Abstract:
A regeneration control device of a hybrid vehicle comprises a comparison unit for, when a regenerative torque generated in the electric motor is used as a braking force during deceleration of the vehicle using only the electric motor, comparing a preset target deceleration with an actual deceleration; and a control unit for, in a case where the result of comparison of the comparison unit indicates that a state in which the actual deceleration is equal to or lower than the target deceleration occurs in a predetermined pattern though the electric motor is generating the maximum regenerative torque, causing the vehicle to travel using both an engine and the electric motor in a cooperative manner during the next deceleration after the current deceleration has been finished, such that the engine braking and the regeneration torque are both used as a braking force.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a U.S. national stage of application No. PCT/JP2011/074181, 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. 2011-005287, filed on Jan. 13, 2011, the disclosure of which are also incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a regeneration control device, a hybrid vehicle, a regeneration control method, and a computer program. 
       BACKGROUND ART 
       [0003]    A hybrid vehicle has an engine and an electric motor, can run by the engine or the electric motor, or can run by the cooperation between the engine and the electric motor, and can regeneratively generate power by the electric motor during deceleration. When the regenerative power generation is performed, regenerative torque is generated in the electric motor. The regenerative torque becomes friction in driving the hybrid vehicle to serve as braking force similarly as an engine brake (see for example, Patent Literature PTL1). Note that the regenerative torque of the electric motor is proportional to regenerative power of the electric motor. That is, as the regenerative power of the electric motor increases, the regenerative torque of the electric motor is also larger. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL1: JP 2007-223421 A 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    As described above, the regenerative torque of the electric motor in the hybrid vehicle serves as the braking force similarly as the engine brake. However, when the regenerative torque of the electric motor is used as the braking force of the hybrid vehicle, the braking force may be insufficient according to a weight of a load of the hybrid vehicle or a state of a downhill grade of a road surface on which the hybrid vehicle is driving, and the like. In this situation, there is a case in which the braking force requested by a driver cannot be satisfied and there is a case in which the driver feels insufficiency in braking force as to cause drivability to deteriorate. 
         [0006]    The present invention is contrived under the above background and an object of the present invention is to provide a regeneration control device, a hybrid vehicle, a regeneration control method and a computer program that can improve drivability when the regenerative torque of the electric motor is used as the braking force. 
       Solution to Problem 
       [0007]    An aspect of the present invention is a regeneration control device. A regeneration control device of the present invention is a regeneration control device of a hybrid vehicle that includes an engine and an electric motor, that is capable of driving by the engine or the electric motor or capable of driving by a cooperation between the engine and the electric motor, and regeneratively generates power by the electric motor at least during deceleration, and uses regenerative torque generated by the regenerative power generation of the electric motor during the driving by only the electric motor as braking force, comprising: a first comparison unit for, when a regenerative torque generated by regenerative power generation in the electric motor is used as a braking force during deceleration of the hybrid vehicle traveling using only the electric motor, comparing a preset target deceleration with an actual deceleration caused by the regenerative torque generated by the regenerative power generation; and a control unit for, in a case where the result of comparison of the comparison unit indicates that a state in which the actual deceleration is equal to or lower than the target deceleration occurs in a predetermined pattern though the electric motor is generating the maximum regenerative torque, causing the vehicle to travel using both an engine and the electric motor in a cooperative manner during the next deceleration after the current deceleration has been finished, such that the engine braking of the engine and the regeneration torque of the electric motor are both used as a braking force. 
         [0008]    For example, the predetermined pattern is a pattern in which the afore-mentioned state is continued for a predetermined time. Alternatively, the predetermined pattern is a pattern in which a state where the afore-mentioned state is continued for the predetermined time is repeated at the predetermined number of times. 
         [0009]    Further, the regeneration control device may further include a second comparison unit for, when both the engine brake of the engine and the regenerative torque of the electric motor are used as the braking force, comparing the actual deceleration obtained by using both the engine brake of the engine and the regenerative torque of the electric motor as the braking force with the target deceleration, wherein the control unit causes, when a comparison result of the second comparison unit indicates that the actual deceleration reaches the target deceleration and the regenerative torque of the electric motor is equal to or less than a predetermined value, the vehicle to travel using only the electric motor to use the regenerative torque of the electric motor as the braking force. 
