Patent Publication Number: US-8985550-B2

Title: Electromagnetic valve

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a divisional of U.S. application Ser. No. 13/365,506, filed Feb. 3, 2012, which claims priority to Japanese Patent Application No. 2011-022509, filed on Feb. 4, 2011, the disclosures of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electromagnetic valve whose valving element is actuated by utilizing magnetic force of an electromagnet. 
     2. Description of Related Art 
     An example of a conventional normally-closed electromagnetic valve will be described. In the normally-closed electromagnetic valve, an electromagnet is produced by energization of a coil. A movable core is attracted to and contacts with a fixed core magnetized by the electromagnet, so that a valving element attached to the movable core is disengaged from a valve seat to cause the electromagnetic valve to be open. When the energization of the coil is stopped and magnetic force of the electromagnet disappears, the movable core is pushed back in an opposite direction from the fixed core by reactive force of a return spring, and the valving element is engaged with the valve seat, thereby to close the electromagnetic valve. In the above-described electromagnetic valve, for example, an elastic body such as rubber can be used for the valving element. In this case, by the repeated opening and closing operation of the valving element for a long period, plastic deformations such as wear and creep are produced in the valving element and the valve seat, so that the valving element and the valve seat are shaped to conform to each other. Thus, a sealing performance when the valving element sits on the valve seat is improved as time passes. However, if the movable core, to which the valving element is attached, rotates while the movable core is attracted and moves to the fixed core, the above-described portions of the valving element and the valve seat, which are conformed in form with each other, are relatively shifted from each other. Therefore, the sealing performance when the valving element sits on the valve seat cannot be maintained. 
     For the measures against this problem, a technology (see, JP2005-214225A, JP2005-98340A) is known. According to this technology, the movable core is attracted on radially one side of a cylindrical core guide part for guiding the movable core when the movable core is attracted and moves to the fixed core. In JP2005-214225A, the center of the return spring which urges the movable core is eccentrically arranged relative to the central axis of the movable core, so that the movable core is pressed against one side of the core guide part and rotation of the movable core is limited. In JP2005-98340A, a gap expansion part, which expands a gap between the movable core and the core guide part, is formed on an outer periphery of the movable core, or an attachment center of an impact absorbing means, which is attached to the movable core, is made eccentric from the center of the movable core. As a result, the movable core is pushed against one side of the core guide part, and rotation of the movable core is prevented. 
     However, in JP2005-214225A, because an action center of urging force of the return spring applied to the movable core is shifted from the center line of the movable core, a sealing load may be one-sided when the valving element sits on the valve seat, and leakage from the electromagnetic valve may occur. In JP2005-98340A, a gravity center of the movable core is shifted from the center line of the movable core, i.e., the gravity center is not located on the same line as the center line of the movable core. Therefore, in the electromagnetic valve disposed in a vehicle, for example, rotation of the movable core may be not limited due to its install direction, an acceleration of the vehicle, a centrifugal force and so on. 
     SUMMARY OF THE INVENTION 
     The present invention addresses at least one of the above disadvantages. 
     According to the present invention, there is provided an electromagnetic valve including a valve seat, a valving element, and a solenoid part. The valve seat has an annular shape and defines a valve hole that opens radially inward of the valve seat. The valving element is provided to be movable between a valve-closing position where the valving element is engaged with the valve seat to close the valve hole and a valve-opening position where the valving element is disengaged from the valve seat to open the valve hole. The solenoid part is configured to drive the valving element by utilizing magnetic force of an electromagnet. The solenoid part includes a coil, a core guide part, a fixed core, and a movable core. The coil becomes the electromagnet upon energization thereof. The core guide part has a cylindrical shape and is arranged radially inward of the coil to form a magnetic circuit. The fixed core is arranged on one end side of the core guide part in an axial direction of the core guide part and is magnetized by the electromagnet. The movable core is accommodated inside the core guide part to be opposed to the fixed core in the axial direction and is reciprocated inside the core guide part in accordance with whether the electromagnet is turned on or off. The valving element moves integrally with the movable core to open or close the valve hole. The core guide part includes a magnetic unbalance part where the magnetic force applied between the core guide part and the movable core is different between on one side and the other side of the core guide part, which are opposed to each other in a radial direction of the core guide part. 
