United States Patent
`
`[19]
`
`[11] Patent Number:
`
`6,003,626
`
`Ibaraki et al.
`
`[45] Date of Patent:
`
`*Dec. 21, 1999
`
`US006003626A
`
`FOREIGN PATENT DOCUMENTS
`
`5—50865
`5—199605
`6—38304
`
`Japan .
`3/1993
`Japan .
`8/1993
`Japan .
`2/1994
`OTHER PUBLICATIONS
`
`Patent Abstracts of Japan, vol. 18, No. 335, (M—1627) Jun.
`24, 1994, JP—6080048, Mar. 22, 1994.
`Patent Abstracts of Japan, vol. 95, No. 3, JP7—067208, Mar.
`10, 1995.
`IEE Proceedings—D/Control Theory and Applications, vol.
`134, No. 6, pp. 373-387, Nov. 1987, J.R. Bumby, et al.,
`“Optimisation and Control of a Hybrid Electric Car”.
`
`Primary Examiner—J. J . Swann
`Assistant Examiner—Frank Vanaman
`
`Attorney, Agent, or Firm—Oblon, Spivak, McClelland,
`Maier & Neustadt, P.C.
`
`[57]
`
`ABSTRACT
`
`Hybrid motor vehicle drive system including an electric
`motor operated by an electric energy generated by an electric
`generator and stored in a storage device, an engine, and a
`controller for controlling the engine and electric generator to
`operate in an electricity generating drive mode when a
`predetermined condition is satisfied, such that the engine is
`operated so as to provide an output greater than a required
`power to drive the motor vehicle, so that the vehicle is driven
`by the engine with the required power while the electric
`generator is operated by the engine with a surplus power to
`charge the storage, and wherein the controller includes a
`special control device operated in the event of a failure of the
`electric generator, for inhibiting the selection of the elec-
`tricity generating drive mode and selecting an engine drive
`mode to operate the engine for driving the motor vehicle
`with the required power, even when the predetermined
`condition is satisfied.
`
`15 Claims, 4 Drawing Sheets
`
`[54] HYBRID DRIVE SYSTEM FOR MOTOR
`VEHICLE, HAVING MEANS FOR
`INHIBITING ELECTRICITY GENERATING
`DRIVE MODE
`
`[75]
`
`Inventors: Ryuji Ibaraki, Toyota; Yutaka Taga,
`Aichi-ken; Atsushi Tabata, Okazaki, all
`of Japan
`
`[73] Assignee: Toyota Jidosha Kabushiki Kaisha,
`Toyota, Japan
`
`[*] Notice:
`
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent
`term provisions of 35 U.S.C.
`154(a)(2).
`
`[21] Appl. No.: 08/725,710
`
`[22]
`
`Filed:
`
`Oct. 4, 1996
`
`[30]
`
`Foreign Application Priority Data
`
`Oct. 5, 1995
`
`[JP]
`
`Japan .................................. .. 7—258400
`
`Int. Cl.5 ..................................................... .. B60K 6/04
`[51]
`[52] U.S. Cl.
`........................................ .. 180/652; 180/65.4
`[58] Field of Search ................................ .. 180/65.2, 65.3,
`180/65.4, 65.6; 701/101
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`................. .. 180/65.2
`4,305,254 12/1981 Kawakatsu et al.
`4,335,429
`6/1982 Kawakatsu ........................... .. 180/65.2
`4,588,040
`5/1986 Albright, Jr. et al.
`.. 180/65.4
`5,343,970
`9/1994 Severinsky ...... ..
`180/65.2
`5,495,906
`3/1996 Furutani
`...... ..
`180/65.4
`5,670,830
`9/1997 Koga et al.
`.......................... .. 180/65.2
`5,697,466
`12/1997 Moroto et al.
`....................... .. 180/65.2
`5,927,415
`7/1999 Ibaraki et al.
`....................... .. 180/65.3
`
`
`
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`
`Page 1 of 11
`
`FORD 1453
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`Page 1 of 11
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`FORD 1453
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`

`
`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 1 014
`
`6,003,626
`
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`Page 2 of 11
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`FORD 1453
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`Page 2 of 11
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`FORD 1453
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`
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`
`
`
`
`
`
`
`
`
`

`
`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 2 of4
`
`6,003,626
`
`F|G.2
`
`
`
`READING 6Ac,
`Ne Ni, No, SOC,
`‘re AND TM
`
`
`
`
`S1
`
`S2
`
`FAILURE
`OF ELECTRIC MOTOR 14 ?
