Patent Publication Number: US-2015060576-A1

Title: Fuel injector

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Japanese Patent Application No. 2013-175817 filed on Aug. 27, 2013, the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a fuel injector controlling to turn on or turn off an injection port by an electric actuator. 
     BACKGROUND 
     Conventionally, a fuel injector generally includes a body which forms an injection port through which fuel is injected, a valve body which opens or closes the valve body, and an electric actuator which opens the valve body using a magnetic attraction force. JP-2006-348842A (US 2006/0283424 A1) discloses the fuel injector that opens the valve body using a smaller magnetic attraction force. The fuel injector further includes a control chamber which applies a pressure of the fuel supplied to the fuel injector to the valve body to a valve-closing direction, a fuel-storing chamber which applies the pressure of the fuel to the valve body to a valve-opening direction, and a control valve which controls a communication state between the control chamber and the injection port. The pressure of the fuel is referred to as a supplying pressure. When the electric actuator is energized, the control valve is opened by the magnetic attraction force, and a pressure in the control chamber is reduced. Therefore, the valve-closing force applied to the valve body is reduced, and the valve body is opened. 
     Since the valve body is opened by a pressure difference between the fuel-storing chamber and the control chamber, the magnetic attraction force is sufficient for a force that is requested for opening the control valve. Comparing with a case where the valve body is opened directly by the magnetic attraction force, the magnetic attraction force that is requested can be reduced. 
     However, when the supplying pressure is low, a fuel pressure in the fuel-storing chamber becomes insufficiently high. Therefore, even though the pressure in the control chamber is reduced, the valve body may not be opened. In addition, even when an extra force other than the magnetic attraction force is used, the valve body may not be opened either. In this case, the extra force may be a stretching force generated by a piezo element. 
     SUMMARY 
     The present disclosure is made in view of the above-mentioned matter, and it is an object to provide a fuel injector which reduces a request force of an electric actuator, and injects fuel even when a supplying pressure is low. 
     According to an aspect of the present disclosure, the fuel injector includes a body, a main valve body, a sub valve body, an electric actuator, and a valve-opening force transmission mechanism. The body houses a main passage through which fuel flows to an injection, and a sub passage branched from the main passage and through which the fuel flows to the injection port. The main valve body opens or closes the main passage, and the sub valve body opens or closes the sub passage. The electric actuator applies a valve-opening force to the sub valve body. The valve-opening force transmission mechanism transmits the valve-opening force of the sub valve body to the main valve body to open the main valve body in a condition that a valve-opening stroke of the sub valve body is greater than or equal to a predetermined amount. 
     When the electric actuator is energized, the sub valve body is opened earlier than the main valve body. The main valve body is opened by the valve-opening force transmission mechanism at a time point that the valve-opening stroke of the sub valve body reaches a predetermined amount. Therefore, the main valve body is opened in a case where a valve-closing force applied to the sub valve body by a fuel pressure is reduced while the sub valve body is opened such that the sub valve body is readily opened. Comparing with a case where the main valve body is opened while the sub valve body is closed, a request force for opening the main valve body can be reduced. In other words, comparing with a fuel injector in which the main valve body and the sub valve body are integrally provided as one member, a request force of the electric actuator can be reduced. 
     Since the valve-opening force generated by the electric actuator is transmitted to the main valve body to open the main valve body, the main valve body can be opened even when the supplying pressure is low. Therefore, the request force of the electric actuator can be reduced, and the supplying fuel can be injected in a case where the supplying pressure is low. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a sectional view showing a fuel injector according to a first embodiment of the present disclosure, while a fuel injection is stopped; 
         FIG. 2  is a sectional view showing the fuel injector of  FIG. 1 , when a sub valve body is opened and a main valve body is closed immediately after the fuel injector is energized; 
         FIG. 3  is a sectional view showing the fuel injector of  FIG. 1 , when the sub valve body and the a main valve body is opened; 
         FIGS. 4A to 4D  are graphs showing results according to valve-opening operations and valve-closing operations in the fuel injector of  FIG. 1 ,  FIG. 4A  is a graph showing a relationship between an attractive force and time,  FIG. 4B  is a graph showing a relationship between a lifting amount and time,  FIG. 4C  is a graph showing a relationship between a pressure and time, and  FIG. 4D  is a graph showing a relationship between a flow rate and time; 
         FIG. 5  is a sectional view showing the fuel injector according to a second embodiment of the present disclosure; and 
         FIG. 6  is a sectional view showing the fuel injector according to a third embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described hereafter referring to drawings. In the embodiments, a part that corresponds to a matter described in a preceding embodiment may be assigned with the same reference numeral, and redundant explanation for the part may be omitted. When only a part of a configuration is described in an embodiment, another preceding embodiment may be applied to the other parts of the configuration. The parts may be combined even if it is not explicitly described that the parts can be combined. The embodiments may be partially combined even if it is not explicitly described that the embodiments can be combined, provided there is no harm in the combination. 
