Hybrid vehicle control with selection of drive mode according to energy conversion efficiency in electric drive

Abstract: A drive control apparatus for an automotive vehicle having an electric motor (14) operated by an electric energy generated by an electric generator (14) and stored in an energy storage device (22), and an engine (12) operated by combustion of a fuel, the apparatus having an engine drive mode and a motor drive mode in which the vehicle is driven by the engine and the motor, respectively, and an electricity generating mode in which the electric generator is operated by the engine to charge the energy storage device, and including a drive source selecting device for selecting one of the engine drive mode and the motor drive mode, on the basis of a first value in the engine drive mode of a physical quantity relating to a condition of the engine and a second value of the same physical quantity, which second value reflects energy conversion efficiencies of the electric generator (14), electric motor (14) and electric energy storage device (22) in the electricity generating mode.

Description: The present invention relates in general to a drive control apparatus for an automotive vehicle, and more particularly to a drive control apparatus for a so-called "hybrid vehicle" equipped with two drive power sources consisting of an electric motor and an engine such as an internal combustion engine. Discussion of the Related Art For the purpose of reducing exhaust gas emissions, for example, there has been proposed a so-called "hybrid vehicle" equipped with two drive power sources, which consist of an electric motor operated by an electric energy stored in an electric energy storage device (electric power supply device), and an engine such as an internal combustion engine operated by combustion of a fuel. One type of such hybrid vehicle is adapted to run in an engine drive mode by operation of the engine, or in a motor drive mode by operation of the electric motor, depending upon the running condition of the vehicle. The hybrid vehicle is further adapted to run in an electricity generating drive mode in which the electric motor is operated as an electric generator or dynamo by a surplus torque of the engine during running of the vehicle with the engine selected as the drive power source, so that an electric energy produced by the electric generator (i.e., electric motor) is stored in the electric energy storage device. An example of a drive control apparatus for controlling such a hybrid vehicle is disclosed in JP-A-6-48222 (published in 1994), wherein the engine is controlled so as to have an ideal torque value which is calculated to minimize the amount of consumption of the fuel by the engine or the amount of exhaust gases produced by the engine. During running of the vehicle with the engine, an instantaneous torque value of the engine which is required for running the vehicle under the current running condition of the vehicle is calculated. If the ideal torque value is larger than the required torque value, that is, if the engine has a surplus torque, the vehicle is driven in the electricity generating drive mode, that is, the electric motor is driven by the surplus torque of the engine to charge the electric energy storage device. It will be understood that the electric motor also functions as the electric generator or dynamo. The vehicle drive control apparatus as described above is further adapted to be placed in a regenerative drive mode wherein the electric generator is driven by a kinetic energy of the running vehicle to generate an electric energy for charging the electric energy storage device as well as to apply a brake to the vehicle. This regenerative drive mode is established when a predetermined regenerative driving condition is satisfied, for example, when the operating amount of the accelerator pedal is zero. In the known vehicle drive control apparatus described above, the operating condition of the engine is controlled on the basis of only the fuel consumption rate (fuel consumption amount g/kWh per unit power) or the exhaust gas rate (exhaust gas amount per unit power) of the engine. This arrangement does not necessarily assure minimization of the fuel consumption amount or exhaust gas amount for a certain amount of electric energy to be stored in the electric energy storage device. Described more particularly, the efficiency of conversion of the kinetic energy into the electric energy by the electric generator varies with the operating speed and torque of the engine, and the conversion efficiency upon storing the electric energy in the electric energy storage device varies with the electric power (electric energy per unit time). Accordingly, the efficiency in charging the electric energy storage device is not sufficiently high under some operating condition (speed, torque, power, etc.) of the engine. A similar problem is encountered in determining whether the vehicle should be run by the engine in the engine drive motor or by the electric motor in the motor drive mode. To effect this determination in order to minimize the fuel consumption amount or the exhaust gas amount, for example, the fuel consumption amount or the exhaust gas amount when the engine is operated in the electricity generating drive mode while driving the electric motor so as to charge the electric energy storage device is compared with that when the engine is operated in the engine drive motor for the sole purpose of driving the vehicle. The vehicle is run in the motor drive mode if the fuel consumption amount or exhaust gas amount in the electricity generating drive mode is smaller than that in the engine drive mode. The vehicle is run in the engine drive mode if the fuel consumption amount or exhaust gas amount in the engine drive motor is smaller. However, the fuel consumption amount or exhaust gas amount could not be sufficiently reduced if the energy conversion efficiencies of the electric generator and the electric energy storage device would not be taken into account in selecting the engine drive mode or the motor drive mode. The rule for selecting the engine drive mode or the motor drive mode remains unchanged even where the vehicle runs on a mountain path. During running of the vehicle on a mountain path, the vehicle load is generally high, and the vehicle is more frequently operated in the engine drive mode, whereby the electric energy storage device tends to be excessively charged by the engine through the electric generator while the vehicle is less frequently operated in the motor drive mode. Excessive charging of the electric energy storage device may lead to reduction of the energy conversion efficiency (charging and discharging efficiencies), and may even cause a failure to charge the electric energy storage device. Thus, the known arrangement for selecting the engine or the electric motor as the drive power source does not permit effective utilization of the electric energy, resulting in an increase in the fuel consumption amount or the exhaust gas amount during running of the vehicle on a mountain path. SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a drive control apparatus for a hybrid vehicle equipped with an electric motor and an engine as drive sources, which apparatus permits effective reduction in the fuel consumption amount or exhaust gas amount of the engine. This object is solved by the drive control apparatus in accordance to claim 1. Advantageous embodiments thereof are defined in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS The above and optional objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: Fig. 1 is a block diagram illustrating an arrangement of a vehicle drive control apparatus constructed according to one embodiment of the present invention; Fig. 2 is a block diagram illustrating the functions achieved by a portion of a controller in the vehicle drive control apparatus of Fig. 1; Fig. 3 is a flow chart illustrating a routine executed by the vehicle drive control apparatus of Fig. 1 for running the vehicle in a selected one of three modes; Fig. 4 is a flow chart for explaining an ELECTRICITY GENERATING DRIVE mode sub-routine in step S7 of the routine of Fig. 3; Fig. 5 is graph showing an example of data map indicating the fuel consumption efficiency of an internal combustion engine, which data map is used by the vehicle drive control apparatus of Fig. 1; Fig. 6 is a graph showing an example of data map indicating the energy conversion efficiency of an electric motor, which data map is used by the vehicle drive control apparatus of Fig. 1; Fig. 7 is a graph indicating a relationship between engine output P ICE and surplus power (P ICE - P L ) used in the sub-routine of Fig. 4; Fig. 8 is a block diagram illustrating an arrangement of a vehicle drive control apparatus constructed according to another embodiment of this invention; Fig. 9 is a block diagram illustrating the functions achieved by a portion of a controller used in the vehicle drive control apparatus of Fig. 8; Fig. 10 is a flow chart showing a routine executed by the vehicle drive control apparatus of Fig. 8; and Fig. 11 is a graph indicating thresholds B1, B2 and C used in the routine of Fig. 10. