Control system of power transmission system

A control system controls a power transmission system located between a motive power source and drive wheels. The power transmission system includes a fluid coupling and an engagement device. The control system includes an electronic control unit configured to: obtain information concerning vibration of the power transmission system; determine whether the vibration of the power transmission system is in a resonance region of the power transmission system; control the engagement device so that the engagement device slips, when the electronic control unit determines that the power transmission system is in the resonance region; and control the motive power source when the electronic control unit determines that the power transmission system is in the resonance region, so that a rotational speed of the motive power source increases as compared with a case where the power transmission system is not in the resonance region.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-206527 filed on Oct. 20, 2015 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The disclosure relates to a control system of a power transmission system that is located between a motive power source and drive wheels, and includes a fluid coupling, and an engagement device that can directly couple the input shaft side and output shaft side of the fluid coupling.

2. Description of Related Art

When a vehicle passes along an undulating road with a markedly uneven road surface, a power transmission system for transmitting driving force from a motive power source, such as an engine and a motor, to drive wheels vibrates along with vertical vibration of the wheels. Further, resonance is generated in a particular portion of the power transmission system, depending on the uneven condition of the road surface and the traveling speed of the vehicle. Under this situation, where the power transmission system includes a hydraulically-operated friction engagement element, the friction engagement element that is hydraulically placed in an engaged state may slip due to variation of transmission torque caused by the vibration, and its durability may deteriorate.

Thus, when resonance is generated in the power transmission system having a friction engagement element or elements for changing the speed ratio, a hydraulic pressure supplied to the friction engagement element that is in an engaged state is further increased, so as to further increase the engaging force of the friction engagement element, according to a technology proposed in Japanese Patent Application Publication No. 2011-220350 (JP 2011-220350 A).

According to the method disclosed in JP 2011-220350 A, the friction engagement element that is in the engaged state is prevented from slipping due to resonance of the power transmission system, thus making it possible to curb reduction of the durability of the friction engagement element.

SUMMARY

However, in the system of JP 2011-220350 A, it is inherently impossible to damp the resonance itself generated in the power transmission system, during traveling of the vehicle, unless the driver intentionally changes the accelerator pedal stroke or position. Therefore, the mechanical strength of the particular portion of the power transmission system which is in a resonant condition may be reduced, or the particular portion may be damaged, due to repeated application of bending stress. Also, the reduction of the durability of the friction engagement element as described above is a phenomenon that may also occur under vehicle operating conditions where excessive transmission torque is continuously applied to the friction engagement element, and some measure against the phenomenon is desired to be taken.

The embodiments provide a control system of a power transmission system, which makes it possible to promptly damp resonance, even when the resonance is generated in a particular portion of the power transmission system.

A control system of a power transmission system according to one aspect relates to a power transmission system located between a motive power source and drive wheels of a vehicle. The power transmission system includes a fluid coupling and an engagement device. The engagement device is configured to control a condition of connection between an input shaft side and an output shaft side of the fluid coupling. The control system includes an electronic control unit. The electronic control unit is configured to obtain information concerning vibration of the power transmission system; determine whether the vibration of the power transmission system is in a resonance region of the power transmission system, control the engagement device so that the engagement device slips, when the electronic control unit determines that the power transmission system is in the resonance region, and control the motive power source when the electronic control unit determines that the power transmission system is in the resonance region, such that a rotational speed of the motive power source increases as compared with a case where the power transmission system is not in the resonance region.

With the control system according to the above aspect, when the power transmission system is in its resonance region, the engagement device is caused to slip, rather than being placed in a directly coupled state, so that fluid that intervenes between the engagement device and the fluid coupling acts as a damper for damping vibration of the power transmission system. As a result, the resonance range of the power transmission system shifts, and its resonant condition settles. Also, at this time, in order to prevent the driver or passenger from feeling uncomfortable or strange about slipping of the engagement device, the electronic control unit increases the rotational speed of the motive power source, so as not to give rise to changes in the traveling state of the vehicle.

