METERING SYSTEM FOR A LIQUID MEDIUM, PARTICULARLY A UREA-WATER SOLUTION

Abstract

The invention relates to a dosing system for a liquid reduction medium enabling precise dosing of the reduction medium in the exhaust gas system of an internal combustion engine. This is carried out particularly by using a quick and safe closing of a pressure-actuated dosing valve. Simultaneously, the dosing system according to the invention also enables the dissipation of the pressure impulses resulting due to the closing movement of the dosing valve.

Description

PRIOR ART

The emissions limit values for nitrogen oxides, in motor vehicles whose weight exceeds a certain limit, require exhaust gas posttreatment devices, which perform selective catalytic reduction (SCR) of the nitrogen oxides contained in the raw emissions from the internal combustion engine. This so-called SCR method for exhaust gas cleaning is known from the prior art, and a detailed explanation of the chemical processes taking place in this method can therefore be dispensed with.

One example of this kind of exhaust gas posttreatment device is known from German Patent Disclosure DE 10 2006 012 855 A1. In it, an aqueous urea solution is stored in a tank and metered as needed by a metering pump and with the aid of a metering valve into an exhaust tube of the engine. The metering valve known from DE 10 2006 012 855 A1 is pressure-actuated. This means that it opens as soon as a predetermined opening pressure at the inlet to the metering valve is exceeded. As soon as the opening pressure is undershot, the metering valve closes again.

The pressure-actuated metering valve is a “passive” component, which does not require its own triggering. Signal lines or control lines from a control unit to the metering valve are not required, either.

DISCLOSURE OF THE INVENTION

It is the object of the invention to further develop a metering system of the type defined at the outset such that the consumption properties, especially the precision with which reducing agent is metered into the exhaust tube, are improved. Moreover, the tightness of the metering valve should be increased, both during operation and after the shutoff of the engine.

This object is attained according to the invention in a metering system for a liquid medium, in particular a liquid reducing agent, such as an aqueous urea-water solution, having a tank, having a metering pump, and having a metering valve, a pressure side of the metering pump and the metering valve communicating with one another through a first line, and the tank and an intake side of the metering pump communicating with one another through a second line, is attained in that a first check valve is provided in the first line.

The first check valve prevents the first line from emptying between two actuations of the metering valve. The consequence of such emptying would be that in the ensuing actuation of the metering pump, the quantity of urea-water solution specified by an engine control unit would not be metered in. As a result, the quality of the exhaust gas posttreatment would suffer, and emissions would rise.

In a further advantageous feature of the invention, it is provided that a reciprocating motion of the metering pump is transmitted at least partially to the valve member of the first check valve.

This means that beginning at a state of repose of the metering pump, the metering pump first executes a pumping stroke and pumps the liquid reducing agent into the first line. In a pumping stroke, the valve member of the first check valve opens as soon as the pressure in the metering pump is greater than the opening pressure of the first check valve, and the pumping of reducing agent to the metering valve begins.

As a result of the transmission according the invention of the reciprocating motion of the metering pump to the valve member, it is in fact ensured that the first check valve remains open during the intake stroke, regardless of the pressure conditions. As a consequence, a predetermined quantity of urea-water solution is reaspirated from the first line back into the metering pump, so that a controlled, rapid pressure reduction takes place in the first line and in the metering valve. As a result, the metering valve closes quickly and tightly, so that the metering of the reducing agent takes place with greater precision.

Just before the metering pump has reached its outset point, and the stroke of the metering pump is equal to zero, the reciprocating motions of the metering pump and of the valve member are uncoupled from one another, so that the first check valve can perform its function known from the prior art and hydraulically disconnects the first line from the metering pump.

Finally, the coupling of the valve member and the stroke of the metering pump makes venting of the metering system easier. When a compressible medium, such as air or vapor, is located in a pumping chamber of the metering pump, it can be ensured by the compulsory opening of the first check valve that the air or vapor located in the pumping chamber of the metering pump is expelled into the first line, and as a result venting of the metering pump takes place. As soon as there is only liquid reducing agent present in the metering pump, the metering pump now pumps fully again, and also, over the course of time, it pushes the vapor bubbles or air bubbles present in the intake line out of the metering system through the metering valve.

