Control system for hybrid vehicle

A control system controls a hybrid vehicle having an engine for rotating a drive axle, an electric motor for assisting the engine in rotating the drive axle, and electric energy storage unit for supplying electric energy to the electric motor. The control system includes a demand drive power calculating unit for calculating a demand drive power for the hybrid vehicle depending on operating conditions of the hybrid vehicle, an engine output power calculating unit for calculating an output power of the engine which corresponds to the demand drive power, a remaining capacity detecting unit for detecting a remaining capacity of the electric energy storage unit, an electric motor output power calculating unit for calculating an output power of the electric motor depending on the demand drive power and the remaining capacity of the electric energy storage unit, and an engine corrective quantity calculating unit for calculating a corrective quantity to reduce the output power of the engine in order to equalize the sum of the calculated output power of the electric motor and the calculated output power of the engine to the demand drive power. An output control unit controls a drive power of the electric motor based on the calculated output power of the electric motor and reducing the output power of the engine based on the calculated corrective quantity.

BACKGROUND OF THE INVENTION 
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. 
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. 
Japanese laid-open patent publication No. 5-229351, for example, discloses 
a drive power control system for propulsion sources on such a hybrid 
vehicle. 
The disclosed drive power control system determines an optimum torque for 
maximizing the efficiency of the engine depending on running conditions of 
the vehicle, detects an actual drive torque of the engine, and determines 
an assistive drive torque based on the optimum torque and the actual 
torque. The drive power control system energizes the electric motor to 
apply an assistive power at suitable times as when the vehicle is to be 
accelerated, depending on the assistive drive torque. 
The electric motor applies the assistive power by simply adding the output 
power of the electric motor depending on the remaining capacity of an 
electric energy storage unit based on the difference between the optimum 
torque and the actual torque which are generated by a throttle valve 
opening that is uniquely determined depending on the driver's action to 
operate the accelerator pedal. For this reason, the drive power control 
system suffers the following problems: 
When the remaining capacity of the electric energy storage unit falls and 
hence the output power of the electric motor drops, the total drive power, 
i.e., the sum of the output power from the engine and the output power 
from the electric motor, is reduced, resulting in a reduction in the 
actual torque. Accordingly, the drivability of the hybrid vehicle is 
impaired. 
One solution is to replace an ordinary throttle valve mechanically linked 
to the accelerator pedal with a throttle valve that is electrically 
controlled by an actuator based on a signal indicative of the amount of 
operation of the accelerator pedal. When the output power from the 
electric motor drops, the opening of the throttle valve is controlled to 
enable the engine to generate an output power commensurate with the drop 
in the output power from the electric motor. The proposal is effective to 
prevent the drivability of the hybrid vehicle from being impaired, but is 
disadvantageous for the following reasons: 
One advantage offered by the assistive drive power provided by the electric 
motor is to reduce the amount of fuel consumption by the engine. 
Specifically, a demand drive power imposed on a motor vehicle which is 
propelled by an engine alone is entirely generated by the engine. However, 
a demand drive power imposed on a hybrid vehicle which is propelled by an 
engine and an electric motor is partly generated by the engine and partly 
generated by the electric motor. The greater the amount of the demand 
drive power shared by the electric motor, the smaller the amount of the 
demand drive power shared by the engine, i.e., the smaller the amount of 
work performed by the engine, resulting in a reduction in the amount of 
fuel consumption by the engine. In the conventional drive power control 
system described above, the torque generated by the engine is maintained 
at an optimum level regardless of the remaining capacity of the electric 
energy storage unit, and hence the advantage of the hybrid vehicle that 
the amount of work performed by the engine can be reduced to reduce the 
amount of fuel consumption by the engine is limited. 
In the above conventional drive power control system, since the output 
power of the engine is determined depending on the throttle valve opening, 
the engine maintains a certain amount of output power at all times except 
when the throttle valve is fully closed. This mode of operation remains 
unchanged even when the electric energy storage unit stores an amount of 
electric energy sufficient enough to enable the electric motor to generate 
all the demand drive power. Therefore, even when the electric energy 
storage unit stores such a sufficient amount of electric energy, the 
electric motor does not generate all the demand drive power, and hence the 
amount of work performed by the engine cannot be eliminated completely. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a control 
system for calculating an output power of an electric motor on a hybrid 
vehicle based on a demand drive power and the remaining capacity of an 
electric energy storage unit, and correcting an output power of an engine 
on the hybrid vehicle based on the demand drive power and the calculated 
output power of the electric motor for increasing drivability of the 
hybrid vehicle and reducing an amount of fuel consumption by the engine. 