    
    
     
       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 sectional view illustrating a solenoid part according to a first embodiment of the invention; 
         FIG. 2  is a sectional view of the solenoid part illustrating unbalance magnetic force applied between a core guide part and a movable core according to the first embodiment; 
         FIG. 3A  is a sectional view illustrating the solenoid part excluding the movable core according to the first embodiment; 
         FIG. 3B  is a cross-sectional view taken along a line IIIB-IIIB of  FIG. 3A  and illustrating a magnetic saturation part provided in the core guide part; 
         FIG. 4  is a schematic diagram illustrating a fuel vapor treatment system according to the first embodiment; 
         FIG. 5A  is a sectional view illustrating a solenoid part excluding a movable core according to a second embodiment of the invention; 
         FIG. 5B  is a cross-sectional view taken along a line VB-VB of  FIG. 5A  and illustrating a magnetic saturation part provided in a core guide part; 
         FIG. 6A  is a sectional view illustrating a solenoid part excluding a movable core according to a third embodiment of the invention; 
         FIG. 6B  is a cross-sectional view taken along a line VIB-VIB of  FIG. 6A  and illustrating a magnetic saturation part provided in a core guide part; 
         FIG. 7A  is a sectional view illustrating a solenoid part excluding a movable core according to a fourth embodiment of the invention; 
         FIG. 7B  is a cross-sectional view taken along a line VIIB-VIIB of  FIG. 7A  and illustrating a magnetic saturation part provided in a core guide part; and 
         FIG. 8  is a perspective view illustrating a movable core according to a fifth embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Modes of the invention will be in detail described based on embodiments below. 
     (First Embodiment) 
     In a first embodiment, an electromagnetic valve of the invention is applied to a fuel vapor treatment system of a vehicle. As shown in  FIG. 4 , the fuel vapor treatment system prevents fuel vapor from emitting into the atmosphere. Fuel vapor is evaporated inside a fuel tank  1  disposed in the vehicle, The fuel vapor treatment system includes a canister  2  which temporarily adsorbs and holds fuel vapor leaked from the tank  1 . The canister  2  is filled with an adsorption agent such as an activated carbon, which adsorbs fuel vapor. The canister  2  includes a vapor port  2   a , a purge port  2   b , and an air port  2   c . The vapor port  2   a  is connected to the tank  1  through a vapor passage  3 , and the purge port  2   b  is connected to an intake pipe  5  of an internal combustion engine through a purge passage  4 . The air port  2   c  opens to the atmosphere through an air passage  6 . 
     A purge valve  7  is provided along the purge passage  4 . The purge valve  7  regulates a flow volume of fuel vapor, which is suctioned into the intake pipe  5  from the canister  2  by intake negative pressure in the internal combustion engine. The electromagnetic valve of the invention is applied to the purge valve  7 . A throttle valve  8  is provided in the intake pipe  5 . The purge passage  4  is connected to the intake pipe  5  on a downstream side (internal combustion engine-side) of the throttle valve  8  in an intake-air flow direction. In the air passage  6 , a filter  9  and a normally-open canister control valve  10  are provided. The filter  9  filtrates air flowing into the canister  2 , and the canister control valve  10  causes the air port  2   c  of the canister  2  to be closed as necessary. The filter  9  can be incorporated into the purge valve  7 , and, in this case, the filter  9  provided in the air passage  6  may be omitted. 
     A structure of the purge valve  7  of the invention will be described referring to  FIG. 1 . The purge valve  7  includes a housing (not shown), a valving element  11 , and a solenoid part  12 . The housing defines a connection passage communicating with the purge passage  4 . The valving element  11  is accommodated inside the housing, and the solenoid part  12  actuates the valving element  11  by utilizing magnetic force of an electromagnet. The connection passage of the housing includes an inflow port, an outflow port, and a communication passage. The inflow port is connected to the purge passage  4  on an upstream side (canister 2-side) of the purge valve  7  in a flow direction of fuel vapor, and the outflow port is connected to the purge passage  4  on a downstream side (intake pipe 5-side) of the purge valve  7  in the flow direction of fuel vapor. The inflow port and the outflow port communicate with each other through the communication passage. An annular valve seat  13  is provided in the communication passage. The valving element  11  is made of, for example, rubber elastic body (e.g., fluorine-contained rubber, silicon rubber) and can cause a valve hole  14  to be opened and closed. The valve hole  14  opens radially inward of the valve seat  13 . When the valving element  11  is engaged with the valve seat  13  to close the valve hole  14 , a communication between the inflow port and the outflow port is closed. When the valving element  11  is disengaged from the valve seat  13  to open the valve hole  14 , the communication between the inflow port and the outflow port is made. 