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to Fig. 1, one embodiment of a vehicle drive control apparatus 10 of the present invention will be described. The apparatus 10 is adapted to control a so-called "hybrid vehicle" which is equipped with two drive sources, one of which is an internal combustion engine 12 such as a gasoline engine operated by combustion of a fuel, and the other of which is a dynamo-electric motor 14 functioning as an electric motor and an electric generator (dynamo). Power of the internal combustion engine 12 and power of the motor 14 are simultaneously or selectively transferred to a transmission 16, and to right and left drive wheels via an output device 18. The transmission 16 may include a speed changing mechanism and a forward/reverse switching mechanism. The speed changing mechanism may be adapted to change the speed ratio either in steps or continuously, or may have a predetermined speed ratio. The forward/reverse switching mechanism has a forward drive position for forward running of the vehicle and a reverse drive position for backward running of the vehicle. The motor 14 may be operated in a reverse direction to run the vehicle backward. Appropriate clutch or clutches may be disposed between the engine 12 and the motor 14, and/or between these drive source or sources 12, 14 and the transmission 16, so that the drive sources 12, 14 and transmission 16 are disconnected from each other as needed. The motor 14 is connected to an electric energy storage device (electric power supply device) 14 in the form of a battery, for example, through a motor/generator control device (hereinafter abbreviated as "M/G control device) 20 so that the motor 14 is selectively placed in a DRIVE state, a CHARGING state, and a NON-LOAD state. In the DRIVE state, the motor 14 is driven by an electric energy supplied from the electric energy storage device 22. In the CHARGING state, the motor 14 functions as an electric generator or dynamo, with regenerative braking (braking torque electrically generated by the motor 14 itself), for storing an electric energy in the electric energy storage device 22. In the NON-LOAD state, the output shaft of the motor 14 is permitted to rotate freely. The engine 12 is controlled by an internal combustion engine control device (hereinafter abbreviated as "ICE control device") 24, which includes an actuator for controlling the amount of a fuel injected into the engine 12, an actuator for controlling a throttle valve of the engine 12, an actuator for controlling the ignition timing of the engine 12, and an actuator for controlling suction and exhaust valves of the engine 12. The M/G control device 20 and the ICE control device 24 are controlled by a controller 26. The controller 26 includes a microcomputer incorporating a central processing unit (CPU), a random-access memory (RAM), and a read-only memory (ROM). The CPU operates according to control programs stored in the ROM while utilizing a temporary data storage function of the RAM. The controller 26 is selectively placed in one of an ENGINE DRIVE mode, a MOTOR DRIVE mode and an ELECTRICITY GENERATING DRIVE mode. In the ENGINE DRIVE mode, the vehicle is driven by the internal combustion engine 12. In the MOTOR DRIVE mode, the vehicle is driven by the electric motor 14. In the ELECTRICITY GENERATING DRIVE mode, the vehicle is driven by the engine 12 while the motor 14 is driven by the engine 12 so as to charge the electric energy storage device 22. The controller 16 is supplied with input signals from various detecting devices. These input signals include signals indicative of a torque T E of the engine 12, a torque T M of the motor 14, an operating amount  A of an accelerator pedal, a speed N E of the engine 12, a speed N M of the motor 14, an output speed N O of the transmission 16, and a charging amount SOC of the electric energy storage device 22. The controller 26 includes drive source selecting means 28 illustrated in the block diagram of Fig. 2. The drive source selecting means 28 selectively establishes the ENGINE DRIVE mode or the MOTOR DRIVE mode. The drive source selecting means 28 includes first calculating means 30 for calculating a fuel consumption amount M fce of the engine 12 in the ENGINE DRIVE mode in which the engine 12 is selected as the drive power source for running the vehicle. This fuel consumption amount M fce is calculated according to the following equation (1): M fce = FCe x P L The drive source selecting means 28 also includes second calculating means 32 for calculating a fuel consumption amount M fcm of the engine 12 in the ELECTRICITY GENERATING DRIVE mode in which the motor 14 is driven by the engine 12 for charging the electric energy storage device 22 with an electric energy necessary to run the vehicle in the MOTOR DRIVE mode (with the motor 14 selected as the drive power source). The calculation of the fuel consumption amount M fcm is calculated according to the following equation (2): M fcm = FCm x P L /(η GEN x η BIN x η BOUT x η MOT ) The drive source selecting means 28 further includes comparing means for comparing the calculated fuel consumption amounts M fce and M fcm , for selecting the ENGINE DRIVE mode or the MOTOR DRIVE mode. It will be understood that the drive source selecting means 28 is adapted to select the ENGINE DRIVE mode or MOTOR DRIVE mode depending upon values of a physical quantity in the form of the fuel consumption amount M fc of the engine 12, according to a predetermined rule, more specifically, on the basis of the physical quantity values in the form of the fuel consumption amounts M fcm and M fce as compared with each other, such that the MOTOR DRIVE mode is selected when the fuel consumption amount M fcm is smaller than the fuel consumption amount M fce while the ENGINE DRIVE mode is selected when the fuel consumption amount M fcm is not smaller than the fuel consumption amount M fce . P L in the above equations (1) and (2) represents instantaneous drive power required for running the vehicle, which power includes components for overcoming an air resistance of the vehicle and a rolling resistance of the tire of each vehicle wheel. The required drive power P L may be determined on the basis of the engine speed N E and torque T E (or the motor speed N M and torque T M ) and the amount or rate of change of the accelerator pedal operating amount  A , and according to a predetermined relationship between the required drive power P L and these parameters. This relationship is represented by a data map stored in memory means 36 also included in the drive source selecting means 28, as indicated in Fig. 2. Other data necessary to calculate the required drive power P L are also stored in the memory means 36. FCe represents a fuel consumption rate (g/kWh) of the internal combustion engine 12 when the required drive power P L is provided by the engine 12. This fuel consumption efficiency FCe may be determined on the basis of the engine torque T E and speed N E , and according to a predetermined relationship between the efficiency FCe and these parameters, which relationship is represented by a data map also stored in the memory means 36. The graph of Fig. 5 shows an iso-fuel consumption rate of the engine 12. In the graph, a hatched area indicates the lowest value of the fuel consumption rate FCe at which the power per unit fuel amount is the highest. The fuel consumption rate FCe increases from the hatched area in the outward direction away from the hatched area. Since the required instantaneous drive power P L is represented by the engine speed N E and torque T E , the fuel consumption rate FCe may be obtained from the data map of Fig. 5, depending upon the required drive power P L . Data necessary to calculate the fuel consumption rate FCe are also stored in the memory means 36. η ICEmax in Fig. 5 represents a maximum value of the fuel consumption efficiency η ICE (reciprocal of the fuel consumption rate FCe). The fuel consumption efficiency η ICE decreases from the hatched area in the outward direction away from the hatched area. The fuel consumption efficiency η ICE may be set with the maximum value η ICEmax being "1". FCm in the above equation (2) represents the fuel consumption rate of the engine 12 when the motor 14 is driven by the engine 12 in the ELECTRICITY GENERATING DRIVE mode to charge the electric energy storage device 22, while η GEN , η BIN , η BOUT and η MOT represent the following: η GEN : electricity generating efficiency, i.e., efficiency of conversion from kinetic energy into electric energy by the motor 14 (functioning as the electric generator) η BIN : charging efficiency, i.e., energy conversion efficiency upon charging of the electric energy storage device 22 η BOUT : discharging efficiency, i.e., energy conversion upon consumption of the electric energy stored in the electric energy storage device 22 η MOT : motor driving efficiency, i.e., efficiency of conversion from electric energy into kinetic energy by the motor 14 The motor driving efficiency η MOT may be represented by the motor torque T M and speed N M , and may be obtained depending upon the operating condition of the motor 14, namely, depending upon the required drive power P L . The graph of Fig. 6 shows an iso-motor driving efficiency. In the graph, a hatched area in the graph indicates the highest value η MOTmax of the motor driving efficiency η MOT at which the power per unit electric energy is the highest. The motor driving efficiency η MOT decreases from the hatched area in the outward direction away from the hatched area. Since the required drive power P L is represented by the motor speed N M and torque T M , the motor driving efficiency η MOT may be obtained from the data map of Fig. 6, depending upon the required drive power P L . The discharging efficiency η BOUT may be represented by the charging amount SOC of the electric energy storage device 22 and the amount of electric energy to be supplied from the device 22 per unit time (electric power). The signal indicative of the charging amount SOC is received from the device 22, and the amount of electric energy to be supplied per unit time corresponds to the required drive power P L . Therefore, the discharging efficiency η BOUT