Injection device

The invention relates to an injection device, in particular for injecting fluid into an exhaust tract of an internal combustion engine, having a valve unit which comprises a valve needle, an injection chamber having at least one injection opening, and a control chamber, wherein the injection device is designed so that a pressure differential between the injection chamber and the control chamber brings about a displacement of the valve needle between an open position in which the valve needle releases a fluid flow through the injection opening, and a closed position in which the valve needle closes off the injection opening. The injection device also has a pump unit integrated in the injection device. The pump unit is designed so as to draw in fluid from the fluid inlet during operation and to provide said fluid to the valve unit under increased pressure.

BACKGROUND OF THE INVENTION

The invention relates to an injection device for fluid, in particular to an injection device for injecting fluid into an exhaust tract of an internal combustion engine.

The demands on the exhaust-gas quality of internal combustion engines, in particular of internal combustion engines for driving motor vehicles, have become ever higher in recent years. In the case of diesel engines in particular, NOxemissions constitute a problem, which is counteracted by means of so-called SCR catalytic converters. In an SCR catalytic converter, environmentally harmful NOxis converted into N2and H2O by means of NH3, which is supplied to the catalytic converter generally in the form of an aqueous urea solution.

In order to supply the urea solution to the exhaust gases of the internal combustion engine, a dosing system is required which conventionally comprises an electrically operated pump and an electrically activated dosing valve. Such known dosing systems are complex and expensive in terms of manufacture, assembly and maintenance.

EP 1 878 920 A1 discloses a liquid pump having an inlet, an outlet, a pump chamber for receiving the liquid, and an actuator which is movable between a first position and a second position and which is designed to pump liquid out of the pump chamber and into the outlet. The inlet and the outlet are fluidically connected to a supply passage when the actuator is in the first position. The supply passage runs around the actuator in order to permit a transfer of heat from the actuator to the liquid.

US 2007/0295003 A1 describes a high-pressure dosing pump which is intended for providing a reducing agent to an exhaust-gas reduction system. The high-pressure dosing pump has an electromagnet for driving a piston which is movably mounted in an inner bore of a valve housing of the pump. The inner bore has a pressure chamber with a one-way inlet valve and a one-way outlet valve. Movement of the piston causes reducing agent at high pressure to be supplied to an injection nozzle. The injection nozzle is arranged at a location which permits a maximum reduction of undesired pollutants in the exhaust gases.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved dosing system which permits an effective injection of fluid and which is inexpensive to manufacture, assemble and maintain.

The object is achieved by means of an injection device according to the invention for a dosing system, as claimed in independent patent claim1. The dependent patent claims describe advantageous embodiments of an injection device according to the invention.

An injection device according to the invention which is designed in particular for injecting fluid into an exhaust tract of an internal combustion engine comprises a valve unit which has a valve needle (nozzle needle), a control chamber and an injection chamber with at least one injection opening. The injection device is designed such that the valve needle can be moved between an open position, in which the valve needle permits a fluid flow through the injection opening, and a closed position, in which the valve needle closes the injection opening, by a pressure difference between the injection chamber and the control chamber.

There is additionally integrated into the injection device a pump unit which is designed to suck fluid out of a fluid supply and discharge said fluid at elevated pressure, that is to say at a pressure higher than the pressure in the fluid supply, to the valve unit. When the valve unit is open, fluid is thus discharged out of the injection device at a pressure higher than the pressure in the fluid supply.

In one embodiment, the valve unit and the pump unit are arranged in a common housing. This permits a particularly compact construction of the injection device. The line paths between the pressure-generating pump unit and the valve unit in which the fluid is under elevated pressure during operation are shorter than in a conventional construction with an external pump unit, and run entirely within the injection device. The risk of an uncontrolled escape of fluid from the injection device (leakage) is reduced, and the hydraulic stability of the system is improved.

An injection unit according to the invention permits a fast release of pressure from the valve needle during the opening process, such that short switching times and a broad range of possible injection quantities can be realized.

A reduced structural size of the injection device permits a high degree of variability during assembly, for example on an exhaust tract, and increases the freedom for the configuration of the fluid tank; in particular, a reduced structural size of the injection device makes it possible to enlarge the usable volume of the fluid tank.

