Abstract:
A control apparatus including a requestedtorque calculating device which calculates a torque requested for driving the hybrid vehicle; and a torque control device which controls a torque generated by an engine and a torque generated by an electric motor of the hybrid vehicle based on the requested torque calculated by the requested torque calculating device. The torque control device controls the electric motor so as to generate a torque obtained by subtracting a torque to be generated by the engine from the requested torque. Acoording to this control apparatus, if the responsiveness in changing the transmission ratio of the transmission is poor and the torque generated by the engine becomes lower than therequested torque, the electric motor is controlled to generate the deficient (supplementary) torque, and smooth acceleration of the vehicle can be performed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a control apparatus for a parallel hybrid vehicle which uses an engine and a motor for driving the vehicle. In particular, the present invention relates to a control technique for transmitting the output of an engine to driving wheels using a CVT (Continuously Variable Transmission) in a parallel hybrid vehicle. 
     2. Description of the Related Art 
     In conventional (non-hybrid) vehicles comprising engines and continuously variable transmissions, by means of detecting the amount of depression of an accelerator pedal using a throttle opening sensor, the driving force required by the drivers of the vehicles is calculated, and based on the calculation results, the engine speed and the transmission ratio are selected to smoothly drive the vehicle. 
     In comparison with vehicles comprising stage transmissions, because vehicles comprising continuously variable transmissions can select the optimum transmission ratio according to the condition of the vehicle, it is possible to maintain the desirable condition in which the engine can provide a higher energy efficiency. Therefore, vehicles comprising continuously variable transmissions are expected to have better fuel consumption. 
     However, in vehicles comprising continuously variable transmissions, the responsiveness in changing the transmission ratio is not good, there is the problem that the acceleration response when a kick down is performed is inferior to that of vehicles comprising stage transmissions. 
     Furthermore, because the acceleration response is not good due to the response delay in changing the transmission ratio, there is a tendency that the driver unduly depresses the accelerator pedal during the delay in the acceleration response, and that the throttle is opened wider than the driver wishes. In such cases, there arise the problems that fuel consumption is increased, exhaust gas is increased, and noise and vibration are increased. 
     The above problems are common to hybrid vehicles comprising continuously variable transmissions. That is, hybrid vehicles can enter into a running mode in which the vehicle is driven by the engine, and when in such a running mode using the engine, the problems due to the inferior responsiveness in changing the transmission ratio occur, similarly to those of conventional vehicles. 
     Also, when the running mode of a hybrid vehicle is shifted from a running mode in which the vehicle is driven by an electric motor to a running mode in which the vehicle is driven by an engine, there is the case that the transmission ratio of the transmission connected to the engine is not set to an optimum ratio. In such a case, due to the inferior responsiveness in changing the transmission ratio, the driving force for the vehicle is temporarily and suddenly changed when the running mode is shifted. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a control apparatus and method for a hybrid vehicle by which smooth acceleration of the vehicle is possible without a delay in generating the driving force when requested by the driver. 
     In order to achieve the above object, the control apparatus according to the present invention comprises a requested torque calculating device which calculates a torque requested for driving the hybrid vehicle; and a torque control device which controls a torque generated by an engine and a torque generated by an electric motor of the hybrid vehicle based on the requested torque calculated by the requested torque calculating device. The torque control device controls the electric motor so as to generate a torque obtained by subtracting a torque to be generated by the engine from the requested torque. 
     According to this control apparatus, if the responsiveness in changing the transmission ratio of the transmission is poor and the torque generated by the engine becomes lower than the requested torque, the electric motor is controlled to generate the deficient (supplementary) torque, and smooth acceleration of the vehicle can be performed. 
     Also, even when the running mode of the hybrid vehicle is shifted from a running mode in which the vehicle is driven by an electric motor to a running mode in which the vehicle is driven by an engine, it is possible to avoid the shock due to the torque change and to shift the running mode smoothly. 
     Furthermore, because the acceleration response can be improved, it is possible to prevent the problem that the driver unduly depresses the accelerator pedal during the delay in the acceleration response, and that the throttle is opened wider than the driver wishes. Therefore, the increases in fuel consumption, exhaust gas, and noise and vibration due to the above problem, can be prevented. 
     When the requested torque is greater than the maximum torque which the engine can generate, the torque control device may control the engine so as to generate the maximum torque and may control the electric motor so as to generate a torque obtained by subtracting the maximum torque from the requested torque. 
