Hybrid vehicle control method and hybrid vehicle control device

Abstract

A controller (20) acquires a required output, which is electric power to be supplied to a drive system (Sysd) in accordance with a required driving force, and compares the required output with three classifications comprising a small output range, a medium output range, and a large output range. When the required output is in the medium output range, the controller (20) controls an engine (11) using a follow operation in which a best fuel consumption line that indicates the best fuel consumption for the engine (11) is followed. When the required output is in the small output range or the large output range, the controller (20) controls the engine (11) using a fixed point operation in which an operating point is fixed.

Description

Hybrid vehicle control method and hybrid vehicle control device

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

[0002] Patent Document 1 discloses a control device for a hybrid system. A target operating line indicating the relationship between engine speed and engine torque is stored in the memory of the control device. The target operating line is created for each of a plurality of different system voltages. The control device selects a target operating line according to the system voltage, and sets a target engine speed and a target engine torque according to the selected target operating line. The control device controls the engine so as to obtain the target engine speed and the target engine torque.

[0003] JP 2021-105382 A

[0004] However, when the internal combustion engine is controlled according to the target operating line, if the required output is low, the generator's power will be surplus to the required output, and the surplus power must be charged into the energy storage system. On the other hand, if the required output is high, the generator's power alone cannot cover the required output, and the insufficient power must be discharged from the energy storage system. This results in an increase in charging and discharging losses in the energy storage system, which causes a problem of worsening fuel efficiency.

[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 improve fuel economy by operating an internal combustion engine at an efficient operating point.

[0006] A control method for a hybrid vehicle according to one embodiment of the present invention includes obtaining a required output, which is the power to be supplied to the drive system according to a required driving force, comparing the required output with three ranges consisting of a small output range, a medium output range, and a large output range, and controlling the internal combustion engine by tracking operation that follows the best fuel efficiency line when the required output is in the medium output range, and controlling the internal combustion engine by fixed-point operation that keeps the operating point constant when the required output is in the small output range or the large output range.

[0007] According to one aspect of the present invention, the internal combustion engine can be operated at an efficient operating point, thereby improving fuel economy.

[0008] FIG. 1 is a schematic diagram showing the configuration of a hybrid vehicle according to a first embodiment. FIG. 2 is a diagram showing a control concept of the hybrid vehicle according to the first embodiment. FIG. 3A is a flowchart showing a control method for a hybrid vehicle related to a required output. FIG. 3B is a flowchart showing a control method for a hybrid vehicle related to operating point determination. FIG. 4 is a time chart showing changes in power generation output and power supply efficiency according to an increase in required output. FIG. 5 is a diagram explaining a situation in which the SOC falls below a lower threshold value when fixed point operation is performed. FIG. 6 is a diagram showing switching from fixed point operation to best fuel economy line tracking operation. FIG. 7 is a diagram explaining operation mode switching based on an SOC threshold value. FIG. 8 is a diagram showing operation mode switching based on an SOC change gradient. FIG. 9 is a flowchart showing a process for determining the rotation speed for lower fixed point operation and the rotation speed for upper fixed point operation.

[0009] A hybrid vehicle (hereinafter referred to as "vehicle") 1 according to this embodiment includes an engine 11, a generator 12, a traction motor 13, a first inverter 14, a second inverter 15, a battery 16, a speed increasing mechanism 17, a speed reduction and differential mechanism 18, and a controller 50.

[0010] The engine 11 is an internal combustion engine such as a gasoline engine or a diesel engine. The engine 11 is connected to a generator 12 via a speed increasing mechanism 17. The generator 12 is driven by the engine 11 to generate electricity. The traction motor 13 is connected to wheels (drive wheels) via a reduction gear and a differential mechanism 18. The traction motor 13 drives the drive wheels and also generates electricity regeneratively during deceleration.

[0011] The first inverter 14 is used to control the generator 12. The second inverter 15 is used to control the traction motor 13. Based on a command from the controller 50, the first inverter 14 generates DC power from the AC power output from the generator 12 and outputs the DC power to the second inverter 15. Based on a command from the controller 50, the second inverter 15 generates AC power from the DC power output from one or both of the battery 16 and the first inverter 14 and supplies the AC power to the traction motor 13. Of the power output from the first inverter 14, any power surplus to the traction motor 13 is stored in the battery 16.