     Embodiments of the present disclosure will be described hereafter referring to drawings. In the embodiments, a part that corresponds to a matter described in a preceding embodiment may be assigned with the same reference numeral, and redundant explanation for the part may be omitted. When only a part of a configuration is described in an embodiment, another preceding embodiment may be applied to the other parts of the configuration. The parts may be combined even if it is not explicitly described that the parts can be combined. The embodiments may be partially combined even if it is not explicitly described that the embodiments can be combined, provided there is no harm in the combination. 
     First embodiment 
     As shown in  FIG. 1 , a fuel injector according to a first embodiment of the present disclosure includes a body  10 , an electromagnetic coil  20 , a stator core  30 , a movable core  40 , a sub valve body  41 , and a main valve body  50 . According to the present embodiment, the fuel injector injects fuel used for a combustion of an internal combustion engine. Specifically, the fuel injector is mounted to the internal combustion engine of a direct injection type to directly inject fuel to a combustion chamber. 
     The body  10  houses the stator core  30 , the movable core  40 , the sub valve body  41 , and the main valve body  50 , and holds the electromagnetic coil  20 . A supplying fuel corresponding to fuel supplied from an exterior of the fuel injector flows through a passage inside of the body  10 , and is injected from an injection port  10   a  provided at an end of the body  10 . 
     The electromagnetic coil  20  generates a magnetic flux when being energized. The stator core  30  is fixed to the body  10 . The movable core  40  is housed in the body  10  and is slidable in an axial direction of the fuel injector. The stator core  30  and the movable core  40  form a magnetic circuit corresponding to a passage of the magnetic flux generated by the electromagnetic coil  20 . When the electromagnetic coil  20  is energized, a magnetic attraction force Fmag is generated, and the movable core  40  is attracted toward the stator core  30 . According to the present embodiment, the electromagnetic coil  20 , the stator core  30  and the movable core  40  correspond to an electric actuator. 
     The stator core  30  has a cylindrical shape. A rod  31  is inserted into a penetrating hole  30   a  provided inside of the stator core  30 . The rod  31  is fixed to the stator core  30  by welding. The movable core  40  has a cylindrical shape. The rod  31  is also inserted into a penetrating hole  40   a  provided inside of the movable core  40 . The movable core  40  is limited by the rod  31  from moving in a radial direction. The movable core  40  is guided by an outer peripheral surface of the rod  31  and is held to be movable in the axial direction. A notation H 1  indicates a first distance between the movable core  40  and the stator core  30 . When the movable core  40  is in contact with the stator core  30  as shown in  FIG. 3 , the first distance H 1  becomes zero. When the main valve body  50  and the sub valve body  41  are completely closed, the first distance H 1  becomes a maximum value MAXH 1  such as 100 μm. 
     A spring SP is provided to deform in a compression direction between the rod  31  and the movable core  40 . An elastic force Fsp of the spring SP is applied to the movable core  40  in a direction opposite to a direction that the movable core  40  is attracted toward the stator core  30 . According to the present embodiment, the spring SP corresponds to a valve-closing side elastic portion. 
     The sub valve body  41  is mounted to the movable core  40  by welding. A part of the sub valve body  41  farther from the injection port  10   a  has a cylindrical shape. The rod  31  is also inserted into an interior of the part of the sub valve body  41 . The sub valve body  41  is guided by the outer peripheral surface of the rod  31  and is held to be movable in the axial direction. 
     The main valve body  50  has a bottomed cylindrical shape. A bottom portion of the main valve body  50  includes an outlet  50   a.  The sub valve body  41  is inserted into the main valve body  50 . The main valve body  50  is fitted to a slidable surface  41   a  of the sub valve body  41 . The sub valve body  41  is provided to be slidable with respect to the main valve body  50 . 
