Abstract:
A fuel injection valve includes a fuel injection port, a needle for cutting the fuel flow into the fuel injection port and a needle mover for moving the needle away from the fuel injection port and allowing the fuel to flow into the fuel injection port. When the needle is moved away from the fuel injection port by the needle mover, the force is applied by a force applicator to the needle away from the fuel injection port only during the period when the needle is moved away from the fuel injection port to less than a predetermined degree.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a fuel injection valve.  
         [0003]     2. Description of the Related Art  
         [0004]     Japanese Unexamined Patent Publication No. 2000-257534 discloses a fuel injection valve for injecting fuel into the combustion chamber of the internal combustion engine. This fuel injection valve comprises a fuel injection port (referred to, in the publication cited above, as “the fuel injection holes” designated by reference numeral  8 ) and a member for closing the fuel injection port (in the cited publication, corresponds to a movable portion  4 A including a plunger  4 , a rod  5  and a valve body  6 , and hereinafter referred to as “the movable portion” as in the cited publication). In this fuel injection valve, the movable portion is subjected to the force generated by the fuel pressure (hereinafter referred to as “the valve opening force due to the fuel pressure”) acting on the movable portion in the direction to open the fuel injection port (hereinafter referred to as “the valve opening direction”), the force generated by the fuel pressure (hereinafter referred to as “the valve closing force due to the fuel pressure”) acting on the movable portion in the direction to close the fuel injection port (hereinafter referred to as “the valve closing direction”) and the force generated by a spring (hereinafter referred to as “the valve closing force due to the spring”) acting on the movable portion in the valve closing direction. Also, this fuel injection valve includes a means (hereinafter referred to as “the electromagnetic means”) for electromagnetically generating the force acting on the movable portion in the valve opening direction.  
         [0005]     In the fuel injection valve disclosed in the publication cited above, as the sum of the two valve closing forces (i.e. the valve closing forces due to the fuel pressure and the spring) is larger than the valve opening force due to the fuel pressure, the fuel injection port is closed by the movable portion in the case where the force from the electromagnetic means (hereinafter referred to as “the valve opening force due to the electromagnetic means”) is not generated. In view of the fact that the total valve opening force due to the electromagnetic means and the fuel pressure is larger than the total valve closing force due to the fuel pressure and the spring, on the other hand, the movable portion is moved away from the fuel injection port thereby to open the fuel injection port and inject the fuel from the fuel injection port when the force is generated by the electromagnetic means.  
         [0006]     In the fuel injection valve disclosed in the cited publication, when the force is generated by the electromagnetic means and the movable portion is moved away from the fuel injection port, the valve opening force due to the fuel pressure increases with the distance covered by the movable portion. When the movable portion is moved to the point farthest from the fuel injection port, the valve opening force due to the fuel pressure assumes a maximum value substantially equal to the valve closing force due to the fuel pressure. In the case where the valve opening force due to the electromagnetic means ceases to be generated under this condition, the movable portion closes the fuel injection port. As the valve opening force due to the fuel pressure is substantially equal to the valve closing force due to the fuel pressure under this condition, as described above, the overall valve closing force cannot be increased by controlling the fuel pressure. In order to cause the movable portion to close the fuel injection port satisfactorily, therefore, the valve closing force due to the spring is required to be correspondingly large.  
         [0007]     The valve opening force due to the fuel pressure is small when the fuel injection port is closed by the movable portion. In order to cause the movable portion to move satisfactorily in the case where the valve closing force due to the spring is excessively large, therefore, it is necessary to use an electromagnetic means of high performance (i.e. an electromagnetic means capable of generating a larger valve opening force). Generally, the electromagnetic means of high performance is large in size. In the case where the electromagnetic means of high performance is required, therefore, the use of an electromagnetic means large in size is unavoidable, thereby leading to a large fuel injection valve. In the case where the fuel injection valve is mounted on the internal combustion engine, for example, the mountability of the fuel injection valve on the internal combustion engine is deteriorated. Also, a bulky electromagnetic means is generally low in responsiveness.  
         [0008]     Accordingly, the object of this invention is to provide a fuel injection valve requiring no large electromagnetic means (generally, no electromagnetic means of high performance).  
       SUMMARY OF THE INVENTION  
       [0009]     In order to solve the problem described above, according to a first aspect of the invention, there is provided a fuel injection valve comprising a fuel injection port, a needle for cutting off the fuel flowing into the fuel injection port, a needle moving means for moving the needle away from the fuel injection port and allowing the fuel to flow into the fuel injection port, and a force application means for applying the force to the needle in the direction away from the fuel injection port only during the period when the degree to which the needle has moved away from the fuel injection port is smaller than a predetermined degree while the needle is moved away from the fuel injection port by the needle moving means.  
