This application is based on German Patent Application 10 2009 000 181.6 filed filed Jan. 13, 2009.
1. Field of the Invention
The invention relates to a fuel injector, in particular a common rail injector, for injecting fuel into a combustion chamber of an internal combustion engine.
2. Description of the Prior Art
The highest priority in the development of internal combustion engines is devoted to adhering to pollutant limit values. Precisely the common rail injection system has made a decisive contribution to reducing pollutants. The advantage of common rail systems is their independence of the injection pressure on the rpm and load. For meeting future exhaust gas limit values, however, a significant increase in the injection pressure is necessary precisely with Diesel engines.
Stroke-controlled common rail injectors are known whose injection valve element is servo-operated. Piezoelectric and magnet valves are used as pressure adjusters and with them the servo circuit is controlled. For fast needle closure, a permanent low-pressure stage is often provided, which exerts a permanent closing hydraulic force on the needle. The disadvantage is the high amount of leakage that ensues between the high-pressure and the low-pressure stage. Leakage unavoidably leads to the necessity of higher pumping power and thus to sacrifices in system efficiency. This situation becomes especially problematic at high pressures. For that reason, the latest injectors are designed to be leak-free at extremely high injection pressures.
In contrast to conventional designs, these so-called leak-free fuel injectors have no permanent low-pressure stage acting in the closing direction, and as a result the attendant leakage points are eliminated. Because of the eliminated low-pressure stage, two-part injection valve elements of the kind used in the fuel injectors with a low-pressure stage that are used in the industry are no longer employed.
While in modern mass-production injectors with a low-pressure stage, both injection valve element parts (control rod and nozzle needle) are pressed against one another because of the resultant pressure forces, in the case of leak-free fuel injectors a separate form- or force-locking connection must be established. For coupling to injection valve elements, it has become known to provide a hydraulic coupler volume between them. The coupler volume is typically realized in the form of a coupler sleeve in which one of the injection valve element parts is guided. The coupler volume is reduced by expelling fuel through the guide gap between the injection valve element part guided in the sleeve and the sleeve itself.
It is the object of the invention to propose a fuel injector of simple construction, in which the coupling of the at least two injection valve element parts is achieved with the least possible number of components.
The invention is based on the fundamental concept of guiding two parts, adjustable relative to one another, of the injection valve element one inside the other, so that it is possible to dispense both with a separate guide sleeve of the kind used in the prior art and a spring subjecting the guide sleeve to spring force. In contrast to the provision of a long, one-piece injection valve element, the at least two-part, and preferably solely two-part, embodiment has the advantage that the production of the individual injection valve element parts is less complicated overall and is thus more economical than the production of a long, one-piece injection valve element. Moreover, existing production lines can be retained along with the existing logistics that are directed to a multi-part injection valve element.
The invention has furthermore recognized the fact that in a version with injection valve element parts guided inside one another, a constant increase in the coupler volume during fuel injector operation would occur if the coupler volume were in communication with an injector volume solely via a guide gap between the two parts, since the flow resistance of the guide gap varies in proportion to the pressure difference applied. The fact that the flow resistance of such a guide gap is in a linear relationship to the magnitude of the pressure difference between the coupler volume and the injector volume would have the result that upon opening of the fuel injector, because of the very low pressure in the coupler volume, a very large amount of fuel would be aspirated from the injector volume, and evacuating the coupler volume in the closing operation would no longer be possible because of the (short) time available. In an extreme case, this would lead to axial wedging of the injection valve element in the fuel injector and thus to a displacement of the fuel injector. To avoid such an effect, in a fuel injector embodied in accordance with the concept of the invention, the coupler volume is made to communicate with the injector volume via at least one throttle arrangement, in addition to or as an alternative to a guide gap, and the throttle arrangement is embodied such that the volumetric fuel flow (flowthrough volume flow) flowing through the throttle arrangement does not vary in proportion to the pressure difference between the coupler volume and the injector volume as in the case of a guide gap but instead varies disproportionately little.
Expressed in other terms, the flowthrough volume flow that flows through the throttle arrangement does not increase to the same extent as a pressure difference between the coupler volume and the injector volume; that is, the flowthrough volume flow and the pressure difference are not in a linear relationship. Expressed in still other terms, it is preferable if the increase in the flowthrough volume flow becomes less and less as the pressure difference becomes greater and greater. Ideally, the flowthrough volume flow is proportional to the root of the pressure difference between the injector volume and the coupler volume.
