Patent Publication Number: US-11648930-B2

Title: Control device of vehicle that controls to vehicle to shift between a plurality of different traveling modes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2020-033433 filed on Feb. 28, 2020. 
     TECHNICAL FIELD 
     The present disclosure relates to a control device of a vehicle. 
     BACKGROUND ART 
     In recent years, a hybrid electrical vehicle has a plurality of traveling modes including a hybrid traveling mode, in which a generator generates electric power based on power of an engine in a state where a clutch is disengaged and an electric motor outputs power at least based on the electric power supplied by the generator to drive a driving wheel, and an engine traveling mode, in which the driving wheel is driven by at least power output by the engine in a state where the clutch is engaged (for example, see WO-A-2019-003443). 
     As a method of efficiently driving a vehicle by power of an internal combustion engine, there may be a method in which a plurality of power transmission paths each having a different reduction ratio are provided between the internal combustion engine and the driving wheel, and a power transmission path to be used is switched according to a traveling state of the vehicle. 
     SUMMARY OF INVENTION 
     However, when the above method is adopted, depending on a difference between the reduction ratio of the power transmission path before switching and the reduction ratio of the power transmission path after the switching, there is a concern that a rotational speed of the internal combustion engine fluctuates greatly when the power transmission path is switched, which may lead to a decrease in a marketability of the vehicle from a viewpoint of vibration and noise, that is, a decrease in commercial value of the vehicle from a so-called noise vibration (NV) viewpoint. 
     If the difference in reduction ratio between the power transmission paths is reduced, the fluctuation of the rotational speed of the internal combustion engine that occurs when the power transmission path is switched can be reduced. 
     However, in such a case, when the vehicle is to be efficiently driven by the power of the internal combustion engine in a wide range of a traveling state, a large number of power transmission paths are required, which leads to an increase in manufacturing cost and weight of the vehicle. 
     The present disclosure provides a control device for a vehicle that can suppress a fluctuation in a rotational speed of an internal combustion engine when switching a power transmission path to be used with a simple configuration. 
     According to the present disclosure, there is provided a control device of a vehicle capable of traveling according to a plurality of traveling modes, the plurality of traveling modes including: a first traveling mode in which power of an internal combustion engine is transmitted to a driving wheel via a first power transmission path having a first reduction ratio to cause the vehicle to travel; a second traveling mode in which the power of the internal combustion engine is transmitted to the driving wheel via a second power transmission path having a second reduction ratio different from the first reduction ratio to cause the vehicle to travel; and a third traveling mode in which power output from an electric motor is transmitted to the driving wheel in accordance with supply of electric power from a generator configured to generate electric power by the power of the internal combustion engine to cause the vehicle to travel, the control device includes: a traveling mode setting unit configured to set the traveling mode of any one of the plurality of traveling modes; and an internal combustion engine control unit configured to control the internal combustion engine based on the traveling mode set by the traveling mode setting unit, in which the traveling mode setting unit configured to cause a shift to shift to the second traveling mode via the third traveling mode based on a traveling state of the vehicle traveling in the first traveling mode, and in which the internal combustion engine control unit includes a derivation unit configured to derive a second rotational speed of the internal combustion engine at the time of transition from the third traveling mode to the second traveling mode based on a first rotational speed of the internal combustion engine at the time of transition from the first traveling mode to the third traveling mode, the traveling state of the vehicle and the second reduction ratio, and the internal combustion engine control unit is configured to control a third rotational speed of the internal combustion engine in the third traveling mode to be a value between the first rotational speed and the second rotational speed. 
     According to the present disclosure, at the time of the transition from the first traveling mode using the first power transmission path having the first reduction ratio to the second traveling mode using the second power transmission path having the second reduction ratio via the third traveling mode, in which the vehicle can travel by the power of the electric motor, the rotational speed of the internal combustion engine in the third traveling mode is controlled to be the third rotational speed that is between the first rotational speed at the time of transition from the first traveling mode to the third traveling mode and the second rotational speed at the time of transition from the third traveling mode to the second traveling mode, so that fluctuation in the rotational speed of the internal combustion engine when switching a power transmission path to be used can be suppressed with a simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a schematic configuration of a vehicle including a control device of a vehicle according to an embodiment of the present disclosure. 
         FIG.  2    is a diagram illustrating the contents of each traveling mode. 
         FIG.  3    is a diagram illustrating a transition example of the traveling mode. 
         FIG.  4    is a block diagram illustrating a functional configuration of the control device. 
         FIG.  5    is a diagram illustrating a control example of an engine rotational speed in a hybrid traveling mode. 
         FIG.  6    is a diagram illustrating a first example of a control performed regarding a transition to a low-speed side engine traveling mode. 
         FIG.  7    is a diagram illustrating a decrease rate of the engine rotational speed in the hybrid traveling mode and a decrease rate of the engine rotational speed in the transition to the low-speed side engine traveling mode. 
         FIG.  8    is a diagram illustrating a second example of a control performed regarding the transition to the low-speed side engine traveling mode. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of a control device of a vehicle according to the present disclosure will be described in detail with reference to the drawings. 
     First, a vehicle including the control device of a vehicle according to the present embodiment will be described with reference to  FIG.  1   . As illustrated in  FIG.  1   , a vehicle  1  of the present embodiment includes a driving device  10  that outputs a driving force of the vehicle  1 , and a control device  100  that controls the entire vehicle  1  including the driving device  10 . 
     [Driving Device] 
     As illustrated in  FIG.  1   , the driving device  10  includes an engine ENG, a generator GEN, a motor MOT, a transmission T, and a case  11  that accommodates the generator GEN, the motor MOT, and the transmission T. The motor MOT and the generator GEN are connected to a battery BAT included in the vehicle  1 , which enables electric power supply from the battery BAT and energy regeneration to the battery BAT. 
     [Transmission] 
     The case  11  is provided with a transmission accommodating chamber  11   a  for accommodating the transmission T and a motor accommodating chamber  11   b  for accommodating the motor MOT and the generator GEN from the engine ENG side along an axial direction. 
     The transmission accommodating chamber  11   a  accommodates an input shaft  21 , a generator shaft  23 , a motor shaft  25 , a counter shaft  27 , and a differential mechanism D arranged in parallel to each other. 
     The input shaft  21  is arranged coaxially with and adjacently to a crankshaft  12  of the engine ENG. A driving force of the crankshaft  12  is transmitted to the input shaft  21  via a damper (not illustrated). The input shaft  21  is provided with a generator drive gear  32  constituting a generator gear train Gg. 
     The input shaft  21  is provided with a low-speed side drive gear  34  constituting a low-speed side engine gear train GLo via a first clutch CL 1  on the engine side with respect to the generator drive gear  32 , and a high-speed side drive gear  36  constituting a high-speed side engine gear train GHi on a side opposite to the engine side (hereinafter, referred to as a motor side). The first clutch CL 1  is a hydraulic clutch for detachably connecting the input shaft  21  and the low-speed side drive gear  34 , and is a so-called multi-plate friction type clutch. 
     The generator shaft  23  is provided with a generator driven gear  40  that meshes with the generator drive gear  32 . The generator drive gear  32  of the input shaft  21  and the generator driven gear  40  of the generator shaft  23  constitute a generator gear train Gg for transmitting a rotation of the input shaft  21  to the generator shaft  23 . The generator GEN is arranged on the motor side of the generator shaft  23 . The generator GEN includes a rotor R fixed to the generator shaft  23  and a stator S fixed to the case  11  and arranged to face an outer diameter side of the rotor R. 
     When the rotation of the input shaft  21  is transmitted to the generator shaft  23  via the generator gear train Gg, the rotor R of the generator GEN rotates due to the rotation of the generator shaft  23 . Accordingly, when the engine ENG is driven, power of the engine ENG input from the input shaft  21  can be converted into electric power by the generator GEN. 
     The motor shaft  25  is provided with a motor drive gear  52  constituting a motor gear train Gm. The motor MOT is arranged on the motor shaft  25  closer to the motor side than the motor drive gear  52 . The motor MOT includes a rotor R fixed to the motor shaft  25  and a stator S fixed to the case  11  and arranged to face an outer diameter side of the rotor R. 