     The solenoid part  12  includes a coil  15 , a magnetic-circuit forming member, a movable core  16 , and a coil spring  17 . The coil  15  is wound around a bobbin (not shown) having an insulation property, and the magnetic-circuit forming member forms a magnetic circuit around the coil  15 . The movable core  16  moves in an axial direction of the coil  15  (in a vertical direction in  FIG. 1 ), and the coil spring  17  urges the movable core  16  in one direction. The coil  15  is energized and controlled by an engine control unit (ECU) via a drive circuit (not shown) and becomes an electromagnet as a result of the supply of an excitation current. The magnetic-circuit forming member includes a yoke  18 , a core guide part  19 , and a fixed core  20 . The yoke  18  is a part of the magnetic circuit located on an outer periphery of the coil  15 . The core guide part  19  is a part of the magnetic circuit located on an inner periphery of the coil  15 . The fixed core  20  is located on one side of the core guide part  19  in the axial direction of the coil  15 . The yoke  18  includes an outer-periphery yoke part and a bottom yoke part. The outer-periphery yoke part covers the outer periphery of the coil  15  along an entire length of the coil  15  in the axial direction of the coil  15 . The bottom yoke part covers an end surface of the coil  15  on the one side of the coil  15  in the axial direction. 
     The core guide part  19  has a cylindrical shape coaxially with the coil  15  and defines a guide hole  19   a  (see  FIG. 3A ) in which the movable core  16  is contained. An inner surface of the guide hole  19   a  is a cylindrical surface which has a constant inner diameter entirely in its longitudinal direction. A core plate  19   b  is integrally formed with the core guide part  19  on an opposite side from the fixed core  20  (on the other side of the core guide part  19 ) in the axial direction. The core plate  19   b  extends radially outward from the core guide part  19  to have a flange-like shape. The core plate  19   b  covers an end surface of the coil  15  on the other side of the coil  15  in the axial direction and is a part of the magnetic circuit which is connected to the yoke  18 . In the core guide part  19 , a magnetic saturation part  21  and a magnetic unbalance part  24  are provided. The fixed core  20  serves as an attraction part, which attracts the movable core  16  in the axial direction due to magnetization of the fixed core  20  by energization of the coil  15 . As shown in  FIG. 1 , the fixed core  20  is integrally provided with the core guide part  19 , but these two can be separately provided from each other. 
     The movable core  16  is contained in the guide hole  19   a  defined by the core guide part  19 , and moves in the guide hole  19   a  in the axial direction (vertical direction in  FIG. 1 ) of the core guide part  19 , facing to the fixed core  20 . The movable core  16  having a cylindrical shape defines a spring chamber  16   a  inside an inner periphery of the movable core  16 . An opening of the movable core  16  on an opposite side thereof from the fixed core  20  is closed by an end board  16   b,  to which the valving element  11  is attached. An outer circumferential surface of the movable core  16  is a cylindrical surface which has a constant outer diameter entirely in its longitudinal direction. The outer diameter is slightly smaller than the inner diameter of the guide hole  19   a , so that the movable core  16  can be reciprocated in the axial direction of the core guide part  19  in the guide hole  19   a.  The coil spring  17  is accommodated in the spring chamber  16   a  of the movable core  16  and located coaxially with the movable core  16 . An end part of the coil spring  17  on its one end side in an axial direction thereof is supported by an end surface of the fixed core  20 , and an end part of the coil spring  17  on its other end side in the axial direction thereof is supported by the end board  16   b  of the movable core  16 . Thus, the coil spring  17  urges the movable core  16  in an opposite direction from the fixed core  20  (in an upper direction in  FIG. 1 ), i.e., in a closing direction in which the valving element  11  is engaged with the valve seat  13  to close the valve hole  14 . 
     The magnetic saturation part  21  and the magnetic unbalance part  24 , which are provided in the core guide part  19 , will be described referring to  FIGS. 3A and 3B . As shown in  FIG. 3A , the magnetic saturation part  21  is provided in the core guide part  19  by forming a recessed part on an entire outer circumference of the core guide part  19  in vicinity to the fixed core  20  to reduce a thickness of the core guide part  19 . Thus, the magnetic saturation part  21 , where magnetic resistance is enhanced by the reduction of a cross-sectional area (thickness) of the magnetic circuit (core guide part  19 ), is provided in an entire circumference of the core guide part  19 . By forming the magnetic saturation part  21  in the core guide part  19 , a magnetic flux flowing directly from the core guide part  19  to the fixed core  20  decreases. Hence, a magnetic force applied between the core guide part  19  and the movable core  16  increases by the decrease of the magnetic flux. Therefore, an attraction force acting between the core guide part  19  and the movable core  16  increases. 
