Patent Publication Number: US-10760521-B2

Title: Control device and control method for fuel pump

Description:
BACKGROUND 
     The present disclosure relates to a control device and a control method for a fuel pump. 
     An internal combustion engine disclosed in US Patent Application Publication No. 2009/0217910 includes a fuel injection valve for injecting fuel into a cylinder, a fuel pipe connected to the fuel injection valve, and a fuel pump for supplying the fuel to the fuel pipe. The fuel pump includes a plunger disposed in the cylinder. The plunger is made of a magnetic material. The plunger is constantly biased in a first direction by a biasing spring provided in the fuel pump. The fuel pump includes a coil for exciting the plunger. When the coil is energized, the plunger is excited by a magnetic field generated around the coil. When the plunger is energized, the plunger moves in a second direction opposite to the first direction against the biasing force of the biasing spring. When energization to the coil is stopped, excitation of the plunger is cancelled and the plunger moves in the first direction according to the biasing force of the biasing spring. In this manner, in the fuel pump, the plunger reciprocates in the cylinder. Each time the plunger reciprocates, the fuel pump executes a suction operation for drawing in fuel and a discharge operation for pressurizing and discharging the drawn fuel. 
     In the control device of the fuel pump described in the above publication, when the fuel injection amount from the fuel injection valve is within a predetermined range, the driving cycle of the fuel injection valve and the driving cycle of the fuel pump are set to be the same. Therefore, one fuel discharge from the fuel pump is performed in response to one fuel injection from the fuel injection valve. With this configuration, in order to allow a sufficient amount of fuel to be supplied to the fuel pipe with respect to the fuel injection amount from the fuel injection valve, it is necessary to design the fuel pump so that the maximum amount of fuel that can be discharged from the fuel pump is increased. 
     On the other hand, there is a demand for downsizing the fuel pump as downsizing of the internal combustion engine is desired. However, with a small-sized fuel pump, the maximum amount of fuel that can be discharged from the fuel pump at one time is small. Accordingly, when the control device described in the above publication is adapted for such a small-sized fuel pump, the amount of fuel discharged from the fuel pump at one time is insufficient for one fuel injection amount from the fuel injection valve, which may not be able to supply a sufficient amount of fuel to the fuel pipe. Therefore, there is room for improving the controllability of the fuel pressure in the fuel pipe. 
     SUMMARY 
     In accordance with one aspect of the present disclosure, a control device for a fuel pump is provided. The fuel pump is a motor-driven fuel pump adapted for an internal combustion engine. The internal combustion engine includes a fuel injection valve configured to inject fuel into a cylinder. The fuel pump is configured to supply fuel to a fuel pipe connected to the fuel injection valve. The fuel pump includes a cylinder, a mover configured to slide in the cylinder, and an electric actuator configured to move the mover. The control device includes processing circuitry that is configured to: perform energization control on the electric actuator to reciprocate the mover so that the fuel pump draws in and discharges fuel; and control a discharge count and a unit discharge amount based on an operating state of the internal combustion engine, the discharge count being a number of times of discharging fuel from the fuel pump to the fuel pipe during a period between a fuel injection from the fuel injection valve and the next fuel injection, and the unit discharge amount being an amount of fuel for one fuel discharge from the fuel pump. 
     In accordance with another aspect of the present disclosure, a control method for a fuel pump is provided. The fuel pump is a motor-driven fuel pump adapted for an internal combustion engine. The internal combustion engine includes a fuel injection valve configured to inject fuel into a cylinder. The fuel pump is configured to supply fuel to a fuel pipe connected to the fuel injection valve. The fuel pump includes a cylinder, a mover configured to slide in the cylinder, and an electric actuator configured to move the mover. The control method includes: performing energization control on the electric actuator to reciprocate the mover so that the fuel pump draws in and discharges fuel; and controlling a discharge count and a unit discharge amount based on an operating state of the internal combustion engine, the discharge count being a number of times of discharging fuel from the fuel pump to the fuel pipe during a period between a fuel injection from the fuel injection valve and the next fuel injection, and the unit discharge amount being an amount of fuel for one fuel discharge from the fuel pump. 
     Other aspects and advantages of the present disclosure will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram showing a configuration of an internal combustion engine including a control device for a fuel pump according to a first embodiment; 
         FIG. 2  is a cross-sectional view of the high-pressure fuel pump in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view showing a state during fuel discharge in the high-pressure fuel pump in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view showing a state during fuel suction in the high-pressure fuel pump in  FIG. 2 ; 
         FIG. 5  is a functional block diagram of the control device in  FIG. 1 ; 
         FIG. 6  is a graph showing a relationship between energization time and discharge amount in the high-pressure fuel pump in  FIG. 2 ; 
         FIG. 7  is a timing diagram showing an example of a manner of fuel injection from a fuel injection valve and a manner of fuel discharge from the high-pressure fuel pump; 
         FIG. 8  is a timing diagram showing an example of a manner of fuel injection from the fuel injection valve and a manner of fuel discharge from the high-pressure fuel pump; 
         FIG. 9  is a timing diagram showing an example of a manner of fuel injection from the fuel injection valve and a manner of fuel discharge from the high-pressure fuel pump according to a second embodiment; 
         FIG. 10  is a functional block diagram of a control device for a fuel pump according to a third embodiment; 
         FIG. 11  is a timing diagram showing an example of a manner of fuel injection from the fuel injection valve and a manner of fuel discharge from the high-pressure fuel pump according to the third embodiment; and 
         FIG. 12  is a timing diagram showing an example of a manner of fuel injection from the fuel injection valve and a manner of fuel discharge from the high-pressure fuel pump according to a modification. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     A control device for a fuel pump according to a first embodiment will now be described with reference to  FIGS. 1 to 6 . 
     As shown in  FIG. 1 , an engine main body  11  of an internal combustion engine  10  mounted on a vehicle includes four cylinders (a first cylinder # 1  to a fourth cylinder # 4 ). To the engine main body  11 , an intake passage  12  is connected. The intake passage  12  includes an intake manifold  13  and an intake pipe  14  connected to the end of the intake manifold  13  on the intake upstream side. The intake manifold  13  includes a surge tank  13 A connected to the intake pipe  14 , an intake introduction section  13 B provided on the intake downstream side of the surge tank  13 A, and an intake branching section  13 C provided on the intake downstream side of the intake introduction section  13 B. The surge tank  13 A has a larger passage cross-sectional area than the intake pipe  14  and the intake introduction section  13 B. The intake branching section  13 C has four end portions branching on the intake downstream side, and the respective four branching end portions are connected to different cylinders. The intake pipe  14  is provided with a throttle valve  21 . By controlling the opening degree of the throttle valve  21 , the flow rate of the intake air flowing through the intake passage  12  is controlled. The air flowing from the intake pipe  14  to the intake manifold  13  is supplied to the respective cylinders # 1  to # 4 . The intake pipe  14  is provided with an air flow meter  90  that detects the flow rate of the intake air flowing through the intake passage  12  on the intake upstream side with respect to the throttle valve  21 . 
     The engine main body  11  is provided with a plurality of fuel injection valves  15 . One fuel injection valve  15  is provided for each cylinder. The fuel injection valve  15  is disposed in the cylinder to inject fuel into the cylinder. In each of the cylinders # 1  to # 4 , an ignition plug  16  is provided. In each of the cylinders # 1  to # 4 , the intake air introduced through the intake passage  12  and the fuel injected from the fuel injection valve  15  are mixed to generate an air-fuel mixture. The mass ratio of intake air to fuel in the air-fuel mixture is referred to as air-fuel ratio. The air-fuel mixture is ignited by the ignition plug  16  and combusted. 
     To the engine main body  11 , an exhaust passage  17  is connected. The exhaust passage  17  includes an exhaust manifold  18  and an exhaust pipe  19  connected to the end of the exhaust manifold  18  on the exhaust downstream side. The exhaust manifold  18  is composed of an exhaust branching section  18 A connected to the engine main body  11  and an exhaust confluence section  18 B provided on the exhaust downstream side of the exhaust branching section  18 A. The exhaust branching section  18 A has four branched ends on the exhaust upstream side, and the respective four branching end portions are connected to different cylinders. In each of the cylinders # 1  to # 4 , exhaust gas generated by combustion of the air-fuel mixture is discharged to the exhaust manifold  18 . In the exhaust passage  17 , a catalyst  20  disposed in the exhaust pipe  19  to purify the exhaust gas is provided. Further, in the exhaust pipe  19 , an air-fuel ratio sensor  91  is disposed on the exhaust upstream side of the catalyst  20 . The air-fuel ratio sensor  91  outputs an electric signal corresponding to the oxygen concentration of the exhaust gas flowing through the exhaust passage  17 , that is, the air-fuel ratio of the air-fuel mixture used for combustion. 
     The internal combustion engine  10  is provided with a fuel supply device  30  for supplying fuel to the fuel injection valve  15  provided in the engine main body  11 . The fuel supply device  30  includes a fuel tank  31  in which fuel is stored. Inside the fuel tank  31 , a low-pressure fuel pump  32  is disposed. To the low-pressure fuel pump  32 , one end of a low-pressure fuel pipe  33  is connected. The low-pressure fuel pump  32  is a motor-driven fuel pump, pumps up the fuel in the fuel tank  31 , and discharges the fuel to the low-pressure fuel pipe  33 . To the other end of the low-pressure fuel pipe  33 , a high-pressure fuel pump  40  is connected. To the high-pressure fuel pump  40 , a high-pressure fuel pipe  34  is connected. The high-pressure fuel pipe  34  is composed of a discharge pipe  34 A connected to the high-pressure fuel pump  40  and a delivery pipe  34 B connected to the discharge pipe  34 A. To the delivery pipe  34 B, the respective fuel injection valves  15  are connected. The fuel discharged from the low-pressure fuel pump  32  to the low-pressure fuel pipe  33  is drawn into the high-pressure fuel pump  40 . In the high-pressure fuel pump  40 , the drawn fuel is pressurized and discharged to the discharge pipe  34 A. The fuel discharged to the discharge pipe  34 A is supplied to the delivery pipe  34 B and injected into the cylinder from the fuel injection valve  15 . In the delivery pipe  34 B, a pressure sensor  92  is provided on a first end portion connected to the discharge pipe  34 A. The pressure sensor  92  detects the fuel pressure Pr in the high-pressure fuel pipe  34 . In the delivery pipe  34 B, a fuel temperature sensor  93  is provided on a second end portion opposite to the first end portion. The fuel temperature sensor  93  detects the temperature of the fuel in the high-pressure fuel pipe  34 . 
     As shown in  FIG. 2 , the high-pressure fuel pump  40  includes a pump section  50  that draws in and pressurizes fuel and a casing  80  to which the pump section  50  is connected. 
     The casing  80  has a box shape. The casing  80  has a lower wall  81  and an upper wall  84  that each have a disc shape, and a peripheral side wall  82  that extends from the circumferential edge of the lower wall  81  to the circumferential edge of the upper wall  84 . At a central portion of the lower wall  81 , a columnar protruded portion  83  that protrudes in the inner space side of the casing  80  is provided. The peripheral side wall  82  is continuously provided over the entire periphery of the circumferential edge of the lower wall  81  and the upper wall  84 , and has a cylindrical shape. The upper wall  84  has a through hole  84 A at a central portion. 
     The pump section  50  includes a housing  51  fixed to the upper end surface of the upper wall  84 . The housing  51  is composed of a main body portion  52  having a cylindrical shape, a flange portion  55  disposed between the main body portion  52  and the upper wall  84 , and an insertion portion  56  extending from the flange portion  55 . The flange portion  55  has a larger diameter than the main body portion  52  and is in contact with the upper wall  84 . The insertion portion  56  extends from the flange portion  55  to the inner space of the casing  80  through the through hole  84 A. The outer diameter of the insertion portion  56  is the same as the inner diameter of the through hole  84 A. Therefore, the outer circumferential surface of the insertion portion  56  is in contact with the inner circumferential surface of the through hole  84 A of the upper wall  84 . The housing  51  has a cylinder bore  57 . The cylinder bore  57  extends from one end face (the lower end face in  FIG. 2 ) of the insertion portion  56  to the inside of the main body portion  52 . Hereinafter, the extending direction (the up-down direction in  FIG. 2 ) of the central axis L of the cylinder bore  57  is simply referred to as the axial direction. 
     The main body portion  52  has a first orthogonal hole  53  and a second orthogonal hole  54  that extend in a direction (the left-right direction in  FIG. 2 ) orthogonal to the axial direction and communicate with the cylinder bore  57 . The first orthogonal hole  53  and the second orthogonal hole  54  extend in opposite directions from the cylinder bore  57 . The first orthogonal hole  53  has a first small diameter portion  53 A that communicates with the cylinder bore  57  and a first large diameter portion  53 B that extends from the first small diameter portion  53 A to the side peripheral surface of the main body portion  52  and opens on the side peripheral surface. In the first large diameter portion  53 B, a suction valve  60  is inserted and fitted. 
