Patent Publication Number: US-2020298824-A1

Title: Hybrid vehicle

Description:
This nonprovisional application is based on Japanese Patent Application No. 2019-054763 filed with the Japan Patent Office on Mar. 22, 2019, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Field 
     The present disclosure relates to a hybrid vehicle, and more specifically to a hybrid vehicle including an internal combustion engine with a forced induction device. 
     Description of the Background Art 
     Japanese Patent Laying-Open No. 2015-058924 discloses a hybrid vehicle having mounted therein an internal combustion engine equipped with a turbo forced induction device, and a motor generator. 
     SUMMARY 
     The above vehicle, however, has a problem: on high land, a delay of a response of boost pressure of the forced induction device and hence a delay of a response of torque that the internal combustion engine generates are larger than on low land. 
     The present disclosure has been made to solve the above-described problem, and an object thereof is to provide a hybrid vehicle capable of reducing a delay of a response of torque generated by an internal combustion engine on high land. 
     According to the present disclosure, a hybrid vehicle comprises: an internal combustion engine; a rotating electric machine; a planetary gear mechanism to which the internal combustion engine, the rotating electric machine and an output shaft are connected; and a controller that controls the internal combustion engine and the rotating electric machine. The internal combustion engine includes a forced induction device that boosts suctioned air to be fed to the internal combustion engine. A boost line is determined on a map representing a relationship between a rotation speed of the internal combustion engine and torque generated by the internal combustion engine, and the forced induction device boosts suctioned air when the torque generated by the internal combustion engine indicated by an operating point on the map exceeds the boost line. The controller controls the internal combustion engine and the rotating electric machine to increase the rotation speed of the internal combustion engine before the torque generated by the internal combustion engine indicated by the operating point exceeds the boost line, and when the controller increases the rotation speed of the internal combustion engine, for lower atmospheric pressure the controller controls the internal combustion engine and the rotating electric machine to increase the rotation speed more than for higher atmospheric pressure. 
     According to this configuration, before the operating point exceeds the boost line, for lower atmospheric pressure the internal combustion engine&#39;s rotation speed is increased more than for higher atmospheric pressure. Atmospheric pressure is lower at high land than low land. Accordingly, the lower the atmospheric pressure, the more the rotation speed is increased. Further, before boosting is started, the rotation speed of the internal combustion engine is increased, which increases the amount of exhaust gas, increases boost pressure, and allows faster increasing torque to be generated. As a result, a hybrid vehicle that is capable of reducing a delay of a response of torque generated by the internal combustion engine on high land can be provided. 
     Preferably, on the map, for lower atmospheric pressure, as compared with higher atmospheric pressure, the controller shifts the boost line toward a side on which the torque generated by the internal combustion engine is smaller. 
     According to this configuration, for low atmospheric pressure, as compared with high atmospheric pressure, the boost line is shifted toward a side on which torque generated is smaller. Atmospheric pressure is lower at high land than low land. For this reason, for high land, boosting is started when torque smaller than that for low land is generated. Further, before boosting is started as faster timed, the rotation speed of the internal combustion engine is increased, which increases the amount of exhaust gas, increases boost pressure, and allows faster increasing torque to be generated. As a result, a delay of a response of torque generated by the internal combustion engine on high land can be reduced to be smaller for lower atmospheric pressure. 
     Preferably, when the controller increases the rotation speed of the internal combustion engine before the torque generated by the internal combustion engine indicated by the operating point exceeds the boost line, for lower atmospheric pressure the controller controls the internal combustion engine and the rotating electric machine to start increasing the rotation speed of the internal combustion engine at smaller generated torque than for higher atmospheric pressure. 
     According to this configuration, for lower atmospheric pressure, rotation speed is increased from when torque generated is still small. As a result, a delay of a response of torque generated by the internal combustion engine on high land can be reduced to be smaller for lower atmospheric pressure. 
     Preferably, the controller increases the rotation speed of the internal combustion engine by controlling the rotating electric machine to increase a rotation speed of the rotating electric machine. This can increase the rotation speed of the internal combustion engine with precision. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention 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 according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram showing an exemplary configuration of an engine including a turbocharger. 
         FIG. 3  is a block diagram showing an exemplary configuration of a controller. 
         FIG. 4  is a diagram for illustrating an operating point of the engine. 
         FIG. 5  is a nomographic chart representing a relationship between rotation speed and torque that the engine, a first MG, and an output element have. 
         FIG. 6  is a nomographic chart representing a relationship between rotation speed and torque that the engine, the first MG, and the output element have. 
         FIG. 7  is a nomographic chart representing a relationship between rotation speed and torque that the engine, the first MG, and the output element have. 
