WO2016015792A1

WO2016015792A1 - Reduction agent dosing system with damped reduction agent feed

Info

Publication number: WO2016015792A1
Application number: PCT/EP2015/001028
Authority: WO
Inventors: Enzo Falbo; Werner Overhoff
Original assignee: Albonair Gmbh
Priority date: 2014-07-28
Filing date: 2015-05-20
Publication date: 2016-02-04
Also published as: DE102014010948A1; CN106460608B; CN106460608A

Abstract

The invention relates to a reduction agent dosing system (10) for the injection of a reduction agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, having a feed pump (20), by means of which a reduction agent is suctioned from a reduction agent tank (40) via a suction line (30) from the tank (40) and fed via a pressure line (50), and introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle (60), wherein the suction line (30) has a bidirectional rubber valve (70).

Description

Reducing agent metering system with damping of

Reducing agent Promotion

The invention relates to a Reduktionsmitteldosiersystem for injection of a reducing agent in the exhaust stream of an internal combustion engine for selective catalytic reduction, with a feed pump, sucked by means of which reducing agent from a reducing agent tank via a suction line from the tank and conveyed via a pressure line and via at least one nozzle in the exhaust stream of the Internal combustion engine is initiated. Furthermore, the invention relates to a method for operating a Reduktionsmitteldosiersystems for injecting a reducing agent into the exhaust stream of an internal combustion engine for selective catalytic reduction, with a magnetic piston pump as a feed pump, sucked by means of which reducing agent from a reducing agent tank via a suction line from the tank and conveyed via a pressure line and over at least one nozzle is introduced into the exhaust gas flow of the internal combustion engine, wherein the piston movement is brought about by driving one or more cylindrical coils. Selective Catalytic Reduction (SCR) catalysts are used to reduce the NOx emissions from diesel engines, combustion plants, incinerators, industrial plants, and the like. For this purpose, a reducing agent in the exhaust system with a Dosing metered. The reducing agent used is ammonia or an ammonia solution or another reducing agent.

Since the entrainment of ammonia in vehicles is safety-critical, urea is used in aqueous solution with usually 32.5% urea content, in particular according to DIN 70070. In the exhaust gas, the urea decomposes at temperatures above 150 ° Celsius into gaseous ammonia and CO ₂ . Parameters for the decomposition of the urea are essentially time (evaporation and reaction time), temperature and droplet size of the injected urea solution. In these SCR catalysts, selective catalytic reduction (SCR) reduces the emission of nitrogen oxides by about 90%.

In the known Reduktionsmitteldosiersystemen for injecting a reducing agent in the exhaust stream of an internal combustion engine for selective catalytic reduction, the reducing agent solution is conveyed by means of a magnetic piston pump to the nozzle. It has been shown that occurs by the pulse in the delivery line after a delivery stroke of the magnetic piston pump uncontrolled subsequent delivery of the reducing agent. Furthermore, it has been shown that due to a high piston speed in a delivery stroke, the hydraulic fluid does not flow as desired through the control bores from the cylinder, but instead hydraulic fluid is conveyed even before closing the control bores, resulting in a promotion of too much reducing agent due to increased deflection of the conveyor diaphragm , which makes an exact dosage difficult.

Furthermore, it has been shown that due to a high piston speed after a delivery stroke, the hydraulic fluid is not supplied as desired via the control bores the cylinder and piston, but instead hydraulic fluid is still sucked from the diaphragm chamber, resulting in a suction of too much reducing agent, which is an exact Dosage difficult. The object of the invention is to form a Reduktionsmitteldosiersystem of the type mentioned on and provide a method for operating a Reduktionsmitteldosiersystems, so that an exact dosage of the reducing agent allows and unwanted over-promotion or Nachförderung after a delivery stroke is reliably prevented.

This object is achieved by a Reduktionsmitteldosiersystem according to claim 1. Advantageous developments of the invention are specified in the dependent claims.

