Patent Publication Number: US-11639694-B2

Title: Vehicle control system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a system for controlling a vehicle including a drive source which generates motive power for traveling and an accelerator pedal to be operated by a driver. 
     BACKGROUND ART 
     Conventionally, a technique has been known which controls a drive source of a vehicle in accordance with an operation of an accelerator pedal. For example, JP2016-176388A discloses a control apparatus including an accelerator opening sensor which detects an opening of an accelerator pedal (accelerator opening), a target acceleration setting unit which sets a target acceleration of a vehicle based on the detected accelerator opening, and an engine controller which controls an engine such that the set target acceleration is realized. 
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     In JP2016-176388A, although a target acceleration is set to increase as an accelerator opening becomes wider, in a range where the accelerator opening is approximately intermediate or wider, a slope of a change in the target acceleration with respect to a change in the accelerator opening (operation gain) tends to decrease toward a wider opening side. This tendency provides natural characteristics in consideration of an output limit of a drive source (engine) but might become a factor that gives discomfort to a driver depending on circumstances. For example, in a circumstance where the driver increases depression of an accelerator pedal, even when the operation gain lowers in a wide opening range along the above-described characteristics of the target acceleration, such lowering of the operation gain does not lead to particular discomfort for the driver who feels the output limit of the drive source. However, in a case where the target acceleration is changed along similar characteristics when the accelerator pedal is subsequently returned, lowering of the operation gain might give discomfort to the driver. That is, the possibility becomes high that although the accelerator pedal is returned, the change (lowering) in the acceleration is maintained at such a low level that the driver has difficulty in perceiving the change. This might cause the driver to feel discomfort (for example, a sense that a vehicle is spontaneously accelerating). 
     The present disclosure has been made in consideration of the above circumstances, and an object thereof is to provide a vehicle control system that is capable of causing a driver to easily perceive a change in an acceleration of a vehicle, the change corresponding to returning of an accelerator pedal, and of thereby improving operability of the vehicle by the accelerator pedal. 
     Means for Solving the Problems 
     To solve the above problems, the present disclosure provides a system for controlling a vehicle including a drive source which generates motive power for traveling and an accelerator pedal to be operated by a driver, the system including an accelerator sensor which detects an accelerator opening as an opening of the accelerator pedal, and a processor configured to execute a target acceleration setting unit which sets a target acceleration of the vehicle based on the accelerator opening detected by the accelerator sensor, a target torque setting unit which sets a target torque of the drive source based on the target acceleration set by the target acceleration setting unit, and a drive source controller which controls the drive source to generate the target torque set by the target torque setting unit. In a case where the target acceleration at a time when the accelerator opening is increased is set as a depression-increasing target acceleration and the target acceleration at a time when the accelerator opening is decreased is set as a pedal-returning target acceleration, under the same condition of the accelerator opening, the target acceleration setting unit sets the target acceleration such that the pedal-returning target acceleration becomes lower than the depression-increasing target acceleration. 
     According to the present disclosure, under the same condition of the accelerator opening, the pedal-returning target acceleration as the target acceleration of the vehicle at a time when the accelerator opening is decreased is set lower than the depression-increasing target acceleration as the target acceleration of the vehicle at a time when the accelerator opening is increased, and an output torque of the drive source is controlled based on the target acceleration having such hysteresis characteristics. Consequently, even in a case where the target acceleration is set such that the operation gain (a slope of a change in the acceleration with respect to a change in the accelerator opening) of the accelerator pedal decreases toward a wider opening side, it becomes easy to cause the driver to perceive the change in the acceleration of the vehicle at a time when the accelerator pedal is returned, and operability of the vehicle by the accelerator pedal can be improved. 
     In a case where a change rate of an acceleration is set as a jerk and a target value of the jerk is set as a target jerk, the target acceleration setting unit preferably calculates the target jerk from the change rate of the accelerator opening and preferably sets the target acceleration based on an integrated value resulting from integration of the calculated target jerk. 
     As described above, in a case where the target jerk is calculated from the change rate of the accelerator opening, the change rate being an index of strength of an acceleration intention of the driver, and the target acceleration is set based on the integrated value resulting from integration of the calculated target jerk, while the above-described hysteresis characteristics (characteristics in which the pedal-returning target acceleration becomes lower than the depression-increasing target acceleration) are appropriately given, the vehicle can be accelerated in a manner more accurate for an intention of the driver. 
     In the above configuration, the target acceleration setting unit, further preferably, sets an upper limit acceleration based on the accelerator opening and sets a lower limit acceleration which becomes lower than the upper limit acceleration only at a specific ratio, sets the integrated value as the target acceleration in a case where the integrated value of the target jerk is less than the upper limit acceleration and greater than the lower limit acceleration, sets the upper limit acceleration as the target acceleration in a case where the integrated value of the target jerk is the upper limit acceleration or more, and sets the lower limit acceleration as the target acceleration in a case where the integrated value of the target jerk is the lower limit acceleration or less. 
     With this configuration, because a difference between the depression-increasing target acceleration and the pedal-returning target acceleration is adequately restricted, the accelerator opening to obtain the same acceleration can be prevented from being largely different between a depression-increasing situation and a pedal-returning situation. Accordingly, while responsiveness (a sense of a change in the acceleration) experienced by the driver when depression of the accelerator pedal is increased and when the accelerator pedal is returned is optimized, discomfort given to the driver due to differences in acceleration characteristics between the depression-increasing situation and the pedal-returning situation can be reduced. 
     In the above configuration, the target acceleration setting unit, further preferably, makes the target jerk closer to zero as the most recently calculated target acceleration becomes closer to the lower limit acceleration when the accelerator opening is decreased and makes the target jerk closer to zero as the most recently calculated target acceleration becomes closer to the upper limit acceleration when the accelerator opening is increased. 
     With this configuration, the operation gain of the accelerator pedal can be prevented from suddenly changing when the target acceleration is increased to the upper limit acceleration due to an increase in depression of the accelerator pedal or when the target acceleration is decreased to the lower limit acceleration due to returning of the accelerator pedal, and riding comfort of the vehicle can suitably be secured. 
     The target acceleration setting unit preferably sets the target jerk to zero when an absolute value of the change rate of the accelerator opening is in a predetermined range including zero. 
     With this configuration, a situation can be avoided where the target acceleration changes even in a case where the accelerator opening unintentionally and minutely fluctuates due to vibration or the like of the vehicle, and the driver can be prevented from feeling discomfort due to an unintended behavior change of the vehicle. 
     Advantageous Effects of Invention 
     As described above, a vehicle control system of the present disclosure can cause a driver to easily perceive a change in an acceleration of a vehicle, the change corresponding to returning of an accelerator pedal, and can thereby improve operability of the vehicle by the accelerator pedal. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram schematically illustrating a specific example of a vehicle to which a vehicle control system of the present disclosure is applied. 
         FIG.  2    is a diagram illustrating an outline configuration of an engine which is installed in the above vehicle. 
         FIG.  3    is a function block diagram illustrating a control system of the above vehicle or the engine. 
