Abstract:
Provided is a method of controlling an electric vehicle capable of preferably performing a regenerative control when a plurality of forward driving ranges are shifted to each other. During travelling on a downhill road, the vehicle executes a downhill regenerative control for adjusting the regenerative amount of an electric motor so that acceleration obtained when an accelerator is closed becomes a downhill acceleration according to a road surface slope. The vehicle also executes an acceleration increase control in which the downhill acceleration obtained when the forward driving ranges are shifted from a second forward driving range to a first forward driving range during travelling on the downhill road is at least temporarily set to an acceleration exceeding the downhill acceleration obtained on the assumption that the vehicle continuously travels on the same road surface slope as that of the currently travelling downhill road in the first forward driving range.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of controlling an electric vehicle having a shift range switching device, e.g., a shift lever, for switching between a first forward driving range and a second forward driving range as shift ranges in response to an input operation performed by the driver of the electric vehicle, and more particularly to a method of controlling an electric vehicle capable of suitably performing a regenerative control process in the first forward driving range and the second forward driving range. 
     BACKGROUND ART 
     U.S. Patent Application Publication No. 2009/0112386 (hereinafter referred to as “US 2009/0112386 A1”) discloses a regenerative control process for an electric motor on an electric vehicle while the electric vehicle is driving on a downhill road. More specifically, US 2009/0112386 A1 has the task of operating the electric motor in a regenerative mode while the electric vehicle is driving on a downhill road, in order to cause the vehicle to generate an acceleration suitable for the gradient of the road surface while at the same time minimizing the influences of disturbance factors (see Abstract and paragraph [0007]). In order to carry out the task, according to US 2009/0112386 A1, the electric vehicle has a device 25 for determining, depending on the gradient of the road surface, a target acceleration of the vehicle 1 if the operational states of an accelerator pedal and a brake pedal of the vehicle 1 are an OFF state when the vehicle 1 is driving on a downhill road, a device 26 for determining a correction quantity ΔTrd of a regenerative torque for bringing the actual acceleration of the vehicle 1 close to the target acceleration, a device 21 for determining a basic target torque Trs of an electric motor 2 , which becomes a regenerative torque at the time the operational state of the accelerator pedal is the OFF state, depending on the operational state of the accelerator pedal, and a device 27 for determining a target torque Trc by correcting the basic target torque Trs with at least the correction quantity ΔTrd, wherein the output torque of the electric motor 2 is controlled depending on the target torque Trc (see Abstract). 
     Electric vehicles include an electric vehicle that is capable of driving forward in a plurality of modes, or stated otherwise, an electric vehicle having a plurality of forward driving ranges {International Publication No. 2010/110098 (hereinafter referred to as “WO 2010/110098 A1”)}. WO 2010/110098 A1 shows a D position and a B position as corresponding to forward driving positions among the shift positions of a shift lever 32 (see  FIG. 2 ). The D position corresponds to a D range which is a forward driving range for transmitting the drive force for moving the vehicle forward to the drive wheels, whereas the B position corresponds to a decelerated forward driving range (engine braking range) for generating an engine braking effect to decelerate the drive wheels by causing the electric motor to generate a regenerative torque, for example, in the D range (see paragraphs [0049] and [0050]). 
     SUMMARY OF INVENTION 
     According to US 2009/0112386 A1, as described above, it is possible to perform a regenerative control process suitable for a downhill road. However, the regenerative control process does not take into account a plurality of forward driving ranges. 
     The present invention has been made in view of the above problems. It is an object of the present invention to provide a method of controlling an electric vehicle which is capable of suitably performing a regenerative control process at the time a plurality of forward driving ranges are switched from one to another. 
     According to the present invention, there is provided a method of controlling an electric vehicle having a range switching device for switching between a first forward driving range and a second forward driving range as shift ranges in response to an input operation performed by the driver of the electric vehicle, the method including, when the driver closes an accelerator of the electric vehicle on a flat road, carrying out an accelerator regenerative control process for applying, to the electric vehicle, a braking force equivalent to engine braking by regenerative electric generation by an electric motor, wherein the deceleration of the electric vehicle in the accelerator regenerative control process at the time the second forward driving range is selected, is greater than the deceleration of the electric vehicle in the accelerator regenerative control process at the time the first forward driving range is selected, when the electric vehicle is driving on a downhill road, carrying out a downhill regenerative control process for adjusting the amount of electric power regenerated by the electric motor such that the acceleration of the electric vehicle at the time the accelerator is closed, is equal to a downhill acceleration depending on a road surface gradient, wherein the downhill acceleration at the time the second forward driving range is selected, is smaller than the downhill acceleration at the time the first forward driving range is selected, throughout a region of the road surface gradient that is established as a control range, and carrying out an acceleration increasing control process for setting the downhill acceleration at the time of switching from the second forward driving range to the first forward driving range while the electric vehicle is driving on the downhill road, at least temporarily to an acceleration in excess of the downhill acceleration obtained on the assumption that the electric vehicle is driving continuously in the first forward driving range at the same road surface gradient as the road surface gradient of the downhill road on which the electric vehicle is currently driving. 
     According to the present invention, at the time the range is switched from the second forward driving range to the first forward driving range while the electric vehicle is driving on the downhill road, the downhill acceleration is set at least temporarily to a downhill acceleration in excess of the normal downhill acceleration for the first forward driving range, i.e., the downhill acceleration obtained on the assumption that the electric vehicle is driving continuously in the first forward driving range at the same road surface gradient as the road surface gradient of the downhill road on which the electric vehicle is currently driving. Therefore, at the time of switching from the second forward driving range to the first forward driving range, the downhill acceleration of the electric vehicle becomes greater than normal even if the downhill regenerative control process is performed in the first forward driving range. Consequently, it is possible to achieve compatibility between an acceleration feel which the driver expects to have by switching from the second forward driving range to the first forward driving range and the downhill regenerative control process that adjusts the downhill acceleration depending on the road surface gradient. 
     The above method may further include carrying out, after the acceleration increasing control process, an acceleration restoring control process for restoring the downhill acceleration to the downhill acceleration obtained on the assumption that the electric vehicle is driving continuously in the first forward driving range at the same road surface gradient as the road surface gradient of the downhill road on which the electric vehicle is currently driving. 
     The absolute value of a rate of change of the downhill acceleration according to the acceleration increasing control process may be greater than the absolute value of a rate of change of the downhill acceleration according to the acceleration restoring control process. 