`
`
`
`
`
`
`NORMAL
`CONTROL
`ROUTINE
`
`SPEC|AL
`CONTROL
`ROUTINE
`
`84
`
`
`
`Page 3 of 11
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`FORD 1453
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`FORD 1453
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`

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`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 3 of4
`
`6,003,626
`
`NORMAL CONTROL ROUTINE
`
`
`
`F7IC5.E3
`
`
`
`CALCULATING
`REQUIRED
`POWER PL
`
`
`
`
`
`
`
`
`ELECTRICITY
`GENERATING
`fi%¥E
`3UB_ROUT|NE
`
`
`
`ENGINE -
`MOTOR
`fl@¥E
`SUBHOUTINE
`
`SUB—ROUTINE
`
`SUB—ROUTINE
`
` RETURN
`
`Page 4 of 11
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`FORD 1453
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`Page 4 of 11
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`FORD 1453
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`

`
`U.S. Patent
`
`Dec. 21, 1999
`
`Sheet 4 of4
`
`6,003,626
`
`
`
`SPECIAL CONTROL ROUTINE
`
`
`
`CALCULATING REQUIRED
`POWER PL
`
` S4-I
`
`OPERATING ENGINE WITH
`THE CALCULATED REQUIRED
` S4-2
`POWER PL
`
`
`RETURN
`
`
`
`DRIVETORQUE
`
`FIG5
`
`VEHICLE SPEED V
`
`Page 5 of 11
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`FORD 1453
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`FORD 1453
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`

`
`6,003,626
`
`1
`HYBRID DRIVE SYSTEM FOR MOTOR
`
`VEHICLE, HAVING MEANS FOR
`INHIBITING ELECTRICITY GENERATING
`DRIVE MODE
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates in general to a so-called
`hybrid drive system for driving a motor vehicle, which drive
`system includes an engine and an electric motor as two drive
`power sources. More particularly, the invention is concerned
`with an improvement of such a hybrid drive system which
`is operable in an electricity generating drive mode wherein
`an electric generator or dynamo is driven by a surplus power
`of the engine during running of the vehicle with the engine
`selected as the effective drive power source, so that an
`electric energy produced by the electric generator is stored
`in an electric energy storage device.
`2. Discussion of the Related Art
`
`JP-A-5-50865 discloses an example of such hybrid drive
`system including two drive power sources consisting of (a)
`an electric motor operated by an electric energy stored in an
`electric energy storage device, and (b) an engine operated by
`combustion of a fuel. The hybrid drive system is generally
`adapted such that the vehicle is run with the electric motor
`under a relatively small load, and is run with the engine
`under a relatively large load. The vehicle is run in an
`electricity generating drive mode when a predetermined
`running condition is satisfied, for example, when the amount
`of the electric energy stored in the electric energy storage
`device has been reduced below a predetermined threshold.
`In the electricity generating drive motor,
`the engine is
`operated by combustion of a fuel so as to provide an output
`which is larger than a required power necessary for running
`the vehicle, so that a suitable electric generator is driven by
`surplus power of the engine. The surplus power is equal to
`the overall engine output minus the required power just
`enough to run the vehicle. In this electricity generating drive
`motor, the vehicle is run by operation of the engine while the
`electric generator is driven by the surplus power of the
`engine to store electric energy in the electric energy storage
`device. Commonly, the electric motor provided as one of the
`two drive power sources is adapted to also function as the
`electric generator. In this case, the hybrid drive system does
`not require an exclusive electric generator or dynamo.
`In the event of some failure of the electric generator in the
`conventional hybrid drive system, the overall output of the
`engine produced in the electricity generating drive mode is
`used as the power to drive the motor vehicle. Since the
`overall output is greater than the power required to run the
`vehicle, the vehicle tends to be accelerated to an excessively
`larger extent than in the normal condition of the hybrid drive
`system. This excessive acceleration of the vehicle is not
`expected by the vehicle operator and is not desirable.
