Expansion body and method for monitoring a pressure sensor in a SCR system with an expansion body

Abstract

The invention relates to an expansion body having an equalization chamber, which is separated by an elastic diaphragm from a fluid which flows through part of the expansion body. A mechanical spring applies an additional spring force to the diaphragm, wherein the spring has mechanical prestress. In addition, the invention relates to an SCR system with such an expansion body, and a method for monitoring a pressure sensor in such a SCR system with an expansion body. The method comprises the following steps: first, an anticipated characteristic pressure at which the spring force of the spring of the expansion body is overcome is defined by setting the prestress of the spring in the expansion body.

Claims

1. A SCR system comprising a feed module with a feed pump, a metering valve connected to the feed module by a pressure line, a pressure sensor, at least one return line, an expansion body having an equalization chamber separated by an elastic diaphragm from a fluid which flows through part of the expansion body, a mechanical spring configured to apply an additional spring force to the diaphragm, the spring having a mechanical prestress, and an electronic controller configured to: define an anticipated characteristic pressure at which the spring force of the spring of the expansion body is overcome, by setting the prestress of the spring in the expansion body; set an initial pressure in the SCR system above the anticipated characteristic pressure; stop the feed pump with the metering valve closed, when the initial pressure is reached; determine a pressure rate in the pressure line over time by means of the pressure sensor; determine a significant change in the pressure rate at which the change in the pressure rate is above a first threshold; determine a measured characteristic pressure at which the significant change in the pressure rate has been determined; and output a fault for the pressure sensor if the measured characteristic pressure is not between thresholds which are dependent on the anticipated characteristic pressure.

2. The SCR system according to claim 1, wherein the mechanical prestress of the spring is applied by means of a mechanical stop.

3. The SCR system according to claim 1, wherein the equalization chamber has a pressure-equalization opening.

4. The SCR system according to claim 1, wherein an orifice is arranged in the return line.

5. The SCR system according to claim 1, wherein a restrictor is arranged in the return line.

6. A method for monitoring a pressure sensor in an SCR system, the system including an expansion body arranged in a pressure line, at least one return line, the expansion body having an equalization chamber separated by an elastic diaphragm from a fluid which flows through a part of the expansion body, a mechanical spring configured to apply an additional spring force to the diaphragm, the spring having a mechanical prestress, the method comprising: defining an anticipated characteristic pressure at which a spring force of the spring of the expansion body is overcome, by setting the prestress of the spring in the expansion body; setting an initial pressure in the SCR system above the anticipated characteristic pressure; stopping a feed pump coupled with a metering valve connected to the feed pump closed, when the initial pressure is reached; determining a pressure rate in the pressure line over time by means of the pressure sensor; determining a significant change in the pressure rate at which the change in the pressure rate is above a first threshold; determining a measured characteristic at which the significant change in the pressure rate has been determined; and outputting a fault for the pressure sensor if the measured characteristic pressure is not between thresholds which are dependent on the anticipated characteristic pressure.

7. The method according to claim 6, wherein the pressure rate is a relative pressure rate.

8. The method according to claim 6, wherein the prestress of the spring is set in such a way that the anticipated characteristic pressure is outside a use range of the SCR system.

9. The method according to claim 8, wherein the prestress of the spring is set in such a way that the anticipated characteristic pressure is above a use range of the SCR system.

10. A non-transitory computer-readable medium storing instructions that, when executed by a processor of a computer, cause the computer to perform operations to control a system that includes an expansion body arranged in a pressure line, the expansion body having an equalization chamber separated by an elastic diaphragm from a fluid which flows through a part of the expansion body, at least one return line, a mechanical spring configured to apply an additional spring force to the diaphragm, the spring having a mechanical prestress, the operations comprising: defining an anticipated characteristic pressure at which a spring force of the spring of the expansion body is overcome, by setting the prestress of the spring in the expansion body; setting an initial pressure in the SCR system above the anticipated characteristic pressure; stopping a feed pump coupled, with a metering valve connected to the feed pump closed, when the initial pressure is reached; determining a pressure rate in the pressure line over time by means of the pressure sensor; determining a significant change in the pressure rate at which the change in the pressure rate is above a first threshold; determining a measured characteristic pressure at which the significant change in the pressure rate has been determined; and outputting a fault for the pressure sensor if the measured characteristic pressure is not between thresholds which are dependent on the anticipated characteristic pressure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

(2) FIG. 1 shows a schematic cross-sectional view through an expansion body according to an exemplary embodiment of the invention.

