Patent ID: 12246695

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the disclosure will be described in detail with reference to the accompanying drawings. In the following embodiment, drawings are simplified or deformed where appropriate, and the scale ratio, shape, and the like of each component are not always drawn accurately.

FIG.1is a diagram that illustrates the schematic configuration of a hybrid electric vehicle10(hereinafter, vehicle10) to which the disclosure is applied and is a diagram that illustrates a relevant part of control functions and control system for various control in the vehicle10. As shown inFIG.1, the vehicle10is a hybrid electric vehicle including an engine12and an electric motor MG that function as a driving force source for propelling the vehicle10. The vehicle10includes a powertrain16provided in a driveline between each of the engine12and the electric motor MG and a pair of drive wheels14.

The engine12is a known internal combustion engine, such as a gasoline engine and a diesel engine. In the engine12, an electronic control unit90(described later) controls an engine controller50to control an engine torque Te that is an output torque of the engine12. The engine controller50includes a throttle actuator, a fuel injection device, an ignition device, and the like provided in the vehicle10.

The electric motor MG is a motor generator that has the function of a motor to generate mechanical power from electric power and the function of a generator to generate electric power from mechanical power. The electric motor MG is connected to a battery54via an inverter52. The inverter52and the battery54are provided in the vehicle10. When the inverter52is controlled by the electronic control unit90(described later), an MG torque Tm that is the output torque of the electric motor MG is controlled. When, for example, the rotation direction of the electric motor MG is a forward rotation direction that is the same rotation direction as that during operation of the engine12, the MG torque Tm is a power running torque for a positive torque on an accelerating side and is a regenerative torque for a negative torque on a decelerating side. The electric motor MG generates power for propelling the vehicle10by using electric power supplied from the battery54via the inverter52instead of the engine12or in addition to the engine12. The electric motor MG generates electric power by using the power of the engine12or driven force input from the drive wheels14. Electric power generated by power generation of the electric motor MG is stored in the battery54via the inverter52. The battery54is an electrical storage device that exchanges electric power with the electric motor MG. The electric power is synonymous with electric energy unless otherwise distinguished. The power is synonymous with torque and force unless otherwise distinguished.

The powertrain16includes a K0clutch20, a torque converter22, an automatic transmission24, and the like in a case18that is a non-rotating member secured to a vehicle body. The K0clutch20is a hydraulic friction engagement device provided between the engine12and the electric motor MG in the driveline between the engine12and the drive wheels14. The torque converter22is connected to the engine12via the K0clutch20.

The automatic transmission24is connected to the torque converter22and is inserted in the driveline between each of the engine12and the electric motor MG and the pair of drive wheels14. In other words, the torque converter22and the automatic transmission24each make up part of the driveline between each of the engine12and the electric motor MG and the pair of drive wheels14. The powertrain16includes a propeller shaft28, a differential gear30, a pair of drive shafts32, and other parts. The propeller shaft28is coupled to a transmission output shaft26that is an output rotating member of the automatic transmission24. The differential gear30is coupled to the propeller shaft28. The drive shafts32are coupled to the differential gear30. The powertrain16includes an engine coupling shaft34and an electric motor coupling shaft36. The engine coupling shaft34couples the engine12to the K0clutch20. The electric motor coupling shaft36couples the K0clutch20to the torque converter22.

The electric motor MG is coupled to the electric motor coupling shaft36in the case18such that power can be transmitted. The electric motor MG is coupled to the driveline between the engine12and the pair of drive wheels14, particularly, the driveline between the K0clutch20and the torque converter22, such that power can be transmitted. In other words, the electric motor MG is connected to the torque converter22and the automatic transmission24such that power can be transmitted, without intervening the K0clutch20. From another viewpoint, the torque converter22and the automatic transmission24each make up part of the driveline between the electric motor MG and the pair of drive wheels14. The torque converter22and the automatic transmission24each transmit driving force from each of the engine12and the electric motor MG to the drive wheels14.

The torque converter22includes a pump impeller22acoupled to the electric motor coupling shaft36and a turbine runner22bcoupled to a transmission input shaft38(an input shaft of a transmission in the disclosure) that is an input rotating member of the automatic transmission24. The pump impeller22ais coupled to the engine12via the K0clutch20and is directly coupled to the electric motor MG. The pump impeller22ais an input rotating member of the torque converter22. The turbine runner22bis an output rotating member of the torque converter22. The electric motor coupling shaft36is also the input rotating member of the torque converter22. The transmission input shaft38is also the output rotating member of the torque converter22, formed integrally with a turbine shaft driven for rotation by the turbine runner22b. The torque converter22is a fluid transmission device that transmits driving force from each of the driving force sources (the engine12and the electric motor MG) to the transmission input shaft38via fluid. The torque converter22includes a lockup clutch40(hereinafter, LU clutch40) that couples the pump impeller22ato the turbine runner22b. The LU clutch40is a known disconnect clutch that connects or disconnects the input and output rotating members of the torque converter22.

