Patent Publication Number: US-2015065297-A1

Title: Control device for hybrid vehicle

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2013-177153 filed on Aug. 28, 2013 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control technique of suppressing slipping of a hybrid vehicle, which includes an engine and an electric motor as a drive source, on an uphill road. 
     2. Description of Related Art 
     A hybrid vehicle is well known which includes an engine, an electric motor, and a hydraulic power transmission that is disposed in a power transmission path between the engine and the electric motor and driving wheels so as to transmit dynamic power via a fluid. An example of such a hybrid vehicle is disclosed in Japanese Patent Application Publication No. 2000-308209 (JP 2000-308209 A). 
     SUMMARY OF THE INVENTION 
     In such a hybrid vehicle, in order to prevent slipping of the vehicle which occurs at the time of changing an applied pressure from a brake pedal to an accelerator pedal to start the vehicle in stop on a slope having a road surface gradient, a state where a torque is transmitted to an axle and driving wheels is set up by causing the electric motor to rotate in advance. However, when the road surface gradient is relatively large and a pressure cannot be applied to the accelerator pedal just after the brake pedal is released, a slipping speed of the vehicle increases and the rotation speed of a pump wheel of the hydraulic power transmission decreases by the negative rotation of a turbine wheel of the hydraulic power transmission. In this way, when the rotation speed of the electric motor decreases and the electric motor is in a locked state where the rotation thereof is hindered, a drive current is limited by a protection circuit provided to protect the temperature of the electric motor. Accordingly, the torque may not be satisfactorily output from the electric motor and the vehicle may slip further. 
     The present invention provides a control device that can suppress slipping of a hybrid vehicle, which includes a hydraulic power transmission between an engine and an electric motor and driving wheels, on a slope and that can prevent a lock of the electric motor. 
     According to a first aspect of the present invention, a hybrid vehicle includes an engine, an electric motor, a hydraulic power transmission that is disposed between the engine and driving wheels, the hydraulic power transmission being disposed between the electric motor and the driving wheels, and an electronic control unit. The electronic control unit is configured to control an output torque of the electric motor so that a rotation speed of an input rotation member of the hydraulic power transmission reaches a predetermined target rotation speed during stopping of the engine. The electronic control unit is configured to start the engine when the output torque of the electric motor is greater than a predetermined torque. 
     According to this aspect, when the output torque of the electric motor at a rotation speed under a slip suppression control is insufficient, the output torque of the engine can be used. Accordingly, it is possible to satisfactorily suppress a slip on a slope and to appropriately prevent the electric motor from being in a locked state. 
     In the aspect, the electronic control unit may be configured to control an output torque of the engine so that the rotation speed of the input rotation member of the hydraulic power transmission reaches the target rotation speed, after the engine is started. According to this aspect, when the output torque of the electric motor at the rotation speed under the slip suppression control using only the output torque of the electric motor is insufficient, the slip suppression control is performed using the output torque of the engine. Accordingly, it is possible to satisfactorily suppress slipping on a slope. 
     In the aspect, the electronic control unit may be configured to increase the rotation speed of the engine using the output torque of the electric motor and to increase the output torque of the engine, when the output torque of the engine exceeds a maximum output torque of the engine at a current rotation speed of the engine. According to this aspect, when the output torque of the engine at the rotation speed under the slip suppression control using the output torque of the engine is insufficient, the rotation speed of the engine increases and thus the output torque of the engine increases by adding the output torque of the electric motor to the output torque of the engine. Accordingly, even when the output torque of the engine is insufficient, it is possible to satisfactorily suppress slipping on a slope. 
     In the aspect, the electronic control unit may be configured to set the target rotation speed based on a predetermined rotation speed of the input rotation member of the hydraulic power transmission corresponding to a target slipping speed and a predetermined rotation speed difference between the input rotation member of the hydraulic power transmission and an output rotation member of the hydraulic power transmission. According to the aspect, the rotation speed of the input rotation member of the hydraulic power transmission is controlled to reach the target rotation speed. Accordingly, slipping on a slope is satisfactorily maintained within the target slipping speed. 
     In the aspect, the electronic control unit may be configured to determine the rotation speed difference based on a road surface gradient on which the hybrid vehicle runs. According to this aspect, slipping on a slope is maintained within the target slipping speed regardless of the road surface gradient. 
     In the aspect, the electronic control unit may be configured to determine the rotation speed difference based on a relationship stored in advance so that the larger the road surface gradient on which the hybrid vehicle runs becomes, the larger the rotation speed difference becomes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a diagram illustrating a schematic configuration of a power transmission path from an engine and an electric motor, which constitute a hybrid vehicle to which the present invention is appropriately applied, to driving wheels along with a control system provided to the vehicle for an output control of the engine serving as a running drive source, a transmission control of an automatic transmission, a drive control of the electric motor, and the like; 
         FIG. 2  is a functional block diagram illustrating principal parts of a slip suppression control function by an electronic control unit illustrated in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating a method of setting a target rotation speed of the electric motor under a slip suppression control; 
         FIG. 4  is a diagram illustrating a method of setting an engine-start threshold value for determining whether to start the engine under the slip suppression control; 
         FIG. 5  is a diagram illustrating a method of setting a target rotation speed when the electric motor outputs a torque under the slip suppression control; and 
         FIG. 6  is a flowchart illustrating principal parts of the slip suppression control by the electronic control unit illustrated in  FIG. 1 , that is, control operations of the slip suppression control of the vehicle. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the following embodiment, the drawings are appropriately simplified or deformed, and the dimensional ratios and the shapes of the constituents thereof are not accurately drawn. 
