GB2401650A

GB2401650A - Method for the multi-actuation of a fuel injector having a two piece nozzle needle

Info

Publication number: GB2401650A
Application number: GB0410169A
Authority: GB
Inventors: Hans-Christoph Magel
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2003-05-09
Filing date: 2004-05-07
Publication date: 2004-11-17
Anticipated expiration: 2024-05-07
Also published as: GB2401650B; FR2854661A1; DE10320980A1; JP2005291195A; GB0410169D0

Abstract

The fuel injector 4 comprises a multi-piece injection valve member 6 having a first needle section 7 and a second needle section 8, via each of which a number of injection apertures 15, 16 on the combustion chamber-side end of the fuel injector 4 can be opened or closed. The fuel injector 4 is actuated by a control valve 32, actuated by an actuator 60. The two needle sections 7, 8 are actuated via control valve 32 which is common to the needle sections 7, 8 in such a manner that a first actuation period 71 of the control valve 32 for opening the first needle section 7 is interrupted for a period of time 80 which permits the first needle section 7 to partly close, which period of time 80 permits force opening the second needle section 8 to build up and is followed by a second actuation period 73 of the control valve 32 so as to open the needle sections 7, 8.

Description

2401 650

DESCRIPTION

METHOD FOR THE MULTI-ACTUATION OF A FUEL INJECTOR

HAVING A VARIONOZZLE

The present invention is concerned with a method for the multi-actuation of a fuel injector having a varionozzle.

In order to supply combustion chambers of auto-ignition internal combustion engines, pressure-controlled as well as stroke-controlled injection systems can be used. In addition to pump-nozzle units and pumpline-nozzle units, accumulator type injection systems (Common Rail) are used as fuel injection systems.

Accumulator-type injection systems for example advantageously permit the injection pressure inlet and the rotational speed of the internal combustion engine to be adjusted. In order to achieve high specific outputs and in order to reduce emissions, an injection pressure which is as high as possible is generally necessary.

DE 102 29 417.8 relates to an accumulator-type injection system having a varionozzle and a pressure converting device. A fuel injector is supplied with fuel via a high pressure fuel source. In accordance with this solution, a pressure amplifier is disposed between an injection valve and the high pressure fuel source.

The pressure amplifier contains a converter piston which separates a pressure at chamber, which can be connected to the high pressure fuel source, from a high pressure chamber which impinges upon a nozzle chamber of the fuel injector. The injection valve of the fuel injector contains a nozzle needle by means of which injection apertures allocated to a combustion chamber can be opened or closed.

The nozzle needle is formed as a coaxial injection valve member comprising a first nozzle needle section and a further second nozzle needle section. The nozzle needle formed in two sections is actuated in dependence upon pressure and closes various injection cross-sections which are disposed on the combustion chamber- side end of the multi-piece injection valve member, and also opens these injection cross-sections. The use of a multi-piece injection valve member formed as a coaxial varionozzle is distinguished by good high pressure resistance. In order to control the two needles of the multi-piece injection valve member in a flexible manner with respect to each other, two control chambers can be formed which can both be controlled independently of each other by means of an actuator. However, this is disadvantageous in terms of costly construction and high production costs.

An actuator which can be actuated in three stages can obviate this disadvantage, wherein the first needle of the multi-piece injection valve member is activated in a first control stage of the actuator which can be actuated in three control stages and the second needle of the multi-piece injection valve member is activated in a second, further control stage. However, an actuator which can be controlled in three stages is also very costly and is difficult to mass-produce owing to the tolerances which have to be respected.

The control principle proposed in accordance with the present invention permits the above-described disadvantage to be circumvented in that the two needle sections of a multi-piece injection valve member are actuated simultaneously via a two-stage valve. The two-stage valve can be formed as a 2 port, 2 position or a 3 port, 2 position multipath valve. In order to be able to actuate the two needle sections of the multi-piece injection valve member independently of each other, the two-stage valve is actuated a number of times.

When the two-stage valve is actuated for the first time, the first needle section of the multi-piece injection valve member opens whilst the second needle section remains closed. If, when the first needle section is open, the two-stage valve is briefly closed and then re-opened, then in addition to the first needle section the second nozzle needle section is also opened. The two-stage valve is only closed for such a short time that as the first needle section is closing, it only partly closes.

