Patent Publication Number: US-2021188241-A1

Title: Hybrid vehicle, drive control system, and method for controlling hybrid vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-229541 filed on Dec. 19, 2019, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to hybrid vehicles, drive control systems, and methods for controlling a hybrid vehicle. 
     2. Description of Related Art 
     Hybrid vehicles (HVs) have been increasingly popular in recent years. Electrically driven vehicles can include hybrid vehicles, electric vehicles (EVs), etc. Typical hybrid vehicles are equipped with separate electronic control units (ECUs) for each function. For example, a hybrid vehicle disclosed in Japanese Unexamined Patent Application Publication No. 2019-156007 (JP 2019-156007 A) includes an engine ECU, a motor ECU, a battery ECU, and an HVECU. The HVECU is connected to the engine ECU, the motor ECU, and the battery ECU via a communication port and sends and receives various control signals and data to and from the engine ECU, the motor ECU, and the battery ECU. 
     SUMMARY 
     It is herein assumed that a hybrid vehicle is equipped with a battery pack and a drive control system. An ECU in the battery pack and an ECU in the drive control system are configured to communicate with each other. 
     The battery pack includes a battery and manages the state of the battery. Specifically, the ECU in the battery pack calculates allowable charging power of the battery and allowable discharging power of the battery and outputs the calculation results to the drive control system. 
     The drive control system includes an engine and two rotating electrical machines (a motor and a generator) and generally controls traveling of the hybrid vehicle. Specifically, the ECU in the drive control system controls the engine or controls the two rotating electrical machines, based on data on the allowable charging power and the allowable discharging power of the battery received from the ECU in the battery pack. 
     The automotive industry is said to have a vertically integrated industrial structure. However, since hybrid vehicles will become more popular around the world, the automotive industry may increasingly shift toward horizontal specialization. The inventors focused on the following possible problem associated with this shift of the industrial structure. 
     There may be a situation where a battery pack company (hereinafter referred to as Company A) and a drive control system company (hereinafter referred to as Company B) are different. For example, Company B sells drive control systems to Company A. Company A develops hybrid vehicles using the driving control systems purchased from Company B and battery packs designed (procured) by Company A. Especially in such a situation, compatibility between the battery pack and the drive control system can be a problem. 
     This will be described more specifically. As described above, the data on the allowable charging power and the allowable discharging power of the battery is sent from the ECU in the battery pack to the ECU in the drive control system. For example, in the case where Company A and Company B have different understandings about the content, timing, etc. of communication or in the case where coordination between Companies A and B about communication is not enough, there is a possibility that data different from data expected by the drive control system may be sent from the ECU in the battery pack to the ECU in the drive control system as the data on the allowable charging power and the allowable discharging power of the battery. Specifically, there is a possibility that the ECU in the battery pack may set the allowable charging power and the allowable discharging power to excessively small values (e.g., values close to 0). In this case, the drive control system may reduce charging and discharging of the battery even though it is still not actually necessary to protect the battery. As a result, the hybrid vehicle may become unable to move. 
     The present disclosure provides a hybrid vehicle, a drive control system, and a method for controlling a hybrid vehicle that can make the hybrid vehicle move even when there is a problem with communication between two ECUs. 
     A hybrid vehicle according to a first aspect of the present disclosure includes: an engine; a first rotating electrical machine; a second rotating electrical machine connected to a drive wheel via an output shaft; a planetary gear set; a battery; a power converter configured to convert electric power among the battery, the first rotating electrical machine, and the second rotating electrical machine; a relay electrically connected between the battery and the power converter; and first and second controllers. The planetary gear set is configured to mechanically couple the engine, the first rotating electrical machine, and the output shaft and is configured to transmit torque among the engine, the first rotating electrical machine, and the output shaft. The first controller is configured to calculate allowable charging power of the battery and allowable discharging power of the battery and output the calculation results. The second controller is configured to control the engine and the power converter according to the allowable charging power and the allowable discharging power received from the first controller. The second controller has, as control modes, a normal mode in which the relay is closed and the battery and the power converter are electrically connected and a batteryless drive mode in which the relay is opened to cause the hybrid vehicle to move with the battery electrically disconnected from the power converter. The second controller is configured to select the batteryless drive mode when at least one of magnitude of the allowable charging power and magnitude of the allowable discharging power become smaller than a first predetermined value. 
     In the hybrid vehicle of the first aspect of the present disclosure, the second controller may be configured to select the batteryless drive mode when at least one of the magnitude of the allowable charging power and the magnitude of the allowable discharging power become smaller than the first predetermined value due to a problem with communication between the first controller and the second controller. 
     According to the hybrid vehicle of the first aspect of the present disclosure, the second controller receives the allowable charging power and the allowable discharging power of the battery from the first controller. When the magnitude of the allowable charging power becomes smaller than the first predetermined value or the magnitude of the allowable discharging power becomes smaller than the first predetermined value during the normal mode of the hybrid vehicle, the second controller switches the control mode of the hybrid vehicle to the batteryless drive mode regardless of whether this decrease in magnitude of the allowable charging power or magnitude of the allowable discharging power is due to the state of the battery (e.g., due to an excessive increase or decrease in temperature of the battery) or due to communication between the first and second controllers. The hybrid vehicle can thus be made to move even when there is a problem with communication between the first and second controllers. 
     In the hybrid vehicle of the first aspect of the present disclosure, the second controller may be configured to cancel the batteryless drive mode when all of the following conditions are satisfied: (i) it is no longer the case that at least one of the magnitude of the allowable charging power and the magnitude of the allowable discharging power are smaller than the first predetermined value, (ii) the hybrid vehicle is stopped, and (iii) an accelerator pedal of the hybrid vehicle is not being operated. 
     When the batteryless drive mode is cancelled (e.g., when the control mode of the hybrid vehicle is switched back to the normal mode), the relay is closed and the battery and the power converter are electrically connected again. It is therefore desirable to avoid canceling the batteryless drive mode while the vehicle is moving (or when there is a possibility that the vehicle may move). According to the hybrid vehicle of the first aspect of the present disclosure, the second controller cancels the batteryless drive mode after confirming that the hybrid vehicle is completely stopped as well as checking the magnitude of the power. This configuration can prevent an abnormality from occurring when, e.g., the control mode of the hybrid vehicle is switched back to the normal mode. 
     The hybrid vehicle of the first aspect of the present disclosure may further include: an auxiliary load; a DC-to-DC converter electrically connected between the relay and the auxiliary load; and an auxiliary battery configured to supply electric power to the auxiliary load. The second controller may be configured to perform constant voltage control of the DC-to-DC converter during the normal mode. The second controller may further have, as the control mode, a constant power mode in which the second controller performs constant power control of the DC-to-DC converter. The second controller may be configured to select the constant power mode when at least one of the magnitude of the allowable charging power and the magnitude of the allowable discharging power becomes smaller than a second predetermined value that is equal to or larger than the first predetermined value. 
     According to the hybrid vehicle of the first aspect of the present disclosure, the second controller further has the constant power mode in which the second controller performs the constant power control of the DC-to-DC converter. As described in detail later, by selecting the constant power mode, the voltage of the auxiliary battery may fluctuate with fluctuations in power consumption of the auxiliary load, but a change in charging and discharging power of the battery with fluctuations in electric power of the auxiliary load is reduced. The battery whose charging and discharging are relatively severely limited due to at least one of the allowable charging power and the allowable discharging power having become smaller than the second predetermined value can thus be more reliably protected. 
     In the hybrid vehicle of the first aspect of the present disclosure, the second controller may be configured to guarantee during the normal mode that a driving force that is output from the hybrid vehicle is controlled to a requested driving force by adjusting torque of the engine, torque of the first rotating electrical machine, and torque of the second rotating electrical machine. The second controller may further have, as the control mode, a reduced guarantee mode in which the second controller less strictly guarantees that the driving force is controlled to the requested driving force than in the normal mode. The second controller may be configured to select the reduced guarantee mode when at least one of the magnitude of the allowable charging power and the magnitude of the allowable discharging power become smaller than a third predetermined value that is equal to or larger than the first predetermined value. 
     According to the hybrid vehicle of the first aspect of the present disclosure, the second controller further has the reduced guarantee mode in which the second controller less strictly guarantees the requested driving force than in the normal mode. As described in detail later, by selecting the reduced guarantee mode, an error in actual driving force may occur due to an error in engine power associated with fluctuations in engine combustion state, but a change in charging and discharging power of the battery associated with the error in engine power is reduced. The battery whose charging and discharging are relatively severely limited due to at least one of the allowable charging power and the allowable discharging power having become smaller than the third predetermined value can thus be more reliably protected. 
