Vehicle control system and vehicle control method

A vehicle control system is configured to include a motive power unit that includes an engine and a rotating electrical machine, a power supply device that is connected to the rotating electrical machine, a pump unit that includes an electric oil pump and a mechanical oil pump for cooling the rotating electrical machine, and a control device. The control device is configured to include a carrier frequency determination unit, a rotating electrical machine determination unit, a vehicle required traveling state determination unit, a motive power unit operation mode determination unit that makes a determination on the operation mode of the motive power unit, and an EOP operation control unit that controls the operation of the electric oil pump on the basis of these determinations.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2011-289159 filed on Dec. 28, 2011 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a vehicle control system and a vehicle control method. In particular, the invention relates to a vehicle control system and a vehicle control method for a vehicle that employs an electric oil pump to cool a rotating electrical machine.

2. Description of Related Art

In a vehicle that is mounted with a rotating electrical machine, an oil pump for circulating a coolant that cools the rotating electrical machine is employed. In a vehicle that is mounted with an engine as well as a rotating electrical machine, a mechanism oil pump that is connected to an output rotary shaft of the engine can be employed. When the engine stops, this mechanical oil pump also stops operating. Thus, an electric oil pump that is driven regardless of the operation of the engine is employed, so that the rotating electrical machine can be cooled while the engine is stopped.

For example, it is described in Japanese Patent Application Publication No. 2004-256063 (JP-2004-256063 A) that, in a control apparatus for a vehicle, when the discharge rate of a mechanical oil pump that is driven by an engine during the cut-off of fuel supply becomes insufficient, an oil pump driven by a motor-generator or an electric oil pump is driven.

Incidentally, as an art related to the invention, it is described in Japanese Patent Application Publication No. 2007-112290 (JP-2007-112290 A) that the optimal rotational speed is set in such a manner as to increase as the carrier frequency used for a drive circuit of an electric motor increases when a vehicle is equipped with an internal combustion engine and the electric motor as motive power output units.

As described in Japanese Patent Application Publication No. 2007-112290 (JP-2007-112290 A), the carrier frequency in the drive circuit of a rotating electrical machine may be changed in accordance with the traveling state of the vehicle. For example, when the rotational speed of the rotating electrical machine is high while the vehicle travels at high speed, the carrier frequency is set high in order to maintain the controllability thereof. Besides, on the contrary, when the rotational speed of the rotating electrical machine is low while the vehicle travels at low speed, the carrier frequency is set low. When the carrier frequency becomes low, the amplitude of ripple noise on a drive signal of the rotating electrical machine becomes large. Then, the loss in the rotating electrical machine increases, and the temperature of the rotating electrical machine rises.

In order to cool the rotating electrical machine, it is possible to employ a mechanical oil pump or an electric oil pump. However, since the mechanical oil pump is driven by an engine, the discharge rate of a coolant is determined in accordance with the rotational speed of the engine. In this manner, the cooling capacity of the mechanical oil pump changes depending on the operation state of the engine. Accordingly, if the mechanical oil pump is employed to cool the rotating electrical machine when the temperature of the rotating electrical machine has risen due to a change in the carrier frequency, the rotating electrical machine is insufficiently cooled in some cases.

SUMMARY OF THE INVENTION

The invention provides a vehicle control system and a vehicle control method that make it possible to appropriately cool a rotating electrical machine even when the carrier frequency is changed.

A vehicle control system of a first aspect of the invention is equipped with a motive power unit that has a rotating electrical machine, a drive circuit that is connected to the rotating electrical machine and operates on the basis of a certain carrier frequency, a pump unit that circulates a coolant that cools the rotating electrical machine, and a control device that increases a cooling capacity of the pump unit for cooling the rotating electrical machine when the carrier frequency is equal to or lower than a preset threshold frequency.

Thus, the rotating electrical machine whose temperature rises when the carrier frequency is changed to a low value can be appropriately cooled.