     The magnetic unbalance part  24  is provided such that the magnetic circuit area of the magnetic saturation part  21  provided in the core guide part  19 , i.e., the cross-sectional area of the thickness-reduced part of the core guide part  19  is different between on one side and the other side of the core guide part  19 , which are opposed to each other in the radial direction of the core guide part  19 . That is, magnetic forces, which act between the core guide part  19  and the movable core  16 , on the one side and on the other side of the core guide part  19  in the radial direction are different from each other. Specifically, as shown in  FIG. 3B , the unbalance part  24  is provided at a position where the outer diameter center Oa of the core guide part  19  (an inner diameter center of the guide hole  19   a ) is eccentrically located relative to an outer diameter center Ob of the magnetic saturation part  21 . As indicated by an arrow in  FIG. 3A , the magnetic saturation part  21  is leaned to the other side (right side in  FIG. 3A ) of the core guide part  19  as a whole in the radial direction of the core guide part  19 . The cross-sectional area of the magnetic saturation part  21  as the magnetic circuit is not constant along the entire circumference of the core guide part  19 , thereby being smaller on a left side than on a right side of the core guide part  19  in  FIG. 3B . 
     Magnetic forces, which act between the core guide part  19  and the movable core  16 , are different between on the one side and the other side of the core guide part  19 , which are opposed to one another in the radial direction. In the present embodiment, the one side of the core guide part  19  has a smaller magnetic circuit area than the other side thereof in the radial direction. Hence, the magnetic force on the one side of the core guide part  19  more strongly acts between the core guide part  19  and the movable core  16  than on the other side of the core guide part  19  in the radial direction. Thus, the movable core  16  is attracted to the one side of the core guide part  19  in the guide hole  19   a . As described above, the magnetic saturation part  21  is leaned entirely to the other side of the core guide part  19  in the radial direction. Therefore, as shown in  FIG. 3B , the cross-sectional area of the magnetic saturation part  21  as the magnetic circuit (the cross-sectional area of the thickness-reduced part) gradually change in a circumferential direction of the core guide part  19  without drastically changing between the smallest area part and the largest area part of the magnetic saturation part  21 . 
     An operation and effect of the purge valve  7  of the first embodiment will be described. The magnetic unbalance part  24  is provided in the core guide part  19 , which is provided in the solenoid part  12  of the purge valve  7  of the present embodiment. More specifically, the cross-sectional magnetic circuit area of the magnetic saturation part  21  (the cross-sectional area of the thickness-reduced part) is smaller on the one side than on the other side of the core guide part  19  which is opposed to the one side in the radial direction. The saturation part  21  functions as the magnetic circuit and is provided for the entire circumference of the core guide part  19 . Consequently, magnetic resistance in the magnetic saturation part  21  becomes large on the one side of the core guide part  19 , which has a smaller cross-sectional area of the magnetic circuit than the other side of the core guide part  19 . Therefore, as indicated in thickness of arrows in  FIG. 2 , the magnetic force, which acts between the core guide part  19  and the movable core  16 , on the one side of the core guide part  19  becomes large relative to the magnetic force on the other side of the core guide part  19 . 
     When the movable core  16  is attracted to the fixed core  20  magnetized by energization of the coil  15 , i.e., when the movable core  16  moves in the guide hole  19   a  in the axial direction of the core guide part  19 , the movable core  16  is attracted to and contacts with the one side of the core guide part  19  in the radial direction, where magnetic force more strongly acts on the movable core  16 . Accordingly, rotation of the movable core  16  is prevented. Because the unbalance part  24  is not provided in the movable core  16 , a gravity center of the movable core  16  is not shifted from a radial center thereof. Therefore, because the gravity center of the movable core  16  is located at the radial center thereof, rotation of the movable core  16  due to, for example, its install direction with respect to the vehicle, an acceleration of the vehicle, or a centrifugal force does not occur. 
