Abstract:
An electric motor can assist an engine at high conversion efficiency. A lower limit is set for the torque shared with an engine by the electric motor when the engine and the electric motor are operated together for traveling, and a hybrid ECU has an assistance control unit that implements control to a traveling mode in which the engine and the electric motor operate together, only when it is estimated that the torque shared with the engine by the electric motor is equal to or greater than the torque lower limit when the electric motor and the engine are operated together for traveling.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. national stage of application No. PCT/JP2011/074142, filed on Oct. 20, 2011. Priority under 35 U.S.C.§119(a) and 35 U.S.C.§365(b) is claimed from Japanese Patent Application No. 2011-012769, filed on Jan. 25, 2011, the disclosure of which are also incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a control device, a hybrid vehicle, a regeneration control method, and a computer program. 
     BACKGROUND ART 
     A hybrid vehicle includes an engine and an electric motor, and is capable of running with the engine or the electric motor or capable of running by the cooperation between the engine and the electric motor. For example, Patent Literature 1 proposes a hybrid vehicle configured to optimize the fuel consumption of the engine and the state of charge (hereinafter, referred to as SOC) of the battery when the vehicle runs by the cooperation between the engine and the electric motor. Note that, in the below description, the state in which the vehicle runs by the cooperation between the engine and the electric motor will be referred to as “the electric motor assists the engine.” 
     CITATION LIST 
     Patent Literature 
     PTL1: JP 2009-107384 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In a hybrid vehicle, the role of the electric motor is to input electric energy from the battery and convert the energy into energy for the rotary motion to rotate a shaft of the electric motor. Generally, when use of the electric motor at relatively low torque is compared to use of the electric motor at relatively high torque, the use at relatively high torque has better conversion efficiency for converting the electric energy into the energy for the rotary motion. Note that, in the below description, the “conversion efficiency” mainly means the conversion efficiency when the electric motor inputs electric energy from the battery and converts the energy into energy for the rotary motion to rotate a shaft of the electric motor. 
     By the way, as for a conventional hybrid vehicle, for example, the optimization of the fuel consumption of the engine and the State of Charge of the battery when the vehicle runs by the cooperation between the engine and the electric motor is taken into consideration. However, the conversion efficiency of the electric motor is not taken into consideration. Thus, the electric motor is likely to be used over a long time while the conversion efficiency of the electric motor is low. It is undesirable because such a state easily causes the decrease in the SOC of the battery. 
     In light of the foregoing, an objective of the present invention is to provide a control device, a hybrid vehicle, a control method, and a computer program where the electric motor can assist the engine with good conversion efficiency. 
     Solution to Problem 
     An aspect of the present invention relates to a control device. The control device of a hybrid vehicle that includes an engine and an electric motor and that is capable of running by the engine or the electric motor or capable of running by cooperation between the engine and the electric motor, includes: besides a lower limit set on torque shared by the electric motor and the engine when the vehicle runs by the cooperation between the engine and the electric motor, control means for performing a control to set a running mode in which the engine cooperates with the electric motor only when it is estimated that the torque shared by the electric motor and the engine is equal to or more than the lower limit or exceeds the lower limit while the vehicle runs by the cooperation between the engine and the electric motor. 
     Alternatively, a control device of a hybrid vehicle that includes an engine and an electric motor and that is capable of running by the engine or the electric motor or capable of running by cooperation between the engine and the electric motor, includes: besides a lower limit set on torque shared by the electric motor and the engine when the vehicle runs by the cooperation between the engine and the electric motor, control means for performing a control to set a running mode in which the engine cooperates with the electric motor only when it is estimated that the time in which the torque shared by the electric motor and the engine is equal to or more than the lower limit or exceeds the lower limit continues for a given length of time while the vehicle runs by the cooperation between the engine and the electric motor. 
     Another aspect of the present invention relates to a hybrid vehicle. The hybrid vehicle of the present invention includes the control device of the present invention. 
     Another aspect of the present invention relates to a control method. The control method of a hybrid vehicle that includes an engine and an electric motor and that is capable of running by the engine or the electric motor or capable of running by cooperation between the engine and the electric motor, includes: besides a lower limit set on torque shared by the electric motor and the engine when the vehicle runs by the cooperation between the engine and the electric motor, a control step for performing a control to set a running mode in which the engine cooperates with the electric motor only when it is estimated that the torque shared by the electric motor and the engine is equal to or more than the lower limit or exceeds the lower limit while the vehicle runs by the cooperation between the engine and the electric motor. 