The required injection pressure is, according to the invention, generated in the injection device itself. A (high-pressure) feed line in which the fluid is at elevated pressure and which must therefore be of particularly stable form, and which is nevertheless susceptible to failure and leakage, can be dispensed with. This increases the operational reliability of the injection system.

An injection device according to the invention has a low voltage and power requirement during operation, and permits a standardized design for multiple applications.

In one embodiment, the valve unit has a control chamber which is delimited by an end of the valve needle, wherein the volume of the control chamber can be varied by movement of the valve needle, or the valve needle can be moved by variation of the pressure in the control chamber. Such an arrangement makes it possible for the valve needle to be actuated by variation of the fluid pressure in the control chamber. It is possible to dispense with a mechanical actuator for activating the valve needle. This simplifies the construction of the injection device, and in particular of the valve unit.

In one embodiment, the control chamber is hydraulically connected to a supply line through which fluid can be supplied to the injection device during operation. The same pressure thus prevails in the control chamber as in the supply line, and the valve needle is pushed by the fluid pressure in the control chamber into a closed position in which it prevents a fluid flow out of the injection device. During operation, the valve unit is reliably closed by the fluid pressure, without the need for an additional actuator.

In one embodiment, the pump unit has a piston chamber and a piston which is movable within the piston chamber. Here, the piston is arranged and designed such that the volume of the piston chamber and the pressure in the piston chamber can be varied by movement of the piston. As a result of such a combination of a piston chamber and a movable piston, a reliable and effective pump unit is provided which is suitable for increasing the pressure of the fluid to be injected.

In one embodiment, the piston chamber is hydraulically connected to the fluid supply by a one-way valve which is formed for example as a non-return ball valve. The one-way valve prevents fluid from flowing out of the piston chamber back into the supply line, and the elevated pressure built up by movement of the piston in the piston chamber thereby being dissipated as a result of a fluid flow out of the piston chamber into the supply line.

In one embodiment, the piston and the valve needle are movable along a common axis. An injection device in which the piston and the valve needle are movable along a common axis can be of particularly simple, space-saving and inexpensive construction. In particular, such an injection device can be constructed in the longitudinal direction of a cylindrical housing, wherein the piston and the valve needle are of cylindrical form and are movable parallel to the axis of the cylinder. A cylindrical injection device of said type is particularly durable and is simple and inexpensive to manufacture.

In one embodiment, the piston can be moved by energization of an electromagnet arranged in the injection device. An electromagnet provides a simple, inexpensive and reliable actuator for moving the piston. The actuator may alternatively be formed as a piezo actuator.

In one embodiment, the piston is supported by an elastic piston-spring element which pushes the piston in the direction of an initial position. An elastic piston-spring element of said type makes it possible to ensure that, when the electromagnet is deactivated, the piston is moved reliably into an initial position.

In one embodiment, the valve needle is supported on a housing of the injection device by an elastic valve needle spring element in such a way that, when the electromagnet is deactivated, that is to say deenergized, the elastic spring element forces the valve needle into the closed position, and the valve unit is reliably closed when the electromagnet is deactivated.

DETAILED DESCRIPTION

In the following description of the figures, statements such as “top” and “bottom” are used for better explanation of the exemplary embodiments of the invention shown in the figures, without restricting the invention to the exemplary embodiments shown or to a particular orientation and/or installation position.

FIG. 1shows a first sectional view of an injection device2according to the invention during a suction process.

An injection device2according to the invention has a for example cylindrical nozzle body4, along the longitudinal axis A of which there is formed a for example substantially cylindrically shaped injection chamber38. At that face end of the injection chamber38which is illustrated at the bottom inFIG. 1there is formed an injection opening8through which fluid emerges from the injection chamber38during an injection process. A lower region, which adjoins the injection opening8, of the injection chamber38has a smaller cross section in a plane perpendicular to the longitudinal axis A of the nozzle body4than an upper region, which is at a greater distance from the injection opening8, of the injection chamber38.

In the injection chamber38there is arranged a substantially cylindrical valve needle6, the longitudinal axis of which is aligned along the longitudinal axis A of the nozzle body4. The valve needle6is of stepped form with a conical lower region6aand a plurality of cylindrical regions6b,6c,6d,6e, wherein the cylindrical regions6b,6c,6d,6ehave, in a plane perpendicular to the longitudinal axis A of the valve needle6, a cross section which is larger the greater the distance thereof from the lower, conical region6a.

