Abstract:
The present invention relates to an electromagnetically driven valve suited for use in an internal combustion engine and aims at achieving appropriate operating characteristics in accordance with operating conditions of the internal combustion engine at the time of opening or closing a valve body. An armature moving together with the valve body is provided and upper and lower cores are disposed on opposed sides of the armature. The upper core and the lower core accommodate upper and lower coils, respectively. An annular protrusion, formed not on the upper core but on the lower core only, has an inner diameter slightly larger than an outer diameter of the armature.

Description:
This is a division of application Ser. No. 09/108,507 filed Jul. 1, 1998, now U.S. Pat. No. 6,125,803. 
    
    
     INCORPORATION BY REFERENCE 
     The disclosed contents of Japanese Patent Applications Nos. HEI 9-257050 filed on Sep. 22, 1997 and HEI 9-305912 filed on Nov. 7, 1997, each including the specification, drawings and abstract are incorporated herein by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to an electromagnetically driven valve for an internal combustion engine and, more particularly, relates to an electromagnetically driven valve suited for use as an intake valve or an exhaust valve of an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     An electromagnetically driven valve employed as an intake valve or an exhaust valve of an internal combustion engine is disclosed, for instance, in Japanese Patent Official Publication No. HEI 4-502048 and Japanese Patent Application Laid-Open No. HEI 7-335437. This electromagnetically driven valve is provided with an armature attached to a valve body. An upper spring and a lower spring are disposed above and below the armature respectively. These springs urge the armature toward its neutral position. 
     An upper core and a lower core are disposed above and below the armature respectively and an upper coil and a lower coil are disposed within the upper core and the lower core respectively. The upper coil and the lower coil, if supplied with an exciting current, generate a magnetic flux circulating therethrough. Upon generation of such a magnetic flux, the armature is attracted toward the upper core or the lower core by an electromagnetic force (hereinafter referred to as an attracting force). Thus, the aforementioned electromagnetically driven valve can displace the valve body to its closed position or its open position by supplying a predetermined exciting current to the upper coil or the lower coil. 
     If supply of an exciting current to the upper coil or the lower coil is stopped after displacement of the valve body to its closed position or its open position, the armature and the valve body are urged by the springs to start a simple harmonic motion. Unless the amplitude of the simple harmonic motion is damped, the armature and the valve body that move from one displacement end toward the other displacement end (hereinafter referred to as a desired displacement end) reach the desired displacement end solely due to urging forces of the springs. However, such displacement of the armature and the valve body causes energy loss resulting from sliding friction or the like. Therefore, the critical position that can be reached by the armature and the valve body due to the urging forces of the springs is not coincident with the desired displacement end. 
     The aforementioned electromagnetically driven valve can compensate for the amount of energy loss resulting from sliding movement and displace the armature and the valve body to the desired displacement end by starting to supply an exciting current to one of the upper coil and the lower coil at a suitable timing after stoppage of supply of an exciting current to the other of the upper coil and the lower coil. The valve body can thereafter be opened and closed by alternately supplying an exciting current to the upper coil and the lower coil at suitable timings. 
     In the aforementioned electromagnetically driven valve, each of the upper core and the lower core is provided with an annular protrusion disposed along an outer periphery thereof. The annular protrusion, which has a predetermined length, protrudes from an end face of the upper core or the lower core. The inner diameter of the annular protrusion is slightly larger than the outer diameter of the armature. 
     When the armature is spaced apart from the desired displacement end, the attracting force acting on the armature (hereinafter referred to as a spaced-state attracting force) is larger in the case where the annular protrusion is provided than in the case where the annular protrusion is not provided. On the other hand, when the armature is close to the desired displacement end, the attracting force acting on the armature (hereinafter referred to as a close-state attracting force) is smaller in the case where the annular protrusion is provided than in the case where the annular protrusion is not provided. Accordingly, as the armature approaches the desired displacement end, the aforementioned electromagnetically driven valve can gradually increase an attracting force acting on the armature. 
     The armature collides with the upper core or the lower core upon arrival of the valve body at its open position or its closed position, thus causing impact noise. In order to reduce impact noise, it is desired to prevent the attracting force acting on the armature from becoming unsuitably large upon arrival of the armature at the desired displacement end. 
     In order to reliably displace the armature to the desired displacement end, it is necessary to ensure a spaced-state attracting force of a certain magnitude. In order to ensure a large spaced-state attracting force and reduce impact noise in the electromagnetically driven valve, it is advantageous to avoid an abrupt increase in the attracting force acting on the armature as the armature approaches the desired displacement end. The aforementioned electromagnetically driven valve can satisfy the aforementioned advantageous condition during both the valve opening operation and the valve closing operation. As a result, the aforementioned electromagnetically driven valve can achieve an enhanced tranquility. 
     In the aforementioned electromagnetically driven valve, the neutral position of the armature is set to the central position between an electromagnet on the valve opening side and an electromagnet on the valve closing side. Thus, there is no change in the amount of energy stored in a pair of springs regardless of whether the armature is positioned on the electromagnet on the valve closing side or on the electromagnet on the valve opening side. In this case, there is no substantial change in the amount of energy required for the springs to urge the armature regardless of whether the valve moves in the valve opening direction or in the valve closing direction. 
     However, the load applied to the valve body in the internal combustion engine may differ depending on whether the valve body moves in the valve opening direction or in the valve closing direction. Hence, a difference in the amount of energy loss may arise depending on whether the valve body of the electromagnetically driven valve moves in the valve opening direction or in the valve closing direction. 
     For example, the exhaust valve is opened when a high combustion pressure remains in a combustion chamber and it is closed when the combustion pressure is released. In this case, the load applied to the exhaust valve is larger during the valve opening operation than during the valve closing operation. 
     Preferably, there should be no substantial difference between the exciting current to be supplied to the electromagnet on the valve opening side and the exciting current to be supplied to the electromagnet on the valve closing side. 
     The aforementioned electromagnetically driven valve is unable to achieve appropriate operating characteristics during the valve opening operation and during the valve closing operation while substantially supplying an equal exciting current to the electromagnets on the valve opening side and on the valve closing side. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the aforementioned background and it is an object of the present invention to provide an electromagnetically driven valve that achieves appropriate operating characteristics in accordance with operating conditions of an internal combustion engine at the time of opening or closing a valve body. 
     Further, it is another object of the present invention to provide an electromagnetically driven valve that achieves substantially the same operating characteristics regardless of whether the valve body moves in the valve opening direction or in the valve closing direction when a pair of electromagnets are substantially supplied with an equal exciting current. 
     In order to achieve the aforementioned objects, a first aspect of the present invention provides an electromagnetically driven valve for an internal combustion engine including an armature coupled to a valve body for reciprocal movement therewith between a first position and a second position, a first electromagnet, a second electromagnet, a first elastic member, and a second elastic member. The first electromagnet is disposed on a first side of the armature adjacent to the first position and the second electromagnet is disposed on a second side of the armature adjacent to the second position. First and second elastic members are coupled to the armature. The first elastic member is biased to urge the armature in a first direction toward the first position and the second elastic member is biased to urge the armature in a second direction opposite the first direction toward the second position. When no electromagnetic force is applied to the armature by the first and second electromagnets, the armature resides in a neutral position between the first and second positions. The neutral position is closer to the first electromagnet than the second electromagnet. 
