CN106460608B

CN106460608B - Reductant metering system with inhibited reductant delivery

Info

Publication number: CN106460608B
Application number: CN201580022569.0A
Authority: CN
Inventors: 恩佐·法尔博; W·奥弗霍夫
Original assignee: Albonair GmbH
Current assignee: Albonair GmbH
Priority date: 2014-07-28
Filing date: 2015-05-20
Publication date: 2020-07-03
Anticipated expiration: 2035-05-20
Also published as: WO2016015792A1; DE102014010948A1; CN106460608A

Abstract

The invention relates to a reducing agent metering system (10) for injecting a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, comprising a delivery pump (20), by means of which the reducing agent of a reducing agent tank (40) is sucked out of the tank (40) via a suction line (30), delivered 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

Reductant metering system with inhibited reductant delivery

The invention relates to a reducing agent dosing system (Reduktionmitteldosiersystem) for injecting a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, having a delivery pump, by means of which the reducing agent in a reducing agent tank is sucked out of the tank via a suction line, is delivered via a pressure line and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle.

The invention further relates to a method for operating a reducing agent metering system for injecting a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, having a magnetic piston pump as a delivery pump, by means of which the reducing agent in a reducing agent tank is sucked out of the tank via a suction line, delivered via a pressure line, and introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, wherein the piston is set in motion by actuating one or more cylindrical coils.

Catalysts for selective catalytic reduction, so-called SCR Catalysts (SCR), are used for reducing nitrogen oxide emissions from diesel engines, combustion plants, waste incineration plants, industrial plants, etc. For this purpose, a metering device is used to inject a reducing agent into the exhaust system. Ammonia or ammonia solutions or other reducing agents are used as reducing agents.

Since it is not safe to carry ammonia in vehicles, aqueous urea solutions with a urea concentration of typically 32.5% are used, in particular according to the DIN 70070 standard. When the temperature exceeds 150 ℃, the urea in the exhaust gas is decomposed into ammonia and CO in gaseous state₂. The parameters of urea decomposition are mainly time (vaporization time and reaction time), temperature and droplet size of the injected urea solution. Among these SCR catalysts, the emission of nitrogen oxides is reduced by about 90% by Selective Catalytic Reduction (SCR).

In known reducing agent metering systems for injecting a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, a reducing agent solution is supplied to an injection nozzle by means of a magnetic piston pump. It is shown here that an uncontrolled resupply of reducing agent occurs as a result of the momentum in the supply line after the supply stroke of the magnetic piston pump. It has furthermore been shown that, due to the high piston speed during the delivery stroke, the hydraulic fluid cannot flow out of the cylinder as desired via the regulating bore, but rather is delivered as early as before the closing of the regulating bore, as a result of which an excess of reducing agent is delivered due to the increased deflection of the delivery membrane, which makes precise metering difficult.

Furthermore, it has been shown that, due to the high piston speed after the delivery stroke, hydraulic fluid is not supplied to the cylinder and the piston via the regulating bore as desired, but rather is also sucked out of the diaphragm chamber, whereby excess reducing agent is sucked in, which makes precise metering difficult.

The object of the present invention is to further develop a reducing agent metering system of the aforementioned type and to provide a method for operating a reducing agent metering system in such a way that precise metering of the reducing agent is possible and undesired overdischarging or resupply after a delivery stroke is effectively prevented.

According to the invention, this object is achieved by a reducing agent metering system as claimed in claim 1. Advantageous developments of the invention are specified in the corresponding dependent claims.

A reducing agent metering system for injecting a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, having a delivery pump, by means of which the reducing agent in a reducing agent tank is sucked out of the tank via a suction line, is delivered via a pressure line and is introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, it being particularly preferred in such a reducing agent metering system that the suction line has a two-way rubber valve.

In particular, the reducing agent metering system can have a pump unit with a delivery pump. In addition to the feed pump, the pump unit can also have further components, in particular a directional control valve and/or a sensor.