         [0010]    Further, the control unit may perform deceleration by only the electric motor in the first deceleration from the time the hybrid vehicle started in order to use regenerative torque generated by the regenerative power generation of the electric motor as the braking force. 
         [0011]    Another aspect of the present invention is a hybrid vehicle. The hybrid vehicle of the present invention has the regeneration control device of the present invention. 
         [0012]    Yet another aspect of the present invention is a regeneration control method. The regenerator control method of the present invention is a regenerator control method of a hybrid vehicle that includes an engine and an electric motor, that is capable of driving by the engine or the electric motor or capable of driving by cooperation of the engine and the electric motor, and regeneratively generates power by the electric motor at least during deceleration, and uses regenerative torque generated by the regenerative power generation of the electric motor during the driving by only the electric motor as braking force including: a first comparison step for, when a regenerative torque generated by regenerative power generation in the electric motor is used as a braking force during deceleration of the hybrid vehicle traveling using only the electric motor, comparing a preset target deceleration with an actual deceleration caused by the regenerative torque generated by the regenerative power generation; a step of controlling for, in a case where the result of comparison of the comparison unit indicates that a state in which the actual deceleration is equal to or lower than the target deceleration occurs in a predetermined pattern though the electric motor is generating the maximum regenerative torque, causing the vehicle to travel using both an engine and the electric motor in a cooperative manner during the next deceleration after the current deceleration has been finished, such that the engine braking of the engine and the regeneration torque of the electric motor are both used as a braking force; a second comparison step for, when both the engine brake of the engine and the regenerative torque of the electric motor are used as the braking force, comparing the actual deceleration obtained by using both the engine brake of the engine and the regenerative torque of the electric motor as the braking force with the target deceleration; and a step of controlling for, when a comparison result of the second comparison unit indicates that the actual deceleration reaches the target deceleration and the regenerative torque of the electric motor is equal to or less than a predetermined value, causing the vehicle to travel using only the electric motor to use the regenerative torque of the electric motor as the braking force. 
         [0013]    Still another aspect of the present invention is a computer program. The computer program of the present invention implements function of the regeneration control device of the present invention in an information processing device. 
       Advantageous Effect of Invention 
       [0014]    According to the present invention, the drivability when the regenerative torque of the electric motor is used as the braking force can be improved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0015]      FIG. 1  is a block diagram illustrating an exemplary structure of a hybrid vehicle according to an embodiment of the present invention. 
           [0016]      FIG. 2  is a block diagram illustrating an exemplary configuration of a function implemented in a hybrid ECU of  FIG. 1 . 
           [0017]      FIG. 3  is a diagram illustrating the relationship between an SOC (state of charge) of a battery and an upper limit value of regenerative power of an electric motor. 
           [0018]      FIG. 4  is a flowchart illustrating process I to be carried out by a regeneration control unit of  FIG. 2 . 
           [0019]      FIG. 5  is a flowchart illustrating process associated with ON/OFF of a key switch in the process of the regeneration control unit (process I) of  FIG. 2 . 
           [0020]      FIG. 6  is a flowchart illustrating process II to be carried out by the regeneration control unit (process II) of  FIG. 2 . 
           [0021]      FIG. 7  is a diagram illustrating the relationship between a deceleration and a rotational speed of the electric motor in a regenerative control of the regeneration control unit of  FIG. 2  and illustrates a case in which an actual deceleration falls short of a target deceleration. 
           [0022]      FIG. 8  is a diagram illustrating the relationship between regenerative torque and the rotational speed of the electric motor in the regenerative control of the regeneration control unit of  FIG. 2  and illustrates a case in which insufficient regenerative torque is generated with respect to maximum regenerative torque. 
           [0023]      FIG. 9  is a diagram illustrating the relationship between the deceleration and the rotational speed of the electric motor in the regenerative control of the regeneration control unit of  FIG. 2  and illustrates a case in which the actual deceleration follows the target deceleration. 
           [0024]      FIG. 10  is a diagram illustrating the relationship between the regenerative torque and the rotational speed of the electric motor in the regenerative control of the regeneration control unit of  FIG. 2  and illustrates a case in which actual regenerative torque is equal to or less than a clutch disconnecting determination regenerative torque threshold value. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    Hereinafter, a hybrid vehicle according to embodiments of the present invention will be described with reference to  FIGS. 1 to 10 . 
