Patent Publication Number: US-6209524-B1

Title: Fuel-injection apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fuel-injection apparatus adapted to direct-injection engines. 
     2. Description of the Prior Art 
     Of the various fuel-injection apparatus that have been conventionally developed, for example, the pressure-accumulated, fuel-injection apparatus is widely used, in which fuel stored in the common rail under pressure is injected into the combustion chambers by the closure and open of the solenoid-operated valves provided on the heads of the injectors. A fuel-injection apparatus, for example, shown in FIG. 5, has a needle valve  59  that is movable in a nozzle body  57  for a reciprocating manner to open and close discharge orifices  58  at the tip of the nozzle body  57 , the needle valve  59  being constantly urged by the action of a closing spring  60  to close the discharge orifices  58 . 
     The nozzle body  57  is provided therein with a fuel passage  62  for allowing the fuel fed under high pressure from the common rail to flow into a fuel sac  61 , a fuel passage  64  for allowing the fuel fed under high pressure from the common rail to flow into a balance chamber  63 , a fuel passage  67  for communicating the balance chamber  63  with a space  66  containing therein a solenoid-operated valve  65  to open and close the fuel passage  67 , a fuel-leak passage  70  for communicating the space  66  with an intermediate chamber  69  surrounding around a slender section  68  of the needle valve  59 , and a fuel discharge passage  71  for communicating the space  66  to a fuel reservoir. 
     The needle valve  59  has upper and lower sections arranged on the axially opposing end of the spender section  68 , the upper section being made greater in diameter, compared with the lower section. The needle valve  59  is subjected to the hydraulic pressure, or fuel pressure, of the high-pressure fuel forced into the fuel sac  61 , which acts so as to open the discharge orifice  58 , and at the same time subjected to the resultant force of the urging force of the closing spring  60  and the fuel pressure of high-pressure fuel in the balance chamber  63 , which acts so as to close the discharge orifice  58 . As the intermediate chamber  69  is exposed to the low fuel pressure acting through the fuel-leak passage  70 , the high-pressure fuel in the fuel sac  61  and balance chamber  63  may leak out through a clearance between the confronting needle valve  59  and the nozzle body  57 . The fuel leaked out in the intermediate chamber  69  is collected in the reservoir through the fuel-leak passage  70 , intermediate chamber  66  and fuel discharge passage  71 . 
     When energizing the solenoid-operated valve  65 , a valve body  72  of the solenoid-operated valve  65  is attracted to an electromagnet against an elastic force of a return spring  73 . At this event, the fuel passage  67  is open to the space  66  whereby the balance chamber  63  is relieved through the fuel passage  67 , resulting in the reduction in the fuel pressure therein. The force of fuel pressure acting on the fuel sac  61  is designed greater than the resultant force of the force of fuel pressure acting on the balance chamber  63  with the spring force and, therefore, the needle valve  59  moves upwards, resulting in opening the discharge orifice  58  as shown in FIG.  5 . In contrast, when deenergizing the solenoid-operated valve  65 , the valve body  72  moves downwards by the action of the return spring  73  to thereby close the space  66  at the fuel passage  67 . As a result, the resultant force of the elastic force of the spring  60  with the force of fuel pressure restored in the balance chamber  63  becomes greater than the force of fuel pressure acting in the fuel sac  61  to make the needle valve  59  move downwards thereby closing the discharge orifice  58 . 
     On the fuel-injection apparatus as described above, however, the fuel leaks constantly out from the intermediate chamber  69  through the fuel passage  67 , namely, the fuel is constantly under the static leakage, in addition to that the fuel leaks out from the balance chamber  63  to the space  66  through the fuel passage  67  at every actuation of the solenoid-operated valve  65 . Moreover, as seen from FIG. 6, the amount of static leakage of fuel increases with the increase of the common rail pressure. That is to say, as the pressure in the intermediate chamber  69  is constantly under the low pressure or the atmospheric pressure, the increase of the fuel pressure results in increasing the amount of the leakage of fuel from the fuel sac  61  and balance chamber  63  to the intermediate chamber  69  through the clearance between the confronting needle valve  59  and nozzle body  57 . 
