Fuel injector with piston restoring of a pressure intensifier piston

The invention relates to a fuel injector for injecting fuel into a combustion chamber of an internal combustion engine, which fuel injector comprises an injection valve element, for opening and closing at least one injection opening and a pressure booster, by way of which fuel which is at system pressure is compressed to injection pressure. The pressure booster is actuated via a first control valve and the injection valve element is actuated via a second control valve. The pressure booster comprises a pressure booster piston which is assigned a spring element which is supported by way of one side on the injector housing and by way of the other side on the pressure booster piston. The pressure booster piston delimits a compression chamber, a differential pressure chamber and a control chamber. The control chamber is arranged at that end of the pressure booster piston which lies opposite the compression chamber, the spring element is received in the control chamber, and the spring element is supported on one side on the injector housing and on the other side on the pressure booster piston.

CROSS-REFERENCE TO RELATED APPLICATION

[0000.4] This application is a 35 USC 371 application of PCT/EP 2007/056865 filed Jul. 6, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is based on a fuel injector for injecting fuel into a combustion chamber of an internal combustion engine.

2. Description of the Prior Art

DE-A 103 35 340 has disclosed a fuel injector that includes a control valve for a pressure booster. The pressure booster has a working chamber that is separated from a differential pressure chamber by a pressure booster piston. The pressure change in the differential pressure chamber of the pressure booster is carried out by means of a servo valve that is activated by an associated switching valve. The differential pressure chamber contains a spring element that encompasses a lower section of the pressure booster piston, which has a diameter that is smaller than the upper section of the pressure booster piston. One end of the spring element is supported on the upper section of the pressure booster piston and the other end is supported on the injector housing. The spring element produces a restoring movement of the pressure booster piston. The disadvantage of this design is that it is not possible to pre-assemble the pressure booster piston and spring element. In addition, this embodiment has spatial disadvantages since it is necessary to select the outer diameter of the spring element to be smaller than the large piston diameter.

SUMMARY OF THE INVENTION

A fuel injector embodied according the invention, which is for injecting fuel into a combustion chamber of an internal combustion engine, includes an injection valve element for opening and closing at least one injection opening and a pressure booster with which fuel at system pressure is compressed to injection pressure. A first control valve triggers the pressure booster and a second control valve triggers the injection valve element; the pressure booster includes a pressure booster piston that is associated with a spring element whose one end is supported on the injector housing and whose other end is supported on the pressure booster piston. The pressure booster piston delimits a compression chamber, a differential pressure chamber, and a control chamber; the control chamber is situated at the end of the pressure booster piston opposite from the compression chamber and the spring element is contained in the control chamber; one end of the spring element is supported on the injector housing and the other is supported on the pressure booster piston.

The spring element is preferably a spiral spring embodied in the form of a compression spring, which tapers conically at the end with which it is supported on the pressure booster piston. The conical tapering at the end with which the spring element is supported on the pressure booster piston prevents the spring element from rubbing against the upper section of the pressure booster piston that it encompasses and thereby contributing to the wear of the pressure booster piston and the spring element.

In order to be able to produce the pressure booster piston in a single piece, a spring plate against which the spring element rests is mounted on the pressure booster piston. The spring plate preferably rests against a ring element that is accommodated in a groove in the pressure booster piston. Preferably, the spring plate has a cylindrical shoulder that encompasses the ring element. The spring plate makes it possible to carry out the installation first by sliding the spring element onto the upper section of the pressure booster piston and then mounting the spring plate on the upper section of the pressure booster piston. The mounting of the spring plate preferably occurs by means of the ring element that is accommodated in the groove in the pressure booster piston. The ring element is preferably a snap ring. In order to install the snap ring on the upper section of the pressure booster piston, preferably a bevel is provided on the piston, which allows the snap ring to be slid on. The snap ring is slid onto the upper section of the pressure booster piston until it snaps into the groove. Then the spring plate is pressed against the ring element with the aid of the spring element so that the cylindrical section slides on around the ring element. This achieves a stable seating of the spring plate.