To achieve the above object, there is provided in accordance with the 
present invention 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, and electric energy storage means for 
supplying electric energy to the electric motor, comprising demand drive 
power calculating means for calculating a demand drive power for the 
hybrid vehicle depending on operating conditions of the hybrid vehicle, 
engine output power calculating means for calculating an output power of 
the engine which corresponds to the demand drive power, remaining capacity 
detecting means for detecting a remaining capacity of the electric energy 
storage means, electric motor output power calculating means for 
calculating an output power of the electric motor depending on the demand 
drive power and the remaining capacity of the electric energy storage 
means, engine corrective quantity calculating means for calculating a 
corrective quantity to reduce the output power of the engine in order to 
equalize the sum of the calculated output power of the electric motor and 
the calculated output power of the engine to the demand drive power, and 
output control means for controlling a drive power of the electric motor 
based on the calculated output power of the electric motor and reducing 
the output power of the engine based on the calculated corrective 
quantity. 
The electric motor output power calculating means comprises means for 
calculating an output power of the electric motor depending on a running 
status quantity determined by the demand drive power, the remaining 
capacity of the electric energy storage means, a vehicle speed of the 
hybrid vehicle, and a running resistance to the hybrid vehicle. 
The output control means comprises means for generating the demand drive 
power solely with the electric motor when the calculated demand drive 
power is at most the calculated output power of the electric motor. 
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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 a multicylinder 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 3 
is connected to rotate the drive axle 2 directly. In addition to the 
ability to rotate the drive axle 2, the electric motor 3 has a 
regenerative ability to convert kinetic energy produced by the rotation of 
the drive axle 2 into electric energy. The electric motor 3 is connected 
to an ultracapacitor (a capacitor having a large electrostatic 
capacitance) 14 serving as an electric energy storage unit through a power 
drive unit 13. 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. 
The control system also has an engine control unit 11 for controlling the 
engine 1, an electric motor control unit 12 for controlling the electric 
motor 3, an energy distribution control unit 15 for carrying out energy 
management based on a determined status of the ultracapacitor 14, and a 
transmission control unit 16 for controlling the transmission mechanism 4. 
The engine control unit 11, the electric motor control unit 12, the energy 
distribution control unit 15, and the transmission control unit 16 are 
connected to each other through a data bus 21 for exchanging detected 
data, flags, and other information. 
FIG. 2 shows the engine 1, the engine control unit 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 of the throttle valve 103 and supplying the 
generated electric signal to the engine control unit 11. A throttle 
actuator 105 for electrically controlling the opening of the throttle 
valve 103 is coupled to the throttle valve 103. The throttle actuator 105 
is controlled for its operated by the engine control unit 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 engine control 
unit 11, which applies signals to the fuel injection valves 106 to control 
times to open and close the fuel injection valves 106. 
An intake pipe absolute pressure (Pba) sensor 108 is connected to the 
intake pipe 102 through a pipe 107 immediately downstream of the throttle 
valve 103. The intake pipe absolute pressure sensor 108 generates an 
electric signal representative of an absolute pressure in the intake pipe 
102, and supplies the generated signal to the engine control unit 11. 
An intake temperature sensor 109 is mounted on the intake pipe 102 
downstream of the intake pipe absolute pressure sensor 108. The intake 
temperature sensor 109 generates an electric signal representative of the 
temperature of intake air flowing in the intake pipe 102 and supplies the 
generated signal to the engine control unit 11. An engine coolant 
temperature sensor 110, which may comprises a thermistor or the like, is 
mounted on the cylinder block of the engine 1. The engine coolant 
temperature sensor 110 generates an electric signal representative of the 
engine coolant temperature and supplies the generated signal to the engine 
control unit 11. 
An engine rotational speed (NE) sensor 111 is mounted near a camshaft or 
crankshaft (not shown) 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 to the engine control unit 11. 