The term suction line is used here to denote a delivery line from the reducing agent tank to the suction opening of the delivery pump. A bidirectional rubber valve is arranged in this suction line. The bi-directional rubber valve is a passive component that automatically opens and releases the flow path when a corresponding pressure differential occurs between the two sides of the bi-directional rubber valve, and in turn automatically closes the flow path when the pressure differential is less than the pressure differential required across the bi-directional rubber valve. This effectively prevents an undesired resupply of reducing agent due to an impulse in the delivery line after the delivery stroke of the delivery pump.

The term pressure line is used here to denote a delivery line into which the pressure side of the delivery pump opens and via which the reducing agent is delivered from the pump to the nozzle.

The terms reductant metering system or metering system are used synonymously in the present invention. The term reducing agent solution or reducing agent encompasses the various reducing agents suitable for selective catalytic reduction, preference being given here to using urea solutions according to DIN 70070. The invention is not so limited.

In a preferred embodiment, the pressure line has a throttle. The results show that the arrangement of a two-way rubber valve in the suction line in combination with a throttle valve in the pressure line makes it possible to meter the reducing agent particularly precisely and effectively prevents undesirable oversupply or re-delivery after the delivery stroke.

Preferably, the system has a component carrier on which the delivery pump is mounted and in which a suction channel leading to the pump inlet and a pressure channel connected to the pump outlet are integrated, wherein the component carrier has an assembly region in which the rubber valve is fluidically connected

Is integrally mounted in the suction channel.

A pump and a bidirectional rubber valve can be arranged particularly advantageously in the suction channel by means of a component carrier of this type. It is possible here to preassemble components arranged on and/or on the component carrier into an assembly. This makes the assembly of the metering system simpler and saves the necessary installation space by combining several components into a space-saving assembly.

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

Particularly preferably, the system has a compressed air supply and the reducing agent is atomized with compressed air inside or outside the nozzle. The compressed air supply can have a directional control valve and/or a pressure control valve. The switching valve is used for controlling, i.e. for switching on and off, the compressed air supply for the entire or parts of the metering system.

Alternatively or additionally, the compressed air supply can have a pressure regulating valve. The compressed air can thus be set to a desired pressure level for atomizing the reducing agent by means of the compressed air. The compressed air itself can originate, for example, from an on-board compressed air system of a commercial vehicle in whose exhaust line a metering system is arranged, in which the prevailing system pressure does not pose a limitation, since the pressure of the compressed air can be reduced to the desired pressure.

In a preferred embodiment of the reducing agent metering system, therefore, a compressed air supply is provided, with the reducing agent being atomized by means of compressed air inside or outside the nozzle. A mixing chamber may be provided for atomizing the reducing agent, which is carried out with compressed air in the mixing chamber as early as before the introduction into the exhaust passage. In a preferred embodiment, however, the nozzle is designed as an externally mixed two-component nozzle, wherein the reducing agent solution flows out of the first nozzle opening and the compressed air is discharged from the second nozzle opening, wherein the two nozzle openings are 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 mixed two-component nozzle and the formation of an aerosol outside the nozzle is achieved. In particular, the second opening of the nozzle is positioned, in particular, at an angle to the injection direction of the first opening of the nozzle in such a way that the reducing agent flowing out of the first opening is atomized by means of compressed air discharged from the second opening.

Preferably, the pressure line is connected to the compressed air supply via a reversing valve or a regulating valve in order to remove the reducing agent of the pressure line and the nozzle by means of compressed air after metering has ended.

In a preferred embodiment of the reducing agent metering system, therefore, a compressed air supply is provided, wherein the pressure line, and thus also the nozzle, which conveys the reducing agent is connected to the compressed air supply via a directional valve, in order to remove the reducing agent from the pressure line and the nozzle after metering has ended by means of compressed air.

The pressure line and the nozzle and/or the metering chamber and/or the metering line for the reducing agent can thus be freed of the reducing agent solution after the metering has ended, by means of compressed air, in order to prevent the reducing agent solution from freezing or crystallizing. Thereby effectively preventing freezing and clogging.