An LU clutch torque Tlu that is the torque capacity of the LU clutch40is changed by a regulated LU hydraulic pressure PRlu supplied from a hydraulic control circuit56provided in the vehicle10. Thus, the operation status, that is, the controlled status, of the LU clutch40is changed. The controlled status of the LU clutch40includes a completely released state that is a state where the LU clutch40is released, a slip state that is a state where the LU clutch40is engaged with a slip, and a completely engaged state that is a state where the LU clutch40is engaged.

The automatic transmission24is, for example, a known planetary gear automatic transmission that includes, for example, a set or multiples sets of planetary gear trains (not shown) and a plurality of engagement devices CB. Each of the engagement devices CB is, for example, a hydraulic friction engagement device that is a multiple disc or single disc clutch or brake that is pressed by a hydraulic actuator, a band brake that is fastened by a hydraulic actuator, or the like. The controlled status, such as an engaged state and a released state, of each of the engagement devices CB is changed by changing a CB torque Tcb that is the torque capacity of each of the engagement devices CB by using a regulated CB hydraulic pressure PRcb supplied from the hydraulic control circuit56. The automatic transmission24may be regarded as a transmission of the disclosure.

The automatic transmission24is a step transmission in which any one of multiple gear stages (also referred to as speed stages) having different gear ratios (also referred to as speed ratios) γat (=AT input shaft rotation speed Ni/AT output shaft rotation speed No) is established by engaging some of the engagement devices CB. The automatic transmission24is configured to be able to shift into 10-speed gear stages of, for example, a first gear stage 1st to a tenth gear stage 10th. In the automatic transmission24, the electronic control unit90(described later) changes the gear stage to be established, that is, selectively establishes any one of multiple gear stages, in accordance with an accelerator operation of a driver, a vehicle speed V, and the like. The AT input shaft rotation speed Ni is the rotation speed of the transmission input shaft38, which is the input shaft rotation speed of the automatic transmission24. The AT input shaft rotation speed Ni is the rotation speed of the output rotating member of the torque converter22. The AT input shaft rotation speed Ni is the same value as the turbine rotation speed Nt that is the output shaft rotation speed of the torque converter22. The AT input shaft rotation speed Ni can be expressed by the turbine rotation speed Nt. The AT output shaft rotation speed No is the rotation speed of the transmission output shaft26, which is the output shaft rotation speed of the automatic transmission24.

The K0clutch20is a wet or dry friction engagement device made up of a multiple disc or single disc clutch that is pressed by a hydraulic actuator. The electronic control unit90(described later) controls the operation status of the hydraulic actuator to change the controlled status, that is, an engaged state, a released state, and the like, of the K0clutch20. In the K0clutch20, when a regulated K0hydraulic pressure PRO is supplied from the hydraulic control circuit56to the hydraulic actuator, a K0torque Tk0that is the torque capacity of the K0clutch20is changed, with the result that the controlled status (engagement status) of the K0clutch20is changed.

In the engaged state of the K0clutch20, the pump impeller22aand the engine12are integrally rotated via the engine coupling shaft34. In other words, when the K0clutch20is engaged, the K0clutch20couples the engine12to the drive wheels14such that power can be transmitted. On the other hand, in the released state of the K0clutch20, transmission of power between the engine12and the pump impeller22ais interrupted. In other words, when the K0clutch20is released, the K0clutch20separates coupling between the engine12and the pair of drive wheels14. Since the electric motor MG is coupled to the pump impeller22a, the K0clutch20is provided in the driveline between the engine12and the electric motor MG and functions as a clutch to disconnect the driveline, that is, a clutch to disconnect the engine12from the electric motor MG. In other words, the K0clutch20is a disconnect clutch that couples the engine12to the electric motor MG when engaged and that separates coupling between the engine12and the electric motor MG when released.

In the powertrain16, power output from the engine12in the case where the K0clutch20is engaged is transmitted from the engine coupling shaft34to the drive wheels14sequentially via the K0clutch20, the electric motor coupling shaft36, the torque converter22, the automatic transmission24, the propeller shaft28, the differential gear30, the drive shafts32, and the like. Power output from the electric motor MG is, regardless of the controlled status of the K0clutch20, transmitted from the electric motor coupling shaft36to the drive wheels14sequentially via the torque converter22, the automatic transmission24, the propeller shaft28, the differential gear30, the drive shafts32, and the like.