         FIG. 8  shows an optimum fuel efficiency line which is an exemplary recommended operation line for the engine. 
         FIG. 9  is a flowchart of an example of a basic computation process for determining operating points for the engine, the first MG, and the second MG. 
         FIG. 10  is a flowchart of an engine command correction process of the present embodiment. 
         FIG. 11  is a diagram for illustrating how an operating point moves according to first and second correction controls. 
         FIGS. 12A to 12C  are timing plots representing how rotation speed, torque generated, and boost pressure change when the presently disclosed correction control is not performed. 
         FIGS. 13A to 13C  is timing plots representing how rotation speed, torque generated, and boost pressure change when the presently disclosed correction control is performed. 
     
    
    
     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;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  according to an embodiment of the present disclosure. 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 travelling. Engine  13  includes a turbo charger  47 . 
     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 is, for example, a permanent magnet type or similar synchronous motor including a rotor having a permanent magnet embedded, or an induction motor. 
     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 . 
     Battery  18  is a rechargeably configured electric power storage component. Battery  18  for example includes a rechargeable battery such as a lithium ion battery, a nickel metal hydride battery or the like, or a power storage element such as an electric double layer capacitor, or the like. 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  can store power generated by first MG  14  and received via first inverter  16  and can supply the stored power to second MG  15  via second inverter  17 . Further, battery  18  can also store power generated by second MG  15  when the vehicle is decelerated, and received via second inverter  17 , and can also supply the stored power to first MG  14  via first inverter  16  when engine  13  is started. 
     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 . 
     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 splitting drive torque into drive torque to first MG  14  and drive torque to an output gear  21 . 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. Engine  13  has output shaft  22  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 . 
     Carrier C to which torque output from engine  13  is transmitted serves as an input element, ring gear R that outputs 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. That is, planetary gear mechanism  20  divides an output of engine  13  for the side of first MG  14  and the side of output gear  21 . First MG  14  is controlled to output torque in accordance with torque output from engine  13 . 
     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, torque output from second MG  15  is added at driven gear  26  to torque output from output gear  21 . Torque thus combined is transmitted to drive wheel  24  with driveshafts  32  and  33  extending laterally from differential gear  28  being interposed. As torque is transmitted to drive wheel  24 , driving force is generated in vehicle  10 . 
     &lt;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  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 intake manifold  46  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 bypass passage  54  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 for 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 catalytic converter  56  and an aftertreatment apparatus  57  provided at prescribed positions in exhaust passage  42 , and thereafter emitted into the atmosphere. Start-up catalytic converter  56  and aftertreatment apparatus  57  contain, for example, a three-way catalyst. 
     Start-up catalytic converter  56  is provided at an upstream portion (a portion closer to the combustion chamber) of exhaust passage  42 , and accordingly, it is heated to activation temperature within a short period of time after engine  13  is started. Furthermore, aftertreatment apparatus  57  located downstream purifies HC, CO and NOx that could not be purified by startup catalytic converter  56 . 
     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 catalytic converter  56  and aftertreatment apparatus  57  to a portion of intake air passage  41  between compressor  48  and air flow meter  50 . 
     &lt;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. 
     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 , and an atmospheric pressure sensor  80  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  80  detects atmospheric pressure. Various sensors 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;Control of Travelling of Vehicle&gt; 
     Vehicle  10  configured as above can be set or switched to such a travelling mode as a hybrid (HV) travelling mode in which engine  13  and second MG  15  serve as motive power sources and an electric (EV) travelling mode in which the vehicle travels 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 travelling mode. 
     The EV travelling 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 travelling mode in which an operation by engine  13  is stopped and second MG  15  outputs driving force. 
     The HV travelling 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 travelling mode in which combined torque of drive torque of engine  13  and drive torque of second MG  15  is output. 
     In the HV travelling 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. 
     Control of engine  13 , first MG  14 , and second MG  15  in coordination while vehicle  10  operates will be described below. 
     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 travelling 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 travelling power as requested system power. 
     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 travelling mode as the travelling mode. When activation of engine  13  has not been requested, HV-ECU  62  sets the EV travelling mode as the travelling mode. 
     When activation of engine  13  has been requested (that is, when the HV travelling mode is set), 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. HV-ECU  62  outputs calculated requested engine power as an engine operation state command to engine ECU  64 . 
     Engine ECU  64  operates in response to the engine operation state command input from HV-ECU  62  to variously control each component of engine  13  such as intake throttle valve  49 , ignition plug  45 , waste gate valve  55 , and EGR valve  60 . 
     HV-ECU  62  sets based on calculated requested engine power, an operating point of engine  13  in a coordinate system defined by an engine rotation speed and engine torque. 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 a 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. 
