Abstract:
To reduce the size, weight, and cost of a motor and peripheral devices of the motor. A hybrid automobile is structured in which a required torque is estimated on the basis of an accelerator operation by a driver, and if it is determined that the running by the motor is possible, and an estimation result exceeds a maximum torque of the motor during execution of a running mode by the motor, a mode switch is carried out to a mode in which the automobile runs by an engine or by the engine and the motor in cooperation with each other, even though the running mode by the motor is being executed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. national stage of application No. PCT/JP2011/074156, 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. 2010-236609, filed on Oct. 21, 2010, the disclosure of which are also incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a driving mode switch control device, a hybrid vehicle, a driving mode switch control method, and a computer program. 
     BACKGROUND ART 
     A hybrid vehicle capable of running by the cooperation between an engine and an electric motor can run with the electric motor, for example, at the start of moving in which relatively large torque is required. This can reduce the gas emission and the fuel consumption. 
     In a conventional vehicle, when the State of Charge (SOC) of the battery is equal to or more than a predetermined value, it is determined that the vehicle can be driven by the electric motor. Then, the vehicle runs with the electric motor (for example, see patent literature PTL1). 
     CITATION LIST 
     Patent Literature 
     PTL1 JP 2005-24049 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional hybrid vehicle, as described above, it is determined based on the SOC of the battery whether the vehicle can run with the electric motor. When it is determined that the vehicle can run with the electric motor, a driving mode with the electric motor is selected. In such a driving mode with the electric motor, it is required that the electric motor has the same driving performance as the engine. Thus, the electric motor and a peripheral device such as an inverter need to secure torque enough not to affect the running in which the electric motor solely operates. In other words, conventional electric motor and peripheral device such as an inverter need to generate the same amount of torque as the engine. This causes the electric motor and the electric motor peripheral device of a conventional hybrid vehicle to become larger and heavier. Furthermore, this also causes the high cost. 
     In light of the foregoing, an objective of the present invention is to provide a driving mode switch control device, a hybrid vehicle, a driving mode switch control method, and a computer program that can reduce the size, weight, and cost of an electric motor and an electric motor peripheral device. 
     Solution to Problem 
     An aspect of the present invention is directed to a driving mode switch control device. According to the present invention, the driving mode switch 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 a cooperation between the engine and the electric motor, includes: 
     request torque estimation means for estimating request torque based on a driver&#39;s accelerator operation; and 
     mode switch means for, when it is determined that the vehicle can run with the electric motor and an estimation result from the request torque estimation means exceeds maximum torque of the electric motor during implementation of a driving mode with the electric motor, performing a mode switch to a mode in which the vehicle runs with the engine or by the cooperation between the engine and the electric motor in spite of implementing the driving mode with the electric motor. 
     Further, the mode switch means can perform a mode switch at a next gear shifting timing even if the estimation result from the request torque estimation means does not exceed the maximum torque of the electric motor during acceleration in the driving mode with the electric motor. 
     Further, the mode switch means can control a rotational speed of the engine to be faster than a rotational speed of the electric motor when performing the switch from the driving mode with the electric motor to the mode in which the vehicle runs with the engine or by the cooperation between the engine and the electric motor. 
     For example, the request torque estimation means determines whether a variation of accelerator opening amount according to the driver&#39;s accelerator operation or accelerator opening amount exceeds a predetermined value. 
     A further aspect of the present invention is directed to a hybrid vehicle. The hybrid vehicle according to the present invention includes the driving mode switch control device according to the present invention. 
     A further aspect of the present invention is directed to a driving mode switch control method. According to the present invention, the driving mode switch 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 a cooperation between the engine and the electric motor, includes: 
     a request torque estimation step for estimating request torque based on a driver&#39;s accelerator operation; and 
     a mode switch step for, when it is determined that the vehicle can run with the electric motor and an estimation result from a process by the request torque estimation step exceeds maximum torque of the electric motor during implementation of a driving mode with the electric motor, performing a mode switch to a mode in which the vehicle runs with the engine or by the cooperation between the engine and the electric motor in spite of implementing the driving mode with the electric motor. 