     A second aspect of the present invention provides an electromagnetically driven valve for an internal combustion engine including an armature coupled to a valve body for reciprocal movement therewith between a first position and a second position, a first elastic member, a second elastic member, a first core, and a second core. The first elastic member is coupled to the armature to bias the armature toward the first position and the second elastic member is coupled to the armature to bias the armature toward the second position. A neutral position of the armature is defined between the first and second positions at the point where the forces applied from the first and second elastic member balance one another. The first core includes a first coil therein and the second core includes a second coil therein. The first and second cores are disposed on opposite sides of the armature and are positioned so that, when the armature is in the neutral position, the first and second cores are spaced apart from the armature. One of the first core and the armature is provided with a first protrusion protruding a predetermined length toward the other of the first core and the armature thereby making a distance between the first core and the armature smaller than a distance between the second core and the armature when the armature is located in the neutral position. The other of the first core and the armature is provided with a protrusion facing side that faces a side of the first protrusion when said armature is in the first position. 
     A third aspect of the present invention provides an electromagnetically driven valve for an internal combustion engine including an armature coupled to a valve body for reciprocal movement therewith between a first position and a second position, a first elastic member, a second elastic member, a first electromagnet, and a second electromagnet. The first elastic member is coupled to the armature to bias the armature toward the first position and the second elastic member is coupled to the armature to bias the armature toward the second position. A neutral position of the armature is defined between the first and second positions at a point in which the forces applied from the first and second elastic member balance one another. The first electromagnet is adjacent to the first position and the second electromagnet is adjacent to the second position. The first and second electromagnets are positioned so that, when the armature is in the neutral position. The first and second electromagnets are spaced apart from the armature. The neutral position is closer to the first electromagnet than the second electromagnet. 
     According to the first aspect of the present invention, whether the valve body is driven in the valve opening direction or in the valve closing direction, the armature can suitably displace the valve body regardless of a difference in load applied thereto or a difference in amplitude of a damping factor thereof. 
     According to the second aspect of the present invention, when the armature is close to the first core, a side of the protrusion disposed on the first core or on the armature faces a protrusion facing side corresponding to the protrusion. In this construction, as the armature approaches the first core, a large spaced-state attracting force acting on the armature tends to increase gradually. As the armature approaches the second core, a relatively small spaced-state attracting force acting on the armature tends to increase abruptly. According to the characteristics of this aspect, in the case where a large load is applied to the valve body when the armature approaches the first core and no large load is applied to the valve body when the armature approaches the second core, the valve body can be suitably operated with a low electric power consumption. 
     According to the third aspect of the present invention, the elastic members generate an urging force that urges the valve body toward its neutral position between first and second electromagnets. The neutral position of the valve body is biased toward the first electromagnet. Hence, more energy is stored in the elastic members when the armature is attracted to the second electromagnet than when the armature is attracted to the first electromagnet. Thus, the elastic members urge the armature away from the second electromagnet with high energy and urge the armature away from the first electromagnet with low energy. In this case, whether the armature moves in the valve opening direction or in the valve closing direction, the armature exhibits substantially the same operating characteristics regardless of a difference in an amplitude of a damping amount. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein: 
     FIG. 1 is a sectional view of an electromagnetically driven valve according to a first embodiment of the present invention; 
     FIG. 2 illustrates flow of a magnetic flux  U  circulating round an upper coil in the electromagnetically driven valve as illustrated in FIG. 1 when an armature is spaced apart from the upper core; 
     FIG. 3 illustrates flow of a magnetic flux  L  circulating round a lower coil in the electromagnetically driven valve as illustrated in FIG. 1 when the armature is spaced apart from the lower core; 
     FIG. 4 illustrates flow of a magnetic flux  U  circulating round the upper coil in the electromagnetically driven valve as illustrated in FIG. 1 when the armature is close to the upper core; 
     FIG. 5 illustrates flow of a magnetic flux  L  circulating round the lower coil in the electromagnetically driven valve as illustrated in FIG. 1 when the armature is close to the lower core; 
     FIG. 6 illustrates flow of a magnetic flux  U  circulating round the upper coil in the electromagnetically driven valve as illustrated in FIG. 1 when the armature abuts the upper core; 
     FIG. 7 illustrates flow of a magnetic flux  L  circulating round the lower coil in the electromagnetically driven valve as illustrated in FIG. 1 when the armature abuts the lower core; 
     FIG. 8 illustrates operating characteristics of the electromagnetically driven valve as illustrated in FIG. 1; 
     FIG. 9 is a sectional view illustrating a part surrounding an armature of an electromagnetically driven valve according to a second embodiment of the present invention; 
     FIG. 10 is a sectional view illustrating a part surrounding an armature of an electromagnetically driven valve according to a third embodiment of the present invention; 
     FIG. 11 is an overall structural view of an electromagnetically driven valve according to a fourth embodiment of the present invention; 
     FIG. 12 is an overall structural view of an electromagnetically driven valve according to a fifth embodiment of the present invention; 
     FIG. 13 is an overall structural view of an electromagnetically driven valve according to a sixth embodiment of the present invention; 
     FIG. 14 is an overall structural view of an electromagnetically driven valve according to a seventh embodiment of the present invention. 
     FIG. 15 is an overall structural view of an electromagnetically driven valve according to a further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a sectional view of an electromagnetically driven valve  10  according to a first embodiment of the present invention. The electromagnetically driven valve  10  is employed as an exhaust valve for an internal combustion engine. The electromagnetically driven valve  10  is attached to a cylinder head  12  in which an exhaust port  14  is formed. Formed in a lower portion of the cylinder head  12  is a combustion chamber  16 . The electromagnetically driven valve  10  is provided with a valve body  18  for bringing the exhaust port  14  into or out of communication with the combustion chamber  16 . A valve seat  19  onto which the valve body moves is disposed in the exhaust port  14 . The exhaust port  14  is brought into communication with the combustion chamber  16  when the valve body  18  moves away from the valve seat  19 , while the exhaust port  14  is brought out of communication with the combustion chamber  16  when the valve body  18  moves onto the valve seat  19 . 
     A valve shaft  20  is formed integrally with the valve body  18 . A valve guide  22  is disposed inside the cylinder head  12 . The valve shaft  20  is slidably held by the valve guide  22 . A lower retainer  24  is attached to an upper end portion of the valve shaft  20 . A lower spring  26  is disposed beneath the lower retainer  24 . The lower spring  26  urges the lower retainer  24  upwards in FIG.  1 . 
     The upper end portion of the valve shaft  20  abuts against an armature shaft  28  made of a non-magnetic material. An armature  30 , which is an annular member made of a magnetic material, is attached to the armature shaft  28 . 
     Upper core  32  and a lower core  34 , each being annular members made of a magnetic material, are disposed above and below the armature  30  respectively. The lower core  34  has an annular protrusion  36 , which has a predetermined length and protrudes from a surface of the lower core  34  toward the upper core  32 . The electromagnetically driven valve  10  according to this embodiment is characterized in that the annular protrusion  36  is formed not on the upper core  32  but only on the lower core  34 . 
     The annular protrusion  36  has a diameter slightly larger than an outer diameter of the armature  30 . Thus, when the armature  30  approaches sufficiently close to the lower core  34 , an inner wall of the annular protrusion  36  faces an outer peripheral surface of the armature  30 . The outer peripheral surface of the armature  30 , which faces the inner peripheral surface of the annular protrusion  36 , will hereinafter be referred to as a protrusion facing side  38 . 
     The upper core  32  and the lower core  34  accommodate an upper coil  40  and a lower coil  42  respectively. Bearings  44 ,  46  are disposed in the vicinity of central axes of the upper core  32  and the lower core  34  respectively. The armature shaft  28  is slidably held by the bearings  44 ,  46 . 
     A core guide  48  surrounds outer peripheral surfaces of the upper core  32  and the lower core  34 . The core guide  48  suitably adjusts a location of the upper core  32  relative to the lower core  34 . An upper case  50  is attached to an upper portion of the upper core  32 , while a lower case  52  is attached to a lower portion of the lower core  34 . 
