Control device for vehicular drive system

A control device for a vehicular drive system including (a) a differential 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, (b) a transmission portion which constitutes a part of the power transmitting path, (c) a coupling device operable to place a power transmitting path between the engine and the drive wheel, selectively in a power-transmitting state or a power-cutoff state, and (d) a shift lever operable between a drive position for the coupling device to select the power-transmitting state, and a non-drive position for the coupling device to select the power-cutoff state, the control device including an engine-speed control device for controlling engine speed NE so as not to exceed a predetermined engine speed value NE′ while the shift lever is placed in the non-drive position, so that the coupling device is engaged while engine torque TE is reduced as a result of an operation of the shift lever from the non-drive position to the drive position, whereby the durability of the coupling device is improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control device for a vehicular drive system, and more particularly to a vehicular drive system which includes a differential mechanism operable to perform a differential function, and a transmission constituting a part of a power transmitting path between the differential mechanism and drive wheels, and which is improved in the durability of coupling devices provided to switch a power transmitting path between an engine and the drive wheels, between a power-cutoff state and a power-transmitting state.

2. Discussion of Related Art

There is known a vehicular drive system including a differential mechanism operable to mechanically synthesize forces and distribute a force, and an electric motor operatively connected to the differential mechanism. Examples of this type of vehicular drive system include a drive system for a hybrid vehicle as disclosed in Patent Document 1. In this hybrid vehicle drive systems, the differential mechanism is constituted by a planetary gear set, and a so-called “electric torque converter” is provided to transmit a drive force from an engine to drive wheels, according to a reaction torque of an electric motor. The vehicular drive system of the Patent Document 1 further includes a step-variable automatic transmission disposed in a power transmitting path between the planetary gear set and the drive wheels. The power transmitting path between the engine and the drive wheels is switched between a power-cutoff state and a power-transmitting state, by controlling releasing and engaging actions of clutches incorporated in the step-variable automatic transmission. The vehicle disclosed in the Patent Document 1 is provided with a manually operable shifting device operable to switch the power transmitting path between a non-drive position for establishing the power-cutoff state, and a drive position for establishing the power-transmitting state.[Patent Document 1] JP-9-308010A[Patent Document 2] JP-1-113531A[Patent Document 3] JP-1-76336A

When the above-described shifting device is manually operated from the non-drive position to the drive position, an output torque of the engine (hereinafter referred to as “engine torque”) is transmitted to the drive wheels through the step-variable automatic transmission.

However, the manual operation from the non-drive position to the drive position has a risk of deterioration of durability of the coupling devices provided for switching of the power transmitting path between the power-cutoff state and the power-transmitting state. The risk of deterioration increases with an increase of the engine torque to be transmitted.

SUMMARY OF THE INVENTION

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 device of a vehicular drive system including a differential mechanism operable to perform a differential function, and a transmission constituting a part of a power transmitting path between the differential mechanism and drive wheels, which control device provides an improvement in the durability of coupling devices provided to switch a power transmitting path between an engine and the drive wheels, between a power-cutoff state and a power-transmitting state.

Namely, the present invention defined in claim1provides a control device for a vehicular drive system including (a) a differential 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, (b) a transmission portion which constitutes a part of the power transmitting path and which functions as a transmission, (c) a coupling device operable to place a power transmitting path between the engine and the drive wheels, selectively in one of a power-transmitting state and a power-cutoff state, and (d) a shifting device operable between a drive position for the coupling device to select the power-transmitting state, and a non-drive position for the coupling device to select the power-cutoff state, said control device comprising engine-speed control means for controlling a speed of the engine so as not to exceed a predetermined value while the shifting device is placed in the non-drive position.

In the above-described drive system including the differential portion having the differential mechanism having the differential function and further including the transmission portion, the coupling device is provided to place the power transmitting path selectively in one of the power-transmitting state and the power-cutoff state, and the shifting device is manually operable between the drive position for the coupling device to place the power transmitting path in the power-transmitting state, and the non-drive position for the coupling device to place the power transmitting path in the power-cutoff state. While the shifting device is placed in the non-drive position, the engine-speed control means controls the engine speed so as not to exceed the predetermined value, for thereby reducing the engine torque to be transmitted to the drive wheel through the coupling device in the process of the engaging action as a result of the manual operation of the shifting device from the non-drive position to the drive position, so that the coupling device is engaged while the engine torque is reduced, whereby the durability of the coupling device is improved.

According to the present invention defined in claim2, the engine-speed control means controls an output of the engine, to thereby control the speed of the engine so as not to exceed the predetermined value, and the control device further comprises electric-motor control means for placing the first electric motor and the second electric motor in a non-load state while the shifting device is placed in the non-drive position. In the present arrangement, the operation of the engine-speed control means to control the engine speed so as not to exceed the predetermined value is performed by controlling the output of the engine, so that there is a reduced necessity to control the engine speed by operating the first electric motor and/or the second electric motor. Therefore, the first electric motor and the second electric motor are placed in the non-load state under the control of the electric-motor control means, so that the loss of electric energy for controlling the electric motors is reduced, and the fuel economy is improved. Further, while the first electric motor and the second electric motor are placed in the non-load state under the control of the electric-motor control means, the differential portion is placed in the electrically neutral state in which the differential portion is not able to transmit the engine torque, that is, the power transmitting path is in the power-cutoff state. Accordingly, upon the manual operation of the shifting device from the non-drive position to the drive position, the coupling device is engaged while the engine torque is not transmitted through the power transmitting path, so that the durability of the coupling device is further improved.

The present invention defined in claim3provides a control device for a vehicular drive system including (a) a differential 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, (b) a transmission portion which constitutes a part of the power transmitting path and which functions as a transmission, (c) a coupling device operable to place a power transmitting path between the engine and the drive wheels, selectively in one of a power-transmitting state and a power-cutoff state, and (d) a shifting device operable between a drive position for the coupling device to select the power-transmitting state, and a non-drive position for the coupling device to select the power-cutoff state, said control device comprising electric-motor means for placing said first electric motor and said second electric motor in a non-load state while said shifting device is placed in said non-drive position.

In the above-described drive system including the differential portion having the differential mechanism having the differential function and further including the transmission portion, the coupling device is provided to place the power transmitting path selectively in one of the power-transmitting state and the power-cutoff state, and the shifting device is manually operable between the drive position for the coupling device to place the power transmitting path in the power-transmitting state, and the non-drive position for the coupling device to place the power transmitting path in the power-cutoff state. While the shifting device is placed in the non-drive position, the electric-motor control means places the first electric motor and the second electric motor in the non-load state, for thereby placing the differential portion in the electrically neutral state, so that the engine torque is not transmitted to the drive wheel upon the manual operation of the shifting device from the non-drive position to the drive position, that is, the engine torque is not transmitted to the coupling device in the process of the engaging action which takes place as a result of the manual operation. Therefore, the coupling device is engaged while the engine torque is not transmitted, whereby the durability of the coupling device is improved, and/or the shifting shock due to the manual operation of the shift lever48is reduced. Further, the fuel economy is improved since the loss of electric energy for controlling the first and second electric motors is reduced while the first and second electric motors are placed in the non-load state under the control of the electric-motor control means.

According to the present invention defined in claim4, the transmission portion is a step-variable automatic transmission, and the coupling device is provided to shift the step-variable automatic transmission. Further, the step-variable automatic transmission is placed into a power-cutoff state by the coupling device when the shifting device is operated to said non-drive position. Accordingly, the power transmitting path can be easily placed into the power-cutoff state when the shifting device is operated to the non-drive position.

According to the present invention defined in claim5, the differential mechanism includes a differential-state switching device operable to place the differential mechanism selectively into a differential state and a locked state, and the differential mechanism is placed into the differential state by the differential-state switching device when the shifting device is operated to the non-drive position. In this arrangement, the differential mechanism is operable between the differential state and the non-differential state. In the differential or non-locked state of the differential mechanism, the rotary elements of the differential mechanism can be freely rotatable, so that the differential portion can be placed in the electrically neutral state with the first electric motor and the second electric motor being placed in the non-load state under the control of the electric-motor control means. Where the transmission portion is a step-variable automatic transmission, a continuously variable transmission is constituted by the transmission portion and the differential mechanism placed in the differential state, while the step-variable transmission is constituted by the transmission portion and the differential mechanism placed in the locked state.

Preferably, the differential mechanism includes a first element fixed to the, a second element fixed to the first electric motor, and a third element fixed to the power distributing member, and the above-indicated differential-state switching device is operable to permit the first, second and third elements to be rotated relative to each other, for thereby placing the differential mechanism in the differential state, and to connect the first, second and third elements for rotation as a unit or to hold the second element stationary, for thereby placing the differential mechanism in the locked state. Thus, the differential mechanism can be switched between the differential and locked states.

Preferably, the differential-state switching device includes a clutch operable to connect at least two of the first, second and third elements to each other for rotation of 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. In this arrangement, the differential mechanism can be easily switched between the differential and locked states.

Preferably, the differential-state switching device is operable to release the clutch and the brake for thereby placing the differential mechanism in the differential state in which the first, second and third elements are rotatable relative to each other and in which the differential mechanism functions as an electrically controlled differential device, and to engage the clutch for thereby enabling the differential mechanism to function as a transmission having a speed ratio of 1, or engage the brake for thereby enabling the differential mechanism to function as a speed-increasing transmission having a speed ratio lower than 1. In this arrangement, the differential mechanism can be switched between the differential and locked state, and is able to function as a transmission having a single gear position with a fixed speed ratio, or a plurality of gear positions with respective fixed speed ratios.

Preferably, the differential mechanism is a planetary gear set, and the first, second and third elements are respective a carrier, a sun gear and a ring gear of the planetary gear set. In this arrangement, the axial dimension of the differential mechanism can be reduced, and the differential mechanism constituted by a single planetary gear set can be simplified in construction.

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 constituted by a single planetary gear set of the single-pinion type can be simplified in construction.

Preferably, the vehicular drive system has an overall speed ratio which is determined by a speed ratio of the transmission portion and a speed ratio of the differential portion. In this case, the vehicle drive force can be obtained over a wide range of speed ratio, by utilizing the speed ratio range of the transmission portion.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the schematic view ofFIG. 1, there is sown a drive mechanism10constituting a part of a drive system for a hybrid vehicle, which drive system is controlled by a control device according to one embodiment of this invention. As shown inFIG. 1, the transmission mechanism10includes an input rotary member in the form of an input shaft14, a differential portion11, an automatic transmission portion20, and an output rotary member in the form of an output shaft22, which are disposed on a common axis in a transmission casing12functioning as a stationary member attached to a body of the vehicle. The differential portion11is connected to the input shaft14either directly, or indirectly via a pulsation absorbing damper (vibration damping device) not shown. The automatic transmission portion20is a transmission portion functioning as a step-variable transmission interposed in a power transmitting path between the differential portion11and drive wheels38, and is connected in series to the differential portion11via a power transmitting member18(power transmitting shaft). The output shaft22is connected to the automatic transmission portion20. This transmission mechanism10is 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 engine8and the pair of drive wheels38, to transmit a vehicle drive force from the engine8to the pair of drive wheels38through a differential gear device36(final speed-reduction device) and a pair of drive axles, as shown inFIG. 5. The engine8may be a gasoline engine or diesel engine and functions as a vehicle drive power source directly connected to the input shaft14or indirectly via the pulsation absorbing damper not shown.

In the present transmission mechanism10, the engine8and the differential portion11are connected to each other directly or indirectly through the pulsation absorbing damper, as described above, but a fluid-operated power transmitting device such as a torque converter or fluid coupling is not interposed between the engine8and the transmission portion11. It is noted that the transmission mechanism10is constructed symmetrically with respect to its axis, and that the lower half of the transmission mechanism is not shown in the schematic view ofFIG. 1. This is also true to the other embodiments of this invention which will be described.