The engine 1 has ignition plugs 113 positioned at the respective cylinders 
and electrically connected to the engine control unit 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 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 (and the shortage of oxygen) in the exhaust gases, and supplies the 
generated signal to the engine control unit 11. The air-fuel ratio sensor 
117 can detect the air-fuel ratio of an air-fuel mixture supplied to 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 the temperature thereof. The catalyst 
temperature sensor 118 supplies an electric signal representative of the 
detected temperature to the engine control unit 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 (hereinafter referred to 
as an "accelerator opening") .theta.ap of the accelerator pedal are 
electrically connected to the engine control unit 11. Electric signals 
generated by the vehicle speed sensor 119 and the accelerator opening 
sensor 120 are supplied to the engine control unit 11. 
A sensor 112 is mounted on the internal combustion engine 1 for generating 
a pulse each time the crankshaft turns through a predetermined angle. A 
pulse signal generated by the sensor 112 is supplied to the engine control 
unit 11, which identifies an engine cylinder into which fuel is to be 
injected, based on the supplied pulse signal. 
The engine control unit 11 comprises an input circuit for shaping the 
waveforms of input signals from the above various sensors, correcting the 
voltage levels thereof into predetermined levels, and converging 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 and 
the ignition plugs 113. The other control units including the electric 
motor control unit 12, the energy distribution control unit 15, and the 
transmission control unit 16 are structurally similar to the engine 
control unit 11. 
FIG. 3 shows a connected arrangement of the electric motor 3, the power 
drive unit 13, the ultracapaci-tor 14, the electric motor control unit 12, 
and the energy distribution control unit 15. 
As shown in FIG. 3, the electric motor 3 is associated with an electric 
motor rotational speed sensor 202 for detecting the rotational speed of 
the electric motor 3. An electric signal generated by the electric motor 
rotational speed sensor 202 as representing the rotational speed of the 
electric motor 3 is supplied to the electric motor control unit 12. The 
power drive unit 13 and the electric motor 3 are interconnected by wires 
connected to a current-voltage sensor 201 which detects a voltage and a 
current supplied to or outputted from the electric motor 3. A temperature 
sensor 203 for detecting the temperature of the power drive unit 13, more 
specifically, the temperature TD of a protective resistor of a drive 
circuit for the electric motor 3, is mounted on the power drive unit 13. 
Detected signals from the sensors 201, 203 are supplied to the electric 
motor control unit 12. 
The ultracapacitor 14 and the power drive unit 13 interconnected by wires 
connected to a current-voltage sensor 204 for detecting a voltage across 
the ultracapacitor 14 and a current outputted from or supplied to the 
ultracapacitor 14. A detected signal from the current-voltage sensor 204 
is supplied to the energy distribution control unit 15. 
FIG. 4 shows a connected arrangement of the transmission mechanism 4 and 
the transmission control unit 16. The transmission mechanism 4 is 
associated with a gear position sensor 301 for detecting a gear position 
of the transmission mechanism 4. A detected signal from the gear position 
sensor 301 is supplied to the transmission control unit 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 transmission control 
unit 16 to change gear positions of the transmission mechanism 4. 
FIGS. 5 and 6 shows a processing sequence for calculating an output power 
to be generated by the electric motor 3 based on a demand drive power, 
i.e., a drive power which the drive of the hybrid vehicle demands, and 
determining output power distributions for the electric motor 3 and the 
engine 1 with respect to the demand drive power. The processing sequence 
shown in FIGS. 5 and 6 is executed by the energy distribution control unit 
15 in each periodic cycle. 
In FIG. 5, the energy distribution control unit 15 detects a remaining 
capacity of the ultracapacitor 14 in STEP1. Specifically, the energy 
distribution control unit 15 integrates an output current from the 
ultracapacitor 14 and an output current (charging current) to the 
ultracapacitor 14 at each periodic interval, and calculates an integrated 
discharged value CAPdis (positive value) and an integrated charged value 
CAPchg (negative value). The energy distribution control unit 15 then 
calculates a remaining capacity CAPrem of the ultracapacitor 14 according 
to the following equation (1): 
EQU CAPrem=CAPful-(CAPdis+CAPchg) (1) 
where CAPful represents a dischargeable quantity when the ultracapacitor 14 
is fully charged. 
