Method for controlling electric vehicle and device for controlling electric vehicle

Abstract

The present invention relates to a method for controlling an electric vehicle provided with an electric motor, a motive power transmission mechanism for transmitting torque of the electric motor to wheels, and a temperature acquisition unit for detecting or estimating a temperature affecting friction of the motive power transmission mechanism. On the basis of the detected temperature, the operation point of the electric motor is changed from an optimal point at which the efficiency of the electric motor is maximized to a low-efficiency point at which the efficiency is lower than the optimal point, or the gear ratio of the motive power transmission mechanism is changed so that the amount of heat generated by the motive power transmission mechanism increases.

Description

Control method for electric vehicle and control device for electric vehicle

[0001] The present invention relates to a control method for an electric vehicle and a control device for an electric vehicle.

[0002] 2. Description of the Related Art A control device for an electric vehicle is known that reduces friction in a power transmission mechanism by warming up the power transmission mechanism that transmits torque from an electric motor to wheels (see, for example, Patent Document 1).

[0003] JP 2011-58586 A

[0004] The warm-up device for an electric vehicle transmission (control device for an electric vehicle) described in Patent Document 1 requires an additional device, for example, a heat storage device in which a container is filled with a latent heat storage material or a nucleation device.

[0005] An object of the present invention is to provide a control method for an electric vehicle and a control device for an electric vehicle that can reduce friction in a power transmission mechanism without adding any additional device.

[0006] In one aspect of the present invention, based on the temperature that affects the friction of a power transmission mechanism, the operating point of an electric motor is changed from an optimal point where the efficiency of the electric motor is maximized to a low-efficiency point where the efficiency is lower than the optimal point, or the gear ratio of the power transmission mechanism is changed so that the amount of heat generated by the power transmission mechanism increases.

[0007] It is a schematic diagram showing a general configuration of an electric vehicle according to an embodiment. It is a control block diagram of a controller according to an embodiment. It is a flowchart of control executed by the controller. It is a diagram showing the relationship between the operating point of an electric motor and the efficiency of the electric motor. It is a diagram showing the relationship between the operating point of an electric motor and the efficiency of the electric motor. It is a diagram showing the relationship between the operating point of an electric motor and the efficiency of the electric motor. It is a diagram showing the relationship between the operating point of an electric motor and the efficiency of the electric motor. It is a diagram showing the relationship between the operating point of an electric motor and the efficiency of the electric motor.

[0008] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that, hereinafter, a driver's operation to request braking / driving force using a braking / driving force request operation means such as an accelerator pedal will be referred to as an "accelerator operation," and the amount of operation will be referred to as an "accelerator operation amount." Braking / driving force refers to driving force and braking force, and is generated by driving torque and braking torque. Braking torque also includes regenerative torque. In a program that executes control, driving torque and braking torque are expressed by positive and negative torque, so torque may be described as a negative value.

[0009] 1 is a diagram showing a schematic configuration of a vehicle 1. The vehicle 1 includes a braking / driving force generating device 2, a control device 3, and sensors 4, 5, 6A, 6B, 7A, and 7B.

[0010] The braking / driving force generating device 2 includes a battery 21, inverters 22, 23, an electric motor 24 for front wheel drive, an electric motor 25 for rear wheel drive, a power transmission mechanism 26 for front wheel drive, a power transmission mechanism 27 for rear wheel drive, wheels 28, and a friction braking mechanism 29.

[0011] Based on a command input from the control device 3, the inverters 22, 23 convert the direct current input from the battery 21 into alternating current and output it to the corresponding electric motors 24, 25. Based on a command input from the control device 3, the inverters 22, 23 also convert the alternating current input from the corresponding electric motors 24, 25 into direct current and input it to the battery 21.

[0012] The electric motors 24, 25 are lubricated with lubricating oil and cooled with a coolant such as water or cooling oil. The electric motor 24 is provided with a rotation speed sensor 7A, and the electric motor 25 is provided with a rotation speed sensor 7B.

