Piston and barrell assembly with stepped top and hydraulically-actuated fuel injector utilizing same

A hydraulically-actuated piston and barrel assembly includes a barrel defining an actuation fluid cavity and a piston bore, which includes an upper bore and a lower bore. A stepped piston is slidably received in the piston bore and moveable between a retracted position and an advanced position. The stepped top of the piston includes a first area that is separate from a second area. The first area and the upper bore define an upper cavity connected to the actuation fluid cavity through a relatively unrestricted flow area, when the piston is in its retracted position. The second area and the lower bore define a lower cavity connected to the actuation fluid cavity through a relatively restricted flow area when the piston is in its retracted position. The first area is exposed to fluid pressure in the upper cavity and the second area is exposed to fluid pressure in the lower cavity over a portion of the piston's movement from its retracted position toward its advanced position. One of the piston or the barrel, or both, further define a restricted passage that includes a restricted flow area connecting the actuation fluid cavity to the lower cavity. The disclosure also includes a description of a hydraulically-actuated fuel injector that incorporates the piston and barrel assembly described above.