Rate shaping plunger/piston assembly for a hydraulically actuated fuel injector

In hydraulically actuated fuel injectors, an intensifier piston is utilized with a plunger for raising fuel pressure to initiate an injection event with a VOP type fuel injector. The present invention lowers the mass flow rate of fuel at the beginning of each injection event by slowing the rate of the plunger relative to the intensifier piston by utilizing a spring to separate the two. As the spring compresses, energy from the high pressure actuation fluid is absorbed rather than transferred to raise fuel pressure. A short time into each injection event, the spring becomes fully compressed and the intensifier piston and plunger begin moving in rigid unison for the remaining of the injection event in a manner typical of prior art fuel injectors. The lowering of injection mass flow rate toward the beginning of each injection event results in improved engine performance and a lowering of undesirable exhaust emissions, especially NOx compounds.