Alternatively or additionally, the reducing agent in the exhaust system is atomized with compressed air and used to clean the line conveying the reducing agent after metering has ended.

Preferably, a pressure sensor is used to detect and monitor the pressure in the pressure line. The correct operation and metering of the delivery pump can also be continuously monitored by said detection and monitoring of the pressure in the pressure line delivering the reducing agent.

Preferably, the system has a heating device for heating the reducing agent solution. The reducing agent solutions customary according to DIN 70070 freeze at temperatures around-11 ℃ owing to their water content. It is therefore necessary, for example, to provide heating devices in the tank and/or in the thermal coupling (thermischer kopplong) to the suction line and/or the pressure line for heating the reducing agent solution at very low ambient temperatures.

Preferably, the delivery pump is a piston pump, in particular a magnetic piston pump. One or more cylindrical coils are arranged in the magnetic piston pump, by means of which a magnetic field is generated. The piston movement of the magnetic piston is controlled by means of a magnetic field generated by means of a coil.

It is particularly advantageous in magnetic piston pumps that they can be operated and controlled precisely by corresponding control of the coils generating the magnetic field.

In a method for operating a reducing agent metering system for injecting a reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, which metering system has a magnetic piston pump as a delivery pump, by means of which the reducing agent from a reducing agent tank is sucked out of the tank via a suction line, delivered via a pressure line and introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle, a magnetic field is generated by means of a cylinder coil, the piston is moved by actuating one or more cylinder coils, and it is provided that the one or more cylinder coils are controlled by pulse width modulation depending on the current position, speed and direction of movement of the piston.

The term control cylinder coil here means that a variable supply of current is applied to the coils over time, whereby a resulting magnetic field is generated in each case, which magnetic field exerts a resulting force on the magnetic piston. The resulting magnetic force moves the piston back and forth in the cylinder of the magnetic piston pump between the top dead center and the bottom dead center and thus accelerates or brakes the piston.

Duty ratioRatio (a)

I.e., the ratio of on time to off time for powering the cylindrical coil during a cycle) is varied by pulse width modulated control of the cylindrical coil, thereby generating a time varying resultant magnetic field of the cylindrical coil. As a result, a resulting magnetic force acting on the piston as a function of time is then generated again, and a variable piston acceleration or piston speed is generated as a result. The arithmetic mean value of the voltage can be varied by varying the duty ratio (Tastgrad). The resulting magnetic force acting on the piston can thus be controlled by pulse-width-modulated control of the cylindrical coil of the magnetic piston pump.

In addition to the development according to the invention, a method for operating a reducing agent metering system can also be used in a reducing agent metering system of this type according to the prior art. However, it is also possible to increase the use of the method for operating the reducing agent metering system in the reducing agent metering system improved according to the invention.

The control of the cylindrical coil is preferably effected in such a way that during the delivery stroke the piston is decelerated to the height of the regulating bore via which the hydraulic fluid is introduced.

The control of the cylindrical coil is preferably effected in such a way that during the suction stroke the piston is decelerated to the height of the regulating bore via which the hydraulic fluid is introduced.

The control of the cylindrical coil is preferably effected in such a way that the piston is braked before top dead center in order to end the delivery stroke.

By decelerating the piston to the height of the adjusting bore, via which the hydraulic fluid is introduced into or out of the cylinder, in which the piston is introduced, the desired flow of hydraulic fluid via the adjusting bore is optimized, since in the event of an excessively high piston speed, an undesired suction of excess hydraulic fluid from the diaphragm chamber during the suction stroke is observed in the suction stroke, while in the delivery stroke an undesired delivery of hydraulic fluid into the diaphragm chamber occurs due to an excessively high piston speed. These undesirable effects in the suction stroke and in the delivery stroke can be significantly reduced by the corresponding pulse-width-modulated control of the cylindrical coil and the braking of the piston in the region of the adjusting bore.