[0012] The battery 16 stores electric power. The battery 16 is charged with electric power generated by the generator 12 and electric power regenerated by the traction motor 13. The electric power of the battery 16 is supplied to the traction motor 13 via the second inverter 15 as needed.

[0013] The controller 50 is a microcomputer including a CPU, a ROM, a RAM, and an input / output interface. The controller 50 is a control device that controls the vehicle 1 including the engine 11, the first inverter 14, the second inverter 15, etc., by executing a program stored in the ROM or RAM by the CPU.

[0014] The controller 50 receives input of state of charge (SOC) information Ssoc from a sensor that detects the SOC of the battery 16. The controller 50 also receives input of information Iv about the vehicle 1 from a higher-level vehicle system. The information Iv about the vehicle 1 includes the vehicle speed, accelerator opening, brake stroke, etc. These pieces of information are used for the control performed by the controller 50.

[0015] In the vehicle 1 configured as described above, the power of the engine 11 is transmitted to the generator 12 via a speed-up mechanism 17. The generator 12 generates AC power from the power transmitted from the engine 11, and this AC power is input to the first inverter 14. The first inverter 14 converts the AC power to DC power and supplies the DC power to the traction motor 13 (second inverter 15). At this time, surplus power is stored in the battery 16. The battery 16 also stores power generated by the traction motor 13 when the vehicle is decelerating. The second inverter 15 is supplied with power from the first inverter 14, but if there is a shortage of power required for traveling, power is also supplied from the battery 16. The second inverter 15 converts the DC power into AC power and supplies it to the traction motor 13. The traction motor 13 generates power from the AC power, and the generated power is transmitted to the drive wheels via a differential mechanism 18.

[0016] The hybrid system of the vehicle 1 is a series hybrid system in which the engine 11 is used to drive the generator 12, and the drive wheels are driven by the traction motor 13. The vehicle 1 has two driving modes: an EV mode and a series hybrid mode. The EV mode is a mode in which the engine 11 is stopped, the traction motor 13 is driven by electric power supplied from the battery 16, and the vehicle travels using the driving force of the traction motor 13. The series hybrid mode is a mode in which the engine 11 is driven to generate electricity using the generator 12, and the traction motor 13 is driven by electric power supplied from the generator 12, and the vehicle travels using the driving force of the traction motor 13.

[0017] In the vehicle 1, the engine 11, the speed increasing mechanism 17, the generator 12, the first inverter 14, and the battery 16 constitute a power supply system Sysp. In the power supply system Sysp, the engine 11, the speed increasing mechanism 17, the generator 12, and the first inverter 14 constitute a power generation system Sysg, and the battery 16 constitutes a power storage system Sysb. In addition, the reduction and differential mechanism 18, the second inverter 15, and the traction motor 13 constitute a drive system Sysd.

[0018] The power supply system Sysp supplies and stores the necessary power according to the output required by the drive system Sysd. The power supply system Sysp allocates the necessary power to the power generation system Sysg and the power storage system Sysb, taking into consideration efficiency, quietness, and SOC. The power generation system Sysg generates the power required by the power supply system at the most efficient operating point. The power storage system Sysb discharges the power required by the power supply system Sysp and charges the regenerative power generated in the drive system Sysd. The drive system Sysd generates driving force from the power supplied by the power supply system Sysp and transmits it to the drive wheels. The drive system Sysd also converts energy into regenerative power during vehicle deceleration and supplies it to the power storage system Sysb.

[0019] An overview of vehicle control according to this embodiment will be described with reference to Fig. 2. Note that the required output and generated output values ​​shown in Fig. 2 are examples, and this embodiment is not limited to these values.

[0020] In this specification, the ratio of the power generated by the generator 12 to the fuel consumed by the engine 11 is referred to as power generation efficiency. Engine efficiency is the dominant factor in this power generation efficiency, and operating the engine 11 at the optimal fuel economy point maximizes power generation efficiency. In contrast, the ratio of the power supplied to the traction motor 13 to the fuel consumed by the engine 11 is referred to as power supply efficiency. In a series hybrid system, the required output (demand) of the traction motor 13 does not necessarily translate directly into the power generated by the generator 12. This is because the battery 16 adjusts the discrepancy between the required output of the traction motor 13 and the power generated by the generator 12. In other words, because the battery 16 charges and discharges, the charge and discharge efficiency of the battery 16 must be taken into consideration. Power supply efficiency differs from power generation efficiency in that the charge and discharge efficiency of the battery 16 is also taken into account.