     The body  10  houses a first passage  31   a,  a second passage  11 , a third passage  12 , and a sack chamber  13 . The sack chamber  13  communicates with the injection port  10   a  and the outlet  50   a.  The third passage  12  communicates with the sack chamber  13 . The second passage  11  communicates with the third passage  12 . The first passage  31   a  communicates with the second passage  11 . The first passage  31   a  is provided between the rod  31  and the stator core  30 . The second passage  11  also functions as a receiver receiving the movable core  40 . The second passage  11  has a ring shape and surrounds the movable core  40  and the sub valve body  41 . The third passage  12  also functions as a receiver receiving the main valve body  50 . The third passage  12  has a ring shape and surrounds the main valve body  50 . 
     According to the present embodiment, the first passage  31   a,  the second passage  11 , the third passage  12  and the sack chamber  13  correspond to a main passage. The main valve body  50  makes or shuts a communication state between the third passage  12  and the sack chamber  13 . Specifically, when a seat surface of the bottom portion of the main valve body  50  is seated on an inner surface of the body  10 , the communication state between the third passage  12  and the sack chamber  13  is shut. The seat surface of the bottom portion of the main valve body  50  is referred to as an outer seat  50   s.  When the outer seat  50   s  is removed from the inner surface of the body  10 , the third passage  12  communicates with the sack chamber  13 , and the supplying fuel is injected from the injection port  10   a  via the main passage. 
     The sub valve body  41  includes a control chamber  42  that is divided by an end of the rod  31 . An outer peripheral surface of the sub valve body  41  and an inner peripheral surface of the main valve body  50  form a fuel-storing chamber  43 . The fuel-storing chamber  43  has a ring shape and surrounds the sub valve body  41 . The sub valve body  41  includes a communication passage  44  that communicates with the control chamber  42  and the fuel-storing chamber  43 . The sub valve body  41  further includes an inlet passage  45  that introduces the fuel in the second passage  11  to the communication passage  44 . An orifice  45   a  is provided in the inlet passage  45  to limit an inlet flow rate of the fuel in the second passage  11 . 
     The fuel-storing chamber  43  communicates with the sack chamber  13  via the outlet  50   a  that is provided in the bottom portion of the main valve body  50 . The inlet passage  45  is branched from the main passage. The inlet passage  45 , the communication passage  44 , the fuel-storing chamber  43  and the outlet  50   a  correspond to a sub passage. Further, the orifice  45   a  corresponds to a sub flow-rate limiter. 
     The sub valve body  41  makes or shuts a communication state between the fuel-storing chamber  43  and the outlet  50   a.  Specifically, when a seat surface of a bottom portion of the sub valve body  41  is seated on the inner peripheral surface of the main valve body  50 , the communication state between the fuel-storing chamber  43  and the outlet  50   a  is shut. The seat surface of the bottom portion of the sub valve body  41  is referred to as an inner seat  41   s.  When the inner seat  41   s  removed from the inner peripheral surface of the main valve body  50 , the fuel-storing chamber  43  communicates with the outlet  50   a , and the supplying fuel is injected from the injection port  10   a  via the main passage and the sack chamber  13 . 
     When the sub valve body  41  is opened while the main valve body  50  is closed, a part of the supplying fuel supplied to the main passage is injected from the injection port  10   a  via the sub passage. When both the main valve body  50  and the sub valve body  41  are opened, the supplying fuel is injected from the injection port  10   a  via both the main passage and the sub passage. When both the main valve body  50  and the sub valve body  41  are completely opened (fully lifted), a throttle level of the inner seat  41   s  is set to a value greater than a throttle level of the outer seat  50   s.  In this case, the supplying fuel is mostly injected from the injection port  10   a  via the main passage. 