     According to the present invention, there is also provided a method of controlling an electric vehicle having a range switching device for switching between a first forward driving range and a second forward driving range as shift ranges in response to an input operation performed by the driver of the electric vehicle, the method including, when the driver closes an accelerator of the electric vehicle on a flat road, carrying out an accelerator regenerative control process for applying, to the electric vehicle, a braking force equivalent to engine braking by regenerative electric generation by an electric motor, wherein the deceleration of the electric vehicle in the accelerator regenerative control process at the time the second forward driving range is selected, is greater than the deceleration of the electric vehicle in the accelerator regenerative control process at the time the first forward driving range is selected, when the electric vehicle is driving on a downhill road, carrying out, in at least the first forward driving range, a downhill regenerative control process for adjusting the amount of electric power regenerated by the electric motor such that the acceleration of the electric vehicle at the time the accelerator is closed, is equal to a downhill acceleration depending on a road surface gradient, and carrying out an acceleration increasing control process for setting the downhill acceleration at the time of switching from the second forward driving range to the first forward driving range while the electric vehicle is driving on the downhill road, at least temporarily to an acceleration in excess of the downhill acceleration obtained on the assumption that the electric vehicle is driving continuously in the first forward driving range at the same road surface gradient as the road surface gradient of the downhill road on which the electric vehicle is currently driving. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of an overall arrangement of an electric vehicle according to an embodiment of the present invention; 
         FIG. 2  is a flowchart of a process of controlling the drive force of a traction motor in a D range or a B range according to the embodiment; 
         FIG. 3  is a timing chart showing by way of example the relationship between the road surface gradient, the vehicle speed, the accelerator pedal opening degree, the brake pedal opening degree, and the target drive force for the traction motor in various control processes according to the embodiment; 
         FIG. 4  is a characteristic diagram showing by way of example the relationship between the road surface gradient and the target acceleration used in a downhill regenerative control process while the D range is being selected; 
         FIG. 5  is a characteristic diagram showing by way of example the relationship between the road surface gradient and the target acceleration for each of the D range and the B range used in the embodiment; 
         FIG. 6  is a characteristic diagram showing by way of example the relationship between the road surface gradient and the target acceleration for each of the D range and the B range used in a comparative example; 
         FIG. 7  is a timing chart illustrating problems caused by continuing the downhill regenerative control process at the time the range is switched from the B range to the D range; 
         FIG. 8  is a timing chart illustrating an acceleration increasing control process and an acceleration restoring control process according to the embodiment; 
         FIG. 9  is a flowchart of a process of calculating a downhill regenerative correction quantity (downhill regenerative control process) (details of S 3  in  FIG. 2 ) according to the embodiment; 
         FIG. 10  is a flowchart of a process of calculating a target acceleration (details of S 12  in  FIG. 9 ) according to the embodiment; and 
         FIG. 11  is a diagram showing by way of example how a timer operates in the acceleration increasing control process and the acceleration restoring control process. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A. Embodiments 
     1. Description of Overall Arrangement 
     [1-1. Overall Arrangement] 
       FIG. 1  shows in block form an overall arrangement of an electric vehicle  10  according to an embodiment of the present invention. The electric vehicle  10  (hereinafter also referred to as “vehicle  10 ”) includes a traction motor (hereinafter referred to as “motor  12 ”), an inverter  14 , a battery  16 , an accelerator pedal  18 , an accelerator pedal opening degree sensor  20  (hereinafter referred to as “AP opening degree sensor  20 ”), a brake pedal  22 , a brake pedal opening degree sensor  24  (hereinafter referred to as “BP opening degree sensor  24 ”), a shift lever  26  (range switching device), a shift position sensor  28 , a vehicle speed sensor  30 , an acceleration sensor  32 , and an electronic control unit  34  (hereinafter referred to as “ECU  34 ”). 
     [1-2. Electric Power System] 
     The motor  12  (electric motor) comprises a three-phase AC brushless motor which generates a drive force (hereinafter referred to as “motor drive force F” or “drive force F”) [N] (or torque [N·m]) for the vehicle  10  based on electric power supplied from the battery  16  through the inverter  14 . The motor  12  also operates in a regenerative mode to regenerate electric power (hereinafter referred to as “regenerated electric power Preg”) [W] and supplies the regenerated electric power Preg to the battery  16  to charge the battery  16 . The motor  12  may supply the regenerated electric power Preg to auxiliaries, not shown. 
     The inverter  14 , which is of the three-phase bridge configuration, performs DC/AC conversion, converts direct current into three-phase alternating current and supplies the three-phase alternating current to the motor  12 . Further, when the motor  12  operates in the regenerative mode, the inverter  14  converts alternating current into direct current and supplies the direct current to the battery  16 . 
     The battery  16  comprises an electric energy storage device (energy storage device) including a plurality of battery cells, and may be a lithium ion secondary battery, a nickel hydrogen secondary battery, a capacitor, or the like. According to the present embodiment, a lithium ion secondary battery is used as the battery  16 . A DC/DC converter, not shown, may be connected between the inverter  14  and the battery  16  for stepping up or down the output voltage from the battery  16  or the output voltage from the motor  12 . 
     [1-3. Various Sensors] 
     The AP opening degree sensor  20  detects a depressed amount (hereinafter referred to as “AP opening degree θap”) [degrees] of the accelerator pedal  18  from its original position, and supplies the detected AP opening degree θap to the ECU  34 . The BP opening degree sensor  24  detects a depressed amount (hereinafter referred to as “BP opening degree θbp”) [degrees] of the brake pedal  22  from its original position, and supplies the detected BP opening degree θbp to the ECU  34 . 
     The shift position sensor  28  detects a position (hereinafter referred to as “shift position Ps”) of the shift lever  26 , and supplies the detected shift position Ps to the ECU  34 . According to the present embodiment, the shift positions Ps include “P” corresponding to a parking range as a shift range, “N” corresponding to a neutral range as a shift range, “R” corresponding to a reverse driving range as a shift range, “D” corresponding to a D range (first forward driving range) as a shift range, and “B” corresponding to a B range (second forward driving range) as a shift range. 
     Each of the D range and the B range is a forward driving range as a shift range. The D range is used when the vehicle  10  is driven normally, i.e., when the vehicle  10  is driving not in the B range. The B range is a shift range in which the amount of electric power regenerated by the motor  12  is made greater than in the D range when the driver of the vehicle  10  wants to increase the amount of electric power regenerated by the vehicle  10 , i.e., the motor  12  (e.g., when the vehicle  10  is driving on a downhill road). 
