Vehicle and vehicle control method

In a hybrid vehicle including an engine, a motor, a generator, and a battery that is electrically connected to the motor and the generator, an ECU performs “battery-less travel control” enabling the vehicle to travel when a fault occurs in the battery by disconnecting the battery from an electrical system including the motor and the generator and driving the motor using power that is generated by the generator using the power of the engine. When the battery-less travel control is underway and a vehicle speed V exceeds a vehicle speed limit Vsh, the ECU implements a vehicle speed limitation. As a result, a control mode of the motor is less likely to shift to a rectangular control mode during the battery-less travel control.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a national phase application based on the PCT International Patent Application No. PCT/IB2012/002250 filed Nov. 7, 2012, claiming priority to Japanese application No. 2011-246504 filed Nov. 10, 2011, the entire contents of both of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a vehicle, and more particularly to a vehicle that travels using electric power from at least one of an engine and a motor.

2. Description of Related Art

Recently, hybrid vehicles that travel using drive power from at least one of an engine and a motor are becoming more widespread. A hybrid vehicle may include a generator provided separately to the motor in order to generate electric power using the power of the engine.

Japanese Patent Application Publication No. 2007-196733 (JP-2007-196733 A) discloses a technique employed in a hybrid vehicle including an engine, a motor, and a generator for performing control (also referred to hereafter as “battery-less travel control”). The hybrid vehicle disconnects the battery from an electrical system including the motor and the generator and drives the motor using power that is generated by the generator using the power of the engine to travel the vehicle when an abnormality occurs in a battery that stores electric power for driving the motor.

Incidentally, a hybrid vehicle is installed with an inverter for driving the motor. Principal methods of controlling the inverter include a pulse width modulation (also referred to as “PWM” hereafter) control method and a rectangular wave voltage control (also referred to simply as “rectangular control” hereafter) method. In the rectangular control, a modulation percentage of a voltage conversion (a value corresponding to a ratio of an output voltage to an input voltage) is larger than that obtained in the PWM control, and therefore a motor output can be increased. However, a control accuracy (a control response) tends to be poor, leading to instability in an inverter output voltage. Therefore, the rectangular control is typically used only in a high vehicle speed region, while the PWM control is used under normal speed region which is lower than the high vehicle speed region.

During the battery-less travel control, meanwhile, the battery cannot be used as a power buffer, and therefore an accurate electric power balance must be achieved between the motor and the generator. When the vehicle speed reaches to a high vehicle speed region during the battery-less travel control, however, the inverter control method shifts from the PWM control to the rectangular control exhibiting poor control precision. Accordingly, the power balance may collapse when a required driving force varies rapidly, and as a result, a voltage (the inverter output voltage) applied to the motor may become unstable.

SUMMARY OF THE INVENTION

The invention has been designed to solve the problem described above, and an object thereof is to suppress instability in a voltage applied to a motor during battery-less travel control.

A vehicle according to a first aspect of the invention is caused to travel by rotating an output shaft coupled to a drive wheel using power from at least one of an engine and a motor, and includes: a generator that generates electric power using the power of the engine; a battery configured to be connectable to the motor and the generator; and a control apparatus that controls the motor and the generator. The control apparatus drives the motor through pulse width modulation control when a vehicle speed is lower than a threshold and drives the motor through rectangular control in which the motor can output a larger torque but poor controllability in comparison with the pulse width modulation control, when the vehicle speed exceeds the threshold. The control apparatus performs battery-less travel control in which the battery is disconnected from the motor and the generator and the motor is driven using the power generated by the generator when an abnormality occurs in the battery. During the battery-less travel control, the control apparatus suppresses execution of the rectangular control by implementing a vehicle speed limitation.

During the battery-less travel control, the control apparatus may control the vehicle speed to be at or below a limit value corresponding to the threshold.

The vehicle may further include a transmission provided between the motor and the output shaft. The control apparatus may modify the limit value in accordance with a speed ratio of the transmission.

The gear speed of the transmission may be set to either a low speed position or a high speed position having a smaller speed ratio than the low speed position. The control apparatus may control the threshold and the limit value to be larger when the gear speed is set at the high speed position than when the gear speed is set at the low speed position.

