Control apparatus for a hybrid vehicle drive system

A control apparatus for a hybrid vehicle drive system including: a differential device provided with rotary elements respectively connected to an engine, first and second electric motors and an output rotary member; a brake for selectively fixing the rotary element connected to the first electric motor, to a stationary member; and a parking-lock mechanism preventing a rotary motion of a parking-lock gear connected to the output rotary member, when a manually-operated shifting device is operated to a parking-lock position. The control apparatus includes an engine starting control-portion configured to start the engine in a starting-mode wherein an operating speed of the engine is raised with a torque generated by the second electric motor in an engaged state of the brake, where the engine is required to be started while the vehicle has been held at rest and the rotary motion of the parking-lock gear is prevented by the parking-lock mechanism.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the priority from Japanese Patent Application No. 2014-118005 filed on Jun. 6, 2014, the disclosure of which is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement of a control apparatus for a drive system of a hybrid vehicle.

2. Description of Related Art

There is known a hybrid vehicle drive system including: a drive system of a hybrid vehicle including: a first differential mechanism provided with a rotary element connected to an engine and a rotary element connected to a first electric motor; a second differential mechanism provided with a rotary element connected to a second electric motor and a rotary element connected to an output rotary member; a brake configured to selectively fix another rotary elements of the second differential mechanism to a stationary member; and a clutch configured to selectively connect the rotary element of the first differential mechanism connected to the engine and the above-indicated another rotary element of the second differential mechanism. Regarding such a hybrid vehicle drive system, techniques for starting the engine have been proposed. JP-2013-133101 A1 discloses an example of such techniques, wherein EV controls of the drive system are implemented so as to improve fuel economy of the hybrid vehicle. According to this technique, the second electric motor is operated to generate a reaction force while the first electric motor is operated to generate a torque to raise an operating speed of the engine, in an engaged state of the brake and in a released state of the clutch, or in a released state of the brake and in an engaged state of the clutch.

According to the prior art techniques described above, however, there is a risk of failure to start the engine while an output of a battery is insufficient, as in a cold state of the engine, so that a sufficiently large amount of electric energy cannot be supplied to the first and second electric motors. This problem was first discovered by the inventor of the present invention in the process of an intensive study in an effort to improve the performance of the hybrid vehicle.

SUMMARY OF THE INVENTION

The present invention was made in view of the background art described above. It is therefore an object of the present invention to provide a control apparatus for a hybrid vehicle drive system, which permits adequate starting of an engine, irrespective of an output of a battery.

The object indicated above is achieved according to a first aspect of the present invention, which provides a control apparatus for a drive system of a hybrid vehicle including: a first differential mechanism provided with a rotary element connected to an engine and a rotary element connected to a first electric motor; a second differential mechanism provided with a rotary element connected to a second electric motor and a rotary element connected to an output rotary member; a brake configured to selectively fix the rotary element of the first differential mechanism connected to the first electric motor, to a stationary member; and a parking lock mechanism configured to prevent a rotary motion of a parking lock gear connected directly or indirectly to the output rotary member, when a manually operated shifting device is operated to a parking lock position, the control apparatus comprising an engine starting control portion configured to start the engine in a first starting mode in which an operating speed of the engine is raised with a torque generated by the second electric motor in an engaged state of the brake, where the engine is required to be started while the rotary motion of the parking lock gear is prevented by the parking lock mechanism and the vehicle has been held at rest.

As described above, the engine starting control portion of the control apparatus according to the first aspect of the invention described above is configured to start the engine in the first starting mode in which the operating speed of the engine is raised with the torque generated by the second electric motor in the engaged state of the brake, where the engine is required to be started while the rotary motion of the parking lock gear is prevented by the parking lock mechanism and the vehicle has been held at rest. Accordingly, the engine can be adequately started even while an output of a battery is insufficient, as in a cold state of the engine. Namely, the present invention provides a control apparatus for a hybrid vehicle drive system, which permits adequate starting of the engine, irrespective of the output of the battery.

According to a second aspect of the invention, the control apparatus according to the first aspect of the invention further comprises a battery sensor configured to detect an output of a battery, and the engine starting control portion starts the engine in the first starting mode when the output of the battery detected by the battery sensor is smaller than a predetermined threshold value. According to this second aspect of the invention, the engine can be adequately started even while the output of the battery is insufficient, as in the cold state of the engine.

According to a third aspect of the invention, the engine starting control portion of the control apparatus according to the second aspect of the invention starts the engine in a second starting mode in which the operating speed of the engine is raised with a torque generated by the first electric motor while the second electric motor is operated to generate a reaction torque, where the engine is required to be started while the output of the battery detected by the battery sensor is not smaller than the predetermined threshold value. According to this third aspect of the invention, the first and second electric motors cooperate with each other to crank the engine for starting the engine where the output of the battery is sufficient. Therefore, the hybrid vehicle can smoothly start running after the engine has been started.

The hybrid vehicle drive system according to any one of the first through third aspects of the invention is preferably configured such that the first differential mechanism is provided with a first rotary element, a second rotary element and a third rotary element, and the second differential mechanism is provided with a first rotary element, a second rotary element and a third rotary element, and such that the first electric motor and the engine are respectively connected to the first and second rotary elements of the first differential mechanism, and the third rotary element of the first differential mechanism and the third rotary element of the second differential mechanism are connected to each other, and further such that the output rotary member is connected to the second rotary element of the second differential mechanism, and the second electric motor is connected to the third rotary element of the second differential mechanism. In this case, the engine starting control portion permits adequate starting of the engine irrespective of the output of the battery in the hybrid vehicle drive system which has a practical arrangement as described above.

Preferably, the above-described brake is provided to selectively fix the first rotary element of the first differential mechanism to the stationary member. In this case, the engine starting control portion permits adequate starting of the engine irrespective of the output of the battery in the hybrid vehicle drive system which has a practical arrangement as described above.

Preferably, the hybrid vehicle drive system further comprises a first clutch configured to selectively connect the first and second rotary elements of the first differential mechanism to each other, a second clutch configured to selectively connect the second rotary element of the first differential mechanism and the first rotary element of the second differential mechanism to each other, and a second brake configured to selectively fix the first rotary element of the second differential mechanism to the stationary member. In this case, the engine starting control portion permits adequate starting of the engine irrespective of the output of the battery in the hybrid vehicle drive system which has a practical arrangement as described above.

The above-indicated threshold value is a predetermined lower limit of the output of the battery above which the operating speed of the engine can be sufficiently raised for starting the engine with the torque generated by the first electric motor while the second electric motor is operated to generate a reaction force. In this case, the first and second electric motors cooperate with each other to crank the engine for starting the engine where the output of the battery is sufficient. Therefore, the hybrid vehicle can smoothly start running after the engine has been started.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to the drawings, a preferred embodiment of the present invention will be described in detail. It is to be understood that the drawings referred to below do not necessarily accurately represent ratios of dimensions of various elements.

