Patent Publication Number: US-2020298854-A1

Title: Hybrid vehicle and method for controlling hybrid vehicle

Description:
This nonprovisional application is based on Japanese Patent Application No. 2019-053049 filed with the Japan Patent Office on Mar. 20, 2019, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Field 
     The present disclosure relates to control of a hybrid vehicle incorporating a motor and an engine including a forced induction device as drive sources. 
     Description of the Background Art 
     A hybrid vehicle that incorporates a generator and an engine, includes a power storage charged by operations of the generator using motive power of the engine, and runs with motive power of the engine has conventionally been known. Some engines mounted on such a hybrid vehicle include a forced induction device such as a turbo charger. 
     For example, Japanese Patent Laying-Open No. 2015-58924 discloses a hybrid vehicle incorporating a motor, a generator, and an engine including a forced induction device. 
     SUMMARY 
     In the hybrid vehicle described above, when a rotation speed of the engine is controlled by using torque generated by the generator, estimation of engine torque may be required. In such a case, for example, engine torque is estimated in consideration of response delay such as first-order delay behind issuance of an output command to the engine. In the engine including a forced induction device, however, responsiveness of engine torque is different between a forced induction range in which forced induction by the forced induction device is performed and a non-forced induction range and hence engine torque may not accurately be estimated when response delay is taken into consideration similarly in both of these regions. 
     An object of the present disclosure is to provide a hybrid vehicle that accurately estimates engine torque in accordance with a state of forced induction by a forced induction device and a method of controlling a hybrid vehicle. 
     A hybrid vehicle according to one aspect of the present disclosure includes an engine including a forced induction device, a motor generator that generates electric power by using motive power of the engine, a power divider that divides motive power output from the engine into motive power to be transmitted to the motor generator and motive power to be transmitted to a drive wheel, and a controller that carries out torque control of the motor generator for setting a rotation speed of the engine to a target value by using engine torque estimated in consideration of responsiveness to an output command to the engine. The controller sets a time constant that determines responsiveness differently between a forced induction range in which forced induction by the forced induction device is performed and a non-forced induction range. 
     By doing so, the time constant that determines responsiveness is set differently between the forced induction range and the non-forced induction range and hence an appropriate time constant can be set in accordance with a state of forced induction by the forced induction device. Therefore, engine torque can accurately be estimated in each of the forced induction range and the non-forced induction range. Accuracy in torque control of the motor generator can thus be improved. 
     In one embodiment, the controller changes the time constant such that a value thereof in the forced induction range is greater than a value thereof in the non-forced induction range. 
     By doing so, an appropriate time constant can be set in each of the forced induction range and the non-forced induction range. Therefore, engine torque can accurately be estimated in each of the forced induction range and the non-forced induction range. 
     Furthermore, in one embodiment, the hybrid vehicle further includes a detector that detects an atmospheric pressure. When the engine torque exceeds a threshold value, the controller determines the engine torque as being in the forced induction range. When the engine torque is lower than the threshold value, the controller determines the engine torque as being in the non-forced induction range. The controller sets the threshold value such that the threshold value when the atmospheric pressure is low is smaller than the threshold value when the atmospheric pressure is high. 
     By doing so, even when responsiveness of engine torque is varied with variation in atmospheric pressure, the time constant can be changed in accordance with the state of forced induction by the forced induction device. Therefore, engine torque can accurately be estimated in each of the forced induction range and the non-forced induction range. 
     A method of controlling a hybrid vehicle according to another aspect of the present disclosure is a method of controlling a hybrid vehicle, the hybrid vehicle including an engine including a forced induction device, a motor generator that generates electric power by using motive power of the engine, and a power divider that divides motive power output from the engine into motive power to be transmitted to the motor generator and motive power to be transmitted to a drive wheel. The method includes carrying out torque control of the motor generator for setting a rotation speed of the engine to a target value by using engine torque estimated in consideration of responsiveness to an output command to the engine and setting a time constant that determines responsiveness differently between a forced induction range in which forced induction by the forced induction device is performed and a non-forced induction range. 
     The foregoing and other objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of the present disclosure when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an exemplary configuration of a drive system of a hybrid vehicle. 
         FIG. 2  is a diagram showing an exemplary configuration of an engine including a turbo charger. 
         FIG. 3  is a block diagram showing an exemplary configuration of a controller. 
         FIG. 4  is a flowchart showing exemplary processing in coordinated control in the hybrid vehicle. 
         FIG. 5  is a diagram for illustrating setting of an operating point on a predetermined operating line. 
         FIG. 6  is a block diagram for illustrating a method of setting a torque command value for a first MG. 
         FIG. 7  is a diagram for illustrating a method of calculating estimated engine torque. 
         FIG. 8  is a flowchart showing exemplary processing for outputting a first MG torque command performed by an HV-ECU. 
         FIG. 9  is a diagram for illustrating an exemplary operation by the HV-ECU. 
         FIG. 10  is a flowchart showing exemplary processing for outputting the first MG torque command performed by the HV-ECU in a modification. 
         FIG. 11  is a diagram for illustrating an exemplary operation by the HV-ECU in the modification. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present disclosure will be described in detail below with reference to the drawings. The same or corresponding elements in the drawings have the same reference characters allotted and description thereof will not be repeated. 