The unbraked impact of the piston against the impact plate at the end of the delivery stroke causes a considerable impulse in the pressure line of the metering system, with the result that, as a result of this impulse, the inlet valve opens and an uncontrolled and undesired delivery of the reducing agent takes place. This undesirable effect can be suppressed by braking the piston near top dead center. The momentum occurring in the pressure line is reduced by avoiding a hard impact of the piston on the impact plate, so that an undesired further supply of reducing agent does not occur after the end of the supply stroke. A corresponding reduction of the duty cycle of the cylindrical coil power supply to brake the piston is achieved.

Embodiments of the invention are illustrated in the figures and described in more detail below. The figures show that:

FIG. 1 shows a schematic view of a reductant metering system;

fig. 2 shows an enlarged view of the metering system according to fig. 1 in a sectional view;

fig. 3 shows the bidirectional rubber valve according to fig. 1 and 2 in different views;

fig. 4 shows a diagrammatic view of the delivery pump according to fig. 2 in a sectional view;

fig. 5 shows a sectional view of the delivery pump during phase B of the delivery stroke according to fig. 9;

fig. 6 shows a sectional view of the delivery pump during phase C of the delivery stroke according to fig. 9;

fig. 7 shows a sectional view of the delivery pump during stage D of the delivery stroke according to fig. 9;

fig. 8 shows a sectional view of the delivery pump during a phase E of the delivery stroke according to fig. 9;

fig. 9 shows a graph for controlling the piston stroke s and the PWM duty cycle of the delivery pump over the time t during the delivery stroke of the delivery pump.

Fig. 1 shows a schematic representation of a reducing agent metering system 10 for injecting a reducing agent into the exhaust gas flow of a combustion engine, not shown, for selective catalytic reduction. The reducing agent solution is sucked out of the tank 40 by means of the delivery pump 20 via the suction line 30 and is delivered to the nozzle 60 via the pressure line 50. The reducing agent is injected into the exhaust gas flow of the internal combustion engine via the injection nozzle 60. According to the invention, a bidirectional rubber valve 70 is arranged in the suction line 30.

Fig. 2 shows an enlarged view of the metering system according to fig. 1 in a sectional view. The pump 20 is arranged here on the component carrier 15. The integrated suction line 30 'and the integrated pressure line 50' are integrated in the component carrier 15. The pump 20 and the component carrier 15 are matched to one another in such a way that the suction line 30 'integrated in the component carrier 15 leads directly to the pump inlet of the pump 20 and, in addition, the pump outlet of the pump 20 leads directly to the pressure line 50' integrated in the component carrier 15.

Fig. 2 also shows a valve seat 16 integrated in the component carrier 15 for receiving a bidirectional rubber valve 70. The two-way rubber valve 70 is held in a valve seat by a valve support 31, which is an integral part of the suction line 30. The suction duct 30 is connected to a tank, not shown in fig. 2. The valve carrier 31 is sealed off from the valve seat 16 in the component carrier 15 by the sealing surfaces of the two-way rubber valve and additionally by an O-ring seal 35.

Fig. 3 shows the bidirectional rubber valve 70 in a top view and the section a-a. The opening gap 71 can be seen in a top view of the rubber valve 70. The opening slit 71 opens automatically when a corresponding pressure difference is applied to the bidirectional rubber valve 70. Conversely, when the necessary pressure difference across the bidirectional rubber valve 70 is lower than the necessary opening pressure, the opening gap 71 automatically closes due to its restoring force.

The arrangement of the two-way rubber valve 70 in the intake line 30 of the metering system 10 leads to a suppression of the reducing agent delivery, in particular at the end of the delivery stroke, since without the integrated two-way rubber valve 70, an impulse in the pressure line 50 of the metering system could lead to an undesired re-delivery of reducing agent from the tank. Such an undesired re-delivery is effectively prevented by the valve 70 integrated into the suction line 30 on the suction side of the pump 20.

