Patent Publication Number: US-2013253749-A1

Title: Hybrid vehicle

Description:
This is a 371 national phase application of PCT/JP2010/073524 filed 27 Dec. 2010, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates a hybrid vehicle, and more particularly to running control of the hybrid vehicle. 
     BACKGROUND OF THE INVENTION 
     A hybrid vehicle provided with an engine and a traction motor each as a driving force source is characterized largely by having excellent fuel efficiency. As one embodiment of a hybrid vehicle, Japanese Patent Laying-Open No. 2009-196415 (PTD 1) discloses a driveline configuration in which an internal combustion engine and a motor generator (MG 1 , MG 2 ) are coupled via a power split device. 
     In such a hybrid vehicle, driving force for the entire vehicle is generated by the sum of a direct torque mechanically transmitted directly from the engine to a drive shaft via the power split device and an output torque of the motor generator (MG 2 ). 
     PTD 1 discloses that, when the temperature of the motor generator (MG 2 ) exceeds a prescribed reference temperature, the torque command value of the motor generator is decreased while the decreased amount of the torque command value is compensated by a direct torque, thereby avoiding a shortage of the driving force for the entire vehicle. 
     Furthermore, Japanese Patent Laying-Open No. 2007-203772 (PTD 2) discloses running control in a hybrid vehicle having a driveline similar to that in PTD 1 for allowing a gradual decrease of the output shaft torque in the torque phase of an automatic transmission. It specifically discloses that, when a decrease in the requested output shaft torque is temporarily corrected prior to the torque phase, at least one of the engine and the motor generator is controlled so as to prevent an increase in the direct torque to the output shaft. 
     CITATION LIST 
     Patent Document 
     
         
         PTD 1: Japanese Patent Laying-Open No. 2009-196415 
         PTD 2: Japanese Patent Laying-Open No. 2007-203772 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In such a hybrid vehicle as disclosed in PTD 1, fuel efficiency is improved by operating the engine at a highly-efficient operating point (torque/rotation speed). In other words, in the state where an operation line obtained as a collection of highly-efficient operating points is set in advance, the engine is controlled such that an operating point is set on this operation line in accordance with the output power from the engine. Then, when the requested driving force for the entire vehicle is excessive or insufficient by the engine output at the operating point, running control is performed so as to allow this excessive or insufficient amount of driving force to be covered by the output torque of the motor generator. 
     Therefore, in the hybrid vehicle, the requested driving force for the entire vehicle may be able to be ensured only by the output from the engine. In this case, the output torque of the motor generator can be set at zero. However, when an inverter performs electric motor control for setting the output torque at zero, switching loss occurs in the inverter. In other words, also in such a situation where the output of the motor generator (traction motor) is not required, loss resulting from electric motor control unnecessarily occurs. This loss causes a decrease in energy efficiency for the entire vehicle, thereby leading to deterioration in fuel efficiency. 
     The present invention has been made in order to solve the above-described problems. An object of the present invention is to reduce the loss resulting from driving control of a traction motor, thereby improving the fuel efficiency of a hybrid vehicle. 
     Solution to Problem 
     According to an aspect of the present invention, a hybrid vehicle includes an internal combustion engine, an electric motor, a first power converter, a power transmission device, and a control device. The electric motor is configured to output a torque to a drive shaft mechanically coupled to a driving wheel. The power transmission device is configured to mechanically transmit a torque originating from an output of the internal combustion engine to the drive shaft. The first power converter is disposed for controlling an output torque of the electric motor. The control device is configured to control the output of each of the internal combustion engine and the electric motor such that requested driving force for an entire vehicle is exerted on the drive shaft. The control device includes a running control unit and an electric motor control unit. The running control unit is configured to selectively apply a first running mode (S/D mode) in which the requested driving force is exerted on the drive shaft by the output of the internal combustion engine in a state where the output torque of the electric motor is set at zero, and a second running mode (normal running mode) in which the requested driving force is exerted on the drive shaft by the output of each of the internal combustion engine and the electric motor. The electric motor control unit is configured to stop an operation of the first power converter in the first running mode. 
     Preferably, during the second running mode, the running control unit calculates a first target rotation speed (NE 1 ) of the internal combustion engine for ensuring the requested driving force in a case where the output torque of the electric motor is set at zero, controls the output of each of the internal combustion engine and the electric motor so as to perform operating-point change control for bringing a rotation speed of the internal combustion engine close to the first target rotation speed, and performs switching from the second running mode to the first running mode when an absolute value of the output torque of the electric motor becomes smaller than a prescribed threshold value. 
     Further preferably, during the second running mode, the running control unit performs the operating-point change control when a difference between a second target rotation speed (NE 2 ) of the internal combustion engine for ensuring the requested driving force in accordance with the second running mode and the first target rotation speed (NE 1 ) is smaller than a prescribed threshold value. 
     Further preferably, during the second running mode, the running control unit performs the operating-point change control when an estimate value (F 1 ) of a fuel consumption in a case where the internal combustion engine operates in accordance with the first running mode in a state where the operation of the first power converter is stopped is smaller than an estimate value (F 2 ) of a fuel consumption in a case where the internal combustion engine operates in accordance with the second running mode. 
     Alternatively preferably, during the first running mode, the running control unit estimates a magnitude of a drag torque (Tm) acting as rotational resistance when the electric motor rotates at zero torque, and controls the output of the internal combustion engine such that a sum of the requested driving force and the estimated drag torque is exerted on the drive shaft. 
     Further preferably, the running control unit estimates the drag torque based on a rotation speed of the electric motor. Alternatively, the running control unit estimates the drag torque based on counter-electromotive force generated in the electric motor. 
     Preferably, the hybrid vehicle further includes a power generator for generating electric power by motive power from the internal combustion engine. The power transmission device includes a three-shaft type power split device. The power split device is mechanically coupled to three shafts including an output shaft of the internal combustion engine, an output shaft of the power generator and the drive shaft; and configured such that when rotation speeds of any two shafts of these three shafts are determined, a rotation speed of remaining one shaft is determined, and configured to, based on the motive power input to and output from any two shafts of these three shafts, input and output the motive power to and from remaining one shaft. 
     Further preferably, the first power converter performs bidirectional power conversion between a power line and the electric motor. The hybrid vehicle further includes a second power converter for performing bidirectional power conversion between the power line and the power generator; and a power storage device electrically connected to the power line. During the second running mode, the running control unit inhibits switching from the second running mode to the first running mode when an SOC of the power storage device is higher than a prescribed first threshold value in a case where the rotation speed of the electric motor falls within a region in which electric power is generated during rotation at zero torque. Further preferably, during the first running mode, the running control unit forcibly performs switching from the first running mode to the second running mode when the SOC of the power storage device is increased above the first threshold value. 
     Alternatively further preferably, during the second running mode, the running control unit controls the output of each of the internal combustion engine, the electric motor and the power generator so as to generate the requested driving force while causing discharge of the power storage device, when the SOC of the power storage device is lower than the first threshold value and higher than a prescribed second threshold value that is lower than the first threshold value. 
     According to another aspect of the present invention, a method of controlling a hybrid vehicle is provided. The hybrid vehicle is equipped with an internal combustion engine, an electric motor configured to output a torque to a drive shaft mechanically coupled to a driving wheel, and a power transmission device for mechanically transmitting a torque originating from an output of the internal combustion engine to the drive shaft. The controlling method includes the steps of: calculating requested driving force for an entire vehicle based on a vehicle state; selecting a first running mode (S/D mode) in which the requested driving force is exerted on the drive shaft by the output of each of the internal combustion engine and the electric motor in a state where an output torque of the electric motor is set at zero and a second running mode (normal running mode) in which the requested driving force is exerted on the drive shaft by the output of each of the internal combustion engine and the electric motor; and stopping an operation of a power converter for controlling the output torque of the electric motor in the first running mode. 