         FIG.  4    is a flowchart illustrating contents of basic control which is executed during traveling of the vehicle. 
         FIG.  5    represents a subroutine illustrating details of control in step S 2  in  FIG.  4   . 
         FIG.  6    represents a subroutine illustrating details of control in step S 12  in  FIG.  5   . 
         FIG.  7    represents a subroutine illustrating details of control in step S 13  in  FIG.  5   . 
         FIG.  8    represents a subroutine illustrating details of control in step S 14  in  FIG.  5   . 
         FIGS.  9 A and  9 B  are diagrams illustrating acceleration characteristic maps which define the relationship between an accelerator opening and a target acceleration for a vehicle speed and each gear stage. 
         FIG.  10    is a graph illustrating the relationship between the accelerator opening and the target acceleration at specific vehicle speed and gear stage. 
         FIG.  11    is a diagram for explaining a setting method of a lower limit acceleration in accordance with a vehicle speed, a gear stage, and a road surface gradient. 
         FIG.  12    is a diagram corresponding to  FIG.  10   , which illustrates one example of the set lower limit acceleration. 
         FIG.  13    is a graph illustrating the relationship between an accelerator opening change rate and a basic jerk. 
         FIG.  14    is a graph illustrating a first correction coefficient which is used for calculation of a target jerk in the relationship with the accelerator opening change rate. 
         FIG.  15    is a diagram illustrating a change in the target acceleration in a case where a driver once increases depression of an accelerator pedal and thereafter returns the accelerator pedal. 
         FIG.  16    is a diagram illustrating the change in the target acceleration in a case where the target acceleration increases to an upper limit acceleration when depression of the accelerator pedal is increased. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     (1) General Configuration of System 
       FIG.  1    is a diagram for explaining a preferable embodiment of the present disclosure and is a diagram schematically illustrating a specific example of a vehicle to which a vehicle control system of the present disclosure is applied. As illustrated in  FIG.  1   , the vehicle includes an engine  1  installed in an engine room ER, a transmission  101  coupled with a crankshaft  20  as an output shaft of the engine  1 , a pair of drive shafts  102  coupled with the transmission  101 , and a pair of wheels  103  mounted on end portions of the respective drive shafts  102  on vehicle-width-direction outer sides. Rotation (output rotation) of the crankshaft  20  of the engine is transmitted to the drive shafts  102  and the wheels  103  while a speed of the rotation is changed by the transmission  101 . That is, the engine  1  installed in the vehicle of the present embodiment is a generation source of motive power (motive power source) for traveling of the vehicle and rotates and drives the wheels  103  via the transmission  101  and the drive shafts  102 . 
     A gear mechanism  101 A is built in the transmission  101 . The gear mechanism  101 A is a mechanism which is capable of achieving a plurality of gear stages (for example, first speed to sixth speed) whose transmission gear ratios are different and couples the crankshaft  20  (output shaft) of the engine  1  and the pair of drive shafts  102  with each other such that they move in an inter-connected manner. Output rotation of the engine  1  is transmitted to the wheels  103  while a speed of the output rotation is changed at the transmission gear ratio corresponding to the gear stage achieved by the gear mechanism  101 A of the transmission  101 . Here, the transmission  101  is a manual transmission (MT) which changes the gear stages while receiving a manual operation by the driver. However, an automatic transmission (AT) which automatically changes gear stages in accordance with a driving condition of the vehicle or the engine  1  may be used as the transmission  101 . 
       FIG.  2    is a system diagram illustrating an outline configuration of the engine  1 . The engine  1  is a four-cycle gasoline engine and includes an engine body  10  which combusts an air-fuel mixture of a fuel (gasoline) and air, an intake passage  30  through which air (intake air) introduced into the engine body  10  flows, and an exhaust passage  40  through which exhaust gas exhausted from the engine body  10  flows. 
     The engine body  10  includes a housing (a cylinder block, a cylinder head, and so forth) in an internal portion of which a plurality of cylinders  11  illustrated in  FIG.  1    are formed and pistons  21  which are housed in the respective cylinders  11  to be capable of reciprocating motions. Below the piston  21 , the above-described crankshaft  20  is disposed. The piston  21  and the crankshaft  20  are coupled together via a connecting rod or the like such that the crankshaft  20  rotates in response to reciprocating motions of the piston  21 . In a lower portion (cylinder block) of the engine body  10 , a crank angle sensor SN 1  is provided which detects an angle of the crankshaft  20  (crank angle) and a rotational speed of the crankshaft  20  (engine speed). 
     Above the pistons  21  of the cylinders  11 , respective combustion chambers  12  are demarcated. In each of the combustion chambers  12 , an intake port  13  and an exhaust port  14  open. In an upper portion (cylinder head) of the engine body  10 , a combination of an injector  15 , a spark plug  16 , an intake valve  17 , and an exhaust valve  18  is provided to each of the cylinders  11 . The injector  15  is an injection valve which injects the fuel (gasoline) into the combustion chamber  12 . The spark plug  16  is a plug which ignites the air-fuel mixture in which the injected fuel and air are mixed together. The intake valve  17  is a valve which opens and closes the intake port  13 . The exhaust valve  18  is a valve which opens and closes the exhaust port  14 . In the upper portion of the engine body  10 , a valve mechanism  19  is provided which drives and opens and closes the intake valves  17  and the exhaust valves  18  of the respective cylinders  11  in response to rotation of the crankshaft  20 . 
     The intake passage  30  is connected with one side surface of the engine body  10  so as to communicate with the intake port  13  of each of the cylinders  11 . The intake passage  30  is provided with an air cleaner  31  which removes foreign objects in intake air, a throttle valve  32  which adjusts a flow amount of intake air and is capable of opening and closing, and a surge tank  33  in this order from an upstream side (a far side from the engine body  10 ). In a section between the air cleaner  31  and the throttle valve  32  in the intake passage  30 , an airflow sensor SN 2  is provided which detects the flow amount of intake air. 
     The exhaust passage  40  is connected with another side surface of the engine body  10  so as to communicate with the exhaust port  14  of each of the cylinders  11 . In the exhaust passage  40 , a plurality of catalysts  41  are provided which purify harmful components in exhaust gas. 
       FIG.  3    is a function block diagram illustrating a control system of the vehicle or the engine  1 . As illustrated in this  FIG.  3    and above  FIG.  1    and  FIG.  2   , the vehicle is provided with an accelerator pedal  60  which is operated by the driver driving the vehicle and an engine control unit (ECU)  50  which controls an output of the engine  1  in accordance with the operation of the accelerator pedal  60  by the driver. Further, the vehicle is provided with an accelerator sensor SN 3  which detects an opening of the accelerator pedal  60  (hereinafter, referred to as accelerator opening), a vehicle speed sensor SN 4  which detects a traveling speed of the vehicle (hereinafter, referred to as vehicle speed), and a gradient sensor SN 5  which detects a gradient of a traveling road (hereinafter, referred to as road surface gradient) on which the vehicle travels. Note that the gradient sensor SN 5  may be a sensor of a type which detects an inclination degree of the vehicle and thereby directly specifies the road surface gradient or may be a sensor of a type which detects an acceleration or the like of the vehicle and thereby indirectly specifies the road surface gradient from an estimation based on the detection results. 