     A further aspect of the present invention relates to a computer program. The computer program according to the present invention causes an information processing apparatus to implement a function of the driving mode switch control device according to the present invention. 
     Advantageous Effects of Invention 
     The present invention can reduce the size, weight, and cost of an electric motor and an electric motor peripheral device of a hybrid vehicle. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram for illustrating an exemplary structure of a hybrid vehicle according to a first embodiment. 
         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 flowchart for illustrating a mode switch process by a mode switch control unit illustrated in  FIG. 2 . 
         FIG. 4  is a timing diagram for illustrating the mode switch process of the mode switch control unit illustrated in  FIG. 2 . 
         FIG. 5  is a flowchart for illustrating a mode switch process according to a second embodiment. 
         FIG. 6  is a timing diagram for illustrating the mode switch process according to the second embodiment. 
         FIG. 7  is a flowchart for illustrating a mode switch process according to a third embodiment. 
         FIG. 8  is a timing diagram for illustrating the mode switch process according to the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Hereinafter, the hybrid vehicle according to the first embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . 
       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  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 , a motor ECU  17 , a hybrid ECU  18 , a wheel  19 , a key switch  20 , and a shift unit  21 . Note that the transmission  16  includes an automated mechanical/manual transmission, and is operated by the shift unit  21  including a drive range (hereinafter, referred to as a D (Drive) range). In that case, 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 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  is a computer working in coordination with the motor ECU  17  according to the instructions from the hybrid ECU  18 , and controls the engine  10 , for example, the amount of fuel injection and the valve timing. 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 hybrid ECU  18 , 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 hybrid ECU  18  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 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 stopping 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 stopping 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 hybrid ECU  18 . 
     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  runs without power, for example, 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 . 
     The inverter  14  is controlled by the motor ECU  17 , 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. 
     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 to shift gears from the hybrid ECU  18  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 motor ECU  17  is a computer working in coordination with the engine ECU  11  according to the instructions from the hybrid ECU  18 , and controls the electric motor  13  by controlling the inverter  14 . For example, the motor 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, the gear position information obtained from the transmission  16 , and the engine rotational speed information obtained from the engine ECU  11  in order to refer to the information, control the clutch  12  and supply the shift instruction signal to shift gears in order to control the transmission  16 . For hybrid running, the hybrid ECU  18  further gives the instructions to the motor ECU  17  to control the electric motor  13  and the inverter  14  based on the obtained SOC information on the battery  15  and other information, and gives the instruction to the engine ECU  11  to control the engine  10 . 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 motor 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  20  activates each unit of the hybrid vehicle  1 , and turning OFF the key switch  20  stops each unit of the hybrid vehicle  1 . 
       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, a request torque estimation unit  30 , a rotational speed synchronization control unit  31 , and a mode switch control unit  32  are implemented. 
     The request torque estimation unit  30  obtains the information on the opening amount of the accelerator operated by the driver from an accelerator opening amount sensor (not illustrated in the drawings), and estimates the driver&#39;s request torque based on the obtained accelerator opening amount information. 
     When the clutch  12  is engaged, the rotational speed synchronization control unit  31  controls the rotational speeds of the engine  10  and the electric motor  13  to almost synchronize with each other. 
     The mode switch control unit  32  controls the switch among an electric motor driving mode in which the vehicle runs with the electric motor  13 , an engine driving mode in which the vehicle runs with the engine  10 , and an assistance driving mode in which the vehicle runs while the electric motor  13  assists the engine  10 . 
     The electric motor driving mode is usually used when the vehicle starts moving (when a 2nd gear is selected) so that the exhaust gas from the engine  10  at the start of moving can be eliminated and the fuel efficiency can be improved. At that time, the clutch  12  is in a disengaged state. 