In the control system according to the above aspect, the information concerning vibration of the power transmission system may include information concerning mechanical vibration of a particular portion of the power transmission system, variation of output torque, and variation of a rotational speed on the output shaft side of the fluid coupling.

With the control system as described above, the presence or absence of resonance in the power transmission system can be grasped with reliability.

In the control system according to the above aspect, the electronic control unit may be configured to control the engagement device so that a slip amount of the engagement device increases as the vibration of the power transmission system increases, when the electronic control unit determines that the power transmission system is in the resonance region.

With the control system as described above, the engagement device is controlled such that its slip amount increases as vibration of the power transmission system increases, so that resonance generated in the power transmission system can be further promptly damped.

In the control system as described above, the electronic control unit may be configured to control the engagement device so that a slip amount of the engagement device becomes equal to 0, when the electronic control unit determines that the power transmission system is no longer in the resonance region after having been in the resonance region.

With the control system as described above, when it is determined that the power transmission system is no longer in the resonance region, the engagement device is controlled so that its slip amount becomes equal to 0. Thus, a loss of power transmission via the fluid coupling can be eliminated.

In the control system as described above, the slip amount of the engagement device may include a fully released state of the engagement device.

With the control system as described above, when the slip amount of the engagement device is controlled so as to place the engagement device in the fully released state, the damping effect of the fluid coupling can be increased to the maximum.

The motive power source may include one internal combustion engine and two rotating electric machines (for example, two motor-generators). In this case, the power transmission system may further include a planetary gear train mounted between the internal combustion engine and a first rotating electric machine, and the planetary gear train and a second rotating electric machine may be connected in parallel to an input shaft of the fluid coupling, while an output shaft of the fluid coupling may be connected to the drive wheels side. In another example, the input shaft of the fluid coupling may be connected to the second rotating electric machine, and a planetary gear train and the output shaft of the fluid coupling may be connected in parallel to the driving wheels side.

With the control system according to the above aspect, when the power transmission system is in its resonance region, the engagement device is caused to slip, so that the fluid present in the fluid coupling acts as a damper for damping vibration of the power transmission system. As a result, the resonance of the power transmission system can be promptly damped, and heat generated due to slipping of the engagement device is dissipated, so that reduction of its durability can be curbed. Also, when the power transmission system is in its resonance region, the electronic control unit increases the rotational speed of the motive power source, so as not to change the traveling state of the vehicle, so that the driver or passenger is prevented from feeling uncomfortable or strange about slipping of the engagement device.

DETAILED DESCRIPTION OF EMBODIMENTS

Some embodiments applied to a hybrid vehicle of front-engine, front-drive type will be described in detail with reference toFIG. 1throughFIG. 9. However, the embodiments are illustrative, not limiting, and the aspects of the disclosure may be applied to any type of vehicle in which a power transmission system including a fluid coupling, and an engagement device that can directly couple the input shaft side and output shaft side of the fluid coupling is located between a motive power source and drive wheels.

FIG. 1schematically shows a gear train of the hybrid vehicle according to one embodiment, andFIG. 2shows control blocks of its principal portion. The hybrid vehicle of this embodiment includes one internal combustion engine, i.e., an engine E, and first rotating electric machine10and second rotating electric machine20, as a motive power source; however, the disclosure is not limited to this arrangement. For example, the hybrid vehicle can have a motive power source that consists of one engine and one rotating electric machine.

In this embodiment, the hybrid vehicle is able to travel in a selected one of an EV mode in which only the rotating electric machines10,20are operated while the engine E is stopped, and an HV mode in which the engine E as well as the rotating electric machines10,20is operated. An on-board ECU (Electronic Control Unit)30switches the traveling mode of the vehicle between the EV mode and the HV mode, based on operating conditions of the vehicle. In either of these modes, the first rotating electric machine10having a small capacity mainly functions as a generator for charging a vehicle-mounted secondary battery B, and the second rotating electric machine20having a large capacity mainly functions as a motor that applies output torque to right and left front wheels (which will be referred to as “drive wheels”) W of the vehicle. While three-phase synchronous motors having high quietness are used as the rotating electric machines10,20in this embodiment, the embodiments are not limited to the use of this type of motors. The ECU is a microcomputer having a central processing unit (CPU) and memory such as ROM and RAM.