Fig. 4 shows a sectional view of the delivery pump 20. The delivery pump 20 is an electromagnetic force piston pump in which the piston 21 is moved by the establishment of a corresponding magnetic field by means of a solenoid 22, i.e. by the magnetic field generated by means of a cylindrical coil 22 and the magnetic force acting on the piston 21 resulting therefrom.

The driving of the piston 21 is achieved by correspondingly controlling the cylindrical coil 22 in the direction of the discharge stroke of the piston 21, i.e. in the direction towards the top dead center during the delivery stroke of the piston 21. As can be seen in fig. 4, the piston 21 is reset by means of a spring 23.

The space surrounding the piston with the cylindrical coil is filled with hydraulic fluid, wherein the lubrication of the piston and the filling of the stroke space take place via the respective adjusting bore 24. The actual delivery volume of the pump 20 is established by the cylinder volume 25.

Suction is achieved via the inlet channel 26. The discharge is effected via an outlet channel 27. A membrane 28 covers the inlet channel 26 and the outlet channel 27.

The control of the cylindrical coil 22 as a function of piston position, piston velocity and direction of movement is described below with reference to the other figures.

The individual piston positions during the delivery cycle can be seen in fig. 4 to 8, which correspond to the individual phases a to G according to fig. 9.

When the cylindrical coil is controlled one hundred percent from the bottom dead center of the piston in the first stage A, the cylindrical coil is controlled one hundred percent to realize pre-magnetization and establish a strong magnetic field. This results in a large acceleration of the piston at the beginning of the delivery stroke, starting from bottom dead center UT, which can be seen by the position of piston 21 shown in fig. 4. At the beginning of phase a, the piston 21 is at bottom dead center UT. By improving the control by means of pulse width modulation of the supplied current, in phase a shown in fig. 9 only the required current i is now set by means of a large pulse width modulation, in order to set the magnetic field in such a way that the piston 21 moves with only a small acceleration.

The first phase a, which involves one hundred percent control, is followed by the second phase B, which has a greater piston velocity. During phase B, the displacement of hydraulic fluid into the regulating bore 24 of the pump provided for this purpose is to be effected, as shown in fig. 5 and schematically indicated by the arrow 24'. During the movement phase B during the delivery stroke, the hydraulic fluid usually flows not only through the preset adjusting opening 24, but also partially into the diaphragm chamber. In order to avoid driving hydraulic fluid ahead of the piston instead of flowing out into the regulating bore 24 provided for this purpose, the piston is kept at a low speed in the movement phase B shown in fig. 9 by means of a reduced pulse-width-modulated duty cycle, as can be seen in fig. 9 by the lower slope of the piston travel curve s over the time t. During phase B, the position of the piston at the height of the adjustment bore 24 is correspondingly shown in the sectional view according to fig. 5. In fig. 5, too, the height plane 25 'is recorded, at which the actual cylinder volume of the piston pump starts and is pressed as the delivery volume against the membrane chamber, as a function of the adjusting edge 25' of the adjusting opening 24.

After the piston has passed beyond the adjusting opening 24 and its adjusting edge 25 'and reached the beginning of the actual cylinder volume 25 at the level of the plane 25', as shown in fig. 6, then in the following time curve is phase C according to fig. 9. In phase C, it is again achieved that the cylindrical coil is controlled with a higher duty cycle, so that the piston is accelerated again, as can be seen from the travel-time diagram of fig. 9 of the piston movement and likewise from the duty cycle recorded in fig. 9. During the movement phase C, the actual delivery of the cylinder volume 25 of the pump 20 is achieved.

The unbraked impact of the piston 21 on the impact plate in the upper dead center OT causes a large impulse in the pressure line 50 of the metering system. This collision and the resulting impulse can be so great in the metering line that the inlet valve opens and the reducing agent flows past the pump until the impulse is eliminated. This may cause undesirable re-delivery and thus uncontrolled metering.