If the two-stage valve remains in its closed position for a longer period of time, then both needle sections of the multi-piece injection valve member close and injection is terminated. If both needle sections of the multi-piece injection valve member remain in their open position, then fuel is injected over a large injection cross-section. The first needle section and also the second needle section are each allocated injection apertures which can be closed or opened only by the first or only by the second needle section of the multi-piece injection valve member. If only the first needle section of the multi-piece injection valve member is in its open position when injection is started, then in an advantageous manner boot injection can be effected, i.e. a first fuel injection phase with a smaller rate of injection. If only the first needle section remains in its open position throughout the entire injection process, then in an advantageous manner a smaller amount of fuel can be injected via a small hydraulic nozzle flow.

The fuel injector which can be operated by the multi-actuation method proposed in accordance with the invention is advantageously formed in such a manner that a leak path is provided which discharges the leaks, which occur during the pause in injection between the injection processes, between the needle sections of the multi-piece injection valve member into the low pressure-side return line. This means that it is possible to avoid an increased hydrocarbon emission owing to fuel leaks to the injection apertures during the pause in injection. The leaks are discharged into a low pressure-side return line without a large increase in costs and the discharging process can be integrated into the construction of the fuel injector in a simple manner.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows schematically the hydraulic construction of a fuel injector configured in accordance with the invention, Figure 2 shows an alternative embodiment of a fuel injector formed in accordance with the invention and Figure 3 shows schematically the pressure or stroke progressions of the two stage control valve, of the first needle section and of the second needle section of the multi-piece injection valve member as well as the present fuel through-closure cross-section, which are all plotted with respect to time.

Figure 1 shows schematically the hydraulic construction of a fuel injector configured in accordance with the invention.

Fuel flows from a pressure accumulator 1 (Common Rail) to a fuel injector 4 via a high pressure line 2. Fuel under high pressure is admitted to a first control chamber 27 via a first branch 2.1 which branches from the high pressure line 2.

An inlet throttle element 3 is accommodated in the first branch 2.1. Fuel under high pressure is also admitted to a nozzle chamber 5 of the fuel injector 4 via a nozzle chamber inlet 2.2 which is connected to the high pressure line 2. The nozzle chamber 5 is disposed in the injector body of the fuel injector 4 in such a manner that it surrounds a multi-piece injection valve 6. The multi-piece injection valve member 6 contains a first outer needle section 7 as well as a second inner needle section 8.

In the region of the nozzle chamber 5, a pressure stage 9 is formed on the first needle section 7, which pressure stage can be formed as a conical surface. An annular passage 10 extends from the nozzle chamber 5 though a nozzle body I 1 of the fuel injector 4 to the combustion chamber 12. A first needle seat 13 for the first needle section 7 as well as a second needle seat 14 for the second needle section 8 are formed in the nozzle body 11 at the combustion chamber-side end of the fuel injector 4. Furthermore, injection apertures which are formed as a first row of holes 15 and as a second row of holes 16 are located at the combustion chamberside end of the nozzle body 11 of the fuel injector 4 in accordance with the illustration in Figure 1. The rows of holes 15, 16 are disposed concentrically with respect to each other, wherein the first row of holes 15 can be opened or closed by the first needle section 7 and the second row of holes 16 can be closed or opened via the inner second needle section 8 of the multi-piece injection valve member 6.

A leak bore 17 which is connected to a transverse bore 18 is located in the second needle section 8 of the multi-piece injection valve member 6. The transverse bore 18 of the second needle section 8 is connected to peripheral slots 19 formed at the outer periphery of the second needle section 8. A leak discharge system is formed by the leak bore 17, the transverse bore 18 as well as the peripheral slots 19 on the second needle section 8 of the multi-piece injection valve member 6, via which system a leak volume, which occurs during a pause in injection on the multi-piece injection valve member 6 between the first needle section 7 and the second needle section 8, can be guided off via a second control chamber 30, an overflow line 33 into a first low pressure-side return line 35. Consequently, an increased hydrocarbon emission owing to fuel leaks to the injection apertures 15, 16 during i the pause in injection is advantageously avoided. The leak system provided by the leak bore 17, the transverse bore 18 and the peripheral slits 19 on the second needle section 8 can be integrated into the fuel injector 4 without additional construction costs.