When an accelerator pedal (not shown) is not depressed during traveling of the vehicle, or when a brake pedal (not shown) is depressed, the second rotating electric machine20functions as a generator according to the SOC (state-of-charge) of the secondary battery B. In this case, regenerative energy is stored in the secondary battery B via the second rotating electric machine20, and thus functions as so-called regenerative braking. The HV mode is selected only when the amount of depression of the accelerator pedal, or the accelerator pedal stroke, is equal to or larger than a predetermined value, e.g., 60%, or the vehicle speed is equal to or higher than a predetermined value, e.g., 60 km/h, or the SOC of the secondary battery B is smaller than a predetermined value, e.g., 40%. In other words, in cases other than these cases, the EV mode is preferably selected. Accordingly, the vehicle travels backward in the EV mode, but the manner of selecting the traveling mode of the vehicle is not limited to this. The output torque delivered from the engine E in the HV mode is applied so as to drive the first rotating electric machine10as the generator, and is also applied as output torque for driving the drive wheels W along with the second rotating electric machine20.

The amount of depression of the accelerator pedal operated by the driver is detected as the accelerator pedal stroke by an accelerator pedal position sensor S1, and its output value is transmitted to the ECU30. The operation to depress the brake pedal that is similarly operated by the driver is detected as an ON/OFF signal of a brake switch S2, and its output value is transmitted to the ECU30. The vehicle speed is detected by a vehicle speed sensor S3, and the information is transmitted to the ECU30. The SOC of the secondary battery B is calculated by an operating state determining unit31of the ECU30.

The engine E is started by use of the first rotating electric machine10. Therefore, the ECU30incorporates a first rotating electric machine controller32that controls operation of the first rotating electric machine10via an inverter I connected to the secondary battery B, and a second rotating electric machine controller33that controls operation of the second rotating electric machine20. The first and second rotating electric machines10,20and the engine E are controlled by the first and second rotating electric machine controllers32,33and an engine controller34of the ECU30, respectively, based on vehicle operating conditions including the accelerator pedal stroke, vehicle speed, SOC of the secondary battery B, and so forth. Along with the control, first and second rotating electric machine speed sensors S4, S5detect the rotational speeds of rotors11,21of the first and second rotating electric machines10,20, respectively, and output these items of information to the ECU30. The engine controller34controls operation and stopping of operation of the engine E in preset timing according to vehicle conditions, and also controls the amount of fuel supplied and the timing of supply of the fuel during operation of the engine E.

A power transmission system TM is mounted between the engine E and two rotating electric machines10,20, and the drive wheels W. The power transmission system TM of this embodiment includes a fluid coupling40, friction clutch FC which functions as an engagement device, planetary gear train (which will be referred to as “first planetary gear train” for the sake of convenience)50, and a differential gear unit60. The friction clutch FC can couple the input shaft41side and output shaft42side of the fluid coupling40, for inhibiting single-phase lock of the second rotating electric machine20, without intervening fluid. Other known engaging devices, such as a magnetic powder clutch, may be used in place of the friction clutch FC. The differential gear unit60connects rotary shafts, i.e., axles WA, of the right and left drive wheels W. In this embodiment, the first planetary gear train50mounted between the engine E and the first rotating electric machine10, and the second rotating electric machine20, are connected in parallel to the input shaft41of the fluid coupling40, and the output shaft42of the fluid coupling40is connected to the right and left drive wheels W side.