     A drive control system according to a second aspect of the present disclosure is configured to control traveling of a hybrid vehicle equipped with a battery. The hybrid vehicle includes a first controller configured to calculate allowable charging power of the battery and allowable discharging power of the battery and output the calculation results. The drive control system includes: an engine; a first rotating electrical machine; a second rotating electrical machine connected to a drive wheel via an output shaft; a planetary gear set; a power converter configured to convert electric power among the battery, the first rotating electrical machine, and the second rotating electrical machine; and a second controller. The planetary gear set is configured to mechanically couple the engine, the first rotating electrical machine, and the output shaft and is configured to transmit torque among the engine, the first rotating electrical machine, and the output shaft. The second controller is configured to control the power converter according to the allowable charging power and the allowable discharging power received from the first controller. The second controller has, as a control mode, a batteryless drive mode in which a relay electrically connected between the battery and the power converter is opened to cause the hybrid vehicle to move with the battery electrically disconnected from the power converter. The second controller is configured to select the batteryless drive mode when at least one of magnitude of the allowable charging power and magnitude of the allowable discharging power become smaller than a predetermined value. 
     According to the drive control system of the second aspect of the present disclosure, as in the first aspect, the hybrid vehicle can be made to move even when there is a problem with communication between the first and second controllers. 
     In a method for controlling a hybrid vehicle according to a third aspect of the present disclosure, the hybrid vehicle includes an engine, a first rotating electrical machine, a second rotating electrical machine connected to a drive wheel via an output shaft, a planetary gear set, a battery, a power converter configured to convert electric power among the battery, the first rotating electrical machine, and the second rotating electrical machine, a relay electrically connected between the battery and the power converter, and first and second controllers. The planetary gear set is configured to mechanically couple the engine, the first rotating electrical machine, and the output shaft and is configured to transmit torque among the engine, the first rotating electrical machine, and the output shaft. The first controller is configured to calculate allowable charging power of the battery and allowable discharging power of the battery and output the calculation results. The second controller is configured to control the engine and the power converter according to the allowable charging power and the allowable discharging power received from the first controller. The second controller is configured to perform batteryless drive control in which the relay is opened to cause the hybrid vehicle to move with the battery electrically disconnected from the power converter. The method includes: performing the batteryless drive control by the second controller when at least one of magnitude of the allowable charging power and magnitude of the allowable discharging power become smaller than a predetermined value. 
     According to the method of the third aspect of the present disclosure, as in the first and second aspects of the present disclosure, the hybrid vehicle can be made to move even when there is a problem with communication between the first and second controllers. 
     According to the first, second, and third aspects of the present disclosure, the hybrid vehicle can be made to move even when there is a problem with communication between the first and second controllers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein: 
         FIG. 1  schematically illustrates the overall configuration of a hybrid vehicle according to a first embodiment; 
         FIG. 2  is a nomographic chart of a power split device; 
         FIG. 3  is a flowchart illustrating an example of a process in a normal mode; 
         FIG. 4  is a flowchart illustrating an example of a process in a batteryless drive mode; 
         FIG. 5  is a nomographic chart illustrating the control states of a first motor generator, a second motor generator, and an engine in the batteryless drive mode; 
         FIG. 6  is a conceptual diagram illustrating limitation of charging and discharging of a battery; 
         FIG. 7  is a state transition diagram illustrating switching of the control mode of the hybrid vehicle in the first embodiment; 
         FIG. 8  is a flowchart illustrating an example of a process for switching of the control mode during the normal mode; 
         FIG. 9  is a flowchart illustrating an example of a process for switching of the control mode during the batteryless drive mode; 
         FIG. 10  is a state transition diagram illustrating switching of the control mode of a hybrid vehicle in a second embodiment; 
         FIG. 11  is a graph illustrating the relationship between the allowable charging power and allowable discharging power of a battery and the control mode in the second embodiment; 
         FIG. 12  is a flowchart illustrating an example of a process for determination of whether a limit condition is satisfied according to the second embodiment; 
         FIG. 13  is a flowchart illustrating an example of a process for determination of whether a return condition is satisfied according to the second embodiment; 
         FIG. 14  is a state transition diagram illustrating switching of the control mode of a hybrid vehicle in a third embodiment; 
         FIG. 15  is a flowchart illustrating an example of a process in a reduced guarantee mode; 
         FIG. 16  is a graph illustrating the relationship between the allowable charging power and allowable discharging power of a battery and the control mode in the third embodiment; 
         FIG. 17  is a flowchart illustrating an example of a process for determination of whether a limit condition is satisfied according to the third embodiment; and 
         FIG. 18  is a flowchart illustrating an example of a process for determination of whether a return condition is satisfied according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described in detail with reference to the drawings. The same or corresponding parts are denoted with the same signs throughout the figures and description thereof will not be repeated. 
     First Embodiment 
     Overall Configuration of Hybrid Vehicle 
       FIG. 1  schematically illustrates the overall configuration of a hybrid vehicle according to the first embodiment. Referring to  FIG. 1 , a vehicle  100  is a hybrid vehicle and includes a battery pack  1  and an HV system  2 . The HV system  2  can be regarded as the “drive control system” according to the present disclosure. 
     The battery pack  1  includes a battery  10 , a battery sensor group  20 , a system main relay (SMR)  30 , and a battery ECU  40 . The HV system  2  includes a power control unit (PCU)  50 , a first motor generator (MG 1 )  61 , a second motor generator (MG 2 )  62 , an engine  63 , a power split device  71 , a drive shaft  72 , drive wheels  73 , a high voltage DC-to-DC converter  81 , a load  82 , an auxiliary DC-to-DC converter  83 , an auxiliary battery  84 , and an HVECU  90 . 
     The battery  10  includes a battery pack composed of a plurality of cells. Each cell is a secondary cell such as a lithium ion cell or a nickel-metal hydride cell. The battery  10  stores electric power for driving the first motor generator  61  and the second motor generator  62  and supplies electric power to the first motor generator  61  and the second motor generator  62  through the PCU  50 . When the first motor generator  61  and the second motor generator  62  generate electric power, the battery  10  is charged with the generated electric power through the PCU  50 . 
     The battery sensor group  20  includes a voltage sensor  21 , a current sensor  22 , and a temperature sensor  23 . The voltage sensor  21  detects the voltage of each cell in the battery  10 . The current sensor  22  detects a current  1 B that is input and output to and from the battery  10 . The temperature sensor  23  detects the temperature TB of the battery  10  (hereinafter also referred to as the “battery temperature TB”). Each sensor outputs its detection results to the battery ECU  40 . 
     The SMR  30  is electrically connected to power lines that connect the battery  10  and the PCU  40 . The SMR  30  electrically connects or disconnects the PCU  40  to and from the battery  10  in response to a control command from the HVECU  90 . The SMR  30  can be regarded as the “relay” according to the present disclosure. 
     The battery ECU  40  includes a processor  41  such as a central processing unit (CPU), a memory  42  such as a read only memory (ROM) and a random access memory (RAM), and input and output ports (not shown) for receiving and outputting various signals. The battery ECU  40  monitors the state of the battery  10  based on signals received from the sensors of the battery sensor group  20  and programs and maps stored in the memory  42 . 
     Main processes that are executed by the battery ECU  40  include a process of calculating allowable charging power Win and allowable discharging power Wout of the battery  10 . The allowable charging power Win is an upper control limit of the charging power of the battery  10  and is set to Win  0 . When the allowable charging power Win is set to Win=0, it means that charging of the battery  10  is prohibited. Similarly, the allowable discharging power Wout is an upper control limit of the discharging power of the battery  10  and is set to Wout  0 . When the allowable discharging power Wout is set to Wout=0, it means that discharging of the battery  10  is prohibited. The process of calculating the allowable charging power Win and the allowable discharging power Wout will be described later in detail with reference to  FIG. 6 . 
     The PCU  50  bidirectionally converts electric power between the battery  10  and the first and second motor generators  61 ,  62  or between the first motor generator  61  and the second motor generator  62  according to a control command from the HVECU  90 . The PCU  50  is configured so that it can independently control the states of the first motor generator  61  and the second motor generator  62 . For example, the PCU  50  can cause the second motor generator  62  to perform power running while causing the first motor generator  61  to regenerate (generate power). The PCU  50  includes, e.g., two inverters (not shown) for the first motor generator  61  and the second motor generator  62  and a converter (not shown) that boosts a direct current (DC) voltage to be supplied to each inverter to a voltage equal to or higher than an output voltage of the battery  10 . The PCU  50  can be regarded as the “power converter” according to the present disclosure. 
