TECHNICAL FIELD

The present invention relates to a coated face of a needle valve member of a diesel fuel injector adapted to be controlled with a close loop strategy.

BACKGROUND OF THE INVENTION

To improve diesel fuel injection in the cylinders of an internal combustion engine, a close loop strategy is implemented wherein a command unit receives signal relevant of the position of the needle valve member of the fuel injectors. Several design of electric switch have been designed a most efficient one utilise the tip end of the needle and the complementary seating face to establish an electrical contact. Unfortunately, said faces wear under pressure and a protective coating having mechanical durability and electrical conductivity is required.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to resolve the above mentioned problems in providing a needle valve member of a fuel injector controlled with a close loop control strategy. The needle has a cylindrical core with a tip end extending along a longitudinal axis, said tip end defining a sloped sealing face adapted to contact, in a closed position, a complementary seat face of a nozzle body and therefore to close an electric circuit enabling an electrical signal relevant of said closed position to be delivered.

Advantageously, said sloped sealing face is coated in a first zone with an electrically isolating wear resistant material adapted to last for the life time of the injector and, remaining electrically conductive in a distinct second zone.

Also, the closed position of the needle generates in said sloped sealing face contact pressures having an uneven spectrum, said pressures being maximum in said first zone.

Also, said conductive second zone is adjacent to said first zone.

Also, a pick contact pressure registers in a pick section of said first zone, the second zone starting approximately 0.2 mm away from said pick section. In an embodiment, said second zone is uncoated.

In another embodiment, said second zone is coated with an electrically conductive coating.

Also, said second zone is arranged between said first zone and a third zone also coated with said wear resistant coating.

Also, the first zone is the upper portion of the sloped face joining the core of the needle and, the third zone extends toward the pointy extremity of the tip face.

Also, said second zone longitudinally extends on a width of about 0.5 mm.

Also, said wear resistant coating is a diamond like coating DLC.

Also, said wear resistant coating is about 3μιη to 4μιη thick.

The invention further extends to a diesel fuel injector adapted to be controlled with a close loop control strategy, said injector comprising a nozzle assembly having a body defining a valve seat face and a needle valve member guided in a bore of the body and adapted to move between an open position and a closed position, said needle being as previously described.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which:

Figure 1 represents the tip end of an injector wherein a needle tip contacts a seat face of a body, said injector tip end being designed in relation with a plot of the contact pressure along the needle tip face.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a diesel fuel injector 10, a needle valve member 12 guided in a bore 14 of a nozzle body 16 between a closed position CP and an open position OP is indirectly controlled by a small electrovalve commanding a strong hydraulic drive. To make this indirect control more accurate, a close loop strategy is implemented wherein an ECU computes a command signal sent to said electrovalve as a function of an input signal representative of the position of the needle 12. More precisely, the needle 12 extends along a longitudinal axis X in an elongated cylindrical core 18 provided at an extremity with a tip end 20 defining a sloped face 22 that, in said closed position CP is in sealing contact against a complementary seating face 24 arranged at the blind end of the bore 14 preventing downstream fuel flow toward spray holes 26 arranged through the wall 28 of said body and that, in said open position OP is lifted away from said seating face 24 enabling said downstream flow and fuel spray.

Although many alternative geometries exist, the example depicted shows a conical seating face 24 and a sloped face 22 comprising several frustoconical portions extending from an upper edge 30 intersection with the needle core 18 to a pointy extremity 32 that is in a small sac 34 defined at the very end of the bore 14, the spray holes 26 opening in said sac 34.

When urged in closed position CP surface contact pressures raise and, on the figure, the plot shows the uneven spectrum of said pressures along said complementary faces. Said pressures reach a pick pressure PP, close to 2000 MPa at a pick section 36 defining a circular line, with almost no width. From said pick section 36, the pressure contacts drop on either sides, the short upper conical portion 38 upwardly extending to the upper edge 30 still experiencing quite high pressures, said upper portion 38 being identified as the area of maximum pressures. In the wider lower conical portion 40 downwardly extending from said pick section 36 toward the pointy extremity 32, the pressures also drop down to zero.

To prevent wear of the faces the sloped face 22 is coated with a diamond like coating (DLC) that is mechanically hard and resistant, and electrically isolating. Said DLC coating layer is of 3 to 4 μιη thickness and is arranged to last for the life of the injector which is to be understood as adapted to last for the life time of the engine the injector equips that, for a vehicle means several hundreds of thousands of kilometers representing for the injector 10 more than 500 million fuel injections or displacements between the open position OP and the closed position CP.

The close loop strategy requires to switch an electric contact that alternatively opens and closes an electric circuit and, to establish said electrical contact the sloped face 22 is provided with an electric conductive zone 42 that is arranged in said lower portion 40, below the pick section 36. The conductive zone 42 is free from DLC and in closed position CP it is in surface contact with an area of the seating face 24. The sloped face 22 therefore defines a first zone 44, DLC coated, then a second zone which is said conductive zone 42 then below a third zone 46, also DLC coated.

To achieve said electric conductivity of said zone 42, the relevant area of the sloped face can be protected when DLC is applied and then remain bear or, alternatively, it can be coated with a specific electrically conductive coating for instance WC-CH (tungsten carbide) or taC (tetragonal amorphous Carbon).

In the example shown, the conductive zone 42 extends over a width W of approximately half a millimeter downwardly extending toward the pointy extremity 32 from an upper limit 48 close to the pick section 36, that is DLC coated, to a lower limit 50. The distance D from said pick section 36 to said upper limit 48 of the conductive zone 42 is of few tenth of a millimeter, 0.2mm for instance. In the shown example the conductive zone 42 is continuous extending all around the needle. In an alternative not represented, said conductive zone can be interrupted and comprise one or more sections enabling said electric contact.

LIST OF REFERENCES

X longitudinal axis

OP open position

CP closed position

D distance

W width

PP pick pressure

10 injector

12 needle valve member

14 bore

16 nozzle body

18 needle core

20 tip end

22 sloped face

24 seating face

26 spray holes

28 wall of the body

30 upper edge

32 pointy extremity

34 sac

36 pick section

38 upper portion

40 lower portion

42 conductive zone - second zone

44 first zone

46 third zone

48 upper limit of the conductive zone

50 lower limit of the conductive zone