The vehicle10includes a mechanical oil pump58(hereinafter, MOP58), an electric oil pump60(hereinafter, EOP60), a pump motor62, and the like. The MOP58is coupled to the pump impeller22a. The MOP58is driven for rotation by the driving force source (the engine12or the electric motor MG) to discharge hydraulic fluid to be used in the powertrain16. The pump motor62is a motor dedicated for the EOP60for driving the EOP60for rotation. The EOP60is driven for rotation by the pump motor62to discharge hydraulic fluid. Hydraulic fluid discharged from the MOP58or the EOP60is supplied to the hydraulic control circuit56. The hydraulic control circuit56supplies the CB hydraulic pressure PRcb, the K0hydraulic pressure PRO, the LU hydraulic pressure PRlu, and the like each regulated from hydraulic fluid discharged from at least one of the MOP58and the EOP60.

The vehicle10further includes the electronic control unit90including a controller concerned with travel control or the like of the vehicle10. The electronic control unit90includes a so-called microcomputer including, for example, a CPU, a RAM, a ROM, input and output interfaces, and other components. The CPU executes various control on the vehicle10by processing signals in accordance with programs stored in the ROM in advance while using the temporary storage function of the RAM. The electronic control unit90is configured to, where necessary, separately include a computer for engine control, a computer for electric motor control, a computer for hydraulic control, and the like. The electronic control unit90may be regarded as a controller of the disclosure.

Various signals and the like based on detected values of various sensors and the like provided in the vehicle10are supplied to the electronic control unit90. The various sensors and the like provided in the vehicle10include, for example, an engine rotation speed sensor70, a turbine rotation speed sensor72, an output shaft rotation speed sensor74, an MG rotation speed sensor76, an accelerator operation amount sensor78, a throttle valve opening degree sensor80, a brake switch82, a battery sensor84, and a fluid temperature sensor86. The various signals and the like include, for example, an engine rotation speed Ne, the turbine rotation speed Nt, the AT output shaft rotation speed No, an MG rotation speed Nm, an accelerator operation amount θacc, a throttle valve opening degree θth, a brake on signal Bon, a battery temperature THbat, a battery charge/discharge current Ibat, and a battery voltage Vbat of the battery54, and a hydraulic fluid temperature THoil. The engine rotation speed Ne is the rotation speed of the engine12. The turbine rotation speed Nt is the same value as the AT input shaft rotation speed Ni. The AT output shaft rotation speed No corresponds to the vehicle speed V. The MG rotation speed Nm is the rotation speed of the electric motor MG. The accelerator operation amount θacc is a driver's accelerator operation amount indicating the magnitude of driver's acceleration operation. The throttle valve opening degree θth is the opening degree of an electronic throttle valve. The brake on signal Bon that is a signal indicating a state where a brake pedal for activating a wheel brake is being operated by the driver. The hydraulic fluid temperature THoil is the temperature of hydraulic fluid in the hydraulic control circuit56.

Various command signals are output from the electronic control unit90to devices provided in the vehicle10. The devices include, for example, the engine controller50, the inverter52, the hydraulic control circuit56, the pump motor62, and the like. The various command signals include, for example, an engine control command signal Se, an MG control command signal Sm, a CB hydraulic control command signal Scb, a K0hydraulic control command signal Sk0, an LU hydraulic control command signal Slu, an EOP control command signal Seop, and the like. The engine control command signal Se is used to control the engine12. The MG control command signal Sm is used to control the electric motor MG. The CB hydraulic control command signal Scb is used to control the engagement devices CB. The K0hydraulic control command signal Sk0is used to control the K0clutch20. The LU hydraulic control command signal Slu is used to control the LU clutch40. The EOP control command signal Seop is used to control the EOP60.

The electronic control unit90includes a hybrid control unit92that functions as a hybrid controller, a clutch control unit94that functions as a clutch controller, a shift control unit96that functions as a shift controller, and the like to implement various control in the vehicle10.

The hybrid control unit92includes the function of an engine control unit92a, that is, an engine controller, that controls the operation of the engine12, and the function of an electric motor control unit92b, that is, an electric motor controller, that controls the operation of the electric motor MG via the inverter52. The hybrid control unit92executes hybrid drive control or the like with the engine12and the electric motor MG by using the control functions.