     A spring guide  54  and an adjuster bolt  56  are disposed in an upper end portion of the upper case  50 . An upper retainer  58  connected with an upper end of the armature shaft  28  is disposed below the spring guide  54 . Disposed between the spring guide  54  and the upper retainer  58  is an upper spring  60  which urges the upper retainer  58  and the armature shaft  28  downwards in FIG.  1 . The adjuster bolt  56  adjusts a neutral position of the armature  30 . In this embodiment, the neutral position of the armature  30  is adjusted to a central portion of a space defined by the upper core  32  and the lower core  34 . 
     The operation of the electromagnetically driven valve  10  will hereinafter be described with reference to FIGS. 2 through 9 as well as FIG.  1 . 
     In the electromagnetically driven valve  10 , when no exciting current is supplied to the upper coil  40  or the lower coil  42 , the armature  30  assumes its neutral position. That is, the armature  30  is held in a central portion of the space defined by the upper core  32  and the lower core  34 . When an exciting current is supplied to the upper coil  40  with the armature  30  assuming its neutral position, an electromagnetic force attracting the armature  30  toward the upper core  32  is generated in a space defined by the armature  30  and the upper core  32 . Hence, the electromagnetically driven valve  10  can displace the armature  30  toward the upper core  32  by supplying a suitable exciting current to the upper coil  40 . The valve body  18  moves onto the valve seat  19  to be completely closed prior to abutment of the armature  30  on the upper core  32 . Thus, the electromagnetically driven valve  10  can completely close the valve body  18  by supplying a suitable exciting current to the upper coil  40 . 
     If supply of an exciting current to the upper coil  40  is stopped with the valve body  18  completely closed, the valve body  18 , the valve shaft  20 , the armature shaft  28  and the armature  30  start to move downwards in FIG. 1 due to urging forces of the upper spring  60  and the lower spring  26 . 
     Displacement of the valve body  18  causes energy loss resulting from sliding friction and the like. The electromagnetically driven valve  10  can compensate for such energy loss by supplying an exciting current to the lower coil  42  to displace the valve body  18  until the armature  30  abuts against the lower core  34 . The valve body  18  becomes completely open when the armature  30  abuts against the lower core  34 . 
     Consequently, the electromagnetically driven valve  10  can completely open the valve body  18  by starting to supply an exciting current to the lower coil  42  at a suitable time after stoppage of the supply of the exciting current to the upper coil  40 . The electromagnetically driven valve  10  can suitably open or close the valve body  18  by supplying at a suitable time thereafter a suitable exciting current to the upper coil  40  or the lower coil  42 . 
     The electromagnetically driven valve  10  according to this embodiment is characterized in that the annular protrusion  36  is formed not on the upper core  32  but only on the lower core  34 . The effect achieved by this feature will be described hereinafter. 
     FIG. 2 illustrates flow of a magnetic flux  U  circulating through the upper core  32  and the armature  30  when a predetermined current I 0  is supplied to the upper coil  40 . The flow of the magnetic flux  U  as illustrated in FIG. 2 is realized when the armature  30  is spaced far apart from the upper core  32 . Provided that N represents the number of turns of the upper coil  40  and R U  represents a reluctance of a magnetic circuit including the upper core  32  and the armature  30  (hereinafter referred to as an upper magnetic circuit  62 ), the magnetic flux  U  circulating through the upper magnetic circuit  62  is expressed as follows. 
     
       
           U =( N I   0 )/ R   U   (1)  
       
     
     FIG. 3 illustrates flow of a magnetic flux  L  circulating through the lower core  34  and the armature  30  when a predetermined current I 0  is supplied to the lower coil  42 . The flow of the magnetic flux  L  as illustrated in FIG. 3 is realized when the armature  30  is spaced far apart from the lower core  34 . Provided that N represents the number of turns of the lower coil  42  and R L  represents a reluctance of a magnetic circuit including the lower core  34  and the armature  30  (hereinafter referred to as a lower magnetic circuit  64 ), the magnetic flux  L  circulating through the lower magnetic circuit  64  is expressed as follows. 
     
       
           L =( N I   0 )/ R   L   (2)  
       
     
     The smaller an air gap formed between the upper core  32  and the armature  30  becomes, the smaller the reluctance R U  of the upper magnetic circuit  62  becomes. Likewise, the smaller an air gap formed between the lower core  34  and the armature  30  becomes, the smaller the reluctance R L  of the lower magnetic circuit  64  becomes. 
     In this embodiment, the annular protrusion  36  protruding toward the armature  30  is formed on the lower core  34 . When the armature  30  is spaced apart from the lower core  34 , the annular protrusion  36  serves to reduce the air gap formed therebetween. Hence, if the armature  30  is equally distant from the upper core  32  and the lower core  34 , the reluctance R L  of the lower magnetic circuit  64  is smaller than the reluctance R U  of the upper magnetic circuit  62 . Accordingly, in this case, the amount of magnetic flux  L  flowing through the lower magnetic circuit  64  is larger than the amount of magnetic flux  U  flowing through the upper magnetic circuit  62 . 
     In the electromagnetically driven valve  10 , when the magnetic flux  U  flows through the upper magnetic circuit  62 , an attracting force is generated between the armature  30  and the upper core  32  to reduce the air gap formed in the upper magnetic circuit  62 . On the other hand, when the magnetic flux  L  flows through the lower magnetic circuit  64 , an attracting force is generated between the armature  30  and the lower core  34  to reduce the air gap formed in the lower magnetic circuit  64 . 
     If the armature  30  is spaced far apart from the upper core  32 , the aforementioned attracting force mainly serves to attract the armature  30  toward the upper core  32 . If the armature  30  is spaced far apart from the lower core  34 , the aforementioned attracting force mainly serves to attract the armature  30  toward the lower core  34 . The larger the amount of magnetic flux flowing through the air gap to be reduced becomes, the larger the aforementioned attracting force becomes. 
     Thus, when the armature  30  is equally distant from the upper core  32  and the lower core  34  and an exciting current I 0  is supplied to both the upper coil  40  and the lower coil  42 , the attracting force generated between the armature  30  and the lower core  34  is larger than the attracting force generated between the armature  30  and the upper core  32 . When the armature  30  is spaced far apart from the upper core  32  or the lower core  34 , an attracting force generated therebetween will hereinafter be referred to as a spaced-state attracting force F F . 
     FIG. 4 illustrates flow of a magnetic flux  U  circulating through the upper core  32  and the armature  30  when a predetermined current I 0  is supplied to the upper coil  40 . The flow of the magnetic flux  U  as illustrated in FIG. 4 is realized when the armature  30  is spaced slightly apart from the upper core  32 . 
     The smaller the air gap formed between the armature  30  and the upper core  32  becomes, the smaller the reluctance R U  of the upper magnetic circuit  62  becomes. As can be seen from the aforementioned formula (1), the smaller the reluctance R U  becomes, the larger the amount of magnetic flux  U  flowing through the upper magnetic circuit  62  becomes. Hence, the amount of magnetic flux  U  flowing through the upper magnetic circuit  62  is larger when the armature  30  is close to the upper core  32  as illustrated in FIG. 4 than when the armature  30  is spaced far apart from the upper core  32  as illustrated in FIG.  2 . 
     The magnetic flux  U , which is transferred between the armature  30  and the upper core  32 , mainly serves as an attracting force that attracts the armature  30  toward the upper core  32  even when the armature  30  is spaced slightly apart from the upper core  32 . Hence, as the armature  30  approaches the upper core  32 , the attracting force that attracts the armature  30  toward the upper core  32  increases in proportion with the magnetic flux  U  flowing through the upper magnetic circuit  62 . When the armature  30  is close to the upper core  32 , an attracting force that attracts the armature  30  toward the upper core  32  will hereinafter be referred to as a close-state attracting force F N . 