     The valving element  11  attached to the end board  16   b  of the movable core  16  is always engaged with the same position of the valve seat  13  at closing time when the valve hole  14  is closed. Hence, as a result of the repeated opening and closing operation of the valving element  11 , the valving element  11  and the valve seat  13  are shaped to conform with each other, and sealing performance at the closing time is improved. The movable core  16  is attracted to and contacts with the one side of the core guide part  19  in the radial direction when the movable core  16  moves in the guide hole  19   a . Thus, sliding resistance is produced between the movable core  16  and the core guide part  19 , and travel speed of the movable core  16  thereby becomes slow. As a result, an impact noise, which is produced when the movable core  16  contacts with the fixed core  20 , is reduced. 
     (Second Embodiment) 
     As shown in  FIGS. 5A and 5B , a second embodiment is an example in which a magnetic unbalance part  24  is provided by positioning an inner diameter center Oc of a guide hole  19   a  eccentrically relative to an outer diameter center Oa of a core guide part  19 . A magnetic saturation part  21  is provided in the core guide part  19  such that a recessed part is formed on an outer peripheral surface of the core guide part  19 , and a depth of the recessed part is constant along the entire circumference of the core guide part  19 . That is, the outer diameter center Oa of the core guide part  19  is located at the same position as a position of an outer diameter center Ob of the magnetic saturation part  21 . As shown in  FIG. 5B , the inner diameter center Oc of the guide hole  19   a  defined by the core guide part  19  is eccentrically positioned on one side (left side in  FIG. 5A ) of the core guide part  19  in a radial direction of the core guide part  19  with respect to the outer diameter center Oa of the core guide part  19 . Therefore, the entire portion of the guide hole  19   a  is formed unevenly on the one side of the core guide part  19  in the radial direction. 
     A magnetic-path cross-sectional area of the magnetic saturation part  21  (a cross-sectional area of a thickness-reduced part) is smaller on the one side than on the other side of the core guide part  19 , which is opposed to the one side in the radial direction. The magnetic saturation part  21  functions as a magnetic circuit and is formed along an entire circumference of the core guide part  19 . Consequently, similar to the first embodiment, magnetic resistance in the magnetic saturation part  21  is large on the one side of the core guide part  19 , which has a smaller cross-sectional area of the magnetic circuit than the other side of the core guide part  19 . Therefore, a magnetic force, which acts between the core guide part  19  and the movable core  16 , on the one side of the core guide part  19  is large relative to a magnetic force on the other side of the core guide part  19 . When the movable core  16  moves in the guide hole  19   a  in an axial direction of the core guide part  19 , the movable core  16  is attracted to and contacts with the one side of the core guide part  19  in the radial direction, where magnetic force more strongly acts on the movable core  16 . Thus, rotation of the movable core  16  is prevented. Accordingly, similar effects (e.g., improvement of sealing performance at valve closing time, and noise abatement) to the first embodiment can be obtained. 
     (Third Embodiment) 
     As shown in  FIGS. 6A and 6B , a third embodiment is an example in which a magnetic unbalance part  24  is provided by boring a through hole  22  on a circumferential wall of a core guide part  19 . If the through hole  22  is bored on the circumferential wall of the core guide part  19  including a magnetic saturation part  21 , a cross-sectional area of the magnetic saturation part  21  as a magnetic circuit is further reduced and magnetic resistance increases. As shown in  FIG. 6B , if several through holes  22  are provided only for one side (left side in  FIG. 6A ) of the core guide part  19  in a radial direction of the core guide part  19 , magnetic resistance on the one side of the magnetic saturation part  21  in the radial direction becomes large relative to magnetic resistance on the other side of the magnetic saturation part  21 , which is opposed to the one side in the radial direction. Thus, when a movable core  16  moves in the guide hole  19   a  in an axial direction of the core guide part  19 , the movable core  16  is attracted to and contacts with the one side of the core guide part  19  in the radial direction, where magnetic force more strongly acts on the movable core  16 . Accordingly, rotation of the movable core  16  is prevented. The through hole  22  can be formed by laser radiation, cutting, water jet cutting, or press working, for example. In  FIGS. 6A and 6B , several through holes  22  are provided only for the one side of the core guide part  19 , which is opposed to the other side in the radial direction. However, a magnetic unbalance part  24  can be provided such that through holes  22  are provided also on the other side of the core guide part  19  in the radial direction and the number of the through holes  22  on the other side is less than the number of the through holes  22  on the one side. 
