Fuel injector and method for the manufacture and/or assembly of a nozzle needle assembly

The invention relates to a fuel injector having a nozzle body (I) and having an injector body (2). In the nozzle body (I) there is formed a high-pressure bore (3) for accommodating a nozzle needle (4) which can perform a stroke movement and via the stroke movement of which at least one injection opening (5) can be opened up or closed off, the fuel injector further including a low pressure chamber (6) coupled to the needle (4) via a coupling device (8) having a first and a second disk-shaped coupler body (9, 10), the low pressure chamber (6) accommodating a piezoelectric actuator (7).

BACKGROUND OF THE INVENTION

The invention concerns a fuel injector for a fuel injection system, in particular a common rail injection system, for injecting fuel into the combustion chamber of an internal combustion engine. The invention furthermore concerns a method for manufacture and/or assembly of a nozzle needle assembly which can be used in particular in such a fuel injector.

A generic fuel injector is disclosed for example in publication DE 10 2008 002 417 A1. The fuel injector described therein comprises a piezoelectric actuator which is accommodated in a relatively pressureless actuator chamber. The piezoelectric actuator is hydraulically coupled to the nozzle needle of the injector such that the nozzle needle assumes its closed position when the piezoelectric actuator is electrically discharged, and transfers to the opening position when the piezoelectric actuator is connected to an electric power source. This means that the opening stroke of the nozzle needle takes place in the opposite direction to the actuator stroke. The coupling device thus achieves a reversal of the movement direction. This has the advantage that the piezoelectric actuator need only be electrically charged during the brief injection phases and is electrically discharged in the longer rest phases of the fuel injector and hence subject to less strain. As a result the life of the piezoelectric actuator provided for activating the nozzle needle is extended. A further measure extending the life of the piezoelectric actuator is the arrangement of the piezoelectric actuator in a relatively pressureless actuator chamber. The actuator is thus not exposed to fuel under high pressure. No high-pressure-resistant seal of the piezoelectric actuator is therefore required.

The device described in the publication for hydraulic coupling of the piezoelectric actuator with the nozzle needle furthermore allows a distance translation between the stroke of the actuator and the stroke of the nozzle needle, in that the cross sections of the two pistons causing the displacement in the coupling device are dimensioned significantly differently. As a result an adequate nozzle needle stroke can be achieved even with a short actuator stroke.

The invention is based on the object of refining a fuel injector of the type described initially in that a greater clearance exists in relation to the surface area design of the coupler pistons to optimize the distance translation. At the same time the structure of the coupling device and the connection of the coupling device to the nozzle needle are simplified to create a simple fuel injector which can be produced at low cost.

SUMMARY OF THE INVENTION

Starting from a generic fuel injector, according to the invention it is proposed that the coupling device comprises a first and second disk-shaped coupler body each with a cylinder bore each accommodating at least one coupler piston delimiting a coupler chamber. The proposed structure of the coupling device with two separate coupler bodies is simple to produce and can therefore be manufactured economically. Also the area ratio of the hydraulically active areas formed on the coupler pistons can be largely freely selected to achieve an optimum distance translation between the actuator stroke and the nozzle needle stroke. For the design of the surface areas, the diameter of the respective cylinder bore can be used in which the respective coupler piston is held. The diameter of the cylinder bore can also be freely selected. With the coupling device, with corresponding arrangement of coupler pistons in the coupler bodies, also a movement reversal can be achieved so that the nozzle needle stroke takes place in the opposite direction to the actuator stroke. This guarantees that the piezoelectric actuator need only be electrically charged to perform an injection, while it is electrically discharged in the phases between two injection processes. As a result the piezoelectric actuator is subject to less strain. In this context it is also favorable that the piezoelectric actuator is arranged in a low-pressure chamber. The piezoelectric actuator can be designed as a “wet” or a “dry” actuator, wherein in the latter case the actuator has a corresponding seal consisting for example of a metal sleeve with a membrane.

Preferably the first and second disk-shaped coupler bodies are arranged lying behind each other in the axial direction between the nozzle body and the injector body. The two disk-shaped coupler bodies thus form housing parts which separate the low-pressure region from the high-pressure region. Furthermore the coupling construction is simple and easy to assemble, and also compact in the axial direction.