         [0026]      FIG. 1  is a block diagram illustrating an exemplary structure of a hybrid vehicle  1 . The hybrid vehicle  1  is one example of an vehicle. The hybrid vehicle  1  is driven by an engine (internal combustion engine)  10  and/or an electric motor  13  through a transmission of a semi-automatic transmission and can generate braking force like an engine brake of an engine  10  by regenerative torque of the electric motor  13  during deceleration. Note that the semiautomatic transmission is a transmission that can automatically shift the gears while having the same structure as a manual transmission. 
         [0027]    The hybrid vehicle  1  includes an engine  10 , an engine ECU (electronic control unit)  11 , a clutch  12 , an electric motor  13 , an inverter  14 , a battery  15 , a transmission  16 , an electric motor ECU  17 , a hybrid ECU  18 , an vehicle wheel  19 , a key switch  20 , an acceleration sensor  21 , and a shift unit  22 . Note that the transmission  16  includes the above-mentioned semiautomatic transmission, and is operated by the shift unit  22  including a drive range (hereinafter, referred to as a D (Drive) range). When the shift unit  22  is in the D range, the shifting operation of the semi-automatic transmission is automated. 
         [0028]    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 . 
         [0029]    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, 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. 
         [0030]    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. 
         [0031]    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 . 
         [0032]    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  stops 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 down grade and the electric motor  13  generates electric power (regenerates electric power) while the engine  10  stops or is in an idling state. 
         [0033]    Note that the clutch  12  differs from the clutch operated by the driver&#39;s operation of a clutch pedal, and is operated by the control of the hybrid ECU  18 . 
         [0034]    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, the hybrid vehicle  1  reduces the speed or runs on the down grade. 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 . As described above, in the state in which the electric motor  13  is power-generated, the hybrid vehicle  1  performs “regeneration to the battery  15 ”. In this state, the electric motor  13  generates regenerative torque having a magnitude depending on the regenerative power. 
         [0035]    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 . 
         [0036]    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. In the battery  15 , a range of an appropriate charge state (hereinafter, referred to as an SOC (state of charge)) is determined and the battery  15  is managed so that the SOC does not deviate from the range. 
         [0037]    The transmission  16  includes a semiautomatic transmission (not shown in the drawings) that selects one of a plurality of gear ratios (change gear ratios) according to the signal to instruct 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 down grade. Note that the semiautomatic transmission can also shift the gear position to a given gear number by the driver&#39;s hand operation of the shift unit  22 . 
         [0038]    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, a DSP, and the like, and internally has an operation unit, a memory, an I/O port, and the like. 
         [0039]    The hybrid ECU  18  is one example of the computer, and controls, for hybrid driving, the clutch  12  and supply the signal to instruct to shift gears in order to control the transmission  16  based on accelerator opening level information, brake operating information, vehicle speed information, gear position information acquired from the transmission  16 , engine rotational speed information acquired from the engine ECU  11  and SOC information acquired from the battery  15 , and instructs the electric motor ECU  17  to control the electric motor  13  and the inverter  14  and instructs the engine ECU  11  to control the engine  10 . The control instructions include also a regenerative control instruction which will be described below. For example, the hybrid ECU  18  includes a CPU, an ASIC, a microprocessor, a DSP, and the like and internally has an arithmetic logical unit, a memory, an I/O port, and the like. 
         [0040]    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. 
         [0041]    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. 
         [0042]    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 . 
         [0043]    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 switch activates each unit of the hybrid vehicle  1 , and turning OFF the key switch  20  stops each unit of the hybrid vehicle  1 . 
         [0044]    The acceleration sensor  21  senses an acceleration of the hybrid vehicle  1  and transfers the sensed acceleration to the hybrid ECU  18  as acceleration information. However, in the embodiment, since the hybrid ECU  18  acquires deceleration information of the hybrid vehicle  1  from the acceleration sensor  21 , in  FIG. 1 , it is illustrated that the deceleration information is transferred from the acceleration sensor  21  to the hybrid ECU  18 . 
         [0045]    The shift unit  22  gives an instruction received from the driver to the semi-automatic transmission of the transmission  16  as described already and when the shift unit  22  is in the D range, the shifting operation of the semi-automatic transmission is automated. 