The differential portion11includes: a first electric motor M1; a power distributing mechanism16functioning as a differential mechanism operable to mechanically distribute an output of the engine8received by the input shaft14, to the first electric motor M1and the power transmitting member18; and a second electric motor M2the output shaft of which is rotated with the power transmitting member18. The second electric motor M2may be disposed at any portion of the power transmitting path between the power transmitting member18and the drive wheels38. Each of the first and second electric motors M1and M2used 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 M1should function at least as an electric generator operable to generate an electric energy and a reaction force, while the second electric motor M2should function at least as a drive power source operable to produce a vehicle drive force.

The power distributing mechanism16includes, as major components, a first planetary gear set24of a single pinion type having a gear ratio ρ1of about 0.418, for example, a switching clutch C0and a switching brake B1. The first planetary gear set24has rotary elements consisting of a first sun gear S1, a first planetary gear P1; a first carrier CA1supporting the first planetary gear P1such that the first planetary gear P1is rotatable about its axis and about the axis of the first sun gear S1; and a first ring gear R1meshing with the first sun gear S1through the first planetary gear P1. Where the numbers of teeth of the first sun gear S1and the first ring gear R1are represented by ZS1and ZR1, respectively, the above-indicated gear ratio ρ1is represented by ZS1/ZRl.

In the power distributing mechanism16, the first carrier CA1is connected to the input shaft14, that is, to the engine8, and the first sun gear S1is connected to the first electric motor M1, while the first ring gear R1is connected to the power transmitting member18. The switching brake B0is disposed between the first sun gear S1and the transmission casing12, and the switching clutch C0is disposed between the first sun gear S1and the first carrier CA1. When the switching clutch C0and brake B0are both released, the power distributing mechanism16is placed in a differential state in which the first sun gear S1, first carrier CA1and first ring gear R1of the first planetary gear set24are rotatable relative to each other, so as to perform a differential function, so that the output of the engine8is distributed to the first electric motor M1and the power transmitting member18, whereby a portion of the output of the engine8is used to drive the first electric motor M1to generate an electric energy which is stored or used to drive the second electric motor M2. Accordingly, the power distributing mechanism16is placed in a continuously-variable shifting state (electrically established CVT state), in which the rotating speed of the power transmitting member18is continuously variable, irrespective of the rotating speed of the engine8, namely, placed in the differential state in which a speed ratio γ0(rotating speed of the input shaft14/rotating speed of the power transmitting member18) of the power distributing mechanism16is 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 mechanism16functions as an electrically controlled continuously variable transmission the speed ratio γ0of which is continuously variable from the minimum value γ0min to the maximum value γ0max.

When the switching clutch C0or brake B0is engaged while the power distributing mechanism16is placed in the continuously-variable shifting state, the mechanism16is brought into a non-differential state in which the differential function is not available. Described in detail, when the switching clutch C0is engaged, the first sun gear S1and the first carrier CA1are connected together, so that the power distributing mechanism16is placed in a locked state or the non-differential state in which the three rotary elements of the first planetary gear set24consisting of the first sun gear S1, first carrier CA1and first ring gear R1are rotatable as a unit, so that the differential portion11is also placed in the non-differential state. In this non-differential state, the rotating speed of the engine8and the rotating speed of the power transmitting member18are made equal to each other, so that the power distributing mechanism16is placed in a fixed-speed-ratio shifting state or step-variable shifting state in which the mechanism16functions as a transmission having a fixed speed ratio γ0equal to 1. When the switching brake B0is engaged in place of the switching clutch C0, the first sun gear S1is fixed to the transmission casing12, so that the power distributing mechanism16is placed in the locked or non-differential state in which the first sun gear S1is not rotatable. Since the rotating speed of the first ring gear R1is made higher than that of the first carrier CA1, the differential portion11is placed in the fixed-speed-ratio shifting state or step-variable shifting state in which the mechanism16functions as a speed-increasing transmission having a fixed speed ratio γ0smaller than 1, for example, about 0.7. Thus, the frictional coupling devices in the form of the switching clutch C0and brake B0function as a differential-state switching device operable to selectively place the differential portion11(power distributing mechanism16) selectively in the differential state and the non-differential state, that is, in the continuously-variable shifting state (differential state) in which the differential portion11(power distributing mechanism16) is operable as an electrically controlled continuously variable transmission, for example, as a continuously variable transmission the speed ratio of which is continuously variable, and in the locked state in which the differential portion11is not operable as the continuously variable transmission but functions as a transmission the speed ratio of which is kept unchanged, that is, in the fixed-speed-ratio shifting state (non-differential state) in which the differential portion11is not operable as the electrically controlled continuously variable transmission, but functions as a transmission having a single gear position with one speed ratio or a plurality of gear positions with respective to or more speed ratios, namely, in the fixed-speed-ration shifting state in which the differential portion11functions as a transmission having one gear position or a plurality of gear positions having fixed speed ratio or ratios.

The automatic transmission portion20includes a single-pinion type second planetary gear set26, a single-pinion type third planetary gear set28and a single-pinion type fourth planetary gear set30. The second planetary gear set26has: a second sun gear S2; a second planetary gear P2; a second carrier CA2supporting the second planetary gear P2such that the second planetary gear P2is rotatable about its axis and about the axis of the second sun gear S2; and a second ring gear R2meshing with the second sun gear S2through the second planetary gear P2. For example, the second planetary gear set26has a gear ratio ρ2of about 0.562. The third planetary gear set28has: a third sun gear S3; a third planetary gear P3; a third carrier CA3supporting the third planetary gear P3such that the third planetary gear P3is rotatable about its axis and about the axis of the third sun gear S3; and a third ring gear R3meshing with the third sun gear S3through the third planetary gear P3. For example, the third planetary gear set28has a gear ratio ρ3of about 0.425. The fourth planetary gear set30has: a fourth sun gear S4; a fourth planetary gear P4; a fourth carrier CA4supporting the fourth planetary gear P4such that the fourth planetary gear P4is rotatable about its axis and about the axis of the fourth sun gear S4; and a fourth ring gear R4meshing with the fourth sun gear S4through the fourth planetary gear P4. For example, the fourth planetary gear set30has a gear ratio ρ4of 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 S4and fourth ring gear R4are represented by ZS2, ZR2, ZS3, ZR3, ZS4and ZR4, respectively, the above-indicated gear ratios ρ2, ρ3and ρ4are represented by ZS2/ZR2. ZS3/ZR3, and ZS4/ZR4, respectively.

In the automatic transmission portion20, the second sun gear S2and the third sun gear S3are integrally fixed to each other as a unit, selectively connected to the power transmitting member18through a second clutch C2, and selectively fixed to the transmission casing12through a first brake B1. The second carrier CA2is selectively fixed to the transmission casing12through a second brake B2, and the fourth ring gear R4is selectively fixed to the transmission casing12through a third brake B3. The second ring gear R2, third carrier CA3and fourth carrier CA4are integrally fixed to each other and fixed to the output shaft22. The third ring gear R3and the fourth sun gear S4are integrally fixed to each other and selectively connected to the power transmitting member18through a first clutch C1. Thus, the automatic transmission portion20and the power transmitting member18are selectively connected to each other through the first clutch C1or second clutch C2, which is used to establish gear positions of the automatic transmission portion20. In other words, the first and second clutches C1, C2cooperate to function as coupling devices operable to switch a power transmitting path connecting the power transmitting member18and the automatic transmission portion20(connecting the differential portion11(power transmitting member18) and the drive wheels38), between a power-transmitting state in which a vehicle drive force can be transmitted through the power transmitting path, and a power-cutoff state in which the vehicle drive force cannot be transmitted through the power transmitting path. That is, the power transmitting path is placed in the power-transmitting state when at least one of the first and second clutches C1, C2is engaged, and is placed in the power-cutoff state when the first and second clutches C1, C2are both released.

The above-described switching clutch C0, first clutch C1, second clutch C2, switching brake B0, first brake B1, second brake B2and third brake B3are 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-C2and brakes B0-B3is selectively engaged for connecting two members between which each clutch or brake is interposed.

In the transmission mechanism10constructed as described above, 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 frictional coupling devices selected from the above-described switching clutch C0, first clutch C1, second clutch C2, switching brake B0, first brake B1, second brake B2and third brake B3, as indicated in the table ofFIG. 2. Those positions have respective speed ratios γ (input shaft speed NIN/output shaft speed NOUT) which change as geometric series. In particular, it is noted that the power distributing mechanism16is provided with the switching clutch C0and brake B0so that the differential portion11can be selectively placed by engagement of the switching clutch C0or switching brake B0, in the fixed-speed-ratio shifting state in which the differential portion11is operable as a transmission having a single gear position with one speed ratio or a plurality of gear positions with respective speed ratios, as well as in the continuously-variable shifting state in which the differential portion11is operable as a continuously variable transmission, as described above. In the present transmission mechanism10, therefore, a step-variable transmission is constituted by the automatic transmission portion20, and the differential portion11which is placed in the fixed-speed-ratio shifting state by engagement of the switching clutch C0or switching brake B0. Further, a continuously variable transmission is constituted by the automatic transmission portion20, and the differential portion11which is placed in the continuously-variable shifting state, with none of the switching clutch C0and brake B0being engaged. In other words, the transmission mechanism10is switched to the step-variable shifting state by engaging one of the switching clutch C0and switching brake B0, and switched to the continuously-variable shifting state by releasing both of the switching clutch C0and brake B0. The differential portion11is also considered to be a transmission switchable between the step-variable shifting state and the continuously-variable shifting state.

Where the transmission mechanism10functions as the step-variable transmission, for example, the first gear position having the highest speed ratio γ1of about 3.357, for example, is established by engaging actions of the switching clutch C0, first clutch C1and third brake B3, and the second gear position having the speed ratio γ2of 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 C1and second brake B2, as indicated inFIG. 2. Further, the third gear position having the speed ratio γ3of 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 C1and first brake B1, and the fourth gear position having the speed ratio γ4of 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 C1and second clutch C2. The fifth gear position having the speed ratio γ5of 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 C2and 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 γ1and γ2, is established by engaging actions of the second clutch C2and the third brake B3. The neutral position N is established by engaging only the switching clutch C0.

Where the transmission mechanism10functions as the continuously-variable transmission, on the other hand, the switching clutch C0and the switching brake B0indicated inFIG. 2are both released, so that the differential portion11functions as the continuously variable transmission, while the automatic transmission portion20connected in series to the differential portion11functions as the step-variable transmission, whereby the speed of the rotary motion transmitted to the automatic transmission portion20placed in one of the first through fourth gear positions, namely, the rotating speed of the power transmitting member18is continuously changed, so that the speed ratio of the drive system when the automatic transmission portion20is placed in one of those gear positions is continuously variable over a predetermined range. Accordingly, the speed ratio of the automatic transmission portion20is continuously variable across the adjacent gear positions, whereby the overall speed ratio γT of the transmission mechanism10is continuously variable.

The collinear chart ofFIG. 3indicates, by straight lines, a relationship among the rotating speeds of the rotary elements in each of the gear positions of the transmission mechanism10, which is constituted by the differential portion11functioning as the continuously-variable shifting portion or first shifting portion, and the automatic transmission portion20functioning as the step-variable shifting portion or second shifting portion. The collinear chart ofFIG. 3is a rectangular two-dimensional coordinate system in which the gear ratios ρ of the planetary gear sets24,26,28,30are 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 X1, X2, XG, that is, the horizontal line X1indicates the rotating speed of 0, while an upper one of the three horizontal lines, that is, the horizontal line X2indicates the rotating speed of 1.0, that is, an operating speed NEof the engine8connected to the input shaft14. The horizontal line XG indicates the rotating speed of the power transmitting member18.