         [0010]     According to a second aspect of the invention, there is provided a fuel injection valve in the first aspect, wherein the force application means includes an elastic member for generating the force to move the needle away from the fuel injection port, and the force generated by the elastic member is applied to the needle until the needle moves away from the fuel injection valve to the aforementioned predetermined degree from the state in which the fuel flow into the fuel injection port is cut off by the needle, and the application of the force generated by the elastic member to the needle is cut off when the needle is moved at least to the predetermined degree away from the fuel injection port.  
         [0011]     According to a third aspect of the invention, there is provided a fuel injection valve in the first aspect, wherein the force application means includes a pressure receiving member for receiving the force from the fuel in the direction away from the fuel injection port, and the force received by the pressure receiving member from the fuel is applied to the needle until the needle moves away from the fuel injection valve to the aforementioned predetermined degree from the state in which the fuel flow into the fuel injection port is cut off by the needle, and the application of the force received by the pressure receiving member from the fuel to the needle is cut off when the needle is moved at least to the predetermined degree away from the fuel injection port.  
         [0012]     According to a fourth aspect of the invention, there is provided a fuel injection valve in any one of the first to third aspects, further comprising a housing for accommodating the needle, wherein the needle, while cutting off the fuel flow into the fuel injection port, is in contact with the inner wall surface of the housing, and when the needle comes away from the inner wall surface of the housing, the fuel is allowed to flow into the fuel injection port, when the needle comes off from the inner wall surface of the housing, the fuel circumvents the needle and flows to the neighborhood of the forward end of the needle through the space between the needle and the inner wall surface of the housing, and the aforementioned predetermined degree corresponds to a point where the fuel flowing between the needle and the inner wall surface of the housing begins to be restricted when the needle comes off from the inner wall surface of the housing. The housing corresponds to the nozzle in the embodiments of the invention described later.  
         [0013]     Generally, the needle of the fuel injection valve, when moved in the direction away from the fuel injection port, is subjected to the force in the direction away from the fuel injection port (valve opening force due to the fuel pressure) by the pressure of the fuel flowing into the forward end of the needle. The valve opening force due to the fuel pressure tends to increase with the degree to which the needles moves away from the fuel injection port. The valve opening force due to the fuel pressure, therefore, assumes a maximum value when the needle is separated farthest from the fuel injection port. In order to move the needle toward the fuel injection port and cut off the fuel flow into the fuel injection port satisfactorily by the needle, therefore, a valve closing force commensurate with the valve opening force due to the fuel pressure (i.e. the force to move the needle toward the fuel injection port) is exerted on the needle. However, the valve opening force due to the fuel pressure is smaller, the smaller the degree to which the needle moves away from the fuel injection port. Especially, the valve opening force due to the fuel pressure assumes a minimum value when the needle cuts off the fuel flow into the fuel injection port. In order to move the needle in the direction away from the fuel injection port while the fuel flow into the fuel injection port is cut off, therefore, a comparatively large valve opening force must be applied to the needle. This force is applied by a needle moving means and for applying such a comparatively large valve opening force, the needle moving means is generally required to be high in performance (or large in size).  
         [0014]     According to this invention, however, when the needle is moved in the direction away from the fuel injection port by the needle moving means, the force application means applies the force to the needle to move away from the fuel injection port during the period when the degree to which the needle is away from the fuel injection port is smaller than a predetermined degree. For this reason, the needle cutting off the fuel flow into the fuel injection port can be moved away from the fuel injection port with a smaller force by the needle moving means. In other words, according to the invention, a high-performance valve opening means (such as a large-sized valve opening means) is not required. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The present invention may be more fully understood from the description of the preferred embodiments of the invention set forth below together with the accompanying drawings, in which:  
         [0016]      FIG. 1  is a longitudinal sectional view showing a fuel injection valve according to a first embodiment of the invention;  
         [0017]      FIG. 2  is a longitudinal sectional view of a nozzle of the fuel injection valve shown in  FIG. 1 ;  
         [0018]      FIG. 3  is a longitudinal sectional view of a needle of the fuel injection valve shown in  FIG. 1 ;  
         [0019]      FIG. 4  is a diagram showing a balance rod of the fuel injection valve shown in  FIG. 1 ;  
         [0020]      FIG. 5  is a longitudinal sectional view of a transmission member of the fuel injection valve shown in  FIG. 1 ;  
         [0021]      FIG. 6  is a longitudinal sectional view of an elastic member shown in  FIG. 1 ;  
         [0022]      FIG. 7  is a longitudinal sectional view of an annular member shown in  FIG. 1 ;  
         [0023]      FIG. 8A  is a diagram showing the relation between the needle lift amount D and the force F (F 1  to F 4 ) exerted on the needle;  
         [0024]      FIG. 8B  is a diagram showing the relation between the needle lift amount D and the total valve closing force Fc exerted on the needle;  
         [0025]      FIG. 9  is a longitudinal sectional view showing the component elements of the fuel injection valve of  FIG. 1  in closed state;  