`
`SUMMARY OF THE INVENTION
`
`It is therefore an object of the present invention to provide
`a hybrid drive system for a motor vehicle, which does not
`suffer from an undesirable change in the running perfor-
`mance of the vehicle even in the event of a failure of the
`electric motor.
`
`The above object may be achieved according to the
`principle of the present invention, which provides a hybrid
`drive system for a motor vehicle, comprising: (a) an electric
`generator for generating an energy; (b) an electric energy
`
`10
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`2
`storage device for storing the electric energy generated by
`the electric generator; (c) an electric motor operated as a first
`drive power source by the electric energy stored in the
`electric energy storage device; (d) an engine operated as a
`second drive power source by combustion of a fuel; and (e)
`a controller for controlling the engine and the electric
`generator to operate in an electricity generating drive mode
`when a predetermined condition is satisfied, such that the
`engine is operated so as to provide an output greater than a
`required power necessary for driving the motor vehicle, so
`that the motor vehicle is driven by the engine with the
`required power while the electric generator is operated by
`the engine with a surplus power to charge the electric energy
`storage device, the surplus power being equal to the output
`minus the required power, wherein the controller includes
`special control means operated in the event of a failure of the
`electric generator, for inhibiting the selection of the elec-
`tricity generating drive mode and selecting an engine drive
`mode to operate the engine for driving the motor vehicle
`with the required power, even when the predetermined
`condition for selecting the electricity generating drive mode
`is satisfied.
`
`invention
`In the hybrid drive system of the present
`constructed as described above, the electricity generating
`drive mode is not selected in the event of a failure of the
`
`electric generator, even when the predetermined condition
`for selecting the electricity generating drive mode is satis-
`fied. In this event, the controller selects the engine drive
`mode in which the vehicle is driven by the engine with the
`required power just enough to run the vehicle. Thus, the
`acceleration value of the vehicle in the above event remains
`
`the same as in the normal state of the electric generator. That
`is,
`the present hybrid drive system does not suffer from
`excessive acceleration of the vehicle unexpected by the
`vehicle operator, even when the electric generator is defec-
`tive.
`
`In the electricity generating drive mode which is normally
`established when the predetermined condition is satisfied,
`the electric generator is operated by the surplus power of the
`engine, which is the overall output of the engine minus the
`required power used to drive the vehicle. The electric
`generator may be provided for the sole purpose of generat-
`ing the electric energy to be consumed by the electric motor.
`However, the electric motor may be adapted to also function
`as the electric generator. In this case, the special control
`means of the controller is activated at least when the electric
`
`motor fails to normally function as the electric generator
`(but is normally operated by the electric energy stored in the
`electric energy storage device).
`The special control means of the controller may be
`adapted to be activated to select the engine drive mode for
`driving the vehicle with the required power, at least when the
`electricity generating drive mode would be selected with the
`predetermined condition being satisfied if the electric gen-
`erator was normal. Described in detail,
`the hybrid drive
`system having the engine drive mode and the electricity
`generating drive mode may have other drive modes such as
`a motor drive mode in which only the electric motor is
`operated to drive the vehicle, and an engine~motor drive
`mode in which the engine and the electric motor are both
`operated to drive the vehicle. The engine drive mode is
`selected when the vehicle running load is relatively high,
`and the motor drive mode is selected when the vehicle
`
`running load is relatively high. The engine~motor drive
`mode is selected when the vehicle running load is consid-
`erably high. In this case, the special control means may be
`activated in the event of a failure of the electric generator, so
`
`Page 6 of 11
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`

`
`6,003,626
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`3
`as to replace at least the electricity generating drive mode
`with the engine drive mode. In other words,
`the special
`control means need not be activated when the engine drive
`mode, motor drive mode or engine~motor drive mode is
`selected by the controller. However,
`the special control
`means is desirably adapted to be activated when the motor
`drive mode or engine~motor drive mode is selected, as well
`as when the electricity generating drive mode is selected, in
`the case where the electric motor also functions as the
`
`electric generator. In this case, the failure of the electric
`generator may influence the operations of the hybrid drive
`system in the motor drive mode and the engine~motor drive
`mode as well as the operation in the electricity generating
`drive mode.