Three vertical lines Y1, Y2and Y3which are arranged in the right direction and which correspond to the differential portion11respectively represent the relative rotating speeds of a second rotary element (second element) RE2in the form of the first sun gear S1, a first rotary element (first element) RE1in the form of the first carrier CA1, and a third rotary element (third element) RE3in the form of the first ring gear R1. The distances between the adjacent ones of the vertical lines Y1, Y2and Y3are determined by the gear ratio ρ1of the first planetary gear set24. That is, the distance between the vertical lines Y1and Y2corresponds to “1”, while the distance between the vertical lines Y2and Y3corresponds to the gear ratio ρ1. Further, five vertical lines Y4, Y5, Y6, Y7and Y8corresponding to the automatic transmission portion20respectively represent the relative rotating speeds of a fourth rotary element (fourth element) RE4in the form of the second and third sun gears S2, S3integrally fixed to each other, a fifth rotary element (fifth element) RE5in the form of the second carrier CA2, a sixth rotary element (sixth element) RE6in the form of the fourth ring gear R4, a seventh rotary element (seventh element) RE7in the form of the second ring gear R2and third and fourth carriers CA3, CA4that are integrally fixed to each other, and an eighth rotary element (eighth element) RE8in the form of the third ring gear R3and fourth sun gear S4integrally fixed to each other. The distances between the adjacent ones of the vertical lines Y4-Y8are determined by the gear ratios ρ2, ρ3and ρ4of the second, third and fourth planetary gear sets26,28,30. That is, the distances between the sun gear and carrier of each of the second, third and fourth planetary gear sets26,28,30corresponds to “1”, while the distances between the carrier and ring gear of each of those planetary gear sets2628,30corresponds to the gear ratio ρ. In the automatic transmission portion20, the distance between the sun gear and carrier of each of the second, third and fourth planetary gear sets26,28,30corresponds to “1”, while the distance between the carrier and the ring gear corresponds to the gear ratio ρ.

Referring to the collinear chart ofFIG. 3, the power distributing mechanism16(differential portion11) of the transmission mechanism10is arranged such that the first rotary element RE1(first carrier CA1) of the first planetary gear set24is integrally fixed to the input shaft14(engine8) and selectively connected to the second rotary element RE2(first sun gear S1) through the switching clutch C0, and this second rotary element RE2is fixed to the first electric motor M1and selectively fixed to the transmission casing12through the switching brake B0, while the third rotary element RE3(first ring gear R1) is fixed to the power transmitting member18and the second electric motor M2, so that a rotary motion of the input shaft14is transmitted to the automatic transmission20(step-variable transmission portion) through the power transmitting member18. A relationship between the rotating speeds of the first sun gear S1and the first ring gear R1is represented by an inclined straight line L0which passes a point of intersection between the lines Y2and X2.

When the transmission mechanism10is brought into the continuously-variable shifting state by releasing actions of the switching clutch C0and brake B0, for instance, the rotating speed of the first sun gear S1represented by a point of intersection between the line L0and the vertical line Y1is raised or lowered by controlling the reaction force generated by an operation of the first electric motor M1to generate an electric energy, so that the rotating speed of the first ring gear R1represented by a point of intersection between the line L1and the vertical line Y3is lowered or raised. When the switching clutch C0is engaged, the first sun gear S1and the first carrier CA1are connected to each other, and the power distributing mechanism16is placed in the non-differential state in which the above-indicated three rotary elements are rotated as a unit, so that the line L0is aligned with the horizontal line X2, so that the power transmitting member18is rotated at a speed equal to the engine speed NE. When the switching brake B0is engaged, on the other hand, the rotation of the first sun gear S1is stopped, and the power distributing mechanism16is placed in the non-differential state and functions as the speed-increasing mechanism, so that the line L0is inclined in the state indicated inFIG. 3, whereby the rotating speed of the first ring gear R1, that is, the rotation of the power transmitting member18represented by a point of intersection between the lines L0and Y3is made higher than the engine speed NEand transmitted to the automatic transmission portion20.

In the automatic transmission portion20, the fourth rotary element RE4is selectively connected to the power transmitting member18through the second clutch C2, and selectively fixed to the transmission casing12through the first brake B1, and the fifth rotary element RE5is selectively fixed to the transmission casing12through the second brake B2, while the sixth rotary element RE6is selectively fixed to the transmission casing12through the third brake B3. The seventh rotary element RE7is fixed to the output shaft22, while the eighth rotary element RE8is selectively connected to the power transmitting member18through the first clutch C1.

When the first clutch C1and the third brake B3are engaged, the automatic transmission portion20is placed in the first gear position. The rotating speed of the output shaft22in the first gear position is represented by a point of intersection between the vertical line Y7indicative of the rotating speed of the seventh rotary element RE7fixed to the output shaft22and an inclined straight line L1which passes a point of intersection between the vertical line Y8indicative of the rotating speed of the eighth rotary element RE8and the horizontal line X2, and a point of intersection between the vertical line Y6indicative of the rotating speed of the sixth rotary element REG and the horizontal line X1, as shown inFIG. 3. Similarly, the rotating speed of the output shaft22in the second gear position established by the engaging actions of the first clutch C1and second brake B2is represented by a point of intersection between an inclined straight line L2determined by those engaging actions and the vertical line Y7indicative of the rotating speed of the seventh rotary element RE7fixed to the output shaft22. The rotating speed of the output shaft22in the third gear position established by the engaging actions of the first clutch C1and first brake B1is represented by a point of intersection between an inclined straight line L3determined by those engaging actions and the vertical line Y7indicative of the rotating speed of the seventh rotary element RE7fixed to the output shaft22. The rotating speed of the output shaft22in the fourth gear position established by the engaging actions of the first clutch C1and second clutch C2is represented by a point of intersection between a horizontal line L4determined by those engaging actions and the vertical line Y7indicative of the rotating speed of the seventh rotary element RE7fixed to the output shaft22. In the first through fourth gear positions in which the switching clutch C0is placed in the engaged state, the eighth rotary element RE8is rotated at the same speed as the engine speed NE, with the drive force received from the power distributing mechanism16. When the switching clutch B0is engaged in place of the switching clutch C0, on the other hand, the eighth rotary element RE8is rotated at a speed higher than the engine speed NE, with the drive force received from the power distributing mechanism16. The rotating speed of the output shaft22in the fifth gear position established by the engaging actions of the first clutch C1, second clutch C2and switching brake B0is represented by a point of intersection between a horizontal line L5determined by those engaging actions and the vertical line Y7indicative of the rotating speed of the seventh rotary element RE7fixed to the output shaft22.

FIG. 4illustrates signals received by an electronic control device40provided to control the transmission mechanism10, and signals generated by the electronic control device40. This electronic control device40includes 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 engine8and electric motors M1and M2, and drive controls such as shifting controls of the automatic transmission portion20.

The electronic control device40is arranged to receive, from various sensors and switches shown inFIG. 4, various signals such as: a signal indicative of a temperature TEMPw of cooling water of the engine; a signal indicative of a presently selected operating position PSHof a shift lever; a signal indicative of the operating speed NEof the engine8; a signal indicative of a value indicating a selected group of forward-drive positions of the transmission mechanism; 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 NOUTof the output shaft22; a signal indicative of a temperature of a working oil of the automatic transmission portion20; 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 operating amount ACCof an accelerator pedal; 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 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 portion11(power distributing mechanism16) in the fixed-speed-ratio shifting state in which the transmission mechanism10functions as a step-variable transmission; a signal indicative of a continuously-variable shifting switch provided to place the differential portion11(power distributing mechanism16) in the continuously variable-shifting state in which the transmission mechanism10functions as the continuously variable transmission; a signal indicative of a rotating speed NM1of the first electric motor M1(hereinafter referred to as “first electric motor speed NM1”); and a signal indicative of a rotating speed NM2of the second electric motor M2(hereinafter referred to as “second electric motor speed NM2”).

The electronic control device40is further arranged to generate various control signals to be applied to an engine-output control device43, to control the engine output, such as: a signal to drive a throttle actuator97for controlling an angle of opening of an electronic throttle valve96disposed in an intake pipe95of the engine8; a signal to control a fuel injection device98for controlling an amount of injection of a fuel into the above-indicated intake pipe95or into the cylinders of the engine8; a signal to control an ignition device99for controlling a timing of ignition of the engine8; a signal to adjust a pressure of a supercharger; a signal to operate the electric air conditioner; signals to operate the electric motors M1and M2; a signal to operate a shift-range indicator for indicating the selected operating position of the shift lever; 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 unit42provided to control the hydraulic actuators of the hydraulically operated frictional coupling devices of the differential portion11and the automatic transmission portion20; a signal to operate an electric oil pump used as a hydraulic pressure source for the hydraulic control unit42; a signal to drive an electric heater; and a signal to be applied to a cruise-control computer.

Reference is now made to the functional block diagram ofFIG. 5for explaining major control functions of the electronic control device40. Step-variable shifting control means54shown inFIG. 5is arranged to determine whether a shifting action of the transmission mechanism10should take place. This determination is made on the basis of a detected state of the vehicle in the form of the detected vehicle speed V and a detected output torque TOUTof the automatic transmission portion20, and according to a shifting boundary line map (shifting control map) which is stored in memory means56and is represented by solid lines and one-dot chain lines inFIG. 6. The step-variable shifting control means54commands the hydraulic control unit42to selectively engage and release the hydraulically operated frictional coupling devices except the switching clutch C0and brake B0, for establishing the determined gear position according to the table ofFIG. 2.

Hybrid control means52is arranged to control the engine8to be operated with high efficiency in the above-described continuously-variable shifting state of the transmission mechanism10, that is, in the differential state of the differential portion11, and to optimize a proportion of drive forces generated by the engine8and the second electric motor M2, and a reaction force generated by the first electric motor M1during its operation as the electric generator, for thereby controlling the speed ratio γ0of the differential portion11operating as the electrically controlled continuously variable transmission. For instance, the hybrid control means52calculates the output as required by the vehicle operator at the present running speed of the vehicle, on the basis of the operating amount ACCof the accelerator pedal and the vehicle running speed V, and calculate a required total vehicle output on the basis of the calculated required output and a required amount of generation of an electric energy by the electric motor. The hybrid control means52calculates a desired engine output, so as to obtain the calculated required total vehicle output, while taking into account of a loss of power transmission, load acting on devices optionally provided, and an assisting torque generated by the second electric motor M2. The hybrid control means52controls the engine8and the amount of generation of electric energy by the first electric motor M1, so as to establish the engine speed NEand torque TEat which the desired engine output is obtained. In other words, the hybrid control means52is able to control the engine speed NEfor a given value of the vehicle running speed V and for a given speed ratio of the automatic transmission portion20, that is, for a given speed of the power transmitting member18, by controlling the amount of generation of the electric energy by the first electric motor M1.

The hybrid control means52is arranged to effect the above-described hybrid control while taking account of the presently selected gear position of the automatic transmission portion20, so as to improve the drivability and fuel economy of the vehicle. In the hybrid control, the differential portion11is controlled to function as the electrically controlled continuously-variable transmission, for optimum coordination of the engine speed NEand vehicle speed V for efficient operation of the engine8, and the rotating speed of the power transmitting member18determined by the selected gear position of the automatic transmission portion20. That is, the hybrid control means52determines a target value of the overall speed ratio γT of the transmission mechanism10, so that the engine8is operated according to a stored highest-fuel-economy curve (fuel economy map or relationship). The target value of the overall speed ratio γT of the transmission mechanism10permits the engine torque TEand speed NEto be controlled so that the engine8provides an output necessary to drive the vehicle with the desired vehicle output. 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 engine8, and is defined in a two-dimensional coordinate system defined by an axis of the engine speed NEand an axis of the engine torque TE. The hybrid control means52controls the speed ratio γ0of the differential portion11, 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.

