Patent Publication Number: US-6712297-B1

Title: Electromagnetic fuel injection device for internal combustion engine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-229906 (corresponding to Japanese Unexamined Patent Publication No. 2002-48031) filed on Jul. 28, 2000 and Japanese Patent Application No. 2002-271626 filed on Sep. 18, 2002. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electromagnetic fuel injection device of an internal combustion engine (hereinafter simply referred to as an engine). 
     2. Description of Related Art 
     In an electromagnetic fuel injection device, it is desirable to control a fuel injection amount in such a manner that the fuel injection amount is proportioned to a length of a corresponding pulse signal applied, for example, from an engine control unit to a coil (or coil winding). To achieve this, fuel injection characteristics of the fuel injection device need to be approximated to a waveform of the pulse signal applied to the coil. That is, a valve opening period of the fuel injection device at time of energization of the coil needs to be reduced, and a valve closing period of the fuel injection device at time of deenergization of the coil needs to be reduced. 
     When a magnetic attractive force for attracting a movable core, which reciprocates together with a valve member, toward a stationary core is increased, a valve opening speed is increased to reduce the valve opening period. However, when a number of turns of the coil is increased to increase the magnetic attractive force, a size of the fuel injection device is disadvantageously increased. 
     Furthermore, when a load of a spring, which urges the valve member toward a valve opening direction, is increased, a valve closing speed is also increased to decrease the valve closing period. However, when the spring load is increased, a valve opening speed of the movable core, which is attracted toward the stationary core against the spring load, and thus of the valve member is disadvantageously reduced. 
     To address the above disadvantages, in a fuel injection device disclosed in Japanese unexamined patent publication No. 11-148437, positions of a movable core, a stationary core and a coil are adjusted to increase a magnetic attractive force, thereby increasing a valve opening speed to reduce a valve opening period without increasing a number of turns of the coil and without increasing a size of a fuel injection valve. 
     However, when the magnetic attractive force is increased, a time period required for reducing the magnetic attractive force at time of deenergization of the coil is increased. That is, a valve closing period is increased. As described above, an increase in a spring load causes an increase in a valve closing speed to reduce a valve closing period. However, as described above, when the spring load is increased, the magnetic attractive force for attracting the movable core against the spring load needs to be increased. A portion of the magnetic attractive force, which is increased by adjusting positions of the movable core, the stationary core and the coil, is used to compensate an increase in the spring load, so that it is difficult to increase a valve opening speed. 
     When the valve opening period or the valve closing period is increased, characteristics of the fuel injection rate become non-proportional to the drive signal applied to the coil (e.g., the signal width of the pulse signal), resulting in variations in the fuel injection amount. Thus, it is difficult to control the fuel injection amount. Particularly, when the signal width of the drive signal applied to the coil is relatively small, for example, during idle operation of the engine, it is difficult to control the fuel injection amount. Thus, in order to provide a required fuel injection amount, the signal width of the corresponding drive signal is increased to inject an excessive amount of fuel. As a result, a fuel consumption is increased, and an amount of noxious components in the exhaust gases increase. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the above disadvantages. Thus, it is an objective of the present invention to provide a compact fuel injection device, which achieves a reduced valve opening period and a reduced valve closing period to more precisely control a fuel injection amount. 
     To address the objective of the present invention, there is provided a fuel injection device including a cylindrical member, a valve body, a valve member, a movable core, a stationary core, an urging means and a coil. The cylindrical member includes a first magnetic portion, a magnetic shield portion and a second magnetic portion arranged in this order from a downstream end of the cylindrical member toward an upstream end of the cylindrical member. The valve body is held by the first magnetic portion of the cylindrical member and includes at least one fuel injection hole and a vale seat arranged on an upstream side of the injection hole. The valve member is reciprocably received in the cylindrical member and includes an abutting portion. The abutting portion is seatable on the valve seat to close the injection hole and is also liftable from the valve seat to release the injection hole. The movable core is arranged on an upstream side of the valve member and is reciprocable together with the valve member. The stationary core is arranged on an upstream side of the movable core and is opposed to the movable core in the cylindrical member. The urging means is for urging the valve member against the valve seat. The coil is arranged radially outward of the cylindrical member to generate a magnetic force for attracting the movable core toward the stationary core upon energization of the coil. The cylindrical member, the stationary core and the coil are sized to satisfy the following conditions: 5 mm≦La≦10 mm, 0 mm≦Lb≦1.0 mm, Lb&lt;Lc, and 0.5≦Ld/La ≦1.5, wherein La is the axial length of the coil, Lb is the axial distance between a downstream end surface of the coil and a downstream end surface of the magnetic shield portion, Lc is the axial distance between the downstream end surface of the coil and a downstream end surface of the stationary core, and Ld is the axial length of the magnetic shield portion. 