The first needle section 7 of the multi-piece injection valve member 6 contains the hereinunder disclosed hydraulic surfaces: A hydraulically active surface Al is formed in the region of the pressure stage 9; a further hydraulically active surface A' 2 iS formed at the combustion chamber- side end of the first outer needle section 7. The end surface of the first needle section 7, which surface abuts the first needle seat 13, represents a further hydraulic surface A:. Furthermore, the first needle section 7 of the multi-piece injection valve member 6 comprises an end side which is active as a hydraulic surface and referenced by Al. In addition, a conical surface is formed on the second needle section 8 of the multi-piece injection valve member 6 on the combustion chamber-side end, which surface extends above the second needle seat 14 and is referenced by A4; furthermore, a further hydraulic surface A5 is located beneath the second needle seat 14 of the second needle section 8, which surface is formed at the tip of the second needle section 8. The end side of the second needle section 8, which runs within the first needle section 7, comprises a hydraulically active surface 6.

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A first control chamber 27 is allocated to the first needle section 7 of the multi- piece injection valve member 6, fuel being admitted to this control chamber via the branch 2.1 branching from the high pressure line 2 with an inlet throttle 3 being interposed. A first spring element 28 can be received in the first control chamber 27, which spring element impinges upon the end side Al of the first needle section 7 in the closing direction. A relief line extends from the first control chamber 27 to a control valve 32, a discharge throttle 29 being accommodated in this relief line. A first low pressure-side return line 35, in which a further throttle element 34 can be disposed, is connected downstream of the control valve 32. On the discharge-side, the outlet of the overflow line 33 is located between the control valve 32 and the first low pressureside return line 35, leaks flowing from a second control chamber 30, via the overflow line 33, into the first low pressure-side return line 35.

The second inner needle section 8 of the multi-piece injection valve member 6 is impinged upon by a second spring element 31 which is disposed in the second control chamber 30. The second spring element 31 is supported on the housing- side and impinges upon the hydraulically active surface A6 at the upper end of the second needle section 8.

The hydraulic mode of operation of the fuel injector 4 in accordance with the illustration in Figure I is as follows: When the control valve 32 is closed, the system pressure level prevailing in the pressure accumulator 1 is admitted into the first control chamber 27. The second control chamber 30, which is disposed above the second needle section 8, is connected to the first low pressure-side return line 35 via the overflow line 33 with a further throttle location 34 being interposed. In this condition the first needle section 7 and also the second needle section 8 are in their closed positions, i.e. the injection apertures 15, 16 formed as the first or second row of holes at the combustion chamber-side end of the nozzle body 11 of the fuel injector 4 are closed. The closure force, by means of which the first needle section 7 is impinged upon at its hydraulically active surface As, is produced by the pressure prevailing in the first control chamber 27. The second needle section 8 is pressed into its closed position by the resilient force of the second spring element 31 which is disposed in the second control chamber 30 since when the first needle section 7 is closed the hydraulic surfaces A4 and AS on the combustion chamber- side end of the second needle section 8 are not influenced with pressure.

In contrast, if the control valve 32 is opened, then a first control pressure level Ps' is present in the first control chamber 27. The first control pressure level Ps' corresponds for example to 70% of the system pressure prevailing in the pressure accumulator 1. There is a lower second control pressure level Ps2 in the further second control chamber 30, which pressure level can correspond for example to approximately 40% of the system pressure prevailing in the pressure accumulator 1. The hydraulic surfaces Al I, A, 2' Al, As, A4 and As as well as A6 of the first needle section 7 and of the second needle section 8 are arranged in such a manner that when the control valve 32 is open, the first needle section 7 opens since the pressure force acting in the opening direction and acting on the hydraulically active surfaces A, ,, A, 2, A2 iS greater than the force acting in the closing direction which acts on the end surface Al of the first needle section 7, possibly supported by a first spring element 28 disposed in the first control chamber 27. In contrast, the second needle section 8 remains closed when the control valve 32 is open since the force of the injection pressure acting in the opening direction and acting on the hydraulically active surface A4 at the combustion chamber-side end of the second needle section 8 is smaller than the force acting in the closing direction and acting on the hydraulically active surface A6 in the second control chamber 30.