The first planetary gear train50has a sun gear51coupled to the rotor11of the first rotating electric machine10, a carrier53that rotatably supports planetary gears52that mesh with the sun gear51, and an internally toothed gear54that meshes with the planetary gears52so as to surround the planetary gears52. An output shaft of the engine E, i.e., a crankshaft EA, is coupled to the carrier53of the first planetary gear train50.

An input gear43provided on the input shaft41of the fluid coupling40meshes with an externally toothed gear55provided integrally with the internally toothed gear54of the first planetary gear train50, and a small gear22formed integrally with the rotor21of the second rotating electric machine20. With this arrangement of gears, a rotation speed of the second rotating electric machine20corresponds to a rotation speed of the motive power source, which includes engine E, the first rotating electric machine10, and the second rotating electric machine20. An output gear44provided on the output shaft42of the fluid coupling40meshes with a final reduction gear61of the differential gear unit60. A pump impeller45provided on the input shaft41of the fluid coupling40, and a turbine runner46provided on the output shaft42of the fluid coupling40are connected via the friction clutch FC. When the vehicle is in an operating state other than predetermined operating states, hydraulic oil whose pressure is regulated to a pressure (which will be referred to as “lock-up hydraulic pressure”) PUfor keeping the friction clutch FC in an engaged state according to the output torque is supplied to the friction clutch FC. The hydraulic oil is supplied via an electric oil pump (not shown) using the vehicle-mounted secondary battery B as a power supply, and a known hydraulic control circuit O. In this embodiment, the above-mentioned predetermined operating states include the case where the second rotating electric machine20is brought into a single-phase locked state, and the case where the power transmission system TM is brought into a resonant condition while the vehicle is traveling on an undulating road, or the like. When the thermal load of the second rotating electric machine20is larger than a threshold value that is set in advance based on single-phase lock of the second rotating electric machine20, the friction clutch FC is controlled from a fully engaged state into a slip state or a fully released state. Similarly, when the power transmission system TM is placed in the resonant condition, the friction clutch FC is controlled from the fully engaged state into the slip state or fully released state, so that the rotational speed of the output shaft42of the fluid coupling40becomes lower than the rotational speed of the input shaft41.

The control content of the friction clutch FC associated with single-phase lock of the second rotating electric machine20is not directly relevant to this disclosure, and therefore, will not be further explained.

A final reduction gear speed sensor S6is mounted to a housing62of the differential gear unit60. The final reduction gear speed sensor S6detects variation of the rotational speed of the final reduction gear61, and outputs the detected information to the ECU30. The final reduction gear speed sensor S6of this embodiment serves to obtain vibration of the power transmission system TM that resonates due to vibration received from the road surface on which the vehicle travels, and is able to grasp the magnitude of changes in the rotational speed that varies due to the vibration, as the amplitude of the vibration. The operating state determining unit31of the ECU30determines whether variation in rotation of the final reduction gear61, namely, the amount of variation of the output torque, has exceeded a predetermined threshold value, based on the detection signal from the final reduction gear speed sensor S6. Namely, in the case where the vehicle travels on an undulating road, for example, and the vehicle body vibrates via the drive wheels due to the roughness of the road surface, the power transmission system TM also vibrates in accordance with the vibration of the vehicle body, and resonance may be generated in a particular portion of the system TM. The variation in the rotational speed of a rotating portion caused by vibration is detected by the final reduction gear speed sensor S6as variation of the output torque. Therefore, when the amount of variation in the output torque exceeds a predetermined value, the operating state determining unit31of the ECU30determines that the particular portion of the power transmission system TM lies in a resonance region. Namely, the operating state determining unit31of the ECU30functions to determine whether the obtained vibration of the power transmission system TM is within its resonance region.

It is possible, as a matter of course, to obtain vibration of the power transmission system TM, using an angular velocity sensor or a vibration sensor, in place of the above-described final reduction gear speed sensor S6. While the vibration of the power transmission system TM is obtained from variation in rotation of the final reduction gear61of the differential unit in this embodiment, the embodiments are not limited to this arrangement. Nonetheless, with regard to the power transmission system TM having a portion, such as a propeller shaft, having a relatively large amplitude of vibration, it may be preferable to obtain information on vibration of the propeller shaft, or the like.