In a preferred embodiment, the end of the spring element opposite from the spring plate is supported on a ring, which rests against a shoulder on the injector housing. The ring has an inner diameter that is larger than the diameter of the upper section of the pressure booster piston and smaller than the diameter of its middle section. The ring makes it possible to pre-assemble the pressure booster piston and the spring element, with the spring element in a prestressed position. To this end, the ring is first slid onto the upper section of the pressure booster piston until it comes to rest against the middle section. Then the spring element is slid onto the upper section of the pressure booster piston. In a subsequent step, the spring plate is placed onto the upper section of the pressure booster piston. The spring element together with the spring plate is prestressed and the ring element is slid onto the upper section of the pressure booster piston until it snaps into the groove. In this way, one end of the prestressed spring element is supported against the ring that rests against the middle section and the other end is supported against the spring plate.

The pressure booster piston is preferably embodied of one piece and includes an upper section that is produced with a first diameter, a middle section that is produced with a second diameter, and a lower section that is produced with a third diameter. The second diameter in which the middle section is produced is larger than both the first diameter of the upper section and the third diameter of the lower section. In a completely assembled fuel injector, the upper section is encompassed by the control chamber, the middle section delimits the control chamber with a first shoulder and delimits the differential pressure chamber with a second shoulder situated at the opposite end from the first shoulder, while the lower section delimits the compression chamber.

To enable actuation of the fuel injector, the control chamber is hydraulically connected to the compression chamber; the connection contains a check valve that prevents a reverse flow of fuel from the compression chamber back into the control chamber. In a preferred embodiment, the hydraulic connection of the control chamber to the compression chamber is embodied in the form of a bore in the pressure booster piston.

To enable actuation of the fuel injector, the first control valve is able to connect the differential pressure chamber to a fuel supply or a fuel return. To this end, the first control valve is embodied in the form of a 3/2-way directional-control valve.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1is a hydraulic diagram of a fuel injector embodied according to the invention.

A pump2draws fuel from a fuel storage tank1and supplies its to a high-pressure reservoir3. From the high-pressure reservoir3, the fuel is supplied to a fuel injector5via a fuel supply line4. To this end, the fuel supply line4feeds into a control chamber6of a pressure booster7. The control chamber6encompasses an upper section8of a pressure booster piston9. The control chamber6is delimited at one end by the injector housing10and that the other end by a first shoulder11of a middle section12of the pressure booster piston9.

Also according to the invention, the control chamber6contains a spring element13that encompasses the upper section8of the pressure booster piston9. One end of the spring element13is supported against a spring plate14in the upper section8of the pressure booster piston9and the other end is supported against a ring15. The ring15here is held by the injector housing10. The spring element13is preferably a spiral spring embodied in the form of a compression spring.

A supply line16connects the control chamber6to a first control valve17. The first control valve17is a 3/2-way directional-control valve. The first control valve17is actuated by means of an electrically triggerable actuator. This can, for example, be an electromagnetic or piezoelectric actuator. Any other rapidly switching actuating unit known to those skilled in the art can also be used as the actuator.

In a first switched position of the first control valve17, the control chamber6is connected to a differential pressure chamber19via the supply line16and a conduit18. The differential pressure chamber19is delimited by a second shoulder20of the middle section12of the pressure booster piston9. The second shoulder20here is situated at the opposite end from the first shoulder11. In addition, the differential pressure chamber19encompasses a lower section21of the pressure booster piston9.

In a second switched position of the first control valve17, the differential pressure chamber19is connected to a fuel return22via the conduit18. The fuel return22preferably feeds into the fuel storage tank1.

The lower section21of the pressure booster piston9delimits a compression chamber23. A conduit24, which is embodied for example in the form of a bore in the pressure booster piston9, fills the compression chamber23with fuel. A check valve25is provided in the conduit24to prevent fuel from flowing out of the compression chamber23via the conduit24and back into the control chamber6.

From the compression chamber23, fuel at injection pressure is supplied via a high-pressure line26into a nozzle chamber27and a second control chamber28. The fuel travels out of the second control chamber28to a second control valve31via an outlet line29that contains an outlet throttle30. The second control valve31is embodied as a 2/2-way directional-control valve that can open or close a connection from the outlet line29into a fuel return32. The second fuel return32is connected, for example, to the fuel return22or directly to the fuel storage tank1. The second control valve31is usually actuated with an electrically triggered actuator. The actuator can, for example, be a solenoid valve or a piezoelectric actuator. As in the first control valve17, any other rapidly switching actuating unit known to those skilled in the art can also be used.

