HYBRID ELECTRIC VEHICLE

A hybrid electric vehicle including: (a) an engagement device disposed between an engine and an electric motor; (b) a transmission disposed between the electric motor and drive wheels; (c) an electric storage device configured to supply an electric power to the electric motor; and (d) a control apparatus. When the engine is to be started, the engagement device is engaged to transmit a torque from the electric motor to the engine, for thereby starting the engine. The control apparatus is configured to inhibit stop of the engine, when an outputtable electric power outputtable from the electric storage device is not larger than a threshold value. The threshold value is not smaller than a start-case-required electric power that is required to start the engine, such that a difference value between the threshold value and the start-case-required electric power is not larger than a predetermined value.

This application claims priority from Japanese Patent Application No. 2021-155966 filed on Sep. 24, 2021, the disclosure of which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a hybrid electric vehicle including an internal combustion engine, an electric motor and a frictional engagement device disposed between the internal combustion engine and the electric motor.

BACKGROUND OF THE INVENTION

There is well-known a hybrid electric vehicle including an internal combustion engine and an electric motor that serve as drive power sources for driving the vehicle. A hybrid electric vehicle disclosed in JP2005-45858A is an example of such a hybrid electric vehicle. This Japanese Patent Application Publication discloses that a maximum output outputtable by a battery is limited as a temperature of the battery is reduced.

SUMMARY OF THE INVENTION

By the way, there is proposed a hybrid electric vehicle including an internal combustion engine, an electric motor and a frictional engagement device disposed between the internal combustion engine and the electric motor. In such a hybrid electric vehicle, when the internal combustion engine is to be started, the electric motor is driven with use of an electric power supplied from a battery, whereby the internal combustion engine is cranked with a cranking torque transmitted from the electric motor through the frictional engagement device. In the hybrid electric vehicle, it becomes more difficult to enable an output required to start the internal combustion engine, to be covered by the electric motor, as a maximum output outputtable by the battery is reduced. Therefore, upon switching from a state in which the vehicle is driven by only the electric motor to another state in which the vehicle is driven by also the internal combustion engine, the output of the electric motor could be insufficient, thereby resulting in a risk of shock caused by the insufficiency of the output of the electric motor. It might be possible to employ an arrangement in which stop of the internal combustion engine is inhibited immediately if the maximum output of the battery is reduced. In this arrangement, although the shock is unlikely to be caused upon start of the internal combustion engine, a time of driving of the internal combustion engine is likely to be increased, thereby resulting in a risk of reduction of fuel economy.

The present invention was made in view of the background art described above. It is therefore an object of the present invention to provide a hybrid electric vehicle including an internal combustion engine, an electric motor and a frictional engagement device disposed between the internal combustion engine and the electric motor, wherein the hybrid electric vehicle is capable of suppressing a shock caused upon start of the internal combustion engine while suppressing reduction of fuel economy.

The object indicated above is achieved according to the following aspects of the present invention.

According to a first aspect of the invention, there is provided a hybrid electric vehicle comprising: (a) an internal combustion engine; (b) an electric motor; (c) drive wheels; (d) a frictional engagement device disposed between the internal combustion engine and the electric motor in a power transmission path through which a power is to be transmitted from the internal combustion engine to the drive wheels; (e) a transmission disposed between the electric motor and the drive wheels in the power transmission path; (f) an electric storage device configured to supply an electric power to the electric motor; and (g) a control apparatus, wherein, when the internal combustion engine is to be started, the frictional engagement device is engaged to transmit a torque from the electric motor to the internal combustion engine, for thereby starting the internal combustion engine, and wherein the control apparatus is configured to inhibit stop of the internal combustion engine, when an outputtable electric power outputtable from the electric storage device is not larger than a threshold value, the threshold value being not smaller than a start-case-required electric power that is required to start the internal combustion engine, such that a difference value between the threshold value and the start-case-required electric power is not larger than a predetermined value.

According to a second aspect of the invention, in the hybrid electric vehicle according to the first aspect of the invention, the threshold value includes an acceleration-case-required electric power required to accelerate the vehicle, when a running speed of the vehicle is in a range not higher than a predetermined speed value.

According to a third aspect of the invention, in the hybrid electric vehicle according to the first or second aspect of the invention, the threshold value is set to a value that is constant or increased as a running speed of the vehicle is increased.

According to a fourth aspect of the invention, in the hybrid electric vehicle according to any one of the first through third aspects of the invention, when the vehicle is being stopped, the threshold value is set to a value that is changed depending on a brake depressing force applied to a brake operating member of the vehicle, such that the threshold value is reduced as the brake depressing force is increased.

According to a fifth aspect of the invention, in the hybrid electric vehicle according to the fourth aspect of the invention, when a shift operation position, which is to be changed by a shift operation device of the vehicle, is a vehicle stop position, the threshold value is set to a minimum value that is irrespective of the brake depressing force.

According to a sixth aspect of the invention, in the hybrid electric vehicle according to any one of the first through fifth aspects of the invention, the start-case-required electric power, which is required to start the internal combustion engine, includes a first required electric power required to increase a rotational speed of the internal combustion engine by the electric motor, a second required electric power required to eliminate backlash present in the power transmission path, and a third required electric power that is to be consumed by auxiliary devices provided in the vehicle.

In the hybrid electric vehicle according to the first aspect of the invention, the stop of the internal combustion engine is inhibited when the outputtable electric power outputtable from the electric storage device is not larger than the threshold value that is not smaller than the start-case-required electric power required to start the internal combustion engine, so that it is possible to suppress the shock caused due to insufficiency of the electric power upon start of the internal combustion engine. Further, since the difference value between the threshold value and the start-case-required electric power is not larger than the predetermined value, it is possible to minimize a length of time for which the internal combustion engine is driven, and accordingly to suppress reduction of the fuel economy.

In the hybrid electric vehicle according to the second aspect of the invention, when the running speed of the vehicle is in the range not higher the predetermined speed value, the threshold value includes the acceleration-case-required electric power required to accelerate the vehicle, so that it is possible to ensure an acceleration performance in a low range of the running speed in which the acceleration performance is required.

In the hybrid electric vehicle according to the third aspect of the invention, the threshold value is set to the value that is constant or increased as the running speed of the vehicle is increased, so that it is possible to suppress frequent switch between start and stop of the internal combustion engine, even in a case in which the start-case-required electric power is fluctuated due to shift-down actions executed in the transmission.

In the hybrid electric vehicle according to the fourth aspect of the invention, when the vehicle is being stopped, the threshold value is changed depending on the brake depressing force, such that the threshold value is reduced as the brake depressing force is increased. Thus, the threshold value is set to a value appropriately dependent on the brake depressing force, so that the reduction of the fuel economy can be further suppressed.

In the hybrid electric vehicle according to the fifth aspect of the invention, when the shift operation position is the vehicle stop position, the threshold value is set to the minimum value that is irrespective of the brake depressing force, so that the internal combustion engine in unlikely to be driven whereby the reduction of the fuel economy can be further suppressed.