       FIG. 1  is a diagram illustrating a schematic configuration of a power transmission path from an engine  14  and an electric motor MG to driving wheels  34 , an engine  14  and an electric motor MG constituting a hybrid vehicle  10  (hereinafter, referred to as vehicle  10 ) to which the present invention is appropriately applied.  FIG. 1  is also a diagram illustrating principal parts of a control system provided to the vehicle  10  for an output control of the engine  14  serving as a running drive source, a transmission control of an automatic transmission  18 , a drive control of the electric motor MG, and the like. 
     In  FIG. 1 , a vehicle power transmission  12  (hereinafter, referred to as power transmission  12 ) includes an engine-coupling/decoupling clutch K 0 , an electric motor MG, a torque converter  16 , an oil pump  22 , and an automatic transmission  18  sequentially from an engine  14  side in a transmission case  20  (hereinafter, referred to as case  20 ). The transmission case  20  is a non-rotation member attached to a vehicle body by fastening with bolts or the like. The power transmission  12  includes a propeller shaft  26  connected to an output shaft  24  as an output rotation member of the automatic transmission  18 , a differential gear  28  connected to the propeller shaft  26 , and a pair of axles  30  connected to the differential gear  28 . The power transmission  12  having this configuration is appropriately used, for example, in a front engine-rear drive (FR) type vehicle  10 . In the power transmission  12 , dynamic power of the engine  14  is transmitted from an engine-coupled shaft  32  to a pair of driving wheels  34  sequentially via the engine-coupling/decoupling clutch K 0 , the torque converter  16 , the automatic transmission  18 , the propeller shaft  26 , the differential gear  28 , and the pair of axles  30  when the engine-coupling/decoupling clutch K 0  engages. The engine-coupled shaft  32  couples the engine  14  to the engine-coupling/decoupling clutch K 0 . 
     The torque converter  16  is a hydraulic power transmission that transmits a driving force input to a pump wheel  16   a  to the automatic transmission  18  via a fluid. The pump wheel  16   a  is connected to the engine  14  sequentially via the engine-coupling/decoupling clutch K 0  and the engine-coupled shaft  32 . The pump wheel  16   a  is an input rotation element to which the driving force is input from the engine  14  and that is rotatable about a shaft core. A turbine wheel  16   b  of the torque converter  16  is an output rotation element of the torque converter  16  and is connected to a transmission input shaft  36  as an input rotation member of the automatic transmission  18  so as not to be relatively rotatable by spline fitting or the like. The torque converter  16  includes a lockup clutch  38 . The lockup clutch  38  is a direct coupling clutch disposed between the pump wheel  16   a  and the turbine wheel  16   b.  The lockup clutch  38  is switched to an engaged state, a slip state, or a disengaged state by an oil pressure control or the like. 
     The electric motor MG is, for example, a synchronous electric motor. The electric motor MG is, for example, a so-called motor-generator set having a function of a motor for generating a mechanical driving force from electric energy and a function of a power generator for generating electric energy from mechanical energy. In other words, the electric motor MG can serve as a running drive source for generating a running driving force instead of the engine  14  as a drive source or along with the engine  14 . The electric motor MG generates electric energy from the driving force generated by the engine  14  or a driving force (mechanical energy) input from the driving wheels  34  side by regeneration. The electric motor MG performs an operation of accumulating the generated electric energy in a battery  46  as a power storage device via an inverter  40 , a step-up converter not illustrated, and the like. The electric motor MG is operably connected to the pump wheel  16   a  and dynamic power is transmitted between the electric motor MG and the pump wheel  16   a.  Accordingly, the electric motor MG is connected to the transmission input shaft  36  so as to enable power transmission, similarly to the engine  14 . The electric motor MG is connected to the battery  46  so as to give and receive electric power to and from the battery  46  via the inverter  40 , the step-up converter not illustrated, and the like. When the vehicle runs using the electric motor MG as the running drive source, the engine-coupling/decoupling clutch K 0  is disengaged. The dynamic power of the electric motor MG is transmitted to the pair of driving wheels  34  sequentially via the torque converter  16 , the automatic transmission  18 , the propeller shaft  26 , the differential gear  28 , the pair of axles  30 , and the like. 
     The oil pump  22  is a mechanical oil pump that is connected to the pump wheel  16   a  and that is rotationally driven by the engine  14  (or the electric motor MG) to generate a working oil pressure for controlling a shift of the automatic transmission  18 , controlling torque capacity of the lockup clutch  38 , controlling engagement and disengagement of the engine-coupling/decoupling clutch K 0 , or supplying a lubricant to the elements of the power transmission path of the vehicle  10 . The power transmission  12  also includes an electric oil pump  52  that is driven by an electric motor not illustrated. The electric oil pump  52  is supplementarily activated to generate an oil pressure, for example, when the oil pump  22  is not activated such as when the vehicle stops. 