     Alternatively, a control method of a hybrid vehicle that includes an engine and an electric motor and that is capable of running by the engine or the electric motor or capable of running by cooperation between the engine and the electric motor, includes: besides a lower limit set on torque shared by the electric motor and the engine when the vehicle runs by the cooperation between the engine and the electric motor, a control step for performing a control to set a running mode in which the engine cooperates with the electric motor only when it is estimated that the time in which the torque shared by the electric motor and the engine is equal to or more than the lower limit or exceeds the lower limit continues for a given length of time while the vehicle runs by the cooperation between the engine and the electric motor. 
     Still another aspect of the present invention relates to a computer program. The computer program causes an information processing apparatus to implement a function of the control device. 
     Advantageous Effects of Invention 
     According to the present invention, the electric motor can assist the engine with good conversion efficiency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram for illustrating an exemplary structure of a hybrid vehicle according to an embodiment of the present invention. 
         FIG. 2  is a block diagram for illustrating an exemplary configuration of a function implemented in a hybrid ECU illustrated in  FIG. 1 . 
         FIG. 3  is a view for illustrating the relationships between the conversion efficiency of the electric motor and torque at both of the assistance side and the regeneration side. 
         FIG. 4  is a flowchart for illustrating a process in an assistance control unit in  FIG. 2 . 
         FIG. 5  is a view for illustrating an example in which the torque required for assistance increases as time passes in the process of the assistance control unit in  FIG. 2 . 
         FIG. 6  is a view for illustrating an example in which the torque required for assistance keeps a constant magnitude for a given length of time in the process of the assistance control unit in  FIG. 2 . 
         FIG. 7  is a view for illustrating an example in which the assistance feasibility determination torque has not come to be fulfilled in the duration determination in the process of the assistance control unit in  FIG. 2 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the hybrid vehicle according to an embodiment of the present invention will be described with reference to  FIGS. 1 to 7 . 
       FIG. 1  is a block diagram for illustrating an exemplary structure of a hybrid vehicle  1 . The hybrid vehicle  1  is an example of a vehicle. The hybrid vehicle  1  is driven by an engine (internal combustion engine)  10  and/or an electric motor  13 . A gearbox including an automated mechanical/manual transmission is placed between the engine  10  and the electric motor  13 . Note that the automated mechanical/manual transmission is a transmission that can automatically perform a gear shifting operation while having the same structure as a manual transmission. 
     The hybrid vehicle  1  includes the engine  10 , an engine Electronic Control Unit (ECU)  11 , a clutch  12 , the electric motor  13 , an inverter  14 , a battery  15 , a transmission  16 , an transmission ECU  17 , a hybrid ECU  18  (referred to as a control device in claims), a wheel  19 , a key switch  20 , and a shift unit  21 . Note that the transmission  16  includes the above-mentioned automated mechanical/manual transmission, and is operated by the shift unit  21  including a drive range (hereinafter, referred to as a D (Drive) range). When the shift unit  21  is at the D range, the gear shifting operation of the automated mechanical/manual transmission is automated. 
     The engine  10  is an example of an internal combustion engine, and is controlled by the engine ECU  11 . The engine  10  internally combusts gasoline, light oil, Compressed Natural Gas (CNG), Liquefied Petroleum Gas (LPG), alternative fuel, or the like in order to generate power for rotating a shaft and transmit the generated power to the clutch  12 . 
     The engine ECU  11  controls the engine  10 , for example, the amount of fuel injection and the valve timing, according to the instructions from the hybrid ECU  18 . For example, the engine ECU  11  includes a Central Processing Unit (CPU), an Application Specific Integrated Circuit (ASIC), a microprocessor (micro-computer), a Digital Signal Processor (DSP), and the like, and internally has an operation unit, a memory, an Input/Output (I/O) port, and the like. 
