WO2015110622A1

WO2015110622A1 - Fuel injector

Info

Publication number: WO2015110622A1
Application number: PCT/EP2015/051450
Authority: WO
Inventors: Guy Hoffmann
Original assignee: Delphi Automotive Systems Luxembourg SA
Current assignee: BorgWarner Luxembourg Automotive Systems SA
Priority date: 2014-01-27
Filing date: 2015-01-26
Publication date: 2015-07-30
Anticipated expiration: 2016-07-27
Also published as: GB201401372D0

Abstract

A fuel injector (8) for a combustion engine comprises a solenoidactuator (11) being lifetime operable reciprocally from a closed position to a fully open position along a main axis (24), the solenoid actuator (11) comprising: an electromagnetic coil (14) wound around the axis (24), a magnetic pole piece (30) associated with the electromagnetic coil (14), and a magneticarmature (28) axiallyspaced apart from themagnetic polepiece (30) by a mechanical air gap (26) maximized when the solenoid actuator (11) is in fully open position. At least one of the pole piece (30) and the armature (28) is made of hard magnetic material having hardness greater than 20 HRC.

Description

Fuel injector TECHNICAL FIELD

This invention relates to a fuel injector of an internal combustion engine, and more particularly to a solenoid actuator of the fuel injector.

BACKGROUND OF THE TNVENTION

Modern direct injection gasoline engines require fuel injectors to operate under extreme conditions of temperature and pressure and with high fuel pressures.

Furthermore, the fuel injector must open and close very rapidly in order to provide multi-pulse injection cycles required for fuel efficiency and low emissions. The injector surfaces, which are subject to sliding and impact contact with other metal surfaces are subject to wear. Electromagnetic fuel injectors such as gasoline solenoid injectors include solenoid actuators. Typically, a solenoid actuator incorporates a solenoid coil and a solenoid armature located between the pole piece of the solenoid and a fixed valve seat. The armature is fixedly arranged to a pintle piloting fuel injection events. The armature typically operates as a movable valve assembly. The injection valve is normally closed due to the closing spring which biases the pintle to the closing position, where the pintle presses against a valve seat of the injection valve and the armature is spaced from the magnetic pole. In order to open the injection valve, i.e. to move the pintle from the closing position to the opening position, the coil of the solenoid actuator is energized so as to generate a magnetic field which attracts the armature towards the magnetic pole against the elastic force exerted by the closing spring. During each step of opening, the stroke of the armature stops when the armature itself impacts against the magnetic pole. Current gasoline direct injectors (GDi), port fuel injectors (PFi) and diesel solenoid injectors as well as high pressure pumps (spill valve) use soft magnetic materials for the armature and pole piece components. These components require coatings like chrome (Cr) or amorphous carbon (a-C) also called DLC (Diamond-Like Carbon) to ensure sufficient wear and impact resistance in areas of contact. These soft materials are typically selected for optimum magnetic actuation performance. In GDi and PFi injectors, the armature and pole piece are typically made of ferritic stainless steels with 12 to 18% Cr.

This coating step within the manufacturing process costs time and money. The future requirements of low emission for combustion engine will increase the speed and frequency of the opening/closing solenoid valve actuation so that the need of wear protection on the pole piece and armature components will also increase. The cost of the coating process will rise in order to maintain a sufficient wear resistance of the components. The repeatability of the thickness of the coating is also considered as a point that is difficult to guarantee without the usage of costly equipments. Furthermore, the coating on the pole piece and on the armature can generate a problem of sticking between the pole piece and the armature after impact.

It is an object of the invention to provide an improved solenoid actuator that overcomes such problems.