In the hybrid control, the hybrid control means52controls an inverter58such that the electric energy generated by the first electric motor M1is supplied to an electric-energy storage device60and the second electric motor M2through the inverter58. That is, a major portion of the drive force produced by the engine8is mechanically transmitted to the power transmitting member18, while the remaining portion of the drive force is consumed by the first electric motor M1to convert this portion into the electric energy, which is supplied through the inverter58to the second electric motor M2, so that the second electric motor M2is operated with the supplied electric energy, to produce a mechanical energy to be transmitted to the power transmitting member18. 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 engine8is converted into a mechanical energy. This electric path includes components associated with the generation of the electric energy and the consumption of the generated electric energy by the second electric motor M2.

It is also noted that the hybrid control means52is capable of establishing a so-called “motor starting and drive” mode in which the vehicle is started and driven by only the electric motor (e.g., second electric motor M2) used as the drive power source, by utilizing the electric CVT function (differential function) of the differential portion11, irrespective of whether the engine8is in the non-operated state. Where the vehicle is started or driven by the electric motor, the hybrid control means52controls the first electric motor M1to operate at a negative speed NM1or to freely operate, for holding the engine speed NE at zero or substantially zero, owing to the differential function of the differential portion11, for reducing the dragging phenomenon of the non-operated engine8, so as to improve the fuel economy. The vehicle starting or running by the electric motor under the hybrid control means52is effected when the output torque TOUTis comparatively low, namely, when the engine torque TEis comparatively low, or when the vehicle speed V is comparatively low, namely, when the vehicle load is comparatively low. Generally, the engine operating efficiency is lower when the engine torque is low than when the engine torque is high.

The hybrid control means52is further capable of holding the engine8in an operated state owing to the electric CVT function of the differential portion11, irrespective of whether the vehicle is stationary or running at a relatively low speed. For example, the first electric motor M1is required to be operated by the drive force of the engine8, to generate an electric energy, when an amount of electric energy SOC stored in the electric-energy storage device60is reduced while the vehicle is stationary. In this case, the speed of the first electric motor M1is raised, so that the differential function of the power distributing mechanism16permits the engine speed NEto be held higher than a lower limit above which the engine8is operable, even if the second electric motor speed NM2determined by the vehicle speed V is lowered to zero (substantially zero) while the vehicle is stationary.

The hybrid control means52is further capable of holding the engine speed NEconstant at a given value owing to the electric CVT function of the differential portion11, by controlling the operating speed NM1of the first electric motor M1and/or the operating speed NM2of the second electric motor M2, irrespective of whether the vehicle is stationary or running. To raise the engine speed NE, for example, the hybrid control means52controls the first electric motor speed NM1to be raised while the second electric motor speed NM2determined by the vehicle speed V is held substantially constant, as is apparent from the collinear chart ofFIG. 3.

The hybrid control means52is further capable of holding the first electric motor M1and the second electric motor M2in the non-load state, by cutting a supply of an electric current from the electric-energy storage device60to the first electric motor M1and the second electric motor M2through the inverter58. In the non-load state of the first electric motor M1and the second electric motor M2, these electric motors can be freely operated, and the differential portion11is not able to transmit a torque, that is placed in a state similar to the power-cutoff state in which the power transmitting path is disconnected within the differential portion11. That is, the hybrid control means52is capable of placing the first electric motor M1and the second electric motor M2in the non-load state, for thereby placing the differential portion11in a neutral state in which the power transmitting path is electrically disconnected.

High-speed-gear determining means62is arranged to determine whether the gear position to which the transmission mechanism10should be shifted according to the shifting boundary line map ofFIG. 6stored in the memory means56is a high-speed-gear position, for example, the fifth gear position. This determination is made to determine which one of the switching clutch C0and the switching brake B0should be engaged to place the transmission mechanism10in the step-variable shifting state.

Switching control portion50is arranged to determine whether the shifting state of the transmission mechanism10should be changed, that is, whether the detected vehicle condition represented by the vehicle speed V and the output torque TOUTis in a continuously variable shifting region for placing the transmission mechanism10in the continuously-variable shifting state, or in a step-variable shifting region for placing the transmission mechanism10in the step-variable shifting state. This determination is made on the basis of the detected vehicle condition and according to a switching boundary line map (switching map or relationship) stored in the memory means56. An example of the switching boundary line map is indicated by broken and two-dot chain lines inFIG. 6. The switching control portion50selectively places the transmission mechanism10in the continuously-variable shifting state or step-variable shifting state, depending upon whether the present vehicle condition is in the continuously-variable shifting region or step-variable shifting region.

Described in detail, when the switching control portion50determines that the detected vehicle condition is in the step-variable shifting region, the switching control portion50disables the hybrid control means52to effect the hybrid control or continuously-variable shifting control, and enables the step-variable shifting control means54to effect a predetermined step-variable shifting control in which the automatic transmission portion20is automatically shifted according to the shifting boundary line map ofFIG. 6stored in the memory means56, for example. In this step-variable shifting control, one of the gear positions of the automatic transmission portion20which is selected according to the shifting boundary line map ofFIG. 6is established by engaging the appropriate combination of the hydraulically operated frictional coupling devices C0, C1, C2, B0, B1, B2and B3, as indicated in the table ofFIG. 2, which indicates a predetermined relationship between each gear position of the transmission mechanism10and the corresponding combination of the frictional coupling devices. This relationship is stored in the memory means56. Namely, the differential portion11and the automatic transmission portion20are operated as a so-called “step-variable automatic transmission” which is automatically shifted according to the predetermined relationship ofFIG. 2.

When the high-speed-gear determining means62has determined that the transmission mechanism10should be shifted to the fifth gear position, for example, the switching control portion50commands the hydraulic control unit42to release the switching clutch C0and engage the switching brake B0, for enabling the differential portion11to function as an auxiliary transmission having a fixed speed ratio γ0of 0.7, for example, so that the transmission mechanism10as 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 means62has not determined that the transmission mechanism10should be shifted to the fifth gear position, the switching control portion50commands the hydraulic control unit42to engage the switching clutch C0and release the switching brake B0, for enabling the differential portion11to function as an auxiliary transmission having a fixed speed ratio γ0 of 1.0, for example, so that the transmission mechanism10as a whole is placed in a speed-reducing gear position having a speed ratio not lower than 1.0. Thus, the transmission mechanism10is switched to the step-variable shifting state by the switching control means50, and is selectively placed in one of the two gear positions while the transmission mechanism10is placed in the step-variable shifting state. Thus, the differential portion11functions as the auxiliary transmission, while the automatic transmission portion20connected in series to the differential portion11functions as the step-variable transmission, so that the transmission mechanism10as a whole functions as the so-called “step-variable automatic transmission”.

When the switching control portion50has determined that the detected vehicle condition is in the continuously-variable shifting region for placing the transmission mechanism10in the continuously-variable shifting state, the switching control portion50commands the hydraulic control unit42to release both of the switching clutch C0and brake B0, for placing the differential portion11in the continuously-variable shifting state, to place the transmission mechanism10as a whole in the continuously-variable shifting state. At the same time, the switching control portion50enables the hybrid control means52to effect the hybrid control, and commands the step-variable shifting control means54to select and hold a predetermined one of the gear positions, or to permit an automatic shifting control according to the shifting boundary line map stored in the memory means56. In the latter case, the variable-step shifting control means54effects the automatic shifting control by suitably selecting the combinations of the operating states of the frictional coupling devices indicated in the table ofFIG. 2, except the combinations including the engagement of the switching clutch C0and brake B0. Thus, the differential portion11functions as the continuously variable transmission while the automatic transmission portion20connected in series to the differential portion11functions as the step-variable transmission, so that the transmission mechanism10provides a sufficient vehicle drive force, such that the speed of the rotary motion transmitted to the automatic transmission portion20placed in one of the first through fourth gear positions, namely, the rotating speed of the power transmitting member18is continuously changed, so that the speed ratio of the transmission mechanism10when the automatic transmission portion20is placed in one of those gear positions is continuously variable over a predetermined range. Accordingly, the speed ratio of the automatic transmission portion20is continuously variable through the adjacent gear positions, whereby the overall speed ratio γT of the transmission mechanism10is continuously variable.

The maps shown inFIG. 6will be described in detail. The shifting boundary line map (shifting map or relationship) shown inFIG. 6by way of example is stored in the memory means56and is used for determining whether the automatic transmission20should be shifted. The shifting boundary line map is represented in a two-dimensional coordinate system defined by axes of control parameters in the form of the vehicle speed V and the required output torque TOUTused as a drive-force-related value. InFIG. 6, the solid lines indicate shift-up boundary lines, while the one-dot chain lines indicate shift-down boundary lines. Further, the broken lines ofFIG. 6indicate an upper vehicle-speed limit V1and an upper output-torque limit T1used by the switching control means50to determine whether the vehicle condition is in the step-variable shifting region and the continuously-variable shifting region. Namely, the broken lines ofFIG. 6are a high-speed-running determining line that is a series of high-speed-running threshold values predetermined as the upper vehicle-speed limit V1used for determining whether the hybrid vehicle is in a high-speed running state, and a high-output-running determining line that is a series of high-output-running threshold values predetermined as the upper output-torque limit T1, which is used as the drive-force-related value relating to the drive force of the hybrid vehicle, for example, the output torque TOUTof the automatic transmission portion20, for determining whether the hybrid vehicle is in a high-output running state. Further, two-dot chain lines inFIG. 6indicate boundary lines offset with respect to the broken lines, by a suitable amount of control hysteresis, so that the broken lines and the two-dot chain lines are selectively used as the switching boundary lines between the step-variable shifting region and the continuously-variable shifting region. Thus,FIG. 6shows the switching boundary line map (switching map or relationship) used by the switching control means50to determine whether the vehicle is in the step-variable shifting state or the continuously-variable shifting state, depending upon whether the vehicle speed V and the output torque TOUTare higher than the predetermined upper limit values V, T1. The shifting boundary line map and the switching boundary line map may be stored in the memory means56, as a complex map. The switching boundary line map may include at least one of the boundary lines representative of the upper vehicle-speed limit V1and the upper output-torque limit T1, and may use only one of the two parameters V and TOUT.

The shifting boundary line map and the switching boundary line map may be replaced by stored equations for comparison of the actual vehicle speed V with the limit value V1and comparison of the actual output torque TOUTwith the limit value T1. In this case, the switching control means50switches the transmission mechanism10in the step-variable shifting state, when the detected actual vehicle speed V has exceeded the upper limit V1, or when the detected output torque TOUTof the automatic transmission portion20has exceeded the upper limit T1. The switching control means50may be arranged to place the transmission mechanism10in the step-variable shifting state even when the vehicle condition is in the continuously-variable shifting region, upon detection of any functional deterioration or defect of the components such as the first and second electric motors M1, M2, inverter58and electric-energy storage device60which are associated with the electric path described above and which are operable to operate the differential portion11as the electrically controlled continuously variable transmission.

The drive-force-related value indicated above is a parameter corresponding to the drive force of the vehicle, which may be the output torque TOUTof the automatic transmission portion20, the engine torque TEor an acceleration value of the vehicle, as well as a drive torque or drive force of drive wheels38. The engine torque TEmay be an actual value calculated on the basis of the accelerator pedal operating amount or the throttle valve opening angle (or intake air quantity, air/fuel ratio or amount of fuel injection) and the engine speed NE, or an estimated value of the engine torque TEor required vehicle drive force which is calculated on the basis of the amount of operation of the accelerator pedal by the vehicle operator or the throttle valve operating angle. The vehicle drive torque may be calculated on the basis of not only the output torque TOUT, etc., but also the ratio of the differential gear device36and the radius of the drive wheels38, or may be directly detected by a torque sensor or the like.

For instance, the upper limit V1of the vehicle speed is determined so that the transmission mechanism10is placed in the step-variable shifting state while the vehicle speed V is higher than the upper limit V1. This determination is effective to minimize a possibility of deterioration of the fuel economy of the vehicle if the transmission mechanism10were placed in the continuously-variable shifting state at a relatively high running speed of the vehicle. The upper limit T1of the output torque TOUTis 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 M1is not so large when the engine output is relatively high in the high-output running state of the vehicle.