     A fuel pressure in the control chamber  42  is applied to the sub valve body  41  as a valve-closing force. The valve-closing force is referred to as a fuel-pressure valve-closing force Ffc. A fuel pressure in the fuel-storing chamber  43  is applied to the sub valve body  41  as a valve-opening force. The valve-opening force is referred to as a fuel-pressure valve-opening force Ffo. A diameter d 1  of the slidable surface  41   a  of the sub valve body  41  is set to a value less than a diameter d 2  of a slidable surface of the rod  31 . That is, a diameter of the control chamber  42  is greater than a diameter of the fuel-storing chamber  43 . When the fuel pressure in the control chamber  42  is equal to the fuel pressure in the fuel-storing chamber  43 , the fuel-pressure valve-closing force Ffc is greater than the fuel-pressure valve-opening force Ffo. When the sub valve body  41  is closed, a part of the fuel pressure is applied to an area S 1  perpendicular to a flow direction of the supplying fuel. In this case, the part of the fuel pressure is referred to as a seat valve-closing force Fsc1. 
     When the main valve body  50  is closed, a part of the fuel pressure is applied to an area S 2  perpendicular to the flow direction of the supplying fuel. 
     A vector difference between the fuel-pressure valve-closing force Ffc, the seat valve-closing force Fsc1 and the fuel-pressure valve-opening Ffo is applied to the sub valve body  41  in a valve-closing direction. The vector difference is referred to as a differential fuel-pressure valve-closing force ΔFfc. 
       Δ Ffc=Ffc+Fsc 1− Ffo    (1)
 
     Since the differential fuel-pressure valve-closing force ΔFfc decreases in accordance with a decrease in fuel pressure in the sub passage, the sub valve body  41  is readily opened. Generally, the differential fuel-pressure valve-closing force ΔFfc and the elastic force Fsp are applied to the sub valve body  41  in the valve-closing direction, and the magnetic attraction force Fmag is applied to the sub valve body  41  in a valve-opening direction. 
     As shown in  FIG. 1 , when a switch SW is turned off to deenergize the electromagnetic coil  20 , the magnetic attraction force Fmag becomes zero. In this case, the inner seat  41   s  is pressed to the main valve body  50  by the differential fuel-pressure valve-closing force ΔFfc and the elastic force Fsp, and the sub valve body  41  is closed. Further, the outer seat  50   s  is pressed to an inner surface of the body  10  by a pressing force Fp, and the main valve body  50  is closed. In this case, the pressing force Fp corresponds to a vector sum of the differential fuel-pressure valve-closing force ΔFfc and the elastic force Fsp. 
         Fp=ΔFfc+Fsp    (2)
 
     The switch SW is controlled by an electric control unit disposed at a position outside of the fuel injector. 
     As the above description, the sub valve body  41  is provided to be slidable with respect to the main valve body  50 . A notation H 2  indicates a movable stroke amount of the sub valve body  41  with respect to the main valve body  50 . As shown in  FIG. 1 , when the sub valve body  41  is closed, a second distance H 2  between the locking portion  51  and the locking portion  41   b  becomes a maximum value MAXH 2 . The main valve body  50  includes a locking portion  51 , and the sub valve body  41  includes a locking portion  41   b . When the second distance H 2  becomes zero, the locking portion  51  and the locking portion  41   b  are in contact with each other. Therefore, the second distance H 2  is limited. As shown in  FIG. 2 , the locking portions  51  and the locking portion  41   b  are in contact with each other, and the second distance H 2  becomes zero. As shown in  FIG. 1 , the main valve body  50  and the sub valve body  41  are completely closed, and the second distance H 2  becomes the maximum value MAXH 2  such as 10 μm. According to the present embodiment, the maximum value MAXH 2  of the second distance H 2  is set to a value less than the maximum value MAXH 1  of the first distance H 1 . 
     Next, valve-opening operations of both the main valve body  50  and the sub valve body  41  will be described. 
     As shown in  FIG. 2 , when the magnetic attraction force Fmag exceeds the pressing force Fp in a case where the switch SW is turned on to generate the magnetic attraction force Fmag, the sub valve body  41  starts to be opened. The main valve body  50  is still closed until the locking portion  41   b  and the locking portion  51  are in contact with each other. In this case, the supplying fuel throttled by the inner seat  41   s  is injected from the injection port  10   a.    
     When the sub valve body  41  is opened, a flow rate of the supplying fuel throttled by the orifice  45   a  and flowing through the inlet passage  45  is set to a value less than a flow rate of the supplying fuel throttled by the inner seat  41   s  and flowing through the injection port  10   a.  Therefore, the fuel pressure in the sub passage is decreased in a case where the main valve body  50  is closed while the sub valve body  41  is opened. 
