The present invention relates generally to servo injector and, more particularly to its piezoelectric control valve.

BACKGROUND OF THE INVENTION

Servo injectors provided with a piezoelectric or magneto restrictive actuator are usually provided with hydraulic lash adjuster arranged between the actuator and the control valve of the injector, this in order to transmit force of displacement from the actuator to the valve. Therefore, the valve commutes between an open position, enabling fuel at high pressure to exit a control chamber and, a closed position forbidding said exit of fuel.

DEI 02009000203 and DEI 02010029106 disclose hydraulic lash adjusters, also identified as hydraulic coupler, of similar embodiments. The adjusters comprise an actuator-side member, connected to the actuator, and a valve-side member connected to the valve. The actuator-side member is solicited toward the actuator by a very stiff cage spring while the valve-side member is solicited by a lesser stiff coil spring toward a closed position of the control valve. Between the actuator-side member and the valve-side member is provided a chamber which in use is filled with fuel, so that the displacement of the actuator is transmitted to the valve-side member.

Recurring problems are the important overall length of the lash adjuster which consequently has a mass detrimental to fast valve movements and also, the complexity of the lash adjuster which adds technical difficulties and cost.

SUMMARY OF THE INVENTION

It is an object of the invention to resolve or at least mitigate the above mentioned problem by providing a piezoelectric control valve for a servo injector, the valve comprising a piezoelectric actuator controlling a valve member. The piezoelectric actuator is adapted to be arranged in a bore provided in the injector body between axially opposed first and second transverse faces in order to control, in use, opening and closing of a return orifice enabling the hydraulic control of the displacements of a needle valve member enabling or forbidding fuel injection. The piezoelectric actuator comprises an encapsulating tube extending along a main axis X from a first extremity to a second extremity closed by a closing member and, a piezoelectric stack axially compressed inside the encapsulating tube. The upper extremity of the stack is in contact with the inner face of the closing member and, the lower extremity of the stack is in contact with a pusher member adapted to cooperate, in use, with the valve member in order to open or close said return orifice.

The piezoelectric control valve further comprises an adjusting member adapted, in use, to permanently maintain a compression force in the encapsulating tube independently of the energizing state of the piezoelectric stack and of the thermal expansion of the injector.

More particularly, the adjusting member is a compression spring arranged in use between the outer face of the closing member and said second transverse face.

In an embodiment, the spring is a leaf spring assembly which may comprise a single or a plurality of individual leaf spring. Each leaf spring is a dome shaped washers.

The invention further extends to a servo injector having a body which peripheral walls internally define a bore extending along a main axis between a first transverse face and a distal second transverse face, the servo injector being provided with a piezoelectric control valve as previously, the piezoelectric actuator of said valve being arranged in the bore.

Particularly, the piezoelectric actuator is arranged so that the compression force generated by the adjusting member is integrally transmitted via the encapsulating tube.

Specifically, the servo injector is arranged to enable fuel injection when energizing the piezoelectric actuator and, to forbid fuel injection when not- energizing the piezoelectric actuator.

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 is an axial section of an injector provided with a

piezoelectric control valve as per the invention.

Figure 2 is a sketch of the piezoelectric control valve of figure 1, valve being energized.

Figure 3 is similar to Figure 2, the valve not being energized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In reference to the figure 1 is presented a fuel injector 10 which extends along a main axis X arbitrarily drawn vertically. To ease and clarify the description said non- limiting orientation may be utilized through words such as "top, bottom, upper, under..."

The fuel injector 10 comprises an injector body 12 constituted of several elements fixedly coaxially maintained together by an injector nut. As it is well known, in the body 12 is arranged a high pressure fuel circuit 14 extending from an inlet 16, usually on the top of the injector, to injection holes 18 arranged in the nozzle 20 of the injector 10, at the lower tip extremity of the injector 10. A lateral branch of said high pressure circuit 14 is diverted toward a control chamber 22 wherefrom depart a return circuit 24 upwardly extending toward an outlet 26 wherefrom the fuel exits the injector 10 and flows at low pressure to return to a fuel tank. In the return circuit 26 is arranged a control valve 30 adapted to open or close a return orifice 42, therefore controlling the pressure in the control chamber 22. In the nozzle 20 is provided a needle valve 28 axially extending between a top extremity protruding in the control chamber 22 and a tip cooperating with the injection holes 18 in order to enable or forbid fuel injection.

The control valve assembly 30 of the present invention, further detailed in reference to schematic figures 2 and 3, comprises a piezoelectric actuator 32 arranged in a bore 44 provided in the injector body 12 and, a control valve member 34 similar to the one described in application WO2015/024692. Although multiple alternatives exist, said control valve member 34 comprises an upper two- way valve 36 normally biased in a closed state CS by a spring 38. The bore 44 defines an internal cylindrical volume limited by a first transverse face 46, at the bottom of the figures, and by a second transverse face 48 drawn at the top of the figures. The stem 40 of said upper two-way valve 36 upwardly protrudes in the center of the first transverse face 46 and, the second transverse face 48, top of the figures, may be provided with a top opening 52 to enable for instance extension of electric wire cables for energizing the piezoelectric actuator 32.