     The engine-coupling/decoupling clutch K 0  is a wet multi-disc hydraulic frictional engagement device in which plural friction plates superimposed on each other are pressed by a hydraulic actuator. The engine-coupling/decoupling clutch K 0  is controlled in engagement and disengagement by an oil pressure control circuit  50  disposed in the power transmission  12  using an oil pressure generated by the oil pump  22  or the electric oil pump  52  as a source pressure. In the engagement and disengagement control, the engaging force of the engine-coupling/decoupling clutch K 0  is, for example, continuously changed with the pressure control of a linear solenoid valve or the like in the oil pressure control circuit  50 . In other words, the engaging force of the engine-coupling/decoupling clutch K 0  may be referred to as power-transmissible torque capacity of the engine-coupling/decoupling clutch K 0 . The engine-coupling/decoupling clutch K 0  includes a pair of clutch rotation members (a clutch hub and a clutch drum) that can relatively rotate in the disengaged state. One (the clutch hub) of the clutch rotation members is connected to the engine-coupled shaft  32  so as not to be relatively rotatable. The other (the clutch drum) of the clutch rotation members is connected to the pump wheel  16   a  of the torque converter  16  so as not to be relatively rotatable. By employing this configuration, the engine-coupling/decoupling clutch K 0  causes the pump wheel  16   a  to rotate together with the engine  14  via the engine-coupled shaft  32 . That is, in the engaged state of the engine-coupling/decoupling clutch K 0 , the driving force from the engine  14  is input to the pump wheel  16   a.  On the other hand, in the disengaged state of the engine-coupling/decoupling clutch K 0 , the dynamic power transmission between the pump wheel  16   a  and the engine  14  is intercepted. Since the electric motor MG is operably connected to the pump wheel  16   a  as described above, the engine-coupling/decoupling clutch K 0  serves as a clutch that is disposed in the power transmission path between the engine  14  and the electric motor MG and that couples and decouples them. In the engine-coupling/decoupling clutch K 0  of this embodiment, the torque capacity (engaging force) increases in proportion to the oil pressure. The engine-coupling/decoupling clutch K 0  of this embodiment is in the disengaged state when an oil pressure is not supplied thereto. The engine-coupling/decoupling clutch K 0  of this embodiment employs a so-called normally-open type clutch. 
     The automatic transmission  18  is connected to the electric motor MG so as to enable power transmission without passing through the engine-coupling/decoupling clutch K 0 . The automatic transmission  18  constitutes a part of the power transmission path from the engine  14  and the electric motor MG to the driving wheels  34 . The automatic transmission  18  transmits dynamic power from the running drive source (the engine  14  and the electric motor MG) to the driving wheels  34  side. For example, the automatic transmission  18  is a planetary gear type multi-stage transmission serving as a stepped automatic transmission in which shifting of a gear stage is performed by switching any one of plural engagement devices, for example, hydraulic frictional engagement devices such as a clutch C and a brake B and plural gear stages (transmission stages) are selectively set up. The switching of any one of the hydraulic frictional engagement devices such as the clutch C and the brake B may be engagement and disengagement of the hydraulic frictional engagement devices. The automatic transmission  18  is a stepped transmission that performs a so-called clutch-to-clutch transmission which is often used in known vehicles, and changes the rotation of the transmission input shaft  36  and outputs the changed rotation from the output shaft  24 . The transmission input shaft  36  is also a turbine shaft that is rotationally driven by the turbine wheel  16   b  of the torque converter  16 . In the automatic transmission  18 , a predetermined gear stage (shift stage) is set up depending on a driver&#39;s accelerator operation, the vehicle speed V, or the like by controlling the engagement and disengagement of the clutch C and the brake B. When both the clutch C and the brake B of the automatic transmission  18  are disengaged, a neutral state is achieved and thus the power transmission path between the driving wheels  34  and the engine  14  and the electric motor MG is intercepted. The automatic transmission  18  is an example of a transmission disposed in the power transmission path between the electric motor and the driving wheels in the present invention. 
     Referring back to  FIG. 1 , the vehicle  10  is provided with an electronic control unit  100  including, for example, a control device related to a hybrid drive control and the like. The electronic control unit  100  is constituted, for example, by a so-called microcomputer including a CPU, a RAM, a ROM, and an input and output interface. The CPU performs various controls on the vehicle  10  by processing signals in accordance with a program stored in advance in the ROM using a temporary memory function of the RAM. For example, the electronic control unit  100  is configured to perform an output control of the engine  14 , a drive control of the electric motor MG including a regeneration control of the electric motor MG, a transmission control of the automatic transmission  18 , a torque capacity control of the lockup clutch  38 , a torque capacity control of the engine-coupling/decoupling clutch K 0 , and the like. The electronic control unit  100  is divided into an engine control section, an electric motor control section, and an oil pressure control (transmission control) section if necessary. 