The energy distribution control unit 15 corrects the calculated remaining 
capacity CAPrem based on an internal resistance of the ultracapacitor 14 
which varies with temperature, etc., thereby determining a final remaining 
capacity of the ultracapacitor 14. 
Instead of calculating the remaining capacity of the ultracapacitor 14 as 
described above, the remaining capacity of the ultracapacitor 14 may be 
determined by detecting an open-circuit voltage across the ultracapacitor 
14. 
In STEP2, the energy distribution control unit 15 determines an output 
power distribution quantity for the electric motor 3, i.e., a drive power 
PRATIO to be generated by the electric motor 3, of a demand drive power 
POWERcom, using an output power distribution ratio table. The drive power 
PRATIO is expressed as a ratio to the demand drive power, and will 
hereinafter be referred to as a "distribution ratio PRATIO". 
FIG. 7 shows the output power distribution ratio table by way of example. 
The output power distribution ratio table is in the form of a graph having 
a horizontal axis which represents the remaining capacity of the 
ultracapacitor 14 and a vertical axis which represents the distribution 
ratio PRATIO. The output power distribution ratio table contains 
predetermined distribution ratios PRATIO with respect to remaining 
capacities, where the charging and discharging efficiency of the 
ultracapacitor 14 is maximum. 
In STEP3, the energy distribution control unit 15 determines a command 
(hereinafter referred to as a "throttle valve opening command") 
.theta.thCOM for the throttle actuator 105, corresponding to an 
accelerator opening .theta.ap detected by the accelerator opening sensor 
120, from an accelerator vs. throttle characteristic table shown in FIG. 
8. 
The accelerator vs. throttle characteristic table shown in FIG. 8 is in the 
form of a graph having a horizontal axis which represents the accelerator 
opening .theta.ap and a vertical axis which represents the throttle valve 
opening command .theta.thCOM. In FIG. 8, values of the accelerator opening 
.theta.ap are equal to corresponding values of the throttle valve opening 
command .theta.thCOM. However, values of the accelerator opening .theta.ap 
may be different from corresponding values of the throttle valve opening 
command .theta.thCOM. 
In STEP4, the energy distribution control unit 15 determines a distribution 
ratio PRATIOth for the electric motor 3 corresponding to the determined 
throttle valve opening command .theta.thCOM from a throttle vs. motor 
output power ratio table shown in FIG. 9. 
The throttle vs. motor output power ratio table shown in FIG. 9 is in the 
form of a graph having a horizontal axis which represents the throttle 
valve opening command .theta.thCOM and a vertical axis which represents 
the distribution ratio PRATIOth. In FIG. 9, the throttle vs. motor output 
power ratio table is established such that the output power, which is 
indicated by the distribution ratio PRATIOth, generated by the electric 
motor 3 is increased when the throttle valve opening command .theta.thCOM 
is 50 degrees or higher, for example. 
While the distribution ratio PRATIOth is determined depending on the 
throttle valve opening command .theta.thCOM in the illustrated embodiment, 
the distribution ratio PRATIOth may be determined depending on one or more 
parameters representing the vehicle speed, the engine rotational speed, 
etc. 
In STEP5, the energy distribution control unit 15 determines a demand drive 
power POWERcom depending on the throttle valve opening command 
.theta.thCOM and the engine rotational speed NE from a demand drive power 
map shown in FIG. 10. 
The demand drive power map shown in FIG. 10 is a map for determining a 
demand drive power POWERcom which the driver of the hybrid vehicle 
demands. The demand drive power map shown in FIG. 10 contains values of 
the demand drive power POWERcom depending on values of the throttle valve 
opening command .theta.thCOM and values of the engine rotational speed NE. 
Since the throttle valve opening command .theta.thcOM is in one-to-one 
correspondence to the accelerator opening .theta.ap in this embodiment, 
the accelerator opening .theta.ap may be used instead of the throttle 
valve opening command .theta.thCOM in the demand drive power map shown in 
FIG. 10. 