(3) FIG. 2 shows a schematic illustration of a reducing agent feed system with the expansion body according to an exemplary embodiment of the invention.

(4) FIG. 3 shows a flowchart of a method according to an exemplary embodiment of the invention.

(5) FIG. 4 shows a diagram of the measured pressure plotted against the actual pressure.

(6) FIG. 5 shows a diagram of a pressure, measured by the pressure sensor, for the SCR system from FIG. 2 plotted against the time.

(7) FIG. 6 shows a diagram of a relative pressure rate, determined from FIG. 5, plotted against the measured pressure.

DETAILED DESCRIPTION

(8) FIG. 1 shows a schematic cross-sectional view through an expansion body 1 of an equalization chamber 10 which is enclosed by a housing 11 except for an opening 12. A diaphragm 20, e.g. an elastomer diaphragm, closes off the opening 12 of the equalization chamber 10 with respect to a line 121 in an air-tight fashion. A pressure equalization opening 13, which connects the equalization chamber 10 to an ambient pressure p.sub.u, is formed in the housing 11. Furthermore, a spring 30, e.g. a spring steel helical spring, is formed in the equalization chamber 10, is prestressed by means of a stop 31 and applies a spring force F to the diaphragm 20. When the pressure p in the line 121 rises, a force which acts on the diaphragm 20 in the direction of the equalization chamber 10, acts until the acting force is larger in the case of a characteristic pressure than the spring force F. The diaphragm 20 is then pressed in the direction of the equalization chamber 10 and compresses the air located therein.

(9) A schematic illustration of a reducing agent feed system 100 of an SCR system (not illustrated) is shown in FIG. 2. It comprises a feed module 110 which has a feed pump 111 which is configured to feed reducing agent from a reducing agent tank 120 to a metering valve 130 via a pressure line 121, at which metering valve 130 the reducing agent is then sprayed into an exhaust gas section (not illustrated). The expansion body 1 (illustrated in FIG. 1) is arranged in the pressure line 121. Furthermore, a pressure sensor 140 is arranged in the reducing agent feed system 100 and configured to measure the pressure p at least in the pressure line 121 over a time period. An electronic controller 150 is connected to the pressure sensor 140 and receives therefrom information about the pressure p in the system 100. Furthermore, the electronic controller 150 is connected to the feed module 110, including the feed pump 111, as well as to the entire metering valve 130, and can control said components.

(10) Furthermore, the reducing agent feed system 100 comprises a return line 160 through which reducing agent is fed back into the reducing agent tank 120 from the system. In this return line 160, an orifice 161 is arranged which provides local flow resistance. In a further exemplary embodiment, a restrictor is arranged in the return line, instead of the orifice 161.

(11) FIG. 3 illustrates a flowchart of an exemplary embodiment of the method according to the invention for monitoring the pressure sensor 140 in the reducing agent feed system 100 with the expansion body 1. In a first step, an anticipated characteristic pressure p.sub.cv at which the spring force F of the spring 30 is overcome is defined 200 by setting the prestress of the spring 30 in the expansion body 1. FIG. 4 illustrates a suitable selection of the anticipated characteristic pressure p.sub.cv.

