Abstract:
Provided are a vehicle such that drive state can be suitably selected in a configuration including an internal combustion engine, and a vehicle control method. In a vehicle and a method of controlling the same, different values are set for a first switching threshold value for switching from a first independent drive state (in which one of a front wheel and a rear wheel is driven) to a combined drive state, and for a second switching threshold value for switching from a second independent drive state (in which the other of the front wheel and the rear wheel is driven by an internal combustion engine) to the combined drive state.

Description:
TECHNICAL FIELD 
     The present invention relates to a vehicle, which is capable of switching between an all-wheel drive mode and at least one of a front wheel drive mode and a rear wheel drive mode, as well as to a method of controlling such a vehicle. 
     BACKGROUND ART 
     U.S. Pat. No. 5,540,299 (hereinafter referred to as “U.S. Pat. No. 5,540,299A”) discloses a method of driving a vehicle having an engine 2 for driving front wheels 1FL, 1FR (primarily driven wheels) and motors ML, MR for driving rear wheels 1RL, 1RR (secondarily driven wheels) (see, Abstract, FIG. 1, and Claim 1). According to U.S. Pat. No. 5,540,299A, when a lateral G (lateral acceleration) is of a predetermined value or greater (W23: YES in FIG. 15), execution of normal driving is inhibited, in view of the fact that it is preferred to leave control over the posture of the vehicle body under the manual operation of the driver (see, W31 in FIG. 15, and column 22, lines 22 through 33). The term “normal driving” implies an assisting drive, and is defined as an antonym of the term “reverse driving”, which implies braking (see, column 8, lines 55 through 59). When the answer to W23 in FIG. 15 is YES, it also is made possible to forcibly execute normal driving in an independent mode, from the viewpoint that it is preferred that stability of the vehicle is improved by operating the vehicle in a four-wheel drive mode (see, column 22, lines 33 through 40). 
     A four-wheel drive vehicle has also been proposed in which the front and rear wheels thereof can be driven independently of each other (see, U.S. Patent Application Publication No. 2012/0015772, hereinafter referred to as “US2012/0015772A1”). According to US2012/0015772A1, a drive unit 6, which comprises an internal combustion engine 4 and an electric motor 5 arranged in series, drives the front wheels Wf, whereas electric motors 2A, 2B drive the rear wheels Wr (see, FIG. 1, and paragraphs [0084], [0085]). 
     SUMMARY OF INVENTION 
     According to U.S. Pat. No. 5,540,299A, the front wheels 1FL, 1FR, which are driven by the engine 2, serve as primarily driven wheels, whereas the rear wheels 1RL, 1RR, which are driven by the motors ML, MR, serve as secondarily driven wheels (see, claim 1). Stated otherwise, the vehicle disclosed in U.S. Pat. No. 5,540,299A is operable in a front-wheel drive mode, in which the front wheels are driven only by the engine 2, and also in a four-wheel drive mode, in which all of the wheels are driven by the engine 2 and the motors ML, MR. However, U.S. Pat. No. 5,540,299A is silent concerning a rear-wheel drive mode, in which the rear wheels are driven only by the motors ML, MR. Furthermore, U.S. Pat. No. 5,540,299A reveals nothing specific in relation to the predetermined value for the lateral G, which is used in step W23 of FIG. 15. 
     The present invention has been made in view of the aforementioned problems. An object of the present invention is to provide a vehicle and a control method for the vehicle, which enable an appropriate drive mode to be selected in an arrangement having an internal combustion engine. 
     According to the present invention, there is provided a vehicle comprising a first drive apparatus configured to drive one of a front wheel and a rear wheel, a second drive apparatus configured to drive another one of the front wheel and the rear wheel, the second drive apparatus including an internal combustion engine, a drive mode controller configured to control the first drive apparatus and the second drive apparatus in order to control drive modes of the front wheel and the rear wheel, and an internal combustion engine controller configured to control an operating state of the internal combustion engine, wherein the drive mode controller switches between a first independent drive mode, in which the vehicle is driven only by a drive force from the first drive apparatus, a second independent drive mode, in which the vehicle is driven only by a drive force from the second drive apparatus, and a composite drive mode, in which the vehicle is driven by a drive force from the first drive apparatus and the second drive apparatus, wherein the drive mode controller switches from the first independent drive mode to the composite drive mode, and from the second independent drive mode to the composite drive mode, based on a lateral acceleration-related value in relation to a lateral acceleration acting on the vehicle, and wherein different values are set as a first switching threshold value, which indicates the lateral acceleration-related value for switching from the first independent drive mode to the composite drive mode, and a second switching threshold value, which indicates the lateral acceleration-related value for switching from the second independent drive mode to the composite drive mode. 
     According to the present invention, different values are set as the first switching threshold value, which is used for switching from the first independent drive mode (a mode in which one of the front and rear wheels is driven) to the composite drive mode, and the second switching threshold value, which is used for switching from the second independent drive mode (a mode in which another one of the front and rear wheels is driven by the internal combustion engine) to the composite drive mode. Stated otherwise, it is possible to switch the threshold value for the lateral acceleration-related value to different values at respective times when the internal combustion engine is in operation, and when the internal combustion engine is stopped. Consequently, it is possible to switch between different drive modes in view of achieving a balance between energy consumption while the internal combustion engine is in operation, and vehicle driving stability, i.e., the capability of the vehicle to be driven as the driver wishes, for example. 
     The second switching threshold value may be less than the first switching threshold value. In this manner, it is possible to switch from the second independent drive mode, in which the vehicle is driven by the internal combustion engine, to the composite drive mode more quickly than switching from the first independent drive mode, in which the vehicle is not driven by the internal combustion engine, to the composite drive mode. Therefore, if the internal combustion engine is in operation prior to switching to the composite drive mode, an increase in driving stability can be achieved more quickly. 
     According to the present invention, there also is provided a vehicle comprising a first drive apparatus configured to drive one of a front wheel and a rear wheel, a second drive apparatus configured to drive another one of the front wheel and the rear wheel, the second drive apparatus including an internal combustion engine, a drive mode controller configured to control the first drive apparatus and the second drive apparatus in order to control drive modes of the front wheel and the rear wheel, and an internal combustion engine controller configured to control an operating state of the internal combustion engine, wherein the drive mode controller switches between a first independent drive mode, in which the vehicle is driven only by a drive force from the first drive apparatus, and a composite drive mode, in which the vehicle is driven by a drive force from the first drive apparatus and the second drive apparatus, wherein the drive mode controller switches from the first independent drive mode to the composite drive mode, based on a lateral acceleration-related value in relation to a lateral acceleration acting on the vehicle, and wherein different values are set as a stopped-state threshold value, which indicates the lateral acceleration-related value for switching from the first independent drive mode to the composite drive mode while the internal combustion engine is stopped, and an operating-state threshold value, which indicates the lateral acceleration-related value for switching from the first independent drive mode to the composite drive mode while the internal combustion engine is in operation. 
     According to the present invention, different values are set as the stopped-state threshold value, which is used for switching from the first independent drive mode (a mode in which one of the front and rear wheels is driven) to the composite drive mode while the internal combustion engine is stopped, and the operating-state threshold value, which is used for switching from the first independent drive mode to the composite drive mode while the internal combustion engine is in operation. Stated otherwise, it is possible to switch the threshold value for the lateral acceleration-related value to different values at respective times when the internal combustion engine is in operation, and when the internal combustion engine is stopped. Consequently, it is possible to switch between different drive modes in view of achieving a balance between energy consumption while the internal combustion engine is in operation, and vehicle driving stability, i.e., the capability of the vehicle to be driven as the driver wishes, for example. 
     The operating-state threshold value may be less than the stopped-state threshold value. In this manner, it is possible to switch from the first independent drive mode to the composite drive mode more quickly when the internal combustion engine is in operation than when the internal combustion engine is stopped. Therefore, if the internal combustion engine is in operation prior to switching from the first independent drive mode to the composite drive mode, an increase in driving stability can be achieved more quickly. 
     In the first independent drive mode, the internal combustion engine may apply a drive force selectively to an electric generator disposed in the vehicle. In the first independent drive mode, therefore, the electric generator can be operated by the drive force from the internal combustion engine in order to supply electric power to components in the vehicle. 
     According to the present invention, there further is provided a method of controlling a vehicle including a first drive apparatus configured to drive one of a front wheel and a rear wheel, a second drive apparatus configured to drive another one of the front wheel and the rear wheel, the second drive apparatus including an internal combustion engine, a drive mode controller configured to control the first drive apparatus and the second drive apparatus in order to control drive modes of the front wheels and the rear wheels, and an internal combustion engine controller configured to control an operating state of the internal combustion engine, the method performed by the drive mode controller comprising switching between a first independent drive mode, in which the vehicle is driven only by a drive force from the first drive apparatus, a second independent drive mode in which the vehicle is driven only by a drive force from the second drive apparatus, and a composite drive mode, in which the vehicle is driven by a drive force from the first drive apparatus and the second drive apparatus, switching from the first independent drive mode to the composite drive mode, and from the second independent drive mode to the composite drive mode, based on a lateral acceleration-related value in relation to a lateral acceleration acting on the vehicle, and setting different values as a first switching threshold value, which indicates the lateral acceleration-related value for switching from the first independent drive mode to the composite drive mode, and a second switching threshold value, which indicates the lateral acceleration-related value for switching from the second independent drive mode to the composite drive mode. 