     The counter shaft  27  is provided with, in order from the engine side, a low-speed side driven gear  60  meshing with the low-speed side drive gear  34 , an output gear  62  meshing with a ring gear  70  of the differential mechanism D, a high-speed side driven gear  64  meshing with the high-speed side drive gear  36  of the input shaft  21  via a second clutch CL 2 , and a motor driven gear  66  meshing with the motor drive gear  52  of the motor shaft  25 . The second clutch CL 2  is a hydraulic clutch for detachably connecting the counter shaft  27  and the high-speed side driven gear  64 , and is a so-called multi-plate friction type clutch. 
     The low-speed side drive gear  34  of the input shaft  21  and the low-speed side driven gear  60  of the counter shaft  27  constitute a low-speed side engine gear train GLo for transmitting the rotation of the input shaft  21  to the counter shaft  27 . The high-speed side drive gear  36  of the input shaft  21  and the high-speed side driven gear  64  of the counter shaft  27  constitute a high-speed side engine gear train GHi for transmitting the rotation of the input shaft  21  to the counter shaft  27 . Here, the low-speed side engine gear train GLo including the low-speed side drive gear  34  and the low-speed side driven gear  60  has a higher reduction ratio than the high-speed side engine gear train GHi including the high-speed side drive gear  36  and the high-speed side driven gear  64 . 
     Therefore, when the first clutch CL 1  is engaged and the second clutch CL 2  is disengaged at the time of driving the engine ENG, the driving force of the engine ENG is transmitted to the counter shaft  27  via the low-speed side engine gear train GLo at a high reduction ratio. Meanwhile, when the first clutch CL 1  is disengaged and the second clutch CL 2  is engaged at the time of driving the engine ENG, the driving force of the engine ENG is transmitted to the counter shaft  27  via the high-speed side engine gear train GHi at a low reduction ratio. Note that the first clutch CL 1  and the second clutch CL 2  are not engaged at the same time. 
     The motor drive gear  52  of the motor shaft  25  and the motor driven gear  66  of the counter shaft  27  constitute the motor gear train Gm for transmitting the rotation of the motor shaft  25  to the counter shaft  27 . When the rotor R of the motor MOT is rotated, the rotation of the motor shaft  25  is transmitted to the counter shaft  27  via the motor gear train Gm. Accordingly, when the motor MOT is driven, the driving force of the motor MOT is transmitted to the counter shaft  27  via the motor gear train Gm. 
     The output gear  62  of the counter shaft  27  and the ring gear  70  of the differential mechanism D constitute a final gear train Gf for transmitting a rotation of the counter shaft  27  to the differential mechanism D. Therefore, the driving force of the motor MOT input to the counter shaft  27  via the motor gear train Gm, the driving force of the engine ENG input to the counter shaft  27  via the low-speed side engine gear train GLo, and the driving force of the engine ENG input to the counter shaft  27  via the high-speed side engine gear train GHi are transmitted to the differential mechanism D via the final gear train Gf and transmitted from the differential mechanism D to an axle DS. As a result, a driving force for the vehicle  1  to travel is output via a pair of driving wheels DW provided at both ends of the axle DS. 
     The driving device  10  configured as described above has a power transmission path for transmitting the driving force of the motor MOT to the axle DS (that is, the driving wheels DW), a low-speed side power transmission path for transmitting the driving force of the engine ENG to the axle DS, and a high-speed side power transmission path for transmitting the driving force of the engine ENG to the axle DS. Thus, as will be described later, the vehicle  1  equipped with the driving device  10  can take a plurality of traveling modes such as an EV traveling mode or a hybrid traveling mode in which the vehicle travels on power output from the motor MOT, and a low-speed side engine traveling mode or a high-speed side engine traveling mode in which the vehicle travels on power output from the engine ENG. 
     The control device  100  acquires vehicle information relating to the vehicle  1  based on detection signals received from various sensors included in the vehicle  1 , and controls the driving device  10  based on the acquired vehicle information. 
     Here, the vehicle information includes information indicating a traveling state of the vehicle  1 . For example, the vehicle information includes, as the information indicating the traveling state of the vehicle  1 , information indicating a speed of the vehicle  1  (hereinafter, also referred to as a vehicle speed), an accelerator pedal (AP) opening degree indicating an operation amount (that is, an accelerator position) with respect to an accelerator pedal provided in the vehicle  1 , a required driving force of the vehicle  1  derived based on the vehicle speed, the AP opening and the like, the rotational speed of the engine ENG (hereinafter referred to as “the engine rotational speed”), and the like. In addition, the vehicle information further includes battery information related to the battery BAT included in the vehicle  1 . The battery information includes, for example, information indicating a state of charge (SOC) of the battery BAT. 
     The control device  100  controls the driving device  10  based on the vehicle information to cause the vehicle  1  to travel in any of the plurality of traveling modes that the vehicle  1  can take. In the control of the driving device  10 , for example, the control device  100  controls the output of power from the engine ENG by controlling the supply of fuel to the engine ENG, controls the output of power from the motor MOT by controlling the supply of electric power to the motor MOT, and controls the generation of electric power (for example, output voltage) of the generator GEN by controlling a field current or the like flowing through a coil of the generator GEN. 
     In the control of the driving device  10 , the control device  100  controls the first clutch CL 1  to be disengaged or engaged by controlling an actuator (not illustrated) that operates the first clutch CL 1 . Similarly, the control device  100  controls the second clutch CL 2  to be disengaged or engaged by controlling an actuator (not illustrated) that operates the second clutch CL 2 . 
     In this way, by controlling the engine ENG, the generator GEN, the motor MOT, the first clutch CL 1  and the second clutch CL 2 , the control device  100  can cause the vehicle  1  to travel in any of the plurality of traveling modes that the vehicle  1  can take. The control device  100  is an example of the control device of a vehicle according to the present disclosure, and is realized by an electronic control unit (ECU) including a processor, a memory, an interface, and the like, for example. 
     [Traveling Mode that Vehicle can Take] 
     Next, a traveling mode that the vehicle  1  can take will be described with reference to  FIG.  2   . In  FIG.  2   , as illustrated in a traveling mode table Ta, the vehicle  1  can take the plurality of traveling modes including the EV traveling mode, the hybrid traveling mode, the low-speed side engine traveling mode, and the high-speed side engine traveling mode. 
     [EV Traveling Model] 
     The EV traveling mode is a traveling mode in which electric power is supplied to the motor MOT from the battery BAT, and the vehicle  1  is driven by the power output from the motor MOT in accordance with the electric power. 
     Specifically, in the EV traveling mode, the control device  100  controls both the first clutch CL 1  and the second clutch CL 2  to be disengaged. In the EV traveling mode, the control device  100  performs a control such that injection of fuel to the engine ENG is stopped (so-called fuel cut), and the output of the power from the engine ENG is stopped. In the EV traveling mode, the control device  100  performs a control such that electric power is supplied to the motor MOT from the battery BAT, and power corresponding to the electric power is output to the motor MOT (illustrated as Motor: “driven by battery”). As a result, in the EV traveling mode, the vehicle  1  travels on power that is output from the motor MOT according the electric power supplied from the battery BAT. 
     In the EV traveling mode, as described above, the output of the power from the engine ENG is stopped, and both the first clutch CL 1  and the second clutch CL 2  are disengaged. Therefore, in the EV traveling mode, no power is input to the generator GEN, and the generator GEN does not generate electric power (illustrated as Generator: “stop power generation”). 
     [Hybrid Traveling Mode] 
     The hybrid traveling mode is an example of a third traveling mode in the present disclosure, and is a traveling mode in which electric power is supplied to the motor MOT at least from the generator GEN, and the vehicle  1  travels on the power output from the motor MOT in accordance with the electric power. 
     Specifically, in the case of the hybrid traveling mode, the control device  100  controls both the first clutch CL 1  and the second clutch CL 2  to be disengaged. In the case of the hybrid traveling mode, the control device  100  causes fuel to inject to the engine ENG to output power from the engine ENG. The power output from the engine ENG is input to the generator GEN via the generator gear train Gg. As a result, electric power generation by the generator GEN is performed. 