In the hybrid electric vehicle according to the sixth aspect of the invention, the start-case-required electric power, which is required to start the internal combustion engine, includes the first required electric power required to increase the rotational speed of the internal combustion engine by the electric motor, the second required electric power required to eliminate the backlash present in the power transmission path, and the third required electric power that is to be consumed by the auxiliary devices provided in the vehicle. Therefore, with the start-case-required electric power being calculated with the first, second and third required electric power being added to one another, it is possible to suppress the shock due to insufficiency of the electric power supplied to the electric power, the shock generated upon elimination of the backlash present in the power transmission path, and influence due to insufficiency of the electric power supplied to the auxiliary devices.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Hereinafter, there will be described preferred embodiment in detail with reference to the accompanying drawings. It is noted that figures of the drawings are simplified or deformed as needed, and each portion is not necessarily precisely depicted in terms of dimension ratio, shape, etc.

Embodiment

FIG.1is a view schematically showing a construction of a vehicle10to which the present invention is applied, for explaining major portions of control functions and control systems that are provided to perform various control operations in the vehicle10. As shown inFIG.1, the vehicle10is a hybrid electric vehicle including an engine12and an electric MG that serve as drive power source for driving the vehicle10. The vehicle10further includes drive wheels14and a power transmission apparatus16that is provided in a power transmission path between the engine12and the drive wheels14.

The engine12is a known internal combustion engine such as gasoline engine and diesel engine. The vehicle10is provided with an engine control device50that includes a throttle actuator, a fuel injection device and an ignition device. With the engine control device50being controlled by an electronic control apparatus90that is described below, an engine torque Te, which is an output torque of the engine12, is controlled.

The electric motor MG is a rotating electric machine having a function serving as a motor configured to generate a mechanical power from an electric power and a function serving as a generator configured to generate an electric power from a mechanical power. That is, the electric motor MG is a so-called “motor generator”. The electric motor MG is connected to a battery54provided in the vehicle10, through an inverter52provided in the vehicle10. The inverter52is controlled by the electronic control apparatus90whereby an MG torque Tm as an output torque of the electric motor MG is controlled. The MG torque Tm serves as a power driving torque when acting as a positive torque for acceleration, with the electric motor MG being rotated in a forward direction that is the same as a direction of rotation of the engine12during operation of the engine12. The MG torque Tm serves as a regenerative torque when acting as a negative torque for deceleration, with the electric motor MG being rotated in the forward direction. Specifically, the electric motor MG receives the electric power from the battery54through the inverter52, and generates the power for driving the vehicle10, in place of or in addition to the engine12. Further, the electric motor MG generates the electric power based on the power of the engine12or a driven power transmitted from the drive wheels14. The electric power generated by the electric motor MG is supplied to the battery54through the inverter52so as to be stored in the battery54. The battery54is an electric storage device to and from which the electric power is supplied from and to the electric motor MG. The electric power corresponds to an electric energy unless they are to be distinguished from each other. The power corresponds to a force or a torque unless they are to be distinguished from each other. It is noted that the battery54corresponds to “electric storage device” recited in the appended claims.

The power transmission apparatus16includes a casing18, a K0 clutch20, a torque converter22and an automatic transmission24. The K0 clutch20, torque converter22and automatic transmission24are disposed inside the casing18as a non-rotary member that is attached to a body of the vehicle10. The K0 clutch20is a hydraulically-operated frictional engagement device that is disposed between the engine12and the electric motor MG in the power transmission path between the engine12and the drive wheels14. The torque converter22is connected to the engine12through the K0 clutch20.

The automatic transmission24is connected to the torque converter22, and is disposed between the torque converter22and the drive wheels14in the power transmission path. The torque converter22and the automatic transmission24are disposed between the electric motor MG and the drive wheels14in the power transmission path. The power transmission apparatus16further includes a propeller shaft28connected to a transmission output shaft26that is an output rotary member of the automatic transmission24, a differential gear device30connected to the propeller shaft28, and a pair of drive shafts32connected to the differential gear device30. The power transmission apparatus16still further includes an engine connection shaft34connecting between the engine12and the K0 clutch20, and an electric-motor connection shaft36connecting between the K0 clutch20and the torque converter22. It is noted that the automatic transmission24corresponds to “transmission” recited in the appended claims.

The electric motor MG is connected to the electric-motor connection shaft36in a power transmittable manner in the casing18. The electric motor MG is disposed in the power transmission path between the engine12and the drive wheels14, so as to be connected to the engine12and the drive wheels14in a power transmittable manner, more specifically, the electric motor MG is disposed in a power transmission path between the K0 clutch20and the torque converter22, so as to be connected to the K0 clutch20and the torque converter22in a power transmittable manner. That is, the electric motor MG is connected to the torque converter22and the automatic transmission24without through the K0 clutch20in a power transmittable manner. From another point of view, the torque converter22and the automatic transmission24constitute respective parts of a power transmission path between the electric motor MG and the drive wheels14. The torque converter22and the automatic transmission24transmit a drive power of the engine12and/or a drive power of the electric motor MG to the drive wheels14.

The torque converter22includes a pump impeller22aconnected to the electric-motor connection shaft36, and a turbine impeller22bconnected to a transmission input shaft38that is an input rotary member of the automatic transmission24. The pump impeller22ais connected to the engine12through the K0 clutch20, and is connected directly to the electric motor MG. The pump impeller22ais an input member of the torque converter22, while the turbine impeller22bis an output member of the torque converter22. The electric-motor connection shaft36serves also as an input rotary member of the torque converter22. The transmission input shaft38serves also as an output rotary member of the torque converter22, which is formed integrally with a turbine shaft that is to be rotated by the turbine impeller22b. The torque converter22is a fluid-type transmission device, and is configured to transmit the drive powers of the drive power sources in the form of the engine12and the electric motor MG, to the transmission input shaft38, through fluid circulating in the torque converter22. The torque converter22includes an LU clutch40configured to connect between the pump impeller22aand the turbine impeller22b. The LU clutch40is a known lockup clutch serving as a direct connection clutch configured to connect between the input and output rotary members of the torque converter22.

The LU clutch40is configured to receive an LU hydraulic pressure PRIu that is a regulated hydraulic pressure supplied from a hydraulic control unit (hydraulic control circuit)56provided in the vehicle10, whereby an LU torque Tlu, i.e., torque capacity of the LU clutch40is changed and its control or operation state is switched. As the operation state of the LU clutch40, there are a fully released state in which the LU clutch40is fully released, a slipped state in which the LU clutch40is engaged with slipping, and a fully engaged state in which the LU clutch40is fully engaged.

The automatic transmission24is a known automatic transmission of a planetary gear type which includes at least one planetary gear device and a plurality of engagement devices CB. Each of the engagement devices CB is a hydraulically-operated frictional engagement device in the form of a multiple-disc type or a single-disc type clutch or brake that is to be pressed by a hydraulic actuator, or a band brake that is to be tightened by a hydraulic actuator. Each of the engagement devices CB is configured to receive a CB hydraulic pressure PRcb that is a regulated hydraulic pressure supplied from the hydraulic control unit56, whereby a CB torque Tcb, i.e., torque capacity of the engagement device CB is changed and its control or operation state is switched between an engaged state and a released state, for example. The plurality of engagement devices CB includes, for example, four clutches C1-C4 and two brakes B1, B2.