The control apparatus may limit the vehicle speed by limiting a torque of the engine or the motor.

The vehicle may further include a planetary gear apparatus that includes a ring gear coupled to the motor, a sun gear coupled to the generator, a pinion gear engaged to the sun gear and the ring gear, and a carrier that is coupled to the engine and supports the pinion gear rotatably.

A vehicle control method according to a second aspect of the invention is a control method for a vehicle that is caused to travel by rotating an output shaft coupled to a drive wheel using power from at least one of an engine and a motor. The vehicle includes: a generator that generates power using the power of the engine; a battery configured to be connectable to the motor and the generator; and a control apparatus that controls the motor and the generator. The control method includes the steps of: driving the motor through pulse width modulation control when a vehicle speed is lower than a threshold, and driving the motor through rectangular control in which the motor can output a larger torque but poor controllability in comparison with the pulse width modulation control, when the vehicle speed exceeds the threshold; performing battery-less travel control in which the battery is disconnected from the motor and the generator and the motor is driven using the power generated by the generator when an abnormality occurs in the battery; and suppressing execution of the rectangular control during the battery-less travel control by implementing a vehicle speed limitation.

According to these aspects of the invention, instability in a voltage, applied to the motor during the battery-less travel control can be suppressed.

DETAILED DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will be described below with reference to the drawings. In the following description, identical reference symbols have been allocated to identical components. Names and functions thereof are also identical. Accordingly, detailed description thereof will not be repeated.

First Embodiment

FIG. 1is an overall block diagram of a vehicle1according to this embodiment. The vehicle1includes an engine100, a first MG200, a power distribution mechanism300, a second MG400, a propeller shaft (output shaft)560, a power control unit (PCU)600, a battery700, a system main relay (SMR)710, and an ECU1000.

The engine100is an internal combustion engine that outputs power by burning a fuel. The power of the engine100is input into the power distribution mechanism300.

The power distribution mechanism300divides the power input from the engine100into power for the output shaft560and power for the first MG200.

The power distribution mechanism300is a planetary gear mechanism having a sun gear (S)310, a ring gear (R)320, a pinion gear (P)340meshed to the sun gear (S)310and the ring gear (R)320, and a carrier (C)330that holds the pinion gear (P)340to be free to rotate and revolve.

The carrier (C)330is coupled to a crankshaft of the engine100. The sun gear (S)310is coupled to a rotor of the first MG200. The ring gear (R)320is coupled to the output shaft560.

The first MG200and the second MG400are alternating current rotating electric machines that respectively function as both a motor and a generator. A rotor of the second MG400is coupled to the output shaft560.

The output shaft560is rotated by at least one of the power of the engine100, which is transmitted thereto via the power distribution mechanism300, and the power of the second MG400. A rotary force of the output shaft560is transmitted to left and right drive wheels82via a reduction gear81. As a result, the vehicle1is caused to travel.

FIG. 2is a collinear diagram showing the power distribution mechanism300. By configuring the power distribution mechanism300as described above, a rotation speed of the sun gear (S)310(=a first MG rotation speed Nm1), a rotation speed of the carrier (C)330(=an engine rotation speed Ne), and a rotation speed of the ring gear (R)320(=a second MG rotation speed Nm2, or in other words a vehicle speed V) have a relationship (a relationship according to which, when any two of the rotation speeds are determined, the remaining rotation speed is also determined) indicated by linked straight lines on the collinear diagram of the power distribution mechanism300.

Returning toFIG. 1, the PCU600converts high voltage direct current power supplied from the battery700into alternating current power and outputs the alternating current power to the first MG200and/or the second MG400. As a result, the first MG200and/or the second MG400are driven. Further, the PCU600converts alternating current power generated by the first MG200and/or the second MG400into direct current power and outputs the direct current power to the battery700. As a result, the battery700is charged.

The battery700is a secondary battery that stores high voltage (approximately 200 V, for example) direct current power for driving the first MG200and/or the second MG400. The battery700is typically configured to include nickel hydrogen or lithium ions. Note that a large capacity capacitor may be employed instead of the battery700.