     (Fourth Embodiment) 
     A fourth embodiment is another case that a magnetic unbalance part  24  is provided by boring a through hole  22  on a circumferential wall of a core guide part  19 . In the example of the above-described third embodiment, the several through holes  22  are provided only for the one side of the core guide part  19 , which is opposed to the other side in the radial direction of the core guide part  19 . 
     However, as shown in  FIGS. 7A and 7B , in the fourth embodiment, the magnetic unbalance part  24  is provided by providing the same number of the through holes  22  both on one side (left side in  FIG. 7A ) and the other side of a magnetic saturation part  21 , which are opposed each other in a radial direction of the core guide part  19 , and by changing diameters of the through holes  22 . In the example as shown in  FIGS. 7A and 7B , the diameters of the through holes  22  are larger on the one side than on the other side of the magnetic saturation part  21  in the radial direction. Accordingly, magnetic resistance is larger on the one side than on the other side of the magnetic saturation part  21  in the radial direction. Therefore, when a movable core  16  moves in the guide hole  19   a  in an axial direction of the core guide part  19 , the movable core  16  is attracted to and contacts with the one side of the core guide part  19  in the radial direction, where a magnetic force more strongly acts on the movable core  16 , and rotation of the movable core  16  is prevented. 
     (Fifth Embodiment) 
     A fifth embodiment is a case that a coating agent  23  is applied to an outer circumferential surface of a movable core  16 , which is attracted to and contacts with one side of a core guide part  19  in a radial direction of a core guide part  19  when the movable core  16  is attracted to a fixed core  20  and moves in a guide hole  19   a  in an axial direction of the core guide part  19 . In the first to fourth embodiments, because the magnetic resistance is larger on the one side than on the other side of the core guide part  19  in the radial direction, the movable core  16  is attracted to and contacts with the one side of the core guide part  19 . Accordingly, rotation of the movable core  16  is prevented. In this case, when the movable core  16  moves in the guide hole  19   a , the outer circumferential surface of the movable core  16  which slides on an inner circumferential surface of the guide hole  19   a , i.e., a sliding surface of the movable core  16  is areally almost fixed. In other words, the sliding surface is within an almost fixed range in a circumferential direction of the movable core  16 . Thus, as shown in  FIG. 8 , it is enough to apply the coating agent  23  only for the sliding surface of the movable core  16 , which is attracted to and contacts with the core guide part  19 , and the coating agent  23  need not be applied to the entire outer circumferential surface of the movable core  16 . Therefore, a consumption amount of the coating agent  23  can be reduced. 
     Modifications of the above embodiments will be described. In the first embodiment, the electromagnetic valve of the invention is applied to the purge valve  7  utilized in the fuel vapor treatment system of the vehicle. However, the electromagnetic valve can be applied to the canister control valve  10 . The electromagnetic valve of the invention can be applied also to, for example, an hydraulic control valve utilized in a valve timing control device of an internal combustion engine, or an hydraulic solenoid utilized in an automatic gear shifting device of an vehicle. 
     To sum up, the electromagnetic valve of the above embodiments may be described as follows. 
     The electromagnetic valve includes the valve seat  13 , the valving element  11 , and the solenoid part  12 . The valve seat  13  has the annular shape and defines the valve hole  14  that opens radially inward of the valve seat  13 . The valving element  11  is movable between the valve-closing position where the valving element  11  is engaged with the valve seat  13  to close the valve hole  14  and the valve-opening position where the valving element  11  is disengaged from the valve seat  13  to open the valve hole  14 . The solenoid part  12  drives the valving element  11  by utilizing magnetic force of the electromagnet. The solenoid part  12  includes the coil  15 , the core guide part  19 , the fixed core  20 , and the movable core  16 . The coil  15  becomes the electromagnet upon energization thereof. The core guide part  19  has the cylindrical shape and is arranged radially inward of the coil  15  to form the magnetic circuit. The fixed core  20  is arranged on one end side of the core guide part  19  in the axial direction of the core guide part  19  and is magnetized by the electromagnet. The movable core  16  is accommodated inside the core guide part  19  to be opposed to the fixed core  20  in the axial direction and is reciprocated inside the core guide part  19  in accordance with whether the electromagnet is turned on or off. The valving element  11  moves integrally with the movable core  16  to open or close the valve hole  14 . The core guide part  19  includes the magnetic unbalance part  24  where the magnetic force applied between the core guide part  19  and the movable core  16  is different between on one side and the other side of the core guide part  19 , which are opposed to each other in the radial direction of the core guide part  19 . 