Particularly advantageous in the Reduktionsmitteldosiersystem for injecting a reducing agent in the exhaust stream of an internal combustion engine for selective catalytic reduction, sucked with a feed pump, by means of which reducing agent from a reducing agent tank via a suction line from the tank and conveyed via a pressure line and via at least one nozzle in the exhaust stream of the It is that the suction line has a bidirectional rubber valve. In particular, the reducing agent metering system may comprise a pump unit containing the feed pump. This pump unit may have additional components, in particular switching valves and / or sensors, in addition to the delivery pump. The term "suction line" designates the delivery line from the reducing agent tank to the suction mouth of the delivery pump. In this suction line a bidirectional rubber valve is arranged. In such a bidirectional rubber valve is a passive component which automatically opens when a corresponding pressure difference between the two sides of the bidirectional rubber valve and releases the flow channel and vice versa automatically closes the flow channel again falls below the required pressure difference on both sides of the bidirectional rubber valve , As a result, an undesirable Nachfördern of Reducing agent reliably prevented due to the pulse in the delivery line after a delivery stroke of the feed pump.

By the term of the pressure line while those conveyor line is designated, in which ^'opens, the pressure side of the pump and through which the reducing agent from the pump is fed to the nozzle.

The terms reducing agent metering system or metering system are used synonymously in the context of the invention. The term reducing agent solution or reducing agent includes any reducing agent suitable for selective catalytic reduction, preferably a urea solution according to DIN 70070 is used for this purpose. However, the invention is not limited thereto. In a preferred embodiment, the pressure line has a throttle. It has been shown that especially the arrangement of a bidirectional rubber valve in the suction line in combination with a throttle in the pressure line allows a particularly accurate dosing of the reducing agent and reliably prevents unwanted over-delivery or Nachförderung after a delivery.

The system preferably has a component carrier on which the delivery pump is mounted, and wherein a suction channel to the pump inlet and a pressure channel adjoining the pump outlet are integrated into the component carrier, wherein the component carrier has a mounting region in which the rubber valve is integrated into the suction channel is.

By such a component carrier, a particularly advantageous arrangement of the pump and the bidirectional rubber valve in the suction line is possible. It is possible to pre-assemble the components arranged on and / or on the component carrier to form an assembly. This results in a simpler installation of the dosing and a saving of the required installation space Summary of multiple components to a space-efficient assembly.

In a preferred overall arrangement, the reducing agent metering system has a tank to which the suction line is connected. In this case, the reducing agent solution is filled into the tank and stored in the tank for the operation of the system. For dosing, the reducing agent solution is removed from the tank and conveyed by means of the feed pump and introduced via at least one nozzle in the exhaust gas stream of the internal combustion engine.

Particularly preferably, the system has a compressed air supply and the reducing agent is atomized inside or outside the nozzle by means of compressed air. In this case, the compressed air supply may have a switching valve and / or a pressure regulating valve. This switching valve is used to control, d. H. the switching on and off of the compressed air supply for all or part of the dosing system.

Alternatively or cumulatively, the compressed air supply may have a pressure regulating valve. In this way, the compressed air can be adjusted to a desired level for atomization of the reducing agent by means of compressed air pressure level. The compressed air itself can be taken from an on-board compressed air system, such as a commercial vehicle, in the exhaust system, the dosing is arranged without the prevailing system pressure in the compressed air system is a limitation, since the pressure of the compressed air can be lowered to the desired pressure.

In a preferred embodiment of the Reduktionsmitteldosiersystems therefore a compressed air supply is provided, wherein the reducing agent is atomized inside or outside the nozzle by means of compressed air. For atomizing the reducing agent, a mixing chamber may be provided, within which an atomization of the reducing agent by means of the compressed air takes place even before the introduction into the exhaust gas tract. In a preferred embodiment, however, the nozzle is an externally mixing two-fluid nozzle in which the reducing agent solution emerges from a first nozzle opening and compressed air emerges from a second nozzle opening, the two nozzle openings being aligned with one another in such a way that the compressed air atomizes the reducing agent outside the nozzle so that the nozzle is designed as an externally mixing two-substance nozzle and forms the aerosol takes place outside the nozzle. In particular, the second opening of the nozzle is positioned in such a way, in particular positioned at an angle relative to the jet direction of the first opening of the nozzle, that by means of the compressed air emerging from the second opening, the reducing agent emerging from the first opening is atomized.

