Control System for Hybrid Vehicle

1. Field of the Invention

The present invention relates to a control system for controlling a hybrid vehicle having an internal combustion engine and an electric motor as separate propulsion sources, and more particularly to a control system for controlling a hybrid vehicle to regenerate electric energy with an electric motor when the hybrid vehicle is decelerated.

2. Description of the Related Art

There have heretofore been known hybrid vehicles each having an internal combustion engine and an electric motor as separate propulsion sources. For accelerating such a hybrid vehicle, the drive axle of the hybrid vehicle is driven by the engine, and stored electric energy is supplied from an electric energy storage unit such as a battery to the electric motor to enable the electric motor to assist in rotating the drive axle. For decelerating the hybrid vehicle, the kinetic energy of the drive axle, i.e., the kinetic energy transmitted from the drive wheels of the hybrid vehicle to the drive axle, is supplied to the electric motor to regenerate electric energy, which is stored in the electric energy storage unit.

One known control system for such a hybrid vehicle is disclosed in Japanese laid-open patent publication No. 7-123509, for example.

According to the disclosed control system, the present capacity (remaining capacity) of a battery as an electric energy storage unit is detected from time to time, and an amount of electric energy that can be regenerated by an electric motor, i.e., an amount of regeneratable electric energy, until the hybrid vehicle comes to a stop is determined from the vehicle speed, etc. Upon acceleration of the hybrid vehicle when the throttle valve of the engine is opened beyond a predetermined opening value, if the sum of the remaining capacity of the battery and the amount of regeneratable electric energy is greater than a minimum capacity of the battery required for the electric motor to restart the engine, then the electric motor is energized to assist in rotating the drive axle.

When the brake pedal of the hybrid vehicle is depressed to brake the hybrid vehicle, if the sum of the remaining capacity of the battery and the amount of regeneratable electric energy is smaller than a reference capacity which is substantially equal to the capacity of the fully charged battery, then since the battery needs to be charged, the electric motor regenerates electric energy, which is stored in the battery. At this time, the amount of regenerated electric energy is controlled in proportion to the depth to which the brake pedal is depressed.

When the opening of the throttle valve is smaller than the predetermined opening value and the brake pedal is not depressed, if the sum of the remaining capacity of the battery and the amount of regeneratable electric energy is smaller than the reference capacity, then electric energy is generated by the electric motor and stored in the battery. The amount of electric energy that is generated by the electric motor is controlled depending on an amount of electric energy to be charged, which is represented by the difference between the sum of the remaining capacity of the battery and the amount of regeneratable electric energy and the reference capacity.

According to the above known control system, when the hybrid vehicle is decelerated by braking, since the amount of regenerated electric energy is proportional to the depth to which the brake pedal is depressed, the amount of regenerated electric energy is small if the depth to which the brake pedal is depressed is relatively small to brake the hybrid vehicle slowly. Therefore, if the hybrid vehicle is temporarily decelerated, but not stopped, by braking, then the kinetic energy of the running hybrid vehicle cannot be converted into a sufficient amount of electric energy for storage in the battery. As a result, when the hybrid vehicle is to be accelerated after such temporary deceleration, no sufficient amount of electric energy can be supplied from the battery to the electric motor, and hence the electric motor fails to provide sufficient assistance in rotating the drive axle.

With the above known control system, while the accelerator pedal is being released in order to decelerate the hybrid vehicle, the electric motor regenerates an amount of electric energy depending on the amount of electric energy to be charged, which is referred to above, until the brake pedal is depressed. If the remaining capacity of the battery is relatively small upon deceleration of the hybrid vehicle, then the amount of electric energy regenerated by the electric motor after the accelerator pedal is released and until the brake pedal is depressed may possibly be greater than the amount of electric energy generated by the electric motor immediately after the brake pedal is depressed. When this happens, a braking force applied to the hybrid vehicle due to regenerative operation of the electric motor, i.e., a regenerative braking torque, is smaller immediately after the brake pedal is depressed than before the brake pedal is depressed, resulting in an unnatural vehicle behavior that will impair the drivability of the hybrid vehicle.

Furthermore, inasmuch as the amount of electric energy regenerated by the electric motor is controlled depending on the depth to which the brake pedal is depressed when the hybrid vehicle is decelerated by braking, the above known control system requires a sensor for detecting the depth to which the brake pedal is depressed, and hence is relatively complex in arrangement and expensive to manufacture.

It is therefore an object of the present invention to provide a control system for controlling a hybrid vehicle to allow the amount of electric energy regenerated by an electric motor upon deceleration of the hybrid vehicle to be controlled adequately depending on the demand of the driver of the hybrid vehicle with a relatively simple arrangement.

According to the present invention, the above object can be achieved by a control system for controlling a hybrid vehicle having an engine for rotating a drive axle, an electric motor for assisting the engine in rotating the drive axle with electric energy and converting kinetic energy of the drive axle into electric energy, and electric energy storage means for supplying electric energy to the electric motor and storing electric energy outputted by the electric motor, comprising deceleration demand detecting means for detecting a deceleration demand for the hybrid vehicle, braking detecting means for detecting whether the hybrid vehicle is braked or not, and decelerating regenerative control means for causing the electric motor to regenerate electric energy when the deceleration demand detecting means detects the deceleration demand, the decelerating regenerative control means comprising means for increasing an amount of electric energy regenerated by the electric motor, by a greater quantity when the hybrid vehicle is braked as detected by the braking detecting means than when the hybrid vehicle is not braked as detected by the braking detecting means, while the deceleration demand detecting means is detecting the deceleration demand.

When the deceleration demand detecting means detects a deceleration demand for the hybrid vehicle due, for example, to the release of an accelerator pedal of the hybrid vehicle, the decelerating regenerative control means causes the electric motor to regenerate electric energy. At this time, the amount of electric energy regenerated by the electric motor is increased by a greater quantity when the hybrid vehicle is braked as detected by the braking detecting means than when the hybrid vehicle is not braked as detected by the braking detecting means. Therefore, braking forces (so-called regenerative braking torque) for the hybrid vehicle which are generated by the regenerative operation of the electric motor are greater when the hybrid vehicle is braked than when hybrid vehicle is not braked. Furthermore, when the hybrid vehicle is braked, since the amount of electric energy regenerated by the electric motor is increased, the storage of the electric energy in the electric energy storage means proceeds quickly. Inasmuch as the amount of electric energy regenerated by the electric motor is increased depending on whether the braking of the hybrid vehicle is detected or not, it is not necessary to detect the depth to which a brake pedal is depressed.

Consequently, the amount of electric energy regenerated by the electric motor when the hybrid vehicle is decelerated can be controlled adequately depending on the demand of the driver with a simple arrangement.

The deceleration demand detecting means comprises means for determining a demand drive power for the hybrid vehicle depending on an accelerator pedal operation of the hybrid vehicle, means for determining a running resistance of the hybrid vehicle depending on a vehicle speed of the hybrid vehicle, and means for detecting a deceleration demand for the hybrid vehicle based on the determined demand drive power and the determined running resistance.