[0021] In the vehicle 1, power corresponding to the required output is supplied to the traction motor 13. Fig. 2 is a diagram showing the relationship between the required output and power supply efficiency when the required output is provided only by the power generation system Sysg. The line connecting the operating points at which fuel efficiency is optimal in the relationship between the torque of the engine 11 and the rotation speed of the engine 11 is called the best fuel efficiency line. The solid line shown in Fig. 2 indicates the relationship between the required output and power supply efficiency when the engine 11 is operated along the best fuel efficiency line. Operating the engine 11 along the best fuel efficiency line is called best fuel efficiency line following operation.

[0022] There are two main approaches to supplying electric power corresponding to the required output to the traction motor 13. The first approach is to change the operating point (rotation speed) of the engine 11 according to the required output and supply electric power corresponding to the required output from the power generation system Sysg (optimal fuel economy line tracking operation). The second approach is to operate the engine 11 at the optimal fuel economy point Pb regardless of the required output, and adjust the surplus or shortage of electric power using the battery 16. In the first approach, if the required output significantly deviates from the optimal fuel economy point Pb, the thermal efficiency of the engine 11 significantly deteriorates, resulting in increased engine loss. This deteriorates fuel economy. On the other hand, in the second approach, the thermal efficiency of the engine 11 is optimal, but if the required output significantly deviates from the output of the generator 12, the charge / discharge amount of the battery 16 increases, resulting in increased battery loss. This deteriorates fuel economy.

[0023] Therefore, in this embodiment, the deterioration of losses is compared and the more efficient option is selected to improve fuel efficiency. For example, when the required output is 10 kW, if the engine 11 is operated at a constant rotation speed so that the generator 12 outputs 17 kW (fixed-point operation OL), the surplus power of 7 kW is charged to the battery 16. The battery loss (BAT loss) at this time is smaller than the engine loss (ENG loss) when generating 10 kW along the optimal fuel efficiency line. In other words, when the required output is in the low-output range, selecting the 17-kW fixed-point operation OL is more efficient. Also, for example, when the vehicle demand fluctuates around 20 kW (17-23 kW), which is the optimal fuel efficiency point Pb of the engine 11, generating power while operating the engine 11 along the optimal fuel efficiency line is relatively efficient. Therefore, when the required output is in the medium-output range, selecting the optimal fuel efficiency line-following operation is more efficient. For example, when the required output is 30 kW, if the engine 11 is operated at a constant rotation speed so that the output of the generator 12 is 23 kW, the 7 kW of power that is insufficient is assisted by the battery 16. The battery loss at this time is smaller than the ENG loss when generating power at 30 kW along the optimum fuel efficiency line. In other words, when the required output is in the high output range, it is more efficient to select fixed-point operation OH, where the required output is 20 kW.

[0024] In this way, when the required output is in the small output range, the controller 50 controls the engine 11 using fixed point operation OL, in which the rotational speed of the engine 11 (the output of the generator 12) is constant (hereinafter referred to as "lower fixed point operation OL"). In the example shown in FIG. 2, the operating point of the lower fixed point operation OL is a rotational speed corresponding to 10 kW, and an operating point with the highest power supply efficiency is selected when performing best fuel economy line tracking operation in the small output range. When the required output is in the medium output range, the controller 50 controls the engine 11 using fixed point operation OH, in which the rotational speed of the engine 11 (the output of the generator 12) is constant (hereinafter referred to as "upper fixed point operation OH"). In the example shown in FIG. 2, the upper fixed point operation OH is 23 kW, and an operating point with the highest power supply efficiency is selected when performing best fuel economy line tracking operation in the large output range. In this way, fuel efficiency can be improved by selectively using, depending on the required output, the operating point that provides the best power supply efficiency, taking into account the charge / discharge loss of the battery 16. The threshold value that separates the low output region from the medium output region and the threshold value that separates the medium output region are set in advance through experiments and simulations.

