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A nozzle assembly for a fuel injector having a needle valve for controlling an injector valve orifice, the needle valve being subjected to a spring force and a fluid pressure force. The needle valve registers with a fuel delivery orifice and is movable into and out of engagement with a valve seat su

A nozzle assembly for a fuel injector having a needle valve for controlling an injector valve orifice, the needle valve being subjected to a spring force and a fluid pressure force. The needle valve registers with a fuel delivery orifice and is movable into and out of engagement with a valve seat surrounding the orifice. The needle valve is normally biased to a valve closing position by a spring located in a spring cage. The spring cage becomes pressurized upon movement of the needle valve toward the needle valve open position, whereby a positive pressure is maintained in the spring chamber throughout the fuel injection event. A calibrated bleed orifice allows the spring chamber pressure to decay to a low value between injection events.

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1. A fuel injector for an internal combustion engine comprising:a cylinder housing defining a cylinder opening;a pump plunger in the cylinder housing defining a high pressure fuel pumping chamber, an engine camshaft follower connected to the plunger;a plunger spring urging the plunger in a direction

1. A fuel injector for an internal combustion engine comprising:a cylinder housing defining a cylinder opening;a pump plunger in the cylinder housing defining a high pressure fuel pumping chamber, an engine camshaft follower connected to the plunger;a plunger spring urging the plunger in a direction to increase the volume of the high pressure fuel pumping chamber, the camshaft follower driving the plunger against the force of the plunger spring to create an injection pressure in the fuel pumping chamber;a needle valve body having a needle valve opening;a fuel delivery orifice in the needle valve body, the needle valve closing the fuel delivery orifice when it is moved to a closed position;a needle valve spring urging the needle valve to the closed position;a spring cage enclosing the needle valve spring, the spring cage being in fluid communication with the needle valve opening when the needle valve is in a closed position;a sealing shoulder on the needle valve that is engageable with the spring cage to trap pressurized fuel in the spring cage during a fuel injection event; anda calibrated fuel vent in communication with the needle valve opening whereby pressurized fuel in the spring cage is discharged to a low pressure region of the injector when a fuel injection event is completed, the presence of pressurized fluid in the spring chamber reducing a tendency for cavitation to occur upon movement of the needle valve. 2. The fuel injector set forth in claim 1 wherein the needle valve, when it is in an open orifice position, has a pressure area that is pressurized by cage pressure in the spring chamber, whereby the needle valve is closed under the force of the needle valve spring supplemented by a pressure force created by cage pressure. 3. The fuel injector set forth in claim 2 wherein the calibrated fuel vent comprises an orifice in the spring cage, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pressure decays following the injection event. 4. The fuel injector set forth in claim 2 wherein the calibrated fuel vent comprises an orifice in the needle valve body, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pressure decays following the injection event. 5. The fuel injector set forth in claim 1 wherein the calibrated fuel vent comprises an orifice in the spring cage, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pressure decays following the injection event. 6. The fuel injector set forth in claim 1 wherein the calibrated fuel vent comprises an orifice in the needle valve body, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pressure decays following the injection event. 7. A fuel injector for an internal combustion engine comprising:a cylinder housing defining a cylinder opening;a pump plunger in the cylinder housing defining a high pressure fuel pumping chamber, an engine camshaft follower connected to the plunger;a plunger spring urging the plunger in a direction to increase the volume of the high pressure fuel pumping chamber, the camshaft follower driving the plunger against the force of the plunger spring to create an injection pressure in the fuel pumping chamber;a needle valve body having a needle valve opening;a fuel delivery orifice in the needle valve body, the needle valve closing the fuel delivery orifice when it is moved to a closed position;a needle valve spring urging the needle valve to the closed position;a spring cage enclosing the needle valve spring, the spring cage being in fluid communication with the needle valve opening whe n the needle valve is in a closed position;a sealing shoulder on the needle valve that is engageable with the spring cage to trap pressurized fuel in the spring cage during a fuel injection event; anda calibrated fuel vent in communication with the needle valve opening whereby pressurized fuel in the spring cage is discharged to a low pressure region of the injector when a fuel injection event is completed, the presence of pressurized fluid in the spring chamber reducing a tendency for cavitation to occur upon movement of the needle valve, the needle valve body comprising a first body portion defining the spring cage and a second body portion defining the needle valve opening;a nozzle nut surrounding the first and second body portions; anda releasable connection between the nozzle nut and the cylinder housing whereby the cylinder housing and the first and second body portions are held in stacked, assembled relationship. 8. The fuel injector set forth in claim 7 wherein the calibrated fuel vent comprises an orifice in the spring cage, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pressure decays following the injection event. 9. The fuel injector set forth in claim 7 wherein the calibrated fuel vent comprises an orifice in the needle valve body, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pressure decays following the injection event. 10. A fuel injector for an internal combustion engine comprising:a cylinder housing defining a cylinder opening;a pump plunger in the cylinder housing defining a high pressure fuel pumping chamber, an engine camshaft follower connected to the plunger;a plunger spring urging the plunger in a direction to increase the volume of the high pressure fuel pumping chamber, the camshaft follower driving the plunger against the force of the plunger spring to create an injection pressure in the fuel pumping chamber;a needle valve body having a needle valve opening;a fuel delivery orifice in the needle valve body, the needle valve closing the fuel delivery orifice when it is moved to a closed position;a needle valve spring urging the needle valve to the closed position;a spring cage enclosing the needle valve spring, the spring cage being in fluid communication with the needle valve opening when the needle valve is in a closed position;a sealing shoulder on the needle valve that is engageable with the spring cage to trap pressurized fuel in the spring cage during a fuel injection event; anda calibrated fuel vent in communication with the needle valve opening whereby pressurized fuel in the spring cage is discharged to a low pressure region of the injector when a fuel injection event is completed, the presence of pressurized fluid in the spring chamber reducing a tendency for cavitation to occur upon movement of the needle valve, the needle valve body comprising a first body portion defining the spring cage and a second body portion defining the needle valve opening;the needle valve, when it is in an open orifice position, having a pressure area that is pressurized by cage pressure in the spring chamber, whereby the needle valve is closed under the force of the needle valve spring supplemented by a pressure force created by cage pressure;a nozzle nut surrounding the first and second body portions anda releasable connection between the nozzle nut and the cylinder housing whereby the cylinder housing and the first and second body portions are held in stacked, assembled relationship. 11. The fuel injector set forth in claim 10 wherein the calibrated fuel vent comprises an orifice in the spring cage, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pres sure decays following the injection event. 12. The fuel injector set forth in claim 10 wherein the calibrated fuel vent comprises an orifice in the needle valve body, which discharges pressurized fluid in the spring chamber at a controlled rate whereby cage pressure is maintained at a high level throughout an injection event and cage pressure decays following the injection event.