1. Field of the Invention
The present invention relates to a fuel injector for use in an internal combustion engine.
2. Description of the Related Art
A fuel injector equipped therein with a pressure chamber connected to nozzle bores via an automatically opened needle is known. In this fuel injector, fuel fed into the pressure chamber is pressurized by a pressurizing device, for example, by a plunger, and then the fuel thus pressurized is injected from the nozzle bores. Such a fuel injector is normally called a unit fuel injector. A unit fuel injector has a high efficiency, and thus unit fuel injectors of various types have been developed. For example, in order to precisely control the fuel injection, a unit fuel injector using a solenoid valve is known (Japanese Unexamined Utility Model publication No. 56-31655). In this unit injector, a solenoid valve is arranged in a fuel discharge passage connected to the pressure chamber. When the fuel injection is carried out, the fuel discharge passage is closed by the solenoid valve to isolate the pressure chamber from the outside. As a result, since the fuel in the pressure chamber is pressurized when the plunger is caused to move towards the pressure chamber, fuel is injected from the nozzle bores. Contrary to this, when the fuel injection is stopped, the solenoid valve opens the fuel discharge passage so that the fuel pressurized by the plunger is discharged from the pressure chamber via the fuel discharge passage.
As mentioned above, in this unit fuel injector, when the fuel injection is carried out, the fuel discharge passage is closed by the solenoid valve. However, at this time, the pressure in the pressure chamber becomes considerably high. Consequently, a high pressure acts on the solenoid valve, and thus there occurs a problem in that a large and strong solenoid valve will be necessary. In addition, if such a large and strong solenoid valve is used, when the solenoid valve is deenergized, a strong residual magnetism remains in the solenoid. As a result, since the solenoid valve does not operate as soon as the solenoid is deenergized, another problem occurs in that a good responsiveness of the solenoid valve can not be obtained. Furthermore, as mentioned above, where the injection of fuel is started by stopping the flow of the pressurized fuel flowing within the fuel discharge passage, and where the injection of fuel is stopped by starting the flow of the pressurized fuel, a certain time must elapse before the flow of the pressurized fuel is started or completely stopped after the solenoid is energized or deenergized. Therefore, a further problem occurs in that the start and stop of the fuel injection will be delayed.