The SMR710is a relay for switching a connection condition between the battery700and an electrical system including the PCU600.

An engine rotation speed sensor10, an output shaft rotation speed sensor15, resolvers21,22, an accelerator position sensor31, and so on are connected to the ECU1000. The engine rotation speed sensor10detects the engine rotation speed Ne (the rotation speed of the engine100). The output shaft rotation speed sensor15detects a rotation speed Np of the output shaft560as the vehicle speed V. The resolvers21,22detect the first MG rotation speed Nm1(the rotation speed of the first MG200) and the second MG rotation speed Nm2(the rotation speed of the second MG400), respectively. The accelerator position sensor31detects an accelerator pedal operation amount A (an amount by which an accelerator pedal is operated by a user). The respective sensors output detection results to the ECU1000.

The ECU1000has an inbuilt central processing unit (CPU) and an inbuilt memory, not shown in the drawings, and executes predetermined calculation processing on the basis of information stored in the memory and information from the respective sensors. The ECU1000controls respective devices installed in the vehicle1on the basis of results of the calculation processing.

FIG. 3is a circuit diagram showing an electrical system for drive-controlling the first MG200and the second MG400. The electrical system is constituted by the first MG200, the second MG400, the PCU600, the battery700, the SMR710, and the ECU1000.

When the SMR710is OFF, the battery700is disconnected from the electrical system. When the SMR710is ON, the battery700is connected to the electrical system. The SMR710is controlled (switched ON and OFF) in response to control signals from the ECU1000. For example, when the user performs an operation to start driving such that a request is issued to activate the electrical system, the ECU1000switches the SMR710ON.

The PCU600includes a converter610and inverters620,630. The converter610has a typical boost chopper circuit configuration constituted by a reactor and two switching elements. An anti-parallel diode is connected to each switching element.

The inverters620,630are connected in parallel to the converter610. The inverter620is connected between the converter610and the first MG200. The inverter620includes a U phase arm, a V phase arm, and a W phase arm. The U phase arm, the V phase arm, and the W phase arm are connected in parallel. The U phase arm, the V phase arm, and the W phase arm respectively include two switching elements (an upper arm and a lower arm) connected in series. Each switching element is provided with an anti-parallel diode.

The inverter630is connected between the converter610and the second MG400. Similarly to the inverter620, the inverter630has a typical three-phase inverter configuration. In other words, the inverter630includes upper and lower arms for three phases (the U phase, the V phase, and the W phase), and an anti-parallel diode provided on each arm.

A direct current voltage (also referred to as a “system voltage VH” hereafter) on a power line54between the converter610and the inverters620,630is detected by a voltage sensor180. A detection result from the voltage sensor180is output to the ECU1000.

The converter610executes a bidirectional direct current voltage conversion between the system voltage VH and a voltage Vb of the battery700. When power discharged from the battery700is to be supplied to the first MG200or the second MG400, the voltage is boosted by the converter610. When, conversely, power generated by the first MG200or the second MG400is to be charged to the battery700, the voltage is reduced by the converter610.

The inverter620converts the system voltage VH into an alternating current voltage by switching the switching elements ON and OFF. The converted alternating current voltage is supplied to the first MG200. The inverter620also converts alternating current power generated by the first MG200into direct current power.

Similarly, the inverter630converts the system voltage VH into an alternating current voltage and supplies the alternating current voltage to the second MG400. The inverter630also converts alternating current power generated by the second MG400into direct current power.

The power line54that electrically connects the converter610to the inverters620,630thus serves as a positive electrode bus bar and a negative electrode bus bar shared by the respective inverters620,630. The power line54is electrically connected to both the first MG200and the second MG400, and therefore power generated by one of the first MG200and the second MG400can be consumed by the other.

Hence, in a condition where the SMR710is ON such that the battery700is connected to the electrical system, the battery700is charged by power generated by one of the first MG200and the second MG400and discharges power to make up for a power deficiency therein. Conversely, in a condition where the SMR710is OFF such that the battery700is disconnected from the electrical system, the battery700cannot be used as a power buffer, and therefore a power balance must be achieved between the first MG200and the second MG400.