The automatic transmission24is a step-variable automatic transmission configured to establish a selected one of a plurality of gear positions, with a corresponding one or ones of the engagement devices CB (clutches C1-C4 and brakes B1, B2) being engaged, wherein the gear positions are different from each other in gear ratio (speed ratios) yat (= AT input rotational speed Ni / AT output rotational speed No). The automatic transmission24is configured to switch from one of the gear positions to another one of the gear positions, namely, to establish one of the gear positions which is selected, by the electronic control apparatus90, depending on, for example, a running speed V of the vehicle10and an accelerator opening degree (accelerator operation degree) θacc representing an amount of accelerating operation of an accelerator pedal42made by a driver (operator) of the vehicle10. A determination as to whether a shifting action is to be executed or not in the automatic transmission24may be made depending on, in place of or in addition to the accelerator opening degree θacc and the running speed V, a throttle opening degree θth or other values each correlated with the accelerator opening degree θacc and/or the AT output rotational speed No or other values each correlated with the running speed V The AT input rotational speed Ni is a rotational speed of the transmission input shaft38, and is an input rotational speed of the automatic transmission24. The AT input rotational speed Ni is also a rotational speed of the output rotary member of the torque converter22, and is equal to a turbine rotational speed Nt that is an output rotational speed of the torque converter22. Therefore, the AT input rotational speed Ni can be represented by the turbine rotational speed Nt. The AT output rotational speed No is a rotational speed of the transmission output shaft26, and is an output rotational speed of the automatic transmission24.

The K0 clutch20is disposed between the engine12and the electric motor MG, and is a wet-type or dry-type frictional engagement device constituted by a multiple-disc type or single-disc type clutch that is to be pressed by a hydraulic clutch actuator (not shown). With an operation state of the clutch actuator being controlled by the electronic control apparatus90, a control or operation state of the K0 clutch20is switched between an engaged state and a released state, for example. In the K0 clutch20, when a regulated K0 hydraulic pressure PRk0 is supplied from the hydraulic control unit56to the hydraulic clutch actuator, a K0 torque Tk0, which is a torque capacity of the K0 clutch20, is changed whereby the operation state of the K0 clutch20is switched. It is noted that the K0 clutch20corresponds to “frictional engagement device” recited in the appended claims.

When the K0 clutch20is engaged, the pump impeller22aand the engine12are to be rotated integrally with each other through the engine connection shaft34. That is, the K0 clutch20connects between the engine12and the drive wheels14in a power transmittable manner, when being engaged. On the other hand, when the K0 clutch20is released, transmission of a power between the engine12and the pump impeller22ais interrupted. That is, the K0 clutch20separates connection between the engine12and the drive wheels14, when being released. The K0 clutch20is disposed in the power transmission path between the engine12and the electric motor MG that is connected to the pump impeller22a, and serves as a clutch configured to cut off the power transmission path between the engine12and the electric motor MG, namely, to disconnect the engine12from the electric motor MG. That is, the K0 clutch20is a clutch configured to connect between the engine12and the electric motor MG, when being engaged, and to separate the connection between the engine12and the electric motor MG, when being released.

In the power transmission apparatus16, the power outputted from the engine12is transmitted, when the K0 clutch20is engaged, to the drive wheels14from the engine connection shaft34through sequentially the K0 clutch20, electric-motor connection shaft36, torque converter22, automatic transmission24, propeller shaft28, differential gear device30and drive shafts32, for example. Further, the power transmitted from the electric motor MG is transmitted, irrespective of the operation state of the K0 clutch20, to the drive wheels14from the electric-motor connection shaft36through sequentially the torque converter22, automatic transmission24, propeller shaft28, differential gear device30and drive shafts32, for example.

The vehicle10further includes an MOP58that is a mechanically-operated oil pump, an EOP60that is an electrically-operated oil pump, and a pump motor62. The MOP58is connected to the pump impeller22a, and is to be rotated and driven by the drive power source or sources (i.e., engine12and/or electric motor MG), so as to output a working fluid OIL that is to be used in the power transmission apparatus16. The pump motor62is a motor serving exclusively to rotate and drive the EOP60. The EOP60outputs the working fluid OIL, when being rotated and driven by the pump motor62. The working fluid OIL outputted by the MOP58and the EOP60is supplied to the hydraulic control unit56. The hydraulic control unit56, which receives the working fluid OIL as an original hydraulic pressure outputted from the MOP58and/or the EOP60, supplies regulated hydraulic pressures that serve as the CB hydraulic pressure PRcb, the K0 hydraulic pressure PRk0 and the LU hydraulic pressure PRlu, for example.

The vehicle10is provided with the electronic control apparatus90as a controller including the control apparatus that is configured to control running of the vehicle10, for example. The electronic control apparatus90includes a so-called microcomputer incorporating a CPU, a ROM, a RAM and an input-output interface. The CPU performs various control operations of the vehicle10, by processing various input signals, according to control programs stored in the ROM, while utilizing a temporary data storage function of the RAM. The electronic control apparatus90includes a plurality of ECUs such as an engine control ECU for controlling an output of the engine12and an electric-motor control ECU for controlling the electric motor MG.

The electronic control apparatus90receives various input signals based on values detected by respective sensors provided in the vehicle10. Specifically, the electronic control apparatus90receives: an output signal of an engine speed sensor70indicative of an engine rotational speed Ne that is a rotational speed of the engine12; an output signal of a turbine speed sensor72indicative of a turbine rotational speed Nt that is equal to the AT input rotational speed Ni; an output signal of an output speed sensor74indicative of the AT output rotational speed No corresponding to the vehicle running speed V; an output signal of an MG speed sensor76indicative of the motor rotational speed Nm; an output signal of an accelerator-opening degree sensor78indicative of the accelerator opening degree (accelerator operation degree) θacc representing the amount of accelerating operation made by the vehicle driver; an output signal of a throttle-opening degree sensor80indicative of a throttle opening degree θth which is an opening degree of an electronic throttle valve; an output signal of a brake switch82indicative of a signal representing a state in which a brake pedal is being operated by the vehicle driver so as to operate wheel brakes; an output signal of a battery sensor84indicative of a battery temperature THbat, a battery charging/discharging electric current Ibat and a battery voltage Vbat; an output signal of a fluid temperature sensor86indicative of a working-fluid temperature THoil that is a temperature of the working fluid OIL in the hydraulic control unit56; and an output signal of a shift position sensor88indicative of a shift operation position POSsh that is to be changed by a shift operation device64.

The electronic control apparatus90generates various output signals to the various devices provided in the vehicle10, such as: an engine control command signal Se that is to be supplied to the engine control device50for controlling the engine12, an MG control command signal Sm that is to be supplied to the inverter52for controlling the electric motor MG; a CB hydraulic control command signal Scb that is to be supplied to the hydraulic control unit56for controlling the operation states of the engagement devices CB; a K0 hydraulic control command signal Sko that is to be supplied to the hydraulic control unit56for controlling the K0 clutch20; an LU hydraulic control command signal Slu that is to be supplied to the hydraulic control unit56for controlling the operation state of the LU clutch40; and an EOP control command signal Seop that is to be supplied to the pump motor62for controlling the EOP60.