     FIG. 5 illustrates flow of a magnetic flux  L  circulating through the lower core  34  and the armature  30  when a predetermined current I 0  is supplied to the lower coil  42 . The flow of the magnetic flux  L  as illustrated in FIG. 5 is realized when the armature  30  is spaced slightly apart from the lower core  34 . 
     The smaller the air gap formed between the armature  30  and the lower core  34  becomes, the smaller the reluctance R L  of the lower magnetic circuit  64  becomes. As can be seen from the aforementioned formula (2), the smaller the reluctance R L  becomes, the larger the amount of magnetic flux  L  flowing through the lower magnetic circuit  64  becomes. Hence, the amount of magnetic flux  L  flowing through the lower magnetic circuit  64  is larger when the armature  30  is close to the lower core  34  as illustrated in FIG. 5 than when the armature  30  is spaced far apart from the lower core  34  as illustrated in FIG.  3 . 
     A magnetic flux is transferred between the armature  30  and the lower core  34  via an air gap formed between the protrusion facing side  38  of the armature  30  and the annular protrusion  36  of the lower core  34  (hereinafter referred to as a radial air gap) as well as an air gap formed between a bottom face of the armature  30  and an upper face of the lower core  34  (hereinafter referred to as an axial air gap). 
     The magnetic flux transferred via the axial air gap serves as an attracting force that always attracts the armature  30  toward the lower core  34 . On the other hand, as illustrated in FIG. 5, when the armature  30  is close to the lower core  34  to such an extent that the protrusion facing side  38  faces the inner wall of the annular protrusion  36 , the magnetic flux transferred via the radial air gap acts on the armature  30  in the radial direction such that the armature  30  is not urged toward the lower core  34 . Therefore, when the armature  30  is close to the lower core  34 , the larger the magnetic flux flowing through the axial air gap becomes, the larger the attracting force (the close-state attracting force F N ) that attracts the armature  30  toward the lower core  34  becomes. 
     As the armature  30  approaches the lower core  34 , the axial air gap decreases in proportion with a displacement amount of the armature  30  and reaches its minimum value of “0” upon abutment of the armature  30  on the lower core  34 . On the other hand, as the armature  30  approaches the lower core  34 , the radial air gap reaches its minimum value G MIN  upon arrival of a lower end portion of the protrusion facing side  38  on an upper end portion of the annular protrusion  36 . Accordingly, the radial air gap is smaller than the axial air gap until the axial air gap becomes smaller than G MIN  after arrival of the lower end portion of the protrusion facing side  38  on the upper end portion of the annular protrusion  36 . 
     The magnetic flux  L  flowing through the lower magnetic circuit  64  tends to follow a route having a small reluctance. Thus, when the radial air gap is smaller than the axial air gap, as the armature  30  approaches the lower core  34 , the magnetic flux  L  flowing through the lower magnetic circuit  64  passes in large part through the radial air gap. In this case, the close-state attracting force F N  assumes a relatively small value for the magnetic flux  L . Further, as the armature  30  approaches the lower core  34 , the close-state attracting force F N  undergoes relatively gradual changes. 
     Consequently, the electromagnetically driven valve  10  ensures that the close-state attracting force F N  generated between the armature  30  and the lower core  34  (hereinafter referred to as a lower close-state attracting force) is smaller than the close-state attracting force F N  generated between the armature  30  and the upper core  32  (hereinafter referred to as an upper close-state attracting force). In addition, the lower close-state attracting force generated as the armature  30  approaches the lower core  34  changes more gradually than the upper close-state attracting force generated as the armature  30  approaches the upper core  32 . 
     FIG. 6 illustrates flow of a magnetic flux  U  circulating through the upper core  32  and the armature  30  when a predetermined current I 0  is supplied to the upper coil  40 . The flow of the magnetic flux  U  as illustrated in FIG. 6 is realized when the armature  30  abuts against the upper core  32 . 
     The reluctance R U  of the upper magnetic circuit  62  assumes its minimum value when the armature  30  abuts against the upper core  32 . In this case, given an exciting current I 0 , the maximum magnetic flux  UMAX  flows through the upper magnetic circuit  62  and the maximum attracting force is generated between the armature  30  and the upper core  32 . This attracting force will hereinafter be referred to as an abutment-state attracting force F C . 
     FIG. 7 illustrates flow of a magnetic flux  L  circulating through the lower core  34  and the armature  30  when a predetermined current I 0  is supplied to the lower coil  42 . The flow of the magnetic flux  L  as illustrated in FIG. 7 is realized when the armature  30  abuts against the lower core  34 . 
     The reluctance R L  of the lower magnetic circuit  64  assumes its minimum value when the armature  30  abuts against the lower core  34 . In this case, given an exciting current I 0 , the maximum magnetic flux  LMAX  flows through the lower magnetic circuit  64 . In this embodiment, the air gap formed between the protrusion facing side  38  of the armature  30  and the annular protrusion  36  of the lower core  34  always exceeds the minimum value G MIN . Thus, when the armature  30  abuts against the lower core  34 , almost all of the magnetic flux  L  is transferred between the bottom face of the armature  30  and the upper face of the lower core  34 . In this case, given an exciting current I 0 , an abutment-state attracting force F C  is generated between the armature  30  and the lower core  34 . This abutment-state attracting force F C  is substantially equal to the abutment-state attracting force F C  generated between the armature  30  and the upper core  32 . 
     FIG. 8 illustrates characteristics of the electromagnetically driven valve  10  in accordance with changes in stroke of the valve body  18 . Referring to FIG. 8, a curve A indicates an attracting force generated between the armature  30  and the upper core  32  when the valve body  18  is displaced between its neutral position and its fully closed position with an exciting current I 0  supplied to the upper coil  40 . Further, a curve B indicates an attracting force generated between the armature  30  and the lower core  34  when the valve body  18  is displaced between its neutral position and its fully closed position with the exciting current I 0  supplied to the lower coil  42 . Still further, a curve C indicates a spring force generated by the upper spring  60  and the lower spring  26  when the valve body  18  is displaced between its neutral position and its fully open position or between its neutral position and its fully closed position. 
     As described above, an exciting current I 0  is supplied to both the upper coil  40  and the lower coil  42 , the spaced-state attracting force F F  is larger between the armature  30  and the lower core  34  than between the armature  30  and the upper core  32 . In this case, the close-state attracting force F N  is smaller between the armature  30  and the lower core  34  than between the armature  30  and the upper core  32 . Further, the abutment-state attracting force F C  generated between the armature  30  and the upper core  32  is substantially equal to the abutment-state attracting force F C  generated between the armature  30  and the lower core  34 . 
     Hence, as the curve A indicates, the attracting force generated between the armature  30  and the upper core  32  is relatively small when the valve body  18  is located in the vicinity of its neutral position. This attracting force tends to increase relatively steeply as the valve body  18  approaches its fully open position. On the other hand, as the curve B indicates, the attracting force generated between the armature  30  and the lower core  34  is relatively large when the valve body  18  is located in the vicinity of its neutral position. This attracting force tends to increase relatively gradually as the valve body  18  approaches its fully open position. 
     As described already, the electromagnetically driven valve  10  is used as an exhaust valve for an internal combustion engine. Hence, the electromagnetically driven valve  10  operates to open the valve body  18  when a high combustion pressure remains in the combustion chamber  16  and close the valve body  18  after release of the combustion pressure. If the valve body  18  is displaced toward its fully open position when a high combustion pressure remains in the combustion chamber  16 , a large load is applied to the valve body  18 . On the other hand, when the valve body  18  is thereafter displaced toward its fully closed position, such a large load is not applied to the valve body. 
