Patent Publication Number: US-6910644-B2

Title: Solenoid-operated fuel injection valve

Description:
INCORPORATION BY REFERENCE 
   The disclosure of Japanese Patent Application No. 2001-394587 filed on Dec. 26, 2001, including the specification, drawings, and abstract is incorporated herein by reference in its entirety. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a solenoid-operated fuel injection valve. 
   2. Description of Related Art 
   Japanese Patent Laid-Open Application No. 8-210217 discloses a solenoid-operated fuel injection valve in accordance with the related art. In this fuel injection valve, a needle member disposed in a container into which fuel is introduced is longitudinally moved by suction forces generated by electromagnetic means, whereby the area of a fuel flow passage defined as a space between an inner surface of the container and an outer surface of the needle member is changed. After having flown through the fuel flow passage, fuel is injected from a nozzle hole. 
   However, the solenoid-operated fuel injection valve in accordance with the related art has the following problems. That is, the suction forces are inappropriate, and the operation of opening and closing the valve cannot be reliably performed. 
   SUMMARY OF THE INVENTION 
   The invention has been made in view of the problems mentioned above. It is an object of the invention to provide a solenoid-operated fuel injection valve that can be reliably opened and closed. 
   In order to solve the problems mentioned above, the invention provides a solenoid-operated fuel injection valve in which an area of a fuel flow passage defined as a space between an inner surface of a container into which fuel is introduced and an outer surface of a needle member disposed in the container is changed by moving the needle member longitudinally by means of suction forces generated by an electromagnetic controller. The electromagnetic controller is provided with first and second magnetic circuits through which the suction forces can be controlled independently of each other. 
   Because this solenoid-operated fuel injection valve employs first and second magnetic circuits through which the suction forces can be controlled independently of each other, the suction forces can be suitably set, and the operation of opening and closing the valve can be reliably performed. In particular, if the first and second magnetic circuits are designed to simultaneously generate suction forces during a predetermined period, the suction forces can be amplified, and the operation of opening and closing the valve can be more reliably performed. 
   If the first and second magnetic circuits are disposed in a longitudinal direction of the needle member, the suction forces can be increased without enlarging the dimension in a direction perpendicular to the longitudinal direction (i.e., radially). 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other objects, features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of the exemplary embodiments of the invention, when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is an explanatory view of a relationship of mechanical connection among internal elements of a solenoid-operated fuel injection valve in accordance with a first embodiment of the invention; 
       FIG. 2  is an explanatory view of a relationship of mechanical connection among internal elements of a solenoid-operated fuel injection valve in accordance with a second embodiment of the invention; 
       FIG. 3  is an explanatory view of a relationship of mechanical connection among internal elements of a solenoid-operated fuel injection valve in accordance with a third embodiment of the invention; 
       FIG. 4  is an explanatory view of a relationship of mechanical connection among internal elements of a solenoid-operated fuel injection valve in accordance with a fourth embodiment of the invention; 
       FIG. 5  is a longitudinal sectional view of the solenoid-operated fuel injection valve in accordance with the fourth embodiment; 
       FIG. 6   a  is a longitudinal sectional view of the solenoid-operated fuel injection valve in accordance with the fourth embodiment in a closed state; 
       FIG. 6   b  is a longitudinal sectional view of the solenoid-operated fuel injection valve in accordance with the fourth embodiment in an open state with a small fuel injection amount; 
       FIG. 6   c  is a longitudinal sectional view of the solenoid-operated fuel injection valve in accordance with the fourth embodiment in an open state with a large fuel injection amount; 
       FIG. 7  is an explanatory view of a function of suppressing secondary fuel injection in the solenoid-operated fuel injection valve in accordance with the fourth embodiment; and 
       FIG. 8  shows timing charts of currents (drive pulses) I 1 , I 2  supplied to first and second coils M 1   c , M 2   c  respectively, suction forces FL, FS, spring forces F 1 , F 2 , a valve-closing force FD resulting from a differential pressure, and a needle valve position. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   In the following description and the accompanying drawings, the invention will be described in more detail in terms of exemplary embodiments. 