     In the electromagnetic valve of the invention, the magnetic unbalance part  24  is provided in the core guide part  19 , which has the cylindrical shape and is a part of the magnetic circuit located radially inward of the coil  15 . More specifically, the magnetic force applied between the movable core  16  and the core guide part  19  is different between on the one side and the other side of the core guide part  19 , which are opposed to each other in the radial direction of the core guide part  19 . Thus, when the movable core  16  is attracted to the fixed core  20  by the action of the electromagnet, i.e., when the movable core  16  moves inside the guide hole  19   a  in the axial direction of the core guide part  19 , the movable core  16  is attracted to and contacts with one side of the core guide  19  in the radial direction of the core guide part  19  (the one side or the other side of the core guide part  19  in the radial direction of the core guide part  19 ) where the magnetic force more strongly acts on the movable core  16 . Hence, rotation of the movable core  16  may be prevented, and accordingly, the valving element  11  moving integrally with the movable core  16  may be always engaged with the same position of the valve seat  13  at closing time when the valve hole  14  is closed. Therefore, the valving element  11  and the valve seat  13  may be shaped to conform to each other, and thereby, the sealing performance at the closing time can be improved. Additionally, the movable core  16  is attracted to and contacts with one side of the core guide part  19  in the radial direction when the movable core  16  moves inside the core guide part  19 . Thus, sliding resistance is produced between the movable core  16  and the core guide part  19 . As a result, the travel speed of the movable core  16  becomes slow, and therefore, an impact noise, which is produced when the movable core  16  contacts with the fixed core  20 , can be reduced. 
     The core guide part  19  may include the magnetic saturation part  21  along the entire outer circumference thereof. The magnetic saturation part  21  includes the recessed part on the outer peripheral surface thereof, to reduce the thickness of the core guide part  19 , thereby decreasing the magnetic-path cross-sectional area of the magnetic saturation part  21 . At the magnetic unbalance part  24 , the magnetic-path cross-sectional area of the magnetic saturation part  21  is different between on the one side and the other side of the core guide part  19 . In the above-described structure, as the magnetic-path cross-sectional area of the magnetic saturation part  21  is smaller, the magnetic resistance of the core guide part  19 , which is the part of the magnetic circuit, may become larger. Thus, the magnetic force applied between the core guide part  19  and the movable core  16  may increase. If a magnetic-path cross-sectional area of one side of the core guide part  19 , which is opposed to the other side of the core guide part  19 , is made smaller than a magnetic-path cross-sectional area of the other side of the core guide part  19 , the magnetic force applied between the core guide part  19  and the movable core  16  may act larger on the one side than on the other side of the core guide part  19 . Therefore, the movable core  16  can be attracted to the one side of the core guide part  19 . 
     The outer diameter center Ob of the magnetic saturation part  21  may be positioned eccentrically with respect to the diameter center Oa of the core guide part  19 . When the outer diameter center Ob of the magnetic saturation part  21  coincides with the diameter center Oa, a magnetic-path cross-sectional area of the magnetic saturation part  21  is a constant in the circumferential direction of the core guide part  19 . However, if the magnetic saturation part  21  is provided such that the outer diameter center Ob of the magnetic saturation part  21  is eccentrically positioned relative to the diameter center Oa of the core guide part  19 , the magnetic unbalance part  24  can be provided, where the magnetic-path cross-sectional area of the magnetic saturation part  21  is different between the one side and the other side of the core guide part  19 , which are opposed to each other. 
     The outer diameter center Ob of the magnetic saturation part  21  may coincide with the outer diameter center Oa of the core guide part  19 . Additionally, the diameter center Oc of the inner peripheral surface of the core guide part  19 , which accommodates the movable core  16 , may be located eccentrically with respect to the outer diameter center Oa of the core guide part  19 . Specifically, the inner peripheral surface of the core guide part  19  may be shifted to either one side in the radial direction of the core guide part  19  relative to the outer peripheral surface of the core guide part  19  (on the one side or the other side of the core guide part  19 ). Accordingly, the magnetic unbalance part  24  can be provided, where the magnetic-path cross-sectional area of the magnetic saturation part  21  is different between the one side and the other side of the core guide part  19 , which are opposed to each other. 