Accordingly, when the control valve 32 is open, fuel is injected into the combustion chamber 12 via the first row of holes 15 which is opened by the first needle section 7 of the multi-piece injection valve member 6, which needle section has moved in the opening direction. If the control valve 32 is then closed, the first control chamber 27 is then connected to the system pressure in the pressure accumulator 1 whilst there is a return pressure level in the second control chamber 30. The first needle section 7 closes whilst the second needle section 8 begins to open. Owing to the closing movement of the first needle section 7, injection is terminated whereupon the second needle section 8 likewise immediately closes. Owing to the needle section 7 moving in the closing direction, all of the injection apertures 15, 16 at the combustion chamber-side end of the fuel injector 4 are closed. Accordingly, in accordance with the injection process described above, injection only having a small injection cross-section, i.e. having the cross-section provided by the first row of holes 15, takes place.

An injection process, during which fuel passes into the combustion chamber 12 via all of the combustion chamber-side injection apertures 15, 16, is as follows: When the control valve 32 is opened, injection occurs via the first row of holes 15 opened by the first needle section 7, as described previously. As a result, the control valve 32 is actuated in such a manner that it is closed for a short period of time so that the first needle section 7 begins to move in the closing direction.

Then the second needle section 8 begins a vertical stroke movement in the opening direction since the pressure acting on the hydraulic surface A4 of the second needle section 8 produces a force in the opening direction when the second control chamber 30 has been discharged. As a result, the injection pressure also acts on the hydraulic surface As, i.e. on the tip of the second needle section 8.

After a short period of time, the control valve 32 is opened. During this period of time, the first needle section 8 travels partly along its stroke in the opening direction so that the injection pressure acts on the tip of the second needle section 8. The first nozzle needle section 7 has conversely moved partly along its stroke in the closing direction, however it is not yet closed.

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During the short closing time period, the first nozzle needle section 7 has only partially travelled in the closing direction and at this point completely opens. The second needle section 8 also opens completely since the pressure force of the injection pressure on the hydraulic surfaces A4 and As at the combustion chamber- side end of the second needle section 8 is larger than the pressure force which acts on the hydraulic surface A6 within the second control chamber 32.

At this point, fuel is injected via the two rows of holes 15 and 16 since the two needle sections 7, 8 of the multi-piece injection valve member 6 are in their open position. If the control valve 32 is closed, then the first needle section 7 closes owing to the system pressure present in the first control chamber 27. After the first needle section 7 closes, no more fuel flows to the second needle section 8 and injection through the first row of holes 15 as well as through the second row of holes 16, i.e. a large injection cross-section, is terminated. The second needle section 8 is closed by the second spring element 31 disposed in the second control chamber 30.

Figure 2 is a constructive embodiment of a fuel injector having a 2 port, 2 position piezoelectric control valve which can be actuated a number of times.

A first closing piston 40 as well as a second closing piston 44 are accommodated in the injector body of the fuel injector 4. The first closing piston 40 contains a collar 41 on which a first closing piston spring 42 is supported. The first closing piston 40 also contains an overflow aperture 43 via which a hollow chamber, which is formed within the first closing piston 40 and in which a second piston spring 40 is accommodated, is connected to the hollow chamber surrounding the first closing piston 40 and accommodating the first spring element 28. A second low pressure-side return line 48 branches from this latter hollow chamber.

The second piston spring 45 accommodated in the hollow chamber of the first closing piston 40 impinges upon an end surface 47 of the second needle section 8.

An end surface 46 of the first needle section 7 is directly impinged upon by an annular surface of the first closing piston 40.

A line containing a discharge throttle 29 extends from the first control chamber 27 and opens into a valve chamber 49 of the control valve 32. The overflow line 33 extends from the second control chamber 30 and, upstream of the further throttle location 34, opens into the first low pressureside return line 35.

A first valve piston 50 is accommodated in the valve chamber 49 of the control valve 32, the end surface 57 of the valve piston 50 being impinged upon by a valve spring 52. The valve spring 52 surrounds a dome- like stop 53. The distance hmaX, also referenced by numeral 51, between the end side 57 of the first valve piston 50 and the stop 53, defines the maximum stroke movement of the first valve piston 50, when it is located in a valve seat 54. The first valve piston 50, which is formed as a hemisphere in the illustration of Figure 2, is for its part impinged upon by a shaft 63 of a second valve piston 55. The shaft 63 is surrounded by a chamber 56, a low pressure-side return line 35 branching from this chamber. The second valve piston 55 comprises an end surface 64 which extends into a hydraulic coupling chamber 58. The hydraulic coupling chamber 58 is also impinged upon by an end surface 62 of a displacement piston 59. The displacement piston 59 is actuated by an actuator 60 which, in the illustration of Figure 2, is formed as a piezoelectric actuator containing a piezoelectric crystal stack 61.