     The clutch  12  is controlled by the transmission ECU  17 , and transmits the shaft output from the engine  10  to the wheel  19  through the electric motor  13  and the transmission  16 . In other words, the clutch  12  mechanically connects the rotating shaft of the engine  10  to the rotating shaft of the electric motor  13  by the control of the transmission ECU  17  in order to transmit the shaft output of the engine  10  to the electric motor  13 . On the other hand, the clutch  12  cuts the mechanical connection between the rotating shaft of the engine  10  and the rotating shaft of the electric motor  13  so that the rotating shaft of the engine  10  and the rotating shaft of the electric motor  13  can rotate at different rotational speeds from each other. 
     For example, the clutch  12  mechanically connects the rotating shaft of the engine  10  to the rotating shaft of the electric motor  13 , for example, when the hybrid vehicle  1  runs by the power of the engine  10  and this causes the electric motor  13  to generate electric power, when the driving force of the electric motor  13  assists the engine  10 , and when the electric motor  13  starts the engine  10 . 
     Further, for example, the clutch  12  cuts the mechanical connection between the rotating shaft of the engine  10  and the rotating shaft of the electric motor  13  when the engine  10  is stopped or is in an idling state and the hybrid vehicle  1  runs by the driving force of the electric motor  13 , and when the hybrid vehicle  1  reduces the speed or runs on the downgrade and the electric motor  13  generates (regenerates) electric power while the engine  10  is stopped or is in an idling state. 
     Note that the clutch  12  differs from a clutch operated by the driver&#39;s operation of a clutch pedal, and is operated by the control of the transmission ECU  17 . 
     The electric motor  13  is a so-called motor generator that supplies a shaft output to the transmission  16  by generating the power for rotating the shaft using the electric power supplied from the inverter  14 , or that supplies electric power to the inverter  14  by generating the electric power using the power for rotating the shaft supplied from the transmission  16 . For example, when the hybrid vehicle  1  gains the speed or runs at a constant speed, the electric motor  13  generates the power for rotating the shaft to supply the shaft output to the transmission  16  in order to cause the hybrid vehicle  1  to run in cooperation with the engine  10 . Further, the electric motor  13  works as an electric generator, for example, when the electric motor  13  is driven by the engine  10 , or when the hybrid vehicle  1  reduces the speed or runs on the downgrade. In that case, electric power is generated by the power for rotating the shaft supplied from the transmission  16  and is supplied to the inverter  14  in order to charge the battery  15 . At that time, the electric motor  13  generates regeneration torque corresponding to the regenerated electric power. 
     The inverter  14  is controlled by the hybrid ECU  18 , and converts the direct voltage from the battery  15  into an alternating voltage or converts the alternating voltage from the electric motor  13  into a direct voltage. When the electric motor  13  generates power, the inverter  14  converts the direct voltage from the battery  15  into an alternating voltage and supplies the electric power to the electric motor  13 . When the electric motor  13  generates electric power, the inverter  14  converts the alternating voltage from the electric motor  13  into a direct voltage. In other words, in that case, the inverter  14  works as a rectifier and a voltage regulator for supplying a direct voltage to the battery  15 . 
     The battery  15  is a secondary cell capable of being charged and discharged. The battery  15  supplies electric power to the electric motor  13  through the inverter  14  when the electric motor  13  generates power. Alternatively, the battery  15  is charged with the electric power generated by the electric motor  13  when the electric motor  13  generates electric power. A proper range of the SOC is determined for the battery  15  and the battery  15  is controlled to maintain the SOC within the range. 
     The transmission  16  includes an automated mechanical/manual transmission (not shown in the drawings) that selects one of a plurality of gear ratios (change gear ratios) according to the shift instruction signal from the transmission ECU  17  in order to shift the change gear ratios and transmit the gear-shifted power of the engine  10  and/or of the electric motor  13  to the wheel  19 . Alternatively, the transmission  16  transmits the power from the wheel  19  to the electric motor  13 , for example, when the vehicle reduces the speed or runs on the downgrade. Note that the automated mechanical/manual transmission can also shift the gear position to a given gear number by the driver&#39;s hand operation of the shift unit  21 . 
     The transmission ECU  17  is a computer for controlling the clutch  12  and the transmission  16 . In other words, the transmission ECU  17  controls the clutch  12  and obtains gear number information from the transmission  16  to supply, to the transmission  16 , the shift instruction signal based on the obtained gear number information in order to control the transmission  16 . For example, the transmission ECU  17  includes a CPU, an ASIC, a microprocessor (micro-computer), a DSP, and the like, and internally has an operation unit, a memory, an I/O port, and the like. 