     Preferably, the controlling method further includes the steps of: calculating a first target rotation speed of the internal combustion engine for ensuring the requested driving force when the output torque of the electric motor is set at zero during the second running mode; and performing operating-point change control for bringing a rotation speed of the internal combustion engine close to the first target rotation speed during the second running mode. Furthermore, the step of selecting selects the first running mode when an absolute value of the output torque of the second electric motor becomes smaller than a prescribed threshold value during the second running mode. 
     Further preferably, the controlling method further includes the steps of: calculating a second target rotation speed of the internal combustion engine for ensuring the requested driving force in accordance with the second running mode; and performing the operating-point change control when a difference between the first target rotation speed and the second target rotation speed is smaller than a prescribed threshold value. 
     Preferably, the controlling method further includes the steps of: estimating a magnitude of a drag torque acting as rotational resistance when the second electric motor rotates at zero torque during the first running mode; and incorporating the drag torque into the requested driving force during the first running mode. 
     Advantageous Effects of Invention 
     According to the present invention, the fuel efficiency of the hybrid vehicle can be improved by reducing the loss resulting from driving control of a traction motor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram for illustrating a configuration example of a hybrid vehicle according to the first embodiment of the present invention. 
         FIG. 2  is a circuit diagram illustrating a configuration example of an electrical system of the hybrid vehicle shown in  FIG. 1 . 
         FIG. 3  is a collinear diagram showing the relation of the rotation speeds among an engine, the first MG and the second MG in the hybrid vehicle shown in  FIG. 1 . 
         FIG. 4  is a collinear diagram during EV (Electric Vehicle) running of the hybrid vehicle shown in  FIG. 1 . 
         FIG. 5  is a collinear diagram at the start of the engine of the hybrid vehicle shown in  FIG. 1 . 
         FIG. 6  is the first flowchart illustrating running control of the hybrid vehicle according to the first embodiment. 
         FIG. 7  is the second flowchart illustrating running control of the hybrid vehicle according to the first embodiment. 
         FIG. 8  is a conceptual diagram for illustrating determination of an engine operating point. 
         FIG. 9  is a collinear diagram under the running control of the hybrid vehicle according to the first embodiment. 
         FIG. 10  shows an operation waveform at the time of switching from a normal running mode to an S/D mode under the running control of the hybrid vehicle according to the present first embodiment. 
         FIG. 11  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the present second embodiment. 
         FIG. 12  is a schematic diagram illustrating a map for calculating a mechanical drag torque. 
         FIG. 13  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the first modification of the present second embodiment. 
         FIG. 14  is a schematic diagram illustrating a map for calculating an electromagnetic drag torque. 
         FIG. 15  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the second modification of the present second embodiment. 
         FIG. 16  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the present third embodiment. 
         FIG. 17  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the present fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The embodiments of the present invention will be hereinafter described in detail with reference to the accompanying drawings, in which the same or corresponding components are designated by the same reference characters, and description thereof will not be basically repeated. 
     First Embodiment 
     Vehicle Configuration 
       FIG. 1  is a block diagram for illustrating a configuration example of a hybrid vehicle according to the first embodiment of the present invention. 
     Referring to  FIG. 1 , a hybrid vehicle includes an engine  100  corresponding to an “internal combustion engine”, a first MG (Motor Generator)  110 , a second MG  120 , a power split device  130 , a reduction gear  140 , a battery  150 , a driving wheel  160 , a PM (Power train Manager)-ECU (Electronic Control Unit)  170 , and an MG (Motor Generator)-ECU  172 . 
     The hybrid vehicle runs by a driving force from at least one of engine  100  and second MG  120 . Engine  100 , first MG  110  and second MG  120  are coupled to one another via power split device  130 . 
     Power split device  130  is typically formed as a planetary gear mechanism. Power split device  130  includes a sun gear  131  that is an external gear, a ring gear  132  that is an internal gear and disposed concentrically with this sun gear  131 , a plurality of pinion gears  133  that engage with sun gear  131  and also with ring gear  132 , and a carrier  134 . Carrier  134  is configured to hold the plurality of pinion gears  133  in a freely rotating and revolving manner. 
     Sun gear  131  is coupled to an output shaft of first MG  110 . Ring gear  132  is rotatably supported coaxially with a crankshaft  102 . Pinion gear  133  is disposed between sun gear  131  and ring gear  132 , and revolves around sun gear  131  while rotating on its axis. Carrier  134  is coupled to the end of crankshaft  102  and supports the rotation shaft of each pinion gear  133 . 
     Sun gear  131  and a ring gear shaft  135  rotate as ring gear  132  rotates. The output shaft of second MG  120  is coupled to ring gear shaft  135 . Ring gear shaft  135  will be hereinafter also referred to as a drive shaft  135 . 
     In addition, the output shaft of second MG  120  may be configured to be coupled to drive shaft  135  through a transmission. In the present embodiment, since the configuration not provided with a transmission is illustrated, the rotation speed ratio between second MG  120  and ring gear (drive shaft)  135  is 1:1. In contrast, in the configuration provided with a transmission, each of the rotation speed ratio and the torque ratio between drive shaft  135  and second MG  120  is determined by the gear ratio. 
     Drive shaft  135  is mechanically coupled to driving wheel  160  through reduction gear  140 . Accordingly, the motive power output by power split device  130  to ring gear  132 , that is, drive shaft  135 , is to be output to driving wheel  160  through reduction gear  140 . Although front wheels are used as driving wheels  160  in the example shown in  FIG. 1 , rear wheels may be used as driving wheels  160  or front wheels and rear wheels may be used as driving wheels  160 . 
     Power split device  130  executes a differential action using sun gear  131 , ring gear  132  and carrier  134  each as a rotating element. These three rotating elements are mechanically coupled to three shafts including crankshaft  102  of engine  100 , the output shaft of first MG  110  and drive shaft  135 . Also, power split device  130  is configured such that when the rotation speeds of any two shafts of these three shafts are determined, the rotation speed of remaining one shaft is determined, and also configured to, based on the motive power input to and output from any two shafts of these three shafts, input and output the motive power to and from remaining one shaft. 
     The motive power generated by engine  100  is split into two paths by power split device  130 . One of the paths serves to drive driving wheel  160  through reduction gear  140  while the other of the paths serves to drive first MG  110  to generate electric power. When first MG  110  functions as a power generator, power split device  130  distributes the motive power, which is input from engine  100  through carrier  134 , to the sun gear  131  side and the ring gear  132  side in accordance with the gear ratio. On the other hand, when first MG  110  functions as an electric motor, power split device  130  combines the motive power input from engine  100  through carrier  134  and the motive power input from first MG  110  through sun gear  131 , and outputs the combined power to ring gear  132 . In this way, power split device  130  functions as a “power transmission device” for mechanically transmitting, to drive shaft  135 , the torque originating from the output of engine  100 . 
     First MG  110  and second MG  120  each are representatively a three-phase alternating-current (AC) rotating electric machine formed of a permanent magnet motor. 
     First MG  110  can mainly operate as a “power generator” to generate electric power by the driving force from engine  100  split by power split device  130 . The electric power generated by first MG  110  is variously used in accordance with the running state of the vehicle and the conditions of an SOC (State Of Charge) of battery  150 . For example, at the time of the normal running of the vehicle, the electric power generated by first MG  110  is used as electric power for driving second MG  120 . On the other hand, when the SOC of battery  150  is lower than a predetermined value, the electric power generated by first MG  110  is converted from the alternating current into a direct current by an inverter described later. Then, the voltage is adjusted by a converter described later and stored in battery  150 . In addition, in the case where engine  100  is monitored at start-up of the engine, and the like, first MG  110  can also operate as an electric motor under the torque control. 