     HV-ECU  62  sets the engine rotation speed corresponding to the set operating point as a target engine rotation speed. 
     As the target engine rotation speed is set, HV-ECU  62  sets a torque command value for first MG  14  for setting a current engine rotation speed to the target engine rotation speed. HV-ECU  62  sets the torque command value for first MG  14 , for example, through feedback control based on a difference between a current engine rotation speed and the target engine rotation speed. 
     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 sets a torque command value for second MG  15  so as to fulfill requested driving force. HV-ECU  62  outputs set torque command values for first MG 14  and second MG  15  as a first MG torque command and 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 signal including the calculated current value and the frequency thereof to PCU  81 . 
     HV-ECU  62  may request increase in boost pressure, for example, when the accelerator position exceeds a threshold value for starting turbo charger  47 , when requested engine power exceeds a threshold value, or when engine torque corresponding to the set operating point exceeds a threshold value. 
     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. 
       FIG. 4  is a diagram for illustrating an operating point of engine  13 . In  FIG. 4 , the vertical axis represents torque Te of engine  13 , and the horizontal axis represents an engine speed Ne of engine  13 . 
     Referring to  FIG. 4 , a line L 1  represents a maximum torque that engine  13  can output. A dotted line L 2  represents a line (a boost line) at which turbocharger  47  starts boosting on low land. When torque Te of engine  13  exceeds boost line L 2  on low land, waste gate valve  55 , having been fully open, is operated in the closing direction. Adjusting the angle of opening of waste gate valve  55  can adjust the flow rate of exhaust air flowing into turbine  53  of turbocharger  47  and the boost pressure for the suctioned air can be adjusted through compressor  48 . When torque Te falls below boost line L 2  on low land, waste gate valve  55  can be fully opened to inactivate turbocharger  47 . 
     In this embodiment, it is assumed that a place having an altitude less than a prescribed elevation (for example, several hundred meters such as  500  m) is low land and a place having an altitude of the prescribed elevation or higher is high land. A dotted line L 2 ′ represents a line (a boost line) at which turbocharger  47  starts boosting. When torque Te of engine  13  exceeds boost line L 2 ′ on high land, waste gate valve  55 , having been fully open, is operated in the closing direction. Adjusting the angle of opening of waste gate valve  55  can adjust the flow rate of exhaust air flowing into turbine  53  of turbocharger  47  and the boost pressure for the suctioned air can be adjusted through compressor  48 . When torque Te falls below boost line L 2 ′ on high land, waste gate valve  55  can be fully opened to inactivate turbocharger  47 . 
     In hybrid vehicle  10 , engine  13  and first MG  14  can be controlled to change the operating point of engine  13 . Also, the final vehicle driving force is adjustable by controlling second MG  15 , and accordingly, the operating point of engine  13  can be moved while the vehicle drive force is adjusted (e.g., maintained). A way of moving the operating point of engine  13  will now be described. 
       FIGS. 5 to 7  are nomographic charts showing the relationship between the rotation speed and torque of engine  13 , first MG  14 , and the output element.  FIG. 5  is a nomographic chart showing the relationship between the rotation speed and torque of the respective elements before the operating point of engine  13  is changed.  FIG. 6  is a nomographic chart showing the relationship between the rotation speed and torque of the respective elements when engine speed Ne of engine  13  is increased from the state shown in  FIG. 5 .  FIG. 7  is a nomographic chart showing the relationship between the rotation speed and torque of the respective elements when torque Te of engine  13  is increased from the state shown in  FIG. 5 . 
     In each of  FIGS. 5 to 7 , the output element is ring gear R coupled to countershaft  25  ( FIG. 1 ). Positions on the vertical axis represent the rotation speeds of the respective elements (engine  13 , first MG  14 , and second MG  15 ), and spacings between the vertical axes represent the gear ratio of planetary gear mechanism  20 . “Te” represents a torque of engine  13 , and “Tg” represents a torque of first MG  14 . “Tep” represents a direct torque of engine  13 , and “Tml” represents a torque obtained by converting torque Tm of second MG  15  on the output element. The sum of Tep and Tml corresponds to a torque output to a drive shaft (countershaft  25 ). The up arrow represents a positive-going torque, a down arrow represents a negative-going torque, and an allow length represents torque magnitude. 
     Referring to  FIGS. 5 and 6 , the dotted line in  FIG. 6  represents the relationship before engine speed Ne is increased, and corresponds to the line shown in  FIG. 5 . The relationship between torque Te of engine  13  and torque Tg of first MG  14  is uniquely determined by the gear ratio of planetary gear mechanism  20 . Thus, first MG  14  can be controlled such that the rotation speed of first MG  14  increases with torque Tg of first MG  14  maintained, thereby increasing engine speed Ne of engine  13  with the driving torque maintained. 
