PURPOSE:To increase the fuel injection while to quicken response by providing more than two steps of plunger having different diameter in a booster plunger fuel injector and feeding the injection fuel to the step section of plunger and the gap section of cylinder. CONSTITUTION:In a booster plunger 3 having a fuel chamber 4 for receiving fuel delivered from a fuel pump 1 and a cylinder chamber 7, the plunger 6 is formed into two steps of large and small diameter plungers 6a, 6b. The gap section 17 between the step section 16 of said plungers 6a, 6b and the large diameter cyliner 3A of booster plunger 3 is communicated through solenoid three-way valve 30 to the fuel chamber 4 of the booster plunger 3. The solenoid three-way valve 30 is constructed such that the gap section 17 is connected to the fuel tank 2 during power supply while connected to the fuel chamber 4 during power interruption. Consequently, the fuel injection can be increased by supplying the fuel to the gap section 17 resulting in reduction of the size of fuel injector and quickening of response.
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