     The suction valve  60  has a cylindrical shape and is attached to the main body portion  52  in a state of protruding from the main body portion  52 . In the suction valve  60 , a suction passage  61  extends through the suction valve  60  in the above-described orthogonal direction is formed. The suction passage  61  is composed of a first suction passage  61 A that is connected to the first small diameter portion  53 A, a second suction passage  61 B that is connected to the first suction passage  61 A and has a larger diameter than the first suction passage  61 A, and a third suction passage  61 C that is connected to the second suction passage  61 B and has the same diameter as the first suction passage  61 A. In the second suction passage  61 B, a first check valve  62  is disposed. The first check valve  62  is composed of a first valve body  63  and a first spring  64  for biasing the first valve body  63  toward the third suction passage  61 C. The first valve body  63  is composed of a first biasing portion  63 A that is in contact with the inner end surface of the second suction passage  61 B on which the third suction passage  61 C opens, and a first bulging portion  63 B that bulges from the central portion of the first biasing portion  63 A toward the first suction passage  61 A. The first bulging portion  63 B has a hemispherical shape. The first spring  64  has a first end that is in contact with the inner end surface of the second suction passage  61 B on which the first suction passage  61 A opens, and a second end that is in contact with the first biasing portion  63 A of the first valve body  63 . To the suction valve  60 , the low-pressure fuel pipe  33  is connected, and to the third suction passage  61 C, fuel is supplied from the low-pressure fuel pipe  33 . 
     The second orthogonal hole  54  has a second small diameter portion  54 A that communicates with the cylinder bore  57  and a second large diameter portion  54 B that extends from the second small diameter portion  54 A to the side peripheral surface of the main body portion  52  and opens on the side peripheral surface. In the second large diameter portion  54 B, a discharge valve  70  is inserted and fitted. The discharge valve  70  has a cylindrical shape and is attached to the main body portion  52  in a state of protruding from the main body portion  52 . The discharge valve  70  and the suction valve  60  are arranged side by side on the same axis extending in the above-described orthogonal direction. In the discharge valve  70 , a discharge passage  71  extending through the discharge valve  70  in the above-described orthogonal direction is formed. The discharge passage  71  is composed of a first discharge passage  71 A that is connected to the second small diameter portion  54 A, a second discharge passage  71 B that is connected to the first discharge passage  71 A and has a larger diameter than the first discharge passage  71 A, and a third discharge passage  71 C that is connected to the second discharge passage  71 B and has the same diameter as the first discharge passage  71 A. In the second discharge passage  71 B, a second check valve  72  is disposed. 
     The second check valve  72  is composed of a second valve body  73  and a second spring  74  for biasing the second valve body  73  toward the first discharge passage  71 A. The second valve body  73  is composed of a second biasing portion  73 A that is in contact with the inner end surface of the second discharge passage  71 B on which the first discharge passage  71 A opens, and a second bulging portion  73 B that bulges from the central portion of the second biasing portion  73 A toward the third discharge passage  71 C. The second bulging portion  73 B has a hemispherical shape. The second spring  74  has a first end that is in contact with the inner end surface of the second discharge passage  71 B on which the third discharge passage  71 C opens, and a second end that is in contact with the second biasing portion  73 A of the second valve body  73 . To the discharge valve  70 , the high-pressure fuel pipe  34  is connected. 
     The pump section  50  includes a plunger  75  serving as a mover that is inserted into the cylinder bore  57  and that is slidable in the cylinder bore  57 . The plunger  75  is made of a magnetic material. The plunger  75  has a columnar rod shape and is inserted into the cylinder bore  57  from the lower end opening of the insertion portion  56 . The lower end portion of the plunger  75  extends from the cylinder bore  57  to the inner space of the casing  80 . The plunger  75  has a groove  75 A at a lower end portion. The groove  75 A extends over the entire circumference in the circumferential direction. Therefore, the plunger  75  has a diameter that is partially reduced at the position in which the groove  75 A is formed. To the groove  75 A, a pedestal  76  having an annular plate shape is connected. The pedestal  76  is composed of a central portion  76 A engaged with the groove  75 A, a curved portion  76 B having a curve and extending outward in the radial direction from the central portion  76 A, and a flat portion  76 C extending outward in the radial direction from the curved portion  76 B. Between the flat portion  76 C and the insertion portion  56  of the housing  51 , a compression spring  77  is disposed. The compression spring  77  biases the pedestal  76  in a direction away from the housing  51 , that is, in a direction of pulling out the plunger  75  from the cylinder bore  57  (downward in  FIG. 2 ). The lower end surface of the plunger  75  is pressed against the upper end surface of the protruded portion  83  of the casing  80  by the biasing force of the compression spring  77 . The plunger  75  has a protrusion  75 B at a lower end portion above the groove  75 A. The protrusion  75 B extends over the entire circumference in the circumferential direction. Therefore, the plunger  75  has a diameter that is partially increased at the position of the protrusion  75 B. The diameter of the protrusion  75 B is larger than the diameter of the cylinder bore  57 . The cylinder bore  57 , the plunger  75 , the first small diameter portion  53 A, the first suction passage  61 A, the second suction passage  61 B, the second small diameter portion  54 A, and the first discharge passage  71 A constitute a pressurizing chamber  78  of the pump section  50 . 
     In the main body portion  52  of the housing  51 , a coil  85  is disposed so as to surround the periphery of the cylinder bore  57 . The coil  85  generates a magnetic field upon energization. When the coil  85  is energized, the plunger  75  is excited by the magnetic field generated around the coil  85 . 
     As indicated by the blank arrow in  FIG. 3 , when the plunger  75  is excited, the plunger  75  moves to a first side (the upper side in  FIG. 3 ) in the axial direction against the biasing force of the compression spring  77 . The plunger  75  moves to the first side until the protrusion  75 B comes into contact with the insertion portion  56 . This movement of the plunger  75  decreases the volume of the pressurizing chamber  78  of the pump section  50  and increases the pressure in the pressurizing chamber  78 . Since the pressurizing chamber  78  is filled with fuel as described later, increasing the pressure of the pressurizing chamber  78  makes the discharge valve  70  open. Specifically, the second valve body  73  of the discharge valve  70  is subjected to the pressure in the pressurizing chamber  78  in the valve opening direction, and is also subjected to the pressure in the high-pressure fuel pipe  34  and the biasing force of the second spring  74  in the valve closing direction. When the pressure in the pressurizing chamber  78  increases and the force of biasing the second valve body  73  in the valve opening direction becomes higher than the force of biasing the second valve body  73  in the valve closing direction, the second valve body  73  is opened. When the second valve body  73  opens, fuel is discharged from the pressurizing chamber  78  to the high-pressure fuel pipe  34  as indicated by the solid line arrow in  FIG. 3 . While the fuel is discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34 , the suction valve  60  is held in a closed state by the pressure in the pressurizing chamber  78 . On the other hand, when the energization to the coil  85  is stopped, the excitation of the plunger  75  is cancelled. 
     As indicated by the blank arrow in  FIG. 4 , when the excitation of the plunger  75  is cancelled, the plunger  75  moves to a second side (the lower side in  FIG. 4 ) in the axial direction by the biasing force of the compression spring  77  so that the plunger  75  is pulled out from the cylinder bore  57 . The plunger  75  moves to the second side until its lower end comes into contact with the protruded portion  83 . This movement of the plunger  75  increases the volume of the pressurizing chamber  78  and decreases the pressure in the pressurizing chamber  78 . The first valve body  63  of the suction valve  60  is subjected to the pressure in the low-pressure fuel pipe  33  in the valve opening direction, and is also subjected to the pressure in the pressurizing chamber  78  and the biasing force of the first spring  64  in the valve closing direction. When the pressure in the pressurizing chamber  78  decreases and the force of biasing the first valve body  63  in the valve closing direction becomes lower than the force of biasing the first valve body  63  in the valve opening direction, the first valve body  63  is opened. When the first valve body  63  opens, fuel is supplied from the low-pressure fuel pipe  33  to the pressurizing chamber  78  as indicated by the solid line arrow in  FIG. 4 . While the high-pressure fuel pump  40  draws in the fuel from the low-pressure fuel pipe  33 , the discharge valve  70  is held in a closed state by the pressure in the high-pressure fuel pipe  34 . 
     In this manner, the plunger  75  reciprocates in the axial direction inside the cylinder bore  57  depending on the energization state of the coil  85 . Accordingly, the coil  85  corresponds to an electric actuator for moving the plunger  75 . Each time the plunger  75  reciprocates, the high-pressure fuel pump  40  performs a suction function of drawing in the fuel and a discharge function of pressurizing and discharging the drawn fuel. Further, in the main body portion  52  of the fuel pump, a coil temperature sensor  94  is provided. The coil temperature sensor  94  detects the temperature of the coil  85 . 
     As shown in  FIG. 1 , the fuel supply device  30  includes a control device  100  for a fuel pump. Further, the internal combustion engine  10  includes a battery  120 . The battery  120  supplies electric power to the respective parts of the internal combustion engine  10 , such as the control device  100  and the electric actuator of the high-pressure fuel pump  40 . 
     To the control device  100 , output signals are input from the air flow meter  90 , the air-fuel ratio sensor  91 , the pressure sensor  92 , the fuel temperature sensor  93 , and the coil temperature sensor  94 . To the control device  100 , an output signal of a crank angle sensor  95  that detects the engine rotational speed NE, which is a rotational speed of a crankshaft of the internal combustion engine  10 , and the crank angle CA, which is a rotation phase of the crankshaft is also input. Further, to the control device  100 , output signals from various sensors such as an accelerator sensor  96  for detecting an accelerator operation amount Acc that is an operation amount of an accelerator pedal, a vehicle speed sensor  97  for detecting a vehicle speed V, etc., are also input. The control device  100  includes a CPU, a ROM, and a RAM. The control device  100  causes the CPU to execute programs stored in the ROM to control driving of the fuel injection valve  15 , driving of the throttle valve  21 , and driving of the high-pressure fuel pump  40 . 
     As shown in  FIG. 5 , the control device  100  includes, as functional sections, a target rotational speed calculation section  101 , a target torque calculation section  102 , a target fuel pressure calculation section  103 , a fuel pressure difference calculation section  104 , an injection feedback amount calculation section  105 , a required injection amount calculation section  106 , an injection time calculation section  107 , an injection start timing calculation section  108 , and an injection valve driving section  109 . Further, the control device  100  includes a target throttle opening degree calculation section  110 , a throttle driving section  111 , an injection interval calculation section  112 , a discharge start timing calculation section  113 , a target discharge amount calculation section  114 , a pump characteristics learning section  115 , a discharge count calculation section  116 , a unit discharge amount calculation section  117 , a driving amount setting section  118 , and a pump driving section  119 . 
     The target rotational speed calculation section  101  calculates a target rotational speed NEt that is a target value of the engine rotational speed NE, based on the engine rotational speed NE detected by the crank angle sensor  95  and the accelerator operation amount Acc detected by the accelerator sensor  96 . 
     The target torque calculation section  102  calculates a target torque TQt that is a target value of the axial torque of the crankshaft of the internal combustion engine  10 , based on the vehicle speed V detected by the vehicle speed sensor  97  and the accelerator operation amount Acc detected by the accelerator sensor  96 . 
     The target fuel pressure calculation section  103  calculates a target fuel pressure Pt that is a target value of the fuel pressure in the high-pressure fuel pipe  34 , based on the target rotational speed NEt calculated by the target rotational speed calculation section  101  and the target torque TQt calculated by the target torque calculation section  102 . In the target fuel pressure calculation section  103 , a map indicating a relationship between a target fuel pressure Pt and each of a target rotational speed NEt and a target torque TQt is stored. This map is previously obtained by experiment and simulation. The target fuel pressure Pt is calculated so as to be higher when the target rotational speed NEt is high than when the target rotational speed NEt is low. Further, the target fuel pressure Pt is calculated so as to be higher when the target torque TQt is large than when the target torque TQt is small. 
     The fuel pressure difference calculation section  104  calculates a fuel pressure difference ΔP (ΔP=Pt−Pr), which is a value obtained by subtracting the fuel pressure Pr in the high-pressure fuel pipe  34  detected by the pressure sensor  92  from the target fuel pressure Pt calculated by the target fuel pressure calculation section  103 . 
     The injection feedback amount calculation section  105  calculates an injection feedback amount FAF for feedback control of feeding the actual air-fuel ratio detected by the air-fuel ratio sensor  91  back to the target air-fuel ratio that is a target value of the air-fuel ratio. The target air-fuel ratio is calculated based on the operating state of the internal combustion engine  10  by the control device  100 . The injection feedback amount calculation section  105  inputs a value obtained by subtracting the actual air-fuel ratio from the target air-fuel ratio to a proportional element, an integral element, and a differential element, and outputs as an injection feedback amount FAF the sum of an output value of the proportional element, an output value of the integral element, and an output value of the differential element. 