Preferably, the pressure line is connected via a switching valve or control valve to a compressed air supply to free the pressure line and the nozzle after completion of the dosage by means of compressed air from reducing agent.

In this preferred embodiment of the Reduktionsmitteldosiersystems thus a compressed air supply is provided, wherein the reducing agent-conveying pressure line and thus the nozzle are connected via a switching valve to the compressed air supply to free them after completion of the dosage by means of compressed air from reducing agent.

As a result, the reducing agent-conveying pressure line and the nozzle and / or a metering chamber and / or the dosing can be freed after completion of the dosage by means of the compressed air from the reducing agent solution to prevent freezing or crystallization of the reducing agent solution. This can effectively prevent frost damage and blockage.

The compressed air can thus be used alternatively or cumulatively for atomizing the reducing agent in the exhaust line and for cleaning the reducing agent-carrying lines after the end of the dosage.

Preferably, the pressure in the pressure line is detected and monitored by means of a pressure sensor. By such detection and monitoring of Pressure in the reducing agent leading pressure line can also be the correct operation of the feed pump and the dosage continuously monitored. The system preferably has a heating device for heating the reducing agent solution. The widely used reducing agent solution according to DIN 70070 freezes due to its water content at about -11 ° C. Therefore, it is necessary to provide a heating device, for example inside the tank and / or in thermal coupling to the suction line and / or pressure line for heating the reducing agent solution in the case of very low ambient temperatures.

The feed pump is preferably a piston pump, in particular a magnetic piston pump. In such a magnetic piston pump, one or more cylindrical coils, by means of which a magnetic field is generated, arranged. The piston movement of the magnetic piston is controlled by the magnetic field generated by the coils.

In the case of a magnetic piston pump, it is particularly advantageous that it can be actuated and controlled precisely by a corresponding control of the coils producing the magnetic field.

Accordingly, in a method for operating a reducing agent metering system for injecting a reducing agent into the exhaust gas stream of a selective catalytic reduction internal combustion engine, it is possible for a reducing agent to be sucked from a reducing agent tank via a suction line from the tank and conveyed via a pressure line and is introduced via at least one nozzle in the exhaust stream of the internal combustion engine, wherein the piston movement is controlled by driving one or more cylindrical coils, by means of which a magnetic field is generated, to provide that a pulse-width modulated control of the / the solenoid / n depending on the current Position, speed and direction of movement of the piston takes place. By the term of the control of the / the cylindrical coil / n is meant the time-variable energization of the coil / n, by each of which a resulting magnetic field is generated, which exerts a resultant force on the magnetic piston. By the resulting magnetic force of the piston is reciprocated in the cylinder of the magnetic piston pump between the top dead center and the bottom dead center and thereby accelerated or decelerated. By a pulse-width modulated control of the / the cylindrical coil / n, the duty cycle, ie the quotient of duty cycle to off duration of energization of the / the cylindrical coil / n during a cycle, varies, resulting in a time-varying resulting magnetic field / of the cylinder / n results. This in turn follows a time-varying resulting on the piston resulting magnetic force and the resulting variable piston acceleration or piston speed. By varying the duty cycle, the arithmetic mean of the electrical voltage can be changed. Accordingly, by a pulse width modulated control of the solenoid / n a magnetic piston pump, the resulting force acting on the piston magnetic force can be controlled.