     To cope with the problem as described just above, a fuel-injection apparatus was developed, which is disclosed in Japanese Patent Laid-Open No.77924/1998. The fuel-injection apparatus, as apparent from FIG. 7, has for its object to make sealing with the use of fuel pressure and is comprised of a body having fuel-discharge orifice equivalent to the reference number  58 , a needle valve  75  movable in a space  74  in the body in a reciprocating manner so as to open and close at its one axial end the fuel-discharge orifice, a balance chamber  76  in which the needle valve  75  is exposed at its axially opposite end serving as a pressure-supporting face to control the amount of lift of the needle valve  75 , a fuel-supply passage  77  for applying the fuel pressure to the balance chamber  76 , a fuel-discharge passage for relief of the fuel pressure in the balance chamber  76 , a valve  79  for opening and closing the fuel-discharge passage  78 , and an actuator for operating the valve  79 . The actuator-operated valve  79  is composed of a valve stem  80  extending through the fuel-discharge passage  78  into the balance chamber  76 , and a valve face provide at the tip of the valve stem  80  so as to make a contact with a valve seat formed at the ingress of the fuel-discharge passage  78 . The actuator-operated valve  79  is made of the valve stem  80  and valve body  81  that are formed integrally with each other. 
     When the actuator is not energized, a return spring  82  forces the valve stem  80  upwards through a spring shoe  83  while the valve face abuts against its associated valve seat and, therefore, the actuator-operated valve  79  closes the fuel-discharge passage  78 . On this event, the high-pressure fuel from the common rail is applied to the fuel sac, shown at  61  in FIG.  5 . The fuel in the sac forces the needle valve  75  to the direction of lift. Moreover, the fuel pressure applied to the balance chamber  76  through the fuel passage  77  acts on a pressure-support face  87 . At this instant, the resultant force of the spring force of a diaphragm spring  88  and the force of fuel pressure acting on the pressure-support face  87  of the needle valve  75  exceeds the force of fuel pressure applied in the fuel sac, which is exerted on the needle valve  75  to open the discharge orifice, so that the needle valve  75  is held closed to stop the fuel injection out of the discharge orifice. 
     The instant the actuator is energized, the valve stem  80  is forced downwards in FIG. 7 against the compressed spring force of the return spring  82 , moving the valve face of the valve body  81  off its seat, whereby the actuator-operated valve  79  opens the fuel-discharge passage  78 . The fuel passage  77  has the effect of a kind of iris, which renders the flow of fuel in the fuel passage  77  smaller than that in the fuel-discharge passage  78 . Therefore, opening the fuel-discharge passage  78  results in relieving the balance chamber  76  of the fuel pressure to the space  74 . The instant the fuel pressure in the balance chamber  76  is relieved, the force to move the needle valve  75  towards opening overcomes the resultant force of the spring force of the diaphragm spring  88  and the fuel force acting on the pressure-support face  87  of the needle valve  75 , which urges the needle valve  75  to the direction of closing. This raises the needle valve  75  off its seat whereby the fuel is injected out of the discharge orifice into the combustion chamber. 
     On deenergizing the actuator, the valve stem  80  is lifted up by the action of the return spring  82  to close the actuator-operated valve  79 . The fuel pressure in the balance chamber  76  is restored by the fuel supply through fuel passage  77  to thereby force downwards the needle valve  75  to close the discharge orifice with the result that the fuel injection ceases. The restored fuel pressure in the balance chamber  76  acts on the valve body  81  and consequently urges, in addition to the force of the return spring  82 , the valve face against the its seat. It will be understood that the higher the fuel pressure in the balance chamber is, the greater is the force opening the actuator-operated valve  79 , which may be thus kept certainly against the fuel leakage. 
     In the meantime, the actuator-operated valve  79  for opening and closing the fuel-discharge passage  78  to the balance chamber  76  has been conventionally produced by forming integrally the valve stem  80  with the valve body  81 . Such integral forming is preferred for producing the large-sized valves in view of mechanical strength and production cost. Nevertheless, as the actuator-operated valve  79  for the fuel-injection apparatus has the valve stem  80  and valve body  81 , which are usually in the range of from about zero point several millimeters to at most several millimeters in their diameter, it becomes much more difficult to achieve the mechanical strength and finish accuracy of the actuator-operated valve  79 , which are durable to the recent high-injection pressure. 