may be obtained on the basis of the charging amount SOC and the required drive power P L and according to a predetermined relationship between the efficiency η BOUT and these values SOC, P L . This relationship is represented by a data map stored in the memory means 36. Other data (such as equations) necessary to calculate the motor driving efficiency η MOT and the discharging efficiency η BOUT are also stored in the memory means 36. On the other hand, the fuel consumption rate FCm, electricity generating efficiency η GEN and charging efficiency η BIN are estimated in the ELECTRICITY GENERATING DRIVE mode in which the motor 14 is driven by the engine 12 to charge the electric energy storage device 22. The thus estimated values FCe, η GEN and η BIN are independent or irrespective of the current running condition of the vehicle. The ELECTRICITY GENERATING DRIVE mode is selected when the charging amount SOC of the electric energy storage device 22 is smaller than a predetermined threshold value "A", as described below by reference to the flow chart of Fig. 4. To calculate the fuel consumption amount M fcm in step S4 of the routine of Fig. 4, average values (e.g., moving averages) of FCm, η GEN , η BIN in previous vehicle runs in the ELECTRICITY GENERATING DRIVE mode, or values of FCm, η GEN , η BIN in the last vehicle run in the ELECTRICITY GENERATING DRIVE mode are stored in the memory means 36. The fuel consumption amount M fcm is calculated on the basis of those values of FCm, η GEN , η BIN stored in the memory means 36 as well as the currently obtained η BOUT and η MOT . In the ELECTRICITY GENERATING DRIVE mode, the values FCm, η GEN and η BIN are obtained as described below. Like the fuel consumption rate FCe, the fuel consumption rate FCm may be obtained on the basis of the engine torque T E and speed N E and according to a predetermined relationship between the rate FCm and these values T E , N E , which relationship is represented by a stored data map. The electricity generating efficiency η GEN may be obtained on the basis of the regenerative braking torque and the motor speed N M and according to a predetermined relationship between the efficiency η GEN and these values, which relationship is represented by a stored data map. The charging efficiency η BIN may be obtained on the basis of the charging amount SOC and the amount of electric energy to be stored in the device 22 per unit time (electric power) and according to a predetermined relationship between the efficiency η BIN and these values, which relationship is represented by a stored data map. The amount of electric energy to be stored per unit time corresponds to surplus power that is consumed by the engine 12 for the purpose of driving the electric motor 14 for charging the electric energy storage device 22. The data maps and equations necessary for calculating the electric generating efficiency η GEN and charging efficiency η BIN are stored in the memory means 36. Referring to the flow chart of Fig. 3, there will be described an operation of the controller 26, by way of example. The routine of Fig. 3 is executed with a predetermined cycle time, and is initiated with step S1 in which the controller 26 reads input signals indicative of the engine speed T E , motor torque T M , accelerator pedal operating amount  A , engine speed N E , motor speed N M , transmission output speed N O and_charging amount SOC. Step S1 is followed by step S2 to determine whether the charging amount SOC is equal to or larger than the predetermined threshold "A", which is the lower limit of the electric energy stored in the electric energy storage device 22, that is necessary to run the vehicle in the MOTOR DRIVE mode in which the motor 14 is selected as the drive power source. If an affirmative decision (YES) is obtained in step S2, the control flow goes to step S3 and the subsequent steps. If a negative decision (NO) is obtained in step S2, the control flow goes to step S7 in which a sub-routine for running the vehicle in the ELECTRICITY GENERATING DRIVE mode is executed as illustrated in the flow chart of Fig. 4 is executed. Step S3 is implemented by the first calculating means 30 and the second calculating means 32 of the drive source selecting means 28 illustrated in Fig. 2, for calculating the fuel consumption amounts M fce and M fcm , respectively. Step S3 is followed by step S4 which is implemented by the comparing means 34 to determine whether the fuel consumption amount M fcm is smaller than the fuel consumption amount M fce . If the fuel consumption amount M fcm is smaller than the fuel consumption amount M fce , the control flow goes to step S6 in which a sub-routine for running the vehicle in the MOTOR DRIVE mode is executed. If the fuel consumption amount M fcm is larger than the fuel consumption amount M fce , the control flow goes to step S5 in which a sub-routine for running the vehicle in the ENGINE DRIVE mode is executed. The selection of the ENGINE DRIVEmode and the MOTOR DRIVE mode in step S4 permits the vehicle to be run with the minimum fuel consumption M fc of the engine 12. Where the fuel consumption amounts M fcm and M fce are identical with each other, either the ENGINE DRIVE mode or the MOTOR DRIVE mode may be selected. In the present embodiment, however, the -ENGINE DRIVE mode is selected. The ELECTRICITY GENERATING DRIVE mode sub-routine in step S7 of the routine of Fig. 3 corresponds to means for determining the output of the engine 12 during charging of the electric energy storage device 22. This sub-routine is executed as illustrated in the flow chart of Fig. 4. The sub-routine is initiated with step R1 in which a candidate value of the output or power P ICE of the engine 12 is determined to be equal to the required drive power P L , in the first cycle of execution of the sub-routine, that is, when the sub-routine is executed for the first time. In the second and subsequent cycles of execution of the sub-routine of Fig. 4, the candidate value of the engine output P ICE is incremented by a predetermined increment amount ▵P ICE in step R1. Since the engine speed N E is held constant depending upon the vehicle running speed, the engine torque T E increases by an amount corresponding to the increment amount ▵P ICE as indicated in Fig. 7. Therefore, the surplus torque value of the engine 12 is obtained as a sum of the increment amounts ▵P ICE in repeated implementation of step R1, that is, as a difference (P ICE - P L ) between the candidate value of the engine output P ICE and the required drive power P L . The graph of Fig. 7 shows iso-drive power lines of the engine 12, which indicate that the engine torque T E for obtaining the same drive power changes with the engine speed NE. The engine output P ICE and the required drive power P L are determined by the specific combination of the current engine speed N E and torque T E . Although the increment amount ▵P ICE is a predetermined fixed value in the present embodiment, the increment amount ▵P ICE may be changed depending upon the running condition of the vehicle, for example, depending upon the required drive power P L . The engine torque T E may be changed by a predetermined increment. Step R1 is followed by step R2 to calculate the fuel consumption rate η ICE on the basis of the engine output P ICE and according to the stored predetermined relationship between h ICE and P ICE as shown in Fig. 5. Then, the control flow goes to step R3 to calculate an overall fuel consumption efficiency η T of a system which is assigned to charge the electric energy storage device 22 by operation of the electric motor 14 by the surplus power (P ICE - P L ) of the engine 12. The calculation of the overall fuel consumption efficiency η T is effected according to the following equation (3), and the calculated efficiency η T is stored in the memory means 36. η T = η GEN x η BIN x η BOUT x η MOT x η ICE + (P L /P ICE ) x η ICE x (1 - η GEN x η BIN x η BOUT x η MOT ) The present embodiment is adapted to obtain the overall fuel consumption efficiency n T as a ratio of the fuel consumption amount Mfc to the output or power of the electric motor 14 when the motor 14 is driven by the electric energy which has been stored in the electric energy storage device 22 by operation of the motor 14 by the surplus power (P ICE - P L ) of the engine 12. The electricity generating efficiency η GEN used in the above equation (3) is obtained according to the data map stored in the memory means 36, depending upon the surplus power (P ICE - P L ), that is, depending upon the speed and surplus torque (regenerative braking torque), while the charging efficiency η BIN is obtained according to the data map also stored in the memory means 36, depending upon the surplus power (P ICE - P L ) and the charging amount SOC. The discharging efficiency η BOUT and the motor driving efficiency η MOT used in the equation (3) are obtained in the MOTOR DRIVE mode and stored in the memory means 36. For instance, averages (e.g., moving averages) of the efficiency η BOUT , η MOT in two or more previous vehicle runs in the MOTOR DRIVE mode are stored in the memory means 36. Alternatively, values of η BOUT , η MOT in the last-vehicle run in the MOTOR DRIVE mode are stored in the memory means 36. These values stored in the memory means 36 are used as the efficiency values η BOUT and η MOT in the equation (3) to calculate the overall fuel consumption efficiency η T . In the MOTOR DRIVE mode, the discharging efficiency η BOUT is obtained on the basis of the required drive power P L and the charging amount SOC and according to a predetermined relationship between the efficiency η BOUT and the values P L and SOC, which relationship is represented by the data map