     In this case, if the responsiveness in changing the transmission ratio of the transmission is poor and the torque generated by the engine becomes lower than the requested torque, the deficient torque can be generated by the electric motor, and smooth acceleration of the vehicle can be performed. 
     The torque control device may comprise an engine torque restricting device which restricts the torque to be generated by the engine in accordance with a predetermined condition. 
     In this case, the engine torque restricting device restricts the torque to be generated by the engine in accordance with a predetermined condition, and the deficient torque is generated by the electric motor, and smooth acceleration of the vehicle can be performed. 
     The engine torque restricting device may restrict the torque to be generated by the engine when the temperature of the catalyst provided in the exhaust system of the engine is higher than a predetermined temperature. 
     In this case, when the temperature of the catalyst provided in the exhaust system of the engine is higher than the predetermined temperature, the engine torque restricting device restricts the torque to be generated by the engine, and the deficient torque is generated by the electric motor. Therefore, smooth acceleration of the vehicle can be performed. 
     The engine torque restricting device may restrict the torque to be generated by the engine when a remaining battery charge of a power storage unit is greater than a predetermined battery charge. 
     In this case, when a remaining battery charge of the power storage unit is greater than the predetermined battery charge, the engine torque restricting device restricts the torque to be generated by the engine, and the deficient torque is generated by the electric motor. Therefore, smooth acceleration of the vehicle can be performed. 
     The engine torque restricting device may stop restricting the torque to be generated by the engine when the requested torque is greater than a predetermined torque. 
     In this case, when the requested torque is greater than a predetermined torque, for example, when the accelerator pedal is fully depressed, the engine torque restricting device stops restricting the torque to be generated by the engine. Therefore, even when quick acceleration is requested, acceleration of the vehicle can be performed smoothly and quickly. 
     The requested torque calculating device may calculate a torque requested for driving the driving wheels of the hybrid vehicle. The torque control device may comprise a conversion device which converts an engine torque to be generated by the engine to a driving wheel torque to be applied to the driving wheels based on at least a transmission ratio in a transmission which transmits the driving force from the engine to the driving wheels. Furthermore, the torque control device may control the electric motor so as to generate a torque obtained by subtracting the driving wheel torque converted by the conversion device from the requested torque. 
     In this case, the requested torque calculating device calculates a torque requested for driving the driving wheels of the hybrid vehicle, and the conversion device converts an engine torque to be generated by the engine to a driving wheel torque to be applied to the driving wheels based on at least the transmission ratio. Then, the torque control device controls the electric motor so as to generate a torque obtained by subtracting the driving wheel torque converted by the conversion device from the requested torque. Therefore, if the relationship between the engine torque generated by the engine and the driving wheel torque is modified due to the change of the transmission ratio, it is possible for the motor to generate a suitable torque in consideration of the modification of the above relationship. That is, the precision in controlling the motor can be improved. 
     Another aspect of the present invention is a hybrid vehicle comprising an engine which generates a torque for driving the hybrid vehicle; a transmission which is connected to an output shaft of the engine; driving wheels which are connected to an output shaft of the transmission; an electric motor which is connected to the driving wheels and assists the torque generated by the engine; a requested torque calculating device which calculates a torque requested for driving the hybrid vehicle; and a torque control device which controls a torque generated by an engine and a torque generated by an electric motor of the hybrid vehicle based on the requested torque calculated by the requested torque calculating device. The torque control device controls the electric motor so as to generate a torque obtained by subtracting a torque to be generated by the engine from the requested torque. 
     According to this hybrid vehicle, if the responsiveness in changing the transmission ratio of the transmission is poor and the torque generated by the engine becomes lower than the requested torque, the electric motor is controlled to generate the deficient torque. 
     Furthermore, another aspect of the present invention is a control method for a hybrid vehicle, comprising calculating a torque requested for driving the hybrid vehicle; controlling a torque generated by an engine and a torque generated by an electric motor of the hybrid vehicle based on the requested torque calculated by the requested torque calculating device; and controlling the electric motor so as to generate a torque obtained by subtracting a torque to be generated by the engine from the requested torque. 
     According to this control method, if the responsiveness in changing the transmission ratio of the transmission is poor and the torque generated by the engine becomes lower than the requested torque, the electric motor is controlled to generate the deficient torque, and the vehicle can be driven smoothly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram illustrating the transmission and control systems of a hybrid vehicle to which a first embodiment of the present invention is applied. 