[0013] The power transmission mechanisms 26, 27 include transmissions 26A, 27A and differential gears 26B, 27B, and transmit the torque of the electric motors 24, 25 to the wheels 28. The power transmission mechanisms 26, 27 are gear boxes in which the respective transmissions 26A, 27A and differential gears 26B, 27B are housed in cases, and share at least one of the lubricating oil and the coolant circulating therein with the corresponding electric motors 24, 25. The power transmission mechanism 26 is provided with a temperature sensor 6A as a temperature acquisition unit, and the power transmission mechanism 27 is provided with a temperature sensor 6B as a temperature acquisition unit. The transmissions 26A, 27A are configured to be switchable between a plurality of gear ratios using a plurality of gears, for example, and transmit the torque of the electric motors 24, 25 to the wheels 28 at a gear ratio based on the control of the control device 3. Although electric vehicles rarely have a configuration that allows switching between a plurality of gear ratios like transmissions 26A and 27A, vehicle 1 is provided with transmissions 26A and 27A.

[0014] The friction braking mechanism 29 includes, for example, a hydraulic or electric disc brake, and generates a braking force on the wheel 28 by friction force.

[0015] The control device 3 includes an accelerator position sensor 4 as an accelerator operation amount sensor, a wheel rotation speed sensor 5, temperature sensors 6A and 6B, rotation speed sensors 7A and 7B, and a controller 8.

[0016] The accelerator position sensor 4 detects the amount of accelerator operation of the vehicle 1. The accelerator position sensor 4 is configured by, for example, a pedal stroke sensor, and detects the amount of operation of an accelerator pedal, which serves as driving force request operation means, as the amount of accelerator operation.

[0017] The wheel rotation speed sensors 5 are provided, for example, at the positions of the wheels 28 and detect the rotation speed of each wheel 28. The rotation speed of the wheels 28 is used, for example, to calculate the speed of the vehicle 1 (hereinafter referred to as vehicle speed) and to determine vehicle behavior such as the difference in rotation speed between the front wheels and the rear wheels.

[0018] The temperature sensors 6A and 6B detect temperatures that affect the friction of the power transmission mechanisms 26 and 27. In this embodiment, the temperature sensors 6A and 6B detect the temperatures of the coolants of the power transmission mechanisms 26 and 27.

[0019] The rotation speed sensors 7A, 7B are configured by, for example, resolvers or encoders, and detect the rotation speed of the rotary shafts of the corresponding electric motors 24, 25 (hereinafter referred to as the rotation speed of the electric motors 24, 25).

[0020] The controller 8 controls the torque generated by the electric motors 24, 25 based on the accelerator operation amount, the vehicle speed, etc. The controller 8 is realized by, for example, a microcomputer including a calculation unit such as a CPU or a GPU, a storage unit 81 such as a ROM and a RAM, and an input / output unit such as an input / output interface. The accelerator position sensor 4, the wheel rotation speed sensor 5, the temperature sensors 6A, 6B, the rotation speed sensors 7A, 7B, and the inverters 22, 23 are electrically or communicatively connected to the controller 8.

[0021] 2 is a control block diagram of the controller 8. As shown in FIG. 2, the controller 8 includes a storage unit 81, a required torque setting unit 82, a required torque distribution unit 83, a warm-up necessity determination unit 84, and an operating point setting unit 85.

[0022] The storage unit 81 stores a computer program for causing the controller 8 to function, various maps used for control, various parameter values, etc. The maps stored in the storage unit 81 include, for example, a map that defines the relationship between the accelerator operation amount and vehicle speed and the required torque, and a map that defines the relationship between the operating points of the electric motors 24, 25 and the efficiency of the electric motors 24, 25. The parameter values ​​stored in the storage unit 81 include, for example, various threshold values ​​such as a temperature threshold value that determines whether or not warm-up is required.

[0023] The required torque setting unit 82 sets the required torque based on the accelerator operation amount and the vehicle speed. The required torque is the torque required by the driver through the operation of the accelerator pedal. The vehicle speed is calculated from the rotational speed of each wheel 28.