[0025] 3A and 3B, a control method for the vehicle 1 according to this embodiment will be described. The flowcharts shown in FIGS.

[0026] The controller 50 acquires the vehicle speed (S10). The controller 50 calculates the upper limit power generation output based on the vehicle speed (S11). The upper limit power generation output is the output of the generator 12 when the engine 11 is operated at the upper limit power generation speed. The upper limit power generation speed is the upper limit power generation speed of the engine 11 allowed at that vehicle speed, taking into account noise and vibration requirements. The controller 50 also acquires the running resistance acting on the drive wheels (S12).

[0027] If the brake is on and the vehicle speed is zero (S13: YES, S14: YES), i.e., if the vehicle 1 is stopped, the controller 50 determines whether the SOC is equal to or greater than a lower threshold (S15). The lower SOC threshold is the minimum SOC required for the vehicle 1 to travel and is set in advance through experiments or simulations. If the SOC is equal to or greater than the lower threshold (S15: YES), the controller 50 stops power generation by the power generation system Sysg (S16) and discharges the power required for the vehicle 1 from the power storage system Sysb (S18). If the SOC is less than the lower threshold (S15: NO), the controller 50 controls the engine 11 by lower fixed-point operation OL (S17) and performs forced power generation by the power generation system Sysg (S19). The power generated by the power generation system Sysg is charged to the power storage system Sysb.

[0028] On the other hand, if the brake is not on (S13: NO), the controller 50 acquires the accelerator opening degree (S20). The controller 50 calculates the required driving force based on the vehicle speed and the accelerator opening degree (S21). The controller 50 calculates the required output, which is the electric power to be supplied to the drive system Sysd, from the running resistance, the required driving force, and the vehicle speed (S22). On the other hand, if the brake is on (S13: YES) but the vehicle speed is not zero (S14: NO), the controller 50 acquires the brake stroke (S23). The controller 50 calculates the required regenerative force based on the vehicle speed and the brake stroke (S24).

[0029] 3B , if the required output is equal to or less than the upper limit power generation output (S25: YES), the controller 50 determines whether the required output is equal to or less than threshold value A (S26). Threshold value A is a value that separates a small output region from a larger medium output region, and is set in advance through experiments or simulations.

[0030] If the required output is equal to or less than threshold A (S26: YES), the controller 50 determines whether the SOC is equal to or less than an upper threshold (S27). The upper SOC threshold is an SOC that ensures a certain amount of free space in the power storage system Sysb for storing regenerative power, and is set in advance through experiments or simulations. If the SOC is greater than the upper threshold (S27: NO), the controller 50 stops power generation by the power generation system Sysg (S28) and supplies the required power to the engine 11 from the power storage system Sysb (S29). If the SOC is equal to or less than the upper threshold (S27: YES), the controller 50 controls the engine 11 using lower fixed-point operation OL (S17). The controller 50 supplies power generated by the power generation system Sysg to the drive system Sysd and charges the surplus power remaining in the drive system Sysd to the power storage system Sysb (S31).

[0031] If the required output is greater than threshold A (S26: NO), the controller 50 determines whether the required output is equal to or less than threshold B (S32). Threshold B is a value that separates a medium output region from a larger high output region, and is set in advance through experiments or simulations. If the required output is equal to or less than threshold B (S32: YES), the controller 50 controls the engine 11 using the best fuel economy line tracking operation (S34) and supplies the power required by the drive system Sysd from the power generation system Sysg (S34). If the required output is greater than threshold B (S32: NO), the controller 50 determines whether the SOC is equal to or greater than a lower threshold (S35). If the SOC is equal to or greater than the lower threshold (S35: YES), the controller 50 controls the engine 11 using upper fixed point operation OH (S36). The controller 50 supplies the electric power generated by the power generation system Sysg to the drive system Sysd, and discharges the electric power that is insufficient in the drive system Sysd from the electric storage system Sysb (S37). If the SOC is lower than the lower threshold (S35: NO), the controller 50 performs the processes of steps S33 and S34 described above.