         [0046]      FIG. 2  is a block diagram illustrating a configuration example of a function implemented in the hybrid ECU  18  that executes the computer program. In other words, when the hybrid ECU  18  executes the computer program, functions of a regeneration control unit  30 , a target deceleration setting unit  31 , and a deceleration comparison unit  32  are implemented. 
         [0047]    The regeneration control unit  30  serves to give an instruction of a regenerative control to the engine ECU  11 , the clutch  12 , the inverter  14 , and the electric motor ECU  17  based on both a comparison result of a target deceleration and an actual deceleration output from the deceleration comparison unit  32  and regenerative torque information transferred from the electric motor ECU  17 . Note that the regeneration control unit  30  may acquire regenerative power information from the electric motor ECU  17  instead of the regenerative torque information and may acquire regenerative torque proportional to regenerative power through calculation. 
         [0048]    The target deceleration setting unit  31  serves to set the target deceleration. The target deceleration is a target value of braking force generated by the regenerative torque which is determined in advance when the electric motor  13  performs regenerative power generation. Setting the target deceleration enables the hybrid ECU  18  to appropriately perform scheduling and the like when the braking force by the regenerative torque of the electric motor  13  and the engine brake are used together. The target deceleration is also transferred to the electric motor ECU  17 , and the electric motor ECU  17  performs a control so that the braking force generated by the regenerative torque of the electric motor  13  approximates to the target deceleration. 
         [0049]    The deceleration comparison unit  32  serves to compare the target deceleration set by the target deceleration setting unit  31  and an actual deceleration of the hybrid vehicle  1  (referred to as an actual deceleration) acquired from the deceleration information of the acceleration sensor  21 . 
         [0050]    Herein, the relationship between the SOC of the battery  15  and the regenerative power will be described below with reference to  FIG. 3 .  FIG. 3  is a diagram illustrating the relationship between the SOC of the battery  15  and an upper limit value of the regenerative power, in which the SOC is set on a horizontal axis and the regenerative power is set on a vertical axis. Note that the target deceleration is a predetermined value and for example, is determined in advance as 50% to 70% of the braking force of the engine brake by the engine  10 . A value of the target deceleration corresponding to the SOC is set in advance by map information and the like, and for example, pre-stored in the memory of the hybrid ECU  18 . 
         [0051]    As illustrated in  FIG. 3 , when the SOC of the battery  15  is in the range of 0% to 80%, the upper limit value of the regenerative power for the battery  15  is almost constant. But when the SOC is more than 80%, the range of the upper limit value of the regenerative power for the battery  15  is narrowed in order to prevent the battery  15  from being overcharged. The regeneration control unit  30  controls the braking force generated by the regenerative torque of the electric motor  13  to achieve a predetermined target deceleration when the SOC is equal to or less than 80%. On the other hand, when the SOC is more than 80%, the range of the regenerative power is slowly narrowed. And as a result, the range of the regenerative torque is also narrowed. Therefore, the situation in which the target deceleration set by the target deceleration setting unit  31  is not reached occurs even though maximum regenerative torque of the electric motor  13  is provided. 
         [0052]    Further, an another situation in which the target deceleration set by the target deceleration setting unit  31  is not reached even though the maximum regenerative torque of the electric motor  13  is provided occurs when the hybrid vehicle  1  is running on the steep downhill grade beyond an assumed range when a load of the hybrid vehicle  1  is in a constant volume state. The regeneration control unit  30  performs a control to supplement insufficiency of the maximum regenerative torque under this situation as described below. That is, the regeneration control unit  30  performs a control so that the deceleration by the engine brake is added to the deceleration by the regenerative torque to satisfy the target deceleration regardless of driver&#39;s manipulation when the deceleration by the regenerative torque of the electric motor  13  does not reach the target deceleration. In addition to the example, when the driver additionally needs the deceleration, the driver steps on a service brake or operates auxiliary brakes (a retarder, an exhaust brake, an engine retarder, and the like). 
         [0053]    Note that in a general driving state in which the SOC of the battery  15  is equal to or less than 80%, the target deceleration setting unit  31  is configured not to excessively set the target deceleration that cannot be reached even though the maximum regenerative torque of the electric motor  13  is provided. Herein, the general driving state denotes the state in which the load of the hybrid vehicle  1  is in the constant volume state and the hybrid vehicle  1  is running on the downhill grade (a degree thereof is various depending on the performance of the vehicle) within the assumed range. 