     The electromagnetically driven valve  10  is constructed such that the valve body  18 , when in its fully closed position after stoppage of supply of an exciting current to the upper coil  40 , is displaced toward its fully open position by urging forces of the upper spring  60  and the lower spring  26 . Likewise, the electromagnetically driven valve  10  is constructed such that the valve body  18 , when in its fully open position after stoppage of supply of an exciting current to the lower coil, is displaced toward its fully closed position by urging forces of the upper spring  60  and the lower spring  26 . 
     In FIG. 8, a critical position that can be reached by the valve body  18  due to urging forces of the upper spring  60  and the lower spring  26  during the valve opening operation of the valve body  18  is marked as D. A critical position that can be reached by the valve body  18  due to urging forces of the upper spring  60  and the lower spring  26  during the valve closing operation of the valve body  18  is marked as E. As described above, the valve body  18  is subjected to a larger load during the valve opening operation than during the valve closing operation. Thus, the critical position D is closer to the neutral position of the valve body  18  than is the critical position E. 
     In order to suitably displace the valve body  18  to its fully open position, when the valve body  18  is located at the critical position D, it is necessary to generate an attracting force that exceeds spring forces generated by the upper spring  60  and the lower spring  26  (the spring forces that urge the valve body  18  toward its neutral position). As the curve B and the straight line C in FIG. 8 indicate, the electromagnetically driven valve  10  satisfies the aforementioned requirement. Hence, the electromagnetically driven valve  10  can suitably displace the valve body  18  to its fully open position. 
     When the valve body  18  is displaced toward the upper core  32  by a distance corresponding to the critical position D, the attracting force generated between the armature  30  and the upper core  32  is smaller than the spring forces generated by the upper spring  60  and the lower spring  26 . Hence, if the lower core  34  is constructed in the same manner as the upper core  32 , that is, unless the lower core  34  is provided with the annular protrusion  36 , the valve body  18  cannot be displaced suitably to its fully closed position by supplying an exciting current I 0  to the lower coil  42 . In view of this respect, the electromagnetically driven valve  10  is constructed such that the valve body  18  can be displaced to its fully closed position with a low electric power consumption. 
     In order to suitably displace the valve body  18  to its fully closed position, when the valve body  18  is located at the critical position E, it is necessary to generate an attracting force that exceeds spring forces generated by the upper spring  60  and the lower spring  26  (the spring forces that urge the valve body  18  toward its neutral position). As the curve A and the straight line C in FIG. 8 indicate, the electromagnetically driven valve  10  satisfies the aforementioned requirement. Hence, the electromagnetically driven valve  10  can suitably displace the valve body  18  to its fully closed position. 
     No matter how small the attracting force generated between the armature  30  and the upper core  32  may be before the valve body  18  of the electromagnetically driven valve  10  reaches the critical position E, if the aforementioned requirement is satisfied when the valve body  18  reaches the critical position E, the valve body  18  will be suitably displaced to its fully closed position. As illustrated in FIG. 8, if an exciting current I 0  is supplied to the upper coil  40 , an attracting force generated between the armature  30  and the upper core  32  when the valve body  18  reaches the critical position E is sufficiently larger than the spring forces generated by the upper spring  60  and the lower spring  26 . Thus, even if the exciting current supplied to the upper coil  40  is smaller than a predetermined value I 0 , the electromagnetically driven valve  10  can suitably displace the valve body  18  to its fully closed position. 
     As the curve A and the curve B in FIG. 8 indicate, the upper core  32  is more suitable in structure than the lower core  34  to generate a close-state attracting force F N  sufficiently large from the exciting current I 0 . Thus, the upper core  32  is more suitable in structure than the lower core  34  to generate an attracting force exceeding the spring forces generated by the upper spring  60  and the lower spring  26  with a low electric power consumption when the valve body  18  is located at the critical position E. In this embodiment, the exciting current supplied to the upper coil  40  is set to such a value that the attracting force generated between the armature  30  and the upper core  32  when the valve body  18  is located at the critical position E slightly exceeds the spring forces generated by the upper spring  60  and the lower spring  26 . As a result, the electromagnetically driven valve  10  makes it possible to drastically economize on electric power in displacing the valve body  18  to its fully closed position. 
     While the internal combustion engine is in operation, the valve body  18  needs to be held either at its fully closed position or at its fully open position. The electromagnetically driven valve  10  can hold the valve body  18  at either its fully closed position or its fully open position by supplying a suitable exciting current to the lower coil  42  or the upper coil  40  after arrival of the valve body  18  at its fully open or closed position that is, after arrival of the armature  30  on the lower core  34  or the upper core  32 . 
     As described previously, given an exciting current I 0 , the abutment-state attracting force F C  generated between the armature  30  and the upper core  32  is substantially equal to the abutment-state attracting force F C  generated between the armature  30  and the lower core  34 . Thus, the electromagnetically driven valve  10  makes it possible to drastically economize on electric power not only in displacing the valve body  18  to its fully closed position but also in displacing the valve body  18  to its fully open position. 
     As described previously, the characteristics of the electromagnetically driven valve  10  according to this embodiment are determined in view of the relationship between timings for opening and closing the valve body  18  and operating conditions of the internal combustion engine. Thus, while the internal combustion engine is in operation, the electromagnetically driven valve  10  can suitably open and close the valve body  18 , while making it possible to drastically economize on electric power. 
     Although the upper core  32  is not provided with a protrusion in this embodiment, the present invention is not limited to such a construction. For example, the upper core  32  may be provided with a protrusion that is smaller than the annular protrusion  36 , as shown in FIG.  15 . 
     An electromagnetically driven valve according to a second embodiment of the present invention will now be described with reference to FIG.  9 . 
     FIG. 9 is a sectional view illustrating a part surrounding the armature of the electromagnetically driven valve according to the second embodiment. In FIGS. 9 and 1, like elements are denoted by like reference numerals. Referring to FIG. 9, the description of those elements constructed in the same manner as in FIG. 1 will be omitted. 
     The electromagnetically driven valve according to this embodiment is realized by substituting a lower core  70  and an armature shaft  72  as illustrated in FIG. 9 for the lower core  34  and the armature shaft  28  as illustrated in FIG.  1 . The lower core  70  has an annular protrusion  74  surrounding the armature shaft  72 . On the other hand, the armature shaft  72  has a recess  76  accommodating the annular protrusion  74 . The armature shaft  72  is connected with the armature  30  at the recess  76 . 
     By providing the armature shaft  72  with the recess  76 , a protrusion facing side  78  is formed on an inner peripheral surface of the armature  30 . When the armature  30  is close to the lower core  70 , the protrusion facing side  78  of the armature  30  faces an outer peripheral surface of the annular protrusion  74 . Since the inner diameter of the armature  30  is slightly larger than the outer diameter of the annular protrusion  74 , a predetermined clearance is always formed between the protrusion facing side  78  and the annular protrusion  74 . 
     In the electromagnetically driven valve according to this embodiment, the annular protrusion  74  and the protrusion facing side  78  operate substantially in the same manner as the annular protrusion  36  and the protrusion facing side  38 . Thus, as is the case with the electromagnetically driven valve  10  according to the first embodiment, while the internal combustion engine is in operation, the electromagnetically driven valve according to this embodiment can suitably open and close the valve body  18 , while making it possible to drastically economize on electric power. 
     An electromagnetically driven valve according to a third embodiment of the present invention will now be described with reference to FIG.  10 . 
     FIG. 10 is a sectional view illustrating a part surrounding the armature of the electromagnetically driven valve according to the third embodiment. In FIGS. 10 and 1, like elements are denoted by like reference numerals. Referring to FIG. 10, the description of those elements constructed in the same manner as in FIG. 1 will be omitted. 