[0024] The required torque distribution unit 83 distributes the required torque to the front and rear wheels in accordance with the torque distribution values ​​for the front and rear wheels.

[0025] The warm-up necessity determining unit 84 determines whether the power transmission mechanisms 26, 27 need to be warmed up based on the temperatures acquired from the temperature sensors 6A, 6B.

[0026] The operating point setting unit 85 independently sets the operating points of the electric motors 24, 25 and the transmissions 26A, 27A. The operating points of the electric motors 24, 25 are represented by the torque and rotation speed of the electric motors 24, 25. The operating points of the transmissions 26A, 27A are represented by the gear ratio of the transmissions 26A, 27A. In this embodiment, the operating point setting unit 85 sets the operating points of the electric motors 24, 25 and the transmissions 26A, 27A based on the required torque, the torque distribution value between the front and rear wheels, the determination result of the warm-up necessity determination unit 84, and a map that defines the relationship between the operating points of the electric motors 24, 25 and their efficiency.

[0027] 3 is a flowchart of the control executed by the controller 8. The control routine shown in the flowchart is pre-programmed, and this program is installed in the controller 8. In accordance with the program, the controller 8 repeatedly executes the following control routine at an operation cycle of, for example, about 10 milliseconds.

[0028] In step S1 of FIG. 3, the controller 8 acquires the outputs of the sensors 4, 5, 6A, 6B, 7A, and 7B, i.e., the accelerator operation amount, the rotational speed of the wheels 28, the temperature of the coolant in the power transmission mechanisms 26 and 27, and the rotational speed of the electric motors 24 and 25.

[0029] In step S2, the required torque setting unit 82 refers to a map that defines the relationship between the accelerator operation amount and the speed of the vehicle 1 and the required torque, and sets the required torque based on the accelerator operation amount and the vehicle speed.

[0030] In step S3, the required torque distribution unit 83 distributes the required torque to the front and rear wheels by multiplying the required torque by a torque distribution value for the front and rear wheels. The torque distribution value for the front and rear wheels is set based on vehicle behavior, such as the difference in rotational speed between the front and rear wheels.

[0031] In step S4, the warm-up necessity determination unit 84 determines whether the temperatures acquired from the temperature sensors 6A, 6B are below a threshold value, thereby determining whether warm-up is necessary for the power transmission mechanisms 26, 27. If it is determined in step S4 that the temperatures are below the threshold value, the process proceeds to step S5, and if it is determined that the temperatures are not below the threshold value, the process proceeds to step S6.

[0032] If the temperature threshold is too low, the warm-up will end before the friction in the power transmission mechanisms 26, 27 has sufficiently decreased, and if the temperature threshold is too high, the warm-up will continue longer than necessary, which will affect the power consumption efficiency of the vehicle 1. For this reason, the temperature threshold is preferably set to a low temperature at which the power consumption efficiency of the vehicle 1 is improved by warming up rather than not warming up, and is desirably determined experimentally from the amount of lubricant and the performance of the lubricant. Note that the power consumption efficiency may be, for example, the distance traveled by the vehicle 1 per 1 kW of power consumption or the amount of power consumed per 1 km traveled by the vehicle 1.

[0033] In step S5, the operating point setting unit 85 sets operating points for the electric motors 24, 25 and the transmissions 26A, 27A during warm-up. In this case, unlike during warm-up, the operating point setting unit 85 changes the operating points of the electric motors 24, 25 from the optimal point to a low-efficiency point, or changes the gear ratio of the transmissions 26A, 27A so that the heat generation amount of the power transmission mechanisms 26, 27 increases, within a range in which the total value of the output torque of the power transmission mechanisms 26, 27 approximately coincides with the required torque. The optimal point is an operating point at which the efficiency of the electric motors 24, 25 is maximized. The low-efficiency point is an operating point at which the efficiency of the electric motors 24, 25 is lower than that at the optimal point. The output torque of the power transmission mechanisms 26, 27 is obtained by multiplying the torque of the electric motors 24, 25 by the gear ratio of the transmissions 26A, 27A.

