the amount of change of the engine speed N_E as a result of a shifting action of the automatic transmission portion 20, irrespective of a change of the input speed N_IN of the automatic transmission portion20, that is, a change of the rotating speed of the power transmitting member 18 (hereinafter referred to as “power-transmitting-member speed N₁₈”) as a result of the shifting action of the automatic transmission portion 20.

[0082] Described in another point of view, the engine 8 is generally operated with a step-variable transmission, following a one-dot chain line indicated in FIG. 7, and with a continuously variable transmission, following the highest-fuel-economy curve indicated by broken line in FIG. 7, or following a line closer to the highest-fuel-economy curve, than when the engine 8 is operated with the step-variable transmission. Accordingly, the engine torque T_E for obtaining the required vehicle drive torque (drive force) is obtained at the engine speed N_E which is closer to the highest-fuel-economy curve, when the engine 8 is operated with the continuously variable transmission than when it is operated with the step-variable transmission. This means that the continuously variable transmission permits a higher degree of fuel economy that the step-variable transmission. Therefore, the hybrid control means 52 is arranged to control the speed ratio y of the differential portion 11 such that the engine 8 is operated following the highest-fuel-economy line indicated by the broken line in FIG. 7, for preventing deterioration of the fuel economy, in spite of a change of the speed ratio of the automatic transmission portion 20 in steps during its shifting action. This arrangement enables the transmission mechanism 10 as a whole to function as a continuously variable transmission, thereby assuring an improved fuel economy.

[0083] As described above, the hybrid control means 52 is arranged to implement a so-called “synchronous speed ratio control” of the differential portion 11 in synchronization with the shifting action of the automatic transmission portion 20. This synchronous speed ratio control is initiated at a moment which is determined by taking account of a response delay from a moment of determination by the step-variable control means 54 of a shifting action of the automatic transmission portion 20 to a moment of initiation of an actual change of the input speed N_IN of the automatic transmission 20 caused by operations of the appropriate hydraulically operated frictional coupling devices, namely, a response delay up to a moment of initiation of a so-called “inertia phase” in which the input speed N_IN of the automatic transmission portion 20, that is, the rotating speed of the power transmitting member 18 changes in the process of the shifting action of the automatic transmission portion 20. For example, the response delay is obtained by experimentation and stored in memory. Alternatively, the hybrid control means 52 initiates the synchronous speed ratio control of the differential portion 11 at a moment of detection of initiation of an actual change of the input speed N_IN of the automatic transmission portion 20.

[0084] The synchronous speed ratio control of the differential portion 11 is terminated at a moment of termination of the inertia phase in the process of the shifting action of the automatic transmission portion 20. For example, a length of duration of the shifting action of the automatic transmission portion 20 is obtained by experimentation and stored in memory. Alternatively, the hybrid control means 52 terminates the synchronous speed ratio control of the differential portion 11 at a moment of detection that the actual change of the input speed N_IN of the automatic transmission portion 20 has been zeroed.

[0085] As described above, the hybrid control means 52 implements the synchronous speed ratio control of the differential portion 11, during the time period (length of time) of the inertia phase in the process of the shifting action of the automatic transmission portion 20, for example, during a time period obtained by experimentation, or during a length of time from the moment of initiation of the actual change of the input speed N_IN of the automatic transmission portion 20 to the moment of detection of zeroing of the actual change of the input speed N_IN.

[0086] The hybrid control means 52 is further arranged to hold the engine speed N_E substantially constant or at a desired value, by controlling the first electric motor speed N_M1 and/or the second electric motor speed N_M2 owing to the electric CVT function of the differential portion 11, irrespective of whether the vehicle is stationary or running. In other words, the hybrid control means 52 is capable of controlling the first electric motor speed N_M1 and/or the second electric motor speed N_M2 as desired while holding the engine speed N_E substantially constant or at a desired value.

[0087] To raise the engine speed N_E during running of the vehicle, for example, the hybrid control means 52 raises the operating speed N_M1 of the first electric motor M1 since the power-transmitting-member speed N₁₈ is determined by the vehicle speed V (speed of the drive wheels 38), as is apparent from the collinear chart of FIG. 3. To hold the engine speed N_E substantially constant during a shifting operation of the automatic transmission portion 20, the hybrid control means 52 changes the first electric motor speed N_M1 in a direction opposite to the direction of change of the power-transmitting-member speed N₁₈ caused by the shifting operation of the automatic transmission portion 20, while the engine speed N_E is held substantially constant.

[0088] The hybrid control means 52 includes engine output control means functioning to control the engine 8, so as to provide a required output, by controlling the throttle actuator 97 to open and close the electronic throttle valve 96, and controlling an amount and time of fuel injection by the fuel injecting device 98 into the engine 8, and/or the timing of ignition of the igniter by the ignition device 99, alone or in combination. For instance, the hybrid control means 52 is basically arranged to control the throttle actuator 97 on the basis of the operating amount A_CC of the accelerator pedal 45 (manually operable vehicle accelerating member) and according to a predetermined stored relationship (not shown) between the operating amount A_CC and the opening angle θ_TH of the electronic throttle valve 96 such that the opening angle θ_TH increases with an increase of the operating amount A_CC. The engine output control device 43 controls the throttle actuator 97 to open and close the electronic throttle valve 96, controls the fuel injecting device 98 to control the fuel injection, and controls the ignition device 99 to control the ignition timing of the igniter, for thereby controlling the torque of the engine 8, according to the commands received from the hybrid control means 52.

[0089] The hybrid control means 52 is capable of establishing a motor-drive mode to drive the vehicle by the electric motor, by utilizing the electric CVT function of the differential portion 11, irrespective of whether the engine 8 is in the non-operated state or in the idling state. Solid line A in FIG. 6 represents an example of a boundary line defining an engine-drive region and a motor-drive region, for switching the vehicle drive power source for starting and driving the vehicle (hereinafter referred to as “drive power source”), between the engine 8 and the electric motor (e.g., second electric motor M2). In other words, the vehicle drive mode is switchable between a so-called “engine drive mode” corresponding to the engine-drive region in which the vehicle is started and driven with the engine 8 used as the drive power source, and the so-called “motor-drive mode” corresponding to the motor-drive region in which the vehicle is driven with the second electric motor M2 used as the drive power source. A predetermined stored relationship representing the boundary line (solid line A) of FIG. 6 for switching between the engine-drive mode and the motor-drive mode is an example of a drive-power-source switching map (drive-power-source map) in a two-dimensional coordinate system defined by control parameters in the form of the vehicle speed V and a drive-force-related value in the form of the output torque T_OUT. This drive-power-source switching map is stored in the memory means 56, together with the shifting boundary line map (shifting map) indicated by solid lines and one-dot chain lines in FIG. 6.

[0090] The hybrid control means 52 determines whether the vehicle condition is in the motor-drive region or engine-drive region, and establishes the motor-drive mode or engine-drive mode. This determination is made on the basis of the vehicle condition represented by the vehicle speed V and the required output torque T_OUT, and according to the drive-power-source switching map of FIG. 6. As is understood from FIG. 6, the motor-drive mode is generally established by the hybrid control means 52, when the output torque T_OUT is in a comparatively low range in which the engine efficiency is comparatively low, namely, when the engine torque T_E is in a comparatively low range, or when the vehicle speed V is in a comparatively low range, that is, when the vehicle load is comparatively low. Usually, therefore, the vehicle is started in the motor-drive mode, rather than in the engine-drive mode. When the vehicle condition upon starting of the vehicle is outside the motor-drive region defined by the drive-power-source switching map of FIG. 6, as a result of an increase of the required output torque T_OUT or engine torque T_E due to an operation of the accelerator pedal 45, the vehicle may be started in the engine-drive mode.

[0091] For reducing a dragging of the engine 8 in its non-operated state and improving the fuel economy in the motor-drive mode, the hybrid control means 52 is arranged to hold the engine speed N_E at zero or substantially zero as needed, owing to the electric CVT function (differential function) of the differential portion 11, that is, by controlling the differential portion 11 to perform its electric CVT function (differential function), so that the first electric motor speed 1 is controlled so as to be freely rotated to have a negative speed N_M1.

[0092] The hybrid control means 52 is further capable of performing a so-called “drive-force assisting” operation (torque assisting operation) to assist the engine 8, by supplying an electric energy from the first electric motor M1 or the electric-energy storage device 60 to the second electric motor M2, so that the second electric motor M2 is operated to transmit a drive torque to the drive wheels 38. Thus, the second electric motor M2 may be used in addition to the engine 8, in the engine-drive mode. The torque assisting operation may be performed to increase the output torque of the second electric motor M2 in the motor drive mode.

[0093] The hybrid control means 52 functions as electric-motor control means for controlling a drive-force assisting operation performed by the second electric motor M2 to increase the vehicle drive force for improving the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator, during a shifting action of the automatic transmission portion 20 under the control of the step-variable control means 54, for thereby compensating for a delayed response to an increase of the vehicle drive torque due to a low shifting response of the automatic transmission portion 20.

[0094] The hybrid control means 52 is arranged to hold the engine 8 in an operated state owing to the electric CVT function of the differential portion 11, irrespective of whether the vehicle is stationary or running at a relatively low speed. When the first electric motor M1 is required to be operated to charge the electric-energy storage device 60 while the vehicle is stationary, in order to charge the electric-energy storage device 60 where the electric energy amount SOS stored in the storage device 60 is reduced, the speed N_E of the engine 8 which is operated to operate the first electric motor M1 at a relatively high speed can be kept high enough to permit the operation of the engine 8 by itself, owing to the differential function of the power distributing mechanism 16, even while the operating speed of the second electric motor M2 determined by the vehicle speed V is zero (substantially zero) when the vehicle is stationary.

[0095] The hybrid control means 52 is further arranged to place the first electric motor M1 in a non-load state by cutting off an electric current applied from the electric energy storage device 60 to the first electric motor M1 through the inverter 58. When the first electric motor M1 is placed in the non-load state, the first electric motor M1 is permitted to be freely rotated, and the differential portion is placed in a state similar to the power cut-off state in which power cannot be transmitted through the power transmitting path within the differential portion 11, and no output can be generated from the differential portion 11. Namely, the hybrid control means 52 is arranged to place the first electric motor M1 in the non-load state, for thereby placing the differential portion 11 in a neutral state in which the power transmitting path is electrically cut off.

[0096] The high-speed-gear determining means 62 is arranged to determine whether the gear position to which the transmission mechanism 10 should be shifted on the basis of the vehicle condition and according to the shifting boundary line map stored in the memory means 56 and indicated in FIG. 6 by way of example is a high-speed-gear position, for example, the fifth gear position. This determination is made by determining whether the gear position selected by the step-variable shifting control means 54 is the fifth gear position or not, for determining which one of the switching clutch C0 and brake B0 should be engaged to place the transmission mechanism 10 in the step-variable shifting state.