In this embodiment, the ECU30appropriately controls operation of the engine E, first and second rotating electric machines10,20, and the friction clutch FC, based on information received from the above-described various sensors S1, S3-S6and the brake switch S2, for example.

A hydraulic controller35of the ECU30functions to control operation of the friction clutch FC, in cooperation with the hydraulic control circuit O. When the operating state determining unit31of the ECU30determines, based on the detection signal from the final reduction gear speed sensor S6, that the power transmission system TM is in its resonance region, the hydraulic controller35changes the slip amount of the friction clutch FC in accordance with the magnitude of vibration of the power transmission system TM. More specifically, the hydraulic controller35of the ECU30stores a map as shown inFIG. 3, which specifies in advance the relationship between the magnitude of variation of the output torque corresponding to the vibration of the power transmission system TM, and the target slip amount of the friction clutch FC. The hydraulic controller35reads a target slip amount ΔN0corresponding to the magnitude of variation of the output torque resulting from resonance of the power transmission system TM, and supplies a hydraulic pressure corresponding to the target slip amount ΔN0, i.e., hydraulic oil whose pressure is set to a target hydraulic pressure P0, to the friction clutch FC. The target hydraulic pressure P0varies according to input torque applied to the input shaft41of the fluid coupling40. The hydraulic controller35stores a map as shown inFIG. 4, which specifies in advance the relationship among the target slip amount, target hydraulic pressure, and the input torque applied to the input shaft41of the fluid coupling40. The hydraulic controller35reads a target hydraulic pressure P0, based on the target slip amount, and the input torque applied to the input shaft41of the fluid coupling40, and supplies the target hydraulic pressure P0thus read to the friction clutch FC. Then, the hydraulic controller35controls the hydraulic pressure supplied to the friction clutch FC, so that the friction clutch FC slips by the target slip amount ΔN0.

In the above manner, the friction clutch FC is shifted from a lock-up state to a slip state, so that the friction clutch FC functions as a damper for absorbing vibration generated in the power transmission system TM. Also, working oil, such as automatic transmission oil, present between the pump impeller45and turbine runner46of the fluid coupling40also functions as a damper. As a result, vibration characteristics of the power transmission system TM shift from a condition indicated by a solid line as shown inFIG. 5by way of example, to a higher vibration frequency range as indicated by a two-dot chain line inFIG. 5, resulting in suppression of the resonance that has been generated.

When the resonance of the power transmission system TM is suppressed or settled by shifting the resonance range in the above manner, the hydraulic controller35of the ECU30supplies hydraulic oil whose pressure is regulated to the lock-up hydraulic pressure PU, to the friction clutch FC again, after a lapse of a predetermined time, so as to avoid hunting of control. In this manner, the friction clutch FC is brought into the lock-up state again. In this embodiment, the hydraulic oil having the lock-up hydraulic pressure PUcorresponding to the input torque is supplied to the friction clutch FC, after a lapse of two seconds from the time when the resonance is eliminated, so that the friction clutch FC is brought into the lock-up state.