The valve needle6is movable along its longitudinal axis A between a closed position, in which the lower end6aof the valve needle6rests on the valve seat8aand closes off the injection opening8in a substantially fluid-tight manner, and an open position, in which the valve needle6opens up the injection opening8.

Around the circumference of an upper region, which is remote from the injection opening8, of the valve needle6there is arranged a cylindrical control chamber sleeve16. Within the control chamber sleeve16there is formed, above the upper face end6bof the valve needle6, a control chamber36whose volume can be varied by movement of the valve needle6in a direction parallel to the longitudinal axis A thereof. Conversely, the valve needle6can be moved parallel to its longitudinal axis A by variation of the difference between the pressure in the injection chamber38and the pressure in the control chamber36.

The control chamber36is delimited on the upper side, which is remote from the valve needle6, by a control plate18. The control plate18is fixed to the nozzle body4by securing pins (poka-yoke pins)46(not visible inFIG. 1) and by a nozzle clamping nut10which surrounds the nozzle body4and the control plate18.

In a central region6eof the valve needle6as viewed in the longitudinal direction, a support ring12is formed around the circumference of the valve needle6. Between the support ring12and the control chamber sleeve16, a cylindrical valve needle spring element14is arranged around the circumference of the valve needle6, which valve needle spring element is supported with its two faces at one side on the control chamber sleeve16and at the other side on the support ring12. The valve needle spring element14pushes the valve needle6elastically into the lower closed position, in which the valve needle6closes off the injection opening8in a substantially fluid-tight manner.

In the control plate18there is formed a fluid duct33with a one-way valve20designed for example as a ball valve or non-return valve. When the one-way valve20is open, the control chamber36is hydraulically connected via the fluid duct33to a piston chamber34which is formed above the one-way valve20in the control plate18.

The volume of the piston chamber34is delimited, on a side facing away from the one-way valve20, by a movable piston28which is arranged above the piston chamber34and which is supported elastically on the control plate18by a piston spring element40. The volume of the piston chamber34can be varied by movement of the piston28in a direction parallel to the longitudinal axis A.

A setting disk22is arranged between a lower face end, which faces toward the valve needle6, of the elastic piston spring element40and the control plate18. The stroke of the piston28can be set through selection of the thickness of the setting disk22.

Around the circumference of the piston28there are formed a metallic inner pole24and a coil30, which together form an electromagnet which is suitable for moving the piston28. At an upper region of the piston28remote from the control plate18, an armature26is formed around the circumference of the piston28. The armature26is magnetically attracted by the inner pole24when an electrical current flows through the coil30.

During a suction process (“suction stroke”) as shown inFIG. 1, with the coil30deactivated, that is to say when no electrical current flows through the coil30, the piston28moves away from the control plate18parallel to the longitudinal axis A under the action of the force exerted by the elastic piston spring element40, such that the spacing between the piston28and the control plate18increases. The volume of the piston chamber34is increased and fluid from the supply32flows through the control chamber36, the fluid duct33and the open one-way valve20into the piston chamber34.

As a result of interaction of the fluid pressure in the control chamber36connected to the supply32and the elastic force of the valve needle spring element14, the valve needle6is forced into the lower closed position, in which the lower end6aof the valve needle6closes the injection opening8in a fluid-tight manner and no fluid can flow out of the injection chamber38through the injection opening8.

FIG. 2shows a section through the injection device2according to the invention, as shown inFIG. 1, in a plane rotated through 90° about the longitudinal axis A of the injection device2.

The components already shown inFIG. 1are denoted by the same reference numerals, and will not be described in detail again.

In the second section plane shown inFIG. 2, the supply32is not visible. Instead, in this plane, it is possible to see a connecting duct48which is formed in the control plate18and which hydraulically connects the piston chamber34to the injection chamber38. The connecting duct48is formed such that a fluid flow between the piston chamber34and the injection chamber38is possible regardless of whether the one-way valve20is open or closed.

The securing pins46already mentioned in conjunction withFIG. 1, by means of which the control plate18is fixed to the nozzle body4, can also be seen inFIG. 2.

