Piezoelectric injector for direct fuel injection

Abstract

A piezo injector for direct fuel injection may comprise an injector body, an actuator unit having a piezoelectric actuator with a head plate and a base plate, a hydraulic coupler having a coupler piston, a coupler cylinder, and a coupler spring, and a nozzle unit having a nozzle needle arranged in a nozzle body. The piezoelectric actuator is surrounded by a corrugated tube braced between the head plate and the base plate such that the piezoelectric actuator has a preload imparted to it. The coupler spring forces the coupler piston against a face side, facing toward the coupler piston, of the nozzle needle.

Claims

1. A piezo injector for direct fuel injection, the piezo injector comprising: an injector body, an actuator unit having a piezoelectric actuator with a head plate and a base plate, wherein the piezoelectric actuator is surrounded by a corrugated tube braced between the head plate and the base plate such that the piezoelectric actuator has a preload imparted to it, a hydraulic coupler having a coupler piston, a coupler cylinder, and a coupler spring, and a nozzle unit having a nozzle needle arranged in a nozzle body, wherein the coupler cylinder is formed in the base plate of the piezoelectric actuator, wherein the coupler spring forces the coupler piston against a face side, facing toward the coupler piston, of the nozzle needle.

2. The piezo injector as claimed in claim 1, wherein the corrugated tube comprises a maraging steel.

3. The piezo injector as claimed in claim 1, wherein the piezo injector does not comprise any sealing material composed of a plastic.

4. The piezo injector as claimed in claim 1, wherein the injector body is a unipartite tube piece fixedly connected at one side to the head plate and at an opposite side to the nozzle body.

5. The piezo injector as claimed in claim 1, wherein the nozzle needle opens outwardly.

6. The piezo injector as claimed in claim 1, wherein the nozzle unit has a nozzle spring for generating a closing force which acts on the nozzle needle.

7. The piezo injector as claimed in claim 6, wherein the nozzle spring, at one side, is fastened to the nozzle body and, at the other side, presses against a spring plate which is attached to the nozzle needle.

8. The piezo injector as claimed in claim 1, wherein a fuel film is arranged in the hydraulic coupler.

9. The piezo injector as claimed in claim 8, wherein the fuel film has a thickness in the range from 0.05 mm to 0.3 mm.

10. An internal combustion engine comprising: at least one combustion chamber one or more piezo injectors for direct fuel injection into the at least one combustion chamber, each piezo injector comprising: an injector body, an actuator unit having a piezoelectric actuator with a head plate and a base plate, a hydraulic coupler having a coupler piston, a coupler cylinder, and a coupler spring, and a nozzle unit having a nozzle needle arranged in a nozzle body, wherein the coupler cylinder is formed in the base plate of the piezoelectric actuator, wherein the piezoelectric actuator is surrounded by a corrugated tube braced between the head plate and the base plate such that the piezoelectric actuator has a preload imparted to it, wherein the coupler spring forces the coupler piston against a face side, facing toward the coupler piston, of the nozzle needle.

11. The internal combustion engine as claimed in claim 10, wherein the corrugated tube comprises a maraging steel.

12. The internal combustion engine as claimed in claim 10, wherein the piezo injector does not comprise any sealing material composed of a plastic.

13. The internal combustion engine as claimed in claim 10, wherein the injector body is a unipartite tube piece fixedly connected at one side to the head plate and at an opposite side to the nozzle body.

14. The internal combustion engine as claimed in claim 10, wherein the nozzle needle opens outwardly.

15. The internal combustion engine as claimed in claim 10, wherein the nozzle unit has a nozzle spring for generating a closing force which acts on the nozzle needle.

16. The internal combustion engine as claimed in claim 15, wherein the nozzle spring, at one side, is fastened to the nozzle body and, at the other side, presses against a spring plate which is attached to the nozzle needle.

17. The internal combustion engine as claimed in claim 10, wherein a fuel film is arranged in the hydraulic coupler.

18. The internal combustion engine as claimed in claim 17, wherein the fuel film has a thickness in the range from 0.05 mm to 0.3 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further embodiments and advantages of the piezo injector will emerge from the following description of an exemplary embodiment and on the basis of FIGS. 1 to 3.

(2) In the Figures:

(3) FIG. 1 is a schematic illustration of a cross section through a piezo injector according to teachings of the present disclosure,

(4) FIG. 2 shows an exterior view of the corrugated tube according to teachings of the present disclosure, and

(5) FIG. 3 is an enlarged illustration of a sub-region of the corrugated tube illustrated in FIG. 2.

DETAILED DESCRIPTION

(6) The illustrated components and the proportions of the components relative to one another are not to be regarded as being true to scale.

(7) The piezo injector 1 illustrated schematically in a cross section in FIG. 1 comprises a piezoelectric actuator 10, which may in particular have a piezo stack. The piezoelectric actuator 10 is advantageously arranged within a corrugated tube 13 which is welded to a head plate 11 and to a base plate 12 of the piezoelectric actuator 10. In this way, the piezoelectric actuator 10 is sealed off with respect to fuel by way of exclusively metallic seals. In particular, no seals composed of plastics, such as for example diaphragms composed of elastomers, are used. In this way, particularly robust and reliable sealing of the piezoelectric actuator 10 is achieved.

(8) The corrugated tube 13 may be produced from a maraging steel. Maraging steels are advantageously of high-strength form, and have a high strength and good ductility. They comprise substantially carbon-free alloys, and are alloyed with a high fraction of nickel, for example greater than 12%. They may also comprise other alloy elements, for example aluminum, molybdenum, copper, niobium, cobalt and/or titanium.