To simplify the construction further, it is furthermore proposed that the first disk-shaped coupler body axially delimits the high-pressure bore formed in the nozzle body. Alternatively or additionally it may be proposed that the second disk-shaped coupler body axially delimits the low-pressure chamber formed in the injector body. Thus the coupling device not only separates the low-pressure region from the high-pressure region but also seals the low-pressure region against the high-pressure region. No additional sealing measures are required so that simple and economic manufacture of the injector is guaranteed.

According to a preferred embodiment of the invention, a connecting piston is formed on the nozzle needle for mechanical connection of the nozzle needle with the first coupler piston held in the first disk-shaped coupler body. The connecting piston is here guided through a guide bore formed in the coupler body. The connecting piston thus extends the nozzle needle into the low-pressure region. The mechanical connection of the connecting piston with the coupler piston can take place for example by welding and/or by press connection.

The connecting piston is guided through the guide bore and through the first coupler chamber at least as far as the first coupler piston. As a result a pressure area formed on the first coupler piston and delimiting the first coupler chamber is reduced by the cross section area of the connecting piston. The necessary needle opening force can thus be reduced via the design of the respective area ratios so that the needle dynamics increase. Also the necessary actuating forces are reduced so that a less powerful actuator can be used.

To seal the guide bore in the first coupler body, which holds the connecting piston formed on the nozzle needle, against the high-pressure bore, the connecting piston can be surrounded in the region of the high-pressure bore by a sleeve lying tightly against the first disk-shaped coupler body. Instead of a separate sealing sleeve, the first coupler body can also be fitted with a cylindrical shoulder to guide the connecting piston and seal the guide bore against the high-pressure bore.

As a refinement it is proposed that the guide bore comprises a low-pressure region for example in the form of a ring groove which is connected via a bore with the low-pressure chamber. This has the advantage that fuel reaching the guide bore due to a leak can be diverted to the low-pressure chamber via the low-pressure region and the bore. The leakage diversion ensures a defined coupler chamber pressure.

To achieve a hydraulic coupling of the nozzle needle with the piezoelectric actuator, the coupler chambers are hydraulically connected via bores in the disk-shaped coupler bodies. If the volume of a coupler chamber changes because of the stroke of a coupler piston held therein, fuel is displaced via the connecting bores from one coupler chamber to the other coupler chamber. Depending on the respective area ratio of the hydraulically active surfaces delimiting the coupler chambers at the respective coupler pistons, a distance translation is achieved. The nozzle needle stroke necessary to clear the injection opening can consequently be achieved even with a short actuator stroke. To improve the hydraulic design a choke is formed preferably in one of the bores connecting the two coupler chambers. The choke causes a damping of the needle speed and a reduction in the characteristic curve gradient.

According to a preferred embodiment of the invention the high-pressure bore formed in the nozzle body has a guide region to guide the nozzle needle. The regions of the high-pressure bore adjacent to the guide region are preferably connected hydraulically via a choke. With this measure the closing speed of the nozzle needle can be optimized. The closing movement of the nozzle needle is here achieved by a closing spring supported on the nozzle needle.

In addition it can be provided that closing forces are also generated by the coupling device. As a refinement it is therefore proposed that the low-pressure chamber is connected with a return circuit via a non-return valve to achieve a pressure rise in the low-pressure chamber. A pressure rise to around 150 bar for example has been found to be sufficient.

According to a further preferred embodiment, as an alternative to a connecting piston formed directly on the nozzle needle, it is proposed that the nozzle needle and the first piston coupler piston held in the first disk-shaped coupler body are coupled together mechanically via a connecting piston which is guided as part of the first coupler piston through a guide bore formed in the coupler body. This means that the connecting piston need not necessarily be part of the nozzle needle but can also be part of the first coupler piston if it is guided through the guide bore on assembly of the injector. For example the connecting piston can be formed as one piece with the first coupler piston or be connected with this such that in a first assembly step the unit, designed as a one-piece unit or constructed from a first coupler piston and connecting piston, is inserted in the guide bore of the coupler body, and then in a second assembly step the connecting piston is connected to the nozzle needle. This has the advantage that the mechanical connecting point is moved from the low-pressure region to the high-pressure region. Problems of fit in the piston guides which can be caused for example by distortions on welding or compression are thus avoided or shifted to a less delicate region. If the high-pressure region is sealed from the low-pressure region via a separate sealing sleeve lying on the first coupler body, it must be ensured that the sealing sleeve is applied before connection of the connecting piston to the nozzle needle.