     Each of the first motor generator  61  and the second motor generator  62  is an alternating current (AC) rotating electrical machine and is, e.g., a three-phase AC synchronous motor having permanent magnets embedded in a rotor. The first motor generator  61  can be regarded as the “first rotating electrical machine” according to the present disclosure. The second motor generator  62  can be regarded as the “second rotating electrical machine” according to the present disclosure. 
     The first motor generator  61  is mainly used as a generator that is driven by the engine  63  via the power split device  71 . Electric power generated by the first motor generator  61  is supplied to the second motor generator  62  or the battery  10  via the PCU  50 . The first motor generator  61  can also crank the engine  63 . 
     The second motor generator  62  mainly operates as an electric motor and drives the drive wheels  73 . The second motor generator  62  is driven by at least one of the electric power from the battery  10  and the electric power generated by the first motor generator  61 , and the driving force of the second motor generator  62  is transmitted to the drive shaft (output shaft)  72 . When braking the vehicle or when reducing acceleration on a downhill slope, the second motor generator  62  operates as a generator for regeneration. The electric power generated by the second motor generator  62  is supplied to the battery  10  via the PCU  50 . 
     The engine  63  outputs power by converting combustion energy generated by combustion of an air-fuel mixture to kinetic energy of a moving element such as a piston or a rotor. 
     The power split device  71  is a planetary gear set. Although not shown in the figure, the power split device  71  includes a sun gear, a ring gear, pinion gears, and a carrier. The carrier is coupled to the engine  63 . The sun gear is coupled to the first motor generator  61 . The ring gear is coupled to the second motor generator  62  and the drive wheels  73  via the drive shaft  72 . The pinion gears mesh with the sun gear and the ring gear. The carrier holds the pinion gears such that the pinion gears can rotate and revolve. 
       FIG. 2  is a nomographic chart of the power split device  71 . Since the power split device  71  is configured as described above, a first MG rotational speed Nm 1  (the rotational speed of the sun gear), an engine rotational speed Ne (the rotational speed of the carrier), and a second MG rotational speed Nm 2  (the rotational speed of the ring gear) are connected by a straight line on the nomographic chart of the power split device  71 . That is, when any two of the first MG rotational speed Nm 1 , the engine rotational speed Ne, and the second MG rotational speed Nm 2  are determined, the remaining rotational speed is also determined. 
     Referring back to  FIG. 1 , the high voltage DC-to-DC converter  81  is a unidirectional DC-to-DC converter electrically connected between the SMR  30  and the load  82 . The high voltage DC-to-DC converter  81  steps down the voltage of electric power transmitted from the battery  10  via the SMR  30  and supplies the stepped-down voltage to the load  82  and the auxiliary DC-to-DC converter  83 , according to the switching operation of transistors (not shown) that is performed based on a control signal from the HVECU  90 . The high voltage DC-to-DC converter  81  can be regarded as the “DC-to-DC converter” according to the present disclosure. 
     The load  82  is electrically connected between the high voltage DC-to-DC converter  81  and the auxiliary DC-to-DC converter  83 . The load  82  is various devices that operate with electric power supplied from at least one of the high voltage DC-to-DC converter  81  and the auxiliary DC-to-DC converter  83 . More specifically, the load  82  includes auxiliaries and by-wire systems (both not shown). The auxiliaries include, e.g., lights (headlights, fog lights, turn signal lights, cornering lights, etc.), an air conditioning system, an audio system, a car navigation system, an antilock brake system (ABS), an oil pump, meters, a defogger, and actuators that drive windshield wipers and power windows. The by-wire systems include electric power steering, an accelerator, and a brake (such as a brake actuator). 
     The auxiliary DC-to-DC converter  83  is electrically connected between the load  82  and the auxiliary battery  84 . The auxiliary DC-to-DC converter  83  is a bidirectional DC-to-DC converter and is, e.g., a choke converter or a flyback converter. The auxiliary DC-to-DC converter  83  is configured so that it can bidirectionally convert DC power between the load  82  and the auxiliary battery  84  according to the switching operation of transistors (not shown) that is performed based on a control signal from the HVECU  90 . More specifically, when the SMR  30  is closed, the auxiliary DC-to-DC converter  83  can step down the electric power supplied from the battery  10  via the high voltage DC-to-DC converter  81  to charge the auxiliary battery  84 . When the SMR  30  is open, the auxiliary DC-to-DC converter  83  can discharge the auxiliary battery  84  to supply a power supply voltage to the load  82  and the HVECU  90 . 
     The auxiliary battery  84  is configured so that it can be charged and discharged by the auxiliary DC-to-DC converter  83 . The output voltage of the auxiliary battery  84  is lower than the output voltage (e.g., about 200V) of the battery  10  and is, e.g., about 12V. The auxiliary battery  84  is, e.g., a lead-acid battery, but the type of the auxiliary battery  84  is not particularly limited. A capacitor such as an electric double layer capacitor may be used instead of the auxiliary battery  84 . 
     Like the battery ECU  40 , the HVECU  90  includes a processor  91  such as a CPU, a memory  92  such as a ROM and a RAM, and input and output ports (not shown). The HVECU  90  performs drive control of the vehicle  100  based on data received from the battery ECU  40  and programs and maps stored in the memory  92 . In the present embodiment, the HVECU  90  switches between control modes of the vehicle  100 . The control modes of the vehicle  100  include a “normal mode” and a “batteryless drive mode.” These control modes will also be described later. 
     The battery ECU  40  can be regarded as the “first controller” according to the present disclosure. The HVECU  90  can be regarded as the “second controller” according to the present disclosure. The HVECU  90  may be comprised of a plurality of ECUs (an engine ECU, an MGECU, etc.) according to the functions, as described in JP 2019-156007 A etc. 
     Normal Mode 
     First, the normal mode, which is a mode used while the vehicle  100  is traveling normally, will be described. 
       FIG. 3  is a flowchart illustrating an example of a process that is executed in the normal mode. Processes represented by the flowcharts of  FIG. 3  and the figures described later ( FIGS. 4, 8, 9 , etc.) are called from a main routine and repeatedly executed in predetermined cycles when a predetermined condition is satisfied. Each step in these flowcharts is basically implemented by software processing by the HVECU  90 , but may be implemented by hardware processing by an electronic circuit provided in the HVECU  90 . In the following description of the flowchart, “S” stands for “step.” 
     Referring to  FIG. 3 , in S 11 , the HVECU  90  calculates a driving force requested to the vehicle  100  by the user (hereinafter also referred to as the “requested driving force P*”). Specifically, the memory  92  of the HVECU  90  has stored therein driving force maps (not shown) prepared in advance for each shift range. The driving force map defines the relationship among the requested driving force P*, the accelerator operation amount (the operation amount of an accelerator pedal) A, and the vehicle speed V (the second MG rotational speed Nm 2 ). The HVECU  90  can calculate the requested driving force P* from the selected shift range, the accelerator operation amount A, and the vehicle speed V by using the driving force maps. 
     In S 12 , the HVECU  90  calculates requested vehicle power that is required to achieve the requested driving force P*. Specifically, the HVECU  90  can calculate the requested vehicle power by multiplying the requested driving force P* by the vehicle speed V and adding predetermined loss power to the resultant product. 
     In S 13 , the HVECU  90  determines whether to operate or stop the engine  63  based on the requested vehicle power. For example, when the requested vehicle power is larger than a predetermined threshold, the HVECU  90  determines to operate the engine  63 . The HVECU  90  performs S 14  and the subsequent steps when it determines to operate the engine  63 . 
     In S 14 , the HVECU  90  calculates a desired engine rotational speed Ne* from requested engine power. More specifically, since the power required by the vehicle  100  is basically output from the engine  63 , it can be said that the requested engine power is equal to the requested vehicle power. The memory  92  of the HVECU  90  has stored therein a recommended operation line indicating how the engine torque Te changes with the engine rotational speed Ne. The recommended operation line is, e.g., an optimal fuel economy line indicating how the engine torque Te change so as to achieve high fuel efficiency. The HVECU  90  can calculate the desired engine rotational speed Ne* by setting the point of intersection of an iso-power line that represents output equal to the requested engine power and the recommended operation line as an operation point of the engine  63 . 
     In S 15 , the HVECU  90  calculates desired torque Tg* of the first motor generator  61  (hereinafter referred to as the “desired first MG torque Tg*”) for bringing the engine rotational speed Ne closer to the desired engine rotational speed Ne*. The HVECU  90  can calculate the desired first MG torque Tg* by feedback control based on the difference between the current engine rotational speed Ne and the desired engine rotational speed Ne*. 