Embodiment

FIG. 1is the schematic view showing an arrangement of a hybrid vehicle drive system10(hereinafter referred to simply as a “drive system10”) to which the present invention is suitably applicable. As shown inFIG. 1, the drive system10according to the present embodiment is of a transversely installed type suitably used for an FF (front-engine front-drive) type vehicle, and is provided with a main vehicle drive power source in the form of an engine12, a first electric motor MG1, a second electric motor MG2, a first differential mechanism in the form of a first planetary gear set14, and a second differential mechanism in the form of a second planetary gear set16, which are disposed on a common axis CE. In the following description of the embodiment, the direction of extension of this axis CE will be referred to as an “axial direction”. The drive system10is constructed substantially symmetrically with respect to the axis CE. InFIG. 1, a lower half of the drive system10is not shown.

The engine12is an internal combustion engine such as a gasoline engine, which is operated to generate a drive force by combustion of a fuel such as a gasoline injected into its cylinders. Each of the first and second electric motors MG1and MG2is a so-called motor/generator having a function of a motor operated to generate a drive force, and a function of an electric generator operated to generate a reaction force, and is provided with a stator18,22connected to a stationary member in the form of a housing (casing)26, and a rotor20,24disposed radially inwardly of the stator18,22.

The first planetary gear set14is a single-pinion type planetary gear set which has a gear ratio ρ1and which includes rotary elements consisting of: a first rotary element in the form of a ring gear R1; a second rotary element in the form of a carrier C1supporting a pinion gear P1such that the pinion gear P1is rotatable about its axis and the axis of the planetary gear set; and a third rotary element in the form of a sun gear S1meshing with the ring gear R1through the pinion gear P1. The second planetary gear set16is a single-pinion type planetary gear set which has a gear ratio ρ2and which includes rotary elements consisting of: a first rotary element in the form of a ring gear R2; a second rotary element in the form of a carrier C2supporting a pinion gear P2such that the pinion gear P2is rotatable about its axis and the axis of the planetary gear set; and a third rotary element in the form of a sun gear S2meshing with the ring gear R2through the pinion gear P2.

In the first planetary gear set14, the ring gear R1is connected to the rotor20of the first electric motor MG1, and the carrier C1is selectively connectable through a clutch CL0to an output shaft of the engine12in the form of a crankshaft12a, while the sun gear S1is fixed to the sun gear S2of the second planetary gear set16and the rotor24of the second electric motor MG2. In the second planetary gear set16, the carrier C2is fixed to an output rotary member in the form of an output gear28. A drive force received by the output gear28is transmitted to a pair of right and left drive wheels (not shown) through a differential gear device and axles (not shown). A torque received by the drive wheels from a roadway surface during running of the hybrid vehicle is transmitted from the output gear28to the drive system10through the differential gear device and axles.

The clutch CL0for selectively connecting and disconnecting the carrier C1of the first planetary gear set14to and from the crankshaft12aof the engine12is disposed between the crankshaft12aand the carrier C1. A clutch CL1for selectively connecting and disconnecting the carrier C1to and from the ring gear R1is disposed between the carrier C1and the ring gear R1. A clutch CL2for selectively connecting and disconnecting the carrier C1to and from the ring gear R2of the second planetary gear set16is disposed between the carrier C1and the ring gear R2. A brake BK1for selectively fixing the ring gear R1to the stationary member in the form of the housing26is disposed between the ring gear R1and the housing26. A brake BK2for selectively fixing the ring gear R2to the housing26is disposed between the ring gear R2and the housing26.

Each of the clutches CL0, CL1and CL2(hereinafter collectively referred to as “clutches CL” unless otherwise specified), and the brakes BK1and BK2(hereinafter collectively referred to as “brakes BK” unless otherwise specified) is preferably a hydraulically operated coupling device the operating state of which is controlled (which is engaged and released) according to a hydraulic pressure applied thereto from a hydraulic control unit54. While wet multiple-disc type frictional coupling devices are preferably used as the clutches CL and brakes BK, meshing type coupling devices, namely, so-called dog clutches (claw clutches) may also be used. Alternatively, the clutches CL and brakes BK may be electromagnetic clutches, magnetic powder clutches and any other clutches the operating states of which are controlled (which are engaged and released) according to electric commands generated from an electronic control device30.

In the drive system10constructed as described above, the carrier C1and the ring gear R1of the first planetary gear set14are connected to each other through the clutch CL1placed in its engaged state, so that the rotary elements of the first planetary gear set14are rotated as a single unit when a rotary motion of the engine12is received by the first planetary gear set14, whereby a ratio of the operating speed of the first planetary gear14to the operating speed of the engine12is held constant. Further, the ring gear R1of the first planetary gear set14is fixed to the housing26through the brake BK1placed in its engaged state, so that the ratio of the operating speed of the first planetary gear set14to the operating speed of the engine12is held constant. In other words, a differential function of the first planetary gear set14with respect to the rotary motion of the engine12is limited in the engaged state of the clutch CL1or the brake BK1, so that a ratio of the output speed to the input speed of the first planetary gear set14is held constant at a predetermined value.

In the drive system10, a differential device comprising the first and second planetary gear sets14and16is provided with four rotary components when the clutch CL2is placed in the engaged state. In other words, the drive system10includes: the differential device comprising the first planetary gear set14and the second planetary gear set16and provided with the four rotary components the relative rotating speeds of which are represented along a vertical axis in each of two-dimensional collinear charts ofFIGS. 5-11referred to below, in which the relative gear ratios of the first and second planetary gear sets14and16are taken along a horizontal axis; and the engine12, the first electric motor MG1, the second electric motor MG2and the output gear28, which are respectively connected to the above-indicated four rotary components. One of the four rotary components is constituted by the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16which are selectively connected to each other through the clutch CL2, and the ring gear R2selectively connected to the carrier C1through the clutch CL2is selectively fixed to the housing26through the brake BK2.

In the present drive system10, the clutch CL0need not be provided. That is, the crankshaft12aof the engine12may be directly connected to the carrier C1of the first planetary gear set14, or indirectly through a damper, for example, without the clutch CL0being disposed therebetween. The clutch CL0is selectively placed in the engaged or released state depending upon the running state of the hybrid vehicle provided with the drive system10. However, the present embodiment will be described on the assumption that the clutch CL0is held in the engaged state.

FIG. 2is the block diagram illustrating major portions of a control system provided to control the drive system10. The electronic control device30shown inFIG. 2is a so-called microcomputer which incorporates a CPU, a ROM, a RAM and an input-output interface and which is operable to perform signal processing operations according to programs stored in the ROM while utilizing a temporary data storage function of the RAM, to implement various drive controls of the drive system10, such as a drive control of the engine12and hybrid drive controls of the first and second electric motors MG1and MG2. In the present embodiment, the electronic control device30serves as a control apparatus for the drive system10. The electronic control device30may be constituted by mutually independent control units as needed for respective controls such as an output control of the engine12and drive controls of the first and second electric motors MG1and MG2. For example, a control unit for a shift switching device58(which will be described) may be independent of the control units for the output control of the engine12and the drive controls of the first and second electric motors MG1and MG2.