     &lt;As to Drive System of Hybrid Vehicle&gt; 
       FIG. 1  is a diagram showing an exemplary configuration of a drive system of a hybrid vehicle (which is simply denoted as a vehicle below)  10 . As shown in  FIG. 1 , vehicle  10  includes as a drive system, a controller  11  as well as an engine  13 , a first motor generator (which is denoted as a first MG below)  14 , and a second motor generator (which is denoted as a second MG below)  15  that serve as motive power sources for running. Engine  13  includes a turbo charger  47  representing one example of a forced induction device. First MG  14  and second MG  15  each perform a function as a motor that outputs torque by being supplied with driving electric power and a function as a generator that generates electric power by being supplied with torque. An alternating current (AC) rotating electric machine is employed for first MG  14  and second MG  15 . The AC rotating electric machine includes, for example, a permanent magnet synchronous motor including a rotor having a permanent magnet embedded. 
     First MG  14  and second MG  15  are electrically connected to a battery  18  with a power control unit (PCU)  81  being interposed. PCU  81  includes a first inverter  16  that supplies and receives electric power to and from first MG  14 , a second inverter  17  that supplies and receives electric power to and from second MG  15 , battery  18 , and a converter  83  that supplies and receives electric power to and from first inverter  16  and second inverter  17 . 
     For example, converter  83  can up-convert electric power from battery  18  and supply up-converted electric power to first inverter  16  or second inverter  17 . Alternatively, converter  83  can down-convert electric power supplied from first inverter  16  or second inverter  17  and supply down-converted electric power to battery  18 . 
     First inverter  16  can convert direct current (DC) power from converter  83  into AC power and supply AC power to first MG  14 . Alternatively, first inverter  16  can convert AC power from first MG  14  into DC power and supply DC power to converter  83 . 
     Second inverter  17  can convert DC power from converter  83  into AC power and supply AC power to second MG  15 . Alternatively, second inverter  17  can convert AC power from second MG  15  into DC power and supply DC power to converter  83 . 
     PCU  81  charges battery  18  with electric power generated by first MG  14  or second MG  15  or drives first MG  14  or second MG  15  with electric power from battery  18 . 
     Battery  18  includes, for example, a lithium ion secondary battery or a nickel metal hydride secondary battery. The lithium ion secondary battery is a secondary battery in which lithium is adopted as a charge carrier, and may include not only a general lithium ion secondary battery containing a liquid electrolyte but also what is called an all-solid-state battery containing a solid electrolyte. Battery  18  should only be a power storage that is at least rechargeable, and for example, an electric double layer capacitor may be employed instead of the secondary battery. 
     Engine  13  and first MG  14  are coupled to a planetary gear mechanism  20 . Planetary gear mechanism  20  transmits drive torque output from engine  13  by dividing drive torque into drive torque to first MG  14  and drive torque to an output gear  21 , and represents an exemplary power divider in the embodiment of the present disclosure. Planetary gear mechanism  20  includes a single-pinion planetary gear mechanism and is arranged on an axis Cnt coaxial with an output shaft  22  of engine  13 . 
     Planetary gear mechanism  20  includes a sun gear S, a ring gear R arranged coaxially with sun gear S, a pinion gear P meshed with sun gear S and ring gear R, and a carrier C holding pinion gear P in a rotatable and revolvable manner. Output shaft  22  is coupled to carrier C. A rotor shaft  23  of first MG  14  is coupled to sun gear S. Ring gear R is coupled to output gear  21 . Output gear  21  represents one of output elements for transmitting drive torque to a drive wheel  24 . 
     In planetary gear mechanism  20 , carrier C to which drive torque output from engine  13  is transmitted serves as an input element, ring gear R that outputs drive torque to output gear  21  serves as an output element, and sun gear S to which rotor shaft  23  is coupled serves as a reaction force element. Planetary gear mechanism  20  divides motive power output from engine  13  into motive power on a side of first MG  14  and motive power on a side of output gear  21 . First MG  14  is controlled to output torque in accordance with an engine rotation speed. 
     A countershaft  25  is arranged in parallel to axis Cnt. Countershaft  25  is attached to a driven gear  26  meshed with output gear  21 . A drive gear  27  is attached to countershaft  25 , and drive gear  27  is meshed with a ring gear  29  in a differential gear  28  representing a final reduction gear. A drive gear  31  attached to a rotor shaft  30  in second MG  15  is meshed with driven gear  26 . Therefore, drive torque output from second MG  15  is added to drive torque output from output gear  21  in a part of driven gear  26 . Drive torque thus combined is transmitted to drive wheel  24  with driveshafts  32  and  33  extending laterally from differential gear  28  being interposed. As drive torque is transmitted to drive wheel  24 , driving force is generated in vehicle  10 . 
     A mechanical oil pump (which is denoted as an MOP below)  36  is provided coaxially with output shaft  22 . MOP  36  delivers lubricating oil with a cooling function, for example, to planetary gear mechanism  20 , first MG  14 , second MG  15 , and differential gear  28 . Vehicle  10  further includes an electric oil pump (which is denoted as an EOP below)  38 . EOP  38  is driven by electric power supplied from battery  18  when operation of engine  13  is stopped, and it delivers lubricating oil to planetary gear mechanism  20 , first MG  14 , second MG  15 , and differential gear  28  in a manner the same as or similar to MOP  36 . 