(12) FIG. 4 shows a pressure p.sub.m, measured by the pressure sensor 140, in the reducing agent feed system 100 plotted against the actual pressure p.sub.t. A use range 300 is indicated in which the reducing agent feed system 100 predominantly operates. The anticipated characteristic pressure p.sub.cv is defined 200 above this use range 300. By using separate monitoring of the pressure sensor 140 below the use range 300, for example at ambient pressure, the pressure sensor in the use range 300 is also monitored, without the prestress of the spring 30 in the expansion body 1 and the associated change in rigidity influencing the reducing agent feed system 100 in the use range 300. The illustrated angle bisector 310 represents an optimum pressure sensor 140, which designates the actual pressure p.sub.t precisely as the measured pressure p.sub.m. In addition to the angle bisector 310, an upper threshold S.sub.2 and a lower threshold S.sub.3 are illustrated which diverge from one another at a relatively high pressure. The thresholds S.sub.2 and S.sub.3 predefine a tolerance range 320 for the pressure sensor 140 and are dependent on the anticipated characteristic pressure p.sub.cv.

(13) In a further step of the flowchart from FIG. 3, said thresholds S.sub.2 and S.sub.3 are defined 201 for the anticipated characteristic pressure p.sub.cv, with the result that the tolerance range 320 is adapted to operating conditions. The anticipated characteristic pressure p.sub.cv and the thresholds S.sub.2 and S.sub.3 can be stored in the electronic controller 150, with the result that when the method for monitoring the pressure sensor 140 is repeated, said pressure p.sub.cv and thresholds S.sub.2 and S.sub.3 do not have to be redefined 200 and 201.

(14) The metering valve 130 is closed 202, subsequent to which an initial pressure P.sub.a above the anticipated characteristic pressure p.sub.cv is set by means of the feed pump 111. If it is determined in an interrogation 204 that the initial pressure p.sub.a has been reached, the feed pump 111 is stopped 205. The pressure p in the pressure line 121 is now decreased via the return line 160 and the orifice 161 arranged therein. The pressure sensor 140 measures 206 the pressure p in the pressure line 121 at this time, and passes on a measured pressure p.sub.m to the electronic controller 150.

(15) FIG. 5 illustrates the measured pressure p.sub.m plotted against the time t. At approximately five seconds, the pressure p reaches the initial pressure p.sub.a of approximately 5.2 bar, subsequent to which the feed pump 111 is stopped 205. In the reciprocal profile of the measured pressure p.sub.m, a significant pressure drop 400 from approximately 3.3 bar to approximately 2.4 bar is detected at approximately eight seconds. In another exemplary embodiment, the characteristic pressure p.sub.cm is determined from this significant change in the pressure rate.

(16) In this exemplary embodiment, a relative pressure rate is determined 207 from the measured pressure p.sub.m as a further step in the flowchart in FIG. 3. The relative pressure rate is defined for the orifice 161 according to formula 1a and for the restrictor according to formula 1b as follows:

(17) $\begin{array}{cc}:=-\frac{1}{\sqrt{p\left(t\right)}}.Math.\frac{\mathrm{dp}\left(t\right)}{\mathrm{dt}}& \left(\mathrm{Formula}1a\right)\\ :=-\frac{1}{p\left(t\right)}.Math.\frac{\mathrm{dp}\left(t\right)}{\mathrm{dt}}& \left(\mathrm{Formula}1b\right)\end{array}$

(18) In this context, dp(t)/dt specifies the change of the pressure p(t) over time, that is to say the pressure rate. The dependence of the pressure rate dp(t)/dt on the rigidity and a change in volume dV(t)/dt is specified by means of formula 2 and formula 3:

(19) $\begin{array}{cc}=V\frac{\mathrm{dp}\left(t\right)}{dV\left(t\right)}& \left(\mathrm{Formula}2\right)\\ \frac{\mathrm{dp}\left(t\right)}{\mathrm{dt}}=\frac{\mathrm{dp}\left(t\right)}{dV\left(t\right)}.Math.\frac{dV\left(t\right)}{dt}=\frac{1}{V}.Math.V.Math.\frac{\mathrm{dp}\left(t\right)}{dV\left(t\right)}.Math.\frac{dV\left(t\right)}{dt}=\frac{}{V}\frac{dV\left(t\right)}{dt}& \left(\mathrm{Formula}3\right)\end{array}$

(20) According to the continuity equation (formula 4), the change in volume dV(t)/dt over time takes place through the entire volume flow Q.sub.ges. In this case, the entire volume flow Q.sub.ges corresponds to the volume flow Q.sub.RL through the orifice 161.