     As the above description, the diameter of the control chamber  42  is greater than the diameter of the fuel-storing chamber  43 . Therefore, the differential fuel-pressure valve-closing force ΔFfc decreases in accordance with a decrease in fuel pressure in the sub passage. When the sub valve body  41  is opened, the seat valve-closing force Fsc1 becomes smaller. Therefore, the differential fuel-pressure valve-closing force ΔFfc becomes remarkably small in a case where the main valve body  50  is closed while the sub valve body  41  is opened. 
     Then, the electromagnetic coil  20  is continuously energized, and the sub valve body  41  is further lifted up. When the second distance H 2  becomes zero, the locking portion  41   b  and the locking portion  51  are in contact with each other, and the main valve body  50  starts to be opened. Therefore, the flow rate of the supplying fuel throttled by the outer seat  50   s  besides the flow rate of the supplying fuel throttled by the inner seat  41   s  are injected from the injection port  10   a.  When the stroke amount of the movable core  40  and a lifting amount of the main valve body  50  sufficiently increase, the throttle level of the outer seat  50   s  becomes less than a throttle level of the injection port  10   a,  and a sufficient amount of the supplying fuel is injected from the injection port  10   a.    
     When a condition that a valve-opening stroke of the sub valve body  41  is greater than or equal to a predetermined amount, a valve-opening force of the sub valve body  41  is transmitted to the main valve body  50  to open the main valve body  50 . The predetermined amount is equal to the maximum value MAXH 2  of the stroke amount H 2 , and the valve-opening force Fso corresponds to a vector difference between the magnetic attraction force Fmag, the differential fuel-pressure valve-closing force ΔFfc and the elastic force Fsp. 
         Fso=Fmag−(ΔFfc+Fsp)    (3)
 
     The main valve body  50  is opened by energizing the electromagnetic coil  20  for a time period according to a target injection amount of the supplying fuel to be injected from the injection port  10   a.  When the target injection amount is less than a specified amount, the electromagnetic coil  20  is deenergized before the second distance H 2  becomes zero, such that the supplying fuel injected only by the sub passage is stopped without being injected by the main passage. 
     Next, valve-closing operations of the main valve body  50  and the sub valve body  41  will be described. 
     When the electromagnetic coil  20  is deenergized in a case where both the main valve body  50  and the sub valve body  41  are opened, the main valve body  50  is held to be completely opened while the sub valve body  41  starts to be closed by the pressing force Fp. When the inner seat  41   s  is in contact with the main valve body  50  such that the sub valve body  41  is closed, the main valve body  50  is pressed by the sub valve body  41  in the valve-closing direction. The main valve body  50  is closed at a time point that both the first distance H 1  and the second distance H 2  become the maximum values MAXH 1  and MAXH 2  after the main valve body  50  starts to be closed. 
     Next, referring to  FIGS. 4A to 4D , variations generated according to valve-opening operations and valve-closing operations of the main valve body  50  and the sub valve body  41  will be described. In addition, horizontal axes indicate time that elapsed since the electromagnetic coil  20  is energized. 
     As shown in  FIG. 4A , the magnetic attraction force Fmag increases with time since the electromagnetic coil  20  is energized. At a time point t 1  that the magnetic attraction force Fmag reaches the pressing force Fp, the sub valve body  41  starts to be opened, and a lifting amount of the sub valve body  41  increases with time. As shown in  FIG. 4B , a line L 1  indicates the lifting amount of the sub valve body  41 . Then, the supplying fuel in the sub passage flows into the sack chamber  13  via the outlet  50   a  and is injected from the injection port  10   a.  Therefore, the fuel pressure in the sack chamber  13  starts to increase with time since the sub valve body  41  starts to be opened. As shown in  FIG. 4C , a line L 3  indicates the fuel pressure in the sack chamber  13 . 
     The fuel pressure in the control chamber  42  is equal to the fuel pressure in the second passage  11  before the time point t 1  that the sub valve body  41  starts to be opened. However, the fuel pressure in the control chamber  42  is less than the fuel pressure in the second passage  11  after the time point t 1 . As shown in  FIG. 4C , a line L 4  indicates the fuel pressure in the control chamber  42 , and a line L 5  indicates the fuel pressure in the second passage  11 . Considering the inlet passage  45  is throttled by the orifice  45   a,  the flow rate flowing from the inlet passage  45  into the communication passage  44  is less than the flow rate flowing from the outlet  50   a.    