     &lt;As to Configuration of Engine&gt; 
       FIG. 2  is a diagram showing an exemplary configuration of engine  13  including turbo charger  47 . Engine  13  is, for example, an in-line four-cylinder spark ignition internal combustion engine. As shown in  FIG. 2 , engine  13  includes, for example, an engine main body  40  formed with four cylinders  40   a,    40   b,    40   c,  and  40   d  being aligned in one direction. 
     One ends of intake ports and one ends of exhaust ports formed in engine main body  40  are connected to cylinders  40   a,    40   b,    40   c,  and  40   d.  One end of the intake port is opened and closed by two intake valves  43  provided in each of cylinders  40   a,    40   b,    40   c,  and  40   d,  and one end of the exhaust port is opened and closed by two exhaust valves  44  provided in each of cylinders  40   a,    40   b,    40   c  and  40   d.  The other ends of the intake ports of cylinders  40   a,    40   b,    40   c,  and  40   d  are connected to an intake manifold  46 . The other ends of the exhaust ports of cylinders  40   a,    40   b,    40   c,  and  40   d  are connected to an exhaust manifold  52 . 
     In the present embodiment, engine  13  is, for example, a direct injection engine and fuel is injected into each of cylinders  40   a,    40   b,    40   c,  and  40   d  by a fuel injector (not shown) provided at the top of each cylinder. An air fuel mixture of fuel and intake air in cylinders  40   a,    40   b,    40   c,  and  40   d  is ignited by an ignition plug  45  provided in each of cylinders  40   a,    40   b,    40   c,  and  40   d.    
       FIG. 2  shows intake valve  43 , exhaust valve  44 , and ignition plug  45  provided in cylinder  40   a  and does not show intake valve  43 , exhaust valve  44 , and ignition plug  45  provided in other cylinders  40   b,    40   c,  and  40   d.    
     Engine  13  is provided with turbo charger  47  that uses exhaust energy to boost suctioned air. Turbo charger  47  includes a compressor  48  and a turbine  53 . 
     An intake air passage  41  has one end connected to intake manifold  46  and the other end connected to an air inlet. Compressor  48  is provided at a prescribed position in intake air passage  41 . An air flow meter  50  that outputs a signal in accordance with a flow rate of air that flows through intake air passage  41  to controller  11  is provided between the other end (air inlet) of intake air passage  41  and compressor  48 . An intercooler  51  that cools intake air pressurized by compressor  48  is disposed in intake air passage  41  provided downstream from compressor  48 . An intake throttle valve (throttle valve)  49  that can regulate a flow rate of intake air that flows through intake air passage  41  is provided between intercooler  51  and one end of intake air passage  41 . 
     An exhaust passage  42  has one end connected to exhaust manifold  52  and the other end connected to a muffler (not shown). Turbine  53  is provided at a prescribed position in exhaust passage  42 . In exhaust passage  42 , a bypass passage  54  that bypasses exhaust upstream from turbine  53  to a portion downstream from turbine  53  and a waste gate valve  55  provided in the bypass passage and capable of regulating a flow rate of exhaust guided to turbine  53  are provided. Therefore, a flow rate of exhaust that flows into turbine  53 , that is, a boost pressure of suctioned air, is regulated by controlling a position of waste gate valve  55 . Exhaust that passes through turbine  53  or waste gate valve  55  is purified by a start-up converter  56  and an aftertreatment apparatus  57  provided at prescribed positions in exhaust passage  42 , and thereafter emitted into the atmosphere. Aftertreatment apparatus  57  contains, for example, a three-way catalyst. 
     Engine  13  is provided with an exhaust gas recirculation (EGR) apparatus  58  that has exhaust flow into intake air passage  41 . EGR apparatus  58  includes an EGR passage  59 , an EGR valve  60 , and an EGR cooler  61 . EGR passage  59  allows some of exhaust to be taken out of exhaust passage  42  as EGR gas and guides EGR gas to intake air passage  41 . EGR valve  60  regulates a flow rate of EGR gas that flows through EGR passage  59 . EGR cooler  61  cools EGR gas that flows through EGR passage  59 . EGR passage  59  connects a portion of exhaust passage  42  between start-up converter  56  and aftertreatment apparatus  57  to a portion of intake air passage  41  between compressor  48  and air flow meter  50 . 
     &lt;As to Configuration of Controller&gt; 
       FIG. 3  is a block diagram showing an exemplary configuration of controller  11 . As shown in  FIG. 3 , controller  11  includes a hybrid vehicle (HV)-electronic control unit (ECU)  62 , an MG-ECU  63 , and an engine ECU  64 . 
     HV-ECU  62  is a controller that controls engine  13 , first MG  14 , and second MG  15  in coordination. MG-ECU  63  is a controller that controls an operation by PCU  81 . Engine ECU  64  is a controller that controls an operation by engine  13 . 
     HV-ECU  62 , MG-ECU  63 , and engine ECU  64  each include an input and output apparatus that supplies and receives signals to and from various sensors and other ECUs that are connected, a storage that serves for storage of various control programs or maps (including a read only memory (ROM) and a random access memory (RAM)), a central processing unit (CPU) that executes a control program, and a counter that counts time. 
     Though  FIG. 3  illustrates a configuration in which HV-ECU  62 , MG-ECU  63 , and engine ECU  64  are separately provided by way of example, the ECUs may be integrated as a single ECU. 