     The electromagnetically driven valve according to this embodiment is realized by substituting a lower core  80  and an armature  82  as illustrated in FIG. 10 for the lower core  34  and the armature  30  as illustrated in FIG.  1 . The lower core  80  has a first annular protrusion  84  and an annular groove  86 . The first annular protrusion  84  is disposed along the outermost periphery of the lower core  80  and the annular groove  86  is located radially inward of the first annular protrusion  84 . A first protrusion facing side  87  is formed on an inner peripheral surface of the first annular protrusion  84 . On the other hand, a second annular protrusion  88  is disposed along the outermost periphery of the armature  82 . A second protrusion facing side  90  is formed on an outer peripheral surface of the second annular protrusion  88 . 
     The second annular protrusion  88  is disposed so as to be fitted with the annular groove  86  of the lower core  80  when the armature  82  is close to the lower core  80 . In this state, the second protrusion facing side  90  faces an inner wall of the first annular protrusion  84 . That is, the outer peripheral surface of the second annular protrusion  88  faces the first protrusion facing side  87 . Since the outer diameter of the armature  82  is slightly smaller than the outer diameter of the first annular protrusion  84 , a predetermined clearance is always formed between the first annular protrusion  84  and the second protrusion facing side  90 . 
     In the electromagnetically driven valve according to this embodiment, the first annular protrusion  84  and the second annular protrusion  88  operate substantially in the same manner as the annular protrusion  36  in the first embodiment. Further, the first protrusion facing side  87  and the second protrusion facing side  90  operate substantially in the same manner as the protrusion facing side  38  in the first embodiment. Thus, as is the case with the electromagnetically driven valve  10  according to the first embodiment, while the internal combustion engine is in operation, the electromagnetically driven valve according to this embodiment can suitably open and close the valve body  18 , while making it possible to drastically economize on electric power. 
     Although the armature  82  is not provided with a protrusion protruding therefrom toward the upper core  32  in this embodiment, the present invention is not limited to such a construction. For example, a protrusion smaller than the second annular protrusion  88  may be formed on the side of the armature  82  that faces the upper core  32 . 
     Although the lower core  80  and the armature  82  are provided with the first annular protrusion  84  and the second annular protrusion  88  respectively in this embodiment, the present invention is not limited to such a construction. It may also be possible to provide only the armature  82  with an annular protrusion. 
     An electromagnetically driven valve according to a fourth embodiment of the present invention will now be described with reference to FIG.  11 . 
     FIG. 11 is an overall structural view of an electromagnetically driven valve  170  according to the fourth embodiment. The electromagnetically driven valve  170  is characterized in that it is provided with an intake valve  172  and an annular protrusion  176  is formed only on an upper core  174 . In FIGS. 11 and 1, like elements are denoted by like reference numerals. Referring to FIG. 11, the description of those elements constructed in the same manner as in FIG. 1 will be omitted or simplified. Formed in the cylinder head  12  is an intake port  180  in which a valve seat  182  is disposed. When the intake valve  172  moves onto the valve seat  182 , the intake port  180  is brought out of communication with the combustion chamber  16 . When the intake valve  172  moves away from the valve seat  182 , the intake port  180  is brought into communication with the combustion chamber  16 . 
     Unlike the case of the exhaust valve, the intake valve  172  is opened when no combustion pressure remains in the combustion chamber  16 . Thus, whether the intake valve  172  is driven to be opened or closed, there is no substantial change in an external force impeding the operation of the intake valve  172 . As a result, the amount of amplitude damped by the external force remains substantially unchanged regardless of whether the intake valve  172  is driven to be opened or closed. 
     The electromagnetically driven valve  170  is constructed such that the intake valve  172  reliably moves onto the valve seat  182  without being adversely affected by thermal expansion of a valve shaft  184  and the like. That is, the electromagnetically driven valve  170  is constructed such that even if the valve shaft  184  and the like thermally expand, the intake valve  172  always reaches the valve seat  182  prior to arrival of the armature  30  on the upper core  174 . Therefore, as the armature  30  is attracted toward the upper coil  40 , the electromagnetically driven valve  170  may bring about circumstances where only the armature  30  and the armature shaft  28  are separated from the valve shaft  184  and move toward the upper coil  40  after arrival of the intake valve  172  on the valve seat  182 . 
     In the electromagnetically driven valve  170 , since the upper retainer  58  is attached to the armature shaft  28 , the spring force of the upper spring  60  is directly transmitted to the armature shaft  28 . On the other hand, since the lower retainer  24  is attached to the valve shaft  184 , the spring force of the lower spring  26  is indirectly transmitted to the armature shaft  28  via the valve shaft  184 . 
     As described above, the electromagnetically driven valve  170  brings about circumstances where the armature shaft  28  is separated from the valve shaft  184  after close approximation of the armature  30  to the upper coil  40 . Under such circumstances, the spring force of the lower spring  26  is not transmitted to the armature shaft  28 , to which only the spring force of the upper spring  60  is transmitted. 
     The upper spring  60  generates a spring force urging the armature  30  toward the lower coil  42 . Hence, when only the spring force generated by the upper spring  60  acts on the armature shaft  28 , the amplitude of the armature  30  moving toward the upper coil  40  is abruptly damped. 
     As the armature  30  moves toward the lower coil  42 , both the spring force of the upper spring  60  and the spring force of the lower spring  26  constantly act on the armature shaft  28  until the armature  30  reaches the lower coil  42  after separation of the armature  30  from the upper coil  40 . Hence, as the armature  30  moves toward the lower coil  42 , the amplitude of the armature  30  is not abruptly damped. 
     As described hitherto, the electromagnetically driven valve  170  ensures that the spring forces of the upper spring  60  and the lower spring  26  damp the amplitude of the armature shaft  28  more drastically when the armature  30  moves toward the upper coil  40  than when the armature  30  moves toward the lower coil  42 . Thus, the amplitude of the intake valve  172  tends to be damped more drastically during the valve closing operation than during the valve opening operation. 
     In the electromagnetically driven valve  170  according to this embodiment, the upper core  174  is provided with the annular protrusion  176  surrounding the armature  30 . Thus, the attracting force generated between the armature  30  and the upper core  174  is relatively large when the intake valve  172  is located in the vicinity of its neutral position, so that the aforementioned difference in damping amount of amplitude can be eliminated. Accordingly, while the internal combustion engine is in operation, the electromagnetically driven valve  170  can suitably open and close the valve body, while making it possible to drastically economize on electric power. 
     FIG. 12 is an overall structural view of an electromagnetically driven valve  100  according to a fifth embodiment of the present invention. The electromagnetically driven valve  100  according to this embodiment is provided with an exhaust valve  102  for an internal combustion engine. The exhaust valve  102  is disposed in a cylinder head  104  such that the exhaust valve  102  is exposed to a combustion chamber in the internal combustion engine. Formed in the cylinder head  104  is an exhaust port  106  in which a valve seat  108  for the exhaust valve  102  is disposed. When the exhaust valve  102  moves away from the valve seat  108 , the exhaust port  106  is brought into communication with the combustion chamber. When the exhaust valve  102  moves onto the valve seat  108 , the exhaust port  106  is brought out of communication with the combustion chamber. 
     A valve shaft  110  is attached to the exhaust valve  102 . The valve shaft  110  is axially slidably held by a valve guide  112  supported by the cylinder head  104 . A lower retainer  114  is attached to an upper end portion of the valve shaft  110 . A lower spring  116  and a spring seat  118  are disposed below the lower retainer  114 . The lower spring  116  urges the lower retainer  114  upwards in FIG.  12 . 
     An armature shaft  120  made of a non-magnetic material is disposed on the valve shaft  110 . An upper retainer  122  is attached to an upper end portion of the armature shaft  120 . An upper spring  124  is disposed on the upper retainer  122 . The upper spring  124  urges the upper retainer  122  downwards in FIG.  12 . 