     At a time point t 2  that the lifting amount of the sub valve body  41  reaches the predetermined amount, the main valve body  50  starts to be opened, the lifting amount of the main valve body  50  increases with time. Further, at the time point t 2 , the second distance H 2  becomes zero. As shown in  FIG. 4B , a line L 2  indicates the lifting amount of the main valve body  50 . When the main valve body  50  starts to be opened, the supplying fuel is injected from the injection port  10   a  via the main passage. In this case, the fuel pressure in the sack chamber  13  and the fuel pressure in the control chamber  42  increase, and the fuel pressure in the second passage  11  decreases. 
     Further, when the main valve body  50  starts to be opened, the flow rate of the injection port  10   a  sharply increases, and the flow rate of the inner seat  41   s  starts to decrease. As shown in  FIG. 4D , a line L 6  indicates the flow rate of the injection port  10   a , and a line L 7  indicates the flow rate of the inner seat  41   s.  The flow rate of the orifice  45   a  increases in a case where the sub valve body  41  starts to be opened, and decreases in a case where the main valve body  50  starts to be opened. As shown in  FIG. 4D , a line L 8  indicates the flow rate of the orifice  45   a.    
     At a time point t 3  that the electromagnetic coil  20  is deenergized, the magnetic attraction force Fmag decreases. At a time point t 4  that the magnetic attraction force Fmag decreases to be equal to the pressing force Fp, the sub valve body  41  starts to be closed, and the lifting amount of the sub valve body  41  starts to decrease. The fuel pressure in the sack chamber  13  and the fuel pressure in the control chamber  42  decrease in accordance with a decrease in lifting amount of the sub valve body  41 . 
     At a time point t 5  that the sub valve body  41  becomes in contact with the main valve body  50  such that the second distance H 2  becomes the maximum value MAXH 2 , the main valve body  50  starts to be closed, and the lifting amount of the main valve body  50  starts to decrease. Further, at the time point t 5 , the sub valve body  41  is completely closed. Then, the fuel pressure in the sack chamber  13  and the flow rate of the injection port  10   a  sharply decrease. At time point t 6 , the main valve body  50  is completely closed. 
     According to the above description, the fuel injector has the following features. Further, effects of the features will be described. 
     (a) A valve body opening or closing the injection port  10   a  includes the main valve body  50  that opens or closes the main passage and the sub valve body  41  that opens or closes the sub passage. When the condition that the valve-opening stroke of the sub valve body  41  is greater than or equal to the predetermined amount, the valve-opening force of the sub valve body  41  is transmitted to the main valve body  50  via a valve-opening force transmission mechanism. In this case, the valve-opening force transmission mechanism corresponds to the locking portion  41  b and the locking portion  51 . 
     When the electromagnetic coil  20  is energized, the sub valve body  41  is opened earlier than the main valve body  50 . When the second distance H 2  becomes zero, the locking portion  41   b  and the locking portion  51  are in contact with each other, the main valve body  50  is lifted up to be opened by the sub valve body  41 . 
     As shown in  FIGS. 4A to 4D , the fuel pressure in the control chamber  42  decreases in a time period from the time point t 1  that the sub valve body  41  starts to be opened to the time point t 2  that the main valve body  50  starts to be opened. The main valve body  50  is opened, in a case where the differential fuel-pressure valve-closing force ΔFfc becomes remarkably small such that the sub valve body  41  is readily opened. Therefore, a request value of the magnetic attraction force Fmag of the electric actuator can be reduced. Since the valve-opening force is transmitted to the main valve body  50  according to the magnetic attraction force Fmag so as to open the main valve body  50 , the main valve body  50  can be opened even though a supplying pressure is low. The supplying pressure corresponds to the fuel pressure of the supplying fuel. Thus, the request value can be reduced, and the supplying fuel can be injected in a case where the supplying pressure is low. 