     The required injection amount calculation section  106  calculates a required fuel injection amount Qt that is a target value of the fuel amount injected from each fuel injection valve  15 . The required injection amount calculation section  106  calculates a base injection amount Qb based on the target rotational speed NEt calculated by the target rotational speed calculation section  101  and the target torque TQt calculated by the target torque calculation section  102 . The base injection amount Qb is calculated so as to be larger when the target rotational speed NEt is high than when the target rotational speed NEt is low. Further, the base injection amount Qb is calculated so as to be larger when the target torque TQt is large than when the target torque TQt is small. The base injection amount Qb is calculated as a fuel injection amount corresponding to the target air-fuel ratio. The required injection amount calculation section  106  calculates the required fuel injection amount Qt by multiplying the base injection amount Qb by the injection feedback amount FAF calculated by the injection feedback amount calculation section  105 . 
     The injection time calculation section  107  calculates an injection time Fi that is a period of time of executing fuel injection for each fuel injection valve  15 , based on the required fuel injection amount Qt calculated by the required injection amount calculation section  106  and the fuel pressure Pr detected by the pressure sensor  92 . 
     The injection start timing calculation section  108  calculates an injection start time such that the fuel injection for the required fuel injection amount Qt calculated by the required injection amount calculation section  106  is completed before the ignition timing of the cylinder where the fuel injection valve  15  is disposed. In the present embodiment, a fixed point in time at which a predetermined crank angle before reaching the compression top dead center is calculated as an injection start timing Fs. 
     The injection valve driving section  109  drives each fuel injection valve  15  based on the crank angle CA detected by the crank angle sensor  95 . At the injection start timing Fs of each fuel injection valve  15  calculated by the injection start timing calculation section  108 , the injection valve driving section  109  controls the fuel injection valve  15  so that fuel injection from the fuel injection valve  15  is started. After the fuel injection is continued during the injection time Fi calculated by the injection time calculation section  107  from the start of the fuel injection, the injection valve driving section  109  ends the fuel injection from the fuel injection valve  15 . 
     The target throttle opening degree calculation section  110  calculates a target throttle opening degree et that is a target value of the opening degree of the throttle valve  21  based on the target torque TQt calculated by the target torque calculation section  102 . 
     The throttle driving section  111  controls the opening degree of the throttle valve  21  to realize the target throttle opening degree et calculated by the target throttle opening degree calculation section  110 . 
     The injection interval calculation section  112  calculates an injection interval Int of fuel based on a fuel injection end timing Fe from the fuel injection valve  15 , the injection start timing Fs calculated by the injection start timing calculation section  108 , and the engine rotational speed NE detected by the crank angle sensor  95 . The injection interval Int of fuel is calculated as a period of time from when the fuel injection is ended at the fuel injection valve  15  provided in any one of the cylinders to when the fuel injection is started at the fuel injection valve  15  provided in the cylinder to be ignited next. For example, in the present embodiment, the first cylinder # 1 , the third cylinder # 3 , the fourth cylinder # 4 , and the second cylinder # 2  are ignited in this order. In this case, the injection interval calculation section  112  calculates as the injection interval Int of fuel each of a period of time from when the fuel injection in the first cylinder # 1  is ended to when the fuel injection in the third cylinder # 3  is started, and a period of time from when the fuel injection in the third cylinder # 3  is ended to when the fuel injection in the fourth cylinder # 4  is started. Further, the injection interval calculation section  112  calculates as the injection interval Int of fuel each of a period of time from when the fuel injection in the fourth cylinder # 4  is ended to when the fuel injection in the second cylinder # 2  is started, and a period of time from when the fuel injection in the second cylinder # 2  is ended to when the fuel injection in the first cylinder # 1  is started. The injection interval calculation section  112  calculates the fuel injection end timing Fe based on the injection time Fi calculated by the injection time calculation section  107  and the injection start timing Fs calculated by the injection start timing calculation section  108 . In the present embodiment, since the injection start timing Fs is set to a fixed point in time at a crank angle, the injection interval Int of fuel becomes shorter as the end timing Fe of fuel injection is later and as the engine rotational speed NE is higher. 
     The discharge start timing calculation section  113  calculates a discharge start timing Ts that is a point in time at which fuel discharge from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  is started. The discharge start timing Ts is calculated based on the timing of fuel injection of the fuel injection valve  15 . In the present embodiment, the discharge start timing Ts is set to the point in time at which a predetermined preparation time has elapsed from the end timing Fe of fuel injection of the fuel injection valve  15 . The fuel injection end timing Fe can be calculated based on the injection time Fi calculated by the injection time calculation section  107  and the injection start timing Fs calculated by the injection start timing calculation section  108 . The preparation time is set to be longer than the time required for the fuel pressure Pr in the high-pressure fuel pipe  34  to become stable after the fuel injection from the fuel injection valve  15  is ended. The preparation time is previously obtained by experiment and simulation and stored in the control device  100 . 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt that is a target value of the fuel discharge amount from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34 . In the present embodiment, the target discharge amount calculation section  114  calculates the target discharge amount TPt at a point in time when a predetermined convergence time has elapsed from the end timing Fe of fuel injection of the fuel injection valve  15 . The convergence time is a time equal to the time required for the fuel pressure Pr in the high-pressure fuel pipe  34  to become stable after the fuel injection from the fuel injection valve  15  is ended, and is set to be shorter than the preparation time. The convergence time is previously obtained by experiment and simulation and stored in the control device  100 . The target discharge amount calculation section  114  calculates a base discharge amount TPb based on the required fuel injection amount Qt calculated by the required injection amount calculation section  106 . The base discharge amount TPb is calculated as an amount equal to the required fuel injection amount Qt. That is, the base discharge amount TPb increases as the required fuel injection amount Qt increases. Further, the target discharge amount calculation section  114  calculates a discharge feedback amount TK based on the fuel pressure difference ΔP calculated by the fuel pressure difference calculation section  104 . In the present embodiment, a value obtained by subtracting from the target fuel pressure Pt the actual fuel pressure Pr after fuel is discharged from the high-pressure fuel pump  40  so as to reach the target fuel pressure Pt is input to a proportional element, an integral element, and an differential element, and the sum of an output value of the proportional element, an output value of the integral element, and an output value of the differential element is calculated as the discharge feedback amount TK. The target discharge amount calculation section  114  calculates the target discharge amount TPt by multiplying the base discharge amount TPb by the discharge feedback amount TK. 
     The pump characteristics learning section  115  learns a relationship between an energization time to the high-pressure fuel pump  40  and the amount of fuel discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  as operation characteristics of the high-pressure fuel pump  40 . The fuel discharge amount from the high-pressure fuel pump  40  is affected by the fuel temperature in the high-pressure fuel pipe  34  detected by the fuel temperature sensor  93 , the temperature of the coil  85  detected by the coil temperature sensor  94 , the battery voltage, etc. 
     As shown in  FIG. 6 , the operation characteristics of the high-pressure fuel pump  40  has a tendency that the fuel discharge amount increases as the energization time is longer. In the high-pressure fuel pump  40 , when energization of the high-pressure fuel pump  40  is started, the plunger  75  moves away from the protruded portion  83  from a state in which the plunger  75  is in contact with the protruded portion  83 . Therefore, as indicated by the solid line in  FIG. 6 , as the elapsed time from the start of energization increases, the moving amount of the plunger  75  increases, the volume of the pressurizing chamber  78  decreases, and thus the amount of fuel discharged from the high-pressure fuel pump  40  increases. Then, when the elapsed time from the start of energization reaches the time (energization time Tik 1 ) required for the plunger  75  to enter the state where the protrusion  75 B of the plunger  75  is in contact with the insertion portion  56  from the state where the plunger  75  is in contact with the protruded portion  83 , the fuel discharge amount from the high-pressure fuel pump  40  becomes a maximum discharge amount TPmax that is the maximum value of fuel discharge amount in one fuel discharge. After that time, the discharge amount does not change even if the energization time becomes longer. A maximum discharge amount TPmax 1  shown in  FIG. 6  is equal to a design maximum discharge amount that is the maximum value of discharge amount that can be realized by design in one fuel discharge from the high-pressure fuel pump  40 . 
     The viscosity of the fuel is higher when the fuel temperature is low than when the fuel temperature is high. Therefore, the resistance in fuel discharge is larger when the fuel temperature is low than when the fuel temperature is high, and the moving speed of the plunger  75  is lowered. Accordingly, as indicated by the long dashed short dashed line in  FIG. 6 , the time (energization time Tik 2 ) required for the discharge amount to reach the maximum discharge amount TPmax 1  tends to become longer when the fuel temperature in the high-pressure fuel pipe  34  is low than when the fuel temperature in the high-pressure fuel pipe  34  is high as indicated by the solid line in  FIG. 6  (Tik 1 &lt;Tik 2 ). 
     Further, the force to move the plunger  75  toward the pressurizing chamber  78  is weaker when the temperature of the coil  85  is high than when the temperature of the coil  85  is low. In addition, the force to move the plunger  75  toward the pressurizing chamber  78  is weaker when the battery voltage is low than when the battery voltage is high. Accordingly, as indicated by the long dashed double-short dashed line in  FIG. 6 , the maximum discharge amount TPmax that can be discharged per one time in the high-pressure fuel pump  40  may be lower when the temperature of the coil  85  is high or the battery voltage is low than when the temperature of the coil  85  is low and the battery voltage is high as indicated by the solid line in  FIG. 6 . Therefore, a maximum discharge amount TPmax 2  in this case is smaller than the design maximum discharge amount (TPmax 1 ). 
     As described above, in the high-pressure fuel pump  40 , the energization time required for discharging a predetermined amount of fuel in one fuel discharge and the maximum value of fuel discharge amount that is possible to be discharged in one fuel discharge change depending on the current state of the high-pressure fuel pump  40 . The pump characteristics learning section  115  calculates a unit discharge amount (as described later) that is a fuel amount in one fuel discharge from the high-pressure fuel pump  40  when the high-pressure fuel pump  40  is driven for the energization time set based on the target discharge amount TPt, on the basis of the fuel pressure difference ΔP calculated by the fuel pressure difference calculation section  104 , and stores the unit discharge amount together with information of the fuel temperature, the temperature of the coil  85 , and the battery voltage. The battery voltage can be obtained from a charge/discharge state of the battery  120 . 
     The discharge count calculation section  116  calculates a necessary discharge count Tnf that is the number of times the high-pressure fuel pump  40  should discharge fuel to the high-pressure fuel pipe  34 , based on the target discharge amount TPt calculated by the target discharge amount calculation section  114 . The target discharge amount TPt is calculated based on the required fuel injection amount Qt and is a parameter correlated with the operating state of the internal combustion engine. That is, the discharge count calculation section  116  calculates the necessary discharge count Tnf based on the operating state of the internal combustion engine  10 . The discharge count calculation section  116  calculates the smallest of the discharge counts necessary for discharging an amount of fuel corresponding to the target discharge amount TPt as the necessary discharge count Tnf. For example, when the target discharge amount TPt is less than the maximum discharge amount TPmax of the high-pressure fuel pump  40  that is the specified amount and the target discharge amount TPt is small, the necessary discharge count Tnf is calculated as one. Further, when the target discharge amount TPt is equal to or larger than the maximum discharge amount TPmax and less than twice the maximum discharge amount TPmax, the necessary discharge count Tnf is calculated as two times. That is, when the target discharge amount TPt is equal to or larger than the maximum discharge amount TPmax that is the specified amount and the target discharge amount TPt is large, the necessary discharge count Tnf is calculated as a plurality of times. The maximum discharge amount TPmax can be calculated based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . 
     The unit discharge amount calculation section  117  sets a target unit discharge amount TPnf that is a target value of the unit discharge amount TPn that is a fuel amount to be discharged from the high-pressure fuel pump  40  per one time, based on the necessary discharge count Tnf set by the discharge count calculation section  116  and the target discharge amount TPt calculated by the target discharge amount calculation section  114 . When the necessary discharge count Tnf is set to one, the unit discharge amount calculation section  117  sets the target discharge amount TPt to the target unit discharge amount TPnf. Further, when the discharge count is set to two times or more, the unit discharge amount calculation section  117  sets the target discharge amount TPnf by an amount obtained by dividing the target discharge amount TPt by the necessary discharge count Tnf (TPt/Tnf). 