SUMMARY OF THE INVENTION

A fuel injector for a combustion engine comprises a solenoid actuator being lifetime operable reciprocally from a closed position to a fully open position along a main axis, the solenoid actuator comprising: an electromagnetic coil wound around the axis, a magnetic pole piece associated with the electromagnetic coil, and a magnetic armature axially spaced apart from the magnetic pole piece by a mechanical air gap maximized when the solenoid actuator is in fully open position. At least one of the pole piece and the armature is made of hard magnetic material having hardness greater than 20 HRC. Alternatively, at least one of the pole piece and the armature is made of hard magnetic material having hardness greater than 30 HRC. At least one of the pole piece and the armature is made of hard martensitic steel. At least one of the pole piece and the armature is made of martensitic steel of the EN 400 series for instance EN 1.4418 steel or EN 1.4313 steel. At least one of the pole piece and the armature is uncoated. Alternatively, both the pole piece and the armature are uncoated such that the size of the mechanical air gap is equal to the size of the magnetic air gap of the solenoid actuator. A method of manufacturing a fuel injector comprises a step of providing a piece of hard magnetic material having hardness greater than 20 HRC, a step of forming at least one of the magnetic armature and the magnetic pole piece into said hard magnetic material, and a step of assembling the magnetic pole piece and the magnetic armature within the fuel injector without an intermediate step of coating at least one of the pole piece and the armature. Alternatively, at least one of the magnetic piece and the armature is made of material having hardness greater than 30 HRC. At least one of the pole piece and the armature is made of hard martensitic or semi-martensitic steel. At least one of the pole piece and the armature is made of martensitic steel of the EN 400 series for instance EN 1.4418 steel or EN 1.4313 steel. The step of assembling occurs immediately after the forming step without an intermediate step of coating both the pole piece and the magnetic armature such that the size of the mechanical air gap is equal to the size of the magnetic air gap of the solenoid actuator.

Further features, uses and advantages of the invention will appear more clearly on a reading of the following detailed description of the embodiments of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be further described by way of example and with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of a conventional solenoid actuator for a fuel injector.

