Hydraulically-actuated fuel injector with variable rate return spring

Abstract

A hydraulically actuated fuel injector includes an injector body defining an actuation fluid chamber that opens to an actuation fluid drain, an actuation fluid inlet and a piston bore, and further defining a nozzle chamber that opens to a plunger bore and a nozzle outlet. A control valve, positioned in the injector body, has a first position that opens the actuation fluid inlet and closes the actuation fluid drain, and a second position that closes the actuation fluid inlet and opens the actuation fluid drain. A piston, positioned in the piston bore. A plunger, positioned in the plunger bore, is moveable between an upper position and a lower position. A needle valve member, positioned in the nozzle chamber, is moveable between an open position in which the nozzle outlet is open and a closed position in which the nozzle outlet is blocked. A portion of the plunger bore and the plunger define a fuel pressurization chamber that opens to the nozzle chamber. A variable rate return spring biases the piston and plunger toward their retracted positions. The variable rate return spring has a relatively low spring rate when the piston is a first distance away from its retracted position and has a relatively high spring rate when the piston is at a second distance away from its retracted position.