     In S 16 , the HVECU  90  calculates engine direct torque Tep. The engine direct torque Tep is torque in the positive direction that is transmitted from the engine  63  to the ring gear of the power split device  71  (that is, to the drive shaft  72 ) with first MG torque Tg as a reaction force (see  FIG. 7  described later). The relationship between the engine direct torque Tep and the desired first MG torque Tg* is uniquely determined according to the gear ratio ρ of the power split device  71  (see the following equation (1)). The engine direct torque Tep can therefore be calculated from the desired first MG torque Tg*. 
         Tep=− 1 /ρ×Tg*   (1)
 
     In S 17 , the HVECU  90  calculates desired torque Tm* of the second motor generator  62  (hereinafter referred to as the “desired second MG torque Tm*). The desired second MG torque Tm* is determined so as to achieve the requested driving force P* calculated in S 11 . Specifically, the HVECU  90  can calculate the desired second MG torque Tm* by subtracting the engine direct torque Tep from the requested driving force P* (see the following equation (2)). 
         Tm*=P*−Tep   (2)
 
     In S 18 , the HVECU  90  controls the PCU  50  so that the torque of the first motor generator  61  (first MG torque Tg) and the torque of the second motor generator  62  (second MG torque Tm) become closer to the desired first MG torque Tg* and the desired second MG torque Tm*, respectively. 
     Batteryless Drive Mode 
     Next, the batteryless drive mode will be described. The batteryless drive mode is a mode in which the SMR  30  is opened and the vehicle  100  moves with the battery  10  disconnected from the electrical system (the PCU  50 , the first motor generator  61 , and the second motor generator  62 ). 
     More specifically, “engine feedback (F/B) control” and “power balance control” are performed in the batteryless drive mode. The engine feedback control is a process of feedback-controlling the engine torque Te to control the engine rotational speed Ne to the desired engine rotational speed Ne*. The power balance control is a process of controlling the PCU  50  so that the driving force requested by the user (requested driving force) is transmitted to the drive wheels  73  and the electric power that is generated by the first motor generator  61  (hereinafter also referred to as the “first MG generation power”) becomes equal to the power that is consumed by the second motor generator  62  (hereinafter also referred to as the “second MG discharge power”). 
       FIG. 4  is a flowchart illustrating an example of a process that is executed in the batteryless drive mode. Referring to  FIG. 4 , in S 21 , the HVECU  90  starts the engine  63  (or keeps the engine  63  in operation if already started). In S 22 , the HVECU  90  opens the SMR  30  to electrically disconnect the battery  10  from the electrical system. 
     The HVECU  90  performs the engine feedback control in S 23  and S 24  and performs the power balance control in S 25  to S 27 . The power balance control and the engine feedback control are performed independently. Although  FIG. 4  illustrates an example in which the power balance control is performed after the engine feedback control, the engine feedback control and the power balance control may be performed in a reverse order. 
     In S 23 , the HVECU  90  sets the desired engine rotational speed Ne*. For example, the HVECU  90  sets the upper limit of the desired engine rotational speed Ne* to a lower value as the vehicle speed V is lower. The HVECU  90  sets the desired engine rotational speed Ne* to a higher value within the set upper limit as the accelerator operation amount A is larger. 
     In S 24 , the HVECU  90  feedback-controls the engine torque Te (specifically, the throttle valve opening degree, the ignition timing, the amount of fuel injection, etc.) so that the engine rotational speed Ne becomes closer to the desired engine rotational speed Ne*. 
     In S 25 , the HVECU  90  calculates the requested driving force P* based on the accelerator operation amount A and the vehicle speed V. 
     In S 26 , the HVECU  90  calculates the desired first MG torque Tg* and the desired second MG torque Tm* so that the power corresponding to the requested driving force P* is transmitted to the drive wheels  73  and the first MG generation power and the second MG discharge power become equal to each other. Specifically, the HVECU  90  can calculate the desired first MG torque Tg* and the desired second MG torque Tm* by solving the following pair of equations (3) and (4) representing the control states of the engine  63 , the first motor generator  61 , and the second motor generator  62  during the batteryless drive mode. 
         P *=(− Tg*/p )× Nm 2+ Tm*×Nm 2  (3)
 
         Tg*×Nm 1+ Tm*×Nm 2=0  (4)
 
       FIG. 5  is a nomographic chart illustrating an example of the control states of the first motor generator  61 , the second motor generator  62 , and the engine  63  in the batteryless drive mode. 
     Referring to  FIGS. 4 and 5 , the engine  63  rotates in the forward direction during batteryless drive control (while the vehicle  100  is moving forward). At this time, the first motor generator  61  rotates in the forward direction and generates power generation torque (torque in the negative direction), and the second motor generator  62  rotates in the forward direction and generates discharge torque (torque in the positive direction). Accordingly, in the equations (3) and (4), the desired first MG torque Tg* has a negative value, and the desired second MG torque Tm*, the first MG rotational speed Nm 1 , and the second MG rotational speed Nm 2  have positive values. 
     In the equation (3), (−Tg*/p) is equal to the engine direct torque Tep (see the equation (2)). Accordingly, the first term on the right side of the equation (3), {(−Tg*/p)× Nm 2 }, represents power that is transmitted from the engine  63  to the drive shaft  72 . The second term on the right side of the equation (3), (Tm*×Nm 2 ), represents power that is transmitted from the second motor generator  62  to the drive shaft  72 , that is, the second MG discharge power. The equation (3) thus indicates that the requested driving force P* is achieved by the sum of the power that is transmitted from the engine  63  to the drive shaft  72  and the second MG discharge power. 
     The first term on the left side of the equation (4), (Tg*×Nm 1 ), represents the first MG generation power. As described above, during the batteryless drive control (while the vehicle  100  is moving forward), the first motor generator  61  rotates in the forward direction and generates power generation torque (Nm1&gt;0 and Tg*&lt;0). Accordingly, (Tg*×Nm 1 ) has a negative value. The second term on the left side of the equation (4), (Tm*×Nm 2 ), represents the second MG discharge power. During the batteryless drive control (while the vehicle  100  is moving forward), the second motor generator  62  rotates in the forward direction and generates discharge torque (Nm2&gt;0 and Tm*&gt;0). Accordingly, (Tm*×Nm 2 ) has a positive value. The equation (4) thus indicates that the magnitude (absolute value) of the first MG generation power and the magnitude (absolute value) of the second MG discharge power are equal. 
     In S 27 , the HVECU  90  controls the PCU  50  so that the first MG torque Tg and the second MG torque Tm become closer to the desired first MG torque Tm* and the desired second MG torque Tm*, respectively. 
     Limitation of Charging and Discharging 
     In the vehicle  100 , charging and discharging of the battery  10  is limited in order to protect the battery  10 . In the present embodiment, the HVECU  90  limits the allowable charging power Win of the battery  10  and the allowable discharging power Wout of the battery  10  according to the temperature TB of the battery  10  (battery temperature TB). 
       FIG. 6  is a conceptual diagram illustrating limitation of charging and discharging of the battery  10 . In  FIG. 6 , the abscissa represents the battery temperature TB, and the upward direction on the ordinate represents the allowable discharging power Wout, and the downward direction on the ordinate represents the allowable charging power Win. 
     Referring to  FIG. 6 , in order to slow degradation of the battery  10 , the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are smaller in a low temperature range (the range of TB&lt;Tlow) and a high temperature range (the range of TB&gt;Tup) than in a normal temperature range (the range of Tlow TB Tup). The HVECU  90  sets the desired first MG torque Tg* and the desired second MG torque Tm* so that the sum of the charging and discharging power of the first motor generator  61  and the charging and discharging power of the second motor generator  62  is within the range between the allowable charging power Win and the allowable discharging power Wout of the battery  10 . 
     The HVECU  90  prohibits charging and discharging of the battery  10  when the battery temperature TB becomes lower than a minimum temperature Tmin or higher than a maximum temperature Tmax. Hereinafter, the temperature range of the minimum temperature Tmin or higher and the maximum temperature Tmax or lower is also referred to as the “normal use range.” 
     Although  FIG. 6  illustrates an example of the temperature dependence of the allowable charging power Win and the allowable discharging power Wout, the allowable charging power Win and the allowable discharging power Wout may be limited according to, e.g., the state of charge (SOC) of the battery  10 . 
     System Compatibility 
     Regarding the vehicle  100  configured as described above, there may be a situation where a company of the battery pack  1  (Company A) and a company of the HV system  2  (Company B) are different. Compatibility (coordination) between the battery pack  1  and the HV system  2  can be a problem in such a situation. 