In a vehicle control method of a second aspect of the invention, a cooling capacity of a pump unit that circulates a coolant that cools a rotating electrical machine is increased, when a carrier frequency preset for a drive circuit that is connected to the rotating electrical machine is equal to or lower than a threshold frequency.

Thus, the rotating electrical machine whose temperature rises when the carrier frequency is changed to a low value can be appropriately cooled.

DETAILED DESCRIPTION OF EMBODIMENT

The embodiment according to the invention will be described hereinafter in detail using the drawings. Hereinafter, a configuration having, as a motive power unit, a motive power transmission mechanism that is provided between an engine and a single rotating electrical machine will be described. However, this is an exemplification for illustration. In this case, any configuration having an engine and a rotating electrical machine is acceptable. In addition, the relationship between the output of the engine and the output of the rotating electrical machine can be appropriately changed in accordance with the specification of a vehicle. Besides, the description will be given on the assumption that a single rotating electrical machine is mounted on the vehicle. However, this is also an exemplification, and there may also be a case where a plurality of rotating electrical machines are mounted on the vehicle. For example, it is appropriate to adopt a configuration in which one rotating electrical machine is employed for driving and the other rotating electrical machine is employed for electric power generation. In addition, separate rotating electrical machines may be employed to drive front wheels and rear wheels respectively.

Besides, a power supply device that is connected to the rotating electrical machine will be described as including an electric storage device, a voltage converter, a smoothing capacitor, and an inverter. However, this is the description of main component elements of the power supply device. Therefore, the power supply device may include other component elements. For example, the power supply device may include a low-voltage inverter circuit, a system main relay, a DC/DC converter, and the like. Besides, a power supply for a drive circuit of an electric oil pump will be described as a low-voltage power supply independent of the power supply device of the rotating electrical machine. However, this is an exemplification for illustration. For example, an electric power obtained through voltage conversion into a low voltage may be supplied from the power supply device of the rotating electrical machine to the drive circuit of the electric oil pump.

Besides, in the following description, the rotating electrical machine and the motive power transmission mechanism are accommodated in a single case body. In addition, the description will be given on the assumption that a coolant circulates between the interior of the case body and a pump unit. However, this is an exemplification for illustration. For example, it is appropriate to adopt a configuration in which the coolant circulates among the rotating electrical machine, the motive power transmission mechanism, and the pump unit, instead of packing the rotating electrical machine and the motive power transmission mechanism into a single case.

Hereinafter, similar elements will be denoted by the same reference symbols respectively in all the drawings, and the same description will be omitted. Besides, in the description in the text, previously mentioned reference symbols are used according to need.

FIG. 1is a diagram showing the configuration of a vehicle control system10for a hybrid vehicle. This vehicle control system10is a system that appropriately controls the operation of an engine14and a rotating electrical machine18, which constitute a motive power unit12, and the driving of a pump unit40for cooling the rotating electrical machine18.

The vehicle control system10includes the engine14and the rotating electrical machine18as the motive power unit12, which serves as a drive source of the hybrid vehicle. Besides, the vehicle control system10includes, as a power supply device30that is connected to the rotating electrical machine18an electric storage device32, a voltage converter34, an inverter36, and smoothing capacitors37and38. The vehicle control system10further includes the pump unit40, which supplies a coolant50into a case body22, which includes the rotating electrical machine18therein, through circulation. The pump unit40is configured to include a mechanical oil pump42and an electric oil pump44. The mechanical oil pump42is driven by the engine14. The electric oil pump44is driven by an EOP drive circuit62that operates by a low-voltage power supply64. In addition, the vehicle control system10is configured to include a control device70that controls these operations as a whole.

The motive power unit12is configured to include the engine14, the rotating electrical machine18, and a motive power transmission mechanism16that is provided between the engine14and the rotating electrical machine18. The engine14is an internal combustion engine. Besides, the rotating electrical machine18is a motor-generator (an M/G) that is mounted on the hybrid vehicle. In addition, the rotating electrical machine18is a three-phase synchronous rotating electrical machine that functions as a motor when the hybrid vehicle is supplied with an electric power from a power supply device30that includes an inverter36as a drive circuit, and that functions as a generator when the hybrid vehicle is driven by the engine14or braked.