     In integral forming of the valve stem  80  with the valve body  81  of the valve  79 , there has been a limit to form the valve stem  80  and valve body  81  slender with high accuracy and, therefore, it could not be helped to make them larger in diameter. For the reasons described above, it has been impossible to make the sufficiently smaller size for the part exposed to the pressure or the area of the pressure-support face of the actuator-operated valve  79 . Increase in high fuel-injection pressure causes increase in thrust force exerted from the pressure-support face on valve opening and consequently the valve opening requires the valve-operating power increasing in proportion to the thrust force on valve opening. This inevitably has resulted in using the actuator relatively large in its size, which has caused a major problem in incorporation into the engine. 
     SUMMARY OF THE INVENTION 
     The present invention is to overcome the above-described shortcomings to be solved, and to provide an improvement in a fuel-injection apparatus in which the fuel pressure applied to the balance chamber through the fuel passage is relieved by opening the actuator-operated valve in the fuel-discharge passage to control a lift of the needle valve exposed at its pressure-support face to the balance chamber, thereby injecting the fuel out of the discharge orifice upon the lift of the needle valve. The present invention more especially provides an improved fuel-injection apparatus in which the fuel pressure in the balance chamber is used for the valve-closing force upon closure of the actuator-operated valve to keep the fuel against leakage through the actuator-operated valve, while a valve stem and valve body of the actuator-operated valve are formed separately from each other and made slender with resulting in miniaturizing the actuator-operated valve whereby the valve-operating power may be made less upon opening of the actuator-operated valve and consequently the actuator may be made smaller in size. 
     The present invention relates to a fuel-injection apparatus comprising, a main body provided with discharge orifice for fuel spray, a needle valve arranged in a space in the main body for reciprocating movement so as to open and close at its one end the discharge orifice, a balance chamber in which the opposite end of the needle valve is exposed so as to provide a fuel pressure-exposed surface to control a lift of the needle valve, a fuel path for supplying a fuel pressure into the balance chamber, a fuel-discharge passage for relieving the fuel pressure in the balance chamber, a valve for opening and closing the fuel-discharge passage, and an actuator for operating the valve, the valve being composed of a valve stem extending through the fuel discharge passage into the balance chamber and a valve body attached to one end of the valve stem and having a valve face that is, on valve closing position, in contact with a valve seat formed at an ingress opening of the fuel discharge passage, and both the valve stem and the valve body being formed separately from each other and united with each other. 
     In the fuel-injection apparatus constructed as described above, when the actuator-operated valve shuts off the fuel-discharge passage, the valve body moves, together with the valve stem extending through the fuel-discharge passage into the balance chamber, towards the egress of the fuel-discharge passage whereby the valve face is brought into a face-to-face contact with the valve seat to close the fuel-discharge passage. At this event, as the fuel pressure in the balance chamber exerts the force to close the actuator-operated valve, the higher the fuel pressure in the balance chamber is, the greater is the force closing the valve. Accordingly, the force sufficient to reseat the valve body to the valve seal may be ensured so that the valve may block certainly the fuel leakage. 
     In production of the actuator-operated valve, moreover, since the valve stem and the valve body are formed separately from each other, both the valve stem and the valve body may be made less in their diameter with high accuracy whereby the valve fabricated by uniting the valve stem with the valve body is made small in its pressure-exposed area. Consequently, less force may be sufficient for opening the valve upon valve-opening phase, resulting in making it possible to employ the miniaturized actuator less in electric power consumption. 
     In one aspect of the present invention, an actuator-operated valve is disclosed wherein a reinforcing plate is fixed to both the ends of the valve stem and the valve body by the use of laser-beam welding. The valve body has a central hole in which one end of the valve stem is inserted. Then a reinforcing plate is abutted to both the inserted foremost end of the valve stem and the end face opposite to the valve face of the valve body. All the valve stem, valve body and reinforcing plate are welded together as a unit, preferably, by laser-beam welding. The reinforcing plate is effective for preventing a crack, which might otherwise occur in the valve stem and the valve body. 
     In another aspect of the present invention, any one of the valve seat and the valve face is formed with concave surfaces for a cancel of pressure. The concave surface is provided for introducing the fuel pressure in the balance chamber thereby to cancel the fuel pressure. Introduction of the fuel pressure into the concave surface on the valve body results in canceling the force that acts in the direction of valve-closing whereby less force may be necessary for pushing the valve to its opening position upon valve-opening phase. 
     In another aspect of the present invention, the concave surfaces are formed on the valve face of the valve body and composed of an annular grooves and radial grooves extending radially from the annular groove so as to communicate with the balance chamber. The concave surfaces has the configuration of, but not limited to, grooves, which is preferable to make a reliable snugly contact of the valve body against the valve seat as well as protect the valve seat from unbalanced abrasion. 