FIG. 6shows the procedure of hydraulic control of the friction clutch FC according to this embodiment, andFIG. 7schematically shows the relationship among variation of the output torque, hydraulic pressure supplied to the friction clutch FC, rotary torques of the first and second rotating electric machines10,20, and the rotational speeds of these rotating electric machines10,20. Initially, it is determined in step S11whether the vehicle is in a region where resonance is generated in the power transmission system TM. If it is determined that the power transmission system TM is in the resonance generation region, namely, there is a need to eliminate resonance of the power transmission system TM (see time t1ofFIG. 7), the control proceeds to step S12. In step S12, the slip amount of the friction clutch FC corresponding to the amplitude of resonance of the output torque, namely, the target slip amount ΔN0, is obtained. Then, in step S13, a hydraulic pressure to be supplied to the friction clutch FC according to the target slip amount ΔN0, namely, a target hydraulic pressure P0, is obtained. Then, in step S14, the target hydraulic pressure P0obtained in step S13is supplied to the friction clutch FC (see time t1to t2inFIG. 7). At this time, in order to prevent the driver or passenger from feeling uncomfortable or strange about slipping of the friction clutch FC, the rotational speed of the first rotating electric machine10is reduced, and the rotational speed of the second rotating electric machine20, which corresponds to the rotational speed of the motive power source, is increased at the same time, so that no changes arise in the traveling state of the vehicle. Then, it is determined again in step S15whether the vehicle is in the region where resonance is generated in the power transmission system TM. Here, if it is determined that the vehicle is still in the resonance generation region of the power transmission system TM, namely, the power transmission system TM is kept in the resonant condition, the control proceeds to step S16, to determine whether the current slip amount ΔN is equal to or smaller than the target slip amount ΔN0. If it is determined in step S16that the current slip amount ΔN is equal to or smaller than the target slip amount ΔN0, namely, the friction clutch FC needs to slip by a further amount, the control proceeds to step S17. Then, after the hydraulic pressure P0to be supplied to the friction clutch FC is reduced by a predetermined value ΔP from the currently supplied hydraulic pressure, so as to increase the slip amount of the friction clutch FC, the control returns to step S15again, to determine whether the vehicle is in the resonance generation region of the power transmission system TM.

If, on the other hand, it is determined in step S16that the current slip amount ΔN exceeds the target slip amount ΔN0, namely, the slip amount of the friction clutch FC needs to be reduced, the control proceeds to step S18. Then, after the hydraulic pressure P0to be supplied to the friction clutch FC is increased by a predetermined value ΔP from the currently supplied hydraulic pressure, so that the slip amount of the friction clutch FC is reduced, the control returns to step S15again, to determine whether the vehicle is in the resonance generation region of the power transmission system TM.

If it is determined in step S15that the vehicle is not in the resonance generation region of the power transmission system TM, namely, the resonance has been eliminated, the control proceeds to step S19, to start counting up the timer (see time t2inFIG. 7). Then, it is determined in step S20whether the count value C of the timer is equal to or larger than a preset threshold value CR. If the count value C of the timer is smaller than the threshold value CR, the control returns to step S19, and continues counting up the timer. Then, if it is determined in step S20that the count value C of the timer is equal to or larger than the threshold value CR, the control proceeds to step S21. In step S21, the lock-up hydraulic pressure PUcorresponding to the input torque applied to the input shaft41of the fluid coupling40is supplied to the friction clutch FC (see time t3to t4inFIG. 7), so as to bring the friction clutch FC back into the normal lock-up state. At the same time, the counter value C of the timer is reset to 0, and then, the control returns to step S11again.

When it is determined in step S15that the resonance has been eliminated, it is possible to immediately proceed to step S21, and bring the friction clutch FC back into the normal lock-up state, without counting up the timer. However, if the friction clutch FC is brought back into the normal lock-up state after a lapse of the predetermined time CR, as in this embodiment, when it is determined that the resonance has been eliminated, hunting of slip control for the friction clutch FC can be avoided with higher reliability.

In the above-described embodiment, the first planetary gear train50and the second rotating electric machine20are connected in parallel to the input shaft41of the fluid coupling40, and the output shaft42of the fluid coupling40is connected to the drive wheels W side. However, the input shaft41of the fluid coupling40may be connected to the second rotating electric machine20, and the first planetary gear train50and the output shaft42of the fluid coupling40may be connected in parallel to the drive wheels W side. Also, a second planetary gear train for multi-speed gearshift and two or more friction engagement elements may be mounted between the first planetary gear train50and the engine E.