(21) $\begin{array}{cc}\frac{dV\left(t\right)}{dt}=-Q_{\mathrm{ges}}=-Q_{RL}& \left(\mathrm{Formula}4\right)\end{array}$

(22) If the formulas 3 and 4 are inserted into the formula 1a and 1b, the dependence of the relative pressure rate on the rigidity is obtained in accordance with formula 5a for the orifice 161 and formula 5b for the restrictor:

(23) $\begin{array}{cc}:=\frac{1}{\sqrt{p\left(t\right)}}.Math.\frac{1}{V}.Math.Q_{RL}.Math.& \left(\mathrm{Formula}5a\right)\\ :=\frac{1}{p\left(t\right)}.Math.\frac{1}{V}.Math.Q_{RL}.Math.& \left(\mathrm{Formula}5b\right)\end{array}$

(24) The rigidity changes significantly, as described at the beginning for the characteristic pressure p.sub.c, and consequently indicates when the spring force F of the spring 30 has been overcome. According to the relationship from formulas 5a and 5b, between the rigidity of the relative pressure rate , the relative pressure rate changes significantly at the characteristic pressure p.sub.c. Such behavior is represented in FIG. 6.

(25) The relative pressure rate , which was determined 205 by means of formula 1a from FIG. 5, is illustrated in FIG. 6 in a diagram plotted against the measured pressure p.sub.m. The measured pressure p.sub.m decreases within the reducing agent feed system 100, in particular within the pressure line 121, starting from the right of the diagram. At approximately 3.3 bar, the relative pressure rate abruptly exceeds a first threshold S.sub.1, with the result that the significant change 401 in the relative pressure rate occurs at this point. It is apparent that the significant change 401 in the relative pressure rate in FIG. 6 can be seen significantly more clearly than the associated significant pressure drop 400 in FIG. 5. In the flowchart in FIG. 3, the significant change 401 in the relative pressure rate is determined 208, as just presented, if the relative pressure rate exceeds the first threshold S.sub.1. The first threshold S.sub.1 is selected according to the conditions in such a way that it is higher than operationally induced changes in the relative pressure rate , but low enough to cover the significant change 401 owing to the expansion body 1. The relative pressure rate depends, as presented above, on the rigidity . The rigidity in turn changes significantly if the spring force F of the prestressed spring 30 in the expansion body 1 has been overcome, and the diaphragm 20 is pressed in the direction of the equalization chamber 10. So that this takes place, the pressure p in the pressure line 121 must correspond to the characteristic pressure p.sub.c. Consequently, a measured characteristic pressure p.sub.cm at which the significant change 401 in the relative pressure rate has been determined 208, is determined 209, as clarified in FIG. 6.

(26) Then, a comparison 210 is carried out between the measured characteristic pressure p.sub.cm and the thresholds S.sub.2 and S.sub.3 which were defined 201 at the beginning, which depend on the anticipated characteristic pressure p.sub.cv and define the tolerance range 320. Reference is made in this respect also to FIG. 4. If the measured characteristic pressure p.sub.cm is above the upper threshold S.sub.2 or below the lower threshold S.sub.3, the measured characteristic pressure p.sub.cm and the anticipated characteristic pressure p.sub.cv do not correspond sufficiently and an error 211 is output for the pressure sensor 140. If the measured characteristic pressure p.sub.cm is, on the other hand, between the thresholds S.sub.2 and S.sub.3, an error 212 is not output.

(27) FIG. 4 illustrates by way of example two measured characteristic pressures p.sub.cm1 and p.sub.cm2 for an anticipated characteristic pressure p.sub.cv. The first measured characteristic pressure p.sub.cm1 is below the lower threshold S.sub.3 at the anticipated characteristic pressure p.sub.cv and therefore outside the tolerance range 320, with the result that the error 211 is output and the pressure sensor 140 is categorized as defective. The second measured characteristic pressure p.sub.cm2 lies directly on the angle bisector 310 for the anticipated characteristic pressure p.sub.cv and therefore represents an optimum pressure sensor 140, for which reason an error 212 is not output.