     More specifically, data on the allowable charging power Win and the allowable discharging power Wout of the battery  10  is sent from the battery ECU  40  in the battery pack  1  to the HVECU  90  in the HV system  2 . For example, in the case where Company A and Company B have different understandings about the content, timing, etc. of communication or in the case where coordination between Company A and Company B about communication is not enough, there is a possibility that data different from data expected by the HVECU  90  may be sent from the battery ECU  40  to the HVECU  90  as the data on the allowable charging power Win and the allowable discharging power Wout of the battery  10 . Specifically, there is a possibility that the battery ECU  40  may set the allowable charging power Win and the allowable discharging power Wout to excessively small values (e.g., values close to 0). In this case, the HVECU  90  may reduce (or prohibit) charging and discharging of the battery  10  even though the temperature of the battery  10  is not actually low or high. As a result, the vehicle  100  may become unable to move. 
     The present embodiment uses a configuration in which, when the magnitude of the allowable charging power Win or the magnitude of the allowable discharging power Wout sent from the battery ECU  40  becomes excessively small during the normal mode, the control mode is switched from the normal mode to the batteryless drive mode regardless of whether this decrease in magnitude of the allowable charging power Win or magnitude of the allowable discharging power Wout is due to the battery temperature TB having fallen out of the normal use range or due to compatibility between the battery pack  1  and the HV system  2 . The batteryless drive mode is a control mode originally intended for limp-home driving (fail-safe driving) of the vehicle  100  in case of an abnormality in the battery  10 . In the present embodiment, however, the batteryless drive mode is not used only for this purpose. The control mode is actively switched to the batteryless drive mode regardless of whether limp-home driving is required, thereby ensuring the driving performance of the vehicle  100 . 
     Switching of Control Mode 
       FIG. 7  is a state transition diagram (state machine diagram) illustrating switching of the control mode of the vehicle  100  in the first embodiment. Referring to  FIG. 7 , the HVECU  90  switches the control mode of the vehicle  100  from the normal mode to the batteryless drive mode when a predetermined “limit condition” is satisfied during the normal mode. The HVECU  90  switches the control mode of the vehicle  100  from the batteryless drive mode back to the normal mode when a predetermined “return condition” is satisfied during the batteryless drive mode. The limit condition and the return condition will be described with reference to flowcharts. 
     Control Flow 
       FIG. 8  is a flowchart illustrating an example of a process for switching of the control mode (determination of whether the limit condition is satisfied) during the normal mode. Referring to  FIG. 8 , in S 31 , the HVECU  90  acquires the battery temperature TB detected by the temperature sensor  23 . The HVECU  90  may acquire the battery temperature TB by communication with the battery ECU  40  or may acquire the battery temperature TB directly from the temperature sensor  23 . The HVECU  90  also acquires the allowable charging power Win and the allowable discharging power Wout of the battery  10  from the battery ECU  40  (S 32 ). 
     Subsequently, in S 33  and S 34 , the HVECU  90  determines whether the magnitude of the allowable charging power Win of the battery  10  is equal to or smaller than a first predetermined value X 1 . The HVECU  90  also determines whether the magnitude of the allowable discharging power Wout of the battery  10  is equal to or smaller than the first predetermined value X 1 . In order to facilitate understanding of the description, both the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are compared to the same value (first predetermined value X 1 ) in this example. However, the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout may be compared to different values from each other. 
     When the magnitude of the allowable charging power Win is larger than the first predetermined value X 1  and the magnitude of the allowable discharging power Wout is larger than the first predetermined value X 1  (NO in S 33 ), the HVECU  90  does not switch the control mode of the vehicle  100  and the process returns to the main routine. That is, the HVECU  90  maintains the normal mode of the vehicle  100 . 
     On the other hand, when the magnitude of the allowable charging power Win is equal to or smaller than the first predetermined value X 1  or the magnitude of the allowable discharging power Wout is equal to or smaller than the first predetermined value X 1  (YES in S 33 ), the process proceeds to S 34  and the HVECU  90  determines whether the battery temperature TB is within the normal use range. As described above, the normal use range is a temperature range in which the allowable charging power Win does not become equal to 0 and the allowable discharging power Wout does not become equal to 0. In the example of  FIG. 6 , the normal use range is a temperature range from TB=Tmin to TB=Tmax. 
     When the battery temperature TB is out of the normal use range (NO in S 34 ), it is highly likely that the magnitude of the allowable charging power Win or the magnitude of the allowable discharging power Wout has become equal to or smaller than the first predetermined value X 1  due to the high or low battery temperature TB. The HVECU  90  therefore non-volatilely records on the memory  92  a history indicating that it has become necessary to reduce the charging and discharging power of the battery  10  due to an increase or decrease in battery temperature TB (hereinafter also referred to as the “temperature change history”) (S 35 ). For example, the HVECU  90  can set a management flag assigned to the memory  92  to on. The process then proceeds to S 36 . 
     When the battery temperature TB is within the normal use range (YES in S 34 ), there is a possibility that it has become necessary to reduce the charging and discharging power of the battery  10  due to compatibility between the battery pack  1  and the HV system  2  rather than due to a change in battery temperature TB. The HVECU  90  therefore does not record the temperature change history, and the process proceeds to S 36 . 
     In S 36 , the HVECU  90  determines that the limit condition is satisfied. The HVECU  90  then switches the control mode of the vehicle  100  from the normal mode to the batteryless drive mode. 
       FIG. 9  is a flowchart illustrating an example of a process for switching of the control mode (determination of whether the return condition is satisfied) during the batteryless drive mode. Referring to  FIG. 9 , the HVECU  90  first acquires the battery temperature TB (S 41 ) and acquires the allowable charging power Win and the allowable discharging power Wout of the battery  10  from the battery ECU  40  (S 42 ). 
     In S 43 , the HVECU  90  determines whether the magnitude of the allowable charging power Win of the battery  10  is larger than the first predetermined value X 1 . The HVECU  90  also determines whether the magnitude of the allowable discharging power Wout of the battery  10  is larger than the first predetermined value X 1 . 
     When the magnitude of the allowable charging power Win is equal to or smaller than the first predetermined value X 1  or when the magnitude of the allowable discharging power Wout is equal to or smaller than the first predetermined value X 1  (NO in S 43 ), it may still necessary to reduce the charging and discharging power of the battery  10 . The process thus returns to the main routine, and the HVECU  90  maintains the batteryless drive mode of the vehicle  100 . 
     When the magnitude of the allowable charging power Win is larger than the first predetermined value X 1  and the magnitude of the allowable discharging power Wout is larger than the first predetermined value X 1  (YES in S 43 ), that is, when it becomes less necessary to reduce the charging and discharging power of the battery  10 , the process proceeds to S 44 , and the HVECU  90  determines whether the memory  92  has the temperature change history (see S 35  in  FIG. 8 ) recorded thereon. 
     When there is no temperature change history (NO in S 44 ), there is a possibility that the charging and discharging power of the battery  10  has been reduced due to compatibility between the battery pack  1  and the HV system  2 . In this case, the HVECU  90  does not switch the control mode of the vehicle  100  back to the normal mode but maintains the batteryless drive mode. 
     When there is the temperature change history (YES in S 44 ), there is a possibility that the charging and discharging power of the battery  10  has been reduced due to a change in battery temperature TB rather than due to compatibility between the battery pack  1  and the HV system  2 . Accordingly, in the case where it has become less necessary to reduce the charging and discharging power of the battery  10  due to the battery temperature TB having gone back into the normal use range, the control mode of the vehicle  100  can be switched back to the normal mode. 
     However, it is desirable to avoid switching the control mode of the vehicle  100  back to the normal mode while the vehicle  100  is moving. Accordingly, in S 45  to S 47 , the HVECU  90  determines whether the vehicle  100  is completely stopped and is ready to return to the normal mode. Specifically, the HVECU  90  determines whether the vehicle speed V=0 (S 45 ), whether the brake of the vehicle  100  is in operation (S 46 ), and whether the accelerator pedal of the vehicle  100  is not being operated (accelerator operation amount A=0) (S 47 ). 
     When at least one of the determination results of S 45  to S 47  is NO, the HVECU  90  determines that the traveling state of the vehicle  100  is not suitable for returning to the normal mode, and the process returns to the main routine (the return condition is not satisfied). When all of the determination results of S 45  to S 47  are YES, the HVECU  90  determines the traveling state of the vehicle  100  is suitable for returning to the normal mode and determines that the return condition is satisfied (S 48 ). 
     As described above, in the first embodiment, the HVECU  90  in the HV system  2  receives the allowable charging power Win and the allowable discharging power Wout of the battery  10  from the battery ECU  40  in the battery pack  1 . When the magnitude of the allowable charging power Win becomes equal to or smaller than the first predetermined value X 1  or the magnitude of the allowable discharging power Wout becomes equal to or smaller than the first predetermined value X 1  during the normal mode of the vehicle  100 , the HVECU  90  switches the control mode of the vehicle  100  to the batteryless drive mode regardless of whether this decrease in magnitude of the allowable charging power Win or magnitude of the allowable discharging power Wout is due to the state of the battery  10  or due to communication between the battery ECU  40  and the HVECU  90 . The vehicle  100  can thus be made to move even when there is a problem with communication between the battery ECU  40  and the HVECU  90 . 