     According to the present invention, there also is provided a method of controlling a vehicle including a first drive apparatus configured to drive one of a front wheel and a rear wheel, a second drive apparatus configured to drive another one of the front wheel and the rear wheel, the second drive apparatus including an internal combustion engine, a drive mode controller configured to control the first drive apparatus and the second drive apparatus in order to control drive modes of the front wheels and the rear wheels, and an internal combustion engine controller configured to control an operating state of the internal combustion engine, the method performed by the drive mode controller comprising switching between a first independent drive mode, in which the vehicle is driven only by a drive force from the first drive apparatus, and a composite drive mode, in which the vehicle is driven by a drive force from the first drive apparatus and the second drive apparatus, switching from the first independent drive mode to the composite drive mode, based on a lateral acceleration-related value in relation to a lateral acceleration acting on the vehicle, and setting different values as a stopped-state threshold value, which indicates the lateral acceleration-related value for switching from the first independent drive mode to the composite drive mode while the internal combustion engine is stopped, and an operating-state threshold value, which indicates the lateral acceleration-related value for switching from the first independent drive mode to the composite drive mode while the internal combustion engine is in operation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic view showing a drive system and related components of a vehicle according to an embodiment of the present invention; 
         FIG. 2  is a diagram showing by way of example a manner in which running modes (drive modes) and drive sources are switched according to the embodiment; 
         FIG. 3  is a first flowchart of a processing sequence for switching the running modes (drive modes) and the drive sources according to the embodiment; 
         FIG. 4  is a second flowchart of the processing sequence for switching the running modes (drive modes) and the drive sources according to the embodiment; 
         FIG. 5  is a diagram showing a relationship between lateral accelerations (hereinafter referred to as “lateral Gs”) and turning radius ratios that take place upon gradual acceleration in different drive modes of the vehicle; 
         FIG. 6  is a diagram showing a relationship between lateral Gs and turning radius ratios, which take place when the vehicle is accelerated with a widely open throttle (WOT) valve in the different drive modes of the vehicle; 
         FIG. 7  is a flowchart of a sequence (details concerning step S 3  of  FIG. 3 ) for setting a first flag and a second flag; 
         FIG. 8  is a view illustrating definitions of various values used in detecting a lateral G; 
         FIG. 9  is a flowchart of a sequence (details concerning step S 32  of  FIG. 7 ) for selecting a drive mode switching inhibition threshold value; 
         FIG. 10  is a diagram showing a first example of a relationship between accelerator openings and drive mode switching inhibition threshold values; 
         FIG. 11  is a diagram showing a second example of a relationship between accelerator openings and drive mode switching inhibition threshold values; 
         FIG. 12  is a schematic view showing a drive system and related components of a vehicle according to a modification of the present invention; 
         FIG. 13  is a diagram showing a first example of a relationship between fore-and-aft accelerations (hereinafter referred to as “fore-and-aft Gs”) and drive mode switching inhibition threshold values; 
         FIG. 14  is a diagram showing a second example of a relationship between fore-and-aft Gs and drive mode switching inhibition threshold values; and 
         FIG. 15  is a diagram showing an example of a relationship between vehicle speeds and drive mode switching inhibition threshold values. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     I. Embodiment 
     A. Arrangement 
     A-1. Overall Arrangement 
       FIG. 1  schematically shows a drive system and related components of a vehicle  10  according to an embodiment of the present invention. As shown in  FIG. 1 , the vehicle  10  has an engine  12  and a first traction motor  14  (hereinafter referred to as a “first motor  14 ” or a “front motor  14 ”), which are disposed in a series-connected layout in a front portion of the vehicle  10 , second and third traction motors  16 ,  18  (hereinafter referred to as “second and third motors  16 ,  18 ” or “rear motors  16 ,  18 ”), which are disposed in a rear portion of the vehicle  10 , a high-voltage battery  20  (hereinafter referred to as a “battery  20 ”), first through third inverters  22 ,  24 ,  26 , and a drive electronic control unit  28  (hereinafter referred to as a “drive ECU  28 ” or simply an “ECU  28 ”). 
     The engine  12  and the first motor  14  transmit a drive force (hereinafter referred to as a “front wheel drive force Ff”) through a transmission  30  to a left front wheel  32   a  and a right front wheel  32   b  (hereinafter referred to collectively as “front wheels  32 ”). The engine  12  and the first motor  14  make up a front wheel drive apparatus  34  (steerable wheel drive device). For example, when the vehicle  10  is under a low load, the front wheels  32  are driven solely by the first motor  14 . When the vehicle  10  is under a medium load, the front wheels  32  are driven solely by the engine  12 . When the vehicle  10  is under a high load, the front wheels  32  are driven by both the engine  12  and the first motor  14 . While the engine  12  and the transmission  30  are disconnected from each other or are connected to each other through a non-illustrated clutch, the engine  12  is capable of driving the first motor  14  so as to enable the first motor  14  to generate electric power, and the generated electric power can be used to charge the battery  20  or is supplied to accessories, not shown. Stated otherwise, the first motor  14  can be used as an electric generator. 
     The second motor  16  includes an output shaft, which is coupled to the rotational shaft of a left rear wheel  36   a,  and transmits a drive force to the left rear wheel  36   a . The third motor  18  includes an output shaft, which is coupled to the rotational shaft of a right rear wheel  36   b , and transmits a drive force to the right rear wheel  36   b . The second and third motors  16 ,  18  make up a rear wheel drive apparatus  38  (i.e., a non-steerable wheel drive device). The left rear wheel  36   a  and the right rear wheel  36   b  will hereinafter be referred to collectively as “rear wheels  36 ”. The drive force, which is transmitted from the rear wheel drive apparatus  38  to the rear wheels  36 , will hereinafter be referred to as a “rear wheel drive force Fr”. 
     The high-voltage battery  20  supplies electric power through the first through third inverters  22 ,  24 ,  26  to the first through third motors  14 ,  16 ,  18 . The high-voltage battery  20  is charged with regenerated electric power Preg from the first through third motors  14 ,  16 ,  18 . 
     The drive ECU  28  controls the engine  12  and the first through third inverters  22 ,  24 ,  26  based on output signals from various sensors and various electronic control units (hereinafter referred to as “ECUs”), for thereby controlling the output power of the engine  12  and the first through third motors  14 ,  16 ,  18 . The drive ECU  28  includes an input/output unit  40 , a processor  42 , and a memory  44 . The drive ECU  28  may comprise a combination of ECUs. For example, plural ECUs, which are associated respectively with the engine  12  and the first through third motors  14 ,  16 ,  18 , and an ECU for managing drive states of the engine  12  and the first through third motors  14 ,  16 ,  18 , may be combined for use as the drive ECU  28 . 
     The various sensors, which supply output signals to the drive ECU  28 , include a vehicle speed sensor  50 , a shift position sensor  52 , an accelerator pedal opening sensor  54 , a yaw rate sensor  56 , and a tire steering angle sensor  58 . 
     A-2. Arrangements and Functions of Various Components 
     The engine  12  comprises a six-cylinder engine, although the engine  12  may be a different type of engine, including a two-cylinder engine, a four-cylinder engine, an eight-cylinder engine, etc. The engine  12  is not limited to a gasoline engine, but may be another type of engine, including a diesel engine, an air engine, etc. 
     Each of the first through third motors  14 ,  16 ,  18  comprises a three-phase AC brushless motor, but may be another type of motor, including a three-phase AC brush motor, a single-phase AC motor, a DC motor, etc. The first through third motors  14 ,  16 ,  18  may have identical or different specifications. Further, the left rear wheel  36   a  and the right rear wheel  36   b  may be driven by a single traction motor. 
     The first through third inverters  22 ,  24 ,  26  each comprises a three-phase bridge design, which serves to convert DC power into three-phase AC power, and to supply the three-phase AC power to the first through third motors  14 ,  16 ,  18 . The first through third inverters  22 ,  24 ,  26  also convert AC power, which is regenerated by the first through third motors  14 ,  16 ,  18 , into DC power, and the DC power is supplied to the high-voltage battery  20 . 
     The high-voltage battery  20  is an energy storage device including a plurality of battery cells, which may be constituted from a lithium ion secondary battery, a nickel hydrogen secondary battery, or a capacitor. According to the present embodiment, the high-voltage battery  20  comprises a lithium ion secondary battery. DC/DC converters, not shown, may be connected between the first through third inverters  22 ,  24 ,  26  and the high-voltage battery  20 , for elevating or stepping down an output voltage from the high-voltage battery  20 , or output voltages from the first through third motors  14 ,  16 ,  18 . 