The actuator 60 illustrated in Figure 2 is formed in such a manner that a further throttle location 34 is formed downstream of the valve seat 54 in the valve discharge, i.e. in the first low pressure-side return line 35. By means of the first closing piston 40, the pressure force is transferred from the first control chamber 27 to the end surface of the first needle section 7 of the multi-piece injection valve member 6. The second control chamber 30 is also formed in the valve body of the fuel injector 4. The pressure force produced by the second closing piston 44 is transferred to the second needle section 8 of the multi-piece injection valve member 6. The second spring element 28 as well as the second piston spring 45 can be used to support the closing movement of the first needle section 7 and of the second needle section 8.

In the non-operational condition of the control valve 32, the valve seat 54 is closed and the flow connection from the first control chamber 27 to the first low '1 pressure-side return line 35 is interrupted by the first valve piston element 50 located in the valve seat 54. The system pressure prevailing in the pressure accumulator 1 acts in the first control chamber 27 whilst the second control chamber 30 is connected to the first low pressure-side return line 35 via the overflow line 33; the two needle sections 7, 8 of the multi-piece injection valve member 6 are closed.

If the actuator 60 formed as a piezoelectric actuator is actuated, then the first valve piston 50 is deflected in the direction of the stop 53, which is formed in the valve chamber 49, by the displacement piston 59 which impinges upon the second valve piston 55 in the hydraulic coupling chamber 58. The hitherto closed valve seat 54 is opened when the maximum stroke hmaX, also referenced by numeral 51, is travelled. Consequently, a flow connection from the first control chamber 27 into the first low pressure-side return line 35 via the discharge throttle 29, the valve chamber 49 and the chamber 56 is opened. As a result, the first control chamber 27 contains a first pressure level Ps' whilst the second control chamber 30 contains the previously mentioned lower second control pressure level Ps2 As a result the first needle section 7 opens whilst the second needle section 8 of the multi-piece injection valve member 6 remains closed. As described above, injection occurs only via the injection apertures 15, i. e. the first row of holes 15, opened by the first needle section 7.

The first valve piston 50 formed as a hemisphere in the illustration of Figure 2 can be formed in various ways. Instead of a first valve piston 50 being spherically formed, as illustrated in Figure 2, the valve member 50 can also be formed as a slide valve, flat-seat valve or as a conical seat valve or any combination of these types of valve. The valve spring 52 illustrated in Figure 2 can be disposed in the valve chamber 50 so as to support the closing movement of the control valve 32 and for definitive positioning when the injection system is in the unpressurised state. A power/stroke converter in the form of the hydraulic coupling chamber 58 is allocated to the actuator 60 formed as a piezoelectric actuator. The small deviation inherent in a piezoelectric actuator can be intensified by correspondingly dimensioning the end surface 62 of the displacement piston 59 as well as the end surface 64 of the second valve piston 55. The hydraulic coupling chamber 58 between the actuator 60 and the first valve piston 50 can be suitably filled by virtue of the fact that filling occurs by means of the guided leakage of the pistons 50, 55. Instead of the actuator 60 being formed as a piezoelectric actuator, as illustrated in Figure 2, solenoid valves can also be used to actuate the control valve 32, as could servo-hydraulic valves.

Figure 3 schematically shows the pressure and stroke progressions of the control valve, of the first needle section as well as of the second needle section of the multi-piece injection valve member and finally the through-closure cross-sections present at the combustion chamber-side end of the fuel injector.