     The engine driving mode is usually used when the vehicle runs (when a 3rd or a 4th gear is selected). Further, at that time, the clutch  12  is in an engaged state and the electric motor  13  regenerates electric power with the output from the engine  10  or, if the SOC of the battery  15  is high, the electric motor  13  is free from the engine  10  in order not to add friction to the engine  10 . 
     The assistance driving mode is usually used when the maximum torque of the electric motor  13  is not sufficient for the vehicle to start moving or to run. At that time, the clutch  12  is in the engaged state and both of the engine  10  and the electric motor  13  produce outputs. 
     Next, the process for a mode switch control implemented in the hybrid ECU  18  executing a computer program will be described with reference to the flowchart in  FIG. 3  and the timing diagram in  FIG. 4 . Note that the process for a mode switch control illustrated in the flowchart of  FIG. 3  is a cycle of the process, and the process is repeatedly performed when the key switch  20  of the hybrid vehicle  1  is in the ON state. 
     At the START, the key switch  20  of the hybrid vehicle  1  is in the ON state. The hybrid ECU  18  has executed a computer program and the request torque estimation unit  30 , the rotational speed synchronization control unit  31 , and the mode switch control unit  32  are implemented in the hybrid ECU  18 . Then, the process goes to step S 1 . 
     In step S 1 , the mode switch control unit  32  determines whether the current mode is the electric motor driving mode. For example, when the 2nd gear is selected, the electric motor driving mode is selected. Note that when the vehicle starts moving, the 2nd gear is selected. As the rotational speed increases, the gear is shifted up into the 3rd gear and the 4th gear. When it is determined in step S 1  that the electric motor driving mode is currently selected, the process goes to step S 2 . On the other hand, when it is determined in step S 1  that the electric motor driving mode is not currently selected, the process of step S 1  is repeated. 
     In step S 2 , the request torque estimation unit  30  determines whether the accelerator opening amount is equal to or more than a predetermined value. At that time, determining whether the accelerator opening amount is equal to or more than a predetermined value is for determining whether the generation of torque equal to or more than the predetermined value is requested while the vehicle runs in the electric motor driving mode. The torque equal to or more than the predetermined value, for example, means the maximum torque that can be generated by the electric motor  13 . 
     In step S 2 , when the accelerator opening is equal to or more than the predetermined value (in other words, when the generation of torque exceeding the maximum torque that can be generated by the electric motor  13  is requested while the vehicle runs in the electric motor driving mode), the process goes to step S 3 . On the other hand, when it is determined in step S 2  that the accelerator opening amount is less than the predetermined value, the process goes back to step S 1 . 
     In step S 3 , the rotational speed synchronization control unit  31  starts a synchronization control in order to almost synchronize the rotational speed of the engine  10  and the rotational speed of the electric motor  13 . 
     In step S 4 , the rotational speed synchronization control unit  31  is on standby until the synchronization of the rotational speed of the engine  10  and the rotational speed of the electric motor  13  is completed. When the synchronization is completed, the rotational speed synchronization control unit  31  provides notification of the fact to the mode switch control unit  32 . Then, the process goes to step S 5 . 
     In step S 5 , the mode switch control unit  32  starts the process for switching the clutch  12  from the disengaged state in the electric motor driving mode to the engaged state. 
     In step S 6 , the mode switch control unit  32  is on standby until the engagement of the clutch  12  is completed. When the engagement of the clutch  12  is completed, the process goes to step S 7 . 
     In step S 7 , the mode switch control unit  32  starts the switch to the assistance driving mode or the engine driving mode. For example, when the mode is switched to the engine driving mode, the power of the engine  10  is transmitted to a tire  20  through the clutch  12  that has been engaged in the process of step S 6 . When the mode is switched to the assistance driving mode, the power of the engine  10  is added to the power of the electric motor  13  and is transmitted to the tire  20 . 