[0097] The switching control means 50 is arranged to selectively switch the transmission mechanism 10 between the continuously-variable shifting state and the step-variable shifting state, that is, between the differential state and the locked state, by engaging and releasing the coupling devices (switching clutch C0 and brake B0) on the basis of the vehicle condition. For example, the switching control means 50 is arranged to determine whether the shifting state of the transmission mechanism 10 (differential portion 11) should be changed, on the basis of the vehicle condition represented by the vehicle speed V and the required output torque T_OUT and according to the switching boundary line map (switching control map or relation) stored in the memory means 56 and indicated by two-dot chain line in FIG. 6 by way of example, namely, whether the vehicle condition is in the continuously-variable shifting region for placing the transmission mechanism 10 in the continuously-variable shifting state, or in the step-variable shifting region for placing the transmission mechanism 10 in the step-variable shifting state. The switching control means 50 places the transmission mechanism 10 in the continuously-variable shifting state or step-variable shifting state, depending upon whether the vehicle condition is in the continuously-variable shifting region or in the step-variable shifting region. Thus, the switching control means 50 limits the electrically controlled differential function of the differential portion 11 by placing the differential portion 11 in the step-variable shifting state by controlling the switching clutch C0 and/or the switching brake B0. That is, the switching control means 50 functions as differential limiting means for limiting the function of the differential portion 11 as the electrically controlled continuously variable transmission.

[0098] Described in detail, when the switching control means 50 determines that the vehicle condition is in the step-variable shifting region, the switching control means 50 disables the hybrid control means 52 to implement a hybrid control or continuously-variable shifting control, and enables the step-variable shifting control means 54 to implement a predetermined step-variable shifting control in which the transmission portion 20 is automatically shifted according to the shifting boundary line map stored in the memory means 56 and indicated in FIG. 6 by way of example. FIG. 2 indicates the combinations of the engaging actions of the hydraulically operated frictional coupling devices C0, C1, C2, B0, B1, B2 and B3, which are stored in the memory means 56 and which are selectively used for automatic shifting of the automatic transmission portion 20. In the step-variable shifting state, the transmission mechanism 10 as a whole constituted by the differential portion 11 and the automatic transmission portion 20 functions as a so-called step-variable automatic transmission which is automatically shifted according to the table of FIG. 2.

[0099] When the high-speed-gear determining means 62 has determined that the transmission mechanism 10 should be shifted to the fifth gear position, the switching control means 50 commands the hydraulic control unit 42 to release the switching clutch C0 and engage the switching brake B0, for enabling the differential portion 11 to function as an auxiliary transmission having a fixed speed ratio γ0 of 0.7, for example, so that the transmission mechanism 10 as a whole is placed in a high-speed gear position so-called “an overdrive gear position” having a speed ratio lower than 1.0. When the high-speed-gear determining means 62 has not determined that the transmission mechanism 10 should be shifted to the fifth gear position, the switching control means 50 commands the hydraulic control unit 42 to engage the switching clutch C0 and release the switching brake B0, for enabling the differential portion 11 to function as an auxiliary transmission having a fixed speed ratio γ0 of 1.0, for example, so that the transmission mechanism 10 as a whole is placed in a speed-reducing gear position having a speed ratio not lower than 1.0. Thus, when the transmission mechanism 10 is switched to the step-variable shifting state by the switching control means 50, the differential portion 11 operable as the auxiliary transmission is placed in a selected one of two gear positions under the control of the switching control means 50 while the automatic transmission portion 20 connected in series to the differential portion 11 functions as a step-variable transmission, so that the transmission mechanism 10 as a whole functions as the so-called step-variable automatic transmission.

[0100] When the switching control means 50 has determined that the vehicle condition is in the continuously-variable shifting region for placing the transmission mechanism 10 in the continuously-variable shifting state, the switching control means 50 commands the hydraulic control unit 42 to release both of the switching clutch C0 and brake B0, for placing the differential portion 11 in the continuously-variable shifting state. At the same time, the switching control means 50 enables the hybrid control means 52 to implement the hybrid control, and commands the step-variable shifting control means 54 to select and hold a predetermined one of the gear positions, or to permit the automatic transmission portion 20 to be automatically shifted according to the shifting boundary line map stored in the map memory 56 and indicated in FIG. 6 by way of example. In the latter case, the variable-step shifting control means 54 implements the automatic shifting control by suitably selecting the combinations of the operating states of the frictional coupling devices indicated in the table of FIG. 2, except the combinations including the engagement of the switching clutch C0 and brake B0. Thus, the differential portion 11 switched to the continuously-variable shifting state under the control of the switching control means 50 functions as the continuously variable transmission while the automatic transmission portion 20 connected in series to the differential portion 11 functions as the step-variable transmission, so that the transmission mechanism 10 provides a sufficient vehicle drive force, such that the input speed N_IN of the automatic transmission portion 20 placed in one of the first through fourth gear positions, namely, the rotating speed N₁₈ of the power transmitting member 18 is continuously changed, so that the speed ratio of the transmission mechanism 10 when the transmission portion 20 is placed in one of those gear positions is continuously variable over a predetermined range. Accordingly, the speed ratio of the automatic transmission portion 20 is continuously variable across the adjacent gear positions, whereby the total speed ratio γT of the transmission mechanism 10 is continuously variable.

[0101] The maps of FIG. 6 will be described in detail. The shifting boundary line map (shifting control map or relation) shown in FIG. 6 by way of example and stored in the memory means 56 is used for determining whether the automatic transmission portion 20 should be shifted, and is defined in a two-dimensional coordinate system by control parameters consisting of the vehicle speed V and the drive-force-related value in the form of the required output torque T_OUT. In FIG. 6, the solid lines indicate the shift-up boundary lines, while the one-dot chain lines indicate the shift-down boundary lines.

[0102] The broken lines in FIG. 6 represent the upper vehicle-speed limit V1 and the upper output-torque limit T1 which are used for the switching control means 50 to determine whether the vehicle condition is in the step-variable shifting region or the continuously-variable shifting region. In other words, the broken lines represent a high-speed-running boundary line indicative of the upper vehicle-speed limit V1 above which it is determined that the hybrid vehicle is in a high-speed running state, and a high-output-running boundary line indicative of the upper output-torque limit T1 of the output torque T_OUT of the automatic transmission portion 20 above which it is determined that the hybrid vehicle is in a high-output running state. The output torque T_OUT is an example of the drive-force-related value which relates to the drive force of the hybrid vehicle. FIG. 6 also shows two-dot chain lines which are offset with respect to the broken lines, by a suitable amount of control hysteresis for determination as to whether the step-variable shifting state is changed to the continuously-variable shifting state or vice versa. Thus, the broken lines and two-dot chain lines of FIG. 6 constitute the stored switching boundary line map (switching control map or relation) used by the switching control means 50 to determine whether the vehicle condition is in the step-variable shifting region or the continuously-variable shifting region, depending upon whether the control parameters in the form of the vehicle speed V and the output torque Tour are higher than the predetermined upper limit values V, T1. This switching boundary line map may be stored in the memory means 56, together with the shifting boundary line map. The switching boundary line map may use at least one of the upper vehicle-speed limit V1 and the upper output-torque limit T1, or at least one of the vehicle speed V and the output torque Tour, as at least one parameter.

[0103] The above-described shifting boundary line map, switching boundary line, and drive-power-source switching map may be replaced by stored equations for comparison of the actual vehicle speed V with the limit value V1 and comparison of the actual output torque T_OUT with the limit value T1. In this case, the switching control means 50 switches the transmission mechanism 10 in the step-variable shifting state by engaging the switching brake B0, when the actual vehicle speed V has exceeded the upper limit V1, or by engaging the switching clutch C0, when the output torque T_OUT of the automatic transmission portion 20 has exceeded the upper limit T1.

[0104] The switching control means 50 may be arranged to place the transmission mechanism 10 in the step-variable shifting state even when the vehicle condition is in the continuously-variable shifting region, upon detection of any functional failure or deterioration of the electric components such as the electric motors which are operable to operate the differential portion 11 as the electrically controlled continuously variable transmission. Those electric components include components such as the first electric motor M1, second electric motor M2, inverter 58, electric-energy storage device 50 and electric lines interconnecting those components, which are associated with the electric path through which an electric energy generated by the first electric motor M1 is converted into a mechanical energy. The functional deterioration of the components may be caused by their failure or a drop of their temperatures.

[0105] The drive-force-related value indicated above is a parameter corresponding to the drive force of the vehicle, which may be the output torque T_OUT of the automatic transmission portion 20, the engine output torque T_E or an acceleration value G of the vehicle, as well as a drive torque or drive force of drive wheels 38. The parameter may be: an actual value calculated on the basis of the operating amount A_CC of the accelerator pedal 45 or the opening angle of the throttle valve (or intake air quantity, air/fuel ratio or amount of fuel injection) and the engine speed N_E;; or any one of estimated values of the required (target) engine torque T_E, required (target) output torque T_OUT of the transmission potion 20 and required vehicle drive force, which are calculated on the basis of the operating amount A_CC of the accelerator pedal 45 or the operating angle of the throttle valve. The above-described vehicle drive torque may be calculated on the basis of not only the output torque T_OUT, etc., but also the ratio of the differential gear device 36 and the radius of the drive wheels 38, or may be directly detected by a torque sensor or the like.

[0106] For instance, the upper vehicle-speed limit V1 is determined so that the transmission mechanism 10 is placed in the step-variable shifting state while the vehicle is in the high-speed running state. This determination is effective to reduce a possibility of deterioration of the fuel economy of the vehicle if the transmission mechanism 10 were placed in the continuously-variable shifting state while the vehicle is in the high-speed running state. On the other hand, the upper output-torque limit T1 is determined depending upon the operating characteristics of the first electric motor M1, which is small-sized and the maximum electric energy output of which is made relatively small so that the reaction torque of the first electric motor M1 is not so large when the engine output is relatively high in the high-output running state of the vehicle.

[0107] Referring to FIG. 8, there is shown a switching boundary line map (switching control map or relation) which is stored in the memory means 56 and which defines engine-output lines serving as boundary lines used by the switching control means 50 to determine whether the vehicle condition is in the step-variable or continuously-variable shifting region. These engine-output lines are defined by control parameters in the form of the engine speed N_E and the engine torque N_T. The switching control means 50 may use the switching boundary line map of FIG. 8 in place of the switching boundary line map of FIG. 6, to determine whether the vehicle condition is in the continuously-variable or step-variable shifting region, on the basis of the engine speed N_E and engine torque T_E. The switching boundary line map of FIG. 6 may be based on the switching boundary line map of FIG. 8. In other words, the broken lines in FIG. 6 may be determined on the basis of the relation (map) of FIG. 8, in the two-dimensional coordinate system defined by the control parameters in the for of the vehicle speed V and the output torque T_OUT.

[0108] The step-variable shifting region defined by the switching boundary line map of FIG. 6 is defined as a high-torque drive region in which the output torque T_OUT is not lower than the predetermined upper limit T1, or a high-speed drive region in which the vehicle speed V is not lower than the predetermined upper limit V1. Accordingly, the step-variable shifting control is implemented when the torque of the engine 8 is comparatively high or when the vehicle speed V is comparatively high, while the continuously-variable shifting control is implemented when the torque of the engine 8 is comparatively low or when the vehicle speed V is comparatively low, that is, when the engine 8 is in a normal output state.

[0109] Similarly, the step-variable shifting region defined by the switching boundary line map of FIG. 8 is defined as a high-torque drive region in which the engine torque T_E is not lower than the predetermined upper limit TE1, or a high-speed drive region in which the engine speed N_E is not lower than the predetermined upper limit NE1, or alternatively defined as a high-output drive region in which the output of the engine 8 calculated on the basis of the engine torque N_T and speed N_E is not lower than a predetermined limit. Accordingly, the step-variable shifting control is implemented when the torque T_E, speed N_E or output of the engine 8 is comparatively high, while the continuously-variable shifting control is implemented when the torque T_E, speed N_E or output of the engine 8 is comparatively low, that is, when the engine 8 is in the normal output state. The boundary lines of the switching boundary switching map of FIG. 8 may be considered as high-speed threshold lines or high-engine-output threshold lines, which define upper limit of the vehicle speed V or engine output.