Since a deceleration demand for the hybrid vehicle is detected based on the demand drive power depending on an accelerator pedal operation of the hybrid vehicle and the running resistance depending on a vehicle speed of the hybrid vehicle, such a deceleration demand for the hybrid vehicle can appropriately be detected. Basically, a deceleration demand for the hybrid vehicle may be detected when the demand drive power is smaller than the running resistance.

Furthermore, the decelerating regenerative control means comprises means for controlling the amount of electric energy regenerated by the electric motor depending on at least a vehicle speed of the hybrid vehicle when the hybrid vehicle is not braked as detected by the braking detecting means, or more preferably, depending on a vehicle speed of the hybrid vehicle or a rotational speed of the engine or the electric motor when the hybrid vehicle is not braked as detected by the braking detecting means.

The decelerating regenerative control means comprises means for controlling the amount of electric energy regenerated by the electric motor such that the amount of electric energy regenerated by the electric motor increases as the vehicle speed of the hybrid vehicle is higher and that the amount of electric energy regenerated by the electric motor increases as the rotational speed of the engine or the electric motor is higher.

Specifically, as the vehicle speed is higher, since the kinetic energy of the hybrid vehicle that can be converted into electric energy by the regenerative operation of the electric motor is greater, the amount of electric energy regenerated by the electric motor when the braking of the hybrid vehicle is not detected, i.e., the basic amount of electric energy regenerated by the electric motor when the hybrid vehicle is decelerated, is increased. Moreover, as the vehicle speed is higher, since greater braking forces are required to decelerate the hybrid vehicle, the amount of electric energy regenerated by the electric motor is increased. As the rotational speed of the engine or the electric motor is higher, since the amount of electric energy regenerated by the electric motor, required to generate desired braking forces (regenerative braking torque), is greater, the amount of electric energy regenerated by the electric motor is increased.

By thus controlling the amount of electric energy regenerated by the electric motor depending on the vehicle speed and the rotational speed of the engine or the electric motor when the braking of the hybrid vehicle is not detected, the kinetic energy of the drive axle produced when the hybrid vehicle is running can effectively be converted into electric energy by the regenerative operation of the electric motor and stored in the electric energy storage means. In addition, braking forces generated by the regenerative operation of the electric motor can be made suitable for an operating status of the hybrid vehicle, such as the vehicle speed and the rotational speed of the engine or the electric motor, thereby improving the drivability of the hybrid vehicle.

The decelerating regenerative control means comprises means for establishing the quantity by which to increase the amount of electric energy regenerated by the electric motor, depending on at least a deceleration of the hybrid vehicle when the hybrid vehicle is braked as detected by the braking detecting means, or more preferably, depending on the deceleration of the hybrid vehicle and a vehicle speed of the hybrid vehicle.

The decelerating regenerative control means comprises means for establishing the quantity by which to increase the amount of electric energy regenerated by the electric motor, such that the quantity increases as the deceleration of the hybrid vehicle is greater and that the quantity increases as the vehicle speed of the hybrid vehicle is higher.

As the deceleration of the hybrid vehicle upon braking of the hybrid vehicle is greater, since braking forces for the hybrid vehicle which are demanded by the driver are greater, the quantity by which to increase the amount of electric energy regenerated by the electric motor is increased in order to increase braking forces (regenerative braking torque) produced by the regenerative operation of the electric motor. As the vehicle speed is higher, since it is preferable to increase braking forces (regenerative braking torque) produced by the regenerative operation of the electric motor in order to keep sufficient braking forces when the hybrid vehicle is braked, the quantity by which to increase the amount of electric energy regenerated by the electric motor is increased.

By thus establishing the quantity by which to increase the amount of electric energy regenerated by the electric motor when the hybrid vehicle is braked, depending on the deceleration of the hybrid vehicle and the vehicle speed, it is possible to maintain braking forces optimum for the running status of the hybrid vehicle and the demand of the driver, and also to store a sufficient amount of electric energy in the electric energy storage means.

The above and other objects, features, and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate a preferred embodiment of the present invention by way of example.

FIG. 1 is a block diagram of a drive apparatus of a hybrid vehicle and a control system therefor according to the present invention;

FIG. 2 is a block diagram of an engine control arrangement of the control system;

FIG. 3 is a block diagram of an electric motor control arrangement of the control system;

FIG. 4 is a block diagram of a transmission control arrangement of the control system;

FIGS. 5 and 6 are a flowchart of a processing sequence of the control system;

FIG. 7 is a diagram showing a data table for determining a basic distribution ratio;

FIG. 8 is a diagram showing a data table for determining a throttle valve opening basic command;

FIG. 9 is a diagram showing a data table for determining a distribution ratio;

FIG. 10 is a diagram showing a map for determining a demand drive power;

FIG. 11 is a diagram showing a map for determining a running status quantity;

FIG. 12 is a diagram showing the relationship between a motor output power POWERmot and a motor demand output power MOTORpower;

FIG. 13 is a flowchart of an decelerating regenerative control process in the processing sequence shown in FIG. 6;

FIG. 14 is a diagram showing data tables used in the decelerating regenerative control process shown in FIG. 13;

FIGS. 15(a) through 15(e) are timing charts illustrative of the decelerating regenerative control process shown in FIG. 13; and

FIG. 16 is a flowchart of an engine control process for controlling the engine of the hybrid vehicle shown in FIG. 1.

FIG. 1 shows in block form a drive apparatus of a hybrid vehicle and a control system therefor according to the present invention. Other components of the hybrid vehicle, including sensors, actuators, etc., are omitted from illustration in FIG. 1.

As shown in FIG. 1, the hybrid vehicle has an internal combustion engine 1 which rotates a drive axle 2 for rotating drive wheels 5 (only one shown) through a transmission mechanism 4. An electric motor, i.e., an electric motor capable of operating also as an electric generator, 3 is connected to rotate the drive axle 2 directly. The electric motor 3 has a rotatable shaft (not shown) coupled coaxially to the output shaft (not shown) of the engine 1. In addition to the ability to rotate the drive axle 2, the electric motor 3 has a regenerative ability to operate as an electric generator to convert kinetic energy produced by the rotation of the drive axle 2 into electric energy. The electric motor 3 is connected to a capacitor 14 serving as an electric energy storage unit through a power drive unit (PDU) 13 of the control system. The electric motor 3 is controlled by the power drive unit 13 to rotate the drive axle 2 and generate electric energy in a regenerative mode. In this embodiment, the capacitor 14 comprises an electric double-layer capacitor having a large electrostatic capacitance.

The control system also has an engine control unit (ENG/ECU) 11 for controlling the engine 1, an electric motor control unit (MOT/ECU) 12 for controlling the electric motor 3, a management control unit (MG/ECU) 15 for carrying out output power distributions of the engine 1 and the electric motor 3 depending on the remaining capacity of the capacitor 14, etc., and a transmission control unit (TM/ECU) 16 for controlling the transmission mechanism 4. These ECUs 11-16 are connected to each other by a data bus 21 for exchanging detected data, flag data, and other data through the data bus 21.