     An upper end portion of the upper spring  124  is held by a spring holder  124  on which an adjuster bolt  126  is disposed. The adjuster bolt  126  is screwed into an upper cap  128  attached to a housing plate  130 . 
     An armature  132 , which is an annular member made of a magnetic material, is connected with the armature shaft  120 . A first electromagnet  134  and a second electromagnet  136  are disposed above and below the armature  132  respectively. The first electromagnet  134  is provided with an upper coil  138  and an upper core  140 , while the second electromagnet  136  is provided with a lower coil  142  and a lower core  144 . The housing plate  130  maintains a predetermined relationship in relative location between the first electromagnet  134  and the second electromagnet  136 . 
     In the electromagnetically driven valve  100 , the armature  132  is urged toward its neutral position by the upper spring  124  urging the armature shaft  120  downwards and the lower spring  116  urging the valve shaft  112  upwards. The neutral position of the armature  132  can be adjusted by the adjuster bolt  126 . 
     In this embodiment, the electromagnetically driven valve  100  is characterized in that the neutral position of the armature  132  is biased a predetermined distance toward the lower core  144  from the central position between the upper core  140  and the lower core  144 . In the following description, the distance between the upper core  140  and the neutral position of the armature  132  will be denoted by XL and the distance between the lower core  144  and the neutral position of the armature  132  will be denoted by XS (&lt;XL). 
     The operation of the electromagnetically driven valve  100  as well as the effect achieved by the aforementioned features will hereinafter be described. 
     In the electromagnetically driven valve  100 , when no exciting current is supplied to the upper coil  138  and the lower coil  142 , the armature  132  is held at its neutral position. In this state, the exhaust valve  102  is located between its fully open position and its fully closed position. If an exciting current is supplied to the upper coil  138  under such circumstances, an attracting force that attracts the armature  132  toward the first electromagnet  134  is generated between the first electromagnet  134  and the armature  132 . 
     Thus, the electromagnetically driven valve  100  can displace the armature  132  toward the first electromagnet  134  by supplying a suitable exciting current to the upper coil  138 . The armature shaft  120  can be displaced toward the first electromagnet  134  until the armature  132  collides with the upper core  140 . The electromagnetically driven valve  100  is constructed such that the exhaust valve  102  reliably moves onto the valve seat  108  prior to arrival of the armature  132  on the upper core  140  without being adversely affected by thermal expansion of the valve shaft  110  and the like. Thus, the electromagnetically driven valve  100  can reliably displace the exhaust valve  102  to its fully closed position by supplying a suitable exciting current to the upper coil  138 . 
     When the armature  132  is magnetically coupled to the first electromagnet  134 , the upper spring  128  contracts in the axial direction by approximately a predetermined length XL and the lower spring  116  expands in the axial direction by approximately the predetermined length XL in comparison with a case where the armature  132  is held at its neutral position. In this state, provided that K represents a spring constant of the upper spring  128  and the lower spring  116 , the amount of energy EU stored in the upper spring  128  and the lower spring  116  is expressed as follows. 
     
       
           EU=K XL   2 /2  (1)  
       
     
     When the armature  132  is magnetically coupled to the first electromagnet  134  and the supply of an exciting current to the upper coil  138  is stopped, the spring forces of the upper spring  124  and the lower spring  116  displace the armature shaft  120 , the valve shaft  110  and the exhaust valve  102  so as to open the exhaust valve  102 . Such displacement causes energy loss resulting from sliding friction or the like. Thus, the amplitude of the exhaust valve  102  is damped to a certain extent as the exhaust valve  102  is displaced toward its fully open position. 
     The electromagnetically driven valve  100  generates an electromagnetic force attracting the armature  132  toward the second electromagnet  136  between the second electromagnet  136  and the armature  132  by supplying an exciting current to the lower coil  142 . Thus, the electromagnetically driven valve  100  can compensate for the aforementioned damping effect and displace the armature  132  to the second electromagnet  136  by supplying an exciting current to the lower coil  142  at a suitable timing after stoppage of supply of an exciting current to the upper coil  134 . 
     The exhaust valve  102  is fully open when the armature  132  abuts against the second electromagnet  136 . Accordingly, the electromagnetically driven valve  100  can displace the exhaust valve  102  from its fully closed position to its fully open position by the supply of an exciting current to the lower coil  142  begun at a suitable timing after stoppage of supply of an exciting current to the upper coil  138 . 
     When the armature  132  is magnetically coupled to the second electromagnet  136 , the upper spring  128  expands in the axial direction by approximately a predetermined length XS and the lower spring  116  contracts in the axial direction by approximately the predetermined length XS in comparison with a case where the armature  132  is held at its neutral position. In this state, provided that K represents the spring constant of the upper spring  128  and the lower spring  116 , the amount of energy EL stored in the upper spring  128  and the lower spring  116  is expressed as follows. 
     
       
           EL=K XS   2 /2  (2)  
       
     
     When the armature  132  is magnetically coupled to the second electromagnet  136 , if supply of an exciting current to the lower coil  142  is stopped, the spring forces of the upper spring  124  and the lower spring  116  displace the armature shaft  120 , the valve shaft  110  and the exhaust valve  102  so as to close the exhaust valve  102 . Such displacement causes energy loss resulting from sliding friction or the like. Thus, the amplitude of the exhaust valve  102  is damped to a certain extent as the exhaust valve  102  is displaced toward its fully closed position. 
     The electromagnetically driven valve  100  can compensate for the aforementioned damping effect and displace the armature  132  to the first electromagnet  134  by supplying an exciting current to the upper coil  138  at a suitable timing after stoppage of supply of an exciting current to the lower coil  142 . Hence, the electromagnetically driven valve  100  can suitably open and close the exhaust valve  102  by alternately supplying an exciting current to the upper coil  124  and the lower coil  130 . 
     In the internal combustion engine, the exhaust valve  102  is opened when a high combustion pressure remains in the combustion chamber. Therefore, the amplitude of the exhaust valve  102  is damped more drastically during the valve opening operation than during the valve closing operation. Accordingly, the achievement of substantially the same operating characteristics in opening and closing the exhaust valve  102  requires that the exhaust valve  102  be urged with more energy during the valve opening operation than during the valve closing operation. 
     As described previously, more energy is stored in the upper spring  124  and the lower spring  116  in the case where the armature  132  is magnetically coupled to the first electromagnet  134  than in the case where the armature  132  is magnetically coupled to the second electromagnet  136 . Thus, the electromagnetically driven valve  100  is constructed such that the upper spring  124  and the lower spring  116  urge the exhaust valve  102  with more energy during the valve opening operation than during the valve closing operation. 
     Since the upper spring  124  and the lower spring  116  urge the exhaust valve  102  as described above, the difference between the amount of energy loss during the valve opening operation and the amount of energy loss during the valve closing operation can be eliminated by the energy generated by the upper spring  124  and the lower spring  116 . Consequently, the electromagnetically driven valve  100  according to this embodiment can achieve substantially the same operating characteristics in opening and closing the exhaust valve  102  without substantially increasing a difference between the exciting current to be supplied to the upper coil  138  and the exciting current to be supplied to the lower coil  142 . 
     Although the neutral position of the armature  132  is always biased toward the second electromagnet  136  in this embodiment, the present invention is not limited to such a construction. For example, an actuator capable of changing the neutral position of the armature  132  may be provided so as to shift the neutral position of the armature  132  toward the second electromagnet  136  only when a high combustion pressure builds up in the combustion chamber, namely, when a high load is applied to the internal combustion engine or when the internal combustion engine rotates at a high speed. 