To achieve secure closure and better operating performance, it is also provided that the first check valve is spring-loaded.

In a further feature of the invention, it is also provided that the first check valve is embodied as a double-acting check valve, and that a second valve seat of the first check valve communicates hydraulically with the tank via a connecting line. This prevents an unwanted new opening of the metering valve after the metering pump has been shut off.

When liquid reducing agent is being delivered by the metering pump, the first check valve opens, and the reducing agent from the metering pump reaches the first line. After the pumping and injection of the reducing agent by the metering valve into the exhaust system, the first check valve closes again. As a result of the closure of the metering valve, a pressure surge occurs in the first line, which is reflected by a single-acting check valve and travels back to the metering valve. There, the pressure surge can trip a brief opening of the metering valve. As a consequence, a not-insignificant quantity of the reducing agent is metered unintentionally into the exhaust pipe. This effect increases with increasing closing speed of the metering valve.

Now if the first check valve is embodied as a double-acting check valve, then during the pumping of the metering pump, the check valve closes a connecting line between the first line and the tank of the metering system. In this position, the metering system behaves no differently from a metering system according to the invention with a single-acting check valve.

However, once the pumping stroke of the metering pump has ended, the valve member of the double-acting check valve is pressed back into the valve seat again, and the hydraulic communication between the metering pump and the metering valve is interrupted. Simultaneously, via the connecting line, a hydraulic communication is made between the first line and the pressureless tank, so that a pressure surge originating at the metering valve is not reflected at the first check valve but rather travels to the tank and is dissipated there. As a result, unwanted after-dribbles inside the metering system that are caused by pressure surges are avoided.

In a further advantageous feature of the invention, it is provided that a pressure maintenance valve, in particular an adjustable pressure maintenance valve, is provided in the connecting line. The opening pressure of the pressure maintenance valve is selected such that it is lower than the opening pressure of the metering valve, since only then can the reflection of pressure surges be reliably avoided. However, because of the maintenance pressure of the pressure maintenance valve, it is possible to suppress the vapor bubble formation inside the first line or inside the metering system and also as a result to further improve the function and the precision with which the reducing agent is metered in and metered.

Further advantages and advantageous features of the invention can be learned from the ensuing drawings, the description, and the claims. All the characteristics disclosed in the drawings, their description, and the claims can be essential to the invention both individually and in arbitrary combination with one another.

FIG. 1 shows the schematic layout of a metering system according to the invention; and

FIGS. 2 and 3 show exemplary embodiments of metering systems according to the invention.

In FIG. 1, an internal combustion engine 1 with an exhaust gas posttreatment device 3 is shown highly simplified and schematically. The exhaust gas posttreatment device 3 includes an exhaust tube 5, an oxidation catalytic converter 7 and an SCR catalytic converter 11. The flow direction of the exhaust gas through the exhaust tube 5 is indicated by arrows (without reference numerals). To supply the SCR catalytic converter 11 with reducing agent, a metering valve 13 for the reducing agent is disposed on the exhaust tube 5, upstream of the SCR catalytic converter 11. The metering valve 13 injects reducing agent as needed into the exhaust tube 5 upstream of the SCR catalytic converter 11.

The metering system of the invention includes the metering valve 13, a metering pump 15, and a storage container 17. The metering pump 15 is shown in FIG. 1 only as a “black box”. Details of it will be explained below in conjunction with FIGS. 2 and 3.

Between the metering pump 15 and the metering valve 13, a first line 19 is provided. Between the tank and the metering pump 15, a second line 21 is provided.

For the sake of completeness, reference should also be made to the sensors disposed in the exhaust system, namely an NOX sensor 25, as well as temperature sensors 23 and 27. These sensors 23, 25 and 27 communicate with an engine control unit 29 via signal lines (without reference numerals). This control unit 29 controls the engine 1 and among other things the metering pump 15 as well. The line connection between the control unit 29 and the metering pump 15 is represented by a dashed-line arrow (without a reference numeral) in FIG. 1.