`
`The controller may further include means for determining
`whether the predetermined condition for selecting the elec-
`tricity generating drive mode is satisfied or not. This means
`may be adapted to determine whether the predetermined
`condition is satisfied or not whether an amount of the electric
`
`energy stored in the electric energy storage device is smaller
`than a predetermined lower limit.
`The controller may further include means for determining
`whether the electric generator fails to normally function due
`to a failure of the electric generator per se, and may further
`include means for determining whether the electric genera-
`tor fails to normally function due to a failure of a motor/
`generator control device which controls the electric motor
`and the electric generator.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The above and optional objects, features, advantages and
`technical and industrial significance of this invention will be
`better understood by reading the following detailed descrip-
`tion of a presently preferred embodiment of the invention,
`when considered in connection with the accompanying
`drawings, in which:
`FIG. 1 is a block diagram illustrating a hybrid drive
`system for a motor vehicle, which is constructed according
`to one embodiment of this invention;
`FIG. 2 is a flow chart for explaining a basic routine
`executed by the hybrid drive system of FIG. 1;
`FIG. 3 is a flow chart showing in detail a normal control
`routine in step S3 of the basic routine of FIG. 2, which is
`executed when the electric motor of the system is normal;
`FIG. 4 is a flow chart showing in detail a special control
`routine in step S4 of the basic routine of FIG. 2, which is
`executed in the event of a failure of the electric motor; and
`FIG. 5 is a graph for explaining threshold values B and C
`used in the normal control routine of FIG. 3.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENT
`
`Referring first to the block diagram of FIG. 1, there is
`diagrammatically illustrated a hybrid drive system 10 for
`driving a motor vehicle. In FIG. 1, thick solid lines indicate
`mechanical connection of components, while thin solid lines
`indicate electrical connection of components. The hybrid
`drive system 10 includes two drive power sources, namely,
`an internal combustion engine 12 such as a gasoline engine
`operated by combustion of a fuel, and an electric motor 14
`operated by an electric energy. Power of the engine 12 and
`the power of the electric motor 14 are simultaneously or
`selectively transmitted to a transmission 16, and transferred
`to drive wheels 20 of the vehicle through a speed reducing
`gear device and a differential gear device. The transmission
`
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`4
`16 includes a forward-reverse switching mechanism and a
`shift mechanism. The switching mechanism has three posi-
`tions: forward position (FWD) for running the vehicle in the
`forward direction; reverse position (REV) for running the
`vehicle in the rearward direction; and neutral position
`The shift mechanism has a plurality of forward-driving
`positions having respective different speed ratios, which are
`selectively established when the forward-reverse switching
`mechanism is placed in the forward position
`The
`transmission 16 is shifted by a shift actuator 24 so that the
`transmission 16 is placed in one of the neutral and reverse
`positions and the forward-driving positions, depending upon
`the currently selected position of a shift lever 22. Described
`in detail, the currently selected position of the shift lever 22
`is detected by a shift position switch 26. The shift actuator
`24 is controlled by a controller 28, according to a SHIFT
`POSITION signal received from the shift position switch 26
`indicative of the currently selected position of the shift lever
`22. The engine 12 and the transmission 16 are connected to
`each other through a clutch 30, which is engaged and
`released by a clutch actuator 32 under the control of the
`controller 28. Normally,
`the clutch 30 is placed in its
`engaged position.
`The electric motor 14 is connected to an electric energy
`storage device 36 such as a battery or condenser, through a
`motor/generator control device 34 (hereinafter referred to as
`“M/G control device 34”). The electric motor 14 is selec-
`tively placed in a DRIVE state, a CHARGING state, and a
`NON-LOAD state by the M/G control device 34 under the
`control of the controller 28. In the DRIVE state, the motor
`14 is driven by an electric energy supplied from the electric
`energy storage device 36. In the CHARGING state, the
`motor 14 functions as an electric generator or dynamo, with
`regenerative braking (braking torque electrically generated
`by the motor 14 itself), for storing an electric energy in the
`electric energy storage device 36. In the NON-LOAD state,
`the output shaft of the motor 14 is permitted to rotate freely.