     At the magnetic unbalance part  24 , the core guide part  19  may include the through hole  22 , which passes through the circumferential wall of the core guide part  19 , only on the one side or the other side of the core guide part  19 . In the above-described structure, because magnetic resistance increases, the magnetic force between the movable core  16  and the core guide part  19  may increase. For example, if the through hole  22  is provided only on one side of the core guide part  19 , magnetic resistance of the one side becomes larger than that of the other side of the core guide part  19 . Accordingly, the movable core  16  can be attracted to the one side of the core guide part  19 , in which the magnetic force applied between the movable core  16  and the core guide part  19  is relatively strong. 
     At the magnetic unbalance part  24 , each of the one side and the other side of the core guide part  19  may include at least one through hole  22 , which passes through the circumferential wall of the core guide part  19 . Moreover, the number of the through holes  22  on the one side of the core guide part  19  may be different from the number of the through holes  22  on the other side of the core guide part  19 . In this instance, the numbers of the through holes  22  passing through the circumferential wall of the core guide part  19  on the one side and on the other side of the core guide part  19  are different from each other. Hence, a magnitude of the magnetic resistance is different between the one side and the other side of the core guide part  19 . For example, when the number of the through holes  22  on the one side of the core guide part  19  is larger than the number of the through holes  22  on the other side of the core guide part  19  (here, all the hole diameters of the through holes  22  are of the same size), the one side is larger than the other side in the magnitude of the magnetic resistance. Therefore, the movable core  16  can be attracted to the one side of the core guide part  19 , where the magnetic force more strongly acts between the movable core  16  and the core guide part  19 . 
     At the magnetic unbalance part  24 , each of the one side and the other side of the core guide part  19  may include a through hole  22 , which passes through the circumferential wall of the core guide part  19 . Additionally, a diameter of the through hole  22  on the one side of the core guide part  19  and a diameter of the through hole  22  on the other side of the core guide part  19  may be different from each other. In this case, the diameters of the through holes  22  passing through the circumferential wall of the core guide part  19  on the one side and the other side of the core guide part  19 , which are opposed to each other, are different from each other. Thus, a magnitude of the magnetic resistance is different between the one side and the other side of the core guide part  19 . For example, when the diameter of the through holes  22  on the one side of the core guide core  19  is larger than the diameter of the through holes  22  on the other side of the core guide core  19 , the one side is larger than the other side in the magnitude of the magnetic resistance. Therefore, the movable core  16  can be attracted to the one side of the core guide part  19 , where the magnetic force more strongly acts between the movable core  16  and the core guide part  19 . 
     The core guide part  19  may include the magnetic saturation part  21  along an entire outer circumference thereof, and the magnetic saturation part  21  includes the recessed part on an outer peripheral surface thereof, to reduce the thickness of the core guide part  19 , thereby decreasing the magnetic-path cross-sectional area of the magnetic saturation part  21 . Furthermore, the through hole  22  on the one side of the core guide part  19  and the through hole  22  on the other side of the core guide part  19  are provided at the magnetic saturation part  21 . By forming the magnetic saturation part  21  in the core guide part  19 , a magnetic flux flowing directly from the core guide part  19  to the fixed core  20  decreases. Hence, the magnetic force applied between the core guide part  19  and the movable core  16  increases by the decrease of the magnetic flux. Therefore, an magnetic attraction force acting between the core guide part  19  and the movable core  16  increases. If the magnetic unbalance part  24  is provided by boring the through holes  22  at the magnetic saturation part  21 , the movable core  16  can be attracted to one side (either side of the one side or the other side of the core guide part  19  in its radial direction) of the core guide part  19  on which the magnetic force strongly affects to the movable core  16 . 
     Only a part of the outer peripheral surface of the movable core  16 , which slides on the inner peripheral surface of the core guide part  19 , may be coated with the coating agent  23 . In the electromagnetic valve of the invention, when the movable core  16  is attracted to the fixed core  20  and moves inside the core guide part  19  due to the action of the electromagnet, the movable core  16  is attracted to and contacts with one side of the core guide part  19  (either side of one side or the other side of the core guide part  19  in the radial direction), the movable core  16  thereby being prevented from rotating. In this case, the outer circumferential surface of the movable core  16  which slides on the inner circumferential surface of the core guide part  19 , i.e., the sliding surface of the movable core  16  is areally almost fixed. In other words, the sliding surface is within an almost fixed range in a circumferential direction of the movable core  16 . Thus, the coating agent  23  need not be applied to the entire outer circumferential surface of the movable core  16 , and it is enough to apply the coating agent  23  only for the sliding surface of the movable core  16 . Therefore, the consumption amount of the coating agent  23  can be reduced. 
     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.