     In a case of the hybrid traveling mode, the control device  100  performs a control such that the electric power generated by the generator GEN is supplied to the motor MOT, and power corresponding to the electric power is output from the motor MOT (illustrated as Motor: “driven by generator”). The electric power supplied from the generator GEN to the motor MOT is greater than the electric power supplied from the battery BAT to the motor MOT. Therefore, in the hybrid traveling mode, the power output from the motor MOT (driving force of the motor MOT) can be increased as compared with the EV traveling mode, and a large driving force can be obtained as the driving force of the vehicle  1 . 
     In the case of the hybrid traveling mode, the control device  100  may perform a control such that the electric power is supplied from the battery BAT to the motor MOT if necessary. That is, in the hybrid traveling mode, the control device  100  may perform a control such that electric power is supplied from both the generator GEN and the battery BAT to the motor MOT. As a result, the electric power supplied to the motor MOT can be increased compared to a case where electric power is supplied to the motor MOT only from the generator GEN, and a greater driving force can be obtained as the driving force of the vehicle  1 . 
     In addition, even in the hybrid traveling mode, in order to provide a driver with a natural feeling in which the vehicle speed and an operating sound of the engine ENG are in conjunction with each other, as will be described later, the control device  100  controls the engine rotational speed such that when the engine rotational speed reaches a predetermined upper limit rotational speed, the engine rotational speed is temporarily lowered to a predetermined lower limit rotational speed and then the engine rotational speed is increased again. A specific control example of the engine rotational speed in the hybrid traveling mode will be described later. 
     [Low-Speed Side Engine Traveling Mode] 
     The low-speed side engine traveling mode is an example of a second traveling mode in the present disclosure, and is a traveling mode in which the power output from the engine ENG is transmitted to the driving wheels DW via the low-speed side power transmission path to cause the vehicle  1  to travel. 
     Specifically, in the case of the low-speed side engine traveling mode, the control device  100  perform a control such that fuel is injected to the engine ENG and power is output from the engine ENG. In addition, in the case of the low-speed side engine traveling mode, the control device  100  controls the first clutch CL 1  to be engaged and the second clutch CL 2  to be disengaged. Thus, in the low-speed side engine traveling mode, the power output from the engine ENG is transmitted to the driving wheels DW via the low-speed side engine gear train GLo, the final gear train Gf, and the differential mechanism D, and the vehicle  1  travels. 
     In the case of the low-speed side engine traveling mode, the power output from the engine ENG is also input to the generator GEN via the generator gear train Gg, but the generator GEN is controlled so as not to generate power. For example, in the low-speed side engine traveling mode, a switching element (for example, a switching element of an inverter device provided between the generator GEN and the battery BAT) provided in an electric power transmission path between the generator GEN and the battery BAT is turned off so that the generator GEN is controlled so as not to generate power. Accordingly, in the low-speed side engine traveling mode, loss caused by electric power generation of the generator GEN can be reduced, and an amount of heat generated by the generator GEN or the like can be reduced. In the low-speed side engine traveling mode, during braking of the vehicle  1 , regenerative electric power generation may be performed by the motor MOT to charge the battery BAT with the generated electric power. 
     In the case of the low-speed side engine traveling mode, for example, the control device  100  stops the supply of electric power to the motor MOT, and stops the output of the power from the motor MOT As a result, in the low-speed side engine traveling mode, a load on the motor MOT can be reduced, and an amount of heat generated by the motor MOT can be reduced. 
     In the case of the low-speed side engine traveling mode, the control device  100  may perform a control such that the electric power is supplied from the battery BAT to the motor MOT if necessary. Thus, in the low-speed side engine traveling mode, the vehicle  1  can travel using the power output from the motor MOT by the electric power supplied from the battery BAT, and as compared with the case where the vehicle  1  travels on only the power of the engine ENG, a greater driving force can be obtained as the driving force of the vehicle  1 . 
     [High-Speed Side Engine Traveling Mode] 
     The high-speed side engine traveling mode is an example of a first traveling mode in the present disclosure, and is a traveling mode in which the power output from the engine ENG is transmitted to the driving wheels DW via the high-speed side power transmission path to cause the vehicle  1  to travel. 
     Specifically, in the case of the high-speed side engine traveling mode, the control device  100  performs a control such that fuel is injected to the engine ENG and power is output from the engine ENG. In addition, in the case of the high-speed side engine traveling mode, the control device  100  controls the second clutch CL 2  to be engaged and the first clutch CL 1  to be disengaged. Thus, in the high-speed side engine traveling mode, the power output from the engine ENG is transmitted to the driving w % heels DW via the high-speed side engine gear train GHi, the final gear train Gf, and the differential mechanism D to cause the vehicle  1  to travel. 
     In the case of the high-speed side engine traveling mode, the power output from the engine ENG is also input to the generator GEN via the generator gear train Gg, but the generator GEN is controlled so as not to generate power. As a result, in the high-speed side engine traveling mode, the loss caused by the electric power generation of the generator GEN can be reduced, and the amount of heat generated by the generator GEN or the like can be reduced. Even in the high-speed side engine traveling mode, during braking of the vehicle  1 , regenerative electric power generation may be performed by the motor MOT to charge the battery BAT with the generated electric power. 
     In the case of the high-speed side engine traveling mode, for example, the control device  100  stops the supply of electric power to the motor MOT, and stops the output of the power from the motor MOT. As a result, in the high-speed side engine traveling mode, the load on the motor MOT can be reduced, and the amount of heat generated by the motor MOT can be reduced. 
     In the case of the high-speed side engine traveling mode, the control device  100  may perform a control such that the electric power is supplied from the battery BAT to the motor MOT if necessary. Thus, in the high-speed side engine traveling mode, the vehicle  1  can also travel using the power output from the motor MOT based on the electric power supplied from the battery BAT, and as compared with the case where the vehicle  1  travels on only the power of the engine ENG, a greater driving force can be obtained as the driving force of the vehicle  1 . 
     [Transition Example of Traveling Model] 
     Next, a transition example of the traveling mode in the vehicle  1  will be described with reference to  FIG.  3   .  FIG.  3    illustrates a relationship between the driving force and the vehicle speed of the vehicle  1  in the hybrid traveling mode, the low-speed side engine traveling mode, and the high-speed side engine traveling mode. In  FIG.  3   , a vertical axis indicates the driving force [N] of the vehicle  1 , and a horizontal axis indicates the vehicle speed [km/h]. 
     A driving force F 1  illustrated in  FIG.  3    is the maximum driving force of the vehicle  1  in the hybrid traveling mode. That is, the driving force F 1  is the maximum driving force of the vehicle  1  obtained by supplying the electric power generated by the generator GEN by the power of the engine ENG to the motor MOT. 
     A driving force F 2  illustrated in  FIG.  3    is the maximum driving force of the vehicle  1  in the low-speed side engine traveling mode. That is, the driving force F 2  is the maximum driving force of the vehicle  1  obtained by transmitting the power of the engine ENG to the axle DS (that is, the driving wheel DW) by the low-speed side power transmission path. 
     A driving force F 3  illustrated in  FIG.  3    is the maximum driving force of the vehicle  1  in the high-speed side engine traveling mode. That is, the driving force F 3  is the maximum driving force of the vehicle  1  obtained by transmitting the power of the engine ENG to the axle DS (that is, the driving wheel DW) by the high-speed side power transmission path. 
     A first applicable range Te 1  illustrated in  FIG.  3    indicates a first traveling state of the vehicle  1  that is applicable to travel in the low-speed side engine traveling mode. Specifically, the first applicable range Te 1  indicates a range in which the vehicle speed is in a range of v 1  to v 2 , and the driving force of the vehicle  1  is equal to or less than a predetermined driving force F 11 . In the present embodiment, when the traveling state of the vehicle  1  is included in the first applicable range Te 1 , the vehicle  1  can travel with less fuel consumption in the low-speed side engine traveling mode as compared with the case where the vehicle  1  travels in the hybrid traveling mode or the high-speed side engine traveling mode. 