In a fuel injector embodied in accordance with the concept of the invention, it is attained that even if there is an extremely great pressure difference between the coupler volume and the injector volume at the onset of the opening event, only a moderate fuel quantity is aspirated through the throttle arrangement into the coupler volume, and the time during the closing event when the injection valve element is moved in the direction of the injection valve element seat, preferably by means of a closing spring, suffices for the previously aspirated coupler volume to be dispensed again by the throttle arrangement into the injector volume in order to restore the original status.
One possibility for embodying the throttle arrangement is to provide at least one and preferably solely one throttle bore, made in particular in the first or the second part; very particularly preferably, the throttle bore is embodied on the order of an outflow throttle restriction from a control chamber, as in known servo circuit fuel injectors. Preferably, the throttle bore is accordingly a graduated bore with a diameter stage that preferably leads to the embodiment of a turbulent, cavitating flow inside the throttle bore.
The aforementioned diameter stage is one possibility for attaining a degressive ratio between the flowthrough volume flow, on the one hand, and the pressure difference between the coupler volume and the injector volume, on the other. In principle, arbitrary throttle stages, in particular hydraulically sharp-edged throttle stages, can be realized for the purpose, preferably with an only slight length in the flow direction. Preferably, the length of the at least one throttle stage is designed such that a turbulent flow develops. The te “hydraulically sharp-edged throttle stage” is understood to mean a length to the hydraulic diameter ratio of less than or equal to 10. For the hydraulic diameter of an annular gap throttle restriction, the following equation applies:
The peripheral length in this equation is the sum of the inner and the outer peripheral length.
It is especially preferable if the throttle arrangement includes a plurality of throttle restrictions connected (disposed) hydraulically in series. Preferably, the throttle restrictions are embodied radially between the two injection valve element parts, preferably in the guide region with which the two parts are guided inside one another. As already indicated, it is especially preferred if the throttle restrictions are embodied in such a way, or in other words have such a slight length in the flow direction, that the guide length is so slight that a turbulent flow develops. If there were a laminar flow, the flowthrough volume flow through the throttle arrangement would be proportional to the pressure difference between the coupler volume and the injector volume, which is what is to be avoided here.
One possibility for embodying the throttle arrangement is to provide a plurality of grooves disposed axially side by side (parallel) on one of the injection valve element parts; the throttle restrictions are formed radially between the lands that define the grooves and the other injection valve element part. Preferably, the lands are at least approximately sharp-edged, in order to achieve a minimal guide length and thus to compel the development of a turbulent flow. Quite particularly preferably, in an embodiment with a plurality of throttle restrictions disposed axially one after the other in the flow direction, a throttle bore in the injection valve element parts is dispensed with.
An embodiment of the fuel injector in which the hydraulic coupler is embodied as a joint is especially expedient; this makes a certain pivotability of the two hydraulically coupled injection valve element parts possible so that in this way, tolerance-caused angular errors and skewed positions can be compensated for.
An especially preferable possibility for embodying such a pivot joint is to contour the guided injection valve element part spherically in the region of the guide in order to enable relative pivoting.
In a refinement of the invention, it is advantageously provided that the fuel injector is embodied as leak-free, except for possible leaks in the region of the control valve element. To that end, a low-pressure stage on the injection valve element that acts in the closing direction on the injection valve element is dispensed with.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings, in which:
In the drawings, identical elements and elements with the same function are identified by the same reference numerals.
In
What in this exemplary embodiment is a two-part injection valve element 18 is disposed axially adjustably inside the injector body 8. The injection valve element 18 protrudes with its lower, first part 19, embodied as a nozzle needle, into a graduated bore of a nozzle body 21. In that body, the first part 19 is guided axially displaceably with a guide portion 22. The axial conduits 13 are embodied radially between the first part 19 and the nozzle body 21 by means of polished sections 9 in the guide portion 22. The nozzle body 21 is screwed to the injector body 8 by means of a union nut, not shown.
The first part 19 (nozzle needle) of the injection valve element 18 is guided in a face-end blind bore 23 of a second part 24 (control rod) of the injection valve element 18.