     FIG. 2 is a block diagram illustrating in detail a control circuit of the first embodiment. 
     FIG. 3 is a block diagram illustrating in detail a control circuit of a second embodiment of the present invention. 
     FIG. 4 is a timing chart for explaining the function of the control circuit shown in FIG.  2 . 
     FIG. 5 is a timing chart for explaining the function of the control circuit shown in FIG.  3 . 
     FIG. 6 is a block diagram illustrating a control circuit of a third embodiment of the present invention. 
     FIG. 7 is a block diagram illustrating a control circuit of a fourth embodiment of the present invention. 
     FIG. 8 is a block diagram illustrating a control circuit of a fifth embodiment of the present invention. 
     FIG. 9 is a block diagram illustrating a control circuit of a sixth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a preferred embodiment of the control apparatus and control method for a hybrid vehicle according to the present invention will be explained referring to the figures. FIG. 1 is a block diagram illustrating the transmission and control system of a hybrid vehicle in which the first embodiment of the present invention is applied. 
     First, the transmission system will be explained. This vehicle comprises an engine E which is activated by the combustion energy of a fuel, and the output shaft of the engine E is connected to an oil pump  2  via a sub motor  1 . The sub motor  1  performs the starting of the engine E and the assistance of the output of the engine E, and so on. The oil pump  2  is rotated by the engine E and supplies oil pressure to an oil pressure controller  24 , and the oil pressure controller  24  controls a continuously variable transmission (hereinafter, referred as CVT)  4  so as to change the transmission ratio thereof. The engine E comprises an exhaust system  29 , and a catalyst  27  is provided in the exhaust system  29 . 
     The output shaft of the engine E is further connected to a planetary gear  3  which changes the running direction of the vehicle to forwards or reverse. A select lever (not shown) is connected to the planetary gear  3 , and the running direction of the vehicle is changed by operating the select lever. 
     The output shaft of the planetary gear  3  is connected to a driving pulley  5  provided in the CVT  4 . The CVT  4  comprises, in addition to the driving pulley  5 , a metal belt  6 , a driven pulley  7 , and a pair of side chambers  8  and  9 , and the metal belt  6  is wound between the driving pulley  5  and the driven pulley  7  so as to transmit the driving force between the pulleys  5  and  7 . 
     The side chambers  8  and  9  are respectively provided at the sides of the driving pulley  5  and the driven pulley  7 , and they are connected to the oil pressure controller  24  via oil passages. The wrapping diameters of the metal belt  6  around the pulleys  8  and  9  can be independently adjusted by changing the widths of the pulleys  8  and  9  by means of controlling the oil pressures applied into the side chambers  8  and  9  from the oil pump  3  via the oil pressure controller  24 . 
     The driven pulley  7  in the CVT  4  is connected to an engaging element  11  provided in a clutch  10 . The clutch  10  comprises, in addition to the engaging element  11 , an engaging element  12  facing the engaging element  11 , and a clutch control actuator  13  for engaging or disengaging the engaging elements  11  and  12 . 
     The engaging element  12  in the clutch  10  is connected to a final reduction gear  14  and a gear  15 . The final reduction gear  14  is engaged with a differential gear  16 , and this differential gear  16  is connected to driving wheels W via a driving shaft  17 . The gear  15  is engaged with a gear  18  which is connected to a rotational shaft of a main motor  19 . 
     Next, the control system in this vehicle will be explained. The control system includes a control circuit  20 , which is electrically connected to a remaining battery charge sensor  22  provided on a power storage unit  21 , a power drive unit  23  for controlling the main motor  19 , the clutch control actuator  13  in the clutch  10 , and the oil pressure controller  24  for controlling the oil pressures applied to the side chambers  8  and  9  in the CVT  4 . The power storage unit  21  consists of a battery, etc., and is a driving source of the main motor  19 . The power storage unit  21  is connected to the main motor  19  via the power drive unit  23 . 
     The control circuit  20  is also connected to an engine sensor  25  which detects the various conditions of the engine E such as a water temperature TW in the engine E, the engine speed Ne, the intake air temperature TA, the intake air pressure PA, and the amount of depression of the accelerator pedal AP. The control circuit  20  is further connected to an engine control device  26  which is connected to the engine E for controlling the engine E. 
     The control circuit  20  is further connected to a catalyst temperature sensor  28  for measuring the temperature of the catalyst  27 , to a vehicle speed sensor  30  for measuring the vehicle speed VCar, and to a ratio sensor for detecting the transmission ration of the CVT  4 . 