FIG. 2 shows the engine 1, the ENG/ECU 11, and ancillary devices thereof. A throttle valve 103 is mounted in an intake pipe 102 connected to the engine 1, and a throttle valve opening sensor 104 is coupled to the throttle valve 103 for generating an electric signal representative of the opening .theta.th of the throttle valve 103 and supplying the generated electric signal to the ENG/ECU 11. A throttle actuator 105 for electrically controlling the opening .theta.th of the throttle valve 103 is coupled to the throttle valve 103, which is of the drive-by-wire (DBW) type. The throttle actuator 105 is controlled for its operation by the ENG/ECU 11.

Fuel injection valves 106 are mounted in the intake pipe 102 at respective positions downstream of the throttle valve 103 and slightly upstream of respective intake valves (not shown) disposed respectively in the cylinders of the engine 1. The fuel injection valves 106 are connected through a pressure regulator (not shown) to a fuel tank (not shown). The fuel injection valves 106 are electrically connected to the ENG/ECU 11, which applies command signals to the fuel injection valves 106 to control times to open and close the fuel injection valves 106. The amount of fuel injected into each of the cylinders of the engine 1 by controlling the time to open the corresponding fuel injection valve 106.

An intake pressure sensor 108 for detecting an intake pressure PBA in the intake pipe 102 (specifically, the absolute pressure of intake air in the intake pipe 102) is connected to the intake pipe 102 through a pipe 107 immediately downstream of the throttle valve 103. The intake pressure sensor 108 generates an electric signal representative of the absolute pressure PBA in the intake pipe 102, and supplies the generated signal to the ENG/ECU 11.

An intake temperature sensor 109 for detecting an intake air temperature TA in the intake pipe 102 is mounted on the intake pipe 102 downstream of the intake pressure sensor 108. The intake temperature sensor 109 generates an electric signal representative of the intake air temperature TA and supplies the generated signal to the ENG/ECU 11.

An engine coolant temperature sensor 110 for detecting a coolant temperature TW of the engine 1 is mounted on the cylinder block of the engine 1. The engine coolant temperature sensor 110, which may comprise a thermistor or the like, generates an electric signal representative of an engine coolant temperature TW and supplies the generated signal to the ENG/ECU 11.

An engine rotational speed sensor 111 for detecting a rotational speed NE of the engine 1 is mounted near a camshaft or crankshaft (output shaft) of the engine 1. The engine rotational speed sensor 111 generates a signal pulse at a predetermined crankshaft angle (hereinafter referred to as a "TDC signal pulse") each time the crankshaft of the engine 1 makes a 180.degree. turn, and supplies the TDC signal pulse as a detected signal representative of the rotational speed NE of the engine 1 to the ENG/ECU 11.

A sensor 112 for detecting a rotational angle of the crankshaft of the engine 1 is mounted on the internal combustion engine 1. The sensor 112 generates a pulse each time the crankshaft turns through a predetermined angle. A pulse signal generated by the sensor 112 is supplied as a signal representative of a rotational angle of the crankshaft of the engine 1, to the ENG/ECU 11, which identifies an engine cylinder into which fuel is to be injected or in which the injected fuel is to be ignited, based on the supplied pulse signal.

The engine 1 has ignition plugs 113 positioned at the respective cylinders for igniting the fuel in the cylinders. The ignition plugs 113 are electrically connected to the ENG/ECU 11, which controls the ignition timing of the ignition plugs 113.

A three-way catalytic converter 115 for purifying toxic components, including HC, CO, NOx, etc. of exhaust gases emitted from the engine 1 is mounted in an exhaust pipe 114 connected to the engine 1. An air-fuel ratio sensor 117 for detecting an air-fuel ratio LAF of an air-fuel mixture burned in the engine 1 is mounted on the exhaust pipe 114 upstream of the three-way catalytic converter 115. The air-fuel ratio sensor 117 generates an electric signal substantially proportional to the concentration of oxygen in the exhaust gases, and supplies the generated signal as a detected signal representative of an air-fuel ratio LAF of an air-fuel mixture to the ENG/ECU 11. The air-fuel ratio sensor 117 can detect the air-fuel ratio LAF of the air-fuel mixture burned in the engine 1 through a wide range of air-fuel ratios ranging from a theoretical air-fuel ratio to lean and rich values.

A catalyst temperature sensor 118 is mounted on the three-way catalytic converter 115 for detecting a temperature TCAT thereof. The catalyst temperature sensor 118 supplies an electric signal representative of the detected temperature TCAT to the ENG/ECU 11. A vehicle speed sensor 119 for detecting the speed Vcar of the hybrid vehicle and an accelerator opening sensor 120 for detecting the depression .theta.ap (hereinafter referred to as an "accelerator opening .theta.ap") of the accelerator pedal are electrically connected to the ENG/ECU 11. Electric signals generated by the vehicle speed sensor 119 and the accelerator opening sensor 120 are supplied to the ENG/ECU 11.

The ENG/ECU 11 comprises an input circuit for shaping the waveforms of input signals (detected signals) from the above various sensors, correcting the voltage levels thereof into predetermined levels, and converting analog signals into digital signals, a central processing unit (hereinafter referred to as a "CPU"), a memory for storing various processing programs to be executed by the CPU and various processed results, and an output circuit for supplying drive signals to the fuel injection valves 106, the ignition plugs 113, and the throttle actuator 105. The other ECUs 12-16 are structurally similar to the ENG/ECU 11.

FIG. 3 shows a connected arrangement of the electric motor 3, the PDU 13, the capacitor 14, the MOT/ECU 12, and the MG/ECU 15.

As shown in FIG. 3, the electric motor 3 is associated with an electric motor rotational speed sensor 202 for detecting a rotational speed NM of the electric motor 3. An electric signal generated by the electric motor rotational speed sensor 202 as representing the detected rotational speed NM of the electric motor 3 is supplied to the MOT/ECU 12. Since the rotational speed NM of the electric motor 3 is equal to the rotational speed NE of the engine 1, the detected signal from the engine rotational speed sensor 111 instead of the detected signal from the electric motor rotational speed sensor 202 may be supplied to the MOT/ECU 12.

The PDU 13 and the electric motor 3 are interconnected by wires connected to a voltage/current sensor 201 which detects a voltage and a current supplied to or outputted from the electric motor 3. A temperature sensor 203 for detecting a temperature TD of the PDU 13, e.g., a temperature TD of a protective resistor of for the electric motor 3 or a temperature TD of an IGBT module (switching circuit), is mounted on the PDU 13. Detected signals from the sensors 201, 203 are supplied to the MOT/ECU 12.

The capacitor 14 and the PDU 13 are interconnected by wires connected to a voltage/current sensor 204 for detecting a voltage across the capacitor 14 and a current outputted from or supplied to the capacitor 14. A detected signal from the voltage/current sensor 204 is supplied to the MG/ECU 15.

The hybrid vehicle has a braking mechanism (not shown) associated with a brake switch 205 which serves as a brake operation detecting mechanism for detecting whether or not the braking mechanism is actuated, specifically, whether a brake pedal thereof is depressed. The brake switch 205 supplies an ON/OFF signal representing whether the braking mechanism is actuated or not is supplied to the MOT/ECU 12.