A temperature detector24that is provided on the rotating electrical machine18is a rotating electrical machine temperature detector that detects a temperature of the rotating electrical machine18. Data detected by the temperature detector24are transmitted to the control device70, using an appropriate signal line.

The motive power transmission mechanism16is a mechanism that has a function of distributing a motive power to be supplied to the hybrid vehicle between an output of the engine14and an output of the rotating electrical machine18. As this motive power transmission mechanism16, it is possible to employ a planetary gear mechanism that is connected to three shafts, namely, an output shaft of the engine14, an output shaft of the rotating electrical machine18, and an output shaft leading to an axle (not shown). A shaft that connects the motive power transmission mechanism16and the engine14to each other inFIG. 1is an output shaft20of the engine14. This output shaft20is connected to a drive shaft of the mechanical oil pump42via a connecting shaft60, and is used to drive the mechanical oil pump42.

The power supply device30is a device for driving the rotating electrical machine18. The electric storage device32that constitutes the power supply device30is a rechargeable high-voltage secondary battery. More specifically, the electric storage device32is a lithium-ion assembled battery that has a terminal voltage ranging from about 200V to about 300 V. The assembled battery is designed to obtain the aforementioned predetermined terminal voltage through the combination of a plurality of batteries called electric cells or battery cells, whose terminal voltage ranges from 1 V to several volts. As the electric storage device32, it is possible to employ a nickel hydride assembled battery, a large-capacity capacitor, or the like.

The voltage converter34is arranged between the electric storage device32and the inverter36. In addition, the voltage converter34is a circuit having a direct-current voltage conversion function. The voltage converter34is configured to include a reactor and a switching element. The voltage converter34has, as the voltage conversion function, a step-up function of stepping up a voltage on the electric storage device32side with the aid of the energy accumulation behavior of the reactor to supply the stepped-up voltage to the inverter36side, and a step-down function of stepping down an electric power from the inverter36side to supply the stepped-down voltage to the electric storage device32side as an electric power for charging.

The inverter36is a drive circuit that is connected to the rotating electrical machine18, and is configured to include a plurality of switching elements, a plurality of reverse connection diodes, and the like. In addition, the inverter36changes direct current power to an alternating-current electric power and vice versa. That is, the inverter36has an alternating-direct conversion function of converting an alternating-current three-phase regenerative electric power from the rotating electrical machine18into a direct-current electric power to supply the direct-current electric power to the electric storage device32side as an electric power for charging, when the rotating electrical machine18is caused to function as a generator. Besides, the inverter36has a direct-alternating conversion function of converting a direct-current electric power from the electric storage device32side into an alternating-current three-phase driving electric power to supply the alternating-current three-phase driving electric power to the rotating electrical machine18as an alternating-current driving electric power.

The inverter36is a circuit that generates a three-phase drive signal through pulse width modulation (PWM) control for appropriately adjusting the timings at which the plurality of the switching elements are turned on and off, and supplies the three-phase driving signal to the rotating electrical machine18. PWM control is control of modulating the pulse width through a comparison between a fundamental wave signal having a cycle corresponding to a rotation period of the rotating electrical machine18and a carrier signal having a sawtooth waveform. The frequency of the carrier signal is called a carrier frequency. In this manner, the carrier frequency is set in accordance with the frequency of fundamental waves in order to ensure the controllability of PWM control. For example, when the rotational speed of the rotating electrical machine18becomes high, the frequency of fundamental waves becomes high, so that a high carrier frequency is set. On the contrary, when the rotational speed of the rotating electrical machine18becomes low, the frequency of fundamental waves becomes low, so that a low carrier frequency is set.