Figure 2 is a schematic diagram of the solenoid actuator for a fuel injector according to one basic example of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 is a schematic diagram of a conventional solenoid actuator 10 for a fuel injector 8 that comprises an axis 24 and further comprises an electromagnetic coil 14 wound around the axis 24, a pole piece 12 associated with the coil 14, an armature 16 axially spaced apart from the pole piece 12, the armature 16 and the pole piece 12 having respectively face to face surfaces 13 and 15, and a compression spring 18 biasing the armature 16. The solenoid actuator 10 as illustrated is in a fully open position. A first coated layer 20 lying on the surface 13 of the pole piece 12 is situated face to face with a second coated layer 22 lying on the other surface 15 of the armature 16. A mechanical air gap 26 having a size ε is formed between the first coated layer 20 and the second coated layer 22. The size of the mechanical air gap ε is the distance between the respective two face to face surfaces 17 and 19 of the two coated layers 20 and 22. Those surfaces 17 and 19 are the reference surfaces for the distance measurement of the size of the mechanical air gap ε as it is where the air stands. The size of the mechanical air gap ε as illustrated is the maximum distance between the two face to face surfaces 17 and 19 of the first coated layer 20 and the second coated layer 22 corresponding to the fully open position of the solenoid actuator 10 of a fuel injector 8. In that position, there is no injection event as the fuel injector 8 is closed. The first coated layer 20 and the second coated layer 22 are made of non- magnetic material such that they do not play a role in the magnetic flux density. As conventional solenoid actuators for fuel injectors use for instance soft magnetic materials for the armature 16 and for the pole piece 12 components, a first effective magnetic air gap having a size s_em is formed between the face to face surfaces 13 and 15 of the pole piece 12 and the armature 16. Soft magnetic materials are usually soft magnetic steels having durability in the range of 70 to 90 HRB (Hardness Rockwell B). Those surfaces 13 and 15 are the reference surfaces for the distance measurement of the size of the first effective magnetic air gap s_em as the size of the effective magnetic air gap s_e is the distance between two magnetic materials. Due to the thickness of the first coated layer 20 and due to the thickness of the second coated layer 22, the size of the first effective magnetic air gap s_em is greater than the size of the mechanical air gap ε. The conventional solenoid actuator 10, wherein the pole piece 12 and the armature 16 are made of soft magnetic material, requires coating layers 20 and 22 on the face to face surfaces of respectively the pole piece 12 and the armature 16 in order to fulfill the wear resistance during lifetime operation as for instance after 1000 million of injection cycles activation or up to 15 years of vehicle lifetime; in other words after 1000 million times of opening the injection valve of an fuel injector wherein the armature 16 impacts the pole piece 12. Figure 2 is a schematic diagram of a solenoid actuator 11 according to the invention for a fuel injector 8 comprising an axis 24 and further comprising an electromagnetic coil 14 wound around the axis 24, a pole piece 30 according to the invention, associated with the coil 14, an armature 28 according to the invention axially spaced apart from the pole piece 30 according to the invention and a compression spring 18 biasing the armature 28 according to the invention. The solenoid actuator 11 according to the invention as illustrated is in a fully open position. The mechanical air gap 26 having a size ε is formed between the two surfaces 32 and 34 respectively of the pole piece 30 according to the invention and the armature 28 according to the invention. In other words, the two surfaces 32 and 34 respectively of the pole piece 30 and the armature 28 are situated face to face so that the mechanical air gap 26 of size ε is formed. The size of the mechanical air gap ε as illustrated is the maximum distance between the two surfaces 32 and 34 respectively of the pole piece 30 and the armature 28 according to the invention corresponding to the full open position of the solenoid actuator 11 according to the invention. In that position, there is no injection event as the fuel injector 8 is closed. It is noted that the pole piece 30 and the armature 28 according to the invention differ from the pole piece 12 and from the armature 16 of the conventional solenoid actuator both illustrated in figure 1 by the fact that the face to face surfaces 32 and 34 of the pole piece 30 and the armature 28 according to the invention are free of coating. In order to withstand 15 years of vehicle lifetime or more than 1000 million times wherein the armature 28 impacts the pole piece 30 during opening phase of the injection valve of a fuel injector for a combustion engine during lifetime operation, the hardness of the material of the other pole piece 30 and the hardness of the other armature 28 have to be in line with the expected wear and impact resistance. In the present invention relative to the solenoid actuator 11 according to the invention for a fuel injector 8 of a combustion engine, as an example, a hardness greater than 20 HRC (Hardness Rockwell C) is considered as satisfying the nominal wear and impact resistance needed for the pole piece 30 and for the armature 28 according to the invention during lifetime operation. Hardness greater than 30 HRC will prevent wear of the pole piece 30 and for the armature 28 according to the invention during lifetime operation under worst case condition. The solenoid actuator 11 according to the invention obviously needs magnetic material for the pole piece 30 and for the armature 28. Thus the material of the armature 28 and the material of the pole piece 30 according to the invention must be a hard magnetic material. In that embodiment, the pole piece 30 and the armature 28 according to the invention are for instance made of hard martensitic steel material that replace the soft magnetic material covered by coating described in the first embodiment illustrated by figure 1. The hard martensitic steel material or the hard semi-martensitic material of the pole piece 30 according to the invention can be different or similar to the hard martensitic steel material or the hard semi-martensitic material of the armature 28 according to the invention. Alternatively the pole piece 30 and the armature 28 are made of semi-martensitic steels. Other parts of a fuel injector can be also made of martensitic or semi-martensitic steel, and more particularly parts that are under impact constraint, such that those parts could be made free of coating. Examples of material having sufficient hardness greater than 30 HRC (Hardness Rockwell C) and sufficient magnetic saturation level and electrical resistivity are the martensitic steels of the EN 400 series as for example the EN 1.4418 steel or the EN 1.4313 steel. The martensitic steels of the EN 400 series family have the advantage of being easily machined with standard lathe and milling equipment. As the solenoid actuator 11 according to the invention for the fuel injector 8 uses for instance hard magnetic materials for the armature 28 and for the pole piece 30 components according to the invention, a second effective magnetic air gap having a size 8eff2 is formed between the two surfaces 32 and 34 respectively of the pole piece 30 and the armature 28. Those two surfaces 32 and 34 are the reference surfaces for the distance measurement of the size of the second effective magnetic air gap 8eff2 as the size of the second magnetic air gap 8eff2 is the distance between two magnetic materials. The size of the second effective magnetic air gap 8eff2 is equal to the size of the mechanical air gap ε as their two reference surfaces 32 and 34 are common.

Alternatively, the solenoid actuator 11 according to the invention for a fuel injector 8 can comprise the pole piece 12 of the conventional solenoid actuator 10 and its first coated layer 20 or the armature 16 of the conventional solenoid actuator 10 and its second coated layer 22 in order to replace the magnetic pole piece 30 or the other magnetic armature 28 according to the invention. In such both cases, the obtained third and fourth sizes of the magnetic air gaps are smaller than the size of the first magnetic air gap seffl obtained with the conventional solenoid actuator 10 illustrated by figure 1.

According to the present invention, a method of manufacturing the solenoid actuator 11 illustrated in figure 2 comprises some innovative steps as for instance a steps providing a hard magnetic material as defined through the embodiment of the solenoid 11 according to the invention, i.e. a hard magnetic material having hardness greater than 20 HRC or greater than 30 HRC to support constraints over life time operation. The hard magnetic material that can be a martensitic or semi-martensitic material can be martensitic steels of the EN 400 series as for example the EN 1.4418 steel or the EN 1.4313 steel. Then a step of forming at least the pole piece 30 or the armature 28 according to the invention or both the pole piece 30 and the armature 28 according to the invention is performed. The assembly of the pole piece 30 and the armature 28 according to the invention within the fuel injector 8 is performed without an intermediate step of coating at least one of the pole piece 30 and the armature 28 or without coating both the pole piece 30 and the armature 28.