`The engine 12 is controlled by various actuators including
`an actuator 42 for controlling a fuel
`injector valve, an
`actuator 44 for controlling a throttle valve, an actuator 46 for
`controlling an ignitor, and an actuator 48 for controlling
`intake and discharge valves. Like the M/G control device 34,
`these actuators 42, 44, 46, 48, are controlled by the control-
`ler 28. The electric energy storage device 36 is electrically
`connected to an electric motor 40 used for driving an
`optionally operated device 38 such as a compressor for an air
`conditioner.
`
`The controller 28 is principally constituted by a micro-
`computer incorporating a central processing unit (CPU), a
`random-access memory (RAM) and a read-only memory
`(ROM). The controller 28 performs data processing opera-
`tions to execute various control routines such as those
`
`illustrated in the flow charts of FIGS. 2-4, according to
`control programs stored in the ROM. The controller 28
`receives output signals of various detectors, which includes
`the SHIFT POSITION signal received from the shift posi-
`tion switch 26. Since an operation of the shift lever 22 can
`be detected on the basis of the SHIFT POSITION signal of
`the shift position switch 26, this signal permits determina-
`tion as to whether the transmission 16 is placed in a position
`in which engine brake is applied to the vehicle. The output
`signals of the detectors received by the controller 28 further
`include: a signal indicative of a rotating speed Ne of the
`engine 12; a signal indicative of a rotating speed Ni of the
`input shaft of the transmission 16 (i.e., rotating speed of the
`drive shaft of the electric motor 14); a signal indicative of a
`rotating speed No of the output shaft of the transmission 16
`
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`6,003,626
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`
`(which can be used to determine a running speed V of the
`vehicle); a signal indicative of an amount of electric energy
`SOC stored in the electric energy storage device 22; an
`ACCELERATOR POSITION signal
`indicative of an
`amount of operation SAC of an accelerator pedal; a BRAKE
`ON signal indicative of an operation of a brake pedal by the
`vehicle operator; and a BRAKE PEDAL FORCE signal
`indicative of a depression force acting on the brake pedal.
`The electric energy amount SOC can be obtained from an
`electric current or charging efficiency of the electric motor
`14 when the motor 14 is operated as the electric generator in
`the CHARGING state.
`
`Referring next to the flow chart of FIG. 2, a basic control
`routine executed by the present hybrid drive system 10 will
`be described. The routine is initiated with step S1 to read the
`amount of operation SAC of the accelerator pedal, the engine
`speed Ne, the input shaft speed Ni and output shaft No of the
`transmission 16, the amount of electric energy SOC stored
`in the electric energy storage device 36, a torque TE of the
`engine 12 and a torque TM of the electric motor 14. The
`engine torque TE may be calculated on the basis of the
`opening angle of the throttle valve, or the amount of fuel
`injection into the engine 12, for example. The motor torque
`TM may be calculated on the basis of an electric current of
`the electric motor 14, for example. Then, the control flow
`goes to step S2 to determine whether the electric motor 14
`fails to normally function as the drive power source (for
`driving the motor vehicle) and/or the electric generator 14
`(for generating an electric energy to be stored in the electric
`energy storage device 36). The failure of the electric motor
`or generator 14 includes a failure of the MG control device
`34, and other defects that prevent normal functioning of the
`electric motor or generator 14. The determination in step S2
`may be effected, for example, on the basis of a relationship
`between the motor torque TM (which is calculated from the
`electric current of the motor 14) and the actual rotating speed
`of the electric motor 14 (i.e., input shaft speed Ni of the
`transmission 16), or on the basis of a relationship between
`the engine speed Ne during operation of the electric motor
`14 as the electric generator and a selected one of the electric
`current of the motor 14, input shaft speed Ni and output shaft
`speed No. If a negative decision (NO) is obtained in step S3,
`the control flow goes to step S3 for executing a normal
`control routine illustrated in detail in the flow chart of FIG.
`
`3. If an affirmative decision (YES) is obtained in step S2,
`that is, if the electric motor 14 fails to normally function, the
`control flow goes to step S4 for executing a special control
`routine illustrated in the flow chart of FIG. 4.
`
`The normal control routine will be described by reference
`to the flow chart of FIG. 3. This routine is initiated with step
`S3-1 to calculate a required power PL necessary to drive the
`motor vehicle in the present running condition of the
`vehicle. This required power PL may be calculated based on
`the detected amount of operation SAC of the accelerator
`pedal or a rate of change of this amount SAC and the vehicle
`running speed V, for example, and according to a predeter-
`mined relationship between the required power PL and the
`amount SAC (or rate of change thereof) and vehicle running
`speed V). This relationship may be represented by an
`equation or data map stored in the ROM of the controller 28.
