Patent ID: 8628031

Claim:
A fuel injector comprising: a source of fluid under pressure proportional to a fuel injection pressure; a needle in a needle chamber moveable between a first needle position engaging a needle seat and a second needle position spaced away from the needle seat; a needle control piston moveable between a first needle control piston position in contact with a needle control piston seat and a second needle control piston position displaced from the needle control piston seat; a linkage between the needle and the needle control piston, the needle reaching the first needle position before the control piston reaches the first control piston position; a needle control valve coupled to the source of fluid under pressure, to a vent and to a first side of the needle control piston; the needle control valve controlling the difference in the fluid pressure between a first and a second side of the needle control piston to controllably force the needle control piston toward the needle control piston seat with a force proportional to the fuel injection pressure when the fluid pressure on the first side of the needle control piston is higher than the fluid pressure on the second side of the needle control piston; the needle control piston, responsive to the control valve, being in an intermediate position between the first and second needle control piston position pushing the needle against the needle seat through the linkage with a force proportional to the fuel injection pressure when the fuel injection pressure is below a threshold injection pressure, and the needle control piston being in the first needle piston position pushing the needle against the needle seat through the linkage with a force not rising with injection pressure for fuel injection pressures above the threshold injection pressure, the force not rising because of the limitation on the compression of the linkage caused by the limitation of the movement of the control piston beyond the first position.