     The electronic control unit  100  is supplied, for example, with a signal indicating an engine rotation speed Ne which is the rotation speed of the engine  14  detected by an engine rotation speed sensor  56 . The electronic control unit  100  is supplied, for example, with a signal indicating the turbine rotation speed Nt of the torque converter  16  as the input rotation speed of the automatic transmission  18  detected by a turbine rotation speed sensor  58 , that is, a transmission input rotation speed Nin which is the rotation speed of the transmission input shaft  36 . The electronic control unit  100  is supplied, for example, with a signal indicating a transmission output rotation speed Nout which is the rotation speed of the output shaft  24  corresponding to the vehicle speed V or the rotation speed of the propeller shaft  26  as the vehicle speed-relevant value detected by an output rotation speed sensor  60 . The electronic control unit  100  is supplied, for example, with a signal indicating a motor rotation speed Nmg which is the rotation speed of the electric motor MG detected by a motor rotation speed sensor  62 . The electronic control unit  100  is supplied, for example, with a signal indicating a throttle valve opening θth which is a degree of opening of an electronic throttle valve (not illustrated) detected by a throttle sensor  64 . The electronic control unit  100  is supplied, for example, with a signal indicating an amount of intake air Qair of the engine  14  detected by an intake air sensor  66 . The electronic control unit  100  is supplied, for example, with a signal indicating a longitudinal acceleration G (or a longitudinal deceleration G) of the vehicle  10  detected by an acceleration sensor  68 . The electronic control unit  100  is supplied, for example, with a signal indicating a coolant temperature THw of the engine  14  detected by a coolant temperature sensor  70 . The electronic control unit  100  is supplied, for example, with a signal indicating a working oil temperature THoil of working oil in the oil pressure control circuit  50  detected by an oil temperature sensor  72 . The electronic control unit  100  is supplied, for example, with a signal indicating an accelerator opening Acc which is a degree of operation of the accelerator pedal  76  as a driving force request quantity (driver-requested output) of the driver for the vehicle  10 , which is detected by an accelerator opening sensor  74 . The electronic control unit  100  is supplied, for example, with a signal indicating a brake pressure Brk which is a degree of operation of the brake pedal  80  as a braking force request quantity (driver-requested deceleration) of the driver for the vehicle  10 , which is detected by a foot brake sensor  78 . The electronic control unit  100  is supplied, for example, with a signal indicating a lever position (a shift operation position, a shift position, or an operation position) Psh of the shift lever  84  such as known “P”, “N”, “D”, “R”, and “S” positions detected by a shift position sensor  82 . The electronic control unit  100  is supplied, for example, with a state of charge (charging capacity or remaining charging capacity) SOC of the battery  46  detected by a battery sensor  86  and the like. The electronic control unit  100  is supplied with electric power from an auxiliary battery  88  that is charged with power dropped by a DCDC converter not illustrated. 
     For example, an engine output control command signal Se for controlling the output of the engine  14  is output from the electronic control unit  100 . For example, an electric motor control command signal Sm for controlling the operation of the electric motor MG is output from the electronic control unit  100 . For example, an oil pressure command signal Sp or the like for activating an electromagnetic valve (solenoid valve) or the electric oil pump  52  included in the oil pressure control circuit  50  is output from the electronic control unit  100  so as to control the engine-coupling/decoupling clutch K 0  or the oil pressure actuators of the clutch C and the brake B of the automatic transmission  18 . 
       FIG. 2  is a functional block diagram illustrating principal parts of the control function of the electronic control unit  100 . In  FIG. 2 , a stepped transmission control unit  102  (stepped transmission control means) serves as a gear shift control unit that performs the gear shift of the automatic transmission  18 . The stepped transmission control unit  102  determines whether to perform the gear shift of the automatic transmission  18  based on the vehicle state indicted by the actual vehicle speed V and the accelerator opening Acc from the known relationship (gear shift diagram, gear shift map) having an up-shift line and a down-shift line stored in advance, for example, using the vehicle speed V and the accelerator opening Acc (or the transmission output torque Tout or the like) as parameters. That is, the stepped transmission, control unit  102  determines whether to shift the gear stage of the automatic transmission  18 , and performs an automatic gear shift control of the automatic transmission  18  so as to set up the determined gear stage. The stepped transmission control unit  102  outputs a command Sp (a transmission output command, an oil pressure command) for causing the engagement device involved in the gear shift of the automatic transmission  18  to engage and/or to be disengaged to the oil pressure control circuit  50 , for example, so as to achieve a gear stage based on a predetermined engagement operation table stored in advance. 
     A hybrid control unit  104  (hybrid control means) has a function of an engine drive control unit that controls driving of the engine  14  and a function of an electric motor operation control unit that controls the operation of the electric motor MG as a drive source or a power generator through the use of the inverter  40  controlling the electric motor MG, and performs a hybrid drive control using the engine  14  and the electric motor MG and the like by the control functions. For example, the hybrid control unit  104  calculates a vehicle request torque from the accelerator opening Ace or the vehicle speed V, and controls the running drive source so as to achieve the output torque of the running drive source (the engine  14  and the electric motor MG) with which the vehicle request torque is obtained in consideration of the transmission loss, the auxiliary device load, the gear stage of the automatic transmission  18 , the state of charge SOC of the battery  46 , and the like. 