A smoothing capacitor37that is provided between the electric storage device32and the voltage converter34is a capacitor element that has a function of smoothing the voltage and electric current on the electric storage device32side. Besides, a smoothing capacitor38that is provided between the voltage converter34and the inverter36is a capacitor element that has a function of smoothing the voltage and electric current on the inverter36side.

The case body22is a housing that includes the motive power transmission mechanism16and the rotating electrical machine18therein. The case body22is stores the coolant50in its interior space. The coolant50lubricates the motive power transmission mechanism16and a movable region of the rotating electrical machine18, and cools the power transmission mechanism16and the rotating electrical machine18. As the coolant, it is possible to employ a lubricating oil called automatic transmission fluid (ATF).

The pump unit40is a unit that includes the mechanical oil pump42and the electric oil pump44. In addition, the pump unit40has a function of supplying the coolant50to the inner space of the case body22through circulation. A coolant discharge channel52is a coolant flow pipe that couples a coolant discharge port, which is provided on a lower side of the case body22along the gravitational direction or at a location close to a bottom of the case body22, and the pump unit40to each other. A coolant supply channel54is a coolant flow pipe that couples the pump unit40and a coolant supply port, which is provided on an upper side of the case body22along the gravitational direction or at a location close to a ceiling portion of the case body22, to each other.

The mechanical oil pump42and the electric oil pump44are connected in parallel with each other between the coolant discharge channel52and the coolant supply channel54. A check valve46is a valve that is provided to prevent the coolant50from flowing backward between the mechanical oil pump42and the coolant supply port of the case body22. By the same token, a check valve48is a valve that is provided to prevent the coolant50from flowing backward between the electric oil pump44and the coolant supply port of the case body22.

The mechanical oil pump42, which is shown as MOP inFIG. 1, is a pump whose drive shaft is connected to the output shaft20of the engine14via the connecting shaft60. The mechanical oil pump42is driven when the engine14operates. That is, the driving of the mechanical oil pump42is started as the engine14is started, and the driving of the mechanical oil pump42is ended as the engine14is stopped.

The electric oil pump44, which is shown as EOP inFIG. 1, is driven by an EOP drive circuit62under a control signal from the control device70. The EOP drive circuit62is supplied with a direct-current electric power from a low-voltage power supply64. The low voltage means a voltage lower than the voltage of the electric storage device32, and it is possible to use a voltage of, for example, about 12 to 16 V. As a motor that rotates the drive shaft of the electric oil pump44, it is possible to employ a three-phase synchronous motor. In this case, the EOP drive circuit62is configured to include an inverter that has a direct current/alternating current conversion function. Incidentally, a single-phase alternating-current motor or a direct-current motor can also be employed instead of the three-phase synchronous motor. The contents of the EOP drive circuit62are changed in accordance with the motor type of a motor that is employed to rotate the drive shaft of the electric oil pump44.

The control device70is a control circuit that has a function of controlling the aforementioned respective elements as a whole. However, in this case in particular, the control device70has a function of performing the control of appropriately cooling the rotating electrical machine18and the like while selectively using the mechanical oil pump42and the electric oil pump44in accordance with the carrier frequency. This control device70can be configured as a computer that is suited to be mounted on a vehicle.

The control device70includes a carrier frequency determination unit72, a rotating electrical machine temperature determination unit74, and a vehicle required traveling state determination unit76. The carrier frequency determination unit72determines whether or not a carrier frequency f is equal to or lower than a threshold frequency f0. The rotating electrical machine temperature determination unit74determines whether or not a temperature θMof the rotating electrical machine18is equal to or higher than a predetermined first threshold temperature θM0. The vehicle required traveling state determination unit76determines whether or not a traveling state required of the hybrid vehicle reaches a predetermined critical traveling state. The critical traveling state is a critical vehicle traveling state of the vehicle in which the cooling of the rotating electrical machine18can be covered by the electric oil pump44alone.