     The HVECU  90  also determines whether to record a temperature change history when switching the control mode of the vehicle  100  to the batteryless drive mode. When there is the temperature change history, the HVECU  90  can switch the control mode back to the normal mode when the allowable charging power Win and the allowable discharging power Wout are restored (when the magnitude of the allowable charging power Win becomes larger than the first predetermined value X 1  and the magnitude of the allowable discharging power Wout becomes larger than the first predetermined value X 1 ), as the HVECU  90  determines that there is no problem with communication between the battery ECU  40  and the HVECU  90  (there is no problem with compatibility between the battery pack  1  and the HV system  2 ). The driving performance of the vehicle  100  can thus be quickly restored. 
     When there is no temperature change history, the HVECU  90  maintains the batteryless drive mode even if the allowable charging power Win and the allowable discharging power Wout are restored, as there is a possibility that there is a problem with communication between the battery ECU  40  and the HVECU  90 . A certain level of driving performance of the vehicle  100  can thus be ensured while protecting the battery  10 . 
     Second Embodiment 
     As described in the first embodiment, the HVECU  90  reduces charging and discharging of the battery  10  when the magnitude of the allowable charging power Win of the battery  10  decreases and the magnitude of the allowable discharging power Wout also decreases. In the second embodiment, a configuration will be described in which the auxiliary battery  84  is used for operation of the load  82  when charging and discharging of the battery  10  is reduced. 
     Since the overall configurations of hybrid vehicles according to the second embodiment and a third embodiment described later are similar to the overall configuration of the vehicle  100  according to the first embodiment (see  FIG. 1 ), description thereof will not be repeated. 
     State Transition Diagram 
       FIG. 10  is a state transition diagram illustrating switching of the control mode of the vehicle  100  in the second embodiment. Referring to  FIG. 10 , in the second embodiment, the HVECU  90  has an “auxiliary constant power mode” in addition to the normal mode and the batteryless drive mode. 
     In the normal mode, the HVECU  90  performs constant voltage control of the high voltage DC-to-DC converter  81 . When power consumption of the load  82  fluctuates during the constant voltage control such as when any of the lights is turned on or off or the windshield wipers are turned on or off, the charging and discharging power of the high voltage DC-to-DC converter  81  fluctuates. However, the voltage of the load  82  can be kept constant by the constant voltage control of the high voltage DC-to-DC converter  81 . 
     The charging and discharging power of the battery  10  may fluctuate with the fluctuations in charging and discharging power of the high voltage DC-to-DC converter  81 . Accordingly, when charging and discharging of the battery  10  has to be reduced due to decreases in allowable charging power Win and allowable discharging power Wout, the fluctuating charging and discharging power of the high voltage DC-to-DC converter  81  may not be able to be covered by the battery  10  alone. 
     In the second embodiment, when the allowable charging power Win and the allowable discharging power Wout of the battery  10  decrease (before switching to the batteryless drive mode), the HVECU  90  switches the control mode of the vehicle  100  from the normal mode to the auxiliary constant power mode. Specifically, the HVECU  90  switches the control of the high voltage DC-to-DC converter  81  from the constant voltage control in the normal mode to constant power control. The input and output power of the high voltage DC-to-DC converter  81  is therefore constant even when the power consumption of the load  82  fluctuates. The influence of the fluctuations in power consumption of the load  82  on the battery  10  is thus reduced. In this case, the voltage of the load  82  may fluctuate, but such voltage fluctuations of the load  82  can be absorbed by the auxiliary battery  84 . 
     When a “first limit condition” is satisfied during the normal mode, the HVECU  90  switches the control mode of the vehicle  100  from the normal mode to the auxiliary constant power mode. When a “second limit condition” is satisfied during the auxiliary constant power mode, the HVECU  90  switches the control mode of the vehicle  100  from the auxiliary constant power mode to the batteryless drive mode. When a “first return condition” is satisfied during the batteryless drive mode, the HVECU  90  switches the control mode of the vehicle  100  from the batteryless drive mode back to the auxiliary constant power mode. When a “second return condition” is satisfied during the auxiliary constant power mode, the HVECU  90  switches the control mode of the vehicle  100  from the auxiliary constant power mode back to the normal mode. 
     Although not shown in the figure, when the “second limit condition” is satisfied during the normal mode, the HVECU  90  may switch the control mode of the vehicle  100  from the normal mode directly to the batteryless drive mode by skipping the auxiliary constant power mode. When the “second return condition” is satisfied during the batteryless drive mode, the HVECU  90  may switch the control mode of the vehicle  100  from the batteryless drive mode directly to the normal mode by skipping the auxiliary constant power mode. 
     Relationship with Win and Wout 
       FIG. 11  is a graph illustrating the relationship between the allowable charging power Win and the allowable discharging power Wout of the battery  10  and the control mode in the second embodiment. In  FIG. 11  and  FIG. 16  described later, the abscissa represents the magnitude (absolute value) of the allowable charging power Win of the battery  10 , and the ordinate represents the magnitude of the allowable discharging power Wout of the battery  10 . 
     Referring to  FIG. 11 , the batteryless drive mode can be selected (1) when the magnitude of the allowable charging power Win is equal to or smaller than the first predetermined value X 1  or (2) when the magnitude of the allowable discharging power Wout is equal to or smaller than the first predetermined value X 1 . Hereinafter, this region of the allowable charging power Win and the allowable discharging power Wout is referred to as the “charging and discharging region R 3 .” 
     The auxiliary constant power mode can be selected (1) when the magnitude of the allowable charging power Win is larger than X 1  and the magnitude of the allowable discharging power Wout is larger than X 1  and equal to or smaller than X 2  or (2) when the magnitude of the allowable charging power Win is larger than X 1  and equal to or smaller than X 2  and the magnitude of the allowable discharging power Wout is larger than X 1 . Hereinafter, this region of the allowable charging power Win and the allowable discharging power Wout is referred to as the “charging and discharging region R 2 .” 
     The normal mode can be selected in cases other than the above, that is, when the magnitude of the allowable charging power Win is larger than X 2  and the magnitude of the allowable discharging power Wout is larger than X 2 . Hereinafter, this region of the allowable charging power Win and the allowable discharging power Wout is referred to as the “charging and discharging region R 1 .” 
     The second embodiment (and the third embodiment described later) also illustrates an example in which the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are compared to the same value (the first predetermined value X 1  or the second predetermined value X 2 ) for each control mode. However, this is for the purpose of avoiding complication of the description, and the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout may be compared to different values from each other. 
     Determination of Whether Limit Condition is Satisfied 
       FIG. 12  is a flowchart illustrating an example of a process for determination of whether the limit condition (the first limit condition or the second limit condition) is satisfied according to the second embodiment. Referring to  FIGS. 12 , S 51  and S 52  are similar to S 31  and S 32  (see  FIG. 8 ) in the first embodiment. 
     In S 53 , the HVECU  90  compares the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout of the battery  10  to a predetermined value (the first predetermined value X 1  or the second predetermined value X 2 ). When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 2  (R 2  in S 53 ), the process proceeds to S 54 . S 54  and S 55  are similar to S 34  and S 35  in the first embodiment. The HVECU  90  then determines that the first limit condition is satisfied and selects the auxiliary constant power mode as the control mode of the vehicle  100  (S 56 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 3  in S 53  (R 3  in S 53 ), the HVECU  90  causes the process to proceed to S 57 . S 57  and S 58  are similar to S 54  and S 55 . The HVECU  90  then determines that the second limit condition is satisfied and selects the batteryless drive mode as the control mode of the vehicle  100  (S 59 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 1  in S 53  (R 1  in S 53 ), the HVECU  90  does not perform S 54  and the subsequent steps or S 57  and the subsequent steps and the process returns to the main routine. In this case, the normal mode is selected as the control mode of the vehicle  100 . 
     Determination of Whether Return Condition is Satisfied 
       FIG. 13  is a flowchart illustrating an example of a process for determination of whether the return condition (the first return condition or the second return condition) is satisfied according to the second embodiment. Referring to  FIGS. 13 , S 601  and S 602  are similar to S 41  and S 42  (see  FIG. 9 ) in the first embodiment. 