The MG/ECU 15 functions as a deceleration request detecting means, and the MG/ECU 15 in combination with the MOT/ECU 12 functions as a decelerating regenerative control means.

FIG. 4 shows a connected arrangement of the transmission mechanism 4 and the TM/ECU 16. The transmission mechanism 4 is associated with a gear position sensor 301 for detecting a gear position (speed reduction ratio) GP of the transmission mechanism 4. A detected signal from the gear position sensor 301 is supplied to the TM/ECU 16. In the illustrated embodiment, the transmission mechanism 4 comprises an automatic transmission mechanism, and is also associated with a transmission actuator 302 which is controlled by the TM/ECU 16 to change gear positions of the transmission mechanism 4.

FIGS. 5 and 6 show a processing sequence for determining output power distributions for the electric motor 3 and the engine 1 with respect to a demand drive power, i.e., how a drive power demanded by the driver of the hybrid vehicle is to be distributed to the electric motor 3 and the engine 1. The processing sequence shown in FIGS. 5 and 6 is executed by the MG/ECU 15 in each periodic cycle (e.g., 1 msec.). However, the processing sequence shown in FIGS. 5 and 6 may be executed by the MOT/ECU 12.

In FIG. 5, the MG/ECU 15 detects a remaining capacity CAPAremc of the capacitor 14 in STEP1.

Specifically, the MG/ECU 15 integrates (accumulates) an output current (discharging current) from the capacitor 14 and an input current (charging current) to the capacitor 14, which are detected by the voltage/current sensor 204, at each periodic interval from a fully charged state of the capacitor 14, and calculates an integrated discharged value CAPAdisch as a total discharged quantity and an integrated charged value CAPAchg as a total charged quantity. In this embodiment, the integrated discharged value CAPAdisch is a positive value, and the integrated charged value CAPAchg is a negative value. The MG/ECU 15 then calculates a basic remaining capacity CAPArem of the capacitor 14 from the integrated discharged value CAPAdisch and the integrated charged value CAPAchg according to the following equation (1):

where CAPAfull represents a dischargeable quantity when the capacitor 14 is fully charged.

The MG/ECU 15 corrects the calculated remaining capacity CAPArem based on the internal resistance of the capacitor 14 which varies with temperature, etc., thereby finally determining a remaining capacity CAPAremc of the capacitor 14. The corrected remaining capacity CAPAremc is represented as a proportion with respect to the dischargeable quantity CAPAfull of the capacitor 14 when it is fully charged.

Instead of calculating the remaining capacity of the capacitor 14 using the integrated discharged value CAPAdisch and the integrated charged value CAPAchg, an opencircuit voltage across the capacitor 14, i.e., an output voltage of the capacitor 14 when it is disconnected, may be detected, and the remaining capacity of the capacitor 14 may be estimated from the detected open-circuit voltage.

In STEP2, the MG/ECU 15 determines an output power to be generated by the electric motor 3 depending on the remaining capacity CAPAremc of the capacitor 14 when the electric motor 3 assists in rotating the drive axle and hence running the hybrid vehicle, more specifically, the proportion PRATIO of a drive power to be generated by the electric motor 3, in a demand drive power POWERcom for the hybrid vehicle which will subsequently be determined in STEP 5, using a predetermined data table shown in FIG. 7. The proportion of the drive power of the electric motor 3 with respect to the demand drive power POWERcom for the hybrid vehicle will hereinafter be referred to as a "distribution ratio", and the distribution ratio PRATIO depending on the remaining capacity CAPAremc of the capacitor 14 will hereinafter be referred to as a "basic distribution ratio PRATIO".

The data table shown in FIG. 7 is in the form of a graph which has a horizontal axis representing the remaining capacity CAPAremc of the capacitor 14 and a vertical axis representing the basic distribution ratio PRATIO. The data table contains values of the basic distribution ratio PRATIO established with respect to values of the remaining capacity CAPAremc such that the capacitor 14 will be discharged most efficiently when the electric motor 3 assists in running the hybrid vehicle.

In STEP3, the MG/ECU 15 searches a predetermined data table shown in FIG. 8 for a basic command .theta.thcom (hereinafter referred to as a "throttle valve opening basic command .theta.thcom") for the throttle valve opening .theta.th to be given to the throttle actuator 105 depending on an accelerator opening .theta.ap.

In FIG. 8, values of the accelerator opening .theta.ap are equal to corresponding values of the throttle valve opening basic command .theta.thcom (.theta.thcom=.theta.ap). However, values of the accelerator opening .theta.ap may be different from corresponding values of the throttle valve opening basic command .theta.thcom.

In STEP4, the MG/ECU 15 searches a predetermined data table shown in FIG. 9 for a distribution ratio PRATIOth for the drive power of the electric motor 3 depending on the determined throttle valve opening basic command .theta.thcom. The distribution ratio PRATIOth serves to correct the distribution ratio of the drive power of the electric motor 3 with respect to the demand drive power POWERcom for the hybrid vehicle by multiplying the basic distribution ratio PRATIO.

In the data table shown in FIG. 9, values of the distribution ratio PRATIOth are established such that the drive power (output power) of the electric motor 3 is increased when the throttle valve opening basic command .theta.thcom is in the vicinity of a fully open throttle valve value (e.g., 50 degrees or higher), i.e., when the hybrid vehicle demands a large acceleration.

While the distribution ratio PRATIOth is determined depending on the throttle valve opening basic command .theta.thcom in the illustrated embodiment, the distribution ratio PRATIOth may be determined depending on the accelerator opening .theta.ap. Alternatively, the distribution ratio PRATIOth may be determined depending on one or more parameters including the vehicle speed and the engine rotational speed.

In STEP5, the MG/ECU 15 searches a map shown in FIG. 10 for a demand drive power POWERcom for the hybrid vehicle, i.e., a drive power POWERcom which the driver of the hybrid vehicle demands by depressing the accelerator pedal, depending on the throttle valve opening basic command .theta.thcom determined in STEP3 and the present engine rotational speed NE detected by the engine rotational speed 111.

The demand drive power POWERcom determined from the map shown in FIG. 10 represents an overall hybrid vehicle drive power, i.e., the sum of the drive power from the engine 1 and the drive power from the electric motor 3, which is demanded depending on the throttle valve opening basic command .theta.thcom and the engine rotational speed NE. The demand drive power POWERcom may not necessarily be the same as a drive power which is outputted by the engine 1 when the engine 1 is actually operated with a throttle valve opening .theta.th equal to the throttle valve opening basic command .theta.thcom (basically, the demand drive power POWERcom is greater than the drive power which is outputted by the engine 1 when the engine 1 is actually operated with the throttle valve opening .theta.th equal to the throttle valve opening basic command .theta.thcom.). The demand drive power POWERcom is "0" when the throttle valve opening basic command .theta.thcom is approximately "0", i.e., when the accelerator opening .theta.ap is approximately "0".

In this embodiment, since the throttle valve opening basic command .theta.thcom is in a one-to-one correspondence to the accelerator opening .theta.ap, the accelerator opening .theta.ap instead of the throttle valve opening basic command .theta.thcom may be used to determine the demand drive power POWERcom.