     To achieve the objective of the present invention, there is also provided a fuel injection device including a body, a cylindrical stationary portion, a valve member, a movable portion, an urging means and a coil. The body includes at least one fuel injection hole and a valve seat arranged on an upstream side of the injection hole. The cylindrical stationary portion exhibits magnetism and is secured to the body. The stationary portion includes a main portion and a reduced size portion, which are arranged in an axial direction of the stationary portion. A cross-sectional area of the reduced size portion measured in a plane perpendicular to an axis of the stationary portion is smaller than a cross sectional area of the main portion measured in a plane perpendicular to the axis of the stationary portion. The valve member is reciprocably received in the body and includes an abutting portion. The abutting portion is seatable on the valve seat to close the injection hole and is also liftable from the valve seat to release the injection hole. The movable portion is arranged on a downstream side of the stationary portion and also on an upstream side of the valve member and is reciprocable together with the valve member. The urging means is for urging the valve member against the valve seat. The coil is coaxial with the stationary portion and is arranged radially outward of the stationary portion to generate a magnetic force for attracting the movable portion toward the stationary portion upon energization of the coil. The stationary portion is sized to satisfy the following condition: 0.2≦Sb/Sa≦0.9, wherein Sa is the cross-sectional area of the main portion, and Sb is the cross-sectional area of the reduced size portion. The stationary portion and the coil are sized to satisfy the following condition: 0.1≦(Lf+Lc)/La≦1.0, wherein La is the axial length of the coil, Lf is the axial length of the reduced size portion, and Lc is the axial distance between a downstream end surface of the coil and a downstream end surface of the stationary portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with additional objectives, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings in which: 
     FIG. 1 is a partial enlarged cross-sectional view of a fuel injection device according to an embodiment of the present invention, showing a structure around a coil; 
     FIG. 2 is a cross-sectional view of the fuel injection device according to the embodiment; 
     FIG. 3A is a graph showing a relationship between an axial length of a coil and a valve opening period; 
     FIG. 3B is a graph showing a relationship between an axial distance between a downstream end surface of the coil and a downstream end surface of a non-magnetic portion and a valve opening period as well as a valve closing period; 
     FIG. 3C is a graph showing a relationship between a ratio of an axial length of the non-magnetic portion/an axial length of the coil and a valve opening time; 
     FIG. 3D is a graph showing a relationship between a wall thickness of a stationary core and a valve opening period; 
     FIG. 4 is a graph showing a relationship between a pulse signal and a coil current with respect to time; 
     FIG. 5 is a graph showing a relationship between a pulse signal and an attractive force with respect to time; 
     FIG. 6 is an enlarged partial cross-sectional view of a fuel injection device according to a second embodiment of the present invention, showing a structure around a coil; 
     FIG. 7 is a cross-sectional view of the fuel injection device according to the second embodiment; 
     FIG. 8A is a graph showing a relationship between a ratio of Sb/Sa and an attractive force; 
     FIG. 8B is a graph showing a relationship between a ratio of (Lf+Lc)/La and an attractive force; 
     FIG. 9 is a graph showing a relationship between a time and an electric current value supplied to the coil; 
     FIG. 10 is a graph showing a relationship between an axial length of the coil and a valve opening time attractive force; 
     FIG. 11 is a graph showing a relationship between a cross-sectional area of a reduced size portion and an attractive force; 
     FIG. 12 is an enlarged partial cross-sectional view of a fuel injection device according to a third embodiment of the present invention, showing a structure around a coil; 
     FIG. 13 is an enlarged partial cross-sectional view of a fuel injection device according to a fourth embodiment of the present invention, showing a structure around a coil; 
     FIG. 14 is an enlarged partial cross-sectional view of a fuel injection device according to a fifth embodiment of the present invention, showing a structure around a coil; 
     FIG. 15 is an enlarged partial cross-sectional view of a fuel injection device according to a sixth embodiment of the present invention, showing a structure around a coil; and 
     FIG. 16 is an enlarged partial cross-sectional view of a fuel injection device according to a seventh embodiment of the present invention, showing a structure around a coil. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Embodiment 
     A fuel injection device according to a first embodiment of the present invention will be described with reference to FIGS. 1 to  5 . 
     With reference to FIGS. 1 and 2, a valve housing (cylindrical member)  11  of the fuel injection device  1  is shaped as a cylindrical body, which includes magnetic portions and a nonmagnetic portion, and is made, for example, of a compound magnetic material. A fuel passage  50  is formed in the valve housing  11 . The fuel passage  50  receives a valve body  15 , a valve member  20 , a spring (serving as an urging means)  25 , a stationary core  30 , an adjusting pipe  31  and a filter  39 . 
     The valve housing  11  has a first magnetic portion  12 , a non-magnetic portion (serving as a magnetic shield portion or magnetic resisting portion)  13  and a second magnetic portion  14 , which are formed as an integral body and are arranged in this order from a downstream end of the valve housing  11  located at a lower end in FIG. 2 toward an upstream end of the valve housing  11  located at an upper end in FIG.  2 . The first magnetic portion  12  and the second magnetic portion  14  are magnetized, and the non-magnetic portion  13  is demagnetized by heating a corresponding portion of the valve housing  11 . The non-magnetic portion  13  restrains a short circuit of the magnetic flux between the first magnetic portion  12  and the second magnetic portion  14 . As shown in FIG. 1, a valve body  15  and an injection hole plate  16  having a cup shape are received in a downstream end of the first magnetic portion  12  (i.e., a lower end of the first magnetic portion  12  in FIG.  1 ). 