     (b) The control chamber  42  is provided in the body  10  to communicate with the sub passage and to apply the fuel pressure to the sub valve body  41  in the valve-closing direction. The sub passage includes the fuel-storing chamber  43  and the communication passage  44 . The fuel pressure in the fuel-storing chamber  43  is applied to the sub valve body  41  in the valve-opening direction. The communication passage  44  communicates with the control chamber  42  and the fuel-storing chamber  43 . 
     The supplying fuel exhausted from the control chamber  42  is injected from the injection port  10   a  via the communication passage  44  and the fuel-storing chamber  43 , such that the sub valve body  41  is readily opened. Therefore, a return passage for returning the supplying fuel exhausted from the control chamber  42  to a fuel tank is unnecessary. 
     (c) The sub passage is provided with the orifice  45   a  corresponding to the sub flow-rate limiter. The orifice  45   a  limits the flow rate of the supplying fuel flowing from the main passage. 
     When the sub valve body  41  is opened in a time period from the time point t 1  to the time point t 2 , the supplying fuel of high pressure which flows from the main passage to the control chamber  42  is limited. Therefore, the fuel pressure in the control chamber  42  is decreased immediately after the sub valve body  41  starts to be opened. The main valve body  50  surely can be opened in a case where the sub valve body  41  is readily opened. 
     The supplying fuel compressed in the control chamber  42  can be introduced by the orifice  45   a  to the main passage according to the lift-up of the sub valve body  41 , immediately after the time point t 1  that the sub valve body  41  starts to be opened. Therefore, it can be prevented that the fuel pressure in the control chamber  42  is temporarily increased such that a valve-opening rate of the sub valve body  41  becomes slow immediately after the sub valve body  41  starts to be opened. 
     (d) The fuel injector includes the spring SP that applies the elastic force to the sub valve body  41  in the valve-closing direction. In the fuel injector, when the sub valve body  41  is closed, the elastic force of the spring SP is applied to the main valve body  50  via the sub valve body  41  in the valve-closing direction. An elastic coefficient of the spring SP is set to a value greater than or equal to a predetermined value such that the sub valve body  41  is closed earlier than the main valve body  50 , in a case where the electric actuator is deenergized. 
     When the elastic coefficient of the spring SP is set to a value less than the predetermined value, the main valve body  50  may be opened earlier than the sub valve body  41  after the electric actuator is deenergized. In this case, the sub valve body  41  is readily opened before the sub valve body  41  is completely closed. Therefore, the sub valve body  41  may be opened while the sub valve body  41  is still in the middle of a valve-closing operation. According to the present embodiment, since the elastic coefficient of the spring SP is set to a value greater than or equal to the predetermined value, it can be prevented that the main valve body  50  is opened earlier than the sub valve body  41 . 
     Second Embodiment 
     As shown in  FIG. 5 , the fuel injector according to a second embodiment of the present disclosure further includes a sub spring SPa that applies an elastic force to the main valve body  50  in the valve-opening direction. The sub spring SPa is pressed to be elastically deformed and is disposed between the main valve body  50  and the body  10 . The elastic force of the sub spring SPa is transmitted to the sub valve body  41  via the inner seat  41   s  in a case where the sub valve body  41  is closed. 
     Therefore, the elastic force of the sub spring SPa is applied to the sub valve body  41  in the valve-opening direction in a case where the sub valve body  41  is closed. Thus, the elastic coefficient of the spring SP is greater than that of the first embodiment to cancel the elastic force of the sub spring SPa. The sub spring SPa corresponds to a valve-opening side elastic portion. 
     In a case where the sub spring SPa is canceled, when the sub valve body  41  and the main valve body  50  are both in the valve-closing operation, it is possible that the main valve body  50  separates from the inner seat  41   s  and is closed earlier than the sub valve body  41 . Then, the fuel pressure in the sub passage is lowered before the sub valve body  41  is completely closed. Thus, the sub valve body  41  is readily opened before the sub valve body  41  is completely closed, and the sub valve body  41  may be opened while the sub valve body  41  is still in the middle of the valve-closing operation. 
     According to the present embodiment, since the main valve body  50  is pressed to the sub valve body  41  by the sub spring SPa, it can be prevented that the main valve body  50  is closed earlier than the sub valve body  41  in the valve-closing operation. 