The ECU1000drive-controls the first MG200and the second MG400by controlling respective switching operations of the inverters620,630. More specifically, the ECU1000sets a first MG torque command value T1com and a second MG torque command value T2com in accordance with the accelerator pedal amount A and the vehicle speed V, and outputs switching control signals to the inverters620,630so that an actual torque of the first MG200and an actual torque of the second MG400respectively match the first MG torque command value T1com and the second MG torque command value T2com.

FIG. 4is a schematic diagram illustrating control modes of the second MG400(i.e. control modes of the inverter630). In the vehicle1according to this embodiment, the control mode of the inverter630is switched to either a PWM control mode or a rectangular control mode.

In the PWM control mode, either sine wave PWM control or overmodulation PWM control is performed.

Sine wave PWM control is used as a typical PWM control method, in which opening/closing of the switching elements in the arms of the respective phases is controlled in accordance with a voltage comparison between a sine wave-shaped voltage command value and a carrier wave (a carrier signal). As a result, a basic wave component of a line-to-line voltage (also referred to simply as an “inverter output voltage” hereafter) applied to the second MG400by the inverter630within a fixed time period forms a pseudo-sine wave. As is well recognized, in sine wave PWM control, an amplitude of the basic wave component can only be increased to approximately 0.61 times an inverter input voltage (a modulation percentage can only be increased to 0.61).

Overmodulation PWM control is similar to the sine wave PWM control described above in that an amplitude of the carrier wave is reduced by being distorted. As a result, the modulation percentage can be increased to a range of 0.61 to 0.78.

In the rectangular control mode, meanwhile, rectangular control is performed. In rectangular control, a switching operation is performed once within the aforesaid fixed time period. As a result, the inverter output voltage within the fixed time period forms a rectangular wave voltage corresponding to a single pulse. Accordingly, the rectangular control exhibits poor control precision (control response) in comparison with the PWM control, but the modulation percentage can be increased to 0.78.

Hence, although the rectangular control exhibits poor control precision (control response) in comparison with the PWM control, the modulation percentage can be increased, enabling an increase in motor output.

Taking into consideration these differences in the characteristics of the control modes, the ECU1000selects the control mode in accordance with a region to which a vehicle operating point, which is determined by a vehicle driving torque (a driving torque of the output shaft560) and the vehicle speed V (the rotation speed Np of the output shaft560, or in other words the second MG rotation speed Nm2), belongs.

FIG. 5is a view showing a correspondence relationship between the vehicle operating point and the control mode of the second MG400(the inverter630). Basically, the PWM control mode exhibiting comparatively favorable controllability is selected in a region A1on a low rotation speed side of a control boundary line L in order to reduce torque variation, while the rectangular control mode is selected in a region A2on a high rotation speed side of the control boundary line L in order to increase the output of the second MG400. Hereafter, the region A1and the region A2will be referred to respectively as a “PWM control region A1” and a “rectangular control region A2”.

Next, battery-less travel control will be described. When an abnormality occurs in the battery700such that charging and discharging are prohibited, the ECU1000performs failsafe control by switching the SMR710OFF so that the vehicle1is caused to travel in a condition where the battery700is disconnected from the electrical system. This failsafe control is “battery-less travel control”.

During the battery-less travel control, the battery700cannot be used as a power buffer, and therefore a power balance must be achieved between the first MG200and the second MG400. In other words, during the battery-less travel control, the second MG400must be driven using the power generated by the first MG200, and therefore a power generation amount of the first MG200and a power consumption amount of the second MG400must be controlled to identical values.

FIG. 6is a flowchart showing processing procedures executed by the ECU1000during the battery-less travel control.

In S10, the ECU1000calculates a vehicle required torque Treq in accordance with the accelerator pedal amount A and the vehicle speed V. For example, a map defining a correspondence relationship between the vehicle required torque Treq and the accelerator pedal amount A and vehicle speed V is stored in the ECU1000in advance, and the ECU1000calculates the vehicle required torque Treq corresponding to the actual accelerator pedal amount A and vehicle speed V using the map.