Referring toFIG. 7, there is shown a shifting-region switching map which is stored in the memory means56and which indicates boundary lines (switching map or relationship) defining the step-variable shifting region and continuously-variable shifting region in a two-dimensional coordinate system which is defined by axes of control parameters in the form of the engine speed NEand the engine torque NT. The boundary lines of the shifting-region switching map are considered to be engine output lines. The switching control means50may use the shifting-region switching map ofFIG. 7in place of the switching boundary line map ofFIG. 6, to determine, on the basis of by the engine speed NEand the engine torque TE, whether the detected vehicle condition represented by the engine speed NEand the engine torque TEis in the continuously-variable or step-variable shifting region. The switching boundary line map ofFIG. 6which is indicated by the broken lines inFIG. 6is based on the map ofFIG. 7. In other words, the broken lines inFIG. 6are switching boundary lines which are represented in the two-dimensional coordinate system defined by the axes of the control parameters in the form of the vehicle speed V and the output torque TOUT, on the basis of the relationship (map) shown inFIG. 7.

The step-variable shifting region defined by the switching boundary line map ofFIG. 6is defined as a high-torque region in which the output torque TOUTis not lower than the predetermined upper limit T1, or a high-speed region in which the vehicle speed V is not lower than the predetermined upper limit V1. Accordingly, the step-variable shifting control is effected when the torque of the engine8is comparatively high or when the vehicle speed V is comparatively high, while the continuously-variable shifting control is effected when the torque of the engine8is comparatively low or when the vehicle speed V is comparatively low, that is, when the engine8is in a normal output state. Similarly, the step-variable shifting region defined by the shifting-region switching map ofFIG. 7is defined as a high-torque region in which the engine torque TEis not lower than the predetermined upper limit TE1, or a high-speed region in which the engine speed NEis not lower than the predetermined upper limit NE1, or alternatively defined as a high-output region in which the output of the engine8calculated on the basis of the engine torque NTand speed NEis not lower than a predetermined limit. Accordingly, the step-variable shifting control is effected when the torque TE, speed NEor output of the engine8is comparatively high, while the continuously-variable shifting control is effected when the torque TE, speed NEor output of the engine8is comparatively low, that is, when the engine8is in the normal output state. The boundary lines of the shifting-region switching map ofFIG. 7may 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 described above

In the present embodiment described above, the transmission mechanism10is 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 hybrid vehicle. In a high-speed running of the vehicle at the vehicle speed V higher than the upper limit V1, the transmission mechanism10is placed in the step-variable shifting state in which the output of the engine8is transmitted to the drive wheels38primarily 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 portion11(power distributing mechanism16) functions as the electrically controlled continuously variable transmission. In a high-output running state of the vehicle with the output torque TOUThigher than the upper limit T1, too, the transmission mechanism10is placed in the step-variable shifting state. Therefore, the transmission mechanism10is 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 M1can be reduced, whereby the required electrical reaction force of the first electric motor M1can be reduced, making it possible to minimize the required sizes of the first electric motor M1and the second electric motor M2, and the required size of the drive system including those electric motors. Alternatively, in the high-output running state of the vehicle, the transmission mechanism10is placed in the step-variable shifting state (fixed-speed-ratio shifting state), so that the engine speed NEchanges with a shift-up action of the automatic transmission portion20, assuring a comfortable rhythmic change of the engine speed NEas the automatic transmission portion20is shifted up, as indicated inFIG. 8. Stated in the other way, when the engine is in the high-output state, it is more important to satisfy a vehicle operator's desire to improve the drivability of the vehicle, than a vehicle operator's desire to improve the fuel economy. In this respect, the transmission mechanism10is switched from the continuously-variable shifting state to the step-variable shifting state (fixed-speed-ratio shifting state) when the engine output becomes relatively high. Accordingly, the vehicle operator is satisfied with a comfortable rhythmic change of the engine speed NEduring the high-output operation of the engine, as indicated inFIG. 8.

FIG. 9shows an example of a manually operable shifting device in the form of a shifting device46including the above-described shift lever48, which is disposed laterally adjacent to an operator's seat, for example, and which is manually operated to select one of a plurality of operating positions consisting of a parking position P for placing the transmission mechanism10(namely, automatic transmission20) in a neutral state in which a power transmitting path is disconnected with both of the clutches C1and C2placed in the released state, while at the same time the output shaft22of the automatic transmission20is in the locked state; a reverse-drive position R for driving the vehicle in the rearward direction; a neutral position N for placing the transmission mechanism10in the neutral state; an automatic forward-drive position D; and a manual forward-drive position M.

When the shift lever48is operated to a selected one of the positions P, R, N, D and M, a manual valve incorporated in the hydraulic control unit42and operatively connected to the shift lever48is operated to establish the corresponding state of the hydraulic control unit42. In the automatic forward-drive position D or the manual forward-drive position M, one of the first through fifth gear positions (1stthrough 5th) indicated in the table ofFIG. 2is established by electrically controlling the appropriate solenoid-operated valves incorporated in the hydraulic control unit42.

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 portion20is in the power-cut-off state established by releasing both of the clutches C1and C2, as shown in the table ofFIG. 2. In the drive positions R, D, M, the power transmitting path in the automatic transmission portion20is in the power-transmitting state established by engaging at least one of the clutches C1and C2, as also shown in the table ofFIG. 2.

Described in detail, a manual operation of the shift lever48from the parking position P or neutral position N to the reverse-drive position R causes the second clutch C2to be engaged for switching the power transmitting path in the automatic transmission portion20from the power-cut-off state to the power-transmitting state. A manual operation of the shift lever48from the neutral position N to the automatic forward-drive position D causes at least the first clutch C1to be engaged for switching the power transmitting path in the automatic transmission portion20from 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.

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 lever48is 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 lever48is 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 lever48is 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 mechanism10is automatically variable, that is, respective different lowest values of the overall speed ratio γT which corresponds to the highest output speed of the transmission mechanism10. Namely, the five positions “D” through “L” select respective different numbers of the speed positions (gear positions) of the automatic transmission portion20which are automatically selectable, so that the lowest overall speed ratio γT available is determined by the selected number of the gear positions. The shift lever48is biased by biasing means such as a spring so that the shift lever48is automatically returned from the shift-up position “+” and shift-down position “−” back to the manual forward-drive position M. The shifting device46is provided with a shift-position sensor49operable to detect the presently selected position of the shift lever48, so that signals indicative of the presently selected operating position of the shift lever48and the number of shifting operations of the shift lever48in the manual forward-shifting position M.

When the shift lever48is operated to the automatic forward-drive position D, the switching control means50effects an automatic switching control of the transmission mechanism10according to the stored switching boundary line map indicated inFIG. 6, and the hybrid control means52effects the continuously-variable shifting control of the power distributing mechanism16, while the step-variable shifting control means54effects an automatic shifting control of the automatic transmission20. When the transmission mechanism10is placed in the step-variable shifting state, for example, the shifting action of the transmission mechanism10is automatically controlled to select an appropriate one of the first through the fifth gear position indicated inFIG. 2. When the drive system is placed in the continuously-variable shifting state, the speed ratio of the power distributing mechanism16is continuously changed, while the shifting action of the automatic transmission20is automatically controlled to select an appropriate one of the first through fourth gear positions, so that the overall speed ratio γT of the transmission mechanism10is 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 mechanism10is automatically shifted.

When the shift lever48is operated to the manual forward-drive position M, on the other hand, the shifting action of the transmission mechanism10is automatically controlled by the switching control means50, hybrid control means52and step-variable shifting control means54, 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 positions “D” through “L”. When the transmission mechanism10is placed in the step-variable shifting state, for example, the shifting action of the transmission mechanism10is automatically controlled within the above-indicated predetermined range of the overall speed ratio γT. When the transmission mechanism10is placed in the step-variable shifting state, the speed ratio of the power distributing mechanism16is continuously changed, while the shifting action of the automatic transmission20is automatically controlled to select an appropriate one of the gear positions the number of which is determined by the manually selected one of the positions “D” through “L”, so that the overall speed ratio γT of the transmission mechanism10is 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 mechanism10are manually selected.

Referring back to the block diagram ofFIG. 5, shift-position determining means80is arranged to determine the presently selected position PSHof the shift lever48, on the basis of the output signal of the shift-position sensor49indicative of the presently selected position PSH. For example, the shift-position determining means80is arranged to determine whether the shift lever48is placed in the parking position P or neutral position N. Further, the shift-position determining means80is arranged to determine whether the shift lever48is operated from the neutral position N or parking position P to the reverse-drive position R or automatic forward-drive position D. In either of these cases, the determination by the shit-position determining means80is made on the basis of the output signal of the shift-position sensor49indicative of the presently selected position PSH.

At least one of the first clutch C1and the second clutch C2is engaged as a result of a manual operation of the shift lever48from the neutral position N or parking position P to the reverse-drive position R or the forward-drive position D, that is, from the non-drive position to the drive position. If the engine is in operation at this time, the engine torque TEis transmitted to the drive wheels38through the automatic transmission portion20. The risk of deterioration of the durability of the first clutch C1and/or the second clutch C2increases with an increase of the engine torque TEto be transmitted to the drive wheels38through the first clutch C1and/or the second clutch C2upon the manual operation of the shift lever48from the non-drive position to the drive position.

There will be described a control operation for reducing the deterioration of the durability of the first clutch C1and/or the second clutch C2due to the manual operation of the shift lever48from the non-drive position to the drive position. The engine8is in operation upon the manual operation of the shift lever48from the non-drive position to the drive position, in the following cases: where a warm-up operation of the engine8is necessary while the temperature of the engine cooling water is lower than a normal operating temperature; where an operation of the first electric motor M1by the engine8is necessary to generate an electric energy for charging the electric-energy storage device60while the electric energy amount SOC stored in the electric-energy storage device60is smaller than a lower limit; where an operation of an optionally provided device such as an air conditioner by the engine8is necessary; and where the vehicle is in the engine-drive mode with the engine8used as the drive power source.

Accelerator-operation determining means82is arranged to determine whether an accelerator pedal45has been operated or not, while it is determined by the shift-position determining means80that the shift lever48is placed in the neutral position N or parking position P. This determination by the accelerator-operation determining means80is made by determining whether the actual operating amount ACCof the accelerator pedal45is larger than a predetermined value ACC′. This predetermined value ACC′ is stored in memory, as a threshold value of the operating amount ACCof the accelerator pedal45above which the accelerator pedal45is considered to have been operated by the user while the shift lever48is placed in the neutral position N or parking position P.

Engine-speed control means84is arranged to control the engine8such that the engine speed NEdoes not exceed a predetermined value NE′, irrespective of the operation of the accelerator pedal45, for reducing the deterioration of durability of the first clutch C1and/or the second clutch C2upon manual operation of the shift lever48from the non-drive position to the drive position, when it is determined by the accelerator-operation determining means82that the accelerator pedal45has been operated while the shift lever48is placed in the non-drive position, that is, while it is determined by the shift-position determining means80that the shift lever48is placed in the neural position N or parking position P.

For example, the engine-speed control means84controls the engine output to prevent the engine speed NEfrom exceeding the predetermined value NE′, irrespective of the operation of the accelerator pedal45. Described in detail, the engine-speed control means84commands the engine-output control device43to perform an operation to reduce the angle of opening of the electronic throttle valve96, an operation to reduce the amount of injection of the fuel by the fuel injection device98, or an operation to retard the timing of ignition of the engine8by the ignition device99, or a combination of those operations, so that the engine speed NEdoes not exceed the predetermined value NE′.

The predetermined engine speed value NE′ indicated above is an upper limit value (e.g., 2000 r.p.m.) which is obtained by experimentation and stored in memory and below which it is possible to reduce an excessive rise of the engine speed NE(a racing phenomenon of the engine8) due to the operation of the accelerator pedal45while the shift lever48is in the neutral position N or parking position P, so that the deterioration of durability of the first clutch C1and/or the second clutch C2due to the manual operation of the shift lever48from the non-drive position to the drive position.