   Solenoid-operated fuel injection valves in accordance with the embodiments will be described hereinafter. It is to be noted herein that like elements are denoted by like reference numerals and that repetition of the same description will be avoided. 
   A first embodiment of the invention will be described with reference to  FIG. 1 , which is an explanatory view of a relationship of mechanical connection among internal elements of an exemplary solenoid-operated fuel injection valve. In this solenoid-operated fuel injection valve, a needle member N disposed in a container H into which fuel is introduced is moved in the longitudinal direction (+Z direction) of the needle member N by suction forces FL, FS generated by an electromagnetic means, whereby the area of a fuel flow passage PS defined as a space between an inner surface IS of the container H and an outer surface of the needle member N changes. The electromagnetic means are provided with first and second magnetic circuits M 1 , M 2  through which the suction forces FL, FS can be controlled independently of each other. 
   This solenoid-operated fuel injection valve assumes a “closed” state when the fuel flow passage PS is closed by the needle member N, and assumes an “open” state when the fuel flow passage PS has been formed. Fuel that has been introduced into the container H is injected from a fuel injection nozzle hole depending on the area of the fuel flow passage PS. Although this nozzle hole can be constructed of the fuel flow passage PS itself, it is also appropriate that the nozzle hole be formed on the rear stage side of the fuel flow passage PS. 
   This solenoid-operated fuel injection valve employs the first and second magnetic circuits M 1 , M 2  through which the suction forces FL, FS can be controlled independently of each other. Therefore, the suction forces (functions of FL and FS) can be suitably set, and the solenoid-operated fuel injection valve can be reliably opened and closed. In particular, if the first and second magnetic circuits M 1 , M 2  are designed to simultaneously generate suction forces for a predetermined period (T 1 , T 2 : see FIG.  8 ), the suction forces can be increased. As a result, the solenoid-operated fuel injection valve can be more reliably opened and closed. 
   The first magnetic circuit M 1  has a pair of magnetic bodies (M 1   a,  M 1   b ) that are opposed and attracted to each other across a gap A 1 . The second magnetic circuit M 2  has a pair of magnetic bodies (M 2   a,  M 2   b ) that are opposed and attracted to each other across a gap A 2 . One of the magnetic bodies M 1   a,  M 1   b  (M 1   a  selected herein) constitutes an electromagnet. One of the magnetic bodies M 2   a,  M 2   b  (M 2   a  selected herein) also constitutes an electromagnet. Either of the magnetic bodies constituting each pair of the magnetic bodies M 1   a,  M 1   b  or M 2   a,  M 2   b  may constitute an electromagnet. 
   It is to be noted herein that an electromagnet is constructed by adding a coil to a magnetic body. For convenience of explanation, however, each of the electromagnets and a corresponding one of the magnetic bodies M 1   a,  M 2   a  are denoted by the same reference numeral. 
   A magnetic body is a metal such as iron, cobalt, nickel, or the like, which is a substance generated by a magnetic pole in a magnetic field. An electromagnet can be constructed by winding a coil around such a substance. If a current is supplied to the coil, a magnetic flux generated by the current flowing through the coil and a magnetic flux generated by the magnetic pole of the magnetic body around which the coil is wound flow through the gaps A 1 , A 2 , whereby strong magnetic fields are formed in the gaps A 1 , A 2 . As a result, the suction forces FL, FS are generated in the magnetic circuits M 1 , M 2  respectively, which include the gaps A 1 , A 2  respectively. 