         [0054]    Next, process I of the regenerative control performed by the hybrid ECU  18  executing the computer program will be described with reference to flowcharts of  FIGS. 4 and 5 . Note that a flow of steps S 1  to S 9  of  FIG. 4  corresponds to process for one cycle and the process is repeatedly executed as long as the key switch  20  is in the ON state. In addition, flow process of steps S 20  to S 23  illustrated in  FIG. 5  is executed together with the flow process of steps S 1  to S 9  illustrated in  FIG. 4 . 
         [0055]    In “START” of  FIG. 4 , the key switch  20  is in the ON state, the hybrid ECU  18  executes the computer program, and the functions of the regeneration control unit  30 , the target deceleration setting unit  31 , and the deceleration comparison unit  32  are implemented in the hybrid ECU  18 , and the process proceeds to step S 1 . Note that at the time of “START”, any one of processes of steps S 4 , S 8 , and S 23  ( FIG. 5 ) described below is executed. At this time, the hybrid vehicle  1  is in a state in which clutch disconnecting regeneration is performed. However, at the time of “START”, in the case where the process of step S 8  is performed, when it is determined that deceleration is achieved in the process of step S 1  just after “START”, the hybrid vehicle  1  is transited from the clutch disconnecting regeneration to clutch connecting regeneration. 
         [0056]    In step S 1 , the deceleration comparison unit  32  determines whether the hybrid vehicle  1  is decelerated according to the deceleration information of the acceleration sensor  21 . In step S 1 , when it is determined that the hybrid vehicle  1  is decelerated, the process proceeds to step S 2 . On the other hand, in step S 1 , when it is determined that the hybrid vehicle  1  is not decelerated, the process in step S 1  is repeated. 
         [0057]    In step S 2 , the deceleration comparison unit  32  compares the target deceleration set by the target deceleration setting unit  31  and the actual deceleration of the hybrid vehicle  1  based on the deceleration information acquired from the acceleration sensor  21  to determine whether the actual deceleration reaches the target deceleration. In step S 2 , when it is determined that the actual deceleration reaches the target deceleration, the process proceeds to step S 3 . On the other hand, in step S 2 , when it is determined that the actual deceleration does not reach the target deceleration, the process proceeds to step S 5 . 
         [0058]    In step S 3 , the regeneration control unit  30  determines whether current regenerative torque of the electric motor  13  is equal to or less than a “clutch disconnecting determination regenerative torque threshold value (a predetermined value disclosed in claims)” according to the regenerative torque information transferred from the electric motor ECU  17 . Note that the “clutch disconnecting determination regenerative torque threshold value” will be described below in detail, and is a threshold which is lower than the maximum regenerative torque by approximately 10% to 20% thereof. In step S 3 , when it is determined that the current regenerative torque of the electric motor  13  is equal to or less than the “clutch disconnecting determination regenerative torque threshold value”, the process proceeds to step S 4 . On the other hand, in step S 3 , when it is determined that the current regenerative torque of the electric motor  13  is more than the “clutch disconnecting determination regenerative torque threshold value”, the process proceeds to step S 11 . 
         [0059]    In step S 4 , the regeneration control unit  30  instructs the engine ECU  11 , the clutch  12 , the inverter  14 , and the electric motor ECU  17  on the “clutch disconnecting regeneration” in which regeneration is performed with the clutch  12  being disconnected to end the process for one cycle (END). 
         [0060]    In step S 5 , the regeneration control unit  30  instructs the inverter  14  and the electric motor ECU  17  to increase the regenerative torque and the process then proceeds to step S 6 . 
         [0061]    In step S 6 , the regeneration control unit  30  determines whether the regenerative torque reaches the maximum regenerative torque as a result of increasing the regenerative torque in step S 5 . Note that the maximum regenerative torque denotes regenerative torque generated by performing regeneration by the electric motor  13  with regenerative power substantially equivalent to an upper limit value of the regenerative power set with respect to the current SOC of the battery  15 . In step S 6 , when it is determined that the regenerative torque reaches the maximum regenerative torque, the process proceeds to step S 7 . On the other hand, in step S 6 , when it is determined that the regenerative torque has not yet reached the maximum regenerative torque, the process returns to step S 2 . 