         [0026]      FIG. 10  is a longitudinal sectional view showing the component elements of the fuel injection valve of  FIG. 1  in a first open state;  
         [0027]      FIG. 11  is a longitudinal sectional view showing the component elements of the fuel injection valve of  FIG. 1  in a second open state;  
         [0028]      FIG. 12  is a longitudinal sectional view showing the fuel injection valve according to a second embodiment;  
         [0029]      FIG. 13  is a longitudinal sectional view of the nozzle of the fuel injection valve shown in  FIG. 12 ;  
         [0030]      FIG. 14  is a longitudinal section view of the needle of the fuel injection valve shown in  FIG. 12 ;  
         [0031]      FIG. 15  is a longitudinal sectional view of a pressure receiving member of the fuel injection valve shown in  FIG. 12 ;  
         [0032]      FIG. 16A  is a diagram showing the relation between the needle lift amount D and the force F (F 1  to F 4 ) exerted on the needle;  
         [0033]      FIG. 16B  is a diagram showing the relation between the needle lift amount D and the total valve closing force Fc exerted on the needle;  
         [0034]      FIG. 17  is a longitudinal sectional view showing the component elements of the fuel injection valve of  FIG. 12  in closed state;  
         [0035]      FIG. 18  is a longitudinal sectional view showing the component elements of the fuel injection valve of  FIG. 12  in a first open state; and  
         [0036]      FIG. 19  is a longitudinal sectional view showing the component elements of the fuel injection valve of  FIG. 12  in a second open state. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     The best mode for embodying the invention will be explained below with reference to the drawings.  FIG. 1  shows a fuel injection valve according to a first embodiment of the invention. In  FIG. 1 , reference numeral  1  designates a nozzle, numeral  2  a needle, numeral  3  an armature, numeral  4  a solenoid, numeral  5  a balance rod, and numeral  6  a coil spring.  
         [0038]      FIG. 2  shows the nozzle  1 . As shown in  FIG. 2 , a space  7  is formed along the longitudinal axis of the fuel injection valve (hereinafter referred to simply as “the longitudinal axis”) of the nozzle  1 . This space  7  is narrowed (the lower side in the drawing is hereinafter referred to as “the forward end side”, and the upper side as “the base end side”) and the inner wall surface  9  for defining the space  7  assumes a conical shape at the forward end side of the nozzle  1 . Also, fuel injection ports  10  are formed at the forward end of the nozzle  1 . The fuel injection ports  10  communicate with the space  7 .  
         [0039]      FIG. 3  shows the needle  2 . Though not described in detail, the needle  2  cuts off or allows the fuel to flow into the fuel injection port  10 . As shown in  FIG. 3 , a base end-side space  11  extending along the longitudinal axis and a forward end-side space  12  similarly extending along the longitudinal axis are formed in the needle  2 . The spaces  11 ,  12  communicate with each other, and the diameter of the base end-side space  11  is larger than the diameter of the forward end-side space  12 . Paths  13  extending in the direction perpendicular to the longitudinal axis are formed at the forward end side of the space  12  of the needle  2 . The paths  13  communicate with the space  12 . Also, the forward end of the needle  2  is narrowed and has a substantially conical outer wall surface  14 . The base end-side portion  15 , the forward end-side portion  16  and the portion  17  between them (hereinafter referred to as “the intermediate portion”) of the needle  2  have different diameters. Specifically, the diameter of the base end-side portion  15  is largest, the diameter of the forward end-side portion  16  is smallest, and the diameter of the intermediate portion  17  assumes a value between these two diameters.  
         [0040]      FIG. 4  shows the balance rod  5 . As shown in  FIG. 4 , a space  18  is formed in and through the balance rod  5  along the longitudinal axis.  
         [0041]     As shown in  FIG. 1 , the portion of the needle  2  including the intermediate portion  17  and the forward end portion  16  is accommodated in the space  7  of the nozzle  1  (hereinafter referred to as “the nozzle space  7 ”). The needle  2  is adapted to slide with respect to the nozzle  1  along the longitudinal axis in the nozzle space  7 . The diameter of the intermediate portion  17  of the needle  1  is substantially equal to the diameter of the nozzle space  7 . Therefore, substantially no gap is formed between the outer wall surface of the intermediate portion  17  of the needle  2  and the wall surface defining the nozzle space  7  (hereinafter referred to simply as “the nozzle inner wall surface”). The diameter of the forward end portion  16  of the needle  2 , on the other hand, is smaller than the diameter of the nozzle space  7 . A gap  19  is formed, therefore, between the outer wall surface of the forward end portion  16  of the needle  2  and the nozzle inner wall surface. The base end-side portion of the gap  19  forms a comparatively large chamber (hereinafter referred to as “the nozzle chamber”)  20 . The paths  13  of the needle  2  are open to the nozzle chamber  20 . The substantially conical outer wall surface  14  of the forward end portion of the needle  2  is adapted to come into contact with the conical inner wall surface  9  defining the forward end area of the nozzle space  7 . In the description that follows, the substantially conical outer wall surface  14  of the needle  2  is hereinafter referred to as “the needle seat wall surface  14 ”, the conical inner wall surface  9  of the nozzle  1  as “the nozzle seat wall surface  9 ”, and the portion of the needle seat wall surface  14  in contact with the nozzle seat wall surface  9  (which surrounds the needle seat wall surface  14 ) as “the seat portion”. While the needle seat wall surface  14  is in contact with the nozzle seat wall surface  9 , the fuel flow into the fuel injection port  10  is cut off. Under this condition, no fuel is injected from the fuel injection valve. Once the needle seat wall surface  14  comes off from the nozzle seat wall surface  9 , on the other hand, the fuel is allowed to flow into the fuel injection port  10 . Under this condition, the fuel is injected from the fuel injection valve.  