stored in the memory means 36. The motor driving efficiency η MOT is obtained on the basis of the required drive power P L , namely, on the basis of the motor speed N M and torque T M and according to a predetermined relationship between the efficiency η MOT and the values N M , T M , which relationship is represented by the data map stored in the memory means 36. Then, the control flow goes to step R4 to determine whether the engine output P ICE is larger than a predetermined threshold P E . Steps R1-R4 are repeatedly implemented until an affirmative decision (YES) is obtained in step R4, that is, until the engine output P ICE exceeds the threshold P E . While this threshold P E may be a predetermined fixed value, it may be changed depending upon the required drive power P L and according to a predetermined relationship between the threshold P E and the power P L , so as to prevent overrunning of the engine 12 and motor 14. This relationship is represented by a data map stored in the memory means 36. Steps R1 through R4 correspond to means for repeatedly calculating the overall fuel consumption efficiency values η T which reflect the electricity generating efficiency η GEN of the motor 14 and the charging efficiency η BIN of the electric energy storage device 22 upon charging of the device 22 by the surplus power (P ICE -P L ), while changing the surplus power (P ICE - P L ) or while changing the candidate value of the engine output P ICE . The overall efficiency values η T calculated in step R3 correspond to the respective candidate values of the engine output P ICE obtained in step R1. As described above, the present embodiment is adapted to obtain the overall fuel consumption efficiency η T while taking into account the efficiency of operation of the motor 14 by the electric energy stored in the electric energy storage device 22, as well as the efficiency of charging of the device 22 by operation of the motor 14 (as the electric generator) by the surplus power (P ICE -P L ) of the engine 12. However, the efficiency of charging of the device 22 may be obtained in step R3, so that the obtained charging efficiency is substituted for the overall fuel consumption efficiency η T . When the affirmative decision (YES) is obtained in step R4, the control flow goes to step R5 to select one of the engine output candidate values P ICE obtained in step R1, which corresponds to the highest overall fuel consumption efficiency value η T . The highest overall fuel consumption efficiency value η T means the lowest ratio of the fuel consumption amount Mfc to the output or power of the motor 14 during operation of the motor 14 by the electric energy which has been stored in the electric energy storage device 22 by operation of the motor 14 by the surplus power (P ICE -P L ). Since the discharging efficiency η BOUT and the motor driving efficiency η MOT in the above equation (3) remain unchanged with a change in the engine output P ICE , the engine output value P ICE whose ratio of the fuel consumption amount Mfc to the amount of the electric energy to be stored in the device 22 is the lowest is selected in step R5. Then, the control flow goes to step R6 wherein the engine 12 is operated so as to provide the selected engine output P ICE , and electricity generating power P GEN given to the motor 14 to charge the device 22 is determined to be equal to the surplus power (P ICE - P L ). Further, the regenerative braking torque of the motor 14 is determined depending upon the surplus power (P ICE - P L ). In the ELECTRICITY GENERATING DRIVE mode, therefore, the vehicle is run by the engine 12 as the drive power source while the motor 14 is operated by the engine 12 to charge the electric energy storage device 22 so as to minimize the ratio of the fuel consumption amount Mfc to the electric energy to be stored. It will be understood that steps R5 and R6 correspond to means for selecting one of the candidate values of the engine output P ICE which corresponds to the highest one of the overall fuel consumption efficiency values η T obtained in step R3, and determining the selected candidate value as the desired engine output P ICE . Thus, the engine output candidate value P ICE which satisfies a predetermined condition is eventually determined as the desired engine output P ICE . The present vehicle drive control apparatus 10 according to the present embodiment of the invention is adapted to select the ENGINE DRIVE mode or the MOTOR DRIVE mode, depending upon whether the fuel consumption amount M fcm is smaller than the fuel consumption amount M fce , more specifically, to select the MOTOR DRIVE mode if the fuel consumption amount M fcm is smaller than the fuel consumption amount M fce , and select the ENGINE DRIVE mode if the fuel consumption amount M fcm is not smaller than the fuel consumption amount M fce . In this respect, it is noted that the fuel consumption amount M fcm in the MOTOR DRIVE mode reflects the electricity generating efficiency η GEN and driving efficiency η MOT of the motor 14 and the charging η BIN and discharging efficiency η BOUT of the electric energy storage device 22. In other words, the fuel consumption amount M fcm can be determined with high accuracy on the basis of the above-indicated efficiency values, whereby the selection of the ENGINE DRIVE or MOTOR DRIVE mode for minimizing the fuel consumption amount Mfc can be achieved with high accuracy. This arrangement permits the vehicle to run with the fuel consumption amount Mfc substantially kept at a minimum value. In the ELECTRICITY GENERATING DRIVE mode in which the vehicle is run by the engine 12 while the motor 14 is operated by the surplus power (P ICE - P L ) of the engine 12 to charge the electric energy storage device 22, the engine output P ICE is determined so as to maximize the overall fuel consumption efficiency η T which reflects the energy conversion efficiencies η GEN and η BIN of the motor 14 and device 22, that is, to minimize the ratio of the fuel consumption amount Mfc to the output or power of the motor 14 during operation of the motor 14 by the electric energy which has been stored in the device 22 by the surplus power (P ICE - P L ). The motor 14 is operated by the electricity generating power P GEN which corresponds to the surplus power (P ICE - P L ), which in turn is equal to the determined engine output P ICE minus the required drive power P L . Since the ratio of the fuel consumption amount Mfc to the output of the motor 14 during operation of the motor 14 by the electric energy stored in the device 22 by the surplus power (P ICE - P L ) is reduced to a minimum, the fuel consumption amount Mfc per unit amount of electric energy to be stored in the device 22 is accordingly minimized, leading to maximum efficiency of charging of the device 22. As explained above, the present vehicle drive control apparatus 10 is adapted to run the vehicle selectively in the ENGINE DRIVE mode or MOTOR DRIVE mode so as to minimize the fuel consumption amount Mfc, and in the ELECTRICITY GENERATING DRIVE mode in which the engine output P ICE (including the surplus power) and the electricity generating power P GEN are determined so as to minimize the fuel consumption amount Mfc per unit amount of electric energy to be stored in the electric energy storage device 22, namely, so as to maximum the overall fuel consumption efficiency η T , so that the overall fuel consumption amount Mfc for running the vehicle is considerably reduced, and the exhaust gas amount corresponding to the fuel consumption amount is accordingly reduced. In the above embodiment, the power transmission efficiency of the transmission 16 is not taken into account for obtaining the fuel consumption amounts M fce , M fcm and the overall fuel consumption efficiency η T . Where the transmission 16 is capable of changing the speed ration in steps or continuously, however, the power transmission efficiency of the transmission 16 varies with the speed ratio, and the fuel consumption amounts M fce , M fcm and the overall fuel consumption efficiency η T vary with the speed ratio and power transmission efficiency of the transmission 16. In the light of this fact, it is desirable to obtain those values M fce , M fcm , η T according to the following equations (4), (5) and (6), which include power transmission efficiency η CVT of the transmission 16 where the transmission 16 is a continuously variable transmission. The power transmission efficiency η CVT is obtained on the basis of appropriate parameters such as the speed ratio, transmission torque and input and output speeds of the transmission 16, and according to a predetermined relationship of the efficiency hCVT and those parameters, which relationship is represented by a data map stored in the memory means 36, together with other data (e.g., equations) necessary to calculate the power transmission efficiency η CVT . M fce = Fce x P L /η CVT M fcm = Fcm x P L /(η GEN x η BIN x η BOUT x η MOT x η CVT ) η T = η GEN x η BIN x η BOUT x η MOT x η CVT * x η ICE + (P L /P ICE ) x η ICE x {1 - (h GEN x η BIN x η BOUT x η MOT x η CVT * x η CV4T *)/η CVT } As described above, the power transmission efficiency η CVT in the above equations (4), (5) and (6) is obtained on the basis of the current running condition of the vehicle such as the transmission torque and speed ration of the transmission 16 which correspond to the required drive power P L , and according to the data map stored in the memory means 36. Like the discharging efficiency η BOUT and the motor driving efficiency η MOT , the power transmission efficiency η CVT * in the above equation (6) is an average (e.g., moving average) of the values stored in the memory means 36 in the previous vehicle runs in