     The hybrid ECU  18  is an example of a computer. For hybrid running, the hybrid ECU  18  obtains accelerator opening amount information, brake operation information, vehicle speed information, engine rotational speed information, and SOC information. Based on the obtained information, the hybrid ECU  18  gives the control instruction to the inverter  14  and gives the control instruction of the engine  10  to the engine ECU  11 . The control instructions also include an assistance control instruction described below. For example, the hybrid ECU  18  includes a CPU, an ASIC, a microprocessor (micro-computer), a DSP, and the like, and internally has an operation unit, a memory, an I/O port, and the like. 
     Note that a computer program to be executed by the hybrid ECU  18  can be installed on the hybrid ECU  18  that is a computer in advance by being stored in a non-volatile memory inside the hybrid ECU  18  in advance. 
     The engine ECU  11 , the transmission ECU  17 , and the hybrid ECU  18  are connected to each other, for example, through a bus complying with the standard of the Control Area Network (CAN) or the like. 
     The wheel  19  is a drive wheel for transmitting the driving force to the road surface. Note that, although only a wheel  19  is illustrated in  FIG. 1 , the hybrid vehicle  1  actually includes a plurality of the wheels  19 . 
     The key switch  20  is a switch that is turned ON/OFF, for example, by insertion of a key by the user at the start of drive. Turning ON the key switch activates each unit of the hybrid vehicle  1 , and turning OFF the key switch  20  stops each unit of the hybrid vehicle  1 . 
     As described above, the shift unit  21  is for giving the instructions from the driver to the automated mechanical/manual transmission in the transmission  16 . When the shift unit  21  is at the D range, the gear shifting operation of the automated mechanical/manual transmission is automated. 
       FIG. 2  is a block diagram for illustrating an exemplary configuration of a function implemented in the hybrid ECU  18  executing a computer program. In other words, when the hybrid ECU  18  executes a computer program, the function of an assistance control unit  30  (referred to as control means in claims) is implemented. 
     The assistance control unit  30  is a function for giving an instruction for an assistance control to the engine ECU  11  based on the SOC information and the accelerator opening amount information. For example, from the SOC information, the assistance control unit  30  determines whether the electric motor  13  can assist the engine  10  in light of the SOC of the battery  15 . From the accelerator opening amount information, the assistance control unit  30  determines how much torque the driver requests. The requested torque is namely the required torque. Based on the required torque, the assistance control unit  30  performs a control to determine the share of torque between the engine  10  and the electric motor  13 . Note that, when the assistance control unit  30  shares the torque between the engine  10  and the electric motor  13 , there is some ways, for example, using the above-mentioned assistance map. 
     Next, the relationship between the conversion efficiency of the electric motor  13  and torque will be described with reference to  FIG. 3 . The role of the electric motor  13  is to input electric energy from the battery  15  and convert the energy into energy for the rotary motion to rotate a shaft of the electric motor  13 . As illustrated in  FIG. 3 , generally, when use of the electric motor  13  at relatively low torque is compared to use at relatively high torque, the use at relatively high torque has better conversion efficiency for converting the electric energy into the energy for the rotary motion. 
     In  FIG. 3 , an assistance side (right side) and a regeneration side (left side) have almost left-right symmetry. At the assistance side, the electric motor  13  works as the power of the hybrid vehicle  1  to input electric energy from the battery  15  and convert the electric energy into energy for the rotary motion to rotate a shaft of the electric motor  13 . On the other hand, at the regeneration side, the electric motor  13  works as an electric generator for charging the battery  15 . The turning force of the wheel  19  rotates the shaft of the electric motor  13  and the energy of the rotary motion of the shaft is converted into electric energy and charges the battery  15 . In  FIG. 3 , for distinction, the torque on the assistance side is shown as active torque and the torque on the regeneration side is shown as passive torque. While the “conversion efficiency” herein relates to the assistance side, use of the electric motor at relatively high torque has better conversion efficiency also on the regeneration side. 
     Next, the process for an assistance control implemented in the hybrid ECU  18  executing a computer program will be described with reference to the flowchart in  FIG. 4 . Note that the outline of the assistance control process will be described here, and the concrete example will be described below with reference to  FIGS. 5 to 7 . The procedures from step S 1  to step S 3  in  FIG. 4  is a cycle of the process and the process is repeated as long as the key switch  20  is in the ON state. 