As indicated inFIG. 2, the electronic control device30is configured to receive various signals from sensors and switches provided in the drive system10. Namely, the electronic control device30receives: an output signal of an accelerator pedal operation amount sensor32indicative of an operation amount or angle ACCof an accelerator pedal (not shown), which corresponds to a vehicle output required by a vehicle operator; an output signal of an engine speed sensor34indicative of an engine speed NE, that is, an operating speed of the engine12; an output signal of a first electric motor speed sensor36indicative of an operating speed NMG1of the first electric motor MG1; an output signal of a second electric motor speed sensor38indicative of an operating speed NMG2of the second electric motor MG2; an output signal of a running speed detector in the form of an output speed sensor40indicative of a rotating speed NOUTof the output gear28, which corresponds to a running speed V of the hybrid vehicle; an output signal of a battery sensor42indicative of an output Pbtof a battery48; and an output signal of a shift position sensor44indicative of a presently selected shift position PSof a manually operated shifting device46. For instance, the battery sensor42detects an electromotive force of the battery48, as a value equivalent to the output Pbt.

The electronic control device30is also configured to generate various control commands to be applied to various portions of the drive system10. Namely, the electronic control device30applies, to an engine control device52, engine output control commands for controlling the output of the engine12, which commands include: a fuel injection amount control signal to control an amount of injection of a fuel by a fuel injecting device into an intake pipe; an ignition control signal to control a timing of ignition of the engine12by an igniting device; and an electronic throttle valve drive control signal to control a throttle actuator for controlling an opening angle θTHof an electronic throttle valve. Further, the electronic control device30applies command signals to an inverter50, for controlling operations of the first and second electric motors MG1and MG2, so that the first and second electric motors MG1and MG2are operated with electric energies supplied thereto from the battery48through the inverter50according to the command signals to control outputs (output torques) of the electric motors MG1and MG2. Electric energies generated by the first and second electric motors MG1and MG2are supplied to and stored in the battery48through the inverter50. Further, the electronic control device30applies command signals for controlling the operating states of the clutches CL0, CL1and CL2and brakes BK1and BK2, to electromagnetic control valves such as linear solenoid valves provided in the hydraulic control unit54, so that hydraulic pressures generated by those electromagnetic control valves are controlled to control the operating states of the clutches CL and brakes BK.

FIG. 3is the perspective view showing an arrangement of the above-indicated shift switching device58provided in the drive system10. As shown inFIG. 3, the shift switching device58includes: a shaft member64rotated by an actuator60; a detent member66which is fixed to a predetermined axial portion of the shaft member64in a non-rotatable manner such that the detent member66is pivoted about an axis of the shaft member64together with the shaft member64, and which has a cam surface68formed along its periphery and provided with a first recess and a second recess respectively defining a parking-lock position and a non-parking-lock position; and an engaging member70in the form of an elongate spring sheet which is provided at its respective opposite end portions with an engaging portion72held in pressing contact with the cam surface68of the detent member66by a biasing force, for selective engagement with the first and second recesses, and a fixing portion74at which the engaging member70is fixed to a fixing member76by fasteners78such as screws. A body of the actuator60, and the fixing member76are fixed to a housing80. The detent member66may also be called a “detent plate”, “parking lever” or “detent lever”, for example, while the engaging member70may also be called a “detent spring”, for example.

As shown inFIG. 1, the drive system10is provided with: a counter shaft102disposed in a power transmitting path between the output gear28and the drive wheels (not shown); and a counter driven gear104, a final drive gear106and a parking lock gear82which are fixed integrally to the counter shaft102such that the counter driven gear104, final drive gear106and parking lock gear82are coaxial with the counter shaft102. Preferably, the parking lock gear82is fixed to one of opposite axial ends of the counter shaft102. The counter driven gear104is held in meshing engagement with the output gear28, so that a drive force is transmitted from the output gear28to the drive wheels through the counter driven gear104, counter shaft102, final drive gear106, etc.

Referring back toFIG. 3, the shift switching device58further includes a parking lock pawl (engaging pawl member)86in the form of an elongate lever which is pivotable between a parking-lock position in which the parking lock pawl86engages external teeth84of the parking lock gear82, and a non-parking-lock position. This parking lock pawl86is supported by the housing80such that the parking lock pawl86is pivotable about a pin88located at its proximal end. The parking lock pawl86has an engaging tooth90formed in its longitudinally intermediate portion, for engagement with the external teeth84, and a sliding portion92formed in its distal or free end portion, for engagement with a parking lock cam96. The parking lock pawl86is normally held by a return spring (not shown) in the non-parking-lock position in which the engaging tooth90is not in engagement with the external teeth84.

The parking lock cam96is fixed to a distal or free end portion of an L-shaped parking rod94which is pivotably connected at its proximal end portion to the detent member66, so that the parking lock cam96is axially movable by the parking rod94. The parking lock cam96has a tapered cam surface, and is biased toward a stopper (not shown), with a preset preloading force generated by a pre-loading spring98in the form of a coil disposed between the parking lock cam96and a spring seat100fixed to a predetermined longitudinal part of the distal end portion of the parking rod94. The parking rod94is supported such that the distal end portion is movable in its longitudinal direction (in a direction perpendicular to the axis about which the parking rod94is pivotably connected to the detent member66), so that the parking lock cam96is movable in sliding contact with the sliding portion92of the parking lock pawl86.

The engaging member70is preferably an elongate sheet spring, so that the engaging portion72provided at the distal end of the engaging member70is normally held in pressing contact with the cam surface68of the detent member66with a predetermined biasing force of the sheet spring. The engaging portion72takes the form of a roller supported at the distal end of the engaging member70rotatably about an axis parallel to the axis of pivoting of the detent member66. In this arrangement, the detent member66is held in the parking-lock position with the engaging portion72held in engagement with the first recess, and in the non-parking-lock position with the engaging portion72held in engagement with the second recess.

In the non-parking-lock position of the detent member66, the engaging tooth90of the parking lock pawl86is not in engagement with the external teeth84of the parking lock gear82. In this non-parking-lock position, a rotary motion of the parking lock gear82is not prevented by the parking lock pawl86. In the parking-lock position of the detent member66, the engaging tooth90of the parking lock pawl86is held in engagement with the external teeth84of the parking lock gear82. In this parking-lock position, the shift switching device58is placed in a parking lock state in which the rotary motion of the parking lock gear82is prevented by the parking lock pawl86. Namely, rotary motions of the drive wheels (not shown) indirectly connected to the parking lock gear82are prevented.

The electronic control device30controls an operation of the actuator60, on the basis of an output signal of an encoder62provided to detect a rotary position of the actuator60, to perform a shift switching control of the shift switching device58for selectively placing the drive system10in a parking position and non-parking positions. The actuator60is constituted by an electric motor such as a switched reluctance motor (SRM), for instance, and functions to actuate the shift switching device58according to command signals received from the electronic control device30. The encoder62is rotated together with a rotary member of the actuator60, and supplies the electronic control device30with the output signal indicative of the rotary position of the actuator60. Preferably, the encoder62is a rotary encoder configured to generate A-phase, B-phase and Z-phase signals.