         [0062]    In step S 7 , the regeneration control unit  30  determines whether a state of the maximum regenerative torque selected as Yes in step S 6  has continued for a predetermined time. Herein, the predetermined time is for example, tens of seconds (for example, 15 seconds). In step S 7 , when it is determined that the corresponding state has continued for the predetermined time, the process proceeds to step S 8 . On the other hand, in step S 7 , when it is determined that the corresponding state has not continued for the predetermined time, the process returns to step S 4 . 
         [0063]    In step S 8 , the regeneration control unit  30  instructs the engine ECU  11 , the clutch  12 , the inverter  14 , and the electric motor ECU  17  on a “clutch connecting regeneration” in which regeneration is performed with the clutch  12  being connected in subsequent deceleration to end the processing for one cycle (END). In other words, the regeneration control unit  30  controls the “clutch disconnecting regeneration” to be continued as it is during current deceleration and the “clutch connecting regeneration” to be performed during the subsequent deceleration. 
         [0064]    In step S 9 , the regeneration control unit  30  determines whether the clutch disconnecting regeneration is performed during the current deceleration. Note that the regeneration control unit  30  stores the state (the clutch connecting regeneration or not) during the current deceleration in a part of a memory area of the hybrid ECU  18 . In step S 9 , when it is determined that the clutch connecting regeneration is performed during the current deceleration, the process returns to step S 8 . On the other hand, in step S 9 , when it is determined that the clutch connecting regeneration is not performed during the current deceleration, the process returns to step S 4 . 
         [0065]    Further, as illustrated in  FIG. 5 , in step S 20 , the regeneration control unit  30  determines whether the key switch  20  of the hybrid vehicle  1  is in the OFF state. In step S 20 , when it is determined that the key switch  20  is in the OFF state, the process proceeds to step S 21 . On the other hand, in step S 20 , when it is determined that the key switch  20  is not in the OFF state, the process in step S 20  is repeated. 
         [0066]    In step S 21 , the function of the regeneration control unit  30  stops and the process proceeds to step S 22 . 
         [0067]    In step S 22 , the regeneration control unit  30  determines whether the key switch  20  of the hybrid vehicle  1  has been turned ON. In step S 22 , when it is determined that the key switch  20  has been turned ON, the process proceeds to step S 23 . On the other hand, in step S 22 , when it is determined that the key switch  20  has not been turned ON, the process returns to step S 21 . 
         [0068]    In step S 23 , the regeneration control unit  30  performs the clutch disconnecting regeneration when the key switch  20  of the hybrid vehicle  1  has been turned ON and in first deceleration after the start of operation of the hybrid vehicle  1 , and then the process returns to step S 20 . 
         [0069]    Note that in the process of step S 7  of  FIG. 4 , it is determined whether the maximum regenerative torque selected as Yes in step S 6  has continued for the predetermined time to determine whether the hybrid vehicle  1  is running on for example, a long downhill grade. That is, when the state of Yes in step S 6  continued for, for example, 15 seconds, it may be determined that the hybrid vehicle  1  is running on the long downhill grade. As a result, the frequent switching of the clutch disconnecting regeneration and the clutch connecting regeneration may be avoided in a road surface environment in which a short downhill grade is off and on. 
         [0070]    Next, process II of the regenerative control performed by the hybrid ECU  18  that executes the computer program will be described with reference to a flowchart of  FIG. 6 . Note that in the flowchart of  FIG. 6 , step S 7  in the flowchart of  FIG. 4  is changed to step S 10 . Therefore, a process of step S 10  will be described and the processes of steps S 1  to S 6 , S 8 , and S 9  will not be described. Further, the flow process of  FIG. 5  is also executed together with the flow process of  FIG. 6 . 
         [0071]    In step S 10 , the regeneration control unit  30  determines whether the state of the maximum regenerative torque selected as Yes in step S 6  is on and off at the predetermined number of times within a predetermined time. Herein, the predetermined time denotes for example, tens of seconds (for example, 30 seconds). Further, the predetermined number of times denotes for example, several times (for example, three times). In step S 10 , when it is determined that the corresponding state is on and off at the predetermined number of times within the predetermined time, the process proceeds to step S 8 . On the other hand, in step S 10 , when it is determined that the corresponding state is not on and off at the predetermined number of times within the predetermined time, the process returns to step S 4 . 