     A vehicle speed sensor  66 , an accelerator position sensor  67 , a first MG rotation speed sensor  68 , a second MG rotation speed sensor  69 , an engine rotation speed sensor  70 , a turbine rotation speed sensor  71 , a boost pressure sensor  72 , a battery monitoring unit  73 , a first MG temperature sensor  74 , a second MG temperature sensor  75 , a first INV temperature sensor  76 , a second INV temperature sensor  77 , a catalyst temperature sensor  78 , a turbine temperature sensor  79 , an atmospheric pressure sensor  90 , and air flow meter  50  are connected to HV-ECU  62 . 
     Vehicle speed sensor  66  detects a speed of vehicle  10  (vehicle speed). Accelerator position sensor  67  detects an amount of pressing of an accelerator pedal (accelerator position). First MG rotation speed sensor  68  detects a rotation speed of first MG  14 . Second MG rotation speed sensor  69  detects a rotation speed of second MG  15 . Engine rotation speed sensor  70  detects a rotation speed of output shaft  22  of engine  13  (engine rotation speed). Turbine rotation speed sensor  71  detects a rotation speed of turbine  53  of turbo charger  47 . Boost pressure sensor  72  detects a boost pressure of engine  13 . First MG temperature sensor  74  detects an internal temperature of first MG  14  such as a temperature associated with a coil or a magnet. Second MG temperature sensor  75  detects an internal temperature of second MG  15  such as a temperature associated with a coil or a magnet. First INV temperature sensor  76  detects a temperature of first inverter  16  such as a temperature associated with a switching element. Second INV temperature sensor  77  detects a temperature of second inverter  17  such as a temperature associated with a switching element. Catalyst temperature sensor  78  detects a temperature of aftertreatment apparatus  57 . Turbine temperature sensor  79  detects a temperature of turbine  53 . Atmospheric pressure sensor  90  detects an atmospheric pressure. Various sensors described above output signals indicating results of detection to HV-ECU  62 . 
     Battery monitoring unit  73  obtains a state of charge (SOC) representing a ratio of a remaining amount of charge to a full charge capacity of battery  18  and outputs a signal indicating the obtained SOC to HV-ECU  62 . 
     Battery monitoring unit  73  includes, for example, a sensor that detects a current, a voltage, and a temperature of battery  18 . Battery monitoring unit  73  obtains an SOC by calculating the SOC based on the detected current, voltage, and temperature of battery  18 . 
     Various known approaches such as an approach by accumulation of current values (coulomb counting) or an approach by estimation of an open circuit voltage (OCV) can be adopted as a method of calculating an SOC. &lt;As to Control of Running of Vehicle&gt; 
     Vehicle  10  configured as above can be set or switched to such a running mode as a hybrid (HV) running mode in which engine  13  and second MG  15  serve as motive power sources and an electric (EV) running mode in which the vehicle runs with engine  13  remaining stopped and second MG  15  being driven by electric power stored in battery  18 . Setting of and switching to each mode is made by HV-ECU  62 . HV-ECU  62  controls engine  13 , first MG  14 , and second MG  15  based on the set or switched running mode. 
     The EV running mode is selected, for example, in a low-load operation region where a vehicle speed is low and requested driving force is low, and refers to a running mode in which an operation by engine  13  is stopped and second MG  15  outputs driving force. 
     The HV running mode is selected in a high-load operation region where a vehicle speed is high and requested driving force is high, and refers to a running mode in which combined torque of drive torque of engine  13  and drive torque of second MG  15  is output. 
     In the HV running mode, in transmitting drive torque output from engine  13  to drive wheel  24 , first MG  14  applies reaction force to planetary gear mechanism  20 . Therefore, sun gear S functions as a reaction force element. In other words, in order to apply engine torque to drive wheel  24 , first MG  14  is controlled to output reaction torque against engine torque. In this case, regenerative control in which first MG  14  functions as a generator can be carried out. 
     HV-ECU  62  further transmits a control signal C 3  based on an operation state including the running mode to EOP  38  and controls drive of EOP  38 . For example, when engine torque corresponding to a set operating point exceeds a threshold value, HV-ECU  62  requests of engine ECU  64  to increase a boost pressure. Though an example in which the threshold value is constant regardless of variation in engine rotation speed is described by way of example in the present embodiment, the threshold value may be set to vary with the engine rotation speed. For example, when the engine rotation speed is in a high rotation speed range, a threshold value smaller than in a low rotation speed range may be set. 
     Control of engine  13 , first MG  14 , and second MG  15  in coordination while vehicle  10  operates will be described below with reference to  FIG. 4 .  FIG. 4  is a flowchart showing exemplary processing in coordinated control in the hybrid vehicle. 
     In a step (a step being denoted as S below)  100 , HV-ECU  62  calculates requested system power. 
     Specifically, HV-ECU  62  calculates requested driving force based on an accelerator position determined by an amount of pressing of the accelerator pedal. HV-ECU  62  calculates requested running power of vehicle  10  based on the calculated requested driving force and a vehicle speed. HV-ECU  62  calculates a value resulting from addition of requested charging and discharging power of battery  18  to requested running power as requested system power. Requested charging and discharging power of battery  18  is set, for example, in accordance with a difference from an SOC of battery  18  and a predetermined control central value. 
     In S 102 , HV-ECU  62  determines whether or not activation of engine  13  has been requested in accordance with calculated requested system power. HV-ECU  62  determines that activation of engine  13  has been requested, for example, when requested system power exceeds a threshold value. 