An injection valve element33extends into the second control chamber28. The injection valve element33is able to open or close at least one injection opening34. When the injection opening34is open, fuel flows out of the nozzle chamber27through the injection opening34and into a combustion chamber35of an internal combustion engine.

In order to initiate the injection event, the first control valve17is initially switched so that the connection from the differential pressure chamber19via the conduit18into the fuel return22is open. As a result, the pressure in the differential pressure chamber19drops. In addition, system pressure continues to prevail in the control chamber6. For this reason, the force of pressure acting on the first shoulder11on the middle section12is greater than the force acting on the second shoulder20and the lower section21of the pressure booster piston9. The pressure booster piston9is slid into the compression chamber23. As a result, the fuel contained in the compression chamber23is compressed to injection pressure. The compressed fuel flows through the high-pressure line26into the nozzle chamber27and second control chamber28. So that the injection valve element33can lift away from its seat and thus open the at least one injection opening34, the second control valve31is switched so that the connection from the outlet line29into the fuel return32is open. As a result, the fuel flows out of the second control chamber28. The pressure in the second control chamber28decreases and the fuel at injection pressure acts on the injection valve element33so that it is lifted away from its seat. Fuel is injected into the combustion chamber35of the internal combustion engine.

In order to terminate the injection event, the second control valve31is initially switched so that the connection from the outlet line29to the fuel return32is closed. As a result, the pressure in the second control chamber28rises to injection pressure. The injection valve element is moved into its seat, thus closing the at least one injection opening34. A spring element36assists the movement of the injection valve element33. The second spring element36is preferably a spiral spring embodied in the form of a compression spring, one end of which is supported against a shoulder on the injection valve element33and the other end of which is supported against the injector housing10. The second spring element36is embodied so that it assists the movement of the injection valve element33into its seat. Next, the first control valve17is switched so that the connection from the control chamber6to the differential pressure chamber19via the supply line16and the conduit18is open. As a result, fuel at system pressure flows out of the control chamber6into the differential pressure chamber19. System pressure builds up in the differential pressure chamber19. Assisted by the spring element13, the pressure booster piston9is moved into its starting position. In other words, the booster piston9is moved into the first control chamber6. At the same time, this also increases the volume of the compression chamber23. The pressure in the compression chamber23decreases. As soon as the pressure in the compression chamber23has fallen below the system pressure, the check valve25opens and fuel flows out of the control chamber6into the compression chamber23via the conduit24in the pressure booster piston9. As soon as the pressure booster piston9has reached its starting position, i.e. the position of the pressure booster piston9in which the volume in the compression chamber23has reached its maximum, the next injection event can begin.

FIG. 2gives a more detailed view of a fuel injector embodied according to the invention.

FIG. 2shows that the first control valve17is embodied in the form of a solenoid valve. In this case, the first control valve17has a coil40that is contained in a magnet core41. To trigger the first control valve17, the coil40can be connected to a control unit that is not shown here via connecting pins42. The first control valve17also has an armature43that is connected to a valve slider44. By means of the valve slider44, the first control valve17can be switched so that either the supply line16from the control chamber6is connected to the conduit18into the differential pressure chamber19or this connection is closed and a connection of the conduit18from the differential pressure chamber19to the fuel return22is open instead.

The second control valve31shown inFIG. 2is also embodied in the form of a solenoid valve. In this case, the second control valve31has a second coil45that is contained in a second magnet core46. The second control valve31also has a second armature47that is connected to a valve element48. The second valve element48can close or open a connection from the outlet line29into the fuel return32.

FIG. 3is an enlarged depiction of the pressure booster7of the fuel injector shown inFIG. 2.

FIG. 3shows that one end of the spring element13is supported against the ring15and the other end is supported against the spring plate14. The ring15rests against a shoulder50embodied on a middle housing section51. The spring force of the spring element13is thus transmitted to the injector housing10. The middle housing section51is connected to the upper housing section52by means of a retaining nut53.