For performing various control operations in the vehicle10, the electronic control apparatus90includes a hybrid control means in the form of a hybrid control portion92, a clutch control means in the form of a clutch control portion94, and a shift control means in the form of a shift control portion96.

The hybrid control portion92has a function serving as an engine control means in the form of an engine control portion92afor controlling operation of the engine12and a function serving as an electric-motor control means in the form of an electric-motor control portion92bfor controlling operation of the electric motor MG through the inverter52, and executes a hybrid-drive control operation, for example, using the engine12and the electric motor MG through these control functions.

The hybrid control portion92calculates a requested drive amount of the vehicle10requested by the vehicle driver, by applying the accelerator opening degree θacc and the vehicle running speed V, for examples, to a requested drive amount map that represents a pre-stored relationship obtained by experimentation or determined by an appropriate design theory. The requested drive amount is, for example, a requested drive torque Trdem of the drive wheels14. From another point of view, the requested drive torque Trdem [Nm] is a requested drive power Proem [W] at the current vehicle running speed V As the requested drive amount, another value such as a requested drive force Frdem [N] of the drive wheels14and a requested AT output torque of the transmission output shaft26may be used, too. In the calculation of the requested drive amount, it is also possible to use, for example, the AT output rotational speed No in place of the vehicle running speed V

The hybrid control portion92calculates a target engine torque Tedem of the engine12and a target MG torque Tmdem of the electric motor MG, which cooperate with each other to realize the requested drive torque Trdem, by taking account of various factors such as a transmission loss, an auxiliary load, the gear ratio yat of the automatic transmission24and a maximum chargeable amount Win and a maximum dischargeable amount Wout of the battery54. The hybrid control portion92outputs an engine control command signal Se by which the engine12is caused to output the calculated target engine torque Tedem, and the outputted engine control command signal Se is supplied to the engine control device50. Further, the hybrid control portion92outputs an MG control command signal Sm by which the electric motor MG is caused to output the calculated target MG torque Tmdem, and the outputted MG control command signal Sm is supplied to the inverter52. The engine control command signal Se is, for example, a command value of an engine power Pe that is the power of the engine12outputting the target engine torque Tedem at the current engine rotational speed Ne. The MG control command signal Sm is, for example, a command value of a consumed electric power Wm of the electric motor MG outputting the target MG torque Tmdem at the current motor rotational speed Nm.

The maximum chargeable amount Win of the battery54is a maximum amount of the electric power that can be charged to the battery54, and represents a limitation of the electric power inputted to the battery54, namely, a limitation of input to the battery54. The maximum dischargeable amount Wout of the battery54is a maximum amount of the electric power that can be discharged from the battery54, and represents a limitation of the electric power outputted from the battery54, namely, a limitation of output of the battery54. The maximum chargeable and dischargeable amounts Win, Wout are calculated by the electronic control apparatus90, for example, based on the battery temperature THbat and a state-of-charge value SOC [%] of the battery54. The state-of-charge value SOC of the battery54is a value indicative of a charged state of the battery54, i.e., an amount of the electric power stored or remaining in the battery54, and is calculated by the electronic control apparatus90, for example, based on the charging/discharging electric current Ibat and the voltage Vbat of the battery54.

When the requested drive torque Trdem can be covered by only the output of the electric motor MG, the hybrid control portion92establishes a motor driving (= BEV driving) mode as a driving mode. When the BEV driving mode is established, the hybrid control portion92causes the vehicle10to perform a BEV driving (electric motor driving) with the K0 clutch20being released and with only the electric motor MG serving as the drive power source. On the other hand, when the requested drive torque Trdem cannot be covered without at least the output of the engine12, the hybrid control portion92establishes another driving mode that is an engine driving mode, i.e., a hybrid driving (= HEV driving) mode.

When the HEV driving mode is established, the hybrid control portion92causes the vehicle10to perform an engine driving, i.e., an HEV driving (hybrid driving) with the K0 clutch20being engaged and with the engine12and the electric motor MG serving as the drive power sources. Further, even when the requested drive torque Trdem can be covered by only the output of the electric motor MG, the hybrid control portion92establishes the HEV driving mode, for example, in a case in which the state-of-charge value SOC of the battery54becomes less than a predetermined engine-start threshold value or in a case in which the engine12or other component needs to be warmed up. The engine-start threshold value is a predetermined threshold value for determining that the state-of-charge value SOC reaches a level at which the engine12must forcibly be started for charging the battery54. Thus, the hybrid control portion92switches between the BEV driving mode and the HEV driving mode, based on, for example, the requested drive torque Trdem, by automatically stopping the engine12during the HEV driving, restarting the engine12after the stop of the engine12, and staring the engine12during the BEV driving.

The clutch control portion94controls the K0 clutch20, depending on the currently established driving mode. For example, when it is determined during the BEV driving that the HEV driving mode is to be established, the clutch control portion94executes a control operation for engaging the K0 clutch20so as to execute a control operation for starting the engine12. For example, when it is determined based on a running state of the vehicle10that start of the engine12is requested, the clutch control portion94outputs the K0 hydraulic control command signal Sko that is supplied to the hydraulic control unit56, wherein the K0 hydraulic control command signal Sko is for switching the K0 clutch20from the released state to the engaged state so as to obtain the K0 torque Tk0 that makes it possible to transmit a cranking torque Tcrn (i.e., torque required to increase the engine rotational speed Ne for cranking the engine12) to the engine12.

The shift control portion96determines whether a shifting action is to be executed in the automatic transmission24, by using, for example, a shifting map that represents a predetermined relationship, and outputs the CB hydraulic control command signal Scb, as needed, which is supplied to the hydraulic control unit56, for executing the shifting action in the automatic transmission24. In the shifting map, the predetermined relationship is represented by shifting lines in two-dimensional coordinates in which the vehicle running speed V and the accelerator opening degree θacc as two variables are taken along respective two axes, wherein the shifting lines are used for the determination as to whether the shifting action is to be executed in the automatic transmission24. In the shifting map, one of the two variables may be the AT output rotational speed No in place of the vehicle running speed V, and the other of the two variables may be any one of the requested drive force Frdem, requested drive torque Trdem and throttle opening degree θth in place of the accelerator opening degree θacc.

The hybrid control portion92further has a function serving as an engine-start control means in the form of an engine-start control portion92cconfigured to start the engine12. The engine-start control portion92ccontrols the engine12and the electric motor MG to execute an engine-start control operation for starting the engine12. For example, when determining that the starting of the engine12is requested, the engine-start control portion92cis configured, upon switching of the K0 clutch20to the engaged state, to supply, to the inverter52, the MG control command signal Sm requesting the electric motor MG to output the required cranking torque Tern. That is, when the engine12is to be started, the engine-start control portion92csupplies, to the inverter52, the MG control command signal Sm by which the electric motor MG is controlled to output the required cranking torque Tcrn, namely, by which the MG torque Tm is increased by an amount corresponding to the required cranking torque Tcrn. With the K0 clutch20being engaged, the cranking torque Tcrn outputted from the electric motor MG is transmitted to the engine12through the engaged K0 clutch20to the engine12whereby the engine rotational speed Ne is increased. Further, when the engine rotational speed Ne has been increased to a predetermined value, the engine-start control portion92ccauses the engine12to be started, by injecting fuel into a combustion chamber of the engine12and then igniting the fuel.