     When activation of engine  13  has been requested, HV-ECU  62  sets the HV running mode as the running mode. When activation of engine  13  has not been requested, HV-ECU  62  sets the EV running mode as the running mode. 
     When it is determined that activation of engine  13  has been requested (YES in S 102 ), the process proceeds to S 104 . Otherwise (NO in S 102 ), the process proceeds to S 112 . 
     In S 104 , HV-ECU  62  calculates power requested of engine  13  (which is denoted as requested engine power below). For example, HV-ECU  62  calculates requested system power as requested engine power. For example, when requested system power exceeds an upper limit value of requested engine power, HV-ECU  62  calculates the upper limit value of requested engine power as requested engine power. 
     In S 106 , HV-ECU  62  outputs calculated requested engine power as an engine operation state command to engine ECU  64 . 
     Engine ECU  64  transmits a control signal C 2  based on the engine operation state command input from HV-ECU  62  and variously controls each component of engine  13  such as intake throttle valve  49 , ignition plug  45 , waste gate valve  55 , and EGR valve  60 . 
     In S 108 , HV-ECU  62  sets based on calculated requested engine power, an operating point of engine  13  on a predetermined operating line set in a coordinate system defined by an engine rotation speed and engine torque. 
     Specifically, HV-ECU  62  sets, for example, an intersection between an equal power line equal in output to requested engine power in the coordinate system and the predetermined operating line as the operating point of engine  13 . 
     The predetermined operating line represents a trace of variation in engine torque with variation in engine rotation speed in the coordinate system, and it is set, for example, by adapting the trace of variation in engine torque high in fuel efficiency through experiments. 
       FIG. 5  is a diagram for illustrating setting of an operating point on a predetermined operating line. The ordinate in  FIG. 5  represents engine torque. The abscissa in  FIG. 5  represents an engine rotation speed.  FIG. 5  shows a predetermined operating line LN 1  (a solid line).  FIG. 5  shows an equal power line LN 2  (a dashed line) of requested engine power calculated in S 104 . 
     In this case, HV-ECU  62  sets as the operating point, an intersection A between the predetermined operating line (LN 1  in  FIG. 5 ) and the equal power line (LN 2  in  FIG. 5 ) of requested engine power. Specifically, intersection A at which the engine rotation speed attains to Ne( 0 ) and engine torque attains to Tq( 1 ) in a coordinate plane of engine torque and the engine rotation speed is set as the operating point. 
     In S 110 , HV-ECU  62  sets the engine rotation speed corresponding to the set operating point as a target engine rotation speed. In the example shown in  FIG. 5 , engine rotation speed Ne( 0 ) corresponding to intersection A set as the operating point is set as the target engine rotation speed. 
     In S 112 , HV-ECU  62  outputs a first MG torque command. Specifically, HV-ECU  62  sets a torque command value for first MG  14  for setting a current engine rotation speed to the set target engine rotation speed. For example, HV-ECU  62  sets as the torque command value for first MG  14 , a sum of first torque of first MG  14  for maintaining the current engine rotation speed and second torque of first MG  14  for changing the current engine rotation speed to the target engine rotation speed. More specifically, HV-ECU  62  sets as the torque command value for first MG  14 , for example, the sum of first torque calculated through feedforward control based on an estimated value of engine torque (which is denoted as estimated engine torque below) and second torque calculated through feedback control based on a difference between the current engine rotation speed and the target engine rotation speed. HV-ECU  62  outputs the set torque command value for first MG  14  as the first MG torque command to MG-ECU  63 . Details of a method of setting a torque command value for first MG 14  will be described later. When it is determined that a request for activation of engine  13  has not been issued (NO in S 102 ), HV-ECU  62  outputs the first MG torque command corresponding to a state that engine  13  is off. 
     In S 114 , HV-ECU  62  outputs a second MG torque command. Specifically, HV-ECU  62  calculates engine torque to be transmitted to drive wheel  24  based on the set torque command value for first MG  14  and a gear ratio of each rotary element of planetary gear mechanism  20  and sets a torque command value for second MG  15  so as to fulfill requested driving force. HV-ECU  62  outputs the set torque command value for second MG  15  as a second MG torque command to MG-ECU  63 . 
     MG-ECU  63  calculates a current value corresponding to torque to be generated by first MG  14  and second MG  15  and a frequency thereof based on the first MG torque command and the second MG torque command input from HV-ECU  62 , and outputs a control signal C  1  including the calculated current value and the frequency thereof to PCU  81 . Torque of first MG  14  and torque of second MG  15  are thus controlled. 
     &lt;As to Setting of Torque Command Value for First MG  14 &gt; 
       FIG. 6  is a block diagram for illustrating a method of setting a torque command value for first MG  14 . As shown in  FIG. 6 , HV-ECU  62  sets as the torque command value for first MG  14 , the sum of a feedforward term Tgff (corresponding to first torque described above) and a feedback term Tgfb (corresponding to second torque described above) in torque control of first MG  14 . 
     HV-ECU  62  calculates, for example, estimated engine torque, converts calculated estimated engine torque into torque to be applied to the output shaft of first MG  14 , and calculates torque that cancels converted torque as feedforward term Tgff. 
     HV-ECU  62  calculates estimated engine torque, for example, in consideration of requested engine power, the target engine rotation speed set in S 110 , and response delay of engine torque. A method of calculating estimated engine torque will be described later. 