The application of the method for operating a Reduktionsmitteldosiersystems can be done alternatively to the development of the invention also in a generic Reduktionsmitteldosiersystem according to the prior art. However, it is also possible to apply the method for operating a reducing agent metering system cumulatively in a reducing agent metering system developed according to the invention. The control of the cylinder coil (s) is preferably carried out such that the piston is slowed down during a delivery stroke at the level of the control bores via which hydraulic fluid is supplied. The control of the cylinder coil (s) is preferably carried out such that the piston is slowed down during a suction stroke at the level of the control bores via which hydraulic fluid is supplied. Preferably, the control of the / of the cylinder / n is such that the piston is decelerated to complete a delivery stroke before top dead center.

By slowing down the piston at the level of the control bores via which hydraulic fluid is supplied and discharged to the cylinder in which the piston is guided, the desired flow of the hydraulic fluid through the control bores is optimized, since at too high piston speeds in the intake stroke, an undesired suction of too much hydraulic fluid is to be observed from the diaphragm chamber during a suction stroke, while during the delivery cycle due to an excessive piston speed, an undesired delivery of hydraulic fluid into the diaphragm chamber. These unwanted effects both during a suction stroke and during a delivery stroke can be significantly reduced by a corresponding pulse-width-modulated control of the / of the cylindrical coil / s and a braking of the piston in the region of the control bores.

An unrestrained impact of the piston on the baffle plate to the end of a delivery stroke leads to a significant impulse in the pressure line of the metering system, with the result that due to this pulse, the inlet valve opens and an uncontrolled and unwanted promotion of reducing agent. This undesirable effect can be suppressed by braking the piston just before top dead center. By avoiding a hard impact of the piston on the baffle plate of the occurring pulse is reduced in the pressure line, so that an undesired further conveying of reducing agent after completion of the delivery stroke is omitted. To decelerate the piston, a corresponding reduction of the duty cycle of the current supply of the / the cylindrical coil / n. An embodiment of the invention is illustrated in the figures and will be explained in more detail below. In the drawing: a schematic representation of a

reductant dosing; an enlarged view of the dosing system of Figure 1 in section. the bidirectional rubber valve according to Figures 1 and 2 in different views; a schematic representation of the feed pump of Figure 2 in section. the section of the feed pump during the phase B of a delivery stroke of FIG. 9; the section of the feed pump during the phase C of a delivery stroke of FIG. 9; the section of the feed pump during the phase D of a delivery stroke of FIG. 9; the section of the feed pump during the phase E of a delivery stroke of FIG. 9; the course of the piston travel s and the PWM duty cycle of the control of the feed pump over the time t during a delivery cycle of the feed pump;

1 shows a schematic representation of the reducing agent metering system 10 for injecting a reducing agent into the exhaust gas flow of an internal combustion engine (not shown) for selective catalytic reduction. By means of the feed pump 20 30 reducing agent solution is sucked from the tank 40 and conveyed via the pressure line 50 to the injection nozzle 60 via the suction line. About the injection nozzle 60, the reducing agent is injected into the exhaust stream of the engine. According to the invention, a bidirectional rubber valve 70 is arranged in the suction line 30.

An enlarged view of the metering system of FIG. 1, Fig. 2 in section. In this case, the pump 20 is arranged on a component carrier 15. Integrated into the component carrier 15 are the integrated suction line 30 'and the integrated pressure line 50'. The pump 20 and the component carrier 15 are adapted to one another in such a way that the suction line 30 'integrated in the component carrier 15 opens directly into the pump inlet of the pump 20 and, furthermore, the pump outlet of the pump 20 discharges directly into the pressure line 50' integrated in the component carrier 15.

Furthermore, FIG. 2 shows the valve seat 16 integrated in the component carrier 15 for receiving the bidirectional rubber valve 70. The bidirectional rubber valve 70 is fixed in the valve seat by the valve carrier 31, which in turn is an integral part of the suction line 30. The suction line 30 is connected to the tank, not shown in FIG. The valve carrier 31 is sealed against the valve seat 16 in the component carrier 15 by means of the sealing surface of the bidirectional rubber valve and additionally by means of an O-ring 35.