The hybrid control unit92calculates a required driving amount of the vehicle10by the driver by, for example, applying an accelerator operation amount θacc and a vehicle speed V to the required driving amount map. The required driving amount map is a relationship obtained empirically or by design in advance and stored, that is, a relationship determined in advance. The required driving amount is, for example, a required driving torque Trdem of the drive wheels14. The required driving torque Trdem [Nm] is a required driving power Prdem [W] at the vehicle speed V at that time from another viewpoint. A required driving force Frdem [N] in the drive wheels14, a required AT output torque in the transmission output shaft26, and the like are able to be used as the required driving amount. In calculating the required driving amount, an AT output shaft rotation speed No or the like may be used instead of the vehicle speed V.

The hybrid control unit92outputs an engine control command signal Se for controlling the engine12and an MG control command signal Sm for controlling the electric motor MG to achieve the required driving power Prdem in consideration of a transmission loss, an auxiliary load, the gear ratio γat of the automatic transmission24, a chargeable power Win and dischargeable power Wout of the battery54, and the like. The engine control command signal Se is, for example, a command value of engine power Pe that is the power of the engine12, that is, an engine torque Te is output at the engine rotation speed Ne at that time. The MG control command signal Sm is, for example, a command value of consumption power Wm of the electric motor MG, that is, an MG torque Tm is output at the MG rotation speed Nm at that time.

The chargeable power Win of the battery54is an inputtable maximum power that defines the limit of input electric power of the battery54and indicates the input limit of the battery54. The dischargeable power Wout of the battery54is an outputtable maximum power that defines the limit of output electric power of the battery54and indicates the output limit of the battery54. The chargeable power Win and dischargeable power Wout of the battery54are calculated by the electronic control unit90based on, for example, a battery temperature THbat and a state of charge SOC [%] of the battery54. A state of charge SOC of the battery54is a value indicating a state of charge of the battery54and is calculated by the electronic control unit90based on, for example, a battery charge/discharge current Ibat, a battery voltage Vbat, and the like.

When only the output power of the electric motor MG is sufficient to provide the required driving torque Trdem, the hybrid control unit92sets the drive mode to a motor drive (hereinafter, BEV drive) mode. In the BEV drive mode, the hybrid control unit92performs BEV driving that the vehicle10runs by using only the electric motor MG as a driving force source in the released state of the K0clutch20. On the other hand, when at least the output of the engine12needs to be used to sufficiently provide the required driving torque Trdem, the hybrid control unit92sets the drive mode to an engine drive mode, that is, a hybrid drive (hereinafter, HEV drive) mode. In the HEV drive mode, the hybrid control unit92performs engine driving, that is, HEV driving, that the vehicle10runs by using at least the engine12as a driving force source in the engaged state of the K0clutch20. On the other hand, even when only the output power of the electric motor MG is able to provide the required driving torque Trdem, but, for example, when the state of charge SOC of the battery54is lower than a predetermined engine start threshold or when warm-up of the engine12or the like is needed, the hybrid control unit92establishes the HEV drive mode. The engine start threshold is a predetermined threshold for determining a state of charge SOC at which the battery54needs to be charged by forcibly starting the engine12. In this way, the hybrid control unit92switches between the BEV drive mode and the HEV drive mode by automatically stopping the engine12during HEV driving, restarting the engine12after the engine stops, or starting the engine12during BEV driving, based on the required driving torque Trdem and the like.

The shift control unit96, for example, determines whether to shift the automatic transmission24by using a shift map that is a predetermined relationship and, where necessary, outputs a CB hydraulic control command signal Scb for executing shift control of the automatic transmission24to the hydraulic control circuit56. The shift map is, for example, a predetermined relationship having shift lines for determining a shift of the automatic transmission24on a two-dimensional coordinate system with vehicle speed V and required driving torque Trdem as variables. In the shift map, an AT output shaft rotation speed No or the like may be used instead of a vehicle speed V, and a required driving force Frdem, an accelerator operation amount θacc, a throttle valve opening degree θth, or the like may be used instead of a required driving torque Trdem.

The hybrid control unit92functionally includes a start-stop control unit92cthat functions as a start-stop controller for controlling the start and stop of the engine12. The start-stop control unit92cmay be regarded as a control unit of the disclosure.