FIG. 8schematically shows a gear train in a control system of a power transmission system as another embodiment. InFIG. 8, the same reference numerals are assigned to elements having the same functions as those of the previous embodiment, and repeated explanation will not be provided for the sake of simplicity. In the embodiment ofFIG. 8, a second planetary gear train70has a sun gear71that can rotate relative to the crankshaft EAof the engine E, a carrier73that rotatably supports planetary gears72that mesh with the sun gear71, and an internally toothed gear74that meshes with the planetary gears72so as to surround the gears72. The sun gear71of the second planetary gear train70and the carrier73of the same train are connected via a first clutch81as a friction engagement element, and a brake82as a friction engagement element is mounted between the sun gear71and a casing (not shown) of the power transmission system TM. The internally toothed gear74of the second planetary gear train70is integrally coupled to the carrier53of the first planetary gear train50. Also, the crankshaft EAof the engine E coupled to the carrier73of the second planetary gear train70is connected to the rotor11of the first rotating electric machine10via a second clutch83as a friction engagement element.

In the meantime, the rotor21of the second rotating electric machine20is coupled to the input shaft41of the fluid coupling40of this embodiment. The output gear44provided on the output shaft42of the fluid coupling40meshes with a driven large gear91provided at one end of a driven shaft90, along with the externally toothed gear55formed integrally with the internally toothed gear54of the first planetary gear train50. Also, the final reduction gear61of the differential gear unit60meshes with a driven small gear92provided at the other end of the driven shaft90.

By selectively switching the first clutch81and the brake82to an engaged state, it is possible to switch the second planetary gear train70between an H position having a small speed reduction ratio, and an L position having a larger speed reduction ratio than that of the H position. In this case, the output torque from the engine E and the output torque of the second rotating electric machine20are transmitted in a parallel condition to the driven large gear91of the driven shaft90. However, when the first clutch81and the brake82are placed in non-engaged states, and the second clutch83is placed in an engaged state, the output torque of the engine E can be applied to the first rotating electric machine10and the input shaft41of the fluid coupling40via the first planetary gear train50. In this case, the second planetary gear train70does not substantially function as a transmission.

FIG. 9shows the relationship between each traveling mode and the operating state of each friction engagement element in this embodiment.

In the EV mode, the vehicle can be switched between a mode in which the first rotating electric machine10functions as a generator, and the second rotating electric machine20functions as a motor, and a mode in which both the first rotating electric machine10and the second rotating electric machine20function as motors, in both of the cases where the vehicle travels forward and backward. The ECU30switches the vehicle between these modes, based on the SOC and the accelerator pedal stroke. When both the first rotating electric machine10and the second rotating electric machine20function as motors, the first clutch81and the brake82are both held in the engaged states. In the EV mode, when the accelerator pedal stroke is returned to 0% during traveling of the vehicle, or when the brake pedal (not shown) is depressed, either one of the first clutch81and the brake82is switched to the engaged state. As a result, the second rotating electric machine20functions as a generator, and its regenerative energy is stored in the secondary battery B via the first rotating electric machine10.

In the HV mode, the vehicle can be switched between a parallel method in which the output torque of the engine E can be applied to the drive wheels W side, and a series method in which the output torque of the engine E is applied to the second rotating electric machine20. In the case of the parallel method, it is possible to switch the gear position between the H position and the L position according to the vehicle speed and the accelerator pedal stroke.

In this embodiment, too, information concerning vibration of the power transmission system TM is obtained by the final reduction gear speed sensor S6, and the friction clutch FC is shifted from the fully engaged state to the slip state when resonance is generated in the power transmission system TM. In this manner, resonance of the power transmission system TM can be promptly eliminated.

While the motive power source consists of one engine E and the first and second rotating electric machines10,20in the above-described embodiments, this disclosure may be applied to a hybrid vehicle in which one engine and one rotating electric machine constitute the motive power source. For example, the arrangement of the first rotating electric machine10and the first planetary gear train50may be eliminated from the embodiment as shown inFIG. 8, such that the output of the engine E is transmitted to the driven large gear91of the driven shaft90directly or via the second planetary gear train70.

The above-described embodiments may be subjected to various modifications.