     In the “START” in  FIG. 4 , the key switch  20  is in the ON state. The hybrid ECU  18  executes a computer program and a function of the assistance control unit  30  is implemented in the hybrid ECU  18 . Then, the process goes to step S 1 . 
     In step S 1 , the assistance control unit  30  determines whether the required torque has reached a level where an assistance determination is feasible. For example, an assistance determination is conventionally performed unless the required torque is zero. However, in the present embodiment, the assistance control unit  30  does not start the assistance determination at a level where the electric motor  13  can operate only with low torque and is forced to operate in an area where the conversion efficiency is bad. For example, once the required torque has entered a “good conversion efficiency area” illustrated in  FIG. 3 , it is determined that the required torque has reached the level where the assistance determination is feasible. When it is determined in step S 1  that the required torque exceeds the level where the assistance determination is feasible, or is equal to more than the level, the process goes to step S 2 . On the other hand, when it is determined in step S 1  that the required torque is equal to or less than the level where the assistance determination is feasible, or is less than the level, the process of step S 1  is repeated. 
     In step S 2 , the assistance control unit  30  determines whether the electric motor  13  can assist the engine  10  with good conversion efficiency in the good conversion efficiency area as illustrated at the assistance side in  FIG. 3 . When it is determined in step S 2  that the assistance with good conversion efficiency is feasible, the process goes to step S 3 . On the other hand, when it is determined in step S 2  that that the assistance with good conversion efficiency is unfeasible, the process goes back to step S 1 . 
     In step S 3 , the assistance control unit  30  determines whether the electric motor  13  can continue assisting the engine  10  for a given length of time with good conversion efficiency in the good conversion efficiency area as illustrated at the assistance side in  FIG. 3 . When it is determined in step S 3  that the assistance continued for a given length of time with good conversion efficiency is feasible, the process goes to step S 4 . On the other hand, when it is determined in step S 3  that the assistance continued for a given length of time with good conversion efficiency is unfeasible, the process goes back to step S 1 . 
     In step S 4 , the assistance control unit  30  causes the electric motor  13  to assist the engine  10  and the process is completed (END). 
     Next, an example in which the torque required for the assistance increases as time passes in the process of the assistance control unit  30  will be described with reference to  FIG. 5 . In  FIG. 5 , the time passage is shown on the horizontal axis and the torque required for the running of the hybrid vehicle  1  is shown on the vertical axis. In the example in  FIG. 5 , required torque tr 0  for the running of the hybrid vehicle  1  increases from a time T 0  as time passes and reaches required torque tr 1  at a time T 1 . In the example in  FIG. 5 , an assistance feasibility determination is started at the required torque tr 1 . Conventionally, the assistance feasibility determination is started at the time T 0 . However, the assistance control unit  30  does not start the assistance feasibility determination as long as the required torque is equal to or less than the required torque tr 1 , or is less than the required torque tr 1  because the electric motor  13  can operate only with low torque and is forced to operate in the bad conversion efficiency area in the range from the time T 0  and the time T 1 . 
     In the example in  FIG. 5 , the required torque reaches required torque tr 2  at a time T 2  and, at that time, the assistance control unit  30  determines that the electric motor  13  can assist the engine  10 . In other words, the required torque tr 1  to tr 2  is the assistance determination torque. 
     Further, in the example in  FIG. 5 , the period from the time T 2  to a time T 3  is shown as a duration determination period. In the example in  FIG. 5 , the required torque continues being equal to or more than the required torque tr 2  or exceeding the required torque tr 2  for a given length of time because the required torque continue increasing even in the period from the time T 2  to the time T 3 . From this, the assistance control unit  30  determines that the duration determination in the period from the time T 2  to the time T 3  is “feasible: OK” and starts the assistance from the time T 3 . At that case, the shaded area in  FIG. 5  shows the area where the electric motor  13  assists the engine  10 . 