In STEP6, the energy distribution control unit 15 calculates a corrective 
term .theta.thADD for the throttle valve opening for generating the demand 
drive power POWERcom (.theta.thADD=.theta.thCOM-.theta.thi (previous 
throttle valve opening)). In STEP7, the energy distribution control unit 
15 determines a running status quantity VSTATUS depending on the vehicle 
speed Vcar detected by the vehicle speed sensor 119 and an extra output 
power POWERex of the engine 1 from a table for establishing running status 
quantities shown in FIG. 11. 
The extra output power POWERex of the engine 1 is calculated according to 
the following equation (2): 
EQU POWERex=POWERcom-RUNRST (2) 
where RUNRST represents a running resistance to the hybrid vehicle, which 
is determined depending on the vehicle speed Vcar from a RUNRST table (not 
shown). The demand drive power POWERcom and the running resistance RUSRST 
are given in the unit of kW (kilowatt), for example. 
The running status quantity VSTATUS determined by the vehicle speed Vcar 
and the extra output power POWERex corresponds to an assistive 
distribution ratio of the electric motor 3 with respect to the extra 
output power POWERex, and may be set to integral values (%) ranging from 0 
to 200. If the running status quantity VSTATUS is "0", then the hybrid 
vehicle is in a running status not to be assisted by the electric motor 3, 
i.e., the hybrid vehicle is decelerating or cruising. If the running 
status quantity VSTATUS is greater than "0", then the hybrid vehicle is in 
a running status to be assisted by the electric motor 3. 
In STEP8, the energy distribution control unit 15 decides whether the 
running status quantity VSTATUS is greater than "0" or not. If VSTATUS&gt;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&lt;0, i.e., if 
the hybrid vehicle is decelerating or cruising, then the hybrid vehicle 
enters a regenerative mode (i.e., a decelerating regenerative mode or a 
cruise charging mode), and control goes from STEP8 to STEP12 shown in FIG. 
6. 
In STEP9, the energy distribution control unit 15 calculates an electric 
motor output power POWERmot according to the following equation (3): 
EQU POWERmot=POWERcom.times.PRATIO.times.PRATIOth.times.VSTATUS(3) 
In STEP10, the energy distribution control unit 15 converts the electric 
motor output power POWERmot as a target with a time constant into an 
electric motor torque command TRQcom. 
FIG. 12 shows the relationship between the electric motor output power 
POWERmot and the electric motor torque command TRQcom. In FIG. 12, the 
solid-line curve illustrates the electric motor output power POWERmot as 
it changes with time, and the dotted-line curve illustrates the electric 
motor torque command TRQcom as it changes with time. 
As can be seen from FIG. 12, the electric motor torque command TRQcom is 
controlled so as to approach the electric motor output power POWERmot as a 
target with a time constant, i.e., with a time delay. If the electric 
motor torque command TRQcom were established such that the electric motor 
3 would generate the electric motor output power POWERmot immediately in 
response to the electric motor torque command TRQcom, then since an 
increase in the output power of the engine 1 would be delayed, the engine 
1 would not be readied to accept the electric motor output power POWERmot 
immediately, with the result that the drivability of the hybrid vehicle 
would be impaired. It is necessary, therefore, to control the electric 
motor 3 to generate the electric motor output power POWERmot until the 
engine 1 becomes ready to accept the electric motor output power POWERmot. 
In STEP11, the energy distribution control unit 15 calculates a corrective 
quantity .theta.thASSIST for controlling a target value .theta.thO for the 
throttle valve opening in a valve closing direction, depending on the 
electric motor torque command TRQcom. Thereafter, control goes from STEP11 
to STEP18. 
The corrective quantity .theta.thASSIST serves to reduce the output power 
of the engine 1 by an amount commensurate with the increase in the output 
power of the electric motor 3 responsive to the electric motor torque 
command TRQcom. The corrective quantity .theta.thASSIST is calculated for 
the following reasons: 
When the target value .theta.thO for the throttle valve opening is 
determined by the corrective term .theta.thADD calculated in STEP6 from 
the throttle valve opening command .theta.thCOM determined in STEP3 and 
the previous throttle valve opening .theta.thi, and the throttle actuator 
105 is controlled by the target value .theta.thO, the demand drive power 
POWERcom is generated solely from the output power of the engine 1. 