Further preferably the connecting piston is connected with the nozzle needle and/or the first coupler piston by force, material and/or form fit. As already mentioned, the connection can take place by welding or pressing. Also a screw connection can be provided. Preferably then at least one end segment of the connecting piston has an external thread and can be inserted in a bore with an internal thread formed in the first piston and/or nozzle needle.

Furthermore the connecting piston can also be indirectly connected with the nozzle needle via a connecting piece. The connecting piece preferably has the same outside diameter as the nozzle needle and is attached axially to the nozzle needle. The connection can take place for example by welding. To accommodate the connecting piece, in the connecting piece can be made a bore, in particular a blind bore, in which an end segment of the connecting piston is inserted. With corresponding choice of diameter, the connection can be a press connection. Alternatively a screw connection or weld connection is feasible.

The object of the invention is furthermore a method for production and/or assembly of a nozzle needle assembly for a fuel injector which comprises a nozzle needle, a coupler piston and a connecting piston, wherein the connecting piston has a smaller outer diameter than the coupler piston and/or the nozzle needle and is part of a one-piece or multi-piece coupler piston. In this method first the connecting piston is guided through a guide bore of a coupler body and then directly or indirectly connected with the nozzle needle by force, material and/or form fit. The method leads to a nozzle needle assembly which can be used particularly advantageously in a fuel injector according to the invention. The nozzle needle assembly is furthermore also suitable for use in a modified design and consequently is not restricted to use in an injector according to the invention.

Preferably the connecting piston with nozzle needle and/or connecting piece for indirect connection of the connecting piece with the nozzle needle is welded, soldered, pressed, screwed and/or glued.

If the connecting piston is connected to the nozzle needle indirectly via a connecting piece, further preferably the connecting piece and the nozzle needle are butt-joined and welded together.

DETAILED DESCRIPTION

The fuel injector shown in longitudinal section inFIG. 1has a nozzle body1to accommodate a nozzle needle4and an injector body2to accommodate a piezoelectric actuator7to activate the nozzle needle4. The nozzle needle4is held mobile in a stroke movement in a high-pressure bore3of the nozzle body1so that via the nozzle needle stroke at least one injection opening5formed in the nozzle body1can be opened or closed. When the nozzle needle4is in its open position, fuel under high pressure is injected via the at least one injection opening5into the combustion chamber of the internal combustion engine. The fuel is supplied to the fuel injector from a high-pressure accumulator34, in the present case from a common rail. For this in the injector body2is formed a supply channel35via which the fuel enters the high-pressure bore3and hence reaches at least one injection opening5.

To activate the nozzle needle4the piezoelectric actuator7can be connected via electrical connections36with an electrical voltage source (not shown). When the piezoelectric actuator7is electrically charged, this undergoes a length expansion constituting the actuator stroke which is converted into a stroke movement of the nozzle needle4because of the coupling device8. The present coupling device8is designed such that a length extension of the piezoelectric actuator7causes a movement of the nozzle needle4opposite the movement direction of the piezoelectric actuator7. This means that the piezoelectric actuator7is electrically charged on the opening stroke of the nozzle needle4while it is discharged between two injection processes or in the closed position of nozzle needle4. This reduces the strain on the piezoelectric actuator7.

It is also favorable for the life of the piezoelectric actuator7that this is accommodated in the low-pressure chamber6of the injector body2. The piezoelectric actuator7is consequently not exposed to high pressure.

Said coupling device8has two disk-shaped coupler bodies9,10which are arranged lying behind each other in the axial direction between the injector body2and the nozzle body1. The two disk-shaped coupler bodies9,10thus separate a low-pressure region allocated to the injector body2from a high-pressure region allocated to the nozzle body1. At the same time the disk-shaped coupler body9lying on the nozzle body1seals the high-pressure bore3, and the disk-shaped body10lying on an injector body2seals the low-pressure chamber6. The coupling device8can thus be shifted completely into the low-pressure region.