Electric-motor control means86is arranged to command the hybrid control means84to inhibit the control of the engine speed NE, when it is not determined by the accelerator-operation determining means82that the accelerator pedal45has been operated while the shift leer48is placed in the non-drive position, that is, while it is determined by the shift-position determining means80that the shift lever48is placed in the non-drive position, or after the engine speed NEhas been controlled by the engine-speed control means84so as not to exceed the predetermined value NE′. In these cases, the electric-motor control means86inhibits the control of the engine speed NE, since it is not necessary for the hybrid control means52to control the engine speed NEso as not to exceed the predetermined value NE′, by controlling the first electric motor M1and/or the second electric motor M2. For instance, the electric-motor control86is arranged to command the hybrid control means52to place the first electric motor M1and the second electric motor M2in the non-load state, for thereby inhibiting the control of the engine speed NE. According to the command from the electric-motor control means86, the hybrid control means52cuts off a supply of the electric currents to the first electric motor M1and the second electric motor M2, for thereby placing the first electric motor M1and the second electric motor M2in the non-load state.

The above-indicated operation to place the first electric motor M1and the second electric motor M2in the non-load state is effective to reduce a loss of electric energy applied to the electric motors, and the load acting on the engine8, resulting in an improvement of the fuel economy of the engine8. Further, the manual operation of the shift lever48from the non-drive position to the drive position while the differential portion11is placed in the electrically neutral state with the first and second electric motors M1, M2being placed in the non-load state causes the first clutch C1and/or the second clutch C2to be engaged while no engine torque TEis transmitted through the drive system, that is, while the torque output from the differential portion11is substantially zero. Accordingly, the durability of the first clutch C1and/or the second clutch C2is further improved.

Transmitting-member-speed control means88is arranged to control the speed of the power transmitting member18by controlling the first electric motor M1and/or the second electric motor M2, so as to permit the first clutch C1and/or the second clutch C2to be engaged with a reduced or restricted relative rotating speed of input and output members of the clutch C1, C2, when the shift lever48is operated from the non-drive position to the drive position, that is, when it is determined by the shift-position determining means80that the shift lever48has been operated from the neutral position N or parking position P to the reverse-drive position R or automatic forward-drive position D.

Described in detail, the above-indicated transmitting-member-speed control means88calculates a target value N18′ of an input speed NINof the automatic transmission portion20(=output shaft speed NOUT×gear ratio γ), that is, the target speed NIS′ of the power transmitting member18in the process of the engaging action of the first clutch C1and/or the second clutch C2, on the basis of the vehicle speed V and the gear ratio γ, so that the first clutch C1and/or the second clutch C2is engaged while the relative rotating speed of its input and output members is restricted. For instance, the transmitting-member-speed control means88is arranged to zero the target speed N18′ of the power transmitting member18when the vehicle is stationary, that is, when the vehicle speed V is zero, and to calculate the target speed N18′ on the basis of the vehicle speed V and the gear ratio of the presently established forward-drive gear position, for example, the gear ratio of the first gear position, when the shift lever48is operated to the forward-drive position D during a forward running of the vehicle.

The transmitting-member-speed control means88commands the hybrid control means52to control the first electric motor M1and/or the second electric motor M2, for synchronous control of the second electric motor speed NM2such that the second electric motor speed NM2coincides with the target speed N18′ of the power transmitting member18. As a result, the first clutch C1and/or the second clutch C2is engaged upon the manual operation of the shift lever48from the non-drive position to the drive position, while the relative rotating speed of the input and output members of the clutch C1, C2is restricted, so that the durability of the first clutch C1and the second clutch C2is improved, and/or the shifting shock upon the engaging action of the clutch C1, C2is reduced. Further, the restriction or reduction of the relative rotating speed of the first clutch C1and/or the second clutch C2during the engaging action permits reduction of deterioration of the durability of the clutch, and/or reduction of the shifting shock, even where the engaging action is effected by fast application of the hydraulic pressure to the first clutch C1and/or the second clutch C2, rather than a gradual increase of the hydraulic pressure of the clutch, by the hydraulic control unit42under the control of the step-variable shifting control means54.

While the differential portion11which is operable in a selected one of its continuously-variable shifting state and its step-variable shifting state (fixed-speed-ratio shifting state) is placed in the step-variable shifting state, the first electric motor speed NM1, second electric motor speed NM2and engine speed NEare bound by each other, and the cannot be freely controlled independently of each other. For instance, even when the first electric motor M1and the second electric motor M2are placed in the non-load state while the differential portion11is placed in the step-variable shifting state, the engine torque TEis transmitted to the power transmitting member18, so that the differential portion11cannot be placed in the electrically neutral state.

In view of the above-described drawback, the switching control means50has not only the function described above, but also a function of commanding the hydraulic control unit42to release the switching clutch C0and the switching brake B0for placing the differential portion11in the continuously-variable shifting state, that is, for placing the power distributing mechanism16in the differential state, when the shift lever48is placed in the non-drive position, namely, when it is determined by the shift-position determining means80that the shift lever48is placed in the neutral position N or parking position P. Accordingly, the differential portion11can be placed in the electrically neutral state under the control of the electric-motor control means86, or the second electric motor speed NM2can be controlled by controlling the first electric motor M1and/or the second electric motor M2under the control of the transmitting-member-speed control means88.

Referring to the flow chart ofFIG. 10, there will be described a major control function of the electronic control device40, that is, a control routine for controlling the differential portion11, so as to reduce the deterioration of durability of the first clutch C1and/or the second clutch C2, when the shift lever48is operated from the presently selected non-drive position to the drive position during an operation of the engine8. This control routine is repeatedly executed with an extremely short cycle time of about several milliseconds to about several tens of milliseconds, for example.FIG. 11is the time chart for explaining the control routine illustrated in the flow chart ofFIG. 10, which is executed upon a manual operation of the shift lever48from the neutral position N to the automatic forward-drive position D, namely, upon the N-to-D shifting operation.

The control routine is initiated with step S1(“step” being hereinafter omitted) corresponding to the shift-position determining means80, to determine whether the shift lever48is placed in the non-drive position N or P. This determination is made on the basis of the output signal of the shift-position sensor49indicative of the presently selected position PSHof the shift lever48. If a negative decision is obtained in S1, the control flow goes to S8wherein the control device40performs various controls other than the present control routine, or terminates one cycle of execution of the present control routine. Up to a point of time t2indicated inFIG. 11, the negative decision is obtained in S1, with the shift lever48being placed in the neutral position N.

If an affirmative decision is obtained in S1, the control flow goes to S2corresponding to the accelerator-operation determining means82, to determine whether the accelerator pedal45has been operated. For example, this determination is made by determining whether the actual accelerator pedal operating amount ACChas exceeded the predetermined value ACC′. After the affirmative decision is obtained in S1and before the determination in S2is made, a step which is not shown and which corresponds to the switching control means50is implemented to command the hydraulic control unit42to release the switching clutch C0and the switching brake B0, for thereby placing the power distributing mechanism16in its differential state.

If an affirmative decision is obtained in S2, the control flow goes to S3corresponding to the engine-speed control means84, to control the output of the engine8such that the engine speed NEdoes not exceed the predetermined value NE′, for example, about 2000 r.p.m., irrespective of the operation of the accelerator pedal45, for inhibiting or reducing a racing phenomenon of the engine8due to the operation of the accelerator pedal45. During a time period between a point of time t1and the point of time t2inFIG. 11, the engine speed NEis kept at a value not higher than the predetermined value NE′, as a result of implementation of S3.

If a negative decision is obtained in S2, or after S3is implemented, the control flow goes to S4corresponding to the electric-motor control means86, to command the hybrid control means52to inhibit the control of the first electric motor M1and/or the second electric motor M2for controlling the engine speed NEso as not to exceed the predetermined value NE′. During the time period between the points of time t1and t2inFIG. 11, the first electric motor speed NM1and the second electric motor speed NM2are not controlled with the differential portion11placed in the neutral state. Subsequently, the control flow goes to S5corresponding to the shift-position determining means80, to determine whether the shift lever48has been operated from the neutral position N or parking position P to the reverse-drive position R or automatic forward-drive position D. This determination is made on the basis of the output signal of the shift-position sensor49indicative of the presently selected position PSHof the shift lever45. If a negative decision is obtained in S5, the control flow goes back to S2. S2through S4are repeatedly implemented until an affirmative decision is obtained in S5. At the point of time t2inFIG. 11, the affirmative decision is obtained in S4, with an operation of the shift lever48from the neutral position N to the forward-drive position D.

If the affirmative decision is obtained in S5, the control flow goes to S6corresponding to the transmitting-member-speed control means88, to calculate the target speed N18′ of the power transmitting member18(=output shaft speed NOUT×gear ratio γ) in the process of the engaging action of the first clutch C1and/or the second clutch C2, on the basis of the vehicle speed V and the gear ratio. and then to command the hybrid control means52to control the first electric motor M1and/or the second electric motor M2, for synchronous control of the second motor speed NM2such that the second electric motor speed NM2coincides with the target speed N18′ of the power transmitting member18. Subsequently, the control flow goes to S7corresponding to the step-variable shifting control means54, to command the hydraulic control unit42to effect the fast application of the hydraulic pressure to the first clutch C1and/or the second clutch C2. During a time period between a point of time t3and a point of time t4inFIG. 11, the fast application of the hydraulic pressure to the first clutch C1is effected after the second electric motor speed NM2becomes equal to the target speed N18′.

In the above-described transmission mechanism10of the present embodiment including the differential portion11having the power distributing mechanism16having the differential function and further including the automatic transmission portion20, the first clutch C1and the second clutch C2are provided as coupling devices to place the power transmitting path selectively in one of the power-transmitting state and the power-cutoff state, and the shift lever48is manually operable between the drive position (position D or R) for at least one of the coupling devices to place the power transmitting path in the power-transmitting state, and the non-drive position for the at least one coupling device to place the power transmitting path in the power-cutoff state. While the shift lever48is placed in the non-drive position, the engine-speed control means84controls the engine speed NEso as not to exceed the predetermined value NE′, for thereby reducing the engine torque TEto be transmitted to the drive wheels38through the first clutch C1and/or the second clutch C2in the process of the engaging action which takes place as a result of the manual operation of the shift lever48from the non-drive position to the drive position, so that the first clutch C1and/or the second clutch2is engaged while the engine torque TEis reduced, whereby the durability of the first clutch C1and/or the second clutch C1is improved, and/or the shifting shock due to the manual operation of the shift lever48is reduced.

The present invention is further arranged such that the operation of the engine-speed control means84to control the engine speed NEso as not to exceed the predetermined value NE′ is performed by controlling the output of the engine8, so that there is a reduced necessity to control the engine speed NEby operating the first electric motor M1and/or the second electric motor M2. Therefore, the first electric motor M1and the second electric motor M2are placed in the non-load state under the control of the electric-motor control means86, so that the loss of electric energy for controlling the electric motors is reduced, and the fuel economy is improved. Further, while the first electric motor M1and the second electric motor M2are placed in the non-load state under the control of the electric-motor control means86, the differential portion11is placed in the electrically neutral state in which the differential portion11is not able to transmit the engine torque TE, that is, the power transmitting path is in the power-cutoff state. Accordingly, upon the manual operation of the shift lever48from the non-drive position to the drive position, the first clutch C1and/or the second clutch C2is engaged while the engine torque TEis not transmitted through the power transmitting path, so that the durability of the first clutch C1and/or the second clutch C2is further improved, and/or the shifting shock is further reduced.