     The driving amount setting section  118  sets a discharge count Tn of fuel discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  between the fuel injection from the fuel injection valve  15  and the next fuel injection, and the unit discharge amount TPn in each discharge. The driving amount setting section  118  first calculates a necessary time Tnes required for discharging the target unit discharge amount TPnf set by the unit discharge amount calculation section  117  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . For example, when the necessary discharge count Tnf is one, the necessary time Tnes is equal to a lift time Ti. Further, when the necessary discharge count Tnf is a plurality of times n (2≤n), the necessary time Tnes is equal to the sum of n times the lift time Ti and n−1 times a standby time. The lift time Ti is a time required from when the plunger  75  in contact with the protruded portion  83  starts to move to when the high-pressure fuel pump  40  discharges the fuel of the target unit discharge amount TPnf. Specifically, when the target unit discharge amount TPnf is equal to the maximum discharge amount TPmax, the moving time of the plunger  75  required from when the plunger  75  in contact with the protruded portion  83  starts to move to when the protrusion  75 B of the plunger  75  comes into contact with the insertion portion  56  is the lift time Ti (for example, the energization time Tik 1  in  FIG. 6 ). Further, the standby time is a time taken for the plunger  75  to move from a first moving end away from the protruded portion  83  to a second moving end in contact with the protruded portion  83 . Specifically, when the high-pressure fuel pump  40  discharges fuel corresponding to the maximum discharge amount TPmax, the standby time is a time for the plunger  75  to enter the state where the plunger  75  is in contact with the protruded portion  83  from the state where the protrusion  75 B of the plunger  75  is in contact with the insertion portion  56 . The lift time Ti and the standby time are calculated based on the operation characteristics of the high-pressure fuel pump  40 . After the necessary time Tnes is calculated in this way, a time obtained by adding the preparation time to the necessary time Tnes is calculated as an execution time Tad. That is, the execution time Tad is a time required from when the fuel injection is ended to when the fuel discharge is completed, for fuel discharge executed between fuel injection and the next fuel injection. When the execution time Tad is equal to or less than the injection interval Int calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn to the same number as the necessary discharge count Tnf. Further, the driving amount setting section  118  sets the unit discharge amount TPn for each discharge to the same amount as the target unit discharge amount TPnf. As a result, when the discharge count Tn of fuel discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at the injection interval Int of the fuel injection valve  15  is a plurality of times, each unit discharge amount TPn in the plurality of times of fuel discharge is set to an amount smaller than the maximum discharge amount TPmax and the unit discharge amounts TPn in the plurality of times of fuel discharge are set to be equal to each other. In this case, the execution time Tad is set based on the discharge count Tn, the unit discharge amount TPn, and the operation characteristics of the high-pressure fuel pump  40 . 
     On the other hand, when the calculated execution time Tad exceeds the injection interval Int calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn and the unit discharge amount TPn based on the injection interval Int such that the execution time Tad that is the time required for the high-pressure fuel pump  40  to complete the fuel discharge does not exceed the injection interval Int. In this case, in the present embodiment, the driving amount setting section  118  sets the discharge count Tn and the unit discharge amount TPn such that the discharge amount from the high-pressure fuel pump  40  at the injection interval Int becomes the maximum discharge amount. The relationship between the discharge count Tn and the unit discharge amount TPn with respect to the injection interval Int is previously obtained by experiment and simulation and stored in the control device  100 . In this way, when the execution time Tad exceeds the injection interval Int, the driving amount setting section  118  calculates and sets the discharge count Tn and the unit discharge amount TPn, so that the upper limit of the execution time Tad is set depending on the injection interval Int. 
     The pump driving section  119  drives the high-pressure fuel pump  40  based on the discharge start timing Ts calculated by the discharge start timing calculation section  113 , and the discharge count Tn and the unit discharge amount TPn that are set by the driving amount setting section  118 . That is, the pump driving section  119  starts energization control for the coil  85  of the high-pressure fuel pump  40  at the discharge start timing Ts. The pump driving section  119  causes the plunger  75  to reciprocate through the energization control, thereby causing the high-pressure fuel pump  40  to execute fuel suction and fuel discharge. One reciprocation of the plunger  75  makes the high-pressure fuel pump  40  execute fuel discharge once. When the lift time Ti set based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115  has elapsed from the start of energization control for the high-pressure fuel pump  40 , the pump driving section  119  ends the energization. Thus, the fuel discharge amount of the high-pressure fuel pump  40  per one time is controlled to be equal to the unit discharge amount TPn. When the discharge count Tn set by the driving amount setting section  118  is two times or more, the pump driving section  119  ends the energization control at the timing when the lift time Ti elapses from the start of the energization control, and executes the energization control again at the timing when a predetermined standby time elapses from the timing of the end. Then, the pump driving section  119  ends the energization control again at the timing when the lift time Ti has elapsed from the start of energization control again. By the repeated energization control, the high-pressure fuel pump  40  executes fuel discharge a plurality of times. 
     An operation and advantages of the present embodiment will now be described with reference to  FIGS. 7 and 8 . In the following description, the point in time of each operation in  FIGS. 7 and 8  is indicated by t followed by three-digit numbers. However, in  FIG. 7 , the symbol t and the first digit  7  of the three digits are omitted. Further, in  FIG. 8 , the symbol t and the first digit  8  of the three digits are omitted. 
     First, an example of a manner of fuel discharge when the engine rotational speed NE of the internal combustion engine is low will be described with reference to  FIG. 7 . 
     The required injection amount calculation section  106  calculates a required fuel injection amount Qt( 1 ) at a point in time t 711 . When the required fuel injection amount Qt( 1 ) is calculated, the injection time calculation section  107  calculates an injection time Fi( 1 ) that is the injection execution time of fuel injection based on the required fuel injection amount Qt( 1 ) and the current fuel pressure Pr detected by the pressure sensor  92 . Then, at a point in time t 712 , which is an injection start timing Fs calculated by the injection start timing calculation section  108  based on the crank angle CA detected by the crank angle sensor  95 , the injection valve driving section  109  starts fuel injection from the fuel injection valve  15 . The injection valve driving section  109  continues the fuel injection during the injection time Fi( 1 ) calculated by the injection time calculation section  107 , and ends the fuel injection at a point in time t 713  when the injection time Fi( 1 ) has elapsed from the point in time t 712 . 
     By executing this fuel injection, the fuel pressure Pr in the high-pressure fuel pipe  34  decreases. At the point in time t 713  when the fuel injection is ended, the fuel injection is ended, but the fuel pressure Pr fluctuates for a while. A period of time from when the point in time t 713 , at which the fuel injection is ended, to when the fuel pressure Pr converges to a constant value is the convergence time described above. 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt( 1 ) at a point in time t 714  when the convergence time has elapsed from the end timing Fe of the fuel injection (point in time t 713 ). The target discharge amount TPt( 1 ) is calculated based on the required fuel injection amount Qt( 1 ) and a discharge feedback amount TK calculated based on a fuel pressure difference ΔP. Before the fuel injection is executed at the point in time t 712 , the difference ΔP (ΔP&gt;0) occurs between the target fuel pressure Pt and the actual fuel pressure Pr. The discharge feedback amount TK is calculated as a value for feedback control to reduce the difference. 
     When the target discharge amount TPt( 1 ) is calculated in this way, the discharge count calculation section  116  calculates a necessary discharge count Tnf for the high-pressure fuel pump  40  to discharge fuel to the high-pressure fuel pipe  34 . In this example, since the target discharge amount TPt( 1 ) is less than the maximum discharge amount TPmax based on the operation characteristics of the high-pressure fuel pump  40 , the necessary discharge count Tnf is calculated as one. Then, the unit discharge amount calculation section  117  calculates the target discharge amount TPt( 1 ) as a target unit discharge amount TPnf (TPnf=TPt( 1 )). When the necessary discharge count Tnf and the target unit discharge amount TPnf are calculated in this way, the driving amount setting section  118  sets a discharge count Tn of fuel for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection, and a unit discharge amount TPn in each discharge. 
     The driving amount setting section  118  first calculates a necessary time Tnes (lift time Ti) required for performing fuel discharge by the discharge amount TPnf set by the unit discharge amount calculation section  117  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . Then, the driving amount setting section  118  calculates a time obtained by adding the necessary time Tnes and the preparation time described above as an execution time Tad. Since the execution time Tad is equal to or less than an injection interval Int( 1 ) calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn to the same number as the necessary discharge count Tnf, and sets the unit discharge amount TPn to be equal to the target unit discharge amount TPnf. 
     After that, the pump driving section  119  drives the high-pressure fuel pump  40  so that fuel discharge is executed at a discharge start timing Ts calculated by the discharge start timing calculation section  113 . In this case, the pump driving section  119  performs fuel discharge once from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 715  when the preparation time described above has elapsed from the point in time t 713 , at which the fuel injection is ended. The fuel discharge is executed during the period between the point in time t 715  and a point in time t 716  when the lift time Ti corresponding to the target unit discharge amount TPnf has elapsed. 
     By executing this fuel discharge, the fuel pressure Pr in the high-pressure fuel pipe  34  increases. At the point in time t 716 , the fuel discharge is ended, however, pressure fluctuations occur in the fuel in the high-pressure fuel pipe  34  for a while. As the unit discharge amount TPn from the high-pressure fuel pump  40  increases, the pressure fluctuations of the fuel tend to increase. The fuel pressure Pr converges to a target fuel pressure Pt when a predetermined time has elapsed from the point in time t 716 , at which the fuel discharge is ended. 
     After that, at a point in time t 717  after the fuel pressure Pr converges to a constant value, the required injection amount calculation section  106  calculates a required fuel injection amount Qt( 2 ) for the next fuel injection. The required fuel injection amount Qt( 2 ) is larger than the required fuel injection amount Qt( 1 ) (Qt( 2 )&gt;Qt( 1 )). When the required fuel injection amount Qt( 2 ) is calculated, the injection time calculation section  107  calculates an injection time Fi( 2 ) that is the injection execution time of fuel injection based on the required fuel injection amount Qt( 2 ) and the current fuel pressure Pr detected by the pressure sensor  92 . At the point in time t 717 , the fuel pressure Pr is equal to the target fuel pressure Pt. The injection valve driving section  109  starts fuel injection from the fuel injection valve  15  at a point in time t 718 , which is the injection start timing Fs. The injection valve driving section  109  continues the fuel injection during the injection time Fi calculated by the injection time calculation section  107 , and ends the fuel injection at a point in time t 719  when the injection time Fi( 2 ) has elapsed from the point in time t 718 . 
     By executing this fuel injection, the fuel pressure Pr in the high-pressure fuel pipe  34  decreases. More fuel is injected in the fuel injection during the period between the point in time t 718  and the point in time t 719  than in the previous fuel injection during the period between the point in time t 712  and the point in time t 713 . Therefore, at the point in time t 719 , at which the fuel injection is ended, the fuel pressure is lower than in the previous fuel injection. Further, since the injected fuel amount is large, the pressure fluctuations of the fuel occurring after the fuel injection are also larger than in the previous fuel injection. Therefore, the convergence time in the subsequent fuel injection is longer than the convergence time in the previous fuel injection. 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt( 2 ) at a point in time t 720  when the convergence time has elapsed from the end timing Fe (point in time t 719 ) of the fuel injection. The target discharge amount TPt( 2 ) is calculated based on the required fuel injection amount Qt( 2 ) and a discharge feedback amount TK calculated based on a fuel pressure difference ΔP. Immediately before the fuel injection is executed at the point in time t 718 , the actual fuel pressure Pr is equal to the target fuel pressure Pt and thus there is no difference. On the other hand, the required fuel injection amount Qt( 2 ) is larger than the required fuel injection amount Qt( 1 ). In the example shown in  FIG. 7 , the target discharge amount TPt( 2 ) is calculated as a value larger than the target discharge amount TPt( 1 ). 
     When the target discharge amount TPt( 2 ) is calculated, the discharge count calculation section  116  calculates a necessary discharge count Tnf. In this example, since the target discharge amount TPt( 2 ) is equal to or larger than the maximum discharge amount TPmax based on the operation characteristics of the high-pressure fuel pump  40  and less than twice the maximum discharge amount TPmax, the necessary discharge count Tnf is calculated as two times. Then, the unit discharge amount calculation section  117  calculates a value obtained by dividing the target discharge amount TPt( 2 ) by 2 as the target unit discharge amount TPnf (TPnf=TPt( 2 )/2). When the necessary discharge count Tnf and the target unit discharge amount TPnf are calculated in this way, the driving amount setting section  118  sets a discharge count Tn of fuel for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection, and a unit discharge amount TPn in each discharge. 
     The driving amount setting section  118  first calculates a necessary time Tnes (Tnes=2×lift time Ti+1×standby time) required for performing fuel discharge by the discharge amount TPnf set by the unit discharge amount calculation section  117  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . Then, the driving amount setting section  118  calculates a time obtained by adding the necessary time Tnes and the preparation time described above as an execution time Tad. Since the execution time Tad is equal to or less than an injection interval Int( 2 ) calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn to the same number as the necessary discharge count Tnf, and sets the unit discharge amount TPn to be equal to the target unit discharge amount TPnf. 
     After that, the pump driving section  119  drives the high-pressure fuel pump  40  so that fuel discharge is started at a discharge start timing Ts calculated by the discharge start timing calculation section  113 . In this case, the pump driving section  119  performs fuel discharge two times from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 721  when the preparation time described above has elapsed from the point in time t 719 , at which the fuel injection is ended. The first fuel discharge is executed during the period between the point in time t 721  and a point in time t 722  when the lift time Ti corresponding to the target unit discharge amount TPnf has elapsed. The pump driving section  119  starts the second fuel discharge at a point in time t 723  when the standby time has elapsed from the point in time t 722 , at which the first fuel discharge is ended. The second fuel discharge is executed during the period between the point in time t 723  and a point in time t 724  when the lift time Ti has elapsed. The first lift time Ti is equal to the second lift time Ti. 