     As the fuel-injection apparatus of this invention is constructed as described just above, the higher the fuel pressure in the balance chamber is, the greater is the force closing the actuator-operated valve, which may thus block certainly the fuel leakage flowing out through the valve. This relieves the fuel injection pump from useless working load, thereby improving specific fuel consumption of engines. Face-to-face contact of the valve face with the valve seat makes it easier to preselect the adequate contact pressure occurring between them, thereby eliminating wear of the valve seal. In case the actuator-operated valve is of a poppet valve and, therefore, the valve seat is formed in a tapered concave face in conformity with the valve body of the poppet valve, the valve may operate reliably and speedy due to the snug fitting of the confronting tapered faces, even if the turbulence eddy flow happens in fuel flow. On the closure position, the reliable sealing may be established between the valve face and the valve seat to prevent certainly the fuel leakage. 
     Moreover, the balance chamber is defined by the recess in the control member while the fuel-discharge passage is also formed in the same control member, whereby the intake and discharge means for the fuel pressure to drive the needle valve may be concentrated on the control member. This is convenient for structure, production and assembly of the fuel-injection apparatus. Forming the fuel passages at the interfaces of the control member and the nozzle body, further, provides the fuel passages by the use of the counterpart of the interfaces, resulting in improving the workability. Moreover, the return spring to bias the needle valve towards its position for closing the discharge orifice is contained in the space in the control member whereby the fuel-injection apparatus may be made compact in size. Adoption of the structure as described above may contribute to production cost saving. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an axial sectional view showing an embodiment of a fuel-injection apparatus according to the present invention: 
     FIG. 2 is a fragmentary axial sectional view showing the essential parts of the fuel-injection apparatus shown in FIG.  1 : 
     FIG. 3 is a plan view showing a valve body of an actuator-operated valve: 
     FIG. 4 is a sectional view taken along the line A—A of FIG.  3 : 
     FIG. 5 is a schematic axially sectioned view illustrating a conventional fuel-injection apparatus: 
     FIG. 6 is a graphic representation of common rail pressure versus amount of static leak of fuel: and 
     FIG. 7 is a fragmentary axially sectioned view showing the essential parts of another conventional fuel-injection apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now in detail to the drawings, a preferred embodiment of the fuel-injection apparatus according to the present invention will be explained below. This fuel-injection apparatus is suitably applicable to the common rail, fuel-injection system or pressure-accumulated, fuel-injection apparatus, not shown in the drawing. The fuel fed in an accumulator, referred to as “common rail” hereinafter, by a fuel injection pump is intensified in pressure in the common rail to the high pressure fuel, which is in turn charged out of injection nozzles of the fuel-injection apparatus into combustion chambers. 
     An injector body  1  of the fuel-injection apparatus, shown in FIG. 1, is hermetically attached in a bore, not shown in figures, provided in a base such as a cylinder head through a sealing member. The injector body  1  is hermetically mounted at the lower end thereof with a nozzle while at the upper shoulder thereof with a high-pressure fuel inlet  2 . The injector  1  includes a middle section  3  provided therein with a space  4  extending axially of the injector  1 . Arranged in the space  4  is a valve  5  for opening and closing a fuel-discharge passage  33 , which will be described below. The valve  5  is designed to be driven by an actuator  6  having an electromagnet  7 , which is accommodated in the injector body  1  with a fixing cap  8  screwed on the middle section  3 . The electromagnet  7  operates in response to control signals issued from a control unit  9  to drive the valve towards its open position or downwards in FIG.  1 . 
     Motion of a plunger  12  with the electromagnet  7  is transmitted to an output shaft  10 , one end of which is formed with a flange  46 , while the opposite end is connected to the actuator-operated valve  5 . The output shaft  10  extends for sliding movement through a guide bore  11  reduced radially with respect to the space  4  in the middle section  3 , and a guide recess  47  in a control member  13 , which will be explained below. A return spring  48  is installed between the flange  46  and a stepped upper edge of the guide bore  11  to urging the output shaft  10  axially upwards. The output shaft  10  may make a high-speed axial reciprocating movement under the operation of the electromagnet  7 . 