     The vehicle speed sensor  30  detects an actual vehicle speed V [km/h] of the vehicle  10  and supplies the detected actual vehicle speed V to the ECU  34 . The acceleration sensor  32  detects an actual acceleration a [m/s/s] of the vehicle  10  and supplies the detected actual acceleration a to the ECU  34 . According to the present embodiment, the acceleration a is used to estimate the gradient (hereinafter referred to as “road surface gradient G” or “gradient G”) of the road on which the vehicle  10  is driving, as described in detail later. 
     [1-4. ECU  34 ] 
     The ECU  34  controls the inverter  14  based on output signals from the various sensors to control the output power of the motor  12  (electric motor output power). The ECU  34  has an input/output unit, a processor, and a memory, all not shown. 
     2. Control Processes According to the Present Embodiment 
     Various control processes according to the present embodiment will be described below. 
     [2-1. Motor Drive Force Control Process] 
       FIG. 2  is a flowchart of a process of controlling the drive force F of the motor  12  in the D range or the B range according to the embodiment. The process shown in  FIG. 2  is applicable where the vehicle  10  is in either a power mode or a regenerative mode. According to the present embodiment, a target value (hereinafter referred to as “target drive force Ftar”) [N] for the drive force F of the motor  12  may be calculated basically by a process similar to the process disclosed in US 2009/0112386 A1. Specifically, whereas US 2009/0112386 A1 uses the torque of the electric motor  2  as a controlled quantity, the present embodiment handles the motor drive force F as controlled quantity in place of the torque. The torque can be calculated by multiplying the drive force F by the radius of a road wheel, not shown. 
     In step S 1  shown in  FIG. 2 , the ECU  34  judges whether or not the shift range corresponding to the shift position Ps detected by the shift position sensor  28  is the D range or the B range. If the shift range is not the D range or the B range, i.e., if the shift range is the P range, the N range, or the R range (S 1 : NO), then the process shown in  FIG. 2  is ended, and a target driver force Ftar is separately established for the motor  12 . If the shift range is the D range or the B range (S 1 : YES), then control goes to step S 2 . 
     In step S 2 , the ECU  34  calculates a basic drive force Fbase. The ECU  34  calculates the basic drive force Fbase based on the AP opening degree θap detected by the AP opening degree sensor  20 , the shift position Ps (or shift range) detected by the shift position sensor  28 , and the vehicle speed V detected by the vehicle speed sensor  30 . Specifically, the ECU  34  calculates the basic drive force Fbase using a map of basic drive forces Fbase in combination with AP opening degrees θap and vehicle speeds V depending on shift positions Ps (shift ranges). For illustrative purposes, it is assumed that a basic drive force Fbase for driving the vehicle  10  in the power mode is of a positive value, whereas a basic drive force Fbase for operating the vehicle  10  in the regenerative mode is of a negative value. 
     The AP opening degree θap and the vehicle speed V are used in the same manner as with US 2009/0112386 A1, and the basic drive force Fbase corresponds to the basic target torque Trs disclosed in US 2009/0112386 A1. Consequently, a basic drive force Fbase can be calculated in the same manner as the basic target torque Trs disclosed in US 2009/0112386 A1 (see paragraphs [0071] and [0082] of US 2009/0112386 A1). 
     However, according to the present embodiment, the vehicle  10  has the D range and the B range each as a shift range for driving the vehicle  10  forward, unlike US 2009/0112386 A1. According to the present embodiment, a map of basic drive forces Fbase in combination with AP opening degrees θap and vehicle speeds V is established in advance for each of the D range and the B range, and a basic drive force Fbase is calculated using the map. As described above, the amount of electric power regenerated by the motor  12  is greater in the B range than in the D range. Consequently, a basic drive force Fbase in the regenerative mode is smaller in the B range (its absolute value is greater because a basic drive force Fbase for operating the vehicle  10  in the regenerative mode is of a negative value). 
     According to the present embodiment, a process for controlling a basic drive force Fbase for operating the vehicle  10  in the regenerative mode will be referred to as “accelerator regenerative control process” as distinguished from “downhill regenerative control process” to be described below. 
     In step S 3 , the ECU  34  performs a downhill regenerative control process and calculates a downhill regenerative correction quantity ΔF 1 . The downhill regenerative correction quantity ΔF 1  is used to accelerate the vehicle  10  gradually while the vehicle  10  is driving on a downhill road, and is similar to the downhill road regenerative correction quantity ΔTrd disclosed in US 2009/0112386 A1 (see paragraph [0074] of US 2009/0112386 A1). According to the present embodiment, however, the downhill regenerative correction quantity ΔF 1  is suitable for the vehicle  10  which has a plurality of forward driving ranges (the D range and the B range). Details of the calculation of the downhill regenerative correction quantity ΔF 1  (downhill regenerative control process) will be described later with reference to  FIGS. 3 ,  9 ,  10 , etc. 
     In step S 4  in  FIG. 2 , the ECU  34  calculates a temporary target drive force Ftar_temp. Specifically, the ECU  34  calculates the sum of the basic drive force Fbase calculated in step S 2  and the downhill regenerative correction quantity ΔF 1  calculated in step S 3  as a temporary target drive force Ftar_temp. 
     In step S 5 , the ECU  34  calculates a braking regenerative correction quantity ΔF 2 . The braking regenerative correction quantity ΔF 2  is a correction quantity for the motor drive force F which is set in response to the brake pedal  22  being depressed, and is used to enhance the drivability of the vehicle  10 . The braking regenerative correction quantity ΔF 2  is similar to the braking regenerative correction quantity ΔTrb disclosed in US 2009/0112386 A1 in that it is established based on the BP opening degree θbp. Therefore, the braking regenerative correction quantity ΔF 2  can be calculated in the same manner as the braking regenerative correction quantity ΔTrb disclosed in US 2009/0112386 A1, for example (see paragraphs [0073] and [0084] of US 2009/0112386 A1). A process of correcting the temporary target drive force Ftar_temp using the braking regenerative correction quantity ΔF 2  is referred to as a drivability filtering process. 
     In step S 6 , the ECU  34  calculates a target drive force Ftar. Specifically, the ECU  34  calculates the sum of the temporary target drive force Ftar_temp calculated in step S 4  and the braking regenerative correction quantity AF 2  calculated in step S 5  as a target drive force Ftar. 