the MOTOR DRIVE mode, or a value stored in the memory means 36 in the last vehicle run in the MOTOR DRIVE mode. Referring to the block diagram of Fig. 8, there will be described a vehicle drive control apparatus 110 constructed according to a second embodiment of the present invention. The apparatus 110 is substantially similar to the apparatus 10 of the first embodiment of Fig. 1. In Fig. 8, thick solid lines indicate mechanical connection of elements, while thin solid lines indicate electrical connection of elements. The vehicle drive control apparatus 110 is arranged to control a hybrid vehicle equipped with an internal combustion engine 112 such as a gasoline engine operated by combustion- of a fuel, and a dynamo-electric motor 114 which functions as an electric motor operated by an electric energy and an electric generator or dynamo for generating electricity. Power of the internal combustion engine 112 and power of the electric motor 114 are simultaneously or selectively transferred to a transmission 116, and to right and left drive wheels 120 via a speed reduction gear 118 and a differential gear. The transmission 116 has a forward/reverse switching mechanism having a forward drive position FWD, a reverse drive position REV, and a neutral position N, and a speed changing mechanism having a plurality of forward drive positions having different speed ratios. With a shift lever 122 operated by the vehicle operator, a shift actuator 124 is operated to place the transmission 116 in a selected one of the forward drive, reverse drive and neutral positions FWD, REV, N. The shift lever 122 has a parking position P, a reverse position R, a neutral position N, a drive position D, a second-speed position 2 and a low-speed position L. A shift position sensor 126 is provided to detect the currently selected position S H of the shift lever 122. An output signal of this shift position sensor 126 is fed to a controller 128, so that the transmission 116 is controlled depending upon the currently selected position S H of the shift lever 122. When the shift lever 122 is placed in the drive position D, the transmission 116 may be placed in any one of the forward drive positions. When the shift lever 122 is placed in the second-speed position 2 or low-speed position L, the forward drive positions in which the transmission may be placed are limited to the positions having relatively large speed ratios. A clutch 130 is disposed between the engine 112 and the transmission 116, for connecting and disconnecting the engine 112 and transmission 116. The clutch 130 is engaged and released by a clutch control actuator 132. The clutch 130 is normally placed in the fully engaged state. The dynamo-electric motor 114 is connected to an electric energy storage device (electric power supply device) 136 in the form of a battery or condenser, for example, through a motor/generator control device (hereinafter abbreviated as "M/G control device) 134 so that the motor 114 is selectively placed in a DRIVE state, a CHARGING state, and a NON-LOAD state. In the DRIVE state, the motor 114 is driven by an electric energy supplied from the electric energy storage device 136. In the CHARGING state, the motor 114 functions as an electric generator or dynamo, with regenerative braking (braking torque electrically generated by the motor 114 itself), for storing an electric energy in the electric energy storage device 136. In the NON-LOAD state, the output shaft of the motor 114 is permitted to rotate freely. The engine 112 is controlled by a fuel injection amount control actuator 142, a throttle valve control actuator 144, an ignition control actuator 146 and a suction and exhaust valve control actuator 148. These actuators 142, 144, 146, 148 and the M/G control device 134 are controlled by a controller 128. The electric energy storage device 136 is also connected to an electric motor 140 for driving an accessory device 138 such as a compressor of an air conditioner. The controller 128 includes a microcomputer incorporating a central processing unit (CPU), a random-access memory (RAM), and a read-only memory (ROM). The CPU operates according to control programs stored in the ROM while utilizing a temporary data storage function of the RAM. The controller 128 has various functions as illustrated in the block diagram of Fig. 9. The controller 128 is supplied with input signals from various detecting devices. These input signals include signals indicative of a speed Ne of the engine 112, an input speed Ni of the transmission 116 (speed of the motor 114), an output speed No of the transmission 116 (which corresponding to the running speed V of the vehicle), and a charging amount SOC of the electric energy storage device 136. The input signals include: an ACCELERATOR signal indicative of an operating amount  A of an accelerator pedal of the vehicle; a BRAKE signal indicating that a brake pedal of the vehicle has been depressed; a BRAKE PEDAL FORCE signal indicative of a force acting on the brake pedal; and ENGINE BRAKING SHIFT POSITION signal indicating that the shift lever 122 is placed in any one of engine braking shift positions, that is, in any one of the drive, second-speed and low-speed positions D, 2, L in which engine braking may be applied to the vehicle. The charging amount SOC of the electric energy storage device 136 may be determined on the basis of a current of the motor 114 during operation thereof as an electric generator to charge the device 136, or on the basis of the charging efficiency of the device 136. The controller 128 includes drive source selecting means 160 illustrated in the block diagram of Fig. 9. The drive source selecting means 160 is adapted to select one or both of the engine 112 and the motor 114 as the drive power source or sources, according to a drive source selecting data map stored in memory means 162. That is, the controller 128 has a MOTOR DRIVE mode in which the motor 114 is selected as the drive power source, an ENGINE DRIVE mode in which the engine 112 is selected as the drive power source, and an ENGINE·MOTOR DRIVE mode in which both the engine 112 and the motor 114 are selected as the drive power sources. An example of the drive source selecting data map is illustrated in the graph of Fig. 11, which represents a predetermined relationship between the vehicle drive torque and running speed V and the above-indicated three drive modes. The drive source selecting data map is provided for each of the drive positions of the transmission 116. A first boundary line B (B1 or B2) and a second boundary line C in Fig. define three ranges corresponding to the MOTOR DRIVE mode, ENGINE DRIVE mode and ENGINE·MOTOR DRIVE mode. Described more specifically, the drive source selecting means 160 selects the MOTOR DRIVE mode when the vehicle running condition as represented by the current vehicle drive torque and speed V is held within the range below the first boundary line B. When the vehicle running condition is held within the range between the first and second boundary lines B and C, the drive source selecting means 160 selects the ENGINE DRIVE mode. When the vehicle running condition is in the range above the second boundary line C, the drive source selecting means 160 selects the ENGINE·MOTOR DRIVE mode. As described below, the first boundary line B is normally located at B1, but may be shifted to B2 so as to enlarge the range in which the MOTOR DRIVE mode is selected. The data map representative of the two boundary lines B1 and C (solid boundary lines) is formulated such that the MOTOR DRIVE mode is selected if the fuel consumption amount M fcm in an ELECTRICITY GENERATING DRIVE mode (which will be described) is smaller than the fuel consumption amount M fce in the ENGINE DRIVE mode, while the ENGINE DRIVE mode is selected if the fuel consumption amount M fce is smaller than the fuel consumption amount M fce , as in the first embodiment. The fuel consumption amount M fcm in the ELECTRICITY GENERATING DRIVE mode is calculated while taking into account the electricity generating efficiency η GEN , charging efficiency η BIN , discharging efficiency η BOUT and motor driving efficiency η MOT , as described above with respect to the first embodiment. It will be understood that the drive source selecting means corresponds to the drive source selecting means 28 of the first embodiment. The controller 128 further includes engine driving means 164 for running the vehicle by operation of the engine 112 in the ENGINE DRIVE mode, motor driving means 166 for running the vehicle by operation of the motor 114 in the MOTOR DRIVE mode, and regenerative driving means 168 for running the vehicle in a REGENERATIVE DRIVE mode in which the motor 114 is driven by a kinetic energy of the running vehicle so as to store an electric energy in the electric energy storage device 136 as well as to apply a brake to the vehicle. The controller 128 further includes motor driving range enlarging means 170 for enlarging the motor driving range within which the vehicle is run in the MOTOR DRIVE mode under the control of the motor driving means 166, if a regenerative charging amount SOC R is larger than a predetermined threshold. The regenerative charging amount SOC R is an amount of electric energy to be stored in the electric energy storage device 136 in the REGENERATIVE DRIVE mode under the control of the regenerative driving means 168. The motor driving range enlarging means 170 includes: regenerative charging amount detecting means 172 for detecting the regenerative charging amount SOC R ; regenerative charging amount memory means 174 in the form of a register, for example, for storing the detected regenerative charging amount values SOC R for a predetermined time