     The control member  13  is arranged interposed between the middle section  3  and a nozzle body  14 . Both the control member  13  and the nozzle body  14  are united together to the middle section  3  to constitute a part of the injector body  1  by screwing a threaded cap  15 , engaged with the nozzle body  14 , onto the mating portion of the middle section  3 . The nozzle body  14  is provided therein with a nozzle bore  16  in which a needle valve  17  is inserted for sliding movement so as to provide an annular clearance  18  therebetween. The clearance around the needle valve  17  forms a high-pressure fuel passage. The nozzle body  14  is formed at its tip with a discharge orifice  19  through which the fuel is injected into the combustion chambers of the internal combustion engine. The needle valve  17  has a tapered end. Axial reciprocating motion of the needle valve  17  causes the its tapered end to raise off and to reseat on a confronting tapered surface  20  at the tip of the nozzle bore  16  in the nozzle body  14 , whereby the fuel flow to be injected out of the discharge orifice  19  may be allowed and blocked. The needle valve  17  has at the midway area thereof an annular tapered surface  21  that forms a pressure-exposed surface for bearing the fuel pressure acting in the direction where the needle valve  17  opens the discharge orifice. The instant the needle valve  17  is raised off the tapered surface  20 , the high-pressure fuel may be injected out of the discharge orifice  19  into the combustion chamber. In contrast, when the needle valve  17  moves back down onto the tapered surface  20 , the fuel flow is blocked and thus the fuel injection ceases. 
     The fuel fed from the common rail for the high-pressure supply source to the fuel inlet  2  flows through a fuel passage  22  in the injector body  1 , a fuel passage  23  in the control member  13  and a fuel passage  24  in the nozzle body  14 , and reaches the fuel sac  25  to which is exposed the tapered surface  21  for the pressure-exposed surface. The instant the needle valve  17  opens the discharge orifice  19 , the fuel in the sac  25  may be injected out of the discharge orifice  19 . 
     As shown in detail in FIG. 2, the control member  13  has a hole  27  that is at the position offset radially outwardly with respect to the center thereof and in alignment with a hole  26  in the injector body  1 . A connector pin  28 , shown in FIG. 1, is inserted in the confronting holes  26 ,  27  to thereby keep the control member  13  in proper position relatively of the middle section  3 . The control member  13  is further provided with a recess  29  opened facing to the nozzle body  14 . The needle vale  17 , explained in detail hereinafter, extends into the recess  29  and has at its end a pressure-exposed surface  31  to the fuel pressure, which cooperates with the recess  29  to define the balance chamber  30 . The control member  13  is bored with a fuel path  32 , which is open to the fuel passage  23  and extends slantwise radially to the center of the control member  13 . The fuel path  32  communicates with the balance chamber  30  to feed the high-pressure fuel into the chamber  30 . The fuel-discharge passage  33  is bored axially at the center of the control member  13  so as to communicate at one end thereof with the balance chamber  30  and at the opposite end with the axially extending space  4  in the middle section  3 . 
     The actuator-operated valve  5  includes a valve stem  34  connected integrally with the output shaft  10  of the actuator  6 , and a valve body  38  bonded with one end of the valve stem  34  by laser-beam welding. The valve stem  34  and the valve body  38  constitute, respectively, the valve stem and valve body structures according to this invention. A return spring  35  of coil spring is abutted at its end against a spring bearing  36  fixed to the output shaft  10  and at the opposite end against a top face  37  of the control member  13 . The return spring  35  is fitted under compression to urge constantly the valve stem  34  upwards, that is, the return spring  35  forces the actuator-operated valve  5  to the closure position. 
     The valve stem  34  extends through the fuel-discharge passage  33 , leaving a small clearance therebetween, into the balance chamber  30 . The valve stem  34  has at its end the valve body  38  to open and close the fuel-discharge passage  33 . 
     Referring to FIGS. 3 and 4, the valve body  38  is provided with a central hole  49  having at the midway portion thereof a tapered step  50 . The valve stem  34  and the valve body  38  are formed separately from each other. In assembly, the valve stem  34  is forced into the central hole  49  of the valve body  38  till the slender nose of the valve stem  34  abuts against the tapered step  50 , where a bottom end  52  of the valve stem  34  is positioned in flush with a bottom surface  51  of the valve body  38 . The assemblage of the valve  5  finishes with laser-beam welding a reinforcing plate  53  to the bottom end  52  of the valve stem  34  and the bottom surface  51  of the valve body  38 . The reinforcing plate  53  is provided for protecting the valve stem  34  and valve body  38  against the high pressure, which might be otherwise cracked under the high pressure. The valve stem  34  and the valve body  38  may be made less in diameter because they are formed separately. This contributes to miniaturization of the actuator-operated valve itself, resulting in reducing the fuel pressure acting on the valve  5 . 