Therefore, if the output power of the engine 1 were controlled with the 
target value .theta.thO not corrected by the corrective quantity 
.theta.thASSIST, and the electric motor 3 were controlled by the electric 
motor torque command TRQcom converted in STEP10, the sum of the output 
power of the engine 1 and the output power of the electric motor 3 would 
exceed the demand drive power POWERcom, resulting in a drive power greater 
than the demand drive power demanded by the driver. To avoid this problem, 
the output power of the engine 1 is reduced by an amount commensurate with 
the output power of the electric motor 3, and the corrective quantity 
.theta.thASSIST is calculated such that the sum of the output power of the 
engine 1 and the output power of the electric motor 3 will be equalized to 
the demand drive power POWERcom. The target value .theta.thO for the 
throttle valve 103 is then determined 
(.theta.thO=.theta.thi+.theta.thADD-.theta.thASSIST), and the throttle 
valve 103 is controlled according to the target value .theta.thO for 
suppressing the output power of the engine 1. 
Furthermore, when the demand drive power POWERcom is smaller than the 
electric motor output power POWERmot, since the target value .theta.thO 
for the throttle valve 103 is equal to or smaller than the difference 
between the throttle valve opening command .theta.thCOM and the corrective 
quantity .theta.thASSIST (.theta.thO.ltoreq..theta.thCOM-.theta.thASSIST), 
the target value .theta.thO becomes nil (.theta.thO=0). The demand drive 
power POWERcom is all generated by the electric motor 3, and the output 
power of the engine 1 is kept at a zero level. 
When the remaining capacity of the ultracapacitor 14 is reduced thereby to 
reduce the output power of the electric motor 3, or the distribution ratio 
of the electric motor 3 based on the running status quantity VSTATUS is 
lowered, the corrective quantity .theta.thASSIST is calculated so as to 
increase the output power of the engine 1 to make up for the reduction in 
the electric motor output power POWERmot. The target value .theta.thO for 
the throttle valve 103 is then determined 
(.theta.thO=.theta.thi+.theta.thADD+.theta.thASSIST), and the output power 
of the engine 1 is controlled according to the target value .theta.thO for 
generating the demand drive power POWERcom. 
In response to a signal representing the target value .theta.thO 
corresponding to the calculated corrective quantity .theta.thASSIST from 
the engine control unit 11, the throttle actuator 105 controls the 
throttle valve 103 to control the output power of the engine 1 
independently of operation of the accelerator pedal. 
In STEP12, the energy distribution control unit 15 decides whether the 
present regenerative mode is the decelerating regenerative mode or the 
cruise charging mode. Specifically, the energy distribution control unit 
15 makes such a mode decision by deciding whether a change Dap 
(=.theta.apj (present value)-.theta.api (previous value) in the 
accelerator opening .theta.ap is smaller than a predetermined negative 
quantity DapD. Alternatively, the energy distribution control unit 15 may 
make such a mode decision based on the extra output power POWERex. 
If Dap&lt;DapD or POWERex&lt;0 in STEP12, then the energy distribution control 
unit 15 judges the present regenerative mode as the decelerating 
regenerative mode, and sets the electric motor output power POWERmot to a 
decelerating regenerative output power POWERreg in STEP13. The 
decelerating regenerative output power POWERreg is calculated according to 
a decelerating regenerative processing routine (not shown). 
In STEP14, the energy distribution control unit 15 reads an optimum target 
value .theta.thO for the throttle valve opening in the decelerating 
regenerative mode, i.e., an optimum target value .theta.thO for the 
throttle valve opening calculated in the decelerating regenerative 
processing routine. Thereafter, control proceeds to STEP19. 
If Dap.gtoreq.DapD or POWERex is nearly zero and VSTATUS=0 in STEP12, then 
the energy distribution control unit 15 judges the present regenerative 
mode as the cruise charging mode, and sets the electric motor output power 
POWERmot to a cruise charging output power POWERcrui in STEP15. The cruise 
charging output power POWERcrui is calculated according to a cruise 
charging processing routine (not shown). 