     The drive system of the vehicle  10  may be arranged as disclosed in US2012/0015772A1, for example. 
     The vehicle speed sensor  50  detects the vehicle speed V [km/h]. The shift position sensor  52  detects respective shift positions, one at a time, at which a shift lever, not shown, is moved, including “P” as a parking range, “N” as a neutral range, “D” as a forward range, and “R” as a reverse range (hereinafter referred to as “shift positions Ps”). The accelerator pedal opening sensor  54  detects an opening of a non-illustrated accelerator pedal (hereinafter referred to as an “accelerator opening θap”). The yaw rate sensor  56  detects a yaw rate Yr of the vehicle  10 . The tire steering angle sensor  58  detects an actual steering angle (hereinafter referred to as a “tire steering angle σ”) of the front wheels  32  as steerable wheels. 
     B. Various Control Processes 
     B-1. Switching Between Drive Modes 
     (1-1. Overview) 
       FIG. 2  shows by way of example the manner in which running modes (drive modes) and drive sources are switched according to the present embodiment. According to the present embodiment, the running modes (drive modes) and the drive sources are switched under the control of the drive ECU  28 . 
     In  FIG. 2 , “RUNNING MODE” indicates whether the vehicle  10  is stopped, is driven forwardly, regenerates electric power, or is driven in reverse, and “DRIVE MODE” indicates whether the vehicle  10  is driven in an “RWD” (Rear Wheel Drive) mode, an “FWD” (Front Wheel Drive) mode, or an “AWD” (All-Wheel Drive) mode. Each of the RWD and FWD modes is a two wheel drive (2WD) mode, whereas the AWD mode is a four wheel drive (4WD) mode. In  FIG. 2 , “REGENERATIVE” indicates that at least one of the first through third motors  14 ,  16 ,  18  is regenerating electric power. 
     In  FIG. 2 , “SHIFT POSITION” indicates a position to which the non-illustrated shift lever is moved. “P” refers to a parking range, “N” refers to a neutral range, “D” refers to the forward range, and “R” refers to a reverse range. 
     In  FIG. 2 , “DRIVE SOURCE” indicates an apparatus for driving the vehicle  10 . “ENG” refers to the engine  12 , “MOT” refers to the rear motors  16 ,  18  when the drive mode is “RWD”, “ENG+MOT” refers to the engine  12  and the front and rear motors  14 ,  16 ,  18  when the drive mode is “AWD”, and “REGENERATE” refers to at least one of the front and rear motors  14 ,  16 ,  18 . 
     According to the present embodiment, as shown in  FIG. 2 , the vehicle speed V is classified into a “LOW SPEED RANGE”, a “MEDIUM SPEED RANGE”, a “HIGH SPEED RANGE”, and a “REVERSE RANGE”, and the drive sources are switched depending on these speed ranges. 
     More specifically, the RWD mode is used when the vehicle  10  is driven forwardly at the vehicle speed V in the low speed range, and when the vehicle  10  is driven in reverse. 
     The FWD mode and the AWD mode are used when the vehicle  10  is driven forwardly at the vehicle speed V in the medium speed range. The FWD mode and the AWD mode are switched in the following manner. Namely, a threshold value is established with respect to the accelerator opening θap (hereinafter referred to as an “accelerator opening threshold value THθ” or a “threshold value THθ”). If the accelerator opening θap is less than the accelerator opening threshold value THθ, the FWD mode is selected. If the accelerator opening θap is greater than the accelerator opening threshold value THθ, the AWD mode is selected. If the vehicle  10  is driven forwardly at the vehicle speed V in the high speed range, the FWD mode is used. 
     The running modes (drive modes) may be switched according to the process shown in  FIG. 13  and the related description thereof in US2012/0015772A1. 
     (1-2. Specific Switching of Drive Modes) 
     (1-2-1. Overall Flow) 
       FIGS. 3 and 4  are first and second flowcharts, respectively, of a processing sequence for switching the running modes (drive modes) and the drive sources according to the present embodiment. In step S 1  of  FIG. 3 , the ECU  28  determines whether or not it is necessary for the vehicle  10  to move in a forward direction. The ECU  28  makes such a judgment by confirming whether or not the shift position Ps, which is sent from the shift position sensor  52 , is a position indicative of forward movement (the forward range D), for example. If it is necessary for the vehicle  10  to move forwardly (step S 1 : YES), control proceeds to step S 2 . 
     In step S 2 , the ECU  28  determines whether or not the rear motors  16 ,  18  are capable of being energized. The ECU  28  makes such a judgment based on the temperatures of the rear motors  16 ,  18 , the occurrence of a failure of the rear motors  16 ,  18 , and an SOC (State Of Charge) of the battery  20 , for example. 
     More specifically, the respective temperatures of the rear motors  16 ,  18  (hereinafter referred to as “rear motor temperatures”) are detected by non-illustrated temperature sensors, and if it is determined that the rear motor temperatures exceed a threshold value indicative of overheating of the rear motors  16 ,  18 , the ECU  28  judges that the rear motors  16 ,  18  cannot be energized. Furthermore, if output signals from various sensors (e.g., voltage sensors, current sensors, and rotational angle sensors) in relation to the rear motors  16 ,  18  exceed a threshold value for judging the occurrence of a failure of the rear motors  16 ,  18 , the ECU  28  judges that the rear motors  16 ,  18  cannot be energized. In addition, if the SOC of the battery  20  is lower than a threshold value for determining if the SOC of the battery  20  is sufficiently large to enable energization of the rear motors  16 ,  18 , the ECU  28  judges that the rear motors  16 ,  18  cannot be energized. As described later, the ECU  28  may also determine whether or not the rear motors  16 ,  18  are capable of being energized based on other judgment criteria apart from the above threshold values. 
     If the rear motors  16 ,  18  can be energized (step S 2 : YES), then in step S 3 , the ECU  28  sets a first flag FLG 1  and a second flag FLG 2  primarily based on a lateral G (lateral acceleration-related value). The first flag FLG 1  is a flag concerning whether or not the drive modes should be inhibited from switching, i.e., a drive mode switching inhibition threshold value, and will be used in step S 4 , to be described below. The second flag FLG 2  is a flag concerning whether or not the engine  12  should be started, regardless of the fact that the drive mode is in any one of the FWD, RWD, and AWD modes, i.e., an engine start judgment flag, and will be used in step S 6 , to be described below. Details of the process for setting the first flag FLG 1  and the second flag FLG 2  will be described later with reference to  FIG. 7 . 
     In step S 4 , the ECU  28  judges whether or not the drive modes should be inhibited from switching based on the first flag FLG 1 . More specifically, if the first flag FLG 1  is 0, the ECU  28  does not inhibit the drive modes from being switched, i.e., allows the drive modes to be switched, and if the first flag FLG 1  is 1, the ECU  28  inhibits the drive modes from being switched. 
     If the first flag FLG 1  is 1 and the drive modes should be inhibited from switching (step S 4 : YES), then in step S 5 , the ECU  28  locks the drive mode in the AWD mode. If the drive modes should not be inhibited from switching (step S 4 : NO), control proceeds to step S 6 . 
     In step S 6 , the ECU  28  judges whether or not the engine  12  should be started based on the second flag FLG 2 . More specifically, if the second flag FLG 2  is 0, the ECU  28  judges at step S 6  that the engine  12  should not be started, and if the second flag FLG 2  is 1, the ECU  28  judges that the engine  12  should be started, regardless of the fact that the drive mode is in any one of the FWD, RWD, and AWD modes. 
     If based on the second flag FLG 2  it is determined that the engine  12  should not be started (S 6 : NO), control proceeds to step S 8 . If based on the second flag FLG 2  it is determined that the engine  12  should be started (step S 6 : YES), then in step S 7 , the ECU  28  starts the engine  12 , and thereafter, control proceeds to step S 8 . 
     As described above, the engine  12  is started in step S 7 , regardless of the fact that the drive mode is in any one of the FWD, RWD, and AWD modes. Stated otherwise, if at the present time, the drive mode is in the FWD or the AWD mode, since the engine  12  has already been operating, the engine  12  is kept in operation. If at the present time, the drive mode is in the RWD mode, then since the rear motors  16 ,  18  are being used as drive sources, the engine  12  is started, but remains in an idling state. The engine  12  is kept idling in order to enable a smooth change to the AWD mode, because if the second flag FLG 2  is 1, the drive modes will subsequently be inhibited from switching and thus it is highly likely that switching to the AWD mode will occur (step S 5 ). 