     Also, referring to  FIGS. 5 and 7 , engine  13  can be controlled such that the output (power) of engine  13  is increased, thereby increasing torque Te of engine  13 . At this time, torque Tg of first MG  14  can be increased such that the rotation speed of first MG  14  does not increase, thereby increasing torque Te of engine  13  with engine speed Ne of engine  13  maintained. Since engine direct torque Tep increases along with an increase in torque Te, second MG  15  can be controlled such that torque Tml decreases, thereby maintaining the torque of the drive shaft. 
     When torque Te of engine  13  is increased, torque Tg of first MG  14  increases, leading to an increase in the power generated by first MG  14 . At this time, if charging of battery  18  is not restricted, battery  18  can be charged with the generated power which has been increased. 
     Although not particularly shown, controlling engine  13  can be controlled such that the output (power) of engine  13  decreases, thereby reducing torque Te of engine  13 . At this time, torque Tg of first MG  14  can be reduced such that the rotation speed of first MG  14  does not decrease, thereby reducing torque Te of engine  13  with engine speed Ne of engine  13  maintained. In this case, torque Tg of first MG  14  decreases, leading to a decrease in the power generated by first MG  14 . At this time, if discharging of battery  18  is not restricted, discharging by battery  18  can be increased to compensate for an amount of the decrease in the power generated by first MG  14 . 
     Referring to  FIG. 4  again, a line L 3  represents a recommended operation line of engine  13 . In other words, engine  13  is usually controlled to move on the recommended operation line (line L 3 ) in which the operating point determined by torque Te and engine speed Ne is set in advance. 
       FIG. 8  shows an optimum fuel efficiency line which is an example recommended operation line of engine  13 . Referring to  FIG. 8 , a line L 5  is an operation line set in advance by initial assessment test or simulation to obtain minimum fuel consumption of engine  13 . The operating point of engine  13  is controlled to be located on line L 5 , leading to optimum (minimum) fuel consumption of engine  13  for the requested power. A dotted line L 6  is an isopower line of engine  13  which corresponds to the requested power. Note that in  FIG. 4 , a dotted line L 41  represents an isopower line. Fuel consumption of engine  13  is optimized (minimized) by controlling engine  13  such that the operating point of engine  13  is a point at intersection EO of dotted line L 6  with line L 5 . A group of closed curves  11  in the figure shows an isoefficiency line of engine  13 , in which the efficiency of engine  13  is higher as closer to the center. 
     &lt;Description of Basic Computation Process of Operating Point&gt; 
       FIG. 9  is a flowchart showing an example basic computation process for determining the operating points of engine  13 , first MG  14 , and second MG  15 . A series of steps shown in this flowchart is repeatedly performed for each prescribed period in HV-ECU  62 . 
     Referring to  FIG. 9 , HV-ECU  62  acquires information on, for example, an accelerator position, a shift range being selected, and a vehicle speed (step S 10 ). The accelerator position is detected by accelerator position sensor  67 , and the vehicle speed is detected by vehicle speed sensor  66 . The rotation speed of a drive shaft or propeller shaft may be used in place of the vehicle speed. 
     HV-ECU  62  then computes a requested driving force (torque) from the information acquired at step S 10  using a drive force map prepared in advance per shift range, which indicates the relationship among requested driving force, accelerator position, and vehicle speed (step S 15 ). HV-ECU  62  then multiplies the computed requested driving force by the vehicle speed and adds prescribed loss power to a result of the multiplication, thereby computing traveling power of the vehicle (step S 20 ). 
     Then, when there is a charge/discharge request (power) of battery  18 , HV-ECU  62  computes a value obtained by adding the charge/discharge request (charge has a positive value) to the computed traveling power as system power (step S 25 ). For example, the charge/discharge request can have a greater positive value as the SOC of battery  18  is lower and have a negative value when the SOC is high. 
     HV-ECU  62  then determines to operate/stop engine  13  in accordance with the computed system power and traveling power (step S 30 ). For example, when system power is greater than a first threshold or when traveling power is greater than a second threshold, HV-ECU  62  determines to operate engine  13 . 
     Then, when determining to operate engine  13 , HV-ECU  62  performs the process of step S 35  and the following processes (HV traveling mode). Although not specifically shown, when determining to stop engine  13  (EV traveling mode), HV-ECU  62  computes torque Tm of second MG  15  based on the requested driving force. 