     Third Embodiment 
     As shown in  FIG. 6 , the fuel injector according to a third embodiment of the present disclosure includes a dividing member  14 . The dividing member  14  is disposed in the body  10  to divide the second passage  11  into an upstream fuel-storing chamber  11   a  and a downstream fuel-storing chamber  11   b.  The dividing member  14  includes a communication hole  15  that communicates with the upstream fuel-storing chamber  11   a  and the downstream fuel-storing chamber  11   b . The communication hole  15  is provided with an orifice  15   a  that limits the flow rate of the supplying fuel. The orifice  15   a  corresponds to a main flow-rate limiter. 
     The main passage includes the upstream fuel-storing chamber  11   a,  the downstream fuel-storing chamber  11   b , and the main flow-rate limiter that limits the flow rate of the supplying fuel flowing from the upstream fuel-storing chamber  11   a  to the downstream fuel-storing chamber  11   b . The upstream fuel-storing chamber  11   a  is disposed at a position upstream of the downstream fuel-storing chamber  11   b . A fuel pressure in the downstream fuel-storing chamber  11   b  is applied to the sub valve body  41  as a valve-opening force corresponding to a fuel-pressure valve-opening force Ffo2. A fuel pressure in the upstream fuel-storing chamber  11   a  is applied to the sub valve body  41  as a valve-closing force corresponding to a fuel-pressure valve-closing force Ffc2. 
     According to the present embodiment, when the main valve body  50  is opened such that the supplying fuel is injected, the fuel pressure in the downstream fuel-storing chamber  11   b  is less than the fuel pressure in the upstream fuel-storing chamber  11   a  according to the orifice  15   a.  Therefore, since the fuel-pressure valve-opening force Ffo2 becomes smaller, rates of the valve-closing operations of the main valve body  50  and the sub valve body  41  become faster. In this case, the valve-closing operations are executed by the spring SP. According to the present embodiment, a valve-closing delay period from a time point that the electromagnetic coil  20  is deenergized to a time point that a fuel injection quantity becomes zero can be shortened, and a responsivity of the valve-closing operation can be improved. 
     Other Embodiment 
     The present disclosure is not limited to the above embodiments, and may change as followings. Further, various combinations of the features of the above embodiments are also within the spirit and scope of the present disclosure. 
     According to the above embodiments, the electric actuator uses an electromagnetic actuator to generate the magnetic attraction force. However, a piezo element may be used in the electric actuator. 
     According to the above embodiments, the lifting amount of the main valve body  50  decreases in accordance with an increase in stroke amount H 2  of the sub valve body  41  with respect to the main valve body  50 . Then, an injection rate corresponding to an injection amount of the supplying fuel injected from the injection port  10   a  per unit time becomes insufficient. When the main valve body  50  is fully lifted up, it is preferable that the stroke amount H 2  is set to a value less than a predetermined upper limit value such that the throttle level of the outer seat  50   s  becomes less than the throttle level of the injection port  10   a.    
     According to the above embodiments, when the sub valve body  41  is in the valve-opening operation from the time point t 1  to the time point t 2 , a decreasing rate of the fuel pressure in the control chamber  42  becomes lower. Then, when the main valve body  50  is opened, the fuel pressure in the control chamber  42  is insufficiently low. Therefore, the request value of the magnetic attraction force Fmag is insufficiently reduced by opening the main valve body  50  in a case where the sub valve body  41  is readily opened according to a decrease of the fuel pressure in the control chamber  42 . It is preferable that the stroke amount H 2  is set to a value greater than or equal to a predetermined lower limit value. 
     According to the above embodiments, the supplying fuel exhausted from the control chamber  42  is injected from the injection port  10   a.  However, the fuel injector may include a return passage through which the supplying fuel exhausted from the control chamber  42  is returned to the fuel tank. In other words, the supplying fuel exhausted from the control chamber  42  is returned to the fuel tank without being injected from the injection port  10   a.    
     According to the above embodiments, the sub passage is provided with the orifice  45   a.  However, a gate valve may be provided in the sub passage to open or close the sub passage. 
     According the above embodiments, the elastic coefficient of the spring SP is set to a value greater than or equal to the predetermined value such that the sub valve body  41  is closed earlier than the main valve body  50 . However, the elastic coefficient of the spring SP may set to any value. 
     While the present disclosure has been described with reference to the embodiments thereof, it is to be understood that the disclosure is not limited to the embodiments and constructions. The present disclosure is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the present disclosure.