     In step S 8 , the ECU  28  judges whether or not the vehicle  10  is decelerating based on the vehicle speed V from the vehicle speed sensor  50 , for example. If the vehicle  10  is decelerating (step S 8 : YES), then in step S 9 , the ECU  28  selects the running mode for regenerating electric power. The ECU  28  controls at least one of the first through third traction motors  14 ,  16 ,  18  in order to regenerate electric power. If the vehicle  10  is not decelerating (step S 8 : NO), then control proceeds to step S 10  of  FIG. 4 . 
     In step S 10  of  FIG. 4 , the ECU  28  judges whether the vehicle  10  is being driven in the low speed range, e.g., from 0 to 30 km/h, based on the vehicle speed V from the vehicle speed sensor  50 . If the vehicle  10  is being driven in the low speed range (step S 10 : YES), then in step S 11 , the ECU  28  selects the RWD mode as the drive mode. At this time, the vehicle  10  is driven by the rear motors  16 ,  18 . If the vehicle  10  is not driven in the low speed range (step S 10 : NO), control proceeds to step S 12 . 
     In step S 12 , the ECU  28  judges whether the vehicle  10  is being driven in the medium speed range, e.g., from 31 to 80 km/h, based on the vehicle speed V from the vehicle speed sensor  50 . If the vehicle  10  is being driven in the medium speed range (step S 12 : YES), then in step S 13 , the ECU  28  judges whether or not the accelerator opening θap is equal to or less than the accelerator opening threshold value THθ. As described above, the accelerator opening threshold value THθ is a threshold value that is used for selecting the FWD mode or the AWD mode. 
     If the accelerator opening θap is equal to or less than the accelerator opening threshold value THθ (step S 13 : YES), then in step S 14 , the ECU  28  selects the FWD mode as the drive mode. At this time, the vehicle  10  is driven by either one or both of the engine  12  and the first motor  14 . If the accelerator opening θap is not equal to or less than the accelerator opening threshold value THθ (step S 13 : NO), then in step S 15 , the ECU  28  selects the AWD mode as the drive mode. At this time, the vehicle  10  is driven by the engine  12  and the first through third motors  14 ,  16 ,  18 . 
     Returning to S 12 , if the vehicle  10  is not being driven in the medium speed range (step S 12 : NO), then the vehicle  10  is judged as being driven in the high speed range, e.g., at a speed of 81 km/h or greater. In this case, in step S 16 , ECU  28  selects the FWD mode as the drive mode. 
     Returning to step S 2  of  FIG. 3 , if the rear motors  16 ,  18  are incapable of being energized (step S 2 : NO), then in step S 17 , the ECU  28  selects the FWD mode as the drive mode. Consequently, the vehicle  10  is prevented from changing to the RWD mode or the AWD mode in a condition in which the rear motors  16 ,  18  cannot be energized. 
     Returning to step S 1 , if the vehicle  10  is not required to move forwardly (step S 1 : NO), then in step S 18 , the ECU  28  judges whether or not the vehicle  10  needs to be driven in reverse. The ECU  28  makes such a judgment by confirming whether or not the shift position Ps, which is sent from the shift position sensor  52 , is a position indicative of reverse movement (reverse range R), for example. If driving of the vehicle  10  in reverse is required (step S 18 : YES), then in step S 19 , the ECU  28  selects the RWD mode as the drive mode. If driving of the vehicle  10  in reverse is not required (step S 18 : NO), then in step S 20 , the ECU  28  selects a running mode for stopping the vehicle  10 , whereupon the engine  12  and the first through third motors  14 ,  16 ,  18  are stopped. 
     (1-2-2. Setting of First Flag FLG 1  and Second Flag FLG 2 ) 
     (1-2-2-1. Concept) 
       FIG. 5  shows a relationship between lateral Gs and turning radius ratios R/R0 upon gradual acceleration, corresponding to the drive modes of the vehicle  10 . The term “gradual acceleration” implies that the vehicle  10  is being gradually accelerated, i.e., that the derivative of the vehicle speed V with respect to time is small, and corresponds to a relatively small accelerator opening θap. The turning radius ratio R/R0 refers to a value indicative of how much the actual turning radius R [m] deviates from a reference turning radius R0 [m]. The turning radius ratio R/R0 is used as an indicator for indicating the turning characteristics of the vehicle  10 . 
     Details of a process for calculating the actual turning radius R and the reference turning radius R0 are disclosed in Japanese Laid-Open Patent Publication No. 2011-252564 or Japanese Laid-Open Patent Publication No. 2008-230513, for example. 
     If the actual turning radius R becomes less than the reference turning radius R0, and hence the turning radius ratio R/R0 becomes smaller, then oversteering of the vehicle  10  tends to occur. Conversely, if the actual turning radius R becomes greater than the reference turning radius R0, and hence the turning radius ratio R/R0 becomes greater, then understeering of the vehicle  10  tends to occur. 
       FIG. 6  shows, for different drive modes of the vehicle, the relationship between lateral Gs and turning radius ratios R/R0 in the case that the vehicle  10  is accelerated with a wide open throttle (WOT) valve. WOT refers to a so-called full throttle with a maximum accelerator opening θap. 
     As shown in  FIGS. 5 and 6 , if the lateral G is less than a first deviation occurrence value Gdiv1 (first lateral G), the turning radius ratios R/R0 in the respective drive modes (FWD, RWD, and AWD modes) are of substantially equal values. If the lateral G becomes greater than the first deviation occurrence value Gdiv1, the turning radius ratio R/R0 in the FWD mode and the turning radius ratios R/R0 in the RWD and AWD modes start to deviate from each other. If the lateral G becomes greater than a second deviation occurrence value Gdiv2 (second lateral G), the turning radius ratio R/R0 in the RWD mode and the turning radius ratio R/R0 in the AWD mode start to deviate from each other. 
     As described above, when the turning radius ratios R/R0 in the drive modes deviate from each other, i.e., if the deviation exceeds a predetermined value at the same lateral G, switching between the drive modes tends to cause an abrupt change in the turning characteristics of the vehicle  10 , thereby leading to the possibility that the driver of the vehicle  10  may feel uncomfortable. According to the present embodiment, if the lateral G exceeds a predetermined threshold value (hereinafter referred to as a “drive mode switching inhibition threshold value G1” or a “first lateral G threshold value G1”), the drive modes are inhibited from being switched. 
     According to the present embodiment, a first lateral G threshold value G1a (a first switching threshold value and a stopped-state threshold value) in the case that the engine  12  is presently stopped (at the processing time), and a first lateral G threshold value G1b (a second switching threshold value and an operating-state threshold value) in the case that the engine  12  is presently in operation, are used selectively as the first lateral G threshold value G1. The first lateral G threshold value G1 will hereinafter be used as a collective term, which is representative of the first lateral G threshold values G1a, G1b, or as one of the first lateral G threshold values G1a, G1b that actually is used for comparison with the lateral G. 
     As shown in  FIG. 5 , the first lateral G threshold value G1a is set to a value equal to the lateral G, i.e., the first deviation occurrence value Gdiv1, at which the turning radius ratios R/R0 in the FWD, RWD, and AWD modes start to deviate from each other. Alternatively, the first lateral G threshold value G1a may be set to a value that is less than the first deviation occurrence value Gdiv1, from the standpoint of reliably inhibiting the drive modes from being switched at the time that the turning radius ratios R/R0 actually start to deviate from each other. Further, alternatively, the first lateral G threshold value G1a may be set to a value that is slightly greater than the first deviation occurrence value Gdiv1, from the standpoint of keeping the deviation less than a predetermined value. 
     Further, as shown in  FIG. 5 , the lateral Gs at which the turning radius ratios R/R0 in the drive modes deviate from each other include the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2. According to the present embodiment, the smaller of such values, i.e., the first deviation occurrence value Gdiv1, is set as the first lateral G threshold value G1a. The smaller of the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2 will hereinafter be referred to as a “deviation reference value Gref”. 
     The first lateral G threshold value G1b, which is used during operation of the engine  12 , is set to a value that is less than the first lateral G threshold value G1a, which is used at the time that the engine  12  is stopped. This is based on the viewpoint that, in the case that the engine  12  is stopped, in terms of engine efficiency, it is preferable to delay starting of the engine  12 . Further, when the engine  12  is in operation, it is preferable for the rear motors  16 ,  18  to be operated promptly, so as to increase maneuvering stability at an early stage. 
     By comparing  FIGS. 5 and 6 , it will be understood that the turning radius ratios R/R0 in the drive modes (and the first deviation occurrence value Gdiv1, the second deviation occurrence value Gdiv2, and the deviation reference value Gref) change depending on how the vehicle  10  is accelerated (for example, whether the vehicle  10  is gradually accelerated or is accelerated with WOT). According to the present embodiment, the first lateral G threshold value G1 (first lateral G threshold values G1a, G1b) varies depending on the accelerator opening θap. As will be described later, the first lateral G threshold value G1 may alternatively be varied depending on another indicator in addition to or instead of the accelerator opening θap. 