[0110] In the present embodiment described above, the transmission mechanism 10 is placed in the continuously-variable shifting state in a low-speed or medium-speed running state of the vehicle or in a low-output or medium-output running state of the vehicle, assuring a high degree of fuel economy of the vehicle. In a high-speed running of the vehicle at the vehicle speed V higher than the upper limit V1, the transmission mechanism 10 is placed in the step-variable shifting state in which the output of the engine 8 is transmitted to the drive wheels 38 primarily through the mechanical power transmitting path, so that the fuel economy is improved owing to reduction of a loss of conversion of the mechanical energy into the electric energy, which would take place when the differential portion 11 functions as the electrically controlled continuously variable transmission.

[0111] In a high-output running state of the vehicle with the output torque T_OUT higher than the upper limit T1, too, the transmission mechanism 10 is placed in the step-variable shifting state. Therefore, the transmission mechanism 10 is placed in the continuously-variable shifting state only when the vehicle speed V is relatively low or medium or when the engine output is relatively low or medium, so that the required amount of electric energy generated by the first electric motor M1, that is, the maximum amount of electric energy that must be transmitted from the first electric motor M1 can be reduced, whereby the required electrical reaction force of the first electric motor M1 can be reduced, making it possible to minimize the required sizes of the first electric motor M1 and the second electric motor M2, and the required size of the drive system including those electric motors.

[0112] Namely, the upper limit TE1 is determined such that the first electric motor M1 can withstand the reaction torque when the engine output T_E is not higher than the upper limit TE1, and the differential portion 11 is placed in the step-variable shifting state when the vehicle is in the high-output running state in which the engine torque T_E is higher than the upper limit TE1. In the step-variable shifting state of the transmission portion 11, therefore, the first electric motor M1 need not withstand the reaction torque with respect to the engine torque T_E as in the continuously-variable shifting state of the transmission portion 11, making it possible to reduce deterioration of durability of the first electric motor M1 while preventing an increase of its required size. In other words, the required maximum output of the first electric motor M1 in the present embodiment can be made smaller than its reaction torque capacity corresponding to the maximum value of the engine output T_E. That is, the required maximum output of the first electric motor M1 can be determined such that its reaction torque capacity is smaller than a value corresponding to the engine torque T_E exceeding the upper limit TE1, so that the first electric motor M1 can be small-sized.

[0113] The maximum output of the first electric motor M1 is a nominal rating of this motor which is determined by experimentation in the environment in which the motor is operated. The above-described upper limit of the engine torque T_E is determined by experimentation such that the upper limit is a value which is equal to or lower than the maximum value of the engine torque T_E, and below which the first electric motor M1 can withstand the reaction torque, so that the deterioration of durability of the first electric motor M1 can be reduced.

[0114] According to the other concept, the transmission mechanism 10 is placed in the step-variable shifting state (fixed-speed-ratio shifting state), rather than in the continuously-variable shifting state, in the high-output running state of the vehicle in which the vehicle operator has a stronger desire for improved drivability of the vehicle rather than improved fuel economy. In this case, the engine speed N_E changes with a shift-up action of the automatic transmission portion 20, assuring a comfortable rhythmic change of the engine speed N_E as the transmission portion 20 is shifted up, as indicated in FIG. 9.

[0115] FIG. 10 shows an example of a manually operable shifting device in the form of a shifting device 46. The shifting device 46 includes the above-described shift lever 48, which is disposed laterally adjacent to an operator's seat, for example, and which is manually operated to select one of a plurality of positions consisting of: a parking position P for placing the drive system 10 (namely, automatic transmission portion 20) in a neutral state in which a power transmitting path is disconnected with both of the switching clutch C0 and brake B0 placed in the released state, and at the same time the output shaft 22 of the automatic transmission portion 20 is in the locked state; a reverse-drive position R for driving the vehicle in the rearward direction; a neutral position N for placing the drive system 10 in the neutral state; an automatic forward-drive shifting position D; and a manual forward-drive shifting position M.

[0116] When the shift lever 46 is operated to a selected one of the shift positions, a manual valve incorporated in the hydraulic control unit 42 and operatively connected to the 4 shift lever 48 is operated to establish the corresponding state of the hydraulic control unit 42. In the automatic forward-drive position D or the manual forward-drive position M, one of the first through fifth gear positions (1^st through 5^th) indicated in the table of FIG. 2 is established by electrically controlling the appropriate solenoid-operated valves incorporated in the hydraulic control unit 42.

[0117] The above-indicated parking position P and the neutral position N are non-drive positions selected when the vehicle is not driven, while the above-indicated reverse-drive position R, and the automatic and manual forward-drive positions D, M are drive positions selected when the vehicle is driven. In the non-drive positions P, N, the power transmitting path in the automatic transmission portion 20 is in the power-cut-off state established by releasing both of the clutches C1 and C2, as shown in the table of FIG. 2. In the drive positions R, D, M, the power transmitting path in the automatic transmission portion 20 is in the power-transmitting state established by engaging at least one of the clutches C1 and C2, as also shown in the table of FIG. 2.

[0118] Described in detail, a manual operation of the shift lever 46 from the parking position P or neutral position N to the reverse-drive position R causes the second clutch C2 to be engaged for switching the power transmitting path in the automatic transmission portion 20 from the power-cut-off state to the power-transmitting state. A manual operation of the shift lever 46 from the neutral position N to the automatic forward-drive position D causes at least the first clutch C1 to be engaged for switching the power transmitting path in the automatic transmission portion 20 from the power-cut-off state to the power-transmitting state. The automatic forward-drive position D provides a highest-speed position, and positions “4” through “L” selectable in the manual forward-drive position M are engine-braking positions in which an engine brake is applied to the vehicle.

[0119] The manual forward-drive position M is located at the same position as the automatic forward-drive position D in the longitudinal direction of the vehicle, and is spaced from or adjacent to the automatic forward-drive position D in the lateral direction of the vehicle. The shift lever 46 is operated to the manual forward-drive position M, for manually selecting one of the above-indicated positions “D” through “L”. Described in detail, the shift lever 46 is movable from the manual forward-drive position M to a shift-up position “+” and a shift-down position “−”, which are spaced from each other in the longitudinal direction of the vehicle. Each time the shift lever 46 is moved to the shift-up position “+” or the shift-down position “−”, the presently selected position is changed by one position. The five positions “D” through “L” have respective different lower limits of a range in which the overall speed ratio γT of the transmission mechanism 10 is automatically variable, that is, respective different lowest values of the overall speed ratio γT which corresponds to the highest output speed of the transmission mechanism 10. Namely, the five positions “D” through “L” select respective different numbers of the speed positions (gear positions) of the automatic transmission portion 20 which are automatically selectable, so that the lowest overall speed ratio γT available is determined by the selected number of the gear positions. The shift lever 46 is biased by biasing means such as a spring so that the shift lever 46 is automatically returned from the shift-up position “+” and shift-down position “−” back to the manual forward-drive position M. The shifting device 46 is provided with a shift-position sensor 49 operable to detect the presently selected position of the shift lever 46, so that signals indicative of the presently selected operating position of the shift lever 48 and the number of shifting operations of the shift lever 46 in the manual forward-shifting position M.

[0120] When the shift lever 46 is operated to the automatic forward-drive position D, the switching control means 50 effects an automatic switching control of the transmission mechanism 10 according to the stored switching boundary line map indicated in FIG. 6, and the hybrid control means 52 effects the continuously-variable shifting control of the power distributing mechanism 16, while the step-variable shifting control means 54 effects an automatic shifting control of the automatic transmission 20. When the transmission mechanism 10 is placed in the step-variable shifting state, for example, the shifting action of the transmission mechanism 10 is automatically controlled to select an appropriate one of the first through the fifth gear position indicated in FIG. 2. When the drive system is placed in the continuously-variable shifting state, the speed ratio of the power distributing mechanism 16 is continuously changed, while the shifting action of the automatic transmission 20 is automatically controlled to select an appropriate one of the first through fourth gear positions, so that the overall speed ratio γT of the transmission mechanism 10 is controlled so as to be continuously variable within the predetermined range. The automatic forward-drive position D is a position selected to establish an automatic shifting mode (automatic mode) in which the transmission mechanism 10 is automatically shifted.

[0121] When the shift lever 46 is operated to the manual forward-drive position M, on the other hand, the shifting action of the transmission mechanism 10 is automatically controlled by the switching control means 50, hybrid control means 52 and step-variable shifting control means 54, such that the overall speed ratio γT is variable within a predetermined range the lower limit of which is determined by the gear position having the lowest speed ratio, which gear position is determined by the manually selected one of the shift positions. When the transmission mechanism 10 is placed in the step-variable shifting state, for example, the shifting action of the transmission mechanism 10 is automatically controlled within the above-indicated predetermined range of the overall speed ratio γT. When the transmission mechanism 10 is placed in the step-variable shifting state, the speed ratio of the power distributing mechanism 16 is continuously changed, while the shifting action of the automatic transmission 20 is automatically controlled to select an appropriate one of the gear positions the number of which is determined by the manually selected one of the shift positions, so that the overall speed ratio γT of the transmission mechanism 10 is controlled so as to be continuously variable within the predetermined range. The manual forward-drive position M is a position selected to establish a manual shifting mode (manual mode) in which the selectable gear positions of the transmission mechanism 10 are manually selected.

[0122] As described above, the transmission mechanism 10 in the present embodiment is provided with the automatic transmission portion 20 in addition to the differential portion 11, and the automatic transmission portion 20 is shifted under the control of the step-variable shifting control means 54, on the basis of the running state of the vehicle and according to the shifting boundary line map shown in FIG. 6 by way of example. Where the shifting action of the automatic transmission portion 20 under the control of the step-variable shifting control means 54 is the power-on shift-down action caused by an operation of the accelerator pedal 45 by the vehicle operator who desires to increase the vehicle drive force for accelerating the vehicle to a relatively large extent, it is required to rapidly increase the vehicle drive torque for improving the vehicle acceleration performance, and the vehicle acceleration drivability as felt by the vehicle operator.

[0123] However, a relatively long time is required for the automatic transmission portion 20 to complete its shift-down action, that is, before the vehicle drive torque has been increased as a result of a change of the speed ratio γ in steps. Thus, the shift-down action has a risk of a low response to an increase of the vehicle drive torque, and deterioration of the vehicle acceleration drivability as felt by the vehicle operator.

[0124] The required shifting time of the automatic transmission 20 is determined by the hydraulic pressures of the appropriate frictional coupling devices to be released and engaged for the shifting action, and the timings of the releasing and engaging actions of those frictional coupling devices. The hydraulic pressures and the timings of the releasing and engaging actions of the frictional coupling devices are determined according to the engine torque T_E, irrespective of whether the differential portion 11 is placed in the continuously-variable shifting state or the step-variable shifting state, while taking account of the durability of the hydraulically operated frictional coupling devices, so that a rate of change N_IN′(=dN_IN/dt) of the input speed N_IN of the automatic transmission portion 20 coincides with a target value, and so as to reduce the shifting shock and shorten the required shifting time, for example. The target value of the rate of change N_IN′ of the input speed N_IN, which is obtained by experimentation and stored in memory, is a value which is a compromise between a relatively high rate of change N_IN′ for a high shifting response effective to improve the vehicle acceleration drivability as felt by the vehicle operator, and a relatively low rate of change N_IN′ for a low shifting response effective to reduce the shifting shock.