     More specifically, for example, when the vehicle request torque is in a range which can be reached by only a motor torque Tmg (electric motor torque) of the electric motor MG, the hybrid control unit  104  sets a running mode to a motor-driven running mode (hereinafter, referred to as EV running mode) and performs motor-driven running (EV running) using the electric motor MG as a running drive source. When the EV running is performed, the hybrid control unit  104  disengages the engine-coupling/decoupling clutch K 0  to intercept the power transmission path between the engine  14  and the torque converter  16  and outputs the motor torque Tmg necessary for the motor-driven running to the electric motor MG. At this time, the hybrid control unit  104  determines a gear stage in which the motor efficiency of the electric motor MG is the highest out of combinations of the operation states (the motor torque Tmg, the motor rotation speed Nmg) of the electric motor MG and the gear stages of the automatic transmission  18  in which the vehicle request driving force is obtained in the EV running, and outputs a command for shifting to the determined gear stage to the stepped transmission control unit  102 . 
     The hybrid control unit  104  switches the running mode from the EV running mode to the engine-driven running mode, starts the engine  14 , and performs the engine-driven running, for example, when the accelerator pedal  76  is pressed deeper to increase the vehicle request torque during the EV running and the motor torque Tmg necessary for the EV running corresponding to the vehicle request torque exceeds a predetermined EV running torque range in which the vehicle can perform the EV running, that is, when the vehicle request torque cannot be achieved without using at least the output torque (engine torque) Te of the engine  14 . The hybrid control unit  104  transmits the engine-start torque Tmgs for starting the engine from the electric motor MG via the engine-coupling/decoupling clutch K 0  to raise the rotation speed of the engine  14  while causing the engine-coupling/decoupling clutch K 0  to engage toward the complete engagement at the time of starting the engine  14 , and raises the engine rotation speed Ne to the rotation speed enabling a self-sustaining operation to control the ignition of the engine, the supply of a fuel, or the like, thereby starting the engine  14 . Then, the hybrid control unit  104  causes the engine-coupling/decoupling clutch K 0  to rapidly completely engage after the engine  14  is started. When the engine-driven running is performed, the hybrid control unit  104  causes the engine-coupling/decoupling clutch K 0  to engage to transmit the driving force from the engine  14  to the pump wheel  16   a,  and outputs an assist torque to the electric motor MG if necessary. When the oil pump  22  is not activated such as when the vehicle stops, the hybrid control unit  104  supplementarily activates the electric oil pump  52  to prevent insufficiency of the working oil. 
     At the time of coast traveling (inertial traveling) with the accelerator turned off, braking by pressing the brake pedal  80 , or the like, the hybrid control unit  104  has a function of the regeneration control means for rotationally driving the electric motor MG with the kinetic energy of the vehicle  10  to cause the electric motor MG to serve as a power generator, for the purpose of improvement of fuel efficiency, and charging the battery  46  with the electric energy via the inverter  40 . The kinetic energy of the vehicle  10  is a reverse driving force transmitted from the driving wheels  34  to the engine  14  side. The regeneration control is performed so as to achieve an amount of power regenerated determined based on the state of charge SOC of the battery  46  or the braking force distribution of the braking force by an oil pressure brake for obtaining a braking force corresponding to the pressure applied to the brake pedal. In this embodiment, the hybrid control unit  104  causes the lockup clutch  38  to engage during regeneration-cost running. 
     When it is determined that the vehicle slips at the time of stopping of the vehicle, a slip suppression control unit  106  calculates a target rotation speed NP* of the pump wheel  16   a,  that is, the target rotation speed N MG * (=N T +ΔN) of the electric motor MG based on an actual rotation speed N T  (for example, a negative value of about −200 rpm) of the turbine wheel  16   b,  which is the output rotation element of the torque converter  16  and which corresponds to the target slipping speed VZ preset to about −2 km/h, and a rotation speed difference ΔN calculated in advance so that the larger the road surface gradient on which the vehicle runs becomes, the larger the rotation speed difference ΔN becomes so as to maintain the target slipping speed VZ. The slip suppression control unit  106  raises the actual rotation speed N P  of the pump wheel  16   a,  that is, the actual rotation speed N MG  of the electric motor MG, so as to be the target rotation speed N P *, that is, the target rotation speed N MG *. The slip suppression control unit  106  increases the torque transmitted to the driving wheels  34  via the torque converter  16  to increase the driving force of the driving wheels  34 , thereby suppressing slipping of the vehicle. In brief, the target rotation speed N MG * of the pump wheel  16   a  of the torque converter  16  is determined on the basis of the rotation speeds of the pump wheel  16   a  and the turbine wheel  16   b  of the torque converter  16  during the stopping of the engine. 
     The slip suppression control unit  106  includes a first slip suppression control unit  108  that increases the driving force of the driving wheels  34  using a feedback rotation speed control based on the output torque T MG  of the electric motor MG, a second slip suppression control unit  110  that increases the driving force of the driving wheels  34  using a feedback control based on the output torque T E  of the engine  14 , and a third slip suppression control unit  112  that increases the driving force of the driving wheels  34  using a feedback rotation speed control based on the output torque T MG  of the electric motor MG and the output torque T E  of the engine  14 . 