     In step S 8 , the mode switch control unit  32  is on standby until the switch of the mode is completed. When the switch of the mode is completed, the mode switch control unit  32  completes the mode switch control. After that, the process goes back to step S 1  at a predetermined timing and the same process is performed. 
       FIG. 4  is a view for illustrating the timings of step S 3  (rotation synchronization), step S 5  (engage clutch), and step S 7  (switch to the assistance running or engine driving mode) in the flowchart of  FIG. 3 .  FIG. 4  illustrates each of the driving mode, the state of the engine, and the rotational speed of the engine and the rotational speed of the electric motor responding to the engagement or disengagement of the clutch from the start of moving. As illustrated in  FIG. 4 , at time of the start of moving, the gear number is shifted into the 2nd and the electric motor driving mode is selected. Intrinsically, after a usual depression of the accelerator increases the rotational speed and the gear is shifted up into 3rd or 4th, the mode is switched to the engine driving mode or the assistance driving mode. However, even if the rotational speed is not increased and the electric motor driving mode is performed, the driving mode switch control is started at the time when it is determined that the accelerator opening amount is equal to or more than the predetermined value. In other words, the control of the synchronization of the rotational speeds of the engine  12  and the electric motor  13  (in step S 3 , referred to as the rotation synchronization) is started. When the rotational speed of the engine  12  is changed from the rotational speed during idling to almost the same speed as the rotational speed of the electric motor  13 , the clutch  12  is engaged (step S 5 ) and then the mode is switched to the assistance driving mode or the engine driving mode (step S 7 ). As a result of that, the necessary power is transmitted to a tire  12 . 
     Effects 
     According to the hybrid vehicle  1 , when it is estimated that the driver&#39;s request torque exceeds the maximum torque of the electric motor  13 , the driving mode is switched to the assistance driving mode or the engine driving mode even if the electric motor driving mode is performed. Thus, the torque corresponding to the request torque can be outputted. In other words, when the request torque exceeds the maximum torque of the electric motor  13 , the torque can be complemented by the power of the engine  12 . Thus, the electric motor  13  and a peripheral device such as the inverter  14  do not need a capability to generate the same amount of torque as the engine  10 . This can reduce the size, weight, and cost of the electric motor  13  and a peripheral device such as the inverter  14 . 
     Second Embodiment 
     A hybrid vehicle  1 A according to the second embodiment of the present invention will be described with reference to the flowchart in  FIG. 5  and the timing diagram in  FIG. 6 . The hybrid vehicle  1 A has the same structure as the hybrid vehicle  1 . The hybrid vehicle  1 A will be described with reference signs in the same line (for example, the hybrid ECU  18 A, or the rotational speed synchronization control unit  31 A). 
     Before the mode is switched from the electric motor driving mode to the assistance driving mode or the engine driving mode, the rotational speed synchronization control unit  31 A of the hybrid vehicle  1 A controls the rotational speed of the engine to be larger than the rotational speed of the electric motor. 
     Note that, in the procedure of step S 3 , the above-mentioned hybrid vehicle  1  almost synchronizes the rotational speed of the engine and the rotational speed of the electric motor. However, a slight error occurs with the synchronization at that time. The range of the error includes an error in which the rotational speed of the engine deviates from the rotational speed of the electric motor in a plus direction and an error in which the rotational speed of the engine deviates from the rotational speed of the electric motor in a minus direction. The deviation cannot be fixed in one direction. On the other hand, the hybrid vehicle  1 A controls the rotational speed of the engine to deviate from the rotational speed of the electric motor necessarily in a plus direction. 
     In the flowchart of  FIG. 5 , step S 10  differs from the flowchart of  FIG. 3  and the other steps are the same as the steps in  FIG. 3 . Hereinafter, a description of the procedures overlapping with the procedures in the flowchart of  FIG. 3  will be omitted. In other words, when the request torque estimation unit  30 A determines in step S 2  that the accelerator opening amount is equal to or more than the predetermined value, the process goes to step S 3 . 