A first exemplary embodiment of a metering system of the invention will be described and explained in conjunction with FIG. 2. Identical components are provided with the same reference numerals, and what has been said with regard to FIG. 1 applies accordingly. The metering valve 13 is schematically shown as a spring-loaded valve.

The metering pump 15 is embodied as a diaphragm pump with a diaphragm 31. The diaphragm 31 of the metering pump 15 is actuated by an electromagnet, which includes a coil 33 and an armature 35. When current is supplied to the coil 33, the armature 35 moves up in FIG. 2, and with it the diaphragm 31 also moves upward, and the armature executes a pumping stroke. The stroke H is shown in FIG. 2. The position of repose of the diaphragm 31 and of the armature 35 shown in FIG. 2 corresponds to a stroke H=0.

A first check valve 39 is disposed between a pumping chamber 37 of the metering pump 15 and the first line 19. The first check valve 39 is shown in FIG. 2 as a reed valve, including a valve member 41 and a valve seat 43. In these reed valves, the valve member 41 is embodied as an elastic diaphragm comprising plastic, such as EPDM, or metal. A spring 44 exerts a force in the closing direction of the first check valve 39 on the valve member 41.

The reciprocating motion of the diaphragm 31 is partially transmitted to the valve member 41 via a tappet 45. This happens as a result of the fact that in the position of repose of the diaphragm 31, and with the first check valve 39 closed, the tappet 45 does not rest on the diaphragm 31.

Between the end of the tappet 45 toward the diaphragm 31 and the diaphragm 31 itself, there is a distance H_MIN in this situation. As soon as the stroke H is greater than H_MIN, the diaphragm, with the aid of the tappet 45, lifts the valve member 41 from the valve seat 43. This coupling of the valve member 41 with the stroke H of the diaphragm 31 of the metering pump causes the first check valve 39 to remain open for a long time during the intake stroke, regardless of the pressure conditions in the first line 19 and in the pumping chamber 37. As a consequence, because of the suction motion of the diaphragm 31, some of the reducing agent located in the first line 19 is reaspirated back into the pumping chamber 37. As a result, a pressure reduction takes place in the line 19, and the metering valve 13 closes quickly and tightly.

Between the pumping chamber 37 and the second line 21, there is a second check valve 46, which prevents reducing agent from being able to flow back out of the pumping chamber 37 into the second line 21 and the tank 17 during the pumping stroke of the metering pump 15. As a rule, this second check valve 46 is not spring-loaded. It is often likewise embodied as a reed valve, although in FIGS. 2 and 3 it is shown symbolically as a ball valve.

In FIG. 3, a further exemplary embodiment of a metering system of the invention is shown. In this exemplary embodiment, the first check valve 39 is embodied as a double-acting check valve and is shown symbolically as a ball valve, although as a rule it is a reed valve or poppet valve. The first check valve 39 has not only the first valve seat 43 but a second valve seat 47 as well, whose outlet communicates hydraulically with the tank 17 via a connecting line 49 and the second line 21. As soon as the metering pump 15 begins to pump reducing agent, the pressure in the pumping chamber 37 rises above the pressure prevailing in the first line 19. As a consequence, the valve member 41 lifts away from the first valve 43 and closes the connecting line 49, because it is pressed sealingly against the second valve seat 47. At the same time, the hydraulic communication between the pumping chamber 37 and the first line 19 is opened up, and the pumping of reducing agent into the first line 19 begins. The pumping stroke in the second exemplary embodiment proceeds in precisely the same way as described in conjunction with the first exemplary embodiment of FIG. 2.

Now once the pumping has ended and the diaphragm 31 has returned to its outset position, the pressure conditions in the first check valve 39 ensure that the valve member 41 is pressed against the first valve seat 43 again, and as a result, the hydraulic communication between the pumping chamber 37 and the first line 19 is interrupted. At the same time, a hydraulic communication is opened between the first line 19, the connecting line 49, the second line 21, and the tank 17, so that any pressure surges originating in the metering valve 13 proceed to the tank 17 and are dissipated there.