     The first slip suppression control unit  108  performs the feedback rotation speed control by adjusting the output torque T MG  of the electric motor MG so that the actual rotation speed NP (=actual rotation speed N MG  of the electric motor MG) of the pump wheel  16   a  as the input rotation member of the torque converter  16  reaches a predetermined constant target rotation speed N P * (=target rotation speed N MG * of the electric motor MG) during the stopping of the engine  14 . As illustrated in  FIG. 3 , when it is assumed that the rotation speed of the axle  30  (the driving wheels  34 ) at a slipping speed of the vehicle of, for example, −2 km/h is −200 rpm, and the rotation speed difference ΔN of the torque converter  16  generating a transmission torque to keep the rotation speed of the axle  30  constant at −200 rpm is 1000 rpm, the target rotation speed N P * is set to 800 rpm. Since the rotation speed difference of the torque converter  16  generating a transmission torque to keep the rotation speed of the axle  30  constant at −200 rpm varies to a certain extent depending on the road surface gradient, the rotation speed difference ΔN of the torque converter  16  may be calculated on the basis of the actual gradient detected by an acceleration sensor or the like from a relationship stored in advance to be equal to a constant slipping speed of, for example, −2 km/h. 
     When the output torque T MG  of the electric motor MG at a current rotation speed of, for example, 800 rpm under the feedback control is greater than a predetermined engine-start threshold value T MGE  illustrated, for example, in  FIG. 4  in the feedback rotation speed control based on the output torque T MG  of the electric motor MG by the first slip suppression control unit  108 , the second slip suppression control unit  110  issues an engine start request to start the engine  14 . After starting the engine  14 , the second slip suppression control unit  110  starts the feedback control based on the output torque T E  of the engine  14  instead of the feedback rotation speed control using the electric motor MG. The second slip suppression control unit  110  performs the feedback rotation speed control by adjusting the output torque T E  of the engine  14  so that the actual rotation speed N P  (=actual rotation speed N MG  of the electric motor MG) of the pump wheel  16   a  as the input rotation member of the torque converter  16  reaches the target rotation speed N P * calculated in advance to be a constant target slipping speed VZ of, for example, −2 km/h. 
     The engine-start threshold value T MGE  is set to a value lower by an engine-start torque margin value β than the maximum torque value T MGmax  of the electric motor MG at the current rotation speed of, for example, 800 rpm under the feedback control, for example, in the maximum torque characteristic diagram of the electric motor MG illustrating in  FIG. 4 . In  FIG. 4 , when the actual torque of the electric motor MG is defined as X and X&gt;(TMGmax−β) is satisfied, the feedback control based on the output torque T E  of the engine  14  is started. That is, when the electric motor MG at the rotation speed under the feedback control requires an output torque equal to or greater than the engine-start threshold value T MGE  in the feedback control using the electric motor MG, the engine  14  is started. 
     In the feedback rotation speed control based on the output torque T E  of the engine  14 , when the output torque T E  of the engine  14  is greater than a predetermined threshold value T ES  in the vicinity of the maximum torque of the engine  14  at the current rotation speed of, for example, 800 rpm under the feedback control, the third slip suppression control unit  112  causes the electric motor MG to output the torque T MG α while maintaining the torque command value for the engine  14 . The third slip suppression control unit  112  performs the feedback control so that the actual rotation speed N P  (=actual rotation speed N MG  of the electric motor MG) of the pump wheel  16   a  as the input rotation member of the torque converter  16  reaches a target rotation speed N MGS * calculated in advance to reach the constant target slipping speed VZ of, for example, −2 km/h. In order to match the actual rotation speed N P  (=actual rotation speed N MG  of the electric motor MG) of the pump wheel  16   a  with the target rotation speed N P * calculated in advance to reach the constant target slipping speed of, for example, −2 km/h, when the maximum output torque of the engine  14  rotating at the current rotation speed of, for example, 800 rpm is insufficient by an insufficient torque α (when the output torque of the engine that performs the feedback control exceeds the maximum output torque of the engine at a current rotation speed of the engine), the slip suppression control unit  106  calculates the engine rotation speed N E α increasing by the insufficient torque α on the basis of the value, which is obtained by adding the insufficient torque α of the output torque of the engine  14  to the actual output torque T E  of the engine  14 , from the previously-stored engine characteristics illustrated in  FIG. 5 . the slip suppression control unit  106  sets the calculated value as the target rotation speed N MG * of the electric motor MG. Accordingly, the torque T MG α from the electric motor MG is added to the engine output torque T E  output from the engine  14  and the feedback control is continuously performed. 
       FIG. 6  is a flowchart illustrating principal parts of the control operation of the electronic control unit  100 , that is, the control operation of the slip suppression control of suppressing slipping of the vehicle on a slope. The control operation of the slip suppression control of suppressing slipping of the vehicle on a slope is repeatedly performed, for example, with a very short cycle of several msec to several tens of msec. 