     Next, referring to the block diagram shown in FIG. 2, the control circuit  20  will be explained in detail. The control circuit  20  comprises a requested driving force calculation device  32 , a requested engine torque calculation device  33 , a transmission efficiency calculation device  34  for calculating the transmission efficiency of the CVT  4  based on the transmission ratio of the CVT  4 , a full throttle torque calculation device  35 , an engine torque restriction device  36 , and a difference calculation device  37 . 
     The requested driving force calculation device  32  receives the amount of depression of the accelerator pedal AP and the vehicle speed VCar from outside of the control circuit  20 . The requested driving force calculation device  32  outputs value of the a requested torque, and transmits this requested torque value to the requested engine torque calculation device  33 . The requested engine torque calculation device  33  also receives the transmission ratio of the CVT  4  from outside of the control circuit  20 . This transmission ratio of the CVT  4  is also input to the transmission efficiency calculation device  34 , and the transmission efficiency calculation device  34  outputs a transmission efficiency to the requested engine torque calculation device  33 . The requested engine torque calculation device  33  outputs a requested engine torque and transmits it to the engine torque restriction device  36  and the difference calculation device  37 . 
     The full throttle torque calculation device  35  receives the engine speed Ne, the intake air temperature TA, the intake air pressure PA, and the water temperature TW from outside of the control circuit  20 , and outputs a full throttle torque value or an engine torque limit to the engine torque restriction device  36 . 
     The engine torque restriction device  36  outputs an engine torque command to the difference calculation device  37 . The engine torque command is also output from the control circuit  20 , and is transmitted to an engine torque control device  38  in the engine control device  26 . The engine torque control device  38  outputs a control signal to the engine E. 
     The difference calculation device  37  outputs a motor torque command to a motor torque control device  39  in the power drive unit  23 , and the motor torque control device  39  outputs a control signal to the main motor  19 . 
     Next, referring to FIG. 1, the function of this embodiment will be explained. First, the function of the control system when the vehicle is driven by the driving force of the engine E will be explained. The driving force output by the engine E is transmitted via the sub motor  1  to the oil pump  2  and the planetary gear  3  to drive them. The oil pump  2  generates oil pressure and transmits it via the oil pressure control device  24  to the side chambers  8  and  9  in the CVT  4 . The planetary gear  3  is operated by the select lever (not shown) so as to change the rotational direction of the output shaft thereof, and changes the running direction of the vehicle. 
     The rotation of the output shaft of the planetary gear  3  is transmitted to the driving pulley  5  in the CVT  4 , and the rotation of the driving pulley  5  is transmitted via the metal belt  6  to the driven pulley  7 . The rotational speed ratio of the driving pulley  5  and the driven pulley  7  is determined by the wrapping diameters of the metal belt  6  around the pulleys  5  and  7 , and the wrapping diameters are adjusted by the oil pressures applied to the side chambers  8  and  9 . That is, the widths of the pulleys  5  and  7  are respectively changed in accordance with the pushing forces generated by the oil pressures applied to the side chambers  8  and  9 , and the contact areas of the metal belt  6  shift along the slopes of the inner side walls of the pulleys  5  and  7  in accordance with the widths of the pulleys  5  and  7 . The oil pressures applied to the side chambers  8  and  9  are respectively controlled by the oil pressure control device  24 . 
     The rotation of the driven pulley  7  in the CVT  4  is transmitted to the engaging element  11  in the clutch  10 . The clutch control actuator  13  can engage or disengage the engaging elements  11  and  12 . When the engaging elements  11  and  12  are disengaged, the rotation of the engaging element  11  is not transmitted to the engaging element  12 . 
     In contrast, when the engaging elements  11  and  12  are engaged to each other, the rotation of the engaging element  11  is transmitted to the engaging element  12 , and is further transmitted to the final reduction gear  14  and the gear  15 . 
     The rotation of the gear  15  is transmitted, via the gear  18  engaging with the gear  15 , to the rotational shaft of the main motor  19 . If the vehicle is in a mode in which the main motor  19  generates electric power so as to charge the power storage unit  21 , electric power generated by the main motor  19  is stored in the power storage unit  23  via the power drive unit  23 . 
     In contrast, if the vehicle is in a mode in which charging of the power storage unit  21  by the main motor  19  is not performed, electric power generated by the main motor  19  is not transmitted to the power storage unit  23 , and the main motor  19  is merely idled. 