         [0072]    Note that in the process of step S 10  of  FIG. 6 , it is determined whether the maximum regenerative torque selected as Yes in step S 6  is on and off at the predetermined number of times within the predetermined time to determine whether the hybrid vehicle  1  is running on for example, the long downhill grade. That is, when the state of Yes in step S 6  is on and off for example, at three times within 30 seconds, it may be determined that the hybrid vehicle  1  is running on the long downhill grade. In addition, it is considered that a cause of the state of Yes in step S 6  being on and off is that since the hybrid vehicle  1  is accelerated due to insufficient braking force, the driver uses the service brake or the auxiliary brake, and as a result, the hybrid vehicle  1  is once decelerated, but when the driver stops using the service brake or the auxiliary brake, a state in which the hybrid vehicle  1  is again accelerated is repeated. As a result, the frequent switching of the clutch disconnecting regeneration and the clutch connecting regeneration may be avoided in the road surface environment in which the short downhill grade is off and on. 
         [0073]    (In Regard to Effects) 
         [0074]    In describing the effects of the embodiment, a case in which the actual deceleration falls short of the target deceleration will be described with reference to  FIG. 7 , a case in which insufficient regenerative torque is generated with respect to the maximum regenerative torque will be described with reference to  FIG. 8 , a case in which the actual deceleration follows the target deceleration will be described with reference to  FIG. 9 , and a case in which actual regenerative torque is equal to or less than the clutch disconnecting determination regenerative torque threshold value will be described with reference to  FIG. 10 . 
         [0075]    In  FIGS. 7 and 9 , a rotational speed of the electric motor is set on a horizontal axis and deceleration is set on a vertical axis. In  FIGS. 8 and 10 , the rotational speed of the electric motor is set on a horizontal axis and regenerative torque is set on a vertical axis. In the states of  FIGS. 7 to 10 , the hybrid vehicle  1  is being decelerated. Therefore, the rotational speed of the electric motor on the horizontal axis of  FIGS. 7 to 10  is changed from the right side (a high side) to a left side (a lower side) as time elapsed. 
         [0076]    As illustrated in  FIG. 7 , when deceleration of the hybrid vehicle  1  is started, the actual deceleration (dotted line) by the regenerative torque of the electric motor  13  is controlled to be approximate to the target deceleration (solid line). In an example of  FIG. 7 , the actual deceleration is lower than the target deceleration, and a deceleration shortfall region is generated as expressed by hatching. Therefore, the driver feels that the deceleration is insufficient. Further, in this case, as illustrated in  FIG. 8 , a region of insufficient regenerative torque (hatching part) in which the actual regenerative torque falls short of required regenerative torque is generated. 
         [0077]    Contrary to this, the regeneration control unit  30  performs a control so that the actual deceleration (dotted line) approximates to the target deceleration (solid line) by, in parallel, using the deceleration by the regenerative torque of the electric motor  13  and the deceleration by the engine brake of the engine  10  together when the hybrid vehicle  1  is decelerated next time, as illustrated in  FIG. 9 . Therefore, the driver may acquire excellent drivability without feeling that the deceleration is insufficient. 
         [0078]    Further, in this case, as illustrated in  FIG. 10 , the braking force by the regenerative torque of the electric motor  13  is added to the braking force by the engine brake of the engine  10  and the actual regenerative torque of the electric motor  13  is equal to or less than the “clutch disconnecting determination regenerative torque threshold value (alternated long and short dash line)”. Therefore, like Yes of step S 3  in the flowcharts of  FIGS. 4 and 6 , the case is in a state in which the clutch disconnecting regeneration is enabled. Note that the “clutch disconnecting determination regenerative torque threshold value” is set to a value which is lower than the maximum regenerative torque by approximately 10% to 20% thereof. When the “clutch disconnecting determination regenerative torque threshold value” is equal to the maximum regenerative torque, a state (referred to as mode hunting) in which the clutch connecting regeneration and the clutch disconnecting regeneration are frequently switched to each other may occur. But the mode hunting may be suppressed by setting the “clutch disconnecting determination regenerative torque threshold value” to the value which is lower than the maximum regenerative torque by approximately 10% to 20% thereof. When the clutch disconnecting regeneration is switched to the clutch connecting regeneration, the engine  10  and the electric motor  13  are synchronized with each other in terms of the rotational speed and the engine  10  is at a higher rotational speed than an idle state to consume fuel, and as a result, the mode hunting is not preferable in terms of improvement of fuel efficiency. 