   It is assumed in the first embodiment that the suction force FL of the first magnetic circuit M 1  is larger than the suction force FS of the second magnetic circuit M 2  (FL&gt;FS) when the solenoid-operated fuel injection valve is closed. The suction force FL of the first magnetic circuit M 1  is inversely proportional to a dimension (air gap) G 1  of the gap A 1 , and the suction force FS of the second magnetic circuit M 2  is inversely proportional to a dimension (air gap) G 2  of the gap A 2 . That is, when the solenoid-operated fuel injection valve is closed, the dimension G 1  of the gap A 1  is smaller than the dimension G 2  of the gap A 2  (G 1 &lt;G 2 ), and thus, the suction force FL is larger than the suction force FS. 
   The magnetic bodies M 1   a,  M 1   b  (M 2   a,  M 2   b ) are disposed between the container H and the needle member N such that the needle member N is moved in the longitudinal direction thereof (+Z direction) by the suction forces FL, FS of the magnetic bodies M 1   a,  M 1   b  (M 2   a,  M 2   b ). The first and second magnetic circuits M 1 , M 2  are compactly disposed. 
   The electromagnets M 1   a,  M 2   a  are fixed to the container H. The magnetic body M 1   b  is connected to a driving force transmission member FX via a first elastic means (spring) S 1 . The magnetic body M 2   b  is fixed to the needle member N. 
   If the suction resulting from the first magnetic circuit M 1  becomes effective when the solenoid-operated fuel injection valve assumes a closed state, the magnetic body M 1   b  moves in the +Z direction. Then, the driving force transmission member connected to the magnetic body M 1   b  moves in the +Z direction due to a spring force of the first elastic means S 1 . Because the driving force transmission member FX is fixed to the needle member N, the needle member N moves in the +Z direction. As a result, the fuel flow passage PS is formed, and the solenoid-operated fuel injection valve is opened. The container H and the needle member N are connected by a second elastic means (spring) S 2 , and the suction force FL generated by the first magnetic circuit M 1  acts against a spring force of the second elastic means S 2 . The second elastic means S 2  is provided if necessary. 
   When the magnetic body M 1   b  is moved by the suction force F L  toward the electromagnet M 1   a  fixed to the container H, the magnetic body M 1   b  abuts on the electromagnet M 1   a . This substantially stops the needle member N from moving. If it is assumed that the needle member N is at a reference position in the Z direction when the solenoid-operated fuel injection valve is closed, the distance between the reference position and the stop position is a limit stroke (first stroke) of the needle member N which is determined by the first magnetic circuit M 1 . In the first embodiment, the first stroke corresponds to the dimension G 1  of the gap. 
   If the suction resulting from the second magnetic circuit M 2  becomes effective when the solenoid-operated fuel injection valve is in a closed state, the magnetic body M 2   b  moves in the +Z direction against a spring force generated by the second elastic means S 2 . Because the magnetic body M 2   b  is fixed to the needle member N, the needle member N moves in the +Z direction. If this suction force acts when the solenoid-operated fuel injection valve is closed, the fuel flow passage PS is formed. Then, the solenoid-operated fuel injection valve is opened. 
   Even after the needle member N has moved in the +Z direction by the first stroke, if the needle member N is further sucked in the +Z direction by the second magnetic circuit M 2 , the magnetic body M 2   b  further moves in the +Z direction. Then, the magnetic body M 2   b  moves in the +Z direction against a spring force of the first elastic means S 1  as well as a spring force of the second elastic means S 2 . Thus, the needle member N fixed to the magnetic body M 2   b  moves in the +Z direction. If the magnetic body M 2   b  abuts on the electromagnet M 2   a,  the second magnetic circuit M 2  stops the needle member N from moving. If it is assumed that the needle member N is at a reference position in the Z direction when the solenoid-operated fuel injection valve is closed, the distance between the reference position and a stop position is a second limit stroke (second stroke) of the needle member N which is determined by the second magnetic circuit M 2 . In the first embodiment, the second stroke corresponds to the dimension G 2  of the gap. It is to be noted herein that there are some cases where the stroke is not equal to the dimension of the gap. 