     The rotation of the final reduction gear  14  is transmitted to the driving wheels via the differential gear  16  and the driving shaft  17 . Thus, the driving wheels are driven by the engine E, and the vehicle runs. 
     On the other hand, in the case where the vehicle is driven by the main motor  19 , while the clutch  10  is disengaged, the power storage unit  21  supplies electric power to the main motor  19  via the power drive unit  23 . The main motor  19  rotates the gear  18 , and the rotation is transmitted to the final reduction gear  14  via the gear  15 . In this case, because the engaging elements  11  and  12  in the clutch  10  are disengaged, the rotation of the final reduction gear  14  is not transmitted to the CVT  4 . Therefore, the rotation of the final reduction gear  14  is transmitted only to the differential gear  16 , and is further transmitted via the driving shaft  17  to the driving wheels W. In this way, the driving wheels W are driven by the main motor  19 , and the vehicle runs. 
     It is also possible to drive the vehicle using both the driving forces of the engine E and of the main motor  19 . In this case, the clutch  10  is engaged, and, while driving the driving wheels W by the engine E, the main motor  19  generates a driving force to assist the driving force of the engine E. 
     Next, the operation for applying engine braking by generating a braking force using the engine E as a load will be explained. In this case, the rotation of the driving wheels W is transmitted to the clutch  10  via the driving shaft  17 , the differential gear  16 , and the final reduction gear  14 . 
     When engine braking is applied, the clutch  10  is engaged. That is, the engaging elements  12  and  11  are engaged to each other by the clutch control actuator  13 . Thus, the rotation of the final reduction gear  14  is transmitted via the clutch  10  to the driven pulley  7  in the CVT  4 . 
     The rotation of the driven pulley  7  is transmitted via the metal belt  6  to the driving pulley  5 , and is further transmitted to the planetary gear  3 . The rotation of the planetary gear  3  is transmitted to the engine E via the oil pump  2  and the sub motor  1 , and the engine E generates a braking force so as to apply engine braking to the vehicle. 
     Next, the operation for obtaining a regenerative braking force by the main motor  19  will be explained. In this case, the clutch  10  is disengaged, and the rotation of the driving wheels W is transmitted via the driving shaft  17 , the differential gear  16 , and the final reduction gear  14 , to the gear  15 . The rotation of the gear  15  is further transmitted via the gear  18  to the rotational shaft of the main motor  19 . 
     When the rotational shaft is rotated, the main motor  19  generates electric power as a generator. The main motor  19  and the power storage unit  21  are electrically connected by the power drive unit  23  when a regeneration operation is performed. Therefore, the electric power generated by the main motor  19  is transmitted via the power drive unit  23  to the power storage unit  21 , and is stored therein. In this way, by drawing electric power from the main motor  19 , the main motor  19  generates a rotational load, and regenerative braking of the vehicle is performed. 
     Next, the operation of the control system will be explained. The control circuit  20  controls the oil pressure control device  24  so that suitable oil pressures are respectively supplied to the side chambers  8  and  9  in the CVT  4  from the oil pump  2 . By controlling the oil pressures in the side chambers  8  and  9 , the widths of the driving pulley  5  and the driven pulley  7  are changed to adjust the wrapping diameters of the metal belt  6 , and the transmission ratio is changed. The transmission ratio of the CVT  4  is continuously detected by the ratio sensor  31  in the CVT  4 , and the measured value is transmitted to the control circuit  20 . 
     The control circuit  20  also controls the clutch control actuator  13  provided in the clutch  10  so as to engage or disengage the engaging elements  11  and  12  in the clutch  10 . 
     Furthermore, the control circuit  20  controls the power drive unit  23  so as to connect or disconnect the main motor  19  and the power storage unit  21 . That is, when the electric power is applied from the power storage unit  21  to the main motor  19  to drive the vehicle using the motor  19 , or when the electric power generated by the main motor  19  is stored in the power storage unit  21  so as to perform regenerative braking, the power drive unit  23  connects the power storage unit  21  to the main motor  19 . In contrast, when the vehicle is driven using only the engine E, or when only engine braking is applied to the vehicle, the power drive unit  23  disconnects the power storage unit  21  from the main motor  19 . 