As also shown in
A control chamber 29 is defined by an upper face 28 of the second part 24 of the injection valve element 18 and by a sleevelike portion, toward the bottom in the plane of the drawing, of the valve body 7 and is supplied with fuel at high pressure from the annular chamber 6 via an inlet throttle restriction 30 extending radially in the sleevelike portion of the valve body 7. The sleevelike portion with the control chamber 29 enclosed in it is surrounded radially on the outside by fuel at high pressure, so that an annular guide gap 31, radially between the sleevelike portion of the valve body 7 and the injection valve element 18, in this case the second part 24, is comparatively fuel-tight.
The control chamber 29 communicates via an axial conduit 32, extending perpendicular in the valve body 7 and having an outlet throttle restriction 33, with a valve chamber 34, which is defined radially on the outside by an axially adjustable, sleevelike control valve element 35 of a control valve 36 (servo valve) that is pressure-compensated in the axial direction in the closed state. From the valve chamber 34, fuel can flow into a low-pressure region 37 of the fuel injector 1 and from there to the injector return connection 16 when the sleevelike control valve element 35 has lifted from its control valve element seat 38 embodied on the valve body 7, or in other words when the control valve 36 is open. For adjusting the sleevelike control valve element 35 upward in the plane of the drawing, an electromagnetic actuator 39 with an electromagnet 40 is provided, which cooperates with an armature plate 41 embodied in one piece with the control valve element 35 and consequently also cooperates with the sleevelike control valve element 35. When current is supplied to the electromagnetic actuator 39, the control valve element 35 lifts from its control valve element seat 38, which is embodied on the valve body 7 and in this exemplary embodiment is embodied as a flat seat. The flow cross sections of the inlet throttle restriction 30 and outlet throttle restriction 33 are adapted to one another such that when the control valve 36 is open, a net outflow of fuel (control quantity) from the control chamber 29 into the low-pressure region 37 of the fuel injector 1, and from there into the tank 3 via the injector return connection 16 and the return line 17, results. As a result, the pressure in the control chamber 29 rapidly drops, and as a result the injection valve element 18, or more precisely the first part 19, lifts from its injection valve element seat 27, so that fuel from the injector volume 12 can flow out into the combustion chamber through the injection port 15.
For terminating the injection event, the current supply to the electromagnetic actuator 39 is discontinued, as a result of which the sleevelike control valve element 35 is adjusted downward, in the plane of the drawing, on its control valve element seat 38 by means of a control spring 42 that is braced on the armature plate 41. The replenishing fuel flowing through the inlet throttle restriction 30 into the control chamber 29 assures a rapid pressure increase in the control chamber 29 and thus assures a closing force acting on the injection valve element 18. The resultant closing motion of the injection valve element 18 is reinforced by a closing spring 43, which is braced on one end on a circumferential collar 44 of the second part 24 and on the other on a lower, annular face end 45 of the valve body 7.
It can also be seen from
As can further be seen from
As already explained, the first part 19 is guided into the second part 24 of the injection valve element 18 and is guided on the inner circumference of the blind bore 23. Axially between what in the plane of the drawing is an upper face end 48 and the base 49, also upper in the plane of the drawing, of the blind bore 23, a hydraulic coupler volume 50 is embodied, which couples the motion of the parts 19, 24. As also seen from
If the current to the actuator 39 is discontinued, then as mentioned above the pressure in the control chamber 29 rises rapidly, and as a result first the second part 24 of the injection valve element 18 moves axially downward in the plane of the drawing. As soon as the first part 19 is in contact with the injection valve element seat 27, the injection is ended, and the coupler volume 50 is pressed empty by means of the closing spring 43 until the original state is regained. The emptying of the coupler volume 50 is possible only because the throttle arrangement 52 is embodied in such a way that the flowthrough volume flow varies disproportionately little, or in other words not linearly to the increase in the pressure difference between the coupler volume 50 and the injector volume 12. There is no linear relationship, as there is in the case of a conventional guide (lubrication gap theory).
In the exemplary embodiment of
The exemplary embodiment of a fuel injector 1 shown in
It can be seen from
The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.
Number | Date | Country | Kind |
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10 2009 000 181.6 | Jan 2009 | DE | national |