In STEP6, the MG/ECU 15 calculates a corrective quantity .theta.thadd for correcting the command for the throttle valve opening .theta.th for enabling the engine 1 to generate the demand drive power POWERcom, from the throttle valve opening basic command .theta.thcom. The corrective quantity .theta.thadd is a corrective quantity for the command for the throttle valve opening .theta.th, to be added to the throttle valve opening basic command .theta.thcom in order to equalize the drive power outputted by the engine 1 to the demand drive power POWERcom. Specifically, the corrective quantity .theta.thadd is determined such that when the throttle valve opening .theta.th is controlled according to a command (=.theta.thcom+.theta.thadd) produced by adding the corrective quantity .theta.thadd to the throttle valve opening basic command .theta.thcom, the drive power outputted by the engine 1 is equalized to the demand drive power POWERcom.

In STEP7, the MG/ECU 15 searches a map shown in FIG. 11, for a running status quantity VSTATUS of the hybrid vehicle depending on the vehicle speed Vcar and an extra output power EXPOWER (described below) of the engine 1 from the present vehicle speed Vcar detected by the vehicle speed sensor 119 and the extra output power EXPOWER.

The extra output power EXPOWER of the engine 1 is calculated from the demand drive power POWERcom determined in STEP5 and a running resistance RUNRST of the hybrid vehicle depending on the vehicle speed Vcar, which is determined from a data table (not shown) from the present vehicle speed Vcar, according to the following equation (2):

The running resistance RUNRST is the same as a drive power which is necessary to keep the hybrid vehicle running at the present vehicle speed Vcar.

The extra output power EXPOWER is produced by subtracting the running resistance RUNRST depending on the present vehicle speed Vcar from the demand drive power POWERcom. If the extra output power EXPOWER is positively greater, then it means that a greater acceleration is required for the hybrid vehicle.

If the extra output power EXPOWER has a negative value, specifically, if it is smaller than a predetermined negative value, then it means that a deceleration is required for the hybrid vehicle. If the extra output power EXPOWER has a value near "0", then it means that a cruise mode (constant-speed mode) is required for the hybrid vehicle.

In this embodiment, each of the demand drive power POWERcom and the running resistance RUNRST is expressed in the unit of KW (kilowatts).

The running status quantity VSTATUS determined from the present vehicle speed Vcar and the extra output power EXPOWER according to the map shown in FIG. 11 serves to determine a distribution ratio for the drive power of the electric motor 3 depending on a loaded state of the hybrid vehicle which corresponds to the extra output power EXPOWER and the present vehicle speed Vcar when the electric motor 3 assists in running the hybrid vehicle, and has a value ranging from 0 to 200%. When the running status quantity VSTATUS multiplies the basic distribution ratio PRATIO, it corrects the distribution ratio for the drive power of the electric motor 3 with respect to the demand drive power POWERcom for the hybrid vehicle, depending on the loaded state of the hybrid vehicle.

Since a larger acceleration is required for the hybrid vehicle as the extra output power EXPOWER is greater, the map shown in FIG. 11 is established such that the running status quantity VSTATUS is greater as the extra output power EXPOWER is greater. Since an acceleration required for the hybrid vehicle with respect to the extra output power EXPOWER is smaller as the vehicle speed Vcar is greater, the map shown in FIG. 11 is established such that the running status quantity VSTATUS is smaller as the vehicle speed Vcar is greater.

The running status quantity VSTATUS also serves to decide whether the electric motor 3 is to assist in running the hybrid vehicle or not. Specifically, the running status quantity VSTATUS is set to VSTATUS=0 when the extra output power EXPOWER calculated according to the equation (2) is of a negative value or a value close to "0" (the demand drive power POWERcom is smaller than or substantially equal to the running resistance RUNRST) as when the accelerator pedal is released (.theta.ap=0). The running status quantity VSTATUS is set to VSTATUS>0 when the extra output power EXPOWER is positively greater than the value close to "0" (the demand drive power POWERcom is greater than the running resistance RUNRST by a certain value).

Therefore, if the running status quantity VSTATUS is "0", it means that the electric motor 3 is not to assist in running the hybrid vehicle (the hybrid vehicle is required to decelerate or cruise), and if the running status quantity VSTATUS is greater than "0", the electric motor 3 is to assist in running the hybrid vehicle (the hybrid vehicle is required to accelerate).

In STEP8, the MG/ECU 15 decides whether the running status quantity VSTATUS is greater than "0" or not. If VSTATUS>0, i.e., if the hybrid vehicle is in a running status to be assisted by the electric motor 3, then the hybrid vehicle enters an assistive mode, and control goes from STEP8 to STEP9 shown in FIG. 6. If VSTATUS.ltoreq.0 in STEP8, i.e., if the hybrid vehicle is to decelerate or cruise, then the hybrid vehicle enters a regenerative mode in which the electric motor 3 regenerates electric energy, and control goes from STEP8 to STEP12 shown in FIG. 6. The cruise mode while the hybrid vehicle is decelerating will hereinafter be referred to as a decelerating regenerative mode, and the cruise mode while the hybrid vehicle is cruising will hereinafter be referred to as or a cruising degenerative mode.

In STEP9 which is executed if the hybrid vehicle is in a running status to be assisted by the electric motor 3 (VSTATUS>0), the MG/ECU 15 calculates a demand output power MOTORpower for the electric motor 3 (hereinafter referred to as a "motor demand output power MOTORpower") from the demand drive power POWERcom determined in STEP5, the basic distribution ratio PRATIO determined in STEP2, the distribution ratio PRATIOth determined in STEP4, and the running status quantity VSTATUS determined in STEP7, according to the following equation (3):

In STEP10, the MG/ECU 15 generates a command MOTORcom for the output power (drive power) of the electric motor 3 (hereinafter referred to as a "motor output power command MOTORcom") in order to follow the motor demand output power MOTORpower with the delay of a given time constant.

FIG. 12 shows the relationship between the motor output power command MOTORcom thus generated and the motor demand output power MOTORpower. In FIG. 12, the solid-line curve illustrates the motor demand output power MOTORpower as it changes with time, and the dotted-line curve illustrates the motor output power command MOTORcom as it changes with time.

As can be seen from FIG. 12, the motor output power command MOTORcom is generated so as to follow the motor demand output power MOTORpower with the delay of a certain time constant. The reason for this is as follows: If the output power of the electric motor 3 were controlled according to the motor demand output power MOTORpower, then when the motor demand output power MOTORpower would change, because a change in the output power (drive power) of the engine 1 would be delayed, the output power of the electric motor 3 and the output power of the engine 1 would not immediately match each other, with the result that the drivability of the hybrid vehicle would be impaired. In order to eliminate the above drawback, in view of the response delay of the output power of the engine 1, the motor output power command MOTORcom is given the delay of a certain time constant compared with the motor demand output power MOTORpower.