     In step S 3 , the rotational speed synchronization control unit  31 A performs a control in order to almost synchronize the rotational speed of the engine  10  and the rotational speed of the electric motor  13 . Note that, in that case, the control to increase the rotational speed of the engine  10  is performed until the rotational speed of the engine  10  becomes larger than the rotational speed of the electric motor  13 . 
     After the rotational speed synchronization control unit  31 A has increased the rotational speed of the engine  10  in step S 3  until the rotational speed of the engine  10  becomes larger than the rotational speed of the electric motor  13 , the process goes to step S 10 . In step S 10 , the rotational speed synchronization control unit  31 A determines whether the rotational speed of the engine  10  is larger than the rotational speed of the electric motor  13  by a predetermined number (for example, Δα rpm). 
     When it is determined in step S 10  that the rotational speed of the engine  10  is larger than the rotational speed of the electric motor  13  by the predetermined number (for example, Δα rpm), the process goes to step S 5 . On the other hand, when it is determined in step S 10  that the rotational speed of the engine  10  is not larger than the rotational speed of the electric motor  13  by Δα rpm, the process goes back to step S 3 . The subsequent procedures are the same as the procedures in the description of  FIG. 3 . 
     Note that the above-mentioned Δα rpm is an example. Thus, the difference between the rotational speed of the engine  10  and the rotational speed of the electric motor  13  when the clutch  12  is engaged can variously be set in consideration of the drivability. 
     The state in step S 10  is illustrated in the circle drawn with a broken line in  FIG. 6 . The state in the upper circle in  FIG. 6  is an enlarged and detailed state of the state in the lower circle. In the example in  FIG. 6 , the rotational speed of the engine deviates from the rotational speed of the electric motor by Δα rpm in a plus direction. 
     Effects 
     According to the hybrid vehicle  1 A, the clutch  12  is shifted from the disengaged state to the engaged state when the rotational speed of the engine  10  is slightly larger than the rotational speed of the electric motor  13  (for example, by Δα rpm). Thus, there is not a feeling of deceleration of the hybrid vehicle  1 A when the clutch  12  is engaged. This can improve the drivability. 
     Further, when the control to almost synchronize the rotational speed of the engine and the rotational speed of the electric motor, as described in the first embodiment, is compared with the control performed so that the rotational speed of the engine deviates from the rotational speed of the electric motor necessarily in a plus direction, as described in the second embodiment, the latter is better than the former because the latter has a larger margin of error than the former. In other words, when the rotational speed of the engine is slower than the rotational speed of the electric motor, the driver gets a great shock at the time when the clutch  12  is engaged. On the other hand, when the rotational speed of the engine is faster than the rotational speed of the electric motor, the driver gets a small shock at the time when the clutch  12  is engaged. This is caused because the friction of the engine  10  is larger than the friction of the electric motor  13 . Further, it seems that the user does not get an uncomfortable feeling much with the shock of acceleration because the user strongly depresses the accelerator. Thus, the accuracy of the latter control can be lower than that of the former control. This can simplify the control of the hybrid ECU  18 A. 
     Third Embodiment 
     A hybrid vehicle  1 B according to the third embodiment of the present invention will be described with reference to the flowchart in  FIG. 7  and the timing diagram in  FIG. 8 . The hybrid vehicle  1 B has the same structure as the hybrid vehicle  1 . The hybrid vehicle  1 B will be described with reference signs in the same line (for example, the request torque estimation unit  30 B, or the mode switch control unit  32 B). 
     Even if the request torque does not reach the torque that requires the switch from the electric motor driving mode to the assistance driving mode, the mode switch control unit  32 B of the hybrid vehicle  1 B controls the mode switch in order to switch the mode from the electric motor driving mode to the assistance driving mode or the engine driving mode while the switch is timed for the next gear shifting timing (in other words, is timed for the timing of the change of the gear number). 