     Therefore, for example, when the traveling state of the vehicle  1  is included in the first applicable range Te 1  due to an increase in the vehicle speed while the vehicle  1  is traveling in the hybrid traveling mode, as illustrated by an arrow A in  FIG.  3   , the control device  100  shifts from the hybrid traveling mode to the low-speed side engine traveling mode. Hereinafter, the state in which the traveling state of the vehicle  1  is included in the first applicable range Te 1  when the vehicle  1  is traveling in the hybrid traveling mode is also referred to as “establishment of a transition condition from the hybrid traveling mode to the low-speed side engine traveling mode”. 
     When the traveling state of the vehicle  1  is included in the first applicable range Te 1 , the control device  100  may intermittently switch the traveling mode between the low-speed side engine traveling mode and the EV traveling mode. For example, when the traveling state of the vehicle  1  is included in the first applicable range Te 1 , based on a remaining capacity of the battery BAT, a temperature of the motor MOT, or the like, the control device  100  can cause the vehicle  1  to travel in a more appropriate traveling mode by intermittently switching the traveling mode between the low-speed side engine traveling mode and the EV traveling mode. 
     A second applicable range Te 2  illustrated in  FIG.  3    indicates the traveling state of the vehicle  1  that is applicable to travel in the high-speed side engine traveling mode. Specifically, the second applicable range Te 2  indicates a range in which the vehicle speed is greater than v 2  and the driving force of the vehicle  1  is equal to or less than a predetermined driving force F 12 . In the present embodiment, when the traveling state of the vehicle  1  is included in the second applicable range Te 2 , the vehicle  1  can travel with less fuel consumption in the high-speed side engine traveling mode as compared with the case where the vehicle  1  travels in the hybrid traveling mode. In addition, in the range of v 2  to v 3  of the vehicle speed included in the second applicable range Te 2 , the vehicle  1  can travel in the low-speed side engine traveling mode, but the vehicle  1  can travel in the high-speed side engine traveling mode with less fuel consumption than in the low-speed side engine traveling mode. Therefore, when the traveling state of the vehicle  1  is the second applicable range Te 2 , the control device  100  causes the vehicle  1  to travel in the high-speed side engine traveling mode. 
     When the traveling state of the vehicle  1  is included in the second applicable range Te 2 , the control device  100  may intermittently switch the traveling mode between the high-speed side engine traveling mode and the EV traveling mode. For example, when the traveling state of the vehicle  1  is included in the second applicable range Te 2 , based on a remaining capacity of the battery BAT, a temperature of the motor MOT, or the like, the control device  100  can cause the vehicle  1  to travel in a more appropriate traveling mode by intermittently switching the traveling mode between the high-speed side engine traveling mode and the EV traveling mode. 
     A third applicable range Te 3  illustrated in  FIG.  3    indicates a second traveling state of the vehicle  1  that is applicable to travel in the low-speed side engine traveling mode. Specifically, the third applicable range Te 3  indicates a range in which the vehicle speed is in a range of v 1  to v 3 , the driving force of the vehicle  1  is equal to or greater than the driving force F 11  and is equal to or less than the driving force F 2  when the vehicle speed is in the range of v 1  to v 2 , and the driving force of the vehicle  1  is equal to or greater than the driving force F 12  and equal to or less than the driving force F 2  when the vehicle speed is in the range of v 2  to v 3 . In the present embodiment, when the traveling state of the vehicle  1  is included in the third applicable range Te 3 , the vehicle  1  can travel in the low-speed side engine traveling mode while suppressing heat generation of the motor MOT, the generator GEN, the engine ENG or the like as compared with the case where the vehicle  1  travels in the hybrid traveling mode or the high-speed side engine traveling mode. 
     Therefore, for example, when the traveling state of the vehicle  1  is included in the third applicable range Te 3  due to an increase in the driving force of the vehicle  1  while the vehicle  1  is traveling in the high-speed side engine traveling mode, as illustrated by an arrow B in  FIG.  3   , the control device  100  shifts the traveling mode from the high-speed side engine traveling mode to the low-speed side engine traveling mode. Specifically, at this time, as will be described later, the control device  100  shifts the traveling mode from the high-speed side engine traveling mode to the hybrid traveling mode, and then shifts the traveling mode from the hybrid traveling mode to the low-speed side engine traveling mode. Hereinafter, the state, in which the traveling state of the vehicle  1  is included in the third applicable range Te 3  when the vehicle  1  is traveling in the high-speed side engine traveling mode, is also referred to as “establishment of a transition condition from the high-speed side engine traveling mode to the low-speed side engine traveling mode”. 
     [Functional Configuration of Control Device] 
     Next, a functional configuration of the control device  100  will be described with reference to  FIGS.  4  to  8   . As illustrated in  FIG.  4   , the control device  100  includes an information acquisition unit  110 , a traveling mode setting unit  120  for setting a traveling mode, and a driving device control unit  130  that controls the driving device  10 . For example, the information acquisition unit  110 , the traveling mode setting unit  120 , and the driving device control unit  130  can realize the functions thereof by executing a program stored in a memory by a processor of an ECU that realizes the control device  100 , or by an interface of the ECU. 
     The information acquisition unit  110  acquires vehicle information related to the vehicle  1  based on detection signals and the like sent from various sensors included in the vehicle  1  to the control device  100 . Then, the information acquisition unit  110  passes the acquired vehicle information to the traveling mode setting unit  120  and the driving device control unit  130 . As described above, the vehicle information includes, for example, information indicating the traveling state of the vehicle  1  such as the vehicle speed, the AP opening degree, the required driving force, and the engine rotational speed. 
     The vehicle speed can be acquired, for example, based on a detection signal from a vehicle speed sensor S 1  that detects a rotational speed of the axle DS. The AP opening degree can be acquired based on a detection signal from an accelerator position sensor (illustrated as AP sensor) S 2  that detects an operation amount of the accelerator pedal provided in the vehicle  1 . The required driving force can be acquired by deriving the driving force based on the vehicle speed acquired based on the detection signal from the vehicle speed sensor S 1  or the AP opening degree acquired based on the detection signal from the AP sensor S 2 . The engine rotational speed can be acquired, for example, based on a detection signal from an engine rotational speed sensor (ENG rotational speed sensor) S 3  that detects the engine rotational speed. 
     As described above, the vehicle information further includes the battery information. The battery information can be acquired, for example, based on a detection signal from a battery sensor S 4  that detects a state of the battery BAT. Specifically, the battery sensor S 4  detects an inter-terminal voltage, a charge/discharge current, a temperature, and the like of the battery BAT, and transmits a detection signal indicating the detected information to the control device  100 . The information acquisition unit  110  derives the SOC of the battery BAT based on the inter-terminal voltage, the charge/discharge current, and the like of the battery BAT detected by the battery sensor S 4 , and acquires the battery information including the derived SOC information. The battery information may include information such as the inter-terminal voltage, the charge/discharge current, the temperature, and the like of the battery BAT detected by the battery sensor S 4 . 
     The traveling mode setting unit  120  sets one of the plurality of traveling modes that the vehicle  1  can take, and notifies the driving device control unit  130  of the set traveling mode. For example, information indicating a setting condition of each traveling mode is stored in advance in the control device  100 . Here, the information indicating the setting condition of each traveling mode is, for example, information in which the traveling state of the vehicle  1  and the traveling mode (that is, the traveling mode to be set) applicable to the traveling state are associated with each other. 
     The traveling mode setting unit  120  sets a traveling mode that is applicable to the traveling state of the vehicle  1  with reference to the vehicle information acquired from the information acquisition unit  110  and the information indicating the setting condition of each traveling mode stored in the control device  100 . 
     For example, when the transition condition from the hybrid traveling mode to the low-speed side engine traveling mode is satisfied, the traveling mode setting unit  120  shifts the hybrid traveling mode to the low-speed side engine traveling mode. Specifically, in this case, the traveling mode setting unit  120  sets the low-speed side engine traveling mode and notifies the driving device control unit  130  that the traveling mode is set to the low-speed side engine traveling mode. As a result, the driving device control unit  130  performs a control such that the first clutch CL 1  is engaged and the traveling mode is shifted to the low-speed side engine traveling mode. 