         [0079]    In addition, since the regeneration control unit  30  performs a control so that a timing for the clutch  12  to be in a connection state does not take place during the deceleration (step S 8 ), there is no case in which the driver feels sudden braking shock by the connection of the clutch  12 . 
         [0080]    Further, according to the processes of step S 7  of the flowchart of  FIG. 4  and step S 10  of the flowchart of  FIG. 6 , the frequent switching of the clutch disconnecting regeneration and the clutch connecting regeneration may be avoided in the road surface environment in which the short downhill grade is on and off as described above. Even as a result, the number of times where the clutch  12  is connected may be decreased and fuel efficiency may be improved by suppressing a fuel consumption amount of the hybrid vehicle  1  to be low. 
         [0081]    Further, according to the processes of steps S 20  to S 23  of the flowchart of  FIG. 5 , in the first deceleration after the start of running of the hybrid vehicle, deceleration by only the electric motor  13  never fail to be performed, and deceleration by the regenerative torque is performed with the clutch  12  being disconnected. As a result, since a chance to connect the clutch  12  may be controlled to be decreased as much as possible during the deceleration, the fuel efficiency may be improved by suppressing the fuel consumption amount of the hybrid vehicle  1  to be low. 
         [0082]    Furthermore, in the case where the engine  10  is a motive power source of an auxiliary machine (a compressor of a cooling machine, and the like), connecting the clutch  12  on the long downhill grade would make it possible to interrupt fuel injection of the engine  10  to acquire a stable motive power source of the auxiliary machine while saving fuel. Even in this case, when the clutch disconnecting regeneration and the clutch connecting regeneration are frequently switched to each other in the road surface environment in which the short downhill grade is on and off, the engine  10  is not appropriate for the motive power source of the auxiliary machine because the rotational speed of the engine  10  becomes unstable. Even in order to avoid the inappropriate state, the processes of step S 7  of the flowchart of  FIG. 4  and step S 10  of the flowchart of  FIG. 6  are valid. 
       Other Embodiments 
       [0083]    In step S 4  of the flowcharts of  FIGS. 4 and 6 , the “clutch disconnecting regeneration” is performed immediately after Yes is selected in step S 3 . Contrary to this, step S 4  may be changed to the “clutch disconnecting regeneration from the next time” so that the clutch disconnecting regeneration is performed when the hybrid vehicle  1  is decelerated next time after Yes in step S 3  is selected. Therefore, since the clutch  12  is not disconnected while the hybrid vehicle  1  is decelerated, it is possible to prevent the driver from feeling shock. Note that even though the clutch  12  is disconnected during the deceleration, since the braking force by the regenerative torque of the electric motor  13  is sufficient, the driver feels very small shock. Accordingly, in general, a need to change step S 4  to the “clutch disconnecting regeneration from the next time” is small. 
         [0084]    Further, in the description of the flowcharts of  FIGS. 4 and 6 , boundary values of determination may be variously changed, such as changing “equal to or less than” to “less than” and “more than” to “equal to or more than”. 
         [0085]    Although it has been described that the engine  10  is the internal combustion engine, the engine  10  may be a heat engine including an external-combustion engine. 
         [0086]    Further, although it has been described that the computer program executed by the hybrid ECU  18  is installed in the hybrid ECU  18  in advance, removable media in which the computer program is recorded (which stores the computer program) may be mounted on a drive (not illustrated in the drawings) and the like, the program read from the removable media may be stored in the nonvolatile memory in the hybrid ECU  18 , or a program transmitted through wired or wireless transmission media may be received from a communication unit (not illustrated in the drawings) and stored in the nonvolatile memory in the hybrid ECU  18  to be installed in the hybrid ECU  18  as the computer. 
         [0087]    In addition, the respective ECUs may be implemented by an ECU formed by organizing the ECUs into one or an ECU having additionally subdivided functions of the respective ECUs may be newly formed. 
         [0088]    Further, the computer program executed by the computer may be a computer program which is processed in time series according to the sequence described in the specification or a computer program which is processed in parallel, or at a needed timing such as the time when calling is received. 
         [0089]    In addition, the embodiments of the present invention are not limited to the afore-mentioned embodiments and may be variously changed without departing from the spirit of the present invention.