At a point in time 70, which represents when the first needle section 7 is actuated, the control valve 32 is deflected by its maximum stroke path hmaX by supplying the actuator 60 with current. The first control valve 32 is actuated during a boot phase 81 of the injection process during a first actuation period 71. The first needle section 7 begins to open immediately after the actuation point in time 1, corresponding to the rising slope 78. During the first actuation period 71, a first flow cross-section Al, also referenced by numeral 76, which is provided by the through-closure cross-sections of the individual bores of the first row of holes 15, is opened so that a corresponding amount of fuel, passing through the first row of holes 15, is injected into the combustion chamber 12 of the internal combustion engine. After the actuation period 71 has finished, actuation of the control valve 32 ends at a point in time 74. At this point in time supply of current to the actuator 60 is interrupted. For a closing duration 80 which continues at the end of actuation 74, supply of current to the actuator 60 is stopped so that the first needle section 7 begins the closing process. Simultaneously with the closing process of the first needle section 7, the second needle section 8 opens corresponding to the rising slope 79 of the second needle section 8. The second needle section 8 opens since the pressure in the second control chamber 30 drops, however the injection pressure is still acting on the hydraulic surface A4. The force acting in the opening direction on the hydraulically active surface A4 is greater than the closing force acting in the closing direction on the hydraulic surface A4.

At a second actuation point in time 72, after the closing time 80 has ended, the control valve 32 is re-actuated by correspondingly supplying the actuator 60 with current. As a result, the partly-complete closing movement of the first needle section 7 is interrupted. The first needle section 7 opens in the opening direction and the second needle section 8 opens further. The total [low cross-section in the form of the two rows of holes 15, 16 at the combustion chamber-side end of the multi-piece injection valve member 6, i.e. the two rows of holes 15, 16, is available in order to inject fuel into the combustion chamber of the internal combustion engine. Consequently, during the boot phase 81 of the injection process, a smaller amount of fuel corresponding to the first row of holes 15 opened by the first needle section 7 is injected into the combustion chamber 12, whereas from the second actuation point in time 72 fuel is injected into the combustion chamber of the internal combustion engine via the two rows of holes 15, 16 at the combustion chamber-side end of the fuel injector 4 during the main injection phase 82.

At a point in time 75, the control valve 32 is re-actuated by the actuator 60 and moves back from its maximum stroke path hmaX, also referenced by numeral 51 in Figure 2, into its valve seat 54, possibly with the aid of a valve spring 52. As a result, first the first needle section 7 of the multi-piece injection valve member 6 closes and after a further length of time the second needle section 8 closes and injection is terminated. List of reference numerals 1 Pressure accumulator 2 High pressure line 2.1

First branch 2.2 Nozzle chamber inlet 3 Inlet throttle 4 Fuel injector Nozzle chamber 6 Multi-piece injection valve member 7 First needle section 8 Second needle section 9 Pressure stage Annular passage 11 Nozzle body 12 Combustion chamber 13 Needle seat of the first needle section 7 14 Needle seat of the second needle section 8 First row of holes 16 Second row of holes 17 Leak bore I Transverse bore 19 Peripheral slit al A' Hydraulically active surface of the pressure stage Al 2 Hydraulically active surface of the end side of the first nozzle needle section 7 A2 Hydraulically active surface of the end side of the first needle section 7 A3 Hydraulically active annular surface of the first needle section 7 A4 Hydraulically active surface of the conical region of the second needle section 8 Hydraulically active surface of the needle tip of the second needle section A6 Hydraulically active surface of the end side of the second needle section 8 27 First control chamber 28 First spring element 29 Discharge throttle Second control chamber 31 Second spring element 32 Control valve 33 Overflow line 34 Further discharge throttle First low pressure-side return line First closing piston 41 Closing piston collar 42 First piston spring 43 Overflow aperture 44 Second closing piston Second piston spring 46 End surface of the first needle section 7 47 End surface of the second needle section 8 48 Second low pressure-side return line 49 Valve chamber of the control valve 32 First valve piston 51 Maximum stroke hmaX 52 Valve spring 53 Stop 54 Valve seat Second valve piston 56 Chamber 57 End surface of the first valve piston 50 58 Hydraulic coupling chamber 59 Displacement piston Actuator 61 Piezoelectric crystal stack 62 End surface of the displacement piston 59 63 Piston shaft 64 Valve piston

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End surface of the second valve piston 55 First actuation point in time 71 First actuation period 72 Second actuation point in time 73 Second actuation period 74 First end of actuation Second end of actuation 76 First through-closure cross section (first row of holes 15) 77 Second flow cross-section (first and second rows of holes 15, 16) 78 Rising slope for the opening of the first needle section 7 79 Rising slope for the second needle section 8 Interruption of actuation of control valve 32 81 Boot phase 82 Main injection phase