     Further, for example, when the transition condition from the high-speed side engine traveling mode to the low-speed side engine traveling mode is satisfied, the traveling mode setting unit  120  shifts the traveling mode from the high-speed side engine traveling mode to the low-speed side engine traveling mode via the hybrid traveling mode. Specifically, in this case, the traveling mode setting unit  120  first sets the hybrid traveling mode and notifies the driving device control unit  130  that the traveling mode is set to the hybrid traveling mode. As a result, the driving device control unit  130  performs a control such that the second clutch CL 2  is disengaged and the traveling mode is shifted to the hybrid traveling mode. Subsequently, the traveling mode setting unit  120  sets the low-speed side engine traveling mode and notifies the driving device control unit  130  that the traveling mode is set to the low-speed side engine traveling mode. As a result, the driving device control unit  130  performs a control such that the first clutch CL 1  is engaged and the traveling mode is shifted to the low-speed side engine traveling mode. 
     The driving device control unit  130  controls the driving device  10  based on the traveling mode set by the traveling mode setting unit  120 , the vehicle information acquired by the information acquisition unit  110 , and the like. The driving device control unit  130  includes, for example, an engine control unit  131  that controls the engine ENG. 
     In the case of the low-speed side engine traveling mode or the high-speed side engine traveling mode, the engine control unit  131  controls the engine ENG such that the engine ENG outputs a driving force for realizing the required driving force indicated by the vehicle information. 
     In the case of the hybrid traveling mode, the engine control unit  131  controls the engine ENG (that is, power generation of the generator in this case) such that the motor MOT outputs a driving force for realizing the required driving force indicated by the vehicle information. Further, in the case of the hybrid traveling mode, the engine control unit  131  performs a control such that the engine rotational speed fluctuates between a predetermined upper limit rotational speed NeH and a lower limit rotational speed NeL. 
     [Engine Rotational Speed in Hybrid Traveling Mode] 
       FIG.  5    illustrates an example of the control of the engine rotational speed performed by the engine control unit  131  in the hybrid traveling mode. In  FIG.  5   , a vertical axis indicates the engine rotational speed [rpm], and a horizontal axis indicates the vehicle speed [km/h]. 
     An engine rotational speed Ne 1  illustrated in  FIG.  5    is an engine rotational speed in the hybrid traveling mode. As indicated by the engine rotational speed Ne 1 , in the case of the hybrid traveling mode, the engine control unit  131  controls the engine rotational speed so as to fluctuate between the predetermined upper limit rotational speed NeH and the lower limit rotational speed NeL. 
     Specifically, in the case of the hybrid traveling mode, the engine control unit  131  first increases the engine rotational speed as the vehicle speed increases at a predetermined increase rate a 1  from a state where both the vehicle speed and the engine rotational speed are 0 (zero). Then, when the engine rotational speed reaches the upper limit rotational speed NeH corresponding to the vehicle speed at that time, the engine rotational speed is decreased to the lower limit rotational speed NeL corresponding to the vehicle speed at that time. After that, the engine control unit  131  increases the engine rotational speed from the lower limit rotational speed NeL as the vehicle speed increases again. However, at this time, the engine rotational speed is increased at an increase rate a 2  smaller than the increase rate a 1 . 
     In the same way thereafter, the engine control unit  131  decreases the engine rotational speed to the lower limit rotational speed NeL when the engine rotational speed reaches the upper limit rotational speed NeH, and as the vehicle speed increases, increases the engine rotational speed while changing the increase rate  2  to an increase rate a 3 , an increase rate a 4 , an increase rate a 5  each time. Here, the increase rate a 2 &gt;the increase rate a 3 &gt;the increase rate a 4 &gt;the increase rate a 5 . 
     In the hybrid traveling mode, since both the first clutch CL 1  and the second clutch CL 2  are disengaged as described above, the engine rotational speed can be set freely regardless of the vehicle speed. However, by controlling the engine rotational speed so as to fluctuate between the upper limit rotational speed NeH and the lower limit rotational speed NeL as the vehicle speed increases, the driver can feel a natural change in an operating sound of the engine ENG in conjunction with the vehicle speed as if the gear is shifted by a stepped transmission even the vehicle  1  is traveling in the hybrid traveling mode. 
     The engine rotational speed Ne 2  illustrated in  FIG.  5    is an engine rotational speed in the low-speed side engine traveling mode. As described above, in the low-speed side engine traveling mode, the engine ENG and the axle DS (that is, the driving wheels DW) are mechanically connected. Therefore, as indicated by the engine rotational speed Ne 2 , the engine rotational speed and the vehicle speed linearly correspond to each other. Specifically, in the present embodiment, in the case of the low-speed side engine traveling mode, the engine rotational speed increases at an increase rate a 11  as the vehicle speed increases. For example, the increase rate a 2 &gt;the increase rate a 11 &gt;the increase rate a 3 . 
     An engine rotational speed Ne 3  illustrated in  FIG.  5    is an engine rotational speed in the high-speed side engine traveling mode. As described above, in the high-speed side engine traveling mode, the engine ENG and the axle DS are mechanically connected in the same manner as the low-speed side engine traveling mode. Therefore, as indicated by the engine rotational speed Ne 3 , the engine rotational speed and the vehicle speed linearly correspond to each other. Specifically, in the present embodiment, in the case of the high-speed side engine traveling mode, the engine rotational speed increases at an increase rate a 12  as the vehicle speed increases. For example, the increase rate a 4 &gt;the increase rate a 12 &gt;the increase rate a 5 . 
     Note that although  FIG.  5    also illustrates the engine rotational speed Ne 2  and the engine rotational speed Ne 3  in a state where the vehicle speed is 0 (zero) for the sake of convenience, the low-speed side engine traveling mode or the high-speed side engine traveling mode does not actually set when the vehicle speed is 0 (zero). 
     [First Example of Control Performed Regarding Transition to Low-Speed Side Engine Traveling Model] 
       FIG.  6    illustrates a first example of a control performed by the control device  100  during the transition to the low-speed side engine traveling mode. The first example is an example in which the transition indicated by the arrow A in  FIG.  3   , that is, the transition from the hybrid traveling mode to the low-speed side engine traveling mode is performed when a transition condition from the hybrid traveling mode to the low-speed side engine traveling mode is satisfied, for example. 
     First, before describing the control performed by the control device  100 , a problem that may occur during the transition from the hybrid traveling mode to the low-speed side engine traveling mode will be described with reference to section A in  FIG.  6   . 
     As illustrated in section A in  FIG.  6   , it is assumed that the engine rotational speed reaches the upper limit rotational speed NeH at a time point t 11  when the vehicle  1  is traveling in the hybrid traveling mode. Therefore, it is assumed that the engine rotational speed is decreased to the lower limit rotational speed NeL at a time point t 12  immediately after the time point t 11 . The engine rotational speed decreased to the lower limit rotational speed NeL is then again controlled to increase toward the upper limit rotational speed NeH as the vehicle speed increases. 
     However, the transition condition from the hybrid traveling mode to the low-speed side engine traveling mode may be satisfied depending on the traveling state of the vehicle  1  at a time point t 13  (for example, a time point immediately after the time point t 12 ) before the engine rotational speed decreased at time t 12  is sufficiently increased. In such a case, when shift to the low-speed side engine traveling mode is performed at the time point t 13 , as illustrated in section A in  FIG.  6   , the engine rotational speed decreased at the time point t 12  may again be decreased due to the transition to the low-speed side engine traveling mode. In this way, when a change in the engine rotational speed that is not intended by the driver occurs a plurality of times within a short period of time, the driver may feel uncomfortable or may misunderstand a failure of the engine ENG. 
     Therefore, when the engine rotational speed is decreased to the lower limit rotational speed NeL in the hybrid traveling mode, the control device  100  prohibits the transition to the low-speed side engine traveling mode for a predetermined period from the time when the lower limit rotational speed NeL is set. 