duration or during running of the vehicle by a predetermined distance; regenerative charging amount determining means 176 for determining whether a sum SOC RT of the regenerative charging amount values SOC R stored-in the memory means 174 is equal to or larger than a predetermined reference value D, or whether a regeneration ratio SOC RT /SOC is equal to or larger than a predetermined reference value E; and shifting means 178 for shifting the first boundary line B from the normal position B1 to enlarging position B2 to enlarge the motor driving range, if an affirmative decision is obtained by the regenerative charging amount determining means 176. The shifted first boundary line B2 indicated by dashed line in Fig. 11 is located below the second boundary line C. Thus, the present motor driving range enlarging means 170 is adapted to enlarge the range in which the vehicle is run in the MOTOR DRIVE mode. The shifted first boundary line B2 may be determined so as to satisfy a predetermined condition, for example, so that the vehicle can be run in the MOTOR DRIVE mode (with the motor 114 selected as the drive power source) at a predetermined velocity Y (km/h) or higher under a given running condition (e.g., running on a road surface having a given gradient X°). Referring next to the flow chart of Fig. 10, there will be described an operation of the controller 128 which includes the various functional means shown in Fig. 9. A control routine of Fig. 10 is initiated with step Q1 to read various input signals including the signals indicative of the operating amount  A of the accelerator pedal, engine speed Ne, transmission input and output speeds Ni, No, charging amount SOC, engine torque T E , motor torque T M and currently selected position S H of the shift lever 126. The engine torque T E may be obtained from the opening angle of the throttle valve or the fuel injection amount, while the motor torque T M may be obtained from a current flowing through the motor 114. Step Q1 is followed by step Q2 to determine whether the accelerator pedal is placed at a fully retracted or non-operated position, namely, whether the operating amount  A of the accelerator pedal is substantially zero or not. If an affirmative decision (YES) is obtained in step Q2, the control flow goes to step Q13 to determine whether the currently selected position S H of the shift lever 126 is any one of the engine braking shift positions D, 2, L. If an affirmative decision (YES) is obtained in step Q13, the control flow goes to step Q14 in which a sub-routine for running the vehicle in the REGENERATIVE DRIVE mode is executed under the control of the regenerative driving means 168 so that the motor 114 is driven by the kinetic energy of the running vehicle to charge the electric energy storage device 136 while a brake is applied to the vehicle. The regenerative braking torque of the motor 114 in this sub-routine is controlled depending upon the running condition of the vehicle, such as the vehicle running speed V and the brake pedal depression force. It will be understood that the REGENERATIVE DRIVE mode is selected when the accelerator pedal is placed at its non-operated position and when the shift lever 122 is placed in any one of the engine braking shift positions. That is, the affirmative decision (YES) in steps Q2 and Q13 is the requirement that should be satisfied to select the REGENERATIVE DRIVE mode. However, the requirement that should be satisfied to select the REGENERATIVE DRIVE mode may be suitably changed or determined. For instance, a result of determination as to whether the brake pedal has been depressed or not may be additionally used to determine whether the REGENERATIVE DRIVE mode should be selected or not. Then, the control flow goes to step Q15, which is implemented by the regenerative charging amount detecting means 172 to detect the regenerative charging amount SOC R of the electric energy storage device 136 in the REGENERATIVE DRIVE mode. The detection of the amount SOC R is based on the current of the motor 114 and the charging efficiency η BIN of the device 136. The detected amount SOC R is stored in the memory means 174. The charging efficiency η BIN may be obtained on the basis of the charging amount SOC and according to a predetermined relationship between the efficiency η BIN and the charging amount SOC, which relationship is represented by a data map stored in a suitable memory means such as the RAM of the controller 128. As indicated above, the regenerative charging amount memory means 174 stores the sum SOC RT of the regenerative charging _ amount values SOC R which have been detected for the predetermined time duration or during vehicle running by the predetermined distance. Namely, a predetermined number of the data values SOC R are always stored in the memory means 174 such that the oldest data value is erased each time the new data value is stored in step Q15, so that the sum SOC RT is updated with the vehicle running time or distance. If a negative decision (NO) is obtained in step Q2, that is, if the operating amount  A of the accelerator pedal is not substantially zero, the control flow goes to step Q3 to calculate the required drive power P L on the basis of the operating amount  A or a rate of change of this operating amount  A and the vehicle running speed V, according to a suitable equation or data map stored in the controller 128, as in the first embodiment. Step Q3 is followed by step Q4 to determine whether the charging amount SOC of the electric energy storage device 136 is equal to or larger than a predetermined lower limit A or not. If an affirmative decision (YES) is obtained in step Q4, the control flow goes to step Q5 and the subsequent steps. If a negative decision (NO) is obtained in step Q4, the control flow goes to step Q16 in which a sub-routine for running the vehicle in the ELECTRICITY GENERATING DRIVE mode is implemented. The lower limit A used in step Q4 is the minimum amount of electric energy stored in the electric energy storage device 136, which minimum amount is required for running the vehicle in the MOTOR DRIVE mode in which the motor 114 is selected as the drive power source. The lower limit A is usually-determined on the basis of the charging and discharging efficiencies η BIN and η BOUT , and is initially set at a standard value A1 which is about 70% of the nominal capacity of the device 136, for example. In the sub-routine of step Q16 implemented if the negative decision (NO) is obtained in step Q4, the engine 112 is operated to provide a total output P ICE not smaller than the required drive power P L , so that the vehicle is driven by the required drive power P L while the motor 114 is operated as the electric generator by the surplus power (P ICE - P L ) to charge the electric energy storage device 136, as in the first embodiment. Step Q16 is implemented in the same manner as step S7 of the first embodiment. If the affirmative decision (YES) is obtained in step Q4, the control flow goes to step Q5, which is implemented by the regenerative charging amount determining means 176, to determine whether the sum SOC RT of the regenerative charging amount values SOC R stored in the memory means 174 is smaller than the reference value D, or whether the ratio SOC RT /SOC is smaller than the reference value E. If an affirmative decision (YES) is obtained in step Q5, that is, if SOC RT < D and/or if SOC RT /SOC < E, step Q6 is implemented to determine the lower limit A at the standard value A1 and locate the first boundary line B at the normal position B1. If a negative decision (NO) is obtained in step Q5, that is if SOC RT ≧ D and/or if SOC RT /SOC ≧ E, step Q7 is implemented to reduce the lower limit A from the standard value A1 to a smaller value A2 and shift the first boundary line B from the normal position B1 to the motor driving range enlarging position B2. Accordingly, the range of the vehicle running condition in which the vehicle is run in the MOTOR DRIVE mode by operation of the motor 114 is enlarged, and the vehicle is more likely to be run in the ENGINE DRIVE, MOTOR DRIVE or ENGINE·MOTOR DRIVE mode in the following step Q10, Q11 or Q12, while the vehicle is less likely to be run in the ELECTRICITY GENERATING DRIVE mode in step Q16, since the lower limit A is reduced to from the standard value A1 to the smaller value A2. This arrangement is advantageous to effectively utilize the electric energy stored in the electric energy storage device 136 during running of the vehicle on a mountain path in which the sum SOC RT of the regenerative charging amount values SOC R tends to be large. In the case where the motor driving range is enlarged in step Q7, it is desirable to change the shift pattern of the transmission 116 so that the changed shift pattern meets the MOTOR DRIVE mode. Step Q6 and Q7 are followed by step Q8 to determine whether a point corresponding to the required drive power P L (determined by the current vehicle drive torque and speed V) is located above the first boundary line B. If an affirmative decision (YES) is obtained in step Q8, the control flow goes to step Q9 to determine whether the point of the required drive power P L is located above the second boundary line C. If a negative decision (NO) is obtained in step Q8, that is, if the point of the required drive power P L is located below the first boundary line B or in the motor driving range, the control flow goes to step Q12 in which a sub-routine for running the vehicle in the MOTOR DRIVE mode is executed. If the point of the required drive power P L is located above the first boundary line B and on or below the second boundary line C, that is, if a negative decision (NO) is obtained in step Q9, the control flow goes