Besides, the control device70is configured to include an EOP operation control unit78that controls the operation of the electric oil pump44, and a motive power unit operation mode determination unit80that determines whether the operation mode of the motive power unit12is an EV operation mode or an I-IV operation mode. It should be noted herein that the EV operation mode is an operation mode in which the rotating electrical machine18is operated without operating the engine14, and that the HV operation mode is an operation mode in which both the engine14and the rotating electrical machine18can be operated.

These functions of the control device70can be realized by running software. More specifically, these functions of the control device70can be realized by executing an oil pump operation control program.

FIG. 2is a flowchart showing the most basic procedures of selectively using the mechanical oil pump42and the electric oil pump44when the carrier frequency f is changed. The respective procedures correspond to the respective processing procedure of the oil pump operation control program.

In this case, first of all, it is determined whether or not the electric oil pump44has been stopped (S10). In the case where means for detecting a rotational speed of the electric oil pump44is provided, this determination can be made on the basis of whether or not the rotational speed of the electric oil pump44is equal to zero. Alternatively, this determination can also be made on the basis of whether or not a drive command signal has been output from the control device70to the EOP drive circuit.

If a positive determination is made in S10, it is then determined whether or not the carrier frequency f is equal to or lower than a predetermined threshold frequency f0(S12). This procedure is executed through the function of the carrier frequency determination unit72of the control device70. More specifically, in the control device70, the value of the carrier frequency f set for the inverter36is compared with the threshold frequency f0. The threshold frequency f0is set in accordance with a rise in a temperature θMof the rotating electrical machine18that results from a decrease in the carrier frequency f.

The carrier frequency f in the inverter36is set in advance according to the frequency of the fundamental wave signal used for control, in order to ensure the controllability of the rotating electrical machine18. When the frequency of the fundamental wave signal is high, the carrier frequency f is set high. When the frequency of the fundamental wave signal is low, the carrier frequency f is set low. The frequency of the fundamental wave signal is a frequency corresponding to the rotational speed of the rotating electrical machine18. In the fundamental wave signal, noise resulting from this carrier frequency emerges as a ripple.

FIGS. 3A and 3Bare diagrams showing the cycle of a fundamental wave signal and the magnitude of ripple noise.FIG. 3Ashows the cycle of the fundamental wave signal and the magnitude of ripple noise in a case wherein the rotational speed of the rotating electrical machine18is high and the vehicle speed of the hybrid vehicle is high in the case where the cycle of the fundamental wave signal is short.FIG. 3Bshows the cycle of the fundamental wave signal and the magnitude of ripple noise in a case wherein the rotational speed of the rotating electrical machine18is low and the vehicle speed of the hybrid vehicle is low in the case where the cycle of the fundamental wave signal is long. In these drawings, the axis of abscissa represents time, and the axis of ordinate represents voltage indicating signal amplitude. In each of the drawings, a fundamental wave signal waveform corresponding to about one cycle is shown, and cyclic ripple noise superimposed on this fundamental wave signal waveform is observable.

A comparison between an amplitude ΔVr1of ripple noise ofFIG. 3Aand an amplitude ΔVr2of ripple noise ofFIG. 3Breveals that the amplitude of ripple noise is larger inFIG. 3Bin which the cycle of the fundamental wave signal is long and the carrier frequency f is low. When the amplitude of ripple noise is large, the loss in the rotating electrical machine18becomes large, the temperature θMof the rotating electrical machine18rises, and the necessity for cooling arises. In this manner, the temperature θMof the rotating electrical machine18rises as the carrier frequency f decreases. Therefore, the carrier frequency f at which the necessity for cooling arises is set as a threshold frequency f0.

Returning toFIG. 2, if a positive determination is made in S12, the stopped electric oil pump44is driven (S14). This procedure is executed through the function of the EOP operation control unit78of the control device70. More specifically, a drive command signal is output from the control device70to the EOP drive circuit62. If a negative determination is made in S12, the electric oil pump44remains stopped without being driven.