     HV-ECU  62  further calculates, for example, a difference between a target rotation speed of first MG  14  and the rotation speed of first MG  14 , and calculates feedback term Tgfb based on the calculated difference (for example, through PI control). 
     HV-ECU  62  calculates the target rotation speed of first MG  14  based on a rotation speed of second MG  15  or a vehicle speed, a target engine rotation speed (a rotation speed of carrier C), and a gear ratio among rotary elements of planetary gear mechanism  20 . 
     &lt;As to Calculation of Estimated Engine Torque&gt; 
     HV-ECU  62  calculates estimated engine torque in consideration of response delay expressed by a certain dead time and a time constant of first-order delay for engine torque calculated by dividing requested engine power by the target engine rotation speed. 
       FIG. 7  is a diagram for illustrating a method of calculating estimated engine torque. The ordinate in  FIG. 7  represents engine power and engine torque. The abscissa in  FIG. 7  represents time.  FIG. 7  shows variation in requested engine power LN 1  (a solid line).  FIG. 7  shows variation in engine torque without taking into account response delay LN 2  (a solid line).  FIG. 7  shows variation in estimated engine torque in consideration of response delay LN 3  (a dashed line). 
     As shown with LN 1  in  FIG. 7 , for instance, an example in which requested engine power is constant is assumed. When the engine rotation speed is also assumed as being constant, engine torque is also constant. 
     When requested engine power is assumed to increase stepwise by a prescribed amount and attains to Pe( 0 ) at time t( 0 ), without response delay being taken into account, engine torque attains to a value Te( 0 ) calculated by dividing requested engine power Pe( 0 ) by the engine rotation speed at time t( 0 ) as shown with LN 2  in  FIG. 7 . 
     Actual variation in engine torque, however, is increase with delay behind increase in requested engine power. Therefore, as shown with LN 3  in  FIG. 7 , HV-ECU  62  calculates estimated engine torque in consideration of response delay expressed by a certain dead time and the time constant of first-order delay for variation in requested engine power. 
     In the example shown in  FIG. 7 , HV-ECU  62  calculates estimated engine torque at the current time point on the assumption that increase in engine torque starts from time t( 1 ) after lapse of the certain dead time since time t( 0 ) at which increase in requested engine power started and engine torque varies at the set time constant. By thus taking into account response delay of engine torque, engine torque can accurately be estimated. 
     In vehicle  10  including turbo charger  47  configured as above, in controlling torque of first MG  14 , calculation of estimated engine torque is required for calculating feedforward term Tgff described above. In such a case, engine torque can accurately be estimated by taking into account response delay of engine torque as described above. 
     In engine  13  including turbo charger  47 , however, responsiveness of engine torque is different between the forced induction range in which forced induction by turbo charger  47  is performed and the non-forced induction range and hence engine torque may not accurately be estimated when response delay is taken into account similarly in both of these regions. 
     In the present embodiment, HV-ECU  62  sets the time constant that determines responsiveness to requested engine power representing an output command, differently between the forced induction range in which forced induction by turbo charger  47  is performed and the non-forced induction range. More specifically, HV-ECU  62  changes the time constant such that a value thereof in the forced induction range where forced induction is performed is greater than a value thereof in the non-forced induction range. 
     By doing so, an appropriate time constant can be set in each of the forced induction range and the non-forced induction range. Therefore, engine torque can accurately be estimated in each of the forced induction range and the non-forced induction range. 
     &lt;As to Processing Performed by HV-ECU  62 &gt; 
     Processing for outputting a first MG torque command performed by HV-ECU  62  will be described below with reference to  FIG. 8 .  FIG. 8  is a flowchart showing exemplary processing for outputting a first MG torque command performed by HV-ECU  62 . 
     In S 200 , HV-ECU  62  calculates estimated engine torque based on the time constant set in previous calculation (which is denoted as the previous value of the time constant below). Since the calculation method is as described above, detailed description thereof will not be repeated. 
     In S 202 , HV-ECU  62  determines whether or not engine torque is in the forced induction range. HV-ECU  62  may determine engine torque as being in the forced induction range, for example, when calculated estimated engine torque is higher than a threshold value. The threshold value is a value for determining whether engine torque is in the forced induction range or the non-forced induction range (a natural aspiration range), and adapted, for example, through experiments. The threshold value may be predetermined, or set, for example, in accordance with an engine rotation speed. The threshold value may be set, for example, such that a value thereof when the engine rotation speed is high is smaller than a value thereof when the engine rotation speed is low. When engine torque is determined as being in the forced induction range (YES in S 202 ), the process proceeds to S 204 . 
     In S 204 , HV-ECU  62  sets a first value corresponding to the forced induction range as the time constant. The first value representing the time constant corresponding to the forced induction range is, for example, a predetermined value adapted through experiments. When engine torque is determined as not being in the forced induction range (that is, in the non-forced induction range) (NO in S 202 ), the process proceeds to S 206 . 
     In S 206 , HV-ECU  62  sets a second value corresponding to non-forced induction as the time constant. The second value representing the time constant corresponding to the non-forced induction range is, for example, a predetermined value adapted through experiments and smaller than the first value. 