to step Q11 in which a sub-routine for running the vehicle in the ENGINE DRIVE mode is executed. If the point of the required drive power P L is located above the second boundary line, that is if an affirmative decision (YES) is obtained in step Q9, the control flow goes to step Q10 in which a sub-routine for running the vehicle in the ENGINE·MOTOR DRIVE mode is executed. The MOTOR DRIVE mode sub-routine of step Q12 is executed by the motor driving means 166 so that the vehicle is driven by operation of only the motor 114. The ENGINE DRIVE mode sub-routine of step Oil is executed by the engine driving means 164 so that the vehicle is driven by operation of the only engine 112. The ENGINE·MOTOR DRIVE sub-routine of step Q10 is executed by the engine driving means 164 and the motor driving means 164 so that the vehicle is driven by operation of both the engine 112 and the motor 114. Steps Q8 and Q9 to select the driving mode of the vehicle depending upon the required drive power P L are implemented by the drive source selecting means 160. When the vehicle is operated in the ENGINE DRIVE mode in step Q11, the motor 114 is placed in a non-load condition. When the vehicle is operated in the MOTOR DRIVE mode in step q12, the clutch 130 is placed in the fully released state to disconnect the engine 112 and the motor 114 from each other. In the present vehicle drive control apparatus 110, the range of the vehicle running condition in which the vehicle is driven in the MOTOR DRIVE mode by operation of only the motor 114 is enlarged by shifting the first boundary line B to the motor driving range enlarging position B2, if the sum SOC RT of the electric energy stored in the electric energy storage device 136 during running of the vehicle in the REGENERATIVE DRIVE mode in step Q14 is equal to or larger than the reference value D, or if the ratio SOC RT /SOC is equal to or larger than the reference value E. As a result of enlargement of the motor driving range, the amount of consumption of the electric energy by the motor 114 is increased. The present arrangement is effective to prevent excessive charging of the electric energy storage device 136 during running of the vehicle on a mountain path, thereby preventing reduction of the energy conversing efficiencies η BIN and η BOUT of the device 136 or a failure to charge the device 136. Further, the enlargement of the motor driving range results in reducing the frequency of operation of the engine 112, and consequent reduction of the fuel consumption by the engine 112 and the amount of exhaust gas emission from the engine 112. The present second embodiment is adapted such that the first boundary line B defining the motor driving range and partially defining the engine driving range is shifted from the standard position B1 to the position B2 toward the second boundary line C, namely, such that a portion of the normal engine driving range in which the fuel economy of the engine 112 is relatively low is replaced by a portion of the enlarged motor driving range. Accordingly, the present arrangement is effective to reduce the fuel consumption and exhaust gas emission of the engine 112. Further, the first and second boundary lines B and C defining the motor driving range and the engine driving range are determined based on the electricity generating efficiency η GEN , charging efficiency η BIN , discharging efficiency η BOUT and motor driving efficiency η MOT , as in the first embodiment, whereby the fuel consumption and exhaust gas amount are further effectively reduced. While the presently preferred embodiments of this invention have been described in detail by reference to the accompanying drawings, it is to be understood that the present invention may be otherwise embodied. In the first embodiment, the ENGINE DRIVE mode or the MOTOR DRIVE mode is selected so as to reduce the fuel consumption amount Mfc, and the engine 12 is controlled in the ELECTRICITY GENERATING DRIVE mode so as to maximize the overall fuel consumption efficiency η T , whereby the fuel consumption amount Mfc is minimized and the exhaust gas amount is accordingly minimized. However, the exhaust gas amount of the engine 12 can be further reduced by replacing the fuel consumption rates Fcm, Fce in the above equations (1) through (6), by an exhaust gas emission rate (amount of the exhaust gases per unit power), or by replacing the fuel consumption efficiency η ICE by an exhaust gas emission efficiency (reciprocal of the exhaust gas emission rate). Further, the selection of the ENGINE DRIVE mode and the MOTOR DRIVE_mode and the determination of the engine output P ICE in the ELECTRICITY GENERATING DRIVE mode may be effected using physical quantities other than the fuel consumption amount Mfc and the exhaust gas amount. The above changes or modifications are equally applicable to the second embodiment. In the first embodiment, the fuel consumption amounts M fcm , M fce are calculated in each cycle of execution of the routine of Fig. 3 depending upon the running condition of the vehicle such as the required drive power P L . In this connection, it is noted that the maximum energy conversion efficiencies η GEN and η MOT (η MOTmax indicated in Fig. 6) of the motor 14 are about 92%, while the input and output efficiency η BIN x η BOUT of the electric energy storage device 22 is about 85%, whereby the overall energy conversion efficiency of the electric control system is about 0.72 = 0.92 x 0.85 x 0.92. In view of this fact, it is possible to select the ENGINE DRIVE mode if the fuel consumption efficiency η ICE for running the vehicle in the ENGINE DRIVE mode with the engine 12 selected as the drive power source is larger than a threshold 0.7η ICEmax indicated in Fig. 5, which threshold-is 70% of the maximum fuel consumption efficiency η ICEmax , and select the MOTOR DRIVE mode if the fuel consumption efficiency η ICE is smaller than the threshold 0.7η ICEmax . This modified arrangement, which also meets the principle of the present invention, facilitates the selection of the ENGINE DRIVE mode and the MOTOR DRIVE mode, by simply obtaining the fuel consumption efficiency η ICEmax in the ENGINE DRIVE mode. The threshold is not limited to 70% of the maximum fuel consumption efficiency η ICEmax , but may be suitably determined depending upon the energy conversion efficiencies of the motor 14 and the electric energy storage device 22. The concept of this modification of the first embodiment is embodied as the data map shown in Fig. 11 used in the second embodiment. Where the overall energy conversion efficiency of the electric control system varies to a comparatively large extent, it is desirable to select the ENGINE DRIVE mode if the fuel consumption efficiency η ICE in the ENGINE DRIVE mode is larger than the threshold, and select the ENGINE DRIVE mode or the MOTOR DRIVE mode by implementing steps S3-S6 of Fig. 3 if the fuel consumption efficiency η ICE is smaller than the threshold. Similar modification is possible where the exhaust gas amount is reduced by replacing the fuel consumption efficiency η ICE by the exhaust gas emission efficiency. In step R1 of the ELECTRICITY GENERATING DRIVE mode sub-routine of Fig. 4, the engine output P ICE is initially set to be equal to the required drive power P L , and is incremented by the predetermined amount ▵P ICE each time the sub-routine is executed, so that the engine output P ICE whose overall fuel consumption efficiency η T is the highest is selected in step R5. However, the manner of determining the engine output P ICE so as to maximize the overall fuel consumption efficiency η T may be modified as needed. For example, the engine output P ICE may be selected within a predetermined range along a line L representative of the minimum fuel consumption rate as indicated by one-dot chain line in the graph of Fig. 5. This predetermined range has a predetermined width in the direction of the engine torque T E on the upper and lower sides of the line L. Although the first embodiment is adapted to select one of the ENGINE DRIVE, MOTOR DRIVE and ELECTRICITY GENERATING DRIVE modes, the modes available in the first embodiment may be suitably changed. For instance, the first embodiment may be modified to have the ENGINE·MOTOR DRIVE mode in which the vehicle is driven by simultaneous operation of the engine 12 and the motor 14 and which is selected when the vehicle load is comparatively high. While the electric motor 14, 114 functions also as the electric generator or dynamo in the illustrated embodiments, a separate electric generator may be provided in addition to the electric motor which is operated for the sole purpose of driving the vehicle. In the second embodiment, the selection of the ENGINE DRIVE, MOTOR DRIVE and ENGINE·MOTOR DRIVE modes is effected according to the boundary lines B and C as represented by the stored data map. However, the selection may be effected using equations similar to the equations (1) and (2) used in the first embodiment. Although the second embodiment uses the clutch 130 between the internal combustion engine 112 and the transmission 116, another clutch may be disposed between the transmission 116 and the electric motor 114, for example. It is also noted that the principle of the present invention is equally applicable to a vehicle drive control apparatus wherein the outputs of the engine 112 and 114 are combined by a suitable device such as a planetary gear device, to provide a single input to the transmission 116. It is to be understood that the present invention may be embodied with various other changes, modifications and improvements, which may occur to those skilled in the art, in the light of the foregoing teachings, according to the pending set of claims.