The present embodiment is further arranged such that the first clutch C1and the second clutch C2are provided to shift the automatic transmission portion20, and the automatic transmission portion20is placed into the power-cutoff state by the releasing actions of the first clutch C1and the second clutch C2, when the shift leer48is operated to the non-drive position. Accordingly, the power transmitting path can be easily placed into the power-cutoff state when the shift lever48is operated to the non-drive position.

The transmission mechanism10of the present embodiment is further arranged such that the differential portion11is provided with the switching clutch C9and the switching brake B0that are operable to place the differential portion11selectively in one of the differential state in which the power distributing mechanism16is operable as the electrically controlled continuously variable transmission, and the non-differential state in which the power distributing mechanism16is not operable as the electrically controlled continuously variable transmission, so that the power distributing mechanism16can be placed into the differential state under the control of the switching control means50when the shift lever48is operated to the non-drive position. In the differential or non-locked state of the power distributing mechanism16, the rotary elements of the power distributing mechanism16are freely rotatable, so that the differential portion11can be placed in the electrically neutral state with the first electric motor M1and the second electric motor M2being placed in the non-load state under the control of the electric-motor control means86.

Then, the other embodiments of this invention will be described. In the following description, the same reference signs as used in the preceding embodiment will be used to identify the same elements, which will not be described.

In the present embodiment, the above-described electric-motor control means86is arranged to command the hybrid control means52to place the first electric motor M1and the second electric motor M2in the non-load state for thereby inhibiting the control of the engine speed NEby controlling the first and second electric motors M1, M2, for the purpose of reducing the deterioration of durability of the first clutch C1and/or the second clutch C2upon the manual operation of the shift lever48from the non-drive position to the drive position, when it is determined by the accelerator-operation determining portion82that the accelerator pedal45has been operated while the shift lever48is placed in the non-drive position, that is, while it is determined by the shift-position determining portion80that the shift lever48is placed in the neutral position N or parking position P. According to the command received from the electric-motor control means86, the hybrid control means52cuts off a supply of the electric currents to the first electric motor M1and the second electric motor M2, for thereby placing the first and second electric motors M1, M2in the non-load state.

By placing the first electric motor M1and the second electric motor M2in the non-load state, the loss of electric energy for controlling those electric motors can be reduced, or the load acting on the engine8can be reduced, to thereby improve the fuel economy of the engine8. Further, when the first electric motor M1and the second electric motor M2are placed in the non-load state to place the differential portion11in the electrically neutral state, the first clutch C1and/or the second clutch C2is engaged as a result of the manual operation of the shift lever48from the non-drive position to the drive position, while the engine torque TEis not transmitted, that is, while the output torque of the differential portion11is substantially zero. Accordingly, the deterioration of durability of the first clutch C1and/or the second clutch C2can be reduced, and/or the shifting shock can be reduced, even if the engine speed NEhas exceeded the predetermined valueNE′as a result of an operation of the accelerator pedal45.

In the present embodiment, the step S3illustrated in the flow chart ofFIG. 10is replaced by a step corresponding to the electric-motor control means86. In this step, the electric-motor control means86commands the hybrid control means52to place the first electric motor M1and the second electric motor M2in the non-load state, for thereby inhibit the control of the engine speed NE by controlling the first electric motor M1and the second electric motor M2.

In the above-described transmission mechanism10of the present embodiment including the differential portion11having the power distributing mechanism16having the differential function and further including the automatic transmission portion20, the first clutch C1and the second clutch C2are provided as coupling devices to place the power transmitting path selectively in one of the power-transmitting state and the power-cutoff state, and the shift lever48is manually operable between the drive position (position D or R) for at least one of the coupling devices to place the power transmitting path in the power-transmitting state, and the non-drive position for the at least one coupling device to place the power transmitting path in the power-cutoff state. While the shift lever48is placed in the non-drive position, the electric-motor control means86places the first electric motor M1and the second electric motor M2in the non-load state, for thereby placing the differential portion11in the electrically neutral state, so that the engine torque TEis not transmitted to the drive wheels38upon the manual operation of the shift lever48from the non-drive position to the drive position, that is, the engine torqueTEis not transmitted to the first clutch C1and/or the second clutch C2in the process of the engaging action which takes place as a result of the manual operation. Therefore, the first clutch C1and/or the second clutch2is engaged while the engine torque TEis not transmitted, whereby the durability of the first clutch C1and/or the second clutch C1is improved, and/or the shifting shock due to the manual operation of the shift lever48is reduced. Further, the fuel economy is improved since the loss of electric energy for controlling the first and second electric motors M1, M2is reduced while the first and second electric motors M1, M2are placed in the non-load state under the control of the electric-motor control means86.

FIG. 12is a schematic view showing an arrangement of a transmission mechanism70according to a further embodiment of the present invention, andFIG. 13is a table indicating gear positions of the transmission mechanism70, and different combinations of engaged states of the hydraulically operated frictional coupling devices for respectively establishing those gear positions, whileFIG. 14is a collinear chart for explaining a shifting operation of the transmission mechanism70.

The transmission mechanism70includes the differential portion11having the first electric motor M1, power distributing mechanism16and second electric motor M2, as in the first embodiment. The transmission mechanism70further includes an automatic transmission portion72having three forward drive positions. The automatic transmission portion72is disposed between the differential portion11and the output shaft22and is connected in series to the differential portion11and output shaft22, through the power transmitting member18. The power distributing mechanism16includes the first planetary gear set24of single-pinion type having a gear ratio ρ1of about 0.418, for example, and the switching clutch C0and the switching brake B0, as in the preceding embodiments. The automatic transmission portion72includes a single-pinion type second planetary gear set26having a gear ratio ρ2of about 0.532, for example, and a single-pinion type third planetary gear set28having a gear ratio ρ3of about 0.418, for example. The second sun gear S2of the second planetary gear set26and the third sun gear S3of the third planetary gear set28are integrally fixed to each other as a unit, selectively connected to the power transmitting member18through the second clutch C2, and selectively fixed to the transmission casing12through the first brake B1. The second carrier CA2of the second planetary gear set26and the third ring gear R3of the third planetary gear set28are integrally fixed to each other and fixed to the output shaft22. The second ring gear R2is selectively connected to the power transmitting member18through the first clutch C1, and the third carrier CA3is selectively fixed to the transmission casing12through the second brake B2.

In the transmission mechanism70constructed 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 B1and second brake B2, as indicated in the table ofFIG. 13. Those gear positions have respective speed ratios y (input shaft speed NIN/output shaft speed NOUT) which change as geometric series. In particular, it is noted that the power distributing mechanism16provided with the switching clutch C0and brake B0can be selectively placed by engagement of the switching clutch C0or switching brake B0, in the fixed-speed-ratio shifting state in which the mechanism16is operable as a transmission having fixed speed ratio or ratios, as well as in the continuously-variable shifting state in which the mechanism16is operable as a continuously variable transmission described above. In the present transmission mechanism70, therefore, a step-variable transmission is constituted by the automatic transmission portion20, and the differential portion11which is placed in the fixed-speed-ratio shifting state by engagement of the switching clutch C0or switching brake B0. Further, a continuously variable transmission is constituted by the automatic transmission portion20, and the differential portion11which is placed in the continuously-variable shifting state, with none of the switching clutch C0and brake B0being engaged. In other words, the transmission mechanism70is switched to the step-variable shifting state, by engaging one of the switching clutch C0and switching brake B0, and to the continuously-variable shifting state by releasing both of the switching clutch C0and switching brake B0.

Where the transmission mechanism70functions as the step-variable transmission, for example, the first gear position having the highest speed ratio γ1of about 2.804, for example, is established by engaging actions of the switching clutch C0, first clutch C1and second brake B2, and the second gear position having the speed ratio γ2of 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 C1and first brake B1, as indicated inFIG. 13. Further, the third gear position having the speed ratio γ3of 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 C1and 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 γ1and γ2, is established by engaging actions of the second clutch C2and the second brake B2. The neutral position N is established by engaging only the switching clutch C0.

When the transmission mechanism70functions as the continuously-variable transmission, on the other hand, the switching clutch C0and the switching brake B0are both released, so that the differential portion11functions as the continuously variable transmission, while the automatic transmission portion72connected in series to the differential portion11functions as the step-variable transmission, whereby the speed of the rotary motion transmitted to the automatic transmission portion72placed in one of the first through third gear positions, namely, the rotating speed of the power transmitting member18is continuously changed, so that the speed ratio of the transmission mechanism10when the automatic transmission portion72is placed in one of those gear positions is continuously variable over a predetermined range. Accordingly, the speed ratio of the automatic transmission portion72is continuously variable across the adjacent gear positions, whereby the overall speed ratio γT of the transmission mechanism70is continuously variable.

The collinear chart ofFIG. 14indicates, by straight lines, a relationship among the rotating speeds of the rotary elements in each of the gear positions of the transmission mechanism70, which is constituted by the differential portion11functioning as the continuously-variable shifting portion or first shifting portion, and the automatic transmission portion72functioning as the step-variable shifting portion or second shifting portion. The collinear chart indicates the rotating speeds of the individual elements of the differential portion11when the switching clutch C0and brake B0are both released, and the rotating speeds of those elements when the switching clutch C0or brake B0is engaged, as in the preceding embodiments

InFIG. 14, four vertical lines Y4, Y5, Y6and Y7corresponding to the automatic transmission portion72respectively represent the relative rotating speeds of a fourth rotary element (fourth element) RE4in the form of the second and third sun gears S2, S3integrally fixed to each other, a fifth rotary element (fifth element) RE5in the form of the third carrier CA3, a sixth rotary element (sixth element) RE6in the form of the second carrier CA2and third ring gear R3that are integrally fixed to each other, and a seventh rotary element (seventh element) RE7in the form of the second ring gear R2. In the automatic transmission portion72, the fourth rotary element RE4is selectively connected to the power transmitting member18through the second clutch C2, and is selectively fixed to the transmission casing12through the first brake B1, and the fifth rotary element RE5is selectively fixed to the transmission casing12through the second brake B2. The sixth rotary element RE6is fixed to the output shaft22of the automatic transmission portion72, and the seventh rotary element RE7is selectively connected to the power transmitting member18through the first clutch C1.

When the first clutch C1and the second brake B2are engaged, the automatic transmission portion72is placed in the first gear position. The rotating speed of the output shaft22in the first gear position is represented by a point of intersection between the vertical line Y6indicative of the rotating speed of the sixth rotary element RE6fixed to the output shaft22and an inclined straight line L1which passes a point of intersection between the vertical line Y7indicative 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 Y5indicative of the rotating speed of the fifth rotary element RE5(CA3) and the horizontal line X1, as indicated inFIG. 14. Similarly, the rotating speed of the output shaft22in the second gear position established by the engaging actions of the first clutch C1and first brake B1is represented by a point of intersection between an inclined straight line L2determined by those engaging actions and the vertical line Y6indicative of the rotating speed of the sixth rotary element RE6(CA2, R3) fixed to the output shaft22. The rotating speed of the output shaft22in the third speed position established by the engaging actions of the first clutch C1and second clutch C2is represented by a point of intersection between an inclined straight line L3determined by those engaging actions and the vertical line Y6indicative of the rotating speed of the sixth rotary element RE6fixed to the output shaft22. In the first through third gear positions in which the switching clutch C0is placed in the engaged state, the seventh rotary element RE7is rotated at the same speed as the engine speed NE, with the drive force received from the differential portion11. When the switching clutch B0is engaged in place of the switching clutch C0, the sixth rotary element RE6is rotated at a speed higher than the engine speed NE, with the drive force received from the differential portion11. The rotating speed of the output shaft22in the fourth gear position established by the engaging actions of the first clutch C1, second clutch C2and switching brake B0is represented by a point of intersection between a horizontal line L4determined by those engaging actions and the vertical line Y6indicative of the rotating speed of the sixth rotary element RE6fixed to the output shaft22.