Claims

I. Method for the multi-actuation of a fuel injector which comprises a multi piece injection valve member having a first needle section and a second needle section, via each of which a number of injection apertures on the combustion chamber-side end of the fuel injector can be opened or closed and the fuel injector is actuated by a control valve which can be actuated by an actuator, and wherein the needle sections can be actuated via a two stage control valve which is common to the two needle sections in such a manner that a first actuation period of the control valve for opening the first needle section is interrupted for a period of time which permits the first needle section to partly close, which period of time permits force acting on the second needle section in the opening direction to build up and which is followed for multiple injection by a second actuation period of the control valve to open the two needle sections.
2. Method as claimed in Claim 1, wherein during the first actuation period of the control valve, the first needle section opens and the second needle section remains in its closed position.
3. Method as claimed in Claim 1, wherein when the actuation of the control valve is interrupted for a period of time which exceeds the period of time permitting the first needle section to partly close, both needle sections are closed.
4. Method as claimed in Claim 1, wherein when the control valve is actuated in the first actuation period, a first pressure level is produced in a first control chamber of the fuel injector and a second pressure level, which is lower than the first pressure level, is produced in a second control chamber of the fuel injector.
5. Method as claimed in Claim 4, wherein an opening force acting on first and second hydraulically active surfaces of the first needle section is greater than a closing force acting on the first needle section which is produced by the first pressure level in the first control chamber.
6. Method as claimed in Claim 4, wherein an opening force acting on a first hydraulically active surface of the second needle section is smaller than the closing force produced by the second pressure level in the second control chamber and acting on a second hydraulic surface of the second needle section.
7. Method as claimed in Claims 4 to 6, wherein fuel is injected into a combustion chamber of an internal combustion engine via a first injection cross-section.
8. Method as claimed in Claim 1, wherein during the period of time when actuation of the control valve is interrupted, while the first needle section is partly closing from its open position, the second needle section moves at least partly in the opening direction when the second control chamber is relieved from pressure and an opening force acting on a first hydraulically active surface of the second needle section builds up.
9. Method as claimed in Claim 8, wherein after the period of time when actuation of the control valve is interrupted has ended, the first needle section opens and an opening force acting on first and second hydraulically active surfaces of the second needle section is greater than a closing force acting on the second needle section in the second control chamber via a hydraulically active surface of the second needle section.
10. Method as claimed in Claims 8 and 9, wherein fuel is injected into a combustion chamber of an internal combustion engine via first and second injection cross-sections.
1 1. Fuel injector for injecting fuel into a combustion chamber of an internal combustion engine, said fuel injector comprising a multi-piece injection valve member having a first needle section and a second needle section, via each of which a number of injection apertures on the combustion chamber-side end of the fuel injector can be opened or closed and the fuel injector is actuated by a two-stage control valve which can be actuated by an actuator and a first control chamber impinging upon the first needle section and a second control chamber impinging upon the second needle section are formed in the fuel injector, and wherein the two-stage control valve comprises a valve piston via which a valve seat can be opened or closed with respect to a low pressure-side return line and the connection from the valve seat to the return line contains a throttle location and a second control chamber is connected between the valve seat and the throttle location.
12. Fuel injector as claimed in Claim 11, wherein a first closing piston transferring the pressure force in the first control chamber to the first needle section and a further second closing piston which is disposed in the second control chamber are provided and are disposed coaxially with respect to each other and the two-stage control valve comprises a first valve piston via which the valve seat can be opened or closed with respect to the low-pressure side return line.
13. Fuel injector as claimed in Claim 11, wherein the actuator is formed as a piezoelectric crystal arrangement.
14. Fuel injector as claimed in Claim I 1, wherein a hydraulic converting device is disposed downstream of the actuator, via which converting device the deviation of the piezoelectric crystal stack can be transferred by interposing a displacement piston at a second valve piston impinging upon the first valve piston.
15. Fuel injector as claimed in Claim I 1, wherein the second needle section of the multi-piece injection valve member comprises a leak channel system via which a leak flow occurring between the needle sections during the pause in injection can be discharged into the second control chamber and via an overflow line into the low pressure-side return line.
16. Fuel injector as claimed in Claim 14, wherein the force/stroke converting device is filled by virtue of the guided leakage at the second valve piston.
17. A method for the multi-actuation of a fuel injector, substantially as hereinbefore described with reference to the accompanying drawings.
18. A fuel injector substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.