     [2-2. Calculation of Downhill Regenerative Correction Quantity ΔF 1  (Downhill Regenerative Control Process)] 
     (2-2-1. Concept of Downhill Regenerative Control Process) 
     The concept of the downhill regenerative control process according to the present embodiment will be described below.  FIG. 3  is a timing chart showing by way of example the relationship between the road surface gradient G, the vehicle speed V, the AP opening degree θap, the BP opening degree θbp, and the target drive force Ftar for the motor  12  in the various control processes according to the present embodiment. For illustrative purposes, the road surface gradient G shown in  FIG. 3  is 0% when the road is flat, is of a positive value when the road is uphill, and is of a negative value when the road is downhill (The same will apply hereinafter). 
     As shown in  FIG. 3 , the gradient of the road on which the vehicle  10  is driving, i.e., the road surface gradient G, is 0% (i.e., flat road) from time t 1  to time t 2 , and becomes lower from time t 2  toward time t 5 , i.e., the gradient G of the downhill road becomes sharper from time t 2  toward time t 5 . At time t 2 , the driver returns the accelerator pedal  18  to its original position, causing the AP opening degree θap to return to zero. When the various control processes according to the present embodiment are used at this time, the vehicle speed V increases more gradually than the road surface gradient G decreases, i.e., the absolute value of the road surface gradient G increases. 
     Specifically, according to the present embodiment, when the AP opening degree θap goes to zero at time t 2 , the basic drive force Fbase reaches a predetermined value (hereinafter referred to as “regenerative basic drive force Freg_base”) (accelerator regenerative control process). The regenerative basic drive force Freg_base is of a fixed value which is set when the AP opening degree θap is zero, and represents a drive force F corresponding to engine braking which is generated by a regenerative electric generation process performed by the motor  12 . After the vehicle  10  has entered the downhill road at time t 2 , the driver does not depress the brake pedal  22 , keeping the BP opening degree θbp at zero. Therefore, if only the regenerative basic drive force Freg_base, which is of a fixed value for the regenerative mode, is applied, then the vehicle speed V tends to increase sharply. 
     According to the present embodiment, the downhill regenerative correction quantity ΔF 1  established according to the downhill regenerative control process is used. Specifically, according to the present embodiment, the downhill regenerative correction quantity ΔF 1  is established depending on the road surface gradient G in addition to the regenerative basic drive force Freg_base. For example, since the road surface gradient G is continuously decreasing, i.e., the absolute value of the road surface gradient G is continuously increasing, after time t 2 , the downhill regenerative correction quantity ΔF 1  is increased. Consequently, while the vehicle  10  is driving on the downhill road, the vehicle  10  can be accelerated gradually without the driver depressing the brake pedal  22 . 
     (2-2-2. Characteristics for Achieving the Downhill Regenerative Control Process) 
       FIG. 4  is a characteristic diagram showing by way of example the relationship between the road surface gradient G and the target acceleration a_tar used in a downhill regenerative control process while the D range is being selected. The characteristics shown in  FIG. 4  are plotted when the vehicle speed V is of a certain value, and may be changed depending on the vehicle speed V. Stated otherwise, characteristic diagrams (maps) as shown in  FIG. 4  are plotted for respective different values of the vehicle speed V, and one of them is selected and used depending on the vehicle speed V. According to the present embodiment, not only the characteristics for the D range shown in FIG.  4 , but also the characteristics for both the D range and the B range (see  FIG. 5 ), are used. 
     In  FIG. 4 , a characteristic curve  100  indicated by the broken line is applied when only the regenerative basic drive force Freg_base (accelerator regenerative control process) is used, and a characteristic curve  102  indicated by the solid line is applied when both the regenerative basic drive force Freg_base (accelerator regenerative control process) and the downhill regenerative correction quantity ΔF 1  (downhill regenerative control process) are used. According to the present embodiment, as described above, since both the regenerative basic drive force Freg_base (accelerator regenerative control process) and the downhill regenerative correction quantity ΔF 1  (downhill regenerative control process) are used, the characteristic curve  102  indicated by the solid line is used in the present embodiment. It should be noted that the characteristic curve  100  indicated by the broken line is illustrated for reference only, and is not used in the present embodiment. 
     As shown in  FIG. 4 , the characteristic curve  102  used in the present embodiment is the same as the characteristic curve  100  when the road surface gradient G changes from zero to a given value G 1 . Stated otherwise, the downhill regenerative control process is inhibited when the road surface gradient G changes from zero to a given value G 1 . Between the value G 1  and a value G 2 , as the road surface gradient G decreases, i.e., the absolute value of the road surface gradient G increases, the characteristic curve  102  deviates from the characteristic curve  100 . That is, the target acceleration a_tar represented by the characteristic curve  102  is lower than the target acceleration a_tar represented by the characteristic curve  100 , thereby making it possible for the vehicle  10  to accelerate gradually on the downhill road. When the road surface gradient G becomes smaller than the value G 2 , i.e., when the absolute value of the road surface gradient G becomes greater than that of the value G 2 , the difference (hereinafter referred to as “difference Δa 1 ”) between the target acceleration a_tar represented by the characteristic curve  102  and the target acceleration a_tar represented by the characteristic curve  100  becomes constant. 
     According to the downhill regenerative control process, as described above, inasmuch as the downhill regenerative correction quantity ΔF 1  is used in addition to the regenerative basic drive force Freg_base, the vehicle is able to accelerate gradually on the downhill road, whereby the manipulation performance by the driver is enhanced. 
     (2-2-3. Road Surface Gradient G Versus Target Acceleration a_tar Characteristics for D Range and B Range) 
     According to the present embodiment, as described above, the vehicle  10  has the B range in addition to the D range as shift ranges for driving the vehicle  10  forward. According to the present embodiment, road surface gradient G versus target acceleration a_tar characteristics are established for respective different values of the vehicle speed V for each of the D range and the B range. 
       FIG. 5  is a characteristic diagram showing by way of example the relationship between the road surface gradient G and the target acceleration a_tar for each of the D range and the B range used in the present embodiment. The characteristics shown in  FIG. 5  are plotted when the vehicle speed V is of a certain value, and may be changed depending on the vehicle speed V. Stated otherwise, characteristic diagrams (maps) as shown in  FIG. 5  are plotted for respective different values of the vehicle speed V, and one of them is selected and used depending on the vehicle speed V. 