     In step S 1  (hereinafter, step will be omitted) of  FIG. 6 , when the engine  14  is stopped, it is determined whether the vehicle slips on the basis of whether the pressure applied to the accelerator pedal is zero (accelerator off), the pressure applied to the brake pedal is zero (brake off), and the vehicle speed V in the D range is negative or the vehicle speed V in the R ranges is positive. When the determination result of S 1  is negative, the engine start request based on the slip suppression control is stopped and the operation request of the electric motor MG for increasing the torque output rotation speed of the engine  14  by the use of the electric motor MG is stopped in S 2 . 
     When it is determined that the vehicle slips at the time of the stopping of the vehicle, the determination result of S 1  is positive. The target rotation speed N P * of the pump wheel  16   a,  that is, the target rotation speed N MG * (=N T +ΔN) of the electric motor MG, is calculated in S 3  on the basis of the actual rotation speed N T  (for example, a negative value of about −200 rpm) of the turbine wheel  16   b  as the output rotation member of the torque converter  16  corresponding to the target slipping speed VZ of, for example, about −2 km/h and the rotation speed difference ΔN between the rotation speed N P  of the pump wheel  16   a  of the torque converter  16  calculated in advance for maintaining the target slipping speed VZ and the rotation speed N T  of the turbine wheel  16   b.    
     Subsequently, in S 4 , whether the engine  14  is in operation is determined on the basis of whether the engine rotation speed N E  is zero. When the determination result of S 4  is negative, the first slip suppression control described with reference to the first slip suppression control unit  108 , that is, the feedback rotation speed control of adjusting the output torque T MG  of the electric motor MG so that the actual rotation speed NP (=actual rotation speed NMG of the electric motor MG) of the pump wheel  16   a  as the input rotation member of the torque converter  16  reaches the constant target rotation speed NP* (=target rotation speed NMG* of the electric motor MG), is performed in S 5  corresponding to the first slip suppression control unit  108 . 
     In S 6 , it is determined whether the output torque TMG of the electric motor MG at the current rotation speed of, for example, 800 rpm under the first slip suppression control is greater than the predetermined engine-start threshold value T MGE  illustrated, for example, in  FIG. 4 . When the determination result of S 6  is negative, the routine up to now is repeated and the first slip suppression control is continuously performed. 
     On the other hand, when the determination result of S 6  is positive, a start request command of the engine  14  is issued to perform the second slip suppression control in S 7  and the engine  14  is started. Accordingly, the determination result of S 4  in a next control cycle is positive. 
     In S 8  which is performed subsequently to the positive determination of S 4  and which corresponds to the second slip suppression control unit  110 , the second slip suppression control is performed instead of the feedback rotation speed control using the electric motor MG as the first slip suppression control. The feedback control using the output torque T E  of the engine  14  is started, and the feedback rotation speed control is performed by adjusting the output torque T E  of the engine  14  so that the actual rotation speed N P  (=actual rotation speed N MG  of the electric motor MG) of the pump wheel  16   a  as the input rotation member of the torque converter  16  reaches the target rotation speed N P * calculated in advance to be the constant target slipping speed VZ of, for example, −2 km/h. 
     Subsequently, in S 9 , it is determined whether the engine torque command value indicating the output torque of the engine  14  under the second slip suppression control is greater than the maximum torque of the engine  14  at the current rotation speed. That is, in the second slip suppression control, whether the maximum output torque of the engine  14  rotating at the current rotation speed of, for example, 800 rpm is insufficient by an insufficient torque a so as to match the actual rotation speed N P  (=actual rotation speed N MG  of the electric motor MG) of the pump wheel  16   a  with the target rotation speed N P * calculated in advance to be the constant slipping speed of, for example, −2 km/h. 
     When the determination result of S 9  is negative, the second slip suppression control is continuously performed instead of the routine performed up to now. On the other hand, when the determination result of S 9  is positive, a request command for increasing the torque output rotation speed of the engine  14  using the electric motor MG is issued to perform the third slip suppression control in S 10 . The torque command value for the engine  14  is maintained so as to prevent transient shortage output torque of the engine  14  at the time of starting the increase in the engine output rotation speed using the electric motor MG in S 11 . 
     Subsequently, S 12  and S 13  corresponding to the third slip suppression control unit  112  are performed. In S 12 , the engine rotation speed N E α increasing by the insufficient torque α is calculated on the basis of the value obtained by adding the insufficient torque α of the output torque of the engine  14  to the actual output torque T E  of the engine  14 , for example, from the previously-stored engine characteristics illustrated in  FIG. 5 . The calculated value is set as the target rotation speed N MGS * of the electric motor MG. Subsequently, in S 13 , when the output torque T E  of the engine  14  is greater than a predetermined threshold value T ES  in the vicinity of the maximum torque of the engine  14  at the current rotation speed of, for example, 800 rpm under the feedback control in the feedback rotation speed control (second slip suppression control) using the output torque T E  of the engine  14 , the torque T MG α is output from the electric motor MG while maintaining the torque command value for the engine  14  up to now. The feedback control, that is, the third slip suppression control, is performed so that the actual rotation speed N P  (=actual rotation speed N MG  of the electric motor MG) of the pump wheel  16   a  as the input rotation member of the torque converter  16  reaches the target rotation speed N MGS * calculated in advance to correspond to the constant target slipping speed VZ of, for example, −2 km/h. 