     In S 208 , HV-ECU  62  calculates feedforward term Tgff. Specifically, HV-ECU  62  calculates estimated engine torque based on the set time constant, converts calculated estimated engine torque into torque to be applied to the rotation shaft of first MG  14 , and calculates first torque that cancels converted torque as feedforward term Tgff. When the set time constant is equal to the previous value of the time constant, feedforward term Tgff may be calculated based on estimated engine torque calculated in S 200 . 
     In S 210 , HV-ECU  62  calculates feedback term Tgfb. Since the method of calculating feedback term Tgfb is as described above, detailed description thereof will not be repeated. 
     In S 212 , HV-ECU  62  calculates the torque command value for first MG  14 . HV-ECU  62  calculates the sum of feedforward term Tgff and feedback term Tgfb as the torque command value for first MG  14 . 
     In S 214 , HV-ECU  62  outputs the calculated torque command value for first MG  14  as the first MG torque command to MG-ECU  63 . 
     &lt;As to Exemplary Operation by HV-ECU  62 &gt; 
     An operation by HV-ECU  62  according to the present embodiment based on the structure and the flowchart as set forth above will be described with reference to  FIG. 9 .  FIG. 9  is a diagram for illustrating an exemplary operation by HV-ECU  62 . The ordinate in  FIG. 9  represents engine torque. The abscissa in  FIG. 9  represents time.  FIG. 9  shows variation in estimated engine torque LN 4 . Requested engine power is assumed to increase stepwise by a prescribed amount and attain to Pe( 0 ) at the time point of time t( 0 ), and thereafter remain constant as seen in variation in requested engine power shown with LN 1  in  FIG. 7  for the sake of convenience of description. 
     When requested system power is calculated (S 100 ) and when it is determined that a request for activation of engine  13  has been issued as calculated requested system power has exceeded the threshold value (YES in S 102 ), requested engine power is calculated (S 104 ) and calculated requested engine power is output to engine ECU  64  as the engine operation state command (S 106 ). Then, the intersection between the predetermined operating point and the equal power line of requested engine power is set as the operating point on the predetermined operating line (S 108 ) and the engine rotation speed corresponding to the set operating point is set as the target engine rotation speed (S 110 ). 
     Estimated engine torque is calculated based on requested system power, the target engine rotation speed, and the previous value of the time constant (S 200 ). When calculated estimated engine torque is equal to or smaller than a threshold value Te( 1 ), engine torque is determined as being in the non-forced induction range (NO in S 202 ) and the second value corresponding to the non-forced induction range is set as the time constant (S 206 ). 
     For example, when requested engine power increases by a prescribed amount at time t( 0 ), estimated engine torque is calculated to start increase at time t( 1 ) after lapse of the dead time since time t( 0 ) as shown with LN 4  in  FIG. 9 . During a period until time t( 2 ) at which estimated engine torque exceeds threshold value Te( 1 ), estimated engine torque increases in a manner of variation of first-order delay with the second value being set as the time constant. 
     When estimated engine torque exceeds threshold value Te( 1 ) at time t( 2 ), engine torque is determined as being in the forced induction range (YES in S 202 ) and the first value corresponding to the forced induction range is set as the time constant (S 204 ). 
     Therefore, when a state that requested engine power is at Pe( 0 ) continues from time t( 2 ), estimated engine torque increases in a manner of variation of first-order delay with the first value being set as the time constant as shown with LN 4  in  FIG. 9 . 
     When estimated engine torque is calculated, feedforward term Tgff is calculated (S 208 ) based on calculated estimated engine torque and feedback term Tgfb is calculated based on a difference between the target rotation speed of first MG  14  and the first MG rotation speed (S 210 ). 
     The sum of calculated feedforward term Tgff and feedback term Tgfb is calculated as the torque command value for first MG  14  (S 212 ), and the first MG torque command is output to MG-ECU  63  (S 112  and S 214 ) and the second MG torque command is output (S 114 ). 
     &lt;As to Function and Effect&gt; 
     As set forth above, according to the hybrid vehicle in the present embodiment, the time constant when engine torque is in the forced induction range is set to be greater than the time constant when engine torque is in the non-forced induction range. Therefore, an appropriate time constant can be set in each of the forced induction range and the non-forced induction range. Therefore, engine torque can accurately be estimated in each of the forced induction range and the non-forced induction range. Accuracy in control of torque of first MG  14  can thus be improved. Therefore, a hybrid vehicle that accurately estimates engine torque in accordance with a state of forced induction by the forced induction device and a method of controlling a hybrid vehicle can be provided. 
     &lt;As to Modification&gt; 
     A modification will be described below. 
     Though intake throttle valve  49  is described as being provided between intercooler  51  and intake manifold  46  in the embodiment above, it may be provided, for example, in intake air passage  41  between compressor  48  and air flow meter  50 . 
     Though the turbo charger is described as an exemplary forced induction device in the embodiment above, the forced induction device is not particularly limited to the turbo charger but may be, for example, a supercharger. 
     Though a boost pressure is regulated by adjusting a position of waste gate valve  55  according to the description of the embodiment above, the boost pressure may be regulated, for example, by providing a motor generator in a shaft that couples compressor  48  and turbine  53  to each other and controlling a turbine rotation speed by means of the motor generator, or the boost pressure may be regulated by adjusting a gap (a vane position) between adjacent vanes among a plurality of vanes arranged around an outer circumference of a blade of turbine  53 . 
     Though torque of first MG  14  when the engine rotation speed is maintained (that is, when the current engine rotation speed is set as the target value) is calculated as feedforward term Tgff according to the description of the embodiment above, the target value is not limited to the current engine rotation speed but may be set to any value between the current engine rotation speed and the target engine rotation speed. 