     A sixth embodiment of the present invention will now be described with reference to FIG.  13 . 
     FIG. 13 is an overall structural view of an electromagnetically driven valve  150  according to the sixth embodiment of the present invention. The electromagnetically driven valve  150  is provided with a first electromagnet  152  instead of the first electromagnet  134  in the electromagnetically driven valve  100  illustrated in FIG.  12 . In FIGS. 13 and 12, like elements are denoted by like reference numerals. Referring to FIG. 13, the description of those elements constructed in the same manner as in FIG. 12 will be omitted or simplified. 
     The first electromagnet  152  has an upper core  154  accommodating the upper coil  138 . An annular protrusion  156  is formed on an end face of the upper core  154  that faces the armature  132 . The inner diameter of the annular protrusion  156  is slightly larger than the outer diameter of the armature  132 . Thus, when the armature  132  is adsorbed on the first electromagnet  152 , a predetermined air gap is formed between the armature  132  and the annular protrusion  156 . 
     In this embodiment, the neutral position of the armature  132  is biased toward the second electromagnet  136  from the central position between the first electromagnet  152  and the second electromagnet  136  by a predetermined distance, as is the case with the fifth embodiment. This construction is advantageous in bringing the exhaust valve  102  close to the second electromagnet  136  by means of the spring forces of the upper spring  124  and the lower spring  116  during the valve opening operation. 
     In such a construction, however, the armature  132  tends to be spaced further apart from the first electromagnet  152  than in the construction in which the neutral position of the armature  132  is set to the central position between the first electromagnet  152  and the second electromagnet  136 . The closer the armature  132  comes to the electromagnet, the more efficiently an electromagnetic force is generated between the armature  132  and the electromagnet. Therefore, it is not always favorable to bias the neutral position of the armature  132  toward the second electromagnet  136  in the light of the efficiency in generating an electromagnetic force between the armature  132  and the first electromagnet  152 . 
     As described previously, the electromagnetically driven valve  150  according to this embodiment has a construction in which the annular protrusion  156  is formed on the upper core  154 . Due to the annular protrusion  156 , the distance between the end face of the upper core  154  and the armature  132  has been reduced. Hence, the first electromagnet  152  efficiently generates an electromagnetic force attracting the armature  132  when the neutral position of the armature  132  is biased toward the second electromagnet  136 . Consequently, the electromagnetically driven valve  150  according to this embodiment makes it possible to further economize on electric power in comparison with the electromagnetically driven valve  100  according to the fifth embodiment. 
     A seventh embodiment of the present invention will now be described with reference to FIG.  14 . 
     FIG. 14 is an overall structural view of an electromagnetically driven valve  160  according to the seventh embodiment. The electromagnetically driven valve  160  is provided with an intake valve  162  and the neutral position of the armature  132  is biased by a predetermined distance toward the first electromagnet  134  from the center point between the first electromagnet  134  and the second electromagnet  136 . In FIGS. 14 and 12, like elements are denoted by like reference numerals. Referring to FIG. 14, the description of those elements constructed in the same manner as in FIG. 12 will be omitted or simplified. 
     Formed in the cylinder head  104  is an intake port  164  in which a valve seat  166  is disposed. When the intake valve  162  moves onto the valve seat  166 , the intake port  164  is brought out of communication with the combustion chamber. When the intake valve  162  moves away from the valve seat  166 , the intake port  164  is brought into communication with the combustion chamber. 
     Unlike the case of the exhaust valve  102 , the intake valve  162  is opened when no combustion pressure remains in the combustion chamber. Hence, whether the intake valve  162  is driven to be opened or closed, there is no substantial change in an external force impeding the operation of the intake valve  162 . Thus, the amount of amplitude damped by the external force remains substantially unchanged regardless of whether the intake valve  162  is driven to be opened or closed. 
     The electromagnetically driven valve  160  is constructed such that the intake valve  162  reliably moves onto the valve seat  166  without being adversely affected by thermal expansion of the valve shaft  110  and the like. In other words, the electromagnetically driven valve  160  is constructed such that even if the valve shaft  110  and the like thermally expand, the intake valve  162  always reaches the valve seat  166  prior to arrival of the armature  132  on the upper core  140 . Hence, as the armature  132  is attracted toward the first electromagnet  134 , the electromagnetically driven valve  160  may bring about circumstances where only the armature  132  and the armature shaft  120  are separated from the valve shaft  110  and move toward the first electromagnet  134  after arrival of the intake valve  162  on the valve seat  166 . 
     In the electromagnetically driven valve  160 , since the upper retainer  122  is attached to the armature shaft  120 , the spring force of the upper spring  124  is directly transmitted to the armature shaft  120 . On the other hand, since the lower retainer  114  is attached to the valve shaft  110 , the spring force of the lower spring  116  is indirectly transmitted to the armature shaft  120  via the valve shaft  110 . 
     As described above, the electromagnetically driven valve  160  brings about circumstances where the armature shaft  120  is separated from the valve shaft  110  after close approximation of the armature  132  to the first electromagnet  134 . Under such circumstances, the spring force of the lower spring  116  is not transmitted to the armature shaft  120 , to which only the spring force of the upper spring  124  is transmitted. 
     The upper spring  124  generates a spring force urging the armature  132  toward the second electromagnet  136 . Hence, when only the spring force generated by the upper spring  124  acts on the armature shaft  120 , the amplitude of the armature  132  moving toward the first electromagnet  134  is abruptly damped. 
     As the armature  132  moves toward the second electromagnet  136 , both the spring force of the upper spring  124  and the spring force of the lower spring  116  act on the armature shaft  120  until the armature  132  reaches the second electromagnet  136  after separation of the armature  132  from the first electromagnet  134  and abutment of the valve shaft  110  on the armature shaft  120 . Hence, as the armature  132  moves toward the second electromagnet  136 , the amplitude of the armature  132  is not abruptly damped. 
     As described hitherto, the electromagnetically driven valve  160  is constructed such that the spring forces of the upper spring  124  and the lower spring  116  damp the amplitude of the armature shaft  120  more drastically when the armature  132  moves toward the first electromagnet  134  than when the armature  132  moves toward the second electromagnet  136 . Thus, the amplitude of the intake valve  162  tends to be damped more drastically during the valve closing operation than during the valve opening operation. 
     As described above, the electromagnetically driven valve  160  has a construction in which the neutral position of the armature  132  is biased toward the first electromagnet  134 . In this construction, the upper spring  124  and the lower spring  116  urge the armature shaft  120  with more energy during the valve closing operation of the intake valve  162  than during the valve opening operation of the intake valve  162 . In this case, the difference between the amount of amplitude damped during the valve opening operation and the amount of amplitude damped during the valve closing operation can be eliminated by the energy generated by the upper spring  124  and the lower spring  116 . Therefore, the electromagnetically driven valve  160  according to this embodiment can achieve substantially the same operating characteristics in opening and closing the intake valve  162  without substantially increasing a difference between the exciting current to be supplied to the upper coil  138  and the exciting current to be supplied to the lower coil  142 . 
     The neutral position of the armature  132  in the electromagnetically driven valve  160  according to this embodiment is different from the neutral position of the armature in the fifth and sixth embodiments. This kind of structural difference can be achieved, for instance, by adjusting the degree to which the adjuster bolt  126  is screwed into the upper cap or by changing the thickness of the spring seat  118 . By changing the thickness of the spring seat  118 , the upper spring  124  and the lower spring  116  can commonly be employed both in the electromagnetically driven valves  100 ,  150  for driving the exhaust valve  102  and in the electromagnetically driven valve  160  for driving the intake valve  162 . 
     While the present invention has been described with reference to what are presently considered to be preferred embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments or constructions. On the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single embodiment, are also within the spirit and scope of the invention.