     After that, at the point in time t 725  after the fuel discharge, the required injection amount calculation section  106  calculates a required fuel injection amount Qt( 3 ) for the next fuel injection, and then the fuel injection is performed. 
     In this way, in the present embodiment, the discharge count Tn of fuel discharged from the high-pressure fuel pump  40  and the unit discharge amount TPn for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection are controlled based on the required fuel injection amount Qt that is correlated with the operating state of the internal combustion engine  10 . Depending on the amount of fuel discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34 , the magnitude of pressure fluctuations of the fuel in the high-pressure fuel pipe  34  changes. The discharge count Tn of the high-pressure fuel pump  40  and the unit discharge amount TPn for the period between the fuel injection and the next fuel injection are controlled based on the operating state of the internal combustion engine  10 . This makes it possible to realize a supply of fuel that makes it difficult for an excess or a shortage of fuel in the high-pressure fuel pipe  34  to occur, while taking into consideration the influence of the pressure fluctuations of the fuel in the high-pressure fuel pipe  34  due to the fuel discharge from the high-pressure fuel pump  40 . In addition, since fuel discharge can be performed a plurality of times during the period between the fuel injection and the next fuel injection depending on the operating state of the internal combustion engine  10 , it is possible to supply an amount of fuel corresponding to the target discharge amount TPt to the high-pressure fuel pipe  34  regardless of the design maximum discharge amount that is the maximum value of discharge amount that can be realized by design in one fuel discharge from the high-pressure fuel pump  40 . Therefore, the controllability of the fuel pressure Pr in the high-pressure fuel pipe  34  can be improved. 
     Further, as shown in  FIG. 7 , in this embodiment, fuel is discharged two times at the point in time t 721 . Since the unit discharge amount TPn in each fuel discharge is smaller than the maximum discharge amount TPmax, it is possible to reduce the pressure fluctuations of the fuel occurring in the high-pressure fuel pipe  34  after the fuel discharge at the point in time t 724 . Further, when fuel is discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34 , the protrusion  75 B of the plunger  75  is prevented from coming into contact with the insertion portion  56 , so that it also contributes to the suppression of sound generated at the high-pressure fuel pump  40 . 
     When fuel discharge is performed a plurality of times, the target unit discharge amount TPnf (TPnf=TPt/Tnf) is set to the amount obtained by dividing the target discharge amount TPt by the necessary discharge count Tnf and the unit discharge amounts TPn of the respective fuel discharges are set to be equal to each other. Accordingly, the amount of fuel supplied in one fuel discharge from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  is constant, and this allows similar pressure fluctuations of the fuel in the high-pressure fuel pipe  34  caused by the fuel discharge to occur in the respective discharges. 
     Next, an example of a manner of fuel discharge when the engine rotational speed NE of the internal combustion engine is high will be described with reference to  FIG. 8 . 
     The required injection amount calculation section  106  calculates a required fuel injection amount Qt at a point in time t 811 . When the required fuel injection amount Qt is calculated, the injection time calculation section  107  calculates an injection time Fi that is the injection execution time of fuel injection based on the required fuel injection amount Qt and the current fuel pressure Pr detected by the pressure sensor  92 . Then, at a point in time t 812 , which is an injection start timing Fs calculated by the injection start timing calculation section  108  based on the crank angle CA detected by the crank angle sensor  95 , the injection valve driving section  109  starts fuel injection from the fuel injection valve  15 . The injection valve driving section  109  continues the fuel injection during the injection time Fi calculated by the injection time calculation section  107 , and ends the fuel injection at a point in time t 813  when the injection time Fi has elapsed from the point in time t 812 . 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt at a point in time t 814  when the convergence time has elapsed from the end timing Fe (point in time t 813 ) of the fuel injection from the fuel injection valve  15 . When the target discharge amount TPt is calculated in this way, the discharge count calculation section  116  calculates a necessary discharge count Tnf for the high-pressure fuel pump  40  to discharge fuel to the high-pressure fuel pipe  34 . In this example, the target discharge amount TPt is 1.2 times the maximum discharge amount TPmax (TPt=1.2×TPmax). Accordingly, the necessary discharge count Tnf is set to two times to calculate the target discharge amount TPt, and the unit discharge amount calculation section  117  calculates a value obtained by dividing the target discharge amount TPt by 2 as the target unit discharge amount TPnf (TPnf=0.6×TPmax). When the necessary discharge count Tnf and the target unit discharge amount TPnf are calculated in this way, the driving amount setting section  118  sets a discharge count Tn of fuel for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection, and a unit discharge amount TPn in each discharge. 
     In order to clarify the advantages of the present embodiment, as a comparative example to the present embodiment, a case where the driving amount setting section  118  first sets the discharge count Tn to the same number of times as the necessary discharge count Tnf, and sets the unit discharge amount TPn to the same amount as the target unit discharge amount TPnf will be described. 
     When the discharge count Tn and the unit discharge amount TPn are set, the pump driving section  119  drives the high-pressure fuel pump  40  so that fuel discharge is started at a discharge start timing Ts calculated by the discharge start timing calculation section  113 . In this case, the pump driving section  119  performs fuel discharge two times from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 815  when the preparation time described above has elapsed from the point in time t 813 , at which the fuel injection is ended. The first fuel discharge is executed during the period between the point in time t 815  and a point in time t 816  when the lift time Ti corresponding to the target unit discharge amount TPnf has elapsed. The pump driving section  119  starts the second fuel discharge at a point in time t 817  when the standby time has elapsed from the point in time t 816 , at which the first fuel discharge is ended. The second fuel discharge is executed during the period between the point in time t 817  and a point in time t 819  when the lift time Ti has elapsed. In this comparative example, the execution time Tad, which is obtained by adding a necessary time Tnes (Tnes=2×lift time Ti+1×standby time) required for performing the fuel discharge of the target unit discharge amount TPnf from the high-pressure fuel pump  40  as many times as the necessary discharge count Tnf and the preparation time described above, exceeds the injection interval Int calculated by the injection interval calculation section  112 . 
     In the present embodiment, when the execution time Tad exceeds the injection interval Int in this way, the discharge count Tn and the unit discharge amount TPn are set such that the execution time Tad does not exceed the injection interval Int. 
     Specifically, the driving amount setting section  118  first calculates a necessary time Tnes (Tnes=2×lift time Ti+1×standby time) required for performing fuel discharge by the discharge amount TPnf set by the unit discharge amount calculation section  117  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . Then, the driving amount setting section  118  calculates a time obtained by adding the necessary time Tnes and the preparation time described above as an execution time Tad. In this case, since the execution time Tad exceeds the injection interval Int calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn and the unit discharge amount TPn such that the amount of fuel discharged from the high-pressure fuel pump  40  is the maximum discharge amount. In this example, the injection interval Int is equal to the sum of the necessary time required for discharging the fuel of the maximum discharge amount TPmax once and the preparation time. Accordingly, the driving amount setting section  118  sets the discharge count Tn to one, and sets the unit discharge amount TPn to the same amount as the maximum discharge amount TPmax. The relationship between the discharge count Tn and the unit discharge amount TPn with respect to the injection interval Int is previously obtained by experiment and simulation and stored in the control device  100 . 
     In this way, when the discharge count Tn and the unit discharge amount TPn are set, the pump driving section  119  drives the high-pressure fuel pump  40  so that fuel discharge is started at a discharge start timing Ts (point in time t 815 ) calculated by the discharge start timing calculation section  113 . In this case, the pump driving section  119  performs fuel discharge once from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 815  when the preparation time described above has elapsed from the point in time t 813 , at which the fuel injection is ended. The fuel discharge is executed during the period between the point in time t 815  and a point in time t 818  when the lift time Ti corresponding to the unit discharge amount TPn (TPn=maximum discharge amount TPmax) has elapsed. The point in time t 818  at which the fuel injection is ended is the same as the injection start timing Fs of the next fuel injection. Accordingly, the fuel discharge is ended when the next fuel injection is started. 
     When the execution time Tad exceeds the injection interval Int in this way, the discharge count Tn and the unit discharge amount TPn are set such that the execution time Tad does not exceed the injection interval Int. In order to discharge fuel from the high-pressure fuel pump  40  once, a time corresponding to the amount of fuel to be discharged is required. Further, the time taken to discharge the fuel from the high-pressure fuel pump  40  also varies depending on the operation characteristics of the high-pressure fuel pump  40  such as the viscosity of the fuel. In the present embodiment, the upper limit of the execution time Tad, which is set based on the discharge count Tn, the unit discharge amount TPn, and the operation characteristics of the high-pressure fuel pump  40 , is shorter when the injection interval Int of fuel is short than when the injection interval Int of fuel is long. Thus, it is possible to prevent the execution time Tad from becoming longer than the injection interval Int of fuel. As a result, it is possible to complete the discharge of fuel within the injection interval Int of fuel, which is a limited period. Therefore, when fuel injection is being executed, fluctuations in the fuel pressure in the high-pressure fuel pipe  34  caused by fuel discharge from the high-pressure fuel pump  40  can be reduced. 
     In the present embodiment, when the target discharge amount TPt is small, the discharge count Tn is set to one; when the target discharge amount TPt is large, the discharge count Tn is set to two times or more. Accordingly, when it is necessary to supply a large amount of fuel to the high-pressure fuel pipe  34 , fuel discharge is performed a plurality of times; when it is unnecessary to supply such a large amount of fuel to the high-pressure fuel pipe  34 , fuel discharge is performed once. Therefore, it is possible to appropriately set the discharge count Tn. 
     Second Embodiment 
     A control device for a fuel pump according to a second embodiment will be described with reference to  FIG. 9 . The second embodiment differs from the first embodiment in the manner in which the unit discharge amount TPn is set. The same constituent elements as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted. In  FIG. 9 , regarding a symbol t indicating the point in time of each operation and three-digit numbers following the symbol, the symbol t and the first digit  9  of the three digits are omitted. 
     As shown in  FIG. 9 , the required injection amount calculation section  106  calculates a required fuel injection amount Qt( 1 ) at a point in time t 911 . When the required fuel injection amount Qt( 1 ) is calculated, the injection time calculation section  107  calculates an injection time Fi( 1 ) that is the injection execution time of fuel injection based on the required fuel injection amount Qt( 1 ) and the current fuel pressure Pr detected by the pressure sensor  92 . Then, at a point in time t 912 , which is an injection start timing Fs calculated by the injection start timing calculation section  108  based on the crank angle CA detected by the crank angle sensor  95 , the injection valve driving section  109  starts fuel injection from the fuel injection valve  15 . The injection valve driving section  109  continues the fuel injection during an injection time Fi calculated by the injection time calculation section  107 , and ends the fuel injection at a point in time t 913  when the injection time Fi( 1 ) has elapsed from the point in time t 912 . 
     By executing this fuel injection, the fuel pressure Pr in the high-pressure fuel pipe  34  decreases. Then, fluctuations occur in the fuel pressure Pr for a while after the point in time t 913 , at which the fuel injection is ended. A period of time from when the point in time t 913 , at which the fuel injection is ended, to when the fuel pressure Pr converges to a constant value is the convergence time described above. 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt( 1 ) at a point in time t 914  when the convergence time has elapsed from the end timing Fe (point in time t 913 ) of the fuel injection. The target discharge amount TPt( 1 ) is calculated based on the required fuel injection amount Qt( 1 ) and a discharge feedback amount TK calculated based on a fuel pressure difference ΔP. 
     When the target discharge amount TPt( 1 ) is calculated, the discharge count calculation section  116  calculates a necessary discharge count Tnf for the high-pressure fuel pump  40  to discharge fuel to the high-pressure fuel pipe  34 . In this example, since the target discharge amount TPt( 1 ) is less than the maximum discharge amount TPmax based on the operation characteristics of the high-pressure fuel pump  40 , the necessary discharge count Tnf is calculated as one. Then, the unit discharge amount calculation section  117  calculates the target discharge amount TPt( 1 ) as a target unit discharge amount TPnf (TPnf=TPt( 1 )). When the necessary discharge count Tnf and the target unit discharge amount TPnf are calculated in this way, the driving amount setting section  118  sets a discharge count Tn of fuel for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection, and a unit discharge amount TPn in each discharge. 
     The driving amount setting section  118  first calculates a necessary time Tnes (lift time Ti) required for performing fuel discharge by the target unit discharge amount TPnf set by the unit discharge amount calculation section  117  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . Then, the driving amount setting section  118  calculates a time obtained by adding the necessary time Tnes and the preparation time described above as an execution time Tad. Since the execution time Tad is equal to or less than an injection interval Int( 1 ) calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn to the same number as the necessary discharge count Tnf, and sets the unit discharge amount TPn to be equal to the target unit discharge amount TPnf. 