`Step S3-1 is followed by step S3-2 to determine whether the
`amount of electric energy SOC stored in the electric energy
`storage device 36 is equal to or larger than a predetermined
`lower limit A. If an affirmative decision (YES) is obtained in
`step S3-2,
`the control flow goes to step S3-3 and the
`following steps. If a negative decision (NO) is obtained in
`step S3-2, the control flow goes to step S3-8 to implement
`
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`an electricity generating drive mode sub-routine. The lower
`limit A is the minimum amount of the electric energy
`required to operate the electric motor 14 for driving the
`motor vehicle in an engine~motor drive mode or a motor
`drive mode in which the electric motor 14 is operated as the
`drive power source, as described below with respect to steps
`S3-5 and S3-7. The lower limit A is determined depending
`upon the charging and discharging efficiencies of the electric
`energy storage device 35. For example, the lower limit A is
`in the neighborhood of 70% of the full capacity of the device
`36. In the electricity generating drive mode sub-routine in
`step S3-8,
`the hybrid drive system 10 is operated in an
`electricity generating drive mode in which the engine 12 is
`operated so as to provide an output which is larger than the
`calculated required power PL, and the electric motor 14 is
`operated as the electric generator with the surplus power
`which is a difference between the engine output and the
`required power PL, so that the electric energy generated by
`the electric generator 14 is stored in the electric energy
`storage device 36. Described more specifically, the control-
`ler 28 controls the M/G control device 34 such that the
`surplus power of the engine is consumed by the electric
`generator 14 so that the calculated required power PL is
`transmitted from the engine 12 to the transmission 16 for
`driving the vehicle. The torque TE and speed Ne of the
`engine 12 and the electric generator 14 are controlled
`depending upon the currently selected position of the trans-
`mission 16 and the expected power loss of the system. It will
`be understood that a portion of the controller assigned to
`implement step S3-8 constitutes means for controlling the
`hybrid drive system 10 in the electricity generating drive
`mode. This mode is selected when the negative decision
`(NO) is obtained in step S3-2, namely, when the amount of
`electric energy SOC currently stored in the electric energy
`storage device 36 is smaller than the predetermined lower
`limit A.
`
`Step S3-3 implemented when the affirmative decision
`(YES) is obtained in step S3-2 is provided to determine
`whether the required power PL is larger than a predeter-
`mined first threshold value B. If an affirmative decision
`
`(YES) is obtained in step S3-3, the control flow goes to step
`S3-4 to determine whether the required power PL is larger
`than a predetermined second threshold value C which is
`larger than the first
`threshold value B. If the negative
`decision (NO) is obtained in step S3-3, that is, if the required
`power PL is equal to or smaller than the first threshold value
`B, it means that the motor vehicle is currently running under
`a relatively low load. In this case, the control flow goes to
`step S3-7 to implement a motor drive mode sub-routine. If
`an affirmative decision (YES) is obtained in step S3-3 while
`a negative decision (NO) is obtained in step S3-4, that is, if
`the required power PL is larger than the first threshold value
`B and is equal to or smaller than the second threshold value
`C, it means that the vehicle is currently running under a
`medium load. In this case, the control flow goes to step S3-6
`to implement an engine drive mode sub-routine. If an
`affirmative decision (YES) is obtained in step S3-4, that is,
`the required power PL is larger than the second threshold
`value C,
`it means that
`the vehicle is running under a
`relatively high load. In this case, the control flow goes to
`step S3-5 to implement an engine~motor drive mode sub-
`routine.