Optionally, a pressure maintenance valve 51 is provided in the connecting line 49. The pressure maintenance valve 51 serves to maintain an adjustable minimum pressure in the first line 19, so that the boiling point of the reducing agent in the first line 19 is raised, and the formation of vapor bubbles is suppressed. Advantageously, the opening pressure of the pressure maintenance valve 49 is lower than the opening pressure of the metering valve 13, so that the closing motion and sealing off of the metering valve 13 are not hindered by the pressure maintenance valve 51. Moreover, pressure surges whose amplitude is greater than the opening pressure of the pressure maintenance valve 51 can proceed onward virtually unhindered into the tank 17 and can be dissipated there.

In an especially advantageous feature of the invention, the pressure maintenance valve 51 is adjustable with regard to its maintenance pressure, so that it can be adjusted to suit the prevailing operating conditions. As a result, it is possible, for instance after the shutoff of the engine, to change the maintenance pressure, so that the first line 19 is completely pressure-relieved, and as a result, the metering valve 13 is also completely tight over a longer period of time. If the risk of vapor bubble formation arises during operation of the engine, the maintenance pressure of the pressure maintenance valve 51 can be increased accordingly.

In FIG. 3b, a block circuit diagram of the exemplary embodiment of FIG. 3a is shown. In addition, various pressure sensors 53.1, 53.2 and 53.3 are shown, which are each installed as needed and connected to the control unit 29.

Claims

1-10. (canceled)

11. A metering system for a liquid medium, in particular a liquid reducing agent, such as an aqueous urea-water solution the metering system, having a tank, a metering pump, and a metering valve, a pressure side of the metering pump and the metering valve communicating with one another through a first line, and the tank and an intake side of the metering pump communicating with one another through a second line, wherein a first check valve is provided in the first line.

12. The metering system as defined by claim 11, wherein a reciprocating motion of the metering pump is transmitted at least partially to a valve member of the first check valve.

13. The metering system as defined by claim 12, wherein the metering pump includes a diaphragm and the reciprocating motion of the diaphragm is transmitted at least partially to the valve member of the first check valve.

14. The metering system as defined by claim 13, wherein the valve member of the first check valve is lifted from a first valve seat as soon as the reciprocating motion of the metering pump exceeds a minimum stroke.

15. The metering system as defined by claim 11, wherein the first check valve is spring-loaded.

16. The metering system as defined by claim 12, wherein the first check valve is spring-loaded.

17. The metering system as defined by claim 13, wherein the first check valve is spring-loaded.

18. The metering system as defined by claim 14, wherein the first check valve is spring-loaded.

19. The metering system as defined by claim 11, wherein the first check valve is embodied as a double-acting check valve, and a second valve seat of the first check valve communicates hydraulically with the tank or the intake side of the metering pump via a connecting line.

20. The metering system as defined by claim 18, wherein the first check valve is embodied as a double-acting check valve, and a second valve seat of the first check valve communicates hydraulically with the tank or the intake side of the metering pump via a connecting line.

21. The metering system as defined by claim 19, wherein a pressure maintenance valve, in particular an adjustable pressure maintenance valve, is provided, in the connecting line.

22. The metering system as defined by claim 20, wherein a pressure maintenance valve, in particular an adjustable pressure maintenance valve, is provided in the connecting line.

23. The metering system as defined by claim 11, wherein a second check valve is provided in the second line.

24. The metering system as defined by claim 12, wherein a second check valve is provided in the second line.

25. The metering system as defined by claim 13, wherein a second check valve is provided in the second line.

26. The metering system as defined by claim 11, wherein the metering valve is pressure-actuated.

27. The metering system as defined by claim 12, wherein the metering valve is pressure-actuated.

28. The metering system as defined by claim 13, wherein the metering valve is pressure-actuated.

29. The metering system as defined by claim 11, wherein the metering pump is actuated by a final control element of an actuator, in particular by an armature of an electromagnet.

30. The metering system as defined by claim 12, wherein the metering pump is actuated by a final control element of an actuator, in particular by an armature of an electromagnet.