[0034] Meanwhile, in step S6, the operating point setting unit 85 sets the operating points of the electric motors 24, 25 and the transmissions 26A, 27A when the electric motors 24, 25 are not warmed up. In this case, the operating point setting unit 85 sets the operating points of the electric motors 24, 25 to optimal points. The operating point setting unit 85 also sets the gear ratios of the transmissions 26A, 27A so that the total value of the output torque of the power transmission mechanisms 26, 27 approximately matches the value of the required torque.

[0035] Examples of how the operating points of the electric motors 24, 25 and the transmissions 26A, 27A are set by the operating point setting unit 85 will be described below with reference to Figs. 4 to 7. Figs. 4 to 7 are diagrams showing the relationship between the operating points of the electric motors 24, 25 and the efficiency of the electric motors 24, 25. In Figs. 4 to 7, the darkest region A1 is the region where the operating points of the electric motors 24, 25 are optimal, and the efficiency of the electric motors 24, 25 decreases as you move from region A1 to region A2 and region A3. In Figs. 4 to 7, the operating points of the electric motors 24, 25 are indicated by "x" marks.

[0036] 4 is a diagram showing how the operating point setting unit 85 changes the operating points of the electric motors 24, 25 and the gear ratios, which are the operating points of the transmissions 26A, 27A, during warm-up when the electric motors 24, 25 are performing a driving operation. The upper diagram in FIG. 4 shows a state in which the operating points of the electric motors 24, 25 are at their optimum points when the gear ratios of the transmissions 26A, 27A are R1 during a non-warm-up period.

[0037] In contrast, the lower diagram in Figure 4 illustrates a state in which, during warm-up, the gear ratios of the transmissions 26A, 27A are changed to R2, which is smaller than R1, and the operating points of the electric motors 24, 25 are changed from the optimal point to a low-efficiency point outside of region A1. In this case, the operating points of the electric motors 24, 25 deviate from the optimal point, reducing the efficiency of the electric motors 24, 25, resulting in increased heat loss and increased heat generation by the electric motors 24, 25. This increases the temperature of the coolant in the electric motors 24, 25, and the heat from the electric motors 24, 25 is transferred to the power transmission mechanisms 26, 27 via the coolant. Furthermore, by changing the gear ratios of the transmissions 26A, 27A to R2, which is smaller than R1, the rotational speeds of the transmissions 26A, 27A increase, raising the temperature of the power transmission mechanisms 26, 27. This increases the temperature of the lubricating oil in the power transmission mechanisms 26, 27, reducing friction in the power transmission mechanisms 26, 27.

[0038] In the example of a non-warmed-up state shown in the upper diagram of Fig. 4, the torque of the electric motors 24, 25 is T1 and the gear ratio is R1, so the output torque of the power transmission mechanisms 26, 27 is T1 x R1, and the sum of the output torques of the power transmission mechanisms 26, 27 is T1 x R1 x 2. In the example of a warmed-up state shown in the upper diagram of Fig. 4, the torque of the electric motors 24, 25 is T2 and the gear ratio is R2, so the sum of the output torques of the power transmission mechanisms 26A, 27 is T2 x R2 x 2. Therefore, the operating point setting unit 85 sets the operating points of the electric motors 24, 25 and the gear ratios of the transmissions 26A, 27A so that T1 x R1 x 2 ≈ T2 x R2 x 2.

[0039] 5 shows a state in which, when electric motors 24, 25 are performing driving operation, operating point setting unit 85 changes the torque distribution between front-wheel drive electric motor 24 and front-wheel drive electric motor 25 and the operating points of electric motors 24, 25 during warm-up, while leaving the gear ratio of transmissions 26A, 27A unchanged at R1. The upper diagram of Fig. 5 shows a state in which the operating point of front-wheel drive electric motor 24 is changed from the optimal point to a low-efficiency point during warm-up. The lower diagram of Fig. 5 shows a state in which the operating point of rear-wheel drive electric motor 25 is changed from the optimal point to a low-efficiency point during warm-up.