     According to the present embodiment, the drive mode is locked in the AWD mode when switching of the drive modes is inhibited accompanying an increase in the lateral G (step S 5  of  FIG. 3 ). Thus, the vehicle  10  is maintained in a stable posture even if the lateral G is large. 
     In the case that the drive mode is locked in the AWD mode, the engine  12  is operated. If up to this time, the drive mode has been in the RWD mode and the engine  12  is operated for the first time after the lateral G has reached the first lateral G threshold value G1, the running state of the vehicle  10  may possibly become unstable until the output power of the engine  12  reaches a required level. According to the present embodiment, the first lateral G threshold value G1a, which is used when the engine  12  is stopped, is used in combination with a certain lateral G threshold value for starting the engine  12  (hereinafter referred to as an “engine starting threshold value G2” or a “second lateral G threshold value G2”). The second lateral G threshold value G2 is set to a value that is less than the first lateral G threshold value G1a. Therefore, the engine  12  can be changed smoothly to the AWD mode from a drive mode in which the engine  12  is not used to drive the vehicle  10  (i.e., the RWD mode). 
     (1-2-2-2. Specific Processing Details) 
       FIG. 7  is a flowchart of a sequence (details of step S 3  of  FIG. 3 ) for setting the first flag FLG 1  and the second flag FLG 2 . In step S 31 , the ECU  28  acquires the accelerator opening θap from the accelerator pedal opening sensor  54 . 
     In step S 32 , based on the accelerator opening θap (see  FIGS. 5 and 6 ), the ECU  28  selects a drive mode switching inhibition threshold value G1 (first lateral G threshold value G1). As described above, the threshold value G1 is selected from among the threshold values G1a, G1b. Details of a process for selecting the first lateral G threshold value G1 will be described later with reference to  FIG. 9 . 
     In step S 33 , based on the accelerator opening θap (see  FIGS. 5 and 6 ), the ECU  28  selects an engine starting threshold value G2 (second lateral G threshold value G2). Details of a process for selecting the second lateral G threshold value G2 will be described later. 
     In step S 34 , the ECU  28  detects a lateral G. The ECU  28  detects a lateral G in the following manner. Namely, the ECU  28  detects (or calculates) a lateral G according to the following equation (1).
 
Lateral  G =( V   2 ×σ)/(1+ A+V   2 )/ L   (1)
 
     In equation (1), V denotes the vehicle speed detected by the vehicle speed sensor  50 , σ denotes the tire steering angle detected by the tire steering angle sensor  58 , A denotes a stability factor, and L denotes the wheelbase of the vehicle  10  (see  FIG. 8 ). 
     According to equation (1), the lateral G increases as the tire steering angle σ increases. Consequently, it is possible to reflect the intention of the driver to turn the vehicle  10 , even on a low-μ road in which the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2 are lower than on a high-μ road. In addition, according to equation (1), it is possible to detect a lateral G on an inclined road or the like. 
     Alternatively, the ECU  28  may detect (or calculate) a lateral G according to the following equation (2).
 
Lateral  G=Yr×V   (2)
 
     In equation (2), Yr denotes the yaw rate detected by the yaw rate sensor  56 , and V denotes the vehicle speed detected by the vehicle speed sensor  50 . According to equation (2), it is possible to detect a lateral G even if the vehicle  10  is spinning. In addition, according to equation (2), it is possible to detect a lateral G on an inclined road or the like. 
     A lateral G may be detected using a lateral G sensor, such as an electrostatic capacitance lateral G sensor, a piezoresistive lateral G sensor, or the like, which independently detects a lateral G. 
     In step S 35  of  FIG. 7 , the ECU  28  judges whether or not the lateral G detected in step S 34  is less than the drive mode switching inhibition threshold value G1 selected in step S 32 . If the lateral G is less than the threshold value G1 (step S 35 : YES), then in step S 36 , the ECU  28  sets the first flag FLG 1  to 0 in order to permit switching of the drive modes. If the lateral G is not less than the threshold value G1 (step S 35 : NO), then in step S 37 , the ECU  28  sets the first flag FLG 1  to 1 in order to inhibit switching of the drive modes. 
     In step S 38 , the ECU  28  judges whether or not the lateral G detected in step S 34  is less than the engine starting threshold value G2 selected in step S 33 . If the lateral G is less than the threshold value G2 (step S 38 : YES), then in step S 39 , the ECU  28  sets the second flag FLG 2  to 0 in order to keep the engine  12  stopped if the present drive mode is the RWD mode. If the lateral G is not less than the threshold value G2 (step S 38 : NO), then in step S 40 , the ECU  28  sets the second flag FLG 2  to 1 in order to start operation of the engine  12  even if the present drive mode is the RWD mode. 
     (1-2-2-3. Setting of Drive Mode Switching Inhibition Threshold Value G1) 
       FIG. 9  is a flowchart of a sequence (details of step S 32  of  FIG. 7 ) for selecting the drive mode switching inhibition threshold value G1. In step S 51 , the ECU  28  judges whether or not the present drive mode (at the processing time) is the FWD mode or the AWD mode. Alternatively, since the drive mode, which is commanded by the ECU  28 , may not necessarily agree with the actual drive mode of the wheels, i.e., the front wheels  32   a ,  32   b  and the rear wheels  36   a ,  36   b , the ECU  28  may determine the drive mode using measured values, e.g., output signals from non-illustrated wheel speed sensors, which are combined with the respective wheels. 
     If the present drive mode is the FWD mode or the AWD mode (step S 51 : YES), then in step S 52 , the ECU  28  sets a first lateral G threshold value G1b, which is used during operation of the engine  12 , depending on the accelerator opening θap (see  FIGS. 5 and 6 ). The relationship between accelerator openings θap and threshold values G1b is stored in advance as a map in the memory  44 , as shown in  FIG. 10 or 11 . The map may contain experimental values or simulated values. 
     As shown in  FIG. 10 , the threshold value G1b decreases as the accelerator opening θap increases. As shown in  FIG. 11 , the threshold value G1b remains constant while the accelerator opening θap changes from 0 to θ1, based on the concept that it is essentially meaningless to change the threshold value G1b as long as the acceleration is low (from 0 to θ1). As shown in  FIG. 11 , the threshold value G1b decreases while the accelerator opening θap changes from θ1 to θ2. Such a feature coincides with the fact that, as described above with reference to  FIGS. 5 and 6 , when the accelerator opening θap increases to thereby increase the fore-and-aft acceleration (fore-and-aft G), the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2 are reduced. 
     In step S 51  of  FIG. 9 , if the present drive mode is not the FWD mode or the AWD mode (step S 51 : NO), then in step S 53 , the ECU  28  judges whether or not the engine  12  is in operation. Even if the drive mode is the RWD mode (the vehicle  10  is driven by the rear motors  16 ,  18 ), the engine  12  may be operated to drive the first motor  14  in order to generate electric power if, for example, the SOC of the battery  20  is lower than a predetermined threshold value (SOC threshold value). Alternatively, the engine  12  may be operated to drive the first motor  14  in order to generate electric power for supplementing the electric power required to energize accessories, not shown. 
     Since step S 51  is used essentially to judge whether or not the engine  12  is in operation, step S 51  may be omitted and only step S 52  may be used. 
     If the engine  12  is in operation (step S 53 : YES), then in step S 52 , as described above, the ECU  28  sets the first lateral G threshold value G1b, which is used when the engine  12  is in operation, depending on the accelerator opening θap. 
     If the engine  12  is not in operation (step S 53 : NO), then in step S 54 , the ECU  28  sets the first lateral G threshold value G1a, which is used when the engine  12  is stopped, depending on the accelerator opening θap (see  FIGS. 5 and 6 ). In the same manner as with the first lateral G threshold values G1b, the relationship between accelerator openings θap and threshold values G1a is stored in advance as the map in the memory  44 , as shown in  FIG. 10 or 11 . The map may contain experimental values or simulated values. 
     As shown in  FIGS. 10 and 11 , at the same accelerator opening θap, the first lateral G threshold value G1a, which is used when the engine  12  is stopped, is greater than the first lateral G threshold value G1b, which is used when the engine  12  is in operation. However, the first lateral G threshold value G1a is not required to be greater than the first lateral G threshold value G1b at all times. If the accelerator opening θap is small (e.g., in a range from 0 to θ1) or if the accelerator opening θap is large (e.g., in a range of θ2 or greater), the first lateral G threshold value G1a and the first lateral G threshold value G1b may be equal to each other. 
     (1-2-2-4. Setting of Engine Starting Threshold Value G2) 
     In step S 33  of  FIG. 7 , the ECU  28  selects an engine starting threshold value G2 in the same manner as with the threshold value G1. The relationship between accelerator openings θap and threshold values G2 is stored in advance as a map in the memory  44 . The map may contain experimental values or simulated values. Alternatively, differences with respect to the threshold values G1 may be preset, and the threshold value G2 may be set based on the threshold value G1. 