     This will be described in detail below with reference to section B in  FIG.  6   . As illustrated in section B in  FIG.  6   , it is assumed that the engine rotational speed reaches the upper limit rotational speed NeH at the time point t 11  when the vehicle  1  is traveling in the hybrid traveling mode, as in the example of section A in  FIG.  6   . Therefore, at the time point t 12  immediately after the time point t 11 , the engine control unit  131  decreases the engine rotational speed to the lower limit rotational speed NeL. When the engine rotational speed is decreased to the lower limit rotational speed NeL, the engine control unit  131  notifies the traveling mode setting unit  120  to that fact. 
     When the traveling mode setting unit  120  receives a notification that the engine rotational speed is decreased from the engine control unit  131 , a transition prohibition setting unit  121  (see  FIG.  4   ) included in the traveling mode setting unit  120  sets Td [s] to a delay timer. Here, the delay timer is, for example, a timing unit that counts down from the set Td [s] toward 0 (zero). Here, td [s] is a predetermined time. As illustrated in section B in  FIG.  6   , during a period in which a count value of the delay timer is greater than 0 (zero), the transition to the low-speed side engine traveling mode is prohibited. 
     For example, it is assumed that the transition condition from the hybrid traveling mode to the low-speed side engine traveling mode is satisfied at the time point t 13  when the count value of the delay timer is greater than 0 (zero). In such a case, the traveling mode setting unit  120  does not perform the transition from the hybrid traveling mode to the low-speed side engine traveling mode at the time point  13 . In this way, the traveling mode setting unit  120  does not perform the transition to the low-speed side engine traveling mode during the predetermined period from the time when the engine rotational speed is decreased to the lower limit rotational speed NeL, so that it is possible to prevent a change in the engine rotational speed that is not intended by the driver from being generated a plurality of times within a short period of time (see a dotted line in section B in  FIG.  6   ). 
     When the transition condition from the hybrid traveling mode to the low-speed side engine traveling mode is satisfied at the time point t 13  when the count value of the delay timer is greater than 0 (zero), the traveling mode setting unit  120  thereafter shifts from the hybrid traveling mode to the low-speed side engine traveling mode at a time point t 14  when the count value of the delay timer becomes 0 (zero). That is, as illustrated in section B in  FIG.  6   , the transition from the hybrid traveling mode to the low-speed side engine traveling mode is shifted (delayed) backward from the time point t 13  to the time point  114 . 
     In this way, when the transition condition from the hybrid traveling mode to the low-speed side engine traveling mode is satisfied during a period in which the transition to the low-speed side engine traveling mode is prohibited, the traveling mode setting unit  120  performs a transition to the low-speed side engine traveling mode when the period ends. Accordingly, after the period during which the transition to the low-speed side engine traveling mode is prohibited ends, the vehicle  1  can be efficiently driven by the low-speed side engine traveling mode suitable for the traveling state of the vehicle  1 . 
     In the above example, the traveling mode setting unit  120  may determine again whether the transition from the hybrid traveling mode to the low-speed side engine traveling mode is performed based on the traveling state of the vehicle  1  at the time point t 14  when the count value of the delay timer becomes 0 (zero), for example. In this way, when the vehicle is no longer suitable for driving in the low-speed side engine traveling mode at the end of the period during which the transition from the hybrid traveling mode to the low-speed side engine traveling mode is prohibited, it is possible to prevent the transition to the low-speed side engine traveling mode. 
     Further, although an example in which the delay timer counts down from Td [s] to 0 (zero) has been described, but the present disclosure is not limited to this, and the delay timer may be counted up from 0 (zero) to Td [s]. Also in this case, when the count value of the delay timer is greater than 0 (zero), the traveling mode setting unit  120  prohibits the transition to the low-speed side engine traveling mode. In this case, the transition prohibition setting unit  121  resets the count value of the delay timer to 0 (zero) when counting up to Td [s] by the delay timer ends, for example. In addition, the delay timer may be provided inside the control device  100  or may be provided outside the control device  100  in a state in which the control device  100  is accessible. 
     Although it is illustrated that the low-speed side engine traveling mode is continuously set from the hybrid traveling mode in  FIG.  6   , but in practice, there is a transition period for engaging the first clutch CL 1  between these. 
     Further, the control device  100  sets the decrease rate of the engine rotational speed in the hybrid traveling mode and the decrease rate of the engine rotational speed at the time of transition from the hybrid traveling mode to the low-speed side engine traveling mode at the same rate, so that it is possible to shift to the low-speed side engine traveling mode without giving the driver a sense of discomfort due to an operating sound of the engine ENG at the time of the transition from the hybrid traveling mode to the low-speed side engine traveling mode. 
     This will be described in detail below with reference to  FIG.  7   . In the following description of  FIG.  7   , the description of the same parts as those in  FIG.  6    will be omitted as appropriate. As illustrated in  FIG.  7   , since the engine rotational speed reaches the upper limit rotational speed NeH at the time point t 11  in the hybrid traveling mode, the engine control unit  131  decreased the engine rotational speed to the lower limit rotational speed NeL at the time point t 12 . 
     As described above, when the engine rotational speed is decreased from the upper limit rotational speed NeH to the lower limit rotational speed NeL in the hybrid traveling mode, a decrease rate calculation unit  132  (see  FIG.  4   ) included in the engine control unit  131  calculates a decrease rate d of the engine rotational speed per unit time at this time. 
     In the example illustrated in  FIG.  7   , the decrease rate d calculated by the decrease rate calculation unit  132  is a decrease rated=ΔNe 1 /ΔT 1 . Here, ΔNe 1  is a value obtained by subtracting the lower limit rotational speed NeL at the time point t 12  from the upper limit rotational speed NeH at the time point t 11 . Here, ΔT 1  is an elapsed time from the time point t 11  to the time point t 12 . That is, the decrease rate calculation unit  132  can obtain ΔT 1  by measuring the elapsed time from the time point t 11  to the time point t 12 . 
     For example, the decrease rate calculation unit  132  calculates the decrease rate d each time the engine rotational speed is lowered from the upper limit rotational speed NeH to the lower limit rotational speed NeL in the hybrid traveling mode, and stores the calculated decrease rate d in a memory of the control device  100  or the like. Note that only the decrease rate d calculated most recently by the decrease rate calculation unit  132  may be stored in the control device  100 . That is, each time the decrease rate d is calculated by the decrease rate calculation unit  132 , the decrease rate d stored in the control device  100  may be updated. 
     In the example illustrated in  FIG.  7   , it is assumed that the hybrid traveling mode is set by the traveling mode setting unit  120  at the time point t 14  after the time point t 12 . In this case, the engine control unit  131  decreases the engine rotational speed from Ne 11  to Ne 12  at the decrease rate d calculated most recently by the decrease rate calculation unit  132  from the time point t 14 . Then, the driving device control unit  130  controls the first clutch CL 1  to be engaged when the engine rotational speed decreases to Ne 12 . As a result, the traveling mode is shifted to the low-speed side engine traveling mode. 
     Here, Ne 12  is a value obtained by, for example, Ne 12 =the number of rotations of the motor MOT at the time point t 14 ×(the number of teeth of the motor drive gear  52 /the number of teeth of the motor driven gear  66 )×(the number of teeth of the low-speed side driven gear  60 /the number of teeth of the low-speed side drive gear  34 ). Accordingly, the rotational speed of the low-speed side drive gear  34  and the rotational speed of the input shaft  21  can be matched. 
     For example, the engine control unit  131  may decrease the engine rotational speed at the decrease rated from the time point t 14  without obtaining the above Ne 12 . In this case, the driving device control unit  130  may control the first clutch CL 1  to be engaged when the engine rotational speed decreased by the engine control unit  131  becomes the engine rotational speed corresponding to the vehicle speed at that time in the low-speed side engine traveling mode. 
     In this way, the control device  100  aligns the decrease rate d of the engine rotational speed in the hybrid traveling mode with the decrease rate d of the engine rotational speed at the time of the transition from the hybrid traveling mode to the low-speed side engine traveling mode. As a result, the control device  100  can provide consistency between a change behavior of the engine rotational speed in the hybrid traveling mode and a change behavior of the engine rotational speed at the time of the transition from the hybrid traveling mode to the low-speed side engine traveling mode. Therefore, when shifting from the hybrid traveling mode to the low-speed side engine traveling mode, it is possible to shift to the low-speed side engine traveling mode without giving the driver a sense of discomfort due to an operating sound of the engine ENG. 