In S11, the ECU1000calculates an engine required power Pereq from the vehicle required torque Treq. More specifically, the ECU1000calculates a product (=a vehicle required power) of the vehicle required torque Treq and the output shaft rotation speed Np as the engine required power Pereq.

In S12, the ECU1000calculates an engine rotation speed target value Nereq and an engine torque target value Tereq for satisfying the engine required power Pereq. For example, an optimum engine operating line determined by the engine rotation speed Ne and an engine torque Te is set in advance in the ECU1000, and the ECU1000calculates the engine rotation speed target value Nereq and the engine torque target value Tereq for satisfying the engine required power Pereq using this optimum engine operating line.

In S13, the ECU1000calculates a first MG torque target value T1req for bearing a reaction force of the engine torque Te from the engine required power Pereq and the engine rotation speed target value Nereq. When a planetary gear ratio of the power distribution mechanism300is set at “ρ”, a relationship of T1=ρ/(1+ρ)×Te is established from a mechanical relationship between a first MG torque T1 and the engine torque Te. The engine torque Te takes a value obtained by dividing an engine power Pe by the engine rotation speed Ne, and therefore the ECU1000calculates the first MG torque target value T1req using a following Equation (1).
T1req=ρ/(1+ρ)×Tereq=ρ/(1+ρ)×(Pereq/Nereq)  (1)

Note that during the battery-less travel control, the first MG torque target value T1req is set at a negative value (T1req<0) in order to set the first MG200in a power generation condition.

In S14, the ECU1000calculates a second MG torque target value T2req such that a first MG target power (=a target value of the power generation amount of the first MG200) and a second MG target power (=a target value of the power consumption amount of the second MG400) take identical values. More specifically, the ECU1000calculates the second MG torque target value T2req using a following Equation (2).
T2req=(T1req×Nm1)/Nm2  (2)

In S15, the ECU1000sets the calculated engine torque target value Tereq, first MG torque target value T1req, and second MG torque target value T2req at an engine torque command value Tecom, the first MG torque command value T1com, and the second MG torque command value T2com, respectively.

Hence, to achieve a power balance between the first MG200and the second MG400during the battery-less travel control, the respective torque command values are set such that the power generation amount of the first MG200and the power consumption amount of the second MG400take identical values. However, when the control mode of the second MG400is switched to the rectangular control mode exhibiting comparatively poor controllability, a control precision of a second MG torque T2 deteriorates. Accordingly, the power generation amount of the first MG200may not match the power consumption amount of the second MG400, and as a result, the inverter output voltage may become unstable.

The inverter output voltage becomes particularly unstable in a pattern where the vehicle required power varies rapidly, typically a pattern where the accelerator is switched ON (accelerator pedal operation amount A>0) and OFF (accelerator pedal operation amount A=0) repeatedly at a high vehicle speed. When the accelerator is switched ON, a negative torque of the first MG200is increased in order to increase the power generation amount of the first MG200(=an output power of the second MG400). To prevent a reduction in the engine rotation speed Ne caused by the increase in the negative torque of the first MG200, the engine torque Te is increased. In actuality, however, the engine rotation speed Ne decreases temporarily due to a control delay in the engine100. This temporary reduction in the engine rotation speed Ne leads to a temporary reduction in the first MG rotation speed Nm1(see the collinear diagram inFIG. 2). As a result, the power generation amount of the first MG200decreases, and therefore the output power of the second MG400must be reduced by an amount corresponding to the reduction in the power generation amount of the first MG200in order to maintain the power balance. In the rectangular control mode exhibiting comparatively poor controllability, however, a delay occurs during reduction of the output power of the second MG400, leading to a reduction in the system voltage VH, and as a result, the inverter output voltage decreases. Hence, when the accelerator is switched ON and OFF repeatedly at a high vehicle speed, the inverter output voltage may be reduced repeatedly, leading to instability therein. In a vehicle where a failsafe is implemented to shut down (stop) the inverters620,630when the system voltage VH falls to or below a predetermined value, a reduction in the system voltage VH may render continued vehicle travel impossible.