     In  FIG. 5 , a characteristic curve  100  indicated by the broken line and a characteristic curve  102  indicated by the solid line are identical to those shown in  FIG. 4 . The characteristic curve  102  is used for the D range according to the present embodiment, and the characteristic curve  100  is shown for reference only, not used in the present embodiment. 
     A characteristic curve  104  indicated by the dot-and-dash line is applied when only the regenerative basic drive force Freg_base (accelerator regenerative control process) is used for the B range, and a characteristic curve  106  indicated by the solid line is applied when both the regenerative basic drive force Freg_base (accelerator regenerative control process) and the downhill regenerative correction quantity ΔF 1  (downhill regenerative control process) are used for the B range. According to the present embodiment, as described above, since both the regenerative basic drive force Freg_base and the downhill regenerative correction quantity ΔF 1  are used also for the B range, the characteristic curve  106  indicated by the solid line is used in the present embodiment. It should be noted that the characteristic curve  104  indicated by the dot-and-dash line is illustrated for reference only, and is not used in the present embodiment. 
     A comparison of the characteristic curve  102  for the D range and the characteristic curve  106  for the B range indicates that the target acceleration a_tar for the B range is smaller than the target acceleration a_tar for the D range at the same road surface gradient G. This is because the amount of electric power regenerated by the motor  12  (regenerated electric power Preg) is greater for the B range than for the D range. Therefore, the amount of charge for the battery  16  is greater when the B range is used. 
     When the road surface gradient G is equal, the target acceleration a_tar represented by the characteristic curve  106  for the B range is set to be always lower than the target acceleration a_tar represented by the characteristic curve  102  for the D range. More specifically, according to the present embodiment, the difference between the target acceleration a_tar represented by the characteristic curve  102  for the D range and the target acceleration a_tar represented by the characteristic curve  106  for the B range is always constant at any value of the road surface gradient G. In other words, the difference (hereinafter referred to as “difference Δa 2 ”) between the characteristic curve  102  and the characteristic curve  106  in a region wherein the road surface gradient G changes from zero to the value G 1 , i.e., a region wherein the downhill regenerative control process is inhibited, and the difference (hereinafter referred to as “difference Δa 3 ”) between the characteristic curve  102  and the characteristic curve  106  in a region wherein the road surface gradient G is lower than the value G 1 , i.e., a region wherein the downhill regenerative control process is permitted, are constant and equal to each other. Consequently, in a case where the range is switched from the D range to the B range when the vehicle  10  is driving on the downhill road, the vehicle  10  is more decelerated than before. 
     Characteristics shown in  FIG. 6  according to a comparative example will be described below in order to illustrate the advantages of the above characteristic curves  102 ,  106 .  FIG. 6  is a characteristic diagram showing by way of example the relationship between the road surface gradient G and the target acceleration a_tar for each of the D range and the B range used in a comparative example. The characteristics shown in  FIG. 6  are plotted at the same vehicle speed V as the characteristics shown in  FIG. 5 . 
     A characteristic curve  200  indicated by the broken line and a characteristic curve  202  indicated by the solid line in  FIG. 6  are identical to the characteristic curves  100 ,  102  shown in  FIG. 5 . A characteristic curve  204  indicated by the dot-and-dash line in  FIG. 6  is identical to the characteristic curve  104  (for reference only) indicated by the dot-and-dash line in  FIG. 5 . In the comparative example, the characteristic curve  204  is actually used for the B range. Stated otherwise, the characteristic curve  106  for the B range indicated by the solid line in  FIG. 5  is not used in the comparative example. 
     According to the comparative example shown in  FIG. 6 , as can be seen from the above description, when the vehicle  10  is driving in the D range, both the regenerative basic drive force Freg_base (accelerator regenerative control process) and the downhill regenerative correction quantity ΔF 1  (downhill regenerative control process) are used, and when the vehicle  10  is driving in the B range, only the regenerative basic drive force Freg_base (accelerator regenerative control process) is used. 
     When the road surface gradient G becomes smaller than the value G 2 , i.e., when the absolute value of the road surface gradient G becomes greater than that of the value G 2 , the characteristic curve  202  for the D range and the characteristic curve  204  for the B range agree with each other in an elliptical region  206  indicated by the broken line. Therefore, even when the range is switched from the D range to the B range while the vehicle  10  is driving on the downhill road whose road surface gradient G is smaller than the value G 2 , i.e., on a downhill road steeper than the road having the value G 2 , the target acceleration a_tar remains unchanged. As a result, the driver tends to feel strange and uncomfortable. 
     In addition, when the range is switched from the D range to the B range, it is considered that the driver is seeking a greater deceleration. According to the comparative example shown in  FIG. 6 , however, even when the driver changes from the D range to the B range, since the target acceleration a_tar remains unchanged, the deceleration of the vehicle  10  does not increase. As a result, the driver is possibly forced to depress the brake pedal  22 , thereby tending to impair the driving performance of the vehicle  10 . 
     In contrast thereto, according to the present embodiment, the above problems can be solved because the characteristic curves  102 ,  106  shown in  FIG. 5  are used. 
     (2-2-4. Acceleration Increasing Control Process) 
     (2-2-4-1. Problems) 
     As described above, the downhill regenerative control process according to the present embodiment uses the characteristic curves  102 ,  106  shown in  FIG. 5 . Therefore, during the downhill regenerative control process, when the range is switched from the D range to the B range, the target acceleration a_tar decreases, and when the range is switched from the B range to the D range, the target acceleration a_tar increases. However, even when the range is switched from the B range to the D range, if the downhill regenerative control process continues, i.e., if the characteristic curve  102  shown in  FIG. 5  is used, then the target acceleration a_tar is lower than if only the accelerator regenerative control process is carried out, i.e., if the characteristic curve  100  shown in  FIG. 5  is used. 
       FIG. 7  is a timing chart illustrating problems caused by continuing the downhill regenerative control process at the time the range is switched from the B range to the D range. More specifically,  FIG. 7  is a timing chart of the vehicle speed V and the acceleration a of each of an electric vehicle (hereinafter referred to as “comparative vehicle”) according to a comparative example which performs the accelerator regenerative control process and the downhill regenerative control process and a general gasoline-powered vehicle whose performance is equivalent to the comparative vehicle, and the target drive force Ftar for the motor  12  of the comparative vehicle. It is assumed that the acceleration a of the comparative vehicle is equal to the target acceleration a_tar.  FIG. 7  does not show any corresponding data of the general gasoline-powered vehicle in the field “TARGET DRIVE FORCE Ftar FOR MOTOR”. 