     The injection hole plate  16  is secured to an outer circumferential wall of the valve body  15  along with a cup shaped support member  17  by laser welding such that the injection hole plate  16  is clamped between the valve body  15  and the support member  17 . The injection hole plate  16  is made of a thin plate material and has a plurality of injection holes  16   a , which extend through the injection hole plate  16  at the center of the injection hole plate  16 . 
     The valve member  20  has a cup shaped cylindrical body and includes an abutting portion  21  at a bottom side of the valve member  20 . The abutting portion  21  of the valve member  20  can be seated against a valve seat  15   a  formed in an inner circumferential wall of the valve body  15 . When the abutting portion  21  of the valve member  20  is seated against the valve seat  15   a , the injection holes  16   a  are closed to stop the fuel injection through the injection holes  16   a . A movable core (or movable portion)  22  is secured to an upstream end of the valve member  20  (i.e., an upper end of the valve member  20  in FIG.  1 ), for example, by laser welding. The valve member  20  includes a plurality of fuel communicating holes  20   a , which penetrate through a lateral wall of the valve member  20  on an upstream side of the abutting portion  21 . Fuel, which is introduced into the vale member  20 , flows outwardly through the fuel communicating holes  20   a  toward a valve arrangement, which is formed by the abutting portion  21  and the valve seat  15   a.    
     The stationary core  30  is shaped as a cylindrical body and is received in the non-magnetic portion  13  and the second magnetic portion  14  such that the stationary core  30  opposes the movable core  22  on an upstream side of the movable core  22 . The adjusting pipe  31  is press fitted into the stationary core  30 . One end of the spring  25 , which serves as the urging means, is engaged to the adjusting pipe  31 , and the other end of the spring  25  is engaged to the movable core  22 . A load of the spring  25  can be adjusted by adjusting an amount of insertion of the adjusting pipe  31  within the stationary core  30 . The spring  25  urges the valve member  20  through the movable core  22  toward the valve seat  15   a.    
     Magnetic members (magnetic housings)  35 ,  36  are arranged radially outward of a coil (or coil winding)  40  and are engaged with the first magnetic portion  12  and the second magnetic portion  14 , respectively. The stationary core  30 , the movable core  22 , the first magnetic portion  12 , the second magnetic portion  14  and the magnetic members  35 ,  36  form a magnetic circuit. 
     The filter  39  is fitted into an upstream side of the valve housing  11 , which is located at an upper side in FIG. 2, to remove foreign particles contained in the fuel. 
     A spool  41 , around which the coil  40  is wound, is attached to an outer circumferential wall of the valve housing  11 . A connector  45 , which is formed by resin molding, covers a radially outer portion of the coil  40  and a radially outer portion of the spool  41 . A terminal  46  is embedded in the connector  45  and is electrically connected to the coil  40 . 
     The fuel, which has passed through the filter  39  into the fuel passage  50  of the valve housing  11 , is discharged from the injection holes  16   a  through a fuel passage of the adjusting pipe  31 , a fuel passage of the stationary core  30 , a fuel passage of the valve member  20 , the fuel communicating holes  20   a  and a space defined between the abutting portion  21  and the valve seat  15   a  when the abutting portion  21  is lifted from the valve seat  15   a.    
     In the fuel injection device  1 , when the coil  40  is deenergized, the valve member  20  is moved in a downward direction (i.e., a valve closing direction) in FIG. 2 by the spring  25 , so that the valve member  20  is seated against the valve seat  15   a  to close the injection holes  16   a , thereby stopping the fuel injection. 
     When the coil  40  is energized, the magnetic flux generated in the coil  40  passes through the magnetic circuit, which surrounds the coil  40 , so that the magnetic attractive force is generated between the stationary core  30  and the movable core  22 . Then, the valve member  20  is attracted toward the stationary core  30 , and the abutting portion  21  is lifted away from the valve seat  15   a . In this way, the fuel is injected through the injection holes  16   a.    
     Next, positions and sizes of the non-magnetic portion  13 , the movable core  22 , the stationary core  30  and the coil  40  will be described. 
     With reference to FIG. 1, an axial length of the coil  40  in a reciprocating direction of the valve member  20  is indicated by “La”. An axial distance between a downstream end surface of the coil  40  and a downstream end surface of the non-magnetic portion  13  is indicated by “Lb”. An axial distance between the downstream end surface of the coil  40  and the downstream end surface of the stationary core  30  is indicated by “Lc”. An axial length of the non-magnetic portion  13  is indicated by “Ld”. A wall thickness of the stationary core  30  is indicated by “Le”. The following conditions need to be satisfied: 5 mm≦La≦10 mm, 0 mm≦Lb≦1.0 mm, Lb&lt;Lc, 0 mm≦Lc≦1.0 mm, 0.5≦Ld/La ≦1.5 and 0.5 mm≦Le≦2.0 mm. When these conditions are satisfied a valve opening period (To) and a valve closing period (Tc) can be advantageously reduced as shown in FIGS. 3A to  3 D. 