     (1-2-3. Processing Details for Switching Between Running Modes (Drive Modes)) 
     Processing details for switching between running modes (drive modes) will be described below. 
     (1-2-3-1. Switching from RWD Mode to FWD Mode) 
     If the drive ECU  28  judges that the running mode (drive mode) should be switched from the RWD mode to the FWD mode, the drive ECU  28  temporarily uses the AWD mode during the process of changing from the RWD mode to the FWD mode. 
     More specifically, while the drive ECU  28  gradually reduces the drive force (rear wheel drive force Fr) transmitted to the rear wheels  36 , which serve as non-steerable wheels, the drive ECU  28  gradually increases the drive force (front wheel drive force Ff) that is transmitted to the front wheels  32 , which serve as steerable wheels. Consequently, the drive ECU  28  uses both the RWD mode and the FWD mode in a combined manner, i.e., in the condition of the AWD mode, temporarily (e.g., for a period of time ranging from 0.1 to 2.0 seconds). 
     However, the AWD mode that is used at the present time (hereinafter referred to as a “transitory AWD mode”) differs from the AWD mode (shown in  FIG. 2 ) that is judged by the drive ECU  28  as having been selected as the running mode (drive mode). Rather, the transitory AWD mode is used to transition from the RWD mode to the FWD mode. Stated otherwise, the AWD mode shown in  FIG. 2  is set according to the processing sequence based on the flowcharts shown in  FIGS. 3 and 4 , whereas the transitory AWD mode is used when it is determined that the RWD mode should switch to the FWD mode according to the processing sequence based on the flowcharts shown in  FIGS. 3 and 4 . The drive modes may be switched based on at least one of the vehicle speed V, a change in the vehicle speed V, i.e., the derivative of the vehicle speed V with respect to time, the accelerator opening θap, a change in the accelerator opening θap, i.e., the derivative of the accelerator opening θap with respect to time, and the yaw rate Yr. 
     In the transitory AWD mode, the total of the front wheel drive force Ff and the rear wheel drive force Fr (hereinafter referred to as a “total drive force Ftotal”) is kept constant. The total drive force Ftotal, which is kept constant, allows the RWD mode to be switched to the FWD mode without causing a change in the behavior of the vehicle  10 , thereby preventing the driver from feeling uncomfortable due to a change in the behavior of the vehicle  10  upon switching from the RWD mode to the FWD mode. 
     Alternatively, in the transitory AWD mode, the total drive force Ftotal may be changed depending on at least one of the accelerator opening θap, a change in the accelerator opening θap, and a change in the vehicle speed V. For example, the total drive force Ftotal may be increased if the accelerator opening θap is large, if a change in the accelerator opening θap is of a positive value, or if a change in the vehicle speed V is of a positive value. Further, the total drive force Ftotal may be reduced if the accelerator opening θap is small, if a change in the accelerator opening θap is of a negative value, or if a change in the vehicle speed V is of a negative value. 
     (1-2-3-2. Switching from FWD Mode to RWD Mode) 
     The FWD mode may be switched to the RWD mode in the same manner as when the RWD mode is switched to the FWD mode. In other words, the transitory AWD is used during a transition from the FWD mode to the RWD mode. Further, the total drive force Ftotal may be controlled during application of the transitory AWD mode. 
     (1-2-3-3. Switching from FWD Mode or RWD Mode to AWD Mode) 
     When switching from the FWD mode to the AWD mode, for example, the front wheel drive force Ff is kept constant, whereas the rear wheel drive force Fr is increased in order to increase the total drive force Ftotal. Alternatively, the front wheel drive force Ff is reduced and the rear wheel drive force Fr is increased in order to increase the total drive force Ftotal, or to keep the total drive force Ftotal constant. Further, alternatively, the front wheel drive force Ff is increased and the rear wheel drive force Fr is increased in order to increase the total drive force Ftotal. 
     Similarly, when switching from the RWD mode to the AWD mode, for example, the rear wheel drive force Fr is kept constant, whereas the front wheel drive force Ff is increased in order to increase the total drive force Ftotal. Alternatively, the rear wheel drive force Fr is reduced and the front wheel drive force Ff is increased in order to increase the total drive force Ftotal, or to keep the total drive force Ftotal constant. Further, alternatively, the rear wheel drive force Fr is increased and the front wheel drive force Ff is increased in order to increase the total drive force Ftotal. 
     (1-2-3-4. Switching from AWD Mode to FWD Mode or RWD Mode) 
     When switching from the AWD mode to the FWD mode, for example, the front wheel drive force Ff is kept constant, whereas the rear wheel drive force Fr is reduced in order to reduce the total drive force Ftotal. Alternatively, the front wheel drive force Ff is increased and the rear wheel drive force Fr is reduced in order to reduce the total drive force Ftotal, or to keep the total drive force Ftotal constant. Further, alternatively, the front wheel drive force Ff is reduced and the rear wheel drive force Fr is reduced in order to reduce the total drive force Ftotal. 
     Similarly, when switching from the AWD mode to the RWD mode, for example, the rear wheel drive force Fr is kept constant, whereas the front wheel drive force Ff is reduced in order to reduce the total drive force Ftotal. Alternatively, the rear wheel drive force Fr is increased and the front wheel drive force Ff is reduced in order to reduce the total drive force Ftotal, or to keep the total drive force Ftotal constant. Further, alternatively, the rear wheel drive force Fr is reduced and the front wheel drive force Ff is reduced in order to reduce the total drive force Ftotal. 
     C. Advantages of the Present Embodiment 
     With respect to switching from the FWD mode to the AWD mode, and switching from the RWD mode to the AWD mode depending on the lateral G (step S 4  of  FIG. 3 : YES→S 5 ), according to the present embodiment, different threshold values are set (see for example  FIG. 5 ), i.e., the first lateral G threshold value G1a (first switching threshold value) for switching from the FWD mode to the AWD mode (step S 51  of  FIG. 9 : NO→S 54 ), and the first lateral G threshold value G1b (second switching threshold value) for switching from the RWD mode to the AWD mode (step S 51 : YES→S 52 ). Stated otherwise, it is possible to switch the first lateral G threshold value G1 to different values in the case that the engine  12  is in operation and in the case that the engine  12  is stopped. Therefore, it is possible to switch between different drive modes in view of achieving a balance between energy consumption while the engine  12  is in operation and the driving stability of the vehicle  10 , i.e., the capability of the vehicle  10  to be driven as the driver wishes. 
     According to the present embodiment, in addition, the first lateral G threshold value G1b (second switching threshold value), which is used in the FWD mode, is less than the first lateral G threshold value G1a (first switching threshold value), which is used in the RWD mode (see  FIG. 5 , etc.). Consequently, switching of the FWD mode to the AWD mode when the engine  12  is in operation occurs more quickly than switching of the RWD mode to the AWD mode when the engine  12  is not in operation. Therefore, if the engine  12  is in operation before switching to the AWD mode, it is possible to increase driving stability at an early stage. 
     With respect to the operating state of the engine  12  when the vehicle  10  is in the RWD mode, according to the present embodiment, different threshold values are set, i.e., the first lateral G threshold value G1a (stopped-state threshold value) for switching from the RWD mode to the AWD mode when the engine  12  is stopped (step S 53  of  FIG. 9 : NO→S 54 ), and the first lateral G threshold value G1b (operating-state threshold value) for switching from the RWD mode to the AWD mode when the engine  12  is in operation (step S 53  of  FIG. 9 : YES→S 52 ). Stated otherwise, the first lateral G threshold value G1 is switched to different values when the engine  12  is in operation and when the engine  12  is stopped. Therefore, it is possible to switch between different drive modes in view of achieving a balance between energy consumption during operation of the engine  12  and driving stability of the vehicle  10 . 
     According to the present embodiment, in addition, the first lateral G threshold value G1b (operating-state threshold value), which is used when the engine  12  is in operation (step S 53 : YES), is less than the first lateral G threshold value G1a (stopped-state threshold value), which is used when the engine  12  is stopped (step S 53 : NO) (see  FIG. 5 , etc.). Consequently, the RWD mode is switched to the AWD mode more quickly while the engine  12  is in operation than when the engine  12  is stopped. Therefore, if the engine  12  is in operation prior to switching from the RWD mode to the AWD mode, it is possible to increase driving stability at an early stage. 
     II. Modifications 
     The present invention is not limited to the above-described embodiment, but various arrangements may be employed based on the above-described disclosure of the present invention. For example, the present invention may employ the following arrangements. 
     A. Vehicle  10  (Objects to which the Invention is Applied) 
     In the above embodiment, the present invention has been described in relation to the vehicle  10  as a self-propelled four-wheeled vehicle ( FIG. 1 ). However, the present invention may also be applied to any type of vehicle that is capable of being switched between at least two of the FWD mode, the RWD mode, and the AWD mode, from the standpoint of a deviation in the turning radius ratios R/R0 in each drive mode, which takes place at the first deviation occurrence value Gdiv1 (first lateral G) or the second deviation occurrence value Gdiv2 (second lateral G). For example, the present invention may be applied to any one of a self-propelled two-wheeled vehicle, a self-propelled three-wheeled vehicle, and a self-propelled six-wheeled vehicle. 