By the way, in a stage of start of the engine12, the maximum dischargeable amount Wout outputtable from the battery54is changed depending on the battery temperature THbat and the state-of-charge value SOC that corresponds to the amount of the electric power stored or remaining in the battery54.FIG.2is a view showing a relationship between the battery temperature THbat and the maximum dischargeable amount Wout. As shown inFIG.2, when the battery temperature THbat is not higher than a predetermined low threshold value TH1, the maximum dischargeable amount Wout is reduced as the battery temperature THbat is reduced. Further, when the battery temperature THbat is not lower than a predetermined high threshold value TH2, the maximum dischargeable amount Wout is reduced as the battery temperature THbat is increased. Still further, although being not shown, the maximum dischargeable amount Wout is reduced as the state-of-charge value SOC of the battery54is reduced.

When the engine12is started in a state in which the maximum dischargeable amount Wout has been reduced, the electric power available for starting the engine12becomes insufficient whereby the MG torque Tm (i.e., cranking torque Tcrn) of the electric motor MG becomes insufficient, so that there is a risk of shock (pull-in shock) caused due to insufficiency of the cranking torque Tcrn in process of starting the engine12. The shock can be avoided by inhibiting intermittent operation of the engine12in event of reduction of the maximum dischargeable amount Wout, for preventing the shock that could be caused in the process of starting the engine12. The intermittent operation of the engine12is an operation in which the engine12is allowed to be stopped and started during operation of the vehicle10.

The hybrid control portion92further has a function serving as an intermittent-operation-inhibition determining means in the form of an intermittent-operation-inhibition determining portion92dconfigured to determine whether the intermittent operation of the engine12is to be inhibited or not. The intermittent-operation-inhibition determining portion92dsets an intermittent-operation-inhibition threshold value α, which is a determination threshold value for determination as to whether the intermittent operation of the engine12is to be inhibited or not. Further, the intermittent-operation-inhibition determining portion92ddetermines whether the current maximum dischargeable amount Wout of the battery54is smaller than the intermittent-operation-inhibition threshold value α or not. The intermittent-operation-inhibition determining portion92ddoes not inhibit the intermittent operation of the engine12, when the maximum dischargeable amount Wout is larger than the intermittent-operation-inhibition threshold value α. That is, the intermittent operation of the engine12is allowed and enabled. It is noted that the intermittent-operation-inhibition threshold value α corresponds to “threshold value” recited in the appended claims.

On the other hand, when the maximum dischargeable amount Wout is not larger than the intermittent-operation-inhibition threshold value α, the intermittent-operation-inhibition determining portion92dinhibits the intermittent operation of the engine12. For example, in a case in which the engine12is in its driving state when the intermittent operation of the engine12is inhibited, the driving state of the engine12is maintained. In a case in which the engine12is in its stopped state when the intermittent operation of the engine12is inhibited, the engine12starts to be driven. Thus, the stop of the engine12is practically inhibited in a case in which the maximum dischargeable amount Wout becomes not larger than the intermittent-operation-inhibition threshold value α and the intermittent operation of the engine12is inhibited.

The intermittent operation of the engine12would be easily inhibited, if the intermittent-operation-inhibition threshold value α, which is the determination threshold value used for the determination as to whether the intermittent operation of the engine12is to be inhibited or not, were large. Consequently, it could be possible to avoid the shock caused upon start of the engine12by inhibiting the engine12from being stopped, but the inhibition of the stop of the engine12increases a time of driving of the engine12, resulting in a risk of reduction of fuel economy. On the other hand, it is possible to suppress the shock caused upon start of the engine12while suppressing the reduction of fuel economy, with the intermittent-operation-inhibition threshold value α being determined as described below.

Prior to determination of the intermittent-operation-inhibition threshold value α, a start-case-required electric power Wneed, which is required to start the engine12, is obtained. The start-case-required electric power Wneed is calculated by adding a cranking electric power Wne, a backlash-eliminating electric power Wgap and an auxiliary-devices electric power Wast to one another (Wneed=Wne+Wgap+Wast), wherein the cranking electric power Wne is an electric power required to increase the engine rotational speed Ne to the MG rotational speed Nm by the electric power MG during a coast running of the vehicle10, namely, required to enable the electric motor MG to generate the cranking torque Tcrn, the backlash-eliminating electric power Wgap is an electric power required to eliminate backlash (gap) between gears that constitute the power transmission path between the drive power sources (engine12and electric motor MG) and the drive wheels14when the vehicle10is switched from a driven state to the driving state, and the auxiliary-devices electric power Wast is an electric power to be consumed by auxiliary devices (such as an air conditioner) provided in the vehicle10. It is noted that the coast running is a running by inertia of the vehicle10with the accelerator pedal42being released from stepping by the vehicle driver. It is noted that the above-described a cranking electric power Wne, backlash-eliminating electric power Wgap and auxiliary-devices electric power Wast correspond to “first required electric power”, “second required electric power” and “third required electric power”, respectively, which are recited in the appended claims.

When the start-case-required electric power Wneed calculated upon start of the engine12is ensured, it is possible to prevent the shock caused due to insufficiency of the cranking torque Tcrn that is outputted from the electric motor MG when the engine rotational speed Ne is to be increased to the MG rotational speed Nm. Further, it is possible to suppress the shock caused when the backlash between the various gears provided in the power transmission apparatus16is eliminated. Still further, it is possible to suppress problem (such as performance reduction of the air conditioner) due to insufficiency of the electric power supplied to the air conditioner and the other auxiliary devices. Therefore, it is preferable the intermittent-operation-inhibition threshold value α is set to be not smaller than the start-case-required electric power Wneed, so that the start-case-required electric power Wneed is ensured upon start of the engine12.

On the other hand, where the intermittent-operation-inhibition threshold value α is made sufficiently larger than the start-case-required electric power Wneed, the start-case-required electric power Wneed can be reliably ensured upon start of the engine12. However, since the intermittent operation of the engine12is easily limited, a region in which the BEV driving can be practiced is practically reduced whereby the fuel economy is reduced. Therefore, the intermittent-operation-inhibition threshold value α is not smaller than a start-case-required electric power Wneed required to start the engine12such that a difference value δ (= |α -Wneed|) between the threshold value α and the start-case-required electric power Wneed is not larger than a predetermined value M. The predetermined value M is pre-obtained by an experimentation or an appropriate design theory, such that the predetermined value M is set to a value that makes it possible to ensure a certain degree of acceleration performance upon start of the engine12and to suppress reduction of the fuel economy which is caused by increase of the intermittent-operation-inhibition threshold value α.