     In the flowchart of  FIG. 7 , step S 20  is added to the flowchart of  FIG. 3  and the other steps are the same as the steps in  FIG. 3 . Hereinafter, a description of the procedures overlapping with the procedures in the flowchart of  FIG. 3  will be omitted. In other words, when the request torque estimation unit  30 B determines in step S 2  that the accelerator opening amount is less than the predetermined value (or, namely, No in step S 2 ), the process goes to step S 20 . On the other hand, when the request torque estimation unit  30 B determines in step S 2  that the accelerator opening amount is equal to or more than the predetermined value, the process goes to step S 3  and the same process as in the flowchart of  FIG. 3  are subsequently performed. 
     In step S 20 , the mode switch control unit  32 B determines whether the present time is the time to shift gears (for example, the time when the rotational speed increases and the gear is shifted into the 3rd). When it is determined in step S 20  that the present time is the time to shift gears, the process goes to step S 3  and the same procedures as in the flowchart of  FIG. 3  are subsequently performed. On the other hand, when it is determined in step S 20  that the present time is not the time to shift gears, the process goes back to step S 1 . 
     The state in which the mode switch is performed at the gear shifting timing is illustrated in the ellipse drawn with a broken line in  FIG. 8 . It is found that only a slight increase in the rotational speed of the engine from the idling speed almost synchronizes the rotational speed of the engine with the rotational speed of the electric motor. 
     Further, instead of step S 4  in  FIG. 7 , step S 10  in  FIG. 5  can be performed. 
     Effects 
     According to the hybrid vehicle  1 B, even if the estimation result from the request torque estimation unit  30  does not exceed the maximum torque of the electric motor  13  during acceleration in the electric motor driving mode, the mode switch is performed at the next gear shifting timing. 
     As described above, even if the request torque does not reach the torque that requires the switch from the electric motor driving mode to the assistance driving mode, the mode switch is performed while timed for the gear shifting timing at a small rotational speed of the engine  10 . This can avoid a mode switch during acceleration to be performed at the next gear number (in other words, except at the gear shifting timing). 
     Thus, only a slight increase in the rotational speed of the engine  10  from the standby state (idling state) is required to synchronize the electric motor  13  and the engine  10 . This can reduce the shock at the time when the clutch  12  connects the engine  10  to the electric motor  13  and save the energy consumption. 
     Other Embodiments 
     In the above-mentioned embodiments, it is determined in the procedure of step S 2  whether the accelerator opening amount is equal to or more than a predetermined value. However, instead of that, it can be determined whether the variation of the accelerator opening amount is equal to or more than a predetermined value. For example, when the variation of the accelerator opening amount is large, the request torque estimation unit  30  can determine that the driver performs a rapid accelerator operation and the driver requires a rapid acceleration, accordingly. Thus, the mode switch control unit  30  controls the mode switch in order to switch the mode from the electric motor driving mode to the engine driving mode. 
     In the second embodiment, the switch such as from the electric motor driving mode to the engine driving mode can be applied to the switch such as from the electric motor driving mode to the engine driving mode not only according to the accelerator opening amount but also according to another condition. For example, the decrease in the SOC of the battery  15  can cause the switch such as from the electric motor driving mode to the engine driving mode. 
     Further, the boundaries of the regions for determination can variously be changed, for example, the “equal to or more than” can be changed into “exceeds” and the “less than” can be changed into “equal to or less than” in the description of the above-mentioned flowchart. 
     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 ,  18 A, or  18 B is installed on the hybrid ECU  18 ,  18 A, or  18 B in advance in the above-mentioned descriptions, the computer program can be installed on the hybrid ECU  18 ,  18 A, or  18 B 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 ,  18 A, or  18 B, or 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 ,  18 A, or  18 B. 
     Further, each ECU can be implemented by an ECU combining some or all of the functions of 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 embodiments, and can variously be modified without departing from the gist of the invention.