     During operation of engine  13  (during the HV traveling mode), HV-ECU  62  computes power Pe of engine  13  from the system power computed at step S 25  (step S 35 ). Power Pe is computed by, for example, making various corrections to or imposing limitations on system power. The computed power Pe of engine  13  is output to engine ECU  64  as a power command of engine  13 . 
     HV-ECU  62  then computes an engine speed Ne (target engine rotation speed) of engine  13  (step S 40 ). In the present embodiment, engine speed Ne is computed such that the operating point of engine  13  is located on line L 3  (recommended operation line) shown in, for example,  FIG. 4 . Specifically, the relationship between power Pe and engine speed Ne in which the operating point of engine  13  is located on line L 3  (recommended operation line) is prepared as a map or the like in advance, and engine speed Ne is computed from power Pe computed at step S 35  using the map. When engine speed Ne is determined, torque Te (target engine torque) of engine  13  is also determined. Consequently, the operating point of engine  13  is determined. 
     HV-ECU  62  then computes torque Tg of first MG  14  (step S 45 ). Torque Te of engine  13  can be estimated from engine speed Ne of engine  13 , and the relationship between torque Te and torque Tg is uniquely determined in accordance with the gear ratio of planetary gear mechanism  20 , and thus, torque Tg can be computed from engine speed Ne. The computed torque Tg is output to MG-ECU  63  as a torque command of first MG  14 . 
     HV-ECU  62  further computes engine direct torque Tep (step S 50 ). Since the relationship between engine direct torque Tep and torque Te (or torque Tg) is uniquely determined in accordance with the gear ratio of planetary gear mechanism  20 , engine direct torque Tep can be computed from the computed torque Te or torque Tg. 
     HV-ECU  62  finally computes torque Tm of second MG  15  (step S 50 ). Torque Tm is determined such that the requested driving force (torque) computed at step S 15  can be obtained, and can be computed by subtracting engine direct torque Tep from the requested driving force converted on the output shaft. The computed torque Tm is output to MG-ECU  63  as the torque command of second MG  15 . 
     As described above, the operating point of engine  13  and the operating points of first MG  14  and second MG  15  are computed. 
     &lt;Control Applied for High Land&gt; 
     Vehicle  10  according to the present disclosure may have a problem, that is, on high land, a delay of a response of boost pressure of turbocharger  47  and hence a delay of a response of torque that engine  13  generates are larger than on low land. 
     Accordingly, HV-ECU  62  according to the present disclosure controls engine  13  and first MG  14  to increase the rotation speed of engine  13  before the torque generated by engine  13  indicated by an operating point exceeds boost lines L 2  and L 2 ′. Boost lines L 2  and L 2 ′ indicate such a line that turbocharger  47  boosts suctioned air when the torque generated by engine  13  indicated by an operating point on the map shown in  FIG. 4  representing a relationship between the rotation speed of engine  13  and the torque generated thereby exceeds boost lines L 2  and L 2 ′. When increasing the rotation speed of engine  13  before the torque generated by engine  13  indicated by the operating point exceeds boost line L 2 ′, for lower atmospheric pressure HV-ECU  62  controls engine  13  and first MG  14  to increase the rotation speed more than for higher atmospheric pressure. This can reduce a delay of a response of torque generated by engine  13  on high land. 
     Hereinafter, control in the present embodiment will be described.  FIG. 10  is a flowchart of an engine command correction process of the present embodiment. This engine command correction process is invoked by a CPU of HV-ECU  62  from a higher-level process periodically as prescribed for control, and thus performed.  FIG. 11  is a diagram for illustrating how an operating point moves according to first and second correction controls. 
     Referring to  FIG. 11 , the first correction control increases the rotation speed while generating torque to be constant, and is applied at horizontal portions of lines kl l and k 12 . The second correction control generates increasing torque while rotation speed is fixed, and is applied at vertical portions of lines kll and k 12 . 
     Referring to  FIG. 10 , HV-ECU  62  obtains an atmospheric pressure from atmospheric pressure sensor  80  (step S 111 ), and determines whether the obtained atmospheric pressure is less than a prescribed value (step S 112 ). The prescribed value is an average atmospheric pressure at a prescribed altitude that is a boundary between low land and high land, as described above, and it is a value used to determine high land with low atmospheric pressure and low land with high atmospheric pressure and predetermined in a design development stage as a value applied to apply control suitable for high land for a value below the prescribed value. 
     If the atmospheric pressure is less than the prescribed value (YES in step S 112 ), that is, when it is determined that the current location is high land, HV-ECU  62  determines whether the first or second engine command correction control, which will be described hereinafter, is currently performed (step S 113 ). 