     Next, an example in which the torque required for assistance keeps a constant magnitude from the time T 2  in the process of the assistance control unit  30  will be described with reference to  FIG. 6 . In the example in  FIG. 6 , similarly to the example in  FIG. 5 , required torque tr 0  for the running of the hybrid vehicle  1  increases from a time T 0  as time passes and reaches required torque tr 1  at a time T 1 . However, required torque tr 2  keeps a constant magnitude after the time T 2 . Even in such a case, the required torque tr 2  at which the assistance is feasible is kept from the time T 2  to a time T 3 . Thus, the assistance control unit  30  determines that the duration determination in the period from the time T 2  to the time T 3  is “feasible: OK” and starts the assistance from the time T 3 . 
     Next, an example in which the assistance feasibility determination torque has come to be unfulfilled in the duration determination in the process of the assistance control unit  30  will be described with reference to  FIG. 7 . In the example in  FIG. 7 , similarly to the example in  FIG. 5 , required torque tr 0  for the running of the hybrid vehicle  1  increases from a time T 0  as time passes and reaches required torque tr 1  at a time T 1 . However, the required torque gradually decreases although temporarily exceeding required torque tr 2  in the period between a time T 2  and a time T 3 , and then is equal to or less than the required torque tr 2  or is less than the required torque tr 2 . Thus, the assistance control unit  30  determines that the duration determination in the period from the time T 2  to the time T 3  is “unfeasible: NG” and does not perform the assistance from the time T 3  because the required torque tr 2  in which the assistance is feasible is not continued in the period between a time T 2  and a time T 3 . 
     Effects 
     The electric motor  13  is controlled not to assist the engine  10  at low torque at which the conversion efficiency of the electric motor  13  is bad. Thus, the electric motor  13  can assist the engine  10  with good conversion efficiency. This causes efficient use of the electric power of the battery  15  so that the SOC of the battery  15  can be kept high. 
     Only when the electric motor  13  can continue assisting the engine  10  with good conversion efficiency for a given length of time, the assistance is actually performed. This can prevent the frequent repetition of the state in which the assistance is performed and the state in which the assistance is not performed. If the repetition occurred, the clutch  12  frequently and repeatedly be engaged and disengaged. At that case, when the clutch  12  is switched from the disengaged state to the engaged state, it is necessary to perform a control to keep the rotational speed of the engine  10  higher than that in the idling state in order to synchronize the rotational speed of the engine  10  with the rotational speed of the electric motor  13 . This causes the fuel consumption to be worse. Such worsening of the fuel consumption can be prevented by the assistance that is actually performed only when the electric motor  13  can continue assisting the engine  10  with good conversion efficiency for a given length of time. 
     Other Embodiments 
     Although the engine  10  has been described as an internal combustion engine, the engine  10  can also be a heat engine including an external combustion engine. 
     Further, while the computer program executed by the hybrid ECU  18  is installed on the hybrid ECU  18  in advance in the above-mentioned descriptions, the computer program can be installed on the hybrid ECU  18  as a computer by attaching removable media recording the computer program (storing the program), for example, to a drive (not shown in the drawings) and storing the computer program read from the removable media in a non-volatile memory inside the hybrid ECU  18 , or by receiving, with a communication unit (not shown in the drawings), a computer program transmitted through a wired or wireless transmission medium and storing the computer program in a non-volatile memory inside the hybrid ECU  18 . 
     Further, each of the ECUs can be implemented by an ECU combining the ECUs. Alternatively, an ECU can newly be provided by the further subdivision of the function of each ECU. 
     Note that the computer program executed by the computer can be for performing the process in chronological order according to the order described herein or can be for performing the process in parallel or at the necessary timing, for example, when the computer program is invoked. 
     Further, the embodiments of the present invention are not limited to the above-mentioned embodiment, and can variously be modified without departing from the gist of the invention. 
     In the above-mentioned description of the embodiment, once the required torque has entered a “good conversion efficiency area (assistance side)” illustrated in  FIG. 3 , it is determined that the required torque has reached the level where the assistance determination is feasible. This means that the required torque has completely entered the “good conversion efficiency area (assistance side)” illustrated in  FIG. 3 . On the other hand, when the required torque often comes in and out of the “good conversion efficiency area (assistance side)” illustrated in  FIG. 3 , it can be determined that the required torque has reached the level where the assistance determination is feasible. To this end, for example, the average value of the required torque in a given length of time is calculated. When the calculated average value of the required torque has entered the “good conversion efficiency area (assistance side)” illustrated in  FIG. 3 , it is determined that the required torque has reached the level where the assistance determination is feasible. 