The piezoelectric actuator 32 comprises an encapsulating tube 54 vertically extending from a circular lower face 56 closed by a diaphragm 50 and which is in abutment against the first transverse face 46 to, an upper extremity that is closed by a closing member 58 that has an inner face 60 inside the tube 54 opposed to an outer face 62, outside the tube 54, arranged facing the second transverse face 48. Between said outer face 62 and the second transverse face 48 is compressed a leaf spring assembly 64.

In the illustrated example, the leaf spring assembly 64 comprises the stack- up of a first leaf spring being a dome shape washer 66 having its large circular edge abutting against the outer face 62 of the closing member 58, then an intermediate flat washer 68, above which a second leak spring dome shape washer 70, reversed relative to the first 66, is. The large circular edge of said second washer 70 abuts against a final flat washer 72 arranged against the second transverse face 48 of the cylindrical bore 44. This stack-up arrangement is compressed between the second transverse face 48 and the closing member 58 and, the deflection of the dome shape washers 66, 70, permanently generates a first vertical force Fl compressing the tube 54 against the first transverse face 46. As will be described, the compression force inside the tube varies during operation of the valve 30.

Alternatively to the illustrated example, multiple other type of compression spring can be arranged and, in the case of leaf spring, a single dome washer or to the contrary, more than two dome washers can be utilized.

Inside the tube 54 are axially X encapsulated a spring tube 74, fixed at its upper extremity to the closing member 58, the spring tube 74, a piezoelectric stack 76 arranged inside the spring tube 74 and, a pusher member 78 fixed to the lower extremity of the spring tube 74. During the assembly process, the spring tube 74 is elongated so that, when it is relaxed after fixing both the closing member 58 and pusher member 78, the piezoelectric stack 74 is axially compressed between said two members 58, 78.

The pusher member 78 is externally adapted to cooperate with the stem 40 of the upper two-way valve in order to open or close said upper two-way valve 36 and to do so, the piezoelectric stack 76 can be electrically actuated via wire cables 80 arranged inside the injector body 12.

At rest when not energized, as represented on figure 3, the piezoelectric stack 76 is axially compressed by the spring tube 74 and its length is minimized so the pusher member 78 is lifted away from the stem 40. Indeed there is a gap G between the pusher member 78 and the stem 40 and, the force Fl generated by the leaf spring assembly 64 is integrally transmitted by the encapsulating tube 54 to the first transverse face 46.

The upper two-way valve 36 not being solicited by the actuator 32 remains in its normally closed state CS and, fuel injection is forbidden.

When energized, as represented on figure 2, the piezoelectric stack 76 axially expends and the pusher member 78 downwardly displaces. At first, it closes the gap G and it comes in contact with the stem 40 then, the pusher member 78 continues its displacement generating on the upper two-way valve 36 an downwardly oriented opening force F2 enabling the upper valve 36 to move to an open state OS for enabling fuel injection. The opening force F2 acts in opposite directions, downwardly on the stem 40 and upwardly on the closing member 58. Therefore the compression force inside the encapsulating tube 54 reduces but, it remains always superior to said opening force F2 so, the encapsulating tube 54 is permanently compressed.

As an example, tests have been successfully completed in using a leaf spring assembly 64 generating a force Fl of 600N and a piezoelectric stack 76 that, when energized generates on the stem 40 an opening force F2 of 250N. The tube 54 is then compressed with an axial force of 350N while, when the stack is not energized the compression force inside the tube 54 is 600N.

It is also known that the injector 10 is subject to heat variations and its dimensions vary in operation. The leaf spring 64 is chosen so it always generates a compression force in the tube 54. For instance, in the extreme condition when the axial X length of the bore 44 is maximized and the stack 76 is energized, the encapsulating tube 54 remains compressed.

LIST OF REFERENCES X main axis

Fl force generated by the leaf spring

F2 opening force

OS open state

CS closed state

G gap

10 fuel injector

12 injector body

14 high pressure circuit

16 inlet

18 injection holes

20 nozzle

22 control chamber

24 return circuit

26 outlet

28 needle valve

30 control valve assembly

32 piezoelectric actuator

34 control valve member

36 upper two-way valve

38 spring

40 stem of the upper two-way valve

42 return orifice

44 bore

46 first transverse face

48 second transverse face

50 diaphragm

52 top opening

54 encapsulating tube

56 lower face of the tube

58 closing member

60 inner face of the closing member

62 outer face of the closing member

64 leaf spring assembly

66 first dome shape washer

68 intermediate flat washer

70 second dome shape washer

72 final flat washer

74 spring tube

76 piezoelectric stack

78 pusher member

80 wire cables