Hence, in this embodiment, the inverter output voltage is prevented from becoming unstable during the battery-less travel control by incorporating control (more specifically, a vehicle speed limitation) that reduces the likelihood of a shift to the rectangular control during the battery-less travel control. This point is the main feature of this embodiment.

FIG. 7is a function block diagram of the ECU1000in a case where a vehicle speed limitation is implemented during the battery-less travel control. Respective function blocks shown inFIG. 7may be realized by hardware or software.

The ECU1000includes a first determination unit1010, a second determination unit1020, and a vehicle speed limitation unit1030.

The first determination unit1010determines whether or not the battery-less travel control is underway. The second determination unit1020determines whether or not the vehicle speed V has exceeded a vehicle speed limit Vsh set in accordance with the control boundary line L (the boundary between the PWM control region A1and the rectangular control region A2).

FIG. 8is a view showing an example of a correspondence relationship between the vehicle speed limit Vsh and the control boundary line L. As shown inFIG. 8, the vehicle speed limit Vsh is set at a value on a minimum speed side of the control boundary line L.

The vehicle speed limitation unit1030implements a vehicle speed limitation when the vehicle speed V exceeds the vehicle speed limit Vsh during the battery-less travel control. More specifically, the vehicle speed limitation unit1030limits the engine torque Te or the second MG torque T2 so that the vehicle speed V falls below the vehicle speed limit Vsh.

FIG. 9is a flowchart showing processing procedures executed by the ECU1000to realize the functions described above.

In S20, the ECU1000determines whether or not the battery-less travel control is underway. When the battery-less travel control is underway (YES in S20), the processing is advanced to S21. When the battery-less travel control is not underway (NO in S20), the processing is advanced to S23.

In S21, the ECU1000determines whether or not the vehicle speed V has exceeded the vehicle speed limit Vsh. When the vehicle speed V exceeds the vehicle speed limit Vsh (YES in S21), the processing is advanced to S22. When the vehicle speed V does not exceed the vehicle speed limit Vsh (NO in S21), the processing is advanced to S23.

In S22, the ECU1000implements the vehicle speed limitation. In other words, the ECU1000limits the engine torque Te or the second MG torque T2 so that the vehicle speed V falls below the vehicle speed limit Vsh.

In S23, the ECU1000does not implement the vehicle speed limitation. In other words, the ECU1000does not limit the engine torque Te or the second MG torque T2.

Hence, in this embodiment, by implementing the vehicle speed limitation during the battery-less travel control, the likelihood of shifting to the rectangular control during the battery-less travel control is reduced. Accordingly, instability in the inverter output voltage during the battery-less travel control can be suppressed, and as a result, the vehicle can travel to a safety area.

Second Embodiment

FIG. 10is an overall block diagram of a vehicle1aaccording to this embodiment. The vehicle1adiffers from the vehicle1shown inFIG. 1in that a transmission500is provided between the second MG400and the output shaft560. All other structures are identical to the vehicle1shown inFIG. 1, and therefore detailed description thereof will not be repeated.

The transmission500shifts the rotation speed of the second MG400and transmits the shifted rotation to the output shaft560. The transmission500is constituted by a set of Ravigneaux planetary gear mechanisms. More specifically, the transmission500includes a first sun gear (S1)510, a second sun gear (S2)520, a first pinion (P1)531meshed to the first sun gear (S1)510, a second pinion (P2)532meshed to the first pinion (P1)531and the second sun gear (S2)520, a ring gear (R1)540meshed to the second pinion (P2)532, and a carrier (C1)550that holds the respective pinions531,532to be free to rotate and revolve. Hence, the first sun gear (S1)510and the ring gear (R1)540constitute, together with the respective pinions531,532, a mechanism corresponding to a double pinion planetary gear mechanism, while the second sun gear (S2)520and the ring gear (R1)540constitute, together with the second pinion (P2)532, a mechanism corresponding to a single pinion planetary gear mechanism.