         [0042]     As shown in  FIG. 1 , the forward end portion of the balance rod  5  is accommodated in the base end-side space  11  of the needle  2 . The needle  2  is slidable with respect to the balance rod  5 . A chamber designated by reference numeral  21  (hereinafter referred to as “the pressure chamber”) is formed between the balance rod  5  and the needle  2 .  
         [0043]     As shown in  FIG. 1 , the armature  3  is mounted at the base end of the needle  2 . The solenoid  4  is arranged in proximity to the armature  3  and adapted to be supplied with power. The solenoid  4 , when supplied with power, generates an electromagnetic force. This electromagnetic force attracts the armature  3  toward the base end. According to this embodiment, the needle  2  is moved in the direction away from the fuel injection port  10 . In this way, the needle seat wall surface  14  comes off from the nozzle seat wall surface  9 .  
         [0044]     The coil spring  6  is arranged between the wall surface, which is formed on the balance rod  5  and faces the forward end side, and the wall surface, which is formed on the armature  3  and faces the base end side. The coil spring  6  urges the needle  2  in the direction toward the fuel injection port  10  at the forward end side (hereinafter referred to as “the valve closing direction”).  
         [0045]     The fuel injection valve includes a tubular member  24 , an elastic member  25  and an annular member  26 .  FIG. 5  shows the tubular member  24 ,  FIG. 6  the elastic member  25  and  FIG. 7  the annular member  26 . The tubular member  24  is for transmitting the elastic force of the elastic member  25  to the needle  2  and is hereinafter referred to as “the transmission member”. The annular member  26  is a part for adjusting the elastic force of the elastic member  25 . As shown in  FIG. 5 , the transmission member  24  includes a tubular body  24   a , and a flange portion  24   b  extending in the direction perpendicular to the center axis (longitudinal axis) of the body  24   a  from the lower outer wall surface of the body  24   a  in the direction away from the center axis of the body  24   a . The transmission member  24  is arranged between the needle  2  and the body  27  of the fuel injection valve as viewed along the diameter in such a form as to accommodate the intermediate portion  17  of the needle  2 . A gap is formed between the inner peripheral surface of the transmission member  24  and the outer peripheral surface of the intermediate portion  17  of the needle  2 . On the other hand, no gap is formed between the outer peripheral surface of the transmission member  24  and the inner peripheral surface of the body  27  of the fuel injection valve. The transmission member  24 , with the outer peripheral surface thereof in contact with the inner peripheral surface of the body  27  of the fuel injection valve, is slidable with respect to the body  27  of the fuel injection valve. Further, in the state shown in  FIG. 1 , a gap is formed between, as viewed along the longitudinal axis, the wall surface of the transmission member  24  facing the base end side (specifically, the wall surface of the flange portion  24  facing the base end side) and the wall surface  27   a  of the fuel injection valve body  27  facing the forward end side (see  FIG. 9  for more detail).  
         [0046]     The elastic member  25  according to this embodiment is an annular disk spring and may be an elastic member such as a wave spring. The elastic member  25  is arranged, as viewed diametrically, between the needle  2  and the fuel injection valve body  27  in the form surrounding the intermediate portion  17  of the needle  2 . The annular member  26  is also arranged, as viewed diametrically, between the needle  2  and the fuel injection valve body  27  in the form surrounding the intermediate portion  17  of the needle  2 .  
         [0047]     As can be understood from  FIG. 1 , as viewed along the longitudinal axis, the annular member  26  is arranged on the base end surface of the nozzle  1 , the elastic member  25  on the annular member  26 , and the transmission member  24  between the elastic member  25  and the surface of the needle  2  facing the forward end side. In the state shown in  FIG. 1  (i.e. the state in which the fuel flow into the fuel injection port  10  is cut off by the needled  2 ), the transmission member  24  is pushed toward the forward end side by the needle  2 , and therefore the elastic member  25  is compressed between the transmission member  24  and the annular member  26 . In other words, under this condition, the elastic member  25  applies the force, through the transmission member  24 , to the needle  2  to move in the direction away from the fuel injection port  10  (hereinafter referred to as “the valve opening direction”).  