[0040] In the example shown in FIG. 5 , during non-warm-up, the operating points of the front-wheel drive electric motor 24 and the rear-wheel drive electric motor 25 are at their optimal points, and the torques of the electric motors 24 and 25 are both distributed at a 50:50 ratio at T1. In contrast, during warm-up, the operating points of the electric motors 24 and 25 are shifted from their optimal points to low-efficiency points outside of region A1, where the torque of the electric motor 24 is greater than T1 at T2 and the torque of the electric motor 25 is less than T1 at T3. In this way, the torque distribution between the front-wheel drive electric motor 24 and the rear-wheel drive electric motor 25 is changed, allowing the operating points of the electric motors 24 and 25 to be flexibly changed. Furthermore, changing the torque distribution between the electric motors 24 and 25 causes the operating points of the electric motors 24 and 25 to deviate from their optimal points, increasing the amount of heat generated by the electric motors 24 and 25 and reducing friction in the power transmission mechanisms 26 and 27.

[0041] 6 shows a state in which, when 100% of torque is distributed to front-wheel drive electric motor 24 and driving operation is performed solely by electric motor 24, the operating point setting unit 85 changes the operating point of electric motor 24 from the optimal point during warm-up and causes electric motor 25 to perform regenerative operation at a low-efficiency point, while the gear ratio of transmissions 26A, 27A remains unchanged at R1. The upper diagram of FIG. 6 shows a state in which the operating point of front-wheel drive electric motor 24 has been changed from the optimal point to a low-efficiency point during warm-up. The lower diagram of FIG. 6 shows a state in which, when rear-wheel drive electric motor 25 is not driven or regeneratively generated during non-warm-up, the operating point of electric motor 25 during regeneration has been changed to a low-efficiency point during warm-up.

[0042] 6 , the torque distribution between the front-wheel drive electric motor 24 and the rear-wheel drive electric motor 25 is changed, so the operating points of the electric motors 24, 25 can be flexibly changed. Furthermore, the torque T4 during regeneration of the rear-wheel drive electric motor 25 is set to the same magnitude as the torque difference T2-T1 before and after the change in the operating point of the front-wheel drive electric motor 24, so the total value of the output torque of the power transmission mechanisms 26, 27 does not change before and after the change in the operating point. Therefore, the regenerative operation of the rear-wheel drive electric motor 25 can increase the amount of heat generated during warm-up while maintaining the total value of the output torque of the power transmission mechanisms 26, 27, and friction in the power transmission mechanisms 26, 27 can be reduced.

[0043] 7 shows a state in which the operating point setting unit 85 changes the gear ratio of the transmissions 26A, 27A from R1 to R2 during warm-up without changing the operating points of the electric motors 24, 25 from the optimal point to the low-efficiency point when the electric motors 24, 25 are performing a driving operation. The upper diagram of FIG. 7 shows a state in which the operating points of the electric motors 24, 25 are at the optimal points when the gear ratio of the transmissions 26A, 27A is R1 during a non-warm-up state. The lower diagram of FIG. 7 shows a state in which the operating points of the electric motors 24, 25 are at the optimal points when the gear ratio of the transmissions 26A, 27A is changed to R2, which is smaller than R1, during warm-up.

[0044] 7, during warm-up, the gear ratio of transmissions 26A, 27A is changed to R2, which is smaller than R1, thereby increasing the rotation speed of transmissions 26A, 27A and raising the temperature of power transmission mechanisms 26, 27. This increases the temperature of the lubricating oil in power transmission mechanisms 26, 27 and reduces friction in power transmission mechanisms 26, 27. Furthermore, because the operating points of electric motors 24, 25 are maintained at optimal points, it is possible to prevent a decrease in the power consumption efficiency of vehicle 1 due to warm-up.