[0032] On the other hand, if the required output is greater than the upper limit power generation output (S25: NO), the controller 50 determines whether the upper limit power generation output is equal to or less than threshold A (S38). If the upper limit power generation output is greater than threshold A (S38: NO), the controller 50 performs the processing from step S32 onward. If the upper limit power generation output is equal to or less than threshold A (S38: YES), the controller 50 determines whether the SOC is equal to or greater than a lower threshold (S39). If the SOC is equal to or greater than the lower threshold (S39: YES), the controller 50 stops power generation by the power generation system Sysg (S40) and discharges the power required for the drive system Sysd from the power storage system Sysb (S41). If the SOC is less than the lower threshold (S39: NO), the controller 50 controls the engine 11 by lower fixed point operation OL (S42) and performs forced power generation by the power generation system Sysg (S19). This supplies power to the power storage system Sysb.

[0033] Referring to FIG. 4 , the transition of control by the controller 50 will be described using an example in which the vehicle 1 accelerates from a stopped state to a certain speed and the required output increases as the vehicle speed increases. The vehicle 1 starts traveling and travels in EV mode until time t1, when the required output reaches a certain value. The controller 50 starts the engine 11 at time t1. The most efficient lower fixed point operation OL is selected as the operating point of the engine 11. Next, at time t2, when the required output reaches threshold value A, the efficiency of the best fuel economy line following operation exceeds that of the lower fixed point operation OL. The controller 50 switches from the lower fixed point operation OL to the best fuel economy line following operation. At time t3, when the required output reaches threshold value B, the efficiency of the upper fixed point operation OH exceeds that of the best fuel economy line following operation. The controller 50 switches from the best fuel economy line following operation to the upper fixed point operation OH.

[0034] If the operating point of the engine 11 is changed in accordance with the required output, an operating point with poor thermal efficiency of the engine 11 may be used. To avoid this, even if the engine 11 is operated while generating power at its best fuel consumption point Pb and charging the surplus power to the battery 16, battery loss occurs. Therefore, according to the control method for the vehicle 1 of this embodiment, the above-described operation switching control is performed taking into account the power supply efficiency of power supplied to the traction motor 13. By performing this operation switching control, the engine 11 can always be operated at an operating point with the highest power supply efficiency. As a result, the engine 11 can be operated at an efficient operating point, thereby improving fuel efficiency.

[0035] In the control method for the vehicle 1 of this embodiment, when the required output is in the low-output range, the engine 11 is controlled by lower fixed-point operation OL using an operating point (rotation speed) that provides the highest fuel efficiency for the engine 11 in the low-output range, i.e., an operating point (best fuel efficiency point) that provides the highest power supply efficiency when the engine 11 is driven in the best fuel efficiency line tracking mode. Also, when the required output is in the high-output range, the engine 11 is controlled by upper fixed-point operation OH using an operating point (rotation speed) that provides the highest fuel efficiency for the engine 11 in the high-output range, i.e., an operating point (best fuel efficiency point) that provides the highest power supply efficiency when the engine 11 is driven in the best fuel efficiency line tracking mode. This method contributes to simplifying control by narrowing down the operating points of the engine 11 to only two: best fuel efficiency line tracking and best fuel efficiency point.

[0036] Second Embodiment A hybrid vehicle according to a second embodiment will be described below with reference to Figures 5 and 6. The hybrid vehicle according to the second embodiment differs from the hybrid vehicle according to the first embodiment in that the driving method is switched in response to a decrease in the SOC of the battery 16.

[0037] As shown in Figure 5, when the upper fixed point operation OH is performed in the high output range and the required output is greater than the output of the power generation system Sysg, the battery 16 assists in making up for the shortfall in power. However, if this situation continues, the SOC of the battery 16 falls below the lower threshold. In this case, power assistance becomes impossible, affecting power performance. The time until the SOC is depleted depends on the output difference between the required output and the output of the power generation system Sysg during the upper fixed point operation OH, the duration, and the SOC. In other words, the longer the required output continues to be high and the lower the SOC, the sooner the SOC will be depleted.