Claims: A drive control apparatus for an automotive hybrid vehicle having: an electric generator (14) generating an electric energy, an electric motor (14) operated as a first drive power source by said electric energy, an electric energy storage device (22) storing the electric energy generated by said electric generator (14), and an engine (12) operated as a second drive power source by combustion of a fuel, said apparatus having an engine drive mode in which the vehicle is driven by said engine (12), a motor drive mode in which the vehicle is driven by said electric motor (14), and an electricity generating drive mode in which said electric generator (14) is operated by said engine (12) to charge said electric energy storage device, said apparatus selecting one of said engine drive mode, said motor drive mode and said electricity generating drive mode, depending upon a running condition of the vehicle, said apparatus being characterised by comprising: drive source selecting means (28) selecting, based on the instantaneous drive power (P L ) required for running the vehicle, one of said engine drive mode and said motor drive mode on the basis of a predetermined rule associated with first and second values of a physical quantity relating to the running condition of said engine (12), wherein said first value is said physical quantity in said engine drive mode while said second value is said physical quantity in said electricity generating drive mode and reflects energy conversion efficiencies of said engine, said electric motor (14), said electric generator (14) and said electric energy storage device (22) in said electricity generating drive mode. A drive control apparatus according to claim 1, wherein said drive source selecting means (28) comprises: first calculating means (30) obtaining said first value of said physical quantity in said engine drive mode; second calculating means (32) obtaining said second value of said physical quantity; and comparing means (34) comparing said first and second values of said physical quantity obtained by said first and second calculating means, and selecting one of said engine drive mode and said motor drive mode depending upon whether one of said first and second values is smaller than the other. A drive control apparatus according to claim 1 or 2, wherein said physical quantity consists of an amount of consumption of said fuel by said engine (12) or an amount of exhaust gas emission from said engine (12). A drive control apparatus according to 3, wherein said first and second calculating means obtain a first value and a second value of said amount of consumption of said fuel in said engine drive mode and said electricity generating drive mode, respectively, said comparing means (34) selecting said engine drive mode if said first value obtained by said first calculating means (30) is smaller than said second value obtained by said second calculating means (32), and selecting said motor drive mode if said first value is not smaller than said second value. A drive control apparatus according to claim 4, wherein said first calculating means calculates said first value of said amount of consumption of said fuel in said engine drive mode, by using a speed and a torque of said engine, and said second calculating means calculates said second value of said amount of consumption of said fuel by said engine in said electricity generating drive mode, by using a speed and a torque of said electric generator. A drive control apparatus according to any one of claims 1 - 5, wherein said energy conversion efficiencies comprise efficiency of conversion of a kinetic energy into said electric energy by said electric generator (14) in said electricity generating drive mode; efficiency of charging of said electric energy storage device (22) with said electric energy generated by said electric generator (14) in said electricity generating drive mode; efficiency of discharging of said electric energy storage device (22) in said motor drive mode; and efficiency of conversion of said electric energy stored in said electric energy storage device (22) into a kinetic energy by said electric motor (14) in said motor drive mode. A drive control apparatus according to any one of claims 1 - 6, further comprising: engine output determining means (26, S7) determining a desired output of said engine (12) in said electricity generating drive mode, on the basis of an overall efficiency of a system comprising said engine (12), said electric motor (14), said electric generator (14) and said electric energy storage device (22), the system being assigned to charge said electric energy storage device (22) in said electricity generating drive mode, said overall efficiency being associated with said physical quantity relating to the condition of said engine, and said overall efficiency reflecting energy conversion efficiencies of said electric energy storage device and said electric generator (14) in said electricity generating drive mode. A drive control apparatus according to claim 7, wherein said engine output determining means (27, S7) determines said desired output of the engine (12) such that said desired output is larger than a value of drive power required for said engine (12) to drive the vehicle in said engine drive mode, by a predetermined amount which provides surplus power by which said electric generator (14) is operated to charge said electric energy storage device (22). A drive control apparatus according to claim 8, wherein said engine output determining means comprises: calculating means (R1-R4) calculating values of said overall efficiency which correspond to different candidate values of said desired output of the engine (12), respectively, said values of said overall efficiency reflecting the energy conversion efficiencies of said electric energy storage device (22) and said electric generator (14) during operation of said electric generator (14) by said surplus power in said electricity generating drive mode; and determining means (R5, R6) selecting one of said different candidate values of said desired output of said engine which corresponds to one of said values of said overall efficiency which satisfies a predetermined condition, and determining the selected candidate value as said desired output of the engine (12). A drive control apparatus according to claim 9, wherein said determining means (R5, R6) determines, as said desired output of the engine (12), one of said candidate values which corresponds to a highest one of said values of said overall efficiency. A drive control apparatus according to one of claims 1 to 10, characterised in that it comprises: a regenerative driving mode in which the vehicle is running in a regenerative drive mode in which said electric generator is driven by a kinetic energy of the running vehicle so as to store an electric energy in said electric energy storage device as well as to apply a brake to the vehicle, said regenerative drive mode is selected when said running condition of the vehicle satisfies a predetermined requirement, wherein enlarging means (170) are provided enlarging the range of the motor drive mode when an amount of the electric energy stored in said electric energy storage device (136) by operation of said electric generator (114) in said regenerative drive mode is larger than a predetermined threshold, wherein said enlarging means (170) enlarges the range of said motor drive mode such that a portion of a range of said engine drive mode in which the operation of said engine is less desirable in terms of said physical quantity than in the other portion of the range of said engine drive mode is replaced by a portion of the enlarged range of said motor drive mode. A drive control apparatus according to claim 11, further comprising memory means storing a data map representative of boundary lines which define said ranges of the engine drive mode and motor drive mode in relation to the running condition of the vehicle, and wherein said enlarging means comprises shifting means (178) shifting one of said boundary lines to enlarge said range of said motor drive mode. A drive control apparatus according to any one of claims 11 or 12, wherein said enlarging means enlarges said range of said motor drive mode such that the amount of enlargement is constant irrespective of the amount of the electric energy stored in said electric energy storage device. A drive control apparatus according to any one of claims 11 to 13, wherein said enlarging means comprises: detecting means (172) detecting the amount of the electric energy stored in said electric energy storage device in said regenerative drive mode; determining means (176) determining whether the amount of the electric energy detected by said detecting means is larger than said predetermined threshold; and means (178) enlarging said range of said motor drive mode if said determining means determines that the amount of the electric energy detected by said detecting means is larger than said predetermined threshold. A drive control apparatus according to one of claims 1 to 14, characterised in that said electric motor also functions as said electric generator.

Inventors: HATA HIROSHI, IBARAKI RYUJI, KUBO SEITOKU, TABATA ATSUSHI, TAGA YUTAKA, HATA, HIROSHI, IBARAKI, RYUJI, KUBO, SEITOKU, TABATA, ATSUSHI, TAGA, YUTAKA, Hata, Hiroshi, c/o Toyota Jidosha K.K., Ibaraki, Ryuji, c/o Toyota Jidosha K.K., Kubo, Seitoku, c/o Toyota Jidosha K.K., Tabata, Atsushi, c/o Toyota Jidosha K.K., Taga, Yutaka, c/o Toyota Jidosha K.K.
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1996
Main classification: B60K6
IPC codes: B60W10, B60L11, F02D29, B60W20, B60K6