Method for adjusting a pressure sensor in an SCR system

Abstract

A method for adjusting a pressure sensor of an SCR system between a delivery pump and a dosing valve, with which the delivery pump is switched off during a switch-off phase of the SCR system and a pressure (p.sub.m) measured by the pressure sensor is detected at least during a section of the switch-off phase in which the delivery pump is switched off, and the adjustment of the pressure sensor is carried out by using the measured pressure (p.sub.m) as the reference pressure for the pressure sensor.

Claims

1. A method for adjustment of a relative pressure sensor (22) of an SCR system between a delivery pump (21) and a dosing valve (32), with which the delivery pump (21) is switched off (101) during a switch-off phase (100) of the SCR system and a pressure (p.sub.m) measured by the relative pressure sensor (22) is detected (104) at least during a section of the switch-off phase (101) in which the delivery pump (21) is switched off, and the adjustment of the relative pressure sensor is carried out by using (105) the measured pressure (p.sub.m) as an ambient pressure for the relative pressure sensor (22), wherein the relative pressure sensor that measures a relative pressure (p) between a pressure of the SCR system and the ambient pressure and the adjustment of the relative pressure sensor (22) is carried out by using (105) the measured pressure (p.sub.m) as the zero point for the relative pressure.

2. A computer program that is arranged to carry out each step of the method according to claim 1.

3. A machine-readable memory medium, on which a computer program according to claim 1 is stored.

4. An electronic control unit (40) that is arranged to adjust the relative pressure sensor (22) by means of a method according to claim 1.

5. The method according to claim 1, characterized in that in a return suction mode, a reduction agent solution is transported back from the SCR system to a tank.

6. A method for adjustment of a pressure sensor (22) of an SCR system between a delivery pump (21) and a dosing valve (32), with which the delivery pump (21) is switched off (101) during a switch-off phase (100) of the SCR system and a pressure (p.sub.m) measured by the pressure sensor (22) is detected (104) at least during a section of the switch-off phase (101) in which the delivery pump (21) is switched off, and the adjustment of the pressure sensor is carried out by using (105) the measured pressure (p) as a reference pressure for the pressure sensor (22), characterized in that the measured pressure (p.sub.m) for the adjustment is detected when the gradient (G.sub.p) of a pressure profile (200) of the pressure sensor lies above a first threshold (S.sub.1).

7. The method according to claim 6, characterized in that the gradient (G.sub.p) of the pressure profile (200) is formed (102) during an adaptation phase.

8. A computer program that is arranged to carry out each step of the method according to claim 6.

9. A machine-readable memory medium, on which a computer program according to claim 6 is stored.

10. An electronic control unit (40) that is arranged to adjust the pressure sensor (22) by means of a method according to claim 6.

11. The method according to claim 6, wherein the reference pressure is an ambient pressure.

12. The method according to claim 6, characterized in that in a return suction mode, a reduction agent solution is transported back from the SCR system to a tank.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the invention are represented in the drawings and described in detail in the following description.

(2) FIG. 1 shows an SCR system with a pressure sensor in a pressure line, for which adjustment is carried out by an embodiment of the method according to the invention.

(3) FIG. 2 shows a flow chart of an embodiment of the method according to the invention.

(4) FIG. 3 shows a diagram of the pressure recorded by the pressure sensor, the actuation of a delivery pump and the actuation of a dosing valve against time, with which the adjustment of the pressure sensor by an embodiment of the method according to the invention is illustrated.

DETAILED DESCRIPTION

(5) An SCR system for an SCR catalytic converter (not shown) of a motor vehicle is shown in FIG. 1. The SCR system contains a reduction agent tank 10 for the reduction agent solution, a delivery module 20 and a dosing module 30. From the reduction agent tank 10, a suction line 11 leads to a delivery pump 21 of the delivery module 20. The liquid reduction agent solution passes via a filter 25 and an outlet in a pressure line 31 to a dosing valve 32 of the dosing module 30 and is metered in a demand-dependent manner into the exhaust gas system of a combustion engine (not shown) upstream of the SCR catalytic converter. Moreover, a return line 12 with a choke 23 leads from the delivery module 20 back into the reduction agent tank 10. Consequently, the SCR system is connected to the surroundings via said return line 12. A 4/2-way valve 24, by means of which the transport direction of the delivery pump 21 can be adjusted, is disposed on the delivery pump 21. In a delivery mode, the delivery pump 21 transports the reduction agent solution from the reduction agent tank 10 to the dosing module 30. In a return suction mode, the reduction agent solution is transported back from the SCR system via the suction line 11 into the reduction agent tank 10.

(6) Furthermore, a pressure sensor 22 is provided on the pressure side of the delivery pump 21 between said pump and the dosing valve 30. In one exemplary embodiment, the pressure sensor 22 is disposed in the delivery module 20 and is associated with the filter 25. The pressure sensor 22 is embodied here as a relative pressure sensor and measures a relative pressure p of the SCR system relative to the ambient pressure. In a further exemplary embodiment, the pressure sensor 22 can also be embodied as an absolute pressure sensor that measures the absolute pressure in the SCR system. The pressure sensor 22 is connected to an electronic control unit 40 and sends the measured pressure values thereof to said electronic control unit 40. The electronic control unit is further connected to the delivery pump 21, the 4/2-way valve 24 and the dosing valve 32 and can control said devices.