     In addition, the control circuit  20  receives the remaining battery charge SOC (state of charge) of the power storage unit  21  detected by the remaining battery charge sensor  22 ; the water temperature TW, the engine speed Ne, the intake air temperature TA, the intake air pressure PA, and the amount of depression of the accelerator pedal AP, which are detected by the engine sensor  25  provided on the engine E; the catalyst temperature TCAT detected by the catalyst temperature sensor  28  provided in the exhaust system  29 ; and the vehicle speed VCar detected by the vehicle speed sensor  30 . 
     The control circuit  20  controls the engine control device  26  so as to perform the control of the engine E such as the torque control of the engine E. 
     Next, referring to FIG. 2, the operation of the control circuit  20  will be explained in detail. First, an explanation will be given for the case that a request for quick acceleration (kick down request) is detected when the vehicle is running using the engine E. A request for a quick acceleration is transmitted to the vehicle when a driver on the vehicle depresses the accelerator pedal. The amount of depression of the accelerator pedal AP is detected by the engine sensor  25 , and the detection result is transmitted to the requested driving force calculation device  32  in the control circuit  20 . 
     Also, the vehicle speed VCar which will be changed in accordance with the operation of the accelerator pedal is detected by the vehicle speed sensor  30 , and the measured value is transmitted to the requested driving force calculation device  32  in the control circuit  20 . 
     The requested driving force calculation device  32  computes a torque requested for driving the vehicle (requested torque) based on the amount of depression of the accelerator pedal AP and the vehicle speed VCar, and transmits the requested torque value to the requested engine torque calculation device  33 . 
     The requested engine torque calculation device  33  also receives the transmission ratio output from the ratio sensor  31  in the CVT  4 . This transmission ratio is also input to the transmission efficiency calculation device  34 , and the transmission efficiency calculation device  34  calculates the transmission efficiency of the CVT  4  at the present transmission ratio based on the input transmission ratio. The transmission ratio is input to the requested engine torque calculation device  33 . In this embodiment, the transmission ratio is a value compensated by multiplying by the gear ratios of the final reduction gear  14  and the differential gear  16 , and the transmission efficiency is a value compensated by multiplying by the transmission efficiencies of the final reduction gear  14  and the differential gear  16 . 
     The requested engine torque calculation device  33  calculates the torque requested on engine E (requested engine torque) based on the requested torque value, the transmission ratio, and the transmission efficiency. The requested engine torque is input to the engine torque restriction device  36 . 
     The full throttle torque calculation device  35  receives the engine speed Ne, the intake air temperature TA, the intake air pressure PA, and the water temperature TW, etc. from the outside of the control circuit  20 . The full throttle torque calculation device  35  calculates the torque which can be generated by the engine E when the throttle is full open (fill throttle torque) based on the above information, and transmits it to the engine torque restriction device  36 . 
     The engine torque restriction device  36  computes an engine torque command based on the requested engine torque, and the fill throttle torque. That is, if the requested engine torque is greater than the full throttle torque, the engine torque restriction device  36  outputs the full throttle torque as an engine torque command. In contrast, if the requested engine torque is no more than the full throttle torque, the engine torque restriction device  36  outputs the requested engine torque as an engine torque command. 
     The engine torque command is transmitted to the engine torque control device  38 , and the engine torque control device  38  controls the torque output by the engine E. 
     The difference calculation device  37  receives the requested engine torque and the engine torque command, and calculates the difference between them to output this as a motor torque command. That is, the value obtained by subtracting the engine torque command from the requested engine torque is output as a motor torque command. In other words, the motor torque command is the deficiency in the engine torque command compared to the requested engine torque. The motor torque command is transmitted to the motor torque control device  39 , and motor torque control device  39  controls the torque generated by the main motor  19  based on the motor torque command. 
     FIG. 4 is a timing chart illustrating the operation of the control circuit  20  of the first embodiment shown in FIG.  2 . At a time t 1 , the accelerator pedal is depressed, and amount of depression of the accelerator pedal AP is increased. However, the vehicle speed VCar is not increased yet at this time t 1 . 
     The amount of depression of the accelerator pedal AP and the vehicle speed VCar are continuously transmitted to the requested driving force calculation device  32 , and the requested driving force calculation device  32  outputs the requested torque. The requested torque is quickly increased at the time t 1  because the vehicle speed VCar is not increased regardless of the increment of the amount of depression of the accelerator pedal AP. 
     On the other hand, in response to the depression of the accelerator pedal, a kick down operation is performed, and the transmission ratio of the CVT  4  is shifted to the lower side (Lo side). However, this shift of the transmission ratio takes a certain time, and in accordance with the change of the transmission ratio, the engine speed Ne is increased for a certain time. 