         [0048]     Next, the fuel flow in the fuel injection valve will be explained. The fuel flows into the fuel injection valve from the opening  22  on the base end side of the balance rod  5 . The fuel that has flowed into the space  18  of the balance rod  5  from the opening  22  flows into the pressure chamber  21  from the forward end-side opening  23  of the balance rod  5 . The fuel that has flowed into the pressure chamber  21  flows into the space  12  of the needle  2 , and through the paths  13  of the needle  2 , flows into the nozzle chamber  20 . The fuel that has flowed into the nozzle chamber  20  flows in the gap  19  and reaches the neighborhood of the forward end portion having the needle seat wall surface  14  (hereinafter referred to simply as “the forward end portion of the needle  2 ”). If the needle seat wall surface  14  comes off from the nozzle seat wall surface  9  in the process, the fuel that has reached the neighborhood of the forward end portion of the needle  2  flows between the needle seat wall surface  14  and the nozzle seat wall surface  9 , and by circumventing the needle  2 , reaches the forward end portion of the needle  2 . Then, the fuel is injected from the fuel injection valve through the fuel injection port  10 .  
         [0049]     Next, the operation of the fuel injection valve will be briefly explained. According to this embodiment, once power is supplied to the solenoid  4 , fuel is injected from the fuel injection valve. Specifically, when power is supplied to the solenoid  4 , the electromagnetic force is generated from the solenoid  4 . This electromagnetic force attracts the armature  3  toward the base end side. The armature  3  is mounted on the needle  2 , and therefore, when the armature  3  is attracted toward the base end side, the needle  2  is also attracted toward the base end side. As a result, the needle seat wall surface  14  is separated from the nozzle seat wall surface  9 . In this way, the fuel that has reached the neighborhood of the forward end portion of the needle  2  circumvents the needle and reaches the forward end portion of the needle  2 . Then, the fuel is injected from the fuel injection port  10 . When power supply to the solenoid  4  is stopped, on the other hand, the generation of the electromagnetic force from the solenoid  4  is stopped. Then, the needle  2  is moved toward the fuel injection port  10  at the forward end side mainly by the urging force of the coil spring  6 , and finally, the needle wall surface  14  comes into contact with the nozzle seat wall surface  9 . Thus, the fuel injection from the fuel injection port  10  is stopped.  
         [0050]     Next, the operation of the fuel injection valve will be explained in detail. Reference is made to  FIGS. 1 and 3 . The forces working on the needle  2  in the valve closing direction (the direction in which the needle  2  is moved toward the fuel injection port  10 ) includes the force attributable to the fuel pressure (the average value of the pressure of the fuel supplied to the fuel injection valve) (hereinafter referred to as “the valve closing force due to the fuel pressure”) and the force attributable to the coil spring (hereinafter referred to as “the valve closing force due to the coil spring”). More specifically, the valve closing force due to the fuel pressure is the force determined by multiplying the diameter of the space  11  (the diameter D 1  in  FIG. 3 ) by the fuel pressure, and the valve closing force due to the coil spring  6  is the urging force of the coil spring  6 . The valve closing force due to the fuel pressure is substantially constant regardless of the lift amount of the needle  2  (which indicates the distance by which the needle seat wall surface  14  is off from the nozzle seat wall surface  9 ). The valve closing force due to the coil spring  6 , on the other hand, though a little varied with the lift amount of the needle  2 , is considered substantially constant regardless of the lift amount of the needle  2 .  FIG. 8A  shows the relation between the lift amount D of the needle  2  and the force F acting on the needle  2 . In  FIG. 8A , the solid line F 1  represents the valve closing force due to the fuel pressure, and the solid line F 2  the valve closing force due to the coil spring  6 .  
         [0051]     The forces acting on the needle  2  in the valve opening direction (the direction in which the needle  2  is moved away from the fuel injection port  10 ) include the force attributable to the fuel pressure (hereinafter referred to as “the valve opening force due to the fuel pressure”) and the force attributable to the elastic member  25  (hereinafter referred to as “the valve opening force due to the elastic member”). More specifically, the valve opening force due to the fuel pressure is the force determined by multiplying the difference between the outer diameter of the intermediate portion  17  (the diameter D 2  in  FIG. 3 ) and the diameter of the seat portion (the diameter D 3  in  FIG. 3 ) by the fuel pressure in the case where the needle seat wall surface  14  is in contact with the nozzle seat wall surface  9  (i.e. in the case where the fuel injection valve is closed). In the case where the needle seat wall surface  14  is separated from the nozzle seat wall surface  9  (i.e. in the case where the fuel injection valve is open), on the other hand, the valve opening force due to the fuel pressure is the force determined by multiplying the outer diameter D 2  of the intermediate portion  17  by the fuel pressure. In  FIG. 8A , the one-dot chain F 3  represents the valve opening force due to the fuel pressure.  
         [0052]     As can be understood also from  FIG. 8A , the force, which is exerted on the nozzle seat wall surface  14  by the fuel that has reached the nozzle seat wall surface  14  nearer to the forward end side of the needle  2  than the seat portion after separation of the needle seat wall surface  14  from the nozzle seat wall surface  9  and the resultant flow of the fuel between the needle seat wall surface  14  and the nozzle seat wall surface  9 , increases with the lift amount of the needle  2 . The valve opening force due to the fuel pressure with the fuel injection valve open, therefore, increases with the lift amount of the needle  2 . In other words, when the lift amount of the needle  2  is small, the fuel flowing between the needle wall surface  14  and the nozzle seat wall surface  9  is reduced, and therefore the valve opening force due to the fuel pressure increases with the lift amount of the needle  2 .  