     The valve body  38  has a tapered conical valve face  39  on a cylindrical major portion  56 . The valve face  39  is in complementary with a valve seat  40  of a conical convex surface, which is formed in the fuel-discharge passage  33  at its end opened facing to the balance chamber  30 . The valve face  39  is formed with an annular groove  54  concentric with the central hole  49 , and radial grooves  55  arranged spaced from each other with a fixed angles and extended radially outwardly from the annular groove  54 . Namely, the radial grooves  55  are arranged symmetric with respect to the center of the valve body  38 . The radial grooves  55  communicate the annular groove  54  with the balance chamber  30  to introduce the fuel pressure in the balance chamber  30  to the annular groove  54 . The fuel pressure in the annular groove  54  acts on the valve body  38  so as to depress the valve body  38 , thereby counteracting the force pressing the valve body  38  against the valve seat  40 . Consequent less force to depress the valve body  38  upon valve opening may be sufficient, so that the miniaturized actuator may provide satisfactory effects. 
     When the electromagnet  7  is in nonconductive state or deenergized, the actuator-operated valve  5  is held by the elastic action of the return spring  35  in closing state where the valve face  38  of the valve body  39  seats against the valve seat  40  in face-to-face contact relation, thereby blocking the fuel-discharge passage  33 . With the electromagnet  7  in energization by the application of electricity, the valve stem  34  of the valve  5  moves downwards in FIG.1, overcoming the elastic forces of the return springs  35 ,  48 . This forces the valve face  39  of the valve body  38  off its valve seat  40  to open the fuel discharge passage  33  at its end of the balance chamber  30 , causing the flow of fuel whereby the fuel pressure in the balance chamber  30  is relieved to the space  4  through the clearance between the confronting fuel-discharge passage  33  and the valve stem  34 . 
     A coil spring  43  for return spring is interposed under compression between a bottom wall  41  of the recess  29  and a spring bearing  42  attached to the axial end  44  of the needle valve  17 . The coil spring  43  forces the needle valve  17  to its closure position where the needle valve  17  blocks the fuel flow to the discharge orifice  19 . The force of the fuel pressure in the balance chamber  30  acting on the pressure-exposed surface  31  of the needle valve  17  may control the lift of the valve body  38  under balance with the fuel pressure exerted on the pressure-exposed surface of the tapered portion  21  of the needle valve  17  and the return force of the coil spring  43  acting on the needle valve  17 . The recess  29  in the control member  13  is partially enlarged to provide a shoulder  45  for accommodate the spring bearing  42 . The shoulder  45  is formed larger in depth by a distance H, compared to the thickness of the spring bearing  42 . The distance H is equal to a distance spanning from the closure position to the open position of the needle valve  17 , which may be thus movable within the range of between the closure and open positions. 
     Since the effective open area provided by moving the valve face  39  of the valve body  38  off the valve seat  40  is designed less than the cross-sectional area of the clearance between the fuel-discharge passage  33  and the valve stem  34  over almost all operating range of the valve  5 , the open degree of the actuator-operated valve  5  upon opening the fuel-discharge passage  33  defines the extent of reduction of the fuel pressure in the balance chamber  30 . 
     The following explains as to the operation of the embodiment constructed as described just above. With the electromagnet  7  being deenergized, the return spring  35 , as shown in FIG. 2, forces the valve stem  34  through the spring bearing  36  upwards in the drawings, whereby the valve face  39  of the valve body  38  seats against the valve seat  40  so that the actuator-operated valve  5  shuts off the fuel-discharge passage  33 . In this event, the high-pressure fuel fed from the common rail is supplied from the high-pressure fuel inlet  2  to the fuel sac  25  through the fuel passages  22 ,  23  and  24 . The fuel in the sac  25  acts on the tapered surface  21  of the needle valve  17 , which is thus urged towards the direction of lift. The fuel reaches the clearance  18  defined between the nozzle body  14  and the periphery of the needle valve  17  thereby to fill the clearance  18 . Moreover, the fuel pressure, which is charged in the balance chamber  30  through the fuel passage  32 , acts on the pressure-exposed surface  31  of the needle valve  17 . Under this phase, the resultant force of the return force of the coil spring  43  with the force of the fuel pressure acting the pressure-exposed surface  31  to force the needle valve  17  to its closing position exceeds the force of the fuel pressure acting on the pressure-exposed surface of the tapered surface  21  to force the needle vale  17  to its open position and, therefore, the needle valve  17  closes the discharge orifice  19  whereby the fuel injection ceases. 