FIG.3is a relationship map showing a relationship between the vehicle running speed V and the determined intermittent-operation-inhibition threshold value α. The relationship map ofFIG.3is pre-obtained by an experimentation or an appropriate design theory, and is stored in the electronic control apparatus90. InFIG.3, its horizontal axis represents the running speed V [km/h] while its vertical axis represents various values of the electric power such as the intermittent-operation-inhibition threshold value α [kW]. Further, inFIG.3, broken line represents the intermittent-operation-inhibition threshold value α used in the coast running, and two-dot chain line represents an intermittent-operation-allowance threshold value β that is a determination threshold value for determination as to whether the intermittent operation of the engine12is to be allowed or not, in a state in which the intermittent operation has been inhibited. As shown inFIG.3, the intermittent-operation-inhibition threshold value α and the intermittent-operation-allowance threshold value β are set such that a gap or hysteresis is provided between the intermittent-operation-inhibition threshold value α and the intermittent-operation-allowance threshold value β. Owing to provision of the hysteresis between the intermittent-operation-inhibition threshold value α and the intermittent-operation-allowance threshold value β, it is possible to prevent frequent switch between inhibition and allowance of the intermittent operation of the engine12and accordingly to prevent a so-called “hunting” that is frequent repeat of start and stop of the engine12.

The start-case-required electric power Wneed, which is represented by solid line inFIG.3, is calculated by adding the above-described cranking electric power Wne, backlash-eliminating electric power Wgap and auxiliary-devices electric power Wast to one another (Wneed = Wne+Wgap+Wast). As shown inFIG.3, the start-case-required electric power Wneed is increased generally in proportion with the running speed V During the coast running, the start-case-required electric power Wneed is fluctuated because the AT input rotational speed Ni of the automatic transmission24is temporarily increased each time when a shift-down action is executed in the automatic transmission24during the coast running. Thus, the AT input rotational speed Ni is temporarily increased each time when the shift-down action is executed in the automatic transmission24, and the cranking electric power Wne required to increase the engine rotational speed Ne to the MG rotational speed Nm is increased with increase of the AT input rotational speed Ni. The start-case-required electric power Wneed is fluctuated as a result of the shift-down actions executed during the coast running.

InFIG.3, the broken line represents the intermittent-operation-inhibition threshold value α that is set based on the start-case-required electric power Wneed represented by the solid line. The intermittent operation of the engine12is more easily inhibited as the intermittent-operation-inhibition threshold value α is set to be larger. Thus, by setting the intermittent-operation-inhibition threshold value α to a larger value, the electric power is more easily ensured thereby making it possible to avoid the shock that could be generated upon start of the engine12. However, with the intermittent-operation-inhibition threshold value α being set to a larger value, the engine12is less likely to be stopped whereby the fuel economy is reduced. On the other hand, by setting the intermittent-operation-inhibition threshold value α to a smaller value, the intermittent operation of the engine12is more easily allowed thereby making it possible to increase the fuel economy owing to increase of time of stop of the engine12. However, with the intermittent-operation-inhibition threshold value α being set to a smaller value, the shock could be more easily generated upon start of the engine12, due to insufficiency of the electric power. For example, if the intermittent-operation-inhibition threshold value α is set to a value smaller than the start-case-required electric power Wneed represented by solid line, there is a risk of the shock upon start of the engine12, due to insufficiency of the electric power. Therefore, it is preferable that the intermittent-operation-inhibition threshold value α is set to the same value as the start-case-required electric power Wneed or set to a value larger than the start-case-required electric power Wneed, as shown inFIG.3in which the intermittent-operation-inhibition threshold value α represented by the broken line is the same as or larger than the start-case-required electric power Wneed in all ranges of the running speed V When the intermittent-operation-inhibition threshold value α is larger than the start-case-required electric power Wneed, the difference value δ (= |α -Wneed|) between the intermittent-operation-inhibition threshold value α and the start-case-required electric power Wneed is not larger than the predetermined value M.

Where the intermittent-operation-inhibition threshold value α is set to the same value as the start-case-required electric power Wneed, the intermittent operation is allowed at least in a range in which the electric power required upon start of the engine12is ensured, whereby the shock is prevented upon start of the engine12. However, inFIG.3, in a range in which the start-case-required electric power Wneed is fluctuated due to the shift-down actions executed in the automatic transmission24, the maximum dischargeable amount Wout could frequently straddle the intermittent-operation-inhibition threshold value α, so that the inhibition and allowance of the intermittent operation of the engine12could be alternately repeated, thereby causing a risk of the above-described hunting that is frequent repeat of start and stop of the engine12. For avoiding such a hunting, the intermittent-operation-inhibition threshold value α is set to a value that is constant or increased as the running speed V is increased, without being reduced as the running speed V is increased, as represented by the broken line inFIG.3. In other words, the intermittent-operation-inhibition threshold value α is generally increased as the running speed V is increased, in all ranges of the running speed V Thus, even in the range in which the start-case-required electric power Wneed is fluctuated due to the shift-down actions executed in the automatic transmission24, there is no risk that maximum dischargeable amount Wout would frequently straddle the intermittent-operation-inhibition threshold value α, so that the above-described hunting is suppressed.

Further, when the vehicle10is to be re-accelerated from a low range of the running speed V in which the vehicle10is about to be stopped, a quick acceleration is desired and accordingly a sufficient acceleration performance is desired. Therefore, for ensuing the acceleration performance of the vehicle10when the vehicle10is to be accelerated from the low range of the running speed V, an acceleration-case-required electric power Wacc (that is required for the acceleration) is added to the intermittent-operation-inhibition threshold value α in the low range of the running speed V in which the running speed V is not higher than a predetermined speed value Vlow, as represented by one-dot chain line inFIG.3. That is, in the low range of the running speed V in which the running speed V is not higher than the predetermined speed value Vlow, the intermittent-operation-inhibition threshold value α is set to an intermittent-operation-inhibition threshold value αvlow in which the acceleration-case-required electric power Wacc is added to the start-case-required electric power Wneed. Thus, even during the BEV driving or upon start of the engine12in the low range of the running speed V, it is possible to ensure the electric power sufficiently for assuring the acceleration performance and to ensure the acceleration performance of the vehicle10. It is noted that the acceleration-case-required electric power Wacc required for the acceleration is pre-obtained by an experimentation or an appropriate design theory, such that the acceleration-case-required electric power Wacc is set to a value that makes it possible to provide the quick acceleration performance that is desired by the vehicle driver when the vehicle10is accelerated from the low range of the running speed V For example, as shown inFIG.3, the intermittent-operation-inhibition threshold value αvlow is set to a value that is obtained by adding the acceleration-case-required electric power Wacc to the start-case-required electric power Wneed when the running speed V is zero. Further, the above-described predetermined speed value Vlow as a threshold value of the low range of the running speed V in which the acceleration-case-required electric power Wacc is added, is also pre-obtained by an experimentation or an appropriate design theory, such that the low range of the running speed V defined by the predetermined speed value Vlow corresponds to a range of the running speed V in which the quick acceleration is required.

Further, when the vehicle10is stopped with the running speed V being zero, the intermittent-operation-inhibition threshold value α is changed depending on a brake depressing force Fbrk applied to a brake pedal44. When the running speed V is zero, it is considered that an intention of the vehicle driver to start running of the vehicle10is smaller as the brake depressing force Fbrk applied to the brake pedal44is larger. Therefore, the intermittent-operation-inhibition threshold value α is set to a value that is made smaller as the brake depressing force Fbrk applied to the brake pedal44is made larger.