[0125] If the hydraulic pressures and the timings of the releasing and engaging actions of the hydraulically operated frictional devices are determined primarily for shortening the required shifting time, there is a possibility of an increase of the shifting shock.

[0126] In view of the above, the hybrid control means 52 is arranged to perform the drive-force assisting operation by the second electric motor M2 to increase the vehicle drive torque (drive force) for improving the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator, during a shifting action of the automatic transmission portion 20 under the control of the step-variable control means 54, for thereby compensating for a delayed response to an increase of the vehicle drive torque due to a low shifting response of the automatic transmission portion 20.

[0127] Accelerator-operating-amount-change-rate determining means 80 is operable upon determination of a shift-down action of the automatic transmission portion 20, to determine whether the shift-down action is a power-on shift-down action indicative of a relatively high degree of vehicle acceleration required by the vehicle operator. This determination is made depending upon whether the rate of change A_CC′(=dA_CC/dt) of the operating amount of the accelerator pedal is higher than a predetermined threshold value A_CC1. This threshold value A_CC1, which is obtained by experimentation, is an upper limit of the speed of operation of the accelerator pedal 45 by the vehicle operator, above which the vehicle acceleration value required by the vehicle operator is so high as to cause the power-on shift down action of the automatic transmission portion 20 that requires the drive-force assisting operation by the second electric motor M2 under the control of the hybrid control means 52.

[0128] Drive-force-assisting feasibility determining means 82 is operable to determine whether the drive-force assisting operation by the second electric motor M2 under the control of the hybrid control means 52 is feasible. For instance, the drive-force-assisting feasibility determining means 82 is arranged to determine that the drive-force assisting operation by the second electric motor M2 is not feasible, in the event of occurrence of any one of: failure or functional deterioration of the second electric motor M2; output limitation of the second electric motor M2 due to a rise of a temperature T_MG2 of the second electric motor M2; functional deterioration of the electric-energy storage device 76 due to reduction of a remaining electric energy amount SOC stored therein, or due to a drop of its temperature; and failure or functional deterioration of an electric system for driving the second electric motor M2.

[0129] When the accelerator-operating-amount-change-rate determining means 80 has determined that the drive-force assisting operation by the second electric motor M2 is not required, or when the drive-force assisting feasibility determining means 82 has determined that the drive-force assisting operation by the second electric motor M2 is not feasible, the hybrid control means 52 does not perform the drive-force assisting operation by the second electric motor M2 under the control of the step-variable shifting control means 54 in the shifting action of the automatic transmission portion 20.

[0130] In the present embodiment, the transmission mechanism 10 (differential portion 11 or power distributing mechanism 16) is switchable between the continuously-variable shifting state (differential state) and the non-continuously-variable shifting state, for example, the step-variable shifting state (locked state), and the switching control means 50 is arranged to select one of those two shifting states of the differential portion 11 on the basis of the vehicle condition, so that the differential portion 11 is placed in the selected one of the continuously-variable shifting state and the step-variable shifting state.

[0131] While the differential portion 11 is placed in the continuously-variable shifting state, for example, the engine speed N_E can be changed (set) as needed by the differential function of the differential portion 11, so that the speed ratio of the differential portion 11 is controlled by the hybrid control means 52 by controlling the first electric motor M1, during a shifting action (transient shifting period) of the automatic transmission portion 20 under the control of the step-variable shifting control means 54, so as to hold the engine speed N_E substantially constant, irrespective of a change of the input speed N_IN of the automatic transmission portion 20 due to its shifting action, for thereby reducing the shifting shock and improve the fuel economy.

[0132] During the power-on shift-down action of the automatic transmission portion 20 caused by an operation of the accelerator pedal 40 to increase the vehicle drive torque, which causes a large amount of change of the target value of the overall speed ratio γT, for example, the speed ratio of the differential portion 11 is first controlled to hold the engine speed N_E substantially constant for permitting a continuous change of the overall speed ratio γT, and is then controlled such that the overall speed ratio γT continuously changes toward the target value.

[0133] In the power-on shift-down action caused by the operation of the operation of the accelerator pedal 45 to increase the vehicle drive torque, however, the vehicle operator feels more comfortable with a change of the overall speed ratio γT in steps (in a non-continuous manner) to rapidly increase the vehicle drive torque (drive force) as in the non-continuously-variable shifting state of the differential portion 11, rather than the continuous change of the overall speed ratio.

[0134] In view of the above, the hybrid control means 52 has, in addition to the above-described function of the synchronous speed ratio control of the differential portion 11, an additional function of controlling the speed ratio of the differential portion 11 to cause a change of the overall speed ratio γT in steps (in the non-continuous manner) toward the target value during the shift-down action of the automatic transmission portion 20 in the continuously-variable shifting state of the differential portion 11, without synchronization with and independently of the shifting action, as in the non-continuously-variable shifting state of the differential portion 11, when the accelerator-operating-amount-change-rate determining means 80 has determined that the drive-force assisting operation by the second electric motor M2 is required. This additional function of controlling the speed ratio of the differential portion 11 is to change the overall speed ratio γT in steps, rather than continuously by controlling the speed ratio γ0 of the differential portion 11 in synchronization with the shifting action of the automatic transmission portion 20 and according to a change of the speed ratio γ of the automatic transmission portion 20. This additional function makes it possible to change the overall speed ratio γT toward the target value, by adding (or subtracting) a change of the speed ratio of the differential portion 11 to (from) the change of the speed ratio of the automatic transmission portion 20 in steps, so that the overall speed ratio γT can be changed in steps during the shift-down action of the automatic transmission portion 20, when the shift-down action is the power-on shift-down action caused by the operation of the accelerator pedal 45 to increase the vehicle drive force, that is, when the target value of the overall speed ratio γT is required to be changed by a large amount during the shifting action of the automatic transmission portion 20.

[0135] The hybrid control means 52 can rapidly increase the vehicle drive force, by rapidly raising the engine speed N_E by the differential function using the first electric motor M1 immediately after the moment of determination of the shift-down action, in place of performing the drive-force assisting operation by the second electric motor M2.

[0136] Described in another point of view, the hybrid control means 52 is operable, upon determination of the shift-down action by the step-variable shifting control means 54 and determination by the accelerator-operating-amount-change-rate determining means 80 that the drive-force assisting operation by the second electric motor M2 is required, in the continuously-variable shifting state of the differential portion 11, to rapidly raise the engine speed N_E by using the first electric motor M1 immediately after the moment of determination of the shift-down action, for increasing the vehicle drive torque in the process of the shift-down action, in place of controlling the speed ratio of the differential portion 11 by using the first electric motor M1 so as to reduce a change of the engine speed N_E irrespective of a change of the input speed N_IN of the automatic transmission 20.

[0137] During a shifting action (a transient shifting period) of the automatic transmission 20 under the control of the step-variable shifting control means 54 in the non-continuously-variable shifting state of the differential portion 11, for example, the speed ratio γ0 of the differential portion 11 is held constant like the speed ratio of the automatic transmission portion 20, and the engine speed NE is changed according to the vehicle speed V and the speed ratio γ of the automatic transmission portion 20, like the input speed N_IN of the automatic transmission portion 20. Namely, the control of the speed ratio of the differential portion 11 as implemented by the hybrid control means 52 in the continuously-variable shifting state is not implemented in the non-continuously-variable shifting state of the differential portion 11. In the non-continuously-variable-shifting state of the differential portion 11, therefore, the hybrid control means 52 is not operable to increase the vehicle drive force by rapidly raising the engine speed N_E by using the first electric motor M1 during the shifting action (transient shifting period) of the automatic transmission portion 20 under the control of the step-variable shifting control means 54, although this manner of increasing the vehicle drive force is possible in the continuously-variable shifting state of the differential portion 11.

[0138] In view of the above, the hybrid control means 52 performs the drive-force assisting operation by the second electric motor M2 during a shifting action of the automatic transmission portion 20, in different manners depending upon whether the differential portion 11 is placed in the continuously-variable shifting state in which the engine speed N_E can be raised as needed owing to the differential function, or in the non-continuously-variable-shifting state in which like the input speed N_IN, the engine speed N_E is changed according to the vehicles peed V and the speed ratio γ of the automatic transmission portion 20.

[0139] Described in detail, the hybrid control means 52 is arranged to perform the drive-force assisting operation by the second electric motor M2 to increase the vehicle drive force for improving the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator, for thereby compensating for a delayed response to an increase of the vehicle drive torque due to a low shifting response of the automatic transmission portion 20, in the non-continuously-variable shifting state of the differential portion 11 in which the response to the increase of the vehicle drive force is relatively low in a shift-down action of the automatic transmission 20, as well as in the continuously-variable shifting state in which the response to the increase of the vehicle drive force can be improved by raising the engine speed N_E to increase the vehicle drive force immediately after initiation of the shifting action of the automatic transmission 20.

[0140] For example, the amount of an assisting drive force to be generated by the drive-force assisting operation by the second electric motor M2 under the control of the hybrid control means 52 is made larger to increase the vehicle drive force by a larger amount in the non-continuously-variable-shifting state of the differential state 11, than in the continuously-variable shifting state, upon determination of the shift-down action by the step-variable shifting control means 54 and determination by the accelerator-operating-amount-change-rate determining means 80 that the drive-force assisting operation by the second electric motor M2 is required to improve the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator.

[0141] Further, the drive-force assisting operation by the second electric motor M2 under the control of the hybrid control means 52 during the shift-down action is initiated at an earlier point of time to more rapidly increase the vehicle drive torque in the non-continuously-variable shifting state of the differential portion 11, than in the continuously-variable shifting state, upon determination of the shift-down action by the step-variable shifting control means 54 and determination by the accelerator-operating-amount-change-rate determining means 80 that the drive-force assisting operation by the second electric motor M2 is required to improve the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator.

[0142] The amount of the assisting drive force to be generated by the drive-force assisting operation by the second electric motor M2, and the moment of initiation of the drive-force assisting operation are determined by the hybrid control means 52, so as to obtain almost the same shifting response, that is, so as to increase the vehicle drive force by almost the same amount in the continuously-variable and non-continuously-variable shifting states of the differential portion 11, for obtaining almost the same vehicle acceleration performance and almost the same vehicle acceleration drivability as felt by the vehicle operator in the continuously-variable and non-continuously-variable shifting states.