     Second MG  120  mainly operates as an “electric motor” and is driven by at least one of the electric power stored in battery  150  and the electric power generated by first MG  110 . The motive power generated by second MG  120  is transmitted to drive shaft  135 , and further transmitted to driving wheel  160  through reduction gear  140 . Accordingly, second MG  120  assists engine  100 , or causes the vehicle to run with the driving force from second MG  120 . 
     During regenerative braking of a hybrid vehicle, second MG  120  is driven by driving wheel  160  through reduction gear  140 . In this case, second MG  120  operates as a power generator. Accordingly, second MG  120  functions as a regenerative brake that converts braking energy into electric power. The electric power generated by second MG  120  is stored in battery  150 . 
     Battery  150  serves as a battery pack having a configuration in which a plurality of battery modules each having a plurality of battery cells integrated with each other are connected in series. The voltage of battery  150  is approximately 200V, for example. 
     Battery  150  can be charged with electric power generated by first MG  110  or second MG  120 . The temperature, voltage and current of battery  150  are detected by a battery sensor  152 . A temperature sensor, a voltage sensor and a current sensor are comprehensively indicated as battery sensor  152 . 
     The charge power to battery  150  is limited so as not to exceed an upper limit value WIN. Similarly, the discharge power of battery  150  is limited so as not to exceed an upper limit value WOUT. Upper limit values WIN and WOUT are determined based on various parameters such as the SOC, the temperature, the change rate of the temperature and the like of battery  150 . 
     PM-ECU  170  and MG-ECU  172  each are configured to incorporate a CPU (Central Processing Unit) and a memory which are not shown, and to perform operation processing based on the value detected by each sensor by means of software processing in accordance with the map and program stored in the memory. Alternatively, at least a part of the ECU may be configured to perform prescribed numerical operation processing and/or logical operation processing by means of hardware processing by a dedicated electronic circuit and the like. 
     Engine  100  is controlled in accordance with a control target value from PM (Power train Manager)-ECU (Electronic Control Unit)  170 . First MG  110  and second MG  120  are controlled by MG-ECU  172 . PM-ECU  170  and MG-ECU  172  are connected so as to allow bidirectional communication with each other. PM-ECU  170  generates a control target value (representatively, a torque target value) for each of engine  100 , first MG  110  and second MG  120  by running control which will be described later. In other words, PM-ECU  170  executes a function of a “running control unit”. 
     Then, MG-ECU  172  controls first MG  110  and second MG  120  in accordance with the control target value transmitted from PM-ECU  170 . In other words, MG-ECU  172  executes a function of an “electric motor control unit”. In addition, engine  100  controls fuel injection quantity, ignition timing and the like in accordance with the operation target value (representatively, a torque target value and a rotation speed target value) from PM-ECU  170 . 
     Although PM-ECU  170  and MG-ECU  172  are formed of separate ECUs in the present embodiment, a single ECU comprehensively having both functions of these ECUs may be provided. 
       FIG. 2  is a circuit diagram illustrating a configuration example of an electrical system of the hybrid vehicle shown in  FIG. 1 . 
     Referring to  FIG. 2 , the electrical system of the hybrid vehicle is provided with a converter  200 , an inverter  210  corresponding to first MG  110  (power generator), an inverter  220  corresponding to second MG  120  (electric motor), and an SMR (System Main Relay)  230 . In other words, inverter  210  corresponds to the “first power converter” while inverter  220  corresponds to the “second power converter”. 
     Converter  200  includes a reactor, two power semiconductor switching elements (which will be also simply referred to as a “switching element”) connected in series, an antiparallel diode provided corresponding to each switching element, and a reactor. As a power semiconductor switching element, an IGBT (Insulated Gate Bipolar Transistor), a power MOS (Metal Oxide Semiconductor) transistor, a power bipolar transistor, and the like may be used as appropriate. The reactor has one end connected to battery  150  on its positive pole and the other end connected to the connection point between two switching elements. Each switching element is controlled by MG-ECU  170  to be turned on or off. 
     When the electric power discharged from battery  150  is supplied to first MG  110  or second MG  120 , the voltage is raised by converter  200 . In contrast, when battery  150  is charged with the electric power generated by first MG  110  or second MG  120 , the voltage is lowered by converter  200 . 
     Converter  200 , inverter  210  and inverter  220  are electrically connected to one another through a power line PL and a ground line GL. A DC voltage (system voltage) VH on power line PL is detected by a voltage sensor  180 . The results detected by voltage sensor  180  are transmitted to MG-ECU  172 . 
     Inverter  210  is formed of a commonly-used three-phase inverter, and includes a U-phase arm, a V-phase arm and a W-phase arm that are connected in parallel. Each of the U-phase arm, the V-phase arm and the W-phase arm has two switching elements (an upper arm element and a lower arm element) connected in series. An antiparallel diode is connected to each switching element. 
     First MG  110  has a U-phase coil, a V-phase coil and a W-phase coil coupled in a star connection as a stator winding. Each phase coil has one end mutually connected at a neutral point  112  and also has the other end connected to a connection point between the switching elements of each phase arm of inverter  210 . 
     During vehicle running, inverter  210  controls the current or voltage of each phase coil of first MG  110  such that first MG  110  operates in accordance with the operation command value (representatively, a torque target value) set for generating the driving force (vehicle driving torque, power generation torque, and the like) requested for vehicle running. 
     As with inverter  210 , inverter  220  is formed of a commonly-used three-phase inverter. As with first MG  110 , second MG  120  has a U-phase coil, a V-phase coil and a W-phase coil coupled in a star connection as a stator winding. Each phase coil has one end mutually connected at a neutral point  122  and also has the other end connected to a connection point between the switching elements of each phase arm of inverter  220 . 
     During vehicle running, inverter  220  controls the current or voltage of each phase coil of second MG  120  such that second MG  120  operates in accordance with the operation command value (representatively, a torque target value) set for generating the driving force (vehicle driving torque, regenerative braking torque, and the like) requested for vehicle running. 
     In addition, for example, PWM (Pulse Width Modulation) control is used for controlling first MG  110  and second MG  120  by inverters  210  and  220 , respectively. Since a well-known and commonly-used technique only has to be employed for PWM control, further detailed description thereof will not be repeated. MG-ECU  172  generates a driving signal for controlling the switching elements forming each of inverters  210  and  220  to be turned on and off in accordance with PWM control. In other words, during operation of inverters  210  and  220 , switching loss occurs as each switching element is turned on or off. 
     An SMR  250  is provided between battery  150  and converter  200 . When SMR  250  is opened, battery  150  is cut off from the electrical system. On the other hand, when SMR  250  is closed, battery  150  is connected to the electrical system. The state of SMR  250  is controlled by PM-ECU  170 . For example, SMR  250  is closed in response to the operation of turning on a power-on switch (not shown) that instructs system startup of the hybrid vehicle while SMR  250  is opened in response to the operation of turning off the power-on switch. 
     As described above, in the hybrid vehicle shown in  FIG. 1 , engine  100 , first MG  110  and second MG  120  are coupled via a planetary gear. This establishes a relation in which the rotation speeds of engine  100 , first MG  110  and second MG  120  are connected with a straight line in a collinear diagram, as shown in  FIG. 3 . 
     According to the hybrid vehicle, PM-ECU  170  executes running control for allowing vehicle running suitable for the vehicle state. For example, at the start of the vehicle and during low speed running, the hybrid vehicle runs with the output from second MG  120  in the state where engine  100  is stopped, as in the collinear diagram shown in  FIG. 4 . In this case, the rotation speed of second MG  120  is rendered positive while the rotation speed of first MG  110  is rendered negative. 