   The first stroke (=G 1 ) of the needle member N which is determined by the first magnetic circuit M 1  is shorter than the second stroke (=G 2 ) of the needle member N which is determined by the second magnetic circuit M 2 . Thus, since the strokes are different from each other, the solenoid-operated fuel injection valve of the first embodiment makes it possible to change the area of the fuel flow passage PS in accordance with the strokes and to perform fuel injection control with high precision. 
   The dimension G 1  of the gap A 1  of the first magnetic circuit M 1  at the time when no suction force has been generated (i.e., when the solenoid-operated fuel injection valve is closed) is smaller than the dimension G 2  of the gap A 2  of the second magnetic circuit M 2  at the time when no suction force has been generated (i.e., when the solenoid-operated fuel injection valve is closed). As described above, the suction force is increased in proportion to a decrease in the gaps. The moment the area of the fuel flow passage PS is increased from zero, the difference between pressures inside and outside the fuel injection nozzle hole strengthens a valve-closing force. Hence, at this moment, the solenoid-operated fuel injection valve requires a suction force larger than the suction force that is applied after the operation of opening the solenoid-operated fuel injection valve has been started. 
   Thus, the moment the area of the fuel flow passage PS is increased from zero, a suction force is generated at least in the first magnetic circuit M 1  in which the needle member has the shorter stroke. If the suction force FL on the side of the first magnetic circuit M 1  during closure of the solenoid-operated fuel injection valve is increased by reducing the dimension G 1  of the gap A 1 , the area of the fuel flow passage PS can be smoothly increased. Once the fuel flow passage PS has been formed, the needle member N can be moved by a relatively small force. Thus, the second stroke can be set as a stroke longer than the first stroke, and fuel injection control can be performed with high precision. 
   Further, the solenoid-operated fuel injection valve is provided with the first and second elastic means S 1 , S 2 , which urge the needle member N against a suction force if the needle member N has moved by the first stroke or more in a direction of application of the suction force. The first elastic means S 1  is disposed in such a manner as to apply a force to the needle member N in the same direction as a suction force if the needle member N moves by a stroke shorter than the first stroke in a direction of application of the suction force. When the needle member N makes a return stroke between the first and second strokes (i.e., when the needle member N is moved by urging forces of the elastic means S 1 , S 2  reversely with respect to the direction of application of the suction force), the needle member N can be moved by a resultant force of the first and second elastic means S 1 , S 2 . Further, if a suction force is applied to the needle member N while a stroke shorter than the first stroke is made, the first elastic means S 1  does not act against the suction force. Thus, the needle member N can be moved at a high speed. 
   One of the magnetic bodies (M 1   a ) of the first magnetic circuit M 1  is fixed to the container H, whereas the other (M 1   b ) is movable with respect to the needle member N and is designed to allow a suction force to be indirectly transmitted to the needle member N. Because the other magnetic body (M 1   b ) indirectly transmits the suction force FL, undesirable resonance of the needle member during a return stroke can be suppressed. 
   The aforementioned solenoid-operated fuel injection valve can be subject to various modifications. 
     FIG. 2  is an explanatory view of a relationship of mechanical connection among internal elements of the solenoid-operated fuel injection valve in accordance with a second embodiment. As in the case of the aforementioned fuel injection valve, the needle member N disposed in the container H into which fuel is introduced is moved in the longitudinal direction (+Z direction) of the needle member N by the suction forces FL, FS generated by the electromagnetic means, whereby the area of the fuel flow passage PS defined as a space between the inner surface IS of the container H and the outer surface of the needle member N changes. This electromagnetic means is provided with the first and second magnetic circuits M 1 , M 2  through which the suction forces FL, FS can be controlled independently of each other. The solenoid-operated fuel injection valve of the second embodiment is different from the one shown in  FIG. 1  in that the first elastic means S 1  is not provided. Additionally, the magnetic body M 1   b  can abut on a stopper member NS fixed to the needle member N at the time of suction. Further, the magnetic body M 1   b  is connected to the container H via a third elastic means (spring) S 3 . The solenoid-operated fuel injection valves of the first and second embodiments are identical in other constructional details. 