The transmission mechanism70is also constituted by the differential portion11functioning as the continuously-variable shifting portion or first shifting portion, and the automatic transmission portion72functioning as the step-variable shifting portion or second shifting portion, so that the present transmission mechanism70has advantages similar to those of the first embodiment.

FIG. 15shows an example of a seesaw switch44(hereinafter referred to as “switch44”) functioning as a shifting-state selecting device manually operable to select the differential state or the non-differential state (locked state) of the power distributing mechanism16, that is, to select the continuously-variable shifting state or the step-variable shifting state of the power distributing mechanism16. The switch44is provided on the vehicle such that the switch44is manually operable by the user, to select the desired shifting state during running of the vehicle. The switch44has a continuously-variable-shifting running pushbutton labeled “CONTINUOUSLY-VARIABLE”, and a step-variable-shifting running pushbutton labeled “STEP-VARIABLE”, as shown inFIG. 15, and is selectively placed in the continuously-variable shifting position (in which the transmission mechanism10is operable as the electrically controlled continuously variable transmission) by depressing the step-variable-shifting running pushbutton, and in the continuously-variable shifting position (in which the transmission mechanism10is operable as the step-variable transmission) by depressing the continuously-variable-shifting running pushbutton.

In the preceding embodiments, the shifting state of the transmission mechanism10is automatically switched on the basis of the detected vehicle condition and according to the switching boundary line map ofFIG. 6. However, the shifting state of the transmission mechanism10may be manually switched by a manual operation of the switch44. Namely, the switching control means50may be arranged to selectively place the transmission mechanism10in the continuously-variable shifting state or the step-variable shifting state, depending upon whether the switch44is placed in its continuously-variable shifting position or step-variable shifting position. For instance, the user of the vehicle manually operates the switch44to place the transmission mechanism10in the continuously-variable shifting state when the user likes the transmission mechanism10to operate as the 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 change of the engine speed as a result of a shifting action of the step-variable transmission.

Where the switch44has a neutral position, the switch44is placed in its neutral position when the user has not selected the desired shifting state or likes the transmission mechanism10to be automatically placed in one of the continuously-variable and step-variable shifting states.

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

The principle of the present invention is applicable to a control device not arranged to implement the step S6of the flow chart ofFIG. 10(not including the transmitting-member-speed control means88) for synchronous control of the second electric motor speed NM2by controlling the first electric motor M1and/or the second electric motor M2, provided step S3of the flow chart ofFIG. 10is implemented (the engine-speed control means84or electric-motor control means86is provided) for controlling the engine speed NEor placing the differential portion11in the neutral state, so as to reduce the deterioration of durability of the first clutch C1and/or the second clutch C2due to the operation of the shift lever48from the non-drive position to the drive position, and/or reduce the shifting shock due to the operation of the shift lever48.

Where the step S6is not implemented or before the synchronous control of the second electric motor speed NM2is completed, the hydraulic pressure of the first clutch C1and/or the second clutch C2may be gradually increased as in a well known transitional hydraulic pressure control of a clutch in the process of an engaging action of the clutch), rather than the fast application of the hydraulic pressure to the first clutch C1and/or the second clutch C2is effected, in the step S7of the flow chart ofFIG. 10(corresponding to the step-variable shifting control means54). This gradual increase of the hydraulic pressure of the first clutch C1and/or the second clutch C2permits more smooth torque transmission through the clutch C1, C2in the process of engaging action, and/or a small amount of the shifting shock, than the fast application of the hydraulic pressure to the first clutch C1and/or the second clutch C2. While the differential portion11is placed in the neutral state, however, the fast application of the hydraulic pressure to the first clutch C1and/or the second clutch C2does not cause a large amount of the shifting shock.

In the above-described step S3(corresponding to the engine-speed control means84), the engine speed NEis controlled while the shift lever48is placed in the neutral position N or parking position P. While the shift lever48is placed in the parking position P, however, the engine speed NE need not be controlled so as not to exceed the predetermined value NE′, in view of a possibility that the user of the vehicle likes to race the engine8while the vehicle is stationary with the shift lever48placed in the parking position P. In other words, it is possible to allow the racing of the engine8or it is not necessary to inhibit the racing of the engine8, while the shift lever48is placed in the parking position P.

The control routine illustrated in the flow chart ofFIG. 10is arranged to implement the step not shown (corresponding to the step-variable means54) to place the power distributing mechanism16in the differential state after the affirmative decision is obtained in the step S1and before the step2is implemented. the power distributing mechanism16may be placed in the differential state when the step S3or the step S4(corresponding to the electric-motor control means86) is implemented to place the differential portion11in the neutral state.

In the illustrated embodiments, the differential portion11is placed selectively in its continuously-variable shifting state or in its fixed-speed-ratio shifting state, to place the transmission mechanism10,70selectively in one of the continuously-variable shifting state in which the differential portion11functions as the electrically controlled continuously variable transmission, and the step-variable shifting state in which the differential portion11functions as the step-variable transmission. However, the principle of the present invention is applicable to a transmission mechanism which cannot be switched to the step-variable shifting state, namely, to a transmission mechanism the differential portion11of which is not provided with the switching clutch C0and switching brake B0and functions only as the electrically controlled continuously variable transmission (electrically controlled differential device).

In the illustrated embodiments, the transmission mechanism10,70is placed selectively in one of the continuously-variable and step-variable shifting states, when the differential portion11(power distributing portion16) is placed selectively in its differential state in which the differential portion11is operable as the electrically controlled continuously variable transmission, and in its non-differential state (locked state) in which the differential portion11is not operable as the electrically controlled continuously variable transmission. However, the transmission mechanism10,70may function as the step-variable transmission while the speed ratio of the differential portion11is variable in steps rather than continuously, while this transmission portion11remains in the differential state. In other words, the differential and non-differential states of the differential portion11need not respectively correspond to the continuously-variable and step-variable shifting states of the transmission mechanism10,70, and the differential portion11need not be switchable between the continuously-variable and step-variable shifting states. The principle of the present invention is applicable to any transmission mechanism (its differential portion11or power distributing mechanism16) which is switchable between the differential state and the non-differential state.

In the transmission mechanisms10,70described above, the power transmitting path is switched between the power-transmitting state and the power-cutoff state, by the frictional coupling devices in the form of the first and second clutches C1, C2which are disposed between the automatic transmission portion20,72and the differential portion11. However, these two clutches C1, C2are not essential and may be replaced by at least one coupling device which is arranged to place the power transmitting path selectively in one of the power-transmitting state and the power-cut-off state, and which may be connected to the output shaft22or to the rotary members of the automatic transmission portion20,72. The coupling device or devices need not constitute a part of the automatic transmission portion20,72, and may be provided separately from the automatic transmission portion20,72.

In the power distributing mechanism16in the illustrated embodiments, the first carrier CA1is fixed to the engine8, and the first sun gear S1is fixed to the first electric motor M1while the first ring gear R1is fixed to the power transmitting member18. However, this arrangement is not essential. The engine8, first electric motor M1and power transmitting member18may be fixed to any other elements selected from the three elements CA1, S1and R1of the first planetary gear set24.

While the engine8is directly fixed to the input shaft14in the illustrated embodiments, the engine8may be operatively connected to the input shaft14through any suitable member such as gears and a belt, and need not be disposed coaxially with the input shaft14.

In the illustrated embodiments, the first electric motor M1and the second electric motor M2are disposed coaxially with the input shaft14, and are fixed to the first sun gear S1and the power transmitting member18, respectively. However, this arrangement is not essential. For example, the first and second electric motors M1, M2may be operatively connected to the first sun gear S1and the power transmitting member18, respectively, through gears or belts.

Although the power distributing mechanism16in the illustrated embodiments is provided with the switching clutch C0and the switching brake B0, the power distributing mechanism16need not be provided with both of the switching clutch C0and brake B0. While the switching clutch C0is provided to selectively connect the first sun gear S1and the first carrier CA1to each other, the switching clutch C0may be provided to selectively connect the first sun gear S1and the first ring gear R1to each other, or selectively connect the first carrier CA1and the first ring gear R1. Namely, the switching clutch C0may be arranged to connect any two elements of the three elements of the first planetary gear set24.

While the switching clutch C0is engaged to establish the neutral position N in the transmission mechanism10,70in the illustrated embodiments, the switching clutch C0need not be engaged to establish the neutral position.

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.

In the illustrated embodiments, the second electric motor M2is fixed to the power transmitting member18. However, the second electric motor M2may be fixed to the output shaft22or to a rotary member of the automatic transmission portion20,72.

In the illustrated embodiments, the automatic transmission portion20,72is disposed in the power transmitting path between the drive wheels38, and the power transmitting member18which is the output member of the differential portion11or power distributing mechanism16. However, the automatic transmission portion20,72may be replaced by any other type of power transmitting device (transmission) such as: a continuously variable transmission (CVT), which is a kind of an automatic transmission; an automatic transmission which is obtained by modifying a permanent-meshing parallel two-axes type transmission well known as a manual transmission, such that the permanent-meshing parallel two-axes type transmission is provided with select cylinders and shift cylinders for automatic selection of one of its gear positions; and a manual transmission of synchronous meshing type manually operated to select one of its gear positions. Where the continuously variable transmission (CVT) is provided, the transmission mechanism as a whole is placed in the step-variable shifting state when the power distributing mechanism16is placed in the fixed-speed-ratio shifting state. The fixed-speed-ratio shifting state is defined as a state in which power is transmitted primarily through a mechanical power transmitting path, without power transmission through an electric path. The continuously variable transmission may be arranged to establish a plurality of predetermined fixed speed ratios corresponding to those of the gear positions of the automatic transmission portion20,72, under the control of a step-variable shifting control portion which stores data indicative of the predetermined speed ratios. The principle of the present invention is applicable to a drive system not including the automatic transmission portion20,72. Where the drive system uses a continuously variable transmission (CVT) in place of the automatic transmission portion20,72or does not include the automatic transmission portion20,73, coupling devices are provided in a power transmitting path between the power transmitting member18and the drive wheels38, so that the power transmitting path is selectively placed in the power-transmitting state and the power-cutoff state by selectively engaging and releasing the coupling devices.

While the automatic transmission portion20,72is connected in series to the differential portion11through the power transmitting member18in the illustrated embodiments, the automatic transmission portion20,72may be mounted on and disposed coaxially with a counter shaft which is parallel to the input shaft14. In this case, the differential portion11and the automatic transmission portion20,72are 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.

The power distributing mechanism16provided as a differential mechanism in the illustrated embodiments may be replaced by a differential gear device including a pinion rotated by the engine8, and a pair of bevel gears which are respectively operatively connected to the first and second electric motors M1, M2.

Although the power distributing mechanism16is constituted by one planetary gear set in the illustrated embodiments, the power distributing mechanism16may be constituted by two or more planetary gear sets and arranged to be operable as a transmission having three or more gear positions when placed in its non-differential state (fixed-speed-ratio shifting state).

In the illustrated embodiments, the manually operable shifting device46is provided with the shift lever48manually operable to select one of a plurality of operating positions. However, the shift lever48may 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 lever48has the manual forward-drive position M for selecting the number of the forward-drive gear positions available for automatic shifting of the automatic transmission portion20,72, the shift lever48placed in the manual forward-drive position M may be used to manually shift up or down the automatic transmission portion20,72, within the range from the first gear position through the fourth gear position, by operating the shift lever48from the position M to the shift-up position “+” or shift-down position “−”.

While the switch44is of a seesaw type switch in the preceding embodiment, the switch44may 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). Where the switch44does not have a neutral position, an additional switch may be provided to enable and disable the switch44. A device not operated by hand but operated in response to a voice of the vehicle operator or operated by foot may be provided in place of, or in addition to the switch44, to select one of the continuously-variable shifting state (differential state) and the step-variable shifting state (non-differential state).

While the embodiments of the present invention have been described for illustrative purpose only, it is to be understood that the present invention may be embodied with various changes and improvements which may occur to those skilled in the art.