     In the above-mentioned embodiment, it is determined in step S 3  whether the state in which “the assistance with good conversion efficiency is feasible” in step S 2  “continues for a given length of time”. In that case, once the state in step S 2  is stopped even only for a short time before the state has continued for the given length of time although “the assistance with good conversion efficiency is feasible” for most of the given length of time, the determination result in step S 3  is “No”. To avoid the result, the average value of the assistance torque of the electric motor  13  in a given length of time is found and, when the average value is in the area in which “the assistance with good conversion efficiency is feasible”, the process can go to step S 4  to perform the assistance. For example, in the flowchart of  FIG. 4 , step S 2  is changed into the procedure for determining “Is the average value of the torque of the electric motor  13  in a given length of time in the state in which the assistance with good conversion efficiency is feasible?” and step S 3  is deleted. This can perform the assistance when the required torque is kept, for most of the given length of time, at the magnitude at which “the assistance with good conversion efficiency is feasible”, so that the number of times to perform the assistance can be increased. 
     Alternatively, even though the required torque has not entered the “good conversion efficiency area (assistance side)” illustrated in  FIG. 3 , it can be determined that the required torque has reached the level where the assistance determination is feasible when the rate of increase in the required torque is large and it is estimated that the required torque rapidly enters the “good conversion efficiency area (assistance side)”. To this end, a threshold is set for the rate of increase in the required torque and it is determined, when the rate of increase in the required torque exceeds the threshold, that the required torque has reached the level where the assistance determination is feasible. 
     Further, in the above-mentioned embodiment, it is determined in step S 3  whether the state in which “the assistance with good conversion efficiency is feasible” in step S 2  “continues for a given length of time”. At that case, when the rate of increase in the required torque is large and it is estimated that the required torque rapidly enters the “good conversion efficiency area (assistance side)” or it is estimated that the state in which “the assistance with good conversion efficiency is feasible” “continues for a given length of time”, the process can also go to step S 4  to perform the assistance. For example, in the flowchart in  FIG. 4 , step S 2  is changed into the procedure for determining “Has the rate of increase in the required torque exceeded the threshold?” and step S 3  is deleted. This can perform the assistance when it is estimated that the required torque rapidly enters the “good conversion efficiency area (assistance side)” or it is estimated that the state in which “the assistance with good conversion efficiency is feasible” “continues for a given length of time”, so that the timing for starting the assistance can be accelerated and the number of times to perform the assistance can be increased. 
     Further, a condition of the assistance termination after the vehicle is shifted to an assistance run has not been described in the above-mentioned embodiment. However, for example, the assistance can be terminated when the required torque enters the “bad conversion efficiency area” illustrated in  FIG. 3 . Further, a threshold is properly set at a value in which the conversion efficiency of the electric motor  13  is low, and the assistance is terminated when the required torque is equal to or less than the threshold. Alternatively, regardless of the conversion efficiency, the assistance can be terminated in order to protect the battery  15  or the inverter  14  when the temperature of the battery  15  or the inverter  14  becomes high. 
     Further, when the SOC of the battery  15  is high, the assistance can be performed to prevent the increase in the SOC, even though the conversion efficiency becomes worse to some degree, in order to continue the state in which regeneration braking is feasible. To respond to such a case, the threshold for the determination in step  51  in  FIG. 4  can be changed according to the SOC of the battery  15 . For example, when the SOC is higher than a predetermined value, the boundary point between the “good conversion efficiency area (assistance side)” and the “bad conversion efficiency area” illustrated in  FIG. 3  is moved toward the zero point (in a left direction of the drawing) by a predetermined amount. This moves the determination criterion in step  51  in a direction to ease the criterion and the number of times to perform the assistance is increased, so that the SOC of the battery  15  can be prevented from increasing and the state in which the regeneration braking is feasible can be continued. 
     Further, even at the same required torque, the torque of the electric motor  13  varies between the assistance at a low gear number and the assistance at a high gear number. At that case, a low gear number has a torque in the “bad conversion efficiency area” illustrated in  FIG. 3  and a high gear number has a torque in the “good conversion efficiency area (assistance side)” illustrated in  FIG. 3 . Then, the assistance can be performed at a high gear number.