   If a suction force of the first magnetic circuit M 1  acts on the magnetic body M 1   b , the stopper member NS that abuts on the magnetic body M 1   b  moves in the +Z direction, and the needle member moves in the +Z direction. In the case where an additional stroke is required, if a suction force generated by the second magnetic circuit M 2  is applied, the needle member N moves by more than the first stroke, whereby the magnetic body M 1   b  and the stopper member NS are separated from each other. The solenoid-operated fuel injection valves of the first and second embodiments are identical in other operational details. 
     FIG. 3  is an explanatory view of a relationship of mechanical connection among internal elements of the solenoid-operated fuel injection valve in accordance with a third embodiment. As in the case of the aforementioned fuel injection valves, the needle member N disposed in the container H into which fuel is introduced is moved in the longitudinal direction (+Z direction) of the needle member N by the suction forces FL, FS generated by the electromagnetic means, whereby the area of the fuel flow passage PS defined as a space between the inner surface IS of the container H and the outer surface of the needle member N changes. This electromagnetic means is provided with the first and second magnetic circuits M 1 , M 2  through which the suction forces FL, FS can be controlled independently of each other. 
   The solenoid-operated fuel injection valve of the third embodiment is different from the one shown in  FIG. 1  in that the first elastic means S 1  is not provided. Additionally the magnetic body M 1   b  can abut on the stopper member NS fixed to the needle member N at the time of suction. Further, the magnetic body M 1   b  is sucked against the second elastic means S 2 . The magnetic body M 1   b  is designed to be slidable with respect to a suitable member. The solenoid-operated fuel injection valves of the first and third embodiments are identical in other constructional details. As is apparent from the foregoing description, the construction of the elastic means and the mode of mechanical connection are abundant in variations. 
     FIG. 4  is an explanatory view of a relationship of mechanical connection among internal elements of the solenoid-operated fuel injection valve in accordance with a fourth embodiment. The solenoid-operated fuel injection valve of the fourth embodiment is basically constructed in the same manner as the solenoid-operated fuel injection valve of a first embodiment shown in  FIG. 1 , but is additionally designed such that the magnetic body M 1   b  can abut on the stopper member NS fixed to the needle member N at the time of suction. If the magnetic body M 1   b  has not moved by the first stroke, the needle N is sucked against the second elastic means. If the magnetic body M 1   b  has moved by the first stroke or more, needle N is sucked against the first and second elastic means S 1 , S 2 . 
   If the needle member N moves too fast when making a return stroke, it bounds upon abutment on the inner surface IS of the container H. As a result, so-called secondary injection occurs. Secondary injection is undesirable in terms of open-close controllability and fuel consumption. 
   The solenoid-operated fuel injection valve of the fourth embodiment is provided with the elastic means S 2  (S 1 ) and the stopper member NS. If the needle member N has moved in the direction of application of a suction force (i.e., in the +Z direction), the elastic means S 2  (S 1 ) urges the needle member N against the suction force. The needle member N is provided with the stopper member NS such that the needle member N hits a certain member (the magnetic body M 1   b  in this embodiment) at a predetermined position between the reference position of the needle member N during closure of the solenoid-operated fuel injection valve and a position corresponding to the first stroke if the needle member N is moved by an urging force of the elastic means S 2  (S 1 ) reversely with respect to the direction of application of the suction force (i.e., in the −Z direction). 
   In this construction, when the needle member N makes a return stroke, the stopper member NS hits the magnetic body M 1   b  fixed to the elastic means S 1 , which performs an impact-absorbing function. Thus, if the needle member N hits the magnetic body M 1   b  during a return stroke, the speed of the needle member N decreases before the needle member N abuts on the inner surface IS of the container. As a result, the amount of secondary injection is reduced. It is also appropriate that the stopper member NS be elastically supported. 
   Next, concrete constructional examples of the aforementioned solenoid-operated fuel injection valves of  FIG. 4  will be described. 