In S14, the electric oil pump44is driven instead of using the mechanical oil pump42for the following reason. That is, the mechanical oil pump is driven by the engine14. In addition, the discharge rate of the coolant50is determined in accordance with the rotational speed of the engine14. In this manner, the cooling capacity of the mechanical oil pump42changes depending on the operation state of the engine14. For example, when the rotational speed of the engine14is low, the coolant50that is needed to cool the rotating electrical machine18is not discharged. On the other hand, the electric oil pump44can discharge a certain amount of the coolant50regardless of the operation state of the engine14. Therefore, the rotating electrical machine18can be appropriately cooled. Due to this difference, if a positive determination is made in S12, the electric oil pump44is driven instead of the mechanical oil pump42. Accordingly, the control device drives the electric oil pump when the carrier frequency is equal to or lower than the threshold frequency.

Thus, the electric oil pump44can discharge a predetermined amount of the coolant50without depending on the rotational speed of the engine14as in the case of the mechanical oil pump42. By using this electric oil pump44, the rotating electrical machine18can be appropriately cooled even when the carrier frequency is changed to a low value.

When the electric oil pump44is driven, an electric power is thereby consumed, so that the fuel economy of the hybrid vehicle as a whole deteriorates. Thus, it is preferable to drive the electric oil pump44only when it is actually necessary to do so.FIG. 4is a flowchart showing procedures for restraining fuel economy from deteriorating, as an improved version of the procedures ofFIG. 2. The respective procedures correspond to the respective processing procedures of the oil pump operation control pro gram.

Since S12and S14inFIG. 4are the same asFIG. 2respectively, detailed description thereof is omitted. In this case, if a positive determination is made in S12, it is then determined whether or not the actual temperature θMof the rotating electrical machine18is equal to or higher than a predetermined first threshold temperature θM0(S16). This procedure is executed through the function of the rotating electrical machine temperature determination unit74of the control device70. More specifically, data detected by the temperature detector24are acquired, transmitted to the control device70, and compared with the first threshold temperature θM0. As the first threshold temperature θM0, it is possible to use a temperature at which the rotating electrical machine18is not overheated.

If a positive determination is made in S16, the stopped electric oil pump44is driven (S14). This procedure is the same as the contents described with reference toFIG. 2, and therefore, detailed description thereof is omitted. If the temperature θMof the rotating electrical machine18is lower than the first threshold temperature θM0, a negative determination is made in S16, the electric oil pump44remains stopped instead of being driven, and a return to S10is made.

A comparison betweenFIG. 4andFIG. 2reveals that the procedure of S16is added inFIG. 4. While a decrease in the carrier frequency f is used as an indicator of a rise in the temperature of the rotating electrical machine18inFIG. 2, it is additionally determined, using the temperature θMof the rotating electrical machine18, whether or not the temperature of the rotating electrical machine18has risen inFIG. 4. Thus, the electric oil pump44can be restrained from being unnecessarily driven, and the fuel economy of the hybrid vehicle as a whole can be restrained from deteriorating. Accordingly, the control device70drives the electric oil pump44when the carrier frequency is equal to or lower than the threshold frequency and also, the temperature of the rotating electrical machine is equal to or higher than the predetermined first threshold temperature θM0. In addition, the control device70prohibits drive of the electric oil pump44when the temperature of the rotating electrical machine18is lower than the predetermined first threshold temperature θM0even if the carrier frequency f is equal to or lower than the threshold frequency.

Although the electric oil pump44discharges a certain amount of the coolant50, the discharge rate of the electric oil pump44may be insufficient to cool the rotating electrical machine18.FIG. 5is a flowchart showing procedures executed when the cooling capacity of the electric oil pump44is insufficient, as an improved version ofFIG. 4. The respective procedures correspond to respective processing procedures of the oil pump operation control program.

In this case, first of all, it is determined whether or not the operation mode of the motive power unit12is the EV operation mode (S18). This determination can be made on the basis of an operation command signal that is output from the control device70to the motive power unit12. Alternatively, in the case where each of the engine14and the rotating electrical machine18is provided with rotational speed detection means, the determination can also be made on the basis of whether or not the engine14is rotating and whether or not the rotating electrical machine18is rotating. That is, if the rotating electrical machine18is rotating and the engine14is not rotating, a positive determination is made in S18.