     More specifically, by setting 0.5≦Ld/La≦1.5, the length of the non-magnetic portion  13  held within the coil  40  is increased, so that an inductance of the coil  40  becomes relatively small. Thus, as shown in FIGS. 4 and 5, when supply of the pulse signal to the coil  40  is turned on, the electric current, which flows through the coil  40 , and an attractive force thus generated are more quickly increased, so that a valve opening period is advantageously reduced. When the supply of the pulse signal to the coil  40  is turned off, the electric current, which flows through the coil  40 , and the attractive force thus generated are quickly reduced, so that a valve closing period is advantageously reduced. Furthermore, the reduction of the valve closing period results in a reduction of power consumption, so that the power consumption of the corresponding drive circuit can be reduced, and the engine can be operated at relatively high rotational speeds. 
     Furthermore, by setting 5 mm≦La≦10 mm, the length of the coil  40  is reduced, so that a length of the magnetic circuit is reduced, and thus a magnetic loss is reduced. With this arrangement, the electric current, which flows through the coil  40 , and the attractive force thus generated are more quickly increased and are also more quickly reduced. 
     Also, by setting 0 mm≦Lc≦1.0 mm, the air gap between the movable core  22  and the stationary core  30 , at which the relatively high magnetic flux density is desired, is more closely positioned to the end of the coil  40 . Since the inductance of the coil  40  is reduced, the electric current, which flows through the coil  40 , and the attractive force thus generated are more quickly increased and are also more quickly reduced. 
     When the electric current, which flows through the coil  40 , and the attractive force thus generated are more quickly increased and are more quickly reduced, the valve opening period and the valve closing period of the fuel injection device  1  are reduced. Thus, the characteristics of the fuel injection rate of the fuel injection device  1  are more approximated to a waveform of the pulse signal applied to the coil  40 . As a result, a pulse duration of the pulse signal applied to the coil  40  and the fuel injection amount are substantially proportioned relative to each other, so that the fuel injection amount can be more precisely controlled. Particularly, when the fuel injection amount is relatively small, for example, during idle operation of the engine, it is not necessary to excessively inject the fuel. Thus, the fuel consumption is improved, and the amount of noxious components contained in the exhaust gases is reduced. 
     It should be noted that even when the settings of 0 mm≦Lc≦1.0 mm and 0.5 mm≦Le≦2.0 among the above described settings are not satisfied, the valve opening period and the valve closing period of the fuel injection device can be still reduced. 
     In the above embodiment of the present invention, the positions of the non-magnetic portion  13 , the movable core  22 , the stationary core  30  and the coil  40  are adjusted, so that the valve opening period is advantageously reduced, and the valve closing period is also advantageously reduced without increasing the urging force of the spring. Furthermore, it is not required to increase the urging force of the spring, and thus it is not required to increase the number of turns of the coil to overcome the increased urging force of the spring, so that the size of the fuel injection device can be advantageously reduce. 
     Furthermore, in the above embodiment, the portion of the compound magnetic material is heated and is thus demagnetized to form the non-magnetic portion  13  serving as the magnetic shield portion. Alternative to this, it is possible to reduce the thickness of the corresponding portion of the magnetic material to form the magnetic shield portion. 
     Second Embodiment 
     A fuel injection device according to a second embodiment of the present invention will be described with reference to FIGS. 6 and 7. In the second embodiment, similar components to those described in the first embodiment are similarly labeled. A resin housing  10  is secured such that the resin housing  10  covers an outer circumferential wall of a cylindrical member  11 . A stationary core  30  and a coil  40  are received radially inward of the resin housing  10 . 
     The cylindrical member  11  is shaped as a generally straight cylindrical body and includes a first magnetic portion  12 , a non-magnetic portion (serving as a magnetic shield portion or magnetic resisting portion)  13  and a second magnetic portion  14 , which are axially arranged in this order from a downstream end of the cylindrical member  11  (i.e., a lower end of the cylindrical member  11  in FIG. 6 or  7 ) to an upstream end of the cylindrical member  11  (i.e., an upper end of the cylindrical member  11  in FIG. 6 or  7 ). The first magnetic portion  12  and the second magnetic portion  14  are magnetized to exhibit magnetism, and the non-magnetic portion  13  is demagnetized. The non-magnetic portion  13  restrains a short circuit of a magnetic flux between the first magnetic portion  12  and the second magnetic portion  14 . The coil  40  surrounds the non-magnetic portion  13  and a downstream end section  14   a  of the second magnetic portion  14  (i.e., a lower end section of the second magnetic portion  14  in FIG.  6 ). In FIG. 6, a boundary between the downstream end section  14   a  of the second magnetic portion  14  and an upstream section of the second magnetic portion  14  (i.e., an upper section of the second magnetic portion  14  other than the downstream end section  14   a  in FIG. 6) is indicated by a dot-dot-dash line. 