The electronic control device30controls the operation of the actuator60in a feedback fashion on the basis of the output signal of the encoder62indicative of the rotary position of the actuator60. When the selection of the parking position of the manually operated shifting device46is detected by the shift position sensor44, for instance, the electronic control device30controls the actuator60so as to place the detent member66in its parking lock position. Namely, the shift switching device58functions as a parking lock mechanism configured to prevent the rotary motion of the parking lock gear82connected indirectly to the output gear28, when the shifting device46is operated to the parking position.

An operating state of the drive system10is controlled through the first and second electric motors MG1and MG2, such that the drive system10functions as an electrically controlled differential portion whose difference of input and output speeds is controllable. For example, an electric energy generated by the first electric motor MG1is supplied to the battery48or the second electric motor MG2through the inverter50. Namely, a major portion of the drive force of the engine12is mechanically transmitted to the output gear28, while the remaining portion of the drive force is consumed by the first electric motor MG1operating as the electric generator, and converted into the electric energy, which is supplied to the second electric motor MG2through the inverter50, so that the second electric motor MG2is operated to generate a drive force to be transmitted to the output gear28. Components associated with the generation of the electric energy and the consumption of the generated electric energy by the second electric motor MG2constitute an electric path through which a portion of the drive force of the engine12is converted into an electric energy which is converted into a mechanical energy.

In the hybrid vehicle provided with the drive system10constructed as described above, a selected one of a plurality of vehicle drive modes is established according to the operating states of the engine12and the first and second electric motors MG1and MG2, and the operating states of the clutches CL and brakes BK.FIG. 4is the table indicating combinations of the operating states of the clutches CL1and CL2and the brakes BK1and BK2, which correspond to the respective eight vehicle drive modes of the drive system10. In this table, “o” marks represent the engaged states of the clutches CL and brakes BK while blanks represent their released states. Drive modes HV1and HV2are hybrid drive modes in which the engine12is operated as the vehicle drive power source while the first and second electric motors MG1and MG2are operated as needed to generate a vehicle drive force and/or an electric energy. In these hybrid drive modes HV1and HV2, at least one of the first and second electric motors MG1and MG2can be operated to generate a reaction force or placed in a non-loaded free state. Drive modes EV1and EV2indicated inFIG. 4are EV drive modes in which the engine12is held at rest while at least one of the first and second electric motors MG1and MG2is used as the vehicle drive power source. Drive modes “1st-speed” through “4th-speed” are constant-speed-ratio drive modes which are established when the differential function of the first and second planetary gear sets14and16is limited, and in which the ratios of the output speeds of the first and second planetary gear sets14and16to the speed of the rotary motion received from the engine12are held constant.

In the drive system10, the clutch CL1and the brake BK1are both placed in the released states, as indicated inFIG. 4, to permit the first planetary gear set14to perform the differential function with respect to the rotary motion received from the engine12, in the hybrid drive modes HV1and HV2in which the engine12is operated as the vehicle drive power source while the first and second electric motors MG1and MG2are operated as needed to generate a drive force and/or an electric energy. The hybrid drive mode HV1is established when the brake BK2is placed in the engaged state while the clutch CL2is placed in the released state, and the hybrid drive mode HV2is established when the brake BK2is placed in the released state while the clutch CL2is placed in the engaged state.

The clutch CL1and the brake BK1are both placed in the released states, to permit the first planetary gear set14to perform the differential function with respect to the rotary motion received from the engine12, also in the EV drive modes in which at least one of the first and second electric motors MG1and MG2is operated as the vehicle drive power source while the engine12is held at rest. The EV drive mode EV1is established when the brake BK2is placed in the engaged state while the clutch CL2is placed in the released state, and the EV drive mode EV2is established when the brake BK2and the clutch CL2are both placed in the engaged states.

In the constant-speed-ratio drive modes in which ratios of the output speeds of the first and second planetary gear sets14and16to the speed of the rotary motion received from the engine12are held constant, either one of the clutch CL1and the brake BK1is placed in the engaged state to limit the differential function of the first planetary gear set14with respect to the rotary motion received from the engine12. The constant-speed-ratio drive mode “1st-speed” which is a first-speed drive mode having the highest speed ratio value is established when the clutch CL1and the brake BK2are placed in the engaged states while the clutch CL2and the brake BK1are placed in the released states. The constant-speed-ratio drive mode “2nd-speed” which is a second-speed drive mode having a speed ratio value lower than that of the constant-speed-ratio drive mode “1st-speed” is established when the clutches CL1and CL2are placed in the released states while the brakes BK1and BK2are placed in the engaged states. The constant-speed-ratio drive mode “3rd-speed” which is a third-speed drive mode having a speed ratio value lower than that of the constant-speed-ratio drive mode “2nd-speed” is established when the clutches CL1and CL2are placed in the engaged states while the brakes BK1and BK2are placed in the released states. The constant-speed-ratio drive mode “4th-speed” which is a fourth-speed drive mode having the lowest speed ratio value is established when the clutch CL1and the brake BK2are placed in the released states while the clutch CL2and the brake BK1are placed in the engaged states.

FIGS. 5-11are the collinear charts each having straight lines which permit indication thereon of the relative rotating speeds of the various rotary components of the drive system10(rotary elements of the first and second planetary gear sets14and16), in respective different states of connection of the rotary elements corresponding to the respective different combinations of the operating states of the clutches CL1and CL2and the brakes BK1and BK2. Each of these collinear charts is defined in a two-dimensional coordinate system having a horizontal axis along which the relative gear ratios ρ of the first and second planetary gear sets14and16are taken, and a vertical axis along which the relative rotating speeds of the rotary elements are taken. The collinear charts indicate the relative rotating speeds when the output gear28is rotated in the positive direction to drive the hybrid vehicle in the forward direction. A horizontal line X1represents the rotating speed of zero, while vertical lines Y1, Y2a, Y2b, Y3, Y4aand Y4barranged in the order of description in the rightward direction represent the respective relative rotating speeds of the various rotary elements. Namely, a solid line Y1represents the rotating speed of the ring gear R1of the first planetary gear set14(first electric motor MG1), and a solid line Y2arepresents the rotating speed of the carrier C1of the first planetary gear set14(engine12), while a broken line Y2brepresents the rotating speed of the ring gear R2of the second planetary gear set16. A broken line Y3represents the rotating speed of the carrier C2of the second planetary gear set16(output gear28), and a solid line Y4arepresents the rotating speed of the sun gear S1of the first planetary gear set14, while a broken line Y4brepresents the rotating speed of the sun gear S2of the second planetary gear set16(second electric motor MG2). InFIGS. 5-11, the vertical lines Y2aand Y2bare superimposed on each other, while the vertical lines Y4aand Y4bare superimposed on each other. Since the sun gears S1and S2are fixed to each other, the relative rotating speeds of the sun gears S1and S2represented by the vertical lines Y4aand Y4bare equal to each other.