     After that, the pump driving section  119  drives the high-pressure fuel pump  40  so that fuel discharge is executed at a discharge start timing Ts calculated by the discharge start timing calculation section  113 . In this case, the pump driving section  119  performs fuel discharge once from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 915  when the preparation time described above has elapsed from the point in time t 913 , at which the fuel injection is ended. The fuel discharge is executed during the period between the point in time t 915  and a point in time t 916  when the lift time Ti corresponding to the target unit discharge amount TPnf has elapsed. 
     By executing this fuel discharge, the fuel pressure Pr in the high-pressure fuel pipe  34  increases. At the point in time t 916 , the fuel discharge is ended, but pressure fluctuations occur in the fuel in the high-pressure fuel pipe  34  for a while. As the unit discharge amount TPn from the high-pressure fuel pump  40  increases, the pressure fluctuations of the fuel tend to increase. The fuel pressure Pr converges to a target fuel pressure Pt when a predetermined time has elapsed from the point in time t 916 , at which the fuel discharge is ended. 
     After that, at a point in time t 917  after the fuel pressure Pr converges to a constant value, the required injection amount calculation section  106  calculates a required fuel injection amount Qt( 2 ) for the next fuel injection. The required fuel injection amount Qt( 2 ) is larger than the required fuel injection amount Qt( 1 ) (Qt( 2 )&gt;Qt( 1 )). When the required fuel injection amount Qt( 2 ) is calculated, the injection time calculation section  107  calculates an injection time Fi( 2 ) that is the injection execution time of fuel injection based on the required fuel injection amount Qt( 2 ) and the current fuel pressure Pr detected by the pressure sensor  92 . At the point in time t 917 , the fuel pressure Pr is equal to the target fuel pressure Pt. The injection valve driving section  109  starts fuel injection from the fuel injection valve  15  at a point in time t 918 , which is the injection start timing Fs. The injection valve driving section  109  continues the fuel injection during an injection time Fi calculated by the injection time calculation section  107 , and ends the fuel injection at a point in time t 919  when the injection time Fi( 2 ) has elapsed from the point in time t 918 . 
     By executing this fuel injection, the fuel pressure Pr in the high-pressure fuel pipe  34  decreases. More fuel is injected in the fuel injection during the period between the point in time t 918  and the point in time t 919  than in the previous fuel injection during the period between the point in time t 912  and the point in time t 913 . Therefore, at the point in time t 919 , at which the fuel injection is ended, the fuel pressure is lower than in the previous fuel injection. Further, since the injected fuel amount is large, the pressure fluctuations of the fuel occurring after the fuel injection are also larger than in the previous fuel injection. Therefore, the convergence time in the subsequent fuel injection is longer than the convergence time in the previous fuel injection. 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt( 2 ) at a point in time t 920  when the convergence time has elapsed from the end timing Fe (point in time t 919 ) of the fuel injection. The target discharge amount TPt( 2 ) is calculated based on the required fuel injection amount Qt( 2 ) and a discharge feedback amount TK calculated based on a fuel pressure difference ΔP. Immediately before the fuel injection is executed at the point in time t 918 , the actual fuel pressure Pr is equal to the target fuel pressure Pt and thus there is no difference. On the other hand, the required fuel injection amount Qt( 2 ) is larger than the required fuel injection amount Qt( 1 ). In the example shown in  FIG. 9 , the target discharge amount TPt( 2 ) is calculated as a value larger than the target discharge amount TPt( 1 ). 
     When the target discharge amount TPt( 2 ) is calculated, the discharge count calculation section  116  calculates a necessary discharge count Tnf. In this example, since the target discharge amount TPt( 2 ) is equal to or larger than the maximum discharge amount TPmax based on the operation characteristics of the high-pressure fuel pump  40  and less than twice the maximum discharge amount TPmax, the necessary discharge count Tnf is calculated as two times. 
     The unit discharge amount calculation section  117  in the present embodiment calculates the target unit discharge amount TPnf as follows. That is, the unit discharge amount calculation section  117  calculates the first target unit discharge amount TPnf in a plurality of times of fuel discharge, that is, a first target unit discharge amount TPnf( 1 ) as the same amount as the maximum discharge amount TPmax (TPnf( 1 )=TPmax). Then, the unit discharge amount calculation section  117  calculates the subsequent target unit discharge amount TPnf in the plurality of times of fuel discharge, that is, a second target unit discharge amount TPnf( 2 ) as the same amount as the amount obtained by subtracting the maximum discharge amount TPmax from the target discharge amount TPt( 2 ) (TPnf( 2 )=TPt−TPmax). 
     When the necessary discharge count Tnf and the target unit discharge amount TPnf are calculated in this way, the driving amount setting section  118  sets a discharge count Tn of fuel for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection, and a unit discharge amount TPn in each discharge. 
     The driving amount setting section  118  first calculates a necessary time Tnes required for discharging the target unit discharge amount TPnf set by the unit discharge amount calculation section  117  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . In this case, the necessary time Tnes is equal to the sum of a lift time Ti( 1 ) taken to discharge the fuel of the target unit discharge amount TPnf( 1 ), the standby time, and a lift time Ti( 2 ) taken to discharge the fuel of the target unit discharge amount TPnf( 2 ). Then, the driving amount setting section  118  calculates a time obtained by adding the necessary time Tnes and the preparation time described above as an execution time Tad. Since the execution time Tad is equal to or less than an injection interval Int( 2 ) calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn to the same number as the necessary discharge count Tnf. In addition, the driving amount setting section  118  sets the unit discharge amount TPn( 1 ) in the first fuel discharge to the same amount as the target unit discharge amount TPnf( 1 ), and sets the unit discharge amount TPn( 2 ) in the second fuel discharge to the same amount as the target unit discharge amount TPnf( 2 ). 
     After that, the pump driving section  119  drives the high-pressure fuel pump  40  so that fuel discharge is started at a discharge start timing Ts calculated by the discharge start timing calculation section  113 . In this case, the pump driving section  119  performs fuel discharge two times from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 921  when the preparation time described above has elapsed from the point in time t 919  at which the fuel injection is ended. The first fuel discharge is executed during the period between the point in time t 921  and a point in time t 922  when the lift time Ti( 1 ) corresponding to the target unit discharge amount TPnf( 1 ) has elapsed. The pump driving section  119  starts the second fuel discharge at a point in time t 923  when the standby time has elapsed from the point in time t 922 , at which the first fuel discharge is ended. The second fuel discharge is executed during the period between the point in time t 923  and a point in time t 924  when the lift time Ti( 2 ) corresponding to the target unit discharge amount TPnf( 2 ) has elapsed. The first lift time Ti( 1 ) is longer than the second lift time Ti( 2 ). 
     After that, at the point in time t 925  after the fuel discharge, the required injection amount calculation section  106  calculates a required fuel injection amount Qt( 3 ) for the next fuel injection, and then the fuel injection is performed. 
     In this way, in the present embodiment, the discharge count Tn of fuel discharged from the high-pressure fuel pump  40  and the unit discharge amount TPn for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection are controlled based on the required fuel injection amount Qt that is correlated with the operating state of the internal combustion engine  10 . Depending on the amount of fuel discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34 , the magnitude of pressure fluctuations of the fuel in the high-pressure fuel pipe  34  changes. The control of the discharge count Tn and the unit discharge amount TPn of the fuel pump for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection based on the operating state of the internal combustion engine  10  makes it possible to realize a supply of fuel that makes it difficult for an excess or a shortage of fuel in the high-pressure fuel pipe  34  to occur when the fuel of the target discharge amount TPt is discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34 , while taking into consideration the influence of the pressure fluctuations of the fuel in the high-pressure fuel pipe  34  due to the fuel discharge from the high-pressure fuel pump  40 . In addition, since fuel discharge can be performed a plurality of times during the period between the fuel injection and the next fuel injection depending on the operating state of the internal combustion engine  10 , it is possible to supply an amount of fuel corresponding to the target discharge amount TPt to the high-pressure fuel pipe  34  regardless of the design maximum discharge amount that is the maximum value of discharge amount that can be realized by design in one fuel discharge from the high-pressure fuel pump  40 . Therefore, the controllability of the fuel pressure Pr in the high-pressure fuel pipe  34  can be improved. 
     The first unit discharge amount TPn is set to the same amount as the maximum discharge amount TPmax when fuel discharge is performed a plurality of times, and the subsequent unit discharge amount is set to be smaller than the maximum discharge amount TPmax. In this case, as shown in  FIG. 9 , the fuel pressure Pr increases relatively greatly after the point in time t 922 , at which the first fuel discharge is ended, so that the fluctuations in the fuel pressure increase. On the other hand, in the second fuel discharge thereafter, the fuel pressure Pr does not increase so much after the point in time t 924 , at which the second fuel discharge is ended, so that the pressure fluctuations of the fuel are less than those in the first fuel discharge. In this way, in the case where fuel discharge is repeated, the reduced magnitude of the pressure fluctuations of the fuel in the last fuel discharge as compared to the magnitude of the pressure fluctuations of the fuel in the first fuel discharge makes it possible to shorten the fluctuation period of the pressure fluctuations in fuel. 
     Third Embodiment 
     A control device for a fuel pump according to a third embodiment will be described with reference to  FIGS. 10 and 11 . The third embodiment differs from the first embodiment in the manner in which the high-pressure fuel pump  40  is driven. The same constituent elements as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted. 
     As shown in  FIG. 10 , a control device  300  for a fuel pump includes, as functional sections, a target rotational speed calculation section  101 , a target torque calculation section  102 , a target fuel pressure calculation section  103 , a fuel pressure difference calculation section  104 , an injection feedback amount calculation section  105 , a required injection amount calculation section  106 , an injection time calculation section  107 , an injection start timing calculation section  108 , and an injection valve driving section  109 . Further, the control device  300  includes a target throttle opening degree calculation section  110 , a throttle driving section  111 , an injection interval calculation section  112 , a discharge start timing calculation section  113 , a target discharge amount calculation section  114 , a pump characteristics learning section  115 , a discharge count calculation section  116 , a first unit discharge amount calculation section  301 , a driving amount setting section  118 , and a first pump driving section  302 . 
     The target rotational speed calculation section  101 , the target torque calculation section  102 , the target fuel pressure calculation section  103 , the fuel pressure difference calculation section  104 , the injection feedback amount calculation section  105 , the required injection amount calculation section  106 , the injection time calculation section  107 , the injection start timing calculation section  108 , and the injection valve driving section  109  each have the same function as those in the first embodiment. Further, the target throttle opening degree calculation section  110 , the throttle driving section  111 , the injection interval calculation section  112 , the discharge start timing calculation section  113 , the target discharge amount calculation section  114 , the pump characteristics learning section  115 , the discharge count calculation section  116 , and the driving amount setting section  118  each have the same function as those in the first embodiment. 
     The first unit discharge amount calculation section  301  has the same function as the unit discharge amount calculation section  117  in the first embodiment. Further, the first pump driving section  302  has the same function as the pump driving section  119  in the first embodiment. 
     The injection interval calculation section  112 , the discharge start timing calculation section  113 , the target discharge amount calculation section  114 , the pump characteristics learning section  115 , the discharge count calculation section  116 , the first unit discharge amount calculation section  301 , the driving amount setting section  118 , and the first pump driving section  302  constitute an inter-injection discharge control executing section  303 . The inter-injection discharge control executing section  303  executes an inter-injection discharge control of executing fuel discharge from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a predetermined point in time within a period between fuel injection from the fuel injection valve  15  and the next fuel injection. 
     The control device  300  also includes an individual control executing section  304  and a control switching section  305 . 
     The individual control executing section  304  executes an individual control of repeatedly discharging fuel from the high-pressure fuel pump  40  in a fixed cycle. In the individual control, fuel discharge is performed irrespective of the timing of fuel injection from the fuel injection valve  15 . The individual control executing section  304  includes a discharge cycle storage section  306 , a second unit discharge amount calculation section  307 , and a second pump driving section  308  as functional sections. 
     The discharge cycle storage section  306  stores an energization cycle that is a cycle of executing energization control for the high-pressure fuel pump  40 . In the present embodiment, the energization cycle is a fixed cycle, and is previously obtained by experiment and simulation such that the cycle is shorter than the driving cycle of the high-pressure fuel pump  40  in the inter-injection discharge control, and the energization cycle is stored. 
     The second unit discharge amount calculation section  307  calculates a unit discharge amount TPn that is the amount of fuel discharged from the high-pressure fuel pump  40  per one time in the individual control. The second unit discharge amount calculation section  307  calculates the unit discharge amount TPn such that when the battery voltage is high, it becomes larger than when the battery voltage is low. In the present embodiment, the unit discharge amount TPn is set to the same amount as the maximum discharge amount TPmax. 
     The second pump driving section  308  drives the high-pressure fuel pump  40  by executing energization control for the high-pressure fuel pump  40  based on the unit discharge amount TPn calculated by the second unit discharge amount calculation section  307  and the energization cycle stored in the discharge cycle storage section  306 , without taking into consideration the timing of fuel injection from the fuel injection valve  15 . 