     The ECU  50  is configured with a microcomputer which includes a processor (e.g., a central processing unit (CPU))  55  performing computation, memory  56  such as ROM and RAM, and various kinds of input-output buses. Detection information by various kinds of sensors is input to the ECU  50 . For example, the ECU  50  is electrically connected with the above-described crank angle sensor SN 1 , airflow sensor SN 2 , accelerator sensor SN 3 , vehicle speed sensor SN 4 , and gradient sensor SN 5 , and various kinds of information detected by those sensors, that is, each piece of information such as the crank angle, the engine speed, an intake air flow amount, the accelerator opening, the vehicle speed, and the road surface gradient is sequentially input to the ECU  50 . 
     The ECU  50  controls actuators of the engine while executing various determinations, computation, and so forth based on input information from the above sensors (SN 1  to SN 5  and so forth). For example, the ECU  50  is electrically connected with plural actuators including the injectors  15 , the spark plugs  16 , and the throttle valves  32  and appropriately outputs control signals based on the above determinations, computation, and so forth to those actuators. 
     The ECU  50  comprises a target acceleration setting unit  51 , a target torque setting unit  52 , an engine controller  53 , and a gear stage estimation unit  54  that are executed by the processor  55  to perform their respective functions. These units are stored in the memory  26  as software modules. The target acceleration setting unit  51  is a control module which sets a target acceleration of the vehicle based on various kinds of information including the accelerator opening detected by the accelerator sensor SN 3 . The target torque setting unit  52  is a control module which sets a target torque (a target value of a rotational torque of the crankshaft  20 ) of the engine  1  based on the target acceleration set by the target acceleration setting unit  51 . The engine controller  53  is a control module which controls the engine  1  to generate the target torque set by the target torque setting unit  52 . The gear stage estimation unit  54  is a control module which estimates the gear stage of the transmission  101  from the relationship between the vehicle speed detected by the vehicle speed sensor SN 4  and the engine speed detected by the crank angle sensor SN 1 . Note that the engine controller  53  is an example of a “drive source controller” in the present disclosure. 
     (2) Basic Control 
     Next, a description will be made about basic control executed by the ECU  50  during traveling of the vehicle with reference to the flowchart of  FIG.  4   . When the control illustrated in  FIG.  4    is started, the ECU  50  acquires various kinds of information which indicate a present state of the vehicle or the engine  1  (step S 1 ). For example, the ECU  50  acquires each of the crank angle and engine speed which are detected by the crank angle sensor SN 1 , the intake air flow amount detected by the airflow sensor SN 2 , the accelerator opening detected by the accelerator sensor SN 3 , the vehicle speed detected by the vehicle speed sensor SN 4 , the road surface gradient detected by the gradient sensor SN 5 , and the gear stage of the transmission  101  which is estimated by the gear stage estimation unit  54 . 
     Next, the target acceleration setting unit  51  of the ECU  50  sets a target acceleration Ac of the vehicle based on information such as the accelerator opening acquired in step S 1  (step S 2 ). Details of a setting method of this target acceleration Ac will be described in a section (3) described later. 
     Next, the target torque setting unit  52  of the ECU  50  sets a target torque Tr as an output torque of the engine  1  which is necessary for realizing the target acceleration Ac set in step S 2  (step S 3 ). Specifically, the target torque setting unit  52  sets the target torque Tr of the engine  1  based on the target acceleration Ac set in step S 2  and the vehicle speed acquired in step S 1 . The vehicle speed is taken into consideration for setting of the target torque Tr because a traveling resistance increases as the vehicle speed becomes higher. In other words, the target torque setting unit  52  estimates the present traveling resistance of the vehicle from information such as the vehicle speed acquired in step S 1 , calculates the output torque of the engine  1  which is necessary for accelerating the vehicle at the target acceleration Ac against the estimated traveling resistance, and thereby sets the calculated output torque as the above target torque Tr. 
     Next, the engine controller  53  of the ECU  50  sets, for the actuators of the engine  1 , control target values for realizing the target torque Tr set in step S 3  (step S 4 ). For example, the engine controller  53  sets respective target values of control amounts including an injection amount and an injection timing of the injector  15 , an ignition timing of the spark plug  16 , and an opening of the throttle valve  32  such that a combustion force corresponding to the above target torque Tr is generated in each of the cylinders  11  of the engine  1 . 
     Next, the engine controller  53  controls the actuators of the engine  1  in accordance with the control target values set in step S 4  (step S 5 ). For example, the engine controller  53  respectively controls the injector  15 , the spark plug  16 , and the throttle valve  32  such that the respective control amounts of the injector  15 , the spark plug  16 , and the throttle valve  32  agree with the control target values set in step S 4 . Accordingly, the output torque equivalent to the target torque Tr set in step S 3  is generated in the engine  1 . This output torque accelerates the vehicle at the acceleration equivalent to the target acceleration Ac set in step S 2 . 
     (3) Basic Flow of Target Acceleration Setting 
     Next, a detailed description will be made about control contents of step S 2  for setting the target acceleration Ac of the vehicle.  FIG.  5    represents a subroutine illustrating details of control in step S 2 . When the control illustrated in  FIG.  5    is started, the target acceleration setting unit  51  of the ECU  50  calculates an upper limit acceleration Amax of the vehicle based on the accelerator opening, the vehicle speed, and the gear stage which are acquired in step S 1  ( FIG.  4   ) (step S 11 ). The upper limit acceleration Amax is an upper limit value of the target acceleration Ac of the vehicle and is a value which can be employed when the driver performs an operation of increasing depression of the accelerator pedal  60  (of increasing the accelerator opening). Note that in the following, information such as the accelerator opening acquired in step S 1  may be rephrased as the present accelerator opening or the like; however, in any case, that means the newest information acquired in a currently progressing processing routine, and its meaning is the same. 
     The upper limit acceleration Amax is decided in accordance with an acceleration characteristic map which defines the relationship between the accelerator opening and the target acceleration (this will hereinafter be referred to as acceleration characteristics also) for the vehicle speed and each of the gear stages.  FIGS.  9 A and  9 B  are diagrams illustrating one example of this acceleration characteristic map.  FIGS.  9 A and  9 B  illustrate, as an example, the acceleration characteristic map which is set in a case where the transmission  101  is a transmission which has six forward stages, and the graphs of  FIGS.  9 A and  9 B  illustrate maps at vehicle speeds of V 1  and V 2 . In this example, the vehicle speed V 2  is greater than the vehicle speed V 1  (V 2 &gt;V 1 ). Six characteristic curves Q 1  to Q 6  in the graphs represent acceleration characteristics at different gear stages, and Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6  represent the acceleration characteristics at a first speed, a second speed, a third speed, a fourth speed, a fifth speed, and a sixth speed, respectively. In other words,  FIG.  9 A  represents the map in which the acceleration characteristics at a vehicle speed of V 1  are defined for each of the gear stages (the first speed to the sixth speed), and  FIG.  9 B  represents the map in which the acceleration characteristics at a vehicle speed of V 2  (&gt;V 1 ) are defined for each of the gear stages. The acceleration characteristics defined in both of the maps are set such that the target acceleration becomes higher as the accelerator opening is wider and are set such that the target acceleration becomes lower as the gear stage is higher under the same condition of the accelerator opening (except a case where the accelerator opening is very narrow). Those acceleration characteristic maps are stored in advance in a storage medium in the ECU  50  together with maps for various vehicle speeds other than V 1  and V 2 . 