[0045] 7, the operating point setting unit 85 also changes the operating points of the electric motors 24, 25 and the gear ratios of the transmissions 26A, 27A so as to maintain the total value of the output torques of the power transmission mechanisms 26, 27 before and after changing the operating points. In the example of Fig. 7, the torque of the electric motors 24, 25 when not warmed up is T5 and the gear ratio is R1, and the torque of the electric motors 24, 25 when warmed up is T6 and the gear ratio is R2, so T5 x R1 x 2 ≈ T6 x R2 x 2.

[0046] According to the above-described embodiment, the operating points of the electric motors 24, 25 are changed from the optimum point where the efficiency of the electric motors 24, 25 is maximized to a low-efficiency point based on the temperature that affects the friction of the power transmission mechanisms 26, 27, or the gear ratio of the transmissions 26A, 27A is changed to a low-efficiency point based on the temperature that affects the friction of the power transmission mechanisms 26, 27.

[0047] According to the embodiment, the operating points of the multiple electric motors 24, 25 are changed independently between the electric motors 24, 25, so that the operating points of the electric motors 24, 25 can be flexibly changed so that the total value of the output torque of the power transmission mechanisms 26, 27 matches the required torque.

[0048] According to the embodiment, the torque distribution between the electric motor 24 for front-wheel drive and the electric motor 25 for rear-wheel drive is changed, so that the operating points of the electric motors 24, 25 can be changed more flexibly than when the vehicle 1 is two-wheel drive.

[0049] As described above, the best configurations, methods, and the like for implementing the present invention have been disclosed in the above description, but the present invention is not limited thereto. That is, although the present invention has been particularly illustrated and described mainly with reference to specific embodiments, those skilled in the art can make various modifications to the above-described embodiments in terms of shape, material, quantity, and other detailed configurations without departing from the scope of the technical idea and purpose of the present invention. Furthermore, the above-disclosed descriptions limiting the shape, material, and the like are provided as examples to facilitate understanding of the present invention and are not intended to limit the present invention. Therefore, descriptions using names of components that are free from some or all of the limitations on shape, material, and the like are included in the present invention.

[0050] The vehicle 1 may be an all-wheel drive vehicle as in the embodiment, or may be a two-wheel drive vehicle.

[0051] The driving / braking force generation device 2 may include an internal combustion engine, which drives a generator to supply electricity to the electric motors 24, 25, thereby driving the wheels 28 with the electric motors 24, 25, or the wheels 28 may be driven by the electric motors 24, 25 and the internal combustion engine. That is, the vehicle 1 may be a so-called series hybrid vehicle or a parallel hybrid vehicle. The power transmission mechanisms 26, 27 may be configured so as to transmit heat from the electric motors 24, 25 to the power transmission mechanisms 26, 27 in a manner other than sharing lubricating oil or coolant with the electric motors 24, 25. For example, if the electric motors 24, 25 and the power transmission mechanisms 26, 27 each have an independent lubricating oil circulation circuit, the oil pans of the electric motors 24, 25 and the power transmission mechanisms 26, 27 may be separated by a partition wall made of a highly thermally conductive material so that heat from the lubricating oil of the electric motors 24, 25 is transferred via the partition wall to the lubricating oil of the power transmission mechanisms 26, 27. The transmissions 26A, 27A may be configured to be able to switch between a plurality of gear ratios in a stepwise manner, or may be configured to be able to switch gear ratios continuously like a CVT (Continuously Variable Transmission).

[0052] In the control device 3, the driving force request operation means is configured by the accelerator pedal, and the accelerator operation amount sensor is configured by the accelerator position sensor 4, but other configurations may be used. For example, the driving force request operation means may be configured by an operation lever, an operation dial, or the like, and the accelerator operation amount sensor may be configured by a sensor such as a stroke sensor or potentiometer that detects the amount of operation of these.

[0053] The temperature sensors 6A, 6B may detect the temperature of the lubricating oil in the power transmission mechanisms 26, 27 or the temperature of the lubricating oil in the electric motors 24, 25, or may detect the outside air temperature, as temperatures that affect the friction of the power transmission mechanisms 26, 27.