     When a start condition of the individual control is satisfied, the control switching section  305  switches the drive control for the high-pressure fuel pump  40  from the inter-fuel discharge control to the individual control. The start condition is set in the control switching section  305 , and indicates that a fuel pressure difference ΔP calculated by the fuel pressure difference calculation section  104  is equal to or higher than a predetermined pressure. Specifically, when the fuel pressure difference ΔP is equal to or higher than the predetermined pressure, the drive control for the high-pressure fuel pump  40  is switched to the individual control. When the fuel pressure difference ΔP is lower than the predetermined pressure, the drive control for the high-pressure fuel pump  40  is switched to the inter-injection discharge control. The predetermined pressure is set to the same value as a fuel pressure difference ΔP for which a time (for example, several seconds) is required for the fuel pressure Pr to reach the target fuel pressure Pt when the inter-injection discharge control is executed. The predetermined pressure is previously obtained by experiment and simulation and stored in the control device  300 . 
     An operation and advantages of the present embodiment will now be described with reference to  FIG. 11 . In the present embodiment, in particular, the following operation and advantages can be obtained. In  FIG. 11 , regarding a symbol t indicating the point in time of each operation and four-digit numbers following the symbol, the symbol t and the first two digits  11  of the four digits are omitted. In  FIG. 11 , a drive control for the fuel injection valve  15  and the high-pressure fuel pump  40  at the time of starting the internal combustion engine  10  will be described as an example. 
     At a point in time t 1111  immediately after the start of the internal combustion engine  10 , the fuel pressure Pr in the high-pressure fuel pipe  34  is low. Accordingly, the fuel pressure difference ΔP between the target fuel pressure Pt and the fuel pressure Pr, which is calculated at the time of starting the internal combustion engine  10 , is equal to or higher than the predetermined pressure. For this reason, the control switching section  305  sets the drive control for the high-pressure fuel pump  40  to the individual control. 
     In the individual control, the second pump driving section  308  performs energization control in the energization cycle stored in the discharge cycle storage section  306  so that the unit discharge amount TPn becomes a unit discharge amount TPn calculated by the second unit discharge amount calculation section  307 . Specifically, the second pump driving section  308  repeatedly executes fuel discharge from the high-pressure fuel pump  40  by using a lift time Ti corresponding to the unit discharge amount TPn at the point in time t 1111 . By the individual control, fuel is discharged from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  without taking into consideration the timing of fuel injection from the fuel injection valve  15 . 
     Therefore, the fuel pressure Pr increases toward the target fuel pressure Pt at an early stage. Then, when the fuel pressure Pr reaches a pressure close to the target fuel pressure Pt, the control switching section  305  switches the drive control for the high-pressure fuel pump  40  from the individual control to the inter-injection discharge control at a point in time t 1112  when the fuel pressure difference ΔP becomes lower than the predetermined pressure. 
     In the inter-injection discharge control, the high-pressure fuel pump  40  is driven as follows. 
     That is, after the start of the internal combustion engine  10 , when the fuel pressure Pr reaches the target fuel pressure Pt, fuel injection from the fuel injection valve  15  is executed at a subsequent point in time t 1113 . By repeating this fuel injection, the fuel pressure Pr decreases. 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt( 1 ) at a point in time t 1115  when the convergence time has elapsed from the end timing Fe (point in time t 1114 ) of the fuel injection. The target discharge amount TPt(l) is calculated based on a required fuel injection amount Qt(l) for the fuel injection during the period between the point in time t 1113  and the point in time t 1114 , and a discharge feedback amount TK calculated based on a fuel pressure difference ΔP. 
     When the target discharge amount TPt(l) is calculated in this way, the discharge count calculation section  116  calculates a necessary discharge count Tnf for the high-pressure fuel pump  40  to discharge fuel to the high-pressure fuel pipe  34 . In this example, since the target discharge amount TPt( 1 ) is less than the maximum discharge amount TPmax based on the operation characteristics of the high-pressure fuel pump  40 , the necessary discharge count Tnf is calculated as one. Then, the first unit discharge amount calculation section  301  calculates the target discharge amount TPt( 1 ) as a target unit discharge amount TPnf (TPnf=TPt( 1 )). When the necessary discharge count Tnf and the target unit discharge amount TPnf are calculated in this way, the driving amount setting section  118  sets a discharge count Tn of fuel for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection, and a unit discharge amount TPn in each discharge. 
     The driving amount setting section  118  first calculates a necessary time Tnes (lift time Ti) required for performing fuel discharge by the target unit discharge amount TPnf set by the first unit discharge amount calculation section  301  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . Then, the driving amount setting section  118  calculates a time obtained by adding the necessary time Tnes and the preparation time described above as an execution time Tad. Since the execution time Tad is equal to or less than an injection interval Int( 1 ) calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn to the same number as the necessary discharge count Tnf, and sets the unit discharge amount TPn to be equal to the target unit discharge amount TPnf. 
     After that, the first pump driving section  302  drives the high-pressure fuel pump  40  so that fuel discharge is executed at a discharge start timing Ts calculated by the discharge start timing calculation section  113 . In this case, the first pump driving section  302  performs fuel discharge once from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 1116  when the preparation time described above has elapsed from the point in time t 1114 , at which the fuel injection is ended. The fuel discharge is executed during the period between the point in time t 1116  and a point in time t 1117  when the lift time Ti corresponding to the target unit discharge amount TPnf has elapsed. Accordingly, fuel discharge from the high-pressure fuel pump  40  is performed at the discharge start timing Ts that is a predetermined point in time within the period between the end of the fuel injection from the fuel injection valve  15  and the start of the next fuel injection. By executing this fuel discharge, the fuel pressure Pr in the high-pressure fuel pipe  34  increases. 
     After that, at a point in time t 1118 , fuel injection from the fuel injection valve  15  is started. By executing the fuel injection, the fuel pressure Pr in the high-pressure fuel pipe  34  decreases. More fuel is injected in the fuel injection during the period between the point in time t 1118  and a point in time t 1119  than in the previous fuel injection during the period between the point in time t 1113  and the point in time t 1114 . Therefore, at the point in time t 1119 , at which the fuel injection is ended, the fuel pressure is lowered than in the previous fuel injection. At the point in time t 119 , since the fuel pressure difference ΔP between the fuel pressure Pr and the target fuel pressure Pt is lower than the predetermined pressure, the inter-injection discharge control is continued. 
     The target discharge amount calculation section  114  calculates a target discharge amount TPt( 2 ) at a point in time t 1120  when the convergence time has elapsed from the end timing Fe (point in time t 1119 ) of the fuel injection. The target discharge amount TPt( 2 ) is calculated based on a required fuel injection amount Qt( 2 ) for the subsequent injection and a discharge feedback amount TK calculated based on a fuel pressure difference ΔP. Since the required fuel injection amount Qt( 2 ) is larger than the required fuel injection amount Qt( 1 ), the target discharge amount TPt( 2 ) is calculated as a value larger than the target discharge amount TPt( 1 ). 
     When the target discharge amount TPt( 2 ) is calculated, the discharge count calculation section  116  calculates a necessary discharge count Tnf. In this example, since the target discharge amount TPt( 2 ) is equal to or larger than the maximum discharge amount TPmax based on the operation characteristics of the high-pressure fuel pump  40  and less than twice the maximum discharge amount TPmax, the necessary discharge count Tnf is calculated as two times. Then, the first unit discharge amount calculation section  301  calculates a value obtained by dividing the target discharge amount TPt( 2 ) by 2 as the target unit discharge amount TPnf (TPnf=TPt( 2 )/2). When the necessary discharge count Tnf and the target unit discharge amount TPnf are calculated in this way, the driving amount setting section  118  sets a discharge count Tn for the period between the fuel injection from the fuel injection valve  15  and the next fuel injection, and a unit discharge amount TPn in each discharge. 
     The driving amount setting section  118  first calculates a necessary time Tnes (Tnes=2×lift time Ti+1×standby time) required for performing fuel discharge by the discharge amount TPnf set by the first unit discharge amount calculation section  301  as many times as the necessary discharge count Tnf, based on the operation characteristics of the high-pressure fuel pump  40  learned by the pump characteristics learning section  115 . Then, the driving amount setting section  118  calculates a time obtained by adding the necessary time Tnes and the preparation time described above is calculated as an execution time Tad. Since the execution time Tad is equal to or less than an injection interval Int( 2 ) calculated by the injection interval calculation section  112 , the driving amount setting section  118  sets the discharge count Tn to the same number as the necessary discharge count Tnf, and sets the unit discharge amount TPn to be equal to the target unit discharge amount TPnf. 
     After that, the first pump driving section  302  drives the high-pressure fuel pump  40  so that fuel discharge is started at a discharge start timing Ts calculated by the discharge start timing calculation section  113 . In this case, the first pump driving section  302  performs fuel discharge two times from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  at a point in time t 1121  when the preparation time described above has elapsed from the point in time t 1119 , at which the fuel injection is ended. The first fuel discharge is executed during the period between the point in time t 1121  and a point in time t 1122  when the lift time Ti corresponding to the target unit discharge amount TPnf has elapsed. The first pump driving section  302  starts the second fuel discharge at a point in time t 1123  when the standby time has elapsed from the point in time t 1122 , at which the first fuel discharge is ended. The second fuel discharge is executed during the period between the point in time t 1123  and a point in time t 1124  when the lift time Ti has elapsed. The first lift time and the second lift time are equal to each other. Accordingly, fuel discharge from the high-pressure fuel pump  40  is performed at the discharge start timing Ts that is a predetermined point in time within the period between the end of the fuel injection from the fuel injection valve  15  and the start of the next fuel injection. 
     When the operating state of the internal combustion engine changes at a point in time t 1125  and the target fuel pressure Pt calculated by the target fuel pressure calculation section  103  increases, the fuel pressure difference ΔP between the target fuel pressure Pt and the fuel pressure Pr increases accordingly. In  FIG. 11 , as the target fuel pressure Pt increases, ΔP becomes equal to or higher than the predetermined pressure. For this reason, the control switching section  305  switches the drive control for the high-pressure fuel pump  40  from the inter-injection discharge control to the individual control. As a result, the second pump driving section  308  performs the energization control for the high-pressure fuel pump  40  until the fuel pressure Pr reaches a pressure close to the target fuel pressure Pt after the point in time t 1125 . As described above, in the individual control, the second pump driving section  308  performs the energization control for the high-pressure fuel pump  40  in the energization cycle stored in the discharge cycle storage section  306  so that the unit discharge amount TPn becomes the unit discharge amount TPn calculated by the second unit discharge amount calculation section  307 . Thus, fuel discharge from the high-pressure fuel pump  40  is repeated irrespective of the timing of fuel injection from the fuel injection valve  15 . The driving cycle of the high-pressure fuel pump  40  in the individual control is shorter than the driving cycle of the high-pressure fuel pump  40  in the inter-injection discharge control. In the inter-injection discharge control, the interval between the starts of the two fuel discharges (for example, the period of time between the point in time t 1116  and the point in time t 1121 ) that are executed via the timing of fuel injection from the fuel injection valve  15  is the driving cycle of the high-pressure fuel pump  40 , while in the individual control, the interval between the starts of fuel discharges (for example, the period of time between the point in time t 1125  and the point in time t 1126 ) that are intermittently executed is the driving cycle of the high-pressure fuel pump  40 . Accordingly, the fuel discharge interval in the individual control is shorter than the fuel discharge interval in the inter-injection discharge control. Therefore, in the individual control, the count of supplying fuel into the high-pressure fuel pipe  34  can be increased as compared to the inter-injection discharge control, and the fuel pressure Pr can be increased to the target fuel pressure Pt at an early stage. 
     In the present embodiment, in the individual control, the unit discharge amount TPn is made larger when the battery voltage is high than when the battery voltage is low. Therefore, when the fuel is repeatedly discharged at fixed intervals by the individual control, it is possible to drive the high-pressure fuel pump  40  with an appropriate discharge amount in consideration of the battery voltage. 
     The above-described embodiments may be modified as follows. The above-described embodiments and the following modifications can be combined as long as the combined modifications remain technically consistent with each other. 
     In the third embodiment, the control switching section  305  is configured to set the drive control of the high-pressure fuel pump  40  to individual control when the start condition indicating that the fuel pressure difference ΔP is equal to or higher than the predetermined pressure is satisfied. The start condition is not limited to this. For example, the start condition may include both the fuel pressure difference ΔP of equal to or higher than the predetermined pressure and the point in time at which the internal combustion engine  10  is started. In this case, the individual control is set when the fuel pressure difference ΔP is equal to or higher than the predetermined pressure and when the internal combustion engine  10  is started. 
     In addition, the control switching section  305  may be configured to set the individual control when the internal combustion engine  10  is started irrespective of whether or not the fuel pressure difference ΔP is equal to or higher than the predetermined pressure. 