[0143] Differential-state determining means 84 is operable upon determination of a shift-down action of the automatic transmission portion 20 by the step-variable shifting control means 54, upon determination by the accelerator-operating-amount-change-rate determining means 80 that the drive-force assisting operation by the second electric motor M2 is required, and upon determination by the drive-force-assisting feasibility determining means 82 that the drive-force assisting operation by the second electric motor M2 is feasible. The differential-state determining means 84 is arranged to determine whether the differential portion 11 is placed in the differential state, that is, in the continuously-variable shifting state. This determination is made for the hybrid driver means 52 to perform the drive-force assisting operation by the second electric motor M2 in the different manners depending upon whether the differential portion 11 is placed in the continuously-variable shifting state or in the non-continuously-variable shifting state. For instance, the differential-state determining means 84 determines whether the differential portion 11 or the power distributing mechanism 16 is placed in the continuously-variable shifting state, by determining whether the vehicle condition represented by the vehicle speed V and the output torque T_OUT is in the continuously-variable shifting region in which the differential portion 11 should be switched to the continuously-variable shifting state. The determination as to whether the vehicle condition is in the continuously-variable shifting region is made according to the switching boundary line map shown in FIG. 6 by way of example, which is formulated to determine whether the transmission mechanism 10 should be switched to the step-variable shifting region for placing the transmission mechanism 10 in the step-variable shifting state, or to the continuously-variable shifting region for placing the transmission mechanism 10 to the continuously-variable shifting state.

[0144] FIG. 11 is a flow chart illustrating a major portion of a control operation of the electronic control device 40, that is, a control routine to improve a vehicle acceleration response of the automatic transmission portion upon a shifting action thereof. This control routine is repeatedly executed with an extremely short cycle time of about several milliseconds to about several tens of milliseconds, for example.

[0145] FIG. 12 is a time chart for explaining the control operation when the automatic transmission portion 20 is commanded to effect a power-on shift-down action from the third gear position to the second gear position while the differential portion 11 is placed in the locked state (step-variable shifting state).

[0146] FIG. 13 is a time chart for explaining the control operation when the automatic transmission portion 20 is commanded to effect the power-on shift-down action from the third gear position to the second gear position while the differential portion 11 is placed in the continuously-variable shifting state.

[0147] The control routine is initiated with step S1 (“step” being hereinafter omitted) corresponding to the accelerator-operating-amount-change-rate determining means 80, to determine whether a shift-down action of the automatic transmission 20 determined by the step-variable shifting control means 54 is a power-on shift-down action which is desired by the vehicle operator to accelerate the vehicle to a relatively large extent and which requires the drive-force assisting operation by the second electric motor M2. The shift-down action is determined by the step-variable shifting control means 54, on the basis of the vehicle condition represented by the vehicle speed V and the output torque T_OUT of the automatic transmission portion 20, and according to the shifting boundary line map shown in FIG. 6 by way of example. For instance, the determination as to whether the shift-down action is the power-on shift-down action is made by determining whether the rate of change A_CC′ of the operating amount of the accelerator pedal is higher than the predetermined threshold value A_CC1.

[0148] In the examples of FIGS. 12 and 13, the automatic transmission 20 is commanded to be shifted down from the third gear position to the second gear position at a point of time t1, and this shift-down action is the power-on shift-down action which is caused by the operation of the accelerator pedal at the change rate A_CC′ higher than the predetermined threshold value A_CC1 and which requires the drive-force assisting operation by the second electric motor M2.

[0149] If an affirmative decision is obtained in said S1, the control flow goes to S2 corresponding to the drive-force-assisting feasibility determining means 82, to determine whether the drive-force assisting operation by the second electric motor M2 is feasible. For instance, the drive-force-assisting feasibility determining means 82 determines that the drive-force assisting operation by the second electric motor M2 is not feasible, in the event of occurrence of any one of: failure or functional deterioration of the second electric motor M2; output limitation of the second electric motor M2 due to a rise of the temperature T_MG2 of the second electric motor M2; functional deterioration of the electric-energy storage device 76 due to reduction of the remaining electric energy amount SOC stored therein, or due to a drop of its temperature; and failure or functional deterioration of the electric system for driving the second electric motor M2.

[0150] If an affirmative decision is obtained in said S2, the control flow goes to S3 corresponding to the differential-state determining means 84, to determine whether the power distributing mechanism 16 is placed in the differential state, that is, whether the differential portion (continuously-variable transmission portion) 11 is placed in the continuously-variable shifting state. For example, this determination is made by determining, according to the switching boundary line map of FIG. 6, whether the vehicle condition is in the continuously-variable shifting region for switching the transmission mechanism 10 to the continuously-variable shifting state.

[0151] If an affirmative decision is obtained in said S3, the control flow goes to S4 corresponding to the hybrid control means 54, to implement the drive-force assisting operation by the second electric motor M2 for the power-on shift-down action, in the continuously-variable shifting state of the differential portion 11. In the case of the power-on shift-down action in the continuously-variable shifting state of the differential portion 11, for example, the engine speed N_E is rapidly raised by the differential function by using the first electric motor M1 immediately after the moment of determination of the shift-down action, and the vehicle drive torque is rapidly increased in the process of the shift-down action. In this case of the power-on shift-down action in the continuously-variable shifting state, the amount of the assisting drive force to be generated by the drive-force assisting operation by the second electric motor M1 is made smaller than that in the case of the power-on shift-down action in the non-continuously-variable shifting state.

[0152] At the point of time t1 indicated in FIG. 13, the automatic transmission portion 20 is commanded to be shifted down to the second gear position in the continuously-variable shifting state (differential state) of the differential portion (continuously-variable transmission portion) 11, and the reduction of a releasing hydraulic pressure P_B1 of the first brake B1 as the coupling device to be released is initiated. During a time period from a point of time t2 to a point of time t5, an engaging hydraulic pressure P_B2 of the second brake B2 as the coupling device to be engaged is increased. At a point of time t4, an engaging action of the second brake B2 is completed, and the shift-down action of the automatic transmission portion 20 is terminated. The transient hydraulic pressure of the coupling device to be released and the transient hydraulic pressure of the coupling device to be engaged, during the time period from the point of time t1 to the point of time t5, are determined so that the rate of change N_IN′ of the input speed N_IN of the automatic transmission portion 20 coincides with the target value in the process of the shifting action. For example, upon initiation of supply of the pressurized working fluid to the coupling device to be engaged, the pressure of the fluid is made relatively high to rapidly eliminate the back clearance of the coupling device, that is, for rapid filling of the coupling device. Then, the fluid pressure is once lowered to prevent a risk of an engaging shock of the coupling device, which would take place if the relatively high initial pressure was maintained for a long time. Subsequently, the fluid pressure to be applied to the coupling device to be engaged is gradually increased to a predetermined value for complete engagement of the coupling device.

[0153] In the example of FIG. 13, a rise of the first electric motor speed N_M1 is initiated substantially at the moment of determination of the shift-down action, and continues during the time period between the points of time t1 and t3, so that the speed ratio γ0 of the differential portion 11 is increased to raise the engine speed N_E. During the time period between the points of time t3 and t5, the input speed N_IN of the automatic transmission portion 20 (power-transmitting-member speed N₁₈) is raised in the process of the shift-down action, while the first electric motor speed N_M1 is kept substantially constant, whereby the engine speed N_E is further raised.

[0154] In the case of the power-on shift-down action of FIG. 13 caused by the rate of change A_CC′ of the operating amount of the accelerator pedal higher than the predetermined threshold value A_CC1, the speed ratio of the differential portion 11 is controlled by the differential function using at least the first electric motor M1, to change the overall speed ratio γT in a non-continuous manner (in steps) during the shift-down action, namely, controlled without synchronization with the shift-down action of the automatic transmission portion 20, to change the overall speed ratio γT finally to the target value, while utilizing a change of the speed ratio of the automatic transmission portion 20 in steps in the process of the shift-down action. Accordingly, the shifting response is improved in the case of FIG. 13, as compared with a case wherein the overall speed ratio γT is continuously changed during the shift-down action.

[0155] During the time period between the points of time t2 and t5 indicated in FIG. 13, the drive-force assisting operation by the second electric motor M2 is performed to improve the vehicle acceleration performance during the shift-down action. Since the engine speed N_E is raised to increase the vehicle drive force immediately after the moment of determination of the shift-down action, however, the assisting drive force to be generated by the drive-force assisting operation is made smaller than in the case of the power-on shift-down action in the non-continuously-variable shifting state of the differential portion 11. Accordingly, the amount of electric power consumption required for driving the second electric motor M2 is made smaller, so that the fuel economy is improved.

[0156] If a negative decision is obtained in said S3, the control flow goes to step S5 corresponding to the hybrid control means 52, to implement the drive-force assisting operation by the second electric motor M2 for the power-on shift-down action, in the non-continuously-variable shifting state of the differential portion 11. In the case of the power-on shift-down action in the non-continuously-variable shifting state of the differential portion 11, for example, a relatively long time is required before the vehicle drive force is increased by a change of the speed ratio γ in steps during the shift-down action, that is, after the moment of determination of the shift-down action and before the vehicle drive force is increased by a rise of the engine speed N_E, so that the response to an increase of the vehicle drive torque during the shift-down action is relatively low. Further, unlike the continuously-variable shifting state of the differential portion 11, the non-continuously-variable shifting state does not permit the first electric motor M1 to raise the engine speed NE to increase the vehicle drive force. In the case of the power-on shift-down action in the non-continuously-variable shifting state, the amount of the assisting drive force to be generated by the drive-force assisting operation by the second electric motor M1 is made larger than that in the case of the power-on shift-down action in the continuously-variable shifting state. Further, the drive-force assisting operation is initiated at an earlier point of time in the non-continuously-variable shifting state.

[0157] At the point of time t1 indicated in FIG. 12, the automatic transmission portion 20 is commanded to be shifted down to the second gear position in the non-continuously-variable shifting state (locked state) of the differential portion (continuously-variable transmission) 11, and the reduction of the releasing hydraulic pressure P_B1 of the first brake B1 as the coupling device to be released is initiated. During a time period from the point of time t1 to the point of time t4, the engaging hydraulic pressure P_B2 of the second brake B2 as the coupling device to be engaged is increased. At the point of time t4, the engaging action of the second brake B2 is completed, and the shift-down action of the automatic transmission portion 20 is terminated. The transient hydraulic pressure of the coupling device to be released and the transient hydraulic pressure of the coupling device to be engaged, during the time period from the point of time t1 to the point of time 4, are determined so that the rate of change N_IN′ of the input speed N_IN of the automatic transmission portion 20 coincides with the target value in the process of the shifting action. For example, upon initiation of supply of the pressurized working fluid to the coupling device to be engaged, the pressure of the fluid is made relatively high. The fluid pressure is then once lowered to prevent the risk of the engaging shock of the coupling device, and is subsequently gradually increased to the predetermined value for complete engagement of the coupling device.

[0158] In the example of FIG. 12 in which the shift-down action takes place in the locked state of the differential portion 11 with the switching clutch C0 placed in the engaged state, the transmission mechanism 10 as a whole functions as a step-variable transmission, and the input speed N_IN of the automatic transmission portion 20 and the engine speed N_E are raised in the process of the shift-down action during the time period from the point of time t2 to the point of time t4. In other words, the vehicle drive torque is not increased during the time period from the point of time t1 at which the shift-down action is determined, to the point of time t2 at which the coupling device to be engaged begins to have an engaging torque capacity.

[0159] In the case of the power-on shift-down action of FIG. 12 caused by the rate of change A_CC′ of the operating amount of the accelerator pedal higher than the predetermined threshold value A_CC1, the drive-force assisting operation by the second electric motor M2 is initiated immediately after the moment of determination of the shift-down action, to improve the vehicle acceleration performance during the shift-down action. Unlike the continuously-variable shifting state of the differential portion 11, the non-continuously-variable shifting state does not permit the engine speed N_E to be raised immediately after the moment of determination of the shift-down action, to increase the vehicle drive torque. Therefore, the amount of the assisting drive force to be generated by the drive-force assisting operation is made larger in the non-continuously-variable shifting state of the differential portion 11, than in the continuously-variable shifting state.