To initiate an injection process, an electrical voltage is applied to the coil30such that an electrical current flows through the coil30. The armature26is attracted in the direction of the inner pole24by the magnetic field generated by the current flow in the coil, and the piston28which is connected to the armature26moves in the direction of the control plate18(“pressure or injection stroke”).

As a result of the movement of the piston28in the direction of the control plate18, the volume of the piston chamber34is reduced, and the fluid pressure in the piston chamber34is increased. The one-way valve20closes and prevents a return of fluid from the piston chamber34into the supply32. Fluid flows out of the piston chamber34into the injection chamber38through the connecting duct48and also increases the fluid pressure in said injection chamber.

When a certain critical value of the fluid pressure in the injection chamber38is exceeded, the fluid pressure in the control chamber36and the force of the valve needle spring element14are no longer sufficient to hold the valve needle6in the closed position counter to the pressure of the fluid which has flowed into the injection chamber38, which fluid acts on the regions6a,6c,6d,6eof the valve needle6and in particular exerts a force, which is directed toward the control chamber36, on the transitions between the regions6a,6c,6dand6e. The valve needle6moves from the closed position into an open position counter to the fluid pressure in the control chamber36, and the lower region6aof the valve needle6moves away from the valve seat8aand opens up the injection opening8.

Fluid which is displaced out of the control chamber36by the opening movement of the valve needle6flows back into the supply32, such that the fluid pressure in the control chamber36does not increase significantly. As a result, the valve needle6rises out of its seat8a, and opens up the injection opening8, particularly quickly.

Fluid at elevated pressure flows out of the injection chamber38through the open injection opening8(injection process) until the fluid pressure in the injection chamber38has fallen to such an extent that it is no longer capable of holding the valve needle6in an open position counter to the combination of the fluid pressure in the control chamber36and the force of the valve needle spring element14. The valve needle6moves back into the lower, closed position again under the action of the fluid pressure in the control chamber36and the force of the valve needle spring element14, in which lower, closed position the lower end6aof the valve needle6is pressed against the valve seat8aand closes off the injection opening8.

By deactivation of the current flow through the coil30, the electromagnet is deactivated and the piston28moves back, under the influence of the piston spring element40, in a direction in which the distance from the piston28to the control plate18and the volume of the piston chamber34increase (“suction stroke”, seeFIG. 1). The one-way valve20opens and fluid flows out of the supply32into the piston chamber34.

By application of an electrical voltage to the coil30again, a further injection process as has been described above can now be initiated.

Below, possible dimensions of the pressure unit and in particular of the coil30of the electromagnet in order to generate a predefined injection pressure will be described, by way of an example, with reference toFIG. 3:

In the case of a pressure of 7 bar in the fluid supply32, it is sought for example to generate an injection pressure of 9.5 bar, such that an additional pressure of 2.5 bar must be generated by the pressure unit.

For an assumed diameter DKof the piston28of 9 mm, that is to say a size of the circular face A of the piston of AK=19.63 mm2, a force to be exerted on the piston28can be calculated as
FK=Δp*AK=2.5 bar*19.63 mm2=4.9 N:

Said force F is to be imparted as a magnetic force which is exerted on the armature26by the coil28:
Fm=B2*AA/8π.

For an assumed magnetic field strength of B=1.8 T generated by the coil28, the required area of the armature AAcan be calculated as:
AA=8π*Fm/B2≈4.5 mm2.

Assuming that the effective area of the armature26pressed onto the piston28has an inner diameter of dA=7 mm, a required outer diameter DAof the armature26can be calculated as:
DA2=AA/4π+dA2
DA≈7.5 mm.

For an assumed magnetomotive force θ of 150 Aw and an assumed maximum current through the coil28of imax=2.2 A, the number of windings of the coil28can be calculated as
N=θ/imax≈68.

If a wire with a diameter dDof 0.45 mm is used for the coil28and the coil28is wound in 6 layers each with approximately 12 windings, then for an inner diameter dSpof the coil28of approx. 5.5 mm, the resulting wire length is approximately 2.5 mm.

A wire conventionally used for such coils has, at this length, and at a temperature of 20° C., an electrical resistance of approximately 5.5Ω. For a supply voltage of 16 V, it can thus be calculated that a current i of
i =U/R≈2.9 A
must flow through the coil30in order to generate the desired injection pressure of 9.5 bar.