         [0053]     The valve opening force due to the elastic member  25  is the urging force of the elastic member  25 . The valve opening force of the elastic member  25  is explained with reference to FIGS.  9  to  11 . FIGS.  9  to  11  are enlarged views of the elastic member  25  and the surrounding parts thereof. In particular,  FIG. 9  shows the state of the elastic member  25 , etc. with the fuel injection valve closed.  FIG. 10  shows the state of the elastic member  25 , etc. with the fuel injection valve open to a predetermined degree (i.e. with the lift amount of the needle  2  assuming a predetermined value). Further,  FIG. 11  shows the state of the elastic member  25 , etc. with the fuel injection valve open to maximum (i.e. with the lift amount of the needle  2  assuming the maximum value).  
         [0054]     In the state shown in  FIG. 9 , the transmission member  24  is pushed toward the forward end side by the end surface  2   a  facing the forward end of the needle  2 . Thus, the elastic member  25  is also compressed by being pushed toward the forward end side. Therefore, the needle  2  is urged in the valve opening direction by the elastic member  25  through the transmission member  24 . Under this condition, the elastic member  25  is compressed to the maximum.  
         [0055]     With power supplied to the solenoid  4 , the electromagnetic force for moving the armature  3  in the valve opening direction is generated by the solenoid  4 , and therefore the force to move the needle  2  in the valve opening direction is exerted on the needle  2  through the armature  3 . The electromagnetic force generated from the solenoid  4  is set at a sufficient value to open the needle  2 , and therefore by supplying power to the solenoid  4 , the fuel injection valve begins to open. At the same time, the needle  2  begins to move in the valve opening direction. For some time after the needle  2  begins to move in the valve opening direction, the elastic member  25  continues to apply the urging force in the valve opening direction to the needle  2  through the transmission member  24 . With the increase in the lift amount, the urging force applied to the needle  2  by the elastic member  25  decreases steadily. Once the lift amount of the needle  2  reaches a predetermined value (D 1  in  FIGS. 8A and 8B ), the flange portion  24   b  of the transmission member  24  comes into contact with the wall surface  27   a  facing the forward end side of the fuel injection valve. This state is shown in  FIG. 10 .  
         [0056]     When the lift amount of the needle  2  exceeds the predetermined value, the transmission member  24  is separated from the needle  2 , and therefore the elastic member  25  no longer applies the urging force in the valve opening direction to the needle  2 . This state is shown in  FIG. 11 .  
         [0057]     To summarize, the valve opening force due to the elastic member  25  decreases with the increase in the lift amount of the needle  2  until the lift amount of the needle  2  reaches a predetermined value (i.e. until the flange portion  24   b  of the transmission member  24  comes into contact with the wall surface  27   a  of the fuel injection valve body  27 ) from zero. After the lift amount of the needle  2  exceeds the same predetermined value (i.e. after the flange portion  24   b  of the transmission member  24  comes into contact with the wall surface  27   a  of the fuel injection valve body  27 ), the valve opening force becomes zero. In  FIG. 8A , the one-dot chain F 4  indicates the valve opening force due to the elastic member  25 .  
         [0058]     According to this embodiment, the valve opening force due to the elastic member  25  described above acts on the needle  2 . The relation between the valve closing force Fc acting on the needle  2  and the lift amount D of the needle  2  is shown in  FIG. 8B . Specifically, according to this embodiment, the total valve closing force Fc, though somewhat varied with the lift amount D of the needle  2 , is substantially constant. The total valve closing force Fc with the lift amount D of the needle  2  at about zero is substantially equal to the total valve closing force Fc associated with a comparatively large lift amount D of the needle  2 . In the case where power is supplied to the solenoid  4  in an attempt to open the fuel injection valve, therefore, the electromagnetic force to be generated by the solenoid  4  is comparatively small. Generally, a large-sized solenoid is required to generate a large electromagnetic force. According to this embodiment, in contrast, the solenoid is so compact that the fuel injection valve can be reduced in size. This improves the mountability of the fuel injection valve on the internal combustion engine. Also, with an increase in the solenoid size, the responsiveness thereof is generally reduced. According to this embodiment, a highly responsive solenoid can be employed, and therefore the operation response of the fuel injection valve is improved. With a small solenoid, only a very short time is required to cut off the valve opening force of the needle  2  after stopping power supply to the solenoid.  
         [0059]     Next, a second embodiment of the invention will be explained.  FIG. 12  shows a fuel injection valve according to the second embodiment. Also in  FIG. 12 , numeral  1  designates a nozzle, numeral  2  a needle, numeral  3  an armature, numeral  4  a solenoid, numeral  5  a balance rod and numeral  6  a coil spring.  