If a negative determination is made in S18, the operation mode of the motive power unit12is the HV operation mode, so that no transition to the subsequent steps is made. Although the same holds true forFIGS. 2 and 4as well, the procedure of S18is shown to draw attention in the case ofFIG. 5, because the processing of making a changeover to the HV operation mode is performed in S26.

If a positive determination is made in S18, the procedures of S10, S12, S16, and S14as described with reference toFIG. 4are sequentially executed in this order. The contents of these procedures are identical to those described with reference toFIG. 4, and therefore, detailed description thereof is omitted.

After the processing of S14is performed, it is determined whether or not the actual temperature θMof the rotating electrical machine18is equal to or higher than a predetermined second threshold temperature θM1(S20). The contents of this procedure are similar to those of S16, but are different therefrom in that the data detected by the temperature detector24are compared not with the first threshold temperature θM0but with the second threshold temperature θM1, which is higher than the first threshold temperature θM0. The second threshold temperature θM1is used to determine whether the cooling by the electric oil pump44is sufficient or insufficient. Therefore, (θM1-θM0) can be set as an appropriate value ranging from several ° C. to about 10° C., in consideration of a measurement error.

If a negative determination is made in S20, the cooling by the electric oil pump44is sufficient. Therefore, it is then determined whether or not the temperature θMof the rotating electrical machine18is lower than the first threshold temperature θM0(S22). The contents of the determination in this procedure are the opposite of those of S16. If a positive determination is made in S22, the cooling by the electric oil pump44is sufficient and the rotating electrical machine18is not overheated, so that the rotating electrical machine18no longer needs to be cooled. Thus, the electric oil pump44is stopped from being driven (S24), and a return to S16is made. If a negative determination is made in S22, the electric oil pump44continues to be driven, and a return to S20is made to monitor a determination on whether or not the temperature θMof the rotating electrical machine18becomes equal to or higher than the second threshold temperature θM1.

If a positive determination is made in S20, the cooling by the electric oil pump44is insufficient. Therefore, the operation mode of the motive power unit12is changed over from the EV operation mode to the HV operation mode (S26). The control device70prohibits operate of the internal combustion engine14when the temperature of the rotating electrical machine18is higher than the predetermined second threshold temperature θM1, which is higher than the predetermined first threshold temperature θM0, after the electric oil pump44is driven. Then, the control device70changes over the operation mode of the motive power unit from the EV operation mode in which the rotating electrical machine18is operated to the HV operation mode in which both the internal combustion engine14and the rotating electrical machine18can be operated, thereby starting the internal combustion engine14to driving the mechanical oil pump42. By changing the operation mode to the HV operation mode, it becomes possible to start the engine14and operate the mechanical oil pump42. The discharge rate of the mechanical oil pump42is increased by raising the rotational speed of the engine14. Thus, a required cooling capacity can be ensured. Incidentally, since the output of the engine14contributes to the output required of the hybrid vehicle, the output of the rotating electrical machine18can be made low. In this manner as well, the temperature of the rotating electrical machine18can be held low.

When the mechanical oil pump42is driven, the electric oil pump44may be left to be driven. At this time, since the cooling capacity of the electric oil pump44and the cooling capacity of the mechanical oil pump42are summated, the rotating electrical machine18is effectively cooled. Because electric power is consumed to drive the electric oil pump44, the electric oil pump44may be stopped from being driven when the mechanical oil pump42is driven. In this case, the mechanical oil pump42begins to be driven after the check valve48is completely closed by stopping the electric oil pump44from being driven. In this manner, the coolant50can be prevented from flowing backward between the mechanical oil pump42and the electric oil pump44.