     The requested engine torque is quickly increased in the same manner as the requested torque. However, because the engine E cannot produce a torque greater than the full throttle torque at that time, the deficiency in torque obtained by subtracting the full throttle torque from the requested engine torque is supplied as the motor torque. 
     SECOND EMBODIMENT 
     Next, a second embodiment of the present invention will be explained referring to FIG. 3 illustrating the control circuit  20  of the second embodiment. 
     In the control circuit  20  shown in FIG. 3, an engine torque limit calculation device  35   a  is provided instead of the full throttle torque calculation device  35  shown in FIG.  2 . This engine torque limit calculation device  35   a  receives the engine speed Ne, the intake air temperature TA, the intake air pressure PA, the water temperature TW, the amount of depression of the accelerator pedal AP, the catalyst temperature TCAT, and the remaining battery charge SOC of the power storage unit  21 , etc., and based on this information, the engine torque limit calculation device  35   a  calculates the engine torque limit. 
     The engine torque limit is input to the engine torque restriction device  36 , and the engine torque restriction device  36  determines whether a restriction should be applied to the requested engine torque. That is, if the requested engine torque is greater than the engine torque limit, the engine torque restriction device  36  outputs the engine torque limit as an engine torque command (a restriction is applied to the requested engine torque). In contrast, if the requested engine torque is no more than the engine torque limit, the engine torque restriction device  36  outputs the requested engine torque as an engine torque command (a restriction is not applied to the requested engine torque). 
     Therefore, if the requested engine torque is greater than the engine torque limit, the torque to be generated by the engine E is limited to the engine torque limit, and the deficiency in torque is supplemented by the main motor  19 . 
     FIG. 5 is a timing chart illustrating the operation of the control circuit  20  of the second embodiment shown in FIG.  3 . The operation in this embodiment is similar to that shown in FIG. 4; however, this operation is different from that of FIG. 4 in that the requested engine torque is restricted based on the engine torque limit. The deficiency in torque obtained by subtracting the engine torque limit from the requested engine torque is supplied as the motor torque. 
     THIRD EMBODIMENT 
     Next, referring to FIG. 6, a third embodiment of the present invention will be explained. FIG. 6 is a block diagram illustrating a control circuit  20  of the third embodiment, and in this figure, the elements corresponding to the elements shown in FIG. 2 have the same reference numbers, and the explanation thereof will be omitted. 
     This control circuit  20  is different from that of FIG. 2 in that a motor torque command output device  37   a  is provided instead of the difference calculation device  37  shown in FIG.  2 . The motor torque command output device  37   a  comprises an engine torque conversion device  40 , a difference calculation device  41 , and a motor torque conversion device  42 . 
     The engine torque conversion device  40  receives the engine torque command output by the engine torque restriction device  36 , the transmission ratio of the CVT  4  input from the outside of the control circuit  20 , and the transmission efficiency output by the transmission efficiency calculation device  34 , and outputs a converted engine torque command. The engine torque command output by the engine torque restriction device  36  is the value of the torque which is directly output by a crank shaft of the engine E. That is, the engine torque command is the value of the torque at the most upstream point of the torque transmission passage through which the torque produced by the engine E is transmitted to the driving wheels W. 
     The engine torque conversion device  40  converts the engine torque command to the converted engine torque command, which is the value of the torque to be applied to the driving wheels W at the most downstream point of the torque transmission passage. This conversion is performed based on the transmission ratio and the transmission efficiency. That is, the relationship between the engine torque command and the converted engine torque command is changed in accordance with the transmission ratio and the transmission efficiency. 
     The difference calculation device  41  calculates the difference between the requested engine torque output by the requested engine torque calculation device  33  and the converted engine torque command output by the engine torque conversion device  40 , and outputs the calculation result as the converted motor torque command. The requested engine torque is the value of the torque which is converted by the requested engine torque calculation device  33  based on the transmission ratio and the transmission efficiency so as to be the value of the torque to be applied to the driving wheels W. Therefore, the value obtained by subtracting the converted engine torque command from the requested engine torque is the converted motor torque command to be applied to the driving wheels W. 