FIG. 3 shows the bidirectional rubber valve 70 in a plan view and the section AA. Recognizable in the plan view of the rubber valve 70 is the opening slot 71. When a corresponding pressure difference across the bidirectional rubber valve 70 is applied, the opening slot 71 opens automatically. Conversely, the opening slit 71 closes automatically due to its restoring forces in the moment when the required pressure difference across the bidirectional rubber valve 70 falls below the required opening pressure. The arrangement of this bi-directional rubber valve 70 in the suction line 30 of the metering 10 leads to a damping of the reducing agent promotion in particular to the end of a delivery stroke, since without this integrated bidirectional rubber valve 70 of the pulse in the pressure line 50 of the metering would cause undesirable Nachförderung of reducing agent from the tank , By integrated into the suction line 30 on the suction side of the pump 20 valve 70, this unwanted Nachförderung is reliably prevented. In Fig. 4 is a view of the feed pump 20 is shown in section. In the feed pump 20 is an electromagnetic piston pump in which the piston 21 by the construction of a corresponding magnetic field by means of a solenoid 22, that is moved by the magnetic field generated by a solenoid 22 and the resulting force acting on the piston 21 magnetic force becomes.

The drive of the piston 21 by a corresponding control of the cylindrical coil 22 takes place in the extension direction of the piston 21, that is, during a delivery stroke of the piston 21 in the direction of the top dead center. The piston 21 is reset by means of a spring 23, as can be seen in FIG.

The space surrounding the piston with the cylindrical coil is filled with hydraulic fluid, wherein a lubrication of the piston and the filling of the displacement takes place via corresponding control bores 24. The actual delivery volume of the pump 20 is formed by the cylinder volume 25th

The suction takes place via the inlet channel 26. The pushing out takes place via the outlet channel 27. The inlet channel 26 and the outlet channel 27 cover the membrane 28. The control of the cylindrical coil 22 as a function of the piston position, piston speed and direction of movement will be explained below with reference to the other figures. FIGS. 4 to 8 show the respective piston positions during a delivery cycle, which correspond to the individual phases A to G according to FIG.

In a 100 percent control of the solenoid during a first phase A starting from the bottom dead center of the piston, the bias and the construction of a strong magnetic field by 100 percent driving the solenoid takes place. This results in a large acceleration of the piston starting from the bottom dead center UT at the beginning of a delivery stroke, as can be seen from the position of the piston 21 shown in Fig. 4. At the beginning of phase A, the piston 21 is at the bottom dead center UT. With improved control by means of a pulse width modulation of the current supply only the current i is set in the phase A shown in Figure 9 with a large pulse width modulation, which is necessary to adjust the magnetic field so that the piston 21 is in motion with only slight acceleration ,

The first phase A with 10O per cent control is followed by a second phase B at high speed of the piston. During phase B, the hydraulic fluid is to be displaced into the control bores 24 of the pump provided for this purpose, as shown in FIG. 5 and indicated schematically by the arrows 24 '. During this movement phase B during the delivery stroke, the hydraulic fluid normally flows not only through the control bores 24 provided in front of it, but also a part also towards the diaphragm chamber. In order to avoid that the hydraulic fluid is driven in front of the piston, instead of draining into the control bores 24 provided therefor, the piston is kept at a low speed in the reduced duty cycle pulse width modulation movement phase B illustrated in FIG. 9, as by the smaller pitch the path course s of the piston over the Time t in Fig. 9 can be seen. The position of the piston at the level of the control bores 24 during the phase B is indicated accordingly in the sectional drawing of FIG. 5. Also entered in Fig. 5 is the height level 25 'corresponding to the control edge 25' of the control bores 24, in which the actual cylinder volume of the piston pump begins and is pressed as a delivery volume to the diaphragm chamber.