         [0060]      FIG. 13  shows the nozzle  1  according to the second embodiment. In  FIG. 13 , numeral  7  designates a space, numeral  9  a nozzle seat wall surface, and numeral  10  fuel injection ports.  FIG. 14  shows the needle  2  according to the second embodiment. In  FIG. 14 , numerals  11 ,  12  designate a space, numeral  13  paths, numeral  14  a needle seat wall surface, numeral  15  a base end-side portion of the needle  2 , numeral  16  a forward end-side portion of the needle  2  and numeral  17  an intermediate portion.  
         [0061]     Referring to  FIG. 12 , the fuel injection valve  1  according to the second embodiment, the transmission member  14 , the elastic member  25  and the annular member  20  in the first embodiment are replaced with a substantially tubular member  28  and an annular spacer  29 . The substantially tubular member  28  is for receiving the fuel pressure and transmitting it to the needle  2 , and is hereinafter referred to as “the pressure receiving member”. As shown in  FIG. 15 , the pressure receiving member  28  includes a tubular body  28   a , and a flange portion  28   b  extending in the direction perpendicular to the center axis (longitudinal axis) of the body  28   a  from the lower outer wall surface of the body  28   a  away from the center axis of the body  28   a . The pressure receiving member  28  accommodates the comparatively base end-side portion of the forward end-side portion  16  of the needle  2  and is arranged between the needle  2  and the fuel injection valve body  27  as viewed along the diameter. Also, no gap is formed between the inner peripheral surface of the pressure receiving member  28  and the outer peripheral surface of the needle  2 , and the inner peripheral surface of the pressure receiving member  28  is in contact with the outer peripheral surface of the needle  2 . The pressure receiving member  28 , however, is slidable with respect to the needle  2 . Also, no gap is formed between the outer peripheral surface of the pressure receiving member  28  and the inner peripheral surface of the fuel injection valve body  27 , and the outer peripheral surface of the pressure receiving member  28  is in contact with the inner peripheral surface of the fuel injection valve body  27 . The pressure receiving member  28  is adapted to slide with respect to the fuel injection valve body. Further, in the state shown in  FIG. 12 , as viewed from the direction of the longitudinal axis, a gap is formed between the wall surface of the pressure receiving member  28  facing the base end-side portion (specifically, the wall surface of the flange portion  28   b  facing the base end-side portion)  28   c  and the wall surface  27   a  of the fuel injection valve body  27  facing the forward end-side portion ( FIG. 17 ).  
         [0062]     As viewed from the direction of the longitudinal axis, on the other hand, the pressure receiving member  28  is arranged between the end surface  31  of the needle  2  facing the forward end-side portion and the end surface  32  of the nozzle  1  facing the base end-side portion (see FIGS.  17  to  19  for more detail). In the state shown in  FIG. 12 , the pressure receiving member  28  is pressed against the end surface  31  facing the forward end-side portion of the needle  2  under the fuel pressure in the space  30  described later, and a gap is formed between the wall surface of the flange portion  28   b  facing the forward end-side portion and the end surface  32  of the nozzle  1  facing the base end-side portion.  
         [0063]     The spacer  29  is arranged between the fuel injection valve body  27  and the nozzle  1 .  
         [0064]     A space  30  is defined between the outer peripheral surface of the needle  2  and the inner peripheral surface of the nozzle  1 . The end surface  33  at the forward end side of the pressure receiving member  28  is exposed to the space  30 . The fuel flows into the space  18  from the base end-side opening  22  of the balance rod  5 , and flows out into the space  30  from the paths  13  of the needle  2  through the pressure chamber  21 . Therefore, the fuel pressure is imposed in the valve opening direction on the end surface  33  at the forward end side of the pressure receiving member  28  exposed to the space  30 . The fuel that has flowed out into the space  30  from the paths  13  of course reaches the neighborhood of the forward end portion. In the case where the needle seat wall surface  14  is separated from the nozzle seat wall surface  9 , the fuel flows through the space between the needle seat wall surface  14  and the nozzle seat wall surface  9 , by circumventing the needle  2 , into the forward end portion of the needle  2 , and is injected from the fuel injection valve through the fuel injection port  10 .  
         [0065]     Next, the operation of the fuel injection valve according to the second embodiment will be briefly explained. Also in this embodiment, once power is supplied to the solenoid  4 , the armature  3  is attracted toward the base end side by the electromagnetic force generated by the solenoid  4 . As a result, the needle  2  is also attracted toward the base end side, and the needle seat wall surface  14  comes off from the nozzle seat wall surface  9 . In this way, the fuel that has reached the neighborhood of the forward end portion of the needle  2  reaches the forward end portion of the needle  2  by circumventing the needle  2 , and is injected from the fuel injection port  10 . Once power supply to the solenoid  4  is stopped, on the other hand, the electromagnetic force also ceases to be generated from the solenoid  4 . Then, the needle  2  is moved toward the fuel injection port  10  at the forward end side mainly by the urging force of the coil spring  6 , and, finally, the needle seat wall surface  14  comes into contact with the nozzle seal wall surface  9 . Thus, the fuel ceases to be injected from the fuel injection port  10 .  