     During normal running, as in the collinear diagram shown in  FIG. 5 , the rotation speed of first MG  110  is rendered positive by operating first MG  110  as a motor such that engine  100  is cranked using first MG  110 . In this case, first MG  110  operates as an electric motor. Then, engine  100  is started to cause the hybrid vehicle to run with the outputs from engine  100  and second MG  120 . As will be described later in detail, a hybrid vehicle is improved in fuel efficiency by operating engine  100  at a highly-efficient operating point. 
     (Control Structure) 
     The running control for the hybrid vehicle according to the present first embodiment will be hereinafter described in detail.  FIGS. 6 and 7  each are a flowchart illustrating running control of the hybrid vehicle according to the first embodiment. The controlling process in accordance with the flowcharts shown in  FIGS. 6 and 7  is, for example, performed by PM-ECU  170  shown in  FIG. 1  for each prescribed control cycle. 
     Referring to  FIG. 6 , in step S 100 , PM-ECU  170  calculates total driving force required in the entire vehicle based on the vehicle state detected based on the sensor output signal. Then, in order to generate this total driving force, PM-ECU  170  calculates a requested driving force Tp* that is to be output to drive shaft  135 . The vehicle state reflected in calculation of the driving force typically includes an accelerator pedal position Acc showing the accelerator pedal operation amount by the user and a vehicle speed V of the hybrid vehicle. 
     For example, PM-ECU  170  stores, in the memory, a map (not shown) in which the relation among accelerator pedal position Acc, vehicle speed V and requested driving force Tp* is set in advance. Then, when accelerator pedal position Acc and vehicle speed V are detected, PM-ECU  170  can calculate requested driving force Tp* by referring to this map. 
     In this way, by adding the torque corresponding to requested driving force Tp* to drive shaft  135 , the hybrid vehicle can generate appropriate vehicle driving force in accordance with the vehicle state. In the following, requested driving force Tp* will also be referred to as a total torque Tp*. 
     In step S 110 , PM-ECU  170  calculates engine requesting power Pe that is output power requested by engine  100  based on total torque Tp* calculated in step S 100 . For example, engine requesting power Pe is set according to the following equation (1) in accordance with total torque Tp*, a drive shaft rotation speed Nr, charge/discharge request power Pchg, and a loss term Loss. 
         Pe=Tp*·Nr+Pchg+Loss   (1)
 
     Charge/discharge request power Pchg is set such that Pchg&gt;0, when battery  150  needs to be charged in accordance with the state (SOC) of battery  150 . On the other hand, when battery  150  is excessively charged and needs to be discharged, charge/discharge request power Pchg is set such that Pchg&lt;0. 
     Furthermore, PM-ECU  170  determines the operating point of engine  100  in step group  5200 . Step group S 200  include steps S 210  to S 250  described below. 
     In step S 210 , PM-ECU  170  calculates engine target rotation speed NE 1  in the normal running mode (second running mode) based on engine requesting power Pe. 
       FIG. 8  is a conceptual diagram for illustrating determination of an engine operating point. 
     Referring to  FIG. 8 , the engine operating point is defined by the combination of engine rotation speed Ne and engine torque Te. The product of engine rotation speed Ne and engine torque Te corresponds to engine output power. 
     An operation line  300  is determined in advance as a collection of engine operating points at which engine  100  can be operated with high efficiency. Operation line  300  corresponds to an optimal fuel efficiency line for suppressing the fuel consumption when the same power is output. 
     In step S 210 , PM-ECU  170  determines an intersection between a predetermined operation line  300  and an equal-power line  310  corresponding to engine requesting power Pe calculated in step S 110  as an engine operating point (target rotation speed Ne* and target torque Te*), as shown in  FIG. 8 . In other words, the engine operating point in the normal running mode is determined as P 2  in the figure. Engine target rotation speed NE 2  calculated in step S 210  is the engine rotation speed at an engine operating point P 2 . 
     Referring back to  FIG. 6 , in step S 220 , PM-ECU  170  calculates engine target rotation speed NE 1  in the shutdown mode (hereinafter described as an S/D mode) in which control of second MG  120  is stopped. The S/D mode corresponds to the first running mode. 
     In the S/D mode, inverter  220  is shut down to stop switching of each switching element (fixed to be off). Thereby, control of second MG  120  is stopped and the output torque of second MG  120  becomes zero. In the S/D mode, no power loss (switching loss) occurs in inverter  220 . Therefore, in the vehicle state in which the output of second MG  120  is not required, the fuel efficiency of the hybrid vehicle can be improved by applying the S/D mode. 
     In the S/D mode, it is necessary to determine an engine operating point such that total torque Tp* can be covered by engine  100  even if the output torque of second MG  120  is set at zero. Therefore, engine target rotation speed NE 1  can be calculated according to the following equation (2) by using a gear ratio ρ in power split device  130 . 
         NE 1 =Pe /( Tp* ·(1+ρ))  (2)
 
     Referring back to  FIG. 8 , a description will be given about a change in the engine operating point from the state where second MG  120  outputs a negative torque. 
     At an operating point P 1  corresponding to engine target rotation speed NE 1 , operating point P 2  is equivalent to the engine output power. On the other hand, since total torque Tp* is output in the state where the output torque of second MG  120  is set at zero, engine torque Te is lower at operating point P 1  than at operating point P 2 . In this case, engine target rotation speed NE 1  is higher than engine target rotation speed NE 2 . 
     Referring back to  FIG. 6 , in steps S 210  and S 220 , engine target rotation speed NE 1  during the S/D mode and engine target rotation speed NE 2  during the normal running mode are calculated. 
     In step S 230 , PM-ECU  170  determines whether the difference (an absolute value) between engine target rotation speeds NE 1  and NE 2  is smaller than a prescribed threshold value α. 
     Then, when the difference between engine target rotation speeds NE 1  and NE 2  is smaller than prescribed threshold value α (determined as YES in S 230 ), PM-ECU  170  proceeds the process to step S 240 , in which a condition is set as follows: engine target rotation speed Ne=NE 1 . On the other hand, when the difference between engine target rotation speeds NE 1  and NE 2  is larger than prescribed threshold value α (determined as NO in S 230 ), PM-ECU  170  proceeds the process to step S 250 , in which a condition is set as follows: engine target rotation speed Ne=NE 2 . 
     Furthermore, in step S  130 , PM-ECU  170  sets a final engine target rotation speed Ne* in the present control cycle based on engine target rotation speed Ne calculated in step S 240  or S 250  and engine target rotation speed Ne* in the previous control cycle. In this case, rate limit processing is applied for setting an upper limit value for the change amount of engine target rotation speed Ne* during the control cycle. 
     Consequently, when the S/D mode is applied, the operating point for continuously applying the S/D mode is set in the case where NE=NE 2  (S 250 ). On the other hand, in the case where NE=NE 1  (S 240 ), the engine operating point is changed so as to shift to the normal running mode. 
     In contrast, when the normal running mode is applied, the operating point for continuously applying the normal running mode is set in the case where NE=NE 1  (S 240 ). On the other hand, in the case where NE=NE 2  (S 250 ), the engine operating-point change control for shifting to the S/D mode is started. Furthermore, in step S 255 , PM-ECU  170  compares the difference (absolute value) between final engine target rotation speed Ne* set in step S 130  and engine target rotation speed NE 1  during the S/D mode with a prescribed threshold value β. Then, when the difference between engine target rotation speed Ne* and engine target rotation speed NE 1  is smaller than prescribed threshold value β (determined as YES in S 250 ), PM-ECU  170  turns on an S/D permission flag in step S 260 . On the other hand, when the difference between engine target rotation speed Ne* and engine target rotation speed NE 1  is larger than prescribed threshold value β (determined as NO in S 250 ), PM-ECU  170  turns off the S/D permission flag in S 270 . 