The carrier (C1)550is coupled to the output shaft560. The second sun gear (S2)520is coupled to the rotor of the second MG400.

The transmission500is further provided with a B1brake561that fixes the first sun gear (S1)510selectively and a B2brake562that fixes the ring gear (R1)540selectively.

The B1brake561generates an engagement force from a frictional force between a friction material fixed to a case side of the transmission500and a friction material fixed to the first sun gear (S1)510side. The B2brake562generates an engagement force from a frictional force between the friction material fixed to the case side of the transmission500and a friction material fixed to the ring gear (R1)540side. The brakes561,562are connected to a speed changing hydraulic circuit (not shown) that outputs oil pressure corresponding to a control signal from the ECU1000, and are engaged and disengaged by the oil pressure output from the speed changing hydraulic circuit.

When the B1brake561is engaged such that the first sun gear (S1)510is fixed and the B2brake562is disengaged such that the ring gear (R1)540is not fixed, a speed position of the transmission500is set in a high speed position Hi. When the B2brake562is engaged such that the ring gear (R1)540is fixed and the B1brake561is disengaged such that the first sun gear (S1)510is not fixed, on the other hand, the speed position of the transmission500is set in a low speed position Lo having a larger speed ratio than the high speed position Hi. Note that the speed ratio is a ratio of an input shaft rotation speed (=the second MG rotation speed Nm2) to an output shaft rotation speed (=the rotation speed Np of the output shaft560) of the transmission500.

The output shaft560is rotated by at least one of the power of the engine100transmitted via the power distribution mechanism300and the power of the second MG400transmitted via the transmission500. The rotary force of the output shaft560is transmitted to the left and right drive wheels82via the reduction gear. As a result, the vehicle1ais caused to travel.

FIG. 11is a collinear diagram showing the power distribution mechanism300and the transmission500. By configuring the power distribution mechanism300as described above, the rotation speed of the sun gear (S)310(=the first MG rotation speed Nm1), the rotation speed of the carrier (C)330(=the engine rotation speed Ne), and the rotation speed of the ring gear (R)320have a relationship (a relationship according to which, when any two of the rotation speeds are determined, the remaining rotation speed is also determined) indicated by linked straight lines on the collinear diagram of the power distribution mechanism300.

Further, by configuring the transmission500as described above, a rotation speed of the first sun gear (S1)510, a rotation speed of the ring gear (R1)540, a rotation speed of the carrier (C1)550, and a rotation speed of the second sun gear (S2)520(=the second MG rotation speed Nm2) have a relationship (a relationship according to which, when any two of the rotation speeds are determined, the remaining two rotation speeds are also determined) indicated by linked straight lines on the collinear diagram of the transmission500.

The carrier (C1)550of the transmission500is connected to the output shaft560, and therefore the rotation speed of the carrier (C1)550matches the rotation speed of the output shaft560(i.e. the vehicle speed V). The ring gear (R)320of the power distribution mechanism300is also connected to the output shaft560, and therefore the rotation speed of the ring gear (R)320also matches the rotation speed of the output shaft560(i.e. the vehicle speed V).

In the low speed position Lo, the B2brake562is engaged such that the ring gear (R1)540is fixed, and therefore the rotation speed of the ring gear (R1)540is zero. Further, in the high speed position Hi, the B1brake561is engaged such that the first sun gear (S1)510is fixed, and therefore the rotation speed of the first sun gear (S1)510is zero. Hence, as shown inFIG. 11, when the second MG rotation speed Nm2is constant, the vehicle speed V during formation of the high speed position Hi is higher than the vehicle speed V during formation of the low speed position Lo in accordance with a relationship between a line of the high speed position Hi (a dot-dash line) and a line of the low speed position Lo (a solid line). Conversely, as shown inFIG. 11, when the vehicle speed V is constant, the second MG rotation speed Nm2during formation of the high speed position Hi is lower than the second MG rotation speed Nm2during formation of the low speed position Lo in accordance with a relationship between a line of the high speed position Hi (a dot-dot-dash line) and a line of the low speed position Lo (a solid line).