     The stationary core  30  is made of a magnetic material, such as an iron material, and is shaped as a generally straight cylindrical body. The stationary core  30  is secured to the resin housing  10  through the cylindrical member  11  at radially inward of the second magnetic portion  14  and the non-magnetic portion  13  and is arranged coaxial with the second magnetic portion  14  and the non-magnetic portion  13 . Thus, an outer circumferential wall of the stationary core  30  is covered by the non-magnetic portion  13  and the second magnetic portion  14 , and a downstream end section  30   a  of the stationary core  30  (i.e., a lower end section of the stationary core  30  in FIG. 6) is surrounded by the coil  40 . In FIG. 6, a boundary between the downstream end section  30   a  of the stationary core  30 , which is surrounded by the coil  40 , and an upstream section of the stationary core  30  (i.e., an upper section of the stationary core  30  other than the downstream end section  30   a  in FIG. 6) is also indicated by the above-described dot-dot-dash line. In the present embodiment, the downstream end section  30   a  of the stationary core  30  and the downstream end section  14   a  of the second magnetic portion  14  cooperatively form a stationary portion  50 , in which a main portion  50   a  and a reduced size portion  50   b  are axially arranged. Specifically, a part of the end section  30   a  of the stationary core  30 , which is covered by the end section  14   a  of the second magnetic portion  14 , and the end section  14   a  of the second magnetic portion  14  cooperatively form the main portion  50   a . Furthermore, a part of the end section  30   a  of the stationary core  30 , which is covered by the non-magnetic portion  13 , forms the reduced size portion  50   b . A cross-sectional area of the reduced size portion  50   b  in a plane perpendicular to an axis O of the stationary core  30  and thus of the cylindrical member  11  is smaller than a cross-sectional area of the main portion  50   a  in a plane perpendicular to the axis O. Here, the reduced size portion  50   b  means that a cross-sectional area Sb of the reduced size portion  50   b  in the plane perpendicular to the axis O is reduced in comparison to a cross-sectional area Sa of the main portion  50   a . A downstream end surface of the stationary core  30 , which also serves as a downstream end surface  50   c  of the stationary portion  50 , is a planar surface that extends in the direction perpendicular to the axis O. 
     A valve body  15  is secured to an inner circumferential surface of a downstream end section of the first magnetic portion  12  (i.e., a lower end section of the first magnetic portion  12  in FIG.  6 ). A cup shaped injection hole plate  16  is secured to an outer circumferential wall of the valve body  15  together with a cup shaped support member  17  by laser welding such that the injection hole plate  16  is clamped between the valve body  15  and the support member  17 . The injection hole plate  16  is made of a thin plate material and has the injection holes  16   a , which extend through the injection hole plate  16  at the center of the injection hole plate  16 . 
     The valve member  20  is shaped as a cup shaped cylindrical body and is axially reciprocably received in the valve body  15  and the cylindrical member  11 . The valve member  20  includes an abutting portion  21  at a bottom wall of the valve member  20  (i.e., a lower end wall of the valve member  20  in FIG.  6 ). The abutting portion  21  can be seated against a valve seat  15   a  formed in an inner circumferential wall of the valve body  15 . When the abutting portion  21  of the valve member  20  is seated against the valve seat  15   a , the injection holes  16   a  are closed to stop the fuel injection through the injection holes  16   a . The valve member  20  includes a plurality of fuel communicating holes  20   a , which penetrate through a lateral wall of the valve member  20  on an upstream side of the abutting portion  21 . Fuel, which is introduced into the vale member  20 , flows outwardly through the fuel communicating holes  20   a  toward a valve arrangement, which is formed by the abutting portion  21  and the valve seat  15   a . 
     A movable core (serving as a movable portion)  22  is made of a magnetic material and is shaped as a cylindrical body. The movable core  22  is secured to an upstream end of the valve member  20  (i.e., an upper end of the valve member  20  in FIG.  6 ), for example, by laser welding. The movable core  22  can reciprocate together with the valve member  20 . The movable core  22  is arranged within the cylindrical member  11  on a downstream side of the downstream end section  30   a  of the stationary core  30 , which serves as a part of the stationary portion  50 . Furthermore, an upstream end section  22   a  of the movable core  22  is opposed to the downstream end section  30   a  of the stationary core  30 . 
     An adjusting pipe  31 , which is demagnetized, is press fitted into the stationary core  30 . One end of a spring  25 , which serves as an urging means, is engaged to the adjusting pipe  31 , and the other end of the spring  25  is engaged to the movable core  22 . A load of the spring  25  can be adjusted by adjusting an amount of insertion of the adjusting pipe  31  within the stationary core  30 . The spring  25  urges the valve member  20  through the movable core  22  toward the valve seat  15   a.    
     Magnetic housings (magnetic members)  35 ,  36  are made of a magnetic material and are secured to a radially outer side of the coil  40 . The magnetic housing  35  is engaged with the first magnetic portion  12 , and the magnetic housing  36  is engaged with the second magnetic portion  14 . In the present embodiment, the resin housing  10 , the valve body  15  and the magnetic housings  35 ,  36  form a body. 