     Though whether engine torque is in the forced induction range or the non-forced induction range is determined based on whether or not estimated engine torque calculated based on the previous value of the time constant is higher than the threshold value according to the description of the embodiment above, engine torque may be determined as being in the forced induction range when the boost pressure detected by boost pressure sensor  72  is higher than the threshold value and as being in the non-forced induction range when the detected boost pressure is equal to or lower than the threshold value. 
     Though whether engine torque is in the forced induction range or the non-forced induction range is determined based on whether or not estimated engine torque is higher than the threshold value according to the description of the embodiment above, the threshold value may be set in accordance with an atmospheric pressure because relation between a state of forced induction and generated engine torque may be varied by the atmospheric pressure, for example, when a vehicle runs at high altitude. 
     Processing performed by HV-ECU  62  in this modification will be described below with reference to  FIG. 10 .  FIG. 10  is a flowchart showing exemplary processing for outputting the first MG torque command performed by HV-ECU  62  in the modification. 
     The process in the flowchart in  FIG. 10  is different from the flowchart in  FIG. 8  in that processing in S 300  is performed after processing in S 200  and before processing in S 202 . Since the process is otherwise the same as the process described in the flowchart in  FIG. 8 , detailed description thereof will not be repeated. 
     In S 300 , HV-ECU  62  sets a threshold value in accordance with an atmospheric pressure detected by atmospheric pressure sensor  90 . HV-ECU  62  may set a threshold value, for example, based on the atmospheric pressure detected by atmospheric pressure sensor  90  and a prescribed map. The prescribed map shows relation between an atmospheric pressure and a threshold value, and has a boundary value between the forced induction range and the non-forced induction range when the atmospheric pressure is varied in experiments set as the threshold value. The prescribed map is created, for example, to set the threshold value such that the threshold value when the atmospheric pressure is low is smaller than the threshold value when the atmospheric pressure is high. 
     An operation by HV-ECU  62  in this embodiment will be described below with reference to  FIG. 11 .  FIG. 11  is a diagram for illustrating an exemplary operation by HV-ECU  62  in the modification. The ordinate in  FIG. 11  represents engine torque. The abscissa in  FIG. 11  represents time.  FIG. 11  shows variation in estimated engine torque LN 5 . Requested engine power is assumed to increase stepwise by a prescribed amount and attain to Pe( 0 ) at the time point of time t( 0 ) and thereafter remain constant as seen in variation in requested engine power shown with LN 1  in  FIG. 7  for the sake of convenience of description. Vehicle  10  running at high altitude (a condition of the low atmospheric pressure) is assumed in the example in  FIG. 11  as compared with the example shown in  FIG. 9 . 
     When the target engine rotation speed is set in accordance with requested system power (S 110 ), estimated engine torque is calculated based on requested system power, the target engine rotation speed, and the previous value of the time constant (S 200 ). Furthermore, a threshold value Te( 2 ) (&lt;Te( 1 )) is set based on the atmospheric pressure detected by atmospheric pressure sensor  90  (S 300 ). 
     When calculated estimated engine torque is equal to or lower than threshold value Te( 2 ), engine torque is determined as being in the non-forced induction range (NO in S 202 ) and the second value corresponding to the non-forced induction range is set as the time constant (S 206 ). 
     For example, when requested engine power increases by a prescribed amount at time t( 0 ), as shown with LN 5  in  FIG. 11 , estimated engine torque is calculated to start increase at time t( 1 ) after lapse of the dead time since time t( 0 ). During a period until time t( 2 ) at which estimated engine torque exceeds threshold value Te( 2 ), estimated engine torque increases in a manner of variation of first-order delay with the second value being set as the time constant. 
     When estimated engine torque exceeds threshold value Te( 2 ) at time t( 2 ), engine torque is determined as being in the forced induction range (YES in S 202 ) and the first value corresponding to the forced induction range is set as the time constant (S 204 ). 
     Therefore, when a state that requested engine power is at Pe( 0 ) continues from time t( 2 ), as shown with LN 5  in  FIG. 11 , estimated engine torque increases in a manner of variation of first-order delay with the first value being set as the time constant. 
     When estimated engine torque is calculated, feedforward term Tgff is calculated based on calculated estimated engine torque (S 208 ) and feedback term Tgfb is calculated based on a difference between the target rotation speed of first MG  14  and the first MG rotation speed (S 210 ). 
     The sum of calculated feedforward term Tgff and feedback term Tgfb is calculated as the torque command value for first MG  14  (S 212 ) and the first MG torque command is output to MG-ECU  63  (S 214 ). 
     By doing so, even though responsiveness of engine torque is varied with variation in atmospheric pressure, the time constant can be changed in accordance with the state of forced induction by turbo charger  47 . Therefore, engine torque can accurately be estimated in each of the forced induction range and the non-forced induction range. The threshold value for determining whether or not engine torque is in the forced induction range is not limited to the threshold value set in accordance with variation in atmospheric pressure as described above but may be set, for example, in accordance with a position of the EGR valve, timing to open and close the intake valve or the exhaust valve, or a state of operation such as an amount of lift. 
     The modification above may be carried out as being combined in its entirety or in part as appropriate. 
     Though an embodiment of the present invention has been described, it should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.