     In step S 11 , the target acceleration setting unit  51  calculates the upper limit acceleration Amax by applying the accelerator opening, the vehicle speed, and the gear stage, which are acquired in step S 1 , to the acceleration characteristic maps illustrated in  FIGS.  9 A and  9 B . For example, in a case where the present vehicle speed is V 1  and the present gear stage is the third speed, the target acceleration setting unit  51  calculates, as the upper limit acceleration Amax, the value which is on a characteristic curve Q 3  in the map of  FIG.  9 A  and corresponds to the present accelerator opening. 
     Next, the target acceleration setting unit  51  calculates a lower limit acceleration Amin of the vehicle based on the accelerator opening, the vehicle speed, the gear stage, and the road surface gradient which are acquired in step S 1  (step S 12 ). The lower limit acceleration Amin is a lower limit value of the target acceleration Ac of the vehicle and is a value which can be employed when the driver performs an operation of returning the accelerator pedal  60  (of decreasing the accelerator opening). Details of a calculation method of this lower limit acceleration Amin will be described in a section (4) described later. 
     Next, the target acceleration setting unit  51  calculates a target jerk Je of the vehicle based on the accelerator opening, the vehicle speed, and the gear stage which are acquired in step S 1  (step S 13 ). In the present specification, a jerk denotes a change rate of an acceleration (a value of a time derivative of the acceleration), and the target jerk Je denotes a target value of the jerk. Details of a calculation method of this target jerk Je will be described in a section (5) described later. 
     Next, the target acceleration setting unit  51  calculates the target acceleration Ac of the vehicle based on the upper limit acceleration Amax, the lower limit acceleration Amin, and the target jerk Je which are calculated in steps S 11  to S 13  (step S 14 ). Details of a calculation method of this target acceleration Ac will be described in a section (6) described later. 
     (4) Calculation Flow of Lower Limit Acceleration 
     Next, a detailed description will be made about control contents of step S 12  for calculating the lower limit acceleration Amin of the vehicle.  FIG.  6    represents a subroutine illustrating details of the control in step S 12 . When the control illustrated in  FIG.  6    is started, the target acceleration setting unit  51  determines whether or not the accelerator opening acquired in step S 1  is included in a hysteresis region Rh decided in advance (step S 21 ). The hysteresis region Rh denotes a region where a difference is provided between the lower limit acceleration Amin and the upper limit acceleration Amax and is decided in advance for each condition of the vehicle speed and the gear stage. 
       FIG.  10    is a graph illustrating one example of the hysteresis region Rh. A characteristic curve Qx of a solid line in this  FIG.  10    represents a characteristic of the target acceleration which conforms to the present vehicle speed and gear stage and is selected from the maps of  FIGS.  9 A and  9 B . For example, in a case where the present vehicle speed is V 1  and the present gear stage is the third speed, as the characteristic curve Qx, the characteristic curve Q 3  in the map of  FIG.  9 A  is selected. As illustrated in  FIG.  10   , the hysteresis region Rh is a region which is positioned between two boundary points X 1  and X 2  on the characteristic curve Qx. In this hysteresis region Rh, the characteristic curve Qx has a characteristic in which the slope of the change in the target acceleration with respect to the change in the accelerator opening (operation gain) decreases toward a wider opening side. In other words, the hysteresis region Rh is set to contain a curved portion of the characteristic curve Qx, the curved portion being curved to draw an arc protruding upward. When between the boundary points X 1  and X 2  of the hysteresis region Rh, the boundary point X 1  on a narrow opening side is set as a first boundary point and the boundary point X 2  on a wide opening side is set as a second boundary point, the first boundary point X 1  is set to a position where the target acceleration becomes about zero, and the second boundary point X 2  is set to a position where the accelerator opening becomes a high opening close to the full opening (for example, around 90%). Note that in the following, the accelerator opening corresponding to the first boundary point X 1  will be referred to as first opening Px 1 , and the accelerator opening corresponding to the second boundary point X 2  will be referred to as second opening Px 2 . 
     In step S 21 , in a case where the target acceleration setting unit  51  compares the accelerator opening acquired in step S 1  with each of the above-described first and second boundary points X 1  and X 2 , in other words, the first opening Px 1  and the second opening Px 2  and where it is confirmed that the accelerator opening is wider than the first opening Px 1  and narrower than the second opening Px 2 , the target acceleration setting unit  51  determines that the accelerator opening is included in the hysteresis region Rh. 
     In a case where the determination is NO in step S 21  and it is confirmed that the present accelerator opening is out of the hysteresis region Rh, the target acceleration setting unit  51  sets the same value as the upper limit acceleration calculated in step S 11  as the lower limit accelerator Amin (step S 26 ). This means that the upper limit acceleration Amax calculated in step S 11  (in other words, a value on the characteristic curve Qx in  FIG.  10   ) is set as the target acceleration Ac without any change. However, because the accelerator opening is out of the hysteresis region Rh, the target acceleration Ac set here becomes a certain value on the characteristic curve Qx positioned on the outside of the hysteresis region Rh. 
     On the other hand, in a case where the determination is YES in step S 21  and it is confirmed that the present accelerator opening is included in the hysteresis region Rh, the target acceleration setting unit  51  calculates a temporary lower limit acceleration Amin 0  based on the accelerator opening acquired in step S 1  (step S 22 ). Specifically, the target acceleration setting unit  51  calculates a value on a temporary lower limit line Qy 0  illustrated in  FIG.  10    as the temporary lower limit acceleration Amin 0 . The temporary lower limit line Qy 0  is defined as a line which linearly connects the first boundary point X 1  as a boundary of the hysteresis region Rh on a narrow opening side with the second boundary point X 2  as a boundary of the hysteresis region Rh on a wide opening side. In step S 22 , the target acceleration setting unit  51  calculates a value on this temporary lower limit line Qy 0 , the value corresponding to the present accelerator opening, as the temporary lower limit acceleration Amin 0 . 
     Next, the target acceleration setting unit  51  calculates a first internal ratio α 1  which defines the relationship between the lower limit acceleration Amin to be set and the upper limit acceleration Amax and temporary lower limit acceleration Amin 0  based on the vehicle speed and the gear stage which are acquired in step S 1  (step S 23 ). That is, the lower limit acceleration Amin is variably set between the upper limit acceleration Amax and the temporary lower limit acceleration Amin 0 . Accordingly, in step S 23 , in order to decide how close to the upper limit acceleration Amax or the temporary lower limit acceleration Amin 0  the value to be set as the lower limit acceleration Amin is, the first internal ratio α 1  based on the vehicle speed and the gear stage is calculated. 