In the case where the temperature of the rotating electrical machine18rises, it is possible to determine whether or not the rotating electrical machine18can be cooled by the electric oil pump44alone by finding out a traveling state of the hybrid vehicle, without needing to determine whether or not the temperature θMof the rotating electrical machine18is equal to or higher than the second threshold temperature θM1.FIG. 6is a flowchart showing procedures of a method of determining whether or not the cooling capacity of the electric oil pump44becomes insufficient on the basis of a traveling state of the hybrid vehicle required by a user. The respective procedures correspond to respective processing procedures of the oil pump operation control program.

In this case, the procedures of S18, S10, S12, S16, and S14are completely the same as inFIG. 5. After the processing of S14is performed, a traveling state required of the hybrid vehicle is acquired (S28). The required traveling state can be acquired as an output and a vehicle speed that are required of the hybrid vehicle, from a degree of depression of an accelerator by the user, an operation of a brake pedal by the user, and the like. It is then determined whether or not the required traveling state thus acquired reaches a critical traveling state (S30). This procedure is executed through the function of the vehicle required traveling state determination unit76of the control device70.

It should be noted herein that the critical traveling state is a vehicle traveling state in which a required cooling capacity of the pump unit40corresponding to a traveling state of the hybrid vehicle as a required cooling capacity corresponding to the traveling state can be covered by the electric oil pump44alone. A value indicating the critical traveling state may be other than the temperature θMof the rotating electrical machine18. For example, it is possible to use a power or a torque as an output of the motive power unit12, a vehicle speed of the hybrid vehicle, an accelerator depression degree corresponding to these values, a time-dependent change rate of the accelerator depression degree, a rotational speed of the axle, a rotational speed of the rotating electrical machine18, a rotational speed of the engine14, or the like.

FIG. 7shows an example in which a threshold rotational speed NE0about the rotational speed NEof the engine14is used as a value indicating the critical traveling state. InFIG. 7, the axis of abscissa represents the rotational speed NEof the engine14, and the axis of ordinate represents the cooling capacity of the mechanical oil pump42or the cooling capacity of the electric oil pump44. In this case, a rectilinear characteristic line stretching rightward and upward with respect to NEis shown as MOP. This is a cooling capacity characteristic line of the mechanical oil pump42. Besides, a characteristic line that is constant with respect to NEis shown as EOP. This is a cooling capacity characteristic line of the electric oil pump44.

The threshold rotational speed NE0is the rotational speed NEof the engine14at which the cooling capacity characteristic line of the mechanical oil pump42and the cooling capacity characteristic line of the electric oil pump44intersect with each other. When the rotational speed NEof the engine14is equal to the threshold rotational speed NE0, the cooling capacity of the mechanical oil pump42is lower than the cooling capacity of the electric oil pump44. It is therefore preferable to drive the electric oil pump44in this range. On the other hand, when the rotational speed NEof the engine14is equal to or higher than the threshold rotational speed NE0, the cooling capacity of the mechanical oil pump42is higher than the cooling capacity of the electric oil pump44. Accordingly, in this range, when the cooling capacity of the electric oil pump44is insufficient to cool the rotating electrical machine18, it is preferable to drive the mechanical oil pump42. In this manner, the threshold rotational speed NE0can be used as a value indicating the critical traveling state.

Returning again toFIG. 6, if a negative determination is made in S30, the electric oil pump44continues to be driven, and a return to S28is made. If a positive determination is made in S30, the operation mode of the motive power unit12is changed over from the EV operation mode to the HV operation mode (S26). Thus, the engine14is started, and the mechanical oil pump42is driven. These contents are the same as those of S26inFIG. 5, and therefore, detailed description thereof is omitted. In this manner, the rotating electrical machine18can be appropriately cooled without comparing the temperature θMof the rotating electrical machine18with the second threshold temperature θM1.

The vehicle control system according to the invention can be utilized for a vehicle that is mounted with a mechanical oil pump and an electric oil pump.

While the disclosure has been explained in conjunction with the specific exemplary embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiment of the disclosure as set forth herein is intended to be illustrative, not limiting. There are changes that may be made without departing from the scope of the disclosure.