     The present invention may also be applied to a vehicle that is capable of switching between a drive mode in which the engine  12  is not in operation, i.e., the RWD mode in the above embodiment, and the AWD mode, from the standpoint of setting the first lateral G threshold value G1 based on the operating state of the engine  12 , i.e., a state in which the engine  12  is in operation or a state in which the engine  12  is stopped. For example, the present invention may be applied to any one of a self-propelled two-wheeled vehicle, a self-propelled three-wheeled vehicle, and a self-propelled six-wheeled vehicle. 
     In the above embodiment, the vehicle  10  includes the single engine  12  and the three traction motors  14 ,  16 ,  18  as drive sources. However, the drive sources are not limited to the above combination. As drive sources, the vehicle  10  may have one or more traction motors for the front wheels  32  and one or more traction motors for the rear wheels  36 . For example, the vehicle  10  may have one traction motor for the front wheels  32  or the rear wheels  36 . In this case, the drive force from the traction motor may be distributed through a differential to the left and right wheels. It is also possible for the vehicle  10  to have an arrangement in which individual traction motors (including so-called in-wheel motors) are assigned respectively to each of the wheels, from the standpoint of accounting for deviations in the turning radius ratio R/R0 in each drive mode at the first deviation occurrence value Gdiv1 (first lateral G) or the second deviation occurrence value Gdiv2 (second lateral G). 
     Furthermore, the present invention may also be applied to a vehicle that has one engine  12  for driving the vehicle and one motor for driving the vehicle, i.e., any one of the first through third motors  14 ,  16 ,  18 , from the standpoint of setting the first lateral G threshold value G1 based on the state of the engine  12 , i.e., during operation of the engine  12  or when the engine  12  is stopped. 
       FIG. 12  schematically shows a drive system and related components of a vehicle  10 A according to a modification of the present invention. The vehicle  10 A includes a front wheel drive apparatus  34   a  and a rear wheel drive apparatus  38   a , in which configurations thereof are in reverse to those of the vehicle  10  according to the aforementioned embodiment. More specifically, the front wheel drive apparatus  34   a  of the vehicle  10 A has second and third traction motors  16   a ,  18   a , which are disposed in a front portion of the vehicle  10 A. Further, the rear wheel drive apparatus  38   a  of the vehicle  10 A has an engine  12   a  and a first traction motor  14   a , which are disposed in a series-connected layout in a rear portion of the vehicle  10 A. 
     According to the above embodiment and the modification shown in  FIG. 12 , the front wheels  32  serve as steerable wheels and the rear wheels  36  serve as non-steerable wheels. However, both the front wheels  32  and the rear wheels  36  may serve as steerable wheels, or the rear wheels  36  may serve as steerable wheels whereas the front wheels  32  may serve as non-steerable wheels. 
     B. First Through Third Traction Motors  14 ,  16 ,  18   
     In the above embodiment, each of the first through third traction motors  14 ,  16 ,  18  comprises a three-phase AC brushless motor. However, the first through third traction motors  14 ,  16 ,  18  may be constituted by three-phase AC brush motors, single-phase AC motors, or DC motors. 
     In the above embodiment, the first through third traction motors  14 ,  16 ,  18  are supplied with electric power from the high-voltage battery  20 . However, the first through third traction motors  14 ,  16 ,  18  may be supplied additionally with electric power from fuel cells. 
     C. Control of Drive Modes of the Vehicle  10   
     C-1. Switching Between Drive Modes 
     In the above embodiment, drive modes are switched according to the processing sequence of the flowcharts shown in  FIGS. 3 and 4 . However, the drive modes may be switched according to other methods. For example, the drive modes may be switched based on at least one of the vehicle speed V, a change in the vehicle speed V, the accelerator opening θap, a change in the accelerator opening θap, and the yaw rate Yr. Alternatively, the running modes (drive modes) may be switched according to the process shown in  FIG. 13  and the related description thereof appearing in US2012/0015772A1. 
     In the above embodiment, the FWD mode, the RWD mode, and the AWD mode serve as respective drive modes of the vehicle  10  that are capable of being switched. However, the present invention may be applied to a vehicle that is capable of switching between at least two of the FWD mode, the RWD mode, and the AWD mode from the standpoint of a deviation in the turning radius ratio R/R0 in each drive mode at the first deviation occurrence value Gdiv1 (first lateral G) or the second deviation occurrence value Gdiv2 (second lateral G). For example, the present invention may be applied to a vehicle that is capable of switching between only the FWD mode and the AWD mode (first switching), as well as to a vehicle that is capable of switching between only the RWD mode and the AWD mode (second switching). The present invention may also be applied to a vehicle having one engine  12  for driving the vehicle  10  and one motor for driving the vehicle  10 , from the standpoint of setting the first lateral G threshold value G1 based on the state of the engine  12 , i.e., when the engine  12  is in operation or when the engine  12  is stopped. 
     In the above embodiment, if the lateral G becomes equal to or greater than the first lateral G threshold value G1 (step S 35  of  FIG. 7 : NO) and the drive modes are inhibited from switching (step S 37  of  FIG. 7 , step S 4  of  FIG. 3 : YES), then the drive mode is locked to the AWD mode (step S 5  of  FIG. 3 ). However, the drive mode selected when the drive modes are inhibited from switching is not limited to the AWD mode. For example, the drive mode selected when the drive modes are inhibited from switching may be the FWD mode or the RWD mode. Alternatively, a preset drive mode may not be selected, but the drive mode may be locked to a drive mode that has been selected when the drive modes are inhibited from switching, i.e., a drive mode immediately before the drive modes are inhibited from switching. 
     C-2. Drive Mode Switching Inhibition Threshold Value G1 (First Lateral G Threshold Value G1) 
     In the above embodiment, a value equal to the first deviation occurrence value Gdiv1, which defines a boundary value at which the turning radius ratio R/R0 of the FWD mode starts to deviate from those of the RWD mode and the AWD mode, is used as the first lateral G threshold value G1a (see  FIG. 5 , etc.). However, the first lateral G threshold value G1a may be set to other values. For example, the first lateral G threshold value G1a may be set to a value that is less than the first deviation occurrence value Gdiv1, from the standpoint of reliably inhibiting the drive modes from switching at the time that the turning radius ratios R/R0 actually start to deviate from each other. Further, alternatively, the first lateral G threshold value G1a may be set to a value that is slightly greater than the first deviation occurrence value Gdiv1, from the standpoint of ensuring that the deviation remains less than a predetermined value. 
     In the above embodiment, the drive mode switching inhibition threshold value G1 is used as the value for the lateral G. However, the drive mode switching inhibition threshold value G1 may be a value that is related to the lateral G (a lateral acceleration-related value). The lateral acceleration-related value includes the lateral G value itself. For example, in view of the fact that, according to the above equation (2), the lateral G is calculated as a product of the yaw rate Yr and the vehicle speed V (lateral G=Yr×V), a quotient produced when the first lateral G threshold value G1 is divided by the vehicle speed V (G1/V) may be compared with the yaw rate Yr, or a quotient produced when the first lateral G threshold value G1 is divided by the yaw rate Yr (G1/Yr) may be compared with the vehicle speed V, thereby providing the same advantages as those of the above embodiment. Stated otherwise, rather than a value that directly indicates a lateral G, a value that indirectly indicates the lateral G, i.e., the yaw rate Yr or the vehicle speed V according to the above embodiment, may be compared with a predetermined threshold value, i.e., a value that indirectly indicates the first lateral G threshold value G1, thereby providing essentially the same advantages as those of the above embodiment. The same technique may be applied to the above equation (1). 
     From the standpoint of setting the first lateral G threshold value G1a based on the state of the engine  12  (when the engine  12  is in operation or when the engine  12  is stopped), the first lateral G threshold value G1a need not necessarily be set based on the first deviation occurrence value Gdiv1 or the deviation reference value Gref. Stated otherwise, the first lateral G threshold values G1a, G1b may be switched when the engine  12  is in operation and when the engine  12  is stopped. 
     In the above embodiment, the first lateral G threshold value G1b, which is used when the engine  12  is in operation, is less than the first lateral G threshold value G1a, which is used when the engine  12  is stopped (see  FIG. 5 , etc.). However, the first lateral G threshold value G1b may be greater than the first lateral G threshold value G1a, for thereby achieving driving stability at an early stage when the vehicle is in the RWD mode. 
     In the above embodiment, the first lateral G threshold value G1 (first lateral G threshold values G1a, G1b) is switched based on the accelerator opening θap (see  FIGS. 5, 6, 10, and 11 ). However, another value, which affects a change (deviation) in the turning radius ratio R/R0, or a similar turning characteristic related value, which depends on switching between drive modes, may be used in addition to or instead of the accelerator opening θap. 