[0054] The controller 8 may estimate the temperature that affects the friction of the power transmission mechanisms 26, 27. That is, the controller 8 may be provided with a temperature acquisition unit that estimates the temperature that affects the friction of the power transmission mechanisms 26, 27, and acquire the temperature by estimation. The temperature acquisition unit may estimate the temperature of the coolant for the power transmission mechanisms 26, 27, the temperature of the lubricating oil for the power transmission mechanisms 26, 27, or the temperature of the lubricating oil for the electric motors 24, 25, for example, based on the outside air temperature or the rotation speed of the electric motors 24, 25 or the power transmission mechanisms 26, 27.

[0055] The controller 8 may change the operating points of the electric motors 24, 25 from an optimal point to a low-efficiency point based on the temperature of the lubricating oil in the power transmission mechanisms 26, 27, the temperature of the lubricating oil in the electric motors 24, 25, or the outside air temperature, or may change the gear ratios of the transmissions 26A, 27A so that the heat generation of the power transmission mechanisms 26, 27 increases.

[0056] The controller 8 may change the gear ratio of the transmissions 26A, 27A to R3, which is greater than R1, using the operating point setting unit 85. In this case, the efficiency of the power transmission mechanisms 26, 27 decreases, resulting in increased heat loss and an increase in the temperature of the power transmission mechanisms 26, 27. As a result, the temperature of the lubricating oil in the power transmission mechanisms 26, 27 increases, and the friction of the power transmission mechanisms 26, 27 decreases.

[0057] If the output torque of the power transmission mechanisms 26, 27 exceeds the required torque after changing the operating point, the controller 8 may brake using the friction braking mechanism 29 to adjust the total value of the output torque of the power transmission mechanisms 26, 27 to the required torque.

[0058] The temperature threshold used by the warm-up necessity determination unit 84 when determining whether warm-up is necessary may be, for example, a lower limit temperature at which the viscosity of the lubricating oil in the power transmission mechanisms 26, 27 remains low enough so as not to affect the operation of the electric motors 24, 25 and the power transmission mechanisms 26, 27.

[0059] 1... Electric vehicle, 3... Control device, 6A, 6B... Temperature sensor, 8... Controller, 24, 25... Electric motor, 26, 27... Power transmission mechanism

Claims

1. A control method for an electric vehicle equipped with an electric motor, a power transmission mechanism that transmits the torque of the electric motor to wheels, and a temperature acquisition unit that detects or estimates temperatures that affect friction in the power transmission mechanism, the control method changing the operating point of the electric motor from an optimal point where the efficiency of the electric motor is maximized to a low-efficiency point where the efficiency is lower than the optimal point, based on the temperature, or changing the gear ratio of the power transmission mechanism so that the amount of heat generated by the power transmission mechanism increases.

2. A control method for an electric vehicle according to claim 1, wherein the operating point when the electric motor is driven is changed from the optimum point to the low-efficiency point.

3. A control method for an electric vehicle according to claim 1, wherein the operating point of the electric motor during regeneration is changed from the optimum point to the low-efficiency point.

4. A control method for an electric vehicle according to claim 2 or 3, wherein the electric vehicle comprises a plurality of electric motors, and the operating points of the plurality of electric motors are changed independently between the electric motors.

5. A control method for an electric vehicle as set forth in claim 4, wherein the plurality of electric motors comprise an electric motor for front-wheel drive and an electric motor for rear-wheel drive, and the control method changes the torque distribution between the electric motor for front-wheel drive and the electric motor for rear-wheel drive, and changes the operating points of the electric motors for front-wheel drive and the electric motor for rear-wheel drive independently between the electric motors.

6. A control device for an electric vehicle comprising an electric motor, a power transmission mechanism that transmits the torque of the electric motor to wheels, and a temperature acquisition unit that detects or estimates temperatures that affect friction in the power transmission mechanism, the control device comprising a controller that, based on the temperature, changes the operating point of the electric motor from an optimum point at which the efficiency of the electric motor is maximized to a low-efficiency point at which the efficiency is lower than the optimum point, or changes the gear ratio of the power transmission mechanism so as to increase the amount of heat generated by the power transmission mechanism.

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