FIG.4is a relationship map showing a relationship between the brake depressing force Fbrk and the intermittent-operation-inhibition threshold value α. The relationship map ofFIG.4is pre-obtained by an experimentation or an appropriate design theory, and is stored in the electronic control apparatus90. InFIG.4, its horizontal axis represents the brake depressing force Fbrk applied to the brake pedal44while its vertical axis represents the intermittent-operation-inhibition threshold value α when the running speed V is zero. As shown inFIG.4, when the brake depressing force Fbrk is larger than a predetermined value K, the intermittent-operation-inhibition threshold value α is smaller as the brake depressing force Fbrk is larger. Further, when the brake depressing force Fbrk is equal to the predetermined value K, the intermittent-operation-inhibition threshold value α is set to, for example, the above-described intermittent-operation-inhibition threshold value αvlow to which the intermittent-operation-inhibition threshold value α is set in the low range of the running speed V. The above-described predetermined value K is pre-obtained by an experimentation or an appropriate design theory, such that the predetermined value K corresponds to a threshold value of a range of the brake depressing force Fbrk in which it is presumed that the vehicle driver has an intention to stop the vehicle10, for example.

When the running speed V is zero, the start-case-required electric power Wneed, which is required upon start of the engine12, corresponds to a sum (= Wne+Wast) of the cranking electric power Wne (that is required to generate the cranking torque Tcrn) and the auxiliary-devices electric power Wast (that is required by the auxiliary devices such as the air conditioner). Therefore, a minimum value αmin of the intermittent-operation-inhibition threshold value α is set to, for example, the above-described sum (= Wne+Wast). Thus, when the running speed V is zero, the intermittent-operation-inhibition threshold value α is changeable within a range between the minimum value αminand the intermittent-operation-inhibition threshold value αvlow in the low range of the running speed V. Specifically described, the intermittent-operation-inhibition threshold value α, which is changeable within the range between the minimum value αminand the intermittent-operation-inhibition threshold value αvlow, is made smaller as the brake depressing force Fbrk applied to the brake pedal44is made larger. Further, by taking account of a possible case that the accelerator pedal42is depressed immediately after the brake pedal44is released from the stopped state of the vehicle10, the electric power required for the acceleration in such a possible case may be added to the intermittent-operation-inhibition threshold value α.

Further, when the shift operation position POSsh that is to be changed by the shift operation device64is a P position (parking position) as a vehicle stop position, the intermittent-operation-inhibition threshold value α is set to a minimum value (to which the intermittent-operation-inhibition threshold value α is to be set when the vehicle10is being stopped), irrespective of the brake depressing force Fbrk applied to the brake pedal44, so that the engine12is likely to be stopped. When the shift operation position Psh is the P position, the vehicle10is unlikely to be started to run, so that reduction of the fuel economy can be suppressed, with the intermittent-operation-inhibition threshold value α being reduced so as to cause the engine12to be easily stopped. The minimum value of the intermittent-operation-inhibition threshold value α, which is used when the vehicle10is being stopped with the shift operation position POSsh being the P position, may be, for example, the above-described minimum value αmin that is used when the brake depressing force Fbrk is sufficiently large.

The intermittent-operation-inhibition determining portion92ddetermines whether the vehicle10is in the coast running or not. When the vehicle10is in the coast running, the intermittent-operation-inhibition determining portion92dselects the intermittent-operation-inhibition threshold value α shown inFIG.3. On the other hand, when the vehicle10is not in the coast running, the intermittent-operation-inhibition determining portion92ddetermines whether the vehicle10is being stopped or not. When the vehicle10is not being stopped, the intermittent-operation-inhibition determining portion92dselects a normal-case intermittent-operation-inhibition threshold value α by which a higher priority is given to prevention of the shock upon start of the engine12, rather than to reduction of the fuel economy due to driving of the engine12. The normal-case intermittent-operation-inhibition threshold value α is set to a value sufficiently larger than the intermittent-operation-inhibition threshold value α that is shown inFIG.3.

On the other hand, when the vehicle10is being stopped, the intermittent-operation-inhibition determining portion92dsets the intermittent-operation-inhibition threshold value α, depending on the shift operation position Psh and the brake depressing force Fbrk. The intermittent-operation-inhibition determining portion92ddetermines whether the shift operation position Psh is the P position or not. When the shift operation position Psh is the P position, the intermittent-operation-inhibition determining portion92dselects the intermittent-operation-inhibition threshold value α for a case in which the shift operation position Psh is the P position. For example, the minimum value αmin, which is used when the brake pedal44is largely depressed, is used as the intermittent-operation-inhibition threshold value α for the case in which the shift operation position Psh is the P position.

When the shift operation position Psh is a position other than the P position, the intermittent-operation-inhibition determining portion92ddetermines whether the vehicle driver is without an intention to accelerate the vehicle10or not, depending on whether the accelerator opening degree θacc (corresponding to an operating mount of the accelerator pedal42) is zero or not. When the accelerator opening degree θacc is larger than zero, the intermittent-operation-inhibition determining portion92ddetermines that the vehicle driver has the intention to accelerate the vehicle10, and selects the above-described normal-case intermittent-operation-inhibition threshold value α. On the other hand, when the accelerator opening degree θacc is zero, the intermittent-operation-inhibition determining portion92ddetermines that the vehicle driver does not have the intention to accelerate the vehicle10, and sets the intermittent-operation-inhibition threshold value α depending on the brake depressing force Fbrk. The intermittent-operation-inhibition determining portion92ddetermines whether the brake depressing force Fbrk is equal or larger than the predetermined value K. When the brake depressing force Fbrk is not smaller than the predetermined value K, the intermittent-operation-inhibition determining portion92dsets the intermittent-operation-inhibition threshold value α, in accordance with the relationship map shown inFIG.4. On the other hand, when the brake depressing force Fbrk is smaller than the predetermined value K, the intermittent-operation-inhibition determining portion92dselects the normal-case intermittent-operation-inhibition threshold value α.

After setting the intermittent-operation-inhibition threshold value α, the intermittent-operation-inhibition determining portion92ddetermines whether the current maximum dischargeable amount Wout is smaller than the set intermittent-operation-inhibition threshold value α or not. When the maximum dischargeable amount Wout is smaller than the set intermittent-operation-inhibition threshold value α, the intermittent-operation-inhibition determining portion92dinhibits the intermittent operation of the engine12. When the maximum dischargeable amount Wout is not smaller than the intermittent-operation-inhibition threshold value α, the intermittent-operation-inhibition determining portion92ddoes not inhibit the intermittent operation of the engine12.

FIG.5is a flow chart showing a main part of a control routine executed by the electronic control apparatus90, namely, a control routine that is executed for setting the intermittent-operation-inhibition threshold value α that makes it possible to suppress reduction of the fuel economy and to prevent the shock caused by insufficiency of the electric power upon start of an engine12. This control routine is executed in a repeated manner during operation of the vehicle10.

The control routine is initiated with step S10corresponding to control function of the intermittent-operation-inhibition determining portion92d, which is implemented to determine whether the vehicle10is in the coast running or not. This determination as to whether the vehicle10is in the coast running or not is made depending on, for example, the running speed V and the accelerator opening degree θacc. When an affirmative determination is made at step S10, step S10is followed by step S20corresponding to control function of the intermittent-operation-inhibition determining portion92d, which is implemented to set the intermittent-operation-inhibition threshold value α, in accordance with the relationship map ofFIG.3that is used during the coast running. On the other hand, when a negative determination is made at step S10, the control flow goes to step S30corresponding to control function of the intermittent-operation-inhibition determining portion92d, which is implemented to determine whether the vehicle10is being stopped or not. When a negative determination is made at step S30, the control flow goes to step S80that is implemented to set the normal-case intermittent-operation-inhibition threshold value α.