     For example, as shown in  FIGS. 13 and 14 , it is possible for the drive mode switching inhibition threshold values G1a, G1b (the first lateral G threshold values G1a, G1b) to be changed based on the fore-and-aft acceleration (fore-and-aft G). The fore-and-aft G may be detected by a non-illustrated fore-and-aft G sensor, for example. As shown in  FIG. 13 , the threshold values G1a, G1b decrease as the for-and-aft G increases. 
     Further, in  FIG. 14 , the threshold values G1a, G1b are kept constant while the fore-and-aft G ranges from 0 to Gf1, based on the concept that it is essentially meaningless to change the threshold values G1a, G1b for the lateral G as long as the fore-and-aft G is low (i.e., ranges from 0 to Gf1). The threshold values G1a, G1b are reduced while the fore-and-aft G ranges from Gf1 to Gf2. This is because, as described above with reference to  FIGS. 5 and 6 , etc., as the fore-and-aft G increases, the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2 are reduced with respect to the lateral G. The threshold values G1a, G1b remain constant when the fore-and-aft G is greater than Gf2, because the threshold values G1a, G1b have reached their minimum values. 
     Alternatively, as shown in  FIG. 15 , the first lateral G threshold values G1a, G1b may be changed based on the vehicle speed V. In  FIG. 15 , the first lateral G threshold values G1a, G1b are kept constant while the vehicle speed V ranges from 0 to V1, based on the concept that it is essentially meaningless to change the threshold values G1a, G1b as long as the vehicle speed V is low (i.e., ranges from 0 to V1). The threshold values G1a, G1b are reduced while the vehicle speed V ranges from V1 to V2. This is because, as described above with reference to  FIGS. 5 and 6 , etc., as the vehicle speed V increases, thereby increasing the fore-and-aft G, the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2 are reduced with respect to the lateral G. The threshold values G1a, G1b remain constant when the vehicle speed V is greater than V2, because the threshold values G1a, G1b have reached their minimum values. 
     Further, alternatively, the first lateral G threshold values G1a, G1b may be changed based on an accelerating intention related value (other than the accelerator opening θap), which indicates the intention of the driver to begin accelerating. For example, the accelerating intention related value other than the accelerator opening θap may be a demand value for the drive force (demand drive force) of the engine  12 , which is set depending on the accelerator opening θap, or a target drive force, which actually is set as a target value for the drive force of the engine  12 , according to various control processes including a feedback control process, a limiting control process, etc., performed on the demand drive force, for example. 
     If the conditions in which the lateral G becomes equal to or greater than the first lateral G threshold values G1a, G1b are very limited, then the first lateral G threshold values G1a, G1b may be fixed during use thereof. 
     In the above-described embodiment, the first lateral G threshold value G1a is set based on the deviation reference value Gref, which is the smaller of the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2. In other words, the first lateral G threshold value G1a has been used irrespective of a switching type of the drive modes. 
     However, in view of the fact that the first deviation occurrence value Gdiv1 and the second deviation occurrence value Gdiv2 differ from each other, as shown in  FIGS. 5 and 6 , the first lateral G threshold value G1a may be made variable depending on the manner in which the drive modes are switched. Stated otherwise, depending on the manner in which the drive modes are switched, it is possible to set the first lateral G threshold value G1a to a different value. For example, upon switching between the FWD mode and the RWD mode or the AWD mode, the first deviation occurrence value Gdiv1 may be used as the first lateral G threshold value G1a, and upon switching between the RWD mode and the AWD mode, the second deviation occurrence value Gdiv2 may be used as the first lateral G threshold value G1a. With respect to the first lateral G threshold value G1, which is used upon switching between the RWD mode and the AWD mode, the first lateral G threshold value G1a may be switched depending on how the engine  12  is operating, similar to the case of the first lateral G threshold values G1a, G1b according to the above embodiment. 
     In the above embodiment, the first lateral G threshold value G1a is set by way of comparison between the first deviation occurrence value Gdiv1 (first lateral G) and the second deviation occurrence value Gdiv2 (second lateral G). However, the first lateral G threshold value G1a remains essentially the same, even if the first lateral G threshold value G1a is set in view of a change in the turning radius ratio R/R0 that occurs upon switching between the drive modes. 
     More specifically, the first lateral G threshold value G1a may be set based on the smaller of a first change, which is a predicted change caused in the turning radius ratio R/R0 upon switching between the FWD mode and the AWD mode while the lateral G stays above the first lateral G threshold value G1a (first switching), and a second change, which is a predicted change caused in the turning radius ratio R/R0 upon switching between the RWD mode and the AWD mode while the lateral G stays above the first lateral G threshold value G1a (second switching). The first switching and the second switching include the transitory AWD mode, which occurs upon switching between the FWD mode and the RWD mode. Alternatively, if the first lateral G threshold value G1a is set depending on how the drive modes are switched, the first lateral G threshold value G1a may be set depending on each of the first change and the second change. 
     In the above embodiment, the first lateral G threshold values G1a, G1b are stored in advance in the memory  44  of the ECU  28 . However, the first lateral G threshold values G1a, G1b may be calculated successively while the vehicle  10  is being driven. If the first lateral G threshold values G1a, G1b are calculated in this manner, the relationship between the lateral Gs and the turning radius ratios R/R0 may be stored for each of the drive modes, and thereafter, the lateral G at which a change in the turning radius ratio R/R0 becomes equal to or greater than a predetermined value may be used as the first lateral G threshold value G1a, whereas the first lateral G threshold value G1b may be calculated from the relationship thereof to the first lateral G threshold value G1a. 
     C-3. Turning Radius Ratio R/R0 (Turning Characteristic Related Value) 
     In the above embodiment, upon switching between the drive modes, the turning radius ratio R/R0 is used as a turning characteristic related value, which deviates in relation to the lateral G. However, the first lateral G threshold value G1 and the second lateral G threshold value G2 may be set based on a different turning characteristic related value, such as the actual turning radius R per se, or a Slip Ratio of any One of the Wheels, for Example. 
     C-4. Engine Starting Threshold Value G2 (Second Lateral G Threshold Value G2) 
     In the above-described embodiment, the second lateral G threshold value G2 is set based on the accelerator opening θap. However, the second lateral G threshold value G2 may be set based on other factors, insofar as the engine  12  can be started based on a judgment that a high possibility exists for the lateral G to become equal to or greater than the first lateral G threshold value G1a in the future. For example, similar to the case of the first lateral G threshold value G1a, the second lateral G threshold value G2 may be set based on another value, such as the fore-and-aft G or the vehicle speed V, in addition to or instead of the accelerator opening θap. Alternatively, if there are very limited conditions in which the lateral G becomes equal to or greater than the first lateral G threshold value G1a, the second lateral G threshold value G2 may be fixed during use thereof, similar to the case of the first lateral G threshold value G1a. 
     Alternatively, the threshold value G2 may be set based on the threshold value G1a. Based on the concept that, if the fore-and-aft G is small, a change in the lateral G, i.e., the derivative of the lateral G with respect to time, is small, the threshold value G2 may be set to a value, the difference of which from the threshold value G1a is small when the fore-and-aft G is small, and the threshold value G2 may be set to a value, the difference of which from the threshold value G1a is large when the fore-and-aft G is large. 
     C-5. Other Features 
     In step S 2  of  FIG. 3 , the ECU  28  judges whether or not the rear motors  16 ,  18  are capable of being energized based on the temperatures of the rear motors  16 ,  18 , the occurrence of a failure of the rear motors  16 ,  18 , and the SOC of the battery  20 . However, the ECU  28  may judge whether or not the rear motors  16 ,  18  are capable of being energized in different ways. For example, the ECU  28  may judge whether or not the rear motors  16 ,  18  are capable of being energized based on only one or two of such conditions, i.e., the temperatures of the rear motors  16 ,  18 , the occurrence of a failure of the rear motors  16 ,  18 , and the SOC of the battery  20 . 
     Alternatively, in addition to or instead of the above indicators, other indicators may be used. For example, a degree of deterioration of the battery  20 , i.e., the number of times that the battery  20  has been charged, the period during which the battery  20  has been used, etc., may be used. 
     According to the flowchart of  FIG. 4 , if the vehicle  10  is driven in the high speed range (step S 12 : NO), the ECU  28  selects the FWD mode and does not energize the rear motors  16 ,  18 . Therefore, based on the vehicle speed V, the ECU  28  actually determines whether or not the rear motors  16 ,  18  are capable of being energized. 
     In the above embodiment, the engine  12  is not idling when the RWD mode is selected, but rather, the engine  12  is stopped, except in step S 7  of  FIG. 3 , and except at times that the first motor  14  is generating electric power under the drive force of the first motor  14 . The engine  12  may remain idling in a standby mode in certain situations other than step S 7  of  FIG. 3 , and other than when the first motor  14  is generating electric power.