In order to suppress this effect, a hard impact of the piston 21 against the impact plate in the upper dead center OT is prevented in that the piston is braked during the movement phase C when the delivery stroke ends, as can be seen in the stroke-time diagram of the piston movement according to fig. 9. Before the top dead center OT, the brake piston 21 is again actuated at the end of the movement phase C in such a way that the duty cycle is reduced to a minimum value when the cylindrical coils are controlled by pulse width modulation. This can be learned from the curve of the duty ratio during the time interval T4 plotted in fig. 9.

After reaching the top dead center OT, the piston 21 is returned to the bottom dead center by means of the spring 23, as a result of which the reducing agent is accordingly sucked out via the suction line 30.

Phase D begins when the piston reaches the top dead center OT, which is shown by the position of the piston 21 according to fig. 7 at the top dead center OT or in fig. 9.

When the piston returns in phase E due to the spring force of the return spring, hydraulic fluid is sucked out of the diaphragm chamber during the working volume stroke. Since the excessively rapid piston movement in the region of the adjusting bore 24 on return to the bottom dead center UT leads to hydraulic fluid continuing to be sucked out of the diaphragm and not just out of the adjusting bore 24, this is illustrated by the arrow 24 ″ in fig. 8, where in phases E and F according to fig. 9 the piston is also braked in the region of the adjusting bore 24 during its return stroke due to the return spring force by correspondingly pulse-width-modulated control of the cylindrical coil, which can be seen in fig. 9 during the movement phases E and F. Fig. 9 clearly shows that the piston speed is lower at the height of the adjusting bore 24 from the lower adjusting edge 25' of the adjusting bore. Once the piston has passed beyond the regulation orifice, a smooth aspiration of hydraulic fluid from the regulation orifice 24 is achieved by slowing the piston speed with a smaller duty cycle in phase F, which is schematically illustrated in fig. 8 by arrow 24 ".

The complete movement cycle is illustrated in fig. 9 in a general overview. In the upper part of fig. 9, a stroke-time diagram of the piston stroke s over the time t during the entire delivery cycle and suction cycle is shown. The bottom dead point UT, the top dead point OT and the adjusting edge 25' of the adjusting orifice 24 of the pump are recorded. In the stroke-time diagram, sections a to D during the delivery stroke and phases E to G following this, which correspond to the suction stroke of the pump, can be seen.

The cylindrical coil is controlled in a pulse-width-modulated manner, i.e. the duty cycle is changed each time, by shifting the phase relative to the individual movement phases a to G, since, on the one hand, a corresponding magnetization must take place and, in addition, a certain reaction time is required until the piston responds correspondingly to the changing magnetic field. For this purpose, the respective duty cycle when controlling the cylindrical coil can be seen in the lower part of fig. 9, i.e. a curve showing the on-time or off-time when the cylindrical coil of the piston pump is supplied with current.

The necessary magnetization is achieved with a high duty cycle during the first period T1, resulting in a correspondingly smaller acceleration and lower movement speed of the piston during phase a. A constant low speed of the piston is achieved immediately during a second period T2 in the movement phase B by control with a reduced duty cycle. The control then takes place with a slightly increased duty cycle during a period T3, for example in the respective movement phase C, i.e. during the actual delivery of the reducing agent, after which the cylinder coil is controlled with the smallest duty cycle during a period T4 for braking the piston at the end of the movement phase C before the top dead center OT is reached. In period T5 of phase D, the duty cycle is again increased at top dead center OT to prevent collision and ensure that the piston is held firmly. Starting from the top dead center OT, the piston is returned in the direction of the bottom dead center UT by the spring force of the return spring, wherein the magnetic influence on the piston is almost suppressed during the subsequent period T6 with a minimum duty cycle, as a result of which the speed of the piston is only approximately limited. In the subsequent time period T7, the braking of the piston at the height of the adjusting bore 24 starting at the height of the adjusting edge 25' is brought about with an increased duty cycle. This is followed by a further period T8 in which the piston is again kept at a lower speed with a slightly reduced duty cycle and returns to bottom dead center UT. When the piston is just about to touch bottom dead center UT, the piston is braked again in time period T9 with an increased duty cycle in order to prevent a collision here too. After the cycle has ended, i.e. after the time period T9 has ended, the conveying cycle is started again from the beginning after the corresponding pause time has elapsed according to the metering requirements.