[0038] Therefore, in the first embodiment described above, when the SOC becomes smaller than the lower threshold, the upper fixed point operation OH is switched to the best fuel economy line tracking operation (S33, S34). In this embodiment, when the controller 50 determines that the SOC has decreased to a predetermined SOC threshold, the upper fixed point operation OH is switched to the best fuel economy line tracking operation. This SOC threshold is a value greater than the lower threshold and is uniquely determined from a map or the like. The map is set so that the SOC threshold is higher the greater the output difference with the required output and the lower the SOC. Furthermore, the map can be adjusted so that the SOC threshold is relatively higher when the required output is on an upward trend and relatively lower when the required output is on a downward trend.

[0039] In this way, switching to the best fuel economy line tracking operation is triggered by the SOC decreasing to the SOC threshold, which stops discharging from the battery 16 and thus suppresses the decrease in SOC. Furthermore, the power required for the drive system Sysd can be supplied from the power generation system Sysg, ensuring power performance. Furthermore, setting the SOC threshold according to the map shown in FIG. 7 allows the timing of the operating point transition to be appropriately controlled.

[0040] In the above-described embodiment, the operating point is switched from the upper fixed point operation OH to the operation following the best fuel economy line. However, the operating point may be changed to a point on the higher torque side of the best fuel economy line. This method switches to an operating point that can obtain an output greater than the required output, allowing the SOC of the battery 16 to be recovered quickly. This contributes to ensuring stable power performance.

[0041] In the above description of the map, the output difference from the demand may be detected using a moving average over a certain period, or the like. This may suppress the influence of disturbances due to temporary changes in the output difference. As shown in FIG. 8, the output difference from the demand may be replaced with the gradient of SOC change. In this case, too, the SOC change may be detected using a moving average over a certain period, or the like, so as to suppress the influence of disturbances due to temporary changes.

[0042] In the above-described embodiment, it is assumed that the rotation speed of the lower side fixed point operation OL and the rotation speed of the upper side fixed point operation OH are determined in advance using a map, etc. However, the power supply efficiency during the best fuel consumption line following operation and the power supply efficiency during the fixed point operation may be compared, and the rotation speed of the lower side fixed point operation OL and the rotation speed of the upper side fixed point operation OH may be determined as needed so that the more efficient operation is selected.

[0043] A method for determining the rotation speed during lower side fixed point operation OL and the rotation speed during upper side fixed point operation OH will be described below with reference to Fig. 9. First, the controller 50 acquires the required output (S50), and acquires each system characteristic according to the environmental conditions (S51). The controller 50 calculates the power generation output of the power generation system Sysg at the best fuel consumption point (S52), and calculates the power supply efficiency during the best fuel consumption line tracking operation (S53).

[0044] The controller 50 determines whether the power generation output at the best fuel economy point is equal to or greater than the required output (S54). If the power generation output is equal to or greater than the required output, the controller 50 performs the processes in steps S55 and onward. On the other hand, if the power generation output is smaller than the required output, the controller 50 performs the processes in steps S59 and onward.

[0045] The controller 50 calculates the maximum power supply efficiency point of the fixed point operation while reducing the rotational speed of the engine 11 (S55). If the power supply efficiency during the best fuel economy line following operation is lower than the maximum power supply efficiency of the fixed point operation (S56: NO), the controller 50 determines the rotational speed at which the maximum power supply efficiency point is achieved as the rotational speed for the lower fixed point operation OL (S57). On the other hand, if the power supply efficiency during the best fuel economy line following operation is equal to or higher than the maximum power supply efficiency of the fixed point operation (S56: YES), the controller 50 determines that the best fuel economy line following operation is being performed (S58).

[0046] The controller 50 calculates the maximum power supply efficiency point of the fixed point operation while increasing the rotational speed of the engine 11 (S59). If the power supply efficiency during the best fuel economy line following operation is lower than the maximum power supply efficiency of the fixed point operation (S60: NO), the controller 50 determines the rotational speed at which the maximum power supply efficiency point is achieved as the rotational speed for the upper fixed point operation OH (S61). On the other hand, if the power supply efficiency during the best fuel economy line following operation is equal to or higher than the maximum power supply efficiency of the fixed point operation (S60: YES), the controller 50 determines that the best fuel economy line following operation is being performed (S58).