The start-stop control unit92cdetermines whether the engine12needs to be started during BEV driving. For example, the start-stop control unit92cdetermines whether the engine12needs to be started based on, in the BEV drive mode, whether the required driving torque Trdem is increased to above the range in which only the MG torque Tm of the electric motor MG is able to provide the required driving torque Trdem, or whether warm-up of the engine12or the like is needed, or whether the state of charge SOC of the battery54is lower than the engine start threshold, or the like. For example, when the required MG torque Tmdem of the electric motor MG for achieving the required driving torque Trdem is greater than a preset determination threshold K during running in the BEV drive mode, the start-stop control unit92cdetermines that the engine12needs to be started. The determination threshold K is obtained empirically or by design in advance. For example, the determination threshold K is set to a value obtained by subtracting a cranking torque Tcrk needed for cranking the engine12at the start of the engine from a maximum torque Tmmx outputtable from the electric motor MG.

When the start-stop control unit92cdetermines that the engine12needs to be started, the clutch control unit94controls the K0clutch20execute control to start the engine12. For example, when the start-stop control unit92cdetermines that the engine12needs to be started, the clutch control unit94outputs a K0hydraulic control command signal Sk0to the hydraulic control circuit56to control the K0clutch20in the released state toward the engaged state such that a K0torque Tk0for transmitting the cranking torque Tcrk to the engine12side is obtained. The cranking torque Tcrk is needed to crank the engine12and is a torque for increasing the engine rotation speed Ne. In other words, in starting the engine12, the clutch control unit94outputs a K0hydraulic control command signal Sk0to the hydraulic control circuit56to control the hydraulic actuator of the K0clutch20such that the controlled status of the K0clutch20is changed from the released state to the engaged state. As a result, as the K0clutch20is engaged, the engine rotation speed Ne is increased to an autonomously operable rotation speed.

The start-stop control unit92ccontrols the engine12and the electric motor MG to execute control to start the engine12. For example, when the start-stop control unit92cdetermines that the engine12needs to be started, the start-stop control unit92coutputs an MG control command signal Sm to the inverter52for the electric motor MG to output the cranking torque Tcrk in synchronization with a change of the K0clutch20into the engaged state by the clutch control unit94. In other words, in starting the engine12, the start-stop control unit92coutputs an MG control command signal Sm to the inverter52to control the electric motor MG such that the electric motor MG outputs the cranking torque Tcrk, that is, the MG torque Tm increases by the amount of cranking torque Tcrk.

When the start-stop control unit92cdetermines that the engine12needs to be started, the start-stop control unit92coutputs an engine control command signal Se to the engine controller50to start fuel supply, engine ignition, and the like in synchronization with cranking of the engine12with the K0clutch20and the electric motor MG. In other words, in starting the engine12, the start-stop control unit92coutputs an engine control command signal Se to the engine controller50to control the engine12such that the engine12starts operation.

During running in the HEV drive mode, when the required MG torque Tmdem of the electric motor MG for achieving the required driving torque Trdem becomes less than a preset determination threshold L, the start-stop control unit92cdetermines that the engine12needs to be stopped. At this time, the start-stop control unit92creleases the K0clutch20and stops fuel supply to the engine12, thus stopping the engine12. Here, the determination threshold L is a value obtained by setting hysteresis to the determination threshold K for determining whether to start the engine12during BEV driving. Specifically, the determination threshold L is varied by a preset stop determination hysteresis H from the determination threshold K. In this way, with a hysteresis set between the determination threshold L and the determination threshold K, a so-called hunting that the engine12is frequently started and stopped is prevented.

Incidentally, for example, it is conceivable that an accelerator pedal is depressed in an extremely low vehicle speed state just before a vehicle stop as a result of steep deceleration of the vehicle10during running in the BEV drive mode or in a state where the vehicle10has decelerated to an extremely low vehicle speed as a result of deceleration of a vehicle running ahead while running on a gentle uphill road in the BEV drive mode. At this time, a higher vehicle speed-side gear stage (for example, the second gear stage 2nd or the third gear stage 3rd) than the first gear stage 1st in the automatic transmission24, the AT input shaft rotation speed Ni of the automatic transmission24decreases, and the vehicle10is accelerated in this state. In accelerating the vehicle10in such a state, if the start of the engine12delays, it is not possible to achieve the required driving torque Trdem by using the MG torque Tm of the electric motor MG, so slow response can occur in acceleration of the vehicle10or backward movement of the vehicle10can occur on an uphill road. To prevent such situations, it is conceivable to start the engine12whenever the vehicle speed V falls within an extremely low vehicle speed range; while, on the other hand, the driving time of the engine12extends to lead to deterioration of fuel efficiency.

In contrast, the electronic control unit90functionally includes a determination threshold correction unit92dthat functions as a determination threshold corrector to correct the determination threshold K for determining whether to start the engine12during running in the BEV drive mode based on the gear stage of the automatic transmission24and the AT input shaft rotation speed Ni of the automatic transmission24.