In this embodiment, each of the motor demand output power MOTORpower and the motor output power command MOTORcom is of a positive value when the electric motor 3 is to assist in running the hybrid vehicle, and of a negative value when the electric motor 3 is to regenerate electric energy. Therefore, the motor demand output power MOTORpower and the motor output power command MOTORcom which are of a positive value serve as commands for the drive power of the electric motor 3, and the motor demand output power MOTORpower and the motor output power command MOTORcom which are of a negative value serve as commands for the amount of electric energy regenerated by the electric motor 3.

The motor output power command MOTORcom thus generated is supplied from the MG/ECU 15 to the MOT/ECU 12. The MOT/ECU 12 controls the output power (drive power) of the electric motor 3 through the PDU 13 according to the supplied motor output power command MOTORcom.

In STEP11, the MG/ECU 15 calculates a corrective quantity (reducing value) .theta.thassist for correcting a command for the throttle valve opening .theta.th in a valve closing direction, depending on the motor output power command MOTORcom. Thereafter, control goes from STEP11 to STEP18.

The corrective quantity .theta.thassist serves to correct a command for the throttle valve opening .theta.th in a valve closing direction in order to reduce the output power of the engine 1 by an output power (drive power) to be generated by the electric motor 3 according to the motor output power command MOTORcom. Specifically, the corrective quantity .theta.thassist is subtracted from the sum (=.theta.thcom+.theta.thadd) of the throttle valve opening basic command .theta.thcom determined in STEP3 and the corrective quantity .theta.thadd determined in STEP6 for thereby reducing a command for the throttle valve opening .theta.th which will finally be given to the throttle actuator 105. The corrective quantity .theta.thassist is calculated for the following reasons:

When a final command for the throttle valve opening .theta.th is determined as the sum of the throttle valve opening basic command .theta.thcom determined in STEP3 and the corrective quantity .theta.thadd determined in STEP6 and the throttle valve opening .theta.th is controlled according to the final command (=.theta.thcom+.theta.thadd), the demand drive power POWERcom is generated solely from the output power of the engine 1. Therefore, if the electric motor 3 were controlled by the motor output power command MOTORcom determined in STEP10, the sum of the output power (drive power) of the engine 1 and the output power (drive power) of the electric motor 3 would exceed the demand drive power POWERcom, resulting in a drive power greater than the drive power demanded by the driver. To avoid this problem, the corrective quantity .theta.thassist is calculated in order to reduce the output power of the engine 1 by an output power to be generated by the electric motor 3 and equalize the sum of the output power of the electric motor 3 and the output power of the engine 1 to the demand drive power POWERcom.

In STEP12, the MG/ECU 15 decides whether the present regenerative mode is the decelerating regenerative mode or not, i.e., whether the hybrid vehicle is required to decelerate or not. Specifically, the MG/ECU 15 makes such a mode decision based on the extra output power EXPOWER used to determine the running quantity status VSTATUS in STEP7, i.e., decides whether EXPOWER<0 or not (or more preferably smaller than a given negative value near 0 or not). Alternatively, the MG/ECU 15 may decide whether a change DAP in the accelerator opening .theta.ap per unit time is smaller than a predetermined negative quantity DAPD or not. According to this alternative, if DAP<DAPD, then the MG/ECU 15 determines that the present regenerative mode is the decelerating regenerative mode, and if DAP.gtoreq.DAPD, then the MG/ECU 15 determines that the present regenerative mode is the cruising regenerative mode.

If the extra output power EXPOWER is smaller than 0 (or smaller than a given negative value near 0) in STEP12, then since the demand drive power POWERcom is smaller than the running resistance RUNRST and the hybrid vehicle is required to decelerate, the MG/ECU 15 judges the present regenerative mode as the decelerating regenerative mode in which the hybrid vehicle decelerates and the electric motor 3 regenerates electric energy, and executes an decelerating regenerative control process shown in FIG. 13 in STEP13.

The decelerating regenerative control process shown in FIG. 13 will be described below. In STEP50 shown in FIG. 13, the MG/ECU 15 decides whether the present rotational speed NM of the electric motor 3 which is detected by the electric motor rotational speed sensor 202 is higher than a predetermined rotational speed NBON (e.g., 1000 rpm, which will hereinafter referred to as a "brake-on permission rotational speed NBON") for deciding whether the amount of electric energy regenerated by the electric motor 3 is to be increased or not when the hybrid vehicle is braked. In this embodiment, since the rotational speed NM of the electric motor 3 is the same as the rotational speed NE of the engine 1, the rotational speed NE of the engine 1 detected by the engine rotational speed sensor 111 may be used instead of the detected rotational speed NM of the electric motor 3 in STEP50.

If NM>NBON, then the MG/ECU 15 decides whether the brake pedal is depressed or not, i.e., if the hybrid vehicle is braked or not, based on an output signal from the brake switch 205 in STEP51. If NM.ltoreq.NBON in STEP50 or the hybrid vehicle is not braked in STEP51, then the MG/ECU 15 sets a braking corrective value BRreg representing an increase in the amount of electric energy regenerated by the electric motor 3 to "0" in STEP58. Then, control goes from STEP58 to STEP59. The braking corrective value BRreg is set to "0" if NM.ltoreq.NBON because the rotational speed NM of the electric motor 3 is too low to increase the amount of electric energy regenerated by the electric motor 3.

If the hybrid vehicle is braked in STEP51, then the MG/ECU 15 decides whether a present deceleration DECV (which is of a positive value when the hybrid vehicle is decelerating) of the hybrid vehicle which is determined as the difference between the vehicle speed Vcar in the preceding control cycle and the vehicle speed Vcar in the present control cycle is greater than a first deceleration DECV1 (e.g., 0.28 m/s.sup.2) or not in STEP52. If DECV.ltoreq.DECV1 indicating that the deceleration of the hybrid vehicle is sufficiently low, then control goes to STEP58. If DECV>DECV1, then the MG/ECU 15 decides whether the present deceleration DECV is greater than a second deceleration DECV2 (e.g., 1.38 m/s.sup.2) which is greater than the first deceleration DECV1 or not in STEP53. If DECV1<DECV.ltoreq.DECV2 indicating that the deceleration of the hybrid vehicle is of an medium level, then the MG/ECU 15 searches a data table shown in FIG. 14 for an increase BRreg 1 (which is of a negative value, and will hereinafter be referred to as a "decelerating regenerative increase BRreg1") in the amount of electric energy regenerated by the electric motor 3, for a medium deceleration depending on the vehicle speed Vcar in STEP56. The MG/ECU 15 sets the braking corrective value BRreg to the decelerating regenerative increase BRreg1 in STEP57, after which control goes to STEP59. The data table shown in FIG. 14 is established such that the absolute value of the decelerating regenerative increase BRreg1 increases as the vehicle speed Vcar increases. The first and second decelerations DECV1, DECV2 correspond respectively to decelerations of 10 rpm/s and 50 rpm/s converted from rotational speeds of the road wheels of the hybrid vehicle, each having a radius of 0.264 m.