[0160] Thus, the drive-force assisting operation by the second electric motor M2 in the non-continuously-variable shifting state of the differential portion 11 in the case of FIG. 12 is initiated at an earlier point of time, to generate the larger assisting drive force, than that in the continuously-variable shifting state of the differential portion 11 in the case of FIG. 13. The amount of the assisting drive force to be generated by the second electric motor M2 and the moment of initiation of the drive-force assisting operation by the second electric motor M2 are determined to obtain substantially the same vehicle acceleration performance (vehicle acceleration drivability as felt by the vehicle operator) and to obtain substantially the same amount of increase of the vehicle drive force, in the case of FIG. 12 (in the non-continuously-variable shifting state) and in the case of FIG. 13 (continuously-variable shifting state).

[0161] If a negative decision is obtained in said S2, the control flow goes to S6 corresponding to the hybrid control means 52, to inhibit the drive-force assisting operation by the second electric motor M2 during the shifting action of the automatic transmission portion 20 under the control of the step-variable shifting control means 54.

[0162] If the power-on shift-down action is caused by the accelerator pedal operation at the rate of change A_CC′ higher than the predetermined threshold value A_CC1 while the differential portion 11 is placed in the continuously-variable shifting state, the shift-down action of the automatic transmission portion 20 is effected under the control of the step-variable shifting control means 54, in the following step S7. In this step, the hybrid control means 44 controls the speed ratio of the differential portion 11 to cause a change of the overall speed ratio γT in steps (in the non-continuous manner) toward the target value during the shift-down action of the automatic transmission portion 20, without synchronization with and independently of the shifting action, rather than to cause a continuous change of the overall speed ratio γT according to a change of the speed ratio γ of the automatic transmission portion 20, in synchronization with the shift-down action. At this time, the engine speed N_E is rapidly raised by the differential function using the first electric motor M1, under the control of the hybrid control means 52, immediately after the moment of determination of the shift-down action, whereby the vehicle drive force is rapidly increased.

[0163] If a skipping power-on shift-down action of the automatic transmission portion 20 from the fourth gear position to the second gear position or from the third gear position to the first gear position, for example, is effected under the control of the step-variable shifting control means 54 while the differential portion 11 is placed in the non-continuously-variable shifting state, on the other hand, a shift-down action to an intermediate gear position is once implemented, and a shift-down action to the target gear position is then implemented, for preventing deterioration of the response to an increase of the drive torque as a result of the power-on shift-down action, which deterioration would take place if the skipping shift-down action to the target gear position were implemented, in the absence of the drive-force assisting operation by the second electric motor M2. Where the skipping power-on shift-down action from the fourth gear position to the second gear position is determined, a shift-down action from the fourth gear position to the third gear position is once implemented, and a shift-down action from the third gear position to the second gear position is then implemented. In this case, the response to the drive torque increase is less likely to be deteriorated even in the absence of the drive-force assisting operation by the second electric motor M2 during the power-on shift-down action from the fourth gear position to the second gear position, than in the case where the automatic transmission portion 20 is shifted down from the fourth gear position directly to the second gear position.

[0164] If a negative decision is obtained in said S1, the control flow goes to S8 corresponding to the hybrid control means 52, to inhibit the drive-force assisting operation by the second electric motor M2 during the shifting action of the automatic transmission portion 20 under the control of the step-variable shifting control means 54.

[0165] In the drive system according to the present embodiment described above, the differential portion 11 is switchable between the continuously-variable shifting state and the non-continuously-variable shifting state, by the differential limiting device in the form of the switching clutch C0 or switching brake B0 operable to limit the differential function of the electrically operated differential device in the form of the differential portion 11. Accordingly, the present drive system has both an advantage of an improvement of the fuel economy of a transmission the speed ratio of which is electrically variable, and an advantage of a high power transmitting efficiency of a gear type power transmitting device constructed for mechanical transmission of power.

[0166] When the differential portion 11 is placed in the continuously-variable shifting state in a normal output state of the engine during a low-speed or medium-speed running or a low-output or medium-output running of the vehicle, for example, the fuel economy of the vehicle is improved. When the differential portion 11 is placed in the non-continuously-variable shifting state during a high-speed running of the vehicle, the output of the engine 8 is transmitted to the drive wheels primarily through the mechanical power transmitting path, the fuel economy is improved owing to reduction of a loss of conversion of a mechanical energy into an electric energy, which would take place when the differential portion 11 is operated as the electrically controlled continuously variable transmission. Where the differential portion 11 is placed in the non-continuously-variable shifting state during a high-output running state of the vehicle, the differential portion 11 is operated as a transmission the speed ratio of which is electrically variable, only when the vehicle speed or output is relatively low or medium, so that the required amount of electric energy generated by the first electric motor M1, that is, the maximum amount of electric energy that must be transmitted from the first electric motor M1 can be reduced, making it possible to minimize the required sizes of the first electric motor M1 and the second electric motor M2 operated by the electric energy generated by the first electric motor M1, and the required size of the transmission mechanism 10 including those electric motors.

[0167] The present embodiment is further arranged such that the hybrid control means 52 performs the drive-force assisting operation by the second electric motor M2 to increase the vehicle drive force during the power-on shift-down action of the automatic transmission portion 20, for thereby improving the vehicle acceleration performance, and the vehicle acceleration drivability as felt by the vehicle operator.

[0168] The present embodiment is further arranged such that the drive-force assisting operation by the second electric motor M2 under the control of the hybrid control means 52 is performed in the respective different manners depending upon whether the differential portion 11 is placed in the continuously-variable shifting state or in the non-continuously-variable shifting state. Accordingly, the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator are improved, not only in the continuously-variable shifting state in which the vehicle drive force can be rapidly increased immediately after the moment of initiation of the shift-down action of the automatic transmission 20, by controlling the engine speed N_E as desired, but also in the non-continuously-variable shifting state in which the response to an increase of the vehicle drive force during the shift-down action of the automatic transmission portion 20 is lower than in the continuously-variable shifting state.

[0169] The amount of the assisting drive force to be generated by the second electric motor M2 under the control of the hybrid control means 52 is made larger in the non-continuously-variable shifting state of the differential portion 11 than in the continuously-variable shifting state, so that the vehicle drive force during the power-on shift-down action of the automatic transmission portion 20 is increased by a larger amount in the non-continuously-variable shifting state than in the continuously-variable shifting state, whereby the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator are improved in the non-continuously-variable shifting state.

[0170] The present embodiment is further arranged such that the drive-force assisting operation by the second electric motor M2 under the control of the hybrid control means 52 is initiated at an earlier point of time when the differential portion 11 is placed in the non-continuously-variable shifting state than when the differential portion 11 is placed in the continuously-variable shifting state. Accordingly, the vehicle drive force is more rapidly increased during the power-on shift-down action of the automatic transmission portion 20, in the non-continuously-variable shifting state than in the continuously-variable shifting state, whereby the vehicle acceleration performance and the vehicle acceleration drivability as felt by the vehicle operator are improved in the non-continuously-variable shifting state.

[0171] The amount of the assisting drive force to be generated by the drive-force assisting operation by the second electric motor M2 under the control of the hybrid control means 52, and the moment of initiation of the drive-force assisting operation are determined so as to obtain substantially the same shifting response in the continuously-variable shifting state and the step-variable shifting state (non-continuously-variable shifting state) of the differential portion 11, that is, to obtain substantially the same amount of increase of the vehicle drive force in the continuously-variable and non-continuously-variable shifting states, so that substantially the same vehicle acceleration performance (vehicle acceleration drivability as felt by the vehicle operator) is obtained in the continuously-variable and non-continuously-variable shifting states of the differential portion 11.

[0172] The, other embodiments of the present invention will be described. In the following descriptions, the same reference as used in the preceding embodiment signs will be used to identify the corresponding elements which will not be described.

[0173] Referring to the schematic view of FIG. 14, there is shown an arrangement of a transmission mechanism 70 in another embodiment of this invention, and FIG. 15 a table indicating a relationship between the gear positions of the transmission mechanism 70 and different combinations of engaged states of the hydraulically operated frictional coupling devices for respectively establishing those gear positions, while FIG. 16 is a collinear chart for explaining a shifting operation of the transmission mechanism 70.

[0174] In the transmission mechanism 10 of the preceding embodiment, the second electric motor M2 provided in the differential portion 11 is disposed so as to be rotated with the output shaft 22. In the transmission mechanism 70 of the present embodiment, the second electric motor M2 is disposed so as to be rotated with the power transmitting member 18. Namely, the second electric motor M2 provided to constitute an electric path for converting an electric energy into a mechanical energy in the transmission mechanism 10 functions to generate the assisting drive force under the control of the hybrid control means 52, for compensating for a delayed response to an increase of the vehicle drive torque due to a low shifting response of the automatic transmission portion 20. In the present transmission mechanism 70, the second electric motor M2 the torque of which is transmitted to the output shaft 22 through the automatic transmission portion 20 does not function to generate the assisting drive force during a shifting action of the automatic transmission portion 20.

[0175] In the present embodiment, a third electric motor M3 is provided such that the third electric motor M3 is rotated with the output shaft 22, so that the drive-force assisting operation is performed under the control of the hybrid control means 52, to compensate for the delayed response to the increase of the vehicle drive force due to the low shifting response of the automatic transmission portion 20. In the transmission mechanism 10 of the preceding embodiment, the second electric motor M2 has both the function of constituting the electric path, and the function to perform the drive-force assisting operation to generate the assisting drive force during a shifting action of the automatic transmission portion 20. In the present embodiment, on the other hand, the second electric motor M2 has the function of constituting the electric path, while the third electric motor M3 has the function of performing the drive-force assisting operation to generate the assisting drive force during the shifting action.

[0176] The present embodiment is different from the preceding embodiment only in that the drive-force assisting operation to generate the assisting drive force during a shifting action of the automatic transmission portion is performed by the third electric motor M3, which is provided in place of the second electric motor M2 in the transmission mechanism 10. Therefore, the drive-force assisting operation by the third electric motor M3 performed under the control of the hybrid control means 52 during the shifting action of the automatic transmission portion 20 will not be described.

[0177] The control functions of the electronic control device 40 relating to the drive-force assisting operation in the present embodiment are different from those in the preceding embodiment, only in that the third electric motor M3 is used in place of the second electric motor M2, for the drive-force assisting operation. For instance, the drive-force-assisting feasibility determining means 82 is arranged to determine whether the drive-force assisting operation by the third electric motor M3 under the control of the hybrid control means 52 is feasible or not. The other control functions of the electronic control device 40 relating to the drive-force assisting operation will not be described.

[0178] The second electric motor M2 of the transmission mechanism 70 has the same functions as the second electric motor M2 of the transmission mechanism 10, except that the formatter second electric motor M2 does not have the function of performing the drive-force assisting operation.