     The first internal ratio α 1  is variably set between zero and one. Specifically, the first internal ratio α 1  is set to zero in a case where the lower limit acceleration Amin agrees with the upper limit acceleration Amax but is set to one in a case where the lower limit acceleration Amin agrees with the temporary lower limit acceleration Amin 0 . Further, the first internal ratio α 1  is set to an intermediate value between zero and one in a case where the lower limit acceleration Amin is lower than the upper limit acceleration Amax and higher than the temporary lower limit acceleration Amin 0 . In other words, the lower limit acceleration Amin is set to a value closer to the upper limit acceleration Amax (farther from the temporary lower limit acceleration Amin 0 ) as the first internal ratio α 1  is closer to zero, and the lower limit acceleration Amin is set to a value closer to the temporary lower limit acceleration Amin 0  (farther from the upper limit acceleration Amax) as the first internal ratio α 1  is closer to one. 
     Next, the target acceleration setting unit  51  calculates a second internal ratio α 2  based on the road surface gradient and the gear stage which are acquired in step S 1  (step S 24 ). Similarly to the above-described first internal ratio α 1 , the second internal ratio α 2  is a value which defines the relationship between the lower limit acceleration Amin to be set and the upper limit acceleration Amax and temporary lower limit acceleration Amin 0  and is variably set between zero and one. The lower limit acceleration Amin is set to a value closer to the upper limit acceleration Amax (farther from the temporary lower limit acceleration Amin 0 ) as the second internal ratio α 2  is closer to zero, and the lower limit acceleration Amin is set to a value closer to the temporary lower limit acceleration Amin 0  (farther from the upper limit acceleration Amax) as the second internal ratio α 2  is closer to one. However, parameters which define the second internal ratio α 2  are the road surface gradient and the gear stage, and in this meaning, the second internal ratio α 2  is different from the first internal ratio α 1  defined based on the vehicle speed and the gear stage. 
     As illustrated in  FIG.  11   , the first internal ratio α 1  and the second internal ratio α 2  which are set in steps S 23  and S 24  are set to become lower as any of the vehicle speed, the gear stage, and the road surface gradient is greater. This means that the lower limit acceleration Amin is set to a value closer to the upper limit acceleration Amax as the vehicle speed becomes higher, the lower limit acceleration Amin is set to a value closer to the upper limit acceleration Amax as the gear stage becomes higher, and the lower limit acceleration Amin is set to a value closer to the upper limit acceleration Amax as the road surface gradient becomes larger. Note that “the road surface gradient becomes large” here is based on the assumption that a gradient of a hill-climbing road is dealt with as a positive gradient. In other words, the road surface gradient being large means that a traveling road of the vehicle is a comparatively steep (hill-climbing) road. 
     Next, the target acceleration setting unit  51  calculates the lower limit acceleration Amin of the vehicle based on the upper limit acceleration Amax calculated in step S 11 , the temporary lower limit acceleration Amin 0  calculated in step S 22 , and the first internal ratio α 1  and the second internal ratio α 2  which are calculated in steps S 23  to S 24  (step S 25 ). Specifically, the target acceleration setting unit  51  calculates the lower limit acceleration Amin by using the following formula (1).
 
 A  min= A  max−min[α1,max {( A  max− Ac ′)/( A  max− A  min 0),α2}]×( A  max− A  min 0)  (1)
 
     Here, Ac′ denotes the previous target acceleration, in other words, a target acceleration calculated in the most recent processing routine which has already been completed. 
     In the above formula (1), in principle, the lower limit acceleration Amin is calculated based on the upper limit acceleration Amax, the temporary lower limit acceleration Amin 0 , and the lesser of the first internal ratio α 1  and the second internal ratio α 2 . That is, the value resulting from multiplication of the difference between the upper limit acceleration Amax and the temporary lower limit acceleration Amin 0  (Amax−Amin 0 ) by the lesser of the first internal ratio α 1  and the second internal ratio α 2  is subtracted from the upper limit acceleration Amax, and the lower limit acceleration Amin is thereby calculated. However, in a case where the ratio (Amax−Ac′)/(Amax−Amin 0 ) obtained by dividing the difference between the upper limit acceleration Amax and the previous target acceleration Ac (Amax−Ac′) by the difference between the upper limit acceleration Amax and the temporary lower limit acceleration Amin 0  (Amax−Amin 0 ) is greater than the second internal ratio α and is smaller than the first internal ratio α 1 , the above ratio is used instead of the above internal ratios α 1  and α 2 . 
       FIG.  12    is a diagram illustrating one example of the lower limit acceleration Amin calculated in step S 25 . In the example illustrated in  FIG.  12   , the lower limit acceleration Amin is set on a lower limit curve Qy which is positioned between the characteristic curve Qx defining the upper limit acceleration Amax and the temporary lower limit line Qy 0  defining the temporary lower limit acceleration Amin 0 . The lower limit curve Qy is a curve which splits a portion between the characteristic curve Qx and the temporary lower limit line Qy 0  at specific ratios, and a value which is on this lower limit curve Qy and corresponds to the present accelerator opening is calculated as the above lower limit acceleration Amin. 
     As illustrated in  FIG.  12   , when the difference between the upper limit acceleration Amax and the lower limit acceleration Amin is set as an upper-lower limit difference HA, this upper-lower limit difference HA decreases toward a position closer to the boundary (the first boundary point X 1  or the second boundary point X 2 ) of the hysteresis region Rh and increases toward a position closer to a center side of the hysteresis region Rh. In other words, the upper-lower limit difference HA is set such that it becomes zero when the accelerator opening is a boundary opening of the hysteresis region Rh, in other words, the first opening Px 1  or the second opening Px 2  and is enlarged as the accelerator opening becomes closer to an intermediate value between the first opening Px 1  and the second opening Px 2 . In step S 25 , the target acceleration setting unit  51  sets the lower limit acceleration Amin such that the upper-lower limit difference HA changes in such a tendency. 
     (5) Calculation Flow of Target Jerk 
     Next, a detailed description will be made about control contents of step S 13  for calculating the target jerk Je of the vehicle.  FIG.  7    represents a subroutine illustrating details of control in step S 13 . When the control illustrated in  FIG.  7    is started, the target acceleration setting unit  51  calculates an opening change rate ΔP as a change rate of the accelerator opening (step S 31 ). The opening change rate ΔP is a value of a time derivative of the accelerator opening and is calculated from a history of the accelerator opening, which is acquired through the most recent predetermined period, for example. In this case, the target acceleration setting unit  51  calculates the opening change rate ΔP based on the change in data of plural accelerator openings including the accelerator opening acquired in a currently progressing processing routine (step S 1 ) and the accelerator openings acquired in the most recent processing routine which has already been completed. The opening change rate ΔP is calculated as a positive value when depression of the accelerator pedal  60  is increased and is calculated as a negative value when the accelerator pedal  60  is returned. Note that in the following, the opening change rate ΔP will appropriately be referred to as accelerator opening change rate ΔP. 