     When it is determined that neither the first nor second correction control is currently performed (NO in step S 113 ), HV-ECU  62  selects one of correction control starting points El, E 2  and the like (see  FIG. 11  described hereinafter) on the recommended operation line, or line L 3 , that corresponds to the atmospheric pressure, and determines whether the operating point has reached the selected starting point (step S 114 ). The starting point, such as starting points El and E 2 , corresponding to an atmospheric pressure is a point with torque smaller and rotation speed lower than boost line L 2 ′, and for high land with higher atmospheric pressure the starting point is predetermined as a point closer to boost line L 2 ′, whereas for high land with lower atmospheric pressure the starting point is predetermined as a point farther away from boost line L 2 ′. Starting points other than starting points Eland E 2  are similarly predetermined. 
     Referring again to  FIG. 11 , starting points El and E 2  are located on the recommended operation line, or line L 3 . Starting point El is farther away from boost line L 2 ′ than starting point E 2  applied for higher atmospheric pressure. 
     Returning to  FIG. 10 , when it is determined that the operating point has not reached starting point El, E 2  or the like corresponding to the atmospheric pressure (NO in step S 114 ), HV-ECU  62  returns to a process higher in level than the engine command correction process. On the other hand, when it is determined that the operating point has reached starting point El, E 2  or the like corresponding to the atmospheric pressure (YES in step S 114 ), HV-ECU  62  starts performing the first correction control (step S 115 ). 
     In the first correction control, HV-ECU  62  outputs a command to MG-ECU  63  for increasing the rotation speed of first MG  14  to thus control the rotation speed of first MG  14  to increase the rotation speed of engine  13  connected to first MG  14  by planetary gear mechanism  20 . Further, HV-ECU  62  outputs a command to engine ECU  64  to control engine  13  to generate constant torque. 
     Referring again to  FIG. 11 , when the first correction control is started from starting point El, the operating point moves on line kl l in a direction in which torque is generated to be constant and rotation speed increases, that is, in a horizontal rightward direction. When the first correction control is started from starting point E 2 , the operating point moves on line k 12  in the horizontal rightward direction. 
     Returning to  FIG. 10 , when it is determined that the first or second correction control is currently performed (YES in step S 113 ), and after step S 115 , HV-ECU  62  determines whether by the first correction control the operating point has reached a rotation speed at which a prescribed boost pressure corresponding to the atmospheric pressure is obtained (step S 116 ). 
     When it is determined that by the first correction control the operating point has reached the rotation speed at which the prescribed boost pressure corresponding to the atmospheric pressure is obtained (YES in step S 116 ), HV-ECU  62  ends the first correction control and starts performing the second correction control. 
     In the second correction control, HV-ECU  62  outputs a command to MG-ECU  63  for fixing the rotation speed of first MG  14  to thus control the rotation speed of first MG  14  to fix the rotation speed of engine  13  connected to first MG  14  by planetary gear mechanism  20 . Further, HV-ECU  62  outputs a command to engine ECU  64  to control engine  13  to generate increasing torque. 
     Referring again to  FIG. 11 , when the first correction control is performed from starting point El and the operating point has reached the rotation speed at which the prescribed boost pressure corresponding to the atmospheric pressure is obtained, the operating point moves on line kl l in a direction in which rotation speed is fixed and increasing torque is generated, that is, in a vertically upward direction. When the first correction control is performed from starting point E 2  and the operating point has reached the rotation speed at which the prescribed boost pressure corresponding to the atmospheric pressure is obtained, the operating point moves on line k 12  in the vertically upward direction. While the operating point is moving on line kl l or line k 12  when boost line L 2 ′ is exceeded turbocharger  47  starts boosting. 
     Returning to  FIG. 10 , when it is determined that the operating point has not reached the rotation speed at which the prescribed boost pressure corresponding to the atmospheric pressure is obtained (NO in step S 116 ), and after step S 117 , HV-ECU  62  determines whether by the second correction control the operating point has reached the recommended operation line, or line L 3  (step S 118 ). 
     When it is determined that the operating point has not reached the recommended operation line or line L 3  (NO in step S 118 ), HV-ECU  62  returns to a process higher in level than the engine command correction process. When it is determined that the operating point has reached the recommended operation line or line L 3  (YES in step S 118 ), HV-ECU  62  proceeds to step  5122 , which will be described hereinafter. 
     Referring to  FIG. 11  again, when the correction control is started from starting point E 1 , the operating point reaches a point E 4  on line L 3 . When the correction control is started from starting point E 2 , the operating point reaches a point E 3  on line L 3 . 
     Returning to  FIG. 10 , when it is determined that the atmospheric pressure is not less than the prescribed value (NO in step S 112 ), that is, the current location is low land, then, HV-ECU  62  determines whether the first or second correction control is currently performed (step S 121 ). When it is determined that neither the first nor second correction control is currently performed (NO in step S 121 ), HV-ECU  62  returns to a process higher in level than the engine command correction process. 