     In S 603 , the HVECU  90  compares the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout of the battery  10  to a predetermined value (the first predetermined value X 1  or the second predetermined value X 2 ). When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 2  (R 2  in S 603 ), the HVECU  90  causes the process to proceed to S 604 . S 604  to S 607  are similar to S 44  to S 47  (see  FIG. 9 ) in the first embodiment. The HVECU  90  then determines that the first return condition is satisfied and selects the auxiliary constant power mode as the control mode of the vehicle  100  (S 608 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 1  in S 603  (R 1  in S 603 ), the HVECU  90  causes the process to proceed to S 609 . In S 609 , the HVECU  90  determines whether there is a temperature change history. When there is a temperature change history (YES in S 609 ), the HVECU  90  determines that the second return condition is satisfied and selects the normal mode as the control mode of the vehicle  100  (S 610 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 3  in S 603  (R 3  in S 603 ), the HVECU  90  does not perform S 604  and the subsequent steps or S 609  and the subsequent steps and the process returns to the main routine. In this case, the batteryless drive mode is maintained as the control mode of the vehicle  100 . 
     As described above, in the second embodiment, the HVECU  90  further has the auxiliary constant power mode in which the HVECU  90  performs the constant power control of the high voltage DC-to-DC converter  81 . In the auxiliary constant power mode, a change in charging and discharging power of the battery  10  with fluctuations in electric power of the load  82  is reduced by allowing voltage fluctuations of the auxiliary battery  84  with fluctuations in power consumption of the load  82  to some extent. The battery  10  whose allowable charging power Win and allowable discharging power Wout are relatively severely limited can thus be more reliably protected. 
     Depending on the SOC of the auxiliary battery  84 , fluctuations in electric power of the load  82  may not be completely absorbed by the auxiliary battery  84 . When the auxiliary battery  84  is close to its fully charged state, the auxiliary battery  84  can receive only a small amount of electric power. When the auxiliary battery  84  almost runs out, the auxiliary battery  84  can supply only a small amount of electric power. It is therefore desirable to set an upper limit on the rate of change in charging and discharging power of the high voltage DC-to-DC converter  81  according to the SOC of the auxiliary battery  84  so that the charging and discharging power of the high voltage DC-to-DC converter  81  does not change suddenly. Specifically, when the auxiliary battery  84  is close to its fully charged state or when the auxiliary battery  84  almost runs out, the rate of change in charging and discharging power of the high voltage DC-to-DC converter  81  can be made as low as a predetermined value or less. 
     The second embodiment is described with respect to an example in which the vehicle  100  has three control modes: the normal mode, the auxiliary constant power mode, and the batteryless drive mode. However, the batteryless drive mode is not an essential control mode. The vehicle  100  may have two control modes: the normal mode and the auxiliary constant power mode, and may be configured to switch the control mode between the normal mode and the auxiliary constant power mode. 
     Third Embodiment 
     Even when the engine torque Te (or engine power Pe) to be output is determined, a slight error in engine power Pe may occur in actual engine control due to excessive fuel supply etc. In the third embodiment, control for handling on the error in engine power Pe will be described. 
     State Transition Diagram 
       FIG. 14  is a state transition diagram illustrating switching of the control mode of the vehicle  100  in the third embodiment. Referring to  FIG. 14 , in the third embodiment, the HVECU  90  has a “reduced guarantee mode” in addition to the normal mode, the auxiliary constant power mode, and the batteryless drive mode. 
     As described above, in the normal mode, each torque (the engine torque Te, the first MG torque Tg, and the second MG torque Tm) is adjusted so as to achieve the requested driving force P* according to the operation of the accelerator pedal by the user. When the engine power Pe changes, the first MG power and the second MG power are controlled so as to compensate for the change. Accordingly, for example, when a desired value of the engine power Pe is 4.0 kW and an actual value of the engine power Pe is 4.5 kW, the battery  10  is charged with the power corresponding to the difference therebetween, which is 4.5 kW-4.0 kW=500 W. When the desired value of the engine power Pe is 4.0 kW and the actual value of the engine power Pe is 3.5 kW, the power of 500 W is discharged from the battery  10 . 
     As described above, in the normal mode, the error in engine power Pe is absorbed by charging or discharging of the battery  10 . This control is based on the design idea of guaranteeing the requested driving force P*, namely giving priority to accurately reflecting the operation of the accelerator pedal by the user in the driving force that is generated by the vehicle  100 . 
     In the batteryless drive mode, the SMR  30  is opened and the battery  10  is electrically disconnected from the PCU  50 . Accordingly, it may be more difficult than in the normal mode to adjust the first MG power and the second MG power so as to achieve the requested driving force P*. That is, it may be difficult to guarantee the requested driving force P*. 
     In the third embodiment, as the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout of the battery  10  decrease, the control mode is switched to the reduced guarantee mode before being switched to the batteryless drive mode. In the reduced guarantee mode, the HVECU  90  stops feedback control of the first MG power and the second MG power for achieving the requested driving force P*, while keeping the SMR  30  closed. In this case, when an error in engine power Pe occurs, the driving force of the vehicle  100  may increase or decrease with the error not sufficiently absorbed by charging or discharging of the battery  10 . As a result, a change in acceleration or deceleration of the vehicle  100  may be larger than in the normal mode. 
     As described above, the reduced guarantee mode is a control mode based on the idea of less strictly guaranteeing the requested driving force P* in a situation where the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout have decreased, but not so much as to switch the control mode to the batteryless drive mode. A slight deviation between the driving force according to the operation of the accelerator pedal by the user (the requested driving force P*) and the actual driving force is allowed in the reduced guarantee mode. 
     In the reduced guarantee mode, the SMR  30  is closed and the battery  10  can be charged and discharged. Accordingly, the driving performance of the vehicle  100  is better in the reduced guarantee mode than in the batteryless drive mode. From another point of view, in conventional vehicles that are not equipped with a traction battery (conventional vehicles such as gasoline vehicles), an error in engine power may directly affect the vehicle driving force. It can also be said that the reduced guarantee mode is a control mode in which the level of guarantee of the requested driving force P* is the same as that in the conventional vehicles. 
     When a “first limit condition” is satisfied during the normal mode, the HVECU  90  switches the control mode of the vehicle  100  from the normal mode to the auxiliary constant power mode. When a “second limit condition” is satisfied during the auxiliary constant power mode, the HVECU  90  switches the control mode of the vehicle  100  from the auxiliary constant power mode to the reduced guarantee mode. When a “third limit condition” is satisfied during the reduced guarantee mode, the HVECU  90  switches the control mode of the vehicle  100  from the reduced guarantee mode to the batteryless drive mode. 
     When a “first return condition” is satisfied during the batteryless drive mode, the HVECU  90  switches the control mode of the vehicle  100  from the batteryless drive mode back to the reduced guarantee mode. When a “second return condition” is satisfied during the reduced guarantee mode, the HVECU  90  switches the control mode of the vehicle  100  from the reduced guarantee mode back to the auxiliary constant power mode. When a “third return condition” is satisfied during the auxiliary constant power mode, the HVECU  90  switches the control mode of the vehicle  100  from the auxiliary constant power mode back to the normal mode. 
     In the third embodiment as well, the HVECU  90  may skip the mode between one mode and another mode. For example, the HVECU  90  may switch the control mode of the vehicle  100  from the normal mode to the reduced guarantee mode by skipping the auxiliary constant power mode. The HVECU  90  may switch the control mode of the vehicle  100  from the normal mode to the batteryless drive mode by skipping the auxiliary constant power mode and the reduced guarantee mode. The HVECU  90  may switch the control mode of the vehicle  100  from the auxiliary constant power mode to the batteryless drive mode by skipping the reduced guarantee mode. Although detailed description will not be repeated, the same applies to when switching the control mode of the vehicle  100  from the batteryless drive mode back to the normal mode. 
     Reduced Guarantee Mode 
       FIG. 15  is a flowchart illustrating an example of a process in the reduced guarantee mode. Referring to  FIG. 15 , since S 71  to S 74  are similar to S 11  to S 14  in the normal mode (see  FIG. 3 ), description thereof will not be repeated. 
     In S 75 , the HVECU  90  calculates the desired first MG torque Tg* according to the desired engine rotational speed Ne* calculated in S 74 . That is, the HVECU  90  does not use feedback based on the difference between the current engine rotational speed Ne and the desired engine rotational speed Ne* to calculate the desired first MG torque Tg*. 
     In S 76 , the HVECU  90  calculates the engine direct torque Tep from the desired first MG torque Tg*. The HVECU  90  also calculates the desired second MG torque Tm* by subtracting the engine direct torque Tep from the requested driving force P* (S 77 ). The HVECU  90  then controls the PCU  50  so that the first MG torque Tg and the second MG torque Tm become closer to the desired first MG torque Tg* and the desired second MG torque Tm*, respectively (S 78 ). These steps are similar to the corresponding steps in the normal mode (S 16  to S 18 ). 