[0179] The transmission mechanism 70 includes the differential portion 11 having the first electric motor M1, power distributing mechanism 16 and second electric motor M2, as in the preceding embodiment. The transmission mechanism 70 further includes an automatic transmission portion 72 having three forward drive positions. The automatic transmission portion 72 is disposed between the differential portion 11 and the output shaft 22 and is connected in series to the differential portion 11 and output shaft 22 through the power transmitting member 18. The power distributing mechanism 16 includes the single-pinion type first planetary gear set 24 having a gear ratio ρ1 of about 0.418, for example, and the switching clutch C0 and the switching brake B0. The automatic transmission portion 72 includes the single-pinion type second planetary gear set 26 having a gear ratio ρ2 of about 0.532, for example, and the single-pinion type third planetary gear set 28 having a gear ratio ρ3 of about 0.418, for example. The second sun gear S2 of the second planetary gear set 26 and the third sun gear S3 of the third planetary gear set 28 are integrally fixed to each other as a unit, selectively connected to the power transmitting member 18 through the second clutch C2, and selectively fixed to the casing 12 through the first brake B1. The second carrier CA2 of the second planetary gear set 26 and the third ring gear R3 of the third planetary gear set 28 are integrally fixed to each other and fixed to the output shaft 22. The second ring gear R2 is selectively connected to the power transmitting member 18 through the first clutch C1, and the third carrier CA3 is selectively fixed to the casing 12 through the second brake B2.

[0180] In the transmission mechanism 70 constructed as described above, one of a first gear position (first speed position) through a fourth gear position (fourth speed position), a reverse gear position (rear-drive position) and a neural position is selectively established by engaging actions of a corresponding combination of the frictional coupling devices selected from the above-described switching clutch C0, first clutch C1, second clutch C2, switching brake B0, first brake B1 and second brake B2, as indicated in the table of FIG. 15. Those gear positions have respective speed ratios γ (input shaft speed N_IN/output shaft speed N_OUT) which change as geometric series. In particular, it is noted that the power distributing mechanism 16 provided with the switching clutch C0 and brake B0 can be selectively placed by engagement of the switching clutch C0 or switching brake B0, in the fixed-speed-ratio shifting state in which the mechanism 16 is operable as a transmission having fixed speed ratio or ratios, as well as in the continuously-variable shifting state in which the mechanism 16 is operable as the continuously variable transmission described above. In the present transmission mechanism 70, therefore, a step-variable transmission is constituted by the transmission portion 20, and the differential portion 11 which is placed in the fixed-speed-ratio shifting state by engagement of the switching clutch C0 or switching brake B0. Further, a continuously variable transmission is constituted by the transmission portion 20, and the differential portion 11 which is placed in the continuously-variable shifting state, with none of the switching clutch C0 and brake B0 being engaged. In other words, the transmission mechanism 70 is switched to the step-variable shifting state, by engaging one of the switching clutch C0 and switching brake B0, and to the continuously-variable shifting state by releasing both of the switching clutch C0 and switching brake B0.

[0181] Where the transmission mechanism 70 functions as the step-variable transmission, for example, the first gear position having the highest speed ratio γ1 of about 2.804, for example, is established by engaging actions of the switching clutch C0, first clutch C1 and second brake B2, and the second gear position having the speed ratio γ2 of about 1.531, for example, which is lower than the speed ratio γ1, is established by engaging actions of the switching clutch C0, first clutch C1 and first brake B1, as indicated in FIG. 15. Further, the third gear position having the speed ratio γ3 of about 1.000, for example, which is lower than the speed ratio γ2, is established by engaging actions of the switching clutch C0, first clutch C1 and second clutch C2, and the fourth gear position having the speed ratio γ4 of about 0.705, for example, which is lower than the speed ratio γ3, is established by engaging actions of the first clutch C1, second clutch C2, and switching brake B0. Further, the reverse gear position having the speed ratio γR of about 2.393, for example, which is intermediate between the speed ratios γ1 andγ2, is established by engaging actions of the second clutch C2 and the second brake B2. The neutral position N is established by engaging only the switching clutch C0.

[0182] When the transmission mechanism 70 functions as the continuously-variable transmission, on the other hand, the switching clutch C0 and the switching brake B0 are both released, as indicated in FIG. 15, so that the differential portion 11 functions as the continuously variable transmission, while the automatic transmission portion 72 connected in series to the differential portion 11 functions as the step-variable transmission, whereby the speed of the rotary motion transmitted to the automatic transmission portion 72 placed in one of the first through third gear positions, namely, the rotating speed of the power transmitting member 18 is continuously changed, so that the speed ratio of the transmission mechanism 10 when the automatic transmission portion 72 is placed in one of those gear positions is continuously variable over a predetermined range. Accordingly, the total speed ratio of the automatic transmission portion 72 is continuously variable across the adjacent gear positions, whereby the overall speed ratio γT of the transmission mechanism 70 a whole is continuously variable.

[0183] The collinear chart of FIG. 16 indicates, by straight lines, a relationship among the rotating speeds of the rotary elements in each of the gear positions of the transmission mechanism 70, which is constituted by the differential portion 11 functioning as the continuously-variable shifting portion or first shifting portion, and the automatic transmission portion 72 functioning as the step-variable shifting portion or second shifting portion. The collinear chart of FIG. 21 indicates the rotating speeds of the individual elements of the power distributing mechanism 16 11 when the switching clutch C0 and brake B0 are both released, and the rotating speeds of those elements when the switching clutch C0 or brake B0 is engaged, as in the preceding embodiment, except in that the third rotary element RE3 (first ring gear R1) is connected to the power transmitting member 18 and the second electric motor M1.

[0184] In FIG. 16, four vertical lines Y4, Y5, Y6 and Y7 corresponding to the automatic transmission portion 72 and arranged in the rightward direction respectively represent the relative rotating speeds of a fourth rotary element (fourth element) RE4 in the form of the second and third sun gears S2, S3 integrally fixed to each other, a fifth rotary element (fifth element) RE5 in the form of the third carrier CA3, a sixth rotary element (sixth element) RE6 in the form of the second carrier CA2 and third ring gear R3 that are integrally fixed to each other, and a seventh rotary element (seventh element) RE7 in the form of the second ring gear R2. In the automatic transmission portion 72, the fourth rotary element RE4 is selectively connected to the power transmitting member 18 through the second clutch C2, and is selectively fixed to the casing 12 through the first brake B1, and the fifth rotary element RE5 is selectively fixed to the casing 12 through the second brake B2. The sixth rotary element RE6 is fixed to the output shaft 22 of the automatic transmission portion 72, and the seventh rotary element RE7 is selectively connected to the power transmitting member 18 through the first clutch C1.

[0185] When the first clutch C1 and the second brake B2 are engaged, the automatic transmission portion 72 is placed in the first gear position. The rotating speed of the output shaft 22 in the first gear position is represented by a point of intersection between the vertical line Y6 indicative of the rotating speed of the sixth rotary element RE6 fixed to the output shaft 22 and an inclined straight line L1 which passes a point of intersection between the vertical line Y7 indicative of the rotating speed of the seventh rotary element RE7 (R2) and the horizontal line X2, and a point of intersection between the vertical line Y5 indicative of the rotating speed of the fifth rotary element RE5 (CA3) and the horizontal line X1, as indicated in FIG. 16. Similarly, the rotating speed of the output shaft 22 in the second gear position established by the engaging actions of the first clutch C1 and first brake B1 is represented by a point of intersection between an inclined straight line L2 determined by those engaging actions and the vertical line Y6 indicative of the rotating speed of the sixth rotary element RE6 (CA2, R3) fixed to the output shaft 22. The rotating speed of the output shaft 22 in the third speed position established by the engaging actions of the first clutch C1 and second clutch C2 is represented by a point of intersection between an inclined straight line L3 determined by those engaging actions and the vertical line Y6 indicative of the rotating speed of the sixth rotary element RE6 fixed to the output shaft 22. In the first through third gear positions in which the switching clutch C0 is placed in the engaged state, the seventh rotary element RE7 is rotated at the same speed as the engine speed N_E, with the drive force received from the differential portion 11. When the switching clutch B0 is engaged in place of the switching clutch C0, the sixth rotary element RE6 is rotated at a speed higher than the engine speed N_E, with the drive force received from the differential portion 11. The rotating speed of the output shaft 22 in the fourth gear position established by the engaging actions of the first clutch C1, second clutch C2 and switching brake B0 is represented by a point of intersection between a horizontal line L4 determined by those engaging actions and the vertical line Y6 indicative of the rotating speed of the sixth rotary element RE6 fixed to the output shaft 22.

[0186] The transmission mechanism 70 according to the present embodiment is also constituted by the differential portion 11 functioning as the continuously-variable shifting portion or first shifting portion, and the automatic transmission portion 72 functioning as the step-variable (automatic) shifting portion or second shifting portion, and the output shaft 22 is provided with the third electric motor M3, so that the present transmission mechanism 70 has advantages similar to those of the first embodiment.

[0187] FIG. 17 shows an example of a seesaw switch 44 (hereinafter referred to as “switch 44”) functioning as a shifting-state selecting device manually operable to select the differential state (non-locked state) and or non-differential state (locked state) of the power distributing mechanism 16, that is, to select the continuously-variable shifting state or step-variable shifting state of the transmission mechanism 10. This switch 44 permits the user to select the desired shifting state during running of the vehicle. The switch 44 has a continuously-variable-shifting running button labeled “STEP-VARIABLE” for running of the vehicle in the continuously-variable shifting state, and a step-variable-shifting running button labeled “CONTINUOUSLY-VARIABLE” for running of the vehicle in the step-variable shifting state, as shown in FIG. 17. When the continuously-variable-shifting running button is depressed by the user, the switch 44 is placed in a continuously-variable shifting position for selecting the continuously-variable shifting state in which the transmission mechanism 10 is operable as the electrically controlled continuously variable transmission. When the step-variable-shifting running button is depressed by the user, the switch 44 is placed in a step-variable shifting position for selecting in the step-variable shifting state in which the transmission mechanism is operable as the step-variable transmission.

[0188] In the preceding embodiments, the shifting state of the transmission mechanism 10 is automatically switched on the basis of the vehicle condition and according to the switching boundary line map shown in FIG. 6 by way of example. However, the shifting state of the transmission mechanism 10, 70 may be switched by a manual operation of the switch 44, in place of or in addition to the automatic switching operation. Namely, the switching control means 50 may be arranged to selectively place the transmission mechanism 10 in the continuously-variable shifting state or the step-variable shifting state, depending upon whether the switch 44 is placed in its continuously-variable shifting position or step-variable shifting position. For instance, the user manually operates the switch 44 to place the transmission mechanism 10 in the continuously-variable shifting state when the user likes the transmission mechanism 10 to operate as a continuously variable transmission or wants to improve the fuel economy of the engine, or alternatively in the step-variable shifting state when the user likes a rhythmical change of the engine speed as a result of a shifting action of the step-variable transmission.

[0189] The switch 44 may have a neutral position in which none of the continuously-variable and step-variable shifting states are selected. In this case, the switch 44 may be placed in its neutral position when the user has not selected the desired shifting state or likes the transmission mechanism 10 to be automatically placed in one of the continuously-variable and step-variable shifting states.

[0190] Where the shifting state of the transmission mechanism 10 is not automatically selected, but is manually selected by a manual operation of the switch 44, step S3 in the flow chart of FIG. 11 is formulated such that the determination as to whether the power distributing mechanism 16 is placed in the differential state, that is, whether the differential portion 11 is placed in the continuously-variable shifting state, is effected depending upon whether the switch 44 has been operated to select the differential state of the power distributing mechanism 16 or the continuously-variable shifting state of the transmission mechanism 10.

[0191] While the preferred embodiments of this invention have been described in detail by reference to the accompanying drawings, it is to be understood that the present invention may be otherwise embodied.