     FIG. 5  is a detailed longitudinal sectional view of the solenoid-operated fuel injection valve in accordance with the fourth embodiment. 
   In this solenoid-operated fuel injection valve, fuel is supplied to the container H from a fuel supply port la located at an end of a fuel supply pipe  1 . The container H is composed of a container body  100  and a nozzle  17  that is fitted to a longitudinal leading end of the container body  100 . The needle member (needle valve) N is disposed in the container body  100 , and extends to the interior of the nozzle  17 . The first and second magnetic circuits M 1 , M 2  are disposed between the container body  100  and the needle valve N. 
   The first magnetic circuit M 1  has a first electromagnet (M 1   a , M 1   c ), which is composed of the cylindrical magnetic body (first core) M 1   a  and a first coil M 1   c  embedded in the first core M 1   a . The first magnetic circuit M 1  is also provided with the annular magnetic body (first armature) M 1   b . The needle valve N, which can slide relative to the first armature M 1   b , is located in an opening of the first armature M 1   b . The first armature M 1   b  is connected to the driving force transmission member FX via the first elastic means (first spring) S 1 , and is elastically coupled to the needle valve N. 
   The second magnetic circuit M 2  has a second electromagnet (M 2   a,  M 2   c ), which is composed of the cylindrical magnetic body (second core) M 2   a  and a second core M 2   c  embedded in the magnetic body M 2   a.  The second magnetic circuit M 2  is also provided with the annular magnetic body (second armature) M 2   b . The needle valve N is securely fitted in an opening of the second armature M 2   b . The second armature M 2   b  is connected to the container H via the second elastic means (second spring) S 2 , and is elastically coupled to the container H. In this constructional example, a sleeve  1   b  in the fuel supply pipe  1  fixed to the container H functions as a stopper, and the second spring S 2  is interposed between the sleeve  1   b  and a base end portion of the needle valve N. 
   A connector  2  for supplying currents to the coils M 1   c , M 2   c  is fitted to the container H. By supplying currents to the coils M 1   c , M 2   c  respectively, the suction forces FL, FS are generated independently of each other. 
   Fuel that has been introduced from the fuel supply port  1   a  flows through an internal region of the second core M 2   a , a fuel passage formed in the armature M 2   b  or the like, an internal region of the first core M 1   a , and a fuel passage formed in the first magnetic body M 1   b  or the like, reaches the interior of the nozzle  17 , and further moves to the extent of almost reaching the fuel flow passage (fuel seal portion) PS. A nozzle hole  19  is formed at a leading end of the nozzle  17 . When the solenoid-operated fuel injection valve is open, fuel is injected from the nozzle hole  19 . Although the operation of the solenoid-operated fuel injection valve has been described above, it will be described hereinafter in more detail. 
     FIG. 6  shows longitudinal sectional views of the solenoid-operated fuel injection valve in respective states. 
   When the solenoid-operated fuel injection valve is closed as shown in  FIG. 6   a , no current is supplied from the connector  2 . In this state, the needle valve N is urged toward the fuel seal portion PS by a second spring force (urging force) F 2  generated by the second spring, and the outer surface of the needle valve N closes the fuel seal portion PS. 
   To shift the solenoid-operated fuel injection valve to an open state with a small fuel injection amount shown in  FIG. 6   b , currents are supplied from the connector  2  to the first and second coils M 1   c , M 2   c , which constitute the first and second magnetic circuits M 1 , M 2  respectively. Thus, a force is applied to the needle valve N in the +Z direction via the first spring S 1  and the stopper member FX, whereby the needle valve N moves away from the fuel seal portion PS against a second spring force F 2  and a valve-closing force FD resulting from a differential pressure. Hence, the solenoid-operated fuel injection valve is opened, and fuel injection is started. 