FIG. 12is a view showing the correspondence relationship between the vehicle operating point and the control mode of the second MG400(the inverter630) according to this embodiment. The correspondence relationship is basically identical to that of the first embodiment described above, but in this embodiment, the control boundary line L is set at different values during formation of the high speed position Hi and the low speed position Lo. More specifically, as shown inFIG. 11, when the vehicle speed V is constant, the second MG rotation speed Nm2is lower during formation of the high speed position Hi than during formation of the low speed position Lo. Taking this point into consideration, the ECU1000sets a control boundary line L2used during formation of the high speed position Hi on a higher vehicle speed side than a control boundary line L1used during formation of the low speed position Lo. As a result, the control mode is less likely to shift to the rectangular control mode during formation of the high speed position Hi than during formation of the low speed position Lo.

In this embodiment, in consideration of this point, the “vehicle speed limit Vsh” (=a vehicle speed at which torque limitation is started during the battery-less travel control) is modified in accordance with the speed ratio of the transmission500. More specifically, as shown inFIG. 12, during formation of the low speed position Lo, the vehicle speed limit Vsh is set at “Vsh1”, whereas during formation of the high speed position Hi having a smaller speed ratio than the low speed position Lo, the vehicle speed limit Vsh is set at “Vsh2”, which is higher than Vsh1.

As shown inFIG. 12, the vehicle speed limit Vsh1 used during formation of the low speed position Lo is set at a value on a minimum speed side of the control boundary line L1used during formation of the low speed position Lo. The vehicle speed limit Vsh2 used during formation of the high speed position Hi is set at a value on a minimum speed side of the control boundary line L2used during formation of the high speed position Hi.

FIG. 13is a function block diagram of the ECU1000according to this embodiment. The ECU1000includes the first determination unit1010, the second determination unit1020, the vehicle speed limitation unit1030, and a switching unit1040. Note that since the first determination unit1010, the second determination unit1020, and the vehicle speed limitation unit1030have already been described in the first embodiment usingFIG. 7, detailed description thereof will not be repeated here.

When the battery-less travel control is underway, the switching unit1040switches the vehicle speed limit Vsh in accordance with the speed position formed by the transmission500(the speed ratio of the transmission500). As described above, the switching unit1040sets the vehicle speed limit Vsh at “Vsh1” during formation of the low speed position Lo, and sets the vehicle speed limit Vsh at “Vsh2”, which is higher than Vsh1, during formation of the high speed position Hi having a smaller speed ratio than the low speed position Lo.

FIG. 14is a flowchart showing processing procedures executed by the ECU1000to realize the functions of the switching unit1040.

In S30, the ECU1000determines whether or not the battery-less travel control is underway. In S31, the ECU1000determines whether or not the speed position formed by the transmission500is the low speed position Lo.

When the low speed position Lo is formed (YES in S31), the ECU1000sets the vehicle speed limit Vsh at “Vsh1” in S32.

When the high speed position Hi is formed (NO in S31), on the other hand, the ECU1000sets the vehicle speed limit Vsh at “Vsh2” in S33.

In this embodiment, as described above, in a case where the low speed position Lo, in which the control mode shifts to the rectangular control at a comparatively low vehicle speed, is formed during the battery-less travel control, the vehicle speed limitation is implemented when the vehicle speed V exceeds comparatively low “Vsh1”. In a case where the high speed position Hi, in which the control mode shifts to the rectangular control at a comparatively high vehicle speed, is formed, on the other hand, the vehicle speed limitation is implemented when the vehicle speed V exceeds comparatively high “Vsh2” but not implemented until the vehicle speed V exceeds “Vsh2”. Therefore, the likelihood of a shift to the rectangular control during the battery-less travel control can be reduced through the vehicle speed limitation, and at the same time, deterioration of a travel performance of the vehicle due to the vehicle speed limitation can be minimized.

The embodiments disclosed herein are in all respects merely examples and should not be considered limiting. The scope of the invention is illustrated by the claims rather than the above description, and equivalent meanings to the claims and all modifications within the scope are included therein.