     A spool  41  is made of a resin material and is arranged such that the spool  41  covers an outer circumferential wall of the cylindrical member  11 . The coil  40  is wound around the spool  41 . Thus, the coil  40  is coaxial with and is positioned radially outward of the stationary portion  50 , which includes the end section  30   a  of the stationary core  30  and the end section  14   a  of the second magnetic portion  14 . A downstream end surface  40   c  of the coil  40  (a lower end surface of the coil  40  in FIG. 6) is arranged in a plane that is perpendicular to the axis O on a downstream side of the downstream end surface  50   c  of the stationary portion  50 . A terminal  46  is embedded in a connector  45  of the resin housing  10  that covers the coil  40  and the spool  41 . The coil  40  is electrically connected to the terminal  46 . When the coil  40  is energized through the terminal  46 , the coil  40 , the stationary portion  50 , the first magnetic portion  12 , the movable core  22  and the magnetic housings  35 ,  36  form a magnetic circuit. 
     A filter  39  is securely fitted into an upstream end section of the cylindrical member  11  (i.e., an upper end section of the cylindrical member  11  in FIG. 7) to remove foreign particles contained in the fuel. The fuel, which has passed through the filter  39  and is introduced into the fuel passage of the cylindrical member  11 , is injected from the injection holes  16   a  through a fuel passage of the adjusting pipe  31 , a fuel passage of the stationary core  30 , a fuel passage of the movable core  22 , a fuel passage of the valve member  20 , the fuel communicating holes  20   a  and a space defined between the abutting portion  21  and the valve seat  15   a  when the abutting portion  21  is lifted from the valve seat  15   a.    
     In the fuel injection device, when the coil  40  is deenergized by turning off supply of the pulse signal to the coil  40 , the valve member  20 , which is urged by the urging force of the spring  25 , is moved together with the movable core  22  in a downward direction (i.e., a valve closing direction) in FIG. 7, so that the abutting portion  21  of the valve member  20  is seated against the valve seat  15   a  to close the injection holes  16   a , thereby stopping the fuel injection. 
     When energization of the coil  40  is initiated by turning on the supply of the pulse signal to the coil  40 , the magnetic flux generated by the coil  40  passes through the magnetic circuit, so that the magnetic attractive force is generated between the end section  30   a  of the stationary core  30  and an opposed end section  22   a  of the movable core  22 . Then, the valve member  20  is magnetically attracted toward the stationary core  30  against the forces, which include the urging force of the spring  25  and the fuel pressure in the fuel passage, so that the valve member  20  moves in an upward direction (i.e., a valve opening direction) in FIG.  7 . Thus, the abutting portion  21  of the valve member  20  is lifted from the valve seat  15   a  to release the injection holes  16   a , so that the fuel is injected from the injection holes  16   a.    
     Next, with reference to FIG. 6, settings of the cross-sectional area Sa of the main portion  50   a , the cross-sectional area Sb of the reduced size portion  50   b , an axial length Lf of the reduced size portion  50   b , an axial length La of the coil  40  and an axial distance Lc between the downstream end surface  40   c  of the coil  40  and the downstream end surface  50   c  of the stationary portion  50  will be described. (1) The cross-sectional area Sa of the main portion  50   a  and the cross-sectional area Sb of the reduced size portion  50   b  are selected such that a ratio of Sb/Sa is in a range of 0.2≦Sb/Sa ≦0.9. Furthermore, the axial length La of the coil  40 , the axial length Lf of the reduced size portion  50   b  and the axial distance Lc between the end surface  40   c  of the coil  40  and the end surface  50   c  of the stationary portion  50  are selected such that a ratio of (Lf+Lc)/La is in a range of 0.1≦(Lf+Lc)/La≦1.0. 
     When the ratio of Sb/Sa is equal to or greater than 0.2, and the ratio of (Lf+Lc)/La is equal to or less than 1.0, a volume of the passing magnetic flux in the magnetic circuit can be increased. In this way, a valve opening time attractive force (i.e., an attractive force exerted at time of valve opening) for attracting the movable core  22  is increased to attract the movable core  22 , which currently places the valve member  20  in the seated state on the valve seat  15   a , against the forces, which include the urging force of the spring  25  and the fuel pressure, at the time of valve opening, as shown in FIGS. 8A and 8B. When the ratio of Sb/Sa becomes less than 0.2 or when the ratio of (Lf+Lc)/La exceeds 1.0, the valve opening time attractive force is reduced below 8 N, as shown in FIGS. 8A and 8B, so that the movable core  22  cannot be attracted and moved toward the stationary core  30 . 
     Furthermore, when the ratio of Sb/Sa is equal to or less than 0.9, and the ratio of (Lf+Lc)/La is equal to or greater than 0.1, an increase in the volume of the passing magnetic flux can be restrained. Thus, increases of the attractive forces, which include a saturation attractive force at time of full lift of the movable core  22  in the valve opening directing and the above-described valve opening time attractive force, are restrained, for example, as shown in FIGS. 8A and 8B. Since the increases of the attractive forces are restrained, an increase of an inductance in the magnetic circuit is also restrained. As a result, when the supply of the pulse signal to the coil  40  is turned on, the electric current, which flows through the coil  40 , is quickly increased, as shown in FIG.  9 . Because of this effect and the increase of the valve opening time attractive force, the movable core  22  is quickly moved in the valve opening direction to lift the valve member  20  from the valve seat  15   a  at the time of energization of the coil  40 , so that a valve opening period is advantageously reduced. Furthermore, since the increase of the saturation attractive force is restrained, the movable core  22  is quickly moved in the valve closing direction to quickly seat the valve member  20  against the valve seat  15   a  at the time of deenergization of the coil  40 , so that a valve closing period is advantageously reduced. When the ratio of Sb/Sa exceeds 0.9, or when the ratio of (Lf+Lc)/La is less than 0.1, the saturation attractive force exceeds 15N, as shown in FIGS. 8A and 8B, so that the valve closing period is disadvantageously increased. (2) The axial length La of the coil  40  is in the range of 4 mm≦La≦12 mm. 