     The motor torque conversion device  42  converts the converted motor torque command output by the difference calculation device  41  to the motor torque command which is the value of the torque to be directly produced by the main motor  19 . This conversion by the motor torque conversion device  42  is performed based on the motor reduction gear ratio (this includes the gear ratios of the final reduction gear  14  and the differential gear  16 ) and the transmission efficiency of the motor torque (this is the transmission efficiency of the transmission passage, including the final reduction gear  14  and the differential gear  16 , from the main motor  19  to the driving shaft  17 ). 
     The motor torque command output by the motor torque conversion device  42  is input to the motor torque control device  39 , and the main motor  19  is controlled in accordance with this motor torque command. 
     According to this embodiment, the motor torque command output device  37   a  calculates the difference between the torque values converted to values at the most downstream point of the torque transmission passage, and further converts the calculated difference to the torque value at the most upstream point of the torque transmission passage so as to output the motor torque command. Therefore, the precision of the motor torque command can be improved. 
     FOURTH EMBODIMENT 
     Next, referring to FIG. 7, a fourth embodiment of the present invention will be explained. The fourth embodiment is a modification of the second embodiment shown in FIG. 3, and FIG. 7 is a block diagram illustrating a control circuit  20  of the fourth embodiment. In FIG. 7, the elements corresponding to the elements shown in FIG. 3 have the same reference numbers, and the explanation thereof will be omitted. 
     This control circuit  20  is different from that of FIG. 3 in that the motor torque command output device  37   a  which is the same as that of the third embodiment shown in FIG. 6 is provided in place of the difference calculation device  37  shown in FIG.  3 . 
     According to the fourth embodiment, similarly to the third embodiment, the motor torque command output device  37   a  calculates the difference between the torque values converted to values at the most downstream point of the torque transmission passage, and further converts the calculated difference to the torque value at the most upstream point of the torque transmission passage so as to output the motor torque command. Therefore, the precision of the motor torque command can be improved. 
     FIFTH EMBODIMENT 
     Next, referring to FIG. 8, a fifth embodiment of the present invention will be explained. The fifth embodiment is a modification of the third embodiment shown in FIG. 6, and FIG. 8 is a block diagram illustrating a control circuit  20  of the fifth embodiment. In FIG. 8, the elements corresponding to the elements shown in FIG. 6 have the same reference numbers, and the explanation thereof will be omitted. 
     This control circuit  20  is different from that of FIG. 6 in that an engine torque estimation device  43  is provided for estimating the torque of the engine E based on the actual operation states of the engine E. This engine torque estimation device  43  receives the engine speed Ne and the air intake passage pressure Pb, estimates the torque which is being output by the engine E at that time based on these information, and outputs the estimated value as the estimated engine torque. This estimated engine torque is the estimated value of the torque to be directly output by a crank shaft of the engine E. 
     In the third embodiment, the engine torque command output by the engine torque restriction device  36  is input to the engine torque conversion device  40 . In contrast, in this fifth embodiment, the estimated engine torque output by the engine torque estimation device  43  is input to the engine torque conversion device  40 . 
     According to the fifth embodiment, the torque being output by the engine E is estimated based on the actual driving states of the engine E, that is, based on the information derived from the actual operation of the engine E. Therefore, it is possible to improve the precision of the motor torque command for controlling the main motor  19  in comparison with the third embodiment in which the engine torque command value is used as the engine torque. 
     SIXTH EMBODIMENT 
     Next, referring to FIG. 9, a sixth embodiment of the present invention will be explained. The sixth embodiment is a modification of the fourth embodiment shown in FIG. 7, and FIG. 9 is a block diagram illustrating a control circuit  20  of the sixth embodiment. In FIG. 9, the elements corresponding to the elements shown in FIG. 7 have the same reference numbers, and the explanation thereof will be omitted. 
     This control circuit  20  is different from that of FIG. 7 in that the engine torque estimation device  43  which is the same as that of the fifth embodiment shown in FIG. 8 is provided for estimating the torque of the engine E based on the actual operation state of the engine E. This engine torque estimation device  43  receives the engine speed Ne and the air intake passage pressure Pb, and outputs the estimated engine torque. Furthermore, the estimated engine torque output by the engine torque estimation device  43  is input to the engine torque conversion device  40 . 
     According to the sixth embodiment, because the torque being output by the engine E is estimated based on the actual driving states of the engine E, it is possible to improve the precision of the motor torque command for controlling the main motor  19  in comparison with the fourth embodiment in which the engine torque command value is used as the engine torque. 
     The present invention is not limited only to the above embodiment, but can be modified within the scope of the present invention.