     [Second Example of Control Performed Regarding Transition to Low-Speed Side Engine Traveling Model] 
       FIG.  8    illustrates a second example of the control performed by the control device  100  during the transition to the low-speed side engine traveling mode. The second example is an example in which the transition indicated by an arrow B in  FIG.  3   , that is, the transition from the high-speed side engine traveling mode to the low-speed side engine traveling mode is performed via the hybrid traveling mode when a transition condition from the high-speed side traveling mode to the low-speed side engine traveling mode is satisfied, for example. 
     The reduction ratio of the low-speed side power transmission path used in the low-speed side engine traveling mode is greater than the reduction ratio of the high-speed side power transmission path used in the high-speed side engine traveling mode. Therefore, due to a difference between the reduction ratio of the low-speed side power transmission path and the reduction ratio of the high-speed side power transmission path, when the high-speed side engine traveling mode is directly shifted to the low-speed side engine traveling mode, an abrupt increase (rise) in the engine rotational speed may occur, which may reduce commercial value of the vehicle  1  from a viewpoint of NV. 
     Therefore, when the transition condition from the high-speed side engine traveling mode to the low-speed side engine traveling mode is satisfied, the control device  100  once shifts from the high-speed side engine traveling mode to the hybrid traveling mode, and then shifts from the hybrid traveling mode to the low-speed side engine traveling mode. As described above, the control device  100  shifts the traveling mode to the low-speed side engine traveling mode via the hybrid traveling mode having a high degree of freedom of the engine rotational speed with respect to the vehicle speed, makes it possible to shift to the low-speed side engine traveling mode while suppressing an abrupt increase in the engine rotational speed during the transition to the low-speed side engine traveling mode. 
     More specifically, at the time point t 21  illustrated in  FIG.  8   , the vehicle  1  is traveling in the high-speed side engine traveling mode. It is assumed that the driver steps on the accelerator pedal at a substantially constant pace from the time point t 21 . In this case, as illustrated in  FIG.  8   , the AP opening degree in the vehicle  1  increases at a substantially constant increase rate from the time point t 21 . Therefore, the engine control unit  131  increases the engine rotational speed up to a time point t 22  after the time point t 21  at a substantially constant increase rate a 21  corresponding to the increase in the AP opening degree. Incidentally, although not illustrated, at this time, the vehicle speed also increases (that is, accelerates) at a substantially constant increase rate as the engine rotational speed increases at the increase rate a 21 . 
     At the time point t 22 , it is assumed that the transition condition from the high-speed side engine traveling mode to the low-speed side engine traveling mode is satisfied. Here, the engine rotational speed at the time point t 22  is set to Ne 21  [rpm]. In this case, first, at the time point t 22 , the control device  100  controls the second clutch CL 2  that has been engaged in the high-speed side engine traveling mode to be disengaged, thereby shifting to the hybrid traveling mode. 
     Further, upon transition to the hybrid traveling mode, the control device  100  increases the engine rotational speed to Ne 22  [rpm]. Here, Ne 22  [rpm] is greater than Ne 21  [rpm] and smaller than Ne 23  [rpm] described later. More specifically, Ne 22  [rpm] is, for example, substantially the average of Ne 21  [rpm] and Ne 23  [rpm]. 
     Thereafter, the control device  100  causes the vehicle  1  to travel in the hybrid traveling mode from the time point t 22  until a time point t 23  when a predetermined time elapses, for example. When the vehicle  1  is traveling in the hybrid traveling mode, the control device  100  increases the engine rotational speed at a substantially constant increase rate a 22  corresponding to the increase in the AP opening degree, as illustrated in  FIG.  8   . Here, the increase rate a 22  is greater than the increase rate a 21  and smaller than an increase rate a 23  described later, for example. 
     Thereafter, at the time point t 23 , the control device  100  causes the first clutch CL 1  that has been disengaged in the hybrid traveling mode to be engaged, thereby shifting to the low-speed side engine traveling mode. As a result, the engine ENG and the driving wheel DW are mechanically connected, and accordingly, as illustrated in  FIG.  8   , the engine rotational speed increases to Ne 23  [rpm] according to the vehicle speed at that time. 
     Then, after shifting to the low-speed side engine traveling mode, as illustrated in  FIG.  8   , the control device  100  increases the engine rotational speed at the substantially constant increase rate a 23  corresponding to the increase in the AP opening degree. Although not illustrated, at this time, the vehicle speed also increases (that is, accelerates) at a substantially constant increase rate as the engine rotational speed increases at the increase rate a 23 . 
     As described above, when the transition condition from the high-speed side engine traveling mode to the low-speed side engine traveling mode is satisfied, the control device  100  performs a series of transitions of traveling modes (hereinafter also simply referred to as “shift”) from the high-speed side engine traveling mode to the low-speed side engine traveling mode via the hybrid traveling mode. 
     The control device  100  controls, at the time of the above shift, the engine rotational speed (for example, Ne 22  described above) in the hybrid traveling mode to be a value between the engine rotational speed (for example, Ne 21  described above) at the time of transition from the high-speed side engine traveling mode to the hybrid traveling mode and the engine rotational speed (for example, Ne 23  described above) at the time of transition from the hybrid traveling mode to the low-speed side engine traveling mode. 
     Specifically, the engine control unit  131  includes a derivation unit  133  (see  FIG.  4   ). At the time of the above transition, the derivation unit  133  derives the engine rotational speed at the time of transition from the hybrid traveling mode to the low-speed side engine traveling mode based on the engine rotational speed at the time of transition from the high-speed side engine traveling mode to the hybrid traveling mode, the traveling state of the vehicle  1  and the reduction ratio of the low-speed side power transmission path. 
     More specifically, as illustrated in  FIG.  8   , when the AP opening degree increases at the substantially constant increase rate, the vehicle  1  is controlled to accelerate at a substantially constant acceleration. Therefore, based on the vehicle speed at the time point t 22  and a predetermined time length from the time point t 22  to the time point t 23 , the derivation unit  133  can predict the vehicle speed at the time point t 23 . Then, the derivation unit  133  can derive Ne 23  [rpm], which is the engine rotational speed w % ben the traveling mode is shifted to the low-speed side engine traveling mode at the time point t 23 , based on the predicted vehicle speed at the time point t 23  and the reduction ratio of the low-speed side power transmission path. 
     As a result, at the time of the above transition, the engine control unit  131  can control the engine rotational speed in the hybrid traveling mode to be the value between the engine rotational speed at the time of transition from the high-speed side engine traveling mode to the hybrid traveling mode and the engine rotational speed at the time of transition from the hybrid traveling mode to the low-speed side engine traveling mode. 
     The control device  100  can roughly predict the increase rate a 23  at the time point t 22  based on the vehicle speed, the AP opening degree, the acceleration of the vehicle  1 , and the like at the predicted time point t 23 . Therefore, when the vehicle is traveling in the hybrid traveling mode, the engine control unit  131  can increase the engine rotational speed at the increase rate a 22  between the increase rate a 21  and the increase rate a 23  based on the predicted increase rate a 23 . 
     The present disclosure is not limited to the embodiments described above, and modifications, improvements, and the like can be made as appropriate. 
     At least the following matters are described in the present specification. Components and the like corresponding to the above-described embodiments are illustrated in parentheses, but the present disclosure is not limited thereto. 