     As shown in FIG. 10, when the axial length La of the coil  40  is equal to or greater than 4 mm and is also equal to or less than 12 mm, the valve opening time attractive force of greater than or equal to 8 N can be reliably achieved without a substantial increase of a radial size of the coil  40 . Thus, the valve opening period is reliably reduced. (3) The cross-sectional area Sb of the reduced size portion  50   b  is in the range of 11 mm 2 ≦Sb≦28 mm 2 . 
     When the cross-sectional area Sb of the reduced size portion  50   b  is equal to or greater than 11 mm 2 , the valve opening time attractive force becomes equal to or greater than 8 N, so that the valve member  20  can be quickly and reliably lifted from the valve seat  15   a , as shown in FIG.  11 . Furthermore, when the cross-sectional area Sb of the reduced size portion  50   b  is equal to or less than 28 mm 2 , the saturation attractive force becomes equal to or less than 15 N, as shown in FIG. 11, so that the valve member  20  can be quickly seated against the vale seat  15   a . As a result, the valve opening period and the valve closing period can be reliably reduced. 
     With the above-described settings, the valve opening period and the valve closing period of the fuel injection device are reduced. Thus, the fuel injection rate characteristics of the fuel injection device are approximated to a waveform of the pulse signal applied to the coil  40 . As a result, a pulse duration of the pulse signal applied to the coil  40  and the fuel injection amount are substantially proportioned relative to each other, so that the fuel injection amount can be more precisely controlled. Particularly, when the fuel injection amount is relatively small, for example, during idle operation of the engine, it is not necessary to excessively inject the fuel. Thus, the fuel consumption of the engine is improved, and the amount of noxious components contained in the exhaust gases is reduced. 
     Among the above settings, the settings described in the above sections (2) and (3) can be changed based on a corresponding specification of the fuel injection device within a range that allows a reduction of the valve closing period and a reduction of the valve opening period. 
     Third to Sixth Embodiments 
     FIGS. 12 to  15  show fuel injection devices according to third to sixth embodiments, respectively, of the present invention. In each of the third to sixth embodiments, components similar to those of the second embodiment are labeled similarly. 
     In each of the fuel injection device according to the third embodiment shown in FIG.  12  and the fuel injection device according to the fourth embodiment shown in FIG. 13, a downstream end section  30   a  of a stationary core  30  includes an annular groove  200 , which extends in a circumferential direction at a part of the downstream end section  30   a  of the stationary core  30 , which is covered by a downstream end section  14   a  of a second magnetic portion  14  and is located adjacent to another part of the downstream end section  30   a  of the stationary core  30 , which is covered by a non-magnetic portion  13 . In the third embodiment, the annular groove  200  is provided in an outer circumferential wall of the stationary core  30 . In the fourth embodiment, the annular groove  200  is provided in an inner circumferential wall of the stationary core  30 . 
     Similar to the second embodiment, in the third and fourth embodiments, the downstream end section  30   a  of the stationary core  30  and the downstream end section  14   a  of the second magnetic portion  14 , which are surrounded by a coil  40 , form a stationary portion  50 . However, unlike the second embodiment, a part of the end section  30   a , which is covered by the second magnetic portion  14  and is other than a base wall  201  of the annular groove  200 , and a part of the end section  14   a , which covers the part of the end section  30   a  other than the base wall  201 , cooperatively form a main body  50   a . Furthermore, a part of the end section  14   a , which covers the base wall  201 , a part of the end section  30   a , which forms the base wall  201 , and a part of the end section  30   a , which is covered by the non-magnetic portion  13 , cooperatively form a reduced size portion  50   b.    
     With reference to FIGS. 12 and 13, in the third and fourth embodiments, a cross-sectional area Sa of the main portion  50   a , a cross-sectional area Sb (Sb 1 , Sb 2 ) of the reduced size portion  50   b , an axial length Lf of the reduced size portion  50   b , an axial length La of the coil  40  and an axial distance Lc between a downstream end surface  40   c  of the coil  40  and a downstream end surface  50   c  of the stationary portion  50  are the same as those of the second embodiment. Here, the cross-sectional area Sb 1  is measured in a plane that extends perpendicular to an axis O through the part of the end section  30   a , which forms the base wall  201 . The cross-sectional area Sb 2  is measured in a plane that extends perpendicular to the axis O through the part of the end section  30   a , which is covered by the non-magnetic portion  13 . It should be noted that as long as 0.2≦Sb/Sa≦0.9 is satisfied, the cross-sectional area Sb of the reduced size portion  50   b  can be selected such that the cross-sectional area Sb 1  and the cross-sectional area Sb 2  are different from each other or are the same. 