     From time t 11  to time t 14  in  FIG. 7 , each broken-line curve shows by way of example data of the comparative vehicle which performs the accelerator regenerative control process and the downhill regenerative control process. From time t 13  to time t 14  in  FIG. 7 , dot-and-dash-line curves representing the vehicle speed V and the acceleration a show by way of example data of the general gasoline-powered vehicle. It should be noted that from time t 11  to time t 13 , no data of the general gasoline-powered vehicle are shown in  FIG. 7 . 
     From time t 11  to time t 14  in  FIG. 7 , the road is downhill, and the road surface gradient G between time t 12  and time t 13  is smaller (i.e., greater in absolute value) and steeper than between time t 11  and time t 12  and between time t 13  and time t 14 . Therefore, the D range is selected between time t 11  and time t 12  and between time t 13  and time t 14 , and the B range is selected between time t 12  and time t 13 . 
     The general gasoline-powered vehicle can achieve a deceleration corresponding to the deceleration according to the accelerator regenerative control process because the gasoline-powered vehicle is capable of performing engine-braking, but does not perform a function or a control process corresponding to the downhill regenerative control process. The comparative vehicle performs both the accelerator regenerative control process and the downhill regenerative control process. Thus, whereas the general gasoline-powered vehicle does not perform a function or a control process corresponding to the downhill regenerative control process, the comparative vehicle performs both the accelerator regenerative control process and the downhill regenerative control process. Consequently, from time t 13  to time t 14 , the comparative vehicle has a smaller acceleration a than the general gasoline-powered vehicle. Thus, even when the driver of the comparative vehicle switches from the B range to the D range, the driver may possibly feel strange and uncomfortable because the acceleration of the electric vehicle does not increase sufficiently and the driver fails to have an acceleration feel as expected. 
     In view of the above problems, when the range is switched from the B range to the D range, the electric vehicle  10  according to the present embodiment carries out an acceleration increasing control process for increasing the target acceleration a_tar temporarily and thereafter carries out an acceleration restoring control process for restoring the target acceleration a_tar to its normal value (the value represented by the characteristic curve  102  shown in  FIG. 5 ). 
     (2-2-4-2. Overview of Acceleration Increasing Control Process and Acceleration Restoring Control Process) 
       FIG. 8  is a timing chart illustrating the acceleration increasing control process and the acceleration restoring control process according to the present embodiment. More specifically,  FIG. 8  is a timing chart of the vehicle speed V and the acceleration a of each of the comparative vehicle (the electric vehicle according to the comparative example which performs the accelerator regenerative control process and the downhill regenerative control process), the general gasoline-powered vehicle (whose performance is equivalent to the comparative vehicle), and the electric vehicle  10  according to the present embodiment, and the target drive force Ftar for the motor  12  of each of the comparative vehicle and the electric vehicle  10  according to the present embodiment. It is assumed that the acceleration a of the comparative vehicle and the electric vehicle  10  is equal to the target acceleration a_tar. 
     In  FIG. 8 , each broken-line-curve represents by way of example data of the comparative example, each dot-and-dash-line curve represents by way of example data of the general gasoline-powered vehicle, and each solid-line curve represents by way of example data of the electric vehicle  10  according to the present embodiment.  FIG. 8  does not show any corresponding data of the general gasoline-powered vehicle in the field “TARGET DRIVE FORCE Ftar FOR MOTOR”. 
     From time t 21  to time t 27  in  FIG. 8 , the road is downhill, and the road surface gradient G between time t 22  and time t 23  is smaller (i.e., greater in absolute value) and steeper than between time t 21  and time t 22  and between time t 23  and time t 27 . Therefore, the D range is selected between time t 21  and time t 22  and between time t 23  and time t 27 , and the B range is selected (a second range corresponding to the B range is selected in the general gasoline-powered vehicle) between time t 22  and time t 23 . 
     In  FIG. 8 , the vehicle speed V of the general gasoline-powered vehicle is higher than the vehicle speed V of the comparative vehicle and the electric vehicle  10 . This is because the general gasoline-powered vehicle is capable of performing engine-braking, but does not perform a function or a control process corresponding to the downhill regenerative control process. 
     From time t 21  to time t 23 , the vehicle speeds V, the accelerations a, and the target drive forces Ftar for the motor  12  of the comparative vehicle and the electric vehicle  10  are identical to each other. From time t 23 , at which the range is switched from the B range to the D range, to time t 26 , the vehicle speed V, the acceleration a, and the target drive force Ftar for the motor  12  of the electric vehicle  10  are greater than those of the comparative vehicle. Stated otherwise, the vehicle speed V, the acceleration a, and the target drive force Ftar for the motor  12  of the electric vehicle  10  increase in the same manner as with the general gasoline-powered vehicle. This is because the electric vehicle  10  according to the present embodiment performs the acceleration increasing control process. 
     As described above, when the range is switched from the B range to the D range, the electric vehicle  10  increases the vehicle speed V, the acceleration a, and the target drive force Ftar for the motor  12 , whereby the vehicle  10  makes it possible to accelerate in the same manner as with the general gasoline-powered vehicle. 
     From time t 25  to time t 26 , the temporary increase in the target acceleration a_tar is gradually reduced (acceleration restoring control process). At time t 26 , the vehicle speeds V, the accelerations a, and the target drive forces Ftar for the motor  12  of the comparative vehicle and the electric vehicle  10  become identical to each other, and thereafter continue to be identical to each other. 
     (2-2-5. Details of Calculation of the Downhill Regenerative Correction Quantity ΔF 1  (Downhill Regenerative Control Process)) 
     Details of the calculation of a downhill regenerative correction quantity ΔF 1  ( FIG. 3 ) (downhill regenerative control process) in step S 3  in  FIG. 2  will be described below.  FIG. 9  is a flowchart of a process of calculating a downhill regenerative correction quantity ΔF 1  (downhill regenerative control process) (details of S 3  in  FIG. 2 ) according to the present embodiment. 
     In step S 11 , the ECU  34  acquires the actual acceleration a of the vehicle  10  from the acceleration sensor  32 . In step S 12 , the ECU  34  calculates a_target acceleration a_tar (see  FIG. 5 ). Details of the calculation of a target acceleration a_tar will be described later with reference to  FIG. 10 . 