InFIGS. 5-11, a solid line L1represents the relative rotating speeds of the three rotary elements of the first planetary gear set14, while a broken line L2represents the relative rotating speeds of the three rotary elements of the second planetary gear set16. Distances between the vertical lines Y1-Y4(Y2b-Y4b) are determined by the gear ratios ρ1and ρ2of the first and second planetary gear sets14and16. Described more specifically, regarding the vertical lines Y1, Y2aand Y4acorresponding to the respective three rotary elements of the first planetary gear set14, a distance between the vertical lines Y2aand Y4arespectively corresponding to the carrier C1and the sun gear S1corresponds to “1”, while a distance between the vertical lines Y1and Y2arespectively corresponding to the ring gear R1and the carrier C1corresponds to the gear ratio “ρ1”. Regarding the vertical lines Y2b, Y3and Y4bcorresponding to the respective three rotary elements of the second planetary gear set16, a distance between the vertical lines Y3and Y4brespectively corresponding to the carrier C2and the sun gear S2corresponds to “1”, while a distance between the vertical lines Y2band Y3respectively corresponding to the ring gear R2and the carrier C2corresponds to the gear ratio “ρ2”. The drive modes of the drive system10will be described by reference toFIGS. 5-11.

The collinear chart ofFIG. 5corresponds to the drive mode HV1of the drive system10, which is preferably the hybrid drive mode in which the engine12is used as the vehicle drive power source while the first and second electric motors MG1and MG2are operated as needed to generate a drive force and/or an electric energy. Described by reference to this collinear chart ofFIG. 5, the differential function of the first planetary gear set14with respect to the rotary motion received from the engine12is permitted in the released states of the clutch CL1and the brake BK1, and the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are rotatable relative to each other in the released state of the clutch CL2. In the engaged state of the brake BK2, the ring gear R2of the second planetary gear set16is fixed to the stationary member in the form of the housing26, so that the rotating speed of the ring gear R2is held zero. In this drive mode HV1, the engine12is operated to generate an output torque by which the output gear28is rotated. At this time, the first electric motor MG1is operated to generate a reaction torque in the first planetary gear set14, so that the output of the engine12can be transmitted to the output gear28. In the second planetary gear set16, the carrier C2, that is, the output gear28is rotated in the positive direction by a positive torque (i.e., a torque acting in a positive direction) generated by the second electric motor MG2in the engaged state of the brake BK2.

The collinear chart ofFIG. 6corresponds to the drive mode HV2of the drive system10, which is preferably the hybrid drive mode in which the engine12is used as the vehicle drive power source while the first and second electric motors MG1and MG2are operated as needed to generate a vehicle drive force and/or an electric energy. Described by reference to this collinear chart ofFIG. 6, the differential function of the first planetary gear set14with respect to the rotary motion received from the engine12is permitted in the released states of the clutch CL1and the brake BK1, and the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are not rotatable relative to each other, in the engaged state of the clutch CL2, that is, the carrier C1and the ring gear R2are integrally rotated as a single rotary component in the engaged state of the clutch CL2. The sun gears S1and S2, which are fixed to each other, are integrally rotated as a single rotary component. Namely, in the drive mode HV2of the drive system10, the first and second planetary gear sets14and16function as a differential device comprising a total of four rotary components. That is, the drive mode HV2is a composite split mode in which the four rotary components are connected to each other in the order of description in the rightward direction as seen inFIG. 6. The four rotary components consist of: the ring gear R1(connected to the first electric motor MG1); a rotary member consisting of the carrier C1and the ring gear R2connected to each other (and connected to the engine12); the carrier C2(connected to the output gear28); and a rotary member consisting of the sun gears S1and S2fixed to each other (and connected to the second electric motor MG2).

In the drive mode HV2, the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are connected to each other in the engaged state of the clutch CL2, so that the carrier C1and the ring gear R2are rotated integrally with each other. Accordingly, either one or both of the first and second electric motors MG1and MG2can receive a reaction force corresponding to the output of the engine12. Namely, one or both of the first and second electric motors MG1and MG2can be operated to receive the reaction force during an operation of the engine12, and each of the first and second electric motors MG1and MG2can be operated at an operating point assuring a relatively high degree of operating efficiency, and/or with a reduced degree of torque limitation due to heat generation.

The collinear chart ofFIG. 5also corresponds to the drive mode EV1of the drive system10, which is preferably the EV drive mode in which the engine12is held at rest while the second electric motor MG2is used as the vehicle drive power source. Described by reference to this collinear chart ofFIG. 5, the differential function of the first planetary gear set14with respect to the rotary motion received from the engine12is permitted in the released states of the clutch CL1and the brake BK1, and the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are rotatable relative to each other in the released state of the clutch CL2. Further, in the engaged state of the brake BK2, the ring gear R2of the second planetary gear set16is fixed to the stationary member in the form of the housing26, so that the rotating speed of the ring gear R2is held zero. In this drive mode EV1, the carrier C2, that is, the output gear28is rotated in the positive direction by a positive torque (i.e., a torque acting in a positive direction) generated by the second electric motor MG2in the second planetary gear set16. Namely, the hybrid vehicle provided with the drive system10can be driven in the forward direction with the positive torque generated by the second electric motor MG2. In this case, the first electric motor MG1is preferably held in a free state.

The collinear chart ofFIG. 7corresponds to the drive mode EV2of the drive system10, which is preferably the EV drive mode in which the engine12is held at rest while at least one of the first and second electric motors MG1and MG2is used as the vehicle drive power source. Described by reference to this collinear chart ofFIG. 7, the differential function of the first planetary gear set14with respect to the rotary motion received from the engine12is permitted in the released states of the clutch CL1and the brake BK1, and the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are not rotatable relative to each other in the engaged state of the clutch CL2. Further, in the engaged state of the brake BK2, the ring gear R2of the second planetary gear set16and the carrier C1of the first planetary gear set14which is connected to the ring gear R2, are fixed to the stationary member in the form of the housing26, so that the rotating speeds of the ring gear R2and the carrier C1are held zero. In this drive mode EV2, the rotating directions of the ring gear R1and the sun gear S1of the first planetary gear set14are opposite to each other. Namely, the carrier C2, that is, the output gear28is rotated in the positive direction by a negative torque (acting in the negative direction) generated by the first electric motor MG1, and/or a positive torque (acting in the positive direction) generated by the second electric motor MG2. That is, the hybrid vehicle provided with the drive system10can be driven in the forward direction when the torque is generated by at least one of the first and second electric motors MG1and MG2.

In the drive mode EV2, at least one of the first and second electric motors MG1and MG2may be operated as the electric generator. In this case, one or both of the first and second electric motors MG1and MG2may be operated to generate a vehicle drive force (torque), at an operating point assuring a relatively high degree of operating efficiency, and/or with a reduced degree of torque limitation due to heat generation. Further, at least one of the first and second electric motors MG1and MG2may be held in a free state, when the generation of an electric energy by a regenerative operation of the electric motors MG1and MG2is inhibited due to full charging of the battery48. Namely, the drive mode EV2can be established under various running conditions of the hybrid vehicle, or may be kept for a relatively long length of time. Accordingly, the drive mode EV2is advantageously provided on a hybrid vehicle such as a plug-in hybrid vehicle, which is frequently placed in an EV drive mode.

The drive modes “1st-speed” through “4th-speed” indicated inFIG. 4are the constant-speed-ratio drive modes which are established when the differential functions of the first and second planetary gear sets14and16are limited and in which the ratio of the output speed of the first or second planetary gear set14,16to the speed of the rotary motion received from the engine12is held constant. In the drive modes “1st-speed” through “4th-speed”, one of the clutch CL1and the brake BK1is placed in the engaged state to hold constant the ratio of the output speed of the first planetary gear set14to the speed of the rotary motion received from the engine12.

