1. Field of the Invention
The present invention relates to an injector, which injects and supplies fuel into an engine.
2. Description of Related Art
In an injector that drives a needle to lift off to open a nozzle hole, it is conventionally considered increasing driving force needed to open the nozzle hole in order to improve injection responsivity. For example, an actuator of the injector includes an element that generates extending force, such as a piezoelectric element or a magnetostrictor, as a means for increasing the driving force.
As shown in FIG. 3, a conventional injector 100, which uses the extending force, includes a needle 102, a piezoelectric actuator 103, a piston 104, and an outer sleeve 106, as described, for example, in WO2005/075811A1. The needle 102 opens or closes a nozzle hole 101. The piezoelectric actuator 103 includes the piezoelectric element and extends or contract in an axial direction of the injector 100. The piston 104 is displaced in the axial direction according to the extension or contraction of the piezoelectric actuator 103. The outer sleeve 106 slidably holds the piston 104 on its outer circumferential side, and defines a pressure chamber 105 of fuel, which is expanded or contracted according to the displacement of the piston 104.
The needle 102 is incorporated into the injector 100 such that fuel pressure in the pressure chamber 105 is applied to the needle 102 in a valve opening direction. More specifically, the needle 102 is incorporated to define the pressure chamber 105 such that the fuel pressure is applied to a front end surface 108 of a rear end portion 107 in a direction toward a rear end side.
The injector 100 leads high-pressure fuel, which is received from a fuel supply source such as a common rail, into a nozzle chamber 109. The injector 100 increases the fuel pressure in the pressure chamber 105 by the extension of the piezoelectric actuator 103. As a result, the needle 102 is driven to lift off in the valve opening direction and to open the nozzle hole 101, so that fuel in the nozzle chamber 109 is injected.
According to the injector 100, fuel in the pressure chamber 105 leaks from sliding portions due to the increase in its pressure. An amount of fuel decreased due to the leak is replenished when the outer sleeve 106 disengages from a front end side body 110.
The outer sleeve 106 supports a spring 111 in the axial direction between the outer sleeve 106 and the piston 104. The pressure chamber 105 is formed when the outer sleeve 106 is urged toward the front end side by the spring 111 to engage the front end side body 110. An outer circumferential side of the outer sleeve 106 is filled with high-pressure fuel received from the fuel supply source. Urging force of the spring 111 applied to the outer sleeve 106 is increased or decreased according to the displacement of the piston 104 in the axial direction. Accordingly, when the urging force of the spring 111 is decreased, the outer sleeve 106 disengages from the front end side body 110, and thereby high-pressure fuel is replenished into the pressure chamber 105 from the outer circumferential side of the outer sleeve 106.
According to the conventional injector 100, a gap between the pressure chamber 105 and the outer circumferential side of the outer sleeve 106 is closed or opened when the outer sleeve 106 engages or disengages from the front end side body 110. Accordingly, the outer sleeve 106 engages the front end side body 110 annularly at a position located far part from the shaft center toward the outer circumferential side of the injector 100, so that an engagement diameter of the outer sleeve 106 is made large. Thus, liquid-tightness of the pressure chamber 105 is difficult to ensure at an engagement position of the outer sleeve 106 with the front end side body 110, so that the spring 111, the urging force of which is strong, needs to be selectively used.