     (1) A control device (control device  100 ) of a vehicle (vehicle  1 ) capable of traveling according to a plurality of traveling modes, 
     the plurality of traveling modes including:
         a first traveling mode (high-speed side engine traveling mode) in which power of an internal combustion engine (engine ENG) is transmitted to a driving wheel (driving wheel DW) via a first power transmission path having a first reduction ratio to cause the vehicle to travel;   a second traveling mode (low-speed side engine traveling mode) in which the power of the internal combustion engine is transmitted to the driving wheel via a second power transmission path having a second reduction ratio different from the first reduction ratio to cause the vehicle to travel; and   a third traveling mode (hybrid traveling mode) in which power output from an electric motor (motor MOT) is transmitted to the driving wheel in accordance with supply of electric power from a generator (generator GEN) configured to generate electric power by the power of the internal combustion engine to cause the vehicle to travel,       

     the control device including:
         a traveling mode setting unit (traveling mode setting unit  120 ) configured to set the traveling mode of any one of the plurality of traveling modes, and   an internal combustion engine control unit (engine control unit  131 ) configured to control the internal combustion engine based on the traveling mode set by the traveling mode setting unit,       

     in which the traveling mode setting unit causes a shift to shift to the second traveling mode via the third traveling mode based on a traveling state of the vehicle traveling in the first traveling mode, and 
     in which the internal combustion engine control unit includes
         a derivation unit (derivation unit  133 ) configured to derive a second rotational speed (Ne 23 ) of the internal combustion engine at the time of transition from the third traveling mode to the second traveling mode based on a first rotational speed (Ne 21 ) of the internal combustion engine at the time of transition from the first traveling mode to the third traveling mode, the traveling state of the vehicle and the second reduction ratio, and       

     the internal combustion engine control unit is configured to control a third rotational speed (Ne 22 ) of the internal combustion engine in the third traveling mode to be a value between the first rotational speed and the second rotational speed. 
     According to (1), at the time of the transition from the first traveling mode using the first power transmission path having the first reduction ratio to the second traveling mode using the second power transmission path having the second reduction ratio via the third traveling mode in which the vehicle can travel by the power of the electric motor, the rotational speed of the internal combustion engine when shifting to the third traveling mode is controlled to be the third rotational speed that is between the first rotational speed of the internal combustion engine in the first traveling mode before the third traveling mode and the second rotational speed of the internal combustion engine in the second traveling mode after the third traveling mode, so that fluctuation in the rotational speed of the internal combustion engine can be suppressed with a simple configuration. 
     (2) The control device of a vehicle according to (1), 
     in which the second reduction ratio is greater than the first reduction ratio, and 
     in which the third rotational speed is greater than the first rotational speed and smaller than the second rotational speed. 
     According to (2), since the second reduction ratio in the second traveling mode is greater than the first reduction ratio in the first traveling mode and the third rotational speed is greater than the first rotational speed and smaller than the second rotational speed, it is possible to shift to the second traveling mode while suppressing an abrupt increase in the rotational speed of the internal combustion engine. 
     (3) The control device of a vehicle according to (2), 
     in which the internal combustion engine control unit is configured to control the rotational speed of the internal combustion engine to increase a predetermined increase rate from the third rotational speed in the third traveling mode, and 
     wherein the predetermined increase rate (increase rate a 22 ) is greater than an increase rate (increase rate a 21 ) of the rotational speed of the internal combustion engine in the first traveling mode before the transition to the third traveling mode, and is smaller than an increase rate (increase rate a 23 ) of the rotational speed of the internal combustion engine after the transition to the second traveling mode predicted based on the traveling state of the vehicle. 
     According to (3), since the increase rate of the rotational speed of the internal combustion engine in the third traveling mode is controlled to be greater than the increase rate of the rotational speed of the internal combustion engine in the first traveling mode and is smaller than the increase rate of the rotational speed of the internal combustion engine in the second traveling mode, the driver can feel a natural change in an operating sound of the internal combustion engine as if the gear is shifted by a stepped transmission. 
     (4) The control device of a vehicle according to any one of (1) to (3), 
     in which the first power transmission path is provided with a first disconnection/connection part (first clutch CL 1 ) configured to disconnect/connect the first power transmission path, 
     in which the second power transmission path is provided with a second disconnection/connection part (second clutch CL 2 ) configured to disconnect/connect the second power transmission path, 
     in which the first traveling mode is a traveling mode in which the first disconnection/connection part is connected and the second disconnection/connection part is disconnected, 
     in which the second traveling mode is a traveling mode in which the first disconnection/connection part is disconnected and the second connection part is connected, and 
     in which the third traveling mode is a traveling mode in which the first disconnection/connection part and the second disconnection/connection part are disconnected. 
     According to (4), since the first traveling mode is a traveling mode in which the first disconnection/connection part of the first power transmission path is connected and the second disconnection/connection part of the second power transmission path is disconnected, the second traveling mode is a traveling mode in which the first disconnection/connection part is disconnected and the second connection part is connected, and the third traveling mode is a traveling mode in which the first disconnection/connection part and the second disconnection/connection part are disconnected, the traveling mode can be shifted between the second traveling mode and the third traveling mode by switching the connection/disconnection of the first disconnection/connection part and the second disconnection/connection part. 
     (5) A control device (control device  100 ) of a vehicle (vehicle  1 ) capable of traveling according to a plurality of traveling modes, 
     the vehicle including:
         an internal combustion engine (engine ENG);   a generator (generator GEN) configured to generate electric power by power of the internal combustion engine;   an electric motor (motor MOT) configured to output power in accordance with the supplied electric power;   a driving wheel (driving wheel DW) driven by power output from at least one of the internal combustion engine and the electric motor;   a first power transmission path (high-speed side power transmission path) provided between the internal combustion engine and the driving wheel, and configured to decelerate the power of the internal combustion engine at a first reduction ratio and transmit the power to the driving wheel;   a first disconnection/connection part (first clutch CL 1 ) configured to disconnect/connect the first power transmission path;   a second power transmission path (low-speed side power transmission path) provided between the internal combustion engine and the driving wheel, and configured to decelerate the power of the internal combustion engine at a second reduction ratio different from the first reduction ratio and transmit the power to the driving wheel; and   a second disconnection/connection part (second clutch CL 2 ) configured to disconnect/connect the second power transmission path,       

     the plurality of traveling modes including:
         a first traveling mode in which the first disconnection/connection part is connected, the second disconnection/connection part is disconnected, and at least the power of the internal combustion engine is transmitted to the driving wheel by the first power transmission path to cause the vehicle to travel;   a second traveling mode in which the second disconnection/connection part is connected, the first disconnection/connection part is disconnected, and at least the power of the internal combustion engine is transmitted to the driving wheel by the second power transmission path to cause the vehicle to travel; and   a third traveling mode in which the first disconnection/connection part and the second disconnection/connection part are disconnected, and the driving wheel is driven by the power output from the electric motor in accordance with at least the electric power supplied from the generator to cause the vehicle to travel,       

     the control device including: 
     a traveling mode setting unit (traveling mode setting unit  120 ) configured to set the traveling mode of any one of the plurality of traveling modes; and 
     an internal combustion engine control unit (engine control unit  131 ) configured to control the internal combustion engine based on the traveling mode set by the traveling mode setting unit, 
     wherein the traveling mode setting unit is configured to cause a shift to shift to the second traveling mode via the third traveling mode based on a traveling state of the vehicle traveling in the first traveling mode, 
     wherein the internal combustion engine control unit includes
         a derivation unit (derivation unit  133 ) configured to derive a second rotational speed (Ne 23 ) of the internal combustion engine at the time of transition from the third traveling mode to the second traveling mode based on a first rotational speed (Ne 21 ) of the internal combustion engine at the time of transition from the first traveling mode to the third traveling mode, the traveling state of the vehicle and the second reduction ratio, and       

     the internal combustion engine control unit is configured to control a third rotational speed (Ne 22 ) of the internal combustion engine in the third traveling mode to be a value between the first rotational speed and the second rotational speed. 
     According to (5), at the time of the transition from the first traveling mode using the first power transmission path having the first reduction ratio to the second traveling mode using the second power transmission path having the second reduction ratio via the third traveling mode in which the vehicle can travel by the power of the electric motor, the rotational speed of the internal combustion engine when shifting to the third traveling mode is controlled to be the third rotational speed that is between the first rotational speed of the internal combustion engine in the first traveling mode before the third traveling mode and the second rotational speed of the internal combustion engine in the second traveling mode after the third traveling mode, so that the fluctuation in the rotational speed of the internal combustion engine can be suppressed with a simple configuration.