The collinear chart ofFIG. 8corresponds to the drive mode “1st-speed”. Described more specifically by reference to this collinear chart ofFIG. 8, the rotary elements of the first planetary gear set14are rotated as a single rotary unit in the engaged state of the clutch CL1. Namely, the operating speeds of the first electric motor MG1connected to the ring gear R1, the engine12connected to the carrier C1and the second electric motor MG2connected to the sun gear S1(sun gear S2) are equal to each other, so that the drive force received from the engine12is transmitted to the sun gear S2of the second planetary gear set16through the first planetary gear set14the rotary elements of which are rotated as a single rotary unit. In the second planetary gear set16, the ring gear R2is fixed to the housing26through the brake BK2placed in the engaged state, so that the speed of the rotary motion of the engine12transmitted to the sun gear S2is reduced by the second planetary gear set16before the rotary motion is transmitted from the carrier C2to the output gear28. In the drive mode “1st-speed”, the speed of the rotary motion of the engine12is changed at a constant ratio corresponding to this drive mode, before the rotary motion is transmitted to the output gear28. Further, a drive force generated by at least one of the first and second electric motors MG1and MG2may be transmitted to the output gear28.

The collinear chart ofFIG. 9corresponds to the drive mode “2nd-speed”. Described more specifically by reference to this collinear chart ofFIG. 9, the ring gear R1of the first planetary gear set14is fixed to the housing26through the brake BK1placed in the engaged state, so that the speed of the rotary motion of the engine12transmitted to the carrier C1is raised by the first planetary gear set14before the rotary motion is transmitted from the carrier C1to the sun gear S2. In the second planetary gear set16, the ring gear R2is fixed to the housing26through the brake BK2placed in the engaged state, so that the speed of the rotary motion of the engine12transmitted to the sun gear S2is reduced by the second planetary gear set16before the rotary motion is transmitted from the carrier C2to the output gear28. In the drive mode “2nd-speed”, the speed of the rotary motion of the engine12is changed at a constant ratio corresponding to this drive mode, before the rotary motion is transmitted to the output gear28. Further, a drive force generated by the second electric motor MG2may be transmitted to the output gear28. In the engaged state of the brake BK1, the operating direction of the second electric motor MG2is the same as the operating direction of the engine12.

The collinear chart ofFIG. 10corresponds to the drive mode “3rd-speed”. Described more specifically by reference to this collinear chart ofFIG. 10, the rotary elements of the first planetary gear set14are rotated as a single rotary unit in the engaged state of the clutch CL1. Namely, the operating speeds of the first electric motor MG1connected to the ring gear R1, the engine12connected to the carrier C1and the second electric motor MG2connected to the sun gear S1(sun gear S2) are equal to each other. Further, the rotary elements of the first and second planetary gear sets14and16are rotated as a single rotary unit in the engaged state of the clutch CL2. Accordingly, the drive force of the engine12transmitted to the carrier C1is transmitted from the carrier C2to the output gear28through the first and second planetary gear sets14and16the rotary elements of which are rotated as the single rotary unit. In the drive mode “3rd-speed”, the speed of the rotary motion of the engine12is changed at a constant ratio (=1) corresponding to this drive mode, before the rotary motion is transmitted to the output gear28. Further, a drive force generated by at least one of the first and second electric motors MG1and MG2may be transmitted to the output gear28.

The collinear chart ofFIG. 11corresponds to the drive mode “4th-speed”. Described more specifically by reference to this collinear chart ofFIG. 11, the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are connected to each other through the clutch CL2placed in the engaged state, so that the rotary elements of the first and second planetary gear sets14and16are rotated as a single rotary unit. Namely, the operating speeds of the carrier C1and the ring gear R2which are connected to each other, and the operating speed of the engine12are equal to each other. Further, the ring gear R1of the first planetary gear set14is fixed to the housing26through the brake BK1placed in the engaged state, so that the speed of the rotary motion of the engine12transmitted to the carrier C1and the ring gear R2connected to each other is raised before the rotary motion is transmitted from the carrier C2to the output gear28. In the drive mode “4th-speed”, the speed of the rotary motion of the engine12is changed at a constant ratio corresponding to this drive mode, before the rotary motion is transmitted to the output gear28. Further, a drive force generated by the second electric motor MG2may be transmitted to the output gear28. In the engaged state of the brake BK1, the operating direction of the second electric motor MG2is the same as the operating direction of the engine12.

FIG. 12is the functional block diagram illustrating major control functions of the electronic control device30. A drive mode switching control portion110shown inFIG. 12is configured to determine the drive mode of the drive system10that should be established. Described more specifically, the drive mode switching control portion110selects one of the drive modes indicated inFIG. 4, on the basis of the accelerator pedal operation amount ACCdetected by the accelerator pedal operation amount sensor32, the vehicle running speed V corresponding to the output speed detected by the output speed sensor40, the output Pbtof the battery48detected by the battery sensor42, etc., and according to a predetermined drive mode switching map.

The drive mode switching control portion110establishes a selected one of the plurality of drive modes ofFIG. 4, namely, selectively establishes one of: the hybrid drive modes HV1and HV2in which the output torque of the engine12and the output torque of at least one of the first and second electric motors MG1and MG2are transmitted to the output gear28; the EV drive mode EV1in which only the output torque of the second electric motor MG2is transmitted to the output gear28; the EV drive mode EV2in which the output torques of the first and second electric motors MG1and MG2are transmitted to the output gear28; and the constant-speed-ratio drive modes “1st-speed” through “4th-speed” in which the differential functions of the first and second planetary gear sets14and16are limited.

A clutch engagement control portion112is configured to control the operating states of the clutches CL1and CL2through the hydraulic control unit54. Described more specifically, the clutch engagement control portion112controls output hydraulic pressures of solenoid control valves provided in the hydraulic control unit54to control the clutches CL1and CL2, for controlling the hydraulic pressures PCL1and PCL2which determine the operating states (torque capacities) of the clutches CL1and CL2. The clutch engagement control portion112is preferably configured to control the operating states of the clutches CL1and CL2, according to the drive mode selected by the drive mode switching control portion110. Namely, the clutch engagement control portion112is basically configured to control the torque capacities of the clutches CL1and CL2, so as to place the clutch CL1in the released state and place the clutch CL2in the engaged state when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode HV2, EV2or “4th-speed”, and so as to place both of the clutches CL1and CL2in the released states when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode HV1, EV1or “2nd-speed”. Further, the clutch engagement control portion112controls the torque capacities of the clutches CL1and CL2so as to place the clutch CL1in the engaged state and place the clutch CL2in the released state when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode “1st-speed”, and so as to place both of the clutches CL1and CL2in the engaged states when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode “3rd-speed”.