     As described above, according to this embodiment, when the vehicle stops in a slope, the output torque of the electric motor MG is controlled so that the rotation speed N P  of the pump wheel (input rotation member)  16   a  of the torque converter  16  (hydraulic power transmission), that is, the rotation speed N MG  of the electric motor MG, reaches the target rotation speed N MG * in the first slip suppression control by the first slip suppression control unit  108 . Then, When the torque necessary for matching the rotation speed N P  of the pump wheel  16   a  of the torque converter  16  with the target rotation speed N MG * is greater than a predetermined torque, the engine  14  is started. Accordingly, when the output torque of the electric motor MG at the rotation speed under the slip suppression control is insufficient, the output torque of the engine  14  can be used and thus the electric motor MG is appropriately prevented from being in the locked state. 
     According to this embodiment, after the engine  14  is started, the second slip suppression control by the second slip suppression control unit  110  is started and the output torque of the engine  14  is controlled so that the rotation speed N P  of the pump wheel  16   a  of the torque converter  16  reaches the target rotation speed N MG *. Accordingly, the output torque of the engine  14  is controlled so that the rotation speed N P  of the pump wheel  16   a  of the torque converter  16  reaches the target rotation speed N MG *. As a result, when the output torque of the electric motor MG at the rotation speed under the slip suppression control using only the output torque of the electric motor MG is insufficient, the slip suppression control is performed using the output torque of the engine  14  and thus the slip in the slope is continuously suppressed. 
     According to this embodiment, when the output torque of the engine  14  at the rotation sped under the second slip suppression control using only the output torque of the engine  14  after the engine  14  is started is insufficient, the third slip suppression control by the third slip suppression control unit  112  is started. When the third slip suppression control is started, the rotation speed of the engine  14  increases and the thus the output torque of the engine increases, by adding the output torque of the electric motor MG to the output torque of the engine  14 . Accordingly, even when the output torque of the engine is insufficient the slip in the slope is suppressed. The electric motor MG adds the torque so that the rotation speed of the engine  14  reaches the rotation speed at which the torque from the engine  14  can be satisfactorily output. Accordingly, the engine  14  rotates at the rotation speed at which the torque from the engine  14  can be satisfactorily output. As a result, a sufficient output torque is output from the engine  14  and thus the slipping of the vehicle is suppressed. 
     According to this embodiment, the target rotation speed N MG * is set on the basis of the rotation speed N P  of the pump wheel  16   a  of the torque converter  16  determined in advance to correspond to the target slipping speed VZ and the predetermined rotation speed difference ΔN of the torque converter  16 . In this way, the target rotation speed NMG* is set on the basis of the rotation speed difference ΔN of the torque converter  16  determined in advance to maintain the target slipping speed VZ. Accordingly, by performing the control so that the rotation speed N P  of the pump wheel  16   a  of the torque converter  16  reaches the target rotation speed N MG *, the slipping on the slope is maintained at the target slipping speed VZ. 
     According to this embodiment, in the slip suppression control unit  106 , the rotation speed difference ΔN used to set the target rotation speed N MG * is determined on the basis of the actual road surface gradient on which the vehicle runs from the relationship stored in advance so that the larger the road surface gradient on which the vehicle runs becomes, the larger the target rotation speed N MG * becomes. Accordingly, the slipping on the slope is maintained at the target slipping speed VZ regardless of the road surface gradient. 
     The embodiment of the present invention has been described in detail with reference to the accompanying drawings. The present invention may be embodied in other aspects. 
     For example, in the above-mentioned embodiment, the target rotation speed N MG * is set by adding the predetermined rotation speed difference ΔN, for example, +1000 rpm, of the torque converter  16  to the rotation speed N P  of, for example, −200 rpm, of the pump wheel  16   a  of the torque converter  16  determined in advance to correspond to the target slipping speed VZ. The target slipping speed VZ and the rotation speed difference ΔN may employ fixed values depending on vehicles. The target rotation speed N MG * may be a fixed value stored in advance. 
     The hybrid vehicle according to the above-mentioned embodiment is equipped with the torque converter  16  as the hydraulic power transmission. A fluid coupling serving as the hydraulic power transmission may be provided instead of, the torque converter  16 . 
     The slip suppression control of the above-mentioned embodiment is applied to an uphill road. The slip suppression control of the above-mentioned embodiment may be applied to a downhill road. 
     In the flowchart of the above-mentioned embodiment, the order of steps may be appropriately changed without causing any contradiction. For example, in the flowchart illustrated in  FIG. 6 , steps S 3  and S 4  may be performed reversely. 
     The automatic transmission  18  of the above-mentioned embodiment is a stepped automatic transmission. The specific structure or the number of transmission stages of the transmission is not particularly limited. 
     In the above-mentioned embodiment, the engine-coupling/decoupling clutch K 0  is disposed between the engine  14  and the electric motor MG. However, the engine-coupling/decoupling clutch K 0  may not be provided necessarily. 
     The above-mentioned embodiment is only an example, and the present invention can be modified and improved in various aspects on the basis of knowledge of those skilled in the art.