After the piston has swept over the control bores 24 and their control edges 25 'and reaches the level of the level 25' at the beginning of the actual cylinder volume 25, as indicated in FIG. 6, the phase C according to FIG. 9. In this phase C, in turn, a control of the solenoid with a higher duty cycle, so that the piston is accelerated again, as can be read from the path-time diagram of Figure 9 of the piston movement and the also entered in Fig. 9 duty cycle can be seen. During the movement phase C, the actual delivery of the cylinder volume 25 of the pump 20 takes place.

An unrestrained impact of the piston 21 on the baffle plate at top dead center OT would cause a significant pulse in the pressure line 50 of the metering system. This impact and also the resulting pulse would in turn be so large in the dosing line that the inlet valve opens and reducing agent flows across the pump until the pulse has dissipated. This would lead to an undesirable Nachförderung and thus to an uncontrolled dosage.

To prevent this effect, a hard impact of the piston 21 is prevented at the baffle plate at top dead center OT, that during the movement phase C at the end of the delivery stroke of the piston is decelerated, as shown in the path-time diagram of the piston movement of FIG. 9 is recognizable. The deceleration of the piston 21 before the top dead center OT is again at the end of the movement phase C characterized in that the duty cycle in the pulse width modulated control of the solenoid extremely on a Minimum is reduced. This can be inferred from the course of the duty cycle plotted in FIG. 9 during the time interval T4.

After reaching the top dead center OT, the piston 21 is moved back to the bottom dead center by means of the spring 23, whereby a corresponding suction of reducing agent via the suction line 30 takes place.

The phase D, indicated by the position of the piston 21 according to FIG. 7 at the top dead center OT or illustrated in FIG. 9, begins when the piston has reached the top dead center OT.

During the return travel of the piston in phase E by the spring force of the return spring, hydraulic fluid is sucked out of the diaphragm chamber during the stroke volume travel. Since too fast a piston movement when returning to the bottom dead center UT in the region of the control bores 24 results in further hydraulic fluid being sucked from the diaphragm and not only from the control bores 24, as shown in FIG. 8 by the arrows 24 " Here, in phases E and F of FIG. 9, the piston is also decelerated during its return by the spring force of the return spring by a corresponding pulse width modulated control of the solenoid in the control bores 24, as can be seen in Fig. 9 during the movement phase E and F. 9 clearly shows the lower piston speed at the level of the control bores 24 from the lower control edge 25 'of the control bores Hydraulic fluid from the control bores 24, as indicated by the arrows 24 "in Fig. 8 schematically. The entire cycle of movement is shown in FIG. 9 in a general overview. In the upper part of Fig. 9 is shown the path-time diagram of the piston stroke s over the time t during an entire delivery and Ansaugzyklus. Entered are the bottom dead center UT, the top dead center OT and the control edge 25 'of the control bores 24 of the pump. recognizable in the path-time diagram are the sections A - D during the delivery stroke and subsequently the phase E to G corresponding to the intake stroke of the pump. The corresponding pulse width modulated control of the solenoid, ie the respective change of the duty cycle, is out of phase with respect to the individual motion phases A to G, since on the one hand the corresponding magnetization must be done and further it requires a certain reaction time until the piston reacts to the changed magnetic field accordingly. For this purpose, the respective duty cycle in the control of the solenoid coil is recognizable in the lower part of Fig. 9, that is, it is the course of the switch-on or shutdown in the energization of the solenoid of the piston pump shown. During a first time period T1, the required magnetization occurs with a high duty cycle resulting in a correspondingly small acceleration and low movement speed of the piston during phase A. This is followed by a constant low speed of the piston by a drive with a reduced duty cycle during a second time period 12 in the movement phase B on. This is followed by a drive with a slightly increased duty cycle during the period T3 approximately corresponding to the movement phase C, that is, during the actual promotion of the reducing agent followed by a control of the solenoid with a minimum duty cycle during a period T4 to brake the piston at the end the movement phase C before reaching the top dead center OT. At the top dead center OT in the period T5 in the phase D, the duty cycle is increased again to prevent a collision and to ensure a secure holding of the piston. Starting from the top dead center OT, the piston is moved back by the spring force of the return spring in the direction of the bottom dead center UT, wherein during a subsequent period T6 with a minimum duty cycle, the magnetic influence of the piston is almost omitted, whereby only slightly limits the speed of the piston becomes. In the following period T7 becomes with increased duty cycle a braking of the piston at the level of the control bores 24 starting at the height of the control edge 25 'brought about. This is followed by another period of time T8, in which the piston is again kept at a low speed with a somewhat reduced duty cycle, and is returned to the bottom dead center UT. Shortly before the impact of the piston at bottom dead center UT, the piston is braked again with an increased duty cycle in the time period T9 in order to prevent an impact here as well. After the end of the cycle, that is to say after the end of the time period T9, the delivery cycle starts again from the beginning after a corresponding pause time, depending on the dosing quantity requirement.