A brake engagement control portion114is configured to control the operating state of the brakes BK1and BK2through the hydraulic control unit54. Described more specifically, the brake engagement control portion114controls output hydraulic pressures of solenoid control valves provided in the hydraulic control unit54to control the brakes BK1and BK2, for controlling the hydraulic pressures PBK1and PBK2which determine the operating states (torque capacities) of the brakes BK1and BK2. The brake engagement control portion114is preferably configured to control the operating states of the brakes BK1and BK2, according to the drive mode selected by the drive mode switching control portion110. Namely, the brake engagement control portion114is basically configured to control the torque capacities of the brakes BK1and BK2, so as to place the brake BK1in the released state and place the brake BK2in the engaged state when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode HV1, EV1, EV2or “1st-speed”, and so as to place both of the brakes BK1and BK2in the released states when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode HV2or “3rd-speed”. Further, the brake engagement control portion114controls the torque capacities of the brakes BK1and BK2, so as to place both of the brakes BK1and BK2in the engaged states when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode “2nd-speed”, and so as to place the brake BK1in the engaged state and place the brake BK2in the released state when the drive mode switching control portion110has determined that the drive system10should be switched to the drive mode “4th-speed”.

An engine drive control portion116is configured to control an operation of the engine12through the engine control device52. For instance, the engine drive control portion116commands the engine control device52to control an amount of supply of a fuel by the fuel injecting device of the engine12into an intake pipe, a timing of ignition (ignition timing) of the engine12by the igniting device, and the opening angle θTHof the electronic throttle valve, so that the engine12generates a required output, that is, a target torque (target engine output).

A first electric motor drive control portion118is configured to control an operation of the first electric motor MG1through the inverter50. For example, the first electric motor drive control portion118controls an amount of an electric energy to be supplied from the battery48to the first electric motor MG1through the inverter50, so that the first electric motor MG1generates a required output, that is, a target torque (target MG1output). A second electric motor drive control portion120is configured to control an operation of the second electric motor MG2through the inverter50. For example, the second electric motor drive control portion120controls an amount of an electric energy to be supplied from the battery48to the second electric motor MG2through the inverter50, so that the second electric motor MG2generates a required output, that is, a target torque (target MG2output).

In the hybrid drive modes in which the engine12is operated while the first and second electric motors MG1and MG2are used as the vehicle drive power source, a required vehicle drive force to be generated by the drive system10(output gear28) is calculated on the basis of the accelerator pedal operation amount ACCdetected by the accelerator pedal operation amount sensor32, and the vehicle running speed V corresponding to the output speed NOUTdetected by the output speed sensor40. The operations of the first and second electric motors MG1and MG2are controlled by the first and second electric motor drive control portions118and120, while the operation of the engine12is controlled by the engine drive control portion116, so that the calculated required vehicle drive force is obtained by the output torque of the engine12and the output torques of the first and second electric motors MG1and MG2.

A parking lock control portion122is configured to control an operation of the parking lock mechanism in the form of the shift switching device58according to the presently selected shift position of the manually operated shifting device46. The parking lock control portion122is basically configured to control an operation of the actuator60for controlling an angular position of the detent member66according to the output signal of the shift position sensor44indicative of the selected shift position PSof the shifting device46. Namely, the parking lock control portion122controls the operation of the actuator60to place the detent member66in the parking lock position when the operation of the shifting device46to the parking position is detected by the shift position sensor44. In the parking lock position of the detent member66, the engaging tooth90of the parking lock pawl86is held in engagement with the external teeth84of the parking lock gear82, so that the shift switching device58is placed in the parking lock position in which the rotary motion of the parking lock gear82is prevented. When the operation of the shifting device46to any non-parking position (any position other than the parking position) is detected by the shift position sensor44, on the other hand, the parking lock control portion122controls the operation of the actuator60to place the detent member66in the non-parking-lock position. In the non-parking-lock position of the detent member66, the engaging tooth90of the parking lock pawl86is not held in engagement with the external teeth84of the parking lock gear82, so that the rotary motion of the parking lock gear82is not prevented (that is, is permitted).

An engine starting control portion124is configured to implement an engine starting control for starting the engine12. Namely, the engine starting control portion124is basically configured to command the engine control device52to start the engine12, when the engine12which has been held at rest is required to be started. For instance, the engine starting control portion124implements the engine starting control for starting the engine,12, when the drive mode switching control portion110has determined that the drive system10should be switched from the EV drive mode EV1or EV2in which the engine12is held at rest, to the hybrid drive mode HV1or HV2.

Where the engine12is required to be started while the rotary motion of the parking lock gear82is prevented by the shift switching device58and the vehicle has been held at rest, the engine starting control portion124starts the engine12in a first starting mode in which the brake BK1is placed in the engaged state, and the operating speed of the engine12is raised with the output torque of the second electric motor MG2while the rotary motion of the parking lock gear82is kept prevented. For example, the engine starting control portion124starts the engine12in the first starting mode, where the engine12is required to be started when the shift position PSdetected by the shift position sensor44is the parking position while the vehicle running speed V detected by the output speed sensor40is zero.

FIG. 13is the table indicating the operating states of the clutches CL1and CL2and the brakes BK1and BK2when the engine12is started in the first starting mode. In the table, a “o” mark and a blank respectively indicate the engaged and released states of the clutches and brakes. In the first starting mode, the brake BK1is placed in the engaged state while the clutches CL1and CL2and the brake BK2are placed in the released states, as indicated inFIG. 13.FIG. 14is the collinear chart having straight lines which permit indication thereon of the relative rotating speeds of the rotary elements of the drive system10when the engine12is started in the first starting mode. When the brake BK1is placed in the engaged state, the operating speed of the first electric motor MG1(the rotating speed of the ring gear R1) is zero, as indicated inFIG. 14. Since the rotary motion of the parking lock gear82is prevented by the shift switching device58, the rotating speed of the output gear28(carrier C2) is held at zero, so that the operating speed NEof the engine12can be raised with the positive torque (indicated by a white arrow inFIG. 14) generated by the second electric motor MG2, without generation of a reaction force by the first electric motor MG1, or without bringing the brake BK2into the engaged state. Namely, the engine12is cranked by operating only the second electric motor MG2while the first electric motor MG1is held at rest, so that an amount of electric energy required to start the engine12can be reduced.

The engine starting control portion124is preferably configured to start the engine12in the above-described first starting mode where the output Pbtof the battery48detected by the battery sensor42is smaller than a predetermined threshold value Po. That is, the engine starting control portion124raises the operating speed of the engine12with the output torque of the second electric motor MG2while the rotary motion of the parking lock gear82is prevented by the shift switching device58and while the brake BK1is placed in the engaged state. Where the output Pbtof the battery48detected by the battery sensor42is not smaller than the predetermined threshold value Po, the engine starting control portion124starts the engine12in a second starting mode in which the operating speed NEof the engine12is raised with the output torque of the first electric motor MG1while the second electric motor MG2is operated to generate a reaction force. When the engine12is started in the second starting mode, the parking lock control portion122permits the rotary motion of the parking lock gear82. That is, the parking lock control portion122places the detent member66in the non-parking-lock position in which the rotary motion of the parking lock gear82is permitted. The above-indicated threshold value Pois a lower limit of the output Pbtof the battery48predetermined by experimentation, above which the operating speed NEof the engine12can be sufficiently raised for starting the engine12with the output torque generated by the first electric motor MG1while the second electric motor MG2is operated to generate the reaction force.