If DECV>DECV2 in STEP53 indicating that the deceleration of the hybrid vehicle is large, then the MG/ECU 15 searches another data table shown in FIG. 14 for an increase BRreg 2 (which is of a negative value, and will hereinafter be referred to as a "decelerating regenerative increase BRreg2") in the amount of electric energy regenerated by the electric motor 3, for a large deceleration depending on the vehicle speed Vcar in STEP54. The MG/ECU 15 sets the braking corrective value BRreg to the decelerating regenerative increase BRreg2 in STEP55, after which control goes to STEP59. The other data table shown in FIG. 14 is established such that the absolute value of the decelerating regenerative increase BRreg2 increases as the vehicle speed Vcar increases. The absolute value of the decelerating regenerative increase BRreg2 for the large deceleration is greater than the absolute value of the decelerating regenerative increase BRreg1 for the medium deceleration with respect to the same vehicle speed Vcar.

In STEP59, the MG/ECU 15 searches a map (not shown) for a basic amount DECreg (which is of a negative value, and will hereinafter be referred to as a "decelerating regenerative quantity DECreg") of electric energy regenerated by the electric motor 3 depending on the present vehicle speed Vcar and the rotational speed NE of the engine 1 (=the rotational speed NM of the electric motor 3). The map is established such that the absolute value of the decelerating regenerative quantity DECreg increases as the vehicle speed Vcar increases and the engine rotational speed NE increases.

Then, the MG/ECU 15 adds the braking corrective value BRreg set in either one of STEP55, STEP57, STEP58 to the decelerating regenerative quantity DECreg determined in STEP59, thus calculating a motor demand output power MOTORpower in STEP60. Thereafter, the MG/ECU 15 generates a motor output power command MOTORcom which is delayed by a given time constant with respect to the motor demand output power MOTORpower in STEP61. The decelerating regenerative control process shown in FIG. 13 now comes to an end.

The motor output power command MOTORcom (<0) thus generated is supplied from the MG/ECU 15 to the MOT/ECU 12. The MOT/ECU 12 controls the output power (the amount of regenerated electric energy) of the electric motor 3 through the PDU 13 according to the supplied motor output power command MOTORcom.

FIGS. 15(a) through 15(e) are timing charts illustrative of the decelerating regenerative control process shown in FIG. 13. FIGS. 15(a) through 15(e) show, respectively, the vehicle speed Vcar, the accelerator opening .theta.ap, the output signal from the brake switch 205, the motor demand output power MOTORpower, and the motor output power command MOTORcom (actual amount of regenerated electric energy) as they change with time. The accelerator pedal is released to start decelerating the hybrid vehicle at a time t1 as shown in FIG. 15(a). The brake pedal starts being depressed at a time t2 as shown in FIG. 15(c). The depth to which the brake pedal is depressed increases at a time t3 as shown in FIG. 15(a).

Between the time t1 and the time t2, since the brake pedal is not depressed, the braking corrective value BRreg is "0". In this period, therefore, the decelerating regenerative quantity DECreg determined from the map based on the vehicle speed Vcar and the engine rotational speed NE is directly used as the motor demand output power MOTORpower. Between the time t2 and the time t3, since the deceleration DECV of the hybrid vehicle is of a medium level (DECV1<DECV.ltoreq.DECV2), the braking corrective value BRreg (=the decelerating regenerative increase BRreg1) is added to the decelerating regenerative quantity DECreg, increasing the absolute value of the motor demand output power MOTORpower (which is of a negative value). At the time t3, the deceleration DECV increases (DECV>DECV2). Inasmuch as the braking corrective value BRreg is set to the decelerating regenerative increase BRreg2 for the large deceleration, the absolute value of the motor demand output power MOTORpower is further increased. At a time t4, the vehicle speed Vcar becomes sufficiently low, reducing the decelerating regenerative quantity DECreg to "0", and the motor rotational speed NM becomes sufficiently low (NM.ltoreq.NBON, see STEP50 in FIG. 13), reducing the braking corrective value BRreg to "0", whereupon the regenerative operation of the electric motor 3 is finished. At this time, the actual amount of regenerated electric energy of the electric motor 3 which is determined by the motor output power command MOTORcom follows the motor demand output power MOTORpower with a slight delay, as shown in FIG. 15(e).

According to the decelerating regenerative control process shown in FIG. 13, as described above, while the hybrid vehicle is decelerating as recognized by the extra output power EXPOWER which is the difference between the demand drive power POWERcom and the running resistance RUNRST, the absolute value of the motor demand output power MOTORpower as the amount of electric energy required to be regenerated by the electric motor 3 is increased by a greater quantity when the brake switch 203 is turned on, i.e., when the hybrid vehicle is braked, than when the brake switch 203 is turned off, i.e., when the hybrid vehicle is not braked. Therefore, the amount of electric energy regenerated by the electric motor 3 when the hybrid vehicle is decelerated can appropriately be controlled depending on the demand of the driver with a simple arrangement.

More specifically, by increasing the amount of electric energy regenerated by the electric motor 3 by a greater quantity when the hybrid vehicle is braked than when the hybrid vehicle is decelerated without being braked, the regenerative braking torque of the electric motor 3 and hence the braking torque of the hybrid vehicle are increased, making it possible to obtain vehicle braking forces that match the demand of the driver for decelerating the hybrid vehicle. At this time, the amount of electric energy regenerated by the electric motor 3 when the hybrid vehicle is not braked, i.e., the decelerating regenerative quantity DECreg which is a basic amount of electric energy regenerated by the electric motor 3 when the hybrid vehicle is decelerated, is established depending on the vehicle speed Vcar and the rotational speed NE of the engine 1 (=the rotational speed NM of the electric motor 3), and the braking corrective value BRreg which is an increase in the amount of electric energy regenerated by the electric motor 3 when the hybrid vehicle is braked is established depending on the vehicle speed Vcar. Therefore, it is possible to obtain vehicle braking forces adequate for the running status of the hybrid vehicle and the operating status of the engine 1 or the electric motor 3. Furthermore, when the hybrid vehicle is decelerated until it is stopped or even when the hybrid vehicle is decelerated temporarily, the kinetic energy of the hybrid vehicle when it is decelerated is effectively converted into electric energy by the regenerative operation of the electric motor 3, and the electric energy is stored in the capacitor 14, i.e., to charge the capacitor 14. In this embodiment, the braking corrective value BRreg is established depending on the deceleration DECV of the hybrid vehicle such that the absolute value of the braking corrective value BRreg increases as the deceleration DECV of the hybrid vehicle increases. Thus, vehicle braking forces generated by the regenerative operation of the electric motor 3 can be brought more effectively into matching the demand for decelerating the hybrid vehicle as intended by the driver, for thereby improving the drivability of the hybrid vehicle.

Referring back to FIG. 6, in STEP14, the MG/ECU 15 establishes a command .theta.tho to be finally given to the throttle actuator 105 as a command for the throttle valve opening .theta.th in the decelerating regenerative mode. Thereafter, control proceeds to STEP19. The command .theta.tho for the throttle valve opening .theta.th is established according to a process not shown, and is basically "0". However, the command .theta.tho may be set to a value greater than "0" in order to increase the amount of electric energy regenerated by the electric motor 3.