     In the third embodiment, the example described above is that only one predetermined pressure is used as a determination value for determining switching of the drive control for the high-pressure fuel pump  40  in the control switching section  305 . However, the manner in which the drive control is switched is not limited to this. That is, a determination value for determining switching from the inter-injection discharge control to the individual control and a determination value for determining switching from the individual control to the inter-injection discharge control may be different. In this case, for example, the control switching section  305  may be configured so that when the fuel pressure difference ΔP is equal to or higher than a first predetermined pressure, the drive control is switched from the inter-injection discharge control to the individual control; when the fuel pressure difference ΔP is lower than a second predetermined pressure that is lower than the first predetermined pressure, the drive control is switched from the individual control to the inter-injection discharge control. 
     In the third embodiment, the second unit discharge amount calculation section  307  is configured to calculate the unit discharge amount TPn for the individual control when the battery voltage is high such that it becomes larger than when the battery voltage is low. However, such a configuration may be omitted. That is, it is also possible to calculate a discharge amount that does not change depending on the battery voltage as the unit discharge amount TPn. 
     In each of the above-described embodiments, the target discharge amount calculation section  114  can also be configured to calculate the target discharge amount TPt based on parameters other than the required fuel injection amount Qt. For example, the target discharge amount calculation section  114  may calculate the target discharge amount TPt based on the engine rotational speed NE of the internal combustion engine  10  detected by the crank angle sensor  95 , the load of the internal combustion engine  10 , etc. Even with such a configuration, the target discharge amount TPt can be calculated based on the operating state of the internal combustion engine. In the case where the target discharge amount TPt is calculated based on the engine rotational speed NE of the internal combustion engine  10 , the target discharge amount TPt can be calculated such that it becomes larger when the engine rotational speed NE is high than when the engine rotational speed NE is low. Further, in the case where the target discharge amount TPt is calculated based on the load of the internal combustion engine  10 , the target discharge amount TPt can be calculated such that it becomes larger when the load is high than when the load is low. 
     Further, the target discharge amount calculation section  114  can appropriately change the point in time at which the target discharge amount TPt is calculated. For example, the target discharge amount calculation section  114  can calculate the target discharge amount TPt at the point in time at which the fuel injection is ended, without taking the convergence time into consideration. 
     In each of the above-described embodiments, the discharge count calculation section  116  can also be configured to calculate the necessary discharge count Tnf based on parameters other than the target discharge amount TPt. For example, the discharge count calculation section  116  can calculate the necessary discharge count Tnf based on the engine rotational speed NE of the internal combustion engine  10  detected by the crank angle sensor  95  or the load of the internal combustion engine  10 . In the case where the necessary discharge count Tnf is calculated based on the engine rotational speed NE of the internal combustion engine  10 , the necessary discharge count Tnf can be calculated such that it becomes larger when the engine rotational speed NE is high than when the engine rotational speed NE is low. Further, in the case where the necessary discharge count Tnf is calculated based on the load of the internal combustion engine  10 , the necessary discharge count Tnf can be calculated such that it becomes larger when the load is high than when the load is low. 
     Further, the discharge count calculation section  116  may be configured to calculate a preset fixed number of times as the necessary discharge count Tnf, instead of calculating the necessary discharge count Tnf depending on the operating state of the internal combustion engine. With this configuration, as in the above-described embodiments, the upper limit of the execution time Tad is restricted by the injection interval Int of fuel in the fuel injection valve  15 . Thus, the discharge count Tn of fuel and the unit discharge amount TPn are controlled based on the operating state of the internal combustion engine. 
     An example of a manner in which the discharge count Tn and the unit discharge amount TPn are set in the case where the necessary discharge count Tnf is calculated as a fixed number will be described below. In the following example, the necessary discharge count Tnf is set to three. 
     As shown in  FIG. 12 , in the discharge count calculation section  116 , the fixed number of times is set to three. In addition, in this example, the target discharge amount TPt is 1.5 times the maximum discharge amount TPmax, and the unit discharge amount TPn when fuel discharge is performed is set to half the maximum discharge amount TPmax (½×TPmax). 
     As shown in  FIG. 12 , the execution time Tad is shorter than an injection interval Int ( 1 ) of fuel in the fuel discharge started at a point in time t 1211 . Accordingly, the discharge count Tn is set to three, and fuel discharge is performed three times. 
     On the other hand, when the rotational speed of the internal combustion engine increases and an injection interval Int( 2 ) becomes shorter than Int( 1 ), as indicated by the long dashed double-short dashed lines in  FIG. 12 , the execution time Tad of the fuel discharge started at a point in time t 1212  becomes longer than the injection interval Int( 2 ) of fuel. In this case, the discharge count Tn and the unit discharge amount TPn are set based on the injection interval Int( 2 ). In this example, the discharge count Tn is set to two times, and the unit discharge amount TPn is set to an amount of 0.75 times the maximum discharge amount TPmax. Then, fuel discharge is performed at the discharge count Tn and the unit discharge amount TPn that are calculated based on the injection interval Int( 2 ). In this way, even when the necessary discharge count Tnf is a fixed number of times, the upper limit of the execution time Tad can be set by the injection interval Int that changes depending on the operating state of the internal combustion engine  10 . Thus, the discharge count Tn and the unit discharge amount TPn are controlled based on the operating state of the internal combustion engine  10 . 
     When the upper limit of the execution time Tad is set according to the injection interval Int of fuel in the fuel injection valve  15 , fuel discharge may be executed with an amount smaller than the target discharge amount TPt. Accordingly, in each of the above-described embodiments, when a restricted upper limit of the execution time Tad causes the execution time Tad to continue to be equal to the injection interval Int of fuel for a predetermined time, a control manner may be adopted in which the individual control for executing fuel discharge is executed irrespective of the timing of fuel injection. In the case of adopting such a configuration, when the individual control is executed and the fuel pressure Pr increases accordingly, the individual control is switched to the inter-injection discharge control to perform fuel discharge based on the timing of fuel injection. With that configuration, even when a configuration to restrict the upper limit of the execution time Tad is adopted, it is possible to suppress a decrease in the fuel pressure Pr in the high-pressure fuel pipe  34 . 
     Each of the above-described embodiments may be configured not to set the upper limit of the execution time Tad according to the injection interval Int of fuel in the fuel injection valve  15 . 
     The operation characteristics of the high-pressure fuel pump  40  does not have to be necessarily learned. 
     In each of the above-described embodiments, the example in which the discharge count Tn is set to one or two has been described. However, it is obvious that the discharge count Tn may be set to three or more. 
     Each of the above-described embodiments does not limit the manner in which the unit discharge amount TPn is set when fuel discharge from the high-pressure fuel pump  40  to the high-pressure fuel pipe  34  is performed a plurality of times during a period between fuel injection from the fuel injection valve  15  and the next fuel injection. For example, the unit discharge amount TPn for the last fuel discharge among a plurality of times of fuel discharge may be set to be the same amount as the maximum discharge amount TPmax of the high-pressure fuel pump  40 , and the unit discharge amount TPn for the other fuel discharges except for the last fuel discharge among the plurality of times of fuel discharge may be set to be smaller than the maximum discharge amount TPmax. Further, when three or more times of fuel discharge is performed during the period between fuel injection from the fuel injection valve  15  and the next fuel injection, the unit discharge amount TPn for the first and last fuel discharges among the plurality of times of fuel discharge may be set to be smaller than the maximum discharge amount TPmax of the high-pressure fuel pump  40 , and the unit discharge amount TPn for the other fuel discharges except for the first and last fuel discharges among the plurality of times of fuel discharge may be set to be equal to the maximum discharge amount TPmax. In addition, among the plurality of times of fuel discharge, the unit discharge amount TPn for fuel discharge at a later point in time may be set to be smaller, or the unit discharge amount TPn for fuel discharge at a later point in time may be set to be larger. Further, the unit discharge amount TPn for each of the plurality of times of fuel discharge can be set to be smaller than the maximum discharge amount TPmax of the high-pressure fuel pump  40 , and the respective unit discharge amounts TPn can be set to be different from each other. 
     Although the injection start timing calculation section  108  calculates a fixed point in time at which the predetermined crank angle before reaching the compression top dead center as the injection start timing Fs, the injection start timing Fs may be set depending on the operating state of the internal combustion engine  10 , instead of the fixed timing. For example, the injection start timing calculation section  108  can calculate the injection start timing Fs, which is a point in time at which fuel injection from each fuel injection valve  15  is started, based on the required fuel injection amount Qt calculated by the required injection amount calculation section  106 , the injection time Fi calculated by the injection time calculation section  107 , and the engine rotational speed NE detected by the crank angle sensor  95 . In this case, each injection start timing Fs in the fuel injection valve  15  can be calculated such that the fuel injection for the required fuel injection amount Qt is completed before the ignition time of the cylinder where the fuel injection valve  15  is disposed. 
     In each of the above-described embodiments, the discharge start timing calculation section  113  calculates a point in time at which the predetermined preparation time has elapsed from the end timing Fe of fuel injection as the discharge start timing Ts. Calculation of the discharge start timing Ts can be changed as appropriate. For example, a point in time at which the convergence time has elapsed from the end timing Fe of fuel injection may be calculated as the discharge start timing Ts. In this case, fuel discharge is executed at the same point in time as the point in time at which the target discharge amount calculation section  114  calculates the target discharge amount TPt. Further, in the configuration in which the target discharge amount TPt is calculated before the fuel injection end timing Fe, the fuel injection end timing Fe may be calculated as the discharge start timing Ts without taking into consideration the preparation time. In this case, the fuel discharge is started at the point in time at which the fuel injection is ended. Furthermore, the discharge start timing calculation section  113  can also calculate a point in time within a period of fuel injection between the start of the fuel injection to the end of the fuel injection as the discharge start timing Ts. 
     In each of the above-described embodiments, the injection interval Int is calculated as a period between the end of fuel injection and the start of the next fuel injection. Calculation of the injection interval Int is not limited to this. For example, a period between the start of fuel injection and the start of the next fuel injection, a period between the start of fuel injection and the end of the next fuel injection, or a period between the end of fuel injection to the end of the next fuel injection may be calculated as the injection interval Int. 
     In each of the above-described embodiments, the example in which the period between fuel injection and the next fuel injection from the fuel injection valve  15  is defined as a period between the end of the fuel injection and the start of the next fuel injection has been described. However, the period between fuel injection from the fuel injection valve  15  and the next fuel injection is not limited to that definition. That is, the period between fuel injection from the fuel injection valve  15  and the next fuel injection means a concept including a period between the end of fuel injection and the end of the next fuel injection, a period between the start of fuel injection and the start of the next fuel injection, and a period between the start of fuel injection and the end of the next fuel injection. 
     The fuel in the fuel tank  31  may be drawn in by the high-pressure fuel pump  40 . In this case, the low-pressure fuel pump  32  and the low-pressure fuel pipe  33  can be omitted. 
     The configuration of the high-pressure fuel pump  40  can be changed as appropriate. For example, the plunger  75  may be composed of a round bar portion that is made of a material different from magnetic material and is inserted in the cylinder bore  57 , and a magnetic portion that is connected to one end of the round bar portion and is made of a magnetic material. Furthermore, a configuration may be provided in which the plunger  75  is displaced by moving the magnetic portion by a magnetic field generated by energizing the coil  85  so that the volume of the pressurizing chamber  78  is changed. Further, a configuration may be provided in which the plunger  75  does not have the protrusion  75 B. In other words, as long as the fuel pump can reciprocate the plunger  75  by energization and has a suction function of drawing in fuel by reciprocating the plunger  75  and a discharge function of pressurizing and discharging the drawn fuel, the same control device as that in each of the above-described embodiments can be adapted for the fuel pump. 
     The control devices  100  and  300  for a fuel pump have a function of controlling the driving of the fuel injection valve  15  and a function of controlling the driving of the throttle valve  21 . These functions may be included in a control section different from the control devices  100  and  300  for the fuel pump. In this case, each of the control devices  100  and  300  may be configured to communicate with the control section to transmit and receive necessary information to and from each other so that the driving of the fuel pump is controlled in the same manner as in each of the above-described embodiments. 
     The control device is not limited to a device that includes a CPU, a ROM, and a RAM and executes software processing. For example, a dedicated hardware circuit (such as an ASIC) may be provided that executes at least part of the software processes executed in each of the above-described embodiments. That is, the controller may be modified as long as it has any one of the following configurations (a) to (c). (a) A configuration including a processor that executes all of the above-described processes according to programs and a program storage device such as a ROM that stores the programs. (b) A configuration including a processor and a program storage device that execute part of the above-described processes according to the programs and a dedicated hardware circuit that executes the remaining processes. (c) A configuration including a dedicated hardware circuit that executes all of the above-described processes. A plurality of software processing circuits each including a processor and a program storage device and a plurality of dedicated hardware circuits may be provided. That is, the above processes may be executed in any manner as long as the processes are executed by processing circuitry that includes at least one of a set of one or more software processing circuits and a set of one or more dedicated hardware circuits.