     In step S 13 , the ECU  34  calculates a downhill regenerative correction quantity ΔF 1 . Specifically, the ECU calculates the difference (hereinafter referred to as “difference D 1 ”) between the actual acceleration a and the target acceleration a_tar, and then calculates a downhill regenerative correction quantity ΔF 1  according to a feedback control process depending on the difference D 1 . Stated otherwise, as the absolute value of the difference D 1  is greater, the absolute value of the downhill regenerative correction quantity ΔF 1  is greater. 
       FIG. 10  is a flowchart of a process of calculating a target acceleration a_tar (details of S 12  in  FIG. 9 ) according to the present embodiment. In step S 21 , the ECU  34  estimates a road surface gradient G. Specifically, the ECU  34  calculates a road surface gradient G using the acceleration a from the acceleration sensor  32 , etc. For example, the ECU  34  may estimate a road surface gradient G according to the process disclosed in US 2009/0112386 A1 (see  FIG. 5 , paragraphs [0076], [0077], [0090] through [0110] of US 2009/0112386 A1). Alternatively, the ECU  34  may estimate a road surface gradient G according to other existing processes. 
     In step S 22 , the ECU  34  judges whether the shift range is the B range or not. If the shift range is the B range (S 22 : YES), then, in step S 23 , the ECU  34  selects the map for the B range, i.e., the characteristic curve  106  in  FIG. 5 . As described above, the map, i.e., the characteristic curve  106 , is selected depending on the vehicle speed V. Therefore, the ECU  34  also uses the vehicle speed V from the vehicle speed sensor  30  in selecting a map. 
     In step S 24 , the ECU  34  selects a target acceleration a_tar corresponding to the road surface gradient G estimated in step S 21  according to the map selected in step S 23 , i.e., the characteristic curve  106  in  FIG. 5 . 
     In step S 25 , the ECU  34  resets a timer  36  (see  FIG. 1 ) of the ECU  34  which is used for the acceleration increasing control process and the acceleration restoring control process.  FIG. 11  shows by way of example how the timer  36  operates. As can be seen from  FIG. 11 , the timer  36 , which is a count-down timer, is kept at an initial value when the B range is selected from time t 32  to time t 33 . When the D range is selected, the timer  36  counts down from time t 33  to time t 34 . Therefore, using the timer  36 , it is possible to measure a prescribed period that has elapsed after switching from the B range to the D range. 
     In step S 22  of  FIG. 10 , if the shift range is not the B range (S 22 : NO), i.e., if the shift range is the D range, then, in step S 26 , the ECU  34  selects the map for the D range, i.e., the characteristic curve  102  in  FIG. 5 . As described above, the map, i.e., the characteristic curve  102 , is selected depending on the vehicle speed V. Therefore, the ECU  34  also uses the vehicle speed V from the vehicle speed sensor  30  in selecting the map. 
     In step S 27 , the ECU  34  selects a target acceleration a_tar corresponding to the road surface gradient G estimated in step S 21  according to the map selected in step S 26 , i.e., the characteristic curve  102  in  FIG. 5 . 
     In step S 28 , the ECU  34  judges whether the count value of the timer  36  has reached zero or not, or stated otherwise, judges whether or not the prescribed period has elapsed after switching from the B range to the D range. If the prescribed period has not elapsed (S 28 : NO), then, in step S 29 , the ECU  34  determines the sum (hereinafter referred to as “target acceleration a_tar (new)”) of the target acceleration a_tar selected in step S 27  (hereinafter referred to as “target acceleration a_tar (old)”) and an acceleration additional value a_add, as a final target acceleration a_tar in the present processing cycle. However, immediately after the range has been switched from the B range to the D range, the ECU  34  gradually increases the target acceleration a_tar as with the acceleration a (=target acceleration a_tar) from time t 23  to time t 24  in  FIG. 8 . After step S 29 , the present processing cycle is ended. Then, a next processing cycle is started from step S 21 . 
     If the prescribed period has elapsed (S 28 : YES), then the present processing cycle is ended. Then, a next processing cycle is started from step S 21 . However, immediately after the prescribed period has elapsed, the ECU  34  gradually decreases the target acceleration a_tar as with the acceleration a (=target acceleration a_tar) from time t 25  to time t 26  in  FIG. 8 . The absolute value of the rate (gradient) at which the target acceleration a_tar is decreased at this time is smaller than the absolute value of the rate (gradient) at which the target acceleration a_tar is increased from time t 23  to time t 24 , i.e., the gradient is lower. 
     3. Advantages of the Present Embodiment 
     According to the present embodiment, as described above, when the road surface gradient G is equal to or smaller than 0%, at the same road surface gradient G, the target acceleration a_tar, i.e., the characteristic curve  106  in  FIG. 5 , for the B range is lower than the target acceleration a_tar, i.e., the characteristic curve  102  in  FIG. 5 , for the D range (see  FIG. 5 ). Stated otherwise, when the road surface gradient G is 0% or less, at the same road surface gradient G, the deceleration for the B range is greater than the deceleration for the D range. Therefore, when the shift range is switched from the D range to the B range, the deceleration of the vehicle  10  is increased irrespectively of whether the vehicle  10  is driving on a flat road or a downhill road. The vehicle  10  thus makes the driver feel decelerated, and prevents the driver from having a feeling of irresponsiveness to the driver&#39;s operation. Further, when the shift range is switched from the B range to the D range, the vehicle  10  makes the driver feel accelerated irrespectively of whether the vehicle  10  is driving on a flat road or a downhill road. Thus, the vehicle  10  prevents the driver from feeling strange and uncomfortable due to the unchanged acceleration a. Consequently, it is possible to perform an appropriate regenerative control process for the vehicle  10  which has a plurality of forward driving ranges, i.e., the D range and the B range. 
     According to the present embodiment, the difference Δa 2  between the characteristic curve  102  and the characteristic curve  106  in the region wherein the road surface gradient G changes from 0% to the value G 1 %, i.e., the region wherein the downhill regenerative control process is inhibited, and the difference Δa 3  between the characteristic curve  102  and the characteristic curve  106  in the region wherein the road surface gradient G is lower than the value G 1 , i.e., the region wherein the downhill regenerative control process is permitted, are constant and equal to each other. 