The determination threshold correction unit92ddetermines a correction amount α for the determination threshold K by applying the current gear stage of the automatic transmission24and the AT input shaft rotation speed Ni to the relational map obtained empirically or by design in advance. Subsequently, when the determination threshold correction unit92ddetermines the correction amount α for the determination threshold K, the determination threshold correction unit92dcorrects the determination threshold K such that the determination threshold K reduces by the determined correction amount a, that is, the engine12is more easily started. Therefore, based on the corrected determination threshold K (hereinafter, determination threshold Kcrt), the engine12is more easily started as compared to the determination threshold K before correction.

FIG.2shows one mode of the relational map for determining the correction amount α for the determination threshold K based on the turbine rotation speed Nt (that is, the AT input shaft rotation speed Ni of the automatic transmission24) and the gear stage of the automatic transmission24. In the relational map shown inFIG.2, the abscissa axis represents turbine rotation speed Nt, that is, AT input shaft rotation speed Ni, and the ordinate axis represents correction amount α. In the relational map ofFIG.2, the continuous line represents the correction amount α in the first gear stage 1st, the dashed line represents the correction amount α in the second gear stage 2nd, and the alternate long and short dashed line represents the correction amount α in the third gear stage 3rd. The relational map is set in a range in which the turbine rotation speed Nt (that is, the AT input shaft rotation speed Ni) is low (for example, a range lower than 1000 rpm).

As shown inFIG.2, in the first gear stage 1st, the correction amount α is set to zero regardless of the turbine rotation speed Nt. In other words, in the first gear stage 1st with the largest gear ratio γat, the determination threshold K is not corrected. In other words, the determination threshold K is corrected when the gear stage is a gear stage other than the first gear stage 1st with the largest gear ratio γat. In relation to this, the determination threshold K before correction is set with reference to the first gear stage 1st. Therefore, the determination threshold K before correction is set to a value with which it is possible to ensure acceleration performance required by the driver when it is determined to start the engine12at appropriate timing in the state of the first gear stage 1st, and backward movement of the vehicle10is suppressed within a possible road gradient.

In the second gear stage 2nd represented by the dashed line and the third gear stage 3rd represented by the alternate long and short dashed line, the correction amount α increases as the turbine rotation speed Nt reduces in a low rotation range of the turbine rotation speed Nt. In other words, when the turbine rotation speed Nt is low, the correction amount α is increased as compared to the case where the turbine rotation speed Nt is high. Therefore, as the turbine rotation speed Nt reduces, the determination threshold Kcrt is corrected so as to reduce. As a result, as the turbine rotation speed Nt reduces, the engine12is more easily started.

When the second gear stage 2nd is compared with the third gear stage 3rd, the correction amount α in the third gear stage 3rd is greater than the correction amount α in the second gear stage 2nd for the same turbine rotation speed Nt. In other words, as the gear stage of the automatic transmission24increases, the correction amount α is increased as compared to when the gear stage is low. Therefore, as the gear stage of the automatic transmission24increases, the determination threshold Kcrt is corrected so as to further reduce as compared to when the gear stage is low. As a result, as the gear stage of the automatic transmission24increases, the engine12is more easily started as compared to when the gear stage is low. In the relational map ofFIG.2, an upper limit value αmax of the correction amount α is defined, and, as the turbine rotation speed Nt becomes lower than or equal to a predetermined value, the correction amount α is set to the upper limit value αmax regardless of the turbine rotation speed Nt or the gear stage.

In the relational map, the values of the correction amount α set in the second gear stage 2nd and the third gear stage 3rd are, for example, set to values with which acceleration performance similar to that in the case of the first gear stage 1st is obtained and backward movement of the vehicle10is suppressed up to the range of road gradient similar to that of the first gear stage 1st. For example, due to a gear ratio difference (=γ1st−γ2nd) between the gear ratio γ1st of the first gear stage 1st and the gear ratio γ2nd of the second gear stage 2nd or a gear ratio step (=γ1st/γ2nd), the correction amount α in the second gear stage 2nd is set such that a shortage of acceleration performance with respect to acceleration operation that can be achieved in the first gear stage 1st during running in the second gear stage 2nd is compensated by the engine torque Te caused by advancing the start timing of the engine12. Similarly, for example, due to a gear ratio difference (=γ1st−γ3rd) between the gear ratio γ1st of the first gear stage 1st and the gear ratio γ3rd of the third gear stage 3rd or a gear ratio step (=γ1st/γ3rd), the correction amount α in the third gear stage 3rd is set such that a shortage of acceleration performance with respect to acceleration operation that can be achieved in the first gear stage 1st during running in the third gear stage 3rd is compensated by the engine torque Te caused by advancing the start timing of the engine12.