     On the other hand, when it is determined that the first or second correction control is currently performed (YES in step S 121 ), and when it is determined that the operating point has reached the recommended operation line or line L 3  (YES in step S 118 ) HV-ECU  62  returns from the currently performed first or second correction control to a normal control in which the correction control is not performed (step S 122 ). 
       FIGS. 12A to 12C  are timing plots representing how rotation speed, torque generated, and boost pressure change when the presently disclosed correction control is not performed. A case where the above-described correction control is not performed will be described with reference to  FIGS. 12A to 12C . As shown in  FIG. 12A  and  FIG. 12B , from time tl rotation speed and torque to be generated start to increase, and as shown in  FIG. 12C , for high land, from time t 2  turbocharger  47  starts boosting, and boost pressure starts to increase, whereas for low land, from time t 3  turbocharger  47  starts boosting and boost pressure starts to increase. 
     However, as shown in  FIG. 12C , high land receives lower atmospheric pressure than low land and the boost pressure does not easily increase, and, as shown in  FIG. 12B , increasing the torque to be generated is delayed, and a target torque to be generated is reached at time t 4 . Thereafter, as shown in  FIG. 12C , at time t 5 , the boost pressure for high land reaches an upper limit. 
       FIGS. 13A to 13C  are timing plots representing how rotation speed, torque generated, and boost pressure change when the presently disclosed correction control is performed. A case where the above-described correction control is performed will be described with reference to  FIGS. 13A to 13C . With reference to  FIG. 13A  and  FIG. 13B , as well as shown in  FIG. 12A  and  FIG. 12B , from time tl rotation speed and torque to be generated start to increase, and with reference to  FIG. 13C , as well as shown in  FIG. 12C , for high land, from time t 2  turbocharger  47  starts boosting and boost pressure starts to increase, whereas for low land, from time t 3  turbocharger  47  starts boosting and boost pressure starts to increase. 
     When performing the correction control, for high land, as has been discussed above, as shown in  FIG. 13A , rotation speed is increased before boosting starts, or before time t 2 , as compared with the  FIG. 12A  case, which is indicated in  FIG. 13A  by a broken line. As a result, as shown in  FIG. 13C , boost pressure rises faster than in the  FIG. 12C  case, which is indicated in  FIG. 13C  by a broken line. Thus, as shown in  FIG. 13B , delay of increase of torque to be generated is alleviated, as compared with the  FIG. 12B  case, which is indicated in  FIG. 13B  by a chain double-dashed line. 
     &lt;Modification&gt; 
     (1) In the above-described embodiment, as shown in  FIGS. 10 and 11 , in the first correction control, rotation speed is increased while constant torque is generated. This is not exclusive, however, and rather than generating constant torque, rotation speed may be increased while torque increasing little by little may be generated. 
     (2) In the above-described embodiment, as shown in  FIGS. 10 and 11 , in the second correction control, increasing torque is generated while rotation speed is fixed. This is not exclusive, however, and rather than fixing rotation speed, increasing torque may be generated while rotation speed is increased little by little. 
     (3) In the above-described embodiment, as shown in  FIGS. 10 and 11 , in the first and second correction controls, rotation speed and torque to be generated are linearly increased from starting points E 1  and E 2  to E 3  and E 4 . This is not exclusive, however, and rotation speed and torque to be generated may be increased from starting points E 1  and E 2  to E 3  and E 4  in a smooth curve. In that case, rotation speed and torque to be generated are increased such that in the first half, rotation speed increases at a larger rate than torque to be generated, and in the second half, torque to be generated increases at a larger rate than rotation speed. 
     (4) In the above-described embodiment, as shown in  FIG. 2 , the forced induction device is a so-called turbocharger,  47 , that is driven by energy of exhaust gas. This is not exclusive, however, and the forced induction device may alternatively be a mechanical forced induction device driven by the rotation of an engine or by a motor. 
     (5) In the above-described embodiment, as shown in  FIG. 4 , the boost line is switched in two stages of boost lines L 2  and L 2 ′ depending on whether the current location is low land or high land. This is not exclusive, however, and boost line L 2  may not be switched to another boost line depending on the altitude. Further, the boost line may be switched in three or more stages depending on the altitude (e.g., for higher altitude, a boost line may be applied to start boosting from smaller generated torque), or the boost line may be gradually shifted (e.g., for higher altitude, the boost line is shifted to be lower toward torque generated). 