     The fuel injection device according to the fifth embodiment shown in FIG. 14 includes an annular groove  200 , which is similar to the annular groove  200  of the third embodiment, in an outer circumferential wall of the stationary core  30 . The fuel injection device according the fifth embodiment also includes a cylindrical member  300 , which is entirely demagnetized, in place of the cylindrical member  11  of the third embodiment. The fuel injection device according to the sixth embodiment shown in FIG. 15 includes an annular groove  200 , which is similar to the annular groove  200  of the fourth embodiment, in an inner circumferential wall of the stationary core  30 . The fuel injection device according to the sixth embodiment also includes a cylindrical member  400 , which is entirely demagnetized, in place of the cylindrical member  11  of the fourth embodiment. 
     In each of the fifth and sixth embodiments, a downstream end section  30   a  of the stationary core  30 , which is surrounded by the coil  40 , forms a stationary portion  50 . Specifically, each part of the end section  30   a  other than a base wall  201  of the annular groove  200  forms a main portion  50   a  of the stationary portion  50 , and a part of the end section  30   a , which forms the base wall  201 , constitutes a reduced size portion  50   b.    
     Furthermore, with reference to FIGS. 14 and 15, in each of the fifth and sixth embodiments, a cross-sectional area Sa (Sa 1 , Sa 2 ) of the main portion  50   a , a cross-sectional area Sb of the reduced size portion  50   b , an axial length Lf of the reduced size portion  50   b , an axial length La of the coil  40  and an axial distance Lc between a downstream end surface  40   c  of the coil  40  and a downstream end surface  50   c  of the stationary portion  50  are the same as those of the second embodiment. Here, the cross-sectional area Sal is measured in a plane that extends perpendicular to an axis O through a part of the end section  30   a , which is located on an upstream side of the base wall  201  (i.e., on an upper side of the base wall  201  in FIG.  14 ). The cross-sectional area Sa 2  is measured in a plane that extends perpendicular to the axis O through a part of the end section  30   a , which is located on a downstream side of the base wall  201  (i.e., on a lower side of the base wall  201  in FIG.  14 ). It should be noted that as long as 0.2≦Sb/Sa≦0.9 is satisfied, the cross-sectional area Sa of the main portion  50   a  can be selected such that the cross-sectional area Sa 1  and the cross-sectional area Sa 2  are the same. 
     Seventh Embodiment 
     FIG. 16 shows a fuel injection device according to a seventh embodiment of the present invention. In the seventh embodiment, components similar to those of the second embodiment are labeled similarly. 
     In the fuel injection device of the seventh embodiment shown in FIG. 16, a cylindrical member  11  includes an annular groove  500 , which extends in a circumferential direction at a part of a downstream end section  14   a  of a second magnetic portion  14 , which is located adjacent to the non-magnetic portion  13 . In the present embodiment, although the annular groove  500  is arranged in an outer circumferential wall of the second magnetic portion  14 , the annular groove  500  can be alternatively provided in an inner circumferential wall of the second magnetic portion  14 . 
     Similar to the second embodiment, in the seventh embodiment, a downstream end section  30   a  of a stationary core  30  and a downstream end section  14   a  of the second magnetic portion  14 , which are surrounded by the coil  40 , form a stationary portion  50 . A part of the downstream end section  14   a , which is other than the base wall  501  of the annular groove  500 , and a part of the end section  30   a , which is covered by the part of the downstream end section  14   a  other than the base wall  501  of the annular groove  500 , cooperatively form a main portion  50   a . Furthermore, a part of the end section  14   a , which forms the base wall  501 , and a part of the end section  30   a , which is surrounded by the base wall  501  and the non-magnetic portion  13 , cooperatively form a reduced size portion  50   b.    
     Furthermore, with reference to FIG. 16, in the seventh embodiment, a cross-sectional area Sa of the main portion  50   a , a cross-sectional area Sb (Sb 1 , Sb 2 ) of the reduced size portion  50   b , an axial length Lf of the reduced size portion  50   b , an axial length La of the coil  40  and an axial distance Lc between a downstream end surface  40   c  of the coil  40  and a downstream end surface  50   c  of the stationary portion  50  are the same as those of the second embodiment. Here, the cross-sectional area Sb 1  is measured in a plane that extends perpendicular to an axis O through the part of the end section  14   a , which forms the base wall  501 . The cross-sectional area Sb 2  is measured in a plane that extends perpendicular to the axis O through the part of the end section  30   a , which is covered by the non-magnetic portion  13 . It should be noted that as long as 0.2≦Sb/Sa≦0.9 is satisfied, the cross-sectional area Sb of the reduced size portion  50   b  can be selected such that the cross-sectional area Sb 1  and the cross-sectional area Sb 2  are different from each other. 
     Additional advantages and modifications will readily occur to those skilled in the art. The invention in its broader terms is therefore, not limited to the specific details, representative apparatus, and illustrative examples shown and described.