[0047] According to this method, the rotation speed during the lower fixed point operation and the rotation speed during the upper fixed point operation are dynamically determined, thereby taking into consideration the characteristics of the engine 11 and the battery 16 that change with environmental conditions. This makes it possible to select the more efficient one between the efficiency during the best fuel economy line following operation and the efficiency during the fixed point operation.

[0048] The present embodiment also includes a control device for the vehicle 1 realized by the above-described controller 50. This control device for the vehicle 1 can operate the engine 11 at an efficient operating point, thereby improving fuel efficiency.

[0049] Although the embodiments of the present invention have been described above, the descriptions and drawings that form part of this disclosure should not be understood to limit the present invention. Various alternative embodiments, examples, and operating techniques will become apparent to those skilled in the art from this disclosure.

[0050] REFERENCE SIGNS LIST 1 Vehicle (hybrid vehicle) 11 Engine (internal combustion engine) 12 Generator 13 Traction motor 14 First inverter 15 Second inverter 16 Battery 50 Controller Sysp Power supply system Sysg Power generation system Sysb Power storage system Sysd Drive system

Claims

1. A control method for a hybrid vehicle equipped with an internal combustion engine and a power generation system including a generator driven by the internal combustion engine to generate electricity, an electricity storage system that stores electric power, and a drive system including an electric motor that uses the electric power to generate driving force for wheels, the control method comprising: obtaining a required output, which is the electric power to be supplied to the drive system according to a required driving force; comparing the required output with three ranges consisting of a small output range, a medium output range larger than the small output range, and a large output range larger than the medium output range; when the required output is in the medium output range, controlling the internal combustion engine by tracking operation that tracks an optimum fuel efficiency line that optimizes the fuel efficiency of the internal combustion engine; and when the required output is in the small output range or the large output range, controlling the internal combustion engine by fixed point operation that keeps the operating point constant.

2. A control method for a hybrid vehicle as described in claim 1, wherein, when the required output is in the small output range, the internal combustion engine is controlled by the fixed point operation using an operating point that provides the best fuel economy for the internal combustion engine in the small output range, and when the required output is in the large output range, the internal combustion engine is controlled by the fixed point operation using an operating point that provides the best fuel economy for the internal combustion engine in the large output range.

3. A control method for a hybrid vehicle according to claim 1 or 2, wherein, even if the required output is in the high output range, when it is determined that the SOC of the storage system has dropped to a predetermined threshold, the control method switches from the fixed point operation to the follow-up operation.

4. A control method for a hybrid vehicle according to claim 1 or 2, wherein, even if the required output is in the high output range, when it is determined that the SOC of the power storage system has dropped to a predetermined threshold, the constant-point operation is switched to an operating point on the higher torque side of the best fuel economy line.

5. The hybrid vehicle control method according to claim 4, wherein the threshold value is set to a higher value as the output difference between the required output and the output during the fixed-point operation becomes larger or as the gradient of change in the SOC of the power storage system becomes larger.

6. The method for controlling a hybrid vehicle according to claim 4 or 5, wherein the threshold value is set to a higher value as the SOC of the power storage system becomes lower.

7. A control method for a hybrid vehicle according to claim 1 or 2, wherein the ratio of the power supplied to the drive system to the fuel consumed by the internal combustion engine is defined as power supply efficiency, and even if the required output is in the small output range or the large output range, if the power supply efficiency by the follow-up operation is higher than the power supply efficiency by the fixed point operation, the internal combustion engine is controlled by the follow-up operation instead of the fixed point operation.

8. A control device for a hybrid vehicle equipped with a power generation system including an internal combustion engine and a generator driven by the internal combustion engine to generate electricity, a storage system that stores electricity, and a drive system including an electric motor that generates driving force for wheels using one or both of the electricity generated by the power generation system and the electricity stored in the storage system, the control device comprising: a controller that controls the power generation system, wherein the controller obtains a required output, which is the power to be supplied to the drive system according to a required driving force, compares the required output with three ranges consisting of a small output range, a medium output range larger than the small output range, and a large output range larger than the medium output range, controls the internal combustion engine by tracking operation that tracks the best fuel efficiency line of the internal combustion engine when the required output is in the medium output range or the large output range, and controls the internal combustion engine by fixed point operation that keeps the operating point constant when the required output is in the small output range or the large output range.

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