     Next, the target acceleration setting unit  51  calculates a basic jerk Je 0  based on the accelerator opening change rate ΔP calculated in step S 31  and the vehicle speed and the gear stage which are acquired in step S 1  (step S 32 ). Specifically, the target acceleration setting unit  51  multiplies the accelerator opening change rate ΔP by a coefficient obtained from the vehicle speed and the gear stage based on a map or the like which is decided in advance and thereby calculates the basic jerk Je 0 . Note that the coefficient used here (the coefficient by which the accelerator opening change rate ΔP is multiplied) can appropriately be decided in accordance with the vehicle speed and the gear stage but is set to become small as the gear stage is higher, for example. 
       FIG.  13    is a graph illustrating the relationship between the accelerator opening change rate ΔP and the basic jerk Je 0 . As described above, because the value obtained by multiplying the accelerator opening change rate ΔP by the coefficient decided from the vehicle speed and the gear stage is the basic jerk Je 0 , this basic jerk Je 0  changes proportionally to the accelerator opening change rate ΔP under a condition where the vehicle speed and the gear stage are the same. That is, the basic jerk Je 0  is calculated such that it takes a positive value when depression of the accelerator pedal  60  is increased (when ΔP is positive) and increases to the positive side as a depression-increasing speed is faster. Conversely, the basic jerk Je 0  is calculated such that it takes a negative value when the accelerator pedal  60  is returned (when ΔP is negative) and increases to the negative side as a returning speed is faster. 
     Next, the target acceleration setting unit  51  calculates a first correction coefficient k1 based on the accelerator opening change rate ΔP calculated in step S 31  (step S 33 ). For example, a map illustrated in  FIG.  14    is applied to calculation of this correction coefficient k1. Accordingly, the first correction coefficient k1 is set to zero when the accelerator opening change rate ΔP is −p1 or more and +p1 or less and is set to one when the accelerator opening change rate ΔP is less than −p1 or more than +p1. Note that p1 (absolute value) is set to a comparatively small value. This is for preventing the change in the target acceleration from occurring even in a case where the accelerator opening unintentionally and minutely fluctuates due to vibration or the like of the vehicle. 
     Next, the target acceleration setting unit  51  calculates a previous internal ratio α′ based on the previous target acceleration Ac′, the upper limit acceleration Amax calculated in step S 11 , and the lower limit acceleration Amin calculated in step S 12  (step S 34 ). The previous internal ratio α′ is a value which defines the relationship between the previous target acceleration Ac′ as the target acceleration calculated in the most recent processing routine which has already been completed and the upper limit acceleration Amax and lower limit acceleration Amin and is variably set between zero and one. The previous internal ratio α′ is set to a value closer to zero as the previous target acceleration Ac′ is closer to the upper limit acceleration Amax (farther from the lower limit acceleration Amin), and the previous internal ratio α′ is set to a value closer to one as the previous target acceleration Ac′ is closer to the lower limit acceleration Amin (farther from the upper limit acceleration Amax). 
     Next, the target acceleration setting unit  51  determines whether or not the accelerator opening change rate ΔP calculated in step S 31  is higher than zero (step S 35 ). The accelerator opening change rate ΔP being higher than zero means that the accelerator opening is increasing, in other words, depression of the accelerator pedal  60  is being increased. Conversely, the accelerator opening change rate ΔP being lower than zero means that the accelerator opening is decreasing, in other words, the accelerator pedal  60  is being returned. 
     In a case where the determination is YES in step S 35  and it is confirmed that the accelerator opening is increasing (depression of the accelerator pedal  60  is being increased), the target acceleration setting unit  51  calculates a second correction coefficient k2 based on the accelerator opening acquired in step S 1  and the previous internal ratio α′ calculated in step S 34  (step S 36 ). For example, the target acceleration setting unit  51  applies the present accelerator opening and the previous internal ratio α′ to a map decided in advance and thereby calculates the second correction coefficient k2. The second correction coefficient k2 is set to decrease as the accelerator opening becomes wider and to decrease as the previous internal ratio α′ becomes lower. 
     Next, the target acceleration setting unit  51  calculates the target jerk Je of the vehicle based on the basic jerk Je 0  calculated in step S 32 , the first correction coefficient k1 calculated in step S 33 , and the second correction coefficient k2 calculated in step S 36  (step S 37 ). Specifically, the target acceleration setting unit  51  calculates the target jerk Je by using the following formula (2).
 
 Je=Je 0× k 1× k 2  (2)
 
     Here, because the accelerator opening change rate ΔP is positive (YES in step S 35 ) as the assumption for reaching the step S 37 , the basic jerk Je 0  in the above formula (2) is positive. Further, as described above, the second correction coefficient k2 is a coefficient which decreases as the previous internal ratio α′ becomes lower. Consequently, by computation of the above formula (2), the target jerk Je is calculated, in a range greater than zero, so as to decrease as the previous internal ratio α′ becomes lower. This means that the target jerk Je decreases (becomes closer to zero) as the previous target acceleration Ac′ becomes closer to the upper limit acceleration Amax. 
     Next, a description will be made about control in a case where the determination is NO in step S 35 , in other words, in a case where it is confirmed that the accelerator opening is decreasing (the accelerator pedal  60  is returned) or the acceleration opening is retained at a constant opening. In this case, the target acceleration setting unit  51  calculates a third correction coefficient k3 based on the accelerator opening and the gear stage which are acquired in step S 1  (step S 39 ). For example, the target acceleration setting unit  51  applies the present accelerator opening and gear stage to a map decided in advance and thereby calculates the third correction coefficient k3. The third correction coefficient k3 is set to decrease as the accelerator opening becomes wider and to decrease as the gear stage becomes higher. 
     Next, the target acceleration setting unit  51  calculates a fourth correction coefficient k4 based on the gear stage acquired in step S 1  and the previous internal ratio α′ calculated in step S 34  (step S 40 ). For example, the target acceleration setting unit  51  applies the present gear stage and the previous internal ratio α′ to a map decided in advance and thereby calculates the fourth correction coefficient k4. The fourth correction coefficient k4 is set to decrease as the gear stage becomes higher and to decrease as the previous internal ratio α′ becomes higher. 
     Next, the target acceleration setting unit  51  calculates the target jerk Je of the vehicle based on the basic jerk Je 0  calculated in step S 32 , the first correction coefficient k1 calculated in step S 33 , the third correction coefficient k3 calculated in step S 39 , and the fourth correction coefficient k4 calculated in step S 40  (step S 41 ). Specifically, the target acceleration setting unit  51  calculates the target jerk Je by using the following formula (3).
 
 Je=Je 0× k 1× k 3× k 4  (3)
 
     Here, because the accelerator opening change rate ΔP is zero or negative (NO in step S 35 ) as the assumption for reaching the step S 41 , the basic jerk Je 0  in the above formula (3) is zero or negative. Further, as described above, the fourth correction coefficient k4 is a coefficient which decreases as the previous internal ratio α′ becomes higher. Consequently, by computation of the above formula (3), the target jerk Je is calculated, in a range of zero or smaller, such that its absolute value decreases as the previous internal ratio α′ becomes higher. This means that the absolute value of the target jerk Je decreases (becomes closer to zero) as the previous target acceleration Ac′ becomes closer to the lower limit acceleration Amin. 