     Therefore, during the normal running mode, engine operating-point change control is started when engine target rotation speed NE 1  obtained during the S/D mode becomes closer to engine target rotation speed NE 2  to some extent (determined as YES in S 230 ). Then, when actual engine target rotation speed Ne* becomes sufficiently close to engine target rotation speed NE 1  by engine operating-point change control (determined as YES in S 255 ), the S/D permission flag is turned on. 
     Referring to  FIG. 7 , subsequent to steps S 250  to S 270 , PM-ECU  170  proceeds the process to step S 140 . In step S 140 , PM-ECU  170  determines target values of the torque and the rotation speed of first MG  110  for implementing final engine target rotation speed Ne* determined in step S 130 . 
       FIG. 9  is a collinear diagram showing the relation of the rotation speed and the torque among first MG  110 , second MG  120  and engine  100  under the running control of the hybrid vehicle according to the present embodiment. 
     Referring to  FIG. 9 , a target rotation speed Nmg 1 * of first MG  110  can be determined according to the following equation (3) by using gear ratio ρ and drive shaft rotation speed Nr of power split device  130 . 
         Nmg 1*=( Ne* ·(1+ρ)− Nr )/ρ  (3)
 
     Then, PM-ECU  170  sets a torque target value Tmg 1 * of first MG  110  such that first MG  110  rotates at target rotation speed Nmg 1 *. For example, torque target value Tmg 1 * can be set according to the following equation (4) so as to sequentially correct torque target value Tmg 1 * based on the deviation between actual rotation speed Nmg 1  and target rotation speed Nmg 1 * of first MG  110  (ΔNmg 1 =Nmg 1 *−Nmg 1 ). In addition, the second term on the right-hand side in the equation (4) shows the calculation result of a PID (Proportional Integral Differential) control based on deviation ΔNmg 1   
         Tmg 1 *=Tmg 1*(previous value)+ PID (Δ Nmg 1)  (4)
 
     When first MG  110  is controlled in accordance with torque target value Tmg 1 *, an engine direct torque Tep(=−Tmg 1 */ρ) is exerted on ring gear  132  (drive shaft  135 ). Engine direct torque Tep corresponds to the torque transmitted to ring gear  132  at the time when engine  100  is operated at each of target rotation speed Ne* and target torque Te* while first MG  110  receives reaction force. 
     The output torque of second MG  120  is exerted on ring gear  132  (drive shaft  135 ). Accordingly, total torque Tp* can be ensured by setting the output torque of second MG  120  so as to compensate for the excessive or insufficient amount of engine direct torque Tep relative to total torque Tp*. 
     Referring back to  FIG. 7 , in step S 150 , PM-ECU  170  calculates engine direct torque Tep based on torque target value Tmg 1 * set in step S 140  and gear ratio ρ. Also as shown in  FIG. 9 , engine direct torque Tep can be calculated by the following equation (5). 
         Tep=−Tmg 1*/ρ  (5)
 
     Furthermore, in step S 160 , PM-ECU  170  calculates a torque target value Tmg 2 * of second MG  120  according to the following equation (6) so as to compensate for the excessive or insufficient amount of engine direct torque Tep relative to total torque Tp*. When a transmission is connected between second MG  120  and drive shaft  135 , the equation (6) only has to be multiplied by the gear ratio. 
         Tmg 2*=( Tp*−Tep /ρ)  (6)
 
     In this way, the output distribution among engine  100 , first MG  110  and second MG  120  for outputting total torque Tp* determined in step S 100  is determined. 
     In step S 280 , PM-ECU  170  determines whether or not torque target value Tmg 2 * calculated in step S 160  is substantially equal to zero. Specifically, in the case where |Tmg 2 *|&lt;ε, a determination is made as YES in step S 280 . In other words, a threshold value ε is a value used for detecting Tmg 2 *≈0 at which no torque step occurs even if the S/D mode is applied to set the output torque of second MG  120  at zero. 
     In the case where |Tmg 2 *|&lt;ε (determined as YES in S 280 ), PM-ECU  170  determines in step S 290  whether the S/D permission flag is turned on or not. When the S/D permission flag is turned on (determined as YES in S 290 ), PM-ECU  170  proceeds the process to step S 300  to perform S/D control for second MG  120 . In other words, inverter  220  is shut down and the output torque of second MG  120  becomes zero. 
     In the S/D mode (first running mode), by setting the engine operating point based on engine target rotation speed NE 1 , the output distribution is determined such that total torque Tp* is exerted on drive shaft  135  by the output from each of first MG  110  and engine  100 , even if the output torque of second MG  120  is set at zero. 
     On the other hand, in the case where |Tmg 2 *|&gt;ε (determined as NO in S 280 ) or while the S/D permission flag is off (determined as NO in S 290 ), PM-ECU  170  proceeds the process to step S 170 . In step S 170 , second MG  120  is controlled in accordance with torque target value Tmg 2 * calculated in step S 160 . In other words, inverter  220  performs DC/AC power conversion by switching control of each switching element. 
     In the normal running mode (second running mode), based on engine target rotation speed NE 2  determined giving priority to engine efficiency, the output distribution is determined such that total torque Tp* is exerted on drive shaft  135  by outputs of first MG  110 , second MG  120  and engine  100 . 
     In this way, under the running control of the hybrid vehicle according to the first embodiment, the S/D mode is applied to thereby allow reduction in the power loss in inverter  220  during vehicle running in which the output torque of second MG  120  is set at zero. Consequently, the fuel efficiency of the hybrid vehicle can be improved. Furthermore, under the engine operating-point change control, variations in the vehicle driving force (torque) at the start of the S/D mode can be suppressed by switching from the normal running mode to the S/D mode after the engine rotation speed is brought close to engine target rotation speed NE 1 . Particularly, variations in the vehicle driving force can be reliably prevented by inhibiting turning-on of the S/D permission flag until the engine rotation speed becomes close to engine target rotation speed NE 1 . 
       FIG. 10  shows an operation waveform during switching from the normal running mode to the S/D mode under running control of the hybrid vehicle according to the present first embodiment. 
     Referring to  FIG. 10 , at time t 1 , |Tmg 2 *|&lt;ε is established for the torque target value of second MG  120 . However, at this stage, since the engine rotation speed is away from engine target rotation speed NE 1 , shutdown control is not started. 
     Then, starting from time t 1 , engine operating-point change control is performed for changing the engine rotation speed so as to be close to engine target rotation speed NE 1 . In this case, since Tmg 2 *&lt;0 is assumed to be established, the engine operating point is changed such that engine torque Te is decreased. 
     At and after time t 1 , engine torque Te gradually decreases as the engine rotation speed is changed so as to be close to engine target rotation speed NE 1 . The output torque (negative value) of second MG  120  also gradually increases and comes closer to zero. 
     At time t 2 , since the engine rotation speed becomes sufficiently close to engine target rotation speed NE 1  (determined as YES in S 255  in  FIG. 6 ), the S/D permission flag is turned on and shutdown control is started. This leads to Tmg 2 *=0. 
     At this point of time, by engine operating-point change control at times t 1  to t 2 , the engine operating point is set, at which total torque Tp* can be exerted on drive shaft  135  by the output of engine  100 , even if the output torque of second MG  120  is set at zero. Accordingly, even when shutdown control is started at time t 2 , variations in total torque (vehicle driving force) Tp exerted on drive shaft  135  can be suppressed. 
     On the other hand, in  FIG. 10 , the behavior at the time when shutdown control is performed at time t 1  is shown by dotted lines. 
     At time t 1 , when shutdown control is started, the output torque of second MG  120  immediately becomes zero by setting torque target value Tmg 2 * of second MG  120  at zero. However, since engine  100  has great inertia, engine rotation speed Ne and engine torque Te change gradually. Consequently, total torque Tp exerted on drive shaft  135  is changed corresponding to the torque change in second MG  120 . It is thus understood that total torque Tp* continuously changes until the engine rotation speed is changed to be engine target rotation speed NE 1 . 