The engine starting control portion124is preferably configured to start the engine12in the second starting mode, by placing the brake BK2in the engaged state while placing the clutches CL1and CL2and the brake BK1in the released states, and by operating the first electric motor MG1to generate the output torque for raising the operating speed NEof the engine12while the second electric motor MG2is operated to generate the reaction force.FIG. 5is the collinear chart having the straight lines which permit indication thereon of the relative rotating speeds of the rotary elements of the drive system10, when the engine12is started in the second starting mode as described above. As is apparent from this collinear chart, the differential function of the first planetary gear set14with respect to the rotary motion received from the engine12is permitted in the released states of the clutch CL1and the brake BK1. In the released state of the clutch CL2, the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are rotatable relative to each other. In the engaged state of the brake BK2, the ring gear R2of the second planetary gear set16is fixed to the stationary member in the form of the housing26, so that the rotating speed of the ring gear R2is held at zero. In this state, the operating speed NEof the engine12can be raised with the positive torque generated by the first electric motor MG1while the reaction force is generated by the second electric motor MG2.

Preferably, the engine starting control portion124is configured to start the engine12in the second starting mode, by placing the clutch CL2in the engaged state while placing the clutch CL1and the brakes BK1and BK2in the released states, and by operating the first electric motor MG1to generate the output torque for raising the operating speed NEof the engine12while the second electric motor MG2is operated to generate the reaction force.FIG. 6is the collinear chart having the straight lines which permit indication thereon of the relative rotating speeds of the rotary elements of the drive system10, when the engine12is started in the second starting mode as described above. As is apparent from this collinear chart, the differential function of the first planetary gear set14with respect to the rotary motion received from the engine12is permitted in the released states of the clutch CL1and the brake BK1. In the engaged state of the clutch CL2, the carrier C1of the first planetary gear set14and the ring gear R2of the second planetary gear set16are rotated as a unit. The sun gears S1and S2which are fixed to each other are rotated as a unit. In this state, the operating speed NEof the engine12can be raised with the positive torque generated by the first electric motor MG1while the reaction force is generated by the second electric motor MG2.

As described above, the engine12is started in the second starting mode where the output Pbtof the battery48is sufficiently large, that is, equal to or larger than the threshold value Poabove which the operating speed NEof the engine12can be sufficiently raised for starting the engine12with the output torque generated by the first electric motor MG1while the second electric motor MG2is operated to generate the reaction force. Accordingly, the vehicle drive mode can be speedily switched from the hybrid drive mode HV1or HV2after the engine12has been started. Thus, the hybrid vehicle can smoothly start running just after the manually operated shifting device46is operated to the drive position D.

FIG. 15is the flow chart illustrating a major portion of one example of an engine starting control implemented by the electronic control device30. The engine starting control is repeatedly implemented with a predetermined cycle time.

The engine starting control is initiated with a step ST1to determine whether the engine12is required to be started while the vehicle has been held at rest. If a negative determination is obtained in the step ST1, the engine starting control is terminated. If an affirmative determination is obtained in the step ST1, the control flow goes to a step ST2to determine whether the output Pbtof the battery48detected by the battery sensor42is smaller than the predetermined threshold value Po. If an affirmative determination is obtained in the step ST2, the control flow goes to a step ST3. If a negative determination is obtained in the step ST2, the control flow goes to a step ST5in which the shift switching device58is controlled to permit the rotary motion of the parking lock gear82, and the operating speed NEof the engine12is raised with the output torque of the first electric motor MG1while the second electric motor MG2is operated to generate the reaction torque. The engine starting control is terminated with the step ST5. In the ST3, the shift switching device58is controlled to prevent (to keep prevention of) the rotary motion of the parking lock gear82, and the brake BK1is placed in the engaged state while the clutches C11and CL2and the brake BK2are placed in the released states. The step ST3is followed by a step ST4in which the operating speed NEof the engine12is raised with the output torque of the second electric motor MG2. The engine starting control is terminated with the step ST4.

In the engine starting control described above, the steps ST2and ST5need not be implemented. That is, the steps ST3and ST4may be always implemented irrespective of the output Pbtof the battery48, when the engine12is required to be started while the rotary motion of the parking lock gear82is prevented by the parking lock mechanism in the form of the shift switching device58and the vehicle has been held at rest. It will be understood from the foregoing description of the engine starting control illustrated inFIG. 15that the step ST1corresponds to an operation of the drive mode switching control portion110, and the step ST3corresponds to operations of the clutch engagement control portion112and the brake engagement control portion114, while the step ST5corresponds to an operation of the first electric motor drive control portion118, and that the steps ST4and ST5correspond to an operation of the second electric motor drive control portion120, and the steps ST3and ST5correspond to an operation of the parking lock control portion122, while the steps ST1-ST5correspond to an operation of the engine starting control portion124.

In the illustrated embodiment, the engine starting control portion124is configured to start the engine12in the first starting mode in which the operating speed NEof the engine12is raised with the torque generated by the second electric motor MG2in the engaged state of the brake BK1, where the engine12is required to be started while the rotary motion of the parking lock gear82is prevented by the parking lock mechanism in the form of the shift switching device58and the vehicle has been held at rest. Accordingly, the engine12can be adequately started even while the output Pbtof the battery48is insufficient, as in a cold state of the engine12. Namely, the illustrated embodiment provides a control apparatus in the form of the electronic control device30for the hybrid vehicle drive system10, which permits adequate starting of the engine12, irrespective of the output Pbtof the battery48.

The illustrated embodiment is further configured such that the engine starting control portion124starts the engine12in the first starting mode when the output Pbtof the battery48detected by the battery sensor42is smaller than the predetermined threshold value Po. Accordingly, the engine12can be adequately started even while the output Pbtof the battery48is insufficient, as in the cold state of the engine.

The illustrated embodiment is also configured such that the engine starting control portion124starts the engine12in the second starting mode in which the operating speed NEof the engine12is raised with the torque generated by the first electric motor MG1while the second electric motor MG2is operated to generate the reaction force, where the engine12is required to be started while the output Pbtof the battery48detected by the battery sensor42is not smaller than the predetermined threshold value Po. Accordingly, the first and second electric motors MG1and MG2cooperate with each other to crank the engine12for starting the engine12where the output Pbtof the battery48is sufficient. Therefore, the hybrid vehicle can smoothly start running after the engine12has been started.

The drive system10has a risk of occurrence of vibrations a whole drive train giving a shock to the hybrid vehicle, due to a torque variation transmitted to the output shaft (output side power transmitting path) when the engine12is started in the engaged state of the clutch CL2or the brake BK2. However, the control apparatus in the form of the electronic control device30according to the illustrated embodiment is configured to start the engine12in the first starting mode in which the above-described risk can be adequately avoided.

While the preferred embodiment of this invention has been described by reference to the drawings, it is to be understood that the invention is not limited to the details of the illustrated embodiments, but may be embodied with various changes which may occur without departing from the spirit of the invention.

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