(7) FIG. 2 shows a flow chart of an embodiment of the method according to the invention. The SCR system is in a switch-off phase 100, in which the delivery module 20, the pressure line 31 and the dosing module 30 are emptied in order to protect said components against damage due to freezing of the reduction agent solution. According to the invention, the delivery pump 12 is switched off 101 during said switch-off phase 100. Following this, a waiting time t.sub.A is waited, in which the pressure in the pressure line 31 reduces from an overpressure and essentially equalizes with the ambient pressure. A profile 200 of the pressure p measured by the pressure sensor 22 against time t is shown in FIG. 3 and is described in detail in the associated description. During an adaptation time t.sub.A, the gradient G.sub.p of the pressure p is formed 102. For this purpose, the pressure difference p that occurs during said adaptation time t.sub.A is recorded by means of the pressure sensor 22 and is divided by the adaptation time t.sub.A. In other exemplary embodiments, the time derivative of the pressure p is formed in order to calculate the gradient G.sub.p. Because the pressure p is decreasing, the gradient G.sub.p is negative. In a comparison 103, the gradient G.sub.p is compared with a first threshold S.sub.1. If the gradient G.sub.p lies above the first threshold S.sub.1, it is assumed that the pressure p has essentially equalized with the ambient pressure. In a further exemplary embodiment, the magnitude of the gradient G.sub.p of the pressure p is formed and then compared with a second threshold in the comparison of the magnitude of the gradient G.sub.p. The second threshold S.sub.2 can for example be the positive value of the first threshold S.sub.1. The pressure sensor 22 now measures 104 the prevailing pressure. Said measured pressure p.sub.m would indicate the ambient pressure as the reference pressure in the case of an ideal pressure sensor 22 without any defects. In the case of a faulty pressure sensor 22, the measured pressure p.sub.m indicates a deviation A from the reference value. The measured pressure p.sub.m is finally used 105 as a new reference pressure for said pressure sensor 22. In other words, the erroneous value of the measured pressure p.sub.m is specified as the new reference value and the pressure sensor 22 is readjusted as a result.

(8) FIG. 3 shows a diagram of a profile 200 of the pressure p that is recorded by the pressure sensor 22 against time t during the switch-off phase 100 of the SCR system. The pressure sensor 22 is embodied as a relative pressure sensor and measures a relative pressure p of the SCR system relative to ambient pressure. On the scale of the pressure, 0 consequently represents the ambient pressure as the reference pressure. In addition, the actuation U.sub.1 of the delivery pump 21 and the actuation U.sub.DV of the dosing valve 32 are represented in said diagram. The actuation U.sub.FP of the delivery pump 21 is scaled in percentage form, wherein 0% means that the delivery pump 21 is switched off. To put it simply, the actuation U.sub.DV of the dosing valve 32 comprises two states 0 and 1, wherein the state 0 means that the dosing valve 32 is closed, and the state 1 means that the dosing valve 32 is open.

(9) At the start, there is an overpressure in the pressure line 31 that results from the operation of the SCR system that is carried out before the switch-off phase 100. As already described, the pressure p initially decreases and essentially equals the ambient pressure. The profile 200 of the pressure p basically describes an exponential decrease. For a detailed description of the following steps, refer to the flow chart in FIG. 2. Relative to the start of the switch-off phase 100, after approx. 0.8 seconds at a first point in time t.sub.1 the delivery pump 21 is stopped 101. A waiting time t.sub.w of approx. 4 seconds is waited, in which the pressure p decreases further. At a second point in time t.sub.2 at approx. 4.8 seconds, an adaptation phase is started, in which the gradient G.sub.p of the pressure p is formed over the adaptation time t.sub.A of approx. 5 seconds 102. It can be seen in said diagram that because of the exponential decrease of the pressure p the gradient G.sub.p is always negative and the magnitude of the gradient G.sub.p decreases at least until the end of the adaptation time t.sub.A. Consequently, the gradient G.sub.p adopts ever decreasing negative values while the pressure p is decreasing, i.e. in other words it is larger. It can be seen that the profile 200 of the pressure p hardly changes in the adaptation phase, in particular at the end thereof, which indicates that the ambient pressure has essentially been reached. If the gradient G.sub.p is above the first threshold S.sub.1, the measured pressure p.sub.m is finally determined 104 and is approx. 50 bar (relative pressure) in this example. The adaptation time t.sub.A can in principle be extended arbitrarily in order to ensure that the gradient G.sub.p exceeds said first threshold S.sub.1. The pressure p.sub.m measured by the pressure sensor 22 has a deviation A of approx. 50 hPa from the actual ambient pressure, which had to be p=0. In order to remove said deviation A, the measured pressure p.sub.m is used 105 as the new reference value for the pressure sensor 22, therefore as the ambient pressure. In other words, the scale for the pressure p is shifted. According to one exemplary embodiment, a characteristic curve of the pressure sensor 22 can be changed by means of an offset with the magnitude of the deviation A for this purpose.

(10) At a third point in time t.sub.3 at approx. 9.8 seconds, the delivery pump 12 is switched on 25 and the pressure p decreases further, so that a reduced pressure is formed in the pressure line. Said reduced pressure is detected by determining a pressure difference p.sub.d from the measured pressure p.sub.m with the delivery pump 21 switched off. I.e., the pressure difference p.sub.d is a pressure difference from the reference pressure. Because in the event of differences of the pressure both the minuend and the subtrahend are affected by the changed reference pressure, the two equalize and the pressure difference remains unchanged, so the real pressure does not have to be known exactly. Thus, this method is suitable both for a relative pressure sensor and for an absolute pressure sensor. If only pressure differences are used in the switch-off phase 100, the change in the characteristic curve can be omitted. If, as in this example, the pressure difference p.sub.d is approx. 250 hPa (between 50 hPa and 200 hPa), at a fourth point in time t.sub.4 after approx. 2.5 seconds, the reduced pressure is large enough for ventilation of the SCR system and the dosing valve 32 can be opened.