According to the invention, the spring element13has a conical section54. This prevents the spring element13from rubbing against the upper section8of the pressure booster piston9. So that the spring element13can function as a return spring for the pressure booster piston9, it is preferably a spiral spring embodied in the form of a compression spring, one end of which acts on the injector housing10and the other end of which acts on the pressure booster piston9. To achieve this, one end of the spring element13is placed against the injector housing10by means of the ring15and the shoulder50and the other end is placed against the upper section8of the pressure booster piston9by means of the spring plate14. To hold the spring plate14in place on the upper section8of the pressure booster piston9, the spring force of the spring element13presses it against a ring element55. The ring element55is prevented from sliding on the upper section8of the pressure booster piston9by being accommodated in a groove56in the upper section8of the pressure booster piston9. The ring element55is preferably a snap ring.

One advantage of installing the spring element13with the ring15and the spring plate14is that the pressure booster piston9can be preassembled together with the spring element13. This also simplifies the subsequent installation in the fuel injector5. For assembly, first the ring15is slid onto the upper section8of the pressure booster piston9. To this end, the inner diameter of the ring15is selected so that it is larger than the diameter d1of the upper section8of the pressure booster piston9and smaller than the second diameter d2of the middle section12of the pressure booster piston9. Because the second diameter d2of the middle section12is larger than the first diameter d1of the upper section8of the pressure booster piston9, this forms the first shoulder11at the transition from the first section8to the middle section12. Since the inner diameter of the ring15is smaller than the second diameter d2of the middle section12of the pressure booster piston9, the ring15rests against the shoulder11when installed. Then, the spring element13is slid onto the upper section8of the pressure booster piston9until it rests against the ring15. The conical region54of the spring element13is embodied so that the last coil of the spring element13has an inner diameter that corresponds to the first diameter d1of the upper section8of the pressure booster piston9. This simultaneously centers the spring element13on the upper section8. In the next assembly step, the spring plate14is slid onto the upper section8of the pressure booster piston9. Lastly, the ring element55is installed. To facilitate installation of the ring element55, a bevel57is provided on the upper section8of the pressure booster piston9. The ring element55is preferably embodied to be expandable. This allows the ring element55to be slid onto the upper section8of the pressure booster piston9via the bevel57. The ring element55expands as it travels along the bevel57. As soon as the ring element55reaches the groove56, it snaps into it. This secures the ring element55firmly to the upper section8of the pressure booster piston9. From this point on, the spring force of the spring element13holds the spring plate14pressed against the ring element55. To prevent the spring plate14from tilting in response to an uneven load exerted on it by the spring element13, the spring plate is preferably provided with a cylindrical section58that encompasses the ring element55. This achieves a firm seating of the spring plate14.

The thus preassembled pressure booster piston9with the spring element13is then inserted into the middle housing part51. So that the ring15rests against the shoulder50on the middle housing part51, the outer diameter of the ring15is larger than the second diameter d2of the middle section of the pressure booster piston9. If the pressure booster piston9is slid further into the middle housing part51after the ring15has come to rest against the shoulder50, this prestresses the spring element13. This prestressing serves to move the pressure booster piston9back into the control chamber6during operation. The movement of the pressure booster piston9finishes when it strikes against an end surface59that is embodied in the upper housing section52and delimits the control chamber6. A lateral bore60that feeds into the conduit24is provided in the upper section8of the pressure booster piston9so that fuel can flow out of the control chamber6and into the compression chamber23even when the pressure booster piston9has come to rest against the end surface59. Fuel at system pressure continuously travels into the conduit24via the lateral bore60.

The conduit24also contains a throttle element61. The throttle element61damps the fuel flow in the conduit24, thus avoiding a pulsation of the check valve25and a resulting wear in the region of the check valve25.

In order to be able to produce an injection pressure in the compression chamber23that is higher than the system pressure with which the fuel is supplied to the fuel injector5, the lower section21of the pressure booster piston9is embodied with a third diameter d3that is smaller than the second diameter d2. The injection pressure to which the fuel in the compression chamber23is compressed is thus a function of the ratio of the second diameter d2of the middle section12of the pressure booster piston9to the third diameter d3of its lower section21.

The foregoing relates to the preferred exemplary embodiment 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.