1. A fuel injector comprising:

an injector body defining a guide bore, a low pressure space, a trapped volume and a fuel pressurization chamber in fluid communication with a nozzle outlet;

a needle valve member positioned in said injector body and being movable between an inject position in which said fuel pressurization chamber is open to said nozzle outlet, and a closed position in which said nozzle outlet is blocked to said fuel pressurization chamber;

said needle valve member including a guide portion and a lifting hydraulic surface exposed to fluid pressure in said fuel pressurization chamber, and a closing hydraulic surface exposed to fluid pressure in said trapped volume;

at least one of said injector body and said needle valve member defining a pressure decay passage extending between said trapped volume and said low pressure space; and

fluid communication between said trapped volume and said fuel pressurization chamber consists essentially of a clearance between said guide portion and said guide bore.

2. The fuel injector of claim 1 wherein said pressure decay passage is sufficiently restrictive to fluid flow that pressure in said trapped volume rises significantly above pressure in said low pressure space by an end of an injection event; but

said pressure decay passage is sufficiently open to fluid flow that pressure in said trapped volume drops to said pressure in said low pressure space before a subsequent injection event.

said needle valve member including a guide portion and a lifting hydraulic surface exposed to fluid pressure in said fuel pressurization chamber, and a closing hydraulic surface exposed to fluid pressure in said trapped volume;

at least one of said injector body and said needle valve member defining a pressure decay passage extending between said trapped volume and said low pressure space; and

fluid communication between said trapped volume and said fuel pressurization chamber consists essentially of a clearance between said guide portion and said guide bore.

2. The fuel injector of claim 1 wherein said pressure decay passage is sufficiently restrictive to fluid flow that pressure in said trapped volume rises significantly above pressure in said low pressure space by an end of an injection event; but

said pressure decay passage is sufficiently open to fluid flow that pressure in said trapped volume drops to said pressure in said low pressure space before a subsequent injection event.

 

3. The fuel injector of claim 1 wherein said pressure decay passage includes a cylindrically shaped pressure decay port defined by said injector body.

4. The fuel injector of claim 1 further comprising a compression spring operably positioned in said trapped volume to bias said needle valve member toward said closed position.

5. The fuel injector of claim 1 wherein;

said pressure decay passage includes a pressure decay port extending between said guide bore and said low pressure space.

6. The fuel injector of claim 5 wherein said injector body defines a nozzle chamber;

said lifting hydraulic surface is positioned in said nozzle chamber; and

said pressure decay port opens into said guide bore at a location that is within 5% of a middle location between said trapped volume and said nozzle chamber.

7. The fuel injector of claim 1 wherein said pressure decay passage is a cylindrically shaped pressure decay port extending between said trapped volume and said low pressure space.

8. The fuel injector of claim 1 wherein said needle valve member includes a spacer positioned in said trapped volume; and

said pressure decay passage includes a pressure decay port extending between said low pressure space and said trapped volume, and further includes a clearance area between said spacer and said trapped volume.

9. A fuel injector comprising:

an injector body defining a guide bore, a low pressure space, a trapped volume and a fuel pressurization chamber in fluid communication with a nozzle outlet;

a needle valve member positioned in said injector body and being movable between an inject position in which said fuel pressurization chamber is open to said nozzle outlet, and a closed position in which said nozzle outlet is blocked to said fuel pressurization chamber;

said needle valve member including a guide portion and a lifting hydraulic surface exposed to fluid pressure in said fuel pressurization chamber, and a closing hydraulic surface exposed to fluid pressure in said trapped volume;

at least one of said injector body and said needle valve member defining a pressure decay passage extending between said trapped volume and said low pressure space, and said pressure decay passage including a cylindrically shaped pressure decay port defined by said injector body;

a compression spring operably positioned in said injector body to bias said needle valve member toward said closed position; and

fluid communication between said trapped volume and said fuel pressurization chamber consists essentially of a clearance between said guide portion and said guide bore.

10. The fuel injector of claim 9 wherein said pressure decay passage is sufficiently restrictive to fluid flow that pressure in said trapped volume rises significantly above pressure in said low pressure space by an end of an injection event; but

said pressure decay passage is sufficiently open to fluid flow that pressure in said trapped volume drops to said pressure in said low pressure space before a subsequent injection event.

11. The fuel injector of claim 10 wherein

said pressure decay passage includes a pressure decay port extending between said guide bore and said low pressure space.

12. The fuel injector of claim 11 wherein said injector body defines a nozzle chamber;

said lifting hydraulic surface is positioned in said nozzle chamber; and

said pressure decay port opens into said guide bore at a location that is within 5% of a middle location between said trapped volume and said nozzle chamber.

13. The fuel injector of claim 10 wherein said pressure decay passage is a cylindrically shaped pressure decay port extending between said trapped volume and said low pressure space.

14. The fuel injector of claim 10 wherein said needle valve member includes a spacer positioned in said trapped volume; and

said pressure decay passage includes a pressure decay port extending between said low pressure space and said trapped volume, and further includes a clearance area between said spacer and said trapped volume.

15. A nozzle assembly comprising:

a body defining a guide bore, a fuel circulation passage, a trapped volume and a nozzle chamber in fluid communication with a nozzle outlet;

a needle valve member positioned in said body and being movable between an inject position in which said nozzle chamber is open to said nozzle outlet, and a closed position in which said nozzle outlet is blocked to said nozzle chamber;

said needle valve member including a guide portion, a lifting hydraulic surface exposed to fluid pressure in said nozzle chamber, and a closing hydraulic surface exposed to fluid pressure in said trapped volume;

at least one of said injector body and said needle valve member defining a pressure decay passage extending between said trapped volume and said fuel circulation passage;

said pressure decay passage being sufficiently restrictive to fluid flow that pressure in said trapped volume rises significantly above pressure in said fuel circulation passage by an end of an injection event, but said pressure decay passage is sufficiently open to fluid flow that pressure in said trapped volume drops to said pressure in said fuel circulation passage before a subsequent injection event; and

fluid communication between said trapped volume and said fuel pressurization chamber consists essentially of a clearance between said guide portion and said guide bore.

16. The nozzle assembly of claim 15 wherein said body includes a casing component, a tip component, a spring cage component and a spacer component;

said trapped volume being defined by said tip component, said spring cage component and said spacer component; and

a portion of said fuel circulation passage being an area between said spring cage component and said casing component.

17. The nozzle assembly of claim 16 wherein said tip component defines said nozzle outlet, said nozzle chamber and said guide bore.

18. The nozzle assembly of claim 16 further comprising a compression spring operably positioned in said trapped volume to bias said needle valve member toward said closed position.

19. The nozzle assembly of claim 16 wherein said pressure decay passage includes a cylindrically shaped bore.

20. The nozzle assembly of claim 15 wherein

said pressure decay passage includes a pressure decay port extending between said guide bore and said fuel circulation passage, and said pressure decay port opens into said guide bore at a location that is within 5% of a middle location between said trapped volume and said nozzle chamber.