As a result of this variable pulse-width-modulated actuation of the solenoid of the piston pump, the piston is moved slowly past and past the control bores 24. As a result, less hydraulic fluid is delivered to the membrane and sucked back from the membrane. This results in a straighter delivery characteristic over the pump back pressure, i. the system becomes counterpressure-independent. Since this control slows the piston, the period of a piston stroke is greater than at a constant unmodulated control of the solenoid. The maximum possible delivery frequency decreases as a result. Since it has been shown that at high frequency, a back pressure in the pressure line of the metering system is built up by the high flow rate, the possibility for reducing the respective duty cycles in the individual periods is given in the vicinity of the maximum delivery frequency of the pump. Thus, the maximum delivery frequency can be achieved. The displayed values of the duty cycles, times and paths are only symbolic.

The special pulse width modulated control increases the dosing accuracy of a diaphragm or piston pump. Further, we reduce the conveying speed of the reducing agent during a delivery stroke of a diaphragm or piston pump by the special pulse width modulated control and thus the formation of a finer spray is given with a two-fluid nozzle.

Claims

claims

1. Reduktionsmitteldosiersystem (10) for injecting a reducing agent in the exhaust stream of an internal combustion engine for selective catalytic reduction, with a feed pump (20) by means of which reducing agent from a reducing agent tank (40) via a suction line (30) from the tank (40) sucked and conveyed via a pressure line (50) and via at least one nozzle (60) is introduced into the exhaust gas stream of the internal combustion engine, characterized in that the suction line (30) has a bidirectional rubber valve (70).

2. dosing system (10) according to claim 1, characterized in that the pressure line (50) has a throttle.

3. dosing system (10) according to claim 1 or 2, characterized in that the system (10) has a component carrier (15) on which the feed pump (20) is mounted and wherein in the component carrier (15) has a suction channel (30 '. ) are integrated to the pump inlet and a pressure channel (50 ') adjoining the pump outlet, wherein the component carrier (15) has a mounting region (16) in which the valve (70) is fluidly mounted in the suction channel (30').

4. dosing system (10) according to any one of the preceding claims, characterized in that the system (10) has a reducing agent tank (40) to which the suction line (30) is connected.

5. dosing system (10) according to one of the preceding claims, characterized in that the system has a compressed air supply and the reducing agent is atomized inside or outside the nozzle by means of compressed air, in particular that the compressed air supply has a switching valve and / or a pressure control valve.

6. dosing system (10) according to any one of the preceding claims, characterized in that it is at the nozzle (60) is an externally mixing two-fluid nozzle in which emerges from at least a first opening reducing agent and exiting compressed air from at least one second opening, wherein the second opening is positioned in such a way, in particular at an angle to the jet direction of the first opening, that the reducing agent emerging from the first opening is atomized by means of the compressed air emerging from the second opening.

7. dosing system (10) according to any one of the preceding claims, characterized in that the pressure line (50) is connected via a switching valve or control valve to a compressed air supply to the pressure line (50) and the nozzle (60) after completion of the dosing by means of compressed air to free from reducing agent.

8. dosing system (10) according to one of the preceding claims, characterized in that it is in the feed pump (20) is a piston pump, in particular a magnetic piston pump.