         [0066]     Next, the operation of the fuel injection valve according to the second embodiment will be explained. As in the first embodiment, the needle  2  is subjected to the valve closing force due to the fuel pressure and the valve closing force due to the coil spring  6 .  FIG. 16A  shows the relation between the lift amount D of the needle  2  and the force F acting on the needle  2 . The solid line F 1  represents the valve closing force due to the fuel pressure and the solid line F 2  represents the valve closing force due to the coil spring  6 . As can be understood from  FIG. 16A , the valve closing force F 1  due to the fuel pressure is substantially constant regardless of the lift amount D of the needle  2 . The valve closing force due to the coil spring  6 , on the other hand, though varied somewhat with the lift amount D of the needle  2 , substantially remains constant regardless of the lift amount D of the needle  2 .  
         [0067]     The valve opening force due to the fuel pressure acts on the needle  2  like in the first embodiment. In  FIG. 16A , the one-dot chain F 3  represents the valve opening force due to the fuel pressure. According to the second embodiment, the needle  2  is subjected to the fuel pressure received from the fuel in the valve opening direction by the pressure receiving member  28  (hereinafter referred to as “the valve opening force from the pressure receiving member”). Next, the valve opening force from the pressure receiving member  28  is explained in detail with reference to FIGS.  17  to  19 .  
         [0068]     FIGS.  17  to  19  are enlarged views showing the pressure receiving member  28  and the neighboring parts. In particular,  FIG. 17  shows the state of the pressure receiving member  28 , etc. when the fuel injection valve is closed.  FIG. 18  shows the state of the pressure receiving member  28 , etc. when the fuel injection valve is open to a predetermined degree (i.e. when the lift amount of the needle  2  reaches a predetermined amount). Further,  FIG. 19  shows the state of the pressure receiving member  28 , etc. when the fuel injection valve is open to maximum (i.e. when the lift amount of the needle  2  reaches a maximum value).  
         [0069]     In the state shown in  FIG. 17 , the fuel pressure acts on the pressure receiving member  28 , and therefore the pressure receiving member  28  urges the needle  2  in the valve opening direction. When power is supplied to the solenoid  4  in this state, the needle  2  is moved in the valve opening direction, so that the fuel injection valve begins to open. For some time after the needle  2  begins to move in the valve opening direction, the fuel pressure acting on the pressure receiving member  28  continues to be transmitted to the needle  2 . When the lift amount of the needle  2  reaches a predetermined value (D 1  in  FIGS. 16A and 16B ), the flange portion  28   b  of the pressure receiving member  28  comes into contact with the wall surface  27   a  facing the forward end side of the fuel injection valve body. This state is shown in  FIG. 18 .  
         [0070]     Once the lift amount of the needle  2  exceeds the predetermined value, the pressure receiving member  28  comes off from the needle  2  and, therefore, the fuel pressure is no longer applied to the needle  2  through the pressure receiving member  28 . This state is shown in  FIG. 19 .  
         [0071]     To summarize, the valve opening force from the pressure receiving member  28  continues to be applied to the needle  2  until the lift amount of the needle  2  reaches a predetermined amount from zero (i.e. until the flange portion  28   b  of the pressure receiving member  28  comes into contact with the wall surface  27   a  of the fuel injection valve body  27 ). After the lift amount of the needle  2  exceeds the predetermined value (i.e. after the flange portion  28  of the pressure receiving member  28  comes into contact with the wall surface  27   a  of the fuel injection valve body), however, the valve opening force from the pressure receiving member  28  is reduced to zero. In  FIG. 16A , the one-dot chain F 4  represents the valve opening force from the pressure receiving member  28 .  
         [0072]     According to this embodiment, the valve opening force from the pressure receiving member  28  described above acts on the needle  2 . The total valve opening force Fc acting on the needle  2  and the lift amount D of the needle  2  thus have the relation as shown in  FIG. 16B . Specifically, according to this embodiment, the total valve closing force Fc remains substantially constant except when the lift amount D of the needle  2  assumes a value approximate to the predetermined value D 1 . When the lift amount D of the needle  2  is about zero, the total valve closing force Fc assumes value substantially equal to the total valve closing force Fc associated with a comparatively large lift amount D of the needle  2 . When power is supplied to the solenoid  4  in an attempt to open the fuel injection valve, therefore, only a comparatively small electromagnetic force is required to be generated from the solenoid  4 . As a result, as in the first embodiment, the fuel injection valve is reduced in size, and therefore the mountability of the fuel injection valve on the internal combustion engine is improved. The operation response of the fuel injection valve is also improved. The compact solenoid greatly shortens the time required from the time point when power stops being supplied to the solenoid to the time point when the valve opening force ceases to act on the needle  2 .  
         [0073]     While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.