     With the above arrangement, the driver is able to recognize a change in the acceleration (deceleration) similarly when the D range and the B range are switched while the vehicle  10  is in the downhill regenerative control process and when the D range and the B range are switched while the vehicle  10  is in the accelerator regenerative control process on a flat road. Therefore, the electric vehicle  10  can prevent the driver from having a feeling of irresponsiveness to the driver&#39;s operation or can make the driver feel more accelerated. 
     According to the present embodiment, when the range is switched from the B range to the D range while the vehicle  10  is driving on a downhill road, the sum of the normal target acceleration a_tar, i.e., the characteristic curve  102  in  FIG. 5 , for the D range and the acceleration additional value a_add is temporarily used as an actual target acceleration a_tar (see  FIG. 8  and S 29  in  FIG. 10 ). Therefore, when the range is switched from the B range to the D range, the acceleration a of the electric vehicle  10  becomes greater than normal even if the downhill regenerative control process is performed in the D range. Consequently, it is possible to achieve compatibility between an acceleration feel which the driver expects to have by switching from the B range to the D range and the downhill regenerative control process that adjusts the target acceleration a_tar depending on the road surface gradient G. 
     According to the present embodiment, after the acceleration increasing control process that adds the acceleration additional value a_add, the acceleration restoring control process that restores the target acceleration a_tar to the normal characteristic curve  102  for the D range. Thus, when the range is switched from the B range to the D range, it is possible to restore the acceleration a to the normal acceleration a (target acceleration a_tar) after having temporarily increased the acceleration a (target acceleration a_tar). Therefore, immediately after switching from the B range to the D range, it is possible to give the driver an acceleration feel as expected and thereafter prevent the electric vehicle  10  from being excessively accelerated using the normal target acceleration a_tar. 
     According to the present embodiment, the absolute value of the rate (gradient) at which the target acceleration a_tar is increased from time t 23  to time t 24  in  FIG. 8  is greater than the absolute value of the rate (gradient) at which the target acceleration a_tar is decreased from time t 25  to time t 26 . Therefore, when the acceleration increasing control process is performed, the acceleration a can be sharply increased to accelerate the electric vehicle  10  quickly. On the other hand, when the acceleration restoring control process is performed, the target acceleration a_tar is gradually decreased and made equal to the normal target acceleration a_tar, i.e., the characteristic curve  102  in  FIG. 5 , thereby changing to the normal downhill regenerative control process without making the driver feel strange and uncomfortable. 
     B. Modifications 
     The present invention is not limited to the above embodiments, but may employ various arrangements based on the disclosure of the above description. For example, the present invention may employ the following arrangements. 
     1. Objects to which the Present Invention is Applicable 
     In the above embodiment, the vehicle  10  which has the single motor  12  and the single battery  16  has been described (see  FIG. 1 ). However, the present invention is not limited to the illustrated vehicle  10 , but is applicable to other objects. For example, the present invention is applicable to an electric vehicle having a propulsive motor and a regenerative motor separately. The present invention is also applicable to a hybrid vehicle having an engine in addition to the motor  12 . Furthermore, the present invention is also applicable to a fuel cell vehicle having a fuel cell in addition to the motor  12  and the battery  16 . Still further, the present invention is applicable not only to the electric vehicle  10  but also to another mobile body or a movable apparatus, e.g., a robot arm, having a plurality of forward driving ranges (driving ranges in one direction). The vehicle  10  is not limited to a four-wheeled vehicle, but may be a truck, a motorcycle, an electric-motor-assisted bicycle, and the like. 
     2. Shift Ranges 
     In the above embodiment, the shift ranges include the P range, the N range, the R range, the D range, and the B range. However, the shift ranges are not limited to these ranges, but may include other ranges insofar as they have a plurality of shift ranges for driving in one direction (the D range and the B range in the above embodiment). 
     In the above embodiment, the driver switches to each of the shift ranges with the shift lever  26 . However, the driver may switch to each of the shift ranges, particularly at least one of the D range and the B range, using another switching device. For example, the driver may switch between the D range and the B range, for example, using a button (not shown) on the steering wheel, in addition to or instead of the shift lever  26 . 
     3. Accelerator Regenerative Control Process, Downhill Regenerative Control Process, Acceleration Increasing Control Process, and Acceleration Restoring Control Process 
     In the above embodiment, both the accelerator regenerative control process and the downhill regenerative control process have been described as being carried out for each of the D range and the B range. However, insofar as the accelerator regenerative control process and the downhill regenerative control process are carried out in effect, these control processes do not need to be formally distinguished from each other. For example, the accelerator regenerative control process may be included in the downhill regenerative control process. Stated otherwise, a regenerative control process that is performed while the vehicle is driving on a downhill road may be collectively called a downhill regenerative control process, and the accelerator regenerative control process may be included in the downhill regenerative control process. 
     In the above embodiment, the downhill regenerative control process is carried out for each of the D range and the B range (the characteristic curves  102 ,  106  in  FIG. 5 ). However, the present invention is not limited to the downhill regenerative control process thus carried out in terms of the acceleration increasing control process and the acceleration restoring control process. For example, the present invention is also applicable to an arrangement where the downhill regenerative control process is carried out only for the D range and only the accelerator regenerative control process is carried out for the B range. More specifically, the acceleration increasing control process and the acceleration restoring control process may be carried out, for example, in the characteristic curves  202 ,  204  of the comparative example shown in  FIG. 6 . Alternatively, the acceleration increasing control process and the acceleration restoring control process may be carried out in a combination of the characteristic curve  202  of the comparative example shown in  FIG. 6  and a characteristic curve that is produced by shifting the characteristic curve  204  downwardly in its entirety in  FIG. 6 . 
     In the above embodiment, the acceleration additional value a_add is used in the acceleration increasing control process. Stated otherwise, the acceleration increasing control process has been described as being independent from the downhill regenerative control process and the accelerator regenerative control process. However, the acceleration increasing control process may be carried out by at least partly stopping or limiting at least one of the downhill regenerative control process and the accelerator regenerative control process, instead of using the acceleration additional value a_add. For example, it is possible to use the characteristic curve  100  instead of the characteristic curve  102  in  FIG. 5  for the acceleration increasing control process performed at the time of switching from the B range to the D range. 
     In the above embodiment, after the acceleration increasing control process is temporarily carried out, the acceleration restoring control process is carried out. However, the acceleration increasing control process may be continuously carried out without the acceleration restoring control process. For example, after the range has been switched from the B range to the D range, the acceleration increasing control process may be continued as long as the D range goes on.