     As described above, in the reduced guarantee mode, even when a slight error in operation point of the engine  63  (the engine rotational speed Ne or the engine power Pe) occurs due to excessive fuel supply etc., no adjustment of the desired first MG torque Tg* (and the desired second MG torque Tm*) is made to absorb the error. In this case, an error in actual driving force from the requested driving force P* may occur. However, this error in driving force is allowed. 
     Relationship with Win and Wout 
       FIG. 16  is a graph illustrating the relationship between the allowable charging power Win and the allowable discharging power Wout of the battery  10  and the control mode in the third embodiment. Referring to  FIG. 16 , the batteryless drive mode can be selected when the magnitude of the allowable charging power Win is equal to or smaller than X 1  or when the magnitude of the allowable discharging power Wout is equal to or smaller than X 1 . Hereinafter, this region of the allowable charging power Win and the allowable discharging power Wout is referred to as the “charging and discharging region R 4 .” 
     The reduced guarantee mode can be selected (1) when the magnitude of the allowable charging power Win is larger than X 1  and the magnitude of the allowable discharging power Wout is larger than X 1  and equal to or smaller than X 2  or (2) when the magnitude of the allowable charging power Win is larger than X 1  and equal to or smaller than X 2  and the magnitude of the allowable discharging power Wout is larger than X 1 . Hereinafter, this region of the allowable charging power Win and the allowable discharging power Wout is referred to as the “charging and discharging region R 3 .” 
     The auxiliary constant power mode can be selected (1) when the magnitude of the allowable charging power Win is larger than X 2  and the magnitude of the allowable discharging power Wout is larger than X 2  and equal to or smaller than X 3  or (2) when the magnitude of the allowable charging power Win is larger than X 2  and equal to or smaller than X 3  and the magnitude of the allowable discharging power Wout is larger than X 2 . Hereinafter, this region of the allowable charging power Win and the allowable discharging power Wout is referred to as the “charging and discharging region R 2 .” 
     The normal mode can be selected in cases other than the above, that is, when the magnitude of the allowable charging power Win is larger than X 3  and the magnitude of the allowable discharging power Wout is larger than X 3 . Hereinafter, this region of the allowable charging power Win and the allowable discharging power Wout is referred to as the “charging and discharging region R 1 .” 
     Determination of Whether Limit Condition is Satisfied 
       FIG. 17  is a flowchart illustrating an example of a process for determination of whether the limit condition is satisfied according to the third embodiment. In  FIG. 17  and  FIG. 18  described later, the steps of acquiring the battery temperature TB and acquiring the allowable charging power Win and the allowable discharging power Wout by the HVECU  90  (see S 51  and S 52  of  FIG. 12  or S 601  and S 602  of  FIG. 13 ) are not shown due to space limitations. 
     Referring to  FIG. 17 , in S 801  (and S 808 ), the HVECU  90  compares the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout of the battery  10  to predetermined values (the first to third predetermined values X 1  to X 3 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 2  (R 2  in S 801 ), the HVECU  90  determines that the first limit condition is satisfied and selects the auxiliary constant power mode (S 804 ). S 802  and S 803  before S 804  are similar to S 54  and S 55  (see  FIG. 12 ) in the second embodiment. S 805 , S 806  and S 809 , S 810  are also similar to S 54  and S 55  in the second embodiment. 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 3  (R 3  in S 801 ), the HVECU  90  determines that the second limit condition is satisfied and selects the reduced guarantee mode (S 807 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 4  (“OTHERS” in S 801  and R 4  in S 808 ), the HVECU  90  determines that the third limit condition is satisfied and selects the batteryless drive mode (S 811 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 1  (“OTHERS” in S 801  and R 1  in S 808 ), the HVECU  90  causes the process to return to the main routine. In this case, none of the first to third limit conditions are satisfied, and the control mode is not switched to any mode in which charging and discharging of the battery  10  is more reduced. 
     Determination of Whether Return Condition is Satisfied 
       FIG. 18  is a flowchart illustrating an example of a process for determination of whether the return condition is satisfied according to the third embodiment. Referring to  FIG. 18 , in S 901  (and S 908 ), the HVECU  90  compares the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout of the battery  10  to predetermined values (the first to third predetermined values X 1  to X 3 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 3  (R 3  in S 901 ), the HVECU  90  determines that the first return condition is satisfied and selects the reduced guarantee mode (S 905 ). S 902  to S 904  before S 905  are similar to S 604  to S 607  (see  FIG. 13 ) in the second embodiment. S 906  and S 909  are similar to S 902 . 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 2  (R 2  in S 901 ), the HVECU  90  determines that the second return condition is satisfied and selects the auxiliary constant power mode (S 907 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 1  (“OTHERS” in S 901  and R 1  in S 908 ), the HVECU  90  determines that the third return condition is satisfied and selects the normal mode (S 910 ). 
     When the magnitude of the allowable charging power Win and the magnitude of the allowable discharging power Wout are included in the charging and discharging region R 4  (“OTHERS” in S 901  and R 4  in S 908 ), the HVECU  90  causes the process to return to the main routine. In this case, none of the first to third return conditions are satisfied, and the control mode is not switched to any mode in which charging and discharging of the battery  10  is less reduced. 
     As described above, in the third embodiment, the HVECU  90  further has the reduced guarantee mode in which the HVECU  90  less strictly guarantees the requested driving force P* than in the normal mode. In the reduced guarantee mode, fluctuations in charging and discharging power of the battery  10  with fluctuations in engine power Pe are reduced by allowing to some extent an error in actual driving force from the requested driving force P* which occurs with fluctuations in engine torque Te. The battery  10  whose allowable charging power Win and allowable discharging power Wout are relatively severely limited can thus be more reliably protected. 
     The following aspect may also be used as a modification of the present disclosure. A hybrid vehicle according to another aspect of the present disclosure includes: an engine; a first rotating electrical machine; a second rotating electrical machine connected to drive wheels via an output shaft; a planetary gear set; a battery; a power converter configured to convert electric power among the battery, the first rotating electrical machine, and the second rotating electrical machine; a relay electrically connected between the battery and the power converter; an auxiliary load; a DC-to-DC converter electrically connected between the relay and the auxiliary load; and an auxiliary battery configured to supply electric power to the auxiliary load; and first and second controllers. The planetary gear set is configured to mechanically couple the engine, the first rotating electrical machine, and the output shaft and is configured to transmit torque among the engine, the first rotating electrical machine, and the output shaft. The first controller is configured to calculate allowable charging power of the battery and allowable discharging power of the battery and output the calculation results. The second controller has a constant voltage control mode in which the second controller performs constant voltage control of the DC-to-DC converter with the relay closed and a constant power mode in which the second controller performs constant power control of the DC-to-DC converter with the relay closed. The second controller is configured to select the constant power mode when at least one of the magnitude of the allowable charging power and the magnitude of the allowable discharging power becomes smaller than a predetermined value. 
     The following aspect may also be used as a modification of the present disclosure. A hybrid vehicle according to still another aspect of the present disclosure includes: an engine; a first rotating electrical machine; a second rotating electrical machine connected to drive wheels via an output shaft; a planetary gear set; a battery; a power converter configured to convert electric power among the battery, the first rotating electrical machine, and the second rotating electrical machine; a relay electrically connected between the battery and the power converter; an auxiliary load; a DC-to-DC converter electrically connected between the relay and the auxiliary load; an auxiliary battery configured to supply electric power to the auxiliary load; and first and second controllers. The planetary gear set is configured to mechanically couple the engine, the first rotating electrical machine, and the output shaft and is configured to transmit torque among the engine, the first rotating electrical machine, and the output shaft. The first controller is configured to calculate allowable charging power of the battery and allowable discharging power of the battery and output the calculation results. The second controller is configured to control the power converter according to the allowable charging power and the allowable discharging power received from the first controller. The second controller has a normal mode and a reduced guarantee mode. The normal mode is a mode in which the second controller guarantees that a driving force that is output from the hybrid vehicle is controlled to a requested driving force by adjusting torque of the engine, torque of the first rotating electrical machine, and torque of the second rotating electrical machine with the relay closed. The reduced guarantee mode is a mode in which the second controller less strictly guarantees that the driving force that is output from the hybrid vehicle is controlled to the requested driving force than in the normal mode with the relay closed. The second controller is configured to select the reduced guarantee mode when at least one of the magnitude of the allowable charging power and the magnitude of the allowable discharging power become smaller than a predetermined value. 
     The embodiments disclosed herein should be considered as illustrative in all respects and not as restrictive. The scope of the present disclosure is shown not by the above description of the embodiments but by the claims and is intended to include all modifications that are made without departing from the spirit and scope of the claims.