For example, with a change in correction amount α due to output of an upshift command from the second gear stage 2nd to the third gear stage 3rd, the determination threshold K is changed and it is, for example, determined whether to start the engine12. Thus, the correction amount α is set such that a phenomenon (that is, hunting) that start and stop of the engine12are repeated is prevented. In addition, for example, when the state of charge SOC of the battery54becomes low and the dischargeable power Wout is limited, the correction amount α is set such that repetition (that is, hunting) of start and stop of the engine12, the engine12is more easily started, with the result that the correction amount α is set such that repetition (that is, hunting) of start and stop of the engine12does not occur.

Since the correction amount α is set as described above, the engine12is more easily started as the gear stage is a higher speed-side gear stage than the first gear stage 1st and the turbine rotation speed Nt reduces. Therefore, acceleration performance similar to that in the case of the first gear stage is obtained, and backward movement of the vehicle10is suppressed within the range of the same road gradient as that in the case of the first gear stage 1st.

FIG.3is a graph showing one mode of the determination threshold K for determining whether to start the engine12, corrected based on the relational map ofFIG.2. InFIG.3, the abscissa axis represents the MG rotation speed Nm of the electric motor MG, and the ordinate axis represents the MG torque Tm of the electric motor MG. InFIG.3, the MG torque Tm of the electric motor MG, represented by the continuous line, represents maximum torque Tmmx that the electric motor MG is able to output. A determination threshold K1represented by the dashed line is a determination threshold K1set during BEV driving in the first gear stage 1st. In other words, the determination threshold K1represents the determination threshold K before correction. A determination threshold K2represented by the alternate long and short dashed line is a determination threshold K2set during BEV driving in the second gear stage 2nd. A determination threshold K3represented by the alternate long and two-short dashed line is a determination threshold K3set during BEV driving in the third gear stage 3rd. On the higher rotation speed side than a predetermined rotation speed Nm1at which the maximum torque Tmmx begins to reduce, the determination threshold K2and the determination threshold K3are set to the same value as the determination threshold K1in the first gear stage 1st and represented by the dashed line.

InFIG.3, the determination threshold K1is set to a value obtained by subtracting the cranking torque Tcrk needed at the start of the engine12from the maximum torque Tmmx of the electric motor MG. Thus, the MG torque Tm for the cranking torque Tcrk used at the start of the engine is ensured, so a reduction in driving force for propelling the vehicle10is prevented at the start of the engine. The determination threshold K2in the second gear stage 2nd represented by the alternate long and short dashed line and the determination threshold K3in the third gear stage 3rd represented by the alternate long and two-short dashed line are values smaller than the determination threshold K1in the first gear stage 1st represented by the dashed line. In other words, the determination threshold K1in the first gear stage 1st is corrected from the determination threshold K1by the correction amount α obtained by the above-described relational map ofFIG.2, with the result that the determination thresholds K2, K3are corrected so as to be lower than the determination threshold K1. Thus, when the MG rotation speed Nm is low and during BEV driving in the second gear stage 2nd or the third gear stage 3rd, the MG torque Tm tends to exceed the determination threshold K2or the determination threshold K3, so the engine is started earlier than during BEV driving in the first gear stage 1st. As a result, the start timing of the engine12is advanced, acceleration response equivalent to that in the case of the first gear stage 1st is obtained, and backward movement of the vehicle10is suppressed.

The determination threshold correction unit92dsets the determination threshold L for determining whether to stop the engine12during HEV driving. The determination threshold correction unit92dconverts the determination threshold K to the determination threshold L for determining whether to stop the engine12during BEV driving by adding or subtracting a preset stop determination hysteresis H to or from the determination threshold K for determining whether to start the engine12during BEV driving. Thus, the determination threshold L is set to a value varied by the stop determination hysteresis H from the determination threshold K.

FIG.4is a flowchart for illustrating a relevant part of control operations of the electronic control unit90and is a flowchart for illustrating control operations for setting the engine start determination threshold K with which backward movement of the vehicle10and slow response of acceleration at the time when the accelerator pedal is depressed in an extremely low vehicle speed range are suppressed. This flowchart is repeatedly executed while the vehicle10is running.