     (6) Calculation Flow of Target Acceleration 
     Next, a detailed description will be made about control contents of step S 14  for calculating the target acceleration Ac of the vehicle.  FIG.  8    represents a subroutine illustrating details of control in step S 14 . When the control illustrated in  FIG.  8    is started, the target acceleration setting unit  51  calculates an integrated value Zj resulting from integration of the target jerk Je (step S 51 ). The integrated value Zj is calculated by adding up the target jerks Je calculated through the most recent predetermined period, for example. In this case, the target acceleration setting unit  51  adds up data of a plurality of target jerks Je including the target jerk Je calculated in a currently progressing processing routine (step S 13 ) and the target jerks Je calculated in the most recent processing routine which has already been completed and thereby calculates the integrated value Zj. 
     Next, the target acceleration setting unit  51  determines whether or not the absolute value of the (present) target jerk Je calculated in step S 13  is less than a predetermined threshold value β, in other words, whether or not the relationship of −β&lt;Je&lt;β holds (step S 52 ). 
     In a case where the determination is YES in step S 52  and it is confirmed that the absolute value of the target jerk Je is less than the threshold value β, the target acceleration setting unit  51  calculates the target acceleration Ac based on the upper limit acceleration Amax calculated in step S 11 , the lower limit acceleration Amin calculated in step S 12 , and the previous internal ratio α′ calculated in step S 34  (step S 53 ). Specifically, the target acceleration setting unit  51  calculates the target acceleration Ac by using the following formula (4).
 
 Ac=A  max−α′×( A  max− A  min)  (4)
 
     As in the above formula (4), in step S 53 , the target acceleration Ac is calculated by using the internal ratio which is the same as the previous internal ratio. That is, in a case where step S 53  is executed, the target acceleration Ac is retained at a value which splits a portion between the upper limit acceleration Amax and the lower limit acceleration Amin at the same ratios. 
     On the other hand, in a case where the determination is YES in step S 52  and it is confirmed that the absolute value of the target jerk Je is the threshold value β or greater, the target acceleration setting unit  51  determines whether or not the integrated value Zj of the target jerk Je which is calculated in step S 51  is the upper limit acceleration Amax calculated in step S 11  or greater (step S 54 ). 
     In a case where the determination is YES in step S 54  and it is confirmed that the integrated value Zj of the target jerk Je is the upper limit acceleration Amax or greater, the target acceleration setting unit  51  sets the upper limit acceleration Amax calculated in step S 11  as the target acceleration Ac (step S 55 ). 
     On the other hand, in a case where the determination is NO in step S 54  and it is confirmed that the integrated value Zj of the target jerk Je is less than the upper limit acceleration Amax, the target acceleration setting unit  51  determines whether or not the integrated value Zj of the target jerk Je which is calculated in step S 51  is the lower limit acceleration Amin calculated in step S 12  or less (step S 56 ). 
     In a case where the determination is YES in step S 56  and it is confirmed that the integrated value Zj of the target jerk Je is the lower limit acceleration Amin or less, the target acceleration setting unit  51  sets the lower limit acceleration Amin calculated in step S 12  as the target acceleration Ac (step S 57 ). 
     On the other hand, in a case where the determination is NO in step S 56  and it is confirmed that the integrated value Zj of the target jerk Je is greater than the lower limit acceleration Amin, in other words, in a case where the relationship of Amin&lt;Zj&lt;Amax holds, the target acceleration setting unit  51  sets the integrated value Zj of the target jerk Je, which is calculated in step S 51 , as the target acceleration Ac (step S 58 ). 
     (7) Operation and Effects 
     As described above, in the present embodiment, the target jerk Je as a target value of a jerk (a change rate of acceleration) of the vehicle is each time calculated from the accelerator opening change rate ΔP, and the target acceleration Ac of the vehicle is set based on the integrated value Zj as a value resulting from integration of the target jerk Je. Thus, in an opening range where the operation gain (the slope of the change in the acceleration with respect to the change in the accelerator opening) of the accelerator pedal  60  decreases toward a wider opening side, in other words, the hysteresis region Rh, the target acceleration Ac at a time when the accelerator pedal  60  is returned (hereinafter, also referred to as pedal-returning target acceleration) can be made lower than the target acceleration Ac at a time when depression of the accelerator pedal  60  is increased (hereinafter, also referred to as depression-increasing target acceleration). 
     That is, the target jerk Je set in accordance with the accelerator opening change rate ΔP (to be proportional to ΔP) becomes less in a pedal-returning situation in which the accelerator opening decreases (ΔP becomes negative) than a depression-increasing situation in which the accelerator opening increases (ΔP becomes positive). Consequently, the integrated value Zj resulting from integration of such a target jerk Je becomes less in the pedal-returning situation than the depression-increasing situation under the same condition of the accelerator opening. This means that the target acceleration Ac calculated based on the integrated value Zj becomes relatively small when the accelerator pedal  60  is returned. Accordingly, in the hysteresis region Rh where the operation gain decreases toward a wider opening side, it becomes easy to cause the driver to perceive the change in the acceleration of the vehicle at a time when the accelerator pedal  60  is returned. 
     For example, because the target acceleration is set such that the operation gain corresponding to the slope of the characteristic curve Qx decreases toward a wider opening side in the above-described hysteresis region Rh, in a case where the target acceleration is evenly set along the characteristic curve Qx, the driver might feel discomfort particularly when the accelerator pedal  60  is returned. For example, in a circumstance where the driver increases depression of the accelerator pedal  60  in the hysteresis region Rh, even when the operation gain lowers in a wide opening range along the above characteristic curve Qx, such lowering of the operation gain does not lead to particular discomfort for the driver who feels an output limit of the engine. However, in a case where the target acceleration is changed along the same characteristic curve Qx when the accelerator pedal is subsequently returned, the driver might feel discomfort due to the above-described lowering of the operation gain. That is, the possibility becomes high that although the accelerator pedal  60  is returned, the change (lowering) in the acceleration is maintained at such a low level that the driver has difficulty in perceiving the change. This might cause the driver to feel discomfort (for example, a sense that the vehicle spontaneously accelerates). On the other hand, in the present embodiment, in the hysteresis region Rh, the pedal-returning target acceleration is made less than the depression-increasing target acceleration. Thus, it becomes easy to cause the driver to perceive the change in the acceleration of the vehicle (behavior change) at a time when the accelerator pedal  60  is returned, and operability of the vehicle by the accelerator pedal  60  can be improved. 