In STEP16, the energy distribution control unit 15 converts the electric 
motor output power POWERmot as a target with a time constant into an 
electric motor torque command TRQcom. In STEP17, the energy distribution 
control unit 15 calculates a corrective quantity .theta.thSUB for 
controlling a target value .theta.thO for the throttle valve opening in a 
valve opening direction, depending on the electric motor torque command 
TRQcom. 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 quantity .theta.thASSIST 
is calculated as described above. 
The electric motor output power POWERmot in the cruise charging mode has a 
sign opposite to the sign of the electric motor output power POWERmot in 
the assistive mode. Specifically, in the cruise charging mode, the 
electric motor 3 is controlled in a direction to reduce the demand drive 
power POWERcom because of the electric motor torque command TRQcom which 
is negative. In order to maintain the demand drive power POWERcom in the 
cruise charging mode, it is necessary to make up for the output power of 
the electric motor 3 reduced by the electric motor torque command TRQcom, 
with the output power of the engine 1. 
In STEP18, the energy distribution control unit 15 calculates the target 
value .theta.thO for the throttle valve 103 according to the following 
equation (4): 
EQU .theta.thO=.theta.thi+.theta.thADD-.theta.thSUB (4) 
In STEP19, the energy distribution control unit 15 decides whether or not 
the calculated target value .theta.thO is equal to or greater than a 
predetermined reference value .theta.thREF. If .theta.thO&lt;.theta.thREF, 
the energy distribution control unit 15 decides whether or not an intake 
pipe absolute pressure Pba is equal to or smaller than a predetermined 
reference value PbaREF in STEP20. 
If Pba&gt;PbaREF, then the processing sequence shown in FIGS. 5 and 6 is 
finished. If .theta.thO&gt;.theta.thREF in STEP19 or if Pba.ltoreq.PbaREF in 
STEP20, then the energy distribution control unit 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 finished. 
When control goes to STEP21, the remaining capacity of the ultracapacitor 
14 is reduced thereby to reduce the electric motor output power POWERmot, 
and the reduction in the electric motor output power POWERmot needs to be 
made up for by the engine 1, but the output power of the engine 1 cannot 
be increased anymore. 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 desired drivability of the 
hybrid vehicle. 
An engine control process carried out by the engine control unit 11 will be 
described below. 
FIG. 13 shows an overall engine control processing sequence, which is 
executed by the engine control unit 11 in each periodic cycle. 
In FIG. 13, the engine control unit 11 detects various engine operating 
parameters including the engine rotational speed NE, the intake pipe 
absolute pressure Pba. etc. in STEP131. Then, the engine control unit 11 
determines an engine operating status in STEP132, controls fuel to be 
supplied to the engine 1 in STEP133, and controls ignition timing of the 
engine 1 in STEP134. 
In STEP133, the engine control unit 11 calculates an amount of fuel to be 
supplied to the engine 1 depending on the read or calculated target value 
.theta.thO for the throttle valve opening. 
While the ultracapacitor is employed as the electric energy storage unit in 
the illustrated embodiment, the electric energy storage unit may instead 
comprise a battery. 
The throttle valve 103 whose opening is controlled by the electrically 
operated actuator 105 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 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. 
With the hybrid vehicle control system according to the present invention, 
a demand drive power is calculated depending on operating conditions of 
the hybrid vehicle, the remaining capacity of the electric energy storage 
unit is detected, an electric motor output power is calculated depending 
on the demand drive power and the remaining capacity, and the output power 
of the engine is corrected on the basis of the electric motor output power 
and the demand drive power. Therefore, when the electric motor output 
power is greater than the demand drive power, the output power of the 
engine can be reduced. Since the fuel consumption by the engine can be 
reduced without impairing the drivability of the hybrid vehicle, exhaust 
gases emitted from the engine can be reduced, and a voltage range and a 
capacity range of the electric energy storage unit for better charging and 
discharging efficiency can primarily be used. 
Inasmuch as the electric motor output power is calculated also in view of a 
running load on the hybrid vehicle, the amount of assistive power can be 
increased when the running load is high and reduced when the running load 
is low. Therefore, the running performance of the hybrid vehicle can be 
improved, and the fuel consumption by the engine can be reduced. 
When the demand drive power for the hybrid vehicle is smaller than the 
calculated electric motor output power, since the demand drive power is 
generated only by the electric motor, the fuel consumption by the engine 
can further be reduced. 
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.