Initially, in step (hereinafter, step is omitted) S10corresponding to the control function of the start-stop control unit92c, a determination threshold K for determining whether to start the engine12, set during BEV driving is obtained. The determination threshold K substantially corresponds to the determination threshold K1set in the first gear stage 1st. Subsequently, in S20corresponding to the control function of the start-stop control unit92c, it is determined whether the engine12is in a stopped state. When the determination of S20is affirmative, the correction amount α is obtained based on the gear stage of the automatic transmission24and the turbine rotation speed Nt (that is, the AT input shaft rotation speed Ni) in S30corresponding to the control function of the determination threshold correction unit92d. In addition, the determination threshold K is calculated (corrected) based on the obtained correction amount α. After that, in S40corresponding to the control function of the start-stop control unit92c, it is determined whether the required MG torque Tmdem of the electric motor MG is greater than the corrected determination threshold Kcrt. When the determination of S40is affirmative, it is determined that the engine12needs to be started in S50corresponding to the control function of the start-stop control unit92c, and control to start the engine12is executed. On the other hand, when the determination of S40is negative, the engine12is maintained in the same state (here, engine stopped state) as that at the time of the last determination in S90corresponding to the control function of the hybrid control unit92.

When the determination of S20is negative, the determination threshold K obtained in S10is corrected by using the stop determination hysteresis H to obtain a determination threshold L for determining whether to stop the engine12in S60corresponding to the control function of the determination threshold correction unit92d. Then, in S70corresponding to the control function of the start-stop control unit92c, it is determined whether the required MG torque Tmdem of the electric motor MG is less than the determination threshold L obtained in S60. When the determination of S70is affirmative, it is determined that the engine12needs to be stopped in S80corresponding to the control function of the start-stop control unit92c, and control to stop the engine12is executed. On the other hand, when the determination of S70is negative, the engine12is maintained in the same state (here, engine driving state) as that at the time of the last determination in S90.

As described above, according to the present embodiment, during BEV driving that the vehicle10is caused to run by using the power of the electric motor MG, the determination threshold K for determining whether to start the engine12is corrected by estimating a shortage of driving force in advance based on the gear stage of the automatic transmission24and the turbine rotation speed Nt (that is, the AT input shaft rotation speed Ni of the automatic transmission24). Therefore, the engine is started at appropriate timing for a change in required MG torque Tmdem, with the result that a shortage of driving force of the vehicle10is suppressed. As a result, backward movement or slow response of acceleration of the vehicle10due to a shortage of driving force of the vehicle10is suppressed.

According to the present embodiment, the determination threshold K is corrected in a gear stage other than the first gear stage 1st with the largest gear ratio γat. Therefore, occurrence of a shortage of driving force in the second gear stage 2nd or the third gear stage 3rd with a smaller gear ratio γat than the first gear stage 1st is estimated in advance, and the engine12is started at appropriate timing, with the result that a shortage of driving force is suppressed. The determination threshold K is corrected so as to reduce as the AT input shaft rotation speed Ni of the automatic transmission24reduces. Therefore, for a situation in which a shortage of driving force tends to occur as the AT input shaft rotation speed Ni of the automatic transmission24reduces, the engine is more easily started early, with the result that a shortage of driving force is suppressed. The determination threshold K is corrected so as to reduce as the gear stage of the automatic transmission24increases as compared to when the gear stage is low. Therefore, for a situation in which a shortage of driving force tends to occur as the gear stage increases, the engine is more easily started early, with the result that a shortage of driving force is suppressed.

The embodiment of the disclosure has been described in detail with reference the drawings; however, the disclosure is also applicable to other embodiments.

For example, in the above-described embodiment, the correction amount α is changed based on the turbine rotation speed Nt, that is, the AT input shaft rotation speed Ni; however, the disclosure is not necessarily limited to the AT input shaft rotation speed Ni. As long as it is a value related to the AT input shaft rotation speed Ni, such as a vehicle speed V, the disclosure is able to be applied as needed.

In the above-described embodiment, the automatic transmission24is a step transmission configured to include a set or multiple sets of planetary gear trains and a plurality of engagement devices CB; however, the disclosure is not necessarily limited to the above-described configuration of the automatic transmission24. The disclosure is able to be applied as needed as long as a transmission is able to be shifted into multiple gear stages.

In the above-described embodiment, the determination threshold K is corrected while the automatic transmission24is in the second gear stage 2nd or the third gear stage 3rd. Alternatively, the determination threshold K may be corrected only in, for example, the second gear stage 2nd. Alternatively, the determination threshold K may be corrected also in a gear stage larger than or equal to the fourth gear stage 4th. However, correction of the determination threshold K is limited to a gear stage established in a low vehicle speed range lower than a vehicle speed range in which the automatic transmission24is shifted.

The above-described embodiment is only illustrative. The disclosure may be implemented in modes including various modifications or improvements based on the knowledge of persons skilled in the art.