     In other words, under the running control of the hybrid vehicle according to the present first embodiment, variations in the vehicle driving force can be suppressed by performing engine operating-point change control when applying the S/D mode for improving the fuel efficiency. 
     Second Embodiment 
     In the second embodiment, the running control for correctly setting the vehicle driving force in the S/D mode will be further described. In the subsequent embodiments including the second embodiment, several controlling processes are further performed in addition to the running control according to the first embodiment. Accordingly, in the following embodiments, the controlling process added to or modified from the first embodiment will be mainly described, but any common parts with the first embodiment will not be basically repeated. 
       FIG. 11  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the present second embodiment. 
     Referring to  FIG. 11 , under the running control according to the second embodiment, PM-ECU  170  further performs a process of steps S 310  to S 330  between steps S 110  and S 220  in  FIG. 6 . 
     In step S 310 , PM-ECU  170  determines whether second MG  120  is under the shutdown control or not. When second MG  120  is under the shutdown control (determined as YES in S 310 ), PM-ECU  170  calculates a drag torque Tm of second MG  120  in step S 320 . 
     Drag torque Tm shows a magnitude of a torque acting as rotational resistance when second MG  120  rotates at an output torque=0. During execution of the torque control of second MG  120 , the output torque can be feedback-controlled so as to compensate for the amount of the drag torque. In contrast, in the S/D mode, since the torque control of second MG  120  is stopped and the output torque becomes zero, the torque (vehicle driving force) exerted on drive shaft  135  may decrease in accordance with the amount of the drag torque of second MG  120 . 
     Typically, the mechanical loss to rotational movement caused by a bearing and the like is exerted as rotational resistance. The torque caused by such mechanical loss changes depending on the rotation speed of second MG  120 . 
     Therefore, as shown in  FIG. 12 , if the corresponding relation of drag torque Tmh to second MG rotation speed Nmg 2  (absolute value) is measured in advance by experiments or the like, a map  320  based on the experimental results can be produced in advance. In addition, when a transmission is disposed between second MG  120  and drive shaft  135 , the converted value to the drive shaft torque in consideration of the gear ratio of the transmission is assumed to be a map value of drag torque Tmh. 
     In step S 320  in  FIG. 11 , drag torque Tm (Tm=Tmh) can be calculated by referring to map  320  based on second MG rotation speed Nmg 2  in the present control cycle. 
     Referring back to  FIG. 11 , in step S 330 , PM-ECU  170  corrects total torque Tp* calculated in step S 100  by reflecting drag torque Tm. In other words, the sum of total torque Tp* calculated in step S 100  and drag torque Tm is newly set as total torque Tp*. Then, in the subsequent control cycle, in step S 220  ( FIG. 6 ), engine target rotation speed NE 1  (that is, engine operating point) in the S/D mode is determined based on total torque Tp* in which drag torque Tm is included. 
     Therefore, the difference resulting from the influence of the drag torque between the vehicle driving force in the S/D mode and the requested total torque Tp* can be suppressed. 
     (First Modification of Second Embodiment) 
     When second MG  120  is a permanent magnet motor, counter-electromotive force is generated by a permanent magnet attached to a rotor. Accordingly, in the S/D mode, an electromagnetic drag torque resulting from this counter-electromotive force is generated. Such an electromagnetic drag torque is reflected in the total torque in the first modification of the second embodiment. 
       FIG. 13  is a flowchart illustrating a controlling process added by the running control according to the first modification of the second embodiment of the present invention. 
     Referring to  FIG. 13 , PM-ECU  170  performs a process of steps S 340  to S 360  after execution of step S 300 . 
     PM-ECU  170  performs a process of detecting counter-electromotive force in step S 340 . Then, in step S 355 , PM-ECU  170  calculates electromagnetic drag torque Tme based on the detected counter-electromotive force. 
     As shown in  FIG. 14 , if the corresponding relation of electromagnetic drag torque Tme to the counter-electromotive force is measured in advance by experiments and the like, a map  330  based on the experimental results can be produced in advance. In addition, when a transmission is disposed between second MG  120  and drive shaft  135 , the converted value to the drive shaft torque in consideration of the gear ratio of the transmission is set as a map value of drag torque Tme. 
     In step S 350  in  FIG. 13 , drag torque Tme can be calculated by referring to map  330  based on the counter-electromotive force detected in step S 340 . 
     In step S 355 , PM-ECU  170  latches drag torque Tme calculated in step S 320 . The drag torque latched in step S 360  is included in drag torque Tm in step S 320  ( FIG. 11 ) in the subsequent (next) control cycle. Consequently, it becomes possible to compensate for the electromagnetic drag torque exerted on a permanent magnet motor and output the requested total torque *Tp. 
     Then, a specific example of the process of detecting counter-electromotive force in step S 340  will be described. 
     As the first detection example, the counter-electromotive force can be detected from the measured value of a line voltage by measuring a two-phase voltage of a three-phase alternating current in second MG  120 . It is to be noted that a voltage sensor needs to be disposed in this detection example. 
     As the second detection example, the counter-electromotive force can also be detected based on the control data under a specified condition for torque control of second MG  120 , without additionally disposing a voltage sensor. 
     The following equations (7) and (8) each are known as a d-q axis voltage equation used for electric motor control. 
         Vd=R·Id−ω·Lq·Iq   (7)
 
         Vq=ω·Ld·Id+R·Iq+ω·φ   (8)
 
     In the equations (7) and (8), Vd and Vq are a d-axis component and a q-axis component, respectively, of the voltage applied to second MG  120 , while Id and Iq are a d-axis component and a q-axis component, respectively, of the voltage applied to second MG  120 . A three-phase voltage and a three-phase current can be mutually converted and inverse-converted from/to Vd, Vq and Id, Iq, respectively, on the d-q axis according to a prescribed conversion matrix. Furthermore, Ld and Lq are a d-axis component and a q-axis component, respectively, of an inductance, and R is a resistance component. Furthermore, ω is an electrical angle speed, and φ is an interlinkage flux. The product of ω·φ corresponds to the counter-electromotive force of second MG  120 . 
     In a high speed rotation region where w becomes relatively large, since resistance R become negligible as compared with ωL, the equations (7) and (8) are transformed into the following equations (9) and (10), respectively. 
         Vd=−ω·Lq·Iq   (9)
 
         Vq=ω·Ld·Id+ω·φ   (10)
 
     In the second detection example, the counter electromotive voltage is detected from the command value of a q-axis voltage Vq obtained when second MG  120  is subjected to zero-current control. 
     Id and Iq command values are set at zero by zero-current control. Then, by feedback-controlling Id and Iq each converted from a three-phase current, Vd and Vq voltage command values for establishing Id=Iq=0 (current command value) are calculated. 
     When Id=Iq=0 is substituted into the equations (9) and (10), a condition is given as follows: Vd=0 and Vq=ω·φ. In other words, the Vq command value during zero-current control corresponds to a counter electromotive voltage. 
     For example, when normal feedback control using a current command value set as Id=Iq=0 is performed for an extremely short period of time before shutting down inverter  220 , a Vq command value during zero-current control can be calculated. 
     In addition, an output torque Trq of the permanent magnet motor is as shown in the following equation (11) using a pole logarithm p. 
         Trq=p ·(φ· Iq +( Ld−Lq )· Id·Iq )  (11)
 
     In other words, during zero-current control (Id=Iq=0), the output torque of second MG  120  is controlled to be zero. Therefore, even if zero-current control is performed for a short period of time before starting shutdown control, the torque does not significantly vary when inverter  220  is shut down. 