One important advantage of the invention compared to the prior art relates to the necessary current to activate the stroke of the armature of the solenoid actuator toward the pole piece. The necessary current II flowing through the electromagnetic coil 14 of the conventional solenoid actuator 10 to generate a constant magnetic flux B allowing the stroke toward the magnetic pole piece 12 of the armature 16 distant from the pole piece 12 by the size of the first magnetic air gap seffl (figure 1) is greater than the necessary current 12 flowing through the electromagnetic coil 14 of the solenoid actuator 11 according to the invention to generate the same constant magnetic flux B allowing the stroke toward the r pole piece 30 of the armature 28 distant from the pole piece 30 by the size of the second magnetic air gap seffZ (figure 2). This is because the size of the second magnetic air gap 8eff2 is smaller than the size of the first magnetic air gap seffl . In other words, the necessary current to execute an injection event with the fuel injector 8 equipped with the solenoid actuator 11 according to the invention having a mechanical air gap of size ε according to the invention is lower than the necessary current to execute an injection event with the fuel injector 8 equipped with the conventional solenoid actuator 10 having the same size of the mechanical air gap ε as the solenoid actuator 11 according to the invention.

Another advantage of the invention is that the use of hard magnetic material without any additional coating layer resolves the problem of sticking between the pole piece and the armature after impact. The use of hard magnetic material can also suppress any kind of abutment stop localized between the face to face surface of the armature and the pole piece that establish a predetermined minimum air gap avoiding the sticking issue during pole piece and armature impact. In addition to this advantage, the absence of coating is also a way to avoid high investment to guarantee the repeatability of the thickness of the coating.

Claims

1. Fuel injector (8) for a combustion engine comprising a solenoid actuator (11) being lifetime operable reciprocally from a closed position to a fully open position along a main axis (24), the solenoid actuator (11) comprising:

an electromagnetic coil (14) wound around the axis (24),

a magnetic pole piece (30) associated with the electromagnetic coil (14), a magnetic armature (28) axially spaced apart from the magnetic pole piece (30) by a mechanical air gap (26) maximized when the solenoid actuator (11) is in fully open position,

characterized in that both the pole piece (30) and the armature (28) are made of hard magnetic material having hardness greater than 20 HRC.

2. Fuel injector (8) according to the preceding claim wherein both the pole piece (30) and the armature (28) are made of hard magnetic material having hardness greater than 30 HRC.

3. Fuel injector (8) according to any one of the preceding claims wherein both the pole piece (30) and the armature (28) are made of hard martensitic steel.

4. Fuel injector (8) according to any one of the preceding claims wherein both the pole piece (30) and the armature (28) are made of martensitic steel of the EN 400 series for instance EN 1.4418 steel or EN 1.4313 steel.

5. Fuel injector (8) according to any one of the preceding claims wherein both the pole piece (30) and the armature (28) are uncoated such that the size of the mechanical air gap (26) is equal to the size of the magnetic air gap of the solenoid actuator (11).

6. A method (100) of manufacturing a fuel injector (8) as set in any one of the preceding claims, the method comprising steps of:

providing a piece of hard magnetic material having hardness greater than

20 HRC, forming both the magnetic armature (28) or the magnetic pole piece (30) into said hard magnetic material,

assembling the magnetic pole piece (30) and the magnetic armature (28) within the fuel injector (8) without an intermediate step of coating at least one of the pole piece (30) and the armature (28).

7. A method (100) according to the preceding claim wherein both the magnetic piece (30) and the armature (28) are made of material having hardness greater than 30 HRC.

8. A method (100) according to any one of claims 6 to 7 wherein both the pole piece (30) and the armature (28) are made of hard martensitic or semi- martensitic steel.

9. A method (100) according to any one of claims 6 to 8 wherein both the pole piece (30) and the armature (28) are made of martensitic steel of the EN 400 series for instance EN 1.4418 steel or EN 1.4313 steel.

10. A method (100) according to any one of claims 6 to 9 wherein the step of assembling occurs immediately after the forming step without an intermediate step of coating both the pole piece (30) and the magnetic armature (28) such that the size of the mechanical air gap (26) is equal to the size of the magnetic air gap of the solenoid actuator (11).