   When the needle valve N is located at a predetermined position before reaching a position corresponding to the first stroke after the start of fuel injection, the supply of current to the second coil M 2   c  is stopped. Then, the first armature M 1   b  abuts on the first core M 1   a , and the needle valve N substantially stops moving. If the supply of current to the first coil M 1   c  is thereafter stopped, the needle valve N is moved in the −Z direction by spring forces F 1 , F 2  of the first and second spring forces. Then, the needle valve N abuts on the inner surface of the container constituting the fuel seal portion PS, and the solenoid-operated fuel injection valve is closed. 
   In this fuel injection, since the amount of fuel injected at a time is small, the degree of dispersion of fuel is high. Hence, fuel is injected in a widely dispersed and atomized state. 
   In the operation of closing the solenoid-operated fuel injection valve in this case, the stopper member NS fitted to the needle valve N hits the inner surface of the container constituting the fuel seal portion PS after having been decelerated by temporarily hitting the first armature M 1   b  before reaching the position corresponding to the first stroke. Therefore, secondary injection is suppressed. 
   To shift the solenoid-operated fuel injection valve to an open state with a large fuel injection amount, currents are first supplied from the connector  2  to the first and second coils M 1   c,  M 2   c  constituting the first and second magnetic circuits M 1 , M 2  at the start of the operation of opening the solenoid-operated fuel injection valve. Thus, a force is applied to the needle valve N in the +Z direction via the first spring S 1  and the stopper member FX, whereby the needle valve N moves away from the fuel seal portion PS against the second spring force F 2  and a valve-opening force FD resulting from a differential pressure. As a result, the solenoid-operated fuel injection valve is opened, and fuel injection is started. 
   Even if the needle valve N has reached the position corresponding to the first stroke after the start of fuel injection, the supply of current to the second coil M 2   c  is continued. While the first armature M 1   b  abuts on the first core M 1   a , the second armature M 2   b  is sucked against the spring force F 1  of the first spring S 1  and the spring force F 2  of the second spring S 2 , and the needle valve N moves further upwards. 
   At this moment, the supply of current to the first coil M 1   c  can be stopped. If the second armature M 2   b  abuts on the second coil M 2   c , the needle valve N stops moving at a position corresponding to the second stroke. If the supply of current to the second coil M 2   c  (to the first coil M 1   c  if necessary) is thereafter stopped, the needle valve N is moved in the −Z direction by the spring forces F 1 , F 2  of the first and second springs. As a result, the needle valve N abuts on the inner surface of the container constituting the fuel seal portion PS, and the solenoid-operated fuel injection valve is closed. 
   In this fuel injection, since the amount of fuel injected at a time is large, the degree of dispersion of fuel is low. Hence, fuel is injected in a highly penetrative manner. 
   In the operation of closing the solenoid-operated fuel injection valve in this case as well, the stopper member NS fitted to the needle valve N hits the fuel seal portion PS after having been decelerated by temporarily hitting the first armature M 1   b  before reaching the position corresponding to the first stroke. Therefore, secondary injection is suppressed. 
     FIG. 7  is an explanatory view of the function of suppressing secondary injection. In the case where the needle valve N has moved from a closed position (reference position) by the second stroke (state C), the second stroke corresponds to the dimension G 2  of the gap A 2 . In closing the solenoid-operated fuel injection valve, if the needle valve N has approached the closed position beyond the position corresponding to the first stroke (the dimension G 1 ) after the lapse of a certain period, the stopper member NS hits the first armature M 1   b . An impact on the stopper member NS is absorbed by the spring S 1  of the first armature M 1   b , and the stopper member NS moves to a position corresponding to closure of the solenoid-operated fuel injection valve while being decelerated together with the first armature M 1   b.    
   That is, the positional change amount (speed) of the needle valve N per unit time decreases immediately before the needle valve N reaches the closed position. The needle valve N hits the inner surface of the container constituting the fuel seal portion PS at a low speed. Accordingly, the bound of the needle valve N is suppressed, and secondary injection is suppressed. 