     In this way, in the first control cycle in which shutdown control is started, zero-current control is performed in a limited extremely short period of time, thereby allowing detection of a counter electromotive voltage used for calculating drag torque Tme. However, since zero-current control cannot be performed during shutdown of inverter  220 , the process of steps S 340  to S 360  is performed only at the start of shutdown control (the first control cycle). In other words, in the subsequent control cycles in which shutdown control is continuously performed, drag torque Tme latched in the first control cycle (S 360 ) is included in common in drag torque Tm in step S 320  ( FIG. 11 ). 
     In the third detection example, as in the second detection example, a counter-electromotive force is detected without additionally disposing a voltage sensor. Zero-current control described in the second detection example cannot be performed in the region where a counter electromotive voltage (ω·φ) becomes large due to high speed rotation. This is because when the counter electromotive voltage is higher than Vq based on zero-current control, Id=Iq=0 cannot be achieved by control. Specifically, zero-current control cannot be applied when the modulation factor ((Vd 2 +Vq 2 ) 1/2 /VH) by inverter  220  is 0.78 or more. 
     In the third detection example, in place of zero-current control, a counter electromotive voltage is detected from the current detection value obtained when second MG  120  is subjected to zero-voltage control (Vd=Vq=0). 
     When Vq=0 is substituted into the equation (10), a condition is given as follows: ω·φ=−ω·Ld·Id. Id can be calculated from the three-phase current detection value of second MG  120 . Furthermore, electrical angle speed ω can also be calculated from the rotation angle detection value of second MG  120 . The three-phase current and the rotation angle each are a detection value used in the normal current feedback control. Furthermore, inductance Ld is a motor constant, and even when saturation during high speed rotation is taken into consideration, a map based on the experimental results and the like can be produced in advance as a function of current Id. 
     The second and third detection examples can be selectively performed in the first control cycle in which shutdown control is started. 
     Therefore, as in the second detection example, in the first control cycle in which shutdown control is performed, zero-voltage control is performed in a limited extremely short period of time, thereby allowing detection of the counter electromotive voltage used for calculating drag torque Tme. Then, in the subsequent control cycles in which shutdown control is continuously performed, drag torque Tme latched in the first control cycle (S 360 ) can be included in common in drag torque Tm in step S 320  ( FIG. 11 ). 
     In this way, according to the first modification of the second embodiment, the electromagnetic drag torque resulting from counter-electromotive force can be calculated by any of the first to third detection examples. Therefore, drag torque Tm in step S 320  in  FIG. 6  can be calculated based on mechanical drag torque Tmh and/or electromagnetic drag torque Tme. 
     By reflecting drag torque Tm in the setting of the engine operating point (engine target rotation speed NE 1 ) in the S/D mode, the difference between the vehicle driving force in the S/D mode and the requested total torque Tp* can be suppressed. 
     (Second Modification of Second Embodiment) 
     In the second modification of the second embodiment, a description will be given about the controlling process of compensating for the torque deviation in engine  100  at the start of the S/D mode. 
       FIG. 15  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the second modification of the present second embodiment. 
     Referring to  FIG. 15 , when performing shutdown control of second MG  120 , PM-ECU  170  further performs steps S 370  and S 380 , subsequent to step S 300 . 
     In step S 370 , PM-ECU  170  determines whether the shutdown control being performed is the first cycle of shutdown control or not. When it is determined as the first cycle of shutdown control (determined as YES in S 370 ), in step S 380 , PM-ECU  130  calculates a torque deviation ΔTp between total torque Tp* calculated in step S 100  and engine direct torque Tep calculated in step S 150  (ΔTp=Tp*−Tep). 
     Accordingly, torque deviation ΔTp is equivalent to an excessive or insufficient amount of engine direct torque Tep ensured at the start of shutdown control, relative to total torque Tp* calculated in step S 100 . As described above, engine direct torque Tep is calculated from torque target value (Tmg 1 *) of first MG  110  used for controlling the engine rotation speed to be equal to engine target rotation speed Ne*. Therefore, torque loss and the like caused by friction loss are included in torque deviation ΔTp at the start of shutdown control. 
     Therefore, during the S/D mode, torque deviation ΔTp calculated at the start of shutdown control is latched. Then, in the subsequent control cycles, it is preferable to include torque deviation ΔTp in total torque Tp* as in the case of drag torque Tm. 
     In this way, in the subsequent control cycles, it becomes possible to determine engine target rotation speed NE 1  (that is, the engine operating point) in the S/D mode by adding torque deviation ΔTp so as to compensate for torque loss and the like in engine  100  (S 220  in  FIG. 6 ). 
     Consequently, it becomes possible to suppress the difference resulting from torque loss and the like in engine  100  between the vehicle driving force in the S/D mode and the requested total torque Tp*. 
     In addition, by setting total torque Tp* to include both of drag torque Tm described in the second embodiment and its first modification and torque deviation ΔTp described in the second modification of the second embodiment, engine target rotation speed NE 1  (engine operating point) in the S/D mode can also be determined. By doing this, it can be expected that the difference between the vehicle driving force in the S/D mode and the requested total torque Tp* is suppressed. 
     Third Embodiment 
     Under the running control described in the first embodiment, engine operating-point change control is performed for applying shutdown control. As a result, in the S/D mode, the engine operating point is deviated from operation line  300  ( FIG. 8 ) that is set based on the optimal fuel efficiency. Therefore, when shutdown control is applied, reduction in power loss in inverter  220  causes improvement in fuel efficiency whereas a change in the engine operating point causes deterioration in fuel efficiency. 
     Under the running control of the hybrid vehicle according to the third embodiment, it is determined whether the engine operating point can be changed or not, based on the prediction about a fuel efficiency improving effect during the S/D mode. 
       FIG. 16  is a flowchart illustrating a controlling process added by the running control of the hybrid vehicle according to the present third embodiment. 
     Referring to  FIG. 16 , under the running control of the hybrid vehicle according to the third embodiment, PM-ECU  170  further performs steps S 400  and S 410  between the process of steps S 110  and S 220 . 
     In step S 400 , PM-ECU  170  reflects loss reduction power Pl (Pl&gt;0) for driving of second MG  120  resulting from shutdown of inverter  220  in engine requesting power Pe at the time when the S/D mode is applied. In other words, by subtracting loss reduction power Pl from engine requesting power Pe calculated in step S 110  ( FIG. 6 ), engine requesting power Pe in the S/D mode is calculated. Loss reduction power P 1  can be set in advance based on the experimental results and the like. 
     PM-ECU  170  calculates total torque Tp* in the S/D mode in step S 410 . As total torque Tp* in the S/D mode, basically, total torque Tp* calculated in step S 100  can be used without change. 
     Furthermore, in step S 220 , based on engine requesting power Pe calculated in step S 400  and total torque Tp* calculated in step S 410 , PM-ECU  170  calculates engine target rotation speed NE 1  in the S/D mode, as has been described with reference to  FIG. 6 . Therefore, it is understood that a loss reducing effect by shutdown of inverter  220  is incorporated in engine target rotation speed NE 1  due to step S 400 . 
     Furthermore, in step S 430 , PM-ECU  170  estimates a fuel consumption F 1  at the engine operating point determined in step S 220 . 
     On the other hand, PM-ECU  170  calculates engine target rotation speed NE 2  (engine operating point) in the normal running mode in step S 210  similar to that in  FIG. 6 , and then, further performs step S 420 . In step S 420 , PM-ECU  170  estimates a fuel consumption F 2  at the engine operating point determined in step S 210 . 
     As to the fuel consumption at each operating point of engine  100 , a map can be produced in advance by measuring each fuel consumption in experiments and the like. Therefore, in steps S 420  and S 430 , fuel consumptions F 1  and F 2  can be estimated by referring to the map based on each engine operating point determined in steps S 210  and S 220 . 