(9) The corrugated tube 13 is illustrated in FIG. 2 in an exterior view and in FIG. 3 in an enlarged detail view. The shaping of the corrugations of the corrugated tube 13 is selected such that a desired spring constant is obtained, without the admissible stresses being exceeded in order to achieve a predefined strength throughout the service life. Available design parameters for the corrugated tube 13 are in particular the wall thickness, the period PER and the amplitude AMP of the corrugations. For example, an outer diameter of the corrugated tube 13 of 9.2 mm, a wall thickness of 0.2 mm, a corrugation amplitude AMP of 0.4 mm and a corrugation period PER of 2 mm give rise to a spring constant of 1.8 N/mm and a possible preload force of 800 N in the case of a length of the corrugated tube 13 of 63 mm.

(10) To obtain a predefined preload, for example of approximately 800 N, the corrugated tube 13 is fixed between the head plate 11 and the base plate 12 such that the piezoelectric actuator 10 is braced with a predefined preload between the head plate 11 and the base plate 12. The fixing of the head plate 11 and of the base plate 12 can be realized for example by way of welding, in particular laser welding. The corrugated tube 13 is in particular connected to the head plate 11 and to the base plate 12 such that the piezoelectric actuator 10 is sealed off in hermetically sealed fashion with respect to the surroundings.

(11) The head plate 11 may provide a feed of electrical connections from an electrical plug connector 43 to the piezoelectric actuator 10. Furthermore, the head plate 11 may comprise a fuel supply 44 for the supply of highly pressurized fuel.

(12) The transmission of forces from the piezoelectric actuator 10 to a nozzle needle 31 is realized by way of a hydraulic coupler, which comprises a coupler cylinder 20, a coupler piston 21 and a coupler spring 22. The coupler spring 20 is advantageously formed in the base plate 12 of the piezoelectric actuator 10. The base plate 12 and the coupler cylinder 20 may thus be formed from a single piece of metal. The coupler piston 21 is inserted into the coupler cylinder 20 with a suitable fit clearance, wherein the fit clearance is preferably selected to be in a range from 2 m to 6 m in order to achieve suitable dynamic transmission characteristics of the hydraulic coupler.

(13) The coupler piston 21 may be of hollow-bored form in order to reduce the moving mass. The coupler piston 21 is forced against a face side of the nozzle needle 31 by way of a coupler spring 22. In this way, an absence of clearances in the drive is ensured. In the exemplary embodiment of FIG. 1, the coupler spring 22 is arranged between the coupler cylinder 20 and the injector body 40 which forms the outer casing of the piezo injector 1. It would alternatively also be possible for the coupler spring 22 to be arranged in the interior of the coupler piston 21, in particular in the intermediate space between the coupler piston 21 and that shell surface of the nozzle needle 31 which faces toward the coupler piston 21.

(14) A thin fuel layer (not illustrated) may be situated in the hydraulic coupler between a face side, facing toward the base plate 12, of the coupler piston 21 and the base plate 12, which is simultaneously formed as coupler cylinder 20. The thickness of said thin fuel layer is selected to be as small as possible in order to achieve the greatest possible stiffness of the coupler. A minimal thickness of said thin fuel layer is determined by the required assembly tolerances and the differences in length expansion between the piezoelectric actuator 10 and the injector body 40 in the event of temperature changes owing to their different coefficients of thermal expansion. The layer thickness of the fuel layer may lie in a range from 0.05 mm to 0.3 mm.

(15) The nozzle needle 31 of the piezo injector 1 is an outwardly opening nozzle needle which is guided in a nozzle body 30. The needle seat of the nozzle needle 31 may for example be in the form of a conical shell surface which, in the nozzle body 30, is forced against a hollow conical surface in order to realize the sealing function. A sealing force which acts on the nozzle needle 31 is generated by a nozzle spring 32 which is braced between the nozzle body 30 and a spring plate 33 which is connected to the nozzle needle 31. The spring plate 33 may in particular be pressed onto the nozzle needle 31.

(16) The spring force of the nozzle spring 32 may be set such that a resultant force composed of the nozzle spring force minus the hydraulically acting needle opening force and minus a contact force of the hydraulic coupler in the presence of the maximum admissible injection pressure (rail pressure) still ensures an adequate needle seat force. The hydraulically acting needle opening force can be calculated from the cross-sectional area of the needle seat and the injection pressure, and the contact force of the hydraulic coupler corresponds to the spring force of the coupler spring 22. The resultant needle seat force is advantageously set to be in the range from 20 N to 50 N in the presence of the maximum admissible injection pressure.

(17) The injector body 40 of the piezo injector 1 regionally forms an outer casing of the piezo injector 1, and may in particular be manufactured from steel. The injector body 40 is preferably formed in one piece, in particular from one tube piece. The injector body 40 may in particular be welded at one side, by way of a weld seam 41, to the head plate 11 of the piezoelectric actuator 10, and at the opposite side, by way of a weld seam 42, to the nozzle body 30. During the process of assembly of the injector body 40, the layer thickness of the thin fuel layer in the coupler piston 21 can be set.

(18) The piezo injector 1 constructed in this way is characterized in particular in that it is relatively easy to produce and is highly robust.

(19) The invention is not restricted by the description based on the exemplary embodiments. Rather, the invention encompasses every novel feature and every combination of features, which in particular includes every combination of features in the patent claims, even if said feature or said combination is itself not explicitly mentioned in the patent claims or exemplary embodiments.