     FIG. 8  shows timing charts of currents I 1 , I 2  (solenoid drive pulses) supplied to the coils M 1   c,  M 2   c,  the suction forces FL, FS, the spring forces F 1 , F 2 , the valve-opening force FD resulting from a differential pressure, and the position of the needle valve (1) when the solenoid-operated fuel injection valve is closed, (2) when the solenoid-operated fuel injection valve is opened with a small fuel injection amount, and (3) when the solenoid-operated fuel injection valve is opened with a large fuel injection amount. 
   When the solenoid-operated fuel injection valve is opened with a small fuel injection amount, the solenoid drive pulses I 1 , I 2  are first supplied simultaneously. After the lapse of a period T 1 , the drive pulse I 2  is stopped (turned OFF). After fuel injection, the drive pulse I 1  is stopped (turned OFF) as well. In this manner, the solenoid-operated fuel injection valve is closed. 
   When the solenoid-operated fuel injection valve is opened with a large fuel injection amount, the solenoid drive pulses I 1 , I 2  are first supplied simultaneously. After the lapse of a period T 2 , the drive pulse I 1  is stopped (turned OFF) while the drive pulse I 2  is still being supplied. After fuel injection, the drive pulse I 2  is stopped (turned OFF) as well. In this manner, the solenoid-operated fuel injection valve is closed. 
   As described above, since the stroke of the needle valve N is variable, atomization of fuel can be carried out as required in accordance with engine load, and fuel consumption can be improved. When a small amount of fuel is injected, atomized fuel is widely dispersed, which is suited for combustion in a low-load range. When a large amount of fuel is injected, fuel is highly rectilinearly injected, which is suited for combustion in a high-load range. 
   It is to be noted herein that the fuel injection amount per unit time is proportional to a duty ratio of the aforementioned drive pulse I 1  or I 2 . In the case where a small amount of fuel is injected, the fuel injection rate can be made relatively low even if the duty ratio is not variable. If the duty ratio is reduced as well, a considerably small amount of fuel can be injected. 
   In the case where a large amount of fuel is injected, the fuel injection rate can be relatively changed even if the duty ratio is not increased. If the duty ratio is increased as well, a considerably large amount of fuel can be injected. Thus, adoption of the aforementioned construction makes it possible to enlarge a dynamic range of the fuel injection rate. 
   To open the valve to which a high fuel pressure is applied, a large suction force is required. In the aforementioned construction, while the suction force FL can be increased by making the dimension G 1  of the gap A 1  relatively narrow, the suction force can be further increased by using a resultant force of the suction forces FL, FS. Thus, it is possible to achieve enhancement of fuel pressure. 
   Because the two springs S 1 , S 2  are employed, the resultant force of the spring forces F 1 , F 2  acts on the needle valve N when the solenoid-operated fuel injection valve is closed. Hence, the responsive characteristic during the operation of closing the solenoid-operated fuel injection valve is improved. When the solenoid-operated fuel injection valve is opened, only one of the spring forces, namely, the spring force F 2  is effective. The degree of contribution of the suction forces to the operation of opening the solenoid-operated fuel injection valve is enhanced. As a result, the responsive characteristic during the operation of opening the solenoid-operated fuel injection valve is improved. 
   In the aforementioned construction, the first and second magnetic circuits M 1 , M 2  are disposed along the longitudinal direction of the needle valve N. Thus, the suction forces can be increased without enlarging the dimension in the direction perpendicular to the longitudinal direction (i.e., radially). 
   Reduction of the amount of transpirable gas has been demanded in supplying fuel to an engine. As a fuel supply system satisfying such a demand, a returnless system has been devised. In the aforementioned solenoid-operated fuel injection valve, a returnless structure or a rail pressure built-in structure can also be adopted by directly operating the needle valve N by means of a solenoid. 
   The solenoid-operated fuel injection valve in accordance with the invention can be reliably opened and closed. 
   While the invention has been described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the exemplary embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the exemplary embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.