`
`In the motor drive mode sub-routine in step S3-7, the
`hybrid drive system 10 is operated in the motor drive mode
`indicated above with respect to the lower limit A. In the
`motor drive mode, only the electric motor 14 is operated as
`the drive power source for running the vehicle. In the engine
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`drive mode sub-routine in step S3-6, the hybrid drive system
`10 is operated in an engine drive mode in which only the
`engine 12 is operated as the drive power source for running
`the vehicle. In the engine~motor drive sub-routine in step
`S3-5,
`the hybrid drive system 10 is operated in the
`engine~motor drive mode indicated above with respect to the
`lower limit A. In the engine~motor drive mode, the engine 12
`and the electric motor 14 are both operated as the drive
`power sources for running the vehicle. In these drive modes
`in steps S3-5, S3-6 and S3-7, the outputs of the engine 12
`and electric motor 14 are controlled depending upon the
`currently selected position of the transmission 16 and the
`expected power loss. In the engine drive mode in step S3-6,
`the electric motor 14 is held in a non-load condition. In the
`
`motor drive mode in step S3-7, the clutch 30 is placed in the
`released state so that the output of the electric motor 14 is
`transmitted to only the transmission 16.
`Each of the first and second threshold values B and C may
`be determined depending upon the current running condition
`of the vehicle, for instance, on the basis of the vehicle drive
`torque and the vehicle speed V and according to a prede-
`termined relationship as shown in FIG. 5 by way of example.
`This relationship is provided for each of the forward-drive
`positions of the transmission 16. When the running condi-
`tion of the vehicle as represented by the drive torque and
`speed V is in an area on a lower load side of a curve
`representative of the first threshold B, namely, on the side
`nearer to the origin “0”, it means that the required power PL
`is equal to or smaller than the first threshold B. In this case,
`step S3-7 is implemented to execute the motor drive mode
`sub-routine. When the running condition is in an area
`between the curve representative of the first threshold B and
`a curve representative of the second threshold C, it means
`that the required power PL is larger than the first threshold
`B and is equal to or smaller than the second threshold C. In
`this case, step S3-6 is implemented to execute the engine
`drive mode sub-routine. When the running condition is in an
`area on a higher load side of the curve representative of the
`second threshold C, it means that the required power PL is
`larger than the second threshold C. In this case, step S3-5 is
`implemented to execute the engine~motor drive mode sub-
`routine. The above relationship may be determined to deter-
`mine the first threshold value B on the basis of the fuel
`
`consumption efficiency (amount of consumption of fuel per
`unit power) and emission gas ratio (amount of the emission
`gas per unit power) of the engine 12 and the energy
`conversion efficiency of the electric motor 14, for minimiz-
`ing the amount of fuel consumption and the amount of
`emission gas of the engine 12.
`The special control routine in step S4 of FIG. 2 which is
`executed in the event of a failure of the electric motor 14 will
`
`be described by reference to the flow chart of FIG. 4. This
`special control routine is initiated with step S4-1 to calculate
`a required power PL necessary to drive the motor vehicle in
`the present running condition of the vehicle, as in step S3-1
`of FIG. 3. Then, the control flow goes to step S4-2 to operate
`the engine 12 with the calculated required power PL for
`driving the vehicle, irrespective of the magnitude of the
`required power PL. In this case, too, the output of the engine
`12 is controlled depending upon the currently selected
`position of the transmission 16 and the expected power loss.
`It will be understood that when the electric motor 14 is not
`
`normally functioning, the hybrid drive system 10 is placed
`in the engine drive mode and the engine 12 is operated so as
`to provide the required power PL for driving the motor
`vehicle, regardless of the current running condition of the
`vehicle as represented by the drive torque and speed V, that
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`is, regardless of the current running load of the vehicle.
`Thus, the vehicle can be driven by the engine 12 with the
`required power PL, in a suitable fashion without excessive
`acceleration, even in the event of a failure of the electric
`motor 14.
`
`In other words, the hybrid drive system 10 would not be
`placed in the electricity generating drive mode even when
`the stored electric energy amount SOC has been reduced to
`the lower limit A, if some failure of the electric motor 14 is
`detected in step S2. In this case, the hybrid drive system 10
`is placed in the engine drive mode in the special control
`routine of FIG. 4 in which the vehicle is run with the engine
`12 being controlled so as to provide the required power PL,
`so that the acceleration value of the vehicle is substantially
`the same as in the normal state of the electric motor 14.
`
`Thus, the present hybrid drive system 10 adapted to execute
`the special control routine of FIG. 4 in the event of a failure
`of the electric motor 24 does not suffer from excessive
`
`acceleration of the vehicle as experienced in the conven-
`tional h