     (6) In the above-described embodiment, as indicated in  FIG. 11  at step S 114 , a starting point corresponding to an atmospheric pressure is selected from a plurality of starting points including starting points El and E 2 , and whether an operating point has reached the selected starting point is determined. This is not exclusive, however, and the starting point may be gradually shifted depending on the atmospheric pressure (e.g., it is shifted to a starting point farther away from boost line L 2 ′ for lower atmospheric pressure) and whether the operating point has reached the shifted starting point may be determined. 
     (7) The above-described embodiment can be regarded as disclosure of a hybrid vehicle such as vehicle  10 . Further, the above-described embodiment can be regarded as disclosure of a controller, such as HV-ECU  62 , for a hybrid vehicle. Further, the above-described embodiment can be regarded as disclosure of a control method in which the controller performs the process shown in  FIG. 10 . Further, the above-described embodiment can be regarded as disclosure of a program of the engine command correction process shown in  FIG. 10  and performed by the controller. 
     &lt;Effect&gt; 
     (1) As shown in  FIGS. 1 to 3 , vehicle  10  includes engine  13 , first MG  14 , planetary gear mechanism  20  to which engine  13 , first MG  14 , and counter shaft  25  are connected, and HV-ECU  62  configured to control engine  13  and first MG  14 . As shown in  FIGS. 1 and 2 , engine  13  includes turbocharger  47  that boosts suctioned air to be fed to engine  13 . As shown in  FIG. 4 , boost lines L 2  and L 2 ′ determined on a map representing a relationship between the rotation speed of engine  13  and torque generated by engine  13  indicate such lines that turbocharger  47  boosts suctioned air when the torque generated by engine  13  indicated by an operating point on the map exceeds boost lines L 2  and L 2 ′, respectively. 
     As shown in  FIGS. 10 and 11 , HV-ECU  62  controls engine  13  and first MG  14  to increase the rotation speed of engine  13  before the torque generated by engine  13  indicated by an operating point exceeds boost line L 2 ′. As shown in  FIG. 11 , when HV-ECU  62  increases the rotation speed of engine  13  before the torque generated by engine  13  indicated by an operating point exceeds boost line L 2 ′, for lower atmospheric pressure (e.g., when a control point moves on line k 11 ) HV-ECU  62  controls engine  13  and first MG  14  to increase the rotation speed more than for higher atmospheric pressure (e.g., when the control point moves on line k 12 ). 
     As a result, before the operating point exceeds boost line L 2 ′, for lower atmospheric pressure engine  13 ′s rotation speed is increased more than for higher atmospheric pressure. Atmospheric pressure is lower at high land than low land. Accordingly, the lower the atmospheric pressure, the more the rotation speed is increased. Further, before boosting is started, the rotation speed of engine  13  is increased, which increases the amount of exhaust gas, increases boost pressure, and allows faster increasing torque to be generated. As a result, a delay of a response of torque generated by engine  13  on high land can be reduced. 
     (2) As shown in  FIGS. 4 and 11 , on the map, for low atmospheric pressure, as compared with high atmospheric pressure, HV-ECU  62  shifts boost line L 2  to boost line L 2 ′ applied for smaller torque generated by engine  13 . 
     Thus, for low atmospheric pressure, as compared with high atmospheric pressure, boost line L 2  is shifted to boost line L 2 ′ applied for smaller torque generated. Atmospheric pressure is lower at high land than low land. For this reason, for high land, boosting is started for torque generated which is smaller than that in the case of low land. Further, before boosting is started as faster timed, the rotation speed of engine  13  is increased, which increases the amount of exhaust gas, increases boost pressure, and allows faster increasing torque to be generated. As a result, a delay of a response of torque generated by engine  13  on high land can be reduced to be smaller for lower atmospheric pressure. 
     (3) As shown in  FIG. 11 , when HV-ECU  62  increases the rotation speed of engine  13  before the torque generated by engine  13  indicated by an operating point exceeds boost line L 2 ′, for lower atmospheric pressure (e.g., when a control point moves on line k 11 ) HV-ECU  62  controls engine  13  and first MG  14  to start increasing the rotation speed of engine  13  at smaller generated torque than for higher atmospheric pressure (e.g., when the control point moves on line k 12 ). 
     Thus, for lower atmospheric pressure, rotation speed is increased from when torque generated is still small. As a result, a delay of a response of torque generated by engine  13  on high land can be reduced to be smaller for lower atmospheric pressure. 
     (4) As shown in  FIG. 10 , HV-ECU  62  increases the rotation speed of engine  13  by controlling the rotation speed of first MG  14  to increase it. This can increase the rotation speed of engine  13  with precision. 
     Although the embodiments of the present invention have been described, it should be considered 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, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.