The piston is slowly guided to and through the regulating orifice 24 by the variable pulse width modulated control of the solenoid of the piston pump. Thereby delivering a small amount of hydraulic fluid to and from the membrane. This results in a straighter delivery profile against the pump back pressure, i.e. the system is less dependent on the back pressure. Since this control slows down the piston, the cycle time of the piston stroke is longer than when the cylindrical coil is controlled constantly unmodulated. Thereby reducing the maximum possible delivery frequency. Since the results show that at high frequencies, high delivery volumes result in back pressure in the pressure line of the metering system, there is the possibility of reducing the respective duty cycle in each time interval near the maximum delivery frequency of the pump. Whereby a maximum transmission frequency can be reached. The values of duty cycle, time and travel shown are merely symbolic.

The metering accuracy of a membrane pump or a piston pump is increased by a specific pulse width modulated control. Furthermore, the reducing agent delivery rate is also reduced by means of a specific pulse-width-modulated control during the delivery stroke of the membrane pump or piston pump, and is therefore considered to form a finer spray through the two-component nozzle.

Claims

1. A reducing agent metering system (10) for injecting reducing agent into the exhaust gas flow of an internal combustion engine for selective catalytic reduction, the metering system (10) having a delivery pump (20) by means of which reducing agent of a reducing agent tank (40) is sucked out of the reducing agent tank (40) via a suction line (30), delivered via a pressure line (50) and introduced into the exhaust gas flow of the internal combustion engine via at least one nozzle (60), characterized in that the suction line (30) has a bidirectional rubber valve (70) and the delivery pump (20) is a piston pump and suction is effected via an inlet channel (26), discharge is effected via an outlet channel (27) and the inlet channel (26) and the outlet channel (27) are covered by a membrane (28), wherein the metering system has a compressed air supply, and atomizing reducing agent by means of compressed air outside the nozzle, wherein the nozzle (60) is an externally mixed two-component nozzle in which reducing agent flows out of at least one first opening and compressed air is discharged from at least one second opening, wherein the second opening is positioned so as to be placed at an angle with respect to the injection direction of the first opening, such that reducing agent flowing out of the first opening is atomized by means of the compressed air discharged from the second opening.

2. The metering system (10) of claim 1, wherein the pressure conduit (50) has a throttle valve.

3. The metering system (10) as claimed in claim 1, characterized in that the metering system (10) has a component carrier (15) on which the delivery pump (20) is mounted, and in that a suction channel (30 ') leading to a pump inlet and a pressure channel (50 ') connected to a pump outlet are integrated in the component carrier (15), wherein the component carrier (15) has a fitting region (16) in which the valve (70) is flow-through integrally mounted in the suction channel (30 ').

4. The metering system (10) as claimed in claim 2, characterized in that the metering system (10) has a component carrier (15) on which the delivery pump (20) is mounted, and in that a suction channel (30 ') leading to a pump inlet and a pressure channel (50 ') connected to a pump outlet are integrated in the component carrier (15), wherein the component carrier (15) has a fitting region (16) in which the valve (70) is flow-through integrally mounted in the suction channel (30 ').

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

6. Metering system (10) according to one of claims 1 to 4, characterized in that the pressure line (50) is connected to the compressed air supply via a reversing or regulating valve in order to remove reducing agent by means of compressed air from the pressure line (50) and the nozzle (60) after metering has ended.

7. Metering system (10) according to claim 5, characterized in that the pressure line (50) is connected to the compressed air supply via a reversing or regulating valve in order to remove the reducing agent by means of compressed air from the pressure line (50) and the nozzle (60) after metering has ended.

8. The metering system (10) of claim 1, wherein the delivery pump (20) is a magnetic piston pump.

9. Metering system (10) according to claim 1, characterized in that the compressed air supply has a reversing valve and/or a pressure regulating valve.