If the extra output power EXPOWER is of a value near "0" (the running status quantity VSTATUS is "0" because of "NO" to STEP8) in STEP12, then the MG/ECU 15 judges the present regenerative mode as the cruising regenerative mode, and sets the motor demand output power MOTORpower to an output power CRUISpower (which is of a negative value) for regenerating electric energy with the electric motor 3 while cruising the hybrid vehicle in STEP15. The output power CRUISpower is determined according to a routine not shown, and may be determined depending on the vehicle speed Vcar, the rotational speed NE of the engine or the rotational speed NM of the electric motor 3, and the remaining capacity CAPAremc of the capacitor 14.

In STEP16, the MG/ECU 15 generates a motor output power command MOTORcom in order to follow the motor demand output power MOTORpower with the delay of a given time constant, as with STEP10.

The motor output power command MOTORcom (<0) thus generated is supplied from the MG/ECU 15 to the MOT/ECU 12. The MOT/ECU 12 controls the output power (the amount of regenerated electric energy) of the electric motor 3 through the PDU 13 according to the supplied motor output power command MOTORcom.

In STEP17, the MG/ECU 15 calculates a corrective quantity (increasing value) .theta.thsub for correcting a command for the throttle valve opening .theta.th in a valve opening direction, depending on the motor output power command MOTORcom. Thereafter, control goes from STEP17 to STEP18.

The corrective quantity .theta.thsub is calculated for the reasons that are opposite to the reasons for which the corrective term .theta.thassist is calculated as described above.

Specifically, the motor demand output power MOTORpower established in the cruising regenerative mode is of a negative value and represents an amount of electric energy regenerated by the electric motor 3. Therefore, when the output power (generated electric energy) of the electric motor 3 is controlled according to the motor demand output power MOTORpower, the electric motor 3 absorbs the drive power (energy) corresponding to the amount of regenerated electric energy according to the motor demand output power MOTORpower, reducing the drive power of the hybrid vehicle, i.e., the electric motor 3 serves as a load on the engine 1. Thus, in order to maintain the demand drive power POWERcom as the drive power of the hybrid vehicle in the cruising regenerative mode, it is necessary to make up for the output power of the electric motor 3 which is absorbed by the regenerative operation of the electric motor 3, with the output power of the engine 1. To meet this requirement, according to the present invention, the corrective quantity .theta.thsub is calculated depending on the motor demand output power MOTORpower in order to increase the output power of the engine 1 by an output power absorbed by the regenerative operation of the electric motor 3 in the cruising regenerative mode.

In STEP18, the MG/ECU 15 calculates a command .theta.tho for the throttle valve opening .theta.th to be finally given to the throttle actuator 105 from the throttle valve opening basic command .theta.thcom determined in STEP3, the corrective quantity .theta.thadd determined in STEP6, the corrective quantity .theta.thassist determined in STEP11, and the corrective quantity .theta.thsub determined in STEP17, according to the following equation (4):

In the assistive mode, the corrective quantity .theta.thsub is set to "0", and in the cruising regenerative mode, the corrective quantity .theta.thassist is set to "0".

In STEP19, the MG/ECU 15 decides whether or not the command .theta.tho for the throttle valve opening .theta.th is equal to or greater than a predetermined value .theta.thref. If .theta.tho<.theta.thref, then the MG/ECU 15 decides whether or not the present intake pressure PBA detected by the intake pressure sensor 108 is equal to or smaller than a predetermined value PBAref in STEP20. If PBA>PBAref, then the processing sequence shown in FIGS. 5 and 6 is brought to an end in the present control cycle.

If .theta.tho.gtoreq..theta.thref in STEP19 or if PBA.ltoreq.PBAref in STEP20, then the MG/ECU 15 changes the speed reduction ratio of the transmission mechanism 4 to a lower speed reduction ratio in STEP21. Thereafter, the processing sequence shown in FIGS. 5 and 6 is brought to an end in the present control cycle.

When control goes to STEP21, the output power of the engine 1 cannot be increased beyond the present output power level. At this time, the speed reduction ratio of the transmission mechanism 4 is changed to a lower speed reduction ratio to keep the torque produced by the drive axle 2 at a constant level, i.e., the same torque as before STEP21, to keep the drivability of the hybrid vehicle. The speed reduction ratio of the transmission mechanism 4 is actually changed by the TM/ECU 16 under the control of the MG/ECU 15.

An engine control process carried out by the ENG/ECU 11 will be described below.

FIG. 16 shows an overall engine control processing sequence, which is executed by the ENG/ECU 11 in each of predetermined control cycles, e.g., control cycles in synchronism with TDC (Top Dead Center).

In FIG. 16, the ENG/ECU 11 detects various engine operating parameters including the engine rotational speed NE, the intake pressure PBA, etc. in STEP131. Then, the ENG/ECU 11 determines an engine operating status in STEP132.

Thereafter, depending on the determined engine operating status, the ENG/ECU 11 executes a fuel control process for controlling the amount of fuel to be injected into the engine 1 and the timing of fuel injection with the fuel injection valves 106 in STEP133. Then, also depending on the determined engine operating status, the ENG/ECU 11 executes an ignition timing control process for controlling the timing of fuel ignition in the engine 1 with the ignition plugs 113 in STEP134, and then executes a throttle valve control process for controlling the throttle valve opening .theta.th with the throttle actuator 105 in STEP135.

Specifically, the ENG/ECU 11 controls the amount of fuel to be injected into the engine 1 and the timing of fuel injection and also controls the timing of fuel ignition depending on the engine rotational speed NE, the intake pressure PBA, etc. in STEP 133 and STEP134. In STEP135, the ENG/ECU 11 also controls the operation of the throttle actuator 105 in order to equalize the actual throttle valve opening .theta.th to the command .theta.tho (which is determined in STEP14 or STEP18 shown in FIG. 6) for the throttle valve opening .theta.th, which is supplied from the MG/ECU 15 to the ENG/ECU 11.

While the capacitor 14 is employed as the electric energy storage unit in the illustrated embodiment, the electric energy storage unit may instead comprise a battery (secondary cell).

The throttle valve 103 of the DBW type may be replaced with an ordinary throttle valve that is mechanically linked to the accelerator pedal. In such a modification, the amount of intake air depending on the output power of the electric motor may be controlled by a passage bypassing the throttle valve and a control valve disposed in the passage. On an engine having a solenoid-operated intake valve which can electromagnetically be operated, rather than by a cam mechanism, the amount of intake air depending on the output power of the electric motor may be controlled by changing the valve opening period of the solenoid-operated intake valve.

The transmission mechanism 4 may comprise a continuously variable transmission mechanism whose speed reduction ratio can be continuously varied. With such a continuously variable transmission mechanism, the speed reduction ratio can be determined from the ratio of the rotational speed of the drive shaft of the continuously variable transmission mechanism to the rotational speed of the driven shaft thereof, rather than by detecting the gear position.

If the braking mechanism is combined with an actuator for hydraulically regulating braking forces, then the actuator may be controlled to reduce the braking forces at the time the amount of electric energy regenerated by the electric motor is increased when the hybrid vehicle is decelerated. This arrangement is effective to improve the drivability of the hybrid vehicle and also to maintain the amount of electric energy regenerated by the electric motor.

Although a certain preferred embodiment of the present invention has been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.