When an affirmative determination is made at step S30, step S40corresponding to control function of the intermittent-operation-inhibition determining portion92dis implemented to determine whether the shift operation position Psh is the P position or not. When an affirmative determination is made at step S40, the control flow goes to step S70corresponding to control function of the intermittent-operation-inhibition determining portion92d, which is implemented to set the intermittent-operation-inhibition threshold value α for the case in which the shift operation position Psh is the P position. When the shift operation position Psh is the P position, the above-described sum (= Wne+Wast) of the cranking electric power Wne (used for cranking the engine12) and the auxiliary-devices electric power Wast (required by the auxiliary devices) is used as the intermittent-operation-inhibition threshold value α, for example.

When a negative determination is made at step S40, step S50corresponding to control function of the intermittent-operation-inhibition determining portion92dis implemented to determine whether the vehicle driver is without the intention to accelerate the vehicle10. This determination regarding the vehicle driver’s invention for the acceleration is made, for example, depending on whether the accelerator opening degree θacc is zero (or a value close to zero) or not. When a negative determination is made at step S50, the control flow goes to step S80that is implemented to set the normal-case intermittent-operation-inhibition threshold value α.

When an affirmative determination is made at step S50, step S60corresponding to control function of the intermittent-operation-inhibition determining portion92dis implemented to determine whether the brake depressing force Fbrk is equal to or larger than the predetermined value K. When it is determined at step S60that the brake depressing force Fbrk is smaller than the predetermined value K, the control flow goes to step S80that is implemented to set the normal-case intermittent-operation-inhibition threshold value α. When it is determined at step S60that the brake depressing force Fbrk is not smaller than the predetermined value K, step S70is implemented to set the intermittent-operation-inhibition threshold value α that is dependent on the brake depressing force Fbrk.

As described above, in the present embodiment, the intermittent operation of the engine12is inhibited, when the maximum dischargeable amount Wout, which is the maximum amount of the electric power that can be discharged from the battery54, is not larger than the intermittent-operation-inhibition threshold value α that is not smaller than the start-case-required electric power Wneed required upon start of the engine12, so that it is possible to suppress the shock caused due to insufficiency of the electric power upon start of the engine12. Further, since the difference value δ between the intermittent-operation-inhibition threshold value α and the start-case-required electric power Wneed is not larger than the predetermined value M, it is possible to minimize a length of time for which the engine12is driven, and accordingly to suppress reduction of the fuel economy.

In the present embodiment, when the vehicle running speed V is in the range not higher the predetermined speed value Vlow, the intermittent-operation-inhibition threshold value α includes the acceleration-case-required electric power Wacc required to accelerate the vehicle10, so that it is possible to ensure the acceleration performance in the low range of the running speed V in which the acceleration performance is required. Further, the intermittent-operation-inhibition threshold value α is set to the value that is constant or increased as the running speed V is increased, so that it is possible to suppress frequent switch between start and stop of the engine12, even in a case in which the start-case-required electric power Wneed is fluctuated due to the shift-down actions executed in the automatic transmission24. Further, when the vehicle10is being stopped, the intermittent-operation-inhibition threshold value α is changed depending on the brake depressing force Fbrk, such that the intermittent-operation-inhibition threshold value α is reduced as the brake depressing force Fbrk is increased. Thus, the intermittent-operation-inhibition threshold value α is set to a value appropriately dependent on the brake depressing force Fbrk, the reduction of the fuel economy can be further suppressed. Further, when the shift operation position Psh is the P position, the intermittent-operation-inhibition threshold value α is set to the minimum value αmin which is determined for when the vehicle10is being stopped and which is irrespective of the brake depressing force Fbrk, so that the engine12in unlikely to be driven whereby the reduction of the fuel economy can be further suppressed.

While the preferred embodiment of this invention has been described in detail by reference to the drawings, it is to be understood that the invention may be otherwise embodied.

For example, in the above-described embodiment, the intermittent-operation-inhibition threshold value α, which is set during the coast running, is set to the same value as the start-case-required electric power Wneed or to a value close to the start-case-required electric power Wneed within a range in which the hunting is not caused, except for the low range of the vehicle running speed V. However, by taking account of a possible case that the vehicle10is re-accelerated from a state of the coast running, the electric power required for the re-acceleration in such a possible case may be added to the intermittent-operation-inhibition threshold value α. That is, the electric power required for the acceleration may be added to the intermittent-operation-inhibition threshold value α, not only when the running speed V is in the low range but also when the running speed V is in other range other than the low range.

In the above-described embodiment, in the low range of the vehicle running speed V in which the running speed V is not higher than the predetermined speed value Vlow, the intermittent-operation-inhibition threshold value α is set to the intermittent-operation-inhibition threshold value αvlow in which the acceleration-case-required electric power Wacc is added to the start-case-required electric power Wneed. However, the acceleration-case-required electric power Wacc does not necessarily added to the start-case-required electric power Wneed, so that the intermittent-operation-inhibition threshold value α may be reduced so as to be changed along with the start-case-required electric power Wneed, as represented by broken line in the low range of the vehicle running speed V inFIG.3.

In the above-described embodiment, when the brake depressing force Fbrk is not smaller than the predetermined value K in a state in which the vehicle10is being stopped, the intermittent-operation-inhibition threshold value α is reduced as the brake depressing force Fbrk is increased. However, this arrangement is not essential. For example, in the state in which the vehicle10is being stopped, the intermittent-operation-inhibition threshold value α may be set to the predetermined minimum value αmin when the brake depressing force Fbrk is not smaller than a predetermined value, and may be set to the intermittent-operation-inhibition threshold value αvlow when the brake depressing force Fbrk is smaller than the predetermined value. Thus, the intermittent-operation-inhibition threshold value α may be changed in a step manner, depending on the brake depressing force Fbrk. In this case, too, the intermittent-operation-inhibition threshold value α is reduced as the brake depressing force Fbrk is increased.

In the above-described embodiment, when the shift operation position Psh is the P position, the sum of the cranking electric power Wne and the auxiliary-devices electric power Wast is used as the intermittent-operation-inhibition threshold value α. However, the intermittent-operation-inhibition threshold value α, when the shift operation position Psh is the P position, may be set to a value that is other than the sum of the cranking electric power Wne and the auxiliary-devices electric power Wast.

In the above-described embodiment, the automatic transmission24is the step-variable automatic transmission including the at least one planetary gear device and the plurality of engagement devices CB. However, this arrangement is not essential. For example, the automatic transmission24may be also a known belt-type continuously variable transmission or a synchronous mesh twin shaft parallel axis-type automatic transmission including a known DCT (Dual Clutch Transmission).

It is to be understood that the embodiment described above is given for illustrative purpose only, and that the present invention may be embodied with various modifications and improvements which may occur to those skilled in the art.

NOMENCLATURE OF ELEMENTS