     In step S 440 , PM-ECU  170  determines whether fuel consumption F 1  during the S/D mode is smaller than fuel consumption F 2  during the normal running mode. In the case where F 1 &lt;F 2 , a determination is made as YES in step S 440 . 
     When determined as N 0  in step S 440 , it cannot be expected to improve fuel efficiency by applying the S/D mode. Accordingly, PM-ECU  170  skips step S 230  and proceeds the process to step S 250 . In step S 250 , engine target rotation speed NE 2  in the normal running mode is assumed to be equal to engine target rotation speed Ne*. In other words, the operating-point change control for applying the S/D mode is inhibited. 
     On the other hand, when determined as YES in step S 440 , it can be expected to improve fuel efficiency by applying the S/D mode. Accordingly, PM-ECU  170  does not inhibit the engine operating-point change control. Therefore, by the process of steps S 230  to S 250  described with reference to  FIG. 6 , it is determined based on the difference between engine target rotation speeds NE 1  and NE 2  whether the operating-point change control can be started or not, as in the first embodiment. 
     Thus, according to the running control of the hybrid vehicle in the third embodiment, by estimating the fuel consumption after changing the engine operating point for applying the S/D mode, it can be determined whether the effect of improving fuel efficiency by shutdown of inverter  220  can be achieved or not. Consequently, since deterioration in fuel efficiency due to application of the S/D mode can be prevented, shutdown control can be efficiently performed. 
     Fourth Embodiment 
     In the fourth embodiment, charge/discharge control of battery  150  related to application of the S/D mode will then be described. 
     When the operation of inverter  220  is stopped, second MG  120  is brought into a power generation state at a high rotation speed. When second MG  120  is brought into a power generation state, a current flows from second MG  120  into inverter  220 . This current is rectified by the antiparallel diode ( FIG. 2 ) in inverter  220  in which its switching operation is stopped, and then, battery  150  is charged. In this case, it is necessary to pay attention to prevent battery  150  from being overcharged. 
       FIG. 17  is a flowchart for illustrating a controlling process added by the running control of the hybrid vehicle according to the present fourth embodiment. 
     Referring to  FIG. 17 , when a determination is made as YES in step S 290  and the S/D mode is applied, PM-ECU  170  further performs the process of steps S 500  to S 550  described below. 
     PM-ECU  170  determines in step S 500  whether rotation speed Nmg 2  of second MG  120  is higher than a reference rotation speed N 0 . Reference rotation speed N 0  is determined from characteristics of second MG  120 . In the case where Nmg 2 &gt;N 0 , second MG  120  is brought into a power generation state, and a current flows from second MG  120  into inverter  220 . 
     At a high rotation speed in which Nmg 2 &gt;N 0  (determined as YES in S 500 ), PM-ECU  170  proceeds the process to step S 510  and determines whether the SOC of battery  150  is higher than a reference value S 1 . For example, reference value S 1  is set in advance at a value that is obtained by subtracting a margin value from the upper limit value of the SOC control range. 
     Then, in the case where SOC&gt;S 1  (determined as YES in S 510 ), PM-ECU  170  proceeds the process to step S 520 , and turns on an S/D inhibit flag for inhibiting switching to shutdown control. Furthermore, in step S 520 , an SOC reducing request for reducing the SOC of battery  150  is turned on. 
     On the other hand, in the case where SOC&lt;S 1  (determined as NO in S 510 ), PM-ECU  170  determines in step S 530  whether the SOC is lower than a reference value S 2 . Reference value S 2  is lower than at least reference value S 1 . Reference value S 2  is set in advance at the SOC level at which a charge current caused by shutdown control is acceptable. 
     In the case where SOC&lt;S 2  (determined as YES in S 530 ), PM-ECU  170  proceeds the process to step S 540 , and then, turns off the S/D inhibit flag and sets the SOC reducing request to be off. On the other hand, while SOC&gt;S 2  (determined as NO in S 530 ), the process of s 540  is skipped. 
     PM-ECU  170  determines in step S 550  whether the S/D inhibit flag is turned on or not. When the S/D inhibit flag is turned on (determined as YES in S 550 ), PM-ECU  170  proceeds the process to step S 170 , and therefore, shutdown control is not performed. In other words, the normal running mode is applied. 
     When the SOC reducing request is set to be on, the output distribution is controlled such that the output of second MG  120  is increased while the output of engine  100  is decreased. For example, the above-described output distribution can be implemented by setting Pchg in the equation (1) at a negative value when calculating engine requesting power Pe in step S 110  ( FIG. 6 ). Consequently, the electric power of battery  150  is consumed by second MG  120 , so that the SOC can be reduced. Then, when the SOC is reduced below reference value S 2  in response to the SOC reducing request, the S/D inhibit flag is turned off, with the result that it becomes possible to apply the S/D mode. 
     When the SOC is increased during shutdown control and becomes greater than reference value S 1 , the S/D inhibit flag is turned on, and thereby, the shutdown control is stopped. Consequently, battery  150  can be prevented from being overcharged. Furthermore, by setting the SOC reducing request to be on, the SOC can be reduced such that shutdown control can be resumed. 
     In this way, according to the running control of the hybrid vehicle in the fourth embodiment, in the case where second MG  120  is brought into a power generation state when the S/D mode is applied, the shutdown control can be prevented from being started when the SOC is relatively high. Therefore, it becomes possible to prevent battery  150  from being overcharged and to apply shutdown control for second MG  120 . 
     Furthermore, when the SOC is relatively high, the output distribution is controlled to reduce the SOC, so that the opportunity of applying shutdown control can be ensured. 
     As has been confirmatively described in the first to fourth embodiments and modifications thereof, in the driveline of the hybrid vehicle, running control of the hybrid vehicle according to the present embodiment can be applied also to the configuration different from that of the hybrid vehicle illustrated in  FIG. 1 . Specifically, including a parallel-type hybrid vehicle, the present invention can be applied to any configuration as long as the driving force (torque of the drive shaft) for the entire vehicle is generated by the sum of the direct torque mechanically transmitted from the engine to the drive shaft and the output torque of the electric motor. In other words, irrespective of the number of electric motors (motor generators) to be disposed and the configuration of the power transmission device, running control according to the first to fourth embodiments and modifications thereof can be applied so as to implement shutdown of a power converter (stop switching) for driving the electric motor whose output torque is set at zero. 
     It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to a hybrid vehicle including an engine and a traction motor each as a driving force source. 
     REFERENCE SIGNS LIST 
       100  engine,  102  crankshaft,  112 ,  122  neutral point,  130  power split device,  131  sun gear,  132  ring gear,  133  pinion gear,  134  carrier,  135  ring gear shaft (drive shaft),  140  reduction gear,  150  battery,  152  battery sensor,  160  driving wheel,  180  voltage sensor,  200  converter,  210 ,  210  inverter,  300  operation line,  310  equal-power line,  320 ,  330  map (drag torque), Acc accelerator pedal position, F 1 , F 2  fuel consumption estimate value, GL ground line, N 0  reference rotation speed (second MG power generation state), NE 1  engine target rotation speed (S/D mode), NE 2  engine target rotation speed (normal running mode), Ne engine rotation speed, Ne* engine target rotation speed, Nmg 1 * target rotation speed (first MG), Nmg 1  first MG rotation speed, Nmg 2  second MG rotation speed, Nr drive shaft rotation speed, P 1 , P 2  engine operating point, PL power line, Pchg charge/discharge request power, Pe engine requesting power, Pl loss reduction power (S/D mode), S 1 , S 2  reference value (SOC), Te engine torque, Tep engine direct torque, Tm, Tme, Tmh drag torque, Tmg 1  torque target value (first MG), Tp* total torque (requested driving force), V vehicle speed.