     Further, in the present embodiment, the upper limit acceleration Amax and the lower limit acceleration Amin are set based on a plurality of pieces of information including the accelerator opening (the accelerator opening, the vehicle speed, the gear stage, and the road surface gradient), and only in a case where the above-described integrated value Zj of the target jerk Je falls between those upper and lower limit accelerations Amax and Amin, the integrated value Zj is employed as the target acceleration Ac. In other words, in a case where the integrated value Zj is the upper limit acceleration Amax or more, the upper limit acceleration Amax is employed as the target acceleration Ac, and in a case where the integrated value Zj is the lower limit acceleration Amin or less, the lower limit acceleration Amin is employed as the target acceleration Ac. In such a configuration, because the difference between the depression-increasing target acceleration and the pedal-returning target acceleration is adequately restricted, the accelerator opening to obtain the same acceleration can be prevented from being largely different between the depression-increasing situation and the pedal-returning situation. Accordingly, while adequate responsiveness (a sense that the acceleration adequately changes) is given to the driver when depression of the accelerator pedal  60  is increased and when the accelerator pedal  60  is returned, discomfort given to the driver due to differences in acceleration characteristics between the depression-increasing situation and the pedal-returning situation can be reduced. 
       FIG.  15    is a diagram for specifically explaining the above operation and effects and is a diagram illustrating the change in the target acceleration in a case where the driver once increases depression of the accelerator pedal  60  and thereafter returns the accelerator pedal  60 . In the example in  FIG.  15   , after the accelerator opening increases from P 1  to P 2  by an increase in depression of the accelerator pedal  60 , the accelerator opening lowers from P 2  to P 0  (&lt;P 1 ) by returning of the accelerator pedal  60 . Such a change in the accelerator opening causes the target acceleration Ac to change in an order of Ac 1 , Ac 2 , Ac 3 , and then Ac 4 . A target acceleration Ac 1  is a target acceleration at a time when the increase in depression of the accelerator pedal  60  is started and is an intermediate value between the upper limit acceleration Amax and the lower limit acceleration Amin at the accelerator opening P 1  at the time point. A target acceleration Ac 2  is a target acceleration at a time when the increase in depression of the accelerator pedal  60  is finished and agrees with the upper limit acceleration Amax at the accelerator opening P 2  at the time point. A target acceleration Ac 3  is a target acceleration at a time when the accelerator pedal  60  is returned until the accelerator opening decreases to P 2  and is lower than the target acceleration Ac 1  at the start of the increase in depression (almost agrees with the lower limit acceleration Amin at the acceleration opening P 2 , here). A target acceleration Ac 4  is a target acceleration at a time when returning of the accelerator pedal  60  is finished and agrees with the lower limit acceleration Amin at the accelerator opening P 0  at the time point. 
     In the above example, comparing the target accelerations Ac in the same accelerator opening zone from P 1  to P 2 , the pedal-returning target acceleration (bold one-dot chain line arrow) as the target acceleration at a time when the accelerator pedal  60  is returned is lower than the depression-increasing target acceleration (bold solid line arrow) as the target acceleration at a time when depression of the accelerator pedal  60  is increased. Because such hysteresis characteristics are provided, in the present embodiment, compared to a hypothetical case where the hysteresis characteristics are not present, the change (lowering) in the acceleration of the vehicle due to returning of the accelerator pedal  60  becomes large, and the driver can be caused to properly perceive the change in the acceleration. Accordingly, because adequate responsiveness can be obtained when the accelerator pedal  60  is returned, operability of the vehicle by the accelerator pedal  60  can be improved. Further, the depression-increasing target acceleration and the pedal-returning target acceleration are set only between the characteristic curve Qx defining the upper limit acceleration Amax and the lower limit curve Qy defining the lower limit acceleration Amin. Thus, the difference between the depression-increasing target acceleration and the pedal-returning target acceleration can be prevented from being unreasonably enlarged, and discomfort of the driver can be reduced. 
     Further, in the present embodiment, when the target jerk Je is calculated from the accelerator opening change rate ΔP, closeness of the previous (most recently calculated) target acceleration Ac′ to the upper limit acceleration Amax or the lower limit acceleration Amin is taken into consideration. That is, when depression of the accelerator pedal  60  is increased (when ΔP&gt;0), the target jerk Je (&gt;0) is calculated by using the second correction coefficient k2 which decreases as the previous target acceleration Ac′ becomes closer to the upper limit acceleration Amax. In addition, when the accelerator pedal  60  is returned (ΔP&lt;0), the target jerk Je (&lt;0) is calculated by using the fourth correction coefficient k4 which decreases as the previous target acceleration Ac′ becomes closer to the lower limit acceleration Amin. In such a configuration, the operation gain (the slope of the change in the acceleration with respect to the change in the accelerator opening) of the accelerator pedal  60  can be prevented from suddenly changing when the target acceleration Ac is increased to the upper limit acceleration Amax due to an increase in depression of the accelerator pedal  60  or when the target acceleration Ac is decreased to the lower limit acceleration Amin due to returning of the accelerator pedal  60 . 
     That is, because the absolute value of the target jerk Je is made less as the target acceleration Ac becomes closer to the upper limit acceleration Amax or the lower limit acceleration Amin and this reduces a changing speed of the target acceleration Ac (=the integrated value Zj of the target jerk Je), the target acceleration Ac can smoothly merge with the upper limit acceleration Amax or the lower limit acceleration Amin. For example, as indicated by a bold solid line arrow Z 1  in  FIG.  16   , the target acceleration Ac at a time when depression of the accelerator pedal  60  is increased changes more gently as the target acceleration Ac becomes closer to the upper limit acceleration Amax and smoothly merges with the upper limit acceleration Amax. A bold broken line arrow Z 2  indicates the change in the target acceleration Ac in a case where the above-described control is not employed, and in this case, the operation gain suddenly changes at a time point when the upper limit acceleration Amax is reached. In the present embodiment in which such an event can be avoided, the acceleration of the vehicle can smoothly be changed, suitable riding comfort can be secured. 
     Further, in the present embodiment, the target jerk Je is calculated by using the first correction coefficient k1 which becomes zero when the accelerator opening change rate ΔP is in a predetermined range including zero (−p1 or more and +p1 or less). Thus, a circumstance can be avoided where the target acceleration Ac changes even in a case where the accelerator opening unintentionally and minutely fluctuates due to vibration or the like of the vehicle, and the driver can be prevented from feeling discomfort due to an unintended behavior change of the vehicle. 
     In the foregoing, the preferable embodiment of the present disclosure has been described; however, the present invention is not limited to the above-described embodiment, and various changes are possible without departing from the scope of the gist of the present disclosure. 
     For example, in the above embodiment, a gasoline engine as a spark ignition type internal combustion engine is used as a drive source of a vehicle; however, a drive source may be an element which can generate motive power for traveling, and a diesel engine may be used as a drive source, for example. Further, a drive source is not limited to an internal combustion engine but may be an electric motor. 
     It should be understood that the embodiments herein are illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof, are therefore intended to be embraced by the claims. 
     REFERENCE CHARACTER LIST 
     
         
         
           
               1  engine (drive source) 
               51  target acceleration setting unit 
               52  target torque setting unit 
               53  engine controller (drive source controller) 
               60  accelerator pedal 
             Amax upper limit acceleration 
             Amin lower limit acceleration 
             SN 3  accelerator sensor