     The instant the controller unit  9  energizes the electromagnet  7 , the valve stem  34  is forced downwards in FIG. 1 against the compression spring force of the return spring  35  to move the valve face  39  of the valve body  38  off the valve seat  40  whereby the valve  5  opens the fuel-discharge passage  33 . The fuel path  32  has the effect of an iris, which renders the flow of fuel in the fuel path  32  smaller than that in the fuel-discharge passage  33 . Therefore, opening the fuel-discharge passage  33  relives the fuel pressure in the balance chamber  30  to the space  4 . Upon relief of the fuel pressure in the balance chamber  30 , the force of the fuel pressure acting on the tapered surface  21  to force the needle valve  17  to its open position overcomes the resultant force of the return force of the coil spring  43  with the force of the fuel pressure acting the pressure-exposed surface  31  to force the needle valve  17  to its closing position to thereby lift the needle vale  17  so that the fuel is injected out of the discharge orifice  19  into the combustion chambers. As the effective open area of the fuel-discharge passage  33  opened by the actuator-operated valve  5  is designed less than the cross-sectional area of any other fuel-discharge passages after the balance chamber  30 , the open degree of the actuator-operated valve  5  defines the magnitude of the fuel pressure in the balance chamber  30 . 
     The instant the control unit  9  ceases the supply of electric current to the electromagnet  7 , the return spring raises the valve stem  34  to close the actuator-operated valve  5 . The balance chamber  30  is applied the fuel pressure from the fuel path  32  thereby restoring the fuel pressure therein and, consequently, the needle valve  17  stops the fuel injection. The restored fuel pressure acts on the valve body  38  to consequently urge, in addition to the force of the return spring  35 , the valve face  39  against the its seat  40 . It will be understood that the higher the fuel pressure in the balance chamber  30  is, the greater is the force closing the actuator-operated valve  5 , which may thus block certainly the fuel leakage. 
     According to the fuel-injection apparatus of the present invention, the valve face  39  is formed with the annular groove  54  concentric with the central hole  49 , and the radial grooves  55  extended radially outwardly from the annular groove  54 , whereby the fuel pressure in the balance chamber  30  is introduced into the annular groove  54 . The fuel pressure in the annular groove  54  acts on the valve body  38  so as to open the actuator-operated valve  5  and, consequently, the valve  5  is easier to open, compared to the prior injectors. This makes it possible to adopt the miniaturized actuator  6  in which less electric power may be sufficient. 
     In the embodiment described above, although both the valve stem  34  and the valve body  38  are integrally united to the actuator-operated valve  5  by the laser-beam welding through the reinforcing plate  53 , the laser-beam welding is not necessarily required and, for example, another assemblage may be employed of press-fitting the valve stem  34  into the valve body  38  and caulking the end of the valve stem. Further, the actuator  6  for the valve  5  may be of piezoelectric element, instead of the electromagnet  7  adopted exemplarily in the embodiment described above. The actuator  6  of piezoelectric element may achieve the rapid operation of the start and stop of the fuel injection with less response lag even the fuel injection cycle is very short in period. Moreover, as the effective open area provided in the fuel-discharge passage by moving the valve body of the actuator-operated valve  5  off the valve seat is less than the minimum cross-sectional area of the clearance between the fuel-discharge passage  33  and the valve stem  34 , the open degree of the actuator-operated valve  5  may affect the magnitude of the fuel pressure relieved from the balance chamber  30 . As the operations of the actuator-operated valve  5  may be altered by changing the timing, conductive duration and voltage of the electric current applied to the piezoelectric element, changing the lift speed of the needle valve  17  in accordance with the engine operating conditions may ensure various fuel-injection rating characteristics, in particular, initial fuel-injection rating characteristics with stability, resulting in reducing NOx emission and noise level. 
     It should be understood that the foregoing relates to only preferred embodiments of the present invention, and that is intended to cover all changes and modifications of the examples of the invention herein chosen for the purposes of the disclosure, which do not constitute departure from the spirit and scope of the invention.