Fuel-injection system for an internal-combustion engine

A fuel-injection system for an internal-combustion engine, comprising:

• at least one fuel injector comprising:
• a) a fuel-metering servo valve comprising:
• (1) an electric actuator,
• (2) a metering chamber, and
• (3) an open/close element, which is movable in response to the action of the electric actuator between a lowered end-of-travel position, in which the open/close element rests against a valve seat for closing a discharge passage coming out of said metering chamber, and a raised end-of-travel position, in which the open/close element leaves said discharge passage open;
• and
• b) an open/close needle, movable in response to the elastic action of a positioning spring and to the pressure in said metering chamber for carrying out fuel injections into a cylinder of the internal-combustion engine;
• and
• an electronic control unit configured to issue, to said electric actuator, a first electrical command, for causing said open/close needle to perform a first opening travel for carrying out a pilot fuel injection, and a second electrical command for causing said open/close needle to perform a second opening travel for carrying out a main fuel injection in a fuel injection event into said cylinder, the first and second electrical commands being separated in time by an electrical dwell time such that the main fuel injection starts without hydraulic dwell time with respect to the end of the pilot fuel injection;
• wherein sizing of the injector and movement of said open/close element for closing said discharge passage are such that the pressure in said metering chamber balances, for a certain time interval, together with the force exerted by said positioning spring, the opening axial force exerted by the fuel pressure on said open/close needle at the end of the pilot fuel injection in such a way that, if no subsequent main injection were commanded, the curve that represents the lift of said open/close needle (7) as a function of time along the closing travel of the pilot fuel injection has an inflection point at a third lift value between a first lift value and a second lift value to enable a temporary stay of the open/close needle substantially at said third lift value;
• wherein considering a characteristic curve that represents the fuel flow rate injected as a function of the lift of the open/close needle at a given reference pressure, said first lift value is assumed equal to a lift value that enables a fuel flow rate of 7% of a value of maximum flow rate to be obtained and said second lift value is assumed as 30% of said first lift value;
• and
• wherein said second electrical command is issued to said electric actuator so as to start the second opening travel of said open/close needle when said open/close needle is between said first and second lift values.