[0192] For example, the second electric motor M2 and the third electric motor M3 provided in the illustrated embodiments may have a function of an electric generator operable to perform a regenerative braking operation for deceleration of the vehicle. Where the second electric motor M2 is connected to the output shaft 22, the output torque of the second electric motor M2 is not amplified by the automatic transmission portion 20, 72, but the second electric motor M2 generates a comparatively large regenerative braking force and is advantageously used for regenerative braking of the vehicle.

[0193] In the illustrated embodiments, the hybrid control means 52 controls the drive-force assisting operation to generate the assisting drive force to drive the vehicle, by using the second electric motor M2 connected to the output shaft 22, or the third electric motor M3 which is connected to the output shaft 22 and which does not constitute the electric path. However, the drive-force assisting operation may be performed by an electric motor M which is provided to drive a wheel other than the drive wheels 38 driven by the output of the output shaft 22. The principle of the present invention is equally applicable to a transmission mechanism provided with the electric motor M described above.

[0194] In the illustrated embodiments, the amount of the assisting drive force generated by the drive-force assisting operation under the control of the hybrid control means 52 during the power-on shift-down operation is made smaller when the differential portion 11 is placed in the continuously-variable shifting state than when the differential portion 11 is placed in the non-continuously-variable shifting state. When the differential portion 11 is placed in the continuously-variable shifting state, however, the drive-force assisting operation need not be performed, since the engine speed N_E can be rapidly raised to increase the vehicle drive force immediately after the power-on shift-down action, in the continuously-variable shifting state of the differential portion 11.

[0195] The switching control means 50 in the illustrated embodiments is arranged to fully engage the switching clutch C0 or switching brake B0 for placing the differential portion 11 in the non-differential state (locked state) in which the differential portion 11 does not have the differential function, for thereby limiting the operation of the differential portion 11 as the electrically controlled continuously variable transmission (differential device). However, the switching control means 50 may be arranged to control the torque capacity of the switching clutch C0 or switching brake B0, for example, to partially engage the switching clutch C0 or switching brake B0 for limiting the operation of the differential portion 11 as the electrically controlled differential device, to thereby place the differential portion 11 in the non-differential state (locked state). Described more specifically, the switching control means 50 may be arranged to partially engage the switching clutch C0 or switching brake B0 to transmit a partial engagement torque, which cooperates with the torque generated by the first electric motor M1, to produce a reaction torque with respect to the engine torque T_E received by the differential portion 11.

[0196] In the arrangement described above, the differential portion 11 can receive the engine torque T_E exceeding the predetermined upper limit TE1 that the torque capacity of the first electric motor M1 can withstand. This arrangement makes it possible to increase the output of the differential portion 11, without having to increase the maximum torque capacity of the first electric motor M1, that is, without an increase of the required size of the first electric motor M1.

[0197] Further, the first electric motor M1 is not required to withstand the reaction torque with respect to the entire engine torque T_E received by the differential portion 11, since the switching clutch C0 or switching brake B0 is not placed in the released state, in the above-described arrangement. Accordingly, it is possible to reduce a percentage of a portion of the engine torque T_E which the first electric motor M1 should withstand, with respect to the entire engine torque T_E received by the differential portion 11, making it possible to reduce the size of the first electric motor M1, improve the durability of the first electric motor M1, or reduce the amount of electric energy supplied from the first electric motor M1 to the second electric motor M2, thereby improving the durability of the second electric motor M2 as well.

[0198] Further, the switching control means 50 may be arranged to partially engage the switching clutch C0 or switching brake B0, irrespective of whether the vehicle condition is in the continuously-variable shifting region in which the differential portion 11 should be placed in the continuously-variable shifting state, or in the step-variable shifting region in which the differential portion 11 should be placed in the non-continuously-variable shifting state.

[0199] In the illustrated embodiments, the transmission mechanism 10, 70 is switchable between its continuously-variable shifting state and the step-variable shifting states by placing the differential portion 11 (power distributing mechanism 16) selectively in one of its differential state in which the differential portion is operable as the electrically controlled continuously variable transmission, and the non-differential state (locked state) in which the differential portion 11 is not operable as the step-variable transmission. However, the differential portion 11 placed in its differential state can be operable as the step-variable transmission the speed ratio of which is variable in steps rather than continuously. In other words, the differential and non-differential states of the differential portion 11 do not respectively correspond to the continuously-variable and step-variable shifting states of the transmission mechanism 10, 70, and therefore the differential portion 11 need not be switchable between the continuously-variable and step-variable shifting states. The principle of this invention is applicable to any transmission mechanism which is switchable between the differential and non-differential states, or wherein the differential portion 11 (power distributing mechanism 16) is switchable between the differential and non-differential states. In the step-variable shifting state, the vehicle drive force is transmitted primarily through the mechanical power transmitting path, without using an electric path.

[0200] In the illustrated embodiments, the speed ratio of the differential portion 11 in its differential state is controlled by the hybrid control means 52 such that the engine speed N_E is kept substantially constant, namely, so as to prevent a change of the overall speed ratio of the transmission mechanism 10, during a shifting action of the automatic transmission portion 20. However, the engine speed N_E need not be kept substantially constant, but may be continuously changed with restriction of its change. This arrangement also follows the principle of the invention.

[0201] In the preceding embodiments, the differential-state determining means 84 (step S3 of FIG. 11) is arranged to determine whether the power distributing mechanism 16 is placed in the differential state or not, by determining whether the vehicle condition is in the continuously-variable shifting region defined by the switching boundary line map indicated in FIG. 6 by way of example. However, the determination by the differential-state determining means 84 as to whether the power distributing mechanism 16 is placed in the differential state or not may be based on the determination by the switching control means 50 as to whether the transmission mechanism 10 is placed in the continuously-variable shifting region or the step-variable shifting region.

[0202] In the power distributing mechanism 16 in the illustrated embodiments, the first carrier CA1 is fixed to the engine 8, and the first sun gear S1 is fixed to the first electric motor M1 while the first ring gear R1 is fixed to the power transmitting member 18. However, this arrangement is not essential. The engine 8, first electric motor M1 and power transmitting member 18 may be fixed to any other elements selected from the three elements CA1, S1 and R1 of the first planetary gear set 24.

[0203] While the engine 8 is directly fixed to the input shaft 14 in the illustrated embodiments, the engine 8 may be operatively connected to the input shaft 14 through any suitable member such as gears and a belt, and need not be disposed coaxially with the input shaft 14.

[0204] In the illustrated embodiments, the first electric motor M1 and the second electric motor M2 are disposed coaxially with the input shaft 14, and are fixed to the first sun gear S1 and the power transmitting member 18, respectively. However, this arrangement is not essential. For example, the first and second electric motors M1, M2 may be operatively connected to the first sun gear S1 and the power transmitting member 18 or output shaft 20, respectively, through gears or belts.

[0205] Although the power distributing mechanism 16 in the illustrated embodiments is provided with the switching clutch C0 and the switching brake B0, the power distributing mechanism 16 need not be provided with both of the switching clutch C0 and brake B0. While the switching clutch C0 is provided to selectively connect the first sun gear S1 and the first carrier CA1 to each other, the switching clutch C0 may be provided to selectively connect the first sun gear S1 and the first ring gear R1 to each other, or selectively connect the first carrier CA1 and the first ring gear R1. Namely, the switching clutch C0 may be arranged to connect any two elements of the three elements of the first planetary gear set 24.

[0206] While the switching clutch C0 is engaged to establish the neutral position N in the transmission mechanism 10, 70 in the illustrated embodiments, the switching clutch C0 need not be engaged to establish the neutral position.

[0207] The hydraulically operated frictional coupling devices used as the switching clutch C0, switching brake B0, etc. in the illustrated embodiments may be replaced by a coupling device of a magnetic-power type, an electromagnetic type or a mechanical type, such as a powder clutch (magnetic powder clutch), an electromagnetic clutch and a meshing type dog clutch.

[0208] While the second electric motor M2 is connected to the power transmitting member 18 or the output shaft 22 in the illustrated embodiments, the second electric motor M2 may be connected to a rotary member of the automatic transmission portion 20, 70.

[0209] In the illustrated embodiments, the automatic transmission portion 20, 72 is disposed in the power transmitting path between the drive wheels 38, and the power transmitting member 18 which is the output member of the differential portion 11 or power distributing mechanism 16. However, the automatic transmission portion 20, 72 may be replaced by any other type of power transmitting device such an automatic transmission which is a permanent-mesh parallel-two-axes type transmission well known as a manual transmission and which is automatically shifted by select cylinders and shift cylinders.

[0210] While the automatic transmission portion 20, 72 in the preceding embodiments is connected in series to the differential portion 11 through the power transmitting member 18, the automatic transmission portion 20, 72 may be mounted on and disposed coaxially with a counter shaft which is parallel to the input shaft 14. In this case, the differential portion 11 and the automatic transmission portion 20, 72 are operatively connected to each other through a suitable power transmitting device or a set of two power transmitting members such as a pair of counter gears, and a combination of a sprocket wheel and a chain.

[0211] The power distributing mechanism 16 provided as a differential mechanism in the preceding embodiments may be replaced by a differential gear device including a pinion rotated by the engine 8, and a pair of bevel gears which mesh with the pinion and which are respectively operatively connected to the first electric motor M1 and the power transmitting member 18.

[0212] While the power distributing mechanism 16 in the illustrated embodiments is constituted by one planetary gear set 24, it may be constituted by two or more planetary gear sets so that the power distributing mechanism 16 is operable as a transmission having three or more gear positions in the non-differential state (fixed-speed-ratio shifting state).

[0213] In the illustrated embodiments, the manually operable shifting device 46 is provided with the shift lever 48 manually operable to select one of a plurality of operating positions. However, the shift lever 48 may be replaced by pushbutton switches, a slide-type or any other type of switch manually operable to select a desired one of a plurality of operating positions, or by devices not operated by hand, such as a device operated in response to a voice of the vehicle operator or operated by foot, to select one of a plurality of operating positions. Although the shift lever 48 has the manual forward-drive position M for selecting the number of the forward-drive gear positions available for automatic shifting of the automatic transmission portion 20, 72, the shift lever 48 placed in the manual forward-drive position M may be used to manually shift up or down the automatic transmission portion 20, 72, within the range from the first gear position through the fourth gear position, by operating the shift lever 48 from the position M to the shift-up position “+” or shift-down position “−”.

[0214] While the switch 44 is of a seesaw type switch in the preceding embodiments, the seesaw switch 44 may be replaced by a single pushbutton switch, two pushbutton switches that are selectively pressed into operated positions, a lever type switch, a slide-type switch or any other type of switch or switching device that is operable to select a desired one of the continuously-variable shifting state (differential state) and the step-variable shifting state (non-differential state). The seesaw switch 44 may or may not have a neutral position. Where the seesaw switch 44 does not have the neutral position, an additional switch may be provided to enable and disable the seesaw switch 44. The function of this additional switch corresponds to the neutral position of the seesaw switch 44. The seesaw switch 44 may be replaced by a switching device operable by a voice generated by the vehicle operator or a foot of the vehicle operator, rather than by hand, to select one of the continuously-variable shifting state (differential state) and the step-variable shifting state (non-differential state).

[0215] It is to be understood that the embodiments of the invention have been descried for illustrative purpose only, and that the present invention may be embodied with various changes and modifications which may occur to those skilled in the art.