In both disk-shaped coupler bodies9,10is formed a cylinder bore11,12which each accommodate a coupler piston15,16, wherein each coupler piston15,16axially delimits a coupler chamber13,14within the respective cylinder bore11,12. The coupler piston surface areas delimiting the respective coupler chambers13,14form pressure areas, the area ratio of which determines the translation ratio between the actuator stroke and the needle stroke. In the present case a significantly larger pressure area is formed on the second coupler piston16allocated to the piezoelectric actuator7to delimit the second coupler chamber14than on the first coupler piston15which is connected via a connecting piston17with the nozzle needle4. The connecting piston17for this is guided through a guide bore18in the first disk-shaped coupler body9and through the first coupler chamber13so that the pressure area19delimiting the coupler chamber13on the first coupler piston15is reduced by the cross-section area of the connecting piston17. Because the first coupler chamber13is arranged between the nozzle needle4and the first coupler piston15, a pressure rise in the first coupler chamber13causes the first coupler piston15and hence the nozzle needle4to be raised. The pressure in the first coupler chamber13rises when, because of the length expansion of the piezoelectric actuator7, the second coupler piston16is immersed more deeply into the second coupler chamber14and thus displaces fuel. Via bores23,24and a choke25formed herein, the fuel displaced from the second coupler chamber14then enters the first coupler chamber13. Because of the area ratio selected i.e. the size of the hydraulically active area formed on a coupler piston15,16, a relatively short actuator stroke can achieve a significantly longer nozzle needle stroke to open the at least one injection opening5. The choke25formed in the bore23or24causes a damping of the needle speed, further improving the hydraulic design.

To seal the guide bore18against the high-pressure bore3, the connecting piston17is surrounded by a sleeve20in the region of the high-pressure bore3. The sleeve20is furthermore supported on the first disk-shaped coupler body9. For this the sleeve20on the face has a supporting surface formed as a sharp edge. Via a closing spring31supported on the nozzle needle4, the sleeve20is held in contact with the disk-shaped coupler body9. The closing spring31also ensures that the nozzle needle4assumes its closed position when piezoelectric actuator7is discharged. Insofar as the arrangement of the sleeve20around the connecting piston17cannot prevent a leakage in the region of the guide bore18, a leakage quantity entering the guide bore18is diverted to a return circuit30via a ring groove21and a bore22which connects the ring groove21with the low-pressure chamber6. In this way a defined coupler chamber pressure is ensured. Between the return circuit30and the low-pressure chamber6can be arranged—as in the present case—a non-return valve29which allows a pressure rise in the low-pressure chamber6. By increasing the fuel pressure in the low-pressure chamber6for example to 150 bar, via the coupling device8closing forces can also be achieved to allow support of a closing movement of the nozzle needle4.

In the low-pressure chamber6is also arranged a pretensioned spring32, by means of which the piezoelectric actuator7is pretensioned against the injector housing2.

For further optimization of the closing movement of the nozzle needle4, the fuel injector shown has a guide region27formed in the high-pressure bore3to guide the nozzle needle4. The regions of the high-pressure bore3adjacent to the guide region27are hydraulically connected via a choke28. The choke28has a damping effect on the movement of the nozzle needle4. The nozzle needle4also has a enlarged diameter in the guide region27forming radially running shoulders26to constitute a pressure step.

Furthermore a needle stop33is provided to delimit the nozzle needle stroke, which in the present case is formed on the end of the sleeve20facing the nozzle needle4. Instead of being arranged in the high-pressure region, the needle stop33can also be arranged in the low-pressure region.

The embodiment of a fuel injector according to the invention shown inFIG. 2differs essentially from that inFIG. 1in that the connecting piston17, by means of which the nozzle needle4and first coupler piston15are mechanically coupled, is part of the coupler piston15. On assembly of the injector the connecting piston17and the first coupler piston15are inserted in the guide bore18as an assembled unit. This means that the connecting piston17is first connected, in the present case welded, with the coupler piston15and then guided through the guide bore18. The sleeve20is then placed on the end of the connecting piston17passed through the guide and seals the high-pressure region against the low-pressure region. Only then is the nozzle needle4with connecting piece37applied and welded to the connecting piston. The connecting piece37forms a unit with the nozzle needle4wherein the connecting piece37and nozzle needle4can also be designed or constructed of one piece. In the present case the connecting piece37is placed axially on the nozzle needle4and welded to this.

With regard to function method, the fuel injector shown inFIG. 2does not differ from that inFIG. 1so that in this connection reference is made to the previous statements. The alternative embodiment shown inFIG. 2substantially facilitates assembly of the fuel injector according to the invention and hence lowers production costs. Also the risk of poor fit in the guide regions is reduced as the mechanical connecting parts are shifted from the low-pressure region to the high-pressure region. Any distortions of the connecting piston17caused by welding or pressing are of secondary importance in the region of the high-pressure bore3so arrangement of the mechanical connecting point in this region has proved advantageous.